Mouse-adapted scrapie infection of SN56 cells: greater efficiency with microsome-associated versus purified PrP-res

Interactions of amyloid coaggregates with biomolecules and its relevance to neurodegeneration

The aggregation of amyloidogenic proteins is a pathological hallmark of various neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. In these diseases, oligomeric intermediates or toxic aggregates of amyloids cause neuronal damage and degeneration. Despite the substantial effort made over recent decades to implement therapeutic interventions, these neurodegenerative diseases are not yet understood at the molecular level. In many cases, multiple disease-causing amyloids overlap in a sole pathological feature or a sole disease-causing amyloid represents multiple pathological features. Various amyloid pathologies can coexist in the same brain with or without clinical presentation and may even occur in individuals without disease. From sparse data, speculation has arisen regarding the coaggregation of amyloids with disparate amyloid species and other biomolecules, which are the same characteristics that make diagnostics and drug development challenging. However, advances in research related to biomolecular condensates and structural analysis have been used to overcome some of these challenges. Considering the development of these resources and techniques, herein we review the cross-seeding of amyloidosis, for example, involving the amyloids amyloid β, tau, α-synuclein, and human islet amyloid polypeptide, and their cross-inhibition by transthyretin and BRICHOS. The interplay of nucleic acid-binding proteins, such as prions, TAR DNA-binding protein 43, fused in sarcoma/translated in liposarcoma, and fragile X mental retardation polyglycine, with nucleic acids in the pathology of neurodegeneration are also described, and we thereby highlight the potential clinical applications in central nervous system therapy.

Genetically engineered cellular models of prion propagation

For over three decades, cultured cells have been a useful tool for dissecting the molecular details of prion replication and the identification of candidate therapeutics for prion disease. A major issue limiting the translatability of these studies has been the inability to reliably propagate disease-relevant, non-mouse strains of prions in cells relevant to prion pathogenesis. In recent years, fueled by advances in gene editing technology, it has become possible to propagate prions from hamsters, cervids, and sheep in immortalized cell lines originating from the central nervous system. In particular, the use of CRISPR-Cas9-mediated gene editing to generate versions of prion-permissive cell lines that lack endogenous PrP expression has provided a blank canvas upon which re-expression of PrP leads to species-matched susceptibility to prion infection. When coupled with the ability to propagate prions in cells or organoids derived from stem cells, these next-generation cellular models should provide an ideal paradigm for identifying small molecules and other biological therapeutics capable of interfering with prion replication in animal and human prion disorders. In this review, we summarize recent advances that have widened the spectrum of prion strains that can be propagated in cultured cells and cutting-edge tissue-based models.

Propagation and Dissemination Strategies of Transmissible Spongiform Encephalopathy Agents in Mammalian Cells

Transmissible spongiform encephalopathies or prion disorders are fatal infectious diseases that cause characteristic spongiform degeneration in the central nervous system. The causative agent, the so-called prion, is an unconventional infectious agent that propagates by converting the host-encoded cellular prion protein PrP into ordered protein aggregates with infectious properties. Prions are devoid of coding nucleic acid and thus rely on the host cell machinery for propagation. While it is now established that, in addition to PrP, other cellular factors or processes determine the susceptibility of cell lines to prion infection, exact factors and cellular processes remain broadly obscure. Still, cellular models have uncovered important aspects of prion propagation and revealed intercellular dissemination strategies shared with other intracellular pathogens. Here, we summarize what we learned about the processes of prion invasion, intracellular replication and subsequent dissemination from ex vivo cell models.

Phase separation of the mammalian prion protein: physiological and pathological perspectives

Abnormal phase transitions have been implicated in the occurrence of proteinopathies. Disordered proteins with nucleic acid binding ability drive the formation of reversible micron-sized condensates capable of controlling nucleic acid processing/transport. This mechanism, achieved via liquid-liquid phase separation (LLPS), underlies the formation of long-studied membraneless organelles (e.g., nucleolus) and various transient condensates formed by driver proteins. The prion protein (PrP) is not a classical nucleic acid-binding protein. However, it binds nucleic acids with high affinity, undergoes nucleocytoplasmic shuttling, contains a long intrinsically disordered region rich in glycines and evenly spaced aromatic residues, among other biochemical/biophysical properties of bona fide drivers of phase transitions. Because of this, our group and others have characterized LLPS of recombinant PrP. In vitro phase separation of PrP is modulated by nucleic acid aptamers, and, depending on the aptamer conformation, the liquid droplets evolve to solid-like species. Herein we discuss recent studies and previous evidence supporting PrP phase transitions. We focus on the central role of LLPS related to PrP physiology and pathology, with a special emphasis on the interaction of PrP with different ligands, such as proteins and nucleic acids, which can play a role in prion disease pathogenesis. Finally, we comment on therapeutic strategies directed at the nonfunctional phase separation that could potentially tackle prion diseases or other protein misfolding disorders.

From Cell Culture to Organoids-Model Systems for Investigating Prion Strain Characteristics

Prion diseases are the hallmark protein folding neurodegenerative disease. Their transmissible nature has allowed for the development of many different cellular models of disease where prion propagation and sometimes pathology can be induced. This review examines the range of simple cell cultures to more complex neurospheres, organoid, and organotypic slice cultures that have been used to study prion disease pathogenesis and to test therapeutics. We highlight the advantages and disadvantages of each system, giving special consideration to the importance of strains when choosing a model and when interpreting results, as not all systems propagate all strains, and in some cases, the technique used, or treatment applied, can alter the very strain properties being studied.

