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

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.

A single protective polymorphism in the prion protein blocks cross-species prion replication in cultured cells

The bank vole (BV) prion protein (PrP) can function as a universal acceptor of prions. However, the molecular details of BVPrP's promiscuity for replicating a diverse range of prion strains remain obscure. To develop a cultured cell paradigm capable of interrogating the unique properties of BVPrP, we generated monoclonal lines of CAD5 cells lacking endogenous PrP but stably expressing either hamster (Ha), mouse (Mo), or BVPrP (M109 or I109 polymorphic variants) and then challenged them with various strains of mouse or hamster prions. Cells expressing BVPrP were susceptible to both mouse and hamster prions, whereas cells expressing MoPrP or HaPrP could only be infected with species-matched prions. Propagation of mouse and hamster prions in cells expressing BVPrP resulted in strain adaptation in several instances, as evidenced by alterations in conformational stability, glycosylation, susceptibility to anti-prion small molecules, and the inability of BVPrP-adapted mouse prion strains to infect cells expressing MoPrP. Interestingly, cells expressing BVPrP containing the G127V prion gene variant, identified in individuals resistant to kuru, were unable to become infected with prions. Moreover, the G127V polymorphic variant impeded the spontaneous aggregation of recombinant BVPrP. These results demonstrate that BVPrP can facilitate cross-species prion replication in cultured cells and that a single amino acid change can override the prion-permissive nature of BVPrP. This cellular paradigm will be useful for dissecting the molecular features of BVPrP that allow it to function as a universal prion acceptor.

Glycans are not necessary to maintain the pathobiological features of bovine spongiform encephalopathy

The role of the glycosylation status of PrPC in the conversion to its pathological counterpart and on cross-species transmission of prion strains has been widely discussed. Here, we assessed the effect on strain characteristics of bovine spongiform encephalopathy (BSE) isolates with different transmission histories upon propagation on a model expressing a non-glycosylated human PrPC. Bovine, ovine and porcine-passaged BSE, and variant Creutzfeldt-Jakob disease (vCJD) isolates were used as seeds/inocula in both in vitro and in vivo propagation assays using the non-glycosylated human PrPC-expressing mouse model (TgNN6h). After protein misfolding cyclic amplification (PMCA), all isolates maintained the biochemical characteristics of BSE. On bioassay, all PMCA-propagated BSE prions were readily transmitted to TgNN6h mice, in agreement with our previous in vitro results. TgNN6h mice reproduced the characteristic neuropathological and biochemical hallmarks of BSE, suggesting that the absence of glycans did not alter the pathobiological features of BSE prions. Moreover, back-passage of TgNN6h-adapted BSE prions to BoTg110 mice recovered the full BSE phenotype, confirming that the glycosylation of human PrPC is not essential for the preservation of the human transmission barrier for BSE prions or for the maintenance of BSE strain properties.

Canine D163-PrP polymorphic variant does not provide complete protection against prion infection in small ruminant PrP context

E/D₁₆₃ polymorphism of dog prion protein (PrP) has been recently proposed as the variant responsible for canid prion resistance. To further investigate the protective role of this variant against prion replication, the transgenic mouse model OvPrP-Tg532 expressing sheep/goat PrP carrying the substitution D₁₆₂ (equivalent to D₁₆₃ position of dog PrP) was generated and intracranially inoculated with a broad collection of small ruminant prion strains. OvPrP-Tg532 mice showed resistance to classical bovine spongiform encephalopathy (BSE) from sheep and some classical scrapie isolates from sheep and goat but were susceptible to ovine atypical L-BSE and numerous classical scrapie isolates. Strikingly, some of these classical scrapie isolates showed a shift in their prion strain properties. These results suggest that other PrP residues apart from E/D₁₆₃ variant of dog PrP or factors distinct than PrP may participate in prion resistance of canids and that different factors may be required for D₁₆₂ sheep PrP to provide effective protection to sheep against ruminant prions.

