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.     

Modulation of microtubule dynamics by tau in living cells: implications for development and neurodegeneration

The neural microtubule-associated protein tau binds to and stabilizes microtubules. Because of alternative mRNA splicing, tau is expressed with either 3 or 4 C-terminal repeats. Two observations indicate that differences between these tau isoforms are functionally important. First, the pattern of tau isoform expression is tightly regulated during development. Second, mutation-induced changes in tau RNA splicing cause neuronal cell death and dementia simply by altering the isoform expression ratio. To investigate whether 3- and 4-repeat tau differentially regulate microtubule behavior in cells, we microinjected physiological levels of these two isoforms into EGFP-tubulin-expressing cultured MCF7 cells and measured the effects on the dynamic instability behavior of individual microtubules by time-lapse microscopy. Both isoforms suppressed microtubule dynamics, though to different extents. Specifically, 4-repeat tau reduced the rate and extent of both growing and shortening events. In contrast, 3-repeat tau stabilized most dynamic parameters about threefold less potently than 4-repeat tau and had only a minimal ability to suppress shortening events. These differences provide a mechanistic rationale for the developmental shift in tau isoform expression and are consistent with a loss-of-function model in which abnormal tau isoform expression results in the inability to properly regulate microtubule dynamics, leading to neuronal cell death and dementia.     

Functional differences of tau isoforms containing 3 or 4 C-terminal repeat regions and the influence of oxidative stress.

We report functional differences between tau isoforms with 3 or 4 C-terminal repeats and a difference in susceptibility to oxidative conditions, with respect to the regulation of microtubule dynamics in vitro and tau-microtubule binding in cultured cells. In the presence of dithiothreitol in vitro, a 3-repeat tau isoform promotes microtubule nucleation, reduces the tubulin critical concentration for microtubule assembly, and suppresses dynamic instability. Under non-reducing conditions, threshold concentrations of 3-repeat tau and tubulin exist below which this isoform still promotes microtubule nucleation and assembly but fails to reduce the tubulin critical concentration or suppress dynamic instability; above these threshold concentrations, amorphous aggregates of 3-repeat tau and tubulin can be produced at the expense of microtubule formation. A 4-repeat tau isoform is less sensitive to the oxidative potential of the environment, behaving under oxidative conditions similarly to the 3-repeat isoform under reducing conditions. Under conditions of oxidative stress, in Chinese hamster ovary cells stably expressing either 3- or 4-repeat tau, 3-repeat tau disassociates from microtubules more readily than the 4-repeat isoform, and tau-containing high molecular weight aggregates are preferentially observed in lysates from the Chinese hamster ovary cells expressing 3-repeat tau, indicating greater susceptibility of 3-repeat tau to oxidative conditions, compared with 4-repeat tau in vivo.     

Regulation of tau isoform expression and dementia

In the central nervous system (CNS), aberrant changes in tau mRNA splicing and consequently in protein isoform ratios cause abnormal aggregation of tau and neurodegeneration. Pathological tau causes neuronal loss in Alzheimer's disease (AD) and a diverse group of disorders called the frontotemporal dementias (FTD), which are two of the most common forms of dementia and afflict more than 10% of the elderly population. Autosomal dominant mutations in the tau gene cause frontotemporal dementia with parkinsonism-chromosome 17 type (FTDP-17). Just over half the mutations affect tau protein function and decrease its affinity for microtubules (MTs) or increase self-aggregation. The remaining mutations occur within exon 10 (E10) and intron 10 sequences and alter complex regulation of E10 splicing by multiple mechanisms. FTDP-17 splicing mutations disturb the normally balanced levels of distinct protein isoforms that result in altered biochemical and structural properties of tau. In addition to FTDP-17, altered tau isoform levels are also pathogenically associated with other FTD disorders such as progressive supranuclear palsy (PSP), corticobasal degeneration and Pick's disease; however, the mechanisms remain undefined and mutations in tau have not been detected. FTDP-17 highlights the association between splicing mutations and the pronounced variability in pathology as well as phenotype that is characteristic of inherited disorders.     

