Single-domain antibodies recognize selectively small oligomeric forms of amyloid beta, prevent Abeta-induced neurotoxicity and inhibit fibril formation

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.

In vivo detection of amyloid-β deposits using heavy chain antibody fragments in a transgenic mouse model for Alzheimer's disease

This study investigated the in vivo properties of two heavy chain antibody fragments (V_HH), ni3A and pa2H, to differentially detect vascular or parenchymal amyloid-β deposits characteristic for Alzheimer's disease and cerebral amyloid angiopathy. Blood clearance and biodistribution including brain uptake were assessed by bolus injection of radiolabeled V_HH in APP/PS1 mice or wildtype littermates. In addition, in vivo specificity for Aβ was examined in more detail with fluorescently labeled V_HH by circumventing the blood-brain barrier via direct application or intracarotid co-injection with mannitol. All V_HH showed rapid renal clearance (10–20 min). Twenty-four hours post-injection ^99mTc-pa2H resulted in a small yet significant higher cerebral uptake in the APP/PS1 animals. No difference in brain uptake were observed for ^99mTc-ni3A or DTPA(¹¹¹In)-pa2H, which lacked additional peptide tags to investigate further clinical applicability. In vivo specificity for Aβ was confirmed for both fluorescently labeled V_HH, where pa2H remained readily detectable for 24 hours or more after injection. Furthermore, both V_HH showed affinity for parenchymal and vascular deposits, this in contrast to human tissue, where ni3A specifically targeted only vascular Aβ. Despite a brain uptake that is as yet too low for in vivo imaging, this study provides evidence that V_HH detect Aβ deposits in vivo, with high selectivity and favorable in vivo characteristics, making them promising tools for further development as diagnostic agents for the distinctive detection of different Aβ deposits.

Engineered antibody approaches for Alzheimer's disease immunotherapy

The accumulation of amyloid-β-peptide (Aβ or A-beta) in the brain is considered to be a key event in the pathogenesis of Alzheimer's disease (AD). Over the last decade, antibody strategies aimed at reducing high levels of Aβ in the brain and or neutralizing its toxic effects have emerged as one of the most promising treatments for AD. Early approaches using conventional antibody formats demonstrated the potential of immunotherapy, but also caused a range of undesirable side effects such meningoencephalitis, vasogenic edema or cerebral microhemorrhages in both murine and humans. This prompted the exploration of alternative approaches using engineered antibodies to avoid adverse immunological responses and provide a safer and more effective therapy. Encouraging results have been obtained using a range of recombinant antibody formats including, single chain antibodies, antibody domains, intrabodies, bispecific antibodies as well as Fc-engineered antibodies in transgenic AD mouse and primate models. This review will address recent progress using these recombinant antibodies against Aβ, highlighting their advantages over conventional monoclonal antibodies and delivery methods.

Generation and characterization of a functional Nanobody against the vascular endothelial growth factor receptor-2; angiogenesis cell receptor

Vascular endothelial growth factor receptor-2 (VEGFR2) is an important tumor-associated receptor and blockade of the VEGF receptor signaling can lead to the inhibition of neovascularization and tumor metastasis. Nanobodies are the smallest intact antigen binding fragments derived from heavy chain-only antibodies occurring in camelids. Here, we describe the identification of a VEGFR2-specific Nanobody, named 3VGR19, from dromedaries immunized with a cell line expressing high levels of VEGFR2. We demonstrate by FACS, that 3VGR19 Nanobody specifically binds VEGFR2 on the surface of 293KDR and HUVECs cells. Furthermore, the 3VGR19 Nanobody potently inhibits formation of capillary-like structures. These data show the potential of Nanobodies for the blockade of VEGFR2 signaling and provide a basis for the development of novel cancer therapeutics.

