Cultured cell sublines highly susceptible to prion infection

A novel ER stress regulator ARL6IP5 induces reticulophagy to ameliorate the prion burden

Prion disease is a fatal and infectious neurodegenerative disorder caused by the trans-conformation conversion of PRNP/PrPC to PRNP/PrPSc. Accumulated PRNP/PrPSc-induced ER stress causes chronic unfolded protein response (UPR) activation, which is one of the fundamental steps in prion disease progression. However, the role of various ER-resident proteins in prion-induced ER stress is elusive. This study demonstrated that ARL6IP5 is compensatory upregulated in response to chronically activated UPR in the cellular prion disease model (RML-ScN2a). Furthermore, overexpression of ARL6IP5 overcomes ER stress by lowering the expression of chronically activated UPR pathway proteins. We discovered that ARL6IP5 induces reticulophagy to reduce the PRNP/PrPSc burden by releasing ER stress. Conversely, the knockdown of ARL6IP5 leads to inefficient macroautophagic/autophagic flux and elevated PRNP/PrPSc burden. Our study also uncovered that ARL6IP5-induced reticulophagy depends on Ca2+-mediated AMPK activation and can induce 3 MA-inhibited autophagic flux. The detailed mechanistic study revealed that ARL6IP5-induced reticulophagy involves interaction with soluble reticulophagy receptor CALCOCO1 and lysosomal marker LAMP1, leading to degradation in lysosomes. Here, we delineate the role of ARL6IP5 as a novel ER stress regulator and reticulophagy inducer that can effectively reduce the misfolded PRNP/PrPSc burden. Our research opens up a new avenue of selective autophagy in prion disease and represents a potential therapeutic target.Abbreviations: ARL6IP5: ADP ribosylation factor-like GTPase 6 interacting protein 5; AMPK: adenosine 5'-monophosphate (AMP)-activated protein kinase; CALCOCO1: calcium binding and coiled-coil domain 1; CQ: chloroquine; DAPI: 4'6-diamino-2-phenylindole; ER: endoplasmic reticulum; ERPHS: reticulophagy/ER-phagy sites; KD: knockdown; KD-CON: knockdown control; LAMP1: lysosomal-associated membrane protein 1; MAP1LC3/LC3, microtubule-associated protein 1 light chain 3; MTOR: mechanistic target of rapamycin kinase; MβCD: methyl beta cyclodextrin; 3 MA: 3-methyladenine; OE: overexpression; OE-CON: empty vector control; PrDs: prion diseases; PRNP/PrPC: cellular prion protein (Kanno blood group); PRNP/PrPSc: infectious scrapie misfolded PRNP; Tm: tunicamycin; UPR: unfolded protein response; UPS: ubiquitin-proteasome system.

Recent advances in cellular models for discovering prion disease therapeutics

INTRODUCTION

Prion diseases are a group of rare and lethal rapidly progressive neurodegenerative diseases arising due to conversion of the physiological cellular prion protein into its pathological counterparts, denoted as "prions". These agents are resistant to inactivation by standard decontamination procedures and can be transmitted between individuals, consequently driving the irreversible brain damage typical of the diseases.

AREAS COVERED

Since its infancy, prion research has mainly depended on animal models for untangling the pathogenesis of the disease as well as for the drug development studies. With the advent of prion-infected cell lines, relevant animal models have been complemented by a variety of cell-based models presenting a much faster, ethically acceptable alternative.

EXPERT OPINION

To date, there are still either no effective prophylactic regimens or therapies for human prion diseases. Therefore, there is an urgent need for more relevant cellular models that best approximate in vivo models. Each cellular model presented and discussed in detail in this review has its own benefits and limitations. Once embarking in a drug screening campaign for the identification of molecules that could interfere with prion conversion and replication, one should carefully consider the ideal cellular model.

Central Residues in Prion Protein PrPC Are Crucial for Its Conversion into the Pathogenic Isoform

Conformational conversion of the cellular prion protein, PrP^C, into the amyloidogenic isoform, PrP^Sc, is a key pathogenic event in prion diseases. However, the conversion mechanism remains to be elucidated. Here, we generated Tg(PrPΔ91-106)-8545/Prnp^0/0 mice, which overexpress mouse PrP lacking residues 91-106. We showed that none of the mice became sick after intracerebral inoculation with RML, 22L, and FK-1 prion strains nor accumulated PrP^ScΔ91-106 in their brains except for a small amount of PrP^ScΔ91-106 detected in one 22L-inoculated mouse. However, they developed disease around 85 days after inoculation with bovine spongiform encephalopathy (BSE) prions with PrP^ScΔ91-106 in their brains. These results suggest that residues 91-106 are important for PrP^C conversion into PrP^Sc in infection with RML, 22L, and FK-1 prions but not BSE prions. We then narrowed down the residues 91-106 by transducing various PrP deletional mutants into RML- and 22L-infected cells and identified that PrP mutants lacking residues 97-99 failed to convert into PrP^Sc in these cells. Our in vitro conversion assay also showed that RML, 22L, and FK-1 prions did not convert PrPΔ97-99 into PrP^ScΔ97-99, but BSE prions did. We further found that PrP mutants with proline residues at positions 97 to 99 or charged residues at positions 97 and 99 completely or almost completely lost their converting activity into PrP^Sc in RML- and 22L-infected cells. These results suggest that the structurally flexible and noncharged residues 97-99 could be important for PrP^C conversion into PrP^Sc following infection with RML, 22L, and FK-1 prions but not BSE prions.

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.

