Pseudo-acetylation of multiple sites on human Tau proteins alters Tau phosphorylation and microtubule binding, and ameliorates amyloid beta toxicity

Targeting tau in Alzheimer's disease: from mechanisms to clinical therapy

ABSTRACT

Alzheimer's disease is the most prevalent neurodegenerative disease affecting older adults. Primary features of Alzheimer's disease include extracellular aggregation of amyloid-β plaques and the accumulation of neurofibrillary tangles, formed by tau protein, in the cells. While there are amyloid-β-targeting therapies for the treatment of Alzheimer's disease, these therapies are costly and exhibit potential negative side effects. Mounting evidence suggests significant involvement of tau protein in Alzheimer's disease-related neurodegeneration. As an important microtubule-associated protein, tau plays an important role in maintaining the stability of neuronal microtubules and promoting axonal growth. In fact, clinical studies have shown that abnormal phosphorylation of tau protein occurs before accumulation of amyloid-β in the brain. Various therapeutic strategies targeting tau protein have begun to emerge, and are considered possible methods to prevent and treat Alzheimer's disease. Specifically, abnormalities in post-translational modifications of the tau protein, including aberrant phosphorylation, ubiquitination, small ubiquitin-like modifier (SUMO)ylation, acetylation, and truncation, contribute to its microtubule dissociation, misfolding, and subcellular missorting. This causes mitochondrial damage, synaptic impairments, gliosis, and neuroinflammation, eventually leading to neurodegeneration and cognitive deficits. This review summarizes the recent findings on the underlying mechanisms of tau protein in the onset and progression of Alzheimer's disease and discusses tau-targeted treatment of Alzheimer's disease.

Acetylated tau exacerbates learning and memory impairment by disturbing with mitochondrial homeostasis

Increased tau acetylation at K274 and K281 has been observed in the brains of Alzheimer's disease (AD) patients and animal models, and mitochondrial dysfunction are noticeable and early features of AD. However, the effect of acetylated tau on mitochondria has been unclear until now. Here, we constructed three type of tau forms, acetylated tau mutant by mutating its K274/K281 into Glutamine (TauKQ) to mimic disease-associated lysine acetylation, the non-acetylation tau mutant by mutating its K274/K281 into Arginine (TauKR) and the wild-type human full-length tau (TauWT). By overexpression of these tau forms in vivo and in vitro, we found that, TauKQ induced more severe cognitive deficits with neuronal loss, dendritic plasticity damage and mitochondrial dysfunctions than TauWT. Unlike TauWT induced mitochondria fusion, TauKQ not only induced mitochondria fission by decreasing mitofusion proteins, but also inhibited mitochondrial biogenesis via reduction of PGC-1a/Nrf1/Tfam levels. TauKR had no significant difference in the cognitive and mitochondrial abnormalities compared with TauWT. Treatment with BGP-15 rescued impaired learning and memory by attenuation of mitochondrial dysfunction, neuronal loss and dendritic complexity damage, which caused by TauKQ. Our data suggested that, acetylation at K274/281 was an important post translational modification site for tau neurotoxicity, and BGP-15 is a potential therapeutic drug for AD.

Tau Lysine Pseudomethylation Regulates Microtubule Binding and Enhances Prion-like Tau Aggregation

Alzheimer's disease (AD) and frontotemporal dementia (FTD) can be classified as tauopathies, which are a group of neurodegenerative diseases that develop toxic tau aggregates in specific brain regions. These pathological tau inclusions are altered by various post-translational modifications (PTMs) that include phosphorylation, acetylation, and methylation. Tau methylation has emerged as a target of interest for its potential involvement in tau pathomechanisms. Filamentous tau aggregates isolated from patients with AD are methylated at multiple lysine residues, although the exact methyltransferases have not been identified. One strategy to study the site-specific effects of methylation is to create methylation mimetics using a KFC model, which replaces lysine (K) with a hydrophobic group such as phenylalanine (F) to approximate the effects of lysine methylation (C or methyl group). In this study, tau methylmimetics were used to model several functional aspects of tau methylation such as effects on microtubule binding and tau aggregation in cell models. Overall, several tau methylmimetics displayed impaired microtubule binding, and tau methylmimetics enhanced prion-like seeded aggregation in the context of the FTD tau mutation P301L. Like other PTMs, tau methylation is a contributing factor to tau pathogenesis and could be a potential therapeutic drug target for the treatment of different tauopathies.

