Distinct FTDP-17 missense mutations in tau produce tau aggregates and other pathological phenotypes in transfected CHO cells

Tau mutation S356T in the three repeat isoform leads to microtubule dysfunction and promotes prion-like seeded aggregation

Tauopathies are a group of neurodegenerative diseases, which include frontotemporal dementia (FTD) and Alzheimer's disease (AD), broadly defined by the development of tau brain aggregates. Both missense and splicing tau mutations can directly cause early onset FTD. Tau protein is a microtubule-associated protein that stabilizes and regulates microtubules, but this function can be disrupted in disease states. One contributing factor is the balance of different tau isoforms, which can be categorized into either three repeat (3R) or four repeat (4R) isoforms based on the number of microtubule-binding repeats that are expressed. Imbalance of 3R and 4R isoforms in either direction can cause FTD and neurodegeneration. There is also increasing evidence that 3R tauopathies such as Pick's disease form tau aggregates predominantly comprised of 3R isoforms and these can present differently from 4R and mixed 3R/4R tauopathies. In this study, multiple mutations in 3R tau were assessed for MT binding properties and prion-like aggregation propensity. Different missense tau mutations showed varying effects on MT binding depending on molecular location and properties. Of the mutations that were surveyed, S356T tau is uniquely capable of prion-like seeded aggregation and forms extensive Thioflavin positive aggregates. This unique prion-like tau strain will be useful to model 3R tau aggregation and will contribute to the understanding of diverse presentations of different tauopathies.

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

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

Systems Medicine as a Strategy to Deal with Alzheimer's Disease

The traits of Alzheimer's disease (AD) include amyloid plaques made of Aβ1-40 and Aβ1-42, and neurofibrillary tangles by the hyperphosphorylation of tau protein. AD is a complex disorder that is heterogenous in genetical, neuropathological, and clinical contexts. Current available therapeutics are unable to cure AD. Systems medicine is a strategy by viewing the body as a whole system, taking into account each individual's unique health profile, provide treatment and associated nursing care clinically for the patient, aiming for precision. Since the onset of AD can lead towards cognitive impairment, it is vital to intervene and diagnose early and prevent further progressive loss of neurons. Moreover, as the individual's brain functions are impaired due to neurodegeneration in AD, it is essential to reconstruct the neurons or brain cells to enable normal brain functions. Although there are different subtypes of AD due to varied pathological lesions, in the majority cases of AD, neurodegeneration and severe brain atrophy develop at the chronic stage. Novel approaches including RNA based gene therapy, stem cell based technology, bioprinting technology, synthetic biology for brain tissue reconstruction are researched in recent decades in the hope to decrease neuroinflammation and restore normal brain function in individuals of AD. Systems medicine include the prevention of disease, diagnosis and treatment by viewing the individual's body as a whole system, along with systems medicine based nursing as a strategy against AD that should be researched further.

Towards a Mechanistic Model of Tau-Mediated Pathology in Tauopathies: What Can We Learn from Cell-Based In Vitro Assays?

Tauopathies are a group of neurodegenerative diseases characterized by the hyperphosphorylation and deposition of tau proteins in the brain. In Alzheimer’s disease, and other related tauopathies, the pattern of tau deposition follows a stereotypical progression between anatomically connected brain regions. Increasing evidence suggests that tau behaves in a “prion-like” manner, and that seeding and spreading of pathological tau drive progressive neurodegeneration. Although several advances have been made in recent years, the exact cellular and molecular mechanisms involved remain largely unknown. Since there are no effective therapies for any tauopathy, there is a growing need for reliable experimental models that would provide us with better knowledge and understanding of their etiology and identify novel molecular targets. In this review, we will summarize the development of cellular models for modeling tau pathology. We will discuss their different applications and contributions to our current understanding of the “prion-like” nature of pathological tau.

Heparan Sulfate Proteoglycans (HSPGs) Serve as the Mediator Between Monomeric Tau and Its Subsequent Intracellular ERK1/2 Pathway Activation

The conversion of soluble tau protein to insoluble, hyperphosphorylated neurofibrillary tangles (NFTs) is a major hallmark leading to neuronal death observed in neurodegenerative tauopathies. Unlike NFTs, the involvement of monomeric tau in the progression of tau pathology has been less investigated. Using live-cell confocal microscopy and flow cytometry, we demonstrate that soluble 0N4R monomers were rapidly endocytosed by SH-SY5Y and C6 glioma cells via actin-dependent macropinocytosis. Further, cellular endocytosis of monomeric tau has been demonstrated to be HSPG-dependent, as shown in C6 glial cells with genetic knockouts of xylosyltransferase-1-a key enzyme in HSPG synthesis-with a reduced level of tau uptake. Tau internalization subsequently triggers ERK1/2 activation and therefore, the upregulation of IL-6 and IL-1β. The role of ERK1/2 in regulating the levels of pro-inflammatory gene transcripts was confirmed by inhibiting the MEK-ERK1/2 signaling pathway, which led to the attenuated IL-6 and IL-1β expressions but not that of TNF-α. Moreover, as a key regulator of tau internalization, LRP1 (low-density lipoprotein receptor-related protein 1) levels were downregulated in response to monomeric tau added to C6 cells, while it was upregulated in HSPG-deficient cells, suggesting that the involvement of LRP1 in tau uptake depends on the presence of HSPGs on the cell surface. The subsequent LRP1 knockdown experiment we performed shows that LRP1 deficiency leads to an attenuated propensity for tau uptake and further elevated IL-6 gene expression. Collectively, our data suggest that tau has multiple extracellular binding partners that mediate its internalization through distinct mechanisms. Additionally, this study demonstrates the important role of both HSPGs and LRP1 in regulating cellular immune responses to tau protein monomers, providing a novel target for alleviating the neuroinflammatory environment before the formation of neurofibrillary tangles.

Pathogenic MAPT mutations Q336H and Q336R have isoform-dependent differences in aggregation propensity and microtubule dysfunction

Tauopathies are a group of heterogeneous neurodegenerative disorders characterized by brain deposition of tau inclusions. These insidious disorders include Alzheimer's disease and frontotemporal dementia, the two leading causes of dementia. Mutations in the microtubule-associated protein tau (MAPT) gene lead to familial forms of frontotemporal dementia. Previously, we used cell-based assays to screen over 20 missense tau mutations and found that decreased microtubule (MT) binding affinity was the most shared property. As a break from this trend, the MAPT mutations Q336H and Q336R are thought to promote MT assembly rather than inhibit it based on in vitro studies. Q336H and Q336R MAPT mutations also cause early onset frontotemporal dementia with Pick bodies, which are mostly composed of 3R tau isoforms. To provide further insights on the pathobiology of these mutations, we assessed Q336H and Q336R tau mutants for aggregation propensity and MT binding in cell-based assays in the context of both 0N3R and 0N4R tau isoforms. Q336R tau was prone to prion-like seeded aggregation but both Q336H and Q336R tau led to increased MT binding. Additionally, we found that different tau isoforms with these mutations heterogeneously regulate different MT subpopulations of tyrosinated and acetylated MTs, markers of newly formed MTs and stable MTs. The Q336H and Q336R tau mutations may exemplify an alternative mechanism where pathogenic tau can bind MTs with higher affinity and hyperstabilize MTs, which prevent proper MT regulation and homeostasis.

