In vitro study of α-synuclein protofibrils by cryo-EM suggests a Cu(2+)-dependent aggregation pathway

Amyloid Accelerator Polyphosphate Implicated as the Mystery Density in α-Synuclein Fibrils

Aberrant aggregation of α-Synuclein is the pathological hallmark of a set of neurodegenerative diseases termed synucleinopathies. Recent advances in cryo-electron microscopy have led to the structural determination of the first synucleinopathy-derived α-Synuclein fibrils, which contain a non-proteinaceous, "mystery density" at the core of the protofilaments, hypothesized to be highly negatively charged. Guided by previous studies that demonstrated that polyphosphate (polyP), a universally conserved polyanion, significantly accelerates α-Synuclein fibril formation, we conducted blind docking and molecular dynamics simulation experiments to model the polyP binding site in α-Synuclein fibrils. Here we demonstrate that our models uniformly place polyP into the lysine-rich pocket, which coordinates the mystery density in patient-derived fibrils. Subsequent in vitro studies and experiments in cells revealed that substitution of the two critical lysine residues K43 and K45 leads to a loss of all previously reported effects of polyP binding on α-Synuclein, including stimulation of fibril formation, change in filament conformation and stability as well as alleviation of cytotoxicity. In summary, our study demonstrates that polyP fits the unknown electron density present in in vivo α-Synuclein fibrils and suggests that polyP exerts its functions by neutralizing charge repulsion between neighboring lysine residues.

Realization of Amyloid-like Aggregation as a Common Cause for Pathogenesis in Diseases

Amyloids were conventionally referred to as extracellular and intracellular accumulation of Aβ42 peptide, which causes the formation of plaques and neurofibrillary tangles inside the brain leading to the pathogenesis in Alzheimer's disease. Subsequently, amyloid-like deposition was found in the etiology of prion diseases, Parkinson's disease, type II diabetes, and cancer, which was attributed to the aggregation of prion protein, α-Synuclein, islet amyloid polypeptide protein, and p53 protein, respectively. Hence, traditionally amyloids were considered aggregates formed exclusively by proteins or peptides. However, since the last decade, it has been discovered that other metabolites, like single amino acids, nucleobases, lipids, glucose derivatives, etc., have a propensity to form amyloid-like toxic assemblies. Several studies suggest direct implications of these metabolite assemblies in the patho-physiology of various inborn errors of metabolisms like phenylketonuria, tyrosinemia, cystinuria, and Gaucher's disease, to name a few. In this review, we present a comprehensive literature overview that suggests amyloid-like structure formation as a common phenomenon for disease progression and pathogenesis in multiple syndromes. The review is devoted to providing readers with a broad knowledge of the structure, mode of formation, propagation, and transmission of different extracellular amyloids and their implications in the pathogenesis of diseases. We strongly believe a review on this topic is urgently required to create awareness about the understanding of the fundamental molecular mechanism behind the origin of diseases from an amyloid perspective and possibly look for a common therapeutic strategy for the treatment of these maladies by designing generic amyloid inhibitors.

Discovery of 4-aminoindole carboxamide derivatives to curtail alpha-synuclein and tau isoform 2N4R oligomer formation

Alzheimer's disease (AD) is a multifactorial, chronic neurodegenerative disease characterized by the presence of extracellular β-amyloid (Aβ) plaques, intraneuronal neurofibrillary tangles (NFTs), activated microglial cells, and an inflammatory state (involving reactive oxygen species production) in the brain. NFTs are comprised of misfolded and hyperphosphorylated forms of the microtubule-binding protein tau. Interestingly, the trimeric form of the 2N4R splice isoform of tau has been found to be more toxic than the trimeric 1N4R isoform in neuron precursor cells. Few drug discovery programs have focused on specific tau isoforms. The present drug discovery project is centered on the anti-aggregation effect of a series of seventeen 4- or 5-aminoindole carboxamides on the 2N4R isoform of tau. The selection of the best compounds was performed using α-synuclein (α-syn). The anti-oligomer and -fibril activities of newly synthesized aminoindole carboxamide derivatives were evaluated with biophysical methods, such as thioflavin T fluorescence assays, photo-induced cross-linking of unmodified proteins, and transmission electron microscopy. To evaluate the reduction of inclusions and cytoprotective effects, M17D neuroblastoma cells expressing inclusion-forming α-syn were treated with the best amide representatives. The 4-aminoindole carboxamide derivatives exhibited a better anti-fibrillar activity compared to their 5-aminoindole counterparts. The amide derivatives 2, 8, and 17 exerted anti-oligomer and anti-fibril activities on α-syn and the 2N4R isoform of tau. At a concentration of 40 μM, compound 8 reduced inclusion formation in M17D neuroblastoma cells expressing inclusion-prone αSynuclein3K::YFP. Our results demonstrate the potential of 4-aminoindole carboxamide derivatives with regard to inhibiting the oligomer formation of α-syn and tau (2N4R isoform) for further optimization prior to pre-clinical studies.

