Prions on the move

Cofactors facilitate bona fide prion misfolding in vitro but are not necessary for the infectivity of recombinant murine prions

Prion diseases, particularly sporadic cases, pose a challenge due to their complex nature and heterogeneity. The underlying mechanism of the spontaneous conversion from PrPC to PrPSc, the hallmark of prion diseases, remains elusive. To shed light on this process and the involvement of cofactors, we have developed an in vitro system that faithfully mimics spontaneous prion misfolding using minimal components. By employing this PMSA methodology and introducing an isoleucine residue at position 108 in mouse PrP, we successfully generated recombinant murine prion strains with distinct biochemical and biological properties. Our study aimed to explore the influence of a polyanionic cofactor in modulating strain selection and infectivity in de novo-generated synthetic prions. These results not only validate PMSA as a robust method for generating diverse bona fide recombinant prions but also emphasize the significance of cofactors in shaping specific prion conformers capable of crossing species barriers. Interestingly, once these conformers are established, our findings suggest that cofactors are not necessary for their infectivity. This research provides valuable insights into the propagation and maintenance of the pathobiological features of cross-species transmissible recombinant murine prion and highlights the intricate interplay between cofactors and prion strain characteristics.

Minor prion substrains overcome transmission barriers

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

IMPORTANCE

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

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

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

Propagation of distinct CWD prion strains during peripheral and intracerebral challenges of gene-targeted mice

Since prion diseases result from infection and neurodegeneration of the central nervous system (CNS), experimental characterizations of prion strain properties customarily rely on the outcomes of intracerebral challenges. However, natural transmission of certain prions, including those causing chronic wasting disease (CWD) in elk and deer, depends on propagation in peripheral host compartments prior to CNS infection. Using gene-targeted GtE and GtQ mice, which accurately control cellular elk or deer PrP expression, we assessed the impact that peripheral or intracerebral exposures play on CWD prion strain propagation and resulting CNS abnormalities. Whereas oral and intraperitoneal transmissions produced identical neuropathological outcomes in GtE and GtQ mice and preserved the naturally convergent conformations of elk and deer CWD prions, intracerebral transmissions generated CNS prion strains with divergent biochemical properties in GtE and GtQ mice that were changed compared to their native counterparts. While CWD replication kinetics remained constant during iterative peripheral transmissions and brain titers reflected those found in native hosts, serial intracerebral transmissions produced 10-fold higher prion titers and accelerated incubation times. Our demonstration that peripherally and intracerebrally challenged Gt mice develop dissimilar CNS diseases which result from the propagation of distinct CWD prion strains points to the involvement of tissue-specific cofactors during strain selection in different host compartments. Since peripheral transmissions preserved the natural features of elk and deer prions, whereas intracerebral propagation produced divergent strains, our findings illustrate the importance of experimental characterizations using hosts that not only abrogate species barriers but also accurately recapitulate natural transmission routes of native strains.

Dysbiosis of the gut microbiota and its effect on α-synuclein and prion protein misfolding: consequences for neurodegeneration

Abnormal behavior of α-synuclein and prion proteins is the hallmark of Parkinson's disease (PD) and prion illnesses, respectively, being complex neurological disorders. A primary cause of protein aggregation, brain injury, and cognitive loss in prion illnesses is the misfolding of normal cellular prion proteins (PrPC) into an infectious form (PrPSc). Aggregation of α-synuclein causes disruptions in cellular processes in Parkinson's disease (PD), leading to loss of dopamine-producing neurons and motor symptoms. Alteration in the composition or activity of gut microbes may weaken the intestinal barrier and make it possible for prions to go from the gut to the brain. The gut-brain axis is linked to neuroinflammation; the metabolites produced by the gut microbiota affect the aggregation of α-synuclein, regulate inflammation and immunological responses, and may influence the course of the disease and neurotoxicity of proteins, even if their primary targets are distinct proteins. This thorough analysis explores the complex interactions that exist between the gut microbiota and neurodegenerative illnesses, particularly Parkinson's disease (PD) and prion disorders. The involvement of the gut microbiota, a complex collection of bacteria, archaea, fungi, viruses etc., in various neurological illnesses is becoming increasingly recognized. The gut microbiome influences neuroinflammation, neurotransmitter synthesis, mitochondrial function, and intestinal barrier integrity through the gut-brain axis, which contributes to the development and progression of disease. The review delves into the molecular mechanisms that underlie these relationships, emphasizing the effects of microbial metabolites such as bacterial lipopolysaccharides (LPS), and short-chain fatty acids (SCFAs) in regulating brain functioning. Additionally, it looks at how environmental influences and dietary decisions affect the gut microbiome and whether they could be risk factors for neurodegenerative illnesses. This study concludes by highlighting the critical role that the gut microbiota plays in the development of Parkinson's disease (PD) and prion disease. It also provides a promising direction for future research and possible treatment approaches. People afflicted by these difficult ailments may find hope in new preventive and therapeutic approaches if the role of the gut microbiota in these diseases is better understood.

Puzzles, challenges, and information reservoir of SARS-CoV-2 quasispecies

Upon the emergence of SARS-CoV-2 in the human population, it was conjectured that for this coronavirus the dynamic intra-host heterogeneity typical of RNA viruses would be toned down. Nothing of this sort is observed. Here we review the main observations on the complexity and diverse composition of SARS-CoV-2 mutant spectra sampled from infected patients, within the framework of quasispecies dynamics. The analyses suggest that the information provided by myriads of genomic sequences within infected individuals may have a predictive value of the genomic sequences that acquire epidemiological relevance. Possibilities to reconcile the presence of broad mutant spectra in the large RNA coronavirus genome with its encoding a 3' to 5' exonuclease proofreading-repair activity are considered. Indeterminations in the behavior of individual viral genomes provide a benefit for the survival of the ensemble. We propose that this concept falls in the domain of "stochastic thinking," a notion that applies also to cellular processes, as a means for biological systems to face unexpected needs.

