An unusual soluble beta-turn-rich conformation of prion is involved in fibril formation and toxic to neuronal cells

Formation of Amyloid-Like Conformational States of β-Structured Membrane Proteins on the Example of OMPF Porin from the Yersinia pseudotuberculosis Outer Membrane

The work presents results of the in vitro and in silico study of formation of amyloid-like structures under harsh denaturing conditions by non-specific OmpF porin of Yersinia pseudotuberculosis (YpOmpF), a membrane protein with β-barrel conformation. It has been shown that in order to obtain amyloid-like porin aggregates, preliminary destabilization of its structure in a buffer solution with acidic pH at elevated temperature followed by long-term incubation at room temperature is necessary. After heating at 95°C in a solution with pH 4.5, significant conformational rearrangements are observed in the porin molecule at the level of tertiary and secondary structure of the protein, which are accompanied by the increase in the content of total β-structure and sharp decrease in the value of characteristic viscosity of the protein solution. Subsequent long-term exposure of the resulting unstable intermediate YpOmpF at room temperature leads to formation of porin aggregates of various shapes and sizes that bind thioflavin T, a specific fluorescent dye for the detection of amyloid-like protein structures. Compared to the initial protein, early intermediates of the amyloidogenic porin pathway, oligomers, have been shown to have increased toxicity to the Neuro-2aCCL-131™ mouse neuroblastoma cells. The results of computer modeling and analysis of the changes in intrinsic fluorescence during protein aggregation suggest that during formation of amyloid-like aggregates, changes in the structure of YpOmpF affect not only the areas with an internally disordered structure corresponding to the external loops of the porin, but also main framework of the molecule, which has a rigid spatial structure inherent to β-barrel.

Extracellular vesicles with diagnostic and therapeutic potential for prion diseases

Prion diseases (PrD) or transmissible spongiform encephalopathies (TSE) are invariably fatal and pathogenic neurodegenerative disorders caused by the self-propagated misfolding of cellular prion protein (PrP^C) to the neurotoxic pathogenic form (PrP^TSE) via a yet undefined but profoundly complex mechanism. Despite several decades of research on PrD, the basic understanding of where and how PrP^C is transformed to the misfolded, aggregation-prone and pathogenic PrP^TSE remains elusive. The primary clinical hallmarks of PrD include vacuolation-associated spongiform changes and PrP^TSE accumulation in neural tissue together with astrogliosis. The difficulty in unravelling the disease mechanisms has been related to the rare occurrence and long incubation period (over decades) followed by a very short clinical phase (few months). Additional challenge in unravelling the disease is implicated to the unique nature of the agent, its complexity and strain diversity, resulting in the heterogeneity of the clinical manifestations and potentially diverse disease mechanisms. Recent advances in tissue isolation and processing techniques have identified novel means of intercellular communication through extracellular vesicles (EVs) that contribute to PrP^TSE transmission in PrD. This review will comprehensively discuss PrP^TSE transmission and neurotoxicity, focusing on the role of EVs in disease progression, biomarker discovery and potential therapeutic agents for the treatment of PrD.

Extracellular Amyloid Deposits in Alzheimer's and Creutzfeldt-Jakob Disease: Similar Behavior of Different Proteins?

Neurodegenerative diseases are characterized by the deposition of specific protein aggregates, both intracellularly and/or extracellularly, depending on the type of disease. The extracellular occurrence of tridimensional structures formed by amyloidogenic proteins defines Alzheimer's disease, in which plaques are composed of amyloid β-protein, while in prionoses, the same term "amyloid" refers to the amyloid prion protein. In this review, we focused on providing a detailed didactic description and differentiation of diffuse, neuritic, and burnt-out plaques found in Alzheimer's disease and kuru-like, florid, multicentric, and neuritic plaques in human transmissible spongiform encephalopathies, followed by a systematic classification of the morphological similarities and differences between the extracellular amyloid deposits in these disorders. Both conditions are accompanied by the extracellular deposits that share certain signs, including neuritic degeneration, suggesting a particular role for amyloid protein toxicity.

Conformation-dependent membrane permeabilization by neurotoxic PrP oligomers: The role of the H2H3 oligomerization domain

The relationship between prion propagation and the generation of neurotoxic species and clinical onset remains unclear. Several converging lines of evidence suggest that interactions with lipids promote various precursors to form aggregation-prone states that are involved in amyloid fibrils. Here, we compared the cytotoxicities of different soluble isolated oligomeric constructs from murine full-length PrP and from the restricted helical H2H3 domain with their effects on lipid vesicles. The helical H2H3 domain is suggested to be the minimal region of PrP involved in the oligomerization process. The discrete PrP oligomers of both the full-length sequence and the H2H3 domain have de novo β-sheeted structure when interacting with the membrane. They were shown to permeabilize synthetic negatively charged vesicles in a dose-dependent manner. Restricting the polymerization domain of the full-length PrP to the H2H3 helices strongly diminished the ability of the corresponding oligomers to associate with the lipid vesicles. Furthermore, the membrane impairment mechanism occurs differently for the full-length PrP oligomers and the H2H3 helices, as shown by dye-release and black lipid membrane experiments. The membrane damage caused by the full-length PrP oligomers is correlated to their neuronal toxicity at submicromolar concentrations, as shown by cell culture assays. Although oligomers of synthetic H2H3 could compromise in vitro cell homeostasis, they followed a membrane-disruptive pattern that was different from the full-length oligomers, as revealed by the role of PrP^C in cell viability assays.