An astrocyte cell line that differentially propagates murine prions

Prion diseases are fatal infectious neurodegenerative disorders in human and animals caused by misfolding of the cellular prion protein (PrP^C) into the pathological isoform PrP^Sc. Elucidating the molecular and cellular mechanisms underlying prion propagation may help to develop disease interventions. Cell culture systems for prion propagation have greatly advanced molecular insights into prion biology, but translation of in vitro to in vivo findings is often disappointing. A wider range of cell culture systems might help overcome these shortcomings. Here, we describe an immortalized mouse neuronal astrocyte cell line (C8D1A) that can be infected with murine prions. Both PrP^C protein and mRNA levels in astrocytes were comparable with those in neuronal and non-neuronal cell lines permitting persistent prion infection. We challenged astrocytes with three mouse-adapted prion strains (22L, RML, and ME7) and cultured them for six passages. Immunoblotting results revealed that the astrocytes propagated 22L prions well over all six passages, whereas ME7 prions did not replicate, and RML prions replicated only very weakly after five passages. Immunofluorescence analysis indicated similar results for PrP^Sc. Interestingly, when we used prion conversion activity as a readout in real-time quaking-induced conversion assays with RML-infected cell lysates, we observed a strong signal over all six passages, comparable with that for 22L-infected cells. These data indicate that the C8D1A cell line is permissive to prion infection. Moreover, the propagated prions differed in conversion and proteinase K–resistance levels in these astrocytes. We propose that the C8D1A cell line could be used to decipher prion strain biology.

Cellular models for discovering prion disease therapeutics: Progress and challenges

Prions, which cause fatal neurodegenerative disorders such as Creutzfeldt-Jakob disease, are misfolded and infectious protein aggregates. Currently, there are no treatments available to halt or even delay the progression of prion disease in the brain. The infectious nature of prions has resulted in animal paradigms that accurately recapitulate all aspects of prion disease, and these have proven to be instrumental for testing the efficacy of candidate therapeutics. Nonetheless, infection of cultured cells with prions provides a much more powerful system for identifying molecules capable of interfering with prion propagation. Certain lines of cultured cells can be chronically infected with various types of mouse prions, and these models have been used to unearth candidate anti-prion drugs that are at least partially efficacious when administered to prion-infected rodents. However, these studies have also revealed that not all types of prions are equal, and that drugs active against mouse prions are not necessarily effective against prions from other species. Despite some recent progress, the number of cellular models available for studying non-mouse prions remains limited. In particular, human prions have proven to be particularly challenging to propagate in cultured cells, which has severely hindered the discovery of drugs for Creutzfeldt-Jakob disease. In this review, we summarize the cellular models that are presently available for discovering and testing drugs capable of blocking the propagation of prions and highlight challenges that remain on the path towards developing therapies for prion disease.

All the Same? The Secret Life of Prion Strains within Their Target Cells

Prions are infectious β-sheet-rich protein aggregates composed of misfolded prion protein (PrP^Sc) that do not possess coding nucleic acid. Prions replicate by recruiting and converting normal cellular PrP^C into infectious isoforms. In the same host species, prion strains target distinct brain regions and cause different disease phenotypes. Prion strains are associated with biophysically distinct PrP^Sc conformers, suggesting that strain properties are enciphered within alternative PrP^Sc quaternary structures. So far it is unknown how prion strains target specific cells and initiate productive infections. Deeper mechanistic insight into the prion life cycle came from cell lines permissive to a range of different prion strains. Still, it is unknown why certain cell lines are refractory to infection by one strain but permissive to another. While pharmacologic and genetic manipulations revealed subcellular compartments involved in prion replication, little is known about strain-specific requirements for endocytic trafficking pathways. This review summarizes our knowledge on how prions replicate within their target cells and on strain-specific differences in prion cell biology.

Engineering a murine cell line for the stable propagation of hamster prions

Prions are infectious protein aggregates that cause several fatal neurodegenerative diseases. Prion research has been hindered by a lack of cellular paradigms for studying the replication of prions from different species. Although hamster prions have been widely used to study prion replication in animals and within in vitro amplification systems, they have proved challenging to propagate in cultured cells. Because the murine catecholaminergic cell line CAD5 is susceptible to a diverse range of mouse prion strains, we hypothesized that it might also be capable of propagating nonmouse prions. Here, using CRISPR/Cas9-mediated genome engineering, we demonstrate that CAD5 cells lacking endogenous mouse PrP expression (CAD5-PrP^-/- cells) can be chronically infected with hamster prions following stable expression of hamster PrP. When exposed to the 263K, HY, or 139H hamster prion strains, these cells stably propagated high levels of protease-resistant PrP. Hamster prion replication required absence of mouse PrP, and hamster PrP inhibited the propagation of mouse prions. Cellular homogenates from 263K-infected cells exhibited prion seeding activity in the RT-QuIC assay and were infectious to naïve cells expressing hamster PrP. Interestingly, murine N2a neuroblastoma cells ablated for endogenous PrP expression were susceptible to mouse prions, but not hamster prions upon expression of cognate PrP, suggesting that CAD5 cells either possess cellular factors that enhance or lack factors that restrict the diversity of prion strains that can be propagated. We conclude that transfected CAD5-PrP^-/- cells may be a useful tool for assessing the biology of prion strains and dissecting the mechanism of prion replication.

Cell biology of prion infection

The development of multiple cell culture models of prion infection over the last two decades has led to a significant increase in our understanding of how prions infect cells. In particular, new techniques to distinguish exogenous from endogenous prions have allowed us for the first time to look in depth at the earliest stages of prion infection through to the establishment of persistent infection. These studies have shown that prions can infect multiple cell types, both neuronal and nonneuronal. Once in contact with the cell, they are rapidly taken up via multiple endocytic pathways. After uptake, the initial replication of prions occurs almost immediately on the plasma membrane and within multiple endocytic compartments. Following this acute stage of prion replication, persistent prion infection may or may not be established. Establishment of a persistent prion infection in cells appears to depend upon the achievement of a delicate balance between the rate of prion replication and degradation, the rate of cell division, and the efficiency of prion spread from cell to cell. Overall, cell culture models have shown that prion infection of the cell is a complex and variable process which can involve multiple cellular pathways and compartments even within a single cell.

Cellular mechanisms responsible for cell-to-cell spreading of prions

Prions are infectious agents that cause fatal neurodegenerative diseases. Current evidence indicates that they are essentially composed of an abnormally folded protein (PrPSc). These abnormal aggregated PrPSc species multiply in infected cells by recruiting and converting the host PrPC protein into new PrPSc. How prions move from cell to cell and progressively spread across the infected tissue is of crucial importance and may provide experimental opportunity to delay the progression of the disease. In infected cells, different mechanisms have been identified, including release of infectious extracellular vesicles and intercellular transfer of PrPSc-containing organelles through tunneling nanotubes. These findings should allow manipulation of the intracellular trafficking events targeting PrPSc in these particular subcellular compartments to experimentally address the relative contribution of these mechanisms to in vivo prion pathogenesis. In addition, such information may prompt further experimental strategies to decipher the causal roles of protein misfolding and aggregation in other human neurodegenerative diseases.

