Tau monoclonal antibody generation based on humanized yeast models: impact on Tau oligomerization and diagnostics

Development of a pan-tau multivalent nanobody that binds tau aggregation motifs and recognizes pathological tau aggregates

Alzheimer's disease and other tauopathies are characterized by the misfolding and aggregation of the tau protein into oligomeric and fibrillar structures. Antibodies against tau play an increasingly important role in studying these neurodegenerative diseases and the generation of tools to diagnose and treat them. The development of antibodies that recognize tau protein aggregates, however, is hindered by complex immunization and antibody selection strategies and limitations to antigen presentation. Here, we have taken a facile approach to identify single-domain antibodies, or nanobodies, that bind to many forms of tau by screening a synthetic yeast surface display nanobody library against monomeric tau and creating multivalent versions of our lead nanobody, MT3.1, to increase its avidity for tau aggregates. We demonstrate that MT3.1 binds to tau monomer, oligomers, and fibrils, as well as pathogenic tau from a tauopathy mouse model, despite being identified through screens against monomeric tau. Through epitope mapping, we discovered binding epitopes of MT3.1 contain the key motif VQIXXK which drives tau aggregation. We show that our bivalent and tetravalent versions of MT3.1 have greatly improved binding ability to tau oligomers and fibrils compared to monovalent MT3.1. Our results demonstrate the utility of our nanobody screening and multivalent design approach in developing nanobodies that bind amyloidogenic protein aggregates. This approach can be extended to the generation of multivalent nanobodies that target other amyloid proteins and has the potential to advance the research and treatment of neurodegenerative diseases.

Alzheimer's Disease: Significant Benefit from the Yeast-Based Models

Alzheimer's disease (AD) is an age-related, multifaceted neurological disorder associated with accumulation of aggregated proteins (amyloid Aβ and hyperphosphorylated tau), loss of synapses and neurons, and alterations in microglia. AD was recognized by the World Health Organization as a global public health priority. The pursuit of a better understanding of AD forced researchers to pay attention to well-defined single-celled yeasts. Yeasts, despite obvious limitations in application to neuroscience, show high preservation of basic biological processes with all eukaryotic organisms and offer great advantages over other disease models due to the simplicity, high growth rates on low-cost substrates, relatively simple genetic manipulations, the large knowledge base and data collections, and availability of an unprecedented amount of genomic and proteomic toolboxes and high-throughput screening techniques, inaccessible to higher organisms. Research reviewed above clearly indicates that yeast models, together with other, more simple eukaryotic models including animal models, C. elegans and Drosophila, significantly contributed to understanding Aβ and tau biology. These models allowed high throughput screening of factors and drugs that interfere with Aβ oligomerization, aggregation and toxicity, and tau hyperphosphorylation. In the future, yeast models will remain relevant, with a focus on creating novel high throughput systems to facilitate the identification of the earliest AD biomarkers among different cellular networks in order to achieve the main goal-to develop new promising therapeutic strategies to treat or prevent the disease.

Non-Rodent Genetic Animal Models for Studying Tauopathy: Review of Drosophila, Zebrafish, and C. elegans Models

Tauopathy refers to a group of progressive neurodegenerative diseases, including frontotemporal lobar degeneration and Alzheimer's disease, which correlate with the malfunction of microtubule-associated protein Tau (MAPT) due to abnormal hyperphosphorylation, leading to the formation of intracellular aggregates in the brain. Despite extensive efforts to understand tauopathy and develop an efficient therapy, our knowledge is still far from complete. To find a solution for this group of devastating diseases, several animal models that mimic diverse disease phenotypes of tauopathy have been developed. Rodents are the dominating tauopathy models because of their similarity to humans and established disease lines, as well as experimental approaches. However, powerful genetic animal models using Drosophila, zebrafish, and C. elegans have also been developed for modeling tauopathy and have contributed to understanding the pathophysiology of tauopathy. The success of these models stems from the short lifespans, versatile genetic tools, real-time in-vivo imaging, low maintenance costs, and the capability for high-throughput screening. In this review, we summarize the main findings on mechanisms of tauopathy and discuss the current tauopathy models of these non-rodent genetic animals, highlighting their key advantages and limitations in tauopathy research.

