The role of heterotrimeric G proteins in mouse egg activation

The signal transduction events that occur following sperm-egg interactions and result in egg activation may be analogous to a ligand-receptor-effector pathway, but the details of this signaling pathway are poorly understood. I have investigated the hypothesis that sperm-induced egg activation occurs via a heterotrimeric G protein-mediated signaling pathway. The human m1 muscarinic receptor, a G protein-coupled receptor, was overexpressed in mouse eggs in order to stimulate endogenous egg G proteins. When treated with the ligand acetylcholine, these eggs displayed the full complement of egg activation events normally induced by the sperm. I next defined the complement of G proteins expressed in the egg. Using an RT-PCR assay, the relative mRNA levels encoding specific G protein {dollar}alpha, beta{dollar}, and {dollar}gamma{dollar} subunits were determined in oocytes, eggs, and preimplantation embryos. mRNA transcripts representing all of the heterotrimeric G protein families are present, and all undergo significant changes in their patterns of expression during these egg and embryo stages. Because pertussis toxin (PT) does not inhibit sperm-induced egg activation and cholera toxin (CT) does not induce egg activation events in the absence of sperm, the most likely heterotrimeric G protein candidates for involvement in egg activation would be CT- and PT-insensitive and would be present at relatively high levels in the oocyte and egg (e.g. Gq and G12 families). I chose to focus on the role of Gq family G proteins since Gq is known to couple to the m1 muscarinic receptor. Microinjection of a function-blocking antibody raised against the common C-terminus of Gq and G11 into mouse eggs results in complete inhibition of the egg activation events that occur in response to acetylcholine in the muscarinic receptor system. However, this same antibody preparation does not have any inhibitory effect on sperm-induced egg activation events during in vitro fertilization. These results suggest that although stimulating the activity of Gq family G proteins can mimic egg activation events, that in fact the sperm most likely does not utilize these G proteins in order to activate the egg.