Functional effects of tau gene mutations deltaN296 and N296H

Mutations in the tau gene cause frontotemporal dementia and parkinsonism linked to chromosome-17 (FTDP-17). Functionally, about half of the known mutations increase the alternative mRNA splicing of exon 10 of the tau gene, resulting in the overproduction of tau isoforms with four microtubule-binding repeats. The other mutations reduce the ability of tau to interact with microtubules, with some mutations also increasing the propensity of tau to assemble into filaments. Here we have examined the functional effects of the recently described tau gene mutations deltaN296 and N296H. Both mutations reduced the ability of tau to promote microtubule assembly, without having a significant effect on tau filament formation. By exon trapping, they increased the splicing of exon 10. DeltaN296 and N296H thus define a class of tau mutations with effects at both the RNA and the protein level.     

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.     

Aberrantly regulated proteins in frontotemporal dementia

Non-Alzheimer's disease of the frontal type, or frontotemporal dementia (FTD), is the second most common form of dementia. Yet, a detailed characterization of the disease has been especially limiting. To identify mechanisms possibly involved in disease pathology or progression, a proteomic analysis of proteins isolated from human frontal cortex with frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) was performed. We used 2D gel electrophoresis and MALDI-TOF to identify a total of 24 proteins differentially expressed in FTDP-17. We identified a ubiquitin C-terminal hydrolase, UCHL1, as well as several proteins involved in oxidative stress to be differentially expressed. Data presented implicate UCHL1 and ubiquitin-mediated degradation as well as oxidative stress response in disease pathology or progression.     

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.     

A minimal length between tau exon 10 and 11 is required for correct splicing of exon 10

Mutations that stimulate exon 10 inclusion into the human tau mRNA cause frontotemporal dementia with parkinsonism, associated with chromosome 17 (FTDP-17), and other tauopathies. This suggests that the ratio of exon 10 inclusion to exclusion in adult brain is one of the factors to determine biological functions of the tau protein. To investigate the underlying splicing mechanism and identify potential therapeutic targets for tauopathies, we generated a series of mini-gene constructs with intron deletions from the full length of tau exons 9-11 mini-gene construct. RT-PCR results demonstrate that there is a minimum distance requirement between exon 10 and 11 for correct splicing of the exon 10. In addition, SRp20, a member of serine-arginine (SR) protein family of splicing factors was found to facilitate exclusion of exon 10 in a dosage-dependent manner. Significantly, SRp20 also induced exon 10 skipping from pre-mRNAs containing mutations identified in FTDP-17 patients. Based on those results, we generated a cell-based system to measure inclusion to exclusion of exon 10 in the tau mRNA using the luciferase reporter. The firefly luciferase was fused into exon 11 in frame, and a stop code was also created in exon 10. Inclusion of exon 10 prevents luciferase expression, whereas exclusion of exon 10 generates luciferase activity. To minimize baseline luciferase expression, our reporter construct also contains a FTDP-17 mutation that increases exon 10 inclusion. We demonstrate that the splicing pattern of our reporter construct mimics that of endogenous tau gene. Co-transfection of SRp20 and SRp55, two SR proteins that promote exon 10 exclusion, increases production of luciferase. We conclude that this cell-based system can be used to identify biological substances that modulate exon 10 splicing.     

Chronic lithium administration to FTDP-17 tau and GSK-3beta overexpressing mice prevents tau hyperphosphorylation and neurofibrillary tangle formation, but pre-formed neurofibrillary tangles do not revert

Glycogen synthase kinase-3 (GSK-3) has been proposed as the main kinase able to aberrantly phosphorylate tau in Alzheimer's disease (AD) and related tauopathies, raising the possibility of designing novel therapeutic interventions for AD based on GSK-3 inhibition. Lithium, a widely used drug for affective disorders, inhibits GSK-3 at therapeutically relevant concentrations. Therefore, it was of great interest to test the possible protective effects of lithium in an AD animal model based on GSK-3 overexpression. We had previously generated a double transgenic model, overexpressing GSK-3beta in a conditional manner, using the Tet-off system and tau protein carrying a triple FTDP-17 (frontotemporal dementia and parkinsonism linked to chromosome 17) mutation. This transgenic line shows tau hyperphosphorylation in hippocampal neurones accompanied by neurofibrillary tangles (NFTs). We used this transgenic model to address two issues: first, whether chronic lithium treatment is able to prevent the formation of aberrant tau aggregates that result from the overexpression of FTDP-17 tau and GSK-3beta; second, whether lithium is able to change back already formed NFTs in aged animals. Our data suggest that progression of the tauopathy can be prevented by administration of lithium when the first signs of neuropathology appear. Furthermore, it is still possible to partially reverse tau pathology in advanced stages of the disease, although NFT-like structures cannot be changed. The same results were obtained after shut-down of GSK-3beta overexpression, supporting the possibility that GSK-3 inhibition is not sufficient to reverse NFT-like aggregates.     

