Scrapie prion protein accumulation by scrapie-infected neuroblastoma cells abrogated by exposure to a prion protein antibody

New implications for prion diseases therapy and prophylaxis

Prion diseases are rare, fatal, progressive neurodegenerative disorders that affect both animal and human. Human prion diseases mainly present as Creutzfeldt-Jakob disease (CJD). However, there are no curable therapies, and animal prion diseases may negatively affect the ecosystem and human society. Over the past five decades, scientists are devoting to finding available therapeutic or prophylactic agents for prion diseases. Numerous chemical compounds have been shown to be effective in experimental research on prion diseases, but with the limitations of toxicity, poor efficacy, and low pharmacokinetics. The earliest clinical treatments of CJD were almost carried out with anti-infectious agents that had little amelioration of the course. With the discovery of pathogenic misfolding prion protein (PrPSc) and increasing insights into prion biology, amounts of novel technologies have attempted to eliminate PrPSc. This review presents new perspectives on clinical and experimental prion diseases, including immunotherapy, gene therapy, small-molecule drug, and stem cell therapy. It further explores the prospects and challenge associated with these emerging therapeutic approaches for prion diseases.

Creutzfeldt-Jakob disease and other prion diseases

Prion diseases share common clinical and pathological characteristics such as spongiform neuronal degeneration and deposition of an abnormal form of a host-derived protein, termed prion protein. The characteristic features of prion diseases are long incubation times, short clinical courses, extreme resistance of the transmissible agent to degradation and lack of nucleic acid involvement. Sporadic and genetic forms of prion diseases occur worldwide, of which genetic forms are associated with mutations in PRNP. Human to human transmission of these diseases has occurred due to iatrogenic exposure, and zoonotic forms of prion diseases are linked to bovine disease. Significant progress has been made in the diagnosis of these disorders. Clinical tools for diagnosis comprise brain imaging and cerebrospinal fluid tests. Aggregation assays for detection of the abnormally folded prion protein have a clear potential to diagnose the disease in peripherally accessible biofluids. After decades of therapeutic nihilism, new treatment strategies and clinical trials are on the horizon. Although prion diseases are relatively rare disorders, understanding their pathogenesis and mechanisms of prion protein misfolding has significantly enhanced the field in research of neurodegenerative diseases.

Large animal models for chronic wasting disease

Chronic wasting disease (CWD) is a fatal neurodegenerative prion disease of cervid species including deer, elk, moose and reindeer. The disease has shown both geographic and species expansion since its discovery in the late 1960's and is now recognized in captive and free-ranging cervid populations in North America, Asia and Europe. The facile transmission of CWD is unique among prion diseases and has resulted in growing concern for cervid populations and human public health. The development of native cervid host models with longitudinal monitoring has revealed new insights about CWD pathogenesis and transmission dynamics. More than 20 years of experimental studies conducted in these models, using biologically relevant routes of infection, have led to better understanding of many aspect of CWD infections. This review addresses some of these insights, including: (i) the temporal intra-host trafficking of CWD prions in tissues and bodily fluids, (ii) the presence of infectivity shed in bodily excretions that may help explain the facile transmission of CWD, (iii) mother-to-offspring CWD transmission, (iv) the influence of some Prnp polymorphisms on CWD susceptibility, and (vi) continued development of vaccine strategies to mitigate CWD.

Recombinant ovine prion protein can be mutated at position 136 to improve its efficacy as an inhibitor of prion propagation

Prion diseases are progressive neurodegenerative disorders with no effective therapeutics. The central event leading to the pathology in the diseases is the conversion of PrP^C into PrP^Sc and its accumulation in the central nervous system. Previous studies demonstrated that recombinant PrP (rPrP) and PrP peptides can inhibit the formation of PrP^Sc. Here, the effectiveness of ovine rPrP mutants at codon 136 and peptides derived from this region were assessed for their ability to inhibit PrP^Sc replication, using protein misfolding cyclic amplification (PMCA). Based on a rPrP VRQ (rVRQ) genotype background (positions 136, 154 and 171) and mutations at position 136, the most effective inhibitors were V136R, V136K and V136P mutants, with IC50 values of 1 to 2 nM; activities much more potent than rVRQ (114 nM). rRRQ and rKRQ were also shown to effectively inhibit multiple ruminant prion amplification reactions that used distinct prion strain seeds and substrate PRNP genotypes. rRRQ, rKRQ and rPRQ were also shown to effectively protect Rov9 cells from scrapie infection when applied at 250 nM. The study demonstrates for the first time that the rPrP sequence can be mutated at sites known to be involved in prion disease susceptibility, to produce inhibitors with improved efficacy.

Prion Propagation is Dependent on Key Amino Acids in Charge Cluster 2 within the Prion Protein

To dissect the N-terminal residues within the cellular prion protein (PrP^C) that are critical for efficient prion propagation, we generated a library of point, double, or triple alanine replacements within residues 23-111 of PrP, stably expressed them in cells silenced for endogenous mouse PrP^C and challenged the reconstituted cells with four common but biologically diverse mouse prion strains. Amino acids (aa) 105-111 of Charge Cluster 2 (CC2), which is disordered in PrP^C, were found to be required for propagation of all four prion strains; other residues had no effect or exhibited strain-specific effects. Replacements in CC2, including aa105-111, dominantly inhibited prion propagation in the presence of endogenous wild type PrP^C whilst other changes were not inhibitory. Single alanine replacements within aa105-111 identified leucine 108 and valine 111 or the cluster of lysine 105, threonine 106 and asparagine 107 as critical for prion propagation. These residues mediate specific ordering of unstructured CC2 into β-sheets in the infectious prion fibrils from Rocky Mountain Laboratory (RML) and ME7 mouse prion strains.

An arrayed genome-wide perturbation screen identifies the ribonucleoprotein Hnrnpk as rate-limiting for prion propagation

A defining characteristic of mammalian prions is their capacity for self-sustained propagation. Theoretical considerations and experimental evidence suggest that prion propagation is modulated by cell-autonomous and non-autonomous modifiers. Using a novel quantitative phospholipase protection assay (QUIPPER) for high-throughput prion measurements, we performed an arrayed genome-wide RNA interference (RNAi) screen aimed at detecting cellular host-factors that can modify prion propagation. We exposed prion-infected cells in high-density microplates to 35,364 ternary pools of 52,746 siRNAs targeting 17,582 genes representing the majority of the mouse protein-coding transcriptome. We identified 1,191 modulators of prion propagation. While 1,151 modified the expression of both the pathological prion protein, PrP^Sc , and its cellular counterpart, PrP^C , 40 genes selectively affected PrP^Sc . Of the latter 40 genes, 20 augmented prion production when suppressed. A prominent limiter of prion propagation was the heterogeneous nuclear ribonucleoprotein Hnrnpk. Psammaplysene A (PSA), which binds Hnrnpk, reduced prion levels in cultured cells and protected them from cytotoxicity. PSA also reduced prion levels in infected cerebellar organotypic slices and alleviated locomotor deficits in prion-infected Drosophila melanogaster expressing ovine PrP^C . Hence, genome-wide QUIPPER-based perturbations can discover actionable cellular pathways involved in prion propagation. Further, the unexpected identification of a prion-controlling ribonucleoprotein suggests a role for RNA in the generation of infectious prions.

