Familial multiple system tauopathy with presenile dementia: a disease with abundant neuronal and glial tau filaments

Mutations in tau gene exon 10 associated with FTDP-17 alter the activity of an exonic splicing enhancer to interact with Tra2 beta

Mutations in the human tau gene leading to aberrant splicing have been identified in FTDP-17, an autosomal dominant hereditary neurodegenerative disorder. Molecular mechanisms by which such mutations cause tau aberrant splicing were not understood. We characterized two mutations in exon 10 of the tau gene, N279K and Del280K. Our results revealed an exonic splicing enhancer element located in exon 10. The activity of this AG-rich splicing enhancer was altered by N279K and Del280K mutations. This exonic enhancer element interacts with human Tra2 beta protein. The interaction between Tra2 beta and the exonic splicing enhancer correlates with the activity of this enhancer element in stimulating splicing. Biochemical studies including in vitro splicing and RNA interference experiments in transfected cells support a role for Tra2 beta protein in regulating alternative splicing of human tau gene. Our results implicate the human tau gene as a target gene for the alternative splicing regulator Tra2 beta, suggesting that Tra2 beta may play a role in aberrant tau exon 10 alternative splicing and in the pathogenesis of tauopathies.

Isoforms changes of tau protein during development in various species

Tau protein is one of the major microtubule-associated proteins of the vertebrate nervous system. Some kinds of isoforms, for example, six isoforms in humans, are generated from a single gene by alternative mRNA splicing. The expression of tau protein is widely believed to be developmentally and pathologically regulated. We examined developmental changes in tau protein from humans, rats, mice, and guinea pigs to determine the universal function of each isoform. Tau isoforms, composed of variants in the amino terminal and carboxyl terminal regions, gradually shifted through development in protein. The developmental changes in the carboxyl terminal region were found to be conserved in all species in which three-repeat tau isoforms were dominant in the fetus or neonate, while four-repeat tau isoforms were dominant in adult brain. On the other hand, the changes in the amino terminal region were not identical in these species. These observations were confirmed using isoform-specific antibodies which could discriminate the numbers of amino-terminus insertions and carboxy-terminus repeat insertions. Developmental regulation of 3- and 4-repeat tau isoforms may contribute to axonal development and neural plasticity.

Tau gene mutations: dissecting the pathogenesis of FTDP-17

Tau is a microtubule-associated protein involved in microtubule assembly and stabilization. Abnormal filamentous tau deposits constitute a major defining characteristic of several neurodegenerative diseases, including Alzheimer's disease. Although the presence of tau pathology correlates with the symptoms of Alzheimer's disease, there was no genetic evidence linking tau to neurodegeneration until recently. However, since 1998, the identification of more than 25 mutations in the tau gene, associated with frontotemporal dementia and parkinsonism linked to chromosome 17, has demonstrated that tau dysfunction can lead to neurodegeneration and the development of clinical symptoms.

Abundant tau filaments and nonapoptotic neurodegeneration in transgenic mice expressing human P301S tau protein

The identification of mutations in the Tau gene in frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) has made it possible to express human tau protein with pathogenic mutations in transgenic animals. Here we report on the production and characterization of a line of mice transgenic for the 383 aa isoform of human tau with the P301S mutation. At 5-6 months of age, homozygous animals from this line developed a neurological phenotype dominated by a severe paraparesis. According to light microscopy, many nerve cells in brain and spinal cord were strongly immunoreactive for hyperphosphorylated tau. According to electron microscopy, abundant filaments made of hyperphosphorylated tau protein were present. The majority of filaments resembled the half-twisted ribbons described previously in cases of FTDP-17, with a minority of filaments resembling the paired helical filaments of Alzheimer's disease. Sarkosyl-insoluble tau from brains and spinal cords of transgenic mice ran as a hyperphosphorylated 64 kDa band, the same apparent molecular mass as that of the 383 aa tau isoform in the human tauopathies. Perchloric acid-soluble tau was also phosphorylated at many sites, with the notable exception of serine 214. In the spinal cord, neurodegeneration was present, as indicated by a 49% reduction in the number of motor neurons. No evidence for apoptosis was obtained, despite the extensive colocalization of hyperphosphorylated tau protein with activated MAP kinase family members. The latter may be involved in the hyperphosphorylation of tau.

Abundant tau filaments and nonapoptotic neurodegeneration in transgenic mice expressing human P301S tau protein

The identification of mutations in the Tau gene in frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) has made it possible to express human tau protein with pathogenic mutations in transgenic animals. Here we report on the production and characterization of a line of mice transgenic for the 383 aa isoform of human tau with the P301S mutation. At 5-6 months of age, homozygous animals from this line developed a neurological phenotype dominated by a severe paraparesis. According to light microscopy, many nerve cells in brain and spinal cord were strongly immunoreactive for hyperphosphorylated tau. According to electron microscopy, abundant filaments made of hyperphosphorylated tau protein were present. The majority of filaments resembled the half-twisted ribbons described previously in cases of FTDP-17, with a minority of filaments resembling the paired helical filaments of Alzheimer's disease. Sarkosyl-insoluble tau from brains and spinal cords of transgenic mice ran as a hyperphosphorylated 64 kDa band, the same apparent molecular mass as that of the 383 aa tau isoform in the human tauopathies. Perchloric acid-soluble tau was also phosphorylated at many sites, with the notable exception of serine 214. In the spinal cord, neurodegeneration was present, as indicated by a 49% reduction in the number of motor neurons. No evidence for apoptosis was obtained, despite the extensive colocalization of hyperphosphorylated tau protein with activated MAP kinase family members. The latter may be involved in the hyperphosphorylation of tau.

Expression patterns of tau mRNA isoforms correlate with susceptible lesions in progressive supranuclear palsy and corticobasal degeneration

Deposition of hyperphosphorylated tau (p-tau) has been observed in several neurodegenerative diseases. The six isoforms of tau are divided into two main groups including three repeat (3R) and four repeat (4R) microtubule-binding domains. Using quantitative RT-PCR method and immunohistochemistry with phosphorylation dependent anti-tau antibody (AT8), we investigated the expression level of tau mRNA isoforms in the frontal cortex and globus pallidus of patients with progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD) to determine whether altered expression patterns of tau mRNA isoforms correlate with p-tau accumulation. The 4R/3R ratios in frontal cortices of CBD and globus pallidus of PSP and CBD were significantly higher than the control (P<0.05). There was no correlation between the expression patterns of tau mRNA isoforms and p-tau accumulation. Our findings suggest that neurodegeneration of PSP and CBD could be regulated by alternative splicing of tau mRNA to yield high 4R/3R ratio. In addition, the lack of correlation between the expression pattern of tau mRNA isoforms and p-tau accumulation suggests that not only alternative splicing of tau mRNA, but also other factors such as post-transcriptional or translational modifications may play a role in the pathogenesis of specific neurodegeneration in PSP and CBD.

