Repeated Disuse Atrophy Imprints a Molecular Memory in Skeletal Muscle: Transcriptional Resilience in Young Adults and Susceptibility in Aged Muscle

Disuse-induced muscle atrophy is common after illness, injury, or falls, and becomes increasingly frequent with ageing. However, whether skeletal muscle retains a memory of disuse remains unknown. We investigated repeated lower-limb immobilization in young adults and a refined aged rat model of repeated disuse, integrating multi-omics, histology, biochemistry, and primary human cell analyses. In young human muscle, a second period of disuse elicited an attenuated transcriptional response in oxidative and mitochondrial pathways, suggesting a protective molecular memory, although this did not translate into attenuated atrophy. In contrast, aged muscle exhibited a detrimental molecular memory, with greater atrophy, exaggerated suppression of aerobic metabolism genes/pathways despite recovery of these genes after initial disuse, depletion of NAD+ and mitochondrial DNA, and activation of proteasomal, extracellular matrix and DNA-damage pathways. Repeated disuse induced DNA hypermethylation and downregulation of aerobic metabolism and mitochondrial genes across species. NR4A1 remained suppressed into recovery after initial disuse in young human muscle, whereas acetylcholine receptor subunit genes (e.g., CHRNA1, CHRND) were epigenetically primed in humans and further upregulated with repeated atrophy in both young and aged muscle. NMRK2 was among the most downregulated genes after both periods of atrophy in humans, and nicotinamide riboside improved myotube size in primary human muscle cells derived post-atrophy. These findings reveal that disuse atrophy imprints a molecular memory in skeletal muscle- attenuated transcriptional resilience in young adults yet exaggerated susceptibility in aged muscle.