Tau Post-translational Modifications Research Articles

Development and characterization of novel anti-acetylated tau monoclonal antibodies to probe pathogenic tau species in Alzheimer’s disease

Tauopathies, including Alzheimer’s disease (AD), are a class of neurodegenerative diseases characterized by the presence of insoluble tau inclusions. Tau phosphorylation has traditionally been viewed as the dominant post-translational modification (PTM) controlling tau function and pathogenesis in tauopathies. However, we and others have identified tau acetylation as a primary PTM regulating both normal tau function as well as abnormal pathogenic features including aggregation. Prior work showed robust tau acetylation in aggregation hotspots located within the 2nd and 3rd repeat regions of tau (residues K280 and K311) in tauopathy brains, including AD, compared to non-tauopathy controls. By screening thousands of hybridoma clones, we generated site-specific and modification-specific monoclonal antibodies targeting acetylated tau at residues K280 or K311. To validate these antibodies in a bona fide neuronal system, we targeted the acetyltransferase CBP to the cytoplasm of neurons to promote tau acetylation. Several antibody clones specifically detected CBP-acetylated tau and co-localized with ac-tau in neurons. Additionally, our lead optimal anti-acetylated-tau monoclonal antibodies detected robust tau pathology in tangles and neuritic plaques of human AD brains. Given the now emerging interest in acetylated tau as critical regulator of tau functions, these sensitive and highly specific tools will allow us to further unravel the tau PTM code and, importantly, could be deployed as diagnostic or disease-modifying agents.

Comprehensive review on Alzheimer's disease: From the posttranslational modifications of Tau to corresponding treatments

AbstractAlzheimer's disease (AD) is a neurodegenerative disease, which is mainly characterized by the abnormal deposition of β‐amyloid peptide (Aβ) and Tau. Since Tau aggregation is more closely associated with synaptic loss, neurodegeneration, and cognitive decline than Aβ, the correlation between Tau and cognitive function in AD has gradually gained attention. The posttranslational modifications (PTMs) of Tau are key factors contributing to its pathological changes, which include phosphorylation, acetylation, ubiquitination, glycosylation, glycation, small ubiquitin‐like modifier mediated modification (SUMOylation), methylation, succinylation, etc. These modifications change the structure of Tau, regulating Tau microtubule interactions, localization, degradation, and aggregation, thereby affecting its propensity to aggregate and leading to neuronal injury and cognitive impairments. Among numerous PTMs, drug development based on phosphorylation, acetylation, ubiquitination, and SUMOylation primarily involves enzymatic reactions, affecting either the phosphorylation or degradation processes of Tau. Meanwhile, methylation, glycosylation, and succinylation are associated with maintaining the structural stability of Tau. Current research is more extensive on phosphorylation, acetylation, ubiquitination, and methylation, with related drugs already developed, particularly focusing on phosphorylation and ubiquitination. In contrast, there is less research on SUMOylation, glycosylation, and succinylation, requiring further basic research, with the potential to become novel drug targets. In conclusion, this review summarized the latest research on PTMs of Tau and related drugs, highlighting the potential of targeting specific PTMs for developing novel therapeutic strategies in AD.

Alzheimer’s disease seeded tau forms paired helical filaments yet lacks seeding potential

Alzheimer’s disease (AD) and many other neurodegenerative diseases are characterized by pathological aggregation of the protein tau. These tau aggregates spread in a stereotypical spatiotemporal pattern in the brain of each disease, suggesting that the misfolded tau can recruit soluble monomers to adopt the same pathological structure. To investigate whether recruited tau indeed adopts the same structure and properties as the original seed, here we template recombinant full-length 0N3R tau, 0N4R tau, and an equimolar mixture of the two using sarkosyl-insoluble tau extracted from AD brain and determine the structures of the resulting fibrils using cryoelectron microscopy. We show that these cell-free amplified tau fibrils adopt the identical molecular structure as the AD paired-helical filament (PHF) but are unable to template additional monomers. Therefore, the PHF structure alone is insufficient for defining the pathological properties of AD tau, and other biochemical components such as tau posttranslational modifications, other proteins, polyanionic cofactors, and salt are required for the prion-like serial propagation 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.

