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.

Abstract

Sperm acrosomal exocytosis is the result of a complex set of signal transduction 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 adenosine monophosphate (cAMP) concentrations as well as to activate proteins of the 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 stimulation of adenylyl cyclase activity is mediated through G proteins. In the present study, we demonstrate that the sperm membrane-bound adenylyl cyclase activity is stimulated by the G protein activators guanosine-5'-O-thiotriphosphate (GTPgammaS) and mastoparan in a concentration-dependent manner. The maximal adenylyl cyclase activity measured with these two G protein activators is similar to the stimulation observed with the zona pellucida, but the effect of GTPgammaS is not additive or synergistic with the effects of mastoparan or the zona pellucida. Pertussis toxin treatment of sperm membranes inhibits the zona pellucida stimulation of adenylyl cyclase activity, while the basal or forskolin-induced activation of the enzyme is not affected. Partial inhibition of the stimulatory effect of the zona pellucida on the adenylyl cyclase activity is observed with guanosine-5'-O-thiodiphosphate (GDPbetaS), another G protein antagonist. To a reconstitution system containing Lubrol-PX, where zona pellucida or GTPgammaS stimulation of the sperm enzyme is not observed, addition of G protein betagamma subunits restores the activation of the sperm adenylyl cyclase by the zona pellucida and GTPgammaS without affecting the enzyme activity under basal or forskolin-stimulated conditions. These results support our hypothesis that mouse sperm adenylyl cyclase is stimulated by the zona pellucida through a pertussis toxin-sensitive pathway involving G proteins of the Gi class.