Direct observation of the interconversion of normal and toxic forms of α-synuclein

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

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

α-Synuclein dimer structures found from computational simulations

Dimer formation is likely the first step in the oligomerization of α-synuclein in Lewy bodies. In order to prevent α-synuclein aggregation, knowledge of the atomistic structures of possible α-synuclein dimers and the interaction affinity between the dimer domains is a necessary prerequisite in the process of rational design of dimerization inhibitors. Using computational methodology, we have investigated several possible α-synuclein dimer structures, focusing on dimers formed from α-helical forms of the protein found when it is membrane-bound, and dimers formed from β-sheet conformations predicted by simulations. Structures and corresponding binding affinities for the interacting monomers in possible α-synuclein dimers, along with properties including the contributions from different interaction energies and the radii of gyration, were found through molecular docking followed by MD simulations and binding-energy calculations. We found that even though α-synuclein is highly charged, hydrophobic contributions play a significant role in stabilizing dimers.

Aggregated α-synuclein and complex I deficiency: exploration of their relationship in differentiated neurons

α-Synuclein becomes misfolded and aggregated upon damage by various factors, for example, by reactive oxygen species. These aggregated forms have been proposed to have differential toxicities and their interaction with mitochondria may cause dysfunction within this organelle that contributes to the pathogenesis of Parkinson's disease (PD). In particular, the association of α-synuclein with mitochondria occurs through interaction with mitochondrial complex I and importantly defects of this protein have been linked to the pathogenesis of PD. Therefore, we investigated the relationship between aggregated α-synuclein and mitochondrial dysfunction, and the consequences of this interaction on cell survival. To do this, we studied the effects of α-synuclein on cybrid cell lines harbouring mutations in either mitochondrial complex I or IV. We found that aggregated α-synuclein inhibited mitochondrial complex I in control and complex IV-deficient cells. However, when aggregated α-synuclein was applied to complex I-deficient cells, there was no additional inhibition of mitochondrial function or increase in cell death. This would suggest that as complex I-deficient cells have already adapted to their mitochondrial defect, the subsequent toxic effects of α-synuclein are reduced.

Synthetic Proteins and Peptides for the Direct Interrogation of α-Synuclein Posttranslational Modifications

α-Synuclein is the aggregation-prone protein associated with Parkinson’s disease (PD) and related neurodegenerative diseases. Complicating both its biological functions and toxic aggregation are a variety of posttranslational modifications. These modifications have the potential to either positively or negatively affect α-synuclein aggregation, raising the possibility that the enzymes that add or remove these modifications could be therapeutic targets in PD. Synthetic protein chemistry is uniquely positioned to generate site-specifically and homogeneously modified proteins for biochemical study. Here, we review the application of synthetic peptides and proteins towards understanding the effects of α-synuclein posttranslational modifications.

Protein misfolding occurs by slow diffusion across multiple barriers in a rough energy landscape

The timescale for the microscopic dynamics of proteins during conformational transitions is set by the intrachain diffusion coefficient, D. Despite the central role of protein misfolding and aggregation in many diseases, it has proven challenging to measure D for these processes because of their heterogeneity. We used single-molecule force spectroscopy to overcome these challenges and determine D for misfolding of the prion protein PrP. Observing directly the misfolding of individual dimers into minimal aggregates, we reconstructed the energy landscape governing nonnative structure formation. Remarkably, rather than displaying multiple pathways, as typically expected for aggregation, PrP dimers were funneled into a thermodynamically stable misfolded state along a single pathway containing several intermediates, one of which blocked native folding. Using Kramers' rate theory, D was found to be 1,000-fold slower for misfolding than for native folding, reflecting local roughening of the misfolding landscape, likely due to increased internal friction. The slow diffusion also led to much longer transit times for barrier crossing, allowing transition paths to be observed directly for the first time to our knowledge. These results open a new window onto the microscopic mechanisms governing protein misfolding.

The First Step of Amyloidogenic Aggregation

The structural and dynamic characterization of the on-pathway intermediates involved in the mechanism of amyloid fibril formation is one of the major remaining biomedical challenges of our time. In addition to mature fibrils, various oligomeric structures are implicated in both the rate-limiting step of the nucleation process and the neuronal toxicity of amyloid deposition. Single-molecule fluorescence spectroscopy (SMFS) is an excellent tool for extracting most of the relevant information on these molecular systems, especially advanced multiparameter approaches, such as pulsed interleaved excitation (PIE). In our investigations of an amyloidogenic SH3 domain of α-spectrin, we have found dynamic oligomerization, even prior to incubation. Our single-molecule PIE experiments revealed that these species are small, mostly dimeric, and exhibit a loose and dynamic molecular organization. Furthermore, these experiments have allowed us to obtain quantitative information regarding the oligomer stability. These pre-amyloidogenic oligomers may potentially serve as the first target for fibrillization-prevention strategies.

Distinct higher-order α-synuclein oligomers induce intracellular aggregation

The cell-to-cell transmission of pathology in Parkinson's disease has been linked to soluble amyloid oligomers. Ion mobility spectrometry (IMS)–MS has been used to show that these soluble oligomers have a compact ring-like conformation.

