Human prion strain selection in transgenic mice

The utility of bank voles for studying prion disease

The transmission of prions between species is typically an inefficient process due to the species barrier, which represents incompatibility between prion seed and substrate molecules. Bank voles (Myodes glareolus) are an exception to this rule, as they are susceptible to a diverse range of prion strains from many different animal species. In particular, bank voles can be efficiently infected with most types of human prions and have played a critical role in validating variably protease-sensitive prionopathy (VPSPr) and certain forms of Gerstmann-Sträussler-Scheinker (GSS) disease as bona fide prion disorders rather than non-transmissible proteinopathies. The bank vole prion protein (BVPrP) confers a "universal prion acceptor" phenotype when expressed in mice and when used as a substrate for in vitro prion amplification assays, indicating that the unique prion transmission properties of bank voles are mediated by BVPrP. Over-expression of BVPrP in mice can also promote the spontaneous development of prion disease, indicating that BVPrP is intrinsically prone to both spontaneous and template-directed misfolding. Here, we discuss the utility of bank voles and BVPrP for prion research and how they have provided new tools for establishing rapid animal bioassays, modeling spontaneous prion disease, standardizing prion diagnostics, and understanding the molecular basis of the species barrier.

Cellular models for discovering prion disease therapeutics: Progress and challenges

Prions, which cause fatal neurodegenerative disorders such as Creutzfeldt-Jakob disease, are misfolded and infectious protein aggregates. Currently, there are no treatments available to halt or even delay the progression of prion disease in the brain. The infectious nature of prions has resulted in animal paradigms that accurately recapitulate all aspects of prion disease, and these have proven to be instrumental for testing the efficacy of candidate therapeutics. Nonetheless, infection of cultured cells with prions provides a much more powerful system for identifying molecules capable of interfering with prion propagation. Certain lines of cultured cells can be chronically infected with various types of mouse prions, and these models have been used to unearth candidate anti-prion drugs that are at least partially efficacious when administered to prion-infected rodents. However, these studies have also revealed that not all types of prions are equal, and that drugs active against mouse prions are not necessarily effective against prions from other species. Despite some recent progress, the number of cellular models available for studying non-mouse prions remains limited. In particular, human prions have proven to be particularly challenging to propagate in cultured cells, which has severely hindered the discovery of drugs for Creutzfeldt-Jakob disease. In this review, we summarize the cellular models that are presently available for discovering and testing drugs capable of blocking the propagation of prions and highlight challenges that remain on the path towards developing therapies for prion disease.

Characterization of goat prions demonstrates geographical variation of scrapie strains in Europe and reveals the composite nature of prion strains

Bovine Spongiform Encephalopathy (BSE) is the only animal prion which has been recognized as a zoonotic agent so far. The identification of BSE in two goats raised the need to reliably identify BSE in small ruminants. However, our understanding of scrapie strain diversity in small ruminants remains ill-defined, thus limiting the accuracy of BSE surveillance and spreading fear that BSE might lurk unrecognized in goats. We investigated prion strain diversity in a large panel of European goats by a novel experimental approach that, instead of assessing the neuropathological profile after serial transmissions in a single animal model, was based on the direct interaction of prion isolates with several recipient rodent models expressing small ruminants or heterologous prion proteins. The findings show that the biological properties of scrapie isolates display different patterns of geographical distribution in Europe and suggest that goat BSE could be reliably discriminated from a wide range of biologically and geographically diverse goat prion isolates. Finally, most field prion isolates showed composite strain features, with discrete strain components or sub-strains being present in different proportions in individual goats or tissues. This has important implications for understanding the nature and evolution of scrapie strains and their transmissibility to other species, including humans.

Artificial strain of human prions created in vitro

The molecular mechanism that determines under physiological conditions transmissibility of the most common human prion disease, sporadic Creutzfeldt-Jakob disease (sCJD) is unknown. We report the synthesis of new human prion from the recombinant human prion protein expressed in bacteria in reaction seeded with sCJD MM1 prions and cofactor, ganglioside GM1. These synthetic human prions were infectious to transgenic mice expressing non-glycosylated human prion protein, causing neurologic dysfunction after 459 and 224 days in the first and second passage, respectively. The neuropathology, replication potency, and biophysical profiling suggest that a novel, particularly neurotoxic human prion strain was created. Distinct biological and structural characteristics of our synthetic human prions suggest that subtle changes in the structural organization of critical domains, some linked to posttranslational modifications of the pathogenic prion protein (PrPSc), play a crucial role as a determinant of human prion infectivity, host range, and targetting of specific brain structures in mice models.

