Hydrogen peroxide production by monoamine oxidase in isolated rat-brain mitochondria: its effect on glutathione levels and Ca2+ efflux

Differential responses to UCP1 ablation in classical brown versus beige fat, despite a parallel increase in sympathetic innervation

In the cold, the absence of the mitochondrial uncoupling protein 1 (UCP1) results in hyper-recruitment of beige fat, but classical brown fat becomes atrophied. Here we examine possible mechanisms underlying this phenomenon. We confirm that in brown fat from UCP1-knockout (UCP1-KO) mice acclimated to the cold, the levels of mitochondrial respiratory chain proteins were diminished; however, in beige fat, the mitochondria seemed to be unaffected. The macrophages that accumulated massively not only in brown fat but also in beige fat of the UCP1-KO mice acclimated to cold did not express tyrosine hydroxylase, the norepinephrine transporter (NET) and monoamine oxidase-A (MAO-A). Consequently, they could not influence the tissues through the synthesis or degradation of norepinephrine. Unexpectedly, in the cold, both brown and beige adipocytes from UCP1-KO mice acquired an ability to express MAO-A. Adipose tissue norepinephrine was exclusively of sympathetic origin, and sympathetic innervation significantly increased in both tissues of UCP1-KO mice. Importantly, the magnitude of sympathetic innervation and the expression levels of genes induced by adrenergic stimulation were much higher in brown fat. Therefore, we conclude that no qualitative differences in innervation or macrophage character could explain the contrasting reactions of brown versus beige adipose tissues to UCP1-ablation. Instead, these contrasting responses may be explained by quantitative differences in sympathetic innervation: the beige adipose depot from the UCP1-KO mice responded to cold acclimation in a canonical manner and displayed enhanced recruitment, while the atrophy of brown fat lacking UCP1 may be seen as a consequence of supraphysiological adrenergic stimulation in this tissue.

Reactive oxygen species formation and its effect on CD4+ T cell-mediated inflammation

Reactive oxygen species (ROS) are produced both enzymatically and non-enzymatically in vivo. Physiological concentrations of ROS act as signaling molecules that participate in various physiological and pathophysiological activities and play an important role in basic metabolic functions. Diseases related to metabolic disorders may be affected by changes in redox balance. This review details the common generation pathways of intracellular ROS and discusses the damage to physiological functions when the ROS concentration is too high to reach an oxidative stress state. We also summarize the main features and energy metabolism of CD4+ T-cell activation and differentiation and the effects of ROS produced during the oxidative metabolism of CD4+ T cells. Because the current treatment for autoimmune diseases damages other immune responses and functional cells in the body, inhibiting the activation and differentiation of autoreactive T cells by targeting oxidative metabolism or ROS production without damaging systemic immune function is a promising treatment option. Therefore, exploring the relationship between T-cell energy metabolism and ROS and the T-cell differentiation process provides theoretical support for discovering effective treatments for T cell-mediated autoimmune diseases.

Inhibition of monoamine oxidase-a increases respiration in isolated mouse cortical mitochondria

Monoamine oxidase (MAO) is an enzyme located on the outer mitochondrial membrane that metabolizes amine substrates like serotonin, norepinephrine and dopamine. MAO inhibitors (MAOIs) are frequently utilized to treat disorders such as major depression or Parkinson's disease (PD), though their effects on brain mitochondrial bioenergetics are unclear. These studies measured bioenergetic activity in mitochondria isolated from the mouse cortex in the presence of inhibitors of either MAO-A, MAO-B, or both isoforms. We found that only 10 μM clorgyline, the selective inhibitor of MAO-A and not MAO-B, increased mitochondrial oxygen consumption rate in State V(CI) respiration compared to vehicle treatment. We then assessed mitochondrial bioenergetics, reactive oxygen species (ROS) production, and Electron Transport Chain (ETC) complex function in the presence of 0, 5, 10, 20, 40, or 80 μM of clorgyline to determine if this change was dose-dependent. The results showed increased oxygen consumption rates across the majority of respiration states in mitochondria treated with 5, 10, or 20 μM with significant bioenergetic inhibition at 80 μM clorgyline. Next, we assessed mitochondrial ROS production in the presence of the same concentrations of clorgyline in two different states: high mitochondrial membrane potential (ΔΨm) induced by oligomycin and low ΔΨm induced by carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP). There were no changes in ROS production in the presence of 5, 10, 20, or 40 μM clorgyline compared to vehicle after the addition of oligomycin or FCCP. There was a significant increase in mitochondrial ROS in the presence of 80 μM clorgyline after FCCP addition, as well as reduced Complex I and Complex II activities, which are consistent with inhibition of bioenergetics seen at this dose. There were no changes in Complex I, II, or IV activities in mitochondria treated with low doses of clorgyline. These studies shed light on the direct effect of MAO-A inhibition on brain mitochondrial bioenergetic function, which may be a beneficial outcome for those taking these medications.

Myocardial ischemia-reperfusion injury is probably due to the excessive production of mitochondrial ROS caused by the activation of 5-HT degradation system mediated by PAF receptor

AIM

Previously, we revealed a crucial role of 5-HT degradation system (5DS), consisting of 5-HT2A receptor (5-HT2AR), 5-HT synthases and monoamine oxidase A (MAO-A), in ischemia-reperfusion (IR)-caused organ injury. Whereas, platelet activating factor receptor (PAFR) also mediates myocardial ischemia-reperfusion injury (MIRI). Here, we try to clarify the relationship between 5DS and PAFR in mediating MIRI.

METHODS

H9c2 cell injury and rat MIRI were caused by hypoxia/reoxygenation (H/R) or PAF, and by ligating the left anterior descending coronary artery then untying, respectively. 5-HT_2AR and PAFR antagonists [sarpogrelate hydrochloride (SH) and BN52021], MAO-A, AKT, mTOR and 5-HT synthase inhibitors (clorgyline, perifosine, rapamycin and carbidopa), and gene-silencing PKCε were used in experiments RESULTS: The mitochondrial ROS production, respiratory chain damage, inflammation, apoptosis and myocardial infarction were significantly prevented by BN52021, SH and clorgyline in H/R and PAF-treated cells and in IR myocardium. BN52021 also significantly suppressed the upregulation of PAFR, 5-HT_2AR, 5-HT synthases and MAO-A expression (mRNA and protein), and Gα_q and PKCε (in plasmalemma) expression induced by H/R, PAF or IR; the effects of SH were similar to that of BN52021 except for no affecting the expression of PAFR and 5-HT_2AR. Gene-silencing PKCε suppressed H/R and PAF-induced upregulation of 5-HT synthases and MAO-A expression in cells; perifosine and rapamycin had not such effects; however, clorgyline suppressed H/R and PAF-induced phosphorylation of AKT and mTOR.

CONCLUSION

MIRI is probably due to PAFR-mediated 5-HT_2AR activation, which further activates PKCε-mediated 5-HT synthesis and degradation, leading to mitochondrial ROS production.

