New insights into the therapeutic approaches for the treatment of tauopathies

Tauopathies are a group of neurological disorders,including Alzheimer’s disease and frontotemporal dementia,which involve progressive neurodegeneration,cognitive deficits,and aberrant tau protein accumulation.The development of tauopathies cannot currently be stopped or slowed down by treatment measures.Given the significant contribution of tau burden in primary tauopathies and the strong association between pathogenic tau accumulation and cognitive deficits,there has been a lot of interest in creating therapies that can alleviate tau pathology and render neuroprotective effects.Recently,small molecules,immunotherapies,and gene therapy have been used to reduce the pathological tau burden and prevent neurodegeneration in animal models of tauopathies.However,the major pitfall of the current therapeutic approach is the difficulty of drugs and gene-targeting modalities to cross the blood-brain barrier and their unintended side effects.In this review,the current therapeutic strategies used for tauopathies including the use of oligonucleotide-based gene therapy approaches that have shown a promising result for the treatment of tauopathies and Alzheimer’s disease in preclinical animal models,have been discussed.

AAV-based gene therapy approaches for genetic forms of tauopathies and related neurogenetic disorders

Tauopathies comprise a spectrum of genetic and sporadic neurodegenerative diseases mainly characterized by the presence of hyperphosphorylated TAU protein aggregations in neurons or glia.Gene therapy,in particular adeno-associated virus(AAV)-based,is an effective medical approach for difficult-to-treat genetic diseases for which there are no convincing traditional therapies,such as tauopathies.Employing AAV-based gene therapy to treat,in particular,genetic tauopathies has many potential therapeutic benefits,but also drawbacks which need to be addressed in order to successfully and efficiently adapt this still unconventional therapy for the various types of tauopathies.In this Viewpoint,we briefly introduce some potentially treatable tauopathies,classify them according to their etiology,and discuss the potential advantages and possible problems of AAV-based gene therapy.Finally,we outline a future vision for the application of this promising therapeutic approach for genetic and sporadic tauopathies.

Is tau a suitable therapeutical target in tauopathies?

Tau is an intracellular protein,found mainly in neurons,but it can also be found in the extracellular space in pathological situations.Here we discuss whether intracellular tau,in aggregated form or modified by phosphorylation,could be toxic inside a neuron.On the other hand,it has been proposed that extracellular tau could be toxic.In this review,we address the question if the elimination of tau would be a possible therapeutic method to avoid tauopathy disorder and we suggest ways to eliminate intracellular and extracellular tau as treatment.

Strategies for translating proteomics discoveries into drug discovery for dementia

Tauopathies,diseases characterized by neuropathological aggregates of tau including Alzheimer's disease and subtypes of fro ntotemporal dementia,make up the vast majority of dementia cases.Although there have been recent developments in tauopathy biomarkers and disease-modifying treatments,ongoing progress is required to ensure these are effective,economical,and accessible for the globally ageing population.As such,continued identification of new potential drug targets and biomarkers is critical."Big data"studies,such as proteomics,can generate information on thousands of possible new targets for dementia diagnostics and therapeutics,but currently remain underutilized due to the lack of a clear process by which targets are selected for future drug development.In this review,we discuss current tauopathy biomarkers and therapeutics,and highlight areas in need of improvement,particularly when addressing the needs of frail,comorbid and cognitively impaired populations.We highlight biomarkers which have been developed from proteomic data,and outline possible future directions in this field.We propose new criteria by which potential targets in proteomics studies can be objectively ranked as favorable for drug development,and demonstrate its application to our group's recent tau interactome dataset as an example.

