Ferulenol specifically inhibits succinate ubiquinone reductase at the level of the ubiquinone cycle

The natural compound ferulenol, a sesquiterpene prenylated coumarin derivative, was purified from Ferula vesceritensis and its mitochondrial effects were studied. Ferulenol caused inhibition of oxidative phoshorylation. At low concentrations, ferulenol inhibited ATP synthesis by inhibition of the adenine nucleotide translocase without limitation of mitochondrial respiration. At higher concentrations, ferulenol inhibited oxygen consumption. Ferulenol caused specific inhibition of succinate ubiquinone reductase without altering succinate dehydrogenase activity of the complex II. This inhibition results from a limitation of electron transfers initiated by the reduction of ubiquinone to ubiquinol in the ubiquinone cycle. This original mechanism of action makes ferulenol a useful tool to study the physiological role and the mechanism of electron transfer in the complex II. In addition, these data provide an additional mechanism by which ferulenol may alter cell function and demonstrate that mitochondrial dysfunction is an important determinant in Ferula plant toxicity.

Dicoumarol impairs mitochondrial electron transport and pyrimidine biosynthesis in human myeloid leukemia HL-60 cells

Dicoumarol, a competitive inhibitor of NAD(P)H:quinone oxidoreductase 1 (NQO1), increases intracellular superoxide and affects cell growth of tumor cells. This work was set to establish a mechanistic link between dicoumarol, superoxide and cell cycle alterations in HL-60 cells. Using ES936, a mechanism-based irreversible inhibitor of NQO1, we demonstrate that NQO1 inhibition is not a major factor involved in superoxide boost. Mitochondrial Complexes II, III and IV were directly inhibited by dicoumarol. Succinate, which inhibits superoxide generation by reversed electron flow in Complex II, significantly decreased superoxide boost in dicoumarol-treated cells and in isolated mitochondria incubated with dicoumarol and decylubiquinol. Superoxide generation in cells was strongly potentiated by blocking the quinone site of Complex II with thenoyltrifluoroacetone, supporting the involvement of cytochrome b560 to drive electrons for increasing superoxide. Simultaneous inhibition of the mitochondrial chain upstream ubiquinone and displacement of succinate from the Complex II active site is proposed as a major mechanism to explain how dicoumarol increases superoxide in HL-60 cells. Dicoumarol-treated cells accumulated in S phase due to the impairment of pyrimidine biosynthesis at dihydroorotate dehydrogenase step because blockade was overcome by addition of exogenous uridine or orotate, but not by dihydroorotate. We demonstrate for the first time that dicoumarol inhibits mitochondrial electron transport, induces superoxide release by reversed electron flow in Complex II, and inhibits pyrimidines biosynthesis. These actions must be taken into account when considering dicoumarol effects on cells.

Mitochondrial Respiratory Dysfunction and Oxidative Stress after Chronic Malathion Exposure

Malathion is a pesticide used on a large scale and with high potential risk for human exposure. However, it is reasonable to hypothesize that while the malathion is metabolizing reactive oxygen species (ROS) can be generated and subsequently there is onset of an oxidative stress in central nervous system (CNS) structures: hippocampus, cortex, striatum and cerebellum of intoxicated rats due to mitochondrial respiratory chain disfunctions. The present study was therefore undertaken to evaluate malathion-induced lipid peroxidation (LPO), superoxide production from sub-mitochondrial particles and the activity of complexes II and IV of the mitochondrial respiratory chain. Malathion was administered in doses of 25, 50, 100 and 150 mg malathion/kg. After malathion administration LPO increased in hippocampus and striatum. This was accompanied by an increase in the formation of superoxide in submitochondrial particles in the hippocampus. Complex IV suffered significant inhibition of its activity. We could demonstrate in this study that malathion induces oxidative stress and it could be due to inactivation of mitochondrial respiratory complexes.

EA. Crystallographic studies of the binding of ligands to the dicarboxylate site of Complex II, and the identity of the ligand in the "oxaloacetate-inhibited" state

Mitochondrial Complex II (succinate:ubiquinone oxidoreductase) is purified in a partially inactivated state, which can be activated by removal of tightly bound oxaloacetate (E.B. Kearney, et al., Biochem. Biophys. Res. Commun. 49 1115-1121). We crystallized Complex II in the presence of oxaloacetate or with the endogenous inhibitor bound. The structure showed a ligand essentially identical to the "malate-like intermediate" found in Shewanella Flavocytochrome c crystallized with fumarate (P. Taylor, et al., Nat. Struct. Biol. 6 1108-1112) Crystallization of Complex II in the presence of excess fumarate also gave the malate-like intermediate or a mixture of that and fumarate at the active site. In order to more conveniently monitor the occupation state of the dicarboxylate site, we are developing a library of UV/Vis spectral effects induced by binding different ligands to the site. Treatment with fumarate results in rapid development of the fumarate difference spectrum and then a very slow conversion into a species spectrally similar to the OAA-liganded complex. Complex II is known to be capable of oxidizing malate to the enol form of oxaloacetate (Y.O. Belikova, et al., Biochim. Biophys. Acta 936 1-9). The observations above suggest it may also be capable of interconverting fumarate and malate. It may be useful for understanding the mechanism and regulation of the enzyme to identify the malate-like intermediate and its pathway of formation from oxaloacetate or fumarate.

