Modification of a Strain of Mouse-adapted Scrapie by Passage through Rats

Alternating anti-prion regimens reduce combination drug resistance but do not further extend survival in scrapie-infected mice

Prion diseases are fatal and infectious neurodegenerative diseases in humans and other mammals caused by templated misfolding of the endogenous prion protein (PrP). Although there is currently no vaccine or therapy against prion disease, several classes of small-molecule compounds have been shown to increase disease-free incubation time in prion-infected mice. An apparent obstacle to effective anti-prion therapy is the emergence of drug-resistant strains during static therapy with either single compounds or multi-drug combination regimens. Here, we treated scrapie-infected mice with dynamic regimens that alternate between different classes of anti-prion drugs. The results show that alternating regimens containing various combinations of Anle138b, IND24 and IND116135 reduce the incidence of combination drug resistance, but do not significantly increase long-term disease-free survival compared to monotherapy. Furthermore, the alternating regimens induced regional vacuolation profiles resembling those generated by a single component of the alternating regimen, suggesting the emergence of strain dominance.

Scrapie in Swine: a Diagnostic Challenge

A naturally occurring prion disease has not been recognized in swine, but the agent of bovine spongiform encephalopathy does transmit to swine by experimental routes. Swine are thought to have a robust species barrier when exposed to the naturally occurring prion diseases of other species, but the susceptibility of swine to the agent of sheep scrapie has not been thoroughly tested. We conducted this experiment to test the susceptibility of swine to U.S. scrapie isolates by intracranial and oral inoculation. Scrapie inoculum was a pooled 10% (w/v) homogenate derived from the brains of clinically ill sheep from the 4^th passage of a serial passage study of the U.S scrapie agent (No. 13-7) through susceptible sheep (homozygous ARQ at prion protein residues 136, 154, and 171, respectively). Pigs were inoculated intracranially (n=19) with a single 0.75 mL dose or orally (n=24) with 15 mL repeated on 4 consecutive days. Necropsies were done on a subset of animals at approximately six months post inoculation (PI): the time the pigs were expected to reach market weight. Remaining pigs were maintained and monitored for clinical signs of transmissible spongiform encephalopathies (TSE) until study termination at 80 months PI or when removed due to intercurrent disease (primarily lameness). Brain samples were examined by immunohistochemistry (IHC), western blot (WB), enzyme immunoassay (EIA), and for a subset of pigs in each inoculation group, bioassay in mice expressing porcine prion protein. At six-months PI, no evidence of scrapie infection was noted by any diagnostic method. However, at 51 months of incubation or greater, 5 animals were positive by one or more methods: IHC (n=4), WB (n=3), or EIA (n=4). Furthermore, positive bioassay results were obtained from all inoculated groups (oral and intracranial; market weight and end of study) suggesting that swine are potential hosts for the agent of scrapie.

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.

Human prion protein sequence elements impede cross-species chronic wasting disease transmission

Chronic wasting disease (CWD) is a fatal prion disease of North American deer and elk and poses an unclear risk for transmission to humans. Human exposure to CWD occurs through hunting activities and consumption of venison from prion-infected animals. Although the amino acid residues of the prion protein (PrP) that prevent or permit human CWD infection are unknown, NMR-based structural studies suggest that the β2-α2 loop (residues 165-175) may impact species barriers. Here we sought to define PrP sequence determinants that affect CWD transmission to humans. We engineered transgenic mice that express human PrP with four amino acid substitutions that result in expression of PrP with a β2-α2 loop (residues 165-175) that exactly matches that of elk PrP. Compared with transgenic mice expressing unaltered human PrP, mice expressing the human-elk chimeric PrP were highly susceptible to elk and deer CWD prions but were concurrently less susceptible to human Creutzfeldt-Jakob disease prions. A systematic in vitro survey of amino acid differences between humans and cervids identified two additional residues that impacted CWD conversion of human PrP. This work identifies amino acids that constitute a substantial structural barrier for CWD transmission to humans and helps illuminate the molecular requirements for cross-species prion transmission.

The aetiology of obesity beyond eating more and exercising less

Although recent increases in availability of energy dense, processed foods and reductions in institutionally driven physical activity have created an environment that is permissible for obesity to occur, several other factors may contribute to the development of obesity in this context. We review evidence for eleven such factors: endocrine disruptors, intrauterine effects, epigenetics, maternal age, differential fecundity and assortative mating by body mass index, microorganisms, reduction in variability of ambient temperatures, smoking cessation, sleep debt, and pharmaceutical iatrogenesis. Evidence for the role of endocrine disruptors, microorganisms, ambient temperatures, sleep and reproductive factors is accumulating, but additional research is needed to confirm the causative role of these factors in human obesity. However, the role of certain pharmaceuticals and smoking cessation in development of human obesity is clear. Practice points for consideration and future research needed are highlighted for each factor.

