Prion replication without host adaptation during interspecies transmissions

Prion diseases: Lessons from historical outbreaks and potential emerging ones

Prion diseases (PrDs) are a unique and fatal class of neurodegenerative disorders caused by misfolded proteinaceous infectious particles, or prions. While the pathogenic form was first documented in humans nearly a century ago, the global monitoring of PrDs only gained momentum after the "Mad Cow" epizootic and its human counterpart of the 1980s and 1990s. Currently, 34 countries track human prion cases annually, with over 27,000 cases. However, true prevalence estimates suggest significantly higher numbers, millions, highlighting the urgency of addressing these enigmatic diseases. Prions are exceptionally resilient, resisting conventional sterilization methods and persisting in environmental reservoirs, such as soil and plants, raising concerns about environmental and cross-species transmission, particularly with the growing prevalence of chronic wasting disease (CWD) in cervids. This review explores the history, pathogenesis, presence, public health implications, and novel innovations in studying and treatment of PrDs. Future priorities should include the development of faster, cost-effective diagnostic tools and systemic therapies to neutralize prions in affected individuals and mitigate environmental risks. Understanding and addressing the challenges posed by prions is critical for global health security in the wake of CWD.

The dynamics of prion spreading is governed by the interplay between the non-linearities of tissue response and replication kinetics

Prion diseases, or transmissible spongiform encephalopathies (TSEs), are neurodegenerative disorders caused by the accumulation of misfolded conformers (PrPSc) of the cellular prion protein (PrPC). During the pathogenesis, the PrPSc seeds disseminate in the central nervous system and convert PrPC leading to the formation of insoluble assemblies. As for conventional infectious diseases, variations in the clinical manifestation define a specific prion strain which correspond to different PrPSc structures. In this work, we implemented the recent developments on PrPSc structural diversity and tissue response to prion replication into a stochastic reaction-diffusion model using an application of the Gillespie algorithm. We showed that this combination of non-linearities can lead prion propagation to behave as a complex system, providing an alternative to the current paradigm to explain strain-specific phenotypes, tissue tropisms, and strain co-propagation while also clarifying the role of the connectome in the neuro-invasion process.

Minor prion substrains overcome transmission barriers

Mammalian prion diseases are infectious neurodegenerative diseases caused by the self-templating form of the prion protein PrPSc. Much evidence supports the hypothesis that prions exist as a mixture of a dominant strain and minor prion strains. While it is known that prions can infect new species, the relative contribution of the dominant prion strain and minor strains in crossing the species barrier is unknown. We previously identified minor prion strains from a biologically cloned drowsy (DY) strain of hamster-adapted transmissible mink encephalopathy (TME). Here we show that these minor prion strains have increased infection efficiency to rabbit kidney epithelial cells that express hamster PrPC compared to the dominant strain DY TME. Using protein misfolding cyclic amplification (PMCA), we found that the dominant strain DY TME failed to convert mouse PrPC to PrPSc, even after several serial passages. In contrast, the minor prion strains isolated from biologically cloned DY TME robustly converted mouse PrPC to PrPSc in the first round of PMCA. This observation indicates that minor prion strains from the mutant spectra contribute to crossing the species barrier. Additionally, we found that the PMCA conversion efficiency for the minor prion strains tested was significantly different from each other and from the short-incubation period prion strain HY TME. This suggests that minor strain diversity may be greater than previously anticipated. These observations further expand our understanding of the mechanisms underlying the species barrier effect and has implications for assessing the zoonotic potential of prions.

IMPORTANCE

Prions from cattle with bovine spongiform encephalopathy have transmitted to humans, whereas scrapie from sheep and goats likely has not, suggesting that some prions can cross species barriers more easily than others. Prions are composed of a dominant strain and minor strains, and the contribution of each population to adapt to new replicative environments is unknown. Recently, minor prion strains were isolated from the biologically cloned prion strain DY TME, and these minor prion strains differed in properties from the dominant prion strain, DY TME. Here we found that these minor prion strains also differed in conversion efficiency and host range compared to the dominant strain DY TME. These novel findings provide evidence that minor prion strains contribute to interspecies transmission, underscoring the significance of minor strain components in important biological processes.

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.

Transmission of Norwegian reindeer CWD to sheep by intracerebral inoculation results in an unusual phenotype and prion distribution

Chronic wasting disease (CWD), a prion disease affecting cervids, has been known in North America (NA) since the 1960s and emerged in Norway in 2016. Surveillance and studies have revealed that there are different forms of CWD in Fennoscandia: contagious CWD in Norwegian reindeer and sporadic CWD in moose and red deer. Experimental studies have demonstrated that NA CWD prions can infect various species, but thus far, there have been no reports of natural transmission to non-cervid species. In vitro and laboratory animal studies of the Norwegian CWD strains suggest that these strains are different from the NA strains. In this work, we describe the intracerebral transmission of reindeer CWD to six scrapie-susceptible sheep. Detection methods included immunohistochemistry (IHC), western blot (WB), enzyme-linked immunosorbent assay (ELISA), real-time quaking-induced conversion (RT-QuIC) and protein misfolding cyclic amplification (PMCA). In the brain, grey matter vacuolation was limited, while all sheep exhibited vacuolation of the white matter. IHC and WB conventional detection techniques failed to detect prions; however, positive seeding activity with the RT-QuIC and PMCA amplification techniques was observed in the central nervous system of all but one sheep. Prions were robustly amplified in the lymph nodes of all animals, mainly by RT-QuIC. Additionally, two lymph nodes were positive by WB, and one was positive by ELISA. These findings suggest that sheep can propagate reindeer CWD prions after intracerebral inoculation, resulting in an unusual disease phenotype and prion distribution with a low amount of detectable prions.

Effect of host and strain factors on α-synuclein prion pathogenesis

Prion diseases are a group of neurodegenerative disorders caused by misfolding of proteins into pathogenic conformations that self-template to spread disease. Although this mechanism is largely associated with the prion protein (PrP) in classical prion diseases, a growing literature indicates that other proteins, including α-synuclein, rely on a similar disease mechanism. Notably, α-synuclein misfolds into distinct conformations, or strains, that cause discrete clinical disorders including multiple system atrophy (MSA) and Parkinson's disease (PD). Because the recognized similarities between PrP and α-synuclein are increasing, this review article draws from research on PrP to identify the host and strain factors that impact disease pathogenesis, predominantly in rodent models, and focuses on key considerations for future research on α-synuclein prions.

