Protease-sensitive scrapie prion protein in aggregates of heterogeneous sizes

Propagation of PrPSc in mice reveals impact of aggregate composition on prion disease pathogenesis

Infectious prions consist of PrPSc, a misfolded, aggregation-prone isoform of the host's prion protein. PrPSc assemblies encode distinct biochemical and biological properties. They harbor a specific profile of PrPSc species, from small oligomers to fibrils in different ratios, where the highest infectivity aligns with oligomeric particles. To investigate the impact of PrPSc aggregate complexity on prion propagation, biochemical properties, and disease pathogenesis, we fractionated elk prions by sedimentation velocity centrifugation, followed by sub-passages of individual fractions in cervidized mice. Upon first passage, different fractions generated PrPSc with distinct biochemical, biophysical, and neuropathological profiles. Notably, low or high molecular weight PrPSc aggregates caused different clinical signs of hyperexcitability or lethargy, respectively, which were retained over passage, whereas other properties converged. Our findings suggest that PrPSc quaternary structure determines an initial selection of a specific replication environment, resulting in transmissible features that are independent of PrPSc biochemical and biophysical properties.

(Dys)functional insights into nucleic acids and RNA-binding proteins modulation of the prion protein and α-synuclein phase separation

Prion diseases are prototype of infectious diseases transmitted by a protein, the prion protein (PrP), and are still not understandable at the molecular level. Heterogenous species of aggregated PrP can be generated from its monomer. α-synuclein (αSyn), related to Parkinson's disease, has also shown a prion-like pathogenic character, and likewise PrP interacts with nucleic acids (NAs), which in turn modulate their aggregation. Recently, our group and others have characterized that NAs and/or RNA-binding proteins (RBPs) modulate recombinant PrP and/or αSyn condensates formation, and uncontrolled condensation might precede pathological aggregation. Tackling abnormal phase separation of neurodegenerative disease-related proteins has been proposed as a promising therapeutic target. Therefore, understanding the mechanism by which polyanions, like NAs, modulate phase transitions intracellularly, is key to assess their role on toxicity promotion and neuronal death. Herein we discuss data on the nucleic acids binding properties and phase separation ability of PrP and αSyn with a special focus on their modulation by NAs and RBPs. Furthermore, we provide insights into condensation of PrP and/or αSyn in the light of non-trivial subcellular locations such as the nuclear and cytosolic environments.

Essential Components of Synthetic Infectious Prion Formation De Novo

Prion diseases are a class of neurodegenerative diseases that are uniquely infectious. Whilst their general replication mechanism is well understood, the components required for the formation and propagation of highly infectious prions are poorly characterized. The protein-only hypothesis posits that the prion protein (PrP) is the only component of the prion; however, additional co-factors are required for its assembly into infectious prions. These can be provided by brain homogenate, but synthetic lipids and non-coding RNA have also been used in vitro. Here, we review a range of experimental approaches, which generate PrP amyloid assemblies de novo. These synthetic PrP assemblies share some, but not necessarily all, properties of genuine infectious prions. We will discuss the different experimental approaches, how a prion is defined, the non-protein requirements of a prion, and provide an overview of the current state of prion amplification and generation in vitro.

Yeast Proteins may Reversibly Aggregate like Amphiphilic Molecules

More than a hundred proteins in yeast reversibly aggregate and phase-separate in response to various stressors, such as nutrient depletion and heat shock. We know little about the protein sequence and structural features behind this ability, which has not been characterized on a proteome-wide level. To identify the distinctive features of aggregation-prone protein regions, we apply machine learning algorithms to genome-scale limited proteolysis-mass spectrometry (LiP-MS) data from yeast proteins. LiP-MS data reveals that 96 proteins show significant structural changes upon heat shock. We find that in these proteins the propensity to phase separate cannot be solely driven by disordered regions, because their aggregation-prone regions (APRs) are not significantly disordered. Instead, the phase separation of these proteins requires contributions from both disordered and structured regions. APRs are significantly enriched in aliphatic residues and depleted in positively charged amino acids. Aggregator proteins with longer APRs show a greater propensity to aggregate, a relationship that can be explained by equilibrium statistical thermodynamics. Altogether, our observations suggest that proteome-wide reversible protein aggregation is mediated by sequence-encoded properties. We propose that aggregating proteins resemble supra-molecular amphiphiles, where APRs are the hydrophobic parts, and non-APRs are the hydrophilic parts.

Asymmetric-flow field-flow fractionation of prions reveals a strain-specific continuum of quaternary structures with protease resistance developing at a hydrodynamic radius of 15 nm

Prion diseases are transmissible neurodegenerative disorders that affect mammals, including humans. The central molecular event is the conversion of cellular prion glycoprotein, PrPC, into a plethora of assemblies, PrPSc, associated with disease. Distinct phenotypes of disease led to the concept of prion strains, which are associated with distinct PrPSc structures. However, the degree to which intra- and inter-strain PrPSc heterogeneity contributes to disease pathogenesis remains unclear. Addressing this question requires the precise isolation and characterization of all PrPSc subpopulations from the prion-infected brains. Until now, this has been challenging. We used asymmetric-flow field-flow fractionation (AF4) to isolate all PrPSc subpopulations from brains of hamsters infected with three prion strains: Hyper (HY) and 263K, which produce almost identical phenotypes, and Drowsy (DY), a strain with a distinct presentation. In-line dynamic and multi-angle light scattering (DLS/MALS) data provided accurate measurements of particle sizes and estimation of the shape and number of PrPSc particles. We found that each strain had a continuum of PrPSc assemblies, with strong correlation between PrPSc quaternary structure and phenotype. HY and 263K were enriched with large, protease-resistant PrPSc aggregates, whereas DY consisted primarily of smaller, more protease-sensitive aggregates. For all strains, a transition from protease-sensitive to protease-resistant PrPSc took place at a hydrodynamic radius (Rh) of 15 nm and was accompanied by a change in glycosylation and seeding activity. Our results show that the combination of AF4 with in-line MALS/DLS is a powerful tool for analyzing PrPSc subpopulations and demonstrate that while PrPSc quaternary structure is a major contributor to PrPSc structural heterogeneity, a fundamental change, likely in secondary/tertiary structure, prevents PrPSc particles from maintaining proteinase K resistance below an Rh of 15 nm, regardless of strain. This results in two biochemically distinctive subpopulations, the proportion, seeding activity, and stability of which correlate with prion strain phenotype.

The Size and Stability of Infectious Prion Aggregates Fluctuate Dynamically during Cellular Uptake and Disaggregation

Prion diseases arise when PrP^Sc, an aggregated, infectious, and insoluble conformer of the normally soluble mammalian prion protein, PrP^C, catalyzes the conversion of PrP^C into more PrP^Sc, which then accumulates in the brain leading to disease. PrP^Sc is the primary, if not sole, component of the infectious prion. Despite the stability and protease insensitivity of PrP^Sc aggregates, they can be degraded after cellular uptake. However, how cells disassemble and degrade PrP^Sc is poorly understood. In this work, we analyzed how the protease sensitivity and size distribution of PrP^Sc aggregates from two different mouse-adapted prion strains, 22L, that can persistently infect cells and 87V, that cannot, changed during cellular uptake. We show that within the first 4 h following uptake large PrP^Sc aggregates from both prion strains become less resistant to digestion by proteinase K (PK) through a mechanism that is dependent upon the acidic environment of endocytic vesicles. We further show that during disassembly, PrP^Sc aggregates from both strains become more resistant to PK digestion through the apparent removal of protease-sensitive PrP^Sc, with PrP^Sc from the 87V strain disassembled more readily than PrP^Sc from the 22L strain. Taken together, our data demonstrate that the sizes and stabilities of PrP^Sc from different prion strains change during cellular uptake and degradation, thereby potentially impacting the ability of prions to infect cells.