Familial Parkinson's point mutation abolishes multiple system atrophy prion replication

In the neurodegenerative disease multiple system atrophy (MSA), α-synuclein misfolds into a self-templating conformation to become a prion. To compare the biological activity of α-synuclein prions in MSA and Parkinson's disease (PD), we developed nine α-synuclein-YFP cell lines expressing point mutations responsible for inherited PD. MSA prions robustly infected wild-type, A30P, and A53T α-synuclein-YFP cells, but they were unable to replicate in cells expressing the E46K mutation. Coexpression of the A53T and E46K mutations was unable to rescue MSA prion infection in vitro, establishing that MSA α-synuclein prions are conformationally distinct from the misfolded α-synuclein in PD patients. This observation may have profound implications for developing treatments for neurodegenerative diseases.

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.

Region-specific protein misfolding cyclic amplification reproduces brain tropism of prion strains

Human prion diseases such as Creutzfeldt-Jakob disease are transmissible brain proteinopathies, characterized by the accumulation of a misfolded isoform of the host cellular prion protein (PrP) in the brain. According to the prion model, prions are defined as proteinaceous infectious particles composed solely of this abnormal isoform of PrP (PrP^Sc). Even in the absence of genetic material, various prion strains can be propagated in experimental models. They can be distinguished by the pattern of disease they produce and especially by the localization of PrP^Sc deposits within the brain and the spongiform lesions they induce. The mechanisms involved in this strain-specific targeting of distinct brain regions still are a fundamental, unresolved question in prion research. To address this question, we exploited a prion conversion in vitro assay, protein misfolding cyclic amplification (PMCA), by using experimental scrapie and human prion strains as seeds and specific brain regions from mice and humans as substrates. We show here that region-specific PMCA in part reproduces the specific brain targeting observed in experimental, acquired, and sporadic Creutzfeldt-Jakob diseases. Furthermore, we provide evidence that, in addition to cellular prion protein, other region- and species-specific molecular factors influence the strain-dependent prion conversion process. This important step toward understanding prion strain propagation in the human brain may impact research on the molecular factors involved in protein misfolding and the development of ultrasensitive methods for diagnosing prion disease.

Bioassays and Inactivation of Prions

The experimental study of prions requires a model for their propagation. However, because prions lack nucleic acids, the simple techniques used to replicate bacteria and viruses are not applicable. For much of the history of prion research, time-consuming bioassays in animals were the only option for measuring infectivity. Although cell models and other in vitro tools for the propagation of prions have been developed, they all suffer limitations, and animal bioassays remain the gold standard for measuring infectivity. A wealth of recent data argues that both β-amyloid (Aβ) and tau proteins form prions that cause Alzheimer's disease, and α-synuclein forms prions that cause multiple system atrophy and Parkinson's disease. Cell and animal models that recapitulate some of the key features of cell-to-cell spreading and distinct strains of prions can now be measured.

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.

Developing Therapeutics for PrP Prion Diseases

The prototypical PrP prion diseases are invariably fatal, and the search for agents to treat them spans more than 30 years, with limited success. However, in the last few years, the application of high-throughput screening, medicinal chemistry, and pharmacokinetic optimization has led to important advances. The PrP prion inoculation paradigm provides a robust assay for testing therapeutic efficacy, and a dozen compounds have been reported that lead to meaningful extension in survival of prion-infected mice. Here, we review the history and recent progress in the field, focusing on studies validated in animal models. Based on screens in cells infected with mouse-passaged PrP prions, orally available compounds were generated that double or even triple the survival of mice infected with the same prion strain. Unfortunately, no compounds have yet shown efficacy against human prions. Nevertheless, the speed of the recent advances brings hope that an effective therapeutic can be developed. A successful treatment for any neurodegenerative disease would be a major achievement, and the growing understanding that the more common neurodegenerative diseases, including Alzheimer's and Parkinson's, progress by an analogous prion mechanism serves to highlight the importance of antiprion therapeutics.

Insights into Mechanisms of Transmission and Pathogenesis from Transgenic Mouse Models of Prion Diseases

Prions represent a new paradigm of protein-mediated information transfer. In the case of mammals, prions are the cause of fatal, transmissible neurodegenerative diseases, sometimes referred to as transmissible spongiform encephalopathies (TSEs), which frequently occur as epidemics. An increasing body of evidence indicates that the canonical mechanism of conformational corruption of cellular prion protein (PrP^C) by the pathogenic isoform (PrP^Sc) that is the basis of prion formation in TSEs is common to a spectrum of proteins associated with various additional human neurodegenerative disorders, including the more common Alzheimer's and Parkinson's diseases. The peerless infectious properties of TSE prions, and the unparalleled tools for their study, therefore enable elucidation of mechanisms of template-mediated conformational propagation that are generally applicable to these related disease states. Many unresolved issues remain including the exact molecular nature of the prion, the detailed cellular and molecular mechanisms of prion propagation, and the means by which prion diseases can be both genetic and infectious. In addition, we know little about the mechanism by which neurons degenerate during prion diseases. Tied to this, the physiological role of the normal form of the prion protein remains unclear and it is uncertain whether or not loss of this function contributes to prion pathogenesis. The factors governing the transmission of prions between species remain unclear, in particular the means by which prion strains and PrP primary structure interact to affect interspecies prion transmission. Despite all these unknowns, advances in our understanding of prions have occurred because of their transmissibility to experimental animals, and the development of transgenic (Tg) mouse models has done much to further our understanding about various aspects of prion biology. In this review, we will focus on advances in our understanding of prion biology that occurred in the past 8 years since our last review of this topic.