Stabilization of a prion strain of synthetic origin requires multiple serial passages

Transmission of prions to a new host is frequently accompanied by strain adaptation, a phenomenon that involves reduction of the incubation period, a change in neuropathological features and, sometimes, tissue tropism. Here we show that a strain of synthetic origin (SSLOW), although serially transmitted within the same species, displayed the key attributes of the strain adaptation process. At least four serial passages were required to stabilize the strain-specific SSLOW phenotype. The biological titration of SSLOW revealed a correlation between clinical signs and accumulation of PrP(Sc) in brains of animals inoculated with high doses (10(-1)-10(-5) diluted brain material), but dissociation between the two processes at low dose inocula (10(-6)-10(-8) diluted brain material). At low doses, several asymptomatic animals harbored large amounts of PrP(Sc) comparable with those seen in the brains of terminally ill animals, whereas one clinically ill animal had very little, if any, PrP(Sc). In summary, the current study illustrates that the phenomenon of prion strain adaptation is more common than generally thought and could be observed upon serial transmission without changing the host species. When PrP(Sc) is seeded by recombinant PrP structures different from that of authentic PrP(Sc), PrP(Sc) properties continued to evolve for as long as four serial passages.

Interspecies transmission of prions

Mammalian prions are infectious agents of proteinaceous nature that cause several incurable neurodegenerative diseases. Interspecies transmission of prions is usually impeded or impossible. Barriers in prion transmission are caused by small interspecies differences in the primary structure of prion proteins. The barriers can also depend on the strain (variant) of a transmitted prion. Interspecies barriers were also shown for yeast prions, which define some heritable phenotypes. Yeast prions reproduce all the main traits of prion transmission barriers observed for mammals. This allowed to show that the barrier in prion transmission can be observed even upon copolymerization of two prionogenic proteins. Available data allow elucidation of the mechanisms that impede prion transmission or make it impossible.

A framework for identification of infections that contribute to human obesity

WHO has declared obesity to be a global epidemic. Obesity management strategies mainly target behavioural components of the disorder, but are only marginally effective. A comprehensive understanding of the causative factors of obesity might provide more effective management approaches. Several microbes are causatively and correlatively linked with obesity in animals and human beings. If infections contribute to human obesity, then entirely different prevention and treatment strategies and public health policies could be needed to address this subtype of the disorder. Ethical reasons preclude experimental infection of human beings with candidate microbes to unequivocally determine their contribution to obesity. As an alternative, the available information about the adipogenic human adenovirus Ad36 has been used to create a template that can be used to examine comprehensively the contributions of specific candidate microbes to human obesity. Clinicians should be aware of infectobesity (obesity of infectious origin), and its potential importance in effective obesity management.

Site-specific structural analysis of a yeast prion strain with species-specific seeding activity

Prion proteins misfold and aggregate into multiple infectious strain variants that possess unique abilities to overcome prion species barriers, yet the structural basis for the species-specific infectivities of prion strains is poorly understood. Therefore, we have investigated the site-specific structural properties of a promiscuous chimeric form of the yeast prion Sup35 from Saccharomyces cerevisiae and Candida albicans. The Sup35 chimera forms two strain variants, each of which selectively infect one species but not the other. Importantly, the N-terminal and middle domains of the Sup35 chimera (collectively referred to as Sup35NM) contain two prion recognition elements (one from each species) that regulate the nucleation of each strain. Mutations in either prion recognition element significantly bias nucleation of one strain conformation relative to the other. Herein, we have investigated the folding of each prion recognition element for the serine-to-arginine mutant at residue 17 of Sup35NM chimera known to promote nucleation of C. albicans strain conformation. Using cysteine-specific labeling analysis, we find that residues in the C. albicans prion recognition element are solvent-shielded, while those outside the recognition sequence (including most of those in the S. cerevisiae recognition element) are solvent-exposed. Moreover, we find that proline mutations in the C. albicans recognition sequence disrupt the prion templating activity of this strain conformation. Our structural findings reveal that differential folding of complementary and non-complementary prion recognition elements within the prion amyloid core of the Sup35NM chimera is the structural basis for its species-specific templating activity.

The prion hypothesis: from biological anomaly to basic regulatory mechanism

Prions are unusual proteinaceous infectious agents that are typically associated with a class of fatal degenerative diseases of the mammalian brain. However, the discovery of fungal prions, which are not associated with disease, suggests that we must now consider the effect of these factors on basic cellular physiology in a different light. Fungal prions are epigenetic determinants that can alter a range of cellular processes, including metabolism and gene expression pathways, and these changes can lead to a range of prion-associated phenotypes. The mechanistic similarities between prion propagation in mammals and fungi suggest that prions are not a biological anomaly but instead could be a newly appreciated and perhaps ubiquitous regulatory mechanism.

A molecular switch controls interspecies prion disease transmission in mice

Transmissible spongiform encephalopathies are lethal neurodegenerative disorders that present with aggregated forms of the cellular prion protein (PrPC), which are known as PrPSc. Prions from different species vary considerably in their transmissibility to xenogeneic hosts. The variable transmission barriers depend on sequence differences between incoming PrPSc and host PrPC and additionally, on strain-dependent conformational properties of PrPSc. The beta2-alpha2 loop region within PrPC varies substantially between species, with its structure being influenced by the residue types in the 2 amino acid sequence positions 170 (most commonly S or N) and 174 (N or T). In this study, we inoculated prions from 5 different species into transgenic mice expressing either disordered-loop or rigid-loop PrPC variants. Similar beta2-alpha2 loop structures correlated with efficient transmission, whereas dissimilar loops correlated with strong transmission barriers. We then classified literature data on cross-species transmission according to the 170S/N polymorphism. Transmission barriers were generally low between species with the same amino acid residue in position 170 and high between those with different residues. These findings point to a triggering role of the local beta2-alpha2 loop structure for prion transmissibility between different species.

