Evidence That Phospholipase C from the Sperm Is Not Responsible for Initiating Ca2+ Release at Fertilization in Mouse Eggs

Abstract

Release of Ca2+ from intracellular stores at fertilization of mammalian eggs is mediated by inositol 1,4,5-trisphosphate (IP3), but the mechanism by which the sperm initiates IP3 production is not yet understood. We tested the hypothesis that phospholipase C (PLC) activity introduced into the mouse egg as a consequence of sperm–egg fusion is responsible for causing Ca2+ release. We demonstrated that microinjecting purified, recombinant PLCγ1 protein into mouse eggs caused Ca2+ oscillations like those seen at fertilization. However, the PLC activity in the minimum amount of purified PLCγ1 protein needed to elicit Ca2+ release when injected into eggs was ∼500–900 times the PLC activity contained in a single sperm. This indicates that a single mouse sperm does not contain enough PLC activity to be responsible for causing Ca2+ release at fertilization. We also examined whether phosphatidylinositol 3-kinase (PI3K) could have a role in this process, and found that several inhibitors of PI3K-mediated signaling had no effect on Ca2+ release at fertilization.