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

Engineered Extracellular Vesicles/Exosomes as a New Tool against Neurodegenerative Diseases

Neurodegenerative diseases are commonly generated by intracellular accumulation of misfolded/aggregated mutated proteins. These abnormal protein aggregates impair the functions of mitochondria and induce oxidative stress, thereby resulting in neuronal cell death. In turn, neuronal damage induces chronic inflammation and neurodegeneration. Thus, reducing/eliminating these abnormal protein aggregates is a priority for any anti-neurodegenerative therapeutic approach. Although several antibodies against mutated neuronal proteins have been already developed, how to efficiently deliver them inside the target cells remains an unmet issue. Extracellular vesicles/exosomes incorporating intrabodies against the pathogenic products would be a tool for innovative therapeutic approaches. In this review/perspective article, we identify and describe the major molecular targets associated with neurodegenerative diseases, as well as the antibodies already developed against them. Finally, we propose a novel targeting strategy based on the endogenous engineering of extracellular vesicles/exosomes constitutively released by cells of the central nervous system.

Muskelin Coordinates PrPC Lysosome versus Exosome Targeting and Impacts Prion Disease Progression

Cellular prion protein (PrP^C) modulates cell adhesion and signaling in the brain. Conversion to its infectious isoform causes neurodegeneration, including Creutzfeldt-Jakob disease in humans. PrP^C undergoes rapid plasma membrane turnover and extracellular release via exosomes. However, the intracellular transport of PrP^C and its potential impact on prion disease progression is barely understood. Here we identify critical components of PrP^C trafficking that also link intracellular and extracellular PrP^C turnover. PrP^C associates with muskelin, dynein, and KIF5C at transport vesicles. Notably, muskelin coordinates bidirectional PrP^C transport and facilitates lysosomal degradation over exosomal PrP^C release. Muskelin gene knockout consequently causes PrP^C accumulation at the neuronal surface and on secreted exosomes. Moreover, prion disease onset is accelerated following injection of pathogenic prions into muskelin knockout mice. Our data identify an essential checkpoint in PrP^C turnover. They propose a novel connection between neuronal intracellular lysosome targeting and extracellular exosome trafficking, relevant to the pathogenesis of neurodegenerative conditions.

Recombinant Antibody Fragments for Neurodegenerative Diseases

BACKGROUND

Recombinant antibody fragments are promising alternatives to full-length immunoglobulins and offer important advantages compared with conventional monoclonal antibodies: extreme specificity, higher affinity, superior stability and solubility, reduced immunogenicity as well as easy and inexpensive large-scale production.

OBJECTIVE

In this article we will review and discuss recombinant antibodies that are being evaluated for neurodegenerative diseases in pre-clinical models and in clinical studies and will summarize new strategies that are being developed to optimize their stability, specificity and potency for advancing their use.

METHODS

Articles describing recombinant antibody fragments used for neurological diseases were selected (PubMed) and evaluated for their significance.

RESULTS

Different antibody formats such as single-chain fragment variable (scFv), single-domain antibody fragments (VHHs or sdAbs), bispecific antibodies (bsAbs), intrabodies and nanobodies, are currently being studied in pre-clinical models of cancer as well as infectious and autoimmune diseases and many of them are being tested as therapeutics in clinical trials. Immunotherapy approaches have shown therapeutic efficacy in several animal models of Alzheimer´s disease (AD), Parkinson disease (PD), dementia with Lewy bodies (DLB), frontotemporal dementia (FTD), Huntington disease (HD), transmissible spongiform encephalopathies (TSEs) and multiple sclerosis (MS). It has been demonstrated that recombinant antibody fragments may neutralize toxic extra- and intracellular misfolded proteins involved in the pathogenesis of AD, PD, DLB, FTD, HD or TSEs and may target toxic immune cells participating in the pathogenesis of MS.

CONCLUSION

Recombinant antibody fragments represent a promising tool for the development of antibody-based immunotherapeutics for neurodegenerative diseases.

