Pharmacological Modulators of Tau Aggregation and Spreading

Shaping the future of preclinical development of successful disease-modifying drugs against Alzheimer's disease: a systematic review of tau propagation models

The transcellular propagation of the aberrantly modified protein tau along the functional brain network is a key hallmark of Alzheimer's disease and related tauopathies. Inoculation-based tau propagation models can recapitulate the stereotypical spread of tau and reproduce various types of tau inclusions linked to specific tauopathy, albeit with varying degrees of fidelity. With this systematic review, we underscore the significance of judicious selection and meticulous functional, biochemical, and biophysical characterization of various tau inocula. Furthermore, we highlight the necessity of choosing suitable animal models and inoculation sites, along with the critical need for validation of fibrillary pathology using confirmatory staining, to accurately recapitulate disease-specific inclusions. As a practical guide, we put forth a framework for establishing a benchmark of inoculation-based tau propagation models that holds promise for use in preclinical testing of disease-modifying drugs.

Native PLGA nanoparticles attenuate Aβ-seed induced tau aggregation under in vitro conditions: potential implication in Alzheimer's disease pathology

Evidence suggests that beta-amyloid (Aβ)-induced phosphorylation/aggregation of tau protein plays a critical role in the degeneration of neurons and development of Alzheimer's disease (AD), the most common cause of dementia affecting the elderly population. Many studies have pursued a variety of small molecules, including nanoparticles conjugated with drugs to interfere with Aβ and/or tau aggregation/toxicity as an effective strategy for AD treatment. We reported earlier that FDA approved PLGA nanoparticles without any drug can attenuate Aβ aggregation/toxicity in cellular/animal models of AD. In this study, we evaluated the effects of native PLGA on Aβ seed-induced aggregation of tau protein using a variety of biophysical, structural and spectroscopic approaches. Our results show that Aβ1-42 seeds enhanced aggregation of tau protein in the presence and absence of heparin and the effect was attenuated by native PLGA nanoparticles. Interestingly, PLGA inhibited aggregation of both 4R and 3R tau isoforms involved in the formation of neurofibrillary tangles in AD brains. Furthermore, Aβ seed-induced tau aggregation in the presence of arachidonic acid was suppressed by native PLGA. Collectively, our results suggest that native PLGA nanoparticles can inhibit the Aβ seed-induced aggregation of different tau protein isoforms highlighting their therapeutic implication in the treatment of AD.

Tau-targeting therapies for Alzheimer disease: current status and future directions

Alzheimer disease (AD) is the most common cause of dementia in older individuals. AD is characterized pathologically by amyloid-β (Aβ) plaques and tau neurofibrillary tangles in the brain, with associated loss of synapses and neurons, which eventually results in dementia. Many of the early attempts to develop treatments for AD focused on Aβ, but a lack of efficacy of these treatments in terms of slowing disease progression led to a change of strategy towards targeting of tau pathology. Given that tau shows a stronger correlation with symptom severity than does Aβ, targeting of tau is more likely to be efficacious once cognitive decline begins. Anti-tau therapies initially focused on post-translational modifications, inhibition of tau aggregation and stabilization of microtubules. However, trials of many potential drugs were discontinued because of toxicity and/or lack of efficacy. Currently, the majority of tau-targeting agents in clinical trials are immunotherapies. In this Review, we provide an update on the results from the initial immunotherapy trials and an overview of new therapeutic candidates that are in clinical development, as well as considering future directions for tau-targeting therapies.