The Involvement of Post-Translational Modifications in Regulating the Development and Progression of Alzheimer's Disease

Post-translational modifications (PTMs) have been recently reported to be involved in the development and progression of Alzheimer's disease (AD). In detail, PTMs include phosphorylation, glycation, acetylation, sumoylation, ubiquitination, methylation, nitration, and truncation, which are associated with pathological functions of AD-related proteins, such as β-amyloid (Aβ), β-site APP-cleavage enzyme 1 (BACE1), and tau protein. In particular, the roles of aberrant PTMs in the trafficking, cleavage, and degradation of AD-associated proteins, leading to the cognitive decline of the disease, are summarized under AD conditions. By summarizing these research progress, the gaps will be filled between PMTs and AD, which will facilitate the discovery of potential biomarkers, leading to the establishment of novel clinical intervention methods against AD.

The Role of Post-Translational Modifications on the Structure and Function of Tau Protein

Involving addition of chemical groups or protein units to specific residues of the target protein, post-translational modifications (PTMs) alter the charge, hydrophobicity, and conformation of a protein, which in tune influences protein function, protein - protein interaction, and protein aggregation. While the occurrence of PTMs is dynamic and subject to regulations, conformational disorder of the target protein facilitates PTMs. The microtubule-associated protein tau is a typical intrinsically disordered protein that undergoes a variety of PTMs including phosphorylation, acetylation, ubiquitination, methylation, and oxidation. Accumulated evidence shows that these PTMs play a critical role in regulating tau-microtubule interaction, tau localization, tau degradation and aggregation, and reinforces the correlation between tau PTMs and pathogenesis of neurodegenerative disease. Here, we review tau PTMs with an emphasis on their influence on tau structure. With available biophysical characterization results, we describe how PTMs induce conformational changes in tau monomer and regulate tau aggregation. Compared to functional analysis of tau PTMs, biophysical characterization of tau PTMs is lagging. While it is challenging, characterizing the specific effects of PTMs on tau conformation and interaction is indispensable to unravel the tau PTM code.

Tau Acetylation in Entorhinal Cortex Induces its Chronic Hippocampal Propagation and Cognitive Deficits in Mice

BACKGROUND

Increased tau acetylation at K174, K274, K280, and K281 has been observed in the brains of Alzheimer's disease (AD) patients or in transgenic mice, but the role of acetylation in tau propagation is elusive.

OBJECTIVE

To study the effect of tau acetylation in entorhinal cortex on tau transmission and learning and memory.

METHODS

Stereotactic brain injection, behavioral test, electrophysiological recording, immunohistochemistry, and immunofluorescence were used.

RESULTS

We constructed the hyperacetylation mimics of tau (AAV-Tau-4Q), the non-acetylation tau mutant (AAV-Tau-4R), and the wild-type tau (AAV-Tau-WT). By overexpressing these different tau proteins in the entorhinal cortex (EC) of 2-month-old mice, we found that overexpressing Tau-4Q in EC for 3 or 6 months (to 5 or 8 months of age) neither induces tau propagation to dentate gyrus (DG) nor glial activation in DG, nor spatial memory deficit. However, overexpressing Tau-WT and Tau-4Q in EC for 13.5 months (15.5 months of age) at 2 months promoted tau propagation respectively to granulosa and hilus of DG with glial activation, synaptic dysfunction, and memory deficit, while overexpressing Tau-4R abolished tau propagation with improved cellular pathologies and cognitive functions. Furthermore, overexpressing Tau-4Q in unilateral DG of 2-month-old mice for 8 weeks also promoted its contralateral transmission with glial activation, and mice with tau (Tau-WT, Tau-4Q, and Tau-4R) overexpression in DG showed cognitive deficits compared with the empty vector controls.

