Effect of the overexpression of wild-type or mutant alpha-synuclein on cell susceptibility to insult

Oxidative stress in Parkinson's disease

Oxidative stress contributes to the cascade leading to dopamine cell degeneration in Parkinson's disease (PD). However, oxidative stress is intimately linked to other components of the degenerative process, such as mitochondrial dysfunction, excitotoxicity, nitric oxide toxicity and inflammation. It is therefore difficult to determine whether oxidative stress leads to, or is a consequence of, these events. Oxidative damage to lipids, proteins, and DNA occurs in PD, and toxic products of oxidative damage, such as 4-hydroxynonenal (HNE), can react with proteins to impair cell viability. There is convincing evidence for the involvement of nitric oxide that reacts with superoxide to produce peroxynitrite and ultimately hydroxyl radical production. Recently, altered ubiquitination and degradation of proteins have been implicated as key to dopaminergic cell death in PD. Oxidative stress can impair these processes directly, and products of oxidative damage, such as HNE, can damage the 26S proteasome. Furthermore, impairment of proteasomal function leads to free radical generation and oxidative stress. Oxidative stress occurs in idiopathic PD and products of oxidative damage interfere with cellular function, but these form only part of a cascade, and it is not possible to separate them from other events involved in dopaminergic cell death.

Attenuation of dopamine transporter activity by alpha-synuclein

Alpha-synuclein accumulates in Lewy bodies in idiopathic Parkinson's disease. Neither the normal function nor contribution of alpha-synuclein to the pathophysiology of neurodegeneration is known. Here we show that a normal function of alpha-synuclein is the negative modulation of human dopamine transporter (hDAT) activity. In cotransfected Ltk(-) cells, alpha-synuclein attenuated the reuptake of dopamine by hDAT, in a manner dependent on expression levels of alpha-synuclein. Alpha-synuclein-mediated inhibition of hDAT activity was independent of expression vectors, cell types and methods of transfection. The alpha-synuclein-mediated decrease in DAT activity occurred through diminished uptake velocity of dopamine, without changes in the affinity of hDAT for dopamine. Co-immunoprecipitation studies confirmed the formation of a stable complex between alpha-synuclein and DAT, through direct protein:protein interactions. Thus, under normal (non-toxic) expression conditions, alpha-synuclein negatively modulates dopamine uptake by DAT.

Modulation of cell death by alpha-synuclein is stimulus-dependent in mammalian cells

alpha-synuclein is a neuronally-expressed protein which is mutated in familial Parkinson's disease. Previous studies have suggested that over-expression of alpha-synuclein can either enhance, reduce or have no effect on the degree of cell death in response to death-inducing stimuli. We resolve this discrepancy by using a well-characterised cell system to demonstrate that wild type alpha-synuclein can enhance cell death in response to ischaemia/reoxygenation or staurosporine treatment whilst protecting against serum removal and dopamine-induced cell death. In contrast, the two mutant forms of alpha-synuclein uniformly enhance cell death. Hence, the disease-associated mutations appear to convert alpha-synuclein from a protein which modulates cell death differently in different circumstances to forms which have a universal damaging effect.

Regulation of alpha-synuclein by bFGF in cultured ventral midbrain dopaminergic neurons

Alpha-synuclein is a neuronal protein that is implicated in the control of synaptic vesicle function and in Parkinson's disease (PD). Consequently, alterations of alpha-synuclein levels may play a role in neurotransmission and in PD pathogenesis. However, the factors that regulate alpha-synuclein levels are unknown. Growth factors mediate neurotrophic and plasticity effects in CNS neurons, and may play a role in disease states. Here we examine the regulation of alpha-synuclein levels in primary CNS neurons, with particular emphasis on dopaminergic neurons. E18 rat cortical neurons and dopaminergic neurons of E14 rat ventral midbrain showed an induction of alpha-synuclein protein levels with maturation in culture. Application of basic Fibroblast growth factor (bFGF) promoted alpha-synuclein expression selectively within dopaminergic, and not GABAergic or cortical neurons. This induction was blocked by actinomycin D, but not by inhibition of bFGF-induced glial proliferation. alpha-Synuclein levels were not altered by glial-derived neurotrophic factor (GDNF), or by apoptotic stimuli. We conclude that bFGF promotes alpha-synuclein expression in cultured ventral midbrain dopaminergic neurons through a direct transcriptional effect. These results suggest that distinct growth factors may thus mediate plasticity responses or influence disease states in ventral midbrain dopaminergic neurons.