Quantitative profiling of posttranslational modifications of pathological tau via sarkosyl fractionation and mass spectrometry.

Tau protein aggregation is associated with posttranslational modifications (PTMs) in 75% of all dementia cases. The distribution of tau pathology and the presence of specific tau phosphorylation sites of interest are typically visualized and measured using antibodies. However, previous knowledge of the target epitopes is required. Additionally, antibodies can be used in a semi-quantitative manner but cannot be used to determine the absolute amount of tau or the extent of the modifications at specific sites or domains. Here we present a discovery assay that characterizes the global qualitative and quantitative tau modification landscape of a sample without a priori knowledge. Our workflow uses sarkosyl fractionation to extract the pathological tau species from sample-limited brain specimens, followed by mass spectrometry (MS) to characterize and quantify tau PTMs. The two-step MS-based proteomics approach includes an exploratory tau PTM analysis and a targeted full-length expressed stable isotope-labeled tau assay, which monitors specific unmodified tau peptides using a heavy isotope-labeled internal standard as a reference. This enables the absolute quantification of the respective tau peptides and the total tau amount in the sample, thus providing the modification extent of tau PTMs. This approach provides precise, comprehensive, qualitative and quantitative tau PTM profiling of the sample. It also enables the detailed molecular comparison of tau across multiple experiments, including a comparison between neurodegenerative diseases, stages of the disease, human patient heterogeneity and characterization of animal models. The approach is useful for studying the molecular features of pathological tau in neurodegeneration. The procedure requires 7-8 d and is suitable for users with expertise in targeted and untargeted MS-based protein analysis.

Palmitic Acid Induces Posttranslational Modifications of Tau Protein in Alzheimer’s Disease–Related Epitopes and Increases Intraneuronal Tau Levels

Metabolic diseases derived from an unhealthy lifestyle have been linked with an increased risk for developing cognitive impairment and even Alzheimer’s disease (AD). Although high consumption of saturated fatty acids such as palmitic acid (PA) has been associated with the development of obesity and type II diabetes, the mechanisms connecting elevated neuronal PA levels and increased AD marker expression remain unclear. Among other effects, PA induces insulin resistance, increases intracellular calcium and reactive oxygen species (ROS) production, and reduces the NAD+/NADH ratio, resulting in decreased activity of the deacetylase Sirtuin1 (SIRT1) in neurons. These mechanisms may affect signaling pathways that impact the posttranslational modifications (PTMs) of the tau protein. To analyze the role played by PA in inducing the phosphorylation and acetylation of tau, we examined PTM changes in human tau in differentiated neurons from human neuroblastoma cells. We found changes in the phosphorylation state of several AD-related sites, namely, S199/202 and S214, that were mediated by a mechanism associated with the dysregulated activity of the kinases GSK3β and mTOR. PA also increased the acetylation of residue K280 and elevated total tau level after long exposure time. These findings provide information about the mechanisms by which saturated fatty acids cause tau PTMs that are similar to those observed in association with AD biochemical changes.Graphical

Quantification of Methylation and Phosphorylation Stoichiometry.

Tauopathies including Alzheimer's disease (AD) are neurodegenerative disorders accompanied by the conversion of functional forms of the microtubule associated protein Tau into non-functional aggregates. A variety of post-translational modifications (PTMs) on Tau precede or accompany the conversion, placing them in position to modulate Tau function as well as its propensity to aggregate. Although Tau PTMs can be characterized by their sites of modification, their total stoichiometry when summed over all sites also is an important metric of their potential impact on function. Here we provide a protocol for rapidly producing recombinant Tau with enzyme-specific PTMs at high stoichiometry in vitro and demonstrate its utility in the context of hyperphosphorylation. Additionally, protocols for estimating phosphorylation and methylation stoichiometry on Tau proteins isolated from any source are presented. Together these methods support experimentation on Tau PTM function over a wide range of experimental conditions.

Western Blot of Tau Protein from Mouse Brains Extracts: How to Avoid Signal Artifacts.

