Endogenous fluctuations of OCT 4 and SOX 2 bias pluripotent cell fate decisions

SOX 2 and OCT 4 are pioneer transcription factors playing a key role in embryonic stem ( ES ) cell self‐renewal and differentiation. How temporal fluctuations in their expression levels bias lineage commitment is unknown. Here, we generated knock‐in reporter fusion ES cell lines allowing to monitor endogenous SOX 2 and OCT 4 protein fluctuations in living cells and to determine their impact on mesendodermal and neuroectodermal commitment. We found that small differences in SOX 2 and OCT 4 levels impact cell fate commitment in G1 but not in S phase. Elevated SOX 2 levels modestly increased neuroectodermal commitment and decreased mesendodermal commitment upon directed differentiation. In contrast, elevated OCT 4 levels strongly biased ES cells towards both neuroectodermal and mesendodermal fates in undirected differentiation. Using ATAC ‐seq on ES cells gated for different endogenous SOX 2 and OCT 4 levels, we found that high OCT 4 levels increased chromatin accessibility at differentiation‐associated enhancers. This suggests that small endogenous fluctuations of pioneer transcription factors can bias cell fate decisions by concentration‐dependent priming of differentiation‐associated enhancers.