Disease-specific tau filaments assemble via polymorphic intermediates

Intermediate species in the assembly of amyloid filaments are believed to play a central role in neurodegenerative diseases and may constitute important targets for therapeutic intervention1,2. However, structural information about intermediate species has been scarce and the molecular mechanisms by which amyloids assemble remain largely unknown. Here we use time-resolved cryogenic electron microscopy to study the in vitro assembly of recombinant truncated tau (amino acid residues 297-391) into paired helical filaments of Alzheimer's disease or into filaments of chronic traumatic encephalopathy3. We report the formation of a shared first intermediate amyloid filament, with an ordered core comprising residues 302-316. Nuclear magnetic resonance indicates that the same residues adopt rigid, β-strand-like conformations in monomeric tau. At later time points, the first intermediate amyloid disappears and we observe many different intermediate amyloid filaments, with structures that depend on the reaction conditions. At the end of both assembly reactions, most intermediate amyloids disappear and filaments with the same ordered cores as those from human brains remain. Our results provide structural insights into the processes of primary and secondary nucleation of amyloid assembly, with implications for the design of new therapies.

Structure challenges in the multivalency of Tau-microtubule interactions

Structural studies aiming to visualize the interaction of Tau with microtubules (MTs) face several challenges, the main concerning the fact that Tau has multiple MT-interacting regions. In particular, the four (or three) pseudo-repeats of Tau bind to identical elements along the MT lattice but do it through non-identical residues. In addition, any given Tau molecule can use all its repeats or just one for its engagement with MTs. Finally, the binding of one Tau is not necessarily in register with respect to the next one. The mismatch in the MT and Tau repeats, therefore, challenges conventional modes of image analysis when visualizing these samples using cryo-electron microscopy. This commentary is dedicated to those challenges and ways to circumvent them while aiming for an atomic description of the Tau-tubulin interaction.

Tau Oligomers as Pathogenic Seeds: Preparation, Characterization, and Propagation In Vitro and In Vivo

Tau oligomers have been shown to be the main toxic tau species in several neurodegenerative disorders. To study tau oligomers, we have developed reagents and established methods for the reliable preparation, isolation, and detection of tau oligomers as well as their seeding and propagation both in vitro and in vivo. Detailed below are methods for isolation of tau oligomers from brain tissues and detection of tau oligomers using tau oligomer-specific antibodies by biochemical, immunohistochemical, and biophysical methods. Further, methods for evaluating the biological activity of the tau oligomers including their effects on synaptic function, seeding, and propagation in cell models and in vivo are also described.

Tau: More than a microtubule-binding protein in neurons

Tau protein was discovered as a microtubule-associated protein nearly 50 years ago, and our understanding of tau has revolved around that role. Even with tau's rise to stardom as a central player in neurodegenerative disease, therapeutic efforts have largely been targeted toward cytoskeletal changes. While some studies hinted toward non-cytoskeletal roles for tau, it is only fairly recently that these ideas have begun to receive considerable attention. Many new binding partners for tau have been identified, including DNA, RNA, RNA-binding proteins, some receptors, and other tau molecules. The diversity of tau binding partners coupled with the discovery of tau other than axonal compartments such as nucleus, dendrites, and synapses have led to the proposal of novel functions for tau in roles such as nuclear stability, cell signaling, transcriptional processing, and protein synthesis. Tau self-assembly in particular has made an impact, leading to the hypothesis that a prion-like function of hyperphosphorylated tau is central to tauopathies. With tau emerging as a multifaceted protein that operates in many parts of the cell and with many molecular partners, the field of tau biology is primed for discoveries that can provide new perspectives on both the unique biochemistry of tau and the nature of devastating neurological diseases.

Travels with tau prions

Tau was originally identified as a microtubule associated protein, and subsequently recognized to constitute the fibrillar assemblies found in Alzheimer disease and related neurodegenerative tauopathies. Point mutations in the microtubule associated protein tau (MAPT) gene cause dominantly inherited tauopathies, and most predispose it to aggregate. This indicates tau aggregation underlies pathogenesis of tauopathies. Our work has suggested that tau functions as a prion, forming unique intracellular pathological assemblies that subsequently move to other cells, inducing further aggregation that underlies disease progression. Remarkably, in simple cells tau forms stably propagating aggregates of distinct conformation, termed strains. Each strain induces a unique and, in some cases, transmissible, neuropathological phenotype upon inoculation into a mouse model. After binding heparan sulfate proteoglycans on the plasma membrane, tau assemblies enter cells via macropinocytosis. From within a vesicle, if not trafficked to the endolysosomal system, tau subsequently enters the cytoplasm, where it becomes a template for its own replication, apparently after processing by valosin containing protein. The smallest seed unit is a stable monomer, which suggests that initial folding events in tau presage subsequent pathological aggregation. The study of tau prions has raised important questions about basic cell biological processes that underlie their replication and propagation, with implications for therapy of tauopathies.