Resveratrol selectively remodels soluble oligomers and fibrils of amyloid Abeta into off-pathway conformers

Misfolded proteins associated with diverse aggregation disorders assemble not only into a single toxic conformer but rather into a suite of aggregated conformers with unique biochemical properties and toxicities. To what extent small molecules can target and neutralize specific aggregated conformers is poorly understood. Therefore, we have investigated the capacity of resveratrol to recognize and remodel five conformers (monomers, soluble oligomers, non-toxic oligomers, fibrillar intermediates, and amyloid fibrils) of the Abeta1-42 peptide associated with Alzheimer disease. We find that resveratrol selectively remodels three of these conformers (soluble oligomers, fibrillar intermediates, and amyloid fibrils) into an alternative aggregated species that is non-toxic, high molecular weight, and unstructured. Surprisingly, resveratrol does not remodel non-toxic oligomers or accelerate Abeta monomer aggregation despite that both conformers possess random coil secondary structures indistinguishable from soluble oligomers and significantly different from their beta-sheet rich, fibrillar counterparts. We expect that resveratrol and other small molecules with similar conformational specificity will aid in illuminating the conformational epitopes responsible for Abeta-mediated toxicity.

Bovine spongiform encephalopathy

Bovine spongiform encephalopathy is an infectious disease of cattle that is transmitted through the consumption of meat-and-bone meal from infected cattle. The etiologic agent is an aberrant isoform of the native cellular prion protein that is a normal component of neurologic tissue. There currently are no approved tests that can detect BSE in live cattle.

Prion agent diversity and species barrier

Mammalian prions are the infectious agents responsible for transmissible spongiform encephalopathies (TSE), a group of fatal, neurodegenerative diseases, affecting both domestic animals and humans. The most widely accepted view to date is that these agents lack a nucleic acid genome and consist primarily of PrP(Sc), a misfolded, aggregated form of the host-encoded cellular prion protein (PrP(C)) that propagates by autocatalytic conversion and accumulates mainly in the brain. The BSE epizooty, allied with the emergence of its human counterpart, variant CJD, has focused much attention on two characteristics that prions share with conventional infectious agents. First, the existence of multiple prion strains that impose, after inoculation in the same host, specific and stable phenotypic traits such as incubation period, molecular pattern of PrP(Sc) and neuropathology. Prion strains are thought to be enciphered within distinct PrP(Sc) conformers. Second, a transmission barrier exists that restricts the propagation of prions between different species. Here we discuss the possible situations resulting from the confrontation between species barrier and prion strain diversity, the molecular mechanisms involved and the potential of interspecies transmission of animal prions, including recently discovered forms of TSE in ruminants.

Highly promiscuous nature of prion polymerization

The primary structure of the prion protein (PrP) is believed to be the key factor in regulating the species barrier of prion transmission. Because the strength of the species barrier was found to be affected by the prion strain, the extent to which the barrier can indeed be attributed to differences in the PrP primary structures of either donor and acceptor species remains unclear. In this study, we exploited the intrinsic property of PrP to polymerize spontaneously into disease-related amyloid conformations in the absence of a strain-specified template and analyzed polymerization of mouse and hamster full-length recombinant PrPs. Unexpectedly, we found no evidence of species specificity in cross-seeding polymerization assays. Even when both recombinant PrP variants were present in mixtures, preformed mouse or hamster fibrils displayed no selectivity in elongation reactions and consumed equally well both homologous and heterologous substrates. Analysis of individual fibrils revealed that fibrils can elongate in a bidirectional or unidirectional manner. Our work revealed that, in the absence of a cellular environment, post-translational modifications, or strain-specified conformational constraints, PrP fibrils are intrinsically promiscuous and capable of utilizing heterologous PrP variants as a substrate in a highly efficient manner. This study suggests that amyloid structures are capable of accommodating local perturbations arising because of a mismatch in amino acid sequences and highlights the promiscuous nature of the self-propagating activity of amyloid fibrils.

The reconstitution of mammalian prion infectivity de novo

The discovery of prion disease transmission in mammals, as well as a non-Mendelian type of inheritance in yeast, has led to the establishment of a new concept in biology, the prion hypothesis. The prion hypothesis postulates that an abnormal protein conformation propagates itself in an autocatalytic manner using the normal isoform of the same protein as a substrate and thereby acts either as a transmissible agent of disease (in mammals), or as a heritable determinant of phenotype (in yeast and fungus). While the prion biology of yeast and fungus supports this idea strongly, the direct proof of the prion hypothesis in mammals, specifically the reconstitution of the disease-associated isoform of the prion protein (PrP(Sc)) in vitro de novo from noninfectious prion protein, has been difficult to achieve despite many years of effort. The present review summarizes our current knowledge about the biochemical nature of the prion infectious agent and structure of PrP(Sc), describes potential strategies for generating prion infectivity de novo and provides some insight on why the reconstitution of infectivity has been difficult to achieve in vitro. Several hypotheses are proposed to explain the apparently low infectivity of the first generation of recently reported synthetic mammalian prions.

