title: Involvement of the calcium-specific protease, calpain, in the fertilizing capacity of human spermatozoa.
abstract: We recently reported the novel finding that human spermatozoa contain the calcium (Ca2+)-dependent protease, calpain. In somatic cells this protease mediates several cellular activities regulated by Ca2+ including membrane fusibility during cell-to-cell interactions. In this paper we examined the participation of sperm calpain in sperm-oocyte penetration, a process that is dependent on Ca2+ and involves membrane fusion between the two cells. Oocyte penetration was assessed using ejaculated spermatozoa from fertile men and zona-free hamster oocytes. Penetration rate was impaired by the presence of the active-site calpain inhibitor, calpain inhibitor-I, in a dose-dependent manner. At 1 mM, penetration scores were reduced by 65% (p < 0.01; n=5). The effects did not involve the oocyte, nor did the inhibitor alter sperm motility. Similar inhibitory effects on sperm penetration capacity were observed with specific antibodies directed either against calpain-I or calpain-II, the two forms of calpains described in somatic cells. At 1:1000 antibody dilution, penetration was inhibited 50 and 60% with anti-calpain-I and anti-calpain-II antibodies, respectively (p < 0.01; n=6). Furthermore, a combination of these two antibodies reduced the penetration rates by 75% (p < 0.01; n=6). We conclude that calpain inhibitor and anti-calpain antibodies impair human sperm capacity to fuse and penetrate the oocyte. These findings suggest that sperm calpain is a novel component of the biochemical processes that regulate the fertilizing capacity of human spermatozoa.
authors.affiliation: Department of Research, Worldwide Medical Corporation, Irvine, California USA.
authors.forename: F J
authors.initials: FJ
authors.lastname: Rojas
authors.forename: I
authors.initials: I
authors.lastname: Moretti-Rojas
mesh_terms: D000818:Animals; D000906:Antibodies / Q000276:immunology; D002154:Calpain / Q000276:immunology / Q000378:metabolism*; D006224:Cricetinae; D005260:Female; D006801:Humans; D008297:Male; D013084:Sperm-Ovum Interactions; D013094:Spermatozoa / Q000276:immunology / Q000378:metabolism / Q000502:physiology*
chemical_list: D000906:Antibodies; D002154:Calpain