alpha-synuclein promotes mitochondrial deficit and oxidative stress

Does cellular iron dysregulation play a causative role in Parkinson's disease?

Selective dopaminergic cell loss in Parkinson's disease is correlated with increased levels of cellular iron. It is still hotly debated as to whether the increase in iron is an upstream event which acts to promote neurodegeneration via formation of oxidative stress or whether iron accumulates as a by-product of the neuronal cell loss. Here we review evidence for loss of iron homeostasis as a causative factor in disease-associated neurodegeneration and the primary players which may be involved. A series of recent studies suggest that iron regulatory proteins (IRPs) coordinate both cellular iron levels and energy metabolism, both of which are disrupted in Parkinson's disease (PD) and may in turn contribute to increased levels of oxidative stress associated with the disease. Iron has also been recently been implicated in promotion of alpha-synuclein aggregation either directly or via increasing levels of oxidative stress suggesting an important role for it in Lewy body formation, another important hallmark of the disease.

Alpha-synuclein, Parkinson's disease, and Alzheimer's disease

Alpha synuclein (alpha-SN) is a ubiquitous protein that is especially abundant in the brain and has been postulated to play a central role in the pathogenesis of Parkinson's disease (PD), Alzheimer's disease, and other neurodegenerative disorders. Here, we show that alpha-SN plays dual role of neuroprotection and neurotoxicity depending on its concentration or level of expression. In addition, our study shows that alpha-synuclein is differentially expressed in human peripheral blood mononuclear cells. PD patients expressed more alpha-synuclein than healthy controls. Thus, the alpha-synuclein expression in the peripheral immune system might be one of the primary causes of immune abnormalities in PD patients.

1-Benzyl-1,2,3,4-tetrahydroisoquinoline, a Parkinsonism-inducing endogenous toxin, increases alpha-synuclein expression and causes nuclear damage in human dopaminergic cells

1-Benzyl-1,2,3,4-tetrahydroisoquinoline (1BnTIQ), an endogenous neurotoxin, is known to cause parkinsonism in rodents and nonhuman primates. The levels of 1BnTIQ in cerebrospinal fluid of patients with Parkinson's disease (PD) were reported to be three times higher than those in control subjects. In the present study, we have evaluated the effects of 1BnTIQ on alpha-synuclein (alpha-syn) expression together with biochemical and morphological changes in human dopaminergic SH-SY5Y cells in culture. 1BnTIQ at lower concentrations (1-50 microM) increased alpha-syn protein expression in a time- and dose-dependent manner in these cells. There was also up-regulation of alpha-syn mRNA by 1BnTIQ. Inhibition of complex I by rotenone and depletion of glutathione by L-buthionine sulfoxamine also correlated with an increase in alpha-syn expression, suggesting that oxidative stress may cause an increase in alpha-syn levels in dopaminergic cells. Furthermore, 1BnTIQ significantly depleted glutathione levels. 1BnTIQ at higher concentrations (500 microM) increased reactive oxygen species levels, decreased ATP levels, and caused nuclear damage in the cells. The 1BnTIQ-induced alpha-syn up-regulation was inhibited by cotreatment with the antioxidants selegiline, coenzyme Q(10), and N-acetylcystein and the caspase inhibitor DEVD-CHO. Taken together, these results suggest that alpha-syn up-regulation and oxidative stress are contributing factors in 1BnTIQ-induced neurotoxicity in dopaminergic neurons in PD.

Alpha-synuclein is expressed in different tissues during human fetal development

Alpha-synuclein is a small presynaptic protein associated with both normal synaptic plasticity and neurodegenerative processes. Its normal cellular function, however, remains unknown. Even though it is highly enriched in the brain, its presence was reported in other human adult tissues. In the present study, we examined tissue expression of alpha-synuclein in human and rat prenatal development. Using Western blot analysis, various peripheral tissues from 15 to 23 gestational weeks, human and E19 rat fetuses, along with human and rat adult tissues, were assayed. alpha-Synuclein expression was observed in all fetal human organs examined. In adult human tissues the high expression of alpha-synuclein was maintained in the brain, whereas in other organs the expression was greatly reduced. In contrast, both in fetal and adult rat tissues, alpha-synuclein was only detected in the brain. In addition to a 19-kDa alpha-synuclein band, 36- and 52-kDa immunoreactive bands were observed in all fetal and adult human organs, with the exception of the brain, but their identity remains to be determined. These findings suggest that apart from its function in development of the nervous system, alpha-synuclein has an important function in peripheral tissues as well during normal human prenatal development.

Degradative organelles containing mislocalized alpha-and beta-synuclein proliferate in presenilin-1 null neurons

Presenilin-1 null mutation (PS1 -/-) in mice is associated with morphological alterations and defects in cleavage of transmembrane proteins. Here, we demonstrate that PS1 deficiency also leads to the formation of degradative vacuoles and to the aberrant translocation of presynaptic alpha- and beta-synuclein proteins to these organelles in the perikarya of primary neurons, concomitant with significant increases in the levels of both synucleins. Stimulation of autophagy in control neurons produced a similar mislocalization of synucleins as genetic ablation of PS1. These effects were not the result of the loss of PS1 gamma-secretase activity; however, dysregulation of calcium channels in PS1 -/- cells may be involved. Finally, colocalization of alpha-synuclein and degradative organelles was observed in brains from patients with the Lewy body variant of AD. Thus, aberrant accumulation of alpha- and beta-synuclein in degradative organelles are novel features of PS1 -/- neurons, and similar events may promote the formation of alpha-synuclein inclusions associated with neurodegenerative diseases.

Drosophila parkinmutants have decreased mass and cell size and increased sensitivity to oxygen radical stress

Mutations in the gene parkin in humans (PARK2) are responsible for a large number of familial cases of autosomal-recessive Parkinson disease. We have isolated a Drosophila homolog of human PARK2 and characterized its expression and null phenotype. parkin null flies have 30% lower mass than wild-type controls which is in part accounted for by a reduced cell size and number. In addition, these flies are infertile, show significantly reduced longevity, and are unable to jump or fly. Rearing mutants on paraquat, which generates toxic free radicals in vivo, causes a further reduction in longevity. Furthermore, loss of parkin results in progressive degeneration of most indirect flight muscle (IFM) groups soon after eclosion, accompanied by apoptosis. However, parkin mutants have normal neuromuscular junction recordings during the third larval instar stage, suggesting that larval musculature is intact and that parkinis required only in pupal and adult muscle. parkin flies do not show an age-dependent dopaminergic neuron loss in the brain, even after aging adults for 3 weeks. Nevertheless, degeneration of IFMs demonstrates the importance of parkin in maintaining specific cell groups, perhaps those with a high-energy demand and the concomitant production of high levels of free radicals. parkin mutants will be a valuable model for future analysis of the mechanisms of cell and tissue degeneration.