TRIM21 and the Function of Antibodies inside Cells

Therapeutic antibodies targeting disease-associated antigens are key tools in the treatment of cancer and autoimmunity. So far, therapeutic antibodies have targeted antigens that are, or are presumed to be, extracellular. A largely overlooked property of antibodies is their functional activity inside cells. The diverse literature dealing with intracellular antibodies emerged historically from studies of the properties of some autoantibodies. The identification of tripartite motif (TRIM) 21 as an intracellular Fc receptor linking cytosolic antibody recognition to the ubiquitin proteasome system brings this research into sharper focus. We review critically the research related to intracellular antibodies, link this to the TRIM21 effector mechanism, and highlight how this work is exposing the previously restricted intracellular space to the potential of therapeutic antibodies.

Specific in vivo knockdown of protein function by intrabodies

Intracellular antibodies (intrabodies) are recombinant antibody fragments that bind to target proteins expressed inside of the same living cell producing the antibodies. The molecules are commonly used to study the function of the target proteins (i.e., their antigens). The intrabody technology is an attractive alternative to the generation of gene-targeted knockout animals, and complements knockdown techniques such as RNAi, miRNA and small molecule inhibitors, by-passing various limitations and disadvantages of these methods. The advantages of intrabodies include very high specificity for the target, the possibility to knock down several protein isoforms by one intrabody and targeting of specific splice variants or even post-translational modifications. Different types of intrabodies must be designed to target proteins at different locations, typically either in the cytoplasm, in the nucleus or in the endoplasmic reticulum (ER). Most straightforward is the use of intrabodies retained in the ER (ER intrabodies) to knock down the function of proteins passing the ER, which disturbs the function of members of the membrane or plasma proteomes. More effort is needed to functionally knock down cytoplasmic or nuclear proteins because in this case antibodies need to provide an inhibitory effect and must be able to fold in the reducing milieu of the cytoplasm. In this review, we present a broad overview of intrabody technology, as well as applications both of ER and cytoplasmic intrabodies, which have yielded valuable insights in the biology of many targets relevant for drug development, including α-synuclein, TAU, BCR-ABL, ErbB-2, EGFR, HIV gp120, CCR5, IL-2, IL-6, β-amyloid protein and p75NTR. Strategies for the generation of intrabodies and various designs of their applications are also reviewed.

Potential candidate camelid antibodies for the treatment of protein-misfolding diseases

Protein-misfolding diseases (PMDs), including Alzheimer's disease would potentially reach epidemic proportion if effective ways to diagnose and treat them were not developed. The quest for effective therapy for PMDs has been ongoing for decades and some of the technologies developed so far show great promise. We report here the development of antibodies by immunization of camelids with prion (PrioV3) and Alzheimer's (PrioAD12, 13 & 120) disease-derived brain material. We show that anti-PrP antibody transmigration across the blood-brain barrier (BBB) was inhibited with phosphatidylinositol-specific phospholipase C (PIPLC). Our camelid anti-prion antibody was also shown to permanently abrogate prion replication in a prion-permissive cell line after crossing the artificial BBB. Furthermore, anti-Aβ/tau antibodies were able to bind their specific immunogens with ELISA and immunohistochemistry. Finally, both PrioV3 and PrioAD12 were shown to co-localize with Lamp-1, a marker of late endosomal/lysosomal compartments. These antibodies could prove to be a valuable tool for the neutralization/clearance of PrP(Sc), Aβ and tau proteins in cellular compartments of affected neurons and could potentially have wider applicability for the treatment of PMDs.

Therapeutic approaches for prion disorders

Prion diseases are lethal for both humans and animals, and affected individuals die after several months following a rapid disease progression. Although researchers have attempted for decades to develop effective therapeutics for the therapy of human prion disorders, until now no efficient drug has been available on the market for transmissible spongiform encephalopathy (TSE) treatment or cure. Approximately 200 patients worldwide have died or suffer from variant Creutzfeldt-Jakob disease (CJD). Incidences for sporadic and familial CJD are approximately 1.5-2 per million per year and one per 10 million per year, respectively, in Europe. This review summarizes classical and modern trials for the development of effective anti-TSE drugs, introduces potential effective delivery systems, such as lentiviral and adeno-associated virus systems for antiprion components, including antibodies and siRNAs, and presents vaccination trials. Most of the antiprion drugs target prion protein PrP(c) and/or PrP(Sc). Alternative targets are receptors and coreceptors for PrP, that is, the 37/67-kDa laminin receptor and heparan sulfate proteoglycanes. We review clinical trials for the treatment of TSEs and describe hindrances and chances for a breakthrough in therapy of prion disorders.

