Novel functions of the alpha-ketoglutarate dehydrogenase complex may mediate diverse oxidant-induced changes in mitochondrial enzymes associated with Alzheimer's disease

The emerging importance of the α-keto acid dehydrogenase complexes in serving as intracellular and intercellular signaling platforms for the regulation of metabolism

The α-keto acid dehydrogenase complex (KDHc) class of mitochondrial enzymes is composed of four members: pyruvate dehydrogenase (PDHc), α-ketoglutarate dehydrogenase (KGDHc), branched-chain keto acid dehydrogenase (BCKDHc), and 2-oxoadipate dehydrogenase (OADHc). These enzyme complexes occupy critical metabolic intersections that connect monosaccharide, amino acid, and fatty acid metabolism to Krebs cycle flux and oxidative phosphorylation (OxPhos). This feature also imbues KDHc enzymes with the heightened capacity to serve as platforms for propagation of intracellular and intercellular signaling. KDHc enzymes serve as a source and sink for mitochondrial hydrogen peroxide (mtH2O2), a vital second messenger used to trigger oxidative eustress pathways. Notably, deactivation of KDHc enzymes through reversible oxidation by mtH2O2 and other electrophiles modulates the availability of several Krebs cycle intermediates and related metabolites which serve as powerful intracellular and intercellular messengers. The KDHc enzymes also play important roles in the modulation of mitochondrial metabolism and epigenetic programming in the nucleus through the provision of various acyl-CoAs, which are used to acylate proteinaceous lysine residues. Intriguingly, nucleosomal control by acylation is also achieved through PDHc and KGDHc localization to the nuclear lumen. In this review, I discuss emerging concepts in the signaling roles fulfilled by the KDHc complexes. I highlight their vital function in serving as mitochondrial redox sensors and how this function can be used by cells to regulate the availability of critical metabolites required in cell signaling. Coupled with this, I describe in detail how defects in KDHc function can cause disease states through the disruption of cell redox homeodynamics and the deregulation of metabolic signaling. Finally, I propose that the intracellular and intercellular signaling functions of the KDHc enzymes are controlled through the reversible redox modification of the vicinal lipoic acid thiols in the E2 subunit of the complexes.

Cigarette smoke causes a bioenergetic crisis in RPE cells involving the downregulation of HIF-1α under normoxia

Age-related macular degeneration (AMD) is the most common blinding disease in the elderly population. However, there are still many uncertainties regarding the pathophysiology at the molecular level. Currently, impaired energy metabolism in retinal pigment epithelium (RPE) cells is discussed as one major hallmark of early AMD pathophysiology. Hypoxia-inducible factors (HIFs) are important modulators of mitochondrial function. Moreover, smoking is the most important modifiable risk factor for AMD and is known to impair mitochondrial integrity. Therefore, our aim was to establish a cell-based assay that enables us to investigate how smoking affects mitochondrial function in conjunction with HIF signaling in RPE cells. For this purpose, we treated a human RPE cell line with cigarette smoke extract (CSE) under normoxia (21% O2), hypoxia (1% O2), or by co-treatment with Roxadustat, a clinically approved HIF stabilizer. CSE treatment impaired mitochondrial integrity, involving increased mitochondrial reactive oxygen species, disruption of mitochondrial membrane potential, and altered mitochondrial morphology. Treatment effects on cell metabolism were analyzed using a Seahorse Bioanalyzer. Mitochondrial respiration and ATP production were impaired in CSE-treated cells under normoxia. Surprisingly, CSE-treated RPE cells also exhibited decreased glycolytic rate under normoxia, causing a bioenergetic crisis, because two major metabolic pathways that provide ATP were impaired by CSE. Downregulation of glycolytic rate was HIF-dependent because HIF-1α, the α-subunit of HIF-1, was downregulated by CSE on the protein level, especially under normoxia. Moreover, hypoxia incubation and treatment with Roxadustat restored glycolytic flux. Taken together, our in vitro model provides interesting insights into HIF-dependent regulation of glycolysis under normoxic conditions, which will enable us to investigate signaling pathways involved in RPE metabolism in health and disease.

Systemic alterations of tricarboxylic acid cycle enzymes in Alzheimer's disease

Mitochondrial dysfunction, especially tricarboxylic acid (TCA) cycle arrest, is strongly associated with Alzheimer's disease (AD), however, its systemic alterations in the central and peripheral of AD patients are not well defined. Here, we performed an integrated analysis of AD brain and peripheral blood cells transcriptomics to reveal the expression levels of nine TCA cycle enzymes involving 35 genes. The results showed that TCA cycle related genes were consistently down-regulated in the AD brain, whereas 11 genes were increased and 16 genes were decreased in the peripheral system. Pearson analysis of the TCA cycle genes with Aβ, Tau and mini-mental state examination (MMSE) revealed several significant correlated genes, including pyruvate dehydrogenase complex subunit (PDHB), isocitrate dehydrogenase subunits (IDH3B, IDH3G), 2-oxoglutarate dehydrogenase complex subunit (DLD), succinyl-CoA synthetase subunit (SUCLA2), malate dehydrogenase subunit (MDH1). In addition, SUCLA2, MDH1, and PDHB were also uniformly down-regulated in peripheral blood cells, suggesting that they may be candidate biomarkers for the early diagnosis of AD. Taken together, TCA cycle enzymes were systemically altered in AD progression, PDHB, SUCLA2, and MDH1 may be potential diagnostic and therapeutic targets.

Real-time analysis of dynamic compartmentalized GSH redox shifts and H2O2 availability in undifferentiated and differentiated cells

BACKGROUND

Glutathione (GSH) is the most abundant, small biothiol antioxidant. GSH redox state (E_h) supports developmental processes, yet with disrupted GSH E_h, poor developmental outcomes may occur. The role of subcellular, compartmentalized redox environments in the context of redox regulation of differentiation is not well understood. Here, using the P19 neurogenesis model of cellular differentiation, kinetics of subcellular H₂O₂ availability and GSH E_h were evaluated following oxidant exposure.

METHODS

Stably transfected P19 cell lines expressing H₂O₂ availability or GSH E_h sensors, Orp1-roGFP or Grx1-roGFP, respectively, targeted to the cytosol, mitochondria, or nucleus were used. Dynamic, compartmentalized changes in H₂O₂ availability and GSH E_h were measured via spectrophotometric and confocal microscopy over 120 min following treatment with H₂O₂ (100 μM) in both differentiated and undifferentiated cells.

RESULTS

Generally, treated undifferentiated cells showed a greater degree and duration of both H₂O₂ availability and GSH E_h disruption than differentiated neurons. In treated undifferentiated cells, H₂O₂ availability was similar in all compartments. Interestingly, in treated undifferentiated cells, mitochondrial GSH E_h was most affected in both the initial oxidation and the rebound kinetics compared to other compartments. Pretreatment with an Nrf2 inducer prevented H₂O₂-induced effects in all compartments of undifferentiated cells.

CONCLUSIONS

Disruption of redox-sensitive developmental pathways is likely stage specific, where cells that are less differentiated and/or are actively differentiating are most affected.

GENERAL SIGNIFICANCE

Undifferentiated cells are more susceptible to oxidant-induced redox dysregulation but are protected by chemicals that induce Nrf2. This may preserve developmental programs and diminish the potential for poor developmental outcomes.

