TDP-43 sustains satellite cells to maintain and regenerate skeletal muscle.

Skeletal muscle satellite cells (SCs), residing between the myofiber plasma membrane and the surrounding basement membrane, continue to maintain and repair skeletal muscle throughout life. Typically, quiescent SCs can transition into a reversible alert state (GAlert) that primes them for rapid activation to maintain or repair muscle. From GAlert, SCs can either re-enter quiescence or commit to the cell cycle, expand, and differentiate to fuse with existing regenerating myofibers. Exit from quiescence requires extensive post-transcriptional remodeling, including changes in RNA processing and RNA-binding protein activity. We show that TDP-43, an RNA-binding protein, is essential for SC maintenance and muscle repair. Conditional deletion of TDP-43 in SCs caused a consistent and progressive loss of GAlert SCs, even in uninjured muscle, leading to depletion of the SC pool. TDP-43 haploinsufficiency was sufficient to impair SC maintenance, indicating that both alleles are required. Integrative analysis suggests that TDP-43 supports the expression of stress response-associated transcripts during the quiescent-to- GAlert transition and that failure to mount this response contributes to SC apoptosis. Thus, we identified TDP-43 as a critical regulator of survival of SCs as these cells activate, and we establish that TDP-43 is required for the maintenance and repair of skeletal muscle.