Template-assisted filament growth by parallel stacking of tau

Effect of Natural Osmolytes on Recombinant Tau Monomer: Propensity of Oligomerization and Aggregation

The pathological misfolding and aggregation of the microtubule associated protein tau (MAPT), a full length Tau2N4R with 441aa, is considered the principal disease relevant constituent in tauopathies including Alzheimer's disease (AD) with an imbalanced ratio in 3R/4R isoforms. The exact cellular fluid composition, properties, and changes that coincide with tau misfolding, seed formation, and propagation events remain obscure. The proteostasis network, along with the associated osmolytes, is responsible for maintaining the presence of tau in its native structure or dealing with misfolding. In this study, for the first time, the roles of natural brain osmolytes are being investigated for their potential effects on regulating the conformational stability of the tau monomer (tauM) and its propensity to aggregate or disaggregate. Herein, the effects of physiological osmolytes myo-inositol, taurine, trimethyl amine oxide (TMAO), betaine, sorbitol, glycerophosphocholine (GPC), and citrulline on tau's aggregation state were investigated. The overall results indicate the ability of sorbitol and GPC to maintain the monomeric form and prevent aggregation of tau, whereas myo-inositol, taurine, TMAO, betaine, and citrulline promote tau aggregation to different degrees, as revealed by protein morphology in atomic force microscopy images. Biochemical and biophysical methods also revealed that tau proteins adopt different conformations under the influence of these osmolytes. TauM in the presence of all osmolytes expressed no toxicity when tested by a lactate dehydrogenase assay. Investigating the conformational stability of tau in the presence of osmolytes may provide a better understanding of the complex nature of tau aggregation in AD and the protective and/or chaotropic nature of osmolytes.

Modulation of Primary and Secondary Processes in Tau Fibril Formation by Salt-Induced Dynamics

The initial stages of amyloid fibrilization begin with the monomers populating aggregation-prone conformers. Characterization of such aggregation-prone conformers is crucial in the study of neurodegenerative diseases. The current study characterizes the aggregation pathway of two tau protein constructs that have been recently demonstrated to form Alzheimer's (AD) fibril structures with divalent ions and chronic traumatic encephalopathy (CTE) fibril structures with monovalent ions. The results highlight the involvement of identical residues in both the primary and secondary processes of both AD and CTE fibril propagation. Nuclear magnetic resonance relaxation experiments reveal increased flexibility of the motifs 321KCGS within R3 and 364PGGGN within R4 in the presence of MgCl2/NaCl, correlating with faster aggregation kinetics and indicating efficient primary nucleation. Notably, the seeded aggregation kinetics of the tau monomers in the presence and absence of metal ions are strikingly different. This correlates with the overall sign of the 15N-ΔR2 profile specifying the dominant mechanism involved in the process of aggregation.

Mirror-Image Phage Display for the Selection of D-Amino Acid Peptide Ligands as Potential Therapeutics

In neurodegenerative diseases like Alzheimer's disease (AD), endogenous proteins or peptides aggregate with themselves. These proteins may lose their function or aggregates and/or oligomers can obtain toxicity, causing injury or death to cells. Aggregation of two major proteins characterizes AD. Amyloid-β peptide (Aβ) is deposited in amyloid plaques within the extracellular space of the brain and Tau in so-called neurofibrillary tangles in neurons. Finding peptide ligands to halt protein aggregation is a promising therapeutical approach. Using mirror-image phage display with a commercially available, randomized 12-mer peptide library, we have selected D-amino acid peptides, which bind to the Tau protein and modulate its aggregation in vitro. Peptides can bind specifically and selectively to a target molecule, but natural L-amino acid peptides may have crucial disadvantages for in vivo applications, as they are sensitive to protease degradation and may elicit immune responses. One strategy to circumvent these disadvantages is the use of non-naturally occurring D-amino acid peptides as they exhibit increased protease resistance and generally do not activate the immune system. To perform mirror-image phage display, the target protein needs to be synthesized as D-amino acid version. If the target protein sequence is too long to be synthesized properly, smaller peptides derived from the full length protein can be used for the selection process. This also offers the possibility to influence the binding region of the selected D-peptides in the full-length target protein. Here we provide the protocols for mirror-image phage display selection on the PHF6* peptide of Tau, based on the commercially available Ph.D.™-12 Phage Display Peptide Library Kit, leading to D-peptides that also bind the full length Tau protein (Tau441), next to PHF6*. In addition, we provide protocols and data for the first characterization of those D-peptides that inhibit Tau aggregation in vitro. © 2024 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Mirror image phage display selection against D-PHF6* fibrils Support Protocol 1: Single phage ELISA Basic Protocol 2: Sequencing and D-peptide generation Basic Protocol 3: Thioflavin-T (ThT) test to control inhibition of Tau aggregation Support Protocol 2: Purification of full-length Tau protein Basic Protocol 4: ELISA to demonstrate the binding of the generated D-peptides to PHF6* and full-length Tau fibrils.

Tau Oligomers as Pathogenic Seeds: Preparation, Characterization, and Propagation In Vitro and In Vivo

Tau oligomers have been shown to be the main toxic tau species in several neurodegenerative disorders. To study tau oligomers, we have developed reagents and established methods for the reliable preparation, isolation, and detection of tau oligomers as well as their seeding and propagation both in vitro and in vivo. Detailed below are methods for isolation of tau oligomers from brain tissues and detection of tau oligomers using tau oligomer-specific antibodies by biochemical, immunohistochemical, and biophysical methods. Further, methods for evaluating the biological activity of the tau oligomers including their effects on synaptic function, seeding, and propagation in cell models and in vivo are also described.

