alpha-Synuclein and neuronal cell death

Implications of protein structure instability: from physiological to pathological secondary structure

Proteins are folded during their synthesis; this process may be spontaneous or assisted. Both phenomena are carefully regulated by the "housekeeping" mechanism and molecular chaperones to avoid the appearance of misfolded proteins. Unfolding process generally occurs during physiological degradation of protein, but in some specific cases it results from genetic or environmental changes and does not correspond to metabolic needs. The main outcome of these phenomena is the appearance of nonfunctional pathologically unfolded proteins with a strong tendency to aggregation. Moreover, for some of these unfolded proteins, the agglomeration that follows initial proteins association may give rise to highly structured soluble aggregates. These aggregates have been identified as the main cause of the so-called amyloidosis or amyloid diseases, such as Alzheimer's, Parkinson's, and Creutzfeldt-Jakob diseases, and type II diabetes mellitus. Although some common mechanisms of amyloid protein aggregation have been identified, the roles of the environmental conditions inducing amyloidosis remain to be clarified. In this review, we will summarize recent studies identifying the origin of amyloid nucleation and will try to predict the therapeutic prospects that may be opened by elucidation of the amyloidosis mechanisms.

A novel, sensitive assay for behavioral defects in Parkinson's disease model Drosophila

Parkinson's disease is a common neurodegenerative disorder with the pathology of α-synuclein aggregation in Lewy bodies. Currently, there is no available therapy that arrests the progression of the disease. Therefore, the need of animal models to follow α-synuclein aggregation is crucial. Drosophila melanogaster has been researched extensively as a good genetic model for the disease, with a cognitive phenotype of defective climbing ability. The assay for climbing ability has been demonstrated as an effective tool for screening new therapeutic agents for Parkinson's disease. However, due to the assay's many limitations, there is a clear need to develop a better behavioral test. Courtship, a stereotyped, ritualized behavior of Drosophila, involves complex motor and sensory functions in both sexes, which are controlled by large number of neurons; hence, behavior observed during courtship should be sensitive to disease processes in the nervous system. We used a series of traits commonly observed in courtship and an additional behavioral trait-nonsexual encounters-and analyzed them using a data mining tool. We found defective behavior of the Parkinson's model male flies that were tested with virgin females, visible at a much younger age than the climbing defects. We conclude that this is an improved behavioral assay for Parkinson's model flies.

Drosophila models of proteinopathies: the little fly that could

Alzheimer’s, Parkinson’s, and Huntington’s disease are complex neurodegenerative conditions with high prevalence characterized by protein misfolding and deposition in the brain. Considerable progress has been made in the last two decades in identifying the genes and proteins responsible for several human ‘proteinopathies’. A wide variety of wild type and mutant proteins associated with neurodegenerative conditions are structurally unstable, misfolded, and acquire conformations rich in ß-sheets (ß-state). These conformers form highly toxic self-assemblies that kill the neurons in stereotypical patterns. Unfortunately, the detailed understanding of the molecular and cellular perturbations caused by these proteins has not produced a single disease-modifying therapy. More than a decade ago, several groups demonstrated that human proteinopathies reproduce critical features of the disease in transgenic flies, including protein misfolding, aggregation, and neurotoxicity. These initial reports led to an explosion of research that has contributed to a better understanding of the molecular mechanisms regulating conformational dynamics and neurotoxic cascades. To remain relevant in this competitive environment, Drosophila models will need to expand their flexible, innovative, and multidisciplinary approaches to find new discoveries and translational applications.

Mitochondrial dysfunction in Parkinson's disease: molecular mechanisms and pathophysiological consequences

Neurons are critically dependent on mitochondrial integrity based on specific morphological, biochemical, and physiological features. They are characterized by high rates of metabolic activity and need to respond promptly to activity-dependent fluctuations in bioenergetic demand. The dimensions and polarity of neurons require efficient transport of mitochondria to hot spots of energy consumption, such as presynaptic and postsynaptic sites. Moreover, the postmitotic state of neurons in combination with their exposure to intrinsic and extrinsic neuronal stress factors call for a high fidelity of mitochondrial quality control systems. Consequently, it is not surprising that mitochondrial alterations can promote neuronal dysfunction and degeneration. In particular, mitochondrial dysfunction has long been implicated in the etiopathogenesis of Parkinson's disease (PD), based on the observation that mitochondrial toxins can cause parkinsonism in humans and animal models. Substantial progress towards understanding the role of mitochondria in the disease process has been made by the identification and characterization of genes causing familial variants of PD. Studies on the function and dysfunction of these genes revealed that various aspects of mitochondrial biology appear to be affected in PD, comprising mitochondrial biogenesis, bioenergetics, dynamics, transport, and quality control.

The Role of p38 MAPK and Its Substrates in Neuronal Plasticity and Neurodegenerative Disease

A significant amount of evidence suggests that the p38-mitogen-activated protein kinase (MAPK) signalling cascade plays a crucial role in synaptic plasticity and in neurodegenerative diseases. In this review we will discuss the cellular localisation and activation of p38 MAPK and the recent advances on the molecular and cellular mechanisms of its substrates: MAPKAPK 2 (MK2) and tau protein. In particular we will focus our attention on the understanding of the p38 MAPK-MK2 and p38 MAPK-tau activation axis in controlling neuroinflammation, actin remodelling and tau hyperphosphorylation, processes that are thought to be involved in normal ageing as well as in neurodegenerative diseases. We will also give some insight into how elucidating the precise role of p38 MAPK-MK2 and p38 MAPK-tau signalling cascades may help to identify novel therapeutic targets to slow down the symptoms observed in neurodegenerative diseases such as Alzheimer's and Parkinson's disease.

Will posttranslational modifications of brain proteins provide novel serological markers for dementias?

Drug development for dementias is significantly hampered by the lack of easily accessible biomarkers. Fluid biomarkers of dementias provide indications of disease stage, but have little prognostic value, cannot detect early pathological changes, and can only be measured in CSF (cerebrospinal fluid) which significantly limits their applicability. In contrast, imaging based biomarkers can provide indications of probability of disease progression, yet are limited in applicability due to cost, radiation and radio-tracers. These aspects highlight the need for other approaches to the development of biomarkers of dementia, which should focus on not only providing information about pathological changes, but also on being measured easily and reproducibly. For other diseases, focus on development of assays monitoring highly specific protease-generated cleavage fragments of proteins has provided assays, which in serum or plasma have the ability to predict early pathological changes. Proteolytic processing of brain proteins, such as tau, APP, and α-synuclein, is a key pathological event in dementias. Here, we speculate that aiming biomarker development for dementias at detecting small brain protein degradation fragments of generated by brain-derived proteases specifically in blood samples could lead to the development of novel markers of disease progression, stage and importantly of treatment efficacy.

