Strain fidelity of chronic wasting disease upon murine adaptation

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.

Neuronal Ndst1 depletion accelerates prion protein clearance and slows neurodegeneration in prion infection

Select prion diseases are characterized by widespread cerebral plaque-like deposits of amyloid fibrils enriched in heparan sulfate (HS), a abundant extracellular matrix component. HS facilitates fibril formation in vitro, yet how HS impacts fibrillar plaque growth within the brain is unclear. Here we found that prion-bound HS chains are highly sulfated, and that the sulfation is essential for accelerating prion conversion in vitro. Using conditional knockout mice to deplete the HS sulfation enzyme, Ndst1 (N-deacetylase / N-sulfotransferase) from neurons or astrocytes, we investigated how reducing HS sulfation impacts survival and prion aggregate distribution during a prion infection. Neuronal Ndst1-depleted mice survived longer and showed fewer and smaller parenchymal plaques, shorter fibrils, and increased vascular amyloid, consistent with enhanced aggregate transit toward perivascular drainage channels. The prolonged survival was strain-dependent, affecting mice infected with extracellular, plaque-forming, but not membrane bound, prions. Live PET imaging revealed rapid clearance of recombinant prion protein monomers into the CSF of neuronal Ndst1- deficient mice, neuronal, further suggesting that HS sulfate groups hinder transit of extracellular prion protein monomers. Our results directly show how a host cofactor slows the spread of prion protein through the extracellular space and identify an enzyme to target to facilitate aggregate clearance.

Mouse models of chronic wasting disease: A review

Animal models are essential tools for investigating and understanding complex prion diseases like chronic wasting disease (CWD), an infectious prion disease of cervids (elk, deer, moose, and reindeer). Over the past several decades, numerous mouse models have been generated to aid in the advancement of CWD knowledge and comprehension. These models have facilitated the investigation of pathogenesis, transmission, and potential therapies for CWD. Findings have impacted CWD management and disease outcomes, though much remains unknown, and a cure has yet to be discovered. Studying wildlife for CWD effects is singularly difficult due to the long incubation time, subtle clinical signs at early stages, lack of convenient in-the-field live testing methods, and lack of reproducibility of a controlled laboratory setting. Mouse models in many cases is the first step to understanding the mechanisms of disease in a shortened time frame. Here, we provide a comprehensive review of studies with mouse models in CWD research. We begin by reviewing studies that examined the use of mouse models for bioassays for tissues, bodily fluids, and excreta that spread disease, then address routes of infectivity and infectious load. Next, we delve into studies of genetic factors that influence protein structure. We then move on to immune factors, possible transmission through environmental contamination, and species barriers and differing prion strains. We conclude with studies that make use of cervidized mouse models in the search for therapies for CWD.

Chronic wasting disease in Europe: new strains on the horizon

Prion diseases are fatal neurodegenerative disorders with known natural occurrence in humans and a few other mammalian species. The diseases are experimentally transmissible, and the agent is derived from the host-encoded cellular prion protein (PrP^C), which is misfolded into a pathogenic conformer, designated PrP^Sc (scrapie). Aggregates of PrP^Sc molecules, constitute proteinaceous infectious particles, known as prions. Classical scrapie in sheep and goats and chronic wasting disease (CWD) in cervids are known to be infectious under natural conditions. In CWD, infected animals can shed prions via bodily excretions, allowing direct host-to-host transmission or indirectly via prion-contaminated environments. The robustness of prions means that transmission via the latter route can be highly successful and has meant that limiting the spread of CWD has proven difficult. In 2016, CWD was diagnosed for the first time in Europe, in reindeer (Rangifer tarandus) and European moose (Alces alces). Both were diagnosed in Norway, and, subsequently, more cases were detected in a semi-isolated wild reindeer population in the Nordfjella area, in which the first case was identified. This population was culled, and all reindeer (approximately 2400) were tested for CWD; 18 positive animals, in addition to the first diagnosed case, were found. After two years and around 25,900 negative tests from reindeer (about 6500 from wild and 19,400 from semi-domesticated) in Norway, a new case was diagnosed in a wild reindeer buck on Hardangervidda, south of the Nordfjella area, in 2020. Further cases of CWD were also identified in moose, with a total of eight in Norway, four in Sweden, and two cases in Finland. The mean age of these cases is 14.7 years, and the pathological features are different from North American CWD and from the Norwegian reindeer cases, resembling atypical prion diseases such as Nor98/atypical scrapie and H- and L-forms of BSE. In this review, these moose cases are referred to as atypical CWD. In addition, two cases were diagnosed in red deer (Cervus elaphus) in Norway. The emergence of CWD in Europe is a threat to European cervid populations, and, potentially, a food-safety challenge, calling for a swift, evidence-based response. Here, we review data on surveillance, epidemiology, and disease characteristics, including prion strain features of the newly identified European CWD agents.

Clinical Use of Improved Diagnostic Testing for Detection of Prion Disease

Prion diseases are difficult to recognize as many symptoms are shared among other neurologic pathologies and the full spectra of symptoms usually do not appear until late in the disease course. Additionally, many commonly used laboratory markers are non-specific to prion disease. The recent introduction of second-generation real time quaking induced conversion (RT-QuIC) has revolutionized pre-mortem diagnosis of prion disease due to its extremely high sensitivity and specificity. However, RT-QuIC does not provide prognostic data and has decreased diagnostic accuracy in some rarer, atypical prion diseases. The objective of this review is to provide an overview of the current clinical utility of fluid-based biomarkers, neurodiagnostic testing, and brain imaging in the diagnosis of prion disease and to suggest guidelines for their clinical use, with a focus on rarer prion diseases with atypical features. Recent advancements in laboratory-based testing and imaging criteria have shown improved diagnostic accuracy and prognostic potential in prion disease, but because these diagnostic tests are not sensitive in some prion disease subtypes and diagnostic test sensitivities are unknown in the event that CWD transmits to humans, it is important to continue investigations into the clinical utility of various testing modalities.

Cervid Prion Protein Polymorphisms: Role in Chronic Wasting Disease Pathogenesis

Chronic wasting disease (CWD) is a prion disease found in both free-ranging and farmed cervids. Susceptibility of these animals to CWD is governed by various exogenous and endogenous factors. Past studies have demonstrated that polymorphisms within the prion protein (PrP) sequence itself affect an animal's susceptibility to CWD. PrP polymorphisms can modulate CWD pathogenesis in two ways: the ability of the endogenous prion protein (PrP^C) to convert into infectious prions (PrP^Sc) or it can give rise to novel prion strains. In vivo studies in susceptible cervids, complemented by studies in transgenic mice expressing the corresponding cervid PrP sequence, show that each polymorphism has distinct effects on both PrP^C and PrP^Sc. It is not entirely clear how these polymorphisms are responsible for these effects, but in vitro studies suggest they play a role in modifying PrP epitopes crucial for PrP^C to PrP^Sc conversion and determining PrP^C stability. PrP polymorphisms are unique to one or two cervid species and most confer a certain degree of reduced susceptibility to CWD. However, to date, there are no reports of polymorphic cervid PrP alleles providing absolute resistance to CWD. Studies on polymorphisms have focused on those found in CWD-endemic areas, with the hope that understanding the role of an animal's genetics in CWD can help to predict, contain, or prevent transmission of CWD.

