Frontotemporal Dementia With Parkinsonism Linked To Chromosome 17 Research Articles

FTDP-17 Mutations Alter the Aggregation and Microtubule Stabilization Propensity of Tau in an Isoform-Specific Fashion.

More than 50 different intronic and exonic autosomal dominant mutations in the tau gene have been linked to the neurodegenerative disorder frontotemporal dementia with Parkinsonism linked to chromosome-17 (FTDP-17). Although the pathological and clinical presentation of this disorder is heterogeneous among patients, the deposition of tau as pathological aggregates is a common feature. Collectively, FTDP-17 mutations have been shown to alter tau's ability to stabilize microtubules, enhance its aggregation, alter mRNA splicing, or induce its hyperphosphorylation, among other effects. Previous in vitro studies from our lab revealed that these mutations differ markedly from each other in the longest 2N4R isoform of tau. However, it is not entirely known whether the effect of a single mutation varies when compared between different isoforms of tau. Differences in the isoelectric points of the N-terminal region of tau isoforms lead to changes in their biochemical properties, raising the possibility that isoforms could also be disproportionately affected by disease-related mechanisms such as mutations. We therefore performed a comparative study of three FTDP-17 mutations present in different regions of tau (R5L, P301L, and R406W) in the three 4R isoforms of tau. We observed significant differences in the effect these mutations exert on the total amount and kinetics of aggregation, aggregate length distributions, and microtubule stabilizing propensity of 4R tau isoforms for all three selected mutants. These results demonstrate that different combinations of FTDP-17 mutations and tau isoforms are functionally distinct and could have important implications for our understanding of disease and animal models of tauopathies.

Frontotemporal dementia with Parkinsonism linked to chromosome-17 mutations enhance tau oligomer formation.

The P301L mutation in tau, a microtubule-associated protein, causes frontotemporal dementia with Parkinsonism linked to chromosome-17 (FTDP-17) that is accompanied by formation of filamentous polymers of tau. The mutation reduces the binding capability of microtubules and enhances tau filament formation. However, it is unclear whether the P301L mutation increases the formation of the intermediates of tau filaments that are suggested to be a toxic species of tau. To determine the amount and structure of the intermediates harboring with the P301L mutation, we purified recombinant versions of wild-type, P301L, and 4 other mutants (i.e., P301S, P301T, V337M, and R406W) tau proteins and analyzed the heparin-induced aggregation of those tau constructs. We found that all of the FTDP-17 mutants increased levels of the intermediate tau oligomers. The sizes were determined by atomic force microscopy and laser light scattering. The V337M and R406W oligomers were similar in size to the wild-type, but the P301L, P301T, and P301S mutants formed smaller oligomers. In a P301L transgenic mouse model, we found tau aggregates that were similar in size to the recombinant tau oligomer. These results indicate that FTDP-17 mutations contribute to the pathogenesis via the increased formation of tau oligomers.

Tau-Induced Pathology in Epilepsy and Dementia: Notions from Patients and Animal Models

Patients with dementia present epilepsy more frequently than the general population. Seizures are more common in patients with Alzheimer’s disease (AD), dementia with Lewy bodies (LBD), frontotemporal dementia (FTD) and progressive supranuclear palsy (PSP) than in other dementias. Missense mutations in the microtubule associated protein tau (MAPT) gene have been found to cause familial FTD and PSP, while the P301S mutation in MAPT has been associated with early-onset fast progressive dementia and the presence of seizures. Brains of patients with AD, LBD, FTD and PSP show hyperphosphorylated tau aggregates, amyloid-β plaques and neuropil threads. Increasing evidence suggests the existence of overlapping mechanisms related to the generation of network hyperexcitability and cognitive decline. Neuronal overexpression of tau with various mutations found in FTD with parkinsonism-linked to chromosome 17 (FTDP-17) in mice produces epileptic activity. On the other hand, the use of certain antiepileptic drugs in animal models with AD prevents cognitive impairment. Further efforts should be made to search for plausible common targets for both conditions. Moreover, attempts should also be made to evaluate the use of drugs targeting tau and amyloid-β as suitable pharmacological interventions in epileptic disorders. The diagnosis of dementia and epilepsy in early stages of those diseases may be helpful for the initiation of treatments that could prevent the generation of epileptic activity and cognitive deterioration.

