Different immunoreactivities of the microtubule-binding region of tau and its molecular basis in brains from patients with Alzheimer's disease, Pick's disease, progressive supranuclear palsy and corticobasal degeneration

Cellular and pathological functions of tau

Tau protein is involved in various cellular processes, including having a canonical role in binding and stabilization of microtubules in neurons. Tauopathies are neurodegenerative diseases marked by the abnormal accumulation of tau protein aggregates in neurons, as seen, for example, in conditions such as frontotemporal dementia and Alzheimer disease. Mutations in tau coding regions or that disrupt tau mRNA splicing, tau post-translational modifications and cellular stress factors (such as oxidative stress and inflammation) increase the tendency of tau to aggregate and interfere with its clearance. Pathological tau is strongly implicated in the progression of neurodegenerative diseases, and the propagation of tau aggregates is associated with disease severity. Recent technological advancements, including cryo-electron microscopy and disease models derived from human induced pluripotent stem cells, have increased our understanding of tau-related pathology in neurodegenerative conditions. Substantial progress has been made in deciphering tau aggregate structures and the molecular mechanisms that underlie protein aggregation and toxicity. In this Review, we discuss recent insights into the diverse cellular functions of tau and the pathology of tau inclusions and explore the potential for therapeutic interventions.

Insights into the Structural Conformations of the Tau Protein in Different Aggregation Status

Tau is a protein characterized by large structural portions displaying extended conformational changes. Unfortunately, the accumulation of this protein into toxic aggregates in neuronal cells leads to a number of severe pathologies, collectively named tauopathies. In the last decade, significant research advancements were achieved, including a better understanding of Tau structures and their implication in different tauopathies. Interestingly, Tau is characterized by a high structural variability depending on the type of disease, the crystallization conditions, and the formation of pathologic aggregates obtained from in vitro versus ex vivo samples. In this review, we reported an up-to-date and comprehensive overview of Tau structures reported in the Protein Data Bank, with a special focus on discussing the connections between structural features, different tauopathies, different crystallization conditions, and the use of in vitro or ex vivo samples. The information reported in this article highlights very interesting links between all these aspects, which we believe may be of particular relevance for a more informed structure-based design of compounds able to modulate Tau aggregation.

Ultrastructural and biochemical classification of pathogenic tau, α-synuclein and TDP-43

Intracellular accumulation of abnormal proteins with conformational changes is the defining neuropathological feature of neurodegenerative diseases. The pathogenic proteins that accumulate in patients' brains adopt an amyloid-like fibrous structure and exhibit various ultrastructural features. The biochemical analysis of pathogenic proteins in sarkosyl-insoluble fractions extracted from patients' brains also shows disease-specific features. Intriguingly, these ultrastructural and biochemical features are common within the same disease group. These differences among the pathogenic proteins extracted from patients' brains have important implications for definitive diagnosis of the disease, and also suggest the existence of pathogenic protein strains that contribute to the heterogeneity of pathogenesis in neurodegenerative diseases. Recent experimental evidence has shown that prion-like propagation of these pathogenic proteins from host cells to recipient cells underlies the onset and progression of neurodegenerative diseases. The reproduction of the pathological features that characterize each disease in cellular and animal models of prion-like propagation also implies that the structural differences in the pathogenic proteins are inherited in a prion-like manner. In this review, we summarize the ultrastructural and biochemical features of pathogenic proteins extracted from the brains of patients with neurodegenerative diseases that accumulate abnormal forms of tau, α-synuclein, and TDP-43, and we discuss how these disease-specific properties are maintained in the brain, based on recent experimental insights.

Deep learning reveals disease-specific signatures of white matter pathology in tauopathies

Although pathology of tauopathies is characterized by abnormal tau protein aggregation in both gray and white matter regions of the brain, neuropathological investigations have generally focused on abnormalities in the cerebral cortex because the canonical aggregates that form the diagnostic criteria for these disorders predominate there. This corticocentric focus tends to deemphasize the relevance of the more complex white matter pathologies, which remain less well characterized and understood. We took a data-driven machine-learning approach to identify novel disease-specific morphologic signatures of white matter aggregates in three tauopathies: Alzheimer disease (AD), progressive supranuclear palsy (PSP), and corticobasal degeneration (CBD). We developed automated approaches using whole slide images of tau immunostained sections from 49 human autopsy brains (16 AD,13 CBD, 20 PSP) to identify cortex/white matter regions and individual tau aggregates, and compared tau-aggregate morphology across these diseases. Tau burden in the gray and white matter for individual subjects strongly correlated in a highly disease-specific fashion. We discovered previously unrecognized tau morphologies for AD, CBD and PSP that may be of importance in disease classification. Intriguingly, our models classified diseases equally well based on either white or gray matter tau staining. Our results suggest that tau pathology in white matter is informative, disease-specific, and linked to gray matter pathology. Machine learning has the potential to reveal latent information in histologic images that may represent previously unrecognized patterns of neuropathology, and additional studies of tau pathology in white matter could improve diagnostic accuracy.

Differential Effects of the Six Human TAU Isoforms: Somatic Retention of 2N-TAU and Increased Microtubule Number Induced by 4R-TAU

In the adult human brain, six isoforms of the microtubule-associated protein TAU are expressed, which result from alternative splicing of exons 2, 3, and 10 of the MAPT gene. These isoforms differ in the number of N-terminal inserts (0N, 1N, 2N) and C-terminal repeat domains (3R or 4R) and are differentially expressed depending on the brain region and developmental stage. Although all TAU isoforms can aggregate and form neurofibrillary tangles, some tauopathies, such as Pick's disease and progressive supranuclear palsy, are characterized by the accumulation of specific TAU isoforms. The influence of the individual TAU isoforms in a cellular context, however, is understudied. In this report, we investigated the subcellular localization of the human-specific TAU isoforms in primary mouse neurons and analyzed TAU isoform-specific effects on cell area and microtubule dynamics in human SH-SY5Y neuroblastoma cells. Our results show that 2N-TAU isoforms are particularly retained from axonal sorting and that axonal enrichment is independent of the number of repeat domains, but that the additional repeat domain of 4R-TAU isoforms results in a general reduction of cell size and an increase of microtubule counts in cells expressing these specific isoforms. Our study points out that individual TAU isoforms may influence microtubule dynamics differentially both by different sorting patterns and by direct effects on microtubule dynamics.

