Regulation of diacylglycerol production and protein kinase C stimulation during sperm- and PLCζ-mediated mouse egg activation

Background information. At fertilization in mammalian eggs, the sperm induces a series of Ca2+ oscillations via the production of inositol 1,4,5-trisphosphate. Increased inositol 1,4,5-trisphosphate production appears to be triggered by a sperm-derived PLC zeta (phospholipase C-zeta) that enters the egg after gamete fusion. The specific phosphatidylinositol 4,5-bisphosphate hydrolytic activity of PLC implies that DAG (diacylglycerol) production, and hence PKC (protein kinase C) stimulation, also occurs during mammalian egg fertilization. Fertilization-mediated increase in PKC activity has been demonstrated; however, its precise role is unclear. Results. We investigated PLC zeta- and fertilization-mediated generation of DAG in mouse eggs by monitoring plasma-membrane translocation of a fluorescent DAG-specific reporter. Consistent plasma-membrane DAG formation at fertilization, or after injection of physiological concentrations of PLC zeta, was barely detectable. However, when PLC zeta is overexpressed in eggs, significant plasma-membrane DAG production occurs in concert with a series of unexpected secondary high-frequency Ca2+ oscillations. We show that these secondary Ca2+ oscillations can be mimicked in a variety of situations by the stimulation of PKC and that they can be prevented by PKC inhibition. The way PKC leads to secondary Ca2+ oscillations appears to involve Ca2+ influx and the loading of thapsigargin-sensitive Ca2+ stores. Conclusions. Our results suggest that overproduction of DAG in PLC zeta-injected eggs can lead to PKC-mediated Ca2+ influx and subsequent overloading of Ca2+ stores. These results suggest that DAG generation in the plasma membrane of fertilizing mouse eggs is minimized since it can perturb egg Ca2+ homoeostasis via excessive Ca2+ influx.