Increase in the relative expression of tau with four microtubule binding repeat regions in frontotemporal lobar degeneration and progressive supranuclear palsy brains

Some cases of familial frontotemporal dementia (FTD) leading to frontotemporal lobar degeneration (FTLD) are caused by mutations in tau on chromosome 17 (FTDP-17). Certain mutations alter the ratio between four (4R tau) and three (3R tau) repeat tau isoforms whereas cases with progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD) mainly have 4R tau brain pathology. We assessed tau mRNA and protein levels in frontal cortex from 15 sporadic FTLD, 21 PSP, 5 CBD, 15 Alzheimer’s disease (AD) and 16 control brains. Moreover, we investigated the disease association and possible tau splicing effects of the tau H1 haplotype. Cases with FTLD and PSP had lower tau mRNA levels than control brains. When analyzing 4R tau and 3R tau mRNA separately, control subjects displayed a 4R tau/3R tau ratio of 0.48. Surprisingly, FTLD brains displayed a more elevated ratio (1.32) than PSP brains (1.12). Also, several FTLD and PSP cases had higher 4R tau/3R tau mRNA than FTDP-17 cases, included as reference tissues, and the ratio increase was seen regardless of underlying histopathology, i.e. both for tau-positive and tau-negative FTLD cases. Furthermore, total tau protein levels were slightly decreased in both FTLD and AD as compared to control subjects. Finally, we confirmed the association of tau H1 with PSP, but could not find any haplotype-related effect on tau exon 10 splicing. In conclusion, we demonstrated increased but largely variable 4R tau/3R tau mRNA ratios in FTLD and PSP cases, suggesting heterogeneous pathophysiological processes within these disorders.     

Tau and neurofilaments in a family with frontotemporal dementia unlinked to chromosome 17q21-22

A Swiss frontotemporal dementia (FTD) kindred with extrapyramidal-like features and without motor neuron disease shows a brain pathology with ubiquitin-positive but tau-negative inclusions. Tau and neurofilament modifications are now studied here in three recently deceased family members. No major and specific decrease of tau was observed as described by others in, e.g., sporadic cases of FTD with absence of tau-positive inclusions. However, a slight decrease of tau, neurofilament, and synaptic proteins, resulting from frontal atrophy was detected. In parallel, polymorphic markers on chromosome 17q21-22, the centromeric region of chromosome 3 and chromosome 9, were tested. Haplotype analysis showed several recombination events for chromosomes 3 and 17, but patients shared a haplotype on chromosome 9q21-22. However as one of the patients exhibited Alzheimer and vascular dementia pathology with uncertain concomitant FTD, this locus is questionable. Altogether, these data indicate principally that the Swiss kindred is unlinked to locus 17q21-22, and that tau is not at the origin of FTD in this family.     

Similar early clinical presentations in familial and non-familial frontotemporal dementia

BACKGROUND: It is unclear whether there are early clinical features that can distinguish between patients with familial and non-familial frontotemporal dementia (FTD). OBJECTIVE: To compare the clinical features of FTD cases who have tau gene mutations with those of cases with a family history of FTD but no tau gene mutation, and with sporadic cases with neither feature. METHODS AND RESULTS: Comparisons of the behavioural, cognitive, and motor features in 32 FTD patients (five positive for tau gene mutations, nine familial but tau negative, and 18 tau negative sporadic) showed that age of onset and duration to diagnosis did not differ between the groups. Apathy was not observed in tau mutation positive cases, and dysexecutive signs were more frequent in familial tau mutation negative cases. Memory deficits and behavioural changes were common in all groups. CONCLUSIONS: In comparison with other neurodegenerative conditions such as Alzheimer's disease and Parkinson's disease, neither tau gene mutations nor strong familial associations confer earlier disease susceptibility.     

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.     

