Straight and paired helical filaments in Alzheimer disease have a common structural unit

Cryo-EM structure of Alzheimer's disease tau filaments with PET ligand MK-6240

Positron Emission Tomography (PET) ligands have advanced Alzheimer's disease (AD) diagnosis and treatment. Using autoradiography and cryo-EM, we identify AD brain tissue with elevated tau burden, purify filaments, and determine the structure of second-generation high avidity PET ligand MK-6240 at 2.31 Å resolution, which bound at a 1:1 ratio within the cleft of tau paired-helical filament (PHF), engaging with glutamine 351, lysine 353, and isoleucine 360. This information elucidates the basis of MK-6240 PET in quantifying PHF deposits in AD and may facilitate the structure-based design of superior ligands against tau amyloids.

Biophysical Studies of Amyloid-Binding Fluorophores to Tau AD Core Fibrils Formed without Cofactors

Tau is an intrinsically disordered protein involved in several neurodegenerative diseases where a common hallmark is the appearance of tau aggregates in the brain. One common approach to elucidate the mechanisms behind the aggregation of tau has been to recapitulate in vitro the self-assembly process in a fast and reproducible manner. While the seeding of tau aggregation is prompted by negatively charged cofactors, the obtained fibrils are morphologically distinct from those found in vivo. The Tau AD core fragment (TADC, tau 306-378) has emerged as a new model and potential solution for the cofactor-free in vitro aggregation of tau. Here, we use TADC to further study this process combining multiple amyloid-detecting fluorophores and fibril bioimaging. We confirmed by transmission electron microscopy that this fragment forms fibrils after quiescent incubation at 37 °C. We then employed a panel of eight amyloid-binding fluorophores to query the formed species by acquiring their emission spectra. The results obtained showed that nearly all dyes detect TADC self-assembled species. However, the successful monitoring of TADC aggregation kinetics was limited to three fluorophores (X-34, Bis-ANS, and pFTAA) which yielded sigmoidal curves but different aggregation half-times, hinting to different species being detected. Altogether, this study highlights the potential of using multiple extrinsic fluorescent probes, alone or in combination, as tools to further clarify mechanisms behind the aggregation of amyloidogenic proteins.

Alzheimer's disease seeded tau forms paired helical filaments yet lacks seeding potential

Alzheimer's disease (AD) and many other neurodegenerative diseases are characterized by pathological aggregation of the protein tau. These tau aggregates spread in a stereotypical spatiotemporal pattern in the brain of each disease, suggesting that the misfolded tau can recruit soluble monomers to adopt the same pathological structure. To investigate whether recruited tau indeed adopts the same structure and properties as the original seed, here we template recombinant full-length 0N3R tau, 0N4R tau, and an equimolar mixture of the two using sarkosyl-insoluble tau extracted from AD brain and determine the structures of the resulting fibrils using cryoelectron microscopy. We show that these cell-free amplified tau fibrils adopt the same molecular structure as the AD paired-helical filament (PHF) tau but are unable to template additional monomers. Therefore, the PHF structure alone is insufficient for defining the pathological properties of AD tau, and other biochemical components such as tau posttranslational modifications, other proteins, polyanionic cofactors, and salt are required for the prion-like serial propagation of tauopathies.

Cryo-EM structures of pathogenic fibrils and their impact on neurodegenerative disease research

Neurodegenerative diseases are commonly associated with the formation of aberrant protein aggregates within the brain, and ultrastructural analyses have revealed that the proteins within these inclusions often assemble into amyloid filaments. Cryoelectron microscopy (cryo-EM) has emerged as an effective method for determining the near-atomic structure of these disease-associated filamentous proteins, and the resulting structures have revolutionized the way we think about aberrant protein aggregation and propagation during disease progression. These structures have also revealed that individual fibril conformations may dictate different disease conditions, and this newfound knowledge has improved disease modeling in the lab and advanced the ongoing pursuit of clinical tools capable of distinguishing and targeting different pathogenic entities within living patients. In this review, we summarize some of the recently developed cryo-EM structures of ex vivo α-synuclein, tau, β-amyloid (Aβ), TAR DNA-binding protein 43 (TDP-43), and transmembrane protein 106B (TMEM106B) fibrils and discuss how these structures are being leveraged toward mechanistic research and therapeutic development.

CryoET of β-amyloid and tau within postmortem Alzheimer's disease brain

A defining pathological feature of most neurodegenerative diseases is the assembly of proteins into amyloid that form disease-specific structures1. In Alzheimer's disease, this is characterized by the deposition of β-amyloid and tau with disease-specific conformations. The in situ structure of amyloid in the human brain is unknown. Here, using cryo-fluorescence microscopy-targeted cryo-sectioning, cryo-focused ion beam-scanning electron microscopy lift-out and cryo-electron tomography, we determined in-tissue architectures of β-amyloid and tau pathology in a postmortem Alzheimer's disease donor brain. β-amyloid plaques contained a mixture of fibrils, some of which were branched, and protofilaments, arranged in parallel arrays and lattice-like structures. Extracellular vesicles and cuboidal particles defined the non-amyloid constituents of β-amyloid plaques. By contrast, tau inclusions formed parallel clusters of unbranched filaments. Subtomogram averaging a cluster of 136 tau filaments in a single tomogram revealed the polypeptide backbone conformation and filament polarity orientation of paired helical filaments within tissue. Filaments within most clusters were similar to each other, but were different between clusters, showing amyloid heterogeneity that is spatially organized by subcellular location. The in situ structural approaches outlined here for human donor tissues have applications to a broad range of neurodegenerative diseases.

Milligram-scale assembly and NMR fingerprint of tau fibrils adopting the Alzheimer's disease fold

In the Alzheimer's disease (AD) brain, the microtubule-associated protein tau aggregates into paired helical filaments in which each protofilament has a C-shaped conformation. In vitro assembly of tau fibrils adopting this fold is highly valuable for both fundamental and applied studies of AD without requiring patient-brain extracted fibrils. To date, reported methods for forming AD-fold tau fibrils have been irreproducible and sensitive to subtle variations in fibrillization conditions. Here, we describe a route to reproducibly assemble tau fibrils adopting the AD fold on the multi-milligram scale. We investigated the fibrillization conditions of two constructs and found that a tau (297-407) construct that contains four AD phospho-mimetic glutamate mutations robustly formed the C-shaped conformation. 2D and 3D correlation solid-state NMR spectra show a single predominant set of chemical shifts, indicating a single molecular conformation. Negative-stain electron microscopy and cryo-EM data confirm that the protofilament formed by 4E-tau (297-407) adopts the C-shaped conformation, which associates into paired, triple, and quadruple helical filaments. In comparison, NMR spectra indicate that a previously reported construct, tau (297-391), forms a mixture of a four-layered dimer structure and the C-shaped structure, whose populations are sensitive to the environmental conditions. The determination of the NMR chemical shifts of the AD-fold tau opens the possibility for future studies of tau fibril conformations and ligand binding by NMR. The quantitative assembly of tau fibrils adopting the AD fold should facilitate the development of diagnostic and therapeutic compounds that target AD tau.

