Evidence for a partially folded intermediate in alpha-synuclein fibril formation

Protein accumulation in traumatic brain injury

Traumatic brain injury (TBI) is one of the most devastating diseases in our society, accounting for a high percentage of mortality and disability. A major consequence of TBI is the rapid and long-term accumulation of proteins. This process largely reflects the interruption of axonal transport as a result of extensive axonal injury. Although many proteins are found accumulating after TBI, three have received particular attention; beta-amyloid precursor protein and its proteolytic products, amyloid-beta (Abeta) peptides, neurofilament proteins, and synuclein proteins. Massive coaccumulations of all of these proteins are found in damaged axons throughout the white matter after TBI. Additionally, these proteins form aggregates in other neuronal compartments and in brain parenchyma after brain trauma. Interestingly, TBI is also an epigenetic risk factor for developing neurodegenerative disorders, such as Alzheimer's disease and Parkinson's disease. Here, the similarities and differences of these accumulations with pathologies of neurodegenerative diseases will be explored. In addition, the potential deleterious roles of protein accumulations on functional outcome and progressive neurodegeneration following TBI will be examined.

α-Synuclein: Normal Function and Role in Neurodegenerative Diseases

Synucleins are a family of small, highly charged proteins expressed predominantly in neurons. Since their discovery and characterization during the last decade, much has been learned about their structure, potential functions, interactions with other proteins, and roles in disease. One of these proteins, alpha-synuclein (alpha-syn), is the major building block of pathological inclusions that characterize many neurodegenerative disorders, including Parkinson's disease (PD), dementia with Lewy bodies (DLB), and neurodegeneration with brain iron accumulation type 1 (NBIA-1), which collectively are termed synucleinopathies. Furthermore, genetic and biological studies support a role for alpha-syn in the pathophysiology of these diseases. Therefore, research must be continued in order to better understand the functions of the synuclein proteins under normal physiological conditions as well as their role in diseases.

Challenges and complexities of alpha-synuclein toxicity: new postulates in unfolding the mystery associated with Parkinson's disease

The discovery of two missense mutations in alpha-synuclein gene and the identification of the alpha-synuclein as the major component of Lewy bodies and Lewy neurites have imparted a new direction in understanding Parkinson's disease. Now that alpha-synuclein has been implicated in several neurodegenerative disorders makes it increasingly clear that aggregation of alpha-synuclein is a hallmark feature in neurodegeneration. Although little has been learned about its normal function, alpha-synuclein appears to be associated with membrane phospholipids and may therefore participate in a number of cell signaling pathways. Here, we review the localization, structure, and function of alpha-synuclein and provide a new hypothesis on, (a) the disruption in the membrane binding ability of synuclein which may be the major culprit leading to the alpha-synuclein aggregation and (b) the complexity associated with nuclear localization of alpha-synuclein and its possible binding property to DNA. Further, we postulated the three possible mechanisms of synuclein induced neuronal degeneration in Parkinson's disease.

The Role of Hydrophobic Interactions in Amyloidogenesis: Example of Prion-Related Polypeptides

Conversion of the non-infectious, cellular form of the prion protein (PrP(C)) to the infectious form (PrP(Sc)) is thought to be driven by an alpha-helical to beta-sheet conformational transition. To reveal the sequence determinants which encourage the transition to beta-fold, we study the synthetic peptides associated with hydrophobic conserved fragments of the N-terminal region of the prion protein. The structure of peptides in solution was probed under various thermodynamic conditions employing circular dichroism and steady state fluorescence spectroscopy as well as dye binding assays. The fluorescence methods utilized pyrene residues covalently attached to the end of the model peptides. In aqueous solutions, the structure assessments indicate the formation of metastable peptide aggregates; the molecular conformations within the peptide micelles are largely coiled. This stage in molecular assembly exists without significant beta-strand formation, i.e., before the appearance of any ordered secondary structure detectable by circular dichroism. At moderate concentrations of trifluoroethanol and/or acetonitrile, the conformational ensemble shifts towards beta-strand formation, and the population of the amorphous aggregates decreases significantly. Overall, the present data indicate that hydrophobic interactions between side chains of the peptide variants prevent, in fact, the formation of the rigid beta-sheet structures. Encouragement of beta-folds requires the destabilization of local interactions in the peptide chain, which in vivo might be possible within cell membranes as well as within partly folded molecular forms.

Natively unfolded C-terminal domain of caldesmon remains substantially unstructured after the effective binding to calmodulin

The structure of C-terminal domain (CaD136, C-terminal residues 636-771) of chicken gizzard caldesmon has been analyzed by a variety of physico-chemical methods. We are showing here that CaD136 does not have globular structure, has low secondary structure content, is essentially noncompact, as it follows from high R(g) and R(S) values, and is characterized by the absence of distinct heat absorption peaks, i.e. it belongs to the family of natively unfolded (or intrinsically unstructured) proteins. Surprisingly, effective binding of single calmodulin molecule (K(d) = 1.4 +/- 0.2 microM) leads only to a very moderate folding of this protein and CaD136 remains substantially unfolded within its tight complex with calmodulin. The biological significance of these observations is discussed.

Neuroprotective strategies in Parkinson's disease : an update on progress

In spite of the extensive studies performed on postmortem substantia nigra from Parkinson's disease patients, the aetiology of the disease has not yet been established. Nevertheless, these studies have demonstrated that, at the time of death, a cascade of events had been initiated that may contribute to the demise of the melanin-containing nigro-striatal dopamine neurons. These events include increased levels of iron and monoamine oxidase (MAO)-B activity, oxidative stress, inflammatory processes, glutamatergic excitotoxicity, nitric oxide synthesis, abnormal protein folding and aggregation, reduced expression of trophic factors, depletion of endogenous antioxidants such as reduced glutathione, and altered calcium homeostasis. To a large extent, the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 6-hydroxydopamine (6-OHDA) animal models of Parkinson's disease confirm these findings. Furthermore, neuroprotection can be afforded in these models with iron chelators, radical scavenger antioxidants, MAO-B inhibitors, glutamate antagonists, nitric oxide synthase inhibitors, calcium channel antagonists and trophic factors. Despite the success obtained with animal models, clinical neuroprotection is much more difficult to accomplish. Although the negative studies obtained with the MAO-B inhibitor selegiline (deprenyl) and the antioxidant tocopherol (vitamin E) may have resulted from an inappropriate choice of drug (selegiline) or an inadequate dose (tocopherol), the niggling problem that still remains is why these drugs, and others, do work in animals while they fail in the clinic. One reason for this may be related to the fact that in normal human brains the number of dopaminergic neurons falls by around 3-5% every decade, while in Parkinson's disease this decline is greater. Brain autopsy studies have shown that by the time the disease is identified, some 70-75% of the dopamine-containing neurons have been lost. More sensitive reliable methods and clinical correlative markers are required to discern between confoundable symptomatic effects versus a possible neuroprotective action of drugs, namely, the ability to delay or forestall disease progression by protecting or rescuing the remaining dopamine neurons or even restoring those that have been lost.A number of other possibilities for the clinical failure of potential neuroprotectants also exist. First, the animal models of Parkinson's disease may not be totally reflective of the disease and, therefore, the chemical pathologies established in the animal models may not cause, or contribute to, the progression of the disease clinically. Second, because of the series of events occurring in neurodegeneration and our ignorance about which of these factors constitutes the primary event in the pathogenic process, a single drug may not be adequate to induce neuroprotection and, as a consequence, use of a cocktail of drugs may be more appropriate. The latter concept receives support from recent complementary DNA (cDNA) microarray gene expression studies, which show the existence of a gene cascade of events occurring in the nigrostriatal pathway of MPTP, 6-OHDA and methamphetamine animal models of Parkinson's disease. Even with the advent of powerful new tools such as genomics, proteomics, brain imaging, gene replacement therapy and knockout animal models, the desired end result of neuroprotection is still beyond our current capability.