Human tauopathy-derived tau strains determine the substrates recruited for templated amplification

Tauopathies are a subset of neurodegenerative diseases characterized by abnormal tau inclusions. Specifically, three-repeat tau and four-repeat tau in Alzheimer’s disease, three-repeat tau in Pick’s disease (PiD) and four-repeat tau in progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD) form amyloid-like fibrous structures that accumulate in neurons and/or glial cells. Amplification and cell-to-cell transmission of abnormal tau based on the prion hypothesis are believed to explain the onset and progression of tauopathies. Recent studies support not only the self-propagation of abnormal tau, but also the presence of conformationally distinct tau aggregates, namely tau strains. Cryogenic electron microscopy analyses of patient-derived tau filaments have revealed disease-specific ordered tau structures. However, it remains unclear whether the ultrastructural and biochemical properties of tau strains are inherited during the amplification of abnormal tau in the brain. In this study, we investigated template-dependent amplification of tau aggregates using a cellular model of seeded aggregation. Tau strains extracted from human tauopathies caused strain-dependent accumulation of insoluble filamentous tau in SH-SY5Y cells. The seeding activity towards full-length four-repeat tau substrate was highest in CBD-tau seeds, followed by PSP-tau and Alzheimer’s disease (AD)-tau seeds, while AD-tau seeds showed higher seeding activity than PiD-tau seeds towards three-repeat tau substrate. Abnormal tau amplified in cells inherited the ultrastructural and biochemical properties of the original seeds. These results strongly suggest that the structural differences of patient-derived tau strains underlie the diversity of tauopathies, and that seeded aggregation and filament formation mimicking the pathogenesis of sporadic tauopathy can be reproduced in cultured cells. Our results indicate that the disease-specific conformation of tau aggregates determines the tau isoform substrate that is recruited for templated amplification, and also influences the prion-like seeding activity.

New Insights Into Drug Discovery Targeting Tau Protein

Microtubule-associated protein tau is characterized by the fact that it is an intrinsically disordered protein due to its lack of a stable conformation and high flexibility. Intracellular inclusions of fibrillar forms of tau with a β-sheet structure accumulate in the brain of patients with Alzheimer's disease and other tauopathies. Accordingly, detachment of tau from microtubules and transition of tau from a disordered state to an abnormally aggregated state are essential events preceding the onset of tau-related diseases. Many reports have shown that this transition is caused by post-translational modifications, including hyperphosphorylation and acetylation. The misfolded tau is self-assembled and forms a tau oligomer before the appearance of tau inclusions. Animal and pathological studies using human samples have demonstrated that tau oligomer formation contributes to neuronal loss. During the progression of tauopathies, tau seeds are released from cells and incorporated into other cells, leading to the propagation of pathological tau aggregation. Accumulating evidence suggests several potential approaches for blocking tau-mediated toxicity: (1) direct inhibition of pathological tau aggregation and (2) inhibition of tau post-translational modifications that occur prior to pathological tau aggregation, (3) inhibition of tau propagation and (4) stabilization of microtubules. In addition to traditional low-molecular-weight compounds, newer drug discovery approaches such as the development of medium-molecular-weight drugs (peptide- or oligonucleotide-based drugs) and high-molecular-weight drugs (antibody-based drugs) provide alternative pathways to preventing the formation of abnormal tau. Of particular interest are recent studies suggesting that tau droplet formation by liquid-liquid phase separation may be the initial step in aberrant tau aggregation, as well results that implicate roles for tau in dendritic and nuclear functions. Here, we review the mechanisms through which drugs can target tau and consider recent clinical trials for the treatment of tauopathies. In addition, we discuss the utility of these newer strategies and propose future directions for research on tau-targeted therapeutics.

An HDAC6-dependent surveillance mechanism suppresses tau-mediated neurodegeneration and cognitive decline

Tauopathies including Alzheimer’s disease (AD) are marked by the accumulation of aberrantly modified tau proteins. Acetylated tau, in particular, has recently been implicated in neurodegeneration and cognitive decline. HDAC6 reversibly regulates tau acetylation, but its role in tauopathy progression remains unclear. Here, we identified an HDAC6-chaperone complex that targets aberrantly modified tau. HDAC6 not only deacetylates tau but also suppresses tau hyperphosphorylation within the microtubule-binding region. In neurons and human AD brain, HDAC6 becomes co-aggregated within focal tau swellings and human AD neuritic plaques. Using mass spectrometry, we identify a novel HDAC6-regulated tau acetylation site as a disease specific marker for 3R/4R and 3R tauopathies, supporting uniquely modified tau species in different neurodegenerative disorders. Tau transgenic mice lacking HDAC6 show reduced survival characterized by accelerated tau pathology and cognitive decline. We propose that a HDAC6-dependent surveillance mechanism suppresses toxic tau accumulation, which may protect against the progression of AD and related tauopathies.

HDAC6 is a tau deacetylase and acetylation of tau inhibits its function and promotes aggregation. Here the authors show that HDAC6 protects against tau accumulation in a mouse model of tauopathy.

