Detection and characterization of proteinase K-sensitive disease-related prion protein with thermolysin

Biochemical analysis of packing and assembling heptad repeat motifs in the coronavirus spike protein trimer

During a coronavirus infection, the spike protein undergoes sequential structural transitions triggered by its receptor and the host protease at the interface between the virus and cell membranes, thereby mediating membrane fusion. After receptor binding, the heptad repeat motif (HR1/HR2) within the viral spike protein bridges the viral and cellular membranes; however, the intermediate conformation adopted by the spike protein when drawing the viral and cellular membranes into close proximity remains unclear due to its transient and unstable nature. Here, we experimentally induced conformational changes in the spike protein of a murine coronavirus by incubating the virus with its receptor, followed by exposure to trypsin. We then treated the virus/receptor complex with proteinase K to probe the tightly packed core structure of the spike protein. The conformations of the spike protein were predicted from the sizes of the protease digestion products detected by western blot analysis. Upon receptor binding, two bands (each showing different reactivity with a fusion-inhibiting HR2-peptide) were detected; we propose that these bands correspond to the packed and unpacked HR1/HR2 motifs. After trypsin-mediated triggering, measurement of temperature and time dependency revealed that packing of the remaining unpacked HR1/HR2 motifs and assembly of three HR1 motifs in a trimer occur almost simultaneously. Thus, the trimeric spike protein adopts an asymmetric-unassembled conformation after receptor binding, followed by direct assembly into the post-fusion form triggered by the host protease. This biochemical study provides mechanistic insight into the previously unknown intermediate structure of the viral fusion protein.IMPORTANCEDuring infection by an enveloped virus, receptor binding triggers fusion between the cellular membrane and the virus envelope, enabling delivery of the viral genome to the cytoplasm. The viral spike protein mediates membrane fusion; however the molecular mechanism underlying this process is unclear. This is because using structural biology methods to track the transient conformational changes induced in the unstable spike trimer is challenging. Here, we harnessed the ability of protease enzymes to recognize subtle differences on protein surfaces, allowing us to detect structural differences in the spike protein before and after conformational changes. Differences in the size of the degradation products were analyzed by western blot analysis. The proposed model explaining the conformational changes presented herein is a plausible candidate that provides valuable insight into unanswered questions in the field of virology.

Species barrier as molecular basis for adaptation of synthetic prions with N-terminally truncated PrP

Mammalian prions are neurotropic pathogens formed from PrPSc assemblies, a misfolded variant of the host-encoded prion protein PrPC. Multiple PrPSc conformations or strains self-propagate in host populations or mouse models of prion diseases, exhibiting distinct biological and biochemical phenotypes. Constrained interactions between PrPSc and PrPC conformations confer species specificity and regulate cross-species transmission. The pathogenicity of fibrillar assemblies derived from bacterially expressed recombinant PrP (rPrP) has been instrumental in demonstrating the protein-only nature of prions. Yet, their ability to encode different strains and transmit between species remains poorly studied, hampering their use in exploring structure-to-strain relationships. Fibrillar assemblies from rPrP with hamster, mouse, human, and bovine primary structures were generated and tested for transmission and adaptation in tg7 transgenic mice expressing hamster PrPC. All assemblies, except the bovine ones, were fully pathogenic on the primary passage, causing clinical disease, PrPSc brain deposition, and spongiform degeneration. They exhibited divergent adaptation processes and strain properties upon subsequent passage. Assemblies of hamster origin propagated without apparent need for adaptation, those of mouse origin adapted abruptly, and those of human origin required serial passages for optimal fitness. Molecular analyses revealed the presence of endogenously truncated PrPSc species in the resulting synthetic strains that lack the 90-140 amino acid region considered crucial for infectivity. In conclusion, rPrP assemblies provide a facile means of generating novel prion strains with adaptative/evolutive properties mimicking genuine prions. The PrP amino acid backbone is sufficient to encode different strains with specific adaptative properties, offering insights into prion transmission and strain diversity.

Convergent generation of atypical prions in knockin mouse models of genetic prion disease

Most cases of human prion disease arise due to spontaneous misfolding of WT or mutant prion protein, yet recapitulating this event in animal models has proven challenging. It remains unclear whether spontaneous prion generation can occur within the mouse lifespan in the absence of protein overexpression and how disease-causing mutations affect prion strain properties. To address these issues, we generated knockin mice that express the misfolding-prone bank vole prion protein (BVPrP). While mice expressing WT BVPrP (I109 variant) remained free from neurological disease, a subset of mice expressing BVPrP with mutations (D178N or E200K) causing genetic prion disease developed progressive neurological illness. Brains from spontaneously ill knockin mice contained prion disease-specific neuropathological changes as well as atypical protease-resistant BVPrP. Moreover, brain extracts from spontaneously ill D178N- or E200K-mutant BVPrP-knockin mice exhibited prion seeding activity and transmitted disease to mice expressing WT BVPrP. Surprisingly, the properties of the D178N- and E200K-mutant prions appeared identical before and after transmission, suggesting that both mutations guide the formation of a similar atypical prion strain. These findings imply that knockin mice expressing mutant BVPrP spontaneously develop a bona fide prion disease and that mutations causing prion diseases may share a uniform initial mechanism of action.

Collateral beauty in the damages: an overview of cosmetics and therapeutic applications of microbial proteases

Microbial proteases are enzymes secreted by a variety of microorganisms, including bacteria and fungi, and have attracted significant attention due to their versatile applications in the food and pharmaceutical industries. In addition, certain proteases have been used in the development of skin health products and cosmetics. This article provides a review of microbial proteases in terms of their classification, sources, properties, and applications. Moreover, different pharmacological and molecular investigations have been reviewed. Various biological activities of microbial proteases, such as Arazyme, collagenase, elastin, and Nattokinase, which are involved in the digestion of dietary proteins, as well as their potential anti-inflammatory, anti-cancer, antithrombotic, and immunomodulatory effects have been included. Furthermore, their ability to control infections and treat various disorders has been discussed. Finally, this review highlights the potential applications and future perspectives of microbial proteases in biotechnology and biomedicine, and proposes further studies to develop new perspectives for disease control and health-promoting strategies using microbial resources.

