Acceleration of oligomerization, not fibrillization, is a shared property of both alpha-synuclein mutations linked to early-onset Parkinson's disease: implications for pathogenesis and therapy

Contribution of two conserved glycine residues to fibrillogenesis of the 106-126 prion protein fragment. Evidence that a soluble variant of the 106-126 peptide is neurotoxic

The fibrillogenic peptide corresponding to the residues 106-126 of the prion protein sequence (PrP 106-126) is largely used to explore the neurotoxic mechanisms underlying the prion disease. However, whether the neuronal toxicity of PrP 106-126 is caused by a soluble or fibrillar form of this peptide is still unknown. The aim of this study was to correlate the structural state of this peptide with its neurotoxicity. Here we show that the two conserved Gly114 and Gly119 residues, in force of their intrinsic flexibility, prevent the peptide assuming a structured conformation, favouring its aggregation in amyloid fibrils. The substitution of both Gly114 and Gly119 with alanine residues (PrP 106-126 AA mutated peptide) reduces the flexibility of this prion fragment and results in a soluble, beta-structured peptide. Moreover, PrP 106-126 AA fragment was highly toxic when incubated with neuroblastoma cells, likely behaving as a neurotoxic protofibrillar intermediate of the wild-type PrP 106-126. These data further confirm that the fibrillar aggregation is not necessary for the induction of the toxic effects of PrP 106-126.

Genes and parkinsonism

Genetic studies in families with mendelian inheritance of Parkinson's disease (PD) have reported the cloning of several disease-associated genes. These studies of rare familial forms of the disease have cast doubt on our understanding of the role of genetics in typical PD and have complicated the classification of the disorder. However, this genetic information might help us to construct a hypothesis for the pathogenetic processes that underlie PD. In this review we describe the molecular genetics of PD as currently understood to help explain the pathways that underlie neurodegeneration.

Polycation-induced oligomerization and accelerated fibrillation of human alpha-synuclein in vitro

The aggregation and fibrillation of alpha-synuclein has been implicated as a causative factor in Parkinson's disease and several other neurodegenerative disorders known as synucleinopathies. The effect of different factors on the process of fibril formation has been intensively studied in vitro. We show here that alpha-synuclein interacts with different unstructured polycations (spermine, polylysine, polyarginine, and polyethyleneimine) to form specific complexes. In addition, the polycations catalyze alpha-synuclein oligomerization. The formation of alpha-synuclein-polycation complexes was not accompanied by significant structural changes in alpha-synuclein. However, alpha-synuclein fibrillation was dramatically accelerated in the presence of polycations. The magnitude of the accelerating effect depended on the nature of the polymer, its length, and concentration. The results illustrate the potential critical role of electrostatic interactions in protein aggregation, and the potential role of naturally occurring polycations in modulating alpha-synuclein aggregation.

In vitro characterization of conditions for amyloid-beta peptide oligomerization and fibrillogenesis

Extensive research causally links amyloid-beta peptide (A beta) to Alzheimer's disease, although the pathologically relevant A beta conformation remains unclear. A beta spontaneously aggregates into the fibrils that deposit in senile plaques. However, recent in vivo and in vitro reports describe a potent biological activity for oligomeric assemblies of A beta. To consistently prepare in vitro oligomeric and fibrillar forms of A beta 1-42, a detailed knowledge of how solution parameters influence structure is required. This manuscript represents the first study using a single chemically and structurally homogeneous unaggregated starting material to demonstrate that the formation of oligomers, fibrils, and fibrillar aggregates is determined by time, concentration, temperature, pH, ionic strength, and A beta species. We recently reported that oligomers inhibit neuronal viability 10-fold more than fibrils and approximately 40-fold more than unaggregated peptide, with oligomeric A beta 1-42-induced neurotoxicity significant at 10 nm. In addition, we were able to differentiate by structure and neurotoxic activity wild-type A beta1-42 from isoforms containing familial mutations (Dahlgren, K. N., Manelli, A. M., Stine, W. B., Jr., Baker, L. K., Krafft, G. A., and LaDu, M. J. (2002) J. Biol. Chem. 277, 32046-32053). Understanding the biological role of specific A beta conformations may define the link between A beta and Alzheimer's disease, re-focusing therapeutic approaches by identifying the pernicious species of A beta ultimately responsible for the cognitive dysfunction that defines the disease.

Motor dysfunction and gliosis with preserved dopaminergic markers in human alpha-synuclein A30P transgenic mice

Alpha-synuclein is a major component of Lewy bodies (LBs) in the substantia nigra and cortex in Parkinson's disease (PD) and dementia with Lewy bodies (DLB), and in glial inclusions in multiple systems atrophy (MSA). Mutations in alpha-synuclein have been associated with autosomal dominant forms of PD. We investigated the clinical and neuropathological effects of overexpression of human alpha-synuclein, alpha-synuclein A30P, and alpha-synuclein A53T under the control of the hamster prion protein (PrP) promoter; 5-15x endogenous levels of protein expression were achieved with widespread neuronal, including nigral, transgene expression. High expression of alpha-synuclein A30P in the Tg5093 line was associated with a progressive motor disorder with rigidity, dystonia, gait impairment, and tremor. Histological analysis of this line showed aberrant expression of the protein in cell soma and progressive CNS gliosis, but no discrete Lewy body-like alpha-synuclein inclusions could be identified. Biochemical analysis demonstrated alpha-synuclein fragmentation. Despite strong expression of the transgene in the nigra, there was no specific deterioration of the nigrostriatal dopaminergic system as assessed by quantitation of nigral tyrosine hydroxylase (TH) containing neurons, striatal TH immunoreactivity, dopamine levels, or dopamine receptor number and function. Lower expressing lines had no specific behavioral or histopathological phenotype. Thus, high expression of mutant human alpha-synuclein resulted in a progressive motor and widespread CNS gliotic phenotype independent of dopaminergic dysfunction in the Tg5093 line.

Pathways to Parkinsonism

A novel gene for Parkinson's disease (PD), DJ-1, has been identified that encodes a multifunctional product with several known protein-protein interactions and effects on gene expression. Here, I outline how it is possible to construct hypotheses that place DJ-1 in different relationships to the other known PD genes, alpha-synuclein and parkin. The identification of multiple genetic causes will provide further impetus to describe the pathway leading to PD.

