Physiological and ultrastructural features of human induced pluripotent and embryonic stem cell-derived skeletal myocytes in vitro

Progress has recently been made toward the production of human skeletal muscle cells from induced pluripotent stem (iPS) cells. However, the functional and ultrastructural characterization, which is crucial for disease modeling and drug discovery, remains to be documented. We show, for the first time to our knowledge, that the electrophysiological properties of human iPS-derived skeletal myocytes are strictly similar to those of their embryonic stem (ES) cell counterparts, and both are typical of aneural mammalian skeletal muscle. In both cell types, intracellular calcium signaling that links membrane depolarization to contraction occurs in the absence of extracellular Ca2+, a unique feature of skeletal muscle. Detailed analysis of the Ca2+ signal revealed diverse kinetics of the rising phase, and hence various rates in the release of Ca2+ from the sarcoplasmic reticulum. This was mirrored by ultrastructural evidence of Ca2+ release units, which varied in location, shape, and size. Thus, the excitation-contraction coupling machinery of both iPS-and ES-derived skeletal myocytes was functional and specific, but did not reach full maturity in culture. This is in contrast with the myofibrillar network, which displayed the same organization as in adult skeletal muscle. Overall, the present study validates the human iPS-based skeletal myocyte model in comparison with the embryonic system, and provides the functional and ultrastructural basis for its application to human skeletal muscle diseases.