The double homeobox transcription factor Duxbl is essential for the exit from totipotency in mice

Abstract

The preimplantation stage is the first phase in mammalian development and starts with fusion of egg and sperm. Thereby, fertilization gives rise to a totipotent single cell, called zygote, which holds the capacity to develop into an entire new organism. Within the zygote, the parental chromatin is reprogrammed to form a new embryonic genome. Activation of this genome results in the first transcriptional event in life, called zygotic genome activation (ZGA). In mice, this transcriptional burst occurs at the 2-cell stage and results in transient upregulation of thousands of genes. Immediately afterwards, at the 4-cell stage, those genes become silenced. Even though ZGA was discovered decades ago, its regulation remains poorly understood. Recently, the double homeobox transcription factor Dux was shown to drive a 2- cell-like conversion of pluripotent embryonic stem cells. In these cells, ectopic expression of Dux induces opening of ZGA-associated gene loci and consequent transcription of encoded genes. Thereby, Dux was discovered as a driver of ZGA. However, genetic depletion of Dux results in negligible impairment of ZGA and is compatible with development. Nevertheless, both activation and rapid decommissioning of the 2-cell-specific transcriptional profile are essential for life. However, how this is regulated has remained unknown. In this thesis, I investigated a potential role for another member of the DUX transcription factor family called Duxbl. Transcriptional profiling of developing murine embryos revealed that Duxbl is part of the ZGA-expression panel. In contrast to Dux, Duxbl does not interact with a histone acetyl transferase and does not induce expression of ZGA-associated genes when overexpressed in embryonic stem cells. Instead, Duxbl suppresses Dux-induced upregulation of 2-cell specific transcripts in vitro. Mechanistically, conservation of the DNA-binding domains of Duxbl and Dux is associated with occupation of Dux-binding sites by Duxbl within ZGA-promoters. Accordingly, preponed Duxbl expression in mouse embryos suppressed a large portion of ZGA- genes and resulted in a 2-cell stage developmental arrest. Inversely, inactivation of Duxbl led to upregulation of ZGA-associated transcripts and a developmental block at the 2- to 4-cell stage. This study identifies Duxbl as a master regulator in preimplantation development that is necessary for decommissioning the totipotency-associated ZGA-burst and thereby enabling the first cell fate decisions.