The tau protein in human cerebrospinal fluid in Alzheimer's disease consists of proteolytically derived fragments

The Enigma of Tau Protein Aggregation: Mechanistic Insights and Future Challenges

Tau protein misfolding and aggregation are pathological hallmarks of Alzheimer's disease and over twenty neurodegenerative disorders. However, the molecular mechanisms of tau aggregation in vivo remain incompletely understood. There are two types of tau aggregates in the brain: soluble aggregates (oligomers and protofibrils) and insoluble filaments (fibrils). Compared to filamentous aggregates, soluble aggregates are more toxic and exhibit prion-like transmission, providing seeds for templated misfolding. Curiously, in its native state, tau is a highly soluble, heat-stable protein that does not form fibrils by itself, not even when hyperphosphorylated. In vitro studies have found that negatively charged molecules such as heparin, RNA, or arachidonic acid are generally required to induce tau aggregation. Two recent breakthroughs have provided new insights into tau aggregation mechanisms. First, as an intrinsically disordered protein, tau is found to undergo liquid-liquid phase separation (LLPS) both in vitro and inside cells. Second, cryo-electron microscopy has revealed diverse fibrillar tau conformations associated with different neurodegenerative disorders. Nonetheless, only the fibrillar core is structurally resolved, and the remainder of the protein appears as a "fuzzy coat". From this review, it appears that further studies are required (1) to clarify the role of LLPS in tau aggregation; (2) to unveil the structural features of soluble tau aggregates; (3) to understand the involvement of fuzzy coat regions in oligomer and fibril formation.

Crosstalk between tau protein autoproteolysis and amyloid fibril formation

Tau cleavage has been shown to have a significant effect on protein aggregation. Tau truncation results in the formation of aggregation-prone fragments leading to toxic aggregates and also causes the formation of harmful fragments that do not aggregate. Thus, targeting proteolysis of tau would be beneficial for the development of therapeutics for Alzheimer's disease and related tauopathies. In this study, amino-terminal quantification and ThT fluorimetry were respectively used to analyze the kinetics of tau fragmentation and fibril formation. SDS-PAGE analysis of tau protein incubated with a disulfide-reducing agent demonstrated that the cysteines of tau have a crucial role in the fibrillation and autoproteolysis. However, the structures converted to amyloid fibrils were different with conformations that led to autoproteolysis. The quantification of the amino terminal indicated that the double-disulfide parallel structures formed in the presence of heparin did not have protease activity. The survey of possible tau disulfide-mediated dimer configurations suggested that the non-register single disulfide bound conformations were involved in the tau autoproteolysis process. Moreover, the inhibition of autoproteolysis resulted in the increment of aggregation rate; hence it seems that the tau auto-cleavage is the cellular defense mechanism against protein fibrillation.

CSF tau368/total-tau ratio reflects cognitive performance and neocortical tau better compared to p-tau181 and p-tau217 in cognitively impaired individuals

INTRODUCTION

Cerebrospinal fluid (CSF) tau biomarkers are reliable diagnostic markers for Alzheimer's disease (AD). However, their strong association with amyloid pathology may limit their reliability as specific markers of tau neurofibrillary tangles. A recent study showed evidence that a ratio of CSF C-terminally truncated tau (tau368, a tangle-enriched tau species), especially in ratio with total tau (t-tau), correlates strongly with tau PET tracer uptake. In this study, we set to evaluate the performance of the tau368/t-tau ratio in capturing tangle pathology, as indexed by a high-affinity tau PET tracer, as well as its association with severity of clinical symptoms.

METHODS

In total, 125 participants were evaluated cross-sectionally from the Translational Biomarkers of Aging and Dementia (TRIAD) cohort (21 young, 60 cognitively unimpaired [CU] elderly [15 Aβ+], 10 Aβ+ with mild cognitive impairment [MCI], 14 AD dementia patients, and 20 Aβ- individuals with non-AD cognitive disorders). All participants underwent amyloid and tau PET scanning, with [18F]-AZD4694 and [18F]-MK6240, respectively, and had CSF measurements of p-tau181, p-tau217, and t-tau. CSF concentrations of tau368 were quantified in all individuals with an in-house single molecule array assay.

RESULTS

CSF tau368 concentration was not significantly different across the diagnostic groups, although a modest increase was observed in all groups as compared with healthy young individuals (all P < 0.01). In contrast, the CSF tau368/t-tau ratio was the lowest in AD dementia, being significantly lower than in CU individuals (Aβ-, P < 0.001; Aβ+, P < 0.01), as well as compared to those with non-AD cognitive disorders (P < 0.001). Notably, in individuals with symptomatic AD, tau368/t-tau correlated more strongly with [18F]-MK6240 PET SUVR as compared to the other CSF tau biomarkers, with increasing associations being seen in brain regions associated with more advanced disease (isocortical regions > limbic regions > transentorhinal regions). Importantly, linear regression models indicated that these associations were not confounded by Aβ PET SUVr. CSF tau368/t-tau also tended to continue to become more abnormal with higher tau burden, whereas the other biomarkers plateaued after the limbic stage. Finally, the tau368/t-tau ratio correlated more strongly with cognitive performance in individuals with symptomatic AD as compared to t-tau, p-tau217 and p-tau181.

CONCLUSION

The tau368/t-tau ratio captures novel aspects of AD pathophysiology and disease severity in comparison to established CSF tau biomarkers, as it is more closely related to tau PET SUVR and cognitive performance in the symptomatic phase of the disease.

Specific binding of Hsp27 and phosphorylated Tau mitigates abnormal Tau aggregation-induced pathology

Amyloid aggregation of phosphorylated Tau (pTau) into neurofibrillary tangles is closely associated with Alzheimer's disease (AD). Several molecular chaperones have been reported to bind Tau and impede its pathological aggregation. Recent findings of elevated levels of Hsp27 in the brains of patients with AD suggested its important role in pTau pathology. However, the molecular mechanism of Hsp27 in pTau aggregation remains poorly understood. Here, we show that Hsp27 partially co-localizes with pTau tangles in the brains of patients with AD. Notably, phosphorylation of Tau by microtubule affinity regulating kinase 2 (MARK2), dramatically enhances the binding affinity of Hsp27 to Tau. Moreover, Hsp27 efficiently prevents pTau fibrillation in vitro and mitigates neuropathology of pTau aggregation in a Drosophila tauopathy model. Further mechanistic study reveals that Hsp27 employs its N-terminal domain to directly interact with multiple phosphorylation sites of pTau for specific binding. Our work provides the structural basis for the specific recognition of Hsp27 to pathogenic pTau, and highlights the important role of Hsp27 in preventing abnormal aggregation and pathology of pTau in AD.

BAG3 Regulation of RAB35 Mediates the Endosomal Sorting Complexes Required for Transport/Endolysosome Pathway and Tau Clearance

BACKGROUND

Declining proteostasis with aging contributes to increased susceptibility to neurodegenerative diseases, including Alzheimer's disease (AD). Emerging studies implicate impairment of the endosome-lysosome pathway as a significant factor in the pathogenesis of these diseases. Previously, we demonstrated that BAG3 regulates phosphorylated tau clearance. However, we did not fully define how BAG3 regulates endogenous tau proteostasis, especially in the early stages of disease progression.

METHODS

Mass spectrometric analyses were performed to identify neuronal BAG3 interactors. Multiple biochemical assays were used to investigate the BAG3-HSP70-TBC1D10B (EPI64B)-RAB35-HRS regulatory networks. Live-cell imaging was used to study the dynamics of the endosomal pathway. Immunohistochemistry and immunoblotting were performed in human AD brains and in P301S tau transgenic mice with BAG3 overexpressed.