Prion protein glycans reduce intracerebral fibril formation and spongiosis in prion disease

Posttranslational modifications (PTMs) are common among proteins that aggregate in neurodegenerative disease, yet how PTMs impact the aggregate conformation and disease progression remains unclear. By engineering knockin mice expressing prion protein (PrP) lacking 2 N-linked glycans (Prnp180Q/196Q), we provide evidence that glycans reduce spongiform degeneration and hinder plaque formation in prion disease. Prnp180Q/196Q mice challenged with 2 subfibrillar, non-plaque-forming prion strains instead developed plaques highly enriched in ADAM10-cleaved PrP and heparan sulfate (HS). Intriguingly, a third strain composed of intact, glycophosphatidylinositol-anchored (GPI-anchored) PrP was relatively unchanged, forming diffuse, HS-deficient deposits in both the Prnp180Q/196Q and WT mice, underscoring the pivotal role of the GPI-anchor in driving the aggregate conformation and disease phenotype. Finally, knockin mice expressing triglycosylated PrP (Prnp187N) challenged with a plaque-forming prion strain showed a phenotype reversal, with a striking disease acceleration and switch from plaques to predominantly diffuse, subfibrillar deposits. Our findings suggest that the dominance of subfibrillar aggregates in prion disease is due to the replication of GPI-anchored prions, with fibrillar plaques forming from poorly glycosylated, GPI-anchorless prions that interact with extracellular HS. These studies provide insight into how PTMs impact PrP interactions with polyanionic cofactors, and highlight PTMs as a major force driving the prion disease phenotype.

Prion protein post-translational modifications modulate heparan sulfate binding and limit aggregate size in prion disease

Many aggregation-prone proteins linked to neurodegenerative disease are post-translationally modified during their biogenesis. In vivo pathogenesis studies have suggested that the presence of post-translational modifications can shift the aggregate assembly pathway and profoundly alter the disease phenotype. In prion disease, the N-linked glycans and GPI-anchor on the prion protein (PrP) impair fibril assembly. However, the relevance of the two glycans to aggregate structure and disease progression remains unclear. Here we show that prion-infected knockin mice expressing an additional PrP glycan (tri-glycosylated PrP) develop new plaque-like deposits on neuronal cell membranes, along the subarachnoid space, and periventricularly, suggestive of high prion mobility and transit through the interstitial fluid. These plaque-like deposits were largely non-congophilic and composed of full length, uncleaved PrP, indicating retention of the glycophosphatidylinositol (GPI) anchor. Prion aggregates sedimented in low density fractions following ultracentrifugation, consistent with oligomers, and bound low levels of heparan sulfate (HS) similar to other predominantly GPI-anchored prions. Collectively, these results suggest that highly glycosylated PrP primarily converts as a GPI-anchored glycoform, with low involvement of HS co-factors, limiting PrP assembly mainly to oligomers. Since PrP^C is highly glycosylated, these findings may explain the high frequency of diffuse, synaptic, and plaque-like deposits in the brain as well as the rapid conversion commonly observed in human and animal prion disease.

Shortening heparan sulfate chains prolongs survival and reduces parenchymal plaques in prion disease caused by mobile, ADAM10-cleaved prions

Cofactors are essential for driving recombinant prion protein into pathogenic conformers. Polyanions promote prion aggregation in vitro, yet the cofactors that modulate prion assembly in vivo remain largely unknown. Here we report that the endogenous glycosaminoglycan, heparan sulfate (HS), impacts prion propagation kinetics and deposition sites in the brain. Exostosin-1 haploinsufficient (Ext1^+/-) mice, which produce short HS chains, show a prolonged survival and a redistribution of plaques from the parenchyma to vessels when infected with fibrillar prions, and a modest delay when infected with subfibrillar prions. Notably, the fibrillar, plaque-forming prions are composed of ADAM10-cleaved prion protein lacking a glycosylphosphatidylinositol anchor, indicating that these prions are mobile and assemble extracellularly. By analyzing the prion-bound HS using liquid chromatography-mass spectrometry (LC-MS), we identified the disaccharide signature of HS differentially bound to fibrillar compared to subfibrillar prions, and found approximately 20-fold more HS bound to the fibrils. Finally, LC-MS of prion-bound HS from human patients with familial and sporadic prion disease also showed distinct HS signatures and higher HS levels associated with fibrillar prions. This study provides the first in vivo evidence of an endogenous cofactor that accelerates prion disease progression and enhances parenchymal deposition of ADAM10-cleaved, mobile prions.

Electron microscopic and confocal laser microscopy analysis of amyloid plaques in chronic wasting disease transmitted to transgenic mice

We report here on the ultrastructure of amyloid plaques in chronic wasting disease (CWD) transmitted to Tg20 transgenic mice overexpressing prion protein (PrP^c). We identified three main types of amyloid deposits in mCWD: large amyloid deposits, unicentric plaques similar to kuru plaques in human prion diseases and multicentric plaques reminiscent of plaques typical of GSS. The most unique type of plaques were large subpial amyloid deposits. They were composed of large areas of amyloid fibrils but did not form "star-like" appearances of unicentric plaques. All types of plaques were totally devoid of dystrophic neuritic elements. However, numerous microglial cells invaded them. The plaques observed by confocal laser microscope were of the same types as those analyzed by electron microscopy. Neuronal processes surrounding the plaques did not show typical features of neuroaxonal dystrophy.