PrPC Governs Susceptibility to Prion Strains in Bank Vole, While Other Host Factors Modulate Strain Features

Bank vole is a rodent species that shows differential susceptibility to the experimental transmission of different prion strains. In this work, the transmission features of a panel of diverse prions with distinct origins were assayed both in bank vole expressing methionine at codon 109 (Bv109M) and in transgenic mice expressing physiological levels of bank vole PrP^C (the BvPrP-Tg407 mouse line). This work is the first systematic comparison of the transmission features of a collection of prion isolates, representing a panel of diverse prion strains, in a transgenic-mouse model and in its natural counterpart. The results showed very similar transmission properties in both the natural species and the transgenic-mouse model, demonstrating the key role of the PrP amino acid sequence in prion transmission susceptibility. However, differences in the PrP^Sc types propagated by Bv109M and BvPrP-Tg407 suggest that host factors other than PrP^C modulate prion strain features.

IMPORTANCE

The differential susceptibility of bank voles to prion strains can be modeled in transgenic mice, suggesting that this selective susceptibility is controlled by the vole PrP sequence alone rather than by other species-specific factors. Differences in the phenotypes observed after prion transmissions in bank voles and in the transgenic mice suggest that host factors other than the PrP^C sequence may affect the selection of the substrain replicating in the animal model.

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.

Guinea Pig Prion Protein Supports Rapid Propagation of Bovine Spongiform Encephalopathy and Variant Creutzfeldt-Jakob Disease Prions

The biochemical and neuropathological properties of bovine spongiform encephalopathy (BSE) and variant Creutzfeldt-Jakob disease (vCJD) prions are faithfully maintained upon transmission to guinea pigs. However, primary and secondary transmissions of BSE and vCJD in guinea pigs result in long incubation periods of ∼450 and ∼350 days, respectively. To determine if the incubation periods of BSE and vCJD prions could be shortened, we generated transgenic (Tg) mice expressing guinea pig prion protein (GPPrP). Inoculation of Tg(GPPrP) mice with BSE and vCJD prions resulted in mean incubation periods of 210 and 199 days, respectively, which shortened to 137 and 122 days upon serial transmission. In contrast, three different isolates of sporadic CJD prions failed to transmit disease to Tg(GPPrP) mice. Many of the strain-specified biochemical and neuropathological properties of BSE and vCJD prions, including the presence of type 2 protease-resistant PrP^Sc, were preserved upon propagation in Tg(GPPrP) mice. Structural modeling revealed that two residues near the N-terminal region of α-helix 1 in GPPrP might mediate its susceptibility to BSE and vCJD prions. Our results demonstrate that expression of GPPrP in Tg mice supports the rapid propagation of BSE and vCJD prions and suggest that Tg(GPPrP) mice may serve as a useful paradigm for bioassaying these prion isolates.

IMPORTANCE

Variant Creutzfeldt-Jakob disease (vCJD) and bovine spongiform encephalopathy (BSE) prions are two of the prion strains most relevant to human health. However, propagating these strains in mice expressing human or bovine prion protein has been difficult because of prolonged incubation periods or inefficient transmission. Here, we show that transgenic mice expressing guinea pig prion protein are fully susceptible to vCJD and BSE prions but not to sporadic CJD prions. Our results suggest that the guinea pig prion protein is a better, more rapid substrate than either bovine or human prion protein for propagating BSE and vCJD prions.

