Fabrication of contractile skeletal muscle tissues using directly converted myoblasts from human fibroblasts.

Transplantation of stem cell-derived myoblasts is a promising approach for the treatment of skeletal muscle function loss. Myoblasts directly converted from somatic cells that bypass any stem cell intermediary stages can avoid the problem of tumor formation after transplantation. Previously, we reported that co-transduction with the myogenic differentiation 1 (MYOD1) gene and the v-myc avian myelocytomatosis viral oncogene lung carcinoma derived homolog (MYCL) gene efficiently converted human fibroblasts into myoblasts. Although the directly converted myoblasts efficiently fused into multinucleated myotubes in vitro and in vivo, it is not clear whether they have the contractile ability, which is the most significant phenotype of the muscle. In the present study, we aimed to examine the in vitro contractile ability of the myotubes differentiated from the directly converted myoblasts by the overexpression of MYOD1 and MYCL. We fabricated three-dimensional (3D) tissues on a microdevice for force measurement. The 3D culture enhanced the differentiation of the myoblasts into myotubes, which were confirmed by gene expression analysis of skeletal muscle-related genes. The tissues started to generate contractile force in response to electrical stimulation after 4 days of culture, which reached approximately 12 μN after 10 days. The addition of IGF-I decreased the contractile force of the 3D tissues, while the use of cryopreserved cells increased it. We confirmed that the tissues fabricated from the cells derived from three different donors generated forces of similar magnitude. Thus, directly converted myoblasts by the overexpression of MYOD1 and MYCL could be a promising cell source for cell therapy.