The association of alpha-synuclein with membranes affects bilayer structure, stability, and fibril formation

The aggregation of alpha-synuclein is believed to be a critical factor in the etiology of Parkinson's disease. alpha-Synuclein is an abundant neuronal protein of unknown function, which is enriched in the presynaptic terminals of neurons. Although alpha-synuclein is found predominantly in the cytosolic fractions, membrane-bound alpha-synuclein has been suggested to play an important role in fibril formation. The effects of alpha-synuclein on lipid bilayers of different compositions were determined using fluorescent environment-specific probes located at various depths. alpha-Synuclein-membrane interactions were found to affect both protein and membrane properties. Our results indicate that in addition to electrostatic interactions, hydrophobic interactions are important in the association of the protein with the bilayer, and lead to disruption of the membrane. The latter was observed by atomic force microscopy and fluorescent dye leakage from vesicles. The kinetics of alpha-synuclein fibril formation were significantly affected by the protein association and subsequent membrane disruption, and reflected the conformation of alpha-synuclein. The ability of alpha-synuclein to disrupt membranes correlated with the binding affinity of alpha-synuclein for the particular membrane composition, and to the induced helical conformation of alpha-synuclein. Protofibrillar or fibrillar alpha-synuclein caused a much more rapid destruction of the membrane than soluble monomeric alpha-synuclein, indicating that protofibrils (oligomers) or fibrils are likely to be significantly neurotoxic.

A protein-chameleon: conformational plasticity of alpha-synuclein, a disordered protein involved in neurodegenerative disorders

Under the physiological conditions in vitro, alpha-synuclein, a conservative presynaptic protein, the aggregation and fibrillation of which is assumed to be involved into the pathogenesis of Parkinson's disease and several other neurodegenerative disorders, known as synucleinopathies, is characterized by the lack of rigid well-defined structure; i.e., it belongs to the class of intrinsically unstructured proteins. Intriguingly, alpha-synuclein is characterized by a remarkable conformational plasticity, adopting a series of different conformations depending on the environment. For example, this protein may either stay substantially unfolded, or adopt an amyloidogenic partially folded conformation, or fold into alpha-helical or beta-structural species, both monomeric and oligomeric. Furthermore, it might form several morphologically different types of aggregates, including oligomers (spheres or doughnuts), amorphous aggregates, and or amyloid-like fibrils. The peculiarities of this astonishing conformational behavior are analyzed to shed light on structural plasticity of this protein-chameleon.

Structural organization of alpha-synuclein fibrils studied by site-directed spin labeling

Despite its importance in Parkinson's disease, a detailed understanding of the structure and mechanism of alpha-synuclein fibril formation remains elusive. In this study, we used site-directed spin labeling and electron paramagnetic resonance spectroscopy to study the structural features of monomeric and fibrillar alpha-synuclein. Our results indicate that monomeric alpha-synuclein, in solution, has a highly dynamic structure, in agreement with the notion that alpha-synuclein is a natively unfolded protein. In contrast, fibrillar aggregates of alpha-synuclein exhibit a distinct domain organization. Our data identify a highly ordered and specifically folded central core region of approximately 70 amino acids, whereas the N terminus is structurally more heterogeneous and the C terminus ( approximately 40 amino acids) is completely unfolded. Interestingly, the central core region of alpha-synuclein exhibits several features reminiscent of those observed in the core region of fibrillar Alzheimer's amyloid beta peptide, including an in-register parallel structure. Although the lengths of the respective core regions differ, fibrils from different amyloid proteins nevertheless appear to be able to take up highly similar, and possibly conserved, structures.

Rapid anionic micelle-mediated alpha-synuclein fibrillization in vitro

Parkinson's disease is characterized by the aggregation of alpha-synuclein into filamentous forms within affected neurons of the basal ganglia. Fibrillization of purified recombinant alpha-synuclein is inefficient in vitro but can be enhanced by the addition of various agents including glycosaminoglycans and polycations. Here we report that fatty acids and structurally related anionic detergents greatly accelerate fibrillization of recombinant alpha-synuclein at low micromolar concentrations with lag times as short as 11 min and apparent first order growth rate constants as fast as 10.4 h-1. All detergents and fatty acids were micellar at active concentrations because of an alpha-synuclein-dependent depression of their critical micelle concentrations. Other anionic surfaces, such as those supplied by anionic phospholipid vesicles, also induced alpha-synuclein fibrillization, with resultant filaments originating from their surface. These data suggest that anionic surfaces presented as micelles or vesicles can serve to nucleate alpha-synuclein fibrillization, that this mechanism underlies the inducer activity of anionic surfactants, and that anionic membranes may serve this function in vivo.

Expansion of polyglutamine induces the formation of quasi-aggregate in the early stage of protein fibrillization

We examined the effects of the expansion of glutamine repeats on the early stage of protein fibrillization. Small-angle x-ray scattering (SAXS) and electron microscopic studies revealed that the elongation of polyglutamine from 35 to 50 repeats in protein induced a large assembly of the protein upon incubation at 37 degrees C and that its formation was completed in approximately 3 h. A bead modeling procedure based on SAXS spectra indicated that the largely assembled species of the protein, quasi-aggregate, is composed of 80 to approximately 90 monomers and a bowl-like structure with long and short axes of 400 and 190 A, respectively. Contrary to fibril, the quasi-aggregate did not show a peak at S = 0.21 A-1 corresponding to the 4.8-A spacing of beta-pleated sheets in SAXS spectra, and reacted with a monoclonal antibody specific to expanded polyglutamine. These results imply that beta-sheets of expanded polyglutamines in the quasi-aggregate are not orderly aligned and are partially exposed, in contrast to regularly oriented and buried beta-pleated sheets in fibril. The formation of non-fibrillary quasi-aggregate in the early phase of fibril formation would be one of the major characteristics of the protein containing an expanded polyglutamine.

