Alpha-synuclein and the Parkinson's disease-related mutant Ala53Thr-alpha-synuclein do not undergo proteasomal degradation in HEK293 and neuronal cells

Chronic Microcystin-LR-Induced α-Synuclein Promotes Neuroinflammation Through Activation of the NLRP3 Inflammasome in Microglia

Microcystin-LR (MC-LR) has been confirmed to cause blood-brain barrier disruption and enter the brain tissue, resulting in non-negligible toxic effects. However, the neurotoxicity of MC-LR is mainly unknown. This study revealed that MC-LR disrupted the function of the ubiquitin-proteasome system in neurons, which inhibited the degradation of α-synuclein (α-syn), leading to its release from neurons for transport into microglia. α-Syn is the main component of Lewy bodies, which has been identified as one of the main pathological features of Parkinson's disease (PD). In vitro, we observed that α-syn mediated by MC-LR activated HMC3 cells and polarized them towards M1 type. In addition, we confirmed that α-syn was transported into HMC3 cells through TLR4 receptors and activated the NLRP3 inflammasome, which in turn enhanced the maturation and release of IL-18 and IL-1β. In the mouse models of chronic MC-LR exposure, a large number of inflammatory factors (IL-6, IL-1β, and TNF-α) were deposited in brain tissue, and activation of NLRP3 in microglia was also observed in the midbrain. Collectively, MC-LR exposure promoted the pathological spread of α-syn from cell to cell, activated NLRP3 inflammasome in microglia, and generated neuroinflammation, in which the TLR4 receptor played a substantial effect.

Chaperon-mediated autophagy can regulate diquat-induced apoptosis by inhibiting α-synuclein accumulation cooperatively with macroautophagy

α-Synuclein, which is associated with Parkinson's disease, is cleared by the ubiquitin-proteasome system and autophagy lysosome system. Chaperon-mediated autophagy (CMA) and macroautophagy are major subtypes of autophagy and play a critical role in pesticide-induced α-synucleinopathy. In this study, we explored the role of CMA in diquat (DQ)-induced α-synucleinopathy and characterized the relationship between CMA and macroautophagy in the clearance of pathologic α-synuclein for the prevention of DQ neurotoxicity. DQ was cytotoxic to SH-SY5Y cells in a concentration-dependent manner, as shown by decreased cell viability and increased cytotoxicity. DQ treatment was also found to induce autophagy such as CMA and macroautophagy by monitoring the expression of Lamp2A and microtubule-associated protein 1A/1B light chain 3B (LC3-II) respectively. Following DQ treatment, SH-SY5Y cells were found to have induced phosphorylated and detergent-insoluble α-synuclein deposits, and MG132, a proteasome inhibitor, effectively potentiated both CMA and macroautophagy for preventing α-synuclein aggregation. Interestingly, CMA impairment by Lamp2A-knock down decreased the LC3II expression compared to in DQ-treated cells transfected with control siRNA. In Lamp2-knock down cells, pathologic α-synuclein was increased 12 h after DQ treatment, but there was no change observed at 24 h. In DQ-treated cells, macroautophagy by 3-methyladenine and bafilomycin inhibition increased Lamp2A expression, indicating an increase in CMA activity. In addition, CMA modulation affected apoptosis, and inhibiting lysosome activity by NH₄Cl increased apoptosis in DQ-treated cells. An increase in autophagy was confirmed to compensate for the decrease in lysosome activity. Pretreatment with z-VAD-fmk, a pan-caspase inhibitor, significantly enhanced the macroautophagy response of DQ-exposed cells without alterations in Lamp2A expression. Our results suggest that CMA can regulate DQ-induced α-synucleinopathy cooperatively with macroautophagy, and crosstalk between macroautophagy and CMA plays an important role in DQ-induced cytotoxicity. Taken together, autophagy modulation may be a useful treatment strategy in pesticide-induced neurodegenerative disorders through preventing α-synucleinopathy.

Hepcidin Decreases Rotenone-Induced α-Synuclein Accumulation via Autophagy in SH-SY5Y Cells

Parkinson’s disease (PD) is a neurodegenerative disorder, and the hallmarks of this disease include iron deposition and α-synuclein (α-syn) aggregation. Hepcidin could reduce iron in the central and peripheral nervous systems. Here, we hypothesized that hepcidin could further decrease α-syn accumulation via reducing iron. Therefore, rotenone or α-syn was introduced into human neuroblastoma SH-SY5Y cells to imitate the pathological progress of PD in vitro. This study investigated the clearance effects of hepcidin on α-syn induced by a relatively low concentration of rotenone exposure or α-syn overexpression to elucidate the potential clearance pathway involved in this process. We demonstrated that SH-SY5Y cell viability was impaired after rotenone treatment in a dose-dependent manner. α-syn expression and iron content increased under a low concentration rotenone (25 nM for 3 days) treatment in SH-SY5Y cells. Pre-treatment with hepcidin peptide suppressed the abovementioned effects of rotenone. However, hepcidin did not affect treatment with rotenone under high iron conditions. Hepcidin also played a role in reducing α-syn accumulation in rotenone and α-syn overexpression conditions. We identified that the probable clearance effect of hepcidin on α-syn was mediated by the autophagy pathway using pretreatment with autophagy inhibitors (3-MA and CQ) and detection of autophagy protein markers (LC3II/I and p62). In conclusion, hepcidin eliminated α-syn expression via the autophagy pathway in rotenone-treated and α-syn overexpression SH-SY5Y cells. This study highlights that hepcidin may offer a potential therapeutic perspective in α-syn accumulation diseases.

Pathways of protein synthesis and degradation in PD pathogenesis

Since the discovery of protein aggregates in the brains of individuals with Parkinson's disease (PD) in the early 20th century, the scientific community has been interested in the role of dysfunctional protein metabolism in PD etiology. Recent advances in the field have implicated defective protein handling underlying PD through genetic, in vitro, and in vivo studies incorporating many disease models alongside neuropathological evidence. Here, we discuss the existing body of research focused on understanding cellular pathways of protein synthesis and degradation, and how aberrations in either system could engender PD pathology with special attention to α-synuclein-related consequences. We consider transcription, translation, and post-translational modification to constitute protein synthesis, and protein degradation to encompass proteasome-, lysosome- and endoplasmic reticulum-dependent mechanisms. Novel findings connecting each of these steps in protein metabolism to development of PD indicate that deregulation of protein production and turnover remains an exciting area in PD research.

The unlikely partnership between LRRK2 and α-synuclein in Parkinson's disease

Our understanding of the mechanisms underlying Parkinson's disease, the once archetypical nongenetic neurogenerative disorder, has dramatically increased with the identification of α-synuclein and LRRK2 pathogenic mutations. While α-synuclein protein composes the aggregates that can spread through much of the brain in disease, LRRK2 encodes a multidomain dual-enzyme distinct from any other protein linked to neurodegeneration. In this review, we discuss emergent datasets from multiple model systems that suggest these unlikely partners do interact in important ways in disease, both within cells that express both LRRK2 and α-synuclein as well as through more indirect pathways that might involve neuroinflammation. Although the link between LRRK2 and disease can be understood in part through LRRK2 kinase activity (phosphotransferase activity), α-synuclein toxicity is multilayered and plausibly interacts with LRRK2 kinase activity in several ways. We discuss common protein interactors like 14-3-3s that may regulate α-synuclein and LRRK2 in disease. Finally, we examine cellular pathways and outcomes common to both mutant α-synuclein expression and LRRK2 activity and points of intersection. Understanding the interplay between these two unlikely partners in disease may provide new therapeutic avenues for PD.