Lysine ε-aminolysis and incorporation of sulfhydryl groups into human brain tau 4R/1N and 306VQIVYK311 enhances the formation of beta structures and toxicity

In the present study, we investigated the effects of N-homocysteine thiolactone (tHcy) modification on expressed and purified tau protein and the synthesized VQIVYK target peptide. The modified constructs were subjected to comprehensive validation using various methodologies, including mass spectrometry. Subsequently, in vivo, in vitro, and in silico characterizations were performed under both reducing and non-reducing conditions, as well as in the presence and absence of heparin as a cofactor. Our results unequivocally confirmed that under reducing conditions and in the presence of heparin, the modified constructs exhibited a greater propensity for aggregation. This enhanced aggregative behavior can be attributed to the disruption of lysine positive charges and the subsequent influence of hydrophobic and p-stacking intermolecular forces. Notably, the modified oligomeric species induced apoptosis in the SH-SY5Y cell line, and this effect was further exacerbated with longer incubation times and higher concentrations of the modifier. These observations suggest a potential mechanism involving reactive oxygen species (ROS). To gain a deeper understanding of the molecular mechanisms underlying the neurotoxic effects, further investigations are warranted. Elucidating these mechanisms will contribute to the development of more effective strategies to counteract aggregation and mitigate neurodegeneration.

Structures of AT8 and PHF1 phosphomimetic tau: Insights into the posttranslational modification code of tau aggregation

The microtubule-associated protein tau aggregates into amyloid fibrils in Alzheimer's disease and other neurodegenerative diseases. In these tauopathies, tau is hyperphosphorylated, suggesting that this posttranslational modification (PTM) may induce tau aggregation. Tau is also phosphorylated in normal developing brains. To investigate how tau phosphorylation induces amyloid fibrils, here we report the atomic structures of two phosphomimetic full-length tau fibrils assembled without anionic cofactors. We mutated key Ser and Thr residues to Glu in two regions of the protein. One construct contains three Glu mutations at the epitope of the anti-phospho-tau antibody AT8 (AT8-3E tau), whereas the other construct contains four Glu mutations at the epitope of the antibody PHF1 (PHF1-4E tau). Solid-state NMR data show that both phosphomimetic tau mutants form homogeneous fibrils with a single set of chemical shifts. The AT8-3E tau rigid core extends from the R3 repeat to the C terminus, whereas the PHF1-4E tau rigid core spans R2, R3, and R4 repeats. Cryoelectron microscopy data show that AT8-3E tau forms a triangular multi-layered core, whereas PHF1-4E tau forms a triple-stranded core. Interestingly, a construct combining all seven Glu mutations exhibits the same conformation as PHF1-4E tau. Scalar-coupled NMR data additionally reveal the dynamics and shape of the fuzzy coat surrounding the rigid cores. These results demonstrate that specific PTMs induce structurally specific tau aggregates, and the phosphorylation code of tau contains redundancy.

Biochemical approaches to assess the impact of post-translational modifications on pathogenic tau conformations using recombinant protein

Tau protein is associated with many neurodegenerative disorders known as tauopathies. Aggregates of tau are thought of as a main contributor to neurodegeneration in these diseases. Increasingly, evidence points to earlier, soluble conformations of abnormally modified monomers and multimeric tau as toxic forms of tau. The biological processes driving tau from physiological species to pathogenic conformations remain poorly understood, but certain avenues are currently under investigation including the functional consequences of various pathological tau changes (e.g. mutations, post-translational modifications (PTMs), and protein-protein interactions). PTMs can regulate several aspects of tau biology such as proteasomal and autophagic clearance, solubility, and aggregation. Moreover, PTMs can contribute to the transition of tau from normal to pathogenic conformations. However, our understating of how PTMs specifically regulate the transition of tau into pathogenic conformations is partly impeded by the relative lack of structured frameworks to assess and quantify these conformations. In this review, we describe a set of approaches that includes several in vitro assays to determine the contribution of PTMs to tau's transition into known pathogenic conformations. The approaches begin with different methods to create recombinant tau proteins carrying specific PTMs followed by validation of the PTMs status. Then, we describe a set of biochemical and biophysical assays that assess the contribution of a given PTM to different tau conformations, including aggregation, oligomerization, exposure of the phosphatase-activating domain, and seeding. Together, these approaches can facilitate the advancement of our understanding of the relationships between PTMs and tau conformations.

Astrocytic accumulation of tau fibrils isolated from Alzheimer's disease brains induces inflammation, cell-to-cell propagation and neuronal impairment

Accumulating evidence highlights the involvement of astrocytes in Alzheimer's disease (AD) progression. We have previously demonstrated that human iPSC-derived astrocytes ingest and modify synthetic tau fibrils in a way that enhances their seeding efficiency. However, synthetic tau fibrils differ significantly from in vivo formed fibrils. To mimic the situation in the brain, we here analyzed astrocytes' processing of human brain-derived tau fibrils and its consequences for cellular physiology. Tau fibrils were extracted from both AD and control brains, aiming to examine any potential differences in astrocyte response depending on the origin of fibrils. Our results show that human astrocytes internalize, but fail to degrade, both AD and control tau fibrils. Instead, pathogenic, seeding capable tau proteoforms are spread to surrounding cells via tunneling nanotubes and exocytosis. Notably, accumulation of AD tau fibrils induces a stronger reactive state in astrocytes, compared to control fibrils, evident by the augmented expression of vimentin and GFAP, as well as by an increased secretion of the pro-inflammatory cytokines IL-8 and MCP-1. Moreover, conditioned media from astrocytes with AD tau fibril deposits induce synapse and metabolic impairment in human iPSC-derived neurons. Taken together, our data suggest that the accumulation of brain-derived AD tau fibrils induces a more robust inflammatory and neurotoxic phenotype in human astrocytes, accentuating the nature of tau fibrils as an important contributing factor to inflammation and neurodegeneration in AD.

Neurodegenerative Disease Tauopathies

Tauopathies are a diverse group of progressive and fatal neurodegenerative diseases characterized by aberrant tau inclusions in the central nervous system. Tau protein forms pathologic fibrillar aggregates that are typically closely associated with neuronal cell death, leading to varied clinical phenotypes including dementia, movement disorders, and motor neuron disease. In this review, we describe the clinicopathologic features of tauopathies and highlight recent advances in understanding the mechanisms that lead to spread of pathologic aggregates through interconnected neuronal pathways. The cell-to-cell propagation of tauopathy is then linked to posttranslational modifications, tau fibril structural variants, and the breakdown of cellular protein quality control.

Docking for Molecules That Bind in a Symmetric Stack with SymDOCK

Discovering ligands for amyloid fibrils, such as those formed by the tau protein, is an area of great current interest. In recent structures, ligands bind in stacks in the tau fibrils to reflect the rotational and translational symmetry of the fibril itself; in these structures, the ligands make few interactions with the protein but interact extensively with each other. To exploit this symmetry and stacking, we developed SymDOCK, a method to dock molecules that follow the protein's symmetry. For each prospective ligand pose, we apply the symmetry operation of the fibril to generate a self-interacting and fibril-interacting stack, checking that doing so will not cause a clash between the original molecule and its image. Absent a clash, we retain that pose and add the ligand-ligand van der Waals energy to the ligand's docking score (here using DOCK3.8). We can check these geometries and energies using an implementation of ANI, a neural-network-based quantum-mechanical evaluation of the ligand stacking energies. In retrospective calculations, symmetry docking can reproduce the poses of three tau PET tracers whose structures have been determined. More convincingly, in a prospective study, SymDOCK predicted the structure of the PET tracer MK-6240 bound in a symmetrical stack to AD PHF tau before that structure was determined; the docked pose was used to determine how MK-6240 fit the cryo-EM density. In proof-of-concept studies, SymDOCK enriched known ligands over property-matched decoys in retrospective screens without sacrificing docking speed and can address large library screens that seek new symmetrical stackers. Future applications of this approach will be considered.

