Fluorescence-coupled CD conformational monitoring of filament formation of tau microtubule-binding repeat domain

Molecular Insights into the Differential Effects of Acetylation on the Aggregation of Tau Microtubule-Binding Repeats

Aggregation of tau protein into intracellular fibrillary inclusions is characterized as the hallmark of tauopathies, including Alzheimer's disease and chronic traumatic encephalopathy. The microtubule-binding (MTB) domain of tau, containing either three or four repeats with sequence similarities, plays an important role in determining tau's aggregation. Previous studies have reported that abnormal acetylation of lysine residues displays a distinct effect on the formation of pathological tau aggregates. However, the underlying molecular mechanism remains mostly elusive. In this study, we performed extensive replica exchange molecular dynamics (REMD) simulations of 144 μs in total to systematically investigate the dimerization of four tau MTB repeats and explore the impacts of Lys280 (K280) or Lys321 (K321) acetylation on the conformational ensembles of the R2 or R3 dimer. Our results show that R3 is the most prone to aggregation among the four repeats, followed by R2 and R4, while R1 displays the weakest aggregation propensity with a disordered structure. Acetylation of K280 could promote the aggregation of R2 peptides by increasing the formation of β-sheet structures and strengthening the interchain interaction. However, K321 acetylation decreases the β-sheet content of the R3 dimer, reduces the ability of R3 peptides to form long β-strands, and promotes the stable helix structure formation. The salt bridge and Y310-Y310 π-π stacking interactions of the R3 dimer are greatly weakened by K321 acetylation, resulting in the inhibition of dimerization. This study uncovers the structural ensembles of tau MTB repeats and provides mechanistic insights into the influences of acetylation on tau aggregation, which may deepen the understanding of the pathogenesis of tauopathies.

Site-specific amino acid D-isomerization of Tau R2 and R3 peptides changes the fibril morphology, resulting in attenuation of Tau aggregation inhibitor potency

Tau, a microtubule-binding protein, is a major component of neurofibrillary tangles in the brains of Alzheimer's disease patients. Tau aggregation following fibril formation induces Alzheimer's disease pathogenesis. The accumulation of D-isomerized amino acids in proteins that occurs in several tissues with aging is thought to be implicated in age-related diseases. D-isomerized Asp accumulation has also been found in Tau in neurofibrillary tangles. We previously demonstrated the effects of D-isomerization of Asp within microtubule-binding repeat peptides of Tau, Tau R2, and R3 on the rates of structural transition and fibril formation. Here, we investigated the potency of Tau aggregation inhibitors on fibril formation of wild-type Tau R2 and R3 peptides and D-isomerized Asp-containing Tau R2 and R3 peptides. D-isomerization of Asp within Tau R2 and R3 peptides attenuated the potency of inhibitors. We next investigated the fibril morphology of D-isomerized Asp-containing Tau R2 and R3 peptides by electron microscopy. D-isomerized Asp-containing Tau R2 and R3 fibrils showed significantly different fibril morphology from that of wild-type peptides. Our results indicate that D-isomerization of Asp within Tau R2 and R3 peptides affects fibril morphology, resulting in attenuation of the potency of Tau aggregation inhibitors.

Phosphorylation of Tau R2 Repeat Destabilizes Its Binding to Microtubules: A Molecular Dynamics Simulation Study

Phosphorylation, the most popular post-translational modification of tau protein, plays an important role in regulating tau physiological functions. However, aberrant phosphorylation attenuates the binding affinity of tau to a microtubule (MT), resulting in MT destabilization followed by accumulation of neurofibrillary tangles in the brain. There are in total 85 potential phosphorylation sites in a full-length tau protein, and about half of them are abnormal as they occur in tau of Alzheimer's disease (AD) brain only. In this work, we investigated the impact of abnormal Ser289, Ser293, and Ser289/Ser293 phosphorylation on tau R2-MT binding and the conformation of tau R2 using molecular dynamics simulation. We found that the phosphorylation significantly affected R2-MT interaction and reduced the binding affinity of tau R2 peptides to MTs. Free energy decomposition analysis suggested that the post-translational modified residues themselves made a significant contribution to destabilize tau repeat R2-MT binding. Therefore, the phosphorylation may attenuate the binding affinity of tau to MTs. Additionally, the phosphorylation also enhanced helix-coil transition of monomeric R2 peptides, which may result in the acceleration of tau aggregation. Since these phosphorylated sites have not been examined in previous experimental studies, our finding through all-atom molecular dynamics simulations and free energy analysis can inspire experimental scientists to investigate the impact of the phosphorylation on MT binding and aggregation of full-length tau and the pathological roles of the phosphorylation at those sites in AD development through in vitro/in vivo assays.

