Glutathione-related enzymes in brain in Parkinson's disease

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.

When should levodopa therapy be initiated in patients with Parkinson's disease?

Levodopa is available in three forms: immediate-release, orally disintegrating and sustained-release tablets. Levodopa is metabolised in the gastrointestinal tract, kidney and liver by aromatic acid dopa decarboxylase using pyridoxine as a cofactor. Approximately 70-80% of the dose is eliminated in the urine. Central conversion of levodopa to dopamine likely occurs at surviving dopaminergic terminals and at serotonergic and adrenergic nerve terminals that contain decarboxylase. Dopamine is metabolised by catechol-O-methyltransferase and monoamine oxidase. The major metabolites of dopamine are homovanillic acid and dihydroxyphenylacetic acid. Levodopa remains the most efficacious pharmacological treatment for the symptoms of Parkinson's disease (PD). Results of current levodopa trials suggest that treatment with levodopa at the onset of disease provides superior motor and functional control compared with dopamine receptor agonists. Moreover, levodopa is generally better tolerated with a lower incidence of gastrointestinal and neuropsychiatric adverse effects. The debate over the role of levodopa in the treatment of PD is fuelled by the results of in vitro studies that show generation of free radicals by levodopa and its toxic effects on cell cultures. Levodopa has also consistently been shown to produce motor fluctuations (in particular dyskinesias) sooner than has been observed in PD patients, especially younger patients, given dopamine agonists initially. However, the cumulative body of knowledge thus far does not show definitive evidence that levodopa is neurotoxic to parkinsonian patients. In older PD patients with lesser risk of motor fluctuations, levodopa may be used initially, and perhaps solely, in demented PD patients and those at higher risk of developing neuropsychiatric adverse effects. In young parkinsonian patients with mild motor dysfunction, use of levodopa may be delayed or the dosage minimised. However, because of levodopa's superior efficacy, when a rapid and sustained symptomatic improvement is required because of significant motor disability, levodopa may be used as the first-line agent regardless of age.

Alpha-lipoic acid differently affects the reserpine-induced oxidative stress in the striatum and prefrontal cortex of rat brain

Antioxidative properties of alpha-lipoic acid (LA) are widely investigated in different in vivo and in vitro models. The aim of this study was to examine whether LA attenuates oxidative stress induced in rats by reserpine, a model substance frequently used to produce Parkinsonism in animals. Male Wistar rats were treated with reserpine (5 mg/kg) and LA (50 mg/kg) separately or in combination. The levels of reduced glutathione (GSH), glutathione disulfide (GSSG), nitric oxide (NO) and S-nitrosothiols as well as activities of glutathione peroxidase (GPx), glutathione-S-transferase (GST) and L-gamma-glutamyl transpeptidase (gamma-GT) were determined in the striatum and prefrontal cortex homogenates. In the striatum and prefrontal cortex a single dose of reserpine significantly enhanced levels of GSSG and NO but not that of S-nitrosothiols when compared with control. In the striatum, LA administered jointly with reserpine markedly increased the concentration of GSH and decreased GSSG level. In the prefrontal cortex, such treatment produced only an increasing tendency in GSH level but caused no changes in GSSG content. In both structures LA injected jointly with reserpine markedly decreased NO concentrations but did not cause significant changes in S-nitrosothiol levels when compared with control. Enzymatic activities of GPx and GST were intensified by LA in the striatum. In the prefrontal cortex, GPx activity was not altered, while that of GST was decreased. Gamma-GT activity was attenuated by reserpine in the striatum while LA reversed this effect. Such changes were not observed in the prefrontal cortex. The mode of LA action in the striatum during the reserpine-evoked oxidative stress strongly suggests that this compound may be of therapeutic value in the treatment of Parkinson's disease.

The Etiopathogenesis of Parkinson Disease and Suggestions for Future Research. Part II

We are at a critical juncture in our knowledge of the etiology and pathogenesis of Parkinson disease (PD). It is clear that PD is not a single entity simply resulting from a dopaminergic deficit; rather it is most likely caused by a combination of genetic and environmental factors. Although there is extensive new information on the etiology and pathogenesis of PD, which may advance its treatment, new syntheses of this information are needed. The second part of this two-part, state-of-the-art review by leaders in PD research critically examines the research field to identify areas for which new knowledge and ideas might be helpful for treatment purposes. Topics reviewed in Part II are genetics, animal models, and oxidative stress.

The medial and lateral substantia nigra in Parkinson's disease: mRNA profiles associated with higher brain tissue vulnerability

Sporadic Parkinson's disease (PD) is characterized by progressive death of dopaminergic neurons within the substantia nigra. However, pathological cell death within this nucleus is not uniform. In PD, the lateral tier of the substantia nigra (SNl) degenerates earlier and more severely than the more medial nigral component (SNm). The cause of this brain regional vulnerability remains unknown. We have used DNA oligonucleotide microarrays to compare gene expression profiles from the SNl to those of the SNm in both PD and control cases. Genes expressed more highly in the PD SNl included the cell death gene, p53 effector related to PMP22, the tumour necrosis factor (TNF) receptor gene, TNF receptor superfamily, member 21, and the mitochondrial complex I gene, NADH dehydrogenase (ubiquinone) 1beta subcomplex, 3, 12 kDa (NDUFbeta3). Genes that were more highly expressed in PD SNm included the dopamine cell signalling gene, cyclic adenosine monophosphate-regulated phosphoprotein, 21 kDa, the activated macrophage gene, stabilin 1, and two glutathione peroxidase (GPX) genes, GPX1 and GPX3. Thus, there is increased expression of genes encoding pro-inflammatory cytokines and subunits of the mitochondrial electron transport chain, and there is a decreased expression of several glutathione-related genes in the SNl suggesting a molecular basis for pathoclisis. Importantly, some of the genes that are differentially regulated in the SNl are known to be expressed highly or predominantely in glial cells. These findings support the view that glial cells can be primarily affected in PD emphasizing the importance of using a whole tissue approach when investigating degenerative CNS disease.