Advances in the cellular and molecular biology of the beta-amyloid protein in Alzheimer's disease

Alzheimer's disease (AD) is a progressive senile dementia characterized by deposition of a 4 kDa peptide of 39-42 residues known as amyloid beta-peptide (Abeta) in the form of senile plaques and the microtubule associated protein tau as paired helical filaments. Genetic studies have identified mutations in the Abeta precursor protein (APP) as the key triggers for the pathogenesis of AD. Other genes such as presenilins 1 and 2 (PS1/2) and apolipoprotein E (APOE) also play a critical role in increased Abeta deposition. Several biochemical and molecular studies using transfected cells and transgenic animals point to mechanisms by which Abeta is generated and aggregated to trigger the neurodegeneration that may cause AD. Three important enzymes collectively known as "secretases" participate in APP processing. An enzymatic activity, beta-secretase, cleaves APP on the amino side of Abeta producing a large secreted derivative, sAPPbeta, and an Abeta-bearing membrane-associated C-terminal derivative, CTFbeta, which is subsequently cleaved by the second activity, gamma-secretase, to release Abeta. Alternatively, a third activity, alpha-secretase, cleaves APP within Abeta to the secreted derivative sAPPalpha and membrane-associated CTFalpha. The predominant secreted APP derivative is sAPPalpha in most cell-types. Most of the secreted Abeta is 40 residues long (Abeta40) although a small percentage is 42 residues in length (Abeta42). However, the longer Abeta42 aggregates more readily and was therefore considered to be the pathologically important form. Advances in our understanding of APP processing, trafficking, and turnover will pave the way for better drug discovery for the eventual treatment of AD. In addition, APP gene regulation and its interaction with other proteins may provide useful drug targets for AD. The emerging knowledge related to the normal function of APP will help in determining whether or not the AD associated changes in APP metabolism affect its function. The present review summarizes our current understanding of APP metabolism and function and their relationship to other proteins involved in AD.

Regulation of tau RNA maturation by thyroid hormone is mediated by the neural RNA-binding protein musashi-1

The tau gene encodes a microtubule-associated protein expressed by neuronal and glial cells. Abnormal deposits of Tau protein are characteristic of several neurodegenerative disorders. Additionally, mutations affecting tau pre-mRNA alternative splicing of exon 10 are associated with frontotemporal dementia and Parkinsonism linked to chromosome 17. In rodents, this process is developmentally regulated by thyroid hormone (T3) causing the predominance of exon 10-containing transcripts. Here we demonstrate that musashi-1 (msi-1) gene is induced by T3 during rat brain development and in N2a cells. T3 increases msi-1 mRNA level in an actinomycin D-sensitive, cycloheximide-resistant fashion without affecting its half-life, which suggests a transcriptional effect. Both ectopic Msi-1 expression and T3 treatment increased the proportion of exon 10-containing tau transcripts. Furthermore, antisense msi-1 expression inhibited T3 action. Our results show that msi-1 mediates the posttranscriptional regulation of tau expression by T3.

Argyrophilic grain disease is a sporadic 4-repeat tauopathy

Argyrophilic grain disease (AGD) was first reported as an adult-onset dementia, but recent studies have emphasized personality change, emotional imbalance, and memory problems as clinical features of AGD. AGD is characterized by spindle- or comma-shaped argyrophilic grains in the neuropil of entorhinal cortex, hippocampus, and amygdala. Immunohistochemistry with monoclonal antibodies specific to tau isoforms with four (4R) or three (3R) repeats in the microtubule-binding domain showed immunostaining of grains with 4R, but not 3R, tau antibodies, suggesting that AGD was a 4R tauopathy. The tau isoform composition of AGD was confirmed with densitometric analysis of Western blots of sarkosyl-insoluble tau from the medial temporal lobe of AGD brains with a range of concurrent neurofibrillary pathology and compared with Alzheimer controls. The 4R/3R ratio was 1 or less for Alzheimer disease; the 4R/3R ratio was more than 1 for AGD, decreasing with increasing neurofibrillary pathology and demonstrating that insoluble tau in AGD was enriched in 4R tau. The frequency of the extended tau haplotype was not different in AGD compared to other sporadic 4R tauopathies, progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD). Furthermore, AGD occurred in PSP and CBD more frequently than in dementia controls, including Alzheimer disease. These results suggest that AGD, PSP and CBD are 4R tauopathies that share common pathologic, biochemical, and genetic characteristics.

Genetics of dementia

Many neurodegenerative diseases are exceedingly complex disorders (Fig. 6). In the past decade, we have made tremendous advances in our understanding [figure: see text] of the genetic basis of these disorders. One common characteristic of these disorders is the existence of rare families in which a given disease is inherited as a Mendelian trait. In this article, we have reviewed the genetics of several common neurodegenerative disorders that are associated with cognitive disturbances and for which causative genes have been identified. Further genetic analysis should clarify the roles of known genes in the pathogenesis of common sporadic forms of these various diseases. Investigation of the normal and aberrant functions of these genes should provide insight into the underlying mechanisms of these disorders. Such research should facilitate new strategies for therapeutic interventions. Although molecular genetics has helped to clarify the etiology of these disorders, clinicians have played a critical role in the careful identification and classification of many families who were involved in the eventual mapping and cloning of causative mutations. The role of the clinician should not be underestimated. Future clinical and molecular genetics findings hold many clinical implications. It is likely that new diagnostic and therapeutic strategies for dementing disorders are just on the horizon.

Neuropathologic variation in frontotemporal dementia due to the intronic tau 10(+16) mutation

BACKGROUND

An increasing number of recently described tau mutations show considerable clinical heterogeneity. The assessment of this phenotypic variation is of vital importance in the differential diagnosis of neurodegenerative diseases.

OBJECTIVE

To assess the neuropathologic heterogeneity in a comprehensive study of 12 brains with a tau mutation at exon 10(+16) (C-to-T) splice site from 9 families.

METHODS

A comprehensive neuropathologic examination has been carried out, using a wide range of tau antibodies.

RESULTS

All brains showed frontotemporal atrophy of varying severity and pallor of the pigmented nuclei of the brainstem. The histologic changes were more extensive to include other cortical areas, the deep gray matter, and the white matter. The hallmark histologic lesions were the tau-positive neuronal and glial inclusions. In neurons, these ranged from typical neurofibrillary tangles through well-circumscribed inclusions to diffuse cytoplasmic staining. This tau pathology was complemented by the presence of large, abnormal achromatic neurons, neuronal loss, astrocytosis, and superficial status spongiosus.

CONCLUSION

The distribution, type, and severity of these histologic abnormalities varied not only from case to case but also within the same brain. These brains with a common tau mutation raise important differential diagnostic problems: cases in the past might have been misdiagnosed as corticobasal degeneration or even atypical Pick disease, disorders with similar, if not identical, phenotypic manifestations.