Novel Polymorphisms and Genetic Features of the Prion Protein Gene (PRNP) in Cats, Hosts of Feline Spongiform Encephalopathy

Prion diseases are fatal neurodegenerative disorders characterized by vacuolation and gliosis in the brain. Prion diseases have been reported in several mammals, and genetic polymorphisms of the prion protein gene (PRNP) play an essential role in the vulnerability of prion diseases. However, to date, investigations of PRNP polymorphisms are rare in cats, which are the major host of feline spongiform encephalopathy (FSE). Thus, we investigated the genetic polymorphisms of the cat PRNP gene and analyzed the structural characteristics of the PrP of cats compared to those of dog, prion disease-resistant animal. To investigate the genetic variations of the cat PRNP gene in 208 cats, we performed amplicon sequencing and examined the genotype, allele and haplotype frequencies of cat PRNP polymorphisms. We evaluated the influence of cat PRNP polymorphisms using PolyPhen-2, PANTHER, PROVEAN and AMYCO. In addition, we carried out structural analysis of cat PrP according to the allele of nonsynonymous single nucleotide polymorphism (SNP) (c.457G > A, Glu153Lys) using Swiss-PdbViewer. Finally, we compared the structural differences between cat and canine PrPs for SNPs associated with prion disease resistance in dogs. We identified a total of 15 polymorphisms, including 14 novel SNPs and one insertion/deletion polymorphism (InDel). Among them, Glu153Lys was predicted to affect the structural stability and amyloid propensity of cat PrP. In addition, asparagine at codon 166 of cat PrP was predicted to have longer hydrogen bond than aspartic acid at codon 163 of canine PrP. Furthermore, substitution to dog-specific amino acids in cat PrP showed an increase in structural stability. To the best of our knowledge, this is the first study regarding the structural characteristics of cat PRNP gene.

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.

Novel Polymorphisms and Genetic Characteristics of the Prion Protein Gene (PRNP) in Dogs-A Resistant Animal of Prion Disease

Transmissible spongiform encephalopathies (TSEs) have been reported in a wide range of species. However, TSE infection in natural cases has never been reported in dogs. Previous studies have reported that polymorphisms of the prion protein gene (PRNP) have a direct impact on the susceptibility of TSE. However, studies on polymorphisms of the canine PRNP gene are very rare in dogs. We examined the genotype, allele, and haplotype frequencies of canine PRNP in 204 dogs using direct sequencing and analyzed linkage disequilibrium (LD) using Haploview version 4.2. In addition, to evaluate the impact of nonsynonymous polymorphisms on the function of prion protein (PrP), we carried out in silico analysis using PolyPhen-2, PROVEAN, and PANTHER. Furthermore, we analyzed the structure of PrP and hydrogen bonds according to alleles of nonsynonymous single nucleotide polymorphisms (SNPs) using the Swiss-Pdb Viewer program. Finally, we predicted the impact of the polymorphisms on the aggregation propensity of dog PrP using AMYCO. We identified a total of eight polymorphisms, including five novel SNPs and one insertion/deletion polymorphism, and found strong LDs and six major haplotypes among eight polymorphisms. In addition, we identified significantly different distribution of haplotypes among eight dog breeds, however, the kinds of identified polymorphisms were different among each dog breed. We predicted that p.64_71del HGGGWGQP, Asp182Gly, and Asp182Glu polymorphisms can impact the function and/or structure of dog PrP. Furthermore, the number of hydrogen bonds of dog PrP with the Glu182 and Gly182 alleles were predicted to be less than those with the Asp182 allele. Finally, Asp163Glu and Asp182Gly showed more aggregation propensity than wild-type dog PrP. These results suggest that nonsynonymous SNPs, Asp182Glu and Asp182Gly, can influence the stability of dog PrP and confer the possibility of TSE infection in dogs.

Molecular dynamics studies of dog prion protein wild-type and its D159N mutant

Prion diseases (e.g. 'mad cow' disease in cattle, chronic wasting disease in deer and elk, Creutzfeldt-Jakob disease in humans) have been a major public health concern affecting humans and almost all animals. However, dogs are strongly resistant to prion diseases. Recently, through transgenic techniques, it was reported that the single (surface) residue D159 is sufficient to confer protection against protein conformational change and pathogenesis, thus provides conformational stability for dog prion protein. This made a big breakthrough in dog prion protein research field. For dog prion protein, another advancement is the produce of its NMR structure in 2005. However, all these breakthroughs are still short of enough structural informatics of dog prion protein. This paper studies dog prion protein wild-type and D159N mutant through molecular dynamics (MD) techniques. Our MD results reveal sufficient structural informatics on the residue at position 159 to understand the mechanism underlying the resistance to prion diseases of dogs. The structural informatics of this paper should be very useful for the medicinal treatment of prion diseases.Communicated by Ramaswamy H. Sarma.