Hypoxic vasoconstriction of partial muscular intra-acinar pulmonary arteries in murine precision cut lung slices

Background

Acute alveolar hypoxia causes pulmonary vasoconstriction (HPV) which serves to match lung perfusion to ventilation. The underlying mechanisms are not fully resolved yet. The major vascular segment contributing to HPV, the intra-acinar artery, is mostly located in that part of the lung that cannot be selectively reached by the presently available techniques, e.g. hemodynamic studies of isolated perfused lungs, recordings from dissected proximal arterial segments or analysis of subpleural vessels. The aim of the present study was to establish a model which allows the investigation of HPV and its underlying mechanisms in small intra-acinar arteries.

Methods

Intra-acinar arteries of the mouse lung were studied in 200 μm thick precision-cut lung slices (PCLS). The organisation of the muscle coat of these vessels was characterized by α-smooth muscle actin immunohistochemistry. Basic features of intra-acinar HPV were characterized, and then the impact of reactive oxygen species (ROS) scavengers, inhibitors of the respiratory chain and Krebs cycle metabolites was analysed.

Results

Intra-acinar arteries are equipped with a discontinuous spiral of α-smooth muscle actin-immunoreactive cells. They exhibit a monophasic HPV (medium gassed with 1% O₂) that started to fade after 40 min and was lost after 80 min. This HPV, but not vasoconstriction induced by the thromboxane analogue U46619, was effectively blocked by nitro blue tetrazolium and diphenyleniodonium, indicating the involvement of ROS and flavoproteins. Inhibition of mitochondrial complexes II (3-nitropropionic acid, thenoyltrifluoroacetone) and III (antimycin A) specifically interfered with HPV, whereas blockade of complex IV (sodium azide) unspecifically inhibited both HPV and U46619-induced constriction. Succinate blocked HPV whereas fumarate had minor effects on vasoconstriction.

Conclusion

This study establishes the first model for investigation of basic characteristics of HPV directly in intra-acinar murine pulmonary vessels. The data are consistent with a critical involvement of ROS, flavoproteins, and of mitochondrial complexes II and III in intra-acinar HPV. In view of the lack of specificity of any of the classical inhibitors used in such types of experiments, validation awaits the use of appropriate knockout strains and siRNA interference, for which the present model represents a well-suited approach.

Differential inhibitory effects of methylmalonic acid on respiratory chain complex activities in rat tissues

Methylmalonic acidemia is an inherited metabolic disorder biochemically characterized by tissue accumulation of methylmalonic acid (MMA) and clinically by progressive neurological deterioration and kidney failure, whose pathophysiology is so far poorly established. Previous studies have shown that MMA inhibits complex II of the respiratory chain in rat cerebral cortex, although no inhibition of complexes I-V was found in bovine heart. Therefore, in the present study we investigated the in vitro effect of 2.5mM MMA on the activity of complexes I-III, II, II-III and IV in striatum, hippocampus, heart, liver and kidney homogenates from young rats. We observed that MMA caused a significant inhibition of complex II activity in striatum and hippocampus (15-20%) at low concentrations of succinate in the medium, but not in the peripheral tissues. We also verified that the inhibitory property of MMA only occurred after exposing brain homogenates for at least 10 min with the acid, suggesting that this inhibition was mediated by indirect mechanisms. Simultaneous preincubation with the nitric oxide synthase inhibitor Nomega-nitro-L-arginine methyl ester (L-NAME) and catalase (CAT) plus superoxide dismutase (SOD) did not prevent MMA-induced inhibition of complex II, suggesting that common reactive oxygen (superoxide, hydrogen peroxide and hydroxyl radical) and nitric (nitric oxide) species were not involved in this effect. In addition, complex II-III (20-35%) was also inhibited by MMA in all tissues tested, and complex I-III only in the kidney (53%) and liver (38%). In contrast, complex IV activity was not changed by MMA in all tissues studied. These results indicate that MMA differentially affects the activity of the respiratory chain pending on the tissues studied, being striatum and hippocampus more vulnerable to its effect. In case our in vitro data are confirmed in vivo in tissues from methylmalonic acidemic patients, it is feasible that that the present findings may be related to the pathophysiology of the tissue damage characteristic of these patients.