Neuroinflammation, Oxidative Stress and the Pathogenesis of Parkinson's Disease

Neuroinflammatory processes play a significant role in the pathogenesis of Parkinson's disease (PD). Epidemiologic, animal, human, and therapeutic studies all support the presence of an neuroinflammatory cascade in disease. This is highlighted by the neurotoxic potential of microglia . In steady state, microglia serve to protect the nervous system by acting as debris scavengers, killers of microbial pathogens, and regulators of innate and adaptive immune responses. In neurodegenerative diseases, activated microglia affect neuronal injury and death through production of glutamate, pro-inflammatory factors, reactive oxygen species, quinolinic acid amongst others and by mobilization of adaptive immune responses and cell chemotaxis leading to transendothelial migration of immunocytes across the blood-brain barrier and perpetuation of neural damage. As disease progresses, inflammatory secretions engage neighboring glial cells, including astrocytes and endothelial cells, resulting in a vicious cycle of autocrine and paracrine amplification of inflammation perpetuating tissue injury. Such pathogenic processes contribute to neurodegeneration in PD. Research from others and our own laboratories seek to harness such inflammatory processes with the singular goal of developing therapeutic interventions that positively affect the tempo and progression of human disease.

Redox control of neural function: background, mechanisms, and significance

The redox environment within neural cells is dependent on a series of redox couples. The glutathione disulfide/ glutathione (GSSG/GSH) redox pair forms the major redox couple in cells and as such plays a critical role in regulating redox-dependent cellular functions. Not only does GSH act as an antioxidant but it also can modulate the activity of a variety of different proteins via S-glutathionylation of cysteine sulfhydryl groups. The thioredoxin system also makes a significant contribution to the redox environment by reducing inter- and intrachain protein disulfide bonds as well as maintaining the activity of important antioxidant enzymes such as peroxiredoxins and methionine sulfoxide reductases. The redox environment affects the activity and function of a number of different protein phosphatases, protein kinases, and transcription factors. The sum of these effects will determine how changes in the redox environment alter overall cellular function, thereby playing a fundamental role in regulating neural cell fate and physiology.

Modulation of microglial pro-inflammatory and neurotoxic activity for the treatment of Parkinson's disease

Parkinson's disease (PD) is a debilitating movement disorder resulting from a progressive degeneration of the nigrostriatal dopaminergic pathway and depletion of neurotransmitter dopamine in the striatum. Molecular cloning studies have identified nearly a dozen genes or loci that are associated with small clusters of mostly early onset and genetic forms of PD. The etiology of the vast majority of PD cases remains unknown, and the precise molecular and biochemical processes governing the selective and progressive degeneration of the nigrostriatal dopaminergic pathway are poorly understood. Current drug therapies for PD are symptomatic and appear to bear little effect on the progressive neurodegenerative process. Studies of postmortem PD brains and various cellular and animal models of PD in the last 2 decades strongly suggest that the generation of pro-inflammatory and neurotoxic factors by the resident brain immune cells, microglia, plays a prominent role in mediating the progressive neurodegenerative process. This review discusses literature supporting the possibility of modulating the activity of microglia as a neuroprotective strategy for the treatment of PD.

Effects of inhaled manganese on biomarkers of oxidative stress in the rat brain

Manganese (Mn) is a ubiquitous and essential element that can be toxic at high doses. In individuals exposed to high levels of this metal, Mn can accumulate in various brain regions, leading to neurotoxicity. In particular, Mn accumulation in the mid-brain structures, such as the globus pallidus and striatum, can lead to a Parkinson's-like movement disorder known as manganism. While the mechanism of this toxicity is currently unknown, it has been postulated that Mn may be involved in the generation of reactive oxygen species (ROS) through interaction with intracellular molecules, such as superoxide and hydrogen peroxide, produced within mitochondria. Conversely, Mn is a required component of an important antioxidant enzyme, Mn superoxide dismutase (MnSOD), while glutamine synthetase (GS), a Mn-containing astrocyte-specific enzyme, is exquisitely sensitive to oxidative stress. To investigate the possible role of oxidative stress in Mn-induced neurotoxicity, a series of inhalation studies was performed in neonatal and adult male and female rats as well as senescent male rats exposed to various levels of airborne-Mn for periods of time ranging from 14 to 90 days. Oxidative stress was then indirectly assessed by measuring glutathione (GSH), metallothionein (MT), and GS levels in several brain regions. MT and GS mRNA levels and regional brain Mn concentrations were also determined. The collective results of these studies argue against extensive involvement of ROS in Mn neurotoxicity in rats of differing genders and ages. There are, however, instances of changes in individual endpoints consistent with oxidative stress in certain brain tissues.

Dynamic modeling of alpha-synuclein aggregation for the sporadic and genetic forms of Parkinson's disease

Excessive accumulation of alpha synuclein (a-syn) in the brain has been implicated in several degenerative neurological disorders, most notably Parkinson's disease. The aggregation of a-syn is the major component of intraneuronal inclusions, Lewy bodies, which are neuropathological features, observed in Parkinson's disease, Lewy body dementia, and other synucleopathies. Diverse cellular events can contribute to a-syn accumulation, aggregation, and to subsequent Lewy body formation. These factors include genetic mutations of synuclein, parkin, or the deubiquitinating enzyme, ubiquitin C-terminal hydrolase (UCH-L1), leading to reduced clearance of a-syn by the ubiquitin proteasomal pathway (UPP). Furthermore, intracellular insults include environmental factors and an age-related decrement in antioxidant defense systems that increase oxidative stress and can affect either the accumulation or clearance of a-syn. We have dynamically modeled a-syn processing in normal and in several disease states; focusing upon alterations in the aggregation and clearance of a-syn as influenced by the UPP and the oxidative stress pathways. Simulation of increased oxidative stress generates a free radical profile analogous to that reported in vivo following exposure to the neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Varying model parameters of oxidative stress, UPP dysfunction, or both pathways, simulate kinetics of a-syn that corresponds with the neuropathology described for the sporadic and genetic forms of Parkinson's disease. This in silico model provides a mathematical framework that enables kinetic appraisal of pathway components to better identify and validate important pharmacological targets.

Paraquat exposure as an etiological factor of Parkinson's disease

Parkinson's disease (PD) is a multifactorial chronic progressive neurodegenerative disease influenced by age, and by genetic and environmental factors. The role of genetic predisposition in PD has been increasingly acknowledged and a number of relevant genes have been identified (e.g., genes encoding alpha-synuclein, parkin, and dardarin), while the search for environmental factors that influence the pathogenesis of PD has only recently begun to escalate. In recent years, the investigation on paraquat (PQ) toxicity has suggested that this herbicide might be an environmental factor contributing to this neurodegenerative disorder. Although the biochemical mechanism through which PQ causes neurodegeneration in PD is not yet fully understood, PQ-induced lipid peroxidation and consequent cell death of dopaminergic neurons can be responsible for the onset of the Parkinsonian syndrome, thus indicating that this herbicide may induce PD or influence its natural course. PQ has also been recently considered as an eligible candidate for inducing the Parkinsonian syndrome in laboratory animals, and can therefore constitute an alternative tool in suitable animal models for the study of PD. In the present review, the recent evidences linking PQ exposure with PD development are discussed, with the aim of encouraging new perspectives and further investigation on the involvement of environmental agents in PD.

