De novo generation of infectious prions in vitro produces a new disease phenotype

Cofactors facilitate bona fide prion misfolding in vitro but are not necessary for the infectivity of recombinant murine prions

Prion diseases, particularly sporadic cases, pose a challenge due to their complex nature and heterogeneity. The underlying mechanism of the spontaneous conversion from PrPC to PrPSc, the hallmark of prion diseases, remains elusive. To shed light on this process and the involvement of cofactors, we have developed an in vitro system that faithfully mimics spontaneous prion misfolding using minimal components. By employing this PMSA methodology and introducing an isoleucine residue at position 108 in mouse PrP, we successfully generated recombinant murine prion strains with distinct biochemical and biological properties. Our study aimed to explore the influence of a polyanionic cofactor in modulating strain selection and infectivity in de novo-generated synthetic prions. These results not only validate PMSA as a robust method for generating diverse bona fide recombinant prions but also emphasize the significance of cofactors in shaping specific prion conformers capable of crossing species barriers. Interestingly, once these conformers are established, our findings suggest that cofactors are not necessary for their infectivity. This research provides valuable insights into the propagation and maintenance of the pathobiological features of cross-species transmissible recombinant murine prion and highlights the intricate interplay between cofactors and prion strain characteristics.

The importance of prion research

Over the past four decades, prion diseases have received considerable research attention owing to their potential to be transmitted within and across species as well as their consequences for human and animal health. The unprecedented nature of prions has led to the discovery of a paradigm of templated protein misfolding that underlies a diverse range of both disease-related and normal biological processes. Indeed, the "prion-like" misfolding and propagation of protein aggregates is now recognized as a common underlying disease mechanism in human neurodegenerative disorders such as Alzheimer's and Parkinson's disease, and the prion principle has led to the development of novel diagnostic and therapeutic strategies for these illnesses. Despite these advances, research into the fundamental biology of prion diseases has declined, likely due to their rarity and the absence of an acute human health crisis. Given the past translational influence, continued research on the etiology, pathogenesis, and transmission of prion disease should remain a priority. In this review, we highlight several important "unsolved mysteries" in the prion disease research field and how solving them may be crucial for the development of effective therapeutics, preventing future outbreaks of prion disease, and understanding the pathobiology of more common human neurodegenerative disorders.

Screening of Anti-Prion Compounds Using the Protein Misfolding Cyclic Amplification Technology

Prion diseases are 100% fatal infectious neurodegenerative diseases affecting the brains of humans and other mammals. The disease is caused by the formation and replication of prions, composed exclusively of the misfolded prion protein (PrPSc). We invented and developed the protein misfolding cyclic amplification (PMCA) technology for in vitro prion replication, which allow us to replicate the infectious agent and it is commonly used for ultra-sensitive prion detection in biological fluids, tissues and environmental samples. In this article, we studied whether PMCA can be used to screen for chemical compounds that block prion replication. A small set of compounds previously shown to have anti-prion activity in various systems, mostly using cells infected with murine prions, was evaluated for their ability to prevent the replication of prions. Studies were conducted simultaneously with prions derived from 4 species, including human, cattle, cervid and mouse. Our results show that only one of these compounds (methylene blue) was able to completely inhibit prion replication in all species. Estimation of the IC50 for methylene blue inhibition of human prions causing variant Creutzfeldt-Jakob disease (vCJD) was 7.7 μM. Finally, we showed that PMCA can be used for structure-activity relationship studies of anti-prion compounds. Interestingly, some of the less efficient prion inhibitors altered the replication of prions in some species and not others, suggesting that PMCA is useful for studying the differential selectivity of potential drugs.

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.

Understanding the key features of the spontaneous formation of bona fide prions through a novel methodology that enables their swift and consistent generation

Among transmissible spongiform encephalopathies or prion diseases affecting humans, sporadic forms such as sporadic Creutzfeldt-Jakob disease are the vast majority. Unlike genetic or acquired forms of the disease, these idiopathic forms occur seemingly due to a random event of spontaneous misfolding of the cellular PrP (PrPC) into the pathogenic isoform (PrPSc). Currently, the molecular mechanisms that trigger and drive this event, which occurs in approximately one individual per million each year, remain completely unknown. Modelling this phenomenon in experimental settings is highly challenging due to its sporadic and rare occurrence. Previous attempts to model spontaneous prion misfolding in vitro have not been fully successful, as the spontaneous formation of prions is infrequent and stochastic, hindering the systematic study of the phenomenon. In this study, we present the first method that consistently induces spontaneous misfolding of recombinant PrP into bona fide prions within hours, providing unprecedented possibilities to investigate the mechanisms underlying sporadic prionopathies. By fine-tuning the Protein Misfolding Shaking Amplification method, which was initially developed to propagate recombinant prions, we have created a methodology that consistently produces spontaneously misfolded recombinant prions in 100% of the cases. Furthermore, this method gives rise to distinct strains and reveals the critical influence of charged surfaces in this process.

Cellular toxicity of scrapie prions in prion diseases; a biochemical and molecular overview

Transmissible spongiform encephalopathies (TSEs) or prion diseases consist of a broad range of fatal neurological disorders affecting humans and animals. Contrary to Watson and Crick's 'central dogma', prion diseases are caused by a protein, devoid of DNA involvement. Herein, we briefly review various cellular and biological aspects of prions and prion pathogenesis focusing mainly on historical milestones, biosynthesis, degradation, structure-function of cellular and scrapie forms of prions .

Essential Components of Synthetic Infectious Prion Formation De Novo

Prion diseases are a class of neurodegenerative diseases that are uniquely infectious. Whilst their general replication mechanism is well understood, the components required for the formation and propagation of highly infectious prions are poorly characterized. The protein-only hypothesis posits that the prion protein (PrP) is the only component of the prion; however, additional co-factors are required for its assembly into infectious prions. These can be provided by brain homogenate, but synthetic lipids and non-coding RNA have also been used in vitro. Here, we review a range of experimental approaches, which generate PrP amyloid assemblies de novo. These synthetic PrP assemblies share some, but not necessarily all, properties of genuine infectious prions. We will discuss the different experimental approaches, how a prion is defined, the non-protein requirements of a prion, and provide an overview of the current state of prion amplification and generation in vitro.

Distinct populations of highly potent TAU seed conformers in rapidly progressing Alzheimer's disease

[Figure: see text].