Prion assemblies: structural heterogeneity, mechanisms of formation, and role in species barrier

Prions are proteinaceous pathogens responsible for a wide range of neurodegenerative diseases in animal and human. Prions are formed from misfolded, ß-sheet rich, and aggregated conformers (PrPSc) of the host-encoded prion protein (PrPC). Prion replication stems from the capacity of PrPSc to self-replicate by templating PrPC conversion and polymerization. The question then arises about the molecular mechanisms of prion replication, host invasion, and capacity to contaminate other species. Studying these mechanisms has gained in recent years further complexity with evidence that PrPSc is a pleiomorphic protein. There is indeed compelling evidence for PrPSc structural heterogeneity at different scales: (i) within prion susceptible host populations with the existence of different strains with specific biological features due to different PrPSc conformers, (ii) within a single infected host with the co-propagation of different strains, and (iii) within a single strain with evidence for co-propagation of PrPSc assemblies differing in their secondary to quaternary structure. This review summarizes current knowledge of prion assembly heterogeneity, potential mechanisms of formation during the replication process, and importance when crossing the species barrier.

Central Residues in Prion Protein PrPC Are Crucial for Its Conversion into the Pathogenic Isoform

Conformational conversion of the cellular prion protein, PrP^C, into the amyloidogenic isoform, PrP^Sc, is a key pathogenic event in prion diseases. However, the conversion mechanism remains to be elucidated. Here, we generated Tg(PrPΔ91-106)-8545/Prnp^0/0 mice, which overexpress mouse PrP lacking residues 91-106. We showed that none of the mice became sick after intracerebral inoculation with RML, 22L, and FK-1 prion strains nor accumulated PrP^ScΔ91-106 in their brains except for a small amount of PrP^ScΔ91-106 detected in one 22L-inoculated mouse. However, they developed disease around 85 days after inoculation with bovine spongiform encephalopathy (BSE) prions with PrP^ScΔ91-106 in their brains. These results suggest that residues 91-106 are important for PrP^C conversion into PrP^Sc in infection with RML, 22L, and FK-1 prions but not BSE prions. We then narrowed down the residues 91-106 by transducing various PrP deletional mutants into RML- and 22L-infected cells and identified that PrP mutants lacking residues 97-99 failed to convert into PrP^Sc in these cells. Our in vitro conversion assay also showed that RML, 22L, and FK-1 prions did not convert PrPΔ97-99 into PrP^ScΔ97-99, but BSE prions did. We further found that PrP mutants with proline residues at positions 97 to 99 or charged residues at positions 97 and 99 completely or almost completely lost their converting activity into PrP^Sc in RML- and 22L-infected cells. These results suggest that the structurally flexible and noncharged residues 97-99 could be important for PrP^C conversion into PrP^Sc following infection with RML, 22L, and FK-1 prions but not BSE prions.

Stability of BSE infectivity towards heat treatment even after proteolytic removal of prion protein

The unconventional infectious agents of transmissible spongiform encephalopathies (TSEs) are prions. Their infectivity co-appears with PrPSc, aberrant depositions of the host's cellular prion protein (PrPC). Successive heat treatment in the presence of detergent and proteolysis by a keratinase from Bacillus licheniformis PWD-1 was shown before to destroy PrPSc from bovine TSE (BSE) and sheep scrapie diseased brain, however data regarding expected reduction of infectivity were still lacking. Therefore, transgenic Tgbov XV mice which are highly BSE susceptible were used to quantify infectivity before and after the bovine brain treatment procedure. Also four immunochemical analyses were applied to compare the levels of PrPSc. After heating at 115 °C with or without subsequent proteolysis, the original BSE infectivity of 106.2-6.4 ID50 g-1 was reduced to a remaining infectivity of 104.6-5.7 ID50 g-1 while strain characteristics were unaltered, even after precipitation with methanol. Surprisingly, PrPSc depletion was 5-800 times higher than the loss of infectivity. Similar treatment was applied on other prion strains, which were CWD1 in bank voles, 263 K scrapie in hamsters and sheep PG127 scrapie in tg338 ovinized mice. In these strains however, infectivity was already destroyed by heat only. These findings show the unusual heat resistance of BSE and support a role for an additional factor in prion formation as suggested elsewhere when producing prions from PrPC. Leftover material in the remaining PrPSc depleted BSE preparation offers a unique substrate for searching additional elements for prion infectivity and improving our concept about the nature of prions.

Allelic Interference in Prion Replication Is Modulated by the Convertibility of the Interfering PrPC and Other Host-Specific Factors

Prion propagation can be interfered with by the expression of a second prion protein in the host. In the present study, we investigated prion propagation in a host expressing two different prion protein genes.

Early studies in transgenic mouse lines have shown that the coexpression of endogenous murine prion protein (PrP^C) and transgenic PrP^C from another species either inhibits or allows the propagation of prions, depending on the infecting prion strain and interacting protein species. The way whereby this phenomenon, so-called “interference,” is modulated remains to be determined. In this study, different transgenic mouse lines were crossbred to produce mice coexpressing bovine and porcine PrP^C, bovine and murine PrP^C, or murine and porcine PrP^C. These animals and their respective hemizygous controls were inoculated with several prion strains from different sources (cattle, mice, and pigs) to examine the effects of the simultaneous presence of PrP^C from two different species. Our results indicate interference with the infection process, manifested as extended survival times and reduced attack rates. The interference with the infectious process was reduced or absent when the potentiality interfering PrP^C species was efficiently converted by the inoculated agent. However, the propagation of the endogenous murine PrP^Sc was favored, allowing us to speculate that host-specific factors may disturb the interference caused by the coexpression of an exogenous second PrP^C.

