An acetylation mimicking mutation, K274Q, in tau imparts neurotoxicity by enhancing tau aggregation and inhibiting tubulin polymerization

EB1 Increases the Dynamics of Tau Droplets and Inhibits Tau Aggregation: Implications in Tauopathies

EB1, a microtubule plus end-tracking protein (+TIP), regulates microtubule dynamics. Recent evidence indicates cross-talk between EB proteins and tau, a microtubule-associated neuronal protein that is important for the growth and stability of microtubules. We investigated the interaction between tau and EB1 and the effect of binding of EB1 on tau function and aggregation. EB1 colocalized with tau in SH-SY5Y cells and coimmunoprecipitated with tau. Further, purified EB1 impaired the ability of adult tau to induce tubulin polymerization in vitro. EB1 bound to tau with a dissociation constant of 2.5 ± 0.7 μM. EB1 reduced heparin-induced tau aggregation with a half-maximal inhibitory concentration of 4.3 ± 0.2 μM, and increased the dynamics of tau in phase-separated droplets. The fluorescence recovery rate in tau droplets increased from 0.02 ± 0.01 to 0.07 ± 0.03 s-1, while the half-time of recovery decreased from 44.5 ± 14 to 13.5 ± 6 s in the presence of 8 μM EB1, suggesting a delay in the transition of tau from the soluble to aggregated form in tau liquid-liquid phase separation. EB1 decreased the rate of aggregation and increased the critical concentration of tau aggregation. Dynamic light scattering, atomic force microscopy, dot blot assays, and SDS-PAGE analysis showed that EB1 inhibited the formation of oligomers and higher-order aggregates of tau. The data suggest a novel role for EB1 as a regulator of tau function and aggregation, and the findings indicated the role of the EB family proteins in neuronal function and neurodegeneration.

Biochemical approaches to assess the impact of post-translational modifications on pathogenic tau conformations using recombinant protein

Tau protein is associated with many neurodegenerative disorders known as tauopathies. Aggregates of tau are thought of as a main contributor to neurodegeneration in these diseases. Increasingly, evidence points to earlier, soluble conformations of abnormally modified monomers and multimeric tau as toxic forms of tau. The biological processes driving tau from physiological species to pathogenic conformations remain poorly understood, but certain avenues are currently under investigation including the functional consequences of various pathological tau changes (e.g. mutations, post-translational modifications (PTMs), and protein-protein interactions). PTMs can regulate several aspects of tau biology such as proteasomal and autophagic clearance, solubility, and aggregation. Moreover, PTMs can contribute to the transition of tau from normal to pathogenic conformations. However, our understating of how PTMs specifically regulate the transition of tau into pathogenic conformations is partly impeded by the relative lack of structured frameworks to assess and quantify these conformations. In this review, we describe a set of approaches that includes several in vitro assays to determine the contribution of PTMs to tau's transition into known pathogenic conformations. The approaches begin with different methods to create recombinant tau proteins carrying specific PTMs followed by validation of the PTMs status. Then, we describe a set of biochemical and biophysical assays that assess the contribution of a given PTM to different tau conformations, including aggregation, oligomerization, exposure of the phosphatase-activating domain, and seeding. Together, these approaches can facilitate the advancement of our understanding of the relationships between PTMs and tau conformations.

Crosstalk between protein post-translational modifications and phase separation

The phenomenon of phase separation is quite common in cells, and it is involved in multiple processes of life activities. However, the current research on the correlation between protein modifications and phase separation and the interference with the tendency of phase separation has some limitations. Here we focus on several post-translational modifications of proteins, including protein phosphorylation modification at multiple sites, methylation modification, acetylation modification, ubiquitination modification, SUMOylation modification, etc., which regulate the formation of phase separation and the stability of phase separation structure through multivalent interactions. This regulatory role is closely related to the development of neurodegenerative diseases, tumors, viral infections, and other diseases, and also plays essential functions in environmental stress, DNA damage repair, transcriptional regulation, signal transduction, and cell homeostasis of living organisms, which provides an idea to explore the interaction between novel protein post-translational modifications and phase separation. Video Abstract.

