A mutation in the microtubule-associated protein tau in pallido-nigro-luysian degeneration

A novel MAPT variant (E342K) as a cause of familial progressive supranuclear palsy

Background

MAPT variants are a known cause of frontotemporal dementia and Parkinsonian syndrome, of which progressive supranuclear palsy syndrome (PSP) is a rare manifestation.

Objective

To report a novel MAPT variant in a PSP pedigree with autosomal dominant inheritance pattern, and to produce a literature review of PSP patients with MAPT variants.

Methods

A comprehensive clinical, genetic, and molecular neuroimaging investigation was conducted on a 61 years-old female proband diagnosed with PSP. We also collected the clinical presentation data and history of the patient's pedigree, and performed further genetic analysis of 4 relatives, from two generations, with and without symptoms.

Results

The proband exhibited typical clinical manifestation of PSP. A cranial MRI revealed midbrain atrophy, and an FDG-PET scan suggested hypo-metabolic changes in caudate nucleus, left prefrontal lobe, both temporal poles, and midbrain. 18F-florzolo-tau-PET revealed tau-protein deposits in the thalamus and brainstem bilaterally. A gene test by whole-exome sequencing identified a novel MAPT variant [NM_005910.6, exon 11, c.1024G > A (p.E342K)], and the same variant was also identified in one affected relative and one asymptomatic relative, a probable pre-symptomatic carrier.

Conclusion

The PSP pedigree caused by the novel MAPT (E342K) variant, expanded the mutational spectrum of MAPT.

Genetics of Multiple System Atrophy and Progressive Supranuclear Palsy: A Systemized Review of the Literature

Multiple system atrophy (MSA) and progressive supranuclear palsy (PSP) are uncommon multifactorial atypical Parkinsonian syndromes, expressed by various clinical features. MSA and PSP are commonly considered sporadic neurodegenerative disorders; however, our understanding is improving of their genetic framework. The purpose of this study was to critically review the genetics of MSA and PSP and their involvement in the pathogenesis. A systemized literature search of PubMed and MEDLINE was performed up to 1 January 2023. Narrative synthesis of the results was undertaken. In total, 43 studies were analyzed. Although familial MSA cases have been reported, the hereditary nature could not be demonstrated. COQ2 mutations were involved in familial and sporadic MSA, without being reproduced in various clinical populations. In terms of the genetics of the cohort, synuclein alpha (SNCA) polymorphisms were correlated with an elevated likelihood of manifesting MSA in Caucasians, but a causal effect relationship could not be demonstrated. Fifteen MAPT mutations were linked with PSP. Leucine-rich repeat kinase 2 (LRRK2) is an infrequent monogenic mutation of PSP. Dynactin subunit 1 (DCTN1) mutations may imitate the PSP phenotype. GWAS have noted many risk loci of PSP (STX6 and EIF2AK3), suggesting pathogenetic mechanisms related to PSP. Despite the limited evidence, it seems that genetics influence the susceptibility to MSA and PSP. MAPT mutations result in the MSA and PSP pathologies. Further studies are crucial to elucidate the pathogeneses of MSA and PSP, which will support efforts to develop novel drug options.

Clinical and genetic analysis of Vietnamese patients diagnosed with early-onset Parkinson's disease

BACKGROUND

Genetic factors play a crucial role in the pathogenesis of Parkinson's disease (PD). However, no comprehensive study has described genetic alterations in Vietnamese patients diagnosed with PD. This study aimed to identify genetic causes and their association with clinical phenotypes in a Vietnamese PD cohort.

METHODS

A total of 83 patients with early-onset PD (disease onset before the age of 50) were recruited for genetic analysis using a combination of multiplex ligation-dependent probe amplification and next-generation sequencing for a panel of 20 PD-associated genes.

RESULTS

It was found that 37 out of 83 patients carried genetic alterations, with 24 pathogenic/likely pathogenic/risk variants and 25 variants of uncertain significance. The pathogenic/likely pathogenic/risk variants were mostly detected in LRRK2, PRKN, and GBA, while the variants of uncertain significance were identified in 12 different genes that were studied. The most common genetic alteration was LRRK2 c.4883G>C (p.Arg1628Pro), and patients with PD carrying this variant were found to have a distinct phenotype. Participants carrying pathogenic/likely pathogenic/risk variants had a significantly higher rate of a family history of PD.

CONCLUSION

These results provide a further understanding of genetic alterations associated with PD in a South-East Asian population.

The Genetic Landscape of Parkinsonism-Related Dystonias and Atypical Parkinsonism-Related Syndromes

In recent decades, genetic research has nominated promising pathways and biological insights contributing to the etiological landscape of parkinsonism-related dystonias and atypical parkinsonism-related syndromes. Several disease-causing mutations and genetic risk factors have been unraveled, providing a deeper molecular understanding of the complex genetic architecture underlying these conditions. These disorders are difficult to accurately diagnose and categorize, thus making genetics research challenging. On one hand, dystonia is an umbrella term linked to clinically heterogeneous forms of disease including dopa-responsive dystonia, myoclonus-dystonia, rapid-onset dystonia-parkinsonism and dystonia-parkinsonism, often viewed as a precursor to Parkinson’s disease. On the other hand, atypical parkinsonism disorders, such as progressive supranuclear palsy, multiple system atrophy and corticobasal degeneration, are rare in nature and represent a wide range of diverse and overlapping phenotypic variabilities, with genetic research limited by sample size availability. The current review summarizes the plethora of available genetic information for these diseases, outlining limits and future directions.

