Prion protein conversions: insight into mechanisms, TSE transmission barriers and strains

Prion protein complexed to a DNA aptamer induce behavioral and synapse dysfunction in mice

Conversion of the cellular prion protein (PrP^C) into the scrapie form (PrP^Sc) is the leading step to the development of transmissible spongiform encephalopathies (TSEs), still incurable neurodegenerative disorders. Interaction of PrP^C with cellular and synthetic ligands that induce formation of scrapie-like conformations has been deeply investigated in vitro. Different nucleic acid (NA) sequences bind PrP and convert it to β-sheet-rich or unfolded species; among such NAs, a 21-mer double-stranded DNA, D67, was shown to induce formation of PrP aggregates that were cytotoxic. However, in vivo effects of these PrP-DNA complexes were not explored. Herein, aggregates of recombinant full-length PrP (rPrP^23-231) induced by interaction with the D67 aptamer were inoculated into the lateral ventricle of Swiss mice and acute effects were investigated. The aggregates had no influence on emotional, locomotor and motor behavior of mice. In contrast, mice developed cognitive impairment and hippocampal synapse loss, which was accompanied by intense activation of glial cells in this brain region. Our results suggest that the i.c.v. injection of rPrP:D67 aggregates is an interesting model to study the neurotoxicity of aggregated PrP in vivo, and that glial cell activation may be an important step for behavioral and cognitive dysfunction in prion diseases.

Association of chronic wasting disease susceptibility with prion protein variation in white-tailed deer (Odocoileus virginianus)

Chronic wasting disease (CWD) is caused by prions, infectious proteinaceous particles, PrP^CWD. We sequenced the PRNP gene of 2,899 white-tailed deer (WTD) from Illinois and southern Wisconsin, finding 38 haplotypes. Haplotypes A, B, D, E, G and 10 others encoded Q₉₅G₉₆S₁₀₀N₁₀₃A₁₂₃Q₂₂₆, designated ‘PrP variant A.’ Haplotype C and five other haplotypes encoded PrP ‘variant C’ (Q₉₅S₉₆S₁₀₀N₁₀₃A₁₂₃Q₂₂₆). Haplotype F and three other haplotypes encoded PrP ‘variant F’ (H₉₅G₉₆S₁₀₀N₁₀₃A₁₂₃Q₂₂₆). The association of CWD with encoded PrP variants was examined in 2,537 tested WTD from counties with CWD. Relative to PrP variant A, CWD susceptibility was lower in deer with PrP variant C (OR = 0.26, p < 0.001), and even lower in deer with PrP variant F (OR = 0.10, p < 0.0001). Susceptibility to CWD was highest in deer with both chromosomes encoding PrP variant A, lower with one copy encoding PrP variant A (OR = 0.25, p < 0.0001) and lowest in deer without PrP variant A (OR = 0.07, p < 0.0001). There appeared to be incomplete dominance for haplotypes encoding PrP variant C in reducing CWD susceptibility. Deer with both chromosomes encoding PrP variant F (FF) or one encoding PrP variant C and the other F (CF) were all CWD negative. Our results suggest that an increased population frequency of PrP variants C or F and a reduced frequency of PrP variant A may reduce the risk of CWD infection. Understanding the population and geographic distribution of PRNP polymorphisms may be a useful tool in CWD management.

In Vitro Conversion Assays Diagnostic for Neurodegenerative Proteinopathies

BACKGROUND

In vitro conversion assays, including real-time quaking-induced conversion (RT-QuIC) and protein misfolding cyclic amplification (PMCA) techniques, were first developed to study the conversion process of the prion protein to its misfolded, disease-associated conformation. The intrinsic property of prion proteins to propagate their misfolded structure was later exploited to detect subfemtogram quantities of the misfolded protein present in tissues and fluids from humans and animals with transmissible spongiform encephalopathies. Currently, conversion assays are used clinically as sensitive and specific diagnostic tools for antemortem diagnosis of prion disease.

CONTENT

In vitro conversion assays are now being applied to the development of diagnostics for related neurodegenerative diseases, including detection of misfolded α-synuclein in Parkinson disease, misfolded amyloid-β in Alzheimer disease, and misfolded tau in Pick disease. Like the predicate prion protein in vitro conversion diagnostics, these assays exploit the ability of endogenously misfolded proteins to induce misfolding and aggregation of their natively folded counterpart in vitro. This property enables biomarker detection of the underlying protein pathology. Herein, we review RT-QuIC and PMCA for (a) prion-, (b) α-synuclein-, (c) amyloid-β-, and (d) tau-opathies.

SUMMARY

Although already in routine clinical use for the detection of transmissible spongiform encephalopathies, in vitro conversion assays for other neurodegenerative disorders require further development and evaluation of diagnostic performance before consideration for clinical implementation.

Oral administration of repurposed drug targeting Cyp46A1 increases survival times of prion infected mice

Prion diseases are fatal, infectious, and incurable neurodegenerative disorders caused by misfolding of the cellular prion protein (PrP^C) into the infectious isoform (PrP^Sc). In humans, there are sporadic, genetic and infectious etiologies, with sporadic Creutzfeldt-Jakob disease (sCJD) being the most common form. Currently, no treatment is available for prion diseases. Cellular cholesterol is known to impact prion conversion, which in turn results in an accumulation of cholesterol in prion-infected neurons. The major elimination of brain cholesterol is achieved by the brain specific enzyme, cholesterol 24-hydroxylase (CYP46A1). Cyp46A1 converts cholesterol into 24(S)-hydroxycholesterol, a membrane-permeable molecule that exits the brain. We have demonstrated for the first time that Cyp46A1 levels are reduced in the brains of prion-infected mice at advanced disease stage, in prion-infected neuronal cells and in post-mortem brains of sCJD patients. We have employed the Cyp46A1 activator efavirenz (EFV) for treatment of prion-infected neuronal cells and mice. EFV is an FDA approved anti-HIV medication effectively crossing the blood brain barrier and has been used for decades to chronically treat HIV patients. EFV significantly mitigated PrP^Sc propagation in prion-infected cells while preserving physiological PrP^C and lipid raft integrity. Notably, oral administration of EFV treatment chronically at very low dosage starting weeks to months after intracerebral prion inoculation of mice significantly prolonged the lifespan of animals. In summary, our results suggest that Cyp46A1 as a novel therapeutic target and that its activation through repurposing the anti-retroviral medication EFV might be valuable treatment approach for prion diseases.

Allelic Interference in Prion Replication Is Modulated by the Convertibility of the Interfering PrPC and Other Host-Specific Factors

Prion propagation can be interfered with by the expression of a second prion protein in the host. In the present study, we investigated prion propagation in a host expressing two different prion protein genes.