Evidence for prion-like mechanisms in several neurodegenerative diseases: potential implications for immunotherapy

Transmissible spongiform encephalopathies (TSEs) are fatal, untreatable neurodegenerative diseases. While the impact of TSEs on human health is relatively minor, these diseases are having a major influence on how we view, and potentially treat, other more common neurodegenerative disorders. Until recently, TSEs encapsulated a distinct category of neurodegenerative disorder, exclusive in their defining characteristic of infectivity. It now appears that similar mechanisms of self-propagation may underlie other proteinopathies such as Alzheimer's disease, Parkinson's disease, Amyotrophic lateral sclerosis, and Huntington's disease. This link is of scientific interest and potential therapeutic importance as this route of self-propagation offers conceptual support and guidance for vaccine development efforts. Specifically, the existence of a pathological, self-promoting isoform offers a rational vaccine target. Here, we review the evidence of prion-like mechanisms within a number of common neurodegenerative disorders and speculate on potential implications and opportunities for vaccine development.

Gene-based antibody strategies for prion diseases

Prion diseases or transmissible spongiform encephalopathies (TSE) are a group of neurodegenerative and infectious disorders characterized by the conversion of a normal cellular protein PrP^C into a pathological abnormally folded form, termed PrP^Sc. There are neither available therapies nor diagnostic tools for an early identification of individuals affected by these diseases. New gene-based antibody strategies are emerging as valuable therapeutic tools. Among these, intrabodies are chimeric molecules composed by recombinant antibody fragments fused to intracellular trafficking sequences, aimed at inhibiting, in vivo, the function of specific therapeutic targets. The advantage of intrabodies is that they can be selected against a precise epitope of target proteins, including protein-protein interaction sites and cytotoxic conformers (i.e., oligomeric and fibrillar assemblies). Herein, we address and discuss in vitro and in vivo applications of intrabodies in prion diseases, focussing on their therapeutic potential.

Immunotherapeutic approaches in prion disease: progress, challenges and potential directions

Therapeutic trials utilizing animal models of prion disease have explored a variety of compounds and a number of approaches with varying success, including several immunotherapeutic strategies, such as passive immunization through the delivery of viruses carrying nucleic acid inserts encoding prion protein-specific immunoglobulin. Targeted, organ-specific cellular production of therapeutic proteins is a relatively unexplored approach in the treatment of neurodegeneration despite many successful experimental outcomes in animal models and human trials of other diseases of the CNS. Emphasizing studies utilizing mouse models of disease, this review outlines developments and limitations of immunological approaches to the treatment of prion diseases. In addition, the authors discuss the potential of an experimental therapeutic strategy, utilizing hybridoma cells injected directly into the CNS to establish long-term production of anti-prion antibodies in vivo within the organ associated with the greatest pathogenic change in prion disease, the brain.

Detection and control of prion diseases in food animals

Transmissible spongiform encephalopathies (TSEs), or prion diseases, represent a unique form of infectious disease based on misfolding of a self-protein (PrP^C) into a pathological, infectious conformation (PrP^Sc). Prion diseases of food animals gained notoriety during the bovine spongiform encephalopathy (BSE) outbreak of the 1980s. In particular, disease transmission to humans, to the generation of a fatal, untreatable disease, elevated the perspective on livestock prion diseases from food production to food safety. While the immediate threat posed by BSE has been successfully addressed through surveillance and improved management practices, another prion disease is rapidly spreading. Chronic wasting disease (CWD), a prion disease of cervids, has been confirmed in wild and captive populations with devastating impact on the farmed cervid industries. Furthermore, the unabated spread of this disease through wild populations threatens a natural resource that is a source of considerable economic benefit and national pride. In a worst-case scenario, CWD may represent a zoonotic threat either through direct transmission via consumption of infected cervids or through a secondary food animal, such as cattle. This has energized efforts to understand prion diseases as well as to develop tools for disease detection, prevention, and management. Progress in each of these areas is discussed.