A general model for amyloid fibril assembly based on morphological studies using atomic force microscopy

Based on atomic force microscopy analysis of the morphology of fibrillar species formed during fibrillation of alpha-synuclein, insulin, and the B1 domain of protein G, a previously described model for the assembly of amyloid fibrils of immunoglobulin light-chain variable domains is proposed as a general model for the assembly of protein fibrils. For all of the proteins studied, we observed two or three fibrillar species that vary in diameter. The smallest, protofilaments, have a uniform height, whereas the larger species, protofibrils and fibrils, have morphologies that are indicative of multiple protofilaments intertwining. In all cases, protofilaments intertwine to form protofibrils, and protofibrils intertwine to form fibrils. We propose that the hierarchical assembly model describes a general mechanism of assembly for all amyloid fibrils.

Certain metals trigger fibrillation of methionine-oxidized alpha-synuclein

The aggregation and fibrillation of alpha-synuclein has been implicated as a key step in the etiology of Parkinson's disease and several other neurodegenerative disorders. In addition, oxidative stress and certain environmental factors, including metals, are believed to play an important role in Parkinson's disease. Previously, we have shown that methionine-oxidized human alpha-synuclein does not fibrillate and also inhibits fibrillation of unmodified alpha-synuclein (Uversky, V. N., Yamin, G., Souillac, P. O., Goers, J., Glaser, C. B., and Fink, A. L. (2002) FEBS Lett. 517, 239-244). Using dynamic light scattering, we show that the inhibition results from stabilization of the monomeric form of Met-oxidized alpha-synuclein. We have now examined the effect of several metals on the structural properties of methionine-oxidized human alpha-synuclein and its propensity to fibrillate. The presence of metals induced partial folding of both oxidized and non-oxidized alpha-synucleins, which are intrinsically unstructured under conditions of neutral pH. Although the fibrillation of alpha-synuclein was completely inhibited by methionine oxidation, the presence of certain metals (Ti3+, Zn2+, Al3+, and Pb2+) overcame this inhibition. These findings indicate that a combination of oxidative stress and environmental metal pollution could play an important role in triggering the fibrillation of alpha-synuclein and thus possibly Parkinson's disease.

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

Filamentous inclusions of alpha-synuclein protein are hallmarks of neurodegenerative diseases collectively known as synucleinopathies. Previous studies have shown that exposure to oxidative and nitrative species stabilizes alpha-synuclein filaments in vitro, and this stabilization may be due to dityrosine cross-linking. To test this hypothesis, we mutated tyrosine residues to phenylalanine and generated recombinant wild type and mutant alpha-synuclein proteins. alpha-Synuclein proteins lacking some or all tyrosine residues form fibrils to the same extent as the wild type protein. Tyrosine residues are not required for protein cross-linking or filament stabilization resulting from transition metal-mediated oxidation, because higher Mr SDS-resistant oligomers and filaments stable to chaotropic agents are detected using all Tyr --> Phe alpha-synuclein mutants. By contrast, cross-linking resulting from exposure to nitrating agents required the presence of one or more tyrosine residues. Furthermore, tyrosine cross-linking is involved in filament stabilization, because nitrating agent-exposed assembled wild type, but not mutant alpha-synuclein lacking all tyrosine residues, was stable to chaotropic treatment. In addition, the formation of stable alpha-synuclein inclusions in intact cells after exposure to oxidizing and nitrating species requires tyrosine residues. These findings demonstrate that nitrative and/or oxidative stress results in distinct mechanisms of alpha-synuclein protein modifications that can influence the formation of stable alpha-synuclein fibrils.

alpha-synuclein aggregation: a link between mitochondrial defects and Parkinson's disease?

Protein aggregation is a shared feature of many human neurodegenerative diseases and appears to be an inevitable consequence of excessive accumulation of misfolded proteins. Recent studies suggest that accumulation of fibrillar alpha-synuclein aggregates is associated with Parkinson's disease and other Lewy body diseases. Furthermore, the missense mutations in alpha-synuclein that are responsible for some early-onset familial types of the disease promote the aggregation process of this protein. Therefore, the mechanism underlying the cellular alpha-synuclein aggregation is of great importance in understanding the pathogenic process of these diseases. This review summarizes recent advances in our understanding of the mechanisms underlying alpha-synuclein aggregation and how the mitochondrial dysfunction plays a role in this process. Protein misfolding and aggregation in vivo can be suppressed and promoted by several factors, such as molecular chaperones, protein degradation systems, and free radicals. Many of these factors are under the control of normal mitochondrial function, prompting the speculation that mitochondrial dysfunction might cause the accumulation of protein aggregates. Recent studies indeed show that mitochondrial defects can lead to the aggregation of alpha-synuclein. In addition, potentially toxic effects of alpha-synuclein have been linked to the aggregated forms rather than the monomers, both in vitro and in cultured cells. Therefore, it is postulated that aggregation of alpha-synuclein might be one of many possible links that connect mitochondrial dysfunction to neurodegeneration.

Amyloid-like fibril formation in an all beta-barrel protein. Partially structured intermediate state(s) is a precursor for fibril formation

Acidic fibroblast growth factor from newt (Notopthalmus viridescens) is a approximately 15-kDa, all beta-sheet protein devoid of disulfide bonds. In the present study, we investigate the effects of 2,2,2-trifluoroethanol (TFE) on the structure of newt acidic fibroblast growth factor (nFGF-1). The protein aggregates maximally in 10% (v/v) TFE. Congo red and thioflavin T binding experiments suggest that the aggregates induced by TFE have properties resembling the amyloid fibrils. Transmission electron microscopy and x-ray fiber diffraction data show that the fibrils (induced by TFE) are straight, unbranched, and have a cross-beta structure with an average diameter of 10-15 A. Preformed fibrils (induced by TFE) of nFGF-1 are observed to seed amyloid-like fibril formation in solutions containing the protein (nFGF-1) in the native beta-barrel conformation. Fluorescence, far-UV CD, anilino-8-napthalene sulfonate binding, multidimensional NMR, and Fourier transformed infrared spectroscopy data reveal that formation of a partially structured intermediate state(s) precedes the onset of the fibrillation process. The native beta-barrel structure of nFGF-1 appears to be disrupted in the partially structured intermediate state(s). The protein in the partially structured intermediate state(s) is found to be "sticky" with a solvent-exposed non-polar surface(s). Amyloid fibril formation appears to occur due to coalescence of the protein in the partially structured intermediate state(s) through solvent-exposed non-polar surfaces and intermolecular beta-sheet formation among the extended, linear beta-strands in the protein.

