Site-specific perturbations of alpha-synuclein fibril structure by the Parkinson's disease associated mutations A53T and E46K

Structure of alpha-synuclein fibrils derived from human Lewy body dementia tissue

The defining feature of Parkinson disease (PD) and Lewy body dementia (LBD) is the accumulation of alpha-synuclein (Asyn) fibrils in Lewy bodies and Lewy neurites. Here we develop and validate a method to amplify Asyn fibrils extracted from LBD postmortem tissue samples and use solid state nuclear magnetic resonance (SSNMR) studies to determine atomic resolution structure. Amplified LBD Asyn fibrils comprise a mixture of single protofilament and two protofilament fibrils with very low twist. The protofilament fold is highly similar to the fold determined by a recent cryo-electron microscopy study for a minority population of twisted single protofilament fibrils extracted from LBD tissue. These results expand the structural characterization of LBD Asyn fibrils and approaches for studying disease mechanisms, imaging agents and therapeutics targeting Asyn.

Early-Onset Parkinson Mutation Remodels Monomer-Fibril Interactions to Allosterically Amplify Synuclein's Amyloid Cascade

Alpha synuclein (αS) aggregates are the main component of Lewy bodies (LBs) associated with Parkinson's disease (PD). A longstanding question about αS and PD pertains to the autosomal dominant E46K αS mutant, which leads to the early onset of PD and LB dementias. The E46K mutation not only promotes αS aggregation but also stabilizes αS monomers in "closed" conformers, which are compact and aggregation-incompetent. Hence, the mechanism of action of the E46K mutation is currently unclear. Here, we show that αS monomers harboring the E46K mutation exhibit more extensive interactions with fibrils compared to those of WT. Such monomer-fibril interactions are sufficient to allosterically drive transitions of αS monomers from closed to open conformations, enabling αS aggregation. We also show that E46K promotes head-to-tail monomer-monomer interactions in early self-association events. This multipronged mechanism provides a new framework to explain how the E46K mutation and possibly other αS variants trigger early-onset PD.

13C and 15N resonance assignments of alpha synuclein fibrils amplified from Lewy Body Dementia tissue

Fibrils of the protein α-synuclein (Asyn) are implicated in the pathogenesis of Parkinson Disease, Lewy Body Dementia, and Multiple System Atrophy. Numerous forms of Asyn fibrils have been studied by solid-state NMR and resonance assignments have been reported. Here, we report a new set of 13C, 15N assignments that are unique to fibrils obtained by amplification from postmortem brain tissue of a patient diagnosed with Lewy Body Dementia.

In silico investigation of the structural stability as the origin of the pathogenicity of α-synuclein protofibrils

α-Synuclein is a presynaptic neuronal protein. The fibril form of α-synuclein is a major constituent of the intraneuronal inclusion called Lewy body, a characteristic hallmark of Parkinson's disease. Recent ssNMR and cryo-EM experiments of wild-type α-synuclein fibrils have shown polymorphism and observed two major polymorphs, rod and twister. To associate the cytotoxicity of α-synuclein fibrils with their structural features, it is essential to understand the origins of their structural stability. In this study, we performed molecular dynamics simulations of the two major polymorphs of wild-type α-synuclein fibrils. The predominance of specific fibril polymorphs was rationalized in terms of relative structural stability in aqueous environments, which was attributed to the cooperative contributions of various stabilizing features. The results of the simulations indicated that highly stable structures in aqueous environments could be maintained by the cooperation of compact sidechain packing in the hydrophobic core, backbone geometry of the maximal β-sheet content wrapping the hydrophobic core, and solvent-exposed sidechains with large fluctuations maximizing the solvation entropy. The paired structure of the two protofilaments provides additional stability, especially at the interface region, by forming steric zipper interactions and hiding the hydrophobic residues from exposure to water. The sidechain interaction analyses and pulling simulations showed that the rod polymorph has stronger sidechain interactions and exhibits higher dissociation energy than the twister polymorph. It is expected that our study will provide a basis for understanding the pathogenic behaviors of diverse amyloid strains in terms of their structural properties.Communicated by Ramaswamy H. Sarma.

13C and 15N Resonance Assignments of Alpha Synuclein Fibrils Amplified from Lewy Body Dementia Tissue

Fibrils of the protein α-synuclein (Asyn) are implicated in the pathogenesis of Parkinson Disease, Lewy Body Dementia, and Multiple System Atrophy. Numerous forms of Asyn fibrils have been studied by solid-state NMR and resonance assignments have been reported. Here, we report a new set of 13C, 15N assignments that are unique to fibrils obtained by amplification from postmortem brain tissue of a patient diagnosed with Lewy Body Dementia.

Structure of alpha-synuclein fibrils derived from human Lewy body dementia tissue

The defining feature of Parkinson disease (PD) and Lewy body dementia (LBD) is the accumulation of alpha-synuclein (Asyn) fibrils in Lewy bodies and Lewy neurites. We developed and validated a novel method to amplify Asyn fibrils extracted from LBD postmortem tissue samples and used solid state nuclear magnetic resonance (SSNMR) studies to determine atomic resolution structure. Amplified LBD Asyn fibrils comprise two protofilaments with pseudo-21 helical screw symmetry, very low twist and an interface formed by antiparallel beta strands of residues 85-93. The fold is highly similar to the fold determined by a recent cryo-electron microscopy study for a minority population of twisted single protofilament fibrils extracted from LBD tissue. These results expand the structural landscape of LBD Asyn fibrils and inform further studies of disease mechanisms, imaging agents and therapeutics targeting Asyn.

Effects of Mutations and Post-Translational Modifications on α-Synuclein In Vitro Aggregation

Fibrillar aggregates of the α-synuclein (αS) protein are the hallmark of Parkinson's Disease and related neurodegenerative disorders. Characterization of the effects of mutations and post-translational modifications (PTMs) on the αS aggregation rate can provide insight into the mechanism of fibril formation, which remains elusive in spite of intense study. A comprehensive collection (375 examples) of mutant and PTM aggregation rate data measured using the fluorescent probe thioflavin T is presented, as well as a summary of the effects of fluorescent labeling on αS aggregation (20 examples). A curated set of 131 single mutant de novo aggregation experiments are normalized to wild type controls and analyzed in terms of structural data for the monomer and fibrillar forms of αS. These tabulated data serve as a resource to the community to help in interpretation of aggregation experiments and to potentially be used as inputs for computational models of aggregation.

