Overexpression of human α-synuclein causes dopamine neuron death in rat primary culture and immortalized mesencephalon-derived cells

Parkinson’s disease (PD) is a neurodegenerative disorder characterized by the appearance of intracytoplasmic inclusions called Lewy bodies (LB) in dopamine neurons in the substantia nigra and the progressive loss of these neurons. Recently, mutations in the α-synuclein gene have been identified in early-onset familial PD, and α-synuclein has been shown to be a major component of LB in all patients. Yet, the pathophysiological function of α-synuclein remains unknown. In this report, we have investigated the toxic effects of adenovirus-mediated α-synuclein overexpression on dopamine neurons in rat primary mesencephalic cultures and in a rat dopaminergic cell line – the large T-antigen immortalized, mesencephalon-derived 1RB3AN27 (N27). Adenovirus-transduced cultures showed high-level expression of α-synuclein within the cells. Overexpression of human mutant α-synuclein (Ala₅₃Thr) selectively induced apoptotic programmed cell death of primary dopamine neurons as well as N27 cells. The mutant protein also potentiated the neurotoxicity of 6-hydroxydopamine (6-OHDA). By contrast, overexpression of wild-type human α-synuclein was not directly neurotoxic but did increase cell death after 6-OHDA. Overexpression of wild-type rat α-synuclein had no effect on dopamine cell survival or 6-OHDA neurotoxicity. These results indicate that overexpression of human mutant α-synuclein directly leads to dopamine neuron death, and overexpression of either human mutant or human wild-type α-synuclein renders dopamine neurons more vulnerable to neurotoxic insults.     

Forebrain circumventricular organs mediate salt appetite induced by intravenous angiotensin II in rats

Mutations in the alpha-synuclein gene have been linked to rare cases of familial Parkinson's disease (PD). Alpha-synuclein is a major component of Lewy bodies (LB), a pathological hallmark of PD. Transgenic mice and Drosophila expressing either wild-type or mutant human alpha-synuclein develop motor deficits, LB-like inclusions in some neurons, and neuronal degeneration. However, the relationship between abnormal aggregates of alpha-synuclein and human dopamine (DA) neuron degeneration remains unclear. In this report, we have investigated the influence of alpha-synuclein expression on DA neurons in primary culture of embryonic human mesencephalon. Two days after culture, human DA cells were transduced with wild-type or mutant human (Ala(53)Thr) alpha-synuclein adenoviruses and maintained for 5 days. Overexpression of mutant and wild-type human alpha-synuclein resulted in 49% (P<0.01) and 27% (P<0.05) loss of DA neurons, respectively, while not affecting viability of other cells in the culture. Overexpression of rat alpha-synuclein or GFP (green fluorescent protein) had no effect on DA neuron survival. Cytoplasmic inclusions of alpha-synuclein were detected immunohistochemically in DA cells transduced with mutant human alpha-synuclein, but not wild-type alpha-synuclein. These results show that overexpression of human alpha-synuclein, particularly the mutant form, can cause human DA neuron death, suggesting that alpha-synuclein may have a primary role in the pathogenesis of PD.     

Wild-type and mutant alpha-synuclein induce a multi-component gene expression profile consistent with shared pathophysiology in different transgenic mouse models of PD

The pathophysiological processes that cause Parkinson's disease (PD) affect dopamine neurons residing in the substantia nigra with devastating consequences for normal movement. One important gene involved in both familial and sporadic PD is alpha-synuclein. We have generated three strains of alpha-synuclein transgenic mice to study the pathologic consequences of the targeted expression of mutant or wild-type human alpha-synuclein in a model system. We have analyzed gene expression patterns in these mice using high throughput microarrays in anatomical regions implicated in disease (substantia nigra and brainstem). Our study reveals gene dosage-dependent dysregulation of several genes important for the dopaminergic phenotype in mice over-expressing wild-type human alpha-synuclein in the substantia nigra at time points preceding neuronal cell death. Analysis of mutant alpha-synuclein mice at a time point when pathology is advanced reveals several new candidate genes that may play a role in neuronal demise and/or protein accumulation.     