Human Tau Aggregates Are Permissive to Protein Synthesis-Dependent Memory in Drosophila Tauopathy Models

Tauopathies including Alzheimer's disease, are characterized by progressive cognitive decline, neurodegeneration, and intraneuronal aggregates comprised largely of the axonal protein Tau. It has been unclear whether cognitive deficits are a consequence of aggregate accumulation thought to compromise neuronal health and eventually lead to neurodegeneration. We use the Drosophila tauopathy model and mixed-sex populations to reveal an adult onset pan-neuronal Tau accumulation-dependent decline in learning efficacy and a specific defect in protein synthesis-dependent memory (PSD-M), but not in its protein synthesis-independent variant. We demonstrate that these neuroplasticity defects are reversible on suppression of new transgenic human Tau expression and surprisingly correlate with an increase in Tau aggregates. Inhibition of aggregate formation via acute oral administration of methylene blue results in re-emergence of deficient memory in animals with suppressed human Tau (hTau)0N4R expression. Significantly, aggregate inhibition results in PSD-M deficits in hTau0N3R-expressing animals, which present elevated aggregates and normal memory if untreated with methylene blue. Moreover, methylene blue-dependent hTau0N4R aggregate suppression within adult mushroom body neurons also resulted in emergence of memory deficits. Therefore, deficient PSD-M on human Tau expression in the Drosophila CNS is not a consequence of toxicity and neuronal loss because it is reversible. Furthermore, PSD-M deficits do not result from aggregate accumulation, which appears permissive, if not protective of processes underlying this memory variant.SIGNIFICANCE STATEMENT Intraneuronal Tau aggregate accumulation has been proposed to underlie the cognitive decline and eventual neurotoxicity that characterizes the neurodegenerative dementias known as tauopathies. However, we show in three experimental settings that Tau aggregates in the Drosophila CNS do not impair but rather appear to facilitate processes underlying protein synthesis-dependent memory within affected neurons.

Nature's Toolbox Against Tau Aggregation: An Updated Review of Current Research

Tau aggregation is a hallmark of several neurodegenerative disorders, such as Alzheimer's disease (AD), frontotemporal dementia, and progressive supranuclear palsy. Hyperphosphorylated tau is believed to contribute to the degeneration of neurons and the development of these complex diseases. Therefore, one potential treatment for these illnesses is to prevent or counteract tau aggregation. In recent years, interest has been increasing in developing nature-derived tau aggregation inhibitors as a potential treatment for neurodegenerative disorders. Researchers have become increasingly interested in natural compounds with multifunctional features, such as flavonoids, alkaloids, resveratrol, and curcumin, since these molecules can interact simultaneously with the various targets of AD. Recent studies have demonstrated that several natural compounds can inhibit tau aggregation and promote the disassembly of pre-formed tau aggregates. Nature-derived tau aggregation inhibitors hold promise as a potential treatment for neurodegenerative disorders. However, it is important to note that more research is needed to fully understand the mechanisms by which these compounds exert their effects and their safety and efficacy in preclinical and clinical studies. Nature-derived inhibitors of tau aggregation are a promising new direction in the research of neurodegenerative complexities. This review focuses on the natural products that have proven to be a rich supply for inhibitors in tau aggregation and their uses in neurodegenerative complexities, including AD.

Expanded Conformations of Monomeric Tau Initiate Its Amyloidogenesis

Understanding early amyloidogenesis is key to rationally develop therapeutic strategies. Tau protein forms well-characterized pathological deposits but its aggregation mechanism is still poorly understood. Using single-molecule force spectroscopy based on a mechanical protection strategy, we studied the conformational landscape of the monomeric tau repeat domain (tau-RD_244-368 ). We found two sets of conformational states, whose frequency is influenced by mutations and the chemical context. While pathological mutations Δ280K and P301L and a pro-amyloidogenic milieu favored expanded conformations and destabilized local structures, an anti-amyloidogenic environment promoted a compact ensemble, including a conformer whose topology might mask two amyloidogenic segments. Our results reveal that to initiate aggregation, monomeric tau-RD_244-368 decreases its polymorphism adopting expanded conformations. This could account for the distinct structures found in vitro and across tauopathies.