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.

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.

D159 and S167 are protective residues in the prion protein from dog and horse, two prion-resistant animals

Prion diseases are fatal neurodegenerative diseases caused by misfolding of the prion protein (PrP). These conditions affect humans and animals, including endemic forms in sheep and deer. Bovine, rodents, and many zoo mammals also developed prion diseases during the "mad-cow" epidemic in the 1980's. Interestingly, rabbits, horses, and dogs show unusual resistance to prion diseases, suggesting that specific sequence changes in the corresponding endogenous PrP prevents the accumulation of pathogenic conformations. In vitro misfolding assays and structural studies have identified S174, S167, and D159 as the key residues mediating the stability of rabbit, horse, and dog PrP, respectively. Here, we expressed the WT forms of rabbit, horse, and dog PrP in transgenic Drosophila and found that none of them is toxic. Replacing these key residues with the corresponding amino acids in hamster PrP showed that mutant horse (S167D) and dog (D159N) PrP are highly toxic, whereas mutant rabbit (S174 N) PrP is not. These results confirm the impact of S167 and D159 in local and long-range structural features in the globular domain of PrP that increase its stability, while suggesting the role of additional residues in the stability of rabbit PrP. Identifying these protective amino acids and the structural features that stabilize PrP can contribute to advance the field towards the development of therapies that halt or reverse the devastating effects of prion diseases.

Protective Effect of Val129-PrP against Bovine Spongiform Encephalopathy but not Variant Creutzfeldt-Jakob Disease

Bovine spongiform encephalopathy (BSE) is the only known zoonotic prion that causes variant Creutzfeldt-Jakob disease (vCJD) in humans. The major risk determinant for this disease is the polymorphic codon 129 of the human prion protein (Hu-PrP), where either methionine (Met₁₂₉) or valine (Val₁₂₉) can be encoded. To date, all clinical and neuropathologically confirmed vCJD cases have been Met₁₂₉ homozygous, with the exception of 1 recently reported Met/Val heterozygous case. Here, we found that transgenic mice homozygous for Val₁₂₉ Hu-PrP show severely restricted propagation of the BSE prion strain, but this constraint can be partially overcome by adaptation of the BSE agent to the Met₁₂₉ Hu-PrP. In addition, the transmission of vCJD to transgenic mice homozygous for Val₁₂₉ Hu-PrP resulted in a prion with distinct strain features. These observations may indicate increased risk for vCJD secondary transmission in Val₁₂₉ Hu-PrP–positive humans with the emergence of new strain features.

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.

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.

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.

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.

Potential approaches for heterologous prion protein treatment of prion diseases

Prion diseases, or transmissible spongiform encephalopathies (TSEs) are progressive, fatal neurodegenerative diseases with no effective treatment. The pathology of these diseases involves the conversion of a protease sensitive form of the cellular prion protein (PrP^C) into a protease resistant infectious form (PrP^res). The efficiency of this conversion is predicated upon a number of factors, most notably a strong homology between cellular PrP^C and PrP^res. In our recently published study, we infected mice with the RML-Chandler strain of scrapie and treated them with heterologous hamster prion proteins. This treatment was seen to reduce clinical signs of prion disease, to delay the onset of clinical symptoms and to prolong survival. In this current article we discuss potential mechanisms of action of treatment with heterologous prion proteins. We also discuss potential extensions of these studies using a heterologous rabbit PrP-based treatment strategy or a peptide based strategy, and improvement of treatment delivery including a lentiviral-based system.

A single amino acid (Asp159) from the dog prion protein suppresses the toxicity of the mouse prion protein in Drosophila

Misfolding of the prion protein (PrP) is the key step in the transmission of spongiform pathologies in humans and several animals. Although PrP is highly conserved in mammals, a few changes in the sequence of endogenous PrP are proposed to confer protection to dogs, which were highly exposed to prion during the mad-cow epidemics. D159 is a unique amino acid found in PrP from dogs and other canines that was shown to alter surface charge, but its functional relevance has never been tested in vivo. Here, we show in transgenic Drosophila that introducing the N159D substitution on mouse PrP decreases its turnover. Additionally, mouse PrP-N159D demonstrates no toxicity and accumulates no pathogenic conformations, suggesting that a single D159 substitution is sufficient to prevent PrP conformational change and pathogenesis. Understanding the mechanisms mediating the protective activity of D159 is likely to lessen the burden of prion diseases in humans and domestic animals.

