A double-labeling immunohistochemical study of tau exon 10 in Alzheimer's disease, progressive supranuclear palsy and Pick's disease

Influence of FTDP-17 mutants on circular tau RNAs

At least 53 mutations in the microtubule associated protein tau gene (MAPT) have been identified that cause frontotemporal dementia. 47 of these mutations are localized between exons 7 and 13. They could thus affect the formation of circular RNAs (circRNAs) from the MAPT gene that occurs through backsplicing from exon 12 to either exon 10 or exon 7. We analyzed representative mutants and found that five FTDP-17 mutations increase the formation of 12➔7 circRNA and three different mutations increase the amount of 12➔10 circRNA. CircRNAs are translated after undergoing adenosine to inosine RNA editing, catalyzed by ADAR enzymes. We found that the interferon induced ADAR1-p150 isoform has the strongest effect on circTau RNA translation. ADAR1-p150 activity had a stronger effect on circTau RNA expression and strongly decreased 12➔7 circRNA, but unexpectedly increased 12➔10 circRNA. In both cases, ADAR-activity strongly promoted translation of circTau RNAs. Unexpectedly, we found that the 12➔7 circTau protein interacts with eukaryotic initiation factor 4B (eIF4B), which is reduced by four FTDP-17 mutations located in the second microtubule domain. These are the first studies of the effect of human mutations on circular RNA formation and translation. They show that point mutations influence circRNA expression levels, likely through changes in pre-mRNA structures. The effect of the mutations is surpassed by editing of the circular RNAs, leading to their translation. Thus, circular RNAs and their editing status should be considered when analyzing FTDP-17 mutations.

Influence of FTDP-17 mutants on circular Tau RNAs

At least 53 mutations in the microtubule associated protein tau gene (MAPT) have been identified that cause frontotemporal dementia. 47 of these mutations are localized between exons 7 and 13. They could thus affect the formation of circular RNAs (circRNAs) from the MAPT gene that occur through backsplicing from exon 12 to either exon 10 or exon 7. We analyzed representative mutants and found that five FTDP-17 mutations increase the formation of 12➔7 circRNA and three different mutations increase the amount of 12➔10 circRNA. CircRNAs are translated after undergoing adenosine to inosine RNA editing, catalyzed by ADAR enzymes. We found that the interferon induced ADAR1-p150 isoform has the strongest effect on circTau RNA translation. ADAR1-p150 activity had a stronger effect on circTau RNA expression and strongly decreased 12➔7 circRNA, but unexpectedly increased 12➔10 circRNA. In both cases, ADAR-activity strongly promoted translation of circTau RNAs. Unexpectedly, we found that the 12➔7 circTau protein interacts with eukaryotic initiation factor 4B (eIF4B), which is reduced by four FTDP-17 mutations located in the second microtubule domain. These are the first studies of the effect of human mutations on circular RNA formation and translation. They show that point mutations influence circRNA expression levels, likely through changes in the secondary pre-mRNA structures. The effect of the mutations is surpassed by editing of the circular RNAs, leading to their translation. Thus, circular RNAs and their editing status should be considered when analyzing FTDP-17 mutations.

Highlights

47/53 known FTDP-17 mutations are located in regions that could influence generation of circular RNAs from the MAPT geneCircular Tau RNAs are translated after adenosine to inosine RNA editing, most effectively caused by ADAR1-p150FTDP-17 mutations influence both circTau RNA and circTau protein expression levelsCircTau protein expression levels do not correlate with circTau RNA expression levelsCircTau proteins bind to eukaryotic initiation factor 4B, which is antagonized by FTDP-17 mutations in exon 10.

Graphic Abstract

α-Synuclein molecular behavior and nigral proteomic profiling distinguish subtypes of Lewy body disorders

Lewy body disorders (LBD), characterized by the deposition of misfolded α-synuclein (α-Syn), are clinically heterogeneous. Although the distribution of α-Syn correlates with the predominant clinical features, the burden of pathology does not fully explain the observed variability in clinical presentation and rate of disease progression. We hypothesized that this heterogeneity might reflect α-Syn molecular diversity, between both patients and different brain regions. Using an ultra-sensitive assay, we evaluated α-Syn seeding in 8 brain regions from 30 LBD patients with different clinical phenotypes and disease durations. Comparing seeding across the clinical phenotypes revealed that hippocampal α-Syn from patients with a cognitive-predominant phenotype had significantly higher seeding capacity than that derived from patients with a motor-predominant phenotype, whose nigral-derived α-Syn in turn had higher seeding capacity than that from cognitive-predominant patients. Interestingly, α-Syn from patients with rapid disease progression (< 3 years to development of advanced disease) had the highest nigral seeding capacity of all the patients included. To validate these findings and explore factors underlying seeding heterogeneity, we performed in vitro toxicity assays, and detailed neuropathological and biochemical examinations. Furthermore, and for the first time, we performed a proteomic-wide profiling of the substantia nigra from 5 high seeder and 5 low seeder patients. The proteomic data suggests a significant disruption in mitochondrial function and lipid metabolism in high seeder cases compared to the low seeders. These observations suggest that distinct molecular populations of α-Syn may contribute to heterogeneity in phenotypes and progression rates in LBD and imply that effective therapeutic strategies might need to be directed at an ensemble of differently misfolded α-Syn species, with the relative contribution of their differing impacts accounting for heterogeneity in the neurodegenerative process.