Infectobesity: Obesity of Infectious Origin

The rapid increase in obesity and the associated health care costs have prompted a search for better approaches for its prevention and management. Such efforts may be facilitated by better understanding the etiology of obesity. Of the several etiological factors, infection, an unusual causative factor, has recently started receiving greater attention. In the last two decades, 10 adipogenic pathogens were reported, including human and nonhuman viruses, scrapie agents, bacteria, and gut microflora. Some of these pathogens are associated with human obesity, but their causative role in human obesity has not been established. This chapter presents information about the natural hosts, signs and symptoms, and pathogenesis of the adipogenic microorganisms. If relevant to humans, "Infectobesity" would be a relatively novel, yet extremely significant concept. A new perspective about the infectious etiology of obesity may stimulate additional research to assess the contribution of hitherto unknown pathogens to human obesity and possibly to prevent or treat obesity of infectious origins.

The expanding universe of prion diseases

Prions cause fatal and transmissible neurodegenerative disease. These etiological infectious agents are formed in greater part from a misfolded cell-surface protein called PrP(C). Several mammalian species are affected by the diseases, and in the case of "mad cow disease" (BSE) the agent has a tropism for humans, with negative consequences for agribusiness and public health. Unfortunately, the known universe of prion diseases is expanding. At least four novel prion diseases--including human diseases variant Creutzfeldt-Jakob disease (vCJD) and sporadic fatal insomnia (sFI), bovine amyloidotic spongiform encephalopathy (BASE), and Nor98 of sheep--have been identified in the last ten years, and chronic wasting disease (CWD) of North American deer (Odocoileus Specis) and Rocky Mountain elk (Cervus elaphus nelsoni) is undergoing a dramatic spread across North America. While amplification (BSE) and dissemination (CWD, commercial sourcing of cervids from the wild and movement of farmed elk) can be attributed to human activity, the origins of emergent prion diseases cannot always be laid at the door of humankind. Instead, the continued appearance of new outbreaks in the form of "sporadic" disease may be an inevitable outcome in a situation where the replicating pathogen is host-encoded.

Converting the prion protein: what makes the protein infectious

The discovery of prion disease transmission in mammals, as well as a non-Mendelian type of inheritance in yeast, has led to the establishment of a new concept in biology, the prion hypothesis. The prion hypothesis postulates that an abnormal protein conformation propagates itself in an autocatalytic manner via recruitment of the normal isoform of the same protein as a substrate, and thereby acts either as a transmissible agent of disease (in mammals) or as a heritable determinant of phenotype (in yeast and fungus). Although reconstitution of fully infectious PrP(Sc)in vitro from synthetic components has not yet been achieved, numerous lines of evidence indicate that the prion protein is the major and essential component, if not the only one, of the prion infectious agent. This article summarizes our current knowledge about the chemical nature of the prion infectious agent, describes potential strategies and challenges related to the generation of prion infectivity de novo, proposes new hypotheses to explain the apparently low infectivity observed in the first synthetic mammalian prions, and describes plausible effects of chemical modifications on prion conversion.

Synthetic prions generated in vitro are similar to a newly identified subpopulation of PrPSc from sporadic Creutzfeldt-Jakob Disease

In recent studies, the amyloid form of recombinant prion protein (PrP) encompassing residues 89-230 (rPrP 89-230) produced in vitro induced transmissible prion disease in mice. These studies showed that unlike "classical" PrP(Sc) produced in vivo, the amyloid fibrils generated in vitro were more proteinase-K sensitive. Here we demonstrate that the amyloid form contains a proteinase K-resistant core composed only of residues 152/153-230 and 162-230. The PK-resistant fragments of the amyloid form are similar to those observed upon PK digestion of a minor subpopulation of PrP(Sc) recently identified in patients with sporadic Creutzfeldt-Jakob disease (CJD). Remarkably, this core is sufficient for self-propagating activity in vitro and preserves a beta-sheet-rich fibrillar structure. Full-length recombinant PrP 23-230, however, generates two subpopulations of amyloid in vitro: One is similar to the minor subpopulation of PrP(Sc), and the other to classical PrP(Sc). Since no cellular factors or templates were used for generation of the amyloid fibrils in vitro, we speculate that formation of the subpopulation of PrP(Sc) with a short PK-resistant C-terminal region reflects an intrinsic property of PrP rather than the influence of cellular environments and/or cofactors. Our work significantly increases our understanding of the biochemical nature of prion infectious agents and provides a fundamental insight into the mechanisms of prions biogenesis.