Inflammasome links traumatic brain injury, chronic traumatic encephalopathy, and Alzheimer's disease

Traumatic brain injury, chronic traumatic encephalopathy, and Alzheimer's disease are three distinct neurological disorders that share common pathophysiological mechanisms involving neuroinflammation. One sequela of neuroinflammation includes the pathologic hyperphosphorylation of tau protein, an endogenous microtubule-associated protein that protects the integrity of neuronal cytoskeletons. Tau hyperphosphorylation results in protein misfolding and subsequent accumulation of tau tangles forming neurotoxic aggregates. These misfolded proteins are characteristic of traumatic brain injury, chronic traumatic encephalopathy, and Alzheimer's disease and can lead to downstream neuroinflammatory processes, including assembly and activation of the inflammasome complex. Inflammasomes refer to a family of multimeric protein units that, upon activation, release a cascade of signaling molecules resulting in caspase-induced cell death and inflammation mediated by the release of interleukin-1β cytokine. One specific inflammasome, the NOD-like receptor protein 3, has been proposed to be a key regulator of tau phosphorylation where it has been shown that prolonged NOD-like receptor protein 3 activation acts as a causal factor in pathological tau accumulation and spreading. This review begins by describing the epidemiology and pathophysiology of traumatic brain injury, chronic traumatic encephalopathy, and Alzheimer's disease. Next, we highlight neuroinflammation as an overriding theme and discuss the role of the NOD-like receptor protein 3 inflammasome in the formation of tau deposits and how such tauopathic entities spread throughout the brain. We then propose a novel framework linking traumatic brain injury, chronic traumatic encephalopathy, and Alzheimer's disease as inflammasomedependent pathologies that exist along a temporal continuum. Finally, we discuss potential therapeutic targets that may intercept this pathway and ultimately minimize long-term neurological decline.

Effects of P301L-TAU on post-translational modifications of microtubules in human iPSC-derived cortical neurons and TAU transgenic mice

TAU is a microtubule-associated protein that promotes microtubule assembly and stability in the axon.TAU is missorted and aggregated in an array of diseases known as tauopathies.Microtubules are essential for neuronal function and regulated via a complex set of post-translational modifications,changes of which affect microtubule stability and dynamics,microtubule interaction with other proteins and cellular structures,and mediate recruitment of microtubule-severing enzymes.As impairment of microtubule dynamics causes neuronal dysfunction,we hypothesize cognitive impairment in human disease to be impacted by impairment of microtubule dynamics.We therefore aimed to study the effects of a disease-causing mutation of TAU(P301L)on the levels and localization of microtubule post-translational modifications indicative of microtubule stability and dynamics,to assess whether P301L-TAU causes stability-changing modifications to microtubules.To investigate TAU localization,phosphorylation,and effects on tubulin post-translational modifications,we expressed wild-type or P301L-TAU in human MAPT-KO induced pluripotent stem cell-derived neurons(i Neurons)and studied TAU in neurons in the hippocampus of mice transgenic for human P301L-TAU(p R5 mice).Human neurons expressing the longest TAU isoform(2N4R)with the P301L mutation showed increased TAU phosphorylation at the AT8,but not the p-Ser-262 epitope,and increased polyglutamylation and acetylation of microtubules compared with endogenous TAU-expressing neurons.P301L-TAU showed pronounced somatodendritic presence,but also successful axonal enrichment and a similar axodendritic distribution comparable to exogenously expressed 2N4R-wildtype-TAU.P301L-TAU-expressing hippocampal neurons in transgenic mice showed prominent missorting and tauopathy-typical AT8-phosphorylation of TAU and increased polyglutamylation,but reduced acetylation,of microtubules compared with non-transgenic littermates.In sum,P301L-TAU results in changes in microtubule PTMs,suggestive of impairment of microtubule stability.This is accompanied by missorting and aggregation of TAU in mice but not in i Neurons.Microtubule PTMs/impairment may be of key importance in tauopathies.

