Balancing potency and differentiation in mouse embryo-derived stem cells and in vivo

Abstract

An inherent challenge for developing organisms is to maintain a critical balance between cell potency and differentiation. This is most obvious in embryonic stem (ES) cells where genetic, epigenetic and cell signalling pathways support ES cell ability to self-renew and to generate all embryonic lineages. Recently, a series of reports revealed how Polycomb-mediated repression might buffer the precocious expression of somatic lineage regulators in ES cells. Notably, these genes carry bivalent chromatin enriched in both repressive (H3K27me3) and permissive (H3K4me2) histone marks. They are targeted by the Polycomb repressive complexes PRC1 (Ring1B) and PRC2, which mediates H3K27me3 and assemble poised RNA polymerase II (RNAP II), conferring silencing of loci primed for future activation (or repression) upon ES cell differentiation. During early development the transition from morula to blastocyst is the starting point for lineage segregation into the inner cell mass (ICM) and trophectoderm (TE). ES cells are derived from the ICM and are pluripotent. By contrast, TE-derived trophoblast stem (TS) cells are multipotent and contribute solely to placenta formation in vivo. To address whether ES cells epigenetic features are unique attributes of pluripotent cells in the early embryo, we compared the epigenetic status of key developmental genes in blastocyst-derived stem cells and in vivo. We provide direct evidence that bivalent histone markings operate in vivo from the eight-cell up to the blastocyst stage. Unexpectedly, we show that bivalent domains are retained at many somatic lineage regulators in extra-embryonic restricted cells. However, and in contrast to pluripotent cells, PRC1 (Ring1B) and poised RNAP are excluded from these PRC2-bound genes, consistent with a loss of gene expression priming. Instead, bivalent genes become selectively targeted by Suv39h1-mediated repression upon trophoblast lineage commitment. Collectively, our results suggest a mutually exclusive role for Ring1b and Suv39h1 in specifying the fate of bivalent genes in a lineage-specific manner upon blastocyst formation and in stem cells. The transcription factor Nanog is a key part of the core genetic network that sustains self-renewal in ES cells. In contrast, the Fgf/Erk signalling pathway primes ES cell for differentiation. How pluripotency-associated intrinsic factors may modulate the effect of extracellular pathways is yet to be investigated. We propose that Nanog can block autocrine Fgf/Erk responses while maintaining paracrine signalling in ES cells through direct repression and activation of Fgfr2 and Fgf4, respectively. Absence of Nanog notably enables an up-regulation of Fgfr2 expression and higher levels of active phosphorylated Erk1/2 forms. Moreover, we demonstrate that ectopic expression of constitutively active Fgfr2 in Nanog over-expressing cells is sufficient to partly bypass ES cell undifferentiated state, further highlighting the delicate balance between self-renewal and differentiation in pluripotent stem cells.