Functional properties of the Ustilago maydis alternative oxidase under oxidative stress conditions

Alternative Oxidase - Aid or obstacle to combat the rise of fungal pathogens?

Fungal pathogens present a growing threat to both humans and global health security alike. Increasing evidence of antifungal resistance in fungal populations that infect both humans and plant species has increased reliance on combination therapies and shown the need for new antifungal therapeutic targets to be investigated. Here, we review the roles of mitochondria and fungal respiration in pathogenesis and discuss the role of the Alternative Oxidase enzyme (Aox) in both human fungal pathogens and phytopathogens. Increasing evidence exists for Aox within mechanisms that underpin fungal virulence. Aox also plays important roles in adaptability that may prove useful within dual targeted fungal-specific therapeutic approaches. As improved fungal specific mitochondrial and Aox inhibitors are under development we may see this as an emerging target for future approaches to tackling the growing challenge of fungal infection.

The Mitochondrial Alternative Oxidase in Ustilago maydis Is Not Involved in Response to Oxidative Stress Induced by Paraquat

It has been shown that the alternative oxidase in mitochondria of fungi and plants has important functions in the response against stress conditions, although their role in some organisms is still unknown. This is the case of Ustilago maydis. There is no evidence of the participation of the U. maydis Aox1 in stressful conditions such as desiccation, high or low temperature, and low pH, among others. Therefore, in this work, we studied the role of the U. maydis Aox1 in cells exposed to oxidative stress induced by methyl viologen (paraquat). To gain insights into the role of this enzyme, we took advantage of four strains: the FB2 wild-type, a strain without the alternative oxidase (FB2aox1Δ), other with the Aox1 fused to the Gfp under the control of the original promoter (FB2aox1-Gfp), and one expressing constitutively de Aox1-Gfp (FB2Potef:aox1-Gfp). Cells were incubated for various times in the presence of 1 mM paraquat and growth, replicative capacities, mitochondrial respiratory activity, Aox1 capacity, and the activities of several antioxidant enzymes (catalase, glutathione peroxidase, glutathione reductase, and superoxide dismutase) were assayed. The results show that (1) the response of U. maydis against oxidative stress was the same in the presence or absence of the Aox1; (2) the activities of the antioxidant enzymes remained constant despite the oxidative stress; and (3) there was a decrease in the GSH/GSSG ratio in U. maydis cells incubated with paraquat.

Targeting the alternative oxidase (AOX) for human health and food security, a pharmaceutical and agrochemical target or a rescue mechanism?

Some of the most threatening human diseases are due to a blockage of the mitochondrial electron transport chain (ETC). In a variety of plants, fungi, and prokaryotes, there is a naturally evolved mechanism for such threats to viability, namely a bypassing of the blocked portion of the ETC by alternative enzymes of the respiratory chain. One such enzyme is the alternative oxidase (AOX). When AOX is expressed, it enables its host to survive life-threatening conditions or, as in parasites, to evade host defenses. In vertebrates, this mechanism has been lost during evolution. However, we and others have shown that transfer of AOX into the genome of the fruit fly and mouse results in a catalytically engaged AOX. This implies that not only is the AOX a promising target for combating human or agricultural pathogens but also a novel approach to elucidate disease mechanisms or, in several cases, potentially a therapeutic cure for human diseases. In this review, we highlight the varying functions of AOX in their natural hosts and upon xenotopic expression, and discuss the resulting need to develop species-specific AOX inhibitors.