RESULTS

The primary group of neuronal BAG3 interactors identified are involved in the endocytic pathway. Among them were key regulators of small GTPases, such as the RAB35 GTPase-activating protein TBC1D10B. We demonstrated that a BAG3-HSP70-TBC1D10B complex attenuates the ability of TBC1D10B to inactivate RAB35. Thus, BAG3 interacts with TBC1D10B to support the activation of RAB35 and recruitment of HRS, initiating endosomal sorting complex required for transport-mediated endosomal tau clearance. Furthermore, TBC1D10B shows significantly less colocalization with BAG3 in AD brains than in age-matched controls. Overexpression of BAG3 in P301S tau transgenic mice increased the colocalization of phosphorylated tau with the endosomal sorting complex required for transport III protein CHMP2B and reduced the levels of the mutant human tau.

CONCLUSIONS

We identified a novel BAG3-TBC1D10B-RAB35 regulatory axis that modulates endosomal sorting complex required for transport-dependent protein degradation machinery and tau clearance. Dysregulation of BAG3 could contribute to the pathogenesis of AD.

An immuno-enrichment free, validated quantification of tau protein in human CSF by LC-MS/MS

Tau protein is a key target of interest in developing therapeutics for neurodegenerative diseases. Here, we sought to develop a method that quantifies extracellular tau protein concentrations in human cerebrospinal fluid (CSF) without antibody-based enrichment strategies. We demonstrate that the fit-for-purpose validated method in Alzheimer's Disease CSF is limited to quasi quantitative measures of tau surrogate peptides. We also provide evidence that CSF total Tau measures by LC-MS are feasible in the presence of monoclonal therapeutic antibodies in human CSF. Our Tau LC-MS/MS method is a translational bioanalytical tool for assaying target engagement and pharmacodynamics for anti-tau antibody drug development campaigns.

Cerebrospinal Fluid Biomarkers Predict Gait Outcomes in Idiopathic Normal Pressure Hydrocephalus

Background and Objectives

The assessment of biomarkers in selecting patients with idiopathic normal pressure hydrocephalus (iNPH) for shunt surgery has been limited to small cohort studies and those with limited follow-up. We assessed the potential for CSF biomarkers in predicting immediate response to CSF tap test (TT) and long-term response after shunt surgery.

Methods

CSF was obtained from patients with iNPH referred for CSF TT after baseline assessment of cognition and gait. CSF neurofilament light (NfL), β-amyloid 42 (Aβ_1–42), β-amyloid 40 (Aβ_1–40), total tau (tTau), and phosphorylated tau 181 (pTau181) and leucine-rich alpha-2-glycoprotein-1 (LRG1) were measured by ELISA. The ability of these measures to predict immediate improvement following CSF TT and long-term improvement following shunt surgery was compared by univariate and adjusted multivariate regression.

Results

Lower NfL, pTau181, tTau, and Aβ_1–40 were individually predictive of long-term improvement in gait outcomes after shunt surgery. A multivariate model of these biomarkers and MRI Evans index, adjusted for age, improved prediction (area under the receiver operating curve 0.76, 95% confidence interval 0.66–0.86). tTau, pTau181, and Aβ_1–40 levels were statistically different in those whose gait improved after CSF TT compared with those who did not. Using a multivariate model, combining these markers with Evans index and transependymal flow did not significantly improve prediction of an immediate response to CSF TT.

Discussion

A combination of CSF biomarkers can predict improvement following shunt surgery for iNPH. However, these measures only modestly discriminate responders from nonresponders following CSF TT. The findings further suggest that abnormal CSF biomarkers in nonresponders may represent comorbid neurodegenerative pathology or a predegenerative phase that presents with an iNPH phenotype.

Revisiting the grammar of Tau aggregation and pathology formation: how new insights from brain pathology are shaping how we study and target Tauopathies

Converging evidence continues to point towards Tau aggregation and pathology formation as central events in the pathogenesis of Alzheimer's disease and other Tauopathies. Despite significant advances in understanding the morphological and structural properties of Tau fibrils, many fundamental questions remain about what causes Tau to aggregate in the first place. The exact roles of cofactors, Tau post-translational modifications, and Tau interactome in regulating Tau aggregation, pathology formation, and toxicity remain unknown. Recent studies have put the spotlight on the wide gap between the complexity of Tau structures, aggregation, and pathology formation in the brain and the simplicity of experimental approaches used for modeling these processes in research laboratories. Embracing and deconstructing this complexity is an essential first step to understanding the role of Tau in health and disease. To help deconstruct this complexity and understand its implication for the development of effective Tau targeting diagnostics and therapies, we firstly review how our understanding of Tau aggregation and pathology formation has evolved over the past few decades. Secondly, we present an analysis of new findings and insights from recent studies illustrating the biochemical, structural, and functional heterogeneity of Tau aggregates. Thirdly, we discuss the importance of adopting new experimental approaches that embrace the complexity of Tau aggregation and pathology as an important first step towards developing mechanism- and structure-based therapies that account for the pathological and clinical heterogeneity of Alzheimer's disease and Tauopathies. We believe that this is essential to develop effective diagnostics and therapies to treat these devastating diseases.

In Vivo Microdialysis in Mice Captures Changes in Alzheimer's Disease Cerebrospinal Fluid Biomarkers Consistent with Developing Pathology

BACKGROUND

Preclinical models of Alzheimer's disease (AD) can provide valuable insights into the onset and progression of the disease, such as changes in concentrations of amyloid-β (Aβ) and tau in cerebrospinal fluid (CSF). However, such models are currently underutilized due to limited advancement in techniques that allow for longitudinal CSF monitoring.

OBJECTIVE

An elegant way to understand the biochemical environment in the diseased brain is intracerebral microdialysis, a method that has until now been limited to short-term observations, or snapshots, of the brain microenvironment. Here we draw upon patient-based findings to characterize CSF biomarkers in a commonly used preclinical mouse model for AD.

METHODS

Our modified push-pull microdialysis method was first validated ex vivo with human CSF samples, and then in vivo in an AD mouse model, permitting assessment of dynamic changes of CSF Aβ and tau and allowing for better translational understanding of CSF biomarkers.

RESULTS

We demonstrate that CSF biomarker changes in preclinical models capture what is observed in the brain; with a decrease in CSF Aβ observed when plaques are deposited, and an increase in CSF tau once tau pathology is present in the brain parenchyma. We found that a high molecular weight cut-off membrane allowed for simultaneous sampling of Aβ and tau, comparable to CSF collection by lumbar puncture in patients.

CONCLUSION

Our approach can further advance AD and other neurodegenerative research by following evolving neuropathology along the disease cascade via consecutive sampling from the same animal and can additionally be used to administer pharmaceutical compounds and assess their efficacy (Bjorkli, unpublished data).

To target Tau pathologies, we must embrace and reconstruct their complexities

The accumulation of hyperphosphorylated fibrillar Tau aggregates in the brain is one of the defining hallmarks of Tauopathy diseases, including Alzheimer's disease. However, the primary events or molecules responsible for initiation of the pathological Tau aggregation and spreading remain unknown. The discovery of heparin as an effective inducer of Tau aggregation in vitro was instrumental to enabling different lines of research into the role of Tau aggregation in the pathogenesis of Tauopathies. However, recent proteomics and cryogenic electron microscopy (cryo-EM) studies have revealed that heparin-induced Tau fibrils generated in vitro do not reproduce the biochemical and ultrastructural properties of disease-associated brain-derived Tau fibrils. These observations demand that we reassess our current approaches for investigating the mechanisms underpinning Tau aggregation and pathology formation. Our review article presents an up-to-date survey and analyses of 1) the evolution of our understanding of the interactions between Tau and heparin, 2) the various structural and mechanistic models of the heparin-induced Tau aggregation, 3) the similarities and differences between brain-derived and heparin-induced Tau fibrils; and 4) emerging concepts on the biochemical and structural determinants underpinning Tau pathological heterogeneity in Tauopathies. Our analyses identify specific knowledge gaps and call for 1) embracing the complexities of Tau pathologies; 2) reassessment of current approaches to investigate, model and reproduce pathological Tau aggregation as it occurs in the brain; 3) more research towards a better understanding of the naturally-occurring cofactor molecules that are associated with Tau brain pathology initiation and propagation; and 4) developing improved approaches for in vitro production of the Tau aggregates and fibrils that recapitulate and/or amplify the biochemical and structural complexity and diversity of pathological Tau in Tauopathies. This will result in better and more relevant tools, assays, and mechanistic models, which could significantly improve translational research and the development of drugs and antibodies that have higher chances for success in the clinic.

