Prion protein amyloid formation under native-like conditions involves refolding of the C-terminal alpha-helical domain

Prion protein amino acid sequence influences formation of authentic synthetic PrPSc

Synthetic prions, generated de novo from minimal, non-infectious components, cause bona fide prion disease in animals. Transmission of synthetic prions to hosts expressing syngeneic PrP^C results in extended, variable incubation periods and incomplete attack rates. In contrast, murine synthetic prions (MSP) generated via PMCA with minimal cofactors readily infected mice and hamsters and rapidly adapted to both species. To investigate if hamster synthetic prions (HSP) generated under the same conditions as the MSP are also highly infectious, we inoculated hamsters with HSP generated with either hamster wild type or mutant (ΔG54, ΔG54/M139I, M139I/I205M) recombinant PrP. None of the inoculated hamsters developed clinical signs of prion disease, however, brain homogenate from HSP^WT- and HSP^ΔG54-infected hamsters contained PrP^Sc, indicating subclinical infection. Serial passage in hamsters resulted in clinical disease at second passage accompanied by changes in incubation period and PrP^Sc conformational stability between second and third passage. These data suggest the HSP, in contrast to the MSP, are not comprised of PrP^Sc, and instead generate authentic PrP^Sc via deformed templating. Differences in infectivity between the MSP and HSP suggest that, under similar generation conditions, the amino acid sequence of PrP influences generation of authentic PrP^Sc.

Molecular insights into the critical role of gallate moiety of green tea catechins in modulating prion fibrillation, cellular internalization, and neuronal toxicity

Transmissible spongiform encephalopathies (TSEs) or prion diseases are fatal neurodegenerative diseases with no approved therapeutics. TSE pathology is characterized by abnormal accumulation of amyloidogenic and infectious prion protein conformers (PrP^Sc) in the central nervous system. Herein, we examined the role of gallate group in green tea catechins in modulating the aggregation of human prion protein (HuPrP) using two green tea constituents i.e., epicatechin 3-gallate (EC3G; with intact gallate ring) and epigallocatechin (EGC; without gallate ring). Molecular docking indicated distinct differences in hydrogen bonding and hydrophobic interactions of EC3G and EGC at the β2-α2 loop of HuPrP. These differences were substantiated by 44-fold higher KD for EC3G as compared to EGC with the former significantly reducing Thioflavin T (ThT) binding aggregates of HuPrP. Conformational alterations in HuPrP aggregates were validated by particle sizing, AFM analysis and A11 and OC conformational antibodies. As compared to EGC, EC3G showed relatively higher reduction in toxicity and cellular internalization of HuPrP oligomers in Neuro-2a cells. Additionally, EC3G also displayed higher fibril disaggregating properties as observed by ThT kinetics and electron microscopy. Our observations were supported by molecular dynamics (MD) simulations that showed markedly reduced α2-α3 and β2-α2 loop mobilities in presence of EC3G that may lead to constriction of HuPrP conformational space with lowered β-sheet conversion. In totality, gallate moiety of catechins play key role in modulating HuPrP aggregation, and toxicity and could be a new structural motif for designing therapeutics against prion diseases and other neurodegenerative disorders.

Structural biology of ex vivo mammalian prions

The structures of prion protein (PrP)–based mammalian prions have long been elusive. However, cryo-EM has begun to reveal the near-atomic resolution structures of fully infectious ex vivo mammalian prion fibrils as well as relatively innocuous synthetic PrP amyloids. Comparisons of these various types of PrP fibrils are now providing initial clues to structural features that correlate with pathogenicity. As first indicated by electron paramagnetic resonance and solid-state NMR studies of synthetic amyloids, all sufficiently resolved PrP fibrils of any sort (n > 10) have parallel in-register intermolecular β-stack architectures. Cryo-EM has shown that infectious brain-derived prion fibrils of the rodent-adapted 263K and RML scrapie strains have much larger ordered cores than the synthetic fibrils. These bona fide prion strains share major structural motifs, but the conformational details and the overall shape of the fibril cross sections differ markedly. Such motif variations, as well as differences in sequence within the ordered polypeptide cores, likely contribute to strain-dependent templating. When present, N-linked glycans and glycophosphatidylinositol (GPI) anchors project outward from the fibril surface. For the mouse RML strain, these posttranslational modifications have little effect on the core structure. In the GPI-anchored prion structures, a linear array of GPI anchors along the twisting fibril axis appears likely to bind membranes in vivo, and as such, may account for pathognomonic membrane distortions seen in prion diseases. In this review, we focus on these infectious prion structures and their implications regarding prion replication mechanisms, strains, transmission barriers, and molecular pathogenesis.

Cellular Prion Protein Expression in the Brain Tissue from Brucella ceti-Infected Striped Dolphins (Stenella coeruleoalba)

Simple Summary

Brucella ceti, a zoonotic bacterial pathogen, is known to exhibit a strong neurotropism and neuropathogenicity for striped dolphins (Stenella coeruleoalba), often leading to their stranding and death. Given the lack of information on B. ceti infection’s neuropathogenesis, we investigated, for the first time, cellular prion protein (PrP^c) expression in the brain tissue from B. ceti-infected, neurobrucellosis-affected striped dolphins. Our study was inspired by previous work, reporting PrP^c as the host cell receptor for B. abortus on the surface of murine macrophages. Immunohistochemistry (IHC) and Western blot (WB) analyses were carried out on brain tissues from 12 striped dolphins found stranded along the coasts of Italy (11 specimens) and the Canary Islands (one individual), five of which served as negative controls. While PrP^c IHC yielded inconclusive results, WB analyses showed a clear-cut PrP^c expression, albeit of different intensity, in the brain tissue of all the herein investigated, B. ceti-infected and neurobrucellosis-affected individuals. In this respect, the aforementioned PrP^c expression patterns could be influenced by a number of intrinsic host-related factors, as well as by several extrinsic factors including simultaneously occurring neuropathies and/or coinfections by other neurotropic pathogens. Additionally, an upregulation of PrP^c mRNA in the brain tissue of striped dolphins could be also hypothesized during the different stages of B. ceti infection, in a similar fashion to what is already shown in murine bone marrow cells challenged with Escherichia coli. In conclusion, much more work is needed in order to properly assess the role of PrP^c, if any, as a host cell receptor for B. ceti in striped dolphins.