Generation of novel neuroinvasive prions following intravenous challenge

Prions typically spread into the central nervous system (CNS), likely via peripheral nerves. Yet prion conformers differ in their capacity to penetrate the CNS; certain fibrillar prions replicate persistently in lymphoid tissues with no CNS entry, leading to chronic silent carriers. Subclinical carriers of variant Creutzfeldt-Jakob (vCJD) prions in the United Kingdom have been estimated at 1:2000, and vCJD prions have been transmitted through blood transfusion, however, the circulating prion conformers that neuroinvade remain unclear. Here we investigate how prion conformation impacts brain entry of transfused prions by challenging mice intravenously to subfibrillar and fibrillar strains. We show that most strains infiltrated the brain and caused terminal disease, however, the fibrillar prions showed reduced CNS entry in a strain-dependent manner. Strikingly, the highly fibrillar mCWD prion strain replicated in the spleen and emerged in the brain as a novel strain, indicating that a new neuroinvasive prion had been generated from a previously non-neuroinvasive strain. The new strain showed altered plaque morphology, brain regions targeted and biochemical properties and these properties were maintained upon intracerebral passage. Intracerebral passage of prion-infected spleen re-created the new strain. Splenic prions resembled the new strain biochemically and intracerebral passage of prion-infected spleen re-created the new strain, collectively suggesting splenic prion replication as a potential source. Taken together, these results indicate that intravenous exposure to prion-contaminated blood or blood products may generate novel neuroinvasive prion conformers and disease phenotypes, potentially arising from prion replication in non-neural tissues or from conformer selection.

Chronic wasting disease: an evolving prion disease of cervids

Chronic wasting disease (CWD) is a relatively new and burgeoning prion epidemic of deer, elk, reindeer, and moose, which are members of the cervid family. While the disease was first described in captive deer, its subsequent discovery in various species of free-ranging animals makes it the only currently recognized prion disorder of both wild and farmed animals. In addition to its expanding range of host species, CWD continues to spread from North America to new geographic areas, including South Korea, and most recently Norway, marking the first time this disease was detected in Europe. Its unparalleled efficiency of contagious transmission, combined with high densities of deer in certain areas, complicates strategies for controlling CWD, raising concerns about its potential for spread to new species. Because there is a high prevalence of CWD in deer and elk, which are commonly hunted and consumed by humans, and since prions from cattle with bovine spongiform encephalopathy have been transmitted to humans causing variant Creutzfeldt-Jakob disease, the possibility of zoonotic transmission of CWD is particularly concerning. Here we review the clinical and pathologic features of CWD and its disturbing epidemiology, and discuss features that affect its transmission, including genetic susceptibility, pathogenesis, and agent strain variability. Finally, we discuss evidence that speaks to the potential for zoonotic transmission of this emerging disease.

Molecular Mechanisms of Chronic Wasting Disease Prion Propagation

Prion disease epidemics, which have been unpredictable recurrences, are of significant concern for animal and human health. Examples include kuru, once the leading cause of death among the Fore people in Papua New Guinea and caused by mortuary feasting; bovine spongiform encephalopathy (BSE) and its subsequent transmission to humans in the form of variant Creutzfeldt-Jakob disease (vCJD), and repeated examples of large-scale prion disease epidemics in animals caused by contaminated vaccines. The etiology of chronic wasting disease (CWD), a relatively new and burgeoning prion epidemic in deer, elk, and moose (members of the cervid family), is more enigmatic. The disease was first described in captive and later in wild mule deer and subsequently in free-ranging as well as captive Rocky Mountain elk, white-tailed deer, and most recently moose. It is therefore the only recognized prion disorder of both wild and captive animals. In addition to its expanding range of hosts, CWD continues to spread to new geographical areas, including recent cases in Norway. The unparalleled efficiency of the contagious transmission of the disease combined with high densities of deer in certain areas of North America complicates strategies for controlling CWD and raises concerns about its potential spread to new species. Because there is a high prevalence of CWD in deer and elk, which are commonly hunted and consumed by humans, the possibility of zoonotic transmission is particularly concerning. Here, we review the current status of naturally occurring CWD and describe advances in our understanding of its molecular pathogenesis, as shown by studies of CWD prions in novel in vivo and in vitro systems.

Scientific opinion on chronic wasting disease (II)

The European Commission asked EFSA for a scientific opinion on chronic wasting disease in two parts. Part one, on surveillance, animal health risk-based measures and public health risks, was published in January 2017. This opinion (part two) addresses the remaining Terms of Reference, namely, 'are the conclusions and recommendations in the EFSA opinion of June 2004 on diagnostic methods for chronic wasting disease still valid? If not, an update should be provided', and 'update the conclusions of the 2010 EFSA opinion on the results of the European Union survey on chronic wasting disease in cervids, as regards its occurrence in the cervid population in the European Union'. Data on the performance of authorised rapid tests in North America are not comprehensive, and are more limited than those available for the tests approved for statutory transmissible spongiform encephalopathies surveillance applications in cattle and sheep. There are no data directly comparing available rapid test performances in cervids. The experience in Norway shows that the Bio-Rad TeSeE™ SAP test, immunohistochemistry and western blotting have detected reindeer, moose and red deer cases. It was shown that testing both brainstem and lymphoid tissue from each animal increases the surveillance sensitivity. Shortcomings in the previous EU survey limited the reliability of inferences that could be made about the potential disease occurrence in Europe. Subsequently, testing activity in Europe was low, until the detection of the disease in Norway, triggering substantial testing efforts in that country. Available data neither support nor refute the conclusion that chronic wasting disease does not occur widely in the EU and do not preclude the possibility that the disease was present in Europe before the survey was conducted. It appears plausible that chronic wasting disease could have become established in Norway more than a decade ago.

Prion Strains and Transmission Barrier Phenomena

Several experimental evidences show that prions are non-conventional pathogens, which physical support consists only in proteins. This finding raised questions regarding the observed prion strain-to-strain variations and the species barrier that happened to be crossed with dramatic consequences on human health and veterinary policies during the last 3 decades. This review presents a focus on a few advances in the field of prion structure and prion strains characterization: from the historical approaches that allowed the concept of prion strains to emerge, to the last results demonstrating that a prion strain may in fact be a combination of a few quasi species with subtle biophysical specificities. Then, we will focus on the current knowledge on the factors that impact species barrier strength and species barrier crossing. Finally, we present probable scenarios on how the interaction of strain properties with host characteristics may account for differential selection of new conformer variants and eventually species barrier crossing.

Enhanced neuroinvasion by smaller, soluble prions

Infectious prion aggregates can propagate from extraneural sites into the brain with remarkable efficiency, likely transported via peripheral nerves. Yet not all prions spread into the brain, and the physical properties of a prion that is capable of transit within neurons remain unclear. We hypothesized that small, diffusible aggregates spread into the CNS via peripheral nerves. Here we used a structurally diverse panel of prion strains to analyze how the prion conformation impacts transit into the brain. Two prion strains form fibrils visible ultrastructurally in the brain in situ, whereas three strains form diffuse, subfibrillar prion deposits and no visible fibrils. The subfibrillar strains had significantly higher levels of soluble prion aggregates than the fibrillar strains. Primary neurons internalized both the subfibrillar and fibril-forming prion strains by macropinocytosis, and both strain types were transported from the axon terminal to the cell body in vitro. However in mice, only the predominantly soluble, subfibrillar prions, and not the fibrillar prions, were efficiently transported from the tongue to the brain. Sonicating a fibrillar prion strain increased the solubility and enabled prions to spread into the brain in mice, as evident by a 40% increase in the attack rate, indicating that an increase in smaller particles enhances prion neuroinvasion. Our data suggest that the small, highly soluble prion particles have a higher capacity for transport via nerves. These findings help explain how prions that predominantly assemble into subfibrillar states can more effectively traverse into and out of the CNS, and suggest that promoting fibril assembly may slow the neuron-to-neuron spread of protein aggregates.

