Electrochemistry of heparin binding to tau protein on Au surfaces

Hematoxylin modulates tau-RD protein fibrillization and ameliorates Alzheimer's disease-like symptoms in a yeast model

Alzheimer's disease (AD) is one of the most serious neurodegenerative diseases with no effective treatment options available. The formation of insoluble amyloid fibrils of the hyperphosphorylated tau protein is intimately associated with AD, hence the tau protein has been a key target for AD drug development. In this work, hematoxylin was discovered as a dual functional compound, that is, acting in the inhibition of repeat domain of tau (tau-RD) protein fibrillogenesis and remodeling of pre-formed tau-RD fibrils in vitro. Meanwhile, hematoxylin was able to reduce the accumulation of tau-RD aggregates in Saccharomyces cerevisiae. Experimental and computational studies indicated that hematoxylin directly interacts with tau-RD protein through hydrophobic forces, hydrogen bonds, π-cation interactions, and π-π stackings. In addition, cellular viability assays showed that hematoxylin greatly reduced cytotoxicity induced by tau-RD aggregates. In summary, hematoxylin might be a promising candidate for further development as a potential therapeutic drug for AD patients.

Chemical Features of Polyanions Modulate Tau Aggregation and Conformational States

The aggregation of tau into insoluble fibrils is a defining feature of neurodegenerative tauopathies. However, tau has a positive overall charge and is highly soluble; so, polyanions, such as heparin, are typically required to promote its aggregation in vitro. There are dozens of polyanions in living systems, and it is not clear which ones might promote this process. Here, we systematically measure the ability of 37 diverse, anionic biomolecules to initiate tau aggregation using either wild-type (WT) tau or the disease-associated P301S mutant. We find that polyanions from many different structural classes can promote fibril formation and that P301S tau is sensitive to a greater number of polyanions (28/37) than WT tau (21/37). We also find that some polyanions preferentially reduce the lag time of the aggregation reactions, while others enhance the elongation rate, suggesting that they act on partially distinct steps. From the resulting structure-activity relationships, the valency of the polyanion seems to be an important chemical feature such that anions with low valency tend to be weaker aggregation inducers, even at the same overall charge. Finally, the identity of the polyanion influences fibril morphology based on electron microscopy and limited proteolysis. These results provide insights into the crucial role of polyanion-tau interactions in modulating tau conformational dynamics with implications for understanding the tau aggregation landscape in a complex cellular environment.

Revisiting the grammar of Tau aggregation and pathology formation: how new insights from brain pathology are shaping how we study and target Tauopathies

Converging evidence continues to point towards Tau aggregation and pathology formation as central events in the pathogenesis of Alzheimer's disease and other Tauopathies. Despite significant advances in understanding the morphological and structural properties of Tau fibrils, many fundamental questions remain about what causes Tau to aggregate in the first place. The exact roles of cofactors, Tau post-translational modifications, and Tau interactome in regulating Tau aggregation, pathology formation, and toxicity remain unknown. Recent studies have put the spotlight on the wide gap between the complexity of Tau structures, aggregation, and pathology formation in the brain and the simplicity of experimental approaches used for modeling these processes in research laboratories. Embracing and deconstructing this complexity is an essential first step to understanding the role of Tau in health and disease. To help deconstruct this complexity and understand its implication for the development of effective Tau targeting diagnostics and therapies, we firstly review how our understanding of Tau aggregation and pathology formation has evolved over the past few decades. Secondly, we present an analysis of new findings and insights from recent studies illustrating the biochemical, structural, and functional heterogeneity of Tau aggregates. Thirdly, we discuss the importance of adopting new experimental approaches that embrace the complexity of Tau aggregation and pathology as an important first step towards developing mechanism- and structure-based therapies that account for the pathological and clinical heterogeneity of Alzheimer's disease and Tauopathies. We believe that this is essential to develop effective diagnostics and therapies to treat these devastating diseases.

Major Differences between the Self-Assembly and Seeding Behavior of Heparin-Induced and in Vitro Phosphorylated Tau and Their Modulation by Potential Inhibitors

