A camelid anti-PrP antibody abrogates PrP replication in prion-permissive neuroblastoma cell lines

Single-domain antibodies and aptamers drive new opportunities for neurodegenerative disease research

Neurodegenerative diseases (NDs) in mammals, such as Alzheimer's disease (AD), Parkinson's disease (PD), and transmissible spongiform encephalopathies (TSEs), are characterized by the accumulation of misfolded proteins in the central nervous system (CNS). Despite the presence of these pathogenic proteins, the immune response in affected individuals remains notably muted. Traditional immunological strategies, particularly those reliant on monoclonal antibodies (mAbs), face challenges related to tissue penetration, blood-brain barrier (BBB) crossing, and maintaining protein stability. This has led to a burgeoning interest in alternative immunotherapeutic avenues. Notably, single-domain antibodies (or nanobodies) and aptamers have emerged as promising candidates, as their reduced size facilitates high affinity antigen binding and they exhibit superior biophysical stability compared to mAbs. Aptamers, synthetic molecules generated from DNA or RNA ligands, present both rapid production times and cost-effective solutions. Both nanobodies and aptamers exhibit inherent qualities suitable for ND research and therapeutic development. Cross-seeding events must be considered in both traditional and small-molecule-based immunodiagnostic and therapeutic approaches, as well as subsequent neurotoxic impacts and complications beyond protein aggregates. This review delineates the challenges traditional immunological methods pose in ND research and underscores the potential of nanobodies and aptamers in advancing next-generation ND diagnostics and therapeutics.

Genetic characterization of the prion protein gene in camels (Camelus) with comments on the evolutionary history of prion disease in Cetartiodactyla

Transmissible spongiform encephalopathies (TSEs) are a fatal neurogenerative disease that include Creutzfeldt-Jakob disease in humans, scrapie in sheep and goats, bovine spongiform encephalopathy (BSE), and several others as well as the recently described camel prion disease (CPD). CPD originally was documented in 3.1% of camels examined during an antemortem slaughterhouse inspection in the Ouargla region of Algeria. Of three individuals confirmed for CPD, two were sequenced for the exon 3 of the prion protein gene (PRNP) and were identical to sequences previously reported for Camelus dromedarius. Given that other TSEs, such as BSE, are known to be capable of cross-species transmission and that there is household consumption of meat and milk from Camelus, regulations to ensure camel and human health should be a One Health priority in exporting countries. Although the interspecies transmissibility of CPD currently is unknown, genotypic characterization of Camelus PRNP may be used for predictability of predisposition and potential susceptibility to CPD. Herein, eight breeds of dromedary camels from a previous genetic (mitochondrial DNA and microsatellites) and morphological study were genotyped for PRNP and compared to genotypes from CPD-positive Algerian camels. Sequence data from PRNP indicated that Ethiopian camels possessed 100% sequence identity to CPD-positive camels from Algeria. In addition, the camel PRNP genotype is unique compared to other members of the Orders Cetartiodactyla and Perissodactyla and provides an in-depth phylogenetic analysis of families within Cetartiodactyla and Perissodactyla that was used to infer the evolutionary history of the PRNP gene.

Applications of nanobodies in brain diseases

Nanobodies are antibody fragments derived from camelids, naturally endowed with properties like low molecular weight, high affinity and low immunogenicity, which contribute to their effective use as research tools, but also as diagnostic and therapeutic agents in a wide range of diseases, including brain diseases. Also, with the success of Caplacizumab, the first approved nanobody drug which was established as a first-in-class medication to treat acquired thrombotic thrombocytopenic purpura, nanobody-based therapy has received increasing attention. In the current review, we first briefly introduce the characterization and manufacturing of nanobodies. Then, we discuss the issue of crossing of the brain-blood-barrier (BBB) by nanobodies, making use of natural methods of BBB penetration, including passive diffusion, active efflux carriers (ATP-binding cassette transporters), carrier-mediated influx via solute carriers and transcytosis (including receptor-mediated transport, and adsorptive mediated transport) as well as various physical and chemical methods or even more complicated methods such as genetic methods via viral vectors to deliver nanobodies to the brain. Next, we give an extensive overview of research, diagnostic and therapeutic applications of nanobodies in brain-related diseases, with emphasis on Alzheimer's disease, Parkinson's disease, and brain tumors. Thanks to the advance of nanobody engineering and modification technologies, nanobodies can be linked to toxins or conjugated with radionuclides, photosensitizers and nanoparticles, according to different requirements. Finally, we provide several perspectives that may facilitate future studies and whereby the versatile nanobodies offer promising perspectives for advancing our knowledge about brain disorders, as well as hopefully yielding diagnostic and therapeutic solutions.