Parkinson's disease

Parkinson's disease (PD) is the most common age-related motoric neurodegenerative disease initially described in the 1800's by James Parkinson as the 'Shaking Palsy'. Loss of the neurotransmitter dopamine was recognized as underlying the pathophysiology of the motor dysfunction; subsequently discovery of dopamine replacement therapies brought substantial symptomatic benefit to PD patients. However, these therapies do not fully treat the clinical syndrome nor do they alter the natural history of this disorder motivating clinicians and researchers to further investigate the clinical phenotype, pathophysiology/pathobiology and etiology of this devastating disease. Although the exact cause of sporadic PD remains enigmatic studies of familial and rare toxicant forms of this disorder have laid the foundation for genome wide explorations and environmental studies. The combination of methodical clinical evaluation, systematic pathological studies and detailed genetic analyses have revealed that PD is a multifaceted disorder with a wide-range of clinical symptoms and pathology that include regions outside the dopamine system. One common thread in PD is the presence of intracytoplasmic inclusions that contain the protein, α-synuclein. The presence of toxic aggregated forms of α-synuclein (e.g., amyloid structures) are purported to be a harbinger of subsequent pathology. In fact, PD is both a cerebral amyloid disease and the most common synucleinopathy, that is, diseases that display accumulations of α-synuclein. Here we present our current understanding of PD etiology, pathology, clinical symptoms and therapeutic approaches with an emphasis on misfolded α-synuclein.

Functional inhibition of transitory proteins by intrabody-mediated retention in the endoplasmatic reticulum

Intrabodies are recombinantly expressed intracellular antibody fragments that can be used to specifically bind and inhibit the function of cellular proteins of interest. Intrabodies can be targeted to various cell compartments by attaching an appropriate localization peptide sequence to them. An efficient strategy with a high success rate is to anchor intrabodies in the endoplasmatic reticulum where they can inhibit transitory target proteins by binding and preventing them to reach their site of action. Intrabodies can be assembled from antibody gene fragments from various sources into dedicated expression vectors. Conventionally, antibody cDNA sequences are derived from selected hybridoma cell clones that express antibodies with the desired specificity. Alternatively, appropriate clones can be isolated by affinity selection from an antibody in vitro display library. Here an evaluation of endoplasmatic reticulum targeted intrabodies with respect to other knockdown approaches is given and the characteristics of various intrabody expression vectors are discussed. A step by step protocol is provided that was repeatedly used to construct intrabodies derived from diverse antibody isotypes producing hybridoma cell clones. The inactivation of the cell surface receptor neural cell adhesion molecule (NCAM) by a highly efficacious novel endoplasmatic reticulum-anchored intrabody is demonstrated.

Intrabody Expression in Mammalian Cells

The intracellular expression of antibodies or antibody fragments (intrabodies) in different compartments of mammalian cells allows to block or modulate the function of endogenous molecules. Intrabodies can alter protein folding, protein-protein, protein-DNA, protein-RNA interactions and protein modification. They can induce a phenotypic knockout and work as neutralizing agents by direct binding to the target antigen, by diverting its intracellular traffic or by inhibiting its association with binding partners. They have been largely employed as research tools and are emerging as therapeutic molecules for the treatment of human diseases as viral pathologies, cancer and misfolding diseases. The fast growing bio-market of recombinant antibodies provides intrabodies with enhanced binding specificity, stability and solubility, together with lower immunogenicity, for their use in therapy. This chapter describes the crucial aspects required to express intrabodies in different intracellular compartments of mammalian cells, their various modes of action and gives an update on the applications of intrabodies in human diseases.