The α-Ketoglutarate Dehydrogenase Complex as a Hub of Plasticity in Neurodegeneration and Regeneration

Abnormal glucose metabolism is central to neurodegeneration, and considerable evidence suggests that abnormalities in key enzymes of the tricarboxylic acid (TCA) cycle underlie the metabolic deficits. Significant recent advances in the role of metabolism in cancer provide new insight that facilitates our understanding of the role of metabolism in neurodegeneration. Research indicates that the rate-limiting step of the TCA cycle, the α-ketoglutarate dehydrogenase complex (KGDHC) and its substrate alpha ketoglutarate (KG), serve as a signaling hub that regulates multiple cellular processes: (1) is the rate-limiting step of the TCA cycle, (2) is sensitive to reactive oxygen species (ROS) and produces ROS, (3) determines whether KG is used for energy or synthesis of compounds to support growth, (4) regulates the cellular responses to hypoxia, (5) controls the post-translational modification of hundreds of cell proteins in the mitochondria, cytosol, and nucleus through succinylation, (6) controls critical aspects of transcription, (7) modulates protein signaling within cells, and (8) modulates cellular calcium. The primary focus of this review is to understand how reductions in KGDHC are translated to pathologically important changes that underlie both neurodegeneration and cancer. An understanding of each role is necessary to develop new therapeutic strategies to treat neurodegenerative disease.

Preparative Biocatalytic Synthesis of α-Ketoglutaramate

α-Ketoglutaramate (KGM) is an underexamined metabolite of L-glutamine in the metabolic pathway of glutaminase II of α-ketoglutarate formation. Presumably, KGM may be a biomarker of hepatic encephalopathy and other hyperammonemic diseases. This metabolite is a substrate for the ω-amidase enzyme and is used to determine its activity in the study of the biochemistry of various types of cancer. However, the commercial unavailability of KGM hinders its widespread use. Methods for the preparative synthesis of KGM are known, but they either do not provide the proper yield or proper purity of the target product. In this work, a detailed description of the procedures is given that allows the production of KGM with a purity above 97% and a yield of the target product above 75% using L-amino acid oxidase from C. adamanteus as a catalyst of L-glutamine conversion. KGM can be obtained both in the form of a highly concentrated aqueous solution and in the form of crystals of sodium salt. The developed methods can be used both for scaling up the synthesis of KGM and for creating economical biocatalytic technologies for the production of other highly purified preparations.

Suppression of a core metabolic enzyme dihydrolipoamide dehydrogenase (dld) protects against amyloid beta toxicity in C. elegans model of Alzheimer's disease

A decrease in energy metabolism is associated with Alzheimer's disease (AD), but it is not known whether the observed decrease exacerbates or protects against the disease. The importance of energy metabolism in AD is reinforced by the observation that variants of dihydrolipoamide dehydrogenase (DLD), is genetically linked to late-onset AD. To determine whether DLD is a suitable therapeutic target, we suppressed the dld-1 gene in Caenorhabditis elegans that express human Aβ peptide in either muscles or neurons. Suppression of the dld-1 gene resulted in significant restoration of vitality and function that had been degraded by Aβ pathology. This included protection of neurons and muscles cells. The observed decrease in proteotoxicity was associated with a decrease in the formation of toxic oligomers rather than a decrease in the abundance of the Aβ peptide. The mitochondrial uncoupler, carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP), which like dld-1 gene expression inhibits ATP synthesis, had no significant effect on Aβ toxicity. Proteomics data analysis revealed that beneficial effects after dld-1 suppression could be due to change in energy metabolism and activation of the pathways associated with proteasomal degradation, improved cell signaling and longevity. Thus, some features unique to dld-1 gene suppression are responsible for the therapeutic benefit. By direct genetic intervention, we have shown that acute inhibition of dld-1 gene function may be therapeutically beneficial. This result supports the hypothesis that lowering energy metabolism protects against Aβ pathogenicity and that DLD warrants further investigation as a therapeutic target.

The Role of the Rare Variants in the Genes Encoding the Alpha-Ketoglutarate Dehydrogenase in Alzheimer's Disease

There is increasing evidence that several mitochondrial abnormalities are present in the brains of patients with Alzheimer's disease (AD). Decreased alpha-ketoglutarate dehydrogenase complex (αKGDHc) activity was identified in some patients with AD. The αKGDHc is a key enzyme in the Krebs cycle. This enzyme is very sensitive to the harmful effect of reactive oxygen species, which gives them a critical role in the Alzheimer and mitochondrial disease research area. Previously, several genetic risk factors were described in association with AD. Our aim was to analyze the associations of rare damaging variants in the genes encoding αKGDHc subunits and AD. The three genes (OGDH, DLST, DLD) encoding αKGDHc subunits were sequenced from different brain regions of 11 patients with histologically confirmed AD and the blood of further 35 AD patients. As a control group, we screened 134 persons with whole-exome sequencing. In all subunits, a one-one rare variant was identified with unknown significance based on American College of Medical Genetics and Genomics (ACMG) classification. Based on the literature research and our experience, R263H mutation in the DLD gene seems likely to be pathogenic. In the different cerebral areas, the αKGDHc mutational profile was the same, indicating the presence of germline variants. We hypothesize that the heterozygous missense R263H in the DLD gene may have a role in AD as a mild genetic risk factor.

Synthesis of α-Ketoglutaramic acid

α-Ketoglutaramic acid (KGM, α-ketoglutaramate), also known as 2-oxoglutaramic acid (OGM, 2-oxoglutaramate), is a substrate of ω-amidase, also known as Nitrilase 2 (NIT2), and is essential for studying the canonical role of ω-amidase, as well as its role in multiple diseases. Until now, KGM used for biological studies has been prepared most often by the enzymatic oxidation of l-glutamine using snake venom l-amino acid oxidase, which provides KGM as an aqueous solution, containing by-products including 5-oxoproline and α-ketoglutarate. The enzymatic method for KGM preparation, therefore, cannot provide pure product or an accurate percent yield evaluation. Here, we report a synthetic method for the preparation of this important substrate, KGM, in 3 steps, from l-2-hydroxyglutaramic acid, in pure form, in 53% overall yield.

The interplay between oxidative stress and bioenergetic failure in neuropsychiatric illnesses: can we explain it and can we treat it?

Nitro-oxidative stress and lowered antioxidant defences play a key role in neuropsychiatric disorders such as major depression, bipolar disorder and schizophrenia. The first part of this paper details mitochondrial antioxidant mechanisms and their importance in reactive oxygen species (ROS) detoxification, including details of NO networks, the roles of H₂O₂ and the thioredoxin/peroxiredoxin system, and the relationship between mitochondrial respiration and NADPH production. The second part highlights and identifies the causes of the multiple pathological sequelae arising from self-amplifying increases in mitochondrial ROS production and bioenergetic failure. Particular attention is paid to NAD⁺ depletion as a core cause of pathology; detrimental effects of raised ROS and reactive nitrogen species on ATP and NADPH generation; detrimental effects of oxidative and nitrosative stress on the glutathione and thioredoxin systems; and the NAD⁺-induced signalling cascade, including the roles of SIRT1, SIRT3, PGC-1α, the FOXO family of transcription factors, Nrf1 and Nrf2. The third part discusses proposed therapeutic interventions aimed at mitigating such pathology, including the use of the NAD⁺ precursors nicotinamide mononucleotide and nicotinamide riboside, both of which rapidly elevate levels of NAD⁺ in the brain and periphery following oral administration; coenzyme Q₁₀ which, when given with the aim of improving mitochondrial function and reducing nitro-oxidative stress in the brain, may be administered via the use of mitoquinone, which is in essence ubiquinone with an attached triphenylphosphonium cation; and N-acetylcysteine, which is associated with improved mitochondrial function in the brain and produces significant decreases in oxidative and nitrosative stress in a dose-dependent manner.

Postsynaptic Proteome of Non-Demented Individuals with Alzheimer's Disease Neuropathology

Some individuals, here referred to as Non-Demented with Alzheimer’s Neuropathology (NDAN), retain their cognitive function despite the presence of amyloid plaques and tau tangles typical of symptomatic Alzheimer’s disease (AD). In NDAN, unlike AD, toxic amyloid-β oligomers do not localize to the postsynaptic densities (PSDs). Synaptic resistance to amyloid-β in NDAN may thus enable these individuals to remain cognitively intact despite the AD-like pathology. The mechanism(s) responsible for this resistance remains unresolved and understanding such protective biological processes could reveal novel targets for the development of effective treatments for AD. The present study uses a proteomic approach to compare the hippocampal postsynaptic densities of NDAN, AD, and healthy age-matched persons to identify protein signatures characteristic for these groups. Subcellular fractionation followed by 2D gel electrophoresis and mass spectrometry were used to analyze the PSDs. We describe fifteen proteins which comprise the unique proteomic signature of NDAN PSDs, thus setting them apart from control subjects and AD patients.

