In Vivo Autoxidation of Dopamine in Guinea Pig Striatum Increases with Age

Catechols are known to react readily with molecular oxygen to form the corresponding quinones together with reduced oxygen species. These products have been shown to be toxic in in vivo and in vitro systems. 5-S-Cysteinyl adducts of catechols are believed to be formed through the spontaneous reaction between quinones and thiol-containing compounds, like cysteine and glutathione (GSH). Thus, the brain levels of these adducts probably indicate the autoxidation rate of catechols in vivo. In the present study, the striatal concentrations of 5-S-cysteinyldopamine (5-S-cysteinyl-DA), 5-S-cysteinyl-3,4-dihydroxyphenylalanine (5-S-cysteinyl-DOPA), and 5-S-cysteinyl-3,4-dihydroxyphenylacetic acid (5-S-cysteinyl-DOPAC) were determined in 2-week-, 2-month- and 3-year-old guinea pigs. In addition, brain levels of DA, the DA metabolite DOPAC, and GSH were assessed. The concentration of 5-S-cysteinyl-DA increased markedly with age. The 3-year-old guinea pigs had the highest level, i.e., 248% of the concentration in the 2-week-old animals and 219% of the concentration in the 2-month-old animals. Furthermore, the striatal 5-S-cysteinyl-DOPA level in the 3-year-old group was 68% higher than in the 2-week-old group and 46% higher than in the 2-month-old group. No age difference in the striatal concentration of DA was found. In contrast, the concentration of DOPAC increased with age: The DOPAC level in the 3-year-old animals was 153% of the level in the 2-week-old animals and 116% of the level in the 2-month-old animals.(ABSTRACT TRUNCATED AT 250 WORDS)     

Reactive Oxygen Species and the Central Nervous System

Radicals are species containing one or more unpaired electrons, such as nitric oxide (NO.). The oxygen radical superoxide (O2.-) and the nonradical hydrogen peroxide (H2O2) are produced during normal metabolism and perform several useful functions. Excessive production of O2.- and H2O2 can result in tissue damage, which often involves generation of highly reactive hydroxyl radical (.OH) and other oxidants in the presence of "catalytic" iron or copper ions. An important form of antioxidant defense is the storage and transport of iron and copper ions in forms that will not catalyze formation of reactive radicals. Tissue injury, e.g., by ischemia or trauma, can cause increased metal ion availability and accelerate free radical reactions. This may be especially important in the brain because areas of this organ are rich in iron and CSF cannot bind released iron ions. Oxidative stress on nervous tissue can produce damage by several interacting mechanisms, including increases in intracellular free Ca2+ and, possibly, release of excitatory amino acids. Recent suggestions that free radical reactions are involved in the neurotoxicity of aluminum and in damage to the substantia nigra in patients with Parkinson's disease are reviewed. Finally, the nature of antioxidants is discussed, it being suggested that antioxidant enzymes and chelators of transition metal ions may be more generally useful protective agents than chain-breaking antioxidants. Careful precautions must be used in the design of antioxidants for therapeutic use.     

Transferrin and Iron in Normal, Alzheimer's Disease, and Parkinson's Disease Brain Regions

Abstract: Oxidant-mediated damage is suspected to be involved in the pathogenesis of several neurodegenerative disorders. Iron promotes conversion of hydrogen peroxide to hydroxyl radical and, thus, may contribute to oxidant stress. We measured iron and its transport protein transferrin in caudate, putamen, globus pallidus, substantia nigra, and frontal cortex of subjects with Alzheimer's disease (n = 14) and Parkinson's disease (n = 14), and in younger adult (n = 8) and elderly (n = 8) normal controls. Although there were no differences between control groups with regard to concentrations of iron and transferrin, iron was significantly increased ( p < 0.05) in Alzheimer's disease globus pallidus and frontal cortex and Parkinson's disease globus pallidus, and transferrin was significantly increased in Alzheimer's disease frontal cortex, compared with elderly controls. The transferrin/iron ratio, a measure of iron mobilization capacity, was decreased in globus pallidus and caudate in both disorders. Regional transferrin and iron concentrations were generally more highly correlated (Pearson's correlation coefficient) in elderly controls than in Alzheimer's and Parkinson's disease. The altered relationship between iron and transferrin provides further evidence that a disturbance in iron metabolism may be involved in both disorders.     