Frontotemporal dementia: tau mutations, deposition, and molecular mechanisms of neuronal cell death]

Exonic and intronic tau mutations have been described in a number of families of frontotemporal dementia and parkinsonism linked to chromosome 17. Most of missense mutations alter the ability of tau to promote microtubule assembly, whereas others influence splicing of exon10 and change the ratio of 3Rtau to 4Rtau isoform. In either case, filamentous hyperphosphorylated tau pathology in neurons and glial cells was observed in affected brains. These observations suggest that the effects of tau mutations may induce its hyperphosphorylation and accumulation, resulting in cell death. In sporadic tauopathies, decreased levels of 3R tau mRNA were detected not only in severely affected cases with progressive supranuclear palsy or corticobasal degeneration but also in cases with Alzheimer's disease or Pick's disease. In addition, levels of 3R tau mRNA were closely correlated with levels of neurofilament mRNA. These results suggest that decreased levels of 3R tau mRNA in sporadic tauopathies may be due to degeneration and loss of neurons that express 3R tau isoforms. In tauopathies, neuronal cell death may occur with multiple defects or abnormalities arisen directly or indirectly from hyperphosphorylation of tau and formation of oligomer or filamentous tau.     

Progranulin mutations in ubiquitin-positive frontotemporal dementia linked to chromosome 17q21

Two genetically distinct types of frontotemporal dementia (FTD) are linked to chromosome 17q21. FTD with parkinsonism (FTDP-17) results from mutations in the gene encoding microtubule associated protein tau (MAPT) and is associated with tau deposition in the patient's brain. An increasing number of FTD families are linked to 17q21 in the absence of a demonstrable MAPT mutation. Brains of these patients do not show tau deposits, but tau-negative intra- and perinuclear inclusions of unknown composition that are immunoreactive to ubiquitin (FTDU-17). These ubiquitin inclusions are located in the cytoplasm or nucleus of predominantly neuronal cells of affected brain regions. By extensive segregation analyses in conclusively linked FTDU-17 families, the candidate region was reduced to a 6.2 Mb segment containing MAPT; however, genomic sequencing of MAPT in FTDU-17 patients excluded disease-causing mutations. Further, the linked region was characterized by the presence of multiple low-copy repeat regions associated with genomic instability. However, we excluded genomic rearrangements as the cause of FTDU-17. Subsequent sequencing of positional candidate genes identified loss-of-function mutations in the gene encoding progranulin (PGRN), a growth factor involved in multiple physiological processes such as cellular proliferation and survival and tissue repair, and pathological processes including tumorigenesis. In a Belgian FTD patient series, the prevalence of PGRN mutations was 3.5 times higher than that of MAPT mutations underscoring a major role for PGRN in FTD pathogenesis. Together, mutation data provided convincing evidence that PGRN haploinsufficiency leads to neurodegeneration because of reduced PGRN-mediated neuronal survival. The PGRN protein is not deposited in the ubiquitin-positive inclusions, the nature of which remains unknown. Due to the functions of PGRN in neuronal survival and the clinicopathological overlaps between FTD and other dementias it is likely that reduced PGRN expression is associated with the progression of other neurodegenerative brain diseases including Alzheimer's disease. These findings open promising novel targets for therapeutic intervention against neurodegeneration.     

Microtubule associated protein (tau) gene variability in patients with frontotemporal dementia

Frontotemporal dementia represents up to 10% of all dementias and is, next to Alzheimer's disease and Lewy body disease, the third most common cause of degenerative dementia. The term "frontotemporal dementia" covers a range of conditions, including Pick's disease, frontal lobe degeneration and dementia associated with motor neurone disease. Neuropathologically FTD is characterised by atrophy of the frontal and temporal lobes of the cerebral cortex, often with additional subcortical changes. Both familial and more frequently sporadic forms of FTD can be recognised. Recently, mutations in the microtubule-associated protein (tau) gene have been found in families with frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). The identification of mutations in the tau gene indicates that the protein plays a central role in the process of neurodegeneration. Epidemiology of frontotemporal dementias in Poland remains still unknown. A prevalence of tau mutations among Polish patients has not been established yet. Here, we report results of a mutational analysis of the tau gene among Polish FTD patients. No pathogenic mutation was found in the analysed sample. The study confirmed that the frequency of tau mutations is very low and depends strongly on the clinical criteria used to select patients. Mutations in the tau gene account only for a small number of FTD cases with a clear autosomal dominant pattern of disease inheritance. Therefore there should exist additionalgenetic and non-genetic factors contributing to the pathogenesis of both familial (linked and non-linked to chromosome 17) and sporadic forms of FTD.     