Emerging Perspectives on Gene Therapy Delivery for Neurodegenerative and Neuromuscular Disorders

Neurodegenerative disorders (NDDs), such as Alzheimer's disease (AD) and Parkinson's Disease (PD), are a group of heterogeneous diseases that mainly affect central nervous system (CNS) functions. A subset of NDDs exhibit CNS dysfunction and muscle degeneration, as observed in Gangliosidosis 1 (GM1) and late stages of PD. Neuromuscular disorders (NMDs) are a group of diseases in which patients show primary progressive muscle weaknesses, including Duchenne Muscular Dystrophy (DMD), Pompe disease, and Spinal Muscular Atrophy (SMA). NDDs and NMDs typically have a genetic component, which affects the physiological functioning of critical cellular processes, leading to pathogenesis. Currently, there is no cure or efficient treatment for most of these diseases. More than 200 clinical trials have been completed or are currently underway in order to establish safety, tolerability, and efficacy of promising gene therapy approaches. Thus, gene therapy-based therapeutics, including viral or non-viral delivery, are very appealing for the treatment of NDDs and NMDs. In particular, adeno-associated viral vectors (AAV) are an attractive option for gene therapy for NDDs and NMDs. However, limitations have been identified after systemic delivery, including the suboptimal capacity of these therapies to traverse the blood-brain barrier (BBB), degradation of the particles during the delivery, high reactivity of the patient's immune system during the treatment, and the potential need for redosing. To circumvent these limitations, several preclinical and clinical studies have suggested intrathecal (IT) delivery to target the CNS and peripheral organs via cerebrospinal fluid (CSF). CSF administration can vastly improve the delivery of small molecules and drugs to the brain and spinal cord as compared to systemic delivery. Here, we review AAV biology and vector design elements, different therapeutic routes of administration, and highlight CSF delivery as an attractive route of administration. We discuss the different aspects of neuromuscular and neurodegenerative diseases, such as pathogenesis, the landscape of mutations, and the biological processes associated with the disease. We also describe the hallmarks of NDDs and NMDs as well as discuss current therapeutic approaches and clinical progress in viral and non-viral gene therapy and enzyme replacement strategies for those diseases.

DOPAnization of tyrosine in α-synuclein by tyrosine hydroxylase leads to the formation of oligomers

Parkinson's disease is a progressive neurodegenerative disorder characterized by the preferential loss of tyrosine hydroxylase (TH)-expressing dopaminergic neurons in the substantia nigra. Although the abnormal accumulation and aggregation of α-synuclein have been implicated in the pathogenesis of Parkinson's disease, the underlying mechanisms remain largely elusive. Here, we found that TH converts Tyr136 in α-synuclein into dihydroxyphenylalanine (DOPA; Y136DOPA) through mass spectrometric analysis. Y136DOPA modification was clearly detected by a specific antibody in the dopaminergic neurons of α-synuclein-overexpressing mice as well as human α-synucleinopathies. Furthermore, dopanized α-synuclein tended to form oligomers rather than large fibril aggregates and significantly enhanced neurotoxicity. Our findings suggest that the dopanization of α-synuclein by TH may contribute to oligomer and/or seed formation causing neurodegeneration with the potential to shed light on the pathogenesis of Parkinson's disease.

DOPAnization of tyrosine in α-synuclein by tyrosine hydroxylase leads to the formation of oligomers

Parkinson's disease is a progressive neurodegenerative disorder characterized by the preferential loss of tyrosine hydroxylase (TH)-expressing dopaminergic neurons in the substantia nigra. Although the abnormal accumulation and aggregation of α-synuclein have been implicated in the pathogenesis of Parkinson's disease, the underlying mechanisms remain largely elusive. Here, we found that TH converts Tyr136 in α-synuclein into dihydroxyphenylalanine (DOPA; Y136DOPA) through mass spectrometric analysis. Y136DOPA modification was clearly detected by a specific antibody in the dopaminergic neurons of α-synuclein-overexpressing mice as well as human α-synucleinopathies. Furthermore, dopanized α-synuclein tended to form oligomers rather than large fibril aggregates and significantly enhanced neurotoxicity. Our findings suggest that the dopanization of α-synuclein by TH may contribute to oligomer and/or seed formation causing neurodegeneration with the potential to shed light on the pathogenesis of Parkinson's disease.

Wide-Line NMR Melting Diagrams, Their Thermodynamic Interpretation, and Secondary Structure Predictions for A30P and E46K α-Synuclein

Parkinson's disease is thought to be caused by aggregation of the intrinsically disordered protein, α-synuclein. Two amyloidogenic variants, A30P, and E46K familial mutants were investigated by wide-line ¹H NMR spectrometry as a completion of our earlier work on wild-type and A53T α-synuclein (Bokor M. et al. WT and A53T α-synuclein systems: melting diagram and its new interpretation. Int. J. Mol. Sci.2020, 21, 3997.). A monolayer of mobile water molecules hydrates A30P α-synuclein at the lowest potential barriers (temperatures), while E46K α-synuclein has here third as much mobile hydration, insufficient for functionality. According to wide-line ¹H NMR results and secondary structure predictions, E46K α-synuclein is more compact than the A30P variant and they are more compact than the wild type (WT) and A53T variants. Linear hydration vs potential barrier sections of A30P α-synuclein shows one and E46K shows two slopes. The different slopes of the latter between potential barriers E _a,1 and E _a,2 reflect a change in water-protein interactions. The 31-32% homogeneous potential barrier distribution of the protein-water bonds refers to a non-negligible amount of secondary structures in all four α-synuclein variants. The secondary structures detected by wide-line ¹H NMR are solvent-exposed α-helices, which are predicted by secondary structure models. β-sheets are only minor components of the protein structures as three- and eight-state predicted secondary structures are dominated by α-helices and coils.

Examining the Toxicity of α-Synuclein in Neurodegenerative Disorders

Simple Summary

Neurodegenerative disorders are complex disorders that display a variety of clinical manifestations. The second-most common neurodegenerative disorder is Parkinson’s disease, and the leading pathological protein of the disorder is considered to be α-synuclein. Nonetheless, α-synuclein accumulation also seems to result in multiple system atrophy and dementia with Lewy bodies. In order to obtain a more proficient understanding in the pathological progression of these synucleinopathies, it is crucial to observe the post-translational modifications of α-synuclein and the conformations of α-synuclein, as well as its role in the dysfunction of cellular pathways.

Abstract

α-synuclein is considered the main pathological protein in a variety of neurodegenerative disorders, such as Parkinson’s disease, multiple system atrophy, and dementia with Lewy bodies. As of now, numerous studies have been aimed at examining the post-translational modifications of α-synuclein to determine their effects on α-synuclein aggregation, propagation, and oligomerization, as well as the potential cellular pathway dysfunctions caused by α-synuclein, to determine the role of the protein in disease progression. Furthermore, α-synuclein also appears to contribute to the fibrilization of tau and amyloid beta, which are crucial proteins in Alzheimer’s disease, advocating for α-synuclein’s preeminent role in neurodegeneration. Due to this, investigating the mechanisms of toxicity of α-synuclein in neurodegeneration may lead to a more proficient understanding of the timeline progression in neurodegenerative synucleinopathies and could thereby lead to the development of potent targeted therapies.

Potent inhibitors of toxic alpha-synuclein identified via cellular time-resolved FRET biosensors

We have developed a high-throughput drug discovery platform, measuring fluorescence resonance energy transfer (FRET) with fluorescent alpha-synuclein (αSN) biosensors, to detect spontaneous pre-fibrillar oligomers in living cells. Our two αSN FRET biosensors provide complementary insight into αSN oligomerization and conformation in order to improve the success of drug discovery campaigns for the treatment of Parkinson's disease. We measure FRET by fluorescence lifetime, rather than traditional fluorescence intensity, providing a structural readout with greater resolution and precision. This facilitates identification of compounds that cause subtle but significant conformational changes in the ensemble of oligomeric states that are easily missed using intensity-based FRET. We screened a 1280-compound small-molecule library and identified 21 compounds that changed the lifetime by >5 SD. Two of these compounds have nanomolar potency in protecting SH-SY5Y cells from αSN-induced death, providing a nearly tenfold improvement over known inhibitors. We tested the efficacy of several compounds in a primary mouse neuron assay of αSN pathology (phosphorylation of mouse αSN pre-formed fibrils) and show rescue of pathology for two of them. These hits were further characterized with biophysical and biochemical assays to explore potential mechanisms of action. In vitro αSN oligomerization, single-molecule FRET, and protein-observed fluorine NMR experiments demonstrate that these compounds modulate αSN oligomers but not monomers. Subsequent aggregation assays further show that these compounds also deter or block αSN fibril assembly.

