An immunohistochemical study of cases of sporadic and inherited frontotemporal lobar degeneration using 3R- and 4R-specific tau monoclonal antibodies

Single-Molecule Characterization and Super-Resolution Imaging of Alzheimer's Disease-Relevant Tau Aggregates in Human Samples

Hyperphosphorylation and aggregation of the protein tau play key roles in the development of Alzheimer's disease (AD). While the molecular structure of the filamentous tau aggregates has been determined to atomic resolution, there is far less information available about the smaller, soluble aggregates, which are believed to be more toxic. Traditional techniques are limited to bulk measures and struggle to identify individual aggregates in complex biological samples. To address this, we developed a novel single-molecule pull-down-based assay (MAPTau) to detect and characterize individual tau aggregates in AD and control post-mortem brain and biofluids. Using MAPTau, we report the quantity, as well as the size and circularity of tau aggregates measured using super-resolution microscopy, revealing AD-specific differences in tau aggregate morphology. By adapting MAPTau to detect multiple phosphorylation markers in individual aggregates using two-color coincidence detection, we derived compositional profiles of the individual aggregates. We find an AD-specific phosphorylation profile of tau aggregates with more than 80 % containing multiple phosphorylations, compared to 5 % in age-matched non-AD controls. Our results show that MAPTau is able to identify disease-specific subpopulations of tau aggregates phosphorylated at different sites, that are invisible to other methods and enable the study of disease mechanisms and diagnosis.

A novel MAPT variant (E342K) as a cause of familial progressive supranuclear palsy

Background

MAPT variants are a known cause of frontotemporal dementia and Parkinsonian syndrome, of which progressive supranuclear palsy syndrome (PSP) is a rare manifestation.

Objective

To report a novel MAPT variant in a PSP pedigree with autosomal dominant inheritance pattern, and to produce a literature review of PSP patients with MAPT variants.

Methods

A comprehensive clinical, genetic, and molecular neuroimaging investigation was conducted on a 61 years-old female proband diagnosed with PSP. We also collected the clinical presentation data and history of the patient's pedigree, and performed further genetic analysis of 4 relatives, from two generations, with and without symptoms.

Results

The proband exhibited typical clinical manifestation of PSP. A cranial MRI revealed midbrain atrophy, and an FDG-PET scan suggested hypo-metabolic changes in caudate nucleus, left prefrontal lobe, both temporal poles, and midbrain. 18F-florzolo-tau-PET revealed tau-protein deposits in the thalamus and brainstem bilaterally. A gene test by whole-exome sequencing identified a novel MAPT variant [NM_005910.6, exon 11, c.1024G > A (p.E342K)], and the same variant was also identified in one affected relative and one asymptomatic relative, a probable pre-symptomatic carrier.

Conclusion

The PSP pedigree caused by the novel MAPT (E342K) variant, expanded the mutational spectrum of MAPT.

Clinical and Pathological Features of FTDP-17 with MAPT p.K298_H299insQ Mutation

BACKGROUND

MAPT is a causative gene in frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17), a hereditary degenerative disease with various clinical manifestations, including progressive supranuclear palsy, corticobasal syndrome, Parkinson's disease, and frontotemporal dementia.

OBJECTIVES

To analyze genetically, biochemically, and pathologically multiple members of two families who exhibited various phenotypes of the disease.

METHODS

Genetic analysis included linkage analysis, homozygosity haplotyping, and exome sequencing. We conducted tau protein microtubule polymerization assay, heparin-induced tau aggregation, and western blotting with brain lysate from an autopsy case. We also evaluated abnormal tau aggregation by using anti-tau antibody and PM-PBB3.

RESULTS

We identified a variant, c.896_897insACA, p.K298_H299insQ, in the MAPT gene of affected patients. Similar to previous reports, most patients presented with atypical parkinsonism. Biochemical analysis revealed that the mutant tau protein had a reduced ability to polymerize microtubules and formed abnormal fibrous aggregates. Pathological study revealed frontotemporal lobe atrophy, midbrain atrophy, depigmentation of the substantia nigra, and four-repeat tau-positive inclusions in the hippocampus, brainstem, and spinal cord neurons. The inclusion bodies also stained positively with PM-PBB3.

CONCLUSIONS

This study confirmed that the insACA mutation caused FTDP-17. The affected patients showed symptoms resembling Parkinson's disease initially and symptoms of progressive supranuclear palsy later. Despite the initial clinical diagnosis of frontotemporal dementia in the autopsy case, the spread of lesions could explain the process of progressive supranuclear palsy. The study of more cases in the future will help clarify the common pathogenesis of MAPT mutations or specific pathogeneses of each mutation.

Genetic forms of tauopathies: inherited causes and implications of Alzheimer's disease-like TAU pathology in primary and secondary tauopathies

Tauopathies are a heterogeneous group of neurologic diseases characterized by pathological axodendritic distribution, ectopic expression, and/or phosphorylation and aggregation of the microtubule-associated protein TAU, encoded by the gene MAPT. Neuronal dysfunction, dementia, and neurodegeneration are common features of these often detrimental diseases. A neurodegenerative disease is considered a primary tauopathy when MAPT mutations/haplotypes are its primary cause and/or TAU is the main pathological feature. In case TAU pathology is observed but superimposed by another pathological hallmark, the condition is classified as a secondary tauopathy. In some tauopathies (e.g. MAPT-associated frontotemporal dementia (FTD), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), and Alzheimer's disease (AD)) TAU is recognized as a significant pathogenic driver of the disease. In many secondary tauopathies, including Parkinson's disease (PD) and Huntington's disease (HD), TAU is suggested to contribute to the development of dementia, but in others (e.g. Niemann-Pick disease (NPC)) TAU may only be a bystander. The genetic and pathological mechanisms underlying TAU pathology are often not fully understood. In this review, the genetic predispositions and variants associated with both primary and secondary tauopathies are examined in detail, assessing evidence for the role of TAU in these conditions. We highlight less common genetic forms of tauopathies to increase awareness for these disorders and the involvement of TAU in their pathology. This approach not only contributes to a deeper understanding of these conditions but may also lay the groundwork for potential TAU-based therapeutic interventions for various tauopathies.

Diagnostic and therapeutic targeting of pathological tau proteins in neurodegenerative disorders

Tauopathies, characterized by fibrillar tau accumulation in neurons and glial cells, constitute a major neuropathological category of neurodegenerative diseases. Neurofibrillary tau lesions are strongly associated with cognitive deficits in these diseases, but the causal mechanisms underlying tau-induced neuronal dysfunction remain unresolved. Recent advances in cryo-electron microscopy examination have revealed various core structures of tau filaments from different tauopathy patients, which can be used to classify tauopathies. In vivo visualization of tau pathology is now available using several tau positron emission tomography tracers. Among these radioprobes, PM-PBB3 allows high-contrast imaging of tau deposits in the brains of patients with diverse disorders and tauopathy mouse models. Selective degradation of pathological tau species by the ubiquitin-proteasome system or autophagy machinery is a potential therapeutic strategy. Alternatively, the non-cell-autonomous clearance of pathological tau species through neuron-glia networks could be reinforced as a disease-modifying treatment. In addition, the development of neuroinflammatory biomarkers is required for understanding the contribution of immunocompetent cells in the brain to preventing neurodegeneration. This review provides an overview of the current research and development of diagnostic and therapeutic agents targeting divergent tau pathologies.

CSF MTBR-tau243 is a specific biomarker of tau tangle pathology in Alzheimer's disease

Aggregated insoluble tau is one of two defining features of Alzheimer's disease. Because clinical symptoms are strongly correlated with tau aggregates, drug development and clinical diagnosis need cost-effective and accessible specific fluid biomarkers of tau aggregates; however, recent studies suggest that the fluid biomarkers currently available cannot specifically track tau aggregates. We show that the microtubule-binding region (MTBR) of tau containing the residue 243 (MTBR-tau243) is a new cerebrospinal fluid (CSF) biomarker specific for insoluble tau aggregates and compared it to multiple other phosphorylated tau measures (p-tau181, p-tau205, p-tau217 and p-tau231) in two independent cohorts (BioFINDER-2, n = 448; and Knight Alzheimer Disease Research Center, n = 219). MTBR-tau243 was most strongly associated with tau-positron emission tomography (PET) and cognition, whereas showing the lowest association with amyloid-PET. In combination with p-tau205, MTBR-tau243 explained most of the total variance in tau-PET burden (0.58 ≤ R2 ≤ 0.75) and the performance in predicting cognitive measures (0.34 ≤ R2 ≤ 0.48) approached that of tau-PET (0.44 ≤ R2 ≤ 0.52). MTBR-tau243 levels longitudinally increased with insoluble tau aggregates, unlike CSF p-tau species. CSF MTBR-tau243 is a specific biomarker of tau aggregate pathology, which may be utilized in interventional trials and in the diagnosis of patients. Based on these findings, we propose to revise the A/T/(N) criteria to include MTBR-tau243 as representing insoluble tau aggregates ('T').

