Prion strain-dependent differences in conversion of mutant prion proteins in cell culture

Chaperone-mediated disaggregation of infectious prions releases particles that seed new prion formation in a strain-specific manner

The mammalian prion protein can form infectious, nonnative, and protease resistant aggregates (PrPD), which cause lethal prion diseases like human Creutzfeldt-Jakob disease. PrPD seeds the formation of new infectious prions by interacting with and triggering the refolding of the normally soluble mammalian prion protein, PrPC, into more PrPD. Refolding of misfolded proteins in the cell is carried out by molecular chaperones such as Grp78. We have recently shown that Grp78 sensitizes PrPD to proteases, indicating structural alterations and leading to its degradation. However, the process of chaperone-mediated PrPD disaggregation, the chaperones involved, and the effect of disaggregation on PrPD seeding activity are unclear. We have now monitored the structural modification, disaggregation, and seeding activity of PrPD from two mouse adapted prion strains, 22L and 87V, in the presence of Grp78 and two forms of the Hsp110 disaggregase chaperone family, Hsp105 and Apg-2. We found that both forms of Hsp110 induced similar amounts of disaggregation and structural change in the protease resistant cores of PrPD from both strains. However, 22L PrPD was more susceptible to destabilization and disaggregation by the chaperones than 87V. Surprisingly, despite disaggregation of both strains, only the 22L PrPD aggregates released by the chaperones had seeding activity, with both forms of Hsp110 enhancing the Grp78 mediated release of these aggregates. Our data show that disassembly of PrPD by Grp78 and Hsp110 chaperones can release seeding particles of PrPD in a strain-specific manner, potentially facilitating prion replication and spread.

Lysine residues are not required for proteasome-mediated proteolysis of cellular prion protein

Cellular prion protein (PrPC) is a glycosylphosphatidylinositol (GPI)-anchored cell-surface protein. The mature cell-surface PrPC is internalized and subsequently degraded by lysosomes. Although, proteasomes are proposed to be involved, the precise mechanism of PrPC degradation remains uncertain. Given that proteins are ubiquitinated primarily on lysine residues, we sought to determine whether lysine residues within PrPC are involved in the ubiquitination and subsequent degradation of PrPC. We generated a plasmid vector expressing a mutant PrPC (called lysine-null PrPC) in which all lysine residues were replaced with arginine residues. Subsequently, we established stably transformed cell lines (designated HpL2-1 PrP-WT and HpL2-1 PrP-K/R, respectively) using the mouse PrPC-deficient neuronal cell line (HpL2-1) and plasmids expressing wild-type (WT) or lysine-null PrPC (PrP-K/R). We found that HpL2-1 PrP-WT and HpL2-1 PrP-K/R cells correctly expressed their respective PrPC which translocated efficiently to the plasma membrane. Subsequently, using immunoblotting and confocal microscopy, we found that treatment with cycloheximide (CHX; a protein synthesis inhibitor) significantly reduced PrPC expression in both these transformed cell lines, indicating that WT and lysine-null PrPC are degraded similarly. Taken together, these results indicate that the lysine residues of PrPC do not regulate its degradation.

Multiple aspects of amyloid dynamics in vivo integrate to establish prion variant dominance in yeast

Prion variants are self-perpetuating conformers of a single protein that assemble into amyloid fibers and confer unique phenotypic states. Multiple prion variants can arise, particularly in response to changing environments, and interact within an organism. These interactions are often competitive, with one variant establishing phenotypic dominance over the others. This dominance has been linked to the competition for non-prion state protein, which must be converted to the prion state via a nucleated polymerization mechanism. However, the intrinsic rates of conversion, determined by the conformation of the variant, cannot explain prion variant dominance, suggesting a more complex interaction. Using the yeast prion system [PSI+ ], we have determined the mechanism of dominance of the [PSI+ ]Strong variant over the [PSI+ ]Weak variant in vivo. When mixed by mating, phenotypic dominance is established in zygotes, but the two variants persist and co-exist in the lineage descended from this cell. [PSI+ ]Strong propagons, the heritable unit, are amplified at the expense of [PSI+ ]Weak propagons, through the efficient conversion of soluble Sup35 protein, as revealed by fluorescence photobleaching experiments employing variant-specific mutants of Sup35. This competition, however, is highly sensitive to the fragmentation of [PSI+ ]Strong amyloid fibers, with even transient inhibition of the fragmentation catalyst Hsp104 promoting amplification of [PSI+ ]Weak propagons. Reducing the number of [PSI+ ]Strong propagons prior to mating, similarly promotes [PSI+ ]Weak amplification and conversion of soluble Sup35, indicating that template number and conversion efficiency combine to determine dominance. Thus, prion variant dominance is not an absolute hierarchy but rather an outcome arising from the dynamic interplay between unique protein conformations and their interactions with distinct cellular proteostatic niches.

The seeding barrier between human and Syrian hamster prion protein amyloid fibrils is determined by β2-α2 loop sequence elements

Transmissive spongiform encephalopathies (TSE) are a group of neurodegenerative diseases caused by infectious protein particles, known as prions. Prions are formed from cellular prion proteins (PrP) and can be transmitted between different mammalian species. Subsequently, the host's PrPs are then converted to prions, followed by the onset of TSE. Interspecies prion infectivity is governed by the amino acid sequence differences of PrPs and prions' inability to replicate in a host is termed a species barrier. Here, we investigated the amino acid sequence determinants of species barrier between recombinant human (rHuPrP) and hamster (rShaPrP) prion protein amyloid fibrils. We discovered that a unidirectional species barrier between rShaPrP and rHuPrP amyloid fibrils exists. This barrier stems from the difference of amino acid sequences in the conserved β2-α2 loop region. Our results revealed that individual amino acids in the β2-α2 loop region are critical for overcoming the barrier between human and hamster prion protein amyloid fibrils in vitro. Furthermore, the barrier was only possible to observe through aggregation kinetics, as the secondary structure rHuPrP fibrils was not affected by the cross-seeding. Overall, we demonstrated the mechanistic pathway behind this interspecies barrier phenomenon, which increases our understanding of prion-related disease development.

On the Structural Diversity and Individuality of Polymorphic Amyloid Protein Assemblies

The prediction of highly ordered three-dimensional structures of amyloid protein fibrils from the amino acid sequences of their monomeric self-assembly precursors constitutes a challenging and unresolved aspect of the classical protein folding problem. Because of the polymorphic nature of amyloid assembly whereby polypeptide chains of identical amino acid sequences under identical conditions are capable of self-assembly into a spectrum of different fibril structures, the prediction of amyloid structures from an amino acid sequence requires a detailed and holistic understanding of its assembly free energy landscape. The full extent of the structure space accessible to the cross-β molecular architecture of amyloid must also be resolved. Here, we review the current understanding of the diversity and the individuality of amyloid structures, and how the polymorphic landscape of amyloid links to biology and disease phenotypes. We present a comprehensive review of structural models of amyloid fibrils derived by cryo-EM, ssNMR and AFM to date, and discuss the challenges ahead for resolving the structural basis and the biological consequences of polymorphic amyloid assemblies.