Optimization of Aryl Amides that Extend Survival in Prion-Infected Mice

Developing therapeutics for neurodegenerative diseases (NDs) prevalent in the aging population remains a daunting challenge. With the growing understanding that many NDs progress by conformational self-templating of specific proteins, the prototypical prion diseases offer a platform for ND drug discovery. We evaluated high-throughput screening hits with the aryl amide scaffold and explored the structure-activity relationships around three series differing in their N-aryl core: benzoxazole, benzothiazole, and cyano. Potent anti-prion compounds were advanced to pharmacokinetic studies, and the resulting brain-penetrant leads from each series, together with a related N-aryl piperazine lead, were escalated to long-term dosing and efficacy studies. Compounds from each of the four series doubled the survival of mice infected with a mouse-passaged prion strain. Treatment with aryl amides altered prion strain properties, as evidenced by the distinct patterns of neuropathological deposition of prion protein and associated astrocytic gliosis in the brain; however, none of the aryl amide compounds resulted in drug-resistant prion strains, in contrast to previous studies on compounds with the 2-aminothiazole (2-AMT) scaffold. As seen with 2-AMTs and other effective anti-prion compounds reported to date, the novel aryl amides reported here were ineffective in prolonging the survival of transgenic mice infected with human prions. Most encouraging is our discovery that aryl amides show that the development of drug resistance is not an inevitable consequence of efficacious anti-prion therapeutics.

Different 2-Aminothiazole Therapeutics Produce Distinct Patterns of Scrapie Prion Neuropathology in Mouse Brains

Because no drug exists that halts or even slows any neurodegenerative disease, developing effective therapeutics for any prion disorder is urgent. We recently reported two compounds (IND24 and IND81) with the 2-aminothiazole (2-AMT) chemical scaffold that almost doubled the incubation times in scrapie prion-infected, wild-type (wt) FVB mice when given in a liquid diet. Remarkably, oral prophylactic treatment with IND24 beginning 14 days prior to intracerebral prion inoculation extended survival from ∼120 days to over 450 days. In addition to IND24, we evaluated the pharmacokinetics and efficacy of five additional 2-AMTs; one was not followed further because its brain penetration was poor. Of the remaining four new 2-AMTs, IND114338 doubled and IND125 tripled the incubation times of RML-inoculated wt and Tg4053 mice overexpressing wt mouse prion protein (PrP), respectively. Neuropathological examination of the brains from untreated controls showed a widespread deposition of self-propagating, β-sheet-rich "scrapie" isoform (PrP(Sc)) prions accompanied by a profound astrocytic gliosis. In contrast, mice treated with 2-AMTs had lower levels of PrP(Sc) and associated astrocytic gliosis, with each compound resulting in a distinct pattern of deposition. Notably, IND125 prevented both PrP(Sc) accumulation and astrocytic gliosis in the cerebrum. Progressive central nervous system dysfunction in the IND125-treated mice was presumably due to the PrP(Sc) that accumulated in their brainstems. Disappointingly, none of the four new 2-AMTs prolonged the lives of mice expressing a chimeric human/mouse PrP transgene inoculated with Creutzfeldt-Jakob disease prions.

Structural determinants of phenotypic diversity and replication rate of human prions

The infectious pathogen responsible for prion diseases is the misfolded, aggregated form of the prion protein, PrP^Sc. In contrast to recent progress in studies of laboratory rodent-adapted prions, current understanding of the molecular basis of human prion diseases and, especially, their vast phenotypic diversity is very limited. Here, we have purified proteinase resistant PrP^Sc aggregates from two major phenotypes of sporadic Creutzfeldt-Jakob disease (sCJD), determined their conformational stability and replication tempo in vitro, as well as characterized structural organization using recently emerged approaches based on hydrogen/deuterium (H/D) exchange coupled with mass spectrometry. Our data clearly demonstrate that these phenotypically distant prions differ in a major way with regard to their structural organization, both at the level of the polypeptide backbone (as indicated by backbone amide H/D exchange data) as well as the quaternary packing arrangements (as indicated by H/D exchange kinetics for histidine side chains). Furthermore, these data indicate that, in contrast to previous observations on yeast and some murine prion strains, the replication rate of sCJD prions is primarily determined not by conformational stability but by specific structural features that control the growth rate of prion protein aggregates.

Sporadic Creutzfeldt-Jakob disease (sCJD) represents ~90% of all human prion diseases worldwide. This neurodegenerative disease, which is transmissible and invariably fatal, is characterized by variable progression rates and remarkable diversity of clinical and pathological traits. The infectious sCJD prions propagating the pathology mainly in the brain are assemblies of abnormally folded isoform (PrP^Sc) of a host-encoded prion protein (PrP^C). The structure and replication mechanisms of human prions are unknown, and whether the PrP^Sc subtypes identified by proteolytic fragmentation represent distinct strains of sCJD prions has been debated. Here, we isolated sCJD prions from patients with two very distant phenotypes of the disease, compared their structural organization using recently developed biophysical techniques, and investigated their replication in vitro. Our data indicate that these sCJD prions are characterized by different secondary structure organization and quaternary packing arrangements, and that these structural differences are responsible for distinct prion replication rates and unique phenotypic characteristics of the disease. Furthermore, our analysis reveals that, contrary to previous observations for yeast prions, the replication tempo of sCJD prions is determined not so much by their conformational stability but by specific structural features that control the growth speed of prion particles.