Enhanced substantia nigra mitochondrial pathology in human α-synuclein transgenic mice after treatment with MPTP111-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine

Recent studies have implicated alpha-synuclein (alpha-S) in the pathogenesis of Parkinson's disease (PD). The mechanisms underlying PD are not completely understood; however, mitochondrial complex I inhibition and oxidative injury may be involved. Because the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is a potent complex I inhibitor that can cause oxidative injury and mimic many aspects of PD in treated animals, we sought to determine whether the overexpression of alpha-S in transgenic (tg) mice (alpha-S-tg) would enhance the substantia nigra (SN) pathology resulting from treatment with MPTP. For this purpose, alpha-S-tg mice were produced expressing high levels of wild-type (wt) human alpha-S under the control of the neuron-specific Thy-1 promoter. Alpha-S-tg mice and non-tg controls were treated with MPTP (15 mg/kg ip, twice a week for 2 weeks) or saline (Sal) and then examined 2 weeks after completion of treatment by transmission electron microscopy (EM). We found that alpha-S-tg mice treated with MPTP had extensive mitochondrial alterations, increases in mitochondrial size, filamentous neuritic aggregations, axonal degeneration, and formation of electron dense perinuclear cytoplasmic inclusions in the SN that did not occur in the hippocampus or neocortex, nor in MPTP-treated non-tg mice or Sal-treated alpha-S-tg mice. These findings support the potential involvement of alpha-S expression in the vulnerability of SN neurons to toxicity from mitochondrial complex I inhibitors and the subsequent development of neurodegenerative pathology.

The Relevance of Iron in the Pathogenesis of Parkinson's Disease

Investigations that revealed increased levels of iron in postmortem brains from patients with Parkinson's disease (PD) as compared to those from individuals not suffering from neurological disorders are reported. The chemical natures in which iron predominates in the brain and the relevance of neuromelanin for neuronal iron binding are discussed. Major findings have been that iron levels increase with the severity of neuropathological changes in PD, presumably due to increased transport through the blood-brain barrier in late stages of parkinsonism. Glial iron is mainly stored as ferric iron in ferritin, while neuronal iron is predominantly bound to neuromelanin. Iron overload may induce progressive degeneration of nigrostriatal neurons by facilitating the formation of reactive biological intermediates, including reactive oxygen species, and the formation of cytotoxic protein aggregates. There are indications that iron-mediated neuronal death in PD proceeds retrogradely. These results are also discussed with respect to their relevance for disease progression in relation to cytotoxic alpha-synuclein protofibril formation.

Tyrosine-to-Cysteine Modification of Human α-Synuclein Enhances Protein Aggregation and Cellular Toxicity

The deposition of alpha-synuclein and other cellular proteins in Lewy bodies in midbrain dopamine neurons is a pathological hallmark of Parkinson's disease. Nitrative and oxidative stress can induce alpha-synuclein protein aggregation, possibly initiated by the formation of stable cross-linking dimers. To determine whether enhanced dimer formation can accelerate protein aggregation and increase cellular toxicity, we have substituted cysteine for tyrosine at positions 39, 125, 133, and 136 in human wild-type (WT) alpha-synuclein, and in A53T and A30P mutant alpha-synuclein. To reduce the likelihood of cross-linking, phenylalanine was substituted for tyrosine at the same sites. We have found that overexpression of Y39C or Y125C mutant proteins leads to increased intracellular inclusions and apoptosis in a rat dopaminergic cell line (N27 cells) and in human embryonic kidney 293 cells. Expression of Y133C, Y136C, and all four Tyr-to-Phe mutations were not more cytotoxic than WT control. Exposure to oxidative stress increased Y39C and Y125C alpha-synuclein aggregation and toxicity. Dimers and oligomers were found in Triton X-100-soluble fractions from adenovirus-mediated overexpression of Y39C and Y125C in N27 cells. In contrast, WT beta-synuclein and all four Tyr-to-Cys mutant beta-synucleins did not cause protein aggregation and cell death. We conclude that cysteine substitution at critical positions in the alpha-synuclein molecule can increase dimer formation and accelerate protein aggregation and cellular toxicity of alpha-synuclein.

Mitochondrial dysfunction and oxidative damage in parkin-deficient mice

Loss-of-function mutations in parkin are the predominant cause of familial Parkinson's disease. We previously reported that parkin-/- mice exhibit nigrostriatal deficits in the absence of nigral degeneration. Parkin has been shown to function as an E3 ubiquitin ligase. Loss of parkin function, therefore, has been hypothesized to cause nigral degeneration via an aberrant accumulation of its substrates. Here we employed a proteomic approach to determine whether loss of parkin function results in alterations in abundance and/or modification of proteins in the ventral midbrain of parkin-/- mice. Two-dimensional gel electrophoresis followed by mass spectrometry revealed decreased abundance of a number of proteins involved in mitochondrial function or oxidative stress. Consistent with reductions in several subunits of complexes I and IV, functional assays showed reductions in respiratory capacity of striatal mitochondria isolated from parkin-/- mice. Electron microscopic analysis revealed no gross morphological abnormalities in striatal mitochondria of parkin-/- mice. In addition, parkin-/- mice showed a delayed rate of weight gain, suggesting broader metabolic abnormalities. Accompanying these deficits in mitochondrial function, parkin-/- mice also exhibited decreased levels of proteins involved in protection from oxidative stress. Consistent with these findings, parkin-/- mice showed decreased serum antioxidant capacity and increased protein and lipid peroxidation. The combination of proteomic, genetic, and physiological analyses reveal an essential role for parkin in the regulation of mitochondrial function and provide the first direct evidence of mitochondrial dysfunction and oxidative damage in the absence of nigral degeneration in a genetic mouse model of Parkinson's disease.