Mice Treated Subcutaneously with Mouse LPS-Converted PrPres or LPS Alone Showed Brain Gene Expression Profiles Characteristic of Prion Disease

Previously, we showed that bacterial lipopolysaccharide (LPS) converts mouse PrP^C protein to a beta-rich isoform (moPrP^res) resistant to proteinase K. In this study, we aimed to test if the LPS-converted PrP^res is infectious and alters the expression of genes related to prion pathology in brains of terminally sick mice. Ninety female FVB/N mice at 5 weeks of age were randomly assigned to 6 groups treated subcutaneously (sc) for 6 weeks either with: (1) Saline (CTR); (2) LPS from Escherichia coli 0111:B4 (LPS), (3) one-time sc administration of de novo generated mouse recombinant prion protein (moPrP; 29-232) rich in beta-sheet by incubation with LPS (moPrP^res), (4) LPS plus one-time sc injection of moPrP^res, (5) one-time sc injection of brain homogenate from Rocky Mountain Lab (RLM) scrapie strain, and (6) LPS plus one-time sc injection of RML. Results showed that all treatments altered the expression of various genes related to prion disease and neuroinflammation starting at 11 weeks post-infection and more profoundly at the terminal stage. In conclusion, sc administration of de novo generated moPrPres, LPS, and a combination of moPrP^res with LPS were able to alter the expression of multiple genes typical of prion pathology and inflammation.

Amplification Techniques for the Detection of Misfolded Prion Proteins in Experimental and Clinical Samples

This article describes two methods for amplifying prions present in experimental and clinical samples: the protein misfolding cyclic amplification (PMCA) assay and the real-time quaking-induced conversion (RT-QuIC) assay. Protocols for preparation of amplification substrate and analysis of results are included in addition to those for the individual assays. For each assay, control and suspect samples are mixed with appropriate amplification substrate, which is whole brains from mice in the case of PMCA and recombinant prion protein produced in bacteria for RT-QuIC, followed by cyclic amplification over a number of cycles of sonication (PMCA) or shaking (RT-QuIC) at a consistent incubation temperature. The resultant amplification products are then assessed either by western blotting (PMCA) or based on fluorescent emissions (RT-QuIC). The equipment and expertise necessary for successfully performing either assay vary and will be important factors for individual laboratories to consider when identifying which assay is more appropriate for their experimental design. © 2020 by John Wiley & Sons, Inc. Basic Protocol 1: Prion amplification via protein misfolding cyclic amplification Support Protocol 1: Collection of whole brains from mice and preparation of normal brain homogenate Basic Protocol 2: Prion amplification via real-time quaking-induced conversion Support Protocol 2: Preparation of recombinant truncated white-tailed-deer prion protein.

Prion Diseases: A Unique Transmissible Agent or a Model for Neurodegenerative Diseases?

The accumulation and propagation in the brain of misfolded proteins is a pathological hallmark shared by many neurodegenerative diseases such as Alzheimer's disease (Aβ and tau), Parkinson's disease (α-synuclein), and prion disease (prion protein). Currently, there is no epidemiological evidence to suggest that neurodegenerative disorders are infectious, apart from prion diseases. However, there is an increasing body of evidence from experimental models to suggest that other pathogenic proteins such as Aβ and tau can propagate in vivo and in vitro in a prion-like mechanism, inducing the formation of misfolded protein aggregates such as amyloid plaques and neurofibrillary tangles. Such similarities have raised concerns that misfolded proteins, other than the prion protein, could potentially transmit from person-to-person as rare events after lengthy incubation periods. Such concerns have been heightened following a number of recent reports of the possible inadvertent transmission of Aβ pathology via medical and surgical procedures. This review will provide a historical perspective on the unique transmissible nature of prion diseases, examining their impact on public health and the ongoing concerns raised by this rare group of disorders. Additionally, this review will provide an insight into current evidence supporting the potential transmissibility of other pathogenic proteins associated with more common neurodegenerative disorders and the potential implications for public health.

Clinical diagnosis of human prion disease

Human prion disease may present in a non-specific way and is often diagnosed at a relatively late stage of the illness. Until recently, clinical diagnosis has been supported by tests that are mostly non-specific and, sometimes, insensitive. Recent laboratory developments have led to a variety of tests that rely on a disease-specific mechanism. One test, the CSF RT-QuIC (Real-Time Quaking-Induced Conversion) test is very sensitive and specific for sporadic CJD and is now used in routine clinical practice. Other tests, based on other tissues, including blood and urine, have been developed and potentially could improve both clinical diagnostic accuracy and lead to earlier diagnosis. While there are yet no proven treatments for prion disease, any treatment to be developed will almost certainly require earlier diagnosis if therapeutic success is to be realized.

Towards an improved early diagnosis of neurodegenerative diseases: the emerging role of in vitro conversion assays for protein amyloids

Tissue accumulation of abnormal aggregates of amyloidogenic proteins such as prion protein, α-synuclein, and tau represents the hallmark of most common neurodegenerative disorders and precedes the onset of symptoms by years. As a consequence, the sensitive and specific detection of abnormal forms of these proteins in patients' accessible tissues or fluids as biomarkers may have a significant impact on the clinical diagnosis of these disorders. By exploiting seeded polymerization propagation mechanisms to obtain cell-free reactions that allow highly amplified detection of these amyloid proteins, novel emerging in vitro techniques, such as the real-time quaking-induced conversion assay (RT-QuIC) have paved the way towards this important goal. Given its high accuracy in identifying misfolded forms of prion protein from Creutzfeldt-Jakob disease (CJD) CSF, RT-QuIC has already been included in the diagnostic criteria for the clinical diagnosis of sporadic CJD, the most common human prion disease. By showing that this assay may also accurately discriminate between Lewy body disorders and other forms of parkinsonisms or dementias, more recent studies strongly suggested that CSF RT-QuIC can also be successfully applied to synucleinopathies. Finally, preliminary encouraging data also suggested that CSF RT-QuIC might also work for tau protein, and accurately distinguish between 3R- and 4R tauopathies, including Pick's disease, progressive supranuclear palsy, and corticobasal degeneration. Here we will review the state of the art of cell-free aggregation assays, their current diagnostic value and putative limitations, and the future perspectives for their expanded use in clinical practice.