Dopaminergic neurotoxicity by 6-OHDA and MPP+: differential requirement for neuronal cyclooxygenase activity

Cyclooxygenase (COX), a key enzymatic mediator of inflammation, is present in microglia and surviving dopaminergic neurons in Parkinson's disease (PD), but its role and place in the chain of neurodegenerative events is unclear. Epidemiologic evidence showed that regular use of nonsteroidal antiinflammatory drugs (NSAIDs), specifically non-aspirin COX inhibitors like ibuprofen, lowers the risk for PD; however, the putative cause-and-effect relationship between COX activity in activated microglia and neuronal loss was challenged recently. We examined whether neuronal COX activity is involved directly in dopaminergic cell death after neurotoxic insult. Using low concentrations of 6-hydroxydopamine (6-OHDA) and 1-methyl-4-phenylpyridium ion (MPP+), neurotoxicants used to model selective dopaminergic cell loss in PD, and cultures of embryonic rat mesencephalic neurons essentially devoid of glia, we tested whether the nonselective COX inhibitor ibuprofen attenuated 6-OHDA and MPP+ neurotoxicity. At levels close to its IC50 for both COX isoforms, ibuprofen protected dopaminergic neurons against 6-OHDA but not MPP+ toxicity. Experiments with selective inhibitors of COX-1 (SC-560) and COX-2 (NS-398 and Cayman 10404), indicated that COX-2, but not COX-1, was involved in 6-OHDA toxicity. Accordingly, 6-OHDA, but not MPP+, increased prostaglandin (PG) levels twofold and this increase was blocked by ibuprofen. At concentrations well above its IC50 for COX, ibuprofen also prevented MPP+ toxicity, but had only limited efficacy against loss of structural complexity. Taken together, our data suggest that selective 6-OHDA toxicity to dopaminergic neurons is associated with neuronal COX-2, whereas MPP+ toxicity is COX independent. This difference may be important for understanding and manipulating mechanisms of dopaminergic cell death.     

Mutant and wild-type alpha-synuclein interact with mitochondrial cytochrome C oxidase

Alpha-synuclein, a presynaptic protein, was found to be the major component in the Lewy bodies (LB) in both inherited and sporadic Parkinson's disease (PD). Furthermore, rare mutations of alpha-synuclein cause autosomal-dominant PD. However, it is unknown how alpha-synuclein is involved in the pathogenesis of nigral degeneration in PD. In this study, we examine the protein-protein interactions of wild-type and mutant (A53T) a-synuclein with adult human brain cDNA expression library using the yeast two-hybrid technique. We found that both normal and mutant alpha-synuclein specifically interact with the mitochondrial complex IV enzyme, cytochrome C oxidase (COX). Wild-type and mutant alpha-synuclein genes were further fused with c-Myc tag and translated in rabbit reticulocyte lysate. Using anti-c-Myc antibody, we demonstrated that both wild-type and mutant alpha-synuclein, coimmunoprecipitated with COX. We also showed that potassium cyanide, a selective COX inhibitor, synergistically enhanced the sensitivity of SH-SY5Y neuroblastoma cells to dopamine-induced cell death. In conclusion, we found specific protein-protein interactions of alpha-synuclein, a major LB protein, to COX, a key enzyme of the mithochondrial respiratory system. This interaction suggests that alpha-synuclein aggregation may contribute to enhance the mitochondrial dysfunction, which might be a key factor in the pathogenesis of PD.     

VPS41, a protein involved in lysosomal trafficking,is protective in Caenorhabditis elegans and mammalian cellular models of Parkinson's disease

VPS41 is a protein identified as a potential therapeutic target for Parkinson's disease (PD) as a result of a high-throughput RNAi screen in Caenorhabditis elegans. VPS41 has a plausible mechanistic link to the pathogenesis of PD, as in yeast it is known to participate in trafficking of proteins to the lysosomal system and several recent lines of evidence have pointed to the importance of lysosomal system dysfunction in the neurotoxicity of alpha-synuclein (α-syn). We found that expression of the human form of VPS41 (hVPS41) prevents dopamine (DA) neuron loss induced by α-syn overexpression and 6-hydroxydopamine (6-OHDA) neurotoxicity in C. elegans. In SH-SY5Y neuroblastoma cell lines stably transfected with hVPS41, we determined that presence of this protein conferred protection against the neurotoxins 6-OHDA and rotenone. Overexpression of hVPS41 did not alter the mitochondrial membrane depolarization induced by these neurotoxins. hVPS41 did, however, block downstream events in the apoptotic cascade including activation of caspase-9 and caspase-3, and PARP cleavage. We also observed that hVPS41 reduced the accumulation of insoluble high-molecular weight forms of α-syn in SH-SY5Y cells after treatment with rotenone. These data show that hVPS41 is protective against both α-syn and neurotoxic-mediated injury in invertebrate and cellular models of PD. These protective functions may be related to enhanced clearance of misfolded or aggregated protein, including α-syn. Our studies indicate that hVPS41 may be a useful target for developing therapeutic strategies for human PD.     