Gene-Edited Cell Models to Study Chronic Wasting Disease

Prion diseases are fatal infectious neurodegenerative disorders affecting both humans and animals. They are caused by the misfolded isoform of the cellular prion protein (PrPC), PrPSc, and currently no options exist to prevent or cure prion diseases. Chronic wasting disease (CWD) in deer, elk and other cervids is considered the most contagious prion disease, with extensive shedding of infectivity into the environment. Cell culture models provide a versatile platform for convenient quantification of prions, for studying the molecular and cellular biology of prions, and for performing high-throughput screening of potential therapeutic compounds. Unfortunately, only a very limited number of cell lines are available that facilitate robust and persistent propagation of CWD prions. Gene-editing using programmable nucleases (e.g., CRISPR-Cas9 (CC9)) has proven to be a valuable tool for high precision site-specific gene modification, including gene deletion, insertion, and replacement. CC9-based gene editing was used recently for replacing the PrP gene in mouse and cell culture models, as efficient prion propagation usually requires matching sequence homology between infecting prions and prion protein in the recipient host. As expected, such gene-editing proved to be useful for developing CWD models. Several transgenic mouse models were available that propagate CWD prions effectively, however, mostly fail to reproduce CWD pathogenesis as found in the cervid host, including CWD prion shedding. This is different for the few currently available knock-in mouse models that seem to do so. In this review, we discuss the available in vitro and in vivo models of CWD, and the impact of gene-editing strategies.

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.

The aminoglycoside G418 hinders de novo prion infection in cultured cells

The study of prions and the discovery of candidate therapeutics for prion disease have been facilitated by the ability of prions to replicate in cultured cells. Paradigms in which prion proteins from different species are expressed in cells with low or no expression of endogenous prion protein (PrP) have expanded the range of prion strains that can be propagated. In these systems, cells stably expressing a PrP of interest are typically generated via coexpression of a selectable marker and treatment with an antibiotic. Here, we report the unexpected discovery that the aminoglycoside G418 (Geneticin) interferes with the ability of stably transfected cultured cells to become infected with prions. In G418-resistant lines of N2a or CAD5 cells, the presence of G418 reduced levels of protease-resistant PrP following challenge with the RML or 22L strains of mouse prions. G418 also interfered with the infection of cells expressing hamster PrP with the 263K strain of hamster prions. Interestingly, G418 had minimal to no effect on protease-resistant PrP levels in cells with established prion infection, arguing that G418 selectively interferes with de novo prion infection. As G418 treatment had no discernible effect on cellular PrP levels or its localization, this suggests that G418 may specifically target prion assemblies or processes involved in the earliest stages of prion infection.

Tackling prion diseases: a review of the patent landscape

Introduction: Prion diseases are a class of rare and fatal neurodegenerative diseases for which no cure is currently available. They are characterized by conformational conversion of cellular prion protein (PrP^C) into the disease-associated 'scrapie' isoform (PrP^Sc). Under an etiological point of view, prion diseases can be divided into acquired, genetic, and idiopathic form, the latter of which are the most frequent.Areas covered: Therapeutic approaches targeting prion diseases are based on the use of chemical and nature-based compounds, targeting either PrP^C or PrP^Sc or other putative player in pathogenic mechanism. Other proposed anti-prion treatments include passive and active immunization strategies, peptides, aptamers, and PrP^C-directed RNA interference techniques. The treatment efficacy has been mainly assessed in cell lines or animal models of the disease testing their ability to reduce prion accumulation.Expert opinion: The assessed strategies focussing on the identification of an efficient anti-prion therapy faced various issues, which go from permeation of the blood brain barrier to immunological tolerance of the host. Indeed, the use of combinatory approaches, which could boost a synergistic anti-prion effect and lower the potential side effects of single treatments and may represent an extreme powerful and feasible way to tackle prion disease.

Global analysis of protein degradation in prion infected cells

Prion diseases are rare, neurological disorders caused by the misfolding of the cellular prion protein (PrP^C) into cytotoxic fibrils (PrP^Sc). Intracellular PrP^Sc aggregates primarily accumulate within late endosomes and lysosomes, organelles that participate in the degradation and turnover of a large subset of the proteome. Thus, intracellular accumulation of PrP^Sc aggregates has the potential to globally influence protein degradation kinetics within an infected cell. We analyzed the proteome-wide effect of prion infection on protein degradation rates in N2a neuroblastoma cells by dynamic stable isotopic labeling with amino acids in cell culture (dSILAC) and bottom-up proteomics. The analysis quantified the degradation rates of more than 4,700 proteins in prion infected and uninfected cells. As expected, the degradation rate of the prion protein is significantly decreased upon aggregation in infected cells. In contrast, the degradation kinetics of the remainder of the N2a proteome generally increases upon prion infection. This effect occurs concurrently with increases in the cellular activities of autophagy and some lysosomal hydrolases. The resulting enhancement in proteome flux may play a role in the survival of N2a cells upon prion infection.

Prion infection, transmission, and cytopathology modeled in a low-biohazard human cell line

Expanding the toolbox of prion research to a low-biohazard, scalable human cell model.

Transmission of prion infectivity to susceptible murine cell lines has simplified prion titration assays and has greatly reduced the need for animal experimentation. However, murine cell models suffer from technical and biological constraints. Human cell lines might be more useful, but they are much more biohazardous and are often poorly infectible. Here, we describe the human clonal cell line hovS, which lacks the human PRNP gene and expresses instead the ovine PRNP VRQ allele. HovS cells were highly susceptible to the PG127 strain of sheep-derived murine prions, reaching up to 90% infected cells in any given culture and were maintained in a continuous infected state for at least 14 passages. Infected hovS cells produced proteinase K–resistant prion protein (PrP^Sc), pelletable PrP aggregates, and bona fide infectious prions capable of infecting further generations of naïve hovS cells and mice expressing the VRQ allelic variant of ovine PrP^C. Infection in hovS led to prominent cytopathic vacuolation akin to the spongiform changes observed in individuals suffering from prion diseases. In addition to expanding the toolbox for prion research to human experimental genetics, the hovS cell line provides a human-derived system that does not require human prions. Hence, the manipulation of scrapie-infected hovS cells may present fewer biosafety hazards than that of genuine human prions.

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.