Structural and functional differences between 3-repeat and 4-repeat tau isoforms. Implications for normal tau function and the onset of neurodegenetative disease

Tau, MAP2, and MAP4 are members of a microtubule-associated protein (MAP) family that are each expressed as "3-repeat" and "4-repeat" isoforms. These isoforms arise from tightly controlled tissue-specific and/or developmentally regulated alternative splicing of a 31-amino acid long "inter-repeat:repeat module," raising the possibility that different MAP isoforms may possess some distinct functional capabilities. Consistent with this hypothesis, regulatory mutations in the human tau gene that disrupt the normal balance between 3-repeat and 4-repeat tau isoform expression lead to a collection of neurodegenerative diseases known as FTDP-17 (fronto-temporal dementias and Parkinsonism linked to chromosome 17), which are characterized by the formation of pathological tau filaments and neuronal cell death. Unfortunately, very little is known regarding structural and functional differences between the isoforms. In our previous analyses, we focused on 4-repeat tau structure and function. Here, we investigate 3-repeat tau, generating a series of truncations, amino acid substitutions, and internal deletions and examining the functional consequences. 3-Repeat tau possesses a "core microtubule binding domain" composed of its first two repeats and the intervening inter-repeat. This observation is in marked contrast to the widely held notion that tau possesses multiple independent tubulin-binding sites aligned in sequence along the length of the protein. In addition, we observed that the carboxyl-terminal sequences downstream of the repeat region make a strong but indirect contribution to microtubule binding activity in 3-repeat tau, which is in contrast to the negligible effect of these same sequences in 4-repeat tau. Taken together with previous work, these data suggest that 3-repeat and 4-repeat tau assume complex and distinct structures that are regulated differentially, which in turn suggests that they may possess isoform-specific functional capabilities. The relevance of isoform-specific structure and function to normal tau action and the onset of neurodegenerative disease are discussed.     

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.     

Inability of tau to properly regulate neuronal microtubule dynamics: a loss-of-function mechanism by which tau might mediate neuronal cell death

Interest in the microtubule-associated protein tau stems from its critical roles in neural development and maintenance, as well as its role in Alzheimer's, FTDP-17 and related neurodegenerative diseases. Under normal circumstances, tau performs its functions by binding to microtubules and powerfully regulating their stability and growing and shortening dynamics. On the other hand, genetic analyses have established a clear cause-and-effect relationship between tau dysfunction/mis-regulation and neuronal cell death and dementia in FTDP-17, but the molecular basis of tau's destructive action(s) remains poorly understood. One attractive model suggests that the intracellular accumulation of abnormal tau aggregates causes cell death, i.e., a gain-of-toxic function model. Here, we describe the evidence and arguments for an alternative loss-of-function model in which tau-mediated neuronal cell death is caused by the inability of affected cells to properly regulate their microtubule dynamic due to mis-regulation by tau. In support of this model, our recent data demonstrate that missense FTDP-17 mutations that alter amino acid residues near tau's microtubule binding region strikingly modify the ability of tau to modulate microtubule dynamics. Additional recent data from our labs support the notion that the same dysfunction occurs in the FTDP-17 regulatory mutations that alter tau RNA splicing patterns. Our model posits that the dynamics of microtubules in neuronal cells must be tightly regulated to enable them to carry out their diverse functions, and that microtubules that are either over-stabilized or under-stabilized, that is, outside an acceptable window of dynamic activity, lead to neurodegeneration. An especially attractive aspect of this model is that it readily accommodates both the structural and regulatory classes of FTDP-17 mutations.     

An SRp75/hnRNPG complex interacting with hnRNPE2 regulates the 5' splice site of tau exon 10, whose misregulation causes frontotemporal dementia

