Recent advances in prion chemotherapeutics

Improvement of anti-prion efficacy with stearoxy conjugation of hydroxypropyl methylcellulose in prion-infected mice

Prion diseases are fatal transmissible neurodegenerative disorders. Among known anti-prions, hydroxypropyl methylcellulose compounds (HPMCs) are unique in their chemical structure and action. They have several excellent anti-prion properties but the effectiveness depends on the prion-infected mouse model. In the present study, we investigated the effects of stearoxy-modified HPMCs on prion-infected cells and mice. Stearoxy modification improved the anti-prion efficacy of HPMCs in prion-infected cells and significantly prolonged the incubation period in a lower HPMC-responding mouse model. However, stearoxy modification showed no improvement over nonmodified HPMCs in an HPMC-responding mouse model. These results offer a new line of inquiry for use with prion-infected mice that do not respond well to HPMCs.

Combination of Styrylbenzoazole Compound and Hydroxypropyl Methylcellulose Enhances Therapeutic Effect in Prion-Infected Mice

Prion diseases are fatal transmissible neurodegenerative disorders. Tremendous efforts have been made for prion diseases; however, no effective treatment is available. Several anti-prion compounds have a preference for which prion strains or prion-infected animal models to target. Styrylbenzoazole compound called cpd-B is effective in RML prion-infected mice but less so in 263K prion-infected mice, whereas hydroxypropyl methylcellulose is effective in 263K prion-infected mice but less so in RML prion-infected mice. In the present study, we developed a combination therapy of cpd-B and hydroxypropyl methylcellulose expecting synergistic effects in both RML prion-infected mice and 263K prion-infected mice. A single subcutaneous administration of this combination had substantially a synergistic effect in RML prion-infected mice but had no additive effect in 263K prion-infected mice. These results showed that the effect of cpd-B was enhanced by hydroxypropyl methylcellulose. The complementary nature of the two compounds in efficacy against prion strains, chemical properties, pharmacokinetics, and physical properties appears to have contributed to the effective combination therapy. Our results pave the way for the strategy of new anti-prion agents.

Transmission, Strain Diversity, and Zoonotic Potential of Chronic Wasting Disease

Chronic wasting disease (CWD) is a prion disease affecting several species of captive and free-ranging cervids. In the past few decades, CWD has been spreading uncontrollably, mostly in North America, resulting in a high increase of CWD incidence but also a substantially higher number of geographical regions affected. The massive increase in CWD poses risks at several levels, including contamination of the environment, transmission to animals cohabiting with cervids, and more importantly, a putative transmission to humans. In this review, I will describe the mechanisms and routes responsible for the efficient transmission of CWD, the strain diversity of natural CWD, its spillover and zoonotic potential and strategies to minimize the CWD threat.

Therapeutic development of polymers for prion disease

Prion diseases, also known as transmissible spongiform encephalopathies, are caused by the accumulation of abnormal isoforms of the prion protein (scrapie isoform of the prion protein, PrPSc) in the central nervous system. Many compounds with anti-prion activities have been found using in silico screening, in vitro models, persistently prion-infected cell models, and prion-infected rodent models. Some of these compounds include several types of polymers. Although the inhibition or removal of PrPSc production is the main target of therapy, the unique features of prions, namely protein aggregation and assembly accompanied by steric structural transformation, may require different strategies for the development of anti-prion drugs than those for conventional therapeutics targeting enzyme inhibition, agonist ligands, or modulation of signaling. In this paper, we first overview the history of the application of polymers to prion disease research. Next, we describe the characteristics of each type of polymer with anti-prion activity. Finally, we discuss the common features of these polymers. Although drug delivery of these polymers to the brain is a challenge, they are useful not only as leads for therapeutic drugs but also as tools to explore the structure of PrPSc and are indispensable for prion disease research.

Strain variation in treatment and prevention of human prion diseases

Transmissible spongiform encephalopathies or prion diseases describe a number of different human disorders that differ in their clinical phenotypes, which are nonetheless united by their transmissible nature and common pathology. Clinical variation in the absence of a conventional infectious agent is believed to be encoded by different conformations of the misfolded prion protein. This misfolded protein is the target of methods designed to prevent disease transmission in a surgical setting and reduction of the misfolded seed or preventing its continued propagation have been the focus of therapeutic strategies. It is therefore possible that strain variation may influence the efficacy of prevention and treatment approaches. Historically, an understanding of prion disease transmission and pathogenesis has been focused on research tools developed using agriculturally relevant strains of prion disease. However, an increased understanding of the molecular biology of human prion disorders has highlighted differences not only between different forms of the disease affecting humans and animals but also within diseases such as Creutzfeldt-Jakob Disease (CJD), which is represented by several sporadic CJD specific conformations and an additional conformation associated with variant CJD. In this chapter we will discuss whether prion strain variation can affect the efficacy of methods used to decontaminate prions and whether strain variation in pre-clinical models of prion disease can be used to identify therapeutic strategies that have the best possible chance of success in the clinic.