Cystamine and cysteamine prevent 3-NP-induced mitochondrial depolarization of Huntington's disease knock-in striatal cells

Abstract Cystamine significantly improved motor deficits and extended survival in mouse models of Huntington's disease (HD); however, the precise mechanism(s) by which cystamine and the related compound cysteamine are beneficial remain to be elucidated. Using clonal striatal cell lines from wild-type (STHdhQ7/HdhQ7) and mutant huntingtin knock-in (STHdhQ111/HdhQ111) mice, we have tested the hypothesis that cystamine and cysteamine could be beneficial by preventing the depolarization of mitochondria in cell cultures. Treatment with 3-nitroproprionic acid (3-NP), a mitochondrial complex II inhibitor, induces mitochondrial depolarization and cell death of mutant HD striatal cells but not of wild-type cells. The 3-NP-mediated decrease in the mitochondrial membrane potential was attenuated by 50 microm cystamine and completely inhibited by 250 microm cystamine. Similar results were obtained using cysteamine (50-500 microm). In addition, both cystamine and cysteamine significantly attenuated the 3-NP-induced cell death. Treatment of mutant HD striatal cells with 3-NP resulted in a robust decrease in the cellular and mitochondrial levels of glutathione (GSH) compared with cells exposed to the vehicle alone. Pre-treatment of the cells with cystamine and cysteamine completely prevented the 3-NP-mediated decrease in cellular and mitochondrial GSH levels. Incubation with L-buthionine (S,R) sulfoximine (BSO) 250 microm in combination with cystamine (250 microm) or cysteamine (250 microm) prior to being treated with 3-NP completely prevented the beneficial effects of cystamine and cysteamine on the 3-NP-mediated mitochondrial depolarization. These results demonstrate that cystamine and cysteamine prevent the 3-NP-induced mitochondrial depolarization of HD striatal cell cultures.

3-nitropropionic acid is a suicide inhibitor of mitochondrial respiration that, upon oxidation by complex II, forms a covalent adduct with a catalytic base arginine in the active site of the enzyme

We report three new structures of mitochondrial respiratory Complex II (succinate ubiquinone oxidoreductase, E.C. 1.3.5.1) at up to 2.1 A resolution, with various inhibitors. The structures define the conformation of the bound inhibitors and suggest the residues involved in substrate binding and catalysis at the dicarboxylate site. In particular they support the role of Arg(297) as a general base catalyst accepting a proton in the dehydrogenation of succinate. The dicarboxylate ligand in oxaloacetate-containing crystals appears to be the same as that reported for Shewanella flavocytochrome c treated with fumarate. The plant and fungal toxin 3-nitropropionic acid, an irreversible inactivator of succinate dehydrogenase, forms a covalent adduct with the side chain of Arg(297). The modification eliminates a trypsin cleavage site in the flavoprotein, and tandem mass spectroscopic analysis of the new fragment shows the mass of Arg(297) to be increased by 83 Da and to have the potential of losing 44 Da, consistent with decarboxylation, during fragmentation.

Inhibition of the electron transport chain and creatine kinase activity by ethylmalonic acid in human skeletal muscle

Ethylmalonic aciduria is a common finding in patients affected by short-chain acyl-CoA dehydrogenase (SCAD) deficiency and other diseases characterized by encephalopathy, muscular symptomatology, and lactic acidemia. Considering that the pathophysiological mechanisms of these disorders are practically unknown and that lactic acidosis suggest an impairment of energy production, the objective of the present work was to investigate the in vitro effect of ethylmalonic acid (EMA), at concentrations varying from 0.25 to 5.0 mM, on important parameters of energy metabolism in human skeletal muscle, such as the activities of the respiratory chain complexes and of creatine kinase, which are responsible for most of the ATP produced and transferred inside the cell. We verified that EMA significantly inhibited the activity of complex I-III at concentrations as low as 0.25 mM, complex II-III at 1 mM and higher concentrations, and complex II at the concentration of 5 mM. In contrast, complex IV was not inhibited by the acid. Finally, we observed that the activity of creatine kinase was significantly inhibited by EMA at the concentrations of 1 and 5 mM. These results suggest that EMA compromises energy metabolism in human skeletal muscle. In case the in vitro effects detected in the present study also occur in vivo, it is tempting to speculate that they may contribute, at least in part, to explain the hypotonia/myopathy, as well as the increased concentrations of lactic acid present in the patients affected by illnesses in which EMA accumulates.

Homocysteine-induced endothelin-1 release is dependent on hyperglycaemia and reactive oxygen species production in bovine aortic endothelial cells

BACKGROUND

Elevated plasma homocysteine (Hcy) is a risk factor for coronary disease. The objective of this study was to investigate whether Hcy either alone or in high glucose conditions induces endothelin-1 (ET-1) synthesis via the production of reactive oxygen species (ROS).

METHODS

Bovine aortic endothelial cells were grown in high (25 mmol/l) and low (5 mmol/l) glucose medium.