Methionine oxidation within the prion protein

Prion diseases are characterized by the self-templated misfolding of the cellular prion protein (PrPC) into infectious aggregates (PrPSc). The detailed molecular basis of the misfolding and aggregation of PrPC remains incompletely understood. It is believed that the transient misfolding of PrPC into partially structured intermediates precedes the formation of insoluble protein aggregates and is a critical component of the prion misfolding pathway. A number of environmental factors have been shown to induce the destabilization of PrPC and promote its initial misfolding. Recently, oxidative stress and reactive oxygen species (ROS) have emerged as one possible mechanism by which the destabilization of PrPC can be induced under physiological conditions. Methionine residues are uniquely vulnerable to oxidation by ROS and the formation of methionine sulfoxides leads to the misfolding and subsequent aggregation of PrPC. Here, we provide a review of the evidence for the oxidation of methionine residues in PrPC and its potential role in the formation of pathogenic prion aggregates.

An astrocyte cell line that differentially propagates murine prions

Prion diseases are fatal infectious neurodegenerative disorders in human and animals caused by misfolding of the cellular prion protein (PrP^C) into the pathological isoform PrP^Sc. Elucidating the molecular and cellular mechanisms underlying prion propagation may help to develop disease interventions. Cell culture systems for prion propagation have greatly advanced molecular insights into prion biology, but translation of in vitro to in vivo findings is often disappointing. A wider range of cell culture systems might help overcome these shortcomings. Here, we describe an immortalized mouse neuronal astrocyte cell line (C8D1A) that can be infected with murine prions. Both PrP^C protein and mRNA levels in astrocytes were comparable with those in neuronal and non-neuronal cell lines permitting persistent prion infection. We challenged astrocytes with three mouse-adapted prion strains (22L, RML, and ME7) and cultured them for six passages. Immunoblotting results revealed that the astrocytes propagated 22L prions well over all six passages, whereas ME7 prions did not replicate, and RML prions replicated only very weakly after five passages. Immunofluorescence analysis indicated similar results for PrP^Sc. Interestingly, when we used prion conversion activity as a readout in real-time quaking-induced conversion assays with RML-infected cell lysates, we observed a strong signal over all six passages, comparable with that for 22L-infected cells. These data indicate that the C8D1A cell line is permissive to prion infection. Moreover, the propagated prions differed in conversion and proteinase K–resistance levels in these astrocytes. We propose that the C8D1A cell line could be used to decipher prion strain biology.

Generation and characterization of two strains of transgene mice expressing chimeric MiniSOG-MusPrP

BACKGROUND

Although the presences of scrapie associated fibril in the brain tissues is a ultrastructural hallmark for prion diseases, the exact morphological structure of prion during the progression of the disease is still unclear. The host prion protein (PrP) is encoded by PrP gene (PRNP) locating on the chromosome 20 in human and the chromosome 2 in mouse. Recently, a novel correlative light and electron microscopy with Mini Singlet Oxygen Generator (miniSOG) was generated. MiniSOG, a small protein of 106 amino acids, can absorb blue light and emit green fluorescence that is detectable under the fluorescence microscope. MiniSOG can also partially catalyze the polymerization of DAB to form black stained structures in the presence of osmium tetroxide, which is able to be observed under transmission electron microscope.

NEW METHODS

Two kinds of miniSOG-PrP expressing recombinant plasmids were generated. Correlative photooxidation and transmission electron microscope were used to detect these plasmids. The plasmids were microinjected into fertilized FVB/NJ eggs and Tg mice expressing miniSOG-PrP fusion proteins were selected after successive bred withPRNP KO Tg mice.

RESULTS

Those two strains of Tg mice, Tg^SOG23 and Tg^231SOG, developed normally and maintained healthy without detectable abnormality after one-year observation. Western blots and immunohistochemical assays with PrP- and miniSOG-specific antibodies confirmed that the chimeric miniSOG-PrP proteins were expressed in the brain tissues of Tg mice. Digital PCR assays proposed that the copy numbers of the inserted external gene in Tg^SOG23 and Tg^231SOG were 2 and 12, respectively.

COMPARISON WITH EXISTING METHOD(S)

Compared with GFP tag miniSOG is significantly smaller, which makes it easy be operated experimentally and possibly has less influence on the biological function of the labeled protein. Additionally, GFP tag is an ideal marker for immunofluorescent assays, but may not be suitable for ultrastructural assays for prion morphology.

CONCLUSION

Those Tg mice may supply novel and useful experimental animals for further study on the potential morphological structure formation and deposits of prion in the brain tissues during prion infection.

Crossing Species Barriers Relies on Structurally Distinct Prion Assemblies and Their Complementation

Prion replication results from the autocatalytic templated assisted conversion of the host-encoded prion protein PrP^C into misfolded, polydisperse PrP^Sc conformers. Structurally distinct PrP^Sc conformers can give rise to multiple prion strains. Within and between prion strains, the biological activity (replicative efficacy and specific infectivity) of PrP^Sc assemblies is size dependent and thus reflects an intrinsic structural heterogeneity. The contribution of such PrP^Sc heterogeneity across species prion adaptation, which is believed to be based on fit adjustment between PrP^Sc template(s) and host PrP^C, has not been explored. To define the structural-to-fitness PrP^Sc landscape, we measured the relative capacity of size-fractionated PrP^Sc assemblies from different prion strains to cross mounting species barriers in transgenic mice expressing foreign PrP^C. In the absence of a transmission barrier, the relative efficacy of the isolated PrP^Sc assemblies to induce the disease is like the efficacy observed in the homotypic context. However, in the presence of a transmission barrier, size fractionation overtly delays and even abrogates prion pathogenesis in both the brain and spleen tissues, independently of the infectivity load of the isolated assemblies. Altering by serial dilution PrP^Sc assembly content of non-fractionated inocula aberrantly reduces their specific infectivity, solely in the presence of a transmission barrier. This suggests that synergy between structurally distinct PrP^Sc assemblies in the inoculum is requested for crossing the species barrier. Our data support a mechanism whereby overcoming prion species barrier requires complementation between structurally distinct PrP^Sc assemblies. This work provides key insight into the "quasispecies" concept applied to prions, which would not necessarily rely on prion substrains as constituent but on structural PrP^Sc heterogeneity within prion population.

Chronic wasting disease (CWD) prion strains evolve via adaptive diversification of conformers in hosts expressing prion protein polymorphisms

Chronic wasting disease (CWD) is caused by an unknown spectrum of prions and has become enzootic in populations of cervid species that express cellular prion protein (PrP^C) molecules varying in amino acid composition. These PrP^C polymorphisms can affect prion transmission, disease progression, neuropathology, and emergence of new prion strains, but the mechanistic steps in prion evolution are not understood. Here, using conformation-dependent immunoassay, conformation stability assay, and protein-misfolding cyclic amplification, we monitored the conformational and phenotypic characteristics of CWD prions passaged through deer and transgenic mice expressing different cervid PrP^C polymorphisms. We observed that transmission through hosts with distinct PrP^C sequences diversifies the PrP^CWD conformations and causes a shift toward oligomers with defined structural organization, replication rate, and host range. When passaged in host environments that restrict prion replication, distinct co-existing PrP^CWD conformers underwent competitive selection, stabilizing a new prion strain. Nonadaptive conformers exhibited unstable replication and accumulated only to low levels. These results suggest a continuously evolving diversity of CWD conformers and imply a critical interplay between CWD prion plasticity and PrP^C polymorphisms during prion strain evolution.

Prion protein PrP nucleic acid binding and mobilization implicates retroelements as the replicative component of transmissible spongiform encephalopathy

The existence of more than 30 strains of transmissible spongiform encephalopathy (TSE) and the paucity of infectivity of purified PrP^Sc, as well as considerations of PrP structure, are inconsistent with the protein-only (prion) theory of TSE. Nucleic acid is a strong contender as a second component. We juxtapose two key findings: (i) PrP is a nucleic-acid-binding antimicrobial protein that is similar to retroviral Gag proteins in its ability to trigger reverse transcription. (ii) Retroelement mobilization is widely seen in TSE disease. Given further evidence that PrP also mediates nucleic acid transport into and out of the cell, a strong case is to be made that a second element – retroelement nucleic acid – bound to PrP constitutes the second component necessary to explain the multiple strains of TSE.

