title: Regulation of neuroendocrine exocytosis by the ARF6 GTPase-activating protein GIT1.
abstract: 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 bilayers. 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 indicated 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-GTPase-activating protein (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.
authors.affiliation: Department of Neurotransmission and Neuroendocrine Secretion, Institut des Neurosciences Cellulaires et Intégratives UMR-7168 CNRS/Université Louis Pasteur, 5 rue Blaise Pascal, 67084 Strasbourg, France.
authors.forename: Maria Zeniou
authors.initials: MZ
authors.lastname: Meyer
authors.forename: Nadine
authors.initials: N
authors.lastname: Déliot
authors.forename: Sylvette
authors.initials: S
authors.lastname: Chasserot-Golaz
authors.forename: Richard T
authors.initials: RT
authors.lastname: Premont
authors.forename: Marie-France
authors.initials: MF
authors.lastname: Bader
authors.forename: Nicolas
authors.initials: N
authors.lastname: Vitale
mesh_terms: D000089982:ADP-Ribosylation Factor 6; D020727:ADP-Ribosylation Factors / Q000378:metabolism*; D000818:Animals; D015153:Blotting, Western; D002395:Catecholamines / Q000378:metabolism; D018797:Cell Cycle Proteins / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D019439:Chromaffin Cells / Q000378:metabolism; D003600:Cytosol / Q000378:metabolism; D004247:DNA / Q000737:chemistry; D004563:Electrochemistry; D005089:Exocytosis; D005786:Gene Expression Regulation; D006153:Guanosine Diphosphate / Q000737:chemistry; D006160:Guanosine Triphosphate / Q000737:chemistry; D006868:Hydrolysis; D008856:Microscopy, Fluorescence; D009479:Neuropeptides / Q000737:chemistry; D009490:Neurosecretory Systems / Q000378:metabolism*; D016716:PC12 Cells; D010750:Phosphoproteins / Q000378:metabolism*; D011485:Protein Binding; D021381:Protein Transport; D034622:RNA Interference; D051381:Rats; D013997:Time Factors; D014162:Transfection
chemical_list: D000089982:ADP-Ribosylation Factor 6; D002395:Catecholamines; D018797:Cell Cycle Proteins; C485648:Git1 protein, rat; D009479:Neuropeptides; D010750:Phosphoproteins; D006153:Guanosine Diphosphate; D006160:Guanosine Triphosphate; D004247:DNA; D020727:ADP-Ribosylation Factors; C000715349:Arf6 protein, rat