Dextran sulphate inhibits an association of prions with plasma membrane at the early phase of infection

The defining characteristic of prion diseases is conversion of a cellular prion protein (PrP^C) to an abnormal prion protein (PrP^Sc). The exogenous attachment of PrP^Sc to the surface of a target cell is critical for infection. However, the initial interaction of PrP^Sc with the cell surface is poorly characterized. In the current study, we specifically focused on the association of PrP^Sc with cells during the early phase of infection, using an acute infection model. First, we treated mouse neuroblastoma N2a-58 cells with prion strain 22 L-infected brain homogenates and revealed that PrP^Sc was associated with membrane fractions within three hours, a short exposure time. These results were also observed in PrP^C-deficient hippocampus cell lines. We also demonstrate here that PrP^Sc from 22 L-infected brain homogenates was associated with lipid rafts during the early phase of infection. Furthermore, we revealed that DS500, a glycosaminoglycan mimetic, inhibited both the attachment of PrP^Sc to membrane fractions and subsequent prion transmission, suggesting that the early association of prions with cell surface is important for prion infection.

Prion protein interacts with the metabotropic glutamate receptor 1 and regulates the organization of Ca2+ signaling

Cellular prion protein (PrP) is a membrane protein that is highly conserved among mammals and mainly expressed on the cell surface of neurons. Despite its reported interactions with various membrane proteins, no functional studies have so far been carried out on it, and its physiological functions remain unclear. Neuronal cell death has been observed in a PrP-knockout mouse model expressing Doppel protein, suggesting that PrP might be involved in Ca²⁺ signaling. In this study, we evaluated the binding of PrP to metabotropic glutamate receptor 1 (mGluR1) and found that wild-type PrP (PrP-wt) and mGluR1 co-immunoprecipitated in dual-transfected Neuro-2a (N2a) cells. Fluorescence resonance energy transfer analysis revealed an energy transfer between mGluR1-Cerulean and PrP-Venus. In order to determine whether PrP can modulate mGluR1 signaling, we performed Ca²⁺ imaging analyses following repetitive exposure to an mGluR1 agonist. Agonist stimulation induced synchronized Ca²⁺ oscillations in cells coexpressing PrP-wt and mGluR1. In contrast, N2a cells expressing PrP-ΔN failed to show ligand-dependent regulation of mGluR1-Ca²⁺ signaling, indicating that PrP can bind to mGluR1 and modulate its function to prevent irregular Ca²⁺ signaling and that its N-terminal region functions as a molecular switch during Ca²⁺ signaling.

Type I interferon protects neurons from prions in in vivo models

Infectious prions comprising abnormal prion protein, which is produced by structural conversion of normal prion protein, are responsible for transmissible spongiform encephalopathies including Creutzfeldt-Jakob disease in humans. Prions are infectious agents that do not possess a genome and the pathogenic protein was not thought to evoke any immune response. Although we previously reported that interferon regulatory factor 3 (IRF3) was likely to be involved in the pathogenesis of prion diseases, suggesting the protective role of host innate immune responses mediated by IRF3 signalling, this remained to be clarified. Here, we investigated the reciprocal interactions of type I interferon evoked by IRF3 activation and prion infection and found that infecting prions cause the suppression of endogenous interferon expression. Conversely, treatment with recombinant interferons in an ex vivo model was able to inhibit prion infection. In addition, cells and mice deficient in type I interferon receptor (subunit interferon alpha/beta receptor 1), exhibited higher susceptibility to 22L-prion infection. Moreover, in in vivo and ex vivo prion-infected models, treatment with RO8191, a selective type I interferon receptor agonist, inhibited prion invasion and prolonged the survival period of infected mice. Taken together, these data indicated that the interferon signalling interferes with prion propagation and some interferon-stimulated genes might play protective roles in the brain. These findings may allow for the development of new strategies to combat fatal diseases.

A Single Amino Acid Substitution, Found in Mammals with Low Susceptibility to Prion Diseases, Delays Propagation of Two Prion Strains in Highly Susceptible Transgenic Mouse Models

Specific variations in the amino acid sequence of prion protein (PrP) are key determinants of susceptibility to prion diseases. We previously showed that an amino acid substitution specific to canids confers resistance to prion diseases when expressed in mice and demonstrated its dominant-negative protective effect against a variety of infectious prion strains of different origins and characteristics. Here, we show that expression of this single amino acid change significantly increases survival time in transgenic mice expressing bank vole cellular prion protein (PrP^C), which is inherently prone to misfolding, following inoculation with two distinct prion strains (the CWD-vole strain and an atypical strain of spontaneous origin). This amino acid substitution hinders the propagation of both prion strains, even when expressed in the context of a PrP^C uniquely susceptible to a wide range of prion isolates. Non-inoculated mice expressing this substitution experience spontaneous prion formation, but showing an increase in survival time comparable to that observed in mutant mice inoculated with the atypical strain. Our results underscore the importance of this PrP variant in the search for molecules with therapeutic potential against prion diseases.

Interrogating the Dimerization Interface of the Prion Protein Via Site-Specific Mutations to p-Benzoyl-L-Phenylalanine

Transmissible spongiform encephalopathies are centered on the conformational transition of the prion protein from a mainly helical, monomeric structure to a β-sheet rich ordered aggregate. Experiments indicate that the main infectious and toxic species in this process are however shorter oligomers, formation of which from the monomers is yet enigmatic. Here, we created 25 variants of the mouse prion protein site-specifically containing one genetically-incorporated para-benzoyl-phenylalanine (pBpa), a cross-linkable non-natural amino acid, in order to interrogate the interface of a prion protein-dimer, which might lie on the pathway of oligomerization. Our results reveal that the N-terminal part of the prion protein, especially regions around position 127 and 107, is integral part of the dimer interface. These together with additional pBpa-containing variants of mPrP might also facilitate to gain more structural insights into oligomeric and fibrillar prion protein species including the pathological variants.

An Amino Acid Substitution Found in Animals with Low Susceptibility to Prion Diseases Confers a Protective Dominant-Negative Effect in Prion-Infected Transgenic Mice

While prion diseases have been described in numerous species, some, including those of the Canidae family, appear to show resistance or reduced susceptibility. A better understanding of the factors underlying prion susceptibility is crucial for the development of effective treatment and control measures. We recently demonstrated resistance to prion infection in mice overexpressing a mutated prion protein (PrP) carrying a specific amino acid substitution characteristic of canids. Here, we show that coexpression of this mutated PrP and wild-type mouse PrP in transgenic mice inoculated with different mouse-adapted prion strains (22 L, ME7, RML, and 301C) significantly increases survival times (by 45 to 113%). These data indicate that this amino acid substitution confers a dominant-negative effect on PrP, attenuating the conversion of PrP^C to PrP^Sc and delaying disease onset without altering the neuropathological properties of the prion strains. Taken together, these findings have important implications for the development of new treatment approaches for prion diseases based on dominant-negative proteins.