A monoclonal antibody against synthetic Aβ dimer assemblies neutralizes brain-derived synaptic plasticity-disrupting Aβ

Diverse lines of evidence indicate that pre-fibrillar, diffusible assemblies of the amyloid β-protein (Aβ) play an important role in Alzheimer's disease pathogenesis. Although the precise molecular identity of these soluble toxins remains unsettled, recent experiments suggest that sodium dodecyl sulfate (SDS)-stable Aβ dimers may be the basic building blocks of Alzheimer's disease-associated synaptotoxic assemblies and as such present an attractive target for therapeutic intervention. In the absence of sufficient amounts of highly pure cerebral Aβ dimers, we have used synthetic disulfide cross-linked dimers (free of Aβ monomer or fibrils) to generate conformation-specific monoclonal antibodies. These dimers aggregate to form kinetically trapped protofibrils, but do not readily form fibrils. We identified two antibodies, 3C6 and 4B5, which preferentially bind assemblies formed from covalent Aβ dimers, but do not bind to Aβ monomer, amyloid precursor protein, or aggregates formed by other amyloidogenic proteins. Monoclonal antibody 3C6, but not an IgM isotype-matched control antibody, ameliorated the plasticity-disrupting effects of Aβ extracted from the aqueous phase of Alzheimer's disease brain, thus suggesting that 3C6 targets pathogenically relevant Aβ assemblies. These data prove the usefulness of covalent dimers and their assemblies as immunogens and recommend further investigation of the therapeutic and diagnostic utility of monoclonal antibodies raised to such assemblies.

Biotechnological applications of recombinant single-domain antibody fragments

BACKGROUND

Single-domain antibody fragments possess structural features, such as a small dimension, an elevated stability, and the singularity of recognizing epitopes non-accessible for conventional antibodies that make them interesting for several research and biotechnological applications.

RESULTS

The discovery of the single-domain antibody's potentials has stimulated their use in an increasing variety of fields. The rapid accumulation of articles describing new applications and further developments of established approaches has made it, therefore, necessary to update the previous reviews with a new and more complete summary of the topic.

CONCLUSIONS

Beside the necessary task of updating, this work analyses in detail some applicative aspects of the single-domain antibodies that have been overseen in the past, such as their efficacy in affinity chromatography, as co-crystallization chaperones, protein aggregation controllers, enzyme activity tuners, and the specificities of the unconventional single-domain fragments.

Nanobody; an old concept and new vehicle for immunotargeting

The use of antibodies in cancer therapy has come a long way since the day Paul Ehrlich described the concept and Kohler and Milstein devised the hybridoma technology to bring this theory to reality. The synthesis of murine monoclonal antibodies (mAbs) was the first success in this field, leading to the invention of chimerization, the production of variable fragments (Fv) with the progression to domain antibodies (dAb) and later humanization technologies to maximize the clinical utility of murine mAbs. It was just by chance that dAbs were found to exist in ?heavy chain? immunoglobulins from Camelidae family and cartilaginous fish. These unique antibody fragments interact with antigen by virtue of only one single variable domain, referred to as VHH or nanobody. Several characteristics make nanobody use superior to the abovementioned antibodies. They are non-immunogenic and show high thermal and chemical stability. There are several reports of raising specific nanobodies against enzymes, haptens, pathogens, toxins and tumor markers, which are outlined in this paper. All these characteristics make them strong candidates as targeting agents for cancer therapy.

Intramuscular delivery of a single chain antibody gene prevents brain Aβ deposition and cognitive impairment in a mouse model of Alzheimer's disease