Tau induces formation of α-synuclein filaments with distinct molecular conformations

Recent structural investigation of amyloid filaments extracted from human patients demonstrated that the ex vivo filaments associated with different disease phenotypes adopt diverse molecular conformations, which are different from those of in vitro amyloid filaments. A very recent cryo-EM structural study also revealed that ex vivo α-synuclein filaments extracted from multiple system atrophy patients adopt distinct molecular structures from those of in vitro α-synuclein filaments, suggesting the presence of co-factors for α-synuclein aggregation in vivo. Here, we report structural characterizations of α-synuclein filaments formed in the presence of a potential co-factor, tau, using cryo-EM and solid-state NMR. Our cryo-EM structure of the tau-promoted α-synuclein filaments reveals some similarities to one of the previously reported polymorphs of in vitro α-synuclein filaments in the core region, while illustrating distinct conformations in the N- and C-terminal regions. The structural study highlights the conformational plasticity of α-synuclein filaments and the importance of the co-factors, requiring additional structural investigation of not only more ex vivo α-synuclein filaments, but also in vitro α-synuclein filaments formed in the presence of diverse co-factors. The comparative structural analyses will help better understand molecular basis of diverse structures of α-synuclein filaments and possible relevance of each structure to the disease phenotype.

Two C-terminal sequence variations determine differential neurotoxicity between human and mouse α-synuclein

BACKGROUND

α-Synuclein (aSyn) aggregation is thought to play a central role in neurodegenerative disorders termed synucleinopathies, including Parkinson's disease (PD). Mouse aSyn contains a threonine residue at position 53 that mimics the human familial PD substitution A53T, yet in contrast to A53T patients, mice show no evidence of aSyn neuropathology even after aging. Here, we studied the neurotoxicity of human A53T, mouse aSyn, and various human-mouse chimeras in cellular and in vivo models, as well as their biochemical properties relevant to aSyn pathobiology.

METHODS

Primary midbrain cultures transduced with aSyn-encoding adenoviruses were analyzed immunocytochemically to determine relative dopaminergic neuron viability. Brain sections prepared from rats injected intranigrally with aSyn-encoding adeno-associated viruses were analyzed immunohistochemically to determine nigral dopaminergic neuron viability and striatal dopaminergic terminal density. Recombinant aSyn variants were characterized in terms of fibrillization rates by measuring thioflavin T fluorescence, fibril morphologies via electron microscopy and atomic force microscopy, and protein-lipid interactions by monitoring membrane-induced aSyn aggregation and aSyn-mediated vesicle disruption. Statistical tests consisted of ANOVA followed by Tukey's multiple comparisons post hoc test and the Kruskal-Wallis test followed by a Dunn's multiple comparisons test or a two-tailed Mann-Whitney test.

RESULTS

Mouse aSyn was less neurotoxic than human aSyn A53T in cell culture and in rat midbrain, and data obtained for the chimeric variants indicated that the human-to-mouse substitutions D121G and N122S were at least partially responsible for this decrease in neurotoxicity. Human aSyn A53T and a chimeric variant with the human residues D and N at positions 121 and 122 (respectively) showed a greater propensity to undergo membrane-induced aggregation and to elicit vesicle disruption. Differences in neurotoxicity among the human, mouse, and chimeric aSyn variants correlated weakly with differences in fibrillization rate or fibril morphology.

CONCLUSIONS

Mouse aSyn is less neurotoxic than the human A53T variant as a result of inhibitory effects of two C-terminal amino acid substitutions on membrane-induced aSyn aggregation and aSyn-mediated vesicle permeabilization. Our findings highlight the importance of membrane-induced self-assembly in aSyn neurotoxicity and suggest that inhibiting this process by targeting the C-terminal domain could slow neurodegeneration in PD and other synucleinopathy disorders.

Parkinson's disease associated mutation E46K of α-synuclein triggers the formation of a distinct fibril structure

Amyloid aggregation of α-synuclein (α-syn) is closely associated with Parkinson's disease (PD) and other synucleinopathies. Several single amino-acid mutations (e.g. E46K) of α-syn have been identified causative to the early onset of familial PD. Here, we report the cryo-EM structure of an α-syn fibril formed by N-terminally acetylated E46K mutant α-syn (Ac-E46K). The fibril structure represents a distinct fold of α-syn, which demonstrates that the E46K mutation breaks the electrostatic interactions in the wild type (WT) α-syn fibril and thus triggers the rearrangement of the overall structure. Furthermore, we show that the Ac-E46K fibril is less resistant to harsh conditions and protease cleavage, and more prone to be fragmented with an enhanced seeding capability than that of the WT fibril. Our work provides a structural view to the severe pathology of the PD familial mutation E46K of α-syn and highlights the importance of electrostatic interactions in defining the fibril polymorphs.

Synaptic vesicle mimics affect the aggregation of wild-type and A53T α-synuclein variants differently albeit similar membrane affinity

α-Synuclein misfolding results in the accumulation of amyloid fibrils in Parkinson’s disease. Missense protein mutations (e.g. A53T) have been linked to early onset disease. Although α-synuclein interacts with synaptic vesicles in the brain, it is not clear what role they play in the protein aggregation process. Here, we compare the effect of small unilamellar vesicles (lipid composition similar to synaptic vesicles) on wild-type (WT) and A53T α-synuclein aggregation. Using biophysical techniques, we reveal that binding affinity to the vesicles is similar for the two proteins, and both interact with the helix long axis parallel to the membrane surface. Still, the vesicles affect the aggregation of the variants differently: effects on secondary processes such as fragmentation dominate for WT, whereas for A53T, fibril elongation is mostly affected. We speculate that vesicle interactions with aggregate intermediate species, in addition to monomer binding, vary between WT and A53T, resulting in different consequences for amyloid formation.

Do Lewy bodies contain alpha-synuclein fibrils? and Does it matter? A brief history and critical analysis of recent reports

Several lines of evidence from neuropathological studies, human genetics, in vitro aggregation studies and cellular and animal models support the hypothesis that aSyn plays a central role in the formation of Lewy pathologies. These are cytoplasmic proteinaceous and lipid-rich inclusions that represent key pathological hallmarks of Parkinson's disease (PD) and other neurodegenerative diseases, collectively referred to as synucleinopathies. For decades, light microscopy and electron microscopy studies of these inclusions have consistently shown that they are rich in filamentous structures that exhibit distinct distribution and organizational patterns depending on where they occur in the brain (e.g., classical brain-stem Lewy bodies (LBs) and cortical LBs) and the type of synucleinopathies. Although the identity of the protein that form these filaments was a subject of debate for decades, the discovery of PD-linked aSyn mutations, the demonstration that LBs are enriched in insoluble forms of aSyn, and the ability of aSyn to form fibrils of similar dimensions have led to convergence on the hypothesis that aSyn fibrils are key components of LBs. In a recent study, Shahmoradian et al used a combination of advanced electron microscopy and immunofluorescence based imaging techniques to investigate the structure, composition, and architecture of LBs from postmortem brain tissues of individuals with PD or other synucleinopathies (Shahmoradian et al., 2019). The paper's main conclusions suggest that "lipid membrane fragments and distorted organelles together with a non-fibrillar form of αSyn are the main structural building blocks for the formation of Lewy pathology". Their proposal that LBs are devoid of aSyn fibrils or that LB formation occurs independently of aSyn fibril formation casts doubts on a substantial body of work that forms the foundation of many of the current basic and translational research programs in academia and industry. In this article, I present a critical analysis of their data and claims in the context of the existing literature In addition, I examine the extent to which their findings and proposed models of the mechanisms of LB formation are consistent with existing data and are supported by other experimental evidence. The results from this analysis caution against overinterpretation of observations from a single report, especially given the limitations of the techniques and experimental approaches used by Shahmoradian et al and for more collaborative and systematic efforts to revisit and characterize LBs and other aSyn pathologies in the brain pathologies at the biochemical, morphological and structural level.