Tau and neurodegeneration

First identified in 1975, tau was implicated in Alzheimer's disease 10 years later. Filamentous tangle inclusions were known to be made of hyperphosphorylated tau by 1991, with similar inclusions gaining recognition for being associated with other neurodegenerative diseases. In 1998, mutations in MAPT, the gene that encodes tau, were identified as the cause of a dominantly inherited form of frontotemporal dementia with abundant filamentous tau inclusions. While this result indicated that assembly of tau into aberrant filaments is sufficient to drive neurodegeneration and dementia, most cases of tauopathy are sporadic. More recent work in experimental systems showed that filamentous assemblies of tau may first form in one brain area, and then spread to others in a prion-like fashion. Beginning in 2017, work on human brains using high-resolution techniques has led to a structure-based classification of tauopathies, which has opened the door to a better understanding of the significance of tau filament formation.

Polymerization of recombinant tau core fragments in vitro and seeding studies in cultured cells

The relative polymerization of specific tau protein cores that define Alzheimer's disease, Pick's disease and corticobasal degeneration were investigated using amyloid fluorometry and electron microscopy. In addition, the relative prion-like activities of polymers comprised of these respective tau protein segments were investigated in a cell-based assay. It is demonstrated that the seeding activities of specific tau core fibrils are affected by the presence of pathogenic tau missense mutations and the microtubule binding domain composition of tau. The unique impact of tau phosphorylation on seeding propensity was also investigated by altering stretches of phospho-mimetic and phospho-null residues in the presence of Alzheimer's disease tau core fibrils. These results have important mechanistic implications for mutation and isoform-specific driven pathogenesis.

A large-scale volumetric correlated light and electron microscopy study localizes Alzheimer's disease-related molecules in the hippocampus

Connectomics is a nascent neuroscience field to map and analyze neuronal networks. It provides a new way to investigate abnormalities in brain tissue, including in models of Alzheimer's disease (AD). This age-related disease is associated with alterations in amyloid-β (Aβ) and phosphorylated tau (pTau). These alterations correlate with AD's clinical manifestations, but causal links remain unclear. Therefore, studying these molecular alterations within the context of the local neuronal and glial milieu may provide insight into disease mechanisms. Volume electron microscopy (vEM) is an ideal tool for performing connectomics studies at the ultrastructural level, but localizing specific biomolecules within large-volume vEM data has been challenging. Here we report a volumetric correlated light and electron microscopy (vCLEM) approach using fluorescent nanobodies as immuno-probes to localize Alzheimer's disease-related molecules in a large vEM volume. Three molecules (pTau, Aβ, and a marker for activated microglia (CD11b)) were labeled without the need for detergents by three nanobody probes in a sample of the hippocampus of the 3xTg Alzheimer's disease model mouse. Confocal microscopy followed by vEM imaging of the same sample allowed for registration of the location of the molecules within the volume. This dataset revealed several ultrastructural abnormalities regarding the localizations of Aβ and pTau in novel locations. For example, two pTau-positive post-synaptic spine-like protrusions innervated by axon terminals were found projecting from the axon initial segment of a pyramidal cell. Three pyramidal neurons with intracellular Aβ or pTau were 3D reconstructed. Automatic synapse detection, which is necessary for connectomics analysis, revealed the changes in density and volume of synapses at different distances from an Aβ plaque. This vCLEM approach is useful to uncover molecular alterations within large-scale volume electron microscopy data, opening a new connectomics pathway to study Alzheimer's disease and other types of dementia.

Structure of the nonhelical filament of the Alzheimer's disease tau core

The microtubule-associated protein tau aggregates into neurofibrillary tangles in Alzheimer's disease (AD). The main type of aggregates, the paired helical filaments (PHF), incorporate about 20% of the full-length protein into the rigid core. Recently, cryo-electron microscopy data showed that a protease-resistant fragment of tau (residues 297-391) self-assembles in vitro in the presence of divalent cations to form twisted filaments whose molecular structure resembles that of AD PHF tau [S. Lövestam et al., Elife 11, e76494 (2022)]. To investigate whether this tau construct is uniquely predisposed to this morphology and structure, we fibrillized tau (297-391) under the reported conditions and determined its structure using solid-state NMR spectroscopy. Unexpectedly, the protein assembled predominantly into nontwisting ribbons whose rigid core spans residues 305-357. This rigid core forms a β-arch that turns at residues 322CGS324. Two protofilaments stack together via a long interface that stretches from G323 to I354. Together, these two protofilaments form a four-layered β-sheet core whose sidechains are stabilized by numerous polar and hydrophobic interactions. This structure gives insight into the fibril morphologies and molecular conformations that can be adopted by this protease-resistant core of AD tau under different pH and ionic conditions.

Docking for molecules that bind in a symmetric stack with SymDOCK

Discovering ligands for amyloid fibrils, such as those formed by the tau protein, is an area of much current interest. In recent structures, ligands bind in stacks in the tau fibrils to reflect the rotational and translational symmetry of the fibril itself; in these structures the ligands make few interactions with the protein but interact extensively with each other. To exploit this symmetry and stacking, we developed SymDOCK, a method to dock molecules that follow the protein's symmetry. For each prospective ligand pose, we apply the symmetry operation of the fibril to generate a self-interacting and fibril-interacting stack, checking that doing so will not cause a clash between the original molecule and its image. Absent a clash, we retain that pose and add the ligand-ligand van der Waals energy to the ligand's docking score (here using DOCK3.8). We can check these geometries and energies using an implementation of ANI, a neural network-based quantum-mechanical evaluation of the ligand stacking energies. In retrospective calculations, symmetry docking can reproduce the poses of three tau PET tracers whose structures have been determined. More convincingly, in a prospective study SymDOCK predicted the structure of the PET tracer MK-6240 bound in a symmetrical stack to AD PHF tau before that structure was determined; the docked pose was used to determine how MK-6240 fit the cryo-EM density. In proof-of-concept studies, SymDOCK enriched known ligands over property-matched decoys in retrospective screens without sacrificing docking speed, and can address large library screens that seek new symmetrical stackers. Future applications of this approach will be considered.

Exploring Methylene Blue and Its Derivatives in Alzheimer's Treatment: A Comprehensive Review of Randomized Control Trials

Methylene blue (MB) and its compounds are investigated for their potential benefits in the management of Alzheimer's disease (AD). AD is a widely seen neuropathological disorder characterized by the gradual decline of cognitive abilities, ultimately leading to the development of severe dementia. It is anticipated that there will be a significant increase in the prevalence of AD due to the aging population. Histopathologically, AD is distinguished by the presence of intracellular tangles of neurofibrillary tissues (NFTs) and extracellular amyloid plaques within the brain. MB is a thiophenazine dye with FDA approval for treating several illnesses. Its ease in crossing the blood-brain barrier and potential therapeutic use in central nervous system diseases have increased interest in its application for treating AD. The literature review includes randomized clinical trials investigating MB's potential benefits in treating AD. The findings of the studies indicate that the administration of MB has demonstrated enhancements in cognitive function, reductions in the accumulation of plaques containing beta-amyloid, improvements in memory and cognitive function in animal subjects, and possesses antioxidant properties that can mitigate oxidative stress and inflammation within the brain. This review evaluates the modern and latest research on the application of MB for treating AD.