Dangerous Liaisons: Tau Interaction with Muscarinic Receptors

The molecular processes underlying neurodegenerative diseases (such as Alzheimer's Disease - AD) remain poorly understood. There is also an imperative need for disease-modifying therapies in AD since the present treatments, acetylcholinesterase inhibitors and NMDA antagonists, do not halt its progression. AD and other dementias present unique pathological features such as that of microtubule associated protein tau metabolic regulation. Tau has numerous binding partners, including signaling molecules, cytoskeletal elements and lipids, which suggests that it is a multifunctional protein. AD has also been associated with severe loss of cholinergic markers in the brain and such loss may be due to the toxic interaction of tau with cholinergic muscarinic receptors. By using specific antagonists of muscarinic receptors it was found in vitro that extracellular tau binds to M1 and M3 receptors and which the increase of intracellular calcium found in neuronal cells upon tau-binding. However, so far, the significance of tau signaling through muscarinic receptor in vivo in tauopathic models remains uncertain. The data reviewed in the present paper highlight the significant effect of M1 receptor/tau interaction in exacerbating tauopathy related pathological features and suggest that selective M1 agonists may serve as a prototype for future therapeutic development toward modification of currently intractable neurodegenerative diseases, such as tauopathies.

A Novel Tau Antibody Detecting the First Amino-Terminal Insert Reveals Conformational Differences Among Tau Isoforms

As human Tau undergoes pathologically relevant post-translational modifications when expressed in yeast, the use of humanized yeast models for the generation of novel Tau monoclonal antibodies has previously been proven to be successful. In this study, human Tau2N4R-ΔK280 purified from yeast was used for the immunization of mice and subsequent selection of high affinity Tau-specific monoclonal antibodies. The characterization of four novel antibodies in different Tau model systems yielded a phosphorylation-dependent antibody (15A10), an antibody directed to the first microtubule-binding repeat domain (16B12), a carboxy-terminal antibody (20G10) and an antibody targeting an epitope on the hinge of the first and second amino-terminal insert (18F12). The latter was found to be conformation-dependent, suggesting structural differences between the Tau splicing isoforms and allowing insight in the roles played by the amino-terminal inserts. As this monoclonal antibody also has the capacity to detect tangle-like structures in different transgenic Tau mice and neurofibrillary tangles in brain sections of patients diagnosed with Alzheimer's disease, we also tested the diagnostic potential of 18F12 in a pilot study and found this monoclonal antibody to have the ability to discriminate Alzheimer's disease patients from control individuals based on increased Tau levels in the cerebrospinal fluid.

Yeast surface display of full-length human microtubule-associated protein tau

Microtubule-associated protein tau is an intrinsically disordered, highly soluble protein found primarily in neurons. Under normal conditions, tau regulates the stability of axonal microtubules and intracellular vesicle transport. However, in patients of neurodegeneration such as Alzheimer's disease (AD), tau forms neurofibrillary deposits, which correlates well with the disease progression. Identifying molecular signatures in tau, such as posttranslational modification, truncation, and conformational change has great potential to detect earliest signs of neurodegeneration and develop therapeutic strategies. Here, we show that full-length human tau, including the longest isoform found in the adult brain, can be robustly displayed on the surface of yeast Saccharomyces cerevisiae. Yeast-displayed tau binds to anti-tau antibodies that cover epitopes ranging from the N-terminus to the 4R repeat region. Unlike tau expressed in the yeast cytosol, surface-displayed tau was not phosphorylated at sites found in AD patients (probed by antibodies AT8, AT270, AT180, and PHF-1). However, yeast-displayed tau showed clear binding to paired helical filament (PHF) tau conformation-specific antibodies Alz-50, MC-1, and Tau-2. Although the tau possessed a conformation found in PHFs, oligomerization or aggregation into larger filaments was undetected. Taken together, yeast-displayed tau enables robust measurement of protein interactions and is of particular interest for characterizing conformational change.