Exosomal secretion of α-synuclein as protective mechanism after upstream blockage of macroautophagy

Accumulation of pathological α-synuclein aggregates plays a major role in Parkinson’s disease. Macroautophagy is a mechanism to degrade intracellular protein aggregates by wrapping them into autophagosomes, followed by fusion with lysosomes. We had previously shown that pharmacological activation of macroautophagy protects against α-synuclein-induced toxicity in human neurons. Here, we hypothesized that inhibition of macroautophagy would aggravate α-synuclein-induced cell death.

Unexpectedly, inhibition of autophagosome formation by silencing of ATG5 protected from α-synuclein-induced toxicity. Therefore, we studied alternative cellular mechanisms to compensate for the loss of macroautophagy. ATG5 silencing did not affect the ubiquitin–proteasome system, chaperone systems, chaperone-mediated autophagy, or the unfolded protein response. However, ATG5 silencing increased the secretion of α-synuclein via exosomes. Blocking exosomal secretion exacerbated α-synuclein-induced cell death.

We conclude that exosomal secretion of α-synuclein is increased after impaired formation of autophagosomes to reduce the intracellular α-synuclein burden. This compensatory mechanism prevents α-synuclein-induced neuronal cell death.

Alzheimer's disease and the autophagic-lysosomal system

Age-related neurodegenerative diseases are of critical concern to the general population and research/medical community due to their health impact and socioeconomic consequences. A feature of most, if not all, neurodegenerative disorders is the presence of proteinopathies, in which misfolded or conformationally altered proteins drive disease progression and are often used as a primary neuropathological marker of disease. In particular, Alzheimer's disease (AD) is characterized by abnormal accumulation of protein aggregates, primarily extracellular plaques composed of the Aβ peptide and intracellular tangles comprised of the tau protein, both of which may indicate a primary defect in protein clearance. Protein degradation is a key cellular mechanism for protein homeostasis and is essential for cell survival but is disrupted in neurodegenerative diseases. Dysregulation in proteolytic pathways - mainly the autophagic-lysosomal system (A-LS) and the ubiquitin-proteasome system (UPS) - has been increasingly associated with proteinopathies in neurodegenerative diseases. Here we review the role of dysfunctional autophagy underlying AD-related proteinopathy and discuss how to model this aspect of disease, as well as summarize recent advances in translational strategies for targeted A-LS dysfunction in AD.

The Proteasome Inhibition Model of Parkinson's Disease

The pathological hallmarks of Parkinson's disease are the progressive loss of nigral dopaminergic neurons and the formation of intracellular inclusion bodies, termed Lewy bodies, in surviving neurons. Accumulation of proteins in large insoluble cytoplasmic aggregates has been proposed to result, partly, from a failure in the function of intracellular protein degradation pathways. Evidence in support for such a hypothesis emerged in the beginning of the years 2000 with studies demonstrating structural and functional deficits in the ubiquitin-proteasome pathway in post-mortem nigral tissue of patients with Parkinson's disease. These fundamental findings have inspired the development of a new generation of animal models based on the use of proteasome inhibitors to disturb protein homeostasis and trigger nigral dopaminergic neurodegeneration. In this review, we provide an updated overview of the current approaches in employing proteasome inhibitors to model Parkinson's disease, with particular emphasis on rodent studies. In addition, the mechanisms underlying proteasome inhibition-induced cell death and the validity criteria (construct, face and predictive validity) of the model will be critically discussed. Due to its distinct, but highly relevant mechanism of inducing neuronal death, the proteasome inhibition model represents a useful addition to the repertoire of toxin-based models of Parkinson's disease that might provide novel clues to unravel the complex pathogenesis of this disorder.

Altered Expression Patterns of Inflammation-Associated and Trophic Molecules in Substantia Nigra and Striatum Brain Samples from Parkinson's Disease, Incidental Lewy Body Disease and Normal Control Cases

Evidence of inflammation has been consistently associated with pathology in Parkinson's disease (PD)-affected brains, and has been suggested as a causative factor. Dopaminergic neurons in the substantia nigra (SN) pars compacta, whose loss results in the clinical symptoms associated with PD, are particularly susceptible to inflammatory damage and oxidative stress. Inflammation in the striatum, where SN dopaminergic neurons project, is also a feature of PD brains. It is not known whether inflammatory changes occur first in striatum or SN. Many animal models of PD have implicated certain inflammatory molecules with dopaminergic cell neuronal loss; however, there have been few studies to validate these findings by measuring the levels of these and other inflammatory factors in human PD brain samples. This study also included samples from incidental Lewy body disease (ILBD) cases, since ILBD is considered a non-symptomatic precursor to PD, with subjects having significant loss of tyrosine hydroxylase-producing neurons. We hypothesized that there may be a progressive change in key inflammatory factors in ILBD samples intermediate between neurologically normal and PD. To address this, we used a quantitative antibody-array platform (Raybiotech-Quantibody arrays) to measure the levels of 160 different inflammation-associated cytokines, chemokines, growth factors, and related molecules in extracts of SN and striatum from clinically and neuropathologically characterized PD, ILBD, and normal control cases. Patterns of changes in inflammation and related molecules were distinctly different between SN and striatum. Our results showed significantly different levels of interleukin (IL)-5, IL-15, monokine induced by gamma interferon, and IL-6 soluble receptor in SN between disease groups. A different panel of 13 proteins with significant changes in striatum, with IL-15 as the common feature, was identified. Although the ability to detect some proteins was limited by sensitivity, patterns of expression indicated involvement of certain T-cell cytokines, vascular changes, and loss of certain growth factors, with disease progression. The results demonstrate the feasibility of profiling inflammatory molecules using diseased human brain samples, and have provided additional targets to validate in relation to PD pathology.

Phosphorylation modulates clearance of alpha-synuclein inclusions in a yeast model of Parkinson's disease

Alpha-synuclein (aSyn) is the main component of proteinaceous inclusions known as Lewy bodies (LBs), the typical pathological hallmark of Parkinson's disease (PD) and other synucleinopathies. Although aSyn is phosphorylated at low levels under physiological conditions, it is estimated that ∼90% of aSyn in LBs is phosphorylated at S129 (pS129). Nevertheless, the significance of pS129 in the biology of aSyn and in PD pathogenesis is still controversial. Here, we harnessed the power of budding yeast in order to assess the implications of phosphorylation on aSyn cytotoxicity, aggregation and sub-cellular distribution. We found that aSyn is phosphorylated on S129 by endogenous kinases. Interestingly, phosphorylation reduced aSyn toxicity and the percentage of cells with cytosolic inclusions, in comparison to cells expressing mutant forms of aSyn (S129A or S129G) that mimic the unphosphorylated form of aSyn. Using high-resolution 4D imaging and fluorescence recovery after photobleaching (FRAP) in live cells, we compared the dynamics of WT and S129A mutant aSyn. While WT aSyn inclusions were very homogeneous, inclusions formed by S129A aSyn were larger and showed FRAP heterogeneity. Upon blockade of aSyn expression, cells were able to clear the inclusions formed by WT aSyn. However, this process was much slower for the inclusions formed by S129A aSyn. Interestingly, whereas the accumulation of WT aSyn led to a marked induction of autophagy, cells expressing the S129A mutant failed to activate this protein quality control pathway. The finding that the phosphorylation state of aSyn on S129 can alter the ability of cells to clear aSyn inclusions provides important insight into the role that this posttranslational modification may have in the pathogenesis of PD and other synucleinopathies, opening novel avenues for investigating the molecular basis of these disorders and for the development of therapeutic strategies.

