Divergent effect of mammalian PLCζ in generating Ca2+ oscillations in somatic cells compared with eggs

Sperm PLC zeta (phospholipase CO zeta is a distinct phosphoinositide-specific PLC isoform that is proposed to be the physiological trigger of egg activation and embryo development at mammalian fertilization. Recombinant PLC zeta has the ability to trigger Ca2+ oscillations when expressed in eggs, but it is not known how PLC zeta activity is regulated in sperm or eggs. In the present study, we have transfected CHO (Chinese-hamster ovary) cells with PLC zeta fused with either YFP (yellow fluorescent protein) or luciferase and found that PLC zeta-transfected cells did not display cytoplasmic Ca2+ oscillations any differently from control cells. PLC zeta expression was not associated with changes in CHO cell resting Ca2+ levels, nor with a significantly changed Ca2+ response to extracellular ATP compared with control cells transfected with either YFP alone, a catalytically inactive PLC zeta or luciferase alone. Sperm extracts containing PLC zeta also failed to cause Ca2+ oscillations in CHO cells. Despite these findings, PLC zeta-transfected CHO cell extracts exhibited high recombinant protein expression and PLC activity. Furthermore, either PL zeta-transfected CHO cells or derived cell extracts could specifically cause cytoplasmic Ca2+ oscillations when microinjected into mouse eggs. These data suggest that PLC zeta-mediated Ca2+ oscillations may require specific factors that are only present within the egg cytoplasm or be inhibited by factors present only in somatic cell lines.