Small ruminant nor98 prions share biochemical features with human gerstmann-sträussler-scheinker disease and variably protease-sensitive prionopathy

Bona fide atypical scrapie faithfully reproduced for the first time in a rodent model

Atypical Scrapie, which is not linked to epidemics, is assumed to be an idiopathic spontaneous prion disease in small ruminants. Therefore, its occurrence is unlikely to be controlled through selective breeding or other strategies as it is done for classical scrapie outbreaks. Its spontaneous nature and its sporadic incidence worldwide is reminiscent of the incidence of idiopathic spontaneous prion diseases in humans, which account for more than 85% of the cases in humans. Hence, developing animal models that consistently reproduce this phenomenon of spontaneous PrP misfolding, is of importance to study the pathobiology of idiopathic spontaneous prion disorders. Transgenic mice overexpressing sheep PrP^C with I112 polymorphism (TgShI112, 1-2 × PrP levels compared to sheep brain) manifest clinical signs of a spongiform encephalopathy spontaneously as early as 380 days of age. The brains of these animals show the neuropathological hallmarks of prion disease and biochemical analyses of the misfolded prion protein show a ladder-like PrP^res pattern with a predominant 7-10 kDa band. Brain homogenates from spontaneously diseased transgenic mice were inoculated in several models to assess their transmissibility and characterize the prion strain generated: TgShI112 (ovine I112 ARQ PrP^C), Tg338 (ovine VRQ PrP^C), Tg501 (ovine ARQ PrP^C), Tg340 (human M129 PrP^C), Tg361 (human V129 PrP^C), TgVole (bank vole I109 PrP^C), bank vole (I109I PrP^C), and sheep (AHQ/ARR and AHQ/AHQ churra-tensina breeds). Our analysis of the results of these bioassays concludes that the strain generated in this model is indistinguishable to that causing atypical scrapie (Nor98). Thus, we present the first faithful model for a bona fide, transmissible, ovine, atypical scrapie prion disease.

A single amino acid residue in bank vole prion protein drives permissiveness to Nor98/atypical scrapie and the emergence of multiple strain variants

Prions are infectious agents that replicate through the autocatalytic misfolding of the cellular prion protein (PrP^C) into infectious aggregates (PrP^Sc) causing fatal neurodegenerative diseases in humans and animals. Prions exist as strains, which are encoded by conformational variants of PrP^Sc. The transmissibility of prions depends on the PrP^C sequence of the recipient host and on the incoming prion strain, so that some animal prion strains are more contagious than others or are transmissible to new species, including humans. Nor98/atypical scrapie (AS) is a prion disease of sheep and goats reported in several countries worldwide. At variance with classical scrapie (CS), AS is considered poorly contagious and is supposed to be spontaneous in origin. The zoonotic potential of AS, its strain variability and the relationships with the more contagious CS strains remain largely unknown. We characterized AS isolates from sheep and goats by transmission in ovinised transgenic mice (tg338) and in two genetic lines of bank voles, carrying either methionine (BvM) or isoleucine (BvI) at PrP residue 109. All AS isolates induced the same pathological phenotype in tg338 mice, thus proving that they encoded the same strain, irrespective of their geographical origin or source species. In bank voles, we found that the M109I polymorphism dictates the susceptibility to AS. BvI were susceptible and faithfully reproduced the AS strain, while the transmission in BvM was highly inefficient and was characterized by a conformational change towards a CS-like prion strain. Sub-passaging experiments revealed that the main strain component of AS is accompanied by minor CS-like strain components, which can be positively selected during replication in both AS-resistant or AS-susceptible animals. These findings add new clues for a better comprehension of strain selection dynamics in prion infections and have wider implications for understanding the origin of contagious prion strains, such as CS.

Prions are transmissible agents responsible for fatal neurodegenerative diseases in humans and animals. Prions exist as strains, exhibiting distinct disease phenotypes and transmission properties. Some prion diseases occur sporadically with a supposedly spontaneous origin, while others are contagious and give rise to epidemics, mainly in animals. We investigated the strain properties of Nor98/atypical scrapie (AS), a sporadic prion disease of small ruminants. We found that AS was faithfully reproduced not only in a homologous context, i.e. ovinised transgenic mice, but also in an unrelated animal species, the bank vole. A natural polymorphism of the bank vole prion protein, coding for methionine (BvM) or for isoleucine (BvI) at codon 109, dictated the susceptibility of voles to AS, with BvI being highly susceptible to AS and BvM rather resistant. Most importantly, the M109I polymorphism mediated the emergence of AS-derived mutant prion strains resembling classical scrapie (CS), a contagious prion disease. Finally, by sub-passages in bank voles, we found that the main strain component of AS is accompanied by minor CS-like strain components, which can be positively selected during replication in both AS-resistant or AS-susceptible vole lines. These findings allow a better understanding of strain selection dynamics and suggest a link between sporadic and contagious prion diseases.