Cycles of aberrant synaptic sprouting and neurodegeneration in Alzheimer's and dementia with Lewy bodies

Alzheimer's disease (AD) and dementia with Lewy bodies (DLB) are the most common neurodegenerative disorders affecting the elderly. The cognitive and motor deficits in these diseases are associated with the disruption of neuritic substructure, loss of synaptic contacts in selectively vulnerable circuitries, and aberrant sprouting. Where as in AD, accumulation of misfolded forms of Abeta triggers neurodegeneration, in DLB accumulation of alpha-synuclein might play a central role. The mechanisms by which oligomeric forms of these proteins might lead to cycles of synapse loss and aberrant sprouting are currently under investigation. Several possibilities are being considered, including mitochondrial damage, caspase activation, lysosomal leakage, fragmentation of the Golgi apparatus, interference with synaptic vesicle transport and function, and interference with gene transcription and signaling. Among them, recent lines of research support the possibility that alterations in signaling pathways such extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK) and p38 relevant to synaptic plasticity and cell survival might play a pivotal role. A wide range of cellular functions are affected by the accumulation of misfolded Abeta and alpha-synuclein; thus it is possible that a more fundamental cellular alteration may underlie the mechanisms of synaptic pathology in these disorders. Among them, one possibility is that scaffold proteins, such as caveolin and JNK-interacting protein (JIP), which are necessary to integrate signaling pathways, are affected, leading to cycles of synapse loss and aberrant sprouting. This is significant because both caveolar dysfunction and altered axonal plasticity might be universally important in the pathogenesis of various neurodegenerative disorders, and therefore these signaling pathways might be common therapeutic targets for these devastating diseases.

Mechanisms of caspase-independent neuronal death: energy depletion and free radical generation

Cultured rat embryonic cortical neurons undergo apoptosis when treated with the topoisomerase-I inhibitor camptothecin. Pharmacological or molecular caspase inhibition prevents apoptosis, but the neurons still die in a delayed nonapoptotic manner. Here we examine the mechanisms leading to such caspase-independent death, focusing on events related to mitochondrial malfunction, which accompanies this delayed death. Given that mitochondria are the major source of ATP in primary neurons, we examined the cellular energy state. Mitochondrially generated ATP was specifically reduced in neurons treated with camptothecin and Boc-aspartyl-fluoromethylketone. Augmentation of cellular ATP by manipulation of the glucose content in the cultures led to an increase in survival specifically in delayed caspase-independent but not early caspase-dependent death. As another possible consequence of mitochondrial malfunction, we found an induction of reactive oxygen species in delayed death. The free radical scavenger Tempol, but not other classes of antioxidants, reduced oxidative stress and promoted survival. Other potential events known to be a direct or indirect consequence of mitochondrial dysfunction, such as the induction of autophagy, release of apoptosis-inducing factor, or opening of the mitochondrial permeability transition pore, were not found to play a significant role in caspase-independent neuronal death. Combining the strategies of increasing intracellular ATP and reducing free radicals led to an additive increase in neuronal survival. We conclude that energy failure and free radical generation contribute to caspase-independent neuronal death. Both could represent potential targets for therapeutic interventions complementary to caspase inhibition.

Mechanism of toxicity in rotenone models of Parkinson's disease

Exposure of rats to the pesticide and complex I inhibitor rotenone reproduces features of Parkinson's disease, including selective nigrostriatal dopaminergic degeneration and alpha-synuclein-positive cytoplasmic inclusions (Betarbet et al., 2000; Sherer et al., 2003). Here, we examined mechanisms of rotenone toxicity using three model systems. In SK-N-MC human neuroblastoma cells, rotenone (10 nm to 1 microm) caused dose-dependent ATP depletion, oxidative damage, and death. To determine the molecular site of action of rotenone, cells were transfected with the rotenone-insensitive single-subunit NADH dehydrogenase of Saccharomyces cerevisiae (NDI1), which incorporates into the mammalian ETC and acts as a "replacement" for endogenous complex I. In response to rotenone, NDI1-transfected cells did not show mitochondrial impairment, oxidative damage, or death, demonstrating that these effects of rotenone were caused by specific interactions at complex I. Although rotenone caused modest ATP depletion, equivalent ATP loss induced by 2-deoxyglucose was without toxicity, arguing that bioenergetic defects were not responsible for cell death. In contrast, reducing oxidative damage with antioxidants, or by NDI1 transfection, blocked cell death. To determine the relevance of rotenone-induced oxidative damage to dopaminergic neuronal death, we used a chronic midbrain slice culture model. In this system, rotenone caused oxidative damage and dopaminergic neuronal loss, effects blocked by alpha-tocopherol. Finally, brains from rotenone-treated animals demonstrated oxidative damage, most notably in midbrain and olfactory bulb, dopaminergic regions affected by Parkinson's disease. These results, using three models of increasing complexity, demonstrate the involvement of oxidative damage in rotenone toxicity and support the evaluation of antioxidant therapies for Parkinson's disease.

Mechanisms of caspase-independent neuronal death: energy depletion and free radical generation

Cultured rat embryonic cortical neurons undergo apoptosis when treated with the topoisomerase-I inhibitor camptothecin. Pharmacological or molecular caspase inhibition prevents apoptosis, but the neurons still die in a delayed nonapoptotic manner. Here we examine the mechanisms leading to such caspase-independent death, focusing on events related to mitochondrial malfunction, which accompanies this delayed death. Given that mitochondria are the major source of ATP in primary neurons, we examined the cellular energy state. Mitochondrially generated ATP was specifically reduced in neurons treated with camptothecin and Boc-aspartyl-fluoromethylketone. Augmentation of cellular ATP by manipulation of the glucose content in the cultures led to an increase in survival specifically in delayed caspase-independent but not early caspase-dependent death. As another possible consequence of mitochondrial malfunction, we found an induction of reactive oxygen species in delayed death. The free radical scavenger Tempol, but not other classes of antioxidants, reduced oxidative stress and promoted survival. Other potential events known to be a direct or indirect consequence of mitochondrial dysfunction, such as the induction of autophagy, release of apoptosis-inducing factor, or opening of the mitochondrial permeability transition pore, were not found to play a significant role in caspase-independent neuronal death. Combining the strategies of increasing intracellular ATP and reducing free radicals led to an additive increase in neuronal survival. We conclude that energy failure and free radical generation contribute to caspase-independent neuronal death. Both could represent potential targets for therapeutic interventions complementary to caspase inhibition.

