Rational targeting for prion therapeutics

Direct interaction of DNMT inhibitors to PrPC suppresses pathogenic process of prion

The conversion of the normal cellular prion protein (PrP^C) to the misfolded pathogenic scrapie prion protein (PrP^Sc) is the biochemical hallmark of prion replication. So far, various chemical compounds that inhibit this conformational conversion have been identified. Here, we report the novel anti-prion activity of SGI-1027 and its meta/meta analogue (M/M), previously known only as potent inhibitors of DNA methyltransferases (DNMTs). These compounds effectively decreased the level of PrP^Sc in cultured cells with permanent prion infection, without affecting PrP^C at the transcriptional or translational levels. Furthermore, SGI-1027 prevented effective prion infection of the cells. In a PrP aggregation assay, both SGI-1027 and M/M blocked the formation of misfolded PrP aggregates, implying that binding of these compounds hinders the PrP conversion process. A series of binding and docking analyses demonstrated that both SGI-1027 and M/M directly interacted with the C-terminal globular domain of PrP^C, but only SGI-1027 bound to a specific region of PrP^C with high affinity, which correlates with its potent anti-prion efficacy. Therefore, we report SGI-1027 and related compounds as a novel class of potential anti-prion agents that preferentially function through direct interaction with PrP^C.

Dual MicroRNA to Cellular Prion Protein Inhibits Propagation of Pathogenic Prion Protein in Cultured Cells

Prion diseases are fatal transmissible neurodegenerative disorders affecting humans and various mammals. In spite of intensive efforts, there is no effective cure or treatment for prion diseases. Cellular forms of prion protein (PrP^C) is essential for propagation of abnormal isoforms of prion protein (PrP^Sc) and pathogenesis. The effect of an artificial dual microRNA (DmiR) on PrP^C suppression and resultant inhibition of prion replication was determined using prion-infectible cell cultures: differentiated C2C12 culture and primary mixed neuronal and glial cells culture (MNGC). Processing of DmiR by prion-susceptible myotubes, but not by reserve cells, in differentiated C2C12 culture slowed prion replication, implying an importance of cell type-specific PrP^C targeting. In MNGC, reduction of PrP^C with DmiR was effective for suppressing prion replication. MNGC lentivirally transduced with non-targeting control miRNAs (scrambled) reduced prion replication at a level similar to that with a synthetic analogue of viral RNA, poly I:C. The results suggest that a synergistic combination of the immunostimulatory RNA duplexes (miRNA) and PrP^C silencing with DmiR might augment a therapeutic potential of RNA interference.

Integrated Organotypic Slice Cultures and RT-QuIC (OSCAR) Assay: Implications for Translational Discovery in Protein Misfolding Diseases

Protein misfolding is a key pathological event in neurodegenerative diseases like prion diseases, synucleinopathies, and tauopathies that are collectively termed protein misfolding disorders. Prions are a prototypic model to study protein aggregation biology and therapeutic development. Attempts to develop anti-prion therapeutics have been impeded by the lack of screening models that faithfully replicate prion diseases and the lack of rapid, sensitive biological screening systems. Therefore, a sensitive model encompassing prion replication and neurotoxicity would be indispensable to the pursuit of intervention strategies. We present an ultra-sensitive screening system coupled to an ex vivo prion organotypic slice culture model to rapidly advance rationale-based high-throughput therapeutic strategies. This hybrid Organotypic Slice Culture Assay coupled with RT-QuIC (OSCAR) permits sensitive, specific and quantitative detection of prions from an infectious slice culture model on a reduced time scale. We demonstrate that the anti-prion activity of test compounds can be readily resolved based on the power and kinetics of seeding activity in the OSCAR screening platform and that the prions generated in slice cultures are biologically active. Collectively, our results imply that OSCAR is a robust model of prion diseases that offers a promising platform for understanding prion proteinopathies and advancing anti-prion therapeutics.

Prion disease: experimental models and reality

The understanding of the pathogenesis and mechanisms of diseases requires a multidisciplinary approach, involving clinical observation, correlation to pathological processes, and modelling of disease mechanisms. It is an inherent challenge, and arguably impossible to generate model systems that can faithfully recapitulate all aspects of human disease. It is, therefore, important to be aware of the potentials and also the limitations of specific model systems. Model systems are usually designed to recapitulate only specific aspects of the disease, such as a pathological phenotype, a pathomechanism, or to test a hypothesis. Here, we evaluate and discuss model systems that were generated to understand clinical, pathological, genetic, biochemical, and epidemiological aspects of prion diseases. Whilst clinical research and studies on human tissue are an essential component of prion research, much of the understanding of the mechanisms governing transmission, replication, and toxicity comes from in vitro and in vivo studies. As with other neurodegenerative diseases caused by protein misfolding, the pathogenesis of prion disease is complex, full of conundra and contradictions. We will give here a historical overview of the use of models of prion disease, how they have evolved alongside the scientific questions, and how advancements in technologies have pushed the boundaries of our understanding of prion biology.