Early studies in transgenic mouse lines have shown that the coexpression of endogenous murine prion protein (PrP^C) and transgenic PrP^C from another species either inhibits or allows the propagation of prions, depending on the infecting prion strain and interacting protein species. The way whereby this phenomenon, so-called “interference,” is modulated remains to be determined. In this study, different transgenic mouse lines were crossbred to produce mice coexpressing bovine and porcine PrP^C, bovine and murine PrP^C, or murine and porcine PrP^C. These animals and their respective hemizygous controls were inoculated with several prion strains from different sources (cattle, mice, and pigs) to examine the effects of the simultaneous presence of PrP^C from two different species. Our results indicate interference with the infection process, manifested as extended survival times and reduced attack rates. The interference with the infectious process was reduced or absent when the potentiality interfering PrP^C species was efficiently converted by the inoculated agent. However, the propagation of the endogenous murine PrP^Sc was favored, allowing us to speculate that host-specific factors may disturb the interference caused by the coexpression of an exogenous second PrP^C.

Cervid Prion Protein Polymorphisms: Role in Chronic Wasting Disease Pathogenesis

Chronic wasting disease (CWD) is a prion disease found in both free-ranging and farmed cervids. Susceptibility of these animals to CWD is governed by various exogenous and endogenous factors. Past studies have demonstrated that polymorphisms within the prion protein (PrP) sequence itself affect an animal's susceptibility to CWD. PrP polymorphisms can modulate CWD pathogenesis in two ways: the ability of the endogenous prion protein (PrP^C) to convert into infectious prions (PrP^Sc) or it can give rise to novel prion strains. In vivo studies in susceptible cervids, complemented by studies in transgenic mice expressing the corresponding cervid PrP sequence, show that each polymorphism has distinct effects on both PrP^C and PrP^Sc. It is not entirely clear how these polymorphisms are responsible for these effects, but in vitro studies suggest they play a role in modifying PrP epitopes crucial for PrP^C to PrP^Sc conversion and determining PrP^C stability. PrP polymorphisms are unique to one or two cervid species and most confer a certain degree of reduced susceptibility to CWD. However, to date, there are no reports of polymorphic cervid PrP alleles providing absolute resistance to CWD. Studies on polymorphisms have focused on those found in CWD-endemic areas, with the hope that understanding the role of an animal's genetics in CWD can help to predict, contain, or prevent transmission of CWD.

Cellulose ether treatment in vivo generates chronic wasting disease prions with reduced protease resistance and delayed disease progression

Chronic wasting disease (CWD) is a prion disease of free-ranging and farmed cervids that is highly contagious because of extensive prion shedding and prion persistence in the environment. Previously, cellulose ether compounds (CEs) have been shown to significantly extend the survival of mice inoculated with mouse-adapted prion strains. In this study, we used CEs, TC-5RW, and 60SH-50, in vitro and in vivo to assess their efficacy to interfere with CWD prion propagation. In vitro, CEs inhibited CWD prion amplification in a dose-dependent manner. Transgenic mice over-expressing elk PrP^C (tgElk) were injected subcutaneously with a single dose of either of the CEs, followed by intracerebral inoculation with different CWD isolates from white tailed deer, mule deer, or elk. All treated groups showed a prolonged survival of up to more than 30 % when compared to the control group regardless of the CWD isolate used for infection. The extended survival in the treated groups correlated with reduced proteinase K resistance of prions. Remarkably, passage of brain homogenates from treated or untreated animals in tgElk mice resulted in a prolonged life span of mice inoculated with homogenates from CE-treated mice (of + 17%) even in the absence of further treatment. Besides the delayed disease onset upon passage in TgElk mice, the reduced proteinase K resistance was maintained but less pronounced. Therefore, these compounds can be very useful in limiting the spread of CWD in captive and wild-ranging cervids.

Implications of peptide assemblies in amyloid diseases

Neurodegenerative disorders and type 2 diabetes are global epidemics compromising the quality of life of millions worldwide, with profound social and economic implications. Despite the significant differences in pathology - much of which are poorly understood - these diseases are commonly characterized by the presence of cross-β amyloid fibrils as well as the loss of neuronal or pancreatic β-cells. In this review, we document research progress on the molecular and mesoscopic self-assembly of amyloid-beta, alpha synuclein, human islet amyloid polypeptide and prions, the peptides and proteins associated with Alzheimer's, Parkinson's, type 2 diabetes and prion diseases. In addition, we discuss the toxicities of these amyloid proteins based on their self-assembly as well as their interactions with membranes, metal ions, small molecules and engineered nanoparticles. Through this presentation we show the remarkable similarities and differences in the structural transitions of the amyloid proteins through primary and secondary nucleation, the common evolution from disordered monomers to alpha-helices and then to β-sheets when the proteins encounter the cell membrane, and, the consensus (with a few exceptions) that off-pathway oligomers, rather than amyloid fibrils, are the toxic species regardless of the pathogenic protein sequence or physicochemical properties. In addition, we highlight the crucial role of molecular self-assembly in eliciting the biological and pathological consequences of the amyloid proteins within the context of their cellular environments and their spreading between cells and organs. Exploiting such structure-function-toxicity relationship may prove pivotal for the detection and mitigation of amyloid diseases.

Real-time Quaking-induced Conversion Assay for Detection of CWD Prions in Fecal Material

The RT-QuIC technique is a sensitive in vitro cell-free prion amplification assay based mainly on the seeded misfolding and aggregation of recombinant prion protein (PrP) substrate using prion seeds as a template for the conversion. RT-QuIC is a novel high-throughput technique which is analogous to real-time polymerase chain reaction (PCR). Detection of amyloid fibril growth is based on the dye Thioflavin T, which fluoresces upon specific interaction with ᵦ-sheet rich proteins. Thus, amyloid formation can be detected in real time. We attempted to develop a reliable non-invasive screening test to detect chronic wasting disease (CWD) prions in fecal extract. Here, we have specifically adapted the RT-QuIC technique to reveal PrP^Sc seeding activity in feces of CWD infected cervids. Initially, the seeding activity of the fecal extracts we prepared was relatively low in RT-QuIC, possibly due to potential assay inhibitors in the fecal material. To improve seeding activity of feces extracts and remove potential assay inhibitors, we homogenized the fecal samples in a buffer containing detergents and protease inhibitors. We also submitted the samples to different methodologies to concentrate PrP^Sc on the basis of protein precipitation using sodium phosphotungstic acid, and centrifugal force. Finally, the feces extracts were tested by optimized RT-QuIC which included substrate replacement in the protocol to improve the sensitivity of detection. Thus, we established a protocol for sensitive detection of CWD prion seeding activity in feces of pre-clinical and clinical cervids by RT-QuIC, which can be a practical tool for non-invasive CWD diagnosis.

