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BackgroundMitochondrial alternative respiratory-chain enzymes are phylogenetically widespread, and buffer stresses affecting oxidative phosphorylation in species that possess them. However, they have been lost in the evolutionary lineages leading to vertebrates and arthropods, raising the question as to what survival or reproductive disadvantages they confer. Recent interest in using them in therapy lends a biomedical dimension to this question.MethodsHere, we examined the impact of the expression of Ciona intestinalis alternative oxidase, AOX, on the reproductive success of Drosophila melanogaster males. Sperm-competition assays were performed between flies carrying three copies of a ubiquitously expressed AOX construct, driven by the α-tubulin promoter, and wild-type males of the same genetic background.ResultsIn sperm-competition assays, AOX conferred a substantial disadvantage, associated with decreased production of mature sperm. Sperm differentiation appeared to proceed until the last stages, but was spatially deranged, with spermatozoids retained in the testis instead of being released to the seminal vesicle. High AOX expression was detected in the outermost cell-layer of the testis sheath, which we hypothesize may disrupt a signal required for sperm maturation.ConclusionsAOX expression in Drosophila thus has effects that are deleterious to male reproductive function. Our results imply that AOX therapy must be developed with caution.Electronic supplementary materialThe online version of this article (doi:10.1186/s12861-017-0151-3) contains supplementary material, which is available to authorized users.

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.

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Alternative oxidases (AOX) are non‐proton‐pumping enzymes present in most groups of organisms, but naturally absent in the mitochondria of vertebrates and insects. However, its xenotopic expression in mammalian and Drosophila models have suggested an immense therapeutic potential, as diverse phenotypes related to mitochondrial diseases have been attenuated. The expression of this enzyme creates an extra pathway for oxygen reduction when the cytochrome c segment of the electron transfer system is overloaded, relieving possible oxidative stress and providing beneficial effects. AOX is known to function in thermogenesis in plants due to its mitochondrial uncoupling activity, so understanding the inherent properties of AOX expressed in metazoans and how they interfere with the metabolism and physiology of model organisms under stress conditions is essential. We have been investigating the effects of temperature and have showed previously that D. melanogaster lines constitutively expressing AOX from Ciona intestinalis develop faster and have larval and pupal viability higher than that of control flies at low temperatures. Here we show that AOX‐expressing larvae are also more active and accumulate ~12,5% more body mass when cultured at the severely stressful temperature of 12°C. Using infrared thermography, we observed that these larvae lose body heat significantly less pronounced, maintaining their body temperature ~0.2°C higher than controls. To understand these effects we have analyzed larval mitochondrial respiration and show a reconfiguration of the electron transfer pathways that sustain oxygen consumption: AOX‐expressing flies have a ~30% decrease in glycerol‐3‐phosphate dehydrogenase (mGPDH)‐driven oxygen consumption, which is compensated by a ~30% increase in complex I (CI)‐driven oxygen consumption, at 12°C. AOX inhibition leads to a ~37% decrease in mGPDH‐driven respiration, suggesting a functional interaction between mGPDH and AOX. Because mGPDH is also a non‐proton‐pumping enzyme, this interaction would ultimately uncouple mitochondria. At low temperatures, this configuration becomes highly functional, especially because mGPDH appears naturally less sensitive to cold, dissipating the energy of electron transfer as heat. Assuming that the increased CI‐driven respiration may stimulate the reactions in the tricarboxylic acid cycle, we speculate the increased body mass and growth in AOX larvae may be explained by increased cataplerosis. In summary, our results suggest the increase in cold‐dependent fitness observed for AOX lines is caused by a combination of heat production via mGPDH‐linked mitochondrial uncoupling and increased CI‐linked cataplerosis.