A New Cell Model for Investigating Prion Strain Selection and Adaptation

Prion diseases are fatal neurodegenerative diseases that affect humans and animals. Prion strains, conformational variants of misfolded prion proteins, are associated with distinct clinical and pathological phenotypes. Host-strain interactions result in the selective damage of distinct brain areas and they are responsible for strain selection and/or adaptation, but the underlying molecular mechanisms are unknown. Prion strains can be distinguished by their cell tropism in vivo and in vitro, which suggests that susceptibility to distinct prion strains is determined by cellular factors. The neuroblastoma cell line PK1 is refractory to the prion strain Me7, but highly susceptible to RML. We challenged a large number of clonal PK1 lines with Me7 and successfully selected highly Me7-susceptible subclones (PME) to investigate whether the prion strain repertoire of PK1 can be expanded. Notably, the Me7-infected PME clones were more protease-resistant when compared to RML-infected PME clones, which suggested that cell-adapted Me7 and RML are distinct prion strains. Strikingly, Me7-refractory cells, including PK1 and astrocytes in cortico-hippocampal cultures, are highly susceptible to prions, being derived from homogenates of Me7-infected PME cells, suggesting that the passage of Me7 in PME cells leads to an extended host range. Thus, PME clones represent a compelling cell model for strain selection and adaptation.

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.

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.

A Bioluminescent Cell Assay to Quantify Prion Protein Dimerization

The prion protein (PrP) is a cell surface protein that in disease misfolds and becomes infectious causing Creutzfeldt-Jakob disease in humans, scrapie in sheep, and chronic wasting disease in deer and elk. Little is known regarding the dimerization of PrP and its role in disease. We developed a bioluminescent prion assay (BPA) to quantify PrP dimerization by bimolecular complementation of split Gaussia luciferase (GLuc) halves that are each fused to PrP. Fusion constructs between PrP and N- and C-terminal GLuc halves were expressed on the surface of RK13 cells (RK13-DC cells) and dimerized to yield a bioluminescent signal that was decreased in the presence of eight different antibodies to PrP. Dimerization of PrP was independent of divalent cations and was induced under stress. Challenge of RK13-DC cells with seven different prion strains did not lead to detectable infection but was measurable by bioluminescence. Finally, we used BPA to screen a compound library for compounds inhibiting PrP dimerization. One of the most potent compounds to inhibit PrP dimerization was JTC-801, which also inhibited prion replication in RML-infected ScN2a and SMB cells with an EC₅₀ of 370 nM and 220 nM, respectively. We show here that BPA is a versatile tool to study prion biology and to identify anti-prion compounds.

Prion-like properties of disease-relevant proteins in amyotrophic lateral sclerosis

The hallmark of age-related neurodegenerative diseases is the appearance of cellular protein deposits and spreading of this pathology throughout the central nervous system. Growing evidence has shown the involvement and critical role of proteins with prion-like properties in the formation of these characteristic cellular aggregates. Prion-like domains of such proteins with their proposed function in the organization of membraneless organelles are prone for misfolding and promoting further aggregation. Spreading of these toxic aggregates between cells and across tissues can explain the progression of clinical phenotypes and pathology in a stereotypical manner, characteristic for almost every neurodegenerative disease. Here, we want to review the current evidence for the role of prion-like mechanisms in classical neurodegenerative diseases and ALS in particular. We will also discuss an intriguingly central role of the protein TDP-43 in the majority of cases of this devastating disease.

The compound (3-{5-[(2,5-dimethoxyphenyl)amino]-1,3,4-thiadiazolidin-2-yl}-5,8-methoxy-2H-chromen-2-one) inhibits the prion protein conversion from PrPC to PrPSc with lower IC50 in ScN2a cells

Prion diseases are fatal neurodegenerative disorders of the central nervous system characterized by the accumulation of a protease resistant form (PrP^Sc) of the cellular prion protein (PrP^C) in the brain. Two types of cellular prion (PrP^C) compounds have been identified that appear to affect prion conversion are known as Effective Binders (EBs) and Accelerators (ACCs). Effective binders shift the balance in favour of PrP^C, whereas Accelerators favour the formation of PrP^Sc. Molecular docking indicates EBs and ACCs both bind to pocket-D of the SHaPrP^C molecule. However, EBs and ACCs may have opposing effects on the stability of the salt bridge between Arg¹⁵⁶ and Glu¹⁹⁶/Glu²⁰⁰. Computational docking data indicate that the hydrophobic benzamide group of the EB, GFP23 and the 1-(3,3-dimethylcyclohexylidene)piperidinium group of the ACC, GFP22 play an important role in inhibition and conversion from SHaPrP^C to SHaPrP^Sc, respectively. Experimentally, NMR confirmed the amide chemical shift perturbations observed upon the binding of GFP23 to pocket-D of SHaPrP^C. Consistent with its role as an ACC, titration of GFP22 resulted in widespread chemical shift changes and signal intensity loss due to protein unfolding. Virtual screening of a ligand database using the molecular scaffold developed from the set of EBs identified six of our compounds (previously studied using fluorescence quenching) as being among the top 100 best binders. Among them, compounds 5 and 6 were found to be particularly potent in decreasing the accumulation SHaPrP^Sc in ScN2a cells with an IC₅₀ of ∼35µM and 20µM.

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.

Bioassays and Inactivation of Prions

The experimental study of prions requires a model for their propagation. However, because prions lack nucleic acids, the simple techniques used to replicate bacteria and viruses are not applicable. For much of the history of prion research, time-consuming bioassays in animals were the only option for measuring infectivity. Although cell models and other in vitro tools for the propagation of prions have been developed, they all suffer limitations, and animal bioassays remain the gold standard for measuring infectivity. A wealth of recent data argues that both β-amyloid (Aβ) and tau proteins form prions that cause Alzheimer's disease, and α-synuclein forms prions that cause multiple system atrophy and Parkinson's disease. Cell and animal models that recapitulate some of the key features of cell-to-cell spreading and distinct strains of prions can now be measured.

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.

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.