Imaging of Glioblastoma Tumor-Associated Myeloid Cells Using Nanobodies Targeting Signal Regulatory Protein Alpha

Glioblastoma (GBM) is the most common malignant primary brain tumor. Glioblastomas contain a large non-cancerous stromal compartment including various populations of tumor-associated macrophages and other myeloid cells, of which the presence was documented to correlate with malignancy and reduced survival. Via single-cell RNA sequencing of human GBM samples, only very low expression of PD-1, PD-L1 or PD-L2 could be detected, whereas the tumor micro-environment featured a marked expression of signal regulatory protein alpha (SIRPα), an inhibitory receptor present on myeloid cells, as well as its widely distributed counter-receptor CD47. CITE-Seq revealed that both SIRPα RNA and protein are prominently expressed on various populations of myeloid cells in GBM tumors, including both microglia- and monocyte-derived tumor-associated macrophages (TAMs). Similar findings were obtained in the mouse orthotopic GL261 GBM model, indicating that SIRPα is a potential target on GBM TAMs in mouse and human. A set of nanobodies, single-domain antibody fragments derived from camelid heavy chain-only antibodies, was generated against recombinant SIRPα and characterized in terms of affinity for the recombinant antigen and binding specificity on cells. Three selected nanobodies binding to mouse SIRPα were radiolabeled with ^99mTc, injected in GL261 tumor-bearing mice and their biodistribution was evaluated using SPECT/CT imaging and radioactivity detection in dissected organs. Among these, Nb15 showed clear accumulation in peripheral organs such as spleen and liver, as well as a clear tumor uptake in comparison to a control non-targeting nanobody. A bivalent construct of Nb15 exhibited an increased accumulation in highly vascularized organs that express the target, such as spleen and liver, as compared to the monovalent format. However, penetration into the GL261 brain tumor fell back to levels detected with a non-targeting control nanobody. These results highlight the tumor penetration advantages of the small monovalent nanobody format and provide a qualitative proof-of-concept for using SIRPα-targeting nanobodies to noninvasively image myeloid cells in intracranial GBM tumors with high signal-to-noise ratios, even without blood-brain barrier permeabilization.

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

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

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Antibody cross-linking neuronal PrPC induces apoptosis.

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Antibody cross-linking microglial PrPC induces neuronal apoptosis.

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

Transportation of Single-Domain Antibodies through the Blood-Brain Barrier

Single-domain antibodies derive from the heavy-chain-only antibodies of Camelidae (camel, dromedary, llama, alpaca, vicuñas, and guananos; i.e., nanobodies) and cartilaginous fishes (i.e., VNARs). Their small size, antigen specificity, plasticity, and potential to recognize unique conformational epitopes represent a diagnostic and therapeutic opportunity for many central nervous system (CNS) pathologies. However, the blood–brain barrier (BBB) poses a challenge for their delivery into the brain parenchyma. Nevertheless, numerous neurological diseases and brain pathologies, including cancer, result in BBB leakiness favoring single-domain antibodies uptake into the CNS. Some single-domain antibodies have been reported to naturally cross the BBB. In addition, different strategies and methods to deliver both nanobodies and VNARs into the brain parenchyma can be exploited when the BBB is intact. These include device-based and physicochemical disruption of the BBB, receptor and adsorptive-mediated transcytosis, somatic gene transfer, and the use of carriers/shuttles such as cell-penetrating peptides, liposomes, extracellular vesicles, and nanoparticles. Approaches based on single-domain antibodies are reaching the clinic for other diseases. Several tailoring methods can be followed to favor the transport of nanobodies and VNARs to the CNS, avoiding the limitations imposed by the BBB to fulfill their therapeutic, diagnostic, and theragnostic promises for the benefit of patients suffering from CNS pathologies.