Mass spectrometry-based methods for robust measurement of Alzheimer's disease biomarkers in biological fluids

Alzheimer's disease (AD) is the most common form of dementia affecting 60%-70% of people afflicted with this disease. Accurate antemortem diagnosis is urgently needed for early detection of AD to enable reliable estimation of prognosis, intervention, and monitoring of the disease. The National Institute on Aging/Alzheimer's Association sponsored the 'Research Framework: towards a biological definition of AD', which recommends using different biomarkers in living persons for a biomarker-based definition of AD regardless of clinical status. Fluid biomarkers represent one of key groups of them. Since cerebrospinal fluid (CSF) is in direct contact with brain and many proteins present in the brain can be detected in CSF, this fluid has been regarded as the best biofluid in which to measure AD biomarkers. Recently, technological advancements in protein detection made possible the effective study of plasma AD biomarkers despite their significantly lower concentrations versus to that in CSF. This and other challenges that face plasma-based biomarker measurements can be overcome by using mass spectrometry. In this review, we discuss AD biomarkers which can be reliably measured in CSF and plasma using targeted mass spectrometry coupled to liquid chromatography (LC/MS/MS). We describe progress in LC/MS/MS methods' development, emphasize the challenges, and summarize major findings. We also highlight the role of mass spectrometry and progress made in the process of global standardization of the measurement of Aβ42/Aβ40. Finally, we briefly describe exploratory proteomics which seek to identify new biomarkers that can contribute to detection of co-pathological processes that are common in sporadic AD.

The informed road map to prevention of Alzheimer Disease: A call to arms

Alzheimer disease (AD) prevention trials hold the promise to delay or prevent cognitive decline and dementia onset by intervening before significant neuronal damage occurs. In recent years, the first AD prevention trials have launched and are yielding important findings on the biology of targeting asymptomatic AD pathology. However, there are limitations that impact the design of these prevention trials, including the translation of animal models that recapitulate key stages and multiple pathological aspects of the human disease, missing target validation in asymptomatic disease, uncertain causality of the association of pathophysiologic changes with cognitive and clinical symptoms, and limited biomarker validation for novel targets. The field is accelerating advancements in key areas including the development of highly specific and quantitative biomarker measures for AD pathology, increasing our understanding of the course and relationship of amyloid and tau pathology in asymptomatic through symptomatic stages, and the development of powerful interventions that can slow or reverse AD amyloid pathology. We review the current status of prevention trials and propose key areas of needed research as a call to basic and translational scientists to accelerate AD prevention. Specifically, we review (1) sporadic and dominantly inherited primary and secondary AD prevention trials, (2) proposed targets, mechanisms, and drugs including the amyloid, tau, and inflammatory pathways and combination treatments, (3) the need for more appropriate prevention animal models and experiments, and (4) biomarkers and outcome measures needed to design human asymptomatic prevention trials. We conclude with actions needed to effectively move prevention targets and trials forward.

Molecular basis of proteinopathies: Etiopathology of dementia and motor disorders

Neurodegenerative diseases are one of the most important medical and social problems affecting elderly people, the percentage of which is significantly increasing in the total world population. The cause of these diseases is the destruction of neurons by protein aggregates that form pathological deposits in neurons, glial cells and in the intercellular space. Proteins whose molecules are easily destabilized by point mutations or endogenous processes are alpha-synuclein (ASN), tau and TDP-43. Pathological forms of these proteins form characteristic aggregates, which accumulate in the neurons and are the cause of various forms of dementia and motor disorders. The most common causes of dementia are tauopathies. In primary tauopathies, which include progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), Pick’s disease (PiD), and frontotemporal dementia (FTD), modified tau molecules disrupt axonal transport and protein distribution in neurons. Ultimately, the helical filaments and neurofibrillary tangles of tau lead to neuron death in various structures of the brain. In Alzheimer’s disease hyperphosphorylated tau tangles along with β amyloid plaques are responsible for the degeneration of the hippocampus, entorhinal cortex and amygdala. The most prevalent synucleinopathies are Parkinson’s disease, multiple system atrophy (MSA) and dementia with Lewy bodies, where there is a degeneration of neurons in the extrapyramidal tracts or, as in MSA, autonomic nerves. TDP-43 inclusions in the cytoplasm cause the degeneration of motor neurons in amyotrophic lateral sclerosis (ALS) and in one of the frontotemporal dementia variant (FTLD-TDP). In this work ASN, tau and TDP-43 structures are described, as well as the genetic and sporadic factors that lead to the destabilization of molecules, their aggregation and incorrect distribution in neurons, which are the causes of neurodegenerative diseases.

Dichotomous role of microtubule associated protein tau as a biomarker of response to and a target for increasing efficacy of taxane treatment in cancers of epithelial origin

Cancer is the second leading cause of death worldwide, and the World Health Organization estimates that one in six deaths globally is due to cancer. Chemotherapy is one of the major modalities used to treat advanced cancers and their metastasis. However, the existence of acquired and intrinsic resistance to anti-cancer drugs often diminishes their therapeutic effect. In order to pre-select patients who could benefit the most from these treatments, the efforts of many research groups have been focused on identification of biomarkers of therapy response. Taxanes paclitaxel (Taxol) and docetaxel (Taxotere) have been introduced as chemotherapy for treatment of cancers of ovary in 1992 and breast in 1996, respectively. Since then, clinical use of taxanes has expanded to include lung, prostate, gastric, head and neck, esophageal, pancreatic, and cervical cancers, as well as Kaposi sarcoma. Several independent molecular mechanisms have been shown to support taxane chemoresistance. One such mechanism is dependent on microtubule associated protein tau. Tau binds to the same site on the inner side of the microtubules that is also occupied by paclitaxel or docetaxel, and several studies have demonstrated that low/no tau expression significantly correlated with better response to the taxane treatment, suggesting that levels of tau expression could have a predictive value in pre-selecting patient cohorts that are likely to benefit from the treatment. However, several other studies have found no correlation between tau expression and taxane response, introducing a controversy and precluding its wide use as a predictive biomarker. Based on the knowledge of tau biology accumulated thus far, in this review we attempt to critically analyze the studies that evaluated tau as a biomarker of taxane response. Further, we identify yet unknown aspects of tau biology understanding of which is necessary for improvement of development of tau as a biomarker of response and a target for increasing response to taxane treatment.

Low Dose Brain Irradiation Reduces Amyloid-β and Tau in 3xTg-AD Mice

We have previously reported that low doses of external beam ionizing irradiation reduced amyloid-β (Aβ) plaques and improved cognition in APP/PS1 mice. In this study we investigated the effects of radiation in an age-matched series of 3xTg-AD mice. Mice were hemibrain-irradiated with 5 fractions of 2 Gy and sacrificed 8 weeks after the end of treatment. Aβ and tau were assessed using immunohistochemistry and quantified using image analysis with Definiens Tissue Studio. We observed a significant reduction in Aβ plaque burden and tau staining; these two parameters were significantly correlated. This preliminary data is further support that low doses of radiation may be beneficial in Alzheimer's disease.