Immunodetection of PrPSc Using Western Immunoblotting Techniques

Western immunoblotting is a workhorse technique used in the prion field to analyze disease-associated forms of the prion protein, termed PrP^Sc. The biochemical stability of PrP^Sc aggregates combined with the increased resistance of prion infectivity to inactivation by various treatments that inactivate most other pathogens complicates the use of Western immunoblotting as a means to characterize PrP^Sc samples. In this chapter, we describe a method for Western immunoblot analysis of PrP^Sc with an emphasis on precautions to address the biochemical and biosafety considerations associated with this procedure.

Generation of Infectious Prions and Detection with the Prion-Infected Cell Assay

Cell lines propagating prions are an efficient and useful means for studying the cellular and molecular mechanisms implicated in prion disease. Utilization of cell-based models has led to the finding that PrP^C and PrP^Sc are released from cells in association with extracellular vesicles known as exosomes. Exosomes have been shown to act as vehicles for infectivity, transferring infectivity between cell lines and providing a mechanism for prion spread between tissues. Here, we describe the methods for generating a prion-propagating cell line with prion-infected brain homogenate, cell lysate, conditioned media, and exosomes and also detection of protease-resistant PrP with the prion-infected cell assay.

Prion strains depend on different endocytic routes for productive infection

Prions are unconventional agents composed of misfolded prion protein that cause fatal neurodegenerative diseases in mammals. Prion strains induce specific neuropathological changes in selected brain areas. The mechanism of strain-specific cell tropism is unknown. We hypothesised that prion strains rely on different endocytic routes to invade and replicate within their target cells. Using prion permissive cells, we determined how impairment of endocytosis affects productive infection by prion strains 22L and RML. We demonstrate that early and late stages of prion infection are differentially sensitive to perturbation of clathrin- and caveolin-mediated endocytosis. Manipulation of canonical endocytic pathways only slightly influenced prion uptake. However, blocking the same routes had drastic strain-specific consequences on the establishment of infection. Our data argue that prion strains use different endocytic pathways for infection and suggest that cell type-dependent differences in prion uptake could contribute to host cell tropism.

Unraveling Prion Protein Interactions with Aptamers and Other PrP-Binding Nucleic Acids

Transmissible spongiform encephalopathies (TSEs) are a group of neurodegenerative disorders that affect humans and other mammals. The etiologic agents common to these diseases are misfolded conformations of the prion protein (PrP). The molecular mechanisms that trigger the structural conversion of the normal cellular PrP (PrP^C) into the pathogenic conformer (PrP^Sc) are still poorly understood. It is proposed that a molecular cofactor would act as a catalyst, lowering the activation energy of the conversion process, therefore favoring the transition of PrP^C to PrP^Sc. Several in vitro studies have described physical interactions between PrP and different classes of molecules, which might play a role in either PrP physiology or pathology. Among these molecules, nucleic acids (NAs) are highlighted as potential PrP molecular partners. In this context, the SELEX (Systematic Evolution of Ligands by Exponential Enrichment) methodology has proven extremely valuable to investigate PrP–NA interactions, due to its ability to select small nucleic acids, also termed aptamers, that bind PrP with high affinity and specificity. Aptamers are single-stranded DNA or RNA oligonucleotides that can be folded into a wide range of structures (from harpins to G-quadruplexes). They are selected from a nucleic acid pool containing a large number (10¹⁴–10¹⁶) of random sequences of the same size (~20–100 bases). Aptamers stand out because of their potential ability to bind with different affinities to distinct conformations of the same protein target. Therefore, the identification of high-affinity and selective PrP ligands may aid the development of new therapies and diagnostic tools for TSEs. This review will focus on the selection of aptamers targeted against either full-length or truncated forms of PrP, discussing the implications that result from interactions of PrP with NAs, and their potential advances in the studies of prions. We will also provide a critical evaluation, assuming the advantages and drawbacks of the SELEX (Systematic Evolution of Ligands by Exponential Enrichment) technique in the general field of amyloidogenic proteins.

PrP Knockout Cells Expressing Transmembrane PrP Resist Prion Infection

Glycosylphosphatidylinositol (GPI) anchoring of the prion protein (PrP^C) influences PrP^C misfolding into the disease-associated isoform, PrP^res, as well as prion propagation and infectivity. GPI proteins are found in cholesterol- and sphingolipid-rich membrane regions called rafts. Exchanging the GPI anchor for a nonraft transmembrane sequence redirects PrP^C away from rafts. Previous studies showed that nonraft transmembrane PrP^C variants resist conversion to PrP^res when transfected into scrapie-infected N2a neuroblastoma cells, likely due to segregation of transmembrane PrP^C and GPI-anchored PrP^res in distinct membrane environments. Thus, it remained unclear whether transmembrane PrP^C might convert to PrP^res if seeded by an exogenous source of PrP^res not associated with host cell rafts and without the potential influence of endogenous expression of GPI-anchored PrP^C To further explore these questions, constructs containing either a C-terminal wild-type GPI anchor signal sequence or a nonraft transmembrane sequence containing a flexible linker were expressed in a cell line derived from PrP knockout hippocampal neurons, NpL2. NpL2 cells have physiological similarities to primary neurons, representing a novel and advantageous model for studying transmissible spongiform encephalopathy (TSE) infection. Cells were infected with inocula from multiple prion strains and in different biochemical states (i.e., membrane bound as in brain microsomes from wild-type mice or purified GPI-anchorless amyloid fibrils). Only GPI-anchored PrP^C supported persistent PrP^res propagation. Our data provide strong evidence that in cell culture GPI anchor-directed membrane association of PrP^C is required for persistent PrP^res propagation, implicating raft microdomains as a location for conversion.