The α-synuclein hereditary mutation E46K unlocks a more stable, pathogenic fibril structure

Aggregation of α-synuclein is a defining molecular feature of Parkinson's disease, Lewy body dementia, and multiple systems atrophy. Hereditary mutations in α-synuclein are linked to both Parkinson's disease and Lewy body dementia; in particular, patients bearing the E46K disease mutation manifest a clinical picture of parkinsonism and Lewy body dementia, and E46K creates more pathogenic fibrils in vitro. Understanding the effect of these hereditary mutations on α-synuclein fibril structure is fundamental to α-synuclein biology. We therefore determined the cryo-electron microscopy (cryo-EM) structure of α-synuclein fibrils containing the hereditary E46K mutation. The 2.5-Å structure reveals a symmetric double protofilament in which the molecules adopt a vastly rearranged, lower energy fold compared to wild-type fibrils. We propose that the E46K misfolding pathway avoids electrostatic repulsion between K46 and K80, a residue pair which form the E46-K80 salt bridge in the wild-type fibril structure. We hypothesize that, under our conditions, the wild-type fold does not reach this deeper energy well of the E46K fold because the E46-K80 salt bridge diverts α-synuclein into a kinetic trap-a shallower, more accessible energy minimum. The E46K mutation apparently unlocks a more stable and pathogenic fibril structure.

Navigating the dynamic landscape of alpha-synuclein morphology: a review of the physiologically relevant tetrameric conformation

N-acetylated α-synuclein (αSyn) has long been established as an intrinsically disordered protein associated with a dysfunctional role in Parkinson's disease. In recent years, a physiologically relevant, higher order conformation has been identified as a helical tetramer that is tailored by buried hydrophobic interactions and is distinctively aggregation resistant. The canonical mechanism by which the tetramer assembles remains elusive. As novel biochemical approaches, computational methods, pioneering purification platforms, and powerful imaging techniques continue to develop, puzzling information that once sparked debate as to the veracity of the tetramer has now shed light upon this new counterpart in αSyn neurobiology. Nuclear magnetic resonance and computational studies on multimeric αSyn structure have revealed that the protein folding propensity is controlled by small energy barriers that enable large scale reconfiguration. Alternatively, familial mutations ablate tetramerization and reconfigure polymorphic fibrillization. In this review, we will discuss the dynamic landscape of αSyn quaternary structure with a focus on the tetrameric conformation.

Structures of fibrils formed by α-synuclein hereditary disease mutant H50Q reveal new polymorphs

Deposits of amyloid fibrils of α-synuclein are the histological hallmarks of Parkinson’s disease, dementia with Lewy bodies, and multiple system atrophy, with hereditary mutations in α-synuclein linked to the first two of these conditions. Seeing the changes to the structures of amyloid fibrils bearing these mutations may help to understand these diseases. To this end, we determined the cryo-EM structures of α-synuclein fibrils containing the H50Q hereditary mutation. We find that the H50Q mutation results in two previously unobserved polymorphs of α-synuclein: Narrow and Wide Fibrils, formed from either one or two protofilaments, respectively. These structures recapitulate conserved features of the wild-type fold but reveal new structural elements including a previously unobserved hydrogen bond network and surprising new protofilament arrangements. The structures of the H50Q polymorphs help to rationalize the faster aggregation kinetics, higher seeding capacity in biosensor cells, and greater cytotoxicity we observe for H50Q compared to wild-type α-synuclein.

A Protofilament-Protofilament Interface in the Structure of Mouse α-Synuclein Fibrils

Fibrillar α-synuclein (AS) is the major component of Lewy bodies, the pathological hallmark of Parkinson's disease. Using solid-state nuclear magnetic resonance (ssNMR), we previously reported a structural characterization of mouse AS (mAS) fibrils and found that the secondary structure of the mAS fibrils is highly similar to a form of human AS (hAS) fibrils. Recently, a three-dimensional structure of these same hAS fibrils was determined by ssNMR and scanning transmission electron microscopy. Using medium- and long-range distance restraints obtained from ssNMR spectra, we found that the single protofilament structure of mAS fibrils is also similar to that of the hAS fibrils. However, residue-specific water accessibility of mAS fibrils probed by water polarization transfer ssNMR measurements indicates that residues S42-T44 and G84-V95 are largely protected from water even though they are located at the edge of the protofilament. Some of the corresponding resonances also exhibit peak doubling. These observations suggest that these residues may be involved in, to our knowledge, a novel protofilament-protofilament interface. We propose a structural model of mAS fibrils that incorporates this dimer interface.

Mechanisms of Strain Diversity of Disease-Associated in-Register Parallel β-Sheet Amyloids and Implications About Prion Strains

The mechanism of prion strain diversity remains unsolved. Investigation of inheritance and diversification of protein-based pathogenic information demands the identification of the detailed structures of abnormal isoforms of the prion protein (PrP^Sc); however, achieving purification is difficult without affecting infectivity. Similar prion-like properties are recognized also in other disease-associated in-register parallel β-sheet amyloids including Tau and α-synuclein (αSyn) amyloids. Investigations into structures of those amyloids via solid-state nuclear magnetic resonance spectroscopy and cryo-electron microscopy recently made remarkable advances due to their relatively small sizes and lack of post-translational modifications. Herein, we review advances regarding pathogenic amyloids, particularly Tau and αSyn, and discuss implications about strain diversity mechanisms of prion/PrP^Sc from the perspective that PrP^Sc is an in-register parallel β-sheet amyloid. Additionally, we present our recent data of molecular dynamics simulations of αSyn amyloid, which suggest significance of compatibility between β-sheet propensities of the substrate and local structures of the template for stability of amyloid structures. Detailed structures of αSyn and Tau amyloids are excellent models of pathogenic amyloids, including PrP^Sc, to elucidate strain diversity and pathogenic mechanisms.

Computational insights into the role of α-strand/sheet in aggregation of α-synuclein

The α-synuclein is a major component of amyloid fibrils found in Lewy bodies, the characteristic intracellular proteinaceous deposits which are pathological hallmarks of neurodegenerative diseases such as Parkinson’s disease (PD) and dementia. It is an intrinsically disordered protein that may undergo dramatic structural changes to form amyloid fibrils. Aggregation process from α-synuclein monomers to amyloid fibrils through oligomeric intermediates is considered as the disease-causative toxic mechanism. However, mechanism underlying aggregation is not well-known despite several attempts. To characterize the mechanism, we have explored the effects of pH and temperature on the structural properties of wild-type and mutant α-synuclein using molecular dynamics (MD) simulation technique. MD studies suggested that amyloid fibrils can grow by monomer. Conformational transformation of the natively unfolded protein into partially folded intermediate could be accountable for aggregation and fibrillation. An intermediate α-strand was observed in the hydrophobic non-amyloid-β component (NAC) region of α-synuclein that could proceed to α-sheet and initiate early assembly events. Water network around the intermediate was analyzed to determine its influence on the α-strand structure. Findings of this study provide novel insights into possible mechanism of α-synuclein aggregation and promising neuroprotective strategy that could aid alleviate PD and its symptoms.

