Cell-free formation of misfolded prion protein with authentic prion infectivity

PMCA screening of retropharyngeal lymph nodes in white-tailed deer and comparisons with ELISA and IHC

Chronic wasting disease (CWD) is a prion disease affecting cervids. CWD diagnosis is conducted through enzyme-linked immunosorbent assay (ELISA) and immunohistochemistry (IHC) in retropharyngeal lymph nodes. Unfortunately, these techniques have limited sensitivity against the biomarker (CWD-prions). Two in vitro prion amplification techniques, real-time quaking-induced conversion (RT-QuIC) and protein misfolding cyclic amplification (PMCA), have shown promise in detecting CWD-prions in tissues and bodily fluids. Recent studies have demonstrated that RT-QuIC yields similar results compared to ELISA and IHC. Here, we analyzed 1003 retropharyngeal lymph nodes (RPLNs) from Texas white-tailed deer. PMCA detected CWD at a higher rate compared to ELISA/IHC, identified different prion strains, and revealed the presence of CWD-prions in places with no previous history. These findings suggest that PMCA exhibits greater sensitivity than current standard techniques and could be valuable for rapid and strain-specific CWD detection.

RT-QuIC detection of CWD prion seeding activity in white-tailed deer muscle tissues

Chronic wasting disease (CWD) is a prion disease circulating in wild and farmed cervid populations throughout North America (United States and Canada), Europe (Finland, Norway, Sweden), and South Korea. CWD is a long-term threat to all cervid populations and to cervid hunting heritage, with the potential to cause substantial economic losses across multiple sectors. In North America, hunting and farming industries focused on the processing and consumption of white-tailed deer (WTD) venison are particularly vulnerable to CWD prion contamination, as millions of WTD are consumed annually. Real-time quaking-induced conversion (RT-QuIC) is a highly sensitive assay amplifying misfolded CWD prions in vitro and has facilitated CWD prion detection in a variety of tissues and excreta. To date, no study has comprehensively examined CWD prion content across bulk skeletal muscle tissues harvested from individual CWD infected WTD. Here, we use RT-QuIC to characterize prion-seeding activity in a variety of skeletal muscles from both wild and farmed CWD-positive WTD. We successfully detected CWD prions in muscles commonly used for consumption (e.g., backstrap, tenderloin, etc.) as well as within tongue and neck samples of WTD. Our results suggest that CWD prions are distributed across the skeletal muscles of infected WTD. We posit that RT-QuIC will be a useful tool for monitoring CWD prions in venison and that the method (with additional protocol optimization and high-throughput functionality) could be used to reduce and/or prevent CWD prions from entering animal and human food chains.

[Prion diseases or transmissible spongiform encephalopathies]

Prion diseases or transmissible spongiform encephalopathies (TSEs) are human and animal diseases naturally or experimentally transmissible with a long incubation period and a fatal course without remission. The nature of the transmissible agent remains debated but the absence of a structure evoking a conventional microorganism led Stanley B. Prusiner to hypothesize that it could be an infectious protein (proteinaceous infectious particle or prion). The prion would be the abnormal form of a normal protein, cellular PrP (PrP^c) which will change its spatial conformation and be converted into scrapie prion protein (PrP^sc) with properties of partial resistance to proteases, aggregation and insolubility in detergents. No inflammatory or immune response are detected in TSEs which are characterized by brain damage combining spongiosis, neuronal loss, astrocytic gliosis, and deposits of PrP^sc that may appear as amyloid plaques. Although the link between the accumulation of PrP^sc and the appearance of lesions remains debated, the presence of PrP^sc is constant during TSE and necessary for a definitive diagnosis. Even if they remain rare diseases (2 cases per million), the identification of kuru, at the end of the 1950s, of iatrogenic cases in the course of the 1970s and of the variant of Creutzfeldt-Jakob disease (CJD) in the mid-1990s explain the interest in these diseases but also the fears they can raise for public health. They remain an exciting research model because they belong both to the group of neurodegenerative diseases with protein accumulation (sporadic CJD), to the group of communicable diseases (iatrogenic CJD, variant of CJD) but also to the group of genetic diseases with a transmission Mendelian dominant (genetic CJD, Gerstmann-Straussler-Scheinker syndrome, fatal familial insomnia).

In Vitro Conversion Assays Diagnostic for Neurodegenerative Proteinopathies

BACKGROUND

In vitro conversion assays, including real-time quaking-induced conversion (RT-QuIC) and protein misfolding cyclic amplification (PMCA) techniques, were first developed to study the conversion process of the prion protein to its misfolded, disease-associated conformation. The intrinsic property of prion proteins to propagate their misfolded structure was later exploited to detect subfemtogram quantities of the misfolded protein present in tissues and fluids from humans and animals with transmissible spongiform encephalopathies. Currently, conversion assays are used clinically as sensitive and specific diagnostic tools for antemortem diagnosis of prion disease.

CONTENT

In vitro conversion assays are now being applied to the development of diagnostics for related neurodegenerative diseases, including detection of misfolded α-synuclein in Parkinson disease, misfolded amyloid-β in Alzheimer disease, and misfolded tau in Pick disease. Like the predicate prion protein in vitro conversion diagnostics, these assays exploit the ability of endogenously misfolded proteins to induce misfolding and aggregation of their natively folded counterpart in vitro. This property enables biomarker detection of the underlying protein pathology. Herein, we review RT-QuIC and PMCA for (a) prion-, (b) α-synuclein-, (c) amyloid-β-, and (d) tau-opathies.

SUMMARY

Although already in routine clinical use for the detection of transmissible spongiform encephalopathies, in vitro conversion assays for other neurodegenerative disorders require further development and evaluation of diagnostic performance before consideration for clinical implementation.

Correlation between Bioassay and Protein Misfolding Cyclic Amplification for Variant Creutzfeldt-Jakob Disease Decontamination Studies

Creutzfeldt-Jakob diseases are neurodegenerative disorders for which transmission linked to medical procedures have been reported in hundreds of patients. As prion diseases, they are characterized by an unusual resistance to conventional decontamination processes. Moreover, their large tissue distribution and the ability of prions to attach to many surfaces raised the risk of transmission in health care facilities. It is therefore of major importance that decontamination procedures applied to medical devices before their reprocessing are thoroughly validated for prion inactivation. We previously described an in vitro assay, which allowed us to classify accurately prion decontamination treatments according to their efficacy on variant Creutzfeldt-Jakob disease. The significance of this study is in demonstrating the concordance between previous in vitro results and infectivity studies in transgenic mice. Furthermore, commercial reagents currently used in hospitals were tested by both protocols, and we observed that most of them were ineffective on human prions.