IMPORTANCE

Mechanisms of prion propagation, and what makes them transmissible, are poorly understood. Glycosylphosphatidylinositol (GPI) membrane anchoring of the prion protein (PrP^C) directs it to specific regions of cell membranes called rafts. In order to test the importance of the raft environment on prion propagation, we developed a novel model for prion infection where cells expressing either GPI-anchored PrP^C or transmembrane-anchored PrP^C, which partitions it to a different location, were treated with infectious, misfolded forms of the prion protein, PrP^res We show that only GPI-anchored PrP^C was able to convert to PrP^res and able to serially propagate. The results strongly suggest that GPI anchoring and the localization of PrP^C to rafts are crucial to the ability of PrP^C to propagate as a prion.

Modulation of Glycosaminoglycans Affects PrPSc Metabolism but Does Not Block PrPSc Uptake

Mammalian prions are unconventional infectious agents composed primarily of the misfolded aggregated host prion protein PrP, termed PrP(Sc). Prions propagate by the recruitment and conformational conversion of cellular prion protein into abnormal prion aggregates on the cell surface or along the endocytic pathway. Cellular glycosaminoglycans have been implicated as the first attachment sites for prions and cofactors for cellular prion replication. Glycosaminoglycan mimetics and obstruction of glycosaminoglycan sulfation affect prion replication, but the inhibitory effects on different strains and different stages of the cell infection have not been thoroughly addressed. We examined the effects of a glycosaminoglycan mimetic and undersulfation on cellular prion protein metabolism, prion uptake, and the establishment of productive infections in L929 cells by two mouse-adapted prion strains. Surprisingly, both treatments reduced endogenous sulfated glycosaminoglycans but had divergent effects on cellular PrP levels. Chemical or genetic manipulation of glycosaminoglycans did not prevent PrP(Sc) uptake, arguing against their roles as essential prion attachment sites. However, both treatments effectively antagonized de novo prion infection independently of the prion strain and reduced PrP(Sc) formation in chronically infected cells. Our results demonstrate that sulfated glycosaminoglycans are dispensable for prion internalization but play a pivotal role in persistently maintained PrP(Sc) formation independent of the prion strain.

IMPORTANCE

Recently, glycosaminoglycans (GAGs) became the focus of neurodegenerative disease research as general attachment sites for cell invasion by pathogenic protein aggregates. GAGs influence amyloid formation in vitro. GAGs are also found in intra- and extracellular amyloid deposits. In light of the essential role GAGs play in proteinopathies, understanding the effects of GAGs on protein aggregation and aggregate dissemination is crucial for therapeutic intervention. Here, we show that GAGs are dispensable for prion uptake but play essential roles in downstream infection processes. GAG mimetics also affect cellular GAG levels and localization and thus might affect prion propagation by depleting intracellular cofactor pools.

Extracellular vesicles--Their role in the packaging and spread of misfolded proteins associated with neurodegenerative diseases

Many cell types, including neurons, are known to release small membranous vesicles known as exosomes. In addition to their protein content these vesicles have recently been shown to contain messenger RNA (mRNA) and micro RNA (miRNA) species. Roles for these vesicles include cell-cell signalling, removal of unwanted proteins, and transfer of pathogens (including prion-like misfolded proteins) between cells, such as infectious prions. Prions are the infectious particles that are responsible for transmissible neurodegenerative diseases such as Creutzfeldt-Jakob disease (CJD) of humans or bovine spongiform encephalopathy (BSE) of cattle. Exosomes are also involved in processing the amyloid precursor protein (APP), which is associated with Alzheimer's disease (AD). As exosomes can be isolated from circulating fluids such as serum, urine, and cerebrospinal fluid (CSF), they provide a potential source of biomarkers for neurological conditions. Here, we review the roles these vesicles play in neurodegenerative disease and highlight their potential in diagnosing these disorders through analysis of their RNA content.

The multivesicular body is the major internal site of prion conversion

The conversion of the properly folded prion protein, PrPc, to its misfolded amyloid form, PrPsc, occurs as the two proteins traffic along the endocytic pathway and PrPc is exposed to PrPsc. To determine the specific site of prion conversion, we knocked down various proteins in the endocytic pathway including Rab7a, Tsg101 and Hrs (also known as HGS). PrPsc was markedly reduced in two chronically infected cell lines by preventing the maturation of the multivesicular body, a process that begins in the early endosome and ends with the sorting of cargo to the lysosome. By contrast, knocking down proteins in the retromer complex, which diverts cargo away from the multivesicular body caused an increase in PrPsc levels. These results suggest that the multivesicular body is the major site for intracellular conversion of PrPc to PrPsc.

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.

Brain iron homeostasis: from molecular mechanisms to clinical significance and therapeutic opportunities

Iron has emerged as a significant cause of neurotoxicity in several neurodegenerative conditions, including Alzheimer's disease (AD), Parkinson's disease (PD), sporadic Creutzfeldt-Jakob disease (sCJD), and others. In some cases, the underlying cause of iron mis-metabolism is known, while in others, our understanding is, at best, incomplete. Recent evidence implicating key proteins involved in the pathogenesis of AD, PD, and sCJD in cellular iron metabolism suggests that imbalance of brain iron homeostasis associated with these disorders is a direct consequence of disease pathogenesis. A complete understanding of the molecular events leading to this phenotype is lacking partly because of the complex regulation of iron homeostasis within the brain. Since systemic organs and the brain share several iron regulatory mechanisms and iron-modulating proteins, dysfunction of a specific pathway or selective absence of iron-modulating protein(s) in systemic organs has provided important insights into the maintenance of iron homeostasis within the brain. Here, we review recent information on the regulation of iron uptake and utilization in systemic organs and within the complex environment of the brain, with particular emphasis on the underlying mechanisms leading to brain iron mis-metabolism in specific neurodegenerative conditions. Mouse models that have been instrumental in understanding systemic and brain disorders associated with iron mis-metabolism are also described, followed by current therapeutic strategies which are aimed at restoring brain iron homeostasis in different neurodegenerative conditions. We conclude by highlighting important gaps in our understanding of brain iron metabolism and mis-metabolism, particularly in the context of neurodegenerative disorders.