Transgenic Rabbits Expressing Ovine PrP Are Susceptible to Scrapie

Transmissible spongiform encephalopathies (TSEs) are a group of neurodegenerative diseases affecting a wide range of mammalian species. They are caused by prions, a proteinaceous pathogen essentially composed of PrPSc, an abnormal isoform of the host encoded cellular prion protein PrPC. Constrained steric interactions between PrPSc and PrPC are thought to provide prions with species specificity, and to control cross-species transmission into other host populations, including humans. Transgenetic expression of foreign PrP genes has been successfully and widely used to overcome the recognized resistance of mouse to foreign TSE sources. Rabbit is one of the species that exhibit a pronounced resistance to TSEs. Most attempts to infect experimentally rabbit have failed, except after inoculation with cell-free generated rabbit prions. To gain insights on the molecular determinants of the relative resistance of rabbits to prions, we generated transgenic rabbits expressing the susceptible V136R154Q171 allele of the ovine PRNP gene on a rabbit wild type PRNP New Zealand background and assessed their experimental susceptibility to scrapie prions. All transgenic animals developed a typical TSE 6-8 months after intracerebral inoculation, whereas wild type rabbits remained healthy more than 700 days after inoculation. Despite the endogenous presence of rabbit PrPC, only ovine PrPSc was detectable in the brains of diseased animals. Collectively these data indicate that the low susceptibility of rabbits to prion infection is not enciphered within their non-PrP genetic background.

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.

Generic amyloidogenicity of mammalian prion proteins from species susceptible and resistant to prions

Prion diseases are lethal, infectious diseases associated with prion protein (PrP) misfolding. A large number of mammals are susceptible to both sporadic and acquired prion diseases. Although PrP is highly conserved and ubiquitously expressed in all mammals, not all species exhibit prion disease. By employing full length recombinant PrP from five known prion susceptible species (human, cattle, cat, mouse and hamster) and two species considered to be prion resistant (pig and dog) the amyloidogenicity of these PrPs has been delineated. All the mammalian PrPs, even from resistant species, were swiftly converted from the native state to amyloid-like structure when subjected to a native condition conversion assay. The PrPs displayed amyloidotypic tinctorial and ultrastructural hallmarks. Self-seeded conversion of the PrPs displayed significantly decreased lag phases demonstrating that nucleation dependent polymerization is a dominating mechanism in the fibrillation process. Fibrils from Aβ1-40, Aβ1-42, Lysozyme, Insulin and Transthyretin did not accelerate conversion of HuPrP whereas fibrils from HuPrP90-231 and HuPrP121-231 as well as full length PrPs of all PrPs efficiently seeded conversion showing specificity of the assay requiring the C-terminal PrP sequence. Our findings have implications for PrP misfolding and could have ramifications in the context of prion resistant species and silent carriers.

Progress on low susceptibility mechanisms of transmissible spongiform encephalopathies

Transmissible spongiform encephalopathies (TSEs), also known as prion diseases, are a group of fatal neurodegenerative diseases detected in a wide range of mammalian species. The "protein-only" hypothesis of TSE suggests that prions are transmissible particles devoid of nucleic acid and the primary pathogenic event is thought to be the conversion of cellular prion protein (PrP(C)) into the disease-associated isoform (PrP(Sc)). According to susceptibility to TSEs, animals can be classified into susceptible species and low susceptibility species. In this review we focus on several species with low susceptibility to TSEs: dogs, rabbits, horses and buffaloes. We summarize recent studies into the characteristics of low susceptibility regarding protein structure, and biochemical and genetic properties.