Small-molecule compound from AlphaScreen disrupts tau-glycan interface

Tauopathies are neurodegenerative diseases characterized by intracellular abnormal tau deposits in the brain. Tau aggregates can propagate from one neuron to another in a prion-like manner, mediated by the interaction between tau and cell surface heparan sulfate proteoglycans. We developed an AlphaScreen assay, with His-tagged tau and biotinylated heparin, to represent the tau-HS interface to target the tau-glycan interface. Using our AlphaScreen assay, with a Z-factor of 0.65, we screened ∼300 compounds and discovered a small-molecule compound (herein referred to as A9), which can disrupt the tau-heparin interaction with micromolar efficacy. A9 also effectively inhibited heparin-induced tau aggregation in Thioflavin T fluorescence assays and attenuated tau internalization by H4 neuroglioma cells. These results strongly suggest that A9 can disrupt the tau-glycan interface in both in vitro molecular and cellular environments. We further determined that A9 interacts with heparin rather than tau and does so with micromolar binding affinity as shown by nuclear magnetic resonance and surface plasmon resonance experiments. A9 binds to heparin in a manner that blocks the sites where tau binds to heparin on the cell surface. These results demonstrate our AlphaScreen method as an effective method for targeting the tau-glycan interface in drug discovery and A9 as a promising lead compound for tauopathies, including Alzheimer's disease.

Effectiveness and safety of anti-tau drugs for Alzheimer's disease: Systematic review and meta-analysis

OBJECTIVE

To assess the cognitive effectiveness and safety of tau-targeting drugs for Alzheimer's disease (AD) METHODS: The MEDLINE, Embase, Cochrane Library, PsycINFO, ClinicalTrials.gov, and WHO International Clinical Trials Registry Platform databases were searched from inception to 22 November 2021. A systematic review and meta-analysis of randomized controlled trials were performed RESULTS: Thirty-four randomized controlled trials comprising 5549 participants, of which fifteen (51.7%) had a low risk of bias, were included. The meta-analysis showed no differences in the cognitive subscale of the AD: Assessment Scale (ADAS-Cog) between anti-tau drugs and placebo (mean difference [MD]: -0.77, 95% CI: -1.64 to 0.10; minimal important difference 3.1-3.8 points, moderate certainty evidence). For ADAS-Cog, the results subgroup analysis suggested a statistical effect of tau posttranslational modifications on drug inhibition (MD: -0.80, 95% CI: -1.43 to -0.17), which was not seen with tau aggregation inhibitors or immunotherapy (interaction p = 0.24). A total of 11.0%, 5.2%, and 4.8% of drugs inhibiting tau aggregation, immunotherapy, and drugs targeting posttranslational modifications, respectively, had a reduced risk of dropouts due to adverse events (AEs).

DISCUSSION

Current evidence suggests that anti-tau drugs are unlikely to have an important impact on slowing cognitive impairment. Although the subgroup analysis suggested that inhibition of tau posttranslational modifications is statistically effective and generally safer because of reduced dropouts due to AEs, the analysis has limited credibility. Additional large-scale and well-designed randomized and placebo-controlled trials will be necessary to explore the benefit of a certain type of anti-tau drug for AD.

Structural study of the recognition mechanism of tau antibody Tau2r3 with the key sequence (VQIINK) in tau aggregation

One of the histopathological features of Alzheimer's disease (AD) is higher order neurofibrillary tangles formed by abnormally aggregated tau protein. The sequence ²⁷⁵VQIINK²⁸⁰ in the microtubule-binding domain of tau plays a key role in tau aggregation. Therefore, an aggregation inhibitor targeting the VQIINK region in tau may be an effective therapeutic agent for AD. We have previously shown that the Fab domain (Fab2r3) of a tau antibody that recognizes the VQIINK sequence can inhibit tau aggregation, and we have determined the tertiary structure of the Fab2r3-VQIINK complex. In this report, we determined the tertiary structure of apo Fab2r3 and analyzed differences in the structures of apo Fab2r3 and Fab2r3-VQIINK to examine the ligand recognition mechanism of Fab2r3. In comparison with the Fab2r3-VQIINK structure, there were large differences in the arrangement of the constant and variable domains in apo Fab2r3. Remarkable structural changes were especially observed in the H3 and L3 loop regions of the complementarity determining regions (CDRs) in apo Fab2r3 and the Fab2r3-VQIINK complex. These structural differences in CDRs suggest that formation of hydrophobic pockets suitable for the antigen is important for antigen recognition by tau antibodies.

