Regulation of zygotic genome activation and DNA damage checkpoint acquisition at the mid-blastula transition

Abstract

Following fertilization, oviparous embryos undergo rapid, mostly transcriptionally silent cleavage divisions until the mid-blastula transition (MBT), when large-scale developmental changes occur, including zygotic genome activation (ZGA) and cell cycle remodeling, via lengthening and checkpoint acquisition. Despite their concomitant appearance, whether these changes are co-regulated is unclear. Three models have been proposed to account for the timing of (ZGA). One model implicates a threshold nuclear to cytoplasmic (N:C) ratio, another stresses the importance cell cycle elongation, while the third model invokes a timer mechanism. We show that precocious Chk1 activity in pre-MBT zebrafish embryos elongates cleavage cycles, thereby slowing the increase in the N:C ratio. We find that cell cycle elongation does not lead to transcriptional activation. Rather, ZGA slows in parallel with the N:C ratio. We show further that the DNA damage checkpoint program is maternally supplied and independent of ZGA. Although pre-MBT embryos detect damage and activate Chk2 after induction of DNA double-strand breaks, the Chk1 arm of the DNA damage response is not activated, and the checkpoint is nonfunctional. Our results are consistent with the N:C ratio model for ZGA. Moreover, the ability of precocious Chk1 activity to delay pre-MBT cell cycles indicate that lack of Chk1 activity limits checkpoint function during cleavage cycles. We propose that Chk1 gain-of-function at the MBT underlies cell cycle remodeling, whereas ZGA is regulated independently by the N:C ratio.

Cohesin facilitates zygotic genome activation in zebrafish

“…Compromising this aspect of the normal gene expression program will almost certainly affect embryogenesis, consistent with mutations in cohesin causing the human developmental disorder Cornelia de Lange syndrome (CdLS). Although cell proliferation is central to early development, in Drosophila and zebrafish, ZGA is independent of, or upstream of, cell cycle number and checkpoint regulators (Blythe and Wieschaus, 2015;Zhang et al, 2014aZhang et al, , 2017. Consistent with these observations, we found that delay in ZGA occurred in Rad21depleted embryos that had the same number of cells as controls.…”

Coordinating Cell Cycle Remodeling with Transcriptional Activation at the Drosophila MBT

“…This failure of α-amanitin to suppress the lethal phenotype of checkpoint-deficient embryos is also in agreement with results in Xenopus and zebrafish (Ikegami, Rivera-Bennetts, Brooker, & Yager, 1997;Newport & Dasso, 1989;Zhang, Kothari, Mullins, & Lampson, 2014) that showed no effect of α-amanitin on checkpoint-dependent S-phase lengthening at the MBT. Instead, the demonstrated link between checkpoint activation and the chromatin transition at the MBT highlights the need to identify the specific changes that occur in chromatin structure preceding the MBT, especially those that delineate euchromatic and heterochromatic regions and may have a major impact on checkpoint activation.…”

Cohesin facilitates zygotic genome activation in zebrafish

“…Although cell proliferation is central to early development, in Drosophila and zebrafish, ZGA is independent of, or upstream of cell cycle number and checkpoint regulators (Blythe and Wieschaus, 2015; Zhang et al, 2014a; Zhang et al, 2017). Consistent with these observations, we found that delay in ZGA occurred in Rad21-depleted embryos that had the same number of cells as controls.…”