Caprine PrP variants harboring Asp-146, His-154 and Gln-211 alleles display reduced convertibility upon interaction with pathogenic murine prion protein in scrapie infected cells

Scrapie, the prion disease of sheep and goats, is a devastating malady of small ruminants. Due to its infectious nature, epidemic outbreaks may occur in flocks/herds consisting of highly susceptible animals. Field studies identified scrapie-protective caprine PrP variants, harboring specific single amino acid changes (Met-142, Arg-143, Asp-146, Ser-146, His-154, Gln-211 and Lys-222). Their effects are under further evaluation, and aim to determine the most protective allele. We assessed some of these variants (Asp-146, His-154, Gln-211 and Lys-222), after their exogenous expression as murine-caprine chimeras in a scrapie- infected murine cell line. We report that exogenously expressed PrPs undergo conformational conversion upon interaction with the endogenous pathological murine prion protein (PrP^SC), which results in the detection of goat-specific and partially PK-resistant moieties. These moieties display a PK-resistance pattern distinct from the one detected in natural goat scrapie cases. Within this cellular model, distinct conformational conversion potentials were assigned to the tested variants. Molecules carrying the Asp-146, His-154 and Gln-211 alleles showed significantly lower conversion levels compared to wild type, confirming their protective effects against scrapie. Although we utilized a heterologous conversion system, this is to our knowledge, the first study of caprine PrP variants in a cellular context of scrapie, that confirms the protective effects of some of the studied alleles.

A dominant-negative mutant inhibits multiple prion variants through a common mechanism

Prions adopt alternative, self-replicating protein conformations and thereby determine novel phenotypes that are often irreversible. Nevertheless, dominant-negative prion mutants can revert phenotypes associated with some conformations. These observations suggest that, while intervention is possible, distinct inhibitors must be developed to overcome the conformational plasticity of prions. To understand the basis of this specificity, we determined the impact of the G58D mutant of the Sup35 prion on three of its conformational variants, which form amyloids in S. cerevisiae. G58D had been previously proposed to have unique effects on these variants, but our studies suggest a common mechanism. All variants, including those reported to be resistant, are inhibited by G58D but at distinct doses. G58D lowers the kinetic stability of the associated amyloid, enhancing its fragmentation by molecular chaperones, promoting Sup35 resolubilization, and leading to amyloid clearance particularly in daughter cells. Reducing the availability or activity of the chaperone Hsp104, even transiently, reverses curing. Thus, the specificity of inhibition is determined by the sensitivity of variants to the mutant dosage rather than mode of action, challenging the view that a unique inhibitor must be developed to combat each variant.

Generating Bona Fide Mammalian Prions with Internal Deletions

Mammalian prions are PrP proteins with altered structures causing transmissible fatal neurodegenerative diseases. They are self-perpetuating through formation of beta-sheet-rich assemblies that seed conformational change of cellular PrP. Pathological PrP usually forms an insoluble protease-resistant core exhibiting beta-sheet structures but no more alpha-helical content, loosing the three alpha-helices contained in the correctly folded PrP. The lack of a high-resolution prion structure makes it difficult to understand the dynamics of conversion and to identify elements of the protein involved in this process. To determine whether completeness of residues within the protease-resistant domain is required for prions, we performed serial deletions in the helix H2 C terminus of ovine PrP, since this region has previously shown some tolerance to sequence changes without preventing prion replication. Deletions of either four or five residues essentially preserved the overall PrP structure and mutant PrP expressed in RK13 cells were efficiently converted into bona fide prions upon challenge by three different prion strains. Remarkably, deletions in PrP facilitated the replication of two strains that otherwise do not replicate in this cellular context. Prions with internal deletion were self-propagating and de novo infectious for naive homologous and wild-type PrP-expressing cells. Moreover, they caused transmissible spongiform encephalopathies in mice, with similar biochemical signatures and neuropathologies other than the original strains. Prion convertibility and transfer of strain-specific information are thus preserved despite shortening of an alpha-helix in PrP and removal of residues within prions. These findings provide new insights into sequence/structure/infectivity relationship for prions.

IMPORTANCE

Prions are misfolded PrP proteins that convert the normal protein into a replicate of their own abnormal form. They are responsible for invariably fatal neurodegenerative disorders. Other aggregation-prone proteins appear to have a prion-like mode of expansion in brains, such as in Alzheimer's or Parkinson's diseases. To date, the resolution of prion structure remains elusive. Thus, to genetically define the landscape of regions critical for prion conversion, we tested the effect of short deletions. We found that, surprisingly, removal of a portion of PrP, the C terminus of alpha-helix H2, did not hamper prion formation but generated infectious agents with an internal deletion that showed characteristics essentially similar to those of original infecting strains. Thus, we demonstrate that completeness of the residues inside prions is not necessary for maintaining infectivity and the main strain-specific information, while reporting one of the few if not the only bona fide prions with an internal deletion.

Structural conservation of prion strain specificities in recombinant prion protein fibrils in real-time quaking-induced conversion

A major unsolved issue of prion biology is the existence of multiple strains with distinct phenotypes and this strain phenomenon is postulated to be associated with the conformational diversity of the abnormal prion protein (PrP^Sc). Real-time quaking-induced conversion (RT-QUIC) assay that uses Escherichia coli-derived recombinant prion protein (rPrP) for the sensitive detection of PrP^Sc results in the formation of rPrP-fibrils seeded with various strains. We demonstrated that there are differences in the secondary structures, especially in the β-sheets, and conformational stability between 2 rPrP-fibrils seeded with either Chandler or 22L strains in the first round of RT-QUIC. In particular, the differences in conformational properties of these 2 rPrP-fibrils were common to those of the original PrP^Sc. However, the strain specificities of rPrP-fibrils seen in the first round were lost in subsequent rounds. Instead, our findings suggest that nonspecific fibrils became the major species, probable owing to their selective growth advantage in the RT-QUIC. This study shows that at least some strain-specific conformational properties of the original PrP^Sc can be transmitted to rPrP-fibrils in vitro, but further conservation appears to require unknown cofactors or environmental conditions or both.

Expression of the Prion Protein Family Member Shadoo Causes Drug Hypersensitivity That Is Diminished by the Coexpression of the Wild Type Prion Protein

The prion protein (PrP) seems to exert both neuroprotective and neurotoxic activities. The toxic activities are associated with the C-terminal globular parts in the absence of the flexible N terminus, specifically the hydrophobic domain (HD) or the central region (CR). The wild type prion protein (PrP-WT), having an intact flexible part, exhibits neuroprotective qualities by virtue of diminishing many of the cytotoxic effects of these mutant prion proteins (PrPΔHD and PrPΔCR) when coexpressed. The prion protein family member Doppel, which possesses a three-dimensional fold similar to the C-terminal part of PrP, is also harmful to neuronal and other cells in various models, a phenotype that can also be eliminated by the coexpression of PrP-WT. In contrast, another prion protein family member, Shadoo (Sho), a natively disordered protein possessing structural features similar to the flexible N-terminal tail of PrP, exhibits PrP-WT-like protective properties. Here, we report that, contrary to expectations, Sho expression in SH-SY5Y or HEK293 cells induces the same toxic phenotype of drug hypersensitivity as PrPΔCR. This effect is exhibited in a dose-dependent manner and is also counteracted by the coexpression of PrP-WT. The opposing effects of Shadoo in different model systems revealed here may be explored to help discern the relationship of the various toxic activities of mutant PrPs with each other and the neurotoxic effects seen in neurodegenerative diseases, such as transmissible spongiform encephalopathy and Alzheimer disease.