Abstract

Brucella ceti, a zoonotic pathogen of major concern to cetacean health and conservation, is responsible for severe meningo-encephalitic/myelitic lesions in striped dolphins (Stenella coeruleoalba), often leading to their stranding and death. This study investigated, for the first time, the cellular prion protein (PrP^c) expression in the brain tissue from B. ceti-infected, neurobrucellosis-affected striped dolphins. Seven B. ceti-infected, neurobrucellosis-affected striped dolphins, found stranded along the Italian coastline (6) and in the Canary Islands (1), were investigated, along with five B. ceti-uninfected striped dolphins from the coast of Italy, carrying no brain lesions, which served as negative controls. Western Blot (WB) and immunohistochemistry (IHC) with an anti-PrP murine monoclonal antibody were carried out on the brain parenchyma of these dolphins. While PrP^c IHC yielded inconclusive results, a clear-cut PrP^c expression of different intensity was found by means of WB analyses in the brain tissue of all the seven herein investigated, B. ceti-infected and neurobrucellosis-affected cetacean specimens, with two dolphins stranded along the Italian coastline and one dolphin beached in Canary Islands also exhibiting a statistically significant increase in cerebral PrP^c expression as compared to the five Brucella spp.-negative control specimens. The significantly increased PrP^c expression found in three out of seven B. ceti-infected, neurobrucellosis-affected striped dolphins does not allow us to draw any firm conclusion(s) about the putative role of PrP^c as a host cell receptor for B. ceti. Should this be the case, an upregulation of PrP^c mRNA in the brain tissue of neurobrucellosis-affected striped dolphins could be hypothesized during the different stages of B. ceti infection, as previously shown in murine bone marrow cells challenged with Escherichia coli. Noteworthy, the inflammatory infiltrates seen in the brain and in the cervico-thoracic spinal cord segments from the herein investigated, B. ceti-infected and neurobrucellosis-affected striped dolphins were densely populated by macrophage/histiocyte cells, often harboring Brucella spp. antigen in their cytoplasm, similarly to what was reported in macrophages from mice experimentally challenged with B. abortus. Notwithstanding the above, much more work is needed in order to properly assess the role of PrP^c, if any, as a host cell receptor for B. ceti in striped dolphins.

High-resolution structure and strain comparison of infectious mammalian prions

Within the extensive range of self-propagating pathologic protein aggregates of mammals, prions are the most clearly infectious (e.g., ∼10⁹ lethal doses per milligram). The structures of such lethal assemblies of PrP molecules have been poorly understood. Here we report a near-atomic core structure of a brain-derived, fully infectious prion (263K strain). Cryo-electron microscopy showed amyloid fibrils assembled with parallel in-register intermolecular β sheets. Each monomer provides one rung of the ordered fibril core, with N-linked glycans and glycolipid anchors projecting outward. Thus, single monomers form the templating surface for incoming monomers at fibril ends, where prion growth occurs. Comparison to another prion strain (aRML) revealed major differences in fibril morphology but, like 263K, an asymmetric fibril cross-section without paired protofilaments. These findings provide structural insights into prion propagation, strains, species barriers, and membrane pathogenesis. This structure also helps frame considerations of factors influencing the relative transmissibility of other pathologic amyloids.

Osmolytes and crowders regulate aggregation of the cancer-related L106R mutant of the Axin protein

Protein aggregation is involved in a variety of diseases, including neurodegenerative diseases and cancer. The cellular environment is crowded by a plethora of cosolutes comprising small molecules and biomacromolecules at high concentrations, which may influence the aggregation of proteins in vivo. To account for the effect of cosolutes on cancer-related protein aggregation, we studied their effect on the aggregation of the cancer-related L106R mutant of the Axin protein. Axin is a key player in the Wnt signaling pathway, and the L106R mutation in its RGS domain results in a native molten globule that tends to form native-like aggregates. This results in uncontrolled activation of the Wnt signaling pathway, leading to cancer. We monitored the aggregation process of Axin RGS L106R in vitro in the presence of a wide ensemble of cosolutes including polyols, amino acids, betaine, and polyethylene glycol crowders. Except myo-inositol, all polyols decreased RGS L106R aggregation, with carbohydrates exerting the strongest inhibition. Conversely, betaine and polyethylene glycols enhanced aggregation. These results are consistent with the reported effects of osmolytes and crowders on the stability of molten globular proteins and with both amorphous and amyloid aggregation mechanisms. We suggest a model of Axin L106R aggregation in vivo, whereby molecularly small osmolytes keep the protein as a free soluble molecule but the increased crowding of the bound state by macromolecules induces its aggregation at the nanoscale. To our knowledge, this is the first systematic study on the effect of osmolytes and crowders on a process of native-like aggregation involved in pathology, as it sheds light on the contribution of cosolutes to the onset of cancer as a protein misfolding disease and on the relevance of aggregation in the molecular etiology of cancer.

Mechanism of misfolding of the human prion protein revealed by a pathological mutation

The misfolding and aggregation of the human prion protein (PrP) is associated with transmissible spongiform encephalopathies (TSEs). Intermediate conformations forming during the conversion of the cellular form of PrP into its pathological scrapie conformation are key drivers of the misfolding process. Here, we analyzed the properties of the C-terminal domain of the human PrP (huPrP) and its T183A variant, which is associated with familial forms of TSEs. We show that the mutation significantly enhances the aggregation propensity of huPrP, such as to uniquely induce amyloid formation under physiological conditions by the sole C-terminal domain of the protein. Using NMR spectroscopy, biophysics, and metadynamics simulations, we identified the structural characteristics of the misfolded intermediate promoting the aggregation of T183A huPrP and the nature of the interactions that prevent this species to be populated in the wild-type protein. In support of these conclusions, POM antibodies targeting the regions that promote PrP misfolding were shown to potently suppress the aggregation of this amyloidogenic mutant.

The cellular and pathologic prion protein

The cellular prion protein, PrP^C, is a small, cell surface glycoprotein with a function that is currently somewhat ill defined. It is also the key molecule involved in the family of neurodegenerative disorders called transmissible spongiform encephalopathies, which are also known as prion diseases. The misfolding of PrP^C to a conformationally altered isoform, designated PrP^TSE, is the main molecular process involved in pathogenesis and appears to precede many other pathologic and clinical manifestations of disease, including neuronal loss, astrogliosis, and cognitive loss. PrP^TSE is also believed to be the major component of the infectious "prion," the agent responsible for disease transmission, and preparations of this protein can cause prion disease when inoculated into a naïve host. Thus, understanding the biochemical and biophysical properties of both PrP^C and PrP^TSE, and ultimately the mechanisms of their interconversion, is critical if we are to understand prion disease biology. Although entire books could be devoted to research pertaining to the protein, herein we briefly review the state of knowledge of prion biochemistry, including consideration of prion protein structure, function, misfolding, and dysfunction.