Cellulose ether treatment in vivo generates chronic wasting disease prions with reduced protease resistance and delayed disease progression

Chronic wasting disease (CWD) is a prion disease of free-ranging and farmed cervids that is highly contagious because of extensive prion shedding and prion persistence in the environment. Previously, cellulose ether compounds (CEs) have been shown to significantly extend the survival of mice inoculated with mouse-adapted prion strains. In this study, we used CEs, TC-5RW, and 60SH-50, in vitro and in vivo to assess their efficacy to interfere with CWD prion propagation. In vitro, CEs inhibited CWD prion amplification in a dose-dependent manner. Transgenic mice over-expressing elk PrP^C (tgElk) were injected subcutaneously with a single dose of either of the CEs, followed by intracerebral inoculation with different CWD isolates from white tailed deer, mule deer, or elk. All treated groups showed a prolonged survival of up to more than 30 % when compared to the control group regardless of the CWD isolate used for infection. The extended survival in the treated groups correlated with reduced proteinase K resistance of prions. Remarkably, passage of brain homogenates from treated or untreated animals in tgElk mice resulted in a prolonged life span of mice inoculated with homogenates from CE-treated mice (of + 17%) even in the absence of further treatment. Besides the delayed disease onset upon passage in TgElk mice, the reduced proteinase K resistance was maintained but less pronounced. Therefore, these compounds can be very useful in limiting the spread of CWD in captive and wild-ranging cervids.

Early stage prion assembly involves two subpopulations with different quaternary structures and a secondary templating pathway

The dynamics of aggregation and structural diversification of misfolded, host-encoded proteins in neurodegenerative diseases are poorly understood. In many of these disorders, including Alzheimer's, Parkinson's and prion diseases, the misfolded proteins are self-organized into conformationally distinct assemblies or strains. The existence of intrastrain structural heterogeneity is increasingly recognized. However, the underlying processes of emergence and coevolution of structurally distinct assemblies are not mechanistically understood. Here, we show that early prion replication generates two subsets of structurally different assemblies by two sequential processes of formation, regardless of the strain considered. The first process corresponds to a quaternary structural convergence, by reducing the parental strain polydispersity to generate small oligomers. The second process transforms these oligomers into larger ones, by a secondary autocatalytic templating pathway requiring the prion protein. This pathway provides mechanistic insights into prion structural diversification, a key determinant for prion adaptation and toxicity.

Enhancement of binding avidity by bivalent binding enables PrPSc-specific detection by anti-PrP monoclonal antibody 132

Anti-prion protein (PrP) monoclonal antibody 132, which recognizes mouse PrP amino acids 119–127, enables us to reliably detect abnormal isoform prion protein (PrP^Sc) in cells or frozen tissue sections by immunofluorescence assay, although treatment with guanidinium salts is a prerequisite. Despite the benefit of this mAb, the mechanism of PrP^Sc-specific detection remains unclear. Therefore, to address this mechanism, we analyzed the reactivities of mono- and bivalent mAb 132 to recombinant mouse PrP (rMoPrP) by enzyme-linked immunosorbent assay (ELISA) and surface plasmon resonance (SPR). In ELISA, binding of the monovalent form was significantly weaker than that of the bivalent form, indicating that bivalent binding confers a higher binding stability to mAb 132. Compared with other anti-PrP mAbs tested, the reactivity of bivalent mAb 132 was easily affected by a decrease in antigen concentration. The binding kinetics of mAb 132 assessed by SPR were consistent with the results of ELISA. The dissociation constant of the monovalent form was approximately 260 times higher than that of the bivalent form, suggesting that monovalent binding is less stable than bivalent binding. Furthermore, the amount of mAb 132 that bound to rMoPrP decreased if the antigen density was too low to allow bivalent binding. If two cellular PrP (PrP^C) are close enough to allow bivalent binding, mAb 132 binds to PrP^C. These results indicate that weak monovalent binding to monomeric PrP^C diminishes PrP^C signals to background level, whereas after exposure of the epitope, mAb 132 binds stably to oligomeric PrP^Sc in a bivalent manner.

Heterogeneity and Architecture of Pathological Prion Protein Assemblies: Time to Revisit the Molecular Basis of the Prion Replication Process?

Prions are proteinaceous infectious agents responsible for a range of neurodegenerative diseases in animals and humans. Prion particles are assemblies formed from a misfolded, β-sheet rich, aggregation-prone isoform (PrP^Sc) of the host-encoded cellular prion protein (PrP^C). Prions replicate by recruiting and converting PrP^C into PrP^Sc, by an autocatalytic process. PrP^Sc is a pleiomorphic protein as different conformations can dictate different disease phenotypes in the same host species. This is the basis of the strain phenomenon in prion diseases. Recent experimental evidence suggests further structural heterogeneity in PrP^Sc assemblies within specific prion populations and strains. Still, this diversity is rather seen as a size continuum of assemblies with the same core structure, while analysis of the available experimental data points to the existence of structurally distinct arrangements. The atomic structure of PrP^Sc has not been elucidated so far, making the prion replication process difficult to understand. All currently available models suggest that PrP^Sc assemblies exhibit a PrP^Sc subunit as core constituent, which was recently identified. This review summarizes our current knowledge on prion assembly heterogeneity down to the subunit level and will discuss its importance with regard to the current molecular principles of the prion replication process.

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.

Modulation of Disordered Proteins with a Focus on Neurodegenerative Diseases and Other Pathologies

Intrinsically disordered proteins (IDPs) do not have rigid 3D structures, showing changes in their folding depending on the environment or ligands. Intrinsically disordered proteins are widely spread in eukaryotic genomes, and these proteins participate in many cell regulatory metabolism processes. Some IDPs, when aberrantly folded, can be the cause of some diseases such as Alzheimer′s, Parkinson′s, and prionic, among others. In these diseases, there are modifications in parts of the protein or in its entirety. A common conformational variation of these IDPs is misfolding and aggregation, forming, for instance, neurotoxic amyloid plaques. In this review, we discuss some IDPs that are involved in neurodegenerative diseases (such as beta amyloid, alpha synuclein, tau, and the “IDP-like” PrP), cancer (p53, c-Myc), and diabetes (amylin), focusing on the structural changes of these IDPs that are linked to such pathologies. We also present the IDP modulation mechanisms that can be explored in new strategies for drug design. Lastly, we show some candidate drugs that can be used in the future for the treatment of diseases caused by misfolded IDPs, considering that cancer therapy has more advanced research in comparison to other diseases, while also discussing recent and future developments in this area of research. Therefore, we aim to provide support to the study of IDPs and their modulation mechanisms as promising approaches to combat such severe diseases.

Recent advances in the histo-molecular pathology of human prion disease

Prion diseases are progressive neurodegenerative disorders affecting humans and other mammalian species. The term prion, originally put forward to propose the concept that a protein could be infectious, refers to PrP^Sc , a misfolded isoform of the cellular prion protein (PrP^C ) that represents the pathogenetic hallmark of these disorders. The discovery that other proteins characterized by misfolding and seeded aggregation can spread from cell to cell, similarly to PrP^Sc , has increased interest in prion diseases. Among neurodegenerative disorders, however, prion diseases distinguish themselves for the broader phenotypic spectrum, the fastest disease progression and the existence of infectious forms that can be transmitted through the exposure to diseased tissues via ingestion, injection or transplantation. The main clinicopathological phenotypes of human prion disease include Creutzfeldt-Jakob disease, by far the most common, fatal insomnia, variably protease-sensitive prionopathy, and Gerstmann-Sträussler-Scheinker disease. However, clinicopathological manifestations extend even beyond those predicted by this classification. Because of their transmissibility, the phenotypic diversity of prion diseases can also be propagated into syngenic hosts as prion strains with distinct characteristics, such as incubation period, pattern of PrP^Sc distribution and regional severity of histopathological changes in the brain. Increasing evidence indicates that different PrP^Sc conformers, forming distinct ordered aggregates, encipher the phenotypic variants related to prion strains. In this review, we summarize the most recent advances concerning the histo-molecular pathology of human prion disease focusing on the phenotypic spectrum of the disease including co-pathologies, the characterization of prion strains by experimental transmission and their correlation with the physicochemical properties of PrP^Sc aggregates.