Strain-Dependent Effect of Macroautophagy on Abnormally Folded Prion Protein Degradation in Infected Neuronal Cells

Prion diseases are neurodegenerative disorders caused by the accumulation of abnormal prion protein (PrP^Sc) in the central nervous system. With the aim of elucidating the mechanism underlying the accumulation and degradation of PrP^Sc, we investigated the role of autophagy in its degradation, using cultured cells stably infected with distinct prion strains. The effects of pharmacological compounds that inhibit or stimulate the cellular signal transduction pathways that mediate autophagy during PrP^Sc degradation were evaluated. The accumulation of PrP^Sc in cells persistently infected with the prion strain Fukuoka-1 (FK), derived from a patient with Gerstmann–Sträussler–Scheinker syndrome, was significantly increased in cultures treated with the macroautophagy inhibitor 3-methyladenine (3MA) but substantially reduced in those treated with the macroautophagy inducer rapamycin. The decrease in FK-derived PrP^Sc levels was mediated, at least in part, by the phosphatidylinositol 3-kinase/MEK signalling pathway. By contrast, neither rapamycin nor 3MA had any apparently effect on PrP^Sc from either the 22L or the Chandler strain, indicating that the degradation of PrP^Sc in host cells might be strain-dependent.

Ubiquitin-specific protease 14 modulates degradation of cellular prion protein

Prion diseases are fatal neurodegenerative disorders characterized by the accumulation of prion protein (PrP^C). To date, there is no effective treatment for the disease. The accumulated PrP, termed PrP^Sc, forms amyloid fibrils and could be infectious. It has been suggested that PrP^Sc is abnormally folded and resistant to proteolytic degradation, and also inhibits proteasomal functions in infected cells, thereby inducing neuronal death. Recent work indicates that the ubiquitin-proteasome system is involved in quality control of PrP^C. To reveal the significance of prion protein ubiqitination, we focused on ubiquitin-specific protease 14 (USP14), a deubiqutinating enzyme that catalyzes trimming of polyubiquitin chains and plays a role in regulation of proteasomal processes. Results from the present study showed that treatment with a selective inhibitor of USP14 reduced PrP^C, as well as PrP^Sc, levels in prion-infected neuronal cells. Overexpression of the dominant negative mutant form of USP14 reduced PrP^Sc, whereas wildtype USP14 increased PrP^Sc in prion-infected cells. These results suggest that USP14 prevents degradation of both normal and abnormal PrP. Collectively, a better understanding about the regulation of PrP^Sc clearance caused by USP14 might contribute greatly to the development of therapeutic strategies for prion diseases.

Human prion protein sequence elements impede cross-species chronic wasting disease transmission

Chronic wasting disease (CWD) is a fatal prion disease of North American deer and elk and poses an unclear risk for transmission to humans. Human exposure to CWD occurs through hunting activities and consumption of venison from prion-infected animals. Although the amino acid residues of the prion protein (PrP) that prevent or permit human CWD infection are unknown, NMR-based structural studies suggest that the β2-α2 loop (residues 165-175) may impact species barriers. Here we sought to define PrP sequence determinants that affect CWD transmission to humans. We engineered transgenic mice that express human PrP with four amino acid substitutions that result in expression of PrP with a β2-α2 loop (residues 165-175) that exactly matches that of elk PrP. Compared with transgenic mice expressing unaltered human PrP, mice expressing the human-elk chimeric PrP were highly susceptible to elk and deer CWD prions but were concurrently less susceptible to human Creutzfeldt-Jakob disease prions. A systematic in vitro survey of amino acid differences between humans and cervids identified two additional residues that impacted CWD conversion of human PrP. This work identifies amino acids that constitute a substantial structural barrier for CWD transmission to humans and helps illuminate the molecular requirements for cross-species prion transmission.

Reduced expression of the presynaptic co-chaperone cysteine string protein alpha (CSPα) does not exacerbate experimentally-induced ME7 prion disease

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CSPα is reduced in ME7-animals during disease progression.

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CSPα heterozygosity does not accelerate behavioural changes in ME7-animals.

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Prion disease pathology is not altered by reduced CSPα expression.

Infection of mice with the ME7 prion agent results in well-characterised neuropathological changes, which includes vacuolation, neurodegeneration and synaptic degeneration. Presynaptic dysfunction and degeneration is apparent through the progressive reduction in synaptic vesicle proteins and eventual loss of synapses. Cysteine string protein alpha (CSPα), which regulates refolding pathways at the synapse, exhibits an early decline during chronic neurodegeneration implicating it as a mediator of disease mechanisms. CSPα null mice develop a progressive neuronal dysfunction through disruption of the integrity of presynaptic function. In this study, we investigated whether reduced expression of CSPα would exacerbate ME7 prion disease. Wild type (+/+) and heterozygous (+/−) mice, which express about a ∼50% reduction in CSPα, were used as a distinct genetic background on which to impose prion disease. +/+ and +/ − mice were inoculated with brain homogenate from either a normal mouse brain (NBH) or from the brain of a mouse which displayed clinical signs of prion disease (ME7). Behavioural tests, western blotting and immunohistochemistry, which resolve key elements of synaptic dysfunction, were used to assess the effect of reduced CSPα on disease. Behavioural tests revealed no change in the progression of disease in ME7–CSPα +/− animals compared to ME7–CSPα +/+ animals. In addition, the accumulation of misfolded PrP^Sc, the diseased associated gliosis or synaptic loss were not different. Thus, the misfolding events that generate synaptic dysfunction and lead to synaptic loss are unlikely to be mediated by a disease associated decrease in the refolding pathways associated with CSPα.

