The Integrated Stress Response Pathway Improves Aged Murine Muscle Stem Cell Activation and in vivo Regeneration.

For efficient regeneration, muscle stem cells (MuSCs) transition out of quiescence through a series of progressively more activated states. During MuSC aging, transition through the earliest steps is the slowest and delayed, with the molecular regulators that govern this transition not well characterized. By analyzing the dynamic changes of MuSCs at the molecular (scRNA-Seq and Cell Painting) and phenotypic (heteromotility) level at single cell resolution we found that the Integrated Stress Response (ISR) Pathway is a critical regulator of MuSC transition states. Aged MuSCs have increased baseline ISR activity in quiescence that does not increase during activation to levels observed in adult MuSCs. Rapid and transient pharmacological ISR activation in vitro was sufficient to increase aged MuSC activation rate and migratory behavior as well as alter the transcriptional states toward a younger phenotype. ISR activation also improved aged MuSC potency and aged mouse muscle regeneration in vivo. Therefore, pharmacological activation of the ISR has therapeutic potential to improve MuSC function and skeletal muscle repair during aging.