Diverse, evolving conformer populations drive distinct phenotypes in frontotemporal lobar degeneration caused by the same MAPT-P301L mutation

Tau protein accumulation is a common denominator of major dementias, but this process is inhomogeneous, even when triggered by the same germline mutation. We considered stochastic misfolding of human tau conformers followed by templated conversion of native monomers as an underlying mechanism and derived sensitive conformational assays to test this concept. Assessments of brains from aged TgTauP301L transgenic mice revealed a prodromal state and three distinct signatures for misfolded tau. Frontotemporal lobar degeneration (FTLD)-MAPT-P301L patients with different clinical phenotypes also displayed three signatures, two resembling those found in TgTauP301L mice. As physicochemical and cell bioassays confirmed diverse tau strains in the mouse and human brain series, we conclude that evolution of diverse tau conformers is intrinsic to the pathogenesis of this uni-allelic form of tauopathy. In turn, effective therapeutic interventions in FTLD will need to address evolving repertoires of misfolded tau species rather than singular, static molecular targets.

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.

The cellular and pathologic prion protein

The cellular prion protein, PrP^C, is a small, cell surface glycoprotein with a function that is currently somewhat ill defined. It is also the key molecule involved in the family of neurodegenerative disorders called transmissible spongiform encephalopathies, which are also known as prion diseases. The misfolding of PrP^C to a conformationally altered isoform, designated PrP^TSE, is the main molecular process involved in pathogenesis and appears to precede many other pathologic and clinical manifestations of disease, including neuronal loss, astrogliosis, and cognitive loss. PrP^TSE is also believed to be the major component of the infectious "prion," the agent responsible for disease transmission, and preparations of this protein can cause prion disease when inoculated into a naïve host. Thus, understanding the biochemical and biophysical properties of both PrP^C and PrP^TSE, and ultimately the mechanisms of their interconversion, is critical if we are to understand prion disease biology. Although entire books could be devoted to research pertaining to the protein, herein we briefly review the state of knowledge of prion biochemistry, including consideration of prion protein structure, function, misfolding, and dysfunction.

Treatment with a non-toxic, self-replicating anti-prion delays or prevents prion disease in vivo

Transmissible spongiform encephalopathies (TSEs) are fatal neurological disorders caused by prions, which are composed of a misfolded protein (PrP^Sc) that self-propagates in the brain of infected individuals by converting the normal prion protein (PrP^C) into the pathological isoform. Here, we report a novel experimental strategy for preventing prion disease based on producing a self-replicating, but innocuous PrP^Sc-like form, termed anti-prion, which can compete with the replication of pathogenic prions. Our results show that a prophylactic inoculation of prion-infected animals with an anti-prion delays the onset of the disease and in some animals completely prevents the development of clinical symptoms and brain damage. The data indicate that a single injection of the anti-prion eliminated ~99% of the infectivity associated to pathogenic prions. Furthermore, this treatment caused significant changes in the profile of regional PrP^Sc deposition in the brains of animals that were treated, but still succumbed to the disease. Our findings provide new insights for a mechanistic understanding of prion replication and support the concept that prion replication can be separated from toxicity, providing a novel target for therapeutic intervention.

Cofactors influence the biological properties of infectious recombinant prions

Prion diseases are caused by a misfolding of the cellular prion protein (PrP) to a pathogenic isoform named PrP^Sc. Prions exist as strains, which are characterized by specific pathological and biochemical properties likely encoded in the three-dimensional structure of PrP^Sc. However, whether cofactors determine these different PrP^Sc conformations and how this relates to their specific biological properties is largely unknown. To understand how different cofactors modulate prion strain generation and selection, Protein Misfolding Cyclic Amplification was used to create a diversity of infectious recombinant prion strains by propagation in the presence of brain homogenate. Brain homogenate is known to contain these mentioned cofactors, whose identity is only partially known, and which facilitate conversion of PrP^C to PrP^Sc. We thus obtained a mix of distinguishable infectious prion strains. Subsequently, we replaced brain homogenate, by different polyanionic cofactors that were able to drive the evolution of mixed prion populations toward specific strains. Thus, our results show that a variety of infectious recombinant prions can be generated in vitro and that their specific type of conformation, i.e., the strain, is dependent on the cofactors available during the propagation process. These observations have significant implications for understanding the pathogenesis of prion diseases and their ability to replicate in different tissues and hosts. Importantly, these considerations might apply to other neurodegenerative diseases for which different conformations of misfolded proteins have been described.

Prion protein amplification techniques

Protein amplification techniques exploit the ability of PrP^TSE to induce a conformational change in prion protein (PrP) in a continuous fashion, so that the small amount of PrP^TSE found in tissues and biologic fluids in prion diseases can be amplified to a point where they are detectable by conventional laboratory techniques. The most widely used protein aggregation assays are protein misfolding cyclic amplification assay (PMCA) and real-time quaking-induced conversion (RT-QuIC). These assays have been used extensively in both animal and human prion disease in studies ranging from the development of diagnostics, understanding disease transmission potential, to investigating mechanisms underlying neurodegeneration. In human prion disease, cerebrospinal fluid (CSF) RT-QuIC analysis has been shown to be a highly sensitive and specific test for sporadic Creutzfeldt-Jakob disease (sCJD) and has now been included in the diagnostic criteria. It is also a useful investigation for some genetic forms of prion disease where other cerebrospinal fluid tests may be negative. PMCA shows great potential for the diagnosis of variant CJD (vCJD) and has the ability to distinguish vCJD from sCJD, which may become increasingly important with emergence of a patient with neuropathologically confirmed vCJD associated with PRNP codon129MV, which indicates that a new wave of vCJD cases is likely and that these may be difficult to distinguish from sCJD.

Experimental transfusion of variant CJD-infected blood reveals previously uncharacterised prion disorder in mice and macaque

Exposure of human populations to bovine spongiform encephalopathy through contaminated food has resulted in <250 cases of variant Creutzfeldt-Jakob disease (vCJD). However, more than 99% of vCJD infections could have remained silent suggesting a long-term risk of secondary transmission particularly through blood. Here, we present experimental evidence that transfusion in mice and non-human primates of blood products from symptomatic and non-symptomatic infected donors induces not only vCJD, but also a different class of neurological impairments. These impairments can all be retransmitted to mice with a pathognomonic accumulation of abnormal prion protein, thus expanding the spectrum of known prion diseases. Our findings suggest that the intravenous route promotes propagation of masked prion variants according to different mechanisms involved in peripheral replication.