Immunotherapy for prion diseases: opportunities and obstacles

Transmissible spongiform encephalopathies (TSEs) represent a unique form of infectious disease based on the misfolding of a self-protein into a pathological conformation. While other human diseases are also attributed to protein misfolding, the TSEs are unique in their zoonotic potential and iatrogenic infectivity. These characteristics are of particular importance in the aftermath of the UK bovine spongiform encephalopathy (BSE) outbreak due to the dual concerns that a subpopulation of individuals exposed to the infectious agent may be serving as asymptomatic carriers, and that TSEs of other food animals may also threaten human health. These potentials, in addition to the ongoing baseline of familial and sporadic human prion diseases, necessitate development of effective treatment options. While TSEs represent a novel paradigm of infection, there is nevertheless the opportunity to apply traditional approaches of medicine for disease treatment and prevention, including vaccines for immunotherapy and immunoprophylaxis. However, vaccine development for TSEs is complicated by the challenges and potential dangers associated with induction of immune responses to a self-epitope, as well as the obstacles to treatment of a chronic infection through immunotherapy. The ongoing threat of TSEs to human health, together with the opportunity to apply information emerging from these investigations to other protein misfolding disorders, justifies the efforts required to overcome these obstacles.

Progress in the development of therapeutic antibodies targeting prion proteins and beta-amyloid peptides

Prion diseases and Alzheimer's disease (AD) are characterized by protein misfolding, and can lead to dementia. However, prion diseases are infectious and transmissible, while AD is not. The similarities and differences between these diseases have led researchers to perform comparative studies. In the last 2 decades, progress has been made in immunotherapy using anti-prion protein and anti-beta-amyloid antibodies. In this study, we review new ideas and strategies for therapeutic antibodies targeting prion diseases and AD through conformation dependence.

PrP-specific camel antibodies with the ability to immunodetect intracellular prion protein

Although there is currently no effective treatment for prion diseases, significant advances have been made in suppressing its progress, using antibodies that block the conversion of PrPC into PrPSc. In order to be effective in treating individuals that have prion diseases, antibodies must be capable of arresting disease in its late stages. This requires the development of antibodies with higher affinity for PrPSc and systems for effective translocation of antibodies across the blood–brain barrier in order to achieve high concentrations of inhibitor at the site of protein replication. An additional advantage is the ability of these antibodies to access the cytosol of affected cells. To this end, we have generated PrP-specific antibodies (known as PrioV) by immunization of camels with murine scrapie material adsorbed to immunomagnetic beads. The PrioV antibodies display a range of specificities with some recognizing the PrP27–30 proteinase K-resistant fragment, others specific for PrPC and a number with dual binding specificity. Independent of their PrP conformation specificity, one of the PrioV antibodies (PrioV3) was shown to bind PrPC in the cytosol of neuroblastoma cells. In marked contrast, conventional anti-PrP antibodies produced in mouse against similar target antigen were unable to cross the neuronal plasma membrane and instead formed a ring around the cells. The PrioV anti-PrP antibodies could prove to be a valuable tool for the neutralization/clearance of PrPSc in intracellular compartments of affected neurons and could potentially have wider applicability for the treatment of so-called protein-misfolding diseases.

Developing intrabodies for the therapeutic suppression of neurodegenerative pathology

Many neurodegenerative diseases have misfolded proteins as a primary occurrence in pathogenesis. A combination of antibody and genetic engineering has emerged as a powerful tool for developing reagents that specifically target the misfolding process itself, and/or abnormal interactions of the misfolded protein species. This review focuses on the selection and testing of intracellular antibody fragments (intrabodies), with a particular focus on Huntington's disease (HD) and Parkinson's disease (PD), both of which show prominent intracellular protein aggregates in affected neurons. The most dramatic advances are in HD, where in vivo efficacy of intrabodies has been demonstrated. Targets in other neurodegenerative disorders, including Alzheimer's disease and prion diseases, are noted more briefly, with an emphasis on the potential for intracellular manipulations. Given the specificity and versatility of antibody-based reagents, the wide range of options for conformational and post-translationally-modified targets, and the recent improvement in gene delivery, this should be a fertile field for 21(st) century pharmacology.