Functional Analysis of the Plasma Membrane H+-ATPases of Ustilago maydis

Plasma membrane H+-ATPases of fungi, yeasts, and plants act as proton pumps to generate an electrochemical gradient, which is essential for secondary transport and intracellular pH maintenance. Saccharomyces cerevisiae has two genes (PMA1 and PMA2) encoding H+-ATPases. In contrast, plants have a larger number of genes for H+-ATPases. In Ustilago maydis, a biotrophic basidiomycete that infects corn and teosinte, the presence of two H+-ATPase-encoding genes has been described, one with high identity to the fungal enzymes (pma1, UMAG_02851), and the other similar to the plant H+-ATPases (pma2, UMAG_01205). Unlike S. cerevisiae, these two genes are expressed jointly in U. maydis sporidia. In the present work, mutants lacking one of these genes (Δpma1 and Δpma2) were used to characterize the role of each one of these enzymes in U. maydis physiology and to obtain some of their kinetic parameters. To approach this goal, classical biochemical assays were performed. The absence of any of these H+-ATPases did not affect the growth or fungal basal metabolism. Membrane potential tests showed that the activity of a single H+-ATPase was enough to maintain the proton-motive force. Our results indicated that in U. maydis, both H+-ATPases work jointly in the generation of the electrochemical proton gradient, which is important for secondary transport of metabolites and regulation of intracellular pH.

Carbon and Nitrogen Sources Have No Impact on the Organization and Composition of Ustilago maydis Respiratory Supercomplexes

Respiratory supercomplexes are found in mitochondria of eukaryotic cells and some bacteria. A hypothetical role of these supercomplexes is electron channeling, which in principle should increase the respiratory chain efficiency and ATP synthesis. In addition to the four classic respiratory complexes and the ATP synthase, U. maydis mitochondria contain three type II NADH dehydrogenases (NADH for reduced nicotinamide adenine dinucleotide) and the alternative oxidase. Changes in the composition of the respiratory supercomplexes due to energy requirements have been reported in certain organisms. In this study, we addressed the organization of the mitochondrial respiratory complexes in U. maydis under diverse energy conditions. Supercomplexes were obtained by solubilization of U. maydis mitochondria with digitonin and separated by blue native polyacrylamide gel electrophoresis (BN-PAGE). The molecular mass of supercomplexes and their probable stoichiometries were 1200 kDa (I₁:IV₁), 1400 kDa (I₁:III₂), 1600 kDa (I₁:III₂:IV₁), and 1800 kDa (I₁:III₂:IV₂). Concerning the ATP synthase, approximately half of the protein is present as a dimer and half as a monomer. The distribution of respiratory supercomplexes was the same in all growth conditions. We did not find evidence for the association of complex II and the alternative NADH dehydrogenases with other respiratory complexes.

On the use of n-octyl gallate and salicylhydroxamic acid to study the alternative oxidase role

The alternative oxidase (AOX) catalyzes the transfer of electrons from ubiquinol to oxygen without the translocation of protons across the inner mitochondrial membrane. This enzyme has been proposed to participate in the regulation of cell growth, sporulation, yeast-mycelium transition, resistance to reactive oxygen species, infection, and production of secondary metabolites. Two approaches have been used to evaluate AOX function: incubation of cells for long periods of time with AOX inhibitors or deletion of AOX gene. However, AOX inhibitors might have different targets. To test non-specific effects of n-octyl gallate (nOg) and salicylhydroxamic acid (SHAM) on fungal physiology we measured the growth and respiratory capacity of two fungal strains lacking (Ustilago maydis-Δaox and Saccharomyces cerevisiae) and three species containing the AOX gene (U. maydis WT, Debaryomyces hansenii, and Aspergillus nidulans). For U. maydis, a strong inhibition of growth and respiratory capacity by SHAM was observed, regardless of the presence of AOX. Similarly, A. nidulans mycelial growth was inhibited by low concentrations of nOg independently of AOX expression. In contrast, these inhibitors had no effect or had a minor effect on S. cerevisiae and D. hansenii growth. These results show that nOg and SHAM have AOX independent effects which vary in different microorganisms, indicating that studies based on long-term incubation of cells with these inhibitors should be considered as inconclusive.

