[Using 2C-like cells to understand embryonic totipotency]. / Comprendre la totipotence embryonnaire à partir des cellules 2C-like.

Totipotency is the ability of a cell to generate a whole organism, a property that characterizes the first embryonic cells, such as the zygote and the blastomeres. This review provides a retrospective on the progress made in the last decade in the study of totipotency, especially with the discovery of mouse ES cells expressing markers of the 2-cell stage (2C-like cells). This model has greatly contributed to a better understanding of the molecular mechanisms involved in totipotency (pioneer factors, epigenetic regulation, splicing, nuclear maturation). 2C-like cells have also paved the way for the development of new cellular models of human totipotency.

Title: Comprendre la totipotence embryonnaire à partir des cellules 2C-like. Abstract: La totipotence est la capacité d'une cellule à générer un organisme entier, une propriété qui caractérise les premières cellules embryonnaires, comme le zygote et les blastomères. Dans cette revue, nous proposons une rétrospective des avancées réalisées au cours de la dernière décennie concernant l'étude de la totipotence avec, notamment, la découverte des cellules ES murines exprimant des marqueurs du stade 2-cellules (2CLC). Ce modèle a considérablement contribué à la meilleure compréhension des mécanismes moléculaires impliqués dans la totipotence (facteurs pionniers, régulation épigénétique, épissage, maturation nucléaire). Les cellules 2CLC ont aussi ouvert la voie au développement de nouveaux modèles cellulaires de totipotence humaine.

A genetic screen identifies BEND3 as a regulator of bivalent gene expression and global DNA methylation.

Epigenetic mechanisms are essential to establish and safeguard cellular identities in mammals. They dynamically regulate the expression of genes, transposable elements and higher-order chromatin structures. Consequently, these chromatin marks are indispensable for mammalian development and alterations often lead to disease, such as cancer. Bivalent promoters are especially important during differentiation and development. Here we used a genetic screen to identify new regulators of a bivalent repressed gene. We identify BEND3 as a regulator of hundreds of bivalent promoters, some of which it represses, and some of which it activates. We show that BEND3 is recruited to a CpG-containg consensus site that is present in multiple copies in many bivalent promoters. Besides having direct effect on the promoters it binds, the loss of BEND3 leads to genome-wide gains of DNA methylation, which are especially marked at regions normally protected by the TET enzymes. DNA hydroxymethylation is reduced in Bend3 mutant cells, possibly as consequence of altered gene expression leading to diminished alpha-ketoglutarate production, thus lowering TET activity. Our results clarify the direct and indirect roles of an important chromatin regulator, BEND3, and, more broadly, they shed light on the regulation of bivalent promoters.

A genome-wide screen reveals new regulators of the 2-cell-like cell state.

In mammals, only the zygote and blastomeres of the early embryo are totipotent. This totipotency is mirrored in vitro by mouse '2-cell-like cells' (2CLCs), which appear at low frequency in cultures of embryonic stem cells (ESCs). Because totipotency is not completely understood, we carried out a genome-wide CRISPR knockout screen in mouse ESCs, searching for mutants that reactivate the expression of Dazl, a gene expressed in 2CLCs. Here we report the identification of four mutants that reactivate Dazl and a broader 2-cell-like signature: the E3 ubiquitin ligase adaptor SPOP, the Zinc-Finger transcription factor ZBTB14, MCM3AP, a component of the RNA processing complex TREX-2, and the lysine demethylase KDM5C. All four factors function upstream of DPPA2 and DUX, but not via p53. In addition, SPOP binds DPPA2, and KDM5C interacts with ncPRC1.6 and inhibits 2CLC gene expression in a catalytic-independent manner. These results extend our knowledge of totipotency, a key phase of organismal life.