CONCLUSION

Tau acetylation induces a time-dependent propagation from EC to DG, and only hippocampus but not EC tau accumulation induces cognitive deficits.

Tau K321/K353 pseudoacetylation within KXGS motifs regulates tau-microtubule interactions and inhibits aggregation

Alzheimer's disease is the leading cause of dementia and a defining hallmark is the progressive brain deposition of tau aggregates. The insidious accumulation of brain tau inclusions is also involved in a group of neurodegenerative diseases termed frontotemporal dementias. In all of these disorders, tau aggregates are enriched in post-translational modifications including acetylation, which has recently been identified at multiple sites. While most evidence suggest that tau acetylation is detrimental and promotes tau aggregation, a few studies support that tau acetylation within the KXGS motif can be protective and inhibit tau aggregation. To model site-specific acetylation at K259, K290, K321, and K353, acetylmimetics were created by mutating lysine to glutamine residues, which approximates size and charge of acetylation. HEK293T cells were transfected to express wild type tau, tau pathogenic mutations (P301L and P301L/S320F) or tau acetylmimetics and assessed by cell-based assays for microtubule binding and tau aggregation. Acetylmimetics within the KXGS motif (K259Q, K290Q, K321Q, K353Q) leads to significant decreased tau-microtubule interactions. Acetylmimetics K321Q and K353Q within the context of the pathogenic P301L tau mutation strongly inhibited prion-like seeded aggregation. This protective effect was confirmed to decrease intrinsic aggregation of P301L/S320F tau double mutation. Surprisingly, K321Q and K353Q acetylmimetics altered the conformational structure of P301L/S320F tau to extensively impair Thioflavin S binding. Site-specific acetylation of tau at K321 and K353 could represent a natural protective mechanism against tau aggregation and could be a potential therapeutic target.

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.

Amyloid Prefibrillar Oligomers: The Surprising Commonalities in Their Structure and Activity

It has been proposed that a “common core” of pathologic pathways exists for the large family of amyloid-associated neurodegenerations, including Alzheimer’s, Parkinson’s, type II diabetes and Creutzfeldt–Jacob’s Disease. Aggregates of the involved proteins, independently from their primary sequence, induced neuron membrane permeabilization able to trigger an abnormal Ca²⁺ influx leading to synaptotoxicity, resulting in reduced expression of synaptic proteins and impaired synaptic transmission. Emerging evidence is now focusing on low-molecular-weight prefibrillar oligomers (PFOs), which mimic bacterial pore-forming toxins that form well-ordered oligomeric membrane-spanning pores. At the same time, the neuron membrane composition and its chemical microenvironment seem to play a pivotal role. In fact, the brain of AD patients contains increased fractions of anionic lipids able to favor cationic influx. However, up to now the existence of a specific “common structure” of the toxic aggregate, and a “common mechanism” by which it induces neuronal damage, synaptotoxicity and impaired synaptic transmission, is still an open hypothesis. In this review, we gathered information concerning this hypothesis, focusing on the proteins linked to several amyloid diseases. We noted commonalities in their structure and membrane activity, and their ability to induce Ca²⁺ influx, neurotoxicity, synaptotoxicity and impaired synaptic transmission.

