Mitochondria in cultured human muscle cells depleted of mitochondrial DNA.

Cultured human muscle cells were depleted of mitochondrial DNA (mtDNA) by prolonged treatment with ethidium bromide (EB). In these respiration-deficient muscle cells neither cytochrome c oxidase activity nor mtDNA were detectable. However, mitochondrial matrix enzymes remained present and were localized in mitochondria-like organelles, as shown by subcellular fractionation. Metabolic labeling showed synthesis of cytochrome c oxidase subunits coded by nuclear DNA (nDNA). These results indicate that depletion of mtDNA in cultured human myoblasts does not inhibit expression of nDNA-coded mitochondrial proteins. The characteristic thread-like pattern of mitochondria was lost in mtDNA-depleted myoblasts, as shown by immunofluorescence with antibodies against cytochrome c oxidase and the F1 part of the mitochondrial ATP synthase (F1-ATPase) and by fluorescence of the carbocyanine dye, 3,3'-dipentyloxacarbocyanine iodide (DiOC5(3)). The organelles visualized by these methods were round and swollen and had a localization different from lysosomes as shown by double-labeling with mitochondrial and lysosomal antibodies. These results indicate that not only synthesis, but also import of mitochondrial proteins into mitochondria-like organelles remains possible in respiration-deficient cells.