Anti-tau scFvs Targeted to the Cytoplasm or Secretory Pathway Variably Modify Pathology and Neurodegenerative Phenotypes

Immunotherapies designed to treat neurodegenerative tauopathies that primarily engage extracellular tau may have limited efficacy as tau is primarily intracellular. We generated tau-targeting single-chain variable fragments (scFvs) and intrabodies (iBs) from the phosphorylated tau-specific antibodies CP13 and PHF1 and the pan-tau antibody Tau5. Recombinant adeno-associated virus (rAAV) was utilized to express these antibody fragments in homozygous JNPL3 P301L tau mice. Two iBs (CP13i, PHF1i) and one scFv (PHF1s) abrogated tau pathology and delayed time to severe hindlimb paralysis. In a second tauopathy model (rTg4510), CP13i and PHF1i reduced tau pathology, but cognate scFvs did not. These data demonstrate that (1) disease-modifying efficacy does not require antibody effector functions, (2) the intracellular targeting of tau with phosphorylated tau-specific iBs is more effective than extracellular targeting with the scFvs, and (3) robust effects on tau pathology before neurodegeneration only resulted in modest disease modification as assessed by delay of severe motor phenotype.

Impact of Tau on Neurovascular Pathology in Alzheimer's Disease

Alzheimer's disease (AD) is a chronic neurodegenerative disorder and the most prevalent cause of dementia. The main cerebral histological hallmarks are represented by parenchymal insoluble deposits of amyloid beta (Aβ plaques) and neurofibrillary tangles (NFT), intracellular filamentous inclusions of tau, a microtubule-associated protein. It is well-established that cerebrovascular dysfunction is an early feature of AD pathology, but the detrimental mechanisms leading to blood vessel impairment and the associated neurovascular deregulation are not fully understood. In 90% of AD cases, Aβ deposition around the brain vasculature, known as cerebral amyloid angiopathy (CAA), alters blood brain barrier (BBB) essential functions. While the effects of vascular Aβ accumulation are better documented, the scientific community has only recently started to consider the impact of tau on neurovascular pathology in AD. Emerging compelling evidence points to transmission of neuronal tau to different brain cells, including astrocytes, as well as to the release of tau into brain interstitial fluids, which may lead to perivascular neurofibrillar tau accumulation and toxicity, affecting vessel architecture, cerebral blood flow (CBF), and vascular permeability. BBB integrity and functionality may therefore be impacted by pathological tau, consequentially accelerating the progression of the disease. Tau aggregates have also been shown to induce mitochondrial damage: it is known that tau impairs mitochondrial localization, distribution and dynamics, alters ATP and reactive oxygen species production, and compromises oxidative phosphorylation systems. In light of this previous knowledge, we postulate that tau can initiate neurovascular pathology in AD through mitochondrial dysregulation. In this review, we will explore the literature investigating tau pathology contribution to the malfunction of the brain vasculature and neurovascular unit, and its association with mitochondrial alterations and caspase activation, in cellular, animal, and human studies of AD and tauopathies.

The role of wild-type tau in Alzheimer's disease and related tauopathies

Tau oligomers have recently emerged as the principal toxic species in Alzheimer's disease (AD) and tauopathies. Tau oligomers are spontaneously self-assembled soluble tau proteins that are formed prior to fibrils, and they have been shown to play a central role in neuronal cell death and in the induction of neurodegeneration in animal models. As the therapeutic paradigm shifts to targeting toxic tau oligomers, this suggests the focus to study tau oligomerization in species that are less susceptible to fibrillization. While truncated and mutation containing tau as well as the isolated repeat domains are particularly prone to fibrillization, the wild-type (WT) tau proteins have been shown to be resistant to fibril formation in the absence of aggregation inducers. In this review, we will summarize and discuss the toxicity of WT tau both in vitro and in vivo, as well as its involvement in tau oligomerization and cell-to-cell propagation of pathology. Understanding the role of WT tau will enable more effective biomarker development and therapeutic discovery for treatment of AD and tauopathies.

Quantification of Tau Protein Lysine Methylation in Aging and Alzheimer's Disease

Tau is a microtubule-associated protein that normally interacts in monomeric form with the neuronal cytoskeleton. In Alzheimer's disease, however, it aggregates to form the structural component of neurofibrillary lesions. The transformation is controlled in part by age- and disease-associated post-translational modifications. Recently we reported that tau isolated from cognitively normal human brain was methylated on lysine residues, and that high-stoichiometry methylation depressed tau aggregation propensity in vitro. However, whether methylation stoichiometry reached levels needed to influence aggregation propensity in human brain was unknown. Here we address this problem using liquid chromatography-tandem mass spectrometry approaches and human-derived tau samples. Results revealed that lysine methylation was present in soluble tau isolated from cognitively normal elderly cases at multiple sites that only partially overlapped with the distributions reported for cognitively normal middle aged and AD cohorts, and that the quality of methylation shifted from predominantly dimethyl-lysine to monomethyl-lysine with aging and disease. However, bulk mol methylation/mol tau stoichiometries never exceeded 1 mol methyl group/mol tau protein. We conclude that lysine methylation is a physiological post-translational modification of tau protein that changes qualitatively with aging and disease, and that pharmacological elevation of tau methylation may provide a means for protecting against pathological tau aggregation.

Molecular and cellular mechanisms underlying the pathogenesis of Alzheimer's disease

Alzheimer’s disease (AD) is the most common neurodegenerative disorder seen in age-dependent dementia. There is currently no effective treatment for AD, which may be attributed in part to lack of a clear underlying mechanism. Studies within the last few decades provide growing evidence for a central role of amyloid β (Aβ) and tau, as well as glial contributions to various molecular and cellular pathways in AD pathogenesis. Herein, we review recent progress with respect to Aβ- and tau-associated mechanisms, and discuss glial dysfunction in AD with emphasis on neuronal and glial receptors that mediate Aβ-induced toxicity. We also discuss other critical factors that may affect AD pathogenesis, including genetics, aging, variables related to environment, lifestyle habits, and describe the potential role of apolipoprotein E (APOE), viral and bacterial infection, sleep, and microbiota. Although we have gained much towards understanding various aspects underlying this devastating neurodegenerative disorder, greater commitment towards research in molecular mechanism, diagnostics and treatment will be needed in future AD research.

Passive immunotherapy for N-truncated tau ameliorates the cognitive deficits in two mouse Alzheimer's disease models

Clinical and neuropathological studies have shown that tau pathology better correlates with the severity of dementia than amyloid plaque burden, making tau an attractive target for the cure of Alzheimer's disease. We have explored whether passive immunization with the 12A12 monoclonal antibody (26-36aa of tau protein) could improve the Alzheimer's disease phenotype of two well-established mouse models, Tg2576 and 3xTg mice. 12A12 is a cleavage-specific monoclonal antibody which selectively binds the pathologically relevant neurotoxic NH226-230 fragment (i.e. NH2htau) of tau protein without cross-reacting with its full-length physiological form(s). We found out that intravenous administration of 12A12 monoclonal antibody into symptomatic (6 months old) animals: (i) reaches the hippocampus in its biologically active (antigen-binding competent) form and successfully neutralizes its target; (ii) reduces both pathological tau and amyloid precursor protein/amyloidβ metabolisms involved in early disease-associated synaptic deterioration; (iii) improves episodic-like type of learning/memory skills in hippocampal-based novel object recognition and object place recognition behavioural tasks; (iv) restores the specific up-regulation of the activity-regulated cytoskeleton-associated protein involved in consolidation of experience-dependent synaptic plasticity; (v) relieves the loss of dendritic spine connectivity in pyramidal hippocampal CA1 neurons; (vi) rescues the Alzheimer's disease-related electrophysiological deficits in hippocampal long-term potentiation at the CA3-CA1 synapses; and (vii) mitigates the neuroinflammatory response (reactive gliosis). These findings indicate that the 20-22 kDa NH2-terminal tau fragment is crucial target for Alzheimer's disease therapy and prospect immunotherapy with 12A12 monoclonal antibody as safe (normal tau-preserving), beneficial approach in contrasting the early Amyloidβ-dependent and independent neuropathological and cognitive alterations in affected subjects.