Evidence for preexisting prion substrain diversity in a biologically cloned prion strain

Prion diseases are a group of inevitably fatal neurodegenerative disorders affecting numerous mammalian species, including Sapiens. Prions are composed of PrPSc, the disease specific conformation of the host encoded prion protein. Prion strains are operationally defined as a heritable phenotype of disease under controlled transmission conditions. Treatment of rodents with anti-prion drugs results in the emergence of drug-resistant prion strains and suggest that prion strains are comprised of a dominant strain and substrains. While much experimental evidence is consistent with this hypothesis, direct observation of substrains has not been observed. Here we show that replication of the dominant strain is required for suppression of a substrain. Based on this observation we reasoned that selective reduction of the dominant strain may allow for emergence of substrains. Using a combination of biochemical methods to selectively reduce drowsy (DY) PrPSc from biologically-cloned DY transmissible mink encephalopathy (TME)-infected brain resulted in the emergence of strains with different properties than DY TME. The selection methods did not occur during prion formation, suggesting the substrains identified preexisted in the DY TME-infected brain. We show that DY TME is biologically stable, even under conditions of serial passage at high titer that can lead to strain breakdown. Substrains therefore can exist under conditions where the dominant strain does not allow for substrain emergence suggesting that substrains are a common feature of prions. This observation has mechanistic implications for prion strain evolution, drug resistance and interspecies transmission.

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

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

Essential Components of Synthetic Infectious Prion Formation De Novo

Prion diseases are a class of neurodegenerative diseases that are uniquely infectious. Whilst their general replication mechanism is well understood, the components required for the formation and propagation of highly infectious prions are poorly characterized. The protein-only hypothesis posits that the prion protein (PrP) is the only component of the prion; however, additional co-factors are required for its assembly into infectious prions. These can be provided by brain homogenate, but synthetic lipids and non-coding RNA have also been used in vitro. Here, we review a range of experimental approaches, which generate PrP amyloid assemblies de novo. These synthetic PrP assemblies share some, but not necessarily all, properties of genuine infectious prions. We will discuss the different experimental approaches, how a prion is defined, the non-protein requirements of a prion, and provide an overview of the current state of prion amplification and generation in vitro.

The existence of Aβ strains and their potential for driving phenotypic heterogeneity in Alzheimer's disease

Reminiscent of the human prion diseases, there is considerable clinical and pathological variability in Alzheimer's disease, the most common human neurodegenerative condition. As in prion disorders, protein misfolding and aggregation is a hallmark feature of Alzheimer's disease, where the initiating event is thought to be the self-assembly of Aβ peptide into aggregates that deposit in the central nervous system. Emerging evidence suggests that Aβ, similar to the prion protein, can polymerize into a conformationally diverse spectrum of aggregate strains both in vitro and within the brain. Moreover, certain types of Aβ aggregates exhibit key hallmarks of prion strains including divergent biochemical attributes and the ability to induce distinct pathological phenotypes when intracerebrally injected into mouse models. In this review, we discuss the evidence demonstrating that Aβ can assemble into distinct strains of aggregates and how such strains may be primary drivers of the phenotypic heterogeneity in Alzheimer's disease.

Prion potentiation after life-long dormancy in mice devoid of PrP

Prions are neurotropic pathogens composed of misfolded assemblies of the host-encoded prion protein PrP^C which replicate by recruitment and conversion of further PrP^C by an autocatalytic seeding polymerization process. While it has long been shown that mouse-adapted prions cannot replicate and are rapidly cleared in transgenic PrP^0/0 mice invalidated for PrP^C, these experiments have not been done with other prions, including from natural resources, and more sensitive methods to detect prion biological activity. Using transgenic mice expressing human PrP to bioassay prion infectivity and RT-QuIC cell-free assay to measure prion seeding activity, we report that prions responsible for the most prevalent form of sporadic Creutzfeldt-Jakob disease in human (MM1-sCJD) can persist indefinitely in the brain of intra-cerebrally inoculated PrP^0/0 mice. While low levels of seeding activity were measured by RT-QuIC in the brain of the challenged PrP^0/0 mice, the bio-indicator humanized mice succumbed at a high attack rate, suggesting relatively high levels of persistent infectivity. Remarkably, these humanized mice succumbed with delayed kinetics as compared to MM1-sCJD prions directly inoculated at low doses, including the limiting one. Yet, the disease that did occur in the humanized mice on primary and subsequent back-passage from PrP^0/0 mice shared the neuropathological and molecular characteristics of MM1-sCJD prions, suggesting no apparent strain evolution during lifelong dormancy in PrP^0/0 brain. Thus, MM1-sCJD prions can persist for the entire life in PrP^0/0 brain with potential disease potentiation on retrotransmission to susceptible hosts. These findings highlight the capacity of prions to persist and rejuvenate in non-replicative environments, interrogate on the type of prion assemblies at work and alert on the risk of indefinite prion persistence with PrP-lowering therapeutic strategies.

Improving the Predictive Value of Prion Inactivation Validation Methods to Minimize the Risks of Iatrogenic Transmission With Medical Instruments

Prions are pathogenic infectious agents responsible for fatal, incurable neurodegenerative diseases in animals and humans. Prions are composed exclusively of an aggregated and misfolded form (PrP^Sc) of the cellular prion protein (PrP^C). During the propagation of the disease, PrP^Sc recruits and misfolds PrP^C into further PrP^Sc. In human, iatrogenic prion transmission has occurred with incompletely sterilized medical material because of the unusual resistance of prions to inactivation. Most commercial prion disinfectants validated against the historical, well-characterized laboratory strain of 263K hamster prions were recently shown to be ineffective against variant Creutzfeldt-Jakob disease human prions. These observations and previous reports support the view that any inactivation method must be validated against the prions for which they are intended to be used. Strain-specific variations in PrP^Sc physico-chemical properties and conformation are likely to explain the strain-specific efficacy of inactivation methods. Animal bioassays have long been used as gold standards to validate prion inactivation methods, by measuring reduction of prion infectivity. Cell-free assays such as the real-time quaking-induced conversion (RT-QuIC) assay and the protein misfolding cyclic amplification (PMCA) assay have emerged as attractive alternatives. They exploit the seeding capacities of PrP^Sc to exponentially amplify minute amounts of prions in biospecimens. European and certain national medicine agencies recently implemented their guidelines for prion inactivation of non-disposable medical material; they encourage or request the use of human prions and cell-free assays to improve the predictive value of the validation methods. In this review, we discuss the methodological and technical issues regarding the choice of (i) the cell-free assay, (ii) the human prion strain type, (iii) the prion-containing biological material. We also introduce a new optimized substrate for high-throughput PMCA amplification of human prions bound on steel wires, as translational model for prion-contaminated instruments.

