Frontotemporal dementia with Parkinsonism linked to chromosome-17 mutations enhance tau oligomer formation

"Prion-like" seeding and propagation of oligomeric protein assemblies in neurodegenerative disorders

Intra- or extracellular aggregates of proteins are central pathogenic features in most neurodegenerative disorders. The accumulation of such proteins in diseased brains is believed to be the end-stage of a stepwise aggregation of misfolded monomers to insoluble cross-β fibrils via a series of differently sized soluble oligomers/protofibrils. Several studies have shown how α-synuclein, amyloid-β, tau and other amyloidogenic proteins can act as nucleating particles and thereby share properties with misfolded forms, or strains, of the prion protein. Although the roles of different protein assemblies in the respective aggregation cascades remain unclear, oligomers/protofibrils are considered key pathogenic species. Numerous observations have demonstrated their neurotoxic effects and a growing number of studies have indicated that they also possess seeding properties, enabling their propagation within cellular networks in the nervous system. The seeding behavior of oligomers differs between the proteins and is also affected by various factors, such as size, shape and epitope presentation. Here, we are providing an overview of the current state of knowledge with respect to the "prion-like" behavior of soluble oligomers for several of the amyloidogenic proteins involved in neurodegenerative diseases. In addition to providing new insight into pathogenic mechanisms, research in this field is leading to novel diagnostic and therapeutic opportunities for neurodegenerative diseases.

Intracellular tau fragment droplets serve as seeds for tau fibrils

Intracellular tau aggregation requires a local protein concentration increase, referred to as "droplets". However, the cellular mechanism for droplet formation is poorly understood. Here, we expressed OptoTau, a P301L mutant tau fused with CRY2olig, a light-sensitive protein that can form homo-oligomers. Under blue light exposure, OptoTau increased tau phosphorylation and was sequestered in aggresomes. Suppressing aggresome formation by nocodazole formed tau granular clusters in the cytoplasm. The granular clusters disappeared by discontinuing blue light exposure or 1,6-hexanediol treatment suggesting that intracellular tau droplet formation requires microtubule collapse. Expressing OptoTau-ΔN, a species of N-terminal cleaved tau observed in the Alzheimer's disease brain, formed 1,6-hexanediol and detergent-resistant tau clusters in the cytoplasm with blue light stimulation. These intracellular stable tau clusters acted as a seed for tau fibrils in vitro. These results suggest that tau droplet formation and N-terminal cleavage are necessary for neurofibrillary tangles formation in neurodegenerative diseases.

Atomic Force Microscopy Applied to the Study of Tauopathies

Atomic force microscopy (AFM) is a scanning probe microscopy technique which has a physical principle, the measurement of interatomic forces between a very thin tip and the surface of a sample, allowing the obtaining of quantitative data at the nanoscale, contributing to the surface study and mechanical characterization. Due to its great versatility, AFM has been used to investigate the structural and nanomechanical properties of several inorganic and biological materials, including neurons affected by tauopathies. Tauopathies are neurodegenerative diseases featured by aggregation of phosphorylated tau protein inside neurons, leading to functional loss and progressive neurotoxicity. In the broad universe of neurodegenerative diseases, tauopathies comprise the most prevalent, with Alzheimer's disease as its main representative. This review highlights the use of AFM as a suitable research technique for the study of cellular damages in tauopathies, even in early stages, allowing elucidation of pathogenic mechanisms of these diseases.

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.