Self-assembly of the microtubule-associated protein tau into neurotoxic oligomers, fibrils, and paired helical filaments, and cell-to-cell spreading of these pathological tau species are critical processes underlying the pathogenesis of Alzheimer’s disease and other tauopathies. Modulating the self-assembly process and inhibiting formation and spreading of such toxic species are promising strategies for therapy development. A challenge in investigating tau self-assembly in vitro is that, unlike most amyloidogenic proteins, tau does not aggregate in the absence of posttranslational modifications (PTM), aggregation inducers, or preformed seeds. The most common induction method is addition of polyanions, such as heparin; yet, this artificial system may not represent adequately tau self-assembly in vivo, which is driven by aberrant phosphorylation and other PTMs, potentially leading to in vitro data that do not reflect the behavior of tau and its interaction with modulators in vivo. To tackle these challenges, methods for in vitro phosphorylation of tau to produce aggregation-competent forms recently have been introduced (Despres et al. (2017) Proc. Natl. Acad. Sci. U.S.A., 114, 9080−908528784767). However, the oligomerization, seeding, and interaction with assembly modulators of the different forms of tau have not been studied to date. To address these knowledge gaps, we compared here side-by-side the self-assembly and seeding activity of heparin-induced tau with two forms of in vitro phosphorylated tau and tested how the molecular tweezer CLR01, a negatively charged compound, affected these processes. Tau was phosphorylated by incubation either with activated extracellular signal-regulated kinase 2 or with a whole rat brain extract. Seeding activity was measured using a fluorescence-resonance energy transfer-based biosensor-cell method. We also used solution-state NMR to investigate the binding sites of CLR01 on tau and how they were impacted by phosphorylation. Our systematic structure–activity relationship study demonstrates that heparin-induced tau behaves differently from in vitro phosphorylated tau. The aggregation rates of the different forms are distinct as is the intracellular localization of the induced aggregates, which resemble brain-derived tau strains suggesting that heparin-induced tau and in vitro phosphorylated tau have different conformations, properties, and activities. CLR01 inhibits aggregation and seeding of both heparin-induced and in vitro phosphorylated tau dose-dependently, although heparin induction interferes with the interaction between CLR01 and tau.

Glycan Determinants of Heparin-Tau Interaction

Tau aggregates into paired helical filaments within neurons, a pathological hallmark of Alzheimer's disease. Heparin promotes tau aggregation and recently has been shown to be involved in the cellular uptake of tau aggregates. Although the tau-heparin interaction has been extensively studied, little is known about the glycan determinants of this interaction. Here, we used surface plasmon resonance (SPR) and NMR spectroscopy to characterize the interaction between two tau fragments, K18 and K19, and several polysaccharides, including heparin, heparin oligosaccharides, chemically modified heparin, and related glycans. Using a heparin-immobilized chip, SPR revealed that tau K18 and K19 bind heparin with a K_D of 0.2 and 70 μM, respectively. In SPR competition experiments, N-desulfation and 2-O-desulfation had no effect on heparin binding to K18, whereas 6-O-desulfation severely reduced binding, suggesting a critical role for 6-O-sulfation in the tau-heparin interaction. The tau-heparin interaction became stronger with longer-chain heparin oligosaccharides. As expected for an electrostatics-driven interaction, a moderate amount of salt (0.3 M NaCl) abolished binding. NMR showed the largest chemical-shift perturbation (CSP) in R2 in tau K18, which was absent in K19, revealing differential binding sites in K18 and K19 to heparin. Dermatan sulfate binding produced minimal CSP, whereas dermatan disulfate, with the additional 6-O-sulfo group, induced much larger CSP. 2-O-desulfated heparin induced much larger CSP in K18 than 6-O-desulfated heparin. Our data demonstrate a crucial role for the 6-O-sulfo group in the tau-heparin interaction, which to our knowledge has not been reported before.

Evaluation of ferritin and transferrin binding to tau protein

Tau protein is a neurodegeneration biomarker. Due to the high concentration of metal ions in the brain, the metallation of tau proteins and their catalytic role in reactive oxygen formation have been identified as a major biochemical pathway of neurodegeneration. High levels of iron ions have been detected in Alzheimer's disease brains. One of biological sources of iron ions are iron-rich proteins, such as transferrin or ferritin. However, the interactions between tau and these metallo-proteins have not been fully characterized. Here, the interactions between the longest form of full-length tau protein (tau441) with iron-rich proteins were detected using electrochemical impedance spectroscopy. Tau441 was immobilized on Au surface, via N-terminal (N-tau-Au film) or Cys-residues (Cys-tau-Au film), and the charge-transfer resistance, R_ct, was monitored prior and post ferritin or transferrin binding. Significant increase in R_ct was observed post transferrin binding above 50μgmL^-1, but not ferritin regardless of concentration with N-tau-Au film. Additionally, the electrochemical trend was linear with respect to transferrin concentration. Electrochemical data indicated low binding by ferritin to N-tau-Au or Cys-tau-Au films. The interaction of apotransferrin or apoferritin with tau films was also evaluated. Electrochemical data may be pointing to the differences in protein binding modes by transferrin compared to ferritin as well as to importance of metal ions in protein-protein interactions.