Mechanistic Insights into Tau Protein-Mediated Regulation of Oxidative Stress.

Abnormal hyperphosphorylation and microtubule-associated protein tau aggregation development in the brain are characteristics of neurodegenerative diseases referred to as tauopathies, which include Alzheimer's disease (AD). The current review summarizes the complex relationships that exist between oxidative stress and tau illness, with particular attention to the roles played by the tau protein, reactive oxygen species and their consequences, and tau phosphorylation and oxidative stress. Two key elements of detrimental cycle that are critical in neurodegenerative tauopathies are tau hyperphosphorylation and oxidative stress. When tau and microtubules are not connected properly, microtubule instability, issues with microtubule transport, and ultimately neuronal death result. While the causes of the more prevalent sporadic late-onset variants and the connections between tau hyperphosphorylation and neurodegeneration remain largely unknown, mutations in the microtubule-associated protein tau (MAPT) gene have been identified in familial cases of early-onset tauopathies. Another detrimental feature of tauopathies is oxidative stress, but the exact role it plays in the development of the disease is unclear. The source of reactive oxygen species (ROS), which lead to oxidative stress within neural tissue, remains an unresolved topic. Although mitochondria have historically been thought to be a primary source of oxidative stress, microglial cells have recently been discovered to create reactive oxygen species in tauopathies. In conclusion, enhancing our comprehension of the impact of oxidative stress on various diseases could facilitate the identification of new disease markers and lead to the formulation of treatment strategies aimed at halting, reversing, or mitigating disease progression.

Identification of tauopathy-associated lipid signatures in Alzheimer's disease mouse brain using label-free chemical imaging.

There is cumulative evidence that lipid metabolism plays a key role in the pathogenesis of various neurodegenerative disorders including Alzheimer's disease (AD). Visualising lipid content in a non-destructive label-free manner can aid in elucidating the AD phenotypes towards a better understanding of the disease. In this study, we combined multiple optical molecular-specific methods, Fourier transform infrared (FTIR) spectroscopic imaging, synchrotron radiation-infrared (SR-IR) microscopy, Raman and stimulated Raman scattering (SRS) microscopy, and optical-photothermal infrared (O-PTIR) microscopy with multivariate data analysis, to investigate the biochemistry of brain hippocampus in situ using a mouse model of tauopathy (rTg4510). We observed a significant difference in the morphology and lipid content between transgenic (TG) and wild type (WT) samples. Immunohistochemical staining revealed some degree of microglia co-localisation with elevated lipids in the brain. These results provide new evidence of tauopathy-related dysfunction in a preclinical study at a subcellular level.

A tau dephosphorylation-targeting chimeraselectively recruits protein phosphatase-1 to ameliorate Alzheimer's disease and tauopathies.

Abnormal accumulation of hyperphosphorylated tau (pTau) is a major cause of neurodegeneration in Alzheimer's disease (AD) and related tauopathies. Therefore, reducing pTau holds therapeutic promise for these diseases. Here, we developed a chimeric peptide, named D20, for selective facilitation of tau dephosphorylation by recruiting protein phosphatase 1 (PP1) to tau. PP1 is one of the active phosphatases that dephosphorylates tau. In both cultured primary hippocampal neurons and mouse models for AD or related tauopathies, we demonstrated that single-dose D20 treatment significantly reduced pTau by dephosphorylation at multiple AD-related sites and total tau (tTau) levels were also decreased. Multiple-dose administration of D20 through tail vein injection in 3xTg AD mice effectively ameliorated tau-associated pathologies with improved cognitive functions. Importantly, at therapeutic doses, D20 did not cause detectable toxicity in cultured neurons, neural cells, or peripheral organs in mice. These results suggest that D20 is a promising drug candidate for AD and related tauopathies.

Tau accumulation is cleared by the induced expression of VCP via autophagy.