Protein aggregation is a common hallmark in neurodegenerative disorders, but is also associated with phenotypic plasticity in a variety of organisms, including yeasts. Alpha-synuclein (aSyn) forms aggregates that are typical of synucleinopathies, and is phosphorylated at S129, but the significance of phosphorylation in the biology and pathophysiology of the protein is still controversial. Exploring the power of budding yeast, we found phosphorylation reduced aSyn toxicity and inclusion formation. While inclusions formed by WT aSyn were homogeneous, those formed by S129A aSyn were larger and heterogeneous. Interestingly, clearance of aSyn inclusions was reduced in cells expressing S129A aSyn, correlating with deficient autophagy activation. The finding that phosphorylation alters the ability of cells to clear aSyn inclusions provides novel insight into the role phosphorylation may have in synucleinopathies, and suggests posttranslational modifications might constitute switches cells use to control the aggregation and clearance of key proteins, opening novel avenues for the development of therapeutic strategies for these devastating disorders.

Linking alpha-synuclein properties with oxidation: a hypothesis on a mechanism underling cellular aggregation

α-Synuclein is a small, natively unstructured protein with propensity to aggregate. α-Synuclein fibrils are major components of Lewy bodies that are hallmarks of many neurodegenerative diseases. The solution properties and aggregation behavior of α-synuclein has been well characterized, but despite numerous studies that address the role of α-synuclein in cells, a clear physiological function of this protein remains a mystery. Over a hundred review articles of α-synuclein have been written in the last decade, making it difficult to list all of the important studies that have added to our insight of α-synuclein physiology. Instead, we briefly review the status of α-synuclein research and propose a model based on the idea that α-synuclein may not have an intrinsic activity in cells but rather, it modifies the function of a group of protein partners that in turn affect cell processes. We propose that it is the loss of its cellular partners under oxidative conditions that promotes α-synuclein aggregation accelerating neuronal death.

A loss in cellular protein partners promotes α-synuclein aggregation in cells resulting from oxidative stress

There is a consensus that oxidative stress promotes neurodegeneration and may be linked to plaque formation. α-Synuclein is the main component of neurodegenerative plaques. We have found that α-synuclein binds strongly to the enzyme phospholipase Cβ1 (PLCβ1) in vitro and in cells affecting both its G protein activation and its degradation. Because PLCβ1 binds to α-synuclein in cells, we tested whether decreasing its level would promote α-synuclein aggregation and whether overproducing PLCβ1 would inhibit aggregation. By imaging fluorescent α-synuclein in living HEK293, PC12, and SK-H-SH cells, we find that α-synuclein aggregation is directly related to the level of PLCβ1. Importantly, we found that oxidative stress does not affect the cellular levels of α-synuclein but results in the down-regulation of PLCβ1 thereby promoting α-synuclein aggregation. A peptide that mimics part of the α-synuclein binding site to PLCβ prevents aggregation. Our studies indicate that PLCβ1 can reduce cell damage under oxidative stress and offers a potential site that might be exploited to prevent α-synuclein aggregation.

Crosstalk between the proteasome system and autophagy in the clearance of α-synuclein

AIM

A growing body of evidence suggests that α-synuclein accumulation may play an important role in the pathogenesis of Parkinson's disease. The aim of this study was to investigate the roles of the proteasome and autophagy pathways in the clearance of wild-type and mutant α-synuclein in PC12 cells.

METHODS

PC12 cells overexpressing either wild-type or A30P mutant α-synuclein were treated with the proteasome inhibitor epoxomicin, the macroautophagy inhibitor 3-MA and the macroautophagy activator rapamycin alone or in combination. The cell viability was assessed using MTT assay. Immunofluorescence and Western blot analysis were used to detect the level of α-synuclein, LAMP-2A, E1 activase, and E2 ligase in the cells. Chymotrypsin-like proteasomal activity was measured using a commercial kit.

RESULTS

When the proteasome and macroautophagy in the wild-type and mutant cells were inhibited with epoxomicin and 3-MA, respectively, the cell viability was significantly decreased, and the α-synuclein level was increased. Both epoxomicin and 3-MA activated the chaperone-mediated autophagy (CMA) by increasing the level of the CMA-limiting enzyme LAMP-2A. Furthermore, 3-MA or epoxomicin significantly decreased chymotrypsin-like proteasomal activity. 3-MA or epoxomicin did not change E1 activase expression in either mutant or wild-type cells, but increased E2 ligase expression, especially when used together. Macroautophagy inducer rapamycin increased the cell viability and reduced epoxomicin-induced α-synuclein accumulation. Interestingly, CMA was also activated by rapamycin.

CONCLUSION

Our results demonstrate the existence of complex crosstalk between different forms of autophagy and between autophagy and the proteasome pathway in the clearance of α-synuclein in PC12 cells.

α-Synuclein and protein degradation systems: a reciprocal relationship

An increasing wealth of data indicates a close relationship between the presynaptic protein alpha-synuclein and Parkinson's disease (PD) pathogenesis. Alpha-synuclein protein levels are considered as a major determinant of its neurotoxic potential, whereas secreted extracellular alpha-synuclein has emerged as an additional important factor in this regard. However, the manner of alpha-synuclein degradation in neurons remains contentious. Both the ubiquitin-proteasome system (UPS) and the autophagy-lysosome pathway (ALP)-mainly macroautophagy and chaperone-mediated autophagy-have been suggested to contribute to alpha-synuclein turnover. Additionally, other proteases such as calpains, neurosin, and metalloproteinases have been also proposed to have a role in intracellular and extracellular alpha-synuclein processing. Both UPS and ALP activity decline with aging and such decline may play a pivotal role in many neurodegenerative conditions. Alterations in these major proteolytic pathways may result in alpha-synuclein accumulation due to impaired clearance. Conversely, increased alpha-synuclein protein burden promotes the generation of aberrant species that may impair further UPS or ALP function, generating thus a bidirectional positive feedback loop leading to neuronal death. In the current review, we summarize the recent findings related to alpha-synuclein degradation, as well as to alpha-synuclein-mediated aberrant effects on protein degradation systems. Identifying the factors that regulate alpha-synuclein association to cellular proteolytic pathways may represent potential targets for therapeutic interventions in PD and related synucleinopathies.

Protein degradation pathways in Parkinson's disease: curse or blessing

Protein misfolding, aggregation and deposition are common disease mechanisms in many neurodegenerative diseases including Parkinson's disease (PD). Accumulation of damaged or abnormally modified proteins may lead to perturbed cellular function and eventually to cell death. Thus, neurons rely on elaborated pathways of protein quality control and removal to maintain intracellular protein homeostasis. Molecular chaperones, the ubiquitin-proteasome system (UPS) and the autophagy-lysosomal pathway (ALP) are critical pathways that mediate the refolding or removal of abnormal proteins. The successive failure of these protein degradation pathways, as a cause or consequence of early pathological alterations in vulnerable neurons at risk, may present a key step in the pathological cascade that leads to spreading neurodegeneration. A growing number of studies in disease models and patients have implicated dysfunction of the UPS and ALP in the pathogenesis of Parkinson's disease and related disorders. Deciphering the exact mechanism by which the different proteolytic systems contribute to the elimination of pathogenic proteins, like α-synuclein, is therefore of paramount importance. We herein review the role of protein degradation pathways in Parkinson's disease and elaborate on the different contributions of the UPS and the ALP to the clearance of altered proteins. We examine the interplay between different degradation pathways and provide a model for the role of the UPS and ALP in the evolution and progression of α-synuclein pathology. With regards to exciting recent studies we also discuss the putative potential of using protein degradation pathways as novel therapeutic targets in Parkinson's disease.

