Glutathione depletion switches nitric oxide neurotrophic effects to cell death in midbrain cultures: implications for Parkinson's disease

In vitro inhibition of glutathione-S-transferase by dopamine and its metabolites, 3,4-dihydroxyphenylacetaldehyde and 3,4-dihydroxyphenylacetic acid

Parkinson's disease is characterized by dopamine dyshomeostasis and oxidative stress. The aldehyde metabolite of dopamine, 3,4-dihydroxyphenylacetaldehyde (DOPAL), has been reported to be cytotoxic and capable of protein modification. Protein modification by DOPAL has been implicated in the pathogenesis of Parkinson's disease, but the complete pathology is unknown. Our findings show that DOPAL modifies glutathione S-transferase (GST), an important enzyme in the antioxidant defense system. DOPAL, dopamine, and the metabolite 3,4-dihydroxyphenylacetic acid (DOPAC), inhibited the activity of GST isolated from N27 dopaminergic cells at an IC₅₀ of 31.46 μM, 82.32 μM, and 260.0 μM, respectively. DOPAL, dopamine, and DOPAC inhibited commercially available equine liver GST at an IC₅₀ of 23.72 μM, 32.17 μM, and 73.70 μM, respectively. This inhibition was time dependent and irreversible. 1 mM ʟ-cysteine or glutathione fully protected GST activity from DOPAL, DA, and DOPAC inhibition. 1 mM carnosine partially protected GST activity from DA inhibition. Furthermore, ʟ-cysteine was found to protect GST by forming a putative thiazolidine conjugate with DOPAL. We conclude that GST inactivation may be a part of the broader etiopathology of Parkinson's disease.

Regulation of Tissue Inflammation by 12-Lipoxygenases

Lipoxygenases (LOXs) are lipid metabolizing enzymes that catalyze the di-oxygenation of polyunsaturated fatty acids to generate active eicosanoid products. 12-lipoxygenases (12-LOXs) primarily oxygenate the 12th carbon of its substrates. Many studies have demonstrated that 12-LOXs and their eicosanoid metabolite 12-hydroxyeicosatetraenoate (12-HETE), have significant pathological implications in inflammatory diseases. Increased level of 12-LOX activity promotes stress (both oxidative and endoplasmic reticulum)-mediated inflammation, leading to damage in these tissues. 12-LOXs are also associated with enhanced cellular migration of immune cells-a characteristic of several metabolic and autoimmune disorders. Genetic depletion or pharmacological inhibition of the enzyme in animal models of various diseases has shown to be protective against disease development and/or progression in animal models in the setting of diabetes, pulmonary, cardiovascular, and metabolic disease, suggesting a translational potential of targeting the enzyme for the treatment of several disorders. In this article, we review the role of 12-LOXs in the pathogenesis of several diseases in which chronic inflammation plays an underlying role.

Glutathione Depletion and Parkinsonian Neurotoxin MPP+-Induced TRPM2 Channel Activation Play Central Roles in Oxidative Cytotoxicity and Inflammation in Microglia

Parkinson's disease (PD) is one of most common neurodegenerative diseases. Environmental stressors such as oxidative stress (OS), calcium ion influx, apoptosis, and inflammation mechanisms are linked to activated microglia in patients with PD. The OS-dependent activated transient receptor potential melastatin 2 (TRPM2) channel is modulated in several neurons by glutathione (GSH). However, the cellular and molecular effects of GSH alteration on TRPM2 activation, OS, apoptosis, and inflammation in the microglia remain elusive. The microglia of TRPM2 wild-type (TRPM2-WT) and knockout (TRPM2-KO) mice were divided into control, PD model (MPP), L-buthionine sulfoximine (BSO), MPP + BSO and MPP + BSO + GSH groups. MPP-induced increases in apoptosis, death, OS, lipid peroxidation, PARP1, caspase-3 and caspase-9, inflammatory cytokines (IL-1β, TNF-α, IL-6), and intracellular free Zn²⁺ and Ca²⁺ levels in the microglia of TRPM2-WT mice were further increased by the BSO treatment, although they were diminished by the GSH treatment. Their levels were further reduced by PARP1 inhibitors (PJ34 and DPQ) and TRPM2 blockers (ACA and 2-APB). However, the effects of MPP and BSO were not observed in the microglia of TRPM2-KO mice. Taken together, our data demonstrate that maintaining GSH homeostasis is not only important for quenching OS in the microglia of patients with PD but also equally critical to modulating TRPM2, thus suppressing inflammatory responses elicited by environmental stressors.

The role of glutathione and glutathione peroxidase in regulating cellular level of reactive oxygen and nitrogen species

Glutathione (GSH) and GSH/glutathione peroxidase (GPX) enzyme system is essential for normal intracellular homeostasis and gets disturbed under pathophysiologic conditions including endothelial dysfunction. Overproduction of reactive oxidative species (ROS) and reactive nitrogen species (RNS) including superoxide (O₂^•-), and the loss of nitric oxide (NO) bioavailability is a characteristic of endothelial dysfunction. The GSH/GPX system play an important role in eliminating ROS/RNS. Studies have provided important information regarding the interactions of ROS/RNS with the GSH/GPX in biological systems; however, it is not clear how this cross talk affect these reactive species and GSH/GPX enzyme system, under physiologic and oxidative/nitrosative stress conditions. In the present study, we developed a detailed endothelial cell kinetic model to understand the relationship amongst the key enzyme systems including GSH, GPX, peroxiredoxin (Prx) and reactive species, such as hydrogen peroxide (H₂O₂), peroxynitrite (ONOO^-), and dinitrogen trioxide (N₂O₃). Our simulation results showed that the alterations in the generation rates of O₂^•- and NO led to the formation of a wide range of ROS and RNS. Simulations performed by varying the ratio of O₂^•- to NO generation rates as well as GSH and GPX concentrations showed that the GPX reducing capacity was dependent on GSH availability, level of oxidative/nitrosative stress, and can be attributed to N₂O₃ levels, but not to H₂O₂ and ONOO^-. Our results showed that N₂O₃ mediated switch-like depletion in GSH and the incorporation of Prx had no considerable effect on the ROS/RNS species other than ONOO^- and H₂O₂. The analysis presented in this study will improve our understanding of vascular diseases in which the levels and oxidation states of GSH, GPX and/or Prx are significantly altered and pharmacological interventions show limited benefits.

5-S-cysteinyl-dopamine, a neurotoxic endogenous metabolite of dopamine: Implications for Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disease worldwide and is characterized for being an idiopathic and multifactorial disease. Extensive research has been conducted to explain the origin of the disease, but it still remains elusive. It is well known that dopamine oxidation, through the endogenous formation of toxic metabolites, is a key process in the activation of a cascade of molecular events that leads to cellular death in the hallmark of PD. Thio-catecholamines, such as 5-S-cysteinyl-dopamine, 5-S-glutathionyl-dopamine and derived benzothiazines, are endogenous metabolites formed in the dopamine oxidative degradation pathway. Those metabolites have been shown to be highly toxic to neurons in the substantia nigra pars compacta, activating molecular mechanisms that ultimately lead to neuronal death. In this review we describe the origin, formation and the toxic effects of 5-S-cysteinyl-dopamine and its oxidative derivatives that cause death to dopaminergic neurons. Furthermore, we correlate the formation of those metabolites with the neurodegeneration progress in PD. In addition, we present the reported neuroprotective strategies of products that protect against the cellular damage of those thio-catecholamines. Finally, we discuss the advantages in the use of 5-S-cysteinyl-dopamine as a potential biomarker for PD.