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.

Alpha-synuclein lowers p53-dependent apoptotic response of neuronal cells. Abolishment by 6-hydroxydopamine and implication for Parkinson's disease

We have examined the influence of alpha-synuclein on the responsiveness of TSM1 neuronal cells to apoptotic stimulus. We show that alpha-synuclein drastically lowers basal and staurosporine-stimulated caspase 3 immunoreactivity and activity. This is accompanied by lower DNA fragmentation and reduced number of terminal deoxynucleotide transferase-mediated dUTP nick end labeling (TUNEL)-positive neurons. Interestingly, alpha-synuclein also diminishes both p53 expression and transcriptional activity. We demonstrate that the antiapoptotic phenotype displayed by alpha-synuclein can be fully reversed by the Parkinson's disease-associated dopamine derivative 6-hydroxydopamine. Thus, 6-hydroxydopamine fully abolishes the alpha-synuclein-mediated reduction of caspase 3 activity and reverses the associated decrease of p53 expression. 6-Hydroxydopamine triggers thioflavin T-positive deposits in alpha-synuclein, but not mock-transfected TSM1 neurons, and drastically increases alpha-synuclein immunoreactivity. Altogether, we suggest that alpha-synuclein lowers the p53-dependent caspase 3 activation of TSM1 in response to apoptotic stimuli and we propose that the natural toxin 6-hydroxydopamine abolishes this antiapoptotic phenotype by triggering alpha-synuclein aggregation, thereby likely contributing to Parkinson's disease neuropathology.

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.

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.

Overexpression of Parkinson's disease-associated alpha-synucleinA53T by recombinant adeno-associated virus in mice does not increase the vulnerability of dopaminergic neurons to MPTP

Mutations in the alpha-synuclein gene are linked to a rare dominant form of familial Parkinson's disease, and alpha-synuclein is aggregated in Lewy bodies of both sporadic and dominant Parkinson's disease. It has been proposed that mutated alpha-synuclein causes dopaminergic neuron loss by enhancing the vulnerability of these neurons to a variety of insults, including oxidative stress, apoptotic stimuli, and selective dopaminergic neurotoxins, such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). To test this hypothesis in vivo, we overexpressed human alpha-synuclein(A53T) in the substantia nigra of normal and MPTP-treated mice by rAAV-mediated gene transfer. Determination of dopaminergic neuron survival, striatal tyrosine hydroxylase fiber density, and striatal content of dopamine and its metabolites in rAAV-injected and uninjected hemispheres demonstrated that alpha-synuclein(A53T) does not increase the susceptibility of dopaminergic neurons to MPTP. Our findings argue against a direct detrimental role for (mutant) alpha-synuclein in oxidative stress and/or apoptotic pathways triggered by MPTP, but do not rule out the possibility that alpha-synuclein aggregation in neurons exposed to oxidative stress for long periods of time may be neurotoxic.

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.

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.

MPP+ increases alpha-synuclein expression and ERK/MAP-kinase phosphorylation in human neuroblastoma SH-SY5Y cells

Alpha-synuclein is a brain presynaptic protein that is linked to familiar early onset Parkinson's disease and it is also a major component of Lewy bodies in sporadic Parkinson's disease and other neurodegenerative disorders. Alpha-synuclein expression increases in substantia nigra of both MPTP-treated rodents and non-human primates, used as animal models of parkinsonism. Here we describe an increase in alpha-synuclein expression in a human neuroblastoma cell line, SH-SY5Y, caused by 5-100 microM MPP+, the active metabolite of MPTP, which induces apoptosis in SH-SY5Y cells after a 4-day treatment. We also analysed the activation of the MAPK family, which is involved in several cellular responses to toxins and stressing conditions. Parallel to the increase in alpha-synuclein expression we observed activation of MEK1,2 and ERK/MAPK but not of SAPK/JNK or p38 kinase. The inhibition of the ERK/MAPK pathway with U0126, however, did not affect the increase in alpha-synuclein. The highest increase in alpha-synuclein (more than threefold) in 4-day cultures was found in adherent cells treated with low concentrations of MPP+ (5 microM). Inhibition of ERK/MAPK reduced the damage caused by MPP+. We suggest that alpha-synuclein increase and ERK/MAPK activation have a prominent role in the cell mechanisms of rescue and damage, respectively, after MPP+ -treatment.