α-Synuclein Shows High Affinity Interaction with Voltage-dependent Anion Channel, Suggesting Mechanisms of Mitochondrial Regulation and Toxicity in Parkinson Disease

Participation of the small, intrinsically disordered protein α-synuclein (α-syn) in Parkinson disease (PD) pathogenesis has been well documented. Although recent research demonstrates the involvement of α-syn in mitochondrial dysfunction in neurodegeneration and suggests direct interaction of α-syn with mitochondria, the molecular mechanism(s) of α-syn toxicity and its effect on neuronal mitochondria remain vague. Here we report that at nanomolar concentrations, α-syn reversibly blocks the voltage-dependent anion channel (VDAC), the major channel of the mitochondrial outer membrane that controls most of the metabolite fluxes in and out of the mitochondria. Detailed analysis of the blockage kinetics of VDAC reconstituted into planar lipid membranes suggests that α-syn is able to translocate through the channel and thus target complexes of the mitochondrial respiratory chain in the inner mitochondrial membrane. Supporting our in vitro experiments, a yeast model of PD shows that α-syn toxicity in yeast depends on VDAC. The functional interactions between VDAC and α-syn, revealed by the present study, point toward the long sought after physiological and pathophysiological roles for monomeric α-syn in PD and in other α-synucleinopathies.

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

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

Colonic mucosal a-synuclein lacks specificity as a biomarker for Parkinson disease

OBJECTIVE

To determine the utility of detecting a-synuclein (aSyn) in colonic mucosal biopsy tissue as a potential diagnostic biomarker for Parkinson disease (PD).

METHODS

We used the paraffin-embedded tissue (PET) blot, which degrades physiologic nonaggregated aSyn using proteinase K and enhances antigen retrieval allowing sensitive and selective detection of remaining protein aggregates, to detect aSyn in colonic mucosal biopsies from 15 patients with early PD (,3 years), 7 patients with later PD (.5 years), and 11 individuals without PD. aSyn and serine 129–phosphorylated aSyn (Ser129p-aSyn) were assessed by PET blot and conventional immunohistochemistry.

RESULTS

PET blot–resistant aggregated aSyn and Ser129p-aSyn was present in 12 of 15 individuals with early PD, 7 of 7 individuals with later PD, and 11 of 11 control subjects. The number of biopsies positive by PET blot relative to conventional immunohistochemistry was significantly lower in both PD groups compared with the control group for both aSyn and Ser129p-aSyn,whereas routine immunohistochemistry was positive more often in PD, but was positive in as many as 9 of 11 control individuals.

CONCLUSION

Strong evidence of the presence of aggregated hyperphosphorylated aSyn in individuals with and without PD, using such a sensitive and specific method as the PET blot, suggests that colonic deposition of aSyn is not a useful diagnostic test for PD. The utility of detecting aSynin the colon as a biomarker in combination with other assessments remains to be determined.

Curcumin Pyrazole and its derivative (N-(3-Nitrophenylpyrazole) Curcumin inhibit aggregation, disrupt fibrils and modulate toxicity of Wild type and Mutant α-Synuclein

Accumulating evidence suggests that deposition of neurotoxic α-synuclein aggregates in the brain during the development of neurodegenerative diseases like Parkinson’s disease can be curbed by anti-aggregation strategies that either disrupt or eliminate toxic aggregates. Curcumin, a dietary polyphenol exhibits anti-amyloid activity but the use of this polyphenol is limited owing to its instability. As chemical modifications in curcumin confiscate this limitation, such efforts are intensively performed to discover molecules with similar but enhanced stability and superior properties. This study focuses on the inhibitory effect of two stable analogs of curcumin viz. curcumin pyrazole and curcumin isoxazole and their derivatives against α-synuclein aggregation, fibrillization and toxicity. Employing biochemical, biophysical and cell based assays we discovered that curcumin pyrazole (3) and its derivative N-(3-Nitrophenylpyrazole) curcumin (15) exhibit remarkable potency in not only arresting fibrillization and disrupting preformed fibrils but also preventing formation of A11 conformation in the protein that imparts toxic effects. Compounds 3 and 15 also decreased neurotoxicity associated with fast aggregating A53T mutant form of α-synuclein. These two analogues of curcumin described here may therefore be useful therapeutic inhibitors for the treatment of α-synuclein amyloidosis and toxicity in Parkinson’s disease and other synucleinopathies.

Activation of MyD88-dependent TLR1/2 signaling by misfolded α-synuclein, a protein linked to neurodegenerative disorders

Synucleinopathies, such as Parkinson's disease and diffuse Lewy body disease, are progressive neurodegenerative disorders characterized by selective neuronal death, abnormal accumulation of misfolded α-synuclein, and sustained microglial activation. In addition to inducing neuronal toxicity, higher-ordered oligomeric α-synuclein causes proinflammatory responses in the brain parenchyma by triggering microglial activation, which may exacerbate pathogenic processes by establishing a chronic neuroinflammatory milieu. We found that higher-ordered oligomeric α-synuclein induced a proinflammatory microglial phenotype by directly engaging the heterodimer TLR1/2 (Toll-like receptor 1 and 2) at the cell membrane, leading to the nuclear translocation of NF-κB (nuclear factor κB) and the increased production of the proinflammatory cytokines TNF-α (tumor necrosis factor-α) and IL-1β (interleukin-1β) in a MyD88-dependent manner. Blocking signaling through the TLR1/2 heterodimer with the small-molecule inhibitor CU-CPT22 reduced the nuclear translocation of NF-κB and secretion of TNF-α from cultured primary mouse microglia. Candesartan cilexetil, a drug approved for treating hypertension and that inhibits the expression of TLR2, reversed the activated proinflammatory phenotype of primary microglia exposed to oligomeric α-synuclein, supporting the possibility of repurposing this drug for synucleinopathies.

