Morphological and biochemical correlations of abnormal tau filaments in progressive supranuclear palsy

Whole-Genome Sequencing Analysis Reveals New Susceptibility Loci and Structural Variants Associated with Progressive Supranuclear Palsy

Background

Progressive supranuclear palsy (PSP) is a rare neurodegenerative disease characterized by the accumulation of aggregated tau proteins in astrocytes, neurons, and oligodendrocytes. Previous genome-wide association studies for PSP were based on genotype array, therefore, were inadequate for the analysis of rare variants as well as larger mutations, such as small insertions/deletions (indels) and structural variants (SVs).

Method

In this study, we performed whole genome sequencing (WGS) and conducted association analysis for single nucleotide variants (SNVs), indels, and SVs, in a cohort of 1,718 cases and 2,944 controls of European ancestry. Of the 1,718 PSP individuals, 1,441 were autopsy-confirmed and 277 were clinically diagnosed.

Results

Our analysis of common SNVs and indels confirmed known genetic loci at MAPT, MOBP, STX6, SLCO1A2, DUSP10, and SP1, and further uncovered novel signals in APOE, FCHO1/MAP1S, KIF13A, TRIM24, TNXB, and ELOVL1. Notably, in contrast to Alzheimer's disease (AD), we observed the APOE ε2 allele to be the risk allele in PSP. Analysis of rare SNVs and indels identified significant association in ZNF592 and further gene network analysis identified a module of neuronal genes dysregulated in PSP. Moreover, seven common SVs associated with PSP were observed in the H1/H2 haplotype region (17q21.31) and other loci, including IGH, PCMT1, CYP2A13, and SMCP. In the H1/H2 haplotype region, there is a burden of rare deletions and duplications (P = 6.73×10-3) in PSP.

Conclusions

Through WGS, we significantly enhanced our understanding of the genetic basis of PSP, providing new targets for exploring disease mechanisms and therapeutic interventions.

Rainwater Charitable Foundation criteria for the neuropathologic diagnosis of progressive supranuclear palsy

Neuropathologic criteria for progressive supranuclear palsy (PSP) proposed by a National Institute of Neurological Disorders and Stroke (NINDS) working group were published in 1994 and based on the presence of neurofibrillary tangles in basal ganglia and brainstem. These criteria did not stipulate detection methods or incorporate glial tau pathology. In this study, a group of 14 expert neuropathologists scored digital slides from 10 brain regions stained with hematoxylin and eosin (H&E) and phosphorylated tau (AT8) immunohistochemistry. The cases included 15 typical and atypical PSP cases and 10 other tauopathies. Blinded to clinical and neuropathological information, raters provided a categorical diagnosis (PSP or not-PSP) based upon provisional criteria that required neurofibrillary tangles or pretangles in two of three regions (substantia nigra, subthalamic nucleus, globus pallidus) and tufted astrocytes in one of two regions (peri-Rolandic cortices, putamen). The criteria showed high sensitivity (0.97) and specificity (0.91), as well as almost perfect inter-rater reliability for diagnosing PSP and differentiating it from other tauopathies (Fleiss kappa 0.826). Most cases (17/25) had 100% agreement across all 14 raters. The Rainwater Charitable Foundation criteria for the neuropathologic diagnosis of PSP feature a simplified diagnostic algorithm based on phosphorylated tau immunohistochemistry and incorporate tufted astrocytes as an essential diagnostic feature.

A critical appraisal of tau-targeting therapies for primary and secondary tauopathies

INTRODUCTION

Primary tauopathies are neurological disorders in which tau protein deposition is the predominant pathological feature. Alzheimer's disease is a secondary tauopathy with tau forming hyperphosphorylated insoluble aggregates. Tau pathology can propagate from region to region in the brain, while alterations in tau processing may impair tau physiological functions.

METHODS

We reviewed literature on tau biology and anti-tau drugs using PubMed, meeting abstracts, and ClnicalTrials.gov.

RESULTS

The past 15 years have seen >30 drugs interfering with tau aggregation, processing, and accumulation reaching the clinic. Initial results with tau aggregation inhibitors and anti-tau monoclonal antibodies have not shown clinical efficacy.

DISCUSSION

The reasons for these clinical failures are unclear but could be linked to the clearing of physiological forms of tau by non-specific drugs. Research is now concentrating efforts on developing reliable translational animal models and selective compounds targeting specific tau epitopes, neurotoxic tau aggregates, and post-translational tau modifications.

Maturation of neuronal AD-tau pathology involves site-specific phosphorylation of cytoplasmic and synaptic tau preceding conformational change and fibril formation

In Alzheimer's disease (AD), tau-protein undergoes a multi-step process involving the transition from a natively unfolded monomer to large, aggregated structures such as neurofibrillary tangles (NFTs). However, it is not yet clear which events initiate the early preclinical phase of AD tauopathy and whether they have impact on the propagation of tau pathology in later disease stages. To address this question, we analyzed the distribution of tau species phosphorylated at T231, S396/S404 and S202/T205, conformationally modified at the MC1 epitope and fibrillary tau detected by the Gallyas method (Gallyas-tau), in the brains of 15 symptomatic and 20 asymptomatic cases with AD pathology as well as of 19 nonAD cases. As initial tau lesions, we identified phosphorylated-T231-tau diffusely distributed within the somatodendritic compartment (IC-tau) and phosphorylated-S396/pS404-tau in axonal lesions of the white matter and in the neuropil (IN-tau). The subcellular localization of pT231-tau in the cell body and pS396/pS404-tau in the presynapse was confirmed in hP301L mutant Drosophila larvae. Phosphorylated-S202/T205-tau, MC1-tau and Gallyas-tau were negative for these lesions. IC- and IN-tau were observed in all analyzed regions of the human brain, including early affected regions in nonAD cases (entorhinal cortex) and late affected regions in symptomatic AD cases (cerebellum), indicating that tau pathology initiation follows similar processes when propagating into previously unaffected regions. Furthermore, a sequence of AD-related maturation of tau-aggregates was observed, initiated by the appearance of IC- and IN-tau, followed by the formation of pretangles exhibiting pT231-tau, pS396/pS404-tau and pS202/pT205-tau, then by MC1-conformational tau, and, finally, by the formation of Gallyas-positive NFTs. Since cases classified as nonAD [Braak NFT stages < I (including a-1b)] already showed IC- and IN-tau, our findings suggest that these lesions are a prerequisite for the development of AD.