Tauopathy-Associated Tau Fragment Ending at Amino Acid 224 Is Generated by Calpain-2 Cleavage

BACKGROUND

Tau aggregation in neurons and glial cells characterizes tauopathies as Alzheimer's disease (AD), progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD). Tau proteolysis has been proposed as a trigger for tau aggregation and tau fragments have been observed in brain and cerebrospinal fluid (CSF). Our group identified a major tau cleavage at amino acid (aa) 224 in CSF; N-terminal tau fragments ending at aa 224 (N-224) were significantly increased in AD and lacked correlation to total tau (t-tau) and phosphorylated tau (p-tau) in PSP and CBD.

OBJECTIVE

Previous studies have shown cleavage from calpain proteases at sites adjacent to aa 224. Our aim was to investigate if calpain-1 or -2 could be responsible for cleavage at aa 224.

METHODS

Proteolytic activity of calpain-1, calpain-2, and brain protein extract was assessed on a custom tau peptide (aa 220-228), engineered with fluorescence resonance energy transfer (FRET) technology. Findings were confirmed with in-gel trypsination and mass spectrometry (MS) analysis of brain-derived bands with proteolytic activity on the FRET substrate. Finally, knock-down of the calpain-2 catalytic subunit gene (CAPN2) was performed in a neuroblastoma cell line (SH-SY5Y).

RESULTS

Calpain-2 and brain protein extract, but not calpain-1, showed proteolytic activity on the FRET substrate. MS analysis of active gel bands revealed presence of calpain-2 subunits, but not calpain-1. Calpain-2 depletion and chemical inhibition suppressed proteolysis of the FRET substrate. CAPN2 knock-down caused a 76.4% reduction of N-224 tau in the cell-conditioned media.

CONCLUSIONS

Further investigation of the calpain-2 pathway in the pathogenesis of tauopathies is encouraged.

N-terminal tau truncation in the pathogenesis of Alzheimer's disease (AD): Developing a novel diagnostic and therapeutic approach

Tau truncation occurs at early stages during the development of human Alzheimer's disease (AD) and other tauopathy dementias. Tau cleavage, particularly in its N-terminal projection domain, is able to drive per se neurodegeneration, regardless of its pro-aggregative pathway(s) and in fragment(s)-dependent way. In this short review, we highlight the pathological relevance of the 20-22 kDa NH₂-truncated tau fragment which is endowed with potent neurotoxic "gain-of-function" action(s), both in vitro and in vivo. An extensive comment on its clinical value as novel progression/diagnostic biomarker and potential therapeutic target in the context of tau-mediated neurodegeneration is also provided.

Tau Secretion: Good and Bad for Neurons

In Alzheimer’s disease (AD), neurofibrillary tangles (NFTs), lesions composed of hyperphosphorylated and aggregated tau, spread from the transentorhinal cortex to the hippocampal formation and neocortex. Growing evidence indicates that tau pathology propagates trans-synaptically, implying that pathological tau released by pre-synaptic neurons is taken up by post-synaptic neurons where it accumulates and aggregates. Observations such as the presence of tau in the cerebrospinal fluid (CSF) from control individuals and in the CSF of transgenic mice overexpressing human tau before the detection of neuronal death indicate that tau can be secreted by neurons. The increase of tau in the CSF in pathological conditions such as AD suggests that tau secretion is enhanced and/or other secretory pathways take place when neuronal function is compromised. In physiological conditions, extracellular tau could exert beneficial effects as observed for other cytosolic proteins also released in the extracellular space. In such a case, blocking tau secretion could have negative effects on neurons unless the mechanism of tau secretion are different in physiological and pathological conditions allowing the prevention of pathological tau secretion without affecting the secretion of physiological tau. Furthermore, distinct extracellular tau species could be secreted in physiological and pathological conditions, species having the capacity to induce tau pathology being only secreted in the latter condition. In the present review, we will focus on the mechanisms and function of tau secretion in both physiological and pathological conditions and how this information can help to elaborate an efficient therapeutic strategy to prevent tau pathology and its propagation.

The Role of Antibodies and Their Receptors in Protection Against Ordered Protein Assembly in Neurodegeneration

Ordered assemblies of proteins are found in the postmortem brains of sufferers of several neurodegenerative diseases. The cytoplasmic microtubule associated protein tau and alpha-synuclein (αS) are found in an assembled state in Alzheimer's disease and Parkinson's disease, respectively. An accumulating body of evidence suggests a "prion-like" mechanism of spread of these assemblies through the diseased brain. Under this hypothesis, assembled variants of these proteins promote the conversion of native proteins to the assembled state. This likely inflicts pathology on cells of the brain through a toxic gain-of-function mechanism. Experiments in animal models of tau and αS pathology have demonstrated that the passive transfer of anti-tau or anti-αS antibodies induces a reduction in the levels of assembled proteins. This is further accompanied by improvements in neurological function and preservation of brain volume. Immunotherapy is therefore considered one of the brightest hopes as a therapeutic avenue in an area currently without disease-modifying therapy. Following a series of disappointing clinical trials targeting beta-amyloid, a peptide that accumulates in the extracellular spaces of the AD brain, attention is turning to active and passive immunotherapies that target tau and αS. However, there are several remaining uncertainties concerning the mechanism by which antibodies afford protection against self-propagating protein conformations. This review will discuss current understanding of how antibodies and their receptors can be brought to bear on proteins involved in neurodegeneration. Parallels will be made to antibody-mediated protection against classical viral infections. Common mechanisms that may contribute to protection against self-propagating protein conformations include blocking the entry of protein "seeds" to cells, clearance of immune complexes by microglia, and the intracellular protein degradation pathway initiated by cytoplasmic antibodies via the Fc receptor TRIM21. As with anti-viral immunity, protective mechanisms may be accompanied by the activation of immune signaling pathways and we will discuss the suitability of such activation in the neurological setting.

Learnings about the complexity of extracellular tau aid development of a blood-based screen for Alzheimer's disease

INTRODUCTION

The tau protein plays a central role in Alzheimer's disease (AD), and there is huge interest in measuring tau in blood and cerebrospinal fluid (CSF).

METHODS

We developed a set of immunoassays to measure tau in specimens from humans diagnosed based on current best clinical and CSF biomarker criteria.

RESULTS

In CSF, mid-region- and N-terminal-detected tau predominated and rose in disease. In plasma, an N-terminal assay (NT1) detected elevated levels of tau in AD and AD-mild cognitive impairment (MCI). Plasma NT1 measurements separated controls from AD-MCI (area under the curve [AUC] = 0.88) and AD (AUC = 0.96) in a discovery cohort and in a Validation Cohort (with AUCs = 0.79 and 0.75, respectively).

DISCUSSION

The forms of tau in CSF and plasma are distinct, but in each specimen type, the levels of certain fragments are increased in AD. Measurement of plasma NT1 tau should be aggressively pursued as a potential blood-based screening test for AD/AD-MCI.

A walk through tau therapeutic strategies

Tau neuronal and glial pathologies drive the clinical presentation of Alzheimer's disease and related human tauopathies. There is a growing body of evidence indicating that pathological tau species can travel from cell to cell and spread the pathology through the brain. Throughout the last decade, physiological and pathological tau have become attractive targets for AD therapies. Several therapeutic approaches have been proposed, including the inhibition of protein kinases or protein-3-O-(N-acetyl-beta-D-glucosaminyl)-L-serine/threonine Nacetylglucosaminyl hydrolase, the inhibition of tau aggregation, active and passive immunotherapies, and tau silencing by antisense oligonucleotides. New tau therapeutics, across the board, have demonstrated the ability to prevent or reduce tau lesions and improve either cognitive or motor impairment in a variety of animal models developing neurofibrillary pathology. The most advanced strategy for the treatment of human tauopathies remains immunotherapy, which has already reached the clinical stage of drug development. Tau vaccines or humanised antibodies target a variety of tau species either in the intracellular or extracellular spaces. Some of them recognise the amino-terminus or carboxy-terminus, while others display binding abilities to the proline-rich area or microtubule binding domains. The main therapeutic foci in existing clinical trials are on Alzheimer's disease, progressive supranuclear palsy and non-fluent primary progressive aphasia. Tau therapy offers a new hope for the treatment of many fatal brain disorders. First efficacy data from clinical trials will be available by the end of this decade.