Neurochemical and Neurogenetic Correlates of Parkinson's Disease

Abstract: We discuss neurochemical and neurogenetic correlates of Parkinson's disease (PD) based on the recent progress in the study of its etiology and pathogenesis. Nigral degeneration with the presence of Lewy bodies in the remaining neurons is the pathologic hallmark of PD, and the resultant loss of striatal dopamine is responsible for most of the clinical manifestations. Although the primary cause is still unknown, mitochondrial respiratory failure and oxidative stress appear to be two major contributors to the nigral cell death. Many endogenous and exogenous compounds with structural similarity to MPTP have been postulated as potential neurotoxins inducing nigral cell death in PD, but there is little evidence of accumulation of such compounds in the nigra. Genetic influence has increasingly been recognized as an important risk factor for PD. In this respect, genetic linkage analysis and molecular cloning of the disease genes in familial parkinsonism are of utmost importance today. Recently, the disease gene for one of the autosomal dominant forms of familial PD was identified, and we cloned the gene for an autosomal recessive type of familial parkinsonism that had been mapped to the long arm of chromosome 6 by our group. Information obtained on familial parkinsonism will contribute to the studies on sporadic PD as well.     

Sensitization of neuronal cells to oxidative stress with mutated human alpha-synuclein

Linkage of alpha-synuclein (alpha-SN) mutations to familial Parkinson's disease (PD) and presence of alpha-SN as a major constituent of Lewy body in both sporadic and familial PD implicate alpha-SN abnormality in PD pathogenesis. Here we demonstrate that overexpression of wild-type or mutant alpha-SN does not cause any deleterious effect on the growth or continued propagation of transfected human cells, but overproduction of mutant alpha-SN heightens their sensitivity to menadione-induced oxidative injury. Such enhanced vulnerability is more pronounced in neuronal transfectants than in their nonneuronal counterparts and is associated with increased production of reactive oxygen species. The data suggest that mutated alpha-SN, especially with an alanine-to-proline substitution at residue 30, sensitizes neuronal cells to oxidative damage.     

Brain Mitochondria Catalyze the Oxidation of 7-(2-Aminoethyl)-3,4-Dihydro-5-Hydroxy-2H-1,4-Benzothiazine-3-Carboxylic Acid (DHBT-1) to Intermediates that Irreversibly Inhibit Complex I and Scavenge Glutathione: Potential Relevance to the Pathogenesis of Parkinson's Disease

Abstract: We have proposed that a very early step in the pathogenesis of idiopathic Parkinson's disease is the elevated translocation of l -cysteine into neuromelanin-pigmented dopaminergic neurons in the substantia nigra. This influx of l -cysteine was proposed to divert the normal neuromelanin pathway by scavenging dopamine-o-quinone, formed by autoxidation of cytoplasmic dopamine, to give initially 5-S-cysteinyldopamine, which is further oxidized to 7 - (2 - aminoethyl) - 3,4 - dihydro - 5 - hydroxy - 2H - 1,4 - benzothiazine-3-carboxylic acid (DHBT-1). In a recent report, it was demonstrated that DHBT-1 evokes inhibition of complex I respiration when incubated with intact rat brain mitochondria and a time-dependent irreversible inhibition of NADH-coenzyme Q₁ (CoQ₁) reductase when incubated with mitochondrial membranes. In this study, it is established that the time dependence of NADH-CoQ₁ reductase inhibition reflects the oxidation of DHBT-1, catalyzed by an unknown constituent of the inner mitochondrial membrane, to an o-quinone imine intermediate that rearranges to 7-(2-aminoethyl) - 5 - hydroxy - 1,4 - benzothiazine - 3 - carboxylic acid (BT-1) and decarboxylates to 7-(2-aminoethyl)-5-hydroxy-1,4-benzothiazine (BT-2), which are further catalytically oxidized to o-quinone imine intermediates. The electrophilic o-quinone imine intermediates formed in these mitochondria-catalyzed oxidations of DHBT-1, BT-1, and BT-2 are proposed to bind covalently to key sulfhydryl residues at the complex I site, thus evoking irreversible inhibition of NADH-CoQ₁ reductase. Evidence for this mechanism derives from the fact that greater than equimolar concentrations of glutathione completely block inhibition of NADH-CoQ₁ reductase by DHBT-1, BT-1, and BT-2 by scavenging their electrophilic o-quinone imine metabolites to form glutathionyl conjugates. The results of this investigation may provide insights into the irreversible loss of glutathione and decreased mitochondrial complex I activity, which are both anatomically specific to the substantia nigra and exclusive to Parkinson's disease.     

