Identification of PLCγ-Dependent and -Independent Events during Fertilization of Sea Urchin Eggs

At fertilization, sea urchin eggs undergo a series of activation events, including a Ca2+action potential, Ca2+release from the endoplasmic reticulum, an increase in intracellular pH, sperm pronuclear formation, MAP kinase dephosphorylation, and DNA synthesis. To examine which of these events might be initiated by activation of phospholipase Cγ (PLCγ), which produces the second messengers inositol trisphosphate (IP3) and diacylglycerol, we used recombinant SH2 domains of PLCγ as specific inhibitors. Sea urchin eggs were co-injected with a GST fusion protein composed of the two tandem SH2 domains of bovine PLCγ and (1) Ca2+green dextran to monitor intracellular free Ca2+, (2) BCECF dextran to monitor intracellular pH, (3) Oregon Green dUTP to monitor DNA synthesis, or (4) fluorescein 70-kDa dextran to monitor nuclear envelope formation. Microinjection of the tandem SH2 domains of PLCγ produced a concentration-dependent inhibition of Ca2+release and also inhibited cortical granule exocytosis, cytoplasmic alkalinization, MAP kinase dephosphorylation, DNA synthesis, and cleavage after fertilization. However, the Ca2+action potential, sperm entry, and sperm pronuclear formation were not prevented by injection of the PLCγSH2 domain protein. Microinjection of a control protein, the tandem SH2 domains of the phosphatase SHP2, had no effect on Ca2+release, cortical granule exocytosis, DNA synthesis, or cleavage. Specificity of the inhibitory action of the PLCγSH2 domains was further indicated by the finding that microinjection of PLCγSH2 domains that had been point mutated at a critical arginine did not inhibit Ca release at fertilization. Additionally, Ca2+release in response to microinjection of IP3, cholera toxin, cADP ribose, or cGMP was not inhibited by the PLCγSH2 fusion protein. These results indicate that PLCγ plays a key role in several fertilization events in sea urchin eggs, including Ca2+release and DNA synthesis, but that the action potential, sperm entry, and male pronuclear formation can occur in the absence of PLCγ activation or Ca2+increase.