To date, approximately 500 iatrogenic Creutzfeldt-Jakob disease cases have been reported worldwide, most of them resulting from cadaveric dura mater graft and from the administration of prion-contaminated human growth hormone. The unusual resistance of prions to decontamination processes, their large tissue distribution, and the uncertainty about the prevalence of variant Creutzfeldt-Jakob disease (vCJD) in the general population lead to specific recommendations regarding identification of tissue at risk and reprocessing of reusable medical devices, including the use of dedicated treatment for prion inactivation. We previously described an in vitro assay, called Surf-PMCA, which allowed us to classify prion decontamination treatments according to their efficacy on vCJD prions by monitoring residual seeding activity (RSA). Here, we used a transgenic mouse line permissive to vCJD prions to study the correlation between the RSA measured in vitro and the in vivo infectivity. Implantation in mouse brains of prion-contaminated steel wires subjected to different decontamination procedures allows us to demonstrate a good concordance between RSA measured by Surf-PMCA (in vitro) and residual infectivity (in vivo). These experiments emphasize the strength of the Surf-PMCA method as a rapid and sensitive assay for the evaluation of prion decontamination procedures and also confirm the lack of efficacy of several marketed reagents on vCJD prion decontamination.

IMPORTANCE Creutzfeldt-Jakob diseases are neurodegenerative disorders for which transmission linked to medical procedures have been reported in hundreds of patients. As prion diseases, they are characterized by an unusual resistance to conventional decontamination processes. Moreover, their large tissue distribution and the ability of prions to attach to many surfaces raised the risk of transmission in health care facilities. It is therefore of major importance that decontamination procedures applied to medical devices before their reprocessing are thoroughly validated for prion inactivation. We previously described an in vitro assay, which allowed us to classify accurately prion decontamination treatments according to their efficacy on variant Creutzfeldt-Jakob disease. The significance of this study is in demonstrating the concordance between previous in vitro results and infectivity studies in transgenic mice. Furthermore, commercial reagents currently used in hospitals were tested by both protocols, and we observed that most of them were ineffective on human prions.

In vitro Modeling of Prion Strain Tropism

Prions are atypical infectious agents lacking genetic material. Yet, various strains have been isolated from animals and humans using experimental models. They are distinguished by the resulting pattern of disease, including the localization of PrPsc deposits and the spongiform changes they induce in the brain of affected individuals. In this paper, we discuss the emerging use of cellular and acellular models to decipher the mechanisms involved in the strain-specific targeting of distinct brain regions. Recent studies suggest that neuronal cultures, protein misfolding cyclic amplification, and combination of both approaches may be useful to explore this under-investigated but central domain of the prion field.

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

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

Peroxymonosulfate Rapidly Inactivates the Disease-Associated Prion Protein

Prions, the etiological agents in transmissible spongiform encephalopathies, exhibit remarkable resistance to most methods of inactivation that are effective against conventional pathogens. Prions are composed of pathogenic conformers of the prion protein (PrP(TSE)). Some prion diseases are transmitted, in part, through environmental routes. The recalcitrance of prions to inactivation may lead to a persistent reservoir of infectivity that contributes to the environmental maintenance of epizootics. At present, few methods exist to remediate prion-contaminated land surfaces. Here we conducted a proof-of-principle study to examine the ability of peroxymonosulfate to degrade PrP(TSE). We find that peroxymonosulfate rapidly degrades PrP(TSE) from two species. Transition-metal-catalyzed decomposition of peroxymonosulfate to produce sulfate radicals appears to enhance degradation. We further demonstrate that exposure to peroxymonosulfate significantly reduced PrP(C) to PrP(TSE) converting ability as measured by protein misfolding cyclic amplification, used as a proxy for infectivity. Liquid chromatography-tandem mass spectrometry revealed that exposure to peroxymonosulfate results in oxidative modifications to methionine and tryptophan residues. This study indicates that peroxymonosulfate may hold promise for decontamination of prion-contaminated surfaces.

Shedding light on prion disease

Proteolytic processing regulates key processes in health and disease. The cellular prion protein (PrP(C)) is subject to at least 3 cleavage events, α-cleavage, β-cleavage and shedding. In contrast to α- and β-cleavage where there is an ongoing controversy on the identity of relevant proteases, the metalloprotease ADAM10 represents the only relevant PrP sheddase. Here we focus on the roles that ADAM10-mediated shedding of PrP(C) and its pathogenic isoform (PrP(Sc)) might play in regulating their physiological and pathogenic functions, respectively. As revealed by our recent study using conditional ADAM10 knockout mice (Altmeppen et al., 2015), shedding of PrP seems to be involved in key processes of prion diseases. These aspects and several open questions arising from them are discussed. Increased knowledge on this topic can shed new light on prion diseases and other neurodegenerative conditions as well.

[Is Parkinson's disease a prion disease?]

The accumulation of a specific protein in aggregated form is a common phenomenon in human neurodegenerative diseases. In Parkinson's disease, this protein is α-synuclein which is a neuronal protein of 143 amino acids. With a monomeric conformation in solution, it also has a natural capacity to aggregate into amyloid structures (dimers, oligomers, fibrils and Lewy bodies or neurites). It therefore fulfils the characteristics of a prion protein (different conformations, seeding and spreading). In vitro and in vivo experimental evidence in transgenic and wild animals indicates a prion-like propagation of Parkinson's disease. The sequential and predictive distribution of α-synuclein demonstrated by Braak et al. and its correlation with non-motor signs are consistent with the prion-like progression. Although the triggering factor causing the misfolding and aggregation of the target protein is unknown, Parkinson's disease is a highly relevant model for the study of these mechanisms and also to test specific treatments targeting the assemblies of α-synuclein and propagation from pre-motor phase of the disease. Despite this prion-like progression, there is currently no argument indicating a risk of human transmission of Parkinson's disease.

