1 1 2 3 Mitogen-activated Protein Kinase Dynamics during the Meiotic G2/MI 4 Transition of Mouse Spermatocytes1 5 6

ABSTRACT 29 Cellular and genetic approaches were used to investigate the requirements for activation during 30 spermatogenesis of the extracellular signal-regulated protein kinases (ERKs), more commonly 31 known as the mitogen-activated protein kinases (MAPKs). The MAPKS and their activating 32 kinases, the MEKs are expressed in specific developmental patterns. The MAPKs and MEK2 33 are expressed in all pre-meiotic germ cells and spermatocytes, while MEK1 is not expressed 34 abundantly in pachytene spermatocytes. Phosphorylated (active) variants of these kinases are 35 diminished in pachytene spermatocytes. Treatment of pachytene spermatocytes with okadaic 36 acid (OA), to induce transition from meiotic prophase to metaphase I (G2/MI), resulted in 37 phosphorylation and enzymatic activation of ERK1/2. However, U0126, an inhibitor of the 38 ERK-activating kinases, MEK1/2, did not inhibit OA-induced MAPK activation or chromosome 39 condensation. Analysis of spermatocytes lacking MOS, a mitogen-activated protein kinase 40 kinase kinase responsible for MEK and MAPK activation, revealed that MOS is not required for 41 OA-induced activation of the MAPKs. OA-induced MAPK activation was inhibited by 42 butyrolactone I, an inhibitor of cyclin-dependent kinases 1 and 2 (CDK1, CDK2); thus these 4