Aquaporin 1 regulates GTP-induced rapid gating of water in secretory vesicles

Abstract

The swelling of secretory vesicles has been implicated in exocytosis, but the underlying mechanism of vesicle swelling remains largely unknown. Zymogen granules (ZGs), the membrane-bound secretory vesicles in exocrine pancreas, swell in response to GTP mediated by a Gαi3 protein. Evidence is presented here that the water channel aquaporin-1 (AQP1) is present in the ZG membrane and participates in rapid GTP-induced vesicular water gating and swelling. Isolated ZGs exhibit low basal water permeability. However, exposure of granules to GTP results in a marked potentiation of water entry. Treatment of ZGs with the known water channel inhibitor Hg2+ is accompanied by a reversible loss in both the basal and GTP-stimulatable water entry and vesicle swelling. Introduction of AQP1-specific antibody raised against the carboxyl-terminal domain of AQP1 blocks GTP-stimulable swelling of vesicles. Our results demonstrate that AQP1 associated at the ZG membrane is involved in basal as well as GTP-induced rapid gating of water in ZGs of the exocrine pancreas

Secretion machinery at the cell plasma membrane

“…Isolated secretory vesicles and reconstituted swelling-competent proteoliposomes have been utilized [13,[20][21][22]49] to determine the mechanism and regulation of vesicle swelling. As previously discussed, isolated ZGs from the exocrine pancreas swell rapidly in response to GTP [20], suggesting rapid water gating into ZGs following exposure to GTP.…”

“…As previously discussed, isolated ZGs from the exocrine pancreas swell rapidly in response to GTP [20], suggesting rapid water gating into ZGs following exposure to GTP. Results from studies demonstrate the presence of the water channel aquaporin-1 (AQP1) at the ZG membrane [21] and its participation in GTP-mediated water entry and vesicle swelling. Further, the molecular regulation of AQP1 at the ZG membrane has been studied [49], providing a general mechanism of secretory vesicle swelling.…”

“…The extent of vesicle swelling dictates the amount of vesicular contents expelled [13]. In the past two decades, besides discovery of the porosome [1][2][3][4][5][6], the discovery of SNAREs [14][15][16], the molecular assembly [7,17 •• ] and disassembly [18,19 • ] of the membrane-associated t-/v-SNARE complex [7,17 •• ,18], in addition to the molecular mechanism of secretory vesicle swelling [20][21][22] required for the regulated release of intravesicular contents, has been determined using, among other approaches, the AFM.…”

Cytoprotective effects of growth factors: BDNF more potent than GDNF in an organotypic culture model of Parkinson's disease

“…In this way, the pro-inflammatory role of AQP4 involves a positive feedback cycle of local brain swelling (cytotoxic edema) and secretion of pro-inflammatory cytokines, which at the molecular level rely on AQP4-dependent osmotic water permeability and astrocyte swelling (338). Evidence for involvement of aquaporins in an analogous process, secretory vesicle exocytosis, has been reported (455,456), though the biophysical mechanisms remain speculative on how aquaporin water transport facilitates fusion of secretory vesicles with the cell plasma membrane. The hypothesis of AQP4-mediated cytokine release fits well with PD, which is intimately linked to inflammation and associated with elevated levels of cytokines, both in post mortem PD brains as well as in animal models (120)(121)(122)(123)(124).…”

A new frontier in cell biology: nanocellbiology

“…The very process that set that in train was thought to be due to an all-or-none process of dehiscence of synaptic vesicles at the presynaptic membrane. Today we know this as a highly regulated event, with a very high level of sensitivity, not only in nerve synapses, but also in endocytotic vesicle fusion in other forms of secretion, as in acinar cells of the pancreas (Cho et al, 2002).…”