Hypoxic Exercise Alleviates Diabetic Skeletal Myopathy in Zebrafish in Association with Hif1a/Foxo3a Signaling and Mitochondrial Quality Control

Type 2 diabetes mellitus (T2DM) is characterized by skeletal muscle insulin resistance, which contributes to systemic metabolic dysregulation and diabetic myopathy (DM), a complication involving the loss of muscle mass and contractile function. Although hypoxic exercise has demonstrated metabolic benefits in chronic diseases, its efficacy against DM remains unclear. We hypothesized that hypoxic exercise would be more effective than normoxic exercise in ameliorating diabetic myopathy-associated skeletal muscle abnormalities. Accordingly, adult zebrafish underwent an 8-week high-glucose intervention combining high-glucose feeding and glucose immersion to establish a T2DM-associated diabetic myopathy model and were subsequently subjected to 5 weeks of normoxic or hypoxic exercise (28.8 cm/s, corresponding to 70% of critical swimming speed (Ucrit) determined under normoxic conditions, 2 h/day, 5 days/week) to compare their effects on skeletal muscle outcomes. Hypoxic exercise produced greater improvements in glucose tolerance and locomotor performance than normoxic exercise, while the effects on skeletal muscle morphological and atrophy-related indices varied across outcomes. Both normoxic exercise and intermittent hypoxic exposure altered the abundance of mitochondrial quality-control-related proteins in the skeletal muscle of zebrafish with diabetic myopathy, whereas hypoxic exercise produced more pronounced changes in several of these proteins. Moreover, hypoxic exercise more markedly increased the abundance of mitochondrial respiratory-chain proteins than aerobic exercise or intermittent hypoxic exposure alone. Marked reactive oxygen species (ROS) accumulation was observed in the skeletal muscle of zebrafish with diabetic myopathy. Both aerobic and hypoxic exercise effectively reduced excessive ROS production, with hypoxic exercise producing the greater effect. Analysis of Foxo3a and its phosphorylation showed that p-Foxo3a expression decreased most prominently in the T2DM hypoxia exercise (DHE) group, indicating that hypoxic exercise is associated with modulation of Hif1a/Foxo3a-related signaling. Hypoxic exercise was associated with greater Hif1a protein abundance and lower Foxo3a Ser253 phosphorylation than normoxic exercise, accompanied by improved mitochondrial quality control and reduced ROS accumulation. In conclusion, hypoxic exercise ameliorated diabetic skeletal myopathy and was accompanied by modulation of Hif1a/Foxo3a-related signaling, more favorable mitochondrial morphology, altered abundance of mitochondrial quality-control-related and respiratory-chain proteins, and reduced dihydroethidium-detectable oxidative signals.