Cross-Linking Cellular Prion Protein Induces Neuronal Type 2-Like Hypersensitivity

Background

Previous reports identified proteins associated with ‘apoptosis’ following cross-linking PrP^C with motif-specific anti-PrP antibodies in vivo and in vitro. The molecular mechanisms underlying this IgG-mediated neurotoxicity and the role of the activated proteins in the apoptotic pathways leading to neuronal death has not been properly defined. Previous reports implicated a number of proteins, including apolipoprotein E, cytoplasmic phospholipase A2, prostaglandin and calpain with anti-PrP antibody-mediated ‘apoptosis’, however, these proteins are also known to play an important role in allergy. In this study, we investigated whether cross-linking PrP^C with anti-PrP antibodies stimulates a neuronal allergenic response.

Methods

Initially, we predicted the allergenicity of the epitope sequences associated with ‘neurotoxic’ anti-PrP antibodies using allergenicity prediction servers. We then investigated whether anti-PrP antibody treatment of mouse primary neurons (MPN), neuroblastoma cells (N2a) and microglia (N11) cell lines lead to a neuronal allergenic response.

Results

In-Silico studies showed that both tail- and globular-epitopes were allergenic. Specifically, binding regions that contain epitopes for previously reported ‘neurotoxic’ antibodies such as ICSM18 (146-159), ICSM35 (91-110), POM 1 (138-147) and POM 3 (95-100) lead to activation of allergenic related proteins. Following direct application of anti-PrP^C antibodies on N2a cells, we identified 4 neuronal allergenic-related proteins when compared with untreated cells. Furthermore, we identified 8 neuronal allergenic-related proteins following treatment of N11 cells with anti-PrP^C antibodies prior to co-culture with N2a cells when compared with untreated cells. Antibody treatment of MPN or MPN co-cultured with antibody-treated N11 led to identifying 10 and 7 allergenic-related proteins when compared with untreated cells. However, comparison with 3F4 antibody treatment revealed 5 and 4 allergenic-related proteins respectively. Of importance, we showed that the allergenic effects triggered by the anti-PrP antibodies were more potent when antibody-treated microglia were co-cultured with the neuroblastoma cell line. Finally, co-culture of N2a or MPN with N11-treated with anti-PrP antibodies resulted in significant accumulation of NO and IL6 but not TNF-α in the cell culture media supernatant.

Conclusions

This study showed for the first time that anti-PrP antibody binding to PrP^C triggers a neuronal hypersensitivity response and highlights the important role of microglia in triggering an IgG-mediated neuronal hypersensitivity response. Moreover, this study provides an important impetus for including allergenic assessment of therapeutic antibodies for neurodegenerative disorders to derive safe and targeted biotherapeutics.

Single domain antibody-based vectors in the delivery of biologics across the blood-brain barrier: a review

Biologics are a promising and effective method for the treatment of central nervous system (CNS) diseases. The blood-brain barrier (BBB) is a natural barrier for the delivery of biologics into the brain, which decreases the effective concentration of drugs in the CNS. A range of strategies has been explored to transport biologics across the BBB endothelium, typically via receptor-mediated transcytosis (RMT), which involving molecules for endogenous BBB receptors to be fused with biologics. This review emphasized a category of novel alternative RMT-targeting vectors: single domain antibodies (sdAb). SdAbs are a unique category of antibodies derived from naturally occurring heavy-chain-only antibodies. Herein, we describe their properties, mechanisms, modifications, and translational perspectives for their ability to transmigrate across the BBB in vitro and in vivo in detail.