Protein Misfolding Cyclic Amplification of Infectious Prions

Transmissible spongiform encephalopathies, or prion diseases, are a group of incurable disorders caused by the accumulation of an abnormally folded prion protein (PrP^Sc) in the brain. According to the "protein-only" hypothesis, PrP^Sc is the infectious agent able to propagate the disease by acting as a template for the conversion of the correctly folded prion protein (PrP^C) into the pathological isoform. Recently, the mechanism of PrP^C conversion has been mimicked in vitro using an innovative technique named protein misfolding cyclic amplification (PMCA). This technology represents a great tool for studying diverse aspects of prion biology in the field of basic research and diagnosis. Moreover, PMCA can be expanded for the study of the misfolding process associated to other neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, and frontotemporal lobar degeneration.

The Prion Concept and Synthetic Prions

Transmissible spongiform encephalopathies or prion diseases are a group of fatal neurodegenerative diseases caused by unconventional infectious agents, known as prions (PrP^Sc). Prions derive from a conformational conversion of the normally folded prion protein (PrP^C), which acquires pathological and infectious features. Moreover, PrP^Sc is able to transmit the pathological conformation to PrP^C through a mechanism that is still not well understood. The generation of synthetic prions, which behave like natural prions, is of fundamental importance to study the process of PrP^C conversion and to assess the efficacy of therapeutic strategies to interfere with this process. Moreover, the ability of synthetic prions to induce pathology in animals confirms that the pathological properties of the prion strains are all enciphered in abnormal conformations, characterizing these infectious agents.

De Novo Generation of a Unique Cervid Prion Strain Using Protein Misfolding Cyclic Amplification

CWD is the only known TSE that affects free-ranging wildlife, specifically cervids such as elk, deer, moose, caribou, and reindeer. CWD has become endemic in both free-ranging and captive herds in North America, South Korea, and, most recently, northern Europe. The prion research community continues to debate the origins of CWD. Original foci of CWD emergence in Colorado and Wyoming coincident with the sheep TSE scrapie suggest that scrapie prions may have adapted to cervids to cause CWD. However, emerging evidence supports the idea that cervid PrP^C may be more prone to misfolding to the pathological isoform. Here we test the hypothesis that cervid PrP^C can spontaneously misfold to create de novo prions. Whether CWD can arise spontaneously as a sporadic TSE or represents a new TSE caused by cervid-adapted scrapie prions profoundly impacts surveillance and mitigation strategies.

Substantial evidence supports the hypothesis that prions are misfolded, infectious, insoluble, and protease-resistant proteins (PrP^RES) devoid of instructional nucleic acid that cause transmissible spongiform encephalopathies (TSEs). Protein misfolding cyclic amplification (PMCA) has provided additional evidence that PrPRes acts as a template that can convert the normal cellular prion protein (PrP^C) present in uninfected normal brain homogenate (NBH) into the infectious misfolded PrP^RES isoform. Human PrP^C has been shown to spontaneously convert to a misfolded pathological state causing sporadic Creutzfeldt-Jakob disease (sCJD). Several investigators have reported spontaneous generation of prions by in vitro assays, including PMCA. Here we tested the rate of de novo generation of cervid prions in our laboratory using our standard PMCA protocol and NBH from transgenic mice expressing cervid PrP^C (TgCerPrP mice). We generated de novo prions in rounds 4, 5, and 7 at low cumulative rates of 1.6, 5.0, and 6.7%, respectively. The prions caused infectious chronic wasting disease (CWD) upon inoculation into normal uninfected TgCerPrP mice and displayed unique biochemical characteristics compared to other cervid prion strains. We conclude that PMCA of cervid PrP^C from normal brain homogenate spontaneously generated a new cervid prion strain. These data support the potential for cervids to develop sporadic CWD.

IMPORTANCE CWD is the only known TSE that affects free-ranging wildlife, specifically cervids such as elk, deer, moose, caribou, and reindeer. CWD has become endemic in both free-ranging and captive herds in North America, South Korea, and, most recently, northern Europe. The prion research community continues to debate the origins of CWD. Original foci of CWD emergence in Colorado and Wyoming coincident with the sheep TSE scrapie suggest that scrapie prions may have adapted to cervids to cause CWD. However, emerging evidence supports the idea that cervid PrP^C may be more prone to misfolding to the pathological isoform. Here we test the hypothesis that cervid PrP^C can spontaneously misfold to create de novo prions. Whether CWD can arise spontaneously as a sporadic TSE or represents a new TSE caused by cervid-adapted scrapie prions profoundly impacts surveillance and mitigation strategies.

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Activation of microglia by retroviral infection correlates with transient clearance of prions from the brain but does not change incubation time

Prion diseases are fatal transmissible diseases, where conversion of the endogenous prion protein (PrP^C ) into a misfolded isoform (PrP^Sc ) leads to neurodegeneration. Microglia, the immune cells of the brain, are activated in neurodegenerative disorders including prion diseases; however, their impact on prion disease pathophysiology is unclear with both beneficial PrP^Sc -clearing and detrimental potentially neurotoxic effects. Moreover, monocytes entering the brain from the periphery during disease course might add to disease pathophysiology. Here, the degree of microglia activation in the brain of prion infected mice with and without an additional intraperitoneal retrovirus infection was studied. Peripheral murine retrovirus infection leads to activation of parenchymal microglia without recruitment of monocytes. This activation correlated with transient clearance or delay in accumulation of infectious prions specifically from the brain at early time points in the diseases course. Microglia expression profiling showed upregulation of genes involved in protein degradation coinciding with prion clearance. This enforces a concept where microglia act beneficial in prion disease if adequately activated. Once microglia activation has ceased, prion disease reemerges leading to disease kinetics undistinguishable from the situation in prion-only infected mice. This might be caused by the loss of microglial homeostatic function at clinical prion disease.