Big tau: What, how, where and why

Over the last 50 years the different isoforms of tau proteins (45-60 kDa) have been a focus of research because of their roles in modulating the dynamic properties of microtubules shaping the structure and function of neurons but also becoming a center of attention in the pathology of neurodegeneration associated with tauopathies. Much less attention has been given to Big tau, a unique isoform containing exon 4a encoding about 250 amino acids to form a much longer projection domain of a protein of 110 kDa. Big tau is expressed in peripheral neurons and selective regions of the central nervous system in a defined transition during postnatal developmental stages. Although Big tau was discovered 30 years ago, there has been a persistent gap of knowledge regarding its physiological properties and pathological implications. This Perspective summarizes the progress so far in defining the structure and expression of Big tau within and outside the nervous system, proposes a role for Big tau in improving axonal transport in projecting axons, considers its potential in averting tau aggregation in tauopathies and highlights the need for further progress.

Molecular Mechanism of Tau Misfolding and Aggregation: Insights from Molecular Dynamics Simulation

Tau dysfunction has a close association with many neurodegenerative diseases, which are collectively referred to as tauopathies. Neurofibrillary tangles (NFTs) formed by misfolding and aggregation of tau are the main pathological process of tauopathy. Therefore, uncovering the misfolding and aggregation mechanism of tau protein will help to reveal the pathogenic mechanism of tauopathies. Molecular dynamics (MD) simulation is well suited for studying the dynamic process of protein structure changes. It provides detailed information on protein structure changes over time at the atomic resolution. At the same time, MD simulation can also simulate various conditions conveniently. Based on these advantages, MD simulations are widely used to study conformational transition problems such as protein misfolding and aggregation. Here, we summarized the structural features of tau, the factors affecting its misfolding and aggregation, and the applications of MD simulations in the study of tau misfolding and aggregation.

Effects of Neighboring Phosphorylation Events on the Affinities of pT181-Tau Antibodies

A tau variant phosphorylated on threonine 181 (pT181-tau) has been widely investigated as a potential Alzheimer's disease (AD) biomarker in cerebrospinal fluid (CSF) and blood. pT181-tau is present in neurofibrillary tangles (NFTs) of AD brains, and CSF levels of pT181-tau correlate with the overall NFT burden. Various immunobased analytical methods, including Western blotting and ELISA, have been used to quantify pT181-tau in human biofluids. The reliability of these methods is dependent on the affinity and binding specificity of the antibodies used to measure pT181-tau levels. Although both of these properties could, in principle, be affected by phosphorylation within or near the antibody's cognate antigen, such effects have not been extensively studied. Here, we developed a biolayer interferometry assay to determine the degree to which the affinity of pT181-tau antibodies is altered by the phosphorylation of serine or threonine residues near the target epitope. Our results revealed that phosphorylation near T181 negatively affected the binding of pT181-tau antibodies to their cognate antigen to varying degrees. In particular, two of three antibodies tested showed a complete loss of affinity for the pT181 target when S184 or S185 was phosphorylated. These findings highlight the importance of selecting antibodies that have been thoroughly characterized in terms of affinity and binding specificity, addressing the potential disruptive effects of post-translational modifications in the epitope region to ensure accurate biomarker quantitation.