Increased infectivity of anchorless mouse scrapie prions in transgenic mice overexpressing human prion protein

Prion protein (PrP) is found in all mammals, mostly as a glycoprotein anchored to the plasma membrane by a C-terminal glycosylphosphatidylinositol (GPI) linkage. Following prion infection, host protease-sensitive prion protein (PrPsen or PrPC) is converted into an abnormal, disease-associated, protease-resistant form (PrPres). Biochemical characteristics, such as the PrP amino acid sequence, and posttranslational modifications, such as glycosylation and GPI anchoring, can affect the transmissibility of prions as well as the biochemical properties of the PrPres generated. Previous in vivo studies on the effects of GPI anchoring on prion infectivity have not examined cross-species transmission. In this study, we tested the effect of lack of GPI anchoring on a species barrier model using mice expressing human PrP. In this model, anchorless 22L prions derived from tg44 mice were more infectious than 22L prions derived from C57BL/10 mice when tested in tg66 transgenic mice, which expressed wild-type anchored human PrP at 8- to 16-fold above normal. Thus, the lack of the GPI anchor on the PrPres from tg44 mice appeared to reduce the effect of the mouse-human PrP species barrier. In contrast, neither source of prions induced disease in tgRM transgenic mice, which expressed human PrP at 2- to 4-fold above normal.

IMPORTANCE

Prion protein (PrP) is found in all mammals, usually attached to cells by an anchor molecule called GPI. Following prion infection, PrP is converted into a disease-associated form (PrPres). While most prion diseases are species specific, this finding is not consistent, and species barriers differ in strength. The amino acid sequence of PrP varies among species, and this variability affects prion species barriers. However, other PrP modifications, including glycosylation and GPI anchoring, may also influence cross-species infectivity. We studied the effect of PrP GPI anchoring using a mouse-to-human species barrier model. Experiments showed that prions produced by mice expressing only anchorless PrP were more infectious than prions produced in mice expressing anchored PrP. Thus, the lack of the GPI anchor on prions reduced the effect of the mouse-human species barrier. Our results suggest that prion diseases that produce higher levels of anchorless PrP may pose an increased risk for cross-species infection.

Protein misfolding cyclic amplification (PMCA): Current status and future directions

Transmissible spongiform encephalopathies (TSEs) most commonly known as prion diseases are invariably fatal neurological disorders that affect humans and animals. These disorders differ from other neurodegenerative conformational diseases caused by the accumulation in the brain of misfolded proteins, sometimes with amyloid properties, in their ability to infect susceptible species by various routes. While the infectious properties of amyloidogenic proteins, other than misfolded prion protein (PrP(TSE)), are currently under scrutiny, their potential to transmit from cell to cell, one of the intrinsic properties of the prion, has been recently shown in vitro and in vivo. Over the decades, various cell culture and laboratory animal models have been developed to study TSEs. These assays have been widely used in a variety of applications but showed to be time consuming and entailed elevated costs. Novel economic and fast alternatives became available with the development of in vitro assays that are based on the property of conformationally abnormal PrP(TSE) to recruit normal cellular PrP(C) to misfold. These include the cell-free conversion assay, protein misfolding cyclic amplification (PMCA) and quaking induced conversion assay (QuIC), of which the PMCA has been the only technology shown to generate infectious prions. Moreover, it allows indefinite amplification of PrP(TSE) with strain-specific biochemical and biological properties of the original molecules and under certain conditions may give rise to new spontaneously generated prions. The method also allows addressing the species barrier phenomena and assessing possible risks of animal-to-animal and animal-to-human transmission. Additionally, its unprecedented sensitivity has made possible the detection of as little as one infectious dose of PrP(TSE) and the biochemical identification of this protein in different tissues and biological fluids, including blood, cerebral spinal fluid (CSF), semen, milk, urine and saliva during the pre-clinical and clinical phases of the disease. The mechanistic similarities between TSEs and other conformational disorders have resulted in the adaptation of the PMCA to the amplification and detection of various amyloidogenic proteins. Here we provide a compelling discussion of the different applications of this technology to the study of TSEs and other neurodegenerative diseases.

Is there a risk of prion-like disease transmission by Alzheimer- or Parkinson-associated protein particles?

The misfolding and aggregation of endogenous proteins in the central nervous system is a neuropathological hallmark of Alzheimer's disease (AD), Parkinson's disease (PD), as well as prion diseases. A molecular mechanism referred to as "nucleation-dependent aggregation" is thought to underlie this neuropathological phenomenon. According to this concept, disease-associated protein particles act as nuclei, or seeds, that recruit cellular proteins and incorporate them, in a misfolded form, into their growing aggregate structure. Experimental studies have shown that the aggregation of the AD-associated proteins amyloid-β (Aβ) and tau, and of the PD-associated protein α-synuclein, can be stimulated in laboratory animal models by intracerebral (i.c.) injection of inocula containing aggregated species of the respective proteins. This has raised the question of whether AD or PD can be transmitted, like certain human prion diseases, between individuals by self-propagating protein particles potentially present on medical instruments or in blood or blood products. While the i.c. injection of inocula containing AD- or PD-associated protein aggregates was found to cause neuronal damage and clinical abnormalities (e.g., motor impairments) in some animal models, none of the studies published so far provided evidence for a transmission of severe or even fatal disease. In addition, available epidemiological data do not indicate a transmissibility of AD or PD between humans. The findings published so far on the effects of experimentally transmitted AD- or PD-associated protein seeds do not suggest specific precautionary measures in the context of hemotherapy, but call for vigilance in transfusion medicine and other medical areas.

Highly potent soluble amyloid-β seeds in human Alzheimer brain but not cerebrospinal fluid

The soluble fraction of brain samples from patients with Alzheimer's disease contains highly biologically active amyloid-β seeds. In this study, we sought to assess the potency of soluble amyloid-β seeds derived from the brain and cerebrospinal fluid. Soluble Alzheimer's disease brain extracts were serially diluted and then injected into the hippocampus of young, APP transgenic mice. Eight months later, seeded amyloid-β deposition was evident even when the hippocampus received subattomole amounts of brain-derived amyloid-β. In contrast, cerebrospinal fluid from patients with Alzheimer's disease, which contained more than 10-fold higher levels of amyloid-β peptide than the most concentrated soluble brain extracts, did not induce detectable seeding activity in vivo. Similarly, cerebrospinal fluid from aged APP-transgenic donor mice failed to induce cerebral amyloid-β deposition. In comparison to the soluble brain fraction, cerebrospinal fluid largely lacked N-terminally truncated amyloid-β species and exhibited smaller amyloid-β-positive particles, features that may contribute to the lack of in vivo seeding by cerebrospinal fluid. Interestingly, the same cerebrospinal fluid showed at least some seeding activity in an in vitro assay. The present results indicate that the biological seeding activity of soluble amyloid-β species is orders of magnitude greater in brain extracts than in the cerebrospinal fluid.