Post-translational modifications in PrP expand the conformational diversity of prions in vivo

Misfolded prion protein aggregates (PrP^Sc) show remarkable structural diversity and are associated with highly variable disease phenotypes. Similarly, other proteins, including amyloid-β, tau, α-synuclein, and serum amyloid A, misfold into distinct conformers linked to different clinical diseases through poorly understood mechanisms. Here we use mice expressing glycophosphatidylinositol (GPI)-anchorless prion protein, PrP^C, together with hydrogen-deuterium exchange coupled with mass spectrometry (HXMS) and a battery of biochemical and biophysical tools to investigate how post-translational modifications impact the aggregated prion protein properties and disease phenotype. Four GPI-anchorless prion strains caused a nearly identical clinical and pathological disease phenotype, yet maintained their structural diversity in the anchorless state. HXMS studies revealed that GPI-anchorless PrP^Sc is characterized by substantially higher protection against hydrogen/deuterium exchange in the C-terminal region near the N-glycan sites, suggesting this region had become more ordered in the anchorless state. For one strain, passage of GPI-anchorless prions into wild type mice led to the emergence of a novel strain with a unique biochemical and phenotypic signature. For the new strain, histidine hydrogen-deuterium mass spectrometry revealed altered packing arrangements of β-sheets that encompass residues 139 and 186 of PrP^Sc. These findings show how variation in post-translational modifications may explain the emergence of new protein conformations in vivo and also provide a basis for understanding how the misfolded protein structure impacts the disease.

Cross-species transmission of CWD prions

Prions cause fatal neurodegenerative diseases in humans and animals and can be transmitted zoonotically. Chronic wasting disease (CWD) is a highly transmissible prion disease of wild deer and elk that affects cervids over extensive regions of the United States and Canada. The risk of cross-species CWD transmission has been experimentally evaluated in a wide array of mammals, including non-human primates and mouse models expressing human cellular prion protein. Here we review the determinants of cross-species CWD transmission, and propose a model that may explain a structural barrier for CWD transmission to humans.

Chronic wasting disease (CWD) in cervids

In April and May of 2016, Norway confirmed two cases of chronic wasting disease (CWD) in a wild reindeer and a wild moose, respectively. In the light of this emerging issue, the European Commission requested EFSA to recommend surveillance activities and, if necessary, additional animal health risk-based measures to prevent the introduction of the disease and the spread into/within the EU, specifically Estonia, Finland, Iceland, Latvia, Lithuania, Norway, Poland and Sweden, and considering seven wild, semidomesticated and farmed cervid species (Eurasian tundra reindeer, Finnish (Eurasian) forest reindeer, moose, roe deer, white-tailed deer, red deer and fallow deer). It was also asked to assess any new evidence on possible public health risks related to CWD. A 3-year surveillance system is proposed, differing for farmed and wild or semidomesticated cervids, with a two-stage sampling programme at the farm/geographically based population unit level (random sampling) and individual level (convenience sampling targeting high-risk animals). The current derogations of Commission Implementing Decision (EU) 2016/1918 present a risk of introduction of CWD into the EU. Measures to prevent the spread of CWD within the EU are dependent upon the assumption that the disease is already present; this is currently unknown. The measures listed are intended to contain (limit the geographic extent of a focus) and/or to control (actively stabilise/reduce infection rates in an affected herd or population) the disease where it occurs. With regard to the zoonotic potential, the human species barrier for CWD prions does not appear to be absolute. These prions are present in the skeletal muscle and other edible tissues, so humans may consume infected material in enzootic areas. Epidemiological investigations carried out to date make no association between the occurrence of sporadic Creutzfeldt-Jakob disease in humans and exposure to CWD prions.

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.

Unique Properties of the Rabbit Prion Protein Oligomer

Prion diseases, also known as transmissible spongiform encephalopathies (TSEs), are a group of fatal neurodegenerative disorders infecting both humans and animals. Recent works have demonstrated that the soluble prion protein oligomer (PrP^O), the intermediate of the conformational transformation from the host-derived cellular form (PrP^C) to the disease-associated Scrapie form (PrP^Sc), exerts the major neurotoxicity in vitro and in vivo. Rabbits show strong resistance to TSEs, the underlying mechanism is unclear to date. It is expected that the relative TSEs-resistance of rabbits is closely associated with the unique properties of rabbit prion protein oligomer which remain to be addressed in detail. In the present work, we prepared rabbit prion protein oligomer (recRaPrP^O) and human prion protein oligomer (recHuPrP^O) under varied conditions, analyzed the effects of pH, NaCl concentration and incubation temperature on the oligomerization, and compared the properties of recRaPrP^O and recHuPrP^O. We found that several factors facilitated the formation of prion protein oligomers, including low pH, high NaCl concentration, high incubation temperature and low conformational stability of monomeric prion protein. RecRaPrP^O was formed more slowly than recHuPrP^O at physiological-like conditions (< 57°C, < 150 mM NaCl). Furthermore, recRaPrP^O possessed higher susceptibility to proteinase K and lower cytotoxicity in vitro than recHuPrP^O. These unique properties of recRaPrP^O might substantially contribute to the TSEs-resistance of rabbits. Our work sheds light on the oligomerization of prion proteins and is of benefit to mechanistic understanding of TSEs-resistance of rabbits.

Transgenic Mouse Bioassay: Evidence That Rabbits Are Susceptible to a Variety of Prion Isolates

Interspecies transmission of prions is a well-established phenomenon, both experimentally and under field conditions. Upon passage through new hosts, prion strains have proven their capacity to change their properties and this is a source of strain diversity which needs to be considered when assessing the potential risks associated with consumption of prion contaminated protein sources. Rabbits were considered for decades to be a prion resistant species until proven otherwise recently. To determine the extent of rabbit susceptibility to prions and to assess the effects of passage of different prion strains through this species a transgenic mouse model overexpressing rabbit PrPC was developed (TgRab). Intracerebral challenges with prion strains originating from a variety of species including field isolates (ovine SSBP/1 scrapie, Nor98- scrapie; cattle BSE, BSE-L and cervid CWD), experimental murine strains (ME7 and RML) and experimentally obtained ruminant (sheepBSE) and rabbit (de novo NZW) strains were performed. On first passage TgRab were susceptible to the majority of prions (Cattle BSE, SheepBSE, BSE-L, de novo NZW, ME7 and RML) tested with the exception of SSBP/1 scrapie, CWD and Nor98 scrapie. Furthermore, TgRab were capable of propagating strain-specific features such as differences in incubation periods, histological brain lesions, abnormal prion (PrPd) deposition profiles and proteinase-K (PK) resistant western blotting band patterns. Our results confirm previous studies proving that rabbits are not resistant to prion infection and show for the first time that rabbits are susceptible to PrPd originating in a number of other species. This should be taken into account when choosing protein sources to feed rabbits.

