Chk1 is activated at the midblastula transition in Xenopus laevis embryos independently of DNA content and the cyclin E/Cdk2 developmental timer

Cell cycle checkpoints that are engaged in response to damaged and unreplicated DNA may serve additional, constitutive functions. In the developing Xenopus laevis embryo, the checkpoint kinase Chk1 is transiently activated at the midblastula transition (MBT), a period of extensive cell cycle remodeling including the acquisition of cell cycle checkpoints. The timing of many cell cycle remodeling events at the MBT, such as the lengthening of cell cycles, depends upon a critical nucleocytoplasmic (N/C) ratio. However, other events, including the degradation of maternal cyclin E, do not depend upon the N/C ratio, and are regulated by an autonomous developmental timer. To better understand what regulates Chk1 activation at the MBT, embryos were treated with aphidicolin, at different developmental times and for different lengths of time, to reduce the DNA content at the MBT. Chk1 was activated at the MBT in these embryos establishing that Chk1 activation occurs independently of the N/C ratio. Cdc25A is normally phosphorylated by Chk1 at the MBT and then degraded. The degradation of Cdc25A demonstrated partial dependence on DNA content, suggesting that factors other than Chk1 regulate its degradation. When the cyclin E developmental timer was disrupted with the Cdk2 inhibitor Δ34-Xic1, Chk1 was still activated at the MBT, indicating that activation of Chk1 at the MBT was not directly linked to the cyclin E timer. Conversely, unreplicated or damaged DNA, delayed the degradation of cyclin E at the MBT, indicating that the cyclin E/Cdk2 timer is sensitive to engagement of cell cycle checkpoints.