Isolation of Exosomes and Microvesicles from Cell Culture Systems to Study Prion Transmission

Extracellular vesicles (EVs) are composed of microvesicles and exosomes. Exosomes are small membrane vesicles (40-120 nm sized) of endosomal origin released in the extracellular medium from cells when multivesicular bodies fuse with the plasma membrane, whereas microvesicles (i.e., shedding vesicles, 100 nm to 1 μm sized) bud from the plasma membrane. Exosomes and microvesicles carry functional proteins and nucleic acids (especially mRNAs and microRNAs) that can be transferred to surrounding cells and tissues and can impact multiple dimensions of the cellular life. Most of the cells, if not all, from neuronal to immune cells, release exosomes and microvesicles in the extracellular medium, and all biological fluids including blood (serum/plasma), urine, cerebrospinal fluid, and saliva contain EVs.Prion-infected cultured cells are known to secrete infectivity into their environment. We characterized this cell-free form of prions and showed that infectivity was associated with exosomes. Since exosomes are produced by a variety of cells, including cells that actively accumulate prions, they could be a vehicle for infectivity in body fluids and could participate to the dissemination of prions in the organism. In addition, such infectious exosomes also represent a natural, simple, biological material to get key information on the abnormal PrP forms associated with infectivity.In this chapter, we describe first a method that allows exosomes and microvesicles isolation from prion-infected cell cultures and in a second time the strategies to characterize the prions containing exosomes and their ability to disseminate the prion agent.

Characterization of mesenchymal stem cells in sheep naturally infected with scrapie

Mesenchymal stem cells (MSCs) can be infected with prions and have been proposed as in vitro cell-based models for prion replication. In addition, autologous MSCs are of interest for cell therapy in neurodegenerative diseases. To the best of our knowledge, the effect of prion diseases on the characteristics of these cells has never been investigated. Here, we analysed the properties of MSCs obtained from bone marrow (BM-MSCs) and peripheral blood (PB-MSCs) of sheep naturally infected with scrapie — a large mammal model for the study of prion diseases. After three passages of expansion, MSCs derived from scrapie animals displayed similar adipogenic, chondrogenic and osteogenic differentiation ability as cells from healthy controls, although a subtle decrease in the proliferation potential was observed. Exceptionally, mesenchymal markers such as CD29 were significantly upregulated at the transcript level compared with controls. Scrapie MSCs were able to transdifferentiate into neuron-like cells, but displayed lower levels of neurogenic markers at basal conditions, which could limit this potential .The expression levels of cellular prion protein (PrPC) were highly variable between cultures, and no significant differences were observed between control and scrapie-derived MSCs. However, during neurogenic differentiation the expression of PrPC was upregulated in MSCs. This characteristic could be useful for developing in vitro models for prion replication. Despite the infectivity reported for MSCs obtained from scrapie-infected mice and Creutzfeldt–Jakob disease patients, protein misfolding cyclic amplification did not detect PrPSc in BM- or PB-MSCs from scrapie-infected sheep, which limits their use for in vivo diagnosis for scrapie.

Transcriptomic Determinants of Scrapie Prion Propagation in Cultured Ovine Microglia

Susceptibility to infection by prions is highly dependent on the amino acid sequence and host expression of the cellular prion protein (PrP^C); however, cellular expression of a genetically susceptible PrP^C is insufficient. As an example, it has been shown in cultured cells that permissive and resistant sublines derived from the same parental population often have similar expression levels of PrP^C. Thus, additional cellular factors must influence susceptibility to prion infection. The aim of this study was to elucidate the factors associated with relative permissiveness and resistance to scrapie prions in cultured cells derived from a naturally affected species. Two closely related ovine microglia clones with different prion susceptibility, but no detectable differences in PrP^C expression levels, were inoculated with either scrapie-positive or scrapie-negative sheep brainstem homogenates. Five passages post-inoculation, the transcriptional profiles of mock and infected clones were sequenced using Illumina technology. Comparative transcriptional analyses identified twenty-two differentially transcribed genes, most of which were upregulated in poorly permissive microglia. This included genes encoding for selenoprotein P, endolysosomal proteases, and proteins involved in extracellular matrix remodeling. Furthermore, in highly permissive microglia, transforming growth factor β–induced, retinoic acid receptor response 1, and phosphoserine aminotranspherase 1 gene transcripts were upregulated. Gene Set Enrichment Analysis identified proteolysis, translation, and mitosis as the most affected pathways and supported the upregulation trend of several genes encoding for intracellular proteases and ribosomal proteins in poorly permissive microglia. This study identifies new genes potentially involved in scrapie prion propagation, corroborates results from other studies, and extends those results into another cell culture model.

Le TN, Aulić S, Bistaffa E, Campagnani I, Virgilio T, Indaco A, Palamara L, Andréoletti O, Tagliavini F, Legname G. Synthetic prions with novel strain-specified properties

Prions are infectious proteins that possess multiple self-propagating structures. The information for strains and structural specific barriers appears to be contained exclusively in the folding of the pathological isoform, PrP^Sc. Many recent studies determined that de novo prion strains could be generated in vitro from the structural conversion of recombinant (rec) prion protein (PrP) into amyloidal structures. Our aim was to elucidate the conformational diversity of pathological recPrP amyloids and their biological activities, as well as to gain novel insights in characterizing molecular events involved in mammalian prion conversion and propagation. To this end we generated infectious materials that possess different conformational structures. Our methodology for the prion conversion of recPrP required only purified rec full-length mouse (Mo) PrP and common chemicals. Neither infected brain extracts nor amplified PrP^Sc were used. Following two different in vitro protocols recMoPrP converted to amyloid fibrils without any seeding factor. Mouse hypothalamic GT1 and neuroblastoma N2a cell lines were infected with these amyloid preparations as fast screening methodology to characterize the infectious materials. Remarkably, a large number of amyloid preparations were able to induce the conformational change of endogenous PrP^C to harbor several distinctive proteinase-resistant PrP forms. One such preparation was characterized in vivo habouring a synthetic prion with novel strain specified neuropathological and biochemical properties.