Quantitative Real-Time Quaking-Induced Conversion Allows Monitoring of Disease-Modifying Therapy in the Urine of Prion-Infected Mice

Prion diseases are fatal neurodegenerative diseases characterized by accumulation of the pathogenic prion protein PrP in the brain. We established quantitative real-time quaking-induced conversion for the measurement of minute amounts of PrP in body fluids such as urine. Using this approach, we monitored the efficacy of antiprion therapy by quantifying the seeding activity of PrP from the brain and urine of mice after prion infection. We found that the aggregation inhibitor anle138b decreased the levels of PrP in the brain and urine. Importantly, variations of PrP levels in the urine closely corresponded to those in the brain. Our findings indicate that quantification of urinary PrP enables measurement of prion disease progression in body fluids and can substitute for immunodetection in brain tissue. We expect PrP quantification biologic fluids (such as urine and cerebrospinal fluid) with quantitative real-time quaking-induced conversion to emerge as a valuable noninvasive diagnostic tool for monitoring disease progression and the efficacy of therapeutic approaches in animal studies and human clinical trials of prion diseases. Moreover, highly sensitive methods for quantifying pathologic aggregate seeds might provide novel molecular biomarkers for other neurodegenerative diseases that may involve prion-like mechanisms (protein aggregation and spreading), such as Alzheimer disease and Parkinson disease.

Multitarget ligands and theranostics: sharpening the medicinal chemistry sword against prion diseases

Prion diseases (PrDs) are fatal neurodegenerative disorders, for which no effective therapeutic and diagnostic tools exist. The main pathogenic event has been identified as the misfolding of a disease-associated prion protein. Nevertheless, pathogenesis seems to involve an intricate array of concomitant processes. Thus, it may be unlikely that drugs acting on single targets can effectively control PrDs. In addition, diagnosis occurs late in the disease process, by which point it is difficult to determine a successful therapeutic intervention. In this context, multitarget ligands (MTLs) and theranostic ligands (TLs) emerge for their potential to effectively cure and diagnose PrDs. In this review, we discuss the medicinal chemistry challenges of identifying novel MTLs and TLs against PrDs, and envision their impact on prion drug discovery.

Prion diseases and adult neurogenesis: how do prions counteract the brain's endogenous repair machinery?

Scientific advances in stem cell biology and adult neurogenesis have raised the hope that neurodegenerative disorders could benefit from stem cell-based therapy. Adult neurogenesis might be part of the physiological regenerative process, however it might become impaired by the disease's mechanism and therefore contribute to neurodegeneration. In prion disorders this endogenous repair system has rarely been studied. Whether adult neurogenesis plays a role or not in brain repair or in the propagation of prion pathology remains unclear. We have recently investigated the status of adult neural stem cells isolated from prion-infected mice. We were able to show that neural stem cells accumulate and replicate prions thus resulting in an alteration of their neuronal destiny. We also reproduced these results in adult neural stem cells, which were infected in vitro. The fact that endogenous adult neurogenesis could be altered by the accumulation of misfolded prion protein represents another great challenge. Inhibiting prion propagation in these cells would thus help the endogenous neurogenesis to compensate for the injured neuronal system. Moreover, understanding the endogenous modulation of the neurogenesis system would help develop effective neural stem cell-based therapies.

Systemic delivery of siRNA down regulates brain prion protein and ameliorates neuropathology in prion disorder

One of the main challenges for neurodegenerative disorders that are principally incurable is the development of new therapeutic strategies, which raises important medical, scientific and societal issues. Creutzfeldt-Jakob diseases are rare neurodegenerative fatal disorders which today remain incurable. The objective of this study was to evaluate the efficacy of the down-regulation of the prion protein (PrP) expression using siRNA delivered by, a water-in-oil microemulsion, as a therapeutic candidate in a preclinical study. After 12 days rectal mucosa administration of Aonys/PrP-siRNA in mice, we observed a decrease of about 28% of the brain PrP^C level. The effect of Aonys/PrP-siRNA was then evaluated on prion infected mice. Several mice presented a delay in the incubation and survival time compared to the control groups and a significant impact was observed on astrocyte reaction and neuronal survival in the PrP-siRNA treated groups. These results suggest that a new therapeutic scheme based an innovative delivery system of PrP-siRNA can be envisioned in prion disorders.

Prion replication occurs in endogenous adult neural stem cells and alters their neuronal fate: involvement of endogenous neural stem cells in prion diseases

Prion diseases are irreversible progressive neurodegenerative diseases, leading to severe incapacity and death. They are characterized in the brain by prion amyloid deposits, vacuolisation, astrocytosis, neuronal degeneration, and by cognitive, behavioural and physical impairments. There is no treatment for these disorders and stem cell therapy therefore represents an interesting new approach. Gains could not only result from the cell transplantation, but also from the stimulation of endogenous neural stem cells (NSC) or by the combination of both approaches. However, the development of such strategies requires a detailed knowledge of the pathology, particularly concerning the status of the adult neurogenesis and endogenous NSC during the development of the disease. During the past decade, several studies have consistently shown that NSC reside in the adult mammalian central nervous system (CNS) and that adult neurogenesis occurs throughout the adulthood in the subventricular zone of the lateral ventricle or the Dentate Gyrus of the hippocampus. Adult NSC are believed to constitute a reservoir for neuronal replacement during normal cell turnover or after brain injury. However, the activation of this system does not fully compensate the neuronal loss that occurs during neurodegenerative diseases and could even contribute to the disease progression. We investigated here the status of these cells during the development of prion disorders. We were able to show that NSC accumulate and replicate prions. Importantly, this resulted in the alteration of their neuronal fate which then represents a new pathologic event that might underlie the rapid progression of the disease.