HS3ST2 expression induces the cell autonomous aggregation of tau

Heparan sulfates have long been known to intracellularly accumulate in Alzheimer's disease neurons, where they colocalize with neurofibrillary tangles made of abnormally phosphorylated and aggregated tau protein. However, the reasons and consequences of the heparan sulfates accumulation in the Alzheimer's cells are not yet well understood. Previously, we showed that the neural heparan sulfate 3-O-sulfotransferase HS3ST2 is critical for the abnormal phosphorylation of tau in Alzheimer's disease-related tauopathy. Using cell models of tauopathy we showed that intracellular 3-O-sulfatated heparan sulfates interact with tau inducing its abnormal phosphorylation. However, it is unknown whether HS3ST2 expression induces the intracellular aggregation of tau in cells. Here, by using replicative pEBV plasmids, we engineered HEK293 cells to stably express HS3ST2 together with human tau carrying or not the P301S mutation. We show that HS3ST2 gain of function induces the cell autonomous aggregation of tau not only in cells expressing tauP301S, but also in cells expressing the wild type tau. Our engineered cells mimicked both the HS intracellular accumulation observed in neurons of Alzheimer's disease and the tau aggregation characteristic of tauopathy development and evolution. These results give evidence that the neural HS3ST2 plays a critical role in the cell autonomous self-aggregation of tau.

Biochemical and biophysical features of disease-associated tau mutants V363A and V363I

The comprehension of pathogenetic mechanisms in tauopathy-associated neurodegenerative diseases can be improved by the knowledge of the biochemical and biophysical features of mutated tau proteins. Here, we used the full-length, wild-type tau, the V363A and V363I mutated species, associated with pathology, and the P301L mutated tau as a benchmark. Using several techniques, including small-angle X-ray scattering, atomic force microscopy, thioflavin T binding, and electrophoretic separation, we compared their course from intrinsically disordered monomers in solution to early-stage recruitment in complexes and then aggregates of increasing size over long periods up to the asymptotic aggregative behavior of full-length tau proteins. We showed that diversity in the kinetics of recruitment and aggregate structure occurs from the beginning and spreads all over their pathway to very large objects. The different extents of conformational changes and types of molecular assemblies among the proteins were also reflected in their in vitro toxicity; this variation could correlate with physiopathology in humans, considering that the P301L mutation is more aggressive than V363A, especially V363I. This study identified the presence of aggregation intermediates and corroborated the oligomeric hypothesis of tauopathies.

Tau mRNA Metabolism in Neurodegenerative Diseases: A Tangle Journey

Tau proteins are known to be mainly involved in regulation of microtubule dynamics. Besides this function, which is critical for axonal transport and signal transduction, tau proteins also have other roles in neurons. Moreover, tau proteins are turned into aggregates and consequently trigger many neurodegenerative diseases termed tauopathies, of which Alzheimer’s disease (AD) is the figurehead. Such pathological aggregation processes are critical for the onset of these diseases. Among the various causes of tau protein pathogenicity, abnormal tau mRNA metabolism, expression and dysregulation of tau post-translational modifications are critical steps. Moreover, the relevance of tau function to general mRNA metabolism has been highlighted recently in tauopathies. In this review, we mainly focus on how mRNA metabolism impacts the onset and development of tauopathies. Thus, we intend to portray how mRNA metabolism of, or mediated by, tau is associated with neurodegenerative diseases.

Neurotoxicity of oligomers of phosphorylated Tau protein carrying tauopathy-associated mutation is inhibited by prion protein

Tauopathies, including Alzheimer's disease (AD), are manifested by the deposition of well-characterized amyloid aggregates of Tau protein in the brain. However, it is rather unlikely that these aggregates constitute the major form of Tau responsible for neurodegenerative changes. Currently, it is postulated that the intermediates termed as soluble oligomers, assembled on the amyloidogenic pathway, are the most neurotoxic form of Tau. However, Tau oligomers reported so far represent a population of poorly characterized, heterogeneous and unstable assemblies. In this study, to obtain the oligomers, we employed the aggregation-prone K18 fragment of Tau protein with deletion of Lys280 (K18Δ280) linked to a hereditary tauopathy. We have described a new procedure of inducing aggregation of mutated K18 which leads either to the formation of nontoxic amyloid fibrils or neurotoxic globular oligomers, depending on its phosphorylation status. We demonstrate that PKA-phosphorylated K18Δ280 oligomers are toxic to hippocampal neurons, which is manifested by loss of dendritic spines and neurites, and impairment of cell-membrane integrity leading to cell death. We also show that N1, the soluble N-terminal fragment of prion protein (PrP), protects neurons from the oligomers-induced cytotoxicity. Our findings support the hypothesis on the neurotoxicity of Tau oligomers and neuroprotective role of PrP-derived fragments in AD and other tauopathies. These observations could be useful in the development of therapeutic strategies for these diseases.