Emerging role of 12/15-Lipoxygenase (ALOX15) in human pathologies

12/15-lipoxygenase (12/15-LOX) is an enzyme, which oxidizes polyunsaturated fatty acids, particularly omega-6 and -3 fatty acids, to generate a number of bioactive lipid metabolites. A large number of studies have revealed the importance of 12/15-LOX role in oxidative and inflammatory responses. The in vitro studies have demonstrated the ability of 12/15-LOX metabolites in the expression of various genes and production of cytokine related to inflammation and resolution of inflammation. The studies with the use of knockout and transgenic animals for 12/15-LOX have further shown its involvement in the pathogenesis of a variety of human diseases, including cardiovascular, renal, neurological and metabolic disorders. This review summarizes our current knowledge on the role of 12/15-LOX in inflammation and various human diseases.

Nitric oxide prevents H2O2-induced apoptosis in SK-N-MC human neuroblastoma cells

Nitric oxide (NO) is a cellular signaling molecule in many physiological and pathological processes including neuroprotector. Here we examined the antiapoptotic effect of NO in SK-N-MC cells. H2O2 treatment (10-200 μM) induced cell death in a dose-dependent manner and pretreatment of cells with 100 μM S-nitroso-N-acetylpenicillamine (SNAP), an NO donor, attenuated the occurrence of H2O2-induced cell death. DAPI staining showed H2O2-induced nuclear fragmentation and NO treatment suppressed it. NO inhibited the proteolytic activation of caspase-3 and mitochondrial cytochrome c release. Treatment of soluble guanylyl cyclase inhibitor ODQ decreased the protective effect of SNAP on H2O2-treated cells and increased caspase 3-like enzyme activity and activation, cytochrome c release, PARP cleavage, and DNA fragmentation, indicating that cGMP is a key mediator in NO-mediated antiapoptosis. The cGMP analog 8-Br-cGMP blocked H2O2-induced apoptotic cell death; reduction of caspase-3 enzyme, cytochrome c release, and caspase-8 and -9. These preventive effects of SNAP and 8-Br-cGMP were suppressed by PKG inhibitor KT5823. Levels of PKGI, PKGII, and p-VASP proteins were increased by SNAP and 8-Br-cGMP and suppressed by KT5823 treatment. These results indicate that PKG is a downstream signal mediator in the suppression of apoptosis by NO and cGMP. Akt activation was inhibited the PI3K inhibitors LY294002 and Wortmannin, resulting in the inhibition of cell viability and increase of cytochrome c release. SNAP induced phosphorylation of Akt and Bad and then increased the interactions between 14-3-3β and p-Bad. These data suggest that the NO suppresses H2O2-induced SK-N-MC cell apoptosis by suppressing apoptosis signal mediating the interaction between 14-3-3β and Bad phosphorylation via PKG/PI3K/Akt.

Involvement of Inositol Biosynthesis and Nitric Oxide in the Mediation of UV-B Induced Oxidative Stress

The involvement of NO-signaling in ultraviolet B (UV-B) induced oxidative stress (OS) in plants is an open question. Inositol biosynthesis contributes to numerous cellular functions, including the regulation of plants tolerance to stress. This work reveals the involvement of inositol-3-phosphate synthase 1 (IPS1), a key enzyme for biosynthesis of myo-inositol and its derivatives, in the response to NO-dependent OS in Arabidopsis. Homozygous mutants deficient for IPS1 (atips1) and wild-type plants were transformed with a reduction- grx1-rogfp2 and used for the dynamic measurement of UV-B-induced and SNP (sodium nitroprusside)-mediated oxidative stresses by confocal microscopy. atips1 mutants displayed greater tissue-specific resistance to the action of UV-B than the wild type. SNP can act both as an oxidant or repairer depending on the applied concentration, but mutant plants were more tolerant than the wild type to nitrosative effects of high concentration of SNP. Additionally, pretreatment with low concentrations of SNP (10, 100 μM) before UV-B irradiation resulted in a tissue-specific protective effect that was enhanced in atips1. We conclude that the interplay between nitric oxide and inositol signaling can be involved in the mediation of UV-B-initiated oxidative stress in the plant cell.

Glutathione: new roles in redox signaling for an old antioxidant

The physiological roles played by the tripeptide glutathione have greatly advanced over the past decades superimposing the research on free radicals, oxidative stress and, more recently, redox signaling. In particular, GSH is involved in nutrient metabolism, antioxidant defense, and regulation of cellular metabolic functions ranging from gene expression, DNA and protein synthesis to signal transduction, cell proliferation and apoptosis. This review will be focused on the role of GSH in cell signaling by analysing the more recent advancements about its capability to modulate nitroxidative stress, autophagy, and viral infection.

Using a holistic approach to assess the impact of engineered nanomaterials inducing toxicity in aquatic systems

In this report, we critically reviewed selected intrinsic physicochemical properties of engineered nanomaterials (ENMs) and their role in the interaction of the ENMs with the immediate surroundings in representative aquatic environments. The behavior of ENMs with respect to dynamic microenvironments at the nano–bio–eco interface level, and the resulting impact on their toxicity, fate, and exposure potential are elaborated. Based on this literature review, we conclude that a holistic approach is urgently needed to fulfill our knowledge gap regarding the safety of discharged ENMs. This comparative approach affords the capability to recognize and understand the potential hazards of ENMs and their toxicity mechanisms, and ultimately to establish a quantitative and reliable system to predict such outcomes.

Glutathione metabolism and Parkinson's disease

It has been established that oxidative stress, defined as the condition in which the sum of free radicals in a cell exceeds the antioxidant capacity of the cell, contributes to the pathogenesis of Parkinson disease. Glutathione is a ubiquitous thiol tripeptide that acts alone or in concert with enzymes within cells to reduce superoxide radicals, hydroxyl radicals, and peroxynitrites. In this review, we examine the synthesis, metabolism, and functional interactions of glutathione and discuss how these relate to the protection of dopaminergic neurons from oxidative damage and its therapeutic potential in Parkinson disease.

Nitric oxide synthase inhibitors protect against rotenone-induced, oxidative stress mediated parkinsonism in rats

Rotenone is known to cause progressive dopaminergic neuronal loss in rodents, but it remains unclear how this mitochondrial complex-I inhibitor mediates neurodegeneration specific to substantia nigra pars compacta (SNpc). One of the proposed mechanisms is increased free radical generation owing to mitochondrial electron transport chain dysfunction following complex-I inhibition. The present study examined the role of nitric oxide (NO) and hydroxyl radicals (OH) in mediating rotenone-induced dopaminergic neurotoxicity. Indications of NO involvement are evidenced by inducible nitric oxide synthase (NOS) over-expression, and increased NADPH-diaphorase staining in SNpc neurons 96h following rotenone administration. Treatment of these animals with specific neuronal NOS inhibitor, 7-nitroindazole (7-NI) and non-specific NOS inhibitor, N-ω-nitro-l-argenine methyl ester (l-NAME) caused reversal of rotenone-induced striatal dopamine depletion, and attenuation of the neurotoxin-induced decrease in the number of tyrosine hydroxylase immunoreactive neurons in SNpc, as well as in apomorphine and amphetamine-induced unilateral rotations. Interestingly, the study also demonstrated the contribution of OH in mediating rotenone nigral toxicity since there appeared a significant generation of the reactive oxygen species in vivo 24h following rotenone administration, a copious loss of reduced and oxidized glutathione, and increased superoxide dismutase and catalase activities in the cytosolic fractions of the ipsilateral SNpc area on the 5th day. An OH scavenging capacity of 7-NI and l-NAME in a Fenton-like reaction, as well as complete reversal of the rotenone-induced increases in the antioxidant enzyme activities, and the loss in reduced and oxidized glutathione contents in the SNpc supported OH involvement in rotenone-induced dopaminergic neurotoxicity. While these results strongly suggest the contribution of both OH and NO, resulting in acute oxidative stress culminating in dopaminergic neurodegeneration caused by rotenone, the course of events indicated generation of OH as the primary event in the neurotoxic processes.