Collapsed state of polyglutamic acid results in amyloid spherulite formation

Self-assembly of proteins and peptides into amyloid fibrils involves multiple distinct intermediates and late-stage fibrillar polymorphs. Understanding the conditions and mechanisms that promote the formation of one type of intermediate and polymorph over the other represents a fundamental challenge. Answers to this question are also of immediate biomedical relevance since different amyloid aggregate species have been shown to have distinct pathogenic potencies. One amyloid polymorph that has received comparatively little attention are amyloid spherulites. Here we report that self-assembly of the intrinsically disordered polymer poly(L-glutamic) acid (PLE) can generate amyloid spherulites. We characterize spherulite growth kinetics, as well as the morphological, optical and tinctorial features of this amyloid polymorph previously unreported for PLE. We find that PLE spherulites share both tinctorial and structural characteristics with their amyloid fibril counterparts. Differences in PLE's molecular weight, polydispersity or chemistry could not explain the selective propensity toward either fibril or spherulite formation. Instead, we provide evidence that PLE polymers can exist in either a collapsed globule or an extended random coil conformation. The collapsed globule consistently produces spherulites while the extended coil assembles into disordered fibril bundles. This results suggests that these 2 PLE conformers directly affect the morphology of the resulting macroscopic amyloid assembly.

Human prion protein sequence elements impede cross-species chronic wasting disease transmission

Chronic wasting disease (CWD) is a fatal prion disease of North American deer and elk and poses an unclear risk for transmission to humans. Human exposure to CWD occurs through hunting activities and consumption of venison from prion-infected animals. Although the amino acid residues of the prion protein (PrP) that prevent or permit human CWD infection are unknown, NMR-based structural studies suggest that the β2-α2 loop (residues 165-175) may impact species barriers. Here we sought to define PrP sequence determinants that affect CWD transmission to humans. We engineered transgenic mice that express human PrP with four amino acid substitutions that result in expression of PrP with a β2-α2 loop (residues 165-175) that exactly matches that of elk PrP. Compared with transgenic mice expressing unaltered human PrP, mice expressing the human-elk chimeric PrP were highly susceptible to elk and deer CWD prions but were concurrently less susceptible to human Creutzfeldt-Jakob disease prions. A systematic in vitro survey of amino acid differences between humans and cervids identified two additional residues that impacted CWD conversion of human PrP. This work identifies amino acids that constitute a substantial structural barrier for CWD transmission to humans and helps illuminate the molecular requirements for cross-species prion transmission.

Prion transmission prevented by modifying the β2-α2 loop structure of host PrPC

Zoonotic prion transmission was reported after the bovine spongiform encephalopathy (BSE) epidemic, when >200 cases of prion disease in humans were diagnosed as variant Creutzfeldt-Jakob disease. Assessing the risk of cross-species prion transmission remains challenging. We and others have studied how specific amino acid residue differences between species impact prion conversion and have found that the β2-α2 loop region of the mouse prion protein (residues 165-175) markedly influences infection by sheep scrapie, BSE, mouse-adapted scrapie, deer chronic wasting disease, and hamster-adapted scrapie prions. The tyrosine residue at position 169 is strictly conserved among mammals and an aromatic side chain in this position is essential to maintain a 310-helical turn in the β2-α2 loop. Here we examined the impact of the Y169G substitution together with the previously described S170N, N174T "rigid loop" substitutions on cross-species prion transmission in vivo and in vitro. We found that transgenic mice expressing mouse PrP containing the triple-amino acid substitution completely resisted infection with two strains of mouse prions and with deer chronic wasting disease prions. These studies indicate that Y169 is important for prion formation, and they provide a strong indication that variation of the β2-α2 loop structure can modulate interspecies prion transmission.

Lesion profiling and subcellular prion localization of cervid chronic wasting disease in domestic cats

Chronic wasting disease (CWD) is an efficiently transmitted, fatal, and progressive prion disease of cervids with an as yet to be fully clarified host range. While outbred domestic cats (Felis catus) have recently been shown to be susceptible to experimental CWD infection, the neuropathologic features of the infection are lacking. Such information is vital to provide diagnostic power in the event of natural interspecies transmission and insights into host and strain interactions in interspecies prion infection. Using light microscopy and immunohistochemistry, we detail the topographic pattern of neural spongiosis (the "lesion profile") and the distribution of misfolded prion protein in the primary and secondary passage of feline CWD (Fel(CWD)). We also evaluated cellular and subcellular associations between misfolded prion protein (PrP(D)) and central nervous system neurons and glial cell populations. From these studies, we (1) describe the novel neuropathologic profile of Fel(CWD), which is distinct from either cervid CWD or feline spongiform encephalopathy (FSE), and (2) provide evidence of serial passage-associated interspecies prion adaptation. In addition, we demonstrate through confocal analysis the successful co-localization of PrP(D) with neurons, astrocytes, microglia, lysosomes, and synaptophysin, which, in part, implicates each of these in the neuropathology of Fel(CWD). In conclusion, this work illustrates the simultaneous role of both host and strain in the development of a unique Fel(CWD) neuropathologic profile and that such a profile can be used to discriminate between Fel(CWD) and FSE.

Bovine spongiform encephalopathy induces misfolding of alleged prion-resistant species cellular prion protein without altering its pathobiological features

Bovine spongiform encephalopathy (BSE) prions were responsible for an unforeseen epizootic in cattle which had a vast social, economic, and public health impact. This was primarily because BSE prions were found to be transmissible to humans. Other species were also susceptible to BSE either by natural infection (e.g., felids, caprids) or in experimental settings (e.g., sheep, mice). However, certain species closely related to humans, such as canids and leporids, were apparently resistant to BSE. In vitro prion amplification techniques (saPMCA) were used to successfully misfold the cellular prion protein (PrP(c)) of these allegedly resistant species into a BSE-type prion protein. The biochemical and biological properties of the new prions generated in vitro after seeding rabbit and dog brain homogenates with classical BSE were studied. Pathobiological features of the resultant prion strains were determined after their inoculation into transgenic mice expressing bovine and human PrP(C). Strain characteristics of the in vitro-adapted rabbit and dog BSE agent remained invariable with respect to the original cattle BSE prion, suggesting that the naturally low susceptibility of rabbits and dogs to prion infections should not alter their zoonotic potential if these animals became infected with BSE. This study provides a sound basis for risk assessment regarding prion diseases in purportedly resistant species.