Impact of APOE on amyloid and tau accumulation in argyrophilic grain disease and Alzheimer's disease

Alzheimer's disease (AD), characterized by the deposition of amyloid-β (Aβ) in senile plaques and neurofibrillary tangles of phosphorylated tau (pTau), is increasingly recognized as a complex disease with multiple pathologies. AD sometimes pathologically overlaps with age-related tauopathies such as four repeat (4R)-tau predominant argyrophilic grain disease (AGD). While AGD is often detected with AD pathology, the contribution of APOE4 to AGD risk is not clear despite its robust effects on AD pathogenesis. Specifically, how APOE genotype influences Aβ and tau pathology in co-occurring AGD and AD has not been fully understood. Using postmortem brain samples (N = 353) from a neuropathologically defined cohort comprising of cases with AD and/or AGD pathology built to best represent different APOE genotypes, we measured the amounts of major AD-related molecules, including Aβ40, Aβ42, apolipoprotein E (apoE), total tau (tTau), and pTau181, in the temporal cortex. The presence of tau lesions characteristic of AD (AD-tau) was correlated with cognitive decline based on Mini-Mental State Examination (MMSE) scores, while the presence of AGD tau lesions (AGD-tau) was not. Interestingly, while APOE4 increased the risk of AD-tau pathology, it did not increase the risk of AGD-tau pathology. Although APOE4 was significantly associated with higher levels of insoluble Aβ40, Aβ42, apoE, and pTau181, the APOE4 effect was no longer detected in the presence of AGD-tau. We also found that co-occurrence of AGD with AD was associated with lower insoluble Aβ42 and pTau181 levels. Overall, our findings suggest that different patterns of Aβ, tau, and apoE accumulation mediate the development of AD-tau and AGD-tau pathology, which is affected by APOE genotype.

Liquid - liquid phase separation of tau: Driving forces, regulation, and biological implications

The past 15 years have witnessed an explosion in the studies of biomolecular condensates that are implicated in numerous biological processes and play vital roles in human health and diseases. Recent findings demonstrate that the microtubule-associated protein tau forms liquid condensates through liquid-liquid phase separation (LLPS) in in vitro experiments using purified recombinant proteins and cell-based experiments. Although in vivo studies are lacking, liquid condensates have emerged as an important assembly state of physiological and pathological tau and LLPS can regulate the function of microtubules, mediate stress granule formation, and accelerate tau amyloid aggregation. In this review, we summarize recent advances in tau LLPS, aiming to unveiling the delicate interactions driving tau LLPS. We further discuss the association of tau LLPS with physiology and disease in the context of the sophisticated regulation of tau LLPS. Deciphering the mechanisms underlying tau LLPS and the liquid-to-solid transition enables rational design of molecules that inhibit or delay the formation of tau solid species, thus providing novel targeted therapeutic strategies for tauopathies.

β-III Tubulin Levels Determine the Neurotoxicity Induced by Colchicine-Site Binding Agent Indibulin

Indibulin, a microtubule-depolymerizing agent, produces minimal neurotoxicity in animals. It is also less cytotoxic toward differentiated neuronal cells than undifferentiated cells. We found that the levels of β-III tubulin, acetylated tubulin, and polyglutamylated tubulin were significantly increased in differentiated neuroblastoma cells (SH-SY5Y). Since neuronal cells express β-tubulin isotypes differently from other cell types, we explored the binding of indibulin to different β-tubulin isotypes. Our molecular docking analysis suggested that indibulin binds to β-III tubulin with lower affinity than to other β-tubulin isotypes. We therefore studied the implications of different β-tubulin isotypes on the cytotoxic effects of indibulin, colchicine, and vinblastine in differentiated SH-SY5Y cells. Upon depletion of β-III tubulin in the differentiated cells, the toxicity of indibulin and colchicine significantly increased, while sensitivity to vinblastine was unaffected. Using biochemical, bioinformatics, and fluorescence spectroscopic techniques, we have identified the binding site of indibulin on tubulin, which had not previously been established. Indibulin inhibited the binding of colchicine and C12 (a colchicine-site binder) to tubulin and also increased the dissociation constant of the interaction between tubulin and colchicine. Indibulin did not inhibit the binding of vinblastine or taxol to tubulin. Interestingly, indibulin antagonized colchicine treatment but synergized with vinblastine treatment in a combination study performed in MDA-MB-231 cells. The results indicate that indibulin is a colchicine-site binder and that the efficacy of colchicine-site binders is affected by the β-III tubulin levels in the cells.