Tauopathy-associated tau modifications selectively impact neurodegeneration and mitophagy in a novel C. elegans single-copy transgenic model

Background

A defining pathological hallmark of the progressive neurodegenerative disorder Alzheimer’s disease (AD) is the accumulation of misfolded tau with abnormal post-translational modifications (PTMs). These include phosphorylation at Threonine 231 (T231) and acetylation at Lysine 274 (K274) and at Lysine 281 (K281). Although tau is recognized to play a central role in pathogenesis of AD, the precise mechanisms by which these abnormal PTMs contribute to the neural toxicity of tau is unclear.

Methods

Human 0N4R tau (wild type) was expressed in touch receptor neurons of the genetic model organism C. elegans through single-copy gene insertion. Defined mutations were then introduced into the single-copy tau transgene through CRISPR-Cas9 genome editing. These mutations included T231E, to mimic phosphorylation of a commonly observed pathological epitope, and K274/281Q, to mimic disease-associated lysine acetylation – collectively referred as “PTM-mimetics” – as well as a T231A phosphoablation mutant. Stereotypical touch response assays were used to assess behavioral defects in the transgenic strains as a function of age. Genetically-encoded fluorescent biosensors were expressed in touch neurons and used to measure neuronal morphology, mitochondrial morphology, mitophagy, and macro autophagy.

Results

Unlike existing tau overexpression models, C. elegans single-copy expression of tau did not elicit overt pathological phenotypes at baseline. However, strains expressing disease associated PTM-mimetics (T231E and K274/281Q) exhibited reduced touch sensation and neuronal morphological abnormalities that increased with age. In addition, the PTM-mimetic mutants lacked the ability to engage neuronal mitophagy in response to mitochondrial stress.

Conclusions

Limiting the expression of tau results in a genetic model where modifications that mimic pathologic tauopathy-associated PTMs contribute to cryptic, stress-inducible phenotypes that evolve with age. These findings and their relationship to mitochondrial stress provides a new perspective into the pathogenic mechanisms underlying AD.

Supplementary information

The online version contains supplementary material available at 10.1186/s13024-020-00410-7.

Phenotype Heterogeneity and Genotype Correlation of MAPT Mutations in a Chinese PUMCH Cohort

Frontotemporal dementia (FTD) is a heterogeneous disease both clinically and pathologically. Genetic mutation in microtubule-associated protein tau (MAPT) is the most common cause of FTD, and the phenotype is related to the mutation location. However, the phenotype and genotype correlation varies somewhat among different cohorts and ethnicities. Whole-genome next-generation sequencing (NGS) was carried out for 1351 patients with dementia at Peking Union Medical College Hospital. MAPT variations classified as pathogenic and of uncertain significance were identified. Demographic information, clinical presentations, and neuroimaging were collected, and the phenotype-genotype correlation was analyzed with a concurrent literature review. Twenty-four patients were enrolled; 8 patients carrying the D177V mutation are discussed separately. The average onset age was young, and most of them had a positive family history. Cognitive dysfunction, behavior, and personality changes as well as aphasia were the most common presentations. Most structural MRIs showed asymmetrical atrophy of the temporal lobe, with/without similar changes in the frontal lobe. L266V carriers presented with youngest onset typical behavior variant FTD or aphasia; P301L carriers presented with behavior variant FTD or aphasia. Functional MRI and molecular imaging also showed that the involved areas were similar to those with structural atrophy. D296N carriers presented atypical parkinsonism and cognitive dysfunction at older ages. Eight D177V carriers had extraordinarily different manifestations. The clinical phenotype of most of them was not FTD, though cerebral vascular lesions were obvious in some of them. MAPT mutation is rare in Chinese dementia patients. The phenotype and genotype correlation is specific and overlaps. The D177V mutation is possibly not directly pathogenic in our cohort. Some of the variants might increase the genetic risk of neurodegenerative diseases.

