Molecular mechanisms mediating exercise-induced maladaptations in vitro

Abstract

Introduction: Physical training stimulates mitochondrial biogenesis and protein synthesis in skeletal muscle. If the training load is too demanding and prolonged, an overtraining syndrome may develop. Reported overtraining symptoms include recurrent respiratory infections, sleep disturbances, with potential alterations at the skeletal muscle level such as soreness, and atrophy. The aim of the present study was to investigate skeletal muscle (mal)adaptations in response to overtraining, using a translational approach combining human data and an in vitro model.

Methods: Electrical stimulation was applied to C2C12 myotubes to mimic endurance exercise. For the simulated training (s-T), stimulation was applied once daily for three days. In the simulated overtraining (s-OT), stimulation was applied three times daily for three days. Proteomic analysis, mitochondrial respiration and imaging were performed in the cellular model. In humans, knee extensor neuromuscular function at rest and in response to a standardized exercise was assessed in overtrained vs. well trained athletes. Muscle biopsies were also collected to assess mitochondrial respiration.

Results: In s-T, we observed beneficial adaptations, including myotube hypertrophy, increased expression of myosin heavy chain proteins, and enhanced mitochondrial respiration. In contrast, s-OT showed myotube atrophy, reduced myosin heavy chain content, and impaired mitochondrial respiration. Consistent with these respiratory findings, proteomics analysis revealed a decreased abundance of proteins involved in the mitochondrial respiratory chain in s-OT. Interestingly, mitochondria appeared aggregated in s-OT myotubes, suggesting possible impairments in mitochondrial quality control. Live-cell imaging revealed possible autophagy alterations in the s-OT condition but not in s-T. Human data - neuromuscular function and muscle samples - are being processed and results will be presented during the conference.

Conclusions: Our in vitro model provides a molecular platform for further investigations of skeletal muscle maladaptations to overtraining. The integrative approach will help to clarify the molecular mechanisms and ameliorate the clinics of muscle maladaptations triggered by excessive exercise in humans.