Phosphorylation of mitogen-activated protein kinase cascade during early embryo development in the mouse

The mitogen-activated protein kinase (MAPK) cascade is one of the most impo rtant signal transduction pathways that regulate the cell cycle in somatic cells. The present study examined the phosphorylation states of components in the MAPK cascade, Raf-1, MEK-1, and extracellular signal regulated kinas es (ERKs), which are activated by mitogens, throughout early mouse embryo d evelopment and in cultured somatic cells generally. In somatic cells, Raf-1 and MEK-1 were phosphorylated at M-phase and dephosphorylated during inter phase. ERKs were not phosphorylated at any stage during the cell cycle. The se results were similar to previous findings for the first and second cell cycles of early mouse embryos. In contrast, after the four-cell stage, not only ERKs, but also Raf-1 and MEK-1, were not phosphorylated at any stage d uring the cell cycle in mouse early embryos. These results suggest that the MAPK cascade in mouse embryos is regulated by the same mechanism as in som atic cells before the two-cell stage, and that regulation is changed to an embryo-specific mechanism after the four-cell stage.