Highly infectious prions are not directly neurotoxic

Prions are infectious agents which cause rapidly lethal neurodegenerative diseases in humans and animals following long, clinically silent incubation periods. They are composed of multichain assemblies of misfolded cellular prion protein. While it has long been assumed that prions are themselves neurotoxic, recent development of methods to obtain exceptionally pure prions from mouse brain with maintained strain characteristics, and in which defined structures-paired rod-like double helical fibers-can be definitively correlated with infectivity, allowed a direct test of this assertion. Here we report that while brain homogenates from symptomatic prion-infected mice are highly toxic to cultured neurons, exceptionally pure intact high-titer infectious prions are not directly neurotoxic. We further show that treatment of brain homogenates from prion-infected mice with sodium lauroylsarcosine destroys toxicity without diminishing infectivity. This is consistent with models in which prion propagation and toxicity can be mechanistically uncoupled.

Isolation of infectious, non-fibrillar and oligomeric prions from a genetic prion disease

Prions are transmissible agents causing lethal neurodegenerative diseases that are composed of aggregates of misfolded cellular prion protein (PrPSc). Despite non-fibrillar oligomers having been proposed as the most infectious prion particles, prions purified from diseased brains usually consist of large and fibrillar PrPSc aggregates, whose protease-resistant core (PrPres) encompasses the whole C-terminus of PrP. In contrast, PrPSc from Gerstmann-Sträussler-Scheinker disease associated with alanine to valine substitution at position 117 (GSS-A117V) is characterized by a small protease-resistant core, which is devoid of the C-terminus. We thus aimed to investigate the role of this unusual PrPSc in terms of infectivity, strain characteristics, and structural features. We found, by titration in bank voles, that the infectivity of GSS-A117V is extremely high (109.3 ID50 U/g) and is resistant to treatment with proteinase K (109.0 ID50 U/g). We then purified the proteinase K-resistant GSS-A117V prions and determined the amount of infectivity and PrPres in the different fractions, alongside the morphological characteristics of purified PrPres aggregates by electron microscopy. Purified pellet fractions from GSS-A117V contained the expected N- and C-terminally cleaved 7 kDa PrPres, although the yield of PrPres was low. We found that this low yield depended on the low density/small size of GSS-A117V PrPres, as it was mainly retained in the last supernatant fraction. All fractions were highly infectious, thus confirming the infectious nature of the 7 kDa PrPres, with infectivity levels that directly correlated with the PrPres amount detected. Finally, electron microscopy analysis of these fractions showed no presence of amyloid fibrils, but only very small and indistinct, non-fibrillar PrPresparticles were detected and confirmed to contain PrP via immunogold labelling. Our study demonstrates that purified aggregates of 7 kDa PrPres, spanning residues ∼90-150, are highly infectious oligomers that encode the biochemical and biological strain features of the original sample. Overall, the autocatalytic behaviour of the prion oligomers reveals their role in the propagation of neurodegeneration in patients with Gerstmann-Sträussler-Scheinker disease and implies that the C-terminus of PrPSc is dispensable for infectivity and strain features for this prion strain, uncovering the central PrP domain as the minimal molecular component able to encode infectious prions. These findings are consistent with the hypothesis that non-fibrillar prion particles are highly efficient propagators of disease and provide new molecular and morphological constraints on the structure of infectious prions.

Native nanodiscs formed by styrene maleic acid copolymer derivatives help recover infectious prion multimers bound to brain-derived lipids

Prions are lipidated proteins that interact with endogenous lipids and metal ions. They also assemble into multimers and propagate into the infectious scrapie form known as PrP^Sc The high-resolution structure of the infectious PrP^Sc state remains unknown, and its analysis largely relies on detergent-based preparations devoid of endogenous ligands. Here we designed polymers that allow isolation of endogenous membrane:protein assemblies in native nanodiscs without exposure to conventional detergents that destabilize protein structures and induce fibrillization. A set of styrene-maleic acid (SMA) polymers including a methylamine derivative facilitated gentle release of the infectious complexes for resolution of multimers, and a thiol-containing version promoted crystallization. Polymer extraction from brain homogenates from Syrian hamsters infected with Hyper prions and WT mice infected with Rocky Mountain Laboratories prions yielded infectious prion nanoparticles including oligomers and microfilaments bound to lipid vesicles. Lipid analysis revealed the brain phospholipids that associate with prion protofilaments, as well as those that are specifically enriched in prion assemblies captured by the methylamine-modified copolymer. A comparison of the infectivity of PrP^Sc attached to SMA lipid particles in mice and hamsters indicated that these amphipathic polymers offer a valuable tool for high-yield production of intact, detergent-free prions that retain in vivo activity. This native prion isolation method provides an avenue for producing relevant prion:lipid targets and potentially other proteins that form multimeric assemblies and fibrils on membranes.

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.

Thermostability as a highly dependent prion strain feature

Prion diseases are caused by the conversion of physiological PrP^C into the pathogenic misfolded protein PrP^Sc, conferring new properties to PrP^Sc that vary upon prion strains. In this work, we analyze the thermostability of three prion strains (BSE, RML and 22L) that were heated at 98 °C for 2 hours. PrP^Sc resistance to proteinase K (PrP^res), residual infectivity by mouse bioassay and in vitro templating activity by protein misfolding cyclic amplification (PMCA) were studied. Heated strains showed a huge loss of PrP^res and a radically different infectivity loss: RML was the most thermolabile strain (6 to 7 log10 infectivity loss), followed by 22L (5 log10) while BSE was the most thermostable strain with low or null infectivity reduction showing a clear dissociation between PrP^res and infectivity. These results indicate that thermostability is a strain-specific feature, measurable by PMCA and mouse bioassay, and a great tool to distinguish prion strains.

PrPC knockdown by liposome-siRNA-peptide complexes (LSPCs) prolongs survival and normal behavior of prion-infected mice immunotolerant to treatment

Prion diseases are members of neurodegenerative protein misfolding diseases (NPMDs) that include Alzheimer's, Parkinson's and Huntington diseases, amyotrophic lateral sclerosis, tauopathies, traumatic brain injuries, and chronic traumatic encephalopathies. No known therapeutics extend survival or improve quality of life of humans afflicted with prion disease. We and others developed a new approach to NPMD therapy based on reducing the amount of the normal, host-encoded protein available as substrate for misfolding into pathologic forms, using RNA interference, a catabolic pathway that decreases levels of mRNA encoding a particular protein. We developed a therapeutic delivery system consisting of small interfering RNA (siRNA) complexed to liposomes and addressed to the central nervous system using a targeting peptide derived from rabies virus glycoprotein. These liposome-siRNA-peptide complexes (LSPCs) cross the blood-brain barrier and deliver PrP siRNA to neuronal cells to decrease expression of the normal cellular prion protein, PrPC, which acts as a substrate for prion replication. Here we show that LSPCs can extend survival and improve behavior of prion-infected mice that remain immunotolerant to treatment. LSPC treatment may be a viable therapy for prion and other NPMDs that can improve the quality of life of patients at terminal disease stages.