Structural mechanisms of oligomer and amyloid fibril formation by the prion protein

Misfolding and aggregation of the prion protein is responsible for multiple neurodegenerative diseases. Works from several laboratories on folding of both the WT and multiple pathogenic mutant variants of the prion protein have identified several structurally dissimilar intermediates, which might be potential precursors to misfolding and aggregation. The misfolded aggregates themselves are morphologically distinct, critically dependent on the solution conditions under which they are prepared, but always β-sheet rich. Despite the lack of an atomic resolution structure of the infectious pathogenic agent in prion diseases, several low resolution models have identified the β-sheet rich core of the aggregates formed in vitro, to lie in the α2-α3 subdomain of the prion protein, albeit with local stabilities that vary with the type of aggregate. This feature article describes recent advances in the investigation of in vitro prion protein aggregation using multiple spectroscopic probes, with particular focus on (1) identifying aggregation-prone conformations of the monomeric protein, (2) conditions which trigger misfolding and oligomerization, (3) the mechanism of misfolding and aggregation, and (4) the structure of the misfolded intermediates and final aggregates.

Self-assembling peptide and protein amyloids: from structure to tailored function in nanotechnology

Self-assembled peptide and protein amyloid nanostructures have traditionally been considered only as pathological aggregates implicated in human neurodegenerative diseases. In more recent times, these nanostructures have found interesting applications as advanced materials in biomedicine, tissue engineering, renewable energy, environmental science, nanotechnology and material science, to name only a few fields. In all these applications, the final function depends on: (i) the specific mechanisms of protein aggregation, (ii) the hierarchical structure of the protein and peptide amyloids from the atomistic to mesoscopic length scales and (iii) the physical properties of the amyloids in the context of their surrounding environment (biological or artificial). In this review, we will discuss recent progress made in the field of functional and artificial amyloids and highlight connections between protein/peptide folding, unfolding and aggregation mechanisms, with the resulting amyloid structure and functionality. We also highlight current advances in the design and synthesis of amyloid-based biological and functional materials and identify new potential fields in which amyloid-based structures promise new breakthroughs.

Human prion diseases: surgical lessons learned from iatrogenic prion transmission

The human prion diseases, or transmissible spongiform encephalopathies, have captivated our imaginations since their discovery in the Fore linguistic group in Papua New Guinea in the 1950s. The mysterious and poorly understood "infectious protein" has become somewhat of a household name in many regions across the globe. From bovine spongiform encephalopathy (BSE), commonly identified as mad cow disease, to endocannibalism, media outlets have capitalized on these devastatingly fatal neurological conditions. Interestingly, since their discovery, there have been more than 492 incidents of iatrogenic transmission of prion diseases, largely resulting from prion-contaminated growth hormone and dura mater grafts. Although fewer than 9 cases of probable iatrogenic neurosurgical cases of Creutzfeldt-Jakob disease (CJD) have been reported worldwide, the likelihood of some missed cases and the potential for prion transmission by neurosurgery create considerable concern. Laboratory studies indicate that standard decontamination and sterilization procedures may be insufficient to completely remove infectivity from prion-contaminated instruments. In this unfortunate event, the instruments may transmit the prion disease to others. Much caution therefore should be taken in the absence of strong evidence against the presence of a prion disease in a neurosurgical patient. While the Centers for Disease Control and Prevention (CDC) and World Health Organization (WHO) have devised risk assessment and decontamination protocols for the prevention of iatrogenic transmission of the prion diseases, incidents of possible exposure to prions have unfortunately occurred in the United States. In this article, the authors outline the historical discoveries that led from kuru to the identification and isolation of the pathological prion proteins in addition to providing a brief description of human prion diseases and iatrogenic forms of CJD, a brief history of prion disease nosocomial transmission, and a summary of the CDC and WHO guidelines for prevention of prion disease transmission and decontamination of prion-contaminated neurosurgical instruments.

Modulation of prion polymerization and toxicity by rationally designed peptidomimetics

Misfolding and aggregation of cellular prion protein is associated with a large array of neurological disorders commonly called the transmissible spongiform encephalopathies. Designing inhibitors against prions has remained a daunting task owing to limited information about mechanism(s) of their pathogenic self-assembly. Here, we explore the anti-prion properties of a combinatorial library of bispidine-based peptidomimetics (BPMs) that conjugate amino acids with hydrophobic and aromatic side chains. Keeping the bispidine unit unaltered, a series of structurally diverse BPMs were synthesized and tested for their prion-modulating properties. Administration of Leu- and Trp-BPMs delayed and completely inhibited the amyloidogenic conversion of human prion protein (HuPrP), respectively. We found that each BPM induced the HuPrP to form unique oligomeric nanostructures differing in their biophysical properties, cellular toxicities and response to conformation-specific antibodies. While Leu-BPMs were found to stabilize the oligomers, Trp-BPMs effected transient oligomerization, resulting in the formation of non-toxic, non-fibrillar aggregates. Yet another aromatic residue, Phe, however, accelerated the aggregation process in HuPrP. Molecular insights obtained through MD (molecular dynamics) simulations suggested that each BPM differently engages a conserved Tyr 169 residue at the α2–β2 loop of HuPrP and affects the stability of α2 and α3 helices. Our results demonstrate that this new class of molecules having chemical scaffolds conjugating hydrophobic/aromatic residues could effectively modulate prion aggregation and toxicity.

Synthetic Aβ peptides acquire prion-like properties in the brain

In transmission studies with Alzheimer's disease (AD) animal models, the formation of Aβ plaques is proposed to be initiated by seeding the inoculated amyloid β (Aβ) peptides in the brain. Like the misfolded scrapie prion protein (PrPSc) in prion diseases, Aβ in AD shows a certain degree of resistance to protease digestion while the biochemical basis for protease resistance of Aβ remains poorly understood. Using in vitro assays, histoblotting, and electron microscopy, we characterize the biochemical and morphological features of synthetic Aβ peptides and Aβ isolated from AD brain tissues. Consistent with previous observations, monomeric and oligomeric Aβ species extracted from AD brains are insoluble in detergent buffers and resistant to digestions with proteinase K (PK). Histoblotting of AD brain tissue sections exhibits an increased Aβ immunoreactivity after digestion with PK. In contrast, synthetic Aβ40 and Aβ42 are soluble in detergent buffers and fully digested by PK. Electron microscopy of Aβ40 and Aβ42 synthetic peptides shows that both species of Aβ form mature fibrils. Those generated from Aβ40 are longer but less numerous than those made of Aβ42. When spiked into human brain homogenates, both Aβ40 and Aβ42 acquire insolubility in detergent and resistance to PK. Our study favors the hypothesis that the human brain may contain cofactor(s) that confers the synthetic Aβ peptides PrPSc-like physicochemical properties.