Clinical features of frontotemporal dementia due to the intronic tau 10(+16) mutation

OBJECTIVE

To describe the clinical features of nine British families with neuropathologically verified frontotemporal dementia (FTD) due to the intronic tau exon 10(+16) mutation.

METHODS

Retrospective chart reviews of family members with FTD belonging to nine tau 10(+16) mutation pedigrees in whom neuropathologic examination had been carried out. APOE genotype was determined for those patients for whom DNA was available.

RESULTS

The median age at onset was 50 years (range 37 to 59 years; n = 30). The median age at death was 61 years (range 42 to 72 years; n = 33). The median duration of the disease was 11 years (range 3 to 22 years; n = 25) for those who have died and is 17 years (range 15 to 23 years; n = 3) for those living. The most common presenting symptom was disinhibition (n = 23). A minority presented with frontal dysexecutive symptoms, apathy, impairment of episodic memory, or depression. All of these patients subsequently developed personality and behavioral change. Memory impairment, language deficits, ritualistic behavior, hyperphagia, and hyperorality were frequent symptoms. Parkinsonism, neuroleptic sensitivity, or primitive reflexes were present in half of the patients, where these data were available. The clinical features of ALS were absent. Neuropathologic examination of 12 patients demonstrated the hallmark tau-positive neuronal and glial inclusions. APOE genotype did not account for the considerable variation in age at onset, age at death, duration of disease, or severity of estimated brain atrophy.

CONCLUSIONS

All cases fulfilled the clinical criteria for a diagnosis of FTD. Despite similar clinical phenotypes, there was considerable variation in age at onset and duration of disease both between and within families, suggesting the presence of an effect due to other genetic or environmental factors.

Inherited frontotemporal dementia in nine British families associated with intronic mutations in the tau gene

Genetic screening of 171 patients with frontotemporal lobar degeneration disclosed 14 patients, across nine pedigrees, with mutations in the intron to exon 10 in the tau gene, a region regulating the splicing of exon 10 via a stem loop mechanism. Thirteen of these patients had the +16 splice site mutation and one had the +13 splice site mutation. Affected members of all nine families presented with changes in behaviour and social conduct that were prototypical of frontotemporal dementia (FTD). In all patients with the +16 splice site mutation, the behavioural profile was characterized by disinhibition, restless overactivity, a fatuous affect, puerile behaviour and verbal and motor stereotypies. The single patient with the +13 mutation presented a contrasting picture of apathy and inertia. In addition, all patients had evidence of semantic loss. Pathologically, five of the six patients so far autopsied shared frontotemporal atrophy with involvement of the substantia nigra. The underlying histology was that of microvacuolar-type cortical degeneration with a few swollen cells. Tau pathology was widespread throughout the brain and present in neurones and glial cells, mostly in the frontal and temporal cortical regions. This was in the form of neurofibrillary tangles and amorphous tau deposits (pre-tangles); Pick bodies were not observed. Ultrastructurally, the tau filaments had a twisted, ribbon-like morphology distinct from the paired helical filaments of Alzheimer's disease. One patient died from an unrelated illness whilst in the early clinical stages of FTD. In this patient, cortical microvacuolar and astrocytic changes were absent, though there were scattered neurones and glial cells, immunoreactive to tau, throughout the cortical and subcortical regions. The disease process underlying the neurodegeneration within these inherited forms of FTD may therefore stem directly from early, primary alterations in the function of tau. All eight families with the +16 mutation seem to be part of a common extended pedigree, possibly originating from a founder member residing within the North Wales region of Great Britain.

Activation of the JNK/p38 pathway occurs in diseases characterized by tau protein pathology and is related to tau phosphorylation but not to apoptosis

JNK and p38, two members of the MAP kinase family, are strongly induced by various stresses including oxidative stress and have been involved in regulation of apoptosis. As both kinases phosphorylate tau protein in vitro, we have investigated their immunohistochemical localization in a group of neurodegenerative diseases characterized by intracellular deposits of hyperphosphorylated tau. Cases included Alzheimer disease, Pick disease, progressive supranuclear palsy, corticobasal degeneration, Gerstmann-Sträussler-Scheinker disease-Indiana kindred, and frontotemporal dementia with parkinsonism linked to chromosome 17. In all tissue samples, strong immunoreactivity for both MAP kinases was found in the same neuronal or glial cells that contained tau-positive deposits. By double immunohistochemistry, JNK and p38 colocalized with tau in the inclusions. Analysis of apoptosis-related changes (DNA fragmentation, activated caspase-3) showed that the expression of JNK and p38 was unrelated to activation of an apoptotic cascade. Our data indicate that phospho-JNK and phospho-p38 are associated with hyperphosphorylated tau in a variety of abnormal tau inclusions, suggesting that these kinases may play a role in the development of degenerative diseases with tau pathology.

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.

Detangling Alzheimer's disease

Alzheimer's disease (AD) afflicts 4 million people in the United States and is expected to strike 14 million by the year 2050, as the population ages. Researchers are scrambling to find genetic risk factors, decipher disease mechanisms, and develop reliable diagnostic tests that detect the illness at its earliest, potentially most treatable stage. Using these findings, they hope to devise new therapeutic approaches. Current clinical trials are testing novel techniques that stall or reverse AD-like neuropathology in mice.

Pick's disease associated with the novel Tau gene mutation K369I

Exonic and intronic mutations in Tau cause neurodegenerative syndromes characterized by frontotemporal dementia and filamentous tau protein deposits. We describe a K369I missense mutation in exon 12 of Tau in a patient with a pathology typical of sporadic Pick's disease. The proband presented with severe personality changes, followed by loss of cognitive function. Detailed postmortem examination of the brain showed atrophy, which was most pronounced in the temporal lobes; and numerous tau-immunoreactive Pick bodies and Pick cells in the neocortex and the hippocampal formation, as well as in subcortical brain regions. Their appearance and staining characteristics were indistinguishable from those of sporadic Pick's disease. However, immunoblot analysis of sarkosyl-insoluble tau showed three major bands of 60, 64, and 68 kDa, consistent with the presence of 3- and 4-repeat tau isoforms, as in Alzheimer's disease. Isolated tau filaments were irregularly twisted ribbons, with a small number of Alzheimer-type paired helical filaments. In the presence of heparin, tau proteins with the K369I mutation formed short, slender filaments. Biochemically, recombinant tau proteins with the K369I mutation showed reduced ability to promote microtubule assembly, suggesting that this may be the primary effect of the mutation by providing a pool of aberrant tau for filament assembly. Taken together, results indicate that the K369I mutation in Tau can cause a dementing disease with a neuropathology like that of Pick's disease.