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

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

A high-content neuron imaging assay demonstrates inhibition of prion disease-associated neurotoxicity by an anti-prion protein antibody

There is an urgent need to develop disease-modifying therapies to treat neurodegenerative diseases which pose increasing challenges to global healthcare systems. Prion diseases, although rare, provide a paradigm to study neurodegenerative dementias as similar disease mechanisms involving propagation and spread of multichain assemblies of misfolded protein ("prion-like" mechanisms) are increasingly recognised in the commoner conditions such as Alzheimer's disease. However, studies of prion disease pathogenesis in mouse models showed that prion propagation and neurotoxicity can be mechanistically uncoupled and in vitro assays confirmed that highly purified prions are indeed not directly neurotoxic. To aid development of prion disease therapeutics we have therefore developed a cell-based assay for the specific neurotoxicity seen in prion diseases rather than to simply assess inhibition of prion propagation. We applied this assay to examine an anti-prion protein mouse monoclonal antibody (ICSM18) known to potently cure prion-infected cells and to delay onset of prion disease in prion-infected mice. We demonstrate that whilst ICSM18 itself lacks inherent neurotoxicity in this assay, it potently blocks prion disease-associated neurotoxicity.

Prion protein monoclonal antibody (PRN100) therapy for Creutzfeldt-Jakob disease: evaluation of a first-in-human treatment programme

BACKGROUND

Human prion diseases, including Creutzfeldt-Jakob disease (CJD), are rapidly progressive, invariably fatal neurodegenerative conditions with no effective therapies. Their pathogenesis involves the obligate recruitment of cellular prion protein (PrP^C) into self-propagating multimeric assemblies or prions. Preclinical studies have firmly validated the targeting of PrP^C as a therapeutic strategy. We aimed to evaluate a first-in-human treatment programme using an anti-PrP^C monoclonal antibody under a Specials Licence.

METHODS

We generated a fully humanised anti-PrP^C monoclonal antibody (an IgG₄κ isotype; PRN100) for human use. We offered treatment with PRN100 to six patients with a clinical diagnosis of probable CJD who were not in the terminal disease stages at the point of first assessment and who were able to readily travel to the University College London Hospital (UCLH) Clinical Research Facility, London, UK, for treatment. After titration (1 mg/kg and 10 mg/kg at 48-h intervals), patients were treated with 80-120 mg/kg of intravenous PRN100 every 2 weeks until death or withdrawal from the programme, or until the supply of PRN100 was exhausted, and closely monitored for evidence of adverse effects. Disease progression was assessed by use of the Medical Research Council (MRC) Prion Disease Rating Scale, Motor Scale, and Cognitive Scale, and compared with that of untreated natural history controls (matched for disease severity, subtype, and PRNP codon 129 genotype) recruited between Oct 1, 2008, and July 31, 2018, from the National Prion Monitoring Cohort study. Autopsies were done in two patients and findings were compared with those from untreated natural history controls.

FINDINGS

We treated six patients (two men; four women) with CJD for 7-260 days at UCLH between Oct 9, 2018, and July 31, 2019. Repeated intravenous dosing of PRN100 was well tolerated and reached the target CSF drug concentration (50 nM) in four patients after 22-70 days; no clinically significant adverse reactions were seen. All patients showed progressive neurological decline on serial assessments with the MRC Scales. Neuropathological examination was done in two patients (patients 2 and 3) and showed no evidence of cytotoxicity. Patient 2, who was treated for 140 days, had the longest clinical duration we have yet documented for iatrogenic CJD and showed patterns of disease-associated PrP that differed from untreated patients with CJD, consistent with drug effects. Patient 3, who had sporadic CJD and only received one therapeutic dose of 80 mg/kg, had weak PrP synaptic labelling in the periventricular regions, which was not a feature of untreated patients with sporadic CJD. Brain tissue-bound drug concentrations across multiple regions in patient 2 ranged from 9·9 μg per g of tissue (SD 0·3) in the thalamus to 27·4 μg per g of tissue (1·5) in the basal ganglia (equivalent to 66-182 nM).

INTERPRETATION

Our academic-led programme delivered what is, to our knowledge, the first rationally designed experimental treatment for human prion disease to a small number of patients with CJD. The treatment appeared to be safe and reached encouraging CSF and brain tissue concentrations. These findings justify the need for formal efficacy trials in patients with CJD at the earliest possible clinical stages and as prophylaxis in those at risk of prion disease due to PRNP mutations or prion exposure.

FUNDING

The Cure CJD Campaign, the National Institute for Health Research UCLH Biomedical Research Centre, the Jon Moulton Charitable Trust, and the UK MRC.

Anchorless risk or released benefit? An updated view on the ADAM10-mediated shedding of the prion protein

The prion protein (PrP) is a broadly expressed glycoprotein linked with a multitude of (suggested) biological and pathological implications. Some of these roles seem to be due to constitutively generated proteolytic fragments of the protein. Among them is a soluble PrP form, which is released from the surface of neurons and other cell types by action of the metalloprotease ADAM10 in a process termed 'shedding'. The latter aspect is the focus of this review, which aims to provide a comprehensive overview on (i) the relevance of proteolytic processing in regulating cellular PrP functions, (ii) currently described involvement of shed PrP in neurodegenerative diseases (including prion diseases and Alzheimer's disease), (iii) shed PrP's expected roles in intercellular communication in many more (patho)physiological conditions (such as stroke, cancer or immune responses), (iv) and the need for improved research tools in respective (future) studies. Deeper mechanistic insight into roles played by PrP shedding and its resulting fragment may pave the way for improved diagnostics and future therapeutic approaches in diseases of the brain and beyond.