Role of monomer arrangement in the amyloid self-assembly

Assembly of amyloid proteins into aggregates requires the ordering of the monomers in oligomers and especially in such highly organized structures as fibrils. This ordering is accompanied by structural transitions leading to the formation of ordered β-structural motifs in proteins and peptides lacking secondary structures. To characterize the effect of the monomer arrangements on the aggregation process at various stages, we performed comparative studies of the yeast prion protein Sup35 heptapeptide (GNNQQNY) along with its dimeric form CGNNQQNY-(d-Pro)-G-GNNQQNY. The (d-Pro)-G linker in this construct is capable of adopting a β-turn, facilitating the assembly of the dimer into the dimeric antiparallel hairpin structure (AP-hairpin). We applied Atomic Force Microscopy (AFM) techniques to follow peptide-peptide interactions at the single molecule level, to visualize the morphology of aggregates formed by both constructs, thioflavin T (ThT) fluorescence to follow the aggregation kinetics, and circular dichroism (CD) spectroscopy to characterize the secondary structure of the constructs. The ThT fluorescence data showed that the AP-hairpin aggregation kinetics is insensitive to the external environment such as ionic strength and pH contrary to the monomers the kinetics of which depends dramatically on the ionic strength and pH. The AFM topographic imaging revealed that AP-hairpins primarily assemble into globular aggregates, whereas linear fibrils are primary assemblies of the monomers suggesting that both constructs follow different aggregation pathways during the self-assembly. These morphological differences are in line with the AFM force spectroscopy experiments and CD spectroscopy measurements, suggesting that the AP-hairpin is structurally rigid regardless of changes of environmental factors.

Persistent prion infection disturbs the function of Oct-1, resulting in the down-regulation of murine interferon regulatory factor-3

As a prompt response against invasion of various viruses, interferon regulatory factor-3 (IRF-3) is initially phosphorylated to become activated and upregulates mainly Type I Interferons (IFN-I) in most cell types. We previously reported that IRF-3-dependent host innate immune responses partially interfere in infection of prions. Here, we found that stable infection of prion suppressed IRF-3 gene-expression. The decreased promoter activity of IRF-3 was significantly restored along with treatment of anti-prion drugs in the prion-infected cells, suggesting that infection of prion directly influence the regulation of IRF-3 transcription. We further investigated promoter activity of 5′- flanking region of murine IRF-3 using a luciferase reporter system and found that the nucleotides -119 to -1 were indispensable for the promoter activity. Within this region, mutations in the Oct-1 binding site significantly reduced the promoter activity and chromatin immunoprecipitation (ChIP) assay revealed that Oct-1 indeed binds to the region. In addition, overexpression of Oct-1 increased the promoter activity of IRF-3. Intriguingly, Oct-1 protein was significantly reduced in prion-infected cells and mice brains compared with uninfected groups. Taken together, we concluded that prion infection could interfere in the function of Oct-1, resulting in the down-regulation of IRF-3.

Conformational properties of prion strains can be transmitted to recombinant prion protein fibrils in real-time quaking-induced conversion

The phenomenon of prion strains with distinct biological characteristics has been hypothesized to be involved in the structural diversity of abnormal prion protein (PrP(Sc)). However, the molecular basis of the transmission of strain properties remains poorly understood. Real-time quaking-induced conversion (RT-QUIC) is a cell-free system that uses Escherichia coli-derived recombinant PrP (rPrP) for the sensitive detection of PrP(Sc). To investigate whether the properties of various prion strains can be transmitted to amyloid fibrils consisting of rPrP (rPrP fibrils) using RT-QUIC, we examined the secondary structure, conformational stability, and infectivity of rPrP fibrils seeded with PrP(Sc) derived from either the Chandler or the 22L strain. In the first round of the reaction, there were differences in the secondary structures, especially in bands attributed to β-sheets, as determined by infrared spectroscopy, and conformational stability between Chandler-seeded (1st-rPrP-fib(Ch)) and 22L-seeded (1st-rPrP-fib(22L)) rPrP fibrils. Of note, specific identifying characteristics of the two rPrP fibril types seen in the β-sheets resembled those of the original PrP(Sc). Furthermore, the conformational stability of 1st-rPrP-fib(Ch) was significantly higher than that of 1st-rPrP-fib(22L), as with Chandler and 22L PrP(Sc). The survival periods of mice inoculated with 1st-rPrP-fib(Ch) or 1st-rPrP-fib(22L) were significantly shorter than those of mice inoculated with mixtures from the mock 1st-round RT-QUIC procedure. In contrast, these biochemical characteristics were no longer evident in subsequent rounds, suggesting that nonspecific uninfected rPrP fibrils became predominant probably because of their high growth rate. Together, these findings show that at least some strain-specific conformational properties can be transmitted to rPrP fibrils and unknown cofactors or environmental conditions may be required for further conservation. Importance: The phenomenon of prion strains with distinct biological characteristics is assumed to result from the conformational variations in the abnormal prion protein (PrP(Sc)). However, important questions remain about the mechanistic relationship between the conformational differences and the strain diversity, including how strain-specific conformations are transmitted. In this study, we investigated whether the properties of diverse prion strains can be transmitted to amyloid fibrils consisting of E. coli-derived recombinant PrP (rPrP) generated by real-time quaking-induced conversion (RT-QUIC), a recently developed in vitro PrP(Sc) formation method. We demonstrate that at least some of the strain-specific conformational properties can be transmitted to rPrP fibrils in the first round of RT-QUIC by examining the secondary structure, conformational stability, and infectivity of rPrP fibrils seeded with PrP(Sc) derived from either the Chandler or the 22L prion strain. We believe that these findings will advance our understanding of the conformational basis underlying prion strain diversity.

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.

A proposed mechanism for the promotion of prion conversion involving a strictly conserved tyrosine residue in the β2-α2 loop of PrPC

The transmission of infectious prions into different host species requires compatible prion protein (PrP) primary structures, and even one heterologous residue at a pivotal position can block prion infection. Mapping the key amino acid positions that govern cross-species prion conversion has not yet been possible, although certain residue positions have been identified as restrictive, including residues in the β2-α2 loop region of PrP. To further define how β2-α2 residues impact conversion, we investigated residue substitutions in PrP(C) using an in vitro prion conversion assay. Within the β2-α2 loop, a tyrosine residue at position 169 is strictly conserved among mammals, and transgenic mice expressing mouse PrP having the Y169G, S170N, and N174T substitutions resist prion infection. To better understand the structural requirements of specific residues for conversion initiated by mouse prions, we substituted a diverse array of amino acids at position 169 of PrP. We found that the substitution of glycine, leucine, or glutamine at position 169 reduced conversion by ∼ 75%. In contrast, replacing tyrosine 169 with either of the bulky, aromatic residues, phenylalanine or tryptophan, supported efficient prion conversion. We propose a model based on a requirement for tightly interdigitating complementary amino acid side chains within specific domains of adjacent PrP molecules, known as "steric zippers," to explain these results. Collectively, these studies suggest that an aromatic residue at position 169 supports efficient prion conversion.

Increased expression of p62/SQSTM1 in prion diseases and its association with pathogenic prion protein

Prion diseases are neurodegenerative disorders characterized by the aggregation of abnormally folded prion protein (PrP^Sc). In this study, we focused on the mechanism of clearance of PrP^Sc, which remains unclear. p62 is a cytosolic protein known to mediate both the formation and degradation of aggregates of abnormal proteins. The levels of p62 protein increased in prion-infected brains and persistently infected cell cultures. Upon proteasome inhibition, p62 co-localized with PrP^Sc, forming a large aggregate in the perinuclear region, hereafter referred to as PrP^Sc-aggresome. These aggregates were surrounded with autophagosome marker LC3 and lysosomes in prion-infected cells. Moreover, transient expression of the phosphomimic form of p62, which has enhanced ubiquitin-binding activity, reduced the amount of PrP^Sc in prion-infected cells, indicating that the activation of p62 could accelerate the clearance of PrP^Sc. Our findings would thus suggest that p62 could be a target for the therapeutic control of prion diseases.

