Effects of oxidative and nitrative challenges on alpha-synuclein fibrillogenesis involve distinct mechanisms of protein modifications

Revisiting the role of 3-nitrotyrosine residues in the formation of alpha-synuclein oligomers and fibrils

Nitration of tyrosine residues in alpha-synuclein (a-syn) has been detected in different synucleinopathies, including Parkinson's disease. The potential role of 3-nitrotyrosine formation in a-syn, as an oxidative post-translational modification, is still elusive. In this work, we generated well-characterized tyrosine nitrated a-syn monomers and studied their capability to form oligomers and fibrils. We constructed tyrosine to phenylalanine mutants, containing a single tyrosine residue, a-syn mutant Y(125/133/136)F and Y(39/125/133)F) and assessed the impact in a-syn biophysical properties. Nitrated wild-type a-syn and the Y-F mutants, with one 3-nitrotyrosine residue in either the protein's N-terminal or C-terminal region, showed inhibition of fibril formation but retained the capacity of oligomer formation. The inhibition of a-syn fibrillation occurs even when an important amount of unmodified a-syn is still present. We characterized oligomers from both nitrated and non-nitrated forms of the wild-type protein and the mutant forms obtained. Our results indicate that the formation of 3-nitrotyrosine in a-syn could induce an off-pathway oligomer formation which may have an important impact in the development of synucleinopathies.

Blood-brain Barrier and Neurovascular Unit Dysfunction in Parkinson's Disease: From Clinical Insights to Pathogenic Mechanisms and Novel Therapeutic Approaches

The blood-brain barrier (BBB) plays a crucial role in the central nervous system by tightly regulating the influx and efflux of biological substances between the brain parenchyma and peripheral circulation. Its restrictive nature acts as an obstacle to protect the brain from potentially noxious substances such as blood-borne toxins, immune cells, and pathogens. Thus, the maintenance of its structural and functional integrity is vital in the preservation of neuronal function and cellular homeostasis in the brain microenvironment. However, the barrier's foundation can become compromised during neurological or pathological conditions, which can result in dysregulated ionic homeostasis, impaired transport of nutrients, and accumulation of neurotoxins that eventually lead to irreversible neuronal loss. Initially, the BBB is thought to remain intact during neurodegenerative diseases, but accumulating evidence as of late has suggested the possible association of BBB dysfunction with Parkinson's disease (PD) pathology. The neurodegeneration occurring in PD is believed to stem from a myriad of pathogenic mechanisms, including tight junction alterations, abnormal angiogenesis, and dysfunctional BBB transporter mechanism, which ultimately causes altered BBB permeability. In this review, the major elements of the neurovascular unit (NVU) comprising the BBB are discussed, along with their role in the maintenance of barrier integrity and PD pathogenesis. We also elaborated on how the neuroendocrine system can influence the regulation of BBB function and PD pathogenesis. Several novel therapeutic approaches targeting the NVU components are explored to provide a fresh outlook on treatment options for PD.

Single residue modulators of amyloid formation in the N-terminal P1-region of α-synuclein

Alpha-synuclein (αSyn) is a protein involved in neurodegenerative disorders including Parkinson’s disease. Amyloid formation of αSyn can be modulated by the ‘P1 region’ (residues 36-42). Here, mutational studies of P1 reveal that Y39A and S42A extend the lag-phase of αSyn amyloid formation in vitro and rescue amyloid-associated cytotoxicity in C. elegans. Additionally, L38I αSyn forms amyloid fibrils more rapidly than WT, L38A has no effect, but L38M does not form amyloid fibrils in vitro and protects from proteotoxicity. Swapping the sequence of the two residues that differ in the P1 region of the paralogue γSyn to those of αSyn did not enhance fibril formation for γSyn. Peptide binding experiments using NMR showed that P1 synergises with residues in the NAC and C-terminal regions to initiate aggregation. The remarkable specificity of the interactions that control αSyn amyloid formation, identifies this region as a potential target for therapeutics, despite their weak and transient nature.

The authors of this work characterize the effect of amino acid substitution on α-synuclein (α-Syn) aggregation. Residues 38 and 42 (in addition to 39) within the P1 region of α-Syn affect amyloid formation. The effect of substitution at position 38 is dependent on the amino-acid introduced, suggesting that specific interactions control α -Syn aggregation.

Nitrosative stress in Parkinson's disease

Parkinson’s Disease (PD) is a neurodegenerative disorder characterized, in part, by the loss of dopaminergic neurons within the nigral-striatal pathway. Multiple lines of evidence support a role for reactive nitrogen species (RNS) in degeneration of this pathway, specifically nitric oxide (NO). This review will focus on how RNS leads to loss of dopaminergic neurons in PD and whether RNS accumulation represents a central signal in the degenerative cascade. Herein, we provide an overview of how RNS accumulates in PD by considering the various cellular sources of RNS including nNOS, iNOS, nitrate, and nitrite reduction and describe evidence that these sources are upregulating RNS in PD. We document that over 1/3 of the proteins that deposit in Lewy Bodies, are post-translationally modified (S-nitrosylated) by RNS and provide a broad description of how this elicits deleterious effects in neurons. In doing so, we identify specific proteins that are modified by RNS in neurons which are implicated in PD pathogenesis, with an emphasis on exacerbation of synucleinopathy. How nitration of alpha-synuclein (aSyn) leads to aSyn misfolding and toxicity in PD models is outlined. Furthermore, we delineate how RNS modulates known PD-related phenotypes including axo-dendritic-, mitochondrial-, and dopamine-dysfunctions. Finally, we discuss successful outcomes of therapeutics that target S-nitrosylation of proteins in Parkinson’s Disease related clinical trials. In conclusion, we argue that targeting RNS may be of therapeutic benefit for people in early clinical stages of PD.

Role of post-translational modifications on the alpha-synuclein aggregation-related pathogenesis of Parkinson’s disease

Together with neuronal loss, the existence of insoluble inclusions of alpha-synuclein (α-syn) in the brain is widely accepted as a hallmark of synucleinopathies including Parkinson’s disease (PD), multiple system atrophy, and dementia with Lewy body. Because the α-syn aggregates are deeply involved in the pathogenesis, there have been many attempts to demonstrate the mechanism of the aggregation and its potential causative factors including post-translational modifications (PTMs). Although no concrete conclusions have been made based on the previous study results, growing evidence suggests that modifications such as phosphorylation and ubiquitination can alter α-syn characteristics to have certain effects on the aggregation process in PD; either facilitating or inhibiting fibrillization. In the present work, we reviewed studies showing the significant impacts of PTMs on α-syn aggregation. Furthermore, the PTMs modulating α-syn aggregation-induced cell death have been discussed. 

Second Sphere Interactions in Amyloidogenic Diseases

Amyloids are protein aggregates bearing a highly ordered cross β structural motif, which may be functional but are mostly pathogenic. Their formation, deposition in tissues and consequent organ dysfunction is the central event in amyloidogenic diseases. Such protein aggregation may be brought about by conformational changes, and much attention has been directed toward factors like metal binding, post-translational modifications, mutations of protein etc., which eventually affect the reactivity and cytotoxicity of the associated proteins. Over the past decade, a global effort from different groups working on these misfolded/unfolded proteins/peptides has revealed that the amino acid residues in the second coordination sphere of the active sites of amyloidogenic proteins/peptides cause changes in H-bonding pattern or protein-protein interactions, which dramatically alter the structure and reactivity of these proteins/peptides. These second sphere effects not only determine the binding of transition metals and cofactors, which define the pathology of some of these diseases, but also change the mechanism of redox reactions catalyzed by these proteins/peptides and form the basis of oxidative damage associated with these amyloidogenic diseases. The present review seeks to discuss such second sphere modifications and their ramifications in the etiopathology of some representative amyloidogenic diseases like Alzheimer's disease (AD), type 2 diabetes mellitus (T2Dm), Parkinson's disease (PD), Huntington's disease (HD), and prion diseases.