Mechanism of toxicity in rotenone models of Parkinson's disease

Exposure of rats to the pesticide and complex I inhibitor rotenone reproduces features of Parkinson's disease, including selective nigrostriatal dopaminergic degeneration and alpha-synuclein-positive cytoplasmic inclusions (Betarbet et al., 2000; Sherer et al., 2003). Here, we examined mechanisms of rotenone toxicity using three model systems. In SK-N-MC human neuroblastoma cells, rotenone (10 nm to 1 microm) caused dose-dependent ATP depletion, oxidative damage, and death. To determine the molecular site of action of rotenone, cells were transfected with the rotenone-insensitive single-subunit NADH dehydrogenase of Saccharomyces cerevisiae (NDI1), which incorporates into the mammalian ETC and acts as a "replacement" for endogenous complex I. In response to rotenone, NDI1-transfected cells did not show mitochondrial impairment, oxidative damage, or death, demonstrating that these effects of rotenone were caused by specific interactions at complex I. Although rotenone caused modest ATP depletion, equivalent ATP loss induced by 2-deoxyglucose was without toxicity, arguing that bioenergetic defects were not responsible for cell death. In contrast, reducing oxidative damage with antioxidants, or by NDI1 transfection, blocked cell death. To determine the relevance of rotenone-induced oxidative damage to dopaminergic neuronal death, we used a chronic midbrain slice culture model. In this system, rotenone caused oxidative damage and dopaminergic neuronal loss, effects blocked by alpha-tocopherol. Finally, brains from rotenone-treated animals demonstrated oxidative damage, most notably in midbrain and olfactory bulb, dopaminergic regions affected by Parkinson's disease. These results, using three models of increasing complexity, demonstrate the involvement of oxidative damage in rotenone toxicity and support the evaluation of antioxidant therapies for Parkinson's disease.

Fe(II)-induced DNA damage in alpha-synuclein-transfected human dopaminergic BE(2)-M17 neuroblastoma cells: detection by the Comet assay

Lewy bodies in the brains of patients with Parkinson's disease (PD) contain aggregates of alpha-synuclein (alpha-syn). Missense mutations (A53T or A30P) in the gene encoding alpha-syn are responsible for rare, inherited forms of PD. In this study, we explored the susceptibility of untransfected human dopaminergic BE(2)-M17 neuroblastoma cells, cells transfected with vector only, or cells transfected with wild-type alpha-syn, A30P alpha-syn or A53T alpha-syn to Fe(II)-induced DNA damage in the form of single-strand breaks (SSBs). DNA SSBs were detected following 2-h treatments with various concentrations of Fe(II) (0.01-100.0 microm), using the alkaline single cell-gel electrophoresis ('Comet') assay and quantified by measuring comet tail length (CTL) microm). Fe(II) treatment induced significant increases in CTL in cells transfected with A30P alpha-syn or A53T alpha-syn, even at the lowest concentrations of Fe(II) tested. In comparison, untransfected cells, vector control cells or cells transfected with wild-type alpha-syn exhibited increases in SSBs only when exposed to concentrations of 1.0 microm Fe(II) and above. Even when exposed to higher concentrations (10.0-100.0 microm) of Fe(II), untransfected cells, vector control cells or cells transfected with wild-type alpha-syn were less susceptible to DNA-damage induction than cells transfected with A30P alpha-syn or A53T alpha-syn. Incorporation of DNA-repair inhibitors, hydroxyurea and cytosine arabinoside, enhanced the sensitivity of DNA damage detection. Susceptibility to Fe(II)-induced DNA damage appeared to be dependent on alpha-syn status because cells transfected with wild-type alpha-syn or A53T alpha-syn were equally susceptible to the damaging effects of the mitochondrial respiratory chain inhibitor rotenone. Overall, our data are suggestive of an enhanced susceptibility to the toxic effects of Fe(II) in neuroblastoma cells transfected with mutant alpha-syn associated with inherited forms of PD.

Alpha-synuclein expression in HEK293 cells enhances the mitochondrial sensitivity to rotenone

Mitochondrial dysfunction has been implicated in the aetiology of sporadic Parkinson's disease but its role in the disease mechanism is not clear. We have investigated the short term effect of G209A mutant or wild-type alpha-synuclein expression upon mitochondrial function using stable inducible cell models. Mitochondrial respiratory chain activities and membrane potential were normal suggesting that increased wild-type or mutant alpha-synuclein expression did not directly affect these parameters. However, both wild-type and mutant G209A alpha-synuclein expression enhanced the fall in mitochondrial membrane potential induced by the complex I inhibitor rotenone. This suggests an indirect interaction between alpha-synuclein expression and mitochondrial function which could render the mitochondria more vulnerable to inhibition by potential endogenous or exogenous factors found in dopaminergic neurones.

General aspects of neurodegeneration

Neurodegenerative diseases are morphologically featured by progressive cell loss in specific vulnerable neuronal populations of the central nervous system, often associated with cytoskeletal protein aggregates forming intracytoplasmic and/or intranuclear inclusions in neurons and/or glial cells. Most neurodegenerative disorders are now classified either according to the hitherto known genetic mechanisms or to the major components of their cellular protein inclusions. The major basic processes inducing neurodegeneration are considered multifactorial ones caused by genetic, environmental, and endogenous factors. They include abnormal protein dynamics with defective protein degradation and aggregation, many of them related to the ubiquitin-proteasomal system, oxidative stress and free radical formation, impaired bioenergetics and mitochondrial dysfunctions, and "neuroinflammatory" processes. These mechanisms that are usually interrelated in complex vitious circles finally leading to programmed cell death cascades are briefly discussed with reference to their pathogenetic role in many, albeit diverse neurodegenerative diseases, like Alzheimer disease, synucleinopathies, tauopathies, and polyglutamine disorders. The impact of protein inclusions on cell dysfunction, activation or prevention of cell death cascades are discussed, but the molecular basis for the underlying disease mechanisms remains to be elucidated.

Gene expression changes presage neurodegeneration in a Drosophila model of Parkinson's disease

Transgenic Drosophila expressing human alpha-synuclein faithfully replicate essential features of human Parkinson's disease, including age-dependent loss of dopaminergic neurons, Lewy-body-like inclusions and locomotor impairment. To define the transcriptional program encoding molecular machinery involved in alpha-synuclein pathology, we characterized expression of the entire Drosophila genome at pre-symptomatic, early and advanced disease stages. Fifty-one signature transcripts, including lipid, energy and membrane transport mRNAs, were tightly associated with alpha-synuclein expression. Most importantly, at the pre-symptomatic stage, when the potential for neuroprotection is greatest, expression changes revealed specific pathology. In age-matched tau transgenic Drosophila, the transcription of alpha-synuclein associated genes was normal, suggesting highly distinct pathways of neurodegeneration. Temporal profiling of progressive gene expression changes in neurodegenerative disease models provides unbiased starting points for defining disease mechanisms and for identifying potential targets for neuroprotective drugs at pre-clinical stages.