Functional and pathological roles of the 12- and 15-lipoxygenases

The 12/15-lipoxygenase enzymes react with fatty acids producing active lipid metabolites that are involved in a number of significant disease states. The latter include type 1 and type 2 diabetes (and associated complications), cardiovascular disease, hypertension, renal disease, and the neurological conditions Alzheimer's disease and Parkinson's disease. A number of elegant studies over the last thirty years have contributed to unraveling the role that lipoxygenases play in chronic inflammation. The development of animal models with targeted gene deletions has led to a better understanding of the role that lipoxygenases play in various conditions. Selective inhibitors of the different lipoxygenase isoforms are an active area of investigation, and will be both an important research tool and a promising therapeutic target for treating a wide spectrum of human diseases.

Altered expression of neuronal nitric oxide synthase in weaver mutant mice

The weaver mouse represents the only genetic animal model of gradual nigrostriatal dopaminergic neurodegeneration which is proposed as a pathophysiological phenotype of Parkinson's disease. The aim of the present study was to analyze the nitric oxide and dopaminergic systems in selected brain regions of homozygous weaver mice at different postnatal ages corresponding to specific stages of the dopamine loss. Structural deficits were evaluated by quantification of tyrosine hydroxylase and neuronal nitric oxide synthase-immunostaining in the cortex, striatum, accumbens nuclei, subthalamic nuclei, ventral tegmental area, and substantia nigra compacta of 10-day, 1- and 2-month-old wild-type and weaver mutant mice. The results confirmed the progressive loss of dopamine during the postnatal development in the adult weaver mainly affecting the substantia nigra pars compacta, striatum, and subthalamic nucleus and slightly affecting the accumbens nuclei and ventral tegmental area. A general decrease in neuronal nitric oxide synthase-immunostaining with age was revealed in both the weaver and wild-type mice, with the decrease being most pronounced in the weaver. In contrast, there was an increase in the substantia nigra pars compacta nitric oxide synthase-immunostaining and a decrease mainly in the subthalamic and accumbens nuclei of the 2-month-old weaver mutant. The decrease in the expression of nNOS may bear functional significance related to the process of aging. DA neurons from the substantia nigra directly modulate the activity of subthalamic nucleus neurons, and their loss may contribute to the abnormal activity of subthalamic nucleus neurons. Although the functional significance of these changes is not clear, it may represent plastic compensating adjustments resulting from the loss of dopamine innervation, highlighting a possible role of nitric oxide in this process.

Involvement of multiple protein kinases in cPLA2 phosphorylation, arachidonic acid release, and cell death in in vivo and in vitro models of 1-methyl-4-phenylpyridinium-induced parkinsonism--the possible key role of PKG

The study was aimed at investigating in vivo and in vitro the involvement of the cGMP/cGMP-dependent protein kinase (PKG) signaling pathway in MPP(+)-induced cytosolic phospholipase A(2) (cPLA(2)) activation of dopaminergic neurons. MPP(+) activated neuronal nitric oxide synthase (NOS)/soluble guanylyl cyclase/cGMP pathway in mouse midbrain and striatum, and in pheochromocytoma cell line 12 cells, and caused an upward shift in [Ca(2+)](i) level in the latter. The activation was accompanied by increases in total and phosphorylated cPLA(2), and increased arachidonic acid release. Effects of selective inhibitors [2-oxo-1,1,1-trifluoro-6,9-12,15-heneicosatetraene (AACOCF(3)), (E)-6-(bromomethylene)tetrahydro-3-(1-naphthalenyl)2h-pyran-2-one (BEL)] indicated the main impact of cPLA(2) on arachidonic acid release in pheochromocytoma cell line 12 cells. Treatment of the cells with the protein kinase inhibitors GF102610x, UO126, and KT5823, and with the nitric oxide synthase (NOS) inhibitor NNLA revealed the involvement of protein kinase C (PKC) and extracellular signal-regulated kinases 1 and 2 (ERK 1/2), with the possible key role of PKG, in cPLA(2) phosphorylation at Ser505. Inhibitors of cPLA(2) and PKG increased viability and reduced MPP(+)-induced apoptosis of the cells. Our results indicate that the neuronal NOS/cGMP/PKG pathway stimulates cPLA(2) phosphorylation at Ser505 by activating PKC and ERK1/2, and suggest that up-regulation of this pathway in experimental models of Parkinson's disease may mediate dopaminergic neuron degeneration and death through activation of cPLA(2).

Acid-base balance and weight gain: are there crucial links via protein and organic acids in understanding obesity?

Obesity is associated with ever increasing social costs posing a general public health challenge. The most obvious reason for obesity, given healthy body functioning, is a positive calorie balance. This article delves into the lesser studied realm of the relationship of weight gain, in particular adipose tissue gain, with increased hydrogen ion concentration, taking protein and organic acids as important caveats in this discussion. The review opens the topic with the contradictory result of various studies reporting a positive relationship between chronic metabolic acidosis and weight loss. It goes to explain a process of weight gain, primarily adipose tissue gain, on acidogenic diets. Insufficient dietary protein could lead to muscle loss, and individual organic acids might indicate if there is any fatty acid oxidation or accumulation of hydrogen ion. The solution to the acid accumulation is discussed not in protein limitation but an increase in the consumption of vegetables and fruits. Finally, this review article based on studies published puts forward a physiological basis including a hypothesis to explain the possible link between hydrogen ion concentration and weight gain. This link could possibly explain the development of diseases and aging partially, and warrants research.

S-Nitrosylation of the epidermal growth factor receptor: a regulatory mechanism of receptor tyrosine kinase activity

Nitric oxide (NO) donors inhibit the epidermal growth factor (EGF)-dependent auto(trans)phosphorylation of the EGF receptor (EGFR) in several cell types in which NO exerts antiproliferative effects. We demonstrate in this report that NO inhibits, whereas NO synthase inhibition potentiates, the EGFR tyrosine kinase activity in NO-producing cells, indicating that physiological concentrations of NO were able to regulate the receptor activity. Depletion of intracellular glutathione enhanced the inhibitory effect of the NO donor 1,1-diethyl-2-hydroxy-2-nitrosohydrazine (DEA/NO) on EGFR tyrosine kinase activity, supporting the notion that such inhibition was a consequence of an S-nitrosylation reaction. Addition of DEA/NO to cell lysates resulted in the S-nitrosylation of a large number of proteins including the EGFR, as confirmed by the chemical detection of nitrosothiol groups in the immunoprecipitated receptor. We prepared a set of seven EGFR(C --> S) substitution mutants and demonstrated in transfected cells that the tyrosine kinase activity of the EGFR(C166S) mutant was completely resistant to NO, whereas the EGFR(C305S) mutant was partially resistant. In the presence of EGF, DEA/NO significantly inhibited Akt phosphorylation in cells transfected with wild-type EGFR, but not in those transfected with C166S or C305S mutants. We conclude that the EGFR can be posttranslationally regulated by reversible S-nitrosylation of C166 and C305 in living cells.