Relationship of the extended tau haplotype to tau biochemistry and neuropathology in progressive supranuclear palsy

Two extended haplotypes of the tau gene (H1 and H2) have been described. The frequency of H1 haplotype is increased in progressive supranuclear palsy (PSP). PSP is associated with filamentous tau lesions in neurons and glia, which are reportedly composed exclusively of tau isoforms with four repeats in the microtubule-binding domain (4R tau). To determine the influence of the tau haplotype on tau isoform composition and neuropathology, we studied 25 PSP cases and 6 Alzheimer's disease patients matched for age, sex, and postmortem delay. In the basal ganglia, tau and amyloid burdens were determined to see if there was an effect of concurrent Alzheimer-type pathology, and the ratio of 4R to 3R tau was measured in detergent-insoluble tau fractions. Insoluble tau from PSP was not composed exclusively of 4R tau. All brains had a mixture of 4R and 3R tau, but the ratio was different in Alzheimer's disease and PSP. In Alzheimer's disease there was less 4R than 3R tau, whereas the ratio was reversed in PSP. In PSP cases with concurrent Alzheimer-type pathology, the ratio of 4R to 3R was intermediate between Alzheimer's disease and PSP. The H1 haplotype had no effect on the 4R to 3R ratio or on tau and amyloid burdens. In summary, the H1 haplotype does not have a major influence on the pathological or biochemical phenotype of PSP.

Neurodegenerative tauopathies

The defining neuropathological characteristics of Alzheimer's disease are abundant filamentous tau lesions and deposits of fibrillar amyloid beta peptides. Prominent filamentous tau inclusions and brain degeneration in the absence of beta-amyloid deposits are also hallmarks of neurodegenerative tauopathies exemplified by sporadic corticobasal degeneration, progressive supranuclear palsy, and Pick's disease, as well as by hereditary frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). Because multiple tau gene mutations are pathogenic for FTDP-17 and tau polymorphisms appear to be genetic risk factors for sporadic progressive supranuclear palsy and corticobasal degeneration, tau abnormalities are linked directly to the etiology and pathogenesis of neurodegenerative disease. Indeed, emerging data support the hypothesis that different tau gene mutations are pathogenic because they impair tau functions, promote tau fibrillization, or perturb tau gene splicing, thereby leading to formation of biochemically and structurally distinct aggregates of tau. Nonetheless, different members of the same kindred often exhibit diverse FTDP-17 syndromes, which suggests that additional genetic or epigenetic factors influence the phenotypic manifestations of neurodegenerative tauopathies. Although these and other hypothetical mechanisms of neurodegenerative tauopathies remain to be tested and validated, transgenic models are increasingly available for this purpose, and they will accelerate discovery of more effective therapies for neurodegenerative tauopathies and related disorders, including Alzheimer's disease.

Selective deposition of mutant tau in the FTDP-17 brain affected by the P301L mutation

Frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) is a familial neurological disorder exhibiting autosomal dominant inheritance. Linkage analyses have led to the identification of many exonic and intronic mutations in the tau gene in affected families. Because FTDP- 17 causes extensive neuronal loss and intracellular tau deposits in affected regions, investigation of this disease should provide an important insight into the significance of tau deposits leading to neurodegeneration. Using site-specific antibodies that distinguish between wild-type and mutant tau, we have analyzed the proportions of wild-type and mutant tau in the soluble and insoluble fractions of the P301L brain. Western blotting showed that mutant tau was selectively deposited in the Sarkosyl-insoluble fraction. Consistent with this, immunocytochemistry showed that intraneuronal tau deposits consisted exclusively of mutant tau. In one case in which abundant senile plaques occurred, in addition to mutant tau, small amounts of wild-type tau were also deposited. On the other hand, the protein levels of mutant tau in the soluble fraction were selectively decreased despite no detectable decrease in the levels of mutant tau mRNA.

Frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17): PPND family. A longitudinal videotape demonstration

Frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17), characterized by an autosomal dominant inheritance pattern, has recently been recognized as a distinct entity that can display a confusingly broad clinical phenotype. The pallido-ponto-nigral degeneration (PPND) variant is the prototypical example of the parkinsonism-predominant pattern of FTDP-17. A longitudinal videotape demonstration of the clinical progression of this entity in a single individual, along with brief videotape segments from three additional affected individuals, is presented in order to facilitate recognition of this disorder.

Tau and transgenic animal models

Advances in genetics and transgenic approaches have a continuous impact on our understanding of Alzheimer's disease (AD) and related disorders, especially as aspects of the histopathology and neurodegeneration can be reproduced in animal models. AD is characterized by extracellular Abeta peptide-containing plaques and neurofibrillary aggregates of hyperphosphorylated isoforms of microtubule-associated protein tau. A causal link between Abeta production, neurodegeneration and dementia has been established with the identification of familial forms of AD which are linked to mutations in the amyloid precursor protein APP, from which the Abeta peptide is derived by proteolysis. No mutations have been identified in the tau gene in AD until today. Tau filament formation, in the absence of Abeta production, is also a feature of several additional neurodegenerative diseases including progressive supranuclear palsy, corticobasal degeneration, Pick's disease, and frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17). The identification of mutations in the tau gene which are linked to FTDP-17 established that dysfunction of tau can, as well as Abeta formation, lead to neurodegeneration and dementia. In this review, newly recognized cellular functions of tau, and the neuropathology and clinical syndrome of FTDP-17 will be presented, as well as recent advances that have been achieved in studies of transgenic mice expressing tau and AD-related kinases and phosphatases. These models link neurofibrillary lesion formation to neuronal loss, provide an in vivo model in which therapies can be assessed, and may contribute to determine the relationship between Abeta production and tau pathology.

Oligodendroglial tau filament formation in transgenic mice expressing G272V tau

Genetic evidence indicates that several mutations in tau, including G272V, are linked to frontotemporal dementia with parkinsonism. We expressed this mutation in mouse brains by combining a prion protein promoter-driven expression system with an autoregulatory transactivator loop that resulted in high expression of human G272V tau in neurons and in oligodendrocytes. We show that G272V tau can form filaments in murine oligodendrocytes. Electron microscopy established that the filaments were either straight or had a twisted structure; these were 17-20 nm wide and had a periodicity of approximately 75 nm. Filament formation was associated with tau phosphorylation at distinct sites, including the AT8 epitope 202/205 in vivo. Immunogold electron microscopy of sarcosyl-extracted spinal cords from G272V transgenic mice using phosphorylation-dependent antibodies AT8 or AT100 identified several sparsely gold-labelled 6-nm filaments. In the spinal cord, fibrillary inclusions were also identified by thioflavin-S fluorescent microscopy in oligodendrocytes and motor neurons. These results establish that expression of the G272V mutation in mice causes oligodendroglial fibrillary lesions that are similar to those seen in human tauopathies.

