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The discovery of the porosome, its isolation, its structure and dynamics at …


Biology Articles » Cell biology » Discovery of the Porosome: revealing the molecular mechanism of secretion and membrane fusion in cells » Regulation of secretory vesicle swelling: Involvement in expulsion of vesicular contents

Regulation of secretory vesicle swelling: Involvement in expulsion of vesicular contents
- Discovery of the Porosome: revealing the molecular mechanism of secretion and membrane fusion in cells

In the last decade, the molecular mechanism of vesicle swelling [31-33] and its involvement in the regulated expulsion of vesicular contents [Jena, et al., unpublished observation] has been established. Secretory vesicle swelling is critical for secretion [34-37], however, the underlying mechanism of vesicle swelling was largely unknown until recently [31-33]. In mast cells, an increase in secretory vesicle volume after stimulation of secretion has previously been suggested from electrophysiological measurements [38]. However, direct evidence of secretory vesicle swelling in live cells was first demonstrated in pancreatic acinar cells using the AFM (Fig. 19) [39]. Isolated zymogen granules (ZGs), the membrane-bound secretory vesicles in exocrine pancreas and parotid glands, possess Cland ATP-sensitive, K+-selective ion channels at the vesicle membrane whose activities have been implicated in vesicle swelling [31]. Additionally, secretion of ZG contents in permeabilized pancreatic acinar cells requires the presence of both K+ and Cl- ions. In vitro ZG-pancreatic plasma membrane fusion assays further demonstrate potentiation of fusion in the presence of GTP [39]. Gai protein has been implicated in the regulation of both K+ and Cl- ion channels in a number of tissues. Analogous to the regulation of K+ and Cl- ion channels at the cell plasma membrane, their regulation at the ZG membrane by a Gai3 protein was demonstrated [31]. Isolated ZGs from exocrine pancreas swell rapidly in response to GTP (Fig. 20) [31]. These studies suggested the involvement of rapid water entry into ZGs following exposure to GTP. Therefore, when the possible involvement of water channels or aquaporins in ZG swelling was explored [32], results from the study demonstrate the presence of aquaporin-1 (AQP1) at the ZG membranes and its participation in GTP-mediated vesicle water entry and swelling (Fig. 21) [32]. To further understand the molecular mechanism of secretory vesicle swelling, the regulation of AQP1 in the ZG was investigated [33]. Detergent-solubilized ZGs immunoprecipitated with monoclonal AQP-1 antibody, co-isolates AQP-1, PLA2, Gai3, potassium channel IRK-8, and the chloride channel ClC-2 [33]. Exposure of ZGs to either the potassium channel blocker glyburide, or the PLA2 inhibitor ONO-RS-082, blocked GTP-induced ZG swelling. RBC, known to possess AQP-1 at the plasma membrane, also swell on exposure to the Gai-agonist mastoparan, and respond similarly to ONO-RS-082 and glyburide, as do ZGs. Additionally, liposomes reconstituted with the AQP-1 immunoisolated complex from solubilized ZGs, also swell in response to GTP. Glyburide or ONO-RS-082 abolished the GTP effect in reconstituted liposomes. Furthermore, immunoisolate-reconstituted planar lipid membrane demonstrate conductance, which is sensitive to glyburide and an AQP-1 specific antibody. These results demonstrate a Gai3-PLA2 mediated pathway and potassium channel involvement in AQP-1 regulation (Fig. 22) [33], contributing to our understanding of the molecular mechanism of ZG swelling. Although secretory vesicle swelling is involved in membrane fusion [37], our recent studies demonstrate that its primary role is in the expulsion of vesicular contents during secretion (unpublished observation).


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