Alpha-synuclein seeding shows a wide heterogeneity in multiple system atrophy

Background

Multiple system atrophy (MSA) is a neurodegenerative condition characterized by variable combinations of parkinsonism, autonomic failure, cerebellar ataxia and pyramidal features. Although the distribution of synucleinopathy correlates with the predominant clinical features, the burden of pathology does not fully explain observed differences in clinical presentation and rate of disease progression. We hypothesized that the clinical heterogeneity in MSA is a consequence of variability in the seeding activity of α-synuclein both between different patients and between different brain regions.

Methods

The reliable detection of α-synuclein seeding activity derived from MSA using cell-free amplification assays remains challenging. Therefore, we conducted a systematic evaluation of 168 different reaction buffers, using an array of pH and salts, seeded with fully characterized brain homogenates from one MSA and one PD patient. We then validated the two conditions that conferred the optimal ability to discriminate between PD- and MSA-derived samples in a larger cohort of 40 neuropathologically confirmed cases, including 15 MSA. Finally, in a subset of brains, we conducted the first multi-region analysis of seeding behaviour in MSA.

Results

Using our novel buffer conditions, we show that the physicochemical factors that govern the in vitro amplification of α-synuclein can be tailored to generate strain-specific reaction buffers that can be used to reliably study the seeding capacity from MSA-derived α-synuclein. Using this novel approach, we were able to sub-categorize the 15 MSA brains into 3 groups: high, intermediate and low seeders. To further demonstrate heterogeneity in α-synuclein seeding in MSA, we conducted a comprehensive multi-regional evaluation of α-synuclein seeding in 13 different regions from 2 high seeders, 2 intermediate seeders and 2 low seeders.

Conclusions

We have identified unexpected differences in seed-competent α-synuclein across a cohort of neuropathologically comparable MSA brains. Furthermore, our work has revealed a substantial heterogeneity in seeding activity, driven by the PBS-soluble α-synuclein, between different brain regions of a given individual that goes beyond immunohistochemical observations. Our observations pave the way for future subclassification of MSA, which exceeds conventional clinical and neuropathological phenotyping and considers the structural and biochemical heterogeneity of α-synuclein present. Finally, our methods provide an experimental framework for the development of vitally needed, rapid and sensitive diagnostic assays for MSA.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40035-022-00283-4.

The Disease Associated Tau35 Fragment has an Increased Propensity to Aggregate Compared to Full-Length Tau

Tau35 is a truncated form of tau found in human brain in a subset of tauopathies. Tau35 expression in mice recapitulates key features of human disease, including progressive increase in tau phosphorylation, along with cognitive and motor dysfunction. The appearance of aggregated tau suggests that Tau35 may have structural properties distinct from those of other tau species that could account for its pathological role in disease. To address this hypothesis, we performed a structural characterization of monomeric and aggregated Tau35 and compared the results to those of two longer isoforms, 2N3R and 2N4R tau. We used small angle X-ray scattering to show that Tau35, 2N3R and 2N4R tau all behave as disordered monomeric species but Tau35 exhibits higher rigidity. In the presence of the poly-anion heparin, Tau35 increases thioflavin T fluorescence significantly faster and to a greater extent than full-length tau, demonstrating a higher propensity to aggregate. By using atomic force microscopy, circular dichroism, transmission electron microscopy and X-ray fiber diffraction, we provide evidence that Tau35 aggregation is mechanistically and morphologically similar to previously reported tau fibrils but they are more densely packed. These data increase our understanding of the aggregation inducing properties of clinically relevant tau fragments and their potentially damaging role in the pathogenesis of human tauopathies.