Aminochrome as a preclinical experimental model to study degeneration of dopaminergic neurons in Parkinson’s disease

Four decades after L-dopa introduction to PD therapy, the cause of Parkinson's disease (PD) remains unknown despite the intensive research and the discovery of a number of gene mutations and deletions in the pathogenesis of familial PD. Different model neurotoxins have been used as preclinical experimental models to study the neurodegenerative process in PD, such as 6-hydroxydopamine (6-OHDA), 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and rotenone. The lack of success in identifying the molecular mechanism for the degenerative process in PD opens the question whether the current preclinical experimental models are suitable to understand the degeneration of neuromelanin-containing dopaminergic neurons in PD. We propose aminochrome as a model neurotoxin to study the neurodegenerative processes occurring in neuromelanin-containing dopaminergic neurons in PD. Aminochrome is an endogenous compound formed during dopamine oxidation and it is the precursor of neuromelanin, a substance whose formation is a normal process in mesencephalic dopaminergic neurons. However, aminochrome itself can induce neurotoxicity under certain aberrant conditions such as (i) one-electron reduction of aminochrome catalyzed by flavoenzymes to leukoaminochrome o-semiquinone radical, which is a highly reactive neurotoxin; or (ii) the formation of aminochrome adducts with alpha-synuclein, enhancing and stabilizing the formation of neurotoxic protofibrils. These two neurotoxic pathways of aminochrome are prevented by DT-diaphorase, an enzyme that effectively reduces aminochrome with two-electrons preventing both aminochrome one-electron reduction or formation alpha synuclein protofibrils. We propose to use aminochrome as a preclinical experimental model to study the neurodegenerative process of neuromelanin containing dopaminergic neurons in PD.     

The A53T alpha-synuclein mutation increases iron-dependent aggregation and toxicity.

Parkinson's disease (PD) is the most common motor disorder affecting the elderly. PD is characterized by the formation of Lewy bodies and death of dopaminergic neurons. The mechanisms underlying PD are unknown, but the discoveries that mutations in alpha-synuclein can cause familial PD and that alpha-synuclein accumulates in Lewy bodies suggest that alpha-synuclein participates in the pathophysiology of PD. Using human BE-M17 neuroblastoma cells overexpressing wild-type, A53T, or A30P alpha-synuclein, we now show that iron and free radical generators, such as dopamine or hydrogen peroxide, stimulate the production of intracellular aggregates that contain alpha-synuclein and ubiquitin. The aggregates can be identified by immunocytochemistry, electron microscopy, or the histochemical stain thioflavine S. The amount of aggregation occurring in the cells is dependent on the amount of alpha-synuclein expressed and the type of alpha-synuclein expressed, with the amount of alpha-synuclein aggregation following a rank order of A53T > A30P > wild-type > untransfected. In addition to stimulating aggregate formation, alpha-synuclein also appears to induce toxicity. BE-M17 neuroblastoma cells overexpressing alpha-synuclein show up to a fourfold increase in vulnerability to toxicity induced by iron. The vulnerability follows the same rank order as for aggregation. These data raise the possibility that alpha-synuclein acts in concert with iron and dopamine to induce formation of Lewy body pathology in PD and cell death in PD.     

Glial cell line derived neurotrophic factor promotes the recovery of dopamine neurons damaged by 6-hydroxydopamine in vitro

Glial cell line derived neurotrophic factor (GDNF) has been shown to be a potent neurotrophic factor for dopamine neurons in culture and to prevent the loss of substantia nigra dopamine neurons following in vivo lesions with 6-hydroxydopamine (6-OHDA). In this study we used mesencephalic cultures containing both neurons and glia to examine whether GDNF protects dopamine neurons from 6-OHDA toxicity in vitro. Our data show that GDNF does not prevent the loss of dopamine neurons caused by treatment with 6-OHDA in vitro. However, continuous exposure to GDNF increases the high affinity dopamine uptake in cultures treated with 6-OHDA, suggesting that it enhances the growth of damaged dopamine neurons. We also show that in vitro treatment with 6-OHDA causes widespread cell death in mesencephalic cultures, which is not restricted to dopamine neurons. The lack of selectivity of 6-OHDA toxicity when applied in vitro may explain the inability of GDNF to prevent the loss of dopamine neurons in mesencephalic cultures. The stimulation of the growth of 6-OHDA damaged dopamine neurons by GDNF, observed in our study, suggests that it may prove beneficial in the treatment of injured dopamine neurons.     

