Hypoxia induces complex I inhibition and ultrastructural damage by increasing mitochondrial nitric oxide in developing CNS

Mitochondrial plasticity in the cerebellum of two anoxia-tolerant sharks: contrasting responses to anoxia/reoxygenation

Exposure to anoxia leads to rapid ATP depletion, alters metabolic pathways and exacerbates succinate accumulation. Upon re-oxygenation, the preferential oxidation of accumulated succinate most often impairs mitochondrial function. Few species can survive prolonged periods of hypoxia and anoxia at tropical temperatures and those that do may rely on mitochondria plasticity in response to disruptions to oxygen availability. Two carpet sharks, the epaulette shark (Hemiscyllium ocellatum; ES) and the grey carpet shark (Chiloscyllium punctatum; GCS) display different adaptive responses to prolonged anoxia: while the ES enters energy conserving metabolic depression, the GCS temporarily elevates its haematocrit prolonging oxygen delivery. High-resolution respirometry was used to investigate mitochondrial function in the cerebellum, a highly metabolically active organ that is oxygen sensitive and vulnerable to injury after anoxia/re-oxygenation (AR).

Succinate was titrated into cerebellar preparations in vitro, with or without pre-exposure to AR, then the activity of mitochondrial complexes was examined. Like most vertebrates, GCS mitochondria significantly increased succinate oxidation rates, with impaired complex I function post-AR. In contrast, ES mitochondria inhibited succinate oxidation rates and both complex I and II capacities were conserved, resulting in preservation of oxidative phosphorylation capacity post-AR.

Divergent mitochondrial plasticity elicited by elevated succinate post A/R parallels the inherently divergent physiological adaptations of these animals to prolonged anoxia, namely the absence (GCS) and presence of metabolic depression (ES). Since anoxia tolerance in these species also occurs at temperatures close to that of humans, examining their mitochondrial responses to AR could provide insights for novel interventions in clinical settings.

Post-Translational Oxidative Modifications of Mitochondrial Complex I (NADH: Ubiquinone Oxidoreductase): Implications for Pathogenesis and Therapeutics in Human Diseases

Mitochondrial complex I (NADH: ubiquinone oxidoreductase; CI) is central to the electron transport chain (ETC), oxidative phosphorylation, and ATP production in eukaryotes. CI is a multi-subunit complex with a complicated yet organized structure that optimally connects electron transfer with proton translocation and forms higher-order supercomplexes with other ETC complexes. Efforts to understand the molecular genetics, expression profile of subunits, and structure-function relationship of CI have increased over the years due to the direct role of the complex in human diseases. Although mutations in the nuclear and mitochondrial genes of CI and altered expression of subunits could potentially lower CI activity leading to mitochondrial dysfunction in many diseases, oxidative post-translational modifications (PTMs) have emerged as an important mechanism contributing to altered CI activity. These mainly include reversible and irreversible cysteine modifications, tyrosine nitration, carbonylation, and tryptophan oxidation that are generated following exposure to reactive oxygen species/reactive nitrogen species. Interestingly, oxidative PTMs could contribute either to CI damage, mitochondrial dysfunction, and ensuing cell death or a response mechanism with potential cytoprotective effects. This has also emerged as a promising field for structural biologists since analysis of PTMs could assist in understanding the structure-function relationship of the complex and correlate electron transfer mechanism with energy production. However, analysis of PTMs of CI and their contribution to CI function are incomplete in many physiological and pathological conditions. This review aims to highlight the role of oxidative PTMs in modulating CI activity with implications toward pathobiology of CNS diseases and novel therapeutics.

Hypoxia induces selective modifications to the acetylome in the brain of zebrafish (Danio rerio)

Reversible protein acetylation is an important regulatory mechanism for modulating protein function. The cellular protein acetylome is in large part dictated by the cellular redox balance, and in particular [NAD⁺]. While the relationship between hypoxia, redox balance, energy charge and resulting mitochondrial dysfunction has been examined in the context of hypoxia-linked pathologies, little is known about the direct effects of decreases in environmental oxygen on reversible lysine acetylation, and the resulting modifications to mitochondrial metabolism. To address this knowledge gap, we exposed zebrafish (Danio rerio) to 16 h of hypoxia (2.21 kPa) and quantified acetylation levels of 1220 proteins using whole-cell proteomics in samples of brain taken from normoxic and hypoxic zebrafish. In addition, we examined the effects of hypoxia on cytoplasmic and mitochondrial redox status, whole-cell energetics, the activity of the mitochondrial NAD⁺-dependent deacetylase SIRT3, and electron transport chain complex activities to determine if there is an association between hypoxia-induced metabolic disturbances, protein acetylation, and mitochondrial function. Our results (1) reveal several key changes in the acetylation status of proteins in the brain, primarily within the mitochondria; (2) show significant fluctuations in cytoplasmic and mitochondrial redox status within the brain during hypoxia exposure; and (3) provide evidence that lysine acetylation may be related to large changes in electron transport and ATP-synthase complex activities and adenylate status in zebrafish exposed to hypoxic stress. Together, these data provide new insights into the role of protein modifications in mitochondrial metabolism during hypoxia.