RESULTS

In high glucose, Hcy caused a time-dependent increase in ET-1 release, which was greatest with 50 micromol/l Hcy at 24 h (p < 0.01). This effect was not seen in low glucose conditions. In high glucose and 50 micromol/l Hcy, ET-1 mRNA levels were maximal after 1 h (p < 0.05). Tissue factor mRNA levels were raised at 4 h (p < 0.05) and functional activity was raised at 6 h (p < 0.01). Intracellular ROS production was increased by 50 micromol/l Hcy after 24 h (p < 0.05) but only in high glucose. To investigate the role of mitochondrial metabolism in ROS production, cells were incubated with thenoyltrifluoroacetone (inhibitor of complex II) or carbonyl cyanide m-chlorophenylhydrazone (uncoupler of oxidative phosphorylation). Both compounds abolished the Hcy-induced increase in ROS production and ET-1 release. There was an alteration in intracellular glutathione (GSH) levels with Hcy treatment with more oxidised GSH present.

CONCLUSION

The combined metabolic burden of Hcy and high glucose stimulates ET-1 synthesis in bovine aortic endothelial cells via a mechanism dependent on the production of mitochondrial ROS, but may not be generalisable to all types of endothelial cells.

Inhibition of mitochondrial complex II affects dopamine metabolism and decreases its uptake into striatal synaptosomes

The mitochondrial toxin, 3-nitropropionic acid (3-NP), is a specific inhibitor of succinate dehydrogenase, complex II in the mitochondrial respiratory chain. The aim of our study was to determine the relationship between inhibition of mitochondrial complex II and dopamine (DA) metabolism and its transport into rat striatal synaptosomes after exposure to 3-NP. The study was carried out using spectrophotometric, radiochemical and HPLC methods. Our data showed that inhibition of succinate dehydrogenase by intraperitoneal (i.p.) injection of 3-NP (cumulated dose 100 mg/kg in 4 days) significantly affected DA metabolism, leading to the accumulation of its metabolites, 3,4-dihydroxylphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the rat striatum. These experimental conditions had no effect on free radical dependent lipid peroxidation in the brain. In vitro experiments revealed that DA and DOPAC significantly decrease lipid peroxidation in the brain homogenate. Moreover, 3-NP significantly inhibited [3H]DA uptake into striatal synaptosomes by specific dopamine transporter (DAT). The scavengers of superoxide radical (O2-) Tempol and Trolox had no effect on DAT function, but the nitric oxide synthase (NOS) inhibitor N w-nitro-L-arginine (100 microM) prevented 3-NP-evoked DAT down-regulation. In summary, our results indicate that inhibition of mitochondrial complex II by 3-NP enhances DA degradation and decreases its uptake into synaptosomes. It is suggested that NO and energy failure are responsible for alteration of the dopaminergic system in the striatum.

Methamphetamine-induced inhibition of mitochondrial complex II: roles of glutamate and peroxynitrite

High-dose methamphetamine (METH) is associated with long-term deficits in dopaminergic systems. Although the mechanism(s) which contributes to these deficits is not known, glutamate and peroxynitrite are likely to play a role. These factors are hypothesized to inhibit mitochondrial function, increasing the free radical burden and decreasing neuronal energy supplies. Previous studies suggest a role for the mitochondrial electron transport chain (ETC) in mediating toxicity of METH. The purpose of the present studies was to determine whether METH administration selectively inhibits complex II of the ETC in rats. High-dose METH administration (10 mg/kg every 2 h x 4) rapidly (within 1 h) decreased complex II (succinate dehydrogenase) activity by approximately 20-30%. In addition, decreased activity of complex II-III, but not complex I-III, of the mitochondrial ETC was also observed 24 h after METH. This inhibition was not due to direct inhibition by METH or METH-induced hyperthermia and was specific to striatal brain regions. METH-induced decreases in complex II-III were prevented by MK-801 and the peroxynitrite scavenger 5,10,15,20-tetrakis (2,4,6-trimethyl-3,5-sulphonatophenyl) porphinato iron III. These findings provide the first evidence that METH administration, via glutamate receptor activation and peroxynitrite formation, selectively alters a specific site of the ETC.

Purification and characterization of succinate:menaquinone oxidoreductase from Corynebacterium glutamicum