In pursuit of degenerative brain disease diagnosis: Dementia biomarkers detected by DNA aptamer-attached portable graphene biosensor

Dementia is a brain disease which results in irreversible and progressive loss of cognition and motor activity. Despite global efforts, there is no simple and reliable diagnosis or treatment option. Current diagnosis involves indirect testing of commonly inaccessible biofluids and low-resolution brain imaging. We have developed a portable, wireless readout-based Graphene field-effect transistor (GFET) biosensor platform that can detect viruses, proteins, and small molecules with single-molecule sensitivity and specificity. We report the detection of three important amyloids, namely, Amyloid beta (Aβ), Tau (τ), and α-Synuclein (αS) using DNA aptamer nanoprobes. These amyloids were isolated, purified, and characterized from the autopsied brain tissues of Alzheimer's Disease (AD) and Parkinson's Disease (PD) patients. The limit of detection (LoD) of the sensor is 10 fM, 1-10 pM, 10-100 fM for Aβ, τ, and αS, respectively. Synthetic as well as autopsied brain-derived amyloids showed a statistically significant sensor response with respect to derived thresholds, confirming the ability to define diseased vs. nondiseased states. The detection of each amyloid was specific to their aptamers; Aβ, τ, and αS peptides when tested, respectively, with aptamers nonspecific to them showed statistically insignificant cross-reactivity. Thus, the aptamer-based GFET biosensor has high sensitivity and precision across a range of epidemiologically significant AD and PD variants. This portable diagnostic system would allow at-home and POC testing for neurodegenerative diseases globally.

Tau protein aggregation associated with SARS-CoV-2 main protease

The primary function of virus proteases is the proteolytic processing of the viral polyprotein. These enzymes can also cleave host cell proteins, which is important for viral pathogenicity, modulation of cellular processes, viral replication, the defeat of antiviral responses and modulation of the immune response. It is known that COVID-19 can influence multiple tissues or organs and that infection can damage the functionality of the brain in multiple ways. After COVID-19 infections, amyloid-β, neurogranin, tau and phosphorylated tau were detected extracellularly, implicating possible neurodegenerative processes. The present study describes the possible induction of tau aggregation by the SARS-CoV-2 3CL protease (3CLpro) possibly relevant in neuropathology. Further investigations demonstrated that tau was proteolytically cleaved by the viral protease 3CL and, consequently, generated aggregates. However, more evidence is needed to confirm that COVID-19 is able to trigger neurodegenerative diseases.

Flow cytometric isolation of drug-like conformational antibodies specific for amyloid fibrils

Antibodies that recognize specific protein conformational states are broadly important for research, diagnostic and therapeutic applications, yet they are difficult to generate in a predictable and systematic manner using either immunization or in vitro antibody display methods. This problem is particularly severe for conformational antibodies that recognize insoluble antigens such as amyloid fibrils associated with many neurodegenerative disorders. Here we report a quantitative fluorescence-activated cell sorting (FACS) method for directly selecting high-quality conformational antibodies against different types of insoluble (amyloid fibril) antigens using a single, off-the-shelf human library. Our approach uses quantum dots functionalized with antibodies to capture insoluble antigens, and the resulting quantum dot conjugates are used in a similar manner as conventional soluble antigens for multi-parameter FACS selections. Notably, we find that this approach is robust for isolating high-quality conformational antibodies against tau and α-synuclein fibrils from the same human library with combinations of high affinity, high conformational specificity and, in some cases, low off-target binding that rival or exceed those of clinical-stage antibodies specific for tau (zagotenemab) and α-synuclein (cinpanemab). This approach is expected to enable conformational antibody selection and engineering against diverse types of protein aggregates and other insoluble antigens (e.g., membrane proteins) that are compatible with presentation on the surface of antibody-functionalized quantum dots.

MAP2 caps tau fibrils and inhibits aggregation

Fibrils of the microtubule-associated protein tau are intimately linked to the pathology of Alzheimer's disease (AD) and related neurodegenerative disorders. A current paradigm for pathology spreading in the human brain is that short tau fibrils transfer between neurons and then recruit naive tau monomers onto their tips, perpetuating the fibrillar conformation with high fidelity and speed. Although it is known that the propagation could be modulated in a cell-specific manner and thereby contribute to phenotypic diversity, there is still limited understanding of how select molecules are involved in this process. MAP2 is a neuronal protein that shares significant sequence homology with the repeat-bearing amyloid core region of tau. There is discrepancy about MAP2's involvement in pathology and its relationship with tau fibrillization. Here, we employed the entire repeat regions of 3R and 4R MAP2, to investigate their modulatory role in tau fibrillization. We find that both proteins block the spontaneous and seeded aggregation of 4R tau, with 4R MAP2 being slightly more potent. The inhibition of tau seeding is observed in vitro, in HEK293 cells, and in AD brain extracts, underscoring its broader scope. MAP2 monomers specifically bind to the end of tau fibrils, preventing recruitment of further tau and MAP2 monomers onto the fibril tip. The findings uncover a new function for MAP2 as a tau fibril cap that could play a significant role in modulating tau propagation in disease and may hold promise as a potential intrinsic protein inhibitor.

Anterograde and Retrograde Propagation of Inoculated Human Tau Fibrils and Tau Oligomers in a Non-Transgenic Rat Tauopathy Model

The tauopathy of Alzheimer's disease (AD) is first observed in the brainstem and entorhinal cortex, spreading trans-synaptically along specific pathways to other brain regions with recognizable patterns. Tau propagation occurs retrogradely and anterogradely (trans-synaptically) along a given pathway and through exosomes and microglial cells. Some aspects of in vivo tau spreading have been replicated in transgenic mice models expressing a mutated human MAPT (tau) gene and in wild-type mice. In this study, we aimed to characterize the propagation of different forms of tau species in non-transgenic 3-4 months old wild-type rats after a single unilateral injection of human tau oligomers and tau fibrils into the medial entorhinal cortex (mEC). We determined whether different variants of the inoculated human tau protein, tau fibrils, and tau oligomers, would induce similar neurofibrillary changes and propagate in an AD-related pattern, and how tau-related pathological changes would correlate with presumed cognitive impairment. We injected human tau fibrils and tau oligomers stereotaxically into the mEC and examined the distribution of tau-related changes at 3 days and 4, 8, and 11 months post-injection using antibodies AT8 and MC1, which reveal early phosphorylation and aberrant conformation of tau, respectively, HT7, anti-synaptophysin, and the Gallyas silver staining method. Human tau oligomers and tau fibrils exhibited some similarities and some differences in their ability to seed and propagate tau-related changes. Both human tau fibrils and tau oligomers rapidly propagated from the mEC anterogradely into the hippocampus and various parts of the neocortex. However, using a human tau-specific HT7 antibody, 3 days post-injection we found inoculated human tau oligomers in the red nucleus, primary motor, and primary somatosensory cortex, a finding not seen in animals inoculated with human tau fibrils. In animals inoculated with human tau fibrils, 3 days post-injection the HT7 antibody showed fibrils in the pontine reticular nucleus, a finding explained only by uptake of human tau fibrils by incoming presynaptic fibers to the mEC and retrograde transport of inoculated human tau fibrils to the brainstem. Rats inoculated with human tau fibrils showed as early as 4 months after inoculation a spread of phosphorylated tau protein at the AT8 epitopes throughout the brain, dramatically faster propagation of neurofibrillary changes than with human tau oligomers. The overall severity of tau protein changes 4, 8, and 11 months after inoculation of human tau oligomers and tau fibrils correlated well with spatial working memory and cognition impairments, as measured by the T-maze spontaneous alternation, novel object recognition, and object location tests. We concluded that this non-trangenic rat model of tauopathy, especially when using human tau fibrils, demonstrates rapidly developing pathologic alterations in neurons, synapses, and identifiable pathways together with cognitive and behavioral changes, through the anterograde and retrograde spreading of neurofibrillary degeneration. Therefore, it represents a promising model for future experimental studies of primary and secondary tauopathies, especially AD.