P301 L, an FTDP-17 Mutant, Exhibits Enhanced Glycation in vitro

BACKGROUND

Frontotemporal dementia and parkinsonism-linked to chromosome-17 are a group of diseases with tau mutations leading to primary tauopathies which include progressive supranuclear palsy, corticobasal syndrome, and frontotemporal lobar degeneration. Alzheimer's disease is a non-primary tauopathy, which displays tau neuropathology of excess tangle formation and accumulation. FTDP-17 mutations are responsible for early onset of AD, which can be attributed to compromised physiological functions due to the mutations. Tau is a microtubule-binding protein that secures the integrity of polymerized microtubules in neuronal cells. It malfunctions owing to various insults and stress conditions-like mutations and post-translational modifications.

OBJECTIVE

In this study, we modified the wild type and tau mutants by methyl glyoxal and thus studied whether glycation can enhance the aggregation of predisposed mutant tau.

METHODS

Tau glycation was studied by fluorescence assays, SDS-PAGE analysis, conformational evaluation, and transmission electron microscopy.

RESULTS

Our study suggests that FTDP-17 mutant P301 L leads to enhanced glycation-induced aggregation as well as advanced glycation end products formation. Glycation forms amorphous aggregates of tau and its mutants without altering its native conformation.

CONCLUSION

The metabolic anomalies and genetic predisposition have found to accelerate tau-mediated neurodegeneration and prove detrimental for the early-onset of Alzheimer's disease.

Distinct microscopic mechanisms for the accelerated aggregation of pathogenic Tau mutants revealed by kinetic analysis

Relating chemical kinetics and physical properties to pathogenicity of disease-related Tau mutants.

The Missing Heritability of Sporadic Frontotemporal Dementia: New Insights from Rare Variants in Neurodegenerative Candidate Genes

Frontotemporal dementia (FTD) is a common form of dementia among early-onset cases. Several genetic factors for FTD have been revealed, but a large proportion of FTD cases still have an unidentified genetic origin. Recent studies highlighted common pathobiological mechanisms among neurodegenerative diseases. In the present study, we investigated a panel of candidate genes, previously described to be associated with FTD and/or other neurodegenerative diseases by targeted next generation sequencing (NGS). We focused our study on sporadic FTD (sFTD), devoid of disease-causing mutations in GRN, MAPT and C9orf72. Since genetic factors have a substantially higher pathogenetic contribution in early onset patients than in late onset dementia, we selected patients with early onset (<65 years). Our study revealed that, in 50% of patients, rare missense potentially pathogenetic variants in genes previously associated with Alzheimer's disease, Parkinson disease, amyotrophic lateral sclerosis and Lewy body dementia (GBA, ABCA7, PARK7, FUS, SORL1, LRRK2, ALS2), confirming genetic pleiotropy in neurodegeneration. In parallel, a synergic genetic effect on FTD is suggested by the presence of variants in five different genes in one single patient. Further studies employing genome-wide approaches might highlight pathogenic variants in novel genes that explain the still missing heritability of FTD.