Anti-IgLON5 disease: a novel topic beyond neuroimmunology

Anti-IgLON5 disease is a recently defined autoimmune disorder of the nervous system associated with autoantibodies against IgLON5. Given its broad clinical spectrum and extremely complex pathogenesis, as well as difficulties in its early diagnosis and treatment, anti-IgLON5 disease has become the subject of considerable research attention in the field of neuroimmunology. Anti-IgLON5 disease has characteristics of both autoimmunity and neurodegeneration due to the unique activity of the antiIgLON5 antibody. Neuropathologic examination revealed the presence of a tauopathy preferentially affecting the hypothalamus and brainstem tegmentum, potentially broadening our understanding of tauopathies. In contrast to that seen with other autoimmune encephalitis-related antibodies, basic studies have demonstrated that IgLON5 antibody-induced neuronal damage and degeneration are irreversible, indicative of a potential link between autoimmunity and neurodegeneration in antiIgLON5 disease. Herein, we comprehensively review and discuss basic and clinical studies relating to anti-IgLON5 disease to better understand this complicated disorder.

Dysfunctional glia:contributors to neurodegenerative disorders

Astrocytes are integral components of the central nervous system,where they are involved in numerous functions critical for neuronal development and functioning,including maintenance of blood-brain barrier,formation of synapses,supporting neurons with nutrients and trophic factors,and protecting them from injury.These roles are markedly affected in the course of chronic neurodegenerative disorders,often before the onset of the disease.In this review,we summarize the recent findings supporting the hypothesis that astrocytes play a fundamental role in the processes contributing to neurodegeneration.We focus onα-synucleinopathies and tauopathies as the most common neurodegenerative diseases.The mechanisms implicated in the development and progression of these disorders appear not to be exclusively neuronal,but are often related to the astrocytic-neuronal integrity and the response of astrocytes to the altered microglial function.A profound understanding of the multifaceted functions of astrocytes and identification of their communication pathways with neurons and microglia in health and in the disease is of critical significance for the development of novel mechanism-based therapies against neurodegenerative disorders.

Neural stem cell transplantation alleviates functional cognitive deficits in a mouse model of tauopathy

The mechanisms of the transplantation of neural stem cells(NSCs)in the treatment of Alzheimer’s disease remain poorly understood.In this study,NSCs were transplanted into the hippocampal CA1 region of the rTg(tau P301L)4510 mouse model,a tauopathy model that is thought to reflect the tau pathology associated with Alzheimer’s disease.The results revealed that NSC transplantation reduced the abnormal aggregation of tau,resulting in significant improvements in the short-term memory of the tauopathy model mice.Compared with wild-type and phosphate-buffered saline(PBS)-treated mice,mice that received NSC transplantations were characterized by changes in the expression of multiple proteins in brain tissue,particularly those related to the regulation of tau aggregation or misfolding.Kyoto Encyclopedia of Genes and Genomes(KEGG)pathway analysis and Gene Ontology(GO)function analysis revealed that these proteins were primarily enriched in pathways associated with long-term potentiation,neurogenesis,and other neurobiological processes.Changes in the expression levels of key proteins were verified by western blot assays.These data provided clues to improve the understanding of the functional capacity associated with NSC transplantation in Alzheimer’s disease treatment.This study was approved by the Beijing Animal Ethics Association and Ethics Committee of Beijing Institute of Technology(approval No.SYXK-BIT-school of life science-2017-M03)in 2017.

Crry silencing alleviates Alzheimer’s disease injury by regulating neuroinflammatory cytokines and the complement system