Selective neurodegeneration, without neurofibrillary tangles, in a mouse model of Niemann-Pick C disease

The BALB/c mouse model of Niemann-Pick type C (NPC) disease exhibits neuropathological similarities to the human condition. There is an age-related cerebral atrophy, demyelination of the corpus callosum, and degeneration of cerebellar Purkinje cells in the NPC mouse. In human NPC, many cortical and subcortical neurons contain neurofibrillary tangles, which are thought by some investigators to play an important role in the neurodegenerative process. The purpose of the present study was to determine whether neurodegeneration occurs in the NPC mouse, in brain regions other than the cerebellum and whether the degeneration is related to the presence of neurofibrillary tangles. Using light microscopic methods with immunohistochemistry, electron microscopy, and cell counting methods, 11-week-old NPC(+/+) and NPC(-/-) animals were examined. In the NPC(-/-) mice, there were 96% fewer Purkinje cells, 28% fewer neurons in the prefrontal cortex, 20% fewer neurons in the thalamus, and 63% fewer glial cells in the corpus callosum. On the other hand, previous studies indicate normal numbers of neurons and glial cells in these same neuroanatomical regions in young NPC(-/-) mice. There were normal numbers of cholinergic neurons in sections assessed in the striatum and basal forebrain in the 11-week-old animals and no evidence of neurofibrillary tangles within cells. The present data indicate that both neurons and glial cells die in the NPC mouse but that all cells are not equally vulnerable. There was no evidence for neurofibrillary tangles in the NPC mouse, and therefore the degenerative process in the mouse is unrelated to the neurofibrillary tangle.

Frontal lobe dementia with novel tauopathy: sporadic multiple system tauopathy with dementia

We present a novel tauopathy in a patient with a 10-yr history of progressive frontal lobe dementia and a negative family history. Autopsy revealed mild atrophy of frontal and parietal lobes and severe atrophy of the temporal lobes. There were occasional filamentous tau-positive inclusions, but more interesting were numerous distinctive globular neuronal and glial tau-positive inclusions in both gray and white matter of the neocortex. Affected subcortical regions included substantia nigra, globus pallidus, subthalamic nucleus, and cerebellar dentate nucleus, in a distribution similar to progressive supranuclear palsy (PSP), but without significant accompanying neuronal loss or gliosis. Predominantly straight filaments were detected by electron microscopy (EM), while other inclusions were similar to fingerprint bodies. No twisted ribbons were detected. Immuno-EM studies revealed that only the filamentous inclusions were composed of tau. Immunoblotting of sarkosyl-insoluble tau revealed 2 major bands of 64 and 68 kDa. Blotting analysis after dephosphorylation revealed predominantly 4-repeat tau. Sequence analysis of tau revealed that there were no mutations in either exons 9-13 or the adjacent intronic sequences. The unique cortical tau pathology in this case of sporadic multiple system tauopathy with dementia adds a new pathologic profile to the spectrum of tauopathies.

Effects of FTDP-17 mutations on the in vitro phosphorylation of tau by glycogen synthase kinase 3beta identified by mass spectrometry demonstrate certain mutations exert long-range conformational changes

In vitro phosphorylation of recombinant wild-type 2N4R tau and FTDP-17 exonic mutant forms P301L, V337M and R406W by glycogen synthase kinase 3beta (GSK3beta) was examined by two dimensional phosphopeptide mapping analysis on thin layer cellulose plates. Comparison of these peptide maps with those generated from wild-type 1N4R tau isoform from which the phosphopeptide constituents and sites of phosphorylation had been determined previously, enabled us to monitor directly changes in phosphorylation of the individual tau proteins. No differences were found in the phosphorylation of wild-type, P301L or V337M tau by GSK3beta but the R406W mutant showed at least two clear differences from the other three tau proteins. The peptides, identified by mass spectrometry corresponding to phosphorylation at both threonine 231 and serine 235 (spot 3), serines 396, 400 and 404 (spot 6a) and serines 195 and 199 (spot 6b) were absent from the R406W peptide map. The findings imply that the R406W mutation in tau exerts long-range conformational effects on the structure of tau.

Organization and functional roles of the cytoskeleton in oligodendrocytes

Mature oligodendrocytes are characterized by their numerous cytoplasmic extensions and flat membranous sheets. These sheets contain an extensive cytoskeletal network of microtubules (MTs) that maintain the cellular morphology, are specifically important for cellular sorting, and provide the rails for organelle trafficking. Mitochondria are localized in the primary and secondary processes and follow the tracks of the MTs in the cytoplasmic extensions. Oligodendrocytes express microtubule associated proteins (MAPs), specifically MAP2 and tau, which might be involved in the regulation and stabilization of the dynamic MT network in the myelin-containing cellular processes. Tau and MAP2 heterogeneity increases during oligodendroglia maturation, and in mature oligodendrocytes tau mRNA with four MT binding domains are more prominent than in progenitor cells. Filamentous cell inclusions are a unifying mechanism underlying a variety of late-onset neurodegenerative disorders and have mainly been viewed as neuron-specific. Recent evidence indicated that glial changes occur in CNS degenerative diseases and seem to be a more common feature than previously thought. Glial fibrillary tangles (GFTs) in oligodendrocytes were observed in familial multiple system tauopathy, and glial cytoplasmic inclusions (GCIs) and oligodendroglia degeneration are the histological hallmark of multiple system atrophy (MSA). GCIs are associated with MTs and contain stress proteins and MAPs. Thus, neurons and glial cells share common cytoskeletal pathologies. During health and disease, MAPs might be important regulators of the structural stability and plasticity of the oligodendroglia cytoskeleton.

Molecular genetics of chromosome 17 tauopathies

The identification of mutations in the gene encoding the microtubule associated protein tau in frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) demonstrated that tau dysfunction can lead to neurodegeneration. At least 11 missense mutations and 1 deletion mutation (delta K280) have been identified in exons 9-13 that encode the microtubule binding domains of tau. In addition, five mutations have been found close to the 5' splice site of exon 10. The different FTDP-17 mutations have multiple effects on the biology and function of tau. These varied pathogenic mechanisms likely explain the wide range of clinical and neuropathological features observed in different families with FTDP-17. In addition to the highly penetrant mutations that are found in large families with FTDP-17, a common extended haplotype in the tau gene also appears to be a risk factor in the development of the apparently sporadic tauopathy, progressive supranuclear palsy (PSP). The mechanism by which this common variability in the tau gene influences the development of PSP is unclear; however, it further suggests a central role for tau in the pathogenesis of several neurodegenerative conditions including Alzheimer's disease (AD).

Coexistent tau and amyloid pathology in hereditary frontotemporal dementia with tau mutations

Hereditary frontotemporal dementia with Parkinsonism linked to chromosome 17 (FTDP-17) is associated with different mutations in the microtubule-associated protein (MAP) tau gene. Pathological changes consist of accumulation of hyperphosphorylated tau protein in frontal and temporal cortex, hippocampus, and some subcortical nuclei. We describe the neuropathological findings in five patients with P301L mutation, and in two affected sibs with R406W mutation. The P301L brains all showed a pretangle-type tauopathy of the frontal and temporal cortices. One of these patients, however, also showed an Alzheimer-type tauopathy with neurofibrillary tangles (NFT), neuritic plaques, and amyloid angiopathy of the temporoparietal cortex. Three tau bands (64, 68, and 72 kDa) were seen in the frontal cortex, while the temporal cortex revealed four bands (60, 64, 68, and 72 kDa), containing all six tau isoforms. The first R406W brain showed many NFT in affected regions with only a few diffuse amyloid plaques. The second R406W brain contained a much higher density of NFT in affected regions, and an extensive amyloid deposition consisting of both diffuse and neuritic plaques with dense cores. An intriguing question is whether the FTD and Alzheimer disease changes are concomitant, or whether there is an interaction between tau and amyloid pathology. An acceleration of NFT formation due to amyloid deposition has been observed in nondemented aging and preclinical AD. The question whether this mechanism occurs in FTD with tau mutations remains to be elucidated.