18F-flortaucipir PET to autopsy comparisons in Alzheimer's disease and other neurodegenerative diseases

Few studies have evaluated the relationship between in vivo18F-flortaucipir PET and post-mortem pathology. We sought to compare antemortem 18F-flortaucipir PET to neuropathology in a consecutive series of patients with a broad spectrum of neurodegenerative conditions. Twenty patients were included [mean age at PET 61 years (range 34-76); eight female; median PET-to-autopsy interval of 30 months (range 4-59 months)]. Eight patients had primary Alzheimer's disease pathology, nine had non-Alzheimer tauopathies (progressive supranuclear palsy, corticobasal degeneration, argyrophilic grain disease, and frontotemporal lobar degeneration with MAPT mutations), and three had non-tau frontotemporal lobar degeneration. Using an inferior cerebellar grey matter reference, 80-100-min 18F-flortaucipir PET standardized uptake value ratio (SUVR) images were created. Mean SUVRs were calculated for progressive supranuclear palsy, corticobasal degeneration, and neurofibrillary tangle Braak stage regions of interest, and these values were compared to SUVRs derived from young, non-autopsy, cognitively normal controls used as a standard for tau negativity. W-score maps were generated to highlight areas of increased tracer retention compared to cognitively normal controls, adjusting for age as a covariate. Autopsies were performed blinded to PET results. There was excellent correspondence between areas of 18F-flortaucipir retention, on both SUVR images and W-score maps, and neurofibrillary tangle distribution in patients with primary Alzheimer's disease neuropathology. Patients with non-Alzheimer tauopathies and non-tau frontotemporal lobar degeneration showed a range of tracer retention that was less than Alzheimer's disease, though higher than age-matched, cognitively normal controls. Overall, binding across both tau-positive and tau-negative non-Alzheimer disorders did not reliably correspond with post-mortem tau pathology. 18F-flortaucipir SUVRs in subcortical regions were higher in autopsy-confirmed progressive supranuclear palsy and corticobasal degeneration than in controls, but were similar to values measured in Alzheimer's disease and tau-negative neurodegenerative pathologies. Quantification of 18F-flortaucipir SUVR images at Braak stage regions of interest reliably detected advanced Alzheimer's (Braak VI) pathology. However, patients with earlier Braak stages (Braak I-IV) did not show elevated tracer uptake in these regions compared to young, tau-negative controls. In summary, PET-to-autopsy comparisons confirm that 18F-flortaucipir PET is a reliable biomarker of advanced Braak tau pathology in Alzheimer's disease. The tracer cannot reliably differentiate non-Alzheimer tauopathies and may not detect early Braak stages of neurofibrillary tangle pathology.

Amyloid Evolution: Antiparallel Replaced by Parallel

Several atomic structures have now been found for micrometer-scale amyloid fibrils or elongated microcrystals using a range of methods, including NMR, electron microscopy, and X-ray crystallography, with parallel β-sheet appearing as the most common secondary structure. The etiology of amyloid disease, however, indicates nanometer-scale assemblies of only tens of peptides as significant agents of cytotoxicity and contagion. By combining solution X-ray with molecular dynamics, we show that antiparallel structure dominates at the first stages of aggregation for a specific set of peptides, being replaced by parallel at large length scales only. This divergence in structure between small and large amyloid aggregates should inform future design of molecular therapeutics against nucleation or intercellular transmission of amyloid. Calculations and an overview from the literature argue that antiparallel order should be the first appearance of structure in many or most amyloid aggregation processes, regardless of the endpoint. Exceptions to this finding should exist, depending inevitably on the sequence and on solution conditions.

Disease-modifying therapies for tauopathies: agents in the pipeline

INTRODUCTION

Tauopathies are heterogeneous clinicopathological entities characterized by abnormal neuronal and/or glial inclusions of the microtubule-binding protein tau. Primary tauopathies considered to be diseases correspond to a major class of frontotemporal lobar degeneration (FTLD) neuropathology (FTLD-Tau), including several forms of frontotemporal dementia (FTD) clinical syndromes. Little progress has been made in the past 20 years in developing effective disease-modifying drugs for primary tauopathies and available symptomatic treatments have limited efficacy. Areas covered: Potential disease-modifying drugs in clinical development to slow neuropathological progression of primary tauopathies. Expert opinion: Since the underlying pathology of primary tauopathies consists of abnormal tau protein aggregates, treatments are being developed to interfere with the aggregation process or to promote the clearance of this protein. Unfortunately, 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. Further evidence will come from ongoing Phase I/II trials on novel drugs and immunotherapeutics with various targets - prevention of deposition or removal of tau aggregates, inhibition of tau phosphorylation/acetylation, modulation of O-GlcNAcylation, activation of autophagy or ubiquitin-proteasome system pathways, and rescue of selected tau loss of function or suppression of tau gene expression.