The Two Cysteines of Tau Protein Are Functionally Distinct and Contribute Differentially to Its Pathogenicity in Vivo

Although Tau accumulation is clearly linked to pathogenesis in Alzheimer's disease and other Tauopathies, the mechanism that initiates the aggregation of this highly soluble protein in vivo remains largely unanswered. Interestingly, in vitro Tau can be induced to form fibrillar filaments by oxidation of its two cysteine residues, generating an intermolecular disulfide bond that promotes dimerization and fibrillization. The recently solved structures of Tau filaments revealed that the two cysteine residues are not structurally equivalent since Cys-322 is incorporated into the core of the fibril, whereas Cys-291 projects away from the core to form the fuzzy coat. Here, we examined whether mutation of these cysteines to alanine affects differentially Tau mediated toxicity and dysfunction in the well-established Drosophila Tauopathy model. Experiments were conducted with both sexes, or with either sex. Each cysteine residue contributes differentially to Tau stability, phosphorylation status, aggregation propensity, resistance to stress, learning, and memory. Importantly, our work uncovers a critical role of Cys-322 in determining Tau toxicity and dysfunction.SIGNIFICANCE STATEMENT Cysteine-291 and Cysteine-322, the only two cysteine residues of Tau present in only 4-Repeat or all isoforms, respectively, have competing functions: as the key residues in the catalytic center, they enable Tau auto-acetylation; and as residues within the microtubule-binding repeat region are important not only for Tau function but also instrumental in the initiation of Tau aggregation. In this study, we present the first in vivo evidence that their substitution leads to differential consequences on Tau's physiological and pathophysiological functions. These differences raise the possibility that cysteine residues play a potential role in determining the functional diversity between isoforms.

Advances and Challenges in Understanding MicroRNA Function in Tauopathies: A Case Study of miR-132/212

In the past decade, several groups have reported that microRNAs (miRNAs) can participate in the regulation of tau protein at different levels, including its expression, alternative splicing, phosphorylation, and aggregation. These observations are significant, since the abnormal regulation and deposition of tau is associated with nearly 30 neurodegenerative disorders. Interestingly, miRNA profiles go awry in tauopathies such as Alzheimer's disease, progressive supranuclear palsy, and frontotemporal dementia. Understanding the role and impact of miRNAs on tau biology could therefore provide important insights into disease risk, diagnostics, and perhaps therapeutics. In this Perspective article, we discuss recent advances in miRNA research related to tau. While proof-of-principle studies hold promise, physiological validation remains limited. To help fill this gap, we describe herein a pure tauopathy mouse model deficient for the miR-132/212 cluster. This miRNA family is strongly downregulated in human tauopathies and shown to regulate tau in vitro and in vivo. No significant differences in survival, motor deficits or body weight were observed in PS19 mice lacking miR-132/212. Age-specific effects were seen on tau expression and phosphorylation but not aggregation. Moreover, various miR-132/212 targets previously implicated in tau modulation were unaffected (GSK-3β, Foxo3a, Mapk1, p300) or, unexpectedly, reduced (Mapk3, Foxo1, p300, Calpain 2) in miR-132/212-deficient PS19 mice. These observations highlight the challenges of miRNA research in living models, and current limitations of transgenic tau mouse models lacking functional miRNA binding sites. Based on these findings, we finally recommend different strategies to better understand the role of miRNAs in tau physiology and pathology.

Tau Protein and Zebrafish Models for Tau-Induced Neurodegeneration

Tauopathies are a specific type of slow and progressive neurodegeneration, which involves intracellular deposition of fibrillar material composed of abnormal hyperphosphorylation of the microtubule associated protein (MAP) tau. Despite many years of intensive research, our understanding of the molecular events that lead to neurodegeneration is far from complete. No effective therapeutic treatments have been defined, and questions surround the validity and utility of existing animal models. It is an urgent need to develop a novel animal model to study the underlying neurodegenerative mechanisms of tauopathies. Zebrafish models of tauopathies could complement existing models by providing an in vivo platform for genetic and chemical screens in order to identify new therapeutic targets and compounds, meanwhile zebrafish models have permitted discovery of unique characteristics of these genes that could have been difficultly observed in other models. Novel transgenic zebrafish models expressing wild-type or mutant forms of human 4R-tau in neurons have recently been reported. These studies show disease-relevant changes including tau hyperphosphorylation, aggregation and somato-dendritic relocalization. This review highlights the availability of transgenic tau zebrafish models that allow more detailed biochemical studies of tau in the zebrafish CNS to characterize solubility, fibril morphology and further clarify phosphorylation proceedings. Furthermore, a deeper knowledge of the zebrafish brain and a better characterization of tau caused by alterations in neurodegenerative disorders are needed.