Highly infectious prions generated by a single round of microplate-based protein misfolding cyclic amplification

Measurements of the presence of prions in biological tissues or fluids rely more and more on cell-free assays. Although protein misfolding cyclic amplification (PMCA) has emerged as a valuable, sensitive tool, it is currently hampered by its lack of robustness and rapidity for high-throughput purposes. Here, we made a number of improvements making it possible to amplify the maximum levels of scrapie prions in a single 48-h round and in a microplate format. The amplification rates and the infectious titer of the PMCA-formed prions appeared similar to those derived from the in vivo laboratory bioassays. This enhanced technique also amplified efficiently prions from different species, including those responsible for human variant Creutzfeldt-Jakob disease. This new format should help in developing ultrasensitive, high-throughput prion assays for cognitive, diagnostic, and therapeutic applications. IMPORTANCE The method developed here allows large-scale, fast, and reliable cell-free amplification of subinfectious levels of prions from different species. The sensitivity and rapidity achieved approach or equal those of other recently developed prion-seeded conversion assays. Our simplified assay may be amenable to high-throughput, automated purposes and serve in a complementary manner with other recently developed assays for urgently needed antemortem diagnostic tests, by using bodily fluids containing small amounts of prion infectivity. Such a combination of assays is of paramount importance to reduce the transfusion risk in the human population and to identify asymptomatic carriers of variant Creutzfeldt-Jakob disease.

Infrared microspectroscopy detects protein misfolding cyclic amplification (PMCA)-induced conformational alterations in hamster scrapie progeny seeds

The self-replicative conformation of misfolded prion proteins (PrP) is considered a major determinant for the seeding activity, infectiousness, and strain characteristics of prions in different host species. Prion-associated seeding activity, which converts cellular prion protein (PrP(C)) into Proteinase K-resistant, infectious PrP particles (PrP(TSE)), can be monitored in vitro by protein misfolding cyclic amplification (PMCA). Thus, PMCA has been established as a valuable analytical tool in prion research. Currently, however, it is under discussion whether prion strain characteristics are preserved during PMCA when parent seeds are amplified in PrP(C) substrate from the identical host species. Here, we report on the comparative structural analysis of parent and progeny (PMCA-derived) PrP seeds by an improved approach of sensitive infrared microspectroscopy. Infrared microspectroscopy revealed that PMCA of native hamster 263K scrapie seeds in hamster PrP(C) substrate caused conformational alterations in progeny seeds that were accompanied by an altered resistance to Proteinase K, higher sedimentation velocities in gradient ultracentrifugations, and a longer incubation time in animal bioassays. When these progeny seeds were propagated in hamsters, misfolded PrP from brain extracts of these animals showed mixed spectroscopic and biochemical properties from both parental and progeny seeds. Thus, strain modifications of 263K prions induced by PMCA seem to have been partially reversed when PMCA products were reinoculated into the original host species.

A specific population of abnormal prion protein aggregates is preferentially taken up by cells and disaggregated in a strain-dependent manner

Prion diseases are characterized by the conversion of the soluble protease-sensitive host-encoded prion protein (PrP(C)) into its aggregated, protease-resistant, and infectious isoform (PrP(Sc)). One of the earliest events occurring in cells following exposure to an exogenous source of prions is the cellular uptake of PrP(Sc). It is unclear how the biochemical properties of PrP(Sc) influence its uptake, although aggregate size is thought to be important. Here we show that for two different strains of mouse prions, one that infects cells (22L) and one that does not (87V), a fraction of PrP(Sc) associated with distinct sedimentation properties is preferentially taken up by the cells. However, while the fraction of PrP(Sc) and the kinetics of uptake were similar for both strains, PrP(Sc) derived from the 87V strain was disaggregated more rapidly than that derived from 22L. The increased rate of PrP(Sc) disaggregation did not correlate with either the conformational or aggregate stability of 87V PrP(Sc), both of which were greater than those of 22L PrP(Sc). Our data suggest that the kinetics of disaggregation of PrP(Sc) following cellular uptake is independent of PrP(Sc) stability but may be dependent upon some component of the PrP(Sc) aggregate other than PrP. Rapid disaggregation of 87V PrP(Sc) by the cell may contribute, at least in part, to the inability of 87V to infect cells in vitro.

Recombinant prion protein refolded with lipid and RNA has the biochemical hallmarks of a prion but lacks in vivo infectivity

During prion infection, the normal, protease-sensitive conformation of prion protein (PrP^C) is converted via seeded polymerization to an abnormal, infectious conformation with greatly increased protease-resistance (PrP^Sc). In vitro, protein misfolding cyclic amplification (PMCA) uses PrP^Sc in prion-infected brain homogenates as an initiating seed to convert PrP^C and trigger the self-propagation of PrP^Sc over many cycles of amplification. While PMCA reactions produce high levels of protease-resistant PrP, the infectious titer is often lower than that of brain-derived PrP^Sc. More recently, PMCA techniques using bacterially derived recombinant PrP (rPrP) in the presence of lipid and RNA but in the absence of any starting PrP^Sc seed have been used to generate infectious prions that cause disease in wild-type mice with relatively short incubation times. These data suggest that lipid and/or RNA act as cofactors to facilitate the de novo formation of high levels of prion infectivity. Using rPrP purified by two different techniques, we generated a self-propagating protease-resistant rPrP molecule that, regardless of the amount of RNA and lipid used, had a molecular mass, protease resistance and insolubility similar to that of PrP^Sc. However, we were unable to detect prion infectivity in any of our reactions using either cell-culture or animal bioassays. These results demonstrate that the ability to self-propagate into a protease-resistant insoluble conformer is not unique to infectious PrP molecules. They suggest that the presence of RNA and lipid cofactors may facilitate the spontaneous refolding of PrP into an infectious form while also allowing the de novo formation of self-propagating, but non-infectious, rPrP-res.