Tau is a neuronal-specific microtubule-associated protein that plays an important role in establishing neuronal polarity and maintaining the axonal cytoskeleton. Aggregated tau is the major component of neurofibrillary tangles (NFTs), structures present in the brains of people affected by neurodegenerative diseases called tauopathies. Tauopathies include Alzheimer's disease (AD), frontotemporal dementia with Parkinsonism (FTDP-17), the early onset dementia observed in Down syndrome (DS; trisomy 21) and the dementia component of myotonic dystrophy type 1 (DM1). Splicing misregulation of adult-specific exon 10, which codes for a microtubule binding domain, results in expression of abnormal ratios of tau isoforms, leading to FTDP-17. Positions 3 to 19 of the intron downstream of exon 10 define a hotspot of splicing regulation: the region diverges between humans and rodents, and point mutations within it result in tauopathies. In this study, we investigated three regulators of exon 10 splicing: serine/arginine-rich protein SRp75 and heterogeneous nuclear ribonucleoproteins hnRNPG and hnRNPE2. SRp75 and hnRNPG inhibit splicing of exon 10 whereas hnRNPE2 activates it. Using co-transfections, co-immunoprecipitations and RNAi we discovered that SRp75 binds to the proximal downstream intron of tau exon 10 at the FTDP-17 hotspot region; and that hnRNPG and hnRNPE2 interact with SRp75. Thus, increased exon 10 inclusion in FTDP mutants may arise from weakened SRp75 binding. This work provides insights into the splicing regulation of the tau gene and into possible strategies for correcting the imbalance in tauopathies caused by changes in the ratio of exon 10.     

Mutations in tau reduce its microtubule binding properties in intact cells and affect its phosphorylation

In vitro evidence has suggested a change in the ability of tau bearing mutations associated with fronto-temporal dementia to promote microtubule assembly. We have used a cellular assay to quantitate the effect of both isoform differences and mutations on the physiological function of tau. Whilst all variants of tau bind to microtubules, microtubule extension is reduced in cells transfected with 3-relative to 4-repeat tau. Mutations reduce microtubule extension with the P301L mutation having a greater effect than the V337M mutation. The R406W mutation had a small effect on microtubule extension but, surprisingly, tau with this mutation was less phosphorylated in intact cells than the other variants.     

The natural osmolyte trimethylamine N-oxide (TMAO) restores the ability of mutant tau to promote microtubule assembly

Coding region and intronic mutations in the gene for microtubule-associated protein tau cause frontotemporal dementia and Parkinsonism linked to chromosome 17 (FTDP-17). Most coding region mutations effect a reduced ability of tau protein to interact with microtubules and lead to the formation of a filamentous pathology made of hyperphosphorylated tau. Here we show that trimethylamine N-oxide (TMAO) restores the ability of tau with FTDP-17 mutations to promote microtubule assembly. To mimic phosphorylation, serine and threonine residues in tau were singly or multiply mutated to glutamic acid, resulting in a reduced ability of tau to promote microtubule assembly. With the exception of the most heavily substituted protein (27 glutamic acid residues), TMAO increased the ability of mutant tau to promote microtubule assembly. However, it had no significant effect on heparin-induced assembly of tau into filaments.     

Correction of alternative splicing of tau in frontotemporal dementia and parkinsonism linked to chromosome 17

Mutations in the human tau gene cause frontotemporal dementia and Parkinsonism associated with chromosome 17 (FTDP-17). One of the major disease mechanisms in FTDP-17 is the increased inclusion of tau exon 10 during pre-mRNA splicing. Here we show that modified oligonucleotides directed against the tau exon 10 splice junctions suppress inclusion of tau exon 10. The effect is mediated by the formation of a stable pre-mRNA-oligonucleotide hybrid, which blocks access of the splicing machinery to the pre-mRNA. Correction of tau splicing occurs in a tau minigene system and in endogenous tau RNA in neuronal pheochromocytoma cells and is specific to exon 10 of the tau gene. Antisense oligonucleotide-mediated exclusion of exon 10 has a physiological effect by increasing the ratio of protein lacking the microtubule-binding domain encoded by exon 10. As a consequence, the microtubule cytoskeleton becomes destabilized and cell morphology is altered. Our results demonstrate that alternative splicing defects of tau as found in FTDP-17 patients can be corrected by application of antisense oligonucleotides. These findings provide a tool to study specific tau isoforms in vivo and might lead to a novel therapeutic strategy for FTDP-17.     