Proteopathic Seed Amplification Assays for Neurodegenerative Disorders

The need for etiological biomarkers for neurodegenerative diseases involving protein aggregation has prompted development of ultrasensitive cellular and cell-free assays based on the prion-like seeding capacity of such aggregates. Among them, prion RT-QuIC assays allow accurate antemortem Creutzfeldt-Jakob disease diagnosis using cerebrospinal fluid and nasal brushings. Analogous assays for synucleinopathies (e.g., Parkinson disease and dementia with Lewy bodies) provide unprecedented diagnostic sensitivity using cerebrospinal fluid. Biosensor cell and tau RT-QuIC assays can detect and discriminate tau aggregates associated with multiple tauopathies (e.g., Alzheimer disease and frontotemporal degeneration). An expanding panel of seed amplification assays should improve diagnostics and therapeutics development.

Structure of NFT: Biochemical Approach

Neurofibrillary tangle (NFT), bundle of paired helical filaments in neurons is one of the defining features of Alzheimer's disease (AD) and their spreads well correlate with disease symptoms and progression of AD. Using the unusual insolubility, NFTs were partially purified and the antibodies were produced. Characterization of these antibodies and biochemical studies of tau in AD revealed that a hyperphosphorylated tau protein is the major component of NFTs. In 1998, mutations in the tau gene were discovered in FTDP-17, demonstrating that abnormalities of tau cause accumulation of tau and neurodegeneration. Abnormal tau pathology occurs not only in AD, but also in other neurodegenerative dementing disorders, such as Pick's disease (PiD), progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD). The tau isoforms accumulated in these inclusions are different among the diseases. Biochemical and proteinchemical analyses of these pathological tau proteins in these tauopathies demonstrated that the protease-resistant cores of the tau aggregates are composed of different microtubule binding regions and distinct between the diseases. Recent Cryo-EM analyses revealed the core structures of tau filaments in AD and PiD, confirming our biochemical observations. Further studies of tau and other abnormal proteins will provide important insights into molecular mechanisms of protein aggregation and prion-like propagation in neurodegenerative diseases.

Roles of tau protein in health and disease

Tau is well established as a microtubule-associated protein in neurons. However, under pathological conditions, aberrant assembly of tau into insoluble aggregates is accompanied by synaptic dysfunction and neural cell death in a range of neurodegenerative disorders, collectively referred to as tauopathies. Recent advances in our understanding of the multiple functions and different locations of tau inside and outside neurons have revealed novel insights into its importance in a diverse range of molecular pathways including cell signalling, synaptic plasticity, and regulation of genomic stability. The present review describes the physiological and pathophysiological properties of tau and how these relate to its distribution and functions in neurons. We highlight the post-translational modifications of tau, which are pivotal in defining and modulating tau localisation and its roles in health and disease. We include discussion of other pathologically relevant changes in tau, including mutation and aggregation, and how these aspects impinge on the propensity of tau to propagate, and potentially drive neuronal loss, in diseased brain. Finally, we describe the cascade of pathological events that may be driven by tau dysfunction, including impaired axonal transport, alterations in synapse and mitochondrial function, activation of the unfolded protein response and defective protein degradation. It is important to fully understand the range of neuronal functions attributed to tau, since this will provide vital information on its involvement in the development and pathogenesis of disease. Such knowledge will enable determination of which critical molecular pathways should be targeted by potential therapeutic agents developed for the treatment of tauopathies.

Molecular Mechanisms in the Pathogenesis of Alzheimer's disease and Tauopathies-Prion-Like Seeded Aggregation and Phosphorylation

Neurofibrillary tau pathology (tangles and threads) and extracellular amyloid-β (Aβ) pathology are defining features of Alzheimer’s disease. For 25 years, most research has focused on the amyloid hypothesis of AD pathogenesis and progression. But, because of failures in clinical trials of Aβ-targeted therapies and the new concept of prion-like propagation of intracellular abnormal proteins, tau has come back into the spotlight as a candidate therapeutic target in AD. Tau pathologies are found in a range of neurodegenerative disorders, but extensive analyses of pathological tau in diseased brains has demonstrated that the abnormal tau protein in each disease is structurally distinct, supporting the idea that progression of the diverse but characteristic tau pathologies occurs through prion-like seed-dependent aggregation. Therefore, intervention in the conversion of normal tau to abnormal forms and in cell-to-cell transmission of tau may be the key to development of disease-modifying therapies for AD and other dementing disorders.

Biochemical classification of tauopathies by immunoblot, protein sequence and mass spectrometric analyses of sarkosyl-insoluble and trypsin-resistant tau

Intracellular filamentous tau pathology is the defining feature of tauopathies, which form a subset of neurodegenerative diseases. We have analyzed pathological tau in Alzheimer’s disease, and in frontotemporal lobar degeneration associated with tauopathy to include cases with Pick bodies, corticobasal degeneration, progressive supranuclear palsy, and ones due to intronic mutations in MAPT. We found that the C-terminal band pattern of the pathological tau species is distinct for each disease. Immunoblot analysis of trypsin-resistant tau indicated that the different band patterns of the 7–18 kDa fragments in these diseases likely reflect different conformations of tau molecular species. Protein sequence and mass spectrometric analyses revealed the carboxyl-terminal region (residues 243–406) of tau comprises the protease-resistant core units of the tau aggregates, and the sequence lengths and precise regions involved are different among the diseases. These unique assembled tau cores may be used to classify and diagnose disease strains. Based on these results, we propose a new clinicopathological classification of tauopathies based on the biochemical properties of tau.

