Overexpression of four-repeat tau mRNA isoforms in progressive supranuclear palsy but not in Alzheimer's disease

MAPT haplotype-associated transcriptomic changes in progressive supranuclear palsy

Progressive supranuclear palsy (PSP) is a neurodegenerative movement and cognitive disorder characterized by abnormal accumulation of the microtubule-associated protein tau in the brain. Biochemically, inclusions in PSP are enriched for tau proteoforms with four microtubule-binding domain repeats (4R), an isoform that arises from alternative tau pre-mRNA splicing. While preferential aggregation and reduced degradation of 4R tau protein is thought to play a role in inclusion formation and toxicity, an alternative hypothesis is that altered expression of tau mRNA isoforms plays a causal role. This stems from the observation that PSP is associated with common variation in the tau gene (MAPT) at the 17q21.31 locus which contains low copy number repeats flanking a large recurrent genomic inversion. The complex genomic structural changes at the locus give rise to two dominant haplotypes, termed H1 and H2, that have the potential to markedly influence gene expression. Here, we explored haplotype-dependent differences in gene expression using a bulk RNA-seq dataset derived from human post-mortem brain tissue from PSP (n = 84) and controls (n = 77) using a rigorous computational pipeline, including alternative pre-mRNA splicing. We found 3579 differentially expressed genes in the temporal cortex and 10,011 in the cerebellum. We also found 7214 differential splicing events in the temporal cortex and 18,802 in the cerebellum. In the cerebellum, total tau mRNA levels and the proportion of transcripts encoding 4R tau were significantly increased in PSP compared to controls. In the temporal cortex, the proportion of reads that expressed 4R tau was increased in cases compared to controls. 4R tau mRNA levels were significantly associated with the H1 haplotype in the temporal cortex. Further, we observed a marked haplotype-dependent difference in KANSL1 expression that was strongly associated with H1 in both brain regions. These findings support the hypothesis that sporadic PSP is associated with haplotype-dependent increases in 4R tau mRNA that might play a causal role in this disorder.

Identification of high-performing antibodies for the reliable detection of Tau proteoforms by Western blotting and immunohistochemistry

Antibodies are essential research tools whose performance directly impacts research conclusions and reproducibility. Owing to its central role in Alzheimer's disease and other dementias, hundreds of distinct antibody clones have been developed against the microtubule-associated protein Tau and its multiple proteoforms. Despite this breadth of offer, limited understanding of their performance and poor antibody selectivity have hindered research progress. Here, we validate a large panel of Tau antibodies by Western blot (79 reagents) and immunohistochemistry (35 reagents). We address the reagents' ability to detect the target proteoform, selectivity, the impact of protein phosphorylation on antibody binding and performance in human brain samples. While most antibodies detected Tau at high levels, many failed to detect it at lower, endogenous levels. By WB, non-selective binding to other proteins affected over half of the antibodies tested, with several cross-reacting with the related MAP2 protein, whereas the "oligomeric Tau" T22 antibody reacted with monomeric Tau by WB, thus calling into question its specificity to Tau oligomers. Despite the presumption that "total" Tau antibodies are agnostic to post-translational modifications, we found that phosphorylation partially inhibits binding for many such antibodies, including the popular Tau-5 clone. We further combine high-sensitivity reagents, mass-spectrometry proteomics and cDNA sequencing to demonstrate that presumptive Tau "knockout" human cells continue to express residual protein arising through exon skipping, providing evidence of previously unappreciated gene plasticity. Finally, probing of human brain samples with a large panel of antibodies revealed the presence of C-term-truncated versions of all main Tau brain isoforms in both control and tauopathy donors. Ultimately, we identify a validated panel of Tau antibodies that can be employed in Western blotting and/or immunohistochemistry to reliably detect even low levels of Tau expression with high selectivity. This work represents an extensive resource that will enable the re-interpretation of published data, improve reproducibility in Tau research, and overall accelerate scientific progress.

Revisiting the grammar of Tau aggregation and pathology formation: how new insights from brain pathology are shaping how we study and target Tauopathies

Converging evidence continues to point towards Tau aggregation and pathology formation as central events in the pathogenesis of Alzheimer's disease and other Tauopathies. Despite significant advances in understanding the morphological and structural properties of Tau fibrils, many fundamental questions remain about what causes Tau to aggregate in the first place. The exact roles of cofactors, Tau post-translational modifications, and Tau interactome in regulating Tau aggregation, pathology formation, and toxicity remain unknown. Recent studies have put the spotlight on the wide gap between the complexity of Tau structures, aggregation, and pathology formation in the brain and the simplicity of experimental approaches used for modeling these processes in research laboratories. Embracing and deconstructing this complexity is an essential first step to understanding the role of Tau in health and disease. To help deconstruct this complexity and understand its implication for the development of effective Tau targeting diagnostics and therapies, we firstly review how our understanding of Tau aggregation and pathology formation has evolved over the past few decades. Secondly, we present an analysis of new findings and insights from recent studies illustrating the biochemical, structural, and functional heterogeneity of Tau aggregates. Thirdly, we discuss the importance of adopting new experimental approaches that embrace the complexity of Tau aggregation and pathology as an important first step towards developing mechanism- and structure-based therapies that account for the pathological and clinical heterogeneity of Alzheimer's disease and Tauopathies. We believe that this is essential to develop effective diagnostics and therapies to treat these devastating diseases.

The Disease Associated Tau35 Fragment has an Increased Propensity to Aggregate Compared to Full-Length Tau

Tau35 is a truncated form of tau found in human brain in a subset of tauopathies. Tau35 expression in mice recapitulates key features of human disease, including progressive increase in tau phosphorylation, along with cognitive and motor dysfunction. The appearance of aggregated tau suggests that Tau35 may have structural properties distinct from those of other tau species that could account for its pathological role in disease. To address this hypothesis, we performed a structural characterization of monomeric and aggregated Tau35 and compared the results to those of two longer isoforms, 2N3R and 2N4R tau. We used small angle X-ray scattering to show that Tau35, 2N3R and 2N4R tau all behave as disordered monomeric species but Tau35 exhibits higher rigidity. In the presence of the poly-anion heparin, Tau35 increases thioflavin T fluorescence significantly faster and to a greater extent than full-length tau, demonstrating a higher propensity to aggregate. By using atomic force microscopy, circular dichroism, transmission electron microscopy and X-ray fiber diffraction, we provide evidence that Tau35 aggregation is mechanistically and morphologically similar to previously reported tau fibrils but they are more densely packed. These data increase our understanding of the aggregation inducing properties of clinically relevant tau fragments and their potentially damaging role in the pathogenesis of human tauopathies.

Human wildtype tau expression in cholinergic pedunculopontine tegmental neurons is sufficient to produce PSP-like behavioural deficits and neuropathology

Progressive Supranuclear Palsy (PSP) is the most common atypical parkinsonism and exhibits hallmark symptomology including motor function impairment and dysexecutive dementia. In contrast to Parkinson's disease, the underlying pathology displays aggregation of the protein tau, which is also seen in disorders such as Alzheimer's disease. Currently, there are no pharmacological treatments for PSP, and drug discovery efforts are hindered by the lack of an animal model specific to PSP. Based on previous results and clinical pathology, it was hypothesized that viral deposition of tau in cholinergic neurons within the hindbrain would produce a tauopathy along neural connections to produce PSP-like symptomology and pathology. By using a combination of ChAT-CRE rats and CRE-dependent AAV vectors, wildtype human tau (the PSP-relevant 1N4R isoform; hTau) was expressed in hindbrain cholinergic neurons. Compared to control subjects (GFP), rats with tau expression displayed deficits in a variety of behavioural paradigms: acoustic startle reflex, marble burying, horizontal ladder and hindlimb motor reflex. Postmortem, the hTau rats had significantly reduced number of cholinergic pedunculopontine tegmentum and dopaminergic substantia nigra neurons, as well as abnormal tau deposits. This preclinical model has multiple points of convergence with the clinical features of PSP, some of which distinguish between PSP and Parkinson's disease.

