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Papers In Press, published online ahead of print January 26, 2006
Department of Neurotransmission and Neuroendocrine Secretion, Institut des Neurosciences Cellulaires et Intégratives, UMR-7168/LC2 CNRS, Strasbourg 67084
Corresponding Author: vitalen{at}neurochem.u-strasbg.fr
Neuroendocrine cells release hormones and neuropeptides by exocytosis, a highly regulated process in which secretory granules fuse with the plasma membrane to release their contents in response to a calcium trigger. Using chromaffin and PC12 cells, we have recently described that the granule-associated GTPase ARF6 plays a crucial role in exocytosis, by activating phospholipase D1 at the plasma membrane and, presumably, promoting the fusion reaction between the two membrane bi-layers. ARF6 is activated by the nucleotide exchange factor ARNO following docking of granules to the plasma membrane. We show here that GIT1, a GTPase-activating protein stimulating GTP hydrolysis on ARF6, is the second molecular partner that turns over the GDP/GTP cycle of ARF6 during cell stimulation. Western blot and immunofluorescence experiments indicate that GIT1 is cytosolic in resting cells but is recruited to the plasma membrane in stimulated cells, where it co-localizes with ARF6 at the granule docking sites. Over-expression of wild-type GIT1 inhibits growth hormone secretion from PC12 cells; this inhibitory effect was not observed in cells expressing a GIT1 mutant impaired in its ARF-GAP activity or in cells expressing other ARF6-GAPs. Conversely reduction of GIT1 by RNA interference increased the exocytotic activity. Using a real time assay for individual chromaffin cells, we found that microinjection of GIT1 strongly reduced the number of exocytotic events. These results provide the first evidence that GIT1 plays a function in calcium-regulated exocytosis in neuroendocrine cells. We propose that GIT1 represents part of the pathway that inactivates ARF6-dependent reactions and thereby negatively regulates and/or terminates exocytotic release.
J. Biol. Chem, 10.1074/jbc.M600081200
Submitted on January 4, 2006
Revised on January 26, 2006
Accepted on January 26, 2006
Regulation of neuroendocrine exocytosis by the ARF6 GTPase-activating protein GIT1
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