Light-Controlled Tyrosine Nitration of Proteins

Tyrosine nitration of proteins is one of the most important oxidative post-translational modifications in vivo. A major obstacle for its biochemical and physiological studies is the lack of efficient and chemoselective protein tyrosine nitration reagents. Herein, we report a generalizable strategy for light-controlled protein tyrosine nitration by employing biocompatible dinitroimidazole reagents. Upon 390 nm irradiation, dinitroimidazoles efficiently convert tyrosine residues into 3-nitrotyrosine residues in peptides and proteins with fast kinetics and high chemoselectivity under neutral aqueous buffer conditions. The incorporation of 3-nitrotyrosine residues enhances the thermostability of lasso peptide natural products and endows murine tumor necrosis factor-α with strong immunogenicity to break self-tolerance. The light-controlled time resolution of this method allows the investigation of the impact of tyrosine nitration on the self-assembly behavior of α-synuclein.

Megadalton-sized Dityrosine Aggregates of α-Synuclein Retain High Degrees of Structural Disorder and Internal Dynamics

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Despite their large size, αSyn dityrosine aggregates are dynamic and disordered.

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αSyn dityrosine aggregates specifically form in complex environments.

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αSyn dityrosine aggregates retain residual membrane binding activity.

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Dityrosine aggregates inhibit amyloid formation of monomeric αSyn.

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αSyn dityrosine aggregates are not cytotoxic.

Heterogeneous aggregates of the human protein α-synuclein (αSyn) are abundantly found in Lewy body inclusions of Parkinson’s disease patients. While structural information on classical αSyn amyloid fibrils is available, little is known about the conformational properties of disease-relevant, non-canonical aggregates. Here, we analyze the structural and dynamic properties of megadalton-sized dityrosine adducts of αSyn that form in the presence of reactive oxygen species and cytochrome c, a proapoptotic peroxidase that is released from mitochondria during sustained oxidative stress. In contrast to canonical cross-β amyloids, these aggregates retain high degrees of internal dynamics, which enables their characterization by solution-state NMR spectroscopy. We find that intermolecular dityrosine crosslinks restrict αSyn motions only locally whereas large segments of concatenated molecules remain flexible and disordered. Indistinguishable aggregates form in crowded in vitro solutions and in complex environments of mammalian cell lysates, where relative amounts of free reactive oxygen species, rather than cytochrome c, are rate limiting. We further establish that dityrosine adducts inhibit classical amyloid formation by maintaining αSyn in its monomeric form and that they are non-cytotoxic despite retaining basic membrane-binding properties. Our results suggest that oxidative αSyn aggregation scavenges cytochrome c’s activity into the formation of amorphous, high molecular-weight structures that may contribute to the structural diversity of Lewy body deposits.

In Search of Effective Treatments Targeting α-Synuclein Toxicity in Synucleinopathies: Pros and Cons

Parkinson's disease (PD), multiple system atrophy (MSA) and Dementia with Lewy bodies (DLB) represent pathologically similar, progressive neurodegenerative disorders characterized by the pathological aggregation of the neuronal protein α-synuclein. PD and DLB are characterized by the abnormal accumulation and aggregation of α-synuclein in proteinaceous inclusions within neurons named Lewy bodies (LBs) and Lewy neurites (LNs), whereas in MSA α-synuclein inclusions are mainly detected within oligodendrocytes named glial cytoplasmic inclusions (GCIs). The presence of pathologically aggregated α-synuclein along with components of the protein degradation machinery, such as ubiquitin and p62, in LBs and GCIs is considered to underlie the pathogenic cascade that eventually leads to the severe neurodegeneration and neuroinflammation that characterizes these diseases. Importantly, α-synuclein is proposed to undergo pathogenic misfolding and oligomerization into higher-order structures, revealing self-templating conformations, and to exert the ability of "prion-like" spreading between cells. Therefore, the manner in which the protein is produced, is modified within neural cells and is degraded, represents a major focus of current research efforts in the field. Given that α-synuclein protein load is critical to disease pathogenesis, the identification of means to limit intracellular protein burden and halt α-synuclein propagation represents an obvious therapeutic approach in synucleinopathies. However, up to date the development of effective therapeutic strategies to prevent degeneration in synucleinopathies is limited, due to the lack of knowledge regarding the precise mechanisms underlying the observed pathology. This review critically summarizes the recent developed strategies to counteract α-synuclein toxicity, including those aimed to increase protein degradation, to prevent protein aggregation and cell-to-cell propagation, or to engage antibodies against α-synuclein and discuss open questions and unknowns for future therapeutic approaches.

Role of mtDNA disturbances in the pathogenesis of Alzheimer's and Parkinson's disease

Neurodegenerative diseases (e.g. Alzheimer's and Parkinson's disease) are becoming increasingly problematic to healthcare systems. Therefore, their underlying mechanisms are trending topics of study in medicinal research. Numerous studies have evidenced a strong association between mitochondrial DNA disturbances (e.g. oxidative damage, mutations, and methylation shifts) and the initiation and progression of neurodegenerative diseases. Therefore, this review discusses the risk and development of neurodegenerative diseases in terms of disturbances in mitochondrial DNA and as a part of a complex ecosystem that includes other important mechanisms (e.g. neuroinflammation and the misfolding and aggregation of amyloid-β peptides, α-synuclein, and tau proteins). In addition, the influence of individual mitochondrial DNA haplogroups on the risk and development of neurodegenerative diseases is also described and discussed.

TLR4 deficiency has a protective effect in the MPTP/probenecid mouse model of Parkinson's disease

Parkinson’s disease (PD) is a multifactorial disorder characterized by progressive loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc) and the presence of Lewy bodies (LBs) consisting of misfolded α-synuclein protein. The etiology of PD is still not clear but systemic inflammation is proved to trigger and exacerbate DA neurons degeneration. Toll-like receptor 4 (TLR4) is a pattern-recognition receptor (PRR) and plays a major role in promoting the host immune. TLR4-mediated signal pathways induce the release of many inflammatory cytokines. It is reasonable to hypothesize that TLR4 is the mediator in microglia contributing to the damage of DA neurons in the SNpc. In this study, we evaluated the role of TLR4 in the chronic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)/probenecid mouse model. Both TLR4-deficient and wild-type (WT) mice were injected with probenecid (250 mg/kg, i.p.) followed by injection of MPTP (25 mg/kg, s.c.) every 4 days for 10 times. From D43 to D47, the behavioral performance in pole test and wire hang test was assessed. Then the mice were euthanized, and SN and striatum were dissected out for biochemical tests. We showed that compared with MPTP-treated WT mice, TLR4 deficiency significantly attenuated MPTP-induced motor deficits and TH-protein expression reduction in SNpc and striatum, suppressed MPTP-induced α-synuclein abnormality and neuroinflammation mediated through oxidative stress, glial activation, NF-κB and the NLRP3 inflammasome signaling pathways. These findings highlight the neuroprotective effect of TLR4-pathways in the chronic MPTP-induced PD mouse model.