Distribution of Catecholamines in the Ventral Mesencephalon of Human Brain, with Special Reference to Parkinson's Disease

Abstract: The high levels of dopamine (DA) detected in the ventral tegmental area (VTA), as well as in the substantia nigra (SN) of human brain, suggest the presence of DA cells in these areas. This favors the possible existence of a mesocortico-limbic system besides the mesostriatal pathway. In Parkinson's disease both DA systems seem to be deficient.     

Increased Formation of Reactive Oxygen Species, but No Changes in Glutathione Peroxidase Activity, in Striata of Mice Transgenic for the Huntington's Disease Mutation

The formation of reactive oxygen species (ROS) and the activities of the antioxidant enzymes glutathione peroxidase (GPx) and catalase (CAT) were measured as a function of age in the striatum of mice transgenic for the Huntington's disease (HD) mutation. Striata from R6/1 transgenic male mice were dissected at different ages (11, 19, and 35 weeks). The amount of dichlorofluorescein (DCF), an index of ROS formation, was significantly increased in R6/1 mice at all ages tested, whereas GPx activity remained unchanged when compared with wild-type control animals in all groups evaluated. CAT activity was very low, just above detection in the striata of both control and transgenic mice. Nineteen and 35-week-old R6/1 mice also developed feet clasping behavior, but only 35-week-old animals showed body weight loss. Our findings support an active role of free radicals in the onset and progression of the neurological phenotype of R6/1 mice. We suggest that changes in ROS formation are due to an age-related increased propensity of the striatum of transgenic animals to generate oxygen radicals as a response to the evolving pathological conditions.     

Initiation of Neuronal Damage by Complex I Deficiency and Oxidative Stress in Parkinson's Disease

Oxidative stress and partial deficiencies of mitochondrial complex I appear to be key factors in the pathogenesis of Parkinson's disease. They are interconnected; complex I inhibition results in an enhanced production of reactive oxygen species (ROS), which in turn will inhibit complex I. Partial inhibition of complex I in nerve terminals is sufficient for in situ mitochondria to generate more ROS. H2O2 plays a major role in inhibiting complex I as well as a key metabolic enzyme, alpha-ketoglutarate dehydrogenase. The vicious cycle resulting from partial inhibition of complex I and/or an inherently higher ROS production in dopaminergic neurons leads over time to excessive oxidative stress and ATP deficit that eventually will result in cell death in the nigro-striatal pathway.     

帕金森病氧化应激机制及抗氧化药物治疗进展

帕金森病(PD)是一种仅次于阿尔兹海默病的第二大神经系统变性疾病,随着社会人口老龄化,PD发病率逐年增高,在65岁以上的老年人,患病率高达1%。PD主要临床表现为静止性震颤、肌强直、运动迟缓、姿势步态异常。目前病因仍未明确,疾病发生与很多因素相关,其主要病理特征为黑质多巴胺能神经元变性缺失。研究发现线粒体功能障碍、钙超载、铁离子的堆积、免疫炎症等均与氧化应激有关,能造成氧化性损伤,促进多巴胺能神经元凋亡,氧化应激在促进PD疾病发展中起到重要作用,因而越来越备受关注,抗氧化治疗在某种程度上为PD的治疗指出新的方向。本文就氧化应激引起DA神经元变性缺失的机制及抗氧化药物的治疗进展进行综述。     

Inhibition of Glutamate Transport in Synaptosomes by Dopamine Oxidation and Reactive Oxygen Species

