Single-Nucleus RNA Sequencing Reveals Muscle Fiber Cell Heterogeneity During Human Skeletal Muscle Aging

Aging impairs skeletal muscle mass and function, but the cell-type-resolved transcriptional states and intercellular signaling changes in human muscle aging remain incompletely mapped. Here, we constructed a single-nucleus RNA sequencing (snRNA-seq) atlas of human vastus lateralis muscle from adult (22-60 years) and elderly (99-101 years) male donors. We identified a comprehensive cellular census and discovered a profound reorganization of the myofiber transcriptional landscape. Aging was characterized by a shift from robust "young" states (MYLK4⁺ type II, LRP1B⁺ type I fibers) to dysfunctional "old" states (RYR3⁺ type II, RYR3⁺ type I fibers), accompanied by a marked emergence of hybrid fiber subtypes. We mechanistically linked these hybrid fibers to key aging pathologies: RUNX1⁺ hybrid fibers displayed a transcriptional signature of denervation, while SAA1⁺ hybrid fibers exhibited features of fatty infiltration, correlated with an expansion of fibro/adipogenic progenitors (FAPs). Pseudotime trajectory analysis supported the progression from young to degenerative or adipogenic fates. The aged microenvironment was globally altered, featuring impaired metabolic activity in muscle stem cells, compromised immune surveillance and function decline of vascular compartments. Critically, cell-cell communication analysis revealed enhanced BMP and Laminin signaling from FAPs to myofibers in aged tissue. Our work provided a high-resolution roadmap of human skeletal muscle aging, establishing denervation and FAP-driven fatty infiltration as key cellular mechanisms driving functional decline, and revealing novel targets for therapeutic intervention.