Prions Ex Vivo: What Cell Culture Models Tell Us about Infectious Proteins

Prions are unconventional infectious agents that are composed of misfolded aggregated prion protein. Prions replicate their conformation by template-assisted conversion of the endogenous prion protein PrP. Templated conversion of soluble proteins into protein aggregates is also a hallmark of other neurodegenerative diseases. Alzheimer's disease or Parkinson's disease are not considered infectious diseases, although aggregate pathology appears to progress in a stereotypical fashion reminiscent of the spreading behavior ofmammalian prions. While basic principles of prion formation have been studied extensively, it is still unclear what exactly drives PrP molecules into an infectious, self-templating conformation. In this review, we discuss crucial steps in the life cycle of prions that have been revealed in ex vivo models. Importantly, the persistent propagation of prions in mitotically active cells argues that cellular processes are in place that not only allow recruitment of cellular PrP into growing prion aggregates but also enable the multiplication of infectious seeds that are transmitted to daughter cells. Comparison of prions with other protein aggregates demonstrates that not all the characteristics of prions are equally shared by prion-like aggregates. Future experiments may reveal to which extent aggregation-prone proteins associated with other neurodegenerative diseases can copy the replication strategies of prions.

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.

Prions and the potential transmissibility of protein misfolding diseases

Prions, or infectious proteins, represent a major frontier in the study of infectious agents. The prions responsible for mammalian transmissible spongiform encephalopathies (TSEs) are due primarily to infectious self-propagation of misfolded prion proteins. TSE prion structures remain ill-defined, other than being highly structured, self-propagating, and often fibrillar protein multimers with the capacity to seed, or template, the conversion of their normal monomeric precursors into a pathogenic form. Purified TSE prions usually take the form of amyloid fibrils, which are self-seeding ultrastructures common to many serious protein misfolding diseases such as Alzheimer's, Parkinson's, Huntington's and Lou Gehrig's (amytrophic lateral sclerosis). Indeed, recent reports have now provided evidence of prion-like propagation of several misfolded proteins from cell to cell, if not from tissue to tissue or individual to individual. These findings raise concerns that various protein misfolding diseases might have spreading, prion-like etiologies that contribute to pathogenesis or prevalence.

Role of lipid in forming an infectious prion?

The infectious agent of the transmissible spongiform encephalopathies, or prion diseases, has been the center of intense debate for decades. Years of studies have provided overwhelming evidence to support the prion hypothesis that posits a protein conformal infectious agent is responsible for the transmissibility of the disease. The recent studies that generate prion infectivity with purified bacterially expressed recombinant prion protein not only provides convincing evidence supporting the core of the prion hypothesis, that a pathogenic conformer of host prion protein is able to seed the conversion of its normal counterpart to the likeness of itself resulting in the replication of the pathogenic conformer and occurrence of disease, they also indicate the importance of cofactors, particularly lipid or lipid-like molecules, in forming the protein conformation-based infectious agent. This article reviews the literature regarding the chemical nature of the infectious agent and the potential contribution from lipid molecules to prion infectivity, and discusses the important remaining questions in this research area.

Cellular aspects of prion replication in vitro

Prion diseases or transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative disorders in mammals that are caused by unconventional agents predominantly composed of aggregated misfolded prion protein (PrP). Prions self-propagate by recruitment of host-encoded PrP into highly ordered β-sheet rich aggregates. Prion strains differ in their clinical, pathological and biochemical characteristics and are likely to be the consequence of distinct abnormal prion protein conformers that stably replicate their alternate states in the host cell. Understanding prion cell biology is fundamental for identifying potential drug targets for disease intervention. The development of permissive cell culture models has greatly enhanced our knowledge on entry, propagation and dissemination of TSE agents. However, despite extensive research, the precise mechanism of prion infection and potential strain effects remain enigmatic. This review summarizes our current knowledge of the cell biology and propagation of prions derived from cell culture experiments. We discuss recent findings on the trafficking of cellular and pathologic PrP, the potential sites of abnormal prion protein synthesis and potential co-factors involved in prion entry and propagation.

Prion propagation and toxicity occur in vitro with two-phase kinetics specific to strain and neuronal type

Prion diseases, or transmissible spongiform encephalopathies (TSEs), are fatal neurodegenerative disorders that occur in humans and animals. The neuropathological hallmarks of TSEs are spongiosis, glial proliferation, and neuronal loss. The only known specific molecular marker of TSEs is the abnormal isoform (PrP(Sc)) of the host-encoded prion protein (PrP(C)), which accumulates in the brain of infected subjects and forms infectious prion particles. Although this transmissible agent lacks a specific nucleic acid component, several prion strains have been isolated. Prion strains are characterized by differences in disease outcome, PrP(Sc) distribution patterns, and brain lesion profiles at the terminal stage of the disease. The molecular factors and cellular mechanisms involved in strain-specific neuronal tropism and toxicity remain largely unknown. Currently, no cellular model exists to facilitate in vitro studies of these processes. A few cultured cell lines that maintain persistent scrapie infections have been developed, but only two of them have shown the cytotoxic effects associated with prion propagation. In this study, we have developed primary neuronal cultures to assess in vitro neuronal tropism and toxicity of different prion strains (scrapie strains 139A, ME7, and 22L). We have tested primary neuronal cultures enriched in cerebellar granular, striatal, or cortical neurons. Our results showed that (i) a strain-specific neuronal tropism operated in vitro; (ii) the cytotoxic effect varied among strains and neuronal cell types; (iii) prion propagation and toxicity occurred in two kinetic phases, a replicative phase followed by a toxic phase; and (iv) neurotoxicity peaked when abnormal PrP accumulation reached a plateau.

Exosomes: vehicles for the transfer of toxic proteins associated with neurodegenerative diseases?

