Loss of XChk1 function triggers apoptosis after the midblastula transition in Xenopus laevis embryos.

Prior to the midblastula transition (MBT), Xenopus laevis embryos do not engage cell cycle checkpoints, although overexpression of the kinase XChk1 arrests cell divisions. At the MBT, XChk1 transiently activates and promotes cell cycle lengthening. In this study, endogenous XChk1 was inhibited by the expression of dominant-negative XChk1 (DN-XChk1). Development appeared normal until the early gastrula stage, when cells lost attachments and chromatin condensed. TUNEL and caspase assays indicated these embryos died by apoptosis during gastrulation. Embryos with unreplicated DNA likewise died by apoptosis. Embryos expressing DN-XChk1 proceeded through additional rapid rounds of DNA replication but initiated zygotic transcription on schedule. Therefore, XChk1 is essential in the early Xenopus embryo for cell cycle remodeling and for survival after the MBT.