11C-CFT-PET in Presymptomatic FTDP-17: A Potential Biomarker Predicting Onset.

Frontotemporal dementia with parkinsonism-linked to chromosome 17 (FTDP-17) is a rare autosomal dominant neurodegenerative disorder. Most patients with FTDP-17 carry the mutation in the microtubule-associated protein tau (MAPT) gene. Striatum is predominantly and early affected in FTDP-17. Five family members (two symptomatic patients and three presymptomatic mutation carriers) from a Chinese pedigree of FTDP-17 with N279K mutation in MAPT were enrolled. Parkinsonism was the initial symptom for symptomatic patients. 2b-carbomethoxy-3b-(4-trimethylstannylphenyl) tropane (11C-CFT) uptake was obviously affected in the putamen of two presymptomatic mutation carriers. Presymptomatic case 3, whose 11C-CFT uptake in the right putamen was normal at baseline, was still free of parkinsonism during follow-up. In conclusion, 11C-CFT-positron emission tomography could be a potential biomarker for the presymptomatic stage of FTDP-17 to predict the disease onset.

FTDP‐17 mutants S320F and S352L have subtle differential effects on 4R tau isoform aggregation in vitro

Frontotemporal Dementia with Parkinsonism linked to chromosome‐17 (FTDP‐17) is a class of inherited neurodegenerative diseases caused by mutations in the tau gene. Tau protein is a microtubule associated protein that stabilizes microtubules in neurons. During neurodegeneration, tau protein aggregates to form toxic filaments. Tau isoforms have either three or four 18‐amino acid microtubule binding repeats, known as 3‐repeat tau (3R tau) or 4‐repeat tau (4R tau). The 4R and 3R tau isoforms are further classified into 0N, 1N and 2N isoforms based on the number of N‐terminal exons. Forty four different mutations in various positions of the tau gene have been reported. FTDP‐17 tau mutants 2N4R S320F and S352L cause neuropathology through different mechanisms of tau aggregation. The mutant S320F forms short tau oligomers while S352L forms long tau filaments in vitro. Here, we determined whether the 4R tau isoforms (0N, 1N, 2N) of S320F and S352L affect tau aggregation differently in vitro. We used right angle laser light scattering, thioflavin S florescence assay and transmission electron microscopy (TEM) to study arachidonic acid induced tau aggregation. All wild type 4R tau isoforms (0N, 1N, 2N) showed the same effect on polymerization of tau into filaments upon addition of arachidonic acid. All S320F isoforms (0N, 1N, 2N) showed a significant decrease (p < 0.01) and S352L isoforms (0N, 1N, 2N) showed no significant change in laser light scattering when compared to the corresponding wild type 4R isoforms. However, S320F isoform 1N showed a significant decrease (p < 0.01) in percentage change from the respective wild type isoform when compared to 0N, 2N S320F by one way ANOVA analysis of laser light scattering. S320F isoforms 0N, 2N showed an increase (p < 0.05, 0.01) and 2N S352L also showed an increase (p < 0.001) in thioflavin S fluorescence when compared to the corresponding wild type 4R isoform. TEM images of all 4R S320F filaments showed very short tau fibrils which could be tau oligomers while all S352L filaments showed an increase in length of tau polymer. The 2N S352L filaments were longer than filaments from 0N and 1N isoforms. Though the mutant isoforms have the same effect on tau polymerization, the extent of polymerization varies. These results show that the presence of N‐terminal exons have small, but significant, alterations in the pro‐aggregation properties of FTDP‐17 4R tau.Support or Funding InformationNIH grant R01NS083391

Multiple signaling factors and drugs alleviate neuronal death induced by expression of human and zebrafish tau proteins in vivo.