Development and assessment of sensitive immuno‐PCR assays for the quantification of cerebrospinal fluid three‐ and four‐repeat tau isoforms in tauopathies

Characteristic tau isoform composition of the insoluble fibrillar tau inclusions define tauopathies, including Alzheimer's disease (AD), progressive supranuclear palsy (PSP) and frontotemporal dementia with parkinsonism linked to chromosome 17/frontotemporal lobar degeneration‐tau (FTDP‐17/FTLD‐tau). Exon 10 splicing mutations in the tau gene, MAPT, in familial FTDP‐17 cause elevation of tau isoforms with four microtubule‐binding repeat domains (4R‐tau) compared to those with three repeats (3R‐tau). On the basis of two well‐characterised monoclonal antibodies against 3R‐ and 4R‐tau, we developed novel, sensitive immuno‐PCR assays for measuring the trace amounts of these isoforms in CSF. This was with the aim of assessing if CSF tau isoform changes reflect the pathological changes in tau isoform homeostasis in the degenerative brain and if these would be relevant for differential clinical diagnosis. Initial analysis of clinical CSF samples of PSP (n = 46), corticobasal syndrome (CBS; n = 22), AD (n = 11), Parkinson's disease with dementia (PDD; n = 16) and 35 controls revealed selective decreases of immunoreactive 4R‐tau in CSF of PSP and AD patients compared with controls, and lower 4R‐tau levels in AD compared with PDD. These decreases could be related to the disease‐specific conformational masking of the RD4‐binding epitope because of abnormal folding and/or aggregation of the 4R‐tau isoforms in tauopathies or increased sequestration of the 4R‐tau isoforms in brain tau pathology.     

Functional characterization of FTDP-17 tau gene mutations through their effects on Xenopus oocyte maturation.

tau gene mutations cause frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). Here we have used Xenopus oocyte maturation as an indicator of microtubule function. We show that wild-type four-repeat Tau protein inhibits maturation in a concentration-dependent manner, whereas three-repeat Tau has no effect. Of the seven four-repeat Tau proteins with FTDP-17 mutations tested, five (G272V, DeltaK280, P301L, P301S, and V337M) failed to interfere significantly with oocyte maturation, demonstrating a greatly reduced ability to interact with microtubules. One mutant protein (R406W) almost behaved like wild-type Tau, and one (S305N) inhibited maturation more strongly than wild-type Tau. With the exception of R406W, wild-type Tau and all the mutants studied were similarly phosphorylated during the Xenopus oocyte maturation, and this was independent of their effects on this process. Data obtained with R406W and S305N may be related to charge changes (phosphorylation and basic amino acids). Our results demonstrate variable effects of FTDP-17 mutations on microtubules in an intact cell situation. Those findings establish Xenopus oocyte maturation as a system allowing the study of the functional effects of tau gene mutations in a quantitative manner.     

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.     

Distinct FTDP-17 missense mutations in tau produce tau aggregates and other pathological phenotypes in transfected CHO cells

Multiple tau gene mutations are pathogenic for hereditary frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17), with filamentous tau aggregates as the major lesions in the CNS of these patients. Recent studies have shown that bacterially expressed recombinant tau proteins with FTDP-17 missense mutations cause functional impairments, i.e., a reduced ability of mutant tau to bind to or promote the assembly of microtubules. To investigate the biological consequences of FTDP-17 tau mutants and assess their ability to form filamentous aggregates, we engineered Chinese hamster ovary cell lines to stably express tau harboring one or several different FTDP-17 mutations and showed that different tau mutants produced distinct pathological phenotypes. For example, delta K, but not several other single tau mutants (e.g., V337 M, P301L, R406W), developed insoluble amorphous and fibrillar aggregates, whereas a triple tau mutant (VPR) containing V337M, P301L, and R406W substitutions also formed similar aggregates. Furthermore, the aggregates increased in size over time in culture. Significantly, the formation of aggregated delta K and VPR tau protein correlated with reduced affinity of these mutants to bind microtubules. Reduced phosphorylation and altered proteolysis was also observed in R406W and delta K tau mutants. Thus, distinct pathological phenotypes, including the formation of insoluble filamentous tau aggregates, result from the expression of different FTDP-17 tau mutants in transfected Chinese hamster ovary cells and implies that these missense mutations cause diverse neurodegenerative FTDP-17 syndromes by multiple mechanisms.     