What Makes a Prion: Infectious Proteins From Animals to Yeast

While philosophers in ancient times had many ideas for the cause of contagion, the modern study of infective agents began with Fracastoro's 1546 proposal that invisible "spores" spread infectious disease. However, firm categorization of the pathogens of the natural world would need to await a mature germ theory that would not arise for 300 years. In the 19th century, the earliest pathogens described were bacteria and other cellular microbes. By the close of that century, the work of Ivanovsky and Beijerinck introduced the concept of a virus, an infective particle smaller than any known cell. Extending into the early-mid-20th century there was an explosive growth in pathogenic microbiology, with a cellular or viral cause identified for nearly every transmissible disease. A few occult pathogens remained to be discovered, including the infectious proteins (prions) proposed by Prusiner in 1982. This review discusses the prions identified in mammals, yeasts, and other organisms, focusing on the amyloid-based prions. I discuss the essential biochemical properties of these agents and the application of this knowledge to diseases of protein misfolding and aggregation, as well as the utility of yeast as a model organism to study prion and amyloid proteins that affect human and animal health. Further, I summarize the ideas emerging out of these studies that the prion concept may go beyond proteinaceous infectious particles and that prions may be a subset of proteins having general nucleating or seeding functions involved in noninfectious as well as infectious pathogenic protein aggregation.

Pathogenic mechanisms of prion protein, amyloid-β and α-synuclein misfolding: the prion concept and neurotoxicity of protein oligomers

Proteinopathies represent a group of diseases characterized by the unregulated misfolding and aggregation of proteins. Accumulation of misfolded protein in the central nervous system (CNS) is associated with neurodegenerative diseases, such as the transmissible spongiform encephalopathies (or prion diseases), Alzheimer's disease, and the synucleinopathies (the most common of which is Parkinson's disease). Of these, the pathogenic mechanisms of prion diseases are particularly striking where the transmissible, causative agent of disease is the prion, or proteinaceous infectious particle. Prions are composed almost exclusively of PrP^Sc ; a misfolded isoform of the normal cellular protein, PrP^C , which is found accumulated in the CNS in disease. Today, mounting evidence suggests other aggregating proteins, such as amyloid-β (Aβ) and α-synuclein (α-syn), proteins associated with Alzheimer's disease and synucleinopathies, respectively, share similar biophysical and biochemical properties with PrP^Sc that influences how they misfold, aggregate, and propagate in disease. In this regard, the definition of a 'prion' may ultimately expand to include other pathogenic proteins. Unifying knowledge of folded proteins may also reveal common mechanisms associated with other features of disease that are less understood, such as neurotoxicity. This review discusses the common features Aβ and α-syn share with PrP and neurotoxic mechanisms associated with these misfolded proteins. Several proteins are known to misfold and accumulate in the central nervous system causing a range of neurodegenerative diseases, such as Alzheimer's, Parkinson's, and the prion diseases. Prions are transmissible misfolded conformers of the prion protein, PrP, which seed further generation of infectious proteins. Similar effects have recently been observed in proteins associated with Alzheimer's disease and the synucleinopathies, leading to the proposition that the definition of a 'prion' may ultimately expand to include other pathogenic proteins. Unifying knowledge of misfolded proteins may also reveal common mechanisms associated with other features of disease that are less understood, such as neurotoxicity.

Are prions transported by plasma exosomes?

Blood has been shown to contain disease-associated misfolded prion protein (PrP(TSE)) in animals naturally and experimentally infected with various transmissible spongiform encephalopathy (TSE) agents, and in humans infected with variant Creutzfeldt-Jakob disease (vCJD). Recently, we have demonstrated PrP(TSE) in extracellular vesicle preparations (EVs) containing exosomes from plasma of mice infected with mouse-adapted vCJD by Protein Misfolding Cyclic Amplification (PMCA). Here we report the detection of PrP(TSE) by PMCA in EVs from plasma of mice infected with Fukuoka-1 (FU), an isolate from a Gerstmann-Sträussler-Scheinker disease patient. We used Tga20 transgenic mice that over-express mouse cellular prion protein, to assay by intracranial injections the level of infectivity in a FU-infected brain homogenate from wild-type mice (FU-BH), and in blood cellular components (BCC), consisting of red blood cells, white blood cells and platelets, plasma EVs, and plasma EVs subjected to multiple rounds of PMCA. Only FU-BH and plasma EVs from FU-infected mice subjected to PMCA that contained PrP(TSE) transmitted disease to Tga20 mice. Plasma EVs not subjected to PMCA and BCC from FU-infected mice failed to transmit disease. These findings confirm the high sensitivity of PMCA for PrP(TSE) detection in plasma EVs and the efficiency of this in vitro method to produce highly infectious prions. The results of our study encourage further research to define the role of EVs and, more specifically exosomes, as blood-borne carriers of PrP(TSE).

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.

A brief history of prions

Proteins were described as distinct biological molecules and their significance in cellular processes was recognized as early as the 18th century. At the same time, Spanish shepherds observed a disease that compelled their Merino sheep to pathologically scrape against fences, a defining clinical sign that led to the disease being named scrapie. In the late 19th century, Robert Koch published his postulates for defining causative agents of disease. In the early 20th century, pathologists Creutzfeldt and Jakob described a neurodegenerative disease that would later be included with scrapie into a group of diseases known as transmissible spongiform encephalopathies (TSEs). Later that century, mounting evidence compelled a handful of scientists to betray the prevailing biological dogma governing pathogen replication that Watson and Crick so convincingly explained by cracking the genetic code just two decades earlier. Because TSEs seemed to defy these new rules, J.S. Griffith theorized mechanisms by which a pathogenic protein could encipher its own replication blueprint without a genetic code. Stanley Prusiner called this proteinaceous infectious pathogen a prion. Here we offer a concise account of the discovery of prions, the causative agent of TSEs, in the wider context of protein biochemistry and infectious disease. We highlight the discovery of prions in yeast and discuss the implication of prions as epigenomic carriers of biological and pathological information. We also consider expanding the prion hypothesis to include other proteins whose alternate isoforms confer new biological or pathological properties.

Prion formation, but not clearance, is supported by protein misfolding cyclic amplification

Prion diseases are fatal transmissible neurodegenerative disorders that affect animals including humans. The kinetics of prion infectivity and PrP(Sc) accumulation can differ between prion strains and within a single strain in different tissues. The net accumulation of PrP(Sc) in animals is controlled by the relationship between the rate of PrP(Sc) formation and clearance. Protein misfolding cyclic amplification (PMCA) is a powerful technique that faithfully recapitulates PrP(Sc) formation and prion infectivity in a cell-free system. PMCA has been used as a surrogate for animal bioassay and can model species barriers, host range, strain co-factors and strain interference. In this study we investigated if degradation of PrP(Sc) and/or prion infectivity occurs during PMCA. To accomplish this we performed PMCA under conditions that do not support PrP(Sc) formation and did not observe either a reduction in PrP(Sc) abundance or an extension of prion incubation period, compared to untreated control samples. These results indicate that prion clearance does not occur during PMCA. These data have significant implications for the interpretation of PMCA based experiments such as prion amplification rate, adaptation to new species and strain interference where production and clearance of prions can affect the outcome.