Inhibitors of alpha-synuclein oligomerization and toxicity: a future therapeutic strategy for Parkinson's disease and related disorders

An abundance of genetic, histopathological, and biochemical evidence has implicated the neuronal protein, alpha-synuclein (alpha-syn) as a key player in the development of several neurodegenerative diseases, the so-called synucleinopathies, of which Parkinson's disease (PD) is the most prevalent. Development of disease appears to be linked to events that increase the intracellular concentration of alpha-syn or cause its chemical modification, either of which can accelerate the rate at which it forms aggregates. Examples of such events include increased copy number of genes, decreased rate of degradation via the proteasome or other proteases, or altered forms of alpha-syn, such as truncations, missense mutations, or chemical modifications by oxidative reactions. Aggregated forms of the protein, especially newly formed soluble aggregates, are toxic to cells, so that one therapeutic strategy would be to reduce the rate at which such oligomerization occurs. We have therefore designed several peptides and also identified small molecules that can inhibit alpha-syn oligomerization and toxicity in vitro. These compounds could serve as lead compounds for the design of new drugs for the treatment of PD and related disorders in the future.

Gamma-(monophenyl)phosphono glutamate analogues as mechanism-based inhibitors of gamma-glutamyl transpeptidase

Gamma-glutamyl transpeptidase (GGT, EC 2.3.2.2) catalyzes the hydrolysis and transpeptidation of extracellular glutathione and plays a central role in glutathione homeostasis. We report here the synthesis and evaluation of a series of hydrolytically stable gamma-(monophenyl)phosphono glutamate analogues with varying electron-withdrawing para substituents on the leaving group phenols as mechanism-based and transition-state analogue inhibitors of Escherichia coli and human GGTs. The monophenyl phosphonates caused time-dependent and irreversible inhibition of both the E. coli and human enzymes probably by phosphonylating the catalytic Thr residue of the enzyme. The inactivation rate of E. coli GGT was highly dependent on the leaving group ability of phenols with electron-withdrawing groups substantially accelerating the rate (Brønsted betalg = -1.4), whereas the inactivation of human GGT was rather slow and almost independent on the nature of the leaving group. The inhibition potency and profiles of the phosphonate analogues were compared to those of acivicin, a classical inhibitor of GGT, suggesting that the phosphonate-based glutamate analogues served as a promising candidate for potent and selective GGT inhibitors.

Up-regulation of gamma-glutamyl transpeptidase activity following glutathione depletion has a compensatory rather than an inhibitory effect on mitochondrial complex I activity: implications for Parkinson's disease

Up-regulation of activity of gamma-glutamyl transpeptidase (GGT) has been reported to occur in the Parkinsonian substantia nigra, the area of the brain affected by the disease. Increased GGT activity has been hypothesized to play a role in subsequent mitochondrial complex I (CI) inhibition by increasing cysteine as substrate for cellular uptake. Intracellular cysteine has been proposed to form toxic adducts with dopamine which can be metabolized to compounds which inhibit CI activity. We have demonstrated that in addition to CI inhibition, GGT activity is up-regulated in dopaminergic cells as a consequence of glutathione depletion. Inhibition of GGT rather than resulting in increased CI inhibition results in exacerbation of this inhibitory effect. This suggests that increased GGT activity is likely an adaptive response to the loss of glutathione to conserve intracellular glutathione content and results in a compensatory effect on CI activity rather than in its inhibition as has been previously widely hypothesized.

Gamma-glutamyl transpeptidase substrate specificity and catalytic mechanism

The enzyme gamma-glutamyltranspeptidase (GGT) is critical to cellular detoxification and leukotriene biosynthesis processes, as well as amino acid transport in kidneys. GGT has also been implicated in many important physiological disorders, including Parkinson's disease and inhibition of apoptosis. It binds glutathione as donor substrate and initially forms a gamma-glutamyl enzyme that can then react with a water molecule or an acceptor substrate (usually an amino acid or a dipeptide) to form glutamate or a product containing a new gamma-glutamyl isopeptide bond, respectively, thus regenerating the free enzyme. Given the importance of GGT in human physiology, we have undertaken studies of its substrate specificity and catalytic mechanism. In the course of these studies, we have developed methods for the indirect evaluation of donor substrate affinity and stereospecificity and applied others for the measurement of steady state and pre-steady state kinetics and linear free-energy relationships. These methods and the pertinent results obtained with them are presented herein.

γ-Glutamyl transpeptidase acylation with peptidic substrates: free energy relationships measured by an HPLC kinetic assay

gamma-Glutamyl transpeptidase (GGT, EC 2.3.2.2) is a highly glycosylated heterodimeric enzyme linked to the external cellular membrane that catalyzes the hydrolysis of glutathione as well as the transfer of its gamma-glutamyl group to amino acids and dipeptides in a transpeptidation reaction. The measurement of both the hydrolytic and transpeptidation activity of this important enzyme is a challenge, since its native substrates are not highly chromogenic. We have developed an HPLC-based method for the quantitative photometric detection of numerous enzyme substrates and products, after their pre-column derivation with dabsyl chloride. The broad applicability of this method was demonstrated in the kinetic investigation of transpeptidation reactions of rat kidney GGT with glutathione, its native substrate, as well as a series of pertinent glutathione analogues. The pH-rate profile constructed for glutathione confirmed the dependence on the ionisation state of at least two residues. Analysis of the free-energy relationships in the series of synthetic peptidic substrate analogues revealed the importance of enzyme-substrate interactions unrelated to amine leaving group basicity during the acylation step. These results are further interpreted in the context of the recently published structure for a similar GGT.

The physiology and pathophysiology of nitric oxide in the brain

Nitric oxide (NO) is a molecule with pleiotropic effects in different tissues. NO is synthesized by NO synthases (NOS), a family with four major types: endothelial, neuronal, inducible and mitochondrial. They can be found in almost all the tissues and they can even co-exist in the same tissue. NO is a well-known vasorelaxant agent, but it works as a neurotransmitter when produced by neurons and is also involved in defense functions when it is produced by immune and glial cells. NO is thermodynamically unstable and tends to react with other molecules, resulting in the oxidation, nitrosylation or nitration of proteins, with the concomitant effects on many cellular mechanisms. NO intracellular signaling involves the activation of guanylate cyclase but it also interacts with MAPKs, apoptosis-related proteins, and mitochondrial respiratory chain or anti-proliferative molecules. It also plays a role in post-translational modification of proteins and protein degradation by the proteasome. However, under pathophysiological conditions NO has damaging effects. In disorders involving oxidative stress, such as Alzheimer's disease, stroke and Parkinson's disease, NO increases cell damage through the formation of highly reactive peroxynitrite. The paradox of beneficial and damaging effects of NO will be discussed in this review.