Heterotypic liquid-liquid phase separation of tau and α-synuclein: Implications for overlapping neuropathologies

Tauopathies and synucleinopathies are characterized by the aggregation of Tau and α-synuclein (AS) into amyloid structures, respectively. Individuals with these neuropathies have an elevated risk of developing subsequent neurodegenerative or comorbid disorders. Intriguingly, post-mortem brain examinations have revealed co-localization of Tau and AS aggregates, suggesting a synergistic pathological relationship with an adverse prognosis. The role of liquid-liquid phase separation (LLPS) in the development of neurodegenerative diseases is currently receiving significant attention, as it can contribute to the aggregation and co-deposition of amyloidogenic proteins. In this study, we investigated the phase separation behavior of Tau and AS under various insults, some of which are implicated in disease progression. Our findings demonstrate the formation of heterotypic droplets composed of Tau and AS at physiologically relevant mole ratios that mimic neurons' soma and terminal buttons. Importantly, these heterotypic droplets exhibit increased resistance to electrostatic screening compared to homotypic condensates. Moreover, we observed that biologically relevant biomolecules, known to be dysregulated in disease, exert different effects on these droplets. Additionally, we provide evidence that phase separation itself influences the amyloid aggregation of Tau and AS, underscoring the significance of this process in the development of aggregopathies.

Structure of the nonhelical filament of the Alzheimer's disease tau core

The microtubule-associated protein tau aggregates into neurofibrillary tangles in Alzheimer's disease (AD). The main type of aggregates, the paired helical filaments (PHF), incorporate about 20% of the full-length protein into the rigid core. Recently, cryo-electron microscopy data showed that a protease-resistant fragment of tau (residues 297-391) self-assembles in vitro in the presence of divalent cations to form twisted filaments whose molecular structure resembles that of AD PHF tau [S. Lövestam et al., Elife 11, e76494 (2022)]. To investigate whether this tau construct is uniquely predisposed to this morphology and structure, we fibrillized tau (297-391) under the reported conditions and determined its structure using solid-state NMR spectroscopy. Unexpectedly, the protein assembled predominantly into nontwisting ribbons whose rigid core spans residues 305-357. This rigid core forms a β-arch that turns at residues 322CGS324. Two protofilaments stack together via a long interface that stretches from G323 to I354. Together, these two protofilaments form a four-layered β-sheet core whose sidechains are stabilized by numerous polar and hydrophobic interactions. This structure gives insight into the fibril morphologies and molecular conformations that can be adopted by this protease-resistant core of AD tau under different pH and ionic conditions.

β-Bracelets: Macrocyclic Cross-β Epitope Mimics Based on a Tau Conformational Strain

The aggregation of misfolded tau into neurotoxic fibrils is linked to the progression of Alzheimer's disease (AD) and related tauopathies. Disease-associated conformations of filamentous tau are characterized by hydrophobic interactions between side chains on unique and distant β-strand modules within each protomer. Here, we report the design and diversity-oriented synthesis of β-arch peptide macrocycles composed of the aggregation-prone PHF6 hexapeptide of tau and the cross-β module specific to the AD tau fold. Termed "β-bracelets", these proteomimetics assemble in a sequence- and macrocycle-dependent fashion, resulting in amyloid-like fibrils that feature in-register parallel β-sheet structure. Backbone N-amination of a selected β-bracelet affords soluble inhibitors of tau aggregation. We further demonstrate that the N-aminated macrocycles block the prion-like cellular seeding activity of recombinant tau as well as mature fibrils from AD patient extracts. These studies establish β-bracelets as a new class of cross-β epitope mimics and demonstrate their utility in the rational design of molecules targeting amyloid propagation and seeding.

Dose-dependent binding behavior of anthraquinone derivative purpurin interacting with tau-derived peptide protofibril

Alzheimer's disease is hallmarked by microtubule-associated protein tau tangles and amyloid-β plaques. The β-structure propensity of tau inclusions is closely related to the hexapeptide motif VQIVYK (termed PHF6), and disruption of this motif prevents tau aggregation. Small-molecule inhibitors are considered a promising therapeutic strategy, but the molecular mechanisms underlying the correlation between dose and inhibitory effects are still unclear. In this work, we investigated the dose-induced influence of purpurin, an anthraquinone derivative, on the structural stability of the PHF6 fibrillar nucleus by performing microsecond all-atom molecular dynamics simulations in explicit water. The stability of PHF6 protofibrils of different sizes was first examined, and it was found that the structural stability of fibrillar oligomers increases with oligomer size, and that the octamer is the minimal stable nucleus for fibril formation. When purpurin molecules were added to the protofibril octamer at a low purpurin/peptide ratio, they bound to the octamer with different coupling states, and the different states may transition to each of the other states through an uncoupling state or directly through a short-time transition. With increasing purpurin/peptide ratio, purpurins tend to self-aggregate rather than bind to the protein surface. Interestingly, the contacts between individual purpurins and the octamer as a function of the purpurin number show a power-law behavior, which may serve as a useful indicator to reflect the binding efficiency of ligands to proteins in drug screening. The interaction analysis reveals that purpurin prefers to bind to the hydrophilic and aromatic Tyr and has the lowest probability with the hydrophobic Val located in the middle of PHF6. Aromatic stacking plays a key role in the octamer-purpurin interaction, in which the three aromatic rings of purpurin have different contributions. In addition, purpurin shows a remarkable disruptive effect on the protofibril octamer when the molar ratio of purpurin to peptide is 1 : 2; above this ratio, the binding mode and disruption effect of purpurin do not change significantly. Our work provides a detailed picture of the dynamics and interactions of purpurin binding to the PHF6 protofibril and expands the understanding of the dose-induced inhibitory mechanism.