Alpha-synuclein and presynaptic function: implications for Parkinson's disease

This article focuses on alpha-synuclein's role in normal and pathological axonal and presynaptic functions and its relationship to Parkinson's disease. It is not possible to mention all the contributions to aspects of this area. Readers interested in alpha-synuclein's relation to aggregation, Lewy lesions, and pathological modifications are referred to the many reviews (see Goldberg and Lansbury 2000; Galvin 2001a; Goedert 2001).

Dopamine- or L-DOPA-induced neurotoxicity: the role of dopamine quinone formation and tyrosinase in a model of Parkinson's disease

Dopamine (DA)- or L-dihydroxyphenylalanine-(L-DOPA-) induced neurotoxicity is thought to be involved not only in adverse reactions induced by long-term L-DOPA therapy but also in the pathogenesis of Parkinson's disease. Numerous in vitro and in vivo studies concerning DA- or L-DOPA-induced neurotoxicity have been reported in recent decades. The reactive oxygen or nitrogen species generated in the enzymatical oxidation or auto-oxidation of an excess amount of DA induce neuronal damage and/or apoptotic or non-apoptotic cell death; the DA-induced damage is prevented by various intrinsic and extrinsic antioxidants. DA and its metabolites containing two hydroxyl residues exert cytotoxicity in dopaminergic neuronal cells mainly due to the generation of highly reactive DA and DOPA quinones which are dopaminergic neuron-specific cytotoxic molecules. DA and DOPA quinones may irreversibly alter protein function through the formation of 5-cysteinyl-catechols on the proteins. For example, the formation of DA quinone-alpha-synuclein consequently increases cytotoxic protofibrils and the covalent modification of tyrosine hydroxylase by DA quinones. The melanin-synthetic enzyme tyrosinase in the brain may rapidly oxidize excess amounts of cytosolic DA and L-DOPA, thereby preventing slowly progressive cell damage by auto-oxidation of DA, thus maintainng DA levels. Since tyrosinase also possesses catecholamine-synthesizing activity in the absence of tyrosine hydroxylase (TH), the double-edged synthesizing and oxidizing functions of tyrosinase in the dopaminergic system suggest its potential for application in the synthesis of DA, instead of TH in the degeneration of dopaminergic neurons, and in the normalization of abnormal DA turnover in the long-term L-DOPA-treated Parkinson's disease patients.

Haploinsufficiency at the alpha-synuclein gene underlies phenotypic severity in familial Parkinson's disease

To date, two point mutations, G209A and G88C, have been reported in the coding region of the alpha-synuclein gene in autosomal dominant familial Parkinson's disease. When translated, these lead to the missense mutations Ala53Thr and Ala30Pro, respectively. Reduced mRNA expression of the G209A allele was reported recently in a Greek-American family. Here, we show that alpha-synuclein mRNA is normally expressed in blood cells and report the results of an analysis of alpha-synuclein mRNA and protein expression in lymphoblastoid cell lines established from kindreds with the G209A and G88C mutations. mRNA expression was characterized using a TaqMan real-time quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) assay. We assessed five affected and three unaffected members of a German family with the G88C mutation and two affected members in different, unrelated Greek families with the G209A mutation. The ratio of wild-type to mutant alpha-synuclein allele expression ranged from 2.2 to 9.2 in the affected individuals with a severe clinical phenotype. The ratios of the expression levels of the wild-type to mutant alleles were only slightly decreased in mild cases and were less than 1.0 in two asymptomatic heterozygotes. Sequence analysis of the RT-PCR products showed only the presence of G in position 88 and G in position 209 in severely affected heterozygotes of the German and Greek families, respectively. High performance liquid chromatography/mass spectrometry demonstrated that, relative to wild-type alpha-synuclein, there is a reduction of Ala30Pro alpha-synuclein in lymphoblastoid cell lines originating from severely affected, but not mildly affected G88C/+ heterozygotes. Taken together, these data indicate that there is haploinsufficiency at the alpha-synuclein gene and that the ratio of expression of the wild-type to mutant alleles correlates with the severity of the clinical phenotype. Furthermore, these findings suggest that haploinsufficiency of alpha-synuclein mutations may contribute to disease progression in these forms of familial Parkinson's disease.

Selective neuronal degeneration induced by soluble oligomeric amyloid beta protein

The prevailing amyloid hypothesis for Alzheimer's disease (AD) holds that amyloid beta-protein (Abeta) causes neuronal degeneration by forming neurotoxic fibrillar structures. Yet, many aspects of AD pathology and symptoms are not well explained by this hypothesis. Here, we present evidence that neurotoxicity of soluble oligomeric Abeta closely corresponds to the selective neurodegeneration so distinctly manifest in AD. Selectivity was first observed in vitro, where only the human central nervous system neuronal cells were susceptible to soluble oligomeric Abeta. Furthermore, in mouse cerebral slice treated with soluble oligomeric Abeta, selective regiospecific toxicity was evident in the hippocampal CA1, a division important for memory, but not in the CA3 subfield. The fibrillar Abeta, however, killed neurons in all regions of the cerebral slice cultures and also in cerebellar slices. Remarkably, even at the highest soluble oligomeric Abeta concentrations, cerebellar neurons were completely spared, consistent with one of the hallmark features of AD pathology. Our observation of the selective neurodegeneration of soluble oligomeric Abeta to neurons involved in cognitive function may provide a new opportunity for the development of an effective AD therapy as well as elucidating the pathological mechanism of AD.

Practical assay and molecular mechanism of aggregation inhibitors of beta-amyloid

Beta-Amyloid peptide (Abeta) is the main protein component of neuritic plaques in the brain of patients of Alzheimer's disease (AD), and its neurotoxicity would be exposed by the formation of aggregates. The aggregation inhibitors composed of an Abeta recognition element (KLVFF) and a hydrophilic moiety are evaluated by a novel fluorescence assay. These compounds inhibit growth of the model aggregates on the KLVFF immobilized surface. In addition, some compounds also possess disrupting activities of preformed aggregates. These compounds could be a key candidate for therapeutic drugs for AD by their novel molecular mechanisms.

