Cyclodextrins inhibit replication of scrapie prion protein in cell culture

Αnti-prion effects of anthocyanins

Prion diseases, also known as Transmissible Spongiform Encephalopathies (TSEs), are protein-based neurodegenerative disorders (NDs) affecting humans and animals. They are characterized by the conformational conversion of the normal cellular prion protein, PrPC, into the pathogenic isoform, PrPSc. Prion diseases are invariably fatal and despite ongoing research, no effective prophylactic or therapeutic avenues are currently available. Anthocyanins (ACNs) are unique flavonoid compounds and interest in their use as potential neuroprotective and/or therapeutic agents against NDs, has increased significantly in recent years. Therefore, we investigated the potential anti-oxidant and anti-prion effects of Oenin and Myrtillin, two of the most common anthocyanins, using the most accepted in the field overexpressing PrPScin vitro model and a cell free protein aggregation model. Our results, indicate both anthocyanins as strong anti-oxidant compounds, upregulating the expression of genes involved in the anti-oxidant response, and reducing the levels of Reactive Oxygen Species (ROS), produced due to pathogenic prion infection, through the activation of the Keap1-Nrf2 pathway. Importantly, they showcased remarkable anti-prion potential, as they not only caused the clearance of pathogenic PrPSc aggregates, but also completely inhibited the formation of PrPSc fibrils in the Cerebrospinal Fluid (CSF) of patients with Creutzfeldt-Jakob disease (CJD). Therefore, Oenin and Myrtillin possess pleiotropic effects, suggesting their potential use as promising preventive and/or therapeutic agents in prion diseases and possibly in the spectrum of neurodegenerative proteinopathies.

Prion therapeutics: Lessons from the past

Prion diseases are a group of incurable zoonotic neurodegenerative diseases (NDDs) in humans and other animals caused by the prion proteins. The abnormal folding and aggregation of the soluble cellular prion proteins (PrPC) into scrapie isoform (PrPSc) in the Central nervous system (CNS) resulted in brain damage and other neurological symptoms. Different therapeutic approaches, including stalling PrPC to PrPSc conversion, increasing PrPSc removal, and PrPC stabilization, for which a spectrum of compounds, ranging from organic compounds to antibodies, have been explored. Additionally, a non-PrP targeted drug strategy using serpin inhibitors has been discussed. Despite numerous scaffolds being screened for anti-prion activity in vitro, only a few were effective in vivo and unfortunately, almost none of them proved effective in the clinical studies, most likely due to toxicity and lack of permeability. Recently, encouraging results from a prion-protein monoclonal antibody, PRN100, were presented in the first human trial on CJD patients, which gives a hope for better future for the discovery of other new molecules to treat prion diseases. In this comprehensive review, we have re-visited the history and discussed various classes of anti-prion agents, their structure, mode of action, and toxicity. Understanding pathogenesis would be vital for developing future treatments for prion diseases. Based on the outcomes of existing therapies, new anti-prion agents could be identified/synthesized/designed with reduced toxicity and increased bioavailability, which could probably be effective in treating prion diseases.

Cyclodextrins: Only Pharmaceutical Excipients or Full-Fledged Drug Candidates?

Cyclodextrins, representing a versatile family of cyclic oligosaccharides, have extensive pharmaceutical applications due to their unique truncated cone-shaped structure with a hydrophilic outer surface and a hydrophobic cavity, which enables them to form non-covalent host-guest inclusion complexes in pharmaceutical formulations to enhance the solubility, stability and bioavailability of numerous drug molecules. As a result, cyclodextrins are mostly considered as inert carriers during their medical application, while their ability to interact not only with small molecules but also with lipids and proteins is largely neglected. By forming inclusion complexes with cholesterol, cyclodextrins deplete cholesterol from cellular membranes and thereby influence protein function indirectly through alterations in biophysical properties and lateral heterogeneity of bilayers. In this review, we summarize the general chemical principles of direct cyclodextrin-protein interactions and highlight, through relevant examples, how these interactions can modify protein functions in vivo, which, despite their huge potential, have been completely unexploited in therapy so far. Finally, we give a brief overview of disorders such as Niemann-Pick type C disease, atherosclerosis, Alzheimer's and Parkinson's disease, in which cyclodextrins already have or could have the potential to be active therapeutic agents due to their cholesterol-complexing or direct protein-targeting properties.

