Disclosures: Jason Silver, PhD: No financial relationships or conflicts of interest

Background and/or Objectives: The skeletal muscle microenvironment transiently remodels and stiffens after exercise and injury, as muscle ages, and in myopathic muscle on variable time scales from hours to years; however, how these changes in stiffness affect resident muscle stem cells (MuSCs) remains understudied. To investigate the mechanisms regulating MuSCs responses to muscle stiffening, we designed and implemented unique mechanically-controlled material microenvironments, identifying key mechanotransduction pathways involved in muscle regeneration.

Design: To isolate the role of stiffness on MuSC behavior, we cultured mouse MuSCs on SPAAC hydrogels capable of in situ stiffening by secondary photocrosslinking of excess cyclooctynes. Using pre- to post-injury stiffness hydrogels, we identified the MuSC response to matrix stiffening and the biochemical pathways responsible. To unequivocally demonstrate a requirement for mechanotransduction in stiffness-dependent proliferation, MuSC conditional YAP and TAZ double knockout mice were generated and monitored during muscle regeneration.

Setting: Laboratory only

Participants: Mice only

Interventions: Not applicable

Main Outcome Measures: MuSC proliferation, migration, and subcellular localization of YAP and TAZ.

Results: Following muscle injury in a mouse, muscle stiffness remained elevated after morphological regeneration was complete, accompanied by activated and proliferative MuSCs. Dynamically stiffening hydrogels induced proliferation, enhanced motility, and localized YAP/TAZ to the cell nucleus in MuSCs. Ablating YAP and TAZ in vivo in mice promotes MuSC quiescence in post-injury muscle and prevents myofiber hypertrophy, demonstrating that exposure to elevated stiffness activates mechanotransduction and maintains cycling MuSCs.

Conclusions: We identify that post-injury stiffening of muscle leads to persistent MuSC proliferation and prevents quiescence acquisition after muscle repair. Using our stiffening SPAAC hydrogels as an in vitro model of the muscle microenvironment, hydrogel stiffening induces proliferative and migratory changes in MuSCs that are mediated by mechanotransduction signaling, which is further supported by in vivo observations. A persistent but gradual increase in muscle elasticity may contribute to regenerative impairment of skeletal muscle in progressive myopathies and during muscle aging.

Level of Evidence: Level IV

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PM&R Meeting Abstracts - https://pmrjabstracts.org/abstract/injury-mediated-stiffening-persistently-activates-muscle-stem-cells-through-yap-and-taz-mechanotransduction/