Tauopathy, including frontotemporal lobar dementia and Alzheimer's disease, describes a class of neurodegenerative diseases characterized by the aberrant accumulation of Tau protein due to defects in proteostasis. Upon generating and characterizing a stable transgenic zebrafish that expresses the human TAUP301L mutant in a neuron-specific manner, we found that accumulating Tau protein was efficiently cleared via an enhanced autophagy activity despite constant Tau mRNA expression; apparent tauopathy-like phenotypes were revealed only when the autophagy was genetically or chemically inhibited. We performed RNA-seq analysis, genetic knockdown, and rescue experiments with clinically relevant point mutations of valosin-containing protein (VCP), and showed that induced expression of VCP, an essential cytosolic chaperone for the protein quality system, was a key factor for Tau degradation via its facilitation of the autophagy flux. This novel function of VCP in Tau clearance was further confirmed in a tauopathy mouse model where VCP overexpression significantly decreased the level of phosphorylated and oligomeric/aggregate Tau and rescued Tau-induced cognitive behavioral phenotypes, which were reversed when the autophagy was blocked. Importantly, VCP expression in the brains of human Alzheimer's disease patients was severely downregulated, consistent with its proposed role in Tau clearance. Taken together, these results suggest that enhancing the expression and activity of VCP in a spatiotemporal manner to facilitate the autophagy pathway is a potential therapeutic approach for treating tauopathy.

The putative contribution of cellular senescence to driving tauopathies.

During mammalian aging, senescent cells accumulate in the body. Recent evidence suggests that senescent cells potentially contribute to age-related neurodegenerative diseases in the central nervous system (CNS), including tauopathies such as Alzheimer's disease (AD). Senescent cells undergo irreversible cell cycle arrest and release an inflammatory 'senescence-associated secretory profile' (SASP), which can exert devastating effects on surrounding cells. Senescent markers and SASP factors have been detected in multiple brain cells in tauopathies, including microglia, astrocytes, and perhaps even post-mitotic neurons, possibly contributing to the initiation as well as progression of these diseases. Here, we discuss the implications of presenting a senescent phenotype in tauopathies and highlight a potential role for the NOD-like receptor protein 3 (NLRP3) inflammasome as a newfound mechanism implicated in senescence and SASP formation.

Early CA2 Tau Inclusions Do Not Distinguish an Age-Related Tauopathy from Early Alzheimer's Disease.

Background: Neuropathologic studies of brains from autopsy series show tau inclusions (pretangles, neuropils threads, neurofibrillary tangles) are detectable more than a decade before amyloid-ß (Aß) deposition in Alzheimer's disease (AD) and develop in a characteristic manner that forms the basis for AD staging. An alternative position views pathological tau without Aß deposition as a 'primary age-related tauopathy' (PART) rather than prodromal AD. Recently, an early focus of tau inclusions in the Ammon's horn second sector (CA2) with relative sparing of CA1 that occurs before tau inclusions develop in the entorhinal cortex (EC) was proposed as an additional feature of PART. Objective: To test the 'definite PART' hypothesis. Methods: We used AT8-immunohistochemistry in 100µm sections to examine the EC, transentorhinal cortex (TRE), and Ammon's horn in 325 brains with tau inclusions lacking Aß deposits (average age at death 66.7 years for females, 66.4 years for males). Results: 100% of cases displayed tau inclusions in the TRE. In 89% of cases, the CA1 tau rating was greater than or equal to that in CA2. In 25%, CA2 was devoid of tau inclusions. Only 4% displayed a higher tau score in CA2 than in the TRE, EC, and CA1. The perforant path also displayed early tau changes. APOE genotyping was available for 199/325 individuals. Of these, 44% had an ɛ4 allele that placed them at greater risk for developing later NFT stages and, therefore, clinical AD. Conclusions: Our new findings call into question the PART hypothesis and are consistent with the idea that our cases represent prodromal AD.

Proteolysis of tau by granzyme A in tauopathies generates fragments that are aggregation prone.