Thin-Plate Superstructures of the Immunogenic 33-mer Gliadin Peptide

Gluten related-disorders have a prevalence of 1-5 % worldwide triggered by the ingestion of gluten proteins in wheat, rye, barley, and some oats. In wheat gluten, the most studied protein is gliadin, whose immunodominant 33-mer amino acid fragment remains after digestive proteolysis and accumulates in the gut mucosa. Here, we report the formation of 33-mer thin-plate superstructures using intrinsic tyrosine (Tyr) steady-state fluorescence anisotropy and cryo-TEM in combination with water tension measurements. Furthermore, we showed that fluorescence decay measurements of 33-mer intrinsic fluorophore Tyr provided information on the early stages of the formation of the thin-plate structures. Finally, conformational analysis of Tyr residues using minimalist models by molecular dynamic simulations (MD) demonstrated that changes in Tyr rotamer states depend on the oligomerization stage. Our findings further advance the understanding of the formation of the 33-mer gliadin peptide superstructures and their relation to health and disease.

A simple, versatile and robust centrifugation-based filtration protocol for the isolation and quantification of α-synuclein monomers, oligomers and fibrils: Towards improving experimental reproducibility in α-synuclein research

Increasing evidence suggests that the process of alpha‐synuclein (α‐syn) aggregation from monomers into amyloid fibrils and Lewy bodies, via oligomeric intermediates plays an essential role in the pathogenesis of different synucleinopathies, including Parkinson's disease (PD), multiple system atrophy and dementia with Lewy bodies (DLB). However, the nature of the toxic species and the mechanisms by which they contribute to neurotoxicity and disease progression remain elusive. Over the past two decades, significant efforts and resources have been invested in studies aimed at identifying and targeting toxic species along the pathway of α‐syn fibrillization. Although this approach has helped to advance the field and provide insights into the biological properties and toxicity of different α‐syn species, many of the fundamental questions regarding the role of α‐syn aggregation in PD remain unanswered, and no therapeutic compounds targeting α‐syn aggregates have passed clinical trials. Several factors have contributed to this slow progress, including the complexity of the aggregation pathways and the heterogeneity and dynamic nature of α‐syn aggregates. In the majority of experiment, the α‐syn samples used contain mixtures of α‐syn species that exist in equilibrium and their ratio changes upon modifying experimental conditions. The failure to quantitatively account for the distribution of different α‐syn species in different studies has contributed not only to experimental irreproducibility but also to misinterpretation of results and misdirection of valuable resources. Towards addressing these challenges and improving experimental reproducibility in Parkinson's research, we describe here a simple centrifugation‐based filtration protocol for the isolation, quantification and assessment of the distribution of α‐syn monomers, oligomers and fibrils, in heterogeneous α‐syn samples of increasing complexity. The protocol is simple, does not require any special instrumentation and can be performed rapidly on multiple samples using small volumes. Here, we present and discuss several examples that illustrate the applications of this protocol and how it could contribute to improving the reproducibility of experiments aimed at elucidating the structural basis of α‐syn aggregation, seeding activity, toxicity and pathology spreading. This protocol is applicable, with slight modifications, to other amyloid‐forming proteins.

