Human tauopathy-derived tau strains determine the substrates recruited for templated amplification

Precision proteoform design for 4R tau isoform selective templated aggregation

Prion-like spread of disease-specific tau conformers is a hallmark of all tauopathies. A 19-residue probe peptide containing a P301L mutation and spanning the R2/R3 splice junction of tau folds and stacks into seeding-competent fibrils and induces aggregation of 4R, but not 3R tau. These tau peptide fibrils propagate aggregated intracellular tau over multiple generations, have a high β-sheet content, a colocalized lipid signal, and adopt a well-defined U-shaped fold found in 4R tauopathy brain-derived fibrils. Fully atomistic replica exchange molecular dynamics (MD) simulations were used to compute the free energy landscapes of the conformational ensemble of the peptide monomers. These identified an aggregation-prohibiting β-hairpin structure and an aggregation-competent U-fold unique to 4R tauopathy fibrils. Guided by MD simulations, we identified that the N-terminal-flanking residues to PHF6, which slightly vary between 4R and 3R isoforms, modulate seeding. Strikingly, when a single amino acid switch at position 305 replaced the serine of 4R tau with a lysine from the corresponding position in the first repeat of 3R tau, the seeding induced by the 19-residue peptide was markedly reduced. Conversely, a 4R tau mimic with three repeats, prepared by replacing those amino acids in the first repeat with those amino acids uniquely present in the second repeat, recovered aggregation when exposed to the 19-residue peptide. These peptide fibrils function as partial prions to recruit naive 4R tau-ten times the length of the peptide-and serve as a critical template for 4R tauopathy propagation. These results hint at opportunities for tau isoform-specific therapeutic interventions.

Probe-dependent Proximity Profiling (ProPPr) Uncovers Similarities and Differences in Phospho-Tau-Associated Proteomes Between Tauopathies

Tauopathies represent a diverse group of neurodegenerative disorders characterized by the abnormal aggregation of the microtubule-associated protein tau. Despite extensive research, the precise mechanisms underlying the complexity of different types of tau pathology remain incompletely understood. Here we describe an approach for proteomic profiling of aggregate-associated proteomes on slides with formalin-fixed, paraffin-embedded (FFPE) tissue that utilizes proximity labelling upon high preservation of aggregate morphology, which permits the profiling of pathological aggregates regardless of their size. To comprehensively investigate the common and unique protein interactors associated with the variety of tau lesions present across different human tauopathies, Alzheimer's disease (AD), corticobasal degeneration (CBD), Pick's disease (PiD), and progressive supranuclear palsy (PSP), were selected to represent the major tauopathy diseases. Implementation of our widely applicable Probe-dependent Proximity Profiling (ProPPr) strategy, using the AT8 antibody, permitted identification and quantification of proteins associated with phospho-tau lesions in well-characterized human post-mortem tissue. The analysis revealed both common and disease-specific proteins associated with phospho-tau aggregates, highlighting potential targets for therapeutic intervention and biomarker development. Candidate validation through high-resolution co-immunofluorescence of distinct aggregates across disease and control cases, confirmed the association of retromer complex protein VPS35 with phospho-tau lesions across the studied tauopathies. Furthermore, we discovered disease-specific associations of proteins including ferritin light chain (FTL) and the neuropeptide precursor VGF within distinct pathological lesions. Notably, examination of FTL-positive microglia in CBD astrocytic plaques indicate a potential role for microglial involvement in the pathogenesis of these tau lesions. Our findings provide valuable insights into the proteomic landscape of tauopathies, shedding light on the molecular mechanisms underlying tau pathology. This first comprehensive characterization of tau-associated proteomes across different tauopathies enhances our understanding of disease heterogeneity and provides a resource for future functional investigation, as well as development of targeted therapies and diagnostic biomarkers.