Oligomers of α-synuclein: picking the culprit in the line-up

In the present chapter, we discuss the key findings on αsyn (α-synuclein) oligomers from a biophysical point of view. Current structural methods cannot provide a high-resolution structure of αsyn oligomers due to their size, heterogeneity and tendency to aggregate. However, a low-resolution structure of a stable αsyn oligomer population is emerging based on compelling data from different research groups. αsyn oligomers are normally observed during the formation of amyloid fibrils and we discuss how they are connected to this process. Another important topic is the interaction of αsyn oligomers and membranes, and we will discuss the evidence which suggests that this interaction might be essential in the pathogenesis of Parkinson's disease and other neurodegenerative disorders. Finally, we present a remarkable example of how small molecules are able to stabilize non-amyloid oligomers and how this might be a potential strategy to inhibit the inherent toxicity of αsyn oligomers. A major challenge is to link the very complex oligomerization pathways seen in clever experiments in vitro with what actually happens in the cell. With the tremendous developments in optical microscopy in mind, we believe that it will be possible to make this link very soon.

Proteinopathies, a core concept for understanding and ultimately treating degenerative disorders?

The current review covers proteinopathies an umbrella term for neurodegenerative disorders that are characterized by the accumulation of specific proteins within neurons or in the brain parenchyma. Most prevalent examples for typical proteinopathies are Alzheimer's disease and Parkinson's disease. In healthy brain, these proteins are unstructured as a monomer, serving most likely as the physiological form. In a disease condition, the unstructured proteins experience a conformational change leading to small oligomers that eventually will aggregate into higher order structures. Prion disease is an exception within the family of proteinopathies as the aggregated prion protein is highly infectious and can self-aggregate and propagate. Recent reports might implicate a prion-like spread of misfolded proteins in Alzheimer's and Parkinson's disease; however there are evident differences in comparison to prion diseases. As proteinopathies are caused by the aggregation of disease-typical proteins with an ordered structure, active and passive immunization protocols have been used to expose model systems to therapeutic antibodies that bind to the aggregates thereby inhibiting the prolongation into higher ordered fibrils or dissolving the existing fibrillar structure. While most of the immunization treatments have been only carried out in preclinical model systems overexpressing the disease-relevant aggregating protein, other approaches are already in clinical testing. Taking the core concept of proteinopathies with conformationally altered protein aggregates into account, immunization appears to be a very promising therapeutic option for neurodegenerative disorders.

A mechanistic model of tau amyloid aggregation based on direct observation of oligomers

Protein aggregation plays a key role in neurodegenerative disease, giving rise to small oligomers that may become cytotoxic to cells. The fundamental microscopic reactions taking place during aggregation, and their rate constants, have been difficult to determine due to lack of suitable methods to identify and follow the low concentration of oligomers over time. Here we use single-molecule fluorescence to study the aggregation of the repeat domain of tau (K18), and two mutant forms linked with familial frontotemporal dementia, the deletion mutant ΔK280 and the point mutant P301L. Our kinetic analysis reveals that aggregation proceeds via monomeric assembly into small oligomers, and a subsequent slow structural conversion step before fibril formation. Using this approach, we have been able to quantitatively determine how these mutations alter the aggregation energy landscape.

Alzheimer's disease is characterized by the aggregation of the Abeta peptide and the tau protein. Here the authors track the formation of wild-type or mutant tau oligomers through to large aggregates and gain insights into the molecular basis of how tau mutations cause disease by altering the aggregation pathway.

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.

pH-induced molecular shedding drives the formation of amyloid fibril-derived oligomers

Amyloid disorders cause debilitating illnesses through the formation of toxic protein aggregates. The mechanisms of amyloid toxicity and the nature of species responsible for mediating cellular dysfunction remain unclear. Here, using β2-microglobulin (β2m) as a model system, we show that the disruption of membranes by amyloid fibrils is caused by the molecular shedding of membrane-active oligomers in a process that is dependent on pH. Using thioflavin T (ThT) fluorescence, NMR, EM and fluorescence correlation spectroscopy (FCS), we show that fibril disassembly at pH 6.4 results in the formation of nonnative spherical oligomers that disrupt synthetic membranes. By contrast, fibril dissociation at pH 7.4 results in the formation of nontoxic, native monomers. Chemical cross-linking or interaction with hsp70 increases the kinetic stability of fibrils and decreases their capacity to cause membrane disruption and cellular dysfunction. The results demonstrate how pH can modulate the deleterious effects of preformed amyloid aggregates and suggest why endocytic trafficking through acidic compartments may be a key factor in amyloid disease.

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.

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.

Lipid peroxidation is essential for α-synuclein-induced cell death

Parkinson's disease is the second most common neurodegenerative disease and its pathogenesis is closely associated with oxidative stress. Deposition of aggregated α‐synuclein (α‐Syn) occurs in familial and sporadic forms of Parkinson's disease. Here, we studied the effect of oligomeric α‐Syn on one of the major markers of oxidative stress, lipid peroxidation, in primary co‐cultures of neurons and astrocytes. We found that oligomeric but not monomeric α‐Syn significantly increases the rate of production of reactive oxygen species, subsequently inducing lipid peroxidation in both neurons and astrocytes. Pre‐incubation of cells with isotope‐reinforced polyunsaturated fatty acids (D‐PUFAs) completely prevented the effect of oligomeric α‐Syn on lipid peroxidation. Inhibition of lipid peroxidation with D‐PUFAs further protected cells from cell death induced by oligomeric α‐Syn. Thus, lipid peroxidation induced by misfolding of α‐Syn may play an important role in the cellular mechanism of neuronal cell loss in Parkinson's disease.

We have found that aggregated α‐synuclein‐induced production of reactive oxygen species (ROS) that subsequently stimulates lipid peroxidation and cell death in neurons and astrocytes. Specific inhibition of lipid peroxidation by incubation with reinforced polyunsaturated fatty acids (D‐PUFAs) completely prevented the effect of α‐synuclein on lipid peroxidation and cell death.