Complement component(3b/4b)receptor 1(CR1)expression is positively related to the abundance of phosphorylated microtubule-associated protein tau(tau),and CR1 expression is associated with susceptibility to Alzheimer’s disease.However,the exact role of CR1 in tau protein-associated neurodegenerative diseases is unknown.In this study,we show that the mouse Cr1-related protein Y(Crry)gene,Crry,is localized to microglia.We also found that Crry protein expression in the hippocampus and cortex was significantly elevated in P301S mice(a mouse model widely used for investigating tau pathology)compared with that in wild-type mice.Tau protein phosphorylation(at serine 202,threonine 205,threonine 231,and serine 262)and expression of the major tau kinases glycogen synthase kinase-3 beta and cyclin-dependent-like kinase 5 were greater in P301S mice than in wild-type mice.Crry silencing by lentivirus-transfected short hairpin RNA led to greatly reduced tau phosphorylation and glycogen synthase kinase-3 beta and cyclin-dependent-like kinase 5 activity.Crry silencing reduced neuronal apoptosis and rescued cognitive impairment of P301S mice.Crry silencing also reduced the levels of the neuroinflammatory factors interleukin-1 beta,tumor necrosis factor alpha,and interleukin-6 and the complement components complement 3 and complement component 3b.Our results suggest that Crry silencing in the P301S mouse model reduces tau protein phosphorylation by reducing the levels of neuroinflammation and complement components,thereby improving cognitive function.

Cornel Iridoid Glycoside Suppresses Hyperactivity Phenotype in rTg4510 Mice through Reducing Tau Pathology and Improving Synaptic Dysfunction

rTg4510 mice are transgenic mice expressing P301L mutant tau and have been developed as an animal model of tauopathies including Alzheimer’s disease(AD).Besides cognitive impairments,rTg4510 mice also show abnormal hyperactivity behavior.Cornel iridoid glycoside(CIG)is an active ingredient extracted from Cornus officinalis,a traditional Chinese herb.The purpose of the present study was to investigate the effects of CIG on the emotional disorders such as hyperactivity,and related mechanisms.The emotional hyperactivity was detected by locomotor activity test and Y maze test.Immunofluorescent and immunohistochemical analyses were conducted to measure neuron loss and phosphorylated tau.Western blotting was used to detect the expression of related proteins.The results showed that intragastric administration of CIG for 3 months decreased the hyperactivity phenotype,prevented neuronal loss,reduced tau hyperphosphorylation and aggregation in the amygdala of rTg4510 mice.Meanwhile,CIG alleviated the synaptic dysfunction by increasing the expression of N-methyl-D-aspartate receptors(NMDARs)subunits GluN1 and GluN2A andαamino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor(AMPAR)subunits GluA1 and GluA2,and increased the level of phosphorylated Ca2+/calmodulin dependent protein kinase IIα(p-CaMK IIα)in the brain of rTg4510 mice.In conclusion,CIG may have potential to treat the emotional disorders in tauopathies such as AD through reducing tau pathology and improving synaptic dysfunction.

What triggers tauopathy in chronic traumatic encephalopathy?

Chronic traumatic encephalopathy（CTE）is a neuropathologically defined tauopathy：CTE is closely related with repetitive,traumatic brain injury.In a most recent study of 202 deceased players of American football from a brain donation program,CTE was neuropathologically diagnosed in 177 players across all levels of play （87%）, including 110 of 111 former National Football League players （99%） （Mez et al., 2017）.

Aberrant Activation of Cdc2/cyclin B1 Is Involved in Initiation of Cytoskeletal Pathology in Murine Niemann-Pick Disease Type C

Niemann-Pick disease type C（NPC） is a fatal, neurovisceral lipid storage disease, neuropathologically characterized by cytoplasmic sequestration of glycolipids in neurons, progressive neuronal loss, neurofibrillary tangles（NFTs） formation, and axonal spheroids（AS）. Cytoskeletal pathology including accumulation of hyperphosphorylated cytoskeletal proteins is a neuropathological hallmark of the mouse model of NPC（npc mice）. With a goal of elucidating the mechanisms underlying the lesion formation, we investigated the temporal and spatial characteristics of cytoskeletal lesions and the roles of cdc2, cdk4, and cdk5 in lesion formation in young npc mice. Cytoskeletal lesions were detectable in npc mice at three weeks of age. Importantly, concomitant activation of cdc2/cyclin B1 kinase and accumulation of a subsequently generated cohort of phospho-epitopes were detected. The activation of cdk4/cyclin D1 and cdk5/p25 kinases was observed during the fourth week of life in npc mice, and this activation contributed to the lesion formation. We concluded that the progression of cytoskeletal pathology in npc mice older than four weeks is accelerated by the cumulative effect of cdc2, cdk4, and cdk5 activation. Furthermore, cdc2/cyclin B1 may act as a key initial player one week earlier. Targeting cell cycle activation may be beneficial to slow down the NPC pathogenesis.