Crosstalk between Oxidative Stress and Tauopathy

Tauopathy is a collective term for neurodegenerative diseases associated with pathological modifications of tau protein. Tau modifications are mediated by many factors. Recently, reactive oxygen species (ROS) have attracted attention due to their upstream and downstream effects on tauopathy. In physiological conditions, healthy cells generate a moderate level of ROS for self-defense against foreign invaders. Imbalances between ROS and the anti-oxidation pathway cause an accumulation of excessive ROS. There is clear evidence that ROS directly promotes tau modifications in tauopathy. ROS is also highly upregulated in the patients’ brain of tauopathies, and anti-oxidants are currently prescribed as potential therapeutic agents for tauopathy. Thus, there is a clear connection between oxidative stress (OS) and tauopathies that needs to be studied in more detail. In this review, we will describe the chemical nature of ROS and their roles in tauopathy.

An Updated View on the Molecular Pathomechanisms of Human Dihydrolipoamide Dehydrogenase Deficiency in Light of Novel Crystallographic Evidence

Dihydrolipoamide dehydrogenase (LADH, E3) deficiency is a rare (autosomal, recessive) genetic disorder generally presenting with an onset in the neonatal age and early death; the highest carrier rate has been found among Ashkenazi Jews. Acute clinical episodes usually involve severe metabolic decompensation and lactate acidosis that result in neurological, cardiological, and/or hepatological manifestations. Clinical severity is due to the fact that LADH is a common E3 subunit to the alpha-ketoglutarate, pyruvate, alpha-ketoadipate, and branched-chain alpha-keto acid dehydrogenase complexes, and is also a constituent in the glycine cleavage system, thus a loss in LADH function adversely affects multiple key metabolic routes. However, the severe clinical pictures frequently still do not parallel the LADH activity loss, which implies the involvement of auxiliary biochemical mechanisms; enhanced reactive oxygen species generation as well as affinity loss for multienzyme complexes proved to be key auxiliary exacerbating pathomechanisms. This review provides an overview and an up-to-date molecular insight into the pathomechanisms of this disease in light of the structural conclusions drawn from the first crystal structure of a disease-causing hE3 variant determined recently in our laboratory.

Protein-protein interactions of human glyoxalase II: findings of a reliable docking protocol

Glyoxalase II (GlxII) is an antioxidant glutathione-dependent enzyme, which catalyzes the hydrolysis of S-d-lactoylglutathione to form d-lactic acid and glutathione (GSH). The last product is the most important thiol reducing agent present in all eukaryotic cells that have mitochondria and chloroplasts. It is generally known that GSH plays a crucial role not only in the cellular redox state but also in various cellular processes. One of them is protein S-glutathionylation, a process that can occur through an oxidation reaction of proteins' thiol groups by GSH. Changes in protein S-glutathionylation have been associated with a range of human diseases such as diabetes, cardiovascular and pulmonary diseases, neurodegenerative diseases and cancer. Within a major project aimed at elucidating the role of GlxII in the mechanism of S-glutathionylation, a reliable computational protocol consisting of a protein-protein docking approach followed by atomistic Molecular Dynamics (MD) simulations was developed and it was applied to the prediction of molecular associations between human GlxII (in the presence and absence of GSH) and some proteins that are known to be S-glutathionylated in vitro, such as actin, malate dehydrogenase (MDH) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). The computational results show a high propensity of GlxII to interact with actin and MDH through its active site and a high stability of the GlxII-protein systems when GSH is present. Moreover, close proximities of GSH with actin and MDH cysteine residues have been found, suggesting that GlxII could be able to perform protein S-glutathionylation by using the GSH molecule present in its catalytic site.

Protein S-glutathionylation: The linchpin for the transmission of regulatory information on redox buffering capacity in mitochondria

Protein S-glutathionylation reactions are a ubiquitous oxidative modification required to control protein function in response to changes in redox buffering capacity. These reactions are rapid and reversible and are, for the most part, enzymatically mediated by glutaredoxins (GRX) and glutathione S-transferases (GST). Protein S-glutathionylation has been found to control a range of cell functions in response to different physiological cues. Although these reactions occur throughout the cell, mitochondrial proteins seem to be highly susceptible to reversible S-glutathionylation, a feature attributed to the unique physical properties of this organelle. Indeed, mitochondria contain a number of S-glutathionylation targets which includes proteins involved in energy metabolism, solute transport, reactive oxygen species (ROS) production, proton leaks, apoptosis, antioxidant defense, and mitochondrial fission and fusion. Moreover, it has been found that conjugation and removal of glutathione from proteins in mitochondria fulfills a number of important physiological roles and defects in these reactions can have some dire pathological consequences. Here, we provide an updated overview on mitochondrial protein S-glutathionylation reactions and their importance in cell functions and physiology.

Low molecular weight galactomannans-based standardized fenugreek seed extract ameliorates high-fat diet-induced obesity in mice via modulation of FASn, IL-6, leptin, and TRIP-Br2

Background: Obesity is a complex, chronic metabolic disorder and its prevalence is increasing throughout most of the world. Low molecular weight galactomannans-based standardized fenugreek seed extract (LMWGAL-TF) has previously shown anti-diabetic and anti-hyperlipidemic potential. Aim: To evaluate the efficacy and mechanism of action of LMWGAL-TF in treating high fat diet (HFD)-induced obesity and hyperlipidemia in mice. Materials and methods: Male C57BL/6 mice were fed the HFD for 12 weeks and were co-administered with LMWGAL-TF (10, 30 and 100 mg kg-1, p.o.). Variables measured were behavioral, biochemical, molecular and histopathological. In a separate in vitro experiment, copper-ascorbate (Cu-As)-induced mitochondrial oxidative damage was evaluated. Results: The HFD-induced increase (p < 0.001) in body weight, fat mass, lean mass, adipose tissue (brown, mesenteric, epididymal and retroperitoneal) and liver weight was significantly attenuated (p < 0.001) by LMWGAL-TF (30 and 100 mg kg-1). The HFD-induced elevated levels of serum lipid, interleukins (ILs)-6 and leptin were significantly decreased (p < 0.001) by LMWGAL-TF (30 and 100 mg kg-1). Elevated fatty acid synthase (FASn), IL-6, leptin and transcriptional regulator interacting with the PHD-bromodomain 2 (TRIP-Br2) mRNA expression in brown adipose tissue (BAT), liver, and epididymal fat were significantly down-regulated (p < 0.001) by LMWGAL-TF (30 and 100 mg kg-1). Additionally, HFD-induced histological alterations in skeletal muscle, liver, white adipose tissue (WAT) and BAT were also reduced by LMWGAL-TF. Furthermore, the Cu-As-induced alteration in mitochondria oxidative stress (lipid peroxidation, protein carbonylation, glutathione, glutathione reductase, glutathione peroxidase, isocitrate dehydrogenase and α-ketoglutarate dehydrogenase) in skeletal muscle and BAT was significantly (p < 0.001) ameliorated by LMWGAL-TF (2, 4 and 6 mg mL-1) treatment. It also reduced the Cu-As-induced mitochondrial swelling. Conclusion: LMWGAL-TF showed its beneficial effect in reducing HFD-induced obesity via down-regulation of FASn, IL-6, leptin, and TRIP-Br2 in mice.

Reductions in the mitochondrial enzyme α-ketoglutarate dehydrogenase complex in neurodegenerative disease - beneficial or detrimental?