Brain Delivery of Single-Domain Antibodies: A Focus on VHH and VNAR

Passive immunotherapy, i.e., treatment with therapeutic antibodies, has been increasingly used over the last decade in several diseases such as cancers or inflammation. However, these proteins have some limitations that single-domain antibodies could potentially solve. One of the main issues of conventional antibodies is their limited brain penetration because of the blood-brain barrier (BBB). In this review, we aim at exploring the different options single-domain antibodies (sDAbs) such as variable domain of heavy-chain antibodies (VHHs) and variable new antigen receptors (VNARs) have already taken to reach the brain allowing them to be used as therapeutic, diagnosis or transporter tools.

Anti-vimentin, anti-TUFM, anti-NAP1L1 and anti-DPYSL2 nanobodies display cytotoxic effect and reduce glioblastoma cell migration

Background:

Glioblastoma is a particularly common and very aggressive primary brain tumour. One of the main causes of therapy failure is the presence of glioblastoma stem cells that are resistant to chemotherapy and radiotherapy, and that have the potential to form new tumours. This study focuses on validation of eight novel antigens, TRIM28, nucleolin, vimentin, nucleosome assembly protein 1-like 1 (NAP1L1), mitochondrial translation elongation factor (EF-TU) (TUFM), dihydropyrimidinase-related protein 2 (DPYSL2), collapsin response mediator protein 1 (CRMP1) and Aly/REF export factor (ALYREF), as putative glioblastoma targets, using nanobodies.

Methods:

Expression of these eight antigens was analysed at the cellular level by qPCR, ELISA and immunocytochemistry, and in tissues by immunohistochemistry. The cytotoxic effects of the nanobodies were determined using AlamarBlue and water-soluble tetrazolium tests. Annexin V/propidium iodide tests were used to determine apoptotsis/necrosis of the cells in the presence of the nanobodies. Cell migration assays were performed to determine the effects of the nanobodies on cell migration.

Results:

NAP1L1 and CRMP1 were significantly overexpressed in glioblastoma stem cells in comparison with astrocytes and glioblastoma cell lines at the mRNA and protein levels. Vimentin, DPYSL2 and ALYREF were overexpressed in glioblastoma cell lines only at the protein level. The functional part of the study examined the cytotoxic effects of the nanobodies on glioblastoma cell lines. Four of the nanobodies were selected in terms of their specificity towards glioblastoma cells and protein overexpression: anti-vimentin (Nb79), anti-NAP1L1 (Nb179), anti-TUFM (Nb225) and anti-DPYSL2 (Nb314). In further experiments to optimise the nanobody treatment schemes, to increase their effects, and to determine their impact on migration of glioblastoma cells, the anti-TUFM nanobody showed large cytotoxic effects on glioblastoma stem cells, while the anti-vimentin, anti-NAP1L1 and anti-DPYSL2 nanobodies were indicated as agents to target mature glioblastoma cells. The anti-vimentin nanobody also had significant effects on migration of mature glioblastoma cells.

Conclusion:

Nb79 (anti-vimentin), Nb179 (anti-NAP1L1), Nb225 (anti-TUFM) and Nb314 (anti-DPYSL2) nanobodies are indicated for further examination for cell targeting. The anti-TUFM nanobody, Nb225, is particularly potent for inhibition of cell growth after long-term exposure of glioblastoma stem cells, with minor effects seen for astrocytes. The anti-vimentin nanobody represents an agent for inhibition of cell migration.

Immunotherapy against Prion Disease

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

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

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

Structural evidence for the critical role of the prion protein hydrophobic region in forming an infectious prion

Prion or PrPSc is the proteinaceous infectious agent causing prion diseases in various mammalian species. Despite decades of research, the structural basis for PrPSc formation and prion infectivity remains elusive. To understand the role of the hydrophobic region in forming infectious prion at the molecular level, we report X-ray crystal structures of mouse (Mo) prion protein (PrP) (residues 89-230) in complex with a nanobody (Nb484). Using the recombinant prion propagation system, we show that the binding of Nb484 to the hydrophobic region of MoPrP efficiently inhibits the propagation of proteinase K resistant PrPSc and prion infectivity. In addition, when added to cultured mouse brain slices in high concentrations, Nb484 exhibits no neurotoxicity, which is drastically different from other neurotoxic anti-PrP antibodies, suggesting that the Nb484 can be a potential therapeutic agent against prion disease. In summary, our data provides the first structure-function evidence supporting a crucial role of the hydrophobic region of PrP in forming an infectious prion.