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.

Amyotrophic lateral sclerosis features predict TDP-43 pathology in frontotemporal lobar degeneration

Clinical and pathological heterogeneity is common in patients with frontotemporal lobar degeneration (FTLD) pathology. This investigated clinical or imaging characteristics that differentiate FTLD-TDP from FTLD-tau, FTLD-TDP subtypes from each other, or pathological stages of FTLD-TDP. Initial clinical, neuropsychological and neuroimaging characteristics were compared between pathologically defined FTLD-tau and FTLD-TDP groups. Voxel-based morphometry analyses contrasted grey matter atrophy patterns. Twenty-six FTLD-TDP, 28 FTLD-tau and 78 controls were included. Amyotrophic lateral sclerosis features, when present, were highly specific FTLD-TDP, which displayed greater cortical and subcortical atrophy than FTLD-tau. FTLD-TDP-43 type B had significantly shorter survival than type A. Type A patients were more cognitively impaired than type B, and basal ganglia atrophy appeared to distinguish type A from type B. Age at onset and survival duration were comparable between stages II and IV. In conclusion, Amyotrophic lateral sclerosis features may be useful in distinguishing FTLD-TDP from FTLD-tau. TDP-43 type A and B appear to present with distinct profiles. The relationship between clinical features and pathological staging in FTLD-TDP-43 is complex, and TDP-43 subtyping may have more clinical utility.

Tau Exon 10 Inclusion by PrPC through Downregulating GSK3β Activity

Tau protein is largely responsible for tauopathies, including Alzheimer’s disease (AD), where it accumulates in the brain as insoluble aggregates. Tau mRNA is regulated by alternative splicing, and inclusion or exclusion of exon 10 gives rise to the 3R and 4R isoforms respectively, whose balance is physiologically regulated. In this sense, one of the several factors that regulate alternative splicing of tau is GSK3β, whose activity is inhibited by the cellular prion protein (PrP^C), which has different physiological functions in neuroprotection and neuronal differentiation. Moreover, a relationship between PrP^C and tau expression levels has been reported during AD evolution. For this reason, in this study we aimed to analyze the role of PrP^C and the implication of GSK3β in the regulation of tau exon 10 alternative splicing. We used AD human samples and mouse models of PrP^C ablation and tau overexpression. In addition, we used primary neuronal cultures to develop functional studies. Our results revealed a paralleled association between PrP^C expression and tau 4R isoforms in all models analyzed. In this sense, reduction or ablation of PrP^C levels induces an increase in tau 3R/4R balance. More relevantly, our data points to GSK3β activity downstream from PrP^C in this phenomenon. Our results indicate that PrP^C plays a role in tau exon 10 inclusion through the inhibitory capacity of GSK3β.

Exploring the role of mitochondrial proteins as molecular target in Alzheimer's disease

Brain is a fully differentiated organ and is sensitive towards oxidative damage of various compounds including lipids, proteins, and DNA that occurs during process of normal aging and is mainly due to its high energy metabolism and reduced activity of anti-oxidative defense mechanism. Mitochondria are dynamic ATP-generating organelles which constitutes cellular functions such as regulation of intracellular calcium, bio-energetic processes, and reduction-oxidation of cells. Such functioning is negatively affected due to the presence of amyloid β peptide (Aβ) which is involved in pathogenesis of Alzheimer disease (AD). Aβ interacts with mitochondria and leads to mitochondrial dysfunction. Mitochondrial dysfunction, abnormal interactions, oxidative stress, and mis-folding of synaptic proteins inside nervous system are explored and regarded as primary or initial features in insurgence of pathology (AD and other neurological disease). The major histopathological hallmarks of AD are characterized by presence of these hallmarks intracellularly, its further progression and exacerbation which leads to excessive accumulation of oligomeric as well as fibrillar-β-amyloid peptides (present extracellularly) and accumulation of neurofibrillary tangles intracellularly. The current review will focus on alterations and variation in mitochondria/mitochondrial DNA (mtDNA) and the rationale for involvement of related abnormalities in pathogenesis of AD.

Molecular and cellular mechanisms underlying the pathogenesis of Alzheimer's disease

Alzheimer’s disease (AD) is the most common neurodegenerative disorder seen in age-dependent dementia. There is currently no effective treatment for AD, which may be attributed in part to lack of a clear underlying mechanism. Studies within the last few decades provide growing evidence for a central role of amyloid β (Aβ) and tau, as well as glial contributions to various molecular and cellular pathways in AD pathogenesis. Herein, we review recent progress with respect to Aβ- and tau-associated mechanisms, and discuss glial dysfunction in AD with emphasis on neuronal and glial receptors that mediate Aβ-induced toxicity. We also discuss other critical factors that may affect AD pathogenesis, including genetics, aging, variables related to environment, lifestyle habits, and describe the potential role of apolipoprotein E (APOE), viral and bacterial infection, sleep, and microbiota. Although we have gained much towards understanding various aspects underlying this devastating neurodegenerative disorder, greater commitment towards research in molecular mechanism, diagnostics and treatment will be needed in future AD research.

Pathogenic Tau Protein Species: Promising Therapeutic Targets for Ocular Neurodegenerative Diseases

Tau is a microtubule-associated protein, which is highly expressed in the central nervous system as well as ocular neurons and stabilizes microtubule structure. It is a phospho-protein being moderately phosphorylated under physiological conditions but its abnormal hyperphosphorylation or some post-phosphorylation modifications would result in a pathogenic condition, microtubule dissociation, and aggregation. The aggregates can induce neuroinflammation and trigger some pathogenic cascades, leading to neurodegeneration. Taking these together, targeting pathogenic tau employing tau immunotherapy may be a promising therapeutic strategy in fighting with cerebral and ocular neurodegenerative disorders.

Promising therapies for the treatment of frontotemporal dementia clinical phenotypes: from symptomatic to disease-modifying drugs

INTRODUCTION

Frontotemporal dementia (FTD) is a heterogeneous clinical entity that includes several disorders characterized by different cellular mechanisms. Distinctive clinical features in FTD include behavioral, affective, and cognitive symptoms. Unfortunately, little progress has been made over the past 20 years in terms of the development of effective disease-modifying drugs with the currently available symptomatic treatments having limited clinical utility.

AREAS COVERED

This article reviews the principal pharmacological intervention studies for FTD. These are predominantly randomized clinical trials and include symptomatic treatments and potential disease-modifying drugs.

