Mammalian eggs are arrested in metaphase II of meiosis until fertilization. Arrest is maintained by cytostatic factor (CSF) activity, which is dependent on the MOS-MEK-MAPK pathway. Inhibition of MEK1/2 with a specific inhibitor, U0126, parthenogenetically activated mouse eggs, producing phenotypes similar to Mos(-/-) parthenogenotes (premature, unequal cleavages and large polar bodies). U0126 inactivated MAPK in eggs within 1 h, in contrast to the 5 h required after fertilization, while the time course of MPF inactivation was similar in U0126-activated and fertilized eggs. We also found that inactivation of MPF by the cdc2 kinase inhibitor roscovitine induced parthenogenetic activation. Inactivation of MPF by roscovitine resulted in the subsequent inactivation of MAPK with a time course similar to that following fertilization. Notably, roscovitine also produced some Mos(-/-)-like phenotypes, indistinguishable from U0126 parthenogenotes. Simultaneous inhibition of both MPF and MAPK in eggs treated with roscovitine and U0126 produced a very high proportion of eggs with the more severe phenotype. These findings confirm that MEK is a required component of CSF in mammalian eggs and imply that the sequential inactivation of MPF followed by MAPK inactivation is required for normal spindle function and polar body emission.
(c) 2002 Elsevier Science (USA).