Interaction of Peptide Aptamers with Prion Protein Central Domain Promotes α-Cleavage of PrPC

Prion diseases are infectious and fatal neurodegenerative diseases affecting humans and animals. Transmission is possible within and between species with zoonotic potential. Currently, no prophylaxis or treatment exists. Prions are composed of the misfolded isoform PrP^Sc of the cellular prion protein PrP^C. Expression of PrP^C is a prerequisite for prion infection, and conformational conversion of PrP^C is induced upon its direct interaction with PrP^Sc. Inhibition of this interaction can abrogate prion propagation, and we have previously established peptide aptamers (PAs) binding to PrP^C as new anti-prion compounds. Here, we mapped the interaction site of PA8 in PrP and modeled the complex in silico to design targeted mutations in PA8 which presumably enhance binding properties. Using these PA8 variants, we could improve PA-mediated inhibition of PrP^Sc replication and de novo infection of neuronal cells. Furthermore, we demonstrate that binding of PA8 and its variants increases PrP^C α-cleavage and interferes with its internalization. This gives rise to high levels of the membrane-anchored PrP-C1 fragment, a transdominant negative inhibitor of prion replication. PA8 and its variants interact with PrP^C at its central and most highly conserved domain, a region which is crucial for prion conversion and facilitates toxic signaling of Aβ oligomers characteristic for Alzheimer's disease. Our strategy allows for the first time to induce α-cleavage, which occurs within this central domain, independent of targeting the responsible protease. Therefore, interaction of PAs with PrP^C and enhancement of α-cleavage represent mechanisms that can be beneficial for the treatment of prion and other neurodegenerative diseases.

A Promising Antiprion Trimethoxychalcone Binds to the Globular Domain of the Cellular Prion Protein and Changes Its Cellular Location

The search for antiprion compounds has been encouraged by the fact that transmissible spongiform encephalopathies (TSEs) share molecular mechanisms with more prevalent neurodegenerative pathologies, such as Parkinson's and Alzheimer's diseases. Cellular prion protein (PrP^C) conversion into protease-resistant forms (protease-resistant PrP [PrP^Res] or the scrapie form of PrP [PrP^Sc]) is a critical step in the development of TSEs and is thus one of the main targets in the screening for antiprion compounds. In this work, three trimethoxychalcones (compounds J1, J8, and J20) and one oxadiazole (compound Y17), previously identified in vitro to be potential antiprion compounds, were evaluated through different approaches in order to gain inferences about their mechanisms of action. None of them changed PrP^C mRNA levels in N2a cells, as shown by reverse transcription-quantitative real-time PCR. Among them, J8 and Y17 were effective in real-time quaking-induced conversion reactions using rodent recombinant PrP (rPrP) from residues 23 to 231 (rPrP^23-231) as the substrate and PrP^Sc seeds from hamster and human brain. However, when rPrP from residues 90 to 231 (rPrP^90-231), which lacks the N-terminal domain, was used as the substrate, only J8 remained effective, indicating that this region is important for Y17 activity, while J8 seems to interact with the PrP^C globular domain. J8 also reduced the fibrillation of mouse rPrP^23-231 seeded with in vitro-produced fibrils. Furthermore, most of the compounds decreased the amount of PrP^C on the N2a cell surface by trapping this protein in the endoplasmic reticulum. On the basis of these results, we hypothesize that J8, a nontoxic compound previously shown to be a promising antiprion agent, may act by different mechanisms, since its efficacy is attributable not only to PrP conversion inhibition but also to a reduction of the PrP^C content on the cell surface.

What Is Our Current Understanding of PrPSc-Associated Neurotoxicity and Its Molecular Underpinnings?

The prion diseases are a collection of fatal, transmissible neurodegenerative diseases that cause rapid onset dementia and ultimately death. Uniquely, the infectious agent is a misfolded form of the endogenous cellular prion protein, termed PrP^Sc. Despite the identity of the molecular agent remaining the same, PrP^Sc can cause a range of diseases with hereditary, spontaneous or iatrogenic aetiologies. However, the link between PrP^Sc and toxicity is complex, with subclinical cases of prion disease discovered, and prion neurodegeneration without obvious PrP^Sc deposition. The toxic mechanisms by which PrP^Sc causes the extensive neuropathology are still poorly understood, although recent advances are beginning to unravel the molecular underpinnings, including oxidative stress, disruption of proteostasis and induction of the unfolded protein response. This review will discuss the diseases caused by PrP^Sc toxicity, the nature of the toxicity of PrP^Sc, and our current understanding of the downstream toxic signaling events triggered by the presence of PrP^Sc.

Comparing the energy landscapes for native folding and aggregation of PrP

Protein sequences are evolved to encode generally one folded structure, out of a nearly infinite array of possible folds. Underlying this code is a funneled free energy landscape that guides folding to the native conformation. Protein misfolding and aggregation are also a manifestation of free-energy landscapes. The detailed mechanisms of these processes are poorly understood, but often involve rare, transient species and a variety of different pathways. The inherent complexity of misfolding has hampered efforts to measure aggregation pathways and the underlying energy landscape, especially using traditional methods where ensemble averaging obscures important rare and transient events. We recently studied the misfolding and aggregation of prion protein by examining 2 monomers tethered in close proximity as a dimer, showing how the steps leading to the formation of a stable aggregated state can be resolved in the single-molecule limit and the underlying energy landscape thereby reconstructed. This approach allows a more quantitative comparison of native folding versus misfolding, including fundamental differences in the dynamics for misfolding. By identifying key steps and interactions leading to misfolding, it should help to identify potential drug targets. Here we describe the importance of characterizing free-energy landscapes for aggregation and the challenges involved in doing so, and we discuss how single-molecule studies can help test proposed structural models for PrP aggregates.