多肽靶向嵌合体通过促进tau蛋白特异性去磷酸化治疗阿尔茨海默病和其他tau蛋白病

Abnormal hyperphosphorylation and accumulation of tau protein play a pivotal role in neurodegeneration in Alzheimer’s disease(AD)and many other tauopathies.Selective elimination of hyperphosphorylated tau is promising for the therapy of these diseases.We have conceptualized a strategy,named dephosphorylation-targeting chimeras(DEPTACs),for specifically hijacking phosphatases to tau to debilitate its hyperphosphorylation.Here,we conducted the step-by-step optimization of each constituent motif to generate DEPTACs with reasonable effectiveness in facilitating the dephosphorylation and subsequent clearance of pathological tau.Specifically,for one of the selected chimeras,D16,we demonstrated its significant efficiency in rescuing the neurodegeneration caused by neurotoxic K18-tau seeds in vitro.Moreover,intravenous administration of D16 also alleviated tau pathologies in the brain and improved memory deficits in AD mice.These results suggested DEPTACs as targeted modulators of tau phosphorylation,which hold therapeutic potential for AD and other tauopathies.

Towards understandings of serine/arginine-rich splicing factors

Serine/arginine-rich splicing factors(SRSFs)refer to twelve RNA-binding proteins which regulate splice site recognition and spliceosome assembly during precursor messenger RNA splicing.SRSFs also participate in other RNA metabolic events,such as transcription,translation and nonsensemediated decay,during their shuttling between nucleus and cytoplasm,making them indispensable for genome diversity and cellular activity.Of note,aberrant SRSF expression and/or mutations elicit fallacies in gene splicing,leading to the generation of pathogenic gene and protein isoforms,which highlights the therapeutic potential of targeting SRSF to treat diseases.In this review,we updated current understanding of SRSF structures and functions in RNA metabolism.Next,we analyzed SRSF-induced aberrant gene expression and their pathogenic outcomes in cancers and non-tumor diseases.The development of some well-characterized SRSF inhibitors was discussed in detail.We hope this review will contribute to future studies of SRSF functions and drug development targeting SRSFs.

Tau-induced neurodegeneration: mechanisms and targets

The accumulation of hyperphosphorylated tau is a common feature of several dementias. Tau is one of the brain microtubule-associated proteins. Here we discuss tau＇s functions in microtubule assembly and stabilization and with regard to its interactions with other proteins. We describe and analyze important post-translational modifications： hyperphosphorylation, ubiquitination, glycation, glycosytation, nitration, polyamination, proteolysis, acetylation, and methylation. We discuss how these post-translational modifications can alter tau＇s biological function. We analyze the role of mitochondrial health in neurodegeneration. We propose that microtubules could be a therapeutic target and review different approaches. Finally, we consider whether tau accumulation or its conformational change is related to tau-induced neurodegeneration, and propose a mechanism of neurodegeneration.

