A Protein Misfolding Shaking Amplification-based method for the spontaneous generation of hundreds of bona fide prions

Prion diseases are a group of rapidly progressing neurodegenerative disorders caused by the misfolding of the endogenous prion protein (PrPC) into a pathogenic form (PrPSc). This process, despite being the central event underlying these disorders, remains largely unknown at a molecular level, precluding the prediction of new potential outbreaks or interspecies transmission incidents. In this work, we present a method to generate bona fide recombinant prions de novo, allowing a comprehensive analysis of protein misfolding across a wide range of prion proteins from mammalian species. We study more than 380 different prion proteins from mammals and classify them according to their spontaneous misfolding propensity and their conformational variability. This study aims to address fundamental questions in the prion research field such as defining infectivity determinants, interspecies transmission barriers or the structural influence of specific amino acids and provide invaluable information for future diagnosis and therapy applications.

Genetic modulation of CWD prion propagation in cervid PrP Drosophila

Chronic wasting disease is a fatal prion condition of cervids such as deer, elk, moose and reindeer. Secretion and excretion of prion infectivity from North American cervids with this condition causes environmental contamination and subsequent efficient lateral transmission in free-ranging and farmed cervids. Variants of cervid PrP exist that affect host susceptibility to chronic wasting disease. Cervid breeding programmes aimed at increasing the frequency of PrP variants associated with resistance to chronic wasting disease may reduce the burden of this condition in animals and lower the risk of zoonotic disease. This strategy requires a relatively rapid and economically viable model system to characterise and support selection of prion disease-modifying cervid PrP variants. Here, we generated cervid PrP transgenic Drosophila to fulfil this purpose. We have generated Drosophila transgenic for S138 wild type cervid PrP, or the N138 variant associated with resistance to chronic wasting disease. We show that cervid PrP Drosophila accumulate bona fide prion infectivity after exposure to cervid prions. Furthermore, S138 and N138 PrP fly lines are susceptible to cervid prion isolates from either North America or Europe when assessed phenotypically by accelerated loss of locomotor ability or survival, or biochemically by accumulation of prion seeding activity. However, after exposure to European reindeer prions, N138 PrP Drosophila accumulated prion seeding activity with slower kinetics than the S138 fly line. These novel data show that prion susceptibility characteristics of cervid PrP variants are maintained when expressed in Drosophila, which highlights this novel invertebrate host in modelling chronic wasting disease.

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

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

Endoproteolysis of cellular prion protein by plasmin hinders propagation of prions

Many questions surround the underlying mechanism for the differential metabolic processing observed for the prion protein (PrP) in healthy and prion-infected mammals. Foremost, the physiological α-cleavage of PrP interrupts a region critical for both toxicity and conversion of cellular PrP (PrP ^C ) into its misfolded pathogenic isoform (PrP ^Sc ) by generating a glycosylphosphatidylinositol (GPI)-anchored C1 fragment. During prion diseases, alternative β-cleavage of PrP becomes prominent, producing a GPI-anchored C2 fragment with this particular region intact. It remains unexplored whether physical up-regulation of α-cleavage can inhibit disease progression. Furthermore, several pieces of evidence indicate that a disintegrin and metalloproteinase (ADAM) 10 and ADAM17 play a much smaller role in the α-cleavage of PrP ^C than originally believed, thus presenting the need to identify the primary protease(s) responsible. For this purpose, we characterized the ability of plasmin to perform PrP α-cleavage. Then, we conducted functional assays using protein misfolding cyclic amplification (PMCA) and prion-infected cell lines to clarify the role of plasmin-mediated α-cleavage during prion propagation. Here, we demonstrated an inhibitory role of plasmin for PrP ^Sc formation through PrP α-cleavage that increased C1 fragments resulting in reduced prion conversion compared with non-treated PMCA and cell cultures. The reduction of prion infectious titer in the bioassay of plasmin-treated PMCA material also supported the inhibitory role of plasmin on PrP ^Sc replication. Our results suggest that plasmin-mediated endoproteolytic cleavage of PrP may be an important event to prevent prion propagation.