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.

Dysfunction of the ubiquitin ligase E3A Ube3A/E6-AP contributes to synaptic pathology in Alzheimer's disease

Synaptic dysfunction and synapse loss are prominent features in Alzheimer's disease. Members of the Rho-family of guanosine triphosphatases, specifically RhoA, and the synaptic protein Arc are implicated in these pathogenic processes. They share a common regulatory molecule, the E3 ligase Ube3A/E6-AP. Here, we show that Ube3A is reduced in an Alzheimer's disease mouse model, Tg2576 mouse, which overexpresses human APP695 carrying the Swedish mutation, and accumulates Aβ in the brain. Depletion of Ube3A precedes the age-dependent behavioral deficits and loss of dendritic spines in these mice, and results from a decrease in solubility following phosphorylation by c-Abl, after Aβ exposure. Loss of Ube3A triggers the accumulation of Arc and Ephexin-5, driving internalization of GluR1, and activation of RhoA, respectively, culminating in pruning of synapses, which is blocked by restoring Ube3A. Taken together, our results place Ube3A as a critical player in Alzheimer's disease pathogenesis, and as a potential therapeutic target.

Divergent brain gene expression patterns associate with distinct cell-specific tau neuropathology traits in progressive supranuclear palsy

Progressive supranuclear palsy (PSP) is a neurodegenerative parkinsonian disorder characterized by tau pathology in neurons and glial cells. Transcriptional regulation has been implicated as a potential mechanism in conferring disease risk and neuropathology for some PSP genetic risk variants. However, the role of transcriptional changes as potential drivers of distinct cell-specific tau lesions has not been explored. In this study, we integrated brain gene expression measurements, quantitative neuropathology traits and genome-wide genotypes from 268 autopsy-confirmed PSP patients to identify transcriptional associations with unique cell-specific tau pathologies. We provide individual transcript and transcriptional network associations for quantitative oligodendroglial (coiled bodies = CB), neuronal (neurofibrillary tangles = NFT), astrocytic (tufted astrocytes = TA) tau pathology, and tau threads and genomic annotations of these findings. We identified divergent patterns of transcriptional associations for the distinct tau lesions, with the neuronal and astrocytic neuropathologies being the most different. We determined that NFT are positively associated with a brain co-expression network enriched for synaptic and PSP candidate risk genes, whereas TA are positively associated with a microglial gene-enriched immune network. In contrast, TA is negatively associated with synaptic and NFT with immune system transcripts. Our findings have implications for the diverse molecular mechanisms that underlie cell-specific vulnerability and disease risk in PSP.

Variation at the TRIM11 locus modifies progressive supranuclear palsy phenotype

Objective

The basis for clinical variation related to underlying progressive supranuclear palsy (PSP) pathology is unknown. We performed a genome‐wide association study (GWAS) to identify genetic determinants of PSP phenotype.

Methods

Two independent pathological and clinically diagnosed PSP cohorts were genotyped and phenotyped to create Richardson syndrome (RS) and non‐RS groups. We carried out separate logistic regression GWASs to compare RS and non‐RS groups and then combined datasets to carry out a whole cohort analysis (RS = 367, non‐RS = 130). We validated our findings in a third cohort by referring to data from 100 deeply phenotyped cases from a recent GWAS. We assessed the expression/coexpression patterns of our identified genes and used our data to carry out gene‐based association testing.

Results

Our lead single nucleotide polymorphism (SNP), rs564309, showed an association signal in both cohorts, reaching genome‐wide significance in our whole cohort analysis (odds ratio = 5.5, 95% confidence interval = 3.2–10.0, p = 1.7 × 10⁻⁹). rs564309 is an intronic variant of the tripartite motif‐containing protein 11 (TRIM11) gene, a component of the ubiquitin proteasome system (UPS). In our third cohort, minor allele frequencies of surrogate SNPs in high linkage disequilibrium with rs564309 replicated our findings. Gene‐based association testing confirmed an association signal at TRIM11. We found that TRIM11 is predominantly expressed neuronally, in the cerebellum and basal ganglia.

Interpretation

Our study suggests that the TRIM11 locus is a genetic modifier of PSP phenotype and potentially adds further evidence for the UPS having a key role in tau pathology, therefore representing a target for disease‐modifying therapies. Ann Neurol 2018;84:485–496

4',6-Diamidino-2-Phenylindole Distinctly Labels Tau Deposits

Tau deposits have distinct biochemical characteristics and vary morphologically based on identification with tau antibodies and several chemical dyes. Here, we report 4',6-diamidino-2-phenylindole (DAPI)-positivity of tau deposits. Furthermore, we investigated the cause for this positivity. DAPI was positive in 3R/4R (3-repeat/4-repeat) tau deposits in Alzheimer's disease, myotonic dystrophy, and neurodegeneration with brain iron accumulation, and in 4R tau deposits in corticobasal degeneration, but negative in 4R tau deposits in frontotemporal dementia with parkinsonism-17 and progressive supranuclear palsy. The peak emission wavelength of DAPI after binding to a tau deposit was similar to that after binding to a nucleus. This DAPI-positivity was conspicuous at the optimum concentration of 2 μg/ml. DAPI-positivity was diminished after formic acid treatment, but preserved after nucleic acid elimination and phosphate moiety blocking. Our results suggest that staining with 2 μg/ml DAPI is a common but useful tool to differentially detect tau deposits in various tauopathies.