Genetics of Progressive Supranuclear Palsy: A Review

Progressive supranuclear palsy (PSP) is an atypical parkinsonism with prominent 4R-tau neuropathology, and the classical clinical phenotype is characterized by vertical supranuclear gaze palsy, unprovoked falls, akinetic-rigid syndrome and cognitive decline. Though PSP is generally regarded as sporadic, there is increasing evidence suggesting that a series of common and rare genetic variants impact on sporadic and familial forms of PSP. To date, more than 10 genes have been reported to show a potential association with PSP. Among these genes, the microtubule-associated protein tau (MAPT) is the risk locus with the strongest effect size on sporadic PSP in the case-control genome-wide association studies (GWAS). Additionally, MAPT mutations are the most common cause of familial PSP while the leucine-rich repeat kinase 2 (LRRK2) is a rare monogenic cause of PSP, and several other gene mutations may mimic the PSP phenotype, like the dynactin subunit 1 (DCTN1). In total, 15 MAPT mutations have been identified in cases with PSP, and the mean age at onset is much earlier than in cases carrying LRRK2 or DCTN1 mutations. GWAS have further identified several risk loci of PSP, proposing molecular pathways related to PSP. The present review focused on genetic studies on PSP and summarized genetic factors of PSP, which may help to elucidate the underlying pathogenesis and provide new perspectives for therapeutic strategies.

Clinical heterogeneity of frontotemporal dementia and Parkinsonism linked to chromosome 17 caused by MAPT N279K mutation in relation to tau positron emission tomography features

BACKGROUND

While mechanistic links between tau abnormalities and neurodegeneration have been proven in frontotemporal dementia and parkinsonism linked to chromosome 17 caused by MAPT mutations, variability of the tau pathogenesis and its relation to clinical progressions in the same MAPT mutation carriers are yet to be clarified.

OBJECTIVES

The present study aimed to analyze clinical profiles, tau accumulations, and their correlations in 3 kindreds with frontotemporal dementia and parkinsonism linked to chromosome 17 attributed to the MAPT N279K mutation.

METHODS

Four patients with N279K mutant frontotemporal dementia and parkinsonism linked to chromosome 17/MAPT underwent [¹¹ C]PBB3-PET to estimate regional tau loads.

RESULTS

Haplotype assays revealed that these kindreds originated from a single founder. Despite homogeneity of the disease-causing MAPT allele, clinical progression was more rapid in 2 kindreds than in the other. The kindred with slow progression showed mild tau depositions, mostly confined to the midbrain and medial temporal areas. In contrast, kindreds with rapid progression showed profoundly increased [¹¹ C]PBB3 binding in widespread regions from an early disease stage.

CONCLUSIONS

[¹¹ C]PBB3-PET can capture four-repeat tau pathologies characteristic of N279K mutant frontotemporal dementia and parkinsonism linked to chromosome 17/MAPT. Our findings indicate that, in addition to the mutated MAPT allele, genetic and/or epigenetic modifiers of tau pathologies lead to heterogeneous clinicopathological features. © 2019 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

Integrative system biology analyses of CRISPR-edited iPSC-derived neurons and human brains reveal deficiencies of presynaptic signaling in FTLD and PSP

Mutations in the microtubule-associated protein tau (MAPT) gene cause autosomal dominant frontotemporal lobar degeneration with tau inclusions (FTLD-tau). MAPT p.R406W carriers present clinically with progressive memory loss and neuropathologically with neuronal and glial tauopathy. However, the pathogenic events triggered by the expression of the mutant tau protein remain poorly understood. To identify the genes and pathways that are dysregulated in FTLD-tau, we performed transcriptomic analyses in induced pluripotent stem cell (iPSC)-derived neurons carrying MAPT p.R406W and CRISPR/Cas9-corrected isogenic controls. We found that the expression of the MAPT p.R406W mutation was sufficient to create a significantly different transcriptomic profile compared with that of the isogeneic controls and to cause the differential expression of 328 genes. Sixty-one of these genes were also differentially expressed in the same direction between MAPT p.R406W carriers and pathology-free human control brains. We found that genes differentially expressed in the stem cell models and human brains were enriched for pathways involving gamma-aminobutyric acid (GABA) receptors and pre-synaptic function. The expression of GABA receptor genes, including GABRB2 and GABRG2, were consistently reduced in iPSC-derived neurons and brains from MAPT p.R406W carriers. Interestingly, we found that GABA receptor genes, including GABRB2 and GABRG2, are significantly lower in symptomatic mouse models of tauopathy, as well as in brains with progressive supranuclear palsy. Genome wide association analyses reveal that common variants within GABRB2 are associated with increased risk for frontotemporal dementia (P < 1 × 10^-3). Thus, our systems biology approach, which leverages molecular data from stem cells, animal models, and human brain tissue can reveal novel disease mechanisms. Here, we demonstrate that MAPT p.R406W is sufficient to induce changes in GABA-mediated signaling and synaptic function, which may contribute to the pathogenesis of FTLD-tau and other primary tauopathies.

FTDP-17 with Pick body-like inclusions associated with a novel tau mutation, p.E372G

Mutations in microtubule-associated protein tau gene (MAPT) cause frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17). Here, we describe a patient with FTDP-17 and a novel missense mutation in exon 13 of MAPT, p.E372G. We compare clinicopathologic features of this patient to two previously unreported patients with another exon 13 mutation, p.G389R. The patient with the p.E372G mutation was a 40-year-old man with behavioral variant frontotemporal dementia (bvFTD), who subsequently developed agrammatic speech and parkinsonism. One of the FTDP-17 patients with p.G389R mutation presented at age 24 with agrammatic variant of primary progressive aphasia, and subsequently behavioral dysfunction. The other presented at age 53 with bvFTD, followed by agrammatic speech and corticobasal syndrome. Neuropathologic features of FTDP-17 due to p.E372G were similar to those of p.G389R, including tau-immunoreactive Pick body-like neuronal inclusions and swollen, tapering thread-like processes in white matter immunoreactive for 3-repeat and 4-repeat tau. Biochemical analysis of insoluble tau showed similar isoform compositions in p.E372G and p.G389R. Functional studies of the p.E372G mutation showed marked increase in tau filament formation and its reduced ability to promote microtubule assembly. Together these findings indicate that p.E372G is a pathogenic MAPT mutation that causes FTDP-17 similar to p.G389R.