Destabilizing polymorphism in cervid prion protein hydrophobic core determines prion conformation and conversion efficiency

Prion diseases are infectious neurodegenerative disorders of humans and animals caused by misfolded forms of the cellular prion protein PrP^C. Prions cause disease by converting PrP^C into aggregation-prone PrP^Sc. Chronic wasting disease (CWD) is the most contagious prion disease with substantial lateral transmission, affecting free-ranging and farmed cervids. Although the PrP primary structure is highly conserved among cervids, the disease phenotype can be modulated by species-specific polymorphisms in the prion protein gene. How the resulting amino-acid substitutions impact PrP^C and PrP^Sc structure and propagation is poorly understood. We investigated the effects of the cervid 116A>G substitution, located in the most conserved PrP domain, on PrP^C structure and conversion and on 116AG-prion conformation and infectivity. Molecular dynamics simulations revealed structural de-stabilization of 116G-PrP, which enhanced its in vitro conversion efficiency when used as recombinant PrP substrate in real-time quaking-induced conversion (RT-QuIC). We demonstrate that 116AG-prions are conformationally less stable, show lower activity as a seed in RT-QuIC and exhibit reduced infectivity in vitro and in vivo. Infectivity of 116AG-prions was significantly enhanced upon secondary passage in mice, yet conformational features were retained. These findings indicate that structurally de-stabilized PrP^C is readily convertible by cervid prions of different genetic background and results in a prion conformation adaptable to cervid wild-type PrP. Conformation is an important criterion when assessing transmission barrier, and conformational variants can target a different host range. Therefore, a thorough analysis of CWD isolates and re-assessment of species-barriers is important in order to fully exclude a zoonotic potential of CWD.

Chronic wasting disease (CWD) is a prion disease which affects wild and captive cervids. Prion diseases are infectious neurodegenerative disorders, and the causative agent consists of abnormally folded prion protein termed PrP^Sc. Prions replicate without genetic information, and their three-dimensional structure is thought to encode heritable information necessary to propagate using the cellular prion protein PrP^C as a substrate for conversion. In this study, we use in vitro and in vivo techniques to analyze the effect of a polymorphism at codon 116 (A>G) of the white-tailed deer prion protein on CWD prion conformation, propagation and pathogenesis. We observed differences in conformation, infectivity and seeding activity in vitro between CWD prions isolated from white-tailed deer encoding wild-type (116AA) PrP^C or 116AG-PrP^C. In mouse bioassays conformational differences are retained, however, 116AG CWD prions resulted in significantly shortened incubation times upon passages. Molecular dynamics simulations suggest that the structure of 116G-PrP^C is more flexible, which is supported by an improved convertibility in an in vitro conversion assay. Altogether these data indicate the importance of a variation in the most conserved PrP domain, and highlight the relationship between PrP^C structural flexibility, prion conformation and conversion, and pathogenesis of prion disease in vivo.

Different Molecular Mechanisms Mediate Direct or Glia-Dependent Prion Protein Fragment 90-231 Neurotoxic Effects in Cerebellar Granule Neurons

Glia over-stimulation associates with amyloid deposition contributing to the progression of central nervous system neurodegenerative disorders. Here we analyze the molecular mechanisms mediating microglia-dependent neurotoxicity induced by prion protein (PrP)90-231, an amyloidogenic polypeptide corresponding to the protease-resistant portion of the pathological prion protein scrapie (PrP^Sc). PrP90-231 neurotoxicity is enhanced by the presence of microglia within neuronal culture, and associated to a rapid neuronal [Ca⁺⁺] _i increase. Indeed, while in "pure" cerebellar granule neuron cultures, PrP90-231 causes a delayed intracellular Ca⁺⁺ entry mediated by the activation of NMDA receptors; when neuron and glia are co-cultured, a transient increase of [Ca⁺⁺] _i occurs within seconds after treatment in both granule neurons and glial cells, then followed by a delayed and sustained [Ca⁺⁺] _i raise, associated with the induction of the expression of inducible nitric oxide synthase and phagocytic NADPH oxidase. [Ca⁺⁺] _i fast increase in neurons is dependent on the activation of multiple pathways since it is not only inhibited by the blockade of voltage-gated channel activity and NMDA receptors but also prevented by the inhibition of nitric oxide and PGE₂ release from glial cells. Thus, Ca⁺⁺ homeostasis alteration, directly induced by PrP90-231 in cerebellar granule cells, requires the activation of NMDA receptors, but is greatly enhanced by soluble molecules released by activated glia. In glia-enriched cerebellar granule cultures, the activation of inducible nitric oxide (iNOS) and NADPH oxidase represents the main mechanism of toxicity since their pharmacological inhibition prevented PrP90-231 neurotoxicity, whereas NMDA blockade by D(-)-2-amino-5-phosphonopentanoic acid is ineffective; conversely, in pure cerebellar granule cultures, NMDA blockade but not iNOS inhibition strongly reduced PrP90-231 neurotoxicity. These data indicate that amyloidogenic peptides induce neurotoxic signals via both direct neuron interaction and glia activation through different mechanisms responsible of calcium homeostasis disruption in neurons and potentiating each other: the activation of excitotoxic pathways via NMDA receptors and the release of radical species that establish an oxidative milieu.

Treatment of Prion Disease with Heterologous Prion Proteins

Prion diseases such as Creutzfeldt-Jakob disease in humans, bovine spongiform encephalopathy in cattle, and scrapie in sheep are fatal neurodegenerative diseases for which there is no effective treatment. The pathology of these diseases involves the conversion of a protease sensitive form of the cellular prion protein (PrP^C) into a protease resistant infectious form (PrP^sc or PrP^res). Both in vitro (cell culture and cell free conversion assays) and in vivo (animal) studies have demonstrated the strong dependence of this conversion process on protein sequence homology between the initial prion inoculum and the host’s own cellular prion protein. The presence of non-homologous (heterologous) proteins is often inhibitory to this conversion process. We hypothesize that the presence of heterologous prion proteins from one species might therefore constitute an effective treatment for prion disease in another species. To test this hypothesis, we infected mice intracerebrally with murine adapted RML-Chandler scrapie and treated them with heterologous prion protein (purified bacterially expressed recombinant hamster prion protein) or vehicle alone. Treated animals demonstrated reduced disease associated pathology, decreased accumulation of protease-resistant disease-associated prion protein, with delayed onset of clinical symptoms and motor deficits. This was concomitant with significantly increased survival times relative to mock-treated animals. These results provide proof of principle that recombinant hamster prion proteins can effectively and safely inhibit prion disease in mice, and suggest that hamster or other non-human prion proteins may be a viable treatment for prion diseases in humans.