Crossing Species Barriers Relies on Structurally Distinct Prion Assemblies and Their Complementation

Prion replication results from the autocatalytic templated assisted conversion of the host-encoded prion protein PrP^C into misfolded, polydisperse PrP^Sc conformers. Structurally distinct PrP^Sc conformers can give rise to multiple prion strains. Within and between prion strains, the biological activity (replicative efficacy and specific infectivity) of PrP^Sc assemblies is size dependent and thus reflects an intrinsic structural heterogeneity. The contribution of such PrP^Sc heterogeneity across species prion adaptation, which is believed to be based on fit adjustment between PrP^Sc template(s) and host PrP^C, has not been explored. To define the structural-to-fitness PrP^Sc landscape, we measured the relative capacity of size-fractionated PrP^Sc assemblies from different prion strains to cross mounting species barriers in transgenic mice expressing foreign PrP^C. In the absence of a transmission barrier, the relative efficacy of the isolated PrP^Sc assemblies to induce the disease is like the efficacy observed in the homotypic context. However, in the presence of a transmission barrier, size fractionation overtly delays and even abrogates prion pathogenesis in both the brain and spleen tissues, independently of the infectivity load of the isolated assemblies. Altering by serial dilution PrP^Sc assembly content of non-fractionated inocula aberrantly reduces their specific infectivity, solely in the presence of a transmission barrier. This suggests that synergy between structurally distinct PrP^Sc assemblies in the inoculum is requested for crossing the species barrier. Our data support a mechanism whereby overcoming prion species barrier requires complementation between structurally distinct PrP^Sc assemblies. This work provides key insight into the "quasispecies" concept applied to prions, which would not necessarily rely on prion substrains as constituent but on structural PrP^Sc heterogeneity within prion population.

Estimating relative CWD susceptibility and disease progression in farmed white-tailed deer with rare PRNP alleles

Chronic wasting disease is a prion disease affecting both free-ranging and farmed cervids in North America and Scandinavia. A range of cervid species have been found to be susceptible, each with variations in the gene for the normal prion protein, PRNP, reportedly influencing both disease susceptibility and progression in the respective hosts. Despite the finding of several different PRNP alleles in white-tailed deer, the majority of past research has focused on two of the more common alleles identified-the 96G and 96S alleles. In the present study, we evaluate both infection status and disease stage in nearly 2100 farmed deer depopulated in the United States and Canada, including 714 CWD-positive deer and correlate our findings with PRNP genotype, including the more rare 95H, 116G, and 226K alleles. We found significant differences in either likelihood of being found infected or disease stage (and in many cases both) at the time of depopulation in all genotypes present, relative to the most common 96GG genotype. Despite high prevalence in many of the herds examined, infection was not found in several of the reported genotypes. These findings suggest that additional research is necessary to more properly define the role that these genotypes may play in managing CWD in both farmed and free-ranging white-tailed deer, with consideration for factors including relative fitness levels, incubation periods, and the kinetics of shedding in animals with these rare genotypes.

Development of a new largely scalable in vitro prion propagation method for the production of infectious recombinant prions for high resolution structural studies

The resolution of the three-dimensional structure of infectious prions at the atomic level is pivotal to understand the pathobiology of Transmissible Spongiform Encephalopathies (TSE), but has been long hindered due to certain particularities of these proteinaceous pathogens. Difficulties related to their purification from brain homogenates of disease-affected animals were resolved almost a decade ago by the development of in vitro recombinant prion propagation systems giving rise to highly infectious recombinant prions. However, lack of knowledge about the molecular mechanisms of the misfolding event and the complexity of systems such as the Protein Misfolding Cyclic Amplification (PMCA), have limited generating the large amounts of homogeneous recombinant prion preparations required for high-resolution techniques such as solid state Nuclear Magnetic Resonance (ssNMR) imaging. Herein, we present a novel recombinant prion propagation system based on PMCA that substitutes sonication with shaking thereby allowing the production of unprecedented amounts of multi-labeled, infectious recombinant prions. The use of specific cofactors, such as dextran sulfate, limit the structural heterogeneity of the in vitro propagated prions and makes possible, for the first time, the generation of infectious and likely homogeneous samples in sufficient quantities for studies with high-resolution structural techniques as demonstrated by the preliminary ssNMR spectrum presented here. Overall, we consider that this new method named Protein Misfolding Shaking Amplification (PMSA), opens new avenues to finally elucidate the three-dimensional structure of infectious prions.

Early stage prion assembly involves two subpopulations with different quaternary structures and a secondary templating pathway

The dynamics of aggregation and structural diversification of misfolded, host-encoded proteins in neurodegenerative diseases are poorly understood. In many of these disorders, including Alzheimer's, Parkinson's and prion diseases, the misfolded proteins are self-organized into conformationally distinct assemblies or strains. The existence of intrastrain structural heterogeneity is increasingly recognized. However, the underlying processes of emergence and coevolution of structurally distinct assemblies are not mechanistically understood. Here, we show that early prion replication generates two subsets of structurally different assemblies by two sequential processes of formation, regardless of the strain considered. The first process corresponds to a quaternary structural convergence, by reducing the parental strain polydispersity to generate small oligomers. The second process transforms these oligomers into larger ones, by a secondary autocatalytic templating pathway requiring the prion protein. This pathway provides mechanistic insights into prion structural diversification, a key determinant for prion adaptation and toxicity.

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.

Heterogeneity and Architecture of Pathological Prion Protein Assemblies: Time to Revisit the Molecular Basis of the Prion Replication Process?

Prions are proteinaceous infectious agents responsible for a range of neurodegenerative diseases in animals and humans. Prion particles are assemblies formed from a misfolded, β-sheet rich, aggregation-prone isoform (PrP^Sc) of the host-encoded cellular prion protein (PrP^C). Prions replicate by recruiting and converting PrP^C into PrP^Sc, by an autocatalytic process. PrP^Sc is a pleiomorphic protein as different conformations can dictate different disease phenotypes in the same host species. This is the basis of the strain phenomenon in prion diseases. Recent experimental evidence suggests further structural heterogeneity in PrP^Sc assemblies within specific prion populations and strains. Still, this diversity is rather seen as a size continuum of assemblies with the same core structure, while analysis of the available experimental data points to the existence of structurally distinct arrangements. The atomic structure of PrP^Sc has not been elucidated so far, making the prion replication process difficult to understand. All currently available models suggest that PrP^Sc assemblies exhibit a PrP^Sc subunit as core constituent, which was recently identified. This review summarizes our current knowledge on prion assembly heterogeneity down to the subunit level and will discuss its importance with regard to the current molecular principles of the prion replication process.