Utilizing NMR and EPR spectroscopy to probe the role of copper in prion diseases

Copper is an essential nutrient for the normal development of the brain and nervous system, although the hallmark of several neurological diseases is a change in copper concentrations in the brain and central nervous system. Prion protein (PrP) is a copper-binding, cell-surface glycoprotein that exists in two alternatively folded conformations: a normal isoform (PrP(C)) and a disease-associated isoform (PrP(Sc)). Prion diseases are a group of lethal neurodegenerative disorders that develop as a result of conformational conversion of PrP(C) into PrP(Sc). The pathogenic mechanism that triggers this conformational transformation with the subsequent development of prion diseases remains unclear. It has, however, been shown repeatedly that copper plays a significant functional role in the conformational conversion of prion proteins. In this review, we focus on current research that seeks to clarify the conformational changes associated with prion diseases and the role of copper in this mechanism, with emphasis on the latest applications of NMR and EPR spectroscopy to probe the interactions of copper with prion proteins.

Hereditary form of prion disease in Poland

BACKGROUND AND PURPOSE

The aim of the study was to perform molecular analysis in a group of patients affected with prion disease. Diagnosis was based on results of clinical and/or histopathological examination of the brain. This is the largest investigation of this type performed so far in Poland.

MATERIAL AND METHODS

Analysed material contained 36 cases of prion disease, including 35 cases of Creutzfeldt-Jakob disease and one case of Gerstmann-Sträussler-Scheinker disease, as well as two familial cases initially suspected of Huntington disease and Alzheimer disease. The control group consisted of 87 subjects. The most frequent known mutations in the PRNP gene were looked for, namely those in codons 102, 117, 178, 200, 217 and OPRI; the polymorphism Met/Val in codon 129 was also analysed. The methods applied were PCR-RFLP and DNA sequencing.

RESULTS

The following mutations were found: E200K in 5 families, P102L in one family (previously identified), D178N in one family and 6OPRI in one family. Overall, mutations were detected in 17 persons (including 8 preclinical ones) from 8 pedigrees. Highly significant difference of codon 129 Met/Val heterozygosity frequencies was found between the affected subjects and the controls. Frequency of the familial form of prion disease in the material analysed was 14%.

CONCLUSIONS

Screening for mutations in the PRNP gene should be performed in all diagnosed cases of prion disease and in cases of familial occurrence of early onset dementia of unknown aetiology. Families with identified mutations should be offered genetic counselling and informed of risks of blood and organs' donation.

Anti-prion activity of an RNA aptamer and its structural basis

Prion proteins (PrPs) cause prion diseases, such as bovine spongiform encephalopathy. The conversion of a normal cellular form (PrP^C) of PrP into an abnormal form (PrP^Sc) is thought to be associated with the pathogenesis. An RNA aptamer that tightly binds to and stabilizes PrP^C is expected to block this conversion and to thereby prevent prion diseases. Here, we show that an RNA aptamer comprising only 12 residues, r(GGAGGAGGAGGA) (R12), reduces the PrP^Sc level in mouse neuronal cells persistently infected with the transmissible spongiform encephalopathy agent. Nuclear magnetic resonance analysis revealed that R12, folded into a unique quadruplex structure, forms a dimer and that each monomer simultaneously binds to two portions of the N-terminal half of PrP^C, resulting in tight binding. Electrostatic and stacking interactions contribute to the affinity of each portion. Our results demonstrate the therapeutic potential of an RNA aptamer as to prion diseases.

Kisspeptin prevention of amyloid-β peptide neurotoxicity in vitro

Alzheimer's disease (AD) onset is associated with changes in hypothalamic-pituitary-gonadal (HPG) function. The 54 amino acid kisspeptin (KP) peptide regulates the HPG axis and alters antioxidant enzyme expression. The Alzheimer's amyloid-β (Aβ) is neurotoxic, and this action can be prevented by the antioxidant enzyme catalase. Here, we examined the effects of KP peptides on the neurotoxicity of Aβ, prion protein (PrP), and amylin (IAPP) peptides. The Aβ, PrP, and IAPP peptides stimulated the release of KP and KP 45-54. The KP peptides inhibited the neurotoxicity of Aβ, PrP, and IAPP peptides, via an action that could not be blocked by kisspeptin-receptor (GPR-54) or neuropeptide FF (NPFF) receptor antagonists. Knockdown of KiSS-1 gene, which encodes the KP peptides, in human neuronal SH-SY5Y cells with siRNA enhanced the toxicity of amyloid peptides, while KiSS-1 overexpression was neuroprotective. A comparison of the catalase and KP sequences identified a similarity between KP residues 42-51 and the region of catalase that binds Aβ. The KP peptides containing residues 45-50 bound Aβ, PrP, and IAPP, inhibited Congo red binding, and were neuroprotective. These results suggest that KP peptides are neuroprotective against Aβ, IAPP, and PrP peptides via a receptor independent action involving direct binding to the amyloid peptides.