Infectious prion diseases in humans: cannibalism, iatrogenicity and zoonoses

In contrast with other neurodegenerative disorders associated to protein misfolding, human prion diseases include infectious forms (also called transmitted forms) such as kuru, iatrogenic Creutzfeldt-Jakob disease and variant Creutzfeldt-Jakob disease. The transmissible agent is thought to be solely composed of the abnormal isoform (PrP(Sc)) of the host-encoded prion protein that accumulated in the central nervous system of affected individuals. Compared to its normal counterpart, PrP(Sc) is β-sheet enriched and aggregated and its propagation is based on an autocatalytic conversion process. Increasing evidence supports the view that conformational variations of PrP(Sc) encoded the biological properties of the various prion strains that have been isolated by transmission studies in experimental models. Infectious forms of human prion diseases played a pivotal role in the emergence of the prion concept and in the characterization of the very unconventional properties of prions. They provide a unique model to understand how prion strains are selected and propagate in humans. Here, we review and discuss how genetic factors interplay with strain properties and route of transmission to influence disease susceptibility, incubation period and phenotypic expression in the light of the kuru epidemics due to ritual endocannibalism, the various series iatrogenic diseases secondary to extractive growth hormone treatment or dura mater graft and the epidemics of variant Creutzfeldt-Jakob disease linked to dietary exposure to the agent of bovine spongiform encephalopathy.

Mouse models for studying the formation and propagation of prions

Prions are self-propagating protein conformers that cause a variety of neurodegenerative disorders in humans and animals. Mouse models have played key roles in deciphering the biology of prions and in assessing candidate therapeutics. The development of transgenic mice that form prions spontaneously in the brain has advanced our understanding of sporadic and genetic prion diseases. Furthermore, the realization that many proteins can become prions has necessitated the development of mouse models for assessing the potential transmissibility of common neurodegenerative diseases. As the universe of prion diseases continues to expand, mouse models will remain crucial for interrogating these devastating illnesses.

Evidence that bank vole PrP is a universal acceptor for prions

Bank voles are uniquely susceptible to a wide range of prion strains isolated from many different species. To determine if this enhanced susceptibility to interspecies prion transmission is encoded within the sequence of the bank vole prion protein (BVPrP), we inoculated Tg(M109) and Tg(I109) mice, which express BVPrP containing either methionine or isoleucine at polymorphic codon 109, with 16 prion isolates from 8 different species: humans, cattle, elk, sheep, guinea pigs, hamsters, mice, and meadow voles. Efficient disease transmission was observed in both Tg(M109) and Tg(I109) mice. For instance, inoculation of the most common human prion strain, sporadic Creutzfeldt-Jakob disease (sCJD) subtype MM1, into Tg(M109) mice gave incubation periods of ∼200 days that were shortened slightly on second passage. Chronic wasting disease prions exhibited an incubation time of ∼250 days, which shortened to ∼150 days upon second passage in Tg(M109) mice. Unexpectedly, bovine spongiform encephalopathy and variant CJD prions caused rapid neurological dysfunction in Tg(M109) mice upon second passage, with incubation periods of 64 and 40 days, respectively. Despite the rapid incubation periods, other strain-specified properties of many prion isolates—including the size of proteinase K–resistant PrP^Sc, the pattern of cerebral PrP^Sc deposition, and the conformational stability—were remarkably conserved upon serial passage in Tg(M109) mice. Our results demonstrate that expression of BVPrP is sufficient to engender enhanced susceptibility to a diverse range of prion isolates, suggesting that BVPrP may be a universal acceptor for prions.

Prions are infectious proteins that cause devastating neurodegenerative diseases in both humans and animals. Unlike other rodents, bank voles are highly susceptible to prions from many different species, suggesting that bank voles do not impose a “species barrier,” which normally restricts the transmission of prions from one species to another. We were curious as to whether the unprecedented promiscuity of bank voles for prions is due to the specific prion protein sequence expressed, or to some other factor inherent to bank vole physiology. To answer this question, we inoculated transgenic mice that express bank vole prion protein [Tg(BVPrP) mice] with a diverse set of prions deriving from eight different species. Like bank voles, Tg(BVPrP) mice were highly susceptible to prions from all species tested, demonstrating that the BVPrP sequence mediates the enhanced susceptibility of bank voles to prions. Because the amino acid sequences of mouse and BVPrP differ at only eight positions, our results demonstrate that alterations to a small subset of residues within PrP can have a profound effect on the susceptibility of an organism to prions from another species.

