A role for cyclin A1 in the activation of MPF and G2-M transition during meiosis of male germ cells in mice

Cell-cycle transition at G2-M is controlled by MPF (M-phase-promoting facto r), a complex consisting of the Cdc2 kinase and a B-type cyclin. We have sh own that in mice, targeted disruption of an A-type cyclin gene, cyclin Al, results in a block of spermatogenesis prior to the entry into metaphase I. The meiotic arrest is accompanied by a defect in Cdc2 kinase activation at the G2-M transition, raising the possibility that a cyclin Al-dependent pro cess dictates the activation of MPF. Here we show that like Cdc2, the expre ssion of B-type cyclins is retained in cyclin Al-deficient spermatocytes, w hile their associated kinases are kept at inactive states. Treatment of arr ested germ cells with the protein phosphatase type-1 and -2A inhibitor okad aic acid restores the MPF activity and induces entry into M phase and the f ormation of normally condensed chromosome bivalents, concomitant with hyper phosphorylation of Cdc25 proteins. Conversely, inhibition of tyrosine phosp hatases, including Cdc25s, by vanadate suppresses the okadaic acid-induced metaphase induction. The highest levels of Cdc25A and Cdc25C expression and their subcellular localization during meiotic prophase coincide with that of cyclin Al, and when overexpressed in HeLa cells, cyclin Al coimmunopreci pitates with Cdc25A. Furthermore, the protein kinase complexes consisting o f cyclin Al and either Cdc2 or Cdk2 phosphorylate both Cdc25A and Cdc25C in vitro. These results suggest that in normal meiotic male germ cells, cycli n Al participates in the regulation of other protein kinases or phosphatase s critical for the G2-M transition. In particular, it may be directly invol ved in the initial amplification of MPF through the activating phosphorylat ion on Cdc25 phosphatases. (C) 2000 Academic Press.