Parkinsonism, movement disorders and genetics in frontotemporal dementia

Frontotemporal dementia (FTD) refers to a group of clinically and genetically heterogeneous neurodegenerative disorders that are a common cause of adult-onset behavioural and cognitive impairment. FTD often presents in combination with various hyperkinetic or hypokinetic movement disorders, and evidence suggests that various genetic mutations underlie these different presentations. Here, we review the known syndromatic-genetic correlations in FTD. Although no direct genotype-phenotype correlations have been identified, mutations in multiple genes have been associated with various presentations. Mutations in the genes that encode microtubule-associated protein tau (MAPT) and progranulin (PGRN) can manifest as symmetrical parkinsonism, including the phenotypes of Richardson syndrome and corticobasal syndrome (CBS). Expansions in the C9orf72 gene are most frequently associated with familial FTD, typically combined with motor neuron disease, but other manifestations, such as symmetrical parkinsonism, CBS and multiple system atrophy-like presentations, have been described in patients with these mutations. Less common gene mutations, such as those in TARDBP, CHMP2B, VCP, FUS and TREM2, can also present as atypical parkinsonism. The most common hyperkinetic movement disorders in FTD are motor and vocal stereotypies, which have been observed in up to 78% of patients with autopsy-proven FTD. Other hyperkinetic movements, such as chorea, orofacial dyskinesias, myoclonus and dystonia, are also observed in some patients with FTD.

Genetic Identification Is Critical for the Diagnosis of Parkinsonism: A Chinese Pedigree with Early Onset of Parkinsonism

BACKGROUND

A number of hereditary neurological diseases display indistinguishable features at the early disease stage. Parkinsonian symptoms can be found in numerous diseases, making it difficult to get a definitive early diagnosis of primary causes for patients with onset of parkinsonism. The accurate and early diagnosis of the causes of parkinsonian patients is important for effective treatments of these patients.

METHODS

We have identified a Chinese family (82 family members over four generations with 21 affected individuals) that manifested the characterized symptoms of parkinsonism and was initially diagnosed as Parkinson's disease. We followed up with the family for two years, during which we carried out clinical observations, Positron Emission Tomography-Computed Tomography neuroimaging analysis, and exome sequencing to correctly diagnose the case.

RESULTS

During the two-year follow-up period, we performed comprehensive medical history collection, physical examination, and structural and functional neuroimaging studies of this Chinese family. We found that the patient exhibited progressive deteriorated parkinsonism with Parkinson disease-like neuropathology and also had a good response to the initial levodopa treatment. However, exome sequencing identified a missense mutation, N279K, in exon 10 of MAPT gene, verifying that the early parkinsonian symptoms in this family are caused by the genetic mutation for hereditary frontotemporal lobar dementia.

CONCLUSIONS

For the inherited parkinsonian patients who even show the neuropathology similar to that in Parkinson's disease and have initial response to levodopa treatment, genetic identification of the molecular basis for the disease is still required for defining the early diagnosis and correct treatment.

Invited review: Frontotemporal dementia caused by microtubule-associated protein tau gene (MAPT) mutations: a chameleon for neuropathology and neuroimaging

Hereditary frontotemporal dementia associated with mutations in the microtubule-associated protein tau gene (MAPT) is a protean disorder. Three neuropathologic subtypes can be recognized, based on the presence of inclusions made of tau isoforms with three and four repeats, predominantly three repeats and mostly four repeats. This is relevant for establishing a correlation between structural magnetic resonance imaging and positron emission tomography using tracers specific for aggregated tau. Longitudinal studies will be essential to determine the evolution of anatomical alterations from the asymptomatic stage to the various phases of disease following the onset of symptoms.

Clinicopathological review of pallidonigroluysian atrophy

Pallidonigroluysian atrophy is a rare neurodegenerative disease characterized by degeneration of the globus pallidus, substantia nigra, and subthalamic nucleus. Few studies have comprehensively documented the clinical and pathological features of pallidonigroluysian atrophy. A systematic review of all published cases of pallidonigroluysian atrophy in English since 1970 was performed. We also report a new case of pallidonigroluysian atrophy. Twenty-five cases of pathologically proven pallidonigroluysian atrophy were reviewed, 24 from the literature and 1 of our own. Average age of onset was 54.3 ± 14.3 years, and average duration of disease was 7.9 ± 5.8 years. The most common first symptom was gait or balance disturbance. Patients had a diversity of movement disorders, including chorea in 5 cases (20%). Nine cases (36%) had coexistent motor neuron disease. Almost all cases had gliosis, and many cases had iron-positive pigments in the pallidonigroluysian system. Tauopathy was absent to rare in this region. Widespread tau-negative, p62-positive glial inclusions, described in 1 previous case, were also present in our patient. As pallidonigroluysian atrophy has a diversity of clinical presentations, it is best defined neuropathologically. The relative lack of tauopathy and the presence of p62-positive glial inclusions or iron-positive pigments in the pallidonigroluysian region may help to distinguish pallidonigroluysian atrophy from similar disease entities.

The clinical spectrum of freezing of gait in atypical parkinsonism

Freezing of gait (FOG), commonly seen in advanced Parkinson's disease (PD), has been classified as its fifth cardinal feature. However, its presence frequently leads to a misdiagnosis of PD. FOG is actually more common in atypical parkinsonism (AP): including vascular Parkinsonism (VP), progressive supranuclear palsy (PSP), multiple system atrophy (MSA), corticobasal degeneration (CBD), dementia with Lewy bodies (DLB), and higher level gait disorders (HLGDs). VP is the result of multiple small vessel infarcts (lacunar state or Binswanger's disease), particularly involving the frontal, parietal, and basal ganglia regions. Approximately 50% have FOG (often referred to as lower body parkinsonism). FOG is also common in neurodegenerative forms of AP, present in 45-57%. Of these, FOG is present in 53% of PSP, 54% MSA, 54% DLB, 25% CBD, and 40% HLGD. It is generally seen in the late stages. There are two syndromes closely associated with AP that are dominated by FOG; pure akinesia (PA) and primary progressive freezing gait (PPFG). PA is characterized by akinesia of gait (including FOG), writing, and speech. Tremor, rigidity, dementia, and response to levodopa are notably absent. PPFG is defined by early FOG (often the initial feature) that progresses to include postural instability. It is accompanied by bradykinesia, rigidity, postural tremor, dementia, and levodopa unresponsiveness. Both syndromes are heterogeneous but PSP seems to be the most common cause. CBD and DLB can also present as PPFG. FOG is a common feature of AP and although typically occurring late in disease may also be an early symptom.