Cholesterol and its reciprocal association with prion infection

Prion diseases are incurable, infectious and fatal neurodegenerative diseases that affect both humans and animals. The pathogenesis of prion disease involves the misfolding of the cellular prion protein, PrP^C, to a disease-causing conformation, PrP^Sc, in the brain. The exact mechanism of conversion of PrP^C to PrP^Sc is not clear; however, there are numerous studies supporting that this process of misfolding requires the association of PrP^C with lipid raft domains of the plasma membrane. An increase in the cellular cholesterol content with prion infection has been observed in both in vivo and in vitro studies. As cholesterol is critical for the formation of lipid rafts, on the one hand, this increase may be related to, or aiding in, the process of prion conversion. On the other hand, increased cholesterol levels may affect neuronal viability. Here, we discuss current literature on the underlying mechanisms and potential consequences of elevated neuronal cholesterol in prion infection and advancements in prion disease therapeutics targeting brain cholesterol homeostasis.

Cyclodextrins Exert a Ligand-like Current Inhibitory Effect on the KV1.3 Ion Channel Independent of Membrane Cholesterol Extraction

Cyclodextrins (CDs) are cyclic oligosaccharides capable of forming water-soluble complexes with a variety of otherwise poorly soluble molecules including cholesterol and different drugs. Consistently, CDs are widely used in research and clinical practice to deplete cholesterol from cellular membranes or to increase solubility and bioavailability of different pharmaceuticals at local concentrations in the millimolar range. Effects of CDs exerted on cellular functions are generally thought to originate from reductions in cholesterol levels. Potential direct, ligand-like CD effects are largely neglected in spite of several recent studies reporting direct interaction between CDs and proteins including AMP-activated protein kinase, β-amyloid peptides, and α-synuclein. In this study, by using patch-clamp technique, time-resolved quantitation of cholesterol levels and biophysical parameters and applying cholesterol-extracting and non-cholesterol-extracting CDs at 1 and 5 mM concentrations, we provide evidence for a previously unexplored ligand-like, cholesterol-independent current inhibitory effect of CDs on K_V1.3, a prototypical voltage-gated potassium channel with pathophysiological relevance in various autoimmune and neurodegenerative disorders. Our findings propose that potential direct CD effects on K_V channels should be taken into consideration when interpreting functional consequences of CD treatments in both research and clinical practice. Furthermore, current-blocking effects of CDs on K_V channels at therapeutically relevant concentrations might contribute to additional beneficial or adverse effects during their therapeutic applications.

Volatile Anesthetic Sevoflurane Precursor 1,1,1,3,3,3-Hexafluoro-2-Propanol (HFIP) Exerts an Anti-Prion Activity in Prion-Infected Culture Cells

Prion disease is a neurodegenerative disorder with progressive neurologic symptoms and accelerated cognitive decline. The causative protein of prion disease is the prion protein (PrP), and structural transition of PrP from the normal helix rich form (PrP^C) to the abnormal β-sheet rich form (PrP^Sc) occurs in prion disease. While so far numerous therapeutic agents for prion diseases have been developed, none of them are still useful. A fluorinated alcohol, hexafluoro isopropanol (HFIP), is a precursor to the inhalational anesthetic sevoflurane and its metabolites. HFIP is also known as a robust α-helix inducer and is widely used as a solvent for highly aggregated peptides. Here we show that the α-helix-inducing activity of HFIP caused the conformational transformation of the fibrous structure of PrP into amorphous aggregates in vitro. HFIP added to the ScN2a cell medium, which continuously expresses PrP^Sc, reduced PrP^Sc protease resistance after 24-h incubation. It was also clarified that ScN2a cells are more susceptible to HFIP than any of the cells being compared. Based on these findings, HFIP is expected to develop as a therapeutic agent for prion disease.

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

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

Human cerebral organoids as a therapeutic drug screening model for Creutzfeldt-Jakob disease

Creutzfeldt-Jakob Disease (CJD) is a fatal, currently incurable, neurodegenerative disease. The search for candidate treatments would be greatly facilitated by the availability of human cell-based models of prion disease. Recently, an induced pluripotent stem cell derived human cerebral organoid model was shown to take up and propagate human CJD prions. This model offers new opportunities to screen drug candidates for the treatment of human prion diseases in an entirely human genetic background. Here we provide the first evidence that human cerebral organoids can be a viable model for CJD drug screening by using an established anti-prion compound, pentosan polysulfate (PPS). PPS delayed prion propagation in a prophylactic-like treatment paradigm and also alleviated propagation when applied following establishment of infection in a therapeutic-like treatment paradigm. This study demonstrates the utility of cerebral organoids as the first human 3D cell culture system for screening therapeutic drug candidates for human prion diseases.