Solid-state NMR of paired helical filaments formed by the core tau fragment tau(297-391)

Aggregation of the tau protein into fibrillar cross-β aggregates is a hallmark of Alzheimer's diseases (AD) and many other neurodegenerative tauopathies. Recently, several core structures of patient-derived tau paired helical filaments (PHFs) have been solved revealing a structural variability that often correlates with a specific tauopathy. To further characterize the dynamics of these fibril cores, to screen for strain-specific small molecules as potential biomarkers and therapeutics, and to develop strain-specific antibodies, recombinant in-vitro models of tau filaments are needed. We recently showed that a 95-residue fragment of tau (from residue 297 to 391), termed dGAE, forms filaments in vitro in the absence of polyanionic co-factors often used for in vitro aggregation of full-length tau. Tau(297-391) was identified as the proteolytic resistant core of tau PHFs and overlaps with the structures characterized by cryo-electron microscopy in ex vivo PHFs, making it a promising model for the study of AD tau filaments in vitro. In the present study, we used solid-state NMR to characterize tau(297-391) filaments and show that such filaments assembled under non-reducing conditions are more dynamic and less ordered than those made in the presence of the reducing agent DTT. We further report the resonance assignment of tau(297-391)+DTT filaments and compare it to existing core structures of tau.

Neuroprotective Action of Coumarin Derivatives through Activation of TRKB-CREB-BDNF Pathway and Reduction of Caspase Activity in Neuronal Cells Expressing Pro-Aggregated Tau Protein

Hyperphosphorylation and aggregation of the microtubule binding protein tau is a neuropathological hallmark of Alzheimer’s disease/tauopathies. Tau neurotoxicity provokes alterations in brain-derived neurotrophic factor (BDNF)/tropomycin receptor kinase B (TRKB)/cAMP-response-element binding protein (CREB) signaling to contribute to neurodegeneration. Compounds activating TRKB may therefore provide beneficial effects in tauopathies. LM-031, a coumarin derivative, has demonstrated the potential to improve BDNF signaling in neuronal cells expressing pro-aggregated ΔK280 tau mutant. In this study, we investigated if LM-031 analogous compounds provide neuroprotection effects through interaction with TRKB in SH-SY5Y cells expressing ΔK280 tau_RD-DsRed folding reporter. All four LMDS compounds reduced tau aggregation and reactive oxygen species. Among them, LMDS-1 and -2 reduced caspase-1, caspase-6 and caspase-3 activities and promoted neurite outgrowth, and the effect was significantly reversed by knockdown of TRKB. Treatment of ERK inhibitor U0126 or PI3K inhibitor wortmannin decreased p-CREB, BDNF and BCL2 in these cells, implying that the neuroprotective effects of LMDS-1/2 are via activating TRKB downstream ERK, PI3K-AKT and CREB signaling. Furthermore, LMDS-1/2 demonstrated their ability to quench the intrinsic fluorescence of tryptophan residues within the extracellular domain of TRKB, thereby consolidating their interaction with TRKB. Our results suggest that LMDS-1/2 exert neuroprotection through activating TRKB signaling, and shed light on their potential application in therapeutics of Alzheimer’s disease/tauopathies.

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.

Second Sphere Interactions in Amyloidogenic Diseases

Amyloids are protein aggregates bearing a highly ordered cross β structural motif, which may be functional but are mostly pathogenic. Their formation, deposition in tissues and consequent organ dysfunction is the central event in amyloidogenic diseases. Such protein aggregation may be brought about by conformational changes, and much attention has been directed toward factors like metal binding, post-translational modifications, mutations of protein etc., which eventually affect the reactivity and cytotoxicity of the associated proteins. Over the past decade, a global effort from different groups working on these misfolded/unfolded proteins/peptides has revealed that the amino acid residues in the second coordination sphere of the active sites of amyloidogenic proteins/peptides cause changes in H-bonding pattern or protein-protein interactions, which dramatically alter the structure and reactivity of these proteins/peptides. These second sphere effects not only determine the binding of transition metals and cofactors, which define the pathology of some of these diseases, but also change the mechanism of redox reactions catalyzed by these proteins/peptides and form the basis of oxidative damage associated with these amyloidogenic diseases. The present review seeks to discuss such second sphere modifications and their ramifications in the etiopathology of some representative amyloidogenic diseases like Alzheimer's disease (AD), type 2 diabetes mellitus (T2Dm), Parkinson's disease (PD), Huntington's disease (HD), and prion diseases.

