For successful lineage reprogramming the target cell program needs to be activated and the donor cell program needs to be silenced. Typically, the same factors are used to reprogram many different donor cell types. This raised the puzzling question how the identical combination of transcription factors can silence so many different donor programs because most transcriptional repressors are considered lineage-specific such as REST which blocks neuronal genes and Groucho which blocks hepatic genes. Therefore, a different set of lineage repressors would have to be induced in different donor cell types.
Here we found that the reprogramming factor Myt1-like (Myt1l) is a pro-neuronal “master repressor” which inhibits scores of lineage programs except the neuronal lineage. Its repressive function is mediated by direct interaction and recruitment of the Sin3/HDAC chromatin complex through a novel protein domain. Myt1l is the only pan-neuronal transcription factor known that is also specifically expressed in neurons. Surprisingly, its physiological targets are almost identical in neurons as in fibroblasts when ectopically expressed suggesting that unrelated lineage programs (such as the fibroblast program) must be silenced by active recruitment of transcriptional repressors rather then by passive epigenetic mechanisms to maintain the neuronal identity.
In this project we also defined the chromatin accessibility changes induced by the neuronal reprogramming factors throughout conversion of fibroblasts into induced neuronal (iN) cells. Thousands of genomic loci are affected as early as 12 hours after reprogramming. Many changes occur between days 2 and 5 of the 3-week reprogramming process. This chromatin switch coincides with robust activation of endogenous neuronal TFs. Integrating chromatin accessibility and transcriptome changes, we built a network model of dynamic TF regulation during reprogramming,andidentifiedseveralkeylineagefactorsthatmediatethereprogrammingprocess.