Phospho-regulation of ASCL1-mediated chromatin opening during cellular reprogramming.

The proneural transcription factor ASCL1 regulates neurogenesis and drives somatic cell reprogramming into neurons. However, not all cell types can be reprogrammed by ASCL1, raising the questions of what provides competence and how we can overcome barriers to enable directed differentiation. Here, we investigate how levels of ASCL1 and its phosphorylation modulate its activity over progressive lineage restriction of embryonic stem cells. We find that inhibition of ASCL1 phosphorylation enhances reprogramming of both mesodermal and neuroectodermal cells, while pluripotent cells remain refractory to ASCL1-directed neuronal differentiation. By performing RNA-seq and ATAC-seq in neuroectoderm, we find that un(der)phosphorylated ASCL1 causes increased chromatin accessibility at sites proximal to neuronal genes, accompanied by their increased expression. Combined analysis of protein stability and proneural function of phosphomutant and phosphomimetic ASCL1 reveals that protein stability plays only a marginal role in regulating activity, while changes in amino acid charge cannot fully explain enhanced activity of the serine-proline mutant variants of ASCL1. Our work provides new insights into proneural factor activity and regulation and suggests ways to optimize reprogramming protocols in cancer and regenerative medicine.