Asparagine residue 368 is involved in Alzheimer's disease tau strain-specific aggregation

In tauopathies, tau forms pathogenic fibrils with distinct conformations (termed "tau strains") and acts as an aggregation "seed" templating the conversion of normal tau into isomorphic fibrils. Previous research showed that the aggregation core of tau fibril covers the C-terminal region (243-406 amino acids (aa)) and differs among the diseases. However, the mechanisms by which distinct fibrous structures are formed and inherited via templated aggregation are still unknown. Here, we sought to identify the key sequences of seed-dependent aggregation. To identify sequences for which deletion reduces tau aggregation, SH-SY5Y cells expressing a series of 10 partial deletion (Del 1-10, covering 244-400 aa) mutants of tau-CTF24 (243-441 aa) were treated with tau seeds prepared from a different tauopathy patient's brain (Alzheimer's disease, progressive supranuclear palsy, and corticobasal degeneration) or recombinant tau, and then seed-dependent tau aggregation was assessed biochemically. We found that the Del 8 mutant lacking 353-368 aa showed significantly decreased aggregation in both cellular and in vitro models. Furthermore, to identify the minimum sequence responsible for tau aggregation, we systematically repeated cellular tau aggregation assays for the delineation of shorter deletion sites and revealed that Asn-368 mutation suppressed tau aggregation triggered by an AD tau seed, but not using other tauopathy seeds. Our study suggested that 353-368 aa is a novel aggregation-responsible sequence other than PHF6 and PHF6*, and within this sequence, the Asn-368 residue plays a role in strain-specific tau aggregation in different tauopathies.

Tau Ser208 phosphorylation promotes aggregation and reveals neuropathologic diversity in Alzheimer's disease and other tauopathies

Tau protein abnormally aggregates in tauopathies, a diverse group of neurologic diseases that includes Alzheimer’s disease (AD). In early stages of disease, tau becomes hyperphosphorylated and mislocalized, which can contribute to its aggregation and toxicity. We demonstrate that tau phosphorylation at Ser208 (pSer208) promotes microtubule dysfunction and tau aggregation in cultured cells. Comparative assessment of the epitopes recognized by antibodies AT8, CP13, and 7F2 demonstrates that CP13 and 7F2 are specific for tau phosphorylation at Ser202 and Thr205, respectively, independently of the phosphorylation state of adjacent phosphorylation sites. Supporting the involvement of pSer208 in tau pathology, a novel monoclonal antibody 3G12 specific for tau phosphorylation at Ser208 revealed strong reactivity of tau inclusions in the brains of PS19 and rTg4510 transgenic mouse models of tauopathy. 3G12 also labelled neurofibrillary tangles in brains of patients with AD but revealed differential staining compared to CP13 and 7F2 for other types of tau pathologies such as in neuropil threads and neuritic plaques in AD, tufted astrocytes in progressive supranuclear palsy and astrocytic plaques in corticobasal degeneration. These results support the hypothesis that tau phosphorylation at Ser208 strongly contributes to unique types of tau aggregation and may be a reliable marker for the presence of mature neurofibrillary tangles.

Unconventional Secretion Mediates the Trans-cellular Spreading of Tau

The progressive deposition of misfolded hyperphosphorylated tau is a pathological hallmark of tauopathies, including Alzheimer's disease. However, the underlying molecular mechanisms governing the intercellular spreading of tau species remain elusive. Here, we show that full-length soluble tau is unconventionally secreted by direct translocation across the plasma membrane. Increased secretion is favored by tau hyperphosphorylation, which provokes microtubule detachment and increases the availability of free protein inside cells. Using a series of binding assays, we show that free tau interacts with components enriched at the inner leaflet of the plasma membrane, finally leading to its translocation across the plasma membrane mediated by sulfated proteoglycans. We provide further evidence that secreted soluble tau species spread trans-cellularly and are sufficient for the induction of intracellular tau aggregation in adjacent cells. Our study demonstrates the mechanistic details of tau secretion and provides insights into the initiation and progression of tau pathology.

Impaired tau-microtubule interactions are prevalent among pathogenic tau variants arising from missense mutations

tau is a microtubule (MT)-associated protein that promotes tubulin assembly and stabilizes MTs by binding longitudinally along the MT surface. tau can aberrantly aggregate into pathological inclusions that define Alzheimer's disease, frontotemporal dementias, and other tauopathies. A spectrum of missense mutations in the tau-encoding gene microtubule-associated protein tau (MAPT) can cause frontotemporal dementias. tau aggregation is postulated to spread by a prion-like mechanism. Using a cell-based inclusion seeding assay, we recently reported that only a few tau variants are intrinsically prone to this type of aggregation. Here, we extended these studies to additional tau mutants and investigated their MT binding properties in mammalian cell-based assays. A limited number of tau variants exhibited modest aggregation propensity in vivo, but most tau mutants did not aggregate. Reduced MT binding appeared to be the most common dysfunction for the majority of tau variants due to missense mutations, implying that MT-targeting therapies could potentially be effective in the management of tauopathies.

β-catenin aggregation in models of ALS motor neurons: GSK3β inhibition effect and neuronal differentiation

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by motor neuron death. A 20% of familial ALS cases are associated with mutations in the gene coding for superoxide dismutase 1 (SOD1). The accumulation of abnormal aggregates of different proteins is a common feature in motor neurons of patients and transgenic ALS mice models, which are thought to contribute to disease pathogenesis. Developmental morphogens, such as the Wnt family, regulate numerous features of neuronal physiology in the adult brain and have been implicated in neurodegeneration. β-catenin is a central mediator of both, Wnt signaling activity and cell-cell interactions. We previously reported that the expression of mutant SOD1 in the NSC34 motor neuron cell line decreases basal Wnt pathway activity, which correlates with cytosolic β-catenin accumulation and impaired neuronal differentiation. In this work, we aimed a deeper characterization of β-catenin distribution in models of ALS motor neurons. We observed extensive accumulation of β-catenin supramolecular structures in motor neuron somas of pre-symptomatic mutant SOD1 mice. In cell-cell appositional zones of NSC34 cells expressing mutant SOD1, β-catenin displays a reduced co-distribution with E-cadherin accompanied by an increased association with the gap junction protein Connexin-43; these findings correlate with impaired intercellular adhesion and exacerbated cell coupling. Remarkably, pharmacological inhibition of the glycogen synthase kinase-3β (GSK3β) in both NSC34 cell lines reverted both, β-catenin aggregation and the adverse effects of mutant SOD1 expression on neuronal differentiation. Our findings suggest that early defects in β-catenin distribution could be an underlying factor affecting the onset of neurodegeneration in familial ALS.

T-complex protein 1-ring complex enhances retrograde axonal transport by modulating tau phosphorylation

The cytosolic chaperonin T-complex protein (TCP) 1-ring complex (TRiC) has been shown to exert neuroprotective effects on axonal transport through clearance of mutant Huntingtin (mHTT) in Huntington's disease. However, it is presently unknown if TRiC also has any effect on axonal transport in wild-type neurons. Here, we examined how TRiC impacted the retrograde axonal transport of brain-derived neurotrophic factor (BDNF). We found that expression of a single TRiC subunit significantly enhanced axonal transport of BDNF, leading to an increase in instantaneous velocity with a concomitant decrease in pauses for retrograde BDNF transport. The transport enhancing effect by TRiC was dependent on endogenous tau expression because no effect was seen in neurons from tau knockout mice. We showed that TRiC regulated the level of cyclin-dependent kinase 5 (CDK5)/p35 positively, contributing to TRiC-mediated tau phosphorylation (ptau). Expression of a single TRiC subunit increased the level of ptau while downregulation of the TRiC complex decreased ptau. We further demonstrated that TRiC-mediated increase in ptau induced detachment of tau from microtubules. Our study has thus revealed that TRiC-mediated increase in tau phosphorylation impacts retrograde axonal transport.

