Evidence for the capacitation-associated membrane priming of mouse spermatozoa

An important feature of male fertility is the physiological priming of mammalian spermatozoa by a multifaceted process referred to as capacitation. It is a prerequisite event before spermatozoa can bind to the egg's extracellular coat, the zona pellucida, and undergo a signal transduction cascade. The net result is the fusion of the plasma membrane (PM) and underlying outer acrosomal membrane at multiple sites and the release of acrosomal contents (i.e., glycohydrolases, proteinases, etc.) at the site of sperm-zona binding. In this study, we have used an indirect immunofluorescence (IIF) assay and other staining approaches to examine capacitation-associated membrane priming of mouse spermatozoa. For IIF studies, we used affinity-purified antibodies against two glycohydrolases that cross-reacted with the acrosomal enzymes only when the uncapacitated spermatozoa were permeabilized. Incubation of spermatozoa in a medium that favors in vitro capacitation induced membrane priming that allowed the antibodies to cross-react with the acrosomal enzymes incapacitating acrosome-intact spermatozoa without permeabilization, as revealed by the appearance of several distinct fluorescent patterns, including an initial immunopositive lining over the acrosome cap to an intense immunopositive reaction throughout the acrosome. These early immunopositive patterns were followed by the appearance of intense fluorescent spots (droplets) that seem to establish contact with the PM in a time-dependent manner. Inclusion of calmodulin, a 17-kDa Ca2+-binding protein which promotes capacitation, in the incubation medium did not alter the overall rate of capacitation; however, its presence accelerated the initial stages of membrane priming. The potential similarities between sperm capacitation and early events of Ca2+-triggered membrane fusion among eukaryotes and among various stations of the secretory and endocytotic pathways are discussed.