The fall of biological maturation promoting factor (MPF) and histone H1 kinase activity during anaphase and telophase in mouse oocytes

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Abstract
Cell fusions have been used to determine the biological activity of the MPF complex in murine oocytes during their progression through anaphase and telophase to metaphase II. Oocytes (1) at metaphase I, (2) during the anaphase‐telophase transition, or (3) at metaphase II were fused to germinal vesicle‐staged (immature) oocytes. The hybrids were cultured for 1 h in the presence of db cAMP before fixation and nuclear evaluation. Metaphase! oocytes invariably induced germinal vesicle breakdown (GVBD) in the immature partner. By contrast, anaphase/telophase oocytes never induced GVBD in immature oocytes. The capacity to induce GVBD reappears after the formation of the second metaphase plate. In a second study, histone H1 kinase activity was measured during mouse oocyte maturation in single oocytes. H1 kinase activity was low in GV oocytes, increased sharply at MI, declined during anaphase and telophase and increased again at MII. After egg activation, H1 kinase activity was reduced to basal levels. These results provide direct evidence that a drop in activity of MPF in murine oocytes occurs concomitantly with the exit from metaphase I; MPF activity remains low until the cell re‐enters metaphase.