Description of the first Spanish case of Gerstmann-Sträussler-Scheinker disease with A117V variant: clinical, histopathological and biochemical characterization

Gerstmann–Sträussler–Scheinker disease (GSS) is a rare neurodegenerative illness that belongs to the group of hereditary or familial Transmissible Spongiform Encephalopathies (TSE). Due to the presence of different pathogenic alterations in the prion protein (PrP) coding gene, it shows an enhanced proneness to misfolding into its pathogenic isoform, leading to prion formation and propagation. This aberrantly folded protein is able to induce its conformation to the native counterparts forming amyloid fibrils and plaques partially resistant to protease degradation and showing neurotoxic properties. PrP with A117V pathogenic variant is the second most common genetic alteration leading to GSS and despite common phenotypic and neuropathological traits can be defined for each specific variant, strikingly heterogeneous manifestations have been reported for inter-familial cases bearing the same pathogenic variant or even within the same family. Given the scarcity of cases and their clinical, neuropathological, and biochemical variability, it is important to characterize thoroughly each reported case to establish potential correlations between clinical, neuropathological and biochemical hallmarks that could help to define disease subtypes. With that purpose in mind, this manuscript aims to provide a detailed report of the first Spanish GSS case associated with A117V variant including clinical, genetic, neuropathological and biochemical data, which could help define in the future potential disease subtypes and thus, explain the high heterogeneity observed in patients suffering from these maladies.

Classical scrapie in small ruminants is caused by at least four different prion strains

The diversity of goat scrapie strains in Europe has recently been studied using bioassays in a wide collection of rodent models, resulting in the classification of classical scrapie into four different categories. However, the sole use of the first passage does not lead to isolate adaptation and identification of the strains involved and might therefore lead to misclassification of some scrapie isolates. Therefore, this work reports the complete transmission study of a wide collection of goat transmissible spongiform encephalopathy (TSE) isolates by intracranial inoculation in two transgenic mouse lines overexpressing either small ruminant (TgGoat-ARQ) or bovine (TgBov) PrP^C. To compare scrapie strains in sheep and goats, sheep scrapie isolates from different European countries were also included in the study. Once the species barrier phenomenon was overcome, an accurate classification of the isolates was attained. Thus, the use of just two rodent models allowed us to fully differentiate at least four different classical scrapie strains in small ruminants and to identify isolates containing mixtures of strains. This work reinforces the idea that classical scrapie in small ruminants is a prion disease caused by multiple different prion strains and not by a single strain, as is the case for epidemic classical bovine spongiform encephalopathy (BSE-C). In addition, the clear dissimilarity between the different scrapie strains and BSE-C does not support the idea that classical scrapie is the origin of epidemic BSE-C.

The protean prion protein

The prion protein, PrP, can adopt at least 2 conformations, the overwhelmingly prevalent cellular conformation (PrP^C) and the scrapie conformation (PrP^Sc). PrP^C features a globular C-terminal domain containing 3 α-helices and a short β-sheet and a long flexible N-terminal tail whose exact conformation in vivo is not yet known and a metastable subdomain with β-strand propensity has been identified within it. The PrP^Sc conformation is very rare and has the characteristics of an amyloid. Furthermore, PrP^Sc is a prion, i.e., it is infectious. This involves 2 steps: (1) PrP^Sc can template PrP^C and coerce it to adopt the PrP^Sc conformation and (2) PrP^Sc can be transmitted between individuals, by oral, parenteral, and other routes and thus propagate as an infectious agent. However, this is a simplification: On the one hand, PrP^Sc is not a single conformation, but rather, a set of alternative similar but distinct conformations. Furthermore, other amyloid conformations of PrP exist with different biochemical and propagative properties. In this issue of PLOS Biology, Asante and colleagues describe the first murine model of familial human prion disease and demonstrate the emergence and propagation of 2 PrP amyloid conformers. Of these, one causes neurodegeneration, whereas the other does not. With its many conformers, PrP is a truly protean protein.

This Primer explores the implications of a study that describes the first murine model of familial human prion disease, demonstrating the emergence and propagation of two PrP amyloid conformers; of these, one causes neurodegeneration while the other does not. With its many conformers, PrP is a truly protean protein.