Reductions in metabolism and excess oxidative stress are prevalent in multiple neurodegenerative diseases. The activity of the mitochondrial enzyme α-ketoglutarate dehydrogenase complex (KGDHC) appears central to these abnormalities. KGDHC is diminished in multiple neurodegenerative diseases. KGDHC can not only be rate limiting for NADH production and for substrate level phosphorylation, but is also a source of reactive oxygen species (ROS). The goal of these studies was to determine how changes in KGDHC modify baseline ROS, the ability to buffer ROS, baseline glutathionylation, calcium modulation and cell death in response to external oxidants. In vivo, reducing KGDHC with adeno virus diminished neurogenesis and increased oxidative stress. In vitro, treatments of short duration increased ROS and glutathionylation and enhanced the ability of the cells to diminish the ROS from added oxidants. However, long-term reductions lessened the ability to diminish ROS, diminished glutathionylation and exaggerated oxidant-induced changes in calcium and cell death. Increasing KGDHC enhanced the ability of the cells to diminish externally added ROS and protected against oxidant-induced changes in calcium and cell death. The results suggest that brief periods of diminished KGDHC are protective, while prolonged reductions are harmful. Furthermore, elevated KGDHC activities are protective. Thus, mitogenic therapies that increase KGDHC may be beneficial in neurodegenerative diseases. Read the Editorial Highlight for this article on Page 689.

Human dihydrolipoamide dehydrogenase (E3) deficiency: Novel insights into the structural basis and molecular pathomechanism

This review summarizes our present view on the molecular pathogenesis of human (h) E3-deficiency caused by a variety of genetic alterations with a special emphasis on the moonlighting biochemical phenomena related to the affected (dihydro)lipoamide dehydrogenase (LADH, E3, gene: dld), in particular the generation of reactive oxygen species (ROS). E3-deficiency is a rare autosomal recessive genetic disorder frequently presenting with a neonatal onset and premature death; the highest carrier rate of a single pathogenic dld mutation (1:94-1:110) was found among Ashkenazi Jews. Patients usually die during acute episodes that generally involve severe metabolic decompensation and lactic acidosis leading to neurological, cardiological, and/or hepatological manifestations. The disease owes its severity to the fact that LADH is the common E3 subunit of the alpha-ketoglutarate (KGDHc), pyruvate (PDHc), and branched-chain α-keto acid dehydrogenase complexes and is also part of the glycine cleavage system, hence the malfunctioning of LADH simultaneously incapacitates several central metabolic pathways. Nevertheless, the clinical pictures are usually not unequivocally portrayed through the loss of LADH activities and imply auxiliary mechanisms that exacerbate the symptoms and outcomes of this disorder. Enhanced ROS generation by disease-causing hE3 variants as well as by the E1-E2 subcomplex of the hKGDHc likely contributes to selected pathogeneses of E3-deficiency, which could be targeted by specific drugs or antioxidants; lipoic acid was demonstrated to be a potent inhibitor of ROS generation by hE3 in vitro. Flavin supplementation might prove to be beneficial for those mutations triggering FAD loss in the hE3 component. Selected pathogenic hE3 variants lose their affinity for the E2 component of the hPDHc, a mechanism which warrants scrutiny also for other E3-haboring complexes.

Protein S-glutathionylation alters superoxide/hydrogen peroxide emission from pyruvate dehydrogenase complex

Pyruvate dehydrogenase (Pdh) is a vital source of reactive oxygen species (ROS) in several different tissues. Pdh has also been suggested to serve as a mitochondrial redox sensor. Here, we report that O₂^•-/ H₂O₂ emission from pyruvate dehydrogenase (Pdh) is altered by S-glutathionylation. Glutathione disulfide (GSSG) amplified O₂^•-/ H₂O₂ production by purified Pdh during reverse electron transfer (RET) from NADH. Thiol oxidoreductase glutaredoxin-2 (Grx2) reversed these effects confirming that Pdh is a target for S-glutathionylation. S-glutathionylation had the opposite effect during forward electron transfer (FET) from pyruvate to NAD⁺ lowering O₂^•-/ H₂O₂ production. Immunoblotting for protein glutathione mixed disulfides (PSSG) following diamide treatment confirmed that purified Pdh can be S-glutathionylated. Similar observations were made with mouse liver mitochondria. S-glutathionylation catalysts diamide and disulfiram significantly reduced pyruvate or 2-oxoglutarate driven O₂^•-/ H₂O₂ production in liver mitochondria, results that were confirmed using various Pdh, 2-oxoglutarate dehydrogenase (Ogdh), and respiratory chain inhibitors. Immunoprecipitation of Pdh and Ogdh confirmed that either protein can be S-glutathionylated by diamide and disulfiram. Collectively, our results demonstrate that the S -glutathionylation of Pdh alters the amount of ROS formed by the enzyme complex. We also confirmed that Ogdh is controlled in a similar manner. Taken together, our results indicate that the redox sensing and ROS forming properties of Pdh and Ogdh are linked to S-glutathionylation.

Antioxidant modulation in restoring mitochondrial function in neurodegeneration

Alzheimer's disease (AD) and Parkinson's disease (PD) are the leading causes of disability associated with neurodegeneration worldwide. These diseases are influenced by multiple genetic and environmental factors and share similar mechanisms as both are characterized by accumulation and aggregation of misfolded proteins - amyloid-beta (Aβ) in AD and α-synuclein in PD. Over the past decade, increasing evidence has shown that mitochondrial dysfunction and the generation of reactive oxygen species (ROS) are involved in the pathology of these diseases, and the contributions of these defects to the cellular and molecular changes that eventually cause neuronal death have been explored. Using mitochondrial protective agents, such as antioxidants, to combat ROS provides a new strategy for neurodegenerative treatment. In this review, we highlight the potential of multiple types of antioxidants, including vitamins, phytochemicals, fatty acids and minerals, as well as synthetic antioxidants specifically targeting the mitochondria, which can restore mitochondrial function, in the treatment of neurodegenerative disorders at both the pre-clinical and clinical stages by focusing on AD and PD.

Comparative Proteomic Identification of Mature and Immature Sperm in the Catfish Cranoglanis bouderius

To understand the molecular responses of mature and immature sperm in the catfish Cranoglanis bouderius, we used the iTRAQ proteomics approach to perform proteomic profiling of spermatogenesis in C. bouderius. As a result, 1,941 proteins were identified, including 361 differentially expressed proteins, 157 upregulated proteins and 204 downregulated proteins in mature sperm relative to immature sperm. All of the identified proteins were categorized into seven types of subcellular localizations and three molecular functions and were found to be involved in nine biological processes. All of the differential proteins were involved in 235 different pathways. Moreover, we found that the tricarboxylic acid (TCA) pathway played an important role in the energy metabolism of sperm and that the EABB pathway was involved in the mechanism of spermatogenesis. Our study is the first to use the iTRAQ-based proteomic approach to analyze the catfish sperm proteome, and the results we obtained using this approach are valuable for understanding the molecular mechanisms of fish spermatogenesis.

Induction of mitochondrial reactive oxygen species production by GSH mediated S-glutathionylation of 2-oxoglutarate dehydrogenase

2-Oxoglutarate dehydrogenase (Ogdh) is an important mitochondria redox sensor that can undergo S-glutathionylation following an increase in H₂O₂ levels. Although S-glutathionylation is required to protect Ogdh from irreversible oxidation while simultaneously modulating its activity it remains unknown if glutathione can also modulate reactive oxygen species (ROS) production by the complex. We report that reduced (GSH) and oxidized (GSSG) glutathione control O2∙-/H₂O₂ formation by Ogdh through protein S-glutathionylation reactions. GSSG (1 mM) induced a modest decrease in Ogdh activity which was associated with a significant decrease in O2∙-/H₂O₂ formation. GSH had the opposite effect, amplifying O2∙-/H₂O₂ formation by Ogdh. Incubation of purified Ogdh in 2.5 mM GSH led to significant increase in O2∙-/H₂O₂ formation which also lowered NADH production. Inclusion of enzymatically active glutaredoxin-2 (Grx2) in reaction mixtures reversed the GSH-mediated amplification of O2∙-/H₂O₂ formation. Similarly pre-incubation of permeabilized liver mitochondria from mouse depleted of GSH showed an approximately ~3.5-fold increase in Ogdh-mediated O2∙-/H₂O₂ production that was matched by a significant decrease in NADH formation which could be reversed by Grx2. Taken together, our results demonstrate GSH and GSSG modulate ROS production by Ogdh through S-glutathionylation of different subunits. This is also the first demonstration that GSH can work in the opposite direction in mitochondria-amplifying ROS formation instead of quenching it. We propose that this regulatory mechanism is required to modulate ROS emission from Ogdh in response to variations in glutathione redox buffering capacity.