Tumor necrosis factor-alpha-induced cell death in U373 cells overexpressing alpha-synuclein

Intracellular alpha-synuclein inclusion formation in glial cells is frequently seen in Parkinson's disease and multiple system atrophy. Microglial activation in these neurodegenerative disorders suggests that neuroinflammatory responses might interact with alpha-synuclein and contribute to the pathogenesis of these disorders. To study the role of tumor necrosis factor-alpha (TNF-alpha), an important proinflammatory cytokine produced by microglia, on cells overexpressing alpha-synuclein we have used the astrocytoma cell line U373 engineered to express C-terminally truncated alpha-synuclein as a fusion protein with red or green fluorescent proteins. We demonstrate that alpha-synuclein overexpression augmented TNF-alpha-induced apoptotic cell death in U373 cells by induction of caspase activation. Furthermore, TNF-alpha exposure was associated with significant cytoskeletal changes characterized by altered inclusion composition with loss of cytoskeletal proteins and elevation of high-molecular-weight alpha-synuclein species. We conclude that alpha-synuclein overexpression significantly increases the vulnerability of U373 cells to apoptosis through TNF-alpha-mediated pathways.

Models of Parkinson's disease

Parkinson's disease (PD) is a heterogenous disease likely to be caused by more than one specific aetiological factor. In rare familial cases of PD with similar clinical features to the idiopathic form of the disease, the underlying genetic cause has been identified. These PD-associated genes have been manipulated to create animal and cell culture models of the disease that have helped to further our understanding of the pathogenesis of PD, particularly concerning causes of the selective loss of dopaminergic neurons at the molecular level. In addition, these models will aid the future development of rational therapeutic strategies. This study briefly reviews toxin-induced models and the genetics of PD. It focuses on recently developed animal models of PD, as well as in vitro approaches to model the disease.

Alpha-synuclein protects naive but not dbcAMP-treated dopaminergic cell types from 1-methyl-4-phenylpyridinium toxicity

The pre-synaptic protein, alpha-synuclein, has been associated with the pathogenesis of Parkinson's disease. The present study indicates that alpha-synuclein, but not its mutants (A53T, A30P), can protect CNS dopaminergic cells from the parkinsonism-inducing drug 1-methyl-4-phenylpyridinium (MPP+), whereas it cannot protect from the dopaminergic toxin, 6-hydroxydopamine, hydrogen-peroxide, or the beta-amyloid peptide, A-beta. Protection from MPP+ was directly correlated with the preservation of mitochondrial function. Specifically, alpha-synuclein rescued cells from MPP+ mediated decreases in mitochondrial dehydrogenase activity and loss of ATP levels by utilizing ketosis. It also prevented toxin-induced activation of the creatine kinase/creatine phosphate system. Similarly, alpha-synuclein protected cells from the complex I inhibitor rotenone and 3-nitroproprionic acid, a complex II inhibitor. Wild-type alpha-synuclein-mediated neuroprotection and subsequent alterations in energy were not found in dbcAMP-differentiated cells. These results suggest that the normal physiological role for alpha-synuclein may change during development.

Dopaminergic neuronal loss and motor deficits in Caenorhabditis elegans overexpressing human alpha-synuclein

Overexpression of human alpha-synuclein in model systems, including cultured neurons, drosophila and mice, leads to biochemical and pathological changes that mimic synucleopathies including Parkinson's disease. We have overexpressed both wild-type (WT) and mutant alanine53-->threonine (A53T) human alpha-synuclein by transgenic injection into Caenorhabditis elegans. Motor deficits were observed when either WT or A53T alpha-synuclein was overexpressed with a pan-neuronal or motor neuron promoter. Neuronal and dendritic loss were accelerated in all three sets of C. elegans dopaminergic neurons when human alpha-synuclein was overexpressed under the control of a dopaminergic neuron or pan-neuronal promoter, but not with a motor neuron promoter. There were no significant differences in neuronal loss between overexpressed WT and A53T forms or between worms of different ages (4 days, 10 days or 2 weeks). These results demonstrate neuronal and behavioral perturbations elicited by human alpha-synuclein in C. elegans that are dependent upon expression in specific neuron subtypes. This transgenic model in C. elegans, an invertebrate organism with excellent experimental resources for further genetic manipulation, may help facilitate dissection of pathophysiologic mechanisms of various synucleopathies.

alpha-synuclein aggregation: a link between mitochondrial defects and Parkinson's disease?

Protein aggregation is a shared feature of many human neurodegenerative diseases and appears to be an inevitable consequence of excessive accumulation of misfolded proteins. Recent studies suggest that accumulation of fibrillar alpha-synuclein aggregates is associated with Parkinson's disease and other Lewy body diseases. Furthermore, the missense mutations in alpha-synuclein that are responsible for some early-onset familial types of the disease promote the aggregation process of this protein. Therefore, the mechanism underlying the cellular alpha-synuclein aggregation is of great importance in understanding the pathogenic process of these diseases. This review summarizes recent advances in our understanding of the mechanisms underlying alpha-synuclein aggregation and how the mitochondrial dysfunction plays a role in this process. Protein misfolding and aggregation in vivo can be suppressed and promoted by several factors, such as molecular chaperones, protein degradation systems, and free radicals. Many of these factors are under the control of normal mitochondrial function, prompting the speculation that mitochondrial dysfunction might cause the accumulation of protein aggregates. Recent studies indeed show that mitochondrial defects can lead to the aggregation of alpha-synuclein. In addition, potentially toxic effects of alpha-synuclein have been linked to the aggregated forms rather than the monomers, both in vitro and in cultured cells. Therefore, it is postulated that aggregation of alpha-synuclein might be one of many possible links that connect mitochondrial dysfunction to neurodegeneration.