Chemical Synthesis of Microtubule-Associated Protein Tau

Deposits of the microtubule-associated protein Tau (MAPT) serve as a hallmark of neurodegenerative diseases known as tauopathies. Numerous studies have demonstrated that in diseases such as Alzheimer's disease (AD), Tau undergoes extensive remodeling. The attachment of post-translational modifications distributed throughout the entire sequence of the protein correlates with clinical presentation. A systematic examination of these protein alterations can shed light on their roles in both healthy and diseased states. However, the ability to access these modifications in the entire protein chain is limited as Tau can only be produced recombinantly or through semisynthesis. In this article, we describe the first chemical synthesis of the longest 2N4R isoform of Tau, consisting of 441 amino acids. The 2N4R Tau was divided into 3 major segments and a total of 11 fragments, all of which were prepared via solid-phase peptide synthesis. The successful chemical strategy has relied on the strategic use of two cysteine sites (C291 and C322) for the native chemical ligations (NCLs). This was combined with modern preparative protein chemistries, such as mercaptothreonine ligation (T205), diselenide-selenoester ligation (D358), and mutations of mercaptoamino acids into native residues via homogeneous radical desulfurization (A40, A77, A119, A157, A246, and A390). The successful completion of the synthesis has established a robust and scalable route to the native protein in multimilligram quantities and high purity. In broader terms, the presented strategy can be applied to the preparation of other shorter isoforms of Tau as well as to introduce all post-translational modifications that are characteristic of tauopathies such as AD.

α-Synuclein Drives Tau's Cytotoxic Aggregates Formation through Hydrophobic Interactions

Tau and α-synuclein are proteins involved in pathologies known as tauopathies and synucleinopathies, respectively. Moreover, evidence shows that there is a crosstalk between them as is seen in the brains of individuals with sporadic neurodegenerative disorders. Based on that, we present data showing that the hydrophobic α-peptide 71 VTGVTAVAQKTV82 induces the aggregation of the full-length tau fragment in the absence of heparin assessed by ThT. Moreover, AFM images reveal the presence of straight filaments and amorphous aggregates of full-length tau in the presence of the α-peptide. Additionally, ITC experiments showed the interaction of the α-peptide with tau full-length (441 amino acids),4R (amino acids from 244 to 369), and both hexapeptides 275 VQIINK280 and 306 VQIVYK311 through hydrophobic interactions. The Raman spectroscopy spectra showed conformational changes in the Amide region in the aggregates formed with full-length tau and α-syn peptide. Furthermore, the incubation of extracellular aggregates with N2a cells showed morphological differences in the cellular body and the nucleus suggesting cell death. Moreover,, the incubation of different types of aggregates in cell culture provokes the release of Lactate dehydrogenase (LDH). Altogether, we found that α-synuclein peptide can drive the aggregation of full-length tau-provoking morphological and structural changes evoking cytotoxic effects.