Regulation of alternative splicing of tau exon 10

The neuronal microtubule-associated protein tau is abnormally hyperphosphorylated and aggregated into neurofibrillary tangles in the brains of individuals with Alzheimer＇s disease and related neurodegenerative disorders. The adult human brain expresses six isoforms of tau generated by alternative splicing of exons 2, 3, and 10 of its pre-mRNA. Exon 10 encodes the second microtubule-binding repeat of tau. Its alternative splicing produces tau isoforms with either three or four microtubule-binding repeats, termed 3R-tau and 4R- tau. In the normal adult human brain, the level of 3R-tau is approximately equal to that of 4R-tau. Several silent and intronic mutations of the tau gene associated with FTDP-17T （frontotemporal dementia with Parkinsonism linked to chromosome 17 and specifically characterized by tau pathology） only disrupt exon 10 splicing, but do not influence the primary sequence of the tau protein. Thus, abnormal exon 10 splicing is sufficient to cause neurodegeneration and dementia. Here, we review the regulation of tau exon 10 splicing by cis-elements and trans-factors and summarize all the mutations associated with FTDP-17T and related tauopathies. The findings suggest that correction of exon 10 splicing may be a potential target for tau exon 10 splicing-related tauopathies.

Recent development in selective Tau tracers for PET imaging in the brain

Abnormal Tau deposition is a crucial pathological hallmark of various neurodegenerative disorders defined as tauopathies,of which Alzheimer’s disease is the most prominent one.To date,a large number of chemical entities with different structures have been developed as Tau imaging tracers for the early diagnosis of tauopathies.Several of them with excellent bio-properties are currently being assessed in clinical trials,and more recently,the Tauvid^(TM)([^(18)F]Flortaucipir,also known as[^(18)F]AV1451 or[^(18)F]T807)as the first Tau tracer was approved by the U.S.Food and Drug Administration in 2020.This review summarized the latest development of Tau tracers and analyzed their chemical structures,with particular attention to the effects of chemical structures on biological properties.In addition,we also discuss the limitations of current Tau imaging tracers,issues that need attention in the development of new tracers,and possible future directions.

In vivo tracking of tau pathology using positron emission tomography (PET) molecular imaging in small animals

Hyperphosphorylation of the tau protein leading to the formation of neurofibrillary tangles(NFTs)is a common feature in a wide range of neurodegenerative diseases known as tauopathies,which include Alzheimer’s disease(AD)and the frontotemporal dementias(FTDs).Although heavily investigated,the mechanisms underlying the pathogenesis and progression of tauopathies have yet to be fully understood.In this context,several rodent models have been developed that successfully recapitulate the behavioral and neurochemical features of tau pathology,aiming to achieve a better understanding of the link between tau and neurodegeneration.To date,behavioral and biochemical parameters assessed using these models have been conducted using a combination of memory tasks and invasive methods such as cerebrospinal fluid(CSF)sampling or post-mortem analysis.Recently,several novel positron emission tomography(PET)radiopharmaceuticals targeting tau tangles have been developed,allowing for non-invasive in vivo quantification of tau pathology.Combined with tau transgenic models and micro-PET,these tracers hold the promise of advancing the development of theoretical models and advancing our understanding of the natural history of AD and non-AD tauopathies.In this review,we briefly describe some of the most important insights for understanding the biological basis of tau pathology,and shed light on the opportunity for improved modeling of tau pathology using a combination of tau-radiopharmaceuticals and animal models.

Advances in the Pathogenesis of Alzheimer’s Disease:Focusing on Tau-Mediated Neurodegeneration

In addition to senile plaques and cerebral amyloid angiopathy,the hyperphosphorylation of tau protein and formation of intraneuronal neurofibrillary tangles(NFTs)represents another neuropathological hallmark in AD brain.Tau is a microtubule-associated protein and localizes predominantly in the axons of neurons with the primary function in maintaining microtubules stability.When the balance between tau phosphorylation and dephosphorylation is changed in favor of the former,tau is hyperphosphorylated and the level of the free tau fractions elevated.The hyperphosphorylation of tau protein and formation of NFTs represent a characteristic neuropathological feature in AD brain.We have discussed the role of Aβin AD in our previous review,this review focused on the recent advances in tau-mediated AD pathology,mainly including tau hyperphosphorylation,propagation of tau pathology and the relationship between tau and Aβ.