Static and dynamic disorder in Aβ40 fibrils

Deposition of Aβ aggregates in the form of amyloid fibrils is a pathological hallmark of Alzheimer's disease. Understanding the structure and dynamics of Aβ fibrils is important for delineating the mechanism of Aβ aggregation and developing effective therapeutic strategies. Here we used site-directed spin labeling and EPR spectroscopy to study the Aβ40 fibril structure and dynamics. We obtained the EPR spectra of 40 spin-labeled Aβ40 fibril samples, with spin labeling coverage of the entire Aβ40 sequence. Analysis of the spin exchange interaction and spin label mobility using spectral simulations suggest that the strength of spin exchange interaction is primarily determined by static disorder in the Aβ40 fibrils. EPR data suggest that the entire Aβ40 sequence except residue D1 is highly ordered and the two hydrophobic regions at residues 17-20 and 31-36 show the lowest static disorder. Dynamic disorder is relatively constant across all reside positions, with residues 22 and 23 having the highest dynamic disorder. Comparison of the EPR data for Aβ40 and Aβ42 fibrils shows overall more ordered packing interactions in Aβ40 fibrils. Another noteworthy difference is the C-terminal residue, which has high static disorder in Aβ42 fibrils, but is ordered in Aβ40 fibrils. The higher static disorder in Aβ42 fibrils may lead to increased fragmentation, monomer dissociation, and structural defects, which may contribute to increased aggregation through secondary nucleation.

Lysine 63-linked ubiquitination of tau oligomers contributes to the pathogenesis of Alzheimer's disease

Ubiquitin-modified tau aggregates are abundantly found in human brains diagnosed with Alzheimer's disease (AD) and other tauopathies. Soluble tau oligomers (TauO) are the most neurotoxic tau species that propagate pathology and elicit cognitive deficits, but whether ubiquitination contributes to tau formation and spreading is not fully understood. Here, we observed that K63-linked, but not K48-linked, ubiquitinated TauO accumulated at higher levels in AD brains compared with age-matched controls. Using mass spectrometry analyses, we identified 11 ubiquitinated sites on AD brain-derived TauO (AD TauO). We found that K63-linked TauO are associated with enhanced seeding activity and propagation in human tau-expressing primary neuronal and tau biosensor cells. Additionally, exposure of tau-inducible HEK cells to AD TauO with different ubiquitin linkages (wild type, K48, and K63) resulted in enhanced formation and secretion of K63-linked TauO, which was associated with impaired proteasome and lysosome functions. Multipathway analysis also revealed the involvement of K63-linked TauO in cell survival pathways, which are impaired in AD. Collectively, our study highlights the significance of selective TauO ubiquitination, which could influence tau aggregation, accumulation, and subsequent pathological propagation. The insights gained from this study hold great promise for targeted therapeutic intervention in AD and related tauopathies.

A novel D-amino acid peptide with therapeutic potential (ISAD1) inhibits aggregation of neurotoxic disease-relevant mutant Tau and prevents Tau toxicity in vitro

Background

Alzheimer’s disease (AD), the most common form of dementia, is a progressive neurodegenerative disorder that mainly affects older adults. One of the pathological hallmarks of AD is abnormally aggregated Tau protein that forms fibrillar deposits in the brain. In AD, Tau pathology correlates strongly with clinical symptoms, cognitive dysfunction, and neuronal death.

Methods

We aimed to develop novel therapeutic D-amino acid peptides as Tau fibrillization inhibitors. It has been previously demonstrated that D-amino acid peptides are protease stable and less immunogenic than L-peptides, and these characteristics may render them suitable for in vivo applications. Using a phage display procedure against wild type full-length Tau (Tau^FL), we selected a novel Tau binding L-peptide and synthesized its D-amino acid version ISAD1 and its retro inversed form, ISAD1rev, respectively.

Results

While ISAD1rev inhibited Tau aggregation only moderately, ISAD1 bound to Tau in the aggregation-prone PHF6 region and inhibited fibrillization of Tau^FL, disease-associated mutant full-length Tau (Tau^FLΔK, Tau^FL-A152T, Tau^FL-P301L), and pro-aggregant repeat domain Tau mutant (Tau^RDΔK). ISAD1 and ISAD1rev induced the formation of large high molecular weight Tau^FL and Tau^RDΔK oligomers that lack proper Thioflavin-positive β-sheet conformation even at lower concentrations. In silico modeling of ISAD1 Tau interaction at the PHF6 site revealed a binding mode similar to those known for other PHF6 binding peptides. Cell culture experiments demonstrated that ISAD1 and its inverse form are taken up by N2a-Tau^RDΔK cells efficiently and prevent cytotoxicity of externally added Tau fibrils as well as of internally expressed Tau^RDΔK.

Conclusions

ISAD1 and related peptides may be suitable for therapy development of AD by promoting off-pathway assembly of Tau, thus preventing its toxicity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13195-022-00959-z.

Antitumour drugs targeting tau R3 VQIVYK and Cys322 prevent seeding of endogenous tau aggregates by exogenous seeds

Emerging experimental evidence suggests tau pathology spreads between neuroanatomically connected brain regions in a prion-like manner in Alzheimer's disease (AD). Tau seeding, the ability of prion-like tau to recruit and misfold naïve tau to generate new seeds, is detected early in human AD brains before the development of major tau pathology. Many antitumour drugs have been reported to confer protection against neurodegeneration, supporting the repurposing of approved and experimental or investigational oncology drugs for AD therapy. In this study, we evaluated whether antitumour drugs that abrogate the generation of seed-competent aggregates of tau Repeat 3 (R3) domain peptides can prevent tau seeding and toxicity in Tau-RD P301S FRET Biosensor cells and Caenorhabditis elegans. We demonstrate that drugs that interact with the N-terminal VQIVYK or the C-terminal region housing the Cys322 prevent R3 dimerisation, abolishing the generation of prion-like R3 seeds. Preformed R3 seeds (fibrils) capped with, or R3 seeds formed in the presence of VQIVYK- or Cys322-targeting drugs have a reduced potency to cause aggregation of naïve tau in biosensor cells and protect worms from aggregate toxicity. These findings indicate that VQIVYK- or Cys322-targeting drugs may act as prophylactic agents against tau seeding.

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.

Functional Integrity of Synapses in the Central Nervous System of Cognitively Intact Individuals with High Alzheimer's Disease Neuropathology Is Associated with Absence of Synaptic Tau Oligomers

BACKGROUND

Certain individuals, here referred to as Non-Demented with Alzheimer Neuropathology (NDAN), do not show overt neurodegeneration (N-) and remain cognitively intact despite the presence of plaques (A+) and tangles (T+) that would normally be consistent with fully symptomatic Alzheimer's disease (AD).