Disease phenotype of classical sheep scrapie is changed upon experimental passage through white-tailed deer

Prion agents occur in strains that are encoded by the structure of the misfolded prion protein (PrPSc). Prion strains can influence disease phenotype and the potential for interspecies transmission. Little is known about the potential transmission of prions between sheep and deer. Previously, the classical US scrapie isolate (No.13-7) had a 100% attack rate in white-tailed deer after oronasal challenge. The purpose of this study was to test the susceptibility of sheep to challenge with the scrapie agent after passage through white-tailed deer (WTD scrapie). Lambs of various prion protein genotypes were oronasally challenged with WTD scrapie. Sheep were euthanized and necropsied upon development of clinical signs or at the end of the experiment (72 months post-inoculation). Enzyme immunoassay, western blot, and immunohistochemistry demonstrated PrPSc in 4 of 10 sheep with the fastest incubation occurring in VRQ/VRQ sheep, which contrasts the original No.13-7 inoculum with a faster incubation in ARQ/ARQ sheep. Shorter incubation periods in VRQ/VRQ sheep than ARQ/ARQ sheep after passage through deer was suggestive of a phenotype change, so comparisons were made in ovinized mice and with sheep with known strains of classical sheep scrapie: No. 13-7 and x-124 (that has a more rapid incubation in VRQ/VRQ sheep). After mouse bioassay, the WTD scrapie and x-124 isolates have similar incubation periods and PrPSc conformational stability that are markedly different than the original No. 13-7 inoculum. Furthermore, brain tissues of sheep with WTD scrapie and x-124 scrapie have similar patterns of immunoreactivity that are distinct from sheep with No. 13-7 scrapie. Multiple lines of evidence suggest a phenotype switch when No. 13-7 scrapie prions are passaged through deer. This represents one example of interspecies transmission of prions resulting in the emergence or selection of new strain properties that could confound disease eradication and control efforts.

Arg177 and Asp159 from dog prion protein slow liquid-liquid phase separation and inhibit amyloid formation of human prion protein

Prion diseases are a group of transmissible neurodegenerative diseases primarily caused by the conformational conversion of prion protein (PrP) from α-helix-dominant cellular prion protein (PrPC) to β-sheet-rich pathological aggregated form of PrPSc in many mammalian species. Dogs exhibit resistance to prion diseases, but the mechanism behind the phenomenon remains poorly understood. Compared with human PrP and mouse PrP, dog PrP has two unique amino acid residues, Arg177 and Asp159. Because PrPC contains a low-complexity and intrinsically disordered region in its N-terminal domain, it undergoes liquid-liquid phase separation (LLPS) in vitro and forms protein condensates. However, little is known about whether these two unique residues modulate the formation of PrPC condensates. Here, using confocal microscopy, fluorescence recovery after photobleaching assays, thioflavin T binding assays, and transmission electron microscopy, we report that Arg177 and Asp159 from the dog PrP slow the LLPS of full-length human PrPC, shifting the equilibrium phase boundary to higher protein concentrations and inhibit amyloid formation of the human protein. In sharp contrast, His177 and Asn159 from the human PrP enhance the LLPS of full-length dog PrPC, shifting the equilibrium phase boundary to lower protein concentrations, and promote fibril formation of the canid protein. Collectively, these results demonstrate how LLPS and amyloid formation of PrP are inhibited by a single residue Arg177 or Asp159 associated with prion disease resistance, and how LLPS and fibril formation of PrP are promoted by a single residue His177 or Asn159. Therefore, Arg177/His177 and Asp159/Asn159 are key residues in modulating PrPC liquid-phase condensation.

Novel polymorphisms in the prion protein gene (PRNP) and stability of the resultant prion protein in different horse breeds

Prion diseases are fatal neurodegenerative disorders in which the main pathogenic event is the conversion of the cellular prion protein (PrPC) into an abnormal and misfolded isoform known as PrPSc. Most prion diseases and their susceptibility and pathogenesis are mainly modulated by the PRNP gene that codes for PrP. Mutations and polymorphisms in the PRNP gene can alter PrPC amino acid sequence, leading to a change in transmission efficiency depending on the place where it occurs. Horses are animals that are considered to be highly resistant to prions. Several studies have attempted to identify polymorphisms in the PRNP gene that explain the reason for this high resistance. In this study, we have analysed 207 horses from 20 different breeds, discovering 3 novel PRNP polymorphisms. By using computer programmes such as PolyPhen-2, PROVEAN, PANTHER, Meta-SNP and PredictSNP, we have predicted the possible impact that these new polymorphisms would have on the horse prion protein. In addition, we measured the propensity for amyloid aggregation using AMYCO and analysed the lack of hydrogen bridges that these changes would entail together with their electrostatic potentials using Swiss-PdbViewer software, showing that an increased amyloid propensity could be due to changes at the level of electrostatic potentials.

Intrinsic determinants of prion protein neurotoxicity in Drosophila: from sequence to (dys)function

Prion diseases are fatal brain disorders characterized by deposition of insoluble isoforms of the prion protein (PrP). The normal and pathogenic structures of PrP are relatively well known after decades of studies. Yet our current understanding of the intrinsic determinants regulating PrP misfolding are largely missing. A 3D subdomain of PrP comprising the β2-α2 loop and helix 3 contains high sequence and structural variability among animals and has been proposed as a key domain regulating PrP misfolding. We combined in vivo work in Drosophila with molecular dynamics (MD) simulations, which provide additional insight to assess the impact of candidate substitutions in PrP from conformational dynamics. MD simulations revealed that in human PrP WT the β2-α2 loop explores multiple β-turn conformations, whereas the Y225A (rabbit PrP-like) substitution strongly favors a 310-turn conformation, a short right-handed helix. This shift in conformational diversity correlates with lower neurotoxicity in flies. We have identified additional conformational features and candidate amino acids regulating the high toxicity of human PrP and propose a new strategy for testing candidate modifiers first in MD simulations followed by functional experiments in flies. In this review we expand on these new results to provide additional insight into the structural and functional biology of PrP through the prism of the conformational dynamics of a 3D domain in the C-terminus. We propose that the conformational dynamics of this domain is a sensitive measure of the propensity of PrP to misfold and cause toxicity. This provides renewed opportunities to identify the intrinsic determinants of PrP misfolding through the contribution of key amino acids to different conformational states by MD simulations followed by experimental validation in transgenic flies.

Prion strains: shining new light on old concepts

Prion diseases are a group of inevitably fatal neurodegenerative disorders affecting numerous mammalian species, including humans. The existence of heritable phenotypes of disease in the natural host suggested that prions exist as distinct strains. Transmission of sheep scrapie to rodent models accelerated prion research, resulting in the isolation and characterization of numerous strains with distinct characteristics. These strains are grouped into categories based on the incubation period of disease in different strains of mice and also by how stable the strain properties were upon serial passage. These classical studies defined the host and agent parameters that affected strain properties, and, prior to the advent of the prion hypothesis, strain properties were hypothesized to be the result of mutations in a nucleic acid genome of a conventional pathogen. The development of the prion hypothesis challenged the paradigm of infectious agents, and, initially, the existence of strains was difficult to reconcile with a protein-only agent. In the decades since, much evidence has revealed how a protein-only infectious agent can perform complex biological functions. The prevailing hypothesis is that strain-specific conformations of PrPSc encode prion strain diversity. This hypothesis can provide a mechanism to explain the observed strain-specific differences in incubation period of disease, biochemical properties of PrPSc, tissue tropism, and subcellular patterns of pathology. This hypothesis also explains how prion strains mutate, evolve, and adapt to new species. These concepts are applicable to prion-like diseases such as Parkinson's and Alzheimer's disease, where evidence of strain diversity is beginning to emerge.