Pyrene conjugation and spectroscopic analysis of hydroxypropyl methylcellulose compounds successfully demonstrated a local dielectric difference associated with in vivo anti-prion activity

Our previous study on prion-infected rodents revealed that hydroxypropyl methylcellulose compounds (HPMCs) with different molecular weights but similar composition and degree of substitution have different levels of long-lasting anti-prion activity. In this study, we searched these HPMCs for a parameter specifically associated with in vivo anti-prion activity by analyzing in vitro chemical properties and in vivo tissue distributions. Infrared spectroscopic and thermal analyses revealed no differences among HPMCs, whereas pyrene conjugation and spectroscopic analysis revealed that the fluorescence intensity ratio of peak III/peak I correlated with anti-prion activity. This correlation was more clearly demonstrated in the anti-prion activity of the 1-year pre-infection treatment than that of the immediate post-infection treatment. In addition, the intensity ratio of peak III/peak I negatively correlated with the macrophage uptake level of HPMCs in our previous study. However, the in vivo distribution pattern was apparently not associated with anti-prion activity and was different in the representative tissues. These findings suggest that pyrene conjugation and spectroscopic analysis are powerful methods to successfully demonstrate local dielectric differences in HPMCs and provide a feasible parameter denoting the long-lasting anti-prion activity of HPMCs in vivo.

An antipsychotic drug exerts anti-prion effects by altering the localization of the cellular prion protein

Prion diseases are neurodegenerative conditions characterized by the conformational conversion of the cellular prion protein (PrP^C), an endogenous membrane glycoprotein of uncertain function, into PrP^Sc, a pathological isoform that replicates by imposing its abnormal folding onto PrP^C molecules. A great deal of evidence supports the notion that PrP^C plays at least two roles in prion diseases, by acting as a substrate for PrP^Sc replication, and as a mediator of its toxicity. This conclusion was recently supported by data suggesting that PrP^C may transduce neurotoxic signals elicited by other disease-associated protein aggregates. Thus, PrP^C may represent a convenient pharmacological target for prion diseases, and possibly other neurodegenerative conditions. Here, we sought to characterize the activity of chlorpromazine (CPZ), an antipsychotic previously shown to inhibit prion replication by directly binding to PrP^C. By employing biochemical and biophysical techniques, we provide direct experimental evidence indicating that CPZ does not bind PrP^C at biologically relevant concentrations. Instead, the compound exerts anti-prion effects by inducing the relocalization of PrP^C from the plasma membrane. Consistent with these findings, CPZ also inhibits the cytotoxic effects delivered by a PrP mutant. Interestingly, we found that the different pharmacological effects of CPZ could be mimicked by two inhibitors of the GTPase activity of dynamins, a class of proteins involved in the scission of newly formed membrane vesicles, and recently reported as potential pharmacological targets of CPZ. Collectively, our results redefine the mechanism by which CPZ exerts anti-prion effects, and support a primary role for dynamins in the membrane recycling of PrP^C, as well as in the propagation of infectious prions.

Insights from Therapeutic Studies for PrP Prion Disease

Although an effective therapy for prion disease has not yet been established, many advances have been made toward understanding its pathogenesis, which has facilitated research into therapeutics for the disease. Several compounds, including flupirtine, quinacrine, pentosan polysulfate, and doxycycline, have recently been used on a trial basis for patients with prion disease. Concomitantly, several lead antiprion compounds, including compound B (compB), IND series, and anle138b, have been discovered. However, clinical trials are still far from yielding significantly beneficial results, and the findings of lead compound studies in animals have highlighted new challenges. These efforts have highlighted areas that need improvement or further exploration to achieve more effective therapies. In this work, we review recent advances in prion-related therapeutic research and discuss basic scientific issues to be resolved for meaningful medical intervention of prion disease.

Neuroradiology of human prion diseases, diagnosis and differential diagnosis

Human transmissible spongiform encephalopathies (TSEs), or prion diseases, are invariably fatal conditions associated with a range of clinical presentations. TSEs are classified as sporadic [e.g. sporadic Creutzfeldt-Jakob disease (sCJD), which is the most frequent form], genetic (e.g. Gerstmann-Straussler-Scheinker disease, fatal familial insomnia, and inherited CJD), and acquired or infectious (e.g. Kuru, iatrogenic CJD, and variant CJD). In the past, brain imaging played a supporting role in the diagnosis of TSEs, whereas nowadays magnetic resonance imaging (MRI) plays such a prominent role that MRI findings have been included in the diagnostic criteria for sCJD. Currently, MRI is required for all patients with a clinical suspicion of TSEs. Thus, MRI semeiotics of TSEs should become part of the cultural baggage of any radiologist. The purposes of this update on the neuroradiology of CJD are to (i) review the pathophysiology and clinical presentation of TSEs, (ii) describe both typical and atypical MRI findings of CJD, and (iii) illustrate diseases mimicking CJD, underlining the MRI key findings useful in the differential diagnosis.