Treatment of microglia with Anti-PrP monoclonal antibodies induces neuronal apoptosis in vitro

Previous reports highlighted the neurotoxic effects caused by some motif-specific anti-PrP^C antibodies in vivo and in vitro. In the current study, we investigated the detailed alterations of the proteome with liquid chromatography–mass spectrometry following direct application of anti-PrP^C antibodies on mouse neuroblastoma cells (N2a) and mouse primary neuronal (MPN) cells or by cross-linking microglial PrP^C with anti-PrP^C antibodies prior to co-culture with the N2a/MPN cells. Here, we identified 4 (3 upregulated and 1 downregulated) and 17 (11 upregulated and 6 downregulated) neuronal apoptosis-related proteins following treatment of the N2a and N11 cell lines respectively when compared with untreated cells. In contrast, we identified 1 (upregulated) and 4 (2 upregulated and 2 downregulated) neuronal apoptosis-related proteins following treatment of MPN cells and N11 when compared with untreated cells. Furthermore, we also identified 3 (2 upregulated and 1 downregulated) and 2 (1 upregulated and 1 downregulated) neuronal apoptosis-related related proteins following treatment of MPN cells and N11 when compared to treatment with an anti-PrP antibody that lacks binding specificity for mouse PrP. The apoptotic effect of the anti-PrP antibodies was confirmed with flow cytometry following labelling of Annexin V-FITC. The toxic effects of the anti-PrP antibodies was more intense when antibody-treated N11 were co-cultured with the N2a and the identified apoptosis proteome was shown to be part of the PrP^C-interactome. Our observations provide a new insight into the prominent role played by microglia in causing neurotoxic effects following treatment with anti-PrP^C antibodies and might be relevant to explain the antibody mediated toxicity observed in other related neurodegenerative diseases such as Alzheimer.

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Antibody cross-linking neuronal PrPC induces apoptosis.

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Antibody cross-linking microglial PrPC induces neuronal apoptosis.

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Different apoptotic pathways were triggered by specific anti-PrP antibody treatments.

Ligands binding to the prion protein induce its proteolytic release with therapeutic potential in neurodegenerative proteinopathies

The prion protein (PrPC) is a central player in neurodegenerative diseases, such as prion diseases or Alzheimer’s disease. In contrast to disease-promoting cell surface PrPC, extracellular fragments act neuroprotective by blocking neurotoxic disease-associated protein conformers. Fittingly, PrPC release by the metalloprotease ADAM10 represents a protective mechanism. We used biochemical, cell biological, morphological, and structural methods to investigate mechanisms stimulating this proteolytic shedding. Shed PrP negatively correlates with prion conversion and is markedly redistributed in murine brain in the presence of prion deposits or amyloid plaques, indicating a sequestrating activity. PrP-directed ligands cause structural changes in PrPC and increased shedding in cells and organotypic brain slice cultures. As an exception, some PrP-directed antibodies targeting repetitive epitopes do not cause shedding but surface clustering, endocytosis, and degradation of PrPC. Both mechanisms may contribute to beneficial actions described for PrP-directed ligands and pave the way for new therapeutic strategies against currently incurable neurodegenerative diseases.

Epitope-specific anti-PrP antibody toxicity: a comparative in-silico study of human and mouse prion proteins

Despite having therapeutic potential, anti-PrP antibodies caused a major controversy due to their neurotoxic effects. For instance, treating mice with ICSM antibodies delayed prion disease onset, but both were found to be either toxic or innocuous to neurons by researchers following cross-linking PrP^C. In order to elucidate and understand the reasons that led to these contradictory outcomes, we conducted a comprehensive in silico study to assess the antibody-specific toxicity. Since most therapeutic anti-PrP antibodies were generated against human truncated recombinant PrP^91-231 or full-length mouse PrP^23-231, we reasoned that host specificity (human vs murine) of PrP^C might influence the nature of the specific epitopes recognized by these antibodies at the structural level possibly explaining the 'toxicity' discrepancies reported previously. Initially, molecular dynamics simulation and pro-motif analysis of full-length human (hu)PrP and mouse (mo)PrP 3D structure displayed conspicuous structural differences between huPrP and moPrP. We identified 10 huPrP and 6 moPrP linear B-cell epitopes from the prion protein 3D structure where 5 out of 10 huPrP and 3 out of 6 moPrP B-cell epitopes were predicted to be potentially toxic in immunoinformatics approaches. Herein, we demonstrate that some of the predicted potentially 'toxic' epitopes identified by the in silico analysis were similar to the epitopes recognized by the toxic antibodies such as ICSM18 (146-159), POM1 (138-147), D18 (133-157), ICSM35 (91-110), D13 (95-103) and POM3 (95-100). This in silico study reveals the role of host specificity of PrP^C in epitope-specific anti-PrP antibody toxicity.

Tackling prion diseases: a review of the patent landscape

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

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.

Novel Compounds Identified by Structure-Based Prion Disease Drug Discovery Using In Silico Screening Delay the Progression of an Illness in Prion-Infected Mice

The accumulation of abnormal prion protein (PrPSc) produced by the structure conversion of PrP (PrPC) in the brain induces prion disease. Although the conversion process of the protein is still not fully elucidated, it has been known that the intramolecular chemical bridging in the most fragile pocket of PrP, known as the "hot spot," stabilizes the structure of PrPC and inhibits the conversion process. Using our original structure-based drug discovery algorithm, we identified the low molecular weight compounds that predicted binding to the hot spot. NPR-130 and NPR-162 strongly bound to recombinant PrP in vitro, and fragment molecular orbital (FMO) analysis indicated that the high affinity of those candidates to the PrP is largely dependent on nonpolar interactions, such as van der Waals interactions. Those NPRs showed not only significant reduction of the PrPSc levels but also remarkable decrease of the number of aggresomes in persistently prion-infected cells. Intriguingly, treatment with those candidate compounds significantly prolonged the survival period of prion-infected mice and suppressed prion disease-specific pathological damage, such as vacuole degeneration, PrPSc accumulation, microgliosis, and astrogliosis in the brain, suggesting their possible clinical use. Our results indicate that in silico drug discovery using NUDE/DEGIMA may be widely useful to identify candidate compounds that effectively stabilize the protein.

Multimodal small-molecule screening for human prion protein binders

Prion disease is a rapidly progressive neurodegenerative disorder caused by misfolding and aggregation of the prion protein (PrP), and there are currently no therapeutic options. PrP ligands could theoretically antagonize prion formation by protecting the native protein from misfolding or by targeting it for degradation, but no validated small-molecule binders have been discovered to date. We deployed a variety of screening methods in an effort to discover binders of PrP, including 19F-observed and saturation transfer difference (STD) NMR spectroscopy, differential scanning fluorimetry (DSF), DNA-encoded library selection, and in silico screening. A single benzimidazole compound was confirmed in concentration-response, but affinity was very weak (Kd > 1 mm), and it could not be advanced further. The exceptionally low hit rate observed here suggests that PrP is a difficult target for small-molecule binders. Whereas orthogonal binder discovery methods could yield high-affinity compounds, non-small-molecule modalities may offer independent paths forward against prion disease.