Tau Antibody Structure Reveals a Molecular Switch Defining a Pathological Conformation of the Tau Protein

Tau antibodies have shown therapeutic potential for Alzheimer's disease and several are in clinical trials. As a microtubule-associated protein, tau relies on dynamic phosphorylation for its normal functions. In tauopathies, it becomes hyperphosphorylated and aggregates into toxic assemblies, which collectively lead to neurodegeneration. Of the phospho-epitopes, the region around Ser396 has received particular attention because of its prominence and stability in tauopathies. Here we report the first structure of a monoclonal tau antibody in complex with the pathologically important phospho-Ser396 residue. Its binding region reveals tau residues Tyr394 to phospho-Ser396 stabilized in a β-strand conformation that is coordinated by a phospho-specific antigen binding site. These details highlight a molecular switch that defines this prominent conformation of tau and ways to target it. Overall, the structure of the antibody-antigen complex clarifies why certain phosphorylation sites in tau are more closely linked to neurodegeneration than others.

Tau alternative splicing in familial and sporadic tauopathies

Six tau isoforms differing in their affinity for microtubules are produced by alternative splicing from the MAPT (microtubule-associated protein tau) gene in adult human brain. Several MAPT mutations causing the familial tauopathy, FTDP-17 (frontotemporal dementia with parkinsonism linked to chromosome 17), affect alternative splicing of exon 10, encoding a microtubule-binding motif. Advanced RNA analysis methods have suggested that levels of exon 10-containing MAPT mRNA are elevated in Alzheimer's disease. Furthermore, the MAPT H1 haplotype, associated with Alzheimer's disease, promotes exon 10 inclusion in MAPT mRNA. Thus an accurate regulation of tau alternative splicing is critical for the maintenance of neuronal viability, and its alteration might be a contributing factor to Alzheimer's disease. Tau alternative splicing could represent a target for therapeutic intervention to delay the progression of pathology in familial as well as sporadic tauopathies.

Mitotic epitopes are incorporated into age-dependent neurofibrillary tangles in Niemann-Pick disease type C

The mechanism underlying neurofibrillary tangles (NFTs) in Alzheimer's disease (AD) and other neurodegenerative disorders remains elusive. Niemann-Pick disease type C (NPC) is a kind of genetic neurovisceral disorder in which the intracellular sequestration of cholesterol and other lipids in neurons, NFT formation and neuronal degeneration in brain are the neuropathology hallmarks. The age of onset and progression of the disease vary dramatically. We have analyzed the hippocampus from 17 NPC cases, aged from 7 months to 55 years, to depict the temporal characteristics of NFT formation. Unexpectedly, classic NFT was observed in about 4-year-old NPC brain, suggesting that NFT is not aging dependent, and that juvenile brain neurons satisfy the requirements for NFT formation. NFT in the hippocampus of NPC was significantly increased in number with the advance of age. More importantly, multiple mitotic phase markers, which are not usually found in normal mature neurons, were abundant in the affected neurons and incorporated into NFT. The unusual activation of cdc2/cyclin B kinase and downstream mitotic indices are closely associated with the age-dependent NFT formation, signifying the contribution of abortive cell cycle to neurodegeneration. The cdc2 inhibitors may be therapeutically used for early intervention of neurodegeneration and NFT formation in NPC.

Monitoring tau-tubulin interactions utilizing second harmonic generation in living neurons

Tau is a microtubule associated protein that is localized to the axon in neurons. During pathological conditions, including frontotemporal dementia (FTD), a shift in tau isoforms occurs that leads to enhanced expression of a form of tau with four (rather than three) microtubule binding repeats; this has been postulated to alter microtubule structure. Second harmonic generation (SHG) is a technique that allows the visualization of intact microtubules in axons of living neurons without the need for labeling or fixing. We examined how the presence of exogenous tau influences SHG in living neurons. Our results show that the presence of tau significantly enhances SHG, specifically in neuronal axons, despite the presence of tau throughout the entire cell. Our data also suggest that the presence or absence of the fourth microtubule binding repeat does not significantly alter tau's ability to enhance SHG. These results provide evidence that SHG is a useful, noninvasive tool to study tau-microtubule interactions in axons; further, it appears that tau overexpression, rather than specific isoforms, is the major contributor to tau-induced changes in axonal microtubule SHG signal.

Differential incorporation of tau isoforms in Alzheimer's disease

There are 6 different isoforms of tau expressed in the adult human brain, and little information is available on the cellular distribution of the isoforms. Tau inclusions are found in neurons and occasionally glia in a variety of diseases. Previous studies conducted on brain homogenates suggested that tau isoforms might be differentially incorporated into inclusions. To further elucidate the complex issue of tau isoform composition in Alzheimer's disease (AD) and other neurodegenerative diseases, monoclonal antibodies that differentiate between tau containing residues encoded by exon 10 (4R tau) and tau lacking exon 10 residues (3R tau) were used in single and double labeling immunohistochemistry as well as biochemical analyses of tau isolated from AD and other neurodegenerative diseases. Immunohistochemical analysis of the hippocampus in 34 AD cases performed with these antibodies showed both 3R and 4R tau isoforms in tangles. While biochemical studies showed that both isoforms were present in insoluble tau aggregates in AD hippocampus and cortex, not all tangles appear to be labeled with the 3R and 4R tau specific monoclonal antibodies. Similar studies in progressive supranuclear palsy and Pick's disease confirmed that these diseases were characterized by incorporation of specific isoforms in fibrillar lesions, but lesions in neither disease were exclusively composed of 3R tau or 4R tau isoforms.