Post-Translational Modifications Control Phase Transitions of Tau

The self-assembly of Tau(297-391) into filaments, which mirror the structures observed in Alzheimer's disease (AD) brains, raises questions about the role of AD-specific post-translational modifications (PTMs) in the formation of paired helical filaments (PHFs). To investigate this, we developed a synthetic approach to produce Tau(291-391) featuring N-acetyllysine, phosphoserine, phosphotyrosine, and N-glycosylation at positions commonly modified in post-mortem AD brains, thus facilitating the study of their roles in Tau pathology. Using transmission electron microscopy (TEM), cryo-electron microscopy (cryo-EM), and a range of optical microscopy techniques, we discovered that these modifications generally hinder the in vitro assembly of Tau into PHFs. Interestingly, while acetylation's effect on Tau assembly displayed variability, either promoting or inhibiting phase transitions in the context of cofactor free aggregation, heparin-induced aggregation, and RNA-mediated liquid-liquid phase separation (LLPS), phosphorylation uniformly mitigated these processes. Our observations suggest that PTMs, particularly those situated outside the fibril's rigid core are pivotal in the nucleation of PHFs. Moreover, in scenarios involving heparin-induced aggregation leading to the formation of heterogeneous aggregates, most AD-specific PTMs, except for K311, appeared to decelerate the aggregation process. The impact of acetylation on RNA-induced LLPS was notably site-dependent, exhibiting both facilitative and inhibitory effects, whereas phosphorylation consistently reduced LLPS across all proteoforms examined. These insights underscore the complex interplay between site-specific PTMs and environmental factors in modulating Tau aggregation kinetics, enhancing our understanding of the molecular underpinnings of Tau pathology in AD and highlighting the critical role of PTMs located outside the ordered filament core in driving the self-assembly of Tau into PHF structures.

Distinct tau folds initiate templated seeding and alter the post-translational modification profile

Pathological tau accumulates in the brain in tauopathies such as Alzheimer's disease, Pick's disease, progressive supranuclear palsy and corticobasal degeneration, and forms amyloid-like filaments incorporating various post-translational modifications (PTMs). Cryo-electron microscopic (cryo-EM) studies have demonstrated that tau filaments extracted from tauopathy brains are characteristic of the disease and share a common fold(s) in the same disease group. Furthermore, the tau PTM profile changes during tau pathology formation and disease progression, and disease-specific PTMs are detected in and around the filament core. In addition, templated seeding has been suggested to trigger pathological tau amplification and spreading in vitro and in vivo, although the molecular mechanisms are not fully understood. Recently, we reported that the cryo-EM structures of tau protofilaments in SH-SY5Y cells seeded with patient-derived tau filaments show a core structure(s) resembling that of the original seeds. Here, we investigated PTMs of tau filaments accumulated in the seeded cells by liquid chromatography/tandem mass spectrometry and compared them with the PTMs of patient-derived tau filaments. Examination of insoluble tau extracted from SH-SY5Y cells showed that numerous phosphorylation, deamidation and oxidation sites detected in the fuzzy coat in the original seeds were well reproduced in SH-SY5Y cells. Moreover, templated tau filament formation preceded both truncation of the N-/C-terminals of tau and PTMs in and around the filament core, indicating these PTMs may predominantly be introduced after the degradation of the fuzzy coat.

Precision Proteoform Design for 4R Tau Isoform Selective Templated Aggregation

Prion-like spread of disease-specific tau conformers is a hallmark of all tauopathies. A 19-residue probe peptide containing a P301L mutation and spanning the R2/R3 splice junction of tau, folds and stacks into seeding-competent fibrils and induces aggregation of 4R, but not 3R tau. These tau peptide fibrils propagate aggregated intracellular tau over multiple generations, have a high β-sheet content, a colocalized lipid signal, and adopt a well-defined U-shaped fold found in 4R tauopathy brain-derived fibrils. Fully atomistic replica exchange molecular dynamics (MD) simulations were used to compute the free energy landscapes of the conformational ensemble of the peptide monomers. These identified an aggregation-prohibiting β-hairpin structure and an aggregation-competent U-fold unique to 4R tauopathy fibrils. Guided by MD simulations, we identified that the N-terminal-flanking residues to PHF6, which slightly vary between 4R and 3R isoforms, modulate seeding. Strikingly, when a single amino acid switch at position 305 replaced the serine of 4R tau with a lysine from the corresponding position in the first repeat of 3R tau, the seeding induced by the 19-residue peptide was markedly reduced. Conversely, a 4R tau mimic with three repeats, prepared by replacing those amino acids in the first repeat with those amino acids uniquely present in the second repeat, recovered aggregation when exposed to the 19-residue peptide. These peptide fibrils function as partial prions to recruit naïve 4R tau-ten times the length of the peptide-and serve as a critical template for 4R tauopathy propagation. These results hint at opportunities for tau isoform-specific therapeutic interventions.