Melatonin and multiple sclerosis: antioxidant, anti-inflammatory and immunomodulator mechanism of action

BACKGROUND

Melatonin is an indole hormone secreted primarily by the pineal gland that showing anti-oxidant, anti-inflammatory and anti-apoptotic capacity. It can play an important role in the pathophysiological mechanisms of various diseases. In this regard, different studies have shown that there is a relationship between Melatonin and Multiple Sclerosis (MS). MS is a chronic immune-mediated disease of the Central Nervous System.

AIM

The objective of this review was to evaluate the mechanisms of action of melatonin on oxidative stress, inflammation and intestinal dysbiosis caused by MS, as well as its interaction with different hormones and factors that can influence the pathophysiology of the disease.

RESULTS

Melatonin causes a significant increase in the levels of catalase, superoxide dismutase, glutathione peroxidase, glutathione and can counteract and inhibit the effects of the NLRP3 inflammasome, which would also be beneficial during SARS-CoV-2 infection. In addition, melatonin increases antimicrobial peptides, especially Reg3β, which could be useful in controlling the microbiota.

CONCLUSION

Melatonin could exert a beneficial effect in people suffering from MS, running as a promising candidate for the treatment of this disease. However, more research in human is needed to help understand the possible interaction between melatonin and certain sex hormones, such as estrogens, to know the potential therapeutic efficacy in both men and women.

Role of 5-HT degradation in acute liver injury induced by carbon tetrachloride

5-hydroxytryptamine (5-HT) is involved in the pathological processes of several liver diseases. Acute liver injury underlies the development of many liver diseases, but the mechanism remains unclear. We aimed to investigate the role of 5-HT in carbon tetrachloride (CCl4)-induced acute liver injury. Acute liver injury was induced with CCl4 (10 mg/kg) in mice pretreated with the 5-HT2A receptor antagonist sarpogrelate hydrochloride (SH) and the 5-HT synthesis inhibitor carbidopa (CDP). LO2 cells were treated with CCl4, 5-HT or 2,5-dimethoxy-4-idopametamine and pretreated with SH, CDP or the monoamine oxidase A (MAO-A) inhibitor clorgyline. Hematoxylin-eosin staining, immunohistochemistry, Real-time quantitative PCR, western blotting, fluorescent probe and biochemical markers were used to evaluate liver compromise. 5-HT2A receptor, 5-HT synthetase and MAO-A were expressed in hepatocytes; their gene and protein expression were upregulated by CCl4, which led to the degradation of mitochondrial 5-HT and overproduction of reactive oxygen species (ROS). Hepatic injury may be aggravated by ROS, which induce oxidative stress and the phosphorylation of p38 mitogen-activated protein kinase, Jun N-terminal kinase, extracellular regulated protein kinase, signal transducer and activator of transcription 3 and nuclear factor kappa-B. 5-HT2A receptor may contribute to acute liver injury by modulating 5-HT synthase and MAO-A expression. The synergistic action of SH and CDP treatment may inhibit CCl4-induced acute liver injury in a dose-dependent manner. Hence, CCl4-induced acute liver injury is due to an increase in mitochondrial ROS production caused by increased 5-HT degradation and probably involves increases in 5-HT2A receptor expression and 5-HT synthesis.

Review: Using isolated mitochondria to investigate mitochondrial hydrogen peroxide metabolism

Mitochondria are recognized as centrally important to cellular reactive oxygen species (ROS), both as a potential source and due to their substantial antioxidant capacity. While much of the initial ROS formed by mitochondria is superoxide, this is rapidly converted to hydrogen peroxide (H₂O₂) which more readily crosses membranes making H₂O₂ important in both redox signalling mechanisms and conditions of oxidative stress. Here I outline our studies on mitochondrial H₂O₂ metabolism with a focus on some of the challenges and strategies involved with developing an integrated model of mitochondria being intrinsic regulators of H₂O₂. This view of mitochondria as regulators of H₂O₂ goes beyond the simpler contention of them being net producers or consumers. Moreover, the integration of both consumption and production can then be tied to a putative mechanism linking energy sensing at the level of the mitochondrial protonmotive force. This mechanism would provide a means of mitochondria communicating their energetic status the extramitochondrial compartment via local H₂O₂ concentrations. I conclude by explaining how these concepts developed using rodent muscle as a model have high relevance and applicability to comparative studies.

New 2-Pyrazoline and Hydrazone Derivatives as Potent and Selective Monoamine Oxidase A Inhibitors

Thirty compounds having 1-[2-(5-substituted-2-benzoxazolinone-3-yl) acetyl]-3,5-disubstitutedphenyl-2-pyrazoline structure and nine compounds having N'-(1,3-disubstitutedphenylallylidene)-2-(5-substituted-2-benzoxazolinone-3-yl)acetohydrazide skeleton were synthesized and evaluated as monoamine oxidase (MAO) inhibitors. All of the compounds exhibited selective MAO-A inhibitor activity in the nanomolar or low micromolar range. The results of the molecular docking for hydrazone derivatives supported the in vitro results. Five compounds, 6 (0.008 μM, Selectivity Index (SI): 9.70 × 10^-4), 7 (0.009 μM, SI: 4.55 × 10^-5), 14 (0.001 μM, SI: 8.00 × 10^-4), 21 (0.009 μM, SI: 1.37 × 10^-5), and 42 (0.010 μM, SI: 5.40 × 10^-6), exhibiting the highest inhibition and selectivity toward hMAO-A and nontoxic to hepatocytes were assessed for antidepressant activity as acute and subchronic in mice. All of these five compounds showed significant antidepressant activity with subchronic administration consistent with the increase in the brain serotonin levels and the compounds crossed the blood-brain barrier according to parallel artificial membrane permeation assay. Compounds 14, 21, and 42 exhibited an ex vivo MAO-A profile, which is highly consistent with the in vitro data.

Adrenaline induces calcium signal in astrocytes and vasoconstriction via activation of monoamine oxidase

Adrenaline or epinephrine is a hormone playing an important role in physiology. It is produced de-novo in the brain in very small amounts compared to other catecholamines, including noradrenaline. Although the effects of adrenaline on neurons have been extensively studied, much less is known about the action of this hormone on astrocytes. Here, we studied the effects of adrenaline on astrocytes in primary co-culture of neurons and astrocytes. Application of adrenaline induced calcium signal in both neurons and astrocytes, but only in neurons this effect was dependent on α- and β-receptor antagonists. The effects of adrenaline on astrocytes were less dependent on adrenoreceptors: the antagonist carvedilol had only moderate effect on the calcium signal and the agonist of adrenoreceptors methoxamine induced a signal only in small proportion of the cells. We found that adrenaline in astrocytes activates phospholipase C and subsequent release of calcium from the endoplasmic reticulum. Calcium signal in astrocytes is initiated by the metabolism of adrenaline by the monoamine oxidase (MAO), which activates reactive oxygen species production and induces lipid peroxidation. Inhibitor of MAO selegiline inhibited both adrenaline-induced calcium signal in astrocytes and the vasoconstriction that indicates an important role for monoamine oxidase in adrenaline-induced signalling and function.