Tau-based treatment strategies in neurodegenerative diseases

Neurofibrillary tangles are a characteristic hallmark of Alzheimer's and other neurodegenerative diseases, such as Pick's disease (PiD), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), and frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). These diseases are summarized as tauopathies, because neurofibrillary tangles are composed of intracellular aggregates of the microtubule-associated protein tau. The molecular mechanisms of tau-mediated neurotoxicity are not well understood; however, pathologic hyperphosphorylation and aggregation of tau play a central role in neurodegeneration and neuronal dysfunction. The present review, therefore, focuses on therapeutic approaches that aim to inhibit tau phosphorylation and aggregation or to dissolve preexisting tau aggregates. Further experimental therapy strategies include the enhancement of tau clearance by activation of proteolytic, proteasomal, or autophagosomal degradation pathways or anti-tau directed immunotherapy. Hyperphosphorylated tau does not bind microtubules, leading to microtubule instability and transport impairment. Pharmacological stabilization of microtubule networks might counteract this effect. In several tauopathies there is a shift toward four-repeat tau isoforms, and interference with the splicing machinery to decrease four-repeat splicing might be another therapeutic option.

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.

Identification of G-protein coupled receptor kinase 2 in paired helical filaments and neurofibrillary tangles

G-protein coupled receptor kinases (GRKs) constitute a serine/threonine kinase family playing a major role in agonist-induced phosphorylation and desensitization of G-protein coupled receptors. Recently, GRK2 and GRK5 have been demonstrated to phosphorylate alpha-synuclein (Ser129) and other synuclein isoforms. We studied colocalization of GRK2, GRK5, alpha-synuclein, and tau in neurodegenerative disorders characterized by fibrillary tau inclusions and/or alpha-synuclein-enriched Lewy bodies. We found that Lewy bodies were negative for both GRK2 and GRK5 in Lewy body disease (LBD) and LBD mixed with Alzheimer disease (AD + LBD). Instead, GRK2 but not GRK5 colocalized with 40% to 50% of neurofibrillary tangles in AD + LBD and AD brains. In disorders with less prominent alpha-synucleinopathy, neuronal and glial fibrillary tau deposits known to contain distinct subsets of tau isoforms were also positive for GRK2. These deposits included tufted astrocytes and coiled bodies in progressive supranuclear palsy, astrocytic plaques in corticobasal degeneration, and Pick bodies in Pick disease. In addition, paired helical filaments isolated from AD and AD + LBD brains were found to immunogold-label for GRK2, suggesting that GRK2 could be a potential tau kinase associated with fibrillary tau. Our studies indicate that GRK2 is a novel component of neuronal and glial fibrillary tau deposits with no preference in tau isoform binding. GRK2 may play a role in hyperphosphorylation of tau in tauopathies.

Region-specific dissociation of neuronal loss and neurofibrillary pathology in a mouse model of tauopathy

Neurofibrillary tangles form in a specific spatial and temporal pattern in Alzheimer's disease. Although tangle formation correlates with dementia and neuronal loss, it remains unknown whether neurofibrillary pathology causes cell death. Recently, a mouse model of tauopathy was developed that reversibly expresses human tau with the dementia-associated P301L mutation. This model (rTg4510) exhibits progressive behavioral deficits that are ameliorated with transgene suppression. Using quantitative analysis of PHF1 immunostaining and neuronal counts, we estimated neuron number and accumulation of neurofibrillary pathology in five brain regions. Accumulation of PHF1-positive tau in neurons appeared between 2.5 and 7 months of age in a region-specific manner and increased with age. Neuron loss was dramatic and region-specific in these mice, reaching over 80% loss in hippocampal area CA1 and dentate gyrus by 8.5 months. We observed regional dissociation of neuronal loss and accumulation of neurofibrillary pathology, because there was loss of neurons before neurofibrillary lesions appeared in the dentate gyrus and, conversely, neurofibrillary pathology appeared without major cell loss in the striatum. Finally, suppressing the transgene prevented further neuronal loss without removing or preventing additional accumulation of neurofibrillary pathology. Together, these results imply that neurofibrillary tangles do not necessarily lead to neuronal death.

