Differential expression of nuclear genes for cytochrome c oxidase during myogenesis

Recent studies of patients with mitochondrial myopathies suggest the existence of both muscle-specific and developmentally regulated isoforms of cytochrome c oxidase (COX), the terminal enzyme complex of the electron transport chain. To investigate the temporal pattern of gene expression of nuclear genes for COX in developing muscle, the steady-state levels of COX mRNA in total RNA from a satellite cell-derived mouse muscle cell line, C2C12, were analyzed and compared with COX mRNA levels in mature rat skeletal muscle. Undifferentiated myoblasts, myotubes just after fusion (early myotubes), and fully differentiated, contractile, striated myotubes (late myotubes) were analyzed for mRNA levels for four of the 10 different nuclear-encoded COX subunits: IV, Vb, Vlc and VIII-liver. Of these, IV, Vb and Vlc are identical in both bovine heart and liver, whereas subunit VIII has heart and liver isoforms. In C2C12 myoblasts, the level of mRNA for subunits IV, Vb, and VIII-liver is equal to or greater than the level in tissues such as brain, skeletal muscle, and liver. As myoblasts fuse and differentiate into myotubes, the levels of mRNA for these subunits undergo radically different changes. Transcripts for subunits IV and Vb accumulate to higher levels during myogenesis. The level of subunit VIII transcripts decreases during myogenesis, providing additional evidence that subunit VIII has tissue-specific isoforms in the rat. Little mRNA for COX Vlc was detected in either the C2C12 cell line or in primary embryonic rat myoblasts or myotubes in culture in spite of high levels in adult skeletal muscles, suggesting that subunit Vlc may have both fetal and adult isoforms in rodents.