EXPERT OPINION

There is insufficient evidence on effective treatments for FTD and studies with better methodological backgrounds are needed. Most studies reporting therapeutic benefits were conducted with selective serotonin reuptake inhibitors, while anti-dementia drugs have been ineffective in FTD. Since the underlying pathology of FTD mostly consists of abnormal tau protein or TDP-43 aggregates, treatments are being developed to interfere with their aggregation process or with the clearance of these proteins. Furthermore, disease-modifying treatments remain years away as demonstrated by the recent negative Phase III findings of a tau aggregation inhibitor (LMTM) for treating the behavioral variant of FTD. The results from current ongoing Phase I/II trials will hopefully give light to future treatment options.

Pathological Changes of Tau Related to Alzheimer's Disease

Alzheimer's disease (AD), the most common form of dementia, is characterized by extracellular β-amyloid plaques and intracellular neurofibrillary tangles (NFTs), which are considered as major targets for AD therapies. However, no effective therapy is available to cure or prevent the progression of AD up until now. Accumulation of NFTs, which consist of abnormally hyperphosphorylated tau, is directly correlated with the degree of dementia in AD patients. Emerging evidence indicates that the prion-like seeding and spreading of tau pathology may be the key driver of AD. In the past decades, greater understanding of tau pathway reveals new targets for the development of specific therapies. Here, we review the recent research progress in the mechanism underlying tau pathology in AD and briefly introduce tau-based therapeutics.

Brainstem tau pathology in Alzheimer's disease is characterized by increase of three repeat tau and independent of amyloid β

INTRODUCTION

Alzheimer-type neuropil threads (NTs) and neurofibrillary tangles (NFTs) are comprised of either 4 repeat (4R)-tau, 3 repeat (3R)-tau, or a mixture of both. In the hippocampus, the number of NFTs, and the proportion of 3R tau progressively increases. If this preferential accumulation of 3R tau also occurs in the brainstem, it may be fundamentally related to progression of Alzheimer pathology.

METHODS

Midbrain and pontine sections of brainstems from 23 cases (Braak-NFT stages I/II: 8, III/IV: 8, and V/VI: 7) were double immunofluorolabeled for 4R and 3R tau. High-resolution (0.645 μm/pixel), in-focus snapshots were tiled to cover entire brain sections using a virtual slide system. Each lesion was classified by size (NT < 200 μm² < NFT) and staining profile (3R/4R). In addition, the localization and quantity of amyloid β (Aβ) deposits were examined in adjacent sections for comparison with tau.

RESULTS

The data sets obtained from approximately 286 gigabytes of image files consisted of 847,763 NTs and 7859 NFTs. The proportion of 3R tau-positive NTs and NFTs in the midbrain, and 3R tau-positive NTs in the pons gradually increased with advancing NFT stages, while the proportion of 3R tau-positive NFTs in the pons was already elevated at early stages. Aβ deposits were absent at NFT stages I/II, and when present at later stages, their regional distribution was different from that of tau. These observations suggest that a progressive increase in the proportion of 3R tau occurs independently of Aβ deposits.

CONCLUSIONS

This is the first quantitative analysis of NFTs and NTs in the human brainstem. We demonstrate that the proportion of 3R tau in the brainstem neurofibrillary changes increases with disease progression. Because this phenomenon is shared between the brainstem and the hippocampus, this increase may be fundamental to the pathogenesis of Alzheimer disease.

PERK activation mitigates tau pathology in vitro and in vivo

The RNA‐like endoplasmic reticulum kinase (PERK) is genetically associated with the tauopathy progressive supranuclear palsy (PSP). To elucidate the functional mechanisms underlying this association, we explored PERK activity in brains of PSP patients and its function in three tauopathy models (cultured human neurons overexpressing 4‐repeat wild‐type tau or treated with the environmental neurotoxin annonacin, and P301S tau transgenic mice). In vitro, treatment with a pharmacological PERK activator CCT020312 or PERK overexpression reduced tau phosphorylation, tau conformational change and 4‐repeat tau isoforms, and increased cell viability. In vivo, the PERK activator significantly improved memory and locomotor function, reduced tau pathology, and prevented dendritic spine and motoneuron loss in P301S tau mice. Importantly, the PERK substrate EIF2A, mediating some detrimental effects of PERK signaling, was downregulated in PSP brains and tauopathy models, suggesting that the alternative PERK–NRF2 pathway accounts for these beneficial effects in the context of tauopathies. In summary, PERK activation may be a novel strategy to treat PSP and eventually other tauopathies.

Optimization of in vitro conditions to study the arachidonic acid induction of 4R isoforms of the microtubule-associated protein tau

The microtubule-associated protein tau exists in six different isoforms that accumulate as filamentous aggregates in a wide spectrum of neurodegenerative diseases classified as tauopathies. One potential source of heterogeneity between these diseases could arise from differential tau isoform aggregation. in vitro assays employing arachidonic acid as an inducer of aggregation have been pivotal in gaining an understanding of the longest four repeat tau isoform (2N4R). These approaches have been less successful for modeling the shorter 1N4R and 0N4R tau isoforms in vitro. Through a careful analysis of in vitro conditions for aggregation, we found that the differences in the acidity of tau isoform N-terminal projection domains determine whether tau filaments cluster into larger assemblies in solution. Beyond the potential biological implications of filament clustering, we provide optimized conditions for the arachidonic acid induction of shorter 4R tau isoforms aggregation in vitro that greatly reduce filament clustering and improved modeling results.

Small-molecule PET Tracers for Imaging Proteinopathies

In this chapter, we provide a review of the challenges and advances in developing successful PET imaging agents for 3 major types of aggregated amyloid proteins: amyloid-beta (Aβ), tau, and alpha-synuclein (α-syn). These 3 amyloids are involved in the pathogenesis of a variety of neurodegenerative diseases, referred to as proteinopathies or proteopathies, that include Alzheimer disease, Lewy body dementias, multiple system atrophy, and frontotemporal dementias, among others. In the Introduction section, we briefly discuss the history of amyloid in neurodegenerative diseases and describe why progress in developing effective imaging agents has been hampered by the failure of crystallography to provide definitive ligand-protein interactions for rational radioligand design efforts. Instead, the field has relied on largely serendipitous, trial-and-error methods to achieve useful and specific PET amyloid imaging tracers for Aβ, tau, and α-syn deposits. Because many of the proteopathies involve more than 1 amyloid protein, it is important to develop selective PET tracers for the different amyloids to help assess the relative contribution of each to total amyloid burden. We use Pittsburgh compound B to illustrate some of the critical steps in developing a potent and selective Aβ PET imaging agent. Other selective Aβ and tau PET imaging compounds have followed similar pathways in their developmental processes. Success for selective α-syn PET imaging agents has not been realized yet, but work is ongoing in multiple laboratories throughout the world. In the tau sections, we provide background regarding 3-repeat (3R) and 4-repeat (4R) tau proteins and how they can affect the binding of tau radioligands in different tauopathies. We review the ongoing efforts to assess the properties of tau ligands, which are useful in 3R, 4R, or combined 3R-4R tauopathies. Finally, we describe in the α-syn sections recent attempts to develop selective tracers to image α-synucleinopathies.

Frontotemporal Dementia

Frontotemporal dementia (FTD) is a heterogeneous disorder with distinct clinical phenotypes associated with multiple neuropathologic entities. Presently, the term FTD encompasses clinical disorders that include changes in behavior, language, executive control, and often motor symptoms. The core FTD spectrum disorders include behavioral variant FTD, nonfluent/agrammatic variant primary progressive aphasia, and semantic variant PPA. Related FTD disorders include frontotemporal dementia with motor neuron disease, progressive supranuclear palsy syndrome, and corticobasal syndrome. In this article, the authors discuss the clinical presentation, diagnostic criteria, neuropathology, genetics, and treatments of these disorders.