Ganglioside Synthase Knockout Reduces Prion Disease Incubation Time in Mouse Models

Localization of the abnormal and normal isoforms of prion proteins to detergent-resistant membrane microdomains, lipid rafts, is important for the conformational conversion. Lipid rafts are enriched in sialic acid-containing glycosphingolipids (namely, gangliosides). Alteration in the ganglioside composition of lipid rafts can affect the localization of lipid raft-associated proteins. To investigate the role of gangliosides in the pathogenesis of prion diseases, we performed intracerebral transmission study of a scrapie prion strain Chandler and a Gerstmann-Sträussler-Scheinker syndrome prion strain Fukuoka-1 using various knockout mouse strains ablated with ganglioside synthase gene (ie, GD2/GM2 synthase, GD3 synthase, or GM3 synthase). After challenge with the Chandler strain, GD2/GM2 synthase knockout mice showed 20% reduction of incubation time, reduced prion protein deposition in the brain with attenuated glial reactions, and reduced localization of prion proteins to lipid rafts. These results raise the possibility that the gangliosides may have an important role in prion disease pathogenesis by affecting the localization of prion proteins to lipid rafts.

Direct Observation of Murine Prion Protein Replication in Vitro

Prions are believed to propagate when an assembly of prion protein (PrP) enters a cell and replicates to produce two or more fibrils, leading to an exponential increase in PrP aggregate number with time. However, the molecular basis of this process has not yet been established in detail. Here, we use single-aggregate imaging to study fibril fragmentation and elongation of individual murine PrP aggregates from seeded aggregation in vitro. We found that PrP elongation occurs via a structural conversion from a PK-sensitive to PK-resistant conformer. Fibril fragmentation was found to be length-dependent and resulted in the formation of PK-sensitive fragments. Measurement of the rate constants for these processes also allowed us to predict a simple spreading model for aggregate propagation through the brain, assuming that doubling of the aggregate number is rate-limiting. In contrast, while α-synuclein aggregated by the same mechanism, it showed significantly slower elongation and fragmentation rate constants than PrP, leading to much slower replication rate. Overall, our study shows that fibril elongation with fragmentation are key molecular processes in PrP and α-synuclein aggregate replication, an important concept in prion biology, and also establishes a simple framework to start to determine the main factors that control the rate of prion and prion-like spreading in animals.

Sporadic Creutzfeldt-Jakob disease

Sporadic Creutzfeldt-Jakob disease (CJD), the most common human prion disease, is generally regarded as a spontaneous neurodegenerative illness, arising either from a spontaneous PRNP somatic mutation or a stochastic PrP structural change. Alternatively, the possibility of an infection from animals or other source remains to be completely ruled out. Sporadic CJD is clinically characterized by rapidly progressive dementia with ataxia, myoclonus, or other neurologic signs and, neuropathologically, by the presence of aggregates of abnormal prion protein, spongiform change, neuronal loss, and gliosis. Despite these common features the disease shows a wide phenotypic variability which was recognized since its early descriptions. In the late 1990s the identification of key molecular determinants of phenotypic expression and the availability of a large series of neuropathologically verified cases led to the characterization of definite clinicopathologic and molecular disease subtypes and to an internationally recognized disease classification. By showing that these disease subtypes correspond to specific agent strain-host genotype combinations, recent transmission studies have confirmed the biologic basis of this classification. The introduction of brain magnetic resonance imaging techniques such as fluid-attenuated inversion recovery and diffusion-weighted imaging sequences and cerebrospinal fluid biomarker assays for the detection of brain-derived proteins as well as real-time quaking-induced conversion assay, allowing the specific detection of prions in accessible biologic fluids and tissues, has significantly contributed to the improved accuracy of the clinical diagnosis of sporadic CJD in recent years.

Overexpression of quality control proteins reduces prion conversion in prion-infected cells

Prion diseases are fatal infectious neurodegenerative disorders in humans and other animals and are caused by misfolding of the cellular prion protein (PrP^C) into the pathological isoform PrP^Sc. These diseases have the potential to transmit within or between species, including zoonotic transmission to humans. Elucidating the molecular and cellular mechanisms underlying prion propagation and transmission is therefore critical for developing molecular strategies for disease intervention. We have shown previously that impaired quality control mechanisms directly influence prion propagation. In this study, we manipulated cellular quality control pathways in vitro by stably and transiently overexpressing selected quality control folding (ERp57) and cargo (VIP36) proteins and investigated the effects of this overexpression on prion propagation. We found that ERp57 or VIP36 overexpression in persistently prion-infected neuroblastoma cells significantly reduces the amount of PrP^Sc in immunoblots and prion-seeding activity in the real-time quaking-induced conversion (RT-QuIC) assay. Using different cell lines infected with various prion strains confirmed that this effect is not cell type– or prion strain–specific. Moreover, de novo prion infection revealed that the overexpression significantly reduced newly formed PrP^Sc in acutely infected cells. ERp57-overexpressing cells significantly overcame endoplasmic reticulum stress, as revealed by expression of lower levels of the stress markers BiP and CHOP, accompanied by a decrease in PrP aggregates. Furthermore, application of ERp57-expressing lentiviruses prolonged the survival of prion-infected mice. Taken together, improved cellular quality control via ERp57 or VIP36 overexpression impairs prion propagation and could be utilized as a potential therapeutic strategy.

Anti-prion Protein Antibody 6D11 Restores Cellular Proteostasis of Prion Protein Through Disrupting Recycling Propagation of PrPSc and Targeting PrPSc for Lysosomal Degradation

PrP^Sc is an infectious and disease-specific conformer of the prion protein, which accumulation in the CNS underlies the pathology of prion diseases. PrP^Sc replicates by binding to the cellular conformer of the prion protein (PrP^C) expressed by host cells and rendering its secondary structure a likeness of itself. PrP^C is a plasma membrane anchored protein, which constitutively recirculates between the cell surface and the endocytic compartment. Since PrP^Sc engages PrP^C along this trafficking pathway, its replication process is often referred to as "recycling propagation." Certain monoclonal antibodies (mAbs) directed against prion protein can abrogate the presence of PrP^Sc from prion-infected cells. However, the precise mechanism(s) underlying their therapeutic propensities remains obscure. Using N2A murine neuroblastoma cell line stably infected with 22L mouse-adapted scrapie strain (N2A/22L), we investigated here the modus operandi of the 6D11 clone, which was raised against the PrP^Sc conformer and has been shown to permanently clear prion-infected cells from PrP^Sc presence. We determined that 6D11 mAb engages and sequesters PrP^C and PrP^Sc at the cell surface. PrP^C/6D11 and PrP^Sc/6D11 complexes are then endocytosed from the plasma membrane and are directed to lysosomes, therefore precluding recirculation of nascent PrP^Sc back to the cell surface. Targeting PrP^Sc by 6D11 mAb to the lysosomal compartment facilitates its proteolysis and eventually shifts the balance between PrP^Sc formation and degradation. Ongoing translation of PrP^C allows maintaining the steady-state level of prion protein within the cells, which was not depleted under 6D11 mAb treatment. Our findings demonstrate that through disrupting recycling propagation of PrP^Sc and promoting its degradation, 6D11 mAb restores cellular proteostasis of prion protein.

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.

Establishment of a simple cell-based ELISA for the direct detection of abnormal isoform of prion protein from prion-infected cells without cell lysis and proteinase K treatment

Prion-infected cells have been used for analyzing the effect of compounds on the formation of abnormal isoform of prion protein (PrP(Sc)). PrP(Sc) is usually detected using anti-prion protein (PrP) antibodies after the removal of the cellular isoform of prion protein (PrP(C)) by proteinase K (PK) treatment. However, it is expected that the PK-sensitive PrP(Sc) (PrP(Sc)-sen), which possesses higher infectivity and conversion activity than the PK-resistant PrP(Sc) (PrP(Sc)-res), is also digested through PK treatment. To overcome this problem, we established a novel cell-based ELISA in which PrP(Sc) can be directly detected from cells persistently infected with prions using anti-PrP monoclonal antibody (mAb) 132 that recognizes epitope consisting of mouse PrP amino acids 119-127. The novel cell-based ELISA could distinguish prion-infected cells from prion-uninfected cells without cell lysis and PK treatment. MAb 132 could detect both PrP(Sc)-sen and PrP(Sc)-res even if all PrP(Sc) molecules were not detected. The analytical dynamic range for PrP(Sc) detection was approximately 1 log. The coefficient of variation and signal-to-background ratio were 7%-11% and 2.5-3.3, respectively, demonstrating the reproducibility of this assay. The addition of a cytotoxicity assay immediately before PrP(Sc) detection did not affect the following PrP(Sc) detection. Thus, all the procedures including cell culture, cytotoxicity assay, and PrP(Sc) detection were completed in the same plate. The simplicity and non-requirement for cell lysis or PK treatment are advantages for the high throughput screening of anti-prion compounds.