Transient multimers modulate conformer abundances of prion protein monomer through conformational selection

Prions are known to be involved in neurodegenerative pathologies such as Creutzfeld-Jakob disease. Current models point to a molecular event which rely on a transmissible structural change that leads to the production of β-sheet-rich prion conformer (PrP^Sc). PrP^Sc itself has the capability to trigger the structural rearrangement of the ubiquitously present prion (PrP^c) substrate in a self-perpetuating cascade. In this article, we demonstrate that recombinant PrP^c exists in a conformational equilibrium. The conformers’ abundances were shown to be dependent on PrP^c concentration through the formation of transient multimers leading to conformational selection. The study of PrP^c mutants that follow dedicated oligomerization pathways demonstrated that the conformers’ relative abundances are modified, thus reinforcing the assertion that the nature of conformers’ interactions orient the oligomerization pathways. Further this result can be viewed as the “signature” of an aborted oligomerization process. This discovery sheds a new light on the possible origin of prion protein diseases, namely that a change in prion protein structure could be transmitted through the formation of transient multimers having different conformer compositions. This could explain the selection of a transient multimeric type that could be viewed as the precursor of PrP^Sc responsible for structural information transmission, and strain apparition.

Direct observation of prion protein oligomer formation reveals an aggregation mechanism with multiple conformationally distinct species

The aggregation of the prion protein (PrP) plays a key role in the development of prion diseases. In the past decade, a similar process has been associated with other proteins, such as Aβ, tau, and α-synuclein, which participate in other neurodegenerative diseases. It is increasingly recognized that the small oligomeric species of aggregates can play an important role in the development of prion diseases. However, determining the nature of the oligomers formed during the aggregation process has been experimentally difficult due to the lack of suitable methods capable of the detection and characterization of the low level of oligomers that may form. To address this problem, we have utilized single-aggregate methods to study the early events associated with aggregation of recombinant murine PrP in vitro to approach the bona fide process in vivo. PrP aggregation resulted in the formation of thioflavin T (ThT)-inactive and ThT-active species of oligomers. The ThT-active oligomers undergo conversion from a Proteinase K (PK)-sensitive to PK-resistant conformer, from which mature fibrils can eventually emerge. Overall, our results show that single-aggregate methods can provide structural and mechanistic insights into PrP aggregation, identify the potential species that mediates cytotoxicity, and reveal that a range of distinct oligomeric species with different properties is formed during prion protein aggregation.

What Is Our Current Understanding of PrPSc-Associated Neurotoxicity and Its Molecular Underpinnings?

The prion diseases are a collection of fatal, transmissible neurodegenerative diseases that cause rapid onset dementia and ultimately death. Uniquely, the infectious agent is a misfolded form of the endogenous cellular prion protein, termed PrP^Sc. Despite the identity of the molecular agent remaining the same, PrP^Sc can cause a range of diseases with hereditary, spontaneous or iatrogenic aetiologies. However, the link between PrP^Sc and toxicity is complex, with subclinical cases of prion disease discovered, and prion neurodegeneration without obvious PrP^Sc deposition. The toxic mechanisms by which PrP^Sc causes the extensive neuropathology are still poorly understood, although recent advances are beginning to unravel the molecular underpinnings, including oxidative stress, disruption of proteostasis and induction of the unfolded protein response. This review will discuss the diseases caused by PrP^Sc toxicity, the nature of the toxicity of PrP^Sc, and our current understanding of the downstream toxic signaling events triggered by the presence of PrP^Sc.

Sequence-dependent dynamical instability of the human prion protein: a comparative simulation study

The present study aimed to explore the most probable regions of the human prion protein backbone for which the initial steps of conformational transitions as a result of intrinsic and extrinsic perturbing factors on the protein structure can be assigned. A total of 0.3-μs molecular dynamics simulations on several analog structures of the protein have been performed. To mimic the impact of the extrinsic and intrinsic destructive parameters on the dynamical characteristics of the protein, mild acidic conditions and R208H mutation have been simulated. The findings indicated that distribution of conformational flexibilities along the protein chain was almost independent of the induced perturbing factors, and was mostly centralized on certain distinct parts of the structure comprising residues 132-145 and 187-203. Analyses also revealed that the segment comprising residues 187-203 may be considered as a peptide sequence, possessing high potential to start the initial steps of conformational rearrangements due to the induced physicochemical alterations. Sequence alignment and molecular dynamics data also revealed that segment 178-203 prefers to accommodate in extended structures rather than α-helices. Region 178-203 may be considered as a peptide switch capable of initiating the conformational transitions due to the introduced modifications and perturbing parameters.

Pathogenic mechanisms of prion protein, amyloid-β and α-synuclein misfolding: the prion concept and neurotoxicity of protein oligomers

Proteinopathies represent a group of diseases characterized by the unregulated misfolding and aggregation of proteins. Accumulation of misfolded protein in the central nervous system (CNS) is associated with neurodegenerative diseases, such as the transmissible spongiform encephalopathies (or prion diseases), Alzheimer's disease, and the synucleinopathies (the most common of which is Parkinson's disease). Of these, the pathogenic mechanisms of prion diseases are particularly striking where the transmissible, causative agent of disease is the prion, or proteinaceous infectious particle. Prions are composed almost exclusively of PrP^Sc ; a misfolded isoform of the normal cellular protein, PrP^C , which is found accumulated in the CNS in disease. Today, mounting evidence suggests other aggregating proteins, such as amyloid-β (Aβ) and α-synuclein (α-syn), proteins associated with Alzheimer's disease and synucleinopathies, respectively, share similar biophysical and biochemical properties with PrP^Sc that influences how they misfold, aggregate, and propagate in disease. In this regard, the definition of a 'prion' may ultimately expand to include other pathogenic proteins. Unifying knowledge of folded proteins may also reveal common mechanisms associated with other features of disease that are less understood, such as neurotoxicity. This review discusses the common features Aβ and α-syn share with PrP and neurotoxic mechanisms associated with these misfolded proteins. Several proteins are known to misfold and accumulate in the central nervous system causing a range of neurodegenerative diseases, such as Alzheimer's, Parkinson's, and the prion diseases. Prions are transmissible misfolded conformers of the prion protein, PrP, which seed further generation of infectious proteins. Similar effects have recently been observed in proteins associated with Alzheimer's disease and the synucleinopathies, leading to the proposition that the definition of a 'prion' may ultimately expand to include other pathogenic proteins. Unifying knowledge of misfolded proteins may also reveal common mechanisms associated with other features of disease that are less understood, such as neurotoxicity.