Anti-prion activities and drug-like potential of functionalized quinacrine analogs with basic phenyl residues at the 9-amino position

In this paper, we report the synthesis and cell-based anti-prion activity of quinacrine analogs derived by replacing the basic alkyl side chain of quinacrine with 4-(4-methylpiperazin-I-yl)phenyl, (1-benzylpiperidin-4-yl) and their structural variants. Several promising analogs were found that have a more favorable anti-prion profile than quinacrine in terms of potency and activity across different prion-infected murine cell models. They also exhibited greater binding affinities for a human prion protein fragment (hPrP(121-231)) than quinacrine, and had permeabilities on the PAMPA-BBB assay that fall within the range of CNS permeant candidates. When evaluated on bidirectional assays on a Pgp overexpressing cell line, one analog was less susceptible to Pgp efflux activity compared to quinacrine. Taken together, the results point to an important role for the substituted 9-amino side chain attached to the acridine, tetrahydroacridine and quinoline scaffolds. The nature of this side chain influenced cell-based potency, PAMPA permeability and binding affinity to hPrP(121-231).

Establishment and characterization of Prnp knockdown neuroblastoma cells using dual microRNA-mediated RNA interference

Prion diseases are fatal transmissible neurodegenerative disorders. In the pathogenesis of the disease, the cellular prion protein (PrPC) is required for replication of abnormal prion (PrPSc), which results in accumulation of PrPSc. Although there have been extensive studies using Prnp knockout systems, the normal function of PrPC remains ambiguous. Compared with conventional germline knockout technologies and transient naked siRNA-dependent knockdown systems, newly constructed durable chained-miRNA could provide a cell culture model that is closer to the disease status and easier to achieve with no detrimental sequelae. The selective silencing of a target gene by RNA interference (RNAi) is a powerful approach to investigate the unknown function of genes in vitro and in vivo. To reduce PrPC expression, a novel dual targeting-microRNA (miRdual) was constructed. The miRdual, which targets N- and C- termini of Prnp simultaneously, more effectively suppressed PrPC expression compared with conventional single site targeting. Furthermore, to investigate the cellular change following PrPC depletion, gene expression analysis of PrPC interacting and/or associating genes and several assays including proliferation, viability and apoptosis were performed. The transcripts 670460F02Rik and Plk3, Ppp2r2b and Csnk2a1 increase in abundance and are reported to be involved in cell proliferation and mitochondrial-mediated apoptosis. Dual-targeting RNAi with miRdual against Prnp will be useful for analyzing the physiological function of PrPC in neuronal cell lines and may provide a potential therapeutic intervention for prion diseases in the future.

Review: contribution of transgenic models to understanding human prion disease

Transgenic mice expressing human prion protein in the absence of endogenous mouse prion protein faithfully replicate human prions. These models reproduce all of the key features of human disease, including long clinically silent incubation periods prior to fatal neurodegeneration with neuropathological phenotypes that mirror human prion strain diversity. Critical contributions to our understanding of human prion disease pathogenesis and aetiology have only been possible through the use of transgenic mice. These models have provided the basis for the conformational selection model of prion transmission barriers and have causally linked bovine spongiform encephalopathy with variant Creutzfeldt-Jakob disease. In the future these models will be essential for evaluating newly identified potentially zoonotic prion strains, for validating effective methods of prion decontamination and for developing effective therapeutic treatments for human prion disease.

Threshold for epileptiform activity is elevated in prion knockout mice

Prion protein (PrP) is abundant in the nervous system, but its role remains uncertain. Prion diseases depend on an aggregation of the protein that is likely to interfere with its normal function. Loss of function does not in itself cause neurodegeneration, but whether it contributes to the clinical features of the disease remains an open question. Patients with classical Creutzfeldt-Jakob disease (CJD) have a higher than expected incidence of epilepsy. To study the mechanisms by which loss of PrP function may underlie changes in vulnerability to epilepsy in disease, we used several acute epilepsy models: we applied a variety of convulsant treatments (zero-magnesium, bicuculline, and pentylenetetrazol) to slices in vitro from PrP knockout (Prnp0/0) and control mice. In all three epilepsy models, we found that longer delays and/or higher concentrations of convulsants were necessary to generate spontaneous epileptiform activity in Prnp0/0 mice. These results together indicate an increased seizure threshold in Prnp0/0 mice, suggesting that loss of PrP function cannot explain a predisposition to seizures initiation in CJD.

Pharmacological chaperone for the structured domain of human prion protein

In prion diseases, the misfolded protein aggregates are derived from cellular prion protein (PrP(C)). Numerous ligands have been reported to bind to human PrP(C) (huPrP), but none to the structured region with the affinity required for a pharmacological chaperone. Using equilibrium dialysis, we screened molecules previously suggested to interact with PrP to discriminate between those which did not interact with PrP, behaved as nonspecific polyionic aggregates or formed a genuine interaction. Those that bind could potentially act as pharmacological chaperones. Here we report that a cationic tetrapyrrole [Fe(III)-TMPyP], which displays potent antiprion activity, binds to the structured region of huPrP. Using a battery of biophysical techniques, we demonstrate that Fe(III)-TMPyP forms a 11 complex via the structured C terminus of huPrP with a K(d) of 4.5 ± 2 μM, which is in the range of its IC(50) for curing prion-infected cells of 1.6 ± 0.4 μM and the concentration required to inhibit protein-misfolding cyclic amplification. Therefore, this molecule tests the hypothesis that stabilization of huPrP(C), as a principle, could be used in the treatment of human prion disease. The identification of a binding site with a defined 3D structure opens up the possibility of designing small molecules that stabilize huPrP and prevent its conversion into the disease-associated form.