Biochemical and morphological classification of disease-associated alpha-synuclein mutants aggregates

Alpha-synuclein (a-syn) aggregation in brain is implicated in several synucleinopathies, including Parkinson's disease (PD), dementia with Lewy bodies (DLB), and multiple system atrophy (MSA). Until date, at least six disease-associated mutations in a-syn (namely A30P, E46K, H50Q, G51D, A53T, and A53E) are known to cause dominantly inherited familial forms of synucleinopathies. Previous studies using recombinant proteins have reported that a subset of disease-associated mutants show higher aggregation propensities and form spectroscopically distinguishable aggregates compared to wild-type (WT). However, morphological and biochemical comparison of the aggregates for all disease-associated a-syn mutants have not yet been performed. In this study, we performed electron microscopic examination, guanidinium hydrochloride (GdnHCl) denaturation, and protease digestion to classify the aggregates from their respective point mutations. Using electron microscopy we observed variations of amyloid fibrillar morphologies among the aggregates of a-syn mutants, mainly categorized into two groups: twisted fibrils observed for both WT and E46K while straight fibrils for the other mutants. GdnHCl denaturation experiments revealed the a-syn mutants except for E46K were more resistant than WT against the denaturation. Mass spectrometry analysis of protease-treated aggregates showed a variety of protease-resistant cores, which may correspond to their morphological properties. The difference of their properties could be implicated in the clinicopathological difference of synucleinopathies with those mutations.

Physiological C-terminal truncation of α-synuclein potentiates the prion-like formation of pathological inclusions

α-Synuclein (αsyn) aggregates into toxic fibrils in multiple neurodegenerative diseases where these fibrils form characteristic pathological inclusions such as Lewy bodies (LBs). The mechanisms initiating αsyn aggregation into fibrils are unclear, but ubiquitous post-translational modifications of αsyn present in LBs may play a role. Specific C-terminally (C)-truncated forms of αsyn are present within human pathological inclusions and form under physiological conditions likely in lysosome-associated pathways, but the roles for these C-truncated forms of αsyn in inclusion formation and disease are not well understood. Herein, we characterized the in vitro aggregation properties, amyloid fibril structures, and ability to induce full-length (FL) αsyn aggregation through prion-like mechanisms for eight of the most common physiological C-truncated forms of αsyn (1-115, 1-119, 1-122, 1-124, 1-125, 1-129, 1-133, and 1-135). In vitro, C-truncated αsyn aggregated more readily than FL αsyn and formed fibrils with unique morphologies. The presence of C-truncated αsyn potentiated aggregation of FL αsyn in vitro through co-polymerization. Specific C-truncated forms of αsyn in cells also exacerbated seeded aggregation of αsyn. Furthermore, in primary neuronal cultures, co-polymers of C-truncated and FL αsyn were potent prion-like seeds, but polymers composed solely of the C-truncated protein were not. These experiments indicated that specific physiological C-truncated forms of αsyn have distinct aggregation properties, including the ability to modulate the prion-like aggregation and seeding activity of FL αsyn. Proteolytic formation of these C-truncated species may have an important role in both the initiation of αsyn pathological inclusions and further progression of disease with strain-like properties.

Role of Sporadic Parkinson Disease Associated Mutations A18T and A29S in Enhanced α-Synuclein Fibrillation and Cytotoxicity

Deposition of presynaptic protein α-synuclein in Lewy bodies and Lewy neurites in the substantia nigra region of brain has been linked with the clinical symptoms of the Parkinson's disease (PD). Proteotoxic stress conditions and mutations that cause abnormal aggregation of α-synuclein have close association with onset of PD and its progression. Therefore, studies pertaining to α-synuclein mutations play important roles in mechanistic understanding of aggregation behavior of the protein and subsequent pathology. Herein, guided by this fact, we have studied the aggregation kinetics, morphology, and neurotoxic effects of the two newly discovered sporadic PD associated mutants A18T and A29S of α-synuclein. Our studies demonstrate that both of the mutants are aggregation prone and undergo rapid aggregation compared to wild-type α-synuclein. Further, it was found that A18T mutant followed faster aggregation kinetics compared to A29S substitution. Additionally, we have designed three point mutations of α-synuclein for better understanding of the effects of substitutions on protein aggregation and demonstrated that substitution of alanine at the 18th position is highly sensitive compared to adjacent positions. Our results provide better understanding of the effects of α-synuclein mutations on its aggregation behavior that may be important in development of PD pathology.

Aggregation and Fibril Structure of AβM01-42 and Aβ1-42

A mechanistic understanding of Aβ aggregation and high-resolution structures of Aβ fibrils and oligomers are vital to elucidating relevant details of neurodegeneration in Alzheimer's disease, which will facilitate the rational design of diagnostic and therapeutic protocols. The most detailed and reproducible insights into structure and kinetics have been achieved using Aβ peptides produced by recombinant expression, which results in an additional methionine at the N-terminus. While the length of the C-terminus is well established to have a profound impact on the peptide's aggregation propensity, structure, and neurotoxicity, the impact of the N-terminal methionine on the aggregation pathways and structure is unclear. For this reason, we have developed a protocol to produce recombinant Aβ1-42, sans the N-terminal methionine, using an N-terminal small ubiquitin-like modifier-Aβ1-42 fusion protein in reasonable yield, with which we compared aggregation kinetics with AβM01-42 containing the additional methionine residue. The data revealed that Aβ1-42 and AβM01-42 aggregate with similar rates and by the same mechanism, in which the generation of new aggregates is dominated by secondary nucleation of monomers on the surface of fibrils. We also recorded magic angle spinning nuclear magnetic resonance spectra that demonstrated that excellent spectral resolution is maintained with both AβM01-42 and Aβ1-42 and that the chemical shifts are virtually identical in dipolar recoupling experiments that provide information about rigid residues. Collectively, these results indicate that the structure of the fibril core is unaffected by N-terminal methionine. This is consistent with the recent structures of AβM01-42 in which M0 is located at the terminus of a disordered 14-amino acid N-terminal tail.

Post translational changes to α-synuclein control iron and dopamine trafficking; a concept for neuron vulnerability in Parkinson's disease

Parkinson's disease is a multifactorial neurodegenerative disorder, the aetiology of which remains elusive. The primary clinical feature of progressively impaired motor control is caused by a loss of midbrain substantia nigra dopamine neurons that have a high α-synuclein (α-syn) and iron content. α-Syn is a neuronal protein that is highly modified post-translationally and central to the Lewy body neuropathology of the disease. This review provides an overview of findings on the role post translational modifications to α-syn have in membrane binding and intracellular vesicle trafficking. Furthermore, we propose a concept in which acetylation and phosphorylation of α-syn modulate endocytic import of iron and vesicle transport of dopamine during normal physiology. Disregulated phosphorylation and oxidation of α-syn mediate iron and dopamine dependent oxidative stress through impaired cellular location and increase propensity for α-syn aggregation. The proposition highlights a connection between α-syn, iron and dopamine, three pathological components associated with disease progression in sporadic Parkinson's disease.