Disease-associated patterns of acetylation stabilize tau fibril formation

Assembly of tau into beta-sheet-rich amyloids dictates the pathology of a diversity of diseases. Lysine acetylation has been proposed to drive tau amyloid assembly, but no direct mechanism has emerged. Using tau fragments, we identify patterns of acetylation that flank amyloidogenic motifs on the tau fragments that promote rapid fibril assembly. We determined a 3.9 Å cryo-EM amyloid fibril structure assembled from an acetylated tau fragment uncovering how lysine acetylation can mediate gain-of-function interactions. Comparison of the structure to an ex vivo tauopathy fibril reveals regions of structural similarity. Finally, we show that fibrils encoding disease-associated patterns of acetylation are active in cell-based tau aggregation assays. Our data uncover the dual role of lysine residues in limiting tau aggregation while their acetylation leads to stabilizing pro-aggregation interactions. Design of tau sequence with specific acetylation patterns may lead to controllable tau aggregation to direct folding of tau into distinct amyloid folds.

Mutation ∆K281 in MAPT causes Pick's disease

Two siblings with deletion mutation ∆K281 in MAPT developed frontotemporal dementia. At autopsy, numerous inclusions of hyperphosphorylated 3R Tau were present in neurons and glial cells of neocortex and some subcortical regions, including hippocampus, caudate/putamen and globus pallidus. The inclusions were argyrophilic with Bodian silver, but not with Gallyas-Braak silver. They were not labelled by an antibody specific for tau phosphorylated at S262 and/or S356. The inclusions were stained by luminescent conjugated oligothiophene HS-84, but not by bTVBT4. Electron cryo-microscopy revealed that the core of tau filaments was made of residues K254-F378 of 3R Tau and was indistinguishable from that of Pick's disease. We conclude that MAPT mutation ∆K281 causes Pick's disease.

Fully co-factor-free ClearTau platform produces seeding-competent Tau fibrils for reconstructing pathological Tau aggregates

Tau protein fibrillization is implicated in the pathogenesis of several neurodegenerative diseases collectively known as Tauopathies. For decades, investigating Tau fibrillization in vitro has required the addition of polyanions or other co-factors to induce its misfolding and aggregation, with heparin being the most commonly used. However, heparin-induced Tau fibrils exhibit high morphological heterogeneity and a striking structural divergence from Tau fibrils isolated from Tauopathies patients' brains at ultra- and macro-structural levels. To address these limitations, we developed a quick, cheap, and effective method for producing completely co-factor-free fibrils from all full-length Tau isoforms and mixtures thereof. We show that Tau fibrils generated using this ClearTau method - ClearTau fibrils - exhibit amyloid-like features, possess seeding activity in biosensor cells and hiPSC-derived neurons, retain RNA-binding capacity, and have morphological properties and structures more reminiscent of the properties of the brain-derived Tau fibrils. We present the proof-of-concept implementation of the ClearTau platform for screening Tau aggregation-modifying compounds. We demonstrate that these advances open opportunities to investigate the pathophysiology of disease-relevant Tau aggregates and will facilitate the development of Tau pathology-targeting and modifying therapies and PET tracers that can distinguish between different Tauopathies.

Domain-specific modulatory effects of phosphomimetic substitutions on liquid-liquid phase separation of tau protein

Aggregation of tau is one of the major pathogenic events in Alzheimer's disease and several other neurodegenerative disorders. Recent reports demonstrated that tau can condense into liquid droplets that undergo time-dependent transition to a solid-like state, suggesting that liquid condensates may be on the pathway to pathological aggregation of tau. While hyperphosphorylation is a key feature of tau isolated from brains of patients with Alzheimer's disease and other tauopathies, the mechanistic role of phosphorylation in tau liquid-liquid phase separation (LLPS) remains largely unexplored. In an attempt to bridge this gap, here we performed systematic studies by introducing phosphomimetic substitutions of Ser/Thr residues with negatively charged Asp/Glu residues in different regions of the protein. Our data indicate that the phosphorylation patterns that increase the polarization of charge distribution in full-length tau (tau441) promote protein LLPS, whereas those that decrease charge polarization have an opposite effect. Overall, this study further supports the notion that tau LLPS is driven by attractive intermolecular electrostatic interactions between the oppositely charged domains. We also show that the phosphomimetic tau variants with low intrinsic propensity for LLPS can be efficiently recruited to droplets formed by the variants with high LLPS propensity. Furthermore, the present data demonstrate that phosphomimetic substitutions have a major effect on time-dependent material properties of tau droplets, generally slowing down their aging. The latter effect is most dramatic for the tau variant with substitutions within the repeat domain, which correlates with the decreased fibrillation rate of this variant.