TO901317 activation of LXR-dependent pathways mitigate amyloid-beta peptide-induced neurotoxicity in 3D human neural stem cell culture scaffolds and AD mice

Alzheimer's disease (AD) is the major cause of neurodegeneration worldwide and is characterized by the accumulation of amyloid beta (Aβ) in the brain, which is associated with neuronal loss and cognitive impairment. Liver X receptor (LXR), a critical nuclear receptor, and major regulator in lipid metabolism and inflammation, is suggested to play a protective role against the mitochondrial dysfunction noted in AD. In our study, our established 3D gelatin scaffold model and a well characterized in vivo (APP/PS1) murine model of AD were used to directly investigate the molecular, biochemical and behavioral effects of neuronal stem cell exposure to Aβ to improve understanding of the in vivo etiology of AD. Herein, human neural stem cells (hNSCs) in our 3D model were exposed to Aβ, and had significantly decreased cell viability, which correlated with decreased mRNA and protein expression of LXR, Bcl-2, CREB, PGC1α, NRF-1, and Tfam, and increased caspase 3 and 9 activities. Cotreatment with a synthetic agonist of LXR (TO901317) significantly abrogated these Aβ-mediated effects in hNSCs. Moreover, TO901317 cotreatment both significantly rescues hNSCs from Aβ-mediated decreases in ATP levels and mitochondrial mass, and significantly restores Aβ-induced fragmented mitochondria to almost normal morphology. TO901317 cotreatment also decreases tau aggregates in Aβ-treated hNSCs. Importantly, TO901317 treatment significantly alleviates the impairment of memory, decreases Aβ aggregates and increases proteasome activity in APP/PS1 mice; whereas, these effects were blocked by cotreatment with an LXR antagonist (GSK2033). Together, these novel results improve our mechanistic understanding of the central role of LXR in Aβ-mediated hNSC dysfunction. We also provide preclinical data unveiling the protective effects of using an LXR-dependent agonist, TO901317, to block the toxicity observed in Aβ-exposed hNSCs, which may guide future treatment strategies to slow or prevent neurodegeneration in some AD patients.

Chemoinformatics Analyses of Tau Ligands Reveal Key Molecular Requirements for the Identification of Potential Drug Candidates against Tauopathies

Tau is a highly soluble protein mainly localized at a cytoplasmic level in the neuronal cells, which plays a crucial role in the regulation of microtubule dynamic stability. Recent studies have demonstrated that several factors, such as hyperphosphorylation or alterations of Tau metabolism, may contribute to the pathological accumulation of protein aggregates, which can result in neuronal death and the onset of a number of neurological disorders called Tauopathies. At present, there are no available therapeutic remedies able to reduce Tau aggregation, nor are there any structural clues or guidelines for the rational identification of compounds preventing the accumulation of protein aggregates. To help identify the structural properties required for anti-Tau aggregation activity, we performed extensive chemoinformatics analyses on a dataset of Tau ligands reported in ChEMBL. The performed analyses allowed us to identify a set of molecular properties that are in common between known active ligands. Moreover, extensive analyses of the fragment composition of reported ligands led to the identification of chemical moieties and fragment combinations prevalent in the more active compounds. Interestingly, many of these fragments were arranged in recurring frameworks, some of which were clearly present in compounds currently under clinical investigation. This work represents the first in-depth chemoinformatics study of the molecular properties, constituting fragments and similarity profiles, of known Tau aggregation inhibitors. The datasets of compounds employed for the analyses, the identified molecular fragments and their combinations are made publicly available as supplementary material.