Drug resistance confounding prion therapeutics

There is not a single pharmaceutical that halts or even slows any neurodegenerative disease. Mounting evidence shows that prions cause many neurodegenerative diseases, and arguably, scrapie and Creutzfeldt-Jakob disease prions represent the best therapeutic targets. We report here that the previously identified 2-aminothiazoles IND24 and IND81 doubled the survival times of scrapie-infected, wild-type mice. However, mice infected with Rocky Mountain Laboratory (RML) prions, a scrapie-derived strain, and treated with IND24 eventually exhibited neurological dysfunction and died. We serially passaged their brain homogenates in mice and cultured cells. We found that the prion strain isolated from IND24-treated mice, designated RML[IND24], emerged during a single passage in treated mice. Although RML prions infect both the N2a and CAD5 cell lines, RML[IND24] prions could only infect CAD5 cells. When passaged in CAD5 cells, the prions remained resistant to high concentrations of IND24. However, one passage of RML[IND24] prions in untreated mice restored susceptibility to IND24 in CAD5 cells. Although IND24 treatment extended the lives of mice propagating different prion strains, including RML, another scrapie-derived prion strain ME7, and chronic wasting disease, it was ineffective in slowing propagation of Creutzfeldt-Jakob disease prions in transgenic mice. Our studies demonstrate that prion strains can acquire resistance upon exposure to IND24 that is lost upon passage in mice in the absence of IND24. These data suggest that monotherapy can select for resistance, thus intermittent therapy with mixtures of antiprion compounds may be required to slow or stop neurodegeneration.

Biaryl amides and hydrazones as therapeutics for prion disease in transgenic mice

The only small-molecule compound demonstrated to substantially extend survival in prion-infected mice is a biaryl hydrazone termed "Compd B" (4-pyridinecarboxaldehyde,2-[4-(5-oxazolyl)phenyl]hydrazone). However, the hydrazone moiety of Compd B results in toxic metabolites, making it a poor candidate for further drug development. We developed a pharmacophore model based on diverse antiprion compounds identified by high-throughput screening; based on this model, we generated biaryl amide analogs of Compd B. Medicinal chemistry optimization led to multiple compounds with increased potency, increased brain concentrations, and greater metabolic stability, indicating that they could be promising candidates for antiprion therapy. Replacing the pyridyl ring of Compd B with a phenyl group containing an electron-donating substituent increased potency, while adding an aryl group to the oxazole moiety increased metabolic stability. To test the efficacy of Compd B, we applied bioluminescence imaging (BLI), which was previously shown to detect prion disease onset in live mice earlier than clinical signs. In our studies, Compd B showed good efficacy in two lines of transgenic mice infected with the mouse-adapted Rocky Mountain Laboratory (RML) strain of prions, but not in transgenic mice infected with human prions. The BLI system successfully predicted the efficacies in all cases long before extension in survival could be observed. Our studies suggest that this BLI system has good potential to be applied in future antiprion drug efficacy studies.

Convergent replication of mouse synthetic prion strains

Prion diseases are neurodegenerative disorders characterized by the aberrant folding of endogenous proteins into self-propagating pathogenic conformers. Prion disease can be initiated in animal models by inoculation with amyloid fibrils formed from bacterially derived recombinant prion protein. The synthetic prions that accumulated in infected organisms are structurally distinct from the amyloid preparations used to initiate their formation and change conformationally on repeated passage. To investigate the nature of synthetic prion transformation, we infected mice with a conformationally diverse set of amyloids and serially passaged the resulting prion strains. At each passage, we monitored changes in the biochemical and biological properties of the adapting strain. The physicochemical properties of each synthetic prion strain gradually changed on serial propagation until attaining a common adapted state with shared physicochemical characteristics. These results indicate that synthetic prions can assume multiple intermediate conformations before converging into one conformation optimized for in vivo propagation.

Changes in prion replication environment cause prion strain mutation

Interspecies prion transmission often leads to stable changes in physical and biological features of prion strains, a phenomenon referred to as a strain mutation. It remains unknown whether changes in the replication environment in the absence of changes in PrP primary structure can be a source of strain mutations. To approach this question, RNA content was altered in the course of amplification of hamster strains in serial protein misfolding cyclic amplification (sPMCAb). On adaptation to an RNA-depleted environment and then readaptation to an environment containing RNA, strain 263K gave rise to a novel PrP(Sc) conformation referred to as 263K(R+), which is characterized by very low conformational stability, high sensitivity to proteolytic digestion, and a replication rate of 10(6)-fold/PMCAb round, which exceeded that of 263K by almost 10(4)-fold. A series of PMCAb experiments revealed that 263K(R+) was lacking in brain-derived 263K material, but emerged de novo as a result of changes in RNA content. A similar transformation was also observed for strain Hyper, suggesting that this phenomenon was not limited to 263K. The current work demonstrates that dramatic PrP(Sc) transformations can be induced by changes in the prion replication environment and without changes in PrP primary structure.

