Evidence for the role of heterotrimeric guanine nucleotide-binding regulatory proteins in the regulation of the mouse sperm adenylyl cyclase by the egg's zona pellucida

Sperm acrosomal exocytosis is the result of a complex set of signal transdu ction pathways activated physiologically by the egg's extracellular matrix, the zona pellucida. In the mouse, the zona pellucida has been demonstrated to induce an increase in sperm intracellular pH, Ca2+, and cyclic adenosin e monophosphate (cAMP) concentrations as well as to activate proteins of th e Gi class (G; guanine nucleotide-binding regulatory proteins). We recently reported that the mouse zona pellucida could activate the adenylyl cyclase of mouse sperm. It is not known, however, whether zona pellucida stimulati on of adenylyl cyclase activity is mediated through G proteins. In the pres ent study, we demonstrate that the sperm membrane-bound adenylyl cyclase ac tivity is stimulated by the G protein activators guanosine-5'-O-thiotriphos phate (GTP gamma S) and mastoparan in a concentration-dependent manner. The maximal adenylyl cyclase activity measured with these two G protein activa tors is similar to the stimulation observed with the zona pellucida, but th e effect of GTP gamma S is not additive or synergistic with the effects of mastoparan or the zona pellucida. Pertussis toxin treatment of sperm membra nes inhibits the zona pellucida stimulation of adenylyl cyclase activity, w hile the basal or forskolin-induced activation of the enzyme is not affecte d. Partial inhibition of the stimulatory effect of the zona pellucida on th e adenylyl cyclase activity is observed with guanosine-5'-O-thiodiphosphate (GDP beta S), another G protein antagonist. To a reconstitution system con taining Lubrol-PX, where zona pellucida or GTP gamma S stimulation of the s perm enzyme is not observed, addition of G protein py subunits restores the activation of the sperm adenylyl cyclase by the zona pellucida and GTP gam ma S without affecting the enzyme activity under basal or forskolin-stimula ted conditions. These results support our hypothesis that mouse sperm adeny lyl cyclase is stimulated by the zona pellucida through a pertussis toxin-s ensitive pathway involving G proteins of the G(l) class.