The elusive tau molecular structures: can we translate the recent breakthroughs into new targets for intervention?

Insights into tau molecular structures have advanced significantly in recent years. This field has been the subject of recent breakthroughs, including the first cryo-electron microscopy structures of tau filaments from Alzheimer’s and Pick’s disease inclusions, as well as the structure of the repeat regions of tau bound to microtubules. Tau structure covers various species as the tau protein itself takes many forms. We will here address a range of studies that help to define the many facets of tau protein structures and how they translate into pathogenic forms. New results shed light on previous data that need now to be revisited in order to up-date our knowledge of tau molecular structure. Finally, we explore how these data can contribute the important medical aspects of this research - diagnosis and therapeutics.

Different tau species lead to heterogeneous tau pathology propagation and misfolding

Tauopathies are a heterogeneous group of pathologies characterized by tau aggregation inside neurons. Most of them are sporadic but certain tauopathies rely on tau gene (MAPT) mutations. They particularly differ from one to another by their different neuropathological signatures e.g. lesion shapes, regions affected and molecular composition of aggregates. Six isoforms of tau exist, but they do not all co-aggregate in each tauopathy but rather have a unique signature for each one. In some tauopathies such as Alzheimer’s disease (AD), tau protein aggregation follows stereotypical anatomical stages. Recent data suggest that this progression is due to an active process of tau protein propagation from neuron-to-neuron. We wondered how tau isoforms or mutations could influence the process of tau aggregation and tau propagation. In human neuropathological material, we found that MAPT mutations induce a faster misfolding compared to tau found in sporadic AD patients. In the rat brain, we observed cell-to-cell transfer of non-pathological tau species irrespective of the tested isoform or presence of a mutation. By contrast, we found that the species of tau impact the propagation of tau pathology markers such as hyperphosphorylation and misfolding. Indeed, misfolding and hyperphosphorylated tau proteins do not spread at the same rate when tau is mutated, or the isoform composition is modified. These results clearly argue for the existence of specific folding properties of tau depending on isoforms or mutations impacting the behavior of pathological tau species.

Directed evolution of a picomolar-affinity, high-specificity antibody targeting phosphorylated tau

Antibodies are essential biochemical reagents for detecting protein post-translational modifications (PTMs) in complex samples. However, recent efforts in developing PTM-targeting antibodies have reported frequent nonspecific binding and limited affinity of such antibodies. To address these challenges, we investigated whether directed evolution could be applied to improve the affinity of a high-specificity antibody targeting phosphothreonine 231 (pThr-231) of the human microtubule-associated protein tau. On the basis of existing structural information, we hypothesized that improving antibody affinity may come at the cost of loss in specificity. To test this hypothesis, we developed a novel approach using yeast surface display to quantify the specificity of PTM-targeting antibodies. When we affinity-matured the single-chain variable antibody fragment through directed evolution, we found that its affinity can be improved >20-fold over that of the WT antibody, reaching a picomolar range. We also discovered that most of the high-affinity variants exhibit cross-reactivity toward the nonphosphorylated target site but not to the phosphorylation site with a scrambled sequence. However, systematic quantification of the specificity revealed that such a tradeoff between the affinity and specificity did not apply to all variants and led to the identification of a picomolar-affinity variant that has a matching high specificity of the original phosphotau antibody. In cell- and tissue-imaging experiments, the high-affinity variant gave significantly improved signal intensity while having no detectable nonspecific binding. These results demonstrate that directed evolution is a viable approach for obtaining high-affinity PTM-specific antibodies and highlight the importance of assessing the specificity in the antibody engineering process.