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

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

Four types of scrapie in goats differentiated from each other and bovine spongiform encephalopathy by biochemical methods

Scrapie in goats has been known since 1942, the archetype of prion diseases in which only prion protein (PrP) in misfolded state (PrPSc) acts as infectious agent with fatal consequence. Emergence of bovine spongiform encephalopathy (BSE) with its zoonotic behaviour and detection in goats enhanced fears that its source was located in small ruminants. However, in goats knowledge on prion strain typing is limited. A European-wide study is presented concerning the biochemical phenotypes of the protease resistant fraction of PrPSc (PrPres) in over thirty brain isolates from transmissible spongiform encephalopathy (TSE) affected goats collected in seven countries. Three different scrapie forms were found: classical scrapie (CS), Nor98/atypical scrapie and one case of CH1641 scrapie. In addition, CS was found in two variants-CS-1 and CS-2 (mainly Italy)-which differed in proteolytic resistance of the PrPres N-terminus. Suitable PrPres markers for discriminating CH1641 from BSE (C-type) appeared to be glycoprofile pattern, presence of two triplets instead of one, and structural (in)stability of its core amino acid region. None of the samples exhibited BSE like features. BSE and these four scrapie types, of which CS-2 is new, can be recognized in goats with combinations of a set of nine biochemical parameters.

A New Cell Model for Investigating Prion Strain Selection and Adaptation

Prion diseases are fatal neurodegenerative diseases that affect humans and animals. Prion strains, conformational variants of misfolded prion proteins, are associated with distinct clinical and pathological phenotypes. Host-strain interactions result in the selective damage of distinct brain areas and they are responsible for strain selection and/or adaptation, but the underlying molecular mechanisms are unknown. Prion strains can be distinguished by their cell tropism in vivo and in vitro, which suggests that susceptibility to distinct prion strains is determined by cellular factors. The neuroblastoma cell line PK1 is refractory to the prion strain Me7, but highly susceptible to RML. We challenged a large number of clonal PK1 lines with Me7 and successfully selected highly Me7-susceptible subclones (PME) to investigate whether the prion strain repertoire of PK1 can be expanded. Notably, the Me7-infected PME clones were more protease-resistant when compared to RML-infected PME clones, which suggested that cell-adapted Me7 and RML are distinct prion strains. Strikingly, Me7-refractory cells, including PK1 and astrocytes in cortico-hippocampal cultures, are highly susceptible to prions, being derived from homogenates of Me7-infected PME cells, suggesting that the passage of Me7 in PME cells leads to an extended host range. Thus, PME clones represent a compelling cell model for strain selection and adaptation.

Variable Protease-Sensitive Prionopathy Transmission to Bank Voles

Variably protease-sensitive prionopathy (VPSPr), a recently described human sporadic prion disease, features a protease-resistant, disease-related prion protein (resPrP^D) displaying 5 fragments reminiscent of Gerstmann-Sträussler-Scheinker disease. Experimental VPSPr transmission to human PrP-expressing transgenic mice, although replication of the VPSPr resPrP^D profile succeeded, has been incomplete because of second passage failure. We bioassayed VPSPr in bank voles, which are susceptible to human prion strains. Transmission was complete; first-passage attack rates were 5%-35%, and second-passage rates reached 100% and survival times were 50% shorter. We observed 3 distinct phenotypes and resPrP^D profiles; 2 imitated sporadic Creutzfeldt-Jakob disease resPrP^D, and 1 resembled Gerstmann-Sträussler-Scheinker disease resPrPD. The first 2 phenotypes may be related to the presence of minor PrP^D components in VPSPr. Full VPSPr transmission confirms permissiveness of bank voles to human prions and suggests that bank vole PrP may efficiently reveal an underrepresented native strain but does not replicate the complex VPSPr PrP^D profile.

Novel Type of Chronic Wasting Disease Detected in Moose (Alces alces), Norway

Chronic wasting disease (CWD) persists in cervid populations of North America and in 2016 was detected for the first time in Europe in a wild reindeer in Norway. We report the detection of CWD in 3 moose (Alces alces) in Norway, identified through a large scale surveillance program. The cases occurred in 13-14-year-old female moose, and we detected an abnormal form of prion protein (PrP^Sc) in the brain but not in lymphoid tissues. Immunohistochemistry revealed that the moose shared the same neuropathologic phenotype, characterized by mostly intraneuronal deposition of PrP^Sc. This pattern differed from that observed in reindeer and has not been previously reported in CWD-infected cervids. Moreover, Western blot revealed a PrP^Sc type distinguishable from previous CWD cases and from known ruminant prion diseases in Europe, with the possible exception of sheep CH1641. These findings suggest that these cases in moose represent a novel type of CWD.

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 (PrP^Sc), 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 PrP^Sc 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.

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

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

iPS Cell Cultures from a Gerstmann-Sträussler-Scheinker Patient with the Y218N PRNP Mutation Recapitulate tau Pathology

Gerstmann-Sträussler-Scheinker (GSS) syndrome is a fatal autosomal dominant neurodegenerative prionopathy clinically characterized by ataxia, spastic paraparesis, extrapyramidal signs and dementia. In some GSS familiar cases carrying point mutations in the PRNP gene, patients also showed comorbid tauopathy leading to mixed pathologies. In this study we developed an induced pluripotent stem (iPS) cell model derived from fibroblasts of a GSS patient harboring the Y218N PRNP mutation, as well as an age-matched healthy control. This particular PRNP mutation is unique with very few described cases. One of the cases presented neurofibrillary degeneration with relevant Tau hyperphosphorylation. Y218N iPS-derived cultures showed relevant astrogliosis, increased phospho-Tau, altered microtubule-associated transport and cell death. However, they failed to generate proteinase K-resistant prion. In this study we set out to test, for the first time, whether iPS cell-derived neurons could be used to investigate the appearance of disease-related phenotypes (i.e, tauopathy) identified in the GSS patient.

