The acrosome reaction in human spermatozoa

During gamete interaction, sperm acrosome reaction (AR) induced by oocyte i nvestment is a prerequisite event for the spermatozoa to pass through the z ona pellucida (ZP), fuse with and penetrate the oocyte. Progesterone (P-4), secreted by cumulus cells, is an important cofactor for the occurrence of this exocytosis event. The AR results from the fusion between outer acrosom al and plasma membranes, leading to inner acrosomal membrane exposure. Bind ing of agonists, P-4 or ZP(3) glycoprotein, to plasma membrane sperm recept ors activates intraspermatic signals and enzymatic pathways involved in the AR. Among the proteins or glycoproteins described as potential sperm recep tors for ZP, G(i)/G(o) protein-coupled and tyrosine kinase receptors have b een described. Sperm receptors for P-4 are poorly characterized, except a p utative GABA(A)-like receptor. ZP- and P-4-promoted AR is mediated by an ob ligatory intracellular calcium increase, appearing first at the acrosome eq uatorial segment and spreading throughout the head. The plasma membrane cha nnels involved in calcium entry are operated by a plasma membrane depolariz ation and protein phosphorylations mediated by protein kinase C and tyrosin e kinase protein. Part of the calcium increase could also be due to intrace llular store release through IP3- and nucleotide (cAMP)-gated channels. Bes ides adenylate cyclase and phospholipase C activations, intracellular calci um increase also stimulates PLA(2) activity and actin depolymerization, lea ding to membrane fusion. Evaluation of AR by staining or fluorescent probes can be useful to predict fertilization success and to direct the therapeut ic strategy in male infertility. (C) 2000 Editions scientifiques et medical es Elsevier SAS.