Glycosylphosphatidylinositol anchoring directs the assembly of Sup35NM protein into non-fibrillar, membrane-bound aggregates

In prion-infected hosts, PrPSc usually accumulates as non-fibrillar, membrane-bound aggregates. Glycosylphosphatidylinositol (GPI) anchor-directed membrane association appears to be an important factor controlling the biophysical properties of PrPSc aggregates. To determine whether GPI anchoring can similarly modulate the assembly of other amyloid-forming proteins, neuronal cell lines were generated that expressed a GPI-anchored form of a model amyloidogenic protein, the NM domain of the yeast prion protein Sup35 (Sup35(GPI)). We recently reported that GPI anchoring facilitated the induction of Sup35(GPI) prions in this system. Here, we report the ultrastructural characterization of self-propagating Sup35(GPI) aggregates of either spontaneous or induced origin. Like membrane-bound PrPSc, Sup35(GPI) aggregates resisted release from cells treated with phosphatidylinositol-specific phospholipase C. Sup35(GPI) aggregates of spontaneous origin were detergent-insoluble, protease-resistant, and self-propagating, in a manner similar to that reported for recombinant Sup35NM amyloid fibrils and induced Sup35(GPI) aggregates. However, GPI-anchored Sup35 aggregates were not stained with amyloid-binding dyes, such as Thioflavin T. This was consistent with ultrastructural analyses, which showed that the aggregates corresponded to dense cell surface accumulations of membrane vesicle-like structures and were not fibrillar. Together, these results showed that GPI anchoring directs the assembly of Sup35NM into non-fibrillar, membrane-bound aggregates that resemble PrPSc, raising the possibility that GPI anchor-dependent modulation of protein aggregation might occur with other amyloidogenic proteins. This may contribute to differences in pathogenesis and pathology between prion diseases, which uniquely involve aggregation of a GPI-anchored protein, versus other protein misfolding diseases.

Emerging prion disease drives host selection in a wildlife population

Infectious diseases are increasingly recognized as an important force driving population dynamics, conservation biology, and natural selection in wildlife populations. Infectious agents have been implicated in the decline of small or endangered populations and may act to constrain population size, distribution, growth rates, or migration patterns. Further, diseases may provide selective pressures that shape the genetic diversity of populations or species. Thus, understanding disease dynamics and selective pressures from pathogens is crucial to understanding population processes, managing wildlife diseases, and conserving biological diversity. There is ample evidence that variation in the prion protein gene (PRNP) impacts host susceptibility to prion diseases. Still, little is known about how genetic differences might influence natural selection within wildlife populations. Here we link genetic variation with differential susceptibility of white-tailed deer to chronic wasting disease (CWD), with implications for fitness and disease-driven genetic selection. We developed a single nucleotide polymorphism (SNP) assay to efficiently genotype deer at the locus of interest (in the 96th codon of the PRNP gene). Then, using a Bayesian modeling approach, we found that the more susceptible genotype had over four times greater risk of CWD infection; and, once infected, deer with the resistant genotype survived 49% longer (8.25 more months). We used these epidemiological parameters in a multi-stage population matrix model to evaluate relative fitness based on genotype-specific population growth rates. The differences in disease infection and mortality rates allowed genetically resistant deer to achieve higher population growth and obtain a long-term fitness advantage, which translated into a selection coefficient of over 1% favoring the CWD-resistant genotype. This selective pressure suggests that the resistant allele could become dominant in the population within an evolutionarily short time frame. Our work provides a rare example of a quantifiable disease-driven selection process in a wildlife population, demonstrating the potential for infectious diseases to alter host populations. This will have direct bearing on the epidemiology, dynamics, and future trends in CWD transmission and spread. Understanding genotype-specific epidemiology will improve predictive models and inform management strategies for CWD-affected cervid populations.

The role of genetics in chronic wasting disease of North American cervids

Chronic wasting disease (CWD) is a major concern for the management of North American cervid populations. This fatal prion disease has led to declines in populations which have high CWD prevalence and areas with both high and low infection rates have experienced economic losses in wildlife recreation and fears of potential spill-over into livestock or humans. Research from human and veterinary medicine has established that the prion protein gene (Prnp) encodes the protein responsible for transmissible spongiform encephalopathies (TSEs). Polymorphisms in the Prnp gene can lead to different prion forms that moderate individual susceptibility to and progression of TSE infection. Prnp genes have been sequenced in a number of cervid species including those currently infected by CWD (elk, mule deer, white-tailed deer, moose) and those for which susceptibility is not yet determined (caribou, fallow deer, sika deer). Over thousands of sequences examined, the Prnp gene is remarkably conserved within the family Cervidae; only 16 amino acid polymorphisms have been reported within the 256 amino acid open reading frame in the third exon of the Prnp gene. Some of these polymorphisms have been associated with lower rates of CWD infection and slower progression of clinical CWD. Here we review the body of research on Prnp genetics of North American cervids. Specifically, we focus on known polymorphisms in the Prnp gene, observed genotypic differences in CWD infection rates and clinical progression, mechanisms for genetic TSE resistance related to both the cervid host and the prion agent and potential for natural selection for CWD-resistance. We also identify gaps in our knowledge that require future research.

Prion seeded conversion and amplification assays

The conversion of the normal prion protein (PrP(C)) into its misfolded, aggregation-prone and infectious (prion) isoform is central to the progression of transmissible spongiform encephalopathies (TSEs) or prion diseases. Since the initial development of a cell free PrP conversion reaction, striking progress has been made in the development of much more continuous prion-induced conversion and amplification reactions. These studies have provided major insights into the molecular underpinnings of prion propagation and enabled the development of ultra-sensitive tests for prions and prion disease diagnosis. This chapter will provide an overview of such reactions and the practical and fundamental consequences of their development.

Prion hypothesis: the end of the controversy?

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

Building Cell Selectivity into CPP-Mediated Strategies

There is a pressing need for more effective and selective therapies for cancer and other diseases. Consequently, much effort is being devoted to the development of alternative experimental approaches based on selective systems, which are designed to be specifically directed against target cells. In addition, a large number of highly potent therapeutic molecules are being discovered. However, they do not reach clinical trials because of their low delivery, poor specificity or their incapacity to bypass the plasma membrane. Cell-penetrating peptides (CPPs) are an open door for cell-impermeable compounds to reach intracellular targets. Putting all these together, research is sailing in the direction of the design of systems with the capacity to transport new drugs into a target cell. Some CPPs show cell type specificity while others require modifications or form part of more sophisticated drug delivery systems. In this review article we summarize several strategies for directed drug delivery involving CPPs that have been reported in the literature.