Cardiolipin Promotes Pore-Forming Activity of Alpha-Synuclein Oligomers in Mitochondrial Membranes

Aggregation of the amyloid-forming α-synuclein (αS) protein is closely associated with the etiology of Parkinson's disease (PD), the most common motor neurodegenerative disorder. Many studies have shown that soluble aggregation intermediates of αS, termed oligomers, permeabilize a variety of phospholipid membranes; thus, membrane disruption may represent a key pathogenic mechanism of αS toxicity. Given the centrality of mitochondrial dysfunction in PD, we therefore probed the formation of ion-permeable pores by αS oligomers in planar lipid bilayers reflecting the complex phospholipid composition of mitochondrial membranes. Using single-channel electrophysiology, we recorded distinct multilevel conductances (100-400 pS) with stepwise current transitions, typical of protein-bound nanopores, in mitochondrial-like membranes. Crucially, we observed that the presence of cardiolipin (CL), the signature phospholipid of mitochondrial membranes, enhanced αS-lipid interaction and the membrane pore-forming activity of αS oligomers. Further, preincubation of isolated mitochondria with a CL-specific dye protected against αS oligomer-induced mitochondrial swelling and release of cytochrome c. Hence, we favor a scenario in which αS oligomers directly porate a local lipid environment rich in CL, for instance outer mitochondrial contact sites or the inner mitochondrial membrane, to induce mitochondrial dysfunction. Pharmacological modulation of αS pore complex formation might thus preserve mitochondrial membrane integrity and alleviate mitochondrial dysfunction in PD.

Mapping Surface Hydrophobicity of α-Synuclein Oligomers at the Nanoscale

Proteins fold into a single structural ensemble but can also misfold into many diverse structures including small aggregates and fibrils, which differ in their toxicity. The aggregate surface properties play an important role in how they interact with the plasma membrane and cellular organelles, potentially inducing cellular toxicity, however, these properties have not been measured to date due to the lack of suitable methods. Here, we used a spectrally resolved, super-resolution imaging method combined with an environmentally sensitive fluorescent dye to measure the surface hydrophobicity of individual aggregates formed by the protein α-synuclein (αS), whose aggregation is associated with Parkinson's disease. We show that the surface of soluble oligomers is more hydrophobic than fibrils and populates a diverse range of coexisting states. Overall, our data show that the conversion of oligomers to fibril-like aggregates and ultimately to fibrils results in a reduction in both hydrophobicity and the variation in hydrophobicity. This funneling characteristic of the energy landscape explains many of the observed properties of αS aggregates and may be a common feature of aggregating proteins.

New Perspectives on Roles of Alpha-Synuclein in Parkinson's Disease

Parkinson’s disease (PD) is one of the synucleinopathies spectrum of disorders typified by the presence of intraneuronal protein inclusions. It is primarily composed of misfolded and aggregated forms of alpha-synuclein (α-syn), the toxicity of which has been attributed to the transition from an α-helical conformation to a β-sheetrich structure that polymerizes to form toxic oligomers. This could spread and initiate the formation of “LB-like aggregates,” by transcellular mechanisms with seeding and subsequent permissive templating. This hypothesis postulates that α-syn is a prion-like pathological agent and responsible for the progression of Parkinson’s pathology. Moreover, the involvement of the inflammatory response in PD pathogenesis has been reported on the excessive microglial activation and production of pro-inflammatory cytokines. At last, we describe several treatment approaches that target the pathogenic α-syn protein, especially the oligomers, which are currently being tested in advanced animal experiments or are already in clinical trials. However, there are current challenges with therapies that target α-syn, for example, difficulties in identifying varying α-syn conformations within different individuals as well as both the cost and need of long-duration large trials.

Cu and Zn coordination to amyloid peptides: From fascinating chemistry to debated pathological relevance

Several diseases share misfolding of different peptides and proteins as a key feature for their development. This is the case of important neurodegenerative diseases such as Alzheimer's and Parkinson's diseases and type II diabetes mellitus. Even more, metal ions such as copper and zinc might play an important role upon interaction with amyloidogenic peptides and proteins, which could impact their aggregation and toxicity abilities. In this review, the different coordination modes proposed for copper and zinc with amyloid-β, α-synuclein and IAPP will be reviewed as well as their impact on the aggregation, and ROS production in the case of copper. In addition, a special focus will be given to the mutations that affect metal binding and lead to familial cases of the diseases. Different modifications of the peptides that have been observed in vivo and could be relevant for the coordination of metal ions are also described.