Misfolding and Self-Assembly Dynamics of Microtubule-Binding Repeats of the Alzheimer-Related Protein Tau

Pathological aggregation of intrinsically disordered tau protein, driven by the interactions between microtubule-binding (MTB) domains, is associated with Alzheimer's disease. The MTB domain contains either three or four repeats with sequence similarities. Compared to amyloid β, many aspects of the misfolding and aggregation mechanisms of tau are largely unknown. In this study, we systematically investigated the dynamics of monomer misfolding and dimerization of each MTB repeat using atomistic discrete molecular dynamic simulations. Our results revealed that all the four repeat monomers (R1-R4) were very dynamic, featuring frequent conformational conversion and lacking stable conformations. While R1, R2, and R4 monomers occasionally adopted partially helical conformations, R3 monomers frequently formed β-sheets. In dimerization simulations, R3 displayed the strongest aggregation propensity with high β-sheet contents, while R1 was the least prone to aggregation. The R2 and R4 dimers contained both helix and β-sheet structures. The β-sheets in R4 assemblies were dominant with β-hairpin conformation. In R2 and R3 dimers, intermolecular β-sheets were mainly driven by residues around the paired helical filament (PHF) regions. Residues around the PHF6* in R2 and PHF6 in R3 had significantly higher intermolecular contacts than other regions, suggesting that these residues play a key role in the amyloid aggregation of tau. Our results on the structural ensembles and early aggregation dynamics of each tau MTB repeat will help understand the nucleation and fibrillization of tau.

Crystal structure of the human tau PHF core domain VQIINK complexed with the Fab domain of monoclonal antibody Tau2r3

Neurofibrillary tangles formed by abnormally aggregated tau protein are a histopathological feature of tauopathies. A tau aggregation inhibitor is a potential therapeutic agent for tauopathies. In this study, we prepared a monoclonal antibody for tau, monoclonal antibody to tau protein (Tau2r3), using as epitope the 272 GGKVQIINKKLD283 peptide in the microtubule-binding domain of tau, the key region mediating tau aggregation. We show that Tau2r3 clearly inhibits tau aggregation. To analyze the inhibition mechanism of Tau2r3, we solved the crystal structure of the Fab domain of Tau2r3 (Fab2r3) in complex with the VQIINK peptide. In the Fab2r3-VQIINK structure, the second and sixth polar residues and the fourth hydrophobic residue of VQIINK are crucial for binding to Fab2r3. The structural data for the Fab2r3-VQIINK complex could contribute to the design of new tau aggregation inhibitors.

Effect of site-specific amino acid D-isomerization on β-sheet transition and fibril formation profiles of Tau microtubule-binding repeat peptides

d-amino acid-containing proteins have been found in several human tissues, and the spontaneous accumulation of d-amino acids in proteins is thought to be involved in age-dependent diseases including dementia. Tau, a microtubule-associated protein, is a major component of neurofibrillary tangles in Alzheimer's disease. Site-specific amino acid D-isomerization in Tau has been observed in the brains of patients with Alzheimer's disease. Here, we conducted amino acid D-isomerization at specific sites in microtubule-binding repeat peptides of Tau (Tau R2 and R3) and examined the effects on Tau structure and fibril formation. Our results demonstrate that amino acid D-isomerization in Tau R2 peptides decreased the rates of β-sheet transition and fibril formation compared with those of the wild-type peptide composed of all l-amino acids. In contrast, Tau R3 peptides that had undergone amino acid D-isomerization at either Asp314, Ser316, or Ser324 showed increased rates of β-sheet transition and fibril formation compared with those of the wild-type Tau R3 peptide.

Expression and purification of tau protein and its frontotemporal dementia variants using a cleavable histidine tag

Recombinant tau protein is widely used to study the biochemical, cellular and pathological aspects of tauopathies, including Alzheimer's disease and frontotemporal dementia with Parkinsonism linked to chromosome 17 (FTPD-17). Pure tau in high yield is a requirement for in vitro evaluation of the protein's physiological and toxic functions. However, the preparation of recombinant tau is complicated by the protein's propensity to aggregate and form truncation products, necessitating the use of multiple, time-consuming purification methods. In this study, we investigated parameters that influence the expression of wild type and FTPD-17 pathogenic tau, in an attempt to identify ways to maximise expression yield. Here, we report on the influence of the choice of host strain, induction temperature, duration of induction, and media supplementation with glucose on tau expression in Escherichia coli. We also describe a straightforward process to purify the expressed tau proteins using immobilised metal affinity chromatography, with favourable yields over previous reports. An advantage of the described method is that it enables high yield production of functional oligomeric and monomeric tau, both of which can be used to study the biochemical, physiological and toxic properties of the protein.

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Factors influencing the expression of wild type and FTPD-17 pathogenic tau were investigated in an attempt to maximise yield.

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Soluble monomeric tau expression level was highest at 37 °C compared to 20 °C and 25 °C.

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Media supplementation with 0.2% glucose did not significantly influence monomeric tau expression levels.

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Circular dichroism confirmed that the purified tau proteins were mostly unfolded, with negative peaks around 200 nm.