Tau is a microtubule-associated protein enriched in the axonal compartment. Its most well-known function is to bind and stabilize microtubules. In Alzheimer's disease and other neurodegenerative diseases known as tauopathies, tau undergoes several abnormal post-translational modifications including hyperphosphorylation, conformational changes, oligomerization, and aggregation. Numerous mouse models of tauopathies have been developed, and Western blotting remains an invaluable tool in studying tau protein physiological and pathological changes in these models. However, many of the antibodies that have been developed to analyze tau post-translational modifications are mouse monoclonal, which are at risk of producing artifactual signals in Western blotting procedures. This risk does not arise due to their lack of specificity, but rather because the secondary antibodies used to detect them will also react with the heavy chain of endogenous mouse immunoglobulins (Igs), leading to a non-specific signal at the same molecular weight as tau protein (around 50kDa). Here, we present the use of anti-light-chain secondary antibodies as a simple and efficient technique to prevent non-specific Ig signals around 50kDa. We demonstrate the efficacy of this method by either eliminating or identifying artifactual signals when using monoclonal antibodies directed at non-phosphorylated epitopes (T49, Tau3R, Tau4R), phosphorylated epitopes (MC6, AT180, CP13), or an abnormal tau conformation (MC1), in wild-type (WT) mice with tau hyperphosphorylation (hypothermic), transgenic mice overexpressing human tau (hTau mice), and tau knockout (TKO) mice.

Tau maturation in clinicopathological spectrum of Lewy body and Alzheimer’s disease

AbstractBackgroundIn Alzheimer’s disease neuropathologic change (ADNC), maturation of neurofibrillary tangles and tau pathology are characterized by posttranslational modifications of tau. ANDC and Lewy body disease (LBD) commonly co‐exist in aging brains and there are overlapping clinical syndromes and heterogeneity in clinical features among cases with mixed ADNC and LBD pathology. Given this clinical and pathological overlap, we examined the postmortem expression of tau conformational epitopes that mark various stages of tangle maturation in clinicopathologic spectrum of Alzheimer’s disease (AD) and LBD.MethodAutopsy‐confirmed cases with diffuse neocortical LBD with/without ADNC (n = 59) were selected (Table1). We used digital histology methods to measure percent area occupied (%AO) with pathology in three regions (middle frontal, anterior cingulate, and superior temporal cortices) immunohistochemically stained with MJF‐R13 (α‐synuclein), NAB228 (beta‐amyloid), and tau monoclonal antibodies marking phosphorylated (AT8), early conformational (MC1, preferentially reactive in pre‐tangles), and C‐terminally truncated tau epitopes (TauC3, preferentially reactive in mature intracellular and ghost tangles). Linear models tested the group comparisons, and correlation between α‐synuclein or beta‐amyloid with outcome of average neocortical tau. In models, %AO were log‐transformed, and age and sex were included as covariates.Resultα‐synuclein was strongly correlated with early tau epitopes labelled with AT8 (β = 1.37, p<0.001) and MC1 (β = 1.20, p<0.001), but not with mature tau epitopes detected by TauC3 (p = 0.38). There were significant positive correlations between all tau epitopes and beta‐amyloid, including TauC3 immunoreactive tau (p≤0.013) (Figure1). In the mixed pathology group, those with clinical LBD had a positive association between α‐synuclein and MC1 (β = 1.61, p = 0.014) and AT8 (β = 1.67, p = 0.005) positive tau, while those with clinical AD had no correlation between α‐synuclein and tau epitopes. There was greater MC1 (β = 3.40, p<0.001), AT8 (β = 3.59, p<0.001), and TauC3 (β = 1.43, p = 0.005) pathology in patients with clinical AD compared to clinical LBD.ConclusionIn LBD, higher α‐synuclein is associated with greater phosphorylated and early tau conformations, but not later conformation of mature tau, which was selectively associated with beta‐amyloid pathology. Our findings suggest that tau pathology may modify the clinical expression of synucleinopathies and C‐terminal truncation of tau may be more closely related to AD pathophysiology than synucleinopathy.