Mechanisms of prion-induced neurodegeneration

Transmissible spongiform encephalopathies (TSEs), or prion diseases, are fatal neurodegenerative disorders characterised by long incubation period, short clinical duration, and transmissibility to susceptible species. Neuronal loss, spongiform changes, gliosis and the accumulation in the brain of the misfolded version of a membrane-bound cellular prion protein (PrP(C)), termed PrP(TSE), are diagnostic markers of these diseases. Compelling evidence links protein misfolding and its accumulation with neurodegenerative changes. Accordingly, several mechanisms of prion-mediated neurotoxicity have been proposed. In this paper, we provide an overview of the recent knowledge on the mechanisms of neuropathogenesis, the neurotoxic PrP species and the possible therapeutic approaches to treat these devastating disorders.

Contrasting Effects of Two Lipid Cofactors of Prion Replication on the Conformation of the Prion Protein

Recent studies introduced two experimental protocols for converting full-length recombinant prion protein (rPrP) purified from E.coli into the infectious prion state (PrP^Sc) with high infectivity titers. Both protocols employed protein misfolding cyclic amplification (PMCA) for generating PrP^Scde novo, but used two different lipids, 1-palmitoyl-2-oleolyl-sn-glycero-3-phospho(1’-rac-glycerol) (POPG) or phosphatidylethanolamine (PE), as conversion cofactors. The current study compares the effect of POPG and PE on the physical properties of native, α-helical full-length mouse rPrP under the solvent conditions used for converting rPrP into PrP^Sc. Surprisingly, the effects of POPG and PE on rPrP physical properties, including its conformation, thermodynamic stability, aggregation state and interaction with a lipid, were found to be remarkably different. PE was shown to have minimal, if any, effects on rPrP thermodynamic stability, cooperativity of unfolding, immediate solvent environment or aggregation state. In fact, little evidence indicates that PE interacts with rPrP directly. In contrast, POPG was found to bind to and induce dramatic changes in rPrP structure, including a loss of α-helical conformation and formation of large lipid-protein aggregates that were resistant to partially denaturing conditions. These results suggest that the mechanisms by which lipids assist conversion of rPrP into PrP^Sc might be fundamentally different for POPG and PE.

Animal models for prion-like diseases

Prion diseases or Transmissible Spongiform Encephalopathies (TSEs) are a group of fatal neurodegenerative disorders affecting several mammalian species being Creutzfeldt-Jacob Disease (CJD) the most representative in human beings, scrapie in ovine, Bovine Spongiform Encephalopathy (BSE) in bovine and Chronic Wasting Disease (CWD) in cervids. As stated by the "protein-only hypothesis", the causal agent of TSEs is a self-propagating aberrant form of the prion protein (PrP) that through a misfolding event acquires a β-sheet rich conformation known as PrP(Sc) (from scrapie). This isoform is neurotoxic, aggregation prone and induces misfolding of native cellular PrP. Compelling evidence indicates that disease-specific protein misfolding in amyloid deposits could be shared by other disorders showing aberrant protein aggregates such as Alzheimer's Disease (AD), Parkinson's Disease (PD), Amyotrophic lateral sclerosis (ALS) and systemic Amyloid A amyloidosis (AA amyloidosis). Evidences of shared mechanisms of the proteins related to each disease with prions will be reviewed through the available in vivo models. Taking prion research as reference, typical prion-like features such as seeding and propagation ability, neurotoxic species causing disease, infectivity, transmission barrier and strain evidences will be analyzed for other protein-related diseases. Thus, prion-like features of amyloid β peptide and tau present in AD, α-synuclein in PD, SOD-1, TDP-43 and others in ALS and serum α-amyloid (SAA) in systemic AA amyloidosis will be reviewed through models available for each disease.

The Unexposed Secrets of Prion Protein Oligomers

According to the "protein-only" hypothesis, the misfolding and conversion of host-derived cellular prion protein (PrP(C)) into pathogenically misfolded PrP are believed to be the key procedure in the pathogenesis of prion diseases. Intermediate, soluble oligomeric prion protein (PrP) aggregates were considered a critical process for prion diseases. Several independent studies on PrP oligomers gained insights into oligomers' formation, biophysical and biochemical characteristics, structure conversion, and neurotoxicity. PrP oligomers are rich in β-sheet structure and slightly resistant to proteinase K digestion. PrP oligomers exhibited more neurotoxicity and induced neuronal apoptosis in vivo and/or in vitro. In this review, we summarized recent studies regarding PrP oligomers and the relationship between misfolded PrP aggregates and neuronal death in the course of prion diseases.

