The xenotopic expression of the alternative oxidase AOX from the tunicate Ciona intestinalis in diverse models of human disease partially alleviates the phenotypic effects of mitochondrial respiratory chain defects. AOX is a non-proton pumping, mitochondrial inner membrane-bound, single-subunit enzyme that can bypass electron transport through the cytochrome segment, providing an additional site for ubiquinone reoxidation and oxygen reduction upon respiratory chain overload. We set out to investigate whether AOX expression in Drosophila could counteract the effects of mitochondrial DNA (mtDNA) replication defects caused by disturbances in the mtDNA helicase or DNA polymerase γ. We observed that the developmental arrest imposed by either the expression of mutant forms of these enzymes or their knockdown was not rescued by AOX. Considering also the inability of AOX to ameliorate the phenotype of tko25t, a fly mutant with mitochondrial translation deficiency, we infer that this alternative enzyme may not be applicable to cases of mitochondrial gene expression defects. Finding the limitations of AOX applicability will help establish the parameters for the future putative use of this enzyme in gene therapies for human mitochondrial diseases.

Although the use of vinasse as a waste helps replenish soil nutrients and improves the quality of the sugarcane crop, it is known that vinasse residues alter the diversity of bacteria naturally present in the soil. The actual impacts of vinasse application on the selection of bacterial taxa are not understood because no studies have addressed this phenomenon directly. Analysis of 16S rRNA gene clone sequences from four soil types showed that the soil planted with sugarcane and fertilized with vinasse has a high diversity of bacteria compared to other biomes, where Acidobacteria were the second most abundant phylum. Although the composition and structure of bacterial communities differ significantly in the four environments (Libshuff's test), forest soils and soil planted with sugarcane without vinasse fertilizer were similar to each other because they share at least 28 OTUs related to Rhizobiales, which are important agents involved in nitrogen fixation. OTUs belonging to Actinomycetales were detected more often in the soil that had vinasse applied, indicating that these groups are more favored by this type of land management.

The mitochondrial respiratory chain in vertebrates and arthropods is different from that of most other eukaryotes because they lack alternative enzymes that provide electron transfer pathways additional to the oxidative phosphorylation (OXPHOS) system. However, the use of diverse experimental models, such as human cells in culture, Drosophila melanogaster and the mouse, has demonstrated that the transgenic expression of these alternative enzymes can impact positively many phenotypes associated with human mitochondrial and other cellular dysfunction, including those typically presented in complex IV deficiencies, Parkinson's, and Alzheimer's. In addition, these enzymes have recently provided extremely valuable data on how, when, and where reactive oxygen species, considered by many as “by‐products” of OXPHOS, can contribute to animal longevity. It has also been shown that the expression of the alternative enzymes is thermogenic in cultured cells, causes reproductive defects in flies, and enhances the deleterious phenotype of some mitochondrial disease models. Therefore, all the reported beneficial effects must be considered with caution, as these enzymes have been proposed to be deployed in putative gene therapies to treat human diseases. Here, we present a brief review of the scientific data accumulated over the past decade that show the benefits and the risks of introducing alternative branches of the electron transport into mammalian and insect mitochondria, and we provide a perspective on the future of this research field.

The expression of the mitochondrial alternative oxidase AOX from Ciona intestinalis (Tunicata: Ascidiacea) has provided clear beneficial effects in a variety of mammalian and insect mitochondrial disease models. Because of its non‐proton pumping terminal oxidase activity, AOX can bypass the cytochrome c segment of the respiratory chain (complexes III and IV), and alleviate the possible overload of electrons that occurs upon oxidative phosphorylation dysfunction, not contributing, though, to the proton‐motive force needed for mitochondrial ATP synthesis. Significant detrimental outcomes have also been reported upon AOX expression, raising concerns regarding its putative deployment as a therapy enzyme for human diseases. In Drosophila, AOX expression is developmentally advantageous at low temperatures when the flies are cultured on a standard, rich diet, but it dramatically compromises adult eclosion when the flies are cultured on a low‐nutrient diet (LN), at 25ºC or above. Here, we applied transcriptomics and metabolomics analyses to show that the interaction between LN and AOX expression causes a general alteration of larval amino acid metabolism, leading to an almost 40% decrease in biomass at the pre‐metamorphosis stage. This reduced nutrient storage impairs development at the late pupa stage with a clear signature for starvation and an overall downregulation of mitochondrial metabolism. Interestingly, lactate dehydrogenase, lactate and 2‐hydroxyglutarate are elevated in AOX‐expressing flies, irrespective of diet. We have also identified that the addition of very low levels of ethanol to LN is sufficient to rescue the lethal phenotype imposed to the flies by the LN‐AOX interaction. We are currently exploring how alcohol dehydrogenase and other enzymes in the ethanol catabolic pathway participate in this rescue phenomenon. Nevertheless, our data points to important roles for two key redox‐regulating enzymes, lactate dehydrogenase and alcohol dehydrogenase, in adjusting the physiological changes induced by AOX function in larvae. As diet is one of the most important external factors that influence metabolism, our work provides important insights into how AOX expression could be safely accomplished, with minimal impact for higher animals.