In vivo regulation of the early embryonic cell cycle in Xenopus.

We report here the first extensive in vivo study of cell cycle regulation in the Xenopus embryo. Cyclin A1, B1, B2, and E1 levels, Cdc2 and Cdk2 kinase activity, and Cdc25C phosphorylation states were monitored during early Xenopus embryonic cell cycles. Cyclin B1 and B2 protein levels were high in the unfertilized egg, declined upon fertilization, and reaccumulated to the same level during the first cell cycle, a pattern repeated during each of the following 11 divisions. Cyclin A1 showed a similar pattern, except that its level was lower in the egg than in the cell cycles after fertilization. Cyclin B1/Cdc2 kinase activity oscillated, peaking before each cleavage, and Cdc25C alternated between a highly phosphorylated and a less phosphorylated form that correlated with high and low cyclin B1/Cdc2 kinase activity, respectively. Unlike the mitotic cyclins, the level of cyclin E1 did not oscillate during embryogenesis, although its associated Cdk2 kinase activity cycled twice for each oscillation of cyclin B1/Cdc2 activity, consistent with a role for cyclin E1 in both S-phase and mitosis. Although the length of the first embryonic cycle is regulated by both the level of cyclin B and the phosphorylation state of Cdc2, cyclin accumulation alone was rate-limiting for later cycles, since overexpression of a mitotic cyclin after the first cycle caused cell cycle acceleration. The activity of Cdc2 closely paralleled the accumulation of cyclin B2, but cell cycle acceleration caused by cyclin B overexpression was not associated with elevation of Cdc2 activity to higher than metaphase levels. Tyrosine phosphorylation of Cdc2, absent during cycles 2-12, reappeared at the midblastula transition coincident with the disappearance of cyclin E1. Cyclin A1 disappeared later, at the beginning of gastrulation. Our results suggest that the timing of the cell cycle in the Xenopus embryo evolves from regulation by accumulation of mitotic cyclins to mechanisms involving periodic G1 cyclin expression and inhibitory tyrosine phosphorylation of Cdc2.