The Structure of PrPSc Prions

PrP^Sc (scrapie isoform of the prion protein) prions are the infectious agent behind diseases such as Creutzfeldt–Jakob disease in humans, bovine spongiform encephalopathy in cattle, chronic wasting disease in cervids (deer, elk, moose, and reindeer), as well as goat and sheep scrapie. PrP^Sc is an alternatively folded variant of the cellular prion protein, PrP^C, which is a regular, GPI-anchored protein that is present on the cell surface of neurons and other cell types. While the structure of PrP^C is well studied, the structure of PrP^Sc resisted high-resolution determination due to its general insolubility and propensity to aggregate. Cryo-electron microscopy, X-ray fiber diffraction, and a variety of other approaches defined the structure of PrP^Sc as a four-rung β-solenoid. A high-resolution structure of PrP^Sc still remains to be solved, but the four-rung β-solenoid architecture provides a molecular framework for the autocatalytic propagation mechanism that gives rise to the alternative conformation of PrP^Sc. Here, we summarize the current knowledge regarding the structure of PrP^Sc and speculate about the molecular conversion mechanisms that leads from PrP^C to PrP^Sc.

Elezgarai S, Bravo S, Vázquez-Fernández E, Rosa I, Eraña H, Gil D, Veiga S, Vidal E, Erickson-Beltran ML, Guitián E, Silva CJ, Nonno R, Ma J, Castilla J, R. Requena J. Recombinant PrPSc shares structural features with brain-derived PrPSc: Insights from limited proteolysis

Very solid evidence suggests that the core of full length PrPSc is a 4-rung β-solenoid, and that individual PrPSc subunits stack to form amyloid fibers. We recently used limited proteolysis to map the β-strands and connecting loops that make up the PrPSc solenoid. Using high resolution SDS-PAGE followed by epitope analysis, and mass spectrometry, we identified positions ~116/118, 133-134, 141, 152-153, 162, 169 and 179 (murine numbering) as Proteinase K (PK) cleavage sites in PrPSc. Such sites likely define loops and/or borders of β-strands, helping us to predict the threading of the β-solenoid. We have now extended this approach to recombinant PrPSc (recPrPSc). The term recPrPSc refers to bona fide recombinant prions prepared by PMCA, exhibiting infectivity with attack rates of ~100%. Limited proteolysis of mouse and bank vole recPrPSc species yielded N-terminally truncated PK-resistant fragments similar to those seen in brain-derived PrPSc, albeit with varying relative yields. Along with these fragments, doubly N- and C-terminally truncated fragments, in particular ~89/97-152, were detected in some recPrPSc preparations; similar fragments are characteristic of atypical strains of brain-derived PrPSc. Our results suggest a shared architecture of recPrPSc and brain PrPSc prions. The observed differences, in particular the distinct yields of specific PK-resistant fragments, are likely due to differences in threading which result in the specific biochemical characteristics of recPrPSc. Furthermore, recombinant PrPSc offers exciting opportunities for structural studies unachievable with brain-derived PrPSc.

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

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

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

Determining the Relative Susceptibility of Four Prion Protein Genotypes to Atypical Scrapie

Atypical scrapie is a sheep prion (PrP^Sc) disease whose epidemiology is consistent with a sporadic origin and is associated with specific polymorphisms of the normal cellular prion protein (PrP^C). To determine the relative amounts of PrP polymorphisms present in atypical scrapie, total PrP was digested with chymotrypsin to generate characteristic peptides spanning relevant polymorphisms at positions 136, 141, 154, 171, and 172 of sheep PrP^C. A multiple reaction monitoring method (MRM), employing ¹⁵N-labeled internal standards, was used to detect and quantify these polymorphisms present in both the PrP^Sc and PrP^C from heterozygous (ALRRY and ALHQY or ALRQD or AFRQY) atypical scrapie-infected or uninfected control sheep. Both polymorphisms of the full length and truncated (C1) natively expressed PrP^C are produced in equal amounts. The overall amount of PrP^C present in the infected or uninfected animals was similar. PrP^Sc isolated from heterozygotes was composed of significant amounts of both PrP polymorphisms, including the ALRRY polymorphism which is highly resistant to classical scrapie. Thus, an atypical scrapie infection does not result from an overexpression of sheep PrP^C. The replication of all atypical scrapie prions occurs at comparable rates, despite polymorphisms at positions 141, 154, 171, or 172.