Role of D278N mutation for stability of prion dimer and tetramer structure

Toxicity of the prion molecule is a result of transmission of conformational change by direct contact with malignant misfolded molecule. The aim of this study is analyze the role of D278N mutation in promoting preferential oligomerization modes. Proteins exist as ensembles in equilibrium between different structural and dynamic states, including functionally relevant conformers as the most populated states as well as malfunctioning conformers as less populated states. Furthermore, the existence of different conformations allows protein oligomerization with condition-specific affinities. The maintenance of a particular role requires specific conversion between multiple stable states. Protein-protein binding may facilitate or may be a necessary condition of structural adaptation. In the case of prion disease, protein-protein interactions, resulting in prion agglomeration, have toxic effect. How exactly increased concentrations of prion oligomers trigger mechanisms leading to neuronal death is not known. Nevertheless, first oligomerization and second aggregate recognition are likely sequence of events that have to happen before any pathological condition may arise. Here, we carry out structural and dynamic analyses of the effect of disease-causing mutations on the dimerization and tetramerization of prion molecule as the first step in aggregate formation. D178N mutation has almost no effect on the monomeric structure but helps to stabilize the dimer, which consequently facilitates tetramer formation and stability.

Protein misfolding cyclic amplification (PMCA): Current status and future directions

Transmissible spongiform encephalopathies (TSEs) most commonly known as prion diseases are invariably fatal neurological disorders that affect humans and animals. These disorders differ from other neurodegenerative conformational diseases caused by the accumulation in the brain of misfolded proteins, sometimes with amyloid properties, in their ability to infect susceptible species by various routes. While the infectious properties of amyloidogenic proteins, other than misfolded prion protein (PrP(TSE)), are currently under scrutiny, their potential to transmit from cell to cell, one of the intrinsic properties of the prion, has been recently shown in vitro and in vivo. Over the decades, various cell culture and laboratory animal models have been developed to study TSEs. These assays have been widely used in a variety of applications but showed to be time consuming and entailed elevated costs. Novel economic and fast alternatives became available with the development of in vitro assays that are based on the property of conformationally abnormal PrP(TSE) to recruit normal cellular PrP(C) to misfold. These include the cell-free conversion assay, protein misfolding cyclic amplification (PMCA) and quaking induced conversion assay (QuIC), of which the PMCA has been the only technology shown to generate infectious prions. Moreover, it allows indefinite amplification of PrP(TSE) with strain-specific biochemical and biological properties of the original molecules and under certain conditions may give rise to new spontaneously generated prions. The method also allows addressing the species barrier phenomena and assessing possible risks of animal-to-animal and animal-to-human transmission. Additionally, its unprecedented sensitivity has made possible the detection of as little as one infectious dose of PrP(TSE) and the biochemical identification of this protein in different tissues and biological fluids, including blood, cerebral spinal fluid (CSF), semen, milk, urine and saliva during the pre-clinical and clinical phases of the disease. The mechanistic similarities between TSEs and other conformational disorders have resulted in the adaptation of the PMCA to the amplification and detection of various amyloidogenic proteins. Here we provide a compelling discussion of the different applications of this technology to the study of TSEs and other neurodegenerative diseases.

Molecular dynamics studies on the NMR structures of rabbit prion protein wild type and mutants: surface electrostatic charge distributions

Prion diseases are invariably fatal and highly infectious neurodegenerative diseases that affect a wide variety of mammalian species such as sheep and goats, cattle, deer and elk, and humans. But for rabbits, studies have shown that they have a low susceptibility to be infected by prion diseases. This paper does molecular dynamics (MD) studies of rabbit NMR structures (of the wild type and its two mutants of two surface residues), in order to understand the specific mechanism of rabbit prion proteins (RaPrP(C)). Protein surface electrostatic charge distributions are specially focused to analyze the MD trajectories. This paper can conclude that surface electrostatic charge distributions indeed contribute to the structural stability of wild-type RaPrP(C); this may be useful for the medicinal treatment of prion diseases.