Aggregate clearance of α-synuclein in Saccharomyces cerevisiae depends more on autophagosome and vacuole function than on the proteasome

Parkinson disease is the second most common neurodegenerative disease. The molecular hallmark is the accumulation of proteinaceous inclusions termed Lewy bodies containing misfolded and aggregated α-synuclein. The molecular mechanism of clearance of α-synuclein aggregates was addressed using the bakers' yeast Saccharomyces cerevisiae as the model. Overexpression of wild type α-synuclein or the genetic variant A53T integrated into one genomic locus resulted in a gene copy-dependent manner in cytoplasmic proteinaceous inclusions reminiscent of the pathogenesis of the disease. In contrast, overexpression of the genetic variant A30P resulted only in transient aggregation, whereas the designer mutant A30P/A36P/A76P neither caused aggregation nor impaired yeast growth. The α-synuclein accumulation can be cleared after promoter shut-off by a combination of autophagy and vacuolar protein degradation. Whereas the proteasomal inhibitor MG-132 did not significantly inhibit aggregate clearance, treatment with phenylmethylsulfonyl fluoride, an inhibitor of vacuolar proteases, resulted in significant reduction in clearance. Consistently, a cim3-1 yeast mutant restricted in the 19 S proteasome regulatory subunit was unaffected in clearance, whereas an Δatg1 yeast mutant deficient in autophagy showed a delayed aggregate clearance response. A cim3-1Δatg1 double mutant was still able to clear aggregates, suggesting additional cellular mechanisms for α-synuclein clearance. Our data provide insight into the mechanisms yeast cells use for clearing different species of α-synuclein and demonstrate a higher contribution of the autophagy/vacuole than the proteasome system. This contributes to the understanding of how cells can cope with toxic and/or aggregated proteins and may ultimately enable the development of novel strategies for therapeutic intervention.

The ubiquitin proteasome system in neurodegenerative diseases: culprit, accomplice or victim?

A shared hallmark for many neurodegenerative disorders is the accumulation of toxic protein species which is assumed to be the cause for these diseases. Since the ubiquitin proteasome system (UPS) is the most important pathway for selective protein degradation it is likely that it is involved in the aetiology neurodegenerative disorders. Indeed, impairment of the UPS has been reported to occur during neurodegeneration. Although accumulation of toxic protein species (amyloid β) are in turn known to impair the UPS the relationship is not necessarily causal. We provide an overview of the most recent insights in the roles the UPS plays in protein degradation and other processes. Additionally, we discuss the role of the UPS in clearance of the toxic proteins known to accumulate in the hallmarks of neurodegenerative diseases. The present paper will focus on critically reviewing the involvement of the UPS in specific neurodegenerative diseases and will discuss if UPS impairment is a cause, a consequence or both of the disease.

Molecular insights into Parkinson's disease

Mutations in SNCA, PINK1, parkin, and DJ-1 are associated with autosomal-dominant or autosomal-recessive forms of Parkinson's disease (PD), the second most common neurodegenerative disorder. Studies on the structural and functional properties of the corresponding gene products have provided significant insights into the molecular underpinnings of familial PD and the much more common sporadic forms of the disease. Here, we review recent advances in our understanding of four PD-related gene products: α-synuclein, parkin, PINK1, and DJ-1. In Part 1, we review new insights into the role of α-synuclein in PD. In Part 2, we summarize the latest developments in understanding the role of mitochondrial dysfunction in PD, emphasizing the role of the PINK1/parkin pathway in regulating mitochondrial dynamics and mitophagy. The role of DJ-1 is also discussed. In Part 3, we point out converging pathways and future directions.

Distinct roles in vivo for the ubiquitin-proteasome system and the autophagy-lysosomal pathway in the degradation of α-synuclein

Increased intracellular levels of α-synuclein are implicated in Parkinson's disease and related disorders and may be caused by alterations in the ubiquitin-proteasome system (UPS) or the autophagy-lysosomal pathway (ALP). A critical question remains how α-synuclein is degraded by neurons in vivo. To address this, our study uses α-synuclein transgenic mice, expressing human α-synuclein or α-synuclein-eGFP under the (h)PDGF-β promoter, in combination with in vivo pharmacologic and multiphoton imaging strategies to systematically test degradation pathways in the living mouse brain. We demonstrate that the UPS is the main degradation pathway for α-synuclein under normal conditions in vivo while with increased α-synuclein burden the ALP is recruited. Moreover, we report alterations of the UPS in α-synuclein transgenic mice and age dependence to the role of the UPS in α-synuclein degradation. In addition, we provide evidence that the UPS and ALP might be functionally connected such that impairment of one can upregulate the other. These results provide a novel link between the UPS, the ALP, and α-synuclein pathology and may have important implications for future therapeutics targeting degradation pathways.

Molecular chaperones and associated cellular clearance mechanisms against toxic protein conformers in Parkinson's disease

Parkinson's disease (PD) is a slowly progressive neurodegenerative disorder marked by the loss of dopaminergic neurons (in particular in the substantia nigra) causing severe impairment of movement coordination and locomotion, associated with the accumulation of aggregated α-synuclein (α-Syn) into proteinaceous inclusions named Lewy bodies. Various early forms of misfolded α-Syn oligomers are cytotoxic. Their formation is favored by mutations and external factors, such as heavy metals, pesticides, trauma-related oxidative stress and heat shock. Here, we discuss the role of several complementing cellular defense mechanisms that may counteract PD pathogenesis, especially in youth, and whose effectiveness decreases with age. Particular emphasis is given to the 'holdase' and 'unfoldase' molecular chaperones that provide cells with potent means to neutralize and scavenge toxic protein conformers. Because chaperones can specifically recognize misfolded proteins, they are key specificity factors for other cellular defenses, such as proteolysis by the proteasome and autophagy. The efficiency of the cellular defenses decreases in stressed or aging neurons, leading to neuroinflammation, apoptosis and tissue loss. Thus, drugs that can upregulate the molecular chaperones, the ubiquitin-proteasome system and autophagy in brain tissues are promising avenues for therapies against PD and other mutation-, stress- or age-dependent protein-misfolding diseases.

Studying protein degradation pathways in vivo using a cranial window-based approach

Understanding how specific proteins are degraded by neurons in living animals is a fundamental question with relevance to many neurodegenerative diseases. Dysfunction in the ubiquitin-proteasome system (UPS) specifically has been implicated in several important neurodegenerative diseases including Alzheimer's Disease, Parkinson's Disease, and amyotrophic lateral sclerosis. Research in this area has been limited by the fact that many inhibitors of the UPS given systemically do not cross the blood-brain barrier (BBB) in appreciable levels. This limits the ability to easily test in vivo specific hypotheses generated in reduced systems, like brain slice or dissociated cell culture, about whether the UPS may degrade a particular protein of interest. Although several techniques including intracerebral application via direct syringe injection, catheter-pump systems and drug-eluting beads are available to introduce BBB-impermeant drugs into brain they each have certain limitations and new approaches could provide further insights into this problem. In order to test the role of the UPS in protein degradation in vivo we have developed a strategy to treat mouse cortex with the UPS inhibitor clasto-lactacystin beta-lactone (CLBL) via a "cranial window" and recover the treated tissue for immunoblot analysis. This approach can be used in several different cranial window configurations including single window and double hemi-window arrangements that are tailored for different applications. We have also developed two different strategies for recovering treated cortical tissue including a vibratome/laser capture microscopy (LCM)-based and a vibratome only-based approach, each with its own specific advantages. We have documented UPS inhibition >600μm deep into the cortex with this strategy. This set of techniques in the living mammalian brain is complementary to previously developed approaches and extends the repertoire of tools that can be used to the study protein degradation pathways relevant to neurodegenerative disease.