In vitro conversion of full-length mammalian prion protein produces amyloid form with physical properties of PrP(Sc)

The "protein only" hypothesis postulates that the infectious agent of prion diseases, PrP(Sc), is composed of the prion protein (PrP) converted into an amyloid-specific conformation. However, cell-free conversion of the full-length PrP into the amyloid conformation has not been achieved. In an effort to understand the mechanism of PrP(Sc) formation, we developed a cell-free conversion system using recombinant mouse full-length PrP with an intact disulfide bond (rPrP). We demonstrate that rPrP will convert into the beta-sheet-rich oligomeric form at highly acidic pH (<5.5) and at high concentrations, while at slightly acidic or neutral pH (>5.5) it assembles into the amyloid form. As judged from electron microscopy, the amyloid form had a ribbon-like assembly composed of two non-twisted filaments. In contrast to the formation of the beta-oligomer, the conversion to the amyloid occurred at concentrations close to physiological and displayed key features of an autocatalytic process. Moreover, using a shortened rPrP consisting of 106 residues (rPrP 106, deletions: Delta23-88 and Delta141-176), we showed that the in vitro conversion mimicked a transmission barrier observed in vivo. Furthermore, the amyloid form displayed a remarkable resistance to proteinase K (PK) and produced a PK-resistant core identical with that of PrP(Sc). Fourier transform infrared spectroscopy analyses showed that the beta-sheet-rich core of the amyloid form remained intact upon PK-digestion and accounted for the extremely high thermal stability. Electron and real-time fluorescent microscopy revealed that proteolytic digestion induces either aggregation of the amyloid ribbons into large clumps or further assembly into fibrils composed of several ribbons. Fibrils composed of ribbons were very fragile and had a tendency to fragment into short pieces. Remarkably, the amyloid form treated with PK preserved high seeding activity. Our work supports the protein only hypothesis of prion propagation and demonstrates that formation of the amyloid form that recapitulates key physical properties of PrP(Sc) can be achieved in vitro in the absence of cellular factors or a PrP(Sc) template.

Autocatalytic conversion of recombinant prion proteins displays a species barrier

The most unorthodox feature of the prion disease is the existence of an abnormal infectious isoform of the prion protein, PrP(Sc). According to the "protein-only" hypothesis, PrP(Sc) propagates its abnormal conformation in an autocatalytic manner using the normal isoform, PrP(C), as a substrate. Because autocatalytic conversion is considered a key element of prion replication, in this study I tested whether in vitro conversion of recombinant PrP into abnormal isoform displays specific features of an autocatalytic process. I found that recombinant human PrP formed two distinct beta-sheet rich isoforms, the beta-oligomer and the amyloid fibrils. The kinetics of the fibrils formation measured at different pH values were consistent with a model in which the beta-oligomer was not on the kinetic pathway to the fibrillar form. As judged by electron microscopy, an acidic pH favored to the long fibrils, whereas short fibrils morphologically similar to "prion rods" were formed at neutral pH. At neutral pH the conversion to the fibrils can be seeded with small aliquots of preformed fibrils. As small as 0.001% aliquot displayed seeding activity. The conversion of human PrP was seeded with high efficacy only with the preformed fibrils of human but not mouse PrP and vice versa. These studies illustrate that in vitro conversion of recombinant PrP displays specific features of an autocatalytic process and mimics the transmission barrier of prion propagation observed in vivo. I speculate that this model can be used as a rapid assay for assessing the intrinsic propensities of prion transmission between different species.

Interactions between heterologous forms of prion protein: binding, inhibition of conversion, and species barriers

The self-induced formation of the disease-associated, protease-resistant prion protein (PrP-res) from the normal protease-sensitive isoform (PrP-sen) appears to be a key event in the pathogenesis of transmissible spongiform encephalopathies. The amino acid sequence specificity of PrP-res formation correlates with, and may account for, the species specificity in transmission of transmissible spongiform encephalopathy agents in vivo. To analyze the mechanism controlling the sequence specificity of PrP-res formation, we compared the binding of PrP-sen to PrP-res with its subsequent acquisition of protease resistance by using cell-free systems consisting of heterologous versus homologous mouse and hamster PrP isoforms. Our studies showed that heterologous PrP-sen can bind to PrP-res with little conversion to the protease-resistant state and, in doing so, can interfere with the conversion of homologous PrP-sen. The interference occurred with molar ratios of homologous to heterologous PrP-sen molecules as low as 1:1. The interference was due primarily to the inhibition of conversion, but not the binding, of the homologous PrP-sen to PrP-res. The results provide evidence that the sequence specificity of PrP-res formation in this model is determined more by the conversion to protease resistance than by the initial binding step. These findings also imply that after the initial binding, further intermolecular interactions between PrP-sen and PrP-res are required to complete the process of conversion to the protease-resistant state.