Vasoconstrictor Mechanisms in Chronic Hypoxia-Induced Pulmonary Hypertension: Role of Oxidant Signaling

Elevated resistance of pulmonary circulation after chronic hypoxia exposure leads to pulmonary hypertension. Contributing to this pathological process is enhanced pulmonary vasoconstriction through both calcium-dependent and calcium sensitization mechanisms. Reactive oxygen species (ROS), as a result of increased enzymatic production and/or decreased scavenging, participate in augmentation of pulmonary arterial constriction by potentiating calcium influx as well as activation of myofilament sensitization, therefore mediating the development of pulmonary hypertension. Here, we review the effects of chronic hypoxia on sources of ROS within the pulmonary vasculature including NADPH oxidases, mitochondria, uncoupled endothelial nitric oxide synthase, xanthine oxidase, monoamine oxidases and dysfunctional superoxide dismutases. We also summarize the ROS-induced functional alterations of various Ca2+ and K+ channels involved in regulating Ca2+ influx, and of Rho kinase that is responsible for myofilament Ca2+ sensitivity. A variety of antioxidants have been shown to have beneficial therapeutic effects in animal models of pulmonary hypertension, supporting the role of ROS in the development of pulmonary hypertension. A better understanding of the mechanisms by which ROS enhance vasoconstriction will be useful in evaluating the efficacy of antioxidants for the treatment of pulmonary hypertension.

Thyroid state affects H2O2 removal by rat heart mitochondria

We investigated the effects of thyroid state on the mechanisms underlying rat heart mitochondrial capacity to remove H₂O₂ produced by an exogenous source. The removal rates were higher in the presence of respiratory substrates independently from thyroid state and were higher in hyperthyroid than in hypothyroid preparations. The thyroid state-linked changes in H₂O₂ removal rates, mirrored those in H₂O₂ release rates, showing that endogenous and exogenous H₂O₂ do not compete for the removing system. Mitochondrial content of coenzyme Q9 and Q10 was lower in hypothyroidism and higher in hyperthyroidism suggesting that the thyroid state-linked changes in the rates of H₂O₂ production are due to changes in the ubiquinone mitochondrial content. The rates of H₂O₂ removal in the presence of antioxidant enzyme inhibitors indicated that the contribution of each antioxidant is dependent on the thyroid state. This was supported by enzymatic activity measurements. Pharmacological inhibition also showed that the overall percentage contribution of the enzymatic processes, as well as that of non-enzymatic processes, is not affected by thyroid state. Cytochrome levels, inferred by light emission measurements, and western blot determination of cytochrome c, were lower in hypothyroid and higher in hyperthyroid preparations supporting the idea that the levels of reducing compounds were modified in opposite way by the changes in thyroid state. Further support was obtained showing that the whole antioxidant capacity, which provides an evaluation of capacity of the systems, different from cytochromes, assigned to H₂O₂ scavenging, was lower in hyperthyroid than in hypothyroid state.

Impact of a novel homozygous mutation in nicotinamide nucleotide transhydrogenase on mitochondrial DNA integrity in a case of familial glucocorticoid deficiency

BACKGROUND

Familial Glucocorticoid Deficiency (FGD) is a rare autosomal recessive disorder that is characterized by isolated glucocorticoid deficiency. Recently, mutations in the gene encoding for the mitochondrial nicotinamide nucleotide transhydrogenase (NNT) have been identified as a causative gene for FGD; however, no NNT activities have been reported in FGD patients carrying NNT mutations.

METHODS

Clinical, biochemical and molecular analyses of lymphocytes from FDG homozygous and heterozygous carriers for the F215S NNT mutation.

RESULTS

In this study, we described an FGD-affected Japanese patient carrying a novel NNT homozygous mutation (c.644T>C; F215S) with a significant loss-of-function (NNT activity = 31% of healthy controls) in peripheral blood cells' mitochondria. The NNT activities of the parents, heterozygous for the mutation, were 61% of controls.

CONCLUSIONS

Our results indicated that (i) mitochondrial biogenesis (citrate synthase activity) and/or mtDNA replication (mtDNA copy number) were affected at ≤60% NNT activity because these parameters were affected in individuals carrying either one or both mutated alleles; and (ii) other outcomes (mtDNA deletions, protein tyrosine nitration, OXPHOS capacity) were affected at ≤30% NNT activity as also observed in murine cerebellar mitochondria from C57BL/6J (NNT^-/-) vs. C57BL/6JN (NNT^+/+) substrains.

GENERAL SIGNIFICANCE

By studying a family affected with a novel point mutation in the NNT gene, a gene-dose response was found for various mitochondrial outcomes providing for novel insights into the role of NNT in the maintenance of mtDNA integrity beyond that described for preventing oxidative stress.

H2O2 release from the very long chain acyl-CoA dehydrogenase

Enhanced mitochondrial generation of oxidants, including hydrogen peroxide (H₂O₂), is related to a large number of pathological conditions, including diet-induced obesity and steatohepatosis. Indeed, we have previously shown that high fat diets increase the generation of H₂O₂ in liver mitochondria energized by activated fatty acids. Here, we further study fatty-acid induced H₂O₂ release in liver mitochondria, and determine the characteristics that regulate it. We find that this production of H₂O₂ is independent of mitochondrial inner membrane integrity and insensitive to purine nucleotides. On the other hand, palmitate-induced H₂O₂ production is strongly enhanced by high fat diets and is pH-sensitive, with a peak at a matrix pH of ~8.5. Using recombinantly expressed human very long chain acyl-CoA dehydrogenase, we are able to demonstrate that palmitate-induced H₂O₂ release may be ascribed to the activity of this enzyme alone, acting as an oxidase. Our results add to a number of findings indicating that sources outside of the electron transport chain can generate significant, physiopathologically relevant, amounts of oxidants in mitochondria.

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High fat diets increase H₂O₂ in palmitoyl CoA-energized liver mitochondria.

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H₂O₂ release is pH-sensitive, with a peak at a matrix pH of ~8.5.

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Recombinantly expressed purified human VLCAD releases H₂O₂, acting as an oxidase.

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VLCAD presents residual oxidase activity, generating significant H₂O₂ in mitochondria.

Enhanced hydrogen peroxide generation accompanies the beneficial bioenergetic effects of methylene blue in isolated brain mitochondria

The redox dye methylene blue (MB) is proven to have beneficial effects in various models of neurodegenerative diseases. Here we investigated the effects of MB (100 nM, 300 nM, and 1 μM) on key bioenergetic parameters and on H2O2 production/elimination in isolated guinea pig brain mitochondria under normal as well as respiration-impaired conditions. As measured by high-resolution Oxygraph the rate of resting oxygen consumption was increased, but the ADP-stimulated respiration was unaffected by MB with any of the substrates (glutamate malate, succinate, or α-glycerophosphate) used for supporting mitochondrial respiration. In mitochondria treated with inhibitors of complex I or complex III MB moderately but significantly increased the rate of ATP production, restored ΔΨm, and increased the rate of Ca(2+) uptake. The effects of MB are consistent with transferring electrons from upstream components of the electron transport chain to cytochrome c, which is energetically favorable when the flow of electrons in the respiratory chain is compromised. On the other hand, MB significantly increased the production of H2O2 measured by Amplex UltraRed fluorimetry under all conditions, in resting, ATP-synthesizing, and respiration-impaired mitochondria, with each substrate combination supporting respiration. Furthermore, it also decreased the elimination of H2O2. Generation of H2O2 without superoxide formation, observed in the presence of MB, is interpreted as a result of reduction of molecular oxygen to H2O2 by the reduced MB. The elevated generation and impaired elimination of H2O2 should be considered for the overall oxidative state of mitochondria treated with MB.