New developments in prion disease research using genetically modified mouse models

While much of what we know about the general principles of protein-based information transfer derives from studies of experimentally adapted rodent prions, these laboratory strains are limited in their ability to recapitulate features of human and animal prions and the diseases they produce. Here, we review how recent approaches using genetically modified mice have informed our understanding of naturally occurring prion diseases, their strain properties, and the factors controlling their transmission and evolution. In light of the increasing importance of chronic wasting disease, the application of mouse transgenesis to study this burgeoning and highly contagious prion disorder, in particular recent insights derived from gene-targeting approaches, will be a major focus of this review.

Novel Prion Strain as Cause of Chronic Wasting Disease in a Moose, Finland

Our previous studies using gene-targeted mouse models of chronic wasting disease (CWD) demonstrated that Norway and North America cervids are infected with distinct prion strains that respond differently to naturally occurring amino acid variation at residue 226 of the prion protein. Here we performed transmissions in gene-targeted mice to investigate the properties of prions causing newly emergent CWD in moose in Finland. Although CWD prions from Finland and Norway moose had comparable responses to primary structural differences at residue 226, other distinctive criteria, including transmission kinetics, patterns of neuronal degeneration, and conformational features of prions generated in the brains of diseased mice, demonstrated that the strain properties of Finland moose CWD prions are different from those previously characterized in Norway CWD. Our findings add to a growing body of evidence for a diverse portfolio of emergent strains in Nordic countries that are etiologically distinct from the comparatively consistent strain profile of North America CWD.

Second passage experiments of chronic wasting disease in transgenic mice overexpressing human prion protein

Chronic wasting disease (CWD) is a prion disease of cervids including deer, elk, reindeer, and moose. Human consumption of cervids is common, therefore assessing the risk potential of CWD transmission to humans is critical. In a previous study, we tested CWD transmission via intracerebral inoculation into transgenic mice (tg66 and tgRM) that over-expressed human prion protein. Mice screened by traditional prion detection assays were negative. However, in a group of 88 mice screened by the ultrasensitive RT-QuIC assay, we identified 4 tg66 mice that produced inconsistent positive RT-QuIC reactions. These data could be false positive reactions, residual input inoculum or indicative of subclinical infections suggestive of cross species transmission of CWD to humans. Additional experiments were required to understand the nature of the prion seeding activity in this model. In this manuscript, second passage experiments using brains from mice with weak prion seeding activity showed they were not infectious to additional recipient tg66 mice. Clearance experiments showed that input CWD prion seeding activity was eliminated by 180 days in tg66 mice and PrPKO mice, which are unable to replicate prion protein, indicating that the weak positive levels of seeding activity detected at later time points was not likely residual inoculum. The failure of CWD prions to cause disease in tg66 after two sequential passages suggested that a strong species barrier prevented CWD infection of mice expressing human prion protein.

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.

Extracellular vesicles with diagnostic and therapeutic potential for prion diseases

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

New Drosophila models to uncover the intrinsic and extrinsic factors that mediate the toxicity of the human prion protein

Misfolding of the prion protein (PrP) is responsible for devastating neurological disorders in humans and other mammals. An unresolved problem in the field is unraveling the mechanisms governing PrP conformational dynamics, misfolding, and the cellular mechanism leading to neurodegeneration. The variable susceptibility of mammals to prion diseases is a natural resource that can be exploited to understand the conformational dynamics of PrP. Here we present a new fly model expressing human PrP with new, robust phenotypes in brain neurons and the eye. By using comparable attP2 insertions, we demonstrated the heightened toxicity of human PrP compared to rodent PrP along with a specific interaction with the amyloid-β peptide. By using this new model, we started to uncover the intrinsic (sequence/structure) and extrinsic (interactions) factors regulating PrP toxicity. We described PERK (officially known as EIF2AK3 in humans) and activating transcription factor 4 (ATF4) as key in the cellular mechanism mediating the toxicity of human PrP and uncover a key new protective activity for 4E-BP (officially known as Thor in Drosophila and EIF4EBP2 in humans), an ATF4 transcriptional target. Lastly, mutations in human PrP (N159D, D167S, N174S) showed partial protective activity, revealing its high propensity to misfold into toxic conformations.

Adaptive selection of a prion strain conformer corresponding to established North American CWD during propagation of novel emergent Norwegian strains in mice expressing elk or deer prion protein

Prions are infectious proteins causing fatal, transmissible neurodegenerative diseases of animals and humans. Replication involves template-directed refolding of host encoded prion protein, PrP^C, by its infectious conformation, PrP^Sc. Following its discovery in captive Colorado deer in 1967, uncontrollable contagious transmission of chronic wasting disease (CWD) led to an expanded geographic range in increasing numbers of free-ranging and captive North American (NA) cervids. Some five decades later, detection of PrP^Sc in free-ranging Norwegian (NO) reindeer and moose marked the first indication of CWD in Europe. To assess the properties of these emergent NO prions and compare them with NA CWD we used transgenic (Tg) and gene targeted (Gt) mice expressing PrP with glutamine (Q) or glutamate (E) at residue 226, a variation in wild type cervid PrP which influences prion strain selection in NA deer and elk. Transmissions of NO moose and reindeer prions to Tg and Gt mice recapitulated the characteristic features of CWD in natural hosts, revealing novel prion strains with disease kinetics, neuropathological profiles, and capacities to infect lymphoid tissues and cultured cells that were distinct from those causing NA CWD. In support of strain variation, PrP^Sc conformers comprising emergent NO moose and reindeer CWD were subject to selective effects imposed by variation at residue 226 that were different from those controlling established NA CWD. Transmission of particular NO moose CWD prions in mice expressing E at 226 resulted in selection of a kinetically optimized conformer, subsequent transmission of which revealed properties consistent with NA CWD. These findings illustrate the potential for adaptive selection of strain conformers with improved fitness during propagation of unstable NO prions. Their potential for contagious transmission has implications for risk analyses and management of emergent European CWD. Finally, we found that Gt mice expressing physiologically controlled PrP levels recapitulated the lymphotropic properties of naturally occurring CWD strains resulting in improved susceptibilities to emergent NO reindeer prions compared with over-expressing Tg counterparts. These findings underscore the refined advantages of Gt models for exploring the mechanisms and impacts of strain selection in peripheral compartments during natural prion transmission.

