Paul S. Brookes

Ischaemic accumulation of succinate controls reperfusion injury through mitochondrial ROS

Ischaemia-reperfusion injury occurs when the blood supply to an organ is disrupted and then restored, and underlies many disorders, notably heart attack and stroke. While reperfusion of ischaemic tissue is essential for survival, it also initiates oxidative damage, cell death and aberrant immune responses through the generation of mitochondrial reactive oxygen species (ROS). Although mitochondrial ROS production in ischaemia reperfusion is established, it has generally been considered a nonspecific response to reperfusion. Here we develop a comparative in vivo metabolomic analysis, and unexpectedly identify widely conserved metabolic pathways responsible for mitochondrial ROS production during ischaemia reperfusion. We show that selective accumulation of the citric acid cycle intermediate succinate is a universal metabolic signature of ischaemia in a range of tissues and is responsible for mitochondrial ROS production during reperfusion. Ischaemic succinate accumulation arises from reversal of succinate dehydrogenase, which in turn is driven by fumarate overflow from purine nucleotide breakdown and partial reversal of the malate/aspartate shuttle. After reperfusion, the accumulated succinate is rapidly re-oxidized by succinate dehydrogenase, driving extensive ROS generation by reverse electron transport at mitochondrial complex I. Decreasing ischaemic succinate accumulation by pharmacological inhibition is sufficient to ameliorate in vivo ischaemia-reperfusion injury in murine models of heart attack and stroke. Thus, we have identified a conserved metabolic response of tissues to ischaemia and reperfusion that unifies many hitherto unconnected aspects of ischaemia-reperfusion injury. Furthermore, these findings reveal a new pathway for metabolic control of ROS production in vivo, while demonstrating that inhibition of ischaemic succinate accumulation and its oxidation after subsequent reperfusion is a potential therapeutic target to decrease ischaemia-reperfusion injury in a range of pathologies.

Nitric oxide, mitochondria and neurological disease.

Damage to the mitochondrial electron transport chain has been suggested to be an important factor in the pathogenesis of a range of neurological disorders, such as Parkinsons disease, Alzheimers disease, multiple sclerosis, stroke and amyotrophic lateral sclerosis. There is also a growing body of evidence to implicate excessive or inappropriate generation of nitric oxide (NO) in these disorders. It is now well documented that NO and its toxic metabolite, peroxynitrite (ONOO-), can inhibit components of the mitochondrial respiratory chain leading, if damage is severe enough, to a cellular energy deficiency state. Within the brain, the susceptibility of different brain cell types to NO and ONOO- exposure may be dependent on factors such as the intracellular reduced glutathione (GSH) concentration and an ability to increase glycolytic flux in the face of mitochondrial damage. Thus neurones, in contrast to astrocytes, appear particularly vulnerable to the action of these molecules. Following cytokine exposure, astrocytes can increase NO generation, due to de novo synthesis of the inducible form of nitric oxide synthase (NOS). Whilst the NO/ONOO- so formed may not affect astrocyte survival, these molecules may diffuse out to cause mitochondrial damage, and possibly cell death, to other cells, such as neurones, in close proximity. Evidence is now available to support this scenario for neurological disorders, such as multiple sclerosis. In other conditions, such as ischaemia, increased availability of glutamate may lead to an activation of a calcium-dependent nitric oxide synthase associated with neurones. Such increased/inappropriate NO formation may contribute to energy depletion and neuronal cell death. The evidence available for NO/ONOO--mediated mitochondrial damage in various neurological disorders is considered and potential therapeutic strategies are proposed.