A Single Subcutaneous Injection of Cellulose Ethers Administered Long before Infection Confers Sustained Protection against Prion Diseases in Rodents

Prion diseases are fatal, progressive, neurodegenerative diseases caused by prion accumulation in the brain and lymphoreticular system. Here we report that a single subcutaneous injection of cellulose ethers (CEs), which are commonly used as inactive ingredients in foods and pharmaceuticals, markedly prolonged the lives of mice and hamsters intracerebrally or intraperitoneally infected with the 263K hamster prion. CEs provided sustained protection even when a single injection was given as long as one year before infection. These effects were linked with persistent residues of CEs in various tissues. More effective CEs had less macrophage uptake ratios and hydrophobic modification of CEs abolished the effectiveness. CEs were significantly effective in other prion disease animal models; however, the effects were less remarkable than those observed in the 263K prion-infected animals. The genetic background of the animal model was suggested to influence the effects of CEs. CEs did not modify prion protein expression but inhibited abnormal prion protein formation in vitro and in prion-infected cells. Although the mechanism of CEs in vivo remains to be solved, these findings suggest that they aid in elucidating disease susceptibility and preventing prion diseases.

Prion diseases are progressive, fatal, neurodegenerative transmissible illnesses in humans and animals caused by prion accumulation in the brain and lymphoreticular system. Because they are prevalent in nature, with atypical forms continuing to emerge, prion diseases are potential threats to both public health and the economy. However, there are no effective methods to prevent these diseases. Here we report that cellulose ethers (CEs), which are non-digestible water-soluble polysaccharides that are commonly used as inactive ingredients in foods and pharmaceuticals, show prophylactic efficacy in prion-infected animals. CEs persist in various tissues and confer sustained preventive efficacy for years, suggesting that they help to prevent prion diseases. Although the enteral absorption of CEs is limited, we found that a portion of the absorbed CEs influences disease progression. Therefore, CEs may be useful to assess disease susceptibility and prevent disease occurrence.

Treatment of SMB-S15 Cells with Resveratrol Efficiently Removes the PrP(Sc) Accumulation In Vitro and Prion Infectivity In Vivo

Prion diseases are transmissible and invariably fatal neurodegenerative disorders, which still lack of efficacious prophylactic and therapeutic tools. Our previous study has proposed that the natural phytoalexin, resveratrol, can reduce the amounts of PrP(Sc) in a scrapie-infected cell line SMB-S15. To address its anti-prion efficacy, the inhibitive activity of resveratrol on prion accumulation in vitro and prion infectivity in vivo was analyzed in the present study. Exposure of SMB-S15 cells to various concentrations of resveratrol (0.25 to 200 μM) reduced and even removed cellular PrP(Sc) in a dose-dependent manner, with EC50 0.61 μM. Meanwhile, PrP(Sc) signals in SMB-S15 cells treated with 5 and 10 μM resveratrol maintained undetectable after drug withdrawal, indicating that the removal of PrP(Sc) in SMB-S15 cells by resveratrol is irreversible. Furthermore, the lysates of SMB-S15 cells exposed to 10 μM resveratrol for 2 and 7 days were intracerebrally inoculated into CD1 mice. All mice (n = 9) infected with SMB-S15 cells without treatment of resveratrol appeared typical experimental scrapie symptoms from 155 to 228 day post inoculation (dpi), while all mice (n = 9) inoculated with SMB-S15 cells treated with resveratrol for 7 days maintained healthy by the end of observations (284 dpi). PrP-specific Western blots and neuropathological tests did not identify PrP(Sc) or prion disease-associated pathological abnormality in the brains of mice inoculated with 7-day resveratrol-treated SMB-S15 cells. It indicates that the prion infectivity of SMB-S15 onto CD1 mice is eradicated by 1-week resveratrol treatment. Sensitivity of PrP(Sc) to resveratrol highlights its potential role in prion therapeutics.

Characterization of four new monoclonal antibodies against the distal N-terminal region of PrP(c)

Prion diseases are a group of fatal neurodegenerative disorders that affect humans and animals. They are characterized by the accumulation in the central nervous system of a pathological form of the host-encoded prion protein (PrP^C). The prion protein is a membrane glycoprotein that consists of two domains: a globular, structured C-terminus and an unstructured N-terminus. The N-terminal part of the protein is involved in different functions in both health and disease. In the present work we discuss the production and biochemical characterization of a panel of four monoclonal antibodies (mAbs) against the distal N-terminus of PrP^C using a well-established methodology based on the immunization of Prnp^0/0 mice. Additionally, we show their ability to block prion (PrP^Sc) replication at nanomolar concentrations in a cell culture model of prion infection. These mAbs represent a promising tool for prion diagnostics and for studying the physiological role of the N-terminal domain of PrP^C.