Prions cause fatal, transmissible neurodegenerative diseases in animals and humans. They are composed of an infectious, neurotoxic protein (PrP) which replicates by imposing pathogenic conformations on its normal, host-encoded counterpart. Chronic wasting disease (CWD) is a contagious prion disorder threatening increasing numbers of free-ranging and captive North American deer, elk, and moose. While CWD detection in Norwegian reindeer and moose in 2016 marked the advent of disease in Europe, its origins and relationship to North American CWD were initially unclear. Here we show, using mice engineered to express deer or elk PrP, that Norwegian reindeer and moose CWD are caused by novel prion strains with properties distinct from those of North American CWD. We found that selection and propagation of North American and Norwegian CWD strains was controlled by a key amino acid residue in host PrP. We also found that particular Norwegian isolates adapted during their propagation in mice to produce prions with characteristics of the North American strain. Our findings defining the transmission profiles of novel Norwegian prions and their unstable potential to produce adapted strains with improved fitness for contagious transmission have implications for risk analyses and management of emergent European CWD.

Efficient interspecies transmission of synthetic prions

Prions are comprised solely of PrP^Sc, the misfolded self-propagating conformation of the cellular protein, PrP^C. Synthetic prions are generated in vitro from minimal components and cause bona fide prion disease in animals. It is unknown, however, if synthetic prions can cross the species barrier following interspecies transmission. To investigate this, we inoculated Syrian hamsters with murine synthetic prions. We found that all the animals inoculated with murine synthetic prions developed prion disease characterized by a striking uniformity of clinical onset and signs of disease. Serial intraspecies transmission resulted in a rapid adaptation to hamsters. During the adaptation process, PrP^Sc electrophoretic migration, glycoform ratios, conformational stability and biological activity as measured by protein misfolding cyclic amplification remained constant. Interestingly, the strain that emerged shares a strikingly similar transmission history, incubation period, clinical course of disease, pathology and biochemical and biological features of PrP^Sc with 139H, a hamster adapted form of the murine strain 139A. Combined, these data suggest that murine synthetic prions are comprised of bona fide PrP^Sc with 139A-like strain properties that efficiently crosses the species barrier and rapidly adapts to hamsters resulting in the emergence of a single strain. The efficiency and specificity of interspecies transmission of murine synthetic prions to hamsters, with relevance to brain derived prions, could be a useful model for identification of structure function relationships between PrP^Sc and PrP^C from different species.

Prions have zoonotic potential as illustrated by the interspecies transmission of bovine spongiform encephalopathy to humans resulting in the emergence of a novel human prion disease. It is unknown if other prion diseases of animals, such as chronic wasting disease, can be transmitted to other species. Models to predict prion zoonotic potential do not exist, in part, due to the lack of understanding of how the structure of PrP^Sc from one species can convert PrP^C from another species. Towards this end, we determined that murine synthetic prions, made from minimal components, can efficiently establish infection in hamsters whose transmission history, clinical features, pathology and biochemical properties of PrP^Sc are consistent with the reisolation of a known prion strain. We conclude that murine synthetic prions can recapitulate interspecies transmission and adaptation allowing for a more detailed mechanistic analysis in a simplified, trackable system.

Rabbit PrP Is Partially Resistant to in vitro Aggregation Induced by Different Biological Cofactors

Prion diseases have been described in humans and other mammals, including sheep, goats, cattle, and deer. Since mice, hamsters, and cats are susceptible to prion infection, they are often used to study the mechanisms of prion infection and conversion. Mammals, such as horses and dogs, however, do not naturally contract the disease and are resistant to infection, while others, like rabbits, have exhibited low susceptibility. Infection involves the conversion of the cellular prion protein (PrP^C) to the scrapie form (PrP^Sc), and several cofactors have already been identified as important adjuvants in this process, such as glycosaminoglycans (GAGs), lipids, and nucleic acids. The molecular mechanisms that determine transmissibility between species remain unclear, as well as the barriers to transmission. In this study, we examine the interaction of recombinant rabbit PrP^C (RaPrP) with different biological cofactors such as GAGs (heparin and dermatan sulfate), phosphatidic acid, and DNA oligonucleotides (A1 and D67) to evaluate the importance of these cofactors in modulating the aggregation of rabbit PrP and explain the animal’s different degrees of resistance to infection. We used spectroscopic and chromatographic approaches to evaluate the interaction with cofactors and their effect on RaPrP aggregation, which we compared with murine PrP (MuPrP). Our data show that all cofactors induce RaPrP aggregation and exhibit pH dependence. However, RaPrP aggregated to a lesser extent than MuPrP in the presence of any of the cofactors tested. The binding affinity with cofactors does not correlate with these low levels of aggregation, suggesting that the latter are related to the stability of PrP at acidic pH. The absence of the N-terminus affected the interaction with cofactors, influencing the efficiency of aggregation. These findings demonstrate that the interaction with polyanionic cofactors is related to rabbit PrP being less susceptible to aggregation in vitro and that the N-terminal domain is important to the efficiency of conversion, increasing the interaction with cofactors. The decreased effect of cofactors in rabbit PrP likely explains its lower propensity to prion conversion.

Transgenic mouse models for the study of prion diseases

Prions are unique agents that challenge the molecular biology dogma by transmitting information on the protein level. They cause neurodegenerative diseases that lack of any cure or treatment called transmissible spongiform encephalopathies. The function of the normal form of the prion protein, the exact mechanism of prion propagation between species as well as at the cellular level and neuron degeneration remains elusive. However, great amount of information known for all these aspects has been achieved thanks to the use of animal models and more precisely to transgenic mouse models. In this chapter, the main contributions of these powerful research tools in the prion field are revised.

Insight From Animals Resistant to Prion Diseases: Deciphering the Genotype - Morphotype - Phenotype Code for the Prion Protein

Prion diseases are a group of neurodegenerative diseases endemic in humans and several ruminants caused by the misfolding of native prion protein (PrP) into pathological conformations. Experimental work and the mad-cow epidemic of the 1980s exposed a wide spectrum of animal susceptibility to prion diseases, including a few highly resistant animals: horses, rabbits, pigs, and dogs/canids. The variable susceptibility to disease offers a unique opportunity to uncover the mechanisms governing PrP misfolding, neurotoxicity, and transmission. Previous work indicates that PrP-intrinsic differences (sequence) are the main contributors to disease susceptibility. Several residues have been cited as critical for encoding PrP conformational stability in prion-resistant animals, including D/E159 in dog, S167 in horse, and S174 in rabbit and pig PrP (all according to human numbering). These amino acids alter PrP properties in a variety of assays, but we still do not clearly understand the structural correlates of PrP toxicity. Additional insight can be extracted from comparative structural studies, followed by molecular dynamics simulations of selected mutations, and testing in manipulable animal models. Our working hypothesis is that protective amino acids generate more compact and stable structures in a C-terminal subdomain of the PrP globular domain. We will explore this idea in this review and identify subdomains within the globular domain that may hold the key to unravel how conformational stability and disease susceptibility are encoded in PrP.