Neuroprotection by hypoxic preconditioning involves upregulation of hypoxia-inducible factor-1 in a prenatal model of acute hypoxia

The molecular pathways underlying the neuroprotective effects of preconditioning are promising, potentially drugable targets to promote cell survival. However, these pathways are complex and are not yet fully understood. In this study we have established a paradigm of hypoxic preconditioning based on a chick embryo model of normobaric acute hypoxia previously developed by our group. With this model, we analyzed the role of hypoxia-inducible factor-1α (HIF-1α) stabilization during preconditioning in HIF-1 signaling after the hypoxic injury and in the development of a neuroprotective effect against the insult. To this end, we used a pharmacological approach, based on the in vivo administration of positive (Fe(2+), ascorbate) and negative (CoCl(2)) modulators of the activity of HIF-prolyl hydroxylases (PHDs), the main regulators of HIF-1. We have found that preconditioning has a reinforcing effect on HIF-1 accumulation during the subsequent hypoxic injury. In addition, we have also demonstrated that HIF-1 induction during hypoxic preconditioning is necessary to obtain an enhancement in HIF-1 accumulation and to develop a tolerance against a subsequent hypoxic injury. We provide in vivo evidence that administration of Fe(2+) and ascorbate modulates HIF accumulation, suggesting that PHDs might be targets for neuroprotection in the CNS.

Altered glucose metabolism and preserved energy charge and neuronal structures in the brain of mouse intermittently exposed to hypoxia

The key for an animal to survive prolonged hypoxia is to avoid rapid decline in ATP levels in vital organs such as the brain. This can be well achieved by a very few of hypoxia-tolerant animals such as freshwater turtles and newborn animals, since these animals can substantially suppress their metabolic levels by coordinated regulation of ATP-producing and ATP-demanding pathways. However, most animals, especially adult mammals, can only tolerate a short period of hypoxia since they are unable to maintain constant ATP levels and energy charge in vital organs during prolonged hypoxic exposure. Here, we described a special mouse model, in which a hypoxia intolerant adult mouse gradually built up an ability to survive prolonged hypoxia after intermittent hypoxic exposures. This increased ability was accompanied by reductions in body temperature and O(2) consumption as well as transient variations in blood pCO(2), pO(2) and pH. The glucose and energy metabolism in the brain of the mouse altered similarly to those reported in the brain of hypoxic turtles. Activities of phosphofructokinase and pyruvate kinase, the two rate-limiting enzymes controlling the rate of glycolysis decreased to baseline levels after a short period of increase. In contrast, the activity of complex I, the major enzyme complex controlling oxidative phosphorylation, was kept inhibited. These alterations in the ATP-producing pathway suggest the occurrence of reverse Pasteur effect, indicating that the animal had entered a hypometabolic state favoring maintenance of ATP level and energy charge in hypoxic conditions. In supporting this idea, the ATP levels and energy charge as well as neuronal structures in the brain were well preserved. This study provides evidence for a possibility that a hypoxic intolerant animal can build up an ability to survive prolonged hypoxia through regulation of its glucose and energy metabolism after an appropriate hypoxic training, which deserves further investigation.

Neuronal death during combined intermittent hypoxia/hypercapnia is due to mitochondrial dysfunction

Breathing-disordered states, such as in obstructive sleep apnea, which are cyclical in nature, have been postulated to induce neurocognitive morbidity in both pediatric and adult populations. The oscillatory nature of intermittent hypoxia, especially when chronic, may mimic the paradigm of ischemia-reperfusion in that tissues and cells are exposed to episodes of low and high O(2) and this may lead to oxidant stress. Therefore, we decided to explore the potential contribution of oxidant stress in our intermittent hypoxia/hypercapnia animal model and the role that mitochondria might play in this stress. Neonatal mice were exposed to intermittent hypoxia/hypercapnia for 10 days and 2 wk. Combined intermittent hypoxia/hypercapnia led to a marked increase in apoptotic cell death in the cerebral cortex. Oxygen consumption studies in isolated mitochondria from intermittent hypoxia/hypercapnia-exposed brains demonstrated significant reductions in both state 4 and state 3 respiratory activities by approximately 60% and 75%, respectively. Electron paramagnetic resonance spectroscopy registered a significant increase in superoxide production during nonphosphorylating state 4 by 37%, although superoxide leakage during state 3 did not increase upon treatment. Neuronal superoxide-specific dihydroethidium oxidation was also greater in exposed animals. These studies indicate that intermittent hypoxia/hypercapnia leads to oxidative stress due to mitochondrial response within the mouse central nervous system.