OBJECTIVE

The existence of NDAN (A + T+N-) subjects suggests that the human brain utilizes intrinsic mechanisms that can naturally evade cognitive decline normally associated with the symptomatic stages of AD (A + T+N+). Deciphering the underlying mechanisms would prove relevant to develop complementing therapeutics to prevent progression of AD-related cognitive decline.

METHODS

Previously, we have reported that NDAN present with preserved neurogenesis and synaptic integrity paralleled by absence of amyloid oligomers at synapses. Using postmortem brain samples from age-matched control subjects, demented AD patients and NDAN individuals, we performed immunofluorescence, western blots, micro transplantation of synaptic membranes in Xenopus oocytes followed by twin electrode voltage clamp electrophysiology and fluorescence assisted single synaptosome-long term potentiation studies.

RESULTS

We report decreased tau oligomers at synapses in the brains of NDAN subjects. Furthermore, using novel approaches we report, for the first time, that such absence of tau oligomers at synapses is associated with synaptic functional integrity in NDAN subjects as compared to demented AD patients.

CONCLUSION

Overall, these results give further credence to tau oligomers as primary actors of synaptic destruction underscoring cognitive demise in AD and support their targeting as a viable therapeutic strategy for AD and related tauopathies.

A thiol-based intramolecular redox switch in four-repeat tau controls fibril assembly and disassembly

Oxidative stress has been implicated in the pathogenesis and progression of several tauopathies, including Alzheimer's disease. The deposition of fibrillar inclusions made of tau protein is one of the pathological hallmarks of these disorders. Although it is becoming increasingly evident that the specific fibril structure may vary from one tauopathy to another and it is recognized that different types of isoforms (three-repeat and four-repeat tau) can be selectively deposited, little is known about the role oxidation may play in aggregation. Four-repeat tau contains two cysteines that can form an intramolecular disulfide bond, resulting in a structurally restrained compact monomer. There is discrepancy as to whether this monomer can aggregate or not. Using isolated four-repeat tau monomers (htau40) with intramolecular disulfide bonds, we demonstrate that these proteins form fibrils. The fibrils are less stable than fibrils formed under reducing conditions but are highly effective in seeding oxidized tau monomers. Conversely, a strong seeding barrier prevents incorporation of reduced tau monomers, tau mimics in which the cysteines have been replaced by alanines or serines, and three-repeat tau (htau23), a single-cysteine isoform. The barrier also holds true when seed and monomer types are reversed, indicating that oxidized and reduced tau are incompatible with each other. Surprisingly, fibrils composed of compact tau disaggregate upon reduction, highlighting the importance of the intramolecular disulfide bond for fibril stability. The findings uncover a novel binary redox switch that controls the aggregation and disaggregation of these fibrils and extend the conformational spectrum of tau aggregates.

Structures of Pathological and Functional Amyloids and Prions, a Solid-State NMR Perspective

Infectious proteins or prions are a remarkable class of pathogens, where pathogenicity and infectious state correspond to conformational transition of a protein fold. The conformational change translates into the formation by the protein of insoluble amyloid aggregates, associated in humans with various neurodegenerative disorders and systemic protein-deposition diseases. The prion principle, however, is not limited to pathogenicity. While pathological amyloids (and prions) emerge from protein misfolding, a class of functional amyloids has been defined, consisting of amyloid-forming domains under natural selection and with diverse biological roles. Although of great importance, prion amyloid structures remain challenging for conventional structural biology techniques. Solid-state nuclear magnetic resonance (SSNMR) has been preferentially used to investigate these insoluble, morphologically heterogeneous aggregates with poor crystallinity. SSNMR methods have yielded a wealth of knowledge regarding the fundamentals of prion biology and have helped to solve the structures of several prion and prion-like fibrils. Here, we will review pathological and functional amyloid structures and will discuss some of the obtained structural models. We will finish the review with a perspective on integrative approaches combining solid-state NMR, electron paramagnetic resonance and cryo-electron microscopy, which can complement and extend our toolkit to structurally explore various facets of prion biology.

Optical chemosensors for the detection of proximally phosphorylated peptides and proteins

Proximal multi-site phosphorylation is a critical post-translational modification in protein biology. The additive effects of multiple phosphosite clusters in close spatial proximity triggers integrative and cooperative effects on protein conformation and activity. Proximal phosphorylation has been shown to modulate signal transduction pathways and gene expression, and as a result, is implicated in a broad range of disease states through altered protein function and/or localization including enzyme overactivation or protein aggregation. The role of proximal multi-phosphorylation events is becoming increasingly recognized as mechanistically important, although breakthroughs are limited due to a lack of detection technologies. To date, there is a limited selection of facile and robust sensing tools for proximal phosphorylation. Nonetheless, there have been considerable efforts in developing optical chemosensors for the detection of proximal phosphorylation motifs on peptides and proteins in recent years. This review provides a comprehensive overview of optical chemosensors for proximal phosphorylation, with the majority of work being reported in the past two decades. Optical sensors, in the form of fluorescent and luminescent chemosensors, hybrid biosensors, and inorganic nanoparticles, are described. Emphasis is placed on the rationale behind sensor scaffolds, relevant protein motifs, and applications in protein biology.

Effect of spin labelling on the aggregation kinetics of yeast prion protein Ure2

Amyloid formation is involved in a wide range of neurodegenerative diseases including Alzheimer's and prion diseases. Structural understanding of the amyloid is critical to delineate the mechanism of aggregation and its pathological spreading. Site-directed spin labelling has emerged as a powerful structural tool in the studies of amyloid structures and provided structural evidence for the parallel in-register β-sheet structure for a wide range of amyloid proteins. It is generally accepted that spin labelling does not disrupt the structure of the amyloid fibrils, the end product of protein aggregation. The effect on the rate of protein aggregation, however, has not been well characterized. Here, we employed a scanning mutagenesis approach to study the effect of spin labelling on the aggregation rate of 79 spin-labelled variants of the Ure2 prion domain. The aggregation of Ure2 protein is the basis of yeast prion [URE3]. We found that all spin-labelled Ure2 mutants aggregated within the experimental timeframe of 15 to 40 h. Among the 79 spin-labelled positions, only five residue sites (N23, N27, S33, I35 and G42) showed a dramatic delay in the aggregation rate as a result of spin labelling. These positions may be important for fibril nucleation, a rate-limiting step in aggregation. Importantly, spin labelling at most of the sites had a muted effect on Ure2 aggregation kinetics, showing a general tolerance of spin labelling in protein aggregation studies.