The Role of Vesicle Trafficking Defects in the Pathogenesis of Prion and Prion-Like Disorders

Prion diseases are fatal and transmissible neurodegenerative diseases in which the cellular form of the prion protein ‘PrP^c’, misfolds into an infectious and aggregation prone isoform termed PrP^Sc, which is the primary component of prions. Many neurodegenerative diseases, like Alzheimer’s disease, Parkinson’s disease, and polyglutamine diseases, such as Huntington’s disease, are considered prion-like disorders because of the common characteristics in the propagation and spreading of misfolded proteins that they share with the prion diseases. Unlike prion diseases, these are non-infectious outside experimental settings. Many vesicular trafficking impairments, which are observed in prion and prion-like disorders, favor the accumulation of the pathogenic amyloid aggregates. In addition, many of the vesicular trafficking impairments that arise in these diseases, turn out to be further aggravating factors. This review offers an insight into the currently known vesicular trafficking defects in these neurodegenerative diseases and their implications on disease progression. These findings suggest that these impaired trafficking pathways may represent similar therapeutic targets in these classes of neurodegenerative disorders.

Characterization of Anchorless Human PrP With Q227X Stop Mutation Linked to Gerstmann-Sträussler-Scheinker Syndrome In Vivo and In Vitro

Alteration in cellular prion protein (PrP^C) localization on the cell surface through mediation of the glycosylphosphatidylinositol (GPI) anchor has been reported to dramatically affect the formation and infectivity of its pathological isoform (PrP^Sc). A patient with Gerstmann-Sträussler-Scheinker (GSS) syndrome was previously found to have a nonsense heterozygous PrP-Q227X mutation resulting in an anchorless PrP. However, the allelic origin of this anchorless PrP^Sc and cellular trafficking of PrP^Q227X remain to be determined. Here, we show that PrP^Sc in the brain of this GSS patient is mainly composed of the mutant but not wild-type PrP (PrP^Wt), suggesting pathological PrP^Q227X is incapable of recruiting PrP^Wt in vivo. This mutant anchorless protein, however, is able to recruit PrP^Wt from humanized transgenic mouse brain but not from autopsied human brain homogenates to produce a protease-resistant PrP^Sc-like form in vitro by protein misfolding cyclic amplification (PMCA). To further investigate the characteristics of this mutation, constructs expressing human PrP^Q227X or PrP^Wt were transfected into neuroblastoma cells (M17). Fractionation of the M17 cells demonstrated that most PrP^Wt is recovered in the cell lysate fraction, while most of the mutant PrP^Q227X is recovered in the medium fraction, consistent with the results obtained by immunofluorescence microscopy. Two-dimensional gel-electrophoresis and Western blotting showed that cellular PrP^Q227X spots clustered at molecular weights of 22–25 kDa with an isoelectric point (pI) of 3.5–5.5, whereas protein spots from the medium are at 18–26 kDa with a pI of 7–10. Our findings suggest that the role of GPI anchor in prion propagation between the anchorless mutant PrP and wild-type PrP relies on the cellular distribution of the protein.

Protective anti-prion antibodies in human immunoglobulin repertoires

Prion immunotherapy may hold great potential, but antibodies against certain PrP epitopes can be neurotoxic. Here, we identified > 6,000 PrP-binding antibodies in a synthetic human Fab phage display library, 49 of which we characterized in detail. Antibodies directed against the flexible tail of PrP conferred neuroprotection against infectious prions. We then mined published repertoires of circulating B cells from healthy humans and found antibodies similar to the protective phage-derived antibodies. When expressed recombinantly, these antibodies exhibited anti-PrP reactivity. Furthermore, we surveyed 48,718 samples from 37,894 hospital patients for the presence of anti-PrP IgGs and found 21 high-titer individuals. The clinical files of these individuals did not reveal any enrichment of specific pathologies, suggesting that anti-PrP autoimmunity is innocuous. The existence of anti-prion antibodies in unbiased human immunological repertoires suggests that they might clear nascent prions early in life. Combined with the reported lack of such antibodies in carriers of disease-associated PRNP mutations, this suggests a link to the low incidence of spontaneous prion diseases in human populations.

Genetic risk factors for Creutzfeldt-Jakob disease

Prion diseases are a group of fatal neurodegenerative disorders of mammals that share a central role for prion protein (PrP, gene PRNP) in their pathogenesis. Prions are infectious agents that account for the observed transmission of prion diseases between humans and animals in certain circumstances. The prion mechanism invokes a misfolded and multimeric assembly of PrP (a prion) that grows by templating of the normal protein and propagates by fission. Aside from the medical and public health notoriety of acquired prion diseases, the conditions have attracted interest as it has been realized that common neurodegenerative disorders share so-called prion-like mechanisms. In this article we will expand on recent evidence for new genetic loci that alter the risk of human prion disease. The most common human prion disease, sporadic Creutzfeldt-Jakob disease (sCJD), is characterized by the seemingly spontaneous appearance of prions in the brain. Genetic variation within PRNP is associated with all types of prion diseases, in particular, heterozygous genotypes at codons 129 and 219 have long been known to be strong protective factors against sCJD. A large number of rare mutations have been described in PRNP that cause autosomal dominant inherited prion diseases. Two loci recently identified by genome-wide association study increase sCJD risk, including variants in or near to STX6 and GAL3ST1. STX6 encodes syntaxin-6, a component of SNARE complexes with cellular roles that include the fusion of intracellular vesicles with target membranes. GAL3ST1 encodes cerebroside sulfotransferase, the only enzyme that sulfates sphingolipids to make sulfatides, a major lipid component of myelin. We discuss how these roles may modify the pathogenesis of prion diseases and their relevance for other neurodegenerative disorders.

Immunotherapy against Prion Disease

The term "prion disease" encompasses a group of neurodegenerative diseases affecting both humans and animals. Currently, there is no effective therapy and all forms of prion disease are invariably fatal. Because of (a) the outbreak of bovine spongiform encephalopathy in cattle and variant Creutzfeldt-Jakob disease in humans; (b) the heated debate about the prion hypothesis; and (c) the availability of a natural prion disease in rodents, the understanding of the pathogenic process in prion disease is much more advanced compared to that of other neurodegenerative disorders, which inspired many attempts to develop therapeutic strategies against these fatal diseases. In this review, we focus on immunotherapy against prion disease. We explain our rationale for immunotherapy as a plausible therapeutic choice, review previous trials using either active or passive immunization, and discuss potential strategies for overcoming the hurdles in developing a successful immunotherapy. We propose that immunotherapy is a plausible and practical therapeutic strategy and advocate more studies in this area to develop effective measures to control and treat these devastating disorders.