Recent Advances in Understanding Mammalian Prion Structure: A Mini Review

Prions are lethal pathogens, which cause fatal neurodegenerative diseases in mammals. They are unique infectious agents and are composed of self-propagating multi-chain assemblies of misfolded host-encoded prion protein (PrP). Understanding prion structure is fundamental to understanding prion disease pathogenesis however to date, the high-resolution structure of authentic ex vivo infectious prions remains unknown. Advances in determining prion structure have been severely impeded by the difficulty in recovering relatively homogeneous prion particles from infected brain and definitively associating infectivity with the PrP assembly state. Recently, however, images of highly infectious ex vivo PrP rods that produce prion-strain specific disease phenotypes in mice have been obtained using cryo-electron microscopy and atomic force microscopy. These images have provided the most detailed description of ex vivo mammalian prions reported to date and have established that prions isolated from multiple strains have a common hierarchical structure. Misfolded PrP is assembled into 20 nm wide rods containing two fibers, each with double helical repeating substructure, separated by a characteristic central gap 8–10 nm in width. Irregularly structured material with adhesive properties distinct to that of the fibers is present within the central gap of the rod. Prions are clearly distinguishable from non-infectious recombinant PrP fibrils generated in vitro and from all other propagating protein structures so far described in other neurodegenerative diseases. The basic architecture of mammalian prions appears to be exceptional and fundamental to their lethal pathogenicity.

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.

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.

Soluble Aβ aggregates can inhibit prion propagation

Mammalian prions cause lethal neurodegenerative diseases such as Creutzfeldt–Jakob disease (CJD) and consist of multi-chain assemblies of misfolded cellular prion protein (PrP^C). Ligands that bind to PrP^C can inhibit prion propagation and neurotoxicity. Extensive prior work established that certain soluble assemblies of the Alzheimer's disease (AD)-associated amyloid β-protein (Aβ) can tightly bind to PrP^C, and that this interaction may be relevant to their toxicity in AD. Here, we investigated whether such soluble Aβ assemblies might, conversely, have an inhibitory effect on prion propagation. Using cellular models of prion infection and propagation and distinct Aβ preparations, we found that the form of Aβ assemblies which most avidly bound to PrP in vitro also inhibited prion infection and propagation. By contrast, forms of Aβ which exhibit little or no binding to PrP were unable to attenuate prion propagation. These data suggest that soluble aggregates of Aβ can compete with prions for binding to PrP^C and emphasize the bidirectional nature of the interplay between Aβ and PrP^C in Alzheimer's and prion diseases. Such inhibitory effects of Aβ on prion propagation may contribute to the apparent fall-off in the incidence of sporadic CJD at advanced age where cerebral Aβ deposition is common.

Prion infectivity is encoded exclusively within the structure of proteinase K-resistant fragments of synthetically generated recombinant PrPSc

Transmissible spongiform encephalopathies, also known as prion diseases, are a group of fatal neurodegenerative disorders affecting both humans and animals. The central pathogenic event in prion disease is the misfolding of normal prion protein (PrP^C) into the pathogenic conformer, PrP^Sc, which self-replicates by converting PrP^C to more of itself. The biochemical hallmark of PrP^Sc is its C-terminal resistance to proteinase K (PK) digestion, which has been historically used to define PrP^Sc and is still the most widely used characteristic for prion detection. We used PK-resistance as a biochemical measure for the generation of recombinant prion from bacterially expressed recombinant PrP. However, the existence of both PK- resistant and -sensitive PrP^Sc forms in animal and human prion disease led to the question of whether the in vitro-generated recombinant prion infectivity is due to the PK-resistant or -sensitive recombinant PrP forms. In this study, we compared undigested and PK-digested recombinant prions for their infectivity using both the classical rodent bioassay and the cell-based prion infectivity assay. Similar levels of infectivity were detected in PK-digested and -undigested samples by both assays. A time course study of recombinant prion propagation showed that the increased capability to seed the conversion of endogenous PrP in cultured cells coincided with an increase of the PK-resistant form of recombinant PrP. Moreover, prion infectivity diminished when recombinant prion was subjected to an extremely harsh PK digestion. These results demonstrated that the infectivity of recombinant prion is encoded within the structure of the PK-resistant PrP fragments. This characteristic of recombinant prion, that a simple PK digestion is able to eliminate all PK-sensitive (non-infectious) PrP species, makes possible a more homogenous material that will be ideal for dissecting the molecular basis of prion infectivity.

Treatment with a non-toxic, self-replicating anti-prion delays or prevents prion disease in vivo

Transmissible spongiform encephalopathies (TSEs) are fatal neurological disorders caused by prions, which are composed of a misfolded protein (PrP^Sc) that self-propagates in the brain of infected individuals by converting the normal prion protein (PrP^C) into the pathological isoform. Here, we report a novel experimental strategy for preventing prion disease based on producing a self-replicating, but innocuous PrP^Sc-like form, termed anti-prion, which can compete with the replication of pathogenic prions. Our results show that a prophylactic inoculation of prion-infected animals with an anti-prion delays the onset of the disease and in some animals completely prevents the development of clinical symptoms and brain damage. The data indicate that a single injection of the anti-prion eliminated ~99% of the infectivity associated to pathogenic prions. Furthermore, this treatment caused significant changes in the profile of regional PrP^Sc deposition in the brains of animals that were treated, but still succumbed to the disease. Our findings provide new insights for a mechanistic understanding of prion replication and support the concept that prion replication can be separated from toxicity, providing a novel target for therapeutic intervention.