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ROS formation by Ogdh is controlled by glutathione.

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GSH amplifies ROS production by Ogdh.

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Ogdh is S-glutathionylated by GSH.

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Grx2 deglutathionylates Ogdh.

Hypoxia and GABA shunt activation in the pathogenesis of Alzheimer's disease

We have previously observed that the conversion of mild cognitive impairment to definitive Alzheimer's disease (AD) is associated with a significant increase in the serum level of 2,4-dihydroxybutyrate (2,4-DHBA). The metabolic generation of 2,4-DHBA is linked to the activation of the γ-aminobutyric acid (GABA) shunt, an alternative energy production pathway activated during cellular stress, when the function of Krebs cycle is compromised. The GABA shunt can be triggered by local hypoperfusion and subsequent hypoxia in AD brains caused by cerebral amyloid angiopathy. Succinic semialdehyde dehydrogenase (SSADH) is a key enzyme in the GABA shunt, converting succinic semialdehyde (SSA) into succinate, a Krebs cycle intermediate. A deficiency of SSADH activity stimulates the conversion of SSA into γ-hydroxybutyrate (GHB), an alternative route from the GABA shunt. GHB can exert not only acute neuroprotective activities but unfortunately also chronic detrimental effects which may lead to cognitive impairment. Subsequently, GHB can be metabolized to 2,4-DHBA and secreted from the brain. Thus, the activation of the GABA shunt and the generation of GHB and 2,4-DHBA can have an important role in the early phase of AD pathogenesis.

Role of glutathione, glutathione transferase, and glutaredoxin in regulation of redox-dependent processes

Over the last decade fundamentally new features have been revealed for the participation of glutathione and glutathione-dependent enzymes (glutathione transferase and glutaredoxin) in cell proliferation, apoptosis, protein folding, and cell signaling. Reduced glutathione (GSH) plays an important role in maintaining cellular redox status by participating in thiol-disulfide exchange, which regulates a number of cell functions including gene expression and the activity of individual enzymes and enzyme systems. Maintaining optimum GSH/GSSG ratio is essential to cell viability. Decrease in the ratio can serve as an indicator of damage to the cell redox status and of changes in redox-dependent gene regulation. Disturbance of intracellular GSH balance is observed in a number of pathologies including cancer. Consequences of inappropriate GSH/GSSG ratio include significant changes in the mechanism of cellular redox-dependent signaling controlled both nonenzymatically and enzymatically with the participation of isoforms of glutathione transferase and glutaredoxin. This review summarizes recent data on the role of glutathione, glutathione transferase, and glutaredoxin in the regulation of cellular redox-dependent processes.

Alpha-ketoglutarate dehydrogenase complex-dependent succinylation of proteins in neurons and neuronal cell lines

Reversible post-translation modifications of proteins are common in all cells and appear to regulate many processes. Nevertheless, the enzyme(s) responsible for the alterations and the significance of the modification are largely unknown. Succinylation of proteins occurs and causes large changes in the structure of proteins; however, the source of the succinyl groups, the targets, and the consequences of these modifications on other proteins remain unknown. These studies focused on succinylation of mitochondrial proteins. The results demonstrate that the α-ketoglutarate dehydrogenase complex (KGDHC) can serve as a trans-succinylase that mediates succinylation in an α-ketoglutarate-dependent manner. Inhibition of KGDHC reduced succinylation of both cytosolic and mitochondrial proteins in cultured neurons and in a neuronal cell line. Purified KGDHC can succinylate multiple proteins including other enzymes of the tricarboxylic acid cycle leading to modification of their activity. Inhibition of KGDHC also modifies acetylation by modifying the pyruvate dehydrogenase complex. The much greater effectiveness of KGDHC than succinyl-CoA suggests that the catalysis owing to the E2k succinyltransferase is important. Succinylation appears to be a major signaling system and it can be mediated by KGDHC. Reversible post-translation modifications of proteins are common and may regulate many processes. Succinylation of proteins occurs and causes large changes in the structure of proteins. However, the source of the succinyl groups, the targets, and the consequences of these modifications on other proteins remains unknown. The results demonstrate that the mitochondrial α-ketoglutarate dehydrogenase complex (KGDHC) can succinylate multiple mitochondrial proteins and alter their function. Succinylation appears to be a major signaling system and it can be mediated by KGDHC.

Mitochondrial DNA sequence associations with dementia and amyloid-β in elderly African Americans

Mitochondrial dysfunction occurs early in the course of several neurodegenerative diseases, and is potentially related to increased oxidative damage and amyloid-β (Aβ) formation in Alzheimer's disease. The goals of this study were to assess mtDNA sequence associations with dementia risk, 10-year cognitive change, and markers of oxidative stress and Aβ among 1089 African-Americans in the population-based Health, Aging, and Body Composition Study. Participants were free of dementia at baseline, and incidence was determined in 187 (18%) cases over 10 to 12 follow-up years. Haplogroup L1 participants were at increased risk for developing dementia (odds ratio = 1.88, 95% confidence interval = 1.23-2.88, p = 0.004), lower plasma Aβ42 levels (p = 0.03), and greater 10-year decline on the Digit Symbol Substitution Test (p = 0.04) when compared with common haplogroup L3. The p.V193I, ND2 substitution was associated with significantly higher Aβ42 levels (p = 0.0012), and this association was present in haplogroup L3 (p = 0.018) but not L1 (p = 0.90) participants. All associations were independent of potential confounders, including APOEε4 status and nuclear genetic ancestry. Identification of mtDNA sequence variation associated with dementia risk and cognitive decline may contribute to the development of new treatment targets and diagnostic tests that identify responders to interventions targeting mitochondria.

Mitochondrial function in human brains is affected by pre- and post mortem factors

AIM

Mitochondrial function and the ensuing ATP synthesis are central to the functioning of the brain and contribute to neuronal physiology. Most studies on neurodegenerative diseases have highlighted that mitochondrial dysfunction is an important event contributing to pathology. However, studies on the human brain mitochondria in various neurodegenerative disorders heavily rely on post mortem samples. As post mortem tissues are influenced by pre- and post mortem factors, we investigated the effect of these variables on mitochondrial function.

METHODS

We examined whether the mitochondrial function (represented by mitochondrial enzymes and antioxidant activities) in post mortem human brains (n=45) was affected by increased storage time (11.8-104.1 months), age of the donor (2 days to 80 years), post mortem interval (2.5-26 h), gender difference and agonal state [based on Glasgow Coma Scale: range=3-15] in the frontal cortex, as a prototype.

RESULTS

We observed that the activities of citrate synthase, succinate dehydrogenase and mitochondrial reductase (MTT) were significantly affected only by gender difference (citrate synthase: P=0.005; succinate dehydrogenase: P=0.01; mitochondrial reductase: P=0.006), being higher in females, but not by any other factor. Mitochondrial complex I activity was significantly inhibited by increasing age (r=-0.40; P=0.05). On the other hand, the mitochondrial antioxidant enzyme glutathione reductase decreased with severe agonal state (P=0.003), while the activity of glutathione-S-transferase declined with increased storage time (P=0.005) and severe agonal state (P=0.02).

CONCLUSION

Our data highlight the influence of pre- and post mortem factors on preservation of mitochondrial function with implications for studies on brain pathology employing stored human samples.