Cryo-EM structure of Alzheimer's disease tau filaments with PET ligand MK-6240

Positron Emission Tomography (PET) ligands have advanced Alzheimer's disease (AD) diagnosis and treatment. Using autoradiography and cryo-EM, we identified AD brain tissue with elevated tau burden, purified filaments, and determined the structure of second-generation high avidity PET ligand MK-6240 at 2.31 Å resolution, which bound at a 1:1 ratio within the cleft of tau paired-helical filament (PHF), engaging with glutamine 351, lysine K353, and isoleucine 360. This information elucidates the basis of MK-6240 PET in quantifying PHF deposits in AD and may facilitate the structure-based design of superior ligands against tau amyloids.

Molecular pathology of neurodegenerative diseases by cryo-EM of amyloids

Abnormal assembly of tau, α-synuclein, TDP-43 and amyloid-β proteins into amyloid filaments defines most human neurodegenerative diseases. Genetics provides a direct link between filament formation and the causes of disease. Developments in cryo-electron microscopy (cryo-EM) have made it possible to determine the atomic structures of amyloids from postmortem human brains. Here we review the structures of brain-derived amyloid filaments that have been determined so far and discuss their impact on research into neurodegeneration. Whereas a given protein can adopt many different filament structures, specific amyloid folds define distinct diseases. Amyloid structures thus provide a description of neuropathology at the atomic level and a basis for studying disease. Future research should focus on model systems that replicate the structures observed in disease to better understand the molecular mechanisms of disease and develop improved diagnostics and therapies.

Protein aggregation and neurodegenerative disease: Structural outlook for the novel therapeutics

Before the controversial approval of humanized monoclonal antibody lecanemab, which binds to the soluble amyloid-β protofibrils, all the treatments available earlier, for Alzheimer's disease (AD) were symptomatic. The researchers are still struggling to find a breakthrough in AD therapeutic medicine, which is partially attributable to lack in understanding of the structural information associated with the intrinsically disordered proteins and amyloids. One of the major challenges in this area of research is to understand the structural diversity of intrinsically disordered proteins under in vitro conditions. Therefore, in this review, we have summarized the in vitro applications of biophysical methods, which are aimed to shed some light on the heterogeneity, pathogenicity, structures and mechanisms of the intrinsically disordered protein aggregates associated with proteinopathies including AD. This review will also rationalize some of the strategies in modulating disease-relevant pathogenic protein entities by small molecules using structural biology approaches and biophysical characterization. We have also highlighted tools and techniques to simulate the in vivo conditions for native and cytotoxic tau/amyloids assemblies, urge new chemical approaches to replicate tau/amyloids assemblies similar to those in vivo conditions, in addition to designing novel potential drugs.

Amyloid fibril structures of tau: Conformational plasticity of the second microtubule-binding repeat

The intrinsically disordered protein tau associates with microtubules in neurons but aggregates into cross-β amyloid fibrils that propagate in neurodegenerative brains. Different tauopathies have different structures for the rigid fibril core. To understand the molecular basis of tau assembly into different polymorphs, here we use solid-state nuclear magnetic resonance (NMR) spectroscopy to determine the structure of a tau protein that includes all microtubule-binding repeats and a proline-rich domain. This P2R tau assembles into well-ordered filaments when induced by heparin. Two- and three-dimensional NMR spectra indicate that R2 and R3 repeats constitute the rigid β-sheet core of the fibril. Unexpectedly, the amino-terminal half of R2 forms a β-arch at ambient temperature (24°C) but a continuous β-strand at 12°C, which dimerizes with the R2 of another protofilament. This temperature-dependent structure indicates that R2 is conformationally more plastic than the R3 domain. The distinct conformational stabilities of different microtubule-binding repeats give insight into the energy landscape of tau fibril formation.

Alzheimer's disease: a mini-review for the clinician

Alzheimer's disease (AD), the most common form of dementia, is a striking example of the connection between neurophysiological abnormalities and higher-order cognitive deficiencies. Since its initial description in 1906, research into the pathophysiology and etiology of AD has led to the illumination of an incredibly complex set of genetic and molecular mechanisms for the disease's progression, characterized by much more than the neuropathological hallmarks of beta-amyloid (Aβ) plaques and neurofibrillary tangles (NFTs). In this review, findings relating the neurodegeneration present in AD to its clinical presentation and treatment are summarized, with an emphasis on the interconnectedness of disease pathophysiology. Further, diagnostic guidelines are provided based on the National Institute on Aging-Alzheimer's Association (NIA-AA) workgroup's clinical recommendations. Through the dissemination of detailed but digestible open access resources such as this one, we can move towards an increase in the equity and accessibility of education for the modern clinician.

The relationship of insulin resistance and diabetes to tau PET SUVR in middle-aged to older adults

BACKGROUND

Insulin resistance (IR) and type 2 diabetes have been found to increase the risk for Alzheimer's clinical syndrome in epidemiologic studies but have not been associated with tau tangles in neuropathological research and have been inconsistently associated with cerebrospinal fluid P-tau181. IR and type 2 diabetes are well-recognized vascular risk factors. Some studies suggest that cardiovascular risk may act synergistically with cortical amyloid to increase tau measured using tau PET. Utilizing data from largely nondemented middle-aged and older adult cohorts enriched for AD risk, we investigated the association of IR and diabetes to tau PET and whether amyloid moderated those relationships.

METHODS

Participants were enrolled in either the Wisconsin Registry for Alzheimer's Prevention (WRAP) or Wisconsin Alzheimer's Disease Research Center (WI-ADRC) Clinical Core. Two partially overlapping samples were studied: a sample characterized using HOMA-IR (n=280 WRAP participants) and a sample characterized on diabetic status (n=285 WRAP and n=109 WI-ADRC). IR was measured using the homeostasis model assessment of insulin resistance (HOMA-IR). Tau PET employing the radioligand 18F-MK-6240 was used to detect AD-specific aggregated tau. Linear regression tested the relationship of IR and diabetic status to tau PET standardized uptake value ratio (SUVR) within the entorhinal cortex and whether relationships were moderated by amyloid assessed by amyloid PET distribution volume ratio (DVR) and amyloid PET positivity status.

RESULTS

Neither HOMA-IR nor diabetic status was significantly associated with tau PET SUVR. The relationship between IR and tau PET SUVR was not moderated by amyloid PET DVR or positivity status. The association between diabetic status and tau PET SUVR was not significantly moderated by amyloid PET DVR but was significantly moderated by amyloid PET positivity status. Among the amyloid PET-positive participants, the estimated marginal tau PET SUVR mean was higher in the diabetic (n=6) relative to the nondiabetic group (n=88).

CONCLUSION

Findings indicate that IR may not be related to tau in generally healthy middle-aged and older adults who are in the early stages of the AD clinicopathologic continuum but suggest the need for additional research to investigate whether a synergistic relationship between type 2 diabetes and amyloid is associated with increased tau levels.