Incomplete glycosylation during prion infection unmasks a prion protein epitope that facilitates prion detection and strain discrimination

The causative factors underlying conformational conversion of cellular prion protein (PrP^C) into its infectious counterpart (PrP^Sc) during prion infection remain undetermined, in part because of a lack of monoclonal antibodies (mAbs) that can distinguish these conformational isoforms. Here we show that the anti-PrP mAb PRC7 recognizes an epitope that is shielded from detection when glycans are attached to Asn-196. We observed that whereas PrP^C is predisposed to full glycosylation and is therefore refractory to PRC7 detection, prion infection leads to diminished PrP^Sc glycosylation at Asn-196, resulting in an unshielded PRC7 epitope that is amenable to mAb recognition upon renaturation. Detection of PRC7-reactive PrP^Sc in experimental and natural infections with various mouse-adapted scrapie strains and with prions causing deer and elk chronic wasting disease and transmissible mink encephalopathy uncovered that incomplete PrP^Sc glycosylation is a consistent feature of prion pathogenesis. We also show that interrogating the conformational properties of the PRC7 epitope affords a direct means of distinguishing different prion strains. Because the specificity of our approach for prion detection and strain discrimination relies on the extent to which N-linked glycosylation shields or unshields PrP epitopes from antibody recognition, it dispenses with the requirement for additional standard manipulations to distinguish PrP^Sc from PrP^C, including evaluation of protease resistance. Our findings not only highlight an innovative and facile strategy for prion detection and strain differentiation, but are also consistent with a mechanism of prion replication in which structural instability of incompletely glycosylated PrP contributes to the conformational conversion of PrP^C to PrP^Sc.

Dogs are resistant to prion infection, due to the presence of aspartic or glutamic acid at position 163 of their prion protein

Unlike other species, prion disease has never been described in dogs even though they were similarly exposed to the bovine spongiform encephalopathy (BSE) agent. This resistance prompted a thorough analysis of the canine PRNP gene and the presence of a negatively charged amino acid residue in position 163 was readily identified as potentially fundamental as it differed from all known susceptible species. In the present study, the first transgenic mouse model expressing dog prion protein (PrP) was generated and challenged intracerebrally with a panel of prion isolates, none of which could infect them. The brains of these mice were subjected to in vitro prion amplification and failed to find even minimal amounts of misfolded prions providing definitive experimental evidence that dogs are resistant to prion disease. Subsequently, a second transgenic model was generated in which aspartic acid in position 163 was substituted for asparagine (the most common in prion susceptible species) resulting in susceptibility to BSE-derived isolates. These findings strongly support the hypothesis that the amino acid residue at position 163 of canine cellular prion protein (PrP^C ) is a major determinant of the exceptional resistance of the canidae family to prion infection and establish this as a promising therapeutic target for prion diseases.

Porcine Prion Protein as a Paradigm of Limited Susceptibility to Prion Strain Propagation

Although experimental transmission of bovine spongiform encephalopathy (BSE) to pigs and transgenic mice expressing pig cellular prion protein (PrPC) (porcine PrP [PoPrP]-Tg001) has been described, no natural cases of prion diseases in pig were reported. This study analyzed pig-PrPC susceptibility to different prion strains using PoPrP-Tg001 mice either as animal bioassay or as substrate for protein misfolding cyclic amplification (PMCA). A panel of isolates representatives of different prion strains was selected, including classic and atypical/Nor98 scrapie, atypical-BSE, rodent scrapie, human Creutzfeldt-Jakob-disease and classic BSE from different species. Bioassay proved that PoPrP-Tg001-mice were susceptible only to the classic BSE agent, and PMCA results indicate that only classic BSE can convert pig-PrPC into scrapie-type PrP (PrPSc), independently of the species origin. Therefore, conformational flexibility constraints associated with pig-PrP would limit the number of permissible PrPSc conformations compatible with pig-PrPC, thus suggesting that pig-PrPC may constitute a paradigm of low conformational flexibility that could confer high resistance to the diversity of prion strains.

The First Report of Genetic and Structural Diversities in the SPRN Gene in the Horse, an Animal Resistant to Prion Disease

Prion diseases are fatal neurodegenerative diseases and are characterized by the accumulation of abnormal prion protein (PrP^Sc) in the brain. During the outbreak of the bovine spongiform encephalopathy (BSE) epidemic in the United Kingdom, prion diseases in several species were reported; however, horse prion disease has not been reported thus far. In previous studies, the shadow of prion protein (Sho) has contributed to an acceleration of conversion from normal prion protein (PrP^C) to PrP^Sc, and the shadow of prion protein gene (SPRN) polymorphisms have been significantly associated with the susceptibility of prion diseases. We investigated the genotype, allele and haplotype frequencies of the SPRN gene using direct sequencing. In addition, we analyzed linkage disequilibrium (LD) and haplotypes among polymorphisms. We also investigated LD between PRNP and SPRN single nucleotide polymorphisms (SNPs). We compared the amino acid sequences of Sho protein between the horse and several prion disease-susceptible species using ClustalW2. To perform Sho protein modeling, we utilized SWISS-MODEL and Swiss-PdbViewer programs. We found a total of four polymorphisms in the equine SPRN gene; however, we did not observe an in/del polymorphism, which is correlated with the susceptibility of prion disease in prion disease-susceptible animals. The SPRN SNPs showed weak LD value with PRNP SNP. In addition, we found 12 horse-specific amino acids of Sho protein that can induce significantly distributional differences in the secondary structure and hydrogen bonds between the horse and several prion disease-susceptible species. To the best of our knowledge, this is the first report regarding the genetic and structural characteristics of the equine SPRN gene.

Absence of single nucleotide polymorphisms (SNPs) in the open reading frame (ORF) of the prion protein gene (PRNP) in a large sampling of various chicken breeds

BACKGROUND

Prion diseases are zoonotic diseases with a broad infection spectrum among mammalian hosts and are caused by the misfolded prion protein (PrPSc) derived from the normal prion protein (PrPC), which encodes the prion protein gene (PRNP). Currently, although several prion disease-resistant animals have been reported, a high dose of prion agent inoculation triggers prion disease infection in these disease-resistant animals. However, in chickens, natural prion disease-infected cases have not been reported, and experimental challenges with prion agents have failed to cause infection. Unlike other prion disease-resistant animals, chickens have shown perfect resistance to prion disease thus far. Thus, investigation of the chicken PRNP gene could improve for understanding the mechanism of perfect prion-disease resistance. Here, we investigated the genetic characteristics of the open reading frame (ORF) of the chicken PRNP gene in a large sampling of various chicken breeds.