A liquid chromatography tandem mass spectroscopy approach for quantification of protein methylation stoichiometry

Post-translational modifications are biologically important and wide-spread modulators of protein function. Although methods for detecting the presence of specific modifications are becoming established, approaches for quantifying their mol modification/mol protein stoichiometry are less well developed. Here we introduce a ratiometric, label-free, targeted liquid chromatography tandem mass spectroscopy-based method for estimating Lys and Arg methylation stoichiometry on post-translationally modified proteins. Methylated Lys and Arg were detected with limits of quantification at low fmol and with linearity extending from 20 to 5000 fmol. This level of sensitivity allowed estimation of methylation stoichiometry from microgram quantities of various proteins, including those derived from either recombinant or tissue sources. The method also disaggregated total methylation stoichiometry into its elementary mono-, di-, and tri-methylated residue components. In addition to being compatible with kinetic experiments of protein methylation, the approach will be especially useful for characterizing methylation states of proteins isolated from cells and tissues.

Pseudo-phosphorylation at AT8 epitopes regulates the tau truncation at aspartate 421

Tau pathology in Alzheimer's disease (AD) includes hyperphosphorylation and truncation of tau. Phosphorylation at S422 is found to suppress truncation of tau at D421 that leading to the generation of ΔTau. However, the interrelation between hyperphosphorylation and generation of ΔTau in AD remains elusive. In current study, staurosporine (Stau) induced ΔTau generation by caspases in SH-SY5Y cells with tau overexpression was found to be accompanied by a dramatic dephosphorylation at S422 and the epitope of the diagnostic antibody AT8 (S199 + S202 + T205), but a moderate dephosphorylation of PHF1 (S396 + S404) epitope. Therefore, to explore the effect of AT8 epitope on tau truncation, the residues in AT8 epitope were mutated to produce "pseudo-phosphorylated" (AT8E) or "pseudo-unphosphorylated" (AT8A) tau constructs. With Stau treatment, the generation of ΔTau from tau-AT8E was significantly attenuated comparing with that from tau-AT8A, which was S422-independent in that addition of S422A mutation still preserved this effect. Interestingly, this modulatory effect was able to be reversed by addition of PHF1E mutation. Moreover, treating the crude tau extracts with recombinant caspase-3 in vitro, also showed that ΔTau level was suppressed by AT8E, and potentiated by AT8E + PHF1E. The results primarily revealed the modulating effects of phosphorylation on ΔTau generation which may have potential implications in tau pathological processes and therapeutic intervention.

The Neurotoxic Role of Extracellular Tau Protein

Tauopathies are a class of neurodegenerative diseases associated with the microtubule-associated protein tau, with Alzheimer’s disease (AD) being the most prevalent related disorder. Neurofibrillary tangles (NFTs) are one of the neuropathological hallmarks present in the brains of AD patients. Because NFTs are aberrant intracellular inclusions formed by hyperphosphorylated tau, it was initially proposed that phosphorylated and/or aggregated intracellular tau protein was causative of neuronal death. However, recent studies suggest a toxic role for non-phosphorylated and non-aggregated tau when it is located in the brain extracellular space. In this work, we will discuss the neurotoxic role of extracellular tau as well its involvement in the spreading of tau pathologies.

AD-Related N-Terminal Truncated Tau Is Sufficient to Recapitulate In Vivo the Early Perturbations of Human Neuropathology: Implications for Immunotherapy

The NH₂tau 26-44 aa (i.e., NH₂htau) is the minimal biologically active moiety of longer 20-22-kDa NH₂-truncated form of human tau-a neurotoxic fragment mapping between 26 and 230 amino acids of full-length protein (htau40)-which is detectable in presynaptic terminals and peripheral CSF from patients suffering from AD and other non-AD neurodegenerative diseases. Nevertheless, whether its exogenous administration in healthy nontransgenic mice is able to elicit a neuropathological phenotype resembling human tauopathies has not been yet investigated. We explored the in vivo effects evoked by subchronic intracerebroventricular (i.c.v.) infusion of NH₂htau or its reverse counterpart into two lines of young (2-month-old) wild-type mice (C57BL/6 and B6SJL). Six days after its accumulation into hippocampal parenchyma, significant impairment in memory/learning performance was detected in NH₂htau-treated group in association with reduced synaptic connectivity and neuroinflammatory response. Compromised short-term plasticity in paired-pulse facilitation paradigm (PPF) was detected in the CA3/CA1 synapses from NH₂htau-impaired animals along with downregulation in calcineurin (CaN)-stimulated pCREB/c-Fos pathway(s). Importantly, these behavioral, synaptotoxic, and neuropathological effects were independent from the genetic background, occurred prior to frank neuronal loss, and were specific because no alterations were detected in the control group infused with its reverse counterpart. Finally, a 2.0-kDa peptide which biochemically and immunologically resembles the injected NH₂htau was endogenously detected in vivo, being present in hippocampal synaptosomal preparations from AD subjects. Given that the identification of the neurotoxic tau species is mandatory to develop a more effective tau-based immunological approach, our evidence can have important translational implications for cure of human tauopathies.

Detection of Aggregation-Competent Tau in Neuron-Derived Extracellular Vesicles

Progressive cerebral accumulation of tau aggregates is a defining feature of Alzheimer's disease (AD). A popular theory that seeks to explain the apparent spread of neurofibrillary tangle pathology proposes that aggregated tau is passed from neuron to neuron. Such a templated seeding process requires that the transferred tau contains the microtubule binding repeat domains that are necessary for aggregation. While it is not clear how a protein such as tau can move from cell to cell, previous reports have suggested that this may involve extracellular vesicles (EVs). Thus, measurement of tau in EVs may both provide insights on the molecular pathology of AD and facilitate biomarker development. Here, we report the use of sensitive immunoassays specific for full-length (FL) tau and mid-region tau, which we applied to analyze EVs from human induced pluripotent stem cell (iPSC)-derived neuron (iN) conditioned media, cerebrospinal fluid (CSF), and plasma. In each case, most tau was free-floating with a small component inside EVs. The majority of free-floating tau detected by the mid-region assay was not detected by our FL assays, indicating that most free-floating tau is truncated. Inside EVs, the mid-region assay also detected more tau than the FL assay, but the ratio of FL-positive to mid-region-positive tau was higher inside exosomes than in free solution. These studies demonstrate the presence of minute amounts of free-floating and exosome-contained FL tau in human biofluids. Given the potential for FL tau to aggregate, we conclude that further investigation of these pools of extracellular tau and how they change during disease is merited.

Prion-like Spreading in Tauopathies

Tau is a microtubule-associated protein that functions in regulating cytoskeleton dynamics, especially in neurons. Misfolded and aggregated forms of tau produce pathological structures in a number of neurodegenerative diseases, including Alzheimer's disease (AD) and tauopathy dementias. These disorders can present with a sporadic etiology, such as in AD, or a familial etiology, such as in some cases of frontotemporal dementia with parkinsonism. Notably, the pathological features of tau pathology in these diseases can be very distinct. For example, the tau pathology in corticobasal degeneration is distinct from that of an AD patient. A wealth of evidence has emerged within the last decade to suggest that the misfolded tau in tauopathies possesses prion-like features and that such features may explain the diverse characteristics of tauopathies. The prion-like concept for tauopathies arose initially from the observation that the progressive accumulation of tau pathology as the symptoms of AD progress seemed to follow anatomically linked pathways. Subsequent studies in cell and animal models revealed that misfolded tau can propagate from cell to cell and from region to region in the brain through direct neuroanatomical connections. Studies in these cell and mouse models have demonstrated that experimentally propagated forms of misfolded tau can exist as conformationally distinct "strains" with unique biochemical, morphological, and neuropathological characteristics. This review discusses the clinical, pathological, and genetic diversity of tauopathies and the discoveries underlying the emerging view that the unique features of clinically distinct tauopathies may be a reflection of the strain of misfolded tau that propagates in each disease.