N-terminal domain of prion protein directs its oligomeric association

The self-association of prion protein (PrP) is a critical step in the pathology of prion diseases. It is increasingly recognized that small non-fibrillar β-sheet-rich oligomers of PrP may be of crucial importance in the prion disease process. Here, we characterize the structure of a well defined β-sheet-rich oligomer, containing ∼12 PrP molecules, and often enclosing a central cavity, formed using full-length recombinant PrP. The N-terminal region of prion protein (residues 23-90) is required for the formation of this distinct oligomer; a truncated form comprising residues 91-231 forms a broad distribution of aggregated species. No infectivity or toxicity was found using cell and animal model systems. This study demonstrates that examination of the full repertoire of conformers and assembly states that can be accessed by PrP under specific experimental conditions should ideally be done using the full-length protein.

C-terminal peptides modelling constitutive PrPC processing demonstrate ameliorated toxicity predisposition consequent to α-cleavage

C-terminal fragments generated through α- and β-cleavage of PrPC appear to harbour different pathogenic potential for host cells. Most significantly, α-cleavage produces a C-terminal fragment that is resistant to folding into soluble β-strand-rich toxic isoforms.

Using protein misfolding cyclic amplification generates a highly neurotoxic PrP dimer causing neurodegeneration

Under the "protein-only" hypothesis, prion-based diseases are proposed to result from an infectious agent that is an abnormal isoform of the prion protein in the scrapie form, PrP(Sc). However, since PrP(Sc) is highly insoluble and easily aggregates in vivo, this view appears to be overly simplistic, implying that the presence of PrP(Sc) may indirectly cause neurodegeneration through its intermediate soluble form. We generated a neurotoxic PrP dimer with partial pathogenic characteristics of PrP(Sc) by protein misfolding cyclic amplification in the presence of 1-palmitoyl-2-oleoylphosphatidylglycerol consisting of recombinant hamster PrP (23-231). After intracerebral injection of the PrP dimer, wild-type hamsters developed signs of neurodegeneration. Clinical symptoms, necropsy findings, and histopathological changes were very similar to those of transmissible spongiform encephalopathies. Additional investigation showed that the toxicity is primarily related to cellular apoptosis. All results suggested that we generated a new neurotoxic form of PrP, PrP dimer, which can cause neurodegeneration. Thus, our study introduces a useful model for investigating PrP-linked neurodegenerative mechanisms.

Prion protein: structural features and related toxicity

Transmissible spongiform encephalopathies, or prion diseases, is a group of infectious neurodegenerative disorders. The conformational conversion from cellular form (PrP(C)) to disease-causing isoform (PrP(Sc)) is considered to be the most important and remarkable event in these diseases, while accumulation of PrP(Sc) is thought to be the main reason for cell death, inflammation and spongiform degeneration observed in infected individuals. Although these rare but unique neurodegenerative disorders have attracted much attention, there are still many questions that remain to be answered. Knowledge of the scrapie agent structures and the toxic species may have significance for understanding the causes of the diseases, and could be helpful for rational design of novel therapeutic and diagnostic methods. In this review, we summarized the available experimental evidence concerning the relationship among the structural features, aggregation status of misfolded PrP and related neurotoxicity in the course of prion diseases development. In particular, most data supports the idea that the smaller oligomeric PrP(Sc) aggregates, rather than the mature amyloid fibers, exhibit the highest toxicity to the host.

Localization of A11-reactive oligomeric species in prion diseases

AIMS

To investigate in prion diseases the in-situ localization of prion protein oligomers sharing a common epitope with amyloid oligomers involved in a range of neurodegenerative diseases.

METHODS AND RESULTS

We performed immunohistochemistry on sporadic Creutzfeldt-Jakob disease (sCJD) (n = 9) and hereditary Gerstmann-Sträussler-Scheinker disease (GSS) (n = 1) specimens with the anti-oligomer antibody A11 to determine the localization of reactive species. We found that A11 reactivity in the sCJD specimens was localized to the cerebral and cerebellar cortices both in spongiform and adjacent, non-spongiform areas, reminiscent of multicentric or diffuse plaques. In the GSS specimens, we found that staining was closely associated with kuru-like plaques, and that A11-reactive species colocalized with protease-resistant prion protein (Prp(Sc)). We also observed sporadic neuronal cytosolic staining in both types of specimen.

CONCLUSIONS

We confirm that intracellular and extracellular A11-reactive species are present in situ in sCJD cases and GSS, and that immunoreactivity for A11 and Prp(Sc) overlaps. We argue that the A11-reactive species are indeed composed of oligomeric Prp(Sc), and suggest that the toxic effects of Prp(Sc) oligomers could be related to the generic oligomeric conformation recognized by A11.

Multiple substitutions of methionine 129 in human prion protein reveal its importance in the amyloid fibrillation pathway

The role of the polymorphism Met or Val in position 129 in the human prion protein is well documented regarding disease susceptibility and clinical manifestations. However, little is known about the molecular background to this phenomenon. We investigated herein the conformational stability, amyloid fibrillation kinetics, and seeding propensity of different 129 mutants, located in β-strand 1 of PrP (Met(129) (WT), M129A, M129V, M129L, M129W, M129P, M129E, M129K, and M129C) in HuPrP(90-231). The mutations M129V, M129L, M129K, and M129C did not affect stability (midpoints of thermal denaturation, T(m) = 65-66 °C), whereas the mutants M129A and M129E and the largest side chain M129W were destabilized by 3-4 °C. The most destabilizing substitution was M129P, which lowered the T(m) by 7.2 °C. All mutants, except for M129C, formed amyloid-like fibrils within hours during fibril formation under near physiological conditions. Fibril-forming mutants showed a sigmoidal kinetic profile and showed shorter lag times during seeding with preformed amyloid fibrils implicating a nucleated polymerization reaction. In the spontaneous reactions, the lag time of fibril formation was rather uniform for the mutants M129A, M129V, and M129L resembling the wild type. When the substituted amino acid had a distinct feature discriminating it from the wild type, such as size (M129W), charge (M129E, M129K), or rotational constraint (M129P), the fibrillation was impeded. M129C did not form ThT/Congo red-positive fibrils, and non-reducing SDS-PAGE of M129C during fibrillation conditions at different time points revealed covalent dimer formation already 15 min after fibrillation reaction initiation. Position 129 appears to be a key site for dictating PrP receptiveness toward recruitment into the amyloid state.