Parkin protects against the toxicity associated with mutant alpha-synuclein: proteasome dysfunction selectively affects catecholaminergic neurons

One hypothesis for the etiology of Parkinson's disease (PD) is that subsets of neurons are vulnerable to a failure in proteasome-mediated protein turnover. Here we show that overexpression of mutant alpha-synuclein increases sensitivity to proteasome inhibitors by decreasing proteasome function. Overexpression of parkin decreases sensitivity to proteasome inhibitors in a manner dependent on parkin's ubiquitin-protein E3 ligase activity, and antisense knockdown of parkin increases sensitivity to proteasome inhibitors. Mutant alpha-synuclein also causes selective toxicity to catecholaminergic neurons in primary midbrain cultures, an effect that can be mimicked by the application of proteasome inhibitors. Parkin is capable of rescuing the toxic effects of mutant alpha-synuclein or proteasome inhibition in these cells. Therefore, parkin and alpha-synuclein are linked by common effects on a pathway associated with selective cell death in catecholaminergic neurons.

Golgi fragmentation occurs in the cells with prefibrillar alpha-synuclein aggregates and precedes the formation of fibrillar inclusion

Amyloid-like fibrillar aggregates of intracellular proteins are common pathological features of human neurodegenerative diseases. However, the nature of pathogenic aggregates and the biological consequences of their formation remain elusive. Here, we describe (i) a model cellular system in which prefibrillar alpha-synuclein aggregates and fibrillar inclusions are naturally formed in the cytoplasm with distinctive kinetics and (ii) a tight correlation between the presence of prefibrillar aggregates and the Golgi fragmentation. Consistent with the structural abnormality of Golgi apparatus, trafficking and maturation of dopamine transporter through the biosynthetic pathway were impaired in the presence of alpha-synuclein aggregates. Reduction in cell viability was also observed in the prefibrillar aggregate-forming condition and before the inclusion formation. The fibrillar inclusions, on the other hand, showed no correlation with Golgi fragmentation and were preceded by these events. Furthermore, at the early stage of inclusion formation, active lysosomes and mitochondria were enriched in the juxtanuclear area and co-aggregate into a compact inclusion body, suggesting that the fibrillar inclusions might be the consequence of an attempt of the cell to remove abnormal protein aggregates and damaged organelles. These results support the hypothesis that prefibrillar alpha-synuclein aggregates are the pathogenic species and suggest that Golgi fragmentation and subsequent trafficking impairment are the specific consequence of alpha-synuclein aggregation.

Characterization of cytoplasmic alpha-synuclein aggregates. Fibril formation is tightly linked to the inclusion-forming process in cells

The alpha-synuclein fibrillation process has been associated with the pathogenesis of several neurodegenerative diseases. Here, we have characterized the cytoplasmic alpha-synuclein aggregates using a fractionation procedure with which different aggregate species can be separated. Overexpression of alpha-synuclein in cells produce two distinct types of aggregates: large juxtanuclear inclusion bodies and small punctate aggregates scattered throughout the cytoplasm. Biochemical fractionation results in an inclusion-enriched fraction and two small aggregate fractions. Electron microscopy and thioflavin S reactivity of the fractions show that the juxtanuclear inclusion bodies are filled with amyloid-like alpha-synuclein fibrils, whereas both the small aggregate fractions contain non-fibrillar spherical aggregates with distinct size distributions. These aggregates appear sequentially, with the smallest population appearing the earliest and the fibrillar inclusions the latest. Based on the structural and kinetic properties, we suggest that the small spherical aggregates are the cellular equivalents of the protofibrils. The proteins that co-exist in the Lewy bodies, such as proteasome subunit, ubiquitin, and hsp70 chaperone, are present in the fibrillar inclusions but absent in the protofibrils, suggesting that these proteins may not be directly involved in the early aggregation stage. As predicted in the aggresome model, disruption of microtubules with nocodazole reduced the number of inclusions and increased the size of the protofibrils. Despite the increased size, the protofibrils remained non-fibrillar, suggesting that the deposition of the protofibrils in the juxtanuclear region is important in fibril formation. This study provides evidence that the cellular fibrillation also involves non-fibrillar intermediate species, and the microtubule-dependent inclusion-forming process is required for the protofibril-to-fibril conversion in cells.

Studies of the aggregation of mutant proteins in vitro provide insights into the genetics of amyloid diseases

Protein aggregation and the formation of highly insoluble amyloid structures is associated with a range of debilitating human conditions, which include Alzheimer's disease, Parkinson's disease, and the Creutzfeldt-Jakob disease. Muscle acylphosphatase (AcP) has already provided significant insights into mutational changes that modulate amyloid formation. In the present paper, we have used this system to investigate the effects of mutations that modify the charge state of a protein without affecting significantly the hydrophobicity or secondary structural propensities of the polypeptide chain. A highly significant inverse correlation was found to exist between the rates of aggregation of the protein variants under denaturing conditions and their overall net charge. This result indicates that aggregation is generally favored by mutations that bring the net charge of the protein closer to neutrality. In light of this finding, we have analyzed natural mutations associated with familial forms of amyloid diseases that involve alteration of the net charge of the proteins or protein fragments associated with the diseases. Sixteen mutations have been identified for which the mechanism of action that causes the pathological condition is not yet known or fully understood. Remarkably, 14 of these 16 mutations cause the net charge of the corresponding peptide or protein that converts into amyloid deposits to be reduced. This result suggests that charge has been a key parameter in molecular evolution to ensure the avoidance of protein aggregation and identifies reduction of the net charge as an important determinant in at least some forms of protein deposition diseases.

Evidence for early cytotoxic aggregates in transgenic mice for human transthyretin Leu55Pro

Familial amyloidotic polyneuropathy (FAP) is a lethal autosomal dominant disorder characterized by systemic extracellular deposition of transthyretin (TTR) amyloid fibrils. Several groups have generated transgenic mice carrying human TTR Val30Met, the most common mutation in FAP. To study amyloidogenicity and cytotoxicity of different TTRs, we produced transgenic mice expressing human TTR Leu55Pro, one of the most aggressive FAP-related mutations. TTR deposition and presence of amyloid fibrils was investigated and compared to animals carrying the human TTR Val30Met gene kept under the same conditions. Deposition in a C57BL/6J background (TTR-Leu55Pro mice) and in a TTR-null background [TTR-Leu55Pro X TTR-knockout (KO) mice] was compared. Animals in a C57BL/6J background presented early (1 to 3 months) nonfibrillar TTR deposition but amyloid was absent. In a TTR-null background, presence of amyloid fibrils was detected starting at 4 to 8 months with a particular involvement of the gastrointestinal tract and skin. This data suggested that TTR homotetramers are more prone to fibril formation than TTR murine wild-type/human mutant heterotetramers. The nature of the deposited material was further investigated by immunocytochemistry. Both amorphous aggregates and small TTR fibrils were present in TTR-Leu55Pro X TTR-KO transgenics. We observed that these TTR deposits mimic the toxic effect of TTR deposits in FAP: animals with TTR deposition, present approximately twofold increased levels of nitrotyrosine in sites related to deposition. The TTR-Leu55Pro X TTR-KO mice here described are an important tool for the dual purpose of investigating factors involved in amyloidogenesis and in cytotoxicity of deposited TTR.

