Acetylation discriminates disease-specific tau deposition

Histone Deacetylation in Alzheimer's Diseases (AD); Hope or Hype

Histone acetylation is the process by which histone acetyltransferases (HATs) add an acetyl group to the N-terminal lysine residues of histones, resulting in a more open chromatin structure. Histone acetylation tends to increase gene expression more than methylation does. In the central nervous system (CNS), histone acetylation is essential for controlling the expression of genes linked to cognition and learning. Histone deacetylases (HDACs), "writing" enzymes (HATs), and "reading" enzymes with bromodomains that identify and localize to acetylated lysine residues are responsible for maintaining histone acetylation. By giving animals HDAC inhibitors (HDACis), it is possible to intentionally control the ratios of "writer" and "eraser" activity, which will change the acetylation of histones. In addition to making the chromatin more accessible, these histone acetylation alterations re-allocate the targeting of "readers," including the transcriptional co-activators, cAMP response element-binding protein (CBP), and bromodomain-containing protein 4 (Brd4) in the CNS. Conclusive evidence has shown that HDACs slow down the progression of Alzheimer's disease (AD) by reducing the amount of histone acetylation, decreasing the activity of genes linked to memory, supporting cognitive decline and Amyloid beta (Aβ) protein accumulation, influencing aberrant tau phosphorylation, and promoting the emergence of neurofibrillary tangles (NFTs). In this review, we have covered the therapeutic targets and functions of HDACs that might be useful in treating AD.

The evolving role of solid state nuclear magnetic resonance methods in studies of amyloid fibrils

Beginning in the 1990s, solid state nuclear magnetic resonance (ssNMR) methods played a major role in elucidating the molecular structures and properties of amyloid fibrils. General principles that explain these structures and properties were uncovered and experimentally-based structural models were first developed from ssNMR data. Since 2017, cryogenic electron microscopy (cryo-EM) techniques have become capable of solving amyloid structures at near-atomic resolution. Although cryo-EM measurements are now the main approach for structural studies of amyloid fibrils, ssNMR measurements remain essential for studies of certain structures and structural features, as well as studies of dynamical and mechanistic aspects. Recent publications from various research groups illustrate the continuing importance of ssNMR and the unique information available from ssNMR measurements in amyloid research.

Shapeshifter W-Tau Peptide Inhibits Tau Aggregation and Disintegrates Paired Helical Filaments

Tauopathies comprise a range of neurodegenerative conditions characterized by the aberrant accumulation of tau protein clumps in the brain. These aggregates are formed by different tau splicing isoforms. Here, we analyzed the role of a specific intron-derived peptide called the W-Tau peptide on the polymerization-depolymerization of tau filaments. This peptide originates from a new isoform of the tau protein, named W-Tau, which is formed due to the retention of intron 12. AlphaFold3 (AF3)-based in silico investigations suggested that the W-Tau peptide interacts with tau monomers. Our in vitro experiments confirmed these predictions and showed that the W-Tau peptide inhibited tau aggregation. In addition, the W-Tau peptide disrupted preexisting paired helical filaments (PHFs) isolated from postmortem brain samples of patients with Alzheimer's disease, thereby supporting its potential therapeutic value. The effectiveness of the W-Tau peptide was demonstrated by the decrease in tau aggregation observed after cotransfection of the W-Tau peptide and PHF seeds, as demonstrated by analysis involving a fluorescence resonance energy transfer (FRET) cell biosensor. The W-Tau peptide breaks PHFs by selectively attaching to their ends, causing the structures to unwind and convert into circle-like formations. Considering the potential neuroprotective effects against tauopathies, the W-Tau isoform and its peptide are interesting candidates for future therapeutic interventions.

Role of saturated fatty acid metabolism in posttranslational modifications of the Tau protein

The relationship between metabolic alterations induced by the consumption of a high-fat diet (HFD) and the risk of developing neurodegenerative diseases such as Alzheimer's disease (AD) has been extensively studied. In particular, the induction of neuronal insulin resistance, endoplasmic reticulum stress, and the production of reactive oxygen species by chronic exposure to high concentrations of saturated fatty acids (sFAs), such as palmitic acid (PA), have been proposed as the cellular and molecular mechanisms underlying cognitive decline. Lipid metabolism affects many processes critical for cellular homeostasis. However, questions remain as to whether neuronal exposure to high sFA levels contributes to the onset and progression of AD features, and how their metabolism plays a role in this process. Therefore, the aim of this work is to review the accumulated evidence for the potential mechanisms by which the neuronal metabolism of sFAs affects signaling pathways that may induce biochemical changes in the AD hallmark protein Tau, ultimately promoting its aggregation and the subsequent generation of neurofibrillary tangles. In particular, the data presented here provide evidence that PA-dependent metabolic stress results in an imbalance in the activities of protein kinases and deacetylases that potentially contribute to the post-translational modifications (PTMs) of Tau.