Mitochondrial reactive oxygen species (ROS) and ROS-induced ROS release

Byproducts of normal mitochondrial metabolism and homeostasis include the buildup of potentially damaging levels of reactive oxygen species (ROS), Ca(2+), etc., which must be normalized. Evidence suggests that brief mitochondrial permeability transition pore (mPTP) openings play an important physiological role maintaining healthy mitochondria homeostasis. Adaptive and maladaptive responses to redox stress may involve mitochondrial channels such as mPTP and inner membrane anion channel (IMAC). Their activation causes intra- and intermitochondrial redox-environment changes leading to ROS release. This regenerative cycle of mitochondrial ROS formation and release was named ROS-induced ROS release (RIRR). Brief, reversible mPTP opening-associated ROS release apparently constitutes an adaptive housekeeping function by the timely release from mitochondria of accumulated potentially toxic levels of ROS (and Ca(2+)). At higher ROS levels, longer mPTP openings may release a ROS burst leading to destruction of mitochondria, and if propagated from mitochondrion to mitochondrion, of the cell itself. The destructive function of RIRR may serve a physiological role by removal of unwanted cells or damaged mitochondria, or cause the pathological elimination of vital and essential mitochondria and cells. The adaptive release of sufficient ROS into the vicinity of mitochondria may also activate local pools of redox-sensitive enzymes involved in protective signaling pathways that limit ischemic damage to mitochondria and cells in that area. Maladaptive mPTP- or IMAC-related RIRR may also be playing a role in aging. Because the mechanism of mitochondrial RIRR highlights the central role of mitochondria-formed ROS, we discuss all of the known ROS-producing sites (shown in vitro) and their relevance to the mitochondrial ROS production in vivo.

Short term exposure to multi-walled carbon nanotubes induce oxidative stress and DNA damage in Xenopus laevis tadpoles

The potential impact of Multiwalled Carbon NanoTubes (MWCNTs) was investigated on Xenopus laevis tadpoles exposed to 0.1, 1 and 10mg/L. Oxidative stress was measured in entire larvae exposed and DNA damage (Comet assay) was carried out in erythrocytes of circulating blood from 2h to 24h according to standardized recommendations. Results showed significant H2O2 production when larvae were exposed to 1mg/L and 10mg/L of MWCNTs after 4h and 2h of exposure, respectively. Antioxidant enzyme activities showed significant induction of catalase (CAT), glutathione reductase (GR) and superoxide dismutase (SOD) from only 2h of exposure to 10mg/L of MWCNTs. In presence of 1mg/L of MWCNTs, only GR and CAT activities were significantly induced at 4h. Enzyme activities do not follow a simple dose-effect relation, but the time of induction is shortened in relation with the tested concentration. The Comet assay results showed significant DNA damages with a dose dependent response. The profiles of DNA damages show fluctuations, in course of time, which are characteristics of oxidative stress response in relation with the continuous balance between damage and compensation process.

The protective role of Bax inhibitor-1 against chronic mild stress through the inhibition of monoamine oxidase A

The anti-apoptotic protein Bax inhibitor-1 (BI-1) is a regulator of apoptosis linked to endoplasmic reticulum (ER) stress. It has been hypothesized that BI-1 protects against neuron degenerative diseases. In this study, BI-1⁻/⁻ mice showed increased vulnerability to chronic mild stress accompanied by alterations in the size and morphology of the hippocampi, enhanced ROS accumulation and an ER stress response compared with BI-1⁺/⁺ mice. BI-1⁻/⁻ mice exposed to chronic mild stress showed significant activation of monoamine oxidase A (MAO-A), but not MAO-B, compared with BI-1⁺/⁺ mice. To examine the involvement of BI-1 in the Ca²⁺-sensitive MAO activity, thapsigargin-induced Ca²⁺ release and MAO activity were analyzed in neuronal cells overexpressing BI-1. The in vitro study showed that BI-1 regulates Ca²⁺ release and related MAO-A activity. This study indicates an endogenous protective role of BI-1 under conditions of chronic mild stress that is primarily mediated through Ca²⁺-associated MAO-A regulation.

Classification of H₂O₂as a neuromodulator that regulates striatal dopamine release on a subsecond time scale

Here we review evidence that the reactive oxygen species, hydrogen peroxide (H(2)O(2)), meets the criteria for classification as a neuromodulator through its effects on striatal dopamine (DA) release. This evidence was obtained using fast-scan cyclic voltammetry to detect evoked DA release in striatal slices, along with whole-cell and fluorescence imaging to monitor cellular activity and H(2)O(2) generation in striatal medium spiny neurons (MSNs). The data show that (1) exogenous H(2)O(2) suppresses DA release in dorsal striatum and nucleus accumbens shell and the same effect is seen with elevation of endogenous H(2)O(2) levels; (2) H(2)O(2) is generated downstream from glutamatergic AMPA receptor activation in MSNs, but not DA axons; (3) generation of modulatory H(2)O(2) is activity dependent; (4) H(2)O(2) generated in MSNs diffuses to DA axons to cause transient DA release suppression by activating ATP-sensitive K(+) (K(ATP)) channels on DA axons; and (5) the amplitude of H(2)O(2)-dependent inhibition of DA release is attenuated by enzymatic degradation of H(2)O(2), but the subsecond time course is determined by H(2)O(2) diffusion rate and/or K(ATP)-channel kinetics. In the dorsal striatum, neuromodulatory H(2)O(2) is an intermediate in the regulation of DA release by the classical neurotransmitters glutamate and GABA, as well as other neuromodulators, including cannabinoids. However, modulatory actions of H(2)O(2) occur in other regions and cell types, as well, consistent with the widespread expression of K(ATP) and other H(2)O(2)-sensitive channels throughout the CNS.

Effects of the thyroid hormone derivatives 3-iodothyronamine and thyronamine on rat liver oxidative capacity

Thyronamines T(0)AM and T(1)AM are naturally occurring decarboxylated thyroid hormone derivatives. Their in vivo administration induces effects opposite to those induced by thyroid hormone, including lowering of body temperature. Since the mitochondrial energy-transduction apparatus is known to be a potential target of thyroid hormone and its derivatives, we investigated the in vitro effects of T(0)AM and T(1)AM on the rates of O(2) consumption and H(2)O(2) release by rat liver mitochondria. Hypothyroid animals were used because of the low levels of endogenous thyronamines. We found that both compounds are able to reduce mitochondrial O(2) consumption and increase H(2)O(2) release. The observed changes could be explained by a partial block, operated by thyronamines, at a site located near the site of action of antimycin A. This hypothesis was confirmed by the observation that thyronamines reduced the activity of Complex III where the site of antimycin action is located. Because thyronamines exerted their effects at concentrations comparable to those found in hepatic tissue, it is conceivable that they can affect in vivo mitochondrial O(2) consumption and H(2)O(2) production acting as modulators of thyroid hormone action.