Analyses of protease resistance and aggregation state of abnormal prion protein across the spectrum of human prions

Prion diseases are characterized by tissue accumulation of a misfolded, β-sheet-enriched isoform (scrapie prion protein (PrP(Sc))) of the cellular prion protein (PrP(C)). At variance with PrP(C), PrP(Sc) shows a partial resistance to protease digestion and forms highly aggregated and detergent-insoluble polymers, two properties that have been consistently used to distinguish the two proteins. In recent years, however, the idea that PrP(Sc) itself comprises heterogeneous species has grown. Most importantly, a putative proteinase K (PK)-sensitive form of PrP(Sc) (sPrP(Sc)) is being increasingly investigated for its possible role in prion infectivity, neurotoxicity, and strain variability. The study of sPrP(Sc), however, remains technically challenging because of the need of separating it from PrP(C) without using proteases. In this study, we have systematically analyzed both PK resistance and the aggregation state of purified PrP(Sc) across the whole spectrum of the currently characterized human prion strains. The results show that PrP(Sc) isolates manifest significant strain-specific differences in their PK digestion profile that are only partially explained by differences in the size of aggregates, suggesting that other factors, likely acting on PrP(Sc) aggregate stability, determine its resistance to proteolysis. Fully protease-sensitive low molecular weight aggregates were detected in all isolates but in a limited proportion of the overall PrP(Sc) (i.e. <10%), arguing against a significant role of slowly sedimenting PK-sensitive PrP(Sc) in the biogenesis of prion strains. Finally, we highlight the limitations of current operational definitions of sPrP(Sc) and of the quantitative analytical measurements that are not based on the isolation of a fully PK-sensitive PrP(Sc) form.

Anle138b: a novel oligomer modulator for disease-modifying therapy of neurodegenerative diseases such as prion and Parkinson's disease

In neurodegenerative diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD) and prion diseases, deposits of aggregated disease-specific proteins are found. Oligomeric aggregates are presumed to be the key neurotoxic agent. Here we describe the novel oligomer modulator anle138b [3-(1,3-benzodioxol-5-yl)-5-(3-bromophenyl)-1H-pyrazole], an aggregation inhibitor we developed based on a systematic high-throughput screening campaign combined with medicinal chemistry optimization. In vitro, anle138b blocked the formation of pathological aggregates of prion protein (PrP^Sc) and of α-synuclein (α-syn), which is deposited in PD and other synucleinopathies such as dementia with Lewy bodies (DLB) and multiple system atrophy (MSA). Notably, anle138b strongly inhibited all prion strains tested including BSE-derived and human prions. Anle138b showed structure-dependent binding to pathological aggregates and strongly inhibited formation of pathological oligomers in vitro and in vivo both for prion protein and α-synuclein. Both in mouse models of prion disease and in three different PD mouse models, anle138b strongly inhibited oligomer accumulation, neuronal degeneration, and disease progression in vivo. Anle138b had no detectable toxicity at therapeutic doses and an excellent oral bioavailability and blood–brain-barrier penetration. Our findings indicate that oligomer modulators provide a new approach for disease-modifying therapy in these diseases, for which only symptomatic treatment is available so far. Moreover, our findings suggest that pathological oligomers in neurodegenerative diseases share structural features, although the main protein component is disease-specific, indicating that compounds such as anle138b that modulate oligomer formation by targeting structure-dependent epitopes can have a broad spectrum of activity in the treatment of different protein aggregation diseases.

Modelling neurodegeneration in prion disease - applications for drug development

Prion diseases are a group of neurodegenerative diseases that affect mammals, including humans and ruminants such as sheep. They are believed to be caused by the conversion of the prion protein (PrP), a host expressed protein, into a toxic form (PrP(sc)). PrP(sc) accumulates in the brain, resulting in neuronal loss and the typical spongiform appearance of the brain. So far, there are no effective therapies available for prion diseases. This review discusses possible therapies for prion diseases and the models available for advancing research into the disease.

Harnessing prions as test agents for the development of broad-range disinfectants

The development of disinfectants with broad-range efficacy against bacteria, viruses, fungi, protozoa and prions constitutes an ongoing challenge. Prions, the causative agents of transmissible spongiform encephalopathies (TSEs) such as Creutzfeldt-Jakob disease (CJD) or its variant (vCJD) rank among the pathogens with the highest resistance to disinfection. Pilot studies have shown that procedures devised for prion disinfection were also highly effective against microbial pathogens. This fueled the idea to systematically exploit prions as test pathogens for the identification of new potential broad-range disinfectants. Prions essentially consist of misfolded, aggregated prion protein (PrP) and putatively replicate by nucleation-dependent, or seeded PrP polymerization. Recently, we have been able to establish PrP seeding activity as a quantitative in vitro indicator for the disinfection of 263K scrapie prions on steel wires used as surrogates for medical instruments. The seeding activity on wires re-processed in different disinfectants could be (1) biochemically determined by quantitative protein misfolding cyclic amplification (qPMCA), (2) biologically detected after qPMCA in a cell assay and (3) correctly translated into residual titres of scrapie infectivity. Our approach will substantially facilitate the identification of disinfectants with efficacy against prions as promising candidates for a further microbiological validation of broad-range activity.

Prion protein aggregation and fibrillogenesis in vitro

This chapter focuses on the structural conversion of natural and recombinant prion proteins in vitro. They key event in prion diseases is the conversion of the cellular prion protein (PrP(C)) into its disease causing isoform PrP(Sc). This conversion is represented by a conformational change from an β-helical dominated isoform into the mostly β-sheeted PrP(Sc). Represented is an overview of in vitro conversion systems that result in β-structured recombinant prion proteins including the current achievements in the generation of synthetic mammalian prions as proof of the protein-only hypothesis. In addition to the conversion of recombinant PrP the chapter features a summary of the protein misfolding cyclic amplification (PMCA) technique which has gained enormous popularity in prion research. Given is a general overview about the technique itself and the broad spectrum of utilization as detection method for prions. The spontaneous generation of prions by the protein misfolding amplification (PMCA) are also discussed.

Lower specific infectivity of protease-resistant prion protein generated in cell-free reactions

Prions are unconventional infectious agents that cause transmissible spongiform encephalopathy (TSE) diseases, or prion diseases. The biochemical nature of the prion infectious agent remains unclear. Previously, using a protein misfolding cyclic amplification (PMCA) reaction, infectivity and disease-associated protease-resistant prion protein (PrPres) were both generated under cell-free conditions, which supported a nonviral hypothesis for the agent. However, these studies lacked comparative quantitation of both infectivity titers and PrPres, which is important both for biological comparison with in vivo-derived infectivity and for excluding contamination to explain the results. Here during four to eight rounds of PMCA, end-point dilution titrations detected a >320-fold increase in infectivity versus that in controls. These results provide strong support for the hypothesis that the agent of prion infectivity is not a virus. PMCA-generated samples caused the same clinical disease and neuropathology with the same rapid incubation period as the input brain-derived scrapie samples, providing no evidence for generation of a new strain in PMCA. However, the ratio of the infectivity titer to the amount of PrPres (specific infectivity) was much lower in PMCA versus brain-derived samples, suggesting the possibility that a substantial portion of PrPres generated in PMCA might be noninfectious.