Alpha-synuclein up-regulates expression of caveolin-1 and down-regulates extracellular signal-regulated kinase activity in B103 neuroblastoma cells: role in the pathogenesis of Parkinson's disease

alpha-Synuclein accumulation plays an important role in the pathogenesis of Lewy body disease (LBD) and Parkinson's disease (PD). Although the mechanisms are not yet clear, it is possible that dysregulation of the extracellular signal-regulated kinase (ERK) might play a role. As caveolins form scaffolds onto which signaling molecules such as ERK can assemble, we propose that signaling alterations associated with alpha-synuclein accumulation and neurodegeneration, might be mediated via caveolae. Therefore, the objective of the present study was to investigate the potential contribution of alterations in the caveolar system in mediating alpha-synuclein effects on the ERK signaling pathway. For this, synuclein-transfected B103 neuroblastoma cells were used as a model system. In this cell line, caveolin-1 expression was up-regulated, whereas, ERK was down-regulated. ERK was weakly but consistently co-immunoprecipitated with alpha-synuclein but caveolin-1 did not co-immunoprecipitate with alpha-synuclein. Moreover, treatment of alpha-synuclein- overexpressing cells with caveolin-1 antisense oligonucleotides resulted in stimulation of ERK activity, with amelioration of the neuritic alterations. Transduction of alpha-synuclein-overexpressing cells, with an adenoviral vector directing the expression of ERK, resulted in suppression of caveolin-1 expression and re-establishment of the normal patterns of neurite outgrowth. These results suggest that alpha-synuclein may also interfere with ERK signaling by dysregulating caveolin-1 expression. Thus, the caveolin-1/ERK pathway could be a therapeutic target for the alpha-synuclein-related neurodegenerative disorders.

Lewy body pathology is a frequent co-pathology in familial Alzheimer's disease

Our institution is currently engaged in ongoing genetic studies of familial Alzheimer's disease (AD), which include clinical ascertainment and brain autopsy of both affected and non-affected family members. Here we describe the analysis of 22 AD families, each with at least one family member with a postmortem diagnosis of dementia with Lewy bodies (DLB). For this study, 47 brains were examined according to NINCDS-Reagan Institute criteria for the diagnosis of AD. Lewy body pathology was evaluated with alpha-synuclein immunohistochemistry. Four families, with either one or two autopsies showing Lewy body pathology, demonstrated linkage to 12p. Five families had two or more autopsies with Lewy body pathology, but their linkage status was unknown. The remaining 13 families had one autopsy demonstrating Lewy bodies. These findings suggest that at least one pathological form of DLB may be familial. In some families, the pathological phenotype is identical in all examined affected family members; but in others, there may be several pathologies that coexist. Careful neuropathological examination of affected family members may prove critical for future genetic analysis of AD and DLB.

Alpha-synuclein: its biological function and role in neurodegenerative diseases

Alpha-synuclein is regarded as a presynaptic protein, which may play an important role in neuronal plasticity. However, the actual physiological function of this protein is not completely clear. Abnormal accumulation of fibrillar alpha-synuclein in Lewy bodies, as well as mutations in the alpha-synuclein gene identified in the familial forms of Parkinson's disease, point to a central role of this protein in the pathophysiology of Lewy body-related disorders. In vivo and in vitro studies showed that overexpression of alpha-synuclein, its aggregation, and interaction with other proteins are the most critical factors affecting the survival of neurons. In Alzheimer's disease, the amount of alpha-synuclein is found to be elevated at synapses, whereas a peptide derived from alpha-synuclein is thought to represent an intrinsic component of amyloid plaques. It is likely that in this disorder alpha-synuclein plays a dual role by being involved not only in synaptic function but also in amyloid beta-fibrillogenesis.

Subcutaneous rotenone exposure causes highly selective dopaminergic degeneration and alpha-synuclein aggregation

Previous studies demonstrated that chronic systemic exposure to the pesticide and mitochondrial toxin rotenone through jugular vein cannulation reproduced many features of Parkinson's disease (PD) in rats, including nigrostriatal dopaminergic degeneration and formation of alpha-synuclein-positive cytoplasmic inclusions in nigral neurons (R. Betarbet et al., 2000, Nat. Neurosci. 3, 1301-1306). Although novel and conceptually important, the rotenone model of PD suffered from being extremely labor-intensive. The current paper demonstrates that these same features of PD can be reproduced by chronic, systemic exposure to rotenone following implantation of subcutaneous osmotic pumps. Chronic subcutaneous exposure to low doses of rotenone (2.0-3.0 mg/kg/day) caused highly selective nigrostriatal dopaminergic lesions. Striatal neurons containing DARPP-32 (dopamine and cAMP-regulated phosphoprotein) remained intact with normal morphology, and NeuN staining revealed normal neuronal nuclear morphology. Neurons of the globus pallidus and subthalamic nucleus were spared. Subcutaneous rotenone exposure caused alpha-synuclein-positive cytoplasmic aggregates in nigral neurons. This new protocol for chronic rotenone administration is a substantial improvement in terms of simplicity and throughput.

The mouse MPTP model: gene expression changes in dopaminergic neurons

Parkinson's disease (PD) is a common neurodegenerative disorder, characterized by the progressive loss of dopaminergic neurons in the substantia nigra. Although valuable animal models have been developed, our knowledge of the aetiology and pathogenic factors implicated in PD is still insufficient to develop causal therapeutic strategies aimed at halting its progression. The neurotoxicity induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is one of the most valuable models for analysing pathological aspects of PD. In this paper we studied the gene expression patterns underlying the pathogenesis of MPTP-induced neurodegeneration. We treated young and old C57BL/6 mice with different schedules of MPTP to induce degenerative processes that vary in intensity and time-course. During the first week after intoxication we used nonradioactive in situ-hybridization to investigate the expression patterns of genes associated with (i) dopamine metabolism and signalling; (ii) familial forms of PD; (iii) protein folding and (iv) energy metabolism. MPTP injections induced different severities of neuronal injury depending on the age of the animals and the schedule of administration as well as a significant degeneration in the striatum. In situ hybridization showed that MPTP intoxication initiated a number of gene expression changes that (i) were restricted to the neurons of the substantia nigra pars compacta; (ii) were correlated in intensity and number of changes with the age of the animals and the severity of histopathological disturbances; (iii) displayed in each a significant down-regulation by the end of one week after the last MPTP injection, but (iv) varied within one MPTP regimen in expression levels during the observation period. The subacute injection of MPTP into one-year-old mice induced the most severe changes in gene expression. All genes investigated were affected. However, alpha-synuclein was the only gene that was exclusively up-regulated in MPTP-treated animals displaying cell death.

Alpha-synuclein regulates neuronal survival via Bcl-2 family expression and PI3/Akt kinase pathway

Alpha-synuclein (alpha-SN) is a ubiquitous protein that is especially abundant in the brain and has been postulated to play a central role in the pathogenesis of Parkinson's disease, Alzheimer's disease, and other neurodegenerative disorders. However, little is known about the neuronal functions of alpha-SN and the molecular and cellular mechanisms underlying neuronal loss. Here, we show that alpha-SN plays dual roles of neuroprotection and neurotoxicity depending on its concentration or level of expression. At nanomolar concentrations, a-SN protected neurons against serum deprivation, oxidative stress, and excitotoxicity through the PI3/Akt signaling pathway, and its protective effect was increased by Bcl-2 overexpression. Conversely, at both low micromolar and overexpressed levels in the cell, alpha-SN resulted in cytotoxicity. This might be related to decreased Bcl-xL expression and increased bax expression, which is subsequently followed by cytochrome c release and caspase activation and also by microglia-mediated inflammatory responses via the NFkappaB and mitogen-activated protein kinase pathways.