Role of alpha-synuclein protein levels in mitochondrial morphology and cell survival in cell lines

α-Synuclein is highly associated with some neurodegeneration and malignancies. Overexpressing wild-type or mutant α-synuclein promotes neuronal death by mitochondrial dysfunction, the underlying mechanisms of which remain poorly defined. It was recently reported that α-synuclein expression could directly lead to mitochondrial fragmentation in vitro and in vivo, which may be due to α-synuclein localization on mitochondria. Here, we applied a double staining method to demonstrate mitochondrial morphogenetic changes in cells overexpressed with α-synuclein. We show that mitochondrial localization of α-synuclein was increased following its overexpression in three distinct cell lines, including HeLa, SH-SY5Y, and PC12 cells, but no alteration in mitochondrial morphology was detected. However, α-synuclein knockdown prevents MPP(+)-induced mitochondrial fragmentation in SH-SY5Y and PC12 cells. These data suggest that α-synuclein protein levels hardly affect mitochondrial morphology in normal cell lines, but may have some influence on that under certain environmental conditions.

GSM-900MHz at low dose temperature-dependently downregulates α-synuclein in cultured cerebral cells independently of chaperone-mediated-autophagy

The expanding use of GSM devices has resulted in public concern. Chaperone-mediated autophagy (CMA) is a way for protein degradation in the lysosomes and increases under stress conditions as a cell defense response. α-synuclein, a CMA substrate, is a component of Parkinson disease. Since GSM might constitute a stress signal, we raised the possibility that GSM could alter the CMA process. Here, we analyzed the effects of chronic exposure to a low GSM-900MHz dose on apoptosis and CMA. Cultured cerebral cortical cells were sham-exposed or exposed to GSM-900MHz at specific absorption rate (SAR): 0.25W/kg for 24 h using a wire-patch cell. Apoptosis was analyzed by DAPI stain of the nuclei and western blot of cleaved caspase-3. The expression of proteins involved in CMA (HSC70, HSP40, HSP90 and LAMP-2A) and α-synuclein were analyzed by western blot. CMA was also quantified in situ by analyzing the cell localization of active lysosomes. 24 h exposure to GSM-900MHz resulted in ∼0.5°C temperature rise. It did not induce apoptosis but increased HSC70 by 26% and slightly decreased HSP90 (<10%). It also decreased α-synuclein by 24% independently of CMA, since the localization of active lysosomes was not altered. Comparable effects were observed in cells incubated at 37.5°C, a condition that mimics the GSM-generated temperature rise. The GSM-induced changes in HSC70, HSP90 and α-synuclein are most likely linked to temperature rise. We did not observe any immediate effect on cell viability. However, the delayed and long term consequences (protective or deleterious) of these changes on cell fate should be examined.

α-Synuclein in central nervous system and from erythrocytes, mammalian cells, and Escherichia coli exists predominantly as disordered monomer

Since the discovery and isolation of α-synuclein (α-syn) from human brains, it has been widely accepted that it exists as an intrinsically disordered monomeric protein. Two recent studies suggested that α-syn produced in Escherichia coli or isolated from mammalian cells and red blood cells exists predominantly as a tetramer that is rich in α-helical structure (Bartels, T., Choi, J. G., and Selkoe, D. J. (2011) Nature 477, 107-110; Wang, W., Perovic, I., Chittuluru, J., Kaganovich, A., Nguyen, L. T. T., Liao, J., Auclair, J. R., Johnson, D., Landeru, A., Simorellis, A. K., Ju, S., Cookson, M. R., Asturias, F. J., Agar, J. N., Webb, B. N., Kang, C., Ringe, D., Petsko, G. A., Pochapsky, T. C., and Hoang, Q. Q. (2011) Proc. Natl. Acad. Sci. 108, 17797-17802). However, it remains unknown whether or not this putative tetramer is the main physiological form of α-syn in the brain. In this study, we investigated the oligomeric state of α-syn in mouse, rat, and human brains. To assess the conformational and oligomeric state of native α-syn in complex mixtures, we generated α-syn standards of known quaternary structure and conformational properties and compared the behavior of endogenously expressed α-syn to these standards using native and denaturing gel electrophoresis techniques, size-exclusion chromatography, and an oligomer-specific ELISA. Our findings demonstrate that both human and rodent α-syn expressed in the central nervous system exist predominantly as an unfolded monomer. Similar results were observed when human α-syn was expressed in mouse and rat brains as well as mammalian cell lines (HEK293, HeLa, and SH-SY5Y). Furthermore, we show that α-syn expressed in E. coli and purified under denaturing or nondenaturing conditions, whether as a free protein or as a fusion construct with GST, is monomeric and adopts a disordered conformation after GST removal. These results do not rule out the possibility that α-syn becomes structured upon interaction with other proteins and/or biological membranes.

Authentically phosphorylated α-synuclein at Ser129 accelerates neurodegeneration in a rat model of familial Parkinson's disease

Parkinson's disease (PD) is characterized by the loss of dopaminergic neurons in the substantia nigra (SN) and the appearance of fibrillar aggregates of insoluble α-synuclein (α-syn) called Lewy bodies (LBs). Approximately 90% of α-syn deposited in LBs is phosphorylated at serine 129 (Ser129). In contrast, only 4% of total α-syn is phosphorylated in normal brain, suggesting that accumulation of Ser129-phosphorylated α-syn is involved in the pathogenesis of PD. However, the role of Ser129 phosphorylation in α-syn neurotoxicity remains unclear. In this study, we coexpressed familial PD-linked A53T α-syn and G-protein-coupled receptor kinase 6 (GRK6) in the rat SN pars compacta using recombinant adeno-associated virus 2. Coexpression of these proteins yielded abundant Ser129-phosphorylated α-syn and significantly exacerbated degeneration of dopaminergic neurons when compared with coexpression of A53T α-syn and GFP. Immunohistochemical analysis revealed that Ser129-phosphorylated α-syn was preferentially distributed to swollen neurites. However, biochemical analysis showed that the increased expression of Ser129-phosphorylated α-syn did not promote accumulation of detergent-insoluble α-syn. Coexpression of catalytically inactive K215R mutant GRK6 failed to accelerate A53T α-syn-induced degeneration. Furthermore, introducing a phosphorylation-incompetent mutation, S129A, into A53T α-syn did not alter the pace of degeneration, even when GRK6 was coexpressed. Our study demonstrates that authentically Ser129-phosphorylated α-syn accelerates A53T α-syn neurotoxicity without the formation of detergent-insoluble α-syn, and suggests that the degenerative process could be constrained by inhibiting the kinase that phosphorylates α-syn at Ser129.