The Role of MAPT Haplotype H2 and Isoform 1N/4R in Parkinsonism of Older Adults

Background and Objective

Recently, we have shown that the Parkinson’s disease (PD) susceptibility locus MAPT (microtubule associated protein tau) is associated with parkinsonism in older adults without a clinical diagnosis of PD. In this study, we investigated the relationship between parkinsonian signs and MAPT transcripts by assessing the effect of MAPT haplotypes on alternative splicing and expression levels of the most common isoforms in two prospective clinicopathologic studies of aging.

Materials and Methods

using regression analysis, controlling for age, sex, study and neuropathology, we evaluated 976 subjects with clinical, genotyping and brain pathology data for haplotype analysis. For transcript analysis, we obtained MAPT gene and isoform-level expression from the dorsolateral prefrontal cortex for 505 of these subjects.

Results

The MAPT H2 haplotype was associated with lower total MAPT expression (p = 1.2x10-14) and global parkinsonism at both study entry (p = 0.001) and proximate to death (p = 0.050). Specifically, haplotype H2 was primarily associated with bradykinesia in both assessments (p<0.001 and p = 0.008). MAPT total expression was associated with age and decreases linearly with advancing age (p<0.001). Analysing MAPT alternative splicing, the expression of 1N/4R isoform was inversely associated with global parkinsonism (p = 0.008) and bradykinesia (p = 0.008). Diminished 1N/4R isoform expression was also associated with H2 (p = 0.001).

Conclusions

Overall, our results suggest that age and H2 are associated with higher parkinsonism score and decreased total MAPT RNA expression. Additionally, we found that H2 and parkinsonism are associated with altered expression levels of specific isoforms. These findings may contribute to the understanding of the association between MAPT locus and parkinsonism in elderly subjects and in some extent to age-related neurodegenerative diseases.

Truncation and Activation of Dual Specificity Tyrosine Phosphorylation-regulated Kinase 1A by Calpain I: A MOLECULAR MECHANISM LINKED TO TAU PATHOLOGY IN ALZHEIMER DISEASE

Hyperphosphorylation and dysregulation of exon 10 splicing of Tau are pivotally involved in pathogenesis of Alzheimer disease (AD) and/or other tauopathies. Alternative splicing of Tau exon 10, which encodes the second microtubule-binding repeat, generates Tau isoforms containing three and four microtubule-binding repeats, termed 3R-Taus and 4R-Taus, respectively. Dual specificity tyrosine-phosphorylation-regulated kinase 1A (Dyrk1A) lies at the Down syndrome critical region of chromosome 21. Overexpression of this kinase may contribute to the early Tau pathology in Down syndrome via phosphorylation of Tau and dysregulation of Tau exon 10. Here, we report that Dyrk1A was truncated at the C terminus and was associated with overactivation of calpain I in AD brain. Calpain I proteolyzed Dyrk1A in vitro first at the C terminus and further at the N terminus and enhanced its kinase activity toward Tau via increased Vmax but not Km. C-terminal truncation of Dyrk1A resulted in stronger activity than its full-length protein in promotion of exon 10 exclusion and phosphorylation of Tau. Dyrk1A was truncated in kainic acid-induced excitotoxic mouse brains and coincided with an increase in 3R-Tau expression and phosphorylation of Tau via calpain activation. Moreover, truncation of Dyrk1A was correlated with an increase in the ratio of 3R-Tau/4R-Tau and Tau hyperphosphorylation in AD brain. Collectively, these findings suggest that truncation/activation of Dyrk1A by Ca(2+)/calpain I might contribute to Tau pathology via promotion of exon 10 exclusion and hyperphosphorylation of Tau in AD brain.

Mitochondrial complex 1 inhibition increases 4-repeat isoform tau by SRSF2 upregulation

Progressive Supranuclear Palsy (PSP) is a neurodegenerative disorder characterised by intracellular aggregation of the microtubule-associated protein tau. The tau protein exists in 6 predominant isoforms. Depending on alternative splicing of exon 10, three of these isoforms have four microtubule-binding repeat domains (4R), whilst the others only have three (3R). In PSP there is an excess of the 4R tau isoforms, which are thought to contribute significantly to the pathological process. The cause of this 4R increase is so far unknown. Several lines of evidence link mitochondrial complex I inhibition to the pathogenesis of PSP. We demonstrate here for the first time that annonacin and MPP⁺, two prototypical mitochondrial complex I inhibitors, increase the 4R isoforms of tau in human neurons. We show that the splicing factor SRSF2 is necessary to increase 4R tau with complex I inhibition. We also found SRSF2, as well as another tau splicing factor, TRA2B, to be increased in brains of PSP patients. Thereby, we provide new evidence that mitochondrial complex I inhibition may contribute as an upstream event to the pathogenesis of PSP and suggest that splicing factors may represent an attractive therapeutic target to intervene in the disease process.

IUTA: a tool for effectively detecting differential isoform usage from RNA-Seq data

Background

Most genes in mammals generate several transcript isoforms that differ in stability and translational efficiency through alternative splicing. Such alternative splicing can be tissue- and developmental stage-specific, and such specificity is sometimes associated with disease. Thus, detecting differential isoform usage for a gene between tissues or cell lines/types (differences in the fraction of total expression of a gene represented by the expression of each of its isoforms) is potentially important for cell and developmental biology.

Results

We present a new method IUTA that is designed to test each gene in the genome for differential isoform usage between two groups of samples. IUTA also estimates isoform usage for each gene in each sample as well as averaged across samples within each group. IUTA is the first method to formulate the testing problem as testing for equal means of two probability distributions under the Aitchison geometry, which is widely recognized as the most appropriate geometry for compositional data (vectors that contain the relative amount of each component comprising the whole). Evaluation using simulated data showed that IUTA was able to provide test results for many more genes than was Cuffdiff2 (version 2.2.0, released in Mar. 2014), and IUTA performed better than Cuffdiff2 for the limited number of genes that Cuffdiff2 did analyze. When applied to actual mouse RNA-Seq datasets from six tissues, IUTA identified 2,073 significant genes with clear patterns of differential isoform usage between a pair of tissues. IUTA is implemented as an R package and is available at http://www.niehs.nih.gov/research/resources/software/biostatistics/iuta/index.cfm.

Conclusions

Both simulation and real-data results suggest that IUTA accurately detects differential isoform usage. We believe that our analysis of RNA-seq data from six mouse tissues represents the first comprehensive characterization of isoform usage in these tissues. IUTA will be a valuable resource for those who study the roles of alternative transcripts in cell development and disease.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-862) contains supplementary material, which is available to authorized users.

Regulation of alternative splicing of tau exon 10

The neuronal microtubule-associated protein tau is abnormally hyperphosphorylated and aggregated into neurofibrillary tangles in the brains of individuals with Alzheimer's disease and related neurodegenerative disorders. The adult human brain expresses six isoforms of tau generated by alternative splicing of exons 2, 3, and 10 of its pre-mRNA. Exon 10 encodes the second microtubule-binding repeat of tau. Its alternative splicing produces tau isoforms with either three or four microtubule-binding repeats, termed 3R-tau and 4Rtau. In the normal adult human brain, the level of 3R-tau is approximately equal to that of 4R-tau. Several silent and intronic mutations of the tau gene associated with FTDP-17T (frontotemporal dementia with Parkinsonism linked to chromosome 17 and specifically characterized by tau pathology) only disrupt exon 10 splicing, but do not influence the primary sequence of the tau protein. Thus, abnormal exon 10 splicing is sufficient to cause neurodegeneration and dementia. Here, we review the regulation of tau exon 10 splicing by cis-elements and trans-factors and summarize all the mutations associated with FTDP-17T and related tauopathies. The findings suggest that correction of exon 10 splicing may be a potential target for tau exon 10 splicing-related tauopathies.