Variations in filament conformation dictate seeding barrier between three- and four-repeat tau

Tau filaments are the pathological hallmark of >20 neurodegenerative diseases including Alzheimer's disease. Six tau isoforms exist that can be grouped into 4-repeat (4R) tau and 3-repeat (3R) tau based on the presence or absence of the second of four microtubule binding repeats. Recent evidence suggests that tau filaments can transfer between cells and spread through the brain. Here we demonstrate in vitro that seeded filament growth, a prerequisite for tau spreading, is crucially dependent on the isoform composition of individual seeds. Seeds of 3R tau and 3R/4R tau recruit both types of isoforms. Seeds of 4R tau recruit 4R tau, but not 3R tau, establishing an asymmetric barrier. Conformational templating of 4R tau onto 3R tau seeds eliminates this barrier, giving rise to a new type of tau filament. These findings provide fundamental mechanistic insights into the seeding, propagation, and diversification of tau filaments.     

Age-dependent emergence and progression of a tauopathy in transgenic mice overexpressing the shortest human tau isoform

Filamentous tau aggregates are hallmarks of tauopathies, e.g., frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17) and amyotrophic lateral sclerosis/parkinsonism-dementia complex (ALS/PDC). Since FTDP-17 tau gene mutations alter levels/functions of tau, we overexpressed the smallest human tau isoform in the CNS of transgenic (Tg) mice to model tauopathies. These mice acquired age-dependent CNS pathology similarto FTDP-17 and ALS/PDC, including insoluble, hyperphosphorylated tau and argyrophilic intraneuronal inclusions formed by tau-immunoreactive filaments. Inclusions were present in cortical and brainstem neurons but were most abundant in spinal cord neurons, where they were associated with axon degeneration, diminished microtubules (MTs), and reduced axonal transport in ventral roots, as well as spinal cord gliosis and motor weakness. These Tg mice recapitulate key features of tauopathies and provide models for elucidating mechanisms underlying diverse tauopathies, including Alzheimer's disease (AD).     

FTDP-17 tau mutations decrease the susceptibility of tau to calpain I digestion

Frontal temporal dementia and Parkinsonism linked to chromosome 17 (FTDP-17) is caused by splice site and missense mutations in the tau gene, and characterized by the accumulation of filamentous tau in cerebral neurons and glia. The missense mutations reduce the ability of tau to promote microtubule assembly and increase the ability of tau to form filaments. In this report we demonstrate that mutants V337M and R406W are less susceptible than mutant P301L or corresponding wild type tau to degradation by calpain I. The differences were at least in part due to changes in accessibility of a cleavage site located about 100 amino acids off the carboxy-terminus. The results suggest that the pathogenesis of some forms of FTDP-17 may involve tau accumulation due to decreased proteolytic degradation.     

Pseudohyperphosphorylation has differential effects on polymerization and function of tau isoforms

The microtubule-associated protein tau exists as six isoforms created through the splicing of the second, third, and tenth exons. The isoforms are classified by their number of N-terminal exons (0N, 1N, or 2N) and by their number of microtubule-binding repeat regions (3R or 4R). Hyperphosphorylated isoforms accumulate in insoluble aggregates in Alzheimer's disease and other tauopathies. These neurodegenerative diseases can be categorized based on the isoform content of the aggregates they contain. Hyperphosphorylated tau has the general characteristics of an upward electrophoretic shift, decreased microtubule binding, and an association with aggregation. Previously we have shown that a combination of seven pseudophosphorylation mutations at sites phosphorylated by GSK-3β, referred to as 7-Phos, induced several of these characteristics in full-length 2N4R tau and led to the formation of fewer but longer filaments. We sought to determine whether the same phosphorylation pattern could cause differential effects in the other tau isoforms, possibly through varied conformational effects. Using in vitro techniques, we examined the electrophoretic mobility, aggregation properties, and microtubule stabilization of all isoforms and their pseudophosphorylated counterparts. We found that pseudophosphorylation affected each isoform, but in several cases certain isoforms were affected more than others. These results suggest that hyperphosphorylation of tau isoforms could play a major role in determining the isoform composition of tau aggregates in disease.