The Rossy Progressive Supranuclear Palsy Centre: Creation and Initial Experience

OBJECTIVE

To describe the development and initial experience of a clinical research program in progressive supranuclear palsy (PSP) and corticobasal syndrome (CBS) in Canada: The Rossy PSP Centre, to share the data acquisition tools adopted, and to report preliminary results.

METHODS

Extensive demographic and longitudinal clinical information is collected every 6 months using standardized forms. Biofluids are collected for biobanking and genetic analysis, and many patients are enrolled in neuroimaging research protocols. Brain donation is an important component of the program, and standardized processing protocols have been established, including very short death to autopsy times in patients undergoing medical assistance in dying.

RESULTS

Between Oct 2019 and Dec 2021, 132 patients were screened, 91 fulfilling criteria for PSP and 19 for CBS; age 71 years; 41% female; duration 5 years, age-of-onset 66 years. The most common symptoms at onset were postural instability and falls (45%), cognitive-behavioral changes (22%), and Parkinsonism (9%). The predominant clinical phenotype was Richardson syndrome (82%). Levodopa and amantadine resulted in partial and short-lasting benefit.

CONCLUSIONS

The Rossy PSP Centre has been established to advance clinical and basic research in PSP and related tauopathies. The extent of the clinical data collected permits deep phenotyping of patients and allows for future clinical and basic research. Preliminary results showed expected distribution of phenotypes, demographics, and response to symptomatic treatments in our cohort. Longitudinal data will provide insight into the early diagnosis and management of PSP. Future steps include enrollment of patients in earlier stages, development of biomarkers, and fast-tracking well-characterized patients into clinical trials.

Human tauopathy strains defined by phosphorylation in R1-R2 repeat domains of tau

Distinctive post-translational modifications (PTM) characterize tau inclusions found in tauopathy patients. Using detergent-insoluble tau isolated from Alzheimer's disease (AD-tau) or Progressive Supranuclear Palsy (PSP-tau) patients, we provide insights into whether phosphorylation of critical residues determine templated tau seeding. Our initial data with phosphorylation-ablating mutations (Ser/Thr → Ala) on select sites of P301L tau showed no changes in seeding efficacy by AD-tau or PSP-tau. Interestingly, when specific sites in the R1-R2 repeat domains (Ser262/Thr263/Ser289/Ser305) were mutated to phosphorylation-mimicking amino acid Glu, it substantially reduced the seeding efficiency of AD-tau, but not PSP-tau seeds. The resultant detergent-insoluble tau shows deficient phosphorylation on AT8, AT100, AT180 and PHF1 epitopes, indicating inter-domain cooperativity. We further identify Ser305 as a critical determinant of AD-tau-specific seeding, whereby the phospho-mimicking Ser305Glu tau abrogates seeding by AD-tau but not PSP-tau. This suggests that phosphorylation on Ser305 could be related to the formation of disease-specific tau strains. Our results highlight the existence of a phospho-PTM code in tau seeding and further demonstrate the distinctive nature of this code in 4R tauopathies.

Cryo-EM structures of tau filaments from SH-SY5Y cells seeded with brain extracts from cases of Alzheimer's disease and corticobasal degeneration

The formation of amyloid filaments through templated seeding is believed to underlie the propagation of pathology in most human neurodegenerative diseases. A widely used model system to study this process is to seed amyloid filament formation in cultured cells using human brain extracts. Here, we report the electron cryo-microscopy structures of tau filaments from  undifferentiated seeded SH-SY5Y cells that transiently expressed N-terminally HA-tagged 1N3R or 1N4R human tau, using brain extracts from individuals with Alzheimer's disease or corticobasal degeneration. Although the resulting filament structures differed from those of the brain seeds, some degrees of structural templating were observed. Studying templated seeding in cultured cells, and determining the structures of the resulting filaments, can thus provide insights into the cellular aspects underlying neurodegenerative diseases.