Investigation of α-Synuclein Amyloid Fibrils Using the Fluorescent Probe Thioflavin T

In this work, α-synuclein amyloid fibrils—the formation of which is a biomarker of Parkinson’s disease—were investigated using the fluorescent probe thioflavin T (ThT). The experimental conditions of protein fibrillogenesis were chosen so that a sufficient number of continuous measurements could be performed to characterize and analyze all stages of this process. The reproducibility of fibrillogenesis and the structure of the obtained aggregates (which is a critical point for further investigation) were proven using a wide range of physical-chemical methods. For the determination of ThT-α-synuclein amyloid fibril binding parameters, the sample and reference solutions were prepared using equilibrium microdialysis. By utilizing absorption spectroscopy of these solutions, the ThT-fibrils binding mode with a binding constant of about 10⁴ M⁻¹ and stoichiometry of ThT per protein molecule of about 1:8 was observed. Fluorescence spectroscopy of the same solutions with the subsequent correction of the recorded fluorescence intensity on the primary inner filter effect allowed us to determine another mode of ThT binding to fibrils, with a binding constant of about 10⁶ M⁻¹ and stoichiometry of about 1:2500. Analysis of the photophysical characteristics of the dye molecules bound to the sites of different binding modes allowed us to assume the possible localization of these sites. The obtained differences in the ThT binding parameters to the amyloid fibrils formed from α-synuclein and other amyloidogenic proteins, as well as in the photophysical characteristics of the bound dye, confirmed the hypothesis of amyloid fibril polymorphism.

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.

Rational design of charged peptides that self-assemble into robust nanofibers as immune-functional scaffolds

Self-assembling peptides programed by sequence design to form predefined nanostructures are useful for a variety of biomedical applications. However, assemblies of classic ionic self-complementary peptides are unstable in neutral pH, while charged peptide hydrogels have low mechanical strength. Here, we report on the rational design of a self-assembling peptide system with optimized charge distribution and density for bioscaffold development. Our designer peptides employs a sequence pattern that undergoes salt triggered self-assembly into β-sheet rich cationic nanofibers in the full pH range (pH 0-14). Our peptides form nanofibrils in physiological condition at a minimum concentration that is significantly lower than has been reported for self-assembly of comparable peptides. The robust fiber-forming ability of our peptides results in the rapid formation of hydrogels in physiological conditions with strong mechanical strength. Moreover, fiber structure is maintained even upon dense conjugation with a model bioactive cargo OVA_257-264 peptide. Nanofibers carrying OVA_257-264 significantly enhanced CD8⁺ T cell activation in vitro. Subcutaneous immunization of our peptide fiber vaccine also elicited robust CD8⁺ T cell activation and proliferation in vivo. Our self-assembling peptides are expected to provide a versatile platform to construct diverse biomaterials.

STATEMENT OF SIGNIFICANCE

This work is an attempt of rational design of materials from molecular level for targeted properties and an exploration in molecular self-assembly. Current widely studied self-assembling peptides do not have stable nanofiber structures and form weak hydrogels under physiological conditions. To address this issue, we develop charged self-assembling peptides with a novel sequence pattern for strong fiber-forming ability under physiological conditions. Our designer peptides can undergo salt-triggered self-assembly into nanofibers that are ultrastable in extreme pH (0-14) and dilute solutions, and into hydrogels with strong mechanical strength. Upon conjugation with a model bioactive cargo, our self-assembled peptides exhibit great potential as bioscaffolds for multiple applications.

Histones facilitate α-synuclein aggregation during neuronal apoptosis

Ample in vitro and in vivo experimental evidence supports the hypothesis that intercellular transmission of α-synuclein (αS) is a mechanism underlying the spread of αS pathology in Parkinson's disease and related disorders. What remains unexplained is where and how initial transmissible αS aggregates form. In a previous study, we demonstrated that αS aggregates rapidly form in neurons with impaired nuclear membrane integrity due to the interaction between nuclear proaggregant factor(s) and αS and that such aggregates may serve as a source for αS seeding. In the present study, we identify histones as a potential nuclear proaggregant factor for αS aggregation in both apoptotic neurons and brains with αS pathology. We further demonstrate that histone-induced aggregates contain a range of αS oligomers, including protofibrils and mature fibrils, and that these αS aggregates can seed additional aggregation. Importantly, we demonstrate transmissibility in mouse brains from stereotaxic injection. This study provides new clues to the mechanism underlying initial pathological aggregation of αS in PD and related disorders, and could lead to novel diagnostic and therapeutic approaches.