Prion protein-coated magnetic beads: synthesis, characterization and development of a new ligands screening method

Prion diseases are characterized by protein aggregation and neurodegeneration. Conversion of the native prion protein (PrP(C)) into the abnormal scrapie PrP isoform (PrP(Sc)), which undergoes aggregation and can eventually form amyloid fibrils, is a critical step leading to the characteristic path morphological hallmark of these diseases. However, the mechanism of conversion remains unclear. It is known that ligands can act as cofactors or inhibitors in the conversion mechanism of PrP(C) into PrP(Sc). Within this context, herein, we describe the immobilization of PrP(C) onto the surface of magnetic beads and the morphological characterization of PrP(C)-coated beads by fluorescence confocal microscopy. PrP(C)-coated magnetic beads were used to identify ligands from a mixture of compounds, which were monitored by UHPLC-ESI-MS/MS. This affinity-based method allowed the isolation of the anti-prion compound quinacrine, an inhibitor of PrP aggregation. The results indicate that this approach can be applied to not only "fish" for anti-prion compounds from complex matrixes, but also to screening for and identify possible cellular cofactors involved in the deflagration of prion diseases.

Prion protein-specific antibodies-development, modes of action and therapeutics application

Prion diseases or Transmissible Spongiform Encephalopathies (TSEs) are lethal neurodegenerative disorders involving the misfolding of the host encoded cellular prion protein, PrP^C. This physiological form of the protein is expressed throughout the body, and it reaches the highest levels in the central nervous system where the pathology occurs. The conversion into the pathogenic isoform denoted as prion or PrP^Sc is the key event in prion disorders. Prominent candidates for the treatment of prion diseases are antibodies and their derivatives. Anti-PrP^C antibodies are able to clear PrP^Sc from cell culture of infected cells. Furthermore, application of anti-PrP^C antibodies suppresses prion replication in experimental animal models. Major drawbacks of immunotherapy are immune tolerance, the risks of neurotoxic side effects, limited ability of compounds to cross the blood-brain barrier and their unfavorable pharmacokinetic. The focus of this review is to recapitulate the current understanding of the molecular mechanisms for antibody mediated anti-prion activity. Although relevant for designing immunotherapeutic tools, the characterization of key antibody parameters shaping the molecular mechanism of the PrP^C to PrP^Sc conversion remains elusive. Moreover, this review illustrates the various attempts towards the development of anti-PrP antibody compounds and discusses therapeutic candidates that modulate PrP expression.

Comparison of the anti-prion mechanism of four different anti-prion compounds, anti-PrP monoclonal antibody 44B1, pentosan polysulfate, chlorpromazine, and U18666A, in prion-infected mouse neuroblastoma cells

Molecules that inhibit the formation of an abnormal isoform of prion protein (PrP^Sc) in prion-infected cells are candidate therapeutic agents for prion diseases. Understanding how these molecules inhibit PrP^Sc formation provides logical basis for proper evaluation of their therapeutic potential. In this study, we extensively analyzed the effects of the anti-PrP monoclonal antibody (mAb) 44B1, pentosan polysulfate (PPS), chlorpromazine (CPZ) and U18666A on the intracellular dynamics of a cellular isoform of prion protein (PrP^C) and PrP^Sc in prion-infected mouse neuroblastoma cells to re-evaluate the effects of those agents. MAb 44B1 and PPS rapidly reduced PrP^Sc levels without altering intracellular distribution of PrP^Sc. PPS did not change the distribution and levels of PrP^C, whereas mAb 44B1 appeared to inhibit the trafficking of cell surface PrP^C to organelles in the endocytic-recycling pathway that are thought to be one of the sites for PrP^Sc formation. In contrast, CPZ and U18666A initiated the redistribution of PrP^Sc from organelles in the endocytic-recycling pathway to late endosomes/lysosomes without apparent changes in the distribution of PrP^C. The inhibition of lysosomal function by monensin or bafilomycin A1 after the occurrence of PrP^Sc redistribution by CPZ or U18666A partly antagonized PrP^Sc degradation, suggesting that the transfer of PrP^Sc to late endosomes/lysosomes, possibly via alteration of the membrane trafficking machinery of cells, leads to PrP^Sc degradation. This study revealed that precise analysis of the intracellular dynamics of PrP^C and PrP^Sc provides important information for understanding the mechanism of anti-prion agents.

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.