Single-Domain Antibodies as Therapeutic and Imaging Agents for the Treatment of CNS Diseases

Antibodies have become one of the most successful therapeutics for a number of oncology and inflammatory diseases. So far, central nervous system (CNS) indications have missed out on the antibody revolution, while they remain 'hidden' behind several hard to breach barriers. Among the various antibody modalities, single-domain antibodies (sdAbs) may hold the 'key' to unlocking the access of antibody therapies to CNS diseases. The unique structural features of sdAbs make them the smallest monomeric antibody fragments suitable for molecular targeting. These features are of particular importance when developing antibodies as modular building blocks for engineering CNS-targeting therapeutics and imaging agents. In this review, we first introduce the characteristic properties of sdAbs compared to traditional antibodies. We then present recent advances in the development of sdAbs as potential therapeutics across brain barriers, including their use for the delivery of biologics across the blood-brain and blood-cerebrospinal fluid (CSF) barriers, treatment of neurodegenerative diseases and molecular imaging of brain targets.

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.

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.

Prions: Beyond a Single Protein

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

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.

Targeting heat shock proteins to modulate α-synuclein toxicity

Parkinson's disease is a slowly progressive neurodegenerative disorder typically characterized by the loss of dopaminergic neurons within the substantia nigra pars compacta, and the intraneuronal deposition of insoluble protein aggregates chiefly comprised of α-synuclein. Patients experience debilitating symptoms including bradykinesia, rigidity and postural instability. No curative treatment currently exists and therapeutic strategies are restricted to symptomatic treatment only. Over the past decade a class of molecular chaperones called the heat shock proteins has emerged as a potentially promising therapeutic target. Heat shock proteins aid in the folding and refolding of proteins, and target denatured proteins to degradation systems. By targeting heat shock proteins through various means including overexpression and pharmacological enhancement, researchers have shown that α-synuclein aggregation and its associated cytotoxicity can be therapeutically modulated in an array of cell and animal models. This review highlights the relevant progress in this field and discusses the relevance of heat shock proteins as therapeutic modulators of α-synuclein toxicity to the rapidly evolving understanding of Parkinson's disease pathogenesis.

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.

Probing the N-terminal β-sheet conversion in the crystal structure of the human prion protein bound to a nanobody

Prions are fatal neurodegenerative transmissible agents causing several incurable illnesses in humans and animals. Prion diseases are caused by the structural conversion of the cellular prion protein, PrP(C), into its misfolded oligomeric form, known as prion or PrP(Sc). The canonical human PrP(C) (HuPrP) fold features an unstructured N-terminal part (residues 23-124) and a well-defined C-terminal globular domain (residues 125-231). Compelling evidence indicates that an evolutionary N-terminal conserved motif AGAAAAGA (residues 113-120) plays an important role in the conversion to PrP(Sc). The intrinsic flexibility of the N-terminal has hampered efforts to obtain detailed atomic information on the structural features of this palindromic region. In this study, we crystallized the full-length HuPrP in complex with a nanobody (Nb484) that inhibits prion propagation. In the complex, the prion protein is unstructured from residue 23 to 116. The palindromic motif adopts a stable and fully extended configuration to form a three-stranded antiparallel β-sheet with the β1 and β2 strands, demonstrating that the full-length HuPrP(C) can adopt a more elaborate β0-β1-α1-β2-α2-α3 structural organization than the canonical β1-α1-β2-α2-α3 prion-like fold. From this structure, it appears that the palindromic motif mediates β-enrichment in the PrP(C) monomer as one of the early events in the conversion of PrP(C) into PrP(Sc).

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.