Succinate:menaquinone oxidoreductase from Corynebacterium glutamicum, a high-G+C, Gram-positive bacterium, was purified to homogeneity. The enzyme contained two heme B molecules and three polypeptides with apparent molecular masses of 67, 29 and 23 kDa, which corresponded to SdhA (flavoprotein), SdhB (iron-sulfur protein), and SdhC (membrane anchor protein), respectively. In non-denaturating polyacrylamide gel electrophoresis, the enzyme migrated as a single band with an apparent molecular mass of 410 kDa, suggesting that it existed as a trimer. The succinate dehydrogenase activity assayed using 2,3-dimethoxy-5-methyl-6-decyl-1,4-benzoquinone and 2,6-dichloroindophenol as the electron acceptor was inhibited by 2-n-heptyl-4-hydroxyquinoline N-oxide (HQNO), and the Dixon plots were biphasic. In contrast, the succinate dehydrogenase activity assayed using phenazine methosulfate and 2,6-dichloroindophenol was inhibited by p-benzoquinone and not by HQNO. These findings suggested that the C. glutamicum succinate:menaquinone oxidoreductase had two quinone binding sites. In the phylogenetic tree of SdhA, Corynebacterium species do not belong to the high-G+C group, which includes Mycobacterium tuberculosis and Streptomyces coelicolor, but are rather close to the group of low-G+C, Gram-positive bacteria such as Bacillus subtilis. This situation may have arisen due to the horizontal gene transfer.

Mitochondrial permeability transition in neuronal damage promoted by Ca2+ and respiratory chain complex II inhibition

Changes in mitochondrial integrity, reactive oxygen species release and Ca2+ handling are proposed to be involved in the pathogenesis of many neurological disorders including methylmalonic acidaemia and Huntington's disease, which exhibit partial mitochondrial respiratory inhibition. In this report, we studied the mechanisms by which the respiratory chain complex II inhibitors malonate, methylmalonate and 3-nitropropionate affect rat brain mitochondrial function and neuronal survival. All three compounds, at concentrations which inhibit respiration by 50%, induced mitochondrial inner membrane permeabilization when in the presence of micromolar Ca2+ concentrations. ADP, cyclosporin A and catalase prevented or delayed this effect, indicating it is mediated by reactive oxygen species and mitochondrial permeability transition (PT). PT induced by malonate was also present in mitochondria isolated from liver and kidney, but required more significant respiratory inhibition. In brain, PT promoted by complex II inhibition was stimulated by increasing Ca2+ cycling and absent when mitochondria were pre-loaded with Ca2+ or when Ca2+ uptake was prevented. In addition to isolated mitochondria, we determined the effect of methylmalonate on cultured PC12 cells and freshly prepared rat brain slices. Methylmalonate promoted cell death in striatal slices and PC12 cells, in a manner attenuated by cyclosporin A and bongkrekate, and unrelated to impairment of energy metabolism. We propose that under conditions in which mitochondrial complex II is partially inhibited in the CNS, neuronal cell death involves the induction of PT.

Shift in the localization of sites of hydrogen peroxide production in brain mitochondria by mitochondrial stress

We have determined the underlying sites of H(2)O(2) generation by isolated rat brain mitochondria and how these can shift depending on the presence of respiratory substrates, electron transport chain modulators and exposure to stressors. H(2)O(2) production was determined using the fluorogenic Amplex red and peroxidase system. H(2)O(2) production was higher when succinate was used as a respiratory substrate than with another FAD-dependent substrate, alpha-glycerophosphate, or with the NAD-dependent substrates, glutamate/malate. Depolarization by the uncoupler p-trifluoromethoxyphenylhydrazone decreased H(2)O(2) production stimulated by all respiratory substrates. H(2)O(2) production supported by succinate during reverse transfer of electrons was decreased by inhibitors of complex I (rotenone and diphenyleneiodonium) whereas in glutamate/malate-oxidizing mitochondria diphenyleneiodonium decreased while rotenone increased H(2)O(2) generation. The complex III inhibitors antimycin and myxothiazol decreased succinate-induced H(2)O(2) production but stimulated H(2)O(2) production in glutamate/malate-oxidizing mitochondria. Antimycin and myxothiazol also increased H(2)O(2) production in mitochondria using alpha-glycerophosphate as a respiratory substrate. In substrate/inhibitor experiments maximal stimulation of H(2)O(2) production by complex I was observed with the alpha-glycerophosphate/antimycin combination. In addition, three forms of in vitro mitochondrial stress were studied: Ca(2+) overload, cold storage for more than 24 h and cytochrome c depletion. In each case we observed (i) a decrease in succinate-supported H(2)O(2) production by complex I and an increase in succinate-supported H(2)O(2) production by complex III, (ii) increased glutamate/malate-induced H(2)O(2) generation by complex I and (iii) increased alpha-glycerophosphate-supported H(2)O(2) generation by complex III. Our results suggest that all three forms of mitochondrial stress resulted in similar shifts in the localization of sites of H(2)O(2) generation and that, in both normal and stressed states, the level and location of H(2)O(2) production depend on the predominant energetic substrate.