Variably protease-sensitive prionopathy

Variably protease-sensitive prionopathy (VPSPr), originally identified in 2008, was further characterized and renamed in 2010. Thirty-seven cases of VPSPr have been reported to date, consistent with estimated prevalence of 0.7-1.7% of all sporadic prion diseases. The lack of gene mutations establishes VPSPr as a sporadic form of human prion diseases, along with sporadic Creutzfeldt-Jakob disease (sCJD) and sporadic fatal insomnia. Like sCJD, VPSPr affects patients harboring any of the three genotypes, MM, MV, and VV at the prion protein (PrP) gene polymorphic codon 129, with VPSPr VV accounting for 65% of all VPSPr cases. Distinguishing clinical features include a median 2-year duration and presentation with psychiatric signs, speech/language impairment, or cognitive decline. Neuropathology comprises moderate spongiform degeneration, PrP amyloid miniplaques, and a target-like or plaque-like PrP deposition. The abnormal PrP associated with VPSPr typically forms an electrophoretic profile of five to seven bands (according to the antibody) presenting variable protease resistance depending on the 129 genotype. The familial prion disease associated with the V180I PrP gene mutation which harbors an abnormal PrP with similar electrophoretic profile might serve as a model for VPSPr. Transmission to animals has definitively established VPSPr as a prion disease. Because of its recent identification, rarity, and the elusiveness of its abnormal PrP, VPSPr remains largely understudied.

Transmissibility of Gerstmann-Sträussler-Scheinker syndrome in rodent models: New insights into the molecular underpinnings of prion infectivity

Prion diseases, or transmissible spongiform encephalopathies, have revealed the bewildering phenomenon of transmissibility in neurodegenerative diseases. Hence, the experimental transmissibility of prion-like neurodegenerative diseases via template directed misfolding has become the focus of intense research. Gerstmann-Sträussler-Scheinker disease (GSS) is an inherited prion disease associated with mutations in the prion protein gene. However, with the exception of a few GSS cases with P102L mutation characterized by co-accumulation of protease-resistant PrP core (PrP^res) of ∼21 kDa, attempts to transmit to rodents GSS associated to atypical misfolded prion protein with ∼8 kDa PrP^res have been unsuccessful. As a result, these GSS subtypes have often been considered as non-transmissible proteinopathies rather than true prion diseases. In a recent study we inoculated bank voles with GSS cases associated with P102L, A117V and F198S mutations and found that they transmitted efficiently and produced distinct pathological phenotypes, irrespective of the presence of 21 kDa PrP^res in the inoculum. This study demonstrates that GSS is a genuine prion disease characterized by both transmissibility and strain variation. We discuss the implications of these findings for the understanding of the heterogeneous clinic-pathological phenotypes of GSS and of the molecular underpinnings of prion infectivity.

Generation of a new infectious recombinant prion: a model to understand Gerstmann-Sträussler-Scheinker syndrome

Human transmissible spongiform encephalopathies (TSEs) or prion diseases are a group of fatal neurodegenerative disorders that include Kuru, Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker syndrome (GSS), and fatal familial insomnia. GSS is a genetically determined TSE caused by a range of mutations within the prion protein (PrP) gene. Several animal models, based on the expression of PrPs carrying mutations analogous to human heritable prion diseases, support that mutations might predispose PrP to spontaneously misfold. An adapted Protein Misfolding Cyclic Amplification methodology based on the use of human recombinant PrP (recPMCA) generated different self-propagating misfolded proteins spontaneously. These were characterized biochemically and structurally, and the one partially sharing some of the GSS PrP^Sc molecular features was inoculated into different animal models showing high infectivity. This constitutes an infectious recombinant prion which could be an invaluable model for understanding GSS. Moreover, this study proves the possibility to generate recombinant versions of other human prion diseases that could provide a further understanding on the molecular features of these devastating disorders.

Prions: Beyond a Single Protein

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

Goat K222-PrPC polymorphic variant does not provide resistance to atypical scrapie in transgenic mice

Host prion (PrP^C) genotype is a major determinant for the susceptibility to prion diseases. The Q/K₂₂₂-PrP^C polymorphic variant provides goats and mice with high resistance against classical scrapie and bovine spongiform encephalopathy (BSE); yet its effect against atypical scrapie is unknown. Here, transgenic mice expressing the goat wild-type (wt) or the K₂₂₂-PrP^C variant were intracerebrally inoculated with several natural cases of atypical scrapie from sheep and goat and their susceptibility to the prion disease was determined. Goat wt and K₂₂₂-PrP^C transgenic mice were 100% susceptible to all the atypical scrapie isolates, showing similar survival times and almost identical disease phenotypes. The capacity of the K₂₂₂-PrP^C variant to replicate specifically the atypical scrapie strain as efficiently as the goat wt PrP^C, but not the classical scrapie or cattle-BSE as previously reported, further suggests the involvement of concrete areas of the host PrP^C in the strain-dependent replication of prions.

