PLCζ Induced Ca2+ Oscillations in Mouse Eggs Involve a Positive Feedback Cycle of Ca2+ Induced InsP3 Formation From Cytoplasmic PIP2

Egg activation at fertilization in mammalian eggs is caused by a series of transient increases in the cytosolic free Ca2+ concentration, referred to as Ca2+ oscillations. It is widely accepted that these Ca2+ oscillations are initiated by a sperm derived phospholipase C isoform, PLC zeta that hydrolyses its substrate PIP2 to produce the Ca2+ releasing messenger InsP(3). However, it is not clear whether PLC zeta induced InsP(3) formation is periodic or monotonic, and whether the PIP2 source for generating InsP(3) from PLC zeta is in the plasma membrane or the cytoplasm. In this study we have uncaged InsP(3) at different points of the Ca2+ oscillation cycle to show that PLC zeta causes Ca2+ oscillations by a mechanism which requires Ca2+ induced InsP(3) formation. In contrast, incubation in Sr2+ media, which also induces Ca2+ oscillations in mouse eggs, sensitizes InsP(3)-induced Ca2+ release. We also show that the cytosolic level Ca2+ is a key factor in setting the frequency of Ca2+ oscillations since low concentrations of the Ca2+ pump inhibitor, thapsigargin, accelerates the frequency of PLC zeta induced Ca2+ oscillations in eggs, even in Ca2+ free media. Given that Ca2+ induced InsP(3) formation causes a rapid wave during each Ca2+ rise, we use a mathematical model to show that InsP(3) generation, and hence PLC zeta's substate PIP2, has to be finely distributed throughout the egg cytoplasm. Evidence for PIP2 distribution in vesicles throughout the egg cytoplasm is provided with a rhodamine-peptide probe, PBP10. The apparent level of PIP2 in such vesicles could be reduced by incubating eggs in the drug propranolol which also reversibly inhibited PLC zeta induced, but not Sr2+ induced, Ca2+ oscillations. These data suggest that the cytosolic Ca2+ level, rather than Ca2+ store content, is a key variable in setting the pace of PLC induced Ca2+ oscillations in eggs, and they imply that InsP(3) oscillates in synchrony with Ca2+ oscillations. Furthermore, they support the hypothesis that PLC zeta and sperm induced Ca2+ oscillations in eggs requires the hydrolysis of PIP2 from finely spaced cytoplasmic vesicles.