H2O2: a dynamic neuromodulator

Increasing evidence implicates hydrogen peroxide (H(2)O(2)) as an intracellular and intercellular signaling molecule that can influence processes from embryonic development to cell death. Most research has focused on relatively slow signaling, on the order of minutes to days, via second messenger cascades. However, H(2)O(2) can also mediate subsecond signaling via ion channel activation. This rapid signaling has been examined most thoroughly in the nigrostriatal dopamine (DA) pathway, which plays a key role in facilitating movement mediated by the basal ganglia. In DA neurons of the substantia nigra, endogenously generated H(2)O(2) activates ATP-sensitive K(+) (K-ATP) channels that inhibit DA neuron firing. In the striatum, H(2)O(2) generated downstream from glutamatergic AMPA receptor activation in medium spiny neurons acts as a diffusible messenger that inhibits axonal DA release, also via K-ATP channels. The source of dynamically generated H(2)O(2) is mitochondrial respiration; thus, H(2)O(2) provides a novel link between activity and metabolism via K-ATP channels. Additional targets of H(2)O(2) include transient receptor potential (TRP) channels. In contrast to the inhibitory effect of H(2)O(2) acting via K-ATP channels, TRP channel activation is excitatory. This review describes emerging roles of H(2)O(2) as a signaling agent in the nigrostriatal pathway and basal ganglia neurons.

Reversible inhibition of hydrogen peroxide elimination by calcium in brain mitochondria

In the present work, the Ca(2+) dependence of mitochondrial H(2) O(2) elimination was investigated. Mitochondria isolated from guinea pig brain were energized by glutamate and malate and incubated with micromolar concentrations of Ca(2+) in the presence of ADP, preventing permeability transition pore formation. After the completion of Ca(2+) uptake, mitochondria were challenged with H(2) O(2) (5 μM), then at various time points residual H(2) O(2) was determined using the Amplex red method and compared with that in mitochondria incubated with H(2) O(2) without Ca(2+) addition. Dose-dependent inhibition of H(2) O(2) elimination by Ca(2+) was detected, which was prevented by the Ca(2+) -uptake inhibitor Ru 360. Stimulation of Ca(2+) release from Ca(2+) -loaded mitochondria by a combined addition of Ru 360 and Na(+) decreased the Ca(2+) -evoked inhibition of H(2) O(2) removal. After Ca(2+) uptake (50 μM), mitochondrial aconitase activity was found to be decreased, which was partially attributable to the impaired elimination of endogenously produced reactive oxygen species. We found that the effects of Ca(2+) and H(2) O(2) on the activity of aconitase were additive. These results confirm that Ca(2+) inhibits elimination of H(2) O(2) in mitochondria and demonstrate that this effect is concentration dependent and reversible. The phenomenon described here can play a role in the modulation of ROS handling under conditions involving excessive cellular Ca(2+) load.

Diminished NADPH transhydrogenase activity and mitochondrial redox regulation in human failing myocardium

Although the functional role of nicotinamide nucleotide transhydrogenase (Nnt) remains to be fully elucidated, there is strong evidence that Nnt plays a critical part in mitochondrial metabolism by maintaining a high NADPH-dependent GSH/GSSG ratio, and thus the control of cellular oxidative stress. Using real-time PCR, spectrophotometric and western blotting techniques, we sought to determine the presence, abundance and activity level of Nnt in human heart tissues and to discern whether these are altered in chronic severe heart failure. Left ventricular levels of the NNT gene and protein expression did not differ significantly between the non-failing donor (NF) and heart failure (HF) group. Notably, compared to NF, Nnt activity rates in the HF group were 18% lower, which coincided with significantly higher levels of oxidized glutathione, lower glutathione reductase activity, lower NADPH and a lower GSH/GSSG ratio. In the failing human heart a partial loss of Nnt activity adversely impacts NADPH-dependent enzymes and the capacity to maintain membrane potential, thus contributing to a decline in bioenergetic capacity, redox regulation and antioxidant defense, exacerbating oxidative damage to cellular proteins.

Redox compartmentalization in eukaryotic cells

Diverse functions of eukaryotic cells are optimized by organization of compatible chemistries into distinct compartments defined by the structures of lipid-containing membranes, multiprotein complexes and oligomeric structures of saccharides and nucleic acids. This structural and chemical organization is coordinated, in part, through cysteine residues of proteins which undergo reversible oxidation-reduction and serve as chemical/structural transducing elements. The central thiol/disulfide redox couples, thioredoxin-1, thioredoxin-2, GSH/GSSG and cysteine/cystine (Cys/CySS), are not in equilibrium with each other and are maintained at distinct, non-equilibrium potentials in mitochondria, nuclei, the secretory pathway and the extracellular space. Mitochondria contain the most reducing compartment, have the highest rates of electron transfer and are highly sensitive to oxidation. Nuclei also have more reduced redox potentials but are relatively resistant to oxidation. The secretory pathway contains oxidative systems which introduce disulfides into proteins for export. The cytoplasm contains few metabolic oxidases and this maintains an environment for redox signaling dependent upon NADPH oxidases and NO synthases. Extracellular compartments are maintained at stable oxidizing potentials. Controlled changes in cytoplasmic GSH/GSSG redox potential are associated with functional state, varying with proliferation, differentiation and apoptosis. Variation in extracellular Cys/CySS redox potential is also associated with proliferation, cell adhesion and apoptosis. Thus, cellular redox biology is inseparable from redox compartmentalization. Further elucidation of the redox control networks within compartments will improve the mechanistic understanding of cell functions and their disruption in disease.

Trace amine-associated receptor 1-Family archetype or iconoclast?

Interest has recently been rekindled in receptors that are activated by low molecular weight, noncatecholic, biogenic amines that are typically found as trace constituents of various vertebrate and invertebrate tissues and fluids. The timing of this resurgent focus on receptors activated by the "trace amines" (TA) beta-phenylethylamine (PEA), tyramine (TYR), octopamine (OCT), synephrine (SYN), and tryptamine (TRYP) is the direct result of 2 publications that appeared in 2001 describing the cloning of a novel G protein-coupled receptor (GPCR) referred to by their discoverers Borowsky et al. as TA1 and Bunzow et al. as TA receptor 1 (TAR1). When heterologously expressed in Xenopus laevis oocytes and various eukaryotic cell lines, recombinant rodent and human TAR dose-dependently couple to the stimulation of adenosine 3',5'-monophosphate (cAMP) production. Structure-activity profiling based on this functional response has revealed that in addition to the TA, other biologically active compounds containing a 2-carbon aliphatic side chain linking an amino group to at least 1 benzene ring are potent and efficacious TA receptor agonists with amphetamine (AMPH), methamphetamine, 3-iodothyronamine, thyronamine, and dopamine (DA) among the most notable. Almost 100 years after the search for TAR began, numerous TA1/TAR1-related sequences, now called TA-associated receptors (TAAR), have been identified in the genome of every species of vertebrate examined to date. Consequently, even though heterologously expressed TAAR1 fits the pharmacological criteria established for a bona fide TAR, a major challenge for those working in the field is to discern the in vivo pharmacology and physiology of each purported member of this extended family of GPCR. Only then will it be possible to establish whether TAAR1 is the family archetype or an iconoclast.