Neural atrophy produced by AAV tau injections into hippocampus and anterior cortex of middle-aged mice

Animal models of tauopathy help in understanding the role of mutations in tau pathobiology. Here, we used adeno-associated viral (AAV) vectors to administer three tau genetic variants (tauwild-type, tauP301L, and tauR406W) intracranially into 12-month-old C57BL/6Nia mice and collected tissue at 16 months. Vectors designed to express green fluorescent protein controlled for surgical procedures and exogenous protein expression by AAV. The tau genetic variants produced considerably different phenotypes. Tauwild-type and tauP301L caused memory impairments. The tauP301L caused increased amounts of aggregated tau, measured both neurochemically and histologically. Tauwild-type produced elevated levels of soluble tau and phosphorylated tau by ELISA and increased staining for phosphorylated forms of tau histologically. However, only the tauwild-type caused localized atrophy of brain tissue at the sites near the injection. The tauR406W had low protein expression and produced no atrophy or memory impairments. This supports the potential use of AAV expressing tauwild-type in aged mice to examine events leading to neurodegeneration in Alzheimer's disease pathology.

Phosphorylated tau in the retina correlates with tau pathology in the brain in Alzheimer's disease and primary tauopathies

The retina is a potential source of biomarkers for the detection of neurodegenerative diseases. Accumulation of phosphorylated tau (p-tau) in the brain is a pathological feature characteristic for Alzheimer's disease (AD) and primary tauopathies. In this study the presence of p-tau in the retina in relation to tau pathology in the brain was assessed. Post-mortem eyes and brains were collected through the Netherlands Brain Bank from donors with AD (n = 17), primary tauopathies (n = 8), α-synucleinopathies (n = 13), other neurodegenerative diseases including non-tau frontotemporal lobar degeneration (FTLD) (n = 9), and controls (n = 15). Retina cross-sections were assessed by immunohistochemistry using antibodies directed against total tau (HT7), 3R and 4R tau isoforms (RD3, RD4), and phospho-epitopes Ser202/Thr205 (AT8), Thr217 (anti-T217), Thr212/Ser214 (AT100), Thr181 (AT270), Ser396 (anti-pS396) and Ser422 (anti-pS422). Retinal tau load was compared to p-tau Ser202/Thr205 and p-tau Thr217 load in various brain regions. Total tau, 3R and 4R tau isoforms were most prominently present in the inner plexiform layer (IPL) and outer plexiform layer (OPL) of the retina and were detected in all cases and controls as a diffuse and somatodendritic signal. Total tau, p-tau Ser202/Thr205 and p-tau Thr217 was observed in amacrine and horizontal cells of the inner nuclear layer (INL). Various antibodies directed against phospho-epitopes of tau showed immunoreactivity in the IPL, OPL, and INL. P-tau Ser202/Thr205 and Thr217 showed significant discrimination between AD and other tauopathies, and non-tauopathy cases including controls. Whilst immunopositivity was observed for p-tau Thr212/Ser214, Thr181 and Ser396, there were no group differences. P-tau Ser422 did not show any immunoreactivity in the retina. The presence of retinal p-tau Ser202/Thr205 and Thr217 correlated with Braak stage for NFTs and with the presence of p-tau Ser202/Thr205 in hippocampus and cortical brain regions. Depending on the phospho-epitope, p-tau in the retina is a potential biomarker for AD and primary tauopathies.

Isoform-specific patterns of tau burden and neuronal degeneration in MAPT-associated frontotemporal lobar degeneration

Frontotemporal lobar degeneration with MAPT pathogenic variants (FTLD-MAPT) has heterogeneous tau pathological inclusions postmortem, consisting of three-repeat (3R) or four-repeat (4R) tau isoforms, or a combination (3R + 4R). Here, we studied grey matter tau burden, its relation to neuronal degeneration, and regional patterns of pathology in different isoform groups of FTLD-MAPT. We included 38 FTLD-MAPT autopsy cases with 10 different MAPT pathogenic variants, grouped based on predominant tau isoform(s). In up to eleven regions (ten cortical and one striatal), we quantified grey matter tau burden using digital histopathological analysis and assigned semi-quantitative ratings for neuronal degeneration (i.e. 0-4) and separate burden of glial and neuronal tau inclusions (i.e. 0-3). We used mixed modelling to compare pathology measures (1) across the entire cohort and (2) within isoform groups. In the total cohort, tau burden and neuronal degeneration were positively associated and most severe in the anterior temporal, anterior cingulate and transentorhinal cortices. Isoform groups showed distinctive features of tau burden and neuronal degeneration. Across all regions, the 3R isoform group had lower tau burden compared to the 4R group (p = 0.008), while at the same time showing more severe neuronal degeneration than the 4R group (p = 0.002). The 3R + 4R group had an intermediate profile with relatively high tau burden along with relatively severe neuronal degeneration. Neuronal tau inclusions were most frequent in the 4R group (p < 0.001 vs. 3R), while cortical glial tau inclusions were most frequent in the 3R + 4R and 4R groups (p ≤ 0.009 vs. 3R). Regionally, neuronal degeneration was consistently most severe in the anterior temporal cortex within each isoform group. In contrast, the regions with the highest tau burden differed in isoform groups (3R: striatum; 3R + 4R: striatum, inferior parietal lobule, middle frontal cortex, anterior cingulate cortex; 4R: transentorhinal cortex, anterior temporal cortex, fusiform gyrus). We conclude that FTLD-MAPT isoform groups show distinctive features of overall neuronal degeneration and regional tau burden, but all share pronounced anterior temporal neuronal degeneration. These data suggest that distinct isoform-related mechanisms of genetic tauopathies, with slightly divergent tau distribution, may share similar regional vulnerability to neurodegeneration within the frontotemporal paralimbic networks.

Disease-associated human genetic variation through the lens of precursor and mature RNA structure

Disease-associated variants (DAVs) are commonly considered either through a genomic lens that describes variant function at the DNA level, or at the protein function level if the variant is translated. Although the genomic and proteomic effects of variation are well-characterized, genetic variants disrupting post-transcriptional regulation is another mechanism of disease that remains understudied. Specific RNA sequence motifs mediate post-transcriptional regulation both in the nucleus and cytoplasm of eukaryotic cells, often by binding to RNA-binding proteins or other RNAs. However, many DAVs map far from these motifs, which suggests deeper layers of post-transcriptional mechanistic control. Here, we consider a transcriptomic framework to outline the importance of post-transcriptional regulation as a mechanism of disease-causing single-nucleotide variation in the human genome. We first describe the composition of the human transcriptome and the importance of abundant yet overlooked components such as introns and untranslated regions (UTRs) of messenger RNAs (mRNAs). We present an analysis of Human Gene Mutation Database variants mapping to mRNAs and examine the distribution of causative disease-associated variation across the transcriptome. Although our analysis confirms the importance of post-transcriptional regulatory motifs, a majority of DAVs do not directly map to known regulatory motifs. Therefore, we review evidence that regions outside these well-characterized motifs can regulate function by RNA structure-mediated mechanisms in all four elements of an mRNA: exons, introns, 5' and 3' UTRs. To this end, we review published examples of riboSNitches, which are single-nucleotide variants that result in a change in RNA structure that is causative of the disease phenotype. In this review, we present the current state of knowledge of how DAVs act at the transcriptome level, both through altering post-transcriptional regulatory motifs and by the effects of RNA structure.

MAPT Q336H mutation: intra-familial phenotypic heterogeneity in a new Italian family

BACKGROUND

Q336H is a rare MAPT mutation, previously found in a single patient with behavioral variant of FTD and tau pathology (Pick bodies). Here, we describe the clinical characteristics of two members of a new family, carrying the Q336H MAPT mutation.

METHODS

Clinical, genetic and neuroradiological assessment and follow-up of the proband.

RESULTS

At age 37, the proband developed naming and objects recognition impairment, due to a lack of knowledge. After 3 years, he developed behavioral disorders. MRI and FDG-PET showed the involvement of the left temporal pole. A diagnosis of semantic variant of primary progressive aphasia (svPPA) was made. At follow-up after 6 and 12 months, a rapid worsening of cognitive deficits occurred. His parent presented, at age 65, slowly progressive memory deficits without behavioral impairment, and, on MRI, evidence of mesial temporal atrophy, consistent with a clinical diagnosis of Alzheimer's disease (AD).