Chronic wasting disease in bank voles: characterisation of the shortest incubation time model for prion diseases

In order to assess the susceptibility of bank voles to chronic wasting disease (CWD), we inoculated voles carrying isoleucine or methionine at codon 109 (Bv109I and Bv109M, respectively) with CWD isolates from elk, mule deer and white-tailed deer. Efficient transmission rate (100%) was observed with mean survival times ranging from 156 to 281 days post inoculation. Subsequent passages in Bv109I allowed us to isolate from all CWD sources the same vole-adapted CWD strain (Bv(109I)CWD), typified by unprecedented short incubation times of 25-28 days and survival times of ∼35 days. Neuropathological and molecular characterisation of Bv(109I)CWD showed that the classical features of mammalian prion diseases were all recapitulated in less than one month after intracerebral inoculation. Bv(109I)CWD was characterised by a mild and discrete distribution of spongiosis and relatively low levels of protease-resistant PrP(Sc) (PrP(res)) in the same brain regions. Despite the low PrP(res) levels and the short time lapse available for its accumulation, end-point titration revealed that brains from terminally-ill voles contained up to 10(8,4) i.c. ID50 infectious units per gram. Bv(109I)CWD was efficiently replicated by protein misfolding cyclic amplification (PMCA) and the infectivity faithfully generated in vitro, as demonstrated by the preservation of the peculiar Bv(109I)CWD strain features on re-isolation in Bv109I. Overall, we provide evidence that the same CWD strain was isolated in Bv109I from the three-cervid species. Bv(109I)CWD showed unique characteristics of "virulence", low PrP(res) accumulation and high infectivity, thus providing exceptional opportunities to improve basic knowledge of the relationship between PrP(Sc), neurodegeneration and infectivity.

A closer look at prion strains: characterization and important implications

Prions are infectious proteins that are responsible for transmissible spongiform encephalopathies (TSEs) and consist primarily of scrapie prion protein (PrP^Sc), a pathogenic isoform of the host-encoded cellular prion protein (PrP^C). The absence of nucleic acids as essential components of the infectious prions is the most striking feature associated to these diseases. Additionally, different prion strains have been isolated from animal diseases despite the lack of DNA or RNA molecules. Mounting evidence suggests that prion-strain-specific features segregate with different PrP^Sc conformational and aggregation states.

Strains are of practical relevance in prion diseases as they can drastically differ in many aspects, such as incubation period, PrP^Sc biochemical profile (e.g., electrophoretic mobility and glycoform ratio) and distribution of brain lesions. Importantly, such different features are maintained after inoculation of a prion strain into genetically identical hosts and are relatively stable across serial passages.

This review focuses on the characterization of prion strains and on the wide range of important implications that the study of prion strains involves.

Naturally prion resistant mammals: a utopia?

Each known abnormal prion protein (PrP (Sc) ) is considered to have a specific range and therefore the ability to infect some species and not others. Consequently, some species have been assumed to be prion disease resistant as no successful natural or experimental challenge infections have been reported. This assumption suggested that, independent of the virulence of the PrP (Sc) strain, normal prion protein (PrP (C) ) from these 'resistant' species could not be induced to misfold. Numerous in vitro and in vivo studies trying to corroborate the unique properties of PrP (Sc) have been undertaken. The results presented in the article "Rabbits are not resistant to prion infection" demonstrated that normal rabbit PrP (C) , which was considered to be resistant to prion disease, can be misfolded to PrP (Sc) and subsequently used to infect and transmit a standard prion disease to leporids. Using the concept of species resistance to prion disease, we will discuss the mistake of attributing species specific prion disease resistance based purely on the absence of natural cases and incomplete in vivo challenges. The BSE epidemic was partially due to an underestimation of species barriers. To repeat this error would be unacceptable, especially if present knowledge and techniques can show a theoretical risk. Now that the myth of prion disease resistance has been refuted it is time to re-evaluate, using the new powerful tools available in modern prion laboratories, whether any other species could be at risk.

Structure of the β2-α2 loop and interspecies prion transmission

Prions are misfolded, aggregated conformers of the prion protein that can be transmitted between species. The precise determinants of interspecies transmission remain unclear, although structural similarity between the infectious prion and host prion protein is required for efficient conversion to the misfolded conformer. The β2-α2 loop region of endogenous prion protein, PrP(C), has been implicated in barriers to prion transmission. We recently discovered that conversion was efficient when incoming and host prion proteins had similar β2-α2 loop structures; however, the roles of primary vs. secondary structural homology could not be distinguished. Here we uncouple the effect of primary and secondary structural homology of the β2-α2 loop on prion conversion. We inoculated prions from animals having a disordered or an ordered β2-α2 loop into mice having a disordered loop or an ordered loop due to a single residue substitution (D167S). We found that prion conversion was driven by a homologous primary structure and occurred independently of a homologous secondary structure. Similarly, cell-free conversion using PrP(C) from mice with disordered or ordered loops and prions from 5 species correlated with primary but not secondary structural homology of the loop. Thus, our findings support a model in which efficient interspecies prion conversion is determined by small stretches of the primary sequence rather than the secondary structure of PrP.

Rabbits are not resistant to prion infection

The ability of prions to infect some species and not others is determined by the transmission barrier. This unexplained phenomenon has led to the belief that certain species were not susceptible to transmissible spongiform encephalopathies (TSEs) and therefore represented negligible risk to human health if consumed. Using the protein misfolding cyclic amplification (PMCA) technique, we were able to overcome the species barrier in rabbits, which have been classified as TSE resistant for four decades. Rabbit brain homogenate, either unseeded or seeded in vitro with disease-related prions obtained from different species, was subjected to serial rounds of PMCA. De novo rabbit prions produced in vitro from unseeded material were tested for infectivity in rabbits, with one of three intracerebrally challenged animals succumbing to disease at 766 d and displaying all of the characteristics of a TSE, thereby demonstrating that leporids are not resistant to prion infection. Material from the brain of the clinically affected rabbit containing abnormal prion protein resulted in a 100% attack rate after its inoculation in transgenic mice overexpressing rabbit PrP. Transmissibility to rabbits (>470 d) has been confirmed in 2 of 10 rabbits after intracerebral challenge. Despite rabbits no longer being able to be classified as resistant to TSEs, an outbreak of "mad rabbit disease" is unlikely.