Prion diseases

Prion diseases are disorders affecting the central nervous system caused by alterations in the conformation of the cellular prion protein. They can be sporadic, hereditary, or acquired and usually present with myoclonus and rapidly progressive dementia in human patients. This article discusses the epidemiology, pathogenesis, diagnosis, and laboratory testing of prion diseases with a primary focus on Creutzfeldt-Jakob disease.

Superoxide dismutase 1 and tgSOD1 mouse spinal cord seed fibrils, suggesting a propagative cell death mechanism in amyotrophic lateral sclerosis

Background

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that specifically affects motor neurons and leads to a progressive and ultimately fatal loss of function, resulting in death typically within 3 to 5 years of diagnosis. The disease starts with a focal centre of weakness, such as one limb, and appears to spread to other parts of the body. Mutations in superoxide dismutase 1 (SOD1) are known to cause disease and it is generally accepted they lead to pathology not by loss of enzymatic activity but by gain of some unknown toxic function(s). Although different mutations lead to varying tendencies of SOD1 to aggregate, we suggest abnormal proteins share a common misfolding pathway that leads to the formation of amyloid fibrils.

Methodology/Principal Findings

Here we demonstrate that misfolding of superoxide dismutase 1 leads to the formation of amyloid fibrils associated with seeding activity, which can accelerate the formation of new fibrils in an autocatalytic cascade. The time limiting event is nucleation to form a stable protein “seed” before a rapid linear polymerisation results in amyloid fibrils analogous to other protein misfolding disorders. This phenomenon was not confined to fibrils of recombinant protein as here we show, for the first time, that spinal cord homogenates obtained from a transgenic mouse model that overexpresses mutant human superoxide dismutase 1 (the TgSOD1^G93A mouse) also contain amyloid seeds that accelerate the formation of new fibrils in both wildtype and mutant SOD1 protein in vitro.

Conclusions/Significance

These findings provide new insights into ALS disease mechanism and in particular a mechanism that could account for the spread of pathology throughout the nervous system. This model of disease spread, which has analogies to other protein misfolding disorders such as prion disease, also suggests it may be possible to design assays for therapeutics that can inhibit fibril propagation and hence, possibly, disease progression.

The physical relationship between infectivity and prion protein aggregates is strain-dependent

Prions are unconventional infectious agents thought to be primarily composed of PrP(Sc), a multimeric misfolded conformer of the ubiquitously expressed host-encoded prion protein (PrP(C)). They cause fatal neurodegenerative diseases in both animals and humans. The disease phenotype is not uniform within species, and stable, self-propagating variations in PrP(Sc) conformation could encode this 'strain' diversity. However, much remains to be learned about the physical relationship between the infectious agent and PrP(Sc) aggregation state, and how this varies according to the strain. We applied a sedimentation velocity technique to a panel of natural, biologically cloned strains obtained by propagation of classical and atypical sheep scrapie and BSE infectious sources in transgenic mice expressing ovine PrP. Detergent-solubilized, infected brain homogenates were used as starting material. Solubilization conditions were optimized to separate PrP(Sc) aggregates from PrP(C). The distribution of PrP(Sc) and infectivity in the gradient was determined by immunoblotting and mouse bioassay, respectively. As a general feature, a major proteinase K-resistant PrP(Sc) peak was observed in the middle part of the gradient. This population approximately corresponds to multimers of 12-30 PrP molecules, if constituted of PrP only. For two strains, infectivity peaked in a markedly different region of the gradient. This most infectious component sedimented very slowly, suggesting small size oligomers and/or low density PrP(Sc) aggregates. Extending this study to hamster prions passaged in hamster PrP transgenic mice revealed that the highly infectious, slowly sedimenting particles could be a feature of strains able to induce a rapidly lethal disease. Our findings suggest that prion infectious particles are subjected to marked strain-dependent variations, which in turn could influence the strain biological phenotype, in particular the replication dynamics.

Safety and efficacy of quinacrine in human prion disease (PRION-1 study): a patient-preference trial

BACKGROUND

The propagation of prions, the causative agents of Creutzfeldt-Jakob disease and other human prion diseases, requires post-translational conversion of normal cellular prion protein to disease-associated forms. The antimalarial drug quinacrine (mepacrine) prevents this conversion in vitro, and was given to patients with various prion diseases to assess its safety and efficacy in changing the course of these invariably fatal and untreatable diseases.

METHODS

Patients with prion disease were recruited via the UK national referral system and were offered a choice between quinacrine (300 mg daily), no quinacrine, or randomisation to immediate quinacrine or deferred quinacrine in an open-label, patient-preference trial. The primary endpoints were death and serious adverse events possibly or probably related to the study drug. This study is registered, ISRCTN 06722585.