Charge neutralization of the central lysine cluster in prion protein (PrP) promotes PrP(Sc)-like folding of recombinant PrP amyloids

The structure of the infectious form of prion protein, PrP(Sc), remains unclear. Most pure recombinant prion protein (PrP) amyloids generated in vitro are not infectious and lack the extent of the protease-resistant core and solvent exclusion of infectious PrP(Sc), especially within residues ∼90-160. Polyanionic cofactors can enhance infectivity and PrP(Sc)-like characteristics of such fibrils, but the mechanism of this enhancement is unknown. In considering structural models of PrP(Sc) multimers, we identified an obstacle to tight packing that might be overcome with polyanionic cofactors, namely, electrostatic repulsion between four closely spaced cationic lysines within a central lysine cluster of residues 101-110. For example, in our parallel in-register intermolecular β-sheet model of PrP(Sc), not only would these lysines be clustered within the 101-110 region of the primary sequence, but they would have intermolecular spacings of only ∼4.8 Å between stacked β-strands. We have now performed molecular dynamics simulations predicting that neutralization of the charges on these lysine residues would allow more stable parallel in-register packing in this region. We also show empirically that substitution of these clustered lysine residues with alanines or asparagines results in recombinant PrP amyloid fibrils with extended proteinase-K resistant β-sheet cores and infrared spectra that are more reminiscent of bona fide PrP(Sc). These findings indicate that charge neutralization at the central lysine cluster is critical for the folding and tight packing of N-proximal residues within PrP amyloid fibrils. This charge neutralization may be a key aspect of the mechanism by which anionic cofactors promote PrP(Sc) formation.

Rational stabilization of helix 2 of the prion protein prevents its misfolding and oligomerization

Designed stabilization of helix 2 of the mouse prion protein is shown to lead to an increase in global stability of the protein. Studies of hydrogen exchange coupled to mass spectrometry confirm that the increase in stability is confined primarily to helix 2, and that it accounts for the global stabilization of the protein. Importantly, such localized stabilization of the protein can completely inhibit its ability to form oligomers and slows down amyloid fibril formation.

Shaking alone induces de novo conversion of recombinant prion proteins to β-sheet rich oligomers and fibrils

The formation of β-sheet rich prion oligomers and fibrils from native prion protein (PrP) is thought to be a key step in the development of prion diseases. Many methods are available to convert recombinant prion protein into β-sheet rich fibrils using various chemical denaturants (urea, SDS, GdnHCl), high temperature, phospholipids, or mildly acidic conditions (pH 4). Many of these methods also require shaking or another form of agitation to complete the conversion process. We have identified that shaking alone causes the conversion of recombinant PrP to β-sheet rich oligomers and fibrils at near physiological pH (pH 5.5 to pH 6.2) and temperature. This conversion does not require any denaturant, detergent, or any other chemical cofactor. Interestingly, this conversion does not occur when the water-air interface is eliminated in the shaken sample. We have analyzed shaking-induced conversion using circular dichroism, resolution enhanced native acidic gel electrophoresis (RENAGE), electron microscopy, Fourier transform infrared spectroscopy, thioflavin T fluorescence and proteinase K resistance. Our results show that shaking causes the formation of β-sheet rich oligomers with a population distribution ranging from octamers to dodecamers and that further shaking causes a transition to β-sheet fibrils. In addition, we show that shaking-induced conversion occurs for a wide range of full-length and truncated constructs of mouse, hamster and cervid prion proteins. We propose that this method of conversion provides a robust, reproducible and easily accessible model for scrapie-like amyloid formation, allowing the generation of milligram quantities of physiologically stable β-sheet rich oligomers and fibrils. These results may also have interesting implications regarding our understanding of prion conversion and propagation both within the brain and via techniques such as protein misfolding cyclic amplification (PMCA) and quaking induced conversion (QuIC).

Protocol for aerosol-free recombinant production and NMR analysis of prion proteins

The central hallmark of prion diseases is the misfolding of cellular prion protein (PrP(C)) into a disease-associated aggregated isoform known as scrapie prion protein (PrP(Sc)). NMR spectroscopy has made many essential contributions to the characterization of recombinant PrP in its folded, unfolded and aggregated states. Recent studies reporting on de novo generation of prions from recombinant PrP and infection of animals using prion aerosols suggest that adjustment of current biosafety measures may be necessary, particularly given the relatively high protein concentrations required for NMR applications that favor aggregation. We here present a protocol for the production of recombinant PrP under biosafety level 2 conditions that avoids entirely exposure of the experimenter to aerosols that might contain harmful PrP aggregates. In addition, we introduce an NMR sample tube setup that allows for safe handling of PrP samples at the spectrometer that usually is not part of a dedicated biosafety level 2 laboratory.

Elongation of mouse prion protein amyloid-like fibrils: effect of temperature and denaturant concentration

Prion protein is known to have the ability to adopt a pathogenic conformation, which seems to be the basis for protein-only infectivity. The infectivity is based on self-replication of this pathogenic prion structure. One of possible mechanisms for such replication is the elongation of amyloid-like fibrils.

We measured elongation kinetics and thermodynamics of mouse prion amyloid-like fibrils at different guanidine hydrochloride (GuHCl) concentrations. Our data show that both increases in temperature and GuHCl concentration help unfold monomeric protein and thus accelerate elongation. Once the monomers are unfolded, further increases in temperature raise the rate of elongation, whereas the addition of GuHCl decreases it.

We demonstrated a possible way to determine different activation energies of amyloid-like fibril elongation by using folded and unfolded protein molecules. This approach separates thermodynamic data for fibril-assisted monomer unfolding and for refolding and formation of amyloid-like structure.

Parameters that affect macromolecular self-assembly of prion protein

Amyloidogenesis of prion protein (PrP) is closely associated with the pathobiology of prion diseases. To understand details on formation of PrP amyloids, we investigated various conditions that influence the process in vitro, using full length and truncated recombinant PrP. Disrupted agitation and fluctuated temperature resulted in prolongation of lag phase during PrP amyloid formation. With the same conditions and material for the assay, fluorescence microplate readers of different manufacturers, which are assumed to have incongruent level of mechanical performance, demonstrated variations for the length of lag phase and the level of fluorescence detection. Presence of preformed amyloid seeds accelerated PrP amyloid formation. Similarly, recombinant proteins of different species affected effectual generation of amyloids. This process was also influenced by the concentrations and truncation of recombinant PrP. By investigating several conditions to perform PrP amyloid formation assay, our study addresses the factors that determine how much and how rapidly PrP amyloids are formed.