New advances in tau imaging in parkinsonism

Currently, the differential diagnosis between atypical parkinsonisms and classical idiopathic Parkinson's disease can be quite difficult because of the significant overlap of clinical presentation and symptoms. Neurodegenerative conditions, including progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), and frontotemporal dementia (FTD), are primarily characterized by accumulation of tau protein in the brain. Recent imaging developments for tau pathology may provide a promising tool for the assessment of diagnosis, prognosis, and progression of these neurodegenerative disorders. This review will survey PET studies to describe the recent advances in the imaging of tau pathology in PSP, CBD, and FTD.

Imaging tau pathology in Parkinsonisms

The recent development of positron emission tomography radiotracers targeting pathological tau in vivo has led to numerous human trials. While investigations have primarily focused on the most common tauopathy, Alzheimer's disease, it is imperative that testing also be performed in parkinsonian tauopathies, such as progressive supranuclear palsy, corticobasal degeneration, and frontotemporal dementia and parkinsonism linked to chromosome 17. Tau aggregates differ in isoforms and conformations across disorders, and as a result one radiotracer may not be appropriate for all tauopathies. In this review, we evaluate the preclinical and clinical reports of current tau radiotracers in parkinsonian disorders. These radiotracers include [18F]FDDNP, [11C]PBB3, [18F]THK-5317, [18F]THK-5351, and [18F]AV-1451 ([18F]T807). There are concerns of off-target binding with [18F]FDDNP and [11C]PBB3, which may increase the signal to noise ratio and thereby decrease the efficacy of these radiotracers. Testing in [18F]THK-5317, [18F]THK-5351, and [18F]AV-1451 has been performed in progressive supranuclear palsy, while [18F]THK-5317 and [18F]AV-1451 have also been tested in corticobasal degeneration patients. [18F]THK-5317 and [18F]THK-5351 have demonstrated binding in brain regions known to be afflicted with pathological tau; however, due to small sample sizes these studies should be replicated before concluding their appropriateness in parkinsonian tauopathies. [18F]AV-1451 has demonstrated mixed results in progressive supranuclear palsy patients and post-mortem analysis shows minimal to no binding to non-Alzheimer's disease tauopathies brain slices.

Therapy and clinical trials in frontotemporal dementia: past, present, and future

Frontotemporal dementia (FTD) is a common form of dementia with heterogeneous clinical presentations and distinct clinical syndromes. This article will review currently available therapies for FTD, its related disorders and their clinical evidence. It will also discuss recent advancements in FTD pathophysiology, treatment development, biomarker advancement and their relation to recently completed or currently ongoing clinical trials as well as future implications. Frontotemporal dementia (FTD) is a type of dementia with distinct clinical syndromes. Current treatments involve off-label use of medications for symptomatic management and cannot modify disease course. Advancements in FTD pathophysiology, genetics, and biomarkers have led to development of small molecules targeting the underlying pathology in hopes of achieving a disease-modifying effect. This article will review current therapies for FTD, discuss advancements in FTD pathophysiology, therapy development, biomarker advancement, their relation to recent clinical trials and future implications. This article is part of the Frontotemporal Dementia special issue.

The MAPT H1 haplotype is associated with tangle-predominant dementia

Tangle-predominant dementia (TPD) patients exhibit cognitive decline that is clinically similar to early to moderate-stage Alzheimer disease (AD), yet autopsy reveals neurofibrillary tangles in the medial temporal lobe composed of the microtubule-associated protein tau without significant amyloid-beta (Aβ)-positive plaques. We performed a series of neuropathological, biochemical and genetic studies using autopsy brain tissue drawn from a cohort of 34 TPD, 50 AD and 56 control subjects to identify molecular and genetic signatures of this entity. Biochemical analysis demonstrates a similar tau protein isoform composition in TPD and AD, which is compatible with previous histological and ultrastructural studies. Further, biochemical analysis fails to uncover elevation of soluble Aβ in TPD frontal cortex and hippocampus compared to control subjects, demonstrating that non-plaque-associated Aβ is not a contributing factor. Unexpectedly, we also observed high levels of secretory amyloid precursor protein α (sAPPα) in the frontal cortex of some TPD patients compared to AD and control subjects, suggesting differences in APP processing. Finally, we tested whether TPD is associated with changes in the tau gene (MAPT). Haplotype analysis demonstrates a strong association between TPD and the MAPT H1 haplotype, a genomic inversion associated with some tauopathies and Parkinson disease (PD), when compared to age-matched control subjects with mild degenerative changes, i.e., successful cerebral aging. Next-generation resequencing of MAPT followed by association analysis shows an association between TPD and two polymorphisms in the MAPT 3' untranslated region (UTR). These results support the hypothesis that haplotype-specific variation in the MAPT 3' UTR underlies an Aβ-independent mechanism for neurodegeneration in TPD.

Paired helical filaments from Alzheimer disease brain induce intracellular accumulation of Tau protein in aggresomes

Abnormal folding of tau protein leads to the generation of paired helical filaments (PHFs) and neurofibrillary tangles, a key neuropathological feature in Alzheimer disease and tauopathies. A specific anatomical pattern of pathological changes developing in the brain suggests that once tau pathology is initiated it propagates between neighboring neuronal cells, possibly spreading along the axonal network. We studied whether PHFs released from degenerating neurons could be taken up by surrounding cells and promote spreading of tau pathology. Neuronal and non-neuronal cells overexpressing green fluorescent protein-tagged tau (GFP-Tau) were treated with isolated fractions of human Alzheimer disease-derived PHFs for 24 h. We found that cells internalized PHFs through an endocytic mechanism and developed intracellular GFP-Tau aggregates with attributes of aggresomes. This was particularly evident by the perinuclear localization of aggregates and redistribution of the vimentin intermediate filament network and retrograde motor protein dynein. Furthermore, the content of Sarkosyl-insoluble tau, a measure of abnormal tau aggregation, increased 3-fold in PHF-treated cells. An exosome-related mechanism did not appear to be involved in the release of GFP-Tau from untreated cells. The evidence that cells can internalize PHFs, leading to formation of aggresome-like bodies, opens new therapeutic avenues to prevent propagation and spreading of tau pathology.