Genetic analysis in patients with familial and sporadic frontotemporal dementia: two tau mutations in only familial cases and no association with apolipoprotein epsilon4

We screened for tau gene mutations among 24 Japanese (6 familial and 18 sporadic cases) and 4 Polish patients with frontotemporal dementia (FTD) using PCR-SSCP analysis followed by DNA sequencing. We identified 2 missense mutations in exon 10: N279K and P301L in 2 Japanese patients with familial FTD. Additionally 3 DNA polymorphisms: 2 known (3' exon 3 + 9, A --> G and exon 7, codon 176, G --> A) and 1 new (exon 8, codon 185, T --> C) were identified in 1 Polish patient. Tau mutations were not found in subjects with a negative family history suggesting that tau mutations do not account for most sporadic cases of FTD. We also found no association of apolipoprotein E4 allele with FTD.     

Cellular tau pathology and immunohistochemical study of tau isoforms in sporadic tauopathies

Pathological inclusions in neurons and glial cells containing fibrillary aggregates of abnormally hyperphosphorylated tau protein are characteristic features in sporadic tauopathies. In the first part of this paper we outline the morphological features of some major sporadic tauopathies. In the second part, to better define the tau isoform composition, we report on the immunohistochemistry of tau isoforms in autopsied brains, including two cases with AD, two with diffuse neurofibrillary tangles with calcification, four with Pick’s disease with Pick bodies (PiD), seven with progressive supranuclear palsy (PSP), six with corticobasal degeneration (CBD) and seven cases with argyrophilic grain disease. We used two monoclonal antibodies, RD3 and RD4, and a polyclonal antibody for exon 10 that effectively distinguish between three‐repeat (3R) tau and four‐repeat (4R) tau. Neuronal neurofibrillary tangles (NFT) in AD and diffuse neurofibrillary tangles with calcification contained both 3R‐tau and 4R‐tau. The Pick bodies were immunopositive for 3R‐tau in two cases; however, in two other cases they were mainly immunopositive for 4R‐tau. Thus, Pick bodies demonstrated heterogeneity. 3R‐tau PiD contained 3R‐tau glial inclusions, and 4R‐tau PiD contained mainly 4R‐tau glial inclusions. Glial inclusions were more abundant in 4R‐tau PiD cases. In progressive supranuclear palsy and CBD, both neuronal and glial tau accumulation forming NFT, pretangles, tuft‐shaped astrocytes, astrocytic plaques, coiled bodies and threads demonstrated 4R‐tau in the cerebral cortices, although in the basal ganglia and brainstem neuronal and glial inclusions were occasionally immunopositive for 3R‐tau in addition to 4R‐tau. Argyrophilic grains (AG) were immunopositive for 4R‐tau, although pretangles were weakly stained for 4R‐tau. Thus the immunoreactivity for 4R‐tau was different between AG and pretangles. Therefore, the isoform composition on immunohistochemical study showed heterogeneity in PiD, and was not uniform in the basal ganglia and brain stem in PSP and CBD. It is suggested that the isoform composition of sporadic tauopathies may have a spectrum in individual cases, and cellular isoform composition may differ in various brain regions.     

