Relationship between nuclear sequestration of PLCζ and termination of PLCζ-induced Ca2+ oscillations in mouse eggs

Phospholipase C-zeta (PLCζ), a strong candidate of the egg-activating sperm factor, causes long-lasting series of Ca2+ spikes and thereby egg activation when expressed in mouse eggs by injection of cRNA. PLCζ moves into the formed pronucleus (PN), and Ca2+ spikes disappear at PN stage. Relationship between nuclear translocation of PLCζ and cessation of Ca2+ oscillations was addressed using various concentrations of wild-type RNA and point mutant K377E RNA having the comparable expression efficiency. PLCζ-induced Ca2+ spikes with 20-30 min intervals similar to those at fertilization ceased between 50 min before and 15 min after the time of complete PN formation (TPN) ∼5 h after the first Ca2+ spike, whereas Ca2+ oscillations induced by K377E lacking nuclear translocation ability continued over 9 h. Formation of the nuclear envelope (NE) began 50-60 min before TPN, visualized by labeling the endoplasmic reticulum network with fluorescent dye DiI and ER-targeting protein ER-DsRed2. PLCζ entered the PN as soon as the NE was formed, and accumulated in enlarging PN. After in vitro fertilization as normal as possible, the last Ca2+ spike occurred between 25 min before and 35 min after initiation of NE formation in most cases. Thus, sequestration of PLCζ into the PN participates in termination of Ca2+ oscillations at the interphase in the mouse 1-cell embryo.