Prions and the potential transmissibility of protein misfolding diseases

Prions, or infectious proteins, represent a major frontier in the study of infectious agents. The prions responsible for mammalian transmissible spongiform encephalopathies (TSEs) are due primarily to infectious self-propagation of misfolded prion proteins. TSE prion structures remain ill-defined, other than being highly structured, self-propagating, and often fibrillar protein multimers with the capacity to seed, or template, the conversion of their normal monomeric precursors into a pathogenic form. Purified TSE prions usually take the form of amyloid fibrils, which are self-seeding ultrastructures common to many serious protein misfolding diseases such as Alzheimer's, Parkinson's, Huntington's and Lou Gehrig's (amytrophic lateral sclerosis). Indeed, recent reports have now provided evidence of prion-like propagation of several misfolded proteins from cell to cell, if not from tissue to tissue or individual to individual. These findings raise concerns that various protein misfolding diseases might have spreading, prion-like etiologies that contribute to pathogenesis or prevalence.

Reversibility of prion misfolding: insights from constant-pH molecular dynamics simulations

The prion protein (PrP) is the cause of a group of diseases known as transmissible spongiform encephalopathies (TSEs). Creutzfeldt-Jakob disease and bovine spongiform encephalopathy are examples of TSEs. Although the normal form of PrP (PrP(C)) is monomeric and soluble, it can misfold into a pathogenic form (PrP(Sc)) that has a high content of β-structure and can aggregate forming amyloid fibrils. The mechanism of conversion of PrP(C) into PrP(Sc) is not known but different triggers have been proposed. It can be catalyzed by a PrP(Sc) sample, or it can be induced by an external factor, such as low pH. The pH effect on the structure of PrP was recently studied by computational methods [Campos et al. J. Phys. Chem. B 2010, 114, 12692-12700], and an evident trend of loss of helical structure was observed with pH decrease, together with a gain of β-structures. In particular, one simulation at pH 2 showed an evident misfolding transition. The main goal of the present work was to study the effects of a change in pH to 7 in several transient conformations of this simulation, in order to draw some conclusions about the reversibility of PrP misfolding. Although the most significant effect caused by the change of pH to 7 was a global stabilization of the protein structure, we could also observe that some conformational transitions induced by pH 2 were reversible in many of our simulations, namely those started from the early moments of the misfolding transition. This observation is in good agreement with experiments showing that, even at pH as low as 1.7, it is possible to revert the misfolding process [Bjorndahl et al. Biochemistry 2011, 50, 1162-1173].

Methionine oxidation perturbs the structural core of the prion protein and suggests a generic misfolding pathway

Oxidative stress and misfolding of the prion protein (PrP^C) are fundamental to prion diseases. We have therefore probed the effect of oxidation on the structure and stability of PrP^C. Urea unfolding studies indicate that H₂O₂ oxidation reduces the thermodynamic stability of PrP^C by as much as 9 kJ/mol. ¹H-¹⁵N NMR studies indicate methionine oxidation perturbs key hydrophobic residues on one face of helix-C as follows: Met-205, Val-209, and Met-212 together with residues Val-160 and Tyr-156. These hydrophobic residues pack together and form the structured core of the protein, stabilizing its ternary structure. Copper-catalyzed oxidation of PrP^C causes a more significant alteration of the structure, generating a monomeric molten globule species that retains its native helical content. Further copper-catalyzed oxidation promotes extended β-strand structures that lack a cooperative fold. This transition from the helical molten globule to β-conformation has striking similarities to a misfolding intermediate generated at low pH. PrP may therefore share a generic misfolding pathway to amyloid fibers, irrespective of the conditions promoting misfolding. Our observations support the hypothesis that oxidation of PrP destabilizes the native fold of PrP^C, facilitating the transition to PrP^Sc. This study gives a structural and thermodynamic explanation for the high levels of oxidized methionine in scrapie isolates.

Dissociation of recombinant prion protein fibrils into short protofilaments: implications for the endocytic pathway and involvement of the N-terminal domain

Fibril dissociation is necessary for efficient conversion of normal prion protein to its misfolded state and continued propagation into amyloid. Recent studies have revealed that conversion occurs along the endocytic pathway. To improve our understanding of the dissociation process, we have investigated the effect of low pH on the stability of recombinant prion fibrils. We show that under conditions that mimic the endocytic environment, amyloid fibrils made from full-length prion protein dissociate both laterally and axially to form protofilaments. Approximately 5% of the protofilaments are short enough to be considered soluble and contain ~100-300 monomers per structure; these also retain the biophysical characteristics of the filaments. We propose that protonation of His residues and charge repulsion in the N-terminal domain trigger fibril dissociation. Our data suggest that lysosomes and late endosomes are competent milieus for propagating the misfolded state not only by destabilizing the normal prion protein but also by accelerating the dissociation of fibrils into smaller structures that may act as seeds.

High-resolution structure of infectious prion protein: the final frontier

Prions are the proteinaceous infectious agents responsible for the transmission of prion diseases. The main or sole component of prions is the misfolded prion protein (PrP(Sc)), which is able to template the conversion of the host's natively folded form of the protein (PrP(C)). The detailed mechanism of prion replication and the high-resolution structure of PrP(Sc) are unknown. The currently available information on PrP(Sc) structure comes mostly from low-resolution biophysical techniques, which have resulted in quite divergent models. Recent advances in the production of infectious prions, using very pure recombinant protein, offer new hope for PrP(Sc) structural studies. This review highlights the importance of, challenges for and recent progress toward elucidating the elusive structure of PrP(Sc), arguably the major pending milestone to reach in understanding prions.