Familial Mutations May Switch Conformational Preferences in α-Synuclein Fibrils

The pathogenesis of Parkinson's disease is closely associated with the aggregation of the α-synuclein protein. Several familial mutants have been identified and shown to affect the aggregation kinetics of α-synuclein through distinct molecular mechanisms. Quantitative evaluation of the relative stabilities of the wild type and mutant fibrils is crucial for understanding the aggregation process and identifying the key component steps. In this work, we examined two topologically different α-synuclein fibril structures that are either determined by solid-state NMR method or modeled based on solid-state NMR data, and characterized their conformational properties and thermodynamic stabilities using molecular dynamics simulations. We show that the two fibril morphologies have comparable size, solvent exposure, secondary structures, and similar molecule/peptide binding modes; but different stabilities. Familial mutations do not significantly alter the overall fibril structures but shift their relative stabilities. Distinct mutations display altered fibril conformational behavior, suggesting different propagation preferences, reminiscent of cross-seeding among prion strains and tau deletion mutants. The simulations quantify the hydrophobic and electrostatic interactions, as well as N-terminal dynamics, that may contribute to the divergent aggregation kinetics that has been observed experimentally. Our results indicate that small molecule and peptide inhibitors may share the same binding region, providing molecular recognition that is independent of fibril conformation.

A sensitive assay reveals structural requirements for α-synuclein fibril growth

The accumulation of α-synuclein (α-syn) fibrils in neuronal inclusions is the defining pathological process in Parkinson's disease (PD). A pathogenic role for α-syn fibril accumulation is supported by the identification of dominantly inherited α-syn (SNCA) gene mutations in rare cases of familial PD. Fibril formation involves a spontaneous nucleation event in which soluble α-syn monomers associate to form seeds, followed by fibril growth during which monomeric α-syn molecules sequentially associate with existing seeds. To better investigate this process, we developed sensitive assays that use the fluorescein arsenical dye FlAsH (fluorescein arsenical hairpin binder) to detect soluble oligomers and mature fibrils formed from recombinant α-syn protein containing an N-terminal bicysteine tag (C2-α-syn). Using seed growth by monomer association (SeGMA) assays to measure fibril growth over 3 h in the presence of C2-α-syn monomer, we observed that some familial PD-associated α-syn mutations (i.e. H50Q and A53T) greatly increased growth rates, whereas others (E46K, A30P, and G51D) decreased growth rates. Experiments with wild-type seeds extended by mutant monomer and vice versa revealed that single-amino acid differences between seed and monomer proteins consistently decreased growth rates. These results demonstrate that α-syn monomer association during fibril growth is a highly ordered process that can be disrupted by misalignment of individual amino acids and that only a subset of familial-PD mutations causes fibril accumulation through increased fibril growth rates. The SeGMA assays reported herein can be utilized to further elucidate structural requirements of α-syn fibril growth and to identify growth inhibitors as a potential therapeutic approach in PD.

The Contribution of α-Synuclein Spreading to Parkinson's Disease Synaptopathy

Synaptopathies are diseases with synapse defects as shared pathogenic features, encompassing neurodegenerative disorders such as Parkinson's disease (PD). In sporadic PD, the most common age-related neurodegenerative movement disorder, nigrostriatal dopaminergic deficits are responsible for the onset of motor symptoms that have been related to α-synuclein deposition at synaptic sites. Indeed, α-synuclein accumulation can impair synaptic dopamine release and induces the death of nigrostriatal neurons. While in physiological conditions the protein can interact with and modulate synaptic vesicle proteins and membranes, numerous experimental evidences have confirmed that its pathological aggregation can compromise correct neuronal functioning. In addition, recent findings indicate that α-synuclein pathology spreads into the brain and can affect the peripheral autonomic and somatic nervous system. Indeed, monomeric, oligomeric, and fibrillary α-synuclein can move from cell to cell and can trigger the aggregation of the endogenous protein in recipient neurons. This novel “prion-like” behavior could further contribute to synaptic failure in PD and other synucleinopathies. This review describes the major findings supporting the occurrence of α-synuclein pathology propagation in PD and discusses how this phenomenon could induce or contribute to synaptic injury and degeneration.

Nanomolar oligomerization and selective co-aggregation of α-synuclein pathogenic mutants revealed by single-molecule fluorescence

Protein aggregation is a hallmark of many neurodegenerative diseases, notably Alzheimer's and Parkinson's disease. Parkinson's disease is characterized by the presence of Lewy bodies, abnormal aggregates mainly composed of α-synuclein. Moreover, cases of familial Parkinson's disease have been linked to mutations in α-synuclein. In this study, we compared the behavior of wild-type (WT) α-synuclein and five of its pathological mutants (A30P, E46K, H50Q, G51D and A53T). To this end, single-molecule fluorescence detection was coupled to cell-free protein expression to measure precisely the oligomerization of proteins without purification, denaturation or labelling steps. In these conditions, we could detect the formation of oligomeric and pre-fibrillar species at very short time scale and low micromolar concentrations. The pathogenic mutants surprisingly segregated into two classes: one group forming large aggregates and fibrils while the other tending to form mostly oligomers. Strikingly, co-expression experiments reveal that members from the different groups do not generally interact with each other, both at the fibril and monomer levels. Together, this data paints a completely different picture of α-synuclein aggregation, with two possible pathways leading to the development of fibrils.

Potential Role of Caffeine in the Treatment of Parkinson's Disease

Parkinson’s disease [PD] is the second most common neurodegenerative disorder after Alzheimer’s disease, affecting 1% of the population over the age of 55. The underlying neuropathology seen in PD is characterised by progressive loss of dopaminergic neurons in the substantia nigra pars compacta with the presence of Lewy bodies. The Lewy bodies are composed of aggregates of α-synuclein. The motor manifestations of PD include a resting tremor, bradykinesia, and muscle rigidity. Currently there is no cure for PD and motor symptoms are treated with a number of drugs including levodopa [L-dopa]. These drugs do not delay progression of the disease and often provide only temporary relief. Their use is often accompanied by severe adverse effects. Emerging evidence from both in vivo and in vitro studies suggests that caffeine may reduce parkinsonian motor symptoms by antagonising the adenosine A_2A receptor, which is predominately expressed in the basal ganglia. It is hypothesised that caffeine may increase the excitatory activity in local areas by inhibiting the astrocytic inflammatory processes but evidence remains inconclusive. In addition, the co-administration of caffeine with currently available PD drugs helps to reduce drug tolerance, suggesting that caffeine may be used as an adjuvant in treating PD. In conclusion, caffeine may have a wide range of therapeutic effects which are yet to be explored, and therefore warrants further investigation in randomized clinical trials.