A novel Gerstmann-Sträussler-Scheinker disease mutation defines a precursor for amyloidogenic 8 kDa PrP fragments and reveals N-terminal structural changes shared by other GSS alleles

To explore pathogenesis in a young Gerstmann-Sträussler-Scheinker Disease (GSS) patient, the corresponding mutation, an eight-residue duplication in the hydrophobic region (HR), was inserted into the wild type mouse PrP gene. Transgenic (Tg) mouse lines expressing this mutation (Tg.HRdup) developed spontaneous neurologic syndromes and brain extracts hastened disease in low-expressor Tg.HRdup mice, suggesting de novo formation of prions. While Tg.HRdup mice exhibited spongiform change, PrP aggregates and the anticipated GSS hallmark of a proteinase K (PK)-resistant 8 kDa fragment deriving from the center of PrP, the LGGLGGYV insertion also imparted alterations in PrP's unstructured N-terminus, resulting in a 16 kDa species following thermolysin exposure. This species comprises a plausible precursor to the 8 kDa PK-resistant fragment and its detection in adolescent Tg.HRdup mice suggests that an early start to accumulation could account for early disease of the index case. A 16 kDa thermolysin-resistant signature was also found in GSS patients with P102L, A117V, H187R and F198S alleles and has coordinates similar to GSS stop codon mutations. Our data suggest a novel shared pathway of GSS pathogenesis that is fundamentally distinct from that producing structural alterations in the C-terminus of PrP, as observed in other prion diseases such as Creutzfeldt-Jakob Disease and scrapie.

Prion diseases can be sporadic, infectious or genetic. The central event of all prion diseases is the structural conversion of the cellular prion protein (PrP^C) to its disease associated conformer, PrP^Sc. Gerstmann-Sträussler-Scheinker Disease (GSS) is a genetic prion disease presenting as a multi-systemic neurological syndrome. A novel mutation, an eight amino acid insertion, was discovered in a young GSS patient. We created transgenic mice expressing this mutation and found that they recapitulate key features of the disease; namely PrP deposition in the brain and a low molecular weight proteinase K (PK) resistant internal PrP fragment. While structural investigations did not reveal a gross alteration in the conformation of this mutant PrP, the insertion lying at the boundary of the globular domain causes alterations in the unstructured amino terminal portion of the protein such that it becomes resistant to digestion by the enzyme thermolysin. We demonstrate by kinetic analysis and sequential digestion that this novel thermolysin resistant species is a precursor to the pathognomonic PK resistant fragment. Analysis of samples from other GSS patients revealed this same signature, suggesting a common molecular pathway.

Detection of prions in blood from patients with variant Creutzfeldt-Jakob disease

Human prion diseases are infectious and invariably fatal neurodegenerative diseases. They include sporadic Creutzfeldt-Jakob disease (sCJD), the most common form, and variant CJD (vCJD), which is caused by interspecies transmission of prions from cattle infected by bovine spongiform encephalopathy. Development of a biochemical assay for the sensitive, specific, early, and noninvasive detection of prions (PrP^Sc) in the blood of patients affected by prion disease is a top medical priority to increase the safety of the blood supply. vCJD has already been transmitted from human to human by blood transfusion, and the number of asymptomatic carriers of vCJD in the U.K. alone is estimated to be 1 in 2000 people. We used the protein misfolding cyclic amplification (PMCA) technique to analyze blood samples from 14 cases of vCJD and 153 controls, including patients affected by sCJD and other neurodegenerative or neurological disorders as well as healthy subjects. Our results showed that PrP^Sc could be detected with 100% sensitivity and specificity in blood samples from vCJD patients. Detection was possible in any of the blood fractions analyzed and could be done with as little as a few microliters of sample volume. The PrP^Sc concentration in blood was estimated to be ~0.5 pg/ml. Our findings suggest that PMCA may be useful for premortem noninvasive diagnosis of vCJD and to identify prion contamination of the blood supply. Further studies are needed to fully validate the technology.

Experimental sheep BSE prions generate the vCJD phenotype when serially passaged in transgenic mice expressing human prion protein

The epizootic prion disease of cattle, bovine spongiform encephalopathy (BSE), causes variant Creutzfeldt-Jakob disease (vCJD) in humans following dietary exposure. While it is assumed that all cases of vCJD attributed to a dietary aetiology are related to cattle BSE, sheep and goats are susceptible to experimental oral challenge with cattle BSE prions and farmed animals in the UK were undoubtedly exposed to BSE-contaminated meat and bone meal during the late 1980s and early 1990s. Although no natural field cases of sheep BSE have been identified, it cannot be excluded that some BSE-infected sheep might have entered the European human food chain. Evaluation of the zoonotic potential of sheep BSE prions has been addressed by examining the transmission properties of experimental brain isolates in transgenic mice that express human prion protein, however to-date there have been relatively few studies. Here we report that serial passage of experimental sheep BSE prions in transgenic mice expressing human prion protein with methionine at residue 129 produces the vCJD phenotype that mirrors that seen when the same mice are challenged with vCJD prions from patient brain. These findings are congruent with those reported previously by another laboratory, and thereby strongly reinforce the view that sheep BSE prions could have acted as a causal agent of vCJD within Europe.

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.

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.

Divergent prion strain evolution driven by PrPC expression level in transgenic mice

Prions induce a fatal neurodegenerative disease in infected host brain based on the refolding and aggregation of the host-encoded prion protein PrP^C into PrP^Sc. Structurally distinct PrP^Sc conformers can give rise to multiple prion strains. Constrained interactions between PrP^C and different PrP^Sc strains can in turn lead to certain PrP^Sc (sub)populations being selected for cross-species transmission, or even produce mutation-like events. By contrast, prion strains are generally conserved when transmitted within the same species, or to transgenic mice expressing homologous PrP^C. Here, we compare the strain properties of a representative sheep scrapie isolate transmitted to a panel of transgenic mouse lines expressing varying levels of homologous PrP^C. While breeding true in mice expressing PrP^C at near physiological levels, scrapie prions evolve consistently towards different strain components in mice beyond a certain threshold of PrP^C overexpression. Our results support the view that PrP^C gene dosage can influence prion evolution on homotypic transmission.