Sequestration of essential proteins causes prion associated toxicity in yeast

Prions are infectious, aggregated proteins that cause diseases in mammals but are not normally toxic in fungi. Excess Sup35p, an essential yeast protein that can exist as the [PSI(+)] prion, inhibits growth of [PSI(+)] but not [psi(-)] cells. This toxicity is rescued by expressing the Sup35Cp domain of Sup35p, which is sufficient for cell viability but not prion propagation. We now show that rescue requires Sup35Cp levels to be proportional to Sup35p overexpression. Overexpression of Sup35p appeared to cause pre-existing [PSI(+)] aggregates to coalesce into larger aggregates, but these were not toxic per se because they formed even when Sup35Cp rescued growth. Overexpression of Sup45p, but not other tested essential Sup35p binding partners, caused rescue. Sup45-GFPp formed puncta that colocalized with large [PSI(+)] Sup35-RFPp aggregates in cells overexpressing Sup35p, and the frequency of the Sup45-GFPp puncta was reduced by rescuing levels of Sup35Cp. In contrast, [PSI(+)] toxicity caused by a high excess of the Sup35p prion domain (Sup35NMp) was rescued by a single copy of Sup35Cp, was not rescued by Sup45p overexpression and was not associated with the appearance of Sup45-GFPp puncta. This suggests [PSI(+)] toxicity caused by excess Sup35p verses Sup35NMp is, respectively, through sequestration/inactivation of Sup45p verses Sup35p.

Protective effect of the AT137RQ and ARQK176 PrP allele against classical scrapie in Sarda breed sheep

The susceptibility of sheep to scrapie is under the control of the host’s prion protein (PrP) gene and is also influenced by the strain of the agent. PrP polymorphisms at codons 136 (A/V), 154 (R/H) and 171 (Q/R/H) are the main determinants of susceptibility/resistance of sheep to classical scrapie. They are combined in four main variants of the wild-type ARQ allele: VRQ, AHQ, ARH and ARR. Breeding programmes have been undertaken on this basis in the European Union and the USA to increase the frequency of the resistant ARR allele in sheep populations. Herein, we report the results of a multi-flock study showing the protective effect of polymorphisms other than those at codons 136, 154 and 171 in Sarda breed sheep. All ARQ/ARQ affected sheep (n = 154) and 378 negative ARQ/ARQ controls from four scrapie outbreaks were submitted to sequencing of the PrP gene. The distribution of variations other than those at the standard three codons, between scrapie cases and negative controls, was statistically different in all flocks. In particular, the AT₁₃₇RQ and ARQK₁₇₆ alleles showed a clear protective effect. This is the first study demonstrating a protective influence of alleles other than ARR under field conditions. If further investigations in other sheep breeds and with other scrapie sources confirm these findings, the availability of various protective alleles in breeding programmes of sheep for scrapie resistance could be useful in breeds with a low frequency of the ARR allele and would allow maintaining a wider variability of the PrP gene.

Can copper binding to the prion protein generate a misfolded form of the protein?

The native prion protein (PrP) has a two domain structure, with a globular folded alpha-helical C-terminal domain and a flexible extended N-terminal region. The latter can selectively bind Cu(2+) via four His residues in the octarepeat (OR) region, as well as two sites (His96 and His111) outside this region. In the disease state, the folded C-terminal domain of PrP undergoes a conformational change, forming amorphous aggregates high in beta-sheet content. Cu(2+) bound to the ORs can be redox active and has been shown to induce cleavage within the OR region, a process requiring conserved Trp residues. Using computational modeling, we have observed that electron transfer from Trp residues to copper can be favorable. These models also reveal that an indole-based radical cation or Cu(+) can initiate reactions leading to protein backbone cleavage. We have also demonstrated, by molecular dynamics simulations, that Cu(2+) binding to the His96 and His111 residues in the remaining PrP N-terminal fragment can induce localized beta-sheet structure, allowing us to suggest a potential mechanism for the initiation of beta-sheet misfolding in the C-terminal domain by Cu(2+).

A potential mechanism for Cu2+ reduction, beta-cleavage, and beta-sheet initiation within the N-terminal domain of the prion protein: insights from density functional theory and molecular dynamics calculations

The N-terminal region of the native human prion protein encompasses four highly conserved octarepeats that each contain a single His, Pro, Gln, and Trp residue as well as several Gly residues. At neutral pH these repeats are capable of individually binding copper (Cu(2+)) ions, involving the His side chain and the backbone amide of the Gly residues. In addition, the two His residues at positions 96 and 111 are also capable of binding Cu(2+). At low concentrations of the metal ion or at low pH, one Cu(2+) may be bound by multiple His residues of the four octarepeats. This complex is known to be redox active, while none of the other Cu(2+)-bound complexes are. Using density functional theory and molecular dynamics calculations data demonstrated how this form of the protein could reduce Cu(2+), through a process involving electron transfer from the Trp side chain. The reduced Cu gives rise to reactive oxygen species (ROS), which can lead to beta-cleavage of the prion protein chain at any of the Gly residues around position 90. Protein fragments of lengths similar to those arising from beta-cleavage are predominantly found in both healthy and Creutzfeldt-Jakob disease (CJD)-affected brains. Models of Cu binding to the His96 and His111 residues also indicate that different modes of Cu(2+) binding result in formation of stable beta-hairpin structures in this region of the protein. It is postulated that through interactions with the C-terminal part of the protein these hairpins may initiate misfolding and yield more stable beta-sheet structures that might associate in the same fashion with additional prion proteins.

Ligand binding promotes prion protein aggregation--role of the octapeptide repeats

Aggregation of the normal cellular prion protein, PrP, is important in the pathogenesis of prion disease. PrP binds glycosaminoglycan (GAG) and divalent cations, such as Cu(2+) and Zn(2+). Here, we report our findings that GAG and Cu(2+) promote the aggregation of recombinant human PrP (rPrP). The normal cellular prion protein has five octapeptide repeats. In the presence of either GAG or Cu(2+), mutant rPrPs with eight or ten octapeptide repeats are more aggregation prone, exhibit faster kinetics and form larger aggregates than wild-type PrP. When the GAG-binding motif, KKRPK, is deleted the effect of GAG but not that of Cu(2+) is abolished. By contrast, when the Cu(2+)-binding motif, the octapeptide-repeat region, is deleted, neither GAG nor Cu(2+) is able to promote aggregation. Therefore, the octapeptide-repeat region is critical in the aggregation of rPrP, irrespective of the promoting ligand. Furthermore, aggregation of rPrP in the presence of GAG is blocked with anti-PrP mAbs, whereas none of the tested anti-PrP mAbs block Cu(2+)-promoted aggregation. However, a mAb that is specific for an epitope at the N-terminus enhances aggregation in the presence of either GAG or Cu(2+). Therefore, although binding of either GAG or Cu(2+) promotes the aggregation of rPrP, their aggregation processes are different, suggesting multiple pathways of rPrP aggregation.

