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J. Biol. Chem., Vol. 277, Issue 51, 49326-49331, December 20, 2002
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From the 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 membrane. In the past few
years we have characterized several aspects of this process using
streptolysin O-permeabilized 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 fluorescent calcium probe, we observed that the
acrosome actually behaves as a calcium store. Depleting calcium from
this compartment by using a light-sensitive chelator prevents secretion
promoted by Rab3A. UV inactivation 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 acrosome 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.
The Intraacrosomal Calcium Pool Plays a Direct Role in
Acrosomal Exocytosis*
,,,,¶
Laboratorio de Biología Celular y
Molecular, Instituto de Histología y Embriología
(IHEM-CONICET), Facultad de Ciencias Médicas, CC 56, Universidad
Nacional de Cuyo, 5500 Mendoza, Argentina and the
§ Departamento de Genética y Fisiología
Molecular, Instituto de Biotecnología, Universidad Nacional
Autónoma de México (UNAM), Cuernavaca,
Morelos 62250, México
*
This work was supported by an International Research Scholar
Award from the Howard Hughes Medical Institute, a grant from CONICET, Argentina, and a Carrillo-Oñativia Scholar Award,
Argentina (to L. S. M.) and by grants of Consejo Nacional de Ciencia
y Tecnología, México, DGAPA-Universidad Nacional
Autónoma de México, and Third World Academy of Sciences (to
C. L. T. and A. D.).The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in
accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
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