Effect of mutant alpha-synuclein on dopamine homeostasis in a new human mesencephalic cell line

Mutations in alpha-synuclein have been linked to rare, autosomal dominant forms of Parkinson's disease. Despite its ubiquitous expression, mutant alpha-synuclein primarily leads to the loss of dopamine-producing neurons in the substantia nigra. alpha-Synuclein is a presynaptic nerve terminal protein of unknown function, although several studies suggest it is important for synaptic plasticity and maintenance. The present study utilized a new human mesencephalic cell line, MESC2.10, to study the effect of A53T mutant alpha-synuclein on dopamine homeostasis. In addition to expressing markers of mature dopamine neurons, differentiated MESC2.10 cells are electrically active, produce dopamine, and express wild-type human alpha-synuclein. Lentivirus-induced overexpression of A53T mutant alpha-synuclein in differentiated MESC2.10 cells resulted in down-regulation of the vesicular dopamine transporter (VMAT2), decreased potassium-induced and increased amphetamine-induced dopamine release, enhanced cytoplasmic dopamine immunofluorescence, and increased intracellular levels of superoxide. These results suggest that mutant alpha-synuclein leads to an impairment in vesicular dopamine storage and consequent accumulation of dopamine in the cytosol, a pathogenic mechanism that underlies the toxicity of the psychostimulant amphetamine and the parkinsonian neurotoxin 1-methyl-4-phenylpyridinium. Interestingly, cells expressing A53T mutant alpha-synuclein were resistant to amphetamine-induced toxicity. Because extravesicular, cytoplasmic dopamine can be easily oxidized into reactive oxygen species and other toxic metabolites, mutations in alpha-synuclein might lead to Parkinson's disease by triggering protracted, low grade dopamine toxicity resulting in terminal degeneration and ultimately cell death.

Alpha-synuclein, especially the Parkinson's disease-associated mutants, forms pore-like annular and tubular protofibrils

Two mutations in the alpha-synuclein gene (A30P and A53T) have been linked to autosomal dominant early-onset Parkinson's disease (PD). Both mutations promote the formation of transient protofibrils (prefibrillar oligomers), suggesting that protofibrils are linked to cytotoxicity. In this work, the effect of these mutations on the structure of alpha-synuclein oligomers was investigated using electron microscopy and digital image processing. The PD-linked mutations (A30P and A53T) were observed to affect both the morphology and the size distribution of alpha-synuclein protofibrils (measured by analytical ultracentrifugation and scanning transmission electron microscopy). The A30P variant was observed to promote the formation of annular, pore-like protofibrils, whereas A53T promotes formation of annular and tubular protofibrillar structures. Wild-type alpha-synuclein also formed annular protofibrils, but only after extended incubation. The formation of pore-like oligomeric structures may explain the membrane permeabilization activity of alpha-synuclein protofibrils. These structures may contribute to the pathogenesis of PD.

Synergistic effects of pesticides and metals on the fibrillation of alpha-synuclein: implications for Parkinson's disease

Aggregation of alpha-synuclein has been implicated in the formation of proteinaceous inclusions in the brain (Lewy bodies, Lewy neurites) that are characteristic of neurodegenerative diseases, such as Parkinson's disease (PD) and dementia with Lewy bodies (DLBs). The etiology of PD is unknown, but recent work has shown that except in rare cases, there appears to be no direct genetic basis. However, several studies have implicated environmental factors, especially pesticides and metals. Here we show that certain pesticides and metals induce a conformational change in alpha-synuclein and directly accelerate the rate of formation of alpha-synuclein fibrils in vitro. In addition, the simultaneous presence of metal and pesticide led to synergistic effects on the rate of fibrillation. We propose a model in which environmentalfactors in conjunction with genetic susceptibility may form the underlying molecular basis for idiopathic PD.

The genetic basis of Parkinson's disease

Although the mechanisms underlying neurodegeneration in Parkinson's disease are not fully understood, considerable evidence suggests that genetic factors can influence susceptibility to the disease. In this article, we critically review this evidence and examine studies estimating patterns of inheritance. In a few families, Parkinson's disease is clearly inherited in a Mendelian fashion, and in some of these the disease causing genes have already been identified. Possible pathogenic mechanisms by which these genes cause Parkinson's disease are discussed. Further candidate genes and systematic efforts to identify genes influencing susceptibility to the disease in general are also summarised. The identification of such susceptibility genes will eventually enable us to more accurately classify this complex disease.

Serpinopathies and the conformational dementias

The serpin superfamily of serine proteinase inhibitors has a central role in controlling proteinases in many biological pathways in a wide range of species. The inhibitory function of the serpins involves a marked conformational transition, but this inherent molecular flexibility also renders the serpins susceptible to point mutations that result in aberrant intermolecular linkage and polymer formation. The effects of such protein aggregation are cumulative, with a progressive loss of cellular function that results in diseases as diverse as cirrhosis and emphysema. The recent recognition that mutations in a serpin can also result in late-onset dementia provides insights into changes that underlie other conformational diseases, such as the amyloidoses, the prion encephalopathies and Huntington and Alzheimer diseases.

Genetics of Parkinson's disease and biochemical studies of implicated gene products

Parkinson's disease was thought, until recently, to have little or no genetic component. This notion has changed with the identification of three genes, and the mapping of five others, that are linked to rare familial forms of the disease (FPD). The products of the identified genes, alpha-synuclein (PARK 1), parkin (PARK 2), and ubiquitin-C-hydrolase-L1 (PARK 5) are the subject of intense cell-biological and biochemical studies designed to elucidate the underlying mechanism of FPD pathogenesis. In addition, the complex genetics of idiopathic PD is beginning to be unraveled. Genetic information may prove to be useful in identifying new therapeutic targets and identifying the preclinical phase of PD, allowing treatment to begin sooner.