Progress in hereditary tauopathies: a mutation in the Tau gene (G389R) causes a Pick disease-like syndrome

We describe the clinical and pathologic phenotypes of the G389R mutation in exon 13 of the Tau gene. Progressive aphasia and memory disturbance are the initial signs and begin in the fourth or fifth decade of life, followed by apathy, indifference, hyperphagia, rigidity, pyramidal signs and dementia. Death occurs after two to five years. Magnetic resonance imaging and neuropathologic studies show frontal and temporal atrophy. Pick body-like and axonal filamentous inclusions found in the neocortex and subcortical white matter, respectively, are tau immunoreactive. Immunoblot analysis of sarkosyl-insoluble tau shows two major bands of 60 and 64 kDa that, upon dephosphorylation, resolve into four bands of three- and four-repeat isoforms. Isolated tau filaments are often straight and occasionally twisted. Recombinant mutant tau protein shows a reduced ability to promote microtubule assembly, suggesting that this may be the primary effect of the mutation. The present findings indicate that the G389R mutation in Tau can cause a dementia similar to that in Pick's disease.

Tau gene mutations in frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). Their relevance for understanding the neurogenerative process

Tau is a microtubule-associated protein that binds to microtubules and promotes microtubule assembly. Six tau isoforms are produced in adult human brain by alternative mRNA splicing from a single gene. Inclusion of a 31 amino acid repeat encoded by exon 10 of the tau gene gives rise to the three isoforms with four microtubule-binding repeats each. The other three tau isoforms have three repeats each. Abundant neurofibrillary lesions made of tau protein constitute a defining neuropathological characteristic of Alzheimer's disease. Filamentous tau protein deposits are also the defining characteristic of other neurodegenerative diseases, many of which are frontotemporal dementias or movement disorders, such as Pick's disease, progressive supranuclear palsy, and corticobasal degeneration. It is well established that the distribution of tau pathology correlates with the presence of symptoms of disease. However, until recently, there was no genetic evidence linking tau to neurodegeneration. This has now changed with the discovery of more than 15 mutations in the tau gene in frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). The new work has shown that dysfunction of tau protein causes neurodegeneration.

From genetics to pathology: tau and alpha-synuclein assemblies in neurodegenerative diseases

The most common degenerative diseases of the human brain are characterized by the presence of abnormal filamentous inclusions in affected nerve cells and glial cells. These diseases can be grouped into two classes, based on the identity of the major proteinaceous components of the filamentous assemblies. The filaments are made of either the microtubule-associated protein tau or the protein alpha-synuclein. Importantly, the discovery of mutations in the tau gene in familial forms of frontotemporal dementia and of mutations in the alpha-synuclein gene in familial forms of Parkinson's disease has established that dysfunction of tau protein and alpha-synuclein can cause neurodegeneration.

Morphological substrates of mental dysfunction in Lewy body disease: an update

Mental dysfunction including cognitive, behavioural changes, mood disorders, and psychosis are increasingly recognized in patients with Parkinson's disease (PD) and related disorders. Their morphological correlates are complex due to multiple system degeneration. CNS changes contributing to cognitive changes in PD include 1. Dysfunction of subcorticocortical networks with neuron losses in a) the dopaminergic nigrostriatal loop, causing striato-(pre)frontal deafferentation and mesocortico-limbic system (medial substantia nigra, ventral tegmentum); b) noradrenergic (locus coeruleus), and serotonergic systems (dorsal raphe nuclei), c) cholinergic forebrain system (nucleus basalis of Meynert, etc), and d) specific nuclei of amygdala and limbic system (thalamic nuclei, hippocampus); 2. Limbic and/or cortical Lewy body and Alzheimer type pathologies with loss of neurons and synapses, 3. Combination of subcortical, cortical, and other pathologies. In general, degeneration of subcortical and striato-frontal networks causes cognitive, executive, behavioural, and mood disorders but less severe dementia than cortical changes which, when present in sufficient numbers, are important factors for overt dementia. In PD, cortical tau pathology with similar or differential patterns than in Alzheimer disease (AD) shows significant linear correlation with cognitive decline. In dementia with Lewy bodies (DLB), the second most frequent cause of dementia in the elderly, cortical Lewy bodies (LB) may or may not be associated with amyloid plaques and neuritic AD lesions. They predominantly affect the limbic system with less frequent isocortical Braak stages, whereas the cholinergic forebrain system is more severely affected than in AD. Both neuritic degeneration in limbic system in PD and DLB and the density of cortical synapse markers correlate with neuritic AD pathology and less with cortical LB counts. Apolipoprotein E epsilon4 allele frequency may represent a common genetic background for both AD and LB pathologies but there are different proportions of plaques between DLB (less Abeta1-40) and AD (more frequent Abeta1-40). Familial parkinsonism with dementia, linked to chromosome 17 (frontotemporal dementia with Parkinsonism (FTDP-17), and other tauopathies pathologically resembling PD plus AD, are often related to mutations of the tau gene, whereas familial PD with alpha-synuclein and Parkin mutations usually show no cognitive impairment. Mood disorders, in particular depression, and psychotic complications in both PD and DLB are related to complex involvement of noradrenergic and serotonergic systems, not confirmed in AD with depression, and both the prefrontal and limbic dopaminergic systems. The specific contributions of cortical and subcortical pathologies to mental dysfunction in PD and related disorders, their relationship to AD, and their genetic and aetiological backgrounds await further elucidation.

Proapoptotic effects of tau cleavage product generated by caspase-3

Using an in vitro translation assay to screen a human brain cDNA library, we isolated the microtubule-associated protein Tau and determined it to be a caspase-3 substrate whose C-terminal cleavage occurred during neuronal apoptosis. DeltaTau, the 50-kDa cleavage product, was detected by Western blot in apoptotic cortical cells probed with anti-PHF-1 and anti-Tau-5 antibodies, but not anti-T-46 antibody which recognizes the C-terminus. Overexpression of DeltaTau in SK-N-BE2(C) cells significantly increased the incidence of cell death. Staurosporine-induced Tau cleavage was blocked by 20 microM z-Asp-Glu-Val-Asp-chloromethylketone, a caspase-3 inhibitor, and in vitro, Tau was selectively cleaved by caspase-3 or calpain, a calcium-activated protease, but not by caspases-1, -8, or -9. (D421E)-Tau, a mutant in which Asp421 was replaced with a Glu, was resistant to cleavage by caspase-3 and tended to suppress staurosporine-induced cell death more efficiently than did wild-type Tau in both transient and stable expression systems. Finally, the incidence of DeltaTau-induced cell death was augmented by expression of Abeta precursor protein (APP) or Swedish APP mutant. Taken together, these results suggest that the caspase-3 cleavage product of Tau may contribute to the progression of neuronal cell death in Alzheimer's disease.