An immunohistochemical study of cases of sporadic and inherited frontotemporal lobar degeneration using 3R- and 4R-specific tau monoclonal antibodies

The pathological distinctions between the various clinical and pathological manifestations of frontotemporal lobar degeneration (FTLD) remain unclear. Using monoclonal antibodies specific for 3- and 4-repeat isoforms of the microtubule associated protein, tau (3R- and 4R-tau), we have performed an immunohistochemical study of the tau pathology present in 14 cases of sporadic forms of FTLD, 12 cases with Pick bodies and two cases without and in 27 cases of familial FTLD associated with 12 different mutations in the tau gene (MAPT), five cases with Pick bodies and 22 cases without. In all 12 cases of sporadic FTLD where Pick bodies were present, these contained only 3R-tau isoforms. Clinically, ten of these cases had frontotemporal dementia and two had progressive apraxia. Only 3R-tau isoforms were present in Pick bodies in those patients with familial FTLD associated with L266V, Q336R, E342V, K369I or G389R MAPT mutations. Patients with familial FTLD associated with exon 10 N279K, N296H or +16 splice site mutations showed tau pathology characterised by neuronal neurofibrillary tangles (NFT) and glial cell tangles that contained only 4R-tau isoforms, as did the NFT in P301L MAPT mutation. With the R406W mutation, NFT contained both 3R- and 4R-tau isoforms. We also observed two patients with sporadic FTLD, but without Pick bodies, in whom the tau pathology comprised only of 4R-tau isoforms. We have therefore shown by immunohistochemistry that different specific tau isoform compositions underlie the various kinds of tau pathology present in sporadic and familial FTLD. The use of such tau isoform specific antibodies may refine pathological criteria underpinning FTLD.

Quantitative analysis of tau isoform transcripts in sporadic tauopathies

A number of neurodegenerative diseases, including Alzheimer's disease (AD), are characterized by intraneuronal accumulation of the tau protein. Some forms of FTDP-17 are caused by mutations in the tau gene affecting exon 10 splicing. Therefore, dysregulation of tau pre-mRNA splicing may be a contributing factor to sporadic tauopathies. To address this question, we devised a real-time RT-PCR strategy based on the use of a single fluorogenic probe to evaluate the ratio between tau isoforms containing or lacking exon 10 (4R/3R ratio) in post-mortem brain samples. We found a two- to six-fold increase in the 4R/3R ratio in cases of FTDP-17 linked to a splice site mutation, hence confirming the validity of the strategy. The difference in the 4R/3R ratio in the superior temporal and superior frontal gyri between AD and control brains was not statistically significant. Similarly, there was no significant difference in the 4R/3R ratio between Pick's disease cases and controls, indicating that the predominance of tau3R protein in PiD reflects post-translational modifications of specific isoforms. This study indicates that post-translational events are likely to be the main factors controlling tau isoform composition in sporadic tauopathies and highlights the benefit of quantitative RT-PCR in the assessment of splicing abnormalities in tauopathies.

Transcriptional and conformational changes of the tau molecule in Alzheimer's disease

Mutations in the tau gene cause frontotemporal dementia with parkinsonism, presumably by affecting the balance between tau isoforms (with either three or four microtubule-binding repeats) or by impairing tau-tubulin binding. Although to date no mutations have been found for Alzheimer's disease, it is plausible that tangle pathology in this disorder is also driven by similar molecular modifications. Investigations of Alzheimer brain tissue with new technologies such as laser capture microscopy, quantitative PCR and fluorescence lifetime imaging will shed light on whether transcriptional or conformational alterations play a role in Alzheimer pathogenesis.

Glial localization of four-repeat tau in atypical progressive supranuclear palsy

In the present case, a patient in whom limb apraxia and asymmetrical parkinsonism developed suggesting corticobasal degeneration, is reported. Neuropathologic examination revealed numerous tufted astrocytes in the precentral cortex in addition to the characteristic pathologic findings of PSP. Therefore, on the basis of clinicopathologic features, atypical progressive supranuclear palsy was diagnosed. In addition, the brain tissue of the present patient was investigated with an antibody specific for four-repeat tau (4R-tau). In the precentral cortex, numerous tau-positive tufted astrocytes, pretangles, and threads were positive for 4R-tau. Using a confocal microscopy we demonstrated that tufted astrocytes positive for 4R-tau were adjacent to astrocytes positive for GFAP. The present findings suggest that accumulation of four-repeat tau in astrocytes is a degenerative process rather than a reactive process.