The Integral Role of RNA in Stress Granule Formation and Function

Stress granules (SGs) are phase-separated, membraneless, cytoplasmic ribonucleoprotein (RNP) assemblies whose primary function is to promote cell survival by condensing translationally stalled mRNAs, ribosomal components, translation initiation factors, and RNA-binding proteins (RBPs). While the protein composition and the function of proteins in the compartmentalization and the dynamics of assembly and disassembly of SGs has been a matter of study for several years, the role of RNA in these structures had remained largely unknown. RNA species are, however, not passive members of RNA granules in that RNA by itself can form homo and heterotypic interactions with other RNA molecules leading to phase separation and nucleation of RNA granules. RNA can also function as molecular scaffolds recruiting multivalent RBPs and their interactors to form higher-order structures. With the development of SG purification techniques coupled to RNA-seq, the transcriptomic landscape of SGs is becoming increasingly understood, revealing the enormous potential of RNA to guide the assembly and disassembly of these transient organelles. SGs are not only formed under acute stress conditions but also in response to different diseases such as viral infections, cancer, and neurodegeneration. Importantly, these granules are increasingly being recognized as potential precursors of pathological aggregates in neurodegenerative diseases. In this review, we examine the current evidence in support of RNA playing a significant role in the formation of SGs and explore the concept of SGs as therapeutic targets.

Inhibition of tau amyloid formation and disruption of its preformed fibrils by Naphthoquinone-Dopamine hybrid

Misfolding and aggregation of tau protein, into pathological amyloids, are hallmarks of a group of neurodegenerative diseases collectively termed tauopathies and their modulation may be therapeutically valuable. Herein, we describe the synthesis and characterization of a dopamine-based hybrid molecule, naphthoquinone-dopamine (NQDA). Using thioflavin S assay, CD, transmission electron microscopy, dynamic light scattering, Congo Red birefringence, and large unilamellar vesicle leakage assays, we demonstrated its efficacy in inhibiting the in vitro aggregation of key tau-derived amyloidogenic fragments, PHF6 (VQIVYK) and PHF6* (VQIINK), prime drivers of aggregation of full-length tau in disease pathology. Isothermal titration calorimetry analysis revealed that the interaction between NQDA and PHF6 is spontaneous and has significant binding efficiency driven by both entropic and enthalpic processes. Furthermore, NQDA efficiently disassembled preformed fibrils of PHF6 and PHF6* into nontoxic species. Molecular dynamic simulations supported the in vitro results and provided a plausible mode of binding of NQDA with PHF6 fibril. NQDA was also capable of inhibiting the aggregation of full-length tau protein and disrupting its preformed fibrils in vitro in a dose-dependent manner. In a comparative study, the IC₅₀ value (50% inhibition of fibril formation) of NQDA in inhibiting the aggregation of PHF6 (25 µm) was ~ 17 µm, which is lower than for other bona fide amyloid inhibitors, naphthoquinone-tryptophan, rosmarinic acid, epigallocatechin gallate, ~ 21, ~ 77, or ~ 19 µm, respectively. Comparable superiority of NQDA was observed for inhibition of PHF6*. These findings suggest that NQDA can be a useful scaffold for designing new therapeutics for Alzheimer's disease and other tauopathies.

Tau Oligomers Neurotoxicity

Although the mechanisms of toxic activity of tau are not fully recognized, it is supposed that the tau toxicity is related rather not to insoluble tau aggregates but to its intermediate forms. It seems that neurofibrillar tangles (NFTs) themselves, despite being composed of toxic tau, are probably neither necessary nor sufficient for tau-induced neuronal dysfunction and toxicity. Tau oligomers (TauOs) formed during the early stages of tau aggregation are the pathological forms that play a key role in eliciting the loss of neurons and behavioral impairments in several neurodegenerative disorders called tauopathies. They can be found in tauopathic diseases, the most common of which is Alzheimer's disease (AD). Evidence of co-occurrence of b-amyloid, α-synuclein, and tau into their most toxic forms, i.e., oligomers, suggests that these species interact and influence each other's aggregation in several tauopathies. The mechanism responsible for oligomeric tau neurotoxicity is a subject of intensive investigation. In this review, we summarize the most recent literature on the damaging effect of TauOs on the stability of the genome and the function of the nucleus, energy production and mitochondrial function, cell signaling and synaptic plasticity, the microtubule assembly, neuronal cytoskeleton and axonal transport, and the effectiveness of the protein degradation system.