Ca2+ oscillations induced by a cytosolic sperm protein factor are mediatedby a maternal machinery that functions only once in mammalian eggs

At fertilization in mammals, the sperm activates the egg by inducing a seri es of oscillations in the intracellular free Ca2+ concentration. There is e vidence showing that this oscillatory event is triggered by a sperm-derived protein factor which diffuses into egg cytoplasm after gamete membrane fus ion, At present the identity of this factor and its precise mechanism of ac tion is unknown. Here, we studied the specificity of action of the sperm fa ctor in triggering Ca2+ oscillations in mammalian eggs. In doing so, we exa mined the patterns of Ca2+ signaling in mouse eggs, zygotes, parthenogeneti c eggs and maturing oocytes following the stimulation of bovine sperm extra cts which contain the sperm factor. It is observed that the sperm factor co uld induce Ca2+ oscillations in metaphase eggs, maturing oocytes and parthe nogenetically activated eggs but not in the zygotes, We present evidence th at Ca2+ oscillations induced by the sperm factor require a maternal machine ry. This machinery functions only once in mammalian oocytes and eggs, and i s inactivated by sperm-derived components but not by parthenogenetic activa tion. In addition, it is found that neither InsP(3) receptor sensitivity to InsP(3) nor Ca2+ pool size are the determinants that cause the fertilized egg to lose its ability to generate sperm-factor-induced Ca2+ oscillations at metaphase, In conclusion, our study suggests that the orderly sequence o f Ca2+ oscillations in mammalian eggs at fertilization is critically depend ent upon the presence of a functional maternal machinery that determines wh ether the sperm-factor-induced Ca2+ oscillations can persist.