The FTDP-17-linked mutation R406W abolishes the interaction of phosphorylated tau with microtubules

The recent finding that several point mutations in the gene encoding for the microtubule-binding protein tau correlate with neurological disorders has heightened interest in the mechanisms of destabilization of this protein. In this study the functional consequences of the tau mutation R406W on the interaction of the protein with microtubules have been analyzed. Mutated tau is less phosphorylated than its normal counterpart at serines 396 and 404. Furthermore, the phosphorylated mutant protein is unable to bind to microtubules, and, as a consequence, microtubules assembled after transient nocodazole treatment in the presence of this tau variant contain only unmodified tau and appear to form more and longer bundles than those assembled in the presence of wild-type tau. We propose that phosphorylated tau, unbound to microtubules, could accumulate in the cytoplasm.     

Differential compartmental processing and phosphorylation of pathogenic human tau and native mouse tau in the line 66 model of frontotemporal dementia

Synapse loss is associated with motor and cognitive decline in multiple neurodegenerative disorders, and the cellular redistribution of tau is related to synaptic impairment in tauopathies, such as Alzheimer''s disease and frontotemporal dementia. Here, we examined the cellular distribution of tau protein species in human tau overexpressing line 66 mice, a transgenic mouse model akin to genetic variants of frontotemporal dementia. Line 66 mice express intracellular tau aggregates in multiple brain regions and exhibit sensorimotor and motor learning deficiencies. Using a series of anti-tau antibodies, we observed, histologically, that nonphosphorylated transgenic human tau is enriched in synapses, whereas phosphorylated tau accumulates predominantly in cell bodies and axons. Subcellular fractionation confirmed that human tau is highly enriched in insoluble cytosolic and synaptosomal fractions, whereas endogenous mouse tau is virtually absent from synapses. Cytosolic tau was resistant to solubilization with urea and Triton X-100, indicating the formation of larger tau aggregates. By contrast, synaptic tau was partially soluble after Triton X-100 treatment and most likely represents aggregates of smaller size. MS corroborated that synaptosomal tau is nonphosphorylated. Tau enriched in the synapse of line 66 mice, therefore, appears to be in an oligomeric and nonphosphorylated state, and one that could have a direct impact on cognitive function.     

Tau phosphorylation and aggregation in Alzheimer's disease pathology

In this article I shall review how tau phosphorylation and aggregation participates in Alzheimer's disease (AD) and other tauopathies. Tau, a microtubule associated protein, is the main component, in phosphorylated form, of the aberrant paired helical filaments found in AD. Tau is present in phosphorylated and aggregated form not only in AD, but in other pathologies (tauopathies). In this review, the phosphorylation of tau, its aggregation, and the possible relation between tau phosphorylation and aggregation is, briefly, described. Also, it is discussed the toxicity of modified tau. In addition, I propose a working model detailing the progression of tau pathologies.     

Interaction between tau and alpha-synuclein proteins is impaired in the presence of P301L tau mutation

Deposits of tau and alpha-synuclein are hallmarks of distinct neurodegenerative diseases: tauopathies and alpha-synucleinopathies. Affinity chromatography experiments demonstrated a direct binding of the two proteins, and alpha-synuclein was shown to induce fibrillization of tau. Here, we verify the presence of this physical interaction by using different cellular systems. This binding was abolished by the most common tau mutation (P301L) associated with frontotemporal dementia. We restored the impaired interaction by inducing heat shock proteins 70 and 90. In addition, we show that P301L tau mutation strongly affects tau and alpha-synuclein neuronal distribution.     

Effects of different anti-tau antibodies on tau fibrillogenesis: RTA-1 and RTA-2 counteract tau aggregation

Tau is the major antigenic component of neurofibrillary pathology in tauopathy, including Alzheimer's disease. Although conversion of soluble tau to an insoluble polymerized fibrillar form is a key factor in the pathogenesis of tauopathy, the mechanism of the change is unclear and no inhibitors of fibril formation are available. Monoclonal antibodies against the 1st or 2nd repeat of the microtubule binding domain, but not the C-terminal 16 residues, completely inhibited tau aggregation into PHF. Furthermore, they did not inhibit tau-induced tubulin assembly. Thus, they are useful to investigate tau protein conversion and will be useful therapeutic lead materials.     