Abstract: Dopamine can form reactive oxygen species and other reactive metabolites that can modify proteins and other cellular constituents. In this study, we tested the effect of dopamine oxidation products, other generators of reactive oxygen species, and a sulfhydryl modifier on the function of glutamate transporter proteins. We also compared any effects with those on the dopamine transporter, a protein whose function we had previously shown to be inhibited by dopamine oxidation. Preincubation with the generators of reactive oxygen species, ascorbate (0.85 m M ) or xanthine (500 µ M ) plus xanthine oxidase (25 mU/ml), inhibited the uptake of [³H]glutamate (10 µ M ) into rat striatal synaptosomes (−54 and −74%, respectively). The sulfhydryl-modifying agent N -ethylmaleimide (50–500 µ M ) also led to a dose-dependent inhibition of [³H]glutamate uptake. Preincubation with dopamine (100 µ M ) under oxidizing conditions inhibited [³H]glutamate uptake by 25%. Exposure of synaptosomes to increasing amounts of dopamine quinone by enzymatically oxidizing dopamine with tyrosinase (2–50 U/ml) further inhibited [³H]glutamate uptake, an effect prevented by the addition of glutathione. The effects of free radical generators and dopamine oxidation on [³H]glutamate uptake were similar to the effects on [³H]dopamine uptake (250 n M ). Our findings suggest that reactive oxygen species and dopamine oxidation products can modify glutamate transport function, which may have implications for neurodegenerative processes such as ischemia, methamphetamine-induced toxicity, and Parkinson's disease.     

The solubility of alpha-synuclein in multiple system atrophy differs from that of dementia with Lewy bodies and Parkinson's disease

Intracellular inclusions containing alpha-synuclein (alpha SN) are pathognomonic features of several neurodegenerative disorders. Inclusions occur in oligodendrocytes in multiple system atrophy (MSA) and in neurons in dementia with Lewy bodies (DLB) and Parkinson's disease (PD). In order to identify disease-associated changes of alpha SN, this study compared the levels, solubility and molecular weight species of alpha SN in brain homogenates from MSA, DLB, PD and normal aged controls. In DLB and PD, substantial amounts of detergent-soluble and detergent-insoluble alpha SN were detected compared with controls in grey matter homogenate. Compared with controls, MSA cases had significantly higher levels of alpha SN in the detergent-soluble fraction of brain samples from pons and white matter but detergent-insoluble alpha SN was not detected. There was an inverse correlation between buffered saline-soluble and detergent-soluble levels of alpha SN in individual MSA cases suggesting a transition towards insolubility in disease. The differences in solubility of alpha SN between grey and white matter in disease may result from different processing of alpha SN in neurons compared with oligodendrocytes. Highly insoluble alpha SN is not involved in the pathogenesis of MSA. It is therefore possible that buffered saline-soluble or detergent-soluble forms of alpha SN are involved in the pathogenesis of other alpha SN-related diseases.     

帕金森病患者血清氧化应激标志物的变化特点

目的:探索帕金森病(Parkinson's disease,PD)患者血清中氧化应激标志物变化情况。方法:募集2014年4月至2015年4月来我院就诊的PD患者62例,正常对照人群59例,采集两组人群的基本临床信息,测定两组血浆中活性氧和丙二醛含量,超氧化物歧化酶、谷胱甘肽过氧化物酶及过氧化氢酶的活性,同时测定血清清除羟自由基的能力,比较两组之间的差异。结果:与正常对照组比较,PD患者血清中活性氧和丙二醛含量,超氧化物歧化酶、谷胱甘肽过氧化物酶及过氧化氢酶的活性,及血清清除羟自由基的能力无明显差异(P0.05)。而Hoehn/Yahr分级≥4 PD患者与正常对照组血清氧化应激标志物比较发现,Hoehn/Yahr分级≥4 PD患者血清丙二醛含量显著高于对照组(P=0.018),其他指标两组之间无显著变化(P0.05)。结论:PD患者血清中氧化应激水平与正常对照组比较无显著差异,运动功能障碍严重的PD患者血清脂质过氧化产物水平高于正常对照组。该研究为PD与氧化应激相关性的研究提供了一定的参考。     

Autoxidation and neurotoxicity of 6-hydroxydopamine in the presence of some antioxidants: potential implication in relation to the pathogenesis of Parkinson's disease

