The E3 Ubiquitin Ligase Nedd4L Acts as a Checkpoint Against Activation in Quiescent Muscle Stem Cells

Adult stem cells play a critical role in tissue repair and maintenance. In tissues with slow turnover, including skeletal muscle, these cells are maintained in a mitotically quiescent state yet remain poised to re-enter the cell cycle to replenish themselves and regenerate the tissue. Using a multiomics approach we identify the PAX7/NEDD4L axis as a checkpoint against muscle stem cell activation in homeostatic skeletal muscle. Our findings demonstrate that PAX7 transcriptionally activates the E3 ubiquitin ligase Nedd4L and that the conditional genetic deletion of Nedd4L impairs muscle stem cell quiescence, with an upregulation of cell cycle and myogenic differentiation genes. Loss of Nedd4L in muscle stem cells results in the expression of DCX which is only expressed during their in vivo activation. Together, this data establishes that the ubiquitin proteasome system, mediated by Nedd4L, is a key regulator of the muscle stem cell quiescent state in non-injured skeletal muscle. Highlights- General inhibition of the ubiquitin proteasome system with MG132 results in muscle stem cells (MuSCs) breaking quiescence. - The E3 ubiquitin ligase Nedd4L is a transcriptional target of Pax7. - The Pax7/Nedd4l axis restricts MuSC activation in homeostatic skeletal muscle. - Genetic deletion of Nedd4L induces MuSCs transition towards activation.