Dysregulation of intracellular calcium homeostasis is responsible for neuronal death in an experimental model of selective hippocampal degeneration induced by trimethyltin

Trimethyltin (TMT) intoxication is considered a suitable experimental model to study the molecular basis of selective hippocampal neurodegeneration as that occurring in several neurodegenerative diseases. We have previously shown that rat hippocampal neurons expressing the Ca(2+)-binding protein calretinin (CR) are spared by the neurotoxic action of TMT hypothetically owing to their ability to buffer intracellular Ca(2+) overload. The present study was aimed at determining whether intracellular Ca(2+) homeostasis dysregulation is involved in the TMT-induced neurodegeneration and if intracellular Ca(2+)-buffering mechanisms may exert a protective action in this experimental model of neurodegeneration. In cultured rat hippocampal neurons, TMT produced time- and concentration-dependent [Ca(2+)](i) increases that were primarily due to Ca(2+) release from intracellular stores although Ca(2+) entry through Ca(v)1 channels also contributed to [Ca(2+)](i) increases in the early phase of TMT action. Cell pre-treatment with the Ca(2+) chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl ester) (2 muM) significantly reduced the TMT-induced neuronal death. Moreover, CR(+) neurons responded to TMT with smaller [Ca(2+)](i) increases. Collectively, these data suggest that the neurotoxic action of TMT is mediated by Ca(2+) homeostasis dysregulation, and the resistance of hippocampal neurons to TMT (including CR(+) neurons) is not homogeneous among different neuron populations and is related to their ability to buffer intracellular Ca(2+) overload.     

T-817MA, a neuroprotective agent, attenuates the motor and cognitive impairments associated with neuronal degeneration in P301L tau transgenic mice

Tau pathology is implicated in mechanisms of neurodegenerative tauopathies, including Alzheimer’s disease (AD) and hereditary frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). It has been reported that transgenic mice expressing FTDP-17 mutation P301L of human tau (P301L mice) display extensive tau pathology and exhibit behavioral deficits with aging. In this study, we investigated the effects of T-817MA, a neuroprotective agent, on the motor and cognitive impairments associated with neuronal degeneration in P301L mice. T-817MA prevented the progression of motor deficit and the loss of spinal cord motor neurons in P301L mice. Furthermore, T-817MA significantly attenuated the spatial memory impairment and the reduction in synaptic terminal density in the hippocampal dentate gyrus of P301L mice. These results indicate that T-817MA improved the motor and cognitive impairments as a result of inhibiting neuronal degeneration derived from tau pathology in the P301L mice. Therefore, it is expected that T-817MA has a therapeutic potential for tau-related neurodegenerative diseases such as AD.     

Cytological evidence for a complex of species within the taxon Bactrocera tau (Diptera: Tephritidae) in Thailand

Analysis of mitotic karyotypes of wild specimens of larvae of the Bactrocera tea-like fruit flies (Diptera: Tephritidae) in Thailand has revealed seven distinct chromosome forms, based on the amount and distribution of heterochromatin in sex chromosomes and autosomes. Such cytological differences are perfecdy correlated with morphological observations and molecular genetics data. These findings clearly suggest that B. tau is a cluster of at least seven closely related species temporarily designated as species A (= B. tau) , B, C, D, E, F and G. On die basis of the gross quantity of heterochromatin accumulation in the genome, three groups of mitotic karyotypes can be recognized. Group 1 comprises species A and E. Species E specifically occurs only in fruits of Strychnos thorelii while species A attacks many kinds of host plants. Group 2 contains species B, F and G. Species B has been found only in Siphonodon celastrineus fruit, whereas species F and G attack die same host species, the medically important plant, Hydnocarpus anthelminthicus , albeit in different localities. Group 3 includes species C and D, each of which comprises larger amounts of pericentric heterochromatin in all chromosomes than die other two groups. Hence, diese two species are cytologically remote from those of groups 1 and 2. Species C and D occur allopatrically but they attack the same host plant species, Momordica cochinchinensis. Thus, genetic differentiation at the chromosomal level to specific host plant species and geographic isolation seem to play an important role in speciation of members of the B. tau complex.     