Exosomes are small membranous vesicles secreted by a number of cell types including neurons and can be isolated from conditioned cell media or bodily fluids such as urine and plasma. Exosome biogenesis involves the inward budding of endosomes to form multivesicular bodies (MVB). When fused with the plasma membrane, the MVB releases the vesicles into the extracellular environment as exosomes. Proposed functions of these vesicles include roles in cell–cell signaling, removal of unwanted proteins, and the transfer of pathogens between cells. One such pathogen which exploits this pathway is the prion, the infectious particle responsible for the transmissible neurodegenerative diseases such as Creutzfeldt–Jakob disease (CJD) of humans or bovine spongiform encephalopathy (BSE) of cattle. Similarly, exosomes are also involved in the processing of the amyloid precursor protein (APP) which is associated with Alzheimer’s disease. Exosomes have been shown to contain full-length APP and several distinct proteolytically cleaved products of APP, including Aβ. In addition, these fragments can be modulated using inhibitors of the proteases involved in APP cleavage. These observations provide further evidence for a novel pathway in which PrP and APP fragments are released from cells. Other proteins such as superoxide dismutase I and alpha-synuclein (involved in amyotrophic lateral sclerosis and Parkinson’s disease, respectively) are also found associated with exosomes. This review will focus on the role of exosomes in neurodegenerative disorders and discuss the potential of these vesicles for the spread of neurotoxicity, therapeutics, and diagnostics for these diseases.

A simple, versatile and sensitive cell-based assay for prions from various species

Detection and quantification of prion infectivity is a crucial step for various fundamental and applied aspects of prion research. Identification of cell lines highly sensitive to prion infection led to the development of cell-based titration procedures aiming at replacing animal bioassays, usually performed in mice or hamsters. However, most of these cell lines are only permissive to mouse-adapted prions strains and do not allow titration of prions from other species. In this study, we show that epithelial RK13, a cell line permissive to mouse and bank vole prion strains and to natural prion agents from sheep and cervids, enables a robust and sensitive detection of mouse and ovine-derived prions. Importantly, the cell culture work is strongly reduced as the RK13 cell assay procedure designed here does not require subcultivation of the inoculated cultures. We also show that prions effectively bind to culture plastic vessel and are quantitatively detected by the cell assay. The possibility to easily quantify a wider range of prions, including rodent experimental strains but also natural agents from sheep and cervids, should prompt the spread of cell assays for routine prion titration and lead to valuable information in fundamental and applied studies.

Prion hypothesis: the end of the controversy?

Forty-three years have passed since it was first proposed that a protein could be the sole component of the infectious agent responsible for the enigmatic prion diseases. Many discoveries have strongly supported the prion hypothesis, but only recently has this once heretical hypothesis been widely accepted by the scientific community. In the past 3 years, researchers have achieved the 'Holy Grail' demonstration that infectious material can be generated in vitro using completely defined components. These breakthroughs have proven that a misfolded protein is the active component of the infectious agent, and that propagation of the disease and its unique features depend on the self-replication of the infectious folding of the prion protein. In spite of these important discoveries, it remains unclear whether another molecule besides the misfolded prion protein might be an essential element of the infectious agent. Future research promises to reveal many more intriguing features about the rogue prions.

Characterization of the role of dendritic cells in prion transfer to primary neurons

TSEs (transmissible spongiform encephalopathies) are neurodegenerative diseases caused by pathogenic isoforms (PrPSc) of the host-encoded PrPc (cellular prion protein). After consumption of contaminated food, PrPSc deposits rapidly accumulate in lymphoid tissues before invasion of the CNS (central nervous system). However, the mechanisms of prion spreading from the periphery to the nervous system are still unclear. In the present study, we investigated the role of DCs (dendritic cells) in the spreading of prion infection to neuronal cells. First, we determined that BMDCs (bone-marrow-derived DCs) rapidly uptake PrPSc after exposure to infected brain homogenate. Next, we observed a progressive catabolism of the internalized prion aggregates. Similar experiments performed with BMDCs isolated from KO (knockout) mice or mice overexpressing PrP (tga20) indicate that both PrPSc uptake and catabolism are independent of PrPc expression in these cells. Finally, using co-cultures of prion-loaded BMDCs and cerebellar neurons, we characterized the transfer of the prion protein and the resulting infection of the neuronal cultures. Interestingly, the transfer of PrPSc was triggered by direct cell-cell contact. As a consequence, BMDCs retained the prion protein when cultured alone, and no transfer to the recipient neurons was observed when a filter separated the two cultures or when neurons were exposed to the BMDC-conditioned medium. Additionally, fixed BMDCs also failed to transfer prion infectivity to neurons, suggesting an active transport of prion aggregates, in accordance with a role of TNTs (tunnelling nanotubes) observed in the co-cultures.

Role of the highly conserved middle region of prion protein (PrP) in PrP-lipid interaction

Converting normal prion protein (PrP(C)) to the pathogenic PrP(Sc) isoform is central to prion disease. We previously showed that, in the presence of lipids, recombinant mouse PrP (rPrP) can be converted into the highly infectious conformation, suggesting a crucial role of lipid-rPrP interaction in PrP conversion. To understand the mechanism of lipid-rPrP interaction, we analyzed the ability of various rPrP mutants to bind anionic lipids and to gain lipid-induced proteinase K (PK) resistance. We found that the N-terminal positively charged region contributes to electrostatic rPrP-lipid binding but does not affect lipid-induced PK resistance. In contrast, the highly conserved middle region of PrP, consisting of a positively charged region and a hydrophobic domain, is essential for lipid-induced rPrP conversion. The hydrophobic domain deletion mutant significantly weakened the hydrophobic rPrP-lipid interaction and abolished the lipid-induced C-terminal PK resistance. The rPrP mutant without positive charges in the middle region reduced the amount of the lipid-induced PK-resistant rPrP form. Consistent with a critical role of the middle region in lipid-induced rPrP conversion, both disease-associated P105L and P102L mutations, localized between lysine residues in the positively charged region, significantly affected lipid-induced rPrP conversion. The hydrophobic domain-localized 129 polymorphism altered the strength of hydrophobic rPrP-lipid interaction. Collectively, our results suggest that the interaction between the middle region of PrP and lipids is essential for the formation of the PK-resistant conformation. Moreover, the influence of disease-associated PrP mutations and the 129 polymorphism on PrP-lipid interaction supports the relevance of PrP-lipid interaction to the pathogenesis of prion disease.

