Effects of MEK inhibitor U0126 on meiotic progression in mouse oocytes: Microtuble organization, asymmetric division and metaphase II arrest

In this study we used U0126, a potent and specific inhibitor of MEK, to study the roles of MEK/ERK/p90 signaling pathway in the meiotic cell cycle of mouse oocytes. The phosphorylation of MAP kinase and p90 in the oocytes treated with 1.5 μ M U0126 was the same as that in oocytes cultured in drug-free medium. With 1.5 μ M U0126 treatment, the spindles appeared normal as they formed in oocytes, but failed to maintain its structure. Instead, the spindle lost one pole or elongated extraordinarily. After further culture, some oocytes extruded gigantic polar bodies (>30 μ m) that later divided into two small ones. Some oocytes underwent symmetric division and produced two equal-size daughter cells in which normal spindles formed. In oocytes with different division patterns, MAP kinase was normally phosphorylated. When the concentration of U0126 was increased to 15 μ M, the phosphorylation of both MAPK and p90 were inhibited, while symmetric division was decreased. When incubating in medium containing 15 μ M U0126 for 14 h, oocytes were activated, but part of them failed to emit polar bodies. MII oocytes were also activated by 15 μ M U0126, at the same time the dephosphorylation of MAP kinase and p90 was observed. Our results indicate that 1) MEK plays important but not indispensable roles in microtubule organization; 2) MEK keeps normal meiotic spindle morphology, targets peripheral spindle positioning and regulates asymmetric division by activating some unknown substrates other than MAP kinase /p90; and 3) activation of MEK/ERK/p90 cascade maintains MII arrest in mouse oocytes.