In Vitro detection of Chronic Wasting Disease (CWD) prions in semen and reproductive tissues of white tailed deer bucks (Odocoileus virginianus)

Chronic Wasting Disease (CWD) is a prion disease affecting several cervid species. Among them, white-tailed deer (WTD) are of relevance due to their value in farming and game hunting. The exact events involved in CWD transmission in captive and wild animals are still unclear. An unexplored mechanism of CWD spread involves transmissions through germplasm, such as semen. Surprisingly, the presence and load of CWD prions in semen and male sexual tissues from WTD has not been explored. Here, we described the detection of CWD prions in semen and sexual tissues of WTD bucks utilizing the Protein Misfolding Cyclic Amplification (PMCA) technology. Samples were obtained post-mortem from farmed pre-clinical, CWD positive WTD bucks possessing polymorphisms at position 96 of the PRNP gene. Our results show that overall CWD detection in these samples had a sensitivity of 59.3%, with a specificity of 97.2%. The data indicate that the presence of CWD prions in male sexual organs and fluids is prevalent in late stage, pre-clinical, CWD-infected WTD (80%-100% of the animals depending on the sample type analyzed). Our findings reveal the presence of CWD prions in semen and sexual tissues of prion infected WTD bucks. Future studies will be necessary to determine whether sexual contact and/or artificial inseminations are plausible means of CWD transmission in susceptible animal species.

Unique Structural Features of Mule Deer Prion Protein Provide Insights into Chronic Wasting Disease

Chronic wasting disease (CWD) is a highly infectious prion disease of cervids. Accumulation of prions, the disease-specific structural conformers of the cellular prion protein (PrP^C), in the central nervous system, is the key pathological event of the disorder. The analysis of cervid PrP^C sequences revealed the existence of polymorphism at position 226, in which deer PrP contains glutamine (Q), whereas elk PrP contains glutamate (E). The effects of this polymorphism on CWD are still unknown. We determined the high-resolution nuclear magnetic resonance structure of the mule deer prion protein that was compared to previously published PrP structures of elk and white-tailed deer. We found that the polymorphism Q226E could influence the long-range intramolecular interactions and packing of the β2-α2 loop and the C-terminus of the α3 helix of cervid PrP structures. This solvent-accessible epitope is believed to be involved in prion conversion. Additional differences were observed at the beginning of the well-defined C-terminus domain, in the α2-α3 region, and in its interactions with the α1 helix. Here, we highlight the importance of the PrP structure in prion susceptibility and how single amino acid differences might influence the overall protein folding.

Destabilizing polymorphism in cervid prion protein hydrophobic core determines prion conformation and conversion efficiency

Prion diseases are infectious neurodegenerative disorders of humans and animals caused by misfolded forms of the cellular prion protein PrP^C. Prions cause disease by converting PrP^C into aggregation-prone PrP^Sc. Chronic wasting disease (CWD) is the most contagious prion disease with substantial lateral transmission, affecting free-ranging and farmed cervids. Although the PrP primary structure is highly conserved among cervids, the disease phenotype can be modulated by species-specific polymorphisms in the prion protein gene. How the resulting amino-acid substitutions impact PrP^C and PrP^Sc structure and propagation is poorly understood. We investigated the effects of the cervid 116A>G substitution, located in the most conserved PrP domain, on PrP^C structure and conversion and on 116AG-prion conformation and infectivity. Molecular dynamics simulations revealed structural de-stabilization of 116G-PrP, which enhanced its in vitro conversion efficiency when used as recombinant PrP substrate in real-time quaking-induced conversion (RT-QuIC). We demonstrate that 116AG-prions are conformationally less stable, show lower activity as a seed in RT-QuIC and exhibit reduced infectivity in vitro and in vivo. Infectivity of 116AG-prions was significantly enhanced upon secondary passage in mice, yet conformational features were retained. These findings indicate that structurally de-stabilized PrP^C is readily convertible by cervid prions of different genetic background and results in a prion conformation adaptable to cervid wild-type PrP. Conformation is an important criterion when assessing transmission barrier, and conformational variants can target a different host range. Therefore, a thorough analysis of CWD isolates and re-assessment of species-barriers is important in order to fully exclude a zoonotic potential of CWD.

