Rab3D Is Not Required for Exocrine Exocytosis but for Maintenance of Normally Sized Secretory Granules

Abstract

Rab3D, a member of the Rab3 subfamily of the Rab/ypt GTPases, is expressed on zymogen granules in the pancreas as well as on secretory vesicles in mast cells and in the parotid gland. To shed light on the function of Rab3D, we have generated Rab3D-deficient mice. These mice are viable and have no obvious phenotypic changes. Secretion of mast cells is normal as revealed by capacitance patch clamping. Furthermore, enzyme content and overall morphology are unchanged in pancreatic and parotid acinar cells of knockout mice. Both the exocrine pancreas and the parotid gland show normal release kinetics in response to secretagogue stimulation, suggesting that Rab3D is not involved in exocytosis. However, the size of secretory granules in both the exocrine pancreas and the parotid gland is significantly increased, with the volume being doubled. We conclude that Rab3D exerts its function during granule maturation, possibly by preventing homotypic fusion of secretory granules.

Rab27b regulates exocytosis of secretory vesicles in acinar epithelial cells from the lacrimal gland

“…Our previous exploration of Rab3D, which is highly expressed in LG acinar cells, found that expression of DN Rab3D only modestly affected secretion and that CA Rab3D did not significantly alter secretion (14). Analogous to work published by Riedel et al in exocrine pancreas (38), our findings in LG from Rab3DKO mouse demonstrated a completely different series of morphological changes relative to those seen for 27bKO, most notably a significant increase in SV size and no other evidence for altered cellular homeostasis (unpublished data; Chiang L, Ngo J, and Hamm-Alvarez SF). Although Rab3D and Rab27b appear to co-localize on LG acinar cells SV (Fig.…”

“…Rabs, which cycle between GTPbound active states and GDP-bound inactive states as assisted by guanine nucleotide exchange factors or GTPase activating proteins, respectively, are implicated in protein trafficking and targeting and fusion of membranous organelles in all eukaryotic cell types (11,12,40). In acinar cells from the LG, pancreas, and parotid gland, the Rab3D isoform is enriched on mature SV and appears to serve as a negative regulator of homotypic fusion (38). In comparison, Rab27 proteins are related but distinct in primary sequence to Rab3 proteins and are one of few Rab proteins directly linked to a human disease, Griscelli Syndrome Type II (1,31).…”

Male NOD mouse external lacrimal glands exhibit profound changes in the exocytotic pathway early in postnatal development

“…Surprisingly, recent generation of a rab3D knockout mouse resulted in no detectable phenotype in release kinetics in the exocrine pancreas or parotid gland (Riedel et al, 2002). However, the size of the secretory granules in the pancreas and parotid gland of this knockout mouse were significantly increased, suggesting that rab3D may act by preventing premature homotypic fusion of mature SVs.…”

Cellular expression of Noc2, a Rab effector protein, in endocrine and exocrine tissues in the mouse

“…Since Rab3D, originally identified in fat cells, is rich in pancreatic and parotid gland acinar cells and gastric chief cells (Ohnishi et al 1996;Tang et al 1996;Valentijin et al 1996), these exocrine cells may well regulate secretion by a combination of Rab and its effector protein, in which Noc2 is a potent candidate for the Rab-binding protein. However, the genetic deletion of Rab3D in mice resulted in normal pancreatic amylase secretion and only a mild change in the morphology of the secretory granules of pancreatic acinar cells (Riedel et al 2002), suggesting that Rab3D…”