Metal Chelation Therapy and Parkinson's Disease: A Critical Review on the Thermodynamics of Complex Formation between Relevant Metal Ions and Promising or Established Drugs

The present review reports a list of approximately 800 compounds which have been used, tested or proposed for Parkinson's disease (PD) therapy in the year range 2014-2019 (April): name(s), chemical structure and references are given. Among these compounds, approximately 250 have possible or established metal-chelating properties towards Cu(II), Cu(I), Fe(III), Fe(II), Mn(II), and Zn(II), which are considered to be involved in metal dyshomeostasis during PD. Speciation information regarding the complexes formed by these ions and the 250 compounds has been collected or, if not experimentally available, has been estimated from similar molecules. Stoichiometries and stability constants of the complexes have been reported; values of the cologarithm of the concentration of free metal ion at equilibrium (pM), and of the dissociation constant Kd (both computed at pH = 7.4 and at total metal and ligand concentrations of 10-6 and 10-5 mol/L, respectively), charge and stoichiometry of the most abundant metal-ligand complexes existing at physiological conditions, have been obtained. A rigorous definition of the reported amounts is given, the possible usefulness of this data is described, and the need to characterize the metal-ligand speciation of PD drugs is underlined.

Exploring the Roles of Post-Translational Modifications in the Pathogenesis of Parkinson's Disease Using Synthetic and Semisynthetic Modified α-Synuclein

Alpha-synuclein (α-syn), a small soluble protein containing 140 amino acids, is associated with the recycling pool of synaptic vesicles in presynaptic terminals. The misfolding and aggregation of α-syn is closely related to a group of neurodegenerative diseases, including Parkinson's disease (PD), which is one of the most common progressive neurodegenerative diseases. Varieties of the post-translational modifications (PTMs) of α-syn, including phosphorylation, ubiquitination, and glycosylation, have been detected in soluble and aggregated α-syn in vivo. These PTMs can have either positive or negative effects on α-syn aggregation and toxicity, which may play critical roles in PD pathogenesis. Herein, we review the advances in synthetic and semisynthetic chemistry to generate homogeneous α-syn variants with site-specific modifications. Using these modified α-syn, we gain insight into the consequences of PTMs on α-syn aggregation and other biophysical properties, which can help elucidate the role of PTMs in the pathogenesis of PD and develop potential therapies to PD.

Living in Promiscuity: The Multiple Partners of Alpha-Synuclein at the Synapse in Physiology and Pathology

Alpha-synuclein (α-syn) is a small protein that, in neurons, localizes predominantly to presynaptic terminals. Due to elevated conformational plasticity, which can be affected by environmental factors, in addition to undergoing disorder-to-order transition upon interaction with different interactants, α-syn is counted among the intrinsically disordered proteins (IDPs) family. As with many other IDPs, α-syn is considered a hub protein. This function is particularly relevant at synaptic sites, where α-syn is abundant and interacts with many partners, such as monoamine transporters, cytoskeletal components, lipid membranes, chaperones and synaptic vesicles (SV)-associated proteins. These protein–protein and protein–lipid membrane interactions are crucial for synaptic functional homeostasis, and alterations in α-syn can cause disruption of this complex network, and thus a failure of the synaptic machinery. Alterations of the synaptic environment or post-translational modification of α-syn can induce its misfolding, resulting in the formation of oligomers or fibrillary aggregates. These α-syn species are thought to play a pathological role in neurodegenerative disorders with α-syn deposits such as Parkinson’s disease (PD), dementia with Lewy bodies (DLB), and multiple system atrophy (MSA), which are referred to as synucleinopathies. Here, we aim at revising the complex and promiscuous role of α-syn at synaptic terminals in order to decipher whether α-syn molecular interactants may influence its conformational state, contributing to its aggregation, or whether they are just affected by it.

TRH Analog, Taltirelin Protects Dopaminergic Neurons From Neurotoxicity of MPTP and Rotenone

Dopaminergic neurons loss is one of the main pathological characters of Parkinson’s disease (PD), while no suitable neuroprotective agents have been in clinical use. Thyrotropin-releasing hormone (TRH) and its analogs protect neurons from ischemia and various cytotoxins, but whether the effect also applies in PD models remain unclear. Here, we showed that Taltirelin, a long-acting TRH analog, exhibited the neuroprotective effect in both cellular and animal models of PD. The in vitro study demonstrated that Taltirelin (5 μM) reduced the generation of reactive oxygen species (ROS) induced by MPP+ or rotenone, alleviated apoptosis and rescued the viability of SH-SY5Y cells and rat primary midbrain neurons. Interestingly, SH-SY5Y cells treated with Taltirelin also displayed lower level of p-tau (S396) and asparagine endopeptidase (AEP) cleavage products, tau N368 and α-synuclein N103 fragments, accompanied by a lower intracellular monoamine oxidase-B (MAO-B) activity. In the subacute MPTP-induced and chronic rotenone-induced PD mice models, we found Taltirelin (1 mg/kg) significantly improved the locomotor function and preserved dopaminergic neurons in the substantia nigra (SN). In accordance with the in vitro study, Taltirelin down-regulated the levels of p-tau (S396), p-α-synuclein (S129) tau N368 and α-synuclein N103 fragments in SN and striatum. Together, this study demonstrates that Taltirelin may exert neuroprotective effect via inhibiting MAO-B and reducing the oxidative stress and apoptosis, preventing AEP activation and its subsequent pathological cleavage of tau and α-synuclein, thus provides evidence for Taltirelin in protective treatment of PD.

Crucial role of protein oligomerization in the pathogenesis of Alzheimer's and Parkinson's diseases

Misfolding and aggregation of the proteins amyloid-β, tau and alpha-synuclein is the predominant pathology underlying the neurodegenerative disorders, Alzheimer's and Parkinson's disease. While end stage insoluble products of aggregation have been well characterised in human and animal models of disease, accumulating evidence from biophysical, cellular and in vivo studies has shown that soluble intermediates of aggregation, or oligomers, may be the key species that mediate toxicity and underlie seeding and spreading in disease. Here, we review the process of protein misfolding, and the intrinsic and extrinsic processes that cause the native states of the key aggregating proteins to undergo conformational change to form oligomers and ultimately fibrils. We discuss the structural features of the key toxic intermediate, and describe the putative mechanisms by which oligomers may cause cell toxicity. Finally, we explore the potential therapeutic approaches raised by the oligomer hypothesis in neurodegenerative disease.

α-Synuclein stimulation of monoamine oxidase-B and legumain protease mediates the pathology of Parkinson's disease

Dopaminergic neurodegeneration in Parkinson's disease (PD) is associated with abnormal dopamine metabolism by MAO-B (monoamine oxidase-B) and intracellular α-Synuclein (α-Syn) aggregates, called the Lewy body. However, the molecular relationship between α-Syn and MAO-B remains unclear. Here, we show that α-Syn directly binds to MAO-B and stimulates its enzymatic activity, which triggers AEP (asparagine endopeptidase; legumain) activation and subsequent α-Syn cleavage at N103, leading to dopaminergic neurodegeneration. Interestingly, the dopamine metabolite, DOPAL, strongly activates AEP, and the N103 fragment of α-Syn binds and activates MAO-B. Accordingly, overexpression of AEP in SNCA transgenic mice elicits α-Syn N103 cleavage and accelerates PD pathogenesis, and inhibition of MAO-B by Rasagiline diminishes α-Syn-mediated PD pathology and motor dysfunction. Moreover, virally mediated expression of α-Syn N103 induces PD pathogenesis in wild-type, but not MAO-B-null mice. Our findings thus support that AEP-mediated cleavage of α-Syn at N103 is required for the association and activation of MAO-B, mediating PD pathogenesis.