Inhibition of mitochondrial complex II alters striatal expression of genes involved in glutamatergic and dopaminergic signaling: possible implications for Huntington's disease

Huntington's disease (HD) is a genetic neurodegenerative disorder characterized by motor abnormalities and cognitive impairment. The irreversible succinate dehydrogenase (SD) inhibitor 3-nitropropionic acid (3NP) causes neurodegeration in the striatum resembling HD when administered to rodents or primates. Using corticostriatal brain slice preparations, we analyzed the pattern of gene expression following 3NP application utilizing cDNA microarrays. Acute 3NP treatment modulates the expression of several genes involved in dopaminergic and glutamatergic signaling in corticostriatal brain slices, and unbalances the downstream serine/threonine protein kinase and phosphatase network affecting the dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32). Our data provide new information about the molecular events possibly underlying neurodegeneration induced by this mitochondrial toxin.

Neuronal vulnerability following inhibition of mitochondrial complex II: a possible ionic mechanism for Huntington's disease

An impaired complex II (succinate dehydrogenase, SD) striatal mitochondrial activity is one of the prominent metabolic alterations in Huntington's disease (HD), and intoxication with 3-nitropropionic acid (3-NP), an inhibitor of mitochondrial complex II, mimics the motor abnormalities and the pathology of HD. We found that striatal spiny neurons responded to this toxin with an irreversible membrane depolarization/inward current, while cholinergic interneurons showed a hyperpolarization/outward current. Both these currents were sensitive to intracellular concentration of ATP. The 3-NP-induced depolarization was associated with an increased release of endogenous GABA, while acetylcholine levels were reduced. Moreover, 3-NP induced a higher depolarization in presymptomatic R6/2 HD transgenic mice compared to wild-type (WT) mice, showing an increased susceptibility to SD inhibition. Conversely, the hyperpolarization did not significantly differ from the one recorded in WT mice. The diverse membrane changes induced by SD inhibition may contribute to the cell-type-specific neuronal death in HD.

Graded reoxygenation with chemical inhibition of oxidative phosphorylation improves posthypoxic recovery in murine hippocampal slices

Rapid and complete tissue reoxygenation is a prime goal of present stroke therapy. However, reoxygenation may trigger detrimental cascades that partially antagonize beneficial effects. It was our goal to investigate selective grading of reoxygenation with targeting of single mitochondrial complexes in murine hippocampal slices. Population spike amplitude (PSAP) and NADH were measured during hypoxic hypoxia (15 min) and recovery (45 min). With onset of reoxygenation, slices were treated for different times with amobarbital (1 mM), malonate (2 mM), or cyanide (1 mM), inhibitors of mitochondrial complex I, II, or IV, respectively. Other slices were treated with nicotinamide (1 mM). Posthypoxic recovery of PSAP increased from 32% +/- 43% of onset in control slices to 52% +/- 59% (P <.05) upon treatment with amobarbital for 1 min and to 62% +/- 37% (P <.05) upon treatment with malonate. With nicotinamide, posthypoxic recovery improved to 73% +/- 25% (P <.05). Oxidation of NADH was prolonged upon treatment with amobarbital, whereas no change in NADH oxidation was observed with malonate and nicotinamide. Thus, grading of reoxygenation with selective targeting of mitochondrial complex I or II but not of complex IV improves outcome upon reoxygenation in murine hippocampal slices.

Effect of oxygen on activation state of complex I and lack of oxaloacetate inhibition of complex II in Langendorff perfused rat heart

Two main entry points for electrons into the mitochondrial respiratory chain are NADH:ubiquinone oxidoreductase (complex I) and succinate:ubiquinone oxidoreductase (complex II). Metabolic regulation of these two respiratory complexes is not understood in detail. It has been suggested that the Krebs cycle metabolic intermediate oxaloacetate (OAA) inhibits complex II in vivo, whereas complex I undergoes a reversible active/de-active transition. In normoxic and anoxic hearts it has been shown that the proportion of complex I in the active and de-active states is different suggesting a possible mode of regulation of the enzyme by oxygen concentration. In the current studies rapid isolation of mitochondrial membranes in a state that preserves the activity of both complex I and complex II has been achieved using Langendorff perfused rat hearts. The findings indicate that the state of activation of complex I is controlled by the oxygen saturation in the perfusate. In addition, these studies show that complex II is fully active in the mitochondrion and not inhibited by OAA regardless of the oxygen concentration.

Striatal cells from mutant huntingtin knock-in mice are selectively vulnerable to mitochondrial complex II inhibitor-induced cell death through a non-apoptotic pathway

