Evidence that phospholipase c from the sperm is not responsible for initiating Ca2+ release at fertilization in mouse eggs

Release of Ca2+ from intracellular stores at fertilization of mammalian egg s is mediated by inositol 1,4,5-trisphosphate (IP3), but the mechanism by w hich the sperm initiates IP, production is not yet understood. We tested th e hypothesis that phospholipase C (PLC) activity introduced into the mouse egg as a consequence of sperm-egg fusion is responsible for causing Ca2+ re lease. We demonstrated that microinjecting purified, recombinant PLC-yl pro tein into mouse eggs caused Ca2+ oscillations like those seen at fertilizat ion. However, the PLC activity in the minimum amount of purified PLC gamma1 protein needed to elicit Ca2+ release when injected into eggs was similar to 500-900 times the PLC activity contained in a single sperm. This indicat es that a single mouse sperm does not contain enough PLC activity to be res ponsible for causing Ca2+ release at fertilization. We also examined whethe r phosphatidylinositol 3-kinase (P13K) could have a role in this process, a nd found that several inhibitors of P13K-mediated signaling had no effect o n Ca2+ release at fertilization. (C) 2001 Academic Press.