Direct visualization of alpha-synuclein oligomers reveals previously undetected pathology in Parkinson's disease brain

Oligomeric forms of alpha-synuclein are emerging as key mediators of pathogenesis in Parkinson's disease. Our understanding of the exact contribution of alpha-synuclein oligomers to disease is limited by the lack of a technique for their specific detection. We describe a novel method, the alpha-synuclein proximity ligation assay, which specifically recognizes alpha-synuclein oligomers. In a blinded study with post-mortem brain tissue from patients with Parkinson's disease (n = 8, age range 73-92 years, four males and four females) and age- and sex-matched controls (n = 8), we show that the alpha-synuclein proximity ligation assay reveals previously unrecognized pathology in the form of extensive diffuse deposition of alpha-synuclein oligomers. These oligomers are often localized, in the absence of Lewy bodies, to neuroanatomical regions mildly affected in Parkinson's disease. Diffuse alpha-synuclein proximity ligation assay signal is significantly more abundant in patients compared to controls in regions including the cingulate cortex (1.6-fold increase) and the reticular formation of the medulla (6.5-fold increase). In addition, the alpha-synuclein proximity ligation assay labels very early perikaryal aggregates in morphologically intact neurons that may precede the development of classical Parkinson's disease lesions, such as pale bodies or Lewy bodies. Furthermore, the alpha-synuclein proximity ligation assay preferentially detects early-stage, loosely compacted lesions such as pale bodies in patient tissue, whereas Lewy bodies, considered heavily compacted late lesions are only very exceptionally stained. The alpha-synuclein proximity ligation assay preferentially labels alpha-synuclein oligomers produced in vitro compared to monomers and fibrils, while stained oligomers in human brain display a distinct intermediate proteinase K resistance, suggesting the detection of a conformer that is different from both physiological, presynaptic alpha-synuclein (proteinase K-sensitive) and highly aggregated alpha-synuclein within Lewy bodies (proteinase K-resistant). These disease-associated conformers represent previously undetected Parkinson's disease pathology uncovered by the alpha-synuclein proximity ligation assay.

Lipid vesicles trigger α-synuclein aggregation by stimulating primary nucleation

α-Synuclein (α-syn) is a 140-residue intrinsically disordered protein that is involved in neuronal and synaptic vesicle plasticity, but its aggregation to form amyloid fibrils is the hallmark of Parkinson's disease (PD). The interaction between α-syn and lipid surfaces is believed to be a key feature for mediation of its normal function, but under other circumstances it is able to modulate amyloid fibril formation. Using a combination of experimental and theoretical approaches, we identify the mechanism through which facile aggregation of α-syn is induced under conditions where it binds a lipid bilayer, and we show that the rate of primary nucleation can be enhanced by three orders of magnitude or more under such conditions. These results reveal the key role that membrane interactions can have in triggering conversion of α-syn from its soluble state to the aggregated state that is associated with neurodegeneration and to its associated disease states.

A molecular chaperone breaks the catalytic cycle that generates toxic Aβ oligomers

Alzheimer's disease is an increasingly prevalent neurodegenerative disorder whose pathogenesis has been associated with aggregation of the amyloid-β peptide (Aβ42). Recent studies have revealed that once Aβ42 fibrils are generated, their surfaces effectively catalyze the formation of neurotoxic oligomers. Here we show that a molecular chaperone, a human Brichos domain, can specifically inhibit this catalytic cycle and limit human Aβ42 toxicity. We demonstrate in vitro that Brichos achieves this inhibition by binding to the surfaces of fibrils, thereby redirecting the aggregation reaction to a pathway that involves minimal formation of toxic oligomeric intermediates. We verify that this mechanism occurs in living mouse brain tissue by cytotoxicity and electrophysiology experiments. These results reveal that molecular chaperones can help maintain protein homeostasis by selectively suppressing critical microscopic steps within the complex reaction pathways responsible for the toxic effects of protein misfolding and aggregation.

NADH fluorescence lifetime is an endogenous reporter of α-synuclein aggregation in live cells

α-Synuclein (aS) aggregation has been amply investigated for its involvement in Parkinson's disease because its amyloid fibrils are the main constituent of Lewy bodies, one of the hallmarks of the disease. aS aggregation was studied here in vitro and in cellular models to correlate aggregation products with toxicity mechanisms. Independent results published elsewhere suggested that aS overexpression and/or aggregation may impair cellular metabolism and cause mitochondrial damage. In this context, we report the characterization of changes in NADH fluorescence properties in vitro and in human embryonic kidney 293 cells upon aS aggregation. The application of the phasor approach to study NADH fluorescence lifetime and emission allowed us to identify changes that correlate with aS aggregation. In particular, the fraction of bound NADH, characterized by longer lifetimes in comparison to free NADH, is increased, and the maximum of the NADH emission is shifted toward shorter wavelengths in the presence of aggregating aS both in vitro and in cells. These data suggest that NADH binds to aggregated aS. NMR experiments in vitro substantiate such binding, which occurs during aggregation. NADH fluorescence is thus useful to detect aS aggregation and by extension the associated oxidative stress.