Extensive striatal neuronal loss occurs in Huntington's disease (HD), which is caused by an expanded polyglutamine tract in huntingtin (htt). Evidence suggests that mutant htt directly or indirectly compromises mitochondrial function, contributing to the neuronal loss. To determine the role of compromised mitochondrial function in the neuronal cell death in HD, immortalized striatal cells established from Hdh(Q7) (wild-type) and Hdh(Q111) (mutant) mouse knock-in embryos were treated with 3-nitropropionic acid (3-NP), a mitochondrial complex II toxin. 3-NP treatment caused significantly greater cell death in mutant striatal cells compared with wild-type cells. In contrast, the extent of cell death induced by rotenone, a complex I inhibitor, was similar in both cell lines. Although evidence of apoptosis was present in 3-NP-treated wild-type striatal cells, it was absent in 3-NP-treated mutant cells. 3-NP treatment caused a greater loss of mitochondrial membrane potential (deltapsim) in mutant striatal cells compared with wild-type cells. Cyclosporine A, an inhibitor of mitochondrial permeability transition pore (PTP), and ruthenium red, an inhibitor of the mitochondrial calcium uniporter, both rescued mutant striatal cells from 3-NP-induced cell death and prevented the loss of deltapsim. These data show that mutant htt specifically increases cell vulnerability to mitochondrial complex II inhibition and further switched the type of cell death induced by complex II inhibition from apoptosis to a non-apoptotic form, caused by mitochondrial membrane depolarization, probably initiated by mitochondrial calcium overload and subsequent PTP opening. These findings suggest that impaired mitochondrial complex II function in HD may contribute to non-apoptotic neuronal cell death.

Mitochondrial calcium, oxidative stress and apoptosis in a neurodegenerative disease model induced by 3-nitropropionic acid

Intracellular calcium homeostasis is important for cell survival. However, increase in mitochondrial calcium (Ca2+m) induces opening of permeability transition pore (PTP), mitochondrial dysfunction and apoptosis. Since alterations of intracellular Ca2+ and reactive oxygen species (ROS) generation are involved in cell death, they might be involved in neurodegenerative processes such as Huntington's disease (HD). HD is characterized by the inhibition of complex II of respiratory chain and increase in ROS production. In this report, we studied the correlation between the inhibitor of the complex II, 3-nitropropionic acid (3NP), Ca2+ metabolism, apoptosis and behavioural alterations. We showed that 3NP (1 mm) is able to release Ca2+m, as neither Thapsigargin (TAP, 2 microm) nor free-calcium medium affected its effect. PTP inhibitors and antioxidants inhibited this process, suggesting an increase in ROS generation and PTP opening. In addition, 3NP (0.1 mm) also induces apoptotic cell death. Behavioural changes in animals treated with 3NP (20 mg/kg/day for 4 days) were also attenuated by pre- and co-treatment with vitamin E (VE, 20 mg/kg/day). Taken together, our results show that complex II inhibition could involve Ca2+m release, oxidative stress and cell death that may precede motor alterations in neurodegenerative processes such as HD.

Hypoxic increase in nitric oxidegeneration of rat sensory neurons requires activation of mitochondrial complex II and voltage-gated calcium channels

Recently, we have demonstrated that sensory neurons of rat lumbar dorsal root ganglia (DRG) respond to hypoxia with an activation of endothelial nitric oxide (NO) synthase (eNOS) resulting in enhanced NO production associated with mitochondria which contributes to resistance against hypoxia. Extracellular calcium is essential to this effect. In the present study on rat DRG slices, we set out to determine what types of calcium channels operate under hypoxia, and which upstream events contribute to their activation, thereby focusing upon mitochondrial complex II. Both the metallic ions Cd2+ and Ni2+, known to inhibit voltage-gated calcium channels and T-type channels, respectively, and verapamil and nifedipine, typical blocker of L-type calcium channels completely prevented the hypoxic neuronal NO generation. Inhibition of complex II by thenoyltrifluoroacetone at the ubiquinon binding site or by 3-nitropropionic acid at the substrate binding site largely diminished hypoxic-induced NO production while having an opposite effect under normoxia. An additional blockade of voltage-gated calcium channels entirely abolished the hypoxic response. The complex II inhibitor malonate inhibited both normoxic and hypoxic NO generation. These data show that complex II activity is required for increased hypoxic NO production. Since succinate dehydrogenase activity of complex II decreased at hypoxia, as measured by histochemistry and densitometry, we propose a hypoxia-induced functional switch of complex II from succinate dehydrogenase to fumarate reductase, which subsequently leads to activation of voltage-gated calcium channels resulting in increased NO production by eNOS.

Molecular Mechanism of Cell Death Induced by the Antioxidant tert-Butylhydroxyanisole in Human Monocytic Leukemia U937 Cells

