Breaking the epigenetic barrier during reversion of post-implantation epiblast cells

Primitive ectoderm cells (PE) in blastocysts represent the foundation of the pluripotent state, which is lost progressively during development. For example, development of epiblast cells from PE in postimplantation embryos is accompanied by transcriptional and epigenetic changes, including DNA methylation and X inactivation (Refs); these changes alter the nature of epiblast cells fundamentally, affecting their responsiveness to signaling molecules, and constitute a robust boundary that prevents their reversion to a PE-like state. Notably, epiblast cells unlike PE, are refractory to leukaemia inhibitory factor (LIF)/STAT3 signalling in vitro; instead, they respond to FGF/Activin to form self-renewing epiblast stem cells (EpiSCs) that are like human ES cells, which differ significantly from mouse embryonic stem cells (ES) derived from PE. However, here we show that under appropriate conditions, epiblast cells from postimplantation embryos can respond to LIF/STAT3/fetal calf serum (FCS), and undergo reprogramming to form embryonic stem cell -like cells (repiES: reprogrammed epiblast ES-like cells). Reprogramming of epiblast cells occurs progressively. First, they form colonies that retain key properties of epiblast cells (cEpi: cultured epiblast), which subsequently show erasure of epigenetic modifications, including DNA demethylation and X-reactivation to generate repiES. Analysis also revealed that repiES progressively acquire a transcriptome profile of ES cells that is distinct from cEpi and EpiSCs. In chimeras, repiES contributed to all the tissues, including germ cells. Thus, we show for the first time that reversion of epiblast cells to repiES phenotype entails progressive loss of phenotypic and epigenetic memory of epiblast cells. Our study provides insights into underlying mechanisms, and a tractable model for how signaling molecules induce epigenetic reprogramming of cells leading to an elemental pluripotent state. We validated our findings concerning reprogramming of epiblast cells by microarray analysis, which showed that repiES are clearly more similar to control ES cells and differ from the cEpi cells, Changes in the transcriptome must also account for the activation of the Oct4-deltaPE-GFP reporter via the distal enhancer. 12 samples