No abstract

Studies on postmortem brains from Parkinson's patients reveal elevated iron in the substantia nigra (SN). Selective cell death in this brain region is associated with oxidative stress, which may be exacerbated by the presence of excess iron. Whether iron plays a causative role in cell death, however, is controversial. Here, we explore the effects of iron chelation via either transgenic expression of the iron binding protein ferritin or oral administration of the bioavailable metal chelator clioquinol (CQ) on susceptibility to the Parkinson's-inducing agent 1-methyl-4-phenyl-1,2,3,6-tetrapyridine (MPTP). Reduction in reactive iron by either genetic or pharmacological means was found to be well tolerated in animals in our studies and to result in protection against the toxin, suggesting that iron chelation may be an effective therapy for prevention and treatment of the disease.

The vesicular monoamine transporter 2 (VMAT2; SLC18A2) is responsible for packaging dopamine into vesicles for subsequent release and has been suggested to serve a neuroprotective role in the dopamine system. Here, we show that mice that express ϳ5% of normal VMAT2 (VMAT2 LO) display age-associated nigrostriatal dopamine dysfunction that ultimately results in neurodegeneration. Elevated cysteinyl adducts to L-DOPA and DOPAC are seen early and are followed by increased striatal protein carbonyl and 3-nitrotyrosine formation. These changes were associated with decreased striatal dopamine and decreased expression of the dopamine transporter and tyrosine hydroxylase. Furthermore, we observed an increase in ␣-synuclein immunoreactivity and accumulation and neurodegeneration in the substantia nigra pars compacta in aged VMAT2 LO mice. Thus, VMAT2 LO animals display nigrostriatal degeneration that begins in the terminal fields and progresses to eventual loss of the cell bodies, ␣-synuclein accumulation, and an L-DOPA responsive behavioral deficit, replicating many of the key aspects of Parkinson's disease. These data suggest that mishandling of dopamine via reduced VMAT2 expression is, in and of itself, sufficient to cause dopamine-mediated toxicity and neurodegeneration in the nigrostriatal dopamine system. In addition, the altered dopamine homeostasis resulting from reduced VMAT2 function may be conducive to pathogenic mechanisms induced by genetic or environmental factors thought to be involved in Parkinson's disease.

␣-Synuclein-containing aggregates represent a feature of a variety of neurodegenerative disorders, including Parkinson's disease (PD). However, mechanisms that promote intraneuronal ␣-synuclein assembly remain poorly understood. Because pesticides, particularly the herbicide paraquat, have been suggested to play a role as PD risk factors, the hypothesis that interactions between ␣-synuclein and these environmental agents may contribute to aggregate formation was tested in this study. Paraquat markedly accelerated the in vitro rate of ␣-synuclein fibril formation in a dosedependent fashion. When mice were exposed to the herbicide, brain levels of ␣-synuclein were significantly increased. This up-regulation followed a consistent pattern, with higher ␣-synuclein at 2 days after each of three weekly paraquat injections and with protein levels returning to control values by day 7 post-treatment. Paraquat exposure was also accompanied by aggregate formation. Thioflavine S-positive structures accumulated within neurons of the substantia nigra pars compacta, and dual labeling and confocal imaging confirmed that these aggregates contained ␣-synuclein. The results suggest that up-regulation of ␣-synuclein as a consequence of toxicant insult and direct interactions between the protein and environmental agents are potential mechanisms leading to ␣-synuclein pathology in neurodegenerative disorders. Parkinson's disease (PD)1 is a common neurodegenerative disorder characterized by the loss of dopaminergic neurons in the nigrostriatal pathway and the formation of intraneuronal inclusions (called Lewy bodies) in different brain regions. Although the etiology of PD remains unknown, several lines of evidence suggest a pathogenetic role of the protein ␣-synuclein. In particular, ␣-synuclein is a major component of Lewy bodies in all PD patients (1, 2), and ␣-synuclein mutations have been associated with clinical and pathological parkinsonism in rare autosomal dominant familial cases (3, 4). It has been hypothesized that a tendency of ␣-synuclein to aggregate may underlie its involvement in Lewy body formation and neurodegeneration. In individuals with ␣-synuclein mutations, abnormal forms of the protein could trigger pathological processes as a result of their enhanced propensity to self-assemble (5-7). However, in the vast majority of patients with idiopathic (nonfamilial) PD, the lack of ␣-synuclein mutations (8, 9) indicates that additional mechanisms may lead to conformational changes and consequent protein aggregation. One such mechanism could be the interaction of ␣-synuclein with other chemical species.The association and fibrillation of ␣-synuclein appear to involve a shift in equilibrium from the natively unfolded to a partially folded protein conformation (10). In a recent study, Uversky and colleagues (11) have shown that incubating ␣-synuclein in the presence of paraquat or other pesticides dramatically accelerates the rate of ␣-synuclein fibrillation in vitro, probably due to the preferential binding of these compound...

The aggregation of alpha-synuclein is believed to play an important role in the pathogenesis of Parkinson's disease as well as other neurodegenerative disorders ("synucleinopathies"). However, the function of alpha-synuclein under physiologic and pathological conditions is unknown, and the mechanism of alpha-synuclein aggregation is not well understood. Here we show that alpha-synuclein forms a tight 2:1 complex with histones and that the fibrillation rate of alpha-synuclein is dramatically accelerated in the presence of histones in vitro. We also describe the presence of alpha-synuclein and its co-localization with histones in the nuclei of nigral neurons from mice exposed to a toxic insult (i.e., injections of the herbicide paraquat). These observations indicate that translocation into the nucleus and binding with histones represent potential mechanisms underlying alpha-synuclein pathophysiology.