Chronic wasting disease (CWD) is a prion disease which affects wild and captive cervids. Prion diseases are infectious neurodegenerative disorders, and the causative agent consists of abnormally folded prion protein termed PrP^Sc. Prions replicate without genetic information, and their three-dimensional structure is thought to encode heritable information necessary to propagate using the cellular prion protein PrP^C as a substrate for conversion. In this study, we use in vitro and in vivo techniques to analyze the effect of a polymorphism at codon 116 (A>G) of the white-tailed deer prion protein on CWD prion conformation, propagation and pathogenesis. We observed differences in conformation, infectivity and seeding activity in vitro between CWD prions isolated from white-tailed deer encoding wild-type (116AA) PrP^C or 116AG-PrP^C. In mouse bioassays conformational differences are retained, however, 116AG CWD prions resulted in significantly shortened incubation times upon passages. Molecular dynamics simulations suggest that the structure of 116G-PrP^C is more flexible, which is supported by an improved convertibility in an in vitro conversion assay. Altogether these data indicate the importance of a variation in the most conserved PrP domain, and highlight the relationship between PrP^C structural flexibility, prion conformation and conversion, and pathogenesis of prion disease in vivo.

Effect of poly-L-arginine in inhibiting scrapie prion protein of cultured cells

Biological effect of poly-L-arginine (PLR), the linear homopolymer comprised of L-arginine, was investigated to determine the activity of suppressing prions. PLR decreased the level of scrapie prion protein (PrP^Sc) in cultured cells permanently infected with prions in a concentration-dependent manner. The PrP^Sc inhibition efficacy of PLR was greater than that of another prion-suppressant poly-L-lysine (PLK) in a molecular mass-dependent fashion. The effective concentration of PLR to inhibit prions was achieved safely below the cytotoxic concentrations, and overall cytotoxicity of PLR was similar to that of PLK. PLR did not alter the cellular prion protein (PrP^C) level and was unable to change the states of preformed recombinant PrP aggregates and PrP^Sc from prion-infected cells. These data eliminate the possibility that the action mechanism of PLR is through removal of PrP^C and pre-existing PrP^Sc. However, PLR formed complexes with plasminogen that stimulates prion propagation via conversion of PrP^C to the misfolded isoform, PrP^Sc. The plasminogen-PLR complex demonstrated the greater positive surface charge values than the similar complex with PLK, raising the possibility that PLR interferes with the role of cofactor for PrP^Sc generation better than PLK.

Mitigation of prion infectivity and conversion capacity by a simulated natural process--repeated cycles of drying and wetting

Prions enter the environment from infected hosts, bind to a wide range of soil and soil minerals, and remain highly infectious. Environmental sources of prions almost certainly contribute to the transmission of chronic wasting disease in cervids and scrapie in sheep and goats. While much is known about the introduction of prions into the environment and their interaction with soil, relatively little is known about prion degradation and inactivation by natural environmental processes. In this study, we examined the effect of repeated cycles of drying and wetting on prion fitness and determined that 10 cycles of repeated drying and wetting could reduce PrP^Sc abundance, PMCA amplification efficiency and extend the incubation period of disease. Importantly, prions bound to soil were more susceptible to inactivation by repeated cycles of drying and wetting compared to unbound prions, a result which may be due to conformational changes in soil-bound PrP^Sc or consolidation of the bonding between PrP^Sc and soil. This novel finding demonstrates that naturally-occurring environmental process can degrade prions.