High-Throughput Screening Methodology to Identify Alpha-Synuclein Aggregation Inhibitors

An increasing number of neurodegenerative diseases are being found to be associated with the abnormal accumulation of aggregated proteins in the brain. In Parkinson's disease, this process involves the aggregation of alpha-synuclein (α-syn) into intraneuronal inclusions. Thus, compounds that inhibit α-syn aggregation represent a promising therapeutic strategy as disease-modifying agents for neurodegeneration. The formation of α-syn amyloid aggregates can be reproduced in vitro by incubation of the recombinant protein. However, the in vitro aggregation of α-syn is exceedingly slow and highly irreproducible, therefore precluding fast high throughput anti-aggregation drug screening. Here, we present a simple and easy-to-implement in-plate method for screening large chemical libraries in the search for α-syn aggregation modulators. It allows us to monitor aggregation kinetics with high reproducibility, while being faster and requiring lower protein amounts than conventional aggregation assays. We illustrate how the approach enables the identification of strong aggregation inhibitors in a library of more than 14,000 compounds.

Self-Assembly of Human Profilin-1 Detected by Carr-Purcell-Meiboom-Gill Nuclear Magnetic Resonance (CPMG NMR) Spectroscopy

Protein oligomerization in the cell has important implications for both health and disease, and an understanding of the mechanisms by which proteins can self-associate is, therefore, of critical interest. Initial stages of the oligomerization process can be hard to detect, as they often involve the formation of sparsely populated and transient states that are difficult to characterize by standard biophysical approaches. Using relaxation dispersion nuclear magnetic resonance spectroscopy, we study the oligomerization of human profilin-1, a protein that regulates the polymerization of actin. We show that in solution and at millimolar concentrations profilin-1 is predominantly monomeric. However, fits of concentration-dependent relaxation data are consistent with the formation of a higher-order oligomer that is generated via a multistep process. Together with crystallographic data for profilin-2, a homologue of the protein studied here, our results suggest that profilin-1 forms a sparsely populated tetrameric conformer in solution.

Endosulfine-alpha inhibits membrane-induced α-synuclein aggregation and protects against α-synuclein neurotoxicity

Neuropathological and genetic findings suggest that the presynaptic protein α-synuclein (aSyn) is involved in the pathogenesis of synucleinopathy disorders, including Parkinson’s disease (PD), dementia with Lewy bodies (DLB) and multiple system atrophy. Evidence suggests that the self-assembly of aSyn conformers bound to phospholipid membranes in an aggregation-prone state plays a key role in aSyn neurotoxicity. Accordingly, we hypothesized that protein binding partners of lipid-associated aSyn could inhibit the formation of toxic aSyn oligomers at membrane surfaces. To address this hypothesis, we characterized the protein endosulfine-alpha (ENSA), previously shown to interact selectively with membrane-bound aSyn, in terms of its effects on the membrane-induced aggregation and neurotoxicity of two familial aSyn mutants, A30P and G51D. We found that wild-type ENSA, but not the non-aSyn-binding S109E variant, interfered with membrane-induced aSyn self-assembly, aSyn-mediated vesicle disruption and aSyn neurotoxicity. Immunoblotting analyses revealed that ENSA was down-regulated in the brains of synucleinopathy patients versus non-diseased individuals. Collectively, these results suggest that ENSA can alleviate neurotoxic effects of membrane-bound aSyn via an apparent chaperone-like activity at the membrane surface, and a decrease in ENSA expression may contribute to aSyn neuropathology in synucleinopathy disorders. More generally, our findings suggest that promoting interactions between lipid-bound, amyloidogenic proteins and their binding partners is a viable strategy to alleviate cytotoxicity in a range of protein misfolding disorders.