The role of α-synuclein in neurodegeneration — An update

Genetic, neuropathological and biochemical evidence implicates α-synuclein, a 140 amino acid presynaptic neuronal protein, in the pathogenesis of Parkinson’s disease and other neurodegenerative disorders. The aggregated protein inclusions mainly containing aberrant α-synuclein are widely accepted as morphological hallmarks of α-synucleinopathies, but their composition and location vary between disorders along with neuronal networks affected. α-Synuclein exists physiologically in both soluble and membran-bound states, in unstructured and α-helical conformations, respectively, while posttranslational modifications due to proteostatic deficits are involved in β-pleated aggregation resulting in formation of typical inclusions. The physiological function of α-synuclein and its role linked to neurodegeneration, however, are incompletely understood. Soluble oligomeric, not fully fibrillar α-synuclein is thought to be neurotoxic, main targets might be the synapse, axons and glia. The effects of aberrant α-synuclein include alterations of calcium homeostasis, mitochondrial dysfunction, oxidative and nitric injuries, cytoskeletal effects, and neuroinflammation. Proteasomal dysfunction might be a common mechanism in the pathogenesis of neuronal degeneration in α-synucleinopathies. However, how α-synuclein induces neurodegeneration remains elusive as its physiological function. Genome wide association studies demonstrated the important role for genetic variants of the SNCA gene encoding α-synuclein in the etiology of Parkinson’s disease, possibly through effects on oxidation, mitochondria, autophagy, and lysosomal function. The neuropathology of synucleinopathies and the role of α-synuclein as a potential biomarker are briefly summarized. Although animal models provided new insights into the pathogenesis of Parkinson disease and multiple system atrophy, most of them do not adequately reproduce the cardinal features of these disorders. Emerging evidence, in addition to synergistic interactions of α-synuclein with various pathogenic proteins, suggests that prionlike induction and seeding of α-synuclein could lead to the spread of the pathology and disease progression. Intervention in the early aggregation pathway, aberrant cellular effects, or secretion of α-synuclein might be targets for neuroprotection and disease-modifying therapy.

Effect of single amino acid substitution on oxidative modifications of the Parkinson's disease-related protein, DJ-1

Mutations in the gene encoding DJ-1 have been identified in patients with familial Parkinson's disease (PD) and are thought to inactivate a neuroprotective function. Oxidation of the sulfhydryl group to a sulfinic acid on cysteine residue C106 of DJ-1 yields the "2O " form, a variant of the protein with enhanced neuroprotective function. We hypothesized that some familial mutations disrupt DJ-1 activity by interfering with conversion of the protein to the 2O form. To address this hypothesis, we developed a novel quantitative mass spectrometry approach to measure relative changes in oxidation at specific sites in mutant DJ-1 as compared with the wild-type protein. Treatment of recombinant wild-type DJ-1 with a 10-fold molar excess of H(2)O(2) resulted in a robust oxidation of C106 to the sulfinic acid, whereas this modification was not detected in a sample of the familial PD mutant M26I exposed to identical conditions. Methionine oxidized isoforms of wild-type DJ-1 were depleted, presumably as a result of misfolding and aggregation, under conditions that normally promote conversion of the protein to the 2O form. These data suggest that the M26I familial substitution and methionine oxidation characteristic of sporadic PD may disrupt DJ-1 function by disfavoring a site-specific modification required for optimal neuroprotective activity. Our findings indicate that a single amino acid substitution can markedly alter a protein's ability to undergo oxidative modification, and they imply that stimulating the conversion of DJ-1 to the 2O form may be therapeutically beneficial in familial or sporadic PD.

SNCA triplication Parkinson's patient's iPSC-derived DA neurons accumulate α-synuclein and are susceptible to oxidative stress

Parkinson's disease (PD) is an incurable age-related neurodegenerative disorder affecting both the central and peripheral nervous systems. Although common, the etiology of PD remains poorly understood. Genetic studies infer that the disease results from a complex interaction between genetics and environment and there is growing evidence that PD may represent a constellation of diseases with overlapping yet distinct underlying mechanisms. Novel clinical approaches will require a better understanding of the mechanisms at work within an individual as well as methods to identify the specific array of mechanisms that have contributed to the disease. Induced pluripotent stem cell (iPSC) strategies provide an opportunity to directly study the affected neuronal subtypes in a given patient. Here we report the generation of iPSC-derived midbrain dopaminergic neurons from a patient with a triplication in the α-synuclein gene (SNCA). We observed that the iPSCs readily differentiated into functional neurons. Importantly, the PD-affected line exhibited disease-related phenotypes in culture: accumulation of α-synuclein, inherent overexpression of markers of oxidative stress, and sensitivity to peroxide induced oxidative stress. These findings show that the dominantly-acting PD mutation is intrinsically capable of perturbing normal cell function in culture and confirm that these features reflect, at least in part, a cell autonomous disease process that is independent of exposure to the entire complexity of the diseased brain.

Mechanism of membrane interaction and disruption by α-synuclein

Parkinson's disease is a common progressive neurodegenerative condition, characterized by the deposition of amyloid fibrils as Lewy bodies in the substantia nigra of affected individuals. These insoluble aggregates predominantly consist of the protein α-synuclein. There is increasing evidence suggesting that the aggregation of α-synuclein is influenced by lipid membranes and, vice versa, the membrane integrity is severely affected by the presence of bound aggregates. Here, using the surface-sensitive imaging technique supercritical angle fluorescence microscopy and Förster resonance energy transfer, we report the direct observation of α-synuclein aggregation on supported lipid bilayers. Both the wild-type and the two mutant forms of α-synuclein studied, namely, the familiar variant A53T and the designed highly toxic variant E57K, were found to follow the same mechanism of polymerization and membrane damage. This mechanism involved the extraction of lipids from the bilayer and their clustering around growing α-synuclein aggregates. Despite all three isoforms following the same pathway, the extent of aggregation and their effect on the bilayers was seen to be variant and concentration dependent. Both A53T and E57K formed cross-β-sheet aggregates and damaged the membrane at submicromolar concentrations. The wild-type also formed aggregates in this range; however, the extent of membrane disruption was greatly reduced. The process of membrane damage could resemble part of the yet poorly understood cellular toxicity phenomenon in vivo.