Glial Cells as Players in Parkinsonism: The “Good,” the “Bad,” and the “Mysterious” Glia

The role of glia in Parkinson's disease (PD) is very interesting because it may open new therapeutic strategies in this disease. Traditionally it has been considered that astrocytes and microglia play different roles in PD: Astroglia are considered the “good” glia and have traditionally been supposed to be neuroprotective due to their capacity to quench free radicals and secrete neurotrophic factors, whereas microglia, considered the “bad” glia, are thought to play a critical role in neuroinflammation. The proportion of astrocytes surrounding dopamine (DA) neurons in the substantia nigra, the target nucleus for neurodegeneration in PD, is the lowest for any brain area, suggesting that DA neurons are more vulnerable in terms of glial support than any neuron in other brain areas. Astrocytes are critical in the modulation of the neurotoxic effects of many toxins that induce experimental parkinsonism and they produce substances in vitro that could modify the effects of L-DOPA from neurotoxic to neurotrophic. There is a great interest in the role of inflammation in PD, and in the brains of these patients there is evidence for microglial production of cytokines and other substances that could be harmful to neurons, suggesting that microglia of the substantia nigra could be actively involved, primarily or secondarily, in the neurodegeneration process. There is, however, evidence in favor of the role of neurotoxic diffusible signals from microglia to DA neurons. More recently a third glial player, oligodendroglia, has been implicated in the pathogenesis of PD. Oligodendroglia play a key role in myelination of the nervous system. Recent neuropathological studies suggested that the nigrostriatal dopamine neurons, which were considered classically as the primary target for neurodegeneration in PD, degenerate at later stages than other neurons with poor myelination. Therefore, the role of oligodendroglia, which also secrete neurotrophic factors, has entered the center of interest of neuroscientists.

Nitric oxide-cyclic GMP signaling pathway limits inflammatory degeneration of midbrain dopaminergic neurons: cell type-specific regulation of heme oxygenase-1 expression

Excessive production of nitric oxide (NO) by microglia is at least in part responsible for the pathogenesis of various neurodegenerative disorders including Parkinson disease, but at the same time NO may also play a distinct role as a signaling molecule such as an activator of soluble guanylyl cyclase. Here we investigated potential roles of the NO-soluble guanylyl cyclase-cyclic GMP signaling pathway in the regulation of dopaminergic neurodegeneration. Activation of microglia by interferon-gamma (IFN-gamma) followed by lipopolysaccharide (LPS) caused dopaminergic cell death in rat midbrain slice cultures, which was dependent on NO production. 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), a soluble guanylyl cyclase inhibitor, as well as KT5823, an inhibitor of cyclic GMP-dependent protein kinase, exacerbated dopaminergic cell death induced by IFN-gamma/LPS. Conversely, 8-bromo-cyclic GMP attenuated IFN-gamma/LPS cytotoxicity on dopaminergic neurons. Notably, although heme oxygenase-1 (HO-1) was expressed prominently in cells other than dopaminergic neurons in control cultures, robust expression of HO-1 was induced in surviving dopaminergic neurons challenged with IFN-gamma/LPS. ODQ and KT5823 decreased, whereas 8-bromo-cyclic GMP increased, the number of dopaminergic neurons expressing HO-1 after IFN-gamma/LPS challenge, without parallel changes in HO-1 expression in other cell populations. An NO donor 3-(4-morpholinyl)sydnonimine hydrochloride also induced HO-1 expression in dopaminergic neurons, which was abolished by ODQ and augmented by 8-bromo-cyclic GMP. Moreover, IFN-gamma/LPS-induced dopaminergic cell death was augmented by zinc protoporphyrin IX, an HO-1 inhibitor. The NO donor cytotoxicity on dopaminergic neurons was also augmented by ODQ and zinc protoporphyrin IX. These results indicate that the NO-cyclic GMP signaling pathway promotes the induction of HO-1 specifically in dopaminergic neurons, which acts as an endogenous protective system to limit inflammatory degeneration of this cell population.

Glial dysfunction in parkin null mice: effects of aging

Parkin mutations in humans produce parkinsonism whose pathogenesis is related to impaired protein degradation, increased free radicals, and abnormal neurotransmitter release. The role of glia in parkin deficiency is little known. We cultured midbrain glia from wild-type (WT) and parkin knock-out (PK-KO) mice. After 18-20 d in vitro, PK-KO glial cultures had less astrocytes, more microglia, reduced proliferation, and increased proapoptotic protein expression. PK-KO glia had greater levels of intracellular glutathione (GSH), increased mRNA expression of the GSH-synthesizing enzyme gamma-glutamylcysteine synthetase, and greater glutathione S-transferase and lower glutathione peroxidase activities than WT. The reverse happened in glia cultured in serum-free defined medium (EF12) or in old cultures. PK-KO glia was more susceptible than WT to transference to EF12 or neurotoxins (1-methyl-4-phenylpyridinium, blockers of GSH synthesis or catalase, inhibitors of extracellular signal-regulated kinase 1/2 and phosphatidylinositol 3 kinases), aging of the culture, or combination of these insults. PK-KO glia was less susceptible than WT to Fe2+ plus H2O2 and less responsive to protection by deferoxamine. Old WT glia increased the expression of heat shock protein 70, but PK-KO did not. Glia conditioned medium (GCM) from PK-KO was less neuroprotective and had lower levels of GSH than WT. GCM from WT increased the levels of dopamine markers in midbrain neuronal cultures transferred to EF12 more efficiently than GCM from PK-KO, and the difference was corrected by supplementation with GSH. PK-KO-GCM was a less powerful suppressor of apoptosis and microglia in neuronal cultures. Our data prove that abnormal glial function is critical in parkin mutations, and its role increases with aging.

Preferential vulnerability of mesencephalic dopamine neurons to glutamate transporter dysfunction

Nigral depletion of the main brain antioxidant GSH is the earliest biochemical event involved in Parkinson's disease pathogenesis. Its causes are completely unknown but increasing number of evidence suggests that glutamate transporters [excitatory amino acid transporters (EAATs)] are the main route by which GSH precursors may enter the cell. In this study, we report that dopamine (DA) neurons, which express the excitatory amino acid carrier 1, are preferentially affected by EAAT dysfunction when compared with non-DA neurons. In rat embryonic mesencephalic cultures, l-trans-pyrrolidine-2,4-dicarboxylate, a substrate inhibitor of EAATs, is directly and preferentially toxic for DA neurons by decreasing the availability of GSH precursors and lowering their resistance threshold to glutamate excitotoxicity through NMDA-receptors. In adult rat, acute intranigral injection of l-trans-pyrrolidine-2,4-dicarboxylate induces a large regionally selective and dose-dependent loss of DA neurons and alpha-synuclein aggregate formation. These data highlight for the first time the importance of excitatory amino acid carrier 1 function for the maintenance of antioxidant defense in DA neurons and suggest its dysfunction as a candidate mechanism for the selective death of DA neurons such as occurring in Parkinson's disease.

Mortality, oxidative stress and tau accumulation during ageing in parkin null mice

Young parkin null (pk-/-) mice have subtle abnormalities of behaviour, dopamine (DA) neurotransmission and free radical production, but no massive loss of DA neurons. We investigated whether these findings are maintained while ageing. Pk-/- mice have reduced life span and age-related reduced exploratory behaviour, abnormal walking and posture, and behaviours similar to those of early Parkinson's disease (PD), reduced number of nigrostriatal DA neurons and proapoptotic shifts in the survival/death proteins in midbrain and striatum. Contrary to young pk-/- animals 24-month-old pk-/- mice do not have compensatory elevation of GSH in striatum, glutathione reductase (GR) and glutathione peroxidase (GPx) activities are increased and catalase unchanged. Aged pk-/- mice accumulate high levels of tau and fail to up-regulate CHIP and HSP70. Our results suggest that aged pk-/- mice lack of the compensatory mechanisms that maintain a relatively normal DA function in early adulthood. This study could help to explain the effects of ageing in patients with genetic risks for Parkinson's disease.

Nitric oxide synthase regulation and diversity: Implications in Parkinson’s disease

Nitric oxide (NO) is a janus faced chemical messenger, which, in the recent years, has been the focus of neurobiologists for its involvement in neurodegenerative disorders in particular, Parkinson's disease (PD). Nitric oxide synthase, the key enzyme involved in NO production exists in three known isoforms. The neuronal and inducible isoforms have been implicated in the pathogenesis of PD. These enzymes are subject to complex expressional and functional regulation involving mRNA diversity, phosphorylation and protein interaction. In the recent years, mRNA diversity and polymorphisms have been identified in the NOS isoforms. Some of these genetic variations have been associated with PD, indicating an etiological role for the NOS genes. This review mainly focuses on the NOS genes - their differential regulation and genetic heterogeneity, highlighting their significance in the pathobiology of PD.

