Progesterone promotes the acrosome reaction in capacitated human spermatozoa as judged by flow cytometry and CD46 staining

The acrosome reaction is a necessary prerequisite for spermatozoa to acquir e fertilizing ability. Several different moieties appear to promote the acr osome reaction through different pathways, including solubilized zona pellu cidae, recombinant zona protein ZP3, follicular fluid, calcium ionophores, and mannosylated bovine serum albumin (BSA). Although many investigators ha ve presented evidence that progesterone also promotes the acrosome reaction through the mediation of a non-genomic cell membrane receptor, this concep t has been challenged. Other workers have suggested that progesterone does not promote an acrosome reaction in human spermatozoa, as judged by the det ection of CD46, a complement regulatory protein present on the inner acroso me membrane, through flow cytometric analysis of large numbers of spermatoz oa. Prior investigations were criticized by the limited numbers of spermato zoa enumerated visually, the use of nonspecific staining techniques, and th e failure to eliminate dead spermatozoa during the scoring of the acrosome reaction. We have repeated these experiments, using both a supravital dye t o eliminate dead spermatozoa from flow cytometric analysis, and anti-CD46 m onoclonal antibody to score acrosome-reacted spermatozoa. Care was taken to validate the adequacy of capacitation conditions, which were proven by the ability of spermatozoa to acrosome react in response to mannosylated BSA a nd to penetrate zona-free hamster eggs. Confocal microscopy was used to con firm that CD46 immunostaining was limited to the acrosomal region of the sp ermatozoon head. Our results indicate that progesterone does promote an acr osome reaction within capacitated spermatozoa.