Activation of p34 cdc2 kinase around the meiotic resumption in bovine oocytes cultured in vitro

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The p34 cdc2 kinase has been identified as a protein factor that is a regulator of meiotic maturation in mammalian oocytes. To investigate the regulatory function of the meiotic resumption in bovine oocytes cultured in vitro, the changes in the phosphorylation states of p34 cdc2 kinase and the histone H1 kinase activity were examined around germinal vesicle breakdown (GVBD). All bovine oocytes just after isolation from their follicles were arrested at the germinal vesicle (GV) stage, and these extracts exhibited two (upper and lower) bands of p34 cdc2 kinase on SDS-PAGE followed by immunoblotting with an antibody against C-terminal peptide of p34 cdc2. When these oocytes were cultured for 24 h in a medium supplemented with 100 μg/ml genistein, tyrosine phosphorylation inhibitor, GVBD was induced in 85% of oocytes, indicating that the upper band of p34 cdc2 kinase in bovine oocytes at the GV stage was already fully phosphorylated tyrosine residue prior to culture. Another (middle) band of p34 cdc2 kinase between the upper and lower bands appeared in the extracts of the oocytes cultured for 4 h, and significant activation of the histone H1 kinase was found in these oocytes (67 ± 18 fmol/ h/ oocyte) as compared to that in oocytes cultured for 0 h (46 ± 11 fmol/ h/ oocyte). The staining intensity of the middle band and the activity of the histone H1 kinase were further increased after the initiation of GVBD at 6 h of culture, but the quantitative changes of upper and lower bands were not detected throughout the 12 h of culture. Thus, it is concluded that the dephosphorylation of p34 cdc2 kinase followed by activation of the histone H1 kinase after the onset of culture plays a key role in the resumption of meiosis in bovine oocytes.