Anti-beta-amyloid (Aβ) immunotherapy is effective in removing brain Aβ, but has shown to be associated with detrimental effects. We have demonstrated that Adeno-associated virus (AAV)-mediated delivery of an anti-Aβ single chain antibody (scFv) gene was effective in clearing brain Aβ without eliciting any inflammatory side effects in old APP(Swe)/PS1dE9 transgenic mice. In the present study, we tested the efficacy and safety of intramuscular delivery of the scFv gene in preventing brain Aβ deposition. The scFv gene was intramuscularly delivered to APP(Swe)/PS1dE9 transgenic mice at 3 months of age, prior to Aβ deposition in the brain. Six months later, we found that the transgenes were expressed in a stable form at the delivered sites, with a small amount of ectopic expression in the liver and olfactory bulb. Brain Aβ plaque formation, Aβ accumulation, AD-type pathologies and cognitive impairment were significantly attenuated in scFv-treated APP(Swe)/PS1dE9 transgenic mice relative to EGFP-treated mice. Intramuscular delivery of scFv gene was well tolerated by the animals, did not cause inflammation or microhemorrhage at the gene expression site and in the brain, and did not induce neutralizing antibodies in the animals. These findings suggest that peripheral application of scFv is effective and safe in preventing the development of Alzheimer's disease (AD), and would be a promising non-inflammatory immunological modality for prevention and treatment of AD.

In vivo imaging biomarkers in mouse models of Alzheimer's disease: are we lost in translation or breaking through?

Identification of biomarkers of Alzheimer's Disease (AD) is a critical priority to efficiently diagnose the patients, to stage the progression of neurodegeneration in living subjects, and to assess the effects of disease-modifier treatments. This paper addresses the development and usefulness of preclinical neuroimaging biomarkers of AD. It is today possible to image in vivo the brain of small rodents at high resolution and to detect the occurrence of macroscopic/microscopic lesions in these species, as well as of functional alterations reminiscent of AD pathology. We will outline three different types of imaging biomarkers that can be used in AD mouse models: biomarkers with clear translational potential, biomarkers that can serve as in vivo readouts (in particular in the context of drug discovery) exclusively for preclinical research, and finally biomarkers that constitute new tools for fundamental research on AD physiopathogeny.

Oral treatment with the d-enantiomeric peptide D3 improves the pathology and behavior of Alzheimer's Disease transgenic mice

Several lines of evidence suggest that the amyloid-β-peptide (Aβ) plays a central role in the pathogenesis of Alzheimer's disease (AD). Not only Aβ fibrils but also small soluble Aβ oligomers in particular are suspected to be the major toxic species responsible for disease development and progression. The present study reports on in vitro and in vivo properties of the Aβ targeting d-enantiomeric amino acid peptide D3. We show that next to plaque load and inflammation reduction, oral application of the peptide improved the cognitive performance of AD transgenic mice. In addition, we provide in vitro data elucidating the potential mechanism underlying the observed in vivo activity of D3. These data suggest that D3 precipitates toxic Aβ species and converts them into nonamyloidogenic, nonfibrillar, and nontoxic aggregates without increasing the concentration of monomeric Aβ. Thus, D3 exerts an interesting and novel mechanism of action that abolishes toxic Aβ oligomers and thereby supports their decisive role in AD development and progression.

Novel amyloid-beta specific scFv and VH antibody fragments from human and mouse phage display antibody libraries

Anti-amyloid immunotherapy has been proposed as an appropriate therapeutic approach for Alzheimer's disease (AD). Significant efforts have been made towards the generation and assessment of antibody-based reagents capable of preventing and clearing amyloid aggregates as well as preventing their synaptotoxic effects. In this study, we selected a novel set of human anti-amyloid-beta peptide 1-42 (Abeta1-42) recombinant monoclonal antibodies in a single chain fragment variable (scFv) and a single-domain (VH) format. We demonstrated that these antibody fragments recognize in a specific manner amyloid-beta deposits in APP/Tg mouse brains, inhibit toxicity of oligomeric Abeta1-42 in neuroblastoma cell cultures in a concentration-dependent manner and reduced amyloid deposits in APP/Tg2576 mice after intracranial administration. These antibody fragments recognize epitopes in the middle/C-terminus region of Abeta, which makes them strong therapeutic candidates due to the fact that most of the Abeta species found in the brains of AD patients display extensive N-terminus truncations/modifications.