Conformational behavior of human alpha-synuclein is modulated by familial Parkinson's disease point mutations A30P and A53T

Structural properties and response to changes in the environment of wild-type (WT), A30P and A53T alpha-synucleins, as well as their propensity to aggregate orform fibrils, were compared by a variety of biophysical methods, including far-UV CD, FTIR, SAXS, static light scattering and Thioflavin T (TFT) fluorescence. All three proteins were natively unfolded under physiological conditions but adopted identical partially-folded conformations under conditions of acidic pH or high temperature. The initial kinetic event in the fibrillation of all three alpha-synucleins was shown to be the formation of a partially-folded intermediate with properties close to those described for these proteins at acidic pH or at high temperatures. Both mutants showed a greater propensity to form non-fibrillar aggregates than wild-type protein. All three proteins formed fibrils faster in the presence of heparin, although substantially higher concentrations were required for the A30P mutant. In contrast to the wild-type and A53T proteins, in which fibrillation was further accelerated by the presence of the pesticide diethyldithiocarbamate (DDC), the A30P mutant was inhibited by DDC. The mutant proteins had significantly lower affinity for DDC than the WT. A model of the effect of mutations on the aggregation behavior of alpha-synuclein is proposed, which explains the different effects of exogenous agents on the three proteins, based on different kinetic partitioning along pathways leading to fibrils and to non-fibrillar aggregates.

Evidence that alpha-synuclein functions as a negative regulator of Ca(++)-dependent alpha-granule release from human platelets

alpha-Synuclein has been implicated in the pathogenesis of Parkinson disease (PD) and related neurodegenerative disorders. More recently, it has been suggested to be an important regulatory component of vesicle transport in neuronal cells. alpha-Synuclein is also highly expressed in platelets and is loosely associated with the membrane of the secretory alpha-granules. However, the functional significance of these observations is unknown. In this study, the possible function of alpha-synuclein in vesicle transport, with particular regard to alpha-granule release from the platelets, was investigated. The results showed that ionomycin- or thrombin-induced alpha-granule secretion was inhibited by exogenous alpha-synuclein addition in a dose-dependent manner. However, [(3)H]5-HT release from the dense granules and hexosaminidase release from the lysosomal granules were not affected. Two point mutants (A30P and A53T) found in some familial types of PD, in addition to beta-synuclein and alpha-synuclein112, effectively inhibited PF4 release from the alpha-granules. However, the deletion mutants, which completely lacked either the N-terminal region or the C-terminal tail, did not affect alpha-granule release. Interestingly, exogenously added alpha-synuclein appeared to enter the platelets but did not change the Ca(++) level in the platelets at the resting state and the increase in the Ca(++) level on stimulation. Electron microscopy also supported that alpha-synuclein inhibits alpha-granule release. These results suggest that alpha-synuclein may function as a specific negative regulator of alpha-granule release in platelets.

Dependence of alpha-synuclein aggregate morphology on solution conditions

Alpha-synuclein is the major component of Lewy bodies and Lewy neurites, which are granular and filamentous protein inclusions that are the defining pathological features of several neurodegenerative conditions such as Parkinson's disease. Fibrillar aggregates formed from alpha-synuclein in vitro resemble brain-derived material, but the role of such aggregates in the etiology of Parkinson's disease and their relation to the toxic molecular species remain unclear. In this study, we investigated the effects of pH and salt concentration on the in vitro assembly of human wild-type alpha-synuclein, particularly with regard to aggregation rate and aggregate morphology. Aggregates formed at pH 7.0 and pH 6.0 in the absence of NaCl and MgCl(2) were fibrillar; the pH 6.0 fibrils displayed a helical twist, as clearly evident by scanning force and electron microscopy. Incubations at pH 7.0 remained transparent during the process of aggregation and exhibited strong thioflavin-T and weak 8-anilino-1-naphthalenesulfonate (ANS) binding; furthermore, they were efficient in seeding fibrillization of fresh solutions. In contrast, incubating alpha-synuclein at low pH (pH 4.0 or pH 5.0) resulted in the rapid formation of turbid suspensions characterized by strong ANS binding, reduced thioflavin-T binding and reduced seeding efficiency. At pH 4.0, fibril formation was abrogated; instead, very large aggregates (dimensions approximately 100 microm) of amorphous appearance were visible by light microscopy. As with acidic conditions, addition of 0.2M NaCl or 10mM MgCl(2) to pH 7.0 incubations led to a shorter aggregation lag time and formation of large, amorphous aggregates. These results demonstrate that the morphology of alpha-synuclein aggregates is highly sensitive to solution conditions, implying that the fibrillar state does not necessarily represent the predominant or most functionally significant aggregated state under physiological conditions.

Paradigm shifts in Alzheimer's disease and other neurodegenerative disorders: the emerging role of oligomeric assemblies

Alzheimer's disease (AD) is a progressive, neurodegenerative disorder characterized by amyloid deposition in the cerebral neuropil and vasculature. These amyloid deposits comprise predominantly fragments and full-length (40 or 42 residue) forms of the amyloid beta-protein (Abeta) organized into fibrillar assemblies. Compelling evidence indicates that factors that increase overall Abeta production or the ratio of longer to shorter forms, or which facilitate deposition or inhibit elimination of amyloid deposits, cause AD or are risk factors for the disease. In vitro studies have demonstrated that fibrillar Abeta has potent neurotoxic effects on cultured neurons. In vivo experiments in non-human primates have demonstrated that Abeta fibrils directly cause pathologic changes, including tau hyperphosphorylation. In concert with histologic studies revealing a lack of tissue injury in areas of the neuropil in which non-fibrillar deposits were found, these data suggested that fibril assembly was a prerequisite for Abeta-mediated neurotoxicity in vivo. Recently, however, both in vitro and in vivo studies have revealed that soluble, oligomeric forms of Abeta also have potent neurotoxic activities, and in fact, may be the proximate effectors of the neuronal injury and death occurring in AD. A paradigm shift is thus emerging that necessitates the reevaluation of the relative importance of polymeric (fibrillar) vs. oligomeric assemblies in the pathobiology of AD. In addition to AD, an increasing number of neurodegenerative disorders, including Parkinson's disease, familial British dementia, familial amyloid polyneuropathy, amyotrophic lateral sclerosis, and prion diseases, are associated with abnormal protein assembly processes. The archetypal features of the assembly-dependent neuropathogenetic effects of Abeta may thus be of relevance not only to AD but to these other disorders as well.