The yin and yang of amyloid: insights from α-synuclein and repeat domain of Pmel17

Amyloid has been traditionally viewed in the context of disease. However, the emerging concept of 'functional amyloid' has taken a new direction into how we view amyloid. Recent studies have identified amyloid fibrils ranging from bacteria to humans that have a beneficial role, instead of being associated with a misfolded state that has been implicated in diseases such as Alzheimer's, Parkinson's and prion diseases. Here, we review our work on two human amyloidogenic polypeptides, one associated with Parkinson's disease, α-synuclein (α-syn), and the other important for melanin synthesis, the repeat domain (RPT) from Pmel17. Particularly, we focused our attention on spectroscopic studies of protein conformation and dynamics and their impact on α-syn amyloid formation and for RPT, we discussed the strict pH dependence of amyloid formation and its role in melanin biosynthesis.

LRRK2 mutant iPSC-derived DA neurons demonstrate increased susceptibility to oxidative stress

Studies of Parkinson's disease (PD) have been hindered by lack of access to affected human dopaminergic (DA) neurons. Here, we report generation of induced pluripotent stem cells that carry the p.G2019S mutation (G2019S-iPSCs) in the Leucine-Rich Repeat Kinase-2 (LRRK2) gene, the most common PD-related mutation, and their differentiation into DA neurons. The high penetrance of the LRRK2 mutation and its clinical resemblance to sporadic PD suggest that these cells could provide a valuable platform for disease analysis and drug development. We found that DA neurons derived from G2019S-iPSCs showed increased expression of key oxidative stress-response genes and α-synuclein protein. The mutant neurons were also more sensitive to caspase-3 activation and cell death caused by exposure to stress agents, such as hydrogen peroxide, MG-132, and 6-hydroxydopamine, than control DA neurons. This enhanced stress sensitivity is consistent with existing understanding of early PD phenotypes and represents a potential therapeutic target.

Circadian dysfunction in a mouse model of Parkinson's disease

Many Parkinson's disease (PD) patients exhibit sleep disorders as part of their symptoms with evidence suggesting that REM sleep disorders may be intimately associated with this disease. Possible dysfunction in the circadian system in PD has received less attention, yet problems in circadian timing are common in neurodegenerative diseases. In the present study, we examined the expression of daily and circadian rhythms in the alpha-synuclein overexpressing (ASO) transgenic line. We found selective deficits in the expression of circadian rhythms of locomotor activity, including lower night-time activity and greater fragmentation in the wheel-running activity in this PD model. These alterations were prominent in young adult (3-4 mo) ASO mice and worsened progressively with age, consistent with prior reports of age-related loss of motor skills. The temporal distribution of sleep was also altered in the ASO mice compared to littermate controls. In the ASO mice, the peak/trough expression of the clock gene PERIOD2 was normal in the master pacemaker of the circadian system: the suprachiasmatic nucleus (SCN); however, the daytime firing rate of SCN neurons was reduced in the mutant mice. Together, this data raises the possibility that a weakening of circadian output is a core feature of PD. The reduction in magnitude of circadian output would be expected to have functional consequences throughout the body.

Alpha-synuclein interacts with Glucocerebrosidase providing a molecular link between Parkinson and Gaucher diseases

The presynaptic protein α-synuclein (α-syn), particularly in its amyloid form, is widely recognized for its involvement in Parkinson disease (PD). Recent genetic studies reveal that mutations in the gene GBA are the most widespread genetic risk factor for parkinsonism identified to date. GBA encodes for glucocerebrosidase (GCase), the enzyme deficient in the lysosomal storage disorder, Gaucher disease (GD). In this work, we investigated the possibility of a physical linkage between α-syn and GCase, examining both wild type and the GD-related N370S mutant enzyme. Using fluorescence and nuclear magnetic resonance spectroscopy, we determined that α-syn and GCase interact selectively under lysosomal solution conditions (pH 5.5) and mapped the interaction site to the α-syn C-terminal residues, 118-137. This α-syn-GCase complex does not form at pH 7.4 and is stabilized by electrostatics, with dissociation constants ranging from 1.2 to 22 μm in the presence of 25 to 100 mm NaCl. Intriguingly, the N370S mutant form of GCase has a reduced affinity for α-syn, as does the inhibitor conduritol-β-epoxide-bound enzyme. Immunoprecipitation and immunofluorescence studies verified this interaction in human tissue and neuronal cell culture, respectively. Although our data do not preclude protein-protein interactions in other cellular milieux, we suggest that the α-syn-GCase association is favored in the lysosome, and that this noncovalent interaction provides the groundwork to explore molecular mechanisms linking PD with mutant GBA alleles.

Transmission of prion strains in a transgenic mouse model overexpressing human A53T mutated α-synuclein

There is a growing interest in the potential roles of misfolded protein interactions in neurodegeneration. To investigate this issue, we inoculated 3 prion strains intracerebrally into transgenic (TgM83) mice that overexpress human A53T α-synuclein. In comparison to nontransgenic controls, there was a striking decrease in the incubation periods of scrapie, classic and H-type bovine spongiform encephalopathies(C-BSE and H-BSE), with conservation of the histopathologic and biochemical features characterizing these 3 prion strains. TgM83 mice died of scrapie or C-BSE prion diseases before accumulating the insoluble and phosphorylated forms of α-synuclein specific to late stages of synucleinopathy. In contrast, the median incubation time for TgM83 mice inoculated with H-BSE was comparable to that observed when these mice were uninfected, thereby allowing the development of molecular alterations of α-synuclein. The last 4 mice of this cohort exhibited early accumulations of H-BSE prion protein along with α-synuclein pathology. The results indicate that a prion disease was triggered concomitantly with an overt synucleinopathy in some transgenic mice overexpressing human A53T α-synuclein after intracerebral inoculation with an H-BSE prion strain.

Toll-like receptor 4 promotes α-synuclein clearance and survival of nigral dopaminergic neurons

Toll-like receptors (TLRs) mediate innate immunity, and their dysregulation may play a role in α-synucleinopathies, such as Parkinson's disease or multiple system atrophy (MSA). The aim of this study was to define the role of TLR4 in α-synuclein-linked neurodegeneration. Ablation of TLR4 in a transgenic mouse model of MSA with oligodendroglial α-synuclein overexpression augmented motor disability and enhanced loss of nigrostriatal dopaminergic neurons. These changes were associated with increased brain levels of α-synuclein linked to disturbed TLR4-mediated microglial phagocytosis of α-synuclein. Furthermore, tumor necrosis factor-α levels were increased in the midbrain and associated with a proinflammatory astroglial response. Our data suggest that TLR4 ablation impairs the phagocytic response of microglia to α-synuclein and enhances neurodegeneration in a transgenic MSA mouse model. The study supports TLR4 signaling as innate neuroprotective mechanism acting through clearance of α-synuclein.