GSPE interferes with tau aggregation in vivo: implication for treating tauopathy

Tauopathies are characterized by progressive neurodegeneration caused by intracellular accumulation of hyperphosphorylated tau protein aggregates in the brain. The present study was designed to test whether a grape seed polyphenolic extract (GSPE) previously shown to inhibit tau protein aggregation in vitro could benefit tau-mediated neuropathology and behavior deficits in JNPL3 transgenic mice expressing a human tau protein containing the P301L mutation. Nine-month-old JNPL3 mice were treated with GSPE delivered through their drinking water for 6 months. We found that GSPE treatment significantly reduced the number of motor neurons immunoreactive for hyperphosphorylated and conformationally-modified tau in the ventral horns of the spinal cord identified using AT100, PHF-1, AT8, and Alz50 tau antibodies. This coincided with a drastically reduced level of hyperphosphorylated and sarcosyl-insoluble tau in spinal cord fractions. Furthermore, the reduction of tau pathology was accompanied by an improvement in the motor function assessed by a wire hang test. Collectively, our results suggest that GSPE can interfere with tau-mediated neurodegenerative mechanisms and ameliorate neurodegenerative phenotype in an animal model of tauopathy. Our studies support further evaluation of GSPE for preventing and/or treating of tauopathies in humans.

Pathogenic forms of tau inhibit kinesin-dependent axonal transport through a mechanism involving activation of axonal phosphotransferases

Aggregated filamentous forms of hyperphosphorylated tau (a microtubule-associated protein) represent pathological hallmarks of Alzheimer's disease (AD) and other tauopathies. While axonal transport dysfunction is thought to represent a primary pathogenic factor in AD and other neurodegenerative diseases, the direct molecular link between pathogenic forms of tau and deficits in axonal transport remain unclear. Recently, we demonstrated that filamentous, but not soluble, forms of wild-type tau inhibit anterograde, kinesin-based fast axonal transport (FAT) by activating axonal protein phosphatase 1 (PP1) and glycogen synthase kinase 3 (GSK3), independent of microtubule binding. Here, we demonstrate that amino acids 2-18 of tau, comprising a phosphatase-activating domain (PAD), are necessary and sufficient for activation of this pathway in axoplasms isolated from squid giant axons. Various pathogenic forms of tau displaying increased exposure of PAD inhibited anterograde FAT in squid axoplasm. Importantly, immunohistochemical studies using a novel PAD-specific monoclonal antibody in human postmortem tissue indicated that increased PAD exposure represents an early pathogenic event in AD that closely associates in time with AT8 immunoreactivity, an early marker of pathological tau. We propose a model of pathogenesis in which disease-associated changes in tau conformation lead to increased exposure of PAD, activation of PP1-GSK3, and inhibition of FAT. Results from these studies reveal a novel role for tau in modulating axonal phosphotransferases and provide a molecular basis for a toxic gain-of-function associated with pathogenic forms of tau.

Ultrastructural alterations of Alzheimer's disease paired helical filaments by grape seed-derived polyphenols

Abnormal folding of the microtubule-associated protein tau leads to aggregation of tau into paired helical filaments (PHFs) and neurofibrillary tangles, the major hallmark of Alzheimer's disease (AD). We have recently shown that grape seed polyphenol extract (GSPE) reduces tau pathology in the TMHT mouse model of tauopathy (Wang et al., 2010). In the present studies we assessed the impact of GSPE exposure on the ultrastructure of PHFs isolated from Alzheimer's disease brain. Transmission electron microscopy revealed that GSPE induced profound dose- and time-dependent alterations in the morphology of PHFs with partial disintegration of filaments. Filaments showed ∼2-fold enlargement in width and displayed numerous protrusions and splayed ends consistent with unfolding of tau and diminished structural stability. In addition, GSPE induced a reduction in immunogold labeling with antibodies against the C-terminal half (12E8, PHF-1) and the middle region of tau (AT8, Tau5, pSer214 tau, and AT180) but not the C-terminal end (Tau46). In comparison, labeling of N-terminus (Alz50) was enhanced. It is unlikely that alterations in immunogold labeling were due to biochemical alterations, e.g., protein phosphatase or proteolytic activities potentially stimulated by GSPE, because western blotting studies have shown the preservation of full length polypeptides of tau and their phospho-epitopes in GSPE-treated samples. The GSPE mechanism may include a noncovalent interaction of polyphenols with proline residues in the proline-rich domain of tau, with Pin1 sites at P213 and P232 most seriously affected as judged by suppression of labeling. Collectively, our results suggest that GSPE has a significant potential for therapeutic development by neutralizing phospho-epitopes and disrupting fibrillary conformation leading to disintegration of PHFs.

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.