X-linked ubiquitin-specific peptidase 11 increases tauopathy vulnerability in women

Although women experience significantly higher tau burden and increased risk for Alzheimer's disease (AD) than men, the underlying mechanism for this vulnerability has not been explained. Here, we demonstrate through in vitro and in vivo models, as well as human AD brain tissue, that X-linked ubiquitin specific peptidase 11 (USP11) augments pathological tau aggregation via tau deubiquitination initiated at lysine-281. Removal of ubiquitin provides access for enzymatic tau acetylation at lysines 281 and 274. USP11 escapes complete X-inactivation, and female mice and people both exhibit higher USP11 levels than males. Genetic elimination of usp11 in a tauopathy mouse model preferentially protects females from acetylated tau accumulation, tau pathology, and cognitive impairment. USP11 levels also strongly associate positively with tau pathology in females but not males. Thus, inhibiting USP11-mediated tau deubiquitination may provide an effective therapeutic opportunity to protect women from increased vulnerability to AD and other tauopathies.

Single domain antibodies targeting pathological tau protein: Influence of four IgG subclasses on efficacy and toxicity

BACKGROUND

Eleven tau immunoglobulin G (IgG) antibodies have entered clinical trials to treat tauopathies, including Alzheimer's disease, but it is unclear which IgG subclass/subtype has the ideal efficacy and safety profile. Only two subtypes, with or without effector function, have been examined in the clinic and not for the same tau antibody. The few preclinical studies on this topic have only compared two subtypes of one antibody each and have yielded conflicting results.

METHODS

We selected two single domain antibodies (sdAbs) derived from a llama immunized with tau proteins and utilized them to generate an array of Fc-(sdAb)2 subclasses containing identical tau binding domains but differing Fc region. Unmodified sdAbs and their IgG subclasses were tested for efficacy in primary cultures and in vivo microdialysis using JNPL3 tauopathy mice.

FINDINGS

Unmodified sdAbs were non-toxic, blocked tau toxicity and promoted tau clearance. However, the efficacy/safety profile of their Fc-(sdAb)2 subclasses varied greatly within and between sdAbs. For one of them, all its subtypes were non-toxic, only those with effector function cleared tau, and were more effective in vivo than unmodified sdAb. For the other sdAb, all its subtypes were toxic in tauopathy cultures but not in wild-type cells, suggesting that bivalent binding of its tau epitope stabilizes a toxic conformation of tau, with major implications for tau pathogenesis. Likewise, its subclasses were less effective than the unmodified sdAb in clearing tau in vivo.

INTERPRETATION

These findings indicate that tau antibodies with effector function are safe and better at clearing pathological tau than effectorless antibodies, Furthermore, tau antibodies can provide a valuable insight into tau pathogenesis, and some may aggravate it.

FUNDING

Funding for these studies was provided by the National Institute of Health (R01 AG032611, R01 NS077239, RF1 NS120488, R21 AG 069475, R21 AG 058282, T32AG052909), and the NYU Alzheimer's Disease Center Pilot Grant Program (via P30 AG008051).

TAR DNA-binding protein 43 oligomers in physiology and pathology

TAR DNA-binding protein 43 (TDP-43) is an RNA/DNA-binding protein involved in RNA regulation and diseases. In 2006, TDP-43 inclusions were found in the disease lesions of several neurodegenerative diseases. It is the pathological hallmark in both amyotrophic lateral sclerosis and frontotemporal lobar dementia. It also presents in a large portion of patients with Alzheimer's disease. TDP-43 is prone to aggregate; however, the role of TDP-43 oligomers remains poorly understood in both physiological and pathological conditions. In this review, we emphasize the role of oligomeric TDP-43 in both physiological and pathological conditions and discuss the potential mechanisms of oligomer formation. Finally, we suggest therapeutic strategies against the TDP-43 oligomers in neurodegenerative diseases.

Shikonin impedes phase separation and aggregation of tau and protects SH-SY5Y cells from the toxic effects of tau oligomers

Tauopathies such as Alzheimer's and Parkinson's diseases involve the abnormal deposition of tau aggregates in the brain and neuronal tissues. We report that a natural naphthoquinone, shikonin, impeded the oligomerization and fibrillization of tau. The compound strongly inhibited heparin, arachidonic acid, and RNA-induced tau aggregation. Atomic force microscopy, dynamic light scattering, SDS-PAGE, and dot blot assays revealed that shikonin diminished tau oligomerization and decreased the mean size of tau oligomers. Transmission electron microscopy and atomic force microscopy analysis further showed that shikonin could suppress tau fibrillization and shorten the tau filaments. Shikonin inhibited tau droplet formation. The compound significantly reduced the aggregation rate of a tryptophan mutant (Y310W-tau) of tau. In addition, shikonin disaggregated preformed tau filaments with a half-maximal disaggregation concentration (DC₅₀) of 6.3 ± 0.4 μM. Pre-treatment of neuroblastoma cells (SH-SY5Y) with shikonin protected the cells from the toxicity induced by tau oligomers and increased their viability. The findings imply that shikonin inhibited several steps in the tau aggregation pathways, especially the early stages, such as liquid-liquid phase separation. Therefore, shikonin is an attractive candidate for developing a therapy against tauopathy.