FINDINGS

107 patients with prion disease (45 sporadic, two iatrogenic, 18 variant, and 42 inherited) were enrolled, 23 in a pilot study and 84 in the main study. Only two patients chose randomisation; 40 took quinacrine during follow-up (37 who chose it at enrollment). Choice of treatment was associated with disease severity, with those least and most severely affected more likely to choose not to receive quinacrine. 78 (73%) patients died: one randomly assigned to deferred treatment, 26 of 38 who chose immediate quinacrine, and 51 of 68 who chose no quinacrine. Although adjusted mortality was lower in those who chose to take quinacrine than in those who did not, this was due to confounding with disease severity, and there was no difference in mortality between groups after adjustment. Four of 40 patients who took quinacrine had a transient response on neurological rating scales. Only two of 14 reported serious adverse events were judged quinacrine-related.

INTERPRETATION

Quinacrine at a dose of 300 mg per day was reasonably tolerated but did not significantly affect the clinical course of prion diseases in this observational study.

Crystal structure of human prion protein bound to a therapeutic antibody

Prion infection is characterized by the conversion of host cellular prion protein (PrP(C)) into disease-related conformers (PrP(Sc)) and can be arrested in vivo by passive immunization with anti-PrP monoclonal antibodies. Here, we show that the ability of an antibody to cure prion-infected cells correlates with its binding affinity for PrP(C) rather than PrP(Sc). We have visualized this interaction at the molecular level by determining the crystal structure of human PrP bound to the Fab fragment of monoclonal antibody ICSM 18, which has the highest affinity for PrP(C) and the highest therapeutic potency in vitro and in vivo. In this crystal structure, human PrP is observed in its native PrP(C) conformation. Interactions between neighboring PrP molecules in the crystal structure are mediated by close homotypic contacts between residues at position 129 that lead to the formation of a 4-strand intermolecular beta-sheet. The importance of this residue in mediating protein-protein contact could explain the genetic susceptibility and prion strain selection determined by polymorphic residue 129 in human prion disease, one of the strongest common susceptibility polymorphisms known in any human disease.

Prions impair bioaminergic functions through serotonin- or catecholamine-derived neurotoxins in neuronal cells

The conversion of the cellular prion protein, PrP(C), to an abnormal isoform, PrP(Sc), is a central event leading to neurodegeneration in prion diseases. Deciphering the molecular and cellular changes imparted by PrP(Sc) accumulation remains an arduous task due to the small number of cell lines supporting prion replication. Here we introduce the 1C11 cell line as a new in vitro model to investigate prion pathogenesis. This cell line is a committed neuroectodermal progenitor able to differentiate into fully functional serotonergic or catecholaminergic neurons. 1C11 cells, which naturally express PrP(C) from the undifferentiated state, can be chronically infected with various prion strains. Prion infection does not promote any noticeable phenotypic change in the progenitor cells nor prevent the onset of the serotonergic and catecholaminergic differentiation programs. Pathogenic prions, however, deviate the overall neurotransmitter-metabolism in both pathways by decreasing bioamine synthesis, storage, and transport, and enhancing catabolism. Noteworthy, oxidized derivatives of both serotonin and catecholamines are selectively detected in the differentiated progenies of infected cells and contribute to irreversible impairment in bioamine synthesis. Finally, the level of PrP(Sc) accumulation, that of infectivity, and the extent of all prion-induced changes in infected cells appear to be correlated. The report of such specific effects of infection on neuronal functions provides a foundation for dissecting the events underlying loss of neuronal homeostasis in prion diseases.

Prion agent diversity and species barrier

Mammalian prions are the infectious agents responsible for transmissible spongiform encephalopathies (TSE), a group of fatal, neurodegenerative diseases, affecting both domestic animals and humans. The most widely accepted view to date is that these agents lack a nucleic acid genome and consist primarily of PrP(Sc), a misfolded, aggregated form of the host-encoded cellular prion protein (PrP(C)) that propagates by autocatalytic conversion and accumulates mainly in the brain. The BSE epizooty, allied with the emergence of its human counterpart, variant CJD, has focused much attention on two characteristics that prions share with conventional infectious agents. First, the existence of multiple prion strains that impose, after inoculation in the same host, specific and stable phenotypic traits such as incubation period, molecular pattern of PrP(Sc) and neuropathology. Prion strains are thought to be enciphered within distinct PrP(Sc) conformers. Second, a transmission barrier exists that restricts the propagation of prions between different species. Here we discuss the possible situations resulting from the confrontation between species barrier and prion strain diversity, the molecular mechanisms involved and the potential of interspecies transmission of animal prions, including recently discovered forms of TSE in ruminants.