Small Molecules with Anti-Prion Activity

Prion pathologies are fatal neurodegenerative diseases caused by the misfolding of the physiological Prion Protein (PrP^C) into a β-structure-rich isoform called PrP^Sc. To date, there is no available cure for prion diseases and just a few clinical trials have been carried out. The initial approach in the search of anti-prion agents had PrP^Sc as a target, but the existence of different prion strains arising from alternative conformations of PrP^Sc, limited the efficacy of the ligands to a straindependent ability. That has shifted research to PrP^C ligands, which either act as chaperones, by stabilizing the native conformation, or inhibit its interaction with PrP^Sc. The role of transition-metal mediated oxidation processes in prion misfolding has also been investigated. Another promising approach is the indirect action via other cellular targets, like membrane domains or the Protein- Folding Activity of Ribosomes (PFAR). Also, new prion-specific high throughput screening techniques have been developed. However, so far no substance has been found to be able to extend satisfactorily survival time in animal models of prion diseases. This review describes the main features of the Structure-Activity Relationship (SAR) of the various chemical classes of anti-prion agents.

Cholesterol Efflux-Independent Modification of Lipid Rafts by AIBP (Apolipoprotein A-I Binding Protein)

OBJECTIVE

AIBP (apolipoprotein A-I binding protein) is an effective and selective regulator of lipid rafts modulating many metabolic pathways originating from the rafts, including inflammation. The mechanism of action was suggested to involve stimulation by AIBP of cholesterol efflux, depleting rafts of cholesterol, which is essential for lipid raft integrity. Here we describe a different mechanism contributing to the regulation of lipid rafts by AIBP. Approach and Results: We demonstrate that modulation of rafts by AIBP may not exclusively depend on the rate of cholesterol efflux or presence of the key regulator of the efflux, ABCA1 (ATP-binding cassette transporter A-I). AIBP interacted with phosphatidylinositol 3-phosphate, which was associated with increased abundance and activation of Cdc42 and rearrangement of the actin cytoskeleton. Cytoskeleton rearrangement was accompanied with reduction of the abundance of lipid rafts, without significant changes in the lipid composition of the rafts. The interaction of AIBP with phosphatidylinositol 3-phosphate was blocked by AIBP substrate, NADPH (nicotinamide adenine dinucleotide phosphate), and both NADPH and silencing of Cdc42 interfered with the ability of AIBP to regulate lipid rafts and cholesterol efflux.

CONCLUSIONS

Our findings indicate that an underlying mechanism of regulation of lipid rafts by AIBP involves PIP-dependent rearrangement of the cytoskeleton.

Therapies for prion diseases

Recent advances in understanding of the molecular biology of prion diseases and improved clinical diagnostic techniques might allow researchers to think about therapeutic trials in Creutzfeldt-Jakob disease (CJD) patients. Some attempts have been made in the past and various compounds have been tested in single case reports and patient series. Controlled trials are rare. However, in the past few years, it has been demonstrated that clinical trials are feasible. The clinicians might face several specific problems when evaluating the efficacy of the drug in CJD, such as rareness of the disease, lack of appropriate preclinical tests and heterogeneous clinical presentation in humans. These problems have to be carefully addressed in future.

Cell Culture Methods for Screening of Prion Therapeutics

It is currently difficult to predict the number of asymptomatic prion carriers who will potentially go on to develop a prion disease or who will unknowingly transmit the prion agent to another individual. As prion therapeutic clinical trials have lacked success, there is a continuous need for novel therapeutics that have the potential to prevent, as for inherited prion disorders; slow, as for all prion disorders; and ultimately stop disease progression. Prion-infected cell models provide an ideal tool to search for new treatment avenues. This chapter describes the use of prion cell culture systems in the identification of prion therapeutics. It also deals with the methods required to validate the potential of an antiprion agent through cell viability and impact on cell growth rate.

Melanin or a Melanin-Like Substance Interacts with the N-Terminal Portion of Prion Protein and Inhibits Abnormal Prion Protein Formation in Prion-Infected Cells