Diagnosis, classification and natural history of degenerative dementias

The release of the first approved medications for the treatment of Alzheimer's disease in Canada has highlighted the renewed need and importance of diagnostic accuracy and understanding of the spectrum of the dementias. The epidemiological scope of the problem of dementia in Canada including risk factors, caregiving patterns and costs of care have been well-characterized through the Canadian Study of Health and Aging (CSHA 1991-1996) with some of the key findings reviewed here. Beyond Alzheimer's disease the phenotypes and genotypes of the other degenerative dementias have been emerging with proposed operational diagnostic criteria that should facilitate their recognition in clinical practice. This paper reviews the clinical phenotypes of the most common causes of dementia with a proposed classification scheme and with discussion of their relevance from a differential treatment standpoint. This paper served as a background document for the working group of the Consensus Conference on Dementia (C3D) in February 1998 and has been revised subsequently for this publication.

Molecular analysis of mutant and wild-type tau deposited in the brain affected by the FTDP-17 R406W mutation

Frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) is a familial neurological disorder, characterized genetically by autosomal dominant inheritance, clinically by behavioral abnormalities and parkinsonism, and neuropathologically by tauopathy. Linkage analyses of affected families have led to identification of several exonic and intronic mutations in the tau gene. In this study, we analyzed molecular species of tau in the soluble and insoluble fractions of brain affected by the FTDP-17 R406W mutation. Protein chemical analysis and Western blotting using site-specific antibodies indicated that almost equal amounts of wild-type and mutant tau were present in the Sarkosyl-insoluble fraction of the R406W brain. Consistent with this, wild-type and mutant tau colocalized in neurofibrillary tangles in the frontal cortex and hippocampus of the R406W brain. In contrast to soluble R406W tau, which was less phosphorylated than soluble wild-type tau, the Sarkosyl-insoluble mutant tau was highly phosphorylated as well as the insoluble wild-type tau.

Familial atypical progressive supranuclear palsy associated with homozigosity for the delN296 mutation in the tau gene

Heterozygous missense and splice-site mutations in the tau gene have been previously identified in familial frontotemporal dementia with autosomal dominant inheritance. Here we report a Spanish kindred in which two brothers born from a third-degree consanguineous marriage were both affected with atypical progressive supranuclear palsy. A homozygous deletion at codon 296 (delN296) was identified in one of the affected siblings. Among the heterozygous carriers, two members with probable Parkinson's disease were identified, but none of heterozygotes developed atypical parkinsonism. The delN296 mutation lies in the sequence corresponding to the second tubulin-binding repeat of tau protein and affects one asparagine residue absolutely conserved in other species. This finding indicates that homozygous mutations in the tau gene may also cause hereditary tauopathies.

Fibers of tau fragments, but not full length tau, exhibit a cross beta-structure: implications for the formation of paired helical filaments

We have used X-ray fiber diffraction to probe the structure of fibers of tau and tau fragments. Fibers of fragments from the microtubule binding domain had a cross beta-structure that closely resembles that reported both for neurofibrillary tangles found in Alzheimer's disease brain and for fibrous lesions from other protein folding diseases. In contrast, fibers of full-length tau had a different, more complex structure. Despite major differences at the molecular level, all fiber types exhibited very similar morphology by electron microscopy. These results have a number of implications for understanding the etiology of Alzheimer's and other tauopathic diseases. The morphology of the peptide fibers suggests that the region in tau corresponding to the peptides plays a critical role in the nucleation of fiber assembly. The dramatically different structure of the full length tau fibers suggests that some region in tau has enough inherent structure to interfere with the formation of cross beta-fibers. Additionally, the similar appearance by electron microscopy of fibrils with varying molecular structure suggests that different molecular arrangements may exist in other samples of fibers formed from tau.

Reduced binding of protein phosphatase 2A to tau protein with frontotemporal dementia and parkinsonism linked to chromosome 17 mutations

Coding region and intronic mutations in the tau gene cause frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). We have previously reported that ABalphaC, a major form of protein phosphatase 2A (PP2A) in brain, binds tightly to tau protein in vitro and is a major tau phosphatase in vivo. Using in vitro assays, we show here that the FTDP-17 mutations G272V, DeltaK280, P301L, P301S, S305N, V337M, G389R, and R406W inhibit by approximately 20-95% the binding of recombinant three-repeat and four-repeat tau isoforms to the ABalphaC holoenzyme and the AC core enzyme of PP2A. Reduction in binding was maximal for tau proteins with the G272V, DeltaK280, and V337M mutations. We also show that tau protein can be specifically coimmunoprecipitated with endogenous PP2A from both rat brain and transfected cell extracts. It is significant that, by using similar coimmunoprecipitation assays, we show that all FTDP-17 mutations tested, including the N279K mutation, alter the ability of tau to associate with cellular PP2A. Taken together, these results indicate that FTDP-17 mutations induce a significant decrease in the binding affinity of tau for PP2A in vivo. We propose that altered protein-protein interactions between PP2A and tau may contribute to FTDP-17 pathogenesis.

Tau gene mutation K257T causes a tauopathy similar to Pick's disease

Exonic and intronic mutations in Tau cause neurodegenerative syndromes characterized by frontotemporal dementia and filamentous tau protein deposits. Here we describe a K257T missense mutation in exon 9 of Tau. The proband, a 47-yr-old male, presented with severe personality changes followed by semantic memory loss. A diagnosis of Pick's disease was made. The symptoms progressed until death at age 51. The proband's brain showed a marked frontotemporal atrophy that was most pronounced in the temporal lobes. Numerous tau-immunoreactive Pick bodies were present in the neocortex and the hippocampal formation, as well as in some subcortical brain regions. Their appearance and staining characteristics were indistinguishable from those of sporadic Pick's disease. Diffuse staining for hyperphosphorylated tau was also observed in some nerve cell bodies. Immunoblot analysis of sarkosyl-insoluble tau showed 2 major bands of 60 and 64 kDa and 2 very minor bands of 68 and 72 kDa. Upon dephosphorylation, these bands resolved into 6 bands consisting of 3-repeat and 4-repeat tau isoforms, with an overall preponderance of 3-repeat tau. Isolated tau filaments were narrow, irregularly twisted ribbons. Biochemically, recombinant tau proteins with the K257T mutation showed a reduced ability to promote microtubule assembly, suggesting that this may be the primary effect of the mutation. In addition, the K257T mutation was found to stimulate heparin-induced assembly of 3-repeat tau into filaments. Taken together, the present findings indicate that the K257T mutation in Tau can cause a dementing condition similar to Pick's disease.