Alterations of the expression and activity of midbrain nitric oxide synthase and soluble guanylyl cyclase in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced Parkinsonism in mice

The study was aimed at investigating the expression and the activity of neuronal nitric oxide synthase, and of soluble guanylyl cyclase and phosphodiesterase activities that regulate guanosine 3',5'-cyclic monophosphate level in the midbrain, in a mouse model of PD using 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine injections. Adult male mice of the C57/BL strain were given three i.p. injections of physiological saline or three i.p. injections of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine solution in physiological saline at 2 h intervals (summary 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine dose: 40 mg/kg), and were killed 3, 7, or 14 days later. mRNA, protein level, and/or activities of neuronal nitric oxide synthase, soluble guanylyl cyclase, phosphodiesterase and guanosine 3',5'-cyclic monophosphate were determined. Immunohistochemistry showed about 75% decrease in the number of tyrosine hydroxylase-positive neurons in the substantia nigra pars compacta. Mice treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine showed increased midbrain guanylyl cyclase and total nitric oxide synthase activities at 3, 7, and 14 days post-treatment. The specific neuronal nitric oxide synthase inhibitor 7-nitroindazole (10 microM) and the specific inducible nitric oxide synthase inhibitor 1400W (10 microM) inhibited the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced excess in nitric oxide synthase activity by 63-70 and 13-25%, respectively. The increases in total midbrain nitric oxide synthase activity were accompanied by elevated guanosine 3',5'-cyclic monophosphate, enhanced expression of neuronal nitric oxide synthase and of the beta1 subunit of guanylyl cyclase at both mRNA and protein levels that persisted up to the end of the observation period, and by enhanced neuronal nitric oxide synthase and guanylyl cyclase beta1 immunoreactivities in substantia nigra pars compacta 7 and 14 days after the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment. The increases in guanylyl cyclase activity were found to occur exclusively due to increased maximal enzyme activity. No 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced change in phosphodiesterase activity has been detected in any brain region studied. 7-Nitroindazole prevented a significant increase in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced midbrain guanosine 3',5'-cyclic monophosphate level and neurodegeneration of dopaminergic neurons. These results raise the possibility that the nitric oxide/guanylyl cyclase/guanosine 3',5'-cyclic monophosphate signaling pathway may play a role in maintaining dopaminergic neurons function in substantia nigra pars compacta.

Midbrain neuronal cultures from parkin mutant mice are resistant to nitric oxide-induced toxicity

Nitric oxide (NO) is a modulator of differentiation and survival of dopamine (DA) neurons. NO may play a role in the pathogenesis of Parkinson's disease (PD) since its levels are increased in parkinsonian brains and it can nitrosylate and alter the function of key proteins involved in the pathogenesis of PD. NO producing neurons are spared in parkinsonian brains suggesting that toxicity by NO can be compensated. Furthermore, the neurotoxic or neurotrophic effects of NO on DA neurons depend on the balance between NO levels and the intracellular levels of glutathione (GSH). We have investigated the effects of NO-donating agents on midbrain neuronal cultures from parkin-deficient mice. Parkin mutations are the most common genetic deficit observed in hereditary parkinsonism. These mice have abnormal DA release and metabolism, increased production of free radicals and a compensatory elevation of GSH. Cultures from parkin knockout (PK-KO) mice were more resistant than those of wild type (WT) to the neurotoxicity by NO, and the difference of susceptibility applied equally to DA, GABA and total number of neurons, and to astrocytes. NO-induced cell death was mainly apoptotic and could be reduced by caspase inhibitors. Cultures from PK-KO had greater levels of GSH than WT and, after treatment with NO, greater levels of S-nitrosoglutathione. The differences in susceptibility disappear when the synthesis of GSH is inhibited or the GSH chelated with diethyl maleate. Our data show that, contrary to the expectations, and related to the enhanced production of GSH in parkin knockout mice, parkin-deficient dopamine neurons are less susceptible to toxicity by NO.

Oxidative stress and pro-apoptotic conditions in a rodent model of Wilson's disease

Wilson's disease (WD) is an inherited disorder, characterized by selective copper deposition in liver and brain, chronic hepatitis and extra-pyramidal signs. In this study, we investigated changes of biochemical markers of oxidative stress and apoptosis in liver, striatum and cerebral cortex homogenates from Long-Evans Cinnamon (LEC) rats, a mutant strain isolated from Long Evans (LE) rats, in whom spontaneous hepatitis develops shortly after birth. LEC and control (LE) rats at 11 and 14 weeks of age were used. We determined tissue levels of glutathione (GSH/GSSG ratio), lipid peroxides, protein-thiols (P-SH), nitric oxide metabolites, activities of caspase-3 and total superoxide-dismutase (SOD), striatal levels of monoamines and serum levels of hepatic amino-transferases. We observed a decrease of protein-thiols, GSH/GSSG ratio and nitrogen species associated to increased lipid peroxidation in the liver and striatum - but not in the cerebral cortex - of LEC rats, accompanied by dramatic increase in serum amino-transferases and decrease of striatal catecholamines. Conversely, SOD and caspase-3 activity increased consistently only in the cortex of LEC rats. Hence, we assume that enhanced oxidative stress may play a central role in the cell degeneration in WD, at the main sites of copper deposition, with discrete pro-apoptotic conditions developing in distal areas.

Acute glutathione depletion restricts mitochondrial ATP export in cerebellar granule neurons

Decreases in GSH pools detected during ischemia sensitize neurons to excitotoxic damage. Thermodynamic analysis predicts that partial GSH depletion will cause an oxidative shift in the thiol redox potential. To investigate the acute bioenergetic consequences, neurons were exposed to monochlorobimane (mBCl), which depletes GSH by forming a fluorescent conjugate. Neurons transfected with redox-sensitive green fluorescent protein showed a positive shift in thiol redox potential synchronous with the formation of the conjugate. Mitochondria within neurons treated with mBCl for 1 h failed to hyperpolarize upon addition of oligomycin to inhibit their ATP synthesis. A decreased ATP turnover was confirmed by monitoring neuronal oxygen consumption in parallel with mitochondrial membrane potential (Deltapsi(m)) and GSH-mBCl formation. mBCl progressively decreased cell respiration, with no effect on mitochondrial proton leak or maximal respiratory capacity, suggesting adequate glycolysis and a functional electron transport chain. This approach to "state 4" could be mimicked by the adenine nucleotide translocator inhibitor bongkrekic acid, which did not further decrease respiration when administered after mBCl. The cellular ATP/ADP ratio was decreased by mBCl, and consistent with mitochondrial ATP export failure, respiration could not respond to an increased cytoplasmic ATP demand by plasma membrane Na(+) cycling; instead, mitochondria depolarized. More prolonged mBCl exposure induced mitochondrial failure, with Deltapsi(m) collapse followed by cytoplasmic Ca(2+) deregulation. The initial bioenergetic consequence of neuronal GSH depletion in this model is thus an inhibition of ATP export, which precedes other forms of mitochondrial dysfunction.