Prion diseases are progressive fatal neurodegenerative illnesses caused by the accumulation of transmissible abnormal prion protein (PrP). To find treatments for prion diseases, we searched for substances from natural resources that inhibit abnormal PrP formation in prion-infected cells. We found that high-molecular-weight components from insect cuticle extracts reduced abnormal PrP levels. The chemical nature of these components was consistent with that of melanin. In fact, synthetic melanin produced from tyrosine or 3-hydroxy-l-tyrosine inhibited abnormal PrP formation. Melanin did not modify cellular or cell surface PrP levels, nor did it modify lipid raft or cellular cholesterol levels. Neither did it enhance autophagy or lysosomal function. Melanin was capable of interacting with PrP at two N-terminal domains. Specifically, it strongly interacted with the PrP region of amino acids 23 to 50 including a positively charged amino acid cluster and weakly interacted with the PrP octarepeat peptide region of residues 51 to 90. However, the in vitro and in vivo data were inconsistent with those of prion-infected cells. Abnormal PrP formation in protein misfolding cyclic amplification was not inhibited by melanin. Survival after prion infection was not significantly altered in albino mice or exogenously melanin-injected mice compared with that of control mice. These data suggest that melanin, a main determinant of skin color, is not likely to modify prion disease pathogenesis, even though racial differences in the incidence of human prion diseases have been reported. Thus, the findings identify an interaction between melanin and the N terminus of PrP, but the pathophysiological roles of the PrP-melanin interaction remain unclear.IMPORTANCE The N-terminal region of PrP is reportedly important for neuroprotection, neurotoxicity, and abnormal PrP formation, as this region is bound by many factors, such as metal ions, lipids, nucleic acids, antiprion compounds, and several proteins, including abnormal PrP in prion disease and the Aβ oligomer in Alzheimer's disease. In the present study, melanin, a main determinant of skin color, was newly found to interact with this N-terminal region and inhibits abnormal PrP formation in prion-infected cells. However, the data for prion infection in mice lacking melanin production suggest that melanin is not associated with the prion disease mechanism, although the incidence of prion disease is reportedly much higher in white people than in black people. Thus, the roles of the PrP-melanin interaction remain to be further elucidated, but melanin might be a useful competitive tool for evaluating the functions of other ligands at the N-terminal region.

Action-FRET of β-cyclodextrin inclusion complexes

Action-FRET is introduced as an original method to probe the structure of gaseous non-covalent complexes.

Cyclodextrins as Protective Agents of Protein Aggregation: An Overview

Cyclodextrins are extensively used in different fields (e.g., catalysis, chromatography, pharma, supramolecular chemistry, bioorganic chemistry, and bioinorganic chemistry), and their applications have been widely reviewed. Their main application in the field of pharmaceutical is as a drug carrier. This review overviews, for the first time, the use of cyclodextrins and their derivatives as antiaggregant agents in a number of proteins (e.g., amyloid-β, insulin, recombinant human growth hormone, prion protein, transthyretin, and α-synuclein) and some multimeric enzymes. There are many diseases that are correlated to protein misfolding and amyloid formation processes affecting numerous organs and tissues. There are over 30 different amyloid proteins and a number of corresponding diseases. Alzheimer's disease is the most common neurodegenerative disease. Treatment of these diseases is still a goal to reach, and many molecules are studied in this perspective. Cyclodextrins have also been studied, and they show great potential; as such, further studies could be very promising. This review aims to be a stimulus for the design of new cyclodextrin derivatives to obtain multifunctional systems with antiaggregant activity.

Proximity of SCG10 and prion protein in membrane rafts

The conversion of normal cellular prion protein (PrPC) into its pathogenic isoform (PrPSc) is an essential event in prion pathogenesis. In culture models, membrane rafts are suggested to play a critical role in PrPSc formation. To identify the candidate molecules capable of interacting with PrPC and facilitating PrPSc formation in membrane rafts, we applied a novel biochemical labeling method termed enzyme-mediated activation of radical sources. Enzyme-mediated activation of radical sources was applied to the Lubrol WX insoluble detergent-resistant membrane fractions from mouse neuroblastoma (N2a) cells in which the surface PrPC was labeled with HRP-conjugated anti-PrP antibody. Two-dimensional western blots of these preparations revealed biotinylated spots of approximately 20 kDa with an isoelectric point of 8.0-9.0. Liquid chromatography-tandem mass spectrometry analysis resulted in the identification of peptides containing SCG10, the neuron-specific microtubule regulator. Proximity of SCG10 and PrPC was confirmed using proximity ligation assay and co-immunoprecipitation assay. Transfection of persistently 22L prion-infected N2a cells with SCG10 small interfering RNA reduced SCG10 expression, but did not prevent PrPSc accumulation, indicating that SCG10 appears to be unrelated to PrPSc formation of 22L prion. Immunofluorescence and western blot analyses showed reduced levels of SCG10 in the hippocampus of prion-infected mice, suggesting a possible association between SCG10 levels and the prion neuropathogenesis. By applying a novel biochemical labeling method against detergent-resistant membrane fractions from mouse neuroblastoma cells, the neuron-specific microtubule-destabilization protein, SCG10 was identified as a novel candidate that is proximate to normal prion protein (PrP) in membrane rafts. SCG10 seemed unrelated to disease-related PrP formation under certain conditions, while there is a possible association between SCG10 levels and prion neuropathogenesis. Cover Image for this issue: doi: 10.1111/jnc.13310.