Porcine prion protein amyloid

Mammalian prions are composed of misfolded aggregated prion protein (PrP) with amyloid-like features. Prions are zoonotic disease agents that infect a wide variety of mammalian species including humans. Mammals and by-products thereof which are frequently encountered in daily life are most important for human health. It is established that bovine prions (BSE) can infect humans while there is no such evidence for any other prion susceptible species in the human food chain (sheep, goat, elk, deer) and largely prion resistant species (pig) or susceptible and resistant pets (cat and dogs, respectively). PrPs from these species have been characterized using biochemistry, biophysics and neurobiology. Recently we studied PrPs from several mammals in vitro and found evidence for generic amyloidogenicity as well as cross-seeding fibril formation activity of all PrPs on the human PrP sequence regardless if the original species was resistant or susceptible to prion disease. Porcine PrP amyloidogenicity was among the studied. Experimentally inoculated pigs as well as transgenic mouse lines overexpressing porcine PrP have, in the past, been used to investigate the possibility of prion transmission in pigs. The pig is a species with extraordinarily wide use within human daily life with over a billion pigs harvested for human consumption each year. Here we discuss the possibility that the largely prion disease resistant pig can be a clinically silent carrier of replicating prions.

Transmission characteristics of variably protease-sensitive prionopathy

This disease is transmissible and thus an authentic prion disease.

Variably protease-sensitive prionopathy (VPSPr), a recently identified and seemingly sporadic human prion disease, is distinct from Creutzfeldt-Jakob disease (CJD) but shares features of Gerstmann-Sträussler-Scheinker disease (GSS). However, contrary to exclusively inherited GSS, no prion protein (PrP) gene variations have been detected in VPSPr, suggesting that VPSPr might be the long-sought sporadic form of GSS. The VPSPr atypical features raised the issue of transmissibility, a prototypical property of prion diseases. We inoculated VPSPr brain homogenate into transgenic mice expressing various levels of human PrP (PrP^C). On first passage, 54% of challenged mice showed histopathologic lesions, and 34% harbored abnormal PrP similar to that of VPSPr. Surprisingly, no prion disease was detected on second passage. We concluded that VPSPr is transmissible; thus, it is an authentic prion disease. However, we speculate that normal human PrP^C is not an efficient conversion substrate (or mouse brain not a favorable environment) and therefore cannot sustain replication beyond the first passage.

Gerstmann-Sträussler-Scheinker disease subtypes efficiently transmit in bank voles as genuine prion diseases

Gerstmann-Sträussler-Scheinker disease (GSS) is an inherited neurodegenerative disorder associated with mutations in the prion protein gene and accumulation of misfolded PrP with protease-resistant fragments (PrP^res) of 6–8 kDa. With the exception of a few GSS cases characterized by co-accumulation of PrP^res of 21 kDa, efforts to transmit GSS to rodents have been unsuccessful. As a result, GSS subtypes exclusively associated with 6–8 kDa PrP^res have often been considered as non-transmissible proteinopathies rather than true prion diseases. We show that GSS with P102L, A117V and F198S mutations transmit efficiently and produce distinct pathological phenotypes in bank voles (M. glareolus), irrespective of the presence of 21 kDa PrP^res in the inoculum, demonstrating that GSS is a genuine prion disease characterized by both transmissibility and strain variation.

Bank Vole Prion Protein As an Apparently Universal Substrate for RT-QuIC-Based Detection and Discrimination of Prion Strains

Prions propagate as multiple strains in a wide variety of mammalian species. The detection of all such strains by a single ultrasensitive assay such as Real Time Quaking-induced Conversion (RT-QuIC) would facilitate prion disease diagnosis, surveillance and research. Previous studies have shown that bank voles, and transgenic mice expressing bank vole prion protein, are susceptible to most, if not all, types of prions. Here we show that bacterially expressed recombinant bank vole prion protein (residues 23-230) is an effective substrate for the sensitive RT-QuIC detection of all of the different prion types that we have tested so far – a total of 28 from humans, cattle, sheep, cervids and rodents, including several that have previously been undetectable by RT-QuIC or Protein Misfolding Cyclic Amplification. Furthermore, comparison of the relative abilities of different prions to seed positive RT-QuIC reactions with bank vole and not other recombinant prion proteins allowed discrimination of prion strains such as classical and atypical L-type bovine spongiform encephalopathy, classical and atypical Nor98 scrapie in sheep, and sporadic and variant Creutzfeldt-Jakob disease in humans. Comparison of protease-resistant RT-QuIC conversion products also aided strain discrimination and suggested the existence of several distinct classes of prion templates among the many strains tested.

