The relationship between calcium, MAP kinase, and DNA synthesis in the seaurchin egg as fertilization

Fertilization releases the brake on the cell cycle and the egg completes me iosis and enters into S phase of the mitotic cell cycle. The MAP kinase pat hway has been implicated in this process, but the precise role of MAP kinas e in meiosis and the first mitotic cell cycle remains unknown and may diffe r according to species. Unlike the eggs of most animals, sea urchin eggs ha ve completed meiosis prior to fertilization and are arrested at the pronucl ear stage. Using both phosphorylation-state-specific antibodies and a MAP k inase activity assay, we observe that MAP kinase is phosphorylated and acti ve in unfertilized sea urchin eggs and then dephosphorylated and inactivate d by 15 min postinsemination. Further, Ca2+ was both sufficient and necessa ry for this MAP kinase inactivation. Treatment of eggs with the Ca2+ ionoph ore A23187 caused MAP kinase inactivation and triggered DNA synthesis. When the rise in intracellular Ca2+ was inhibited by injection of a chelator, B APTA or EGTA, the activity of MAP kinase remained high. Finally, inhibition of the MAP kinase signaling pathway by the specific MEK inhibitor PD98059 triggered DNA synthesis in unfertilized eggs. Thus, whenever MAP kinase act ivity is retained, DNA synthesis is inhibited while inactivation of MAP kin ase correlates with initiation of DNA synthesis. (C) 2000 Academic Press.