REDOX REGULATION BY INTRINSIC SPECIES AND EXTRINSIC NUTRIENTS IN NORMAL AND CANCER CELLS

Cells in multicellular organisms are exposed to both endogenous oxidative stresses generated metabolically and to oxidative stresses that originate from neighboring cells and from other tissues. To protect themselves from oxidative stress, cells are equipped with reducing buffer systems (glutathione/GSH and thioredoxin/thioredoxin reductase) and have developed several enzymatic mechanisms against oxidants that include catalase, superoxide dismutase, and glutathione peroxidase. Other major extrinsic defenses (from the diet) include ascorbic acid, beta-carotene and other carotenoids, and selenium. Recent evidence indicates that in addition to their antioxidant function, several of these redox species and systems are involved in regulation of biological processes, including cellular signaling, transcription factor activity, and apoptosis in normal and cancer cells. The survival and overall well-being of the cell is dependent upon the balance between the activity and the intracellular levels of these antioxidants as well as their interaction with various regulatory factors, including Ref-1, nuclear factor-kappaB, and activating protein-1.

Differential effects of l-DOPA on monoamine metabolism, cell survival and glutathione production in midbrain neuronal-enriched cultures from parkin knockout and wild-type mice

l-DOPA is the most effective treatment for Parkinson's disease but in isolated neuronal cultures it is neurotoxic for dopamine (DA) neurones. Experiments in vivo and clinical studies have failed to show toxicity of l-DOPA in animals or patients but that does not exclude the possibility of a toxic effect of l-DOPA on patients with certain genetic risk factors. Mutations of the parkin gene are the most frequent cause of hereditary parkinsonism. Parkin null mice have a mild phenotype that could be modified by different neurotoxins. The aim of this study was to investigate whether the toxic effects of l-DOPA on DA neurones are amplified in parkin null mice. We have measured the effects of l-DOPA on cell viability, tyrosine hydroxylase (TH) expression, DA metabolism and glutathione levels of parkin knockout (PK-KO) midbrain cultures. Neuronal-enriched cultures from PK-KO mice have similar proportions of the different cell types with the exception of a significant increment of microglial cells. l-DOPA (400 microm for 24 h) reduced the number of TH-immunoreactive cells to 50% of baseline and increased twofold the percentage of apoptotic cells in cultures of wild-type (WT) animals. The PK-KO mice, however, are not only resistant to the l-DOPA-induced pro-apoptotic effects but they have an increased number of TH-immunoreactive neurones after treatment with l-DOPA, suggesting that l-DOPA is toxic for neurones of WT mice but not those of parkin null mice. MAPK and phosphatidylinositol-3 kinase signalling pathways are not involved in the differential l-DOPA effects in WT and PK-KO cultures. Intracellular levels of l-DOPA were not different in WT and parkin null mice but the intracellular and extracellular levels of DA and 3-4-dihydroxyphenylacetic acid, however, were significantly increased in parkin null animals. Furthermore, monoamine oxidase activity was significantly increased in parkin null mice, suggesting that these animals have an increased metabolism of DA. The levels of glutathione were further increased in parkin null mice than in controls both with and without treatment with l-DOPA, suggesting that a compensatory mechanism may protect DA neurones from neuronal death. This study opens new avenues for understanding the mechanisms of action of l-DOPA on DA neurones in patients with Park-2 mutations.

Treatment with 1,2,3,4-tetrahydroisoquinolone affects the levels of nitric oxide, S-nitrosothiols, glutathione and the enzymatic activity of γ-glutamyl transpeptidase in the dopaminergic structures of rat brain

Depletion of glutathione (GSH), nitrosative stress and chronic intoxication with some neurotoxins have been postulated to play a major role in the pathogenesis of Parkinson's disease. This study aimed to examine the effects of acute and chronic treatments with 1,2,3,4-tetrahydroisoquinoline (TIQ), an endo-/exogenous substance suspected of producing Parkinsonism in human, on the levels of nitric oxide (NO), S-nitrosothiols and glutathione (GSH) in the whole rat brain and in its dopaminergic structures. TIQ administered at a dose of 50 mg/kg i.p. significantly increased the tissue concentrations of NO and GSH in the substantia nigra (SN), striatum (STR) and cortex (CTX) of rats receiving this compound both acutely and chronically. Moreover, it decreased the level of oxidized glutathione (GSSG) and enhanced GSH:GSSG ratio affecting in this way the redox state of brain cells. TIQ also increased the level of S-nitrosothiols when measured in the whole rat brain and CTX, although it markedly decreased their level in the STR after both treatments. Inhibition of the constitutive NO synthase by l-NAME in the presence of TIQ caused decreases in GSH and S-nitrosothiol levels in the brain. The latter effect shows that the TIQ-mediated increases in GSH and S-nitrosothiol concentrations were dependent on the enhanced NO level. The above-described results suggest that TIQ can act as a modulator of GSH, NO and S-nitrosothiol levels but not as a parkinsonism-inducing agent in the rat brain.

The effects of stress and aging on glutathione metabolism

Glutathione plays a critical role in many biological processes both directly as a co-factor in enzymatic reactions and indirectly as the major thiol-disulfide redox buffer in mammalian cells. Glutathione also provides a critical defense system for the protection of cells from many forms of stress. However, mild stress generally increases glutathione levels, often but not exclusively through effects on glutamate cysteine ligase, the rate-limiting enzyme for glutathione biosynthesis. This upregulation in glutathione provides protection from more severe stress and may be a critical feature of preconditioning and tolerance. In contrast, during aging, glutathione levels appear to decline in a number of tissues, thereby putting cells at increased risk of succumbing to stress. The evidence for such a decline is strongest in the brain where glutathione loss is implicated in both Parkinson's disease and in neuronal injury following stroke.

Glutathione depletion resulting in selective mitochondrial complex I inhibition in dopaminergic cells is via an NO-mediated pathway not involving peroxynitrite: implications for Parkinson's disease

An early biochemical change in the Parkinsonian substantia nigra (SN) is reduction in total glutathione (GSH + GSSG) levels in affected dopaminergic neurons prior to depletion in mitochondrial complex I activity, dopamine loss, and cell death. We have demonstrated using dopaminergic PC12 cell lines genetically engineered to inducibly down-regulate glutathione synthesis that total glutathione depletion in these cells results in selective complex I inhibition via a reversible thiol oxidation event. Here, we demonstrate that inhibition of complex I may occur either by direct nitric oxide (NO) but not peroxinitrite-mediated inhibition of complex I or through H2O2-mediated inhibition of the tricarboxylic acid (TCA) cycle enzyme alpha-ketoglutarate dehydrogenase (KGDH) which supplies NADH as substrate to the complex; activity of both enzymes are reduced in PD. While glutathione depletion causes a reduction in spare KGDH enzymatic capacity, it produces a complete collapse of complex I reserves and significant effects on mitochondrial function. Our data suggest that NO is likely the primary agent involved in preferential complex I inhibition following acute glutathione depletion in dopaminergic cells. This may have major implications in terms of understanding mechanisms of dopamine cell death associated with PD especially as they relate to complex I inhibition.

Upregulation of guanylyl cyclase expression and activity in striatum of MPTP-induced parkinsonism in mice

The aim of our study was to investigate the expression and the activity of soluble guanylyl cyclase (GC) and phosphodiesterase (PDE) activities that regulate cGMP level in the striatum, hippocampus, and brain cortex in an animal model of PD, induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). We observed the increase of total activity and protein level of GC in striatum after MPTP injection. It was accompanied by an enhancement of both mRNA expression and protein level of GCbeta1 subunit. MPTP induces mRNA expression and elevates protein concentration of GCbeta1 in striatum up to 14 days after its injection, which in turn causes a marked enhancement of cGMP formation. Furthermore, the activation of GC occurs through change of maximal enzyme activity (V(max)). Simultaneously, no change in PDE activity has been detected in all investigated regions of the brain after MPTP. MPTP injection caused the elevation of GCbeta1 protein level in both the membrane and cytosol fractions being significantly higher in cytosol. Western blot analysis demonstrated about 45-67% decrease of tyrosine hydroxylase protein content in striatum. These data suggest that NO/cGMP signaling pathway may at least partially contribute to dopaminergic fiber degeneration in the striatum, the damage attributed to PD.