Glia cell number modulates sensitivity to MPTP in mice

Free radical damage has been shown to play a significant role in the pathogenesis of a number of neurodegenerative diseases including Parkinson's disease. One model of experimental parkinsonism is the loss of substantia nigra cells following administration of MPTP. Previously, it has been shown that a number of inbred strains of mice have differential responses to this toxin, and this difference is dependent on glial cells. In this study, the number of glial cells in the substantia nigra pars compacta of C57Bl/6J (MPTP-sensitive) and Swiss Webster (MPTP-resistant) strains of mice was examined. The C57Bl/6J mice have an approximately 50% lower number of GFAP+ and S-100beta glial cells than the Swiss Webster mice. C57Bl/6J mice have a 25% increased number of resident nonactivated microglial cells. To determine whether this difference in cell number has functional significance, we used an in vitro SN culture system that allowed us to manipulate the number of glial cells. When C57Bl/6 neurons were grown on a glial mat plated with twice the number of cells, we were able to rescue the MPTP-sensitive neurons from toxin-induced cell death. This suggests that the number of glial cells in the SNpc may be an important factor in the survival of dopaminergic neurons following exposure to xenobiotics.

The bipyridyl herbicide paraquat produces oxidative stress-mediated toxicity in human neuroblastoma SH-SY5Y cells: relevance to the dopaminergic pathogenesis

Paraquat (PQ) is a cationic nonselective bipyridyl herbicide widely used to control weeds and grasses in agriculture. Epidemiologic studies indicate that exposure to pesticides can be a risk factor in the incidence of Parkinson's disease (PD). A strong correlation has been reported between exposure to paraquat and PD incidence in Canada, Taiwan, and the United States. This correlation is supported by animal studies showing that paraquat produces toxicity in dopaminergic neurons of the rat and mouse brain. However, it is unclear how paraquat triggers toxicity in dopaminergic neurons. Based on the prooxidant properties of paraquat, it was hypothesized that paraquat may induce oxidative stress-mediated toxicity in dopaminergic neurons. To explore this possibility, dopaminergic SH-SY5Y cells were treated with paraquat, and several biomarkers of oxidativestress were measured. First, a specific dopamine transporter inhibitor GBR12909 significantly protected SY5Y cells against the toxicity of paraquat, indicating that paraquat exerts its toxicity by a mechanism involving the dopamine transporter (DAT). Second, paraquat increased intracellular levels of reactive oxygen species (ROS), but decreased the levels of glutathione. Third, paraquat inhibited glutathione peroxidase activity, but did not affect glutathione reductase activity. On the other hand, paraquat increased GST activity by 24 h, after which GST activity returned to the control value at 48 h. Fourth, paraquat dissipated mitochondrial transmembrane potential (MTP). Fifth, paraquat produced increases of malondialdehyde (MDA) and protein carbonyls, as well as DNA fragmentation, indicating oxidative damage to major cellular components. Sixth, paraquat increased the protein level of heme oxygenase-1 (HO-1). Taken together, these findings verify our hypothesis that paraquat produces oxidative stress-mediated toxicity in SH-SY5Y cells. Thus, current findings suggest that paraquat may induce the pathogenesis of dopaminergic neurons through oxidative stress.

Lack of glutathione peroxidase-1 exacerbates Abeta-mediated neurotoxicity in cortical neurons

The aetiologies of Alzheimer's disease (AD) are complex and multifactorial. Current therapies are largely ineffective, as the pathophysiological pathways are poorly understood. Observations in AD autopsies, as well as in vivo and in vitro observations in transgenic mice, have implicated oxidative stress as pathogenic in AD. This study used the Glutathione Peroxidase-1 knockout mouse (Gpx1--/--) model to investigate the role of antioxidant disparity in neuropathologies. Cultured neurons from control and Gpx1--/-- embryos were treated with AD-related peptides and the degree of cell loss compared. Results show that antioxidant disparity makes Gpx1--/-- cells more susceptible to Abeta toxicity. Surrogate replacement of Gpx1 with the reactive oxygen species scavenger N-acetyl cysteine and the Gpx1 mimetic ebselen, reverses the Gpx1--/-- increased susceptibility to Abeta toxicity. Such results support a role for oxidative stress in AD-related neuronal loss. This study is the first to report such findings using the Gpx1--/-- model, and supports a role for oxidative stress as one of the contributing factors, in development of AD-like pathologies.

α-Synuclein aggregation in neurodegenerative diseases and its inhibition as a potential therapeutic strategy

There is strong evidence for the involvement of α-synuclein in the pathologies of several neurodegenerative disorders, including PD (Parkinson's disease). Development of disease appears to be linked to processes that increase the rate at which α-synuclein forms aggregates. These processes include increased protein concentration (via either increased rate of synthesis or decreased rate of degradation), and altered forms of α-synuclein (such as truncations, missense mutations, or chemical modifications by oxidative reactions). Aggregated forms of the protein are toxic to cells and one therapeutic strategy would be to reduce the rate at which aggregation occurs. To this end we have designed several peptides that reduce α-synuclein aggregation. A cell-permeable version of one such peptide was able to inhibit the DNA damage induced by Fe(II) in neuronal cells transfected with α-synuclein (A53T), a familial PD-associated mutation.

Mitochondrial inhibition and oxidative stress: reciprocating players in neurodegeneration

Although the etiology for many neurodegenerative diseases is unknown, the common findings of mitochondrial defects and oxidative damage posit these events as contributing factors. The temporal conundrum of whether mitochondrial defects lead to enhanced reactive oxygen species generation, or conversely, if oxidative stress is the underlying cause of the mitochondrial defects remains enigmatic. This review focuses on evidence to show that either event can lead to the evolution of the other with subsequent neuronal cell loss. Glutathione is a major antioxidant system used by cells and mitochondria for protection and is altered in a number of neurodegenerative and neuropathological conditions. This review also addresses the multiple roles for glutathione during mitochondrial inhibition or oxidative stress. Protein aggregation and inclusions are hallmarks of a number of neurodegenerative diseases. Recent evidence that links protein aggregation to oxidative stress and mitochondrial dysfunction will also be examined. Lastly, current therapies that target mitochondrial dysfunction or oxidative stress are discussed.