Biochemical properties of highly neuroinvasive prion strains

Infectious prions propagate from peripheral entry sites into the central nervous system (CNS), where they cause progressive neurodegeneration that ultimately leads to death. Yet the pathogenesis of prion disease can vary dramatically depending on the strain, or conformational variant of the aberrantly folded and aggregated protein, PrP^Sc. Although most prion strains invade the CNS, some prion strains cannot gain entry and do not cause clinical signs of disease. The conformational basis for this remarkable variation in the pathogenesis among strains is unclear. Using mouse-adapted prion strains, here we show that highly neuroinvasive prion strains primarily form diffuse aggregates in brain and are noncongophilic, conformationally unstable in denaturing conditions, and lead to rapidly lethal disease. These neuroinvasive strains efficiently generate PrP^Sc over short incubation periods. In contrast, the weakly neuroinvasive prion strains form large fibrillary plaques and are stable, congophilic, and inefficiently generate PrP^Sc over long incubation periods. Overall, these results indicate that the most neuroinvasive prion strains are also the least stable, and support the concept that the efficient replication and unstable nature of the most rapidly converting prions may be a feature linked to their efficient spread into the CNS.

Prion diseases are fatal neurodegenerative disorders that are also infectious. Prions are composed of a misfolded, aggregated form of a normal cellular protein that is highly expressed in neurons. Prion- infected individuals show variability in the clinical signs and brain regions that selectively accumulate prions, even within the same species expressing the same prion protein sequence. The basis of these divergent disease phenotypes is unclear, but is thought to be due to different conformations of the misfolded prion protein, known as strains. Here we characterized the neuropathology and biochemical properties of prion strains that efficiently or poorly invade the CNS from their peripheral entry site. We show that prion strains that efficiently invade the CNS also cause a rapidly terminal disease after an intracerebral exposure. These rapidly lethal strains were unstable when exposed to denaturants or high temperatures, and efficiently accumulated misfolded prion protein over a short incubation period in vivo. Our findings indicate that the most invasive, rapidly spreading strains are also the least conformationally stable.

Selection and characterization of DNA aptamers against PrP(Sc)

Transmissible spongiform encephalopathies (TSEs) are a group of zoonotic and fatal neurodegenerative disorders that affect humans and animals. The pathogenesis of TSEs involves a conformational conversion of the cellular prion protein (PrP) into abnormal isoforms. Currently, cellular and pathological forms of PrP are differentiated using specific antibody-based analyses that are resource intensive and not applicable to all species and strains. Thus, there is an urgent need for sensitive and efficient assays that can detect pathological forms of PrP. Using systematic evolution of ligands by exponential enrichment, we developed DNA aptamers that can differentiate normal and abnormal PrP isoforms. These aptamers represent the first reagents that can identify pathological isoforms of PrP across multiple host species. Second, they are able to distinguish different strains of prions. Third, they can be used to detect prions in peripheral blood cells, which are otherwise undetectable using conventional antibody-based detection methods. Thus, DNA aptamers offer promise for the development of presymptomatic screens of tissue, blood and other body fluids for prion contamination.

Transgenic mouse models and prion strains

Here we review the known strain profiles of various prion diseases of animals and humans, and how transgenic mouse models are being used to elucidate basic molecular mechanisms of prion propagation and strain variation and for assessing the zoonotic potential of various animal prion strains.

Nucleic acid-free mutation of prion strains

While prions share the ability to propagate strain information with nucleic acid-based pathogens, it is unclear how they mutate and acquire fitness in the absence of this informational component. Because prion diseases occur as epidemics, understanding this mechanism is of paramount importance for implementing control strategies to limit their spread and for evaluating their zoonotic potential. Here we review emerging evidence indicating how prion protein primary structures, in concert with PrP(Sc) conformational compatibility, determine prion strain mutation.

Prion strain interactions are highly selective

Various misfolded and aggregated neuronal proteins commonly coexist in neurodegenerative disease, but whether the proteins coaggregate and alter the disease pathogenesis is unclear. Here, we used mixtures of distinct prion strains, which are believed to differ in conformation, to test the hypothesis that two different aggregates interact and change the disease in vivo. We tracked two prion strains in mice histopathologically and biochemically, as well as by spectral analysis of plaque-bound PTAA (polythiophene acetic acid), a conformation-sensitive fluorescent amyloid ligand. We found that prion strains interacted in a highly selective and strain-specific manner, with (1) no interaction, (2) hybrid plaque formation, or (3) blockage of one strain by a second (interference). The hybrid plaques were maintained on additional passage in vivo and each strain seemed to maintain its original conformational properties, suggesting that one strain served only as a scaffold for aggregation of the second strain. These findings not only further our understanding of prion strain interactions but also directly demonstrate interactions that may occur in other protein aggregate mixtures.

A molecular switch controls interspecies prion disease transmission in mice

Transmissible spongiform encephalopathies are lethal neurodegenerative disorders that present with aggregated forms of the cellular prion protein (PrPC), which are known as PrPSc. Prions from different species vary considerably in their transmissibility to xenogeneic hosts. The variable transmission barriers depend on sequence differences between incoming PrPSc and host PrPC and additionally, on strain-dependent conformational properties of PrPSc. The beta2-alpha2 loop region within PrPC varies substantially between species, with its structure being influenced by the residue types in the 2 amino acid sequence positions 170 (most commonly S or N) and 174 (N or T). In this study, we inoculated prions from 5 different species into transgenic mice expressing either disordered-loop or rigid-loop PrPC variants. Similar beta2-alpha2 loop structures correlated with efficient transmission, whereas dissimilar loops correlated with strong transmission barriers. We then classified literature data on cross-species transmission according to the 170S/N polymorphism. Transmission barriers were generally low between species with the same amino acid residue in position 170 and high between those with different residues. These findings point to a triggering role of the local beta2-alpha2 loop structure for prion transmissibility between different species.

Prion strain mutation determined by prion protein conformational compatibility and primary structure

Prions are infectious proteins composed of the abnormal disease-causing isoform PrPSc, which induces conformational conversion of the host-encoded normal cellular prion protein PrPC to additional PrPSc. The mechanism underlying prion strain mutation in the absence of nucleic acids remains unresolved. Additionally, the frequency of strains causing chronic wasting disease (CWD), a burgeoning prion epidemic of cervids, is unknown. Using susceptible transgenic mice, we identified two prevalent CWD strains with divergent biological properties but composed of PrPSc with indistinguishable biochemical characteristics. Although CWD transmissions indicated stable, independent strain propagation by elk PrPC, strain coexistence in the brains of deer and transgenic mice demonstrated unstable strain propagation by deer PrPC. The primary structures of deer and elk prion proteins differ at residue 226, which, in concert with PrPSc conformational compatibility, determines prion strain mutation in these cervids.

Susceptibilities of nonhuman primates to chronic wasting disease

A species barrier may protect humans from this disease.