D-amino acid-based peptide inhibitors as early or preventative therapy in Alzheimer disease

Beta amyloid (Aβ) accumulation is recognized as a hallmark of Alzheimer disease (AD) pathology and the aggregation of Aβ peptide is hypothesized to drive pathogenesis. As such, Aβ is a logical target for therapeutic intervention and there have been many studies looking at diverse classes of drugs that target Aβ. Of concern is the recent failure of several clinical trials, highlighting the need for earlier, possibly preventative intervention, and raising the question of what form of Aβ is the best target. The Aβ oligomers are considered to be the toxic species, but many therapies, such as antibody therapies, target monomers, removing them as substrates for aggregation. Peptide inhibitors, in contrast, are able to interfere with the aggregation process itself. Designing peptide inhibitors requires some knowledge of Aβ structure; while there is structural information about the amyloid core of Aβ fibrils, the transient nature of oligomers makes them difficult to characterize. Fortunately, some interaction sites have been identified between monomers and oligomers of Aβ and these, plus known aggregation-prone sequences in Aβ, can serve as a basis for inhibitor design. In this mini-review we focus on D-amino acid based peptide inhibitors and discuss how their non-toxic and stable nature can be beneficial, while they specifically target aggregation-prone sequences within the Aβ peptide. Many peptide inhibitors have been designed using the LVFFA domain within Aβ to disrupt the self-assembly of Aβ peptide. While this may be sufficient to stop aggregation in vitro, other aggregation sites at the C-terminus may promote aggregation independently and the flexible N terminus may be a good target to induce clearance of aggregates. Ultimately, it may be a combination of targets that provides the best therapeutic strategy.

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.

Interactions between misfolded protein oligomers and membranes: A central topic in neurodegenerative diseases?

The deposition of amyloid material has been associated with many different diseases. Although these diseases are very diverse the amyloid material share many common features such as cross-β-sheet structure of the backbone of the proteins deposited. Another common feature of the aggregation process for a wide variety of proteins is the presence of prefibrillar oligomers. These oligomers are linked to the cytotoxicity occurring during the aggregation of proteins. These prefibrillar oligomers interact extensively with lipid membranes and in some cases leads to destabilization of lipid membranes. This interaction is however highly dependent on the nature of both the oligomer and the lipids. Anionic lipids are often required for interaction with the lipid membrane while increased exposure of hydrophobic patches from highly dynamic protein oligomers are structural determinants of cytotoxicity of the oligomers. To explore the oligomer lipid interaction in detail the interaction between oligomers of α-synuclein and the 4th fasciclin-1 domain of TGFBIp with lipid membranes will be examined here. For both proteins the dynamic species are the ones causing membrane destabilization and the membrane interaction is primarily seen when the lipid membranes contain anionic lipids. Hence the dynamic nature of oligomers with exposed hydrophobic patches alongside the presence of anionic lipids could be essential for the cytotoxicity observed for prefibrillar oligomers in general. This article is part of a Special Issue entitled: Lipid-protein interactions.

Single fibril growth kinetics of α-synuclein

Neurodegenerative disorders associated with protein misfolding are fatal diseases that are caused by fibrillation of endogenous proteins such as α-synuclein (α-syn) in Parkinson's disease (PD) or amyloid-β in Alzheimer's disease. Fibrils of α-syn are a major pathological hallmark of PD and certain aggregation intermediates are postulated to cause synaptic failure and cell death of dopaminergic neurons in the substantia nigra. For the development of therapeutic approaches, the mechanistic understanding of the fibrillation process is essential. Here we report real-time observation of α-syn fibril elongation on a glass surface, imaged by total internal reflection fluorescence microscopy using thioflavin T fluorescence. Fibrillation on the glass surface occurred in the same time frame and yielded fibrils of similar length as fibrillation in solution. Time-resolved imaging of fibrillation on a single fibril level indicated that α-syn fibril elongation follows a stop-and-go mechanism; that is, fibrils either extend at a homogenous growth rate or stop to grow for variable time intervals. The fibril growth kinetics were compatible with a model featuring two states, a growth state and a stop state, which were approximately isoenergetic and interconverted with rate constants of ~1.5×10(-4) s(-1). In the growth state, α-syn monomers were incorporated into the fibril with a rate constant of 8.6×10(3) M(-1) s(-1). Fibril elongation of α-syn is slow compared to other amyloidogenic proteins.

Bioactive TTR105-115-based amyloid fibrils reduce the viability of mammalian cells

A growing number of protein-based fibrous biomaterials have been produced with a cross-β amyloid core yet the long-term effect of these materials on cell viability and the influence of core and non-core protein sequences on viability is not well understood. Here, synthetic bioactive TTR1-RGD and control TTR1-RAD or TTR1 fibrils were used to test the response of mammalian cells. At high fibril concentrations cell viability was reduced, as assessed by mitochondrial reduction assays, lactate dehydrogenase membrane integrity assays and apoptotic biomarkers. This reduction occurred despite the high density of RGD cell adhesion ligands and use of cells displaying integrin receptors. Cell viability was affected by fibril size, maturity and whether fibrils were added to the cell media or as a pre-coated surface layer. These findings show that while cells initially interact well with synthetic fibrils, cellular integrity can be compromised over longer periods of time, suggesting a better understanding of the role of core and non-core residues in determining cellular interactions is required before TTR1-based fibrils are used as biomaterials.

Brain injection of α-synuclein induces multiple proteinopathies, gliosis, and a neuronal injury marker

Intracerebral injection of amyloidogenic α-synuclein (αS) has been shown to induce αS pathology in the CNS of nontransgenic mice and αS transgenic mice, albeit with varying efficiencies. In this study, using wild-type human αS transgenic mice (line M20), we demonstrate that intracerebral injection of recombinant amyloidogenic or soluble αS induces extensive αS intracellular inclusion pathology that is associated with robust gliosis. Near the injection site, a significant portion of αS inclusions are detected in neurons but also in astrocytes and microglia. Aberrant induction of expression of the intermediate filament protein peripherin, which is associated with CNS neuronal injury, was also observed predominantly near the site of injection. In addition, many pSer129 αS-induced inclusions colocalize with the low-molecular-mass neurofilament subunit (NFL) or peripherin staining. αS inclusion pathology was also induced in brain regions distal from the injection site, predominantly in neurons. Unexpectedly, we also find prominent p62-immunoreactive, αS-, NFL-, and peripherin-negative inclusions. These findings provide evidence that exogenous αS challenge induces αS pathology but also results in the following: (1) a broader disruption of proteostasis; (2) glial activation; and (3) a marker of a neuronal injury response. Such data suggest that induction of αS pathology after exogenous seeding may involve multiple interdependent mechanisms.