Targeting antioxidants to mitochondria by conjugation to lipophilic cations

Mitochondrial oxidative damage contributes to a range of degenerative diseases. Consequently, the selective inhibition of mitochondrial oxidative damage is a promising therapeutic strategy. One way to do this is to invent antioxidants that are selectively accumulated into mitochondria within patients. Such mitochondria-targeted antioxidants have been developed by conjugating the lipophilic triphenylphosphonium cation to an antioxidant moiety, such as ubiquinol or alpha-tocopherol. These compounds pass easily through all biological membranes, including the blood-brain barrier, and into muscle cells and thus reach those tissues most affected by mitochondrial oxidative damage. Furthermore, because of their positive charge they are accumulated several-hundredfold within mitochondria driven by the membrane potential, enhancing the protection of mitochondria from oxidative damage. These compounds protect mitochondria from damage following oral delivery and may therefore form the basis for mitochondria-protective therapies. Here we review the background and work to date on this class of mitochondria-targeted antioxidants.

H2O2 signaling in the nigrostriatal dopamine pathway via ATP-sensitive potassium channels: issues and answers

The role of reactive oxygen species (ROS) as signaling agents is increasingly appreciated. Studies of ROS functions in the central nervous system, however, are only in their infancy. Using fast-scan cyclic voltammetry and fluorescence imaging in brain slices, the authors discovered that hydrogen peroxide (H2O2) is an endogenous regulator of dopamine release in the dorsal striatum. Given the key role of dopamine in motor, reward, and cognitive pathways, regulation by H2O2 has implications for normal dopamine function, as well as for dysfunction of dopamine transmission. In this review, data are summarized to show that H2O2 is a diffusible messenger in the striatum, generated downstream from glutamate receptor activation, and an intracellular signal in dopamine neurons of the substantia nigra, generated during normal pacemaker activity. The mechanism by which H2O2 inhibits dopamine release and dopamine cell activity is activation of ATP-sensitive K+ (KATP) channels. Characteristics of the neuronal and glial antioxidant networks required to permit H2O2 signaling, yet prevent oxidative damage, are also considered. Lastly, estimates of physiological H2O2 levels are discussed, and strengths and limitations of currently available methods for H2O2 detection, including fluorescence imaging using dichlorofluorescein (DCF) and the next generation of fluorescent probes, are considered.

Mitochondrial retrograde regulation in plants

Plant cells must react to a variety of adverse environmental conditions that they may experience on a regular basis. Part of this response centers around (1) ROS as damaging molecules and signaling molecules; (2) redox status, which can be influenced by ROS production; and (3) availability of metabolites. All of these are also likely to interface with changes in hormone levels [Desikan, R., Hancock, J., Neill, S., 2005. Reactive oxygen species as signalling molecules. In: Smirnoff, N. (ed.), Antioxidants and reactive oxygen species in plants. Blackwell Pub. Ltd., Oxford, pp. 169-196; Kwak, J.M., Nguyen, V., Schroeder, J.I., 2006. The role of reactive oxygen species in hormonal responses. Plant Physiol. 141, 323-329]. Each of these areas can be strongly influenced by changes in mitochondrial function. Such changes trigger altered nuclear gene expression by a poorly understood process of mitochondrial retrograde regulation (MRR), which is likely composed of several distinct signaling pathways. Much of what is known about plant MRR centers around the response to a dysfunctional mtETC and subsequent induction of genes encoding proteins involved in recovery of mitochondrial functions, such as AOX and alternative NAD(P)H dehydrogenases, and genes encoding enzymes aimed at regaining ROS level/redox homeostasis, such as glutathione transferases, catalases, ascorbate peroxidases and superoxide dismutases. However, as evidence of new and interesting targets of MRR emerge, this picture is likely to change and the complexity and importance of MRR in plant responses to stresses and the decision for cells to either recover or switch into programmed cell death mode is likely to become more apparent.

Altered expression of a limited number of genes contributes to cardiac decompensation during chronic ventricular tachypacing in dogs

Our aim was to determine the changes in the gene expression profile occurring during the transition from compensated dysfunction (CD) to decompensated heart failure (HF) in pacing-induced dilated cardiomyopathy. Twelve chronically instrumented dogs underwent left ventricular pacing at 210 beats/min for 3 wk and at 240 beats thereafter, and four normal dogs were used as control. The transition from CD to HF occurred between the 3rd and 4th wk of pacing, with end-stage HF at 28 +/- 1 days. RNA was extracted from left ventricular tissue at control and 3 and 4 wk of pacing (n = 4) and tested with the Affymetrix Canine Array. We found 509 genes differentially expressed in CD vs. control (P < or = 0.05, fold change > or = +/-2), with 362 increasing and 147 decreasing; 526 genes were differentially expressed in HF vs. control (P < or = 0.05; fold change > or = +/-2), with 439 increasing and 87 decreasing. To better understand the transition, we compared gene alterations at 3 vs. 4 wk pacing and found that only 30 genes differed (P < or = 0.05; fold change of +/-2). We conclude that a number of processes including normalization of gene regulation during decompensation, appearance of new upregulated genes and maintenance of gene expression all contribute to the transition to overt heart failure with an unexpectedly small number of genes differentially regulated.

Monoamine oxidase inhibitors l-deprenyl and clorgyline protect nonmalignant human cells from ionising radiation and chemotherapy toxicity

l-Deprenyl (R-(−)-deprenyl, selegiline) is an inhibitor of monoamine oxidase-B (MAO-B) that is known to protect nerve cells from a variety of chemical and physical insults. As apoptosis is a common mechanism of radiation-induced cell death, the effect of l-deprenyl on the survival of cultured cells and tissue explants was studied following exposure to gamma radiation. The results obtained were compared with the effects of the less-selective MAO-B inhibitor pargyline and the MAO-A inhibitor clorgyline. l-Deprenyl at a concentration of 10⁻⁹ M protected the nontumorigenic cell line (HaCaT) and normal human urothelial explants from the effects of cobalt-60 gamma radiation, but did not protect tumorigenic human cell lines HaCaT-ras, HPV-transfected human keratinocytes (HPV-G cells), or PC3. Human bladder carcinoma explants were not protected. Clorgyline showed a smaller protective effect of normal cells, whereas pargyline had no effect. Radiation-induced delayed effects (genomic instability measured as delayed cell death) were prevented in normal cells by l-deprenyl but, interestingly, deprenyl appeared to increase the amount of delayed death in the tumorigenic cell lines. Studies using l-deprenyl prior to the exposure of nonmalignant cells to cisplatin showed that cell death due to this agent was also reduced. Treatment of cultures of nontumorigenic cells with l-deprenyl or clorgyline significantly increased the levels of the protein Bcl-2 following irradiation, but there was no such effect on the already-elevated levels of this protein in the tumour samples. Since the Bcl-2 has been shown to be an inhibitor of apoptosis or programmed cell death, this would imply that the protective effects of l-deprenyl and clorgyline involve activation of antiapoptotic pathways within the normal cell. This hypothesis is supported by data showing reduced levels of apoptosis in HaCAT cells and in normal bladder explant cultures following treatment with l-deprenyl.