Phosphorylated alpha-synuclein at Ser-129 is targeted to the proteasome pathway in a ubiquitin-independent manner

α-Synuclein (a-Syn) is a major component of fibrillar aggregates in Lewy bodies (LBs), a characteristic hallmark of Parkinson disease. Almost 90% of a-Syn deposited in LBs is phosphorylated at Ser-129. However, the role of Ser-129-phosphorylated a-Syn in the biogenesis of LBs remains unclear. Here, we investigated the metabolism of Ser-129-phosphorylated a-Syn. In SH-SY5Y cells, inhibition of protein phosphatase 2A/1 by okadaic acid, and inhibition of the proteasome pathway by MG132 or lactacystin accumulated Ser-129-phosphorylated a-Syn. However, these inhibitions did not alter the amounts of total a-Syn within the observation time. Inhibition of the autophagy-lysosome pathway by 3-methyladenine or chloroquine accumulated Ser-129-phosphorylated a-Syn in parallel to total a-Syn during longer incubations. Experiments using cycloheximide showed that Ser-129-phosphorylated a-Syn diminished rapidly (t(½) = 54.9 ± 6.4 min), in contrast to the stably expressed total a-Syn. The short half-life of Ser-129-phosphorylated a-Syn was blocked by MG132 to a greater extent than okadaic acid. In rat primary cortical neurons, either MG132, lactacystin, or okadaic acid accumulated Ser-129-phosphorylated a-Syn. Additionally, we did not find that phosphorylated a-Syn was ubiquitinated in the presence of proteasome inhibitors. These data show that Ser-129-phosphorylated a-Syn is targeted to the proteasome pathway in a ubiquitin-independent manner, in addition to undergoing dephosphorylation. The proteasome pathway may play a role in the biogenesis of Ser-129-phosphorylated a-Syn-rich LBs.

The Relationship between Parkin and Protein Aggregation in Neurodegenerative Diseases

The most prominent changes in neurodegenerative diseases are protein accumulation and inclusion formation. Several neurodegenerative diseases, including Alzheimer's, the Synucleinopathies and Tauopathies share several overlapping clinical symptoms manifest in Parkinsonism, cognitive decline and dementia. As degeneration progresses in the disease process, clinical symptoms suggest convergent pathological pathways. Biochemically, protein cleavage, ubiquitination and phosphorylation seem to play fundamental roles in protein aggregation, inclusion formation and inflammatory responses. In the following we provide a synopsis of the current knowledge about protein accumulation and astrogliosis as a common denominator in neurodegenerative diseases, and we propose insights into protein degradation and anti-inflammation. We review the E3-ubiquitin ligase and other possible functions of parkin as a suppressant of inflammatory signs and a strategy to clear amyloid proteins in neurodegenerative diseases.

A critical evaluation of the ubiquitin-proteasome system in Parkinson's disease

The evidence for impairment in the ubiquitin proteasome system (UPS) in Parkinson's disease (PD) is mounting and becoming increasingly more convincing. However, it is presently unclear whether UPS dysfunction is a cause or result of PD pathology, a crucial distinction which impedes both the understanding of disease pathogenesis and the development of effectual therapeutic approaches. Recent findings discussed within this review offer new insight and provide direction for future research to conclusively resolve this debate.

Neurodegeneration in Parkinson's disease: genetics enlightens physiopathology

Parkinson's disease (PD) is a severe neurodegenerative disorder of complex etiology and enigmatic physiopathology. In the past decade, the identification of genes involved in rare familial Parkinsonian syndromes has brought hope that understanding the functions of their products will provide insight into the molecular mechanisms responsible for neurodegeneration. The knowledge accumulated thus far has delineated two putative, potentially interconnected, disease-causing pathways: alpha-synuclein accumulation may be central to Parkinsonism due to alpha-synuclein gene defects, but possibly also to sporadic PD and other genetic forms presenting with Lewy bodies; altered mitochondrial physiology may be pivotal to Parkinsonian syndromes caused by parkin, PINK1, and possibly DJ-1 gene mutations. Adding new pieces to this fragmentary picture to determine to what extent sporadic PD and Parkinsonism due to distinct genetic causes share common pathogenic mechanisms remains a major challenge toward the development of future therapeutic strategies for these disabling disorders.

Controlling the mass action of alpha-synuclein in Parkinson's disease

Parkinson's disease (PD) is an age-related neurodegenerative disease with unknown etiology. Growing evidence from genetic, pathologic, animal modeling, and biochemical studies strongly support the theory that abnormal aggregation of alpha-synuclein plays a critical role in the pathogenesis of PD. Protein aggregation is an alternative folding process that competes with the native folding pathway. Whether or not a protein is subject to the aggregation process is determined by the concentration of the protein as well as thermodynamic properties inherent to each polypeptide. An increase in cellular concentration of alpha-synuclein has been associated with the disease in both familial and sporadic forms of PD. Thus, maintenance of the intraneuronal steady state levels of alpha-synuclein below the critical concentration is a key challenge neuronal cells are facing. Expression of the alpha-synuclein gene is under the control of environmental factors and aging, the two best-established risk factors for PD. Studies also suggest that the degradation of this protein is mediated by proteasomal and autophagic pathways, which are two mechanisms that are related to the pathogenesis of PD. Recently, vesicle-mediated exocytosis has been suggested as a novel mechanism for disposal of neuronal alpha-synuclein. Relocalization of the protein to specific compartments may be another method for increasing its local concentration. Regulation of the neuronal steady state levels of alpha-synuclein has significant implications in the development of PD, and understanding the mechanism may disclose potential therapeutic targets for PD and other related diseases.

Wild type alpha-synuclein is degraded by chaperone-mediated autophagy and macroautophagy in neuronal cells

Alpha-synuclein (ASYN) is crucial in Parkinson disease (PD) pathogenesis. Increased levels of wild type (WT) ASYN expression are sufficient to cause PD in humans. The manner of post-transcriptional regulation of ASYN levels is controversial. Previously, we had shown that WT ASYN can be degraded by chaperone-mediated autophagy (CMA) in isolated liver lysosomes. Whether this occurs in a cellular and, in particular, in a neuronal cell context is unclear. Using a mutant ASYN form that lacks the CMA recognition motif and RNA interference against the rate-limiting step in the CMA pathway, Lamp2a, we show here that CMA is indeed involved in WT ASYN degradation in PC12 and SH-SY5Y cells, and in primary cortical and midbrain neurons. However, the extent of involvement varies between cell types, potentially because of differences in compensatory mechanisms. CMA inhibition leads to an accumulation of soluble high molecular weight and detergent-insoluble species of ASYN, suggesting that CMA dysfunction may play a role in the generation of such aberrant species in PD. ASYN and Lamp2a are developmentally regulated in parallel in cortical neuron cultures and in vivo in the central nervous system, and they physically interact as indicated by co-immunoprecipitation. In contrast to previous reports, inhibition of macroautophagy, but not the proteasome, also leads to WT ASYN accumulation, suggesting that this lysosomal pathway is also involved in normal ASYN turnover. These results indicate that CMA and macroautophagy are important pathways for WT ASYN degradation in neurons and underline the importance of CMA as degradation machinery in the nervous system.

Cell-produced alpha-synuclein oligomers are targeted to, and impair, the 26S proteasome

Proteasomal dysfunction may play a role in neurodegenerative conditions and protein aggregation. Overexpression in neuronal cells of alpha-synuclein, a molecule linked to Parkinson's Disease, may lead to proteasomal dysfunction. Using PC12 cells stably expressing wild-type or mutant alpha-synuclein and gel filtration, we demonstrate that soluble, intermediate size oligomers of alpha-synuclein co-elute with the 26S proteasome. These soluble oligomers associate with the 26S proteasome and are significantly increased following treatment with proteasomal, but not lysosomal, inhibitors, indicating specific degradation of these particular species by the 26S proteasome. Importantly, expression of alpha-synuclein resulted in a significant inhibition of all proteasomal activities without affecting the levels or assembly of the 26S proteasome. Pharmacological dissociation of alpha-synuclein oligomers restored proteasomal function and reduced polyubiquitinated protein load in intact cells. Our findings suggest a model where only a subset of specific soluble cell-derived alpha-synuclein oligomers is targeted to the 26S proteasome for degradation, and simultaneously inhibit its function, likely by impeding access of other proteasomal substrates.