Nitrite augments tolerance to ischemia/reperfusion injury via the modulation of mitochondrial electron transfer

Nitrite (NO2 −) is an intrinsic signaling molecule that is reduced to NO during ischemia and limits apoptosis and cytotoxicity at reperfusion in the mammalian heart, liver, and brain. Although the mechanism of nitrite-mediated cytoprotection is unknown, NO is a mediator of the ischemic preconditioning cell-survival program. Analogous to the temporally distinct acute and delayed ischemic preconditioning cytoprotective phenotypes, we report that both acute and delayed (24 h before ischemia) exposure to physiological concentrations of nitrite, given both systemically or orally, potently limits cardiac and hepatic reperfusion injury. This cytoprotection is associated with increases in mitochondrial oxidative phosphorylation. Remarkably, isolated mitochondria subjected to 30 min of anoxia followed by reoxygenation were directly protected by nitrite administered both in vitro during anoxia or in vivo 24 h before mitochondrial isolation. Mechanistically, nitrite dose-dependently modifies and inhibits complex I by posttranslational S-nitrosation; this dampens electron transfer and effectively reduces reperfusion reactive oxygen species generation and ameliorates oxidative inactivation of complexes II–IV and aconitase, thus preventing mitochondrial permeability transition pore opening and cytochrome c release. These data suggest that nitrite dynamically modulates mitochondrial resilience to reperfusion injury and may represent an effector of the cell-survival program of ischemic preconditioning and the Mediterranean diet.

Concentration-dependent effects of nitric oxide on mitochondrial permeability transition and cytochrome c release.

The mitochondrial permeability transition pore (PTP) and associated release of cytochrome c are thought to be important in the apoptotic process. Nitric oxide (NO⋅) has been reported to inhibit apoptosis by acting on a variety of extra-mitochondrial targets. The relationship between cytochromec release and PTP opening, and the effects of NO⋅are not clearly established. Nitric oxide, S-nitrosothiols and peroxynitrite are reported to variously inhibit or promote PTP opening. In this study the effects of NO⋅ on the PTP were characterized by exposing isolated rat liver mitochondria to physiological and pathological rates of NO⋅ released from NONOate NO⋅donors. Nitric oxide reversibly inhibited PTP opening with an IC50 of 11 nm NO⋅/s, which can be readily achieved in vivo by NO⋅ synthases. The mechanism involved mitochondrial membrane depolarization and inhibition of Ca2+ accumulation. At supraphysiological release rates (>2 μm/s) NO⋅ accelerated PTP opening. Substantial cytochrome c release occurred with only a 20% change in mitochondrial swelling, was an early event in the PTP, and was also inhibited by NO⋅. Furthermore, NO⋅ exposure resulted in significantly lower cytochrome c release for the same degree of PTP opening. It is proposed that this pathway represents an additional mechanism underlying the antiapoptotic effects of NO⋅.

Mitochondria: regulators of signal transduction by reactive oxygen and nitrogen species.

The functional role of mitochondria in cell physiology has previously centered around metabolism, with oxidative phosphorylation playing a pivotal role. Recently, however, this perspective has changed significantly with the realization that mitochondria are active participants in signal transduction pathways, not simply the passive recipients of injunctions from the rest of the cell. In this review the emerging role of the mitochondrion in cell signaling is discussed in the context of cytochrome c release, hydrogen peroxide formation from the respiratory chain, and the nitric oxide-cytochrome c oxidase signaling pathway.

The basal proton conductance of mitochondria depends on adenine nucleotide translocase content

The basal proton conductance of mitochondria causes mild uncoupling and may be an important contributor to metabolic rate. The molecular nature of the proton-conductance pathway is unknown. We show that the proton conductance of muscle mitochondria from mice in which isoform 1 of the adenine nucleotide translocase has been ablated is half that of wild-type controls. Overexpression of the adenine nucleotide translocase encoded by the stress-sensitive B gene in Drosophila mitochondria increases proton conductance, and underexpression decreases it, even when the carrier is fully inhibited using carboxyatractylate. We conclude that half to two-thirds of the basal proton conductance of mitochondria is catalysed by the adenine nucleotide carrier, independently of its ATP/ADP exchange or fatty-acid-dependent proton-leak functions.

Direct evidence for S-nitrosation of mitochondrial complex I.