Prion pathogenesis is unaltered in a mouse strain with a permeable blood-brain barrier

Transmissible spongiform encephalopathies (TSEs) are caused by the prion, which consists essentially of PrPSc, an aggregated, conformationally modified form of the cellular prion protein (PrPC). Although TSEs can be experimentally transmitted by intracerebral inoculation, most instances of infection in the field occur through extracerebral routes. The epidemics of kuru and variant Creutzfeldt-Jakob disease were caused by dietary exposure to prions, and parenteral administration of prion-contaminated hormones has caused hundreds of iatrogenic TSEs. In all these instances, the development of postexposure prophylaxis relies on understanding of how prions propagate from the site of entry to the brain. While much evidence points to lymphoreticular invasion followed by retrograde transfer through peripheral nerves, prions are present in the blood and may conceivably cross the blood-brain barrier directly. Here we have addressed the role of the blood-brain barrier (BBB) in prion disease propagation using Pdgfbret/ret mice which possess a highly permeable BBB. We found that Pdgfbret/ret mice have a similar prion disease incubation time as their littermate controls regardless of the route of prion transmission. These surprising results indicate that BBB permeability is irrelevant to the initiation of prion disease, even when prions are administered parenterally.

Synthetic Prion Selection and Adaptation

Prion pathologies are characterized by the conformational conversion of the cellular prion protein (PrP^C) into a pathological infectious isoform, known as PrP^Sc. The latter acquires different abnormal conformations, which are associated with specific pathological phenotypes. Recent evidence suggests that prions adapt their conformation to changes in the context of replication. This phenomenon is known as either prion selection or adaptation, where distinct conformations of PrP^Sc with higher propensity to propagate in the new environment prevail over the others. Here, we show that a synthetically generated prion isolate, previously subjected to protein misfolding cyclic amplification (PMCA) and then injected in animals, is able to change its biochemical and biophysical properties according to the context of replication. In particular, in second transmission passage in vivo, two different prion isolates were found: one characterized by a predominance of the monoglycosylated band (PrP^Sc-M) and the other characterized by a predominance of the diglycosylated one (PrP^Sc-D). Neuropathological, biochemical, and biophysical assays confirmed that these PrP^Sc possess distinctive characteristics. Finally, PMCA analysis of PrP^Sc-M and PrP^Sc-D generated PrP^Sc (PrP^Sc-PMCA) whose biophysical properties were different from those of both inocula, suggesting that PMCA selectively amplified a third PrP^Sc isolate. Taken together, these results indicate that the context of replication plays a pivotal role in either prion selection or adaptation. By exploiting the ability of PMCA to mimic the process of prion replication in vitro, it might be possible to assess how changes in the replication environment influence the phenomenon of prion selection and adaptation.

Effects of peptidyl-prolyl isomerase 1 depletion in animal models of prion diseases

Pin1 is a peptidyl-prolyl isomerase that induces the cis-trans conversion of specific Ser/Thr-Pro peptide bonds in phosphorylated proteins, leading to conformational changes through which Pin1 regulates protein stability and activity. Since down-regulation of Pin1 has been described in several neurodegenerative disorders, including Alzheimer's Disease (AD), Parkinson's Disease (PD) and Huntington's Disease (HD), we investigated its potential role in prion diseases. Animals generated on wild-type (Pin1^+/+), hemizygous (Pin1^+/-) or knock-out (Pin1^-/-) background for Pin1 were experimentally infected with RML prions. The study indicates that, neither the total depletion nor reduced levels of Pin1 significantly altered the clinical and neuropathological features of the disease.

Quasispecies and virus

Quasispecies theory has been instrumental in the understanding of RNA virus population dynamics because it considered for the first time mutation as an integral part of the replication process. The key influences of quasispecies theory on experimental virology have been: (1) to disclose the mutant spectrum nature of viral populations and to evaluate its consequences; (2) to unveil collective properties of genome ensembles that can render a mutant spectrum a unit of selection; and (3) to identify new vulnerability points of pathogenic RNA viruses on three fronts: the need to apply multiple selective constraints (in the form of drug combinations) to minimize selection of treatment-escape variants, to translate the error threshold concept into antiviral designs, and to construct attenuated vaccine viruses through alterations of viral polymerase copying fidelity or through displacements of viral genomes towards unfavorable regions of sequence space. These three major influences on the understanding of viral pathogens preceded extensions of quasispecies to non-viral systems such as bacterial and tumor cell collectivities and prions. These developments are summarized here.

Cofactors influence the biological properties of infectious recombinant prions

Prion diseases are caused by a misfolding of the cellular prion protein (PrP) to a pathogenic isoform named PrP^Sc. Prions exist as strains, which are characterized by specific pathological and biochemical properties likely encoded in the three-dimensional structure of PrP^Sc. However, whether cofactors determine these different PrP^Sc conformations and how this relates to their specific biological properties is largely unknown. To understand how different cofactors modulate prion strain generation and selection, Protein Misfolding Cyclic Amplification was used to create a diversity of infectious recombinant prion strains by propagation in the presence of brain homogenate. Brain homogenate is known to contain these mentioned cofactors, whose identity is only partially known, and which facilitate conversion of PrP^C to PrP^Sc. We thus obtained a mix of distinguishable infectious prion strains. Subsequently, we replaced brain homogenate, by different polyanionic cofactors that were able to drive the evolution of mixed prion populations toward specific strains. Thus, our results show that a variety of infectious recombinant prions can be generated in vitro and that their specific type of conformation, i.e., the strain, is dependent on the cofactors available during the propagation process. These observations have significant implications for understanding the pathogenesis of prion diseases and their ability to replicate in different tissues and hosts. Importantly, these considerations might apply to other neurodegenerative diseases for which different conformations of misfolded proteins have been described.