Assessment of common variability and expression quantitative trait loci for genome-wide associations for progressive supranuclear palsy

Progressive supranuclear palsy is a rare parkinsonian disorder with characteristic neurofibrillary pathology consisting of hyperphosphorylated tau protein. Common variation defining the microtubule associated protein tau gene (MAPT) H1 haplotype strongly contributes to disease risk. A recent genome-wide association study (GWAS) revealed 3 novel risk loci on chromosomes 1, 2, and 3 that primarily implicate STX6, EIF2AK3, and MOBP, respectively. Genetic associations, however, rarely lead to direct identification of the relevant functional allele. More often, they are in linkage disequilibrium with the causative polymorphism(s) that could be a coding change or affect gene expression regulatory motifs. To identify any such changes, we sequenced all coding exons of those genes directly implicated by the associations in progressive supranuclear palsy cases and analyzed regional gene expression data from control brains to identify expression quantitative trait loci within 1 Mb of the risk loci. Although we did not find any coding variants underlying the associations, GWAS-associated single-nucleotide polymorphisms at these loci are in complete linkage disequilibrium with haplotypes that completely overlap with the respective genes. Although implication of EIF2AK3 and MOBP could not be fully assessed, we show that the GWAS single-nucleotide polymorphism rs1411478 (STX6) is a strong expression quantitative trait locus with significantly lower expression of STX6 in white matter in carriers of the risk allele.

Variation in tau isoform expression in different brain regions and disease states

Progressive supranuclear palsy (PSP) is the most common atypical parkinsonian disorder. Abnormal tau inclusions, in selected regions of the brain, are a hallmark of the disease and the H1 haplotype of MAPT, the gene encoding tau, is the major risk factor in PSP. A 3-repeat and 4-repeat (4R) tau isoform ratio imbalance has been strongly implicated as a cause of disease. Thus, understanding tau isoform regional expression in disease and pathology-free states is crucial to elucidating the mechanisms involved in PSP and other tauopathies. We used a tau isoform-specific fluorescent assay to investigate relative 4R-tau expression in 6 different brain regions in PSP cases and healthy control samples. We identified a marked difference in 4R-tau relative expression, across brain regions and between MAPT haplotypes. Highest 4R-tau expression levels were identified in the globus pallidus compared with pons, cerebellum, and frontal cortex. 4R-tau expression levels were related to the MAPT H1 and H1c haplotypes. Similar regional variation was seen in PSP case and in control samples.

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.

Role of tau protein in neuronal damage in Alzheimer's disease and Down syndrome

Neurodegenerative disorders constitute a growing concern worldwide. Their incidence has increased steadily, in particular among the elderly, a high-risk population that is becoming an important segment of society. Neurodegenerative mechanisms underlie many ailments such as Parkinson's disease, Huntington's disease, Alzheimer's disease (AD) and Down syndrome (DS, trisomy 21). Interestingly, there is increasing evidence suggesting that many such diseases share pathogenic mechanisms at the cellular and subcellular levels. These include altered protein misfolding, impaired autophagy, mitochondrial dysfunction, membrane damage, and altered axonal transport. Regarding AD and DS, the first common link comes from observations that DS patients undergo AD-like pathology early in adulthood. Also, the gene encoding for the amyloid precursor protein is present in human autosome 21 and in murine chromosome 16, an animal model of DS. Important functions related to preservation of normal neuronal architecture are impaired in both conditions. In particular, the stable assembly of microtubules, which is critical for the cytoskeleton, is impaired in AD and DS. In this process, tau protein plays a pivotal role in controlling microtubule stability. Abnormal tau expression and hyperphosphorylation are common features in both conditions, yet the mechanisms leading to these phenomena remain obscure. In the present report we review possible common mechanisms that may alter tau expression and function, in particular in relation to the effect of certain overexpressed DS-related genes, using cellular models of human DS. The latter contributes to the identification of possible therapeutic targets that could aid in the treatment of both AD and DS.

The role of MAPT sequence variation in mechanisms of disease susceptibility

The microtubule-associated protein tau (MAPT or tau) is of great interest in the field of neurodegeneration as there is a well-established genetic link between the MAPT gene locus and tauopathies, a diverse group of neurodegenerative dementias and movement disorders. The genomic architecture in the region spanning the MAPT locus contains a ~1.8 Mb block of linkage disequilibrium characterized by two major haplotypes: H1 and H2. Recent studies have established strong genetic association between the MAPT locus and neurodegenerative disease and uncovered haplotype-specific differences in expression and alternative splicing of MAPT transcripts. Integrating genetic association data and gene expression data to understand how non-coding genetic variation at a gene locus affects gene expression and leads to susceptibility to disease is a high priority in disease genetics, and the MAPT locus provides an excellent paradigm for this. In the absence of protein-coding changes caused by haplotype sequence variation, altered levels of protein expression or altered ratios of isoform expression are excellent candidate mechanisms to link the MAPT genetic disease association with biological function. The use of novel transgenic and endogenous genetic models are required to understand the role of MAPT sequence variation in mechanisms of disease susceptibility.

Hsp70 alters tau function and aggregation in an isoform specific manner

Tauopathies are characterized by abnormal aggregation of the microtubule associated protein tau. This aggregation is thought to occur when tau undergoes shifts from its native conformation to one that exposes hydrophobic areas on separate monomers, allowing contact and subsequent association into oligomers and filaments. Molecular chaperones normally function by binding to exposed hydrophobic stretches on proteins and assisting in their refolding. Chaperones of the heat shock protein 70 (Hsp70) family have been implicated in the prevention of abnormal tau aggregation in adult neurons. Tau exists as six alternatively spliced isoforms, and all six isoforms appear capable of forming the pathological aggregates seen in Alzheimer's disease. Because tau isoforms differ in primary sequence, we sought to determine whether Hsp70 would differentially affect the aggregation and microtubule assembly characteristics of the various tau isoforms. We found that Hsp70 inhibits tau aggregation directly and not through inducer-mediated effects. We also determined that Hsp70 inhibits the aggregation of each individual tau isoform and was more effective at inhibiting the three repeat isoforms. Finally, all tau isoforms robustly induced microtubule formation while in the presence of Hsp70. The results presented herein indicate that Hsp70 affects tau isoform dysfunction while having very little impact on the normal function of tau to mediate microtubule assembly. This indicates that targeting Hsp70 to tau may provide a therapeutic approach for the treatment of tauopathies that avoids disruption of normal tau function.