6-Hydroxydopamine (6-OHDA) is a dopaminergic neurotoxin putatively involved in the pathogenesis of Parkinson's disease (PD). Its neurotoxicity has been related to the production of reactive oxygen species. In this study we examine the effects of the antioxidants ascorbic acid (AA), glutathione (GSH), cysteine (CySH), and N-acetyl-CySH (NAC) on the autoxidation and neurotoxicity of 6-OHDA. In vitro, the autoxidation of 6-OHDA proceeds rapidly with the formation of H2O2 and with the participation of the H2O2 produced in the reaction. The presence of AA induced a reduction in the consumption of O2 during the autoxidation of 6-OHDA and a negligible presence of the p-quinone, which demonstrates the efficiency of AA to act as a redox cycling agent. The presence of GSH, CySH, and NAC produced a significant reduction in the autoxidation of 6-OHDA. In vivo, the presence of sulfhydryl antioxidants protected against neuronal degeneration in the striatum, which was particularly remarkable in the case of CySH and was attributed to its capacity to remove the H2O2 produced in the autoxidation of 6-OHDA. These results corroborate the involvement of oxidative stress as the major mechanism in the neurotoxicity of 6-OHDA and the putative role of CySH as a scavenger in relation to PD.     

Overexpression of alpha-synuclein in rat substantia nigra results in loss of dopaminergic neurons, phosphorylation of alpha-synuclein and activation of caspase-9: resemblance to pathogenetic changes in Parkinson's disease

To elucidate the role of alpha-synuclein in the pathogenesis of Parkinson's disease, both human alpha-synuclein transgenic mice and targeted overexpression of human alpha-synuclein in rat substantia nigra using viral vector-based methods have been studied, however, little is known about the pathogenetic changes of dopaminergic neuron loss. Therefore, it is necessary to address whether the pathogenetic changes in brains with Parkinson's disease are recapitulated in these models. Here, we used the recombinant adeno-associated viral (rAAV) vector system for human alpha-synuclein gene transfer to rat substantia nigra and observed approximately 50% loss of dopaminergic neurons at 13 weeks after infection, which was comparably slower than the progression of neurodegeneration reported in other studies. In the slower progression of neurodegeneration, we identified several important features in common with the pathogenesis of Parkinson's disease, such as phosphorylation of alpha-synuclein at Ser129 and activation of caspase-9. Both findings were also evident in cortical tissues overexpressing alpha-synuclein via rAAV. Our results indicate that overexpression of alpha-synuclein via rAAV apparently recapitulates several important features of brains with Parkinson's disease and dementia with Lewy bodies, and thus alpha-synucleinopathy described here is likely to be an ideal model for the study of the pathogenesis of Parkinson's disease and dementia with Lewy bodies.     

Depolarization of In Situ Mitochondria Due to Hydrogen Peroxide-Induced Oxidative Stress in Nerve Terminals

Mitochondrial membrane potential (delta psi(m)) was determined in intact isolated nerve terminals using the membrane potential-sensitive probe JC-1. Oxidative stress induced by H2O2 (0.1-1 mM) caused only a minor decrease in delta psi(m). When complex I of the respiratory chain was inhibited by rotenone (2 microM), delta psi(m) was unaltered, but on subsequent addition of H2O2, delta psi(m) started to decrease and collapsed during incubation with 0.5 mM H2O2 for 12 min. The ATP level and [ATP]/[ADP] ratio were greatly reduced in the simultaneous presence of rotenone and H2O2. H2O2 also induced a marked reduction in delta psi(m) when added after oligomycin (10 microM), an inhibitor of F0F1-ATPase. H2O2 (0.1 or 0.5 mM) inhibited alpha-ketoglutarate dehydrogenase and decreased the steady-state NAD(P)H level in nerve terminals. It is concluded that there are at least two factors that determine delta psi(m) in the presence of H2O2: (a) The NADH level reduced owing to inhibition of alpha-ketoglutarate dehydrogenase is insufficient to ensure an optimal rate of respiration, which is reflected in a fall of delta psi(m) when the F0F1-ATPase is not functional. (b) The greatly reduced ATP level in the presence of rotenone and H2O2 prevents maintenance of delta psi(m) by F0F1-ATPase. The results indicate that to maintain delta psi(m) in the nerve terminal during H2O2-induced oxidative stress, both complex I and F0F1-ATPase must be functional. Collapse of delta psi(m) could be a critical event in neuronal injury in ischemia or Parkinson's disease when H2O2 is generated in excess and complex I of the respiratory chain is simultaneously impaired.     