DISCUSSION

and conclusion This is the second family carrying the MAPT Q336H mutation reported so far. We showed that svPPA and AD-like phenotype can be associated with this mutation. A wide clinical variability exists at intra-familial level for Q336H MAPT mutation, pointing to genetic and/or environmental influencing factors on disease expression. We also confirmed that svPPA can be associated with MAPT mutations, suggesting that this gene should be analyzed also in patients with svPPA, especially with early onset. In addition, an AD-like phenotype may be associated with this mutation, suggesting its different effects on protein misfolding and aggregation.

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.

Cellular and pathological heterogeneity of primary tauopathies

Microtubule-associated protein tau is abnormally aggregated in neuronal and glial cells in a range of neurodegenerative diseases that are collectively referred to as tauopathies. Multiple studies have suggested that pathological tau species may act as a seed that promotes aggregation of endogenous tau in naïve cells and contributes to propagation of tau pathology. While they share pathological tau aggregation as a common feature, tauopathies are distinct from one another with respect to predominant tau isoforms that accumulate and the selective vulnerability of brain regions and cell types that have tau inclusions. For instance, primary tauopathies present with glial tau pathology, while it is mostly neuronal in Alzheimer's disease (AD). Also, morphologies of tau inclusions can greatly vary even within the same cell type, suggesting distinct mechanisms or distinct tau conformers in each tauopathy. Neuropathological heterogeneity across tauopathies challenges our understanding of pathophysiology behind tau seeding and aggregation, as well as our efforts to develop effective therapeutic strategies for AD and other tauopathies. In this review, we describe diverse neuropathological features of tau inclusions in neurodegenerative tauopathies and discuss what has been learned from experimental studies with mouse models, advanced transcriptomics, and cryo-electron microscopy (cryo-EM) on the biology underlying cell type-specific tau pathology.

MAPT R406W increases tau T217 phosphorylation in absence of amyloid pathology

Objective

Tau hyperphosphorylation at threonine 217 (pT217) in cerebrospinal fluid (CSF) has recently been linked to early amyloidosis and could serve as a highly sensitive biomarker for Alzheimer’s disease (AD). However, it remains unclear whether other tauopathies induce pT217 modifications. To determine if pT217 modification is specific to AD, CSF pT217 was measured in AD and other tauopathies.

Methods

Using immunoprecipitation and mass spectrometry methods, we compared CSF T217 phosphorylation occupancy (pT217/T217) and amyloid‐beta (Aβ) 42/40 ratio in cognitively normal individuals and those with symptomatic AD, progressive supranuclear palsy, corticobasal syndrome, and sporadic and familial frontotemporal dementia.

Results

Individuals with AD had high CSF pT217/T217 and low Aβ42/40. In contrast, cognitively normal individuals and the majority of those with 4R tauopathies had low CSF pT217/T217 and normal Aβ 42/40. We identified a subgroup of individuals with increased CSF pT217/T217 and normal Aβ 42/40 ratio, most of whom were MAPT R406W mutation carriers. Diagnostic accuracies of CSF Aβ 42/40 and CSF pT217/T217, alone and in combination were compared. We show that CSF pT217/T217 × CSF Aβ 42/40 is a sensitive composite biomarker that can separate MAPT R406W carriers from cognitively normal individuals and those with other tauopathies.

Interpretation

MAPT R406W is a tau mutation that leads to 3R+4R tauopathy similar to AD, but without amyloid neuropathology. These findings suggest that change in CSF pT217/T217 ratio is not specific to AD and might reflect common downstream tau pathophysiology common to 3R+4R tauopathies.

Distinct Neurotoxic Effects of Extracellular Tau Species in Primary Neuronal-Glial Cultures

Recent data from various experimental models support the link between extracellular tau and neurodegeneration; however, the exact mechanisms by which extracellular tau or its modified forms or aggregates cause neuronal death remain unclear. We have previously shown that exogenously applied monomers and oligomers of the longest tau isoform (2N4R) at micromolar concentrations induced microglial phagocytosis of stressed-but-viable neurons in vitro. In this study, we investigated whether extracellular phosphorylated tau2N4R (p-tau2N4R), isoform 1N4R (tau1N4R) and K18 peptide can induce neuronal death or loss in primary neuronal-glial cell cultures. We found that p-tau2N4R at 30 nM concentration induced loss of viable neurons; however, 700 nM p-tau2N4R caused necrosis of both neurons and microglia, and this neuronal death was partially glial cell-dependent. We also found that extracellular tau1N4R oligomers, but not monomers, at 3 μM concentration caused neuronal death in mixed cell cultures: self-assembly tau1N4R dimers-tetramers induced neuronal necrosis and apoptosis, whereas Aβ-promoted tau1N4R oligomers caused glial cell-dependent loss of neurons without signs of increased cell death. Monomeric and pre-aggregated tau peptide containing 4R repeats (K18) had no effect in mixed cultures, suggesting that tau neurotoxicity might be dependent on N-terminal part of the protein. Taken together, our results show that extracellular p-tau2N4R is the most toxic form among investigated tau species inducing loss of neurons at low nanomolar concentrations and that neurotoxicity of tau1N4R is dependent on its aggregation state.

Genetic Factors That Could Affect Concussion Risk in Elite Rugby

Elite rugby league and union have some of the highest reported rates of concussion (mild traumatic brain injury) in professional sport due in part to their full-contact high-velocity collision-based nature. Currently, concussions are the most commonly reported match injury during the tackle for both the ball carrier and the tackler (8-28 concussions per 1000 player match hours) and reports exist of reduced cognitive function and long-term health consequences that can end a playing career and produce continued ill health. Concussion is a complex phenotype, influenced by environmental factors and an individual's genetic predisposition. This article reviews concussion incidence within elite rugby and addresses the biomechanics and pathophysiology of concussion and how genetic predisposition may influence incidence, severity and outcome. Associations have been reported between a variety of genetic variants and traumatic brain injury. However, little effort has been devoted to the study of genetic associations with concussion within elite rugby players. Due to a growing understanding of the molecular characteristics underpinning the pathophysiology of concussion, investigating genetic variation within elite rugby is a viable and worthy proposition. Therefore, we propose from this review that several genetic variants within or near candidate genes of interest, namely APOE, MAPT, IL6R, COMT, SLC6A4, 5-HTTLPR, DRD2, DRD4, ANKK1, BDNF and GRIN2A, warrant further study within elite rugby and other sports involving high-velocity collisions.

MAPT subhaplotypes in corticobasal degeneration: assessing associations with disease risk, severity of tau pathology, and clinical features

The microtubule-associated protein tau (MAPT) H1 haplotype is the strongest genetic risk factor for corticobasal degeneration (CBD). However, the specific H1 subhaplotype association is not well defined, and it is not clear whether any MAPT haplotypes influence severity of tau pathology or clinical presentation in CBD. Therefore, in the current study we examined 230 neuropathologically confirmed CBD cases and 1312 controls in order to assess associations of MAPT haplotypes with risk of CBD, severity of tau pathology (measured as semi-quantitative scores for coiled bodies, neurofibrillary tangles, astrocytic plaques, and neuropil threads), age of CBD onset, and disease duration. After correcting for multiple testing (P < 0.0026 considered as significant), we confirmed the strong association between the MAPT H2 haplotype and decreased risk of CBD (Odds ratio = 0.26, P = 2 × 10⁻¹²), and also observed a novel association between the H1d subhaplotype and an increased CBD risk (Odds ratio = 1.76, P = 0.002). Additionally, although not statistically significant after correcting for multiple testing, the H1c haplotype was associated with a higher risk of CBD (Odds ratio = 1.49, P = 0.009). No MAPT haplotypes were significantly associated with any tau pathology measures, age of CBD onset, or disease duration. Though replication will be important and there is potential that population stratification could have influenced our findings, these results suggest that several MAPT H1 subhaplotypes are primarily responsible for the strong association between MAPT H1 and risk of CBD, but that H1 subhaplotypes are unlikely to play a major role in driving tau pathology or clinical features. Our findings also indicate that similarities in MAPT haplotype risk-factor profile exist between CBD and the related tauopathy progressive supranuclear palsy, with H2, H1d, and H1c displaying associations with both diseases.