Tauopathies, including Alzheimer's disease, corticobasal degeneration and progressive supranuclear palsy, are characterised by the aggregation of tau into insoluble neurofibrillary tangles in the brain. Tau is subject to a range of post-translational modifications, including proteolysis, that can promote its aggregation. Neuroinflammation is a hallmark of tauopathies and evidence is growing for a role of CD8+ T cells in disease pathogenesis. CD8+ T cells release granzyme proteases but what role these proteases play in neuronal dysfunction is currently lacking. Here, we identified that granzyme A (GzmA) is present in brain tissue and proteolytically cleaves tau. Mass spectrometric analysis of tau fragments produced on digestion of tau with GzmA identified three cleavage sites at R194-S195, R209-S210 and K240-S241. Mutation of the critical Arg or Lys residues at the cleavage sites in tau or chemical inhibition of GzmA blocked the proteolysis of tau by GzmA. Development of a semi-targeted mass spectrometry approach identified peptides in tauopathy brain tissue corresponding to proteolysis by GzmA at R209-S210 and K240-S241 in tau. When expressed in cells the GzmA-cleaved C-terminal fragments of tau were highly phosphorylated and aggregated upon incubation of the cells with tauopathy brain seed. The C-terminal fragment tau195-441 was able to transfer between cells and promote aggregation of tau in acceptor cells, indicating the propensity for such tau fragments to propagate between cells. Collectively, these results raise the possibility that GzmA, released from infiltrating cytotoxic CD8+ T cells, proteolytically cleaves tau into fragments that may contribute to its pathological properties in tauopathies.

Live-cell visualization of tau aggregation in human neurons.

Alzheimer's disease (AD) and more than twenty other dementias, termed tauopathies, are pathologically defined by insoluble aggregates of the microtubule-associated protein tau (MAPT). Although tau aggregation correlates with AD symptomology, the specific tau species, i.e., monomers, soluble oligomers, and insoluble aggregates that induce neurotoxicity are incompletely understood. We developed a light-responsive tau protein (optoTAU) and used viscosity-sensitive AggFluor probes to investigate the consequence(s) of tau aggregation in human neurons and identify modifiers of tau aggregation in AD and other tauopathies. We determined that optoTAU reproduces biological and structural properties of tau aggregation observed in human brains and the pathophysiological transition in tau solubility in live cells. We also provide proof-of-concept for the utilization of optoTAU as a pharmacological platform to identify modifiers of tau aggregation. These findings have broad implications for the characterization of aggregation-prone proteins and investigation of the complex relationship between protein solubility, cellular function, and disease progression.

[Tau Positron Emission Tomography: Applications in Diagnosis and Prognosis of Various Diseases].

Tau positron emission tomography (PET) is a neuroimaging technique that visualizes tau deposition using PET tracers that selectively bind to tau aggregates. Studies have reported the diagnostic and prognostic value of tau PET in Alzheimer's disease and other tauopathies. However, the binding profiles of tau PET drugs vary widely across tauopathies; therefore, an accurate understanding of the disease-specific characteristics is essential for interpretation of tau PET findings. In this review, we discuss the properties of tau-PET agents and their applications in various diseases.

Regulation of proteostasis by sleep through autophagy in Drosophila models of Alzheimer's disease.

Sleep and circadian rhythm dysfunctions are common clinical features of Alzheimer's disease (AD). Increasing evidence suggests that in addition to being a symptom, sleep disturbances can also drive the progression of neurodegeneration. Protein aggregation is a pathological hallmark of AD; however, the molecular pathways behind how sleep affects protein homeostasis remain elusive. Here we demonstrate that sleep modulation influences proteostasis and the progression of neurodegeneration in Drosophila models of tauopathy. We show that sleep deprivation enhanced Tau aggregational toxicity resulting in exacerbated synaptic degeneration. In contrast, sleep induction using gaboxadol led to reduced toxic Tau accumulation in neurons as a result of modulated autophagic flux and enhanced clearance of ubiquitinated Tau, suggesting altered protein processing and clearance that resulted in improved synaptic integrity and function. These findings highlight the complex relationship between sleep and regulation of protein homeostasis and the neuroprotective potential of sleep-enhancing therapeutics to slow the progression or delay the onset of neurodegeneration.

Alzheimer's Disease as a Membrane Dysfunction Tauopathy? New Insights into the Amyloid Cascade Hypothesis.