From high-throughput cell culture screening to mouse model: identification of new inhibitor classes against prion disease

Transmissible spongiform encephalopathies (TSE) or prion diseases belong to a category of fatal and so far untreatable neurodegenerative conditions. All prion diseases are characterized by both degeneration in the central nervous system (CNS) in humans and animals and the deposition and accumulation of Proteinase K-resistant prion protein (PrP(res)). Until now, no pharmaceutical product has been available to cure these diseases or to alleviate their associated symptoms. Here, a cell-culture screening system is described that allows for the large-scale analysis of the PrP(res) inhibitory potential of a library of compounds and the identification of structural motifs leading potent compounds able to cause PrP(res) clearance at the cellular level. Based on different scrapie-infected cell lines, 10,000 substances were tested, out of which 530 potential inhibitors were identified. After re-screening and validation using a series of dilutions, 14 compounds were identified as the most effective. These 14 compounds were then used for therapeutic studies in a mouse bioassay to test and verify their in vivo potency. Two compounds exhibited therapeutic potential in the mouse model by significantly extending the survival time of intracerebrally infected mice, when treated 90 days after infection with scrapie.

Quantitative detection and biological propagation of scrapie seeding activity in vitro facilitate use of prions as model pathogens for disinfection

Prions are pathogens with an unusually high tolerance to inactivation and constitute a complex challenge to the re-processing of surgical instruments. On the other hand, however, they provide an informative paradigm which has been exploited successfully for the development of novel broad-range disinfectants simultaneously active also against bacteria, viruses and fungi. Here we report on the development of a methodological platform that further facilitates the use of scrapie prions as model pathogens for disinfection. We used specifically adapted serial protein misfolding cyclic amplification (PMCA) for the quantitative detection, on steel wires providing model carriers for decontamination, of 263K scrapie seeding activity converting normal protease-sensitive into abnormal protease-resistant prion protein. Reference steel wires carrying defined amounts of scrapie infectivity were used for assay calibration, while scrapie-contaminated test steel wires were subjected to fifteen different procedures for disinfection that yielded scrapie titre reductions of ≤10(1)- to ≥10(5.5)-fold. As confirmed by titration in hamsters the residual scrapie infectivity on test wires could be reliably deduced for all examined disinfection procedures, from our quantitative seeding activity assay. Furthermore, we found that scrapie seeding activity present in 263K hamster brain homogenate or multiplied by PMCA of scrapie-contaminated steel wires both triggered accumulation of protease-resistant prion protein and was further propagated in a novel cell assay for 263K scrapie prions, i.e., cerebral glial cell cultures from hamsters. The findings from our PMCA- and glial cell culture assays revealed scrapie seeding activity as a biochemically and biologically replicative principle in vitro, with the former being quantitatively linked to prion infectivity detected on steel wires in vivo. When combined, our in vitro assays provide an alternative to titrations of biological scrapie infectivity in animals that substantially facilitates the use of prions as potentially highly indicative test agents in the search for novel broad-range disinfectants.

Presence and seeding activity of pathological prion protein (PrP(TSE)) in skeletal muscles of white-tailed deer infected with chronic wasting disease

Chronic wasting disease (CWD) is a contagious, rapidly spreading transmissible spongiform encephalopathy (TSE), or prion disease, occurring in cervids such as white tailed-deer (WTD), mule deer or elk in North America. Despite efficient horizontal transmission of CWD among cervids natural transmission of the disease to other species has not yet been observed. Here, we report for the first time a direct biochemical demonstration of pathological prion protein PrP^TSE and of PrP^TSE-associated seeding activity, the static and dynamic biochemical markers for biological prion infectivity, respectively, in skeletal muscles of CWD-infected cervids, i. e. WTD for which no clinical signs of CWD had been recognized. The presence of PrP^TSE was detected by Western- and postfixed frozen tissue blotting, while the seeding activity of PrP^TSE was revealed by protein misfolding cyclic amplification (PMCA). Semi-quantitative Western blotting indicated that the concentration of PrP^TSE in skeletal muscles of CWD-infected WTD was approximately 2000-10000 -fold lower than in brain tissue. Tissue-blot-analyses revealed that PrP^TSE was located in muscle-associated nerve fascicles but not, in detectable amounts, in myocytes. The presence and seeding activity of PrP^TSE in skeletal muscle from CWD-infected cervids suggests prevention of such tissue in the human diet as a precautionary measure for food safety, pending on further clarification of whether CWD may be transmissible to humans.

Prion seeded conversion and amplification assays

The conversion of the normal prion protein (PrP(C)) into its misfolded, aggregation-prone and infectious (prion) isoform is central to the progression of transmissible spongiform encephalopathies (TSEs) or prion diseases. Since the initial development of a cell free PrP conversion reaction, striking progress has been made in the development of much more continuous prion-induced conversion and amplification reactions. These studies have provided major insights into the molecular underpinnings of prion propagation and enabled the development of ultra-sensitive tests for prions and prion disease diagnosis. This chapter will provide an overview of such reactions and the practical and fundamental consequences of their development.

Detection of RNA in the Plasma of Patients with Sporadic Creutzfeldt–Jakob Disease, Gerstmann–Straüssler Syndrome and Other Non-Transmissible Spongiform Encephalopathy Brain Disorders

The infectious agent of transmissible spongiform encephalopathy (TSE) was assumed to be the aggregate of abnormal prion protein isoform (PrPsc). We observed that lowering the pH of 3% SDS-inoculated plasma or brain homogenate after PK digestion to 4.5 (acidic SDS condition) enabled to precipitate proteinase K-resistant prion protein (PrPres) in plasma as well as PrPres in the brain with synthetic poly-A RNA as affinity aggregate. Therefore, we determined if RNA molecules could be used for discriminating TSE patients from healthy individuals. We also examined the plasma of patients with classical Creutzfeldt–Jakob disease (CJD) and other brain disorders who were not diagnosed with TSE. The results indicated that RNA approximately 1.5–2.0 kb in length was commonly observed in the plasma of patients with brain disorders but was not detected in the plasma of healthy volunteers. Enhanced expression of RNA and its protection from endogenous nucleases might occur in the former group of patients. Moreover, we speculate that the non-transmissible neuronal disorders overlap with prion diseases.