The Antioxidative Role of Chaperone-Mediated Autophagy as a Downstream Regulator of Oxidative Stress in Human Diseases

Chaperone-mediated autophagy (CMA) plays an important role in regulating a variety of cellular functions by selectively degrading damaged or functional proteins in the cytoplasm. One of the cellular processes in which CMA participates is the oxidative stress response. Oxidative stress regulates CMA activity, while CMA protects cells from oxidative damage by degrading oxidized proteins and preventing the accumulation of excessive reactive oxygen species (ROS). Changes in CMA activity have been found in many human diseases, and oxidative stress is also involved. Therefore, understanding the interaction mechanism of ROS and CMA will provide new targets for disease treatment. In this review, we discuss the role of CMA in combatting oxidative stress during the development of different conditions, such as aging, neurodegeneration, liver diseases, infections, pulmonary disorders, and cancers.

Unraveling the Influence of K280 Acetylation on the Conformational Features of Tau Core Fragment: A Molecular Dynamics Simulation Study

Abnormal aggregation of the microtubule-associated protein Tau is closely associated with tauopathies, including Alzheimer's disease and chronic traumatic encephalopathy. The hexapeptide 275VQIINK280 (PHF6*), a fibril-nucleating core motif of Tau, has been shown to play a vital role in the aggregation of Tau. Mounting experiment evidence demonstrated the acetylation of a single-lysine residue K280 in the PHF6* was a critical event for the formation of pathological Tau amyloid deposits. However, the underlying mechanisms by which K280 acetylation affects Tau aggregation at the atomic level remain elusive. In this work, we performed replica exchange molecular dynamics simulations to investigate the influence of acetylation of K280 on the aggregation of PHF6*. Our simulations show that acetylation of K280 not only enhances the self-assembly capability of PHF6* peptides but also increases the β-sheet structure propensity of the PHF6*. The inter-molecular interactions among PHF6* peptides are strengthened by the acetylation of K280, resulting in an increased ordered β-sheet-rich conformations of the PHF6* assemblies along with a decrease of the structural diversity. The residue-pairwise contact frequency analysis shows that K280 acetylation increases the interactions among the hydrophobic chemical groups from PHF6* peptides, which promotes the aggregation of PHF6*. This study offers mechanistic insights into the effects of acetylation on the aggregation of PHF6*, which will be helpful for an in-depth understanding of the relationship between acetylation and Tau aggregation at the molecular level.

Regulation of liquid-liquid phase separation with focus on post-translational modifications

Liquid-liquid phase separation (LLPS), a type of phase transition that is important in organisms, is a unique means of forming biomolecular condensates. LLPS plays a significant role in transcription, genome organisation, immune response and cell signaling, and its dysregulation may cause neurodegenerative diseases and cancers. Exploring the regulatory mechanism of LLPS contributes to the understanding of the pathogenic mechanism of abnormal phase transition and enables potential therapeutic targets to be proposed. Many factors have been found to regulate LLPS, of which post-translational modification (PTM) is among the most important. PTMs can change the structure, charge, hydrophobicity and other properties of the proteins involved in phase separation and thereby affect the phase transition behaviour. In this review, we discuss LLPS and the regulatory effects of PTMs, RNA and molecular chaperones in a phase separation system. We introduce several common PTMs (including phosphorylation, arginine methylation, arginine citrullination, acetylation, ubiquitination and poly(ADP-ribosyl)ation), highlight recent advances regarding their roles in LLPS and describe the regulatory mechanisms behind these features. This review provides a detailed overview of the field that will help further the understanding of and interventions in LLPS.