Stem-forming regions that are essential for the amyloidogenesis of prion proteins

Prion diseases represent fatal neurodegenerative disorders caused by the aggregation of prion proteins. With regard to the formation of the amyloidogenic cross-β-structure, the initial mechanism in the conversion to a β-structure is critically important. To explore the core regions forming a stem of the amyloid, we designed and prepared a series of peptides comprised of two native sequences linked by a turn-inducing dipeptide moiety and examined their ability to produce amyloids. A sequence alignment of the peptides bearing the ability to form amyloid structures revealed that paired strands consisting of VNITI (residues 180-184) and VTTTT (residues 189-193) are the core regions responsible for initiating the formation of cross-β-structures and for further ordered aggregation. In addition, most of the causative mutations responsible for inherited prion diseases were found to be located in these stem-forming regions on helix H2 and their counterpart on helix H3. Moreover, the volume effect of the nonstem domain, which contains ~200 residues, was deduced to be a determinant of the nature of the association such as oligomerization, because the stem-forming domain is only a small part of a prion protein. Taken together, we conclude that the mechanism underlying the initial stage of amyloidogenesis is the exposure of a newly formed intramolecular β-sheet to a solvent through the partial transition of a native structure from an α-helix to a β-structure. Our results also demonstrate that prion diseases caused by major prion proteins except the prions of some fungi such as yeast are inherent only in mammals, as evidenced by a comparison of the corresponding sequences to the stem-forming regions among different animals.

Massive conformation change in the prion protein: Using dual-basin structure-based models to find misfolding pathways

We employ all-atom structure-based models with a force field with multiple energetic basins for the C-terminal (residues 166-226) of the mammalian prion protein. One basin represents the known alpha-helical (αH) structure while the other represents the same residues in a left-handed beta-helical (LHBH) conformation. The LHBH structure has been proposed to help describe one class of in vitro grown fibrils, as well as possibly self-templating the conversion of normal cellular prion protein to the infectious form. Yet, it is unclear how the protein may make this global rearrangement. Our results demonstrate that the conformation changes are not strongly limited by large-scale geometry modification and that there may exist an overall preference for the LHBH conformation. Furthermore, our model presents novel intermediate trapping conformations with twisted LHBH structure.

Prion protein aggregation and fibrillogenesis in vitro

This chapter focuses on the structural conversion of natural and recombinant prion proteins in vitro. They key event in prion diseases is the conversion of the cellular prion protein (PrP(C)) into its disease causing isoform PrP(Sc). This conversion is represented by a conformational change from an β-helical dominated isoform into the mostly β-sheeted PrP(Sc). Represented is an overview of in vitro conversion systems that result in β-structured recombinant prion proteins including the current achievements in the generation of synthetic mammalian prions as proof of the protein-only hypothesis. In addition to the conversion of recombinant PrP the chapter features a summary of the protein misfolding cyclic amplification (PMCA) technique which has gained enormous popularity in prion research. Given is a general overview about the technique itself and the broad spectrum of utilization as detection method for prions. The spontaneous generation of prions by the protein misfolding amplification (PMCA) are also discussed.

Mutation directional selection sheds light on prion pathogenesis

As mutations in the PRNP gene account for human hereditary prion diseases (PrDs), it is crucial to elucidating how these mutations affect the central pathogenic conformational transition of normal cellular prion protein (PrP(C)) to abnormal scrapie isoform (PrP(Sc)). Many studies proposed that these pathogenic mutations may make PrP more susceptible to conformational change through altering its structure stability. By evaluating the most recent observations regarding pathogenic mutations, it was found that the pathogenic mutations do not exert a uniform effect on the thermodynamic stability of the human PrP's structure. Through analyzing the reported PrDs-related mutations, we found that 25 out of 27 mutations possess strong directional selection, i.e., enhancing hydrophobicity or decreasing negative and increasing positive charge. Based on the triggering role reported by previous studies of facilitating factors in PrP(C) conversion, e.g., lipid and polyanion, we proposed that the mutation-induced changes may strengthen the interaction between PrP and facilitating factors, which will accelerate PrP conversion and cause PrDs.

Probing structural differences between PrP(C) and PrP(Sc) by surface nitration and acetylation: evidence of conformational change in the C-terminus

We used two chemical modifiers, tetranitromethane (TNM) and acetic anhydride (Ac(2)O), which specifically target accessible tyrosine and lysine residues, respectively, to modify recombinant Syrian hamster PrP(90-231) [rSHaPrP(90-231)] and SHaPrP 27-30, the proteinase K-resistant core of PrP(Sc) isolated from brain of scrapie-infected Syrian hamsters. Our aim was to find locations of conformational change. Modified proteins were subjected to in-gel proteolytic digestion with trypsin or chymotrypsin and subsequent analysis by mass spectrometry (MALDI-TOF). Several differences in chemical reactivity were observed. With TNM, the most conspicuous reactivity difference seen involves peptide E(221)-R(229) (containing Y(225) and Y(226)), which in rSHaPrP(90-231) was much more extensively modified than in SHaPrP 27-30; peptide H(111)-R(136), containing Y(128), was also more modified in rSHaPrP(90-231). Conversely, peptides Y(149)-R(151), Y(157)-R(164), and R(151)-Y(162) suffered more extensive modification in SHaPrP 27-30. Acetic anhydride modified very extensively peptide G(90)-K(106), containing K(101), K(104), K(106), and the amino terminus, in both rSHaPrP(90-231) and SHaPrP 27-30. These results suggest that (1) SHaPrP 27-30 exhibits important conformational differences in the C-terminal region with respect to rSHaPrP(90-231), resulting in the loss of solvent accessibility of Y(225) and Y(226), very solvent-exposed in the latter conformation; because other results suggest preservation of the two C-terminal helices, this might mean that these are tightly packed in SHaPrP 27-30. (2) On the other hand, tyrosines contained in the stretch spanning approximately Y(149)-R(164) are more accessible in SHaPrP 27-30, suggesting rearrangements in α-helix H1 and the short β-sheet of rSHaPrP(90-231). (3) The amino-terminal region of SHaPrP 27-30 is very accessible. These data should help in the validation and construction of structural models of PrP(Sc).

Detailed biophysical characterization of the acid-induced PrP(c) to PrP(β) conversion process

Prions are believed to spontaneously convert from a native, monomeric highly helical form (called PrP(c)) to a largely β-sheet-rich, multimeric and insoluble aggregate (called PrP(sc)). Because of its large size and insolubility, biophysical characterization of PrP(sc) has been difficult, and there are several contradictory or incomplete models of the PrP(sc) structure. A β-sheet-rich, soluble intermediate, called PrP(β), exhibits many of the same features as PrP(sc) and can be generated using a combination of low pH and/or mild denaturing conditions. Studies of the PrP(c) to PrP(β) conversion process and of PrP(β) folding intermediates may provide insights into the structure of PrP(sc). Using a truncated, recombinant version of Syrian hamster PrP(β) (shPrP(90-232)), we used NMR spectroscopy, in combination with other biophysical techniques (circular dichroism, dynamic light scattering, electron microscopy, fluorescence spectroscopy, mass spectrometry, and proteinase K digestion), to characterize the pH-driven PrP(c) to PrP(β) conversion process in detail. Our results show that below pH 2.8 the protein oligomerizes and conversion to the β-rich structure is initiated. At pH 1.7 and above, the oligomeric protein can recover its native monomeric state through dialysis to pH 5.2. However, when conversion is completed at pH 1.0, the large oligomer "locks down" irreversibly into a stable, β-rich form. At pH values above 3.0, the protein is amenable to NMR investigation. Chemical shift perturbations, NOE, amide line width, and T(2) measurements implicate the putative "amylome motif" region, "NNQNNF" as the region most involved in the initial helix-to-β conversion phase. We also found that acid-induced PrP(β) oligomers could be converted to fibrils without the use of chaotropic denaturants. The latter finding represents one of the first examples wherein physiologically accessible conditions (i.e., only low pH) were used to achieve PrP conversion and fibril formation.

