The Role of Chk2 and Wee1 Protein Kinases during the Early Embryonic Development of Xenopus laevis
Wroble, Brian Noel
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In somatic cells, when DNA is damaged or incompletely replicated, checkpoint pathways arrest the cell cycle prior to M or S phases by inhibiting cyclin-dependent kinases (Cdks). In Xenopus laevis, embryonic cellular divisions (2-12) consist of rapid cleavage cycles in which gap phases, checkpoint engagement, and apoptosis are absent. Upon the completion of the 12th cellular division, the midblastula transition (MBT) begins and the cell cycle lengthens, acquiring gap phases. In addition, cell cycle checkpoint pathways and an apoptotic program become functional. The studies described here were performed to better understand the roles of two protein kinases, Chk2/Cds1 and Wee1, during checkpoint signaling in the developing embryo. The DNA damage checkpoint is mediated by the Chk2/Cds1 kinase. Conflicting evidence implicates Chk2 as an inhibitor or promoter of apoptosis. To better understand the developmental function of Chk2 and its role in apoptosis, we expressed wild-type (wt) and dominant-negative (DN) Chk2 in Xenopus embryos. Wt-Chk2 created a pre-MBT checkpoint by promoting degradation of Cdc25A and phosphorylation of Cdks. Embryos expressing DN-Chk2 developed normally until gastrulation and then underwent apoptosis. Conversely, low doses of wt-Chk2 blocked radiation-induced apoptosis. These data indicate that Chk2 inhibits apoptosis in the early embryo. Therefore, Chk2 operates as a switch between cell cycle arrest and apoptosis in response to genomic assaults. In Xenopus laevis, Wee1 kinase phosphorylates and inhibits Cdks. To determine the role of Wee1 in cell cycle checkpoint signaling and remodeling at the MBT, exogenous Wee1 was expressed in one-cell stage embryos. Modest overexpression of Wee1 created a pre-MBT cell cycle checkpoint, similar to Chk2, characterized by cell cycle delay and phosphorylation of Cdks. Furthermore, overexpression of Wee1 disrupted remodeling events that normally occur at the MBT, including degradation of Cdc25A, cyclin E, and Wee1. Interestingly, overexpression of Wee1 also resulted in post-MBT apoptosis. Taken together, these data suggest the importance of Wee1 as not only a Cdk inhibitory kinase, but also potentially as a promoter of apoptosis during early development of Xenopus laevis. The studies described here provide evidence that Chk2 and Wee1 have both similar and distinct roles in the developing embryo.
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