PrP^C protein plays a key role in prion transmission across species. Here, the authors compare transmission of a representative scrapie isolate to transgenic mice expressing variable levels of the same Prnp allele as the donor sheep, and find divergent strain propagation regulated by PrP^C gene dosage.

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.

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.

Mammalian prions and their wider relevance in neurodegenerative diseases

Prions are notorious protein-only infectious agents that cause invariably fatal brain diseases following silent incubation periods that can span a lifetime. These diseases can arise spontaneously, through infection or be inherited. Remarkably, prions are composed of self-propagating assemblies of a misfolded cellular protein that encode information, generate neurotoxicity and evolve and adapt in vivo. Although parallels have been drawn with Alzheimer's disease and other neurodegenerative conditions involving the deposition of assemblies of misfolded proteins in the brain, insights are now being provided into the usefulness and limitations of prion analogies and their aetiological and therapeutic relevance.

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.

A systematic investigation of production of synthetic prions from recombinant prion protein

According to the protein-only hypothesis, infectious mammalian prions, which exist as distinct strains with discrete biological properties, consist of multichain assemblies of misfolded cellular prion protein (PrP). A critical test would be to produce prion strains synthetically from defined components. Crucially, high-titre ‘synthetic' prions could then be used to determine the structural basis of infectivity and strain diversity at the atomic level. While there have been multiple reports of production of prions from bacterially expressed recombinant PrP using various methods, systematic production of high-titre material in a form suitable for structural analysis remains a key goal. Here, we report a novel high-throughput strategy for exploring a matrix of conditions, additives and potential cofactors that might generate high-titre prions from recombinant mouse PrP, with screening for infectivity using a sensitive automated cell-based bioassay. Overall, approximately 20 000 unique conditions were examined. While some resulted in apparently infected cell cultures, this was transient and not reproducible. We also adapted published methods that reported production of synthetic prions from recombinant hamster PrP, but again did not find evidence of significant infectious titre when using recombinant mouse PrP as substrate. Collectively, our findings are consistent with the formation of prion infectivity from recombinant mouse PrP being a rare stochastic event and we conclude that systematic generation of prions from recombinant PrP may only become possible once the detailed structure of authentic ex vivo prions is solved.

Prions efficiently cross the intestinal barrier after oral administration: Study of the bioavailability, and cellular and tissue distribution in vivo

Natural forms of prion diseases frequently originate by oral (p.o.) infection. However, quantitative information on the gastro-intestinal (GI) absorption of prions (i.e. the bioavailability and subsequent biodistribution) is mostly unknown. The main goal of this study was to evaluate the fate of prions after oral administration, using highly purified radiolabeled PrP(Sc). The results showed a bi-phasic reduction of PrP(Sc) with time in the GI, except for the ileum and colon which showed sustained increases peaking at 3-6 hr, respectively. Plasma and whole blood (125)I-PrP(Sc) reached maximal levels by 30 min and 3 hr, respectively, and blood levels were constantly higher than plasma. Upon crossing the GI-tract (125)I-PrP(Sc) became associated to blood cells, suggesting that binding to cells decreased the biological clearance of the agent. Size-exclusion chromatography revealed that oligomeric (125)I-PrP(Sc) were transported from the intestinal tract, and protein misfolding cyclic amplification showed that PrP(Sc) in organs and blood retained the typical prion self-replicating ability. Pharmacokinetic analysis found the oral bioavailability of (125)I-PrP(Sc) to be 33.6%. Interestingly, (125)I-PrP(Sc) reached the brain in a quantity equivalent to the minimum amount needed to initiate prion disease. Our findings provide a comprehensive and quantitative study of the fate of prions upon oral infection.

A critical appraisal of the pathogenic protein spread hypothesis of neurodegeneration

There has been an explosion in the number of papers discussing the hypothesis of 'pathogenic spread' in neurodegenerative disease - the idea that abnormal forms of disease-associated proteins, such as tau or α-synuclein, physically move from neuron to neuron to induce disease progression. However, whether inter-neuronal spread of protein aggregates actually occurs in humans and, if so, whether it causes symptom onset remain uncertain. Even if pathogenic spread is proven in humans, it is unclear how much this would alter the specific therapeutic approaches that are in development. A critical appraisal of this increasingly popular hypothesis thus seems both important and timely.

Ex vivo mammalian prions are formed of paired double helical prion protein fibrils

Mammalian prions are hypothesized to be fibrillar or amyloid forms of prion protein (PrP), but structures observed to date have not been definitively correlated with infectivity and the three-dimensional structure of infectious prions has remained obscure. Recently, we developed novel methods to obtain exceptionally pure preparations of prions from mouse brain and showed that pathogenic PrP in these high-titre preparations is assembled into rod-like assemblies. Here, we have used precise cell culture-based prion infectivity assays to define the physical relationship between the PrP rods and prion infectivity and have used electron tomography to define their architecture. We show that infectious PrP rods isolated from multiple prion strains have a common hierarchical assembly comprising twisted pairs of short fibres with repeating substructure. The architecture of the PrP rods provides a new structural basis for understanding prion infectivity and can explain the inability to systematically generate high-titre synthetic prions from recombinant PrP.

Physical, chemical and kinetic factors affecting prion infectivity

The mouse-adapted scrapie prion strain RML is one of the most widely used in prion research. The introduction of a cell culture-based assay of RML prions, the scrapie cell assay (SCA) allows more rapid and precise prion titration. A semi-automated version of this assay (ASCA) was applied to explore a range of conditions that might influence the infectivity and properties of RML prions. These include resistance to freeze-thaw procedures; stability to endogenous proteases in brain homogenate despite prolonged exposure to varying temperatures; distribution of infective material between pellet and supernatant after centrifugation, the effect of reducing agents and the influence of detergent additives on the efficiency of infection. Apparent infectivity is increased significantly by interaction with cationic detergents. Importantly, we have also elucidated the relationship between the duration of exposure of cells to RML prions and the transmission of infection. We established that the infection process following contact of cells with RML prions is rapid and followed an exponential time course, implying a single rate-limiting process.