Alzheimer’s disease‐associated post‐translational modifications of tau influence tau propagation

AbstractBackgroundIn Alzheimer’s disease (AD), tau accumulates with a stereotyped progressive involvement of brain regions. The cellular mechanism for this is the neuron‐to‐neuron propagation of tau, but the factors that influence tau propagation are not fully understood. We hypothesize that AD‐associated post‐translational modifications (PTMs) of tau reduce its propagation since lysines are critical for LRP1‐mediated tau uptake.MethodA construct was designed to model tau propagation in Chinese hamster ovary (CHO) cells and allow identification of donor and recipient cells by immunocytochemistry and flow cytometry. In addition to the wildtype (WT) tau sequence, constructs were made with pseudophosphorylation and pseudoacetylation mutations corresponding to the phosphorylations and acetylations found on low molecular weight (LMW) and high molecular weight (HMW) tau species in AD brains. Additionally, individual PTM mimic mutations corresponding to PTMs of the LMW tau species were made. The constructs were transfected into CHO cells and tau propagation was assessed by flow cytometry after 48 hours. Statistical analysis was performed by ANOVA with adjustment for multiple comparisons.ResultTau propagation using this cell culture model was demonstrated by immunocytochemistry and flow cytometry. Comparison of the WT, LMW‐mimic (8D2Q mutant), and HMW‐mimic (20D3Q mutant) tau propagation constructs revealed that the 8D2Q and 20D3Q mutants displayed reduced tau propagation compared to WT by 23.7% (p = 0.0186) and 26.5% (p = 0.0087), respectively. Comparing the ten single mutant constructs to WT, T181D and T231D showed increased tau propagation by 20.3% (p = 0.0002) and 15.9% (p = 0.0094), respectively. Compared to WT, T217D, K281Q, K353Q, and S404D showed reduced tau propagation by 17.3% (p = 0.0030), 15.3% (p = 0.0123), 23.7% (p <0.0001), and 16.6% (p = 0.0046), respectively.ConclusionThe mimics of AD‐associated tau PTMs found on LMW and HMW tau reduce tau propagation, consistent with the initial hypothesis. The impact of individual PTM mimics on tau propagation is variable, but notably both pseudoacetylation mimics reduce propagation, possibly due to lower affinity with LRP1. Understanding how AD‐associated tau PTMs impact tau propagation may suggest novel targets to slow the progression of AD pathology.

Neuronal populations in the inferior temporal gyrus with truncated tau inclusions occur as early as those with hyperphosphorylated tau inclusions in Alzheimer’s disease, however they do not completely overlap

AbstractBackgroundAlzheimer’s disease (AD) features stereotypical spread of hyperphosphorylated tau (p‐tau) and beta‐amyloid. Although other pathological tau posttranslational modifications (PTMs) have been described in AD, a prevalent disease model preconizes that other tau PTMs always overlap with p‐tau, making the latter an excellent marker of tau pathology burden. We recently showed in experimental studies that truncated tau (tr‐tau), a pathological tau PTM generated via cleavage by active caspases, is as common as p‐tau in neurons at late AD stages; however, only about 40% of tr‐tau positive neurons also show p‐tau positivity. This makes tr‐tau a potential marker of AD previously invisible at neuropathological investigation or a potential target of diagnostic tool development. We sought to determine how early in AD tr‐tau is detected compared to p‐tau and to what degree tr‐tau and p‐tau neuronal populations overlap at early AD stages.MethodsWe used multiplex immunofluorescence to probe tr‐tau (D13, D402, D418, TauC3) and p‐tau (PHF1) species in the same tissue slides of postmortem human brain tissue of healthy controls and subjects at progressive AD stages. We quantified neuronal tau pathology and colocalization in digital images of inferior temporal gyrus (ITG) as representative of an area involved in p‐tau pathology in intermediate stages.ResultsOur preliminary analysis included 14 cases (4 controls, 10 AD) from all Braak stages (1:1:2:3:3:2:2 for Braak 0 through 6). Of the cases used in the study, 71% were male and the mean (SD) age of death was 82.3 (10.9). As expected, p‐tau was detected from Braak 4; however, we detected tr‐tau species from Braak 3 (Fig. 1). Both p‐tau and tr‐tau burden increased with Braak stage (Fig. 2). The overlap between tr‐tau and p‐tau positive neuron populations was 30.8% at Braak 4 (Fig. 3A) and increased to 54.6% at Braak 6 (Fig. 3B).ConclusionNeuronal tr‐tau deposits develop in AD as early as p‐tau pathology. Even in earlier stages, the % of overlapping is moderate. This corroborates tr‐tau as a potential biomarker of tau pathology in AD not covered by p‐tau screening. We are currently finalizing analysis of a much larger cohort including other brain regions.