Experimental Transmission of the Chronic Wasting Disease Agent to Swine after Oral or Intracranial Inoculation

Chronic wasting disease (CWD) is a naturally occurring, fatal neurodegenerative disease of cervids. The potential for swine to serve as hosts for the agent of CWD is unknown. The purpose of this study was to investigate the susceptibility of swine to the CWD agent following experimental oral or intracranial inoculation. Crossbred piglets were assigned to three groups, intracranially inoculated (n = 20), orally inoculated (n = 19), and noninoculated (n = 9). At approximately the age at which commercial pigs reach market weight, half of the pigs in each group were culled ("market weight" groups). The remaining pigs ("aged" groups) were allowed to incubate for up to 73 months postinoculation (mpi). Tissues collected at necropsy were examined for disease-associated prion protein (PrPSc) by Western blotting (WB), antigen capture enzyme immunoassay (EIA), immunohistochemistry (IHC), and in vitro real-time quaking-induced conversion (RT-QuIC). Brain samples from selected pigs were also bioassayed in mice expressing porcine prion protein. Four intracranially inoculated aged pigs and one orally inoculated aged pig were positive by EIA, IHC, and/or WB. By RT-QuIC, PrPSc was detected in lymphoid and/or brain tissue from one or more pigs in each inoculated group. The bioassay was positive in four out of five pigs assayed. This study demonstrates that pigs can support low-level amplification of CWD prions, although the species barrier to CWD infection is relatively high. However, detection of infectivity in orally inoculated pigs with a mouse bioassay raises the possibility that naturally exposed pigs could act as a reservoir of CWD infectivity.IMPORTANCE We challenged domestic swine with the chronic wasting disease agent by inoculation directly into the brain (intracranially) or by oral gavage (orally). Disease-associated prion protein (PrPSc) was detected in brain and lymphoid tissues from intracranially and orally inoculated pigs as early as 8 months of age (6 months postinoculation). Only one pig developed clinical neurologic signs suggestive of prion disease. The amount of PrPSc in the brains and lymphoid tissues of positive pigs was small, especially in orally inoculated pigs. Regardless, positive results obtained with orally inoculated pigs suggest that it may be possible for swine to serve as a reservoir for prion disease under natural conditions.

Biochemical Characterization of Prions

Prion disease or transmissible spongiform encephalopathies are characterized by the presence of the abnormal form of the prion protein (PrP^Sc). The pathological and transmissible properties of PrP^Sc are enciphered in its secondary and tertiary structures. Since it's well established that different strains of prions are linked to different conformations of PrP^Sc, biochemical characterization of prions seems a preliminary but reliable approach to detect, analyze, and compare prion strains. Experimental biochemical procedures might be helpful in distinguishing PrP^Sc physicochemical properties and include resistance to proteinase K (PK) digestion, insolubility in nonionic detergents, PK-resistance under denaturing conditions and sedimentation properties in sucrose gradients. This biochemical approach has been extensively applied in human prion disorders and subsequently expanded for PrP^Sc characterization in animals. In particular, in sporadic Creutzfedlt-Jakob disease (sCJD) PrP^Sc is characterized by two main glycotypes conventionally named Type 1 and Type 2, based on the apparent gel migration at 21 and 19kDa of the PrP^Sc PK-resistant fragment. An additional PrP^Sc type was identified in sCJD characterized by an unglycosylated dominant glycoform pattern and in 2010 a variably protease-sensitive prionopathy (VPSPr) was reported showing a PrP^Sc with an electrophoretic ladder like pattern. Additionally, the presence of PrP^Sc truncated fragments completes the electrophoretic characterization of different prion strains. By two-dimensional (2D) electrophoretic analysis additional PrP^Sc pattern was identified, since this procedure provides information about the isoelectric point and the different peptides length related to PK cleavage, as well as to glycosylation extent or GPI anchor presence. We here provide and extensive review on PrP^Sc biochemical analysis in human and animal prion disorders. Further, we show that PrP^Sc glycotypes observed in CJD share similarities with PrP^Sc in bovine spongiform encephalopathy forms (BSE).

Characterization of physiochemical properties of caveolin-1 from normal and prion-infected human brains

Caveolin-1 is a major component protein of the caveolae—a type of flask shaped, 50-100 nm, nonclathrin-coated, microdomain present in the plasma membrane of most mammalian cells. Caveolin-1 functions as a scaffolding protein to organize and concentrate signaling molecules within the caveolae, which may be associated with its unique physicochemical properties including oligomerization, acquisition of detergent insolubility, and association with cholesterol. Here we demonstrate that caveolin-1 is detected in all brain areas examined and recovered in both detergent-soluble and -insoluble fractions. Surprisingly, the recovered molecules from the two different fractions share a similar molecular size ranging from 200 to 2,000 kDa, indicated by gel filtration. Furthermore, both soluble and insoluble caveolin-1 molecules generate a proteinase K (PK)-resistant C-terminal core fragment upon the PK-treatment, by removing ˜36 amino acids from the N-terminus of the protein. Although it recognizes caveolin-1 from A431 cell lysate, an antibody against the C-terminus of caveolin-1 fails to detect the brain protein by Western blotting, suggesting that the epitope in the brain caveolin-1 is concealed. No significant differences in the physicochemical properties of caveolin-1 between uninfected and prion-infected brains are observed.

Aβ seeds and prions: How close the fit?

The prion paradigm is increasingly invoked to explain the molecular pathogenesis of neurodegenerative diseases involving the misfolding and aggregation of proteins other than the prion protein (PrP). Extensive evidence from in vitro and in vivo studies indicates that misfolded and aggregated Aβ peptide, which is the probable molecular trigger for Alzheimer's disease, manifests all of the key characteristics of canonical mammalian prions. These features include a β-sheet rich architecture, tendency to polymerize into amyloid, templated corruption of like protein molecules, ability to form structurally and functionally variant strains, systematic spread by neuronal transport, and resistance to inactivation by heat and formaldehyde. In addition to Aβ, a growing body of research supports the view that the prion-like molecular transformation of specific proteins drives the onset and course of a remarkable variety of clinicopathologically diverse diseases. As such, the expanded prion paradigm could conceptually unify fundamental and translational investigations of these disorders.

Prion Strain Characterization of a Novel Subtype of Creutzfeldt-Jakob Disease

In 2007, we reported a patient with an atypical form of Creutzfeldt-Jakob disease (CJD) heterozygous for methionine-valine (MV) at codon 129 who showed a novel pathological prion protein (PrP^TSE) conformation with an atypical glycoform (AG) profile and intraneuronal PrP deposition. In the present study, we further characterize the conformational properties of this pathological prion protein (PrP^TSE MV^AG), showing that PrP^TSE MV^AG is composed of multiple conformers with biochemical properties distinct from those of PrP^TSE type 1 and type 2 of MV sporadic CJD (sCJD). Experimental transmission of CJD-MV^AG to bank voles and gene-targeted transgenic mice carrying the human prion protein gene (TgHu mice) showed unique transmission rates, survival times, neuropathological changes, PrP^TSE deposition patterns, and PrP^TSE glycotypes that are distinct from those of sCJD-MV1 and sCJD-MV2. These biochemical and experimental data suggest the presence of a novel prion strain in CJD-MV^AGIMPORTANCE Sporadic Creutzfeldt-Jakob disease is caused by the misfolding of the cellular prion protein, which assumes two different major conformations (type 1 and type 2) and, together with the methionine/valine polymorphic codon 129 of the prion protein gene, contribute to the occurrence of distinct clinical-pathological phenotypes. Inoculation in laboratory rodents of brain tissues from the six possible combinations of pathological prion protein types with codon 129 genotypes results in the identification of 3 or 4 strains of prions. We report on the identification of a novel strain of Creutzfeldt-Jakob disease isolated from a patient who carried an abnormally glycosylated pathological prion protein. This novel strain has unique biochemical characteristics, does not transmit to humanized transgenic mice, and shows exclusive transmission properties in bank voles. The identification of a novel human prion strain improves our understanding of the pathogenesis of the disease and of possible mechanisms of prion transmission.

