Overexpression of insulin‐like growth factor‐1 attenuates skeletal muscle damage and accelerates muscle regeneration and functional recovery after disuse

New Findings What is the central question of this study? Insulin‐like growth factor‐1 (IGF‐1) promotes muscle hypertrophy, but no studies have investigated the effect of IGF‐1 on the susceptibility of atrophied muscles to reloading‐induced muscle injury. What is the main finding and what is its importance? We employed a comprehensive set of methods, including muscle physiological measurements, molecular biology techniques and non‐invasive magnetic resonance imaging. The results concurrently demonstrate that local overexpression of IGF‐1 in a primary antigravity muscle protects the muscle from reloading‐induced muscle damage and accelerates muscle regeneration and functional recovery following cast immobilization. These findings add new physiological significance to the benefits of IGF‐1 on skeletal muscle mass and force generation during varied loading conditions. Skeletal muscle is a highly dynamic tissue that responds to endogenous and external stimuli, including alterations in mechanical loading and growth factors. In particular, the antigravity soleus muscle experiences significant muscle atrophy during disuse and extensive muscle damage upon reloading. Given that insulin‐like growth factor‐1 (IGF‐1) has been implicated as a central regulator of muscle repair and modulation of muscle size, we examined the effect of virally mediated overexpression of IGF‐1 on the soleus muscle following hindlimb cast immobilization and upon reloading. Recombinant IGF‐1 cDNA virus was injected into one of the posterior hindlimbs of the mice, while the contralateral limb was injected with saline (control). At 20 weeks of age, both hindlimbs were immobilized for 2 weeks to induce muscle atrophy in the soleus and ankle plantarflexor muscle group. Subsequently, the mice were allowed to reambulate, and muscle damage and recovery were monitored over a period of 2–21 days. The primary finding of this study was that IGF‐1 overexpression attenuated reloading‐induced muscle damage in the soleus muscle, and accelerated muscle regeneration and force recovery. Muscle T 2 assessed by magnetic resonance imaging, a non‐specific marker of muscle damage, was significantly lower in IGF‐1‐injected compared with contralateral soleus muscles at 2 and 5 days reambulation ( P < 0.05). The reduced prevalence of muscle damage in IGF‐1‐injected soleus muscles was confirmed on histology, with a lower fractional area of abnormal muscle tissue in IGF‐1‐injected muscles at 2 days reambulation (33.2 ± 3.3 versus 54.1 ± 3.6%, P < 0.05). Evidence of the effect of IGF‐1 on muscle regeneration included timely increases in the number of central nuclei (21% at 5 days reambulation), paired‐box transcription factor 7 (36% at 5 days), embryonic myosin (37% at 10 days) and elevated MyoD mRNA (7‐fold at 2 days) in IGF‐1‐injected limbs ( P < 0.05). These findings demonstrate a potential role of IGF‐1 in protecting unloaded skeletal muscles from damage and accelerating muscle repair and regeneration.