Prion transmission prevented by modifying the β2-α2 loop structure of host PrPC

Zoonotic prion transmission was reported after the bovine spongiform encephalopathy (BSE) epidemic, when >200 cases of prion disease in humans were diagnosed as variant Creutzfeldt-Jakob disease. Assessing the risk of cross-species prion transmission remains challenging. We and others have studied how specific amino acid residue differences between species impact prion conversion and have found that the β2-α2 loop region of the mouse prion protein (residues 165-175) markedly influences infection by sheep scrapie, BSE, mouse-adapted scrapie, deer chronic wasting disease, and hamster-adapted scrapie prions. The tyrosine residue at position 169 is strictly conserved among mammals and an aromatic side chain in this position is essential to maintain a 310-helical turn in the β2-α2 loop. Here we examined the impact of the Y169G substitution together with the previously described S170N, N174T "rigid loop" substitutions on cross-species prion transmission in vivo and in vitro. We found that transgenic mice expressing mouse PrP containing the triple-amino acid substitution completely resisted infection with two strains of mouse prions and with deer chronic wasting disease prions. These studies indicate that Y169 is important for prion formation, and they provide a strong indication that variation of the β2-α2 loop structure can modulate interspecies prion transmission.

Effect of electrostatics on aggregation of prion protein Sup35 peptide

Self-assembly of misfolded proteins into ordered fibrillar structures is a fundamental property of a wide range of proteins and peptides. This property is also linked with the development of various neurodegenerative diseases such as Alzheimer's and Parkinson's. Environmental conditions modulate the misfolding and aggregation processes. We used a peptide, CGNNQQNY, from yeast prion protein Sup35, as a model system to address effects of environmental conditions on aggregate formation. The GNNQQNY peptide self-assembles in fibrils with structural features that are similar to amyloidogenic proteins. Atomic force microscopy (AFM) and thioflavin T (ThT) fluorescence assay were employed to follow the aggregation process at various pHs and ionic strengths. We also used single molecule AFM force spectroscopy to probe interactions between the peptides under various conditions. The ThT fluorescence data showed that the peptide aggregates fast at pH values approaching the peptide isoelectric point (pI = 5.3) and the kinetics is 10 times slower at acidic pH (pH 2.0), suggesting that electrostatic interactions contribute to the peptide self-assembly into aggregates. This hypothesis was tested by experiments performed at low (11 mM) and high (150 mM) ionic strengths. Indeed, the aggregation lag time measured at pH 2 at low ionic strength (11 mM) is 195 h, whereas the lag time decreases ~5 times when the ionic strength is increased to 150 mM. At conditions close to the pI value, pH 5.6, the aggregation lag time is 12 ± 6 h under low ionic strength, and there is minimal change to the lag time at 150 mM NaCl. The ionic strength also influences the morphology of aggregates visualized with AFM. In pH 2.0 and at high ionic strength, the aggregates are twofold taller than those formed at low ionic strength. In parallel, AFM force spectroscopy studies revealed minimal contribution of electrostatics to dissociation of transient peptide dimers.

Integrity of helix 2-helix 3 domain of the PrP protein is not mandatory for prion replication

The process of prion conversion is not yet well understood at the molecular level. The regions critical for the conformational change of PrP remain mostly debated and the extent of sequence change acceptable for prion conversion is poorly documented. To achieve progress on these issues, we applied a reverse genetic approach using the Rov cell system. This allowed us to test the susceptibility of a number of insertion mutants to conversion into prion in the absence of wild-type PrP molecules. We were able to propagate several prions with 8 to 16 extra amino acids, including a polyglycine stretch and His or FLAG tags, inserted in the middle of the protease-resistant fragment. These results demonstrate the possibility to increase the length of the loop between helices H2 and H3 up to 4-fold, without preventing prion replication. They also indicate that this loop probably remains unstructured in PrP(Sc). We also showed that bona fide prions can be produced following insertion of octapeptides in the two C-terminal turns of H2. These insertions do not interfere with the overall fold of the H2-H3 domain indicating that the highly conserved sequence of the terminal part of H2 is not critical for the conversion. Altogether these data showed that the amplitude of modifications acceptable for prion conversion in the core of the globular domain of PrP is much greater than one might have assumed. These observations should help to refine structural models of PrP(Sc) and elucidate the conformational changes underlying prions generation.

Protective role of interferon regulatory factor 3-mediated signaling against prion infection

Abnormal prion protein (PrP(Sc)) generated from the cellular isoform of PrP (PrP(C)) is assumed to be the main or sole component of the pathogen, called prion, of transmissible spongiform encephalopathies (TSE). Because PrP is a host-encoded protein, acquired immune responses are not induced in TSE. Meanwhile, activation of the innate immune system has been suggested to partially block the progression of TSE; however, the mechanism is not well understood. To further elucidate the role of the innate immune system in prion infection, we investigated the function of interferon regulatory factor 3 (IRF3), a key transcription factor of the MyD88-independent type I interferon (IFN) production pathway. We found that IRF3-deficient mice exhibited significantly earlier onset with three murine TSE strains, namely, 22L, FK-1, and murine bovine spongiform encephalopathy (mBSE), following intraperitoneal transmission, than with wild-type controls. Moreover, overexpression of IRF3 attenuated prion infection in the cell culture system, while PrP(Sc) was increased in prion-infected cells treated with small interfering RNAs (siRNAs) against IRF3, suggesting that IRF3 negatively regulates PrP(Sc) formation. Our findings provide new insight into the role of the host innate immune system in the pathogenesis of prion diseases.

Direct evidence of generation and accumulation of β-sheet-rich prion protein in scrapie-infected neuroblastoma cells with human IgG1 antibody specific for β-form prion protein

We prepared β-sheet-rich recombinant full-length prion protein (β-form PrP) (Jackson, G. S., Hosszu, L. L., Power, A., Hill, A. F., Kenney, J., Saibil, H., Craven, C. J., Waltho, J. P., Clarke, A. R., and Collinge, J. (1999) Science 283, 1935-1937). Using this β-form PrP and a human single chain Fv-displaying phage library, we have established a human IgG1 antibody specific to β-form but not α-form PrP, PRB7 IgG. When prion-infected ScN2a cells were cultured with PRB7 IgG, they generated and accumulated PRB7-binding granules in the cytoplasm with time, consequently becoming apoptotic cells bearing very large PRB7-bound aggregates. The SAF32 antibody recognizing the N-terminal octarepeat region of full-length PrP stained distinct granules in these cells as determined by confocal laser microscopy observation. When the accumulation of proteinase K-resistant PrP was examined in prion-infected ScN2a cells cultured in the presence of PRB7 IgG or SAF32, it was strongly inhibited by SAF32 but not at all by PRB7 IgG. Thus, we demonstrated direct evidence of the generation and accumulation of β-sheet-rich PrP in ScN2a cells de novo. These results suggest first that PRB7-bound PrP is not responsible for the accumulation of β-form PrP aggregates, which are rather an end product resulting in the triggering of apoptotic cell death, and second that SAF32-bound PrP lacking the PRB7-recognizing β-form may represent so-called PrP(Sc) with prion propagation activity. PRB7 is the first human antibody specific to β-form PrP and has become a powerful tool for the characterization of the biochemical nature of prion and its pathology.