Iatrogenic Creutzfeldt-Jakob disease with Amyloid-β pathology: an international study

The presence of pathology related to the deposition of amyloid-β (Aβ) has been recently reported in iatrogenic Creutzfeldt-Jakob disease (iCJD) acquired from inoculation of growth hormone (GH) extracted from human cadaveric pituitary gland or use of cadaveric dura mater (DM) grafts.To investigate this phenomenon further, a cohort of 27 iCJD cases - 21 with adequate number of histopathological sections - originating from Australia, France, Italy, and the Unites States, were examined by immunohistochemistry, amyloid staining, and Western blot analysis of the scrapie prion protein (PrPSc), and compared with age-group matched cases of sporadic CJD (sCJD), Alzheimer disease (AD) or free of neurodegenerative diseases (non-ND).Cases of iCJD and sCJD shared similar profiles of proteinase K-resistant PrPSc with the exception of iCJD harboring the "MMi" phenotype. Cerebral amyloid angiopathy (CAA), either associated with, or free of, Thioflavin S-positive amyloid core plaques (CP), was observed in 52% of 21 cases of iCJD, which comprised 37.5% and 61.5% of the cases of GH- and DM-iCJD, respectively. If only cases younger than 54 years were considered, Aβ pathology affected 41%, 2% and 0% of iCJD, sCJD and non-ND, respectively. Despite the patients' younger age CAA was more severe in iCJD than sCJD, while Aβ diffuse plaques, in absence of Aβ CP, populated one third of sCJD. Aβ pathology was by far most severe in AD. Tau pathology was scanty in iCJD and sCJD.In conclusion, (i) despite the divergences in the use of cadaveric GH and DM products, our cases combined with previous studies showed remarkably similar iCJD and Aβ phenotypes indicating that the occurrence of Aβ pathology in iCJD is a widespread phenomenon, (ii) CAA emerges as the hallmark of the Aβ phenotype in iCJD since it is observed in nearly 90% of all iCJD with Aβ pathology reported to date including ours, and it is shared by GH- and DM-iCJD, (iii) although the contributions to Aβ pathology of other factors, including GH deficiency, cannot be discounted, our findings increase the mounting evidence that this pathology is acquired by a mechanism resembling that of prion diseases.

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.

Fatal familial insomnia and sporadic fatal insomnia

Fatal familial insomnia (FFI) and sporadic fatal insomnia (sFI), or thalamic form of sporadic Creutzfeldt-Jakob disease MM2 (sCJDMM2T), are prion diseases originally named and characterized in 1992 and 1999, respectively. FFI is genetically determined and linked to a D178N mutation coupled with the M129 genotype in the prion protein gene (PRNP) at chromosome 20. sFI is a phenocopy of FFI and likely its sporadic form. Both diseases are primarily characterized by progressive sleep impairment, disturbances of autonomic nervous system, and motor signs associated with severe loss of nerve cells in medial thalamic nuclei. Both diseases harbor an abnormal disease-associated prion protein isoform, resistant to proteases with relative mass of 19 kDa identified as resPrP^TSE type 2. To date at least 70 kindreds affected by FFI with 198 members and 18 unrelated carriers along with 25 typical cases of sFI have been published. The D178N-129M mutation is thought to cause FFI by destabilizing the mutated prion protein and facilitating its conversion to PrP^TSE. The thalamus is the brain region first affected. A similar mechanism triggered spontaneously may underlie sFI.

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.

Strains of Pathological Protein Aggregates in Neurodegenerative Diseases

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

Species-dependent structural polymorphism of Y145Stop prion protein amyloid revealed by solid-state NMR spectroscopy

One of the most puzzling aspects of the prion diseases is the intricate relationship between prion strains and interspecies transmissibility barriers. Previously we have shown that certain fundamental aspects of mammalian prion propagation, including the strain phenomenon and species barriers, can be reproduced in vitro in seeded fibrillization of the Y145Stop prion protein variant. Here, we use solid-state nuclear magnetic resonance spectroscopy to gain atomic level insight into the structural differences between Y145Stop prion protein amyloids from three species: human, mouse, and Syrian hamster. Remarkably, we find that these structural differences are largely controlled by only two amino acids at positions 112 and 139, and that the same residues appear to be key to the emergence of structurally distinct amyloid strains within the same protein sequence. The role of these residues as conformational switches can be rationalized based on a model for human Y145Stop prion protein amyloid, providing a foundation for understanding cross-seeding specificity.

Prion diseases can be transmitted across species. Here the authors use solid-state NMR to study prion protein (PrP) amyloids from human, mouse and Syrian hamster and show that their structural differences are mainly governed by two residues, which helps to understand interspecies PrP propagation on a molecular level.

The physical dimensions of amyloid aggregates control their infective potential as prion particles

Transmissible amyloid particles called prions are associated with infectious prion diseases in mammals and inherited phenotypes in yeast. All amyloid aggregates can give rise to potentially infectious seeds that accelerate their growth. Why some amyloid seeds are highly infectious prion particles while others are less infectious or even inert, is currently not understood. To address this question, we analyzed the suprastructure and dimensions of synthetic amyloid fibrils assembled from the yeast (Saccharomyces cerevisiae) prion protein Sup35NM. We then quantified the ability of these particles to induce the [PSI⁺] prion phenotype in cells. Our results show a striking relationship between the length distribution of the amyloid fibrils and their ability to induce the heritable [PSI⁺] prion phenotype. Using a simple particle size threshold model to describe transfection activity, we explain how dimensions of amyloid fibrils are able to modulate their infectious potential as prions.

Biology and Genetics of PrP Prion Strains

Prion diseases are a group of fatal neurodegenerative disorders caused by the misfolding of the cellular prion protein (PrP^C) into a pathogenic conformation (PrP^Sc). PrP^Sc is capable of folding into multiple self-replicating prion strains that produce phenotypically distinct neurological disorders. Evidence suggests that the structural heterogeneity of PrP^Sc is the molecular basis of strain-specific prion properties. The self-templating of PrP^Sc typically ensures that prion strains breed true upon passage. However, prion strains also have the capacity to conformationally transform to maximize their rate of replication in a given environment. Here, we provide an overview of the prion-strain phenomenon and describe the role of strain adaptation in drug resistance. We also describe recent evidence that shows the presence of distinct conformational strains in other neurodegenerative disorders.