Parkinson's genetics--creating exciting new insights

Parkinson's disease is a complex disorder in which the genetic aspects are only just being realized. The underlying cause for the degeneration of dopaminergic substantia nigra neurons and the formation of Lewy bodies in Parkinson's disease is unknown. The identification of clear inherited forms of the disease has provided important clues as to how this complex process may be occurring. Mutations have now been identified in the alpha-synuclein (4q21.3-23), parkin (6q25.2-27), and ubiquitin carboxy terminal hydrolase-L1 (4p16.3) genes in families with Parkinson's disease. Four additional chromosomal locations; 2p13, 4p14-15, 1p35-36, and 12p11.2-q13.1 have been linked to Parkinson's disease families but no pathologic gene mutations have been identified to date. As additional Parkinson's disease loci are mapped and their genes identified we will continue to add to our understating of the critical biochemical pathways involved and be able to develop effective disease altering treatments.

Oligomeric and fibrillar species of amyloid-beta peptides differentially affect neuronal viability

Genetic evidence predicts a causative role for amyloid-beta (A beta) in Alzheimer's disease. Recent debate has focused on whether fibrils (amyloid) or soluble oligomers of A beta are the active species that contribute to neurodegeneration and dementia. We developed two aggregation protocols for the consistent production of stable oligomeric or fibrillar preparations of A beta-(1-42). Here we report that oligomers inhibit neuronal viability 10-fold more than fibrils and approximately 40-fold more than unaggregated peptide, with oligomeric A beta-(1-42)-induced inhibition significant at 10 nm. Under A beta-(1-42) oligomer- and fibril-forming conditions, A beta-(1-40) remains predominantly as unassembled monomer and had significantly less effect on neuronal viability than preparations of A beta-(1-42). We applied the aggregation protocols developed for wild type A beta-(1-42) to A beta-(1-42) with the Dutch (E22Q) or Arctic (E22G) mutations. Oligomeric preparations of the mutations exhibited extensive protofibril and fibril formation, respectively, but were not consistently different from wild type A beta-(1-42) in terms of inhibition of neuronal viability. However, fibrillar preparations of the mutants appeared larger and induced significantly more inhibition of neuronal viability than wild type A beta-(1-42) fibril preparations. These data demonstrate that protocols developed to produce oligomeric and fibrillar A beta-(1-42) are useful in distinguishing the structural and functional differences between A beta-(1-42) and A beta-(1-40) and genetic mutations of A beta-(1-42).

alpha -Synucleinopathy and selective dopaminergic neuron loss in a rat lentiviral-based model of Parkinson's disease

Parkinson's disease (PD) is characterized by the progressive loss of substantia nigra dopaminergic neurons and the presence of cytoplasmic inclusions named Lewy bodies. Two missense mutations of the alpha-synuclein (alpha-syn; A30P and A53T) have been described in several families with an autosomal dominant form of PD. alpha-Syn also constitutes one of the main components of Lewy bodies in sporadic cases of PD. To develop an animal model of PD, lentiviral vectors expressing different human or rat forms of alpha-syn were injected into the substantia nigra of rats. In contrast to transgenic mice models, a selective loss of nigral dopaminergic neurons associated with a dopaminergic denervation of the striatum was observed in animals expressing either wild-type or mutant forms of human alpha-syn. This neuronal degeneration correlates with the appearance of abundant alpha-syn-positive inclusions and extensive neuritic pathology detected with both alpha-syn and silver staining. Lentiviral-mediated expression of wild-type or mutated forms of human alpha-syn recapitulates the essential neuropathological features of PD. Rat alpha-syn similarly leads to protein aggregation but without cell loss, suggesting that inclusions are not the primary cause of cell degeneration in PD. Viral-mediated genetic models may contribute to elucidate the mechanism of alpha-syn-induced cell death and allow the screening of candidate therapeutic molecules.

Protein misfolding, amyloid formation, and neurodegeneration: a critical role for molecular chaperones?

The most conspicuous feature of many neurodegenerative disorders, including Alzheimer's, Parkinson's, and Huntington's disease, is the occurrence of protein aggregates in ordered fibrillar structures known as amyloid found inside and outside of brain cells. The appearance of aggregates in diseased brains implies an underlying incapacity in the cellular machinery of molecular chaperones that normally functions to prevent the accumulation of misfolded proteins. Here we review recent studies that have revealed a critical role for molecular chaperones in several neurodegenerative disorders.

Familial conformational diseases and dementias

Familial conformational diseases occur when a mutation alters the conformation of a protein resulting in abnormal intermolecular interactions, protein aggregation, and consequent tissue damage. The molecular mechanisms of conformational disease are best understood for the serine protease inhibitor (serpin) superfamily of proteins. The serpinopathies include alpha(1)-antitrypsin (SERPINA1) deficiency and the newly characterized familial encephalopathy with neuroserpin inclusion bodies (FENIB) resulting from mutations in the neuroserpin (SERPINI1) gene. This review discusses how insights gained from the study of the serpins may be used to guide our research into other common diseases such as Alzheimer disease, Huntington disease, and Parkinson disease.

The ceruloplasmin and hydrogen peroxide system induces alpha-synuclein aggregation in vitro

Alpha-synuclein is a key component of Lewy bodies in the brain of patients with Parkinson's disease (PD) and recent studies suggest that oxidative stress reactions might contribute to abnormal aggregation of this molecule. Since hydrogen peroxide-mediated ceruloplasmin (CP) modification can induce the formation of free radicals and release of copper ions, we investigated the role of CP in the aggregation of alpha-synuclein. When alpha-synuclein was incubated with both CP and H(2)O(2), alpha-synuclein concomitantly was induced to be aggregated. Thioflavin-S staining of alpha-synuclein aggregates showed that they displayed characteristic fibrillar structures. Hydroxyl radical scavengers and spin-trapping agent such as 5,5'-dimethyl 1-pyrolline N-oxide and tert-butyl-alpha-phenylnitrone significantly inhibited the aggregation of alpha-synuclein. Copper chelator, penicillamine also inhibited the CP/H(2)O(2) system-induced alpha-synuclein aggregation. This indicates that the aggregation of alpha-synuclein can be mediated by the CP/H(2)O(2) system via the generation of hydroxyl radical. The CP/H(2)O(2) system-induced alpha-synuclein aggregation resulted in the generation of protein carbonyl derivatives. Antioxidant molecules, carnosine, homocarnosine and anserine significantly inhibited the CP/H(2)O(2) system-induced aggregation of alpha-synuclein. These results suggest that the CP/H(2)O(2) system may be related to abnormal aggregation of alpha-synuclein which may be involved in the pathogenesis of PD and related disorders.

