Altered expression of Chk1 disrupts cell cycle remodeling at the midblastula transition in Xenopus laevis embryos

Studies in several model systems, including Xenopus laevis oocytes and embryos, have indicated that the checkpoint kinase, Chk1, is required for early development, even in the absence of damaged or unreplicated DNA. Chk1 is transiently activated at the midblastula transition (MBT) in Xenopus, a time when the cell cycle remodels from rapid embryonic cleavage cycles to longer, more regulated somatic cell cycles. To better understand the role of Chk1 in cell cycle remodeling, mRNA encoding Chk1 was microinjected into 1-cell stage embryos, and the effects on both the MBT and on the expression of several cell cycle regulators were examined. Zygotic transcription, a hallmark of the MBT that depends upon the nucleocytoplasmic (N/C) ratio, was blocked, as was degradation of maternal cyclin E, an event of the MBT that occurs independent of the N/C ratio. Levels of mitotic cyclins were elevated throughout early development, consistent with cell cycle arrest at G2/M. In these embryos, Cdc25A level was low, whereas Cdc25C level was not affected. Furthermore, the level of Wee1 increased at 6 hrs post-fertilization (pf), the time at which the MBT normally occurs, even though these embryos did not demonstrate any known markers of the MBT. These studies suggest that in addition to targeting Cdc25A for degradation, Chk1 may also function in cell cycle remodeling at the MBT by stabilizing Wee1 until it is replaced by the somatic Wee2 protein during gastrulation.