Lipid binding inhibits alpha-synuclein fibril formation

Parkinson's disease is the second most common neurodegenerative disorder, and the cause is unknown; however, substantial evidence implicates the aggregation of alpha-synuclein as a critical factor in the etiology of the disease. alpha-Synuclein is a relatively abundant brain protein of unknown function, and the purified protein is intrinsically unfolded. The amino acid sequence has seven repeats with an apolipoprotein lipid-binding motif, which are predicted to form amphiphilic helices. We have investigated the interaction of alpha-synuclein with lipid vesicles of different sizes and properties by monitoring the effects on the conformation of the protein and the kinetics of fibrillation. The nature of the interaction of alpha-synuclein with vesicles was highly dependent on the phospholipid composition, the ratio of alpha-synuclein to phospholipid, and the size of the vesicles. The strongest interactions were between alpha-synuclein and vesicles composed of 1,2-dipalmitoyl-sn-glycero-3-phosphate/1,2-dipalmitoyl-sn-glycero-3-phosphocholine and 1,2-dipalmitoyl-sn-glycero-3-phospho-RAC-(1-glycerol)/1,2-dipalmitoyl-sn-glycero-3-phosphocholine and involved formation of helical structure in alpha-synuclein. A strong correlation was observed between the induction of alpha-helix in alpha-synuclein and the inhibition of fibril formation. Thus, helical, membrane-bound alpha-synuclein is unlikely to contribute to aggregation and fibrillation. Given that a significant fraction of alpha-synuclein is membrane-bound in dopaminergic neurons, this observation has significant physiological significance.

Nitration inhibits fibrillation of human alpha-synuclein in vitro by formation of soluble oligomers

The aggregation of alpha-synuclein in dopaminergic neurons is a critical factor in the etiology of Parkinson's disease (PD). Oxidative and nitrative stress is also implicated in PD. We examined the effect of nitration on the propensity of alpha-synuclein to fibrillate in vitro. Fibril formation of alpha-synuclein was completely inhibited by nitration, due to the formation of stable soluble oligomers (apparently octamers). More importantly the presence of sub-stoichiometric concentrations of nitrated alpha-synuclein led to inhibition of fibrillation of non-modified alpha-synuclein. These observations suggest that nitration of soluble alpha-synuclein may be a protective factor in PD, rather than a causative one.

High pressure induces the formation of aggregation-prone states of proteins under reducing conditions

The pressure stability of ribonuclease A and bovine pancreatic trypsin inhibitor has been investigated with Fourier transform infrared spectroscopy in the presence of the disulfide bond reducing agent 2-mercaptoethanol. The secondary structure of the reduced proteins at high pressure (1 GPa) is not significantly different from the pressure-induced conformation of the native form. Upon decompression under reducing conditions, amorphous aggregates are formed. Such aggregates are not formed upon decompression of the native proteins. Our data demonstrate that high pressure populates, and thus allows the potential characterization of highly aggregation-prone conformations. The relevance of these findings with regard to fibril formation is discussed and the possible role of conformational fluctuations of intermediates on the aggregation pathway is emphasized.

Lewy body pathology is a frequent co-pathology in familial Alzheimer's disease

Our institution is currently engaged in ongoing genetic studies of familial Alzheimer's disease (AD), which include clinical ascertainment and brain autopsy of both affected and non-affected family members. Here we describe the analysis of 22 AD families, each with at least one family member with a postmortem diagnosis of dementia with Lewy bodies (DLB). For this study, 47 brains were examined according to NINCDS-Reagan Institute criteria for the diagnosis of AD. Lewy body pathology was evaluated with alpha-synuclein immunohistochemistry. Four families, with either one or two autopsies showing Lewy body pathology, demonstrated linkage to 12p. Five families had two or more autopsies with Lewy body pathology, but their linkage status was unknown. The remaining 13 families had one autopsy demonstrating Lewy bodies. These findings suggest that at least one pathological form of DLB may be familial. In some families, the pathological phenotype is identical in all examined affected family members; but in others, there may be several pathologies that coexist. Careful neuropathological examination of affected family members may prove critical for future genetic analysis of AD and DLB.

Polycation-induced oligomerization and accelerated fibrillation of human alpha-synuclein in vitro

The aggregation and fibrillation of alpha-synuclein has been implicated as a causative factor in Parkinson's disease and several other neurodegenerative disorders known as synucleinopathies. The effect of different factors on the process of fibril formation has been intensively studied in vitro. We show here that alpha-synuclein interacts with different unstructured polycations (spermine, polylysine, polyarginine, and polyethyleneimine) to form specific complexes. In addition, the polycations catalyze alpha-synuclein oligomerization. The formation of alpha-synuclein-polycation complexes was not accompanied by significant structural changes in alpha-synuclein. However, alpha-synuclein fibrillation was dramatically accelerated in the presence of polycations. The magnitude of the accelerating effect depended on the nature of the polymer, its length, and concentration. The results illustrate the potential critical role of electrostatic interactions in protein aggregation, and the potential role of naturally occurring polycations in modulating alpha-synuclein aggregation.

Characterization of cytoplasmic alpha-synuclein aggregates. Fibril formation is tightly linked to the inclusion-forming process in cells

The alpha-synuclein fibrillation process has been associated with the pathogenesis of several neurodegenerative diseases. Here, we have characterized the cytoplasmic alpha-synuclein aggregates using a fractionation procedure with which different aggregate species can be separated. Overexpression of alpha-synuclein in cells produce two distinct types of aggregates: large juxtanuclear inclusion bodies and small punctate aggregates scattered throughout the cytoplasm. Biochemical fractionation results in an inclusion-enriched fraction and two small aggregate fractions. Electron microscopy and thioflavin S reactivity of the fractions show that the juxtanuclear inclusion bodies are filled with amyloid-like alpha-synuclein fibrils, whereas both the small aggregate fractions contain non-fibrillar spherical aggregates with distinct size distributions. These aggregates appear sequentially, with the smallest population appearing the earliest and the fibrillar inclusions the latest. Based on the structural and kinetic properties, we suggest that the small spherical aggregates are the cellular equivalents of the protofibrils. The proteins that co-exist in the Lewy bodies, such as proteasome subunit, ubiquitin, and hsp70 chaperone, are present in the fibrillar inclusions but absent in the protofibrils, suggesting that these proteins may not be directly involved in the early aggregation stage. As predicted in the aggresome model, disruption of microtubules with nocodazole reduced the number of inclusions and increased the size of the protofibrils. Despite the increased size, the protofibrils remained non-fibrillar, suggesting that the deposition of the protofibrils in the juxtanuclear region is important in fibril formation. This study provides evidence that the cellular fibrillation also involves non-fibrillar intermediate species, and the microtubule-dependent inclusion-forming process is required for the protofibril-to-fibril conversion in cells.

Amyloidogenicity and cytotoxicity of islet amyloid polypeptide

Insoluble amyloid formation by islet amyloid polypeptide (IAPP) in the islets of Langerhans of the pancreas is a major pathophysiological feature of noninsulin dependent diabetes mellitus (NIDDM) or type II diabetes. Because in vivo formed amyloid colocalizes with areas of cell degeneration and IAPP amyloid aggregates are cytotoxic per se, the process of IAPP amyloid formation has been strongly associated with the progressive pancreatic cell degeneration and thus much of the pathology of type II diabetes. IAPP is a pancreatic polypeptide of 37 residues that, in its soluble form, is believed to play a role as a regulator of glucose homeostasis. The molecular cause and mechanism of the conversion of soluble IAPP into insoluble amyloid aggregates in vivo and its role in disease progress still remain to be clarified. Nevertheless, in the past few years significant progress has been made in understanding the amyloidogenesis pathway of IAPP in vitro and gaining insight into the structural and conformational "requirements" of IAPP amyloidogenesis and related cytotoxic effects. Importantly, several of the studies have revealed significant similarities of the above features of IAPP to other amyloidogenic polypeptides such as the beta-amyloid polypeptide Abeta. This suggests that, at the molecular level, amyloidogenesis, and possibly related cell degeneration and disease pathogenesis by completely different polypeptide sequences, may obey to common structural and conformational "rules" and follow similar molecular pathways. This review describes studies on the structural and conformational features of IAPP amyloid formation and cytotoxicity, and the application of the obtained knowledge for the understanding of the molecular mechanism of the IAPP amyloidogenesis pathway and the related cytotoxicity.