Novel antibody derivatives for proteome and high-content analysis

The understanding of cellular processes and their pathophysiological alterations requires comprehensive data on the abundance, distribution, modification, and interaction of all cellular components. On the one hand, artificially introduced fluorescent fusion proteins provide information about their distribution and dynamics in living cells but not about endogenous factors. On the other hand, antibodies can detect endogenous proteins, posttranslational modifications, and other cellular components but mostly in fixed and permeabilized cells. Here we highlight a new technology based on the antigen-binding domain of heavy-chain antibodies (V(H)H) from Camelidae. These extremely stable V(H)H domains can be produced in bacteria, coupled to matrices, and used for affinity purification and proteome studies. Alternatively, these V(H)H domains can be fused with fluorescent proteins and expressed in living cells. These fluorescent antigen-binding proteins called "chromobodies" can be used to detect and trace proteins and other cellular components in vivo. Chromobodies can, in principle, detect any antigenic structure, including posttranslational modifications, and thereby dramatically expand the quality and quantity of information that can be gathered in high-content analysis. Depending on the epitope chosen, chromobodies can also be used to modulate protein function in living cells.

Basic mechanisms of neurodegeneration: a critical update

Neurodegenerative diseases are characterized by progressive dysfunction of specific populations of neurons, determining clinical presentation. Neuronal loss is associated with extra and intracellular accumulation of misfolded proteins, the hallmarks of many neurodegenerative proteinopathies. Major basic processes include abnormal protein dynamics due to deficiency of the ubiquitin-proteosome-autophagy system, oxidative stress and free radical formation, mitochondrial dysfunction, impaired bioenergetics, dysfunction of neurotrophins, 'neuroinflammatory' processes and (secondary) disruptions of neuronal Golgi apparatus and axonal transport. These interrelated mechanisms lead to programmed cell death is a long run over many years. Neurodegenerative disorders are classified according to known genetic mechanisms or to major components of protein deposits, but recent studies showed both overlap and intraindividual diversities between different phenotypes. Synergistic mechanisms between pathological proteins suggest common pathogenic mechanisms. Animal models and other studies have provided insight into the basic neurodegeneration and cell death programs, offering new ways for future prevention/treatment strategies.

RNA aptamers generated against oligomeric Abeta40 recognize common amyloid aptatopes with low specificity but high sensitivity

Aptamers are useful molecular recognition tools in research, diagnostics, and therapy. Despite promising results in other fields, aptamer use has remained scarce in amyloid research, including Alzheimer's disease (AD). AD is a progressive neurodegenerative disease believed to be caused by neurotoxic amyloid β-protein (Aβ) oligomers. Aβ oligomers therefore are an attractive target for development of diagnostic and therapeutic reagents. We used covalently-stabilized oligomers of the 40-residue form of Aβ (Aβ40) for aptamer selection. Despite gradually increasing the stringency of selection conditions, the selected aptamers did not recognize Aβ40 oligomers but reacted with fibrils of Aβ40, Aβ42, and several other amyloidogenic proteins. Aptamer reactivity with amyloid fibrils showed some degree of protein-sequence dependency. Significant fibril binding also was found for the naïve library and could not be eliminated by counter-selection using Aβ40 fibrils, suggesting that aptamer binding to amyloid fibrils was RNA-sequence-independent. Aptamer binding depended on fibrillogenesis and showed a lag phase. Interestingly, aptamers detected fibril formation with ≥15-fold higher sensitivity than thioflavin T (ThT), revealing substantial β-sheet and fibril formation undetected by ThT. The data suggest that under physiologic conditions, aptamers for oligomeric forms of amyloidogenic proteins cannot be selected due to high, non-specific affinity of oligonucleotides for amyloid fibrils. Nevertheless, the high sensitivity, whereby aptamers detect β-sheet formation, suggests that they can serve as superior amyloid recognition tools.