Prion diseases such as chronic wasting disease and scrapie are emerging in North America at an increasing rate. Infectious prions are introduced into the environment from both living and dead animals where they can bind to soil. Little information is available on the effect of prion inactivation under conditions that would be found in the natural environment. In this study, we exposed both unbound and soil-bound prions to repeated cycles of drying and wetting to simulate ambient environmental conditions. We found evidence of prion inactivation in both unbound and soil bound prions. The influence of repeated cycles of drying and wetting are dependent on the prion strain and soil type used and, interestingly, prions bound to soil were more susceptible to inactivation. This is the first report of natural environmental processes mitigating prion infectivity. This data suggests that the total environmental prion load is a balance between input and natural clearance.

PrP conformational transitions alter species preference of a PrP-specific antibody

The epitope of the 3F4 antibody most commonly used in human prion disease diagnosis is believed to consist of residues Met-Lys-His-Met (MKHM) corresponding to human PrP-(109-112). This assumption is based mainly on the observation that 3F4 reacts with human and hamster PrP but not with PrP from mouse, sheep, and cervids, in which Met at residue 112 is replaced by Val. Here we report that, by brain histoblotting, 3F4 did not react with PrP of uninfected transgenic mice expressing elk PrP; however, it did show distinct immunoreactivity in transgenic mice infected with chronic wasting disease. Compared with human PrP, the 3F4 reactivity with the recombinant elk PrP was 2 orders of magnitude weaker, as indicated by both Western blotting and surface plasmon resonance. To investigate the molecular basis of these species- and conformer-dependent preferences of 3F4, the epitope was probed by peptide membrane array and antigen competition experiments. Remarkably, the 3F4 antibody did not react with MKHM but reacted strongly with KTNMK (corresponding to human PrP-(106-110)), a sequence that is also present in cervids, sheep, and cattle. 3F4 also reacted with elk PrP peptides containing KTNMKHV. We concluded that the minimal sequence for the 3F4 epitope consists of residues KTNMK, and the species- and conformer-dependent preferences of 3F4 arise largely from the interactions between Met(112) (human PrP) or Val(115) (cervid PrP) and adjacent residues.

Abbreviated incubation times for human prions in mice expressing a chimeric mouse-human prion protein transgene

Transgenic (Tg) mouse lines that express chimeric mouse-human prion protein (PrP), designated MHu2M, are susceptible to prions from patients with sporadic Creutzfeldt-Jakob disease (sCJD). With the aim of decreasing the incubation time to fewer than 200 days, we constructed transgenes in which one or more of the nine human residues in MHu2M were changed to mouse. The construct with murine residues at positions 165 and 167 was expressed in Tg(MHu2M,M165V,E167Q) mice and resulted in shortening the incubation time to approximately 110 days for prions from sCJD patients. The construct with a murine residue at position 96 resulted in lengthening the incubation time to more than 280 days for sCJD prions. When murine residues 96, 165, and 167 were expressed, the abbreviated incubation times for sCJD prions were abolished. Variant CJD prions showed prolonged incubation times between 300 and 700 days in Tg(MHu2M) mice on first passage and incubation times of approximately 350 days in Tg(MHu2M,M165V,E167Q) mice. On second and third passages of variant CJD prions in Tg(MHu2M) mice, multiple strains of prions were detected based on incubation times and the sizes of the protease-resistant, deglycosylated PrP(Sc) fragments. Our discovery of a previously undescribed chimeric transgene with abbreviated incubation times for sCJD prions should facilitate studies on the prion species barrier and human prion diversity.

