Negative Elongation Factor (NELF) regulates muscle progenitor expansion for efficient myofiber repair and stem cell pool repopulation [S5_RNApolII_Cut&Tag]

Negative Elongation Factor (NELF) is a critical transcriptional regulator that works through stabilizing paused RNA Polymerase to permit rapid gene expression changes in response to environmental cues. Whilst NELF is essential for embryonic development, its role in adult stem cells remains unclear. Here, through a muscle stem cell-specific deletion, we show that NELF is required for efficient muscle regeneration and replenishing the stem cell pool. Mechanistic studies using PRO-seq, single cell trajectory analyses and myofiber cultures revealed that NELF works at a specific stage of muscle regeneration whereby it mediates massive expansion of muscle progenitors. Strikingly, transplantation experiments indicate that these progenitors are also necessary for stem cell pool repopulation implying they are able to return to quiescence. Thus, we identified a critical role for NELF in expansion of muscle progenitors in response to injury and revealed that progenitors returning to quiescence are major contributors to stem cell pool repopulation.