Engineering of human myotubes towards a mature metabolic and contractile phenotype

Skeletal muscle mediates the beneficial effects of exercise, thereby improving insulin sensitivity and reducing the risk for type 2 diabetes. Current skeletal-muscle-models in vitro are incapable of fully recapitulating its physiological functions especially regarding exercise. Supplementation of IGF1, a growth factor secreted by myofibers in vivo, might help to overcome these limitations. Primary human CD56-positive myoblasts were differentiated into myotubes in the presence/absence of IGF1 in serum-free medium for 10 days. Daily collected samples were analyzed by proteomics, qRT-PCR and myotube contractibility via electrical-pulse-stimulation (EPS). Mitochondrial respiration and glucose uptake were measured. IGF1-supported differentiation formed thicker multinucleated myotubes showing physiological contraction upon EPS following day 6 while myotubes without IGF1 were almost incapable of contraction. IGF1-treatment upregulated particularly muscle-specific proteins that contribute to myofibril and sarcomere assembly, striated muscle contraction, and ATP production. Elevated PPARGC1A, MYH7 and reduced MYH1/2 suggest a switch towards a more oxidative phenotype in line with elevated mitochondrial respiration. IGF1-treatment also upregulated expression of GLUT4 and increased insulin-dependent glucose uptake. To conclude, utilizing IGF1, we engineered human myotubes that recapitulate the physiological traits of skeletal muscle in vivo vastly superior to established protocols. This novel model enables investigation of exercise on a molecular level, drug screening and interorgan-crosstalk by transfer to organ-on-chip.