Efficacy and mechanism of a glycoside compound inhibiting abnormal prion protein formation in prion-infected cells: implications of interferon and phosphodiesterase 4D-interacting protein

A new type of antiprion compound, Gly-9, was found to inhibit abnormal prion protein formation in prion-infected neuroblastoma cells, in a prion strain-independent manner, when the cells were treated for more than 1 day. It reduced the intracellular prion protein level and significantly modified mRNA expression levels of genes of two types: interferon-stimulated genes were downregulated after more than 2 days of treatment, and the phosphodiesterase 4D-interacting protein gene, a gene involved in microtubule growth, was upregulated after more than 1 day of treatment. A supplement of interferon given to the cells partly restored the abnormal prion protein level but did not alter the normal prion protein level. This interferon action was independent of the Janus activated kinase-signal transducer and activator of transcription signaling pathway. Therefore, the changes in interferon-stimulated genes might be a secondary effect of Gly-9 treatment. However, gene knockdown of phosphodiesterase 4D-interacting protein restored or increased both the abnormal prion protein level and the normal prion protein level, without transcriptional alteration of the prion protein gene. It also altered the localization of abnormal prion protein accumulation in the cells, indicating that phosphodiesterase 4D-interacting protein might affect prion protein levels by altering the trafficking of prion protein-containing structures. Interferon and phosphodiesterase 4D-interacting protein had no direct mutual link, demonstrating that they regulate abnormal prion protein levels independently. Although the in vivo efficacy of Gly-9 was limited, the findings for Gly-9 provide insights into the regulation of abnormal prion protein in cells and suggest new targets for antiprion compounds.

IMPORTANCE

This report describes our study of the efficacy and potential mechanism underlying the antiprion action of a new antiprion compound with a glycoside structure in prion-infected cells, as well as the efficacy of the compound in prion-infected animals. The study revealed involvements of two factors in the compound's mechanism of action: interferon and a microtubule nucleation activator, phosphodiesterase 4D-interacting protein. In particular, phosphodiesterase 4D-interacting protein was suggested to be important in regulating the trafficking or fusion of prion protein-containing vesicles or structures in cells. The findings of the study are expected to be useful not only for the elucidation of cellular regulatory mechanisms of prion protein but also for the implication of new targets for therapeutic development.

Antiprion compounds that reduce PrP(Sc) levels in dividing and stationary-phase cells

During prion diseases, a normally benign, host protein, denoted PrP(C), undergoes alternative folding into the aberrant isoform, PrP(Sc). We used ELISA to identify and confirm hits in order to develop leads that reduce PrP(Sc) in prion-infected dividing and stationary-phase mouse neuroblastoma (ScN2a-cl3) cells. We tested 52,830 diverse small molecules in dividing cells and 49,430 in stationary-phase cells. This led to 3100 HTS and 970 single point confirmed (SPC) hits in dividing cells, 331 HTS and 55 confirmed SPC hits in stationary-phase cells as well as 36 confirmed SPC hits active in both. Fourteen chemical leads were identified from confirmed SPC hits in dividing cells and three in stationary-phase cells. From more than 682 compounds tested in concentration-effect relationships in dividing cells to determine potency (EC50), 102 had EC50 values between 1 and 10 μM and 50 had EC50 values of <1 μM; none affected cell viability. We observed an excellent correlation between EC50 values determined by ELISA and Western immunoblotting for 28 representative compounds in dividing cells (R(2)=0.75; p <0.0001). Of the 55 confirmed SPC hits in stationary-phase cells, 23 were piperazine, indole, or urea leads. The EC50 values of one indole in stationary-phase and dividing ScN2a-cl3 cells were 7.5 and 1.6 μM, respectively. Unexpectedly, the number of hits in stationary-phase cells was ~10% of that in dividing cells. The explanation for this difference remains to be determined.

Therapies for human prion diseases

The pathological foundation of human prion diseases is a result of the conversion of the physiological form of prion protein (PrP(c)) to the pathological protease resistance form PrP(res). Most patients with prion disease have unknown reasons for this conversion and the subsequent development of a devastating neurodegenerative disorder. The conversion of PrP(c) to PrP(res), with resultant propagation and accumulation results in neuronal death and amyloidogenesis. However, with increasing understanding of neurodegenerative processes it appears that protein-misfolding and subsequent propagation of these rouge proteins, is a generic phenomenon shared with diseases caused by tau, α-synucleins and β-amyloid proteins. Consequently, effective anti-prion agents may have wider implications. A number of therapeutic approaches include polyanionic, polycyclic drugs such as pentosan polysulfate (PPS), which prevent the conversion of PrP(c) to PrP(res) and might also sequester and down-regulate PrP(res). Polyanionic compounds might also help to clear PrP(res). Treatments aimed at the laminin receptor, which is an important accessory molecule in the conversion of PrP(c) to PrP(res) - neuroprotection, immunotherapy, siRNA and antisense approaches have provided some experimental promise.

Laboratory activities involving transmissible spongiform encephalopathy causing agents: risk assessment and biosafety recommendations in Belgium

Since the appearance in 1986 of epidemic of bovine spongiform encephalopathy (BSE), a new form of neurological disease in cattle which also affected human beings, many diagnostic and research activities have been performed to develop detection and therapeutic tools. A lot of progress was made in better identifying, understanding and controlling the spread of the disease by appropriate monitoring and control programs in European countries. This paper reviews the recent knowledge on pathogenesis, transmission and persistence outside the host of prion, the causative agent of transmissible spongiform encephalopathies (TSE) in mammals with a particular focus on risk (re)assessment and management of biosafety measures to be implemented in diagnostic and research laboratories in Belgium. Also, in response to the need of an increasing number of European diagnostic laboratories stopping TSE diagnosis due to a decreasing number of TSE cases reported in the last years, decontamination procedures and a protocol for decommissioning TSE diagnostic laboratories is proposed.