Phenotypic effects of dietary stress in combination with a respiratory chain bypass in mice

The alternative oxidase (AOX) from Ciona intestinalis was previously shown to be expressible in mice and to cause no physiological disturbance under unstressed conditions. Because AOX is known to become activated under some metabolic stress conditions, resulting in altered energy balance, we studied its effects in mice subjected to dietary stress. Wild‐type mice (Mus musculus, strain C57BL/6JOlaHsd) fed a high‐fat or ketogenic (high‐fat, low‐carbohydrate) diet show weight gain with increased fat mass, as well as loss of performance, compared with chow‐fed animals. Unexpectedly, AOX‐expressing mice fed on these metabolically stressful, fat‐rich diets showed almost indistinguishable patterns of weight gain and altered body composition as control animals. Cardiac performance was impaired to a similar extent by ketogenic diet in AOX mice as in nontransgenic littermates. AOX and control animals fed on ketogenic diet both showed wide variance in weight gain. Analysis of the gut microbiome in stool revealed a strong correlation with diet, rather than with genotype. The microbiome of the most and least obese outliers reared on the ketogenic diet showed no consistent trends compared with animals of normal body weight. We conclude that AOX expression in mice does not modify physiological responses to extreme diets.

Hypothesized Evolutionary Consequences of the Alternative Oxidase (AOX) in Animal Mitochondria

The environment in which eukaryotes first evolved was drastically different from what they experience today, and one of the key limiting factors was the availability of oxygen for mitochondrial respiration. During the transition to a fully oxygenated Earth, other compounds such as sulfide posed a considerable constraint on using mitochondrial aerobic respiration for energy production. The ancestors of animals, and those that first evolved from the simpler eukaryotes have mitochondrial respiratory components that are absent from later-evolving animals. Specifically, mitochondria of most basal metazoans have a sulfide-resistant alternative oxidase (AOX), which provides a secondary oxidative pathway to the classical cytochrome pathway. In this essay, I argue that because of its resistance to sulfide, AOX respiration was critical to the evolution of animals by enabling oxidative metabolism under otherwise inhibitory conditions. I hypothesize that AOX allowed for metabolic flexibility during the stochastic oxygen environment of early Earth which shaped the evolution of basal metazoans. I briefly describe the known functions of AOX, with a particular focus on the decreased production of reactive oxygen species (ROS) during stress conditions. Then, I propose three evolutionary consequences of AOX-mediated protection from ROS observed in basal metazoans: 1) adaptation to stressful environments, 2) the persistence of facultative sexual reproduction, and 3) decreased mitochondrial DNA mutation rates. Recognizing the diversity of mitochondrial respiratory systems present in animals may help resolve the mechanisms involved in major evolutionary processes such as adaptation and speciation.

Alternative Oxidase Is Involved in the Pathogenicity, Development, and Oxygen Stress Response of Botrytis cinerea

Alternative oxidase (AOX) is a ubiquinol terminal oxidase that is involved in fungal mitochondrial oxidative phosphorylation. In this study, we analyzed the roles of AOX in Botrytis cinerea by generating BcAOX deletion mutants. The mutants exhibited defects in mycelial growth, sporulation, spore germination, and virulence. Furthermore, the sensitivity of the mutants to quinone outside inhibitor fungicides and oxidative stress were increased. All phenotypic variations could be restored in the complemented strain. In summary, these results showed that BcAOX is involved in the regulation for vegetative development, adaptation to environmental stress, and virulence of B. cinerea.

Alternative respiratory chain enzymes: Therapeutic potential and possible pitfalls