Recent Insights on Alzheimer's Disease Originating from Yeast Models

In this review article, yeast model-based research advances regarding the role of Amyloid-β (Aβ), Tau and frameshift Ubiquitin UBB+1 in Alzheimer’s disease (AD) are discussed. Despite having limitations with regard to intercellular and cognitive AD aspects, these models have clearly shown their added value as complementary models for the study of the molecular aspects of these proteins, including their interplay with AD-related cellular processes such as mitochondrial dysfunction and altered proteostasis. Moreover, these yeast models have also shown their importance in translational research, e.g., in compound screenings and for AD diagnostics development. In addition to well-established Saccharomyces cerevisiae models, new upcoming Schizosaccharomyces pombe, Candida glabrata and Kluyveromyces lactis yeast models for Aβ and Tau are briefly described. Finally, traditional and more innovative research methodologies, e.g., for studying protein oligomerization/aggregation, are highlighted.

Digital ELISA for the quantification of attomolar concentrations of Alzheimer's disease biomarker protein Tau in biological samples

The close correlation between Tau pathology and Alzheimer's disease (AD) progression makes this protein a suitable biomarker for diagnosis and monitoring of the disorder evolution. However, the use of Tau in diagnostics has been hampered, as it currently requires collection of cerebrospinal fluid (CSF), which is an invasive clinical procedure. Although measuring Tau-levels in blood plasma would be favorable, the concentrations are below the detection limit of a conventional ELISA. In this work, we developed a digital ELISA for the quantification of attomolar protein Tau concentrations in both buffer and biological samples. Individual Tau molecules were first captured on the surface of magnetic particles using in-house developed antibodies and subsequently isolated into the femtoliter-sized wells of a 2 × 2 mm² microwell array. Combination of high-affinity antibodies, optimal assay conditions and a digital quantification approach resulted in a 24 ± 7 aM limit of detection (LOD) in buffer samples. Additionally, a dynamic range of 6 orders of magnitude was achieved by combining the digital readout with an analogue approach, allowing quantification from attomolar to picomolar levels of Tau using the same platform. This proves the compatibility of the presented assay with the wide range of Tau concentrations encountered in different biological samples. Next, the developed digital assay was applied to detect total Tau levels in spiked blood plasma. A similar LOD (55 ± 29 aM) was obtained compared to the buffer samples, which was 5000-fold more sensitive than commercially available ELISAs and even outperformed previously reported digital assays with 10-fold increase in sensitivity. Finally, the performance of the developed digital ELISA was assessed by quantifying protein Tau in three clinical CSF samples. Here, a high correlation (i.e. Pearson coefficient of 0.99) was found between the measured percentage of active particles and the reference protein Tau values. The presented digital ELISA technology has great capacity in unlocking the potential of Tau as biomarker for early AD diagnosis.

Preparation and Characterization of Tau Oligomer Strains

An increasing number of studies have demonstrated the existence of multiple conformational entities of tau, as have been observed for prion protein. We have developed and optimized techniques to isolate and study oligomeric tau strains both in vitro and ex vivo. Moreover, we have modified protocols that demonstrate the seeding properties of oligomeric tau strains that are capable of propagating in vivo. These methods and protocols are explained in this chapter.

NMR Meets Tau: Insights into Its Function and Pathology

In this review, we focus on what we have learned from Nuclear Magnetic Resonance (NMR) studies on the neuronal microtubule-associated protein Tau. We consider both the mechanistic details of Tau: the tubulin relationship and its aggregation process. Phosphorylation of Tau is intimately linked to both aspects. NMR spectroscopy has depicted accurate phosphorylation patterns by different kinases, and its non-destructive character has allowed functional assays with the same samples. Finally, we will discuss other post-translational modifications of Tau and its interaction with other cellular factors in relationship to its (dys)function.