Inherent anti-amyloidogenic activity of human immunoglobulin gamma heavy chains

We have previously shown that a subpopulation of naturally occurring human IgGs were cross-reactive against conformational epitopes on pathologic aggregates of Abeta, a peptide that forms amyloid fibrils in the brains of patients with Alzheimer disease, inhibited amyloid fibril growth, and dissociated amyloid in vivo. Here, we describe similar anti-amyloidogenic activity that is a general property of free human Ig gamma heavy chains. A gamma(1) heavy chain, F1, had nanomolar binding to an amyloid fibril-related conformational epitope on synthetic oligomers and fibrils as well as on amyloid-laden tissue sections. F1 did not bind to native Abeta monomers, further indicating the conformational nature of its binding site. The inherent anti-amyloidogenic activity of Ig gamma heavy chains was demonstrated by nanomolar amyloid fibril and oligomer binding by polyclonal and monoclonal human heavy chains that were isolated from inert or weakly reactive antibodies. Most importantly, the F1 heavy chain prevented in vitro fibril growth and reduced in vivo soluble Abeta oligomer-induced impairment of rodent hippocampal long term potentiation, a cellular mechanism of learning and memory. These findings demonstrate that free human Ig gamma heavy chains comprise a novel class of molecules for developing potential therapeutics for Alzheimer disease and other amyloid disorders. Moreover, establishing the molecular basis for heavy chain-amyloidogenic conformer interactions should advance understanding on the types of interactions that these pathologic assemblies have with biological molecules.

Llama VHH antibody fragments against GFAP: better diffusion in fixed tissues than classical monoclonal antibodies

Camelids produce antibodies made of homodimeric heavy chains, and the antigen-binding region being composed of a single domain called VHH. These VHHs are much smaller than complete IgG. They are also more thermostable and more soluble in water; they should, therefore, diffuse more readily in the tissues. VHHs, expressed in bacteria, are easier to produce than conventional monoclonal antibodies. Because of these special characteristics, these antibody fragments could have interesting developments in immunohistochemistry and in the development of biomarkers. To test the possibility of their use in immunohistochemistry (IHC), we selected the glial fibrillary acidic protein (GFAP), a well-known marker of astrocytes. One alpaca (Lama pacos) was immunized against GFAP. Lymphocytes were isolated; the DNA was extracted; the VHH-coding sequences were selectively amplified. Three VHHs with a high affinity for GFAP and their corresponding mRNA were selected by ribosome display. Large quantities of the recombinant VHHs coupled with different tags were harvested from transfected bacteria. One of them was shown to immunolabel strongly and specifically to GFAP of human astrocytes in tissue sections. The quality of the IHC was comparable or, in some aspects, superior to the quality obtained with conventional IgG. The VHH was shown to diffuse on a longer distance than conventional monoclonal antibodies in fixed cortical tissue: a property that may be useful in immunolabeling of thick sections.

Single domain antibodies: promising experimental and therapeutic tools in infection and immunity

Antibodies are important tools for experimental research and medical applications. Most antibodies are composed of two heavy and two light chains. Both chains contribute to the antigen-binding site which is usually flat or concave. In addition to these conventional antibodies, llamas, other camelids, and sharks also produce antibodies composed only of heavy chains. The antigen-binding site of these unusual heavy chain antibodies (hcAbs) is formed only by a single domain, designated VHH in camelid hcAbs and VNAR in shark hcAbs. VHH and VNAR are easily produced as recombinant proteins, designated single domain antibodies (sdAbs) or nanobodies. The CDR3 region of these sdAbs possesses the extraordinary capacity to form long fingerlike extensions that can extend into cavities on antigens, e.g., the active site crevice of enzymes. Other advantageous features of nanobodies include their small size, high solubility, thermal stability, refolding capacity, and good tissue penetration in vivo. Here we review the results of several recent proof-of-principle studies that open the exciting perspective of using sdAbs for modulating immune functions and for targeting toxins and microbes.