Common and Specific Marks of Different Tau Strains Following Intra-Hippocampal Injection of AD, PiD, and GGT Inoculum in hTau Transgenic Mice

Heterozygous hTau mice were used for the study of tau seeding. These mice express the six human tau isoforms, with a high predominance of 3Rtau over 4Rtau. The following groups were assessed: (i) non-inoculated mice aged 9 months (n = 4); (ii) Alzheimer's Disease (AD)-inoculated mice (n = 4); (iii) Globular Glial Tauopathy (GGT)-inoculated mice (n = 4); (iv) Pick's disease (PiD)-inoculated mice (n = 4); (v) control-inoculated mice (n = 4); and (vi) inoculated with vehicle alone (n = 2). AD-inoculated mice showed AT8-immunoreactive neuronal pre-tangles, granular aggregates, and dots in the CA1 region of the hippocampus, dentate gyrus (DG), and hilus, and threads and dots in the ipsilateral corpus callosum. GGT-inoculated mice showed unique or multiple AT8-immunoreactive globular deposits in neurons, occasionally extended to the proximal dendrites. PiD-inoculated mice showed a few loose pre-tangles in the CA1 region, DG, and cerebral cortex near the injection site. Coiled bodies were formed in the corpus callosum in AD-inoculated mice, but GGT-inoculated mice lacked globular glial inclusions. Tau deposits in inoculated mice co-localized active kinases p38-P and SAPK/JNK-P, thus suggesting active phosphorylation of the host tau. Tau deposits were absent in hTau mice inoculated with control homogenates and vehicle alone. Deposits in AD-inoculated hTau mice contained 3Rtau and 4Rtau; those in GGT-inoculated mice were mainly stained with anti-4Rtau antibodies, but a small number of deposits contained 3Rtau. Deposits in PiD-inoculated mice were stained with anti-3Rtau antibodies, but rare neuronal, thread-like, and dot-like deposits showed 4Rtau immunoreactivity. These findings show that tau strains produce different patterns of active neuronal seeding, which also depend on the host tau. Unexpected 3Rtau and 4Rtau deposits after inoculation of homogenates from 4R and 3R tauopathies, respectively, suggests the regulation of exon 10 splicing of the host tau during the process of seeding, thus modulating the plasticity of the cytoskeleton.

Tau interactome and RNA binding proteins in neurodegenerative diseases

Pathological tau aggregation is a primary neuropathological feature of many neurodegenerative diseases. Intriguingly, despite the common presence of tau aggregates in these diseases the affected brain regions, clinical symptoms, and morphology, conformation, and isoform ratio present in tau aggregates varies widely. The tau-mediated disease mechanisms that drive neurodegenerative disease are still unknown. Tau interactome studies are critically important for understanding tauopathy. They reveal the interacting partners that define disease pathways, and the tau interactions present in neuropathological aggregates provide potential insight into the cellular environment and protein interactions present during pathological tau aggregation. Here we provide a combined analysis of 12 tau interactome studies of human brain tissue, human cell culture models and rodent models of disease. Together, these studies identified 2084 proteins that interact with tau in human tissue and 1152 proteins that interact with tau in rodent models of disease. Our combined analysis of the tau interactome revealed consistent enrichment of interactions between tau and proteins involved in RNA binding, ribosome, and proteasome function. Comparison of human and rodent tau interactome studies revealed substantial differences between the two species. We also performed a second analysis to identify the tau interacting proteins that are enriched in neurons containing granulovacuolar degeneration or neurofibrillary tangle pathology. These results revealed a timed dysregulation of tau interactions as pathology develops. RNA binding proteins, particularly HNRNPs, emerged as early disease-associated tau interactors and therefore may have an important role in driving tau pathology.