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

In brain mitochondria, state 4 respiration supported by the NAD-linked substrates glutamate/malate in the presence of EGTA promotes a high rate of exogenous H2O2 removal. Omitting EGTA decreases the H2O2 removal rate by almost 80%. The decrease depends on the influx of contaminating Ca2+, being prevented by the Ca2+ uniporter inhibitor ruthenium red. Arsenite is also an inhibitor (maximal effect approximately 40%, IC50, 12 microm). The H2O2 removal rate (EGTA present) is decreased by 20% during state 3 respiration and by 60-70% in fully uncoupled conditions. H2O2 removal in mitochondria is largely dependent on glutathione peroxidase and glutathione reductase. Both enzyme activities, as studied in disrupted mitochondria, are inhibited by Ca2+. Glutathione reductase is decreased by 70% with an IC50 of about 0.9 microm, and glutathione peroxidase is decreased by 38% with a similar IC50. The highest Ca2+ effect with glutathione reductase is observed in the presence of low concentrations of H2O2. With succinate as substrate, the removal is 50% less than with glutamate/malate. This appears to depend on succinate-supported production of H2O2 by reverse electron flow at NADH dehydrogenase competing with exogenous H2O2 for removal. Succinate-dependent H2O2 is inhibited by rotenone, decreased DeltaPsi, as described previously, and by ruthenium red and glutamate/malate. These agents also increase the measured rate of exogenous H2O2 removal with succinate. Succinate-dependent H2O2 generation is also inhibited by contaminating Ca2+. Therefore, Ca2+ acts as an inhibitor of both H2O2 removal and the succinate-supported H2O2 production. It is concluded that mitochondria function as intracellular Ca2+-modulated peroxide sinks.

Brain antioxidant regulation in mammals and anoxia-tolerant reptiles: balanced for neuroprotection and neuromodulation

Reactive oxygen species (ROS) generated by mitochondrial respiration and other processes are often viewed as hazardous substances. Indeed, oxidative stress, defined as an imbalance between oxidant production and antioxidant protection, has been linked to several neurological disorders, including cerebral ischemia-reperfusion and Parkinson's disease. Consequently, cells and organisms have evolved specialized antioxidant defenses to balance ROS production and prevent oxidative damage. Research in our laboratory has shown that neuronal levels of ascorbate, a low molecular weight antioxidant, are ten-fold higher than those in much less metabolically active glial cells. Ascorbate levels are also selectively elevated in the CNS of anoxia-tolerant reptiles compared to mammals; moreover, plasma and CSF ascorbate concentrations increase markedly in cold-adapted turtles and in hibernating squirrels. Levels of the related antioxidant, glutathione, vary much less between neurons and glia or among species. An added dimension to the role of the antioxidant network comes from recent evidence that ROS can act as neuromodulators. One example is modulation of dopamine release by endogenous hydrogen peroxide, which we describe here for several mammalian species. Together, these data indicate adaptations that prevent oxidative stress and suggest a particularly important role for ascorbate. Moreover, they show that the antioxidant network must be balanced precisely to provide functional levels of ROS, as well as neuroprotection.

Regulation of JNK/ERK activation, cell apoptosis, and tissue regeneration by monoamine oxidases after renal ischemia-reperfusion

Reactive oxygen species (ROS) contribute to the ischemia-reperfusion injury. In kidney, the intracellular sources of ROS during ischemia-reperfusion are still unclear. In the present study, we investigated the role of the catecholamine-degrading enzyme monoamine oxidases (MAOs) in hydrogen peroxide (H2O2) generation after reperfusion and their involvement in cell events leading to tissue injury and recovery. In a rat model of renal ischemia-reperfusion, we show concomitant MAO-dependent H2O2 production and lipid peroxidation in the early reperfusion period. Rat pretreatment with the irreversible MAO inhibitor pargyline resulted in the following: i) prevented H2O2 production and lipid peroxidation; ii) decreased tubular cell apoptosis and necrosis, measured by TUNEL staining and histomorphological criteria; and iii) increased tubular cell proliferation as determined by proliferating cell nuclear antigen expression. MAO inhibition also prevented Jun N-terminal kinase phosphorylation and promoted extracellular signal-regulated kinase activation, two mitogen-activated protein kinases described as a part of a "death" and "survival" pathway after ischemia-reperfusion. This work demonstrates the crucial role of MAOs in mediating the production of injurious ROS, which contribute to acute apoptotic and necrotic cell death induced by renal ischemia-reperfusion in vivo. Targeted inhibition of these oxidases could provide a new avenue for therapy to prevent renal damage and promote renal recovery after ischemia-reperfusion.

Phenylethylamine induces an increase in cytosolic Ca2+ in yeast

Beta-phenylethylamine (PEA) induced an increase in cytosolic free calcium ion concentration ([Ca2+]c) in Saccharomyces cerevisiae cells monitored with transgenic aequorin, a Ca2+-dependent photoprotein. The PEA-induced [Ca2+]c increase was dependent on the concentrations of PEA applied, and the Ca2+ mostly originated from an extracellular source. Preceding the Ca2+ influx, H2O2 was generated in the cells by the addition of PEA. Externally added H2O2 also induced a [Ca2+]c increase. These results suggest that PEA induces the [Ca2+]c increase via H2O2 generation. The PEA-induced [Ca2+]c increase occurred in the mid1 mutant with a slightly smaller peak than in the wild-type strain, indicating that Mid1, a stretch-activated nonselective cation channel, may not be mainly involved in the PEA-induced Ca2+ influx. When PEA was applied, the MATa mid1 mutant was rescued from alpha-factor-induced death in a Ca2+-limited medium, suggesting that the PEA-induced [Ca2+]c increase can reinforce calcium signaling in the mating pheromone response pathway.

H(2)O(2) is a novel, endogenous modulator of synaptic dopamine release

Recent evidence suggests that reactive oxygen species (ROS) might act as modulators of neuronal processes, including synaptic transmission. Here we report that synaptic dopamine (DA) release can be modulated by an endogenous ROS, H(2)O(2). Electrically stimulated DA release was monitored in guinea pig striatal slices using carbon-fiber microelectrodes with fast-scan cyclic voltammetry. Exogenously applied H(2)O(2) reversibly inhibited evoked release in the presence of 1.5 mM Ca(2+). The effectiveness of exogenous H(2)O(2), however, was abolished or decreased by conditions that enhance Ca(2+) entry, including increased extracellular Ca(2+) concentration ([Ca(2+)](o); to 2.4 mM), brief, high-frequency stimulation, and blockade of inhibitory D(2) autoreceptors. To test whether DA release could be modulated by endogenous H(2)O(2), release was evoked in the presence of the H(2)O(2)-scavenging enzyme, catalase. In the presence of catalase, evoked [DA](o) was 60% higher than after catalase washout, demonstrating that endogenously generated H(2)O(2) can also inhibit DA release. Importantly, the Ca(2+) dependence of the catalase-mediated effect was opposite to that of H(2)O(2): catalase had a greater enhancing effect in 2.4 mM Ca(2+) than in 1.5 mM, consistent with enhanced H(2)O(2) generation in higher [Ca(2+)](o). Together these data suggest that H(2)O(2) production is Ca(2+) dependent and that the inhibitory mechanism can be saturated, thus preventing further effects from exogenous H(2)O(2). These findings show for the first time that endogenous H(2)O(2) can modulate vesicular neurotransmitter release, thus revealing an important new signaling role for ROS in synaptic transmission.