The Sensitivity of Tau Tracers for the Discrimination of Alzheimer's Disease Patients and Healthy Controls by PET

Hyperphosphorylated tau aggregates, also known as neurofibrillary tangles, are a hallmark neuropathological feature of Alzheimer's disease (AD). Molecular imaging of tau by positron emission tomography (PET) began with the development of [18F]FDDNP, an amyloid β tracer with off-target binding to tau, which obtained regional specificity through the differing distributions of amyloid β and tau in AD brains. A concerted search for more selective and affine tau PET tracers yielded compounds belonging to at least eight structural categories; 18F-flortaucipir, known variously as [18F]-T807, AV-1451, and Tauvid®, emerged as the first tau tracer approved by the American Food and Drug Administration. The various tau tracers differ concerning their selectivity over amyloid β, off-target binding at sites such as monoamine oxidase and neuromelanin, and degree of uptake in white matter. While there have been many reviews of molecular imaging of tau in AD and other conditions, there has been no systematic comparison of the fitness of the various tracers for discriminating between AD patient and healthy control (HC) groups. In this narrative review, we endeavored to compare the binding properties of the various tau tracers in vitro and the effect size (Cohen's d) for the contrast by PET between AD patients and age-matched HC groups. The available tracers all gave good discrimination, with Cohen's d generally in the range of two-three in culprit brain regions. Overall, Cohen's d was higher for AD patient groups with more severe illness. Second-generation tracers, while superior concerning off-target binding, do not have conspicuously higher sensitivity for the discrimination of AD and HC groups. We suppose that available pharmacophores may have converged on a maximal affinity for tau fibrils, which may limit the specific signal imparted in PET studies.

Quantitative proteomics of tau and Aβ in detergent fractions from Alzheimer's disease brains

The two hallmarks of Alzheimer's disease (AD) are amyloid-β (Aβ) plaques and neurofibrillary tangles marked by phosphorylated tau. Increasing evidence suggests that aggregating Aβ drives tau accumulation, a process that involves synaptic degeneration leading to cognitive impairment. Conversely, there is a realization that non-fibrillar (oligomeric) forms of Aβ mediate toxicity in AD. Fibrillar (filamentous) aggregates of proteins across the spectrum of the primary and secondary tauopathies were the focus of recent structural studies with a filament structure-based nosologic classification, but less emphasis was given to non-filamentous co-aggregates of insoluble proteins in the fractions derived from post-mortem human brains. Here, we revisited sarkosyl-soluble and -insoluble extracts to characterize tau and Aβ species by quantitative targeted mass spectrometric proteomics, biochemical assays, and electron microscopy. AD brain sarkosyl-insoluble pellets were greatly enriched with Aβ₄₂ at almost equimolar levels to N-terminal truncated microtubule-binding region (MTBR) isoforms of tau with multiple site-specific post-translational modifications (PTMs). MTBR R3 and R4 tau peptides were most abundant in the sarkosyl-insoluble materials with a 10-fold higher concentration than N-terminal tau peptides. This indicates that the major proportion of the enriched tau was the aggregation-prone N-terminal and proline-rich region (PRR) of truncated mixed 4R and 3R tau with more 4R than 3R isoforms. High concentration and occupancies of site-specific phosphorylation pT₁₈₁ (~22%) and pT₂₁₇ (~16%) (key biomarkers of AD) along with other PTMs in the PRR and MTBR indicated a regional susceptibility of PTMs in aggregated tau. Immunogold labelling revealed that tau may exist in globular non-filamentous form (N-terminal intact tau) co-localized with Aβ in the sarkosyl-insoluble pellets along with tau filaments (N-truncated MTBR tau). Our results suggest a model that Aβ and tau interact forming globular aggregates, from which filamentous tau and Aβ emerge. These characterizations contribute towards unravelling the sequence of events which lead to end-stage AD changes.

Aβ and Tau Interact with Metal Ions, Lipid Membranes and Peptide-Based Amyloid Inhibitors: Are These Common Features Relevant in Alzheimer’s Disease?

In the last two decades, the amyloid hypothesis, i.e., the abnormal accumulation of toxic Aβ assemblies in the brain, has been considered the mainstream concept sustaining research in Alzheimer’s Disease (AD). However, the course of cognitive decline and AD development better correlates with tau accumulation rather than amyloid peptide deposition. Moreover, all clinical trials of amyloid-targeting drug candidates have been unsuccessful, implicitly suggesting that the amyloid hypothesis needs significant amendments. Accumulating evidence supports the existence of a series of potentially dangerous relationships between Aβ oligomeric species and tau protein in AD. However, the molecular determinants underlying pathogenic Aβ/tau cross interactions are not fully understood. Here, we discuss the common features of Aβ and tau molecules, with special emphasis on: (i) the critical role played by metal dyshomeostasis in promoting both Aβ and tau aggregation and oxidative stress, in AD; (ii) the effects of lipid membranes on Aβ and tau (co)-aggregation at the membrane interface; (iii) the potential of small peptide-based inhibitors of Aβ and tau misfolding as therapeutic tools in AD. Although the molecular mechanism underlying the direct Aβ/tau interaction remains largely unknown, the arguments discussed in this review may help reinforcing the current view of a synergistic Aβ/tau molecular crosstalk in AD and stimulate further research to mechanism elucidation and next-generation AD therapeutics.

Tau as a Biomarker of Neurodegeneration

Less than 50 years since tau was first isolated from a porcine brain, its detection in femtolitre concentrations in biological fluids is revolutionizing the diagnosis of neurodegenerative diseases. This review highlights the molecular and technological advances that have catapulted tau from obscurity to the forefront of biomarker diagnostics. Comprehensive updates are provided describing the burgeoning clinical applications of tau as a biomarker of neurodegeneration. For the clinician, tau not only enhances diagnostic accuracy, but holds promise as a predictor of clinical progression, phenotype, and response to drug therapy. For patients living with neurodegenerative disorders, characterization of tau dysregulation could provide much-needed clarity to a notoriously murky diagnostic landscape.

Deciphering the Role of ATP on PHF6 Aggregation

The aggregation of Tau protein, which are involved in Alzheimer's disease, are associated with the self-assembly of the hexapeptide sequence, paired helical filament 6 (PHF6) from repeat 3 of Tau. In order to treat Alzheimer's disease and other such tauopathies, one of the therapeutic strategies is to inhibit aggregation of Tau and its nucleating segments. Therefore, we have studied the effect of adenosine triphosphate (ATP) on the aggregation of PHF6. ATP has, interestingly, demonstrated its ability to inhibit and dissolve protein aggregates. Using classical molecular dynamics simulations, we observed that the hydrophobic core of PHF6 segment displays extended β-sheet conformation, which stabilizes PHF6 aggregates. However, the distribution of ATP around the vicinity of the peptides enables PHF6 to remain discrete and attain random coil conformers. The interpeptide interactions are substituted by PHF6-ATP interactions through hydrogen bonding and hydrophobic interactions (including π-π stacking). Furthermore, the adenosine moiety of ATP contributes more than the triphosphate chain toward PHF6-ATP interaction. Ultimately, this work establishes the inhibitory activity of ATP against Tau aggregation; hence, the therapeutic effect of ATP should be explored further in regard to the effective treatment of Alzheimer's disease.

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.