Prions are infectious proteins capable of acquiring multiple self-propagating structures. The information for strains and structural specific barriers appears to be contained exclusively in the folding of the pathological isoform, designated as PrP^Sc. During propagation, disease-associated conformer PrP^Sc coerces the physiological form, denoted as PrP^C, to adopt the pathological isoform conformation. We describe here the generation of an array of infectious materials with different structural, morphological, biochemical and cell biological characteristics. After producing purified recombinant prion protein of the wild-type mouse full-length sequence in Escherichia coli, we polymerized the protein into various amyloid fibril conformations based on different amyloid preparations. We also applied a build-in methodology for screening amyloid preparations and generate infectious materials using an amyloid-infected cell culture assay. Some of the amyloid fibrils preparations were able to efficiently amplify in PMCA (Protein Misfolding Cyclic Amplification), and to induce endogenous PrP^C to convert into PrP^Sc in both murine hypothalamic GT1 and neuroblastoma N2a cell lines. One such protocol lead to the generation of a novel synthetic prion strain in mice.

Review: Prion-like mechanisms of transactive response DNA binding protein of 43 kDa (TDP-43) in amyotrophic lateral sclerosis (ALS)

Amyotrophic lateral sclerosis (ALS) is a fatal devastating neurodegenerative disorder which predominantly affects the motor neurons in the brain and spinal cord. The death of the motor neurons in ALS causes subsequent muscle atrophy, paralysis and eventual death. Clinical and biological evidence now demonstrates that ALS has many similarities to prion disease in terms of disease onset, phenotype variability and progressive spread. The pathognomonic ubiquitinated inclusions deposited in the neurons and glial cells in brains and spinal cords of patients with ALS and fronto-temporal lobar degeneration with ubiquitinated inclusions contain aggregated transactive response DNA binding protein of 43 kDa (TDP-43), and evidence now suggests that TDP-43 has cellular prion-like properties. The cellular mechanisms of prion protein misfolding and aggregation are thought to be responsible for the characteristics of prion disease. Therefore, there is a strong mechanistic basis for a prion-like behaviour of the TDP-43 protein being responsible for some characteristics of ALS. In this review, we compare the prion-like mechanisms of TDP-43 to the clinical and biological nature of ALS in order to investigate how this protein could be responsible for some of the characteristic properties of the disease.

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.

hTERT-immortalized ovine microglia propagate natural scrapie isolates

Ex vivo propagation of natural prion isolates (i.e., propagated solely in the natural host) is crucial for the characterization and study of transmissible spongiform encephalopathies (TSEs). Several well-established, prion-permissive cell culture systems are available; however, only a few cell lines are permissive to natural prion isolates and these cells are not pathophysiologically relevant (e.g., renal epithelium and fibroblast-like cells). Therefore, a pathophysiologically relevant cell line derived from a natural TSE host could be used for propagation of natural prion isolates. In this study, ovine brain macrophages (microglia) were immortalized by transfection with the human telomerase reverse transcriptase (hTERT) gene to identify cell lines (hTERT-microglia) permissive to natural scrapie prion isolates. Following transfection, hTERT-microglia were passaged up to 100 times and their lifespan was significantly longer compared to parental cells (Fisher's exact test, P<0.001). Multiple sublines were permissive to cell culture-adapted prions; two sublines were also permissive to natural scrapie isolates (i.e., derived from brain homogenates of sheep infected with scrapie). Prion infectivity and partial protease resistance of the prion protein were maintained in hTERT-microglia. Comparisons between scrapie-permissive and non-permissive hTERT-microglia sublines revealed that overall quantity of the normal cellular prion protein was not associated with prion permissiveness. The use of hTERT-microglia in future TSE studies may be more germane to the characterization of the cellular and subcellular pathophysiology of natural scrapie prion isolates and to investigate host-specific factors involved in prion replication.

A bovine cell line that can be infected by natural sheep scrapie prions

Cell culture systems represent a crucial part in basic prion research; yet, cell lines that are susceptible to prions, especially to field isolated prions that were not adapted to rodents, are very rare. The purpose of this study was to identify and characterize a cell line that was susceptible to ruminant-derived prions and to establish a stable prion infection within it. Based on species and tissue of origin as well as PrP expression rate, we pre-selected a total of 33 cell lines that were then challenged with natural and with mouse propagated BSE or scrapie inocula. Here, we report the successful infection of a non-transgenic bovine cell line, a sub-line of the bovine kidney cell line MDBK, with natural sheep scrapie prions. This cell line retained the scrapie infection for more than 200 passages. Selective cloning resulted in cell populations with increased accumulation of PrP^res, although this treatment was not mandatory for retaining the infection. The infection remained stable, even under suboptimal culture conditions. The resulting infectivity of the cells was confirmed by mouse bioassay (Tgbov mice, Tgshp mice). We believe that PES cells used together with other prion permissive cell lines will prove a valuable tool for ongoing efforts to understand and defeat prions and prion diseases.

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.

Assessing proteinase K resistance of fish prion proteins in a scrapie-infected mouse neuroblastoma cell line

The key event in prion pathogenesis is the structural conversion of the normal cellular protein, PrP(C), into an aberrant and partially proteinase K resistant isoform, PrP(Sc). Since the minimum requirement for a prion disease phenotype is the expression of endogenous PrP in the host, species carrying orthologue prion genes, such as fish, could in theory support prion pathogenesis. Our previous work has demonstrated the development of abnormal protein deposition in sea bream brain, following oral challenge of the fish with natural prion infectious material. In this study, we used a prion-infected mouse neuroblastoma cell line for the expression of three different mature fish PrP proteins and the evaluation of the resistance of the exogenously expressed proteins to proteinase K treatment (PK), as an indicator of a possible prion conversion. No evidence of resistance to PK was detected for any of the studied recombinant proteins. Although not indicative of an absolute inability of the fish PrPs to structurally convert to pathogenic isoforms, the absence of PK-resistance may be due to supramolecular and conformational differences between the mammalian and piscine PrPs.

Defined α-synuclein prion-like molecular assemblies spreading in cell culture

Background

α-Synuclein (α-syn) plays a central role in the pathogenesis of synucleinopathies, a group of neurodegenerative disorders that includes Parkinson disease, dementia with Lewy bodies and multiple system atrophy. Several findings from cell culture and mouse experiments suggest intercellular α-syn transfer.