MAPT S305I mutation: implications for argyrophilic grain disease

Frontotemporal lobar degeneration (FTLD) with mutations in the tau gene (MAPT) causes familial frontotemporal dementia with tau pathology. Many of these mutations result in morphological phenotypes resembling sporadic tauopathies, although, to date, no such cases mimicking argyrophilic grain disease (AgD) have been documented. We now present a case with a novel S305I MAPT mutation and a morphological phenotype showing resemblance to AgD. At the age of 39, the patient developed behavioural and personality changes and lack of verbal fluency with later poor performance on naming tasks and rigidity in the extremities. After a short disease course of 1.5 years, the patient died. A unique neuropathological phenotype with neuronal diffuse cytoplasmic tau immunoreactivity, oligodendroglial-coiled bodies, argyrophilic grains, and non-argyrophilic, but tau-immunopositive and ubiquitin-immunonegative pre-grains were observed, whereas classical neurofibrillary tangles, Pick bodies, and neuritic plaques were absent. The tau-positive abnormal structures were composed only of 4R-tau isoforms and, ultrastructurally, straight filaments. Neuronal loss was greatest in the medial temporal cortex, hippocampus, and amygdala. These pathological features resemble AgD. The novel S305I substitution has a strong effect on MAPT exon 10 splicing, thereby causing a striking increase in 4R-tau isoforms. Our observation not only widens the phenotypic spectrum of FTLD with MAPT mutation but also underpins the notion that the predominance of similar neuropathological findings in sporadic AgD cases may be viewed as features of a distinct disease entity.

Clinical and neuropathologic features of progressive supranuclear palsy with severe pallido-nigro-luysial degeneration and axonal dystrophy

Pallido-nigro-luysial atrophy (PNLA) is a rare disorder that in many cases has histopathological features similar to progressive supranuclear palsy (PSP). In a pathological series of over 400 cases of PSP, eight cases were noted to have features similar to those described in PNLA, including severe atrophy and neuronal loss in the globus pallidus, substantia nigra and subthalamic nucleus, in addition to many axonal spheroids in the globus pallidus and substantia nigra. These eight cases of PSP-PNLA were compared to 11 typical PSP cases with quantitative neuropathologic indices and assessment of demographics, clinical features and the timing of clinical features. PSP-PNLA cases were younger, had longer disease duration and more often were not initially diagnosed with PSP; in the end, they did not differ from PSP with respect to any major clinical feature. The clinical course of PSP-PNLA, however, was different, with earlier gait abnormalities and difficulty with handwriting, but later falls, rigidity and dysphagia than PSP. Pathologically, the same types of lesions were detected in both PSP and PSP-PNLA, but there were differences in the distribution and density of tau-pathology, with less tau-pathology in motor cortex, striatum, pontine nuclei and cerebellum in PSP-PNLA. These clinical and pathological findings suggest that PSP-PNLA should be considered a variant of PSP.

The tau N279K exon 10 splicing mutation recapitulates frontotemporal dementia and parkinsonism linked to chromosome 17 tauopathy in a mouse model

Intracellular tau deposits are characteristic of several neurodegenerative disorders called tauopathies. The tau protein regulates the stability and assembly of microtubules by binding to microtubules through three or four microtubule-binding repeats (3R and 4R). The number of microtubule-binding repeats is determined by the inclusion or exclusion of the second microtubule-binding repeat encoded by exon 10 of the TAU gene. TAU gene mutations that alter the inclusion of exon 10, and hence the 4R:3R ratio, are causal in the tauopathy frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). A mutation located in exon 10 has been identified in several FTDP-17 families that present with increased exon 10 inclusion in both mRNA and protein, parkinsonism, movement disorders, and dementia. We have engineered a human tau minigene construct that was designed to allow alternative splicing of the tau exon 10. Here we demonstrate that transgenic mice expressing human tau protein with this mutation develop neurodegeneration as result of aberrant splicing. The mice recapitulate many of the disease hallmarks that are seen in patients with this mutation, including increased tau exon 10 inclusion in both mRNA and protein, motor and behavioral deficits, and tau protein accumulation in neurons and tufted astrocytes. Furthermore, these mice present with degeneration of the nigrostriatal dopaminergic pathway, suggesting a possible mechanism for parkinsonism in FTDP-17. Additionally, activated caspase-3 immunoreactivity in both neurons and astrocytes implicates the involvement of the apoptotic pathway in the pathology of these mice.

Hereditary frontotemporal dementia caused by Tau gene mutations

Tau protein is involved in microtubule assembly and stabilization. Filamentous deposits made of tau constitute a defining characteristic of several neurodegenerative diseases. The relevance of tau dysfunction for neurodegeneration has been clarified through the identification of mutations in the Tau gene in cases with frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). Although the mechanisms by which these mutations lead to nerve cell death are only incompletely understood, it is clear that they cause the formation of tau filaments with distinct morphologies and isoform compositions. The range of tau pathology identified in FTDP-17 recapitulates that in sporadic tauopathies, indicating a major role for tau dysfunction in these diseases.

A decade of tau transgenic animal models and beyond

The first tau transgenic mouse model was established more than a decade ago. Since then, much has been learned about the role of tau in Alzheimer's disease and related disorders. Animal models, both in vertebrates and invertebrates, were significantly improved and refined as a result of the identification of pathogenic mutations in Tau in human cases of frontotemporal dementia. They have been instrumental for dissecting the cross-talk between tau and the second hallmark lesion of Alzheimer's disease, the Abeta peptide-containing amyloid plaque. We discuss how the tau models have been used to unravel the pathophysiology of Alzheimer's disease, to search for disease modifiers and to develop novel treatment strategies. While tau has received less attention than Abeta, it is rapidly acquiring a more prominent position and the emerging view is one of a synergistic action of Abeta and tau in Alzheimer's disease. Moreover, the existence of a number of neurodegenerative diseases with tau pathology in the absence of extracellular deposits underscores the relevance of research on tau.