Neuropathological background of phenotypical variability in frontotemporal dementia

Frontotemporal lobar degeneration (FTLD) is the umbrella term encompassing a heterogeneous group of pathological disorders. With recent discoveries, the FTLDs have been show to classify nicely into three main groups based on the major protein deposited in the brain: FTLD-tau, FTLD-TDP and FTLD-FUS. These pathological groups, and their specific pathologies, underlie a number of well-defined clinical syndromes, including three frontotemporal dementia (FTD) variants [behavioral variant frontotemporal dementia (bvFTD), progressive non-fluent aphasia, and semantic dementia (SD)], progressive supranuclear palsy syndrome (PSPS) and corticobasal syndrome (CBS). Understanding the neuropathological background of the phenotypic variability in FTD, PSPS and CBS requires large clinicopathological studies. We review current knowledge on the relationship between the FTLD pathologies and clinical syndromes, and pool data from a number of large clinicopathological studies that collectively provide data on 544 cases. Strong relationships were identified as follows: FTD with motor neuron disease and FTLD-TDP; SD and FTLD-TDP; PSPS and FTLD-tau; and CBS and FTLD-tau. However, the relationship between some of these clinical diagnoses and specific pathologies is not so clear cut. In addition, the clinical diagnosis of bvFTD does not have a strong relationship to any FTLD subtype or specific pathology and therefore remains a diagnostic challenge. Some evidence suggests improved clinicopathological association of bvFTD by further refining clinical characteristics. Unlike FTLD-tau and FTLD-TDP, FTLD-FUS has been less well characterized, with only 69 cases reported. However, there appears to be some associations between clinical phenotypes and FTLD-FUS pathologies. Clinical diagnosis is therefore promising in predicting molecular pathology.

Cyclic AMP-dependent protein kinase regulates the alternative splicing of tau exon 10: a mechanism involved in tau pathology of Alzheimer disease

Hyperphosphorylation and deposition of tau into neurofibrillary tangles is a hallmark of Alzheimer disease (AD). Alternative splicing of tau exon 10 generates tau isoforms containing three or four microtubule binding repeats (3R-tau and 4R-tau), which are equally expressed in adult human brain. Dysregulation of exon 10 causes neurofibrillary degeneration. Here, we report that cyclic AMP-dependent protein kinase, PKA, phosphorylates splicing factor SRSF1, modulates its binding to tau pre-mRNA, and promotes tau exon 10 inclusion in cultured cells and in vivo in rat brain. PKA-Cα, but not PKA-Cβ, interacts with SRSF1 and elevates SRSF1-mediated tau exon 10 inclusion. In AD brain, the decreased level of PKA-Cα correlates with the increased level of 3R-tau. These findings suggest that a down-regulation of PKA dysregulates the alternative splicing of tau exon 10 and contributes to neurofibrillary degeneration in AD by causing an imbalance in 3R-tau and 4R-tau expression.

Tau gene promoter rs242557 and allele-specific protein binding

The H1 haplotype clade of the tau gene (MAPT) is associated with increased risk of the sporadic disorders, progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD) and to a lesser extent, Parkinson’s disease (PD). The H1c sub-haplotype drives this association in PSP and CBD, and is also weakly associated with Alzheimer’s disease (AD), suggesting involvement in common pathogenic pathway(s). The rs242557 single-nucleotide polymorphism (SNP) that defines H1c resides in a highly conserved repressor domain in the MAPT promoter. Previously, in cellular reporter assays, we showed significant rs242557 allele-specific differences in transcriptional repression, with the H1c-specific rs242557/A allele contributing a significantly higher MAPT promoter activity compared to the non-H1c rs242557/G allele. With evidence of allele-specific differences in protein binding to this repressor domain, we set out to identify those proteins that bind to this region. Electrophoretic mobility shift assay (EMSA) analysis strongly suggested allele-specific differences in protein affinities. In order to identify nuclear proteins that differentially bind to this repressor domain, we carried out a promoter-trap assay and analysed the bound proteins by SDS-PAGE and HPLC ESI-QTOF mass spectrometry. We identified 37 proteins and used bioinformatic tools such as STRING and Reactome to analyse and stratify the results. These included U2AF65, hnRNPU, PTBP1, hnRNPD0, U5 snRNP 116, ALY, HMGB2, H1 and actin and provide the basis for further studies of the role of the MAPT repressor domain and the binding proteins in regulating MAPT gene transcription and splicing.

Link between DYRK1A overexpression and several-fold enhancement of neurofibrillary degeneration with 3-repeat tau protein in Down syndrome

Triplication of chromosome 21 in Down syndrome (DS) results in overexpression of the minibrain kinase/dual-specificity tyrosine phosphorylated and regulated kinase 1A gene (DYRK1A). DYRK1A phosphorylates cytoplasmic tau protein and appears in intraneuronal neurofibrillary tangles (NFTs). We have previously shown significantly more DYRK1A-positive NFTs in DS brains than in sporadic Alzheimer disease (AD) brains. This study demonstrates a gene dosage-proportional increase in the level of DYRK1A in DS in the cytoplasm and the cell nucleus, and enhanced cytoplasmic and nuclear immunoreactivity of DYRK1A in DS. The results suggest that overexpressed DYRK1A may alter both phosphorylation of tau and alternative splicing factor (ASF). Two-dimensional electrophoresis revealed modification of ASF phosphorylation in DS/AD and AD in comparison to controls. Altered phosphorylation of ASF by overexpressed nuclear DYRK1A may contribute to the alternative splicing of the tau gene and an increase by 2.68 × of the 3R/4R ratio in DS/AD, and a several-fold increase in the number of 3R tau-positive NFTs in DS/AD subjects compared with that in sporadic AD subjects. These data support the hypothesis that phosphorylation of ASF by overexpressed DYRK1A may contribute to alternative splicing of exon 10, increased expression of 3R tau, and early onset of neurofibrillary degeneration in DS.

Using non-uniform read distribution models to improve isoform expression inference in RNA-Seq

MOTIVATION

RNA-Seq technology based on next-generation sequencing provides the unprecedented ability of studying transcriptomes at high resolution and accuracy, and the potential of measuring expression of multiple isoforms from the same gene at high precision. Solved by maximum likelihood estimation, isoform expression can be inferred in RNA-Seq using statistical models based on the assumption that sequenced reads are distributed uniformly along transcripts. Modification of the model is needed when considering situations where RNA-Seq data do not follow uniform distribution.

RESULTS

We proposed two curves, the global bias curve (GBC) and the local bias curves (LBCs), to describe the non-uniformity of read distributions for all genes in a transcriptome and for each gene, respectively. Incorporating the bias curves into the uniform read distribution (URD) model, we introduced non-URD (N-URD) models to infer isoform expression levels. On a series of systematic simulation studies, the proposed models outperform the original model in recovering major isoforms and the expression ratio of alternative isoforms. We also applied the new model to real RNA-Seq datasets and found that its inferences on expression ratios of alternative isoforms are more reasonable. The experiments indicate that incorporating N-URD information can improve the accuracy in modeling and inferring isoform expression in RNA-Seq.