Cleavage of normal and pathological forms of alpha-synuclein by neurosin in vitro

Neurosin is one of the serine proteases predominantly expressed in the central nervous system. Neurosin is presumed to play an important role in the degradation of alpha-synuclein (alpha-syn), since a previous study showed that neurosin degrades alpha-syn, inhibits polymerization of alpha-syn in vitro, and exists in Lewy bodies. However, the details of alpha-syn degradation by neurosin are little known. We investigated neurosin-mediated cleavage of alpha-syn by immunoblotting and liquid chromatography-ion trap mass spectrometry (LC/MS/MS). We also compared alpha-syn degradation by neurosin between phosphorylated and non-phosphorylated forms of alpha-syn, and between mutant and wild-type alpha-syn. Neurosin cleaved alpha-syn at specific sites. The major cleavage site was localized between Lys80 and Thr81 within the NAC region (E61 to V95), which is important for alpha-syn aggregation, and accordingly may preclude alpha-syn polymerization. Meanwhile, alternative, minor forms of processing also occur. They conserve the NAC region with truncation of the C-terminal region, and accordingly may contribute to alpha-syn polymerization. Phosphorylated alpha-syn was more resistant to degradation by neurosin than non-phosphorylated alpha-syn. The A30P mutant was more resistant to degradation than the wild-type and other alpha-syn mutants. This resistance to neurosin-mediated degradation of phosphorylated alpha-syn and the A30P mutant, which are, respectively, posttranslational and genetic factors related to the development of Parkinson's disease (PD), provides supporting evidence that neurosin is involved in the pathogenesis of PD.

Calpain-cleavage of alpha-synuclein: connecting proteolytic processing to disease-linked aggregation

Parkinson's disease (PD) and dementia with Lewy bodies (DLB) are both characterized pathologically by the presence of neuronal inclusions termed Lewy bodies (LBs). A common feature found in LBs are aggregates of alpha-synuclein (alpha-Syn), and although it is now recognized that alpha-Syn is the major building block for these toxic filaments, the mechanism of how this occurs remains unknown. In the present study, we demonstrate that proteolytic processing of alpha-Syn by the protease calpain I leads to the formation of aggregated high-molecular weight species and adoption of a beta-sheet structure. To determine whether calpain-cleavage of alpha-Syn occurs in PD and DLB, we designed site-directed calpain-cleavage antibodies to alpha-Syn and tested their utility in several animal model systems. Detection of calpain-cleaved alpha-Syn was evident in mouse models of cerebral ischemia and PD and in a Drosophila model of PD. In the human PD and DLB brain, calpain-cleaved alpha-Syn antibodies immunolabeled LBs and neurites in the substantia nigra. Moreover, calpain-cleaved alpha-Syn fragments identified within LBs colocalized with activated calpain in neurons of the PD and DLB brains. These findings suggest that calpain I may participate in the disease-linked aggregation of alpha-Syn in various alpha-synucleinopathies.

Synuclein activates microglia in a model of Parkinson's disease

Parkinson's disease (PD) is the most common neurodegenerative movement disorder afflicting >500,000 patients in the United States alone. This age-related progressive disorder is typified by invariant loss of dopaminergic substantia nigra neurons (DAN), dystrophic neurites, the presence of alpha-synuclein (SYN) positive intracytoplasmic inclusions (Lewy bodies) in the remaining DAN, and activated microglia. As such, microglial activation and resultant increase in proinflammatory molecules have moved to the forefront of PD research as a potential pathobiologic mechanism of disease. Herein, we present data demonstrating early microglial activation in mice that over-express wild-type SYN, the release of SYN from a SYN overexpressing MN9D cell line, and dose-dependent SYN-mediated activation of primary microglial cultures with consequent increases in proinflammatory molecules. Furthermore, we provide evidence that the CD36 scavenger receptor and downstream kinases are involved in SYN-mediated microglial activation. Together, our data suggest an early role for SYN and inflammation in PD pathogenesis.

Zinc protects SK-N-SH cells from methamphetamine-induced alpha-synuclein expression

Methamphetamine (METH) is a well-known drug of abuse and neurotoxin that may cause temporary or permanent disturbances in the dopaminergic systems of the brain, predisposing individuals to Parkinsonism. Previously, we have shown that METH causes dopaminergic cell death by increasing the production of reactive oxygen species (ROS) and by depleting cellular ATP levels. These effects were abolished by pretreatment with ZnCl(2) which enhanced expression of the zinc binding protein, metallothionein. In the present study, the effects of ZnCl(2) on alpha-synuclein expression were examined further in METH-treated SK-N-SH cells in culture. We show that METH significantly increased alpha-synuclein expression in a dose-dependent manner after inducing oxidative stress. Pretreatment with ZnCl(2) (50microM) reversed this stimulatory effect. We propose that zinc mediates this neuroprotective response via the production of metallothionein.

The neurotoxin, MPP+, induces hyperphosphorylation of Tau, in the presence of alpha-Synuclein, in SH-SY5Y neuroblastoma cells

Alzheimer's disease (AD) is characterized, in part, by intracellular neurofibrillary tangles composed of hyperphosphorylated filamentous aggregates of the microtubule-associated protein, Tau. Such hyperphosphorylated Tau is also found in Lewy bodies (LBs), and cytoplasmic inclusion bodies in certain forms of Parkinson's disease (PD). Further, LBs also contain aggregates of alpha-synuclein (alpha-Syn), also a microtubule-associated protein, which has been linked to the genesis of PD. To investigate a specific correlation between Tau phosphorylation and alpha-Syn, we generated a SH-SY5Y cell line that stably expresses human wild type alpha-Syn. Protein expression levels in the stably transfected cell line (SHalpha-Syn) were within the physiological range of alpha-Syn expression found in Substantia nigra. We show here, in the MPP+ (1-methyl-4-phenylpyridinium ion) cell model of parkinsonism, a time- and dose-dependent increase in the hyperphosphorylation of Tau at pSer396/404 (PHF-1-reactive Tau, p-Tau), concomitant with increased accumulation of alpha-Syn, upon treatment of cells with the neurotoxin. This increase in p-Tau was strictly dependent on the presence of alphaSyn, since in transfected cells not expressing any alpha-Syn, MPP+ failed to induce an increase in PHF-1-reactive Tau. The production of p-Tau caused increased cytotoxicity as indexed by reduced cell viability. Moreover, in the absence of alpha-Syn, the cells were more resistant to MPP+ -induced cell death. The increased levels of both p-Tau and alpha-Syn led to diminished levels of these proteins associated with the cytoskeleton, which was accompanied by enhanced presence of the proteins in the cytoskeletal-free fractions. These data indicate that alpha-Syn and p-Tau modulate the pathogenicity of one another, suggesting a novel convergent mechanism of neurodegeneration.

Oxidative stress induces nuclear translocation of C-terminus of α-synuclein in dopaminergic cells

Growing evidence suggests that oxidative stress is involved in the neuronal degeneration and can promote the aggregation of alpha-synuclein. However, the role of alpha-synuclein under physiological and pathological conditions remains poorly understood. In the present study, we examined the possible interaction between the alpha-synuclein and oxidative stress. In a dopaminergic cell line MES23.5, we have found that the 200microM H(2)O(2) treatment induced the translocation of alpha-synuclein from cytoplasm to nuclei at 30min post-treatment. The immunoactivity of alpha-synuclein became highly intensive in the nuclei after 2h treatment. The protein translocated to nucleus was a 10kDa fragment of C-terminus region of alpha-synuclein, while full-length alpha-synuclein remained in cytoplasm. Thioflavine-S staining suggested that the C-terminal fragment in the nuclei has no beta-sheet structures. Our present results indicated that 200microM H(2)O(2) treatment induces the intranuclear accumulation of the C-terminal fragment of alpha-synuclein in dopaminergic neurons, whose role remains to be investigated.