Cellular models for discovering prion disease therapeutics: Progress and challenges

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

The role of prion strain diversity in the development of successful therapeutic treatments

Prions are a self-propagating misfolded conformation of a cellular protein. Prions are found in several eukaryotic organisms with mammalian prion diseases encompassing a wide range of disorders. The first recognized prion disease, the transmissible spongiform encephalopathies (TSEs), affect several species including humans. Alzheimer's disease, synucleinopathies, and tauopathies share a similar mechanism of self-propagation of the prion form of the disease-specific protein reminiscent of the infection process of TSEs. Strain diversity in prion disease is characterized by differences in the phenotype of disease that is hypothesized to be encoded by strain-specific conformations of the prion form of the disease-specific protein. Prion therapeutics that target the prion form of the disease-specific protein can lead to the emergence of drug-resistant strains of prions, consistent with the hypothesis that prion strains exist as a dynamic mixture of a dominant strain in combination with minor substrains. To overcome this obstacle, therapies that reduce or eliminate the template of conversion are efficacious, may reverse neuropathology, and do not result in the emergence of drug resistance. Recent advancements in preclinical diagnosis of prion infection may allow for a combinational approach that treats the prion form and the precursor protein to effectively treat prion diseases.

PrPC knockdown by liposome-siRNA-peptide complexes (LSPCs) prolongs survival and normal behavior of prion-infected mice immunotolerant to treatment

Prion diseases are members of neurodegenerative protein misfolding diseases (NPMDs) that include Alzheimer's, Parkinson's and Huntington diseases, amyotrophic lateral sclerosis, tauopathies, traumatic brain injuries, and chronic traumatic encephalopathies. No known therapeutics extend survival or improve quality of life of humans afflicted with prion disease. We and others developed a new approach to NPMD therapy based on reducing the amount of the normal, host-encoded protein available as substrate for misfolding into pathologic forms, using RNA interference, a catabolic pathway that decreases levels of mRNA encoding a particular protein. We developed a therapeutic delivery system consisting of small interfering RNA (siRNA) complexed to liposomes and addressed to the central nervous system using a targeting peptide derived from rabies virus glycoprotein. These liposome-siRNA-peptide complexes (LSPCs) cross the blood-brain barrier and deliver PrP siRNA to neuronal cells to decrease expression of the normal cellular prion protein, PrPC, which acts as a substrate for prion replication. Here we show that LSPCs can extend survival and improve behavior of prion-infected mice that remain immunotolerant to treatment. LSPC treatment may be a viable therapy for prion and other NPMDs that can improve the quality of life of patients at terminal disease stages.

Mammalian prion propagation in PrP transgenic Drosophila

Mammalian prions propagate by template-directed misfolding and aggregation of normal cellular prion related protein PrP^C as it converts into disease-associated conformers collectively referred to as PrP^Sc. Mammalian species may be permissive for prion disease because these hosts have co-evolved specific co-factors that assist PrP^C conformational change and prion propagation. We have tested this hypothesis by examining whether faithful prion propagation occurs in the normally PrP^C-null invertebrate host Drosophila melanogaster. Ovine PrP transgenic Drosophila exposed at the larval stage to ovine scrapie showed a progressive accumulation of transmissible prions in adult flies. Strikingly, the biological properties of distinct ovine prion strains were maintained during their propagation in Drosophila. Our observations show that the co-factors necessary for strain-specific prion propagation are not unique to mammalian species. Our studies establish Drosophila as a novel host for the study of transmissible mammalian prions.

In vitro Modeling of Prion Strain Tropism

Prions are atypical infectious agents lacking genetic material. Yet, various strains have been isolated from animals and humans using experimental models. They are distinguished by the resulting pattern of disease, including the localization of PrPsc deposits and the spongiform changes they induce in the brain of affected individuals. In this paper, we discuss the emerging use of cellular and acellular models to decipher the mechanisms involved in the strain-specific targeting of distinct brain regions. Recent studies suggest that neuronal cultures, protein misfolding cyclic amplification, and combination of both approaches may be useful to explore this under-investigated but central domain of the prion field.

Prionoid Proteins in the Pathogenesis of Neurodegenerative Diseases

There is a growing body of evidence that prionoid protein behaviors are a core element of neurodegenerative diseases (NDs) that afflict humans. Common elements in pathogenesis, pathological effects and protein-level behaviors exist between Alzheimer's Disease (AD), Parkinson's Disease (PD), Huntington's Disease (HD) and Amyotrophic Lateral Sclerosis (ALS). These extend beyond the affected neurons to glial cells and processes. This results in a complicated system of disease progression, which often takes advantage of protective processes to promote the propagation of pathological protein aggregates. This review article provides a current snapshot of knowledge on these proteins and their intrinsic role in the pathogenesis and disease progression seen across NDs.

Cell biology of prion infection

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

Anti-prion Protein Antibody 6D11 Restores Cellular Proteostasis of Prion Protein Through Disrupting Recycling Propagation of PrPSc and Targeting PrPSc for Lysosomal Degradation

PrP^Sc is an infectious and disease-specific conformer of the prion protein, which accumulation in the CNS underlies the pathology of prion diseases. PrP^Sc replicates by binding to the cellular conformer of the prion protein (PrP^C) expressed by host cells and rendering its secondary structure a likeness of itself. PrP^C is a plasma membrane anchored protein, which constitutively recirculates between the cell surface and the endocytic compartment. Since PrP^Sc engages PrP^C along this trafficking pathway, its replication process is often referred to as "recycling propagation." Certain monoclonal antibodies (mAbs) directed against prion protein can abrogate the presence of PrP^Sc from prion-infected cells. However, the precise mechanism(s) underlying their therapeutic propensities remains obscure. Using N2A murine neuroblastoma cell line stably infected with 22L mouse-adapted scrapie strain (N2A/22L), we investigated here the modus operandi of the 6D11 clone, which was raised against the PrP^Sc conformer and has been shown to permanently clear prion-infected cells from PrP^Sc presence. We determined that 6D11 mAb engages and sequesters PrP^C and PrP^Sc at the cell surface. PrP^C/6D11 and PrP^Sc/6D11 complexes are then endocytosed from the plasma membrane and are directed to lysosomes, therefore precluding recirculation of nascent PrP^Sc back to the cell surface. Targeting PrP^Sc by 6D11 mAb to the lysosomal compartment facilitates its proteolysis and eventually shifts the balance between PrP^Sc formation and degradation. Ongoing translation of PrP^C allows maintaining the steady-state level of prion protein within the cells, which was not depleted under 6D11 mAb treatment. Our findings demonstrate that through disrupting recycling propagation of PrP^Sc and promoting its degradation, 6D11 mAb restores cellular proteostasis of prion protein.

