Benjamin Falcon

Cryo-EM structures of tau filaments from Alzheimer's disease.

Alzheimer’s disease is the most common neurodegenerative disease, and there are no mechanism-based therapies. The disease is defined by the presence of abundant neurofibrillary lesions and neuritic plaques in the cerebral cortex. Neurofibrillary lesions comprise paired helical and straight tau filaments, whereas tau filaments with different morphologies characterize other neurodegenerative diseases. No high-resolution structures of tau filaments are available. Here we present cryo-electron microscopy (cryo-EM) maps at 3.4–3.5 Å resolution and corresponding atomic models of paired helical and straight filaments from the brain of an individual with Alzheimer’s disease. Filament cores are made of two identical protofilaments comprising residues 306–378 of tau protein, which adopt a combined cross-β/β-helix structure and define the seed for tau aggregation. Paired helical and straight filaments differ in their inter-protofilament packing, showing that they are ultrastructural polymorphs. These findings demonstrate that cryo-EM allows atomic characterization of amyloid filaments from patient-derived material, and pave the way for investigation of a range of neurodegenerative diseases.

Conformation Determines the Seeding Potencies of Native and Recombinant Tau Aggregates

Background: Characteristics of seed-competent Tau are unknown. Results: Native Tau aggregates have a higher seeding potency than recombinant Tau aggregates. Recombinant Tau acquires the conformation and potency of native Tau aggregates by seeded assembly. Conclusion: Conformation determines the seeding potencies of Tau aggregates. Significance: Understanding the properties of seed-competent Tau gives insight into disease mechanisms. Intracellular Tau inclusions are a pathological hallmark of several neurodegenerative diseases, collectively known as the tauopathies. They include Alzheimer disease, tangle-only dementia, Pick disease, argyrophilic grain disease, chronic traumatic encephalopathy, progressive supranuclear palsy, and corticobasal degeneration. Tau pathology appears to spread through intercellular propagation, requiring the formation of assembled “prion-like” species. Several cell and animal models have been described that recapitulate aspects of this phenomenon. However, the molecular characteristics of seed-competent Tau remain unclear. Here, we have used a cell model to understand the relationships between Tau structure/phosphorylation and seeding by aggregated Tau species from the brains of mice transgenic for human mutant P301S Tau and full-length aggregated recombinant P301S Tau. Deletion of motifs 275VQIINK280 and 306VQIVYK311 abolished the seeding activity of recombinant full-length Tau, suggesting that its aggregation was necessary for seeding. We describe conformational differences between native and synthetic Tau aggregates that may account for the higher seeding activity of native assembled Tau. When added to aggregated Tau seeds from the brains of mice transgenic for P301S Tau, soluble recombinant Tau aggregated and acquired the molecular properties of aggregated Tau from transgenic mouse brain. We show that seeding is conferred by aggregated Tau that enters cells through macropinocytosis and seeds the assembly of endogenous Tau into filaments.

Like prions: the propagation of aggregated tau and α-synuclein in neurodegeneration

The abnormal aggregation of a small number of known proteins underlies the most common human neurodegenerative diseases. In tauopathies and synucleinopathies, the normally soluble intracellular proteins tau and &agr;-synuclein become insoluble and filamentous. In recent years, non-cell autonomous mechanisms of aggregate formation have come to the fore, suggesting that nucleation-dependent aggregation may occur in a localized fashion in human tauopathies and synucleinopathies, followed by seed-dependent propagation. There is a long prodromal phase between the formation of protein aggregates and the appearance of the first clinical symptoms, which manifest only after extensive propagation, opening novel therapeutic avenues.

Short Fibrils Constitute the Major Species of Seed-Competent Tau in the Brains of Mice Transgenic for Human P301S Tau