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.

Induction of the Protective Antioxidant Response Element Pathway by 6-Hydroxydopamine In Vivo and In Vitro

Parkinson's disease, a progressive neurodegenerative disorder, is characterized by loss of midbrain dopaminergic neurons. The etiology of sporadic Parkinson's disease is unknown; however, oxidative stress is thought to play a major role in disease pathogenesis. Little is known regarding the transcriptional changes that occur in Parkinson's disease. The antioxidant response element is a cis-acting enhancer sequence that is upstream of many phase II detoxification and antioxidant genes. Here we show that 6-hydroxydopamine, a mitochondrial inhibitor used to model Parkinson's disease, activates the antioxidant response element both in cultured neurons and in the striatum and brainstem of 6-OHDA-lesioned mice. Pretreatment with antioxidants or NMDA receptor antagonists reduced but did not abolish activation. Further induction of this pathway with tert-butylhydroquinone was able to significantly reduce cell death due to 6-OHDA in vitro. These observations indicate that 6-OHDA activates the antioxidant response element through components of oxidative stress, excitotoxicity, and potential structural factors. Further induction of this endogenous defense mechanism may suggest a novel therapeutic venue in Parkinson's disease.

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.

Synthesis of S-alkyl L-homocysteine analogues of glutathione and their kinetic studies with gamma-glutamyl transpeptidase

A series of S-alkyl L-homocysteine analogues of glutathione was synthesized with varied oxidation state of the sulfur and tested for inhibition of rat kidney gamma-glutamyl transpeptidase (GGT). The strong selectivity of the enzyme with respect to the sulfur oxidation state reveals important information for the development of powerful competitive inhibitors.

Increased extracellular glutamate evoked by 1-methyl-4-phenylpyridinium [MPP(+)] in the rat striatum is not essential for dopaminergic neurotoxicity and is not derived from released glutathione

A number of studies have implicated the interactions of the excitatory amino acid L-glutamate (Glu) with its ionotropic and metabotropic receptors as important components of the mechanism underlying the dopaminergic neurotoxicity of 1-methyl-4-phenylpyridinium [MPP(+)]. Furthermore, microdialysis experiments have demonstrated that perfusion of relatively high concentrations of MPP(+) into the rat striatum evoke a delayed, massive release of Glu. Interestingly, perfusion of MPP(+) also mediates a similar release of glutathione (GSH). Together, these observations raise the possibility that the rise of extracellular Glu mediated by MPP(+) may be the result of hydrolysis of released GSH by gamma-glutamyl transpeptidase (gamma-GT). In the present investigation it is demonstrated that perfusions of solutions of 0.7 and 1.3 mM MPP(+) dissolved in artificial cerebrospinal fluid into the rat striatum evoke neurotoxic damage to dopaminergic terminals, assessed by both a two-day test/challenge procedure and tyrosine hydroxylase immunoreactivity, but without the release of Glu. Perfusions of 2.5 mM MPP(+) cause more extensive dopaminergic neurotoxicity and a dose-dependent release of Glu. However, neither this release of Glu nor MPP(+)-induced dopaminergic neurotoxicity are blocked by the irreversible gamma-GT inhibitor acivicin. Together, these observations indicate that a rise of extracellular levels of Glu is not essential for the dopaminergic neurotoxicity of MPP(+). Furthermore, the rise of extracellular Glu caused by perfusion of 2.5 mM MPP(+) is not the result of the gamma-GT-mediated hydrolysis of released GSH. It is possible that the rise of extracellular levels of Glu, L-aspartate, L-glycine and L-taurine evoked by perfusions of 2.5 mM MPP(+) into the rat striatum may reflect, at least in part, the release of these amino acids from astrocytes.

Pharmacological manipulation of ataxia-telangiectasia kinase activity as a treatment for Parkinson’s disease

Parkinson's disease (PD) is a major cause of morbidity and mortality among older individuals. Although the causes of Parkinson's disease are multifactorial, considerable evidence indicates that elevated labile iron in the substantia nigra pars compacta plays an important role in producing oxyradicals which subsequently damage nigro-striatal neurons. Based on this several researchers have suggested that blood-brain barrier crossing iron chelators might have clinical efficacy in treating PD. Work demonstrating that iron chelators protect nigro-striatal neurons in the N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and 6-hydroxydopamine-induced rodent PD models supports this hypothesis. Recently, we found that the ATM gene product (mutated in ataxia-telangiectasia, A-T), is required for cell survival and genomic stability maintenance following exposure to low labile iron concentrations. Iron chelators (desferal, quercetin, and apoferritin) also increase A-T cell genomic stability and viability, and activate ATM-dependent cellular events in normal cells. Additionally Atm-deficient mice exhibit a selective loss of dopaminergic nigro-striatal neurons. Based on this, we propose that iron chelators protect the substantia nigra pars compacta not only by chelating labile iron and reducing oxyradical formation, but also by inducing ATM activity, leading to increased oxidative stress resistance and DNA repair. Support for this hypothesis comes from the recent observation that the iron chelating flavonoid quercetin both directly activates ATM and protects neuronal cells from the toxic effects of the N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Therefore since; (1) ATM is required for iron toxicity resistance, (2) iron chelators such as quercetin, desferal, and apoferritin induce ATM activity and/or ATM-dependent events, and (3), Atm-deficient mice preferentially lose dopaminergic nigro-striatal neurons, we propose that ATM activity has an important function in PD. Furthermore, pharmacological manipulation of ATM activity via iron chelation might have clinical efficacy in PD treatment.