Alpha-Synuclein: From Early Synaptic Dysfunction to Neurodegeneration

Over the last two decades, many experimental and clinical studies have provided solid evidence that alpha-synuclein (α-syn), a small, natively unfolded protein, is closely related to Parkinson's disease (PD) pathology. To provide an overview on the different roles of this protein, here we propose a synopsis of seminal and recent studies that explored the many aspects of α-syn. Ranging from the physiological functions to its neurodegenerative potential, the relationship with the possible pathogenesis of PD will be discussed. Close attention will be paid on early cellular and molecular alterations associated with the presence of α-syn aggregates.

Clearing Extracellular Alpha-Synuclein from Cerebrospinal Fluid: A New Therapeutic Strategy in Parkinson's Disease

This concept article aims to show the rationale of targeting extracellular α-Synuclein (α-Syn) from cerebrospinal fluid (CSF) as a new strategy to remove this protein from the brain in Parkinson's disease (PD). Misfolding and intracellular aggregation of α-synuclein into Lewy bodies are thought to be crucial in the pathogenesis of PD. Recent research has shown that small amounts of monomeric and oligomeric α-synuclein are released from neuronal cells by exocytosis and that this extracellular alpha-synuclein contributes to neurodegeneration, progressive spreading of alpha-synuclein pathology, and neuroinflammation. In PD, extracellular oligomeric-α-synuclein moves in constant equilibrium between the interstitial fluid (ISF) and the CSF. Thus, we expect that continuous depletion of oligomeric-α-synuclein in the CSF will produce a steady clearance of the protein in the ISF, preventing transmission and deposition in the brain.

Cell Biology and Pathophysiology of α-Synuclein

α-Synuclein is an abundant neuronal protein that is highly enriched in presynaptic nerve terminals. Genetics and neuropathology studies link α-synuclein to Parkinson's disease (PD) and other neurodegenerative disorders. Accumulation of misfolded oligomers and larger aggregates of α-synuclein defines multiple neurodegenerative diseases called synucleinopathies, but the mechanisms by which α-synuclein acts in neurodegeneration are unknown. Moreover, the normal cellular function of α-synuclein remains debated. In this perspective, we review the structural characteristics of α-synuclein, its developmental expression pattern, its cellular and subcellular localization, and its function in neurons. We also discuss recent progress on secretion of α-synuclein, which may contribute to its interneuronal spread in a prion-like fashion, and describe the neurotoxic effects of α-synuclein that are thought to be responsible for its role in neurodegeneration.

Acrolein-mediated neuronal cell death and alpha-synuclein aggregation: Implications for Parkinson's disease

Growing evidence suggests that oxidative stress plays a critical role in neuronal destruction characteristic of Parkinson's disease (PD). However, the molecular mechanisms of oxidative stress-mediated dopaminergic cell death are far from clear. In the current investigation, we tested the hypothesis that acrolein, an oxidative stress and lipid peroxidation (LPO) product, is a key factor in the pathogenesis of PD. Using a combination of in vitro, in vivo, and cell free models, coupled with anatomical, functional, and behavioral examination, we found that acrolein was elevated in 6-OHDA-injected rats, and behavioral deficits associated with 6-OHDA could be mitigated by the application of the acrolein scavenger hydralazine, and mimicked by injection of acrolein in healthy rats. Furthermore, hydralazine alleviated neuronal cell death elicited by 6-OHDA and another PD-related toxin, rotenone, in vitro. We also show that acrolein can promote the aggregation of alpha-synuclein, suggesting that alpha-synuclein self-assembly, a key pathological phenomenon in human PD, could play a role in neurotoxic effects of acrolein in PD models. These studies suggest that acrolein is involved in the pathogenesis of PD, and the administration of anti-acrolein scavengers such as hydralazine could represent a novel strategy to alleviate tissue damage and motor deficits associated with this disease.

Insights into the Molecular Mechanisms of Alzheimer's and Parkinson's Diseases with Molecular Simulations: Understanding the Roles of Artificial and Pathological Missense Mutations in Intrinsically Disordered Proteins Related to Pathology

Amyloid-β and α-synuclein are intrinsically disordered proteins (IDPs), which are at the center of Alzheimer's and Parkinson's disease pathologies, respectively. These IDPs are extremely flexible and do not adopt stable structures. Furthermore, both amyloid-β and α-synuclein can form toxic oligomers, amyloid fibrils and other type of aggregates in Alzheimer's and Parkinson's diseases. Experimentalists face challenges in investigating the structures and thermodynamic properties of these IDPs in their monomeric and oligomeric forms due to the rapid conformational changes, fast aggregation processes and strong solvent effects. Classical molecular dynamics simulations complement experiments and provide structural information at the atomic level with dynamics without facing the same experimental limitations. Artificial missense mutations are employed experimentally and computationally for providing insights into the structure-function relationships of amyloid-β and α-synuclein in relation to the pathologies of Alzheimer's and Parkinson's diseases. Furthermore, there are several natural genetic variations that play a role in the pathogenesis of familial cases of Alzheimer's and Parkinson's diseases, which are related to specific genetic defects inherited in dominant or recessive patterns. The present review summarizes the current understanding of monomeric and oligomeric forms of amyloid-β and α-synuclein, as well as the impacts of artificial and pathological missense mutations on the structural ensembles of these IDPs using molecular dynamics simulations. We also emphasize the recent investigations on residual secondary structure formation in dynamic conformational ensembles of amyloid-β and α-synuclein, such as β-structure linked to the oligomerization and fibrillation mechanisms related to the pathologies of Alzheimer's and Parkinson's diseases. This information represents an important foundation for the successful and efficient drug design studies.

Opposed Effects of Dityrosine Formation in Soluble and Aggregated α-Synuclein on Fibril Growth

Parkinson's disease is the second most common neurodegenerative disease. It is characterized by aggregation of the protein α-synuclein (α-syn) in Lewy bodies, mitochondrial dysfunction, and increased oxidative stress in the substantia nigra. Oxidative stress leads to several modifications of biomolecules including dityrosine (DiY) crosslinking in proteins, which has recently been detected in α-syn in Lewy bodies from Parkinson's disease patients. Here we report that α-syn is highly susceptible to ultraviolet-induced DiY formation. We investigated DiY formation of α-syn and nine tyrosine-to-alanine mutants and monitored its effect on α-syn fibril formation in vitro. Ultraviolet irradiation of intrinsically disordered α-syn generates DiY-modified monomers and dimers, which inhibit fibril formation of unmodified α-syn by interfering with fibril elongation. The inhibition depends on both the DiY group and its integration into α-syn. When preformed α-syn fibrils are crosslinked by DiY formation, they gain increased resistance to denaturation. DiY-stabilized α-syn fibrils retain their high seeding efficiency even after being exposed to denaturant concentrations that completely depolymerize non-crosslinked seeds. Oxidative stress-associated DiY crosslinking of α-syn therefore entails two opposing effects: (i) inhibition of aggregation by DiY-modified monomers and dimers, and (ii) stabilization of fibrillar aggregates against potential degradation mechanisms, which can lead to promotion of aggregation, especially in the presence of secondary nucleation.

The Genetics of Parkinson Disease

The 200years of research efforts on Parkinson disease (PD) form the basis of our understanding of the second most common neurodegenerative disorder after Alzheimer disease. This journey has been marked by the revolutionary discovery of a neurotransmitter replacement therapy that provides a longer and healthier life to patients. Since 1997, the advances in the genetics of PD have expanded our understanding of this neurodegenerative disorder and they are opening up new ways to search for disease-modifying therapies. This chapter is a summary of the historical discoveries and latest progress in PD research.