O-GlcNAc modification differentially regulates microtubule binding and pathological conformations of tau isoforms in vitro

Tau proteins undergo several posttranslational modifications in physiological and disease conditions. In Alzheimer's disease, O-GlcNAcylation modification of serine/threonine (S/T) residues in tau is reduced. In mouse models of tauopathy, O-GlcNAcase inhibitors lead to increased O-GlcNAcylation and decreased filamentous aggregates of tau. However, various nonfilamentous tau conformations, linked to toxicity and neurodegeneration in tauopathies, involve processes like oligomerization, misfolding, and greater exposure of the phosphatase-activating domain in the amino terminus of tau. Additionally, it is becoming clearer that posttranslational modifications may differently regulate tau pathobiology in an isoform-dependent manner. Therefore, it is crucial to investigate the effects of O-GlcNAcylation on nonfilamentous conformations of both the four-repeat (4R, e.g., hT40) and three-repeat (3R, e.g., hT39) tau isoforms. In this study, we assessed how O-GlcNAcylation impacts pathological tau conformations of the longest 4R and 3R tau isoforms (hT40 and hT39, respectively) using recombinant proteins. Mass spectrometry showed that tau is modified with O-GlcNAc at multiple S/T residues, primarily in the proline-rich domain and the C-terminal region. O-GlcNAcylation of hT40 and hT39 does not affect microtubule polymerization but has opposite effects on hT40 (increases) and hT39 (decreases) binding to preformed microtubules. Although O-GlcNAcylation interferes with forming filamentous hT40 aggregates, it does not alter the formation of pathological nonfilamentous tau conformations. On the other hand, O-GlcNAcylation increases the formation of pathological nonfilamentous hT39 conformations. These findings suggest that O-GlcNAcylation differentially modulates microtubule binding and the adoption of pathological tau conformations in the longest 4R and 3R tau isoforms.

Amyloid-motif-dependent tau self-assembly is modulated by isoform sequence context

The microtubule-associated protein tau is implicated in neurodegenerative diseases characterized by amyloid formation. Mutations associated with frontotemporal dementia increase tau aggregation propensity and disrupt its endogenous microtubule-binding activity. However, the structural relationship between aggregation propensity and biological activity remains unclear. We employed a multi-disciplinary approach, including computational modeling, NMR, cross-linking mass spectrometry, and cell models to engineer tau sequences that modulate its structural ensemble. Our findings show that substitutions near the conserved 'PGGG' β-turn motif informed by tau isoform context reduce tau aggregation in vitro and cells and can even counteract aggregation induced by turn destabilizing disease-associated proline-to-serine mutations. Engineered tau sequences maintain microtubule binding and explain why 3R isoforms exhibit reduced pathogenesis compared to 4R. We propose a simple mechanism to reduce the formation of pathogenic species while preserving biological function, thus offering insights for therapeutic strategies aimed at reducing tau protein misfolding in neurodegenerative diseases.

Abstract Figure

Non-enzymatic posttranslational protein modifications in protein aggregation and neurodegenerative diseases

Highly reactive metabolic intermediates and other small molecules frequently react with amino acid side chains, leading to non-enzymatic posttranslational modifications (nPTMs) of proteins. The abundance of these modifications increases under high metabolic activity or stress conditions and can dramatically impact protein structure and function. Although protein quality control mechanisms typically mitigate the effects of these impaired proteins, in long-lived and degradation-resistant proteins, nPTMs accumulate. In some cases, such as cataract development and diabetes, clear links between nPTMs, aging, and disease progression have been established. In neurodegenerative diseases such as Alzheimer's and Parkinson's disease, a key question is whether accumulation of nPTMs is a cause or consequence of protein aggregation. This review focuses on major nPTMs found on proteins with central roles in neurodegenerative diseases such as α-synuclein, β-amyloid, and tau. We summarize current knowledge on the formation of these modifications and discuss their potential impact on disease onset and progression. Additionally, we examine what is known to date about how nPTMs impair cellular detoxification, repair, and degradation systems. Finally, we critically discuss the available methodologies to systematically investigate nPTMs at the molecular level and outline suitable approaches to study their effects on protein aggregation. We aim to foster more research into the role of nPTMs in neurodegeneration by adapting methodologies that have proven successful in studying enzymatic posttranslational modifications. Specifically, we advocate for site-specific incorporation of these modifications into target proteins using advanced chemical and molecular biology techniques.