Influence of pH on the human prion protein: insights into the early steps of misfolding

Transmissible spongiform encephalopathies, or prion diseases, are caused by misfolding and aggregation of the prion protein PrP. Conversion from the normal cellular form (PrP(C)) or recombinant PrP (recPrP) to a misfolded form is pH-sensitive, in that misfolding and aggregation occur more readily at lower pH. To gain more insight into the influence of pH on the dynamics of PrP and its potential to misfold, we performed extensive molecular-dynamics simulations of the recombinant PrP protein (residues 90-230) in water at three different pH regimes: neutral (or cytoplasmic) pH (∼7.4), middle (or endosomal) pH (∼5), and low pH (<4). We present five different simulations of 50 ns each for each pH regime, amounting to a total of 750 ns of simulation time. A detailed analysis and comparison with experiment validate the simulations and lead to new insights into the mechanism of pH-induced misfolding. The mobility of the globular domain increases with decreasing pH, through displacement of the first helix and instability of the hydrophobic core. At middle pH, conversion to a misfolded (PrP(Sc)-like) conformation is observed. The observed changes in conformation and stability are consistent with experimental data and thus provide a molecular basis for the initial steps in the misfolding process.

Molecular dynamics as an approach to study prion protein misfolding and the effect of pathogenic mutations

Computer simulation of protein dynamics offers unique high-resolution information that complements experiment. Using experimentally derived structures of the natively folded prion protein (PrP), physically realistic dynamics and conformational changes can be simulated, including the initial steps of misfolding. By introducing mutations in silico, the effect of pathogenic mutations on PrP conformation and dynamics can be assessed. Here, we briefly introduce molecular dynamics methods and review the application of molecular dynamics simulations to obtain insight into various aspects of the PrP, including the mechanism of misfolding, the response to changes in the environment, and the influence of disease-related mutations.

Prion protein and its conformational conversion: a structural perspective

The key molecular event in the pathogenesis of prion diseases is the conformational conversion of a cellular prion protein, PrP(C), into a misfolded form, PrP(Sc). In contrast to PrP(C) that is monomeric and α-helical, PrP(Sc) is oligomeric in nature and rich in β-sheet structure. According to the "protein-only" model, PrP(Sc) itself represents the infectious prion agent responsible for transmissibility of prion disorders. While this model is supported by rapidly growing experimental data, detailed mechanistic and structural aspects of prion protein conversion remain enigmatic. In this chapter we describe recent advances in understanding biophysical and biochemical aspects of prion diseases, with a special focus on structural underpinnings of prion protein conversion, the structural basis of prion strains, and generation of prion infectivity in vitro from bacterially-expressed recombinant PrP.

Prion fibrillization is mediated by a native structural element that comprises helices H2 and H3

Aggregation and misfolding of the prion protein (PrP) are thought to be the cause of a family of lethal neurodegenerative diseases affecting humans and other animals. Although the structures of PrP from several species have been solved, still little is known about the mechanisms that lead to the misfolded species. Here, we show that the region of PrP comprising the hairpin formed by the helices H2 and H3 is a stable independently folded unit able to retain its secondary and tertiary structure also in the absence of the rest of the sequence. We also prove that the isolated H2H3 is highly fibrillogenic and forms amyloid fibers morphologically similar to those obtained for the full-length protein. Fibrillization of H2H3 but not of full-length PrP is concomitant with formation of aggregates. These observations suggest a "banana-peeling" mechanism for misfolding of PrP in which H2H3 is the aggregation seed that needs to be first exposed to promote conversion from a helical to a beta-rich structure.

Generation of prions in vitro and the protein-only hypothesis

Prions are self-propagating proteinaceous infectious agents capable of transmitting disease in the absence of nucleic acids. The nature of the infectious agent in prion diseases has been at the center of passionate debate for the past 30 years. However, recent reports on the in vitro generation of prions have settled all doubts that the misfolded prion protein (PrP(Sc)) is the key component in propagating infectivity. However, we still do not understand completely the mechanism of prion replication and whether or not other cellular factors besides PrP(Sc) are required for infectivity. In this article, we discuss these recent reports under the context of the protein-only hypothesis and their implications.

beta-sheet constitution of prion proteins

Structural information regarding normal prion protein (PrP(C)) and the scrapie isoform (PrP(Sc)) is of vital importance for elucidating the pathogenesis of prion diseases (PDs). Despite successful determination of the three-dimensional structures of PrP(C), the structural details of PrP(Sc) remain elusive. Nevertheless, accumulated evidence indicates that beta-sheets comprise the basic building blocks of PrP(Sc). Consensus has been reached about the beta-sheet constitution of the N-terminus of PrP, but the constitution of C-terminal beta-sheets is heavily debated. By evaluating the most recent observations regarding the dynamics and structures of PrP, we propose that helix 2 is more likely than helices 1 and 3 to participate in beta-sheet formation. This hypothesis also provides clues to explaining an intriguing phenomenon in prion biology-the lack of PDs in non-mammals.

Differential stability of the bovine prion protein upon urea unfolding

Prion diseases, or transmissible spongiform encephalopathies, are a group of infectious neurological diseases associated with the structural conversion of an endogenous protein (PrP) in the central nervous system. There are two major forms of this protein: the native and noninfectious cellular form, PrP(C); and the misfolded, infectious, and proteinase K-resistant form, PrP(Sc). The C-terminal domain of PrP(C) is mainly alpha-helical in structure, whereas PrP(Sc) in known to aggregate into an assembly of beta-sheets, forming amyloid fibrils. To identify the regions of PrP(C) potentially involved in the initial steps of the conversion to the infectious conformation, we have used high-resolution NMR spectroscopy to characterize the stability and structure of bovine recombinant PrP(C) (residues 121 to 230) during unfolding with the denaturant urea. Analysis of the 800 MHz (1)H NMR spectra reveals region-specific information about the structural changes occurring upon unfolding. Our data suggest that the dissociation of the native beta-sheet of PrP(C) is a primary step in the urea-induced unfolding process, while strong hydrophobic interactions between helices alpha1 and alpha3, and between alpha2 and alpha3, stabilize these regions even at very high concentrations of urea.