Camelid single-domain antibody-fragment engineering for (pre)clinical in vivo molecular imaging applications: adjusting the bullet to its target

INTRODUCTION

Molecular imaging is a fast developing field and there is a growing need for specific imaging tracers in the clinic. Camelid single-domain antibody-fragments (sdAbs) recently emerged as a new class of molecular imaging tracers.

AREAS COVERED

We review the importance of molecular imaging in the clinic and the use of camelid sdAbs as in vivo molecular imaging tracers. Interest in imaging tracers based on antibody fragments or man-made protein scaffolds expanded over the last years. Camelid sdAbs are small, monomeric binding fragments that are derived from unique heavy-chain-only antibodies. In vivo imaging studies with sdAbs targeting various cell membrane receptors in different disease models have been reported and more sdAb imaging tracers are under development. The first clinical trial with a camelid sdAb as a molecular imaging tracer targeting the breast cancer marker Human Epidermal growth factor Receptor 2 is currently ongoing.

EXPERT OPINION

We expect that the development and use of sdAbs as tracers for both preclinical and clinical molecular imaging applications will become widespread.

Nanobodies as structural probes of protein misfolding and fibril formation

The deposition of peptides and proteins as amyloid fibrils is a common feature of nearly 50 medical -disorders affecting the brain or a variety of other organs and tissues. These disorders, which include Alzheimer's disease, Parkinson's disease, the prion diseases, and type II diabetes, have an enormous impact on the public health and economy of the modern world. Extensive research is therefore taking place to determine the underlying molecular mechanisms and determinants of the pathological conversion of amyloidogenic proteins from their soluble forms into fibrillar structures. The use of molecular probes and biophysical techniques, such as X-ray crystallography and particularly NMR spectroscopy, are allowing detailed analysis of the mechanism of fibril formation and of the underlying structural and chemical features of the associated pathogenicity. Nanobodies, the antigen-binding domains derived from camelid heavy-chain antibodies, are excellent tools to probe protein aggregation as a result of their exquisite specificity and high affinity and stability, along with their ease of expression and small size; the latter in particular allows them to be used very efficiently in combination with NMR spectroscopy and X-ray crystallography. In this chapter we present an overview of how nanobodies are being used to obtain detailed information on the mechanisms of amyloid formation and on the nature and origin of their links with human diseases.

Cell-penetrating anti-GFAP VHH and corresponding fluorescent fusion protein VHH-GFP spontaneously cross the blood-brain barrier and specifically recognize astrocytes: application to brain imaging

Antibodies normally do not cross the blood-brain barrier (BBB) and cannot bind an intracellular cerebral antigen. We demonstrate here for the first time that a new class of antibodies can cross the BBB without treatment. Camelids produce native homodimeric heavy-chain antibodies, the paratope being composed of a single-variable domain called VHH. Here, we used recombinant VHH directed against human glial fibrillary acidic protein (GFAP), a specific marker of astrocytes. Only basic VHHs (e.g., pI=9.4) were able to cross the BBB in vitro (7.8 vs. 0% for VHH with pI=7.7). By intracarotid and intravenous injections into live mice, we showed that these basic VHHs are able to cross the BBB in vivo, diffuse into the brain tissue, penetrate into astrocytes, and specifically label GFAP. To analyze their ability to be used as a specific transporter, we then expressed a recombinant fusion protein VHH-green fluorescent protein (GFP). These "fluobodies" specifically labeled GFAP on murine brain sections, and a basic variant (pI=9.3) of the fusion protein VHH-GFP was able to cross the BBB and to label astrocytes in vivo. The potential of VHHs as diagnostic or therapeutic agents in the central nervous system now deserves attention.