Highly neurotoxic monomeric α-helical prion protein

Prion diseases are infectious and belong to the group of protein misfolding neurodegenerative diseases. In these diseases, neuronal dysfunction and death are caused by the neuronal toxicity of a particular misfolded form of their cognate protein. The ability to specifically target the toxic protein conformer or the neuronal death pathway would provide powerful therapeutic approaches to these diseases. The neurotoxic forms of the prion protein (PrP) have yet to be defined but there is evidence suggesting that at least some of them differ from infectious PrP (PrP(Sc)). Herein, without making an assumption about size or conformation, we searched for toxic forms of recombinant PrP after dilution refolding, size fractionation, and systematic biological testing of all fractions. We found that the PrP species most neurotoxic in vitro and in vivo (toxic PrP, TPrP) is a monomeric, highly α-helical form of PrP. TPrP caused autophagy, apoptosis, and a molecular signature remarkably similar to that observed in the brains of prion-infected animals. Interestingly, highly α-helical intermediates have been described for other amyloidogenic proteins but their biological significance remains to be established. We provide unique experimental evidence that a monomeric α-helical form of an amyloidogenic protein represents a cytotoxic species. Although toxic PrP has yet to be purified from prion-infected brains, TPrP might be the equivalent of one highly neurotoxic PrP species generated during prion replication. Because TPrP is a misfolded, highly neurotoxic form of PrP reproducing several features of prion-induced neuronal death, it constitutes a useful model to study PrP-induced neurodegenerative mechanisms.

Protein homeostasis and aging in neurodegeneration

Genetic and environmental factors responsible for numerous neurodegenerative diseases vary between disorders, yet age remains a universal risk factor. Age-associated decline in protein homeostasis, or proteostasis, enables disease-linked proteins to adopt aberrant tertiary structures, accumulate as higher-ordered aggregates, and cause a myriad of cellular dysfunctions and neuronal death. However, recent findings suggest that the assembly of disease proteins into tightly ordered aggregates can significantly delay proteotoxic onset. Furthermore, manipulation of metabolic pathways through key signaling components extends lifespan, bolsters proteostasis networks, and delays the onset of proteotoxicity. Thus, understanding the relationship between proteostasis and aging has provided important insights into neurodegeneration.

Structural requirements for efficient prion protein conversion: cofactors may promote a conversion-competent structure for PrP(C)

To understand why cross-species infection of prion disease often results in inefficient transmission and reduced protein conversion, most research has focused on defining the effect of variations in PrP primary structures, including sequence compatibility of substrate and seed. By contrast, little research has been aimed at investigating structural differences between different variants of PrP(C) and secondary structural requirements for efficient conversion. This is despite a clear role for molecular chaperones in formation of prions in non-mammalian systems, indicating the importance of secondary/tertiary structure during the conversion process. Recent data from our laboratory on the cellular location of disease-specific prion cofactors supports the critical role of specific secondary structural motifs and the stability of these motifs in determining the efficiency of disease-specific prion protein conversion. In this paper we summarize our recent results and build on the hypothesis previously suggested by Wuthrich and colleagues, that stability of certain regions of the prion protein is crucial for protein conversion to abnormal isoforms in vivo. It is suggested that one role for molecular cofactors in the conversion process is to stabilize PrP(C) structure in a form that is amenable for conversion to PrP(Sc).

Complement protein C1q forms a complex with cytotoxic prion protein oligomers

A growing number of studies have investigated the interaction between C1q and PrP, but the oligomeric form of PrP involved in this interaction remains to be determined. Aggregation of recombinant full-length murine PrP in the presence of 100 mm NaCl allowed us to isolate three different types of oligomers by size-exclusion chromatography. In contrast to PrP monomers and fibrils, these oligomers activate the classical complement pathway, the smallest species containing 8-15 PrP protomers being the most efficient. We used Thioflavine T fluorescence to monitor PrP aggregation and showed that, when added to the reaction, C1q has a cooperative effect on PrP aggregation and leads to the formation of C1q-PrP complexes. In these complexes, C1q interacts through its globular domains preferentially with the smallest oligomers, as shown by electron microscopy, and retains the ability to activate the classical complement pathway. Using two cell lines, we also provide evidence that C1q inhibits the cytotoxicity induced by the smallest PrP oligomers. The cooperative interaction between C1q and PrP could represent an early step in the disease, where it prevents elimination of the prion seed, leading to further aggregation.

Macromolecular crowding converts the human recombinant PrPC to the soluble neurotoxic beta-oligomers

Prion diseases are fatal neurodegenerative disorders and are linked with the conversion of the cellular isoform of the prion protein (PrP(C)) into the abnormal beta-sheet-rich isoform. It is widely accepted that the soluble oligomers of beta-PrP are neurotoxic and that they are more pathologically significant. To unravel the molecular mechanism under the conversion process, it is critical to identify the factors that can promote the conversion from PrP(C) to the beta-oligomers. By recording circular dichroism spectra and performing a size-exclusion HPLC assay, we found that the conformation of the recombinant human prion protein (rPrP(C)) was converted from an alpha-helical conformation into beta-sheet oligomers under a macromolecular crowding condition. The soluble beta-oligomers of rPrP were resistant to proteinase K digestion and could bind to the dyes thioflavin T and 8-anilino-1-naphthalene sulfonate. Furthermore, by the 3-(4, 5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium assay, we showed that the soluble beta-oligomers were neurotoxic. These results suggest that macromolecular crowding, which has not been considered before, is a key intracellular factor in the formation of soluble neurotoxic beta-oligomers in prion diseases.