New Insights Into Drug Discovery Targeting Tau Protein

Microtubule-associated protein tau is characterized by the fact that it is an intrinsically disordered protein due to its lack of a stable conformation and high flexibility. Intracellular inclusions of fibrillar forms of tau with a β-sheet structure accumulate in the brain of patients with Alzheimer's disease and other tauopathies. Accordingly, detachment of tau from microtubules and transition of tau from a disordered state to an abnormally aggregated state are essential events preceding the onset of tau-related diseases. Many reports have shown that this transition is caused by post-translational modifications, including hyperphosphorylation and acetylation. The misfolded tau is self-assembled and forms a tau oligomer before the appearance of tau inclusions. Animal and pathological studies using human samples have demonstrated that tau oligomer formation contributes to neuronal loss. During the progression of tauopathies, tau seeds are released from cells and incorporated into other cells, leading to the propagation of pathological tau aggregation. Accumulating evidence suggests several potential approaches for blocking tau-mediated toxicity: (1) direct inhibition of pathological tau aggregation and (2) inhibition of tau post-translational modifications that occur prior to pathological tau aggregation, (3) inhibition of tau propagation and (4) stabilization of microtubules. In addition to traditional low-molecular-weight compounds, newer drug discovery approaches such as the development of medium-molecular-weight drugs (peptide- or oligonucleotide-based drugs) and high-molecular-weight drugs (antibody-based drugs) provide alternative pathways to preventing the formation of abnormal tau. Of particular interest are recent studies suggesting that tau droplet formation by liquid-liquid phase separation may be the initial step in aberrant tau aggregation, as well results that implicate roles for tau in dendritic and nuclear functions. Here, we review the mechanisms through which drugs can target tau and consider recent clinical trials for the treatment of tauopathies. In addition, we discuss the utility of these newer strategies and propose future directions for research on tau-targeted therapeutics.

18F-flortaucipir uptake patterns in clinical subtypes of primary progressive aphasia

We analyzed ¹⁸F-flortaucipir uptake patterns and structural changes in patients with subtypes of primary progressive aphasia (PPA) using ¹⁸F-flortaucipir positron emission tomography and volumetric magnetic resonance imaging. We enrolled 34 consecutive patients with PPA (10 nonfluent/agrammatic PPA [nfvPPA], 18 semantic variant PPA [svPPA], and 6 logopenic variant PPA [lvPPA], as well as 20 healthy controls, and 20 patients with Alzheimer's disease. ¹⁸F-flortaucipir uptake was increased in the frontal cortex and underlying white matter, and subcortical nuclei in the 10 nfvPPA and 8 nfvPPA-amyloid-β (Aβ)- subgroup patients. In the svPPA patients (both the 13 svPPA-Aβ- and 5 svPPA-Aβ+), uptake generally increased in the widespread neocortex with left anterior temporal predominance. ¹⁸F-flortaucipir uptake patterns in the 6 lvPPA and the 5 lvPPA-Aβ+ subgroup patients were similar to those seen in the patients with Alzheimer's disease with mild predominance in the left lateral temporal cortex. Cortical thinning in each PPA subtype corresponded with increased ¹⁸F-flortaucipir uptake. ¹⁸F-flortaucipir uptake patterns and cortical atrophy were distinct and corresponded to areas related to the specific language functions that are impaired in each subtype of PPA.

Predominant subcortical accumulation of 18F-flortaucipir binding in behavioral variant frontotemporal dementia

Behavioral variant frontotemporal dementia (bvFTD) is the most common form of frontotemporal dementia, and tau pathology can be found in 40%-50% of bvFTD patients. In this study, we sought to investigate ¹⁸F-flortaucipir-binding patterns and their correlates in clinically diagnosed bvFTD patients by comparing with results for Alzheimer's disease (AD) patients. We enrolled 20 bvFTD, 20 AD, and 20 age-matched healthy subjects who underwent neuropsychological tests, magnetic resonance imaging, and tau positron emission tomography scans with ¹⁸F-flortaucipir. Regional standardized uptake value ratios for the cerebral cortex and underlying white matter were compared between the 2 groups. The bvFTD patients showed increased ¹⁸F-flortaucipir binding in the putamen and globus pallidus when compared to the healthy controls. In addition, bvFTD was associated with increased binding in the white matter regions underlying the frontal, anterior cingulate, and insula cortices. The bvFTD patients may exhibit predominantly subcortical ¹⁸F-flortaucipir-binding pattern that is distinct from the patterns seen in AD patients. We hypothesize that the clinical characteristics of bvFTD patients may be attributable to the dysfunctional frontal-subcortical networks. However, concerns remain regarding unknown "off-target" binding in the white matter and the basal ganglia.

Tau Filaments and the Development of Positron Emission Tomography Tracers

A pathological pathway leading from soluble, monomeric to insoluble, filamentous Tau, is believed to underlie human Tauopathies. Cases of frontotemporal dementia are caused by dominantly inherited mutations in MAPT, the Tau gene. They show that dysfunction of Tau protein is sufficient to cause neurodegeneration and dementia. Extrapolation to the more common sporadic Tauopathies leads one to conclude that the pathological pathway is central to the development of all cases of disease, even if there are multiple reasons for Tau assembly. These findings are conceptually similar to those reported for beta-amyloid, alpha-synuclein and prion protein. Here, we provide an overview of Tau filaments and their positron emission tomography ligands.

Slowly progressive dementia caused by MAPT R406W mutations: longitudinal report on a new kindred and systematic review

Background

The MAPT c.1216C > T (p.Arg406Trp; R406W) mutation is a known cause of frontotemporal dementia with Parkinsonism linked to chromosome 17 tau with Alzheimer’s disease-like clinical features.

Methods

We compiled clinical data from a new Swedish kindred with R406W mutation. Seven family members were followed longitudinally for up to 22 years. Radiological examinations were performed in six family members and neuropathological examinations in three. We systematically reviewed the literature and compiled clinical, radiological, and neuropathological data on 63 previously described R406W heterozygotes and 3 homozygotes.

Results

For all cases combined, the median age of onset was 56 years and the median disease duration was 13 years. Memory impairment was the most frequent symptom, behavioral disturbance and language impairment were less common, and Parkinsonism was rare. Disease progression was most often slow. The most frequent clinical diagnosis was Alzheimer’s disease. R406W homozygotes had an earlier age at onset and a higher frequency of behavioral symptoms and Parkinsonism than heterozygotes. In the new Swedish kindred, a consistent imaging finding was ventromedial temporal lobe atrophy, which was evident also in early disease stages as a widening of the collateral sulcus with ensuing atrophy of the parahippocampal gyrus. Unlike previously published R406W carriers, all three autopsied patients from the novel family showed neuropathological similarities with progressive supranuclear palsy, with predominant four-repeat (exon 10+) tau isoform (4R) tauopathy and neurofibrillary tangles accentuated in the basal-medial temporal lobe. Amyloid-β pathology was absent.

Conclusions

Dominance of 4R over three-repeat (exon 10−) tau isoforms contrasts with earlier reports of R406W patients and was not sufficiently explained by the presence of H1/H2 haplotypes in two of the autopsied patients. R406W patients often show a long course of disease with marked memory deficits. Both our neuropathological results and our imaging findings revealed that the ventromedial temporal lobes were extensively affected in the disease. We suggest that this area may represent the point of origin of tau deposition in this disease with relatively isolated tauopathy.

Electronic supplementary material

The online version of this article (doi:10.1186/s13195-017-0330-2) contains supplementary material, which is available to authorized users.

Familial tauopathy with P364S MAPT mutation: clinical course, neuropathology and ultrastructure of neuronal tau inclusions

AIMS

This report presents the clinical course, neuropathology and ultrastructure of neuronal tau inclusions of four Slovene relatives with P364S MAPT mutation.

METHODS

The clinical history of three out of four P364S MAPT mutation carriers was taken. After formalin fixation, thorough sampling of the central nervous system was followed by paraffin embedding, H&E, Gallyas, Bielschowsky and immunostaining with AT8, anti-3R, anti-4R tau, anti-amyloid-β, anti-TDP43 and anti-alpha-synuclein antibodies. The distribution and density of different types of neuronal tau inclusions were semiquantitatively assessed. In addition, the ultrastructure of neuronal tau inclusions was analysed.

RESULTS

Macroscopic examination of the brains was unremarkable. Microscopically, neuronal tau inclusions of almost all known types were widespread and distributed fairly uniformly in all cases. Pick bodies and swollen neurones were found in only one family member. Mutant tau was composed of 3R and 4R isoforms, with a slight predominance of 3R tau. Composite neuronal tau inclusion (CNTI), found in all four relatives, was a hallmark of the P364S MAPT mutation. CNTI showed compartmental differences in H&E and Gallyas staining, tau isoforms immunolabelling and ultrastructure, displaying fuzzy fibrils in the core and paired twisted tubules at the periphery.

