Characterizing the Role of the Negative Elongation Factor Complex in Myogenic Cell State Changes

Abstract

The robust regenerative capacity of muscle stem cells (MuSCs) and their progenitors depends on their ability to undergo rapid and vast changes to their transcriptome during cell state changes. While transcription factors and epigenetic remodelling proteins are critical to render genes permissive for transcription, often these genes are found to have paused promoter proximal RNA Polymerase II (Pol II) which remains in a rate-limiting poised state. Indeed, while prior studies have shown poised Pol II is often regulated by the Negative Elongation Factor (NELF) to induce rapid changes in gene expression, the specific need for NELF in somatic stem cell populations has not been previously examined. In this thesis, we identify a specific requirement for NELF-dependent promoter proximal Pol II pausing in proliferating myogenic progenitors. Here, NELF stabilizes nascent transcripts associated with the paused RNA Pol II at genes required to maintain muscle progenitors in cell cycle. This promotes expansion of the pool of myogenic progenitors required to adequately repair damaged skeletal muscle. Our molecular analysis suggests that in proliferating progenitors, NELF-bound Pol II ensures the stabilization of transcripts, and continued expression of genes that prevent p53-mediated cell cycle withdrawal and terminal differentiation. Unexpectedly, this work revealed a previously unappreciated contribution of proliferating myogenic progenitors to replenish the stem cell niche in support of MuSC self-renewal during skeletal muscle regeneration. Based on our results, new therapeutic avenues which could treat muscle wasting disease are also discussed.