Retrotranslocation of prion proteins from the endoplasmic reticulum by preventing GPI signal transamidation

Neurodegeneration in diseases caused by altered metabolism of mammalian prion protein (PrP) can be averted by reducing PrP expression. To identify novel pathways for PrP down-regulation, we analyzed cells that had adapted to the negative selection pressure of stable overexpression of a disease-causing PrP mutant. A mutant cell line was isolated that selectively and quantitatively routes wild-type and various mutant PrPs for ER retrotranslocation and proteasomal degradation. Biochemical analyses of the mutant cells revealed that a defect in glycosylphosphatidylinositol (GPI) anchor synthesis leads to an unprocessed GPI-anchoring signal sequence that directs both ER retention and efficient retrotranslocation of PrP. An unprocessed GPI signal was sufficient to impart ER retention, but not retrotranslocation, to a heterologous protein, revealing an unexpected role for the mature domain in the metabolism of misprocessed GPI-anchored proteins. Our results provide new insights into the quality control pathways for unprocessed GPI-anchored proteins and identify transamidation of the GPI signal sequence as a step in PrP biosynthesis that is absolutely required for its surface expression. As each GPI signal sequence is unique, these results also identify signal recognition by the GPI-transamidase as a potential step for selective small molecule perturbation of PrP expression.

Prnp knockdown in transgenic mice using RNA interference

RNA interference has become a widely used approach to perform gene knockdown experiments in cell cultures and more recently transgenic animals. A designed miRNA targeting the prion protein mRNA was built and expressed using the human PRNP promoter. Its efficiency was confirmed in transfected cells and it was used to generate several transgenic mouse lines. Although expressed at low levels, it was found to downregulate the endogenous mouse Prnp gene expression to an extent that appears to be directly related with the transgene expression level and that could reach up to 80% inhibition. This result highlights the potential and limitations of the RNA interference approach when applied to disease resistance.

Hsp104: a weapon to combat diverse neurodegenerative disorders

Many of the fatal neurodegenerative disorders that plague humankind, including Alzheimer's and Parkinson's disease, are connected with the misfolding of specific proteins into a surprisingly generic fibrous conformation termed amyloid. Prior to amyloid fiber assembly, many proteins populate a common oligomeric conformation, which may be severely cytotoxic. Therapeutic innovations are desperately sought to safely reverse this aberrant protein aggregation and return proteins to normal function. Whether mammalian cells possess any such endogenous activity remains unclear. By contrast, fungi, plants and bacteria all express Hsp104, a protein-remodeling factor, which synergizes with the Hsp70 chaperone system to resolve aggregated proteins and restore their functionality. Surprisingly, amyloids can also be adaptive. In yeast, Hsp104 directly regulates the amyloidogenesis of several prion proteins, which can confer selective advantages. Here, I review the modus operandi of Hsp104 and showcase efforts to unleash Hsp104 on the protein-misfolding events connected to disparate neurodegenerative amyloidoses.

Immunisation strategies against prion diseases: prime-boost immunisation with a PrP DNA vaccine containing foreign helper T-cell epitopes does not prevent mouse scrapie

Vaccination against prion diseases constitutes a promising approach for the treatment and prevention of the disease. Passive immunisation with antibodies binding to the cellular prion protein (PrP(C)) can protect against prion disease. However, immunotherapeutic strategies with active immunisation are limited due to the immune tolerance against the self-antigen. In order to develop an anti-prion vaccine, we designed a novel DNA fusion vaccine composed of mouse PrP and immune stimulatory helper T-cell epitopes of the tetanus toxin that have previously been reported to break tolerance to other self-antigens. This approach provoked a strong PrP(C)-specific humoral and cellular immune response in PrP null mice, but only low antibody titres were found in vaccinated wild-type mice. Furthermore, prime-boost immunisation with the DNA vaccine and recombinant PrP protein increased antibody titres in PrP null mice, but failed to protect wild-type mice from mouse scrapie.

Update on human prion disease

The recognition that variant Creutzfeldt-Jakob disease (vCJD) is caused by the same prion strain as bovine spongiform encephalopathy in cattle has dramatically highlighted the need for a precise understanding of the molecular biology of human prion diseases. Detailed clinical, pathological and molecular data from a large number of human prion disease patients indicate that phenotypic diversity in human prion disease relates in part to the propagation of disease-related PrP isoforms with distinct physicochemical properties. Incubation periods of prion infection in humans can exceed 50 years and therefore it will be some years before the extent of any human vCJD epidemic can be predicted with confidence.

Clinical presentation and pre-mortem diagnosis of variant Creutzfeldt-Jakob disease associated with blood transfusion: a case report

BACKGROUND

Concerns have been raised that variant Creutzfeldt-Jakob disease (vCJD) might be transmissible by blood transfusion. Two cases of prion infection in a group of known recipients of transfusion from donors who subsequently developed vCJD were identified post-mortem and reported in 2004. Another patient from this at-risk group developed neurological signs and was referred to the National Prion Clinic.

METHODS

The patient was admitted for investigation and details of blood transfusion history were obtained from the National Blood Service and Health Protection Agency; after diagnosis of vCJD, the patient was enrolled into the MRC PRION-1 trial. When the patient died, brain and tonsil tissue were obtained at autopsy and assessed for the presence of disease-related PrP by immunoblotting and immunohistochemistry.

FINDINGS

A clinical diagnosis of probable vCJD was made; tonsil biopsy was not done. The patient received experimental therapy with quinacrine, but deteriorated and died after a clinical course typical of vCJD. Autopsy confirmed the diagnosis and showed prion infection of the tonsils.