CONCLUSIONS

P364S MAPT mutation is characterized clinically by a variable combination of frontotemporal dementia, parkinsonism and motor neurone disease of short duration, and neuropathologically by a widespread uniform distribution of all known neuronal tau inclusions in one family member. Two-compartment CNTI is a unique characteristic of the P364S MAPT mutation.

Intrafamilial phenotypic heterogeneity in a Taiwanese family with a MAPT p.R5H mutation: a case report and literature review

Background

Frontotemporal degeneration (FTD) is a clinically and genetically heterogeneous neurodegenerative disorder characterized by deficits in executive function that frequently overlaps with parkinsonism and motor neuron disorders. Several genes have been identified to cause autosomal dominant forms of FTD, including the gene coding for the protein associated with microtubule tau (MAPT). While most reported pathogenic mutations in MAPT occur in exons 9–13, few families have been reported with mutations outside of this region. Herein, we report a first Taiwanese family having the exon 1 p.Arg5His mutation in MAPT with intrafamilial phenotype heterogeneity.

Case presentation

A 63-year-old man presented with progressive non-fluent speech and impaired memory for 3 years. He then developed apraxia, myoclonus and parkinsonism feature at his right hand. Extensive neurologic and neurocognitive examination lead to a diagnosis of FTD mixed with corticobasal syndrome. Magnetic resonance imaging revealed asymmetric atrophy in the left frontal and temporal lobes and single-photon emission computed tomography indicated decreased metabolism in the same areas as well as the left basal ganglia. The patient’s mother had been diagnosed with amyotrophic lateral sclerosis (ALS) at the age of 60 and was deceased 10 years later due to respiratory failure. The patient’s younger sister had persistent depressive disorder in her early forties and did not have any prominent cognitive or motor dysfunctions. We performed genetic analysis applying a targeted next generation sequencing (NGS) panel covering MAPT, GRN, VCP, FUS, CHMP2B, and TARDBP on the proband, followed by Sanger sequencing of candidate genes in eight family members. Hexanucleotide repeat expansion of C9Orf72 was determined by repeat-primed PCR. We identified a missense mutation in exon 1 of MAPT gene, c.14G > A (p.R5H), which was previously reported in only two Japanese patients in a literature review. This substitution co-segregated with the disease phenotypes in the family.

Conclusions

This is the first report of the occurrence of the MAPT p.R5H mutation in the Taiwanese population. Our findings extend the current knowledge of phenotypic heterogeneity among family members carrying the MAPT p.R5H mutation.

Distinct binding of PET ligands PBB3 and AV-1451 to tau fibril strains in neurodegenerative tauopathies

Diverse neurodegenerative disorders are characterized by deposition of tau fibrils composed of conformers (i.e. strains) unique to each illness. The development of tau imaging agents has enabled visualization of tau lesions in tauopathy patients, but the modes of their binding to different tau strains remain elusive. Here we compared binding of tau positron emission tomography ligands, PBB3 and AV-1451, by fluorescence, autoradiography and homogenate binding assays with homologous and heterologous blockades using tauopathy brain samples. Fluorescence microscopy demonstrated intense labelling of non-ghost and ghost tangles with PBB3 and AV-1451, while dystrophic neurites were more clearly detected by PBB3 in brains of Alzheimer's disease and diffuse neurofibrillary tangles with calcification, characterized by accumulation of all six tau isoforms. Correspondingly, partially distinct distributions of autoradiographic labelling of Alzheimer's disease slices with 11C-PBB3 and 18F-AV-1451 were noted. Neuronal and glial tau lesions comprised of 4-repeat isoforms in brains of progressive supranuclear palsy, corticobasal degeneration and familial tauopathy due to N279K tau mutation and 3-repeat isoforms in brains of Pick's disease and familial tauopathy due to G272V tau mutation were sensitively detected by PBB3 fluorescence in contrast to very weak AV-1451 signals. This was in line with moderate 11C-PBB3 versus faint 18F-AV-1451 autoradiographic labelling of these tissues. Radioligand binding to brain homogenates revealed multiple binding components with differential affinities for 11C-PBB3 and 18F-AV-1451, and higher availability of binding sites on progressive supranuclear palsy tau deposits for 11C-PBB3 than 18F-AV-1451. Our data indicate distinct selectivity of PBB3 compared to AV-1451 for diverse tau fibril strains. This highlights the more robust ability of PBB3 to capture wide-range tau pathologies.

Roles of tau protein in health and disease

Tau is well established as a microtubule-associated protein in neurons. However, under pathological conditions, aberrant assembly of tau into insoluble aggregates is accompanied by synaptic dysfunction and neural cell death in a range of neurodegenerative disorders, collectively referred to as tauopathies. Recent advances in our understanding of the multiple functions and different locations of tau inside and outside neurons have revealed novel insights into its importance in a diverse range of molecular pathways including cell signalling, synaptic plasticity, and regulation of genomic stability. The present review describes the physiological and pathophysiological properties of tau and how these relate to its distribution and functions in neurons. We highlight the post-translational modifications of tau, which are pivotal in defining and modulating tau localisation and its roles in health and disease. We include discussion of other pathologically relevant changes in tau, including mutation and aggregation, and how these aspects impinge on the propensity of tau to propagate, and potentially drive neuronal loss, in diseased brain. Finally, we describe the cascade of pathological events that may be driven by tau dysfunction, including impaired axonal transport, alterations in synapse and mitochondrial function, activation of the unfolded protein response and defective protein degradation. It is important to fully understand the range of neuronal functions attributed to tau, since this will provide vital information on its involvement in the development and pathogenesis of disease. Such knowledge will enable determination of which critical molecular pathways should be targeted by potential therapeutic agents developed for the treatment of tauopathies.

Wild-Type, but Not Mutant N296H, Human Tau Restores Aβ-Mediated Inhibition of LTP in Tau-/- mice

Microtubule associated protein tau (MAPT) is involved in the pathogenesis of Alzheimer's disease and many forms of frontotemporal dementia (FTD). We recently reported that Aβ-mediated inhibition of hippocampal long-term potentiation (LTP) in mice requires tau. Here, we asked whether expression of human MAPT can restore Aβ-mediated inhibition on a mouse Tau^−/− background and whether human tau with an FTD-causing mutation (N296H) can interfere with Aβ-mediated inhibition of LTP. We used transgenic mouse lines each expressing the full human MAPT locus using bacterial artificial chromosome technology. These lines expressed all six human tau protein isoforms on a Tau^−/− background. We found that the human wild-type MAPT H1 locus was able to restore Aβ₄₂-mediated impairment of LTP. In contrast, Aβ₄₂ did not reduce LTP in slices in two independently generated transgenic lines expressing tau protein with the mutation N296H associated with frontotemporal dementia (FTD). Basal phosphorylation of tau measured as the ratio of AT8/Tau5 immunoreactivity was significantly reduced in N296H mutant hippocampal slices. Our data show that human MAPT is able to restore Aβ₄₂-mediated inhibition of LTP in Tau^−/− mice. These results provide further evidence that tau protein is central to Aβ-induced LTP impairment and provide a valuable tool for further analysis of the links between Aβ, human tau and impairment of synaptic function.

Frontotemporal lobar degeneration: Pathogenesis, pathology and pathways to phenotype

Frontotemporal Lobar Degeneration (FTLD) is a clinically, pathologically and genetically heterogeneous group of disorders that affect principally the frontal and temporal lobes of the brain. There are three major associated clinical syndromes, behavioral variant frontotemporal dementia (bvFTD), semantic dementia (SD) and progressive non-fluent aphasia (PNFA); three principal histologies, involving tau, TDP-43 and FUS proteins; and mutations in three major genes, MAPT, GRN and C9orf72, along with several other less common gene mutations. All three clinical syndromes can exist separately or in combination with Amyotrophic Lateral Sclerosis (ALS). SD is exclusively a TDP-43 proteinopathy, and PNFA may be so, with both showing tight clinical, histological and genetic inter-relationships. bvFTD is more of a challenge with overlapping histological and genetic features, involvement of any of the three aggregating proteins, and changes in any of the three major genes. However, when ALS is present, all cases show a clear histological phenotype with TDP-43 aggregated proteins, and familial forms are associated with expansions in C9orf72. TDP-43 and FUS are nuclear carrier proteins involved in the regulation of RNA metabolism, whereas tau protein - the product of MAPT - is responsible for the assembly/disassembly of microtubules, which are vital for intracellular transport. Mutations in TDP-43 and FUS genes are linked to clinical ALS rather than FTLD (with or without ALS), suggesting that clinical ALS may be a disorder of RNA metabolism. Conversely, the protein products of GRN and C9orf72, along with those of the other minor genes, appear to form part of the cellular protein degradation machinery. It is possible therefore that FTLD is a reflection of dysfunction within lysosomal/proteasomal systems resulting in failure to remove potentially neurotoxic (TDP-43 and tau) aggregates, which ultimately overwhelm capacity to function. Spread of aggregates along distinct pathways may account for the different clinical phenotypes, and patterns of progression of disease.