Development of new treatments for Parkinson's disease in transgenic animal models: a role for beta-synuclein

Neuronal death in Parkinson's disease (PD), one of the most common neurodegenerative disorders in the adult and aging population is probably caused by misfolding of synaptic proteins such as alpha-synuclein. Although, some treatments are currently available to control some of the symptoms of PD, none of these approaches directly addresses the mechanisms of disease. With the advent of new experimental animal models for this disorder, the potential for development and discovery of new treatment has been significantly bolstered. Among them, overexpression of alpha-synuclein results in motor deficits. dopaminergic loss and formation of inclusion bodies. Co-expression of mutant amyloid precursor protein, accelerates alpha-synuclein aggregation and enhances the neurodegenerative pathology in these mice, providing a unique model where to investigate the interactions between Abeta1-42 and alpha-synuclein and to develop treatments for combined Alzheimer's disease and PD. Development of anti-parkinsonian treatments based on these models includes: (i) anti-aggregation or pro-degradation compounds, (ii) neuroprotective compounds, and (iii) neurotrophic agents. Among them, we characterized beta-synuclein, the non-amyloidogenic homologue of alpha-synuclein, as an inhibitor of aggregation of alpha-synuclein. Our results raise the intriguing possibility that beta-synuclein might be a natural negative regulator of alpha-synuclein aggregation, and that a similar class of endogenous factors might regulate the aggregation state of other molecules involved in neurodegeneration. Such an anti-amyloidogenic property of beta-synuclein might also provide a novel strategy for the treatment of neurodegenerative disorders.

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.

Gamma-synuclein promotes cancer cell survival and inhibits stress- and chemotherapy drug-induced apoptosis by modulating MAPK pathways

Synucleins are a family of highly conserved small proteins predominantly expressed in neurons. Recently we and others have found that gamma-synuclein is dramatically up-regulated in the vast majority of late-stage breast and ovarian cancers and that gamma-synuclein over-expression can enhance tumorigenicity. In the current study, we have found that gamma-synuclein is associated with two major mitogen-activated kinases (MAPKs), i.e. extracellular signal-regulated protein kinases (ERK1/2) and c-Jun N-terminal kinase 1 (JNK1), and have shown that over-expression of gamma-synuclein leads to constitutive activation of ERK1/2 and down-regulation of JNK1 in response to a host of environmental stress signals, including UV, arsenate, and heat shock. We also tested the effects of gamma-synuclein on apoptosis and activation of JNK and ERK in response to several chemotherapy drugs. We have found that gamma-synuclein-expressing cells are significantly more resistant to the chemotherapeutic drugs paclitaxel and vinblastine as compared with the parental cells. The resistance to paclitaxel can be partially obliterated when ERK activity is inhibited using a MEK1/2 inhibitor. Activation of JNK and its downstream caspase-3 by paclitaxel or vinblastine is significantly down-regulated in gamma-synuclein-expressing cells, indicating that the paclitaxel- or vinblastine-activated apoptosis pathway is blocked by gamma-synuclein. In contrast to paclitaxel and vinblastine, etoposide does not activate JNK, and gamma-synuclein over-expression has no apparent effect on this drug-induced apoptosis. Taken together, our data indicate that oncogenic activation of gamma-synuclein contributes to the development of breast and ovarian cancer by promoting tumor cell survival under adverse conditions and by providing resistance to certain chemotherapeutic drugs.

Fibroblast growth factor 1 regulates signaling via the glycogen synthase kinase-3beta pathway. Implications for neuroprotection

We hypothesize that in neurodegenerative disorders such as Alzheimer's disease and human immunodeficiency virus encephalitis the neuroprotective activity of fibroblast growth factor 1 (FGF1) against several neurotoxic agents might involve regulation of glycogen synthase kinase-3beta (GSK3beta), a pathway important in determining cell fate. In primary rat neuronal and HT22 cells, FGF1 promoted a time-dependent inactivation of GSK3beta by phosphorylation at serine 9. Blocking FGF1 receptors with heparinase reduced this effect. The effects of FGF1 on GSK3beta were dependent on phosphatidylinositol 3-kinase (PI3K)-protein kinase B (Akt) because inhibitors of this pathway or infection with dominant negative Akt adenovirus blocked inactivation. Furthermore, treatment of neuronal cells with FGF1 resulted in ERK-independent Akt phosphorylation and beta-catenin translocation into the nucleus. On the other hand, infection with wild-type GSK3beta recombinant adenovirus-associated virus increased activity of GSK3beta and cell death, both of which were reduced by FGF1 treatment. Moreover, FGF1 protection against glutamate toxicity was dependent on GSK3beta inactivation by the PI3K-Akt but was independent of ERK. Taken together these results suggest that neuroprotective effects of FGF1 might involve inactivation of GSK3beta by a pathway involving activation of the PI3K-Akt cascades.

An in vitro model of Parkinson's disease: linking mitochondrial impairment to altered alpha-synuclein metabolism and oxidative damage

Chronic systemic complex I inhibition caused by rotenone exposure induces features of Parkinson's disease (PD) in rats, including selective nigrostriatal dopaminergic degeneration and formation of ubiquitin- and alpha-synuclein-positive inclusions (Betarbet et al., 2000). To determine underlying mechanisms of rotenone-induced cell death, we developed a chronic in vitro model based on treating human neuroblastoma cells with 5 nm rotenone for 1-4 weeks. For up to 4 weeks, cells grown in the presence of rotenone had normal morphology and growth kinetics, but at this time point, approximately 5% of cells began to undergo apoptosis. Short-term rotenone treatment (1 week) elevated soluble alpha-synuclein protein levels without changing message levels, suggesting that alpha-synuclein degradation was retarded. Chronic rotenone exposure (4 weeks) increased levels of SDS-insoluble alpha-synuclein and ubiquitin. After a latency of >2 weeks, rotenone-treated cells showed evidence of oxidative stress, including loss of glutathione and increased oxidative DNA and protein damage. Chronic rotenone treatment (4 weeks) caused a slight elevation in basal apoptosis and markedly sensitized cells to further oxidative challenge. In response to H2O2, there was cytochrome c release from mitochondria, caspase-3 activation, and apoptosis, all of which occurred earlier and to a much greater extent in rotenone-treated cells; caspase inhibition provided substantial protection. These studies indicate that chronic low-grade complex I inhibition caused by rotenone exposure induces accumulation and aggregation of alpha-synuclein and ubiquitin, progressive oxidative damage, and caspase-dependent death, mechanisms that may be central to PD pathogenesis.