Development of antibody fragments for immunotherapy of prion diseases

Prions are infectious proteins responsible for a group of fatal neurodegenerative diseases called TSEs (transmissible spongiform encephalopathies) or prion diseases. In mammals, prions reproduce themselves by recruiting the normal cellular protein PrP(C) and inducing its conversion into the disease-causing isoform denominated PrP(Sc). Recently, anti-prion antibodies have been shown to permanently cure prion-infected cells. However, the inability of full-length antibodies and proteins to cross the BBB (blood-brain barrier) hampers their use in the therapy of TSEs in vivo. Alternatively, brain delivery of prion-specific scFv (single-chain variable fragment) by AAV (adeno-associated virus) transfer delays the onset of the disease in infected mice, although protection is not complete. We investigated the anti-prion effects of a recombinant anti-PrP (D18) scFv by direct addition to scrapie-infected cell cultures or by infection with both lentivirus and AAV-transducing vectors. We show that recombinant anti-PrP scFv is able to reduce proteinase K-resistant PrP content in infected cells. In addition, we demonstrate that lentiviruses are more efficient than AAV in gene transfer of the anti-PrP scFv gene and in reducing PrP(Sc) content in infected neuronal cell lines. Finally, we have used a bioinformatic approach to construct a structural model of the D18scFv-PrP(C) complex. Interestingly, according to the docking results, Arg(PrP)(151) (Arg(151) from prion protein) is the key residue for the interactions with D18scFv, anchoring the PrP(C) to the cavity of the antibody. Taken together, these results indicate that combined passive and active immunotherapy targeting PrP might be promising strategies for therapeutic intervention in prion diseases.

Oral pravastatin prolongs survival time of scrapie-infected mice

Statins are potent inhibitors of HMG-CoA (3-hydroxy-3-methylglutaryl coenzyme A) reductase in the cholesterol-biosynthesis pathway. They are either lipophilic (e.g. simvastatin) or hydrophilic [e.g. pravastatin (PRV)] compounds, considered mainly for long-term treatment of hypercholesterolaemic individuals. Beneficial effects of statins are not related exclusively to their lipid-lowering action; they also possess cholesterol-independent, pleiotropic effects (e.g. anti-inflammatory and antioxidant). Recent studies revealed that simvastatin treatment increased survival significantly in scrapie-infected mice. Although PRV treatment results in measurable drug levels in the mouse brain, the anti-prion effect of this compound has not been investigated. Therefore, we aimed to test the potential therapeutic action of PRV in a murine scrapie model. Our study showed that high-dose and long-term oral PRV treatment prolonged survival times of strain 139A scrapie-infected mice significantly (194 versus 177 days) in the absence of any obvious toxicity, suggesting that protective effects of statins may be independent of absolute solvent or water solubility of the drug.

Targeting prion proteins in neurodegenerative disease

BACKGROUND

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

OBJECTIVE

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

METHODS

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

RESULTS/CONCLUSIONS

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

Intracellular antibodies (intrabodies) and their therapeutic potential

Combining exquisite specificity and high antigen-binding affinity, intrabodies have been used as a biotechnological tool to interrupt, modulate, or define the functions of a wide range of target antigens at the posttranslational level. An intrabody is an antibody that has been designed to be expressed intracellularly and can be directed to a specific target antigen present in various subcellular locations including the cytosol, nucleus, endoplasmic reticulum (ER), mitochondria, peroxisomes, plasma membrane and trans-Golgi network (TGN) through in frame fusion with intracellular trafficking/localization peptide sequences. Although intrabodies can be expressed in different forms, the most commonly used format is a singlechain antibody (scFv Ab) created by joining the antigen-binding variable domains of heavy and light chain with an interchain linker (ICL), most often the 15 amino acid linker (GGGGS)(3) between the variable heavy (VH) and variable light (VL) chains. Intrabodies have been used in research of cancer, HIV, autoimmune disease, neurodegenerative disease, and transplantation. Clinical application of intrabodies has mainly been hindered by the availability of robust gene delivery system(s) including target cell directed gene delivery. This review will discuss several methods of intrabody selection, different strategies of cellular targeting, and recent successful examples of intrabody applications. Taking advantage of the high specificity and affinity of an antibody for its antigen, and of the virtually unlimited diversity of antigen-binding variable domains available for molecular targeting, intrabody techniques are emerging as promising tools to generate phenotypic knockouts, to manipulate biological processes, and to obtain a more thorough understanding of functional genomics.

