Calcium release at fertilization of Xenopus eggs requires type IIP3 receptors, but not SH2 domain-mediated activation of PLCγ or Gq-mediated activation of PLCβ

Elevation of intracellular Ca2+ at fertilization is essential for the initiation of development in the Xenopus egg, but the pathway between sperm-egg interaction and Ca2+ release from the egg's endoplasmic reticulum is not well understood. Here we show that injection of an inhibitory antibody against the type I IP3 receptor reduces Ca2+ release at fertilization, indicating that the Ca2+ release requires IP3. We then examine how IP3 production is initiated. Xenopus eggs were injected with specific inhibitors of the activation of two phospholipase C isoforms, PLC gamma and PLC beta. The Src-homology 2 (SH2) domains of PLC gamma were used to inhibit SH2-mediated activation of PLC gamma, and an antibody against G(q) family G-proteins was used to inhibit G(q)-mediated activation of PLC beta. Though the PLC gamma SH2 domains inhibited platelet-derived growth factor (PDGF)-induced Ca2+ release in eggs with exogenously expressed PDGF receptors, they did not inhibit the Ca2+ rise at fertilization. Similarly, the G(q) family antibody blocked serotonin-induced Ca2+ release in eggs with exogenously expressed serotonin 2C receptors, but not the Ca2+ rise at fertilization. A mixture of PLC gamma SH2 domains and the G(q) antibody also did not inhibit the Ca2+ rise at fertilization. These results indicate that Ca2+ release at fertilization of Xenopus eggs requires type I IP3-gated Ca2+ channels, but not SH2 domain-mediated activation of PLC gamma or G(q)-mediated activation of PLC beta. (C) 1999 Academic Press.