Tau aggregation

Here we revisit tau protein aggregation at primary, secondary, tertiary and quaternary structures. In addition, the presence of non-aggregated tau protein, which has been recently discovered, is also commented on.

Elongation of Fibrils Formed by a Tau Fragment is Inhibited by a Transient Dimeric Intermediate

The formation and propagation of aggregates of the tau protein in the brain are associated with the tauopathy group of neurodegenerative diseases. Different tauopathies have been shown to be associated with structurally distinct aggregates of tau. However, the mechanism by which different structural folds arise remains poorly understood. In this study of fibril formation by the fragment tau-K18 of tau, it is shown that the Lys 280 → Glu mutation in the variant tau-K18 K280E forms fibrils that are morphologically distinct from those formed by wild-type (wt) tau-K18. The mutant fibrils appear to have two protofilaments twisted around each other, whereas the wt fibrils are straight and appear to have a single protofilament. Modeling the kinetics of seeded aggregation, using a simple Michaelis-Menten-like mechanism, reveals that the two morphologically distinct fibrils are elongated with different catalytic efficiencies. Surprisingly, when the elongation of monomeric tau-K18 is seeded with tau-K18 K280E fibrils, it is seen to be inhibited at high monomer concentrations. Such inhibition is not seen when elongation is seeded with tau-K18 fibrils. The mechanism of inhibition is shown to be describable as uncompetitive inhibition, in which a transient dimeric form of tau-K18 acts as an uncompetitive inhibitor. Importantly, a dimeric form of tau-K18 is seen to be populated to a detectable extent early during aggregation. A covalently linked tau dimer, with an inter-molecular disulphide linkage, is shown to be capable of acting as an inhibitor. In summary, a quantitative kinetic approach has provided an understanding of how the formation of distinct structural folds of tau fibrils can be modulated by mutation and how the elongation of one fibril type, but not the other, is inhibited by a transiently formed dimer.

Structure and Polymorphism of Amyloid and Amyloid-Like Aggregates

Amyloids are protein aggregates with the cross-β structure. The interest in amyloids is explained, on the one hand, by their role in the development of socially significant human neurodegenerative diseases, and on the other hand, by the discovery of functional amyloids, whose formation is an integral part of cellular processes. To date, more than a hundred proteins with the amyloid or amyloid-like properties have been identified. Studying the structure of amyloid aggregates has revealed a wide variety of protein conformations. In the review, we discuss the diversity of protein folds in the amyloid-like aggregates and the characteristic features of amyloid aggregates that determine their unusual properties, including stability and interaction with amyloid-specific dyes. The review also describes the diversity of amyloid aggregates and its significance for living organisms.

Nuclear and cellular, micro and nano calcification in Alzheimer's disease patients and correlation to phosphorylated Tau

Brain calcification (calcium phosphate mineral formation) has been reported in the past 100 years in the brains of Alzheimer's disease (AD) patients. However, the association between calcification and AD, the triggers for calcification, and its role within the disease are not clear. On the other hand, hyperphosphorylated Tau protein (pTau) tangles have been widely studied and recognized as an essential factor in developing AD. In this work, calcification in the brains of AD patients is characterized by advanced electron microscopy and fluorescence microscopy. Results are then compared to samples from cognitively healthy, age-matched donors, and the colocalization of calcification and pTau is investigated. Here, we show that AD patients' brains present microcalcification associated with the neural cell nuclei and cell projections, and that these are strongly related to the presence of pTau. The link between microcalcification and pTau suggests a potential mechanism of brain cell damage. Together with the formation of amyloid plaques and neurofibrillary tangles, microcalcification in neuronal cells adds to a better understanding of the pathology of AD. Finally, the presence of microcalcification in the neuronal cells of AD patients may assist in AD diagnosis, and may open avenues for developing intervention strategies based on inhibition of calcification. STATEMENT OF SIGNIFICANCE: Brain calcification has been reported in the past 100 years in the brains of Alzheimer's disease (AD) patients. However, the association between calcification and AD is not clear. Hyperphosphorylated Tau protein (pTau) has been studied and recognized as a key factor in developing AD. We show here that AD patients' brains present microcalcification associated with the neuronal cell nuclei and cell projections, and that these are related to the presence of pTau. The study of calcification in brain cells can contribute to a better understanding of the biochemical mechanisms associated with AD and might also reveal that calcification is part of the full disease mechanism. Moreover, this work opens the possibility for using calcification as a biomarker to identify AD.

Cryo-EM structures of τ filaments from human brain

Electron cryo-microscopy (cryo-EM) has made it possible to determine near-atomic structures of τ filaments from human brain. Previous work had shown that the cores of paired helical and straight filaments of Alzheimer's disease are made of two identical, but differently arranged C-shaped protofilaments. In recent years, cryo-EM has shown that the Alzheimer τ fold is 79 amino acids long. Five of the eight β-strands give rise to two antiparallel β-sheets, with the other three forming a β-helix. High-affinity binding sites of positron emission tomography ligand APN-1607 (PM-PBB3) are in the β-helix region. The Alzheimer fold contrasts with the 94 amino acid-long Pick fold, which is J-shaped and comprises nine β-strands that give rise to four antiparallel β-sheets, in the absence of a β-helix. Chronic traumatic encephalopathy τ fold is similar to the Alzheimer fold, but differs in the β-helix region, which is larger and contains a non-proteinaceous density that is probably hydrophobic. These folds are mostly two-layered. By contrast, the 107 amino acid τ fold of the 4R tauopathy corticobasal degeneration is four-layered and comprises 11 β-strands. It contains an internal, probably hydrophilic, density that is surrounded by τ. The τ folds described here share the presence of microtubule-binding repeats 3 and 4, as well as 10-13 amino acids after repeat 4.

Insights into neurodegeneration from electron microscopy studies

Neurodegenerative diseases, such as Alzheimer's disease and Parkinson's disease, pose an increasingly severe burden for individuals and society in an ageing population. The causes and mechanisms of the diseases are poorly understood and as yet there are no effective treatments. Some of the molecular complexes involved in degeneration have been identified and electron microscopy has provided an essential tool in the investigations. The focus of this review is to show how electron microscopy has contributed historically to the understanding of disease and to summarize the most striking current advances. It does not seek to cover in detail the recent technical developments in microscopy, involving better microscopes, better electron detectors and more powerful image processing techniques, which have made possible the new insights. In many instances pathological filament assemblies are associated with brain cells that die in the disease, causing the observed symptoms such as dementia or movement disorders. Using electron microscopy it is now possible to go beyond morphological descriptions to produce atomic structures of many of the filaments. This information may help to understand the seeding and assembly of the filaments, with the aim of finding small molecule inhibitors that could potentially provide a form of treatment for the diseases.