Frontotemporal dementia and tauopathy

The presence of abundant neurofibrillary lesions made of hyperphosphorylated tau proteins is the characteristic neuropathology of a subset of neurodegenerative disorders classified as "tauopathies." The discovery of mutations in the tau gene in frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) constitutes convincing evidence that tau proteins play a key role in the pathogenesis of neurodegenerative disorders. Moreover, it now is known that the most common form of sporadic frontotemporal dementia (FTD), which is characterized by frontotemporal neuron loss, gliosis, and microvacuolar change, also is a tauopathy caused by a loss of tau protein expression. Thus, these discoveries have begun to change the classification and the neuropathologic diagnosis of FTD and tauopathies, as well as current understanding of the disease mechanisms underlying them. Although transgenic mice expressing wild-type human tau or variants thereof with an FTDP-17 mutation result in tau pathologies and brain degeneration similar to that seen in human tauopathies, the precise mechanisms leading to the onset and progression of neurodegenerative disorders remain incompletely understood. Here, we review current understanding of human neurodegenerative tauopathies and prospects for translative recent insights about these into therapeutic interventions to prevent or ameliorate them.     

Neuropathologic, biochemical, and molecular characterization of the frontotemporal dementias

The frontotemporal dementias (FTDs) are a heterogeneous group of neurodegenerative disorders that are characterized clinically by dementia, personality changes, language impairment, and occasionally extrapyramidal movement disorders. Historically, the diagnosis and classification of FTDs has been fraught with difficulties, especially with regard to establishing a consensus on the neuropathologic diagnosis. Recently, an international group of scientists participated in a consensus conference to develop such neuropathologic criteria. They recommended a diagnostic classification scheme that incorporated a biochemical analysis of the insoluble tau isoform composition, as well as ubiquitin immunohistochemistry. The use and reliability of this classification system has yet to be examined. In this study, we evaluated 21 cases of FTD. Using traditional histochemical stains and tau protein and ubiquitin immunohistochemistry, we separated each case into one of the following categories: classic Pick disease (PiD; n = 7), corticobasal degeneration (CBD; n = 5), dementia lacking distinctive histopathologic features (DLDH; n = 4), progressive supranuclear palsy (PSP; n = 2), frontotemporal lobar degeneration with motor neuron disease or motor neuron disease-type inclusions (FTLD-MND/MNI; n = 2), and neurofibrillary tangle dementia (NFTD; n = 1). Additionally, we independently categorized each case by the insoluble tau isoform pattern, including 3R (n = 5), 4R (n = 7), 3R/4R (n = 3), and no insoluble tau (n = 6). As suggested by the proposed diagnostic scheme, we found that the insoluble tau isoform patterns correlated strongly with the independently derived histopathologic diagnoses (p < 0.001). The data show that cases containing predominantly 3R tau were classic PiD (100%). Cases with predominantly 4R tau were either CBD (71%) or PSP (29%). Cases with both 3R and 4R tau were either a combination of PiD and Alzheimer disease (67%) or NFTD (33%). Finally, cases with no insoluble tau were either DLDH (67%) or FTLD-MND/MNI (33%). To further characterize these cases, we also performed quantitative Western blots for soluble tau, APOE genotyping, and, in selected cases, tau gene sequencing. We show that soluble tau is reduced in DLDH and FTLD-MND/MNI and that APOE4 is overrepresented in PiD and DLDH. We also identified a new family with the R406W mutation and pathology consistent with NFTD. This study validates the recently proposed diagnostic criteria and forms a framework for further refinement of this classification scheme.     

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.     