Kinetic studies of amyloid beta-protein fibril assembly. Differential effects of alpha-helix stabilization

Amyloid beta-protein (Abeta) fibril assembly is a defining characteristic of Alzheimer's disease. Fibril formation is a complex nucleation-dependent polymerization process characterized in vitro by an initial lag phase. To a significant degree, this phase is a consequence of the energy barrier that must be overcome in order for Abeta monomers to fold and oligomerize into fibril nuclei. Here we show that low concentrations of 2,2,2-trifluoroethanol (TFE) convert predominately unstructured Abeta monomers into partially ordered, quasistable conformers. Surprisingly, this results in a temporal decrease in the lag phase for fibril formation and a significant increase in the rate of fibril elongation. The TFE effect is concentration dependent and is maximal at approximately 20% (v/v). In the presence of low concentrations of TFE, fibril formation is observed in Abeta samples at nanomolar concentration, well below the critical concentration for Abeta fibril formation in the absence of TFE. As the amount of TFE is increased above 20%, helix content progressively rises to approximately 80%, a change paralleled first by a decrease in elongation rate and then by a complete cessation of fibril growth. These findings are consistent with the hypothesis that a partially folded helix-containing conformer is an intermediate in Abeta fibril assembly. The requirement that Abeta partially folds in order to assemble into fibrils contrasts with the mechanism of amyloidogenesis of natively folded proteins such as transthyretin and lysozyme, in which partial unfolding is a prerequisite. Our results suggest that in vivo, factors that affect helix formation and stability will have significant effects on the kinetics of Abeta fibril formation.

Synergistic effects of pesticides and metals on the fibrillation of alpha-synuclein: implications for Parkinson's disease

Aggregation of alpha-synuclein has been implicated in the formation of proteinaceous inclusions in the brain (Lewy bodies, Lewy neurites) that are characteristic of neurodegenerative diseases, such as Parkinson's disease (PD) and dementia with Lewy bodies (DLBs). The etiology of PD is unknown, but recent work has shown that except in rare cases, there appears to be no direct genetic basis. However, several studies have implicated environmental factors, especially pesticides and metals. Here we show that certain pesticides and metals induce a conformational change in alpha-synuclein and directly accelerate the rate of formation of alpha-synuclein fibrils in vitro. In addition, the simultaneous presence of metal and pesticide led to synergistic effects on the rate of fibrillation. We propose a model in which environmentalfactors in conjunction with genetic susceptibility may form the underlying molecular basis for idiopathic PD.

Genetics of Parkinson's disease and biochemical studies of implicated gene products

Parkinson's disease was thought, until recently, to have little or no genetic component. This notion has changed with the identification of three genes, and the mapping of five others, that are linked to rare familial forms of the disease (FPD). The products of the identified genes, alpha-synuclein (PARK 1), parkin (PARK 2), and ubiquitin-C-hydrolase-L1 (PARK 5) are the subject of intense cell-biological and biochemical studies designed to elucidate the underlying mechanism of FPD pathogenesis. In addition, the complex genetics of idiopathic PD is beginning to be unraveled. Genetic information may prove to be useful in identifying new therapeutic targets and identifying the preclinical phase of PD, allowing treatment to begin sooner.

Conformational behavior of human alpha-synuclein is modulated by familial Parkinson's disease point mutations A30P and A53T

Structural properties and response to changes in the environment of wild-type (WT), A30P and A53T alpha-synucleins, as well as their propensity to aggregate orform fibrils, were compared by a variety of biophysical methods, including far-UV CD, FTIR, SAXS, static light scattering and Thioflavin T (TFT) fluorescence. All three proteins were natively unfolded under physiological conditions but adopted identical partially-folded conformations under conditions of acidic pH or high temperature. The initial kinetic event in the fibrillation of all three alpha-synucleins was shown to be the formation of a partially-folded intermediate with properties close to those described for these proteins at acidic pH or at high temperatures. Both mutants showed a greater propensity to form non-fibrillar aggregates than wild-type protein. All three proteins formed fibrils faster in the presence of heparin, although substantially higher concentrations were required for the A30P mutant. In contrast to the wild-type and A53T proteins, in which fibrillation was further accelerated by the presence of the pesticide diethyldithiocarbamate (DDC), the A30P mutant was inhibited by DDC. The mutant proteins had significantly lower affinity for DDC than the WT. A model of the effect of mutations on the aggregation behavior of alpha-synuclein is proposed, which explains the different effects of exogenous agents on the three proteins, based on different kinetic partitioning along pathways leading to fibrils and to non-fibrillar aggregates.

Early and late gene changes in MPTP mice model of Parkinson's disease employing cDNA microarray

Recently, we reported specific brain gene expression changes in the chronic MPTP model inthe late stage of degeneration, employing cDNA expression array, which indicate a "domino" cascade of events involved in neuronal cell death. In an attempt to elucidate early gene expression profile in the region of the substantia nigra (SN) and the striatum of acute MPTP-treated mice (3-24 h), we elected a restricted number of genes affected by the long-term MPTP treatment, and their expression was examined. Specifically, we detected alterations in the expression of genes implicated in oxidative-stress, inflammatory processes, signal transduction and glutamate toxicity. These pro-toxic genes appear to be compensated by the elevated expression in trophic factors and antioxidant defenses, which are also activated by short exposure to MPTP. The time course of these gene expression changes indicates the importance of investigating the early gene cascade of events occurring prior to late nigrostriatal dopamine neuronal cell death.

Tryptophanyl substitutions in apomyoglobin determine protein aggregation and amyloid-like fibril formation at physiological pH

Myoglobin is an alpha-helical globular protein that contains two highly conserved tryptophan residues located at positions 7 and 14 in the N-terminal region of the protein. Replacement of both indole residues with phenylalanine residues, i.e. W7F/W14F, results in the expression of an unstable, not correctly folded protein that does not bind the prosthetic group. Here we report data (Congo red and thioflavine T binding assay, birefringence, and electron microscopy) showing that the double Trp/Phe replacements render apomyoglobin molecules highly susceptible to aggregation and amyloid-like fibril formation under physiological conditions in which most of the wild-type protein is in the native state. In refolding experiments, like the wild-type protein, the W7F/W14F apomyoglobin mutant formed a soluble, partially folded helical state between pH 2.0 and pH 4.0. A pH increase from 4.0 to 7.0 restored the native structure only in the case of the wild-type protein and determined aggregation of W7F/W14F. The circular dichroism spectrum recorded immediately after neutralization showed that the polypeptide consists mainly of beta-structures. In conclusion, under physiological pH conditions, some mutations that affect folding may cause protein aggregation and the formation of amyloid-like fibrils.