Getting a grip on prions: oligomers, amyloids, and pathological membrane interactions

The prion (infectious protein) concept has evolved with the discovery of new self-propagating protein states in organisms as diverse as mammals and fungi. The infectious agent of the mammalian transmissible spongiform encephalopathies (TSE) has long been considered the prototypical prion, and recent cell-free propagation and biophysical analyses of TSE infectivity have now firmly established its prion credentials. Other disease-associated protein aggregates, such as some amyloids, can also have prion-like characteristics under certain experimental conditions. However, most amyloids appear to lack the natural transmissibility of TSE prions. One feature that distinguishes the latter from the former is the glycophosphatidylinositol membrane anchor on prion protein, the molecule that is corrupted in TSE diseases. The presence of this anchor profoundly affects TSE pathogenesis, which involves major membrane distortions in the brain, and may be a key reason for the greater neurovirulence of TSE prions relative to many other autocatalytic protein aggregates.

Differential display detects host nucleic acid motifs altered in scrapie-infected brain

The transmissible spongiform encephalopathies (TSEs) including scrapie have been attributed to an infectious protein or prion. Infectivity is allied to conversion of the endogenous nucleic-acid-binding protein PrP to an infectious modified form known as PrP(sc). The protein-only theory does not easily explain the enigmatic properties of the agent including strain variation. It was previously suggested that a short nucleic acid, perhaps host-encoded, might contribute to the pathoetiology of the TSEs. No candidate host molecules that might explain transmission of strain differences have yet been put forward. Differential display is a robust technique for detecting nucleic acid differences between two populations. We applied this technique to total nucleic acid preparations from scrapie-infected and control brain. Independent RNA preparations from eight normal and eight scrapie-infected (strain 263K) hamster brains were randomly amplified and visualized in parallel. Though the nucleic acid patterns were generally identical in scrapie-infected versus control brain, some rare bands were differentially displayed. Molecular species consistently overrepresented (or underrepresented) in all eight infected brain samples versus all eight controls were excised from the display, sequenced, and assembled into contigs. Only seven ros contigs (RNAs over- or underrepresented in scrapie) emerged, representing <4 kb from the transcriptome. All contained highly stable regions of secondary structure. The most abundant scrapie-only ros sequence was homologous to a repetitive transposable element (LINE; long interspersed nuclear element). Other ros sequences identified cellular RNA 7SL, clathrin heavy chain, visinin-like protein-1, and three highly specific subregions of ribosomal RNA (ros1-3). The ribosomal ros sequences accurately corresponded to LINE; retrotransposon insertion sites in ribosomal DNA (p<0.01). These differential motifs implicate specific host RNAs in the pathoetiology of the TSEs.

Self-propagating beta-sheet polypeptide structures as prebiotic informational molecular entities: the amyloid world

The idea is advanced that under the extreme earth conditions for ~3.9 billions years ago, protein-based beta-sheet molecular structures were the first self-propagating and information-processing biomolecules that evolved. The amyloid structure of these aggregates provided an effective protection against the harsh conditions known to decompose both polyribonucleotides and natively folded polypeptides. In the prebiotic amyloid world, both the replicative and informational functions were carried out by structurally stable beta-sheet protein aggregates in a prion-like mode involving templated self-propagation and storage of information in the beta-sheet conformation. In this amyloid (protein)-first, hybrid replication-metabolism view, the synthesis of RNA, and the evolvement of an RNA-protein world, were later, but necessary events for further biomolecular evolution to occur. I further argue that in our contemporary DNA<-->RNA-->protein world, the primordial beta-conformation-based information system is preserved in the form of a cytoplasmic epigenetic memory.

PrP(Sc) of scrapie 263K propagates efficiently in spleen and muscle tissues with protein misfolding cyclic amplification

Transmissible spongiform encephalopathies (TSEs), or prion diseases, are transmissible neurodegenerative disorders of protein conformation. This group of diseases is caused by infectious agents, termed prions, which can convert normal conformation (PrP(C)) into misfolded protein (PrP(Sc)). The infectivity of non-neuronal tissues has been wildly addressed, but the propagating features and the biochemical properties of prion generated from these tissues are only partially settled. In this study, utilizing protein misfolding cyclic amplification (PMCA), the in vitro conversion of PrP(C) into PrP(Sc) in spleen and muscle tissues can be induced by PrP(Sc) produced in vivo. The further propagation of newly formed PrP(Sc) in normal brain and some of the biochemical properties of new PrP(Sc) are similar as the brain-derived prions, implying the naturally infectious pathway of prion from peripheral generation to neuro-invasion. However, compared with the brain-derived PrP(Sc), the weaker resistance of new PrP(Sc) to some inactivated agents, i.e. sodium hydroxide and thermal inactivation, are observed. Our data provide the reliable evidence that the brain-derived PrP(Sc) can utilize the PrP(C) from non-neuronal tissues for its propagation. Similarity of the replicative ability in PMCA in vitro and the infectivity in vivo highlights the possibility to use PMCA instead of bioassay to investigate the propagation of prion.

Chemical and biophysical insights into the propagation of prion strains

Transmissible spongiform encephalopathies (TSEs) are lethal infectious neurodegenerative diseases. TSEs are caused by prions, infectious agents lacking informational nucleic acids, and possibly identical with higher-order aggregates of the cellular glycolipoprotein PrP(C). Prion strains are derived from TSE isolates that, even after inoculation into genetically identical hosts, cause disease with distinct patterns of protein aggregate deposition, incubation times, morphology of the characteristic brain damage, and cellular tropism. Most of these traits are relatively stable across serial passages. Here we review current techniques for studying prion strain differences in vivo and in cells, and discuss the strain phenomena in the general context of the knowledge gained from modeling prion fibril growth in vitro and in simple organisms.

