Abstract
Contractile activity is a fundamental property of skeletal muscles. We describe the establishment of a "feeder-supported in vitro exercise model" using human-origin primary satellite cells, allowing highly-developed contractile myotubes to readily be generated by applying electrical pulse stimulation (EPS). The use of murine fibroblasts as the feeder cells allows biological responses to EPS in contractile human myotubes to be selectively evaluated with species-specific analyses such as RT-PCR. We successfully applied this feeder-supported co-culture system to myotubes derived from primary satellite cells obtained from sporadic inclusion body myositis (sIBM) patients who are incapable of strenuous exercise testing. Our results demonstrated that sIBM myotubes possess essentially normal muscle functions, including contractility development, de novo sarcomere formation, and contraction-dependent myokine upregulation, upon EPS treatment. However, we found that some of sIBM myotubes, but not healthy control myotubes, often exhibit abnormal cytoplasmic TDP-43 accumulation upon EPS-evoked contraction, suggesting potential pathogenic involvement of the contraction-inducible TDP-43 distribution peculiar to sIBM. Thus, our "feeder-supported in vitro exercise model" enables us to obtain contractile human-origin myotubes, potentially utilizable for evaluating exercise-dependent intrinsic and pathogenic properties of patient muscle cells. Our approach, using feeder layers, further expands the usefulness of the "in vitro exercise model".