A closer look at prion strains: characterization and important implications

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

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

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

Chimeric elk/mouse prion proteins in transgenic mice

Chronic wasting disease (CWD) of deer and elk is a highly communicable neurodegenerative disorder caused by prions. Investigations of CWD are hampered by slow bioassays in transgenic (Tg) mice. Towards the development of Tg mice that will be more susceptible to CWD prions, we created a series of chimeric elk/mouse transgenes that encode the N terminus of elk PrP (ElkPrP) up to residue Y168 and the C terminus of mouse PrP (MoPrP) beyond residue 169 (mouse numbering), designated Elk3M(SNIVVK). Between codons 169 and 219, six residues distinguish ElkPrP from MoPrP: N169S, T173N, V183I, I202V, I214V and R219K. Using chimeric elk/mouse PrP constructs, we generated 12 Tg mouse lines and determined incubation times after intracerebral inoculation with the mouse-passaged RML scrapie or Elk1P CWD prions. Unexpectedly, one Tg mouse line expressing Elk3M(SNIVVK) exhibited incubation times of <70 days when inoculated with RML prions; a second line had incubation times of <90 days. In contrast, mice expressing full-length ElkPrP had incubation periods of >250 days for RML prions. Tg(Elk3M,SNIVVK) mice were less susceptible to CWD prions than Tg(ElkPrP) mice. Changing three C-terminal mouse residues (202, 214 and 219) to those of elk doubled the incubation time for mouse RML prions and rendered the mice resistant to Elk1P CWD prions. Mutating an additional two residues from mouse to elk at codons 169 and 173 increased the incubation times for mouse prions to >300 days, but made the mice susceptible to CWD prions. Our findings highlight the role of C-terminal residues in PrP that control the susceptibility and replication of prions.

Review: Creutzfeldt-Jakob disease: prion protein type, disease phenotype and agent strain

The human transmissible spongiform encephalopathies or human prion diseases are one of the most intensively investigated groups of rare human neurodegenerative conditions. They are generally held to be unique in terms of their complex epidemiology and phenotypic variability, but they may also serve as a paradigm with which other more common protein misfolding disorders might be compared and contrasted. The clinico-pathological phenotype of human prion diseases appears to depend on a complex interaction between the prion protein genotype of the affected individual and the physico-chemical properties of the neurotoxic and transmissible agent, thought to comprise of misfolded prion protein. A major focus of research in recent years has been to define the phenotypic heterogeneity of the recognized human prion diseases, correlate this with molecular-genetic features and then determine whether this molecular-genetic classification of human prion disease defines the biological properties of the agent as determined by animal transmission studies. This review seeks to survey the field as it currently stands, summarize what has been learned, and explore what remains to be investigated in order to obtain a more complete scientific understanding of prion diseases and to protect public health.

Homology, similarity, and identity in peptide epitope immunodefinition

The tendency to use the terms homology, similarity, and identity interchangeably persists in comparative biology. When translated to immunology, overlapping the concepts of homology, similarity, and identity complicates the exact definition of the self-nonself dichotomy and, in particular, affects immunopeptidomics, an emerging field aimed at cataloging and distinguishing immunoreactive peptide epitopes from silent nonreactive amino acid sequences. The definition of similar/dissimilar peptides in immunology is discussed with special attention to the analysis of immunological (dis)similarity between two or more protein sequences that equates to measuring sequence similarity with the use of a proper measurement unit such as a length determinant.

Identification of I137M and other mutations that modulate incubation periods for two human prion strains

We report here the transmission of human prions to 18 new transgenic (Tg) mouse lines expressing 8 unique chimeric human/mouse prion proteins (PrP). Extracts from brains of two patients, who died of sporadic Creutzfeldt-Jakob disease (sCJD), contained either sCJD(MM1) or sCJD(VV2) prion strains and were used for inocula. Mice expressing chimeric PrP showed a direct correlation between expression level and incubation period for sCJD(MM1) prions irrespective of whether the transgene encoded methionine (M) or valine (V) at polymorphic residue 129. Tg mice expressing chimeric transgenes encoding V129 were unexpectedly resistant to infection with sCJD(VV2) prions, and when transmission did occur, it was accompanied by a change in strain type. The transmission of sCJD(MM1) prions was modulated by single amino acid reversions of each human PrP residue in the chimeric sequence. Reverting human residue 137 in the chimeric transgene from I to M prolonged the incubation time for sCJD(MM1) prions by more than 100 days; structural analyses suggest a profound change in the orientation of amino acid side chains with the I→M mutation. These findings argue that changing the surface charge in this region of PrP greatly altered the interaction between PrP isoforms during prion replication. Our studies contend that strain-specified replication of prions is modulated by PrP sequence-specific interactions between the prion precursor PrP(C) and the infectious product PrP(Sc).