Hypothesis: proteasomal dysfunction: a primary event in neurogeneration that leads to nitrative and oxidative stress and subsequent cell death

It is proposed that a primary mechanism leading to neuronal cell death in common neurodegenerative diseases is interference with proteasome function. This can involve genetic defects, direct inactivation of the proteasome (e.g., by reactive oxygen species), or overloading with proteins. The latter can be caused by excessive production of normal proteins or by the formation of poorly degradable proteins as a result of genetic mutations, faulty posttranslational modification, or protein modification by reactive oxygen or nitrogen species. Blockage of the proteasome leads to increased oxidative and nitrative stress, the latter apparently due to upregulation of nitric oxide synthase. Thus, agents that increase proteasome function may be generally neuroprotective, as may be NOS inhibitors. Proteasome inhibitors should be used with caution as therapeutic agents.

Parkinson's disease: one biochemical pathway to fit all genes?

Although originally discounted, hereditary factors have emerged as the focus of research in Parkinson's disease (PD). Genetic studies have identified mutations in alpha-synuclein and ubiquitin C-terminal hydrolase as rare causes of autosomal dominant PD and mutations in parkin as a cause of autosomal recessive PD. Functional characterization of the identified disease genes implicates the ubiquitin-mediated protein degradation pathway in these hereditary forms of PD and also in the more common sporadic forms of PD. Subsequent identification of further loci in familial PD and diverse genetic factors modulating the risk for sporadic PD point to substantial genetic heterogeneity in the disease. Thus, new candidate genes are expected to encode proteins either involved in ubiquitin-mediated protein degradation or sequestrated in intracytoplasmic protein aggregations. Future identification of disease genes is required to confirm this hypothesis, thereby unifying the clinical and genetic heterogeneity of PD, including the common sporadic form of the disease, by one biochemical pathway.

Impairment of the ubiquitin-proteasome system causes dopaminergic cell death and inclusion body formation in ventral mesencephalic cultures

Mutations in alpha-synuclein, parkin and ubiquitin C-terminal hydrolase L1, and defects in 26/20S proteasomes, cause or are associated with the development of familial and sporadic Parkinson's disease (PD). This suggests that failure of the ubiquitin-proteasome system (UPS) to degrade abnormal proteins may underlie nigral degeneration and Lewy body formation that occur in PD. To explore this concept, we studied the effects of lactacystin-mediated inhibition of 26/20S proteasomal function and ubiquitin aldehyde (UbA)-induced impairment of ubiquitin C-terminal hydrolase (UCH) activity in fetal rat ventral mesencephalic cultures. We demonstrate that both lactacystin and UbA caused concentration-dependent and preferential degeneration of dopaminergic neurons. Inhibition of 26/20S proteasomal function was accompanied by the accumulation of alpha-synuclein and ubiquitin, and the formation of inclusions that were immunoreactive for these proteins, in the cytoplasm of VM neurons. Inhibition of UCH was associated with a loss of ubiquitin immunoreactivity in the cytoplasm of VM neurons, but there was a marked and localized increase in alpha-synuclein staining which may represent the formation of inclusions bodies in VM neurons. These findings provide direct evidence that impaired protein clearance can induce dopaminergic cell death and the formation of proteinaceous inclusion bodies in VM neurons. This study supports the concept that defects in the UPS may underlie nigral pathology in familial and sporadic forms of PD.

alpha-Synuclein protects against oxidative stress via inactivation of the c-Jun N-terminal kinase stress-signaling pathway in neuronal cells

