SH2 Domain-Mediated Activation of Phospholipase Cγ Is Not Required to Initiate Ca2+Release at Fertilization of Mouse Eggs

Abstract

The initiation of Ca2+release at fertilization of mammalian eggs requires inositol trisphosphate (Miyazakiet al.,1992,Science257, 251–255), indicating that an enzyme of the phospholipase C family is probably activated. Because Ca2+release at fertilization in echinoderm eggs is initiated by SH2 domain-mediated activation of phospholipase Cγ (Carrollet al.,1997,J. Cell Biol.138, 1303–1311), we examined the possible role of PLCγ in initiating Ca2+release at fertilization in mouse eggs. Both PLCγ isoforms, PLCγ1 and PLCγ2, are present in mouse eggs and sperm, and stimulation of these enzymes in the egg by way of an exogenously expressed PDGF receptor causes Ca2+release. Recombinant SH2 domains of PLCγ1 and PLCγ2 inhibit PLCγ1 and PLCγ2 activation by the PDGF receptor, completely preventing Ca2+release in response to PDGF when injected at an ≈20- to 40-fold excess over the concentrations of endogenous proteins. However, even at an ≈100- to 400-fold excess over endogenous protein levels, PLCγ1 and PLCγ2 SH2 domains do not inhibit Ca2+release at fertilization. These findings indicate that Ca2+release at fertilization of mouse eggs does not require SH2-domain-mediated activation of PLCγ. However, activation of PLCγ in the egg by an alternative pathway, or introduction of activated PLCγ from the sperm, may be important.