A new case of frontotemporal dementia and parkinsonism resulting from an intron 10 +3-splice site mutation in the tau gene: clinical and pathological features

Hereditary frontotemporal dementia and parkinsonism (FTDP) linked to chromosome 17 (FTDP-17) constitutes a new form of tauopathy, and mutations in the tau gene have recently been reported in some affected families. This report presents clinical and neuropathological data from a member of a British family (SOT 254) with a history of dementia and movement disorder. The medical history of the affected patient, a woman aged 44 years, was reviewed, and a detailed post-mortem examination of the brain was undertaken. A panel of well characterized phosphorylation-dependent and independent anti-tau antibodies was used to assess tau pathology, and inclusions were examined by electron microscopy. Neuronal loss and gliosis were widely distributed, but most severe in neocortical regions, and were associated with abundant neuronal and glial tau inclusions which consisted of a mixture of paired helical filaments (PHFs), similar to those in Alzheimer's disease, and distinct twisted ribbon-like filaments. Genomic DNA was obtained from post-mortem tissue from the index patient, and blood from two unaffected members of the same family. For the index case only, sequencing of intronic sequences flanking exon 10 of the tau gene identified a G to A transition at position +3 of the splice-donor site downstream of exon 10, identical to that reported in multiple system tauopathy with presenile dementia (MSTD). The clinical, neuropathological and genetic findings strongly suggest that SOT 254 represents a new example of FTDP-17 resulting from a mutation in the tau gene. These results are compared with those reported for other FTDP-17 families, i.e. for MSTD.

Structure of tau protein and assembly into paired helical filaments

Over the past few years the systematic investigation of paired helical filament assembly from tau protein in vitro has become feasible. We review our current understanding of the structure and conformations of tau protein and how this affects tau's assembly into the pathological paired helical filaments in Alzheimer's disease.

Tau mutations in frontotemporal dementia FTDP-17 and their relevance for Alzheimer's disease

Alzheimer's disease is characterised by the degeneration of selected populations of nerve cells that develop filamentous inclusions prior to degeneration. The neuronal inclusions of Alzheimer's disease are made of the microtubule-associated protein tau, in a hyperphosphorylated state. Abundant filamentous tau inclusions are not limited to Alzheimer's disease. They are the defining neuropathological characteristic of frontotemporal dementias, such as Pick's disease, and of progressive supranuclear palsy and corticobasal degeneration. The discovery of mutations in the tau gene in familial frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) has provided a direct link between tau dysfunction and dementing disease. Known mutations produce either a reduced ability of tau to interact with microtubules, or an overproduction of tau isoforms with four microtubule-binding repeats. This leads in turn to the assembly of tau into filaments similar or identical to those found in Alzheimer's disease brain. Several missense mutations also have a stimulatory effect on heparin-induced tau filament formation. Assembly of tau into filaments may be the gain of toxic function that is believed to underlie the demise of affected brain cells.

Determinants of 4-repeat tau expression. Coordination between enhancing and inhibitory splicing sequences for exon 10 inclusion

Mutations in the tau gene are pathogenic causing autosomal dominant frontotemporal dementia with Parkinsonism-chromosome 17 type (FTDP-17). Some mutations in tau exon 10 (E10) and immediately adjacent sequences cause disease by altering E10 splicing. To determine the mechanism of normal E10 splicing regulation and how FTDP-17 mutations alter splicing, mutational analysis of E10 was performed. The results show that E10 contains a complex array of both enhancer and inhibitor cis-acting elements that modulate usage of a weak 5' splice site. The 5' end of E10 contains a previously unrecognized multipartite exon splicing enhancer (ESE) composed of an SC35-like binding sequence, a purine-rich sequence, and an AC-rich element. Downstream of this ESE is a purine-rich exon splicing inhibitor. Intronic sequences immediately downstream of E10 also are inhibitory. The results support an alternative model in which I10 inhibitory sequences appear to function as a linear sequence. The cis-elements described are not redundant, and all appear required for normal E10 splicing. Results with double mutations demonstrate that the ESE and the intronic inhibitory element collaborate to regulate splicing. Thus splicing of tau E10 is regulated by a complex set of cis-acting elements that span nearly the entire exon and also include intronic sequences.

Aberrant splicing of tau pre-mRNA caused by intronic mutations associated with the inherited dementia frontotemporal dementia with parkinsonism linked to chromosome 17

Frontotemporal dementia accounts for a significant fraction of dementia cases. Frontotemporal dementia with parkinsonism linked to chromosome 17 is associated with either exonic or intronic mutations in the tau gene. This highlights the involvement of aberrant pre-mRNA splicing in the pathogenesis of neurodegenerative disorders. Little is known about the molecular mechanisms of the splicing defects underlying these diseases. To establish a model system for studying the role of pre-mRNA splicing in neurodegenerative diseases, we have constructed a tau minigene that reproduces tau alternative splicing in both cultured cells and in vitro biochemical assays. We demonstrate that mutations in a nonconserved intronic region of the human tau gene lead to increased splicing between exon 10 and exon 11. Systematic biochemical analyses indicate the importance of U1 snRNP and, to a lesser extent, U6 snRNP in differentially recognizing wild-type versus intron mutant tau pre-mRNAs. Gel mobility shift assays with purified U1 snRNP and oligonucleotide-directed RNase H cleavage experiments support the idea that the intronic mutations destabilize a stem-loop structure that sequesters the 5' splice site downstream of exon 10 in tau pre-mRNA, leading to increases in U1 snRNP binding and in splicing between exon 10 and exon 11. Thus, mutations in nonconserved intronic regions that increase rather than decrease alternative splicing can be an important pathogenic mechanism for the development of human diseases.

Abnormal tau-containing filaments in neurodegenerative diseases

It has been known for some time that the neurofibrillary pathology in Alzheimer's disease consists of so-called paired helical and straight filaments made up of the microtubule-associated protein tau. The degree of dementia observed in the disease correlates better with the extent of neurofibrillary pathology than with the Abeta amyloid deposits, the other characteristic defining pathological fibrous deposit in Alzheimer's disease. However, no familial cases of Alzheimer's disease have been genetically linked to the tau protein locus. Recently a group of frontotemporal dementias with parkinsonism linked to chromosome 17 has been shown to be caused by mutations in the tau gene. Some are missense mutations giving altered tau proteins, whereas others affect the splicing of the pre-mRNA and change the balance between different tau isoforms. Histologically these diseases are all characterised by various kinds of filamentous tau protein deposits, mostly in the complete absence of Abeta deposits. The abnormal tau filaments show different morphologies, depending on the nature of the tau mutation. These diseases show that tau mutations can be a prime cause of inherited dementing illness and may throw some light on the pathological process in the much larger number of sporadic cases of Alzheimer's disease.