Reaction of complement factors varies with prion strains in vitro and in vivo

Roles of complement factors in prion infection of the central nervous system remain unclear. In this study, we assessed the strain-dependent reactivity of complement factors in prion infections of Neuro2a (N2a) cells and mouse brains. N2a cells persistently infected with either Chandler or 22L scrapie strains were cultured in the presence of normal mouse serum (NMS), followed by staining with phosphatidylserine binding protein and early apoptosis marker Annexin V. The proportion of Annexin V positive cells was increased both in Chandler- and 22L-infected cells. Preincubation of NMS with anti-C1q, C3 and/or C9 antibodies reduced Annexin V positive cells in Chandler-infected cells, while only anti-C3 antibodies were effective on 22L-infected cells. The immunohistochemistry showed that deposition of C1q and C3 was different between Chandler- and 22L-infected mouse brains. These results indicate that the reactivity of complement factors differs between prion strains both in vitro and in vivo.

Effects of a brain-engraftable microglial cell line expressing anti-prion scFv antibodies on survival times of mice infected with scrapie prions

We first verified that a single chain Fv fragment against prion protein (anti-PrP scFv) was secreted by HEK293T cells and prevented prion replication in infected cells. We then stably expressed anti-PrP scFv in brain-engraftable murine microglial cells and intracerebrally injected these cells into mice before or after infection with prions. Interestingly, the injection before or at an early time point after infection attenuated the infection marginally but significantly prolonged survival times of the mice. These suggest that the ex vivo gene transfer of anti-PrP scFvs using brain-engraftable cells could be a possible immunotherapeutic approach against prion diseases.

Effect of hydrophobic mutations in the H2-H3 subdomain of prion protein on stability and conversion in vitro and in vivo

Prion diseases are fatal neurodegenerative diseases, which can be acquired, sporadic or genetic, the latter being linked to mutations in the gene encoding prion protein. We have recently described the importance of subdomain separation in the conversion of prion protein (PrP). The goal of the present study was to investigate the effect of increasing the hydrophobic interactions within the H2-H3 subdomain on PrP conversion. Three hydrophobic mutations were introduced into PrP. The mutation V209I associated with human prion disease did not alter protein stability or in vitro fibrillization propensity of PrP. The designed mutations V175I and T187I on the other hand increased protein thermal stability. V175I mutant fibrillized faster than wild-type PrP. Conversion delay of T187I was slightly longer, but fluorescence intensity of amyloid specific dye thioflavin T was significantly higher. Surprisingly, cells expressing V209I variant exhibited inefficient proteinase K resistant PrP formation upon infection with 22L strain, which is in contrast to cell lines expressing wild-type, V175I and T187I mPrPs. In agreement with increased ThT fluorescence at the plateau T187I expressing cell lines accumulated an increased amount of the proteinase K-resistant prion protein. We showed that T187I induces formation of thin fibrils, which are absent from other samples. We propose that larger solvent accessibility of I187 in comparison to wild-type and other mutants may interfere with lateral annealing of filaments and may be the underlying reason for increased conversion efficiency.

Effect of glycans and the glycophosphatidylinositol anchor on strain dependent conformations of scrapie prion protein: improved purifications and infrared spectra

Mammalian prion diseases involve conversion of normal prion protein, PrP(C), to a pathological aggregated state (PrP(res)). The three-dimensional structure of PrP(res) is not known, but infrared (IR) spectroscopy has indicated high, strain-dependent β-sheet content. PrP(res) molecules usually contain a glycophosphatidylinositol (GPI) anchor and large Asn-linked glycans, which can also vary with strain. Using IR spectroscopy, we tested the conformational effects of these post-translational modifications by comparing wild-type PrP(res) with GPI- and glycan-deficient PrP(res) produced in GPI-anchorless PrP transgenic mice. These analyses required the development of substantially improved purification protocols. Spectra of both types of PrP(res) revealed conformational differences between the 22L, ME7, and Chandler (RML) murine scrapie strains, most notably in bands attributed to β-sheets. These PrP(res) spectra were also distinct from those of the hamster 263K scrapie strain. Spectra of wild-type and anchorless 22L PrP(res) were nearly indistinguishable. With ME7 PrP(res), modest differences between the wild-type and anchorless spectra were detected, notably an ∼2 cm(-1) shift in an apparent β-sheet band. Collectively, the data provide evidence that the glycans and anchor do not grossly affect the strain-specific secondary structures of PrP(res), at least relative to the differences observed between strains, but can subtly affect turns and certain β-sheet components. Recently reported H-D exchange analyses of anchorless PrP(res) preparations strongly suggested the presence of strain-dependent, solvent-inaccessible β-core structures throughout most of the C-terminal half of PrP(res) molecules, with no remaining α-helix. Our IR data provide evidence that similar core structures also comprise wild-type PrP(res).

Dominant prion mutants induce curing through pathways that promote chaperone-mediated disaggregation

Protein misfolding underlies many neurodegenerative diseases, including the transmissible spongiform encephalopathies (prion diseases). Although cells typically recognize and process misfolded proteins, prion proteins evade protective measures by forming stable, self-replicating aggregates. However, coexpression of dominant-negative prion mutants can overcome aggregate accumulation and disease progression through currently unknown pathways. Here we determine the mechanisms by which two mutants of the Saccharomyces cerevisiae Sup35 protein cure the [PSI(+)] prion. We show that both mutants incorporate into wild-type aggregates and alter their physical properties in different ways, diminishing either their assembly rate or their thermodynamic stability. Whereas wild-type aggregates are recalcitrant to cellular intervention, mixed aggregates are disassembled by the molecular chaperone Hsp104. Thus, rather than simply blocking misfolding, dominant-negative prion mutants target multiple events in aggregate biogenesis to enhance their susceptibility to endogenous quality-control pathways.

Insights into prion biology: integrating a protein misfolding pathway with its cellular environment

Protein misfolding and assembly into ordered, self-templating aggregates (amyloid) has emerged as a novel mechanism for regulating protein function. For a subclass of amyloidogenic proteins known as prions, this process induces transmissible changes in normal cellular physiology, ranging from neurodegenerative disease in animals and humans to new traits in fungi. The severity and stability of these altered phenotypic states can be attenuated by the conformation or amino-acid sequence of the prion, but in most of these cases, the protein retains the ability to form amyloid in vitro. Thus, our ability to link amyloid formation in vitro with its biological consequences in vivo remains a challenge. In two recent studies, we have begun to address this disconnect by assessing the effects of the cellular environment on traits associated with the misfolding of the yeast prion Sup35. Remarkably, the effects of quality control pathways and of limitations on protein transfer in vivo amplify the effects of even slight differences in the efficiency of Sup35 misfolding, leading to dramatic changes in the associated phenotype. Together, our studies suggest that the interplay between protein misfolding pathways and their cellular context is a crucial contributor to prion biology.