The neuropathology of frontotemporal lobar degeneration with respect to the cytological and biochemical characteristics of tau protein

Pathological examinations, using a panel of tau and other antibodies, were performed on the brains from 55 consecutively acquired cases of frontotemporal lobar degeneration (FTLD). Clinically, these comprised 31 cases of frontotemporal dementia (FTD), 10 cases of motor neurone disease inclusion dementia (MNDID), seven cases of progressive aphasia (PA), four cases of semantic dementia (SD) and three cases of progressive apraxia (PAX). Tau pathology, in the form of neurofibrillary tangles (NFTs) and glial cell tangles, was present in six cases of FTD with parkinsonism linked to chromosome 17, five of these cases resulting from +16 splice-site mutation and one from +13 mutation in the tau gene. The insoluble tau proteins were comprised mostly of four-repeat (4-R) isoforms. Eight other cases of FTD, one of PA and all three cases of PAX showed tau-positive inclusions (Pick bodies) and swollen cells (Pick cells), characteristic of Pick's disease. In these cases, the insoluble tau proteins were present in most instances as three-repeat (3-R) tau isoforms, although two cases with a mixture of 3-R and 4-R isoforms were seen. One other case of FTD showed an unusual pathology characterized by massive extracellular deposition of tau protein, composed of 4-R tau isoforms, within white matter without neuronal or glial cell inclusions. However, 33 (60%) of 55 FTLD cases showed no tau pathology in the brain, except for the rare NFTs, composed of a mix of 3-R and 4-R isoforms, in some of the more elderly cases. Of these 33 cases, 13 had FTD, 10 had MNDID, six had PA and four had SD. The pathological changes present were those of a superficial cortical laminar microvacuolation with mild subpial and subcortical gliosis; the 10 MNDID cases had ubiquitin-positive inclusions in the cerebral cortex and hippocampus. These 33 nontau FTLD cases, along with five Alzheimer's disease (AD) and six Huntington's disease (HD) cases with severe pathology, showed a variable loss of soluble tau proteins, broadly comparable with the extent of neuronal loss from the cortex and loss of the intracortical perikaryal marker, NeuN, but unrelated to proteins within afferent projection fibres such as neurofilament and alpha-synuclein. Levels of tau mRNA were decreased in parallel in the tau-negative FTLD cases and in the severe AD and HD cases. Hence, the loss of tau from these 33 nontau FTLD cases is just one aspect of a neurodegenerative process that destroys many components of the nerve cell machinery and does not represent a specific disordering of the cell's ability to form tau proteins or incorporate these into microtubules.

Tau protein expression in adult bovine oligodendrocytes: functional and pathological significance

In tauopathies, overexpression of tau exon 10 is linked to degeneration and abnormal tau deposition in neurons and oligodendroglia (OLGs). To compare exon 10 expression in normal neurons and OLGs, adult bovine brain was examined for the expression of tau in gray matter and cultured OLGs isolated from white matter. Using exon-specific antibodies, we found that both types of tissues abundantly expressed exon 2 but isolated OLGs had a lower expression of exons 3 and 10 when compared to gray matter. Relative expression of exons 3 and 10 did not change significantly during the in vitro maturation of OLGs for 39 days. Using a panel of well-characterized antibodies against tau, we determined that isolated OLGs contained tau phosphorylated at the Tau-1, 12E8, and PHF-1 but not the AT8, AT100, AT180, and AT270 epitopes. Tau phosphorylation status diminished during in vitro maturation, suggesting that healthy OLG processes require regulated phosphorylation of tau at specific sites. We propose that the tau isoform profile and phosphorylation status contribute to the vulnerability of OLGs in degenerative diseases linked to overexpression of exon 10.

Hyperphosphorylation and aggregation of tau in mice expressing normal human tau isoforms

Neurofibrillary tangles are composed of insoluble aggregates of the microtubule-associated protein tau. In Alzheimer's disease the accumulation of neurofibrillary tangles occurs in the absence of tau mutations. Here we present mice that develop pathology from non-mutant human tau, in the absence of other exogenous factors, including beta-amyloid. The pathology in these mice is Alzheimer-like, with hyperphosphorylated tau accumulating as aggregated paired helical filaments. This pathologic tau accumulates in the cell bodies and dendrites of neurons in a spatiotemporally relevant distribution.