Transmissible and genetic prion diseases share a common pathway of neurodegeneration

Prion diseases can be infectious, sporadic and genetic. The infectious forms of these diseases, including bovine spongiform encephalopathy and Creutzfeldt-Jakob disease, are usually characterized by the accumulation in the brain of the transmissible pathogen, an abnormally folded isoform of the prion protein (PrP) termed PrPSc. However, certain inherited PrP mutations appear to cause neurodegeneration in the absence of PrPSc, working instead by favoured synthesis of CtmPrP, a transmembrane form of PrP. The relationship between the neurodegeneration seen in transmissible prion diseases involving PrPSc and that associated with ctmPrP has remained unclear. Here we find that the effectiveness of accumulated PrPSc in causing neurodegenerative disease depends upon the predilection of host-encoded PrP to be made in the ctmPrP form. Furthermore, the time course of PrPSc accumulation in transmissible prion disease is followed closely by increased generation of CtmPrP. Thus, the accumulation of PrPSc appears to modulate in trans the events involved in generating or metabolising CtmPrP. Together, these data suggest that the events of CtmPrP-mediated neurodegeneration may represent a common step in the pathogenesis of genetic and infectious prion diseases.

Creutzfeldt-Jakob disease, new variant creutzfeldt-jakob disease, and bovine spongiform encephalopathy

Creutzfeldt-Jakob disease (CJD) is a subacute spongiform encephalopathy (SSE) that is manifested by a variety of neurologic signs that usually include dementia, myoclonus, and an abnormal electroencephalogram (EEG). In 1996, a new variant of CJD (nvCJD) with a somewhat distinctive clinical presentation and neuropathology was reported in adolescents and young adults, a cohort of patients not normally affected with CJD. The appearance of nvCJD coincided temporally and geographically with the emergence of an SSE in cattle known as bovine spongiform encephalopathy (BSE), or mad cow disease. This article discusses the clinical syndrome, pathology, and pathogenesis of classical CJD, nvCJD, and other human SSEs, as well as the link between BSE and nvCJD.

Prions

Prions are unprecedented infectious pathogens that cause a group of invariably fatal neurodegenerative diseases by an entirely novel mechanism. Prion diseases may present as genetic, infectious, or sporadic disorders, all of which involve modification of the prion protein (PrP). Bovine spongiform encephalopathy (BSE), scrapie of sheep, and Creutzfeldt-Jakob disease (CJD) of humans are among the most notable prion diseases. Prions are transmissible particles that are devoid of nucleic acid and seem to be composed exclusively of a modified protein (PrPSc). The normal, cellular PrP (PrPC) is converted into PrPSc through a posttranslational process during which it acquires a high beta-sheet content. The species of a particular prion is encoded by the sequence of the chromosomal PrP gene of the mammals in which it last replicated. In contrast to pathogens carrying a nucleic acid genome, prions appear to encipher strain-specific properties in the tertiary structure of PrPSc. Transgenetic studies argue that PrPSc acts as a template upon which PrPC is refolded into a nascent PrPSc molecule through a process facilitated by another protein. Miniprions generated in transgenic mice expressing PrP, in which nearly half of the residues were deleted, exhibit unique biological properties and should facilitate structural studies of PrPSc. While knowledge about prions has profound implications for studies of the structural plasticity of proteins, investigations of prion diseases suggest that new strategies for the prevention and treatment of these disorders may also find application in the more common degenerative diseases.

The prion diseases

The human prion diseases are fatal neurodegenerative maladies that may present as sporadic, genetic, or infectious illnesses. The sporadic form is called Creutzfeldt-Jakob disease (CJD) while the inherited disorders are called familial (f) CJD, Gerstmann-Straussler-Scheinker (GSS) disease and fatal familial insomnia (FFI). Prions are transmissible particles that are devoid of nucleic acid and seem to be composed exclusively of a modified protein (PrPSc). The normal, cellular PrP (PrPC) is converted into PrPSc through a posttranslational process during which it acquires a high beta-sheet content. In fCJD, GSS, and FFI, mutations in the PrP gene located on the short arm of chromosome 20 are the cause of disease. Considerable evidence argues that the prion diseases are disorders of protein conformation.

Prion diseases and the BSE crisis

Bovine spongiform encephalopathy (BSE) and human Creutzfeldt-Jakob disease (CJD) are among the most notable central nervous system degenerative disorders caused by prions. CJD may present as a sporadic, genetic, or infectious illness. Prions are transmissible particles that are devoid of nucleic acid and seem to be composed exclusively of a modified protein (PrPSc). The normal, cellular prion protein (PrPC) is converted into PrPSc through a posttranslational process during which it acquires a high beta-sheet content. It is thought that BSE is a result of cannibalism in which faulty industrial practices produced prion-contaminated feed for cattle. There is now considerable concern that bovine prions may have been passed to humans, resulting in a new form of CJD.

Propagation of prions with artificial properties in transgenic mice expressing chimeric PrP genes

Transgenic mice expressing chimeric prion protein (PrP) genes derived from Syrian hamster (SHa) and mouse (Mo) PrP genes were constructed. One SHa/MoPrP gene, designated MH2M PrP, contains five amino acid substitutions encoded by SHaPrP, while another construct, designated MHM2 PrP, has two substitutions. Transgenic (Tg) (MH2M PrP) mice were susceptible to both Syrian hamster and mouse prions, whereas three lines expressing MHM2 PrP were resistant to Syrian hamster prions. The brains of Tg(MH2M PrP) mice dying of scrapie contained chimeric PrPSc and prions with an artificial host range favoring propagation in mice that express the corresponding chimeric PrP and were also transmissible, at reduced efficiency, to nontransgenic mice and hamsters. Our findings provide genetic evidence for homophilic interactions between PrPSc in the inoculum and PrPc synthesized by the host.

