State-Dependent FGF Signaling in Satellite Cell-Mediated Skeletal Muscle Regeneration and Pathological Remodeling.

Skeletal muscle regeneration depends on coordinated transitions of muscle stem cells (MuSCs), also known as satellite cells, from quiescence through activation and proliferative expansion to differentiation and fusion, while self-renewal replenishes the quiescent MuSC pool within a dynamically remodeled niche. Fibroblast growth factor (FGF) signaling regulates these transitions, but its effects vary as MuSCs and their niche change across regenerative stages. FGF output is shaped by ligand availability and extracellular presentation, fibroblast growth factor receptor (FGFR) isoform expression and coreceptor availability, receptor trafficking, intracellular feedback, and the state of the responding cell. Following acute injury, FGF inputs can support MuSC activation and expansion; signaling is subsequently reconfigured during differentiation, fusion, self-renewal, and return to quiescence. Aging-associated regenerative decline, chronic injury and dystrophic remodeling, denervation, and metabolic dysfunction disrupt this coordination and can uncouple FGF activity from productive repair. Rhabdomyosarcoma provides a distinct malignant context in which the FGF network is rewired to sustain oncogenic myogenic cell states. Here, we integrate molecular, cellular, and niche-level evidence across these settings to explain why FGF signaling produces divergent outcomes and to clarify how cellular context and timing should inform therapeutic modulation.