Assessing the role of cellular senescence on skeletal muscle fiber type-specific atrophy in older adults

Age-related reductions in whole-muscle force and power occur largely due to factors within skeletal muscle, particularly the pronounced atrophy of the muscle fibers expressing the fast myosin heavy chain (MyHC) II isoforms. Cellular senescence, a state of irreversible cell cycle arrest that can be characterized by DNA damage (presence of γH2AX) and chromatin remodeling reflected by a loss of nuclear HMGB1, has emerged as a potential contributor to skeletal muscle aging. Muscle nuclei (i.e. myonuclei) are essential for maintaining fiber size and function, and accumulating evidence indicates they can exhibit senescence-associated features. However, whether senescent myonuclei contribute to fast fiber atrophy in older adults, and whether this differs between males and females, remains unknown. We tested the hypotheses that the prevalence of senescent myonuclei would: 1) be greater in fast fibers from older compared to young adults with no age differences in slow fibers, and 2) not differ between males and females. Vastus lateralis biopsies from 20 young (19-34 yr; 10 females) and 20 older (65-84 yr; 10 females) adults were analyzed for myonuclei staining positive for γH2AX (γH2AX + ) and negative for HMGB1 (HMGB1 - ) via immunohistochemistry of tissue cross-sections. MyHC I fiber cross-sectional area (CSA) did not differ between young (4679 ± 1182 µm 2 ) and old (4524 ± 1002 µm 2 ; P = 0.635), but males (5000 ± 1023 µm 2 ) had ~19% larger MyHC I fibers than females (4203 ± 1016 µm 2 ; P = 0.019). MyHC II CSA was ~70% larger in young (5026 ± 1613 µm 2 ) compared with old (2959 ± 1095 µm 2 ; P < 0.001) and ~37% larger in males (4622 ± 1515 µm 2 ) than females (3363 ± 1709 µm 2 ; P = 0.003). The relative abundance of γH2AX + /HMGB1 - myonuclei in MyHC I fibers did not differ between young (5.2 ± 2.1%) and old (4.9 ± 3.2%; P = 0.765), but females (6.1 ± 2.8%) exhibited a higher percentage than males (4.1 ± 2.0 %; P = 0.015). In the MyHC II fibers, the percentage of γH2AX + /HMGB1 - myonuclei did not differ between young (4.6 ± 2.5%) and old (4.6 ± 2.4%; P = 0.967) or between males (4.4 ± 2.4%) and females (4.8 ± 2.5%; P = 0.539). Myonuclei per fiber for MyHC I fibers did not differ between young (2.3 ± 0.7) and old (2.3 ± 0.6; P = 0.828), but males (2.6 ± 0.6) had ~30% more myonuclei per fiber than females (2.0 ± 0.6; P = 0.013). For MyHC II fibers, the young (2.6 ± 1.0) had ~30% more myonuclei per fiber than old (2.0 ± 0.6; P = 0.022), with no differences between males (2.6 ± 0.7) and females (2.1 ± 1.0; P = 0.052). The prevalence of γH2AX + /HMGB1 - myonuclei was not associated with CSA in either MyHC I (R 2 = 0.07; P = 0.095) or MyHC II fibers (R 2 = 0.02; P = 0.367). In contrast, myonuclear content was closely associated with CSA in both MyHC I and (R 2 = 0.40; P < 0.001) and MyHC II (R 2 = 0.67; P < 0.001) fibers. These data suggest that age-related fast fiber atrophy cannot be attributed to an increased prevalence of senescent myonuclei, but instead, supports the concept that myonuclear content is an important regulator of muscle fiber size in aging humans. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.