Tau mis-splicing in the pathogenesis of neurodegenerative disorders

Tau proteins, which stabilize the structure and regulate the dynamics of microtubules, also play important roles in axonal transport and signal transduction. Tau proteins are missorted, aggregated, and found as tau inclusions under many pathological conditions associated with neurodegenerative disorders, which are collectively known as tauopathies. In the adult human brain, tau protein can be expressed in six isoforms due to alternative splicing. The aberrant splicing of tau pre-mRNA has been consistently identified in a variety of tauopathies but is not restricted to these types of disorders as it is also present in patients with non-tau proteinopathies and RNAopathies. Tau mis-splicing results in isoform-specific impairments in normal physiological function and enhanced recruitment of excessive tau isoforms into the pathological process. A variety of factors are involved in the complex set of mechanisms underlying tau mis-splicing, but variation in the cis-element, methylation of the MAPT gene, genetic polymorphisms, the quantity and activity of spliceosomal proteins, and the patency of other RNA-binding proteins, are related to aberrant splicing. Currently, there is a lack of appropriate therapeutic strategies aimed at correcting the tau mis-splicing process in patients with neurodegenerative disorders. Thus, a more comprehensive understanding of the relationship between tau mis-splicing and neurodegenerative disorders will aid in the development of efficient therapeutic strategies for patients with a tauopathy or other, related neurodegenerative disorders. [BMB Reports 2016; 49(8): 405-413]

FTDP-17 with Pick body-like inclusions associated with a novel tau mutation, p.E372G

Mutations in microtubule-associated protein tau gene (MAPT) cause frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17). Here, we describe a patient with FTDP-17 and a novel missense mutation in exon 13 of MAPT, p.E372G. We compare clinicopathologic features of this patient to two previously unreported patients with another exon 13 mutation, p.G389R. The patient with the p.E372G mutation was a 40-year-old man with behavioral variant frontotemporal dementia (bvFTD), who subsequently developed agrammatic speech and parkinsonism. One of the FTDP-17 patients with p.G389R mutation presented at age 24 with agrammatic variant of primary progressive aphasia, and subsequently behavioral dysfunction. The other presented at age 53 with bvFTD, followed by agrammatic speech and corticobasal syndrome. Neuropathologic features of FTDP-17 due to p.E372G were similar to those of p.G389R, including tau-immunoreactive Pick body-like neuronal inclusions and swollen, tapering thread-like processes in white matter immunoreactive for 3-repeat and 4-repeat tau. Biochemical analysis of insoluble tau showed similar isoform compositions in p.E372G and p.G389R. Functional studies of the p.E372G mutation showed marked increase in tau filament formation and its reduced ability to promote microtubule assembly. Together these findings indicate that p.E372G is a pathogenic MAPT mutation that causes FTDP-17 similar to p.G389R.

Glial Tau Pathology in Tauopathies: Functional Consequences

Tauopathies are a class of neurodegenerative diseases characterized by the presence of hyperphosphorylated and aggregated tau pathology in neuronal and glial cells. Though the ratio of neuronal and glial tau aggregates varies across diseases, glial tau aggregates can populate the same degenerating brain regions as neuronal tau aggregates. While much is known about the deleterious consequences of tau pathology in neurons, the relative contribution of glial tau pathology to these diseases is less clear. Recent studies using a number of model systems implicate glial tau pathology in contributing to tauopathy pathogenesis. This review aims to highlight the functional consequences of tau overexpression in glial cells and explore the potential contribution of glial tau pathology in the pathogenesis of neurodegenerative tauopathies.

A Novel Tau Mutation in Exon 12, p.Q336H, Causes Hereditary Pick Disease

Pick disease (PiD) is a frontotemporal lobar degeneration with distinctive neuronal inclusions (Pick bodies) that are enriched in 3-repeat (3R) tau. Although mostly sporadic, mutations in the tau gene (MAPT) have been reported. We screened 24 cases of neuropathologically confirmed PiD for MAPT mutations and found a novel mutation (c.1008G>C, p.Q336H) in 1 patient. Pathogenicity was confirmed on microtubule assembly and tau filament formation assays. The patient was compared with sporadic PiD and PiD associated with MAPT mutations from a review of the literature. The patient had behavioral changes at 55 years of age, followed by reduced verbal fluency, parkinsonism, and death at 63 years of age. His mother and maternal uncle had similar symptoms. Recombinant tau with p.Q336H mutation formed filaments faster than wild-type tau, especially with 3R tau. It also promoted more microtubule assembly than wild-type tau. We conclude that mutations in MAPT, including p.Q336H, can be associated with clinical, pathologic, and biochemical features that are similar to those in sporadic PiD. The pathomechanism of p.Q336H, and another previously reported variant at the same codon (p.Q336R), seems to be unique to MAPT mutations in that they not only predispose to abnormal tau filament formation but also facilitate microtubule assembly in a 3R tau-dependent manner.

Propagation of tau pathology: hypotheses, discoveries, and yet unresolved questions from experimental and human brain studies

Tau is a microtubule-associated protein and a key regulator of microtubule stabilization as well as the main component of neurofibrillary tangles-a principle neuropathological hallmark of Alzheimer's disease (AD)-as well as pleomorphic neuronal and glial inclusions in neurodegenerative tauopathies. Cross-sectional studies of neurofibrillary pathology in AD reveal a stereotypic spatiotemporal pattern of neuronal vulnerability that correlates with disease severity; however, the relationship of this pattern to disease progression is less certain and exceptions to the typical pattern have been described in a subset of AD patients. The basis for the selective vulnerability of specific populations of neurons to tau pathology and cell death is largely unknown, although there have been a number of hypotheses based upon shared properties of vulnerable neurons (e.g., degree of axonal myelination or synaptic plasticity). A recent hypothesis for selective vulnerability takes into account the emerging science of functional connectivity based upon resting state functional magnetic resonance imaging, where subsets of neurons that fire synchronously define patterns of degeneration similar to specific neurodegenerative disorders, including various tauopathies. In the past 6 years, the concept of tau propagation has emerged from numerous studies in cell and animal models suggesting that tau moves from cell-to-cell and that this may trigger aggregation and region-to-region spread of tau pathology within the brain. How the spread of tau pathology relates to functional connectivity is an area of active investigation. Observations of templated folding and propagation of tau have prompted comparisons of tau to prions, the pathogenic proteins in transmissible spongiform encephalopathies. In this review, we discuss the most compelling studies in the field, discuss their shortcomings and consider their implications with respect to human tauopathies as well as the controversy that tauopathies may be prion-like disorders.

The Abundance of Nonphosphorylated Tau in Mouse and Human Tauopathy Brains Revealed by the Use of Phos-Tag Method

Tauopathies are neurodegenerative diseases characterized by aggregates of hyperphosphorylated tau. Previous studies have identified many disease-related phosphorylation sites on tau. However, it is not understood how tau is hyperphosphorylated and what extent these sites are phosphorylated in both diseased and normal brains. Most previous studies have used phospho-specific antibodies to analyze tau phosphorylation. These results are useful but do not provide information about nonphosphorylated tau. Here, we applied the method of Phos-tag SDS-PAGE, in which phosphorylated tau was separated from nonphosphorylated tau in vivo. Among heterogeneously phosphorylated tau species in adult mouse brains, the nonphosphorylated 0N4R isoform was detected most abundantly. In contrast, perinatal tau and tau in cold water-stressed mice were all phosphorylated with a similar extent of phosphorylation. In normal elderly human brains, nonphosphorylated 0N3R and 0N4R tau were most abundant. A slightly higher phosphorylation of tau, which may represent the early step of hyperphosphorylation, was increased in Alzheimer disease patients at Braak stage V. Tau with this phosphorylation state was pelleted by centrifugation, and sarkosyl-soluble tau in either Alzheimer disease or corticobasal degeneration brains showed phosphorylation profiles similar to tau in normal human brain, suggesting that hyperphosphorylation occurs in aggregated tau. These results indicate that tau molecules are present in multiple phosphorylation states in vivo, and nonphosphorylated forms are highly expressed among them.