The interneuronal propagation of aggregated tau is believed to play an important role in the pathogenesis of human tauopathies. It requires the uptake of seed-competent tau into cells, seeding of soluble tau in recipient neurons and release of seeded tau into the extracellular space to complete the cycle. At present, it is not known which tau species are seed-competent. Here, we have dissected the molecular characteristics of seed-competent tau species from the TgP301S tau mouse model using various biochemical techniques and assessed their seeding ability in cell and animal models. We found that sucrose gradient fractions from brain lysates seeded cellular tau aggregation only when large (>10 mer) aggregated, hyperphosphorylated (AT8- and AT100-positive) and nitrated tau was present. In contrast, there was no detectable seeding by fractions containing small, oligomeric (<6 mer) tau. Immunodepletion of the large aggregated AT8-positive tau strongly reduced seeding; moreover, fractions containing these species initiated the formation and spreading of filamentous tau pathology in vivo, whereas fractions containing tau monomers and small oligomeric assemblies did not. By electron microscopy, seed-competent sucrose gradient fractions contained aggregated tau species ranging from ring-like structures to small filaments. Together, these findings indicate that a range of filamentous tau aggregates are the major species that underlie the spreading of tau pathology in the P301S transgenic model. SIGNIFICANCE STATEMENT The spread of tau pathology from neuron to neuron is postulated to account for, or at least to contribute to, the overall propagation of tau pathology during the development of human tauopathies including Alzheimers disease. It is therefore important to characterize the native tau species responsible for this process of seeding and pathology spreading. Here, we use several biochemical techniques to dissect the molecular characteristics of native tau protein conformers from TgP301S tau mice and show that seed-competent tau species comprise small fibrils capable of seeding tau pathology in cell and animal models. Characterization of seed-competent tau gives insight into disease mechanisms and therapeutic interventions.

Cytosolic Fc receptor TRIM21 inhibits seeded tau aggregation

Significance The mammalian cell cytoplasm contains numerous proteins with direct antimicrobial activity. Although these have been extensively studied in the context of viral and bacterial infection, it is unknown whether pathogenic self-propagating proteins, proposed to underlie common neurodegenerative diseases, can be targeted in a similar manner. We studied the ability of tripartite motif protein 21 (TRIM21), a newly identified intracellular antibody receptor, to intercept assemblies of misfolded tau, a cytoplasmic protein that aggregates in patients with Alzheimer’s disease. We developed tau “seeding” assays in human cells and found that TRIM21 could intercept and potently neutralize antibody-labeled tau assemblies. These findings demonstrate that the intracellular immune system can act against self-propagating misfolded proteins, with implications for ongoing attempts to develop antibody-based therapies for neurodegenerative disorders. Alzheimer’s disease (AD) and other neurodegenerative disorders are associated with the cytoplasmic aggregation of microtubule-associated protein tau. Recent evidence supports transcellular transfer of tau misfolding (seeding) as the mechanism of spread within an affected brain, a process reminiscent of viral infection. However, whereas microbial pathogens can be recognized as nonself by immune receptors, misfolded protein assemblies evade detection, as they are host-derived. Here, we show that when misfolded tau assemblies enter the cell, they can be detected and neutralized via a danger response mediated by tau-associated antibodies and the cytosolic Fc receptor tripartite motif protein 21 (TRIM21). We developed fluorescent, morphology-based seeding assays that allow the formation of pathological tau aggregates to be measured in situ within 24 h in the presence of picomolar concentrations of tau seeds. We found that anti-tau antibodies accompany tau seeds into the cell, where they recruit TRIM21 shortly after entry. After binding, TRIM21 neutralizes tau seeds through the activity of the proteasome and the AAA ATPase p97/VCP in a similar manner to infectious viruses. These results establish that intracellular antiviral immunity can be redirected against host-origin endopathogens involved in neurodegeneration.

Galectin-8–mediated selective autophagy protects against seeded tau aggregation

Assembled tau can transfer between cells and seed the aggregation of soluble tau. This process is thought to underlie the amplification and propagation of tau inclusions throughout the brain in neurodegenerative diseases, including Alzheimers disease. An understanding of the mechanisms involved may provide strategies for limiting assembled tau propagation. Here, we sought to determine how assembled tau seeds gain access to the cytosol and whether this access triggers cellular defenses. We show that tau assemblies enter cells through clathrin-independent endocytosis and escape from damaged endomembranes into the cytosol, where they seed the aggregation of soluble tau. We also found that the danger receptor galectin-8 detects damaged endomembranes and activates autophagy through recruitment of the cargo receptor nuclear dot protein 52 (NDP52). Inhibition of galectin-8– and NDP52-dependent autophagy increased seeded tau aggregation, indicating that autophagy triggered by damaged endomembranes during the entry of assembled tau seeds protects against tau aggregation, in a manner similar to cellular defenses against cytosol-dwelling microorganisms. A second autophagy cargo receptor, p62, then targeted seeded tau aggregates. Our results reveal that by monitoring endomembrane integrity, cells reduce entry of tau seeds into the cytosol and thereby prevent seeded aggregation. The mechanisms described here may help inform the development of therapies aimed at inhibiting the propagation of protein assemblies in neurodegenerative diseases.