Chimeric prion protein expression in cultured cells and transgenic mice

The efficient expression of exogenous prion protein (PrP) molecules in mouse neuroblastoma cells that are chronically infected with murine scrapie prions (ScN2a cells; Butler, D.A., et al., 1988, J. Virol. 62, 1558-1564) and in transgenic mice is described. This technology allows investigation of the PrP molecule for structural regions involved in determining species specificity, as well as ablation experiments designed to address the functionality of particular regions of the PrP molecule. Previous reports demonstrated that the PrP gene specifies the host range for susceptibility of transgenic animals to prions (Scott, M., et al., 1989, Cell 59, 847-857; Prusiner, S.B., et al., 1990, Cell 63, 673-686). Consistent with these results, we showed that Syrian hamster (SHa) PrP is ineligible for efficient conversion to PrPSc in ScN2a cells. By constructing a series of chimeric mouse (Mo)/SHaPrP genes, we developed an epitopically tagged functional variant of the MoPrP gene, which can efficiently form protease-resistant PrP molecules upon expression in ScN2a cells. The presence of a defined epitope for an SHa-specific monoclonal antibody allows the products of this chimeric gene to be discriminated from endogenous MoPrP and creates a useful reagent for exploring structure/function relationships via targeted mutagenesis. In addition, we developed a transgenic mouse expression vector by manipulation of an SHaPrP cosmid clone. This vector permits the efficient expression of foreign PrP genes in the brains of transgenic animals, enabling pathological consequences of in vitro mutagenesis to be studied.

Transmissable spongiform encephalopathy (TSE) agents as crystalline forms of the prion protein (PrP) that multiply by allowing normal metabolic forms of PrP to join the crystal

The prion protein (PrP), is found only in the brain of animals infected with TSE. It is a modified form of a normal protein (PrPn) produced from the genome of the animal. The modification prevents breakdown by proteinases and hence its total chemical or physical structure is unknown. The finding of fibre-like structures in microglia and neurones that cross-react with antibodies produced against PrP and the rapid turnover of PrPn may mean that normal biochemical pathway PrPn forms can join a crystal seed of PrP to produce these fibres. This hypothesis, that the modification of PrP is physical rather than chemical, avoids the major problems with theories of PrP as the infective agent.

Beef and bovine spongiform encephalopathy: the risk persists

Bovine spongiform encephalopathy (BSE) is one of the transmissible spongiform encephalopathies (TSE) that are currently known to the authors to affect only mammals, including man. The diseases are progressive, fatal paralyses and dementias, for which there are no methods of certain diagnosis and no treatment. In this review the disease in cattle, the mode of transfer of these TSEs between animals by mouth, the possible presence of infective agents in the food that we eat, the resistance of BSE to cooking, and the likelihood that humans may become infected are discussed. The origins of BSE, whether from sheep, from cows, or as a mutation are considered. Whatever the origin of BSE, a substantial danger for man exists. Creutzfeld-Jakob disease (CJD), a TSE of man, may have been derived from eating infected animal tissue in the past. The possibility that this was of bovine origin and the implications that this would have for BSE infected meat in human food are discussed.

Molecular biology and transgenetics of prion diseases

Considerable progress has been made deciphering the role of an abnormal isoform of the prion protein (PrP) in scrapie of animals and Gerstmann-Sträussler syndrome (GSS) of humans. Some transgenic (Tg) mouse (Mo) lines that carry and express a Syrian hamster (Ha) PrP gene developed scrapie 75 d after inoculation with Ha prions; non-Tg mice failed to show symptoms after greater than 500 d. Brains of these infected Tg(HaPrP) mice featured protease-resistant HaPrPSc, amyloid plaques characteristic for Ha scrapie, and 10(9) ID50 units of Ha-specific prions upon bioassay. Studies on Syrian, Armenian, and Chinese hamsters suggest that the domain of the PrP molecule between codons 100 and 120 controls both the length of the incubation time and the deposition of PrP in amyloid plaques. Ataxic GSS in families shows genetic linkage to a mutation in the PrP gene, leading to the substitution of Leu for Pro at codon 102. Discovery of a point mutation in the Prp gene from humans with GSS established that GSS is unique among human diseases--it is both genetic and infectious. These results have revised thinking about sporadic Creutzfeldt-Jakob disease, suggesting it may arise from a somatic mutation. These findings combined with those from many other studies assert that PrPSc is a component of the transmissible particle, and the PrP amino acid sequence controls the neuropathology and species specificity of prion infectivity. The precise mechanism of PrPSc formation remains to be established. Attempts to demonstrate a scrapie-specific nucleic acid within highly purified preparations of prions have been unrewarding to date. Whether transmissible prions are composed only of PrPSc molecules or do they also contain a second component such as small polynucleotide remains uncertain.