Optimization of Arylamides as Novel, Potent and Brain-penetrant Antiprion Lead Compounds

The prion diseases caused by PrP^Sc, an alternatively folded form of the cellular prion protein (PrP^C), are rapidly progressive, fatal, and untreatable neurodegenerative syndromes. We employed HTS ELISA assays to identify compounds that lower the level of PrP^Sc in prion-infected mouse neuroblastoma (ScN2a-cl3) cells and identified a series of arylamides. SAR studies indicated that small amides with one aromatic, or heteroaromatic ring, on each side of the amide bond are of modest potency. Of note, benzamide (7), with an EC₅₀ of 2200 nM, was one of only a few arylamide hits with a piperazine group on its aniline moiety. The basic piperazine nitrogen can be protonated at physiologic pH, improving solubility, and therefore we wanted to exploit this feature in our search for a drug candidate. An SAR campaign resulted in several key analogs, including a set with biaryl groups introduced on the carbonyl side for improved potency. Several of these biaryl analogs have submicromolar potency, with the most potent analog 17 having an EC₅₀ = 22 nM. More importantly, 17 and several biarylamides (20, 24, 26, 27) were able to traverse the BBB and displayed excellent drug levels in the brains of mice following oral dosing. These biarylamides may represent good starting points for further lead optimization for the identification of potential drug candidates for the treatment of prion diseases.

Synthesis of an (11) C-labeled antiprion GN8 derivative and evaluation of its brain uptake by positron emission tomography

A radiolabeled PET! A (11) C-labeled derivative of N,N'-(methylenedi-4,1-phenylene)bis[2-(1-pyrrolidinyl) acetamide] (GN8), an antiprion agent currently under development, was synthesized by palladium-catalyzed rapid methylation of aryltributylstannane and assessed for brain penetration and organ distribution in rats by positron emission tomography (PET).

Pharmacokinetics and metabolism of 2-aminothiazoles with antiprion activity in mice

PURPOSE

To discover drugs lowering PrP(Sc) in prion-infected cultured neuronal cells that achieve high concentrations in brain to test in mouse models of prion disease and then treat people with these fatal diseases.

METHODS

We tested 2-AMT analogs for EC50 and PK after a 40 mg/kg single dose and 40-210 mg/kg/day doses for 3 days. We calculated plasma and brain AUC, ratio of AUC/EC50 after dosing. We reasoned that compounds with high AUC/EC50 ratios should be good candidates going forward.

RESULTS

We evaluated 27 2-AMTs in single-dose and 10 in 3-day PK studies, of which IND24 and IND81 were selected for testing in mouse models of prion disease. They had high concentrations in brain after oral dosing. Absolute bioavailability ranged from 27-40%. AUC/EC50 ratios after 3 days were >100 (total) and 48-113 (unbound). Stability in liver microsomes ranged from 30->60 min. Ring hydroxylated metabolites were observed in microsomes. Neither was a substrate for the MDR1 transporter.

CONCLUSIONS

IND24 and IND81 are active in vitro and show high AUC/EC50 ratios (total and unbound) in plasma and brain. These will be evaluated in mouse models of prion disease.

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.

Influence of Mabs on PrP(Sc) formation using in vitro and cell-free systems

PrP^Sc is believed to serve as a template for the conversion of PrP^C to the abnormal isoform. This process requires contact between the two proteins and implies that there may be critical contact sites that are important for conversion. We hypothesized that antibodies binding to either PrP^cor PrP^Sc would hinder or prevent the formation of the PrP^C–PrP^Sc complex and thus slow down or prevent the conversion process. Two systems were used to analyze the effect of different antibodies on PrP^Sc formation: (i) neuroblastoma cells persistently infected with the 22L mouse-adapted scrapie stain, and (ii) protein misfolding cyclic amplification (PMCA), which uses PrP^Sc as a template or seed, and a series of incubations and sonications, to convert PrP^C to PrP^Sc. The two systems yielded similar results, in most cases, and demonstrate that PrP-specific monoclonal antibodies (Mabs) vary in their ability to inhibit the PrP^C–PrP^Sc conversion process. Based on the numerous and varied Mabs analyzed, the inhibitory effect does not appear to be epitope specific, related to PrP^C conformation, or to cell membrane localization, but is influenced by the targeted PrP region (amino vs carboxy).

Synthesis of 9-substituted 2,3,4,9-tetrahydro-1H-carbazole derivatives and evaluation of their anti-prion activity in TSE-infected cells

2,3,4,9-Tetrahydro-9-[2-hydroxy-3-(1-piperidinyl)propyl]-6-methyl-1H-carbazol-1-one (GJP14) is a novel anti-prion compound that we previously discovered by in silico screening and cellular assay. In this study, a variety of GJP14 derivatives were prepared using pyrrole derivatives, (haloalkyl)oxiranes, and amines, and their anti-prion activity was evaluated in TSE-infected cells. It was found that the tricyclic aromatic ring, a hydroxy group at the 2-position and an amino group at the 3-position of the N-propyl group were the basic requirements for anti-prion activity. The derivatives bearing an N-ortho-halobenzyl group exhibited an improved activity, and the most potent derivative was 8 times as effective as the original lead compound, GJP14.