Seeding and transgenic overexpression of alpha-synuclein triggers dendritic spine pathology in the neocortex

Although misfolded and aggregated α-synuclein (α-syn) is recognized in the disease progression of synucleinopathies, its role in the impairment of cortical circuitries and synaptic plasticity remains incompletely understood. We investigated how α-synuclein accumulation affects synaptic plasticity in the mouse somatosensory cortex using two distinct approaches. Long-term in vivo imaging of apical dendrites was performed in mice overexpressing wild-type human α-synuclein. Additionally, intracranial injection of preformed α-synuclein fibrils was performed to induce cortical α-syn pathology. We find that α-synuclein overexpressing mice show decreased spine density and abnormalities in spine dynamics in an age-dependent manner. We also provide evidence for the detrimental effects of seeded α-synuclein aggregates on dendritic architecture. We observed spine loss as well as dystrophic deformation of dendritic shafts in layer V pyramidal neurons. Our results provide a link to the pathophysiology underlying dementia associated with synucleinopathies and may enable the evaluation of potential drug candidates on dendritic spine pathology in vivo.

Protease resistance of infectious prions is suppressed by removal of a single atom in the cellular prion protein

Resistance to proteolytic digestion has long been considered a defining trait of prions in tissues of organisms suffering from transmissible spongiform encephalopathies. Detection of proteinase K-resistant prion protein (PrP^Sc) still represents the diagnostic gold standard for prion diseases in humans, sheep and cattle. However, it has become increasingly apparent that the accumulation of PrP^Sc does not always accompany prion infections: high titers of prion infectivity can be reached also in the absence of protease resistant PrP^Sc. Here, we describe a structural basis for the phenomenon of protease-sensitive prion infectivity. We studied the effect on proteinase K (PK) resistance of the amino acid substitution Y169F, which removes a single oxygen atom from the β2–α2 loop of the cellular prion protein (PrP^C). When infected with RML or the 263K strain of prions, transgenic mice lacking wild-type (wt) PrP^C but expressing MoPrP^169F generated prion infectivity at levels comparable to wt mice. The newly generated MoPrP^169F prions were biologically indistinguishable from those recovered from prion-infected wt mice, and elicited similar pathologies in vivo. Surprisingly, MoPrP^169F prions showed greatly reduced PK resistance and density gradient analyses showed a significant reduction in high-density aggregates. Passage of MoPrP^169F prions into mice expressing wt MoPrP led to full recovery of protease resistance, indicating that no strain shift had taken place. We conclude that a subtle structural variation in the β2–α2 loop of PrP^C affects the sensitivity of PrP^Sc to protease but does not impact prion replication and infectivity. With these findings a specific structural feature of PrP^C can be linked to a physicochemical property of the corresponding PrP^Sc.

PrP Knockout Cells Expressing Transmembrane PrP Resist Prion Infection

Glycosylphosphatidylinositol (GPI) anchoring of the prion protein (PrP^C) influences PrP^C misfolding into the disease-associated isoform, PrP^res, as well as prion propagation and infectivity. GPI proteins are found in cholesterol- and sphingolipid-rich membrane regions called rafts. Exchanging the GPI anchor for a nonraft transmembrane sequence redirects PrP^C away from rafts. Previous studies showed that nonraft transmembrane PrP^C variants resist conversion to PrP^res when transfected into scrapie-infected N2a neuroblastoma cells, likely due to segregation of transmembrane PrP^C and GPI-anchored PrP^res in distinct membrane environments. Thus, it remained unclear whether transmembrane PrP^C might convert to PrP^res if seeded by an exogenous source of PrP^res not associated with host cell rafts and without the potential influence of endogenous expression of GPI-anchored PrP^C To further explore these questions, constructs containing either a C-terminal wild-type GPI anchor signal sequence or a nonraft transmembrane sequence containing a flexible linker were expressed in a cell line derived from PrP knockout hippocampal neurons, NpL2. NpL2 cells have physiological similarities to primary neurons, representing a novel and advantageous model for studying transmissible spongiform encephalopathy (TSE) infection. Cells were infected with inocula from multiple prion strains and in different biochemical states (i.e., membrane bound as in brain microsomes from wild-type mice or purified GPI-anchorless amyloid fibrils). Only GPI-anchored PrP^C supported persistent PrP^res propagation. Our data provide strong evidence that in cell culture GPI anchor-directed membrane association of PrP^C is required for persistent PrP^res propagation, implicating raft microdomains as a location for conversion.

IMPORTANCE

Mechanisms of prion propagation, and what makes them transmissible, are poorly understood. Glycosylphosphatidylinositol (GPI) membrane anchoring of the prion protein (PrP^C) directs it to specific regions of cell membranes called rafts. In order to test the importance of the raft environment on prion propagation, we developed a novel model for prion infection where cells expressing either GPI-anchored PrP^C or transmembrane-anchored PrP^C, which partitions it to a different location, were treated with infectious, misfolded forms of the prion protein, PrP^res We show that only GPI-anchored PrP^C was able to convert to PrP^res and able to serially propagate. The results strongly suggest that GPI anchoring and the localization of PrP^C to rafts are crucial to the ability of PrP^C to propagate as a prion.

Insights into Mechanisms of Transmission and Pathogenesis from Transgenic Mouse Models of Prion Diseases

Prions represent a new paradigm of protein-mediated information transfer. In the case of mammals, prions are the cause of fatal, transmissible neurodegenerative diseases, sometimes referred to as transmissible spongiform encephalopathies (TSEs), which frequently occur as epidemics. An increasing body of evidence indicates that the canonical mechanism of conformational corruption of cellular prion protein (PrP^C) by the pathogenic isoform (PrP^Sc) that is the basis of prion formation in TSEs is common to a spectrum of proteins associated with various additional human neurodegenerative disorders, including the more common Alzheimer's and Parkinson's diseases. The peerless infectious properties of TSE prions, and the unparalleled tools for their study, therefore enable elucidation of mechanisms of template-mediated conformational propagation that are generally applicable to these related disease states. Many unresolved issues remain including the exact molecular nature of the prion, the detailed cellular and molecular mechanisms of prion propagation, and the means by which prion diseases can be both genetic and infectious. In addition, we know little about the mechanism by which neurons degenerate during prion diseases. Tied to this, the physiological role of the normal form of the prion protein remains unclear and it is uncertain whether or not loss of this function contributes to prion pathogenesis. The factors governing the transmission of prions between species remain unclear, in particular the means by which prion strains and PrP primary structure interact to affect interspecies prion transmission. Despite all these unknowns, advances in our understanding of prions have occurred because of their transmissibility to experimental animals, and the development of transgenic (Tg) mouse models has done much to further our understanding about various aspects of prion biology. In this review, we will focus on advances in our understanding of prion biology that occurred in the past 8 years since our last review of this topic.

Prions: Beyond a Single Protein

Since the term protein was first coined in 1838 and protein was discovered to be the essential component of fibrin and albumin, all cellular proteins were presumed to play beneficial roles in plants and mammals. However, in 1967, Griffith proposed that proteins could be infectious pathogens and postulated their involvement in scrapie, a universally fatal transmissible spongiform encephalopathy in goats and sheep. Nevertheless, this novel hypothesis had not been evidenced until 1982, when Prusiner and coworkers purified infectious particles from scrapie-infected hamster brains and demonstrated that they consisted of a specific protein that he called a "prion." Unprecedentedly, the infectious prion pathogen is actually derived from its endogenous cellular form in the central nervous system. Unlike other infectious agents, such as bacteria, viruses, and fungi, prions do not contain genetic materials such as DNA or RNA. The unique traits and genetic information of prions are believed to be encoded within the conformational structure and posttranslational modifications of the proteins. Remarkably, prion-like behavior has been recently observed in other cellular proteins-not only in pathogenic roles but also serving physiological functions. The significance of these fascinating developments in prion biology is far beyond the scope of a single cellular protein and its related disease.

