Activation of mouse sperm phosphatidylinositol-4,5 bisphosphate-phospholipase C by zona pellucida is modulated by tyrosine phosphorylation

Many cellular responses to the occupancy of membrane receptors include the hydrolysis of phosphatidylinositol-4,5 bisphosphate (PIP₂) by phospholipase C (PLC) and the subsequent generation of inositol 1,4,5-triphosphate (IP₃) and diacylglycerol (DAG). In the gamete interaction system, sperm respond to binding to the egg's extracellular matrix, the zona pellucida (zp), by exocytosis of the acrosome in a process known as the acrosome reaction (AR). Under physiological conditions, zp binding stimulates ARs only after sperm have undergone a final maturation phase, known as capacitation. One of the zp glycoproteins, ZP3, serves as the ligand for sperm plasma membrane receptors and as the trigger for this regulated exocytosis. Both phosphoinositide-linked and tyrosine kinase-mediated pathways participate in the signalling cascade triggered by sperm-zp interaction. This paper reports that stimulation with solubilized zp increased PIP₂-PLC enzymatic activity from mouse sperm. ZP3 is the zp component responsible for this stimulation. The effect was abolished by tyrphostin, suggesting that zp activation of PLC was mediated by tyrosine phosphorylation and that γ was the PLC isoform involved. We show the presence and distribution of PLCγ1 in mouse sperm. Immunostaining studies indicate that PLCγ1 is restricted to the sperm head. Sperm capacitation induced translocation of PLCγ1 from the soluble to the particulate fraction. These data suggest that PLCγ1 constitutes a component in the cascade that couples sperm binding to the egg's extracellular matrix with acrosomal exocytosis, a regulated secretory response upon which fertilization depends absolutely. © 1996 Wiley-Liss, Inc.