Drug Discovery and Screening Tool Development for Tauopathies by Focusing on Pathogenic Tau Repeat 3 Oligomers

Comprehending early amyloidogenesis is essential for the development of effective therapeutic strategies. In tauopathies like Alzheimer's disease (AD), the abnormal accumulation of tau protein is initiated by pathological tau seeds. Mounting evidence implies that the microtubule binding domain, consisting of three to four repeats, plays a pivotal role in this process, yet the exact region driving the formation of pathogenic species needs to be further scrutinized. Here, we chemically synthesized individual tau repeats to identify those exhibiting pathogenic prion-like characteristics. Notably, repeat 3 (R3) displayed a remarkable propensity to polymerize, form toxic filaments, and induce cognitive impairment, even in the absence of an aggregation-promoting inducer, highlighting its physiological relevance. Additionally, oligomeric R3 was identified as a particularly pathological form, prompting the establishment of a screening platform. Through screening, tolcapone was found to possess therapeutic efficacy against pathological tau aggregates in PS19 transgenic mice. This screening platform provides a valuable avenue for identifying compounds that selectively interact with peptides implicated in the progression of tauopathies.

Cellular and pathological functions of tau

Tau protein is involved in various cellular processes, including having a canonical role in binding and stabilization of microtubules in neurons. Tauopathies are neurodegenerative diseases marked by the abnormal accumulation of tau protein aggregates in neurons, as seen, for example, in conditions such as frontotemporal dementia and Alzheimer disease. Mutations in tau coding regions or that disrupt tau mRNA splicing, tau post-translational modifications and cellular stress factors (such as oxidative stress and inflammation) increase the tendency of tau to aggregate and interfere with its clearance. Pathological tau is strongly implicated in the progression of neurodegenerative diseases, and the propagation of tau aggregates is associated with disease severity. Recent technological advancements, including cryo-electron microscopy and disease models derived from human induced pluripotent stem cells, have increased our understanding of tau-related pathology in neurodegenerative conditions. Substantial progress has been made in deciphering tau aggregate structures and the molecular mechanisms that underlie protein aggregation and toxicity. In this Review, we discuss recent insights into the diverse cellular functions of tau and the pathology of tau inclusions and explore the potential for therapeutic interventions.

Stable ubiquitin conjugation for biological interrogation of ubiquitinated tau repeat domain

Protein semisynthesis approaches are key for gaining insights into the effects of post-translational modifications (PTMs) on the structure and function of modified proteins. Among PTMs, ubiquitination involves the conjugation of a small protein modifier to a substrate amino acid residue and is unique in controlling a variety of cellular processes. Interest has grown in understanding the role of ubiquitination in neurodegenerative conditions, including tauopathies. The latter are characterized by the accumulation of the intrinsically disordered protein tau in the form of neurofibrillary tangles in the brains of patients. The presence of ubiquitinated tau in the pathological aggregates suggests that ubiquitination might play a role in the formation of abnormal protein deposits. In this study, we developed a new strategy, based on dehydroalanine chemistry, to install wild type ubiquitin on a tau repeat domain construct with site-specificity. We optimized a three-step reaction which yielded a good amount of highly pure tau repeat domain ubiquitinated in position 353. The structural features of the conjugate were examined by circular dichroism and NMR spectroscopy. The ubiquitinated tau was challenged in a number of assays: fibrils formation under aggregating conditions in vitro, chemical stability upon exposure to a variety of biological media including cell extracts, and internalization into astrocytes. The results demonstrated the wide applicability of the new semisynthetic strategy for the investigation of ubiquitinated substrates in vitro or in cell, and in particular for studying if ubiquitination has a role in the molecular mechanisms that underlie the aberrant transition of tau into pathological aggregates.

Dysregulated expression of cholesterol biosynthetic genes in Alzheimer's disease alters epigenomic signatures of hippocampal neurons

Aging is the main risk factor of cognitive neurodegenerative diseases such as Alzheimer's disease, with epigenome alterations as a contributing factor. Here, we compared transcriptomic/epigenomic changes in the hippocampus, modified by aging and by tauopathy, an AD-related feature. We show that the cholesterol biosynthesis pathway is severely impaired in hippocampal neurons of tauopathic but not of aged mice pointing to vulnerability of these neurons in the disease. At the epigenomic level, histone hyperacetylation was observed at neuronal enhancers associated with glutamatergic regulations only in the tauopathy. Lastly, a treatment of tau mice with the CSP-TTK21 epi-drug that restored expression of key cholesterol biosynthesis genes counteracted hyperacetylation at neuronal enhancers and restored object memory. As acetyl-CoA is the primary substrate of both pathways, these data suggest that the rate of the cholesterol biosynthesis in hippocampal neurons may trigger epigenetic-driven changes, that may compromise the functions of hippocampal neurons in pathological conditions.