In Silico Evaluation of Acetylation Mimics in the 27 Lysine Residues of Human Tau Protein

BACKGROUND

Various neurodegenerative diseases, including Alzheimer's disease (AD), are related to abnormal hyperphosphorylated microtubule-associated protein tau accumulation in brain lesions. Recent studies have focused on toxicity caused by another post-translational modification (PTM), acetylation of the lysine (K) residues of tau protein. Because there are numerous acetylation sites, several studies have introduced mimics of tau acetylation using amino acid substitutions from lysine to glutamine (Q). However, human tau protein contains over 20 acetylation sites; thus, investigation of the effects of an acetylated tau is difficult.

OBJECTIVE

Here, the authors in silico evaluated acetylation effects using SIFT, PolyPhen-2 and PROVEAN which can estimate the effects of amino acid substitutions based on the sequence homology or protein structure in tau isoforms. In addition, they also investigated 27 acetylation effects on the amyloid formation of tau proteins using Waltz.

RESULTS

15 acetylation mimics were estimated to be the most detrimental, which indicates that there may be novel pathogenic acetylation sites in the human tau protein. Interestingly, the deleterious effect of acetylation mimics was different according to the type of isoforms. Furthermore, all acetylation mimics were predicted to be a region of amyloid formation at the codons 274-279 of human tau protein. Notably, acetylation mimic of codon 311 (K311Q) induced the formation of an additional amyloid region located on codons 306-311 of the human tau protein.

CONCLUSION

To the best of our knowledge, this is the first simultaneous in-silico evaluation of the acetylation state of 27 human tau protein residues.

AMPK Ameliorates Tau Acetylation and Memory Impairment Through Sirt1

Alzheimer's disease (AD) is the most common neurodegenerative disease, but its underlying mechanism is still unclear and the identities of drugs for AD also lack. Tau acetylation has become potentially important post-translational modification of tau. Levels of tau acetylation are significantly enhanced in AD patients and transgenic mouse models of AD, but the underlying mechanism and roles of tau hyperacetylation in AD onset maintain elusive. In the current study, we found that tau acetylation is obviously enhanced and the activities of AMP-activated protein kinase (AMPK) and sirtuin1 (Sirt1) are significantly decreased in APP/PS1 and streptozotocin (STZ) mice and high glucose (HG)-treated cells. Moreover, we demonstrated that activation of AMPK reduces the level of tau acetylation and ameliorates memory impairment, and its mechanism is associated with activation of Sirt1. Taken together, AMPK might be a crucial upstream molecular to regulate acetylation of tau and become a new target for AD therapy in the future.

Acetylation of Aβ40 Alters Aggregation in the Presence and Absence of Lipid Membranes