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).

Prions

The discovery of infectious proteins, denoted prions, was unexpected. After much debate over the chemical basis of heredity, resolution of this issue began with the discovery that DNA, not protein, from pneumococcus was capable of genetically transforming bacteria (Avery et al. 1944). Four decades later, the discovery that a protein could mimic viral and bacterial pathogens with respect to the transmission of some nervous system diseases (Prusiner 1982) met with great resistance. Overwhelming evidence now shows that Creutzfeldt-Jakob disease (CJD) and related disorders are caused by prions. The prion diseases are characterized by neurodegeneration and lethality. In mammals, prions reproduce by recruiting the normal, cellular isoform of the prion protein (PrP(C)) and stimulating its conversion into the disease-causing isoform (PrP(Sc)). PrP(C) and PrP(Sc) have distinct conformations: PrP(C) is rich in α-helical content and has little β-sheet structure, whereas PrP(Sc) has less α-helical content and is rich in β-sheet structure (Pan et al. 1993). The conformational conversion of PrP(C) to PrP(Sc) is the fundamental event underlying prion diseases. In this article, we provide an introduction to prions and the diseases they cause.

Calcineurin inhibition at the clinical phase of prion disease reduces neurodegeneration, improves behavioral alterations and increases animal survival

Prion diseases are fatal neurodegenerative disorders characterized by a long pre-symptomatic phase followed by rapid and progressive clinical phase. Although rare in humans, the unconventional infectious nature of the disease raises the potential for an epidemic. Unfortunately, no treatment is currently available. The hallmark event in prion diseases is the accumulation of a misfolded and infectious form of the prion protein (PrP(Sc)). Previous reports have shown that PrP(Sc) induces endoplasmic reticulum stress and changes in calcium homeostasis in the brain of affected individuals. In this study we show that the calcium-dependent phosphatase Calcineurin (CaN) is hyperactivated both in vitro and in vivo as a result of PrP(Sc) formation. CaN activation mediates prion-induced neurodegeneration, suggesting that inhibition of this phosphatase could be a target for therapy. To test this hypothesis, prion infected wild type mice were treated intra-peritoneally with the CaN inhibitor FK506 at the clinical phase of the disease. Treated animals exhibited reduced severity of the clinical abnormalities and increased survival time compared to vehicle treated controls. Treatment also led to a significant increase in the brain levels of the CaN downstream targets pCREB and pBAD, which paralleled the decrease of CaN activity. Importantly, we observed a lower degree of neurodegeneration in animals treated with the drug as revealed by a higher number of neurons and a lower quantity of degenerating nerve cells. These changes were not dependent on PrP(Sc) formation, since the protein accumulated in the brain to the same levels as in the untreated mice. Our findings contribute to an understanding of the mechanism of neurodegeneration in prion diseases and more importantly may provide a novel strategy for therapy that is beneficial at the clinical phase of the disease.

Prion protein interaction with stress-inducible protein 1 enhances neuronal protein synthesis via mTOR

Transmissible spongiform encephalopathies are fatal neurodegenerative diseases caused by the conversion of prion protein (PrP(C)) into an infectious isoform (PrP(Sc)). How this event leads to pathology is not fully understood. Here we demonstrate that protein synthesis in neurons is enhanced via PrP(C) interaction with stress-inducible protein 1 (STI1). We also show that neuroprotection and neuritogenesis mediated by PrP(C)-STI1 engagement are dependent upon the increased protein synthesis mediated by PI3K-mTOR signaling. Strikingly, the translational stimulation mediated by PrP(C)-STI1 binding is corrupted in neuronal cell lines persistently infected with PrP(Sc), as well as in primary cultured hippocampal neurons acutely exposed to PrP(Sc). Consistent with this, high levels of eukaryotic translation initiation factor 2alpha (eIF2alpha) phosphorylation were found in PrP(Sc)-infected cells and in neurons acutely exposed to PrP(Sc). These data indicate that modulation of protein synthesis is critical for PrP(C)-STI1 neurotrophic functions, and point to the impairment of this process during PrP(Sc) infection as a possible contributor to neurodegeneration.

[Innovation of therapeutics and prophylaxis for prion diseases]

There is no established treatment for prion diseases; however, recently several drug candidates, including pentosan polysulfate and doxycycline, have been clinically used on a trial basis to prevent accumulation of abnormal prion protein in the brain. So far the outcome of the trials is still very far from the goal where a complete cure of the diseases is expected. In order to bridge the gap between the reality and the ideal, the followings are suggested. First, combination therapy needs to be developed against multi-targets: inhibition of prion replication; degradation and scavengery of prion; inhibition of prion-related neurodegeneration. Secondly, preclinical diagnostic means, by which healthy prion-carriers can be revealed before the onset of the diseases, should be explored for earlier therapeutic interventions. The last is to disclose intrinsic disease susceptibility factors and environmental factors, both of which could solely or jointly facilitate in suppressing prion replication and disease progress. Exploitation of these items should be tough but will be deserved for overcoming the fatal diseases.