Frontotemporal dementia and parkinsonism linked to chromosome 17 with the N279K tau mutation

We present a case of frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) harboring the N279K mutation in the MAPT gene from the family known as pallido-ponto-nigral degeneration (PPND). This 49-year-old man was followed for 17 years. He presented at age 41 years with left leg stiffness and en-bloc turning. During the course of his illness he developed a constellation of symptoms including parkinsonism, pyramidal signs, vertical gaze palsy, dysphagia, dystonia, personality and cognitive dysfunction, weight loss and mutism. Gross neuropathological examination showed mild atrophy of the cerebral cortex, hippocampal formation, amygdala, thalamus, subthalamic nucleus and depigmentation of the substantia nigra. Microscopy revealed neuronal loss and gliosis in the same regions. Tau immunohistochemistry showed pretangles, numerous threads, grain-like structures and oligodendroglial tau-positive inclusions ("coiled bodies"). In the spinal cord the tau pathology was more abundant in gray than white matter. Pretangles and threads were present in the anterior and, to a lesser extent, in the posterior horns. FTDP-17 should be suspected in patients with a history of familial parkinsonism combined with behavioral and cognitive changes, onset before age 65 years and an aggressive clinical course.

Autopsy case of pure akinesia showing pallidonigro-luysian atrophy

A 60-year-old man developed levodopa-resistant pure akinesia. The patient gradually became more akinetic without accompanying gaze palsies, nuchal dystonia, or other parkinsonian features such as rigidity or tremor. At the age of 71, he died of bronchopneumonia. Neuropathologically, bilateral marked neuronal loss and gliosis were restrictedly observed in the globus pallidus, substantia nigra and corpus luysii, whereas mild gliosis without neuronal loss was found in the brain stem. With Gallyas-Braak silver stain, numerous argyrophilic fibrous structures partly surrounding glial nuclei were observed in the three major affected regions. With Bodian stain, however, they were rarely recognized. The structures were partly positive for tau protein. Rare neurofibrillary tangles were found in the three areas and brain stem. They were relatively more numerous but still sparse in the hippocampus and the parahippocampus. The present case was diagnosed as having pallidonigro-luysian atrophy based on two characteristic findings: (i) the distribution of lesions showing neuronal loss with gliosis; and (ii) significant presence of tau-positive argyrophilic fibrous structures related to glia but with the absence of neurofibrillary tangles in the major affected regions and the brain stem. As our present case uniquely showed pure akinesia for the whole clinical course, it is noteworthy to report it here with a full neuropathological evaluation. In addition, a moderate number of diffuse plaques positive for beta-amyloid were distributed in the thalamus.

Transgenic mice expressing mutant (N279K) human tau show mutation dependent cognitive deficits without neurofibrillary tangle formation

Mutations in the tau gene, which is located on chromosome 17, were found causative for autosomal dominantly inherited frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). To determine if cognitive deficits could be caused by tau mutations, two transgenic mouse lines were generated expressing a four-repeat isoform of human tau or its mutant, containing one of the FTDP-17 mutations (WILD mice and N279K mice). In open field test, N279K mice showed hyperactivity in locomotion and rearing. In prepulse inhibition test, N279K mice but not Wild mice showed significant deficits. Both transgenic mice, especially N279K mice, showed impairment in acquisition of spatial learning in Morris water maze. Although both N279K mice and Wild mice acquired passive avoidance as well as non-transgenic mice, N279K mice but not Wild mice showed severe deficits in acquisition of active avoidance. Histological analysis of the present mutant mice did not show any signs of neurofibrillary tangle formations in the brain, and cognitive dysfunction seemed to precede such neuropathological changes or occur independently from them. The behavioral phenotype of N279K mice mimics features of human FTDP-17 and provides a basic model for elucidating mechanisms underlying cognitive deficits in not only FTDP-17, but also diverse tauopathies.

Tau alternative splicing and frontotemporal dementia

A number of neurodegenerative diseases are characterized by the presence of abundant deposits containing Tau protein. Expression of the human tau gene is under complex regulation. Mutations in the tau gene have been identified in patients with frontotemporal lobe dementia. These mutations affect either biochemical/biophysical properties or the delicate balance of different splicing isoforms. In this review, we summarize recent advances in our understanding of genetics and molecular pathogenesis of tauopathies with the focus on frontotemporal lobe dementia. We review published studies on tau pre-mRNA splicing regulation. Understanding molecular mechanisms of tauopathies may help in developing effective therapies for neurodegenerative tauopathies and related disorders, including Alzheimer disease.

Tau protein as a differential biomarker of tauopathies

Microtubule-associated Tau proteins are the basic component of intraneuronal and glial inclusions observed in many neurological disorders, the so-called tauopathies. Many etiological factors, phosphorylation, splicing, and mutations, relate Tau proteins to neurodegeneration. Molecular analysis has revealed that hyperphosphorylation and abnormal phosphorylation might be one of the important events in the process leading to tau intracellular aggregation. Specific set of pathological tau proteins exhibiting a typical biochemical pattern, and a different regional and laminar distribution, could characterize five main classes of tauopathies. A direct correlation has been established between the regional brain distribution of tau pathology and clinical symptoms; for instance progressive involvement of neocortical areas is well correlated to the severity of dementia in Alzheimer's disease, overall suggesting that pathological tau proteins are reliable marker of the neurodegenerative process. Recent discovery of tau gene mutations in frontotemporal dementia with parkinsonism linked to chromosome 17 has reinforced the predominant role attributed to tau proteins in the pathogenesis of neurodegenerative disorders, and underlined the fact that distinct sets of tau isoforms expressed in different neuronal populations could lead to different pathologies. Overall, a better knowledge of the etiological factors responsible for the aggregation of tau proteins in brain diseases is essential for development of future differential diagnosis and therapeutic strategies. They would hopefully find their application against Alzheimer's disease but also in all neurological disorders for which a dysfunction of Tau biology has been identified.