Which way does the citric acid cycle turn during hypoxia? The critical role of α-ketoglutarate dehydrogenase complex

The citric acid cycle forms a major metabolic hub and as such it is involved in many disease states involving energetic imbalance. In spite of the fact that it is being branded as a "cycle", during hypoxia, when the electron transport chain does not oxidize reducing equivalents, segments of this metabolic pathway remain operational but exhibit opposing directionalities. This serves the purpose of harnessing high-energy phosphates through matrix substrate-level phosphorylation in the absence of oxidative phosphorylation. In this Mini-Review, these segments are appraised, pointing to the critical importance of the α-ketoglutarate dehydrogenase complex dictating their directionalities.

Deficits in the mitochondrial enzyme α-ketoglutarate dehydrogenase lead to Alzheimer's disease-like calcium dysregulation

Understanding the molecular sequence of events that culminate in multiple abnormalities in brains from patients that died with Alzheimer's disease (AD) will help to reveal the mechanisms of the disease and identify upstream events as therapeutic targets. The activity of the mitochondrial α-ketoglutarate dehydrogenase complex (KGDHC) in homogenates from autopsy brain declines with AD. Experimental reductions in KGDHC in mouse models of AD promote plaque and tangle formation, the hallmark pathologies of AD. We hypothesize that deficits in KGDHC also lead to the abnormalities in endoplasmic reticulum (ER) calcium stores and cytosolic calcium following K(+) depolarization that occurs in cells from AD patients and transgenic models of AD. The activity of the mitochondrial enzyme KGDHC was diminished acutely (minutes), long-term (days), or chronically (weeks). Acute inhibition of KGDHC produced effects on calcium opposite to those in AD, while the chronic or long-term inhibition of KGDHC mimicked the AD-related changes in calcium. Divergent changes in proteins released from the mitochondria that affect endoplasmic reticulum calcium channels may underlie the selective cellular consequences of acute versus longer term inhibition of KGDHC. The results suggest that the mitochondrial abnormalities in AD can be upstream of those in calcium.

Redox regulation of mitochondrial function

Redox-dependent processes influence most cellular functions, such as differentiation, proliferation, and apoptosis. Mitochondria are at the center of these processes, as mitochondria both generate reactive oxygen species (ROS) that drive redox-sensitive events and respond to ROS-mediated changes in the cellular redox state. In this review, we examine the regulation of cellular ROS, their modes of production and removal, and the redox-sensitive targets that are modified by their flux. In particular, we focus on the actions of redox-sensitive targets that alter mitochondrial function and the role of these redox modifications on metabolism, mitochondrial biogenesis, receptor-mediated signaling, and apoptotic pathways. We also consider the role of mitochondria in modulating these pathways, and discuss how redox-dependent events may contribute to pathobiology by altering mitochondrial function.

Changes in mitochondrial glutathione levels and protein thiol oxidation in ∆yfh1 yeast cells and the lymphoblasts of patients with Friedreich's ataxia

Friedreich's ataxia (FRDA) is a neurodegenerative disease caused by low levels of the mitochondrial protein frataxin. The main phenotypic features of frataxin-deficient human and yeast cells include iron accumulation in mitochondria, iron-sulfur cluster defects and high sensitivity to oxidative stress. Frataxin deficiency is also associated with severe impairment of glutathione homeostasis and changes in glutathione-dependent antioxidant defenses. The potential biological consequences of oxidative stress and changes in glutathione levels associated with frataxin deficiency include the oxidation of susceptible protein thiols and reversible binding of glutathione to the SH of proteins by S-glutathionylation. In this study, we isolated mitochondria from frataxin-deficient ∆yfh1 yeast cells and lymphoblasts of FRDA patients, and show evidence for a severe mitochondrial glutathione-dependent oxidative stress, with a low GSH/GSSG ratio, and thiol modifications of key mitochondrial enzymes. Both yeast and human frataxin-deficient cells had abnormally high levels of mitochondrial proteins binding an anti-glutathione antibody. Moreover, proteomics and immunodetection experiments provided evidence of thiol oxidation in α-ketoglutarate dehydrogenase (KGDH) or subunits of respiratory chain complexes III and IV. We also found dramatic changes in GSH/GSSG ratio and thiol modifications on aconitase and KGDH in the lymphoblasts of FRDA patients. Our data for yeast cells also confirm the existence of a signaling and/or regulatory process involving both iron and glutathione.

Mitochondrial DNA sequence variation associated with dementia and cognitive function in the elderly

Mitochondrial dysfunction is a prominent hallmark of Alzheimer's disease (AD). Mitochondrial DNA (mtDNA) damage may be a major cause of abnormal reactive oxidative species production in AD or increased neuronal susceptibility to oxidative injury during aging. The purpose of this study was to assess the influence of mtDNA sequence variation on clinically significant cognitive impairment and dementia risk in the population-based Health, Aging, and Body Composition (Health ABC) Study. We first investigated the role of common mtDNA haplogroups and individual variants on dementia risk and 8-year change on the Modified Mini-Mental State Examination (3MS) and Digit Symbol Substitution Test (DSST) among 1,631 participants of European genetic ancestry. Participants were free of dementia at baseline and incidence was determined in 273 cases from hospital and medication records over 10-12 follow-up years. Participants from haplogroup T had a statistically significant increased risk of developing dementia (OR = 1.86, 95% CI = 1.23, 2.82, p = 0.0008) and haplogroup J participants experienced a statistically significant 8-year decline in 3MS (β = -0.14, 95% CI = -0.27, -0.03, p = 0.0006), both compared with common haplogroup H. The m.15244A>G, p.G166G, CytB variant was associated with a significant decline in DSST score (β = -0.58, 95% CI -0.89, -0.28, p = 0.00019) and the m.14178T>C, p.I166V, ND6 variant was associated with a significant decline in 3MS score (β = -0.87, 95% CI -1.31, -3.86, p = 0.00012). Finally, we sequenced the complete ~16.5 kb mtDNA from 135 Health ABC participants and identified several highly conserved and potentially functional nonsynonymous variants unique to 22 dementia cases and aggregate sequence variation across the hypervariable 2-3 regions that influences 3MS and DSST scores.

Reversible and irreversible protein glutathionylation: biological and clinical aspects

INTRODUCTION

Depending in part on the glutathione:glutathione disulfide ratio, reversible protein glutathionylation to a mixed disulfide may occur. Reversible glutathionylation is important in protecting proteins against oxidative stress, guiding correct protein folding, regulating protein activity and modulating proteins critical to redox signaling. The potential also exists for irreversible protein glutathionylation via Michael addition of an -SH group to a dehydroalanyl residue, resulting in formation of a stable, non-reducible thioether linkage.

AREAS COVERED

This article reviews factors contributing to reversible and irreversible protein glutathionylation and their biomedical implications. It also examines the possibility that certain drugs such as busulfan may be toxic by promoting irreversible glutathionylation. The reader will gain an appreciation of the protective nature and control of function resulting from reversible protein glutathionylation. The reader is also introduced to the recently identified phenomenon of irreversible protein glutathionylation and its possible deleterious effects.

EXPERT OPINION

The process of reversible protein glutathionylation is now well established but these findings need to be substantiated at the tissue and organ levels, and also with disease state. That being said, irreversible protein glutathionylation can also occur and this has implications in disease and aging. Toxicologists should consider this when evaluating the possible side effects of certain drugs such as busulfan that may generate a glutathionylating species in vivo.

