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This 96-well-plate ‘real-time quaking-induced conversion’ assay allows the detection of abnormal prion protein in human brain and CSF samples. It can be applied to study many protein misfolding diseases, as well as for drug screening and prion strain discrimination. The development and adaption of in vitro misfolded protein amplification systems has been a major innovation in the detection of abnormally folded prion protein scrapie (PrP Sc ) in human brain and cerebrospinal fluid (CSF) samples. Herein, we describe a fast and efficient protein amplification technique, real-time quaking-induced conversion (RT-QuIC), for the detection of a PrP Sc seed in human brain and CSF. In contrast to other in vitro misfolded protein amplification assays—such as protein misfolding cyclic amplification (PMCA)—which are based on sonication, the RT-QuIC technique is based on prion seed–induced misfolding and aggregation of recombinant prion protein substrate, accelerated by alternating cycles of shaking and rest in fluorescence plate readers. A single RT-QuIC assay typically analyzes up to 32 samples in triplicate, using a 96-well-plate format. From sample preparation to analysis of results, the protocol takes ∼87 h to complete. In addition to diagnostics, this technique has substantial generic analytical applications, including drug screening, prion strain discrimination, biohazard screening (e.g., to reduce transmission risk related to prion diseases) and the study of protein misfolding; in addition, it can potentially be used for the investigation of other protein misfolding diseases such as Alzheimer's and Parkinson's disease.

Prions are infectious proteins composed of the abnormal disease-causing isoform PrPSc, which induces conformational conversion of the host-encoded normal cellular prion protein PrPC to additional PrPSc. The mechanism underlying prion strain mutation in the absence of nucleic acids remains unresolved. Additionally, the frequency of strains causing chronic wasting disease (CWD), a burgeoning prion epidemic of cervids, is unknown. Using susceptible transgenic mice, we identified two prevalent CWD strains with divergent biological properties but composed of PrPSc with indistinguishable biochemical characteristics. Although CWD transmissions indicated stable, independent strain propagation by elk PrPC, strain coexistence in the brains of deer and transgenic mice demonstrated unstable strain propagation by deer PrPC. The primary structures of deer and elk prion proteins differ at residue 226, which, in concert with PrPSc conformational compatibility, determines prion strain mutation in these cervids.

Key Points Transmissible spongiform encephalopathies (TSEs) are neurodegenerative diseases of humans and many animal species that are caused by prions. The main constituent of prions is scrapie prion protein (PrP Sc ), an aggregated moiety of the host-derived membrane glycolipoprotein cellular prion protein (PrP C ). Although PrP C is encoded by the host genome, prions were found to encipher many phenotypic TSE variants, known as prion strains. Prion strains are TSE isolates that, when inoculated into new hosts, consistently cause disease with specific characteristics, such as incubation period, patterns of PrP Sc distribution and relative severity of spongiform changes in the brain (the lesion profile).The agent-specified information of prion strains is thought to be contained within distinct conformations of various PrP Sc isotypes. Prions exert their destructive effects predominantly, if not exclusively, within the central nervous system. However, the direct cause of neurotoxicity remains unclear. PrP C is required for prion replication because mice that lack PrP C are resistant to prions. The presence of PrP C on neurons is a prerequisite for prion-induced neurotoxicity. A series of transgenic mice that express various prion protein mutants indicate that deletion of specific regions of PrP C can render it neurotoxic. This toxicity is modulated by co-expression of wild-type PrP C . Currently, there is no reagent allowing non-invasive, pre-mortem diagnosis of prion diseases. In view of recent unfortunate cases of Creutzfeldt–Jakob disease infection through blood transfusion, reliable, specific and, most importantly, sensitive reagents are urgently needed. Although it is now accepted that the infectious agent that causes transmissible spongiform encephalopathies is PrP Sc , recent insights into the existence of prion strains pose a fascinating challenge to prion research. What is the nature of prion strains? And how can they be discriminated? Transmissible spongiform encephalopathies (TSEs) are neurodegenerative diseases that are caused by prions and affect humans and many animal species. It is now widely accepted that the infectious agent that causes TSEs is PrP Sc , an aggregated moiety of the host-derived membrane glycolipoprotein PrP C . Although PrP C is encoded by the host genome, prions themselves encipher many phenotypic TSE variants, known as prion strains. Prion strains are TSE isolates that, after inoculation into distinct hosts, cause disease with consistent characteristics, such as incubation period, distinct patterns of PrP Sc distribution and spongiosis and relative severity of the spongiform changes in the brain. The existence of such strains poses a fascinating challenge to prion research.