Characterization of tau oligomeric seeds in progressive supranuclear palsy

Background

Progressive supranuclear palsy (PSP) is a neurodegenerative tauopathy which is primarily defined by the deposition of tau into globose-type neurofibrillary tangles (NFT). Tau in its native form has important functions for microtubule dynamics. Tau undergoes alternative splicing in exons 2, 3, and 10 which results in six different isoforms. Products of splicing on exon 10 are the most prone to mutations. Three repeat (3R) and four repeat (4R) tau, like other disease-associated amyloids, can form oligomers which may then go on to further aggregate and form fibrils. Recent studies from our laboratory and others have provided evidence that tau oligomers, not NFTs, are the most toxic species in neurodegenerative tauopathies and seed the pathological spread of tau.

Results

Analysis of PSP brain sections revealed globose-type NFTs, as well as both phosphorylated and unphosphorylated tau oligomers. Analysis of PSP brains via Western blot and ELISA revealed the presence of increased levels of tau oligomers compared to age-matched control brains. Oligomers were immunoprecipitated from PSP brain and were capable of seeding the oligomerization of both 3R and 4R tau isoforms.

Conclusions

This is the first time tau oligomers have been characterized in PSP. These results indicate that tau oligomers are an important component of PSP pathology, along with NFTs. The ability of PSP brain-derived tau oligomers to seed 3R and 4R tau suggests that these oligomers represent the pathological species responsible for disease propagation and the presence of oligomers in a pure neurodegenerative tauopathy implies a common neuropathological process for tau seen in diseases with other amyloid proteins.

Acetylated tau, a novel pathological signature in Alzheimer's disease and other tauopathies

The microtubule-binding protein, tau, is the major component of neurofibrillary inclusions characteristic of Alzheimer's disease and related neurodegenerative tauopathies. When tau fibrillizes, it undergoes abnormal post-translational modifications resulting in decreased solubility and altered microtubule-stabilizing properties. Recently, we reported that the abnormal acetylation of tau at lysine residue 280 is a novel, pathological post-translational modification. Here, we performed detailed immunohistochemistry to further examine acetylated-tau expression in Alzheimer's disease and other major tauopathies. Immunohistochemistry using a polyclonal antibody specific for acetylated-tau at lysine 280 was conducted on 30 post-mortem central nervous system regions from patients with Alzheimer's disease (10 patients), corticobasal degeneration (5 patients), and progressive supranuclear palsy (5 patients). Acetylated-tau pathology was compared with the sequential emergence of other tau modifications in the Alzheimer's disease hippocampus using monoclonal antibodies to multiple well-characterized tau epitopes. All cases studied showed significant acetylated-tau pathology in a distribution pattern similar to hyperphosphorylated-tau. Acetylated-tau pathology was largely in intracellular, thioflavin-S-positive tau inclusions in Alzheimer's disease, and also thioflavin-S-negative pathology in corticobasal degeneration and progressive supranuclear palsy. Acetylated-tau was present throughout all stages of Alzheimer's disease pathology, but was more prominently associated with pathological tau epitopes in moderate to severe-stage cases. These temporal and morphological immunohistochemical features suggest acetylation of tau at this epitope is preceded by early modifications, including phosphorylation, and followed by later truncation events and cell death in Alzheimer's disease. Acetylation of tau at lysine 280 is a pathological modification that may contribute to tau-mediated neurodegeneration by both augmenting losses of normal tau properties (reduced solubility and microtubule assembly) as well as toxic gains of function (increased tau fibrillization). Thus, inhibiting tau acetylation could be a disease-modifying target for drug discovery target in tauopathies.

Tauopathies: one disease or many?

Tauopathies are a group of disorders that have in common abnormal accumulation of tau protein in the brain. Although the different tauopathies have long been considered to be separate diseases, it is now clear that progressive supranuclear palsy, corticobasal degeneration and some forms of tau-positive frontotemporal lobar degeneration share clinical, pathological and genetic features. The important overlap between these disorders suggest they may represent different phenotypes of a single disease process, the clinical result depending on the topography of pathological lesions as well as other unknown factors.

Tau as a biomarker of neurodegenerative diseases

The microtubule-associated protein Tau is mainly expressed in neurons of the CNS and is crucial in axonal maintenance and axonal transport. The rationale for Tau as a biomarker of neurodegenerative diseases is that it is a major component of abnormal intraneuronal aggregates observed in numerous tauopathies, including Alzheimer's disease. The molecular diversity of Tau is very useful when analyzing it in the brain or in the peripheral fluids. Immunohistochemical and biochemical characterization of Tau aggregates in the brain allows the postmortem classification and differential diagnosis of tauopathies. As peripheral biomarkers of Alzheimer's disease in the cerebrospinal fluid, Tau proteins are now validated for diagnosis and predictive purposes. For the future, the detailed characterization of Tau in the brain and in peripheral fluids will lead to novel promising biomarkers for differential diagnosis of dementia and monitoring of therapeutics.

Phosphorylated TDP-43 in Alzheimer's disease and dementia with Lewy bodies

Phosphorylated and proteolytically cleaved TDP-43 is a major component of the ubiquitin-positive inclusions in the most common pathological subtype of frontotemporal lobar degeneration (FTLD-U). Intracellular accumulation of TDP-43 is observed in a subpopulation of patients with other dementia disorders, including Alzheimer's disease (AD) and dementia with Lewy bodies (DLB). However, the pathological significance of TDP-43 pathology in these disorders is unknown, since biochemical features of the TDP-43 accumulated in AD and DLB brains, especially its phosphorylation sites and pattern of fragmentation, are still unclear. To address these issues, we performed immunohistochemical and biochemical analyses of AD and DLB cases, using phosphorylation-dependent anti-TDP-43 antibodies. We found a higher frequency of pathological TDP-43 in AD (36-56%) and in DLB (53-60%) than previously reported. Of the TDP-43-positive cases, about 20-30% showed neocortical TDP-43 pathology resembling the FTLD-U subtype associated with progranulin gene (PGRN) mutations. Immunoblot analyses of the sarkosyl-insoluble fraction from cases with neocortical TDP-43 pathology showed intense staining of several low-molecular-weight bands, corresponding to C-terminal fragments of TDP-43. Interestingly, the band pattern of these C-terminal fragments in AD and DLB also corresponds to that previously observed in the FTLD-U subtype associated with PGRN mutations. These results suggest that the morphological and biochemical features of TDP-43 pathology are common between AD or DLB and a specific subtype of FTLD-U. There may be genetic factors, such as mutations or genetic variants of PGRN underlying the co-occurrence of abnormal deposition of TDP-43, tau and alpha-synuclein.