Prion diseases are neurodegenerative disorders that propagate as multiple strains in a variety of mammalian species. The detection of all such prion types by a single ultrasensitive assay, such as the Real Time Quaking-induced Conversion (RT-QuIC) assay, would facilitate prion disease diagnosis, surveillance, and research. Here we show detection of minute amounts of 28 different prion types from humans, cattle, sheep, cervids and rodents, some of which were previously undetectable, using a single recombinant bank vole prion protein substrate. We also demonstrate the generation of prion type-dependent RT-QuIC conversion products which may help with prion strain discrimination and the characterization of distinct classes of prion templates. Finally, we describe a practical strategy for prion strain discrimination, e.g. classical and atypical L-type bovine spongiform encephalopathy; classical and atypical Nor98 sheep scrapie; and human sporadic and variant Creutzfeldt-Jakob disease. Thus, our study provides a basis for wide-ranging prion detection and strain discrimination.

Detection and discrimination of classical and atypical L-type bovine spongiform encephalopathy by real-time quaking-induced conversion

Statutory surveillance of bovine spongiform encephalopathy (BSE) indicates that cattle are susceptible to both classical BSE (C-BSE) and atypical forms of BSE. Atypical forms of BSE appear to be sporadic and thus may never be eradicated. A major challenge for prion surveillance is the lack of sufficiently practical and sensitive tests for routine BSE detection and strain discrimination. The real-time quaking-induced conversion (RT-QuIC) test, which is based on prion-seeded fibrillization of recombinant prion protein (rPrPSen), is known to be highly specific and sensitive for the detection of multiple human and animal prion diseases but not BSE. Here, we tested brain tissue from cattle affected by C-BSE and atypical L-type bovine spongiform encephalopathy (L-type BSE or L-BSE) with the RT-QuIC assay and found that both BSE forms can be detected and distinguished using particular rPrPSen substrates. Specifically, L-BSE was detected using multiple rPrPSen substrates, while C-BSE was much more selective. This substrate-based approach suggests a diagnostic strategy for specific, sensitive, and rapid detection and discrimination of at least some BSE forms.

The prion protein protease sensitivity, stability and seeding activity in variably protease sensitive prionopathy brain tissue suggests molecular overlaps with sporadic Creutzfeldt-Jakob disease

INTRODUCTION

Variably protease sensitive prionopathy (VPSPr) is a recently described, sporadic human prion disease that is pathologically and biochemically distinct from the currently recognised sporadic Creutzfeldt-Jakob disease (sCJD) subtypes. The defining biochemical features of the abnormal form of the prion protein (PrPSc) in VPSPr are increased sensitivity to proteolysis and the presence of an N- and C-terminally cleaved ~8 kDa protease resistant PrPSc (PrPres) fragment. The biochemical and neuropathological profile of VPSPr has been proposed to resemble either Gerstmann-Sträussler-Scheinker syndrome (GSS) or familial CJD with the PRNP-V180I mutation. However, in some cases of VPSPr two protease resistant bands have been observed in Western blots that co-migrate with those of type 2 PrPres, suggesting that a proportion of the PrPSc present in VPSPr has properties similar to those of sCJD.

RESULTS

Here, we have used conformation dependent immunoassay to confirm the presence of PrPSc in VPSPr that is more protease sensitive compared with sCJD. However, CDI also shows that a proportion of PrPSc in VPSPr resists PK digestion of its C-terminus, distinguishing it from GSS associated with ~8 kDa PrPres, and showing similarity to sCJD. Intensive investigation of a single VPSPr case with frozen tissue from multiple brain regions shows a broad, region-specific spectrum of protease sensitivity and differential stability of PrPSc in the absence of PK treatment. Finally, using protein misfolding cyclic amplification and real-time quaking induced conversion, we show that VPSPr PrPSc has the potential to seed conversion in vitro and that seeding activity is dispersed through a broad range of aggregate sizes. We further propose that seeding activity is associated with the ~19 and ~23 kDa PrPres rather than the ~8 kDa fragment.

CONCLUSIONS

Therefore, PrPSc in VPSPr is heterogeneous in terms of protease sensitivity and stability to denaturation with the chaotrope GdnHCl and includes a proportion with similar properties to that found in sCJD.

Prion protein-specific antibodies that detect multiple TSE agents with high sensitivity

This paper describes the generation, characterisation and potential applications of a panel of novel anti-prion protein monoclonal antibodies (mAbs). The mAbs were generated by immunising PRNP null mice, using a variety of regimes, with a truncated form of recombinant ovine prion protein spanning residues 94-233. Epitopes of specific antibodies were mapped using solid-phase Pepscan analysis and clustered to four distinct regions within the PrP molecule. We have demonstrated the utility of these antibodies by use of Western blotting and immunohistochemistry in tissues from a range of different species affected by transmissible spongiform encephalopathy (TSE). In comparative tests against extensively-used and widely-published, commercially available antibodies, similar or improved results can be obtained using these new mAbs, specifically in terms of sensitivity of detection. Since many of these antibodies recognise native PrPC, they could also be applied to a broad range of immunoassays such as flow cytometry, DELFIA analysis or immunoprecipitation. We are using these reagents to increase our understanding of TSE pathogenesis and for use in potential diagnostic screening assays.

