Abstract
BackgroundThe cell cycles of the Xenopus laevis embryo undergo extensive remodeling beginning at the midblastula transition (MBT) of early development. Cell divisions 2–12 consist of rapid cleavages without gap phases or cell cycle checkpoints. Some remodeling events depend upon a critical nucleo-cytoplasmic ratio, whereas others rely on a maternal timer controlled by cyclin E/Cdk2 activity. One key event that occurs at the MBT is the degradation of maternal Wee1, a negative regulator of cyclin-dependent kinase (Cdk) activity.ResultsIn order to assess the effect of Wee1 on embryonic cell cycle remodeling, Wee1 mRNA was injected into one-cell stage embryos. Overexpression of Wee1 caused cell cycle delay and tyrosine phosphorylation of Cdks prior to the MBT. Furthermore, overexpression of Wee1 disrupted key developmental events that normally occur at the MBT such as the degradation of Cdc25A, cyclin E, and Wee1. Overexpression of Wee1 also resulted in post-MBT apoptosis, tyrosine phosphorylation of Cdks and persistence of cyclin E/Cdk2 activity. To determine whether Cdk2 was required specifically for the survival of the embryo, the cyclin E/Cdk2 inhibitor, Δ34-Xic1, was injected in embryos and also shown to induce apoptosis.ConclusionTaken together, these data suggest that Wee1 triggers apoptosis through the disruption of the cyclin E/Cdk2 timer. In contrast to Wee1 and Δ34-Xic1, altering Cdks by expression of Chk1 and Chk2 kinases blocks rather than promotes apoptosis and causes premature degradation of Cdc25A. Collectively, these data implicate Cdc25A as a key player in the developmentally regulated program of apoptosis in X. laevis embryos.
Highlights
The cell cycles of the Xenopus laevis embryo undergo extensive remodeling beginning at the midblastula transition (MBT) of early development
To determine whether the cell cycle lengthening induced by exogenous Wee1 coincided with the phosphorylation of cyclin-dependent kinase (Cdk), embryos expressing Wee1 or luciferase were assayed by Western analysis using a phosphoCdk primary antibody (Figure 1C)
Overexpression of Wee1 triggers apoptosis after the MBT In previous studies, we showed that X. laevis embryos expressing exogenous checkpoint kinases, Chk1 or Chk2, exhibited a similar cell cycle delay and premature Cdk phosphorylation [28,29]
Summary
The cell cycles of the Xenopus laevis embryo undergo extensive remodeling beginning at the midblastula transition (MBT) of early development. Cell divisions 2–12 consist of rapid cleavages without gap phases or cell cycle checkpoints. The first twelve cleavage cycles following fertilization consist of rapid oscillations between S and M phase without intervening gap phases. These cell cycles do not engage checkpoints in response to damaged or unreplicated DNA [1,2,3]. Beginning at the midblastula transition (MBT), cell cycles lengthen, acquiring gap phases and operable cell cycle checkpoints [5,6]. The molecular players in cell cycle remodeling during the early development of X. laevis have been (page number not for citation purposes)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.