Vesicle permeabilization by purified soluble oligomers of prion protein: a comparative study of the interaction of oligomers and monomers with lipid membranes

The conversion of normal cellular prion protein (PrP) into its pathological isoform, scrapie PrP, may occur at the cell surface or, more probably, in late endosomes. The early events leading to the structural conversion of PrP appear to be related to the presence of more or less stable soluble oligomers, which might mediate neurotoxicity. In the current study, we investigate the interaction of alpha-rich PrP monomers and beta-rich size-exclusion-chromatography-purified PrP oligomers with lipid membranes. We compare their structural properties when associated with lipid bilayers and study their propensities to permeabilize the membrane at physiological pH. We also study the influence of the N-terminal flexible region (residues 24-103) by comparing full-length PrP(24-234) and N-terminally truncated PrP(104-234) oligomers. We showed that both 12-subunit oligomers cause an immediate and large increase in the permeability of the membrane, whereas equivalent amounts of monomeric forms cause no detectable leakage. Although the two monomeric PrP constructs undergo an alpha-to-beta conformational change when bound to the negatively charged membrane, only the full-length form of monomeric PrP has a weak fusogenic effect. Finally, the oligomers affect the integrity of the membrane differently from the monomers, independently of the presence of the N-terminal flexible domain. As for other forms of amyloidogenesis, a reasonable mechanism for the toxicity arising from PrP fibrillization must be associated with low-molecular-weight oligomeric intermediates, rather than with mature fibrils. Knowledge of the mechanism of action of these soluble oligomers would have a high impact on the development of novel therapeutic targets.

Misfolding of the prion protein: linking biophysical and biological approaches

Prion diseases are a group of neurodegenerative diseases that can arise spontaneously, be inherited, or acquired by infection in mammals. The propensity of the prion protein to adopt different structures is a clue to its pathological and perhaps biological role too. While the normal monomeric PrP is well characterized, the misfolded conformations responsible for neurodegeneration remain elusive despite progress in this field. Both structural dynamics and physico-chemical approaches are thus fundamental for a better knowledge of the molecular basis of this pathology. Indeed, multiple misfolding pathways combined with extensive posttranslational modifications of PrP and probable interaction(s) with cofactors call for a combination of approaches. In this review, we outline the current physico-chemical knowledge explaining the conformational diversities of PrP in relation with postulated or putative cellular partners such as proteic or non-proteic ligands.

Globular and pre-fibrillar prion aggregates are toxic to neuronal cells and perturb their electrophysiology

Prion diseases are characterised at autopsy by neuronal loss and accumulation of amorphous protein aggregates and/or amyloid fibrils in the brains of humans and animals. These protein deposits result from the conversion of the cellular, mainly alpha-helical prion protein (PrP(C)) to the beta-sheet-rich isoform (PrP(Sc)). Although the pathogenic mechanism of prion diseases is not fully understood, it appears that protein aggregation is itself neurotoxic and not the product of cell death. The precise nature of the neurotoxic species and mechanism of cell death are yet to be determined, although recent studies with other amyloidogenic proteins suggest that ordered pre-fibrillar or oligomeric forms may be responsible for cellular dysfunction. In this study we have refolded recombinant prion protein (rPrP) to two distinct forms rich in beta-sheet structure with an intact disulphide bond. Here we report on the structural properties of globular aggregates and pre-fibrils of rPrP and show that both states are toxic to neuronal cells in culture. We show that exogenous rPrP aggregates are internalised by neuronal cells and found in the cytoplasm. We also measured the changes in electrophysiological properties of cultured neuronal cells on exposure to exogenous prion aggregates and discuss the implications of these findings.

Methods for conversion of prion protein into amyloid fibrils

Misfolding and aggregation of prion protein (PrP) is related to several neurodegenerative diseases in humans such as Creutzfeldt-Jacob disease, fatal familial insomnia, and Gerstmann-Straussler-Sheinker disease. Amyloid fibrils prepared from recombinant PrP in vitro share many features of the infectious prions. These fibrils can be used as a synthetic surrogate of PrP(Sc) for development of prion diagnostics, including generation of PrP(Sc)-specific antibody, for screening of antiprion drugs, or for development of antiprion decontamination procedures. Here, we describe the methods of preparation of prion protein fibrils in vitro and biochemical assays for assessing physical properties and the quality of fibrils.

ERK1/2 and p38 MAP kinases control prion protein fragment 90-231-induced astrocyte proliferation and microglia activation

Astrogliosis and microglial activation are a common feature during prion diseases, causing the release of chemoattractant and proinflammatory factors as well as reactive free radicals, involved in neuronal degeneration. The recombinant protease-resistant domain of the prion protein (PrP90-231) displays in vitro neurotoxic properties when refolded in a beta-sheet-rich conformer. Here, we report that PrP90-231 induces the secretion of several cytokines, chemokines, and nitric oxide (NO) release, in both type I astrocytes and microglial cells. PrP90-231 elicited in both cell types the activation of ERK1/2 MAP kinase that displays, in astrocytes, a rapid kinetics and a proliferative response. Conversely, in microglia, PrP90-231-dependent MAP kinase activation was delayed and long lasting, inducing functional activation and growth arrest. In microglial cells, NO release, dependent on the expression of the inducible NO synthase (iNOS), and the secretion of the chemokine CCL5 were Ca(2+) dependent and under the control of the MAP kinases ERK1/2 and p38: ERK1/2 inhibition, using PD98059, reduced iNOS expression, while p38 blockade by PD169316 inhibited CCL5 release. In summary, we demonstrate that glial cells are activated by extracellular misfolded PrP90-231 resulting in a proliferative/secretive response of astrocytes and functional activation of microglia, both dependent on MAP kinase activation. In particular, in microglia, PrP90-231 activated a complex signalling cascade involved in the regulation of NO and chemokine release. These data argue in favor of a causal role for misfolded prion protein in sustaining glial activation and, possibly, glia-mediated neuronal death.