Pathological inclusion bodies in tauopathies contain distinct complements of tau with three or four microtubule-binding repeat domains as demonstrated by new specific monoclonal antibodies

Pathological inclusions containing fibrillar aggregates of hyperphosphorylated tau protein are a characteristic feature in the tauopathies, which include Alzheimer's disease, frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17), progressive supranuclear palsy, corticobasal degeneration and Pick's disease. Tau isoform composition and cellular and regional distribution as well as morphology of these inclusions vary in each disorder. Recently, several pathological missense and exon 10 splice-donor site mutations of the tau gene were identified in FTDP-17. Exon 10 codes for the second of four microtubule-binding repeat domains. The splice-site mutations result in increased inclusion of exon 10 which causes a relative increase in tau isoforms containing four microtubule-binding repeat domains over those containing three repeat domains. This could be a central aetiological mechanism in FTDP-17 and, perhaps, other related tauopathies. We have investigated changes in the ratio and distribution of three-repeat and four-repeat tau in the different tauopathies as a basis of the phenotypic range of these disorders and the selective vulnerability of different subsets of neurones. In this study, we have developed two monoclonal antibodies, RD3 and RD4 that effectively distinguish these closely related tau isoforms. These new isoform-specific antibodies are useful tools for analysing tau isoform expression and distribution as well as pathological changes in the human brain.

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

The microtubule-associated protein tau accumulates as cytoplasmic inclusions in Alzheimer's disease (AD), Pick's disease (PiD), progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD). We investigated the immunoreactivity of tau-positive structures using a panel of antibodies to epitopes spanning the entire length of the tau molecule. In ethanol-fixed brain tissues, most antibodies to the microtubule-binding domain (MBD) required formic acid (FA) treatment to stain tau inclusions in PSP and CBD. This is in contrast with the intense labeling of neurofibrillary tangles in AD without FA treatment. Pick bodies (PiB) in PiD showed an intermediate pattern with respect to the immunoreactivity of the MBD because accumulated tau in PiB mostly lacks the insertion of exon 10, and the proportion of tau phosphorylated at Ser262 is smaller than in other abnormal tau structures. Such immunohistochemical profiles appeared to correlate with the occurrence of the smeared tau on immunoblot analysis of brain homogenate. The smeared tau was more abundant in AD and PiD than in PSP and CBD. Since the smeared tau was N-terminally truncated and was characteristic of advanced forms of modified tau, these findings suggest that tau accumulated in AD and PiD was processed more markedly than that in PSP and CBD. The MBD of tau may be masked in the presence of the intact N terminus and require FA treatment for antibody recognition in tissue sections. Advanced modification may expose the MBD in brain tissues of AD and PiD. It is suggested that the processing of abnormally accumulated tau characterizes the pathophysiology of each tauopathy.

P301L tauopathy: confocal immunofluorescence study of perinuclear aggregation of the mutated protein

The clinical and neuropathological features in the P301L tauopathy have been described in several kindreds. In this study, we present findings in two previously unreported patients, evaluated both genetically, neuropathologically, and with multiparametric confocal immunofluorescence. The patients were female, with age 65 and 75 years old, respectively. Both exhibited clinical symptoms of frontotemporal dementia (FTD). Marked atrophy of the frontal and temporal lobes with moderate atrophy of the remaining cerebral and brain stem structures was present. The substantia nigra was pale. The atrophic neocortical regions exhibited neuronal loss, marked gliosis, status spongiosus, and occasional ballooned neurons. By light microscopy, the most striking findings were argyrophilic perinuclear rings, frequently with an attached small inclusion (mini Pick-like body), especially prominent in dentate granule cells, entorhinal and temporal cortices, and to a lesser extent in CA1. These structures were immunopositive for tau protein (Tau-2, AT-8, PHF-1, MC-1). Numerous astrocytic plaques, tuft-shaped astrocytes, coiled bodies, and dystrophic neurites were also present. Confocal immunofluorescence with a P301L-specific antibody directly demonstrated the presence of the mutated protein in the PHF-1 positive aggregates. The mutated tau protein (4-repeat tau) was detected in the mini Pick-like bodies, indicating an important biochemical difference between these inclusions and classical Pick bodies (3-repeat tau). Additionally, since 4-repeat tau protein is not normally present in dentate granule cells, this result also suggests an abnormality in the mRNA splicing mechanisms. The structural features of the involvement of proteolytic systems in this tauopathy were assessed by immunohistochemistry for the active form of calpain II (C-27) and ubiquitin. Colocalization of PHF-1 positive aggregates with C-27 points to the possible involvement of calpain in tau protein hyperphosphorylation. Absence of immunostaining for ubiquitin indicates possible dysfunction of the ubiquitin-proteasome system in this tauopathy.