Peptide Aptamers Expressed in the Secretory Pathway Interfere with Cellular PrPSc Formation

Prion diseases are rare and obligatory fatal neurodegenerative disorders caused by the accumulation of a misfolded isoform (PrPSc) of the host-encoded prion protein (PrPc). Prophylactic and therapeutic regimens against prion diseases are very limited. To extend such strategies we selected peptide aptamers binding to PrP from a combinatorial peptide library presented on the Escherichia coli thioredoxin A (trxA) protein as a scaffold. In a yeast two-hybrid screen employing full-length murine PrP (aa 23-231) as a bait we identified three peptide aptamers that reproducibly bind to PrP. Treatment of prion-infected cells with recombinantly expressed aptamers added to the culture medium abolished PrPSc conversion with an IC50 between 350 and 700 nM. For expression in eukaryotic cells, peptide aptamers were fused to an N-terminal signal peptide for entry of the secretory pathway. The C terminus was modified by a glycosyl-phosphatidyl-inositol-(GPI) anchoring signal, a KDEL retention motif and the transmembrane and cytosolic domain of LAMP-I, respectively. These peptide aptamers retained their binding properties to PrPc and, depending on peptide sequence and C-terminal modification, interfered with endogenous PrPSc conversion upon expression in prion-infected cells. Notably, infection of cell cultures could be prevented by expression of KDEL peptide aptamers. For the first time, we show that trxA-based peptide aptamers can be targeted to the secretory pathway, thereby not losing the affinity for their target protein. Beside their inhibitory effect on prion conversion, these molecules could be used as fundament for rational drug design.

Selective re-routing of prion protein to proteasomes and alteration of its vesicular secretion prevent PrPScformation

Conversion of the cellular prion protein (PrP(C)) into the abnormal scrapie isoform (PrP(Sc)) is the hallmark of prion diseases, which are fatal and transmissible neurodegenerative disorders. ER-retained anti-prion recombinant single-chain Fv fragments have been proved to be an effective tool for inhibition of PrP(C) trafficking to the cell surface and antagonize PrP(Sc) formation and infectivity. In the present study, we have generated the secreted version of 8H4 intrabody (Sec-8H4) in order to compel PrP(C) outside the cells. The stable expression of the Sec-8H4 intrabodies induces proteasome degradation of endogenous prion protein but does not influence its glycosylation profile and maturation. Moreover, we found a dramatic diverting of PrP(C) traffic from its vesicular secretion and, most importantly, a total inhibition of PrP(Sc) accumulation in NGF-differentiated Sec-8H4 PC12 cells. These results confirm that perturbing the intracellular traffic of endogenous PrP(C) is an effective strategy to inhibit PrP(Sc) accumulation and provide convincing evidences for application of intracellular antibodies in prion diseases.

Blocking translocation of cell surface molecules from the ER to the cell surface by intracellular antibodies targeted to the ER

Intracellular antibodies (intrabodies) constitute a potent tool to neutralize the function of target proteins inside specific cell compartments (cytosol, nucleus, mitochondria and ER). The intrabody technology is an attractive alternative to the generation of gene-targeted knockout animals and complements or replaces knockdown techniques such as antisense-RNA, RNAi and RNA aptamers. This article focuses on intrabodies targeted to the ER. Intracellular anti-bodies expressed and retained inside the ER (ER intrabodies) are shown to be highly efficient in blocking the translocation of secreted and cell surface molecules from the ER to the cell surface.The advantage of ER intrabodies over cytoplasmic intrabodies is that they are correctly folded and easier to select. A particular advantage of the intrabody technology over existing ones is the possibility of inhibiting selectively post-translational modifications of proteins.The main applications of ER intrabodies so far have been (i) inactivation of oncogenic receptors and (ii) functional inhibition of virus envelope proteins and virus-receptor molecules on the surface of host cells.In cancer research, the number of in vivo mouse models for evaluation of the therapeutic potential of intrabodies is increasing.In the future, endosomal localized receptors involved in bacterial and viral infections, intracellular oncogenic receptors and enzymes involved in glycosylation of tumour antigens might be new targets for ER intrabodies.