Intraperitoneal Infection of Wild-Type Mice with Synthetically Generated Mammalian Prion

The prion hypothesis postulates that the infectious agent in transmissible spongiform encephalopathies (TSEs) is an unorthodox protein conformation based agent. Recent successes in generating mammalian prions in vitro with bacterially expressed recombinant prion protein provide strong support for the hypothesis. However, whether the pathogenic properties of synthetically generated prion (rec-Prion) recapitulate those of naturally occurring prions remains unresolved. Using end-point titration assay, we showed that the in vitro prepared rec-Prions have infectious titers of around 10⁴ LD₅₀ / μg. In addition, intraperitoneal (i.p.) inoculation of wild-type mice with rec-Prion caused prion disease with an average survival time of 210 – 220 days post inoculation. Detailed pathological analyses revealed that the nature of rec-Prion induced lesions, including spongiform change, disease specific prion protein accumulation (PrP-d) and the PrP-d dissemination amongst lymphoid and peripheral nervous system tissues, the route and mechanisms of neuroinvasion were all typical of classical rodent prions. Our results revealed that, similar to naturally occurring prions, the rec-Prion has a titratable infectivity and is capable of causing prion disease via routes other than direct intra-cerebral challenge. More importantly, our results established that the rec-Prion caused disease is pathogenically and pathologically identical to naturally occurring contagious TSEs, supporting the concept that a conformationally altered protein agent is responsible for the infectivity in TSEs.

The transmissible spongiform encephalopathies (TSEs) are a group of infectious neurodegenerative diseases affecting both humans and animals. The prion hypothesis postulates that prions are protein conformation based infectious agents responsible for TSE infectivity. Prions have been synthetically generated in vitro, but it remains unclear whether the properties of synthetically generated prion are the same as those of TSE agents and whether the disease caused by synthetically generated prion is identical to naturally occurring TSEs. In this study, we demonstrated that similar to the classical TSE agents, the synthetically generated prion has a titratable infectivity and is able to cause prion disease in wild-type mice via routes other than direct intra-cerebral inoculation. More importantly, we showed that the synthetically generated prion induced pathological changes, including the dissemination of disease-specific prion protein accumulation and the route and mechanism of neuroinvasion, were all typical of classical TSEs. These results demonstrate the similarity of synthetically generated prion to the infectious agent in TSEs, providing strong evidence supporting the prion hypothesis.

Implications of prion adaptation and evolution paradigm for human neurodegenerative diseases

There is a growing body of evidence indicating that number of human neurodegenerative diseases, including Alzheimer disease, Parkinson disease, fronto-temporal dementias, and amyotrophic lateral sclerosis, propagate in the brain via prion-like intercellular induction of protein misfolding. Prions cause lethal neurodegenerative diseases in humans, the most prevalent being sporadic Creutzfeldt-Jakob disease (sCJD); they self-replicate and spread by converting the cellular form of prion protein (PrP^C) to a misfolded pathogenic conformer (PrP^Sc). The extensive phenotypic heterogeneity of human prion diseases is determined by polymorphisms in the prion protein gene, and by prion strain-specific conformation of PrP^Sc. Remarkably, even though informative nucleic acid is absent, prions may undergo rapid adaptation and evolution in cloned cells and upon crossing the species barrier. In the course of our investigation of this process, we isolated distinct populations of PrP^Sc particles that frequently co-exist in sCJD. The human prion particles replicate independently and undergo competitive selection of those with lower initial conformational stability. Exposed to mutant substrate, the winning PrP^Sc conformers are subject to further evolution by natural selection of the subpopulation with the highest replication rate due to the lowest stability. Thus, the evolution and adaptation of human prions is enabled by a dynamic collection of distinct populations of particles, whose evolution is governed by the selection of progressively less stable, faster replicating PrP^Sc conformers. This fundamental biological mechanism may explain the drug resistance that some prions gained after exposure to compounds targeting PrP^Sc. Whether the phenotypic heterogeneity of other neurodegenerative diseases caused by protein misfolding is determined by the spectrum of misfolded conformers (strains) remains to be established. However, the prospect that these conformers may evolve and adapt by a prion-like mechanism calls for the reevaluation of therapeutic strategies that target aggregates of misfolded proteins, and argues for new therapeutic approaches that will focus on prior pathogenetic steps.

Elucidating the role of cofactors in mammalian prion propagation

Perhaps the most intriguing scientific question about mammalian prions is how these proteinaceous entities encode and propagate infectivity. Over the past decade, our laboratory has taken a reductionist biochemical approach to study this challenging question. Our studies have resulted in the identification of endogenous phospholipid and polyanionic cofactor molecules that facilitate prion formation in vitro. Using these cofactor molecules, we have been able to produce purified, chemically defined prions with high levels of specific infectivity for wild type animal hosts. Our most recent studies suggest that cofactor molecules may also play crucial roles in maintaining the infectious conformation and strain properties of mammalian prions. The ability to produce high titer prions in vitro using cofactors provides an unprecedented opportunity to study the structural mechanism of infectious prion formation directly.

The standard scrapie cell assay: development, utility and prospects

Prion diseases are a family of fatal neurodegenerative diseases that involve the misfolding of a host protein, PrP^C. Measuring prion infectivity is necessary for determining efficacy of a treatment or infectivity of a prion purification procedure; animal bioassays are, however, very expensive and time consuming. The Standard Scrapie Cell Assay (SSCA) provides an alternative approach. The SSCA facilitates quantitative in vitro analysis of prion strains, titres and biological properties. Given its robust nature and potential for high throughput, the SSCA has substantial utility for in vitro characterization of prions and can be deployed in a number of settings. Here we provide an overview on establishing the SSCA, its use in studies of disease dissemination and pathogenesis, potential pitfalls and a number of remaining challenges.