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The circular dichroism peaks shifted towards 220 nm following the preparation of Alzheimer-like filaments, suggesting secondary structure re-orientation towards β-sheets.

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The tau proteins adopted classical fibrillisation similar to filaments isolated from the brains of Alzheimer's disease patients.

Molecular “light switch” [Ru(phen)2dppzidzo]2+monitoring the aggregation of tau

The luminescence of [Ru(phen)₂dppzidzo]²⁺has a linear response to the amounts of tau filaments. It may have a possible binding mode as depicted in the right diagram.

Modulation of tau protein fibrillization by oleocanthal

Among the phenolic compounds extracted from extra virgin olive oil, oleocanthal (1) has attracted considerable attention in the modulation of many human diseases, such as inflammation and Alzheimer's disease (AD). Indeed, 1 is capable of altering the fibrillization of tau protein, which is one of the key factors at the basis of neurodegenerative diseases, and of covalently reacting with lysine ε-amino groups of the tau fragment K18 in an unspecific fashion. In the present study, an investigation of the recognition process and the reaction profile between 1 and the wild-type tau protein has been conducted by a circular dichroism, surface plasmon resonance, fluorescence, and mass spectrometry combined approach. As a result, 1 has been found to interact with tau-441, inducing stable conformational modifications of the protein secondary structure and also interfering with tau aggregation. These findings provide experimental support for the potential reduced risk of AD and related neurodegenerative diseases associated with olive oil consumption and may offer a new chemical scaffold for the development of AD-modulating agents.

[Overview of structural study on conformations and intermolecular interactions of biomolecules]

Information on the conformational feature and specific intermolecular interaction of biomolecules is important to understand the biological function and to develop device for treating disorder caused by the abnormal function. Thus the 3D structures of the biologically active molecules and the specific interactions with their target molecules at the atomic level have been investigated by various physicochemical approaches. Herein, the following five subjects are reviewed: (1) function-linked conformations of biomolecules including natural annular products, opioid peptides and neuropeptides; (2) π-π stacking interactions of tryptophan derivatives with coenzymes and nucleic acid bases; (3) mRNA cap recognition of eukaryotic initiation factor 4E and its regulation by 4E-binding protein; (4) conformational feature of histamine H2 receptor antagonists and design of cathepsin B inhibitors; (5) self-aggregation mechanism of tau protein and its inhibition.

Structural studies of specific intermolecular interactions and self-aggregation of biomolecules and their application to drug design

Information on the structural basis of intermolecular recognition or self-aggregation of biomolecules at the atomic level is important to understand biological functions and to develop devices for treating disorders caused by abnormal functions. Thus structural analysis of specific intermolecular or intramolecular interactions of biomolecules has been performed using various physicochemical approaches. Herein, the following three subjects are reviewed: (1) structural analyses of mRNA cap structure recognition by eukaryotic initiation factor 4E and its functional regulation by endogenous 4E-binding protein; (2) structural studies of self-aggregation mechanism of microtubule-binding domain in tau protein and aggregation inhibitor; and (3) molecular design of cathepsin B-specific inhibitor.

Linkage-dependent contribution of repeat peptides to self-aggregation of three- or four-repeat microtubule-binding domains in tau protein

Although one of the priorities in Alzheimer's research is to clarify the filament formation mechanism for the tau protein, it is still unclear how it is transformed from a normal structure in a neuron. To examine the linkage-dependent contribution of each repeat peptide (R1-R4) to filament formation of the three- or four-repeat microtubule-binding domain (MBD) in the tau protein, four two-repeat peptides (R12, R13, R23 and R34) and two three-repeat peptides (R123 and R234) were prepared, and their in vitro self-aggregation was investigated by thioflavin S fluorescence and circular dichroism measurements, and by electron microscopy in neutral buffer (pH 7.6). Comparison of these aggregation behaviors with previous results for single-repeat peptides and wild-type 3RMBD (R134) and 4RMBD (R1234) indicated that (a) the two-repeat R23, not the R2 or R3 single repeat, forms the core structure in self-aggregation of 4RMBD, whereas that of 3RMBD comprises the R3 single repeat, (b) co-existence of R1 and R4 repeats is necessary for the aggregation behavior inherent in 3RMBD and 4RMBD, whereas the R1 or R4 repeat alone functions as a repressor or modifier of the filament formation, (c) 4RMBD aggregation is accompanied by R1-driven transition from random and alpha-helix structures to a beta-sheet structure, whereas 3RMBD aggregation involves three-repeat R134-specific transition from a random structure to an alpha-helix structure without the participation of a beta-sheet structure, and (d) the peptides that include the R1 repeat form a long filament irrespective of the absence or presence of the R4 repeat, whereas those that include the R4 repeat, but not the R1 repeat, form a relatively short filament. To the best of our knowledge, a systematic study of the linkage-dependent contribution of each repeat peptide to the paired helical filament formation of tau MBD has not been carried out previously, and thus the present information is useful for understanding the essence of the filament formation of tau MBD.