Interactions of Lipid Membranes with Fibrillar Protein Aggregates

Amyloid fibrils are an intriguing class of protein aggregates with distinct physicochemical, structural and morphological properties. They display peculiar membrane-binding behavior, thus adding complexity to the problem of protein-lipid interactions. The consensus that emerged during the past decade is that amyloid cytotoxicity arises from a continuum of cross-β-sheet assemblies including mature fibrils. Based on literature survey and our own data, in this chapter we address several aspects of fibril-lipid interactions, including (i) the effects of amyloid assemblies on molecular organization of lipid bilayer; (ii) competition between fibrillar and monomeric membrane-associating proteins for binding to the lipid surface; and (iii) the effects of lipids on the structural morphology of fibrillar aggregates. To illustrate some of the processes occurring in fibril-lipid systems, we present and analyze fluorescence data reporting on lipid bilayer interactions with fibrillar lysozyme and with the N-terminal 83-residue fragment of amyloidogenic mutant apolipoprotein A-I, 1-83/G26R/W@8. The results help understand possible mechanisms of interaction and mutual remodeling of amyloid fibers and lipid membranes, which may contribute to amyloid cytotoxicity.

Conformational dynamics of α-synuclein: insights from mass spectrometry

Different mass spectrometry approaches are combined to investigate the conformational flexibility of α-synuclein.

β2-microglobulin amyloid fibrils are nanoparticles that disrupt lysosomal membrane protein trafficking and inhibit protein degradation by lysosomes

Fragmentation of amyloid fibrils produces fibrils that are reduced in length but have an otherwise unchanged molecular architecture. The resultant nanoscale fibril particles inhibit the cellular reduction of the tetrazolium dye 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), a substrate commonly used to measure cell viability, to a greater extent than unfragmented fibrils. Here we show that the internalization of β2-microglobulin (β2m) amyloid fibrils is dependent on fibril length, with fragmented fibrils being more efficiently internalized by cells. Correspondingly, inhibiting the internalization of fragmented β2m fibrils rescued cellular MTT reduction. Incubation of cells with fragmented β2m fibrils did not, however, cause cell death. Instead, fragmented β2m fibrils accumulate in lysosomes, alter the trafficking of lysosomal membrane proteins, and inhibit the degradation of a model protein substrate by lysosomes. These findings suggest that nanoscale fibrils formed early during amyloid assembly reactions or by the fragmentation of longer fibrils could play a role in amyloid disease by disrupting protein degradation by lysosomes and trafficking in the endolysosomal pathway.

Models of α-synuclein aggregation in Parkinson's disease

Parkinson's disease (PD) is not only characterized by motor disturbances but also, by cognitive, sensory, psychiatric and autonomic dysfunction. It has been proposed that some of these symptoms might be related to the widespread pathology of α-synuclein (α-syn) aggregation in different nuclei of the central and peripheral nervous system. However, the pathogenic formation of α-syn aggregates in different brain areas of PD patients is poorly understood. Most experimental models of PD are valuable to assess specific aspects of its pathogenesis, such as toxin-induced dopaminergic neurodegeneration. However, new models are required that reflect the widespread and progressive formation of α-syn aggregates in different brain areas. Such α-syn aggregation is induced in only a few animal models, for example perikaryon inclusions are found in rats administered rotenone, aggregates with a neuritic morphology develop in mice overexpressing either mutated or wild-type α-syn, and in Smad3 deficient mice, aggregates form extensively in the perikaryon and neurites of specific brain nuclei. In this review we focus on α-syn aggregation in the human disorder, its genetics and the availability of experimental models. Indeed, evidences show that dopamine (DA) metabolism may be related to α-syn and its conformational plasticity, suggesting an interesting link between the two pathological hallmarks of PD: dopaminergic neurodegeneration and Lewy body (LB) formation.

Aldehyde Dehydrogenase 1 making molecular inroads into the differential vulnerability of nigrostriatal dopaminergic neuron subtypes in Parkinson's disease

A preferential dysfunction/loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc) accounts for the main motor symptoms of Parkinson’s disease (PD), the most common degenerative movement disorder. However, the neuronal loss is not stochastic, but rather displays regionally selectivity, indicating the existence of different DA subpopulations in the SNpc. To identify the underlying molecular determinants is thereby instrumental in understanding the pathophysiological mechanisms of PD-related neuron dysfunction/loss and offering new therapeutic targets. Recently, we have demonstrated that aldehyde dehydrogenase 1 (ALDH1A1) is one such molecular determinant that defines and protects an SNpc DA neuron subpopulation preferentially affected in PD. In this review, we provide further analysis and discussion on the roles of ALDH1A1 in the function and survival of SNpc DA neurons in both rodent and human brains. We also explore the feasibility of ALDH1A1 as a potential biomarker and therapeutic target for PD.