Prevention of 1-methyl-4-phenylpyridinium- and 6-hydroxydopamine-induced nitration of tyrosine hydroxylase and neurotoxicity by EUK-134, a superoxide dismutase and catalase mimetic, in cultured dopaminergic neurons

Oxidative stress has been implicated in the selective degeneration of dopaminergic (DAergic) neurons in Parkinson's disease (PD). In this study, we tested the efficacy of EUK-134, a superoxide dismutase (SOD) and catalase mimetic, on the nitration of tyrosine hydroxylase (TH), a marker of oxidative stress, and neurotoxicity produced by 1-methyl-4-phenylpyridinium (MPP(+)) and 6-hydroxydopamine (6-OHDA) in primary DAergic neuron cultures. Exposure of cultures to 10 microM MPP(+) reduced dopamine (DA) uptake and the number of tyrosine hydroxylase immunoreactive (THir) neurons to 56 and 52% of control, while exposure to 30 microM 6-OHDA reduced DA uptake and the number of THir neurons to 58 and 59% of control, respectively. Pretreatment of cultures with 0.5 microM EUK-134 completely protected DAergic neurons against MPP(+)- and 6-OHDA-induced neurotoxicity. Exposure of primary neuron cultures to either MPP(+) or 6-OHDA produced nitration of tyrosine residues in TH. Pretreatment of cultures with 0.5 microM EUK-134 completely prevented MPP(+)- or 6-OHDA-induced nitration of tyrosine residues in TH. Taken together, these results support the idea that reactive oxygen species (ROS) are critically involved in MPP(+)- and 6-OHDA-induced neurotoxicity and suggest a potential therapeutic role for synthetic catalytic scavengers of ROS, such as EUK-134, in the treatment of PD.     

Strengths and limitations of genetic mouse models of Parkinson's disease

Genetic mouse models based on alpha-synuclein overexpression are particularly compelling because abnormal accumulation of alpha-synuclein occurs in sporadic Parkinson's disease (PD). Our laboratory has characterized a mouse overexpressing wild-type human alpha-synuclein under the Thy1 promoter, which confers broad expression of the transgene in neurons. These mice show progressive sensorimotor anomalies starting at 2 months of age, as well as olfactory and digestive deficits similar to those observed in patients at early stages of PD. Patterns of gene expression examined in nigrostriatal neurons isolated by single-cell laser capture microdissection in these mice at 6 months of age show an upregulation of defence mechanisms including increased levels of genes involved in proteasome and mitochondrial function, as well as cholesterol biosynthesis. At the same time, numerous alterations in genes encoding ion channels suggest that changes in the cellular function of these neurons occur independently of cell death. These data provide information on the early effects--in a mammalian brain--of a mutation known to cause PD, and they identify a number of useful end points for evaluating potential neuroprotective therapies that could interfere with the pathophysiological mechanisms of PD upstream of neuronal cell death.     

Progressive degeneration of dopamine neurons in 6-hydroxydopamine rat model of Parkinson's disease does not involve activation of caspase-9 and caspase-3

6-Hydroxydopamine (6-OHDA), a neurotoxin that causes the death of dopamine (DA) neurons, is commonly used to produce experimental models of Parkinson's disease (PD) in rodents. In the rat model of PD first described by Sauer and Oertel, DA neurons progressively die over several weeks following a striatal injection of 6-OHDA. It is generally assumed that DA neurons die through apoptosis after exposure to 6-OHDA, but data supporting activation of a caspase enzymatic cascade are lacking. In this study, we sought to determine if caspases involved in the intrinsic apoptotic cascade play a role in the initial stages of 6-OHDA-induced death of DA neurons in the progressively lesioned rat model of PD. We found that injection of 6-OHDA into adult rat striatum did not activate caspase-9 or caspase-3 or increase levels of caspase-dependent cleavage products in the substantia nigra at various survival times up to 7 days after the lesion, even though this paradigm produced DA neuronal loss. These data suggest that in the adult rat brain DA neurons whose terminals are challenged with 6-OHDA do not die through a classical caspase-dependent apoptotic mechanism.     