Probing the conformation of a prion protein fibril with hydrogen exchange

A fragment of the prion protein, PrP(89-143, P101L), bearing a mutation implicated in familial prion disease, forms fibrils that have been shown to induce prion disease when injected intracerebrally into transgenic mice expressing full-length PrP containing the P101L mutation. In this study, we utilize amide hydrogen exchange measurements to probe the organization of the peptide in its fibrillar form. We determined the extent of hydrogen exchange first by tandem proteolysis, liquid chromatography, and mass spectrometry (HXMS) and then by exchange-quenched NMR. Although single amide resolution is afforded by NMR measurements, HXMS is well suited to the study of natural prions because it does not require labeling with NMR active isotopes. Thus, natural prions obtained from infected animals can be compared with model systems such as PrP(89-143, P101L) studied here. In our study, we find two segments of sequence that display a high level of protection from exchange, residues 102-109 and 117-136. In addition, there is a region that displays exchange behavior consistent with the presence of a conformationally heterogeneous turn. We discuss our data with respect to several structural models proposed for infectious PrP aggregates and highlight HXMS as one of the few techniques well suited to studying natural prions.

Neuronal low-density lipoprotein receptor-related protein 1 binds and endocytoses prion fibrils via receptor cluster 4

For infectious prion protein (designated PrP(Sc)) to act as a template to convert normal cellular protein (PrP(C)) to its distinctive pathogenic conformation, the two forms of prion protein (PrP) must interact closely. The neuronal receptor that rapidly endocytoses PrP(C) is the low-density lipoprotein receptor-related protein 1 (LRP1). We show here that on sensory neurons LRP1 is also the receptor that binds and rapidly endocytoses smaller oligomeric forms of infectious prion fibrils, and recombinant PrP fibrils. Although LRP1 binds two molecules of most ligands independently to its receptor clusters 2 and 4, PrP(C) and PrP(Sc) fibrils bind only to receptor cluster 4. PrP(Sc) fibrils out-compete PrP(C) for internalization. When endocytosed, PrP(Sc) fibrils are routed to lysosomes, rather than recycled to the cell surface with PrP(C). Thus, although LRP1 binds both forms of PrP, it traffics them to separate fates within sensory neurons. The binding of both to ligand cluster 4 should enable genetic modification of PrP binding without disrupting other roles of LRP1 essential to neuronal viability and function, thereby enabling in vivo analysis of the role of this interaction in controlling both prion and LRP1 biology.

Generating a prion with bacterially expressed recombinant prion protein

The prion hypothesis posits that a misfolded form of prion protein (PrP) is responsible for the infectivity of prion disease. Using recombinant murine PrP purified from Escherichia coli, we created a recombinant prion with the attributes of the pathogenic PrP isoform: aggregated, protease-resistant, and self-perpetuating. After intracerebral injection of the recombinant prion, wild-type mice developed neurological signs in approximately 130 days and reached the terminal stage of disease in approximately 150 days. Characterization of diseased mice revealed classic neuropathology of prion disease, the presence of protease-resistant PrP, and the capability of serially transmitting the disease; these findings confirmed that the mice succumbed to prion disease. Thus, as postulated by the prion hypothesis, the infectivity in mammalian prion disease results from an altered conformation of PrP.

GPI anchoring facilitates propagation and spread of misfolded Sup35 aggregates in mammalian cells

Prion diseases differ from other amyloid-associated protein misfolding diseases (e.g. Alzheimer's) because they are naturally transmitted between individuals and involve spread of protein aggregation between tissues. Factors underlying these features of prion diseases are poorly understood. Of all protein misfolding disorders, only prion diseases involve the misfolding of a glycosylphosphatidylinositol (GPI)-anchored protein. To test whether GPI anchoring can modulate the propagation and spread of protein aggregates, a GPI-anchored version of the amyloidogenic yeast protein Sup35NM (Sup35GPI) was expressed in neuronal cells. Treatment of cells with Sup35NM fibrils induced the GPI anchor-dependent formation of self-propagating, detergent-insoluble, protease-resistant, prion-like aggregates of Sup35GPI. Live-cell imaging showed intercellular spread of Sup35GPI aggregation to involve contact between aggregate-positive and aggregate-negative cells and transfer of Sup35GPI from aggregate-positive cells. These data demonstrate GPI anchoring facilitates the propagation and spread of protein aggregation and thus may enhance the transmissibility and pathogenesis of prion diseases relative to other protein misfolding 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.

The role of the prion protein membrane anchor in prion infection

Normal cellular and abnormal disease-associated forms of prion protein (PrP) contain a C-terminal glycophosphatidyl-inositol (GPI) membrane anchor. The importance of the GPI membrane anchor in prion diseases is unclear but there are data to suggest that it both is and is not required for abnormal prion protein formation and prion infection. Utilizing an in vitro model of prion infection we have recently demonstrated that, while the GPI anchor is not essential for the formation of abnormal prion protein in a cell, it is necessary for the establishment of persistent prion infection. In combination with previously published data, our results suggest that GPI anchored PrP is important in the amplification and spread of prion infectivity from cell to cell.

Tunnelling nanotubes: a highway for prion spreading?

The discovery of tunnelling nanotubes (TNTs) and their proposed role in long intercellular transport of organelles, bacteria and viruses have led us to examine their potential role during prion spreading. We have recently shown that these membrane bridges can form between neuronal cells, as well as between dendritic cells and primary neurons and that both endogenous and exogenous PrP(Sc) appear to traffic through these structures between infected and non-infected cells. Furthermore, prion infection can be efficiently transmitted from infected dendritic cells to primary neurons only in co-culture conditions permissive for TNT formation. Therefore, we propose a role for TNTs during prion spreading from the periphery to the central nervous system (CNS). Here, we discuss some of the key steps where TNTs might play a role during prion neuroinvasion.