Inducible expression of chimpanzee prion protein (PrP) in murine PrP knock-out cells

In transmissible spongiform encephalopathy (TSE) pathogenesis the cellular prion protein (PrP(C)) is converted into its pathogenic PrP(Sc) isoform. Prion protein gene (Prnp) deficient mice (PrP(0/0)) are resistant to PrP(Sc) infection, but following reconstitution of Prnp they regain their susceptibility to infection. Therefore, it is challenging to simulate this natural situation in a cell culture model. We have previously reported the inducible stable expression of a human PrP(C) in murine 3T3 cells. In this study, we used murine PrP(0/0) cells stably expressing exemplarily the chimpanzee Prnp under the control of inducible tetracycline (Tet) system. The Prnp was integrated using a lentiviral vector. Its expression in the engineered PrP(0/0)Chimp1/Tet-Off cell line was analyzed by Western blot (Wb) and fluorescence activated cell sorting (FACS) analyses. PrP(C) was partially purified by using immobilized metal affinity chromatography (IMAC). Compared to all the other cell systems which possess an endogenous PrP(C) expression, here described cell line contains only an overexpressing species specific PrP(C) expression which is tightly regulated and can be turned-off at any time without showing any endogenous host PrP(C) expression. Consequently, a contamination of the isolated PrP(C) is impossible. This cell line potentially offers a new tool for simulation of mice bioassays widely used in TSE infection studies.

Trans-dominant inhibition of prion propagation in vitro is not mediated by an accessory cofactor

Previous studies identified prion protein (PrP) mutants which act as dominant negative inhibitors of prion formation through a mechanism hypothesized to require an unidentified species-specific cofactor termed protein X. To study the mechanism of dominant negative inhibition in vitro, we used recombinant PrP^C molecules expressed in Chinese hamster ovary cells as substrates in serial protein misfolding cyclic amplification (sPMCA) reactions. Bioassays confirmed that the products of these reactions are infectious. Using this system, we find that: (1) trans-dominant inhibition can be dissociated from conversion activity, (2) dominant-negative inhibition of prion formation can be reconstituted in vitro using only purified substrates, even when wild type (WT) PrP^C is pre-incubated with poly(A) RNA and PrP^Sc template, and (3) Q172R is the only hamster PrP mutant tested that fails to convert into PrP^Sc and that can dominantly inhibit conversion of WT PrP at sub-stoichiometric levels. These results refute the hypothesis that protein X is required to mediate dominant inhibition of prion propagation, and suggest that PrP molecules compete for binding to a nascent seeding site on newly formed PrP^Sc molecules, most likely through an epitope containing residue 172.

Over the past two decades, various investigators have observed that heterozygous animals possessing two different forms of the gene encoding the prion protein (PrP) are more difficult to infect with some strains of infectious prions than homozygous animals possessing only the most commonly occurring form of the gene encoding PrP for that species. In 1995, it was hypothesized that the inhibition of prion infection in heterozygous animals might be caused by competition between the two different types of PrP molecules for binding to a common cofactor required for prion propagation, provisionally named “protein X,” through a specific portion of the PrP molecule. Here, we report that mixing different purified PrP molecules together in test tube reactions lacking accessory proteins can also interfere with prion propagation. We also found that some mutations of the putative protein X binding site do not inhibit the formation of hamster prions in chemical reactions. Our work suggests that different PrP molecules most likely compete for binding to newly formed prions rather than an accessory protein cofactor, and argues against the existence of protein X.

Heterozygous inhibition in prion infection: the stone fence model

The human PrP gene (PRNP) has two major polymorphic codons: 129 for methionine (M) or valine (V) and 219 for glutamate (E) or lysine (K). The PRNP heterozygotes appear to be protected from sporadic CJD compared to the PRNP homozygotes. The molecular mechanism responsible for these protective effects of PRNP heterozygosity has remained elusive. In this review, we describe the inhibition of PrP conversion observed in a series of transmission studies using PRNP heterozygous animal models. In vCJD infection, the conversion incompetent human PrP 129V molecules showed an inhibitory effect on the conversion of human PrP 129M molecules in the 129M/V heterozygous mice. Furthermore, though the human PrP 219E and PrP 219K were both conversion competent in vCJD infection, these conversion competent PrP molecules showed an inhibitory effect in the 219E/K heterozygous animals. To explain this heterozygous inhibition, we propose a possible mechanism designated as the stone fence model.

Cell models of prion infection

Due to recent renewal of interest and concerns in prion diseases, a number of cell systems permissive to prion multiplication have been generated in the last years. These include established cell lines, neuronal stem cells and primary neuronal cultures. While most of these models are permissive to experimental, mouse-adapted strains of prions, the propagation of natural field isolates from sheep scrapie and chronic wasting disease has been recently achieved. These models have improved our knowledge on the molecular and cellular events controlling the conversion of the PrP(C) protein into abnormal isoforms and on the cell-to-cell spreading of prions. Infected cultured cells will also facilitate investigations on the molecular basis of strain identity and on the mechanisms that lead to neurodegeneration. The ongoing development of new cell models with improved characteristics will certainly be useful for a number of unanswered critical issues in the prion field.

Scrapie infection of prion protein-deficient cell line upon ectopic expression of mutant prion proteins

Expression of the cellular prion protein (PrP(C)) is crucial for susceptibility to prions. In vivo, ectopic expression of PrP(C) restores susceptibility to prions and transgenic mice that express heterologous PrP on a PrP knock-out background have been used extensively to study the role of PrP alterations for prion transmission and species barriers. Here we report that prion protein knock-out cells can be rendered permissive to scrapie infection by the ectopic expression of PrP. The system was used to study the influence of sheep PrP-specific residues in mouse PrP on the infection process with mouse adapted scrapie. These studies reveal several critical residues previously not associated with species barriers and demonstrate that amino acid residue alterations at positions known to have an impact on the susceptibility of sheep to sheep scrapie also drastically influence PrP(Sc) formation by mouse-adapted scrapie strain 22L. Furthermore, our data suggest that amino acid polymorphisms located on the outer surfaces of helix 2 and 3 drastically impact conversion efficiency. In conclusion, this system allows for the fast generation of mutant PrP(Sc) that is entirely composed of transgenic PrP and is, thus, ideally suited for testing if artificial PrP molecules can affect prion replication. Transmission of infectivity generated in HpL3-4 cells expressing altered PrP molecules to mice could also help to unravel the potential influence of mutant PrP(Sc) on host cell tropism and strain characteristics in vivo.