Evidence That GqFamily G Proteins Do Not Function in Mouse Egg Activation at Fertilization

Abstract

Embryonic development is initiated after the fertilizing sperm contacts the egg and triggers a process termed “egg activation,” resulting in calcium release, cortical granule exocytosis, recruitment of maternal mRNAs, and cell cycle resumption. Heterotrimeric guanine nucleotide-binding proteins (G proteins) may be involved in mouse egg activation since inhibition of G protein βγ subunits partially inhibits sperm-induced cell cycle resumption. In addition, specific events of egg activation can be initiated in the absence of sperm by acetylcholine stimulation of mouse eggs overexpressing the human m1 muscarinic receptor, a G protein-coupled receptor. In somatic cells, G proteins in the Gqfamily couple ligand stimulation of the m1 muscarinic receptor to activation of phospholipase C, resulting in the production of inositol 1,4,5-trisphosphate (IP3) and IP3-mediated release of intracellular calcium. Since IP3-mediated calcium release is involved in egg activation at fertilization, we have examined the role of Gqfamily G proteins in both sperm-independent (muscarinic receptor-mediated) and sperm-induced egg activation using a function-blocking antibody raised against the common C-terminal region of Gqand G11proteins. We show that this antibody effectively inhibits Gqfamily G proteins in mouse eggs by demonstrating that the antibody inhibits egg activation in response to stimulation of the m1 muscarinic receptor. This same antibody, however, does not inhibit sperm-induced egg activation events. These results indicate that although activation of Gqfamily G proteins can result in egg activation in the mouse, it is unlikely that these proteins are used by the sperm to initiate egg activation at fertilization.