Discovery of Novel Anti-prion Compounds Using In Silico and In Vitro Approaches

Prion diseases are associated with the conformational conversion of the physiological form of cellular prion protein (PrP^C) to the pathogenic form, PrP^Sc. Compounds that inhibit this process by blocking conversion to the PrP^Sc could provide useful anti-prion therapies. However, no suitable drugs have been identified to date. To identify novel anti-prion compounds, we developed a combined structure- and ligand-based virtual screening system in silico. Virtual screening of a 700,000-compound database, followed by cluster analysis, identified 37 compounds with strong interactions with essential hotspot PrP residues identified in a previous study of PrP^C interaction with a known anti-prion compound (GN8). These compounds were tested in vitro using a multimer detection system, cell-based assays, and surface plasmon resonance. Some compounds effectively reduced PrP^Sc levels and one of these compounds also showed a high binding affinity for PrP^C. These results provide a promising starting point for the development of anti-prion compounds.

Structure-activity analysis and antiprion mechanism of isoprenoid compounds

The prion strain-specific mechanism by which normal prion protein is converted to abnormal prion protein remains largely unknown. This study found that insect juvenile hormone III reduced abnormal prion protein levels only in cells infected with the RML prion. We conducted a structure-activity analysis using juvenile hormone III biosynthetic intermediates in the isoprenoid pathway. Both farnesol and geranylgeraniol, the most potent inhibitors of abnormal prion protein formation, behaved in an RML prion-dependent fashion. Neither of them modified cellular and cell surface prion protein levels. Events downstream of this pathway include cholesterol biosynthesis and protein prenylation. However, neither of these isoprenoid compounds modified lipid raft microdomains and cellular cholesterol levels and neither affected the representative prenylated protein expression levels of prenylation pathways. Therefore, these isoprenoid compounds are a new class of prion strain-dependent antiprion compounds. They are useful for exploring strain-specific prion biology.

The effects of the cellular and infectious prion protein on the neuronal adaptor protein X11α

BACKGROUND

The neuronal adaptor protein X11α is a multidomain protein with a phosphotyrosine binding (PTB) domain, two PDZ (PSD_95, Drosophila disks-large, ZO-1) domains, a Munc Interacting (MI) domain and a CASK interacting region. Amongst its functions is a role in the regulation of the abnormal processing of the amyloid precursor protein (APP). It also regulates the activity of Cu/Zn Superoxide dismutase (SOD1) through binding with its chaperone the copper chaperone for SOD1. How X11α production is controlled has remained unclear.

METHODS

Using the neuroblastoma cell line, N2a, and knockdown studies, the effect of the cellular and infectious prion protein, PrP(C) and PrP(Sc), on X11α is examined.

RESULTS

We show that X11α expression is directly proportional to the expression of PrP(C), whereas its levels are reduced by PrP(Sc). We also show PrP(Sc) to affect X11α at a functional level. One of the effects of prion infection is lowered cellular SOD1 levels, here by knockdown of X11α we identify that the effect of PrP(Sc) on SOD1 can be reversed indicating that X11α is involved in prion disease pathogenesis.

CONCLUSIONS

A role for the cellular and infectious prion protein, PrP(C) and PrP(Sc), respectively, in regulating X11α is identified in this work.

GENERAL SIGNIFICANCE

Due to the multiple interacting partners of X11α, dysfunction or alteration in X11α will have a significant cellular effect. This work highlights the role of PrP(C) and PrP(Sc) in the regulation of X11α, and provides a new target pathway to control X11α and its related functions.

Cyclodextrins, blood-brain barrier, and treatment of neurological diseases

Biological barriers are the main defense systems of the homeostasis of the organism and protected organs. The blood-brain barrier (BBB), formed by the endothelial cells of brain capillaries, not only provides nutrients and protection to the central nervous system but also restricts the entry of drugs, emphasizing its importance in the treatment of neurological diseases. Cyclodextrins are increasingly used in human pharmacotherapy. Due to their favorable profile to form hydrophilic inclusion complexes with poorly soluble active pharmaceutical ingredients, they are present as excipients in many marketed drugs. Application of cyclodextrins is widespread in formulations for oral, parenteral, nasal, pulmonary, and skin delivery of drugs. Experimental and clinical data suggest that cyclodextrins can be used not only as excipients for centrally acting marketed drugs like antiepileptics, but also as active pharmaceutical ingredients to treat neurological diseases. Hydroxypropyl-β-cyclodextrin received orphan drug designation for the treatment of Niemann-Pick type C disease. In addition to this rare lysosomal storage disease with neurological symptoms, experimental research revealed the potential therapeutic use of cyclodextrins and cyclodextrin nanoparticles in neurodegenerative diseases, stroke, neuroinfections and brain tumors. In this context, the biological effects of cyclodextrins, their interaction with plasma membranes and extraction of different lipids are highly relevant at the level of the BBB.