Wild-type alpha-synuclein interacts with pro-apoptotic proteins PKCdelta and BAD to protect dopaminergic neuronal cells against MPP+-induced apoptotic cell death

Alpha-synuclein is a pre-synaptic protein of unknown function that has been implicated in the pathogenesis of Parkinson's disease (PD). Recently, we demonstrated that 1-methyl-4-phenylpyridinium (MPP+) induces caspase-3-dependent proteolytic activation of PKCdelta, which subsequently contributes to neuronal apoptotic cell death in mesencephalic dopaminergic neuronal cells. In the present study, we examined whether PKCdelta interacts with alpha-synuclein to modulate MPP+-induced dopaminergic degeneration. Over-expression of wild-type human alpha-synuclein in mesencephalic dopaminergic neuronal cells (N27 cells) attenuated MPP+-induced (300 microM) cytotoxicity, release of mitochondrial cytochrome c, and subsequent caspase-3 activation, without affecting reactive oxygen species (ROS) generation. Wild-type alpha-synuclein over-expression also dramatically reduced MPP+-induced caspase-3-mediated proteolytic cleavage of PKCdelta, whereas over-expression of the mutant human alpha-synucleinA53T did not alter the PKCdelta cleavage under similar conditions. Immunoprecipitation-kinase assay revealed reduced PKCdelta kinase activity in wild-type alpha-synuclein over-expressing cells in response to MPP+ treatment. Wild-type alpha-synuclein over-expression also rescued mesencephalic dopaminergic neuronal cells from MPP+-induced apoptotic cell death, while alpha-synucleinA53T exacerbated the MPP+-induced DNA fragmentation. Furthermore, co-immunoprecipitation studies revealed that alpha-synuclein interacts with the pro-apoptotic proteins PKCdelta and BAD, but not with the anti-apoptotic protein Bcl-2 following MPP+ treatment. We also observed that the interaction between PKCdelta and alpha-synuclein does not involve direct phosphorylation. Together, our results demonstrate that wild-type alpha-synuclein interacts with the pro-apoptotic molecules BAD and PKCdelta to protect dopaminergic neuronal cells against neurotoxic insults.     

Glia-dependent neurotoxicity and neuroprotection in mesencephalic cultures

Dopaminergic neurotoxicities of 6-hydroxydopamine (6-OHDA) and the lipopolysaccharide (LPS) were compared in rat mesencephalic cultures plated on poly-l-lysine or on glial monolayers. In the neuron-enriched cultures plated on polylysine, 6-OHDA killed 89% of the tyrosine hydroxylase (TH)-immunopositive neurons, but LPS was not neurotoxic. Conversely, in mixed neuron/glial cultures, 6-OHDA killed only 27% of the TH-immunopositive neurons while LPS killed 70%. The mixed neuronal/glial mesencephalic culture offers a better in vitro model for studying possible mechanisms involved in Parkinson's disease.     

Expression of mutant alpha-synucleins enhances dopamine transporter-mediated MPP+ toxicity in vitro

Mutations in the alpha-synuclein gene (A30P and A53T) are reported to cause familial Parkinson's disease (PD), but it is not known how they result in selective dopaminergic cell death. Here we report on effects of mutant alpha-synucleins on dopamine transporter (DAT)-mediated toxicity of the selective dopaminergic neurotoxin 1-methyl-4-phenylpyridinium ion (MPP+) in vitro. We established human embryonic kidney HEK-293 cell lines stably co-expressing each alpha-synuclein isoform and the human DAT. We demonstrate that expression of all alpha-synuclein isoforms enhances toxicity of general complex I inhibition (rotenone), but only the expression of mutant alpha-synucleins induces significant increased DAT-dependent toxicity of very low concentrations of MPP+ compared to wild-type protein. Proteasomal inhibition by lactacystin does not alter MPP+-toxicity in all cell lines. Our data suggest a new mechanism of MPP+-induced dopaminergic toxicity by an interaction between mutant alpha-synucleins and the DAT, which is independent of the function of the proteasome.     