The structure and phase of tau: from monomer to amyloid filament

Tau is a microtubule-associated protein involved in regulation of assembly and spatial organization of microtubule in neurons. However, in pathological conditions, tau monomers assemble into amyloid filaments characterized by the cross-β structures in a number of neurodegenerative diseases known as tauopathies. In this review, we summarize recent progression on the characterization of structures of tau monomer and filament, as well as the dynamic liquid droplet assembly. Our aim is to reveal how post-translational modifications, amino acid mutations, and interacting molecules modulate the conformational ensemble of tau monomer, and how they accelerate or inhibit tau assembly into aggregates. Structure-based aggregation inhibitor design is also discussed in the context of dynamics and heterogeneity of tau structures.

Molecular mechanism of amyloidogenicity and neurotoxicity of a pro-aggregated tau mutant in the presence of histidine tautomerism via replica-exchange simulation

Considering ΔK280 tau mutation, δε isomer with highest sheet content may accelerate aggregation; generating small compounds to inhibit this would help tp prevent tauopathies.

TDP-43 and Tau Oligomers in Alzheimer's Disease, Amyotrophic Lateral Sclerosis, and Frontotemporal Dementia

Proteinaceous aggregates are major hallmarks of several neurodegenerative diseases. Aggregates of post-translationally modified transactive response (TAR)-DNA binding protein 43 (TDP-43) in cytoplasmic inclusion bodies are characteristic features in frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). Recent studies have also reported TDP-43 aggregation in Alzheimer's disease (AD). TDP-43 is an RNA/DNA binding protein (RBP) mainly present in the nucleus. In addition to several RBPs, TDP-43 has also been reported in stress granules in FTD and ALS pathologies. Despite knowledge of cytoplasmic mislocalization of TDP-43, the cellular effects of TDP-43 aggregates and their cytotoxic mechanism(s) remain to be clarified. We hypothesize that TDP-43 forms oligomeric assemblies that associate with tau, another key protein involved in ALS and FTD. However, no prior studies have investigated the interactions between TDP-43 oligomers and tau. It is therefore important to thoroughly investigate the cross-seeding properties and cellular localization of both TDP-43 and tau oligomers in neurodegenerative diseases. Here, we demonstrate the effect of tau on the cellular localization of TDP-43 in WT and P301L tau-inducible cell models (iHEK) and in WT HEK-293 cells treated exogenously with soluble human recombinant tau oligomers (Exo-rTauO). We observed cytoplasmic TDP-43 accumulation o in the presence of tau in these cell models. We also studied the occurrence of TDP-43 oligomers in AD, ALS, and FTD human brain tissue using novel antibodies generated against TDP-43 oligomers as well as generic TDP-43 antibodies. Finally, we examined the cross-seeding property of AD, ALS, and FTD brain-derived TDP-43 oligomers (BDT43Os) on tau aggregation using biochemical and biophysical assays. Our results allow us to speculate that TDP-43/tau interactions might play a role in AD, ALS, and FTD.

Intrinsic Origin of Tau Protein Aggregation: Effects of Histidine Tautomerism on Tau267-312 Monomer

Histidine tautomerism is considered a crucial component that affects the constitutional and accumulation characteristics of the tau_267-312 monomer in the neutral condition, which are connected with the pathobiology of Alzheimer's disease (AD). Interpreting the organizational characteristics and accumulation procedure is a challenging task because two tautomeric conformations (the N^ε-H or N^δ-H tautomer) can occur in the open neutral condition. In the current work, replica-exchange molecular dynamics (REMD) simulations were performed to investigate the structural properties of the tau_267-312 monomer considering the histidine tautomeric effect. Based on the simulation outcomes, the histidine 268 (H268) (δ)-H299 (δ) (δδ) isomer had the highest β-sheet content with a value of 26.2%, which acquires a sheet-governing toxic conformer with the first abundant conformational state of 22.6%. In addition, δδ displayed notable antiparallel β-sheets between lysine 8 (K8)-asparagine 13 (N13) and valine 40 (V40)-tyrosine 44 (Y44) as well as between K32-H33 and V40-Y44 (β-meander supersecondary structure), indicating this tautomeric isomer may exist to stimulate tau oligomerization. Furthermore, H299 was found to play an essential role in the structural stabilization of the δδ isomer compared with H268. The present research will aid in obtaining insight into the organizational and accumulation properties of tau protein in the presence of histidine tautomerism to control AD.

Extensive Plasmid Library to Prepare Tau Protein Variants and Study Their Functional Biochemistry

Tau neurofibrillary tangles are key pathological features of Alzheimer's disease and other tauopathies. Recombinant protein technology is vital for studying the structure and function of tau in physiology and aggregation in pathophysiology. However, open-source and well-characterized plasmids for efficiently expressing and purifying different tau variants are lacking. We generated 44 sequence-verified plasmids including those encoding full length (FL) tau-441, its four-repeat microtubule-binding (K18) fragment, and their respective selected familial pathological variants (N279K, V337M, P301L, C291R, and S356T). Moreover, plasmids for expressing single (C291A), double (C291A/C322A), and triple (C291A/C322A/I260C) cysteine-modified variants were generated to study alterations in cysteine content and locations. Furthermore, protocols for producing representative tau forms were developed. We produced and characterized the aggregation behavior of the triple cysteine-modified tau-K18, often used in real-time cell internalization and aggregation studies because it can be fluorescently labeled on a cysteine outside the microtubule-binding core. Similar to the wild type (WT), triple cysteine-modified tau-K18 aggregated by progressive β-sheet enrichment, albeit at a slower rate. On prolonged incubation, cysteine-modified K18 formed paired helical filaments similar to those in Alzheimer's disease, sharing morphological phenotypes with WT tau-K18 filaments. Nonetheless, cysteine-modified tau-K18 filaments were significantly shorter (p = 0.002) and mostly wider than WT filaments, explainable by their different principal filament elongation pathways: vertical (end-to-end) and lateral growth for WT and cysteine-modified, respectively. Cysteine rearrangement may therefore induce filament polymorphism. Together, the plasmid library, the protein production methods, and the new insights into cysteine-dependent aggregation should facilitate further studies and the design of antiaggregation agents.