The alternative respiratory chain (aRC), comprising the alternative NADH dehydrogenases (NDX) and quinone oxidases (AOX), is found in microbes, fungi and plants, where it buffers stresses arising from restrictions on electron flow in the oxidative phosphorylation system. The aRC enzymes are also found in species belonging to most metazoan phyla, including some chordates and arthropods species, although not in vertebrates or in Drosophila. We postulated that the aRC enzymes might be deployed to alleviate pathological stresses arising from mitochondrial dysfunction in a wide variety of disease states. However, before such therapies can be contemplated, it is essential to understand the effects of aRC enzymes on cell metabolism and organismal physiology. Here we report and discuss new findings that shed light on the functions of the aRC enzymes in animals, and the unexpected benefits and detriments that they confer on model organisms. In Ciona intestinalis, the aRC is induced by hypoxia and by sulfide, but is unresponsive to other environmental stressors. When expressed in Drosophila, AOX results in impaired survival under restricted nutrition, in addition to the previously reported male reproductive anomalies. In contrast, it confers cold resistance to developing and adult flies, and counteracts cell signaling defects that underlie developmental dysmorphologies. The aRC enzymes may also influence lifespan and stress resistance more generally, by eliciting or interfering with hormetic mechanisms. In sum, their judicious use may lead to major benefits in medicine, but this will require a thorough characterization of their properties and physiological effects.

Expression of alternative NADH dehydrogenases (NDH-2) in the phytopathogenic fungus Ustilago maydis

Type 2 alternative NADH dehydrogenases (NDH‐2) participate indirectly in the generation of the electrochemical proton gradient by transferring electrons from NADH and NADPH into the ubiquinone pool. Due to their structural simplicity, alternative NADH dehydrogenases have been proposed as useful tools for gene therapy of cells with defects in the respiratory complex I. In this work, we report the presence of three open reading frames, which correspond to NDH‐2 genes in the genome of Ustilago maydis. These three genes were constitutively transcribed in cells cultured in YPD and minimal medium with glucose, ethanol, or lactate as carbon sources. Proteomic analysis showed that only two of the three NDH‐2 were associated with isolated mitochondria in all culture media. Oxygen consumption by permeabilized cells using NADH or NADPH was different for each condition, opening the possibility of posttranslational regulation. We confirmed the presence of both external and internal NADH dehydrogenases, as well as an external NADPH dehydrogenase insensitive to calcium. Higher oxygen consumption rates were observed during the exponential growth phase, suggesting that the activity of NADH and NADPH dehydrogenases is coupled to the dynamics of cell growth.

Streptozotocin-Induced Adaptive Modification of Mitochondrial Supercomplexes in Liver of Wistar Rats and the Protective Effect of Moringa oleifera Lam

The increasing prevalence of diabetes continues to be a major health issue worldwide. Alteration of mitochondrial electron transport chain is a recognized hallmark of the diabetic-associated decline in liver bioenergetics; however, the molecular events involved are only poorly understood. Moringa oleifera is used for the treatment of diabetes. However, its role on mitochondrial functionality is not yet established. This study was aimed to evaluate the effect of M. oleifera extract on supercomplex formation, ATPase activity, ROS production, GSH levels, lipid peroxidation, and protein carbonylation. The levels of lipid peroxidation and protein carbonylation were increased in diabetic group. However, the levels were decreased in Moringa-treated diabetic rats. Analysis of in-gel activity showed an increase in all complex activities in the diabetic group, but spectrophotometric determinations of complex II and IV activities were unaffected in this treatment. However, we found an oxygen consumption abolition through complex I-III-IV pathway in the diabetic group treated with Moringa. While respiration with succinate feeding into complex II-III-IV was increased in the diabetic group. These findings suggest that hyperglycemia modifies oxygen consumption, supercomplexes formation, and increases ROS levels in mitochondria from the liver of STZ-diabetic rats, whereas M. oleifera may have a protective role against some alterations.