What Is a Quasispecies? Historical Origins and Current Scope

The quasispecies concept is introduced by means of a simple theoretical model that uses as little chemical kinetics and mathematics as possible but fully in the spirit of Albert Einstein who said: "Things should be made as simple as possible but not simpler." More elaborate treatments follow in the forthcoming chapters. It is shown that the most important results of the theory, in particular the existence of error thresholds, are not dependent on simplifying assumptions concerning the distribution of fitness values. Error thresholds are regularly found on landscapes with large and irregular scatter of fitness. After the introduction to theory, it will be shown how experimental data on the evolution of molecules or viruses may be fit to the theoretical model.

Evaluation of infective property of recombinant prion protein amyloids in cultured cells overexpressing cellular prion protein

Misfolded isoform of prion protein (PrP), termed scrapie PrP (PrP(Sc)), tends to aggregate into various fibril forms. Previously, we reported various conditions that affect aggregation of recombinant PrP into amyloids. Because amyloidogenesis of PrP is closely associated with transmissible spongiform encephalopathies such as Creutzfeldt-Jakob disease in humans, we investigated infectivity of recombinant PrP amyloids generated in vitro. Using cultured cell lines which overexpress cellular PrP of different species, we measured the level of de novo synthesized PrP(Sc) in cells inoculated with recombinant mouse PrP amyloids. While PrP-overexpressing cells were susceptible to mouse-adapted scrapie prions used as the positive control, demonstrating the species barrier effect, infection with amyloids made of truncated recombinant PrP (PrP[89-230]) failed to form and propagate PrP(Sc) even in the cells that express mouse cellular PrP. This suggests that infectivity of PrP amyloids generated in vitro is different from that of natural prions. Recombinant PrP (89-230) amyloids tested in the current study retain no or a minute level, if any, of prion infectivity.

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.

Pathology of SSLOW, a transmissible and fatal synthetic prion protein disorder, and comparison with naturally occurring classical transmissible spongiform encephalopathies

AIMS

Naturally occurring transmissible spongiform encephalopathies (TSEs) accumulate disease-specific forms of prion protein on cell membranes in association with pathognomonic lesions. We wished to determine whether synthetic prion protein disorders recapitulated these and other subcellular TSE-specific changes.

METHODS

SSLOW is a TSE initiated with refolded synthetic prion protein. Five terminally sick hamsters previously intracerebrally inoculated with third passage SSLOW were examined using light and immunogold electron microscopy.

RESULTS

SSLOW-affected hamsters showed widespread abnormal prion protein (PrP(SSLOW) ) and amyloid plaques. PrP(SSLOW) accumulated on plasma lemmas of neurites and glia without pathological changes. PrP(SSLOW) also colocalized with increased coated vesicles and pits, coated spiral membrane invaginations and membrane microfolding. PrP(SSLOW) was additionally observed in lysosomes of microglial cells but not of neurones or astrocytes.

CONCLUSIONS

PrP(SSLOW) is propagated by cell membrane conversion of normal PrP and lethal disease may be linked to the progressive growth of amyloid plaques. Cell membrane changes present in SSLOW are indistinguishable from those of naturally occurring TSEs. However, some lesions found in SSLOW are absent in natural animal TSEs and vice versa. SSLOW may not entirely recapitulate neuropathological features previously described for natural disease. End-stage neuropathology in SSLOW, particularly the nature and distribution of amyloid plaques may be significantly influenced by the early redistribution of seeds within the inoculum and its recirculation following interstitial, perivascular and other drainage pathways. The way in which seeds are distributed and aggregate into plaques in SSLOW has significant overlap with murine APP overexpressing mice challenged with Aβ.

Synthetic prions and other human neurodegenerative proteinopathies

Transmissible spongiform encephalopathies (TSE) are a heterogeneous group of neurodegenerative disorders. The common feature of these diseases is the pathological conversion of the normal cellular prion protein (PrP(C)) into a β-structure-rich conformer-termed PrP(Sc). The latter can induce a self-perpetuating process leading to amplification and spreading of pathological protein assemblies. Much evidence suggests that PrP(Sc) itself is able to recruit and misfold PrP(C) into the pathological conformation. Recent data have shown that recombinant PrP(C) can be misfolded in vitro and the resulting synthetic conformers are able to induce the conversion of PrP(C) into PrP(Sc)in vivo. In this review we describe the state-of-the-art of the body of literature in this field. In addition, we describe a cell-based assay to test synthetic prions in cells, providing further evidence that synthetic amyloids are able to template conversion of PrP into prion inclusions. Studying prions might help to understand the pathological mechanisms governing other neurodegenerative diseases. Aggregation and deposition of misfolded proteins is a common feature of several neurodegenerative disorders, such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and other disorders. Although the proteins implicated in each of these diseases differ, they share a common prion mechanism. Recombinant proteins are able to aggregate in vitro into β-rich amyloid fibrils, sharing some features of the aggregates found in the brain. Several studies have reported that intracerebral inoculation of synthetic aggregates lead to unique pathology, which spread progressively to distal brain regions and reduced survival time in animals. Here, we review the prion-like features of different proteins involved in neurodegenerative disorders, such as α-synuclein, superoxide dismutase-1, amyloid-β and tau.

Comparison of 2 synthetically generated recombinant prions

Prion is a protein-conformation-based infectious agent causing fatal neurodegenerative diseases in humans and animals. Our previous studies revealed that in the presence of cofactors, infectious prions can be synthetically generated in vitro with bacterially expressed recombinant prion protein (PrP). Once initiated, the recombinant prion is able to propagate indefinitely via serial protein misfolding cyclic amplification (sPMCA). In this study, we compared 2 separately initiated recombinant prions. Our results showed that these 2 recombinant prions had distinct biochemical properties and caused different patterns of spongiosis and PrP deposition in inoculated mice. Our findings indicate that various recombinant prions can be initiated in vitro and potential reasons for this variability are discussed.

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.

Ethics in prion disease

This paper is intended to discuss some of the scientific and ethical issues that are created by increased research efforts towards earlier diagnosis, as well as to treatment of, human prion diseases (and related dementias), including the resulting consequences for individuals, their families, and society. Most patients with prion disease currently are diagnosed when they are about 2/3 of the way through their disease course (Geschwind et al., 2010a; Paterson et al., 2012b), when the disease has progressed so far that even treatments that stop the disease process would probably have little benefit. Although there are currently no treatments available for prion diseases, we and others have realized that we must diagnose patients earlier and with greater accuracy so that future treatments have hope of success. As approximately 15% of prion diseases have a autosomal dominant genetic etiology, this further adds to the complexity of ethical issues, particularly regarding when to conduct genetic testing, release of genetic results, and when or if to implement experimental therapies. Human prion diseases are both infectious and transmissible; great care is required to balance the needs of the family and individual with both public health needs and strained hospital budgets. It is essential to proactively examine and address the ethical issues involved, as well as to define and in turn provide best standards of care.