The Structure of Mammalian Prions and Their Aggregates

Prion diseases, such as Creutzfeldt-Jakob disease in humans, bovine spongiform encephalopathy in cattle, chronic wasting disease in cervids (i.e., deer, elk, moose, and reindeer), and sheep scrapie, are caused by the misfolding of the cellular prion protein (PrP^C) into a disease-causing conformer (PrP^Sc). PrP^C is a normal, GPI-anchored protein that is expressed on the surface of neurons and other cell types. The structure of PrP^C is well understood, based on studies of recombinant PrP, which closely mimics the structure of native PrP^C. In contrast, PrP^Sc is prone to aggregate into a variety of quaternary structures, such as oligomers, amorphous aggregates, and amyloid fibrils. The propensity of PrP^Sc to assemble into these diverse forms of aggregates is also responsible for our limited knowledge about its structure. Then again, the repeating nature of certain regular PrP^Sc aggregates has allowed (lower resolution) insights into the structure of the infectious conformer, establishing a four-rung β-solenoid structure as a key element of its architecture.

Infectivity-associated PrP(Sc) and disease duration-associated PrP(Sc) of mouse BSE prions

Disease-related prion protein (PrP^Sc), which is a structural isoform of the host-encoded cellular prion protein, is thought to be a causative agent of transmissible spongiform encephalopathies. However, the specific role of PrP^Sc in prion pathogenesis and its relationship to infectivity remain controversial. A time-course study of prion-affected mice was conducted, which showed that the prion infectivity was not simply proportional to the amount of PrP^Sc in the brain. Centrifugation (20,000 ×g) of the brain homogenate showed that most of the PrP^Sc was precipitated into the pellet, and the supernatant contained only a slight amount of PrP^Sc. Interestingly, mice inoculated with the obtained supernatant showed incubation periods that were approximately 15 d longer than those of mice inoculated with the crude homogenate even though both inocula contained almost the same infectivity. Our results suggest that a small population of fine PrP^Sc may be responsible for prion infectivity and that large, aggregated PrP^Sc may contribute to determining prion disease duration.

The Prion-Like Properties of Amyloid-β Assemblies: Implications for Alzheimer's Disease

Since the discovery that prion diseases can be transmitted to experimental animals by inoculation with afflicted brain matter, researchers have speculated that the brains of patients suffering from other neurodegenerative diseases might also harbor causative agents with transmissible properties. Foremost among these disorders is Alzheimer's disease (AD), the most common cause of dementia in the elderly. A growing body of research supports the concept that the pathogenesis of AD is initiated and sustained by the endogenous, seeded misfolding and aggregation of the protein fragment amyloid-β (Aβ). At the molecular level, this mechanism of nucleated protein self-assembly is virtually identical to that of prions consisting of the prion protein (PrP). The formation, propagation, and spread of Aβ seeds within the brain can thus be considered a fundamental feature of AD pathogenesis.

Analysis of Conformational Stability of Abnormal Prion Protein Aggregates across the Spectrum of Creutzfeldt-Jakob Disease Prions

The wide phenotypic variability of prion diseases is thought to depend on the interaction of a host genotype with prion strains that have self-perpetuating biological properties enciphered in distinct conformations of the misfolded prion protein PrP(Sc) This concept is largely based on indirect approaches studying the effect of proteases or denaturing agents on the physicochemical properties of PrP(Sc) aggregates. Furthermore, most data come from studies on rodent-adapted prion strains, making current understanding of the molecular basis of strains and phenotypic variability in naturally occurring diseases, especially in humans, more limited. To fill this gap, we studied the effects of guanidine hydrochloride (GdnHCl) and heating on PrP(Sc) aggregates extracted from 60 sporadic Creutzfeldt-Jakob disease (CJD) and 6 variant CJD brains. While denaturation curves obtained after exposure of PrP(Sc) to increasing GdnHCl concentrations showed similar profiles among the 7 CJD types analyzed, PrP(Sc) exposure to increasing temperature revealed significantly different and type-specific responses. In particular, MM1 and VV2, the most prevalent and fast-replicating CJD types, showed stable and highly resistant PrP(Sc) aggregates, whereas VV1, a rare and slowly propagating type, revealed unstable aggregates that easily dissolved at low temperature. Taken together, our results indicate that the molecular interactions mediating the aggregation state of PrP(Sc), possibly enciphering strain diversity, are differently targeted by GdnHCl, temperature, and proteases. Furthermore, the detected positive correlation between the thermostability of PrP(Sc) aggregates and disease transmission efficiency makes inconsistent the proposed hypothesis that a decrease in conformational stability of prions results in an increase in their replication efficiency.

IMPORTANCE

Prion strains are defined as infectious isolates propagating distinctive phenotypic traits after transmission to syngeneic hosts. Although the molecular basis of prion strains is not fully understood, it is largely accepted that variations in prion protein conformation drive the molecular changes leading to the different phenotypes. In this study, we exposed abnormal prion protein aggregates encompassing the spectrum of human prion strains to both guanidine hydrochloride and thermal unfolding. Remarkably, while exposure to increasing temperature revealed significant strain-specific differences in the denaturation profile of the protein, treatment with guanidine hydrochloride did not. The findings suggest that thermal and chemical denaturation perturb the structure of prion protein aggregates differently. Moreover, since the most thermostable prion protein types were those associated with the most prevalent phenotypes and most rapidly and efficiently transmitting strains, the results suggest a direct correlation between strain replication efficiency and the thermostability of prion protein aggregates.

PrPSc-Specific Antibody Reveals C-Terminal Conformational Differences between Prion Strains

Understanding the structure of PrP(Sc) and its strain variation has been one of the major challenges in prion disease biology. To study the strain-dependent conformations of PrP(Sc), we purified proteinase-resistant PrP(Sc) (PrP(RES)) from mouse brains with three different murine-adapted scrapie strains (Chandler, 22L, and Me7) and systematically tested the accessibility of epitopes of a wide range of anti-PrP and anti-PrP(Sc) specific antibodies by indirect enzyme-linked immunosorbent assay (ELISA). We found that epitopes of most anti-PrP antibodies were hidden in the folded structure of PrP(RES), even though these epitopes are revealed with guanidine denaturation. However, reactivities to a PrP(Sc)-specific conformational C-terminal antibody showed significant differences among the three different prion strains. Our results provide evidence for strain-dependent conformational variation near the C termini of molecules within PrP(Sc) multimers.

IMPORTANCE

It has long been apparent that prion strains can have different conformations near the N terminus of the PrP(Sc) protease-resistant core. Here, we show that a C-terminal conformational PrP(Sc)-specific antibody reacts differently to three murine-adapted scrapie strains. These results suggest, in turn, that conformational differences in the C terminus of PrP(Sc) also contribute to the phenotypic distinction between prion strains.

Strain-dependent profile of misfolded prion protein aggregates

Prions are composed of the misfolded prion protein (PrP^Sc) organized in a variety of aggregates. An important question in the prion field has been to determine the identity of functional PrP^Sc aggregates. In this study, we used equilibrium sedimentation in sucrose density gradients to separate PrP^Sc aggregates from three hamster prion strains (Hyper, Drowsy, SSLOW) subjected to minimal manipulations. We show that PrP^Sc aggregates distribute in a wide range of arrangements and the relative proportion of each species depends on the prion strain. We observed a direct correlation between the density of the predominant PrP^Sc aggregates and the incubation periods for the strains studied. The relative presence of PrP^Sc in fractions of different sucrose densities was indicative of the protein deposits present in the brain as analyzed by histology. Interestingly, no association was found between sensitivity to proteolytic degradation and aggregation profiles. Therefore, the organization of PrP molecules in terms of the density of aggregates generated may determine some of the particular strain properties, whereas others are independent from it. Our findings may contribute to understand the mechanisms of strain variation and the role of PrP^Sc aggregates in prion-induced neurodegeneration.