Recent High-Resolution Structures of Amyloids Involved in Neurodegenerative Diseases

Amyloids are highly ordered aggregates composed of proteins or peptides. They are involved in several pathologies, including hallmark neurodegenerative disorders such as Alzheimer’s (AD) and Parkinson’s (PD). Individuals affected by these diseases accumulate in their brains amyloids inclusions composed of misfolded forms of a peptide (Aβ) and a protein (Tau) in AD and α-synuclein protein (α-Sn) in PD. Tau and α-Sn aggregates are also present in other neurodegenerative diseases. The insoluble nature and heterogeneity of amyloids have hampered their study at the molecular level. However, the use of solid state NMR and Cryogenic-electron microscopy along with fine-tuned modulation of the aggregation in vitro and improved isolation methods of brain-derived amyloids has allowed the elucidation of these elusive conformations at high resolution. In this work, we review the latest progress on the recent amyloid structures reported for Aβ, Tau, and α-Sn. The two-fold symmetry emerges as a convergent feature in the tridimensional arrangement of the protofilaments in the fibrillary structure of these pathological amyloids, with many of them exhibiting a Greek-key topology as part of their overall architecture. These specific features can serve as novel guides to seek potential molecular targets in drug design efforts.

The role of amyloids in Alzheimer's and Parkinson's diseases

With varying clinical symptoms, most neurodegenerative diseases are associated with abnormal loss of neurons. They share the same common pathogenic mechanisms involving misfolding and aggregation, and these visible aggregates of proteins are deposited in the central nervous system. Amyloid formation is thought to arise from partial unfolding of misfolded proteins leading to the exposure of hydrophobic surfaces, which interact with other similar structures and give rise to form dimers, oligomers, protofibrils, and eventually mature fibril aggregates. Accumulating evidence indicates that amyloid oligomers, not amyloid fibrils, are the most toxic species that causes Alzheimer's disease (AD) and Parkinson's disease (PD). AD has recently been recognized as the 'twenty-first century plague', with an incident rate of 1% at 60 years of age, which then doubles every fifth year. Currently, 5.3 million people in the US are afflicted with this disease, and the number of cases is expected to rise to 13.5 million by 2050. PD, a disorder of the brain, is the second most common form of dementia, characterized by difficulty in walking and movement. Keeping the above views in mind, in this review we have focused on the roles of amyloid in neurodegenerative diseases including AD and PD, the involvement of amyloid in mitochondrial dysfunction leading to neurodegeneration, are also considered in the review.

To target Tau pathologies, we must embrace and reconstruct their complexities

The accumulation of hyperphosphorylated fibrillar Tau aggregates in the brain is one of the defining hallmarks of Tauopathy diseases, including Alzheimer's disease. However, the primary events or molecules responsible for initiation of the pathological Tau aggregation and spreading remain unknown. The discovery of heparin as an effective inducer of Tau aggregation in vitro was instrumental to enabling different lines of research into the role of Tau aggregation in the pathogenesis of Tauopathies. However, recent proteomics and cryogenic electron microscopy (cryo-EM) studies have revealed that heparin-induced Tau fibrils generated in vitro do not reproduce the biochemical and ultrastructural properties of disease-associated brain-derived Tau fibrils. These observations demand that we reassess our current approaches for investigating the mechanisms underpinning Tau aggregation and pathology formation. Our review article presents an up-to-date survey and analyses of 1) the evolution of our understanding of the interactions between Tau and heparin, 2) the various structural and mechanistic models of the heparin-induced Tau aggregation, 3) the similarities and differences between brain-derived and heparin-induced Tau fibrils; and 4) emerging concepts on the biochemical and structural determinants underpinning Tau pathological heterogeneity in Tauopathies. Our analyses identify specific knowledge gaps and call for 1) embracing the complexities of Tau pathologies; 2) reassessment of current approaches to investigate, model and reproduce pathological Tau aggregation as it occurs in the brain; 3) more research towards a better understanding of the naturally-occurring cofactor molecules that are associated with Tau brain pathology initiation and propagation; and 4) developing improved approaches for in vitro production of the Tau aggregates and fibrils that recapitulate and/or amplify the biochemical and structural complexity and diversity of pathological Tau in Tauopathies. This will result in better and more relevant tools, assays, and mechanistic models, which could significantly improve translational research and the development of drugs and antibodies that have higher chances for success in the clinic.

A critical appraisal of tau-targeting therapies for primary and secondary tauopathies

INTRODUCTION

Primary tauopathies are neurological disorders in which tau protein deposition is the predominant pathological feature. Alzheimer's disease is a secondary tauopathy with tau forming hyperphosphorylated insoluble aggregates. Tau pathology can propagate from region to region in the brain, while alterations in tau processing may impair tau physiological functions.

METHODS

We reviewed literature on tau biology and anti-tau drugs using PubMed, meeting abstracts, and ClnicalTrials.gov.

RESULTS

The past 15 years have seen >30 drugs interfering with tau aggregation, processing, and accumulation reaching the clinic. Initial results with tau aggregation inhibitors and anti-tau monoclonal antibodies have not shown clinical efficacy.

DISCUSSION

The reasons for these clinical failures are unclear but could be linked to the clearing of physiological forms of tau by non-specific drugs. Research is now concentrating efforts on developing reliable translational animal models and selective compounds targeting specific tau epitopes, neurotoxic tau aggregates, and post-translational tau modifications.

The Structure Biology of Tau and Clue for Aggregation Inhibitor Design

Tau is a microtubule-associated protein that is mainly expressed in central and peripheral nerve systems. Tau binds to tubulin and regulates assembly and stabilization of microtubule, thus playing a critical role in neuron morphology, axon development and navigation. Tau is highly stable under normal conditions; however, there are several factors that can induce or promote aggregation of tau, forming neurofibrillary tangles. Neurofibrillary tangles are toxic to neurons, which may be related to a series of neurodegenerative diseases including Alzheimer's disease. Thus, tau is widely accepted as an important therapeutic target for neurodegenerative diseases. While the monomeric structure of tau is highly disordered, the aggregate structure of tau is formed by closed packing of β-stands. Studies on the structure of tau and the structural transition mechanism provide valuable information on the occurrence, development, and therapy of tauopathies. In this review, we summarize recent progress on the structural investigation of tau and based on which we discuss aggregation inhibitor design.

Mechanistic roles for altered O-GlcNAcylation in neurodegenerative disorders

Neurodegenerative diseases such as Alzheimer's and Parkinson's remain highly prevalent and incurable disorders. A major challenge in fully understanding and combating the progression of these diseases is the complexity of the network of processes that lead to progressive neuronal dysfunction and death. An ideal therapeutic avenue is conceivably one that could address many if not all of these multiple misregulated mechanisms. Over the years, chemical intervention for the up-regulation of the endogenous posttranslational modification (PTM) O-GlcNAc has been proposed as a potential strategy to slow down the progression of neurodegeneration. Through the development and application of tools that allow dissection of the mechanistic roles of this PTM, there is now a growing body of evidence that O-GlcNAc influences a variety of important neurodegeneration-pertinent mechanisms, with an overall protective effect. As a PTM that is appended onto numerous proteins that participate in protein quality control and homeostasis, metabolism, bioenergetics, neuronal communication, inflammation, and programmed death, O-GlcNAc has demonstrated beneficence in animal models of neurodegenerative diseases, and its up-regulation is now being pursued in multiple clinical studies.

