Respiration-dependent removal of exogenous H2O2 in brain mitochondria: inhibition by Ca2+

Glutathione Redox State Regulates Mitochondrial Reactive OxygenProduction

Oxidative stress induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; dioxin) is poorly understood. Following one dose of TCDD (5 microg/kg body weight), mitochondrial succinate-dependent production of superoxide and H2O2 in mouse liver doubled at 7-28 days, then subsided by day 56; concomitantly, levels of GSH and GSSG increased in both cytosol and mitochondria. Cytosol displayed a typical oxidative stress response, consisting of diminished GSH relative to GSSG, decreased potential to reduce protein-SSG mixed disulfide bonds (type 1 thiol redox switch) or protein-SS-protein disulfide bonds (type 2 thiol redox switch), and a +10 mV change in GSSG/2GSH reduction potential. In contrast, mitochondria showed a rise in reduction state, consisting of increased GSH relative to GSSG, increases in type 1 and type 2 thiol redox switches, and a -25 mV change in GSSG/2GSH reduction potential. Comparing Ahr(-/-) knock-out and wild-type mice, we found that TCDD-induced thiol changes in both cytosol and mitochondria were dependent on the aromatic hydrocarbon receptor (AHR). GSH was rapidly taken up by mitochondria and stimulated succinate-dependent H2O2 production. A linear dependence of H2O2 production on the reduction potential for GSSG/2GSH exists between -150 and -300 mV. The TCDD-stimulated increase in succinate-dependent and thiol-stimulated production of reactive oxygen paralleled a four-fold increase in formamidopyrimidine DNA N-glycosylase (FPG)-sensitive cleavage sites in mitochondrial DNA, compared with a two-fold increase in nuclear DNA. These results suggest that TCDD produces an AHR-dependent oxidative stress in mitochondria, with concomitant mitochondrial DNA damage mediated, at least in part, by an increase in the mitochondrial thiol reduction state.

Ca2+-induced permeabilization promotes free radical release from rat brain mitochondria with partially inhibited complex I

Mitochondrial complex I dysfunction has been implicated in a number of brain pathologies, putatively owing to an increased rate of reactive oxygen species (ROS) release. However, the mechanisms regulating the ROS burden are poorly understood. In this study we investigated the effect of Ca2+ loads on ROS release from rat brain mitochondria with complex I partially inhibited by rotenone. The addition of 20 nm rotenone to brain mitochondria increased ROS release. Ca2+ (100 microm) alone had no effect on ROS release, but greatly potentiated the effects of rotenone. The effect of Ca2+ was decreased by ruthenium red. Ca2+-challenged mitochondria lose about 88% of their glutathione and 46% of their cytochrome c under these conditions, although this depends only on Ca2+ loading and not complex I inhibition. ADP in combination with oligomycin decreased the loss of glutathione and cytochrome c and free radical generation. Cyclosporin A alone was ineffective in preventing these effects, but augmented the protection provided by ADP and oligomycin. Non-specific permeabilization of mitochondria with alamethicin also increased the ROS signal, but only when combined with partial inhibition of complex I. These results demonstrate that Ca2+ can greatly increase ROS release by brain mitochondria when complex I is impaired.

Dopamine-derived Dopaminochrome Promotes H2O2 Release at Mitochondrial Complex I

Inhibitors of Complex I of the mitochondrial respiratory chain, such as rotenone, promote Parkinson disease-like symptoms and signs of oxidative stress. Dopamine (DA) oxidation products may be implicated in such a process. We show here that the o-quinone dopaminochrome (DACHR), a relatively stable DA oxidation product, promotes concentration (0.1-0.2 mum)- and respiration-dependent generation of H(2)O(2) at Complex I in brain mitochondria, with further stimulation by low concentrations of rotenone (5-30 nm). The rotenone effect required that contaminating Ca(2+) (8-10 mum) was not removed. DACHR apparently extracts an electron from the constitutively autoxidizable site in Complex I, producing a semiquinone, which then transfers an electron to O(2), generating O(2)(.) and then H(2)O(2). Mitochondrial removal of H(2)O(2) monoamine, formed by either oxidase activity or DACHR, was performed largely by glutathione peroxidase and glutathione reductase, which were negatively regulated by low intramitochondrial Ca(2+) levels. Thus, the H(2)O(2) formed accumulated in the medium if contaminating Ca(2+) was present; in the absence of Ca(2+), H(2)O(2) was completely removed if it originated from monoamine oxidase, but was less completely removed if it originated from DACHR. We propose that the primary action of rotenone is to promote extracellular O(2)(.) release via activation of NADPH oxidase in the microglia. In turn, O(2)(.) oxidizes DA to DACHR extracellularly. (The reaction is favored by the lack of GSH, which would otherwise preferably produce GSH adducts of dopaminoquinone.) Once formed, DACHR (which is resistant to GSH) enters neurons to activate the rotenone-stimulated redox cycle described.