The mitochondrial alternative oxidase Aox1 is needed to cope with respiratory stress but dispensable for pathogenic development in Ustilago maydis

The mitochondrial alternative oxidase is an important enzyme that allows respiratory activity and the functioning of the Krebs cycle upon disturbance of the respiration chain. It works as a security valve in transferring excessive electrons to oxygen, thereby preventing potential damage by the generation of harmful radicals. A clear biological function, besides the stress response, has so far convincingly only been shown for plants that use the alternative oxidase to generate heat to distribute volatiles. In fungi it was described that the alternative oxidase is needed for pathogenicity. Here, we investigate expression and function of the alternative oxidase at different stages of the life cycle of the corn pathogen Ustilago maydis (Aox1). Interestingly, expression of Aox1 is specifically induced during the stationary phase suggesting a role at high cell density when nutrients become limiting. Studying deletion strains as well as overexpressing strains revealed that Aox1 is dispensable for normal growth, for cell morphology, for response to temperature stress as well as for filamentous growth and plant pathogenicity. However, during conditions eliciting respiratory stress yeast-like growth as well as hyphal growth is strongly affected. We conclude that Aox1 is dispensable for the normal biology of the fungus but specifically needed to cope with respiratory stress.

Comparative morphological characteristics of three Brettanomyces bruxellensis wine strains in the presence/absence of sulfur dioxide

The red wine spoilage yeast Brettanomyces bruxellensis has been the subject of numerous investigations. Some of these studies focused on spoilage mechanisms, sulfur dioxide tolerance and nutrient requirements. Pseudomycelium formation, although a striking feature of this species, has however been poorly investigated. Furthermore, literature regarding the induction mechanism of pseudomycelium formation in this yeast is limited and lacks clarity, as results published are contradictory. This study elucidates this phenomenon among strains from geographically different areas. Potential environmental cues were investigated, to attain a better understanding of this mechanism and its role as a survival strategy. SO₂ was previously reported to induce this morphological change however results obtained in this study did not support this. Nevertheless, the results obtained using scanning and transmission electron microscopy illustrate, for the first time in this yeast, deformity to the cell membrane and alterations to the fibrillar layers in SO₂ treated cells. In addition, the SO₂ exposed cultures displayed cell size variations, with cells displaying a decrease in length as well as delayed growth, with a prolonged lag phase. Fluorescence microscopy demonstrated a decrease in metabolic activity and the appearance of inclusion body-like structures in the cells following exposure to SO_2.

Physiological role of alternative oxidase (from yeasts to plants)

Mitochondria of all so far studied organisms, with the exception of Archaea, mammals, some yeasts, and protists, contain, along with the classical phosphorylating cytochrome pathway, a so-called cyanide-insensitive alternative oxidase (AOX) localized on the matrix side of the mitochondrial inner membrane, and electron transport through which is not coupled with ATP synthesis and energy accumulation. Mechanisms underlying plentiful functions of AOX in organisms at various levels of organization ranging from yeasts to plants are considered. First and foremost, AOX provides a chance of cell survival after inhibiting the terminal components of the main respiratory chain or losing the ability to synthesize these components. The vitally important role of AOX is obvious in thermogenesis of thermogenic plant organs where it becomes the only terminal oxidase with a very high activity, and the energy of substrate oxidation by this respiratory pathway is converted into heat, thus promoting evaporation of volatile substances attracting pollinating insects. AOX plays a fundamentally significant role in alleviating or preventing oxidative stress, thus ensuring the defense against a wide range of stresses and adverse environmental conditions, such as changes in temperature and light intensities, osmotic stress, drought, and attack by incompatible strains of bacterial pathogens, phytopathogens, or their elicitors. Participation of AOX in pathogen survival during its existence inside the host, in antivirus defense, as well as in metabolic rearrangements in plants during embryogenesis and cell differentiation is described. Examples are given to demonstrate that AOX might be an important tool to overcome the adverse aftereffects of restricted activity of the main respiratory chain in cells and whole animals.

