ASCL1 can reprogram somatic cells into neurons by opening chromatin at inaccessible sites. This reprogramming ability can be enhanced by preventing ASCL1 from being phosphorylated. However, the molecular basis of this enhancement is unclear, and it is not known why some cell types are more responsive to ASCL1-mediated reprogramming than others. Here, Roberta Azzarelli, Anna Philpott and colleagues explore these questions by overexpressing ASCL1 in different murine cell types. They show that reprogramming efficiency is enhanced in mesodermal and neuroectodermal cells when they use a phosphomutant form of ASCL1, which cannot be phosphorylated. This mutant protein is also expressed at higher levels in the mouse cells than wild-type ASCL1, which indicates that preventing ASCL1 from being phosphorylated enhances its stability. However, increasing the level of wild-type ASCL1 in the cells does not lead to an increase in reprogramming efficiency, suggesting that increased stability does not explain the enhanced reprogramming capabilities of the phosphomutant. Instead, ATAC-seq shows that the phosphomutant form of ASCL1 promotes increased chromatin remodelling in regions that are associated with neuronal gene expression. Overall, this work enhances our understanding of the molecular mechanisms underpinning ASCL1-mediated reprogramming, which could have implications for improving current reprogramming approaches.
