Mitochondrial morphology and function in mitochondrial disease

Mitochondria play a crucial role in maintaining cellular health. It is interesting that the shape of mitochondria can vary depending on the type of cell, mitochondrial function, and other cellular conditions. The morphology of this network is maintained by the balance between mitochondrial fusion and fission, necessary for the proper functioning of the mitochondrial energetics as well as the maintenance of mitochondria DNA (mtDNA) (Chan, 2006; Friedman and Nunnari, 2014; Glancy et al., 2015). If the balance between the two processes is slightly perturbed, dramatic changes in mitochondrial morphology can occur. However, there are limited studies that link functional assessment with mitochondrial morphology evaluation at high magnification, even fewer that do so in situ and none in human muscle biopsies. Therefore, we have developed a method which combines functional assessment of mitochondria through Cytochrome c Oxidase (COX) histochemistry, with a 3D electron microscopy (EM) technique, serial block-face scanning electron microscopy (SBFSEM). Using this technique, we examine the relationship between 3D mitochondrial morphology and mitochondrial activity in human skeletal muscle. Here we apply COX-SBFSEM to muscle samples from patients with single, large-scale mtDNA deletions, a cause of mitochondrial disease. These deletions cause oxidative phosphorylation deficiency, which can be observed through changes in COX activity, and a typical mosaic pattern of enzyme deficiency with both COX normal and deficient fibres present in the same biopsy. Using COX-SBFSEM, we can distinguish three distinct mitochondrial populations within muscle fibres: COX normal, COX intermediate and COX deficient mitochondria. COX normal mitochondria were larger and more complex than the COX deficient mitochondria, However, in fibres that were generally COX deficient, there was no apparent difference between the three types of mitochondria and all mitochondria in fibres that are COX deficient overall are fragmented and spherical. One of the main advantages of combining 3D-EM with the COX reaction is the ability to look at how per-mitochondrion oxidative phosphorylation status is spatially distributed within muscle fibres. Here we show a robust spatial pattern in fibres that are generally COX normal and that the spatial pattern is less clear in fibres that are predominantly COX intermediate and COX deficient. ### Competing Interest Statement The authors have declared no competing interest.