Prions in skeletal muscle

Considerable evidence argues that consumption of beef products from cattle infected with bovine spongiform encephalopathy (BSE) prions causes new variant Creutzfeldt-Jakob disease. In an effort to prevent new variant Creutzfeldt-Jakob disease, certain "specified offals," including neural and lymphatic tissues, thought to contain high titers of prions have been excluded from foods destined for human consumption [Phillips, N. A., Bridgeman, J. & Ferguson-Smith, M. (2000) in The BSE Inquiry (Stationery Office, London), Vol. 6, pp. 413-451]. Here we report that mouse skeletal muscle can propagate prions and accumulate substantial titers of these pathogens. We found both high prion titers and the disease-causing isoform of the prion protein (PrP(Sc)) in the skeletal muscle of wild-type mice inoculated with either the Me7 or Rocky Mountain Laboratory strain of murine prions. Particular muscles accumulated distinct levels of PrP(Sc), with the highest levels observed in muscle from the hind limb. To determine whether prions are produced or merely accumulate intramuscularly, we established transgenic mice expressing either mouse or Syrian hamster PrP exclusively in muscle. Inoculating these mice intramuscularly with prions resulted in the formation of high titers of nascent prions in muscle. In contrast, inoculating mice in which PrP expression was targeted to hepatocytes resulted in low prion titers. Our data demonstrate that factors in addition to the amount of PrP expressed determine the tropism of prions for certain tissues. That some muscles are intrinsically capable of accumulating substantial titers of prions is of particular concern. Because significant dietary exposure to prions might occur through the consumption of meat, even if it is largely free of neural and lymphatic tissue, a comprehensive effort to map the distribution of prions in the muscle of infected livestock is needed. Furthermore, muscle may provide a readily biopsied tissue from which to diagnose prion disease in asymptomatic animals and even humans.

Analyzing the influence of PrP primary structure on prion pathogenesis in transgenic mice

Expression of prion protein (PrP) genes in transgenic (Tg) mice has been an extremely effective means of studying human and animal prion diseases. Indeed, much of what we currently understand about the molecular basis of prion pathogenesis derives from such studies. Despite these advances, the emergence of a new variant of Creutzfeldt-Jakob disease (vCJD), apparently the human manifestation of bovine spongiform encephalopathy (BSE), demonstrates that our understanding of the factors controlling prion transmission is far from complete. We review studies in Tg mice that have addressed issues of prion strains and species barriers and have provided insights into mechanisms of prion propagation. The goal of future investigation will be to determine the interplay between PrP primary structure and conformation in determining prion transmission barriers and we discuss some ongoing transgenic studies designed to address these issues.

Prions and the prion disorders

One of us remembers sitting in a high school biology class in 1977 being taught about scrapie, a naturally occurring disorder of sheep. The teacher had no particular interest in agriculture, but was pointing out some peculiar characteristics of this disease as a biological curiosity on a wet Friday afternoon. The prion disorders captured the imagination of a range of biologists (including that teacher) well before the epidemic of bovine spongiform encephalopathy (BSE) and the appearance of a new variant of the human prion disease, Creutzfeldt Jakob disease (CJD), in the UK, because of their extraordinary biology and the unique properties of the infectious agent. We review the results of studies leading to a convergence of evidence that the causative infectious agent, the 'prion', is devoid of nucleic acid and is composed of an abnormal isoform of a host-encoded protein, the prion protein (PrP).

Identification of a prion protein epitope modulating transmission of bovine spongiform encephalopathy prions to transgenic mice

There is considerable concern that bovine prions from cattle with bovine spongiform encephalopathy (BSE) may have been passed to humans (Hu), resulting in a new form of Creutzfeldt-Jakob disease (CJD). We report here the transmission of bovine (Bo) prions to transgenic (Tg) mice expressing BoPrP; one Tg line exhibited incubation times of approximately 200 days. Like most cattle with BSE, vacuolation and astrocytic gliosis were confined in the brainstems of these Tg mice. Unexpectedly, mice expressing a chimeric Bo/Mo PrP transgene were resistant to BSE prions whereas mice expressing Hu or Hu/Mo PrP transgenes were susceptible to Hu prions. A comparison of differences in Mo, Bo, and Hu residues within the C terminus of PrP defines an epitope that modulates conversion of PrPC into PrPSc and, as such, controls prion transmission across species. Development of susceptible Tg(BoPrP) mice provides a means of measuring bovine prions that may prove critical in minimizing future human exposure.