Glutamate and glutathione interplay in a motor neuronal model of amyotrophic lateral sclerosis reveals altered energy metabolism

Impairment of mitochondrial function might contribute to oxidative stress associated with neurodegeneration in amyotrophic lateral sclerosis (ALS). Glutamate levels in tissues of ALS patients are sometimes altered. In neurons, mitochondrial metabolism of exogenous glutamine is mainly responsible for the net synthesis of glutamate, which is a neurotransmitter, but it is also necessary for the synthesis of glutathione, the main endogenous antioxidant. We investigated glutathione synthesis and glutamine/glutamate metabolism in a motor neuronal model of familial ALS. In standard culture conditions (with glutamine) or restricting glutamine or cystine, the level of glutathione was always lower in the cell line expressing the mutant (G93A) human Cu, Zn superoxide dismutase (G93ASOD1) than in the line expressing wild-type SOD1. With glutamine the difference in glutathione was associated with a lower glutamate and impairment of the glutamine/glutamate metabolism as evidenced by lower glutaminase and cytosolic malate dehydrogenase activity. d-β-hydroxybutyrate, as an alternative to glutamine as energy substrate in addition to glucose, reversed the decreases of cytosolic malate dehydrogenase activity and glutamate and glutathione. However, in the G93ASOD1 cell line, in all culture conditions the expression of pyruvate dehydrogenase kinase l protein, which down-regulates pyruvate dehydrogenase activity, was induced, together with an increase in lactate release in the medium. These findings suggest that the glutathione decrease associated with mutant SOD1 expression is due to mitochondrial dysfunction caused by the reduction of the flow of glucose-derived pyruvate through the TCA cycle; it implies altered glutamate metabolism and depends on the different mitochondrial energy substrates.

A mitocentric view of Alzheimer's disease suggests multi-faceted treatments

Alzheimer's disease (AD) is defined by senile plaques made of amyloid-beta peptide (Abeta), neurofibrillary tangles made of hyperphosphorylated tau proteins, and memory deficits. Thus, the events initiating the cascade leading to these end points may be more effective therapeutic targets than treating each facet individually. In the small percentage of cases of AD that are genetic (or animal models that reflect this form of AD), the factor initiating AD is clear (e.g., genetic mutations lead to high Abeta1-42 or hyperphosphorylated tau proteins). In the vast majority of AD cases, the cause is unknown. Substantial evidence now suggests that abnormalities in glucose metabolism/mitochondrial function/oxidative stress (GMO) are an invariant feature of AD and occur at an early stage of the disease process in both genetic and non-genetic forms of AD. Indeed, decreases in brain glucose utilization are diagnostic for AD. Changes in calcium homeostasis also precede clinical manifestations of AD. Abnormal GMO can lead to plaques, tangles, and the calcium abnormalities that accompany AD. Abnormalities in GMO diminish the ability of the brain to adapt. Therapies targeting mitochondria may ameliorate abnormalities in plaques, tangles, calcium homeostasis, and cognition that comprise AD.

Potential therapeutic benefits of strategies directed to mitochondria

The mitochondrion is the most important organelle in determining continued cell survival and cell death. Mitochondrial dysfunction leads to many human maladies, including cardiovascular diseases, neurodegenerative disease, and cancer. These mitochondria-related pathologies range from early infancy to senescence. The central premise of this review is that if mitochondrial abnormalities contribute to the pathological state, alleviating the mitochondrial dysfunction would contribute to attenuating the severity or progression of the disease. Therefore, this review will examine the role of mitochondria in the etiology and progression of several diseases and explore potential therapeutic benefits of targeting mitochondria in mitigating the disease processes. Indeed, recent advances in mitochondrial biology have led to selective targeting of drugs designed to modulate and manipulate mitochondrial function and genomics for therapeutic benefit. These approaches to treat mitochondrial dysfunction rationally could lead to selective protection of cells in different tissues and various disease states. However, most of these approaches are in their infancy.

Functional aspects of redox control during neuroinflammation

Neuroinflammation is a CNS reaction to injury in which some severe pathologies, regardless of their origin, converge. The phenomenon emphasizes crosstalk between neurons and glia and reveals a complex interaction with oxidizing agents through redox sensors localized in enzymes, receptors, and transcription factors. When oxidizing pressures cause reversible molecular changes, such as minimal or transitory proinflammatory cytokine overproduction, redox couples provide a means of translating the presence of reactive oxygen or nitrogen species into useful signals in the cell. Additionally, thiol-based redox sensors convey information about localized changes in redox potential induced by physiologic or pathologic situations. They are susceptible to oxidative changes and become key events during neuroinflammation, altering the course of a signaling response or the behavior of specific transcription factors. When oxidative stress augments the pressure on the intracellular environment, the effective reduction potential of redox pairs diminishes, and cell signaling shifts toward proinflammatory and proapoptotic signals, creating a vicious cycle between oxidative stress and neuroinflammation. In addition, electrophilic compounds derived from the oxidative cascade react with key protein thiols and interfere with redox signaling. This article reviews the relevant functional aspects of redox control during the neuroinflammatory process.

Sustained hydrogen peroxide stress decreases lactate production by cultured astrocytes

Oxidative stress and disrupted energy metabolism are common to many pathological conditions of the brain. Because astrocytes play an important role in the glucose metabolism of the brain, we have investigated whether sustained oxidative stress affects astroglial glucose metabolism with cultured primary rat astrocytes as a model system. Cultured astrocytes were exposed to a sustained concentration of approximately 50 muM H(2)O(2) in the presence of [U-(13)C]glucose, and cellular and extracellular contents of lactate and glucose were analysed by enzymatic assays and NMR spectroscopy. Exposure of the cells to sustained H(2)O(2) stress for up to 120 min significantly lowered the rate of lactate accumulation in the media to 61% +/- 14% of that in cultures incubated without peroxide. In addition, the ratio of lactate release to glucose consumption was lowered in peroxide-treated astrocytes to 77% +/- 13% of that in control cells, and the specific activity of glyceraldehyde-3-phosphate dehydrogenase had declined to about 10% of control cells within 90 min. In addition, the (13)C enrichment of intracellular and extracellular [(13)C]lactate was about 30% and 95%, respectively, and was not affected by the presence of peroxide, demonstrating that two metabolic pools of lactate are present in cultured astrocytes. The decreased rate of lactate production by astrocytes that have been exposed to peroxide stress is a new example of an alteration by oxidative stress of an important metabolic pathway in astrocytes. Such alterations could contribute to the pathological conditions that have been connected with oxidative stress and disrupted energy metabolism in the brain.

Cause and consequence: mitochondrial dysfunction initiates and propagates neuronal dysfunction, neuronal death and behavioral abnormalities in age-associated neurodegenerative diseases

Age-related neurodegenerative diseases are associated with mild impairment of oxidative metabolism and accumulation of abnormal proteins. Within the cell, the mitochondria appears to be a dominant site for initiation and propagation of disease processes. Shifts in metabolism in response to mild metabolic perturbations may decrease the threshold for irreversible injury in response to ordinarily sublethal metabolic insults. Mild impairment of metabolism accrue from and lead to increased reactive oxygen species (ROS). Increased ROS change cell signaling via post-transcriptional and transcriptional changes. The cause and consequences of mild impairment of mitochondrial metabolism is one focus of this review. Many experiments in tissues from humans support the notion that oxidative modification of the alpha-ketoglutarate dehydrogenase complex (KGDHC) compromises neuronal energy metabolism and enhances ROS production in Alzheimer's Disease (AD). These data suggest that cognitive decline in AD derives from the selective tricarboxylic acid (TCA) cycle abnormalities. By contrast in Huntington's Disease (HD), a movement disorder with cognitive features distinct form AD, complex II+III abnormalities may dominate. These distinct mitochondrial abnormalities culminate in oxidative stress, energy dysfunction, and aberrant homeostasis of cytosolic calcium. Cytosolic calcium, elevations even only transiently, leads to hyperactivity of a number of enzymes. One calcium-activated enzyme with demonstrated pathophysiological import in HD and AD is transglutaminase (TGase). TGase is a crosslinking enzymes that can modulate transcription, inactivate metabolic enzymes, and cause aggregation of critical proteins. Recent data indicate that TGase can silence expression of genes involved in compensating for metabolic stress. Altogether, our results suggest that increasing KGDHC via inhibition of TGase or via a host of other strategies to be described would be effective therapeutic approaches in age-associated neurodegenerative diseases.