Antibodies in infectious diseases: polyclonals, monoclonals and niche biotechnology

Antibody preparations have a long history of providing protection from infectious diseases. Although antibodies remain the only natural host-derived defense mechanism capable of completely preventing infection, as products, they compete against inexpensive therapeutics such as antibiotics, small molecule inhibitors and active vaccines. The continued discovery in the monoclonal antibody (mAb) field of leads with broadened cross neutralization of viruses and demonstrable synergy of antibody with antibiotics for bacterial diseases, clearly show that innovation remains. The commercial success of mAbs in chronic disease has not been paralleled in infectious diseases for several reasons. Infectious disease immunotherapeutics are limited in scope as endemic diseases necessitate active vaccine development. Also, the complexity of these small markets draws the interest of niche companies rather than big pharmaceutical corporations. Lastly, the cost of goods for mAb therapeutics is inherently high for infectious agents due to the need for antibody cocktails, which better mimic polyclonal immunoglobulin preparations and prevent antigenic escape. In cases where vaccine or convalescent populations are available, current polyclonal hyperimmune immunoglobulin preparations (pIgG), with modern and highly efficient purification technology and standardized assays for potency, can make economic sense. Recent innovations to broaden the potency of mAb therapies, while reducing cost of production, are discussed herein. On the basis of centuries of effective use of Ab treatments, and with growing immunocompromised populations, the question is not whether antibodies have a bright future for infectious agents, but rather what formats are cost effective and generate safe and efficacious treatments to satisfy regulatory approval.

PrP(Sc)-specific antibodies with the ability to immunodetect prion oligomers

The development of antibodies with binding capacity towards soluble oligomeric forms of PrPSc recognised in the aggregation process in early stage of the disease would be of paramount importance in diagnosing prion diseases before extensive neuropathology has ensued. As blood transfusion appears to be efficient in the transmission of the infectious prion agent, there is an urgent need to develop reagents that would specifically recognize oligomeric forms of the abnormally folded prion protein, PrPSc.

To that end, we show that anti-PrP monoclonal antibodies (called PRIOC mAbs) derived from mice immunised with native PrP-coated microbeads are able to immunodetect oligomers/multimers of PrPSc. Oligomer-specific immunoreactivity displayed by these PRIOC mAbs was demonstrated as large aggregates of immunoreactive deposits in prion-permissive neuroblastoma cell lines but not in equivalent non-infected or prn-p^0/0 cell lines. In contrast, an anti-monomer PrP antibody displayed diffuse immunoreactivity restricted to the cell membrane. Furthermore, our PRIOC mAbs did not display any binding with monomeric recombinant and cellular prion proteins but strongly detected PrPSc oligomers as shown by a newly developed sensitive and specific ELISA. Finally, PrioC antibodies were also able to bind soluble oligomers formed of Aβ and α-synuclein. These findings demonstrate the potential use of anti-prion antibodies that bind PrPSc oligomers, recognised in early stage of the disease, for the diagnosis of prion diseases in blood and other body fluids.

Combining in-situ proteolysis and microseed matrix screening to promote crystallization of PrPc-nanobody complexes

Prion proteins (PrPs) are difficult to crystallize, probably due to their inherent flexibility. Several PrPs structures have been solved by nuclear magnetic resonance (NMR) techniques; however, only three structures were solved by X-ray crystallography. Here we combined in-situ proteolysis with automated microseed matrix screening (MMS) to crystallize two different PrP(C)-nanobody (Nb) complexes. Nanobodies are single-domain antibodies derived from heavy-chain-only antibodies of camelids. Initial crystallization screening conditions using in-situ proteolysis of mouse prion (23-230) in complex with a nanobody (Nb_PrP_01) gave thin needle aggregates, which were of poor diffraction quality. Next, we used these microcrystals as nucleants for automated MMS. Good-quality crystals were obtained from mouse PrP (89-230)/Nb_PrP_01, belonged to the monoclinic space group P 1 21 1, with unit-cell parameters a = 59.13, b = 63.80, c = 69.79 Å, β = 101.96° and diffracted to 2.1 Å resolution using synchrotron radiation. Human PrP (90-231)/Nb_PrP_01 crystals belonged to the monoclinic space group C2, with unit-cell parameters a = 131.86, b = 45.78, c = 45.09 Å, β = 96.23° and diffracted to 1.5 Å resolution. This combined strategy benefits from the power of the MMS technique without suffering from the drawbacks of the in-situ proteolysis. It proved to be a successful strategy to crystallize PrP-nanobodies complexes and could be exploited for the crystallization of other difficult antigen-antibody complexes.