Polymorphisms in the tau gene in sporadic frontotemporal dementia and other neurodegenerative disorders

The tau gene on chromosome 17 is fundamental in the pathogenesis of a number of neurodegenerative disorders. Mutations in tau are found in familial frontotemporal dementia (FTD) and the A0/A0 genotype associated with progressive supranuclear palsy (PSP). This study investigates the hypothesis that polymorphisms in the tau gene are associated with sporadic FTD. Western Australian populations of patients with sporadic frontotemporal dementia, PSP, Alzheimer's disease (AD), Huntington's disease (HD) and normal controls were studied. A new method was developed using fluorescently labelled probes to determine polymorphisms in the GT repeat region of intron 9. The A0/A0 genotype was found in 95% of PSP patients (n=20), 58.3% of FTD patients (n=48), 60.8% of AD patients (n=52), 75% of HD patients (n=40), and 75% of normal controls (n=40). None of these differences in genotype frequency were found to be significant by the Fisher exact test (P > 0.05). There were no significant differences in the frequencies of A0/A3 and A0/A1 haplotypes. We have not observed a significant increase in the A0/A0 genotype frequency in sporadic frontotemporal dementia suggesting that this polymorphism is unlikely to be related to the development of this condition. Furthermore, we have observed an increase in the A0/A0 genotype in PSP which did not reach statistical significance, suggesting that there may be population differences in the role of genetic factors in conferring risks to neurodegenerative disorders. Our work does not exclude that tau may interact with other genetic factors.     

Regulation and differential expression of tau mRNA isoforms as oligodendrocytes mature in vivo: implications for myelination

Oligodendrocytes and neurons derive from the same cell type but develop distinct morphologic and functional properties as they mature in vivo. Both cells express tau protein, a developmentally regulated protein in the central nervous system. The regulation of tau has been investigated extensively in neurons but not in oligodendrocytes, so we studied regulation of tau in oligodendrocytes in vivo. The amino-derived tau isoforms consist of isoforms with zero (A0), one (A1), or two (A2) inserts. We examined the developmental regulation of tau mRNA isoforms at the amino domain by comparing tau expression in oligodendrocytes (OLGs) isolated from 1- and 20-day-old rat brain and in age-matched cortex, which abounds in neurons. In the rat brain, myelination peaks at 20 days. By using semiquantitative RT-PCR, we found that OLGs and cortex from 1-day-old rat brain largely had amino-derived tau isoforms with no insert, whereas OLGs from 20-day-old rat brain had similar levels of amino-derived tau isoforms with no insert or with one insert. We also found that 20-day-old OLGs had twofold more tau mRNA levels than younger OLGs. In contrast to OLGs from 20-day-old rat brain, age-matched cortex had comparable levels of A0, A1, and A2 tau amino-derived isoforms. Further, younger and older OLGs had a reciprocal pattern of expression of both carboxy-derived tau mRNA isoforms with either three (3R) or four (4R) repeats. In contrast, younger and older cortex expressed either 3R or 4R tau. This study showed an upregulation of tau mRNA and cell-specific tau mRNA isoform expression in OLGs forming myelin.     

Phospho-tau/total tau ratio in cerebrospinal fluid discriminates Creutzfeldt-Jakob disease from other dementias

Early clinical symptoms of sporadic Creutzfeldt-Jakob disease (CJD) may overlap with other neurodegenerative diseases like Alzheimer's disease (AD) and frontotemporal degeneration (FTD). On entering an era in which pharmaceutical treatment of CJD occurs, reliable diagnostic markers like immunodetection of 14-3-3 proteins in the cerebrospinal fluid (CSF) are required. However, false negative results in autopsy-proven, sporadic CJD cases, as well as false positive results in several other disorders including AD and FTD showing high CSF tau protein levels, limit the potential of this marker. Due to neuronal lysis the cytosolic fraction of total tau containing phosphorylated and non-phosphorylated isoforms is partially liberated into the CSF. Since hyperphosphorylation of tau may specifically occur in neurodegenerative diseases associated with neurofibrillary changes, we hypothesized that the phospho-tau (P-tau)/total tau ratio in CSF may be a useful marker to discriminate CJD from other neurodegenerative disorders. The P-tau/total tau ratio discriminated patients with CJD from all other neuro-degenerative disorders including patients with AD and FTD without any overlap. Although the results have to be confirmed in a larger sample, the preliminary data suggest that simultaneous measurement of total tau and P-tau in CSF may be useful to identify patients with CJD.