Conformational changes and cleavage of tau in Pick bodies parallel the early processing of tau found in Alzheimer pathology

Neuronal protein inclusions are a common feature in Alzheimer disease (AD) and Pick disease. Even though the inclusions are morphologically different, flame-shape structure for AD vs. spherical structure for Pick disease, both have filaments mainly composed of tau protein. In AD, a well-defined pattern of conformational changes and truncation has been described. In this study, we used laser scanning confocal microscopy to characterize and compare the processing of tau protein during Pick disease with that found in AD. We found that tau protein of Pick disease preserves most of the relevant epitopes found in AD, the conformational foldings labelled by Alz-50 and Tau-66, the cleavage sites D(421) and E(391), as well as many phosphorylated sites, such as Ser(199/202), Thr(205) and Ser(396/404). We found a strong pattern of association between phosphorylation and cleavage at site D(421), as well as the phosphorylation and the conformational Alz-50 epitope. When we used late AD markers such as the conformational Tau-66 epitope and MN423 (cleavage at site E(391)) in Pick bodies (PBs), the overlap was significantly less. Furthermore, following morphological quantification, we found significantly higher numbers of phosphorylated tau in PBs. Overall, our findings suggest that phosphorylation is an early event, likely preceding the cleavage of tau at D(421). Despite this consistency with AD, we found a major distinction, namely that PBs lack beta-sheet conformation. We propose a scheme of early tau processing in these structures, similar to neurofibrillary tangles of AD.

Phosphorylation and cleavage of tau in non-AD tauopathies

The tau protein, well known as the primary component of neurofibrillary tangles, also comprises the Pick bodies found in Pick's disease (PiD) and the glial lesions associated with progressive supranuclear palsy (PSP) and cortico-basal ganglionic degeneration (CBD). Many of the tau alterations that are characteristic of Alzheimer's disease have also been identified in PSP and CBD. In this report, we examine three non-AD tauopathies (PSP, CBD, and PiD) for the presence of two specific tau alterations, phosphorylation at Ser422 and truncation at Asp421. We find that truncation at Asp421 is an alteration that is unique to neuronal lesions, occurring in Pick bodies as well as in neurofibrillary tangles, but not in lesions associated with glia. Conversely, phosphorylation at Ser422 is not only present in all these lesions, but identifies additional glial and neuronal pathology in disease-susceptible cortical regions. These results suggest that the molecular alterations of tau that occur during the initial process of tangle formation in AD are similar in non-AD tauopathies, but the middle and later changes are not common to all diseases.

Neurofibrillary tangle-predominant dementia: comparison with classical Alzheimer disease

Neurofibrillary tangle predominant dementia (NFTPD) is a subset of late onset dementia, clinically different from traditional "plaque and tangle" Alzheimer disease (AD): later onset, shorter duration, less severe cognitive impairment, and almost absence of ApoE epsilon4. Neuropathology reveals abundant allocortical neurofibrillary pathology with no or few isocortical tau lesions, absence of neuritic plaques, absence or scarcity of amyloid deposits, but neurofibrillary changes comprising both 3 and 4 repeat (3R and 4R) tau immunohistochemistry are not significantly different from those in classical AD. Comparing 51 autopsy cases of NFTPD with 244 classical AD subjects, the nosology of NFTPD and its differences from AD are discussed.

Adenosine A2A receptors are up-regulated in Pick's disease frontal cortex

Adenosine A2A receptors (A2AR) are highly expressed in striatum. However, they are also present in extrastriatal structures. A2AR were studied in post-mortem human frontal cortex from Pick's disease (PiD) and age-matched non-demented controls by radioligand binding assays, Western-blotting, real-time PCR and adenylyl cyclase activity determination. Saturation binding assay using [3H]ZM 241385, a selective A2A antagonist, as radioligand revealed a significant increase in total adenosine A2AR numbers (Bmax) in frontal cortex from PiD samples (191% of control Bmax), suggesting up-regulation of this receptor. A significant increase in the level of A2AR was also detected by Western-blotting. Furthermore, expression of mRNA coding A2AR determined by quantitative real-time PCR was enhanced. In agreement, stimulation of adenylyl cyclase by CGS 21680, a selective A2A receptor agonist, was significantly strengthened. Up-regulation of A2B receptors and their corresponding mRNA was also observed. These results show that A2A adenosine receptor/adenylyl cyclase transduction pathway is up-regulated and sensitized in frontal cortex brain from PiD.

