Protein Kinase C and Exocytosis in Mammalian Spermatozoa

Abstract

The recent papers by O'Toole et al. (1996)and Liu and Baker (1997)are further steps towards ascribing a role for protein kinase C (PKC) in the acrosome reaction (Breitbart et al., 1992, Rotem et al., 1992). The source of Ca2+ (intracellular versus extracellular pools) and diacylglycerol (DAG) [phosphatidylinositol (4,5)-bisphosphate (PIP2) versus phosphatidylcholine (PC)] during sperm activation requires extensive and accurate measurements of lipid messenger molecules and inositol (1,4,5)-trisphosphate (IP3) formation, which are under investigation. Indeed, it was demonstrated that IP3-sensitive Ca2+ pools are present in activated mammalian sperm (Walensky and Snyder, 1995). We still favour the model that phosphoinositide turnover is an early event in sperm activation that provides IP3 for Ca2+ mobilization and DAG for PKC activation (Naor and Breitbart, 1997). Concerning the signal transduction events leading to phospholipase A2 (PLA2) activation, we favour the DAG–PKC–mitogen activated protein kinase–PLA2 pathway during sperm activation, and studies are in progress to confirm this pathway. Experiments are also underway to identify further mammalian sperm PKC subspecies, their endogenous substrates, the modes of their respective activation and their relative physiological roles in sperm–egg interaction.