A phenolic antioxidant 3-tert-butyl-4-hydroxyanisole (BHA) is a widely used food additive. BHA had cytotoxicity in human monocytic leukemia U937 cells. BHA at 0.75 mM caused nuclear condensation and fragmentation, structural damage in mitochondria, decrease in mitochondrial transmembrane potential, and internucleosomal DNA cleavage. It induced the activities of caspase-3 and/or -7, -6, -8 and -9, especially high when DEVD-MCA was the substrate (caspase-3 and/or -7). DEVDase activity increased in time- and dose-dependent manner and high activity was observed in lysates of cells treated for 3 h at 0.75 mM. Addition of GSH (reduced glutathione) during the treatment of cells with BHA inhibited the induction of DEVDase activity, and the intracellular GSH level decreased as the concentration of BHA was raised. Intracellular ATP levels decreased in time- and dose-dependent manner when the cells were treated with BHA in the presence or absence of glucose. Enzyme activities involved in the respiratory chain were assayed with the mitochondrial fraction prepared from U937 cells. BHA distinctly inhibited NADH-ubiquinone oxidoreductase (complex I) and cytochrome c oxidase (complex IV) at low concentrations. Succinate-ubiquinone oxidoreductase (complex II) was also inhibited, but to somewhat less extent. Without mitochondrial enzymes, BHA stimulated the ubiquinol-dependent reduction of cytochrome c (complex III), but it might have some detrimental effects on the mitochondrial enzyme reaction of complex III. The inhibition of mitochondrial oxidative phosphorylation might corroborate the mechanistic evidence for apoptosis of leukemia cells by BHA. Cell death induced by BHA is primarily ascribable to apoptosis.

Apoptotic death in Leishmania donovani promastigotes in response to respiratory chain inhibition: complex II inhibition results in increased pentamidine cytotoxicity

The biochemical changes consequent to respiratory chain inhibition and their relationship to cell death in Leishmania spp. remain elusive. Inhibitors of respiratory chain complexes I, II, and III were able to induce apoptotic death of the bloodstream form of Leishmania donovani. Complex I inhibition resulted in mitochondrial hyperpolarization that was preceded by increased superoxide production. Limitation of electron transport by thenoyltrifluoroacetone and antimycin A, inhibitors of complexes II and III, respectively, resulted in dissipation of mitochondrial membrane potential that was sensitive to cyclosporin A, a blocker of mitochondrial permeability transition pore. Further studies conducted with thenoyltrifluoroacetone showed maximal generation of hydrogen peroxide with a moderate elevation of superoxide levels. Complex III inhibition provoked superoxide generation only. Interference with complex II but not complexes I and III increased intracellular Ca(2+). A tight link between Ca(2+) and reactive oxygen species was demonstrated by antioxidant-induced diminution of the Ca(2+) increase. However, chelation of extracellular Ca(2+) could not abrogate the early increase of reactive oxygen species, providing evidence that Ca(2+) elevation was downstream to reactive oxygen species generation. Ca(2+) influx occurred through nonselective cation and L-type channels and Na(+)/Ca(2+) exchanger-like pathways. Antioxidants such as glutathione and Ca(2+) channel blockers reduced apoptotic death. This study provides a new possibility that concurrent inhibition of respiratory chain complex II with pentamidine administration increases cytotoxicity of the drug. This increased cytotoxicity was connected to a 4-fold elevation in intracellular Ca(2+) that was pooled only from intracellular sources. Therefore, inhibition of complexes I, II, and III leads to apoptosis and complex II inhibition in parallel with pentamidine administration-enhanced drug efficacy.

Adenosinergic protection of dopaminergic and GABAergic neurons against mitochondrial inhibition through receptors located in the substantia nigra and striatum, respectively

Mitochondrial dysfunction may contribute to dopaminergic (DAergic) cell death in Parkinson's disease and GABAergic cell death in Huntington's disease. In the present work, we tested whether blocking A1 receptors could enhance the damage to DAergic and GABAergic neurons caused by mitochondrial inhibition, and whether blocking A2a receptors could protect against damage in this model. Animals received an intraperitoneal injection of 8-cyclopentyl-1,3-dipropylxanthine (CPX) (A1 antagonist) or 3,7-dimethyl-1-propargylxanthine (DMPX) (A2a antagonist) 30 min before intrastriatal infusion of malonate (mitochondrial complex II inhibitor). Damage was assessed 1 week later by measuring striatal dopamine, tyrosine hydroxylase (TH), and GABA content. In mice and rats, malonate-induced depletion of striatal dopamine, TH, or GABA was potentiated by pretreatment with 1 mg/kg CPX and attenuated by pretreatment with 5 mg/kg DMPX. To determine the location of the A1 and A2a receptors mediating these effects, CPX or DMPX was infused directly into the striatum or substantia nigra of rats 30 min before intrastriatal infusion of malonate. When infused into the striatum, CPX (20 ng) potentiated, whereas DMPX (50 ng) prevented malonate-induced GABA loss, but up to 100 ng of CPX or 500 ng of DMPX did not alter malonate-induced striatal dopamine loss. Intranigral infusion of CPX (100 ng) or DMPX (500 ng), however, did exacerbate and protect, respectively, against malonate-induced striatal dopamine loss. Thus, A1 receptor blockade enhances and A2a receptor blockade protects against damage to DAergic and GABAergic neurons caused by mitochondrial inhibition. Interestingly, these effects are mediated by A1 and A2a receptors located in the substantia nigra for DAergic neurons and in the striatum for GABAergic neurons.