Targeting prion proteins in neurodegenerative disease

BACKGROUND

Spongiform neurodegeneration is the pathological hallmark of individuals suffering from prion disease. These disorders, whose manifestation is sporadic, familial or acquired by infection, are caused by accumulation of the aberrantly folded isoform of the cellular prion protein (PrP(c)), termed PrP(Sc). Although usually rare, prion disorders are inevitably fatal and transferrable by infection.

OBJECTIVE

Pathology is restricted to the central nervous system and premortem diagnosis is usually not possible. Yet, promising approaches towards developing therapeutic regimens have been made recently.

METHODS

The biology of prion proteins and current models of neurotoxicity are discussed and prophylactic and therapeutic concepts are introduced.

RESULTS/CONCLUSIONS

Although various promising drug candidates with antiprion activity have been identified, this proof-of-concept cannot be transferred into translational medicine yet.

Prion protein amino acid determinants of differential susceptibility and molecular feature of prion strains in mice and voles

The bank vole is a rodent susceptible to different prion strains from humans and various animal species. We analyzed the transmission features of different prions in a panel of seven rodent species which showed various degrees of phylogenetic affinity and specific prion protein (PrP) sequence divergences in order to investigate the basis of vole susceptibility in comparison to other rodent models. At first, we found a differential susceptibility of bank and field voles compared to C57Bl/6 and wood mice. Voles showed high susceptibility to sheep scrapie but were resistant to bovine spongiform encephalopathy, whereas C57Bl/6 and wood mice displayed opposite features. Infection with mouse-adapted scrapie 139A was faster in voles than in C57Bl/6 and wood mice. Moreover, a glycoprofile change was observed in voles, which was reverted upon back passage to mice. All strains replicated much faster in voles than in mice after adapting to the new species. PrP sequence comparison indicated a correlation between the transmission patterns and amino acids at positions 154 and 169 (Y and S in mice, N and N in voles). This correlation was confirmed when inoculating three additional rodent species: gerbils, spiny mice and oldfield mice with sheep scrapie and 139A. These rodents were chosen because oldfield mice do have the 154N and 169N substitutions, whereas gerbil and spiny mice do not have them. Our results suggest that PrP residues 154 and 169 drive the susceptibility, molecular phenotype and replication rate of prion strains in rodents. This might have implications for the assessment of host range and molecular traceability of prion strains, as well as for the development of improved animal models for prion diseases.

Prions are unconventional infectious agents that cause fatal neurodegenerative diseases in animals and humans. A pathological form of the cellular prion protein (PrP^C), named PrP^Sc, appears to be the major or the sole component of prions. These agents are transmitted by inducing the conversion of host PrP^C into PrP^Sc that accumulates in the brain of affected individuals. Different factors are believed to modulate such events, which explains the variable transmission efficiency observed under inter-species experimental inoculation. These factors are still fairly unknown, although evidence exists that some kind of structural compatibility between PrP^Sc of the infectious inoculum and PrP^C of the host has a role in making transmission more or less efficient. We investigated the transmission of prions to different rodents and showed that specific amino acid substitutions (Y154N and S169N) in the prion protein are major determinants of susceptibility to prions. In particular, we showed that these specific variations i) direct the transmission rate of prions between different species in a way that is dependent on the prion strain, ii) affect the molecular characteristics of prions, and iii) influence their replication efficiency.

Glycosylation of prion strains in transmissible spongiform encephalopathies

This is a review of prion replication in the context of the cell biology of membrane proteins especially folding quality control in the endoplasmic reticulum (ER). Transmissible spongiform encephalopathies, such as scrapie and BSE, are infectious lethal diseases of mammalian neurons characterised by conversion of the normal membrane protein PrPC to the disease-associated conformational isomer called PrPSc. PrPSc, apparently responsible for infectivity, forms a number of different conformations and specific N-glycosylation site occupancies that correlate with TSE strain differences. Dimerisation and specific binding of PrPc and PrPSc seems critical in PrPSc biosynthesis and is influenced by N-glycosylation and disulfide bond formation. PrPsc can be amplified in vitro but new glycosylation cannot occur in cell free environments without the special conditions of microsome mediated in vitro translation, thus strain specific glycosylation of PrPSc formed in vitro in the absence of these conditions must take place by imprintation of PrPc from existing glycosylation site-occupancies. PrPSc formed in cell free homogenates is not infectious pointing to events necessary for infectivity that only occur in intact cells. Such events may include glycosylation site occupancy and ER folding chaperone activity. In the biosynthetic pathway of PrPSc, early acquisition of sensitivity of the GPI anchor to phospholipase C can be distinguished from the later acquisition of protease resistance and detergent insolubility. By analogy to the co-translational formation of the MHC I loading complex, it is postulated that PrPSc or its specific peptides could imprint nascent PrPc chains thereby ensuring its own folds and the observed glycosylation site occupancy ratios of strains.

Globular and pre-fibrillar prion aggregates are toxic to neuronal cells and perturb their electrophysiology

Prion diseases are characterised at autopsy by neuronal loss and accumulation of amorphous protein aggregates and/or amyloid fibrils in the brains of humans and animals. These protein deposits result from the conversion of the cellular, mainly alpha-helical prion protein (PrP(C)) to the beta-sheet-rich isoform (PrP(Sc)). Although the pathogenic mechanism of prion diseases is not fully understood, it appears that protein aggregation is itself neurotoxic and not the product of cell death. The precise nature of the neurotoxic species and mechanism of cell death are yet to be determined, although recent studies with other amyloidogenic proteins suggest that ordered pre-fibrillar or oligomeric forms may be responsible for cellular dysfunction. In this study we have refolded recombinant prion protein (rPrP) to two distinct forms rich in beta-sheet structure with an intact disulphide bond. Here we report on the structural properties of globular aggregates and pre-fibrils of rPrP and show that both states are toxic to neuronal cells in culture. We show that exogenous rPrP aggregates are internalised by neuronal cells and found in the cytoplasm. We also measured the changes in electrophysiological properties of cultured neuronal cells on exposure to exogenous prion aggregates and discuss the implications of these findings.

Sodium valproate does not augment Prpsc in murine neuroblastoma cells

Sodium valproate (VPA) has been reported to increase the accumulation of the pathologic isoform of prion protein (PrPsc) in scrapie-infected murine neuroblastoma cells. In this study, the effect of VPA on PrPsc accumulation was investigated in murine N2a neuroblastoma cells chronically infected with scrapie strain 22L (N2a-22L). No accumulation of PrPsc was detected after short-term (3 days) or long-term (21 days) treatment of N2a-22L cells with 4.8, 12, 18 or 24 microM VPA. Higher VPA concentrations (240 and 600 microM) also failed to augment PrPsc expression. In conclusion, in our experimental conditions, no deleterious effect was induced by VPA on prions replication.

Experimental treatments for human transmissible spongiform encephalopathies: is there a role for pentosan polysulfate?