Transgenetic studies implicate interactions between homologous PrP isoforms in scrapie prion replication

Transgenic (Tg) mice expressing both Syrian hamster (Ha) and mouse (Mo) prion protein (PrP) genes were used to probe the mechanism of scrapie prion replication. Four Tg lines expressing HaPrP exhibited distinct incubation times ranging from 48 to 277 days, which correlated inversely with HaPrP mRNA and HaPrPC. Bioassays of Tg brain extracts showed that the prion inoculum dictates which prions are synthesized de novo. Tg mice inoculated with Ha prions had approximately 10(9) ID50 units of Ha prions per gram of brain and less than 10 units of Mo prions. Conversely, Tg mice inoculated with Mo prions synthesized Mo prions but not Ha prions. Similarly, Tg mice inoculated with Ha prions exhibited neuropathologic changes characteristic of hamsters with scrapie, while Mo prions produced changes similar to those in non-Tg mice. Our results argue that species specificity of scrapie prions resides in the PrP sequence and prion synthesis is initiated by a species-specific interaction between PrPSc in the inoculum and homologous PrPC.

Transgenic mice expressing hamster prion protein produce species-specific scrapie infectivity and amyloid plaques

Three transgenic mouse lines designated Tg 69, 71, and 81 were produced harboring a Syrian hamster (Ha) prion protein (PrP) gene; all expressed the cellular HaPrP isoform in their brains. Inoculation of Tg 81 mice or hamsters with Ha prions caused scrapie in integral of 75 days; nontransgenic control mice failed to develop scrapie after greater than 500 days. Tg 71 mice inoculated with Ha prions developed scrapie in integral of 170 days. Both Tg 71 and Tg 81 mice exhibited spongiform degeneration and reactive astrocytic gliosis, and they produced the scrapie HaPrP isoform in their brains. Tg 81 brains also showed HaPrP amyloid plaques characteristic of Ha scrapie and contained integral of 10(9) ID50 units of Ha prions based on Ha bioassays. Our findings argue that the PrP gene modulates scrapie susceptibility, incubation times, and neuropathology; furthermore, they demonstrate synthesis of infectious scrapie prions programmed by a recombinant DNA molecule.

Unraveling prion diseases through molecular genetics

Prions are transmissible pathogens that cause degenerative diseases in humans and animals. Unique attributes of prion diseases include infectious, sporadic and genetic manifestations, as well as progression to death, all in the absence of a detectable immune response. Prions are resistant to chemical procedures that modify or destroy nucleic acids and are composed largely of a protein, designated PrPSc. Molecular cloning of a cognate cDNA established a cellular host origin for PrPSc protein and a convergence with the genetics of host susceptibility. The murine PrP gene is linked to the Prn-i gene which determines incubation times in experimental scrapie. Mice with long incubation times have unusual PrP alleles encoding phenylalanine and valine at codons 108 and 189. Moreover, the ataxic form of Gerstmann-Sträussler syndrome (a rare human neurodegenerative disorder) has been defined as an autosomal dominant disorder with a PrP mis-sense mutation at codon 102 linked to the predisposition locus. These studies argue that amino acid substitutions in 'PrP' genes may modulate initiation and development of prion diseases.

Scrapie-infected murine neuroblastoma cells produce protease-resistant prion proteins

Scrapie and Creutzfeldt-Jakob disease are transmissible, degenerative neurological diseases caused by prions. Considerable evidence argues that prions contain protease-resistant sialoglycoproteins, designated PrPSc, encoded by a cellular gene. The prion protein (PrP) gene also encodes a normal cellular protein designated PrPC. We established clonal cell lines which support the replication of mouse scrapie or Creutzfeldt-Jakob disease prions. Mouse neuroblastoma N2a cells were exposed to mouse scrapie prions and subsequently cloned. After limited proteinase K digestion, three PrP-immunoreactive proteins with apparent molecular masses ranging between 20 and 30 kilodaltons were detected in extracts of scrapie-infected N2a cells by Western (immuno-) blotting. The authenticity of these PrPSc molecules was established by using monospecific antiserum raised against a synthetic peptide corresponding to a portion of the prion protein. Those clones synthesizing PrPSc molecules possessed scrapie prion infectivity as measured by bioassay; clones without PrPSc failed to demonstrate infectivity. Detection of PrPSc molecules in scrapie-infected N2a cells supports the contention that PrPSc is a component of the infectious scrapie particle and opens new approaches to the study of prion diseases.

Scrapie: a review of its relation to human disease and ageing

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A comparison of some biological characteristics of the mouse-passaged scrapie agents, 22A and ME7

Two mouse-adapted scrapie agents of different sheep origin were compared. The titre, reached in the brains of mice in the terminal stage of scrapie, is of the same order for both agents. There is a threefold difference between the incubation periods of the two agents in some mouse strains, of which C57 is one, and in this strain incubation of the 22A agent, given as a large dose by a peripheral route, occupies almost the whole life-span.

The most fundamental difference between the agents concerns the reversal of the ranking of incubation periods, typically in the VM and C57 mouse strams: incubation of ME7 in VM takes almost twice as long as in C57, whereas most sub-lines of 22A take half as long in VM as in C57. The implications of this type of host-genotype, agent-strain interaction are discussed in terms of the possible nature of agent differences, the possibility of latent infection and the consequences for scrapie eradication programmes.