Myonuclear loss, rather than senescent myonuclei, associates with fiber type-specific atrophy in aging human skeletal muscle.

Age-related reductions in whole-muscle function are attributed, in part, to pronounced atrophy of muscle fibers expressing the fast myosin heavy chain (MyHC) II isoforms. Senescence, a state of irreversible cell cycle arrest that can be characterized by DNA damage (γH2AX) and chromatin remodeling (loss of nuclear HMGB1), may contribute to skeletal muscle aging. Muscle nuclei (myonuclei) maintain fiber size and function and could exhibit senescence-associated features; however, the prevalence of senescent myonuclei and whether they contribute to fast fiber atrophy in older adults remains unknown. Vastus lateralis biopsies from 20 young (19-34yr; 10 females) and 20 older (65-84yr; 10 females) adults were analyzed via immunohistochemistry for myonuclei positive for γH2AX (γH2AX(+)) and negative for HMGB1 (HMGB1(-)). MyHC II cross-sectional area (CSA) was ~70% larger in young compared with old, whereas MyHC I CSA did not differ with age. The relative abundance of γH2AX(+)/HMGB1(-) myonuclei did not differ with age and was not associated with CSA in either fiber type. Single-nucleus RNA-sequencing corroborated no age-related difference in the prevalence of myonuclei with senescence-associated features. Myonuclear content of MyHC II fibers was ~30% higher in young compared with old and was closely associated with CSA in both fiber types. Size-cluster analysis revealed a pronounced age-related leftward shift in MyHC II CSA that paralleled the reductions in myonuclear number, consistent with myonuclear loss. These data suggest that age-related fast fiber atrophy is not attributed to an increased prevalence of senescent myonuclei but instead occurs concomitantly with fiber type-specific myonuclear loss across the lifespan.