Formulated Chinese Medicine Shaoyao Gancao Tang Reduces Tau Aggregation and Exerts Neuroprotection through Anti-Oxidation and Anti-Inflammation

Misfolded tau proteins induce accumulation of free radicals and promote neuroinflammation by activating microglia-releasing proinflammatory cytokines, leading to neuronal cell death. Traditional Chinese herbal medicines (CHMs) have been widely used in clinical practice to treat neurodegenerative diseases associated with oxidative stress and neuroinflammation. This study examined the neuroprotection effects of formulated CHMs Bai-Shao (made of Paeonia lactiflora), Gan-Cao (made of Glycyrrhiza uralensis), and Shaoyao Gancao Tang (SG-Tang, made of P. lactiflora and G. uralensis at 1 : 1 ratio) in cell model of tauopathy. Our results showed that SG-Tang displayed a greater antioxidative and antiaggregation effect than Bai-Shao and Gan-Cao and a stronger anti-inflammatory activity than Bai-Shao but similar to Gan-Cao. In inducible 293/SH-SY5Y cells expressing proaggregant human tau repeat domain (ΔK280 tau_RD), SG-Tang reduced tau misfolding and reactive oxygen species (ROS) level in ΔK280 tau_RD 293 cells and promoted neurite outgrowth in ΔK280 tau_RD SH-SY5Y cells. Furthermore, SG-Tang displayed anti-inflammatory effects by reducing nitric oxide (NO) production in mouse BV-2 microglia and increased cell viability of ΔK280 tau_RD-expressing SH-SY5Y cells inflamed by BV-2 conditioned medium. To uncover the neuroprotective mechanisms of SG-Tang, apoptosis protein array analysis of inflamed tau expressing SH-SY5Y cells was conducted and the suppression of proapoptotic proteins was confirmed. In conclusion, SG-Tang displays neuroprotection by exerting antioxidative and anti-inflammatory activities to suppress neuronal apoptosis in human tau cell models. The study results lay the base for future applications of SG-Tang on tau animal models to validate its effect of reducing tau misfolding and potential disease modification.

Minimalistic in vitro systems for investigating tau pathology

Neurofibrillary tangles, formed of hyperphosphorylated, misfolded tau accumulations, are a pathological hallmark of neurodegenerative diseases such as Alzheimer's disease (AD) and frontotemporal dementia. The neuroanatomical localisation of tau pathology in AD brains of different disease stages suggests that tau tangle pathology is spreading throughout the brain along connected neuronal circuits. Pathogenic tau can act as a prion-like seed, inducing the misfolding of native tau and leading to disease propagation throughout the brain. However, it is not yet fully understood how tau spreads between individual neurons or brain regions. Here, we review the models for investigating tau propagation in vitro, and summarise the findings from key studies into the mechanisms of tau pathology propagation in disease.

Tau Fibril Formation in Cultured Cells Compatible with a Mouse Model of Tauopathy

Neurofibrillary tangles composed of hyperphosphorylated tau protein are primarily neuropathological features of a number of neurodegenerative diseases collectively termed tauopathy. To understand the mechanisms underlying the cause of tauopathy, precise cellular and animal models are required. Recent data suggest that the transient introduction of exogenous tau can accelerate the development of tauopathy in the brains of non-transgenic and transgenic mice expressing wild-type human tau. However, the transmission mechanism leading to tauopathy is not fully understood. In this study, we developed cultured-cell models of tauopathy representing a human tauopathy. Neuro2a (N2a) cells containing propagative tau filaments were generated by introducing purified tau fibrils. These cell lines expressed full-length (2N4R) human tau and the green fluorescent protein (GFP)-fused repeat domain of tau with P301L mutation. Immunocytochemistry and super-resolution microscopic imaging revealed that tau inclusions exhibited filamentous morphology and were composed of both full-length and repeat domain fragment tau. Live-cell imaging analysis revealed that filamentous tau inclusions are transmitted to daughter cells, resulting in yeast-prion-like propagation. By a standard method of tau preparation, both full-length tau and repeat domain fragments were recovered in sarkosyl insoluble fraction. Hyperphosphorylation of full-length tau was confirmed by the immunoreactivity of phospho-Tau antibodies and mobility shifts by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). These properties were similar to the biochemical features of P301L mutated human tau in a mouse model of tauopathy. In addition, filamentous tau aggregates in cells barely co-localized with ubiquitins, suggesting that most tau aggregates were excluded from protein degradation systems, and thus propagated to daughter cells. The present cellular model of tauopathy will provide an advantage for dissecting the mechanisms of tau aggregation and degradation and be a powerful tool for drug screening to prevent tauopathy.

Bidirectional interplay of HSF1 degradation and UPR activation promotes tau hyperphosphorylation

The unfolded protein response (UPR) in the endoplasmic reticulum (ER) and the cytoplasmic heat stress response are two major stress response systems necessary for maintaining proteostasis for cellular health. Failure of either of these systems, such as in sustained UPR activation or in insufficient heat shock response activation, can lead to the development of neurodegeneration. Alleviation of ER stress and enhancement of heat shock response through heat shock factor 1 (HSF1) activation have previously been considered as attractive potential therapeutic targets for Alzheimer’s disease (AD)—a prevalent and devastating tauopathy. Understanding the interplay of the two aforementioned systems and their cooperative role in AD remain elusive. Here we report studies in human brain and tau pathogenic mouse models (rTg4510, PS19, and rTg21221), identifying HSF1 degradation and UPR activation as precursors of aberrant tau pathogenesis. We demonstrate that chemical ER stress inducers caused autophagy-lysosomal HSF1 degradation, resulting in tau hyperphosphorylation in rat primary neurons. In addition, permanent HSF1 loss reversely causes chronic UPR activation, leading to aberrant tau phosphorylation and aggregation in the hippocampus of aged HSF1 heterozygous knock-out mice. The deleterious interplay of UPR activation and HSF1 loss is exacerbated in N2a cells stably overexpressing a pro-aggregation mutant Tau_RD ΔK280 (N2a-Tau_RD ΔK280). We provide evidence of how these two stress response systems are intrinsically interweaved by showing that the gene encoding C/EBP-homologous protein (CHOP) activation in the UPR apoptotic pathway facilitates HSF1 degradation, which likely further contributes to prolonged UPR via ER chaperone HSP70 a5 (BiP/GRP78) suppression. Upregulating HSF1 relieves the tau toxicity in N2a-Tau_RD ΔK280 by reducing CHOP and increasing HSP70 a5 (BiP/GRP78). Our work reveals how the bidirectional crosstalk between the two stress response systems promotes early tau pathology and identifies HSF1 being one likely key player in both systems.

Tauopathy including Alzheimer’s disease (AD) is characterized by a build-up of tau aggregates in the brain, highly associated with failure of cellular protein homeostasis. Proteostasis can be achieved by protein quality control system to cope with numerous stresses such as proteotoxic stress from misfolded proteins. This cellular protective system includes heat shock response regulated by heat shock factor 1 (HSF1) activation and unfolded protein response in ER. Despite the importance of stress response in maintaining proteostasis, their role in neurodegenerative diseases like tauopathy is not clearly understood. The current study reports how the interplay between the two stress response systems, unfolded protein response and HSF1 promotes early tau pathology and identifies HSF1 protein degradation being one likely key player in both human AD and tau transgenic mouse AD models. We identify aging-associated AD-like neuropathological changes in the hippocampus of HSF1 heterozygous knock-out mice. We speculate that that HSF1 loss may constitute a mechanistic connection between ER stress and tau hyperphosphorylation in tau pathology. This study demonstrates the potential therapeutic significance of stabilizing HSF1 protein in treating AD.

The Distance between N and C Termini of Tau and of FTDP-17 Mutants Is Modulated by Microtubule Interactions in Living Cells

The microtubule (MT)-associated protein Tau is a natively unfolded protein, involved in a number of neurodegenerative disorders, collectively called tauopathies, aggregating in neurofibrillary tangles (NFT). It is an open question how the conversion from a MT bound molecule to an aggregation-prone Tau species occurs and, also, if and how tauopathy-related mutations affect its behavior in the cell. To address these points, we exploited a genetically encoded FRET sensor based on the full length Tau protein, to monitor in real time Tau conformational changes in different conditions in live cells. By studying the FRET signal we found that soluble Tau molecules, detached from MTs, display an unfolded structure. On the contrary, we observed an increased FRET signal generated by Tau monomers bound to MT, indicating that the association with MTs induced a folding of Tau protein, decreasing the distance between its N and C termini. We exploited the FRET sensor to investigate the impact of FTDP-17 mutations and of phosphorylation-site mutations on Tau folding and mobility in live cells. We demonstrated that the FTDP-17 Tau mutations weaken the interaction of Tau with cellular MTs, shifting the equilibrium towards the soluble pool while, conversely, phosphorylation site mutations shift the equilibrium of Tau towards the MT-bound state and a more closed conformation.