Tau filaments from multiple cases of sporadic and inherited Alzheimer’s disease adopt a common fold

The ordered assembly of tau protein into abnormal filaments is a defining characteristic of Alzheimer’s disease (AD) and other neurodegenerative disorders. It is not known if the structures of tau filaments vary within, or between, the brains of individuals with AD. We used a combination of electron cryo-microscopy (cryo-EM) and immuno-gold negative-stain electron microscopy (immuno-EM) to determine the structures of paired helical filaments (PHFs) and straight filaments (SFs) from the frontal cortex of 17 cases of AD (15 sporadic and 2 inherited) and 2 cases of atypical AD (posterior cortical atrophy). The high-resolution structures of PHFs and SFs from the frontal cortex of 3 cases of AD, 2 sporadic and 1 inherited, were determined by cryo-EM. We also used immuno-EM to study the PHFs and SFs from a number of cortical and subcortical brain regions. PHFs outnumbered SFs in all AD cases. By cryo-EM, PHFs and SFs were made of two C-shaped protofilaments with a combined cross-β/β-helix structure, as described previously for one case of AD. The higher resolution structures obtained here showed two additional amino acids at each end of the protofilament. The immuno-EM findings, which indicated the presence of repeats 3 and 4, but not of the N-terminal regions of repeats 1 and 2, of tau in the filament cores of all AD cases, were consistent with the cryo-EM results. These findings show that there is no significant variation in tau filament structures between individuals with AD. This knowledge will be crucial for understanding the mechanisms that underlie tau filament formation and for developing novel diagnostics and therapies.

Measurement of Tau Filament Fragmentation Provides Insights into Prion-like Spreading

The ordered assembly of amyloidogenic proteins causes a wide spectrum of common neurodegenerative diseases, including Alzheimers and Parkinsons diseases. These diseases share common features with prion diseases, in which misfolded proteins can self-replicate and transmit disease across different hosts. Deciphering the molecular mechanisms that underlie the amplification of aggregates is fundamental for understanding how pathological deposits can spread through the brain and drive disease. Here, we used single-molecule microscopy to study the assembly and replication of tau at the single aggregate level. We found that tau aggregates have an intrinsic ability to amplify by filament fragmentation, and determined the doubling times for this replication process by kinetic modeling. We then simulated the spreading time for aggregates through the brain and found this to be in good agreement with both the observed time frame for spreading of pathological tau deposits in Alzheimers disease and in experimental models of tauopathies. With this work we begin to understand the physical parameters that govern the spreading rates of tau and other amyloids through the human brain.

CRYO-EM STRUCTURES OF TAU FILAMENTS FROM ALZHEIMER'S DISEASE BRAIN

not available. TUESDAY, JULY 18, 2017

CHARACTERISATION OF TAU SPECIES INVOLVED IN TAU SEEDING AND SPREAD IN CELLULAR AND ANIMAL MODELS

Background:The neurofibrillary tangle is a pathological hallmark of Alzheimer’s disease (AD) and primarily consists of hyperphosphorylated tau protein (pTau). pTau first appears in the entorhinal cortex in the presymptomatic stage, then gradually disseminates to the hippocampal region around the onset of clinical symptoms of AD. Halting this tau spread in the asympomatic stage is a promising therapeutic approach for AD. The exosome is a small vesicle of 50-100 nm in diameter, enriched in ceramide, and is suggested to contain neuropathogenic proteins, such as prion, a-synuclein, and recently tau proteins. A growing body of evidence suggests that microglia contribute to tauopathy-related pathogenesis in both human and animal models. We hypothesize that microglia transduce tau aggregates into nearby neuronal cells via exosomal secretion, and that inhibition of the exosome synthesis or secretory pathway reduces tau dissemination. Methods: Adeno-associated virus serotype 6 expressing FTDP-17-linked mutation of tau protein was stereotaxically injected into the entorhinal cortical region of C57BL/6 mice, and the animals were sacrificed at 7 and 28 days post injection. The brain specimens were examined for tau accumulation in the hippocampal region. The animals were also systemically treated with specific inhibitors of neutral sphingomyelinase-2 to block exosome synthesis, or colony stimulating factor 1 receptor (CSF1R) to deplete microglia. Results:We found that human tau propagate from entorhinal cortical neurons to dentate granular cells after AAV injection, and that this propagation is sensitive to the inhibition of exosome synthesis or microglial depletion. We also found that tau-containing exosomes isolated from microglia efficiently transduce tau protein to neurons in vitro and in vivo. Finally, these results were reproduced in P301S tau mice (PS19) treated with these compounds. Conclusions: These results demonstrate that exosome secretion from microglia play a significant role in propagation of tau protein from entorhinal cortex to hippocampal neurons. Our findings could lead to an entirely novel paradigm for delaying the progression of disease in AD and other tauopathies such as frontotemporal dementia and chronic traumatic encephalopathy.