Human alpha-synuclein-harboring familial Parkinson's disease-linked Ala-53 --> Thr mutation causes neurodegenerative disease with alpha-synuclein aggregation in transgenic mice

Mutations in alpha-synuclein (alpha-Syn) cause Parkinson's disease (PD) in a small number of pedigrees with familial PD. Moreover, alpha-Syn accumulates as a major component of Lewy bodies and Lewy neurites, intraneuronal inclusions that are neuropathological hallmarks of PD. To better understand the pathogenic relationship between alterations in the biology of alpha-Syn and PD-associated neurodegeneration, we generated multiple lines of transgenic mice expressing high levels of either wild-type or familial PD-linked Ala-30 --> Pro (A30P) or Ala-53 --> Thr (A53T) human alpha-Syns. The mice expressing the A53T human alpha-Syn, but not wild-type or the A30P variants, develop adult-onset neurodegenerative disease with a progressive motoric dysfunction leading to death. Pathologically, affected mice exhibit neuronal abnormalities (in perikarya and neurites) including pathological accumulations of alpha-Syn and ubiquitin. Consistent with abnormal neuronal accumulation of alpha-Syn, brain regions with pathology exhibit increases in detergent-insoluble alpha-Syn and alpha-Syn aggregates. Our results demonstrate that the A53T mutant alpha-Syn causes significantly greater in vivo neurotoxicity as compared with other alpha-Syn variants. Further, alpha-Syn-dependent neurodegeneration is associated with abnormal accumulation of detergent-insoluble alpha-Syn.

The cell biology of alpha-synuclein: a sticky problem?

Parkinson's disease (PD) is the most common neurodegenerative motor disorder, marked by chronic progressive loss of neurons in the substantia nigra, thereby damaging purposeful control of movement. For decades, it was believed that PD was caused solely by environmental causes. However, the discovery of genetic factors involved in PD has revolutionized our attempts to understand the disease's pathology. PD now appears to be more polygenetic than previously thought and is most likely caused by a complex interaction of genetic risks and environmental exposures. The first gene found to be mutated in PD encodes for the presynaptic protein alpha-synuclein, which is also a major component of Lewy bodies and Lewy neurites, the neuropathological hallmarks of the disease. While these findings provide a classic example of how rare genetic mutations in disease can point to important pathways in idiopathic disease pathologies, much of the study of alpha-synuclein has focused on understanding how this protein undergoes the transition from an unfolded monomer to amorphous aggregates or Lewy body-like filaments rather than addressing what its fundamental function might be. Since alterations in synuclein function may predispose to the disease pathology of PD, regardless of the presence of genetic mutations, a more thorough understanding of the cellular regulation and function of alpha-synuclein may be of crucial importance to our understanding of this degenerating disorder.

The relationship between oxidative/nitrative stress and pathological inclusions in Alzheimer's and Parkinson's diseases

Alzheimer's (AD) and Parkinson's diseases (PD) are late-onset neurodegenerative diseases that have tremendous impact on the lives of affected individuals, their families, and society as a whole. Remarkable efforts are being made to elucidate the dominant factors that result in the pathogenesis of these disorders. Extensive postmortem studies suggest that oxidative/nitrative stresses are prominent features of these diseases, and several animal models support this notion. Furthermore, it is likely that protein modifications resulting from oxidative/nitrative damage contribute to the formation of intracytoplasmic inclusions characteristic of each disease. The frequent presentation of both AD and PD in individuals and the co-occurrence of inclusions characteristic of AD and PD in several other neurodegenerative diseases suggests the involvement of a common underlying aberrant process. It can be surmised that oxidative/nitrative stress, which is cooperatively influenced by environmental factors, genetic predisposition, and senescence, may be a link between these disorders.

Genetics of Parkinson's disease and biochemical studies of implicated gene products

Parkinson's disease was thought, until recently, to have little or no genetic component. This notion has changed with the identification of three genes, and the mapping of five others, that are linked to rare familial forms of the disease (FPD). The products of the identified genes, alpha-synuclein (PARK 1), parkin (PARK 2), and ubiquitin-C-hydrolase-L1 (PARK 5) are the subject of intense cell-biological and biochemical studies designed to elucidate the underlying mechanism of FPD pathogenesis. In addition, the complex genetics of idiopathic PD is beginning to be unraveled. Genetic information may prove to be useful in identifying new therapeutic targets and identifying the preclinical phase of PD, allowing treatment to begin sooner.

Dopamine-dependent neurotoxicity of alpha-synuclein: a mechanism for selective neurodegeneration in Parkinson disease

The mechanism by which dopaminergic neurons are selectively lost in Parkinson disease (PD) is unknown. Here we show that accumulation of alpha-synuclein in cultured human dopaminergic neurons results in apoptosis that requires endogenous dopamine production and is mediated by reactive oxygen species. In contrast, alpha-synuclein is not toxic in non-dopaminergic human cortical neurons, but rather exhibits neuroprotective activity. Dopamine-dependent neurotoxicity is mediated by 54 83-kD soluble protein complexes that contain alpha-synuclein and 14-3-3 protein, which are elevated selectively in the substantia nigra in PD. Thus, accumulation of soluble alpha-synuclein protein complexes can render endogenous dopamine toxic, suggesting a potential mechanism for the selectivity of neuronal loss in PD.

Biochemical characterization of the core structure of alpha-synuclein filaments

Intracellular filamentous aggregates comprised of alpha-synuclein such as Lewy bodies and glial cytoplasmic inclusions are the defining hallmarks of a subset of neurodegenerative diseases including Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy. We have analyzed biochemical and structural properties of alpha-synuclein filaments assembled in vitro or extracted from brains of patients with multiple system atrophy and found that both types of filaments are insoluble to detergents and partially resistant to proteinase K digestion. Immunoelectron microscopy and immunoblot analysis showed that both amino and carboxyl termini of alpha-synuclein in in vitro assembled filaments were degraded by proteinase K treatment, whereas the central portion of alpha-synuclein is resistant to proteinase K and retains filamentous structures. Protein sequencing and mass spectrometric analyses of the proteinase K-resistant, minimal fragment of 7 kDa revealed that amino acid residues 31-109 of alpha-synuclein constitute the core unit of the filaments. These observations suggest that the central half of the alpha-synuclein polypeptide, containing five tandem repeats as well as a part of the carboxyl-terminal acidic region, forms the core structure of alpha-synuclein filaments, which is coated by the amino- and carboxyl-terminal portions at the periphery.