Conserved Lysine Acetylation within the Microtubule-Binding Domain Regulates MAP2/Tau Family Members

Lysine acetylation has emerged as a dominant post-translational modification (PTM) regulating tau proteins in Alzheimer’s disease (AD) and related tauopathies. Mass spectrometry studies indicate that tau acetylation sites cluster within the microtubule-binding region (MTBR), a region that is highly conserved among tau, MAP2, and MAP4 family members, implying that acetylation could represent a conserved regulatory mechanism for MAPs beyond tau. Here, we combined mass spectrometry, biochemical assays, and cell-based approaches to demonstrate that the tau family members MAP2 and MAP4 are also subject to reversible acetylation. We identify a cluster of lysines in the MAP2 and MAP4 MTBR that undergo CBP-catalyzed acetylation, many of which are conserved in tau. Similar to tau, MAP2 acetylation can occur in a cysteine-dependent auto-regulatory manner in the presence of acetyl-CoA. Furthermore, tubulin reduced MAP2 acetylation, suggesting tubulin binding dictates MAP acetylation status. Taken together, these results uncover a striking conservation of MAP2/Tau family post-translational modifications that could expand our understanding of the dynamic mechanisms regulating microtubules.

Stabilizing the Hsp70-Tau Complex Promotes Turnover in Models of Tauopathy

Heat shock protein 70 (Hsp70) is a chaperone that normally scans the proteome and initiates the turnover of some proteins (termed clients) by linking them to the degradation pathways. This activity is critical to normal protein homeostasis, yet it appears to fail in diseases associated with abnormal protein accumulation. It is not clear why Hsp70 promotes client degradation under some conditions, while sparing that protein under others. Here, we used a combination of chemical biology and genetic strategies to systematically perturb the affinity of Hsp70 for the model client, tau. This approach revealed that tight complexes between Hsp70 and tau were associated with enhanced turnover while transient interactions favored tau retention. These results suggest that client affinity is one important parameter governing Hsp70-mediated quality control.

Mutant Tau knock-in mice display frontotemporal dementia relevant behaviour and histopathology

Models of Tau pathology related to frontotemporal dementia (FTD) are essential to determine underlying neurodegenerative pathologies and resulting tauopathy relevant behavioural changes. However, existing models are often limited in their translational value due to Tau overexpression, and the frequent occurrence of motor deficits which prevent comprehensive behavioural assessments. In order to address these limitations, a forebrain-specific (CaMKIIα promoter), human mutated Tau (hTauP301L+R406W) knock-in mouse was generated out of the previously characterised PLB1Triple mouse, and named PLB2Tau. After confirmation of an additional hTau species (~60kDa) in forebrain samples, we identified age-dependent progressive Tau phosphorylation which coincided with the emergence of FTD relevant behavioural traits. In line with the non-cognitive symptomatology of FTD, PLB2Tau mice demonstrated early emerging (~6months) phenotypes of heightened anxiety in the elevated plus maze, depressive/apathetic behaviour in a sucrose preference test and generally reduced exploratory activity in the absence of motor impairments. Investigations of cognitive performance indicated prominent dysfunctions in semantic memory, as assessed by social transmission of food preference, and in behavioural flexibility during spatial reversal learning in a home cage corner-learning task. Spatial learning was only mildly affected and task-specific, with impairments at 12months of age in the corner learning but not in the water maze task. Electroencephalographic (EEG) investigations indicated a vigilance-stage specific loss of alpha power during wakefulness at both parietal and prefrontal recording sites, and site-specific EEG changes during non-rapid eye movement sleep (prefrontal) and rapid eye movement sleep (parietal). Further investigation of hippocampal electrophysiology conducted in slice preparations indicated a modest reduction in efficacy of synaptic transmission in the absence of altered synaptic plasticity. Together, our data demonstrate that the transgenic PLB2Tau mouse model presents with a striking behavioural and physiological face validity relevant for FTD, driven by the low level expression of mutant FTD hTau.

Tau Hyperphosphorylation and Oxidative Stress, a Critical Vicious Circle in Neurodegenerative Tauopathies?

Hyperphosphorylation and aggregation of the microtubule-associated protein tau in brain, are pathological hallmarks of a large family of neurodegenerative disorders, named tauopathies, which include Alzheimer's disease. It has been shown that increased phosphorylation of tau destabilizes tau-microtubule interactions, leading to microtubule instability, transport defects along microtubules, and ultimately neuronal death. However, although mutations of the MAPT gene have been detected in familial early-onset tauopathies, causative events in the more frequent sporadic late-onset forms and relationships between tau hyperphosphorylation and neurodegeneration remain largely elusive. Oxidative stress is a further pathological hallmark of tauopathies, but its precise role in the disease process is poorly understood. Another open question is the source of reactive oxygen species, which induce oxidative stress in brain neurons. Mitochondria have been classically viewed as a major source for oxidative stress, but microglial cells were recently identified as reactive oxygen species producers in tauopathies. Here we review the complex relationships between tau pathology and oxidative stress, placing emphasis on (i) tau protein function, (ii) origin and consequences of reactive oxygen species production, and (iii) links between tau phosphorylation and oxidative stress. Further, we go on to discuss the hypothesis that tau hyperphosphorylation and oxidative stress are two key components of a vicious circle, crucial in neurodegenerative tauopathies.

Frontotemporal lobar degeneration: old knowledge and new insight into the pathogenetic mechanisms of tau mutations

Frontotemporal lobar degeneration (FTLD) is a group of heterogeneous neurodegenerative diseases which includes tauopathies. In the central nervous system (CNS) tau is the major microtubule-associated protein (MAP) of neurons, promoting assembly and stabilization of microtubules (MTs) required for morphogenesis and axonal transport. Primary tauopathies are characterized by deposition of abnormal fibrils of tau in neuronal and glial cells, leading to neuronal death, brain atrophy and eventually dementia. In genetic tauopathies mutations of tau gene impair the ability of tau to bind to MTs, alter the normal ratio among tau isoforms and favor fibril formation. Recently, additional functions have been ascribed to tau and different pathogenetic mechanisms are then emerging. In fact, a role of tau in DNA protection and genome stability has been reported and chromosome aberrations have been found associated with tau mutations. Furthermore, newly structurally and functionally characterized mutations have suggested novel pathological features, such as a tendency to form oligomeric rather than fibrillar aggregates. Tau mutations affecting axonal transport and plasma membrane interaction have also been described. In this article, we will review the pathogenetic mechanisms underlying tau mutations, focusing in particular on the less common aspects, so far poorly investigated.

The twenty-four KDa C-terminal tau fragment increases with aging in tauopathy mice: implications of prion-like properties

The truncated tau protein is a component of the neurofibrillary tangles found in the brains with tauopathies. However, the molecular mechanisms by which the truncated tau fragment causes neurodegeneration remain unknown. Tau pathology was recently suggested to spread through intercellular propagation, and required the formation of 'prion-like' species. We herein identified a new fragment of the tau protein that consisted of four binding domains and a C-terminal tail (Tau-CTF24), but lacked the N-terminal projection domain, and found that it increased with aging in tauopathy model mice (Tg601). Tau-CTF24-like fragments were also present in human brains with tauopathies. A mass spectroscopic analysis revealed that Tau-CTF24 was cleaved behind R242. The digestion of full-length tau (Tau-FL) by calpain produced Tau-CTF24 in vitro and calpain activity increased in old Tg601. Recombinant Tau-CTF24 accelerated heparin-induced aggregation and lost the ability to promote microtubule assembly. When insoluble tau from diseased brains or aggregated recombinant tau was introduced as seeds into SH-SY5Y cells, a larger amount of insoluble tau was formed in cells overexpressing Tau-CTF24 than in those overexpressing Tau-FL. Furthermore, lysates containing the Tau-CTF24 inclusion propagated to naive tau-expressing cells more efficiently than those containing the Tau-FL inclusion. Immunoblot and confocal microscopic analyses revealed that aggregated Tau-CTF24 bound to cells more rapidly and abundantly than aggregated Tau-FL. Our results suggest that Tau-CTF24 contributes to neurodegeneration by enhancing prion-like propagation as well as deteriorating the mechanisms involved in microtubule function.