Modulating disease-relevant tau oligomeric strains by small molecules

The pathological aggregation of tau plays an important role in Alzheimer's disease and many other related neurodegenerative diseases, collectively referred to as tauopathies. Recent evidence has demonstrated that tau oligomers, small and soluble prefibrillar aggregates, are highly toxic due to their strong ability to seed tau misfolding and propagate the pathology seen across different neurodegenerative diseases. We previously showed that novel curcumin derivatives affect preformed tau oligomer aggregation pathways by promoting the formation of more aggregated and nontoxic tau aggregates. To further investigate their therapeutic potential, we have extended our studies o disease-relevant brain-derived tau oligomers (BDTOs). Herein, using well-characterized BDTOs, isolated from brain tissues of different tauopathies, including Alzheimer's disease, progressive supranuclear palsy, and dementia with Lewy bodies, we found that curcumin derivatives modulate the aggregation state of BDTOs by reshaping them and rescue neurons from BDTO-associated toxicity. Interestingly, compound CL3 showed an effect on the aggregation pattern of BDTOs from different tauopathies, resulting in the formation of less neurotoxic larger tau aggregates with decreased hydrophobicity and seeding propensity. Our results lay the groundwork for potential investigations of the efficacy and beneficial effects of CL3 and other promising compounds for the treatment of tauopathies. Furthermore, CL3 may aid in the development of tau imaging agent for the detection of tau oligomeric strains and differential diagnosis of the tauopathies, thus enabling earlier interventions.

Truncation of Tau selectively facilitates its pathological activities

Neurofibrillary tangles of abnormally hyperphosphorylated Tau are a hallmark of Alzheimer's disease (AD) and related tauopathies. Tau is truncated at multiple sites by various proteases in AD brain. Although many studies have reported the effect of truncation on the aggregation of Tau, these studies mostly employed highly artificial conditions, using heparin sulfate or arachidonic acid to induce aggregation. Here, we report for the first time the pathological activities of various truncations of Tau, including site-specific phosphorylation, self-aggregation, binding to hyperphosphorylated and oligomeric Tau isolated from AD brain tissue (AD O-Tau), and aggregation seeded by AD O-Tau. We found that deletion of the first 150 or 230 amino acids (aa) enhanced Tau's site-specific phosphorylation, self-aggregation, and binding to AD O-Tau and aggregation seeded by AD O-Tau, but deletion of the first 50 aa did not produce a significant effect. Deletion of the last 50 aa was found to modulate Tau's site-specific phosphorylation, promote its self-aggregation, and cause it to be captured by and aggregation seeded by AD O-Tau, whereas deletion of the last 20 aa had no such effects. Among the truncated Taus, Tau_151-391 showed the highest pathological activities. AD O-Tau induced aggregation of Tau_151-391 in vitro and in cultured cells. These findings suggest that the first 150 aa and the last 50 aa protect Tau from pathological characteristics and that their deletions facilitate pathological activities. Thus, inhibition of Tau truncation may represent a potential therapeutic approach to suppress Tau pathology in AD and related tauopathies.

Tau and Alpha Synuclein Synergistic Effect in Neurodegenerative Diseases: When the Periphery Is the Core

In neuronal cells, tau is a microtubule-associated protein placed in axons and alpha synuclein is enriched at presynaptic terminals. They display a propensity to form pathologic aggregates, which are considered the underlying cause of Alzheimer's and Parkinson's diseases. Their functional impairment induces loss of axonal transport, synaptic and mitochondrial disarray, leading to a "dying back" pattern of degeneration, which starts at the periphery of cells. In addition, pathologic spreading of alpha-synuclein from the peripheral nervous system to the brain through anatomical connectivity has been demonstrated for Parkinson's disease. Thus, examination of the extent and types of tau and alpha-synuclein in peripheral tissues and their relation to brain neurodegenerative diseases is of relevance since it may provide insights into patterns of protein aggregation and neurodegeneration. Moreover, peripheral nervous tissues are easily accessible in-vivo and can play a relevant role in the early diagnosis of these conditions. Up-to-date investigations of tau species in peripheral tissues are scant and have mainly been restricted to rodents, whereas, more evidence is available on alpha synuclein in peripheral tissues. Here we aim to review the literature on the functional role of tau and alpha synuclein in physiological conditions and disease at the axonal level, their distribution in peripheral tissues, and discuss possible commonalities/diversities as well as their interaction in proteinopathies.

Mechanisms of secretion and spreading of pathological tau protein

Accumulation of misfolded and aggregated forms of tau protein in the brain is a neuropathological hallmark of tauopathies, such as Alzheimer's disease and frontotemporal lobar degeneration. Tau aggregates have the ability to transfer from one cell to another and to induce templated misfolding and aggregation of healthy tau molecules in previously healthy cells, thereby propagating tau pathology across different brain areas in a prion-like manner. The molecular mechanisms involved in cell-to-cell transfer of tau aggregates are diverse, not mutually exclusive and only partially understood. Intracellular accumulation of misfolded tau induces several mechanisms that aim to reduce the cellular burden of aggregated proteins and also promote secretion of tau aggregates. However, tau may also be released from cells physiologically unrelated to protein aggregation. Tau secretion involves multiple vesicular and non-vesicle-mediated pathways, including secretion directly through the plasma membrane. Consequently, extracellular tau can be found in various forms, both as a free protein and in vesicles, such as exosomes and ectosomes. Once in the extracellular space, tau aggregates can be internalized by neighboring cells, both neurons and glial cells, via endocytic, pinocytic and phagocytic mechanisms. Importantly, accumulating evidence suggests that prion-like propagation of misfolding protein pathology could provide a general mechanism for disease progression in tauopathies and other related neurodegenerative diseases. Here, we review the recent literature on cellular mechanisms involved in cell-to-cell transfer of tau, with a particular focus in tau secretion.