Contrasting Effects of Ferric and Ferrous Ions on Oligomerization and Droplet Formation of Tau: Implications in Tauopathies and Neurodegeneration

The dysregulation of metal homeostasis is reported to enhance the aggregation of tau, a key neuronal microtubule-associated protein. Herein, we found that ferric (Fe³⁺) ions enhanced tau aggregation. Fe³⁺ and Al³⁺ induced tau aggregation while several trivalent metal ions such as Cr³⁺, La³⁺, and V³⁺ had no discernable effect on tau aggregation. Fe³⁺ reduced the critical concentration of tau required for the liquid-liquid phase separation (LLPS); however, Cr³⁺, La³⁺, and V³⁺ did not affect tau droplet formation. Dynamic light scattering, atomic force microscopic, and transmission electron microscopic analysis suggested that Fe³⁺ significantly increased the formation of tau oligomers and fibrils. In contrast, Fe²⁺ neither enhanced tau droplet formation nor increased the heparin-induced aggregation of tau. Using a tryptophan mutant (Y310W-tau) of tau, Fe³⁺ was found to bind to tau with four times higher affinity than Fe²⁺. Acrylamide quenching of the tryptophan fluorescence of Y310W-tau, 1-anilino-8-naphthalene sulfonate (ANS) fluorescence experiment, and far-UV circular dichroism analysis indicated that Fe³⁺ decreased the solvent exposure of the tryptophan residue, perturbed the hydrophobic surface arrangement, and disrupted the secondary structure of tau, respectively. The increase in the β-sheet content and a subsequent decrease in the disordered content of tau due to the binding of Fe³⁺ may favor tau aggregation. Fe³⁺ may enhance and stabilize the non-covalent interactions between disordered domains of tau molecules leading to tau aggregation. The data highlighted the relationship between the dysregulation of ferric ions and neurodegenerative disorders.

The Structure Biology of Tau and Clue for Aggregation Inhibitor Design

Tau is a microtubule-associated protein that is mainly expressed in central and peripheral nerve systems. Tau binds to tubulin and regulates assembly and stabilization of microtubule, thus playing a critical role in neuron morphology, axon development and navigation. Tau is highly stable under normal conditions; however, there are several factors that can induce or promote aggregation of tau, forming neurofibrillary tangles. Neurofibrillary tangles are toxic to neurons, which may be related to a series of neurodegenerative diseases including Alzheimer's disease. Thus, tau is widely accepted as an important therapeutic target for neurodegenerative diseases. While the monomeric structure of tau is highly disordered, the aggregate structure of tau is formed by closed packing of β-stands. Studies on the structure of tau and the structural transition mechanism provide valuable information on the occurrence, development, and therapy of tauopathies. In this review, we summarize recent progress on the structural investigation of tau and based on which we discuss aggregation inhibitor design.

The acetylation modification regulates the stability of Bm30K-15 protein and its mechanism in silkworm, Bombyx mori

The 30 K proteins are the major silkworm hemolymph proteins and are involved in a variety of physiological processes, such as nutrient and energy storage, embryogenesis, immune response, and inhibition of apoptosis. The Bm30K-15 protein is one of the 30 K proteins and is abundant in the hemolymph of fifth instar silkworm larva. We previously found that the Bm30K-15 protein can be acetylated. In the present study, we found that acetylation can improve the protein stability of Bm30K-15. Further exploration confirmed that the increase in protein stability by acetylation was caused by competition between acetylation and ubiquitination. In summary, these findings aim to provide insight into the effect of acetylation modification on the protein level and stability of the Bm30K-15 and the possible molecular mechanism of its existence in silkworm, Bombyx mori.

C1 Inhibits Liquid-Liquid Phase Separation and Oligomerization of Tau and Protects Neuroblastoma Cells against Toxic Tau Oligomers

The pathological aggregation of tau is one of the major contributing factors for several neurodegenerative tauopathies, including Alzheimer's disease. Here, we report that C1, a synthetic derivative of curcumin, strongly inhibited both the aggregation and filament formation of purified tau and protected neuroblastoma cells from the deleterious effects of the tau oligomers. Using confocal microscopy, C1 was found to reduce both the size and number of the tau droplets and increased the critical concentration of tau required for the droplet formation in vitro indicating that C1 suppressed the liquid-liquid phase separation of tau. C1 inhibited the aggregation of tau with a half-maximal inhibitory concentration of 1.5 ± 0.1 μM. An analysis of the aggregation kinetics data indicated that C1 strongly reduced the initial rate of the aggregation of tau. A dot blot analysis using tau-oligomer-specific antibody indicated that C1 inhibited the oligomerization of tau. Furthermore, dynamic light scattering experiments suggested that C1 strongly reduced the mean diameter of the tau oligomers. Atomic force microscopy experiments showed that C1 treatment reduced both the size and number of tau oligomers, suppressed the transition of tau oligomers into filaments, and also disintegrated preformed tau filaments. Also, the binding interaction of C1 with tau was monitored using absorbance and fluorescence spectroscopy. C1 bound to Y310W-tau with a dissociation constant of 2.0 ± 0.5 μM. The findings suggested that C1 is a potent inhibitor of tau aggregation and provided insights into the inhibitory mechanism of C1 on the oligomerization and fibril formation of tau.