Potential of active and passive immunizations for the prevention and therapy of transmissible spongiform encephalopathies

Transmissible spongiform encephalopathies are fatal neurodegenerative disorders that affect humans and certain animals and are caused by prions. In most cases, infection occurs by ingestion of prions. Their long-time persistence in the environment creates a reservoir of potentially infectious matter that renders the eradication of the disease problematic. Unfortunately, no cure is available to date. Yet, for both the treatment of infected and the protection of uninfected individuals, active and passive immunizations have been shown to have a beneficial effect on the course of the disease. The current review provides an overview of such antibody-based approaches and assesses their feasibility and potential in prophylaxis and therapy of transmissible spongiform encephalopathies.

Expression and detection strategies for an scFv fragment retaining the same high affinity than Fab and whole antibody: Implications for therapeutic use in prion diseases

Since antibodies currently constitute the most rapidly growing class of human therapeutics, the high-yield production of recombinant antibodies and antibody fragments is a real challenge. Using as model a monoclonal antibody directed against the human prion protein that we prepared previously and tested for its therapeutic value, we describe here experimental conditions allowing the production of large quantities (up to 35 mg/l of bacterial culture) of correctly refolded and totally functional single chain fragment variable (scFv). These quantities were sufficient to characterize the binding properties of this small recombinant fragment through in vitro and ex vivo approaches. Interestingly, this scFv retains full binding capacity for its antigen, i.e. the human prion protein, when compared with the corresponding Fab or whole antibody, and recognizes soluble, solid-phase-adsorbed, and membrane-bound prion protein. This strongly suggests that from the mAb cloning step to the refolding of the recombinant fragment, each stage is well controlled, leading to almost 100% functional scFv. These results are of interest not only in view of possible immunotherapy for prion diseases, but also more generally in emphasizing the great promise of these small recombinant molecules in the context of targeted therapies.

T-cell protection and enrichment through lentiviral CCR5 intrabody gene delivery

CCR5 is the chemokine co-receptor for R5-tropic human immunodeficiency virus type 1 (HIV-1) isolates most often associated with primary infection. We have developed an HIV-1 self-inactivating vector, CAD-R5, containing a CCR5 single-chain antibody (intrabody) gene, which when expressed in T-cell lines and primary CD4+ T cells disrupts CCR5 cell surface expression and provides protection from R5-tropic isolate exposure. Furthermore, CAD-R5 intrabody expression in primary CD4+ T cells supports significant growth and enrichment over time during HIV-1-pulsed dendritic cell-T-cell interactions. These results indicate that CCR5 intrabody-expressing CD4+ T cells are refractory against this highly efficient primary route of infection. CD34+ cells transduced with the CAD-R5 vector gave rise to CD4+ and CD8+ thymocytes in non-obese diabetic (NOD)/ severely combined-immunodeficient (SCID)-human thymus/liver (hu thy/liv) mice, suggesting that CCR5 intrabody expression can be maintained throughout differentiation without obvious cellular effects. CD4+ T cells isolated from NOD/SCID-hu thy/liv mice were resistant to R5-tropic HIV-1 challenge demonstrating the maintenance of protection. Our findings demonstrate delivery of anti-HIV-1 activity through CCR5 intrabodies in primary CD4+ T cells and CD34+ cell-derived T-cell progeny. Thus, gene delivery strategies that provide a selective survival and growth advantage for T effector cells may provide a therapeutic benefit for HIV-1-infected individuals who have failed conventional therapies.