Protein Posttranslational Modifications: Roles in Aging and Age-Related Disease

Aging is characterized by the progressive decline of biochemical and physiological function in an individual. Consequently, aging is a major risk factor for diseases like cancer, obesity, and type 2 diabetes. The cellular and molecular mechanisms of aging are not well understood, nor is the relationship between aging and the onset of diseases. One of the hallmarks of aging is a decrease in cellular proteome homeostasis, allowing abnormal proteins to accumulate. This phenomenon is observed in both eukaryotes and prokaryotes, suggesting that the underlying molecular processes are evolutionarily conserved. Similar protein aggregation occurs in the pathogenesis of diseases like Alzheimer's and Parkinson's. Further, protein posttranslational modifications (PTMs), either spontaneous or physiological/pathological, are emerging as important markers of aging and aging-related diseases, though clear causality has not yet been firmly established. This review presents an overview of the interplay of PTMs in aging-associated molecular processes in eukaryotic aging models. Understanding PTM roles in aging could facilitate targeted therapies or interventions for age-related diseases. In addition, the study of PTMs in prokaryotes is highlighted, revealing the potential of simple prokaryotic models to uncover complex aging-associated molecular processes in the emerging field of microbiogerontology.

Sirtuin-2 Protects Neural Cells from Oxidative Stress and Is Elevated in Neurodegeneration

Sirtuins are highly conserved lysine deacetylases involved in ageing, energy production, and lifespan extension. The mammalian SIRT2 has been implicated in Parkinson's disease (PD) where studies suggest SIRT2 promotes neurodegeneration. We therefore evaluated the effects of SIRT2 manipulation in toxin treated SH-SY5Y cells and determined the expression and activity of SIRT2 in postmortem brain tissue from patients with PD. SH-SY5Y viability in response to oxidative stress induced by diquat or rotenone was measured following SIRT2 overexpression or inhibition of deacetylase activity, along with α-synuclein aggregation. SIRT2 in human tissues was evaluated using Western blotting, immunohistochemistry, and fluorometric activity assays. In SH-SY5Y cells, elevated SIRT2 protected cells from rotenone or diquat induced cell death and enzymatic inhibition of SIRT2 enhanced cell death. SIRT2 protection was mediated, in part, through elevated SOD2 expression. SIRT2 reduced the formation of α-synuclein aggregates but showed minimal colocalisation with α-synuclein. In postmortem PD brain tissue, SIRT2 activity was elevated compared to controls but also elevated in other neurodegenerative disorders. Results from both in vitro work and brain tissue suggest that SIRT2 is necessary for protection against oxidative stress and higher SIRT2 activity in PD brain may be a compensatory mechanism to combat neuronal stress.

Effects of Intrinsic and Extrinsic Factors on Aggregation of Physiologically Important Intrinsically Disordered Proteins

Misfolding and aggregation of proteins and peptides play an important role in a number of diseases as well as in many physiological processes. Many of the proteins that misfold and aggregate in vivo are intrinsically disordered. Protein aggregation is a complex multistep process, and aggregates can significantly differ in morphology, structure, stability, cytotoxicity, and self-propagation ability. The aggregation process is influenced by both intrinsic (e.g., mutations and expression levels) and extrinsic (e.g., polypeptide chain truncation, macromolecular crowding, posttranslational modifications, as well as interaction with metal ions, other small molecules, lipid membranes, and chaperons) factors. This review examines the effect of a variety of these factors on aggregation of physiologically important intrinsically disordered proteins.

Oxidative stress: A major pathogenesis and potential therapeutic target of antioxidative agents in Parkinson's disease and Alzheimer's disease

Oxidative stress reflects an imbalance between the overproduction and incorporation of free radicals and the dynamic ability of a biosystem to detoxify reactive intermediates. Free radicals produced by oxidative stress are one of the common features in several experimental models of diseases. Free radicals affect both the structure and function of neural cells, and contribute to a wide range of neurodegenerative diseases, including Parkinson's disease and Alzheimer's disease. Although the precise mechanisms that result in the degeneration of neurons and the relevant pathological changes remain unclear, the crucial role of oxidative stress in the pathogenesis of neurodegenerative diseases is associated with several proteins (such as α-synuclein, DJ-1, Amyloid β and tau protein) and some signaling pathways (such as extracellular regulated protein kinases, phosphoinositide 3-kinase/Protein Kinase B pathway and extracellular signal-regulated kinases 1/2) that are tightly associated with the neural damage. In this review, we present evidence, gathered over the last decade, concerning a variety of pathogenic proteins, their important signaling pathways and pathogenic mechanisms associated with oxidative stress in Parkinson's disease and Alzheimer's disease. Proper control and regulation of these proteins' functions and the related signaling pathways may be a promising therapeutic approach to the patients. We also emphasizes antioxidative options, including some new neuroprotective agents that eliminate excess reactive oxygen species efficiently and have a certain therapeutic effect; however, controversy surrounds some of them in terms of the dose and length of therapy. These agents require further investigation by clinical application in patients suffering Parkinson's disease and Alzheimer's disease.

α-synuclein aggregation and its modulation

Parkinson's disease (PD) is a neurological disorder marked by the presence of cytoplasmic inclusions, Lewy bodies (LBs) and Lewy neurites (LNs) as well as the degeneration of dopamine producing neurons in the substantia nigra region of the brain. The LBs and LNs in PD are mainly composed of aggregated form of a presynaptic protein, α-synuclein (α-Syn). However, the mechanisms of α-Syn aggregation and actual aggregated species responsible for the degeneration of dopaminergic neurons have not yet been resolved. Despite the fact that α-Syn aggregation in LBs and LNs is crucial and mutations of α-Syn are associated with early onset PD, it is really a challenging task to establish a correlation between α-Syn aggregation rate and PD pathogenesis. Regardless of strong genetic contribution, PD is mostly sporadic and familial forms of the disease represent only a minor part (<10%) of all cases. The complexity in PD further increases due to the involvement of several cellular factors in the pathogenesis of the disease as well as the environmental factors associated with the risk of developing PD. Therefore, effect of these factors on α-Syn aggregation pathway and how these factors modulate the properties of wild type (WT) as well as mutated α-Syn should be collectively taken into account. The present review specifically provides an overview of recent research on α-Syn aggregation pathways and its modulation by several cellular factors potentially relevant to PD pathogenesis. We also briefly discuss about effect of environmental risk factors on α-Syn aggregation.

Alpha-Synuclein in Parkinson's Disease: From Pathogenetic Dysfunction to Potential Clinical Application

Parkinson's disease is a neurodegenerative disease/synucleinopathy that develops slowly; however, there is no efficient method of early diagnosis, nor is there a cure. Progressive dopaminergic neuronal cell loss in the substantia nigra pars compacta and widespread aggregation of the α-synuclein protein (encoded by the SNCA gene) in the form of Lewy bodies and Lewy neurites are the neuropathological hallmarks of Parkinson's disease. The SNCA gene has undergone gene duplications, triplications, and point mutations. However, the specific mechanism of α-synuclein in Parkinson's disease remains obscure. Recent research showed that various α-synuclein oligomers, pathological aggregation, and propagation appear to be harmful in certain areas in Parkinson's disease patients. This review summarizes our current knowledge of the pathogenetic dysfunction of α-synuclein associated with Parkinson's disease and highlights current approaches that seek to develop this protein as a possible diagnostic biomarker and therapeutic target.