Cholesterol balance in prion diseases and Alzheimer's disease

Prion diseases are transmissible and fatal neurodegenerative disorders of humans and animals. They are characterized by the accumulation of PrP^Sc, an aberrantly folded isoform of the cellular prion protein PrP^C, in the brains of affected individuals. PrP^C is a cell surface glycoprotein attached to the outer leaflet of the plasma membrane by a glycosyl-phosphatidyl-inositol (GPI) anchor. Specifically, it is associated with lipid rafts, membrane microdomains enriched in cholesterol and sphinoglipids. It has been established that inhibition of endogenous cholesterol synthesis disturbs lipid raft association of PrP^C and prevents PrP^Sc accumulation in neuronal cells. Additionally, prion conversion is reduced upon interference with cellular cholesterol uptake, endosomal export, or complexation at the plasma membrane. Altogether, these results demonstrate on the one hand the importance of cholesterol for prion propagation. On the other hand, growing evidence suggests that prion infection modulates neuronal cholesterol metabolism. Similar results were reported in Alzheimer’s disease (AD): whereas amyloid β peptide formation is influenced by cellular cholesterol, levels of cholesterol in the brains of affected individuals increase during the clinical course of the disease. In this review, we summarize commonalities of alterations in cholesterol homeostasis and discuss consequences for neuronal function and therapy of prion diseases and AD.

Interaction of pathogens with host cholesterol metabolism

PURPOSE OF REVIEW

Pathogens of different taxa, from prions to protozoa, target cellular cholesterol metabolism to advance their own development and to impair host immune responses, but also causing metabolic complications, for example, atherosclerosis. This review describes recent findings of how pathogens do it.

RECENT FINDINGS

A common theme in interaction between pathogens and host cholesterol metabolism is pathogens targeting lipid rafts of the host plasma membrane. Many intracellular pathogens use rafts as an entry gate, taking advantage of the endocytic machinery and high abundance of outward-looking molecules that can be used as receptors. At the same time, disruption of the rafts' functional capacity, achieved by the pathogens through a number of various means, impairs the ability of the host to generate immune response, thus helping pathogen to thrive. Pathogens cannot synthesize cholesterol, and salvaging host cholesterol helps pathogens build advanced cholesterol-containing membranes and assembly platforms. Impact on cholesterol metabolism is not limited to the infected cells; proteins and microRNAs secreted by infected cells affect lipid metabolism systemically.

SUMMARY

Given an essential role that host cholesterol metabolism plays in pathogen development, targeting this interaction may be a viable strategy to fight infections, as well as metabolic complications of the infections.

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.

Prion infection impairs cholesterol metabolism in neuronal cells

Conversion of prion protein (PrP(C)) into a pathological isoform (PrP(Sc)) during prion infection occurs in lipid rafts and is dependent on cholesterol. Here, we show that prion infection increases the abundance of cholesterol transporter, ATP-binding cassette transporter type A1 (ATP-binding cassette transporter type A1), but reduces cholesterol efflux from neuronal cells leading to the accumulation of cellular cholesterol. Increased abundance of ABCA1 in prion disease was confirmed in prion-infected mice. Mechanistically, conversion of PrP(C) to the pathological isoform led to PrP(Sc) accumulation in rafts, displacement of ABCA1 from rafts and the cell surface, and enhanced internalization of ABCA1. These effects were abolished with reversal of prion infection or by loading cells with cholesterol. Stimulation of ABCA1 expression with liver X receptor agonist or overexpression of heterologous ABCA1 reduced the conversion of prion protein into the pathological form upon infection. These findings demonstrate a reciprocal connection between prion infection and cellular cholesterol metabolism, which plays an important role in the pathogenesis of prion infection in neuronal cells.

A cell-biased effect of estrogen in prion infection

Prion disorders are associated with the accumulation of a misfolded form (PrP(Sc)) of the normal prion protein, PrP(C). Here, we show that estrogen acts as a regulator of the processes of both prion infection and prion maintenance. Estrogen was found to be cell biased in its effect; it protected cells against prion infection in a prevention mode and enabled prion maintenance in a treatment mode. These processes were regulated by the estrogen receptor subtypes Erα and Erβ. By using specific receptor agonists, Erα was found to be the main receptor active in slowing prion infection, whereas in chronically infected cells, although Erα allowed partial maintenance of PrP(Sc) levels, Erβ was the main receptor involved in maintaining PrP(Sc) in a treatment paradigm. A cell-biased effect of estrogen has been reported for other neurodegenerative disorders, including Alzheimer's disease. Estrogen's effect is dependent on the cell's health status, which impacts the use of estrogen. This work also identified that by targeting the estrogen receptors with the selective estrogen receptor modulators tamoxifen (Tam) and 4-hydroxy-tamoxifen (OHT), PrP(Sc) could be cleared from prion-infected cell culture. Tam and OHT had half-maximal inhibitory concentrations for clearance of PrP(Sc) of 0.47 μM and 0.14 nM, respectively. This work identifies further factors involved in the prion disease process, and through antagonism of the estrogen system, we demonstrate that the estrogen system is a target for controlling PrP(Sc) levels.