Sustained deficiency of mitochondrial complex I activity during long periods of survival after seizures induced in immature rats by homocysteic acid

Our previous work demonstrated the marked decrease of mitochondrial complex I activity in the cerebral cortex of immature rats during the acute phase of seizures induced by bilateral intracerebroventricular infusion of dl-homocysteic acid (600 nmol/side) and at short time following these seizures. The present study demonstrates that the marked decrease ( approximately 60%) of mitochondrial complex I activity persists during the long periods of survival, up to 5 weeks, following these seizures, i.e. periods corresponding to the development of spontaneous seizures (epileptogenesis) in this model of seizures. The decrease was selective for complex I and it was not associated with changes in the size of the assembled complex I or with changes in mitochondrial content of complex I. Inhibition of complex I was accompanied by a parallel, up to 5 weeks lasting significant increase (15-30%) of three independent mitochondrial markers of oxidative damage, 3-nitrotyrosine, 4-hydroxynonenal and protein carbonyls. This suggests that oxidative modification may be most likely responsible for the sustained deficiency of complex I activity although potential role of other factors cannot be excluded. Pronounced inhibition of complex I was not accompanied by impaired ATP production, apparently due to excess capacity of complex I documented by energy thresholds. The decrease of complex I activity was substantially reduced by treatment with selected free radical scavengers. It could also be attenuated by pretreatment with (S)-3,4-DCPG (an agonist for subtype 8 of group III metabotropic glutamate receptors) which had also a partial antiepileptogenic effect. It can be assumed that the persisting inhibition of complex I may lead to the enhanced production of reactive oxygen and/or nitrogen species, contributing not only to neuronal injury demonstrated in this model of seizures but also to epileptogenesis.

An improved method to obtain a soluble nuclear fraction from embryonic brain tissue

This paper describes modifications of the standard methods for obtaining a soluble nuclear fraction from embryonic brain tissue. The main improvements are: (1) the inclusion of a low speed centrifugation step to prevent the appearance of high density contaminants, (2) a sucrose density gradient to remove perinuclear mitochondria and ER membranes and (3) a protein extraction approach which significantly enhances protein yield. To demonstrate the effectiveness of the method, pellets were analyzed by light and electron microscopy and purity of the soluble extracts was immunologically tested. Finally, to illustrate the applicability of this approach, the induction of the transcription factor HIF-1 (hypoxia-inducible factor-1) was assessed by Western blot using soluble nuclear fractions and by immuno-electron microscopy using purified nuclear fractions, both obtained from the optic lobes of chick embryos. In conclusion, the procedure presently described appears to be reliable and convenient for obtaining a pure soluble nuclear fraction from a discrete amount of embryonic brain tissue.

Methylglyoxal-induced mitochondrial dysfunction in vascular smooth muscle cells

The effects of methylglyoxal (MG) on mitochondria with specific foci on peroxynitrite (ONOO(-)) production, manganese superoxide dismutase (MnSOD) activity, and mitochondrial functions in vascular smooth muscle A-10 cells were investigated. Mitochondrial MG content was significantly increased after A-10 cells were treated with exogenous MG, and so did advanced glycated endproducts (AGEs) formation, indicated by the appearance of N(epsilon)-(carboxyethyl) lysine, in A-10 cells. The levels of mitochondrial reactive oxygen species (mtROS) and ONOO(-) were significantly increased by MG treatment. Application of ONOO(-) specific scavenger uric acid lowered the level of mtROS. MG significantly enhanced the production of mitochondrial superoxide (O(2)(-)) and nitric oxide (NO), which were inhibited by SOD mimic 4-hydroxy-tempo and mitochondrial nitric oxide synthase (mtNOS) specific inhibitor 7-nitroindazole, respectively. The activity of MnSOD was decreased by MG treatment. Furthermore, MG decreased respiratory complex III activity and ATP synthesis in mitochondria, indicating an impaired mitochondrial respiratory chain. AGEs cross-link breaker alagebrium reversed all aforementioned mitochondrial effects of MG. Our data demonstrated that mitochondrial function is under the control of MG. By inhibiting Complex III activity, MG induces mitochondrial oxidative stress and reduces ATP production. These discoveries will help unmask molecular mechanisms for various MG-induced mitochondrial dysfunction-related cellular disorders.

The formation of peroxynitrite in the applied physiology of mitochondrial nitric oxide

Mitochondria require nitric oxide ((.)NO) to exert a delicate control of metabolic rate as well as to regulate life functions, cell cycle activation and arrest, and apoptosis. All activities depend on the matrical (.)NO steady state concentration as provided by mitochondrial (mtNOS) and cytosolic sources (eNOS) and reduced by forming superoxide anion and H2O2 and a low peroxynirite (ONOO(-)) yield. We review herein the biochemical pathways involved in the control of (.)NO mitochondrial level and its biological and physiological significance in hormone effects and aging. At high ()NO, the cost of this physiological regulation is that ONOO(-) excess will lead to nitrosation/nitration and oxidization of mitochondrial and cell proteins and lipids. The disruption of (.)NO modulation of mitochondrial respiration supports then, a platform for prevalent neurodegenerative and metabolic diseases.