BackgroundThe axonal tau protein is a tubulin-binding protein, which plays important roles in the formation and stability of the microtubule. Mutations in the tau gene are associated with familial forms of frontotemporal dementia with Parkinsonism linked to chromosome-17 (FTDP-17). Paired helical filaments of tau and extracellular plaques containing beta-amyloid are found in the brain of Alzheimer’s disease (AD) patients.ResultsTransgenic models, including those of zebrafish, have been employed to elucidate the mechanisms by which tau protein causes neurodegeneration. In this study, a transient expression system was established to express GFP fusion proteins of zebrafish and human tau under the control of a neuron-specific HuC promoter. Approximately ten neuronal cells expressing tau-GFP in zebrafish embryos were directly imaged and traced by time-lapse recording, in order to evaluate the neurotoxicity induced by tau-GFP proteins. Expression of tau-GFP was observed to cause high levels of neuronal death. However, multiple signaling factors, such as Bcl2-L1, Nrf2, and GDNF, were found to effectively protect neuronal cells expressing tau-GFP from death. Treatment with chemical compounds that exert anti-oxidative or neurotrophic effects also resulted in a similar protective effect and maintained human tau-GFP protein in a phosphorylated state, as detected by antibodies pT212 and AT8.ConclusionsThe novel finding of this study is that we established an expression system expressing tau-GFP in zebrafish embryos were directly imaged and traced by time-lapse recording to evaluate the neurotoxicity induced by tau-GFP proteins. This system may serve as an efficient in vivo imaging platform for the discovery of novel drugs against tauopathy.

Tau alternative splicing in familial and sporadic tauopathies

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

Hrd1 Facilitates Tau Degradation and Promotes Neuron Survival

Intraneuronal accumulation of abnormal phosphorylated tau (p-tau) is a molecular pathology in many neurodegenerative tauopathies, including Alzheimer's disease (AD) and frontotemporal dementia with parkinsonism-linked to chromosome 17 (FTDP-17). However, the underlying mechanism remains unclear. Here, we showed an inverse relationship between endoplasmic reticulum membrane ubiquitin ligase (E3) Hrd1 expression and p-tau accumulation in the hippocampal neurons of AD, and proposed that Hrd1 may be a negative regulator of p-tau. This notion was further supported by in vitro study demonstrating that Hrd1 interacted with tau and promoted the degradation of total tau and p-tau as well. The degradation of tau depended on its Hrd1 E3 activity. Knockdown of endogenous Hrd1 with siRNA stabilized tau levels. In addition, inhibition of proteasome maintained tau level and increased Hrd1-mediated tau ubiquitination, suggesting the proteasome was involved in tau/p-tau degradation. Over-expression of Hrd1 significantly alleviated tau cytotoxicity and promoted cell survival. These results indicated that Hrd1 functions as an E3 targeting tau or abnormal p-tau for proteasome degradation. The study provides an important insight into the molecular mechanisms of human tauopathies.

Alternative Splicing in the Postgenomic Era, edited by Benjamin J. Blencowe and Brenton R. Graveley. 2007, XXIV, Springer, New York. ISBN: 978-0-387-77373-5.

<i>Alternative Splicing in the Postgenomic Era</i>, edited by Benjamin J. Blencowe and Brenton R. Graveley. 2007, XXIV, Springer, New York. ISBN: 978-0-387-77373-5.

Stepwise proteolysis liberates tau fragments that nucleate the Alzheimer-like aggregation of full-length tau in a neuronal cell model

Tau is a highly soluble protein, yet it aggregates abnormally in Alzheimer's disease. Here, we address the question of proteolytic processing of tau and the nucleation of aggregates by tau fragments. We show in neuronal cell models that fragments of the repeat domain of tau containing mutations of FTDP17 (frontotemporal dementia with parkinsonism linked to chromosome 17), produced by endogenous proteases, can induce the aggregation of full-length tau. Fragments are generated by successive cleavages, first N-terminally between K257 and S258, then C-terminally around residues 353-364; conversely, when the N-terminal cleavage is inhibited, no fragmentation and aggregation takes place. The C-terminal truncation and the coaggregation of fragments with full-length tau depends on the propensity for beta-structure. The aggregation is modulated by phosphorylation but does not depend on it. Aggregation but not fragmentation as such is toxic to cells; conversely, toxicity can be prevented by inhibiting either aggregation or proteolysis. The results reveal a novel pathway of abnormal tau aggregation in neuronal cells.