The amyloid cascade hypothesis postulates that extracellular deposits of amyloid ß (Aß) are the primary and initial cause leading to the full development of Alzheimer's disease (AD) with intracellular neurofibrillary tangles; however, the details of this mechanism have not been fully described until now. Our preliminary data, coming from our day-to-day neuropathology practice, show that the primary location of the hyperphosphorylated tau protein is in the vicinity of the cell membrane of dystrophic neurites. This observation inspired us to formulate a hypothesis that presumes an interaction between low-density lipoprotein receptor-related protein 1 (LRP1) and fibrillar aggregates of, particularly, Aß42 anchored at the periphery of neuritic plaques, making internalization of the LRP1-Aß42 complex infeasible and, thus, causing membrane dysfunction, leading to the tauopathy characterized by intracellular accumulation and hyperphosphorylation of the tau protein. Understanding AD as a membrane dysfunction tauopathy may draw attention to new treatment approaches not only targeting Aß42 production but also, perhaps paradoxically, preventing the formation of LRP1-Aß42.

Loss of DNA glycosylases improves health and cognitive function in a C. elegans model of human tauopathy.

Alzheimer's disease (AD) is a neurodegenerative disorder representing a major burden on families and society. Some of the main pathological hallmarks of AD are the accumulation of amyloid plaques (Aß) and tau neurofibrillary tangles. However, it is still unclear how Aß and tau aggregates promote specific phenotypic outcomes and lead to excessive oxidative DNA damage, neuronal cell death and eventually to loss of memory. Here we utilized a Caenorhabditis elegans (C. elegans) model of human tauopathy to investigate the role of DNA glycosylases in disease development and progression. Transgenic nematodes expressing a pro-aggregate form of tau displayed altered mitochondrial content, decreased lifespan, and cognitive dysfunction. Genetic ablation of either of the two DNA glycosylases found in C. elegans, NTH-1 and UNG-1, improved mitochondrial function, lifespan, and memory impairment. NTH-1 depletion resulted in a dramatic increase of differentially expressed genes, which was not apparent in UNG-1 deficient nematodes. Our findings clearly show that in addition to its enzymatic activity, NTH-1 has non-canonical functions highlighting its modulation as a potential therapeutic intervention to tackle tau-mediated pathology.

Chronic Neuronal Hyperexcitation Exacerbates Tau Propagation in a Mouse Model of Tauopathy.

The intracerebral spread of tau is a critical mechanism associated with functional decline in Alzheimer's disease (AD) and other tauopathies. Recently, a hypothesis has emerged suggesting that tau propagation is linked to functional neuronal connections, specifically driven by neuronal hyperactivity. However, experimental validation of this hypothesis remains limited. In this study, we investigated how tau propagation from the entorhinal cortex to the hippocampus, the neuronal circuit most susceptible to tau pathology in AD, is affected by the selective stimulation of neuronal activity along this circuit. Using a mouse model of seed-induced propagation combined with optogenetics, we found that the chronic stimulation of this neuronal connection over a 4-week period resulted in a significant increase in insoluble tau accumulation in both the entorhinal cortex and hippocampus. Importantly, the ratio of tau accumulation in the hippocampus relative to that in the entorhinal cortex, serving as an indicator of transcellular spreading, was significantly higher in mice subjected to chronic stimulation. These results support the notion that abnormal neuronal activity promotes tau propagation, thereby implicating it in the progression of tauopathy.

Tau in neurodegenerative diseases: molecular mechanisms, biomarkers, and therapeutic strategies.

The deposition of abnormal tau protein is characteristic of Alzheimer's disease (AD) and a class of neurodegenerative diseases called tauopathies. Physiologically, tau maintains an intrinsically disordered structure and plays diverse roles in neurons. Pathologically, tau undergoes abnormal post-translational modifications and forms oligomers or fibrous aggregates in tauopathies. In this review, we briefly introduce several tauopathies and discuss the mechanisms mediating tau aggregation and propagation. We also describe the toxicity of tau pathology. Finally, we explore the early diagnostic biomarkers and treatments targeting tau. Although some encouraging results have been achieved in animal experiments and preclinical studies, there is still no cure for tauopathies. More in-depth basic and clinical research on the pathogenesis of tauopathies is necessary.

PM2.5 triggers tau aggregation in a mouse model of tauopathy.

