SH2 domain-mediated activation of phospholipase Cγ is not required to initiate Ca2+ release at fertilization of mouse eggs

The initiation of Ca2+ release at fertilization of mammalian eggs requires inositol trisphosphate (Miyazaki et al., 1992, Science 257, 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 C gamma (Carroll et nl., 1997, T Cell Biol. 138, 1303-1311), we examined the possible role of PLC gamma in initiating Ca2+ release at fertilization in mouse eggs. Both PLC gamma isoforms, PLC gamma 1 and PLC gamma 2, 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 PLC gamma 1 and PLC gamma 2 inhibit PLC gamma 1 and PLC gamma 2 activation by the PDGF receptor, completely preventing Ca2+ release in response to PDGF when injected at an approximate to 20- to 40-fold excess over the concentrations of endogenous proteins. However, even at an approximate to 100- to 400-fold excess over endogenous protein levels, PLC gamma 1 and PLC gamma 2 3112 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 PLC gamma. However, activation of PLC gamma in the egg by an alternative pathway, or introduction of activated PLC gamma from the sperm, may be important. (C) 1998 Academic Press.