Delineation of early changes in cases with progressive supranuclear palsy-like pathology. Astrocytes in striatum are primary targets of tau phosphorylation and GFAP oxidation

Progressive supranuclear palsy (PSP) is a complex tauopathy usually confirmed at post-mortem in advanced stages of the disease. Early PSP-like changes that may outline the course of the disease are not known. Since PSP is not rarely associated with argyrophilic grain disease (AGD) of varible intensity, the present study was focused on AGD cases with associated PSP-like changes in an attempt to delineate early PSP-like pathology in this category of cases. Three were typical clinical and pathological PSP. Another case presented with cognitive impairment, abnormal behavior and two falls in the last three months. One case suffered from mild cognitive impairment, and two had no evidence of neurological abnormality. Neuropathological study revealed, in addition to AGD, increased intensity and extent of lesion in three groups of regions, striatum, pallidus/subthalamus and selected nuclei of the brain stem, correlating with neurological impairment. Biochemical studies disclosed oxidative damage in the striatum and amygdala. Together the present observations suggest (i) early PSP-like lesions in the striatum, followed by the globus pallidus/subthalamus and selected nuclei of the brain stem; (ii) early involvement of neurons and astrocytes, but late appearance of tufted astrocytes; and (iii) oxidative damage of glial acidic protein in the striatum.

Mitogen-activated protein kinase activity may not be necessary for the neuropathology of Niemann-Pick type C mice

Hyperphosphorylation of neurofilament and tau, and formation of cytoskeletal lesions, are notable features of several human neurodegenerative diseases, including Niemann-Pick Disease Type C (NPC). Previous studies suggested that the MAPKs, extracellular signal regulated kinase 1 and 2 (ERK1/2) may play a significant role in this aspect of NPC. To test this idea, we treated npc mice with PD98059, a specific and potent inhibitor of MAPK activation. Although activity of ERK1/2 was inhibited by 40%, a 2-week intracerebroventricular infusion of PD98059 just prior to onset of cytoskeletal pathology and symptoms in npc mice did not delay or inhibit prominent hallmarks of NPC. Unexpectedly, ERK1/2 inhibition led to aggravation of tau hyperphosphorylation, particularly in oligodendroctyes, in a manner similar to that of certain human tauopathies. Our results suggest that ERK1/2 does not play a major role in NPC neuropathology, and therefore, that MAPK inhibition is unlikely to be a useful strategy for managing the disease.

Astrocytic tau pathology positively correlates with neurofibrillary tangle density in progressive supranuclear palsy

Tufted astrocytes (TAs) are considered reliable, specific markers for the neuropathologic diagnosis of progressive supranuclear palsy (PSP). It is known that neurofibrillary tangles (NFTs) may relate directly to neurodegeneration, but the role of glial tau pathology is not well determined. To examine the hypothesis that TAs are as pathogenic as NFTs and that both might have a common accumulation, we evaluated the topographic relationship between TAs and NFTs in 12 cases of PSP. The sections of 13 different parts of the brain were stained using the Gallyas-Braak method, and TAs and NFTs were counted and compared statistically. The number of TAs significantly correlated with that of NFTs in the central gray matter, pontine nuclei, and tegmentum, which are responsible for the main symptoms in PSP. In the examined allocortex, however, NFTs were abundant without accompanying TAs. Staining with the specific antibody for 4-repeat tau (RD4) and 3-repeat tau (RD3) was performed to clarify this discrepancy from the standpoint of tau isoforms. NFTs in the entorhinal cortex were stained with both RD3 and RD4, but NFTs in the premotor cortex were stained with only RD4. The nature of NFTs in the allocortical area was different from that of the isocortex in PSP. TAs in the isocortex may share the same pathologic cascade with NFTs stained only by RD4. These results suggest that TAs are part of the same pathologic process as NFTs in PSP.

Identification of G-protein coupled receptor kinase 2 in paired helical filaments and neurofibrillary tangles

G-protein coupled receptor kinases (GRKs) constitute a serine/threonine kinase family playing a major role in agonist-induced phosphorylation and desensitization of G-protein coupled receptors. Recently, GRK2 and GRK5 have been demonstrated to phosphorylate alpha-synuclein (Ser129) and other synuclein isoforms. We studied colocalization of GRK2, GRK5, alpha-synuclein, and tau in neurodegenerative disorders characterized by fibrillary tau inclusions and/or alpha-synuclein-enriched Lewy bodies. We found that Lewy bodies were negative for both GRK2 and GRK5 in Lewy body disease (LBD) and LBD mixed with Alzheimer disease (AD + LBD). Instead, GRK2 but not GRK5 colocalized with 40% to 50% of neurofibrillary tangles in AD + LBD and AD brains. In disorders with less prominent alpha-synucleinopathy, neuronal and glial fibrillary tau deposits known to contain distinct subsets of tau isoforms were also positive for GRK2. These deposits included tufted astrocytes and coiled bodies in progressive supranuclear palsy, astrocytic plaques in corticobasal degeneration, and Pick bodies in Pick disease. In addition, paired helical filaments isolated from AD and AD + LBD brains were found to immunogold-label for GRK2, suggesting that GRK2 could be a potential tau kinase associated with fibrillary tau. Our studies indicate that GRK2 is a novel component of neuronal and glial fibrillary tau deposits with no preference in tau isoform binding. GRK2 may play a role in hyperphosphorylation of tau in tauopathies.

Unexpected abundance of pathological tau in progressive supranuclear palsy white matter

OBJECTIVE

To investigate whether biochemical insoluble tau with 4 (4R) and/or 3 (3R) microtubule-binding repeats accumulate in white as well as gray matter in progressive supranuclear palsy (PSP), a neurodegenerative tauopathy.