Results

Through a methodology used to obtain synthetic mammalian prions, we tested whether recombinant human α-syn amyloids can promote prion-like accumulation in neuronal cell lines in vitro. A single exposure to amyloid fibrils of human α-syn was sufficient to induce aggregation of endogenous α-syn in human neuroblastoma SH-SY5Y cells. Remarkably, endogenous wild-type α-syn was sufficient for the formation of these aggregates, and overexpression of the protein was not required.

Conclusions

Our results provide compelling evidence that endogenous α-syn can accumulate in cell culture after a single exposure to exogenous α-syn short amyloid fibrils. Importantly, using α-syn short amyloid fibrils as seed, endogenous α-syn aggregates and accumulates over several passages in cell culture, providing an excellent tool for potential therapeutic screening of pathogenic α-syn aggregates.

The potential of mesenchymal stem cell in prion research

Scrapie and bovine spongiform encephalopathy are fatal neurodegenerative diseases caused by the accumulation of a misfolded protein (PrP(res)), the pathological form of the cellular prion protein (PrP(C)). For the last decades, prion research has greatly progressed, but many questions need to be solved about prion replication mechanisms, cell toxicity, differences in genetic susceptibility, species barrier or the nature of prion strains. These studies can be developed in murine models of transmissible spongiform encephalopathies, although development of cell models for prion replication and sample titration could reduce economic and timing costs and also serve for basic research and treatment testing. Some murine cell lines can replicate scrapie strains previously adapted in mice and very few show the toxic effects of prion accumulation. Brain cell primary cultures can be more accurate models but are difficult to develop in naturally susceptible species like humans or domestic ruminants. Stem cells can be differentiated into neuron-like cells and be infected by prions. However, the use of embryo stem cells causes ethical problems in humans. Mesenchymal stem cells (MSCs) can be isolated from many adult tissues, including bone marrow, adipose tissue or even peripheral blood. These cells differentiate into neuronal cells, express PrP(C) and can be infected by prions in vitro. In addition, in the last years, these cells are being used to develop therapies for many diseases, including neurodegenerative diseases. We review here the use of cell models in prion research with a special interest in the potential use of MSCs.

Identification of a gene regulatory network associated with prion replication

Prions consist of aggregates of abnormal conformers of the cellular prion protein (PrP^C). They propagate by recruiting host-encoded PrP^C although the critical interacting proteins and the reasons for the differences in susceptibility of distinct cell lines and populations are unknown. We derived a lineage of cell lines with markedly differing susceptibilities, unexplained by PrP^C expression differences, to identify such factors. Transcriptome analysis of prion-resistant revertants, isolated from highly susceptible cells, revealed a gene expression signature associated with susceptibility and modulated by differentiation. Several of these genes encode proteins with a role in extracellular matrix (ECM) remodelling, a compartment in which disease-related PrP is deposited. Silencing nine of these genes significantly increased susceptibility. Silencing of Papss2 led to undersulphated heparan sulphate and increased PrP^C deposition at the ECM, concomitantly with increased prion propagation. Moreover, inhibition of fibronectin 1 binding to integrin α8 by RGD peptide inhibited metalloproteinases (MMP)-2/9 whilst increasing prion propagation. In summary, we have identified a gene regulatory network associated with prion propagation at the ECM and governed by the cellular differentiation state.

Membrane-anchored Aβ accelerates amyloid formation and exacerbates amyloid-associated toxicity in mice

Pathological, genetic, and biochemical hallmarks of Alzheimer's disease (AD) are linked to amyloid-β (Aβ) peptide aggregation. Especially misfolded Aβ42 peptide is sufficient to promote amyloid plaque formation. However, the cellular compartment facilitating the conversion of monomeric Aβ to aggregated toxic Aβ species remains unknown. In vitro models suggest lipid membranes to be the driving force of Aβ conversion. To this end, we generated two novel mouse models, expressing either membrane-anchored or nonanchored versions of the human Aβ42 peptide. Strikingly, membrane-anchored Aβ42 robustly accelerated Aβ deposition and exacerbated amyloid-associated toxicity upon crossing with Aβ precursor protein transgenic mice. These in vivo findings support the hypothesis that Aβ-membrane interactions play a pivotal role in early-onset AD as well as neuronal damage and provide evidence to study Aβ-membrane interactions as therapeutic targets.

Novel compounds lowering the cellular isoform of the human prion protein in cultured human cells

PURPOSE

Previous studies showed that lowering PrP(C) concomitantly reduced PrP(Sc) in the brains of mice inoculated with prions. We aimed to develop assays that measure PrP(C) on the surface of human T98G glioblastoma and IMR32 neuroblastoma cells. Using these assays, we sought to identify chemical hits, confirmed hits, and scaffolds that potently lowered PrP(C) levels in human brains cells, without lethality, and that could achieve drug concentrations in the brain after oral or intraperitoneal dosing in mice.

METHODS

We utilized HTS ELISA assays to identify small molecules that lower PrP(C) levels by ≥30% on the cell surface of human glioblastoma (T98G) and neuroblastoma (IMR32) cells.

RESULTS

From 44,578 diverse chemical compounds tested, 138 hits were identified by single point confirmation (SPC) representing 7 chemical scaffolds in T98G cells, and 114 SPC hits representing 6 scaffolds found in IMR32 cells. When the confirmed SPC hits were combined with structurally related analogs, >300 compounds (representing 6 distinct chemical scaffolds) were tested for dose-response (EC₅₀) in both cell lines, only studies in T98G cells identified compounds that reduced PrP(C) without killing the cells. EC₅₀ values from 32 hits ranged from 65 nM to 4.1 μM. Twenty-eight were evaluated in vivo in pharmacokinetic studies after a single 10 mg/kg oral or intraperitoneal dose in mice. Our results showed brain concentrations as high as 16.2 μM, but only after intraperitoneal dosing.

CONCLUSIONS

Our studies identified leads for future studies to determine which compounds might lower PrP(C) levels in rodent brain, and provide the basis of a therapeutic for fatal disorders caused by PrP prions.