Induction of PrPSc-specific systemic and mucosal immune responses in white-tailed deer with an oral vaccine for chronic wasting disease

The ongoing epidemic of chronic wasting disease (CWD) within cervid populations indicates the need for novel approaches for disease management. A vaccine that either reduces susceptibility to infection or reduces shedding of prions by infected animals, or a combination of both, could be of benefit for disease control. The development of such a vaccine is challenged by the unique nature of prion diseases and the requirement for formulation and delivery in an oral format for application in wildlife settings. To address the unique nature of prions, our group targets epitopes, termed disease specific epitopes (DSEs), whose exposure for antibody binding depends on disease-associated misfolding of PrP^C into PrP^Sc. Here, a DSE corresponding to the rigid loop (RL) region, which was immunogenic following parenteral vaccination, was translated into an oral vaccine. This vaccine consists of a replication-incompetent human adenovirus expressing a truncated rabies glycoprotein G recombinant fusion with the RL epitope (hAd5:tgG-RL). Oral immunization of white-tailed deer with hAd5:tgG-RL induced PrP^Sc-specific systemic and mucosal antibody responses with an encouraging safety profile in terms of no adverse health effects nor prolonged vector shedding. By building upon proven strategies of formulation for wildlife vaccines, these efforts generate a particular PrP^Sc-specific oral vaccine for CWD as well as providing a versatile platform, in terms of carrier protein and biological vector, for generation of other oral, peptide-based CWD vaccines.

Targeting of Disordered Proteins by Small Molecules in Neurodegenerative Diseases

The formation of protein aggregates and inclusions in the brain and spinal cord is a common neuropathological feature of a number of neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and many others. These are commonly referred as neurodegenerative proteinopathies or protein-misfolding diseases. The main characteristic of protein aggregates in these disorders is the fact that they are enriched in amyloid fibrils. Since protein aggregation is considered to play a central role for the onset of neurodegenerative proteinopathies, research is ongoing to develop strategies aimed at preventing or removing protein aggregation in the brain of affected patients. Numerous studies have shown that small molecule-based approaches may be potentially the most promising for halting protein aggregation in neurodegenerative diseases. Indeed, several of these compounds have been found to interact with intrinsically disordered proteins and promote their clearing in experimental models. This notwithstanding, at present small molecule inhibitors still awaits achievements for clinical translation. Hopefully, if we determine whether the formation of insoluble inclusions is effectively neurotoxic and find a valid biomarker to assess their protein aggregation-inhibitory activity in the human central nervous system, the use of small molecule inhibitors will be considered as a cure for neurodegenerative protein-misfolding diseases.

Vaccination strategies

Currently all prion diseases are without effective treatment and are universally fatal. It is increasingly being recognized that the pathogenesis of many neurodegenerative diseases, such as Alzheimer disease (AD), includes "prion-like" properties. Hence, any effective therapeutic intervention for prion disease could have significant implications for other neurodegenerative diseases. Conversely, therapies that are effective in AD might also be therapeutically beneficial for prion disease. AD-like prion disease has no effective therapy. However, various vaccine and immunomodulatory approaches have shown great success in animal models of AD, with numerous ongoing clinical trials of these potential immunotherapies. More limited evidence suggests that immunotherapies may be effective in prion models and in naturally occurring prion disease. In particular, experimental data suggest that mucosal vaccination against prions can be effective for protection against orally acquired prion infection. Many prion diseases, including natural sheep scrapie, bovine spongiform encephalopathy, chronic wasting disease, and variant Creutzfeldt-Jakob disease, are thought to be acquired peripherally, mainly by oral exposure. Mucosal vaccination would be most applicable to this form of transmission. In this chapter we review various immunologically based strategies which are under development for prion infection.

A role for astroglia in prion diseases

In this issue of JEM, Krejciova et al. (https://doi.org/10.1084/jem.20161547) report that astrocytes derived from human iPSCs can replicate human CJD prions. These observations provide a new, potentially very valuable model for studying human prions in cellula and for identifying antiprion compounds that might serve as clinical candidates. Furthermore, they add to the evidence that astrocytes may not be just innocent bystanders in prion diseases.

Novel amplification mechanism of prions through disrupting sortilin-mediated trafficking

Conformational conversion of the cellular prion protein, PrP^C, into the abnormally folded isoform of prion protein, PrP^Sc, which leads to marked accumulation of PrP^Sc in brains, is a key pathogenic event in prion diseases, a group of fatal neurodegenerative disorders caused by prions. However, the exact mechanism of PrP^Sc accumulation in prion-infected neurons remains unknown. We recently reported a novel cellular mechanism to support PrP^Sc accumulation in prion-infected neurons, in which PrP^Sc itself promotes its accumulation by evading the cellular inhibitory mechanism, which is newly identified in our recent study. We showed that the VPS10P sorting receptor sortilin negatively regulates PrP^Sc accumulation in prion-infected neurons, by interacting with PrP^C and PrP^Sc and trafficking them to lysosomes for degradation. However, PrP^Sc stimulated lysosomal degradation of sortilin, disrupting the sortilin-mediated degradation of PrP^C and PrP^Sc and eventually evoking further accumulation of PrP^Sc in prion-infected neurons. These findings suggest a positive feedback amplification mechanism for PrP^Sc accumulation in prion-infected neurons.

Toward Therapy of Human Prion Diseases

Three decades after the discovery of prions as the cause of Creutzfeldt-Jakob disease and other transmissible spongiform encephalopathies, we are still nowhere close to finding an effective therapy. Numerous pharmacological interventions have attempted to target various stages of disease progression, yet none has significantly ameliorated the course of disease. We still lack a mechanistic understanding of how the prions damage the brain, and this situation results in a dearth of validated pharmacological targets. In this review, we discuss the attempts to interfere with the replication of prions and to enhance their clearance. We also trace some of the possibilities to identify novel targets that may arise with increasing insights into prion biology.

PrP Knockout Cells Expressing Transmembrane PrP Resist Prion Infection

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

IMPORTANCE

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

Prions: Beyond a Single Protein

Since the term protein was first coined in 1838 and protein was discovered to be the essential component of fibrin and albumin, all cellular proteins were presumed to play beneficial roles in plants and mammals. However, in 1967, Griffith proposed that proteins could be infectious pathogens and postulated their involvement in scrapie, a universally fatal transmissible spongiform encephalopathy in goats and sheep. Nevertheless, this novel hypothesis had not been evidenced until 1982, when Prusiner and coworkers purified infectious particles from scrapie-infected hamster brains and demonstrated that they consisted of a specific protein that he called a "prion." Unprecedentedly, the infectious prion pathogen is actually derived from its endogenous cellular form in the central nervous system. Unlike other infectious agents, such as bacteria, viruses, and fungi, prions do not contain genetic materials such as DNA or RNA. The unique traits and genetic information of prions are believed to be encoded within the conformational structure and posttranslational modifications of the proteins. Remarkably, prion-like behavior has been recently observed in other cellular proteins-not only in pathogenic roles but also serving physiological functions. The significance of these fascinating developments in prion biology is far beyond the scope of a single cellular protein and its related disease.