Dependence of alpha-synuclein aggregate morphology on solution conditions

Alpha-synuclein is the major component of Lewy bodies and Lewy neurites, which are granular and filamentous protein inclusions that are the defining pathological features of several neurodegenerative conditions such as Parkinson's disease. Fibrillar aggregates formed from alpha-synuclein in vitro resemble brain-derived material, but the role of such aggregates in the etiology of Parkinson's disease and their relation to the toxic molecular species remain unclear. In this study, we investigated the effects of pH and salt concentration on the in vitro assembly of human wild-type alpha-synuclein, particularly with regard to aggregation rate and aggregate morphology. Aggregates formed at pH 7.0 and pH 6.0 in the absence of NaCl and MgCl(2) were fibrillar; the pH 6.0 fibrils displayed a helical twist, as clearly evident by scanning force and electron microscopy. Incubations at pH 7.0 remained transparent during the process of aggregation and exhibited strong thioflavin-T and weak 8-anilino-1-naphthalenesulfonate (ANS) binding; furthermore, they were efficient in seeding fibrillization of fresh solutions. In contrast, incubating alpha-synuclein at low pH (pH 4.0 or pH 5.0) resulted in the rapid formation of turbid suspensions characterized by strong ANS binding, reduced thioflavin-T binding and reduced seeding efficiency. At pH 4.0, fibril formation was abrogated; instead, very large aggregates (dimensions approximately 100 microm) of amorphous appearance were visible by light microscopy. As with acidic conditions, addition of 0.2M NaCl or 10mM MgCl(2) to pH 7.0 incubations led to a shorter aggregation lag time and formation of large, amorphous aggregates. These results demonstrate that the morphology of alpha-synuclein aggregates is highly sensitive to solution conditions, implying that the fibrillar state does not necessarily represent the predominant or most functionally significant aggregated state under physiological conditions.

Structural transformation and aggregation of human alpha-synuclein in trifluoroethanol: non-amyloid component sequence is essential and beta-sheet formation is prerequisite to aggregation

Amyloid-like aggregation of alpha-synuclein and deposit in Lewy bodies are thought to be the major cause of Parkinson's disease. Here we describe the secondary structural transformation and aggregation of human alpha-synuclein and its C-terminus truncated fragments in trifluoroethanol. Proteins containing the NAC (non-amyloid component) segment undergo a three-state transition: from native random coil to beta-sheet and to alpha-helical structure, while the NAC deficient fragment and gamma-synuclein undergo a typical two-state coil-to-alpha transition. The beta-sheet form is highly hydrophobic that strongly binds to 1-anilinonaphthalene-8-sulfonic acid (ANS) and is prone to self-aggregation. The results suggest that the NAC sequence is essential to beta-sheet formation and the aggregation originates from the beta-sheet intermediate, which may be implicated in the pathogenesis of Parkinson's disease.

Amyloid-fibril formation. Proposed mechanisms and relevance to conformational disease

The phenomenon of the transformation of proteins into amyloid-fibrils is of interest, firstly, because it is closely connected to the so-called conformational diseases, many of which are hitherto incurable, and secondly, because it remains to be explained in physical terms (energetically and structurally). The process leads to fibrous aggregates in the form of extracellular amyloid plaques, neuro-fibrillary tangles and other intracytoplasmic or intranuclear inclusions. In this review, basic principles common to the field of amyloid fibril formation and conformational disease are underlined. Existing models for the mechanism need to be tested by experiment. The kinetic and energetic bases of the process are reviewed. The main controversial issue remains the coexistence of more than one protein conformation. The possible role of oligomeric intermediates, and of domain-swapping is also discussed. Mechanisms for cellular defence and novel therapies are considered.

Genetics of Parkinson's disease and biochemical studies of implicated gene products

Parkinson's disease was thought, until recently, to have little or no genetic component. This notion has changed with the identification of three genes, and the mapping of five others, that are linked to rare familial forms of the disease (FPD). The products of the identified genes, alpha-synuclein (PARK 1), parkin (PARK 2), and ubiquitin-C-hydrolase-L1 (PARK 5) are the subject of intense cell-biological and biochemical studies designed to elucidate the underlying mechanism of FPD pathogenesis. In addition, the complex genetics of idiopathic PD is beginning to be unraveled. Genetic information may prove to be useful in identifying new therapeutic targets and identifying the preclinical phase of PD, allowing treatment to begin sooner.

Biochemical characterization of the core structure of alpha-synuclein filaments

Intracellular filamentous aggregates comprised of alpha-synuclein such as Lewy bodies and glial cytoplasmic inclusions are the defining hallmarks of a subset of neurodegenerative diseases including Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy. We have analyzed biochemical and structural properties of alpha-synuclein filaments assembled in vitro or extracted from brains of patients with multiple system atrophy and found that both types of filaments are insoluble to detergents and partially resistant to proteinase K digestion. Immunoelectron microscopy and immunoblot analysis showed that both amino and carboxyl termini of alpha-synuclein in in vitro assembled filaments were degraded by proteinase K treatment, whereas the central portion of alpha-synuclein is resistant to proteinase K and retains filamentous structures. Protein sequencing and mass spectrometric analyses of the proteinase K-resistant, minimal fragment of 7 kDa revealed that amino acid residues 31-109 of alpha-synuclein constitute the core unit of the filaments. These observations suggest that the central half of the alpha-synuclein polypeptide, containing five tandem repeats as well as a part of the carboxyl-terminal acidic region, forms the core structure of alpha-synuclein filaments, which is coated by the amino- and carboxyl-terminal portions at the periphery.

Methionine oxidation inhibits fibrillation of human alpha-synuclein in vitro

We examined the effect of methionine oxidation of human recombinant alpha-synuclein on its structural properties and propensity to fibrillate. Both oxidized and non-oxidized alpha-synucleins were natively unfolded under conditions of neutral pH, with the oxidized protein being slightly more disordered. Both proteins adopted identical partially folded conformations under conditions of acidic pH. The fibrillation of alpha-synuclein at neutral pH was completely inhibited by methionine oxidation. This inhibitory effect was eliminated at low pH. The addition of oxidized alpha-synuclein to the unoxidized form led to a substantial inhibition of alpha-synuclein fibrillation.