The expression of alpha-synuclein, a synaptic molecule implicated in the pathogenesis of neurodegenerative disorders such as Parkinson's disease and Lewy body disease is increased upon injury to the nervous system, indicating that it might play a role in regeneration and plasticity; however, the mechanisms are unclear. Because c-Jun N-terminal kinase (JNK), a member of the mitogen-activated protein kinase family, plays an important role in stress response, the main objective of the present study was to better understand the involvement of this pathway in the signaling responses associated with resistance to injury in cells expressing alpha-synuclein. For this purpose, the JNK-signaling pathway was investigated in alpha-synuclein-transfected neuronal cell line glucose transporter (GT) 1-7 under oxidative stress conditions. Although hydrogen peroxide challenge resulted in JNK activation and cell death in cells transfected with vector control or beta-synuclein, alpha-synuclein-transfected cells were resistant to hydrogen peroxide, and JNK was not activated. The inactivation of JNK in the alpha-synuclein-transfected cells was associated with increased expression and activity of JNK-interacting protein (JIP)-1b/islet-brain (IB)1, the scaffold protein for the JNK pathway. Similarly, cells transfected with JIP-1b/IB1 were resistant to hydrogen peroxide associated with inactivation of the JNK pathway. In these cells, expression of endogenous alpha-synuclein was significantly increased at the protein level. Furthermore, alpha-synuclein was co-localized with JIP-1b/IB1 in the growth cones. Taken together, these results suggest that increased alpha-synuclein expression might protect cells from oxidative stress by inactivation of JNK via increased expression of JIP-1b/IB1. Furthermore, interactions between alpha-synuclein and JIP-1b/IB1 may play a mutual role in the neuronal response to injury and neurodegeneration.

Expression of A53T mutant but not wild-type alpha-synuclein in PC12 cells induces alterations of the ubiquitin-dependent degradation system, loss of dopamine release, and autophagic cell death

Alpha-synuclein mutations have been identified in certain families with Parkinson's disease (PD), and alpha-synuclein is a major component of Lewy bodies. Other genetic data indicate that the ubiquitin-dependent proteolytic system is involved in PD pathogenesis. We have generated stable PC12 cell lines expressing wild-type or A53T mutant human alpha-synuclein. Lines expressing mutant but not wild-type alpha-synuclein show: (1) disruption of the ubiquitin-dependent proteolytic system, manifested by small cytoplasmic ubiquitinated aggregates and by an increase in polyubiquitinated proteins; (2) enhanced baseline nonapoptotic death; (3) marked accumulation of autophagic-vesicular structures; (4) impairment of lysosomal hydrolysis and proteasomal function; and (5) loss of catecholamine-secreting dense core granules and an absence of depolarization-induced dopamine release. Such findings raise the possibility that the primary abnormality in these cells may involve one or more deficits in the lysosomal and/or proteasomal degradation pathways, which in turn lead to loss of dopaminergic capacity and, ultimately, to death. These cells may serve as a model to study the effects of aberrant alpha-synuclein on dopaminergic cell function and survival.

The herbicide paraquat causes up-regulation and aggregation of alpha-synuclein in mice: paraquat and alpha-synuclein

alpha-Synuclein-containing aggregates represent a feature of a variety of neurodegenerative disorders, including Parkinson's disease (PD). However, mechanisms that promote intraneuronal alpha-synuclein assembly remain poorly understood. Because pesticides, particularly the herbicide paraquat, have been suggested to play a role as PD risk factors, the hypothesis that interactions between alpha-synuclein and these environmental agents may contribute to aggregate formation was tested in this study. Paraquat markedly accelerated the in vitro rate of alpha-synuclein fibril formation in a dose-dependent fashion. When mice were exposed to the herbicide, brain levels of alpha-synuclein were significantly increased. This up-regulation followed a consistent pattern, with higher alpha-synuclein at 2 days after each of three weekly paraquat injections and with protein levels returning to control values by day 7 post-treatment. Paraquat exposure was also accompanied by aggregate formation. Thioflavine S-positive structures accumulated within neurons of the substantia nigra pars compacta, and dual labeling and confocal imaging confirmed that these aggregates contained alpha-synuclein. The results suggest that up-regulation of alpha-synuclein as a consequence of toxicant insult and direct interactions between the protein and environmental agents are potential mechanisms leading to alpha-synuclein pathology in neurodegenerative disorders.

alpha-Synuclein affects the MAPK pathway and accelerates cell death

Insoluble alpha-synuclein accumulates in Parkinson's disease, diffuse Lewy body disease, and multiple system atrophy. However, the relationship between its accumulation and pathogenesis is still unclear. Recently, we reported that overexpression of alpha-synuclein affects Elk-1 phosphorylation in cultured cells, which is mainly performed by mitogen-activated protein kinases (MAPKs). We further examined the relationship between MAPK signaling and the effects of alpha-synuclein expression on ecdysone-inducible neuro2a cell lines and found that cells expressing alpha-synuclein had less phosphorylated MAPKs. Moreover, they showed significant cell death when the concentration of serum in the culture medium was reduced. Under normal serum conditions, the addition of the MAPK inhibitor U0126 also caused cell death in alpha-synuclein-expressing cells. Transfection of constitutively active MEK-1 resulted in MAPK phosphorylation in alpha-synuclein-expressing cells and improved cell viability even under reduced serum conditions. Thus, we conclude that alpha-synuclein regulates the MAPK pathway by reducing the amount of available active MAPK. Our findings suggest a mechanism for pathogenesis and thus offer therapeutic insight into synucleinopathies.