Oxidative stress in neurodegeneration: cause or consequence?

Oxidative stress has long been linked to the neuronal cell death that is associated with certain neurodegenerative conditions. Whether it is a primary cause or merely a downstream consequence of the neurodegenerative process is still an open question, however. The advent of a growing number of in vitro and in vivo models that emulate human disease pathology is aiding scientists in deciphering just where oxidative stress intersects with other cellular events in the emerging roadmap leading to neurodegeneration. Here I review the evidence for oxidative stress in neurodegeneration and how this relates to other cellular events.

Selective and persistent activation of extracellular signal-regulated protein kinase by nitric oxide in glial cells induces neuronal degeneration in glutathione-depleted midbrain cultures

Intracellular glutathione (GSH) levels determine whether nitric oxide (NO) is neurotrophic for dopamine neurons or triggers a cell death cascade in primary midbrain cultures. We have investigated herein the role of the extracellular-signal regulated protein kinase (ERK) 1/2 pathway in this GSH switching effect. The short-lived NO donor DEA/NO induces a transient activation of ERK-1/2 that totally disappears 2 h after NO administration. The depletion of GSH increases and the supplementation of GSH suppresses ERK-1/2 activation in response to NO treatment. More interestingly, GSH depletion changes the kinetic of phosphorylation leading to a second prolonged phase of ERK-1/2 activation from 2 to 16 h after NO addition. This change of kinetic is ultimately responsible for NO toxicity under GSH-depleted conditions, because selective blockade of the second and persistent phase of activation prevents cell death. In addition, the only transient ERK activation, induced by NO under normal GSH conditions, did not cause ERK-dependent cell death. Immunocytochemical colocalization studies demonstrate that ERK activation takes place exclusively in glial cells, mainly in astrocytes and less frequently in oligodendrocytes and glial progenitors. Furthermore, glial cell elimination or inactivation in the culture, by gliotoxic drugs, abrogates NO-induced ERK activation. Our results indicate that neurotrophism of NO switches into neurotoxicity after GSH depletion due to persistent activation of the ERK-1/2 signaling pathway in glial cells. The implication of these results in pathological conditions like Parkinson's disease, where GSH depletion and NO overproduction have been documented, are discussed.

Green tea polyphenols enhance sodium nitroprusside-induced neurotoxicity in human neuroblastoma SH-SY5Y cells

Oxidative stress is a main mediator in nitric oxide (NO) -induced neurotoxicity and has been implicated in the pathogenesis of many neurodegenerative disorders. Green tea polyphenols are usually expected as potent chemo-preventive agents due to their ability of scavenging free radicals and chelating metal ions. However, not all the actions of green tea polyphenols are necessarily beneficial. In the present study, we demonstrated that higher-concentration green tea ployphenols significantly enhanced the neurotoxicity by treatment of sodium nitroprusside (SNP), a nitric oxide donor. SNP induced apoptosis in human neuroblastoma SH-SY5Y cells in a concentration and time-dependent manner, as estimated by cell viability assessment, FACScan analysis and DNA fragmentation assay, whereas treatment with green tea polyphenols alone had no effect on cell viability. Pre-treatment with lower-dose green tea polyphenols (50 and 100 microm) had only a slightly deleterious effect in the presence of SNP, while higher-dose green tea polyphenols (200 and 500 microm) synergistically damaged the cells severely. Further research showed that co-incubation of green tea polyphenols and SNP caused loss of mitochondrial membrane potential, depletion of intracellular GSH and accumulation of reactive oxygen species, and exacerbated NO-induced neuronal apoptosis via a Bcl-2 sensitive pathway.

Paraquat leads to dopaminergic neural vulnerability in organotypic midbrain culture

Paraquat (1,1'-dimethyl-4,4'-bipyridinium, PQ) is a herbicide to possibly induce Parkinson's disease (PD), since a strong correlation has been found between the incidence of the disease and the amount of PQ used. In this study, we examined PQ toxicity in rat organotypic midbrain slice cultures. PQ dose dependently reduced the number of dopaminergic neurons in cultured slices. Since this damage was prevented by GBR-12909, the dopamine transporter could be an initial step of the PQ induced dopaminergic neurotoxicity. The sequential treatments with lower PQ and 1-methyl-4-phenyl pyridinium (MPP+) doses, where each dose alone was not lethal, markedly killed dopamine neurons, suggesting that the exposure of a lower dose of PQ could lead to the vulnerability of dopaminergic neurons. This cell death was prevented by the inhibitors of NMDA, nitric oxide synthase (NOS), cycloheximide and caspase cascade. Neurons expressing NOS were identified inside and around the regions where dopamine neurons were packed. The cell death induced by the sequential treatments with PQ and MPP+ was also rescued by L-deprenyl and dopamine D2/3 agonists. These results strongly support that the constant exposure to low levels of PQ would lead to the vulnerability of dopaminergic neurons in the nigrostriatal system by the excitotoxic pathway, and might potentiate neurodegeneration caused by the exposure of other substances and aging.

Paraquat induces long-lasting dopamine overflow through the excitotoxic pathway in the striatum of freely moving rats

The herbicide paraquat is an environmental factor that could be involved in the etiology of Parkinson's disease. We have previously shown that paraquat penetrates through the blood-brain barrier and is taken up by neural cells. In this study, we examined the in vivo toxic mechanism of paraquat to dopamine neurons. GBR-12909, a selective dopamine transporter inhibitor, reduced paraquat uptake into the striatal tissue including dopaminergic terminals. The subchronic treatment with systemic paraquat significantly decreased brain dopamine content in the striatum and slightly in the midbrain and cortex, and was accompanied by the diminished level of its acidic metabolites in rats. When paraquat was administered through a microdialysis probe, a transitory increase in the extracellular levels of glutamate, followed by long-lasting elevations of the extracellular levels of NO(x)(-) (NO(2)(-) plus NO(3)(-)) and dopamine were detected in the striatum of freely moving rats. This dopamine overflow lasted for more than 24 h after the paraquat treatment. Dopamine overflow was inhibited by N(G)-nitro-L-arginine methyl ester, dizocilpine, 6,7-dinitroquinoxaline-2,3-dione and L-deprenyl. The toxic mechanism of paraquat involves glutamate induced activation of non-NMDA receptors, resulting in activation of NMDA receptor-channels. The influx of Ca(2+) into cells stimulates nitric oxide synthase. Released NO would diffuse to dopaminergic terminals and further induce mitochondrial dysfunction by the formation of peroxynitrite, resulting in continuous and long-lasting dopamine overflow. The constant exposure to low levels of paraquat may lead to the vulnerability of dopaminergic terminals in humans, and might potentiate neurodegeneration caused by the exposure of other substances, such as endogenous dopaminergic toxins.