Elucidation of the molecular mechanism during the early events in immunoglobulin light chain amyloid fibrillation. Evidence for an off-pathway oligomer at acidic pH

Light chain amyloidosis involves the systemic pathologic deposition of monoclonal light chain variable domains of immunoglobulins as insoluble fibrils. The variable domain LEN was obtained from a patient who had no overt amyloidosis; however, LEN forms fibrils in vitro, under mildly destabilizing conditions. The in vitro kinetics of fibrillation were investigated using a wide variety of probes. The rate of fibril formation was highly dependent on the initial protein concentration. In contrast to most amyloid systems, the kinetics became slower with increasing LEN concentrations. At high protein concentrations a significant lag in time was observed between the conformational changes and the formation of fibrils, consistent with the formation of soluble off-pathway oligomeric species and a branched pathway. The presence of off-pathway species was confirmed by small angle x-ray scattering. At low protein concentrations the structural rearrangements were concurrent with fibril formation, indicating the absence of formation of the off-pathway species. The data are consistent with a model for fibrillation in which a dimeric form of LEN (at high protein concentration) inhibits fibril formation by interaction with an intermediate on the fibrillation pathway and leads to formation of the off-pathway intermediate.

Effect of association state and conformational stability on the kinetics of immunoglobulin light chain amyloid fibril formation at physiological pH

Light chain amyloidosis involves the systemic deposition of fibrils in patients overproducing monoclonal immunoglobulin light chains. The kinetics of fibril formation of LEN, a benign light chain variable domain, were investigated at physiological pH in the presence of urea. Despite the lack of in vivo fibril formation, LEN readily forms fibrils in vitro under mildly destabilizing conditions. The effect of low to moderate concentrations of urea on the conformation, association state, stability, and kinetics of fibrillation of LEN were investigated. The conformation of LEN was only slightly affected by the addition of up to 4 m urea. The fibrillation kinetics were highly dependent on protein and urea concentrations, becoming faster with decreasing protein concentration and increasing urea concentration. Changes in spectral probes were concomitant to fibril formation throughout the protein and urea concentration ranges, indicating the absence of off-pathway oligomeric species or amorphous aggregates prior to fibril formation. Reducing the amount of dimers initially present in solution by either decreasing the protein concentration or adding urea resulted in faster fibril formation. Thus, increasing concentrations of urea, by triggering dissociation of dimeric LEN, lead to increased rates of fibrillation.

Biophysical properties of the synucleins and their propensities to fibrillate: inhibition of alpha-synuclein assembly by beta- and gamma-synucleins

The pathological hallmark of Parkinson's disease is the presence of intracellular inclusions, Lewy bodies, and Lewy neurites, in the dopaminergic neurons of the substantia nigra and several other brain regions. Filamentous alpha-synuclein is the major component of these deposits and its aggregation is believed to play an important role in Parkinson's disease and several other neurodegenerative diseases. Two homologous proteins, beta- and gamma-synucleins, are also abundant in the brain. The synucleins are natively unfolded proteins. beta-Synuclein, which lacks 11 central hydrophobic residues compared with its homologs, exhibited the properties of a random coil, whereas alpha- and gamma-synucleins were slightly more compact and structured. gamma-Synuclein, unlike its homologs, formed a soluble oligomer at relatively low concentrations, which appears to be an off-fibrillation pathway species. Here we show that, although they have similar biophysical properties to alpha-synuclein, beta- And gamma-synucleins inhibit alpha-synuclein fibril formation. Complete inhibition of alpha-synuclein fibrillation was observed at 4:1 molar excess of beta- and gamma-synucleins. No significant incorporation of beta-synuclein into the fibrils was detected. The lack of fibrils formed by beta-synuclein is most readily explained by the absence of a stretch of hydrophobic residues from the middle region of the protein. A model for the inhibition is proposed.

Natively unfolded proteins: a point where biology waits for physics

The experimental material accumulated in the literature on the conformational behavior of intrinsically unstructured (natively unfolded) proteins was analyzed. Results of this analysis showed that these proteins do not possess uniform structural properties, as expected for members of a single thermodynamic entity. Rather, these proteins may be divided into two structurally different groups: intrinsic coils, and premolten globules. Proteins from the first group have hydrodynamic dimensions typical of random coils in poor solvent and do not possess any (or almost any) ordered secondary structure. Proteins from the second group are essentially more compact, exhibiting some amount of residual secondary structure, although they are still less dense than native or molten globule proteins. An important feature of the intrinsically unstructured proteins is that they undergo disorder-order transition during or prior to their biological function. In this respect, the Protein Quartet model, with function arising from four specific conformations (ordered forms, molten globules, premolten globules, and random coils) and transitions between any two of the states, is discussed.

Unraveling multistate unfolding of rabbit muscle creatine kinase

GdmCl-induced unfolding of rabbit muscle creatine kinase, CK, has been studied by a variety of physico-chemical methods including near and far UV CD, SEC, intrinsic fluorescence (intensity, anisotropy and lifetime) as well as intensity and lifetime of bound ANS fluorescence. The formation of several stable unfolding intermediates, some of which were not observed previously, has been established. This was further confirmed by representation of fluorescence data in terms of "phase diagram", i.e. I(lambda1) versus I(lambda2) dependence, where I(lambda1) and I(lambda2) are fluorescence intensity values measured on wavelengths lambda(1) and lambda(2) under the different experimental conditions for a protein undergoing structural transformations. The unfolding behavior of CK was shown to be strongly affected by association of partially folded intermediates. A model of CK unfolding, which takes into account both structural perturbations and association of partially folded intermediates has been elaborated.

Accelerated alpha-synuclein fibrillation in crowded milieu

Parkinson's disease is the second most common age-related neurodegenerative disease, resulting from loss of dopaminergic neurons in the substantia nigra. The aggregation and fibrillation of alpha-synuclein has been implicated as a causative factor in the disease, and the process of fibril formation has been intensively studied in vitro with dilute protein solutions. However, the intracellular environment of proteins is crowded with other macromolecules, whose concentration can reach 400 g/l. To address this discrepancy, the effect of molecular crowding on alpha-synuclein fibrillation has being studied. The addition of high concentrations of different polymers (proteins, polysaccharides and polyethylene glycols) dramatically accelerated alpha-synuclein fibrillation in vitro. The magnitude of the accelerating effect depended on the nature of the polymer, its length and concentration. Our results suggest that the major factor responsible for the accelerated fibrillation under crowded conditions is the excluded volume.

The chicken-egg scenario of protein folding revisited

What is the first step in protein folding - hydrophobic collapse (compaction) or secondary structure formation? It is still not clear if the major driving force in protein folding is hydrogen bonding or hydrophobic interactions or both. We analyzed data on the conformational characteristics of 41 globular proteins in native and partially folded conformational states. Our analysis shows that a good correlation exists between relative decrease in hydrodynamic volume and increase in secondary structure content. No compact equilibrium intermediates lacking secondary structure, or highly ordered non-compact species, were found. This correlation provides experimental support for the hypothesis that hydrophobic collapse occurs simultaneously with formation of secondary structure in the early stages of the protein folding.