Intracellular accumulation of a mild-denatured monomer of the human PrP fragment 90-231, as possible mechanism of its neurotoxic effects

Because of high tendency of the prion protein (PrP) to aggregate, the exact PrP isoform responsible for prion diseases as well as the pathological mechanism that it activates remains still controversial. In this study, we show that a pre-fibrillar, monomeric or small oligomeric conformation of the human PrP fragment 90-231 (hPrP90-231), rather than soluble or fibrillar large aggregates, represents the neurotoxic species. In particular, we demonstrate that monomeric mild-denatured hPrP90-231 (incubated for 1 h at 53 degrees C) induces SH-SY5Y neuroblastoma cell death, while, when structured in large aggregates, it is ineffective. Using spectroscopic and cellular techniques we demonstrate that this toxic conformer is characterized by a high exposure of hydrophobic regions that favors the intracellular accumulation of the protein. Inside the cells hPrP90-231 is mainly compartmentalized into the lysosomes where it may trigger pro-apoptotic 'cell death' signals. The PrP toxic conformation, which we have obtained inducing a controlled in vitro conformational change of the protein, might mimic mild-unfolding events occurring in vivo, in the presence of specific mutations, oxidative reactions or proteolysis. Thus, in light of this model, we propose that novel therapeutic strategies, designed to inhibit the interaction of the toxic PrP with the plasmamembrane, could be beneficial to prevent the formation of intracellular neurotoxic aggregates and ultimately the neuronal death.

In vitro and in vivo neurotoxicity of prion protein oligomers

The mechanisms underlying prion-linked neurodegeneration remain to be elucidated, despite several recent advances in this field. Herein, we show that soluble, low molecular weight oligomers of the full-length prion protein (PrP), which possess characteristics of PrP to PrPsc conversion intermediates such as partial protease resistance, are neurotoxic in vitro on primary cultures of neurons and in vivo after subcortical stereotaxic injection. Monomeric PrP was not toxic. Insoluble, fibrillar forms of PrP exhibited no toxicity in vitro and were less toxic than their oligomeric counterparts in vivo. The toxicity was independent of PrP expression in the neurons both in vitro and in vivo for the PrP oligomers and in vivo for the PrP fibrils. Rescue experiments with antibodies showed that the exposure of the hydrophobic stretch of PrP at the oligomeric surface was necessary for toxicity. This study identifies toxic PrP species in vivo. It shows that PrP-induced neurodegeneration shares common mechanisms with other brain amyloidoses like Alzheimer disease and opens new avenues for neuroprotective intervention strategies of prion diseases targeting PrP oligomers.

Amino-terminally truncated prion protein PrP90-231 induces microglial activation in vitro

The conversion of the prion protein (PrP) into a protease-resistant isoform (PrP(Res)) is considered the pathogenic event responsible for prion encephalopathies. Microglia activation accompanies PrP(Res) deposition representing an early event in the progression of these diseases. It is now believed that microglial cells play a worsening, if not causative, role in prion-induced neuronal death, through the release of proinflammatory and neurotoxic molecules. Indeed, in vitro observations have demonstrated that PrP(Res) and the synthetic prion fragment PrP106-126 induce neuronal death by activating microglial to migrate in the lesion area and secrete cytokines. Recently, we and others have demonstrated that the recombinant peptide, corresponding to the protease-resistant portion of PrP encompassing the amino acids 90-231 (PrP90-231), when beta-structured, is toxic for neuronal cells, in vitro. Here we report that PrP90-231 induces activation of N9 microglial cells, characterized by cell proliferation arrest and increased secretion of different cytokines (RANTES, GCSF, and IL-12). Moreover, the treatment of N9 cells with PrP90-231 elicited inducible nitric oxide synthase (i-NOS) expression, nitric oxide release, and a delayed (15 min to 1 h of treatment) extracellular signal-regulated kinases 1/2 (ERK1/2) phosphorylation/activation. Although ERK1/2 is known to regulate proliferative and differentiative events, we show that its blockade, using the specific MEK inhibitor PD98059, did not prevent PrP90-231-induced inhibition of N9 cell proliferation. To our knowledge, this is the first evidence that a recombinant PrP(Res)-like peptide elicits microglial activation in vitro, thus representing a potentially important tool to develop possible therapeutic strategies to target prion-induced brain inflammation.

Diversity in prion protein oligomerization pathways results from domain expansion as revealed by hydrogen/deuterium exchange and disulfide linkage

The prion protein (PrP) propensity to adopt different structures is a clue to its biological role. PrP oligomers have been previously reported to bear prion infectivity or toxicity and were also found along the pathway of in vitro amyloid formation. In the present report, kinetic and structural analysis of ovine PrP (OvPrP) oligomerization showed that three distinct oligomeric species were formed in parallel, independent kinetic pathways. Only the largest oligomer gave rise to fibrillar structures at high concentration. The refolding of OvPrP into these different oligomers was investigated by analysis of hydrogen/deuterium exchange and introduction of disulfide bonds. These experiments revealed that, before oligomerization, separation of contacts in the globular part (residues 127-234) occurred between the S1-H1-S2 domain (residues 132-167) and the H2-H3 bundle (residues 174-230), implying a conformational change of the S2-H2 loop (residues 168-173). The type of oligomer to be formed depended on the site where the expansion of the OvPrP monomer was initiated. Our data bring a detailed insight into the earlier conformational changes during PrP oligomerization and account for the diversity of oligomeric entities. The kinetic and structural mechanisms proposed here might constitute a physicochemical basis of prion strain genesis.