Appoptosin-Mediated Caspase Cleavage of Tau Contributes to Progressive Supranuclear Palsy Pathogenesis

Progressive supranuclear palsy (PSP) is a movement disorder characterized by tau neuropathology where the underlying mechanism is unknown. An SNP (rs1768208 C/T) has been identified as a strong risk factor for PSP. Here, we identified a much higher T-allele occurrence and increased levels of the pro-apoptotic protein appoptosin in PSP patients. Elevations in appoptosin correlate with activated caspase-3 and caspase-cleaved tau levels. Appoptosin overexpression increased caspase-mediated tau cleavage, tau aggregation, and synaptic dysfunction, whereas appoptosin deficiency reduced tau cleavage and aggregation. Appoptosin transduction impaired multiple motor functions and exacerbated neuropathology in tau-transgenic mice in a manner dependent on caspase-3 and tau. Increased appoptosin and caspase-3-cleaved tau were also observed in brain samples of patients with Alzheimer's disease and frontotemporal dementia with tau inclusions. Our findings reveal a novel role for appoptosin in neurological disorders with tau neuropathology, linking caspase-3-mediated tau cleavage to synaptic dysfunction and behavioral/motor defects.

Zinc Modulates Self-Assembly of Bacillus thermocatenulatus Lipase

Thermoalkalophilic lipases are prone to aggregation from their dimer interface to which structural zinc is very closely located. Structural zinc sites have been shown to induce protein aggregation, but the interaction between zinc and aggregation tendency in thermoalkalophilic lipases remains elusive. Here we delineate the interplay between zinc and aggregation of the lipase from Bacillus thermocatenulatus (BTL2), which is taken to be a representative of thermoalkalophilic lipase. Results showed that zinc removal disrupted the BTL2 dimer, leading to monomer formation and reduced thermostability manifesting as a link between zinc and dimerization that leads to thermostability, while zinc addition induced aggregation. Biochemical and kinetic characterizations of zinc-induced aggregates showed that the aggregates obtained from the early and late stages of aggregation had differential characteristics. In the early stages, the aggregates were soluble and possessed native-like structures, while in the late stages, the aggregates became insoluble and showed fibrillar characteristics with binding affinities for Congo red and thioflavin T. The impact of temperature on zinc-induced aggregation was further investigated, and it was found that the native-like early aggregates could completely dissociate into functional lipase forms at high temperatures while dissociation of the late aggregates was limited. To this end, we report that the zinc-induced aggregation of BTL2 can be reversed by temperature switches and initiated by ordered aggregates in the early stages that gain fibrillar-like features over time. Insights revealed by this work contributes to the knowledge of aggregation mechanisms that exist in thermophilic proteins, reflecting the potential use of metal addition and/or removal to fine-tune aggregation tendency.

Passive immunization with phospho-tau antibodies reduces tau pathology and functional deficits in two distinct mouse tauopathy models

In Alzheimer’s disease (AD), an extensive accumulation of extracellular amyloid plaques and intraneuronal tau tangles, along with neuronal loss, is evident in distinct brain regions. Staging of tau pathology by postmortem analysis of AD subjects suggests a sequence of initiation and subsequent spread of neurofibrillary tau tangles along defined brain anatomical pathways. Further, the severity of cognitive deficits correlates with the degree and extent of tau pathology. In this study, we demonstrate that phospho-tau (p-tau) antibodies, PHF6 and PHF13, can prevent the induction of tau pathology in primary neuron cultures. The impact of passive immunotherapy on the formation and spread of tau pathology, as well as functional deficits, was subsequently evaluated with these antibodies in two distinct transgenic mouse tauopathy models. The rTg4510 transgenic mouse is characterized by inducible over-expression of P301L mutant tau, and exhibits robust age-dependent brain tau pathology. Systemic treatment with PHF6 and PHF13 from 3 to 6 months of age led to a significant decline in brain and CSF p-tau levels. In a second model, injection of preformed tau fibrils (PFFs) comprised of recombinant tau protein encompassing the microtubule-repeat domains into the cortex and hippocampus of young P301S mutant tau over-expressing mice (PS19) led to robust tau pathology on the ipsilateral side with evidence of spread to distant sites, including the contralateral hippocampus and bilateral entorhinal cortex 4 weeks post-injection. Systemic treatment with PHF13 led to a significant decline in the spread of tau pathology in this model. The reduction in tau species after p-tau antibody treatment was associated with an improvement in novel-object recognition memory test in both models. These studies provide evidence supporting the use of tau immunotherapy as a potential treatment option for AD and other tauopathies.

Cellular factors modulating the mechanism of tau protein aggregation

Pathological accumulation of the microtubule-associated protein tau, in the form of neurofibrillary tangles, is a major hallmark of Alzheimer's disease, the most prevalent neurodegenerative condition worldwide. In addition to Alzheimer's disease, a number of neurodegenerative diseases, called tauopathies, are characterized by the accumulation of aggregated tau in a variety of brain regions. While tau normally plays an important role in stabilizing the microtubule network of the cytoskeleton, its dissociation from microtubules and eventual aggregation into pathological deposits is an area of intense focus for therapeutic development. Here we discuss the known cellular factors that affect tau aggregation, from post-translational modifications to molecular chaperones.

Cell-based Models To Investigate Tau Aggregation

Accumulation of abnormal tau aggregates in neuron is an important pathological signature in multiple neurodegenerative disorders including Alzheimer's disease. Tau is a neuron specific microtubule-associated protein that regulates microtubule stability, which is critical for axonal outgrowth and synaptic plasticity. In a pathological condition, tau dissociates from microtubules and forms insoluble aggregates called neurofibrillary tangles (NFTs). The accumulation of NFTs in neuron directly correlates with microtubule dysfunction and neuronal degeneration. Due to the pathophysiological importance of tau, great efforts have been made to understand tau aggregation processes and find therapeutics to halt or reverse the processes. However, progress has been slow due to the lack of a suitable method for monitoring tau aggregation. In this mini-review, we will review the conventional methods for studying tau aggregation, and introduce recent cell-based sensor approaches that allow monitoring tau aggregation in living cells.

Inhibition of tau aggregation by a rosamine derivative that blocks tau intermolecular disulfide cross-linking

Abnormal tau aggregates are presumed to be neurotoxic and are an important therapeutic target for multiple neurodegenerative disorders including Alzheimer's disease. Growing evidence has shown that tau intermolecular disulfide cross-linking is critical in generating tau oligomers that serve as a building block for higher-order aggregates. Here we report that a small molecule inhibitor prevents tau aggregation by blocking the generation of disulfide cross-linked tau oligomers. Among the compounds tested, a rosamine derivative bearing mild thiol reactivity selectively labeled tau and effectively inhibited oligomerization and fibrillization processes in vitro. Our data suggest that controlling tau oxidation status could be a new therapeutic strategy for prevention of abnormal tau aggregation.

Hirano bodies differentially modulate cell death induced by tau and the amyloid precursor protein intracellular domain

BACKGROUND

Hirano bodies are actin-rich paracrystalline inclusions found in brains of patients with Alzheimer's disease (AD), frontotemporal dementia (FTD), and in normal aged individuals. Although studies of post-mortem brain tissue provide clues of etiology, the physiological function of Hirano bodies remains unknown. A cell culture model was utilized to study the interactions of mutant tau proteins, model Hirano bodies, and GSK3β in human astrocytoma cells.

RESULTS

Most tau variants showed co-localization with model Hirano bodies. Cosedimentation assays revealed this interaction may be direct, as recombinant purified forms of tau are all capable of binding F-actin. Model Hirano bodies had no effect or enhanced cell death induced by tau in the absence of amyloid precursor protein intracellular domain (AICD). In the presence of AICD and tau, synergistic cell death was observed in most cases, and model Hirano bodies decreased this synergistic cell death, except for forms of tau that caused significant cell death in the presence of Hirano bodies only. A role for the kinase GSK3β is suggested by the finding that a dominant negative form of GSK3β reduces this synergistic cell death. A subset of Hirano bodies in brain tissue of both Alzheimer's disease and normal aged individuals was found to contain tau, with some Hirano bodies in Alzheimer's disease brains containing hyperphosphorylated tau.

