GSE126413
Pro-neuronal activity of Myod1 due to promiscuous binding [ATAC-seq]
Basic helix-loop-helix (bHLH) pioneer transcription factors Myod1 and Ascl1 are biochemically related but produce fundamentally different outcomes when expressed in fibroblasts: Myod1 produces muscle cells and Ascl1 induces neurons. Here, we sought to investigate the molecular mechanisms explaining the differential activity. Surprisingly, we found a large overlap in the overall binding patterns of Ascl1 and Myod1 in fibroblasts, with both transcription factors accessing both neuronal and myogenic targets. We also observed similar changes in chromatin accessibility and transcriptional activation.  Yet, Myod1 predominantly induced a muscle program and Ascl1 a neuronal program. We found that differences in binding affinity at key targets resulted in largely distinct reprogramming outcomes. Accordingly, exchanging Myod1's C-terminal protein-protein interacting domain and DNA-binding basic domain with those of Ascl1 induces an Ascl1-like binding and converts Myod1 into a pro-neuronal factor. Finally, we found that co-expression of Myod1 with the transcriptional repressor Myt1l inhibits induction of the muscle program and yields functional neuronal cells. Our findings are compatible with the notion that pioneer factor activity is associated with high-affinity protein-DNA and suggest that promiscuous binding of pioneer factors can induce unspecific lineage features which need to be kept in check by co-factor interactions. Overall design: ATAC-seq was performed on MEF+Myod1 48hr post-dox induction. New data was analyzed together with existing data on MEF alone, MEF+rtTA and MEF+Ascl1 (GSE101397) to determine differences in chromatin accessibility in MEFs when overexpressing either Myod1 or Ascl1