Infectious prions accumulate to high levels in non proliferative C2C12 myotubes

Prion diseases are driven by the strain-specific, template-dependent transconformation of the normal cellular prion protein (PrP^C) into a disease specific isoform PrP^Sc. Cell culture models of prion infection generally use replicating cells resulting in lower levels of prion accumulation compared to animals. Using non-replicating cells allows the accumulation of higher levels of PrP^Sc and, thus, greater amounts of infectivity. Here, we infect non-proliferating muscle fiber myotube cultures prepared from differentiated myoblasts. We demonstrate that prion-infected myotubes generate substantial amounts of PrP^Sc and that the level of infectivity produced in these post-mitotic cells, 10^5.5 L.D.₅₀/mg of total protein, approaches that observed in vivo. Exposure of the myotubes to different mouse-adapted agents demonstrates strain-specific replication of infectious agents. Mouse-derived myotubes could not be infected with hamster prions suggesting that the species barrier effect is intact. We suggest that non-proliferating myotubes will be a valuable model system for generating infectious prions and for screening compounds for anti-prion activity.

This manuscript describes the generation of a new cell culture system to study the replication of infectious prions. While numerous cell lines exist that can replicate prions, these systems are usually based upon proliferating cells. As mammalian cell cultures double approximately every day, prions established in the culture must also, at least, double to be maintained. This is problematic, however, as prions replicate relatively slowly and cell replication may outpace prion replication. In fact, many cell culture systems do not replicate prions and those that do often do not replicate all strains of prions. Here we describe the use of differentiated non-proliferative muscle cells to replicate prions without the interfering effect of cell division. We observed that prions accumulate to very high levels in this muscle cell culture with infectivity approaching that observed in animals.

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.

Drug resistance confounding prion therapeutics

There is not a single pharmaceutical that halts or even slows any neurodegenerative disease. Mounting evidence shows that prions cause many neurodegenerative diseases, and arguably, scrapie and Creutzfeldt-Jakob disease prions represent the best therapeutic targets. We report here that the previously identified 2-aminothiazoles IND24 and IND81 doubled the survival times of scrapie-infected, wild-type mice. However, mice infected with Rocky Mountain Laboratory (RML) prions, a scrapie-derived strain, and treated with IND24 eventually exhibited neurological dysfunction and died. We serially passaged their brain homogenates in mice and cultured cells. We found that the prion strain isolated from IND24-treated mice, designated RML[IND24], emerged during a single passage in treated mice. Although RML prions infect both the N2a and CAD5 cell lines, RML[IND24] prions could only infect CAD5 cells. When passaged in CAD5 cells, the prions remained resistant to high concentrations of IND24. However, one passage of RML[IND24] prions in untreated mice restored susceptibility to IND24 in CAD5 cells. Although IND24 treatment extended the lives of mice propagating different prion strains, including RML, another scrapie-derived prion strain ME7, and chronic wasting disease, it was ineffective in slowing propagation of Creutzfeldt-Jakob disease prions in transgenic mice. Our studies demonstrate that prion strains can acquire resistance upon exposure to IND24 that is lost upon passage in mice in the absence of IND24. These data suggest that monotherapy can select for resistance, thus intermittent therapy with mixtures of antiprion compounds may be required to slow or stop neurodegeneration.

Cytosolic caspases mediate mislocalised SOD2 depletion in an in vitro model of chronic prion infection

Oxidative stress as a contributor to neuronal death during prion infection is supported by the fact that various oxidative damage markers accumulate in the brain during the course of this disease. The normal cellular substrate of the causative agent, the prion protein, is also linked with protective functions against oxidative stress. Our previous work has found that, in chronic prion infection, an apoptotic subpopulation of cells exhibit oxidative stress and the accumulation of oxidised lipid and protein aggregates with caspase recruitment. Given the likely failure of antioxidant defence mechanisms within apoptotic prion-infected cells, we aimed to investigate the role of the crucial antioxidant pathway components, superoxide dismutases (SOD) 1 and 2, in an in vitro model of chronic prion infection. Increased total SOD activity, attributable to SOD1, was found in the overall population coincident with a decrease in SOD2 protein levels. When apoptotic cells were separated from the total population, the induction of SOD activity in the infected apoptotic cells was lost, with activity reduced back to levels seen in mock-infected control cells. In addition, mitochondrial superoxide production was increased and mitochondrial numbers decreased in the infected apoptotic subpopulation. Furthermore, a pan-caspase probe colocalised with SOD2 outside of mitochondria within cytosolic aggregates in infected cells and inhibition of caspase activity was able to restore cellular levels of SOD2 in the whole unseparated infected population to those of mock-infected control cells. Our results suggest that prion propagation exacerbates an apoptotic pathway whereby mitochondrial dysfunction follows mislocalisation of SOD2 to cytosolic caspases, permitting its degradation. Eventually, cellular capacity to maintain oxidative homeostasis is overwhelmed, thus resulting in cell death.

Anti-prion drug mPPIg5 inhibits PrP(C) conversion to PrP(Sc)

Prion diseases, also known as transmissible spongiform encephalopathies, are a group of fatal neurodegenerative diseases that include scrapie in sheep, bovine spongiform encephalopathy (BSE) in cattle and Creutzfeldt-Jakob disease (CJD) in humans. The ‘protein only hypothesis’ advocates that PrP^Sc, an abnormal isoform of the cellular protein PrP^C, is the main and possibly sole component of prion infectious agents. Currently, no effective therapy exists for these diseases at the symptomatic phase for either humans or animals, though a number of compounds have demonstrated the ability to eliminate PrPSc in cell culture models. Of particular interest are synthetic polymers known as dendrimers which possess the unique ability to eliminate PrP^Sc in both an intracellular and in vitro setting. The efficacy and mode of action of the novel anti-prion dendrimer mPPIg5 was investigated through the creation of a number of innovative bio-assays based upon the scrapie cell assay. These assays were used to demonstrate that mPPIg5 is a highly effective anti-prion drug which acts, at least in part, through the inhibition of PrP^C to PrP^Sc conversion. Understanding how a drug works is a vital component in maximising its performance. By establishing the efficacy and method of action of mPPIg5, this study will help determine which drugs are most likely to enhance this effect and also aid the design of dendrimers with anti-prion capabilities for the future.

