Filamentous Tau Pathology Research Articles

Data-driven approaches for tau-PET imaging biomarkers in Alzheimer's disease.

Previous positron emission tomography (PET) studies have quantified filamentous tau pathology using regions-of-interest (ROIs) based on observations of the topographical distribution of neurofibrillary tangles in post-mortem tissue. However, such approaches may not take full advantage of information contained in neuroimaging data. The present study employs an unsupervised data-driven method to identify spatial patterns of tau-PET distribution, and to compare these patterns to previously published "pathology-driven" ROIs. Tau-PET patterns were identified from a discovery sample comprised of 123 normal controls and patients with mild cognitive impairment or Alzheimer's disease (AD) dementia from the Swedish BioFINDER cohort, who underwent [18 F]AV1451 PET scanning. Associations with cognition were tested in a separate sample of 90 individuals from ADNI. BioFINDER [18 F]AV1451 images were entered into a robust voxelwise stable clustering algorithm, which resulted in five clusters. Mean [18 F]AV1451 uptake in the data-driven clusters, and in 35 previously published pathology-driven ROIs, was extracted from ADNI [18 F]AV1451 scans. We performed linear models comparing [18 F]AV1451 signal across all 40 ROIs to tests of global cognition and episodic memory, adjusting for age, sex, and education. Two data-driven ROIs consistently demonstrated the strongest or near-strongest effect sizes across all cognitive tests. Inputting all regions plus demographics into a feature selection routine resulted in selection of two ROIs (one data-driven, one pathology-driven) and education, which together explained 28% of the variance of a global cognitive composite score. Our findings suggest that [18 F]AV1451-PET data naturally clusters into spatial patterns that are biologically meaningful and that may offer advantages as clinical tools.

DATA-DRIVEN TAU-PET COVARIANCE NETWORKS ENHANCE PREDICTION OF RETROSPECTIVE COGNITIVE CHANGE IN ALZHEIMER’S DISEASE

Positron emission tomography (PET) studies investigating filamentous tau pathology in vivo have used theory-derived regions-of-interest (ROIs) based on post-mortem studies. However, hypothesis-free data-driven approaches may yield ROIs optimized specifically for tau-PET data, resulting in better clinical utility. Demographic information, MMSE scores, AV1451-PET images and T1-weighted magnetic resonance images closest to PET were downloaded from the ADNI LONI website for 90 individuals (43 controls, 37 MCI, 10 AD dementia). AV1451 images were coregistered to each subject's T1 image and intensity normalized to the cerebellar gray matter. Images were then spatially normalized to the MNI152 template, stacked into a 4D image and masked with a liberal cortical mask including subcortical regions. This image stack was entered into a previously validated voxelwise clustering algorithm, which includes a procedure for defining optimal clustering solutions. The algorithm produces ROIs that represent patterns of AV1451 signal covariance across subjects. Linear mixed models were run assessing relationships between AV1451 signal within these ROIs and retrospective change in MMSE scores (575 observations). Corrected Aikake's information criterion (AICc) was used to compare models using our data-derived ROIs to models using theory-derived ROIs from three tau-PET studies published in 2016. All models were adjusted for age, gender and education. The clustering algorithm identified a six-cluster solution to be the best solution (Figure 1). These clusters included a medial/inferior temporal ROI, a temporo-parietal predominant ROI, a frontal ROI, a dorsal ROI, a “partial-volume” ROI encircling the cortex, and an “off-target” ROI encompassing the subcortex, medial occipital lobe and the hippocampus. Across all ROIs tested, the model using the data-driven temporo-parietal ROI to predict retrospective change in MMSE (Figure 2) showed the best model fit (AICc=2312.56). With the exception of the stage IV ROI from Cho et al. (AICc=2320.54), no other ROIs demonstrated competitive model fit (ΔAICc > 10; Figure 3). Our clustering approach yielded a data-driven temporo-parietal ROI that is known to be highly AD specific and was identified as the best predictor of retrospective cognitive decline. The results suggest that hypothesis-free data-derived ROIs may offer enhanced predictive utility compared to theory-driven ROIs by utilizing information specific to tau-PET signal. Six-cluster solution of tau-PET covariance networks. A. Voxelwise images underwent dimension reduction with a region-growing algorithm for computation purposes. These small regions (atoms) were entered into a clustering analysis (Bootstrapping Analysis of Stable Clusters, Bellec et al., 2010, Neuroimage), which identified six clusters. The top image shows a stability matrix comparing the stability clustering between each atom and every other atom, organized by cluster membership. The bottom image shows the cluster partitioning. B. Kernel density estimate plots showcasing the distribution of AV1451 SUVR across subjects within each cluster. C. The six clusters identified by the clustering algorithm. SUVR = Standard uptake value ratio. Effect of AV1451 standard uptake value ratio (SUVR) within the data-driven temporo-parietal ROI on retrospective longitudinal change in MMSE scores. From left to right, the three SUVR groups represent the lowest quintile, middle three quintiles, and highest quintile of AV1451 SUVR in the temporo-parietal ROI. Statistics in the bottom-right corner refer to the adjusted Temporo-parietal SUVR × Time interaction on MMSE. Linear mixed models comparing AVI451 signal to retrospective change in MMSE were run, controlling for age, sex and education. For each model, a different AVI451 ROI was used. ROIs included the six clusters identified in our analysis, as well as Braak stage ROIs taken from three different papers: Schöll, Lockhart et al., 2016 Neuron; Cho et al., 2016 Ann. Neurol.; Schwarz et al., 2016 Brain. Two versions of each Braak ROIs were created, one using regions from that stage only (e.g. Stage 3), and one combining all regions from that stage with all regions from previous stages (e.g. Stage 1+2+3). The fit of each model was measured using AICc; AICc was inverted for the graph so that higher value corresponds to better model fit. ΔAICc indicates differences in model fit, such that ΔAICc>10 suggests one model can be rejected in favor of the other. The data-driven temporoparietal ROI (red arrow) demonstrated the best model fit, and was superior to all other models except for the Braak Stage 4 ROI from Cho et al. (black arrow). Dotted lines indicate models where the ROI SUVR × Time interaction parameter within the model exceeds the given alpha level.