Acetylated Ubiquitin Modulates the Catalytic Activity of the E1 Enzyme Uba1

Ubiquitin (Ub) signaling requires the covalent passage of Ub among E1, E2, and E3 enzymes. The choice of E2 and E3 enzymes combined with multiple rounds of the cascade leads to the formation of polyubiquitin chains linked through any one of the seven lysines on Ub. The linkage type and length act as a signal to trigger important cellular processes such as protein degradation or the DNA damage response. Recently, proteomics studies have identified that Ub can be acetylated at six of its seven lysine residues under various cell stress conditions. To understand the potential differences in Ub signaling caused by acetylation, we synthesized all possible acetylated ubiquitin (acUb) variants and examined the E1-mediated formation of the corresponding E2∼acUb conjugates in vitro using kinetic methods. A Förster resonance energy transfer assay was optimized in which the Ub constructs were labeled with a CyPet fluorophore and the E2 UBE2D1 was labeled with a YPet fluorophore to monitor the formation of E2∼Ub conjugates. Our methods enable the detection of small differences that may otherwise be concealed in steady-state ubiquitination experiments. We determined that Ub, acetylated at K11, K27, K33, K48, or K63, has altered turnover numbers for E2∼Ub conjugate formation by the E1 enzyme Uba1. This work provides evidence that acetylation of Ub can alter the catalysis of ubiquitination early on in the pathway.

The structure and phase of tau: from monomer to amyloid filament

Tau is a microtubule-associated protein involved in regulation of assembly and spatial organization of microtubule in neurons. However, in pathological conditions, tau monomers assemble into amyloid filaments characterized by the cross-β structures in a number of neurodegenerative diseases known as tauopathies. In this review, we summarize recent progression on the characterization of structures of tau monomer and filament, as well as the dynamic liquid droplet assembly. Our aim is to reveal how post-translational modifications, amino acid mutations, and interacting molecules modulate the conformational ensemble of tau monomer, and how they accelerate or inhibit tau assembly into aggregates. Structure-based aggregation inhibitor design is also discussed in the context of dynamics and heterogeneity of tau structures.

The role of wild-type tau in Alzheimer's disease and related tauopathies

Tau oligomers have recently emerged as the principal toxic species in Alzheimer's disease (AD) and tauopathies. Tau oligomers are spontaneously self-assembled soluble tau proteins that are formed prior to fibrils, and they have been shown to play a central role in neuronal cell death and in the induction of neurodegeneration in animal models. As the therapeutic paradigm shifts to targeting toxic tau oligomers, this suggests the focus to study tau oligomerization in species that are less susceptible to fibrillization. While truncated and mutation containing tau as well as the isolated repeat domains are particularly prone to fibrillization, the wild-type (WT) tau proteins have been shown to be resistant to fibril formation in the absence of aggregation inducers. In this review, we will summarize and discuss the toxicity of WT tau both in vitro and in vivo, as well as its involvement in tau oligomerization and cell-to-cell propagation of pathology. Understanding the role of WT tau will enable more effective biomarker development and therapeutic discovery for treatment of AD and tauopathies.

Tauopathies: Deciphering Disease Mechanisms to Develop Effective Therapies

Tauopathies are neurodegenerative diseases characterized by the pathological accumulation of microtubule-associated protein tau (MAPT) in the form of neurofibrillary tangles and paired helical filaments in neurons and glia, leading to brain cell death. These diseases include frontotemporal dementia (FTD) and Alzheimer's disease (AD) and can be sporadic or inherited when caused by mutations in the MAPT gene. Despite an incredibly high socio-economic burden worldwide, there are still no effective disease-modifying therapies, and few tau-focused experimental drugs have reached clinical trials. One major hindrance for therapeutic development is the knowledge gap in molecular mechanisms of tau-mediated neuronal toxicity and death. For the promise of precision medicine for brain disorders to be fulfilled, it is necessary to integrate known genetic causes of disease, i.e., MAPT mutations, with an understanding of the dysregulated molecular pathways that constitute potential therapeutic targets. Here, the growing understanding of known and proposed mechanisms of disease etiology will be reviewed, together with promising experimental tau-directed therapeutics, such as recently developed tau degraders. Current challenges faced by the fields of tau research and drug discovery will also be addressed.