Site-Specific Ubiquitination of Tau Amyloids Promoted by the E3 Ligase CHIP.

Post-translational modifications of Tau are emerging as key players in determining the onset and progression of different tauopathies such as Alzheimer's disease, and are recognized to mediate the structural diversity of the disease-specific Tau amyloids. Here we show that the E3 ligase CHIP catalyzes the site-specific ubiquitination of Tau filaments both in vitro and in cellular models, proving that also Tau amyloid aggregates are direct substrate of PTMs. Transmission electron microscopy and mass spectrometry analysis on ubiquitin-modified Tau amyloids revealed that the conformation of the filaments restricts CHIP-mediated ubiquitination to specific positions of the repeat domain, while only minor alterations in the structure of the fibril core were inferred using seeding experiments in vitro and in a cell-based tauopathy model. Overexpression of CHIP significantly increased the ubiquitination of exogenous PHF, proving that the ligase can interact and modify Tau aggregates also in a complex cellular environment.

Untangling Tau: Molecular Insights into Neuroinflammation, Pathophysiology, and Emerging Immunotherapies.

Neuroinflammation, a core pathological feature observed in several neurodegenerative diseases, including Alzheimer's disease (AD), is rapidly gaining attention as a target in understanding the molecular underpinnings of these disorders. Glial cells, endothelial cells, peripheral immune cells, and astrocytes produce a variety of pro-inflammatory mediators that exacerbate the disease progression. Additionally, microglial cells play a complex role in AD, facilitating the clearance of pathological amyloid-beta peptide (Aβ) plaques and aggregates of the tau protein. Tau proteins, traditionally associated with microtubule stabilization, have come under intense scrutiny for their perturbed roles in neurodegenerative conditions. In this narrative review, we focus on recent advances from molecular insights that have revealed aberrant tau post-translational modifications, such as phosphorylation and acetylation, serving as pathological hallmarks. These modifications also trigger the activation of CNS-resident immune cells, such as microglia and astrocytes substantially contributing to neuroinflammation. This intricate relationship between tau pathologies and neuroinflammation fosters a cascading impact on neural pathophysiology. Furthermore, understanding the molecular mechanisms underpinning tau's influence on neuroinflammation presents a frontier for the development of innovative immunotherapies. Neurodegenerative diseases have been relatively intractable to conventional pharmacology using small molecules. We further comprehensively document the many alternative approaches using immunotherapy targeting tau pathological epitopes and structures with a wide array of antibodies. Clinical trials are discussed using these therapeutic approaches, which have both promising and disappointing outcomes. Future directions for tau immunotherapies may include combining treatments with Aβ immunotherapy, which may result in more significant clinical outcomes for neurodegenerative diseases.

Unraveling the tau puzzle: a review of mechanistic targets and therapeutic interventions to prevent tau pathology in Alzheimer’s disease

Alzheimer’s disease (AD) is a prevalent neurodegenerative disease characterized by irreversible neural degeneration and cognitive decline. The prion-like propagation of the β-amyloid (Aβ) and tau proteins leads to the formation of protein plaques and, subsequently, neuronal dysfunction, contributing significantly to AD pathogenesis. Although effective AD treatments remain elusive, targeting tau protein aggregation has emerged as a promising therapeutic approach. However, recent anti-tau antibody trials have shown limited success in improving cognition, underscoring the need for a more advanced, multifaceted approach to address multiple mechanisms of tau pathology. This review examines the role of tau protein in the context of AD, with a particular focus on potential therapeutic interventions. Emphasis is placed on the modulation of tau protein expression, tau post-translational modifications and aggregation, receptor-mediated uptake and extracellular release pathways, neural inflammatory response pathways, intercellular organelle exchange, mitochondrial function, microtubule stability, and nuclear factor expression as critical intervention points. Despite the challenges faced in ongoing anti-tau clinical efforts, a comprehensive strategy targeting multiple pathways involved in tau pathology, by using either combinations of existing drugs or novel multitarget drugs, holds promise. By gaining a deeper understanding of the complex mechanisms underlying tau pathology, researchers can develop innovative therapeutic strategies to combat AD.