Caveolin-1 interacts with alpha-synuclein and mediates toxic actions of cellular alpha-synuclein overexpression

Caveolin-1 (Cav-1) is a transmembrane protein which clusters proteins and lipids at the cell membrane into a subclass of lipid rafts named caveolae. To increase our understanding about putative functions of Cav-1 in neuronal cells, we used mouse brain extracts and a novel technology coupling surface plasmon resonance to mass spectrometry to find binding partners to Cav-1. An interaction between Cav-1 and alpha-synclein was found and confirmed in reciprocal pulldown experiments. Genetic overexpression of alpha-synclein in mouse neuroblastoma Neuro2A cells (N2A) expectedly decreased cell survival, but also significantly increased the levels of Cav-1. Furthermore, si-RNA-mediated knockdown of Cav-1 counteracted cell death induced by overexpression of alpha-synuclein. We also used an inhibitor of proteasome (MG132) to induce cell death in a Parkinson's disease context. Cav-1 knockdown had no effect on cell death induced by MG132. Conversely, treating the cells with mevastatin, an inhibitor of cholesterol synthesis, inhibits cell death induced by MG132, but not by alpha synuclein overexpression. It can be concluded that Cav-1 may play a functional role in neuronal cells by virtue of its physical interaction with alpha-synuclein and regulate alpha synuclein-mediated actions on cell death, processes known to be involved in synucleinopathies including Parkinson's disease.

Drug discovery in Parkinson's disease-Update and developments in the use of cellular models

Parkinson's disease (PD) is the second most common neurodegenerative disorder and is characterized by the degeneration of dopaminergic (DA) neurons within the substantia nigra. Dopamine replacement drugs remain the most effective PD treatment but only provide temporary symptomatic relief. New therapies are urgently needed, but the search for a disease-modifying treatment and a definitive understanding of the underlying mechanisms of PD has been limited by the lack of physiologically relevant models that recapitulate the disease phenotype. The use of immortalized cell lines as in vitro model systems for drug discovery has met with limited success, since efficacy and safety too often fail to translate successfully in human clinical trials. Drug discoverers are shifting their focus to more physiologically relevant cellular models, including primary neurons and stem cells. The recent discovery of induced pluripotent stem (iPS) cell technology presents an exciting opportunity to derive human DA neurons from patients with sporadic and familial forms of PD. We anticipate that these human DA models will recapitulate key features of the PD phenotype. In parallel, high-content screening platforms, which extract information on multiple cellular features within individual neurons, provide a network-based approach that can resolve temporal and spatial relationships underlying mechanisms of neurodegeneration and drug perturbations. These emerging technologies have the potential to establish highly predictive cellular models that could bring about a desperately needed revolution in PD drug discovery.

Changes in the solubility and phosphorylation of α-synuclein over the course of Parkinson's disease

Lewy bodies are made from insoluble, phosphorylated α-synuclein, but the earliest changes that precipitate such pathology still remain conjecture. In this study, we quantify and identify relationships between the levels of the main pathologic form of phosphorylated α-synuclein over the course of Parkinson's disease in regions affected early through to end-stage disease. Brain tissue samples from 33 cases at different disease stages and 13 controls were collected through the Australian Network of Brain Banks. 500 mg of frozen putamen (affected preclinically) and frontal cortex (affected late) was homogenized, fractionated and α-synuclein levels evaluated using specific antibodies (syn-1, BD Transduction Laboratories; S129P phospho-α-synuclein, Elan Pharmaceuticals) and quantitative western blotting. Statistical analyses assessed the relationship between the different forms of α-synuclein, compared levels between groups, and determined any changes over the disease course. Soluble S129P was detected in controls with higher levels in putamen compared with frontal cortex. In contrast, insoluble α-synuclein occurred in Parkinson's disease with a significant increase in soluble and lipid-associated S129P, and a decrease in soluble frontal α-synuclein over the disease course. Increasing soluble S129P in the putamen correlated with increasing S129P in other fractions and regions. These data show that soluble non-phosphorylated α-synuclein decreases over the course of Parkinson's disease, becoming increasingly phosphorylated and insoluble. The finding that S129P α-synuclein normally occurs in vulnerable brain regions, and in Parkinson's disease has the strongest relationships to the pathogenic forms of α-synuclein in other brain regions, suggests a propagating role for putamenal phospho-α-synuclein in disease pathogenesis.

Genetic basis of Parkinson's disease: inheritance, penetrance, and expression

Parkinson’s disease can be caused by rare familial genetic mutations, but in most cases it is likely to result from an interaction between multiple genetic and environmental risk factors. Over recent years, many variants in a growing number of genes involved in the pathogenesis of Parkinson’s disease have been identified. Mutations in several genes have been shown to cause familial parkinsonism. In this review, we discuss 12 of them (SNCA, LRRK2, Parkin, PINK1, DJ1, ATP13A2, PLA2G6, FBXO7, UCHL1, GIGYF2, HTRA2, and EIF4G1). Additionally, six genes have been shown conclusively to be risk factors for sporadic Parkinson’s disease, and are also discussed (GBA, MAPT, BST1, PARK16, GAK, and HLA). Many more genes and genetic loci have been suggested, but need confirmation. There is evidence that pathways involved in the rare familial forms also play a role in the sporadic form, and that the respective genes might also be risk factors for sporadic Parkinson’s disease. The identification of genes involved in the development of Parkinson’s disease will improve our understanding of the underlying molecular mechanisms, and will hopefully lead to new drug targets and treatment strategies.

Compact conformations of α-synuclein induced by alcohols and copper

The intrinsically disordered protein α-synuclein aggregates into amyloid fibrils, a process known to be implicated in several neurodegenerative states. Partially folded forms of the protein are thought to trigger the aggregation process. Here, α-synuclein conformers are characterized by analysis of the charge-state distributions observed in electrospray-ionization mass spectrometry under negative-ion mode. It is found that, even at neutral pH, a small fraction of the molecular population is in a compact conformation. Several distinct partially folded forms are then identified under conditions that promote α-synuclein aggregation, such as solutions of simple and fluorinated alcohols. Specific intermediates accumulate at increasing concentrations of ethanol, hexafluoro-2-propanol, and trifluoroethanol. Finally, extensive folding induced by Cu(2+) binding is revealed by titrations in the presence of Cu(2+)-glycine. The data confirm the existence of a single, high-affinity binding site for Cu(2+). Because accumulation of this partially folded form correlates with enhancement of fibrillation kinetics, it is likely to represent an amyloidogenic intermediate in α-synuclein conformational transitions.

