Sperm-induced activation of mammalian eggs is associated with a transient increase in the concentration of intracellular Ca2+. The role of inositol 1,4,5-trisphosphate (IP3)mediated release of Ca2+ from intracellular 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 ZP2f conversion that is involved in the zona pellucida block to polyspermy, as well as the ZP2 to ZP2f conversion promoted by microinjected IP3 in non-inseminated eggs. As anticipated, inseminated eggs that had been microinjected with the antibody were polyspermic. In addition, the antibody inhibited the fertilization-associated decrease in H1 kinase activity and pronucleus formation, and the concentration dependence for inhibition of these events was similar to that observed for inhibiting the ZP2 to ZP2f conversion. Last, the antibody inhibited the fertilization-induced recruitment of maternal mRNAs and post-translational modifications of proteins. In each case, eggs microinjected with the mAb 4C11, which also binds to the IP3 receptor but does not inhibit IP3-induced Ca2+ release, had no inhibitory effect on fertilization and egg activation. Results of these studies suggest that IP3-mediated Ca2+ release is essential for both early and late events of mouse egg activation.

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It should be noted that a brief treatment of hamster eggs with biologically active phorbol diesters results in emission of the second polar body and possibly pronucleus formation (Gallicano et al., 1993). The experiments reported by Endo et al. (1989) exposed mouse eggs to phorbol diesters for 1 hour. This difference in experimental protocols is unlikely to account for the differences in egg activation in hamster and mouse eggs. We have confirmed the report of Gallicano et al. (1993) and have shown that a brief treatment of hamster eggs results in emission of the second polar body. In contrast, a similar treatment of mouse eggs does not (Moore, Kopf and Schultz, unpublished observations). Thus, species differences may in fact account for the reported differences of these eggs in response to activators of protein kinase C.

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