Solid-state NMR structure of a pathogenic fibril of full-length human α-synuclein

Misfolded α-synuclein amyloid fibrils are the principal components of Lewy bodies and neurites, hallmarks of Parkinson's disease (PD). We present a high-resolution structure of an α-synuclein fibril, in a form that induces robust pathology in primary neuronal culture, determined by solid-state NMR spectroscopy and validated by EM and X-ray fiber diffraction. Over 200 unique long-range distance restraints define a consensus structure with common amyloid features including parallel, in-register β-sheets and hydrophobic-core residues, and with substantial complexity arising from diverse structural features including an intermolecular salt bridge, a glutamine ladder, close backbone interactions involving small residues, and several steric zippers stabilizing a new orthogonal Greek-key topology. These characteristics contribute to the robust propagation of this fibril form, as supported by the structural similarity of early-onset-PD mutants. The structure provides a framework for understanding the interactions of α-synuclein with other proteins and small molecules, to aid in PD diagnosis and treatment.

ɑ-Synuclein strains and the variable pathologies of synucleinopathies

Several decades ago, a mysterious transmissible agent was found responsible for a group of progressive and lethal encephalopathies affecting the nervous system of both animals and humans. This infectious agent showed a strain-encoded manner of inheritance even though it lacked nucleic acids. The identification of infectious proteins resolved this apparent conundrum. Misfolded infectious protein particles, or prions, were found to exist as conformational isomers with a unique fingerprint that can be faithfully passaged to next generations. Protein-based strain-encoded inheritance is characterized by strain-specific infectivity and symptomatology. It is found in diverse organisms, such as yeast, fungi, and mammals. Now, this concept is revisited to examine the pathological role of amyloid proteins involved in neurodegenerative diseases where it might underlie certain types of dementia and motor-related neurodegenerative disorders. Given the discovery of the SNCA gene and the identification of its gene product, ɑ-synuclein (ɑ-SYN), as the main histopathological component of Parkinson's disease, dementia with Lewy bodies and multiple system atrophy, the scientific community was left puzzled by the fact that a single protein appeared to be involved in different diseases with diverging clinical phenotypes. Recent studies are now indicating that ɑ-SYN may act in a way similar to prions and that ɑ-SYN misfolded structural variants may behave as strains with distinct biochemical and functional properties inducing specific phenotypic traits, which might finally provide an explanation for the clinical heterogeneity observed between Parkinson's disease, MSA, and dementia with Lewy bodies patients. These crucial new findings may pave the way for unexplored therapeutic avenues and identification of new potential biomarkers. Parkinson's disease and other synucleinopathies share ɑ-synuclein deposits as a common histopathological hallmark. New and ongoing developments are now showing that variations in the aggregation process and the formation of ɑ-synuclein strains may be paralleled by the development of distinct synucleinopathies. Here, we review the recent developments and the role of strains in synucleinopathies. This article is part of a special issue on Parkinson disease.

MpUL-multi: Software for Calculation of Amyloid Fibril Mass per Unit Length from TB-TEM Images

Structure determination for amyloid fibrils presents many challenges due to the high variability exhibited by fibrils and heterogeneous morphologies present, even in single samples. Mass per unit length (MPL) estimates can be used to differentiate amyloid fibril morphologies and provide orthogonal evidence for helical symmetry parameters determined by other methods. In addition, MPL data can provide insight on the arrangement of subunits in a fibril, especially for more complex fibrils assembled with multiple parallel copies of the asymmetric unit or multiple twisted protofilaments. By detecting only scattered electrons, which serve as a relative measure of total scattering, and therefore protein mass, dark field imaging gives an approximation of the total mass of protein present in any given length of fibril. When compared with a standard of known MPL, such as Tobacco Mosaic Virus (TMV), MPL of the fibrils in question can be determined. The program suite MpUL-multi was written for rapid semi-automated processing of TB-TEM dark field data acquired using this method. A graphical user interface allows for simple designation of fibrils and standards. A second program averages intensities from multiple TMV molecules for accurate standard determination, makes multiple measurements along a given fibril, and calculates the MPL.

Preparation of Amyloid Fibrils for Magic-Angle Spinning Solid-State NMR Spectroscopy

Solid-state NMR spectroscopy (SSNMR) is an established and invaluable tool for the study of amyloid fibril structure with atomic-level detail. Optimization of the homogeneity and concentration of fibrils enhances the resolution and sensitivity of SSNMR spectra. Here, we present a fibrillization and fibril processing protocol, starting from purified monomeric α-synuclein, that enables the collection of high-resolution SSNMR spectra suitable for site-specific structural analysis. This protocol does not rely on any special features of α-synuclein and should be generalizable to any other amyloid protein.

Single-molecule FRET studies on alpha-synuclein oligomerization of Parkinson's disease genetically related mutants

Oligomers of alpha-synuclein are toxic to cells and have been proposed to play a key role in the etiopathogenesis of Parkinson's disease. As certain missense mutations in the gene encoding for alpha-synuclein induce early-onset forms of the disease, it has been suggested that these variants might have an inherent tendency to produce high concentrations of oligomers during aggregation, although a direct experimental evidence for this is still missing. We used single-molecule Förster Resonance Energy Transfer to visualize directly the protein self-assembly process by wild-type alpha-synuclein and A53T, A30P and E46K mutants and to compare the structural properties of the ensemble of oligomers generated. We found that the kinetics of oligomer formation correlates with the natural tendency of each variant to acquire beta-sheet structure. Moreover, A53T and A30P showed significant differences in the averaged FRET efficiency of one of the two types of oligomers formed compared to the wild-type oligomers, indicating possible structural variety among the ensemble of species generated. Importantly, we found similar concentrations of oligomers during the lag-phase of the aggregation of wild-type and mutated alpha-synuclein, suggesting that the properties of the ensemble of oligomers generated during self-assembly might be more relevant than their absolute concentration for triggering neurodegeneration.

Amyloid polymorphism: structural basis and neurobiological relevance

Our understanding of the molecular structures of amyloid fibrils that are associated with neurodegenerative diseases, of mechanisms by which disease-associated peptides and proteins aggregate into fibrils, and of structural properties of aggregation intermediates has advanced considerably in recent years. Detailed molecular structural models for certain fibrils and aggregation intermediates are now available. It is now well established that amyloid fibrils are generally polymorphic at the molecular level, with a given peptide or protein being capable of forming a variety of distinct, self-propagating fibril structures. Recent results from structural studies and from studies involving cell cultures, transgenic animals, and human tissue provide initial evidence that molecular structural variations in amyloid fibrils and related aggregates may correlate with or even produce variations in disease development. This article reviews our current knowledge of the structural and mechanistic aspects of amyloid formation, as well as current evidence for the biological relevance of structural variations.

High resolution structural characterization of Aβ42 amyloid fibrils by magic angle spinning NMR

The presence of amyloid plaques composed of amyloid beta (Aβ) fibrils is a hallmark of Alzheimer’s disease (AD). The Aβ peptide is present as several length variants with two common alloforms consisting of 40 and 42 amino acids, denoted Aβ_1–40 and Aβ_1–42, respectively. While there have been numerous reports that structurally characterize fibrils of Aβ_1–40, very little is known about the structure of amyloid fibrils of Aβ_1–42, which are considered the more toxic alloform involved in AD. We have prepared isotopically ¹³C/¹⁵N labeled Aβ_M01–42 fibrils in vitro from recombinant protein and examined their ¹³C–¹³C and ¹³C–¹⁵N magic angle spinning (MAS) NMR spectra. In contrast to several other studies of Aβ fibrils, we observe spectra with excellent resolution and a single set of chemical shifts, suggesting the presence of a single fibril morphology. We report the initial structural characterization of Aβ_M01–42 fibrils utilizing ¹³C and ¹⁵N shift assignments of 38 of the 43 residues, including the backbone and side chains, obtained through a series of cross-polarization based 2D and 3D ¹³C–¹³C, ¹³C–¹⁵N MAS NMR experiments for rigid residues along with J-based 2D TOBSY experiments for dynamic residues. We find that the first ∼5 residues are dynamic and most efficiently detected in a J-based TOBSY spectrum. In contrast, residues 16–42 are easily observed in cross-polarization experiments and most likely form the amyloid core. Calculation of ψ and φ dihedral angles from the chemical shift assignments indicate that 4 β-strands are present in the fibril’s secondary structure.