Acute cellular uptake of abnormal prion protein is cell type and scrapie-strain independent

Transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative diseases that include Creutzfeldt-Jakob disease, bovine spongiform encephalopathy and sheep scrapie. Although one of the earliest events during TSE infection is the cellular uptake of protease resistant prion protein (PrP-res), this process is poorly understood due to the difficulty of clearly distinguishing input PrP-res from either PrP-res or protease-sensitive PrP (PrP-sen) made by the cell. Using PrP-res tagged with a unique antibody epitope, we examined PrP-res uptake in neuronal and fibroblast cells exposed to three different mouse scrapie strains. PrP-res uptake was rapid and independent of scrapie strain, cell type, or cellular PrP expression, but occurred in only a subset of cells and was influenced by PrP-res preparation and aggregate size. Our results suggest that PrP-res aggregate size, the PrP-res microenvironment, and/or host cell-specific factors can all influence whether or not a cell takes up PrP-res following exposure to TSE infectivity.

Endocytosis of prion protein is required for ERK1/2 signaling induced by stress-inducible protein 1

The secreted cochaperone STI1 triggers activation of protein kinase A (PKA) and ERK1/2 signaling by interacting with the cellular prion (PrP(C)) at the cell surface, resulting in neuroprotection and increased neuritogenesis. Here, we investigated whether STI1 triggers PrP(C) trafficking and tested whether this process controls PrP(C)-dependent signaling. We found that STI1, but not a STI1 mutant unable to bind PrP(C), induced PrP(C) endocytosis. STI1-induced signaling did not occur in cells devoid of endogenous PrP(C); however, heterologous expression of PrP(C) reconstituted both PKA and ERK1/2 activation. In contrast, a PrP(C) mutant lacking endocytic activity was unable to promote ERK1/2 activation induced by STI1, whereas it reconstituted PKA activity in the same condition, suggesting a key role of endocytosis in the former process. The activation of ERK1/2 by STI1 was transient and appeared to depend on the interaction of the two proteins at the cell surface or shortly after internalization. Moreover, inhibition of dynamin activity by expression of a dominant-negative mutant caused the accumulation and colocalization of these proteins at the plasma membrane, suggesting that both proteins use a dynamin-dependent internalization pathway. These results show that PrP(C) endocytosis is a necessary step to modulate STI1-dependent ERK1/2 signaling involved in neuritogenesis.

Host PrP glycosylation: a major factor determining the outcome of prion infection

The expression of the prion protein (PrP) is essential for transmissible spongiform encephalopathy (TSE) or prion diseases to occur, but the underlying mechanism of infection remains unresolved. To address the hypothesis that glycosylation of host PrP is a major factor influencing TSE infection, we have inoculated gene-targeted transgenic mice that have restricted N-linked glycosylation of PrP with three TSE strains. We have uniquely demonstrated that mice expressing only unglycosylated PrP can sustain a TSE infection, despite altered cellular location of the host PrP. Moreover we have shown that brain material from mice infected with TSE that have only unglycosylated PrP^Sc is capable of transmitting infection to wild-type mice, demonstrating that glycosylation of PrP is not essential for establishing infection within a host or for transmitting TSE infectivity to a new host. We have further dissected the requirement of each glycosylation site and have shown that different TSE strains have dramatically different requirements for each of the glycosylation sites of host PrP, and moreover, we have shown that the host PrP has a major role in determining the glycosylation state of de novo generated PrP^Sc.

In prion infection, disease requires the presence of the endogenous host-encoded prion protein, PrP. PrP is a glycoprotein (modified by the addition of sugar molecules) with two consensus sites for sugars to attach. Different PrP forms are usually observed: one diglycosylated, two different monoglycosylated, and one unglycosylated. How PrP glycosylation influences prion infection remains obscure. We have used three different murine transgenic models, developed with the gene-replacement technique, to investigate each glycotype of PrP contribution to prion diseases, or transmissible spongiform encephalopathies (TSEs). For this purpose, mice expressing mono- or unglycosylated PrP were challenged with different prion strains. Remarkably, we found that glycosylation of host PrP is not mandatory for TSE infection, because mice expressing only unglycosylated PrP were susceptible to infection and able to transmit the disease to other animals. However, we also show that host PrP glycosylation can modulate the infectious process, since strains differ in their ability to infect hosts with restricted PrP glycosylation. These results elucidate the role of glycosylation in prion infection and in particular demonstrate that strains need sugars at specific sites of host PrP to successfully induce prion disease.

The primary determinant of the prion infectious process is the glycosylation status of the host prion protein, PrP.

A cell line infectible by prion strains from different species

It has been shown previously that ovine prion protein (PrP(C)) renders rabbit epithelial RK13 cells permissive to the multiplication of ovine prions, thus providing evidence that species barriers can be crossed in cultured cells through the expression of a relevant PrP(C). The present study significantly extended this observation by showing that mouse and bank vole prions can be propagated in RK13 cells that express the corresponding PrP(C). Importantly, the respective molecular patterns of abnormal PrP (PrP(res)) and, where examined, the neuropathological features of the infecting strains appeared to be maintained during the propagation in cell culture. These findings indicate that RK13 cells can be genetically engineered to replicate prion strains faithfully from different species. Such an approach may facilitate investigations of the molecular basis of strain identity and prion diversity.

Susceptibility of cell substrates to PrPSc infection and safety control measures related to biological and biotherapeutical products

Concerns over the potential for infectious prion proteins to contaminate human biologics and biotherapeutics have been raised from time to time. Transmission of the pathogenic form of prion protein (PrP(Sc)) through veterinary vaccines has been observed, yet no human case through the use of vaccine products has been reported. However, iatrogenic transmissions of PrP(Sc) in humans through blood components, tissues and growth hormone have been reported. These findings underscore the importance of reliable detection or diagnostic methods to prevent the transmission of prion diseases, given that the number of asymptomatic infected individuals remains unknown, the perceived incubation time for human prion diseases could be decades, and no cure of the diseases has been found yet. A variety of biochemical and molecular methods can selectively concentrate PrP(Sc) to facilitate its detection in tissues and cells. Furthermore, some methods routinely used in the manufacturing process of biological products have been found to be effective in reducing PrP(Sc) from the products. Questions remain unanswered as to the validation criteria of these methods, the minimal infectious dose of the PrP(Sc) required to cause infection and the susceptibility of cells used in gene therapy or the manufacturing process of biological products to PrP(Sc) infections. Here, we discuss some of these challenging issues.