CONCLUSION

The results demonstrate a complex interaction between tau and AICD involving activation of GSK3β in promoting cell death, and the ability of Hirano bodies to modulate this process.

Phosphorylation of α-tubulin by protein kinase C stimulates microtubule dynamics in human breast cells

Protein kinase C (PKC) engenders motility through phosphorylation of α-tubulin at Ser-165 in nontransformed MCF-10A cells. Live cell imaging explored the impact of PKC-mediated phosphorylation on microtubule (MT) dynamics. MTs fluorescently labeled with GFP-α-tubulin were treated with diacylglycerol (DAG)-lactone (a membrane-permeable PKC activator), or cotransfected with a pseudophosphorylated S165D-α6-tubulin mutant. Each condition increased the dynamicity of MTs by stimulating the rate and duration of the growth phase and decreasing the frequency of catastrophe. In MDA-MB-231 metastatic breast cells where the intrinsic PKC activity is high, these MT growth parameters were also high but could be suppressed by expression of phosphorylation-resistant S165N-α6-tubulin or by treatment with a pan-PKC inhibitor (bis-indoleylmaleimide). Subcellular fractionation and immunofluorescence of MCF-10A cells showed that phosphorylation (via DAG-lactone) or pseudophosphorylation of α6-tubulin increased its partitioning into MTs as compared to controls, and produced longer, more stable MTs. Following expression of the plus-end binding protein GFP-EB1, DAG-lactone accelerated the formation and increased the number of nascent MTs. Expression of S165D-α6-tubulin promoted Rac1 activation and Rac1-dependent cell motility. These findings call attention to PKC-mediated phosphorylation of α-tubulin as a novel mechanism for controlling the dynamics of MTs that result in cell movement.

Tau pathology is present in vivo and develops in vitro in sensory neurons from human P301S tau transgenic mice: a system for screening drugs against tauopathies

Intracellular tau aggregates are the neuropathological hallmark of several neurodegenerative diseases, including Alzheimer's disease, progressive supranuclear palsy, and cases of frontotemporal dementia, but the link between these aggregates and neurodegeneration remains unclear. Neuronal models recapitulating the main features of tau pathology are necessary to investigate the molecular mechanisms of tau malfunction, but current models show little and inconsistent spontaneous tau aggregation. We show that dorsal root ganglion (DRG) neurons in transgenic mice expressing human P301S tau (P301S-htau) develop tau pathology similar to that found in brain and spinal cord and a significant reduction in mechanosensation occurs before detectable fibrillar tau formation. DRG neuronal cultures established from adult P301S-htau mice at different ages retained the pattern of aberrant tau found in vivo. Moreover, htau became progressively hyperphosphorylated over 2 months in vitro beginning with nonsymptomatic neurons, while hyperphosphorylated P301S-htau-positive neurons from 5-month-old mice cultured for 2 months died preferentially. P301S-htau-positive neurons grew aberrant axons, including spheroids, typically found in human tauopathies. Neurons cultured at advanced stages of tau pathology showed a 60% decrease in the fraction of moving mitochondria. SEG28019, a novel O-GlcNAcase inhibitor, reduced steady-state pSer396/pSer404 phosphorylation over 7 weeks in a significant proportion of DRG neurons showing for the first time the possible beneficial effect of prolonged dosing of O-GlcNAcase inhibitor in vitro. Our system is unique in that fibrillar tau forms without external manipulation and provides an important new tool for understanding the mechanisms of tau dysfunction and for screening of compounds for treatment of tauopathies.

Overexpression of 14-3-3z promotes tau phosphorylation at Ser262 and accelerates proteosomal degradation of synaptophysin in rat primary hippocampal neurons

β-amyloid peptide accumulation, tau hyperphosphorylation, and synapse loss are characteristic neuropathological symptoms of Alzheimer’s disease (AD). Tau hyperphosphorylation is suggested to inhibit the association of tau with microtubules, making microtubules unstable and causing neurodegeneration. The mechanism of tau phosphorylation in AD brain, therefore, is of considerable significance. Although PHF-tau is phosphorylated at over 40 Ser/Thr sites, Ser²⁶² phosphorylation was shown to mediate β-amyloid neurotoxicity and formation of toxic tau lesions in the brain. In vitro, PKA is one of the kinases that phosphorylates tau at Ser²⁶², but the mechanism by which it phosphorylates tau in AD brain is not very clear. 14-3-3ζ is associated with neurofibrillary tangles and is upregulated in AD brain. In this study, we show that 14-3-3ζ promotes tau phosphorylation at Ser²⁶² by PKA in differentiating neurons. When overexpressed in rat hippocampal primary neurons, 14-3-3ζ causes an increase in Ser²⁶² phosphorylation, a decrease in the amount of microtubule-bound tau, a reduction in the amount of polymerized microtubules, as well as microtubule instability. More importantly, the level of pre-synaptic protein synaptophysin was significantly reduced. Downregulation of synaptophysin in 14-3-3ζ overexpressing neurons was mitigated by inhibiting the proteosome, indicating that 14-3-3ζ promotes proteosomal degradation of synaptophysin. When 14-3-3ζ overexpressing neurons were treated with the microtubule stabilizing drug taxol, tau Ser²⁶² phosphorylation decreased and synaptophysin level was restored. Our data demonstrate that overexpression of 14-3-3ζ accelerates proteosomal turnover of synaptophysin by promoting the destabilization of microtubules. Synaptophysin is involved in synapse formation and neurotransmitter release. Our results suggest that 14-3-3ζ may cause synaptic pathology by reducing synaptophysin levels in the brains of patients suffering from AD.

Prions Ex Vivo: What Cell Culture Models Tell Us about Infectious Proteins

Prions are unconventional infectious agents that are composed of misfolded aggregated prion protein. Prions replicate their conformation by template-assisted conversion of the endogenous prion protein PrP. Templated conversion of soluble proteins into protein aggregates is also a hallmark of other neurodegenerative diseases. Alzheimer's disease or Parkinson's disease are not considered infectious diseases, although aggregate pathology appears to progress in a stereotypical fashion reminiscent of the spreading behavior ofmammalian prions. While basic principles of prion formation have been studied extensively, it is still unclear what exactly drives PrP molecules into an infectious, self-templating conformation. In this review, we discuss crucial steps in the life cycle of prions that have been revealed in ex vivo models. Importantly, the persistent propagation of prions in mitotically active cells argues that cellular processes are in place that not only allow recruitment of cellular PrP into growing prion aggregates but also enable the multiplication of infectious seeds that are transmitted to daughter cells. Comparison of prions with other protein aggregates demonstrates that not all the characteristics of prions are equally shared by prion-like aggregates. Future experiments may reveal to which extent aggregation-prone proteins associated with other neurodegenerative diseases can copy the replication strategies of prions.

Interactions between Aβ and mutated Tau lead to polymorphism and induce aggregation of Aβ-mutated tau oligomeric complexes

One of the main hallmarks of the fronto-temporal dementia with Parkinsonism linked to chromosome 17 (FTDP-17) is the accumulation of neurofibrillary tangles in the brain as an outcome of the aggregation of mutated tau protein. This process occurs due to a number of genetic mutations in the MAPT gene. One of these mutations is the ∆K280 mutation in the tau R2 repeat domain, which promotes the aggregation vis-à-vis that for the wild-type tau. Experimental studies have shown that in Alzheimer's disease Aβ peptide forms aggregates both with itself and with wild-type tau. By analogy, in FTDP-17, it is likely that there are interactions between Aβ and mutated tau, but the molecular mechanisms underlying such interactions remain to be elucidated. Thus, to investigate the interactions between Aβ and mutated tau, we constructed fourteen ∆K280 mutated tau-Aβ17-42 oligomeric complexes. In seven of the mutated tau-Aβ17-42 oligoemric complexes the mutated tau oligomers exhibited hydrophobic interactions in their core domain, and in the other seven mutated tau-Aβ17-42 oligoemric complexes the mutated tau oligomers exhibited salt-bridge interactions in their core domain. We considered two types of interactions between mutated tau oligomers and Aβ oligomers: interactions of one monomer of the Aβ oligomer with one monomer of the mutated tau oligomer to form a single-layer conformation, and interactions of the entire Aβ oligomer with the entire mutated tau oligomer to form a double-layer conformation. We also considered parallel arrangements of Aβ trimers alternating with mutated tau trimers in a single-layer conformation. Our results demonstrate that in the interactions of Aβ and mutated tau oligomers, polymorphic mutated tau-Aβ17-42 oligomeric complexes were observed, with a slight preference for the double-layer conformation. Aβ trimers alternating with mutated tau trimers constituted a structurally stable confined β-structure, albeit one that was energetically less stable than all the other constructed models.