A hallmark of Alzheimer's disease (AD) is the formation of senile plaques comprised of the β-amyloid (Aβ) peptide. Aβ fibrillization is a complex nucleation-dependent process involving a variety of metastable intermediate aggregates and features the formation of inter- and intramolecular salt bridges involving lysine residues, K16 and K28. Cationic lysine residues also mediate protein-lipid interactions via association with anionic lipid headgroups. As several toxic mechanisms attributed to Aβ involve membrane interactions, the impact of acetylation on Aβ₄₀ aggregation in the presence and absence of membranes was determined. Using chemical acetylation, varying mixtures of acetylated and nonacetylated Aβ₄₀ were produced. With increasing acetylation, fibril and oligomer formation decreased, eventually completely arresting fibrillization. In the presence of total brain lipid extract (TBLE) vesicles, acetylation reduced the interaction of Aβ₄₀ with membranes; however, fibrils still formed at near complete levels of acetylation. Additionally, the combination of TBLE and acetylated Aβ promoted annular aggregates. Finally, toxicity associated with Aβ₄₀ was reduced with increasing acetylation in a cell culture assay. These results suggest that in the absence of membranes that the cationic character of lysine plays a major role in fibril formation. However, acetylation promotes unique aggregation pathways in the presence of lipid membranes.

Drosophila Tau Negatively Regulates Translation and Olfactory Long-Term Memory, But Facilitates Footshock Habituation and Cytoskeletal Homeostasis

Although the involvement of pathological tau in neurodegenerative dementias is indisputable, its physiological roles have remained elusive in part because its abrogation has been reported without overt phenotypes in mice and Drosophila This was addressed using the recently described Drosophila tau^KO and Mi{MIC} mutants and focused on molecular and behavioral analyses. Initially, we show that Drosophila tau (dTau) loss precipitates dynamic cytoskeletal changes in the adult Drosophila CNS and translation upregulation. Significantly, we demonstrate for the first time distinct roles for dTau in adult mushroom body (MB)-dependent neuroplasticity as its downregulation within α'β'neurons impairs habituation. In accord with its negative regulation of translation, dTau loss specifically enhances protein synthesis-dependent long-term memory (PSD-LTM), but not anesthesia-resistant memory. In contrast, elevation of the protein in the MBs yielded premature habituation and depressed PSD-LTM. Therefore, tau loss in Drosophila dynamically alters brain cytoskeletal dynamics and profoundly affects neuronal proteostasis and plasticity.SIGNIFICANCE STATEMENT We demonstrate that despite modest sequence divergence, the Drosophila tau (dTau) is a true vertebrate tau ortholog as it interacts with the neuronal microtubule and actin cytoskeleton. Novel physiological roles for dTau in regulation of translation, long-term memory, and footshock habituation are also revealed. These emerging insights on tau physiological functions are invaluable for understanding the molecular pathways and processes perturbed in tauopathies.

The complexity of tau in Alzheimer's disease

Alzheimer's disease (AD) is characterized by two major pathological lesions in the brain, amyloid plaques and neurofibrillary tangles (NFTs) composed mainly of amyloid-β (Aβ) peptides and hyperphosphorylated tau, respectively. Although accumulation of toxic Aβ species in the brain has been proposed as one of the important early events in AD, continued lack of success of clinical trials based on Aβ-targeting drugs has triggered the field to seek out alternative disease mechanisms and related therapeutic strategies. One of the new approaches is to uncover novel roles of pathological tau during disease progression. This review will primarily focus on recent advances in understanding the contributions of tau to AD.

Drugs for Targeted Therapies of Alzheimer's Disease

Alzheimer's disease (AD) is one type of neurodegenerative diseases, which is prevalent in the elderly. Beta-amyloid (Aβ) plaques and phosphorylated tau-induced neurofibrillary tangles are two pathological hallmarks of this disease and the corresponding pathological pathways of these hallmarks are considered as the therapeutic targets. There are many drugs scheduled for pre-clinical and clinical trial that target to inhibit the initiators of pathological Aβ and tau aggregates as well as critical Aβ secretases and kinases in tau hyperphosphorylation. In addition, studies in disease gene variations, and detection of key prognostic effectors in early development are also important for AD control. The discovery of potential drug targets contributed to targeted therapy in a stage-dependent manner, However, there are still some issues that cause concern such as the low bioavailability and low efficacy of candidate drugs from clinical trial reports. Therefore, modification of drug candidates and development of delivery agents are essential and critical. With other medical advancements like cell replacement therapy, there is hope for the cure of Alzheimer's disease in the foreseeable future.