C-Terminal Tyrosine Residue Modifications Modulate the Protective Phosphorylation of Serine 129 of α-Synuclein in a Yeast Model of Parkinson's Disease

Parkinson´s disease (PD) is characterized by the presence of proteinaceous inclusions called Lewy bodies that are mainly composed of α-synuclein (αSyn). Elevated levels of oxidative or nitrative stresses have been implicated in αSyn related toxicity. Phosphorylation of αSyn on serine 129 (S129) modulates autophagic clearance of inclusions and is prominently found in Lewy bodies. The neighboring tyrosine residues Y125, Y133 and Y136 are phosphorylation and nitration sites. Using a yeast model of PD, we found that Y133 is required for protective S129 phosphorylation and for S129-independent proteasome clearance. αSyn can be nitrated and form stable covalent dimers originating from covalent crosslinking of two tyrosine residues. Nitrated tyrosine residues, but not di-tyrosine-crosslinked dimers, contributed to αSyn cytotoxicity and aggregation. Analysis of tyrosine residues involved in nitration and crosslinking revealed that the C-terminus, rather than the N-terminus of αSyn, is modified by nitration and di-tyrosine formation. The nitration level of wild-type αSyn was higher compared to that of A30P mutant that is non-toxic in yeast. A30P formed more dimers than wild-type αSyn, suggesting that dimer formation represents a cellular detoxification pathway in yeast. Deletion of the yeast flavohemoglobin gene YHB1 resulted in an increase of cellular nitrative stress and cytotoxicity leading to enhanced aggregation of A30P αSyn. Yhb1 protected yeast from A30P-induced mitochondrial fragmentation and peroxynitrite-induced nitrative stress. Strikingly, overexpression of neuroglobin, the human homolog of YHB1, protected against αSyn inclusion formation in mammalian cells. In total, our data suggest that C-terminal Y133 plays a major role in αSyn aggregate clearance by supporting the protective S129 phosphorylation for autophagy and by promoting proteasome clearance. C-terminal tyrosine nitration increases pathogenicity and can only be partially detoxified by αSyn di-tyrosine dimers. Our findings uncover a complex interplay between S129 phosphorylation and C-terminal tyrosine modifications of αSyn that likely participates in PD pathology.

Parkinson’s disease is characterized by loss of dopaminergic neurons in midbrain and the presence of αSyn protein inclusions. Human αSyn mimics the disease pathology in yeast resulting in cytotoxicity and aggregate formation. αSyn is abundantly phosphorylated at serine S129 and possesses four tyrosines (Y39, Y125, Y133, and Y136) that can be posttranslationally modified by nitration or phosphorylation. The consequence of each of these possible modifications is still unclear. Nitration as consequence of oxidative stress is a hallmark for neurodegenerative diseases. Here, we addressed the molecular mechanism, how tyrosine posttranslational modifications affect αSyn cytotoxicity. Tyrosine nitration can contribute to αSyn toxicity or can be part of a cellular salvage pathway when di-tyrosine-crosslinked dimers are formed. The Y133 residue, which can be either phosphorylated or nitrated, determines whether S129 is protectively phosphorylated and αSyn inclusions are cleared. This interplay with S129 phosphorylation demonstrates a dual role for C-terminal tyrosine residues. Yeast flavohemoglobin Yhb1 and its human counterpart neuroglobin NGB protect cells against cytotoxicity and aggregate formation. These novel insights into the molecular pathways responsible for αSyn cytotoxicity indicate NGB as a potential target for therapeutic intervention in PD.

Formation of covalent di-tyrosine dimers in recombinant α-synuclein

Parkinson's disease is associated with fibril deposition in the diseased brain. Misfolding events of the intrinsically disordered synaptic protein α-synuclein are suggested to lead to the formation of transient oligomeric and cytotoxic species. The etiology of Parkinson's disease is further associated with mitochondrial dysfunction and formation of reactive oxygen species. Oxidative stress causes chemical modification of native α-synuclein, plausibly further influencing misfolding events. Here, we present evidence for the spontaneous formation of covalent di-tyrosine α-synuclein dimers in standard recombinant protein preparations, induced without extrinsic oxidative or nitrative agents. The dimers exhibit no secondary structure but advanced SAXS studies reveal an increased structural definition, resulting in a more hydrophobic micro-environment than the highly disordered monomer. Accordingly, monomers and dimers follow distinct fibrillation pathways.

Amphetamine-related drugs neurotoxicity in humans and in experimental animals: Main mechanisms

Amphetamine-related drugs, such as 3,4-methylenedioxymethamphetamine (MDMA) and methamphetamine (METH), are popular recreational psychostimulants. Several preclinical studies have demonstrated that, besides having the potential for abuse, amphetamine-related drugs may also elicit neurotoxic and neuroinflammatory effects. The neurotoxic potentials of MDMA and METH to dopaminergic and serotonergic neurons have been clearly demonstrated in both rodents and non-human primates. This review summarizes the species-specific cellular and molecular mechanisms involved in MDMA and METH-mediated neurotoxic and neuroinflammatory effects, along with the most important behavioral changes elicited by these substances in experimental animals and humans. Emphasis is placed on the neuropsychological and neurological consequences associated with the neuronal damage. Moreover, we point out the gap in our knowledge and the need for developing appropriate therapeutic strategies to manage the neurological problems associated with amphetamine-related drug abuse.

Mitochondrial impairment and oxidative stress compromise autophagosomal degradation of α-synuclein in oligodendroglial cells

α-Synuclein (α-syn)-containing glial cytoplasmic inclusions originating in oligodendrocytes are characteristically observed in multiple system atrophy. The mechanisms of glial cytoplasmic inclusion formation remain rather elusive. α-Syn over-expression, uptake from the environment, oxidative stress or impairment of the proteolytic degradation systems have been discussed. Here, we investigated whether in oligodendrocytes autophagy plays a major role in the degradation and aggregation of endogenously expressed α-syn and of α-syn taken up from the extracellular environment. Furthermore, we studied whether in cells with impaired mitochondria the accumulation and aggregation of exogenously added α-syn is promoted. Using primary cultures of rat brain oligodendrocytes and an oligodendroglial cell line, genetically engineered to express green fluorescent protein-microtubule-associated light chain 3 with or without α-syn to monitor the autophagic flux, we demonstrate that both exogenously applied α-syn and α-syn stably expressed endogenously are effectively degraded by autophagy and do not affect the autophagic flux per se. Mitochondrial impairment with the protonophore carbonyl cyanide 3-chlorophenylhydrazone or 3-nitropropionic acid disturbs the autophagic pathway and leads to the accumulation of exogenously applied α-syn and enhances its propensity to form aggregates intracellularly. Thus, mitochondrial dysfunction and oxidative stress, which occur over time and are significant pathological features in synucleinopathies, have an impact on the autophagic pathway and participate in pathogenesis. Glial cytoplasmic inclusions are characteristically observed in multiple system atrophy, their occurrence might be related to failure in protein degradation systems. Here, we show that in oligodendrocytes autophagy is the major route of α-synuclein degradation which is either endogenously expressed or added exogenously (1, 2). Mitochondrial impairment (3) disturbs the autophagic flux and leads to the accumulation of exogenously applied α-synuclein, and enhances its propensity to form aggregates intracellulary (4).