Biochemical classification of tauopathies by immunoblot, protein sequence and mass spectrometric analyses of sarkosyl-insoluble and trypsin-resistant tau.

Intracellular filamentous tau pathology is the defining feature of tauopathies, which form a subset of neurodegenerative diseases. We have analyzed pathological tau in Alzheimer’s disease, and in frontotemporal lobar degeneration associated with tauopathy to include cases with Pick bodies, corticobasal degeneration, progressive supranuclear palsy, and ones due to intronic mutations in MAPT. We found that the C-terminal band pattern of the pathological tau species is distinct for each disease. Immunoblot analysis of trypsin-resistant tau indicated that the different band patterns of the 7–18 kDa fragments in these diseases likely reflect different conformations of tau molecular species. Protein sequence and mass spectrometric analyses revealed the carboxyl-terminal region (residues 243–406) of tau comprises the protease-resistant core units of the tau aggregates, and the sequence lengths and precise regions involved are different among the diseases. These unique assembled tau cores may be used to classify and diagnose disease strains. Based on these results, we propose a new clinicopathological classification of tauopathies based on the biochemical properties of tau.Electronic supplementary materialThe online version of this article (doi:10.1007/s00401-015-1503-3) contains supplementary material, which is available to authorized users.

Role of Cysteine-291 and Cysteine-322 in the Polymerization of Human Tau into Alzheimer-like Filaments