Tauopathies: A Distinct Class of Neurodegenerative Diseases

Tauopathies: A Distinct Class of Neurodegenerative DiseasesNeurodegenerative diseases are characterized by neuronal loss and intraneuronal accumulation of fibrillary materials, of which, neurofibrillary tangles (NFT) are the most common. Neurofibrillary tangles also occur in normal aging and contain the hyperphosphorylated microtubule-associated protein tau. A detailed presentation is made of the molecular bases of Alzheimer's disease (AD), postencephalitic parkinsonism, amyotrophic lateral sclerosis/parkinsonism-dementia complex (ALS/PDC) of Guam, progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), Pick's disease, frontotemporal dementia (FTD), Down's syndrome, myotonic dystrophy (DM) and Niemann-Pick Type C (NPC) disease, which are considered to be common tauopathies. The unique human tau gene extends over 100 kb of the long arm of chromosome 17 and contains 16 exons. The human brain contains six tau isoforms that contain from 352 to 441 amino acids. To date, 34 pathogenic tau mutations have been described among 101 families affected by FTD with parkinsonism linked to chromosome 17 (FTDP-17). These mutations may involve alternative splicing of exon 10 that lead to changes in the proportion of 4-repeat- and 3-repeat-tau isoforms, or may modify tau interactions with microtubules. Tau aggregates differ in degree of phosphorylation and in content of tau isoforms. Five classes of tauopathies have been defined depending on the type of tau aggregates. The key event in tauopathies is the disorganization of the cytoskeleton, which is based on mutations/polymorphisms in the tau gene and lead to nerve cell degeneration. In this review, tauopathies as a distinct class of neurodegenerative diseases are discussed with emphasis on their molecular pathology and genetics.

FTDP-17 Mutations Compromise the Ability of Tau to Regulate Microtubule Dynamics in Cells

The neural microtubule-associated protein Tau binds directly to microtubules and regulates their dynamic behavior. In addition to being required for normal development, maintenance, and function of the nervous system, Tau is associated with several neurodegenerative diseases, including Alzheimer disease. One group of neurodegenerative dementias known as FTDP-17 (fronto-temporal dementia with Parkinsonism linked to chromosome 17) is directly linked genetically to mutations in the tau gene, demonstrating that Tau misfunction can cause neuronal cell death and dementia. These mutations result either in amino acid substitutions in Tau or in altered Tau mRNA splicing that skews the expression ratio of wild-type 3-repeat and 4-repeat Tau isoforms. Because wild-type Tau regulates microtubule dynamics, one possible mechanism underlying Tau-mediated neurodegeneration is aberrant regulation of microtubule behavior. In this study, we microinjected normal and mutated Tau protein into cultured cells expressing fluorescent tubulin and measured the effects on the dynamic instability of individual microtubules. We found that the FTDP-17 amino acid substitutions G272V (in both 3-repeat and 4-repeat Tau contexts), DeltaK280, and P301L all exhibited markedly reduced abilities to regulate dynamic instability relative to wild-type Tau. In contrast, the FTDP-17 R406W mutation (which maps in a regulatory region outside the microtubule binding domain of Tau) did not significantly alter the ability of 3-repeat or 4-repeat Tau to regulate microtubule dynamics. Overall, these data are consistent with a loss-of-function model in which both amino acid substitutions and altered mRNA splicing in Tau lead to neurodegeneration by diminishing the ability of Tau to properly regulate microtubule dynamics.

Proteomic and Functional Analyses Reveal a Mitochondrial Dysfunction in P301L Tau Transgenic Mice

Transgenic mice overexpressing the P301L mutant human tau protein exhibit an accumulation of hyperphosphorylated tau and develop neurofibrillary tangles. The consequences of tau pathology were investigated here by proteomics followed by functional analysis. Mainly metabolism-related proteins including mitochondrial respiratory chain complex components, antioxidant enzymes, and synaptic proteins were identified as modified in the proteome pattern of P301L tau mice. Significantly, the reduction in mitochondrial complex V levels in the P301L tau mice revealed using proteomics was also confirmed as decreased in human P301L FTDP-17 (frontotemporal dementia with parkinsonism linked to chromosome 17) brains. Functional analysis demonstrated a mitochondrial dysfunction in P301L tau mice together with reduced NADH-ubiquinone oxidoreductase activity and, with age, impaired mitochondrial respiration and ATP synthesis. Mitochondrial dys-function was associated with higher levels of reactive oxygen species in aged transgenic mice. Increased tau pathology as in aged homozygous P301L tau mice revealed modified lipid peroxidation levels and the up-regulation of antioxidant enzymes in response to oxidative stress. Furthermore, P301L tau mitochondria displayed increased vulnerability toward beta-amyloid (Abeta) peptide insult, suggesting a synergistic action of tau and Abeta pathology on the mitochondria. Taken together, we conclude that tau pathology involves a mitochondrial and oxidative stress disorder possibly distinct from that caused by Abeta.