Tau Proteolysis in the Pathogenesis of Tauopathies: Neurotoxic Fragments and Novel Biomarkers

With predictions showing that 131.5 million people worldwide will be living with dementia by 2050, an understanding of the molecular mechanisms underpinning disease is crucial in the hunt for novel therapeutics and for biomarkers to detect disease early and/or monitor disease progression. The metabolism of the microtubule-associated protein tau is altered in different dementias, the so-called tauopathies. Tau detaches from microtubules, aggregates into oligomers and neurofibrillary tangles, which can be secreted from neurons, and spreads through the brain during disease progression. Post-translational modifications exacerbate the production of both oligomeric and soluble forms of tau, with proteolysis by a range of different proteases being a crucial driver. However, the impact of tau proteolysis on disease progression has been overlooked until recently. Studies have highlighted that proteolytic fragments of tau can drive neurodegeneration in a fragment-dependent manner as a result of aggregation and/or transcellular propagation. Proteolytic fragments of tau have been found in the cerebrospinal fluid and plasma of patients with different tauopathies, providing an opportunity to develop these fragments as novel disease progression biomarkers. A range of therapeutic strategies have been proposed to halt the toxicity associated with proteolysis, including reducing protease expression and/or activity, selectively inhibiting protease-substrate interactions, and blocking the action of the resulting fragments. This review highlights the importance of tau proteolysis in the pathogenesis of tauopathies, identifies putative sites during tau fragment-mediated neurodegeneration that could be targeted therapeutically, and discusses the potential use of proteolytic fragments of tau as biomarkers for different tauopathies.

Remodeling Tau and Prion Proteins Using Nanochaperons

There is increasing evidence that tau protein behaves in a prion-like manner in tauopathy. The stabilization of tau protein using a small molecular compound can limit tauopathy associated morbidity that advances with ageing. Here, a lab-on-a-chip experiment is reported, where gold citrate nanoparticles (5 nm, AuNPs) can remodel mutant tau protein (P301L) and prion, thus resolving the mutant tau- and prion-mediated impairment of kinesin cargo transport on microtubules. It is found that tau protein is overexpressed in Alzheimer's disease (AD) patient serum samples and the tau conformational change can also be affected in human serum samples of AD when treated with AuNPs ex vivo. Similarly, AuNPs reorganizing the prion protein and inducing conformational changes of prions in AD serum have been observed, while having no effect on alpha-synuclein in Parkinson patient serum. The mapping of AD serum mediated traffic jams, using particle tracking and mean square displacement analysis, and the observed recovery of uninterrupted processive motor functions by AuNP treatment show that kinesin cargo assays might be a useful method for future ex vivo validation of a targeted therapy against tauopathy before administration, a viable option to combat various neurodegenerative disorders arising from the susceptibility of amyloidogenic proteins toward aggregation.

Amyloidogenesis of Tau protein

The role of microtubule-associated protein Tau in neurodegeneration has been extensively investigated since the discovery of Tau amyloid aggregates in the brains of patients with Alzheimer's disease (AD). The process of formation of amyloid fibrils is known as amyloidogenesis and attracts much attention as a potential target in the prevention and treatment of neurodegenerative conditions linked to protein aggregation. Cerebral deposition of amyloid aggregates of Tau is observed not only in AD but also in numerous other tauopathies and prion diseases. Amyloidogenesis of intrinsically unstructured monomers of Tau can be triggered by mutations in the Tau gene, post-translational modifications, or interactions with polyanionic molecules and aggregation-prone proteins/peptides. The self-assembly of amyloid fibrils of Tau shares a number of characteristic features with amyloidogenesis of other proteins involved in neurodegenerative diseases. For example, in vitro experiments have demonstrated that the nucleation phase, which is the rate-limiting stage of Tau amyloidogenesis, is shortened in the presence of fragmented preformed Tau fibrils acting as aggregation templates ("seeds"). Accordingly, Tau aggregates released by tauopathy-affected neurons can spread the neurodegenerative process in the brain through a prion-like mechanism, originally described for the pathogenic form of prion protein. Moreover, Tau has been shown to form amyloid strains-structurally diverse self-propagating aggregates of potentially various pathological effects, resembling in this respect prion strains. Here, we review the current literature on Tau aggregation and discuss mechanisms of propagation of Tau amyloid in the light of the prion-like paradigm.

Extracellular truncated tau causes early presynaptic dysfunction associated with Alzheimer's disease and other tauopathies

The largest part of tau secreted from AD nerve terminals and released in cerebral spinal fluid (CSF) is C-terminally truncated, soluble and unaggregated supporting potential extracellular role(s) of NH2 -derived fragments of protein on synaptic dysfunction underlying neurodegenerative tauopathies, including Alzheimer's disease (AD). Here we show that sub-toxic doses of extracellular-applied human NH2 tau 26-44 (aka NH 2 htau) -which is the minimal active moiety of neurotoxic 20-22kDa peptide accumulating in vivo at AD synapses and secreted into parenchyma- acutely provokes presynaptic deficit in K+ -evoked glutamate release on hippocampal synaptosomes along with alteration in local Ca2+ dynamics. Neuritic dystrophy, microtubules breakdown, deregulation in presynaptic proteins and loss of mitochondria located at nerve endings are detected in hippocampal cultures only after prolonged exposure to NH 2 htau. The specificity of these biological effects is supported by the lack of any significant change, either on neuronal activity or on cellular integrity, shown by administration of its reverse sequence counterpart which behaves as an inactive control, likely due to a poor conformational flexibility which makes it unable to dynamically perturb biomembrane-like environments. Our results demonstrate that one of the AD-relevant, soluble and secreted N-terminally truncated tau forms can early contribute to pathology outside of neurons causing alterations in synaptic activity at presynaptic level, independently of overt neurodegeneration.

Immunological memory to hyperphosphorylated tau in asymptomatic individuals

Several reports have described the presence of antibodies against Alzheimer’s disease-associated hyperphosphorylated forms of tau in serum of healthy individuals. To characterize the specificities that can be found, we interrogated peripheral IgG⁺ memory B cells from asymptomatic blood donors for reactivity to a panel of phosphorylated tau peptides using a single-cell screening assay. Antibody sequences were recovered, cloned, and expressed as full-length IgGs. In total, 52 somatically mutated tau-binding antibodies were identified, corresponding to 35 unique clonal families. Forty-one of these antibodies recognize epitopes in the proline-rich and C-terminal domains, and binding of 26 of these antibodies is strictly phosphorylation dependent. Thirteen antibodies showed inhibitory activity in a P301S lysate seeded in vitro tau aggregation assay. Two such antibodies, CBTAU-7.1 and CBTAU-22.1, which bind to the proline-rich and C-terminal regions of tau, respectively, were characterized in more detail. CBTAU-7.1 recognizes an epitope that is similar to that of murine anti-PHF antibody AT8, but has different phospho requirements. Both CBTAU-7.1 and CBTAU-22.1 detect pathological tau deposits in post-mortem brain tissue. CBTAU-7.1 reveals a similar IHC distribution pattern as AT8, immunostaining (pre)tangles, threads, and neuritic plaques. CBTAU-22.1 shows selective detection of neurofibrillary changes by IHC. Taken together, these results suggest the presence of an ongoing antigen-driven immune response against tau in healthy individuals. The wide range of specificities to tau suggests that the human immune repertoire may contain antibodies that can serve as biomarkers or be exploited for therapy.