High hydrostatic pressure as a tool to study protein aggregation and amyloidosis

Aggregation of proteins is a serious problem, affecting both industrial production of proteins and human health. Despite recent advances in the theories and experimental techniques available to address understanding of protein aggregation processes, mechanisms of aggregate formation have proved challenging to study. This is in part because the typical irreversibility of protein aggregation processes at atmospheric conditions complicates analysis of their kinetics and thermodynamics. Because high hydrostatic pressures act to disfavor the hydrophobic and electrostatic interactions that cause protein aggregation, studies conducted under high hydrostatic pressures may allow protein aggregates to be formed reversibly, enabling thermodynamic and kinetic parameters to be measured in greater detail. Although application of high hydrostatic pressures to protein aggregation problems is rather recent, a growing literature, reviewed herein, suggests that high pressure may be a useful tool for both understanding protein aggregation and reversing it in industrial applications.

Formation and removal of alpha-synuclein aggregates in cells exposed to mitochondrial inhibitors

Mitochondrial dysfunction has been associated with Parkinson's disease. However, the role of mitochondrial defects in the formation of Lewy bodies, a pathological hallmark of Parkinson's disease has not been addressed directly. In this report, we investigated the effects of inhibitors of the mitochondrial electron-transport chain on the aggregation of alpha-synuclein, a major protein component of Lewy bodies. Treatment with rotenone, an inhibitor of complex I, resulted in an increase of detergent-resistant alpha-synuclein aggregates and a reduction in ATP level. Another inhibitor of the electron-transport chain, oligomycin, also showed temporal correlation between the formation of aggregates and ATP reduction. Microscopic analyses showed a progressive evolution of small aggregates of alpha-synuclein to a large perinuclear inclusion body. The inclusions were co-stained with ubiquitin, 20 S proteasome, gamma-tubulin, and vimentin. The perinuclear inclusion bodies, but not the small cytoplasmic aggregates, were thioflavin S-positive, suggesting the amyloid-like conformation. Interestingly, the aggregates disappeared when the cells were replenished with inhibitor-free medium. Disappearance of aggregates coincided with the recovery of mitochondrial metabolism and was partially inhibited by proteasome inhibitors. These results suggest that the formation of alpha-synuclein inclusions could be initiated by an impaired mitochondrial function and be reversed by restoring normal mitochondrial metabolism.

The herbicide paraquat causes up-regulation and aggregation of alpha-synuclein in mice: paraquat and alpha-synuclein

alpha-Synuclein-containing aggregates represent a feature of a variety of neurodegenerative disorders, including Parkinson's disease (PD). However, mechanisms that promote intraneuronal alpha-synuclein assembly remain poorly understood. Because pesticides, particularly the herbicide paraquat, have been suggested to play a role as PD risk factors, the hypothesis that interactions between alpha-synuclein and these environmental agents may contribute to aggregate formation was tested in this study. Paraquat markedly accelerated the in vitro rate of alpha-synuclein fibril formation in a dose-dependent fashion. When mice were exposed to the herbicide, brain levels of alpha-synuclein were significantly increased. This up-regulation followed a consistent pattern, with higher alpha-synuclein at 2 days after each of three weekly paraquat injections and with protein levels returning to control values by day 7 post-treatment. Paraquat exposure was also accompanied by aggregate formation. Thioflavine S-positive structures accumulated within neurons of the substantia nigra pars compacta, and dual labeling and confocal imaging confirmed that these aggregates contained alpha-synuclein. The results suggest that up-regulation of alpha-synuclein as a consequence of toxicant insult and direct interactions between the protein and environmental agents are potential mechanisms leading to alpha-synuclein pathology in neurodegenerative disorders.

Membrane-bound alpha-synuclein has a high aggregation propensity and the ability to seed the aggregation of the cytosolic form

Alpha-synuclein exists as at least two structural isoforms: a helix-rich, membrane-bound form and a disordered, cytosolic form. Here, we investigated the role of membrane-bound alpha-synuclein in the aggregation process. In a cell-free system consisting of isolated brain fractions, spontaneous and progressive aggregation of alpha-synuclein was observed in membranes starting at day 1, whereas no aggregation was observed in the cytosolic fraction in a 3-day period. The addition of antioxidants reduced the aggregation in the membrane fraction, implicating the role of oxidative modifications. When excess cytosolic alpha-synuclein was added to brain membranes, the rate of aggregation was increased, while the lag time was unaffected. Incorporation of cytosolic alpha-synuclein into membrane-associated aggregates was demonstrated by fractionation and co-immunoprecipitation experiments. In our recent study, we showed that mitochondrial inhibitors such as rotenone, induced alpha-synuclein aggregation in cells. In the present study using rotenone-treated cells, the earliest appearance of alpha-synuclein oligomeric species was observed in membranous compartments. Furthermore, alpha-synuclein-positive inclusions were co-stained with DiI, a membrane-partitioning fluorescent dye, confirming the presence of lipid components in alpha-synuclein aggregates. These results suggest that membrane-bound alpha-synuclein can generate nuclei that seed the aggregation of the more abundant cytosolic form.

What does it mean to be natively unfolded?

Natively unfolded or intrinsically unstructured proteins constitute a unique group of the protein kingdom. The evolutionary persistence of such proteins represents strong evidence in the favor of their importance and raises intriguing questions about the role of protein disorders in biological processes. Additionally, natively unfolded proteins, with their lack of ordered structure, represent attractive targets for the biophysical studies of the unfolded polypeptide chain under physiological conditions in vitro. The goal of this study was to summarize the structural information on natively unfolded proteins in order to evaluate their major conformational characteristics. It appeared that natively unfolded proteins are characterized by low overall hydrophobicity and large net charge. They possess hydrodynamic properties typical of random coils in poor solvent, or premolten globule conformation. These proteins show a low level of ordered secondary structure and no tightly packed core. They are very flexible, but may adopt relatively rigid conformations in the presence of natural ligands. Finally, in comparison with the globular proteins, natively unfolded polypeptides possess 'turn out' responses to changes in the environment, as their structural complexities increase at high temperature or at extreme pH.

Residual structure and dynamics in Parkinson's disease-associated mutants of alpha-synuclein

alpha-Synuclein (alpha S) is a pre-synaptic protein that has been implicated as a possible causative agent in the pathogenesis of Parkinson's disease (PD). Two autosomal dominant missense mutations in the alpha S gene are associated with early onset PD. Because alpha S is found in an aggregated fibrillar form in the Lewy body deposits characteristic of Parkinson's patients, aggregation of the protein is believed to be related to its involvement in the disease process. The wild type (WT) and early onset mutants A30P and A53T display diverse in vitro aggregation kinetics even though the gross physicochemical and morphological properties of the mutants are highly similar. We used high resolution solution NMR spectroscopy to compare the structural and dynamic properties of the A53T and A30P mutants with those of WT alpha S in the free state. We found that the A30P mutation disrupts a region of residual helical structure that exists in the WT protein, whereas the A53T mutation results in a slight enhancement of a small region around the site of mutation with a preference for extended conformations. Based on these results and on the anticipated effects of these mutations on elements of secondary structure, we proposed a model of how these two PD-linked mutations influence alpha S fibril formation that is consistent with the documented differences in the fibrillization kinetics of the two mutants.