Role of neurotoxicants and traumatic brain injury in α-synuclein protein misfolding and aggregation

Protein misfolding and aggregation are key pathological features of many neurodegenerative diseases including Parkinson's disease (PD) and other forms of human Parkinsonism. PD is a complex and multifaceted disorder whose etiology is not fully understood. However, several lines of evidence support the multiple hit hypothesis that genetic vulnerability and environmental toxicants converge to trigger PD pathology. Alpha-synuclein (α-Syn) aggregation in the brain is an important pathophysiological characteristic of synucleinopathies including PD. Epidemiological and experimental studies have shown that metals and pesticides play a crucial role in α-Syn aggregation leading to the onset of various neurodegenerative diseases including PD. In this review, we will emphasize key findings of several epidemiological as well as experimental studies of metal- and pesticide-induced α-Syn aggregation and neurodegeneration. We will also discuss other factors such as traumatic brain injury and oxidative insult in the context of α-Syn-related neurodegenerative processes.

α-Synuclein Amyloid Fibrils with Two Entwined, Asymmetrically Associated Protofibrils

Parkinson disease and other progressive neurodegenerative conditions are characterized by the intracerebral presence of Lewy bodies, containing amyloid fibrils of α-synuclein. We used cryo-electron microscopy and scanning transmission electron microscopy (STEM) to study in vitro-assembled fibrils. These fibrils are highly polymorphic. Focusing on twisting fibrils with an inter-crossover spacing of 77 nm, our reconstructions showed them to consist of paired protofibrils. STEM mass per length data gave one subunit per 0.47 nm axial rise per protofibril, consistent with a superpleated β-structure. The STEM images show two thread-like densities running along each of these fibrils, which we interpret as ladders of metal ions. These threads confirmed the two-protofibril architecture of the 77-nm twisting fibrils and allowed us to identify this morphotype in STEM micrographs. Some other, but not all, fibril morphotypes also exhibit dense threads, implying that they also present a putative metal binding site. We propose a molecular model for the protofibril and suggest that polymorphic variant fibrils have different numbers of protofibrils that are associated differently.

Cu(II) promotes amyloid pore formation

The aggregation of α-synuclein is associated with dopamine neuron death in Parkinson's disease. There is controversy in the field over the question of which species of the aggregates, fibrils or protofibrils, are toxic. Moreover, compelling evidence suggested the exposure to heavy metals to be a risk of PD. Nevertheless, the mechanism of metal ions in promoting PD remains unclear. In this research, we investigated the structural basis of Cu(II) induced aggregation of α-synuclein. Using transmission electron microscopy experiments, Cu(II) was found to promote in vitro aggregation of α-synuclein by facilitating annular protofibril formation rather than fibril formation. Furthermore, neuroprotective baicalein disaggregated annular protofibrils accompanied by considerable decrease of β-sheet content. These results strongly support the hypothesis that annular protofibrils are the toxic species, rather than fibrils, thereby inspiring us to search novel therapeutic strategies for the suppression of the toxic annular protofibril formation.

Generation and characterization of novel conformation-specific monoclonal antibodies for α-synuclein pathology

α-Synuclein (α-syn), a small protein that has the intrinsic propensity to aggregate, is implicated in several neurodegenerative diseases including Parkinson's disease (PD), dementia with Lewy bodies (DLB), and multiple system atrophy (MSA), which are collectively known as synucleinopathies. Genetic, pathological, biochemical, and animal modeling studies provided compelling evidence that α-syn aggregation plays a key role in the pathogenesis of PD and related synucleinopathies. It is therefore of utmost importance to develop reliable tools that can detect the aggregated forms of α-syn. We describe here the generation and characterization of six novel conformation-specific monoclonal antibodies that recognize specifically α-syn aggregates but not the soluble, monomeric form of the protein. The antibodies described herein did not recognize monomers or fibrils generated from other amyloidogenic proteins including β-syn, γ-syn, β-amyloid, tau protein, islet amyloid polypeptide and ABri. Interestingly, the antibodies did not react to overlapping linear peptides spanning the entire sequence of α-syn, confirming further that they only detect α-syn aggregates. In immunohistochemical studies, the new conformation-specific monoclonal antibodies showed underappreciated small micro-aggregates and very thin neurites in PD and DLB cases that were not observed with generic pan antibodies that recognize linear epitope. Furthermore, employing one of our conformation-specific antibodies in a sandwich based ELISA, we observed an increase in levels of α-syn oligomers in brain lysates from DLB compared to Alzheimer's disease and control samples. Therefore, the conformation-specific antibodies portrayed herein represent useful tools for research, biomarkers development, diagnosis and even immunotherapy for PD and related pathologies.