Nitric oxide triggers the toxicity due to glutathione depletion in midbrain cultures through 12-lipoxygenase

Glutathione (GSH) depletion is the earliest biochemical alteration shown to date in brains of Parkinson's disease patients. However, data from animal models show that GSH depletion by itself is not sufficient to induce nigral degeneration. We have previously shown that non-toxic inhibition of GSH synthesis with l-buthionine-(S,R)-sulfoximine in primary midbrain cultures transforms a nitric oxide (NO) neurotrophic effect, selective for dopamine neurons, into a toxic effect with participation of guanylate cyclase (GC) and cGMP-dependent protein kinase (PKG) (Canals, S., Casarejos, M. J., de Bernardo, S., Rodríguez-Martín, E., and Mena, M. A. (2001) J. Neurochem. 79, 1183-1195). Here we demonstrate that arachidonic acid (AA) metabolism through the 12-lipoxygenase (12-LOX) pathway is also central for this GSH-NO interaction. LOX inhibitors (nordihydroguaiaretic acid and baicalein), but not cyclooxygenase (indomethacin) or epoxygenase (clotrimazole) ones, prevent cell death in the culture, even when added 10 h after NO treatment. Furthermore, the addition of AA to GSH-depleted cultures precipitates a cell death process that is indistinguishable from that initiated by NO in its morphology, time course, and 12-LOX, GC, and PKG dependence. The first AA metabolite through the 12-LOX enzyme, 12-hydroperoxyeicosatetraenoic acid, induces cell death in the culture, and its toxicity is greatly enhanced by GSH depletion. In addition we show that if GSH synthesis inhibition persists for up to 4 days without any additional treatment, it will induce a cell death process that also depends on 12-LOX, GC, and PKG activation. In this study, therefore, we show that the signaling pathway AA/12-LOX/12-HPETE/GC/PKG may be important in several pathologies in which GSH decrease has been documented, such as Parkinson's disease. The potentiating effect of NO over such a signaling pathway may be of relevance as part of the cascade of events leading to and sustaining nerve cell death.

Phosphorylation of NF-kappaB proteins by cyclic GMP-dependent kinase. A noncanonical pathway to NF-kappaB activation

The transcription factor NF-kappaB is activated in cellular stress responses. This requires rapid regulation of its function, which is accomplished, in part, by various modes of phosphorylation. Even though diverse DNA binding subunits of NF-kappaB proteins may transactivate from distinct recognition sequences, the differential regulation of transcription from the large number of NF-kappaB responsive sites in various gene promoters and enhancers has been incompletely understood. The cyclic GMP-dependent kinase (PKG) is an important mediator of signal transduction that may induce gene expression through cAMP response element binding protein (CREB) and through other, yet undefined, mechanisms. We have previously characterized a signal transduction pathway that leads to activation-induced cell death in T-lymphocytes and involves the activation of PKG. Here we demonstrate that the NF-kappaB proteins p65, p49 (also called p52), and p50 are specific substrates for this kinase. PKG dose-dependently increases the transactivating activity of p65 from the NF-kappaB consensus sequence. It also mediates dose-dependently an increase in transcriptional activity by p49 or p50 from a unique CCAAT/enhance binding protein (C/EBP)-associated NF-kappaB site, but not from the consensus site. Phosphorylation of p65, p50, or p49 does not alter their subcellular distribution. Because the release of cytosolic p65/p50 heterodimers into the nucleus is by itself insufficient to differentiate all the numerous NF-kappaB promoter sequences, phosphorylation of the DNA-binding subunits reveals a form of differential regulation of NF-kappaB activity and it implies a novel pathway for PKG-induced gene transcription. These observations may bear on mechanisms of programmed cell death in T-lymphocytes. They may also be relevant to ongoing efforts to induce cancer cell apoptosis through activation of PKG.

Zinc toxicity on neonatal cortical neurons: involvement of glutathione chelation

Several mechanisms have been implicated in pathological neuronal death including zinc neurotoxicity, calcium excitotoxicity and oxidative injury. Glutathione (GSH) serves to provide reducing equivalents for the maintenance of oxidant homeostasis, and also plays roles in intracellular and intercellular signaling in the brain. We investigated the role of GSH homeostasis in the neurotoxic action of zinc using both mixed cortical cultures containing neurons and glia, and cortical neurons prepared from 1-day-old rats. Zinc caused neuronal cell death in a concentration-dependent manner. In parallel, a high concentration of zinc depleted GSH, in a time-dependent manner, preceding the onset of neuronal damage. Depletion of GSH by diethylmaleate injured neurons and exacerbated zinc-induced death. In contrast, replenishment of GSH attenuated zinc neurotoxicity. The thiol-containing compounds N-acetylcysteine and GSH chemically chelated zinc leading to decreases in the influx of zinc, the fall in GSH level and neuronal death. Interestingly, the glycolytic substrate pyruvate, but not lactate, chelated zinc concentration dependently and prevented its toxicity. On the other hand, pyrrolidine dithiocarbamate, serving as a zinc chaperon, enhanced its entry and toxicity. The results suggest that zinc non-enzymatically depleted GSH, an intrinsic factor for neuron survival, leading to activation of the cellular death signal and eventually neuronal death.

Cytosolic and mitochondrial glutathione in microglial cells are differentially affected by oxidative/nitrosative stress

Glutathione (GSH), the major cellular protectant against reactive oxygen and nitrogen species, is compartmentalized in a cytosolic (c) and a mitochondrial (mt) pool. We investigated how c-GSH and mt-GSH are differentially affected by endogenously produced nitric oxide (NO). Microglial cell line (N9) cultures were immunostimulated with lipopolysaccharide/interferon-gamma to elicit the inducible isoform of NO synthase (iNOS). Despite a significant reduction in total GSH, the mt-GSH remained nearly unaffected by iNOS-mediated NO production. To investigate possible consequences of GSH depletion on the mitochondrial membrane potential, we used buthionine sulfoximine (BSO) to reduce separately the c-GSH, whereas ethacrynic acid (EA) was applied to deplete both mt-GSH and c-GSH. The mitochondrial membrane potential was more vulnerable to NO exposure in EA-pretreated cultures than in BSO-pretreated cultures, indicated by a potentiated release of tetramethylrhodamine from mitochondria into the cytosol. To relate the EA-mediated decrease in mitochondrial membrane potential to the oxidant buildup after GSH depletion, we loaded the cells with the oxidant-sensitive fluorochrome 2',7'-dihydrodichlorofluorescein (DCF) diacetate. EA treatment caused an increase in DCF fluorescence over time that was potentiated when the iNOS expression was stimulated. Inhibition of NO production abolished this effect. We conclude that endogenous NO production in microglial cells does not compromise the mt-GSH pool which, in turn, might explain the ability of these cells to combat high-output NO production.

Thiolic antioxidants protect from nitric oxide-induced toxicity in fetal midbrain cultures

Nitric oxide (NO) may act as a neuroprotector or neurotoxic agent in dopamine neurons, depending on cell redox status. We have investigated the effect of several thiolic antioxidants, glutathione (GSH), its cell permeable analog GSH ethyl ester (GSHEE), and the GSH synthesis precursor L-N-acetyl cysteine (L-NAC), as well as non-thiolic antioxidants like ascorbic acid (AA) and uric acid, on NO-induced toxicity in fetal midbrain cultures. The cultures were treated for 8-24 h with neurotoxic doses of the NO donor diethylamine/nitric oxide complex sodium DEA/NO (200-400 micro M) and/or antioxidants. Thiolic antioxidants, at equimolar concentrations, added at the same time or previous to DEA/NO, protected from cell death, from tyrosine hydroxylase (TH) positive cell number decrease and from intracellular GSH depletion, induced by DEA/NO, without increasing intracellular GSH content. In these conditions, S-nitrosothiol compound formation was detected in the culture media. Protection disappeared when antioxidants were supplied 30 min after NO treatment. Nevertheless, non-thiolic antioxidants, AA and uric acid, with similar peroxynitrite scavenging activity to thiolic antioxidants, and free radical-scavenging enzymes as catalase and Cu/Zn-superoxide dismutase, which prevent extracellular peroxynitrite ion formation, and 4,5-dihydroxy-1,3-benzene-disulfonic acid (Tiron), which prevents intracellular peroxynitrite ion formation, did not rescue cell cultures from neurotoxicity induced by NO. In addition, AA exacerbated DEA/NO-induced toxicity in a dose-dependent manner from 200 micro M AA. The present results suggest that only antioxidants with thiol group exert neuroprotection from NO-induced toxicity in fetal midbrain cultures, probably by direct interaction of NO and thiol groups, resulting in NO blocking. On the other hand, some classical antioxidants, like AA, exacerbate neurotoxicity due to NO.