Aromatic monoamine-induced immediate oxidative burst leading to an increase in cytosolic Ca2+ concentration in tobacco suspension culture

Aromatic monoamines may contribute to both chemical and physical protection of plants. Addition of phenylethylamine (PEA) and benzylamine to tobacco suspension culture (cell line BY-2) induced a very rapid and transient generation of two active oxygen species (AOS), H2O2 and superoxide anion, both detected with chemiluminescence. Electron spin resonance spectroscopy revealed that hydroxy radicals are also produced. With laser-scanning confocal microscopy, fluorescence spectroscopy and microplate fluorescence reading, intracellular H2O2 production was detected using dichlorofluorescin diacetate as a fluorescent probe. Following AOS production, cytosolic Ca2+ concentration ([Ca2+]c) of the tobacco cells, monitored with luminescence of transgenic aequorin, increased and attained to a peak level 12 s after PEA addition. The PEA-induced increase in [Ca2+]c was inhibited by a Ca2+ chelator, Ca2+ antagonists and AOS scavengers, suggesting that PEA-induced AOS triggered a Ca2+ influx across the plasma membrane.

Phenylethylamine-induced generation of reactive oxygen species and ascorbate free radicals in tobacco suspension culture: mechanism for oxidative burst mediating Ca2+ influx

In the previous paper [Kawano et al. (2000a) Plant Cell Physiol. 41: 1251], we demonstrated that addition of phenylethylamine (PEA) and benzylamine can induce an immediate and transient burst of active oxygen species (AOS) in tobacco suspension culture. Detected AOS include H2O2, superoxide anion and hydroxyl radicals. Use of several inhibitors suggested the presence of monoamine oxidase-like H2O2-generating activity in the cellular soluble fraction. It was also suggested that peroxidase(s) or copper amine oxidase(s) are involved in the extracellular superoxide production as a consequence of H2O2 production. Since more than 85% of the PEA-dependent AOS generating activity was localized in the extracellular space (extracellular fluid + cell wall), extracellularly secreted enzymes, probably peroxidases, may largely contribute to the oxidative burst induced by PEA. The PEA-induced AOS generation was also observed in the horseradish peroxidase (HRP) reaction mixture, supporting the hypothesis that peroxidases catalyze the oxidation of PEA leading to AOS generation. In addition to AOS production, we observed that PEA induced an increase in monodehydroascorbate radicals (MDA) in the cell suspension culture and in HRP reaction mixture using electron spin resonance spectroscopy and the newly invented MDA reductase-coupled method. Here we report that MDA production is an indicator of peroxidase-mediated generation of PEA radical species in tobacco suspension culture.

Peroxynitrite formed by mitochondrial NO synthase promotes mitochondrial Ca2+ release

Mitochondria contribute to the maintenance of the intracellular Ca2+ homeostasis by taking up and releasing the cation via separate and specific pathways. The molecular details of the release pathway are elusive but its stimulation by the cross-linking of some vicinal thiols and consequently NAD+ hydrolysis are known. Thiol cross-linking and NAD+ hydrolysis can be achieved by addition of peroxynitrite (ONOO-), the product of the reaction between superoxide (O2-) and nitric oxide (nitrogen monoxide, NO*) to mitochondria. Mitochondria contain an NO synthase (mtNOS), which is stimulated by Ca2+, and are a copious source of O2-. We show here that intramitochondrially formed ONOO- stimulates the specific, NAD+-linked Ca2+ release from mitochondria. Our findings that upon Ca2+ uptake mtNOS is stimulated, that ONOO- is formed, and that Ca2+ is subsequently released from intact mitochondria suggest the existence of a feedback loop, which prevents overloading of mitochondria with Ca2+.

Monoamine oxidase and mitochondrial respiration

Mitochondrial defects encompassing complexes I-IV of the electron transport chain characterize a relatively large number of neurodegenerative diseases. The relationships between mitochondrial lesions and recently described genetic alterations have not yet been defined. We describe a general mechanism whereby the enzymatic metabolism of neurotransmitters by monoamine oxidase (MAO) damages mitochondria, altering their protein thiol status and suppressing respiration. In these experiments, incubation of rat brain mitochondria with tyramine (a mixed MAO-A/MAO-B substrate) for 15 min at 27 degrees C suppressed state 3 respiration by 32.8% and state 5 respiration by 40.1%. These changes were accompanied by a 10-fold rise in protein-glutathione mixed disulfides. Direct comparison of effects on respiration and MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] dye reduction during electron flow gave similar results. It is suggested that certain mitochondrial lesions may derive from the natural turnover of monoamine neurotransmitters in susceptible individuals.

The regeneration of reduced glutathione in rat forebrain mitochondria identifies metabolic pathways providing the NADPH required

Metabolic pathways underlying the regeneration of reduced glutathione were investigated in acutely isolated metabolically active mitochondria from rat forebrain. The application of hydrogen peroxide to the organelles was accompanied by a transient increase in glutathione disulfide. The recovery of reduced glutathione was significantly improved in the presence of alternatively succinate, malate, citrate, isocitrate, or beta-hydroxybutyrate. Inhibition of succinate dehydrogenase by malonate abolished the beneficial effect of succinate on the reduction of glutathione disulfide but did not influence the effect of isocitrate. Fluorocitrate, an inhibitor of aconitase, blocked the effect exerted by citrate but did not inhibit the effects of malate or beta-hydroxybutyrate. Uncoupling of the respiratory chain by carbonyl cyanide m-chlorophenylhydrazone prevented the beneficial effect of beta-hydroxybutyrate but did not abolish the improved reduction of mitochondrial glutathione disulfide in the presence of malate and isocitrate. These results suggest that NADP+-dependent isocitrate dehydrogenase as well as malic enzyme and nicotinamide nucleotide transhydrogenase contribute to the regeneration of NADPH required for the reduction of glutathione disulfide in brain mitochondria.

Tyrosine hydroxylase and Parkinson's disease

A consistent neurochemical abnormality in Parkinson's disease (PD) is degeneration of dopaminergic neurons in substantia nigra, leading to a reduction of striatal dopamine (DA) levels. As tyrosine hydroxylase (TH) catalyses the formation of L-DOPA, the rate-limiting step in the biosynthesis of DA, the disease can be considered as a TH-deficiency syndrome of the striatum. Similarly, some patients with hereditary L-DOPA-responsive dystonia, a neurological disorder with clinical similarities to PD, have mutations in the TH gene and decreased TH activity and/or stability. Thus, a logical and efficient treatment strategy for PD is based on correcting or bypassing the enzyme deficiency by treatment with L-DOPA, DA agonists, inhibitors of DA metabolism, or brain grafts with cells expressing TH. A direct pathogenetic role of TH has also been suggested, as the enzyme is a source of reactive oxygen species (ROS) in vitro and a target for radical-mediated oxidative injury. Recently, it has been demonstrated that L-DOPA is effectively oxidized by mammalian TH in vitro, possibly contributing to the cytotoxic effects of DOPA. This enzyme may therefore be involved in the pathogenesis of PD at several different levels, in addition to being a promising candidate for developing new treatments of this disease.