Magnesium inhibits spontaneous and iron-induced aggregation of alpha-synuclein

Multiple studies implicate metals in the pathophysiology of neurodegenerative diseases. Disturbances in brain iron metabolism are linked with synucleinopathies. For example, in Parkinson's disease, iron levels are increased and magnesium levels are reduced in the brains of patients. To understand how changes in iron and magnesium might affect the pathophysiology of Parkinson's disease, we investigated binding of iron to alpha-synuclein, which accumulates in Lewy bodies. Using fluorescence of the four tyrosines in alpha-synuclein as indicators of metal-related conformational changes in alpha-synuclein, we show that iron and magnesium both interact with alpha-synuclein. alpha-Synuclein exhibits fluorescence peaks at 310 and 375 nm. Iron lowers both fluorescence peaks, while magnesium increases the fluorescence peak only at 375 nm, which suggests that magnesium affects the conformation of alpha-synuclein differently than iron. Consistent with this hypothesis, we also observe that magnesium inhibits alpha-synuclein aggregation, measured by immunoblot, cellulose acetate filtration, or thioflavine-T fluorescence. In each of these studies, iron increases alpha-synuclein aggregation, while magnesium at concentrations >0.75 mm inhibits the aggregation of alpha-synuclein induced either spontaneously or by incubation with iron. These data suggest that the conformation of alpha-synuclein can be modulated by metals, with iron promoting aggregation and magnesium inhibiting aggregation.

Methionine oxidation inhibits fibrillation of human alpha-synuclein in vitro

We examined the effect of methionine oxidation of human recombinant alpha-synuclein on its structural properties and propensity to fibrillate. Both oxidized and non-oxidized alpha-synucleins were natively unfolded under conditions of neutral pH, with the oxidized protein being slightly more disordered. Both proteins adopted identical partially folded conformations under conditions of acidic pH. The fibrillation of alpha-synuclein at neutral pH was completely inhibited by methionine oxidation. This inhibitory effect was eliminated at low pH. The addition of oxidized alpha-synuclein to the unoxidized form led to a substantial inhibition of alpha-synuclein fibrillation.

Biophysical properties of the synucleins and their propensities to fibrillate: inhibition of alpha-synuclein assembly by beta- and gamma-synucleins

The pathological hallmark of Parkinson's disease is the presence of intracellular inclusions, Lewy bodies, and Lewy neurites, in the dopaminergic neurons of the substantia nigra and several other brain regions. Filamentous alpha-synuclein is the major component of these deposits and its aggregation is believed to play an important role in Parkinson's disease and several other neurodegenerative diseases. Two homologous proteins, beta- and gamma-synucleins, are also abundant in the brain. The synucleins are natively unfolded proteins. beta-Synuclein, which lacks 11 central hydrophobic residues compared with its homologs, exhibited the properties of a random coil, whereas alpha- and gamma-synucleins were slightly more compact and structured. gamma-Synuclein, unlike its homologs, formed a soluble oligomer at relatively low concentrations, which appears to be an off-fibrillation pathway species. Here we show that, although they have similar biophysical properties to alpha-synuclein, beta- And gamma-synucleins inhibit alpha-synuclein fibril formation. Complete inhibition of alpha-synuclein fibrillation was observed at 4:1 molar excess of beta- and gamma-synucleins. No significant incorporation of beta-synuclein into the fibrils was detected. The lack of fibrils formed by beta-synuclein is most readily explained by the absence of a stretch of hydrophobic residues from the middle region of the protein. A model for the inhibition is proposed.

Inherent toxicity of aggregates implies a common mechanism for protein misfolding diseases

A range of human degenerative conditions, including Alzheimer's disease, light-chain amyloidosis and the spongiform encephalopathies, is associated with the deposition in tissue of proteinaceous aggregates known as amyloid fibrils or plaques. It has been shown previously that fibrillar aggregates that are closely similar to those associated with clinical amyloidoses can be formed in vitro from proteins not connected with these diseases, including the SH3 domain from bovine phosphatidyl-inositol-3'-kinase and the amino-terminal domain of the Escherichia coli HypF protein. Here we show that species formed early in the aggregation of these non-disease-associated proteins can be inherently highly cytotoxic. This finding provides added evidence that avoidance of protein aggregation is crucial for the preservation of biological function and suggests common features in the origins of this family of protein deposition diseases.

Accelerated alpha-synuclein fibrillation in crowded milieu

Parkinson's disease is the second most common age-related neurodegenerative disease, resulting from loss of dopaminergic neurons in the substantia nigra. The aggregation and fibrillation of alpha-synuclein has been implicated as a causative factor in the disease, and the process of fibril formation has been intensively studied in vitro with dilute protein solutions. However, the intracellular environment of proteins is crowded with other macromolecules, whose concentration can reach 400 g/l. To address this discrepancy, the effect of molecular crowding on alpha-synuclein fibrillation has being studied. The addition of high concentrations of different polymers (proteins, polysaccharides and polyethylene glycols) dramatically accelerated alpha-synuclein fibrillation in vitro. The magnitude of the accelerating effect depended on the nature of the polymer, its length and concentration. Our results suggest that the major factor responsible for the accelerated fibrillation under crowded conditions is the excluded volume.

Dopaminergic cell loss induced by human A30P alpha-synuclein gene transfer to the rat substantia nigra

Somatic cell gene transfer was used to express a mutant form of alpha-synuclein (alpha-syn) that is associated with Parkinson's disease (PD) in the rat substantia nigra (SN), a brain region that, in humans, degenerates during PD. DNA encoding the A30P mutant of human alpha-syn linked to familial PD was incorporated into an adeno-associated virus vector, which was injected into the adult rat midbrain. The cytomegalovirus/chicken beta-actin promoter was used to drive transgene expression. Over a 1-year time course, this treatment produced three significant features relevant to PD: (1) accumulation of alpha-syn in SN neuron perikarya, (2) Lewy-like dystrophic neurites in the SN and the striatum, and (3) a 53% loss of SN dopamine neurons. However, motor dysfunction was not found in either rotational or rotating rod testing. The lack of behavioral deficits, despite the significant cell loss, may reflect pathogenesis similar to that of PD, where greater than 50% losses occur before motor behavior is affected.