Peroxide detoxification by brain cells

Peroxides are generated continuously in cells that consume oxygen. Among the different peroxides, hydrogen peroxide is the molecule that is formed in highest quantities. In addition, organic hydroperoxides are synthesized as products of cellular metabolism. Generation and disposal of peroxides is a very important process in the human brain, because cells of this organ consume 20% of the oxygen used by the body. To prevent cellular accumulation of peroxides and damage generated by peroxide-derived radicals, brain cells contain efficient antioxidative defense mechanisms that dispose of peroxides and protect against oxidative damage. Cultured brain cells have been used frequently to investigate peroxide metabolism of neural cells. Efficient disposal of exogenous hydrogen peroxide was found for cultured astrocytes, oligodendrocytes, microglial cells, and neurons. Comparison of specific peroxide clearance rates revealed that cultured oligodendrocytes dispose of the peroxide quicker than the other neural cell cultures. Both catalase and the glutathione system contribute to the clearance of hydrogen peroxide by brain cells. For efficient glutathione-dependent reduction of peroxides, neural cells contain glutathione in high concentration and have substantial activity of glutathione peroxidase, glutathione reductase, and enzymes that supply the NADPH required for the glutathione reductase reaction. This article gives an overview on the mechanisms involved in peroxide detoxification in brain cells and on the capacity of the different types of neural cells to dispose of peroxides.

AIF deficiency compromises oxidative phosphorylation

Apoptosis-inducing factor (AIF) is a mitochondrial flavoprotein that, after apoptosis induction, translocates to the nucleus where it participates in apoptotic chromatinolysis. Here, we show that human or mouse cells lacking AIF as a result of homologous recombination or small interfering RNA exhibit high lactate production and enhanced dependency on glycolytic ATP generation, due to severe reduction of respiratory chain complex I activity. Although AIF itself is not a part of complex I, AIF-deficient cells exhibit a reduced content of complex I and of its components, pointing to a role of AIF in the biogenesis and/or maintenance of this polyprotein complex. Harlequin mice with reduced AIF expression due to a retroviral insertion into the AIF gene also manifest a reduced oxidative phosphorylation (OXPHOS) in the retina and in the brain, correlating with reduced expression of complex I subunits, retinal degeneration, and neuronal defects. Altogether, these data point to a role of AIF in OXPHOS and emphasize the dual role of AIF in life and death.

Calcium, ATP, and ROS: a mitochondrial love-hate triangle

The mitochondrion is at the core of cellular energy metabolism, being the site of most ATP generation. Calcium is a key regulator of mitochondrial function and acts at several levels within the organelle to stimulate ATP synthesis. However, the dysregulation of mitochondrial Ca2+homeostasis is now recognized to play a key role in several pathologies. For example, mitochondrial matrix Ca2+overload can lead to enhanced generation of reactive oxygen species, triggering of the permeability transition pore, and cytochrome c release, leading to apoptosis. Despite progress regarding the independent roles of both Ca2+and mitochondrial dysfunction in disease, the molecular mechanisms by which Ca2+can elicit mitochondrial dysfunction remain elusive. This review highlights the delicate balance between the positive and negative effects of Ca2+and the signaling events that perturb this balance. Overall, a “two-hit” hypothesis is developed, in which Ca2+plus another pathological stimulus can bring about mitochondrial dysfunction.

The characterization of mitochondrial permeability transition in clonal pancreatic beta-cells. Multiple modes and regulation

Mitochondrial permeability transition (MPT), which contributes substantially to the regulation of normal mitochondrial metabolism, also plays a crucial role in the initiation of cell death. It is known that MPT is regulated in a tissue-specific manner. The importance of MPT in the pancreatic beta-cell is heightened by the fact that mitochondrial bioenergetics serve as the main glucose-sensing regulator and energy source for insulin secretion. In the present study, using MIN6 and INS-1 beta-cells, we revealed that both Ca(2+)-phosphate- and oxidant-induced MPT is remarkably different from other tissues. Ca(2+)-phosphate-induced transition is accompanied by a decline in mitochondrial reactive oxygen species production related to a significant potential dependence of reactive oxygen species formation in beta-cell mitochondria. Hydroperoxides, which are indirect MPT co-inducers active in liver and heart mitochondria, are inefficient in beta-cell mitochondria, due to the low mitochondrial ability to metabolize them. Direct cross-linking of mitochondrial thiols in pancreatic beta-cells induces the opening of a low conductance ion permeability of the mitochondrial membrane instead of the full scale MPT opening typical for liver mitochondria. Low conductance MPT is independent of both endogenous and exogenous Ca(2+), suggesting a novel type of nonclassical MPT in beta-cells. It results in the conversion of electrical transmembrane potential into DeltapH instead of a decrease in total protonmotive force, thus mitochondrial respiration remains in a controlled state. Both Ca(2+)- and oxidant-induced MPTs are phosphate-dependent and, through the "phosphate flush" (associated with stimulation of insulin secretion), are expected to participate in the regulation in beta-cell glucose-sensing and secretory activity.