Alternative oxidase: distribution, induction, properties, structure, regulation, and functions

The respiratory chain in the majority of organisms with aerobic type metabolism features the concomitant existence of the phosphorylating cytochrome pathway and the cyanide- and antimycin A-insensitive oxidative route comprising a so-called alternative oxidase (AOX) as a terminal oxidase. In this review, the history of AOX discovery is described. Considerable evidence is presented that AOX occurs widely in organisms at various levels of organization and is not confined to the plant kingdom. This enzyme has not been found only in Archaea, mammals, some yeasts and protists. Bioinformatics research revealed the sequences characteristic of AOX in representatives of various taxonomic groups. Based on multiple alignments of these sequences, a phylogenetic tree was constructed to infer their possible evolution. The ways of AOX activation, as well as regulatory interactions between AOX and the main respiratory chain are described. Data are summarized concerning the properties of AOX and the AOX-encoding genes whose expression is either constitutive or induced by various factors. Information is presented on the structure of AOX, its active center, and the ubiquinone-binding site. The principal functions of AOX are analyzed, including the cases of cell survival, optimization of respiratory metabolism, protection against excess of reactive oxygen species, and adaptation to variable nutrition sources and to biotic and abiotic stress factors. It is emphasized that different AOX functions complement each other in many instances and are not mutually exclusive. Examples are given to demonstrate that AOX is an important tool to overcome the adverse aftereffects of restricted activity of the main respiratory chain in cells and whole animals. This is the first comprehensive review on alternative oxidases of various organisms ranging from yeasts and protists to vascular plants.

Overexpression of ALTERNATIVE OXIDASE1a alleviates mitochondria-dependent programmed cell death induced by aluminium phytotoxicity in Arabidopsis

Alternative oxidase (AOX) is a terminal oxidase found in all plants, and functions to maintain the electron flux and reduce the production of reactive oxygen species (ROS). Our previous study demonstrated that aluminium (Al) treatment could induce increased expression of the AOX1a gene, but the mechanism of how AOX1a participates in the regulation of Al-induced programmed cell death (PCD) is still not clear. To investigate the possible mechanism, mitochondrial ROS production and the behaviour of mitochondria, as well as caspase-3-like activation were monitored under Al treatment in wild-type (WT), AOX1a-lacking (aox1a), and AOX1a-overexpressing (AOX1a-OE) Arabidopsis. Our results showed that Al treatment increased the expression of AOX1a at both the transcriptional and translational levels. Overexpression of AOX1a reduced mitochondrial ROS production by maintaining the mitochondrial electron flux, and alleviated subsequent mitochondrial dysfunction and caspase-3-like activation in Al-induced PCD. Moreover, it was found that a change in AOX1a level could influence the expression levels of downstream functional genes that play protective roles in Al-induced PCD. Experiments using mutants and inhibitors demonstrated that superoxide anion (O2 (-)) derived from mitochondria was involved in Al-induced upregulation of AOX1a gene expression. Taken together, these results indicated that overexpression of AOX1a alleviated Al-induced PCD by maintaining mitochondrial function and promoting the expression of protective functional genes, providing new insights into the signalling cascades that modulate the Al phytotoxicity mechanism.

Effects of chitosan and oligochitosan on development and mitochondrial function of Rhizopus stolonifer

The antifungal activities of chitosan and oligochitosan have been used to control postharvest decay of the fruits. The effect of chitosan and oligochitosan on mycelium growth, spore germination, and mitochondrial function of Rhizopus stolonifer was evaluated in order to establish a connection between fungus development and the main organelle in charge to provide energy to the cell. The mycelium growth of R. stolonifer was significantly reduced on minimum media amended with chitosan or oligochitosan. The highest antifungal indexes were obtained on media containing chitosan or oligochitosan at 2.0 mg ml(-1). Microscopic observation showed that chitosan and oligochitosan affected the spore germination and hyphae morphology. Both polymers increased oxygen consumption of R. stolonifer. Respiratory activity was restored with NADH in permeabilized treated and untreated cells, and was inhibited with rotenone and flavones. Complex III and IV were inhibited by antimycin A and cyanide, respectively, in treated and untreated cells. Chitosan and oligochitosan increased NADH dehydrogenase activity in isolated mitochondria. However, there were not changes in the cytochrome c oxidase and ATPase activities by effect of these polymers. These results suggest that both chitosan and oligochitosan affect the development of R. stolonifer and might be implicated in the mitochondrial dysfunction.