The reconstitution of mammalian prion infectivity de novo

The discovery of prion disease transmission in mammals, as well as a non-Mendelian type of inheritance in yeast, has led to the establishment of a new concept in biology, the prion hypothesis. The prion hypothesis postulates that an abnormal protein conformation propagates itself in an autocatalytic manner using the normal isoform of the same protein as a substrate and thereby acts either as a transmissible agent of disease (in mammals), or as a heritable determinant of phenotype (in yeast and fungus). While the prion biology of yeast and fungus supports this idea strongly, the direct proof of the prion hypothesis in mammals, specifically the reconstitution of the disease-associated isoform of the prion protein (PrP(Sc)) in vitro de novo from noninfectious prion protein, has been difficult to achieve despite many years of effort. The present review summarizes our current knowledge about the biochemical nature of the prion infectious agent and structure of PrP(Sc), describes potential strategies for generating prion infectivity de novo and provides some insight on why the reconstitution of infectivity has been difficult to achieve in vitro. Several hypotheses are proposed to explain the apparently low infectivity of the first generation of recently reported synthetic mammalian prions.

Towards a structural understanding of the fibrillization pathway in Machado-Joseph's disease: trapping early oligomers of non-expanded ataxin-3

Machado-Joseph's disease is caused by a CAG trinucleotide repeat expansion that is translated into an abnormally long polyglutamine tract in the protein ataxin-3. Except for the polyglutamine region, proteins associated with polyglutamine diseases are unrelated, and for all of these diseases aggregates containing these proteins are the major components of the nuclear proteinaceous deposits found in the brain. Aggregates of the expanded proteins display amyloid-like morphological and biophysical properties. Human ataxin-3 containing a non-pathological number of glutamine residues (14Q), as well as its Caenorhabditis elegans (1Q) orthologue, showed a high tendency towards self-interaction and aggregation, under near-physiological conditions. In order to understand the discrete steps in the assembly process leading to ataxin-3 oligomerization, we have separated chromatographically high molecular mass oligomers as well as medium mass multimers of non-expanded ataxin-3. We show that: (a) oligomerization occurs independently of the poly(Q)-repeat and it is accompanied by an increase in beta-structure; and (b) the first intermediate in the oligomerization pathway is a Josephin domain-mediated dimer of ataxin-3. Furthermore, non-expanded ataxin-3 oligomers are recognized by a specific antibody that targets a conformational epitope present in soluble cytotoxic species found in the fibrillization pathway of expanded polyglutamine proteins and other amyloid-forming proteins. Imaging of the oligomeric forms of the non-pathological protein using electron microscopy reveals globular particles, as well as short chains of such particles that likely mimic the initial stages in the fibrillogenesis pathway occurring in the polyglutamine-expanded protein. Thus, they constitute potential targets for therapeutic approaches in Machado-Joseph's disease, as well as valuable diagnostic markers in disease settings.

Gerstmann-Sträussler-Scheinker Disease Amyloid Protein Polymerizes According to the “Dock-and-Lock” Model

Prion protein (PrP) amyloid formation is a central feature of genetic and acquired prion diseases such as Gerstmann-Sträussler-Scheinker disease (GSS) and variant Creutzfeldt-Jakob disease. The major component of GSS amyloid is a PrP fragment spanning residues approximately 82-146, which when synthesized as a peptide, readily forms fibrils featuring GSS amyloid. The present study employed surface plasmon resonance (SPR) to characterize the binding events underlying PrP82-146 oligomerization at the first stages of fibrillization, according to evidence suggesting a pathogenic role of prefibrillar oligomers rather than mature amyloid fibrils. We followed in real time the binding reactions occurring during short term (seconds) addition of PrP82-146 small oligomers (1-5-mers, flowing species) onto soluble prefibrillar PrP82-146 aggregates immobilized on the sensor surface. SPR data confirmed very efficient aggregation/elongation, consistent with the hypothesis of nucleation-dependent polymerization process. Much lower binding was observed when PrP82-146 flowed onto the scrambled sequence of PrP82-146 or onto prefibrillar Abeta42 aggregates. As previously found with Abeta40, SPR data could be adequately fitted by equations modeling the "dock-and-lock" mechanism, in which the "locking" step is due to sequential conformational changes, each increasing the affinity of the monomer for the fibril until a condition of irreversible binding is reached. However, these conformational changes (i.e. the locking steps) appear to be faster and easier with PrP82-146 than with Abeta40. Such differences suggest that PrP82-146 has a greater propensity to polymerize and greater stability of the aggregates.

Structuring the puzzle of prion propagation

Of all the prion proteins identified to date, the agent responsible for transmissible spongiform encephalopathies is one of the least characterized. Nevertheless, recent advances in the prion field should lead to important progress in our knowledge of mammalian prions. First, the demonstration that PrP aggregates generated in vitro infect animals and cause neuronal death is a considerable breakthrough. Second, new structural data provide direct insight into the structure of the infectious agent. Third, the study of yeast prions unveiled what might be the structural basis for the strain phenomena in transmissible spongiform encephalopathies.