Phenotypic heterogeneity within a new family with theMAPT p301s mutation

Mutations in the gene encoding the microtubule-associated protein tau (MAPT) cause frontotemporal dementia and parkinsonism linked to chromosome 17. Clinical variability is seen not only among families with different mutations, but also among family members with the same mutation. We investigated a newly identified familial frontotemporal dementia and parkinsonism family. The disease was of early onset and was inherited as an autosomal dominant trait. Clinically, parkinsonism was the prominent and often early feature, and it preceded dementia. Three autopsied cases shared involvement predominantly in the frontal and temporal lobes and also in the subcortical nuclei, including substantia nigra, globus pallidus, and subthalamic nucleus, that microscopically consisted of neuronal loss, microvacuolation, and astrocytic fibrosis. Immunohistochemistry demonstrated neuropil threads, ballooned cells, and glial fibrillary tangles. Sequencing analysis of the MAPT gene showed an alteration in one allele, resulting in a P301S substitution. These findings suggest that the MAPT P301S mutation can cause pathologically subcortical-predominant, neuropil thread-rich, tau-containing lesions, which could result in consistent parkinsonism. Our study confirms the notion that the phenotype observed in affected individuals from P301S MAPT mutation families is heterogeneous and is broader than the phenotypes seen to date in affected family members carrying other MAPT mutations.

The cytoskeleton in neurodegenerative diseases

Abundant abnormal aggregates of cytoskeletal proteins are neuropathological signatures of many neurodegenerative diseases that are broadly classified by filamentous aggregates of neuronal intermediate filament (IF) proteins, or by inclusions containing the microtubule-associated protein (MAP) tau. The discovery of mutations in neuronal IF and tau genes firmly establishes the importance of neuronal IF proteins and tau in the pathogenesis of neurodegenerative diseases. Multiple IF gene mutations are pathogenic for Charcot-Marie-Tooth (CMT) disease and amyotrophic lateral sclerosis (ALS)--in addition to those in the copper/zinc superoxide dismutase-1 (SOD1) gene. Tau gene mutations are pathogenic for frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17), and tau polymorphisms are genetic risk factors for sporadic progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD). Thus, IF and tau abnormalities are linked directly to the aetiology and pathogenesis of neurodegenerative diseases. In vitro and transgenic animal models are being used to demonstrate that different mutations impair protein function, promote tau fibrilization, or perturb tau gene splicing, leading to aberrant and distinct tau aggregates. For recognition of these disorders at neuropathological examination, immunohistochemistry is needed, and this may be combined with biochemistry and molecular genetics to properly determine the nosology of a particular case. As reviewed here, the identification of molecular genetic defects and biochemical alterations in cytoskeletal proteins of human neurodegenerative diseases has facilitated experimental studies and will promote the development of assays of molecules which inhibit abnormal neuronal IF and tau protein inclusions.

Amyloid‐induced neurofibrillary tangle formation in Alzheimer's disease: insight from transgenic mouse and tissue‐culture models

Of all forms of dementia, Alzheimer's disease is the most prevalent. It is histopathologically characterized by beta-amyloid-containing plaques, tau-containing neurofibrillary tangles, reduced synaptic density and neuronal loss in selected brain areas. For the rare familial forms of Alzheimer's disease, pathogenic mutations have been identified in both the gene encoding the precursor of the Abeta peptide, APP, itself and in the presenilin genes which encode part of the APP-protease complex. For the more frequent sporadic forms of Alzheimer's disease, the pathogenic trigger has not been unambiguously identified. Whether Abeta is again the main cause remains to be heavily discussed. In a related disorder termed frontotemporal dementia, which is characterized by tangles in the absence of beta-amyloid deposition, mutations have been identified in tau which also lead to neurodegeneration and dementia. For Alzheimer's disease the existence of familial forms lead to the proposition of the amyloid cascade hypothesis, which claims that beta-amyloid causes or enhances the tangle pathology. In this review, we describe tau transgenic mouse models in which aspects of the tau-associated pathology, including tangle formation, has been achieved. Moreover, tau transgenic mouse and tissue-culture models were used to test the amyloid cascade hypothesis. In addition, we discuss alternative hypotheses to explain the sporadic forms. The animal and tissue-culture models will provide insight into the underlying biochemical mechanisms of tau aggregation and nerve cell degeneration. These mechanisms may be partially shared between sporadic Alzheimer's disease, the familial forms and frontotemporal dementia. Eventually, Alzheimer's disease may be redefined based on biochemical events rather than phenotype.

Retarded axonal transport of R406W mutant tau in transgenic mice with a neurodegenerative tauopathy

Intracellular accumulations of filamentous tau inclusions are neuropathological hallmarks of neurodegenerative diseases known as tauopathies. The discovery of multiple pathogenic tau gene mutations in many kindreds with familial frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17) unequivocally confirmed the central role of tau abnormalities in the etiology of neurodegenerative disorders. To examine the effects of tau gene mutations and the role of tau abnormalities in neurodegenerative tauopathies, transgenic (Tg) mice were engineered to express the longest human tau isoform (T40) with or without the R406W mutation (RW and hWT Tg mice, respectively) that is pathogenic for FTDP-17 in several kindreds. RW but not hWT tau Tg mice developed an age-dependent accumulation of insoluble filamentous tau aggregates in neuronal perikarya of the cerebral cortex, hippocampus, cerebellum, and spinal cord. Significantly, CNS axons in RW mice contained reduced levels of tau when compared with hWT mice, and this was linked to retarded axonal transport and increased accumulation of an insoluble pool of RW but not hWT tau. Furthermore, RW but not hWT mice demonstrated neurodegeneration and a reduced lifespan. These data indicate that the R406W mutation causes reduced binding of this mutant tau to microtubules, resulting in slower axonal transport. This altered tau function caused by the RW mutation leads to increased accumulation and reduced solubility of RW tau in an age-dependent manner, culminating in the formation of filamentous intraneuronal tau aggregates similar to that observed in tauopathy patients.