Mammalian prions and their wider relevance in neurodegenerative diseases

Prions are notorious protein-only infectious agents that cause invariably fatal brain diseases following silent incubation periods that can span a lifetime. These diseases can arise spontaneously, through infection or be inherited. Remarkably, prions are composed of self-propagating assemblies of a misfolded cellular protein that encode information, generate neurotoxicity and evolve and adapt in vivo. Although parallels have been drawn with Alzheimer's disease and other neurodegenerative conditions involving the deposition of assemblies of misfolded proteins in the brain, insights are now being provided into the usefulness and limitations of prion analogies and their aetiological and therapeutic relevance.

Neurotoxic Antibodies against the Prion Protein Do Not Trigger Prion Replication

Prions are the infectious agents causing transmissible spongiform encephalopathies (TSE), progressive, inexorably lethal neurological diseases. Antibodies targeting the globular domain (GD) of the cellular prion protein PrP^C trigger a neurotoxic syndrome morphologically and molecularly similar to prion disease. This phenomenon raises the question whether such antibodies induce infectious prions de novo. Here we exposed cerebellar organotypic cultured slices (COCS) to the neurotoxic antibody, POM1. We then inoculated COCS homogenates into tga20 mice, which overexpress PrP^C and are commonly utilized as sensitive indicators of prion infectivity. None of the mice inoculated with COCS-derived lysates developed any signs of disease, and all mice survived for at least 200 days post-inoculation. In contrast, all mice inoculated with bona fide prions succumbed to TSE after 55–95 days. Post-mortem analyses did not reveal any signs of prion pathology in mice inoculated with POM1-COCS lysates. Also, lysates from POM1-exposed COCS were unable to convert PrP by quaking. Hence, anti-GD antibodies do not catalyze the generation of prion infectivity. These data indicate that prion replication can be separated from prion toxicity, and suggest that anti-GD antibodies exert toxicity by acting downstream of prion replication.

Sialylation of Glycosylphosphatidylinositol (GPI) Anchors of Mammalian Prions Is Regulated in a Host-, Tissue-, and Cell-specific Manner

Prions or PrP(Sc) are proteinaceous infectious agents that consist of misfolded, self-replicating states of the prion protein or PrP(C) PrP(C) is posttranslationally modified with N-linked glycans and a sialylated glycosylphosphatidylinositol (GPI) anchor. Conformational conversion of PrP(C) gives rise to glycosylated and GPI-anchored PrP(Sc) The question of the sialylation status of GPIs within PrP(Sc) has been controversial. Previous studies that examined scrapie brains reported that both sialo- and asialo-GPIs were present in PrP(Sc), with the majority being asialo-GPIs. In contrast, recent work that employed cultured cells claimed that only PrP(C) with sialylo-GPIs could be recruited into PrP(Sc), whereas PrP(C) with asialo-GPIs inhibited conversion. To resolve this controversy, we analyzed the sialylation status of GPIs within PrP(Sc) generated in the brain, spleen, or cultured N2a or C2C12 myotube cells. We found that recruiting PrP(C) with both sialo- and asialo-GPIs is a common feature of PrP(Sc) The mixtures of sialo- and asialo-GPIs were observed in PrP(Sc) universally regardless of prion strain as well as host, tissue, or type of cells that produced PrP(Sc) Remarkably, the proportion of sialo- versus asialo-GPIs was found to be controlled by host, tissue, and cell type but not prion strain. In summary, this study found no strain-specific preferences for selecting PrP(C) with sialo- versus asialo-GPIs. Instead, this work suggests that the sialylation status of GPIs within PrP(Sc) is regulated in a cell-, tissue-, or host-specific manner and is likely to be determined by the specifics of GPI biosynthesis.

Structure-Based Drug Discovery for Prion Disease Using a Novel Binding Simulation

The accumulation of abnormal prion protein (PrP(Sc)) converted from the normal cellular isoform of PrP (PrP(C)) is assumed to induce pathogenesis in prion diseases. Therefore, drug discovery studies for these diseases have focused on the protein conversion process. We used a structure-based drug discovery algorithm (termed Nagasaki University Docking Engine: NUDE) that ran on an intensive supercomputer with a graphic-processing unit to identify several compounds with anti-prion effects. Among the candidates showing a high-binding score, the compounds exhibited direct interaction with recombinant PrP in vitro, and drastically reduced PrP(Sc) and protein-aggresomes in the prion-infected cells. The fragment molecular orbital calculation showed that the van der Waals interaction played a key role in PrP(C) binding as the intermolecular interaction mode. Furthermore, PrP(Sc) accumulation and microgliosis were significantly reduced in the brains of treated mice, suggesting that the drug candidates provided protection from prion disease, although further in vivo tests are needed to confirm these findings. This NUDE-based structure-based drug discovery for normal protein structures is likely useful for the development of drugs to treat other conformational disorders, such as Alzheimer's disease.

Prion diseases: immunotargets and therapy

Transmissible spongiform encephathalopathies or prion diseases are a group of neurological disorders characterized by neuronal loss, spongiform degeneration, and activation of astrocytes or microglia. These diseases affect humans and animals with an extremely high prevalence in some species such as deer and elk in North America. Although rare in humans, they result in a devastatingly swift neurological progression with dementia and ataxia. Patients usually die within a year of diagnosis. Prion diseases are familial, sporadic, iatrogenic, or transmissible. Human prion diseases include Kuru, sporadic, iatrogenic, and familial forms of Creutzfeldt–Jakob disease, variant Creutzfeldt–Jakob disease, Gerstmann–Sträussler–Scheinker disease, and fatal familial insomnia. The causative agent is a misfolded version of the physiological prion protein called PrP^Sc in the brain. There are a number of therapeutic options currently under investigation. A number of small molecules have had some success in delaying disease progression in animal models and mixed results in clinical trials, including pentosan polysulfate, quinacrine, and amphotericin B. More promisingly, immunotherapy has reported success in vitro and in vivo in animal studies and clinical trials. The three main branches of immunotherapy research are focus on antibody vaccines, dendritic cell vaccines, and adoptive transfer of physiological prion protein-specific CD4⁺ T-lymphocytes. Vaccines utilizing antibodies generally target disease-specific epitopes that are only exposed in the misfolded PrP^Sc conformation. Vaccines utilizing antigen-loaded dendritic cell have the ability to bypass immune tolerance and prime CD4⁺ cells to initiate an immune response. Adoptive transfer of CD4⁺ T-cells is another promising target as this cell type can orchestrate the adaptive immune response. Although more research into mechanisms and safety is required, these immunotherapies offer novel therapeutic targets for prion diseases.

Shedding light on prion disease

Proteolytic processing regulates key processes in health and disease. The cellular prion protein (PrP(C)) is subject to at least 3 cleavage events, α-cleavage, β-cleavage and shedding. In contrast to α- and β-cleavage where there is an ongoing controversy on the identity of relevant proteases, the metalloprotease ADAM10 represents the only relevant PrP sheddase. Here we focus on the roles that ADAM10-mediated shedding of PrP(C) and its pathogenic isoform (PrP(Sc)) might play in regulating their physiological and pathogenic functions, respectively. As revealed by our recent study using conditional ADAM10 knockout mice (Altmeppen et al., 2015), shedding of PrP seems to be involved in key processes of prion diseases. These aspects and several open questions arising from them are discussed. Increased knowledge on this topic can shed new light on prion diseases and other neurodegenerative conditions as well.