De novo generation of infectious prions with bacterially expressed recombinant prion protein

The prion hypothesis is strongly supported by the fact that prion infectivity and the pathogenic conformer of prion protein (PrP) are simultaneously propagated in vitro by the serial protein misfolding cyclic amplification (sPMCA). However, due to sPMCA's enormous amplification power, whether an infectious prion can be formed de novo with bacterially expressed recombinant PrP (rPrP) remains to be satisfactorily resolved. To address this question, we performed unseeded sPMCA with rPrP in a laboratory that has never been exposed to any native prions. Two types of proteinase K (PK)-resistant and self-perpetuating recombinant PrP conformers (rPrP-res) with PK-resistant cores of 17 or 14 kDa were generated. A bioassay revealed that rPrP-res(17kDa) was highly infectious, causing prion disease in wild-type mice with an average survival time of about 172 d. In contrast, rPrP-res(14kDa) completely failed to induce any disease. Our findings reveal that sPMCA is sufficient to initiate various self-perpetuating PK-resistant rPrP conformers, but not all of them possess in vivo infectivity. Moreover, generating an infectious prion in a prion-free environment establishes that an infectious prion can be formed de novo with bacterially expressed rPrP.

Identification of misfolded proteins in body fluids for the diagnosis of prion diseases

Transmissible spongiform encephalopathy (TSE) or prion diseases are fatal rare neurodegenerative disorders affecting man and animals and caused by a transmissible infectious agent. TSE diseases are characterized by spongiform brain lesions with neuronal loss and the abnormal deposition in the CNS, and to less extent in other tissues, of an insoluble and protease resistant form of the cellular prion protein (PrP^C), named PrP^TSE. In man, TSE diseases affect usually people over 60 years of age with no evident disease-associated risk factors. In some cases, however, TSE diseases are unequivocally linked to infectious episodes related to the use of prion-contaminated medicines, medical devices, or meat products as in the variant Creutzfeldt-Jakob disease (CJD). Clinical signs occur months or years after infection, and during this silent period PrP^TSE, the only reliable marker of infection, is not easily measurable in blood or other accessible tissues or body fluids causing public health concerns. To overcome the limit of PrP^TSE detection, several highly sensitive assays have been developed, but attempts to apply these techniques to blood of infected hosts have been unsuccessful or not yet validated. An update on the latest advances for the detection of misfolded prion protein in body fluids is provided.

Using protein misfolding cyclic amplification generates a highly neurotoxic PrP dimer causing neurodegeneration

Under the "protein-only" hypothesis, prion-based diseases are proposed to result from an infectious agent that is an abnormal isoform of the prion protein in the scrapie form, PrP(Sc). However, since PrP(Sc) is highly insoluble and easily aggregates in vivo, this view appears to be overly simplistic, implying that the presence of PrP(Sc) may indirectly cause neurodegeneration through its intermediate soluble form. We generated a neurotoxic PrP dimer with partial pathogenic characteristics of PrP(Sc) by protein misfolding cyclic amplification in the presence of 1-palmitoyl-2-oleoylphosphatidylglycerol consisting of recombinant hamster PrP (23-231). After intracerebral injection of the PrP dimer, wild-type hamsters developed signs of neurodegeneration. Clinical symptoms, necropsy findings, and histopathological changes were very similar to those of transmissible spongiform encephalopathies. Additional investigation showed that the toxicity is primarily related to cellular apoptosis. All results suggested that we generated a new neurotoxic form of PrP, PrP dimer, which can cause neurodegeneration. Thus, our study introduces a useful model for investigating PrP-linked neurodegenerative mechanisms.

The structure of prion: is it enough for interpreting the diverse phenotypes of prion diseases?

Prion diseases, or transmissible spongiform encephalopathies, are neurodegenerative diseases, which affect human and many species of animals with 100% fatality rate. The most accepted etiology for prion disease is 'prion', which arises from the conversion from cellular PrP(C) to the pathological PrP(Sc). This review discussed the characteristic structure of PrP, including PRNP gene, PrP(C), PrP(Sc), PrP amyloid, and prion strains.

Evidence of effective scrapie transmission via colostrum and milk in sheep

Background

Evidence for scrapie transmission from VRQ/VRQ ewes to lambs via milk was first reported in 2008 but in that study there were concerns that lateral transmission may have contributed to the high transmission rate observed since five control lambs housed with the milk recipients also became infected. This report provides further information obtained from two follow-up studies, one where milk recipients were housed separately after milk consumption to confirm the validity of the high scrapie transmission rate via milk and the second to assess any difference in infectivity from colostrum and subsequent milk. Protein misfolding cyclic amplification (PMCA) was also used to detect prion protein in milk samples as a comparison with the infectivity data and extended to milk samples from ewes without a VRQ allele.

Results

Seven pairs of lambs fed colostrum and milk individually from seven scrapie-affected sheep (pre-clinical or clinical) presented with disease-associated prion protein, PrP^d, in rectal lymphoid tissue at 4–5 months of age. Five further pairs of lambs fed either colostrum or subsequent milk from five pre-clinical scrapie-affected sheep equally presented with PrP^d in lymphoid tissue by 9 months of age. Nine sheep were lost due to intercurrent diseases but all remaining milk or colostrum recipients, including those in the original study with the lateral transmission controls, developed clinical signs of scrapie from 19 months of age and scrapie was confirmed by brain examination. Unexposed control sheep totalling 19 across all three studies showed no evidence of infection.

Scrapie PrP was amplified repeatedly by PMCA in all tested milk samples from scrapie-affected VRQ/VRQ sheep, and in one scrapie-affected ARQ/ARQ sheep. By contrast, milk samples from five VRQ/VRQ and 11 ARQ/ARQ scrapie-free sheep did not have detectable scrapie PrP on repeated tests.

Conclusions

Feeding of milk from scrapie-affected sheep results in a high transmission rate in VRQ/VRQ sheep and both colostrum and milk transmit scrapie. Detection of scrapie prion protein in individual milk samples from scrapie-affected ewes confirms PMCA as a valuable in vitro test.