Chronic wasting disease (CWD) is a transmissible spongiform encephalopathy, or prion disease, that affects deer, elk, and moose. Human susceptibility to CWD remains unproven despite likely exposure to CWD-infected cervids. We used 2 nonhuman primate species, cynomolgus macaques and squirrel monkeys, as human models for CWD susceptibility. CWD was inoculated into these 2 species by intracerebral and oral routes. After intracerebral inoculation of squirrel monkeys, 7 of 8 CWD isolates induced a clinical wasting syndrome within 33–53 months. The monkeys’ brains showed spongiform encephalopathy and protease-resistant prion protein (PrPres) diagnostic of prion disease. After oral exposure, 2 squirrel monkeys had PrPres in brain, spleen, and lymph nodes at 69 months postinfection. In contrast, cynomolgus macaques have not shown evidence of clinical disease as of 70 months postinfection. Thus, these 2 species differed in susceptibility to CWD. Because humans are evolutionarily closer to macaques than to squirrel monkeys, they may also be resistant to CWD.

In vitro studies of the transmission barrier

Protein Misfolding Cyclic Amplification (PMCA) has proved to be an efficient method mimicking in vitro some of the fundamental steps involved in prion replication in vivo. Thus, it can be used to efficiently replicate a variety of prion strains/species. The in vitro generated prions possess key prion features, i.e., they are infectious in vivo and maintain their strain specificity. One of the big challenges is its use for studying prion transmission barriers. PMCA has been efficiently used for adapting different prion species through a range of species barriers; however its capacity for overcoming purportedly unbreakable species barriers compels us to adapt it in order to use it as a reliable technique. In addition, this in vitro method might be a crucial tool in evaluating the potential risks of different prion strains (natural or experimentally generated in vitro) to humans and animals.

Prions: Protein Aggregation and Infectious Diseases

Transmissible spongiform encephalopathies (TSEs) are inevitably lethal neurodegenerative diseases that affect humans and a large variety of animals. The infectious agent responsible for TSEs is the prion, an abnormally folded and aggregated protein that propagates itself by imposing its conformation onto the cellular prion protein (PrPC) of the host. PrPCis necessary for prion replication and for prion-induced neurodegeneration, yet the proximal causes of neuronal injury and death are still poorly understood. Prion toxicity may arise from the interference with the normal function of PrPC, and therefore, understanding the physiological role of PrPCmay help to clarify the mechanism underlying prion diseases. Here we discuss the evolution of the prion concept and how prion-like mechanisms may apply to other protein aggregation diseases. We describe the clinical and the pathological features of the prion diseases in human and animals, the events occurring during neuroinvasion, and the possible scenarios underlying brain damage. Finally, we discuss potential antiprion therapies and current developments in the realm of prion diagnostics.

Spontaneous generation of prion infectivity in fatal familial insomnia knockin mice

A crucial tenet of the prion hypothesis is that misfolding of the prion protein (PrP) induced by mutations associated with familial prion disease is, in an otherwise normal mammalian brain, sufficient to generate the infectious agent. Yet this has never been demonstrated. We engineered knockin mice to express a PrP mutation associated with a distinct human prion disease, fatal familial insomnia (FFI). An additional substitution created a strong transmission barrier against pre-existing prions. The mice spontaneously developed a disease distinct from that of other mouse prion models and highly reminiscent of FFI. Unique pathology was transmitted from FFI mice to mice expressing wild-type PrP sharing the same transmission barrier. FFI mice were highly resistant to infection by pre-existing prions, confirming infectivity did not arise from contaminating agents. Thus, a single amino acid change in PrP is sufficient to induce a distinct neurodegenerative disease and the spontaneous generation of prion infectivity.

Trans-species amplification of PrP(CWD) and correlation with rigid loop 170N

Chronic wasting disease (CWD) is an efficiently transmitted spongiform encephalopathy of cervids. Whether CWD could represent a threat to non-cervid species remains speculative. Here we show that brain homogenates from several CWD-susceptible non-cervid species, such as ferrets and hamsters, support amplification of PrP(CWD) by sPMCA, whereas brain homogenates from CWD-resistant species, such as laboratory mice and transgenic mice expressing human PrP(C) [Tg(HuPrP) mice], do not. We also investigated whether several North American species that share the environment with cervids would support amplification of PrP(CWD) by sPMCA. Three native rodent species, including voles and field mice, supported PrP(CWD) amplification, whereas other species (e.g. prairie dog, coyote) did not. Analysis of PrP sequences suggests that an ability to support amplification of PrP(CWD) in trans-species sPMCA is correlated with the presence of asparagine at position 170 of the substrate species PrP. Serial PMCA may offer insights into species barriers to transmission of CWD.

Bacterial colitis increases susceptibility to oral prion disease

Dietary exposure to prion-contaminated materials has caused kuru and variant Creutzfeldt-Jakob disease in humans and transmissible spongiform encephalopathies (TSEs) in cattle, mink, and felines. The epidemiology of dietary prion infections suggests that host genetic modifiers and possibly exogenous cofactors may play a decisive role in determining disease susceptibility. However, few cofactors influencing susceptibility to prion infection have been identified. In the present study, we investigated whether colitis might represent one such cofactor. We report that moderate colitis caused by an attenuated Salmonella strain more than doubles the susceptibility of mice to oral prion infection and modestly accelerates the development of disease after prion challenge. The prion protein was up-regulated in intestines and mesenteric lymph nodes of mice with colitis, providing a possible mechanism for the effect of colitis on the pathogenesis of prion disease. Therefore, moderate intestinal inflammation at the time of prion exposure may constitute one of the elusive risk factors underlying the development of TSE.

The comprehensive native interactome of a fully functional tagged prion protein

The enumeration of the interaction partners of the cellular prion protein, PrP(C), may help clarifying its elusive molecular function. Here we added a carboxy proximal myc epitope tag to PrP(C). When expressed in transgenic mice, PrP(myc) carried a GPI anchor, was targeted to lipid rafts, and was glycosylated similarly to PrP(C). PrP(myc) antagonized the toxicity of truncated PrP, restored prion infectibility of PrP(C)-deficient mice, and was physically incorporated into PrP(Sc) aggregates, indicating that it possessed all functional characteristics of genuine PrP(C). We then immunopurified myc epitope-containing protein complexes from PrP(myc) transgenic mouse brains. Gentle differential elution with epitope-mimetic decapeptides, or a scrambled version thereof, yielded 96 specifically released proteins. Quantitative mass spectrometry with isotope-coded tags identified seven proteins which co-eluted equimolarly with PrP(C) and may represent component of a multiprotein complex. Selected PrP(C) interactors were validated using independent methods. Several of these proteins appear to exert functions in axomyelinic maintenance.