The mechanism of monomer transfer between two structurally distinct PrP oligomers

In mammals, Prion pathology refers to a class of infectious neuropathologies whose mechanism is based on the self-perpetuation of structural information stored in the pathological conformer. The characterisation of the PrP folding landscape has revealed the existence of a plethora of pathways conducing to the formation of structurally different assemblies with different biological properties. However, the biochemical interconnection between these diverse assemblies remains unclear. The PrP oligomerisation process leads to the formation of neurotoxic and soluble assemblies called O1 oligomers with a high size heterodispersity. By combining the measurements in time of size distribution and average size with kinetic models and data assimilation, we revealed the existence of at least two structurally distinct sets of assemblies, termed O^a and O^b, forming O1 assemblies. We propose a kinetic model representing the main processes in prion aggregation pathway: polymerisation, depolymerisation, and disintegration. The two groups interact by exchanging monomers through a disintegration process that increases the size of O^a. Our observations suggest that PrP oligomers constitute a highly dynamic population.

Internal Disequilibria and Phenotypic Diversification during Replication of Hepatitis C Virus in a Noncoevolving Cellular Environment

Viral quasispecies evolution upon long-term virus replication in a noncoevolving cellular environment raises relevant general issues, such as the attainment of population equilibrium, compliance with the molecular-clock hypothesis, or stability of the phenotypic profile. Here, we evaluate the adaptation, mutant spectrum dynamics, and phenotypic diversification of hepatitis C virus (HCV) in the course of 200 passages in human hepatoma cells in an experimental design that precluded coevolution of the cells with the virus. Adaptation to the cells was evidenced by increase in progeny production. The rate of accumulation of mutations in the genomic consensus sequence deviated slightly from linearity, and mutant spectrum analyses revealed a complex dynamic of mutational waves, which was sustained beyond passage 100. The virus underwent several phenotypic changes, some of which impacted the virus-host relationship, such as enhanced cell killing, a shift toward higher virion density, and increased shutoff of host cell protein synthesis. Fluctuations in progeny production and failure to reach population equilibrium at the genomic level suggest internal instabilities that anticipate an unpredictable HCV evolution in the complex liver environment.IMPORTANCE Long-term virus evolution in an unperturbed cellular environment can reveal features of virus evolution that cannot be explained by comparing natural viral isolates. In the present study, we investigate genetic and phenotypic changes that occur upon prolonged passage of hepatitis C virus (HCV) in human hepatoma cells in an experimental design in which host cell evolutionary change is prevented. Despite replication in a noncoevolving cellular environment, the virus exhibited internal population disequilibria that did not decline with increased adaptation to the host cells. The diversification of phenotypic traits suggests that disequilibria inherent to viral populations may provide a selective advantage to viruses that can be fully exploited in changing environments.

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

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

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

Novel strain properties distinguishing sporadic prion diseases sharing prion protein genotype and prion type

In most human sporadic prion diseases the phenotype is consistently associated with specific pairings of the genotype at codon 129 of the prion protein gene and conformational properties of the scrapie PrP (PrP^Sc) grossly identified types 1 and 2. This association suggests that the 129 genotype favours the selection of a distinct strain that in turn determines the phenotype. However, this mechanism cannot play a role in the phenotype determination of sporadic fatal insomnia (sFI) and a subtype of sporadic Creutzfeldt-Jakob disease (sCJD) identified as sCJDMM2, which share 129 MM genotype and PrP^Sc type 2 but are associated with quite distinct phenotypes. Our detailed comparative study of the PrP^Sc conformers has revealed major differences between the two diseases, which preferentially involve the PrP^Sc component that is sensitive to digestion with proteases (senPrP^Sc) and to a lesser extent the resistant component (resPrP^Sc). We conclude that these variations are consistent with two distinct strains in sFI and sCJDMM2, and that the rarer sFI is the result of a variant strain selection pathway that might be favoured by a different brain site of initial PrP^Sc formation in the two diseases.

The activities of amyloids from a structural perspective

The aggregation of proteins into structures known as amyloids is observed in many neurodegenerative diseases, including Alzheimer's disease. Amyloids are composed of pairs of tightly interacting, many stranded and repetitive intermolecular β-sheets, which form the cross-β-sheet structure. This structure enables amyloids to grow by recruitment of the same protein and its repetition can transform a weak biological activity into a potent one through cooperativity and avidity. Amyloids therefore have the potential to self-replicate and can adapt to the environment, yielding cell-to-cell transmissibility, prion infectivity and toxicity.

Isolation of a Defective Prion Mutant from Natural Scrapie

It is widely known that prion strains can mutate in response to modification of the replication environment and we have recently reported that prion mutations can occur in vitro during amplification of vole-adapted prions by Protein Misfolding Cyclic Amplification on bank vole substrate (bvPMCA). Here we exploited the high efficiency of prion replication by bvPMCA to study the in vitro propagation of natural scrapie isolates. Although in vitro vole-adapted PrP^Sc conformers were usually similar to the sheep counterpart, we repeatedly isolated a PrP^Sc mutant exclusively when starting from extremely diluted seeds of a single sheep isolate. The mutant and faithful PrP^Sc conformers showed to be efficiently autocatalytic in vitro and were characterized by different PrP protease resistant cores, spanning aa ∼155–231 and ∼80–231 respectively, and by different conformational stabilities. The two conformers could thus be seen as different bona fide PrP^Sc types, putatively accounting for prion populations with different biological properties. Indeed, once inoculated in bank vole the faithful conformer was competent for in vivo replication while the mutant was unable to infect voles, de facto behaving like a defective prion mutant. Overall, our findings confirm that prions can adapt and evolve in the new replication environments and that the starting population size can affect their evolutionary landscape, at least in vitro. Furthermore, we report the first example of “authentic” defective prion mutant, composed of brain-derived PrP^C and originating from a natural scrapie isolate. Our results clearly indicate that the defective mutant lacks of some structural characteristics, that presumably involve the central region ∼90–155, critical for infectivity but not for in vitro replication. Finally, we propose a molecular mechanism able to account for the discordant in vitro and in vivo behavior, suggesting possible new paths for investigating the molecular bases of prion infectivity.

Prions are unique infectious agents, consisting of PrP^Sc, a self-propagating aggregated conformer of the host-encoded prion protein PrP^C. Despite the absence of any nucleic acid information, prions exist as distinct strains that share the same amino acid sequence but differ in their conformation. Moreover, prions can mutate and are thus heterogeneous populations able to evolve and adapt to new replication environments. During in vitro amplification of sheep scrapie, we found that a prion mutant could be obtained from one natural isolate. The prion mutant identified was characterized in vivo and in vitro, showing unusual biochemical and biological features: a smaller than usual C-terminal proteinase resistant core of PrP^Sc, which spans aa ∼155–231, and the inability to propagate in vivo despite an efficient autocatalytic replication in vitro. With such a signature, we denoted the mutant as a “defective” prion mutant. We thus postulate a new hypothesis for the discrepancy between the in vitro and in vivo behavior of the defective mutant and suggest that the central PrP^Sc domain ∼90–160 might have a key role in prion replication. This work provides important new insights into the mechanism underpinning prion replication and has numerous implications for understanding the molecular requirements indispensable for prion infectivity.