Filamentous tau pathology is central to a large number of dementing disorders, including Alzheimer's disease in which polymerized tau is hyperphosphorylated. Previous studies on heparin-dependent tau polymerization, using recombinant tau isoforms lacking Cys-291, suggest that tau dimerization via Cys-322 is critical for initiation of assembly of soluble tau into filaments. We report heparin-dependent in vitro polymerization of human recombinant tau (1–383 isoform), containing both Cys-291 and Cys-322, into paired helical filaments as characterized by electron microscopy. Tau polymerization, under physiological tau concentrations in the presence of dithiothreitol (DTT), was followed by a Thioflavine S fluorescence assay. To understand the molecular basis for heparin-induced tau polymerization, we expressed and purified C291A, C322A, and C291A/C322A tau mutants. The DTT requirement for tau polymerization was abolished using either the C291A or C322A tau mutant and polymerization was not observed with the C291A/C322A tau double mutant. Analysis by sodium dodecyl sulfate gel electrophoresis showed that, unlike wild type tau, a significant amount of the C291A mutant and the C322A mutant is present as a disulfide bonded dimer. Taken together these results suggest that, in isoforms containing both Cys-291 and Cys-322, a dimeric tau with an intermolecular disulfide bond through either Cys-291 or Cys-322 is presumably acting as a seed for initiation of tau polymerization.

Transgenic mouse models of tauopathies: prospects for animal models of Pick's disease.

Because filamentous neuronal tau inclusions are neuropathologic hallmarks of Pick's disease and a number of other neurodegenerative disorders known as tauopathies, the authors generated lines of transgenic (Tg) mice that overexpress the shortest human tau isoform in the CNS. These Tg mice accumulated argyrophilic and filamentous tau immunoreactive neuronal inclusions with age in cortex and brainstem as well as in spinal cord, where these inclusions were most abundant and associated with gliosis. The ventral roots of affected spinal cord segments showed axonal degeneration, whereas residual axons exhibited reduced microtubules and reduced fast axonal transport. The inclusions were composed of 10 to 20 nm tau immunopositive straight filaments. In addition, the Tg mice developed age-related motor weakness as well as progressive hyperphosphorylation and decreased solubility of brain and spinal cord tau proteins. Thus, these Tg mice recapitulate phenotypic features of human tauopathies. In this article the authors review the phenotype of these Tg mice and discuss how the availability of relevant animal model of tauopathies will facilitate discovery of more effective therapies to treat Pick's disease and related disorders characterized by filamentous tau pathology in selectively vulnerable regions of the CNS.

Tau gene mutations and neurodegeneration.

Abundant neurofibrillary lesions made of the microtubule-associated protein tau constitute a defining neuropathological characteristic of Alzheimer's disease. Filamentous tau protein deposits are also the defining neuropathological characteristic of other neurodegenerative diseases, many of which are frontotemporal dementias or movement disorders, such as Pick's disease, progressive supranuclear palsy and corticobasal degeneration. It is well established that the distribution of tau pathology correlates with the presence of symptoms of disease. However, until recently, there was no genetic evidence linking dysfunction of tau protein to neurodegeneration and dementia. This has now changed with the discovery of close to 20 mutations in the tau gene in frontotemporal dementia with Parkinsonism linked to chromosome 17. All cases with tau mutations examined to date have shown an abundant filamentous tau pathology in brain cells. Pathological heterogeneity is determined to a large extent by the location of mutations in tau. Known mutations are either coding region or intronic mutations located close to the splice-donor site of the intron downstream of exon 10. Most coding region mutations produce a reduced ability of tau to interact with microtubules. Several of these mutations also promote sulphated glycosaminoglycan-induced assembly of tau into filaments. Intronic mutations and some coding region mutations produce increased splicing in of exon 10, resulting in an overexpression of four-repeat tau isoforms. Thus a normal ratio of three-repeat to four-repeat tau isoforms is essential for preventing the development of tau pathology. The new work has shown that dysfunction of tau protein can cause neurodegeneration and dementia.

New insights into genetic and molecular mechanisms of brain degeneration in tauopathies

Abundant neurofibrillary lesions consisting of the microtubule associated protein tau and amyloid β peptide deposits are the defining lesions of Alzheimer's disease. Prominent filamentous tau pathology and brain degeneration in the absence of extracellular amyloid deposition characterize a number of other neurodegenerative disorders (i.e. progressive supranuclear palsy, corticobasal degeneration, Pick's disease) collectively referred to as tauopathies. The discovery of multiple tau gene mutations that are pathogenic for hereditary frontotemporal dementia and parkinsonism linked to chromosome 17 in many kindreds, as well as the demonstration that tau polymorphisms are genetic risk factors for sporadic tauopathies, directly implicate tau abnormalities in the onset/progression of neurodegenerative disease. Different tau gene mutations may be pathogenic by impairing the functions of tau or by perturbing the splicing of the tau gene, thereby resulting in biochemically and structurally distinct tau aggregates. However, since specific polymorphisms and mutations in the tau gene lead to diverse phenotypes, it is plausible that additional genetic or epigenetic factors influence the clinical and pathological manifestations of both familial and sporadic tauopathies. Thus, efforts to develop animal models of tau-mediated neurodegeneration should provide further insights into the onset and progression of tauopathies as well as Alzheimer's disease, and they could accelerate research to discover more effective therapies for these disorders.