Signaling pathways and posttranslational modifications of tau in Alzheimer's disease: the humanization of yeast cells

In the past decade, yeast have been frequently employed to study the molecular mechanisms of human neurodegenerative diseases, generally by means of heterologous expression of genes encoding the relevant hallmark proteins. However, it has become evident that substantial posttranslational modifications of many of these proteins are required for the development and progression of potentially disease relevant changes. This is exemplified by the neuronal tau proteins, which are critically involved in a class of neuro-degenerative diseases collectively called tauopathies and which includes Alz-heimer's disease (AD) as its most common representative. In the course of the disease, tau changes its phosphorylation state and becomes hyperphosphory-lated, gets truncated by proteolytic cleavage, is subject to O-glycosylation, sumoylation, ubiquitinylation, acetylation and some other modifications. This poses the important question, which of these posttranslational modifications are naturally occurring in the yeast model or can be reconstituted by heterol-ogous gene expression. Here, we present an overview on common modifica-tions as they occur in tau during AD, summarize their potential relevance with respect to disease mechanisms and refer to the native yeast enzyme orthologs capable to perform these modifications. We will also discuss potential approaches to humanize yeast in order to create modification patterns resembling the situation in mammalian cells, which could enhance the value of Saccharomyces cerevisiae and Kluyveromyces lactis as disease models.

Signaling pathways and posttranslational modifications of tau in Alzheimer's disease: the humanization of yeast cells

In the past decade, yeast have been frequently employed to study the molecular mechanisms of human neurodegenerative diseases, generally by means of heterologous expression of genes encoding the relevant hallmark proteins. However, it has become evident that substantial posttranslational modifications of many of these proteins are required for the development and progression of potentially disease relevant changes. This is exemplified by the neuronal tau proteins, which are critically involved in a class of neuro-degenerative diseases collectively called tauopathies and which includes Alz-heimer's disease (AD) as its most common representative. In the course of the disease, tau changes its phosphorylation state and becomes hyperphosphory-lated, gets truncated by proteolytic cleavage, is subject to O-glycosylation, sumoylation, ubiquitinylation, acetylation and some other modifications. This poses the important question, which of these posttranslational modifications are naturally occurring in the yeast model or can be reconstituted by heterol-ogous gene expression. Here, we present an overview on common modifica-tions as they occur in tau during AD, summarize their potential relevance with respect to disease mechanisms and refer to the native yeast enzyme orthologs capable to perform these modifications. We will also discuss potential approaches to humanize yeast in order to create modification patterns resembling the situation in mammalian cells, which could enhance the value of Saccharomyces cerevisiae and Kluyveromyces lactis as disease models.

Tau in physiology and pathology

Tau is a microtubule-associated protein that has a role in stabilizing neuronal microtubules and thus in promoting axonal outgrowth. Structurally, tau is a natively unfolded protein, is highly soluble and shows little tendency for aggregation. However, tau aggregation is characteristic of several neurodegenerative diseases known as tauopathies. The mechanisms underlying tau pathology and tau-mediated neurodegeneration are debated, but considerable progress has been made in the field of tau research in recent years, including the identification of new physiological roles for tau in the brain. Here, we review the expression, post-translational modifications and functions of tau in physiology and in pathophysiology.

tau protein, ischemic injury and vascular dementia

Clinical, neuroimaging and neuropathological studies have confirmed overlap between Alzheimer's disease (AD) and vascular dementia (VaD). Classical neuropathological changes of AD (plaques and tangles) can be present in VaD. We review neuroimaging, biochemical and animal studies to consider the role of tau protein in ischemic injury and VaD pathogenesis. The evidence comes largely from transgenic animal studies that confirm that tau transgenes influence cerebral vasculature. Clinicobiochemical studies in the cerebrospinal fluid (CSF) have, similarly, confirmed alterations in both total and phosphorylated tau protein in VaD. These data suggest that tau protein not only serves as a potential diagnostic tool for differential diagnosis of VaD from other types of dementia, but may also be a therapeutic target in ischemic stroke.