海马脑片锥体神经元钙离子通道的盲法膜片钳研究

目的和方法 :采用大鼠海马脑片盲法膜片钳全细胞记录技术研究CA1区锥体神经元电压门控性Ca2 通道的动力学特征。结果 :大鼠海马脑片CA1区锥体神经元电压门控性Ca2 通道电流具有如下特点 :①激活的阈电位偏低 ,为 (- 4 9.3± 8.6 )mV ,范围为 - 6 5～ - 30mV(n =2 3)。②衰减时间常数τ值较大 ,且变化范围大 (10 0～ 70 0ms) (n =12 ) ,并且衰减具有Ca2 电流幅值的依赖性 ,③稳态失活呈现电压依赖性 ,半失活电压为 (- 5 5 .4± 9.7)mV ,斜率因子为 (5 .3± 0 .9)mV(n =10 )。④当细胞外Ca2 浓度为 2 .5mmol/L时 ,Ca2 通道的反转电位为 (5 5±13)mV(n =10 )。⑤尾电流成分较为单一 ,不表现电压依赖性。另外 ,Ca2 电流对戊脉胺及双氢吡啶类化合物硝苯地平均不敏感。结论 :根据上述Ca2 电流特征 ,海马脑片CA1区锥体神经元上的Ca2 通道主要以N型为主     

Physiological modulation of voltage-dependent inactivation in the cardiac muscle L-type calcium channel: a modelling study

The inactivation of the L-type Ca²⁺ current is composed of voltage-dependent and calcium-dependent mechanisms. The relative contribution of these processes is still under dispute and the idea that the voltage-dependent inactivation could be subject to further modulation by other physiological processes had been ignored. This study sought to model physiological modulation of inactivation of the current in cardiac ventricular myocytes, based upon the recent detailed experimental data that separated total and voltage-dependent inactivation (VDI) by replacing extracellular Ca²⁺ with Mg²⁺ and monitoring L-type Ca²⁺ channel behaviour by outward K⁺ current flowing through the channel in the absence of inward current flow. Calcium-dependent inactivation (CDI) was based upon Ca²⁺ influx and formulated from data that was recorded during β-adrenergic stimulation of the myocytes. Ca²⁺ influx and its competition with non-selective monovalent cation permeation were also incorporated into channel permeation in the model. The constructed model could closely reproduce the experimental Ba²⁺ and Ca²⁺ current results under basal condition where no β-stimulation was added after a slight reduction of the development of fast voltage-dependent inactivation with depolarization. The model also predicted that under β-adrenergic stimulation voltage-dependent inactivation is lost and calcium-dependent inactivation largely compensates it. The developed model thus will be useful to estimate the respective roles of VDI and CDI of L-type Ca²⁺ channels in various physiological and pathological conditions of the heart which would otherwise be difficult to show experimentally.     

Central role for p62/SQSTM1 in the elimination of toxic tau species in a mouse model of tauopathy

Intracellular accumulation of filamentous tau aggregates with progressive neuronal loss is a common characteristic of tauopathies. Although the neurodegenerative mechanism of tau‐associated pathology remains unclear, molecular elements capable of degrading and/or sequestering neurotoxic tau species may suppress neurodegenerative progression. Here, we provide evidence that p62/SQSTM1, a ubiquitinated cargo receptor for selective autophagy, acts protectively against neuronal death and neuroinflammation provoked by abnormal tau accumulation. P301S mutant tau transgenic mice (line PS19) exhibited accumulation of neurofibrillary tangles with localization of p62 mostly in the brainstem, but neuronal loss with few neurofibrillary tangles in the hippocampus. In the hippocampus of PS19 mice, the p62 level was lower compared to the brainstem, and punctate accumulation of phosphorylated tau unaccompanied by co‐localization of p62 was observed. In PS19 mice deficient in p62 (PS19/p62‐KO), increased accumulation of phosphorylated tau, acceleration of neuronal loss, and exacerbation of neuroinflammation were observed in the hippocampus as compared with PS19 mice. In addition, increase of abnormal tau and neuroinflammation were observed in the brainstem of PS19/p62‐KO. Immunostaining and dot‐blot analysis with an antibody selectively recognizing tau dimers and higher‐order oligomers revealed that oligomeric tau species in PS19/p62‐KO mice were significantly accumulated as compared to PS19 mice, suggesting the requirement of p62 to eliminate disease‐related oligomeric tau species. Our findings indicated that p62 exerts neuroprotection against tau pathologies by eliminating neurotoxic tau species, suggesting that the manipulative p62 and selective autophagy may provide an intrinsic therapy for the treatment of tauopathy.