Why kiss-and-hop explains that tau does not stabilize microtubules and does not interfere with axonal transport (at physiological conditions).

Tau is a microtubule-associated protein that is enriched in the axonal process of neurons. Post-translational modifications of tau have been implicated in the development of tauopathies characterized by defects in axonal transport, neuronal atrophy, and microtubule disassembly. Although tau is almost quantitatively bound to microtubules under physiological conditions, it does not significantly affect axonal transport. Furthermore, acute or chronic tau deficiency does not result in significant destabilization of neuronal microtubules, challenging the classical view that disease-related tau modifications directly cause axonal microtubule collapse. Here, we discuss how the rapid interaction kinetics of the tau-microtubule interaction, which we previously termed the kiss-and-hop interaction, explains why tau does not affect microtubule-dependent axonal transport but still allows tau to modulate microtubule polymerization. In contrast, tau modifications that slow down the kinetics of the tau-microtubule interaction and increase the residence time of tau at a microtubule interaction site can disrupt axonal transport and cause dendritic atrophy. We discuss the consequences of such a gain-of-toxicity mechanism in terms of the development of disease-modulating drugs that target the tau protein.

Distinct tau folds initiate templated seeding and alter the post-translational modification profile.

Pathological tau accumulates in the brain in tauopathies such as Alzheimer's disease, Pick's disease, progressive supranuclear palsy and corticobasal degeneration, and forms amyloid-like filaments incorporating various post-translational modifications (PTMs). Cryo-electron microscopic (cryo-EM) studies have demonstrated that tau filaments extracted from tauopathy brains are characteristic of the disease and share a common fold(s) in the same disease group. Furthermore, the tau PTM profile changes during tau pathology formation and disease progression, and disease-specific PTMs are detected in and around the filament core. In addition, templated seeding has been suggested to trigger pathological tau amplification and spreading in vitro and in vivo, although the molecular mechanisms are not fully understood. Recently, we reported that the cryo-EM structures of tau protofilaments in SH-SY5Y cells seeded with patient-derived tau filaments show a core structure(s) resembling that of the original seeds. Here, we investigated PTMs of tau filaments accumulated in the seeded cells by liquid chromatography/tandem mass spectrometry and compared them with the PTMs of patient-derived tau filaments. Examination of insoluble tau extracted from SH-SY5Y cells showed that numerous phosphorylation, deamidation and oxidation sites detected in the fuzzy coat in the original seeds were well reproduced in SH-SY5Y cells. Moreover, templated tau filament formation preceded both truncation of the N-/C-terminals of tau and PTMs in and around the filament core, indicating these PTMs may predominantly be introduced after the degradation of the fuzzy coat.

Activity-dependent tau cleavage by caspase-3 promotes neuronal dysfunction and synaptotoxicity

Tau-mediated toxicity is associated with cognitive decline and Alzheimer's disease (AD) progression. In particular, tau post-translational modifications (PTMs) are thought to generate aberrant tau species resulting in neuronal dysfunction. Despite being well characterized in postmortem AD brain, it is unclear how caspase-mediated C-terminal tau cleavage promotes neurodegeneration, as few studies have developed the models to dissect this pathogenic mechanism. Here, we show that proteasome impairment results in cleaved tau accumulation at the post-synaptic density (PSD), a process that is modulated by neuronal activity. Cleaved tau (at residue D421) impairs neuronal firing and causes inefficient initiation of network bursts, consistent with reduced excitatory drive. We propose that reduced neuronal activity, or silencing, is coupled to proteasome dysfunction, which drives cleaved tau accumulation at the PSD and subsequent synaptotoxicity. Our study connects three common themes in the progression of AD: impaired proteostasis, caspase-mediated tau cleavage, and synapse degeneration.