Nonselenium glutathione peroxidase in human brain : elevated levels in Parkinson's disease and dementia with lewy bodies

Nonselenium glutathione peroxidase (NSGP) is a new member of the antioxidant family. Using antibodies to recombinant NSGP we have examined the distribution of this enzyme in normal, Parkinson's disease (PD), and dementia with Lewy body disease (DLB) brains. We have also co-localized this enzyme with alpha-synuclein as a marker for Lewy bodies. In normal brains there was a very low level of NSGP staining in astrocytes. In PD and DLB there were increases in the number and staining intensity of NSGP-positive astrocytes in both gray and white matter. Cell counting of NSGP cells in PD and DLB frontal and cingulated cortices indicated there was 10 to 15 times more positive cells in gray matter and three times more positive cells in white matter than in control cortices. Some neurons were positive for both alpha-synuclein and NSGP in PD and DLB, and double staining indicated that NSGP neurons contained either diffuse cytoplasmic alpha-synuclein deposits or Lewy bodies. In concentric Lewy bodies, alpha-synuclein staining was peripheral whereas NSGP staining was confined to the central core. Immunoprecipitation indicated there was direct interaction between alpha-synuclein and NSGP. These results suggest oxidative stress conditions exist in PD and DLB and that certain cells have responded by up-regulating this novel antioxidant enzyme.

Generation of reactive oxygen species and activation of NF-kappaB by non-Abeta component of Alzheimer's disease amyloid

Non-amyloid beta (Abeta) component of Alzheimer's disease (AD) amyloid (NAC) coexists with Abeta protein in senile plaques. After exposure to NAC fibrils, cortical neurons of rat brain primary culture became apoptotic, while astrocytes were activated with extension of their processes. NAC fibrils decreased the activity of reducing 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) in cortical neurons more markedly (IC(50) = 5.6 microm) than in astrocytes (IC(50) approximately 50 microm). The neuron-specific toxicity of NAC fibrils was indicated also by an increased release of lactate dehydrogenase from the cells. Neuronal apoptosis was suppressed by pre-treatment with the antioxidants, propyl gallate (PG) and N-t-butyl-phenylnitrone (BPN), or overexpression of human Bcl-2. Exposure to NAC fibrils enhanced generation of reactive oxygen species (ROS) in neurons and less efficiently in astrocytes, as demonstrated by oxidation of 2',7'-dichlorofluorescin. The site of ROS generation was shown to be mitochondria by oxidation of chloromethyl-tetramethyl rosamine. Exposure to NAC fibrils increased also the nuclear translocation of nuclear factor kappa B (NF-kappaB) and enhanced its DNA-binding activity, which was inhibited by PG and BPN more efficiently in neurons than in astrocytes. These results suggest that NAC fibrils increase mitochondrial ROS generation and activate NF-kappaB, thereby causing a differential change in gene expression between neurons and astrocytes in the AD brain.

Expression of alpha-synuclein, the precursor of non-amyloid beta component of Alzheimer's disease amyloid, in human cerebral blood vessels

The non-amyloid beta component of Alzheimer's disease amyloid (NAC) is detected in cerebral amyloid angiopathy; and the precursor of NAC is now known to be identical to alpha-synuclein (alpha-S), a major component of Lewy bodies in Parkinson's disease. We studied if cerebral vascular cells express alpha-S. Immunohistochemical studies of human cerebral tissues from control and cerebral amyloid angiopathy patients revealed the expression of alpha-S in vascular endothelial and smooth muscle cells. Then we studied the expression of alpha-S in vitro using cultures of vascular cells. Cultures of human umbilical vein endothelial cells and umbilical artery smooth muscle cells were found to constitutively express alpha-S messenger RNA and protein. alpha-S is normally expressed in vascular cells and may play some physiological role in the vascular wall.

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.

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.

Oxidatively modified proteins in aging and disease

There is a large body of evidence implicating oxidative damage in the pathogenesis of both normal aging and neurodegenerative diseases. Oxidative damage to proteins has been well established. Although there are a large number of potential oxidative modifications only a few have been systematically studied. The most frequently studied marker of oxidative damage to proteins is protein carbonyl groups. 3-Nitrotyrosine is thought to be a relatively specific marker of oxidative damage mediated by peroxynitrite. Increased concentrations of both protein carbonyls and 3-nitrotyrosine have been documented in both normal aging as well as in Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS). These findings help to provide a rationale for trials of antioxidants in neurodegenerative diseases.

Parkinson-like neurodegeneration induced by targeted overexpression of alpha-synuclein in the nigrostriatal system

Recombinant adeno-associated viral vectors display efficient tropism for transduction of the dopamine neurons of the substantia nigra. Taking advantage of this unique property of recombinant adeno-associated viral vectors, we expressed wild-type and A53T mutated human alpha-synuclein in the nigrostriatal dopamine neurons of adult rats for up to 6 months. Cellular and axonal pathology, including alpha-synuclein-positive cytoplasmic inclusions and swollen, dystrophic neurites similar to those seen in brains from patients with Parkinson's disease, developed progressively over time. These pathological alterations occurred preferentially in the nigral dopamine neurons and were not observed in other nondopaminergic neurons transduced by the same vectors. The degenerative changes were accompanied by a loss of 30-80% of the nigral dopamine neurons, a 40-50% reduction of striatal dopamine, and tyrosine hydroxylase levels that was fully developed by 8 weeks. Significant motor impairment developed in those animals in which dopamine neuron cell loss exceeded a critical threshold of 50-60%. At 6 months, signs of cell body and axonal pathology had subsided, suggesting that the surviving neurons had recovered from the initial insult, despite the fact that alpha-synuclein expression was maintained at a high level. These results show that nigral dopamine neurons are selectively vulnerable to high levels of either wild-type or mutant alpha-synuclein, pointing to a key role for alpha-synuclein in the pathogenesis of Parkinson's disease. Targeted overexpression of alpha-synuclein in the nigrostriatal system may provide a new animal model of Parkinson's disease that reproduces some of the cardinal pathological, neurochemical, and behavioral features of the human disease.