Acceleration of α-synuclein aggregation by exosomes

Exosomes are small vesicles released from cells into extracellular space. We have isolated exosomes from neuroblastoma cells and investigated their influence on the aggregation of α-synuclein, a protein associated with Parkinson disease pathology. Using cryo-transmission electron microscopy of exosomes, we found spherical unilamellar vesicles with a significant protein content, and Western blot analysis revealed that they contain, as expected, the proteins Flotillin-1 and Alix. Using thioflavin T fluorescence to monitor aggregation kinetics, we found that exosomes catalyze the process in a similar manner as a low concentration of preformed α-synuclein fibrils. The exosomes reduce the lag time indicating that they provide catalytic environments for nucleation. The catalytic effects of exosomes derived from naive cells and cells that overexpress α-synuclein do not differ. Vesicles prepared from extracted exosome lipids accelerate aggregation, suggesting that the lipids in exosomes are sufficient for the catalytic effect to arise. Using mass spectrometry, we found several phospholipid classes in the exosomes, including phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, phosphatidylinositol, and the gangliosides GM2 and GM3. Within each class, several species with different acyl chains were identified. We then prepared vesicles from corresponding pure lipids or defined mixtures, most of which were found to retard α-synuclein aggregation. As a striking exception, vesicles containing ganglioside lipids GM1 or GM3 accelerate the process. Understanding how α-synuclein interacts with biological membranes to promote neurological disease might lead to the identification of novel therapeutic targets.

Hydrogen/deuterium exchange-protected oligomers populated during Aβ fibril formation correlate with neuronal cell death

The aggregation of the amyloid-β peptide (Aβ) to form fibrils and plaques is strongly associated with Alzheimer's disease (AD). Although it is well established that this process generates neurotoxicity, it is also heterogeneous with a variety of species being formed during the conversion process. This heterogeneity makes it difficult to detect and characterize each of the aggregates formed, which precludes establishing the specific features responsible for the neurotoxicity observed. Here we use pulse-labeling hydrogen-deuterium exchange experiments analyzed by electrospray ionization mass spectrometry (PL-HDX-ESI-MS) to distinguish three ensembles populated during the aggregation of the 40 and 42 residue forms of the Aβ peptide, Aβ40 and Aβ42, on the basis of differences in their persistent structure. Noticeably, two of them are more abundant at the beginning and at the end of the lag phase and are therefore not detectable by conventional assays such as Thioflavin T (ThT). The ensembles populated at different stages of the aggregation process have a surprisingly consistent average degree of exchange, indicating that there are definite structural transitions between the different stages of aggregation. To determine whether an ensemble of species with a given hydrogen exchange pattern correlates with neurotoxicity, we combined PL-HDX-ESI-MS experiments with parallel measurements of the neurotoxicity of the samples under study. The results of this dual approach show that the maximum toxicity correlates with the ensemble comprising HDX protected oligomers, indicating that development of persistent structure within Aβ oligomers is a determinant of neurotoxicity.

α-Synuclein forms non-selective cation channels and stimulates ATP-sensitive potassium channels in hippocampal neurons

KEY POINTS

In Parkinson's disease, the protein α-synuclein (αS) is produced within neurons and also appears in the extracellular fluid. In this study in hippocampal neurons, αS formed non-selective cation channels with multiple levels of conductance and rectification depending on their insertion site. αS channels induced local spontaneous increases in intracellular Na(+) and Ca(2+), depolarized neurons, augmented bursting activity and stimulated the opening of ATP-sensitive K(+) channels. Non-selective channels were also observed in neurons transfected with either wild-type or mutant A53T αS, and after extracellular application of these proteins. The properties of αS channels in neuronal membranes suggest that extracellular αS is more toxic than αS produced within neurons. In Parkinson's disease and several other neurodegenerative diseases, the protein α-synuclein (αS) is produced within neurons and accumulates in the extracellular fluid. Several mechanisms of αS action are proposed, one of which is the formation of cation-permeable pores that may mediate toxicity. αS induces non-selective cation channels in lipid bilayers, but whether this occurs in living neurons and which properties the channels possess have not yet been examined. In this study the properties of αS channels in dissociated hippocampal neurons are documented. In cell-attached recordings the incorporation of αS into membranes was driven by applied negative potentials. These channels exhibited multiple levels of conductance (30, 70 and 120 pS at -100 mV) and inward rectification. The persistent activity of αS channels induced local changes in intracellular Na(+) and Ca(2+), depolarized neurons and augmented bursting activity. αS channels formed by adding αS to the intracellular membrane in inside-out patches exhibited outward rectification. αS channels were equally permeable to Na(+), K(+) and Ca(2+). These channels were also observed in neurons transfected with wild-type or mutant A53T αS, and after extracellular application of wild-type or mutant A53T αS proteins. Opening of αS channels stimulated opening of ATP-sensitive K(+) (KATP ) channels and did not interfere with the activity of delayed rectifier K(+) channels. The properties of αS channels in neuronal membranes suggest stronger toxicity of extracellularly applied αS than intracellular αS. Enhancement of neuronal excitability and distortions in ion homeostasis may underlie the toxic effects of αS that can be dampened by KATP channels.

Prion-like mechanisms in the pathogenesis of tauopathies and synucleinopathies

Neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease, are characterized by the abnormal aggregation of a small number of intracellular proteins, with tau and α-synuclein being the most commonly affected. Until recently, the events leading to aggregate formation were believed to be entirely cell-autonomous, with protein misfolding occurring independently in many cells. It is now believed that protein aggregates form in a small number of brain cells, from which they propagate intercellularly through templated recruitment, reminiscent of the mechanisms by which prions spread through the nervous system.