An energy systems approach to Parkinson's disease

The cause of Parkinson's disease (PD) remains unknown despite it being the second most prevalent neurodegenerative condition. Indeed, there is a growing consensus that there is no single cause, and that PD is a multifactorial systemic condition, in which a number of factors may determine its etiopathogenesis. We describe a systems approach that addresses the multifactorial aspects of PD and overcomes constraints on conventional experimentation imposed by PD's causal complexity, its long temporal duration, and its uniqueness to human brains. Specifically, a mathematical model of brain energy metabolism is used as a core module to which other modules describing cellular processes thought to be associated with PD can be attached and studied in an integrative environment. Employing brain energy usage as the core of a systems approach also enables the potential role that compromised energy metabolism may have in the etiology of PD. Although developed for PD, it has not escaped our attention that the energy systems approach outlined here could also be applied to other neurodegenerative disorders-most notably Alzheimer's disease.

Dysregulation of autophagy and Parkinson's disease: the MEF2D link

Autophagy refers to the process by which lysosomes degrade intracellular components. Three basic forms of it, macro-, micro-, and chaperon-mediated autophagy, exist in cells. Several studies have shown that dysregulation of macroautophagy compromises the viability of neurons. Recent evidence indicates that chaperone-mediated autophagy plays a role in direct degradation of neuronal transcription factor MEF2D, a protein known to promote neuronal survival. Disruption of this regulatory pathway by α-synuclein leads to neuronal stress, which may underlie neuronal loss in Parkinson's disease.

A microRNA embedded AAV α-synuclein gene silencing vector for dopaminergic neurons

Alpha-synuclein (SNCA), an abundantly expressed presynaptic protein, is implicated in Parkinson's disease (PD). Since over-expression of human SNCA (hSNCA) leads to death of dopaminergic (DA) neurons in human, rodent and fly brain, hSNCA gene silencing may reduce levels of toxic forms of SNCA and ameliorate degeneration of DA neurons in PD. To begin to develop a gene therapy for PD based on hSNCA gene silencing, two AAV gene silencing vectors were designed, and tested for efficiency and specificity of silencing, as well as toxicity in vitro. The same hSNCA silencing sequence (shRNA) was used in both vectors, but in one vector, the shRNA was embedded in a microRNA backbone and driven by a pol II promoter, and in the other the shRNA was not embedded in a microRNA and was driven by a pol III promoter. Both vectors silenced hSNCA to the same extent in 293T cells transfected with hSNCA. In DA PC12 cells, neither vector decreased expression of rat SNCA, tyrosine hydroxylase (TH), dopamine transporter (DAT) or the vesicular monoamine transporter (VMAT). However, the mir30 embedded vector was significantly less toxic to both PC12 and SH-SY5Y cells. Our in vitro data suggest that this miRNA-embedded silencing vector may be ideal for chronic in vivo SNCA gene silencing in DA neurons.

Specificity and kinetics of alpha-synuclein binding to model membranes determined with fluorescent excited state intramolecular proton transfer (ESIPT) probe

Parkinson disease is characterized cytopathologically by the deposition in the midbrain of aggregates composed primarily of the presynaptic neuronal protein α-synuclein (AS). Neurotoxicity is currently attributed to oligomeric microaggregates subjected to oxidative modification and promoting mitochondrial and proteasomal dysfunction. Unphysiological binding to membranes of these and other organelles is presumably involved. In this study, we performed a systematic determination of the influence of charge, phase, curvature, defects, and lipid unsaturation on AS binding to model membranes using a new sensitive solvatochromic fluorescent probe. The interaction of AS with vesicular membranes is fast and reversible. The protein dissociates from neutral membranes upon thermal transition to the liquid disordered phase and transfers to vesicles with higher affinity. The binding of AS to neutral and negatively charged membranes occurs by apparently different mechanisms. Interaction with neutral bilayers requires the presence of membrane defects; binding increases with membrane curvature and rigidity and decreases in the presence of cholesterol. The association with negatively charged membranes is much stronger and much less sensitive to membrane curvature, phase, and cholesterol content. The presence of unsaturated lipids increases binding in all cases. These findings provide insight into the relation between membrane physical properties and AS binding affinity and dynamics that presumably define protein localization in vivo and, thereby, the role of AS in the physiopathology of Parkinson disease.

Defects in very long chain fatty acid synthesis enhance alpha-synuclein toxicity in a yeast model of Parkinson's disease

We identified three S. cerevisiae lipid elongase null mutants (elo1Δ, elo2Δ, and elo3Δ) that enhance the toxicity of alpha-synuclein (α-syn). These elongases function in the endoplasmic reticulum (ER) to catalyze the elongation of medium chain fatty acids to very long chain fatty acids, which is a component of sphingolipids. Without α-syn expression, the various elo mutants showed no growth defects, no reactive oxygen species (ROS) accumulation, and a modest decrease in survival of aged cells compared to wild-type cells. With (WT, A53T or E46K) α-syn expression, the various elo mutants exhibited severe growth defects (although A30P had a negligible effect on growth), ROS accumulation, aberrant protein trafficking, and a dramatic decrease in survival of aged cells compared to wild-type cells. Inhibitors of ceramide synthesis, myriocin and FB1, were extremely toxic to wild-type yeast cells expressing (WT, A53T, or E46K) α-syn but much less toxic to cells expressing A30P. The elongase mutants and ceramide synthesis inhibitors enhance the toxicity of WT α-syn, A53T and E46K, which transit through the ER, but have a negligible effect on A30P, which does not transit through the ER. Disruption of ceramide-sphingolipid homeostasis in the ER dramatically enhances the toxicity of α-syn (WT, A53T, and E46K).

UV-laser microdissection and mRNA expression analysis of individual neurons from postmortem Parkinson's disease brains

Cell specificity of gene expression analysis is essential to avoid tissue sample related artifacts, in particular when the relative number of target cells present in the compared tissues varies dramatically, e.g., when comparing dopamine neurons in midbrain tissues from control subjects with those from Parkinson's disease (PD) cases. Here, we describe a detailed protocol that combines contact-free UV-laser microdissection and quantitative PCR of reverse-transcribed RNA of individual neurons from postmortem human midbrain tissue from PD patients and unaffected controls. Among expression changes in a variety of dopamine neuron marker, maintenance, and cell-metabolism genes, we found that α-synuclein mRNA levels were significantly elevated in individual neuromelanin-positive dopamine midbrain neurons from PD brains when compared to those from matched controls.