Prion diseases and their biochemical mechanisms

Prion diseases, also known as the transmissible spongiform encephalopathies (TSEs), are a group of fatal neurodegenerative disorders that affect humans and animals. These diseases are intimately associated with conformational conversion of the cellular prion protein, PrP(C), into an oligomeric beta-sheet-rich form, PrP(Sc). A growing number of observations support the once heretical hypothesis that transmission of TSE diseases does not require nucleic acids and that PrP(Sc) alone can act as an infectious agent. The view that misfolded proteins can be infectious is also supported by recent findings regarding prion phenomena in yeast and other fungi. One of the most intriguing facets of prions is their ability to form different strains, leading to distinct phenotypes of TSE diseases. Within the context of the "protein-only" model, prion strains are believed to be encoded in distinct conformations of misfolded prion protein aggregates. In this review, we describe recent advances in biochemical aspects of prion research, with a special focus on the mechanism of conversion of prion protein to the pathogenic form(s), the emerging structural knowledge of fungal and mammalian prions, and our rapidly growing understanding of the molecular basis of prion strains and their relation to barriers of interspecies transmissibility.

Amyloid fibrils of human prion protein are spun and woven from morphologically disordered aggregates

Propagation and infectivity of prions in human prionopathies are likely associated with conversion of the mainly alpha-helical human prion protein, HuPrP, into an aggregated form with amyloid-like properties. Previous reports on efficient conversion of recombinant HuPrP have used mild to harsh denaturing conditions to generate amyloid fibrils in vitro. Herein we report on the in vitro conversion of four forms of truncated HuPrP (sequences 90-231 and 121-231 with and without an N-terminal hexa histidine tag) into amyloid-like fibrils within a few hours by using a protocol (phosphate buffered saline solutions at neutral pH with intense agitation) close to physiological conditions. The conversion process monitored by thioflavin T, ThT, revealed a three stage process with lag, growth and equilibrium phases. Seeding with preformed fibrils shortened the lag phase demonstrating the classic nucleated polymerization mechanism for the reaction. Interestingly, comparing thioflavin T kinetics with solubility and turbidity kinetics it was found that the protein initially formed nonthioflavionophilic, morphologically disordered aggregates that over time matured into amyloid fibrils. By transmission electron microscopy and by fluorescence microscopy of aggregates stained with luminescent conjugated polythiophenes (LCPs); we demonstrated that HuPrP undergoes a conformational conversion where spun and woven fibrils protruded from morphologically disordered aggregates. The initial aggregation functioned as a kinetic trap that decelerated nucleation into a fibrillation competent nucleus, but at the same time without aggregation there was no onset of amyloid fibril formation. The agitation, which was necessary for fibril formation to be induced, transiently exposes the protein to the air-water interface suggests a hitherto largely unexplored denaturing environment for prion conversion.

Antimicrobial activity of human prion protein is mediated by its N-terminal region

Background

Cellular prion-related protein (PrP^c) is a cell-surface protein that is ubiquitously expressed in the human body. The multifunctionality of PrP^c, and presence of an exposed cationic and heparin-binding N-terminus, a feature characterizing many antimicrobial peptides, made us hypotesize that PrP^c could exert antimicrobial activity.

Methodology and Principal Findings

Intact recombinant PrP exerted antibacterial and antifungal effects at normal and low pH. Studies employing recombinant PrP and N- and C-terminally truncated variants, as well as overlapping peptide 20mers, demonstrated that the antimicrobial activity is mediated by the unstructured N-terminal part of the protein. Synthetic peptides of the N-terminus of PrP killed the Gram-negative bacteria Escherichia coli and Pseudomonas aeruginosa, and the Gram-positive Bacillus subtilis and Staphylococcus aureus, as well as the fungus Candida parapsilosis. Fluorescence studies of peptide-treated bacteria, paired with analysis of peptide effects on liposomes, showed that the peptides exerted membrane-breaking effects similar to those seen after treatment with the “classical” human antimicrobial peptide LL-37. In contrast to LL-37, however, no marked helix induction was detected for the PrP-derived peptides in presence of negatively charged (bacteria-mimicking) liposomes. PrP furthermore showed an inducible expression during wounding of human skin ex vivo and in vivo, as well as stimulation of keratinocytes with TGF-α in vitro.

Conclusions

The demonstration of an antimicrobial activity of PrP, localisation of its activity to the N-terminal and heparin-binding region, combined with results showing an increased expression of PrP during wounding, indicate that PrPs could have a previously undisclosed role in host defense.

Aggregation and amyloid fibril formation induced by chemical dimerization of recombinant prion protein in physiological-like conditions

Prion diseases are caused by the conversion of a cellular protein (PrP(C)) into a misfolded, aggregated isoform (PrP(Res)). Misfolding of recombinant PrP(C) in the absence of PrP(Res) template, cellular factors, denaturing agents, or at neutral pH has not been achieved. A number of studies indicate that dimerization of PrP(C) may be a key step in the aggregation process. In an effort to understand the molecular event that may activate misfolding of PrP(C) in more relevant physiological conditions, we tested if enforced dimerization of PrP(C) may induce a conformational change reminiscent of the conversion of PrP(C) to PrP(Res). We used a well described inducible dimerization strategy whereby a chimeric PrP(C) composed of a modified FK506-binding protein (Fv) fused with PrP(C) and termed Fv-PrP is incubated in the presence of a monomeric FK506 or dimerizing AP20187 ligand. Addition of AP20187 but not FK506 to recombinant Fv-PrP (rFv-PrP) in physiological-like conditions resulted in a rapid conformational change characterized by an increase in beta-sheet structure and simultaneous aggregation of the protein. Aggregates were partially resistant to proteinase K and induced the conversion of soluble rFv-PrP in serial seeding experiments. As judged from thioflavin T binding and electron microscopy, aggregates converted to amyloid fibers. Aggregates were toxic to cultured cells, whereas soluble rFv-PrP and amyloid fibers were harmless. This study strongly supports the proposition that dimerization of PrP(C) is a key pathological primary event in the conversion of PrP(C) and may initiate the pathogenesis of prion diseases.