Induction of alpha-synuclein aggregation by intracellular nitrative insult

Brain lesions containing filamentous and aggregated alpha-synuclein are hallmarks of neurodegenerative synucleinopathies. Oxidative stress has been implicated in the formation of these lesions. Using HEK 293 cells stably transfected with wild-type and mutant alpha-synuclein, we demonstrated that intracellular generation of nitrating agents results in the formation of alpha-synuclein aggregates. Cells were exposed simultaneously to nitric oxide- and superoxide-generating compounds, and the intracellular formation of peroxynitrite was demonstrated by monitoring the oxidation of dihydrorhodamine 123 and the nitration of alpha-synuclein. Light microscopy using antibodies against alpha-synuclein and electron microscopy revealed the presence of perinuclear aggregates under conditions in which peroxynitrite was generated but not when cells were exposed to nitric oxide- or superoxide-generating compounds separately. alpha-Synuclein aggregates were observed in 20-30% of cells expressing wild-type or A53T mutant alpha-synuclein and in 5% of cells expressing A30P mutant alpha-synuclein. No evidence of synuclein aggregation was observed in untransfected cells or cells expressing beta-synuclein. In contrast, selective inhibition of the proteasome resulted in the formation of aggregates detected with antibodies to ubiquitin in the majority of the untransfected cells and cells expressing alpha-synuclein. However, alpha-synuclein did not colocalize with these aggregates, indicating that inhibition of the proteasome does not promote alpha-synuclein aggregation. In addition, proteasome inhibition did not alter the steady-state levels of alpha-synuclein, but addition of the lysosomotropic agent ammonium chloride significantly increased the amount of alpha-synuclein, indicating that lysosomes are involved in degradation of alpha-synuclein. Our data indicate that nitrative and oxidative insult may initiate pathogenesis of alpha-synuclein aggregates.

Lewy body pathology in Alzheimer's disease

Lewy bodies, the characteristic pathological lesion of substantia nigra neurons in Parkinson's disease (PD), are frequently observed to accompany the amyloid plaque and neurofibrillary tangle pathology of Alzheimer's disease (AD). However the typical anatomic distribution of Lewy bodies in AD is distinct from PD. The most common site of occurrence is the amygdala, where Lewy bodies are observed in approximately 60% of both sporadic and familial AD. Other common sites of occurrence include the periamygdaloid and entorhinal cortex, while neocortical and brainstem areas develop Lewy bodies in a lower percentage of cases. In contrast, dementia with Lewy bodies (DLB), defined by widespread neocortical and brainstem Lewy bodies but frequently accompanied by variable levels of AD-type pathology, represents the other end of a spectrum of pathology associated with dementia. The observation of Lewy bodies in familial AD cases suggests that like neurofibrillary tangles, the formation of Lewy bodies can be induced by the pathological state caused by Abeta-amyloid overproduction. The role of Lewy body formation in the dysfunction and degeneration of neurons remains unclear. The protein alpha-synuclein appears to be an important structural component of Lewy bodies, an observation spurred by the discovery of point mutations in the alpha-synuclein gene linked to rare cases of autosomal dominant PD. Further investigation of alpha-synuclein and its relationship to pathological conditions promoting Lewy body formation in AD, PD, and DLB may yield further insight into pathogenesis of these diseases.

Proteasomal inhibition leads to formation of ubiquitin/alpha-synuclein-immunoreactive inclusions in PC12 cells

Proteasomal dysfunction has been recently implicated in the pathogenesis of several neurodegenerative diseases, including Parkinson's disease and diffuse Lewy body disease. We have developed an in vitro model of proteasomal dysfunction by applying pharmacological inhibitors of the proteasome, lactacystin or ZIE[O-tBu]-A-leucinal (PSI), to dopaminergic PC12 cells. Proteasomal inhibition caused a dose-dependent increase in death of both naive and neuronally differentiated PC12 cells, which could be prevented by caspase inhibition or CPT-cAMP. A percentage of the surviving cells contained discrete cytoplasmic ubiquitinated inclusions, some of which also contained synuclein-1, the rat homologue of human alpha-synuclein. However the total level of synuclein-1 was not altered by proteasomal inhibition. The ubiquitinated inclusions were present only within surviving cells, and their number was increased if cell death was prevented. We have thus replicated, in this model system, the two cardinal pathological features of Lewy body diseases, neuronal death and the formation of cytoplasmic ubiquitinated inclusions. Our findings suggest that inclusion body formation and cell death may be dissociated from one another.