Protein aggregation and neurodegeneration in prototypical neurodegenerative diseases: Examples of amyloidopathies, tauopathies and synucleinopathies

Alzheimer's and Parkinson's diseases are the most prevalent neurodegenerative diseases that generate important health-related direct and indirect socio-economic costs. They are characterized by severe neuronal losses in several disease-specific brain regions associated with deposits of aggregated proteins. In Alzheimer's disease, β-amyloid peptide-containing plaques and intraneuronal neurofibrillary tangles composed of hyperphosphorylated microtubule-associated protein tau are the two main neuropathological lesions, while Parkinson's disease is defined by the presence of Lewy Bodies that are intraneuronal proteinaceous cytoplasmic inclusions. α-Synuclein has been identified as a major protein component of Lewy Bodies and heavily implicated in the pathogenesis of Parkinson's disease. In the past few years, evidence has emerged to explain how these aggregate-prone proteins can undergo spontaneous self-aggregation, propagate from cell to cell, and mediate neurotoxicity. Current research now indicates that oligomeric forms are probably the toxic species. This article discusses recent progress in the understanding of the pathogenesis of these diseases, with a focus on the underlying mechanisms of protein aggregation, and emphasizes the pathophysiological molecular mechanisms leading to cellular toxicity. Finally, we present the putative direct link between β-amyloid peptide and tau in causing toxicity in Alzheimer's disease as well as α-synuclein in Parkinson's disease, along with some of the most promising therapeutic strategies currently in development for those incurable neurodegenerative disorders.

Targeting α-synuclein for treatment of Parkinson's disease: mechanistic and therapeutic considerations

Progressive neuronal cell loss in a small subset of brainstem and mesencephalic nuclei and widespread aggregation of the α-synuclein protein in the form of Lewy bodies and Lewy neurites are neuropathological hallmarks of Parkinson's disease. Most cases occur sporadically, but mutations in several genes, including SNCA, which encodes α-synuclein, are associated with disease development. The discovery and development of therapeutic strategies to block cell death in Parkinson's disease has been limited by a lack of understanding of the mechanisms driving neurodegeneration. However, increasing evidence of multiple pivotal roles of α-synuclein in the pathogenesis of Parkinson's disease has led researchers to consider the therapeutic potential of several strategies aimed at reduction of α-synuclein toxicity. We critically assess the potential of experimental therapies targeting α-synuclein, and discuss steps that need to be taken for target validation and drug development.

Posttranslational Modifications and Clearing of α-Synuclein Aggregates in Yeast

The budding yeast Saccharomyces cerevisiae represents an established model system to study the molecular mechanisms associated to neurodegenerative disorders. A key-feature of Parkinson’s disease is the formation of Lewy bodies, which are cytoplasmic protein inclusions. Misfolded α-synuclein is one of their main constituents. Expression of α-synuclein protein in yeast leads to protein aggregation and cellular toxicity, which is reminiscent to Lewy body containing human cells. The molecular mechanism involved in clearance of α-synuclein aggregates is a central question for elucidating the α-synuclein-related toxicity. Cellular clearance mechanisms include ubiquitin mediated 26S proteasome function as well as lysosome/vacuole associated degradative pathways as autophagy. Various modifications change α-synuclein posttranslationally and alter its inclusion formation, cytotoxicity and the distribution to different clearance pathways. Several of these modification sites are conserved from yeast to human. In this review, we summarize recent findings on the effect of phosphorylation and sumoylation of α-synuclein to the enhanced channeling to either the autophagy or the proteasome degradation pathway in yeast model of Parkinson’s disease.

Elucidating the Role of Site-Specific Nitration of α-Synuclein in the Pathogenesis of Parkinson's Disease via Protein Semisynthesis and Mutagenesis

Parkinson's disease (PD) is characterized by the loss of dopaminergic neurons in the substantia nigra and the presence of intraneuronal inclusions consisting of aggregated and post-translationally modified α-synuclein (α-syn). Despite advances in the chemical synthesis of α-syn and other proteins, the generation of site-specifically nitrated synthetic proteins has not been reported. Consequently, it has not been possible to determine the roles of nitration at specific residues in regulating the physiological and pathogenic properties of α-syn. Here we report, for the first time, the site-specific incorporation of 3-nitrotyrosine at different regions of α-syn using native chemical ligation combined with a novel desulfurization strategy. This strategy enabled us to investigate the role of nitration at single or multiple tyrosine residues in regulating α-syn structure, membrane binding, oligomerization, and fibrils formation. We demonstrate that different site-specifically nitrated α-syn species exhibit distinct structural and aggregation properties and exhibit reduced affinity to negatively charged vesicle membranes. We provide evidence that intermolecular interactions between the N- and C-terminal regions of α-syn play critical roles in mediating nitration-induced α-syn oligomerization. For example, when Y39 is not available for nitration (Y39F and Y39/125F), the extent of cross-linking is limited mostly to dimer formation, whereas mutants in which Y39 along with one or multiple C-terminal tyrosines (Y125F, Y133F, Y136F and Y133/136F) can still undergo nitration readily to form higher-order oligomers. Our semisynthetic strategy for generating site-specifically nitrated proteins opens up new possibilities for investigating the role of nitration in regulating protein structure and function in health and disease.

Mitochondrial Dysfunction and α-Synuclein Synaptic Pathology in Parkinson's Disease: Who's on First?

Parkinson's disease (PD) is the most common neurodegenerative movement disorder. Its characteristic neuropathological features encompass the loss of dopaminergic neurons of the nigrostriatal system and the presence of Lewy bodies and Lewy neurites. These are intraneuronal and intraneuritic proteinaceous insoluble aggregates whose main constituent is the synaptic protein α-synuclein. Compelling lines of evidence indicate that mitochondrial dysfunction and α-synuclein synaptic deposition may play a primary role in the onset of this disorder. However, it is not yet clear which of these events may come first in the sequel of processes leading to neurodegeneration. Here, we reviewed data supporting either that α-synuclein synaptic deposition precedes and indirectly triggers mitochondrial damage or that mitochondrial deficits lead to neuronal dysfunction and α-synuclein synaptic accumulation. The present overview shows that it is still difficult to establish the exact temporal sequence and contribution of these events to PD.

Protective effect of alpha-synuclein knockdown on methamphetamine-induced neurotoxicity in dopaminergic neurons

The over-expression of α-synuclein is a major factor in the death of dopaminergic neurons in a methamphetamine-induced model of Parkinson's disease. In the present study, α-synuclein knockdown rats were created by injecting α-synuclein-shRNA lentivirus stereotaxically into the right striatum of experimental rats. At 2 weeks post-injection, the rats were injected intraperitoneally with methamphetamine to establish the model of Parkinson's disease. Expression of α-synuclein mRNA and protein in the right striatum of the injected rats was significantly downregulated. Food intake and body weight were greater in α-synuclein knockdown rats, and water intake and stereotyped behavior score were lower than in model rats. Striatal dopamine and tyrosine hydroxylase levels were significantly elevated in α-synuclein knockdown rats. Moreover, superoxide dismutase activity was greater in α-synuclein knockdown rat striatum, but the levels of reactive oxygen species, malondialdehyde, nitric oxide synthase and nitrogen monoxide were lower compared with model rats. We also found that α-synuclein knockdown inhibited methamphetamine-induced neuronal apoptosis. These results suggest that α-synuclein has the capacity to reverse methamphetamine-induced apoptosis of dopaminergic neurons in the rat striatum by inhibiting oxidative stress and improving dopaminergic system function.