Lewy bodies

Lewy bodies (LB) in the substantia nigra are a cardinal pathological feature of Parkinson's disease, but they occur in a number of neurodegenerative diseases and can be widespread in the nervous system. The characteristics, locations, and composition of LB are reviewed, with particular attention to alpha-synuclein (alpha-SYN), which appears to be the major component of LB. The propensity for alpha-SYN, a presynaptic protein widely expressed in the brain, to aggregate is because of an amyloidogenic central region. The factors that favor the aggregation of alpha-SYN and mechanisms of toxicity are examined, and a mechanism through which aggregates of alpha-SYN could induce mitochondrial dysfunction and/or release of proapoptotic molecules is proposed.

Dopamine transporter-mediated cytotoxicity of 6-hydroxydopamine in vitro depends on expression of mutant alpha-synucleins related to Parkinson's disease

6-Hydroxydopamine (6-OHDA) is widely used to produce animal models of Parkinson's disease (PD) by selectively destroying the nigro-striatal dopaminergic systems, but selective toxicity of 6-OHDA towards dopaminergic cells in vitro remains controversial. Mutant (A30P and A53T) alpha-synuclein isoforms cause increased vulnerability of cells towards various toxic insults and enhance dopamine transporter (DAT)-mediated toxicity of the selective dopaminergic neurotoxin and mitochondrial complex I inhibitor MPP(+) in vitro. Here we extend our recent studies on DAT-mediated toxicity to elucidate the mechanisms involved in selective dopaminergic toxicity of 6-OHDA. We studied the cytotoxicity as well as the toxic mechanisms of 6-OHDA in human embryonic kidney HEK-293 cells ectopically co-expressing mutant alpha-synucleins and the human DAT protein. 6-OHDA showed half-maximal toxic concentration (TC(50)) of 88 microM in HEK-hDAT cells without alpha-synuclein expression after 24 h, whereas the TC(50) values significantly decreased to 58 and 39 microM by expression of A30P and A53T alpha-synuclein, respectively. alpha-Synuclein expression did not affect 6-OHDA toxicity in HEK-293 cells not expressing the DAT. Analysis of intracellular parameters of cellular energy metabolism revealed that the co-expression of mutant alpha-synucleins in HEK-hDAT cells accelerates the reduction of intracellular net ATP levels and ATP/ADP ratios induced by 6-OHDA. Uptake function of the DAT was not altered by expression of alpha-synuclein isoforms. Our data suggest a mechanism of 6-OHDA-induced dopaminergic toxicity involving an interaction of mutant alpha-synucleins with the DAT molecule and subsequent acceleration of cellular energy depletion that might be relevant for the pathogenesis of PD.

The role of chaperones in Parkinson's disease and prion diseases

The etiologies of neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, polyglutamine diseases, or prion diseases may be diverse; however, aberrations in protein folding, processing, and/or degradation are common features of these entities, implying a role of quality control systems, such as molecular chaperones and the ubiquitin-proteasome pathway. There is substantial evidence for a causal role of protein misfolding in the pathogenic process coming from neuropathology, genetics, animal modeling, and biophysics. The presence of protein aggregates in all neurodegenerative diseases gave rise to the hypothesis that protein aggregates, be it intracellular or extracellular deposits, may perturb the cellular homeostasis and disintegrate neuronal function (Table 1). More recently, however, an increasing number of studies have indicated that protein aggregates are not toxic per se and might even serve a protective role by sequestering misfolded proteins. Specifically, experimental models of polyglutamine diseases, Alzheimer's disease, and Parkinson's disease revealed that the appearance of aggregates can be dissociated from neuronal toxicity, while misfolded monomers or oligomeric intermediates seem to be the toxic species. The unique features of molecular chaperones to assist in the folding of nascent proteins and to prevent stress-induced misfolding was the rationale to exploit their effects in different models of neurodegenerative diseases. This chapter concentrates on two neurodegenerative diseases, Parkinson's disease and prion diseases, with a special focus on protein misfolding and a possible role of molecular chaperones.

Synuclein, dopamine and oxidative stress: co-conspirators in Parkinson's disease?

The etiology of Parkinson's disease (PD) is presently unknown. The unifying hallmark of disease is depletion of dopamine and loss of nigrostriatal dopamine neurons. Familial and sporadic forms of the disease are described. The familial mutations occur within alpha-synuclein and molecules involved in protein degradation and mitochondrial function. Sporadic PD is thought to involve the interplay of genetic and environmental factors. Despite disparate initiating triggers, a convergent pathobiologic model for this common neurodegenerative disease has been proposed. Likely players have emerged that may form the basis for this common pathway model of disease. In this review, we examine the role of three most implicated PD pathogenic conspirators: synuclein, dopamine and oxidative stress.

Characterization of alpha-synuclein aggregation and synergistic toxicity with protein tau in yeast

A yeast model was generated to study the mechanisms and phenotypical repercussions of expression of alpha-synuclein as well as the coexpression of protein tau. The data show that aggregation of alpha-synuclein is a nucleation-elongation process initiated at the plasma membrane. Aggregation is consistently enhanced by dimethyl sulfoxide, which is known to increase the level of phospholipids and membranes in yeast cells. Aggregation of alpha-synuclein was also triggered by treatment of the yeast cells with ferrous ions, which are known to increase oxidative stress. In addition, data are presented in support of the hypothesis that degradation of alpha-synuclein occurs via autophagy and proteasomes and that aggregation of alpha-synuclein disturbs endocytosis. Reminiscent of observations in double-transgenic mice, coexpression of alpha-synuclein and protein tau in yeast cells is synergistically toxic, as exemplified by inhibition of proliferation. Taken together, the data show that these yeast models recapitulate major aspects of alpha-synuclein aggregation and cytotoxicity, and offer great potential for defining the underlying mechanisms of toxicity and synergistic actions of alpha-synuclein and protein tau.

The role of α-synuclein in neurodegenerative diseases

Alpha-synuclein is a 140 amino acid neuronal protein that has been associated with several neurodegenerative diseases. A point mutation in the gene coding for the alpha-synuclein protein was the first discovery linking this protein to a rare familial form of Parkinson's disease (PD). Subsequently, other mutations in the alpha-synuclein gene have been identified in familial PD. The aggregated proteinaceous inclusions called Lewy bodies found in PD and cortical Lewy body dementia (LBD) were discovered to be predominantly alpha-synuclein. Aberrant aggregation of alpha-synuclein has been detected in an increasing number of neurodegenerative diseases, collectively known as synucleopathies. Alpha-synuclein exists physiologically in both soluble and membrane-bound states, in unstructured and alpha-helical conformations, respectively. The physiological function of alpha-synuclein appears to require its translocation between these subcellular compartments and interconversion between the 2 conformations. Abnormal processing of alpha-synuclein is predicted to lead to pathological changes in its binding properties and function. In this review, genetic and environmental risk factors for alpha-synuclein pathology are described. Various mechanisms for in vitro and in vivo alpha-synuclein aggregation and neurotoxicity are summarized, and their relevance to neuropathology is explored.

Phosphorylated tau and the neurodegenerative foldopathies

Many studies have implicated phosphorylated tau in the Alzheimer disease process. However, the cellular fate of phosphorylated tau has only recently been described. Recent work has shown that tau phosphorylation at substrate sites for the kinases Cdk5 and GSK3-beta can trigger the binding of tau to the chaperones Hsc70 and Hsp27. The binding of phosphorylated tau to Hsc70 implied that the complex may be a substrate for the E3 ligase CHIP and this possibility was experimentally verified. The presence of this system in cells suggests that phosphorylated tau may hold toxic dangers for cell viability, and the response of the cell is to harness a variety of protective mechanisms. These include binding to chaperones, which may prevent more toxic conformations of the protein, ubiquitination which will direct the protein to the proteasome, segregation of tau aggregates from the cellular machinery, and recruitment of Hsp27 which will confer anti-apoptotic properties to the cell.

