Wee1 kinase alters cyclin E/Cdk2 and promotes apoptosis during the early embryonic development of Xenopus laevis.
BACKGROUND
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. 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.


RESULTS
In 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, Delta34-Xic1, was injected in embryos and also shown to induce apoptosis.


CONCLUSIONS
Taken together, these data suggest that Wee1 triggers apoptosis through the disruption of the cyclin E/Cdk2 timer. In contrast to Wee1 and Delta34-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.
Wroble
Brian N
BN
Department of Biological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA, USA. bwroble@vt.edu
Finkielstein
Carla V
CV
Sible
Jill C
JC
D000818:Animals; D017209:Apoptosis / Q000235:genetics*; D036703:Blastula / Q000196:embryology; D015153:Blotting, Western; D018797:Cell Cycle Proteins / Q000235:genetics*; D019927:Cyclin E / Q000235:genetics*; D051357:Cyclin-Dependent Kinase 2 / Q000037:antagonists & inhibitors / Q000235:genetics*; D004625:Embryo, Nonmammalian / Q000166:cytology / Q000201:enzymology; D018507:Gene Expression Regulation, Developmental; D011505:Protein-Tyrosine Kinases / Q000235:genetics*; D012333:RNA, Messenger / Q000235:genetics; D029867:Xenopus Proteins / Q000037:antagonists & inhibitors / Q000235:genetics*; D014982:Xenopus laevis / Q000196:embryology* / Q000235:genetics; D020687:cdc25 Phosphatases / Q000235:genetics
D018797:Cell Cycle Proteins; D019927:Cyclin E; D012333:RNA, Messenger; D029867:Xenopus Proteins; C466028:Wee2 protein, Xenopus; D011505:Protein-Tyrosine Kinases; C495891:Cdk2 protein, Xenopus; D051357:Cyclin-Dependent Kinase 2; C102266:Cdc25A protein, Xenopus; D020687:cdc25 Phosphatases