The E3 Ubiquitin Ligase, CHIP/STUB1, Inhibits Aggregation of Phosphorylated Proteoforms of Microtubule-associated Protein Tau (MAPT)

Hyper-phosphorylated tau accumulates as insoluble fibrils in Alzheimer's disease (AD) and related dementias. The strong correlation between phosphorylated tau and disease has led to an interest in understanding how cellular factors discriminate it from normal tau. Here, we screen a panel of chaperones containing tetratricopeptide repeat (TPR) domains to identify those that might selectively interact with phosphorylated tau. We find that the E3 ubiquitin ligase, CHIP/STUB1, binds 10-fold more strongly to phosphorylated tau than unmodified tau. The presence of even sub-stoichiometric concentrations of CHIP strongly suppresses aggregation and seeding of phosphorylated tau. We also find that CHIP promotes rapid ubiquitination of phosphorylated tau, but not unmodified tau, in vitro. Binding to phosphorylated tau requires CHIP's TPR domain, but the binding mode is partially distinct from the canonical one. In cells, CHIP restricts seeding by phosphorylated tau, suggesting that it could be an important barrier in cell-to-cell spreading. Together, these findings show that CHIP recognizes a phosphorylation-dependent degron on tau, establishing a pathway for regulating the solubility and turnover of this pathological proteoform.

Cryo-EM Structures of Chronic Traumatic Encephalopathy Tau Filaments with PET Ligand Flortaucipir

Positron emission tomography (PET) imaging allows monitoring the progression of amyloid aggregation in the living brain. [18F]-Flortaucipir is the only approved PET tracer compound for the visualisation of tau aggregation. Here, we describe cryo-EM experiments on tau filaments in the presence and absence of flortaucipir. We used tau filaments isolated from the brain of an individual with Alzheimer's disease (AD), and from the brain of an individual with primary age-related tauopathy (PART) with a co-pathology of chronic traumatic encephalopathy (CTE). Unexpectedly, we were unable to visualise additional cryo-EM density for flortaucipir for AD paired helical or straight filaments (PHFs or SFs), but we did observe density for flortaucipir binding to CTE Type I filaments from the case with PART. In the latter, flortaucipir binds in a 1:1 molecular stoichiometry with tau, adjacent to lysine 353 and aspartate 358. By adopting a tilted geometry with respect to the helical axis, the 4.7 Å distance between neighbouring tau monomers is reconciled with the 3.5 Å distance consistent with π-π-stacking between neighbouring molecules of flortaucipir.

Big tau aggregation disrupts microtubule tyrosination and causes myocardial diastolic dysfunction: from discovery to therapy

BACKGROUND

Amyloid plaques and neurofibrillary tangles, the molecular lesions that characterize Alzheimer's disease (AD) and other forms of dementia, are emerging as determinants of proteinopathies 'beyond the brain'. This study aims to establish tau's putative pathophysiological mechanistic roles and potential future therapeutic targeting of tau in heart failure (HF).

METHODS AND RESULTS

A mouse model of tauopathy and human myocardial and brain tissue from patients with HF, AD, and controls was employed in this study. Tau protein expression was examined together with its distribution, and in vitro tau-related pathophysiological mechanisms were identified using a variety of biochemical, imaging, and functional approaches. A novel tau-targeting immunotherapy was tested to explore tau-targeted therapeutic potential in HF. Tau is expressed in normal and diseased human hearts, in contradistinction to the current oft-cited observation that tau is expressed specifically in the brain. Notably, the main cardiac isoform is high-molecular-weight (HMW) tau (also known as big tau), and hyperphosphorylated tau segregates in aggregates in HF and AD hearts. As previously described for amyloid-beta, the tauopathy phenotype in human myocardium is of diastolic dysfunction. Perturbation in the tubulin code, specifically a loss of tyrosinated microtubules, emerged as a potential mechanism of myocardial tauopathy. Monoclonal anti-tau antibody therapy improved myocardial function and clearance of toxic aggregates in mice, supporting tau as a potential target for novel HF immunotherapy.

CONCLUSION

The study presents new mechanistic evidence and potential treatment for the brain-heart tauopathy axis in myocardial and brain degenerative diseases and ageing.