An assessment of the translational relevance of Drosophila in drug discovery

INTRODUCTION

Drosophila melanogaster offers a powerful expedient and economical system with facile genetics. Because of the high sequence and functional conservation with human disease-associated genes, it has been cardinal in deciphering disease mechanisms at the genetic and molecular level. Drosophila are amenable to and respond well to pharmaceutical treatment which coupled to their genetic tractability has led to discovery, repositioning, and validation of a number of compounds. Areas covered: This review summarizes the generation of fly models of human diseases, their advantages and use in elucidation of human disease mechanisms. Representative studies provide examples of the utility of this system in modeling diseases and the discovery, repositioning and testing on pharmaceuticals to ameliorate them. Expert opinion: Drosophila offers a facile and economical whole animal system with many homologous organs to humans, high functional conservation and established methods of generating and validating human disease models. Nevertheless, it remains relatively underused as a drug discovery tool probably because its relevance to mammalian systems remains under question. However, recent exciting success stories using Drosophila disease models for drug screening, repositioning and validation strongly suggest that fly models should figure prominently in the drug discovery pipeline from bench to bedside.

Tauopathy: A common mechanism for neurodegeneration and brain aging

Tau, a microtubule-associated protein promotes assembly and stability of microtubules which is related to axoplasmic flow and critical neuronal activities upon physiological conditions. Under neurodegenerative condition such as in Alzheimer's Disease (AD), tau-microtubule binding dynamics and equilibrium are severely affected due to its aberrant post-translational modifications including acetylation and hyperphosphorylation. This event results in its conformational changes to form neurofibrillary tangles (NFT) after aggregation in the cytosol. The formation of NFT is more strongly correlated with cognitive decline than the distribution of senile plaque, which is formed by polymorphous beta-amyloid (Aβ) protein deposits, another pathological hallmark of AD. In neurodegenerative conditions, other than AD, the disease manifestation is correlated with mutations of the MAPT gene. In Primary age-related tauopathy (PART), which is commonly observed in the brains of aged individuals, tau deposition is directly correlated with cognitive deficits even in the absence of Aβ deposition. Thus, tauopathy has been considered as an essential hallmark in neurodegeneration and normal brain aging. In this review, we highlighted the recent progress about the tauopathies in the light of its posttranslational modifications and its implication in AD and the aged brain.

An evaluation of Drosophila as a model system for studying tauopathies such as Alzheimer's disease

Work spanning almost two decades using the fruit fly, Drosophila melanogaster, to study tau-mediated neurodegeneration has provided valuable and novel insights into the causes and mechanisms of tau-mediated toxicity and dysfunction in tauopathies such as Alzheimer's disease (AD). The fly has proven to be an excellent model for human diseases because of its cost efficiency, and the availability of powerful genetic tools for use in a comparatively less-complicated, but evolutionarily conserved, in vivo system. In this review, we provide a critical evaluation of the insights provided by fly models, highlighting both the advantages and limitations of the system. The fly has contributed to a greater understanding of the causes of tau abnormalities, the role of these abnormalities in mediating toxicity and/or dysfunction, and the nature of causative species mediating tau-toxicity. However, it is not possible to perfectly model all aspects of human degenerative diseases. What sets the fly apart from other animal models is its genetic tractability, which makes it highly amenable to overcoming experimental limitations. The explosion of genetic technology since the first fly disease models were established has translated into fly lines that allow for greater temporal control in restricting tau expression to single neuron types, and lines that can label and monitor the function of subcellular structures and components; thus, fly models offer an unprecedented view of the neurodegenerative process. Emerging genetic technology means that the fly provides an ever-evolving experimental platform for studying disease.