Protective effect of TAT‐delivered α‐synuclein: relevance of the C‐terminal domain and involvement of HSP70

Alpha-synuclein (alpha-syn) is a 140-amino acid presinaptic protein whose mutations A30P and A53T have been linked to familiar Parkinson's disease (PD). Many data suggest that alpha-syn aggregation is the key event that triggers alpha-syn-mediated neurotoxicity. Nevertheless, other lines of evidence proposed a protective role of alpha-syn against oxidative stress (a major feature of PD), even if the exact mechanism of this protective action and the role of the pathogenetic mutations to this respect have not been elucidated yet. To address these points, we developed an in vitro model of oxidative stress by exposing PC12 cells to hydrogen peroxide (H2O2) (150 microM) for 72 h, and we evaluated alpha-syn-mediated protection delivering increasing amounts of alpha-syn (wild type [WT] or mutated) inside cells using the fusion proteins TAT-alpha-syn (WT, A30P, and A53T). We found that nanomolar amounts of TAT-alpha-syn-mediated protected against oxidative stress and other cellular injuries (6-hydroxydopamine and serum deprivation), whereas micromolar amounts of the fusion proteins were intrinsically toxic to cells. The protective effect was independent from the presence of the mutations A30P and A53T, but no protection occurred when cells were challenged with the proteasome inhibitors lactacystin and MG132. We verified that the protection mechanism required the presence of the C-terminal domain of alpha-syn, as nanomolar amounts of the C-terminal truncated fusion protein TAT-alpha-syn (WT[1-97]) failed in preventing H2O2 toxicity. To further characterize the molecular mechanisms at the basis of alpha-syn protection, we investigated the possible involvement of the chaperone protein HSP70 that is widely implicated in neuroprotection. We found that, at nanomolar concentrations, TAT-alpha-syn was able to increase HSP70 protein level, whereas at the micromolar scale, TAT-alpha-syn decreased HSP70 at the protein level. These effects on HSP70 were independent from the presence of alpha-syn pathogenetic mutations but required the alpha-syn C-terminal domain. The implications for alpha-syn-mediated neurotoxicity and for PD pathogenesis and progression are discussed.

Impaired degradation of mutant alpha-synuclein by chaperone-mediated autophagy

Aberrant alpha-synuclein degradation is implicated in Parkinson's disease pathogenesis because the protein accumulates in the Lewy inclusion bodies associated with the disease. Little is known, however, about the pathways by which wild-type alpha-synuclein is normally degraded. We found that wild-type alpha-synuclein was selectively translocated into lysosomes for degradation by the chaperone-mediated autophagy pathway. The pathogenic A53T and A30P alpha-synuclein mutants bound to the receptor for this pathway on the lysosomal membrane, but appeared to act as uptake blockers, inhibiting both their own degradation and that of other substrates. These findings may underlie the toxic gain-of-function by the mutants.

A critical review of the development and importance of proteinaceous aggregates in animal models of Parkinson's disease: new insights into Lewy body formation

The pace of development of new animal models of Parkinson's disease (PD) has increased dramatically in the recent past, primarily because of the identification of the protein, alpha-synuclein, in Lewy bodies in both idiopathic and familial PD. This discovery has allowed the production of transgenic models that incorporate a form of human, mutant alpha-synuclein from rare familial cases, and has enabled the search for Lewy-body-like aggregations of this protein in toxin-induced models. Indeed, alpha-synuclein-positive inclusions, some of which bear strong resemblance to Lewy bodies, have now been recognized and their formation investigated in several different, environmentally-induced and transgenic models. Nevertheless, these data have yet to provide a uniform theory of inclusion pathogenesis for PD. The aim of this review is not only to summarize the findings to date on alpha-synuclein-immunopositive inclusion bodies, including some new information on Lewy bodies, but also provide a concise viewpoint on their origin and formation in animal models. We will provide evidence for a predicted series of intracellular events that underlie inclusion formation. Triggered by oxidative and metabolic stress, chronic, toxin-treated animals, rather than transgenic models transfected with human alpha-synuclein, eventually produce inclusion bodies that most closely resemble early stages of Lewy bodies. Elucidating the common mechanisms in animal models is a first step towards understanding the role of Lewy bodies and their formation in Parkinson's disease.

Ubiquitylation of synphilin-1 and alpha-synuclein by SIAH and its presence in cellular inclusions and Lewy bodies imply a role in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disease characterized by Lewy body formation and death of dopaminergic neurons. Mutations in alpha-synuclein and parkin cause familial forms of PD. Synphilin-1 was shown to interact with alpha-synuclein and to promote the formation of cytosolic inclusions. We now report that synphilin-1 interacts with the E3 ubiquitin-ligases SIAH-1 and SIAH-2. SIAH proteins ubiquitylate synphilin-1 both in vitro and in vivo, promoting its degradation by the ubiquitin-proteasome system. Inability of the proteasome to degrade synphilin-1/SIAH complex leads to a robust formation of ubiquitylated cytosolic inclusions. Ubiquitylation is required for inclusion formation, because a catalytically inactive mutant of SIAH-1, which still binds to synphilin-1, fails to promote inclusions. Like synphilin-1, alpha-synuclein associates with SIAH in intact cells, but the interaction with SIAH-2 was much stronger that with SIAH-1. In vitro experiments show that SIAH-2 monoubiquitylates alpha-synuclein. Further evidence that SIAH proteins may play a role in inclusion formation comes from the demonstration of SIAH immunoreactivity in Lewy bodies of PD patients.

Proteasome inhibition and aggregation in Parkinson's disease: a comparative study in untransfected and transfected cells

Dysfunction of the ubiquitin-proteasome system (UPS) has been implicated in Parkinson's disease (PD) and other neurodegenerative disorders. We have investigated the effect of UPS inhibition on the metabolism of alpha-synuclein (SYN) and parkin, two proteins genetically and histopathologically associated to PD. Pharmacological inhibition of proteasome induced accumulation of both parkin and SYN in transfected PC12 cells. We found that this effect was caused by increased protein synthesis rather than impairment of protein degradation, suggesting that inhibition of the UPS might lead to non-specific up-regulation of cytomegalovirus (CMV)-driven transcription. To investigate whether endogenous parkin and SYN can be substrate of the UPS, untransfected PC12 cells and primary mesencephalic neurones were exposed to proteasome inhibitors, and parkin and SYN expression was evaluated at both protein and mRNA level. Under these conditions, we found that proteasome inhibitors did not affect the level of endogenous parkin and SYN. However, we confirmed that dopaminergic neurones were selectively vulnerable to the toxicity of proteasome inhibitors. Our results indicate that studies involving the use of proteasome inhibitors, particularly those in which proteins are expressed from a heterologous promoter, are subjected to potential artefacts that need to be considered for the interpretation of the role of UPS in PD pathogenesis.

Mechanism of neurodegenerative disease: role of the ubiquitin proteasome system

Many neurodegenerative disorders such as Alzheimer's disease (AD) Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS) and Huntington's disease (HD) are characterized by neuronal damage that may be caused by toxic, abnormal, aggregation-prone proteins. The purpose of this review is threefold: 1) to provide the reader with an overview of the genes involved in the abnormal processing and accumulation of misfolded proteins in neurodegenerative diseases using PD as a model disease; 2) to understand the cellular mechanisms for disposal of abnormal proteins, and the effects of toxic protein accumulation on ubiquitin proteasome system (UPS) and neuronal survival and 3) to discuss the development and challenges of cell culture and animal models for a rational and effective treatment for these disorders.