Liquid-liquid phase separation induces pathogenic tau conformations in vitro

Formation of membrane-less organelles via liquid-liquid phase separation is one way cells meet the biological requirement for spatiotemporal regulation of cellular components and reactions. Recently, tau, a protein known for its involvement in Alzheimer’s disease and other tauopathies, was found to undergo liquid–liquid phase separation making it one of several proteins associated with neurodegenerative diseases to do so. Here, we demonstrate that tau forms dynamic liquid droplets in vitro at physiological protein levels upon molecular crowding in buffers that resemble physiological conditions. Tau droplet formation is significantly enhanced by disease-associated modifications, including the AT8 phospho-epitope and the P301L tau mutation linked to an inherited tauopathy. Moreover, tau droplet dynamics are significantly reduced by these modified forms of tau. Extended phase separation promoted a time-dependent adoption of toxic conformations and oligomerization, but not filamentous aggregation. P301L tau protein showed the greatest oligomer formation following extended phase separation. These findings suggest that phase separation of tau may facilitate the formation of non-filamentous pathogenic tau conformations.

Tau plays an important role in tauopathies and undergoes liquid-liquid phase separation (LLPS). The authors show that disease-related P301L mutant and phosphomimic (S199E/S202E/T205E) tau enhance LLPS in vitro at physiological levels, and using specific antibodies, that tau LLPS leads to pathological conformations such as N-terminal exposure and oligomeric species.

Phase Separation and Cytotoxicity of Tau are Modulated by Protein Disulfide Isomerase and S-nitrosylation of this Molecular Chaperone

Cells have evolved molecular chaperones that modulate phase separation and misfolding of amyloidogenic proteins to prevent neurodegenerative diseases. Protein disulfide isomerase (PDI), mainly located at the endoplasmic reticulum and also present in the cytosol, acts as both an enzyme and a molecular chaperone. PDI is observed to be S-nitrosylated in the brain of Alzheimer's disease patients, but the mechanism has remained elusive. We herein report that both wild-type PDI and its quadruple cysteine mutant only having chaperone activity, significantly inhibit pathological phosphorylation and abnormal aggregation of Tau in cells, and significantly decrease the mitochondrial damage and Tau cytotoxicity resulting from Tau aberrant aggregation, highlighting the chaperone property of PDI. More importantly, we show that wild-type PDI is selectively recruited by liquid droplets of Tau, which significantly inhibits phase separation and stress granule formation of Tau, whereas S-nitrosylation of PDI abrogates the recruitment and inhibition. These findings demonstrate how phase separation of Tau is physiologically regulated by PDI and how S-nitrosylation of PDI, a perturbation in this regulation, leads to disease.

The Acetyl Mimicking Mutation, K274Q in Tau, Enhances the Metal Binding Affinity of Tau and Reduces the Ability of Tau to Protect DNA

The aggregation of tau, a microtubule-associated protein, is known to play an important role in several neurological disorders including Alzheimer's disease. Alzheimer's disease is considered to be associated with the dyshomeostasis of metal ions such as aluminum, zinc, copper, and ferric ions. Tau is predominately acetylated at the K274 residue in Alzheimer's disease, and the acetylation of the K274 residue is thought to be linked with dementia. Using acetyl mimicking K274Q mutation in tau, we have examined the effects of the acetylation at K274 residue of tau on the interactions of tau with metal ions and also on the ability of tau to protect DNA from the heat and other stressors. We found that Zn²⁺ and Al³⁺ increased the liquid-liquid phase separation of tau, an initial stage of tau aggregation. Further, Zn²⁺ and Al³⁺ considerably reduced the critical concentration for the phase separation of K274Q tau. Using far-UV circular dichroism and fluorescence spectroscopy, we provide evidence suggesting that the binding of Zn²⁺ and Al³⁺ induces conformational changes in tau. The K274Q mutation enhanced the binding affinity of tau for Zn²⁺, Al³⁺, Cu²⁺, and Fe³⁺ ions. In addition, Zn²⁺, Al³⁺, Cu²⁺, and Fe³⁺ significantly enhanced the aggregation propensity of K274Q tau in comparison to tau. Interestingly, tau binds to DNA with a higher affinity than K274Q tau. Tau protects DNA from the DNase treatment in vitro as well as from the heat stress in neuroblastoma cells more efficiently than K274Q tau. The results indicated that the acetylation of K274 residue of tau may increase metal ion-induced toxicity and diminish the ability of tau to protect DNA.