Tau gene mutations in frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17).

Tau is a microtubule-associated protein that binds to microtubules and promotes microtubule assembly. Six tau isoforms are produced in adult human brain by alternative mRNA splicing from a single gene. Inclusion of a 31-amino acid repeat encoded by exon 10 of the tau gene gives rise to the three isoforms with four microtubule-binding repeats each. The other three tau isoforms have three repeats each. Abundant neurofibrillary lesions made of tau protein constitute a defining neuropathological characteristic of Alzheimer's disease. Filamentous tau protein deposits are also the defining characteristic of other neurodegenerative diseases, many of which are frontotemporal dementias or movement disorders, such as Pick's disease, progressive supranuclear palsy and corticobasal degeneration. It is well established that the distribution of tau pathology correlates with the presence of symptoms of disease. However, until recently, there was no genetic evidence linking dysfunction of tau protein to neurodegeneration. This has now changed with the discovery of more than 15 mutations in the tau gene in "frontotemporal dementia and parkinsonism linked to chromosome 17" (FTDP-17). Clinically, this condition is characterised by profound personality changes, progressive dementia and extrapyramidal symptoms. Neuropathologically, all cases with tau mutations examined to date have shown an abundant filamentous tau pathology in brain cells. Pathological heterogeneity is determined to a large extent by the location of mutations in tau. Known mutations are either coding region or intronic mutations located close to the splice-donor site of the intron following exon 10. Most coding region mutations produce a reduced ability of tau to interact with microtubules, thus probably setting in motion the mechanisms that lead to the formation of tau filaments. Several of these mutations also promote sulphated glycosaminoglycan-induced assembly of tau into filaments. Intronic mutations and some coding region mutations produce increased splicing in of exon 10, resulting in an overexpression of four-repeat tau isoforms. Thus, a normal ratio of three-repeat to four-repeat tau isoforms is essential for preventing the development of tau pathology. Taken together, the new work has shown that dysfunction of tau protein causes neurodegeneration and dementia.

Filamentous tau pathology in nerve cells, astrocytes, and oligodendrocytes of aged baboons.

Intracellular filamentous inclusions containing abnormally phosphorylated tau protein are hallmarks of several human neurodegenerative disorders. This study reveals tau-positive cytoskeletal abnormalities in neurons and glial cells of aged baboons. The brains of four baboons (Papio hamadryas, 20-30 yr of age) were examined using the Gallyas silver technique for neurofibrillary changes and phosphorylation-dependent anti-tau antibodies (AT8, AT100, AT270, PHF-1, TG-3). Conspicuous changes were noted in two animals, 26 and 30 yr of age. In both animals, a combination of neuronal and glial cytoskeletal pathology was seen preferentially affecting limbic brain areas, including the hippocampal formation. In the 30-yr-old animal, numerous tau-positive inclusions were seen in the granule cells of the fascia dentata. These cells even exhibited an accumulation of argyrophilic neurofibrillary tangles. The glial changes affected both astrocytes and oligodendrocytes. Tau-positive astrocytes were seen in perivascular, subpial, and subependymal locations. Tau-positive oligodendrocytes preferentially occurred in limbic fiber tracts including the entorhinal perforant path. Ultrastructurally, tau-positive straight filaments (10-14 nm) in both neurons and glial cells were revealed by anti-tau immunoelectron microscopy. This study thus indicates the potential usefulness of aged baboons for experimental investigation of neuronal and glial filamentous tau pathology. This nonhuman primate species may provide valuable information pertinent to the broad spectrum of human tauopathies.