Tau Prion Strains Dictate Patterns of Cell Pathology, Progression Rate, and Regional Vulnerability In Vivo

Tauopathies are neurodegenerative disorders that affect distinct brain regions, progress at different rates, and exhibit specific patterns of tau accumulation. The source of this diversity is unknown. We previously characterized two tau strains that stably maintain unique conformations in vitro and in vivo, but did not determine the relationship of each strain to parameters that discriminate between tauopathies such as regional vulnerability or rate of spread. We have now isolated and characterized 18 tau strains in cells based on detailed biochemical and biological criteria. Inoculation of PS19 transgenic tau (P301S) mice with these strains causes strain-specific intracellular pathology in distinct cell types and brain regions, and induces different rates of network propagation. In this system, strains alone are sufficient to account for diverse neuropathological presentations, similar to those that define human tauopathies. Further study of these strains can thus establish a structural logic that governs these biological effects.

Effects of Intrinsic and Extrinsic Factors on Aggregation of Physiologically Important Intrinsically Disordered Proteins

Misfolding and aggregation of proteins and peptides play an important role in a number of diseases as well as in many physiological processes. Many of the proteins that misfold and aggregate in vivo are intrinsically disordered. Protein aggregation is a complex multistep process, and aggregates can significantly differ in morphology, structure, stability, cytotoxicity, and self-propagation ability. The aggregation process is influenced by both intrinsic (e.g., mutations and expression levels) and extrinsic (e.g., polypeptide chain truncation, macromolecular crowding, posttranslational modifications, as well as interaction with metal ions, other small molecules, lipid membranes, and chaperons) factors. This review examines the effect of a variety of these factors on aggregation of physiologically important intrinsically disordered proteins.

Calcium dysregulation contributes to neurodegeneration in FTLD patient iPSC-derived neurons

Mutations in the gene MAPT encoding tau, a microtubules-associated protein, cause a subtype of familial neurodegenerative disorder, known as frontotemporal lobar degeneration tauopathy (FTLD-Tau), which presents with dementia and is characterized by atrophy in the frontal and temporal lobes of the brain. Although induced pluripotent stem cell (iPSC) technology has facilitated the investigation of phenotypes of FTLD-Tau patient neuronal cells in vitro, it remains unclear how FTLD-Tau patient neurons degenerate. Here, we established neuronal models of FTLD-Tau by Neurogenin2-induced direct neuronal differentiation from FTLD-Tau patient iPSCs. We found that FTLD-Tau neurons, either with an intronic MAPT mutation or with an exonic mutation, developed accumulation and extracellular release of misfolded tau followed by neuronal death, which we confirmed by correction of the intronic mutation with CRISPR/Cas9. FTLD-Tau neurons showed dysregulation of the augmentation of Ca²⁺ transients evoked by electrical stimulation. Chemogenetic or pharmacological control of neuronal activity-relevant Ca²⁺ influx by the introduction of designer receptors exclusively activated by designer drugs (DREADDs) or by the treatment with glutamate receptor blockers attenuated misfolded tau accumulation and neuronal death. These data suggest that neuronal activity may regulate neurodegeneration in tauopathy. This FTLD-Tau model provides mechanistic insights into tauopathy pathogenesis and potential avenues for treatments.

Glycoform-independent prion conversion by highly efficient, cell-based, protein misfolding cyclic amplification

Prions are formed of misfolded assemblies (PrP^Sc) of the variably N-glycosylated cellular prion protein (PrP^C). In infected species, prions replicate by seeding the conversion and polymerization of host PrP^C. Distinct prion strains can be recognized, exhibiting defined PrP^Sc biochemical properties such as the glycotype and specific biological traits. While strain information is encoded within the conformation of PrP^Sc assemblies, the storage of the structural information and the molecular requirements for self-perpetuation remain uncertain. Here, we investigated the specific role of PrP^C glycosylation status. First, we developed an efficient protein misfolding cyclic amplification method using cells expressing the PrP^C species of interest as substrate. Applying the technique to PrP^C glycosylation mutants expressing cells revealed that neither PrP^C nor PrP^Sc glycoform stoichiometry was instrumental to PrP^Sc formation and strainness perpetuation. Our study supports the view that strain properties, including PrP^Sc glycotype are enciphered within PrP^Sc structural backbone, not in the attached glycans.

Structural conservation of prion strain specificities in recombinant prion protein fibrils in real-time quaking-induced conversion

A major unsolved issue of prion biology is the existence of multiple strains with distinct phenotypes and this strain phenomenon is postulated to be associated with the conformational diversity of the abnormal prion protein (PrP^Sc). Real-time quaking-induced conversion (RT-QUIC) assay that uses Escherichia coli-derived recombinant prion protein (rPrP) for the sensitive detection of PrP^Sc results in the formation of rPrP-fibrils seeded with various strains. We demonstrated that there are differences in the secondary structures, especially in the β-sheets, and conformational stability between 2 rPrP-fibrils seeded with either Chandler or 22L strains in the first round of RT-QUIC. In particular, the differences in conformational properties of these 2 rPrP-fibrils were common to those of the original PrP^Sc. However, the strain specificities of rPrP-fibrils seen in the first round were lost in subsequent rounds. Instead, our findings suggest that nonspecific fibrils became the major species, probable owing to their selective growth advantage in the RT-QUIC. This study shows that at least some strain-specific conformational properties of the original PrP^Sc can be transmitted to rPrP-fibrils in vitro, but further conservation appears to require unknown cofactors or environmental conditions or both.