NO* (nitric oxide) is a pleiotropic signalling molecule, with many of its effects on cell function being elicited at the level of the mitochondrion. In addition to the well-characterized binding of NO* to the Cu(B)/haem-a3 site in mitochondrial complex IV, it has been proposed by several laboratories that complex I can be inhibited by S-nitrosation of a cysteine. However, direct molecular evidence for this is lacking. In this investigation we have combined separation techniques for complex I (blue-native gel electrophoresis, Superose 6 column chromatography) with sensitive detection methods for S-nitrosothiols (chemiluminescence, biotin-switch assay), to show that the 75 kDa subunit of complex I is S-nitrosated in mitochondria treated with S-nitrosoglutathione (10 microM-1 mM). The stoichiometry of S-nitrosation was 7:1 (i.e. 7 mol of S-nitrosothiols per mol of complex I) and this resulted in significant inhibition of the complex. Furthermore, S-nitrosothiols were detected in mitochondria isolated from hearts subjected to ischaemic preconditioning. The implications of these results for the physiological regulation of respiration, for reactive oxygen species generation and for a potential role of S-nitrosation in cardioprotection are discussed.

The Proton Permeability of the Inner Membrane of Liver Mitochondria from Ectothermic and Endothermic Vertebrates and from Obese Rats: Correlations with Standard Metabolic Rate and Phospholipid Fatty Acid Composition

We measured the proton leak across the inner membrane of liver mitochondria isolated from six different vertebrate species and from obese and control Zucker rats. Proton leak at 37 degrees C was similar in rat and pigeon, and in obese and control Zucker rats. Compared to rat, it was lower in cane toad, shingleback lizard, and the Madeiran lizard Lacerta dugessi. Proton leak at 20 degrees C was similar in xenopus toad and higher in rainbow trout, compared to rat. In general, proton permeability and substrate oxidation activity were greater in liver mitochondria from endotherms than those from ectotherms. Analysis of this and previous data showed that proton leak per milligram of mitochondrial protein correlated with standard metabolic rate, and proton leak per milligram of inner membrane phospholipid correlated with 11 phospholipid fatty acid compositional parameters, including unsaturation index.

Nitric oxide partitioning into mitochondrial membranes and the control of respiration at cytochrome c oxidase

An emerging and important site of action for nitric oxide (NO) within cells is the mitochondrial inner membrane, where NO binds to and inhibits members of the electron transport chain, complex III and cytochrome c oxidase. Although it is known that inhibition of cytochrome c oxidase by NO is competitive with O2, the mechanisms that underlie this phenomenon remain unclear, and the impact of both NO and O2 partitioning into biological membranes has not been considered. These properties are particularly interesting because physiological O2 tensions can vary widely, with NO having a greater inhibitory effect at low O2 tensions (<20 μM). In this study, we present evidence for a consumption of NO in mitochondrial membranes in the absence of substrate, in a nonsaturable process that is O2 dependent. This consumption modulates inhibition of cytochrome c oxidase by NO and is enhanced by the addition of exogenous membranes. From these data, it is evident that the partition of NO into mitochondrial membranes has a major impact on the ability of NO to control mitochondrial respiration. The implications of this conclusion are discussed in the context of mitochondrial lipid:protein ratios and the importance of NO as a regulator of respiration in pathophysiology.

Nutrient-sensitized screening for drugs that shift energy metabolism from mitochondrial respiration to glycolysis

Most cells have the inherent capacity to shift their reliance on glycolysis relative to oxidative metabolism, and studies in model systems have shown that targeting such shifts may be useful in treating or preventing a variety of diseases ranging from cancer to ischemic injury. However, we currently have a limited number of mechanistically distinct classes of drugs that alter the relative activities of these two pathways. We screen for such compounds by scoring the ability of >3,500 small molecules to selectively impair growth and viability of human fibroblasts in media containing either galactose or glucose as the sole sugar source. We identify several clinically used drugs never linked to energy metabolism, including the antiemetic meclizine, which attenuates mitochondrial respiration through a mechanism distinct from that of canonical inhibitors. We further show that meclizine pretreatment confers cardioprotection and neuroprotection against ischemia-reperfusion injury in murine models. Nutrient-sensitized screening may provide a useful framework for understanding gene function and drug action within the context of energy metabolism.