Viral capsid assembly as a model for protein aggregation diseases: Active processes catalyzed by cellular assembly machines comprising novel drug targets

Viruses can be conceptualized as self-replicating multiprotein assemblies, containing coding nucleic acids. Viruses have evolved to exploit host cellular components including enzymes to ensure their replicative life cycle. New findings indicate that also viral capsid proteins recruit host factors to accelerate their assembly. These assembly machines are RNA-containing multiprotein complexes whose composition is governed by allosteric sites. In the event of viral infection, the assembly machines are recruited to support the virus over the host and are modified to achieve that goal. Stress granules and processing bodies may represent collections of such assembly machines, readily visible by microscopy but biochemically labile and difficult to isolate by fractionation. We hypothesize that the assembly of protein multimers such as encountered in neurodegenerative or other protein conformational diseases, is also catalyzed by assembly machines. In the case of viral infection, the assembly machines have been modified by the virus to meet the virus' need for rapid capsid assembly rather than host homeostasis. In the case of the neurodegenerative diseases, it is the monomers and/or low n oligomers of the so-called aggregated proteins that are substrates of assembly machines. Examples for substrates are amyloid β peptide (Aβ) and tau in Alzheimer's disease, α-synuclein in Parkinson's disease, prions in the prion diseases, Disrupted-in-schizophrenia 1 (DISC1) in subsets of chronic mental illnesses, and others. A likely continuum between virus capsid assembly and cell-to-cell transmissibility of aggregated proteins is remarkable. Protein aggregation diseases may represent dysfunction and dysregulation of these assembly machines analogous to the aberrations induced by viral infection in which cellular homeostasis is pathologically reprogrammed. In this view, as for viral infection, reset of assembly machines to normal homeostasis should be the goal of protein aggregation therapeutics. A key basis for the commonality between viral and neurodegenerative disease aggregation is a broader definition of assembly as more than just simple aggregation, particularly suited for the crowded cytoplasm. The assembly machines are collections of proteins that catalytically accelerate an assembly reaction that would occur spontaneously but too slowly to be relevant in vivo. Being an enzyme complex with a functional allosteric site, appropriated for a non-physiological purpose (e.g. viral infection or conformational disease), these assembly machines present a superior pharmacological target because inhibition of their active site will amplify an effect on their substrate reaction. Here, we present this hypothesis based on recent proof-of-principle studies against Aβ assembly relevant in Alzheimer's disease.

Oligomer formation of tau protein hyperphosphorylated in cells

Abnormal phosphorylation (“hyperphosphorylation”) and aggregation of Tau protein are hallmarks of Alzheimer disease and other tauopathies, but their causative connection is still a matter of debate. Tau with Alzheimer-like phosphorylation is also present in hibernating animals, mitosis, or during embryonic development, without leading to pathophysiology or neurodegeneration. Thus, the role of phosphorylation and the distinction between physiological and pathological phosphorylation needs to be further refined. So far, the systematic investigation of highly phosphorylated Tau was difficult because a reliable method of preparing reproducible quantities was not available. Here, we generated full-length Tau (2N4R) in Sf9 cells in a well defined phosphorylation state containing up to ∼20 phosphates as judged by mass spectrometry and Western blotting with phospho-specific antibodies. Despite the high concentration in living Sf9 cells (estimated ∼230 μm) and high phosphorylation, the protein was not aggregated. However, after purification, the highly phosphorylated protein readily formed oligomers, whereas fibrils were observed only rarely. Exposure of mature primary neuronal cultures to oligomeric phospho-Tau caused reduction of spine density on dendrites but did not change the overall cell viability.

Toxicity of protein oligomers is rationalized by a function combining size and surface hydrophobicity

The misfolding and aberrant assembly of peptides and proteins into fibrillar aggregates is the hallmark of many pathologies. Fibril formation is accompanied by oligomeric species thought to be the primary pathogenic agents in many of these diseases. With the aim of identifying the structural determinants responsible for the toxicity of misfolded oligomers, we created 12 oligomeric variants from the N-terminal domain of the E. coli HypF protein (HypF-N) by replacing one or more charged amino acid residues with neutral apolar residues and allowing the mutated proteins to aggregate under two sets of conditions. The resulting oligomeric species have different degrees of cytotoxicity when added to the extracellular medium of the cells, as assessed by the extent of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) reduction, apoptosis, and influx of Ca2+ into the cells. The structural properties of the oligomeric variants were characterized by evaluating their surface hydrophobicity with 8-anilinonaphthalene-1-sulfonate (ANS) binding and by measuring their size by means of turbidimetry as well as light scattering. We find that increases in the surface hydrophobicity of the oligomers following mutation can promote the formation of larger assemblies and that the overall toxicity correlates with a combination of both surface hydrophobicity and size, with the most toxic oligomers having high hydrophobicity and small size. These results have allowed the relationships between these three parameters to be studied simultaneously and quantitatively, and have enabled the generation of an equation that is able to rationalize and even predict toxicity of the oligomers resulting from their surface hydrophobicity and size.

High stability and cooperative unfolding of α-synuclein oligomers

Many neurodegenerative diseases are linked with formation of amyloid aggregates. It is increasingly accepted that not the fibrils but rather oligomeric species are responsible for degeneration of neuronal cells. Strong evidence suggests that in Parkinson's disease (PD), cytotoxic α-synuclein (αSN) oligomers are key to pathogenicity. Nevertheless, insight into the oligomers' molecular properties remains scarce. Here we show that αSN oligomers, despite a large amount of disordered structure, are remarkably stable against extreme pH, temperature, and even molar amounts of chemical denaturants, though they undergo cooperative unfolding at higher denaturant concentrations. Mutants found in familial PD lead to slightly larger oligomers whose stabilities are very similar to that of wild-type αSN. Isolated oligomers do not revert to monomers but predominantly form larger aggregates consisting of stacked oligomers, suggesting that they are off-pathway relative to the process of fibril formation. We also demonstrate that 4-(dicyanovinyl)julolidine (DCVJ) can be used as a specific probe for detection of αSN oligomers. The high stability of the αSN oligomer indicates that therapeutic strategies should aim to prevent the formation of or passivate rather than dissociate this cytotoxic species.