Fracture and Growth Are Competing Forces Determining the Fate of Conformers in Tau Fibril Populations

Tau fibrils are pathological aggregates that can transfer between neurons and then recruit soluble Tau monomers by template-assisted conversion. The propagation of different fibril polymorphs is thought to be a contributing factor to phenotypic diversity in Alzheimer disease and other Tauopathies. We found that a homogeneous population of Tau fibrils composed of the truncated version K18 (residues 244-372) gradually converted to a new set of fibril conformers when subjected to multiple cycles of seeding and growth. Using double electron-electron resonance (DEER) spectroscopy, we observed that the distances between spin labels at positions 311 and 328 in the fibril core progressively decreased. The findings were corroborated by changes in turbidity, morphology, and protease sensitivity. Fibrils that were initially formed under stirring conditions exhibited an increased fragility compared with fibrils formed quiescently after multiple cycles of seeding. The quiescently formed fibrils were marked by accelerated growth. The difference in fragility and growth between the different conformers explains how the change in incubation condition could lead to the amplification of a minor subpopulation of fibrils. Under quiescent conditions where fibril breakage is minimal, faster growing fibrils have a selective advantage. The findings are of general importance as they suggest that changes in selective pressures during fibril propagation in the human brain could result in the emergence of new fibril conformers with varied clinicopathological consequences.

Mechanisms of prion-induced neurodegeneration

Transmissible spongiform encephalopathies (TSEs), or prion diseases, are fatal neurodegenerative disorders characterised by long incubation period, short clinical duration, and transmissibility to susceptible species. Neuronal loss, spongiform changes, gliosis and the accumulation in the brain of the misfolded version of a membrane-bound cellular prion protein (PrP(C)), termed PrP(TSE), are diagnostic markers of these diseases. Compelling evidence links protein misfolding and its accumulation with neurodegenerative changes. Accordingly, several mechanisms of prion-mediated neurotoxicity have been proposed. In this paper, we provide an overview of the recent knowledge on the mechanisms of neuropathogenesis, the neurotoxic PrP species and the possible therapeutic approaches to treat these devastating disorders.

Emergence of two prion subtypes in ovine PrP transgenic mice infected with human MM2-cortical Creutzfeldt-Jakob disease prions

Introduction

Mammalian prions are proteinaceous pathogens responsible for a broad range of fatal neurodegenerative diseases in humans and animals. These diseases can occur spontaneously, such as Creutzfeldt-Jakob disease (CJD) in humans, or be acquired or inherited. Prions are primarily formed of macromolecular assemblies of the disease-associated prion protein PrP^Sc, a misfolded isoform of the host-encoded prion protein PrP^C. Within defined host-species, prions can exist as conformational variants or strains. Based on both the M/V polymorphism at codon 129 of PrP and the electrophoretic signature of PrP^Sc in the brain, sporadic CJD is classified in different subtypes, which may encode different strains. A transmission barrier, the mechanism of which remains unknown, limits prion cross-species propagation. To adapt to the new host, prions have the capacity to ‘mutate’ conformationally, leading to the emergence of a variant with new biological properties. Here, we transmitted experimentally one rare subtype of human CJD, designated cortical MM2 (129 MM with type 2 PrP^Sc), to transgenic mice overexpressing either human or the VRQ allele of ovine PrP^C.

Results

In marked contrast with the reported absence of transmission to knock-in mice expressing physiological levels of human PrP, this subtype transmitted faithfully to mice overexpressing human PrP, and exhibited unique strain features. Onto the ovine PrP sequence, the cortical MM2 subtype abruptly evolved on second passage, thereby allowing emergence of a pair of strain variants with distinct PrP^Sc biochemical characteristics and differing tropism for the central and lymphoid tissues. These two strain components exhibited remarkably distinct replicative properties in cell-free amplification assay, allowing the ‘physical’ cloning of the minor, lymphotropic component, and subsequent isolation in ovine PrP mice and RK13 cells.

Conclusions

Here, we provide in-depth assessment of the transmissibility and evolution of one rare subtype of sporadic CJD upon homologous and heterologous transmission. The notion that the environment or matrix where replication is occurring is key to the selection and preferential amplification of prion substrain components raises new questions on the determinants of prion replication within and between species. These data also further interrogate on the interplay between animal and human prions.

Electronic supplementary material

The online version of this article (doi:10.1186/s40478-016-0284-9) contains supplementary material, which is available to authorized users.

Sheep prions with molecular properties intermediate between classical scrapie, BSE and CH1641-scrapie

Efforts to differentiate bovine spongiform encephalopathy (BSE) from scrapie in prion infected sheep have resulted in effective methods to decide about the absence of BSE. In rare instances uncertainties remain due to assumptions that BSE, classical scrapie and CH1641–a rare scrapie variant–could occur as mixtures. In field samples including those from fallen stock, triplex Western blotting analyses of variations in the molecular properties of the proteinase K resistant part of the disease‑associated form of prion protein (PrP^res) represents a powerful tool for quick discrimination purposes. In this study we examined 7 deviant ovine field cases of scrapie for some typical molecular aspects of PrP^res found in CH1641‑scrapie, classical scrapie and BSE. One case was most close to scrapie with respect to molecular mass of its non-glycosylated fraction and N-terminally located 12B2‑epitope content. Two cases were unlike classical scrapie but too weak to differentiate between BSE or CH1641. The other 4 cases appeared intermediate between scrapie and CH1641 with a reduced molecular mass and 12B2‑epitope content, together with the characteristic presence of a second PrP^res population. The existence of these 2 PrP^res populations was further confirmed through deglycosylation by PNGaseF. The findings indicate that discriminatory diagnosis between classical scrapie, CH1641 and BSE can remain inconclusive with current biochemical methods. Whether such intermediate cases represent mixtures of TSE strains should be further investigated e.g. in bioassays with rodent lines that are varying in their susceptibility or other techniques suitable for strain typing.

Use of a 2-aminothiazole to Treat Chronic Wasting Disease in Transgenic Mice

Treatment with the 2-aminothiazole IND24 extended the survival of mice infected with mouse-adapted scrapie but also resulted in the emergence of a drug-resistant prion strain. Here, we determined whether IND24 extended the survival of transgenic mice infected with prions that caused scrapie in sheep or prions that caused chronic wasting disease (CWD; hereafter "CWD prions") in deer, using 2 isolates for each disease. IND24 doubled the incubation times for mice infected with CWD prions but had no effect on the survival of those infected with scrapie prions. Biochemical, neuropathologic, and cell culture analyses were used to characterize prion strain properties following treatment, and results indicated that the CWD prions were not altered by IND24, regardless of survival extension. These results suggest that IND24 may be a viable candidate for treating CWD in infected captive cervid populations and raise questions about why some prion strains develop drug resistance whereas others do not.