Development of a grape seed polyphenolic extract with anti-oligomeric activity as a novel treatment in progressive supranuclear palsy and other tauopathies

A diverse group of neurodegenerative diseases - including progressive supranuclear palsy (PSP), corticobasal degeneration and Alzheimer's disease among others, collectively referred to as tauopathies - are characterized by progressive, age-dependent intracellular formations of misfolded protein aggregates that play key roles in the initiation and progression of neuropathogenesis. Recent studies from our laboratory reveal that grape seed-derived polyphenolic extracts (GSPE) potently prevent tau fibrillization into neurotoxic aggregates and therapeutically promote the dissociation of preformed tau aggregates [J. Alzheimer's Dis. (2009) vol. 16, pp. 433]. Based on our extensive bioavailability, bioactivity and functional preclinical studies, combined with the safety of GSPE in laboratory animals and in humans, we initiated a series of studies exploring the role of GSPE (Meganatural-Az(®) GSPE) as a potential novel botanical drug for the treatment of certain forms of tauopathies including PSP, a neurodegenerative disorder involving the accumulation and deposition of misfolded tau proteins in the brain characterized, in part, by abnormal intracellular tau inclusions in specific anatomical areas involving astrocytes, oligodendrocytes and neurons [J. Neuropathol. Exp. Neurol. (2002) vol. 61, pp. 33]. In this mini-review article, we discuss the biochemical characterization of GSPE in our laboratory and its potential preventative and therapeutic role in model systems of abnormal tau processing pertinent to PSP and related tauopathies.

Tau and neurodegenerative disorders

The mechanisms that render tau a toxic agent are still unclear, although increasing evidence supports the assertion that alterations of tau can directly cause neuronal degeneration. In addition, it is unclear whether neurodegeneration in various tauopathies occurs via a common mechanism or that specific differences exist. The aim of this review is to provide an overview of tauopathies from bench to bedside. The review begins with clinicopathological findings of familial and sporadic tauopathies. It includes a discussion of the similarities and differences between these two conditions. The second part concentrates on biochemical alterations of tau such as phosphorylation, truncation and acetylation. Although pathological phosphorylation of tau has been studied for many years, recently researchers have focused on the physiological role of tau during development. Finally, the review contains a summary of the significance of tauopathy model mice for research on neurofibrillary tangles, axonopathies, and synaptic alteration.

Brain tau isoform mRNA and protein correlation in PSP brain

Insoluble aggregates of the microtubule associated protein, tau are pathological hallmarks of several neurodegenerative diseases, including Alzheimer’s disease (AD), called tauopathies. The tau gene (MAPT) is alternatively spliced and the composition of resulting protein isoforms in aggregates is disease specific. Progressive supranuclear palsy (PSP) is characterised by tangles predominantly containing isoforms with four microtubule binding repeat domains (4R-tau) suggesting that changes in isoform-specific mRNA expression play a role the pathogenesis of the disease. This is supported by the genetics of MAPT. In this study, we quantified expression of 3R- and 4R-tau isoforms at both the mRNA and protein levels in the caudate nucleus, a region severely affected by tau pathology. Results from real-time qPCR and a recently developed ELISA showed statistically significant increase in 4R-tau isoforms in PSP samples compared to controls. In addition, we measured soluble and insoluble hyperphosphorylated tau protein fractions in each PSP sample and compared to the corresponding mRNA transcript levels. No strong correlations were observed with either 3R- or 4R-tau. These findings confirmed the increased ratio of 4R-tau:3R-tau isoforms in PSP. However, we did not find a direct quantitative relationship between individual mRNA and protein levels suggesting a more complex regulation of isoform expression at the post-transcriptional level.

Different MAPT haplotypes are associated with Parkinson's disease and progressive supranuclear palsy

The H1 MAPT haplotype in the 17q21 chromosomal region has been associated with several neurodegenerative diseases. Some reports have suggested that there is an association between genetic variants within the H1 haplotype with Parkinson's disease (PD), progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD). Here we report a genetic association study using seven SNPs located along the 17q21 region, in PD patients and controls. In addition, we compared these results with a dataset of previously published PSP/CBD patients from the same population. Our results show that the H1-rs242557(G) allele sub-haplotype is increased in PD (p=0.005), while the H1-rs242557(A) allele sub-haplotype is increased in PSP/CBD (p=0.0002), comparing to controls. The rs242557 polymorphism could act modulating the phenotypic expressivity of the H1 risk on these parkinsonisms. The location of this polymorphism in the 5' regulatory region of MAPT gene suggests the presence of a functional mechanism involved in the variation of MAPT expression levels.

The role of tau in neurodegeneration

Since the identification of tau as the main component of neurofibrillary tangles in Alzheimer's disease and related tauopathies, and the discovery that mutations in the tau gene cause frontotemporal dementia, much effort has been directed towards determining how the aggregation of tau into fibrillar inclusions causes neuronal death. As evidence emerges that tau-mediated neuronal death can occur even in the absence of tangle formation, a growing number of studies are focusing on understanding how abnormalities in tau (e.g. aberrant phosphorylation, glycosylation or truncation) confer toxicity. Though data obtained from experimental models of tauopathies strongly support the involvement of pathologically modified tau and tau aggregates in neurodegeneration, the exact neurotoxic species remain unclear, as do the mechanism(s) by which they cause neuronal death. Nonetheless, it is believed that tau-mediated neurodegeneration is likely to result from a combination of toxic gains of function as well as from the loss of normal tau function. To truly appreciate the detrimental consequences of aberrant tau function, a better understanding of all functions carried out by tau, including but not limited to the role of tau in microtubule assembly and stabilization, is required. This review will summarize what is currently known regarding the involvement of tau in the initiation and development of neurodegeneration in tauopathies, and will also highlight some of the remaining questions in need of further investigation.

Knock-out and transgenic mouse models of tauopathies

Tauopathies, characterized by the dysfunction and aggregation of the microtubule-associated protein tau (MAPT), represent some of the most devastating neurodegenerative disorders afflicting the elderly, including Alzheimer's disease and progressive supranuclear palsy. Here we review the range of Mapt knock-out and MAPT transgenic mouse models which have proven successful at providing insights into the molecular mechanisms of neurodegenerative disease. In this overview we highlight several themes, including the insights such models provide into the cellular and molecular mechanisms of tauopathy, the direct relationship between neuropathology and behaviour, and the use of mouse models to help provide a platform for testing novel therapies. Mouse models have helped clarify the relationship between pathological forms of tau, cell death, and the emergence of disease, as well as the interaction between tau and other disease-associated molecules, such as the A beta peptide. Finally, we discuss potential future MAPT genomic DNA models to investigate the importance of alternative splicing of the MAPT locus and its role in sporadic tauopathies.

mTOR-dependent signalling in Alzheimer's disease

Neurodegeneration and neurofibrillary degeneration are the two main pathological mechanisms of cognitive impairments in Alzheimer's disease (AD). It is not clear what factors determine the fates of neurons during the progress of the disease. Emerging evidence has suggested that mTOR-dependent signalling is involved in the two types of degeneration in AD brains. This review focuses on the roles of mTOR-dependent signalling in the pathogenesis of AD. It summarizes the recent advancements in the understanding of its roles in neurodegeneration and neurofibrillary degeneration, as well as the evidence achieved when mTOR-related signalling components were tested as potential biomarkers of cognitive impairments in the clinical diagnosis of AD.