INTERPRETATION

This case of transfusion-associated vCJD infection, identified ante-mortem, is the third instance from a group of 23 known recipients who survived at least 5 years after receiving a transfusion from donors who subsequently developed vCJD. The risk to the remaining recipients of such transfusions is probably high, and these patients should be offered specialist follow-up and investigation. Tonsil biopsy will allow early and pre-symptomatic diagnosis in other iatrogenically exposed individuals at high risk, as in those with primary infection with bovine spongiform encephalopathy prions.

Current and future molecular diagnostics for prion diseases

It is now widely held that the infectious agents underlying the transmissible spongiform encephalopathies are prions, which are primarily composed of a misfolded, protease-resistant isoform of the host prion protein. Untreatable prion disorders include some human diseases, such as Creutzfeldt-Jakob disease, and diseases of economically important animals, such as bovine spongiform encephalopathy (cattle) and chronic wasting disease (deer and elk). Detection and diagnosis of prion disease (and presymptomatic incubation) is contingent upon developing novel assays, which exploit properties uniquely possessed by this misfolded protein complex, rather than targeting an agent-specific nucleic acid. This review highlights some of the conventional and disruptive technologies developed to respond to this challenge.

Prions and their partners in crime

Prions, the infectious agents of transmissible spongiform encephalopathies (TSEs), have defied full characterization for decades. The dogma has been that prions lack nucleic acids and are composed of a pathological, self-inducing form of the host's prion protein (PrP). Recent progress in propagating TSE infectivity in cell-free systems has effectively ruled out the involvement of foreign nucleic acids. However, host-derived nucleic acids or other non-PrP molecules seem to be crucial. Interactions between TSE-associated PrP and its normal counterpart are also pathologically important, so the physiological functions of normal PrP and how they might be corrupted by TSE infections have been the subject of recent research.

A yeast-based assay to isolate drugs active against mammalian prions

Recently, we have developed a yeast-based (Saccharomyces cerevisiae) assay to isolate drugs active against mammalian prions. The initial assumption was that mechanisms controlling prion appearance and/or propagation could be conserved from yeast to human, as it is the case for most of the major cell biology regulatory mechanisms. Indeed, the vast majority of drugs we isolated as active against both [PSI(+)] and [URE3] budding yeast prions turned out to be also active against mammalian prion in three different mammalian cell-based assays. These results strongly argue in favor of common prion controlling mechanisms conserved in eukaryotes, thus validating our yeast-based assay and also the use of budding yeast to identify antiprion compounds and to study the prion world.

Neural stem cell model for prion propagation

The study of prion transmission and targeting is a major scientific issue with important consequences for public health. Only a few cell culture systems that are able to convert the cellular isoform of the prion protein into the pathologic scrapie isoform of the prion protein (PrP(Sc)) have been described. We hypothesized that central nervous system neural stem cells (NSCs) could be the basis of a new cell culture model permissive to prion infection. Here, we report that monolayers of differentiated fetal NSCs and adult multipotent progenitor cells isolated from mice were able to propagate prions. We also demonstrated the large influence of neural cell fate on the production of PrP(Sc), allowing the molecular study of prion neuronal targeting in relation with strain differences. This new stem cell-based model, which is applicable to different species and to transgenic mice, will allow thoughtful investigations of the molecular basis of prion diseases, and will open new avenues for diagnostic and therapeutic research.

Prion disease genetics

Prion diseases have stimulated intense scientific scrutiny since it was proposed that the infectious agent was devoid of nucleic acid. Despite this finding, genetics has played a key role in understanding the pathobiology and clinical aspects of prion disease through the effects of a series of polymorphisms and mutations in the prion protein gene (PRNP). The advent of variant Creutzfeldt-Jakob disease has confirmed one of the most powerful human genetic susceptibility factors, as all tested patients have an identical genotype at polymorphic codon 129 of PRNP. This review will also consider the accrued reports of inherited prion disease and attempt a genotype-phenotype correlation. The prospects for detection of novel genetic susceptibility factors using mouse models and human genetic association studies will be explored.

Mefloquine, an antimalaria drug with antiprion activity in vitro, lacks activity in vivo

In view of the effectiveness of antimalaria drugs inhibiting abnormal protease-resistant prion protein (PrP-res) formation in scrapie agent-infected cells, we tested other antimalarial compounds for similar activity. Mefloquine (MF), a quinoline antimalaria drug, was the most active compound tested against RML and 22L mouse scrapie agent-infected cells, with 50% inhibitory concentrations of approximately 0.5 and approximately 1.2 microM, respectively. However, MF administered to mice did not delay the onset of intraperitoneally inoculated scrapie agent, the result previously observed with quinacrine. While most anti-scrapie agent compounds inhibit PrP-res formation in vitro, many PrP-res inhibitors have no activity in vivo. This underscores the importance of testing promising candidates in vivo.

Precise hit: adeno-associated virus in gene targeting

Vectors based on the adeno-associated virus (AAV) have attracted much attention as potent gene-delivery vehicles, mainly because of the persistence of this non-pathogenic virus in the host cell and its sustainable therapeutic gene expression. However, virus infection can be accompanied by potentially mutagenic random vector integration into the genome. A novel approach to AAV-mediated gene therapy based on gene targeting through homologous recombination allows efficient, high-fidelity, non-mutagenic gene repair in a host cell.