Amino Acids and Biogenic Amines in Cerebrospinal Fluid of Patients with Parkinson's Disease

To study changes in amino acid metabolism and biogenic amines in Parkinson's disease, we set up a prospective study and measured biogenic amines, their main metabolites, and 22 different amino acids, in cerebrospinal fluid of Parkinson's disease patients (n = 24) and age-matched controls (n = 30). A trend toward higher dopamine levels in Parkinson's disease patients was interpreted as an effect of treatment with levodopa and/or selegiline. Significantly lower concentrations of the dopamine metabolite 3,4-dihydroxyphenylacetic acid in the Parkinson's disease group might reflect dopaminergic cell loss. Our results revealed decreased serotonin catabolism that was interpreted as an effect of treatment with selegiline. Whereas all amino acid levels were unchanged, taurine was significantly lower in Parkinson's disease patients. Studies showed that taurine exerts a trophic action on the central nervous system. In this view, decreased taurine in a neurodegenerative disorder as Parkinson's disease deserves attention.     

散发性帕金森病蛋白酶体功能障碍及其所致的路易(小)体形成

散发性帕金森病(sporadic Parkinson's disease, sPD)的主要病理特征之一是中脑黑质致密部(substantia nigra pars compacta, SNpc)残存多巴胺能神经元内核周路易(小)体(Lewy body, LB)形成.LB发生的具体原因和确切过程有待进一步阐释.来自遗传学、尸体解剖和实验科学的报道提示,蛋白酶体功能障碍及其所致的LB形成可能是按照聚集体形成途径(process of aggresomes)进行的.在聚集体形成途径过程中,异常蛋白质聚集基本上经历了非纤维化分子聚集过程(molecular crowding)以及后续的纤维化聚集过程(fibrilation of aggregation).其间,蛋白酶体功能障碍(dysfunction of proteasome)、内质网相关降解丧失(loss of endoplasmic reticulum associated degradation)、非纤维化聚集物(nonfibrilar aggregates)、聚集体(aggresomes)及至纤维化LB (fibrilar LB)等构成了sPD病变过程的主要事件.这提示在sPD病变过程中,蛋白酶体功能障碍及其所致的LB形成过程实质上是细胞信号的转导过程,其间涉及了众多的蛋白质分子.     

Dual Effect of Pyruvate in Isolated Nerve Terminals: Generation of Reactive Oxygen Species and Protection of Aconitase

Generation of reactive oxygen species (ROS) in synaptosomes was investigated in the presence of different substrates. When pyruvate was used as a substrate an increased rate of hydrogen peroxide formation was detected by the Amplex Red fluorescent assay, but aconitase, which is known to be a highly sensitive enzyme to ROS was not inhibited. In contrast, pyruvate exerted a partial protection on aconitase against a time-dependent inactivation that occurred when synaptosomes were incubated in the absence of substrates. Disruption of synaptosomal membranes with Triton X-100 prevented the protective effect of pyruvate. It is suggested that citrate and/or isocitrate formed in the metabolism of pyruvate could be responsible for a partial protection of aconitase. Therefore while pyruvate could have a prooxidant effect it could also exert a protective effect on the aconitase. Special issue dedicated to Dr. Bernd Hamprecht.     

Dose-dependent protective effect of selenium in rat model of Parkinson's disease: neurobehavioral and neurochemical evidences

Normal cellular metabolism produces oxidants that are neutralized within cells 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 selenium, an antioxidant, can prevent or slowdown neuronal injury in a 6-hydroxydopamine (6-OHDA) model of Parkinsonism. Rats were pre-treated with sodium selenite (0.1, 0.2 and 0.3 mg/kg body weight) for 7 days. On day 8, 2 micro L 6-OHDA (12.5 micro g in 0.2% ascorbic acid in normal saline) was infused in the right striatum. Two weeks after 6-OHDA infusion, rats were tested for neurobehavioral activity, and were killed 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. Selenium was found to be successful in upregulating the antioxidant status and lowering the dopamine loss, and functional recovery returned close to the baseline dose-dependently. This study revealed that selenium, which is an essential part of our diet, may be helpful in slowing down the progression of neurodegeneration in parkinsonism.