High throughput lipidomic profiling of schizophrenia and bipolar disorder brain tissue reveals alterations of free fatty acids, phosphatidylcholines, and ceramides

A mass spectrometry based high throughput approach was employed to profile white and gray matter lipid levels in the prefrontal cortex (Brodmann area 9) of 45 subjects including 15 schizophrenia and 15 bipolar disorder patients as well as 15 controls samples. We found statistically significant alterations in levels of free fatty acids and phosphatidylcholine in gray and white matter of both schizophrenia and bipolar disorder samples compared to controls. Also, ceramides were identified to be significantly increased in white matter of both neuropsychiatric disorders as compared to control levels. The patient cohort investigated in this study includes a number of drug naive as well as untreated patients, allowing the assessment of drug effects on lipid levels. Our findings indicate that while gray matter phosphatidylcholine levels were influenced by antipsychotic medication, this was not the case for phosphatidylcholine levels in white matter. Changes in free fatty acids or ceramides in either white or gray matter also did not appear to be influenced by antipsychotic treatment. To assess lipid profiles in the living patient, we also profiled lipids of 40 red blood cell samples, including 7 samples from drug naive first onset patients. We found significant alterations in the concentrations of free fatty acids as well as ceramide. Overall, our findings suggest that lipid abnormalities may be a disease intrinsic feature of both schizophrenia and bipolar disorder reflected by significant changes in the central nervous system as well as peripheral tissues.

Tau exon 10 alternative splicing and tauopathies

Abnormalities of microtubule-associated protein tau play a central role in neurofibrillary degeneration in several neurodegenerative disorders that collectively called tauopathies. Six isoforms of tau are expressed in adult human brain, which result from alternative splicing of pre-mRNA generated from a single tau gene. Alternative splicing of tau exon 10 results in tau isoforms containing either three or four microtubule-binding repeats (3R-tau and 4R-tau, respectively). Approximately equal levels of 3R-tau and 4R-tau are expressed in normal adult human brain, but the 3R-tau/4R-tau ratio is altered in the brains in several tauopathies. Discovery of silence mutations and intronic mutations of tau gene in some individuals with frontotemporal dementia with Parkinsonism linked to chromosome 17 (FTDP-17), which only disrupt tau exon 10 splicing but do not alter tau's primary sequence, demonstrates that dysregulation of tau exon 10 alternative splicing and consequently of 3R-tau/4R-tau balance is sufficient to cause neurodegeneration and dementia. Here, we review the gene structure, transcripts and protein isoforms of tau, followed by the regulation of exon 10 splicing that determines the expression of 3R-tau or 4R-tau. Finally, dysregulation of exon 10 splicing of tau in several tauopathies is discussed. Understanding the molecular mechanisms by which tau exon 10 splicing is regulated and how it is disrupted in tauopathies will provide new insight into the mechanisms of these tauopathies and help identify new therapeutic targets to treat these disorders.

Tau-based treatment strategies in neurodegenerative diseases

Neurofibrillary tangles are a characteristic hallmark of Alzheimer's and other neurodegenerative diseases, such as Pick's disease (PiD), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), and frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). These diseases are summarized as tauopathies, because neurofibrillary tangles are composed of intracellular aggregates of the microtubule-associated protein tau. The molecular mechanisms of tau-mediated neurotoxicity are not well understood; however, pathologic hyperphosphorylation and aggregation of tau play a central role in neurodegeneration and neuronal dysfunction. The present review, therefore, focuses on therapeutic approaches that aim to inhibit tau phosphorylation and aggregation or to dissolve preexisting tau aggregates. Further experimental therapy strategies include the enhancement of tau clearance by activation of proteolytic, proteasomal, or autophagosomal degradation pathways or anti-tau directed immunotherapy. Hyperphosphorylated tau does not bind microtubules, leading to microtubule instability and transport impairment. Pharmacological stabilization of microtubule networks might counteract this effect. In several tauopathies there is a shift toward four-repeat tau isoforms, and interference with the splicing machinery to decrease four-repeat splicing might be another therapeutic option.

Increase in the relative expression of tau with four microtubule binding repeat regions in frontotemporal lobar degeneration and progressive supranuclear palsy brains

Some cases of familial frontotemporal dementia (FTD) leading to frontotemporal lobar degeneration (FTLD) are caused by mutations in tau on chromosome 17 (FTDP-17). Certain mutations alter the ratio between four (4R tau) and three (3R tau) repeat tau isoforms whereas cases with progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD) mainly have 4R tau brain pathology. We assessed tau mRNA and protein levels in frontal cortex from 15 sporadic FTLD, 21 PSP, 5 CBD, 15 Alzheimer's disease (AD) and 16 control brains. Moreover, we investigated the disease association and possible tau splicing effects of the tau H1 haplotype. Cases with FTLD and PSP had lower tau mRNA levels than control brains. When analyzing 4R tau and 3R tau mRNA separately, control subjects displayed a 4R tau/3R tau ratio of 0.48. Surprisingly, FTLD brains displayed a more elevated ratio (1.32) than PSP brains (1.12). Also, several FTLD and PSP cases had higher 4R tau/3R tau mRNA than FTDP-17 cases, included as reference tissues, and the ratio increase was seen regardless of underlying histopathology, i.e. both for tau-positive and tau-negative FTLD cases. Furthermore, total tau protein levels were slightly decreased in both FTLD and AD as compared to control subjects. Finally, we confirmed the association of tau H1 with PSP, but could not find any haplotype-related effect on tau exon 10 splicing. In conclusion, we demonstrated increased but largely variable 4R tau/3R tau mRNA ratios in FTLD and PSP cases, suggesting heterogeneous pathophysiological processes within these disorders.

Single molecule profiling of tau gene expression in Alzheimer's disease

Tau is a microtubule-associated protein that is important for establishing and maintaining neuronal morphology. In addition to its role in normal cells, tau protein is involved in many neurodegenerative diseases, e.g. Alzheimer's disease (AD) and frontotemporal dementia, as the main component of intraneuronal aggregates. Alternative splicing of tau gene in the brain can give rise to at least six protein variants. A causative role of skewed tau exon 10 inclusion has been defined in frontotemporal dementia; however, no link was established between the aberrant splicing of tau and AD. Here, we applied a single-molecule-based technology, polymerase colony or polony, to simultaneously monitor tau splicing variant and haplotype profile in sporadic AD and normal brains. We found that the coordinated expression of tau exons 2 and 10 is altered in AD. Additional investigations of cis and trans mechanisms of this observation revealed a decreased protein expression of a known tau splicing factor, htra2-beta-1 in AD, thereby implicating a trans mechanism. Our results demonstrate that dysregulation of combinatorial splicing might serve as a signature for aging-related diseases, and the polony assay could be widely adapted for the study of other tauopathies. Furthermore, splicing-based therapeutics is an emerging area of drug development, and a well-defined and quantitative assay for monitoring single-gene transcriptome will be relevant for such development.

Progressive supranuclear palsy (PSP): a quantitative study of the pathological changes in cortical and subcortical regions of eight cases

In eight cases of progressive supranuclear palsy (PSP), neurofibrillary tangles (NFT) were numerous in the substantia nigra (SN), red nucleus (RN), locus caeruleus (LC), pontine nuclei (PN), and inferior olivary nucleus (ION) and abnormally enlarged neurons (EN) in the ION, LC and PN. Loss of Purkinje cells was evident in the cerebellum. Tufted astrocytes (TA) were abundant in the striatum, SN and RN and glial inclusions ('coiled bodies') (GI) in the midbrain (SN, RN) and pons (LC). Neuritic plaques were frequent in one case. NFT, GI, and TA densities were uncorrelated in most areas. NFT and EN densities were positively correlated in the midbrain and surviving neurons and disease duration in several areas. These results suggest: 1) predominantly subcortical pathology in PSP with widespread NFT while TA and GI have a more localized distribution, 2) little correlation between neuronal and glial pathologies, and 3) shorter duration cases may be more likely to develop cortical pathology.