Biological basis and psychiatric symptoms in frontotemporal dementia

Frontotemporal dementia is a neurodegenerative disease characterized by focal degeneration of the frontal and temporal lobes, clinically presenting with disinhibited behavior, personality changes, progressive non-fluent aphasia and/or impaired semantic memory. Research progress has been made in re-organizing the clinical concept of frontotemporal dementia and neuropathological classification based on multiple accumulating proteins. Alongside this progress a list of genetic mutations or variants that are causative or increase the risk of frontotemporal dementia have been identified and some of these gene products are extensively studied. However, there are still a lot of points that need to be overcome, including lack of specific diagnostic biomarker which enable antemortem diagnosis of underlying neurodegenerative process, and lack of disease modifying therapy which could prevent disease progression. Early and precise diagnosis of frontotemporal dementia is urgently required. In this context, how to define prodromal frontotemporal dementia and early differential diagnosis from primary psychiatric disorders are also important issues. In this review we will summarize and discuss current understanding of biological basis and psychiatric symptoms in frontotemporal dementia.

Ultrastructural and biochemical classification of pathogenic tau, α-synuclein and TDP-43

Intracellular accumulation of abnormal proteins with conformational changes is the defining neuropathological feature of neurodegenerative diseases. The pathogenic proteins that accumulate in patients' brains adopt an amyloid-like fibrous structure and exhibit various ultrastructural features. The biochemical analysis of pathogenic proteins in sarkosyl-insoluble fractions extracted from patients' brains also shows disease-specific features. Intriguingly, these ultrastructural and biochemical features are common within the same disease group. These differences among the pathogenic proteins extracted from patients' brains have important implications for definitive diagnosis of the disease, and also suggest the existence of pathogenic protein strains that contribute to the heterogeneity of pathogenesis in neurodegenerative diseases. Recent experimental evidence has shown that prion-like propagation of these pathogenic proteins from host cells to recipient cells underlies the onset and progression of neurodegenerative diseases. The reproduction of the pathological features that characterize each disease in cellular and animal models of prion-like propagation also implies that the structural differences in the pathogenic proteins are inherited in a prion-like manner. In this review, we summarize the ultrastructural and biochemical features of pathogenic proteins extracted from the brains of patients with neurodegenerative diseases that accumulate abnormal forms of tau, α-synuclein, and TDP-43, and we discuss how these disease-specific properties are maintained in the brain, based on recent experimental insights.

Relationship Between 18F-Flortaucipir Uptake and Histologic Lesion Types in 4-Repeat Tauopathies

Progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD) are 4-repeat (4R) tauopathies with overlapping, but also morphologically distinct, tau immunoreactive lesions that vary in count by brain region. 18F-flortaucipir PET uptake has been reported to correlate with overall tau burden, and-in 1 CBD case-to have greater affinity to threads than tangles. We determined whether 18F-flortaucipir uptake is associated with histologic lesion type in 4R tauopathies. Methods: We performed semiquantitative regional lesion counts on pretangles/neurofibrillary tangles, threads, oligodendroglial coiled bodies, tufted astrocytes, and astrocytic plaques in 29 cases of autopsied 4R tauopathy (PSP, 16; CBD, 13). Regression models were used for statistical analyses. Results:18F-flortaucipir uptake marginally correlated with threads in the precentral cortex (P = 0.04) and with astrocytic lesions in the red nucleus (P = 0.05). Conclusion: The findings do not support 18F-flortaucipir's having differential affinity to any 4R tau lesion type.

