The dynamics of MAPK inactivation at fertilization in mouse eggs

Egg activation at fertilization in mammals is initiated by prolonged Ca2+ oscillations that trigger the completion of meiosis and formation of pronuclei. A late fall in MAPK activity is essential for pronuclear formation, but the precise timing and mechanism of decline are unknown. Here, we have measured the dynamics of MAPK inactivation in fertilizing mouse eggs using novel chemiluminescent MAPK activity reporters. This reveals that the MAPK activity decrease begins during the Ca2+ oscillations, but MAPK does not completely inactivate until after pronuclear formation. MAPK in eggs consists of Mos, MEK and ERK1/2. Notably, the MAPK activity decline at fertilization is not explained by upstream destruction of Mos, because a decrease in Mos-luciferase signal is not associated with egg activation. Further, Mos over-expression does not affect the timing of MAPK inactivation or pronuclear formation. However, the late decrease in MAPK could be rapidly reversed by the protein phosphatase inhibitor, okadaic acid. These data suggest that the completion of meiosis in mouse zygotes is driven by an increased phosphatase activity and not by a decline in Mos levels, or MEK activity.