Role of Tau Protein in Neurodegenerative Diseases and Development of Its Targeted Drugs: A Literature Review

Tau protein is a microtubule-associated protein that is widely distributed in the central nervous system and maintains and regulates neuronal morphology and function. Tau protein aggregates abnormally and forms neurofibrillary tangles in neurodegenerative diseases, disrupting the structure and function of neurons and leading to neuronal death, which triggers the initiation and progression of neurological disorders. The aggregation of tau protein in neurodegenerative diseases is associated with post-translational modifications, which may affect the hydrophilicity, spatial conformation, and stability of tau protein, promoting tau protein aggregation and the formation of neurofibrillary tangles. Therefore, studying the role of tau protein in neurodegenerative diseases and the mechanism of aberrant aggregation is important for understanding the mechanism of neurodegenerative diseases and finding therapeutic approaches. This review describes the possible mechanisms by which tau protein promotes neurodegenerative diseases, the post-translational modifications of tau protein and associated influencing factors, and the current status of drug discovery and development related to tau protein, which may contribute to the development of new therapeutic approaches to alleviate or treat neurodegenerative diseases.

Hyperacetylation mimetics within the tau filament core inhibits prion-like propagation of misfolded tau

Acetylation of key Lysine residues characterizes aggregates of the microtubule-associated protein tau constituting the neuropathological hallmark of many neurodegenerative diseases, such as Alzheimer's disease (AD) and Progressive Supranuclear Palsy (PSP). This has led to the idea that acetylation influences tau aggregation. Using a HEK293 cell-based aggregation assay, we tested whether acetylation-mimicking substitutions (K→Q) on five AD-associated acetyl-modified sites (AcK-311, 353, 369, 370, 375) influenced its propensity to aggregate when exposed to tau seeds derived from two clinically distinctive diseases - AD and PSP. In combination, the presence of 5K→Q sites ablated tau aggregation induced by seeds from both AD and PSP patients, indicating that acetylation within the filament core domain of tau could have an inhibitory effect on seed-mediated aggregation. We had previously identified that a phosphorylation-mimetic on Ser305 (S→E) abrogated tau aggregation by seeds from AD patients, without affecting seeding by PSP patients. Combining the S305→E to the 5K→Q acetyl-modified sites, we found that this tau could now be seeded only by PSP patients, but not by AD patients, confirming Ser305 as a critical determinant of strain-specific tau seeding. On the other hand, acetylation-nullifying substitutions (K→R or K→A) on these same Lys sites did not alter tau seeding abilities compared to the parental tau construct. Notably, the combined acetylation-nullifying Alanine substitutions on these 5 Lys sites resulted in spontaneous self-aggregation, with the filaments resembling amorphous deposits. All together, we demonstrate that cooperative acetyl-occupancy in the tau filament core influences seeded propagation of misfolded tau as well as drives self-aggregation.

Disentangling tau: One protein, many therapeutic approaches

The tauopathies encompass over 20 adult neurodegenerative diseases and are characterized by the dysfunction and accumulation of insoluble tau protein. Among them, Alzheimer's disease, frontotemporal dementia, and progressive supranuclear palsy collectively impact millions of patients and their families worldwide. Despite years of drug development using a variety of mechanisms of action, no therapeutic directed against tau has been approved for clinical use. This raises important questions about our current model of tau pathology and invites thoughtful consideration of our approach to nonclinical models and clinical trial design. In this article, we review what is known about the biology and genetics of tau, placing it in the context of current and failed clinical trials. We highlight potential reasons for the lack of success to date and offer suggestions for new pathways in therapeutic development. Overall, our viewpoint to the future is optimistic for this important group of neurodegenerative diseases.

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.

Ensemble-based design of tau to inhibit aggregation while preserving biological activity

The microtubule-associated protein tau is implicated in neurodegenerative diseases characterized by amyloid formation. Mutations associated with frontotemporal dementia increase tau aggregation propensity and disrupt its endogenous microtubule-binding activity. The structural relationship between aggregation propensity and biological activity remains unclear. We employed a multi-disciplinary approach, including computational modeling, NMR, cross-linking mass spectrometry, and cell models to design tau sequences that stabilize its structural ensemble. Our findings reveal that substitutions near the conserved 'PGGG' beta-turn motif can modulate local conformation, more stably engaging in interactions with the 306VQIVYK311 amyloid motif to decrease aggregation in vitro and in cells. Designed tau sequences maintain microtubule binding and explain why 3R isoforms of tau exhibit reduced pathogenesis over 4R isoforms. We propose a simple mechanism to reduce the formation of pathogenic species while preserving biological function, offering insights for therapeutic strategies aimed at reducing protein misfolding in neurodegenerative diseases.