FTDP-17 tau mutations induce distinct effects on aggregation and microtubule interactions

FTDP-17 mutations in the tau gene lead to early onset frontotemporal dementias characterized by the pathological aggregation of the microtubule-associated protein tau. Tau aggregation is closely correlated with the progression and severity of localized atrophy of certain regions in the brain. These mutations are primarily located in or near the microtubule-binding repeat regions of tau and can have vastly different effects on the protein. Some mutations have been linked to effects such as increased levels of aggregation, hyperphosphorylation, defects in mRNA splicing, and weakened interaction with microtubules. Given the differential effects of the mutations, it may not be surprising that the pathology associated with FTDP-17 can vary widely as well. Despite this variety, several of the mutations are commonly used interchangeably as aggregation inducers for in vitro and in vivo models of tauopathies. We generated recombinant forms of 12 FTDP-17 mutations chosen for their predicted effects on the charge, hydrophobicity, and secondary structure of the protein. We then examined the effects that the mutations had on the properties of in vitro aggregation of the protein and its ability to stabilize microtubule assembly. The group of mutations induced very different effects on the total amount of aggregation, the kinetics of aggregation, and filament morphology. Several of the mutations inhibited the microtubule stabilization ability of tau, while others had very little effect compared to wild-type tau. These results indicate that the mechanisms of disease progression may differ among FTDP-17 mutations and that the effects of the varying mutations may not be equal in all model systems.

Trans-cellular propagation of Tau aggregation by fibrillar species

Aggregation of the microtubule associated protein Tau is associated with several neurodegenerative disorders, including Alzheimer disease and frontotemporal dementia. In Alzheimer disease, Tau pathology spreads progressively throughout the brain, possibly along existing neural networks. However, it is still unclear how the propagation of Tau misfolding occurs. Intriguingly, in animal models, vaccine-based therapies have reduced Tau and synuclein pathology by uncertain mechanisms, given that these proteins are intracellular. We have previously speculated that trans-cellular propagation of misfolding could be mediated by a process similar to prion pathogenesis, in which fibrillar Tau aggregates spread pathology from cell to cell. However, there has been little evidence to demonstrate true trans-cellular propagation of Tau misfolding, in which Tau aggregates from one cell directly contact Tau protein in the recipient cell to trigger further aggregation. Here we have observed that intracellular Tau fibrils are directly released into the medium and then taken up by co-cultured cells. Internalized Tau aggregates induce fibrillization of intracellular Tau in these naive recipient cells via direct protein-protein contact that we demonstrate using FRET. Tau aggregation can be amplified across several generations of cells. An anti-Tau monoclonal antibody blocks Tau aggregate propagation by trapping fibrils in the extracellular space and preventing their uptake. Thus, propagation of Tau protein misfolding among cells can be mediated by release and subsequent uptake of fibrils that directly contact native protein in recipient cells. These results support the model of aggregate propagation by templated conformational change and suggest a mechanism for vaccine-based therapies in neurodegenerative diseases.

Increasing O-GlcNAc slows neurodegeneration and stabilizes tau against aggregation

Oligomerization of tau is a key process contributing to the progressive death of neurons in Alzheimer's disease. Tau is modified by O-linked N-acetylglucosamine (O-GlcNAc), and O-GlcNAc can influence tau phosphorylation in certain cases. We therefore speculated that increasing tau O-GlcNAc could be a strategy to hinder pathological tau-induced neurodegeneration. Here we found that treatment of hemizygous JNPL3 tau transgenic mice with an O-GlcNAcase inhibitor increased tau O-GlcNAc, hindered formation of tau aggregates and decreased neuronal cell loss. Notably, increases in tau O-GlcNAc did not alter tau phosphorylation in vivo. Using in vitro biochemical aggregation studies, we found that O-GlcNAc modification, on its own, hinders tau oligomerization. O-GlcNAc also inhibits thermally induced aggregation of an unrelated protein, TAK-1 binding protein, suggesting that a basic biochemical function of O-GlcNAc may be to prevent protein aggregation. These results also suggest O-GlcNAcase as a potential therapeutic target that could hinder progression of Alzheimer's disease.

Induction of intracellular tau aggregation is promoted by α-synuclein seeds and provides novel insights into the hyperphosphorylation of tau

Intracytoplasmic proteinaceous inclusions, primarily composed of tau or α-synuclein (α-syn), are predominant pathological features of Alzheimer's disease (AD) and Parkinson's disease (PD), respectively. However, the coexistence of these pathological aggregates is identified in many neurodegenerative disorders, including spectrum disorders of AD and PD. Whereas α-syn can spontaneously polymerize into amyloidogenic fibrils, in vitro, tau polymerization requires an inducing agent. The current study presents a human-derived cellular model, in which recombinant, preformed α-syn fibrils cross-seed intracellular tau to promote the formation of neurofibrillary tangle-like aggregates. These aggregates were hyperphosphorylated, Triton insoluble, and thioflavin-S positive, either comingling with endogenously expressed α-syn aggregates or induced by only exogenously applied recombinant α-syn fibrils. Furthermore, filamentous, amyloidogenic tau took over the cellular soma, displacing the nucleus and isolating or displacing organelles, likely preventing cellular function. Although a significant proportion of wild-type tau formed these cellular inclusions, the P301L mutation in tau increased aggregation propensity resulting from α-syn seeds to over 50% of total tau protein. The role of phosphorylation on the development of these tau aggregates was investigated by coexpressing glycogen synthase kinase 3 β or microtubule-associated protein/microtubule affinity-regulating kinase 2. Expression of either kinase inhibited the formation of α-syn-induced tau aggregates. Analyses of phosphorylation sites suggest that multiple complex factors may be associated with this effect and that Triton-soluble versus Triton-insoluble tau may be independently targeted by kinases. The current work not only provides an exceptional cellular model of tau pathology, but also examines α-syn-induced tau inclusion formation and provides novel insights into hyperphosphorylation observed in disease.

The acetylation of tau inhibits its function and promotes pathological tau aggregation

The microtubule associated protein tau promotes neuronal survival through binding and stabilization of MTs. Phosphorylation regulates tau-microtubule interactions and hyperphosphorylation contributes to the aberrant formation of insoluble tau aggregates in Alzheimer's disease (AD) and related tauopathies. However, other pathogenic post-translational tau modifications have not been well characterized. Here we demonstrate that tau acetylation inhibits tau function via impaired tau-microtubule interactions and promotes pathological tau aggregation. Mass spectrometry analysis identified specific lysine residues, including lysine 280 (K280) within the microtubule-binding motif as the major sites of tau acetylation. Immunohistochemical and biochemical studies of brains from tau transgenic mice and patients with AD and related tauopathies showed that acetylated tau pathology is specifically associated with insoluble, Thioflavin-positive tau aggregates. Thus, tau K280 acetylation in our studies was only detected in diseased tissue, suggesting it may have a role in pathological tau transformation. This study suggests that tau K280 acetylation is a potential target for drug discovery and biomarker development for AD and related tauopathies.