The Role of Alzheimer’s disease relevant Tau modifications in Neurodegeneration and Mitochondrial dysfunction

AbstractBackgroundAlzheimer’s disease (AD) is a progressive neurodegenerative disorder whose pathological hallmarks include intraneuronal neurofibrillary tangles (NFTs) composed of the microtubule‐associated protein Tau. Tau isolated from AD brain exhibits abnormally high levels of post‐translational modifications (PTMs) including phosphorylation and acetylation at specific epitopes that increase with disease severity and age. In addition, mitochondrial dysfunction is an early feature of AD, and abnormal, toxic tau PTMs may contribute to disease pathogenesis. A major bottleneck in understanding the mechanisms behind the neurotoxicity of pathological forms of Tau is the lack of genetically tractable models that can recapitulate the effects of Tau PTMs in a short time frame without artifacts associated with Tau overexpression.MethodHuman 0N4R Tau (wild type) was expressed in touch receptor neurons through single‐copy gene insertion. Mutations were introduced into the single‐copy tau transgene through CRISPR‐Cas9 genome editing, including T231E and T231A, to mimic phosphorylation and phospho‐ablation of a commonly observed pathological epitope, respectively, and K274/281Q, to mimic disease‐associated lysine acetylation. We then assessed their impact on age‐dependent response to light touch, neurodegeneration, and mitochondrial parameters such as abundance, morphology, trafficking, and turnover, using fluorescent biosensors including mito‐mKeima.ResultUnlike existing tau overexpression models, C. elegans single‐copy expression of wild type human tau did not elicit overt pathological phenotypes at baseline. However, strains expressing disease associated PTM‐mimetics (T231E and K274/281Q) exhibited reduced touch sensation and neuronal morphological abnormalities that increased with age. Remarkably, the PTM‐mimetics selectively impaired mitophagy following mitochondrial oxidative stress, but had no effect on macroautophagy, and furthermore reduced mitolysosomal trafficking.ConclusionSingle copy expression limits pathological phenotypes to strains expressing disease‐associated Tau mutants. In addition to overt pathology, these mutants eliminate oxidative stress‐induced mitophagy and reduce trafficking of mitolysosomes. Our findings highlight a selective mechanism through which disease‐associated Tau PTMs may suppress compensatory responses to mitochondrial stress that occur with age and provide a new perspective into the pathogenic mechanisms underlying AD.

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

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). 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.

Caspase-cleaved tau is senescence-associated and induces a toxic gain of function by putting a brake on axonal transport

The microtubule-associated protein tau plays a central role in tauopathies such as Alzheimer’s disease (AD). The exact molecular mechanisms underlying tau toxicity are unclear, but aging is irrefutably the biggest risk factor. This raises the question of how cellular senescence affects the function of tau as a microtubule regulator. Here we report that the proportion of tau that is proteolytically cleaved at the caspase-3 site (TauC3) doubles in the hippocampus of senescent mice. TauC3 is also elevated in AD patients. Through quantitative live-cell imaging, we show that TauC3 has a drastically reduced dynamics of its microtubule interaction. Single-molecule tracking of tau confirmed that TauC3 has a longer residence time on axonal microtubules. The reduced dynamics of the TauC3-microtubule interaction correlated with a decreased transport of mitochondria, a reduced processivity of APP-vesicle transport and an induction of region-specific dendritic atrophy in CA1 neurons of the hippocampus. The microtubule-targeting drug Epothilone D normalized the interaction of TauC3 with microtubules and modulated the transport of APP-vesicles dependent on the presence of overexpressed human tau. The results indicate a novel toxic gain of function, in which a post-translational modification of tau changes the dynamics of the tau-microtubule interaction and thus leads to axonal transport defects and neuronal degeneration. The data also introduce microtubule-targeting drugs as pharmacological modifiers of the tau-microtubule interaction with the potential to restore the physiological interaction of pathologically altered tau with microtubules.