Prion protein misfolding, strains, and neurotoxicity: an update from studies on Mammalian prions

Prion diseases, also known as transmissible spongiform encephalopathies (TSEs), are a group of fatal neurodegenerative disorders affecting humans and other mammalian species. The central event in TSE pathogenesis is the conformational conversion of the cellular prion protein, PrP^C, into the aggregate, β-sheet rich, amyloidogenic form, PrP^Sc. Increasing evidence indicates that distinct PrP^Sc conformers, forming distinct ordered aggregates, can encipher the phenotypic TSE variants related to prion strains. Prion strains are TSE isolates that, after inoculation into syngenic hosts, cause disease with distinct characteristics, such as incubation period, pattern of PrP^Sc distribution, and regional severity of histopathological changes in the brain. In analogy with other amyloid forming proteins, PrP^Sc toxicity is thought to derive from the existence of various intermediate structures prior to the amyloid fiber formation and/or their specific interaction with membranes. The latter appears particularly relevant for the pathogenesis of TSEs associated with GPI-anchored PrP^Sc, which involves major cellular membrane distortions in neurons. In this review, we update the current knowledge on the molecular mechanisms underlying three fundamental aspects of the basic biology of prions such as the putative mechanism of prion protein conversion to the pathogenic form PrP^Sc and its propagation, the molecular basis of prion strains, and the mechanism of induced neurotoxicity by PrP^Sc aggregates.

Identifying critical sites of PrP(c)-PrP(Sc) interaction in prion-infected cells by dominant-negative inhibition

A direct physical interaction of the prion protein isoforms is a key element in prion conversion. Which sites interact first and which parts of PrP^c are converted subsequently is presently not known in detail. We hypothesized that structural changes induced by PrP^Sc interaction occur in more than one interface and subsequently propagate within the PrP^C substrate, like epicenters of structural changes. To identify potential interfaces we created a series of systematically-designed mutant PrPs and tested them in prion-infected cells for dominant-negative inhibition (DNI) effects. This showed that mutant PrPs with deletions in the region between first and second α-helix are involved in PrP-PrP interaction and conversion of PrP^C into PrP^Sc. Although some PrPs did not reach the plasma membrane, they had access to the locales of prion conversion and PrP^Sc recycling using autophagy pathways. Using other series of mutant PrPs we already have identified additional sites which constitute potential interaction interfaces. Our approach has the potential to characterize PrP-PrP interaction sites in the context of prion-infected cells. Besides providing further insights into the molecular mechanisms of prion conversion, this data may help to further elucidate how prion strain diversity is maintained.

Prions in variably protease-sensitive prionopathy: an update

Human prion diseases, including sporadic, familial, and acquired forms such as Creutzfeldt-Jakob disease (CJD), are caused by prions in which an abnormal prion protein (PrP^Sc) derived from its normal cellular isoform (PrP^C) is the only known component. The recently-identified variably protease-sensitive prionopathy (VPSPr) is characterized not only by an atypical clinical phenotype and neuropathology but also by the deposition in the brain of a peculiar PrP^Sc. Like other forms of human prion disease, the pathogenesis of VPSPr also currently remains unclear. However, the findings of the peculiar features of prions from VPSPr and of the possible association of VPSPr with a known genetic prion disease linked with a valine to isoleucine mutation at residue 180 of PrP reported recently, may be of great importance in enhancing our understanding of not only this atypical human prion disease in particular, but also other prion diseases in general. In this review, we highlight the physicochemical and biological properties of prions from VPSPr and discuss the pathogenesis of VPSPr including the origin and formation of the peculiar prions.

Biochemical characterization of prion strains in bank voles

Prions exist as different strains exhibiting distinct disease phenotypes. Currently, the identification of prion strains is still based on biological strain typing in rodents. However, it has been shown that prion strains may be associated with distinct PrP^Sc biochemical types. Taking advantage of the availability of several prion strains adapted to a novel rodent model, the bank vole, we investigated if any prion strain was actually associated with distinctive PrP^Sc biochemical characteristics and if it was possible to univocally identify strains through PrP^Sc biochemical phenotypes. We selected six different vole-adapted strains (three human-derived and three animal-derived) and analyzed PrP^Sc from individual voles by epitope mapping of protease resistant core of PrP^Sc (PrP^res) and by conformational stability and solubility assay. Overall, we discriminated five out of six prion strains, while two different scrapie strains showed identical PrP^Sc types. Our results suggest that the biochemical strain typing approach here proposed was highly discriminative, although by itself it did not allow us to identify all prion strains analyzed.