The aggregation and prion-like propagation of tau are the hallmarks of Alzheimer's disease (AD) and other tauopathies. However, the molecular mechanisms underlying the assembly and spread of tau pathology remain elusive. Epidemiological data show that exposure to fine particulate matter (PM2.5) is associated with an increased risk of AD. However, the molecular mechanisms remain unknown. Here, we showed that PM2.5 triggered the aggregation of tau and promoted the formation of tau fibrils. Injection of PM2.5-induced tau preformed fibrils (PFFs) into the hippocampus of tau P301S transgenic mice promoted the aggregation of tau and induced cognitive deficits and synaptic dysfunction. Furthermore, intranasal administration of PM2.5 exacerbated tau pathology and induced cognitive impairment in tau P301S mice. In conclusion, our results indicated that PM2.5 exposure promoted tau pathology and induced cognitive impairments. These results provide mechanistic insight into how PM2.5 increases the risk of AD.

Neuropathological hallmarks in the post-mortem retina of neurodegenerative diseases.

The retina is increasingly recognised as a potential source of biomarkers for neurodegenerative diseases. Hallmark protein aggregates in the retinal neuronal tissue could be imaged through light non-invasively. Post-mortem studies have already shown the presence of specific hallmark proteins in Alzheimer's disease, primary tauopathies, synucleinopathies and frontotemporal lobar degeneration. This study aims to assess proteinopathy in a post-mortem cohort with different neurodegenerative diseases and assess the presence of the primary pathology in the retina. Post-mortem eyes were collected in collaboration with the Netherlands Brain Bank from donors with Alzheimer's disease (n = 17), primary tauopathies (n = 8), synucleinopathies (n = 27), frontotemporal lobar degeneration (n = 8), mixed pathology (n = 11), other neurodegenerative diseases (n = 6), and cognitively normal controls (n = 25). Multiple cross sections of the retina and optic nerve tissue were immunostained using antibodies against pTau Ser202/Thr205 (AT8), amyloid-beta (4G8), alpha-synuclein (LB509), pTDP-43 Ser409/410 and p62-lck ligand (p62) and were assessed for the presence of aggregates and inclusions. pTau pathology was observed as a diffuse signal in Alzheimer's disease, primary tauopathies and controls with Alzheimer's disease neuropathological changes. Amyloid-beta was observed in the vessel wall and as cytoplasmic granular deposits in all groups. Alpha-synuclein pathology was observed as Lewy neurites in the retina in synucleinopathies associated with Lewy pathology and as oligodendroglial cytoplasmic inclusions in the optic nerve in multiple system atrophy. Anti-pTDP-43 generally showed typical neuronal cytoplasmic inclusion bodies in cases with frontotemporal lobar degeneration with TDP-43 and also in cases with later stages of limbic-associated TDP-43 encephalopathy. P62 showed inclusion bodies similar to those seen with anti-pTDP-43. Furthermore, pTau and alpha-synuclein pathology were significantly associated with increasing Braak stages for neurofibrillary tangles and Lewy bodies, respectively. Mixed pathology cases in this cohort consisted of cases (n = 6) with high Braak LB stages (> 4) and low or moderate AD pathology, high AD pathology (n = 1, Braak NFT 6, Thal phase 5) with moderate LB pathology, or a combination of low/moderate scores for different pathology scores in the brain (n = 4). There were no cases with advanced co-pathologies. In seven cases with Braak LB ≥ 4, LB pathology was observed in the retina, while tau pathology in the retina in the mixed pathology group (n = 11) could not be observed. From this study, we conclude that the retina reflects the presence of the major hallmark proteins associated with neurodegenerative diseases. Although low or moderate levels of copathology were found in the brains of most cases, the retina primarily manifested protein aggregates associated with the main neurodegenerative disease. These findings indicate that with appropriate retinal imaging techniques, retinal biomarkers have the potential to become highly accurate indicators for diagnosing the major neurodegenerative diseases of the brain.

Severe neurodegeneration in brains of transgenic rats producing human tau prions.