Effect of proteasome inhibition on cellular oxidative damage, antioxidant defences and nitric oxide production

The ubiquitin/proteasome pathway plays an essential role in protein turnover in vivo, and contributes to removal of oxidatively damaged proteins. We examined the effects of proteasome inhibition on viability, oxidative damage and antioxidant defences in NT-2 and SK-N-MC cell lines. The selective proteasome inhibitor, lactacystin (1 microM) caused little loss of viability, but led to significant increases in levels of oxidative protein damage (measured as protein carbonyls), ubiquitinated proteins, lipid peroxidation and 3-nitrotyrosine, a biomarker of the attack of reactive nitrogen species (such as peroxynitrite, ONOO(-)) upon proteins. Higher levels (25 microM) of lactacystin did not further increase the levels of carbonyls, lipid peroxidation, 3-nitrotyrosine, or ubiquitinated proteins, but produced increases in the levels of 8-hydroxyguanine (a biomarker of oxidative DNA damage) and falls in levels of GSH. Lactacystin (25 microM) caused loss of viability, apparently by apoptosis, and also increased production of nitric oxide (NO.) (measured as levels of NO2- plus NO3-) by the cells; this was inhibited by N-nitro-L-arginine methyl ester (L-NAME), which also decreased cell death induced by 25 microM lactacystin and decreased levels of 3-nitrotyrosine. The NO. production appeared to involve nNOS; iNOS or eNOS were not detectable in either cell type. Another proteasome inhibitor, epoxomicin, had similar effects.

Effect of overexpression of wild-type and mutant Cu/Zn-superoxide dismutases on oxidative stress and cell death induced by hydrogen peroxide, 4-hydroxynonenal or serum deprivation: potentiation of injury by ALS-related mutant superoxide dismutases and protection by Bcl-2

Mutations in Cu/Zn-superoxide dismutase (SOD1) are associated with some cases of familial amyotrophic lateral sclerosis (ALS). We overexpressed Bcl-2, wild-type SOD1 or mutant SOD1s (G37R and G85R) in NT-2 and SK-N-MC cells. Overexpression of Bcl-2 rendered cells more resistant to apoptosis induced by serum withdrawal, H2O2 or 4-hydroxy-2-trans-nonenal (HNE). Overexpression of Bcl-2 had little effect on levels of protein carbonyls, lipid peroxidation, 8-hydroxyguanine (8-OHG) or 3-nitrotyrosine. Serum withdrawal or H2O2 raised levels of protein carbonyls, lipid peroxidation, 8-OHG and 3-nitrotyrosine, changes that were attenuated in cells overexpressing Bcl-2. Overexpression of either SOD1 mutant tended to increase levels of lipid peroxidation, protein carbonyls, and 3-nitrotyrosine and accelerated viability loss induced by serum withdrawal, H2O2 or HNE, accompanied by greater rises in oxidative damage parameters. The effects of mutant SOD1s were attenuated by Bcl-2. By contrast, expression of wild-type SOD1 rendered cells more resistant to loss of viability induced by serum deprivation, HNE or H2O2. The levels of lipid peroxidation in wild-type SOD1 transfectants were elevated. Overexpression of mutant SOD1s makes cells more predisposed to undergo apoptosis in response to several insults. Our cellular systems appear to mimic events in patients with ALS or transgenic mice overexpressing mutant SOD1.

Experimental models of Parkinson's disease

Research into the pathogenesis of Parkinson's disease has been rapidly advanced by the development of animal models. Initial models were developed by using toxins that specifically targeted dopamine neurons, the most successful of which used 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, a toxin that causes parkinsonism in man. More recently, the identification of alpha-synuclein mutations as a rare cause of Parkinson's disease has led to the development of alpha-synuclein transgenic mice and Drosophila. Here, I discuss the merits and limitations of these different animal models in our attempts to understand the physiology of Parkinson's disease and to develop new therapies.