Thiol redox state and oxidative stress in midbrain and striatum of weaver mutant mice, a genetic model of nigrostriatal dopamine deficiency

In this study we measured thiol redox state (TRS) and the oxidative stress indicator lipid peroxidation in midbrain and striatum of adult (4 months old) male control (+/+) and weaver (wv/wv) mice in order to relate them with oxidative stress in conditions of progressive and severe (approximately 70%) nigrostriatal dopaminergic neurodegeneration. Specifically, we measured the specific TRS components glutathione (GSH), glutathione disulfide (GSSG), cysteine (CSH), and the general classes of TRS components. The latter are the protein thiols (PSH) and the disulfides between (a) protein (P) and protein thiols (PSSP), (b) protein and non-protein (NP/R) thiols (PSSR, PSSC) and (c) non-protein and non-protein thiols (NPSSR, NPSSC). In addition, the main product of lipid peroxidation malonyl dialdehyde (MDA) was estimated. In the midbrain of wv/wv, GSH and NPSSC levels are decreased (44% and 64%, respectively) and GSSG, NPSSR, CSH, PSH, PSSP, PSSR and MDA levels are increased (23%, 660%, 110%, 51%, 68%, 18% and 44%, respectively). In the striatum of male wv/wv, protein and non-protein thiol/disulfide and MDA levels do not change, possibly due to the high decrease in striatal dopamine level versus midbrain. Our data show that the high degeneration of the dopaminergic nigrostriatal neurons in male adult wv/wv mice is accompanied by significant changes in TRS and an increase in lipid peroxidation in the midbrain, suggesting involvement of oxidative stress in the degeneration of dopaminergic neurons. They also strengthen the possible use of thiol antioxidants for the development of new neuroprotective therapeutic strategies for neurodegenerative diseases, such as Parkinson's disease.

Redox imbalance

Substantial evidence implies that redox imbalance attributable to an overproduction of reactive oxygen species or reactive nitrogen species that overwhelm the protective defense mechanism of cells contributes to all forms of Parkinson's disease. Factors such as dopamine, neuromelanin, and transition metals may, under certain circumstances, contribute to the formation of oxygen species such as H(2)O(2), superoxide radicals, and hydroxyl radicals and react with reactive nitrogen species such as nitric oxide or peroxinitrite. Mitochodrial dysfunction and excitotoxicity may be a cause and a result of oxidative stress. Consequences of this redox imbalance are lipid peroxidation, oxidation of proteins, DNA damage, and interference of reactive oxygen species with signal transduction pathways. These consequences become even more harmful when genetic variations impair the normal degradation of altered proteins. Therefore, therapeutic strategies must aim at reducing free-radical formation and scavenging free-radicals.

Neuroprotection in Parkinson's disease: an elusive goal

Parkinson's disease is a chronic progressive condition that causes disability and reduction of quality of life. Symptomatic treatments are effective in the early disease; however, with time, most patients develop motor complications. Neuroprotective therapies are those that can slow disease progression; unfortunately, these agents are not available. Advances in the knowledge of the possible pathogenic events that can lead to nigral cell death have increased dramatically. These mechanisms include oxidative stress, mitochondrial dysfunction, inflammation, excitotoxicity, alterations in protein degradation, and ultimately apoptosis. Based on these laboratory scientific findings, a number of agents have been studied in clinical trials. However, how to assess disease evolution and establish reliable endpoints is still an unresolved issue. The monoamine oxidase inhibitors selegiline and rasagiline have been shown to be neuroprotective in vitro and in animal models, but so far this property was not demonstrated in clinical trials. Other agents have been studied and still others are undergoing clinical investigation. These include antiexcitotoxicity drugs like riluzole, the bioenergetic agent coenzyme Q10, trophic factors, and antiapoptotic drugs. Laboratory and clinical data suggest that dopamine agonists may have a neuroprotective action, but this has yet to be proven. However, as our basic and clinical knowledge on Parkinson's disease increases, it is likely that a neuroprotective drug will be found.

Brain antioxidant systems in human methamphetamine users

Animal data suggest that the widely abused psychostimulant methamphetamine can damage brain dopamine neurones by causing dopamine-dependent oxidative stress; however, the relevance to human methamphetamine users is unclear. We measured levels of key antioxidant defences [reduced (GSH) and oxidized (GSSG) glutathione, six major GSH system enzymes, copper-zinc superoxide dismutase (CuZnSOD), uric acid] that are often altered after exposure to oxidative stress, in autopsied brain of human methamphetamine users and matched controls. Changes in the total (n = 20) methamphetamine group were limited to the dopamine-rich caudate (the striatal subdivision with the most severe dopamine loss) in which only activity of CuZnSOD (+ 14%) and GSSG levels (+ 58%) were changed. In the six methamphetamine users with severe (- 72 to - 97%) caudate dopamine loss, caudate CuZnSOD activity (+ 20%) and uric acid levels (+ 63%) were increased with a trend for decreased (- 35%) GSH concentration. Our data suggest that brain levels of many antioxidant systems are preserved in methamphetamine users and that GSH depletion, commonly observed during severe oxidative stress, might occur only with severe dopamine loss. Increased CuZnSOD and uric acid might reflect compensatory responses to oxidative stress. Future studies are necessary to establish whether these changes are associated with oxidative brain damage in human methamphetamine users.

cDNA microarray analysis of changes in gene expression associated with MPP+ toxicity in SH-SY5Y cells

cDNA microarray analysis of 1-methyl-4-phenyl-pyridinium (MPP+) toxicity (1 mM, 72 h) in undifferentiated SH-SY5Y cells identified 48 genes that displayed a signal intensity greater than the mean of all differentially expressed genes and a two-fold or greater difference in normalized expression. RT-PCR analysis of a subset of genes showed that c-Myc and RNA-binding protein 3 (RMB3) expression decreased by approximately 50% after 72 h of exposure to MPP+ (1 mM) but did not change after 72 h of exposure to 6-hydroxydopamine (25 microM), rotenone (50 nM), and hydrogen peroxide (600 microM). Exposure of retinoic acid (RA)-differentiated SH-SY5Y cells to MPP+ (1 mM, 72 h) also resulted in a decrease in RMB3 expression and an increase in GADD153 expression. In contrast, c-Myc expression was slightly increased in RA-differentiated cells. Collectively, these data provide new insights into the molecular mechanisms of MPP+ toxicity and show that MPP+ can elicit distinct patterns of gene expression in undifferentiated and RA-differentiated SH-SY5Y cells.

Potential therapeutic properties of green tea polyphenols in Parkinson's disease

Tea is one of the most frequently consumed beverages in the world. It is rich in polyphenols, a group of compounds that exhibit numerous biochemical activities. Green tea is not fermented and contains more catechins than black tea or oolong tea. Although clinical evidence is still limited, the circumstantial data from several recent studies suggest that green tea polyphenols may promote health and reduce disease occurrence, and possibly protect against Parkinson's disease and other neurodegenerative diseases. Green tea polyphenols have demonstrated neuroprotectant activity in cell cultures and animal models, such as the prevention of neurotoxin-induced cell injury. The biological properties of green tea polyphenols reported in the literature include antioxidant actions, free radical scavenging, iron-chelating properties, (3)H-dopamine and (3)H-methyl-4-phenylpyridine uptake inhibition, catechol-O-methyltransferase activity reduction, protein kinase C or extracellular signal-regulated kinases signal pathway activation, and cell survival/cell cycle gene modulation. All of these biological effects may benefit patients with Parkinson's disease. Despite numerous studies in recent years, the understanding of the biological activities and health benefits of green tea polyphenols is still very limited. Further in-depth studies are needed to investigate the safety and efficacy of green tea in humans and to determine the different mechanisms of green tea in neuroprotection.