Prion-mediated neurodegeneration is associated with early impairment of the ubiquitin-proteasome system

Prion diseases are a group of fatal neurodegenerative disorders characterised by the accumulation of misfolded prion protein (PrP(Sc)) in the brain. The critical relationship between aberrant protein misfolding and neurotoxicity currently remains unclear. The accumulation of aggregation-prone proteins has been linked to impairment of the ubiquitin-proteasome system (UPS) in a variety of neurodegenerative disorders, including Alzheimer's, Parkinson's and Huntington's diseases. As the principal route for protein degradation in mammalian cells, this could have profound detrimental effects on neuronal function and survival. Here, we determine the temporal onset of UPS dysfunction in prion-infected Ub(G76V)-GFP reporter mice, which express a ubiquitin fusion proteasome substrate to measure in vivo UPS activity. We show that the onset of UPS dysfunction correlates closely with PrP(Sc) deposition, preceding earliest behavioural deficits and neuronal loss. UPS impairment was accompanied by accumulation of polyubiquitinated substrates and found to affect both neuronal and astrocytic cell populations. In prion-infected CAD5 cells, we demonstrate that activation of the UPS by the small molecule inhibitor IU1 is sufficient to induce clearance of polyubiquitinated substrates and reduce misfolded PrP(Sc) load. Taken together, these results identify the UPS as a possible early mediator of prion pathogenesis and promising target for development of future therapeutics.

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.

Nanoscale insights into full-length prion protein aggregation on model lipid membranes

The aggregates of the full-length human recombinant prion protein (PrP) (23–231) on model membranes were investigated by combining the atomic force microscopy (AFM) measurements and theoretical calculations at pH 5.0, showing the great effect of PrP concentration on its supramolecular assemblies on the lipid bilayer.

Preclinical detection of infectivity and disease-specific PrP in blood throughout the incubation period of prion disease

Variant Creutzfeldt-Jakob disease (vCJD) is a fatal neurodegenerative disorder characterised by accumulation of pathological isoforms of the prion protein, PrP. Although cases of clinical vCJD are rare, there is evidence there may be tens of thousands of infectious carriers in the United Kingdom alone. This raises concern about the potential for perpetuation of infection via medical procedures, in particular transfusion of contaminated blood products. Accurate biochemical detection of prion infection is crucial to mitigate risk and we have previously reported a blood assay for vCJD. This assay is sensitive for abnormal PrP conformers at the earliest stages of preclinical prion disease in mice and precedes the maximum infectious titre in blood. Not only does this support the possibility of screening asymptomatic individuals, it will also facilitate the elucidation of the complex relationship that exists between the ensemble of abnormal PrP conformers present in blood and the relationship to infectivity.

Highly infectious CJD particles lack prion protein but contain many viral-linked peptides by LC-MS/MS

It is widely believed that host prion protein (PrP), without nucleic acid, converts itself into an infectious form (PrP‐res) that causes transmissible encephalopathies (TSEs), such as human sporadic CJD (sCJD), endemic sheep scrapie, and epidemic BSE. There are many detailed investigations of PrP, but proteomic studies of other proteins in verified infectious TSE particles have not been pursued, even though brain homogenates without PrP retain their complete infectious titer. To define proteins that may be integral to, process, or protect an agent genome, we developed a streamlined, high‐yield purification of infectious FU‐CJD mouse brain particles with minimal PrP. Proteinase K (PK) abolished all residual particle PrP, but did not reduce infectivity, and viral‐size particles lacking PrP were ∼70S (vs. 90–120S without PK). Furthermore, over 1,500 non‐PrP proteins were still present and positively identified in high titer FU‐CJD particles without detectable PrP by mass spectrometry (LC‐MS/MS); 114 of these peptides were linked to viral motifs in the environmental–viral database, and not evident in parallel uninfected controls. Host components were also identified in both PK and non‐PK treated particles from FU‐CJD mouse brain and human sCJD brain. This abundant cellular data had several surprises, including finding Huntingtin in the sCJD but not normal human brain samples. Similarly, the neural Wiskott–Aldrich sequence and multivesicular and endosome components associated with retromer APP (Alzheimer amyloid) processing were only in sCJD. These cellular findings suggest that new therapies directed at retromer–vesicular trafficking in other neurodegenerative diseases may also counteract late‐onset sCJD PrP amyloid pathology. J. Cell. Biochem. 115: 2012–2021, 2014. © 2014 Wiley Periodicals, Inc.

Genetics of Prion Disease in Cattle

Bovine spongiform encephalopathy (BSE) is a prion disease that is invariably fatal in cattle and has been implicated as a significant human health risk. As a transmissible disease of livestock, it has impacted food safety, production practices, global trade, and profitability. Genetic polymorphisms that alter the prion protein in humans and sheep are associated with transmissible spongiform encephalopathy susceptibility or resistance. In contrast, there is no strong evidence that nonsynonymous mutations in the bovine prion gene (PRNP) are associated with classical BSE (C-BSE) disease susceptibility, though two bovine PRNP insertion/deletion polymorphisms, in the putative region, are associated with susceptibility to C-BSE. However, these associations do not explain the full extent of BSE susceptibility, and loci outside of PRNP appear to be associated with disease incidence in some cattle populations. This article provides a review of the current state of genetic knowledge regarding prion diseases in cattle.

Molecular Mechanism of the Misfolding and Oligomerization of the Prion Protein: Current Understanding and Its Implications

Prion diseases, also known as transmissible spongiform encephalopathies, make up a group of fatal neurodegenerative disorders linked with the misfolding and aggregation of the prion protein (PrP). Although it is not yet understood how the misfolding of PrP induces neurodegeneration, it is widely accepted that the formation of misfolded prion protein (termed PrP(Sc)) is both the triggering event in the disease and the main component of the infectious agent responsible for disease transmission. Despite the clear involvement of PrP(Sc) in prion diseases, the exact composition of PrP(Sc) is not yet well-known. Recent studies show that misfolded oligomers of PrP could, however, be responsible for neurotoxicity and/or infectivity in the prion diseases. Hence, understanding the molecular mechanism of formation of the misfolded oligomers of PrP is critical for developing an understanding about the prion diseases and for developing anti-prion therapeutics. This review discusses recent advances in understanding the molecular mechanism of misfolded oligomer formation by PrP and its implications for the development of anti-prion therapeutics.