Effects of impaired membrane interactions on α-synuclein aggregation and neurotoxicity

The post-mortem brains of individuals with Parkinson's disease (PD) and other synucleinopathy disorders are characterized by the presence of aggregated forms of the presynaptic protein α-synuclein (aSyn). Understanding the molecular mechanism of aSyn aggregation is essential for the development of neuroprotective strategies to treat these diseases. In this study, we examined how interactions between aSyn and phospholipid vesicles influence the protein's aggregation and toxicity to dopaminergic neurons. Two-dimensional NMR data revealed that two familial aSyn mutants, A30P and G51D, populated an exposed, membrane-bound conformer in which the central hydrophobic region was dissociated from the bilayer to a greater extent than in the case of wild-type aSyn. A30P and G51D had a greater propensity to undergo membrane-induced aggregation and elicited greater toxicity to primary dopaminergic neurons compared to the wild-type protein. In contrast, the non-familial aSyn mutant A29E exhibited a weak propensity to aggregate in the presence of phospholipid vesicles or to elicit neurotoxicity, despite adopting a relatively exposed membrane-bound conformation. Our findings suggest that the aggregation of exposed, membrane-bound aSyn conformers plays a key role in the protein's neurotoxicity in PD and other synucleinopathy disorders.

Structural characterization of toxic oligomers that are kinetically trapped during α-synuclein fibril formation

We describe the isolation and detailed structural characterization of stable toxic oligomers of α-synuclein that have accumulated during the process of amyloid formation. Our approach has allowed us to identify distinct subgroups of oligomers and to probe their molecular architectures by using cryo-electron microscopy (cryoEM) image reconstruction techniques. Although the oligomers exist in a range of sizes, with different extents and nature of β-sheet content and exposed hydrophobicity, they all possess a hollow cylindrical architecture with similarities to certain types of amyloid fibril, suggesting that the accumulation of at least some forms of amyloid oligomers is likely to be a consequence of very slow rates of rearrangement of their β-sheet structures. Our findings reveal the inherent multiplicity of the process of protein misfolding and the key role the β-sheet geometry acquired in the early stages of the self-assembly process plays in dictating the kinetic stability and the pathological nature of individual oligomeric species.

Expression and Transport of α-Synuclein at the Blood-Cerebrospinal Fluid Barrier and Effects of Manganese Exposure

The choroid plexus maintains the homeostasis of critical molecules in the brain by regulating their transport between the blood and cerebrospinal fluid (CSF). The current study was designed to investigate the potential role of the blood-CSF barrier (BCSFB) in α-synuclein (a-Syn) transport in the brain as affected by exposure to manganese (Mn), the toxic metal implicated in Parkinsonian disorders. Immunohistochemistry was used to identify intracellular a-Syn expression at the BCSFB. Quantitative real-time PCR was used to quantify the change in a-Syn mRNA expression following Mn treatments at the BCSFB in vitro. ELISA was used to quantify a-Syn levels following in vivo and in vitro treatments of Mn, copper (Cu), and/or external a-Syn. Thioflavin-T assay was used to investigate a-Syn aggregation after incubating with Mn and/or Cu in vitro. A two-chamber Transwell system was used to study a-Syn transport by BCSFB monolayer. Data revealed the expression of endogenous a-Syn in rat choroid plexus tissue and immortalized choroidal epithelial Z310 cells. The cultured primary choroidal epithelia from rats showed the ability to take up a-Syn from extracellular medium and transport a-Syn across the cellular monolayer from the donor to receiver chamber. Exposure of cells with Mn induced intracellular a-Syn accumulation without causing any significant changes in a-Syn mRNA expression. A significant increase in a-Syn aggregation in a cell-free system was observed with the presence of Mn. Moreover, Mn exposure resulted in a significant uptake of a-Syn by primary cells. These data indicate that the BCSFB expresses a-Syn endogenously and is capable of transporting a-Syn across the BCSFB monolayer; Mn exposure apparently increases a-Syn accumulation in the BCSFB by facilitating its uptake and intracellular aggregation.