Synaptic dysfunction in Alzheimer's disease: Mechanisms and therapeutic strategies

Alzheimer's disease (AD), the most prevalent neurodegenerative disease in the elderly population, is characterized by progressive cognitive decline and pathological hallmarks of amyloid plaques and neurofibrillary tangles. However, its pathophysiological mechanisms are poorly understood, and diagnostic tools and interventions are limited. Here, we review recent research on the amyloid hypothesis and beta-amyloid-induced dysfunction of neuronal synapses through distinct cell surface receptors. We also review how tau protein leads to synaptotoxicity through pathological modification, localization, and propagation. Finally, we discuss experimental therapeutics for AD and propose potential applications of disease-modifying strategies targeting synaptic failure for improved treatment of AD.

Degradation or aggregation: the ramifications of post-translational modifications on tau

Tau protein is encoded in the microtubule-associated protein tau (MAPT) gene and contributes to the stability of microtubules in axons. Despite of its basic isoelectric point and high solubility, tau is often found in intraneuronal filamentous inclusions such as paired helical filaments (PHFs), which are the primary constituent of neurofibrillary tangles (NFTs). This pathological feature is the nosological entity termed "tauopathies" which notably include Alzheimer's disease (AD). A proteinaceous signature of all tauopathies is hyperphosphorylation of the accumulated tau, which has been extensively studied as a major pharmacological target for AD therapy. However, in addition to phosphorylation events, tau undergoes a number of diverse posttranslational modifications (PTMs) which appear to be controlled by complex crosstalk. It remains to be elucidated which of the PTMs or their combinations have pro-aggregation or anti-aggregation properties. In this review, we outline the consequences of and communications between several key PTMs of tau, such as acetylation, phosphorylation, and ubiquitination, focusing on their roles in aggregation and degradation. We place emphasis on the structure of tau protofilaments from the human AD brain, which may be good targets to modulate etiological PTMs which cause tau aggregation. [BMB Reports 2018; 51(6): 265-273].

Role of Tau Acetylation in Alzheimer's Disease and Chronic Traumatic Encephalopathy: The Way Forward for Successful Treatment

Progressive neurodegenerative diseases plague millions of individuals both in the United States and across the world. The current pathology of progressive neurodegenerative tauopathies, such as Alzheimer's disease (AD), Pick's disease, frontotemporal dementia (FTD), and progressive supranuclear palsy, primarily revolves around phosphorylation and hyperphosphorylation of the tau protein. However, more recent evidence suggests acetylation of tau protein at lysine 280 may be a critical step in molecular pathology of these neurodegenerative diseases prior to the tau hyperphosphorylation. Secondary injury cascades such as oxidative stress, endoplasmic reticulum stress, and neuroinflammation contribute to lasting damage within the brain and can be induced by a number of different risk factors. These injury cascades funnel into a common pathway of early tau acetylation, which may serve as the catalyst for progressive degeneration. The post translational modification of tau can result in production of toxic oligomers, contributing to reduced solubility as well as aggregation and formation of neurofibrillary tangles, the hallmark of AD pathology. Chronic Traumatic Encephalopathy (CTE), caused by repetitive brain trauma is also associated with a hyperphosphorylation of tau. We postulated acetylation of tau at lysine 280 in CTE disease could be present prior to the hyperphosphorylation and tested this hypothesis in CTE pathologic specimens. We also tested for ac-tau 280 in early stage Alzheimer's disease (Braak stage 1). Histopathological examination using the ac tau 280 antibody was performed in three Alzheimer's cases and three CTE patients. Presence of ac-tau 280 was confirmed in all cases at early sites of disease manifestation. These findings suggest that tau acetylation may precede tau phosphorylation and could be the first "triggering" event leading to neuronal loss. To the best of our knowledge, this is the first study to identify acetylation of the tau protein in CTE. Prevention of tau acetylation could possibly serve as a novel target for stopping neurodegeneration before it fully begins. In this study, we highlight what is known about tau acetylation and neurodegeneration.