Amyotrophic lateral sclerosis with dementia: an autopsy case showing many Bunina bodies, tau-positive neuronal and astrocytic plaque-like pathologies, and pallido-nigral degeneration

We report the case of a 54-year-old woman with mental retardation who developed frontotemporal dementia and amyotrophic lateral sclerosis (ALS) in the presenium. She presented with dementia at age 48, and motor neuron signs developed at age 53. She had no family history of dementia or ALS. Postmortem examination disclosed histopathological features of ALS, including pyramidal tract degeneration, mild loss of motor neurons, and many Bunina bodies immunoreactive for cystatin C, but not ubiquitin-positive inclusions. Unusual features of this case included severe neuronal loss in the substantia nigra and medial globus pallidus. The subthalamic nucleus, limbic system, and cerebral cortex were well preserved. In addition, neurofibrillary tangles (NFTs) were found in the frontal, temporal, insular, and cingulate cortices, nucleus basalis of Meynert, and locus coeruleus, and to a lesser degree, in the dentate nucleus, cerebellum, hippocampus, and amygdala. No ballooned neurons, tufted astrocytes, or astrocytic plaques were found. Tau immunostaining demonstrated many pretangles rather than NFTs and glial lesions resembling astrocytic plaques in the frontal and temporal cortices. This glial tau pathology predominantly developed in the middle to deep layers in the primary motor cortex, and was frequently associated with the walls of blood vessels. NFTs were immunolabeled with 3-repeat and 4-repeat specific antibodies against tau, respectively. Although the pathophysiological relationship between tau pathology and the selective involvement of motor neurons, substantia nigra, and globus pallidus was unclear, we considered that it might be more than coincidental.

TDP-43 is a component of ubiquitin-positive tau-negative inclusions in frontotemporal lobar degeneration and amyotrophic lateral sclerosis

Ubiquitin-positive tau-negative neuronal cytoplasmic inclusions and dystrophic neurites are common pathological features in frontotemporal lobar degeneration (FTLD) with or without symptoms of motor neuron disease and in amyotrophic lateral sclerosis (ALS). Using biochemical and immunohistochemical analyses, we have identified a TAR DNA-binding protein of 43 kDa (TDP-43), a nuclear factor that functions in regulating transcription and alternative splicing, as a component of these structures in FTLD. Furthermore, skein-like inclusions, neuronal intranuclear inclusions, and glial inclusions in the spinal cord of ALS patients are also positive for TDP-43. Dephosphorylation treatment of the sarkosyl insoluble fraction has shown that abnormal phosphorylation takes place in accumulated TDP-43. The common occurrence of intracellular accumulations of TDP-43 supports the hypothesis that these disorders represent a clinicopathological entity of a single disease, and suggests that they can be newly classified as a proteinopathy of TDP-43.

Glial fibrillary acidic protein is a major target of glycoxidative and lipoxidative damage in Pick's disease

Pick's disease is a subset of fronto-temporal dementia characterised by severe atrophy of the temporal and frontal lobes due to marked neuronal loss accompanied by astrocytic gliosis enriched in glial acidic protein. The remaining neurones have intracytoplasmic inclusions composed of hyperphosphorylated tau, called Pick bodies, in addition to hyperphosphorylated tau in astrocytes and oligodendrocytes. Gel electrophoresis and western blotting using markers of glycoxidation (advanced glycation end products, N-carboxyethyl-lysine and N-carboxymethyl-lysine: AGE, CEL, CML, respectively) and lipoxidation (4-hydroxy-2-nonenal: HNE, and malondialdehyde-lysine: MDAL) were used in the frontal and occipital cortex in three Pick's disease cases and three age-matched controls. In Pick's disease, increased AGE, CML, CEL, HNE and MDAL bands of about 50 kDa were observed in the frontal cortex (but not in the occipital cortex) in association with increased density of glial acidic protein bands. Bi-dimensional gel electrophoresis and western blotting also disclosed increased amounts and numbers of glial acidic protein isoforms in the frontal cortex in Pick's disease. Moreover, redox proteomics showed glycoxidation, as revealed with anti-CEL antibodies and lipoxidation using anti-HNE antibodies, of at least three glial acidic protein isoforms. The present results demonstrate that glial acidic protein is a target of oxidative damage in the frontal cortex in Pick's disease.

Single cell gene expression profiling in Alzheimer's disease

Development and implementation of microarray techniques to quantify expression levels of dozens to hundreds to thousands of transcripts simultaneously within select tissue samples from normal control subjects and neurodegenerative diseased brains has enabled scientists to create molecular fingerprints of vulnerable neuronal populations in Alzheimer's disease (AD) and related disorders. A goal is to sample gene expression from homogeneous cell types within a defined region without potential contamination by expression profiles of adjacent neuronal subpopulations and nonneuronal cells. The precise resolution afforded by single cell and population cell RNA analysis in combination with microarrays and real-time quantitative polymerase chain reaction (qPCR)-based analyses allows for relative gene expression level comparisons across cell types under different experimental conditions and disease progression. The ability to analyze single cells is an important distinction from global and regional assessments of mRNA expression and can be applied to optimally prepared tissues from animal models of neurodegeneration as well as postmortem human brain tissues. Gene expression analysis in postmortem AD brain regions including the hippocampal formation and neocortex reveals selectively vulnerable cell types share putative pathogenetic alterations in common classes of transcripts, for example, markers of glutamatergic neurotransmission, synaptic-related markers, protein phosphatases and kinases, and neurotrophins/neurotrophin receptors. Expression profiles of vulnerable regions and neurons may reveal important clues toward the understanding of the molecular pathogenesis of various neurological diseases and aid in identifying rational targets toward pharmacotherapeutic interventions for progressive, late-onset neurodegenerative disorders such as mild cognitive impairment (MCI) and AD.

