Displaced myonuclei are attributable to both resident myonuclear migration and stem cell fusion during mechanical loading in adult skeletal muscle

Non-peripheral (displaced) myonuclei are characteristic of skeletal muscle pathology and severe injury but also appear after exercise and with aging. Displaced myonuclei are typically attributed to the activity of muscle stem cells, or satellite cells. We sought to address whether displaced myonuclei in adult skeletal muscle are exclusively from an exogenous source such as satellite cells or can result from resident myonuclear migration. To address this question, we used a murine recombination-independent muscle fibre-specific doxycycline-inducible fluorescent myonuclear labelling approach, EdU stem cell fate tracking, two durations of plantaris muscle mechanical overload (MOV, 3 days and 7 days), and fluorescent histology. Our findings show that: 1) displaced myonuclei emerge early during MOV in adult mice, 2) resident myonuclear movement occurs rapidly during MOV, and 3) the contribution of resident versus exogenous displaced myonuclei depends on the preferential effects of MOV for specific fibre types or fibre sizes with a given MOV duration. These observations provide fundamental insights on myonuclear motility in response to stress in vivo and reframe our understanding of how a recognized feature of mammalian skeletal muscle can emerge in response to stressors such as mechanical loading.