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.

Influence of surface groups on poly(propylene imine) dendrimers antiprion activity

Prion diseases are characterized by the accumulation of PrP(Sc), an aberrantly folded isoform of the host protein PrP(C). Specific forms of synthetic molecules known as dendrimers are able to eliminate protease-resistant PrP(Sc) in both an intracellular and in vitro setting. The properties of a dendrimer which govern this ability are unknown. We addressed the issue by comparing the in vitro antiprion ability of numerous modified poly(propylene-imine) dendrimers, which varied in size, structure, charge, and surface group composition. Several of the modified dendrimers, including an anionic glycodendrimer, reduced the level of protease resistant PrP(Sc) in a prion strain-dependent manner. This led to the formulation of a new working model for dendrimer/prion interactions which proposes dendrimers eliminate PrP(Sc) by destabilizing the protein and rendering it susceptible to proteolysis. This ability is not dependent on any particular charge of dendrimer, but does require a high density of reactive surface groups.

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.

Photodynamic inactivation of prions by disulfonated hydroxyaluminium phthalocyanine

Sulfonated phthalocyanines (Pcs) are cyclic tetrapyrroles that constitute a group of photosensitizers. In the presence of visible light and diatomic oxygen, Pcs produce singlet oxygen and other reactive oxygen species that have known degradation effects on lipids, proteins and/or nucleic acids. Pcs have been used successfully in the treatment of bacterial, yeast and fungal infections, but their use in the photodynamic inactivation of prions has never been reported. Here, we evaluated the photodynamic activity of the disodium salt of disulfonated hydroxyaluminium phthalocyanine (PcDS) against mouse-adapted scrapie RML prions in vitro. PcDS treatment of RML brain homogenate resulted in a time- and dose-dependent inactivation of prions. The photodynamic potential of Pcs offers a new way to inactivate prions using biodegradable compounds at room temperature and normal pressure, which could be useful for treating thermolabile materials and liquids.

Prion inhibition with multivalent PrPSc binding compounds

Quinacrine and related heterocyclic compounds have antiprion activity. Since the infectious pathogen of prion diseases is composed of multimeric PrP(Sc) assemblies, we hypothesized that this antiprion property could be enhanced by attaching multiple quinacrine-derived chloroquinoline or acridine moieties to a scaffold. In addition to exploring Congo red dye and tetraphenylporphyrin tetracarboxylic acid scaffolds, which already possess intrinsic prion-binding ability; trimesic acid was used in this role. In practice, Congo red itself could not be modified with chloroquinoline or acridine units, and a modified dicarboxyl analog was also unreactive. The latter also lacked antiprion activity in infected cultured cells. While addition of chloroquinoline to a tetraphenylporphyrin tetracarboxylic acid scaffold resulted in some reduction of PrP(Sc), moieties attached to a trimesic acid scaffold exhibited sub-micromolar IC(50)'s as well as a toxicity profile superior to quinacrine. Antiprion activity of these molecules was influenced by the length, polarity, and rigidity associated with the variable linear or cyclic polyamine tethers, and in some instances was modulated by host-cell and/or strain type. Unexpectedly, several compounds in our series increased PrP(Sc) levels. Overall, inhibitory and enhancing properties of these multivalent compounds offer new avenues for structure-based investigation of prion biology.

Cofactor molecules maintain infectious conformation and restrict strain properties in purified prions

Prions containing misfolded prion protein (PrP(Sc)) can be formed with cofactor molecules using the technique of serial protein misfolding cyclic amplification. However, it remains unknown whether cofactors materially participate in maintaining prion conformation and infectious properties. Here we show that withdrawal of cofactor molecules during serial propagation of purified recombinant prions caused adaptation of PrP(Sc) structure accompanied by a reduction in specific infectivity of >10(5)-fold, to undetectable levels, despite the ability of adapted "protein-only" PrP(Sc) molecules to self-propagate in vitro. We also report that changing only the cofactor component of a minimal reaction substrate mixture during serial propagation induced major changes in the strain properties of an infectious recombinant prion. Moreover, propagation with only one functional cofactor (phosphatidylethanolamine) induced the conversion of three distinct strains into a single strain with unique infectious properties and PrP(Sc) structure. Taken together, these results indicate that cofactor molecules can regulate the defining features of mammalian prions: PrP(Sc) conformation, infectivity, and strain properties. These findings suggest that cofactor molecules likely are integral components of infectious prions.

A brain aggregate model gives new insights into the pathobiology and treatment of prion diseases

Brain aggregates (BrnAggs) derived from fetal mouse brains contain mature neurons and glial cells. We determined that BrnAggs are consistently infected with Rocky Mountain Laboratory scrapie strain prions and produce increasing levels of the pathogenic form of the prion protein (PrP). Their abundant dendrites undergo degeneration shortly after prion infection. Treatment of prion-infected BrnAggs with drugs, such as a γ-secretase inhibitors and quinacrine (Qa), which stop PrP formation and dendritic degeneration, mirrors the results from rodent studies. Because PrP is trafficked into lysosomes by endocytosis and autophagosomes by phagocytosis in neurons of prion strain-infected BrnAggs, we studied the effects of drugs that modulate subcellular trafficking. Rapamycin (Rap), which activates autophagy, markedly increased light-chain 3-II (LC3-II)-positive autophagosomes and cathepsin D-positive lysosomes in BrnAggs but could not eliminate the intracellular PrP within them. Adding Qa to Rap markedly reduced the number of LC3-II-positive autolysosomes. Rap + Qa created a competition between Rap increasing and Qa decreasing LC3-II. Rapamycin + Qa decreased total PrP by 56% compared with that of Qa alone, which reduced PrP by 37% relative to Rap alone. We conclude that the decrease was dominated by the ability of Qa to decrease the formation of PrP. Therefore, BrnAggs provide an efficient in vitro tool for screening drug therapies and studying the complex biology of prions.