Patterns of hippocampal tau pathology differentiate neurodegenerative dementias

BACKGROUND/AIMS

Deposits of phosphorylated tau protein and convergence of pathology in the hippocampus are the hallmarks of neurodegenerative tauopathies. Thus we aimed to evaluate whether regional and cellular vulnerability patterns in the hippocampus distinguish tauopathies or are influenced by their concomitant presence.

METHODS

We created a heat map of phospho-tau (AT8) immunoreactivity patterns in 24 hippocampal subregions/layers in individuals with Alzheimer's disease (AD)-related neurofibrillary degeneration (n = 40), Pick's disease (n = 8), progressive supranuclear palsy (n = 7), corticobasal degeneration (n = 6), argyrophilic grain disease (AGD, n = 18), globular glial tauopathy (n = 5), and tau-astrogliopathy of the elderly (n = 10). AT8 immunoreactivity patterns were compared by mathematical analysis.

RESULTS

Our study reveals disease-specific hot spots and regional selective vulnerability for these disorders. The pattern of hippocampal AD-related tau pathology is strongly influenced by concomitant AGD. Mathematical analysis reveals that hippocampal involvement in primary tauopathies is distinguishable from early-stage AD-related neurofibrillary degeneration.

CONCLUSION

Our data demonstrate disease-specific AT8 immunoreactivity patterns and hot spots in the hippocampus even in tauopathies, which primarily do not affect the hippocampus. These hot spots can be shifted to other regions by the co-occurrence of tauopathies like AGD. Our observations support the notion that globular glial tauopathies and tau-astrogliopathy of the elderly are distinct entities.

A novel triple repeat mutant tau transgenic model that mimics aspects of pick's disease and fronto-temporal tauopathies

Tauopathies are a group of disorders leading to cognitive and behavioral impairment in the aging population. While four-repeat (4R) Tau is more abundant in corticobasal degeneration, progressive supranuclear palsy, and Alzheimer’s disease, three-repeat (3R) Tau is the most abundant splice, in Pick's disease. A number of transgenic models expressing wild-type and mutant forms of the 4R Tau have been developed. However, few models of three-repeat Tau are available. A transgenic mouse model expressing three-repeat Tau was developed bearing the mutations associated with familial forms of Pick's disease (L266V and G272V mutations). Two lines expressing high (Line 13) and low (Line 2) levels of the three-repeat mutant Tau were analyzed. By Western blot, using antibodies specific to three-repeat Tau, Line 13 expressed 5-times more Tau than Line 2. The Tau expressed by these mice was most abundant in the frontal-temporal cortex and limbic system and was phosphorylated at residues detected by the PHF-1, AT8, CP9 and CP13 antibodies. The higher-expressing mice displayed hyperactivity, memory deficits in the water maze and alterations in the round beam. The behavioral deficits started at 6-8 months of age and were associated with a progressive increase in the accumulation of 3R Tau. By immunocytochemistry, mice from Line 13 displayed extensive accumulation of 3R Tau in neuronal cells bodies in the pyramidal neurons of the neocortex, CA1-3 regions, and dentate gyrus of the hippocampus. Aggregates in the granular cells had a globus appearance and mimic Pick’s-like inclusions. There were abundant dystrophic neurites, astrogliosis and synapto-dendritic damage in the neocortex and hippocampus of the higher expresser line. The hippocampal lesions were moderately argyrophilic and Thioflavin-S negative. By electron microscopy, discrete straight filament aggregates were detected in some neurons in the hippocampus. This model holds promise for better understanding the natural history and progression of 3R tauopathies and their relationship with mitochondrial alterations and might be suitable for therapeutical testing.

Invited review: Frontotemporal dementia caused by microtubule-associated protein tau gene (MAPT) mutations: a chameleon for neuropathology and neuroimaging

Hereditary frontotemporal dementia associated with mutations in the microtubule-associated protein tau gene (MAPT) is a protean disorder. Three neuropathologic subtypes can be recognized, based on the presence of inclusions made of tau isoforms with three and four repeats, predominantly three repeats and mostly four repeats. This is relevant for establishing a correlation between structural magnetic resonance imaging and positron emission tomography using tracers specific for aggregated tau. Longitudinal studies will be essential to determine the evolution of anatomical alterations from the asymptomatic stage to the various phases of disease following the onset of symptoms.

Hypoxia alters expression of zebrafish microtubule-associated protein tau (mapta, maptb) gene transcripts

BACKGROUND

Microtubule-associated protein tau (MAPT) is abundant in neurons and functions in assembly and stabilization of microtubules to maintain cytoskeletal structure. Human MAPT transcripts undergo alternative splicing to produce 3R and 4R isoforms normally present at approximately equal levels in the adult brain. Imbalance of the 3R-4R isoform ratio can affect microtubule binding and assembly and may promote tau hyperphosphorylation and neurofibrillary tangle formation as seen in neurodegenerative diseases such as frontotemporal dementia (FTD) and Alzheimer's disease (AD). Conditions involving hypoxia such as cerebral ischemia and stroke can promote similar tau pathology but whether hypoxic conditions cause changes in MAPT isoform formation has not been widely explored. We previously identified two paralogues (co-orthologues) of MAPT in zebrafish, mapta and maptb.

RESULTS

In this study we assess the splicing of transcripts of these genes in adult zebrafish brain under hypoxic conditions. We find hypoxia causes increases in particular mapta and maptb transcript isoforms, particularly the 6R and 4R isoforms of mapta and maptb respectively. Expression of the zebrafish orthologue of human TRA2B, tra2b, that encodes a protein binding to MAPT transcripts and regulating splicing, was reduced under hypoxic conditions, similar to observations in AD brain.

CONCLUSION

Overall, our findings indicate that hypoxia can alter splicing of zebrafish MAPT co-orthologues promoting formation of longer transcripts and possibly generating Mapt proteins more prone to hyperphosphorylation. This supports the use of zebrafish to provide insight into the mechanisms regulating MAPT transcript splicing under conditions that promote neuronal dysfunction and degeneration.

Therapeutic and diagnostic challenges for frontotemporal dementia

In the search for therapeutic modifiers, frontotemporal dementia (FTD) has traditionally been overshadowed by other conditions such as Alzheimer's disease (AD). A clinically and pathologically diverse condition, FTD has been galvanized by a number of recent discoveries such as novel genetic variants in familial and sporadic forms of disease and the identification of TAR DNA binding protein of 43 kDa (TDP-43) as the defining constituent of inclusions in more than half of cases. In combination with an ever-expanding knowledge of the function and dysfunction of tau-a protein which is pathologically aggregated in the majority of the remaining cases-there exists a greater understanding of FTD than ever before. These advances may indicate potential approaches for the development of hypothetical therapeutics, but FTD remains highly complex and the roles of tau and TDP-43 in neurodegeneration are still wholly unclear. Here the challenges facing potential therapeutic strategies are discussed, which include sufficiently accurate disease diagnosis and sophisticated technology to deliver effective therapies.

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.

Microtubule-associated protein tau as a therapeutic target in Alzheimer's disease

INTRODUCTION

Alzheimer's disease (AD) is a major public health problem in modern society and as yet, other than a few symptomatic drugs, there is no disease-modifying treatment for this disease available.

AREAS COVERED

Neurofibrillary pathology, which is made up from abnormally hyperphosphorylated microtubule-associated protein tau, is both a hallmark and key lesion of AD and related tauopathies. The density of neurofibrillary pathology in the cerebral cortex correlates with the degree of dementia. Both experimental and transgenic animal studies have consistently shown that abnormal hyperphosphorylation of tau causes cognitive impairment. Abnormal hyperphosphorylation of tau converts it from a microtubule assembly-promoting to a microtubule-disrupting protein and promotes its self-assembly into paired helical filaments. To date, the bulk of studies have shown that abnormal hyperphosphorylation is the key gain of toxic function step though some cell culture and transgenic mouse studies have also reported that aggregated tau can lead to neurodegeneration. In this article, we have reviewed data from our lab and that from PubMed search on the molecular mechanism of tau pathology and the potential of tau as a therapeutic target for AD and related disorders.

EXPERT OPINION

In our opinion, inhibition of abnormal hyperphosphorylation of tau is the most rational therapeutic target. Therapeutic approaches include restoration of the activity of protein phosphatase-2A, which is the major regulator of tau phosphorylation and the activity of which is compromised in AD brain, inhibition of one or more tau protein kinases which include GSK-3β, cyclin-dependent protein kinase-5, dual-specificity tyrosine phosphorylated-regulated kinase 1A, Ca(2+)/calmodulin-activated protein kinase II and casein kinase I, enhancement of O-GlcNAcylation of tau, and tau immunization.

