2020_BSDB_2.pdf

Mouse embryonic stem cells (mESCs) represent are the primary in vitro model for many aspects of early embryonic development. They are the descendants of the cells of the cells inner cell mass (ICM) of early blastocyst and whilst they maintain the potency of their forerunner cells, they acquire the ability to self-renew during ex vivo culture.

Over the past decades considerable work has focused in understanding the molecular basis of this ex vivo self-renewal and how it relates to cellular potency. The dominant model is that the combined activities of a discrete network of transcription factors (TFs), referred to as the pluripotency network, act cooperatively to maintain their own expression and promote proliferation whilst simultaneously blocking differentiation. However useful, this model has yet to adequately explain why such a complex, self-sustaining and robust self-renewal network would have evolved in a population that exhibits negligible self-renewal in vivo. In the context of Waddingtonian lineage transitions, as a cell traverses developmental landscapes, the gene regulatory network (GRN) that establishes each discernible cell state must also define the range of lineage trajectories available to it, incorporating various inputs like signalling and positional information to choose one trajectory over the other.

Here we present data that argues that the canonical pluripotency network can be divided into at least two distinct subgroups; those that promote lineage and those that balance lineage decisions, with both classes of TFs functioning in creating a state of lineage competency, similar to what is seen in the ICM. Specifically, we see that the combined activities of ESRRB and NANOG promote a gene expression signature similar to the ICM when expressed at a near 1:1 ratio. However if ESRRB expression persists in the absence of NANOG it is no longer tethered to epiblast associated loci and acts to drive the expression of primitive endoderm (PrE) associated genes. We formalise the kinetics of this interaction and show that oscillation of ERK signalling result in asynchronous fluctuations in NANOG and ESRRB ratios and a concomitant stepwise increase in PrE determinants, similar to what is seen in the ICM. From this we propose that the ex vivo culture environment selects for ratios of TFs that, whilst maintaining the expression of ICM-like genes, and as such potency, prevent cells from progressing further in development.