Inclusion complex of a Bcl-2 inhibitor with cyclodextrin: characterization, cellular accumulation, and in vivo antitumor activity

Small molecule inhibitors always exhibit poor water solubility due to the inherent hydrophobic property. It is an important challenge when they are developed as a real drug. S1, a structure-specific Bcl-2 inhibitor encountered this issue when moved forward in preclinical development. Herein, we prepared a 1:1 type of S1-gamma-cyclodextrin (S1-gamma-CD) inclusion complex to enhance the solubility. Bioevaluation of this new formulation was carried out totally in water solution. The cell internalization and cellular accumulation of S1-gamma-CD was illustrated by its fluorescence analogue S2. Disruption of Bcl-2/Bax heterodimerization in MCF-7 cells further revealed S1-gamma-CD could reach the subcellular function site. Moreover, the even stronger disruption by S1-gamma-CD than free S1 was found due to the higher local concentrations. Furthermore, the in vivo antitumor activity of S1-gamma-CD was evaluated in the H22 xenograft model. Results showed it exhibited significant antitumor activity with a decrease of tumor size (average tumor volume = 234 +/- 76 mm(3) vs control group, 398 +/- 121 mm(3), P < 0.01, and S1 group, 296 +/- 65 mm(3), P < 0.05), and a much longer survival time (the median time to the end point = 39.9 days vs control group, 29.2 days, P < 0.01). More importantly, the similar disruption of Bcl-2/Bax was found in S1-gamma-CD treated mice and free S1 treated ones. It demonstrated that S1-gamma-CD not only obtains a pharmaceutical level in vivo but also maintains the mechanism-based antitumor ability of S1 itself. It has been identified that cyclodextrin is appropriate to deliver a structure-specific molecule to its subcellular function site without any adverse effects on its mechanism-based potency, in either cultured cells or animals.

Chemical induction of misfolded prion protein conformers in cell culture

Prion-infected cells accumulate a heterogeneous population of aberrantly folded PrP conformers, including the disease-causing isoform (PrP(Sc)). We found that specific chemicals can modulate the levels of various PrP conformers in cultured cells. Positively charged polyamidoamines (dendrimers) eliminated protease-resistant (r) PrP(Sc) from prion-infected cells and induced the formation of insoluble, protease-sensitive PrP aggregates (designated PrP(A)). Larger, positively charged polyamidoamines more efficaciously induced the formation of PrP(A) and cleared rPrP(Sc), whereas negatively charged polyamidoamines neither induced PrP(A) nor cleared rPrP(Sc). Although the biochemical properties of PrP(A) were shown to be similar to protease-sensitive (s) PrP(Sc), bioassays of PrP(A) indicated that it is not infectious. Our studies argue that PrP(A) represents an aggregated PrP species that is off-pathway relative to the formation of rPrP(Sc). It remains to be established whether the formation of PrP(A) inhibits the formation of rPrP(Sc) by sequestering PrP(C) in the form of benign, insoluble aggregates.

Inhibition of cholesterol recycling impairs cellular PrP(Sc) propagation

The infectious agent in prion diseases consists of an aberrantly folded isoform of the cellular prion protein (PrP(c)), termed PrP(Sc), which accumulates in brains of affected individuals. Studies on prion-infected cultured cells indicate that cellular cholesterol homeostasis influences PrP(Sc) propagation. Here, we demonstrate that the cellular PrP(Sc) content decreases upon accumulation of cholesterol in late endosomes, as induced by NPC-1 knock-down or treatment with U18666A. PrP(c) trafficking, lipid raft association, and membrane turnover are not significantly altered by such treatments. Cellular PrP(Sc) formation is not impaired, suggesting that PrP(Sc) degradation is increased by intracellular cholesterol accumulation. Interestingly, PrP(Sc) propagation in U18666A-treated cells was partially restored by overexpression of rab 9, which causes redistribution of cholesterol and possibly of PrP(Sc) to the trans-Golgi network. Surprisingly, rab 9 overexpression itself reduced cellular PrP(Sc) content, indicating that PrP(Sc) production is highly sensitive to alterations in dynamics of vesicle trafficking.