Computational Insights Into the Inhibition Mechanism of Proanthocyanidin B2 on Tau Hexapeptide (PHF6) Oligomer

The formation of amyloid fibrils from Tau is a key pathogenic feature of Alzheimer's disease (AD). To disturb the formation of Tau aggregates is considered as a promising therapeutic strategy for AD. Recently, a natural product proanthocyanidin B2 (PB2) was confirmed to not only inhibit Tau aggregation, but also disaggregate Tau fibrils. Herein, to explore the inhibition mechanism of PB2 against Tau fibril and to provide the useful information for drug design and discovery, all-atom molecular dynamics simulations were carried out for the ordered Tau hexapeptide PHF6 oligomer in the presence and absence of PB2. The obtained result shows that PB2 can transform PHF6 oligomer from the ordered β-sheet structure into disordered one. Moreover, the clustering analysis and binding free energy calculations identify that S3 site is the most potential binding site. At S3 site, by hydrophobic and hydrogen bond interactions, the residues V309, Y310 and K311 are essential for binding with PB2, especially K311. In a word, our study reveals the molecular mechanism of PB2 inhibiting PHF6 aggregation and it will provide some valuable information for the development of Tau aggregation inhibitors.

Tau strains shape disease

Tauopathies consist of over 25 different neurodegenerative diseases that include argyrophilic grain disease (AGD), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), and Pick’s disease (PiD). Tauopathies are defined by brain accumulation of microtubule-associated protein tau in fibrillar aggregates, whose prevalence strongly correlates with dementia. Dominant mutations in tau cause neurodegenerative diseases, and most increase its aggregation propensity. Pathogenesis of tauopathies may involve pathological tau conformers that serve as templates to recruit native protein into growing assemblies and also move between brain cells to cause disease progression, similar to prions. Prions adopt pathological conformations, termed “strains,” that stably propagate in living systems, and create unique patterns of neuropathology. Data from multiple laboratories now suggest that tau acts as a prion. It propagates unique strains indefinitely in cultured cells, and when these are inoculated into mouse models, they create defined neuropathological patterns, which establish a direct link between conformation and disease. In humans, distinct fibril structures are associated with different diseases, but causality has not been established as in mice. Cryo-EM structures of tau fibrils isolated from tauopathy brains reveal distinct fibril cores across disease. Interestingly, the conformation of the tau monomer unit within different fibril subtypes from the same patient appears relatively preserved. This is consistent with data that the tau monomer samples an ensemble of conformations that act as distinct pathologic templates in the formation of restricted numbers of strains. The propensity of a tau monomer to adopt distinct conformations appears to be linked to defined local motifs that expose different patterns of amyloidogenic amino acid sequences. The prion hypothesis, which predicts that protein structure dictates resultant disease, has proved particularly useful to understand the diversity of human tauopathies. The challenge now is to develop methods to rapidly classify patients according to the structure of the underlying pathological protein assemblies to achieve more accurate diagnosis and effective therapy.

The Cryo-EM Effect: Structural Biology of Neurodegenerative Disease Aggregates

Neurogenerative diseases are characterized by diverse protein aggregates with a variety of microscopic morphologic features. Although ultrastructural studies of human neurodegenerative disease tissues have been conducted since the 1960s, only recently have near-atomic resolution structures of neurodegenerative disease aggregates been described. Solid-state nuclear magnetic resonance spectroscopy and X-ray crystallography have provided near-atomic resolution information about in vitro aggregates but pose logistical challenges to resolving the structure of aggregates derived from human tissues. Recent advances in cryo-electron microscopy (cryo-EM) have provided the means for near-atomic resolution structures of tau, amyloid-β (Aβ), α-synuclein (α-syn), and transactive response element DNA-binding protein of 43 kDa (TDP-43) aggregates from a variety of diseases. Importantly, in vitro aggregate structures do not recapitulate ex vivo aggregate structures. Ex vivo tau aggregate structures indicate individual tauopathies have a consistent aggregate structure unique from other tauopathies. α-syn structures show that even within a disease, aggregate heterogeneity may correlate to disease course. Ex vivo structures have also provided insight into how posttranslational modifications may relate to aggregate structure. Though there is less cryo-EM data for human tissue-derived TDP-43 and Aβ, initial structural studies provide a basis for future endeavors. This review highlights structural variations across neurodegenerative diseases and reveals fundamental differences between experimental systems and human tissue derived protein inclusions.

Inclusion of the C-Terminal Domain in the β-Sheet Core of Heparin-Fibrillized Three-Repeat Tau Protein Revealed by Solid-State Nuclear Magnetic Resonance Spectroscopy

Many neurodegenerative diseases such as Alzheimer's disease are characterized by pathological β-sheet filaments of the tau protein, which spread in a prion-like manner in patient brains. To date, high-resolution structures of tau filaments obtained from patient brains show that the β-sheet core only includes portions of the microtubule-binding repeat domains and excludes the C-terminal residues, indicating that the C-terminus is dynamically disordered. Here, we use solid-state NMR spectroscopy to identify the β-sheet core of full-length 0N3R tau fibrillized using heparin. Assignment of 13C and 15N chemical shifts of the rigid core of the protein revealed a single predominant β-sheet conformation, which spans not only the R3, R4, R' repeats but also the entire C-terminal domain (CT) of the protein. This massive β-sheet core qualitatively differs from all other tau fibril structures known to date. Using long-range correlation NMR experiments, we found that the R3 and R4 repeats form a β-arch, similar to that seen in some of the brain-derived tau fibrils, but the R1 and R3 domains additionally stack against the CT, reminiscent of previously reported transient interactions of the CT with the microtubule-binding repeats. This expanded β-sheet core structure suggests that the CT may have a protective effect against the formation of pathological tau fibrils by shielding the amyloidogenic R3 and R4 domains, preventing side-on nucleation. Truncation and post-translational modification of the CT in vivo may thus play an important role in the progression of tauopathies.

A Mechanism for the Inhibition of Tau Neurotoxicity: Studies with Artificial Membranes, Isolated Mitochondria, and Intact Cells

It is currently believed that molecular agents that specifically bind to and neutralize the toxic proteins/peptides, amyloid β (Aβ42), tau, and the tau-derived peptide PHF6, hold the key to attenuating the progression of Alzheimer's disease (AD). We thus tested our previously developed nonaggregating Aβ42 double mutant (Aβ42_DM) as a multispecific binder for three AD-associated molecules, wild-type Aβ42, the tau_K174Q mutant, and a synthetic PHF6 peptide. Aβ42_DM acted as a functional inhibitor of these molecules in in vitro assays and in neuronal cell-based models of AD. The double mutant bound both cytotoxic tau_K174Q and synthetic PHF6 and protected neuronal cells from the accumulation of tau in cell lysates and mitochondria. Aβ42_DM also reduced toxic intracellular levels of calcium and the overall cell toxicity induced by overexpressed tau, synthetic PHF6, Aβ42, or a combination of PHF6and Aβ42. Aβ42_DM inhibited PHF6-induced overall mitochondrial dysfunction: In particular, Aβ42_DM inhibited PHF6-induced damage to submitochondrial particles (SMPs) and suppressed PHF6-induced elevation of the ζ-potential of inverted SMPs (proxy for the inner mitochondrial membrane, IMM). PHF6 reduced the lipid fluidity of cardiolipin/DOPC vesicles (that mimic the IMM) but not DOPC (which mimics the outer mitochondrial membrane), and this effect was inhibited by Aβ42_DM. This inhibition may be explained by the conformational changes in PHF6 induced by Aβ42_DM in solution and in membrane mimetics. On this basis, the paper presents a mechanistic explanation for the inhibitory activity of Aβ42_DM against Aβ42- and tau-induced membrane permeability and cell toxicity and provides confirmatory evidence for its protective function in neuronal cells.