Accelerated high fidelity prion amplification within and across prion species barriers

Experimental obstacles have impeded our ability to study prion transmission within and, more particularly, between species. Here, we used cervid prion protein expressed in brain extracts of transgenic mice, referred to as Tg(CerPrP), as a substrate for in vitro generation of chronic wasting disease (CWD) prions by protein misfolding cyclic amplification (PMCA). Characterization of this infectivity in Tg(CerPrP) mice demonstrated that serial PMCA resulted in the high fidelity amplification of CWD prions with apparently unaltered properties. Using similar methods to amplify mouse RML prions and characterize the resulting novel cervid prions, we show that serial PMCA abrogated a transmission barrier that required several hundred days of adaptation and subsequent stabilization in Tg(CerPrP) mice. While both approaches produced cervid prions with characteristics distinct from CWD, the subtly different properties of the resulting individual prion isolates indicated that adaptation of mouse RML prions generated multiple strains following inter-species transmission. Our studies demonstrate that combined transgenic mouse and PMCA approaches not only expedite intra- and inter-species prion transmission, but also provide a facile means of generating and characterizing novel prion strains.

Differential effects of prion particle size on infectivity in vivo and in vitro

The conversion of cellular prion protein to the disease-associated isoform (PrP(Sc)) has been suggested to follow a mechanism of seeded aggregation. Here, we show that fragmentation of PrP(Sc) aggregates by sonication increases converting activity in cell culture in a way similar to in vitro conversion assays. In contrast, under the same conditions the infectious titer of sonicated samples in vivo was reduced. We modified the size distribution of PrP(Sc) by adsorption to nitrocellulose, which resulted in a reduction of the infectious titer in non-sonicated samples and an increase in sonicated samples. Our results indicate that NC-adsorption can (i) block some active sites of PrP(Sc) aggregates and (ii) reduce the rate of clearance from the brain. For large particles with low clearance the effect of NC-particles on the number of available active sites may dominate, whereas for smaller particles (i.e. sonicated samples) the effect of NC-adsorption on clearance dominates resulting in increased infectivity.

Branched chain mechanism of polymerization and ultrastructure of prion protein amyloid fibrils

The discovery of prion disease and the establishment of the protein only hypothesis of prion propagation raised substantial interest in the class of maladies referred to as conformational diseases. Although significant progress has been made in elucidating the mechanisms of polymerization for several amyloidogenic proteins and peptides linked to conformational disorders and solving their fibrillar 3D structures, studies of prion protein amyloid fibrils and their polymerization mechanism have proven to be very difficult. The present minireview introduces the mechanism of branched-chain reaction for describing the peculiar kinetics of prion polymerization and summarizes our current knowledge about the substructure of prion protein amyloid fibrils.

Vaccination with prion peptide-displaying papillomavirus-like particles induces autoantibodies to normal prion protein that interfere with pathologic prion protein production in infected cells

Prion diseases are fatal neurodegenerative disorders caused by proteinaceous infectious pathogens termed prions (PrP(Sc)). To date, there is no prophylaxis or therapy available for these transmissible encephalopathies. Passive immunization with monclonal antibodies recognizing the normal host-encoded prion protein (PrP(C)) has been reported to abolish PrP(Sc) infectivity and to delay onset of disease. Because of established immunologic tolerance against the widely expressed PrP(C), active immunization appears to be difficult to achieve. To overcome this limitation, papillomavirus-like particles were generated that display a nine amino acid B-cell epitope, DWEDRYYRE, of the murine/rat prion protein in an immunogenic capsid surface loop, by insertion into the L1 major capsid protein of bovine papillomavirus type 1. The PrP peptide was selected on the basis of its previously suggested central role in prion pathogenesis. Immunization with PrP-virus-like particles induced high-titer antibodies to PrP in rabbit and in rat, without inducing overt adverse effects. As determined by peptide-specific ELISA, rabbit immune sera recognized the inserted murine/rat epitope and also cross-reacted with the homologous rabbit/human epitope differing in one amino acid residue. In contrast, rat immune sera recognized the murine/rat peptide only. Sera of both species reacted with PrP(C) in its native conformation in mouse brain and on rat pheochromocytoma cells, as determined by immunoprecipitation and fluorescence-activated cell sorting analysis. Importantly, rabbit anti-PrP serum contained high-affinity antibody that inhibited de novo synthesis of PrP(Sc) in prion-infected cells. If also effective in vivo, PrP-virus-like particle vaccination opens a unique possibility for immunologic prevention of currently fatal and incurable prion-mediated diseases.

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.

In vitro amplification and detection of variant Creutzfeldt–Jakob disease PrPSc

Variant Creutzfeldt-Jakob disease (vCJD) poses a serious risk of secondary transmission and the need to detect infectivity in asymptomatic individuals is therefore of major importance. Following infection, it is assumed that minute amounts of disease-associated prion protein (PrP(Sc)) replicate by conversion of the host cellular prion protein (PrP(C)). Therefore, methods of rapidly reproducing this conversion process in vitro would be valuable tools in the development of such tests. We show that one such technique, protein misfolding cyclic amplification (PMCA), can amplify vCJD PrP(Sc) from human brain tissue, and that the degree of amplification is dependent upon the substrate PRNP codon 129 polymorphism. Both human platelets and transgenic mouse brain are shown to be suitable alternative substrate sources, and amplified PrP(Sc) can be detected using a conformation-dependent immunoassay (CDI), allowing the detection of putative proteinase K sensitive forms of PrP(Sc).

The use of conformation-specific ligands and assays to dissect the molecular mechanisms of neurodegenerative diseases

The use of conformation-specific ligands has been closely linked to progress in the molecular characterization of neurodegenerative diseases. Deposition of misfolded or misprocessed proteins is now recognized as a hallmark of all neurodegenerative diseases. Initially, dyes like Congo red and thioflavin T were used as crudely conformation-specific ligands for staining the beta-sheeted protein components of amyloid deposits in neurodegenerative diseases such as Alzheimer disease (AD) and prion disease, the two diseases in which protein conformations were distinguished early on. This conformational characterization of extracellular protein deposits with dyes ultimately led to the identification of key players in the disease processes. The recent discovery of intermediate conformational species, i.e., soluble oligomers for AD and PK-sensitive PrP(Sc) for prion disease, whose conformation and assembly are thought to be distinct from both the physiological and the fibrillar conformational states, replaced the former notion that the microscopic protein deposits themselves caused disease. This insight and the generation of conformation-specific monoclonal antibodies to these conformers further advanced diagnosis and the understanding of molecular mechanisms of AD and are likely to do so in other neurodegenerative diseases. Here we review how conformer distinction performed by a variety of different techniques, including biophysical, biochemical, and antibody-based methods, led to the current molecular concepts of AD and the prion diseases. We provide an outlook on the application of these techniques in advancing the understanding of molecular mechanisms of other neurodegenerative diseases or degenerative brain conditions.