Neuropathology and pathogenesis of extrapyramidal movement disorders: a critical update-I. Hypokinetic-rigid movement disorders

Extrapyramidal movement disorders include hypokinetic rigid and hyperkinetic or mixed forms, most of them originating from dysfunction of the basal ganglia (BG) and their information circuits. The functional anatomy of the BG, the cortico-BG-thalamocortical, and BG-cerebellar circuit connections are briefly reviewed. Pathophysiologic classification of extrapyramidal movement disorder mechanisms distinguish (1) parkinsonian syndromes, (2) chorea and related syndromes, (3) dystonias, (4) myoclonic syndromes, (5) ballism, (6) tics, and (7) tremor syndromes. Recent genetic and molecular-biologic classifications distinguish (1) synucleinopathies (Parkinson's disease, dementia with Lewy bodies, Parkinson's disease-dementia, and multiple system atrophy); (2) tauopathies (progressive supranuclear palsy, corticobasal degeneration, FTLD-17; Guamian Parkinson-dementia; Pick's disease, and others); (3) polyglutamine disorders (Huntington's disease and related disorders); (4) pantothenate kinase-associated neurodegeneration; (5) Wilson's disease; and (6) other hereditary neurodegenerations without hitherto detected genetic or specific markers. The diversity of phenotypes is related to the deposition of pathologic proteins in distinct cell populations, causing neurodegeneration due to genetic and environmental factors, but there is frequent overlap between various disorders. Their etiopathogenesis is still poorly understood, but is suggested to result from an interaction between genetic and environmental factors. Multiple etiologies and noxious factors (protein mishandling, mitochondrial dysfunction, oxidative stress, excitotoxicity, energy failure, and chronic neuroinflammation) are more likely than a single factor. Current clinical consensus criteria have increased the diagnostic accuracy of most neurodegenerative movement disorders, but for their definite diagnosis, histopathological confirmation is required. We present a timely overview of the neuropathology and pathogenesis of the major extrapyramidal movement disorders in two parts, the first one dedicated to hypokinetic-rigid forms and the second to hyperkinetic disorders.

Assembly of transgenic human P301S Tau is necessary for neurodegeneration in murine spinal cord

A pathological pathway leading from soluble monomeric to insoluble filamentous Tau is characteristic of many human neurodegenerative diseases, which also exhibit dysfunction and death of brain cells. However, it is unknown how the assembly of Tau into filaments relates to cell loss. To study this, we first used a mouse line transgenic for full-length human mutant P301S Tau to investigate the temporal relationship between Tau assembly into filaments, assessed using anti-Tau antibody AT100, and motor neuron numbers, in the lumbar spinal cord. AT100 immunoreactivity preceded nerve cell loss. Murine Tau did not contribute significantly to either Tau aggregation or neurodegeneration. To further study the relevance of filament formation for neurodegeneration, we deleted hexapeptides ²⁷⁵VQIINK²⁸⁰ and ³⁰⁶VQIVYK³¹¹, either singly or in combination, from human 0N4R Tau with the P301S mutation. These hexapeptides are essential for the assembly of Tau into filaments. Homozygous mice transgenic for P301S Tau with the hexapeptide deletions, which expressed Tau at a similar level to the heterozygous line transgenic for P301S Tau, had a normal lifespan, unlike mice from the P301S Tau line. The latter had significant levels of sarkosyl-insoluble Tau in brain and spinal cord, and exhibited neurodegeneration. Mice transgenic for P301S Tau with the hexapeptide deletions failed to show significant levels of sarkosyl-insoluble Tau or neurodegeneration. Recombinant P301S Tau with the hexapeptide deletions failed to form β-sheet structure and filaments following incubation with heparin. Taken together, we conclude that β-sheet assembly of human P301S Tau is necessary for neurodegeneration in transgenic mice.

Tau Oligomers

The accumulation of tau filaments in neurons is a pathological hallmark of various neurodegenerative diseases, including Alzheimer's disease. However, it is not the filamentous aggregates themselves, but non-filamentous tau species, tau oligomer, that is thought to be the culprit in tau-mediated neurodegeneration. The definition of and methodology for isolating tau oligomers vary among researchers. Here we describe how tau oligomers are identified, summarize the differences of tau oligomers among research groups, and discuss their hypothesized functions.