Transgenic animal models of Alzheimer's disease and related disorders: histopathology, behavior and therapy

Alzheimer's disease (AD) is a devastating neurodegenerative disease that affects more than 15 million people worldwide. Within the next generation, these numbers will more than double. To assist in the elucidation of pathogenic mechanisms of AD and related disorders, such as frontotemporal dementia (FTDP-17), genetically modified mice, flies, fish and worms were developed, which reproduce aspects of the human histopathology, such as beta-amyloid-containing plaques and tau-containing neurofibrillary tangles (NFT). In mice, the tau pathology caused selective behavioral impairment, depending on the distribution of the tau aggregates in the brain. Beta-amyloid induced an increase in the numbers of NFT, whereas the opposite was not observed in mice. In beta-amyloid-producing transgenic mice, memory impairment was associated with increased levels of beta-amyloid. Active and passive beta-amyloid-directed immunization caused the removal of beta-amyloid plaques and restored memory functions. These findings have since been translated to human therapy. This review aims to discuss the suitability and limitations of the various animal models and their contribution to an understanding of the pathophysiology of AD and related disorders.

The role of tau (MAPT) in frontotemporal dementia and related tauopathies

Tau is a multifunctional protein that was originally identified as a microtubule-associated protein. In patients diagnosed with frontotemporal dementia and parkinsonism linked to chromosome 17, mutations in the gene encoding tau (MAPT) have been identified that disrupt the normal binding of tau to tubulin resulting in pathological deposits of hyperphosphorylated tau. Abnormal filamentous tau deposits have been reported as a pathological characteristic in several other neurodegenerative diseases, including frontotemporal dementia, Pick Disease, Alzheimer disease, argyrophilic grain disease, progressive supranuclear palsy, and corticobasal degeneration. In the last five years, extensive research has identified 34 different pathogenic MAPT mutations in 101 families worldwide. In vitro, cell-free and transfected cell studies have provided valuable information on tau dysfunction and transgenic mice carrying human MAPT mutations are being generated to study the influence of MAPT mutations in vivo. This mutation update describes the considerable differences in clinical and pathological presentation of patients with MAPT mutations and summarizes the effect of the different mutations on tau functioning. In addition, the role of tau as a genetic susceptibility factor is discussed, together with the genetic evidence for additional causal genes for tau-positive as well as tau-negative dementia.

Tau phosphorylation and kinase activation in familial tauopathy linked to deln296 mutation

Tau phosphorylation has been examined by immunohistochemistry in the brain of a patient affected with familial tauopathy with progressive supranuclear palsy-like phenotype linked to the delN296 mutation in the tau gene. Phospho-specific tau antibodies Thr181, Ser202, Ser214, Ser396 and Ser422, and antibodies to glycogen synthase kinase-3alpha/beta (GSK-3alpha/beta) and to phosphorylated (P) mitogen-activated protein kinase/extracellular signal-regulated kinases (MAPK/ERK), stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK), p38 kinase (p38) and GSK-3betaSer9 have been used to gain understanding of the identification of phosphorylation sites, as well as of the specific kinases that regulate tau phosphorylation at those specific sites, in a familial tauopathy. The neuropathological examination disclosed atrophy of the right precentral gyrus and the brainstem. Neurone loss and gliosis were observed in the substantia nigra, several nuclei of the brainstem and diencephalon. Hyper-phosphorylated tau accumulated in neurones with neurofibrillary tangles and in neurones with pretangles in the substantia nigra, locus ceruleus, peri-aqueductal grey matter, reticular formation, motor nuclei of the brainstem, and thalamus, amygdala and hippocampus. tau-immunoreactive astrocytes and, particularly, oligodendrocytes with coiled bodies were widespread in the brainstem, diencephalons, cerebral white matter and cerebral cortex. Increased expression of MAPK/ERK-P, SAPK/JNK-P, p-38-P and GSK-3beta-P was observed in select subpopulations of neurones with neurofibrillary tangles and in neurones with pretangles. MAPK/ERK-P, SAPK/JNK-P, p38-P and GSK-3beta-P were also expressed in tau-containing astrocytes and in oligodendrocytes with coiled bodies. These findings show, for the first time, activation of precise kinases that regulate tau phosphorylation at specific sites in familial tauopathy.

Clinical-electrophysiological correlation of tremor and myoclonus in a kindred with the N279K tau mutation

We used electrophysiological methods to study the hyperkinetic movement disorders in a pallido-ponto-nigral degeneration (PPND) family, which harbors the N279K tau gene mutation. Our purpose was to: (1). characterize the tremor patterns, (2). characterize the myoclonus physiology, (3). determine whether electrophysiology can detect abnormalities in asymptomatic cases. In PPND, we found that the activation tremors correlated with a semi-rhythmic 6-10 Hz electromyography (EMG) pattern, and the rest tremors showed a reciprocal 4-6 Hz pattern. At least two different myoclonus physiology patterns exist in PPND, most notably contrasted by the presence or absence of a demonstrable electroencephalography (EEG) correlate. Electrophysiology yielded remarkable findings in those asymptomatic at-risk individuals that carried the N279K tau mutation.

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.

Progressive supranuclear palsy: clinical and genetic aspects

PURPOSE OF THE REVIEW

This review focuses on the recent additions to the literature in the clinical and genetic aspects of progressive supranuclear palsy.

RECENT FINDINGS

Clinical features of progressive supranuclear palsy are reasonably well established and known to be quite characteristic. Recent epidemiological studies suggest that the disorder is more common than previously considered and that it is frequently misdiagnosed. New laboratory and novel imaging techniques are being tested and cerebrospinal fluid levels of tau protein have been found helpful in diagnosis. Pathological and biochemical studies in progressive supranuclear palsy brains have shown the predominance of hyperphosphorylated tau isoforms which contain the sequence encoded by exon 10 (4R) aggregated into filaments. Familial tauopathies linked to tau gene mutations showing clinical and neuropathological overlap with sporadic progressive supranuclear palsy have been described. Despite recent discoveries of the strong genetic association of sporadic progressive supranuclear palsy with tau gene polymorphisms, a specific risk allele for developing the palsy has not yet been identified yet.