Human transmissible spongiform encephalopathies (TSEs), also known as prion diseases, are caused by the accumulation of an abnormal isoform of the prion protein in the CNS. Creutzfeldt-Jakob disease in its sporadic form is the most frequent type of human TSE. At present, there is no proven specific or effective treatment available for any form of TSE. Pentosan polysulfate (PPS) has been shown to prolong the incubation period when administered to the cerebral ventricles in a rodent TSE model. Cerebroventricular administration of PPS has been carried out in 26 patients with TSEs and has been shown to be well tolerated in doses < or = 220 microg/kg/day. Proof of efficacy has been difficult because the specific and objective criteria for measurement of response have not been established yet. Preliminary clinical experience confirms extended survival in patients with variant Creutzfeldt-Jakob disease receiving intraventricular PPS; however, it is still not clear if this is due to PPS itself. Further prospective investigations of long-term intraventricular PPS administration are essential for the assessment of its effects.

Human prion proteins with pathogenic mutations share common conformational changes resulting in enhanced binding to glycosaminoglycans

Mutation in the prion gene PRNP accounts for 10-15% of human prion diseases. However, little is known about the mechanisms by which mutant prion proteins (PrPs) cause disease. Here we investigated the effects of 10 different pathogenic mutations on the conformation and ligand-binding activity of recombinant human PrP (rPrP). We found that mutant rPrPs react more strongly with N terminus-specific antibodies, indicative of a more exposed N terminus. The N terminus of PrP contains a glycosaminoglycan (GAG)-binding motif. Binding of GAG is important in prion disease. Accordingly, all mutant rPrPs bind more GAG, and GAG promotes the aggregation of mutant rPrPs more efficiently than wild-type recombinant normal cellular PrP (rPrP(C)). Furthermore, point mutations in PRNP also cause conformational changes in the region between residues 109 and 136, resulting in the exposure of a second, normally buried, GAG-binding motif. Importantly, brain-derived PrP from transgenic mice, which express a pathogenic mutant with nine extra octapeptide repeats, also binds more strongly to GAG than wild-type PrP(C). Thus, several rPrPs with distinct pathogenic mutations have common conformational changes, which enhance binding to GAG. These changes may contribute to the pathogenesis of inherited prion diseases.

A 3-disulfide mutant of mouse prion protein expression, oxidative folding, reductive unfolding, conformational stability, aggregation and isomerization

The structure of wild-type mouse prion protein mPrP(23-231) consists of two distinctive segments with approximately equal size, a disordered and flexible N-terminal domain encompassing residues 23-124 and a largely structured C-terminal domain containing about 40% of helical structure and stabilized by one disulfide bond (Cys(178)-Cys(213)). We have expressed a mPrP mutant with 4 Ala/Ser-->Cys replacements, two each at the N-(Cys(36), Cys(112)) and C-(Cys(134), Cys(169)) domains. Our specific aims are to study the interaction between N- and C-domains of mPrP during the oxidative folding and to produce stabilized isomers of mPrP for further analysis. Oxidative folding of fully reduced mutant, mPrP(6C), generates one predominant 3-disulfide isomer, designated as N-mPrP(3SS), which comprises the native disulfide (Cys(178)-Cys(213)) and two non-native disulfide bonds (Cys(36)-Cys(134) and Cys(112)-Cys(169)) that covalently connect the N- and C-domains. In comparison to wild-type mPrP(23-231), N-mPrP(3SS) exhibits an indistinguishable CD spectra, a similar conformational stability in the absence of thiol and a reduced ability to aggregate. In the presence of thiol catalyst and denaturant, N-mPrP(3SS) unfolds and generates diverse isomers that are amenable to further isolation, structural and functional analysis.

Strain fidelity of chronic wasting disease upon murine adaptation

Chronic wasting disease (CWD), a prion disease of deer and elk, is highly prevalent in some regions of North America. The establishment of mouse-adapted CWD prions has proven difficult due to the strong species barrier between mice and deer. Here we report the efficient transmission of CWD to transgenic mice overexpressing murine PrP. All mice developed disease 500 +/- 62 days after intracerebral CWD challenge. The incubation period decreased to 228 +/- 103 days on secondary passage and to 162 +/- 6 days on tertiary passage. Mice developed very large, radially structured cerebral amyloid plaques similar to those of CWD-infected deer and elk. PrP(Sc) was detected in spleen, indicating that murine CWD was lymphotropic. PrP(Sc) glycoform profiles maintained a predominantly diglycosylated PrP pattern, as seen with CWD in deer and elk, across all passages. Therefore, all pathological, biochemical, and histological strain characteristics of CWD appear to persist upon repetitive serial passage through mice. These findings indicate that the salient strain-specific properties of CWD are encoded by agent-intrinsic components rather than by host factors.

CJD PrPsc removal by nanofiltration process: Application to a therapeutic immunoglobulin solution (Lymphoglobuline®)

The characteristic of transmissible spongiform encephalopathies (TSE) is an accumulation of partially protease resistant (PrP(res)) abnormal prion protein (PrP(sc)). This pathological prion protein is very resistant to conventional inactivation methods. The risk of transmission of TSE, such as Creutzfeldt-Jakob disease (CJD), by biopharmaceutical products prepared from human cells must be taken into account. The nanofiltration process has been proved to be effective in removing viruses and scrapie agent. The major advantages of this technique are flexibility and efficacy in removing infectious particles without altering biopharmaceutical characteristics and properties. This study focused on the removal of human PrP(sc) by means of a nanofiltration method after spiking a Lymphoglobuline solution with a CJD brain homogenate. Lymphoglobuline equine anti-human thymocyte immunoglobulin is a selective immunosuppressive agent acting mainly on human T lymphocytes. The therapeutic indications are: immunosuppression for transplantation: prevention and treatment of graft rejection; treatment of aplastic anemia. In our study, CJD homogenate was spiked at three different dilutions (low, moderate and high) in the Lymphoglobuline product. The nanofiltration process was performed on each sample. Using the western blot technique, the PrP(res) signal detected in nanofiltrates was compared to that obtained with a reference scale (dilution series of CJD brain homogenate in Lymphoglobuline detected by western blot and elaborated on 3.3 log). After nanofiltration, the PrP(res) western blot signal was detected with a significant reduction in the less dilute sample, whereas the signal was undetectable in the two other samples. These are the first data in CJD demonstrating a clearance between 1.6 and 3.3 log with a Lymphoglobuline recovery of over 93%. The nanofiltration process confirms its relative efficacy in removing human CJD PrP(sc).

Amyloids, prions and the inherent infectious nature of misfolded protein aggregates

Misfolded aggregates present in amyloid fibrils are associated with various diseases known as "protein misfolding" disorders. Among them, prion diseases are unique in that the pathology can be transmitted by an infectious process involving an unprecedented agent known as a "prion". Prions are infectious proteins that can transmit biological information by propagating protein misfolding and aggregation. The molecular mechanism of prion conversion has a striking resemblance to the process of amyloid formation, suggesting that misfolded aggregates have an inherent ability to be transmissible. Intriguing recent data suggest that other protein misfolding disorders might also be transmitted by a prion-like infectious process.