Nanopore analysis reveals differences in structural stability of ovine PrP(C) proteins corresponding to scrapie susceptible (VRQ) and resistance (ARR) genotypes

Species, as well as individuals within species, have unique susceptibilities to prion infection that are likely based on sequence differences in cellular prion protein (PrP(C)). Species barriers to transmission also reflect PrP(C) sequence differences. Defining the structure-activity relationship of PrP(C)/PrP(Sc) with respect to infectivity/susceptibility will benefit disease understanding and assessment of transmission risks. Here, nanopore analysis is employed to investigate genotypes of sheep PrP(C) corresponding to differential susceptibilities to scrapie infection. Under non-denaturing conditions scrapie resistant (ARR) and susceptible (VRQ) genotypes display similar, type I (bumping) predominant event profiles, suggesting a conserved folding pattern. Under increasingly denaturing conditions both proteins shift to type II (intercalation/translocation) events but with different sensitivities to unfolding. Specifically, when pre-incubated in 2M Gdn-HCl, the VRQ variant had more of type II events as compared with the ARR protein, suggesting a more flexible unfolding pattern. Addition of PrP(Sc)-specific polyclonal antibody (YML) to the ARR variant, pre-incubated in 2M Gdn-HCl, reduced the number of type II events with no clear intercalation/translocation peak, whereas for VRQ, type II events above blockades of 90 pA bound YML. A second PrP(Sc)-specific antibody (SN6b) to a different cryptic epitope reduced type II events for VRQ but not the ARR variant. Collectively, the event patterns associated with sequential denaturation, as well as interactions with PrP(Sc)-specific antibodies, support unique patterns and/or propensities of misfolding between the genotypes. Overall, nanopore analysis identifies intermediate conformations that occur during the unfolding pathways of ARR and VRQ genotypes and may help to understand the correlation of structural properties that induce protein misfolding.

Exploring the risks of a putative transmission of BSE to new species

The prion responsible for the Bovine Spongiform Encephalopathy (BSE) shows unique features when compared with other prions. One of these features is its ability to infect almost all experimentally tested animal models. In the paper published in The Journal of Neuroscience (1) we describe a series of experiments directed toward elucidating which would be the in vivo behavior of BSE if it would infect dogs and rabbits, two alleged prion resistant species. Protein misfolding cyclic amplification (PMCA) was used to generate canidae and leporidae in vitro adapted BSE prions. A characterization of their in vivo pathobiological properties showed that BSE prions were capable not only of adapting to new species but they maintained, in the case of rabbits, their ability to infect transgenic mice expressing human PrP. The remarkable adaptation ability of certain prions implies that any new host species could lead to the emergence of new infectious agents with unpredictable transmission potential. Our results suggest that caution must be taken when considering the use of any mammal derived protein in feedstuffs.

Molecular dynamics studies on the NMR and X-ray structures of rabbit prion proteins

Prion diseases, traditionally referred to as transmissible spongiform encephalopathies (TSEs), are invariably fatal and highly infectious neurodegenerative diseases that affect a wide variety of mammalian species, manifesting as scrapie in sheep and goats, bovine spongiform encephalopathy (BSE or mad-cow disease) in cattle, chronic wasting disease in deer and elk, and Creutzfeldt-Jakob diseases, Gerstmann-Sträussler-Scheinker syndrome, fatal familial insomnia, and kulu in humans, etc. These neurodegenerative diseases are caused by the conversion from a soluble normal cellular prion protein (PrP(C)) into insoluble abnormally folded infectious prions (PrP(Sc)), and the conversion of PrP(C) to PrP(Sc) is believed to involve conformational change from a predominantly α-helical protein to one rich in β-sheet structure. Such a conformational change may be amenable to study by molecular dynamics (MD) techniques. For rabbits, classical studies show that they have a low susceptibility to be infected by PrP(Sc), but recently it was reported that rabbit prions can be generated through saPMCA (serial automated Protein Misfolding Cyclic Amplification) in vitro and the rabbit prion is infectious and transmissible. In this paper, we first do a detailed survey on the research advances of rabbit prion protein (RaPrP) and then we perform MD simulations on the NMR and X-ray molecular structures of rabbit prion protein wild-type and mutants. The survey shows to us that rabbits were not challenged directly in vivo with other known prion strains and the saPMCA result did not pass the test of the known BSE strain of cattle. Thus, we might still look rabbits as a prion resistant species. MD results indicate that the three α-helices of the wild-type are stable under the neutral pH environment (but under low pH environment the three α-helices have been unfolded into β-sheets), and the three α-helices of the mutants (I214V and S173N) are unfolded into rich β-sheet structures under the same pH environment. In addition, we found an interesting result that the salt bridges such as ASP201-ARG155, ASP177-ARG163 contribute greatly to the structural stability of RaPrP.