INVOLVEMENT OF INOSITOL 1,4,5-TRISPHOSPHATE-MEDIATED CA2+ RELEASE IN EARLY AND LATE EVENTS OF MOUSE EGG ACTIVATION

Sperm-induced activation of mammalian eggs is associated with a transi ent increase in the concentration of intracellular Ca2+. The role of i nositol 1,4,5-trisphosphate (IP3)-mediated release of Ca2+ from intrac ellular stores during mouse egg activation was examined in the present study by determining the effects of microinjected monoclonal antibody (mAb) 18A10, which binds to the IP3 receptor and inhibits IP3-induced Ca2+ release, on endpoints of egg activation following insemination. The antibody inhibited in a concentration-dependent manner the ZP2 to ZP2(f) conversion that is involved in the zona pellucida block to poly spermy, as well as the ZP2 to ZP2(f) conversion promoted by microinjec ted IP3 in non-inseminated eggs. As anticipated, inseminated eggs that had been microinjected with the antibody were polyspermic. In additio n, the antibody inhibited the fertilization-associated decrease in H1 kinase activity and pronucleus formation, and the concentration depend ence for inhibition of these events was similar to that observed for i nhibiting the ZP2 to ZP2(f) conversion. Last, the antibody inhibited t he fertilization-induced recruitment of maternal mRNAs and post-transl ational modifications of proteins. In each case, eggs microinjected wi th the mAb 4C11, which also binds to the IP3 receptor but does not inh ibit IP3-induced Ca2+ release, had no inhibitory effect on fertilizati on and egg activation. Results of these studies suggest that IP3-media ted Ca2+ release is essential for both early and late events of mouse egg activation.