A pilot trial of the microtubule-interacting peptide (NAP) in mice overexpressing alpha-synuclein shows improvement in motor function and reduction of alpha-synuclein inclusions

Abnormal accumulation of α-synuclein is associated with several neurodegenerative disorders (synucleinopathies), including sporadic Parkinson's disease (PD). Genetic mutations and multiplication of α-synuclein cause familial forms of PD and polymorphisms in the α-synuclein gene are associated with PD risk. Overexpression of α-synuclein can impair essential functions within the cell such as microtubule-dependent transport, suggesting that compounds that act on the microtubule system may have therapeutic benefit for synucleinopathies. In this study, mice overexpressing human wildtype α-synuclein under the Thy1 promoter (Thy1-aSyn) and littermate wildtype control mice were administered daily the microtubule-interacting peptide NAPVSIPQ (NAP; also known as davunetide or AL-108) intranasally for 2 months starting at 1 month of age, in a regimen known to produce effective concentrations of the peptide in mouse brain. Motor performance, coordination, and activity were assessed at the end of treatment. Olfactory function, which is altered in PD, was measured 1 month later. Mice were sacrificed at 4.5 months of age, and their brains examined for proteinase K-resistant α-synuclein inclusions in the substantia nigra and olfactory bulb. NAP-treated Thy1-aSyn mice showed a 38% decrease in the number of errors per step in the challenging beam traversal test and a reduction in proteinase K-resistant α-synuclein inclusions in the substantia nigra compared to vehicle treated transgenics. The data indicate a significant behavioral benefit and a long lasting improvement of α-synuclein pathology following administration of a short term (2 months) NAP administration in a mouse model of synucleinopathy.

Raised calcium promotes α-synuclein aggregate formation

Parkinson's and Parkinson's-plus diseases are associated with abnormal, aggregated forms of the protein, α-synuclein. We have investigated the effects of calcium on α-synuclein aggregation in vitro and in vivo. We treated monomeric α-synuclein with calcium in vitro and used fluorescence imaging, fluorescence correlation and scanning electron microscopy to investigate protein aggregation. Incubation of fluorescent-labelled monomeric α-synuclein (24h) at low concentration (10 μM) with calcium resulted in surface aggregates (1.5±0.7 μm(2)) detected by fluorescence microscopy saturating at a half-maximum calcium concentration of 80 μM, whilst incubations without calcium showed few protein aggregates. Scanning electron microscopy revealed that α-synuclein surface plaques (0.5-1 μm) form in the presence of calcium and comprise 10-20 nm globular particles. Incubation of α-synuclein at high concentration (75 μM; 6h) resulted in soluble oligomeric aggregates detected by fluorescence correlation spectroscopy in a calcium dependent process, saturating at a half maximum calcium concentration of 180 μM. In cell culture experiments, we used thapsigargin or calcium ionophore A23187 to induce transient increases of intracellular free calcium in human 1321N1 cells expressing an α-synuclein-GFP construct and observed calcium flux and α-synuclein aggregation by fluorescence microscopy. The cell culture data shows that a transient increase in intracellular free calcium significantly increased the proportion of cells bearing cytoplasmic α-synuclein aggregates 6 and 12h post-treatment (P, 0.01). Our data indicates that calcium accelerates α-synuclein aggregation on surfaces, in free solution and in cultured cells and suggests that surface adsorption may play an important role in the calcium-dependent aggregation mechanism.

Gaucher disease and parkinsonism, a molecular link theory

Mutant GBA was found recently to be the most prevalent risk factor for familial parkinsonism. The two diseases do not share common symptoms and there is no direct pathway to explain the mechanism by which GBA mutations can confer the risk. Increased burden on the degradative pathway caused by defective glucocerebrosidase, or toxic side effects of glycosylated lipids accumulation were proposed to explain brain damage. Both hypotheses are not sufficient to explain the linkage. In order to develop a more inclusive theory we introduced into the model the prion theory and the second hit. Other possibilities are also brought into consideration.

The role of leucine-rich repeat kinase 2 (LRRK2) in Parkinson's disease

Parkinson's disease, like many common age-related conditions, is now recognized to have a substantial genetic component. Here, I discuss how mutations in a large complex gene--leucine-rich repeat kinase 2 (LRRK2)--affect protein function, and I review recent evidence that LRRK2 mutations affect pathways that involve other proteins that have been implicated in Parkinson's disease, specifically α-synuclein and tau. These concepts can be used to understand disease processes and to develop therapeutic opportunities for the treatment of Parkinson's disease.

Inhibiting α-synuclein oligomerization by stable cell-penetrating β-synuclein fragments recovers phenotype of Parkinson's disease model flies

The intracellular oligomerization of α-synuclein is associated with Parkinson's disease and appears to be an important target for disease-modifying treatment. Yet, to date, there is no specific inhibitor for this aggregation process. Using unbiased systematic peptide array analysis, we identified molecular interaction domains within the β-synuclein polypeptide that specifically binds α-synuclein. Adding such peptide fragments to α-synuclein significantly reduced both amyloid fibrils and soluble oligomer formation in vitro. A retro-inverso analogue of the best peptide inhibitor was designed to develop the identified molecular recognition module into a drug candidate. While this peptide shows indistinguishable activity as compared to the native peptide, it is stable in mouse serum and penetrates α-synuclein over-expressing cells. The interaction interface between the D-amino acid peptide and α-synuclein was mapped by Nuclear Magnetic Resonance spectroscopy. Finally, administering the retro-inverso peptide to a Drosophila model expressing mutant A53T α-synuclein in the nervous system, resulted in a significant recovery of the behavioral abnormalities of the treated flies and in a significant reduction in α-synuclein accumulation in the brains of the flies. The engineered retro-inverso peptide can serve as a lead for developing a novel class of therapeutic agents to treat Parkinson's disease.

Glial innate immunity generated by non-aggregated alpha-synuclein in mouse: differences between wild-type and Parkinson's disease-linked mutants

BACKGROUND

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized pathologically by the presence in the brain of intracellular protein inclusions highly enriched in aggregated alpha-synuclein (α-Syn). Although it has been established that progression of the disease is accompanied by sustained activation of microglia, the underlying molecules and factors involved in these immune-triggered mechanisms remain largely unexplored. Lately, accumulating evidence has shown the presence of extracellular α-Syn both in its aggregated and monomeric forms in cerebrospinal fluid and blood plasma. However, the effect of extracellular α-Syn on cellular activation and immune mediators, as well as the impact of familial PD-linked α-Syn mutants on this stimulation, are still largely unknown.

METHODS AND FINDINGS

In this work, we have compared the activation profiles of non-aggregated, extracellular wild-type and PD-linked mutant α-Syn variants on primary glial and microglial cell cultures. After stimulation of cells with α-Syn, we measured the release of Th1- and Th2- type cytokines as well as IP-10/CXCL10, RANTES/CCL5, MCP-1/CCL2 and MIP-1α/CCL3 chemokines. Contrary to what had been observed using cell lines or for the case of aggregated α-Syn, we found strong differences in the immune response generated by wild-type α-Syn and the familial PD mutants (A30P, E46K and A53T).

