Elevated myonuclear density during skeletal muscle hypertrophy in response to training is reversed during detraining

Abstract

Myonuclei gained during exercise-induced skeletal muscle hypertrophy may be long-lasting and could facilitate future muscle adaptability after deconditioning, a concept colloquially termed "muscle memory." The evidence for this is limited, mostly due to the lack of a murine exercise-training paradigm that is nonsurgical and reversible. To address this limitation, we developed a novel progressive weighted-wheel-running (PoWeR) model of murine exercise training to test whether myonuclei gained during exercise persist after detraining. We hypothesized that myonuclei acquired during training-induced hypertrophy would remain following loss of muscle mass with detraining. Singly housed female C57BL/6J mice performed 8 wk of PoWeR, while another group performed 8 wk of PoWeR followed by 12 wk of detraining. Age-matched sedentary cage-dwelling mice served as untrained controls. Eight weeks of PoWeR yielded significant plantaris muscle fiber hypertrophy, a shift to a more oxidative phenotype, and greater myonuclear density than untrained mice. After 12 wk of detraining, the plantaris muscle returned to an untrained phenotype with fewer myonuclei. A finding of fewer myonuclei simultaneously with plantaris deconditioning argues against a muscle memory mechanism mediated by elevated myonuclear density in primarily fast-twitch muscle. PoWeR is a novel, practical, and easy-to-deploy approach for eliciting robust hypertrophy in mice, and our findings can inform future research on the mechanisms underlying skeletal muscle adaptive potential and muscle memory.

Keywords: exercise; muscle memory; myonuclear accretion; satellite cell; weighted-wheel running.

Fig. 1.

Muscle fiber type and size adaptations to progressive weighted-wheel-running (PoWeR) training and detraining. A: PoWeR wheel-loading strategy. Inset: magnet (black box) placement on the wheel. B: fiber type distribution data for untrained (n = 14), PoWeR-trained (n = 8), and detrained (n = 7) mice. C: average muscle fiber cross-sectional area (CSA) in untrained, PoWeR-trained, and detrained mice. D: fiber type (FT)-specific CSA data for untrained, PoWeR-trained, and detrained mice. Values are means ± SE. *P < 0.05 vs. untrained; #P < 0.05 vs. PoWeR-trained. E–G: representative images of fiber type and dystrophin staining on entire muscle cross sections from untrained (E), PoWeR-trained (F), and detrained (G) mice. Pink, type I; green, type IIa; red, type IIb; unstained/black, type IIx; blue, dystrophin. Scale bar = 200 µm.

Fig. 2.

Myonuclear and satellite cell density adaptations to progressive weighted-wheel-running (PoWeR) training and detraining. A–C: representative images of longitudinal muscle fibers and myonuclei in untrained, PoWeR-trained, and detrained mice. Blue, DAPI⁺ myonuclei. Scale bar = 100 µm. D: single-fiber myonuclear density data analysis for untrained (n = 8), PoWeR-trained (n = 7), and detrained mice (n = 8). E: representative image of dystrophin and DAPI staining for immunohistochemical (IHC) myonuclear density analysis. Red, dystrophin; blue, DAPI. White arrows point to myonuclei. Scale bar = 20 µm. F: IHC myonuclear density analysis for untrained (n = 10), PoWeR-trained (n = 7), and detrained (n = 6) mice. G: representative image of IHC satellite cell identification. Green, laminin; blue, DAPI; pink, Pax7. White arrows point to Pax7⁺/DAPI⁺ satellite cells located beneath the laminin border. Scale bar = 50 µm. H: satellite cell density in untrained (n = 14), PoWeR-trained (n = 7), and detrained (n = 8) mice. Values are means ± SE. *P < 0.05 vs. untrained; #P < 0.05 vs. PoWeR-trained.

Fig. 3.

Schematic illustrating muscle fiber adaptations to progressive weighted-wheel running (PoWeR) and detraining.