Nigral glutathione deficiency is not specific for idiopathic Parkinson's disease

The consistent findings of decreased levels of the major antioxidant glutathione in substantia nigra of patients with idiopathic Parkinson's disease (PD) has provided most of the basis for the oxidative stress hypothesis of the etiology of PD. To establish whether a nigral glutathione deficiency is unique to PD, as is generally assumed, or is present in other Parkinsonian conditions associated with nigral damage, we compared levels of reduced glutathione (GSH) in postmortem brain of patients with PD to those with progressive supranuclear palsy (PSP) and multiple system atrophy (MSA). As compared with the controls, nigral GSH levels were decreased in the PD and PSP patient groups (P < 0.05 for PD [-30%], PSP [-21%]), whereas a similar decrease in the MSA patient group did not reach statistical significance (P = 0.078, MSA [-20%]). GSH levels were normal in all examined normal and degenerating extra-nigral brain areas in PSP and MSA. A trend for decreased levels of uric acid (antioxidant and product of purine catabolism) also was observed in nigra of all patient groups (-19 to -30%). These data suggest that glutathione depletion, possibly consequent to overutilisation in oxidative stress reactions, could play a causal role in nigral degeneration in all nigrostriatal dopamine deficiency disorders, and that antioxidant therapeutic approaches should not be restricted to PD.

TNF? increases activity of ?-glutamyl transpeptidase in cultured rat astroglial cells

To investigate the presence of gamma-glutamyl transpeptidase (gammaGT) in brain cells, cultures enriched for astroglial cells, neurons, oligodendroglial cells, and microglial cells were studied. Astroglial cultures contained a specific gammaGT activity of 2.3 +/- 0.9 nmol/min/mg protein. A similar specific gammaGT activity was measured for oligodendroglial cultures, whereas microglial cells and neurons contained less than 30% of the specific gammaGT activity of astroglial cultures. The activity of gammaGT in astroglial cultures was elevated strongly by the presence of tumor necrosis factor-alpha (TNFalpha) in a time- and concentration-dependent manner. Maximal activity of gammaGT was observed after incubation of astroglial cultures for 3 days with 30 ng/mL TNFalpha. Under these conditions the specific gammaGT activity was increased by threefold compared to controls. Presence of the gammaGT-inhibitor acivicin completely inhibited gammaGT activity both in TNFalpha-treated and in control cells. In addition, the increase in astroglial gammaGT activity after application of TNFalpha was prevented completely by the presence of the protein synthesis inhibitor cycloheximide. gammaGT is involved in extracellular processing of glutathione (GSH) that is exported by astroglial cells. After TNFalpha-treatment the concentration of GSH in the medium of astroglial cells was reduced significantly compared to control cells. In conclusion, the data presented demonstrate that TNFalpha stimulates gammaGT synthesis in astroglial cells and thereby improves the capacity to process GSH exported by these cells.

Protective effect of adenosine in rat model of Parkinson's disease: neurobehavioral and neurochemical evidences

Normal cellular metabolism produces oxidants which are neutralized within the cell by antioxidant enzymes and other antioxidants. An imbalance between oxidant and antioxidant has been postulated to lead the degeneration of dopaminergic neurons in Parkinson's disease. In this study, we examined whether adenosine, an antioxidant, can prevent or slowdown neuronal injury in 6-hydroxydopamine (6-OHDA) model of Parkinsonism. Rats were treated with adenosine (500, 250, 125 mg/kg b.wt.) once before surgery and five times after surgery (1 h interval). 2 microl 6-OHDA (12.5 microg in 0.2% ascorbic acid in normal saline) was infused in the right striatum. Two weeks after 6-OHDA infused rats were tested for neurobehavioral activity and sacrificed after 3 weeks of 6-OHDA infusion, for the estimation of glutathione peroxidase, glutathione-S-transferase, glutathione reductase, glutathione content, lipid peroxidation and dopamine and its metabolites. Adenosine was found to be successful in up-regulating the antioxidant status, lowering the dopamine loss and functional recovery returned close to the baseline dose. This study revealed that adenosine, which is an essential part of our body, might be helpful in slowing down the progression of neurodegeneration in Parkinsonism.

Differential effect of nitric oxide on glutathione metabolism and mitochondrial function in astrocytes and neurones: implications for neuroprotection/neurodegeneration?

Primary culture rat astrocytes exposed to the long acting nitric oxide donor (Z)-1-[2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (DETA-NO) for 24 h approximately double their concentration of glutathione (GSH) and show no sign of cell death. In contrast, GSH was depleted by 48%, and significant loss of mitochondrial respiratory chain complex activity and cell death were observed in primary culture rat neurones subjected to DETA-NO for 18 h. Northern blot analysis suggested that mRNA amounts of both subunits of glutamate-cysteine ligase (GCL), the rate-limiting enzyme in GSH synthesis, were elevated in astrocytes following nitric oxide (NO) exposure. This correlated with an increase in astrocytic GCL activity. Neurones on the other hand did not exhibit increased GCL activity when exposed to NO. In addition, the rate of GSH efflux was doubled and gamma-glutamyltranspeptidase (gamma-GT) activity was increased by 42% in astrocytes treated with NO for 24 h. These results suggest that astrocytes, but not neurones, up-regulate GSH synthesis as a defence mechanism against excess NO. It is possible that the increased rate of GSH release and activity of gamma-GT in astrocytes may have important implications for neuroprotection in vivo by optimizing the supply of GSH precursors to neurones in close proximity.

