Cholesterol efflux-mediated signal transduction in mammalian sperm - beta-cyclodextrins initiate transmembrane signaling leading to an increase in protein tyrosine phosphorylation and capacitation

Sperm capacitation in vitro is highly correlated with an increase in protei n tyrosine phosphorylation that is regulated by cAMP through a unique mode of signal transduction cross-talk. The activation of this signaling pathway , as well as capacitation, requires bovine serum albumin (BSA) in the incub ation medium. BSA is hypothesized to modulate capacitation through its abil ity to remove cholesterol from the sperm plasma membrane. Here we demonstra te that the cholesterol-binding heptasaccharides, methyl-beta-cyclodextrin and OH-propyl-beta-cyclodextrin, promote the release of cholesterol from th e mouse sperm plasma membrane in media devoid of BSA Both of these beta-cyc lodextrins were also demonstrated to increase protein tyrosine phosphorylat ion in the absence of BSA in both mouse and bull sperm, and the patterns of phosphorylation were similar to those induced by media containing BSA The potency of the different beta-cyclodextrins to increase protein tyrosine ph osphorylation in sperm was correlated with their cholesterol binding effici encies, and preincubation of the beta-cyclodextrins with cholesterol-SO4- t o saturate their cholesterol-binding sites blocked the ability of these com pounds to stimulate protein tyrosine phosphorylation, The beta-cyclodextrin effect on protein tyrosine phosphorylation was both NaHCO3 and protein kin ase A-dependent. The beta-cyclodextrins were also able to capacitate mouse sperm in the absence of BSA, as measured by the ability of the zona pelluci da to induce the acrosome reaction and by successful fertilization in vitro . In summary, beta-cyclodextrins can completely replace BSA in media to sup port signal transduction leading to capacitation. These data further suppor t the coupling of cholesterol efflux to the activation of membrane and tran smembrane signaling events leading to the activation of a unique signaling pathway involving the cross-talk between cAMP and tyrosine kinase second me ssenger systems, thus defining a new mode of cellular signal transduction i nitiated by cholesterol release.