METHODS

To assess tau pathology in PSP white matter, we combined Western blot (WB) and immunohistochemical methods to analyze 23 autopsy-confirmed PSP brains.

RESULTS

WBs showed an unexpected abundance of insoluble tau in white and gray matter of PSP brains, but biochemical tau pathology in white matter was not correlated with immunohistochemistry using the same panel of epitope-specific anti-tau antibodies used for WB. Despite heterogeneity in the representation of pathological 3R and 4R tau isoforms in cortical versus subcortical regions, biochemically detectable white matter tau pathology is a constant feature of PSP.

INTERPRETATION

These studies show additional similarities between PSP and corticobasal degeneration, but unlike corticobasal degeneration, more abundant white matter tau pathology in PSP is detectable by WB than by immunohistochemistry. The differential detection of abnormal tau by biochemistry versus microscopy in PSP may reflect distinct pathological mechanisms, and elucidation of these processes will augment efforts to develop better strategies for the diagnosis and treatment of PSP and related neurodegenerative tauopathies.

Interactions between volitional and automatic breathing during respiratory apraxia

The sites and forms of interactions between voluntary breathing acts and automatic respiratory rhythm generation are the subject of considerable research interest. We report here observations of the control of breathing in a patient suffering from an advanced form of progressive supranuclear palsy (Steele-Richardson-Olszewski syndrome). This patient demonstrated a severely compromised ability to perform volitional respiratory acts upon command, despite exacerbated behavioural and automatic control of respiration. The presence of residual volitional control of breathing in this patient provided interesting insights concerning the interaction between the automatic and the voluntary control of respiration. We observed that (1) when the subjects was asked to inspire voluntarily he could at best mobilize a volume similar to spontaneous VT and only very slowly; (2) automatic breathing movements persisted, superimposed onto the active voluntary movements, with an amplitude that decreased when the inspiratory activity, albeit weak, reached its maximal level; (3) during breath holding both the amplitude and the frequency of the basal spontaneous rhythmic activity were depressed. This observation therefore supports the idea of a strong interaction between volitional and automatic breathing in the form of an inhibition of automatic activity during voluntary breathing. Although, the site of interaction (spinal versus supraspinal) could not be determined during volitional inspiration, the effect of breath holding on the frequency of the spontaneous breathing activity supports the view that a volitional breathing arrest has some inhibitory effects on the respiratory oscillator at the medullary level. Finally, in an attempt to reconcile the persistence of a rhythmic activity during voluntary inspiration and expiration with previous data from the literature, it is proposed that the normal suppression of the automatic activity during voluntary inspiration relies on cortical and sub-cortical structures involved in the planning, i.e. the praxic component, of a respiratory task rather than on projections originating from the primary motor cortex.

Progressive supranuclear palsy and corticobasal degeneration: lumping versus splitting

Progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD) are both sporadic disorders with tau pathology. Criteria have been defined that in most instances allow for adequate diagnosis of the two disorders both clinically and neuropathologically; however, overlap is not uncommon. For example, patients with PSP may present with severe unilateral apraxia and supranuclear gaze palsy can occur in CBD. Pathological overlap also occurs and pathologically "mixed" cases are encountered. Common to both these two tauopathies is that isoforms of four-repeat tau due to splicing of exon 10 define the tau filamentous aggregates. This is in contrast to other tau disorders such as Pick's with three-repeat tau aggregates. Additional evidence for a causal link between PSP and CBD is the finding that both disorders are homozygous for the H1 tau haplotype. Furthermore, in some families with parkinsonism linked to defined mutations of the tau gene (FTDP-17), involved relatives have presented with PSP whereas others with the CBD phenotype. Although PSP and CBD frequently can be clearly separated clinically and pathologically, the degree of clinicopathological and genetic overlap is important and suggests that they represent different phenotypes of the same disorder, with differences occurring perhaps in relation to different genetic background. That PSP and CBD are distinct nosological entities occurring in patients with similar genetic predisposition cannot be ruled out.

Evidence for independent mechanisms and a multiple-hit model of tau assembly

Formation of inclusions containing polymerized tau protein is a hallmark of Alzheimer's disease and related disorders. In vitro studies have demonstrated the ability of polyglycosaminoglycans and fatty acids to promote tau polymerization. In this report, we examined their impact on tau polymerization separately and together. Tau assembly with only arachidonic acid was faster than that with only heparin. The presence of dithiothreitol reduced heparin-promoted tau assembly while enhancing arachidonic acid reactions. However, simultaneous use of these molecules increased the rate of filament assembly substantially, negated the effects of the reducing agent, and has very little effect on the morphology of filaments. The increases in polymerization resulted from accelerated nucleation. Finally, a FTDP-17 mutation was identified that could complement heparin to generate assembly kinetics similar to that of wild-type tau with both inducers. Our results support a multiple-hit model where several induced changes in tau can function independently to promote tau assembly.

Individual and regional variations of phospho-tau species in progressive supranuclear palsy

The purpose of this study was to learn about possible variations in phospho-tau profiles in terms of case-to-case differences, regional modifications and diversification of tau phosphorylation sites in five PSP cases with moderate to severe frontosubcortical dysfunction. Gel electrophoresis of sarkosyl-insoluble fractions and Western blotting with five anti-tau phospho-specific antibodies directed to phosphorylation sites Thr181, Ser202, Ser214, Ser396 and Ser422 were used to study four brain regions including frontal cortex, area 8, subcortical white matter of the frontal lobe, caudate/putamen: striatum, and basis pontis: pons. Although two bands of 66 and 62 kDa were observed in almost every region in each case, the intensity of the bands depends on the anti-tau phospho-specific antibody. More importantly, bands of 72, 50/55 and 37 kDa were commonly found in PSP brains, whereas other bands of about 60, 42, 33 and 29 kDa were irregularly observed. The pattern of bands differed slightly from one case to another and from one region to another. Moreover, the phospho-tau profile differed depending on the anti-tau phospho-specific antibody used. These data suggest that several species of tau are variably phosphorylated at a given time in a given region (and probably in a given cell), and that tau aggregates are composed of several phosphorylated truncated or cleaved tau molecules, in addition to phosphorylated complete tau isoforms.