Expression of transcription factors c-Fos, c-Jun, CREB-1 and ATF-2, and caspase-3 in relation with abnormal tau deposits in Pick's disease

Hyper-phosphorylated tau deposition in Pick bodies and neuron loss are major hallmarks of Pick's disease (PiD). However, there is no regional correlation between neuron loss and Pick bodies, as illustrated in dentate gyrus, where Pick bodies are present in almost every neuron, whereas cell death, if present, is not a major event. In order to better understand the possible role of selected transcription factors and members of the caspase family in cell death and cell survival, immunohistochemistry to c-Fos, c-Jun, CREB-1, ATF-2; c-Fos(P), c-Jun(P) and CREB-1(P); and procaspase-8, procaspase-3 and active (cleaved) caspase-3 immunohistochemistry was carried out in the frontal cortex and hippocampus. Increased expression of c-Fos, c-Jun, CREB-1 and ATF-2 was observed in PiD cases. Increased c-Fos(P), c-Jun(P) and CREB-1(P) was also found in the nuclei of neurons in diseased brains. Interestingly, c-Fos but not c-Fos(P) co-localized in many Pick bodies, as observed by double labelling-immunofluorescence and confocal microscopy. Pro-caspase-8 and pro-caspase-3 were increased in PiD. Moreover, granular active caspase-3 was observed in the nuclei as was aggregated active caspase-3 in the cytoplasm of neurons in PiD. Finally, double-labelling immunofluorescence and confocal microscopy disclosed co-localization of cytoplasmic active caspase-3 only in neurons with Pick bodies. Together, these findings show an increased expression of selected transcription factors and active (phosphorylated) forms in PiD, c-Fos sequestration in Pick bodies, and increased active caspase-3 expression in relation with Pick bodies. Since all these findings were observed equally in neurons of both vulnerable regions (frontal cortex) and resistant regions (dentate gyrus), it may be suggested that transcription factors are only barely related with cell death. Active caspase-3 is associated with tau deposition in Pick bodies, but it is not a marker of cell death in the dentate gyrus in PiD. The present findings are in line with the previous studies showing tau products cleaved by caspase-3, as recognized by specific tau-cleaved antibodies, in Alzheimer's disease and other tauopathies.

Relationship between microtubule-binding repeats and morphology of neurofibrillary tangle in Alzheimer's disease

OBJECTIVE

The present study was performed to compare the distributions of three-repeat (3R) and four-repeat (4R) neurofibrillary tangles (NFT) with those of pretangles (p-NFT), intracellular NFT (i-NFT), and extracellular NFT (e-NFT) in the hippocampus of Alzheimer's disease brains.

METHODS

NFT labeling was performed using anti-tau antibodies: pSer262 for p-NFT, pSer422 for i-NFT, AT8 for e-NFT, RD3 for 3R, and RD4 for 4R tau, and Gallyas impregnation for the NFT population. RD4- and pSer422-positive NFT were detected predominantly in sectors from CA2 to CA4, while RD3- and pSer262-positive NFT were predominantly present in CA1, the entorhinal cortex, and the subiculum. The tau epitope recognized by pSer262 belongs to 4R tau but it showed a strong correlation with RD3- and AT8-positive NFT.

CONCLUSIONS

Sectors CA2-CA4 showed predominantly 4R-NFT containing the pSer422 epitope. pSer262 may detect the process of transformation from p-NFT to i-NFT, and e-NFT consisted predominantly of 3R tau.

Individual and regional variations of phospho-tau species in progressive supranuclear palsy

The purpose of this study was to learn about possible variations in phospho-tau profiles in terms of case-to-case differences, regional modifications and diversification of tau phosphorylation sites in five PSP cases with moderate to severe frontosubcortical dysfunction. Gel electrophoresis of sarkosyl-insoluble fractions and Western blotting with five anti-tau phospho-specific antibodies directed to phosphorylation sites Thr181, Ser202, Ser214, Ser396 and Ser422 were used to study four brain regions including frontal cortex, area 8, subcortical white matter of the frontal lobe, caudate/putamen: striatum, and basis pontis: pons. Although two bands of 66 and 62 kDa were observed in almost every region in each case, the intensity of the bands depends on the anti-tau phospho-specific antibody. More importantly, bands of 72, 50/55 and 37 kDa were commonly found in PSP brains, whereas other bands of about 60, 42, 33 and 29 kDa were irregularly observed. The pattern of bands differed slightly from one case to another and from one region to another. Moreover, the phospho-tau profile differed depending on the anti-tau phospho-specific antibody used. These data suggest that several species of tau are variably phosphorylated at a given time in a given region (and probably in a given cell), and that tau aggregates are composed of several phosphorylated truncated or cleaved tau molecules, in addition to phosphorylated complete tau isoforms.

Hereditary Pick's disease with the G272V tau mutation shows predominant three-repeat tau pathology

Frontotemporal dementia and parkinsonism linked to chromosome 17 have been associated with mutations in the microtubule associated protein tau (MAPT or tau) gene. This disorder is characterized by a large spectrum of neuronal and glial tau lesions in different brain regions. Pick bodies were found in a family with hereditary Pick's disease with the G272V mutation and in several families with other tau mutations in exons 9 and 11-13. The biochemical composition of Pick bodies varies between these mutations. Until recently, no detailed biochemical characterization of G272V brain material was done owing to unavailability of fresh frozen brain material. We now report a detailed study using the immunohistochemistry, western blots and electron microscopy of two brains with the G272V mutation that recently became available. Both brains showed severe neuronal loss in the temporal cortex, whereas in the frontal cortex the loss was less; and abundant Pick bodies in the dentate gyrus of the hippocampus, and caudate nucleus. The Pick bodies consisted exclusively of three-repeat (3R) isoforms, as was demonstrated by isoform-specific antibodies and supported by western blot analysis of sarkosyl-insoluble tau. These observations confirm that this family diagnosed with hereditary Pick disease meets all the criteria for this condition, including the presence of Pick bodies that are unphosphorylated at Ser262 and contain twisted filaments with long periodicity consisting only of 3R tau.