Yeast biopanning against site-specific phosphorylations in tau

The detection of site-specific phosphorylation in the microtubule-associated protein tau is emerging as a means to diagnose and monitor the progression of Alzheimer's Disease and other neurodegenerative diseases. However, there is a lack of phospho-specific monoclonal antibodies and limited validation of their binding specificity. Here, we report a novel approach using yeast biopanning against synthetic peptides containing site-specific phosphorylations. Using yeast cells displaying a previously validated phospho-tau (p-tau) single-chain variable region fragment (scFv), we show selective yeast cell binding based on single amino acid phosphorylation on the antigen. We identify conditions that allow phospho-specific biopanning using scFvs with a wide range of affinities (KD = 0.2 to 60 nM). Finally, we demonstrate the capability of screening large libraries by performing biopanning in 6-well plates. These results show that biopanning can effectively select yeast cells based on phospho-site specific antibody binding, opening doors for the facile identification of high-quality monoclonal antibodies.

A Validated Method to Prepare Stable Tau Oligomers

There is growing evidence that tau oligomers are a major pathological species in a number of tauopathies including Alzheimer's disease. However, it is still unclear what exact mechanisms underlie tau oligomer-mediated dysfunction. Studies of tau oligomers in vitro are limited by the high propensity for aggregation and consequent changes in the aggregation state of the produced tau samples over time. In this protocol, we provide a step-by-step description of a validated method for producing stable and structurally characterized oligomers of tau that can be used in biochemical, cellular, and animal model studies to evaluate mechanisms of action of tau in tauopathies.

Characterization of microtubule-associated protein tau isoforms and Alzheimer's disease-like pathology in normal sheep (Ovis aries): relevance to their potential as a model of Alzheimer's disease

Alzheimer's disease is a chronic neurodegenerative disease that accounts for up to 80% of all dementias. Characterised by deteriorations of memory and cognitive function, the key neuropathological features are accumulations of β-amyloid and hyperphosphorylated tau, as 'plaques' and 'tangles', respectively. Despite extensive study, however, the exact mechanism underlying aggregate formation in Alzheimer's disease remains elusive, as does the contribution of these aggregates to disease progression. Importantly, a recent evaluation of current Alzheimer's disease animal models suggested that rodent models are not able to fully recapitulate the pathological intricacies of the disease as it occurs in humans. Therefore, increasing attention is being paid to species that might make good alternatives to rodents for studying the molecular pathology of Alzheimer's disease. The sheep (Ovis aries) is one such species, although to date, there have been few molecular studies relating to Alzheimer's disease in sheep. Here, we investigated the Alzheimer's disease relevant histopathological characteristics of 22 sheep, using anti-β-amyloid (Abcam 12267 and mOC64) and phosphorylation specific anti-tau (AT8 and S396) antibodies. We identified numerous intraneuronal aggregates of both β-amyloid and tau that are consistent with early Alzheimer's disease-like pathology. We confirmed the expression of two 3-repeat (1N3R, 2N3R) and two 4-repeat (1N4R, 2N4R) tau isoforms in the ovine brain, which result from the alternative splicing of two tau exons. Finally, we investigated the phosphorylation status of the serine396 residue in 30 sheep, and report that the phosphorylation of this residue begins in sheep aged as young as 2 years. Together, these data show that sheep exhibit naturally occurring β-amyloid and tau pathologies, that reflect those that occur in the early stages of Alzheimer's disease. This is an important step towards the validation of the sheep as a feasible large animal species in which to model Alzheimer's disease.

Structure-based discovery of small molecules that disaggregate Alzheimer's disease tissue derived tau fibrils in vitro

Alzheimer's disease (AD) is the consequence of neuronal death and brain atrophy associated with the aggregation of protein tau into fibrils. Thus disaggregation of tau fibrils could be a therapeutic approach to AD. The small molecule EGCG, abundant in green tea, has long been known to disaggregate tau and other amyloid fibrils, but EGCG has poor drug-like properties, failing to fully penetrate the brain. Here we have cryogenically trapped an intermediate of brain-extracted tau fibrils on the kinetic pathway to EGCG-induced disaggregation and have determined its cryoEM structure. The structure reveals that EGCG molecules stack in polar clefts between the paired helical protofilaments that pathologically define AD. Treating the EGCG binding position as a pharmacophore, we computationally screened thousands of drug-like compounds for compatibility for the pharmacophore, discovering several that experimentally disaggregate brain-derived tau fibrils in vitro. This work suggests the potential of structure-based, small-molecule drug discovery for amyloid diseases.