RESULTS

We found only tandem repeat deletion polymorphisms of the chicken PRNP ORF in the 4 chicken breeds including 106 Dekalb White, 100 Ross, 98 Ogolgye and 100 Korean native chickens. In addition, the distribution of chicken insertion/deletion polymorphisms was significantly different among the 4 chicken breeds. Finally, we found significant differences in the number of PRNP SNPs between prion disease-susceptible species and prion disease-resistant species. Notably, chickens lack SNPs in the ORF of the prion protein.

CONCLUSION

In this study, we found that the absence of SNPs in the chicken PRNP ORF is a notable feature of animals with perfect resistant to prion disease.

Prion disease is accelerated in mice lacking stress-induced heat shock protein 70 (HSP70)

Prion diseases are a group of incurable neurodegenerative disorders that affect humans and animals via infection with proteinaceous particles called prions. Prions are composed of PrP^Sc, a misfolded version of the cellular prion protein (PrP^C). During disease progression, PrP^Sc replicates by interacting with PrP^C and inducing its conversion to PrP^Sc As PrP^Sc accumulates, cellular stress mechanisms are activated to maintain cellular proteostasis, including increased protein chaperone levels. However, the exact roles of several of these chaperones remain unclear. Here, using various methodologies to monitor prion replication (i.e. protein misfolding cyclic amplification and cellular and animal infectivity bioassays), we studied the potential role of the molecular chaperone heat shock protein 70 (HSP70) in prion replication in vitro and in vivo Our results indicated that pharmacological induction of the heat shock response in cells chronically infected with prions significantly decreased PrP^Sc accumulation. We also found that HSP70 alters prion replication in vitro More importantly, prion infection of mice lacking the genes encoding stress-induced HSP70 exhibited accelerated prion disease progression compared with WT mice. In parallel with HSP70 being known to respond to endogenous and exogenous stressors such as heat, infection, toxicants, and ischemia, our results indicate that HSP70 may also play an important role in suppressing or delaying prion disease progression, opening opportunities for therapeutic intervention.

Primary structural differences at residue 226 of deer and elk PrP dictate selection of distinct CWD prion strains in gene-targeted mice

Our gene-targeting strategy addresses several critical issues relating to chronic wasting disease (CWD), a contagious, lethal neurodegenerative prion disease affecting wild as well as captive cervids, which poses an uncertain risk to humans. First, we show that residue 226, the sole primary structural difference between deer and elk prion protein, dictates the selection and propagation of different CWD strains. Because the strain properties of prions affect their host-range potential, these findings suggest that CWD from elk and deer pose distinct risks to humans and other species. Second, we show that these gene-targeted mice offer an unprecedented means to address distinctive aspects of CWD peripheral pathogenesis and horizontal transmission that are not afforded by previously generated transgenic mouse models.

Although the unifying hallmark of prion diseases is CNS neurodegeneration caused by conformational corruption of host prion protein (PrP) to its infective counterpart, contagious transmission of chronic wasting disease (CWD) results from shedding of prions produced at high titers in the periphery of diseased cervids. While deer and elk PrP primary structures are equivalent except at residue 226, which is glutamate in elk and glutamine in deer, the effect of this difference on CWD pathogenesis is largely unknown. Using a gene-targeting approach where the mouse PrP coding sequence was replaced with elk or deer PrP, we show that the resulting GtE226 and GtQ226 mice had distinct kinetics of disease onset, prion conformations, and distributions of prions in the brains of diseased mice following intracerebral CWD challenge. These findings indicate that amino acid differences at PrP residue 226 dictate the selection and propagation of divergent strains in deer and elk with CWD. Because prion strain properties largely dictate host-range potential, our findings suggest that prion strains from elk and deer pose distinct risks to sympatric species or humans exposed to CWD. GtE226 and GtQ226 mice were also highly susceptible to CWD prions following intraperitoneal and oral exposures, a characteristic that stood in stark contrast to previously produced transgenic models. Remarkably, disease transmission was effective when infected mice were cohoused with naïve cagemates. Our findings indicate that gene-targeted mice provide unprecedented opportunities to accurately investigate CWD peripheral pathogenesis, CWD strains, and mechanisms of horizontal CWD transmission.

Understanding Prion Strains: Evidence from Studies of the Disease Forms Affecting Humans

Prion diseases are a unique group of rare neurodegenerative disorders characterized by tissue deposition of heterogeneous aggregates of abnormally folded protease-resistant prion protein (PrPSc), a broad spectrum of disease phenotypes and a variable efficiency of disease propagation in vivo. The dominant clinicopathological phenotypes of human prion disease include Creutzfeldt⁻Jakob disease, fatal insomnia, variably protease-sensitive prionopathy, and Gerstmann⁻Sträussler⁻Scheinker disease. Prion disease propagation into susceptible hosts led to the isolation and characterization of prion strains, initially operatively defined as "isolates" causing diseases with distinctive characteristics, such as the incubation period, the pattern of PrPSc distribution, and the regional severity of neuropathological changes after injection into syngeneic hosts. More recently, the structural basis of prion strains has been linked to amyloid polymorphs (i.e., variant amyloid protein conformations) and the concept extended to all protein amyloids showing polymorphic structures and some evidence of in vivo or in vitro propagation by seeding. Despite the significant advances, however, the link between amyloid structure and disease is not understood in many instances. Here we reviewed the most significant contributions of human prion disease studies to current knowledge of the molecular basis of phenotypic variability and the prion strain phenomenon and underlined the unsolved issues from the human disease perspective.

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

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

A Single Amino Acid Substitution, Found in Mammals with Low Susceptibility to Prion Diseases, Delays Propagation of Two Prion Strains in Highly Susceptible Transgenic Mouse Models

Specific variations in the amino acid sequence of prion protein (PrP) are key determinants of susceptibility to prion diseases. We previously showed that an amino acid substitution specific to canids confers resistance to prion diseases when expressed in mice and demonstrated its dominant-negative protective effect against a variety of infectious prion strains of different origins and characteristics. Here, we show that expression of this single amino acid change significantly increases survival time in transgenic mice expressing bank vole cellular prion protein (PrP^C), which is inherently prone to misfolding, following inoculation with two distinct prion strains (the CWD-vole strain and an atypical strain of spontaneous origin). This amino acid substitution hinders the propagation of both prion strains, even when expressed in the context of a PrP^C uniquely susceptible to a wide range of prion isolates. Non-inoculated mice expressing this substitution experience spontaneous prion formation, but showing an increase in survival time comparable to that observed in mutant mice inoculated with the atypical strain. Our results underscore the importance of this PrP variant in the search for molecules with therapeutic potential against prion diseases.