SUMMARY

Recent clinical studies and clinicopathological correlations are contributing significantly to the delineation of the clinical features of progressive supranuclear palsy. These features and the appropriate use of laboratory tests allow for an earlier identification of the disease and a more accurate premortem diagnosis. However, no specific biological markers for the disorder are available yet, and consequently diagnosis in the early stages or when some of the characteristic signs and symptoms are missing, remains a major challenge. Despite the recent advances in the understanding of genetic factors involved in progressive supranuclear palsy, the cause of the disease still remains unknown. Biochemical studies in brains from progressive supranuclear palsy patients provide a potential helpful instrument to improve the characterization of this disorder.

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.

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.

Recent advances in the understanding of tau protein and movement disorders

Tau plays an important role in movement disorders. The accumulation of pathological tau is a major substrate of frontotemporal dementia and parkinsonism linked to chromosome 17, progressive supranuclear palsy, and corticobasal degeneration. Over the past year, several new mutations on the tau gene have been found. These mutations have been classified into three groups: (i) mutations in constitutively spliced exons; (ii) mutations in the alternatively spliced exon 10; and (iii) mutations of the exon 10 5' splice site. Some patients presenting with frontotemporal dementia and parkinsonism linked to chromosome 17 transiently respond to levodopa therapy. The significance of Pick bodies was recognized by a recent study on kindred with the Glu342Val tau mutation. In sporadic cases of progressive supranuclear palsy, the presence of the H1 haplotype was found to be a risk factor. Corticobasal degeneration shares a common genetic background with progressive supranuclear palsy. This opens the question of whether corticobasal degeneration represents a separate disorder or a spectrum of disease with progressive supranuclear palsy. However, distinguishing features are observed, and include oculomotor abnormalities, which may help to differentiate these two disorders on clinical grounds. Despite recent advances in the understanding of the tauopathies, there are still no curative therapies available. It is hoped that studies in transgenic tau animal models will lead to the development of successful treatments.

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.

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.

Going new places using an old MAP: tau, microtubules and human neurodegenerative disease

The microtubule-associated protein tau was originally identified as a protein that co-purified with tubulin in vitro, stimulated assembly of tubulin into microtubules and strongly stabilized microtubules. Recognized now as one of the most abundant axonal microtubule-associated proteins, a convergence of evidence implicates an overlapping in vivo role of tau with other axonal microtubule-associated proteins (e.g. MAP1B) in establishing microtubule stability, axon elongation and axonal structure. Missense and splice-site mutations in the human tau gene are now known to be causes of inherited frontotemporal dementia and parkinsonism linked to chromosome 17, a cognitive disorder of aging. This has provided direct evidence for the hypothesis that aberrant, filamentous assembly of tau, a frequent hallmark of a series of human cognitive diseases, including Alzheimer's disease, can directly provoke neurodegeneration.

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.

Competition for microtubule-binding with dual expression of tau missense and splice isoforms

How tau mutations lead to neurodegeneration is unknown but may be related to altered microtubule binding properties of mutant tau protein. The tendency for the mutations to cluster around the microtubule-binding domain of tau or to alter the ratios of those splice isoforms that affect binding supports the view that the tau/microtubule interaction is critical and finely regulated. In cells transfected with both mutant and wild-type tau isoforms fused to either yellow fluorescent protein or cyan fluorescent protein we can observe tau fusion proteins that differ by a single amino acid or by the inclusion or exclusion of exon 10. With coexpression of mutant and wild-type tau, the mutant isoform appears diffuse throughout the cytoplasm; however, when mutant tau is expressed alone, it appears mostly bound to the microtubules. Dual imaging of the three- and four-repeat tau isoforms indicated that the expression of four-repeat tau displaced three-repeat tau from the microtubules. These results suggest that altered kinetic competition among the isoforms for microtubule binding could be a disease precipitant.

New insights into genetic and molecular mechanisms of brain degeneration in tauopathies

Abundant neurofibrillary lesions consisting of the microtubule associated protein tau and amyloid beta peptide deposits are the defining lesions of Alzheimer's disease. Prominent filamentous tau pathology and brain degeneration in the absence of extracellular amyloid deposition characterize a number of other neurodegenerative disorders (i.e. progressive supranuclear palsy, corticobasal degeneration, Pick's disease) collectively referred to as tauopathies. The discovery of multiple tau gene mutations that are pathogenic for hereditary frontotemporal dementia and parkinsonism linked to chromosome 17 in many kindreds, as well as the demonstration that tau polymorphisms are genetic risk factors for sporadic tauopathies, directly implicate tau abnormalities in the onset/progression of neurodegenerative disease. Different tau gene mutations may be pathogenic by impairing the functions of tau or by perturbing the splicing of the tau gene, thereby resulting in biochemically and structurally distinct tau aggregates. However, since specific polymorphisms and mutations in the tau gene lead to diverse phenotypes, it is plausible that additional genetic or epigenetic factors influence the clinical and pathological manifestations of both familial and sporadic tauopathies. Thus, efforts to develop animal models of tau-mediated neurodegeneration should provide further insights into the onset and progression of tauopathies as well as Alzheimer's disease, and they could accelerate research to discover more effective therapies for these disorders.

Tau pathology: a marker of neurodegenerative disorders

Tau is not only a basic component of neurofibrillary degeneration, but is also an aetiological factor, as demonstrated by mutations on the tau gene responsible for frontotemporal dementias with parkinsonism linked to chromosome 17. Polymorphisms on the tau gene and the hierarchical invasion of neocortical areas by tau pathology in numerous sporadic neurodegenerative diseases also suggest that tau pathology is a primary pathogenic event in non-familial dementing diseases and a lead for solid diagnostic and therapeutic approaches.