Insoluble wild-type and protease-resistant mutant prion protein in brains of patients with inherited prion disease

We studied prion proteins (PrP) in skin and brains of Libyan Jews carrying the E200K mutation who died of familial Creutzfeldt-Jakob disease (CJD). Unexpectedly, studies with brain showed that PrP molecules encoded both by the wild-type (wt) and mutant alleles exhibit altered properties characteristic of the prion protein associated with prion diseases (PrPSc). Using monospecific antisera, we found that wtPrP was insoluble in the brains of three patients who were heterozygous for the E200K mutation, whereas mutant PrP was both insoluble and protease-resistant. Our results argue that both wild-type and mutant PrP undergo conformational changes and are particularly intriguing, because the normal isoform PrPc is soluble in nondenaturing detergents and is readily digested by proteases, whereas PrPSc is insoluble and resistant to proteolytic digestion. Our findings indicate that insoluble wtPrP represents a conformational intermediate, the first to be identified, within a pathway in which PrPc is converted to PrPSc.

Prion isolate specified allotypic interactions between the cellular and scrapie prion proteins in congenic and transgenic mice

Different prion isolates, often referred to as "strains," present an enigma because considerable evidence argues that prions are devoid of nucleic acid. To investigate prion diversity, we inoculated three "strains" of prions into congenic and transgenic mice harboring variable numbers of two different alleles, designated a and b, of the prion protein (PrP) structural gene, Prn-p. The length of the incubation time was inversely related to the number of Prn-p(a) genes in mice inoculated with the Rocky Mountain Laboratory (RML) prion strain. Results with mice lacking this locus (Prn-p0/0) and transgenic mice argue that long incubation times are not a dominant trait as thought for many years, but rather they are due to reduced levels of the substrate PrPC-A (cellular isoform of PrP, allotype A) in (Prn-p(a) x Prn-pb)F1 mice. In contrast, the Prn-p(a) gene extended incubation times in mice inoculated with the 87V and 22A prion strains, whereas the Prn-pb gene was permissive. Experiments with the 87V isolate suggest that a genetic locus distinct from Prn-p controls deposition of the scrapie isoform of PrP (PrPSc) and attendant neuropathology. Each prion isolate produced distinguishable patterns of PrPSc accumulation in brain; of note, the patterns in Prn-p(a) and Prn-pb congenic mice inoculated with RML prions were more different than those in congenic Prn-pb mice with RML or 22A prions. Our results suggest that scrapie "strain-specific" incubation times can be explained by differences in the relative efficiency of allotypic interactions that lead to conversion of PrPC into PrPSc.

Release of the cellular prion protein from cultured cells after loss of its glycoinositol phospholipid anchor

Secreted forms of the sialoglycoprotein designated cellular prion protein (PrPC) have been identified that cannot be explained by alternative splicing. We report that secreted forms of PrPC derive from precursors that are bound to the plasma membrane by glycoinositol phospholipid (GPI) anchors. Secreted PrPC slowly appeared in the culture medium of metabolically radiolabelled cells after incubations of 8-24 h. Digestion of nascent PrPC with phosphatidylinositol-specific phospholipase C (PIPLC) prevented the appearance of secreted PrPC. Secreted PrPC partitioned into the aqueous phase of Triton X-114 like PrpC-released PrPC. While the M(r) of PIPLC-released PrPC was reduced 2-4 kDa after treatment with aqueous hydroflouric acid, which removes the entire GPI anchor modification, the M(r) of secreted PrPC was unchanged. Both PIPLC-released and secreted PrPC were recognized by antiserum raised against a synthetic C-terminal peptide corresponding to residues 220-233 (amino acid 231 is the site of GPI attachment). We conclude that GPI-anchored PrPC is post-translationally processed to remove most, if not all, of the GPI modification and then shed into culture medium. Whether PrPC is shed after proteolysis near the C-terminus remains to be established. A minority of PrPC in normal Syrian hamster brain partitioned into the aqueous phase of Triton X-114 like shed PrPC, suggesting physiological significance.