Phagocytosis of full-length Tau oligomers by Actin-remodeling of activated microglia

BACKGROUND

Alzheimer's disease is associated with the accumulation of intracellular Tau tangles within neurons and extracellular amyloid-β plaques in the brain parenchyma, which altogether results in synaptic loss and neurodegeneration. Extracellular concentrations of oligomers and aggregated proteins initiate microglial activation and convert their state of synaptic surveillance into a destructive inflammatory state. Although Tau oligomers have fleeting nature, they were shown to mediate neurotoxicity and microglial pro-inflammation. Due to the instability of oligomers, in vitro experiments become challenging, and hence, the stability of the full-length Tau oligomers is a major concern.

METHODS

In this study, we have prepared and stabilized hTau40^WT oligomers, which were purified by size-exclusion chromatography. The formation of the oligomers was confirmed by western blot, thioflavin-S, 8-anilinonaphthaalene-1-sulfonic acid fluorescence, and circular dichroism spectroscopy, which determine the intermolecular cross-β sheet structure and hydrophobicity. The efficiency of N9 microglial cells to phagocytose hTau40^WT oligomer and subsequent microglial activation was studied by immunofluorescence microscopy with apotome. The one-way ANOVA was performed for the statistical analysis of fluorometric assay and microscopic analysis.

RESULTS

Full-length Tau oligomers were detected in heterogeneous globular structures ranging from 5 to 50 nm as observed by high-resolution transmission electron microscopy, which was further characterized by oligomer-specific A11 antibody. Immunocytochemistry studies for oligomer treatment were evidenced with A11⁺ Iba1^high microglia, suggesting that the phagocytosis of extracellular Tau oligomers leads to microglial activation. Also, the microglia were observed with remodeled filopodia-like actin structures upon the exposure of oligomers and aggregated Tau.

CONCLUSION

The peri-membrane polymerization of actin filament and co-localization of Iba1 relate to the microglial movements for phagocytosis. Here, these findings suggest that microglia modified actin cytoskeleton for phagocytosis and rapid clearance of Tau oligomers in Alzheimer's disease condition.

Revisiting the intersection of amyloid, pathologically modified tau and iron in Alzheimer's disease from a ferroptosis perspective

The complexity of Alzheimer's disease (AD) complicates the search for effective treatments. While the key roles of pathologically modified proteins has occupied a central role in hypotheses of the pathophysiology, less attention has been paid to the potential role for transition metals overload, subsequent oxidative stress, and tissue injury. The association of transition metals, the major focus heretofore iron and amyloid, the same can now be said for the likely pathogenic microtubular associated tau (MAPT). This review discusses the interplay between iron, pathologically modified tau and oxidative stress, and connects many related discoveries. Basic principles of the transition to pathological MAPT are discussed. Iron, its homeostatic mechanisms, the recently described phenomenon of ferroptosis and purported, although still controversial roles in AD are reviewed as well as considerations to overcome existing hurdles of iron-targeted therapeutic avenues that have been attempted in AD. We summarize the involvement of multiple pathological pathways at different disease stages of disease progression that supports the potential for a combinatorial treatment strategy targeting multiple factors.

Pan-HDAC Inhibitors Promote Tau Aggregation by Increasing the Level of Acetylated Tau

Epigenetic remodeling via histone acetylation has become a popular therapeutic strategy to treat Alzheimer's disease (AD). In particular, histone deacetylase (HDAC) inhibitors including M344 and SAHA have been elucidated to be new drug candidates for AD, improving cognitive abilities impaired in AD mouse models. Although emerged as a promising target for AD, most of the HDAC inhibitors are poorly selective and could cause unwanted side effects. Here we show that tau is one of the cytosolic substrates of HDAC and the treatment of HDAC inhibitors such as Scriptaid, M344, BML281, and SAHA could increase the level of acetylated tau, resulting in the activation of tau pathology.

Role of Tau as a Microtubule-Associated Protein: Structural and Functional Aspects

Microtubules (MTs) play a fundamental role in many vital processes such as cell division and neuronal activity. They are key structural and functional elements in axons, supporting neurite differentiation and growth, as well as transporting motor proteins along the axons, which use MTs as support tracks. Tau is a stabilizing MT associated protein, whose functions are mainly regulated by phosphorylation. A disruption of the MT network, which might be caused by Tau loss of function, is observed in a group of related diseases called tauopathies, which includes Alzheimer’s disease (AD). Tau is found hyperphosphorylated in AD, which might account for its loss of MT stabilizing capacity. Since destabilization of MTs after dissociation of Tau could contribute to toxicity in neurodegenerative diseases, a molecular understanding of this interaction and its regulation is essential.