Tau R2 and R3 are essential regions for tau aggregation, seeding and propagation

Tauopathies are characterised by intracellular deposits of fibrillar tau tangles. However, the interneuronal spread of pathological tau species precedes the development of major tau burdens. Two amyloid motifs, VQIINK in repeat 2 and VQIVYK in repeat 3, of tau repeat domain, assemble into β-sheet-rich fibrils on their own but alone do not form seed-competent fibrils. In contrast, the entire R3 region self-aggregates and forms seed-competent fibrils. Our study aimed to identify the minimal regions in the tau repeat domain that define seeding and its impact on intracellular tau phosphorylation and aggregation. Using peptides of individual repeats, we show that R2, like R3, forms seed-competent fibrils when assembled in the presence of heparin. However, R3, but not R2, forms seed-competent fibrils when assembled without heparin, even though both R2 and R3 have identical N-terminal hexapeptide and cysteine residue sequences. Moreover, cysteine to alanine substitution in R3 abrogates its self-aggregation and seeding potency. Tau RD P301S biosensor cells and Tau P301L (0N4R)-expressing HEK293 cells seeded with R2 and R3 fibrils show the induction of pathological phosphorylation of tau at Ser262/Ser396/Ser404 positions and oligomerisation of native tau. Protein fractions of biosensor cells seeded with R2 and R3 fibrils reseed endogenous tau aggregation when introduced into a fresh set of biosensor cells. Our findings suggest that R3 may be the minimal region for pathological seed generation under physiological conditions, whereas R2 might need polyanionic cofactors to generate pathogenic seeds. Lastly, R2 and R3 fibrils induce template-induced misfolding and pathological hyperphosphorylation of intracellular tau, making intracellular tau seed-competent.

Biofluid Biomarkers of Alzheimer’s Disease: Progress, Problems, and Perspectives

Since the establishment of the biomarker-based A-T-N (Amyloid/Tau/Neurodegeneration) framework in Alzheimer’s disease (AD), the diagnosis of AD has become more precise, and cerebrospinal fluid tests and positron emission tomography examinations based on this framework have become widely accepted. However, the A-T-N framework does not encompass the whole spectrum of AD pathologies, and problems with invasiveness and high cost limit the application of the above diagnostic methods aimed at the central nervous system. Therefore, we suggest the addition of an “X” to the A-T-N framework and a focus on peripheral biomarkers in the diagnosis of AD. In this review, we retrospectively describe the recent progress in biomarkers based on the A-T-N-X framework, analyze the problems, and present our perspectives on the diagnosis of AD.

Deciphering the prion-like behavior of pathogenic protein aggregates in neurodegenerative diseases

Neurodegenerative diseases are hitherto classified based on their core clinical features, the anatomical distribution of neurodegeneration, and the cell populations mainly affected. On the other hand, the wealth of neuropathological, genetic, molecular and biochemical studies have identified the existence of distinct insoluble protein aggregates in the affected brain regions. These findings have spread the use of a collective term, proteinopathy, for neurodegenerative disorders with particular type of structurally altered protein accumulation. Particularly, a recent breakthrough in this field came with the discovery that these protein aggregates can transfer from one cell to another, thereby converting normal proteins to potentially toxic, misfolded species in a prion-like manner. In this review, we focus specifically on the molecular and cellular basis that underlies the seeding activity and transcellular spreading phenomenon of neurodegeneration-related protein aggregates, and discuss how these events contribute to the disease progression.

Delivery of Intravenously Administered Antibodies Targeting Alzheimer’s Disease-Relevant Tau Species into the Brain Based on Receptor-Mediated Transcytosis

Alzheimer’s disease (AD) is a neurodegenerative disease that causes memory loss, cognitive decline, and eventually dementia. The etiology of AD and its pathological mechanisms remain unclear due to its complex pathobiology. At the same time, the number of patients with AD is increasing worldwide. However, no therapeutic agents for AD are currently available for definitive care. Several phase 3 clinical trials using agents targeting amyloid β (Aβ) and its related molecules have failed, with the exception of aducanumab, an anti-Aβ monoclonal antibody (mAb), clinically approved by the US Food and Drug Administration in 2021, which could be modified for AD drug development due to controversial approval. Neurofibrillary tangles (NFTs) composed of tau rather than senile plaques composed of Aβ are correlated with AD pathogenesis. Moreover, Aβ and tau pathologies initially proceed independently. At a certain point in the progression of AD symptoms, the Aβ pathology is involved in the alteration and spreading of the tau pathology. Therefore, tau-targeting therapies have attracted the attention of pharmaceutical scientists, as well as Aβ-targeting therapies. In this review, I introduce the implementations and potential of AD immunotherapy using intravenously administered anti-tau and anti-receptor bispecific mAbs. These cross the blood-brain barrier (BBB) based on receptor-mediated transcytosis and are subsequently cleared by microglia based on Fc-mediated endocytosis after binding to tau and lysosomal degradation.