Prion Protein-Antibody Complexes Characterized by Chromatography-Coupled Small-Angle X-Ray Scattering

Aberrant self-assembly, induced by structural misfolding of the prion proteins, leads to a number of neurodegenerative disorders. In particular, misfolding of the mostly α-helical cellular prion protein (PrP(C)) into a β-sheet-rich disease-causing isoform (PrP(Sc)) is the key molecular event in the formation of PrP(Sc) aggregates. The molecular mechanisms underlying the PrP(C)-to-PrP(Sc) conversion and subsequent aggregation remain to be elucidated. However, in persistently prion-infected cell-culture models, it was shown that treatment with monoclonal antibodies against defined regions of the prion protein (PrP) led to the clearing of PrP(Sc) in cultured cells. To gain more insight into this process, we characterized PrP-antibody complexes in solution using a fast protein liquid chromatography coupled with small-angle x-ray scattering (FPLC-SAXS) procedure. High-quality SAXS data were collected for full-length recombinant mouse PrP [denoted recPrP(23-230)] and N-terminally truncated recPrP(89-230), as well as their complexes with each of two Fab fragments (HuM-P and HuM-R1), which recognize N- and C-terminal epitopes of PrP, respectively. In-line measurements by fast protein liquid chromatography coupled with SAXS minimized data artifacts caused by a non-monodispersed sample, allowing structural analysis of PrP alone and in complex with Fab antibodies. The resulting structural models suggest two mechanisms for how these Fabs may prevent the conversion of PrP(C) into PrP(Sc).

Mechanisms of prion-induced neurodegeneration

Transmissible spongiform encephalopathies (TSEs), or prion diseases, are fatal neurodegenerative disorders characterised by long incubation period, short clinical duration, and transmissibility to susceptible species. Neuronal loss, spongiform changes, gliosis and the accumulation in the brain of the misfolded version of a membrane-bound cellular prion protein (PrP(C)), termed PrP(TSE), are diagnostic markers of these diseases. Compelling evidence links protein misfolding and its accumulation with neurodegenerative changes. Accordingly, several mechanisms of prion-mediated neurotoxicity have been proposed. In this paper, we provide an overview of the recent knowledge on the mechanisms of neuropathogenesis, the neurotoxic PrP species and the possible therapeutic approaches to treat these devastating disorders.

Detection of misfolded protein aggregates from a clinical perspective

Neurodegenerative Protein Misfolding Diseases (PMDs), such as Alzheimer's (AD), Parkinson's (PD) and prion diseases, are generally difficult to diagnose before irreversible damage to the central nervous system damage has occurred. Detection of the misfolded proteins that ultimately lead to these conditions offers a means for providing early detection and diagnosis of this class of disease. In this review, we discuss recent developments surrounding protein misfolding diseases with emphasis on the cytotoxic oligomers implicated in their aetiology. We also discuss the relationship of misfolded proteins with biological membranes. Finally, we discuss how far techniques for providing early diagnoses for PMDs have advanced and describe promising clinical approaches. We conclude that antibodies with specificity towards oligomeric species of AD and PD and lectins with specificity for particular glycosylation, show promise. However, it is not clear which approach may yield a reliable clinical test first. Relevance for patients: Individuals suffering from protein misfolding diseases will likely benefit form earlier, less- or even non-invasive diagnosis techniques. The current state and possible future directions for these are subject of this review.

Design and Optimization of Anti-amyloid Domain Antibodies Specific for β-Amyloid and Islet Amyloid Polypeptide

Antibodies with conformational specificity are important for detecting and interfering with polypeptide aggregation linked to several human disorders. We are developing a motif-grafting approach for designing lead antibody candidates specific for amyloid-forming polypeptides such as the Alzheimer peptide (Aβ). This approach involves grafting amyloidogenic peptide segments into the complementarity-determining regions (CDRs) of single-domain (VH) antibodies. Here we have investigated the impact of polar mutations inserted at the edges of a large hydrophobic Aβ42 peptide segment (Aβ residues 17-42) in CDR3 on the solubility and conformational specificity of the corresponding VH domains. We find that VH expression and solubility are strongly enhanced by introducing multiple negatively charged or asparagine residues at the edges of CDR3, whereas other polar mutations are less effective (glutamine and serine) or ineffective (threonine, lysine, and arginine). Moreover, Aβ VH domains with negatively charged CDR3 mutations show significant preference for recognizing Aβ fibrils relative to Aβ monomers, whereas the same VH domains with other polar CDR3 mutations recognize both Aβ conformers. We observe similar behavior for a VH domain grafted with a large hydrophobic peptide from islet amyloid polypeptide (residues 8-37) that contains negatively charged mutations at the edges of CDR3. These findings highlight the sensitivity of antibody binding and solubility to residues at the edges of CDRs, and provide guidelines for designing other grafted antibody fragments with hydrophobic binding loops.

Identification of a Compound That Disrupts Binding of Amyloid-β to the Prion Protein Using a Novel Fluorescence-based Assay

The prion protein (PrP) has been implicated both in prion diseases such as Creutzfeldt-Jakob disease, where its monomeric cellular isoform (PrP^C) is recruited into pathogenic self-propagating polymers of misfolded protein, and in Alzheimer disease, where PrP^C may act as a receptor for synaptotoxic oligomeric forms of amyloid-β (Aβ). There has been considerable interest in identification of compounds that bind to PrP^C, stabilizing its native fold and thereby acting as pharmacological chaperones to block prion propagation and pathogenesis. However, compounds binding PrP^C could also inhibit the binding of toxic Aβ species and may have a role in treating Alzheimer disease, a highly prevalent dementia for which there are currently no disease-modifying treatments. However, the absence of a unitary, readily measurable, physiological function of PrP makes screening for ligands challenging, and the highly heterogeneous nature of Aβ oligomer preparations makes conventional competition binding assays difficult to interpret. We have therefore developed a high-throughput screen that utilizes site-specifically fluorescently labeled protein to identify compounds that bind to PrP and inhibit both Aβ binding and prion propagation. Following a screen of 1,200 approved drugs, we identified Chicago Sky Blue 6B as the first small molecule PrP ligand capable of inhibiting Aβ binding, demonstrating the feasibility of development of drugs to block this interaction. The interaction of Chicago Sky Blue 6B was characterized by isothermal titration calorimetry, and its ability to inhibit Aβ binding and reduce prion levels was established in cell-based assays.