Nitrotetrazolium Blue Exclusion Assay (NBTx). Demonstration of a novel assay to quantify cytochrome c oxidase deficiency.

Respiratory chain impairment, interfering with energy production in the cell, is a major underlying cause of mitochondrial diseases. Yet, the surprising variety of clinical symptoms and the wide gap between ages of onset, as well as the involvement of mitochondrial impairment in ageing and age-related diseases continues to challenge our understanding of the pathogenic processes. The explanation for this complexity likely lays in the multiple roles of mitochondria in cellular function, a unique set of requirements for each cell types, and a convergence of a complex dual genetic predispositions and environmental factors. As the defined gene defect poorly predicts clinical manifestations in humans it remains essential to carefully assess the etiopathology of mitochondrial diseases. Given the heterogeneity of mitochondrial activity in cells and their functional relevance, clarifying the contribution of mitochondrial metabolic dysfunction at the cellular level is fundamental. For this purpose, we developed a novel enzyme histochemical method that enables precise quantification of COX deficiency in fresh-frozen tissues. I demonstrate that the loss of oxidative activity is detected at very low levels – an achievement unequaled by previous techniques and opens up new opportunities for the study of early disease processes or comparative investigations. Moreover, human biopsy samples of several genotypic origins were used and the successful detection of COX-deficient cells suggests a broad application for this new method. Lastly, the assay can be adapted to a wide range of tissues in the mouse and extends to other animal models, illustrated here with the fruit fly, Drosophila melanogaster. Overall, the new assay enables the quantification and precise mapping of single cells presenting impaired COX activity with the full extent of COX deficiency in tissues being made visible. This work makes the demonstration that the new enzyme histochemistry assay is a reliable tool for exposing COX-deficient cells and thereby expends new possibilities for future investigation