Calcium Release at Fertilization of Xenopus Eggs Requires Type I IP3 Receptors, but Not SH2 Domain-Mediated Activation of PLCγ or Gq-Mediated Activation of PLCβ

AbstractElevation 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, PLCγ and PLCβ. The Src-homology 2 (SH2) domains of PLCγ were used to inhibit SH2-mediated activation of PLCγ, and an antibody against Gq family G-proteins was used to inhibit Gq-mediated activation of PLCβ. Though the PLCγ 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 Gq 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 PLCγ SH2 domains and the Gq 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 PLCγ or Gq-mediated activation of PLCβ