Clinicopathologic assessment and imaging of tauopathies in neurodegenerative dementias

Microtubule-associated protein tau encoded by the MAPT gene binds to microtubules and is important for maintaining neuronal morphology and function. Alternative splicing of MAPT pre-mRNA generates six major tau isoforms in the adult central nervous system resulting in tau proteins with three or four microtubule-binding repeat domains. In a group of neurodegenerative disorders called tauopathies, tau becomes aberrantly hyperphosphorylated and dissociates from microtubules, resulting in a progressive accumulation of intracellular tau aggregates. The spectrum of sporadic frontotemporal lobar degeneration associated with tau pathology includes progressive supranuclear palsy, corticobasal degeneration, and Pick’s disease. Alzheimer’s disease is considered the most prevalent tauopathy. This review is divided into two broad sections. In the first section we discuss the molecular classification of sporadic tauopathies, with a focus on describing clinicopathologic relationships. In the second section we discuss the neuroimaging methodologies that are available for measuring tau pathology (directly using tau positron emission tomography ligands) and tau-mediated neuronal injury (magnetic resonance imaging and fluorodeoxyglucose positron emission tomography). Both sections have detailed descriptions of the following neurodegenerative dementias – Alzheimer’s disease, progressive supranuclear palsy, corticobasal degeneration and Pick’s disease.

Extensive deamidation at asparagine residue 279 accounts for weak immunoreactivity of tau with RD4 antibody in Alzheimer's disease brain

Background

Intracytoplasmic inclusions composed of filamentous tau proteins are defining characteristics of neurodegenerative tauopathies, but it remains unclear why different tau isoforms accumulate in different diseases and how they induce abnormal filamentous structures and pathologies. Two tau isoform-specific antibodies, RD3 and RD4, are widely used for immunohistochemical and biochemical studies of tau species in diseased brains.

Results

Here, we show that extensive irreversible post-translational deamidation takes place at asparagine residue 279 (N279) in the RD4 epitope of tau in Alzheimer’s disease (AD), but not corticobasal degeneration (CBD) or progressive supranuclear palsy (PSP), and this modification abrogates the immunoreactivity to RD4. An antiserum raised against deamidated RD4 peptide specifically recognized 4R tau isoforms, regardless of deamidation, and strongly stained tau in AD brain. We also found that mutant tau with N279D substitution showed reduced ability to bind to microtubules and to promote microtubule assembly.

Conclusion

The biochemical and structural differences of tau in AD from that in 4R tauopathies found in this study may therefore have implications for prion-like propagation of tau.

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.

Cognitive impairment in lacunar strokes: the SPS3 trial

OBJECTIVE

Lacunar strokes are a leading cause of cognitive impairment and vascular dementia. However, adequate characterization of cognitive impairment is lacking. The aim of this study was to estimate the prevalence and characterize the neuropsychological impairment in lacunar stroke patients.

METHODS

All English-speaking participants in the Secondary Prevention of Small Subcortical Strokes (SPS3) trial (National Clinical Trial 00059306) underwent neuropsychological testing at baseline. Raw scores were converted to z scores using published norms. Those with impairment (z ≤ -1.5) in memory and/or nonmemory domains were classified as having mild cognitive impairment (MCI).

RESULTS

Among the 1,636 participants, average z scores on all tests were < 0, with the largest deficits seen on tests of episodic memory (range of means, -0.65 to -0.92), verbal fluency (mean, -0.89), and motor dexterity (mean, -2.5). Forty-seven percent were classified as having MCI (36% amnestic, 37% amnestic multidomain, 28% nonamnestic). Of those with modified Rankin score 0-1 and Barthel score = 100, 41% had MCI. Younger age (odds ratio [OR] per 10-year increase, 0.87), male sex (OR, 1.3), less education (OR, 0.13-0.66 for higher education levels compared to 0-4 years education), poststroke disability (OR, 1.4), and impaired activities of daily living (OR, 1.8) were independently associated with MCI.

INTERPRETATION

In this large, well-characterized cohort of lacunar stroke patients, MCI was present in nearly half, including many with minimal or no physical disabilities. Cognitive dysfunction in lacunar stroke patients may commonly be overlooked in clinical practice but may be as important as motor and sensory sequelae.

Selective tau tyrosine nitration in non-AD tauopathies

Previously, we reported the characterization of two novel antibodies that react with tau nitrated at tyrosine 197 (Tau-nY197) and tyrosine 394 (Tau-nY394) in Alzheimer's disease (AD). In this report, we examined whether tau nitration at these sites also occurs in corticobasal degeneration (CBD), progressive supranuclear palsy (PSP) and Pick's disease (PiD), three neurodegenerative tauopathies that contain abundant tau deposits within glial and neuronal cell types but lack amyloid deposition. The reactivity of these antibodies was also compared to two previously characterized antibodies Tau-nY18 and Tau-nY29, specific for tau nitrated at tyrosine 18 and tyrosine 29, respectively. In the present experiments, Tau-nY18 did not label the classical pathological lesions of CBD or PSP but did label the neuronal lesions associated with PiD to a limited extent. In contrast, Tau-nY29 revealed some, but not all classes of tau inclusions associated with both CBD and PSP but did label numerous Pick body inclusions in PiD. Tau-nY197 was restricted to the neuropil threads in both CBD and PSP; however, similar to Tau-nY29, extensive Pick body pathology was clearly labeled. Tau-nY394 did not detect any of the lesions associated with these disorders. In contrast, extensive neuronal and glial tau pathology within these diseases was labeled by Tau-Y197, a monoclonal antibody that reacts within the Y-197-containing proline-rich region of the molecule. Based on our Western and IHC experiments, it appears that nitration of tau at tyrosine 29 is a pathological modification that might be associated with neurodegeneration. Collectively, our data suggest that site-specific tau tyrosine nitration events occur in a disease and lesion-specific manner, indicating that nitration appears to be a highly controlled modification in AD and non-AD tauopathies.

Clinical and neuroanatomical signatures of tissue pathology in frontotemporal lobar degeneration

Relating clinical symptoms to neuroanatomical profiles of brain damage and ultimately to tissue pathology is a key challenge in the field of neurodegenerative disease and particularly relevant to the heterogeneous disorders that comprise the frontotemporal lobar degeneration spectrum. Here we present a retrospective analysis of clinical, neuropsychological and neuroimaging (volumetric and voxel-based morphometric) features in a pathologically ascertained cohort of 95 cases of frontotemporal lobar degeneration classified according to contemporary neuropathological criteria. Forty-eight cases (51%) had TDP-43 pathology, 42 (44%) had tau pathology and five (5%) had fused-in-sarcoma pathology. Certain relatively specific clinicopathological associations were identified. Semantic dementia was predominantly associated with TDP-43 type C pathology; frontotemporal dementia and motoneuron disease with TDP-43 type B pathology; young-onset behavioural variant frontotemporal dementia with FUS pathology; and the progressive supranuclear palsy syndrome with progressive supranuclear palsy pathology. Progressive non-fluent aphasia was most commonly associated with tau pathology. However, the most common clinical syndrome (behavioural variant frontotemporal dementia) was pathologically heterogeneous; while pathologically proven Pick's disease and corticobasal degeneration were clinically heterogeneous, and TDP-43 type A pathology was associated with similar clinical features in cases with and without progranulin mutations. Volumetric magnetic resonance imaging, voxel-based morphometry and cluster analyses of the pathological groups here suggested a neuroanatomical framework underpinning this clinical and pathological diversity. Frontotemporal lobar degeneration-associated pathologies segregated based on their cerebral atrophy profiles, according to the following scheme: asymmetric, relatively localized (predominantly temporal lobe) atrophy (TDP-43 type C); relatively symmetric, relatively localized (predominantly temporal lobe) atrophy (microtubule-associated protein tau mutations); strongly asymmetric, distributed atrophy (Pick's disease); relatively symmetric, predominantly extratemporal atrophy (corticobasal degeneration, fused-in-sarcoma pathology). TDP-43 type A pathology was associated with substantial individual variation; however, within this group progranulin mutations were associated with strongly asymmetric, distributed hemispheric atrophy. We interpret the findings in terms of emerging network models of neurodegenerative disease: the neuroanatomical specificity of particular frontotemporal lobar degeneration pathologies may depend on an interaction of disease-specific and network-specific factors.