Abnormal Group I Metabotropic Glutamate Receptor Expression and Signaling in the Frontal Cortex in Pick Disease

Group I metabotropic glutamate receptors (mGluR1) regulate synaptic transmission through the stimulation of phospholipase Cbeta1 (PLCbeta1) and then by the activation of protein kinase C (PKC). Considering these properties, it is conceivable that major cortical functional deficits may be attributed to abnormal mGluR processing and signaling. The present work examines mGluRI expression and signaling in the frontal cortex (area 8) of 3 cases with Pick disease (PiD), a neurodegenerative disease with abnormal phospho-tau accumulation, in comparison with 3 age-matched controls by means of glutamate binding assays, enzymatic activity, gel electrophoresis and Western blotting, solubility and immunoprecipitation assays, and confocal microscopy. Reduced expression levels of PLCbeta1 and reduced PLCbeta1 activity have been found in PiD. The expression levels of the nonrelated phospholipase PLCgamma, a substrate of tyrosine kinase, are also reduced in PiD. This is accompanied by a marked decrease in the expression of cPKCalpha and increased expression of the inner band (76 kDa) of the nPKCdelta doublet at the expense of a decrease of the phosphorylated (active) form (78 kDa). In contrast, L-[3H]glutamate-specific binding to mGluRs is augmented in PiD cases, mainly because of the higher mGluR1 and mGluRs expression levels detected. No modifications in PLCbeta1 solubility have been observed in PiD and no interactions between PLCbeta1 and tau have been demonstrated in diseased and control cases. Moreover, double-labeling immunofluorescence and confocal microscopy have shown no colocalization of phospho-tau (AT8 antibody) and PLCbeta1 in phospho-tau inclusions, including Pick bodies. These results demontrate for the first time abnormal mGluR signaling in the cerebral cortex in PiD and selective vulnerability of phospholipases and PKC to PiD.

SEREX identification of new tumor antigens linked to melanoma-associated retinopathy

Metastatic melanoma still has a very poor prognosis since it withstands conventional therapies like surgery or chemotherapy. A paraneoplastic autoimmune manifestation of this disease is melanoma-associated retinopathy (MAR). MAR has been associated with prolonged survival and may be an early marker of tumor progression. By screening a retina and a melanoma cDNA phage library by SEREX using sera of patients suffering from melanoma and, in some cases, clinical symptoms of MAR, we identified 20 new antigens (HD-MM-28-47), of which 14 clones had high homology to well-known genes. Six of these genes had previously been associated with retina: rhodopsin, visual arrestin, MEK1, SRPX, BBS1 and galectin-3. Individual clones were recognized by up to 43% of patients' sera, while sera of healthy volunteers were negative except in 2 cases. The expression profile of the antigens identified on the basis of homologous EST database entries in healthy tissues was ubiquitous to differential. Using RT-PCR, we found frequent expression of preselected antigens in melanoma cell lines. For rhodopsin, this could be quantified by quantitative PCR. Retinal proteins were recognized by serum antibodies of melanoma patients but not healthy controls. The role of these antigens in MAR awaits further investigation. (Supplementary material for this article can be found on the International Journal of Cancer website at http://www.interscience.wiley.com/jpages/0020-7136/suppmat/index.html.)

Proteolysis of non-phosphorylated and phosphorylated tau by thrombin

The microtubule-associated protein tau aggregates intracellularly by unknown mechanisms in Alzheimer's disease and other tauopathies. A contributing factor may be a failure to break down free cytosolic tau, thus creating a surplus for aggregation, although the proteases that degrade tau in brain remain unknown. To address this issue, we prepared cytosolic fractions from five normal human brains and from perfused rat brains and incubated them with or without protease inhibitors. D-Phenylalanyl-L-prolylarginyl chloromethyl ketone, a thrombin-specific inhibitor, prevented tau breakdown in these fractions, suggesting that thrombin is a brain protease that processes tau. We next exposed human recombinant tau to purified human thrombin and analyzed the fragments by N-terminal sequencing. We found that thrombin proteolyzed tau at multiple arginine and lysine sites. These include Arg(155)-Gly(156), Arg(209)-Ser(210), Arg(230)-Thr(231), Lys(257)-Ser(258), and Lys(340)-Ser(341) (numbering according to the longest human tau isoform). Temporally, the initial cleavage occurred at the Arg(155)-Gly(156) bond. Proteolysis of the resultant C-terminal tau fragment then proceeded bidirectionally. When tau was phosphorylated by glycogen synthase kinase-3beta, most of these proteolytic processes were inhibited, except for the first cleavage at the Arg(155)-Gly(156) bond. Furthermore, paired helical filament tau prepared from Alzheimer's disease brain was more resistant to thrombin proteolysis than following dephosphorylation by alkaline phosphatase. The results suggest a possible role for thrombin in proteolysis of tau under physiological and/or pathological conditions in human brains. They are consistent with the hypothesis that phosphorylation of tau inhibits proteolysis by thrombin or other endogenous proteases, leading to aggregation of tau into insoluble fibrils.

Identification of amino-terminally cleaved tau fragments that distinguish progressive supranuclear palsy from corticobasal degeneration

Progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD) are neurodegenerative diseases that are characterized by intracytoplasmic aggregates of hyperphosphorylated tau with four microtubule-binding repeats. Although PSP and CBD have distinctive pathological features, no biochemical difference in aggregated tau has been identified. In this study, we examined the brains of eight patients with PSP, six patients with CBD, and one atypical case with pathological features of both CBD and PSP. On immunoblots of sarkosyl-insoluble brain extracts, a 33kDa band predominated in the low molecular weight tau fragments in PSP, whereas two closely related bands of approximately 37kDa predominated in CBD. Immunoblots of the atypical case showed both the 33kDa band and the 37kDa doublet. Protein sequencing and immunochemical analyses showed that the 33kDa band and the 37kDa doublet consisted of the carboxyl half of tau with different amino termini. These results suggest that, despite the identical composition of tau isoforms, different proteolytic processing of abnormal tau takes place in these two diseases. Such a biochemical divergence may be related to the neuropathological features of these diseases.