The biological determinants of the phenotypic variation in sporadic Creutzfeldt-Jakob disease (sCJD) are unknown. To categorize sCJD cases, the prion protein (PrP) codon 129 genotype and the biochemical characteristics of the disease-associated form of PrP (PrP Sc ) can be combined to form six subgroups (MM1, MM2, MV1, MV2, VV1, and VV2). This classification largely correlates with the known variation in the clinical and pathological features of sCJD, with the MM1 and MV1 cases representing the \"classic\" phenotype of sCJD. To address how this classification relates to different strains of sCJD we have inoculated each subgroup of sCJD to a panel of mice expressing different forms of the human PRNP gene (129MM, 129VV, and 129MV). We have established that all subtypes are transmissible to at least one genotype of mouse, and both agent and host factors determine transmission efficiency and the form of PrP Sc deposited in the brain. Moreover, we have identified four distinct strains of sCJD using our in vivo strain typing panel.

Prions are infectious proteins consisting mainly of PrPSc, a β sheet-rich conformer of the normal host protein PrPC, and occur in different strains. Strain identity is thought to be encoded by PrPSc conformation. We found that biologically cloned prion populations gradually became heterogeneous by accumulating \"mutants,\" and selective pressures resulted in the emergence of different mutants as major constituents of the evolving population. Thus, when transferred from brain to cultured cells, \"cell-adapted\" prions outcompeted their \"brain-adapted\" counterparts, and the opposite occurred when prions were returned from cells to brain. Similarly, the inhibitor swainsonine selected for a resistant substrain, whereas, in its absence, the susceptible substrain outgrew its resistant counterpart. Prions, albeit devoid of a nucleic acid genome, are thus subject to mutation and selective amplification.

Prions are unusual protein assemblies that propagate their conformationally-encoded information in absence of nucleic acids. The first prion identified, the scrapie isoform (PrPSc) of the cellular prion protein (PrPC), caused epidemic and epizootic episodes [1]. Most aggregates of other misfolding-prone proteins are amyloids, often arranged in a Parallel-In-Register-β-Sheet (PIRIBS) [2] or β-solenoid conformations [3]. Similar folding models have also been proposed for PrPSc, although none of these have been confirmed experimentally. Recent cryo-electron microscopy (cryo-EM) and X-ray fiber-diffraction studies provided evidence that PrPSc is structured as a 4-rung β-solenoid (4RβS) [4, 5]. Here, we combined different experimental data and computational techniques to build the first physically-plausible, atomic resolution model of mouse PrPSc, based on the 4RβS architecture. The stability of this new PrPSc model, as assessed by Molecular Dynamics (MD) simulations, was found to be comparable to that of the prion forming domain of Het-s, a naturally-occurring β-solenoid. Importantly, the 4RβS arrangement allowed the first simulation of the sequence of events underlying PrPC conversion into PrPSc. This study provides the most updated, experimentally-driven and physically-coherent model of PrPSc, together with an unprecedented reconstruction of the mechanism underlying the self-catalytic propagation of prions.

Prions are lethal mammalian pathogens composed of aggregated conformational isomers of a host-encoded glycoprotein and which appear to lack nucleic acids. Their unique biology, allied with the public-health risks posed by prion zoonoses such as bovine spongiform encephalopathy, has focused much attention on the molecular basis of prion propagation and the \"species barrier\" that controls cross-species transmission. Both are intimately linked to understanding how multiple prion \"strains\" are encoded by a protein-only agent. The underlying mechanisms are clearly of much wider importance, and analogous protein-based inheritance mechanisms are recognized in yeast and fungi. Recent advances suggest that prions themselves are not directly neurotoxic, but rather their propagation involves production of toxic species, which may be uncoupled from infectivity.