Cytosolic Fc receptor TRIM21 inhibits seeded tau aggregation

Alzheimer's disease (AD) and other neurodegenerative disorders are associated with the cytoplasmic aggregation of microtubule-associated protein tau. Recent evidence supports transcellular transfer of tau misfolding (seeding) as the mechanism of spread within an affected brain, a process reminiscent of viral infection. However, whereas microbial pathogens can be recognized as nonself by immune receptors, misfolded protein assemblies evade detection, as they are host-derived. Here, we show that when misfolded tau assemblies enter the cell, they can be detected and neutralized via a danger response mediated by tau-associated antibodies and the cytosolic Fc receptor tripartite motif protein 21 (TRIM21). We developed fluorescent, morphology-based seeding assays that allow the formation of pathological tau aggregates to be measured in situ within 24 h in the presence of picomolar concentrations of tau seeds. We found that anti-tau antibodies accompany tau seeds into the cell, where they recruit TRIM21 shortly after entry. After binding, TRIM21 neutralizes tau seeds through the activity of the proteasome and the AAA ATPase p97/VCP in a similar manner to infectious viruses. These results establish that intracellular antiviral immunity can be redirected against host-origin endopathogens involved in neurodegeneration.

Formation, release, and internalization of stable tau oligomers in cells

Tau is a neuronal microtubule-binding protein that, in Alzheimer's disease and other neurodegenerative diseases, can form oligomeric and large fibrillar aggregates, which deposit in neurofibrillary tangles. Tau's physiological state of multimerization appears to vary across conditions, and a stable dimeric form of soluble tau has been suggested from experiments using recombinant tau in vitro. We tested if tau dimerization or oligomerization, also occurs in cells, and if soluble tau oligomers are relevant for the release and internalization of tau. We developed a sensitive tau split-luciferase assay to show the rapid intracellular formation of stable tau dimers that are released and taken up by cells. Our data further suggest that tau dimerization can be accelerated slightly by aggregation catalysts. We conclude that tau oligomers are a stable physiological form of tau, and that tau oligomerization does not necessarily lead to tau aggregation.

Tunneling nanotube (TNT)-mediated neuron-to neuron transfer of pathological Tau protein assemblies

A given cell makes exchanges with its neighbors through a variety of means ranging from diffusible factors to vesicles. Cells use also tunneling nanotubes (TNTs), filamentous-actin-containing membranous structures that bridge and connect cells. First described in immune cells, TNTs facilitate HIV-1 transfer and are found in various cell types, including neurons. We show that the microtubule-associated protein Tau, a key player in Alzheimer’s disease, is a bona fide constituent of TNTs. This is important because Tau appears beside filamentous actin and myosin 10 as a specific marker of these fine protrusions of membranes and cytosol that are difficult to visualize. Furthermore, we observed that exogenous Tau species increase the number of TNTs established between primary neurons, thereby facilitating the intercellular transfer of Tau fibrils. In conclusion, Tau may contribute to the formation and function of the highly dynamic TNTs that may be involved in the prion-like propagation of Tau assemblies.

Expanding the Repertoire of Biomarkers for Alzheimer's Disease: Targeted and Non-targeted Approaches

The first biofluid markers developed for Alzheimer’s disease (AD) used targeted approaches for discovery. These initial biomarkers were directed at key protein constituents of the hallmark brain lesions in AD. Biomarkers for plaques targeted the amyloid beta protein (Aβ) and for tangles, the microtubule-associated protein tau. Cerebrospinal fluid levels of Aβ and tau have excellent diagnostic utility and can be used to monitor aspects of therapeutic development. Recent research has extended our current concepts of AD, which now include a slow buildup of pathology during a long pre-symptomatic period, a complex cascade of pathological pathways in the brain that may accelerate once symptoms develop, the potential of aggregated proteins to spread across brain pathways, and interactions with vascular and other age-associated brain pathologies. There are many potential roles for biomarkers within this landscape. A more diverse set of biomarkers would provide a better picture of the staging and state of pathological events in the brain across the stages of AD. The aim of this review is to focus on methods of biomarker discovery that may help to expand the currently accepted biomarkers. Opportunities and approaches for targeted and non-targeted (or −omic) biomarker discovery are highlighted, with examples from recent studies. How biomarker discoveries can be developed and integrated to become useful tools in diagnostic and therapeutic efforts is discussed.

Tau in physiology and pathology

Tau is a microtubule-associated protein that has a role in stabilizing neuronal microtubules and thus in promoting axonal outgrowth. Structurally, tau is a natively unfolded protein, is highly soluble and shows little tendency for aggregation. However, tau aggregation is characteristic of several neurodegenerative diseases known as tauopathies. The mechanisms underlying tau pathology and tau-mediated neurodegeneration are debated, but considerable progress has been made in the field of tau research in recent years, including the identification of new physiological roles for tau in the brain. Here, we review the expression, post-translational modifications and functions of tau in physiology and in pathophysiology.

Extracellular Membrane Vesicles as Vehicles for Brain Cell-to-Cell Interactions in Physiological as well as Pathological Conditions

Extracellular vesicles are involved in a great variety of physiological events occurring in the nervous system, such as cross talk among neurons and glial cells in synapse development and function, integrated neuronal plasticity, neuronal-glial metabolic exchanges, and synthesis and dynamic renewal of myelin. Many of these EV-mediated processes depend on the exchange of proteins, mRNAs, and noncoding RNAs, including miRNAs, which occurs among glial and neuronal cells. In addition, production and exchange of EVs can be modified under pathological conditions, such as brain cancer and neurodegeneration. Like other cancer cells, brain tumours can use EVs to secrete factors, which allow escaping from immune surveillance, and to transfer molecules into the surrounding cells, thus transforming their phenotype. Moreover, EVs can function as a way to discard material dangerous to cancer cells, such as differentiation-inducing proteins, and even drugs. Intriguingly, EVs seem to be also involved in spreading through the brain of aggregated proteins, such as prions and aggregated tau protein. Finally, EVs can carry useful biomarkers for the early diagnosis of diseases. Herein we summarize possible roles of EVs in brain physiological functions and discuss their involvement in the horizontal spreading, from cell to cell, of both cancer and neurodegenerative pathologies.

C-Terminally Truncated Forms of Tau, But Not Full-Length Tau or Its C-Terminal Fragments, Are Released from Neurons Independently of Cell Death

Recent evidence suggests that tau aggregation may spread via extracellular release and subsequent uptake by synaptically connected neurons, but little is known about the processes by which tau is released or the molecular forms of extracellular tau. To gain insight into the nature of extracellular tau, we used highly sensitive ELISAs, which, when used in tandem, are capable of differentiating between full-length (FL) tau, mid-region-bearing fragments, and C-terminal (CT) fragments. We applied these assays to the systematic study of the conditioned media of N2a cells, induced pluripotent stem cell-derived human cortical neurons, and primary rat cortical neurons, each of which was carefully assessed for viability. In all three neuronal models, the bulk of extracellular tau was free-floating and unaggregated and <0.2% was encapsulated in exosomes. Although most intracellular tau was FL, the majority of extracellular tau was CT truncated and appeared to be released both actively by living neurons and passively by dead cells. In contrast, only a small amount of extracellular tau was aggregation-competent tau (i.e., contained the microtubule-binding regions) and this material appears to be released solely due to a low level of cell death that occurs in all cell culture systems. Importantly, amyloid β-protein (Aβ)-induced neuronal compromise significantly increased the quantity of all forms of extracellular tau, but the presence of Aβ before detectable cell compromise did not increase extracellular tau. Collectively, these results suggest that factors that induce neuronal death are likely to be necessary to initiate the extracellular spread of tau aggregation.

SIGNIFICANCE STATEMENT

Recent studies suggest that the transfer of tau between neurons underlies the characteristic spatiotemporal progression of neurofibrillary pathology. We searched for tau in the conditioned medium of N2a cells, induced pluripotent stem cell-derived human cortical neurons, and primary rat cortical neurons and analyzed the material present using four different tau ELISAs. We demonstrate that the majority of tau released from healthy neurons is C-terminally truncated and lacks the microtubule-binding region (MTBR) thought necessary for self-aggregation. A small amount of MTBR-containing tau is present outside of cells, but this appears to be solely due to cell death. Therefore, if propagation of tau aggregation is mediated by extracellular tau, our findings suggest that neuronal compromise is required to facilitate this process.