CONCLUSIONS

These findings might contribute to explain the differences in the onset and progression of this highly debilitating disease, which could be of value in the development of rational approaches towards effective control of immune responses that are associated with PD.

Alpha-synuclein suppression by targeted small interfering RNA in the primate substantia nigra

The protein α-synuclein is involved in the pathogenesis of Parkinson's disease and other neurodegenerative disorders. Its toxic potential appears to be enhanced by increased protein expression, providing a compelling rationale for therapeutic strategies aimed at reducing neuronal α-synuclein burden. Here, feasibility and safety of α-synuclein suppression were evaluated by treating monkeys with small interfering RNA (siRNA) directed against α-synuclein. The siRNA molecule was chemically modified to prevent degradation by exo- and endonucleases and directly infused into the left substantia nigra. Results compared levels of α-synuclein mRNA and protein in the infused (left) vs. untreated (right) hemisphere and revealed a significant 40–50% suppression of α-synuclein expression. These findings could not be attributable to non-specific effects of siRNA infusion since treatment of a separate set of animals with luciferase-targeting siRNA produced no changes in α-synuclein. Infusion with α-synuclein siRNA, while lowering α-synuclein expression, had no overt adverse consequences. In particular, it did not cause tissue inflammation and did not change (i) the number and phenotype of nigral dopaminergic neurons, and (ii) the concentrations of striatal dopamine and its metabolites. The data represent the first evidence of successful anti-α-synuclein intervention in the primate substantia nigra and support further development of RNA interference-based therapeutics.

Missing pieces in the Parkinson's disease puzzle

Parkinson's disease is a neurodegenerative process characterized by numerous motor and nonmotor clinical manifestations for which effective, mechanism-based treatments remain elusive. Here we discuss a series of critical issues that we think researchers need to address to stand a better chance of solving the different challenges posed by this pathology.

Alpha-synuclein sequesters arachidonic acid to modulate SNARE-mediated exocytosis

Alpha-synuclein is a synaptic modulatory protein implicated in the pathogenesis of Parkinson disease. The precise functions of this small cytosolic protein are still under investigation. alpha-Synuclein has been proposed to regulate soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins involved in vesicle fusion. Interestingly, alpha-synuclein fails to interact with SNARE proteins in conventional protein-binding assays, thus suggesting an indirect mode of action. As the structural and functional properties of both alpha-synuclein and the SNARE proteins can be modified by arachidonic acid, a common lipid regulator, we analysed this possible tripartite link in detail. Here, we show that the ability of arachidonic acid to stimulate SNARE complex formation and exocytosis can be controlled by alpha-synuclein, both in vitro and in vivo. Alpha-synuclein sequesters arachidonic acid and thereby blocks the activation of SNAREs. Our data provide mechanistic insights into the action of alpha-synuclein in the modulation of neurotransmission.

Lewy body pathology in fetal grafts

Although fetal nigral transplants have been shown to survive grafting into the striatum, increased [(18)F]6-fluroro-L-3,4-dihydroxyphenylalanine ((18)F-DOPA) uptake and improved motor function in open-label assessments have failed to establish any clinical benefits in double-blind, sham-controlled studies. To understand morphological and neurochemical alterations of grafted neurons, we performed postmortem analyses on six Parkinson's disease (PD) patients who had received fetal tissue transplantation 18-19 months, 4 years, and 14 years previously. These studies revealed robust neuronal survival with normal dopaminergic phenotypes in 18-month-old grafts and decreased dopamine transporter and increased cytoplasmic alpha-synuclein in 4-year-old grafts. We also found a decline of both dopamine transporter and tyrosine hydroxylase and the formation of Lewy body-like inclusions in 14-year-old grafts, which stained positive for alpha-synuclein and ubiquitin proteins. These pathological changes suggest that PD is an ongoing process that affects grafted cells in the striatum in a manner similar to how resident dopamine neurons are affected in the substantia nigra.

Upregulation of alpha-synuclein expression in the rat cerebellum in experimental hepatic encephalopathy

AIMS

The overexpression of alpha-synuclein has been associated with neurodegenerative diseases, especially when the protein aggregates to form insoluble structures. The present study examined the effect of chronic hyperammonaemia on alpha-synuclein expression in the rat cerebellum following portacaval anastomosis (PCA).

METHODS

Immunohistochemical and western blot determinations were performed 1 month and 6 months after the PCA procedure.

RESULTS

A time-dependent increase in alpha-synuclein expression was seen in the cerebellar grey matter compared with the controls. At 1 month post PCA, alpha-synuclein-immunopositive material was observed in the molecular layer, while the Purkinje cells showed weak alpha-synuclein expression, and alpha-synuclein aggregates were observed throughout the granular layer. At 6 months post PCA, alpha-synuclein expression was significantly increased compared with the controls. alpha-synuclein-immunostained astroglial cells were also found; the Bergmann glial cells showed alpha-synuclein-positive processes in the molecular layer of PCA-exposed rats, and in the granular layer, perivascular astrocytes showed intense alpha-synuclein immunoreactivity, as indicated by colocalization of alpha-synuclein with glial fibrillary acidic protein (GFAP). In addition, ubiquitin-immunoreactive inclusions were present in PCA-exposed rats, although they did not colocalize with alpha-synuclein. Western blotting performed at 6 months post PCA showed a reduction in the level of soluble alpha-synuclein compared with 1 month post PCA and the controls; this reduction was concomitant with an increase in the insoluble form of alpha-synuclein.

CONCLUSIONS

Although the precise mechanism by which alpha-synuclein aggregates in PCA-treated rats remains unknown, the present data suggest an important role for this protein in the onset and progression of hepatic encephalopathy, probably via its expression in astroglial cells.

Basic mechanisms of neurodegeneration: a critical update

Neurodegenerative diseases are characterized by progressive dysfunction of specific populations of neurons, determining clinical presentation. Neuronal loss is associated with extra and intracellular accumulation of misfolded proteins, the hallmarks of many neurodegenerative proteinopathies. Major basic processes include abnormal protein dynamics due to deficiency of the ubiquitin-proteosome-autophagy system, oxidative stress and free radical formation, mitochondrial dysfunction, impaired bioenergetics, dysfunction of neurotrophins, 'neuroinflammatory' processes and (secondary) disruptions of neuronal Golgi apparatus and axonal transport. These interrelated mechanisms lead to programmed cell death is a long run over many years. Neurodegenerative disorders are classified according to known genetic mechanisms or to major components of protein deposits, but recent studies showed both overlap and intraindividual diversities between different phenotypes. Synergistic mechanisms between pathological proteins suggest common pathogenic mechanisms. Animal models and other studies have provided insight into the basic neurodegeneration and cell death programs, offering new ways for future prevention/treatment strategies.