Both wild-type and mutant tau proteins can misfold into prions and self-propagate in the central nervous system of animals and people. To extend the work of others, we investigated the molecular basis of tau prion-mediated neurodegeneration in transgenic (Tg) rats expressing mutant human tau (P301S); this line of Tg rats is denoted Tg12099. We used the rat Prnp promoter to drive the overexpression of mutant tau (P301S) in the human 0N4R isoform. In Tg12099(+/+) rats homozygous for the transgene, ubiquitous expression of mutant human tau resulted in the progressive accumulation of phosphorylated tau inclusions, including silver-positive tangles in the frontal cortices and limbic system. Signs of central nervous system dysfunction were found in terminal Tg12099(+/+) rats exhibiting severe neurodegeneration and profound atrophy of the amygdala and piriform cortex. The greatest increases in tau prion activity were found in the corticolimbic structures. In contrast to the homozygous Tg12099(+/+) rats, we found lower levels of mutant tau in the hemizygous rats, resulting in few neuropathologic changes up to 2 years of age. Notably, these hemizygous rats could be infected by intracerebral inoculation with recombinant tau fibrils or precipitated tau prions from the brain homogenates of sick, aged homozygous Tg12099(+/+) rats. Our studies argue that the regional propagation of tau prions and neurodegeneration in the Tg12099 rats resembles that found in human primary tauopathies. These findings seem likely to advance our understanding of human tauopathies and may lead to effective therapeutics for Alzheimer's disease and other tau prion disorders.

Ligand-Based Virtual Screening as a Path to New Chemotypes for Candidate PET Radioligands for Imaging Tauopathies.

Ligand-based virtual screening (LBVS) has rarely been tested as a method for discovering new structural scaffolds for PET radioligand development. This study used LBVS to discover potential chemotype leads for developing radioligands for PET imaging of tauopathies. ZINC12, a free database of over 12 million commercially available compounds, was searched to discover novel scaffolds based on similarities to four query compounds. Thirteen high-ranking hits were purchased and assayed for their ability to compete against three tritiated radioligands at their distinct binding sites in Alzheimer's disease brain tissue. Three hits were 2-substituted 6-methoxy naphthalenes. Synthetic elaboration of this new chemotype yielded three new ligands (25, 26, and 28) with high affinity for the [3H]6 (flortaucipur) neurofibrillary tangle binding site. Compound 28 showed remarkably high affinity (Ki, 7 nM) and other desirable properties for a candidate PET radioligand, including low topological polar surface area, moderate computed log D, and amenability for labeling with carbon-11. LBVS appears to be uniquely valuable for discovering new chemotypes for candidate PET radioligands.

Synaptic gene expression changes in frontotemporal dementia due to the MAPT 10 + 16 mutation.

AIMS: Mutations in the MAPT gene encoding tau protein can cause autosomal dominant neurodegenerative tauopathies including frontotemporal dementia (often with Parkinsonism). In Alzheimer's disease, the most common tauopathy, synapse loss is the strongest pathological correlate of cognitive decline. Recently, Positron Emission Tomography (PET) imaging with synaptic tracers revealed clinically relevant loss of synapses in primary tauopathies; however, the molecular mechanisms leading to synapse degeneration in primary tauopathies remain largely unknown. In this study, we examined post-mortem brain tissue from people who died with frontotemporal dementia with tau pathology (FTDtau) caused by the MAPT intronic exon 10 + 16 mutation, which increases splice variants containing exon 10 resulting in higher levels of tau with four microtubule-binding domains. METHODS: We used RNA sequencing and histopathology to examine temporal cortex and visual cortex, to look for molecular phenotypes compared to age, sex and RNA integrity matched participants who died without neurological disease (n = 12 FTDtau10 + 16 and 13 controls). RESULTS: Bulk tissue RNA sequencing reveals substantial downregulation of gene expression associated with synaptic function. Upregulated biological pathways in human MAPT 10 + 16 brain included those involved in transcriptional regulation, DNA damage response and neuroinflammation. Histopathology confirmed increased pathological tau accumulation in FTDtau10 + 16 cortex as well as a loss of presynaptic protein staining and region-specific increased colocalization of phospho-tau with synapses in temporal cortex. CONCLUSIONS: Our data indicate that synaptic pathology likely contributes to pathogenesis in FTDtau10 + 16 caused by the MAPT 10 + 16 mutation.