Full restoration of specific infectivity and strain properties from pure mammalian prion protein

The protein-only hypothesis predicts that infectious mammalian prions are composed solely of PrPSc, a misfolded conformer of the normal prion protein, PrPC. However, protein-only PrPSc preparations lack significant levels of prion infectivity, leading to the alternative hypothesis that cofactor molecules are required to form infectious prions. Here, we show that prions with parental strain properties and full specific infectivity can be restored from protein-only PrPSc in vitro. The restoration reaction is rapid, potent, and requires bank vole PrPC substrate, post-translational modifications, and cofactor molecules. To our knowledge, this represents the first report in which the essential properties of an infectious mammalian prion have been restored from pure PrP without adaptation. These findings provide evidence for a unified hypothesis of prion infectivity in which the global structure of protein-only PrPSc accurately stores latent infectious and strain information, but cofactor molecules control a reversible switch that unmasks biological infectivity.

Mechanisms of Strain Diversity of Disease-Associated in-Register Parallel β-Sheet Amyloids and Implications About Prion Strains

The mechanism of prion strain diversity remains unsolved. Investigation of inheritance and diversification of protein-based pathogenic information demands the identification of the detailed structures of abnormal isoforms of the prion protein (PrPSc); however, achieving purification is difficult without affecting infectivity. Similar prion-like properties are recognized also in other disease-associated in-register parallel β-sheet amyloids including Tau and α-synuclein (αSyn) amyloids. Investigations into structures of those amyloids via solid-state nuclear magnetic resonance spectroscopy and cryo-electron microscopy recently made remarkable advances due to their relatively small sizes and lack of post-translational modifications. Herein, we review advances regarding pathogenic amyloids, particularly Tau and αSyn, and discuss implications about strain diversity mechanisms of prion/PrPSc from the perspective that PrPSc is an in-register parallel β-sheet amyloid. Additionally, we present our recent data of molecular dynamics simulations of αSyn amyloid, which suggest significance of compatibility between β-sheet propensities of the substrate and local structures of the template for stability of amyloid structures. Detailed structures of αSyn and Tau amyloids are excellent models of pathogenic amyloids, including PrPSc, to elucidate strain diversity and pathogenic mechanisms.

Transmission studies of chronic wasting disease to transgenic mice overexpressing human prion protein using the RT-QuIC assay

Chronic wasting disease (CWD) is a fatal prion disease which infects deer, elk and moose. CWD was first described as a wasting syndrome in captive deer in Colorado and Wyoming wildlife facilities from 1967 to 1979. Currently, CWD has been reported in 26 states of the USA, three Canadian provinces, South Korea, Norway and Finland. Since human consumption of cervids is common, it is critical to determine if CWD can infect humans. Published research, including epidemiologic studies and transmission studies using animal models, including transgenic mice that express human prion protein, have suggested existence of a strong species barrier between cervid CWD and humans. In the current study, we tested CWD transmission into two additional strains of transgenic mice (tg66 and tgRM). These mice over-express human prion protein at high levels and are highly sensitive to infection by human-tropic prions. One hundred and eight mice were inoculated intracerebrally with three different sources of CWD. After long periods of observation, brain tissues from CWD-inoculated mice were screened for evidence of prion infection by RT-QuIC, immunohistochemistry (IHC) and immunoblot. No IHC or immunoblot evidence was found to suggest transmission had occurred, and most mice were negative by RT-QuIC assay. However, four mice with inconsistent positive RT-QuIC reactions were detected. The seeding activity detected in these mice may represent a low level of CWD agent, suggesting a possible transfer of CWD infection. Alternatively, these results might be due to false positive reactions or residual CWD inoculum.

The first report of polymorphisms and genetic characteristics of the prion protein gene (PRNP) in horses

Prion diseases have a wide host range, but prion-infected cases have never been reported in horses. Genetic polymorphisms that can directly impact the structural stability of horse prion protein have not been investigated thus far. In addition, we noticed that previous studies focusing on horse-specific amino acids and secondary structure predictions of prion protein were performed for limited parts of the protein. In this study, we found genetic polymorphisms in the horse prion protein gene (PRNP) in 201 Thoroughbred horses. The identified polymorphism was assessed to determine whether this polymorphism impedes stability of protein using PolyPhen-2, PROVEAN and PANTHER. In addition, we evaluated horse-specific amino acids in horse and mouse prion proteins using same methods. We found only one single nucleotide polymorphism (SNP) in the horse prion protein, and three annotation tools predicted that the SNP is benign. In addition, horse-specific amino acids showed different effects on horse and mouse prion proteins, respectively. Abbreviations: PRNP: prion protein gene; SNP: single nucleotide polymorphism; CJD: Creutzfeldt-Jakob disease; CWD: chronic wasting disease; TME: transmissible mink encephalopathy; FSE: feline spongiform encephalopathy; MD: molecular dynamics; ER: endoplasmic reticulum; GPI: glycosylphosphatidylinositol; NMR: nuclear magnetic resonance; ORF: open reading frame; GWAS: genome-wide association study; NAPA: non-adaptive prion amplification; HMM: hidden Markov model; NCBI: National Center for Biotechnology Information.

Comparative analysis of prions in nervous and lymphoid tissues of chronic wasting disease-infected cervids

The prevalence, host range and geographical bounds of chronic wasting disease (CWD), the prion disease of cervids, are expanding. Horizontal transmission likely contributes the majority of new CWD cases, but the mechanism by which prions are transmitted among CWD-affected cervids remains unclear. To address the extent to which prion amplification in peripheral tissues contributes to contagious transmission, we assessed the prion levels in central nervous and lymphoreticular system tissues in white-tailed deer (Odocoileus virginianus), red deer (Cervus elaphus elaphus) and elk (Cervus canadensis). Using real-time quaking-induced conversion, cervid prion cell assay and transgenic mouse bioassay, we found that the retropharyngeal lymph nodes of red deer, white-tailed deer and elk contained similar prion titres to brain from the same individuals. We propose that marked lymphotropism is essential for the horizontal transmission of prion diseases and postulate that shed CWD prions are produced in the periphery.

Behind the potential evolution towards prion resistant species

Historically, the observation of naturally occurring cases of prion disease led to the classification of different susceptibility grades and to the designation of prion resistant species. However, the development of highly efficient in vitro prion propagation systems and the generation of ad hoc transgenic models allowed determining that leporidae and equidae families have been erroneously considered resistant to prion infection. On the contrary, similar approaches revealed an unexpected high level of resistance of the canidae family. In PLoS Pathogens [ 1 ], we describe experiments directed toward elucidating which are the determinants of the alleged prion resistance of this family. Studies based on the sequence of the canine prion protein coupled with structural in silico analysis identified a key residue probably implicated in this resistance. Cell and brain-based PMCA highlighted that the presence of aspartic or glutamic acid at codon 163 of the canid PrP, strongly inhibits prion replication in vitro. Transgenic animals carrying this substitution in mouse PrP were resistant to prion infection after intracerebral challenge with different mouse prion strains. The confirmation of the importance of this substitution and its exclusivity in this family, suggests it could have been evolutionarily favored, due to their diet based on carrion and small ruminants.