Phosphorylation of tau by fyn: implications for Alzheimer's disease

The abnormal phosphorylation of tau protein on serines and threonines is a hallmark characteristic of the neurofibrillary tangles of Alzheimer's disease (AD). The discovery that tau could be phosphorylated on tyrosine and evidence that Abeta signal transduction involved tyrosine phosphorylation led us to question whether tyrosine phosphorylation of tau occurred during the neurodegenerative process. In this study we determined that human tau tyr18 was phosphorylated by the src family tyrosine kinase fyn. By developing both polyclonal and monoclonal probes specific for phospho-tyr18, we found that the phosphorylation of tau at tyr18 occurred at early developmental stages in mouse but was absent in the adult. Our phosphospecific probes also revealed that paired helical filament preparations exhibited phospho-tyr18 reactivity that was sensitive to phosphotyrosine-specific protein phosphatase treatment. Moreover, immunocytochemical studies indicated that tyrosine phosphorylated tau was present in the neurofibrillary tangles in AD brain. However, the staining pattern excluded neuropil threads and dystrophic neurites indicating that tyrosine phosphorylated tau was distributed in AD brain in a manner dissimilar from other abnormally phosphorylated tau. We also found evidence suggesting that differentially phosphorylated tau existed within degenerating neurons. Our data add new support for a role for fyn in the neurodegenerative process.

General aspects of neurodegeneration

Neurodegenerative diseases are morphologically featured by progressive cell loss in specific vulnerable neuronal populations of the central nervous system, often associated with cytoskeletal protein aggregates forming intracytoplasmic and/or intranuclear inclusions in neurons and/or glial cells. Most neurodegenerative disorders are now classified either according to the hitherto known genetic mechanisms or to the major components of their cellular protein inclusions. The major basic processes inducing neurodegeneration are considered multifactorial ones caused by genetic, environmental, and endogenous factors. They include abnormal protein dynamics with defective protein degradation and aggregation, many of them related to the ubiquitin-proteasomal system, oxidative stress and free radical formation, impaired bioenergetics and mitochondrial dysfunctions, and "neuroinflammatory" processes. These mechanisms that are usually interrelated in complex vitious circles finally leading to programmed cell death cascades are briefly discussed with reference to their pathogenetic role in many, albeit diverse neurodegenerative diseases, like Alzheimer disease, synucleinopathies, tauopathies, and polyglutamine disorders. The impact of protein inclusions on cell dysfunction, activation or prevention of cell death cascades are discussed, but the molecular basis for the underlying disease mechanisms remains to be elucidated.

Tau protein expression in adult bovine oligodendrocytes: functional and pathological significance

In tauopathies, overexpression of tau exon 10 is linked to degeneration and abnormal tau deposition in neurons and oligodendroglia (OLGs). To compare exon 10 expression in normal neurons and OLGs, adult bovine brain was examined for the expression of tau in gray matter and cultured OLGs isolated from white matter. Using exon-specific antibodies, we found that both types of tissues abundantly expressed exon 2 but isolated OLGs had a lower expression of exons 3 and 10 when compared to gray matter. Relative expression of exons 3 and 10 did not change significantly during the in vitro maturation of OLGs for 39 days. Using a panel of well-characterized antibodies against tau, we determined that isolated OLGs contained tau phosphorylated at the Tau-1, 12E8, and PHF-1 but not the AT8, AT100, AT180, and AT270 epitopes. Tau phosphorylation status diminished during in vitro maturation, suggesting that healthy OLG processes require regulated phosphorylation of tau at specific sites. We propose that the tau isoform profile and phosphorylation status contribute to the vulnerability of OLGs in degenerative diseases linked to overexpression of exon 10.

Pathological inclusion bodies in tauopathies contain distinct complements of tau with three or four microtubule-binding repeat domains as demonstrated by new specific monoclonal antibodies

Pathological inclusions containing fibrillar aggregates of hyperphosphorylated tau protein are a characteristic feature in the tauopathies, which include Alzheimer's disease, frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17), progressive supranuclear palsy, corticobasal degeneration and Pick's disease. Tau isoform composition and cellular and regional distribution as well as morphology of these inclusions vary in each disorder. Recently, several pathological missense and exon 10 splice-donor site mutations of the tau gene were identified in FTDP-17. Exon 10 codes for the second of four microtubule-binding repeat domains. The splice-site mutations result in increased inclusion of exon 10 which causes a relative increase in tau isoforms containing four microtubule-binding repeat domains over those containing three repeat domains. This could be a central aetiological mechanism in FTDP-17 and, perhaps, other related tauopathies. We have investigated changes in the ratio and distribution of three-repeat and four-repeat tau in the different tauopathies as a basis of the phenotypic range of these disorders and the selective vulnerability of different subsets of neurones. In this study, we have developed two monoclonal antibodies, RD3 and RD4 that effectively distinguish these closely related tau isoforms. These new isoform-specific antibodies are useful tools for analysing tau isoform expression and distribution as well as pathological changes in the human brain.