A Specific Inhibitor of p34cdc2/Cyclin B Suppresses Fertilization-Induced Calcium Oscillations in Mouse Eggs1

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Abstract
Fertilization-induced Ca2+ oscillations in mouse eggs cease at the time of pronuclear formation when maturation-promoting factor (MPF) is inactivated, but the Ca2+ oscillations are ceaseless if eggs are arrested at metaphase by colcemid, which maintains the activity of MPF. To determine the possible role of MPF in regulation of cytoplasmic Ca2+ excitability, roscovitine, a specific inhibitor of p34cdc2/cyclin B kinase, was used to inactivate MPF, and its effect on fertilization-induced Ca2+ oscillations was investigated. Our results showed that roscovitine at ≥ 50 μM suppressed fertilization-induced Ca2+ oscillations in normal and colcemid-treated metaphase II (MII) eggs after the first 1–2 Ca2+ spikes. Roscovitine inhibition of fertilization-induced Ca2+ oscillations could be reversed by extensive washing of the eggs. Histone H1 kinase activity in colcemid-treated MII eggs was similarly inhibited by roscovitine, which suggested that the cessation of fertilization-induced Ca2+ oscillations is due to the inactivation of MPF. Thimerosal-induced Ca2+ oscillations in Ca2+-, Mg2+-free medium was also suppressed by roscovitine, suggesting a general inhibitory effect of roscovitine on Ca2+ oscillations. The inhibition may be achieved by disruption of Ca2+ release and refilling of the calcium store. Thapsigargin, an inhibitor of the endoplasmic reticulum Ca-ATPase, induced significantly less Ca2+ release in roscovitine-treated eggs than in the non-drug-treated eggs. Taken together, our results suggest that MPF plays an important role in regulation of the cytoplasmic Ca2+ excitability in mouse eggs.