Lysosomes and α-synuclein form a dangerous duet leading to neuronal cell death

Neurodegenerative diseases are (i) characterized by a selective neuronal vulnerability to degeneration in specific brain regions; and (ii) likely to be caused by disease-specific protein misfolding. Parkinson's disease (PD) is characterized by the presence of intraneuronal proteinacious cytoplasmic inclusions, called Lewy Bodies (LB). α-Synuclein, an aggregation prone protein, has been identified as a major protein component of LB and the causative for autosomal dominant PD. Lysosomes are responsible for the clearance of long-lived proteins, such as α-synuclein, and for the removal of old or damaged organelles, such as mitochondria. Interestingly, PD-linked α-synuclein mutants and dopamine-modified wild-type α-synuclein block its own degradation, which result in insufficient clearance, leading to its aggregation and cell toxicity. Moreover, both lysosomes and lysosomal proteases have been found to be involved in the activation of certain cell death pathways. Interestingly, lysosomal alterations are observed in the brains of patients suffering from sporadic PD and also in toxic and genetic rodent models of PD-related neurodegeneration. All these events have unraveled a causal link between lysosomal impairment, α-synuclein accumulation, and neurotoxicity. In this review, we emphasize the pathophysiological mechanisms connecting α-synuclein and lysosomal dysfunction in neuronal cell death.

Oxidative stress promotes uptake, accumulation, and oligomerization of extracellular α-synuclein in oligodendrocytes

The accumulation and aggregation of α-synuclein (α-Syn) in glial cytoplasmic inclusions originating in oligodendrocytes is a characteristic hallmark of multiple system atrophy, a progressive adult onset neurodegenerative disorder. The origin of α-Syn deposition in oligodendrocytes in multiple system atrophy is still unclear, but the uptake of α-Syn from the environment after neuronal secretion has been discussed. The present study was undertaken to investigate the consequences of α-Syn uptake from the environment in cultured oligodendroglial cells and its localization and potential to form intracellular aggregates in the absence or presence of the microtubule-associated protein tau, which has been demonstrated to act synergistically with α-Syn. Primary rat brain oligodendrocytes and clonal oligodendroglial OLN-93 cells were incubated with human recombinant soluble and pre-aggregated α-Syn. The data show that oligodendrocytes are capable to take up and internalize soluble and pre-aggregated α-Syn from their growth medium. In a time-dependent manner, α-Syn oligomerizes and small intracellular aggregates are formed. These do not exert cytotoxic responses or mitochondrial impairment. Oxidative stress exerted by hydrogen peroxide further promotes α-Syn oligomer formation and leads to an enlargement of the aggregates. This process is not affected or modified by the presence of tau in OLN-93 cells. Furthermore, membrane lipid modification by docosahexaenoic acid promotes α-Syn uptake and oligomerization, indicating that changing the membrane lipid composition and structure contributes to the protein aggregation process and pathological events. Hence, although α-Syn taken up by oligodendrocytes from the environment is not toxic per se, under conditions of oxidative stress, which might occur during chronic disease progression and aging, aggregates are enlarged and eventually may contribute to cytotoxicity and cellular death.

Exploring the accessible conformations of N-terminal acetylated α-synuclein

Alpha synuclein (αsyn) fibrils are found in the Lewy Bodies of patients with Parkinson's disease (PD). The aggregation of the αsyn monomer to soluble oligomers and insoluble fibril aggregates is believed to be one of the causes of PD. Recently, the view of the native state of αsyn as a monomeric ensemble was challenged by a report suggesting that αsyn exists in its native state as a helical tetramer. This review reports on our current understanding of αsyn within the context of these recent developments and describes the work performed by a number of groups to address the monomer/tetramer debate. A number of in depth studies have subsequently shown that both non-acetylated and acetylated αsyn purified under mild conditions are primarily monomer. A description of the accessible states of acetylated αsyn monomer and the ability of αsyn to self-associate is explored.

Toxicity of extracellular secreted alpha-synuclein: Its role in nitrosative stress and neurodegeneration

It has been demonstrated that both oligomerisation and accumulation of α-synuclein (ASN) are the key molecular processes involved in the pathophysiology of neurodegenerative diseases such as Parkinson's disease, Alzheimer's disease and other synucleinopathies. Alterations of ASN expression and impairment of its degradation can lead to the formation of intracellular deposits of this protein, called Lewy bodies. Overexpressed or misfolded ASN could be secreted to the extracellular space. Today the prion-like transmission of ASN oligomers to neighbouring cells is believed to be responsible for protein modification and propagation of neurodegeneration in the brain. It was presented that oxidative/nitrosative stress may play a key role in ASN secretion and spread of ASN pathology. Moreover, ASN-evoked protein oxidation, nitration and nitrosylation lead to disturbances in synaptic transmission and cell death. The interaction of secreted ASN with other amyloidogenic proteins and its involvement in irreversible mitochondrial disturbances and oxidative stress were also described. A better understanding of the mechanisms of ASN secretion and dysfunction may help to explain the molecular mechanisms of neurodegeneration and may be the basis for the development of novel therapeutic strategies.

Biophysical properties and cellular toxicity of covalent crosslinked oligomers of α-synuclein formed by photoinduced side-chain tyrosyl radicals

Alpha-synuclein (αS), a 140 amino acid presynaptic protein, is the major component of the fibrillar aggregates (Lewy bodies) observed in dopaminergic neurons of patients affected by Parkinson's disease. It is currently believed that noncovalent oligomeric forms of αS, arising as intermediates in its aggregation, may constitute the major neurotoxic species. However, attempts to isolate and characterize such oligomers in vitro, and even more so in living cells, have been hampered by their transient nature, low concentration, polymorphism, and inherent instability. In this work, we describe the preparation and characterization of low molecular weight covalently bound oligomeric species of αS obtained by crosslinking via tyrosyl radicals generated by blue-light photosensitization of the metal coordination complex ruthenium (II) tris-bipyridine in the presence of ammonium persulfate. Numerous analytical techniques were used to characterize the αS oligomers: biochemical (anion-exchange chromatography, SDS-PAGE, and Western blotting); spectroscopic (optical: UV/Vis absorption, steady state, dynamic fluorescence, and dynamic light scattering); mass spectrometry; and electrochemical. Light-controlled protein oligomerization was mediated by formation of Tyr-Tyr (dityrosine) dimers through -C-C- bonds acting as covalent bridges, with a predominant involvement of residue Y39. The diverse oligomeric species exhibited a direct effect on the in vitro aggregation behavior of wild-type monomeric αS, decreasing the total yield of amyloid fibrils in aggregation assays monitored by thioflavin T (ThioT) fluorescence and light scattering, and by atomic force microscopy (AFM). Compared to the unmodified monomer, the photoinduced covalent oligomeric species demonstrated increased toxic effects on differentiated neuronal-like SH-SY5Y cells. The results highlight the importance of protein modification induced by oxidative stress in the initial molecular events leading to Parkinson's disease.

The Aggregation of Huntingtin and α-Synuclein

Huntington's and Parkinson's diseases are neurodegenerative disorders associated with unusual protein interactions. Although the origin and evolution of these diseases are completely different, characteristic deposits of protein aggregates (huntingtin and α-synuclein resp.), are a common feature in both diseases. After these observations, many studies are performed with both proteins. Some of them try to understand the nature and driving forces of the aggregation process; others try to find a correlation between the genetic and failure in protein function. Finally with the combination of both approaches, it was proposed that possible strategies deal with pathologic aggregation. Unfortunately, if protein aggregation is a cause or a consequence of the neurodegeneration observed in these pathologies, it is still debatable. This paper describes the process of aggregation of two proteins: huntingtin and α synuclein. The characteristics of the aggregation reaction of these proteins have been followed with novel methods both in vivo and in vitro; these studies include both the combination with other proteins and the presence of various chemical compounds. The ultimate goal of this study was to summarize recent findings on protein aggregation and its possible role as a therapeutic target in neurodegenerative diseases and their role in biomaterial science.