Rab3D localizes to zymogen granules in rat pancreatic acini and other exocrine glands

Abstract

Rab3 proteins are members of the family of Ras-like monomeric GTP-binding proteins that have been implicated in secretion in neuronal cells. Although an isoform of Rab3 has been assumed to exist in pancreatic acini, its identity has not yet been established. We now report that Rab3D is present in rat pancreatic acini and is localized to the zymogen granule membrane. Reverse transcription-polymerase chain reaction (PCR) was used with primers based on mouse Rab3D to amplify Rab3D from rat pancreas. The PCR product without primer sites consisted of 580 base pairs and was 94% identical to the mouse Rab3D cDNA sequence previously cloned from adipocytes. Western blotting with a polyclonal antiserum raised against Rab3D-specific carboxyterminal amino acids identified Rab3D in rat pancreatic acini and revealed its concentration on zymogen granule membranes. Immunocytochemistry of pancreatic lobules showed that Rab3D localized to the apical region in a pattern similar to amylase. Confocal fluorescence microscopy of lobules double immunolabeled with antibodies to Rab3D and the granule membrane marker protein glycoprotein-2 (GP-2) revealed a similar localization of these proteins to zymogen granules. Immunocytochemistry also revealed the presence of Rab3D in chief and enterochromaffin-like cells in the stomach, acinar cells in lacrimal and parotid gland, and Paneth cells in the intestine. These results show that Rab3D is expressed in rat pancreatic acini and other exocrine secretory cells. Its location implies it may be involved in regulated exocytosis.

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

“…In accord with the morphological changes, there was no amylase secretion from pancreatic acinar cells of Noc2 -/-mice in response to secretagogues. 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…”

Adenoviral capsid modulates secretory compartment organization and function in acinar epithelial cells from rabbit lacrimal gland

“…[25][26][27] Stimulation of lacrimal acini causes the loss of subapical rab3D concomitant with the fusion of mature secretory vesicles with the apical plasma membrane. 27 As shown in the triply-labeled nontransduced and Ad-GFP transduced lacrimal acini in Figure 3, rab3D (red) in resting, nontransduced acini is enriched in the subapical cytoplasm beneath the actin-enriched (blue) apical plasma membrane surrounding the lumen (*) in association with large (1-3 mm) vesicular structures (arrows).…”

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

“…In lacrimal acini, secretory products are sorted into immature secretory vesicles in the trans-Golgi network and acquire external proteins characteristic of mature secretory vesicles (SVs) as they move towards the apical membrane. Mature SVs localized in the subapical cytoplasm of lacrimal acini are enriched in the small GTPase, rab3D (Ohnishi et al, 1996;Wang et al, 2003). Although the precise function of rab3D in acinar secretion is still in question, its association with mature SVs is decreased concomitant with apical exocytosis (Wang et al, 2003).…”