Sporadic Pick's disease: a tauopathy characterized by a spectrum of pathological tau isoforms in gray and white matter

Pick's disease is characterized neuropathologically by distinct tau-immunoreactive intraneuronal inclusions known as Pick bodies and by insoluble tau proteins with predominantly three microtubule-binding repeat tau isoforms. However, recent immunohistochemical studies showed that the antibody specific for exon 10, which encodes the fourth microtubule-binding repeat, detected other tau lesions in Pick's disease. To better define the spectrum of tau pathology in Pick's disease, we used biochemical, immunohistochemical, and ultrastructural techniques to analyze the tau isoform composition in 14 Pick's disease brains. Western blot analysis showed that both three and four microtubule-binding repeat pathological tau isoforms are present in gray and white matter of various brain regions. Using phosphorylation-dependent anti-tau antibodies, we show that major tau phosphoepitopes are present in sarcosyl-insoluble gray and white matter regions of Pick's disease brains. Also, for the first time to our knowledge, we demonstrated that isoforms with four microtubule-binding repeat tau isoforms are present in Pick bodies from selected brains. Isolated tau filaments were straight or twisted and formed by three microtubule-binding repeat or four microtubule-binding repeat tau isoforms. Major tau phosphorylation-dependent and exon 10-specific epitopes were present in filaments. Therefore, Pick's disease is characterized by an accumulations of Pick bodies in the hippocampal region and cortex as well as the presence of three and four microtubule-binding repeat tau pathology in both cortical gray and white matter that distinguish this tauopathy from other neurodegenerative disorders.

Morphological and biochemical correlations of abnormal tau filaments in progressive supranuclear palsy

Progressive supranuclear palsy (PSP) is characterized by specific filamentous tau inclusions present in 3 types of cells including oligodendrocytes (coiled bodies), astrocytes (tufted astrocytes), and neurons (neurofibrillary tangles; NFTs). To correlate the morphological features and biochemical composition of tau in the inclusions, we examined tau filament-enriched fractions isolated from selected brain regions. Frontal and cerebellar white matter manifested a predominance of coiled bodies. The isolated fractions contained straight, 14-nm-wide filaments of relatively smooth appearance. Caudate nucleus and motor cortex with numerous tufted astrocytes contained mostly straight, but irregular, 22-nm-wide filaments with jagged contours. Perirhinal cortex and hippocampus, rich in NFTs, contained 22-nm-wide filaments that were twisted at 80-nm intervals. Among the regions, those with tufted astrocytes showed the most heterogeneity in the ultrastructure of filaments. In all regions, isolated filaments were immunolabeled with PHF-1, Tau 46, and AT8. Fractions from all regions showed 2 PHF-1 immunoreactive bands of 64 and 68 kDa, while an additional band of 60 kDa was detected in NFT-enriched regions. All fractions, in varying extents, showed Tau-1-immunoreactive bands between 45-64 kDa. The results indicate that the 3 types of PSP tau inclusions vary in the ultrastructure although with some overlapping features. Neuronal and glial inclusions also vary in the biochemical profile of tau protein. These differences may depend on the metabolism of tau in the diseased oligodendrocytes, astrocytes, and neurons.

Correction of alternative splicing of tau in frontotemporal dementia and parkinsonism linked to chromosome 17

Mutations in the human tau gene cause frontotemporal dementia and Parkinsonism associated with chromosome 17 (FTDP-17). One of the major disease mechanisms in FTDP-17 is the increased inclusion of tau exon 10 during pre-mRNA splicing. Here we show that modified oligonucleotides directed against the tau exon 10 splice junctions suppress inclusion of tau exon 10. The effect is mediated by the formation of a stable pre-mRNA-oligonucleotide hybrid, which blocks access of the splicing machinery to the pre-mRNA. Correction of tau splicing occurs in a tau minigene system and in endogenous tau RNA in neuronal pheochromocytoma cells and is specific to exon 10 of the tau gene. Antisense oligonucleotide-mediated exclusion of exon 10 has a physiological effect by increasing the ratio of protein lacking the microtubule-binding domain encoded by exon 10. As a consequence, the microtubule cytoskeleton becomes destabilized and cell morphology is altered. Our results demonstrate that alternative splicing defects of tau as found in FTDP-17 patients can be corrected by application of antisense oligonucleotides. These findings provide a tool to study specific tau isoforms in vivo and might lead to a novel therapeutic strategy for FTDP-17.

Pathological glial tau accumulations in neurodegenerative disease: review and case report

Abnormal deposits of tau protein accumulate in glia in many neurodegenerative diseases. This suggests that in some instances the disease process may target glial tau, with neuronal degeneration a secondary consequence of this process. In this report, we summarize the pattern of glial tau pathology in various neurodegenerative disorders and add original findings from a case of sporadic frontotemporal dementia that exhibits astrocytic tau pathology. The neurodegenerative diseases span the spectrum of relative neuronal and glial tau involvement, from disorders affecting only neuronal tau to those in which abnormal tau deposits are found only in glia. From this, we conclude that glial tau can be a primary target of the disease process, and that this can lead to neuronal degeneration.