Antiprion action of new cyclodextrin analogues

BACKGROUND

Prion disorders are characterised by the accumulation of a misfolded isoform (PrPSc) of the host encoded prion protein (PrPC). This paper examines the antiprion potential of cyclodextrin (CD) analogues and it identifies sulphated-beta-cyclodextrin, with a half-maximal inhibitory concentration (IC50) of 2.4 microM, as having 31-fold greater antiprion activity than that previously reported for beta-cyclodextrin (betaCD).

METHODS

Scrapie infected cells were treated with a range of betaCD analogues. This enabled a CD structure to antiprion activity analysis to be carried out. The metachromatic activity of each of the cyclodextrins was determined, this test is employed to mimic complexation of glycosaminogylcans to a cell membrane.

RESULTS

Sulphated-betaCD had an IC50 of 2.4 microM and it was the only CD found to have metachromatic activity. Its activity was equivalent to that of heparin and heparin sulphate, this may account for sulphated-betaCD's superior antiprion action.

GENERAL SIGNIFICANCE

In solution heparin can form a helical structure with a hydrophobic interior, the hydrophobic interior of cyclic CDs is vital for CD molecule encapsulation. The controlled CD structure, however, restricts degradation by human enzymes; consequently sulphated-CDs could be ideal candidates in the search for prion therapeutics. Sulphated-CDs may open up avenues for the treatment of TSEs.

New inhibitors of prion replication that target the amyloid precursor

At present, there is no effective therapy for any of the neurodegenerative amyloidoses, despite renewed efforts to identify compounds active against the various implicated pathogenetic molecules. We have screened a library of 2960 natural and synthetic compounds in two cell lines chronically infected with mouse prions, and have identified eight new inhibitors of prion replication in vitro. They belong to two distinct chemical families that have not previously been recognised as effective in the field of transmissible spongiform encephalopathies: seven are 3-aminosteroids and one is a derivative of erythromycin A with an oxime functionality. Our results suggest that these aminosteroids inhibit prion replication by triggering a common target, possibly implicated in the regulatory pathways of cellular prion protein metabolism. Furthermore, using a quantitative approach for the study of protein stability, it was shown that the erythromycin A derivative altered prion protein stability by direct interaction. Such direct targeting of this amyloid precursor might provide new clues for the understanding of prion diseases and, more importantly, help to define new molecules that are active against prion diseases.

Mucoadhesive microspheres for nasal administration of cyclodextrins

The aim of this study was to examine the in vitro capacity of cyclodextrins to interfere on the beta-amyloid fibril formation; then, mucoadhesive microspheres containing cyclodextrins were prepared and characterised as nasal delivery system for brain targeting. Eight batches of microspheres containing chitosan or alginate loaded with beta-cyclodextrin or hydroxypropyl-beta-cyclodextrin in two different cyclodextrin to polymer ratios were produced by spray drying. The results show that none of the tested CDs has direct cellular toxicity and they protect the cell viability from beta-peptide. The microspheres prepared are characterised by small particle sizes, ability to absorb water and to delay the in vitro dissolution rate of the CDs; good ex vivo mucoadhesive properties of the formulations are assessed. The microsphere properties are influenced by the kind of polymer, of cyclodextrin and by cyclodextrin to polymer ratio used. In particular, the alginate formulation containing the higher cyclodextrin content shows the best performance.

Identification and functional analysis of a novel cyclin e/cdk2 substrate ankrd17

Cyclin E/Cdk2 is a key regulator in G(1)-S transition. We have identified a novel cyclin E/Cdk2 substrate called Ankrd17 (ankyrin repeat protein 17) using the TAP tag purification technique. Ankrd17 protein contains two clusters of a total 25 ankyrin repeats at its N terminus, one NES (nuclear exporting signal) and one NLS (nuclear localization signal) in the middle, and one RXL motif at its C terminus. Ankrd17 is expressed in various tissues and associates with cyclin E/Cdk2 in an RXL-dependent manner. It can be phosphorylated by cyclin E/Cdk2 at 3 phosphorylation sites (Ser(1791), Ser(1794), and Ser(2150)). Overexpression of Ankrd17 promotes S phase entry, whereas depletion of Ankrd17 expression by small interfering RNA inhibits DNA replication and blocks cell cycle progression as well as up-regulates the expression of p53 and p21. Ankrd17 is localized to the nucleus and interacts with DNA replication factors including MCM family members, Cdc6 and PCNA. Depletion of Ankrd17 results in decreased loading of Cdc6 and PCNA onto DNA suggesting that Ankrd17 may be directly involved in the DNA replication process. Taken together, these data indicate that Ankrd17 is an important downstream effector of cyclin E/Cdk2 and positively regulates G(1)/S transition.