Transmission Properties of Human PrP 102L Prions Challenge the Relevance of Mouse Models of GSS

Inherited prion disease (IPD) is caused by autosomal-dominant pathogenic mutations in the human prion protein (PrP) gene (PRNP). A proline to leucine substitution at PrP residue 102 (P102L) is classically associated with Gerstmann-Sträussler-Scheinker (GSS) disease but shows marked clinical and neuropathological variability within kindreds that may be caused by variable propagation of distinct prion strains generated from either PrP 102L or wild type PrP. To-date the transmission properties of prions propagated in P102L patients remain ill-defined. Multiple mouse models of GSS have focused on mutating the corresponding residue of murine PrP (P101L), however murine PrP 101L, a novel PrP primary structure, may not have the repertoire of pathogenic prion conformations necessary to accurately model the human disease. Here we describe the transmission properties of prions generated in human PrP 102L expressing transgenic mice that were generated after primary challenge with ex vivo human GSS P102L or classical CJD prions. We show that distinct strains of prions were generated in these mice dependent upon source of the inoculum (either GSS P102L or CJD brain) and have designated these GSS-102L and CJD-102L prions, respectively. GSS-102L prions have transmission properties distinct from all prion strains seen in sporadic and acquired human prion disease. Significantly, GSS-102L prions appear incapable of transmitting disease to conventional mice expressing wild type mouse PrP, which contrasts strikingly with the reported transmission properties of prions generated in GSS P102L-challenged mice expressing mouse PrP 101L. We conclude that future transgenic modeling of IPDs should focus exclusively on expression of mutant human PrP, as other approaches may generate novel experimental prion strains that are unrelated to human disease.

Inherited prion disease (IPD) is caused by pathogenic mutations in the human prion protein (PrP) gene leading to the formation of lethal prions in the brain. To-date the properties of prions causing IPD and their similarities to prions causing other forms of human prion disease remain ill-defined. In the present study we have investigated the properties of prions seen in patients with Gerstmann-Sträussler-Scheinker (GSS) disease associated with the substitution of leucine for proline at amino acid position 102 (GSS P102L). We examined the ability of these prions to infect transgenic mice expressing human mutant 102L PrP, human wild-type PrP or wild-type mice. We found that GSS-102L prions have properties distinct from other types of human prions by showing that they can only infect transgenic mice expressing human PrP carrying the same mutation. Mice expressing wild-type human PrP or wild-type mouse PrP were entirely resistant to infection with GSS-102L prions. We conclude that accurate modeling of inherited prion disease requires the expression of authentic mutant human PrP in transgenic models, as other approaches may generate results that do not mirror the human disease.

A novel and rapid method for obtaining high titre intact prion strains from mammalian brain

Mammalian prions exist as multiple strains which produce characteristic and highly reproducible phenotypes in defined hosts. How this strain diversity is encoded by a protein-only agent remains one of the most interesting and challenging questions in biology with wide relevance to understanding other diseases involving the aggregation or polymerisation of misfolded host proteins. Progress in understanding mammalian prion strains has however been severely limited by the complexity and variability of the methods used for their isolation from infected tissue and no high resolution structures have yet been reported. Using high-throughput cell-based prion bioassay to re-examine prion purification from first principles we now report the isolation of prion strains to exceptional levels of purity from small quantities of infected brain and demonstrate faithful retention of biological and biochemical strain properties. The method's effectiveness and simplicity should facilitate its wide application and expedite structural studies of prions.

The Unexposed Secrets of Prion Protein Oligomers

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

Proteinase K and the structure of PrPSc: The good, the bad and the ugly

Infectious proteins (prions) are, ironically, defined by their resistance to proteolytic digestion. A defining characteristic of the transmissible isoform of the prion protein (PrP(Sc)) is its partial resistance to proteinase K (PK) digestion. Diagnosis of prion disease typically relies upon immunodetection of PK-digested PrP(Sc) by Western blot, ELISA or immunohistochemical detection. PK digestion has also been used to detect differences in prion strains. Thus, PK has been a crucial tool to detect and, thereby, control the spread of prions. PK has also been used as a tool to probe the structure of PrP(Sc). Mass spectrometry and antibodies have been used to identify PK cleavage sites in PrP(Sc). These results have been used to identify the more accessible, flexible stretches connecting the β-strand components in PrP(Sc). These data, combined with physical constraints imposed by spectroscopic results, were used to propose a qualitative model for the structure of PrP(Sc). Assuming that PrP(Sc) is a four rung β-solenoid, we have threaded the PrP sequence to satisfy the PK proteolysis data and other experimental constraints.

N-terminal domain of prion protein directs its oligomeric association

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

Prion neuropathology follows the accumulation of alternate prion protein isoforms after infective titre has peaked

Prions are lethal infectious agents thought to consist of multi-chain forms (PrP^Sc) of misfolded cellular prion protein (PrP^C). Prion propagation proceeds in two distinct mechanistic phases: an exponential phase 1, which rapidly reaches a fixed level of infectivity irrespective of PrP^C expression level, and a plateau (phase 2), which continues until clinical onset with duration inversely proportional to PrP^C expression level. We hypothesized that neurotoxicity relates to distinct neurotoxic species produced following a pathway switch when prion levels saturate. Here we show a linear increase of proteinase K-sensitive PrP isoforms distinct from classical PrP^Sc at a rate proportional to PrP^C concentration, commencing at the phase transition and rising until clinical onset. The unaltered level of total PrP during phase 1, when prion infectivity increases a million-fold, indicates that prions comprise a small minority of total PrP. This is consistent with PrP^C concentration not being rate limiting to exponential prion propagation and neurotoxicity relating to critical concentrations of alternate PrP isoforms whose production is PrP^C concentration dependent.

Prions (PrP) are infectious agents that cause lethal neurodegenerative diseases. Here the authors study the kinetics of prion propagation in mice and show that the onset of neuropathology occurs during the late phase of disease and is hypothesized to be due to increases in a toxic isoform of PrP that is different from the infectious species.