Mitotic memory of active transcription in mouse ES cells – MitMAT

The preservation of cell identity relies on the maintenance of gene expression programs through mitosis. Generally, this is mediated by epigenetic systems of repression involving chromatin modifications. This mechanism provides a simple mean to mitotically inherit silent transcription states, which, by default, define the set of active genes underlying cellular identity. Although this model of gene regulation has been widely and successfully applied to the vast majority of mammalian systems, it does not provide a satisfactory explanation to the properties of mouse Embryonic Stem (ES) cells - a pluripotent population of cells derived from the inner cell mass of the blastocyst. In-vitro expanded ES cells display extraordinary abilities: they are captured in an undifferentiated and self-renewing state, yet they are capable of multi-lineage differentiation even after virtually infinite cell generations. To proliferate and maintain the undifferentiated state, ES cells need to keep silent a large number of genes that trigger cell differentiation. However, the genetic invalidation of the main systems of epigenetic repression (Polycomb, CpG methylation, H3K9 methylation) does not impair self-renewal. Hence, ES cells constitute an exception regarding the general mechanism of epigenetic inheritance of gene expression profiles.

The observation that chromatin-mediated epigenetic silencing seems not operative in ES cells suggests that silencing of developmental genes may be de-novo established after each cell division owing to the highly efficient action of a hand-full of sequence-specific pluripotency transcription factors. In this context, it is noteworthy that ES cells rely on the permanent activity of pluripotency genes (such as Oct4 and Sox2). However, mitosis represents a dramatic event for the cell regarding the interactions established between transcription factors and their binding DNA targets: the high compaction to which the chromatin is subjected during mitosis leads to the general loss of transcription factor binding. Therefore, after each cell division, ES cells need to rapidly re-establish the pluripotency network in order to avoid expression of differentiation genes and maintain their undifferentiated and pluripotent identity: the existence of a memory of gene activation (rather than repression), could in this context be particularly interesting.

Over the last few years new observations are reinforcing the potential role of a memory of gene activation: in several biological contexts it has been shown that certain sequence-specific transcription factors are able to retain their binding to their genomic targets during mitosis. This so-called “mitotic bookmarking” mechanism represents a simple mean to maintain active gene transcription throughout cell generations: it acts as a memory of gene expression that instructs transcription programs from mother to daughter cells. We have hypothesized that mitotic bookmarking by pluripotency factors mediates the efficient re-establishment of the network upon division in order to preserve the identity of ES cells. Over the last two years we have demonstrated that Esrrb, a pluripotency transcription factor, acts indeed as a mitotic bookmarking regulator of ES cells. Here, we describe the strategies that will be implemented to study mitotic bookmarking by Esrrb and other pluripotency transcription factors in ES cells.