The intraacrosomal calcium pool plays a direct role in acrosomal exocytosis

DE BLAS, GERARDO; MICHAUT, MARCELA; TREVIÑO, CLAUDIA L; TOMES, CLAUDIA N; YUNES, ROBERTO; DARSZON, ALBERTO; MAYORGA, LUIS SEGUNDO

JOURNAL OF BIOLOGICAL CHEMISTRY

The American Society for Biochemistry and Molecular Biology, Inc.

Año: 2002 vol. 277 p. 49326 - 49331

0021-9258

The acrosome reaction is a unique type of regulated exocytosis. The single secretory granule of the sperm fuses at multiple points with the overlying plasma mem-brane. In the past few years we have characterized sev-eral aspects of this process using streptolysin O-perme-abilized human spermatozoa. Here we show that Rab3A triggers acrosomal exocytosis in the virtual absence of calcium in the cytosolic compartment. Interestingly, exocytosis is blocked when calcium is depleted from intracellular stores. By using a membrane-permeant flu-orescent calcium probe, we observed that the acrosome actually behaves as a calcium store. Depleting calcium from this compartment by using a light-sensitive chela-tor prevents secretion promoted by Rab3A. UV inactiva-tion of the chelator restores exocytosis. Rab3A-triggered exocytosis is blocked by calcium pump and inositol 1,4,5-trisphosphate (IP3)-sensitive calcium channel inhibitors. Calcium measurements inside and outside the ac-rosome showed that Rab3A promotes a calcium efflux from the granule. Interestingly, release of calcium through IP3-sensitive calcium channels was necessary even when exocytosis was initiated by increasing free calcium in the extraacrosomal compartment in both permeabilized and intact spermatozoa. Our results show that a calcium efflux from the acrosome through IP3-sensitive channels is necessary downstream Rab3A activation during the membrane fusion process leading to acrosomal exocytosis.