A Fluorescent Oligothiophene-Bis-Triazine ligand interacts with PrP fibrils and detects SDS-resistant oligomers in human prion diseases

Background

Prion diseases are characterized by the accumulation in the central nervous system of an abnormally folded isoform of the prion protein, named PrP^Sc. Aggregation of PrP^Sc into oligomers and fibrils is critically involved in the pathogenesis of prion diseases. Oligomers are supposed to be the key neurotoxic agents in prion disease, so modulation of prion aggregation pathways with small molecules can be a valuable strategy for studying prion pathogenicity and for developing new diagnostic and therapeutic approaches. We previously identified thienyl pyrimidine compounds that induce SDS-resistant PrP^Sc (rSDS-PrP^Sc) oligomers in prion-infected samples.

Results

Due to the low effective doses of the thienyl pyrimidine hits, we synthesized a quaterthiophene-bis-triazine compound, called MR100 to better evaluate their diagnostic and therapeutic potentials. This molecule exhibits a powerful activity inducing rSDS-PrP^Sc oligomers at nanomolar concentrations in prion-infected cells. Fluorescence interaction studies of MR100 with mouse PrP fibrils showed substantial modification of the spectrum, and the interaction was confirmed in vitro by production of rSDS-oligomer species upon incubation of MR100 with fibrils in SDS-PAGE gel. We further explored whether MR100 compound has a potential to be used in the diagnosis of prion diseases. Our results showed that: (i) MR100 can detect rSDS-oligomers in prion-infected brain homogenates of various species, including human samples from CJD patients; (ii) A protocol, called “Rapid Centrifugation Assay” (RCA), was developed based on MR100 property of inducing rSDS-PrP^Sc oligomers only in prion-infected samples, and avoiding the protease digestion step. RCA allows the detection of both PK-sensitive and PK-resistant PrP^Sc species in rodents samples but also from patients with different CJD forms (sporadic and new variant); (iii) A correlation could be established between the amount of rSDS-PrP^Sc oligomers revealed by MR100 and the duration of the symptomatic phase of the disease in CJD patients; and (iv) Bioassay experiments showed that MR100 can trap prion infectivity more efficiently than P30 drug.

Conclusions

MR100 is a powerful tool not only for studying the prion aggregation pathways regarding oligomeric and sPrP^Sc species, but also for developing alternative methods for the detection of prion-infected samples. Considering our bioassay results, MR100 is a promising molecule for the development of prion decontamination approaches.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-016-0074-7) contains supplementary material, which is available to authorized users.

Insights into Mechanisms of Chronic Neurodegeneration

Chronic neurodegenerative diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD), and prion diseases are characterised by the accumulation of abnormal conformers of a host encoded protein in the central nervous system. The process leading to neurodegeneration is still poorly defined and thus development of early intervention strategies is challenging. Unique amongst these diseases are Transmissible Spongiform Encephalopathies (TSEs) or prion diseases, which have the ability to transmit between individuals. The infectious nature of these diseases has permitted in vivo and in vitro modelling of the time course of the disease process in a highly reproducible manner, thus early events can be defined. Recent evidence has demonstrated that the cell-to-cell spread of protein aggregates by a “prion-like mechanism” is common among the protein misfolding diseases. Thus, the TSE models may provide insights into disease mechanisms and testable hypotheses for disease intervention, applicable to a number of these chronic neurodegenerative diseases.

Le TN, Aulić S, Bistaffa E, Campagnani I, Virgilio T, Indaco A, Palamara L, Andréoletti O, Tagliavini F, Legname G. Synthetic prions with novel strain-specified properties

Prions are infectious proteins that possess multiple self-propagating structures. The information for strains and structural specific barriers appears to be contained exclusively in the folding of the pathological isoform, PrP^Sc. Many recent studies determined that de novo prion strains could be generated in vitro from the structural conversion of recombinant (rec) prion protein (PrP) into amyloidal structures. Our aim was to elucidate the conformational diversity of pathological recPrP amyloids and their biological activities, as well as to gain novel insights in characterizing molecular events involved in mammalian prion conversion and propagation. To this end we generated infectious materials that possess different conformational structures. Our methodology for the prion conversion of recPrP required only purified rec full-length mouse (Mo) PrP and common chemicals. Neither infected brain extracts nor amplified PrP^Sc were used. Following two different in vitro protocols recMoPrP converted to amyloid fibrils without any seeding factor. Mouse hypothalamic GT1 and neuroblastoma N2a cell lines were infected with these amyloid preparations as fast screening methodology to characterize the infectious materials. Remarkably, a large number of amyloid preparations were able to induce the conformational change of endogenous PrP^C to harbor several distinctive proteinase-resistant PrP forms. One such preparation was characterized in vivo habouring a synthetic prion with novel strain specified neuropathological and biochemical properties.

Prions are infectious proteins capable of acquiring multiple self-propagating structures. The information for strains and structural specific barriers appears to be contained exclusively in the folding of the pathological isoform, designated as PrP^Sc. During propagation, disease-associated conformer PrP^Sc coerces the physiological form, denoted as PrP^C, to adopt the pathological isoform conformation. We describe here the generation of an array of infectious materials with different structural, morphological, biochemical and cell biological characteristics. After producing purified recombinant prion protein of the wild-type mouse full-length sequence in Escherichia coli, we polymerized the protein into various amyloid fibril conformations based on different amyloid preparations. We also applied a build-in methodology for screening amyloid preparations and generate infectious materials using an amyloid-infected cell culture assay. Some of the amyloid fibrils preparations were able to efficiently amplify in PMCA (Protein Misfolding Cyclic Amplification), and to induce endogenous PrP^C to convert into PrP^Sc in both murine hypothalamic GT1 and neuroblastoma N2a cell lines. One such protocol lead to the generation of a novel synthetic prion strain in mice.

Distinct effects of Cu2+-binding on oligomerization of human and rabbit prion proteins

The cellular prion protein (PrP(C)) is a kind of cell-surface Cu(2+)-binding glycoprotein. The oligomerization of PrP(C) is highly related to transmissible spongiform encephalopathies (TSEs). Cu(2+) plays a vital role in the oligomerization of PrP(C), and participates in the pathogenic process of TSE diseases. It is expected that Cu(2+)-binding has different effects on the oligomerization of TSE-sensitive human PrP(C) (HuPrP(C)) and TSE-resistant rabbit PrP(C) (RaPrP(C)). However, the details of the distinct effects remain unclear. In the present study, we measured the interactions of Cu(2+) with HuPrP(C) (91-230) and RaPrP(C) (91-228) by isothermal titration calorimetry, and compared the effects of Cu(2+)-binding on the oligomerization of both PrPs. The measured dissociation constants (Kd) of Cu(2+) were 11.1 ± 2.1 μM for HuPrP(C) and 21.1 ± 3.1 μM for RaPrP(C). Cu(2+)-binding promoted the oligomerization of HuPrP(C) more significantly than that of RaPrP(C). The far-ultraviolet circular dichroism spectroscopy experiments showed that Cu(2+)-binding induced more significant secondary structure change and increased more β-sheet content for HuPrP(C) compared with RaPrP(C). Moreover, the urea-induced unfolding transition experiments indicated that Cu(2+)-binding decreased the conformational stability of HuPrP(C) more distinctly than that of RaPrP(C). These results suggest that RaPrP(C) possesses a low susceptibility to Cu(2+), potentially weakening the risk of Cu(2+)-induced TSE diseases. Our work sheds light on the Cu(2+)-promoted oligomerization of PrP(C), and may be helpful for further understanding the TSE-resistance of rabbits.

Sensitive detection of aggregated prion protein via proximity ligation

The DNA assisted solid-phase proximity ligation assay (SP-PLA) provides a unique opportunity to specifically detect prion protein (PrP) aggregates by investigating the collocation of 3 or more copies of the specific protein. We have developed an SP-PLA that can detect PrP aggregates in brain homogenates from infected hamsters even after a 10(7)-fold dilution. In contrast, brain homogenate from uninfected animals did not generate a detectable signal at 100-fold higher concentration. Using either of the 2 monoclonal anti-PrP antibodies, 3F4 and 6H4, we successfully detected low concentrations of aggregated PrP. The presented results provide a proof of concept that this method might be an interesting tool in the development of diagnostic approaches of prion diseases.