Oxidative stress in Parkinson's disease

Oxidative stress contributes to the cascade leading to dopamine cell degeneration in Parkinson's disease (PD). However, oxidative stress is intimately linked to other components of the degenerative process, such as mitochondrial dysfunction, excitotoxicity, nitric oxide toxicity and inflammation. It is therefore difficult to determine whether oxidative stress leads to, or is a consequence of, these events. Oxidative damage to lipids, proteins, and DNA occurs in PD, and toxic products of oxidative damage, such as 4-hydroxynonenal (HNE), can react with proteins to impair cell viability. There is convincing evidence for the involvement of nitric oxide that reacts with superoxide to produce peroxynitrite and ultimately hydroxyl radical production. Recently, altered ubiquitination and degradation of proteins have been implicated as key to dopaminergic cell death in PD. Oxidative stress can impair these processes directly, and products of oxidative damage, such as HNE, can damage the 26S proteasome. Furthermore, impairment of proteasomal function leads to free radical generation and oxidative stress. Oxidative stress occurs in idiopathic PD and products of oxidative damage interfere with cellular function, but these form only part of a cascade, and it is not possible to separate them from other events involved in dopaminergic cell death.

Alterations in m-RNA expression for Cu,Zn-superoxide dismutase and glutathione peroxidase in the basal ganglia of MPTP-treated marmosets and patients with Parkinson's disease

Alterations occurring in the antioxidant enzymes, copper, zinc-dependent superoxide dismutase (Cu,Zn-SOD) and glutathione peroxidase (GPX) following nigral dopaminergic denervation are unclear. We now report on the distribution and levels of m-RNA for Cu,Zn-SOD and GPX in basal ganglia of normal and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated common marmosets, and in normal individuals and patients with Parkinson's disease (PD) using in situ hybridization histochemistry and oligodeoxynucleotide (single-stranded DNA) probes. Cu,Zn-SOD and GPX m-RNA was present throughout basal ganglia (nucleus accumbens, caudate-putamen, globus pallidus, substantia nigra) in the common marmoset, with the highest levels being in substantia nigra (SN). Following MPTP induced nigral cell loss, Cu,Zn-SOD m-RNA levels were decreased in all areas but the SNr, and particularly in SNc (71%, P<0.001). MPTP-treatment had no effect on GPX m-RNA expression in any area of basal ganglia. Cu,Zn-SOD and GPX m-RNA was also present in the normal human SN. In PD, however, Cu,Zn-SOD m-RNA was significantly decreased (89%, P<0.005) in SNc, and there was a near-complete loss of GPX m-RNA in both SNc (100%, P<0.005) and SNr (88%, P<0.005). The loss of Cu,Zn-SOD m-RNA in SNc in MPTP-treated marmosets and patients with PD suggests that it is primarily located in dopaminergic neuronal cell bodies. The loss of GPX m-RNA in SNc in PD also suggests a localisation to dopaminergic cell bodies, but the similar change in SNr may indicate its presence in dopaminergic neurites. In contrast, the absence of change in GPX m-RNA in MPTP-treated primates appears to rule out its presence in dopaminergic cells in this species, but this may only be apparent and may reflect increased expression in glial cells following acute toxin treatment.

Synthesis of aza and oxaglutamyl-p-nitroanilide derivatives and their kinetic studies with gamma-glutamyltranspeptidase

A new series of L-glutamic acid p-nitroanilide analogues has been synthesized and tested as substrates and inhibitors of rat kidney gamma-glutamyltranspeptidase (GGT). Kinetic parameters (K(m) and k(cat)) were determined for each analogue and provide insight into the scope and limits of GGT catalytic efficiency.

Etiology of Parkinson's disease

There is growing recognition that Parkinson's disease (PD) is likely to arise from the combined effects of genetic predisposition as well as largely unidentified environmental factors. The relative contribution of each varies from one individual to another. Even in situations where more than one family member is affected, the predominant influence may be environmental. Although responsible for only a small minority of cases of PD, recently identified genetic mutations have provided tremendous insights into the basis for neurodegeneration and have led to growing recognition of the importance of abnormal protein handling in Parkinson's as well as other neurodegenerative disorders. Abnormal protein handling may increase susceptibility to oxidative stress; conversely, numerous other factors, including oxidative stress and impaired mitochondrial function can lead to impaired protein degradation. A limited number of environmental factors are known to be toxic to the substantia nigra; in contrast, some factors such as caffeine intake and cigarette smoking may protect against the development of PD, although the mechanisms are not established. We review the various genetic and environmental factors thought to be involved in PD, as well as the mechanisms that contribute to selective nigral cell death.

Endoplasmic reticulum stress and the unfolded protein response in cellular models of Parkinson's disease

6-hydroxydopamine, 1-methyl-4-phenyl-pyridinium (MPP+), and rotenone cause the death of dopaminergic neurons in vitro and in vivo and are widely used to model Parkinson's disease. To identify regulated genes in such models, we performed serial analysis of gene expression on neuronal PC12 cells exposed to 6-hydroxydopamine. This revealed a striking increase in transcripts associated with the unfolded protein response. Immunoblotting confirmed phosphorylation of the key endoplasmic reticulum stress kinases IRE1alpha and PERK (PKR-like ER kinase) and induction of their downstream targets. There was a similar response to MPP+ and rotenone, but not to other apoptotic initiators. As evidence that endoplasmic reticulum stress contributes to neuronal death, sympathetic neurons from PERK null mice in which the capacity to respond to endoplasmic reticulum stress is compromised were more sensitive to 6-hydroxydopamine. Our findings, coupled with evidence from familial forms of Parkinson's disease, raise the possibility of widespread involvement of endoplasmic reticulum stress and the unfolded protein response in the pathophysiology of this disease.

Mapping of the active site of rat kidney gamma-glutamyl transpeptidase using activated esters and their amide derivatives

The enzyme gamma-glutamyl transpeptidase (GGT), implicated in many physiological processes, catalyses the transfer of a gamma-glutamyl from a donor substrate to an acyl acceptor substrate, usually an amino acid or a peptide. In order to investigate which moieties of the donor substrate are necessary for recognition by GGT, the structure of the well-recognized substrate L-gamma-glutamyl-p-nitroanilide was modified. Several activated esters and their amide derivatives were synthesized and used as substrates. Kinetic (K(m) and V(max)) and inhibition constants (K(i)) were measured and reveal that almost the entire gamma-glutamyl moiety is necessary for recognition in the binding site of the donor substrate. The implied presence of certain complementary amino acids in this substrate binding site will allow the more rational design of various substrate analogues and inhibitors.

Dopamine thioethers: formation in brain and neurotoxicity

Dopamine (DA) oxidation is proposed to be a significant contributor to some forms of neurodegeneration, although the mechanisms are not fully resolved. Recent results from in vitro and in vivo models have suggested that some products of oxidized DA metabolized along the mercapturic acid pathway (MAP) may contribute to dopaminergic neurodegeneration. Here we review recent findings on the localization of MAP enzymes in human brain, as well as the concentration and neurotoxicity of their DA thioether products.