The frontotemporal dementia mutation R406W blocks tau's interaction with the membrane in an annexin A2-dependent manner

Changes of the microtubule-associated protein tau are central in Alzheimer's disease (AD) and frontotemporal dementia with Parkinsonism linked to chromosome 17 (FTDP-17). However, the functional consequence of the FTDP-17 tau mutation R406W, which causes a tauopathy clinically resembling AD, is not well understood. We find that the R406W mutation does not affect microtubule interaction but abolishes tau's membrane binding. Loss of binding is associated with decreased trapping at the tip of neurites and increased length fluctuations during process growth. Tandem affinity purification tag purification and mass spectrometry identify the calcium-regulated plasma membrane-binding protein annexin A2 (AnxA2) as a potential interaction partner of tau. Consistently, wild-type tau but not R406W tau interacts with AnxA2 in a heterologous yeast expression system. Sequestration of Ca(2+) or knockdown of AnxA2 abolishes the differential trapping of wild-type and R406W tau. We suggest that the pathological effect of the R406W mutation is caused by impaired membrane binding, which involves a functional interaction with AnxA2 as a membrane-cytoskeleton linker.

Seeding of normal Tau by pathological Tau conformers drives pathogenesis of Alzheimer-like tangles

Neurofibrillary tangles (NFTs) in Alzheimer disease and related tauopathies are composed of insoluble hyperphosphorylated Tau protein, but the mechanisms underlying the conversion of highly soluble Tau into insoluble NFTs remain elusive. Here, we demonstrate that introduction of minute quantities of misfolded preformed Tau fibrils (Tau pffs) into Tau-expressing cells rapidly recruit large amounts of soluble Tau into filamentous inclusions resembling NFTs with unprecedented efficiency, suggesting a "seeding"-recruitment process as a highly plausible mechanism underlying NFT formation in vivo. Consistent with the emerging concept of prion-like transmissibility of disease-causing amyloidogenic proteins, we found that spontaneous uptake of Tau pffs into cells is likely mediated by endocytosis, suggesting a potential mechanism for the propagation of Tau lesions in tauopathy brains. Furthermore, sequestration of soluble Tau by pff-induced Tau aggregates attenuates microtubule overstabilization in Tau-expressing cells, supporting the hypothesis of a Tau loss-of-function toxicity in cells harboring NFTs. In summary, our study establishes a cellular system that robustly develops authentic NFT-like Tau aggregates, which provides mechanistic insights into NFT pathogenesis and a potential tool for identifying Tau-based therapeutics.

Serine-409 phosphorylation and oxidative damage define aggregation of human protein tau in yeast

Unraveling the biochemical and genetic alterations that control the aggregation of protein tau is crucial to understand the etiology of tau-related neurodegenerative disorders. We expressed wild type and six clinical frontotemporal dementia with parkinsonism (FTDP) mutants of human protein tau in wild-type yeast cells and cells lacking Mds1 or Pho85, the respective orthologues of the tau kinases GSK3β and cdk5. We compared tau phosphorylation with the levels of sarkosyl-insoluble tau (SinT), as a measure for tau aggregation. The deficiency of Pho85 enhanced significantly the phosphorylation of serine-409 (S409) in all tau mutants, which coincided with marked increases in SinT levels. FTDP mutants tau-P301L and tau-R406W were least phosphorylated at S409 and produced the lowest levels of SinT, indicating that S409 phosphorylation is a direct determinant for tau aggregation. This finding was substantiated by the synthetic tau-S409A mutant that failed to produce significant amounts of SinT, while its pseudophosphorylated counterpart tau-S409E yielded SinT levels higher than or comparable to wild-type tau. Furthermore, S409 phosphorylation reduced the binding of protein tau to preformed microtubules. The highest SinT levels were found in yeast cells subjected to oxidative stress and with mitochondrial dysfunction. Under these conditions, the aggregation of tau was enhanced although the protein is less phosphorylated, suggesting that additional mechanisms are involved. Our results validate yeast as a prime model to identify the genetic and biochemical factors that contribute to the pathophysiology of human tau.

Transgenic mouse and cell culture models demonstrate a lack of mechanistic connection between endoplasmic reticulum stress and tau dysfunction

In vivo aggregation of tau protein is a hallmark of many neurodegenerative disorders, including Alzheimer's disease (AD). Recent evidence has also demonstrated activation of the unfolded protein response (UPR), a cellular response to endoplasmic reticulum (ER) stress, in AD, although the role of the UPR in disease pathogenesis is not known. Here, three model systems were used to determine whether a direct mechanistic link could be demonstrated between tau aggregation and the UPR. The first model system used was SH-SY5Y cells, a neuronal cultured cell line that endogenously expresses tau. In this system, the UPR was activated using chemical stressors, tunicamycin and thapsigargin, but no changes in tau expression levels, solubility, or phosphorylation were observed. In the second model system, wild-type 4R tau and P301L tau, a variant with increased aggregation propensity, were heterologously overexpressed in HEK 293 cells. This overexpression did not activate the UPR. The last model system examined here was the PS19 transgenic mouse model. Although PS19 mice, which express the P301S variant of tau, display severe neurodegeneration and formation of tau aggregates, brain tissue samples did not show any activation of the UPR. Taken together, the results from these three model systems suggest that a direct mechanistic link does not exist between tau aggregation and the UPR.

Microtubule-associated protein tau in human prostate cancer cells: isoforms, phosphorylation, and interactions

Tau is a microtubule-associated protein whose function has been investigated primarily in neurons. Recently, tau expression has been correlated with increased drug resistance in various cancers of non-neuronal tissues. In this report, we investigate the tau expressed in cancerous prostate lines ALVA-31, DU 145, and PC-3. Prostate cancer tau is heat-stable and highly phosphorylated, containing many of the modifications identified in Alzheimer's disease brain tau. RT-PCR and phosphatase treatment indicated that all six alternatively spliced adult brain tau isoforms are expressed in ALVA-31 cells, and isoforms containing exon 6 as well as high molecular weight tau isoforms containing either exon 4A or a larger splice variant of exon 4A are also present. Consistent with its hyperphosphorylated state, a large proportion of ALVA-31 tau does not bind to microtubules, as detected by confocal microscopy and biochemical tests. Finally, endogenous ALVA-31 tau can interact with the p85 subunit of phosphatidylinositol 3-kinase, as demonstrated by co-immunoprecipitations and in vitro protein-binding assays. Our results suggest that tau in prostate cancer cells does not resemble that from normal adult brain and support the hypothesis that tau is a multifunctional protein.

Inhibition of tau polymerization with a cyanine dye in two distinct model systems

In a host of neurodegenerative diseases Tau, a microtubule-associated protein, aggregates into insoluble lesions within neurons. Previous studies have utilized cyanine dyes as Tau aggregation inhibitors in vitro. Herein we utilize cyanine dye 3,3'-diethyl-9-methyl-thiacarbocyanine iodide (C11) to modulate Tau polymerization in two model systems, an organotypic slice culture model derived from Tau transgenic mice and a split green fluorescent protein complementation assay in Tau-expressing cells. In slice cultures, submicromolar concentrations (0.001 microm) of C11 produced a significant reduction of aggregated Tau and a corresponding increase in unpolymerized Tau. In contrast, treatment with a 1 microm dose promoted aggregation of Tau. These results were recapitulated in the complementation assay where administration of 1 microm C11 produced a significant increase in polymerized Tau relative to control, whereas treatment of cells with 0.01 microm C11 resulted in a marked reduction of aggregated Tau. In the organotypic slice cultures, modulation of Tau aggregation was independent of changes in phosphorylation at disease and microtubule binding relevant epitopes for both dosing regimes. Furthermore, treatment with 0.001 microm C11 resulted in a decrease in both total filament mass and number. There was no evidence of apoptosis or loss of synaptic integrity at either dose, however, whereas submicromolar concentrations of C11 did not interfere with microtubule binding, higher doses resulted in a decrease in the levels of microtubule-bound Tau. Overall, a cyanine dye can dissociate aggregated Tau in an ex vivo model of tauopathy with little toxicity and exploration of the use of these type of dyes as therapeutic agents is warranted.