Analysis of prion strains by PrPSc profiling in sporadic Creutzfeldt-Jakob disease

BACKGROUND

Prion diseases are a group of invariably fatal neurodegenerative disorders affecting humans and a wide range of mammals. An essential part of the infectious agent, termed the prion, is composed of an abnormal isoform (PrPSc) of a host-encoded normal cellular protein (PrPC). The conversion of PrPC to PrPSc is thought to play a crucial role in the development of prion diseases and leads to PrPSc deposition, mainly in the central nervous system. Sporadic Creutzfeldt-Jakob disease (sCJD), the most common form of human prion disease, presents with a marked clinical heterogeneity. This diversity is accompanied by a molecular signature which can be defined by histological, biochemical, and genetic means. The molecular classification of sCJD is an important tool to aid in the understanding of underlying disease mechanisms and the development of therapy protocols. Comparability of classifications is hampered by disparity of applied methods and inter-observer variability.

METHODS AND FINDINGS

To overcome these difficulties, we developed a new quantification protocol for PrPSc by using internal standards on each Western blot, which allows for generation and direct comparison of individual PrPSc profiles. By studying PrPSc profiles and PrPSc type expression within nine defined central nervous system areas of 50 patients with sCJD, we were able to show distinct PrPSc distribution patterns in diverse subtypes of sCJD. Furthermore, we were able to demonstrate the co-existence of more than one PrPSc type in individuals with sCJD in about 20% of all patients and in more than 50% of patients heterozygous for a polymorphism on codon 129 of the gene encoding the prion protein (PRNP).

CONCLUSION

PrPSc profiling represents a valuable tool for the molecular classification of human prion diseases and has important implications for their diagnosis by brain biopsy. Our results show that the co-existence of more than one PrPSc type might be influenced by genetic and brain region-specific determinants. These findings provide valuable insights into the generation of distinct PrPSc types.

KDEL-tagged anti-prion intrabodies impair PrP lysosomal degradation and inhibit scrapie infectivity

Transmissible spongiform encephalopathy or prion diseases are fatal neurodegenerative disorders characterized by the conversion of the cellular prion protein (PrPC) into the infectious scrapie isoform (PrPSc). We have recently demonstrated that anti-prion intrabodies targeted to the lumen of the endoplasmic reticulum provide a simple and effective means to inhibit the transport of PrPC to the cell surface. Here, we report that they completely block the traffic of mature full-length PrPC molecules, impair prion lysosomal degradation, and interfere with the early phase of scrapie formation. Since anti-prion intrabodies efficiently block PrPSc accumulation in vitro, we investigated whether they could also antagonize scrapie infectivity in vivo. We found that mice intracerebrally injected with KDEL-8H4-NGF-differentiated PC12 cells infected with scrapie neither develop scrapie clinical signs nor brain damage. Furthermore, no protease-resistant PrPSc is detectable in brains of inoculated animals. These results indicate that anti-prion intrabody strategy may be effective against prion infection.

Analysis of the polymorphic prion protein gene codon 129 in idiopathic Parkinson's disease

Idiopathic Parkinson's disease (IPD) is a neurodegenerative disorder of unknown aetiology. Histopathological similarities between IPD and Creutzfeldt-Jakob prion disease (CJD) have been suggested. Homozygosity at polymorphic prion protein gene codon 129 (PRNP129) is a risk factor for developing CJD. Therefore we investigated a putative genetic link between CJD and IPD by studying PRNP129 genotype segregation in 81 patients with IPD. We did not ascertain a different PRNP129 genotype distribution in IPD patients compared to healthy Germans. We found a significant difference in PRNP129 genotype in dependence of the clinical predominance type of IPD. Patients with tremor-dominant IPD presented less frequent a methionine homozygosis at PRNP129 than hypokinetic-rigid IPD patients (30% versus 62.5%; p<0.033). In conclusion, genotype distribution at codon 129 is obviously not essential in determining IPD. But our results may provide first evidence of an association between certain PRNP129 polymorphisms and the clinical presentation of IPD.

Scrapie pathogenesis in brain and retina: Effects of prion protein expression in neurons and astrocytes

Brain damage in the transmissible spongiform encephalopathies or prion diseases is associated with the conversion of normal host prion protein to an abnormal protease-resistant isoform, and expression of prion protein is required for susceptibility to these diseases. This article reviews the data on studies using transgenic mice expressing prion protein in specific individual cell types to study the roles of these cell types in prion disease pathogenesis. Surprisingly damage to neurons in brain and retina appeared to require different prion protein-expressing cells, suggesting that different pathogenic mechanisms operate in these two neuronal tissues.

Trapping Prion Protein in the Endoplasmic Reticulum Impairs PrPC Maturation and Prevents PrPSc Accumulation

The conversion of the normal cellular prion protein (PrP(C)) into the abnormal scrapie isoform (PrP(Sc)) is a key feature of prion diseases. The pathogenic mechanisms and the subcellular sites of the conversion are complex and not completely understood. In particular, little is known on the role of the early compartment of the secretory pathway in the processing of PrP(C) and in the pathogenesis of prion diseases. In order to interfere with the intracellular traffic of endogenous PrP(C) we have generated two anti-prion single chain antibody fragments (scFv) directed against different epitopes, each fragment tagged either with a secretory leader or with the ER retention signal KDEL. The stable expression of these constructs in PC12 cells allowed us to study their specific effects on the synthesis, maturation, and processing of endogenous PrP(C) and on PrP(Sc) formation. We found that ER-targeted anti-prion scFvs retain PrP(C) in the ER and inhibit its translocation to the cell surface. Retention in the ER strongly affects the maturation and glycosylation state of PrP(C), with the appearance of a new aberrant endo-H sensitive glycosylated species. Interestingly, ER-trapped PrP(C) acquires detergent insolubility and proteinase K resistance. Furthermore, we show that ER-targeted anti-prion antibodies prevent PrP(Sc) accumulation in nerve growth factor-differentiated PC12 cells, providing a new tool to study the molecular pathology of prion diseases.

Propagating Prions in Fungi and Mammals

Prions constitute a rare class of protein, which can switch to a robust amyloid form and then propagate that form in the absence of a nucleic acid determinant, thereby creating a unique, protein-only infectious agent. Details of the mechanism that drives conversion to the prion form and then subsequent propagation of that form are beginning to emerge using a range of in vivo and in vitro approaches. Recent studies on both mammalian and fungal prions are providing a greater understanding of the structural features that distinguish prions from non-transmissible amyloids.