Inhibition of the alpha-ketoglutarate dehydrogenase-mediated reactive oxygen species generation by lipoic acid

Dihydrolipoamide dehydrogenase (LADH) is a flavo-enzyme that serves as a subunit of alpha-ketoglutarate dehydrogenase complex (alpha-KGDHC). Reactive oxygen species (ROS) generation by alpha-KGDHC has been assigned to LADH (E3 subunit) and explained by the diaphorase activity of E3. Dysfunctions of alpha-KGDHC and concurrent ROS production have been implicated in neurodegeneration, ischemia-reperfusion, and other pathological conditions. In this work we investigated the in-depth details of ROS generation by isolated LADH and alpha-KGDHC. We found a parallel generation of superoxide and hydrogen peroxide by the E3 subunit of alpha-KGDHC which could be blocked by lipoic acid (LA) acting on a site upstream of the E3 subunit. The pathologically relevant ROS generation (at high NADH/NAD+ ratio and low pH) in the reverse mode of alpha-KGDHC could also be inhibited by LA. Our results contradict the previously proposed mechanism for pH-dependent ROS generation by LADH, showing no disassembling of the E3 functional homodimer at acidic pH using a physiologically relevant method for the examination. It is also suggested that LA could be beneficial in reducing the cell damage related to excessive ROS generation under pathological conditions.

Alpha-ketoglutarate dehydrogenase and glutamate dehydrogenase work in tandem to modulate the antioxidant alpha-ketoglutarate during oxidative stress in Pseudomonas fluorescens

Alpha-ketoglutarate (KG) is a crucial metabolite in all living organisms, as it participates in a variety of biochemical processes. We have previously shown that this keto acid is an antioxidant and plays a key role in the detoxification of reactive oxygen species (ROS). In an effort to further confirm this intriguing phenomenon, Pseudomonas fluorescens was exposed to menadione-containing media, with various amino acids as the sources of nitrogen. Here, we demonstrate that KG dehydrogenase (KGDH) and NAD-dependent glutamate dehydrogenase (GDH) work in tandem to modulate KG homeostasis. While KGDH was sharply decreased in cells challenged with menadione, GDH was markedly increased in cultures containing arginine (Arg), glutamate (Glu), and proline (Pro). When ammonium (NH(4)) was utilized as the nitrogen source, both KGDH and GDH levels were diminished. These enzymatic profiles were reversed when control cells were incubated in menadione media. (13)C nuclear magnetic resonance and high-performance liquid chromatography studies revealed how KG was utilized to eliminate ROS with the concomitant formation of succinate. The accumulation of KG in the menadione-treated cells was dependent on the redox status of the lipoic acid residue in KGDH. Indeed, the treatment of cellular extracts from the menadione-exposed cells with dithiothreitol, a reducing agent, partially restored the activity of KGDH. Taken together, these data reveal that KG is pivotal to the antioxidative defense strategy of P. fluorescens and also point to the ROS-sensing role for KGDH.

The impact of surfactant protein-A on ozone-induced changes in the mouse bronchoalveolar lavage proteome

Background

Ozone is a major component of air pollution. Exposure to this powerful oxidizing agent can cause or exacerbate many lung conditions, especially those involving innate immunity. Surfactant protein-A (SP-A) plays many roles in innate immunity by participating directly in host defense as it exerts opsonin function, or indirectly via its ability to regulate alveolar macrophages and other innate immune cells. The mechanism(s) responsible for ozone-induced pathophysiology, while likely related to oxidative stress, are not well understood.

Methods

We employed 2-dimensional difference gel electrophoresis (2D-DIGE), a discovery proteomics approach, coupled with MALDI-ToF/ToF to compare the bronchoalveolar lavage (BAL) proteomes in wild type (WT) and SP-A knockout (KO) mice and to assess the impact of ozone or filtered air on the expression of BAL proteins. Using the PANTHER database and the published literature most identified proteins were placed into three functional groups.

Results

We identified 66 proteins and focused our analysis on these proteins. Many of them fell into three categories: defense and immunity; redox regulation; and protein metabolism, modification and chaperones. In response to the oxidative stress of acute ozone exposure (2 ppm; 3 hours) there were many significant changes in levels of expression of proteins in these groups. Most of the proteins in the redox group were decreased, the proteins involved in protein metabolism increased, and roughly equal numbers of increases and decreases were seen in the defense and immunity group. Responses between WT and KO mice were similar in many respects. However, the percent change was consistently greater in the KO mice and there were more changes that achieved statistical significance in the KO mice, with levels of expression in filtered air-exposed KO mice being closer to ozone-exposed WT mice than to filtered air-exposed WT mice.

Conclusion

We postulate that SP-A plays a role in reactive oxidant scavenging in WT mice and that its absence in the KO mice in the presence or absence of ozone exposure results in more pronounced, and presumably chronic, oxidative stress.

Glutamine homeostasis and mitochondrial dynamics

Glutamine is a multifaceted amino acid that plays key roles in many metabolic pathways and also fulfils essential signaling functions. Although classified as non-essential, recent evidence suggests that glutamine is a conditionally essential amino acid in several physiological situations. Glutamine homeostasis must therefore be exquisitely regulated and mitochondria represent a major site of glutamine metabolism in numerous cell types. Glutaminolysis is mostly a mitochondrial process with repercussions in organelle structure and dynamics suggesting a tight and mutual control between mitochondrial form and cell bioenergetics. In this review we describe an updated account focused on the critical involvement of glutamine in oxidative stress, mitochondrial dysfunction and tumour cell proliferation, with special emphasis in the initial steps of mitochondrial glutamine pathways: transport into the organelle and hydrolytic deamidation through glutaminase enzymes. Some controversial issues about glutamine catabolism within mitochondria are also reviewed.

Oxidant-induced changes in mitochondria and calcium dynamics in the pathophysiology of Alzheimer's disease

Considerable data support the hypothesis that mitochondrial abnormalities link gene defects and/or environmental insults to the neurodegenerative process. The interaction of oxidants with calcium and the mitochondrial enzymes of the tricarboxylic acid cycle are central to that relationship. Abnormalities that were discovered in brains or fibroblasts from patients with Alzheimer's disease (AD) have been modeled in vitro and in vivo to assess their pathophysiological importance and to determine how they might be reversed. The conclusions are consistent with the hypothesis that the AD-related abnormalities result from oxidative stress. The selection of compounds for reversal is complex because the actions of the relevant compounds vary under different conditions, such as cell redox states and acute versus chronic changes. However, the models that have been developed are useful for testing the effectiveness of the potential medications. The results suggest that the reversal of mitochondrial deficits and a reduction in oxidative stress will reduce clinical and pathological changes and benefit patients.

Molecular mechanisms and clinical implications of reversible protein S-glutathionylation

Sulfhydryl chemistry plays a vital role in normal biology and in defense of cells against oxidants, free radicals, and electrophiles. Modification of critical cysteine residues is an important mechanism of signal transduction, and perturbation of thiol-disulfide homeostasis is an important consequence of many diseases. A prevalent form of cysteine modification is reversible formation of protein mixed disulfides (protein-SSG) with glutathione (GSH). The abundance of GSH in cells and the ready conversion of sulfenic acids and S-nitroso derivatives to S-glutathione mixed disulfides suggests that reversible S-glutathionylation may be a common feature of redox signal transduction and regulation of the activities of redox sensitive thiol-proteins. The glutaredoxin enzyme has served as a focal point and important tool for evolution of this regulatory mechanism, because it is a specific and efficient catalyst of protein-SSG deglutathionylation. However, mechanisms of control of intracellular Grx activity in response to various stimuli are not well understood, and delineation of specific mechanisms and enzyme(s) involved in formation of protein-SSG intermediates requires further attention. A large number of proteins have been identified as potentially regulated by reversible S-glutathionylation, but only a few studies have documented glutathionylation-dependent changes in activity of specific proteins in a physiological context. Oxidative stress is a hallmark of many diseases which may interrupt or divert normal redox signaling and perturb protein-thiol homeostasis. Examples involving changes in S-glutathionylation of specific proteins are discussed in the context of diabetes, cardiovascular and lung diseases, cancer, and neurodegenerative diseases.