The conformational change of a host protein, PrPC, into a disease-associated isoform, PrPSc, appears to play a critical role in the pathogenesis of prion diseases such as Creutzfeldt-Jakob disease and scrapie. However, the fundamental mechanism by which infectious prions are produced in neurons remains unknown. To investigate the mechanism of prion formation biochemically, we conducted a series of experiments using the protein misfolding cyclic amplification (PMCA) technique with a preparation containing only native PrPC and copurified lipid molecules. These experiments showed that successful PMCA propagation of PrPSc molecules in a purified system requires accessory polyanion molecules. In addition, we found that PrPSc molecules could be formed de novo from these defined components in the absence of preexisting prions. Inoculation of samples containing either prion-seeded or spontaneously generated PrPSc molecules into hamsters caused scrapie, which was transmissible on second passage. These results show that prions able to infect wild-type hamsters can be formed from a minimal set of components including native PrPC molecules, copurified lipid molecules, and a synthetic polyanion.

Two-stage prion infection Prion infections have a clinically silent incubation period that can go on for years or even decades, followed by an aggressive, short clinical phase. Experiments in the RML mouse model of prion disease now show that prion propagation in the brain proceeds by two distinct phases: a relatively brief exponential phase that is not rate-limited by prion protein concentration, followed by a plateau phase. Surprisingly, it is the latter that can be very prolonged, accounting for the majority of the clinically silent incubation period. The similar levels of infectivity at the end of the first and second phase suggest that there is a separation between prion infectivity and toxicity. The authors suggest that the prions are not neurotoxic themselves, but catalyse the formation of such species from host-cell-encoded cellular prion protein. Here it is shown that during the silent phase of prion infection, prions first exponentially propagate until a defined limit is reached. Then a plateau phase follows. Prion propagation is independent of prion concentration, whereas in the plateau phase the time to clinical onset is inversely correlated to prion concentration. The similar levels of infectivity at the end of the first and second phase suggests that there is a separation between prion infectivity and toxicity. Moreover, something seems to limit prion production. It is suggested that the prions are not neurotoxic themselves but catalyse the formation of such species from PrP C . Production of neurotoxic species is triggered when prion propagation saturates, leading to a switch from autocatalytic production of infectivity to a toxic pathway. Mammalian prions cause fatal neurodegenerative conditions including Creutzfeldt–Jakob disease in humans and scrapie and bovine spongiform encephalopathy in animals 1 . Prion infections are typically associated with remarkably prolonged but highly consistent incubation periods followed by a rapid clinical phase. The relationship between prion propagation, generation of neurotoxic species and clinical onset has remained obscure. Prion incubation periods in experimental animals are known to vary inversely with expression level of cellular prion protein. Here we demonstrate that prion propagation in brain proceeds via two distinct phases: a clinically silent exponential phase not rate-limited by prion protein concentration which rapidly reaches a maximal prion titre, followed by a distinct switch to a plateau phase. The latter determines time to clinical onset in a manner inversely proportional to prion protein concentration. These findings demonstrate an uncoupling of infectivity and toxicity. We suggest that prions themselves are not neurotoxic but catalyse the formation of such species from PrP C . Production of neurotoxic species is triggered when prion propagation saturates, leading to a switch from autocatalytic production of infectivity (phase 1) to a toxic (phase 2) pathway.

Prion diseases are progressive, incurable and fatal neurodegenerative conditions. The term ‘prion’ was first nominated to express the revolutionary concept that a protein could be infectious. We now know that prions consist of PrPSc, the pathological aggregated form of the cellular prion protein PrPC. Over the years, the term has been semantically broadened to describe aggregates irrespective of their infectivity, and the prion concept is now being applied, perhaps overenthusiastically, to all neurodegenerative diseases that involve protein aggregation. Indeed, recent studies suggest that prion diseases (PrDs) and protein misfolding disorders (PMDs) share some common disease mechanisms, which could have implications for potential treatments. Nevertheless, the transmissibility of bona fide prions is unique, and PrDs should be considered as distinct from other PMDs.