Biochemical and strain properties of CJD prions: complexity versus simplicity

Prions, the agents responsible for transmissible spongiform encephalopathies, are infectious proteins consisting primarily of scrapie prion protein (PrP(Sc)), a misfolded, β-sheet enriched and aggregated form of the host-encoded cellular prion protein (PrP(C)). Their propagation is based on an autocatalytic PrP conversion process. Despite the lack of a nucleic acid genome, different prion strains have been isolated from animal diseases. Increasing evidence supports the view that strain-specific properties may be enciphered within conformational variations of PrP(Sc). In humans, sporadic Creutzfeldt-Jakob disease (sCJD) is the most frequent form of prion diseases and has demonstrated a wide phenotypic and molecular spectrum. In contrast, variant Creutzfeldt-Jakob disease (vCJD), which results from oral exposure to the agent of bovine spongiform encephalopathy, is a highly stereotyped disease, that, until now, has only occurred in patients who are methionine homozygous at codon 129 of the PrP gene. Recent research has provided consistent evidence of strain diversity in sCJD and also, unexpectedly enough, in vCJD. Here, we discuss the puzzling biochemical/pathological diversity of human prion disorders and the relationship of that diversity to the biological properties of the agent as demonstrated by strain typing in experimental models.

Conserved properties of human and bovine prion strains on transmission to guinea pigs

The first transmissions of human prion diseases to rodents used guinea pigs (Gps, Cavia porcellus). Later, transgenic mice expressing human or chimeric human/mouse PrP replaced Gps, but the small size of the mouse limits some investigations. To investigate the fidelity of strain-specific prion transmission to Gps, we inoculated 'type 1' and 'type 2' prion strains into Gps, and we measured the incubation times and determined the strain-specified size of the unglycosylated, protease-resistant (r) PrP(Sc) fragment. Prions passaged once in Gps from cases of sporadic (s) Creutzfeldt-Jakob disease (CJD) and Gerstmann-Sträussler-Scheinker (GSS) disease caused by the P102L mutation were used, as well as human prions from a variant (v) CJD case, bovine prions from bovine spongiform encephalopathy (BSE) and mouse-passaged scrapie prions. Variant CJD and BSE prions transmitted to all the inoculated Gps with incubation times of 367 ± 4 and 436 ± 28 days, respectively. On second passage in Gps, vCJD and BSE prions caused disease in 287 ± 4 and 310 ± 4 days, whereas sCJD and GSS prions transmitted in 237 ± 4 and 279 ± 19 days, respectively. Although hamster Sc237 prions transmitted to two of three Gps after 574 and 792 days, mouse-passaged RML and 301V prion strains, the latter derived from BSE prions, failed to transmit disease to Gps. Those Gps inoculated with vCJD or BSE prions exhibited 'type 2' unglycosylated, rPrP(Sc) (19 kDa), whereas those receiving sCJD or GSS prions displayed 'type 1' prions (21 kDa), as determined by western blotting. Such strain-specific properties were maintained in Gps as well as mice expressing a chimeric human/mouse transgene. Gps may prove particularly useful in further studies of novel human prions such as those causing vCJD.

Conformational transformation and selection of synthetic prion strains

Prion protein is capable of folding into multiple self-replicating prion strains that produce phenotypically distinct neurological disorders. Although prion strains often breed true upon passage, they can also transform or "mutate" despite being devoid of nucleic acids. To dissect the mechanism of prion strain transformation, we studied the physicochemical evolution of a mouse synthetic prion (MoSP) strain, MoSP1, after repeated passage in mice and cultured cells. We show that MoSP1 gradually adopted shorter incubation times and lower conformational stabilities. These changes were accompanied by structural transformation, as indicated by a shift in the molecular mass of the protease-resistant core of MoSP1 from approximately 19 kDa [MoSP1(2)] to 21 kDa [MoSP1(1)]. We show that MoSP1(1) and MoSP1(2) can breed with fidelity when cloned in cells; however, when present as a mixture, MoSP1(1) preferentially proliferated, leading to the disappearance of MoSP1(2). In culture, the rate of this transformation process can be influenced by the composition of the culture media and the presence of polyamidoamines. Our findings demonstrate that prions can exist as a conformationally diverse population of strains, each capable of replicating with high fidelity. Rare conformational conversion, followed by competitive selection among the resulting pool of conformers, provides a mechanism for the adaptation of the prion population to its host environment.

Transgenic mouse models and prion strains

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