Myt1l safeguards neuronal identity by actively repressing many non-neuronal fates

Abstract

Normal differentiation and induced reprogramming require the activation of target cell programs and silencing of donor cell programs. In reprogramming, the same factors are often used to reprogram many different donor cell types. As most developmental repressors, such as RE1-silencing transcription factor (REST) and Groucho (also known as TLE), are considered lineage-specific repressors, it remains unclear how identical combinations of transcription factors can silence so many different donor programs. Distinct lineage repressors would have to be induced in different donor cell types. Here, by studying the reprogramming of mouse fibroblasts to neurons, we found that the pan neuron-specific transcription factor Myt1-like (Myt1l) exerts its pro-neuronal function by direct repression of many different somatic lineage programs except the neuronal program. The repressive function of Myt1l is mediated via recruitment of a complex containing Sin3b by binding to a previously uncharacterized N-terminal domain. In agreement with its repressive function, the genomic binding sites of Myt1l are similar in neurons and fibroblasts and are preferentially in an open chromatin configuration. The Notch signalling pathway is repressed by Myt1l through silencing of several members, including Hes1. Acute knockdown of Myt1l in the developing mouse brain mimicked a Notch gain-of-function phenotype, suggesting that Myt1l allows newborn neurons to escape Notch activation during normal development. Depletion of Myt1l in primary postmitotic neurons de-repressed non-neuronal programs and impaired neuronal gene expression and function, indicating that many somatic lineage programs are actively and persistently repressed by Myt1l to maintain neuronal identity. It is now tempting to speculate that similar 'many-but-one' lineage repressors exist for other cell fates; such repressors, in combination with lineage-specific activators, would be prime candidates for use in reprogramming additional cell types.

Rapid Chromatin Switch in the Direct Reprogramming of Fibroblasts to Neurons

“…Since both Ascl1 and Myod1 (a myogenic bHLH TF) bind a common E-box sequence (CAGCTG) (Castro et al, 2011; Fong et al, 2015), we speculate that the strong over expression of Ascl1 led to the initiation of an off-target myogenic program that is not normally observed in a more tightly controlled endogenous environment. This is also consistent with our recent finding that Myt1l represses non-neuronal fates, including myogenic genes, suggesting potential synergy between Ascl1 and Myt1l (Mall et al, 2017). …”

Programming and reprogramming the brain: a meeting of minds in neural fate

“…Although Myt1l bound to similar sites in both MEFs and neurons, it alone was not sufficient to elicit neuronal reprogramming. Structure-function experiments suggested that Myt1l acts as a transcriptional repressor, which is consistent with its interaction with Sin3B (Mall et al, 2017). Transcriptomic data suggest that many genes and possibly somatic expression programmes, including genes representing a fibroblast signature, are downregulated by Myt1l, and that neural genes are impoverished in Myt1l binding sites.…”

The novel tool of cell reprogramming for applications in molecular medicine

“…A recent study on iPS cell reprogramming suggested that the reprogramming factors themselves initially bind and decommission fibroblast enhancers and gradually activate pluripotency enhancers [70]. Investigating the iN cell reprogramming mechanism, we found that Myt1l, one of the three reprogramming factors, appears to be dedicated to suppress the fibroblast and many other non-neuronal programs whereas activation of the neuronal program is accomplished by the “on target pioneer” factor Ascl1 [69, 79]. Therefore, repressing alternative cell fates along with concomitant induction of cell type specific programs enable faithful and efficient binary decisions during cell fate reprogramming.…”