Lentiviral nigral delivery of GDNF does not prevent neurodegeneration in a genetic rat model of Parkinson's disease

Viral delivery of glial cell line-derived neurotrophic factor (GDNF) currently represents one of the most promising neuroprotective strategies for Parkinson's Disease (PD). However, the effect of this neurotrophic factor has never been tested in the newly available genetic models of PD based on the viral expression of mutated alpha-synuclein. In this study, we evaluated the ability of lentiviral vectors coding for GDNF (lenti-GDNF) to prevent nigral dopaminergic degeneration associated with the lentiviral mediated expression of the A30P mutant human alpha-synuclein (lenti-A30P). This virally based rat model develops a progressive and selective loss of dopamine neurons associated with the appearance of alpha-synuclein containing inclusions, thus recapitulating the major hallmarks of PD. Lenti-GDNF was injected in the substantia nigra 2 weeks before nigral administration of lenti-A30P. Although a robust expression of GDNF was observed in the whole nigrostriatal pathway due to retrograde and/or anterograde transport, lenti-GDNF did not prevent the alpha-synuclein-induced dopaminergic neurodegeneration in the lentiviral-based genetic rat model of PD. These results suggest that sustained GDNF treatment cannot modulate the cellular toxicity related to abnormal folded protein accumulation as mutated human alpha-synuclein.     

Effects of CDNF on 6-OHDA-induced apoptosis in PC12 cells via modulation of Bcl-2/Bax and caspase-3 activation

Progressive dopamine neuron degeneration in the substantia nigra pars compacta is considered the most prominent pathological characteristic of Parkinson’s disease (PD). Currently, there is no cure, but only the capability to relieve the symptoms of PD. The conserved dopamine neurotrophic factor (CDNF) protects and rescues dopamine neurons in vivo. However, the molecular function of CDNF in PD remains unclear. In present study, we investigated the role and intrinsic mechanism of CDNF in preventing and reversing rat pheochromocytoma (PC12) cells from apoptosis induced by 6-hydroxydopamine (6-OHDA). We demonstrate that 6-OHDA induces cell death in PC12 cells, but that CDNF attenuates this effect in a dose-dependent manner. Further study shows that upregulation of the Bcl-2/Bax ratio and downregulation of caspase-3 activity are observed in a dose-dependent manner upon pre-treatment or post-treatment with CDNF, suggesting a pathway of regulation of apoptosis by CDNF. These data demonstrate that CDNF prevents the apoptosis of PC12 cells induced by 6-OHDA by modulating Bcl-2/Bax and caspase-3 activation.     

RNA interference-mediated knockdown of alpha-synuclein protects human dopaminergic neuroblastoma cells from MPP(+) toxicity and reduces dopamine transport

The critical observation in the pathology of Parkinson's disease (PD) is that neurodegeneration is largely restricted to dopaminergic neurons that develop cytoplasmic inclusions called Lewy bodies. These aggregations contain the protein alpha-synuclein. Furthermore, it is becoming apparent that alpha-synuclein expression levels are a major factor in PD pathogenesis. Patients with additional copies of the alpha-synuclein gene develop PD with a severity proportional to levels of alpha-synuclein overexpression. Similarly, overexpression of alpha-synuclein in in vitro and in vivo models has been shown to be toxic. However, little is known about the effects of reducing alpha-synuclein expression in human neurons. To investigate this, we have developed a system in which levels of alpha-synuclein can be acutely suppressed by using RNA interference (RNAi) in a physiologically relevant human dopaminergic cellular model. By using small interfering RNA (siRNA) molecules targeted to endogenous alpha-synuclein, we achieved 80% protein knockdown. We show that alpha-synuclein knockdown has no effect on cellular survival either under normal growth conditions over 5 days or in the presence of the mitochondrial inhibitor rotenone. Knockdown does, however, confer resistance to the dopamine transporter (DAT)-dependent neurotoxin N-methyl-4-phenylpyridinium (MPP(+)). We then demonstrate for the first time that alpha-synuclein suppression decreases dopamine transport in human cells, reducing the maximal uptake velocity (V(max)) of dopamine and the surface density of its transporter by up to 50%. These results show that RNAi-mediated alpha-synuclein knockdown alters cellular dopamine homeostasis in human cells and may suggest a mechanism for the increased survival in the presence of MPP(+), a toxin used extensively to model Parkinson's disease.