An Amino Acid Substitution Found in Animals with Low Susceptibility to Prion Diseases Confers a Protective Dominant-Negative Effect in Prion-Infected Transgenic Mice

While prion diseases have been described in numerous species, some, including those of the Canidae family, appear to show resistance or reduced susceptibility. A better understanding of the factors underlying prion susceptibility is crucial for the development of effective treatment and control measures. We recently demonstrated resistance to prion infection in mice overexpressing a mutated prion protein (PrP) carrying a specific amino acid substitution characteristic of canids. Here, we show that coexpression of this mutated PrP and wild-type mouse PrP in transgenic mice inoculated with different mouse-adapted prion strains (22 L, ME7, RML, and 301C) significantly increases survival times (by 45 to 113%). These data indicate that this amino acid substitution confers a dominant-negative effect on PrP, attenuating the conversion of PrP^C to PrP^Sc and delaying disease onset without altering the neuropathological properties of the prion strains. Taken together, these findings have important implications for the development of new treatment approaches for prion diseases based on dominant-negative proteins.

In Vitro Approach To Identify Key Amino Acids in Low Susceptibility of Rabbit Prion Protein to Misfolding

Prion diseases, or transmissible spongiform encephalopathies (TSEs), are a group of rare progressive neurodegenerative disorders caused by an abnormally folded prion protein (PrP^Sc). This is capable of transforming the normal cellular prion protein (PrP^C) into new infectious PrP^Sc Interspecies prion transmissibility studies performed by experimental challenge and the outbreak of bovine spongiform encephalopathy that occurred in the late 1980s and 1990s showed that while some species (sheep, mice, and cats) are readily susceptible to TSEs, others are apparently resistant (rabbits, dogs, and horses) to the same agent. To study the mechanisms of low susceptibility to TSEs of certain species, the mouse-rabbit transmission barrier was used as a model. To identify which specific amino acid residues determine high or low susceptibility to PrP^Sc propagation, protein misfolding cyclic amplification (PMCA), which mimics PrP^C-to-PrP^Sc conversion with accelerated kinetics, was used. This allowed amino acid substitutions in rabbit PrP and accurate analysis of misfolding propensities. Wild-type rabbit recombinant PrP could not be misfolded into a protease-resistant self-propagating isoform in vitro despite seeding with at least 12 different infectious prions from diverse origins. Therefore, rabbit recombinant PrP mutants were designed to contain every single amino acid substitution that distinguishes rabbit recombinant PrP from mouse recombinant PrP. Key amino acid residue substitutions were identified that make rabbit recombinant PrP susceptible to misfolding, and using these, protease-resistant misfolded recombinant rabbit PrP was generated. Additional studies characterized the mechanisms by which these critical amino acid residue substitutions increased the misfolding susceptibility of rabbit PrP.IMPORTANCE Prion disorders are invariably fatal, untreatable diseases typically associated with long incubation periods and characteristic spongiform changes associated with neuronal loss in the brain. Development of any treatment or preventative measure is dependent upon a detailed understanding of the pathogenesis of these diseases, and understanding the mechanism by which certain species appear to be resistant to TSEs is critical. Rabbits are highly resistant to naturally acquired TSEs, and even under experimental conditions, induction of clinical disease is not easy. Using recombinant rabbit PrP as a model, this study describes critical molecular determinants that confer this high resistance to transmissible spongiform encephalopathies.

Unraveling the key to the resistance of canids to prion diseases

One of the characteristics of prions is their ability to infect some species but not others and prion resistant species have been of special interest because of their potential in deciphering the determinants for susceptibility. Previously, we developed different in vitro and in vivo models to assess the susceptibility of species that were erroneously considered resistant to prion infection, such as members of the Leporidae and Equidae families. Here we undertake in vitro and in vivo approaches to understand the unresolved low prion susceptibility of canids. Studies based on the amino acid sequence of the canine prion protein (PrP), together with a structural analysis in silico, identified unique key amino acids whose characteristics could orchestrate its high resistance to prion disease. Cell- and brain-based PMCA studies were performed highlighting the relevance of the D163 amino acid in proneness to protein misfolding. This was also investigated by the generation of a novel transgenic mouse model carrying this substitution and these mice showed complete resistance to disease despite intracerebral challenge with three different mouse prion strains (RML, 22L and 301C) known to cause disease in wild-type mice. These findings suggest that dog D163 amino acid is primarily, if not totally, responsible for the prion resistance of canids.

Detection of individuals or whole species resistant to any infectious disease is vital to understand the determinants of susceptibility and to develop appropriate therapeutic and preventative strategies. Canids have long been considered resistant to prion infection given the absence of clinical disease despite exposure to the causal agent. Through extensive analysis of the canine prion protein we have detected a key amino acid that might be responsible for their universal resistance to prion disease. Using in vitro and in vivo models we demonstrated that the presence of this residue confers resistance to prion infection when introduced to susceptible animals, opening the way to develop a new therapeutic approach against these, at present, untreatable disorders.

Strains of Pathological Protein Aggregates in Neurodegenerative Diseases

The presence of protein aggregates in the brain is a hallmark of neurodegenerative disorders such as Alzheimer’s disease (AD) and Parkinson’s disease (PD). Considerable evidence has revealed that the pathological protein aggregates in many neurodegenerative diseases are able to self-propagate, which may enable pathology to spread from cell-to-cell within the brain. This property is reminiscent of what occurs in prion diseases such as Creutzfeldt-Jakob disease. A widely recognized feature of prion disorders is the existence of distinct strains of prions, which are thought to represent unique protein aggregate structures. A number of recent studies have pointed to the existence of strains of protein aggregates in other, more common neurodegenerative illnesses such as AD, PD, and related disorders. In this review, we outline the pathobiology of prion strains and discuss how the concept of protein aggregate strains may help to explain the heterogeneity inherent to many human neurodegenerative disorders.

The Ecology of Prions

Chronic wasting disease (CWD) affects cervids and is the only known prion disease readily transmitted among free-ranging wild animal populations in nature. The increasing spread and prevalence of CWD among cervid populations threaten the survival of deer and elk herds in North America, and potentially beyond. This review focuses on prion ecology, specifically that of CWD, and the current understanding of the role that the environment may play in disease propagation. We recount the discovery of CWD, discuss the role of the environment in indirect CWD transmission, and consider potentially relevant environmental reservoirs and vectors. We conclude by discussing how understanding the environmental persistence of CWD lends insight into transmission dynamics and potential management and mitigation strategies.