Seeking a mechanism for the toxicity of oligomeric α-synuclein

In a number of neurological diseases including Parkinson’s disease (PD), α‑synuclein is aberrantly folded, forming abnormal oligomers, and amyloid fibrils within nerve cells. Strong evidence exists for the toxicity of increased production and aggregation of α-synuclein in vivo. The toxicity of α-synuclein is popularly attributed to the formation of “toxic oligomers”: a heterogenous and poorly characterized group of conformers that may share common molecular features. This review presents the available evidence on the properties of α-synuclein oligomers and the potential molecular mechanisms of their cellular disruption. Toxic α-synuclein oligomers may impact cells in a number of ways, including the disruption of membranes, mitochondrial depolarization, cytoskeleton changes, impairment of protein clearance pathways, and enhanced oxidative stress. We also examine the relationship between α-synuclein toxic oligomers and amyloid fibrils, in the light of recent studies that paint a more complex picture of α-synuclein toxicity. Finally, methods of studying and manipulating oligomers within cells are described.

Molecular determinants of α-synuclein mutants' oligomerization and membrane interactions

Parkinson's disease (PD) is associated with the formation of toxic α-synuclein oligomers that can penetrate the cell membrane. Familial forms of PD are caused by the point mutations A53T, A30P, E46K, and H50Q. Artificial point mutations E35K and E57K also increase oligomerization and pore formation. We generated structural conformations of α-synuclein and the above-mentioned mutants using molecular dynamics. We elucidated four main regions in these conformers contacting the membrane and found that the region including residues 39-45 (Zone2) may have maximum membrane penetration. E57K mutant had the highest rate of interaction with the membrane, followed by A53T, E46K, and E35K mutants and wild type (wt) α-synuclein. The mutant A30P had the smallest percentage of conformers that contact the membrane by Zone 2 than all other mutants and wt α-synuclein. These results were confirmed experimentally in vitro. We identified the key amino acids that can interact with the membrane (Y38, E62, and N65 (first hydrophilic layer); E104, E105, and D115 (second hydrophilic layer), and V15 and V26 (central hydrophobic layer)) and the residues that are involved in the interprotein contacts (L38, V48, V49, Q62, and T64). Understanding the molecular interactions of α-synuclein mutants is important for the design of compounds blocking the formation of toxic oligomers.

Insights on the interaction of alpha-synuclein and metals in the pathophysiology of Parkinson's disease

The interaction of different metals with the Parkinson's disease-associated protein alpha-synuclein results in oxidative stress, protein aggregation and pathology progression.

The H50Q mutation enhances α-synuclein aggregation, secretion, and toxicity

Over the last two decades, the identification of missense mutations in the α-synuclein (α-Syn) gene SNCA in families with inherited Parkinson disease (PD) has reinforced the central role of α-Syn in PD pathogenesis. Recently, a new missense mutation (H50Q) in α-Syn was described in patients with a familial form of PD and dementia. Here we investigated the effects of this novel mutation on the biophysical properties of α-Syn and the consequences for its cellular function. We found that the H50Q mutation affected neither the structure of free or membrane-bound α-Syn monomer, its interaction with metals, nor its capacity to be phosphorylated in vitro. However, compared with the wild-type (WT) protein, the H50Q mutation accelerated α-Syn fibrillization in vitro. In cell-based models, H50Q mutation did not affect α-Syn subcellular localization or its ability to be phosphorylated by PLK2 and GRK6. Interestingly, H50Q increased α-Syn secretion from SHSY5Y cells into culture medium and induced more mitochondrial fragmentation in hippocampal neurons. Although the transient overexpression of WT or H50Q did not induce toxicity, both species induced significant cell death when added to the culture medium of hippocampal neurons. Strikingly, H50Q exhibited more toxicity, suggesting that the H50Q-related enhancement of α-Syn aggregation and secretion may play a role in the extracellular toxicity of this mutant. Together, our results provide novel insight into the mechanism by which this newly described PD-associated mutation may contribute to the pathogenesis of PD and related disorders.