In vitro aggregation assays for the characterization of α-synuclein prion-like properties

Aggregation of α-synuclein plays a crucial role in the pathogenesis of synucleinopathies, a group of neurodegenerative diseases including Parkinson disease (PD), dementia with Lewy bodies (DLB), diffuse Lewy body disease (DLBD) and multiple system atrophy (MSA). The common feature of these diseases is a pathological deposition of protein aggregates, known as Lewy bodies (LBs) in the central nervous system. The major component of these aggregates is α-synuclein, a natively unfolded protein, which may undergo dramatic structural changes resulting in the formation of β-sheet rich assemblies. In vitro studies have shown that recombinant α-synuclein protein may polymerize into amyloidogenic fibrils resembling those found in LBs. These aggregates may be uptaken and propagated between cells in a prion-like manner. Here we present the mechanisms and kinetics of α-synuclein aggregation in vitro, as well as crucial factors affecting this process. We also describe how PD-linked α-synuclein mutations and some exogenous factors modulate in vitro aggregation. Furthermore, we present a current knowledge on the mechanisms by which extracellular aggregates may be internalized and propagated between cells, as well as the mechanisms of their toxicity.

Irisflorentin improves α-synuclein accumulation and attenuates 6-OHDA-induced dopaminergic neuron degeneration, implication for Parkinson's disease therapy

Parkinson’s disease (PD) is a degenerative disorder of the central nervous system that is characterized by progressive loss of dopaminergic neurons in the substantia nigra pars compacta as well as motor impairment. Aggregation of α-synuclein in neuronal cells plays a key role in this disease. At present, therapeutics for PD provides moderate symptomatic benefits, but it is not able to delay the development of the disease. Current efforts toward the treatment of PD are to identify new drugs that slow or arrest the progressive course of PD by interfering with a disease-specific pathogenetic process in PD patients. Irisflorentin derived from the roots of Belamcanda chinensis (L.) DC. is an herb which has been used for the treatment of inflammatory disorders in traditional Chinese medicine. The purpose of the present study was to assess the potential for irisflorentin to ameliorate PD in Caenorhabditis elegans models. Our data reveal that irisflorentin prevents α-synuclein accumulation in the transgenic Caenorhabditis elegans model and also improves dopaminergic neuron degeneration, food-sensing behavior, and life-span in a 6-hydroxydopamine-induced Caenorhabditis elegans model, thus indicating its potential as a anti-parkinsonian drug candidate. Irisflorentin may exert its effects by promoting rpn-3 expression to enhance the activity of proteasomes and down-regulating egl-1 expression to block apoptosis pathways. These findings encourage further investigation on irisflorentin as a possible potent agent for PD treatment.

The H50Q mutation induces a 10-fold decrease in the solubility of α-synuclein

The conversion of α-synuclein from its intrinsically disordered monomeric state into the fibrillar cross-β aggregates characteristically present in Lewy bodies is largely unknown. The investigation of α-synuclein variants causative of familial forms of Parkinson disease can provide unique insights into the conditions that promote or inhibit aggregate formation. It has been shown recently that a newly identified pathogenic mutation of α-synuclein, H50Q, aggregates faster than the wild-type. We investigate here its aggregation propensity by using a sequence-based prediction algorithm, NMR chemical shift analysis of secondary structure populations in the monomeric state, and determination of thermodynamic stability of the fibrils. Our data show that the H50Q mutation induces only a small increment in polyproline II structure around the site of the mutation and a slight increase in the overall aggregation propensity. We also find, however, that the H50Q mutation strongly stabilizes α-synuclein fibrils by 5.0 ± 1.0 kJ mol⁻¹, thus increasing the supersaturation of monomeric α-synuclein within the cell, and strongly favors its aggregation process. We further show that wild-type α-synuclein can decelerate the aggregation kinetics of the H50Q variant in a dose-dependent manner when coaggregating with it. These last findings suggest that the precise balance of α-synuclein synthesized from the wild-type and mutant alleles may influence the natural history and heterogeneous clinical phenotype of Parkinson disease.

α-Synuclein Misfolding Versus Aggregation Relevance to Parkinson's Disease: Critical Assessment and Modeling

α-Synuclein, an abundant and conserved presynaptic brain protein, is implicated as a critical factor in Parkinson's disease (PD). The aggregation of α-synuclein is believed to be a critical event in the disease process. α-Synuclein is characterized by a remarkable conformational plasticity, adopting different conformations depending on the environment. Therefore, it is classified as an "intrinsically disordered protein." Recently, a debate has challenged the view on the intrinsically disordered behavior of α-synuclein in the cell. It has been proposed that α-synuclein is a stable tetramer with a low propensity for aggregation; however, its destabilization leads to protein misfolding and its aggregation kinetics. In our critical analysis, we discussed about major issues: (i) why α-synuclein conformational behavior does not fit into the normal secondary structural characteristics of proteins, (ii) potential amino acids involved in the complexity of misfolding in α-synuclein that leads to aggregation, and (iii) the role of metals in misfolding and aggregation. To evaluate the above critical issues, we developed bioinformatics models related to secondary and tertiary conformations, Ramachandran plot, free energy change, intrinsic disordered prediction, solvent accessibility, and FoldIndex pattern. To the best of our knowledge, this is a novel critical assessment to understand the misfolding biology of synuclein and its relevance to Parkinson's disease.

The effects of pdr1, djr1.1 and pink1 loss in manganese-induced toxicity and the role of α-synuclein in C. elegans

Parkinson's disease (PD) is a neurodegenerative brain disorder characterized by selective dopaminergic (DAergic) cell loss that results in overt motor and cognitive deficits. Current treatment options exist to combat PD symptomatology, but are unable to directly target its pathogenesis due to a lack of knowledge concerning its etiology. Several genes have been linked to PD, including three genes associated with an early-onset familial form: parkin, pink1 and dj1. All three genes are implicated in regulating oxidative stress pathways. Another hallmark of PD pathophysiology is Lewy body deposition, associated with the gain-of-function genetic risk factor α-synuclein. The function of α-synuclein is poorly understood, as it shows both neurotoxic and neuroprotective activities in PD. Using the genetically tractable invertebrate Caenorhabditis elegans (C. elegans) model system, the neurotoxic or neuroprotective role of α-synuclein upon acute Mn exposure in the background of mutated pdr1, pink1 or djr1.1 was examined. The pdr1 and djr1.1 mutants showed enhanced Mn accumulation and oxidative stress that was reduced by α-synuclein. Moreover, DAergic neurodegeneration, while unchanged with Mn exposure, returned to wild-type (WT) levels for pdr1, but not djr1.1 mutants expressing α-synuclein. Taken together, this study uncovers a novel, neuroprotective role for WT human α-synuclein in attenuating Mn-induced toxicity in the background of PD-associated genes, and further supports the role of extracellular dopamine in exacerbating Mn neurotoxicity.