Polymorphic α-Synuclein Strains Modified by Dopamine and Docosahexaenoic Acid Interact Differentially with Tau Protein

The pathological hallmark of synucleinopathies, including Parkinson’s disease (PD), is the aggregation of α-synuclein (α-Syn) protein. Even so, tau protein pathology is abundantly found in these diseases. Both α-Syn and tau can exist as polymorphic aggregates, a phenomenon that has been widely studied, mostly in their fibrillar assemblies. We have previously discovered that in addition to α-Syn oligomers, oligomeric tau is also present in the brain tissues of patients with PD and dementia with Lewy bodies (DLB). However, the effect of interaction between polymorphic α-Syn oligomers and tau has not been scrupulously studied. Here, we have explored the structural and functional diversity of distinct α-Syn oligomers, prepared by modifying the protein with dopamine (DA) and docosahexaenoic acid (DHA). The two α-Syn oligomers differed in aggregate size, conformation, sensitivity to proteinase K digestion, tryptic digestion, and toxicity, suggesting them as distinct α-Syn oligomeric strains. We examined their internalization mechanisms in primary neurons and seeding propensity in inducing α-Syn aggregation. Using a combined approach of molecular and cellular techniques, we observed that the tau aggregates cross-seeded with the individual α-Syn oligomeric strains differed in their biochemical and biological properties, suggesting two distinct tau strains. The tau aggregate cross-seeded with the DA-modified α-Syn oligomeric strain possessed a potent intracellular tau seeding propensity. This study provides a comprehensive analysis of unique strain-specific interaction between oligomeric α-Syn and tau. Furthermore, this study allows us to speculate that distinct α-Syn-tau interactions inducing tau aggregation might be an underlying mechanism of neurodegeneration in PD.

Polymorphic Aβ42 fibrils adopt similar secondary structure but differ in cross-strand side chain stacking interactions within the same β-sheet

Formation of polymorphic amyloid fibrils is a common feature in neurodegenerative diseases involving protein aggregation. In Alzheimer’s disease, different fibril structures may be associated with different clinical sub-types. Structural basis of fibril polymorphism is thus important for understanding the role of amyloid fibrils in the pathogenesis and progression of these diseases. Here we studied two types of Aβ42 fibrils prepared under quiescent and agitated conditions. Quiescent Aβ42 fibrils adopt a long and twisted morphology, while agitated fibrils are short and straight, forming large bundles via lateral association. EPR studies of these two types of Aβ42 fibrils show that the secondary structure is similar in both fibril polymorphs. At the same time, agitated Aβ42 fibrils show stronger interactions between spin labels across the full range of the Aβ42 sequence, suggesting a more tightly packed structure. Our data suggest that cross-strand side chain packing interactions within the same β-sheet may play a critical role in the formation of polymorphic fibrils.

Tau-Cofactor Complexes as Building Blocks of Tau Fibrils

The aggregation of the human tau protein into neurofibrillary tangles is directly diagnostic of many neurodegenerative conditions termed tauopathies. The species, factors and events that are responsible for the initiation and propagation of tau aggregation are not clearly established, even in a simplified and artificial in vitro system. This motivates the mechanistic study of in vitro aggregation of recombinant tau from soluble to fibrillar forms, for which polyanionic cofactors are the most commonly used external inducer. In this study, we performed biophysical characterizations to unravel the mechanisms by which cofactors induce fibrillization. We first reinforce the idea that cofactors are the limiting factor to generate ThT-active tau fibrils, and establish that they act as templating reactant that trigger tau conformational rearrangement. We show that heparin has superior potency for recruiting monomeric tau into aggregation-competent species compared to any constituent intermediate or aggregate "seeds." We show that tau and cofactors form intermediate complexes whose evolution toward ThT-active fibrils is tightly regulated by tau-cofactor interactions. Remarkably, it is possible to find mild cofactors that complex with tau without forming ThT-active species, except when an external catalyst (e.g., a seed) is provided to overcome the energy barrier. In a cellular context, we propose the idea that tau could associate with cofactors to form a metastable complex that remains "inert" and reversible, until encountering a relevant seed that can trigger an irreversible transition to β-sheet containing species.

Toxic Tau Oligomers Modulated by Novel Curcumin Derivatives

The pathological aggregation and accumulation of tau, a microtubule-associated protein, is a common feature amongst more than 18 different neurodegenerative diseases that are collectively known as tauopathies. Recently, it has been demonstrated that the soluble and hydrophobic tau oligomers are highly toxic in vitro due to their capacity towards seeding tau misfolding, thereby propagating the tau pathology seen across different neurodegenerative diseases. Modulating the aggregation state of tau oligomers through the use of small molecules could be a useful therapeutic strategy to target their toxicity, regardless of other factors involved in their formation. In this study, we screened and tested a small library of newly synthesized curcumin derivatives against preformed recombinant tau oligomers. Our results show that the curcumin derivatives affect and modulate the tau oligomer aggregation pathways, converting to a more aggregated non-toxic state as assessed in the human neuroblastoma SH-SY5Y cell line and primary cortical neuron cultures. These results provide insight into tau aggregation and may become a basis for the discovery of new therapeutic agents, as well as advance the diagnostic field for the detection of toxic tau oligomers.

Spontaneous Rupture and Entanglement of Human Neuronal Tau Protein Induced by Piconewton Compressive Force

Mechanical force vector fluctuations in living cells can have a significant impact on protein behavior and functions. Here we report that a human tau protein tertiary structure can abruptly and spontaneously rupture, like a balloon, under biologically available piconewton compressive force, using a home-modified atomic force microscopy single-molecule manipulation. The rupture behavior is dependent on the physiological level of presence of ions, such as K⁺ and Mg²⁺. We observed rupture events in the presence of K⁺ but not in the presence of Mg²⁺ ions. We have also explored the entangled protein state formed following the events of the multiple and simultaneous protein ruptures under crowding. Crowded proteins simultaneously rupture and then spontaneously refold to an entangled folding state, different from either folded and unfolded states of the tau protein, which can be a plausible pathway for the tau protein aggregation that is related to a number of neurodegenerative diseases.

Therapeutic antibody targeting microtubule-binding domain prevents neuronal internalization of extracellular tau via masking neuron surface proteoglycans

Pathologically altered tau protein is a common denominator of neurodegenerative disorders including Alzheimer’s disease (AD) and other tauopathies. Therefore, promising immunotherapeutic approaches target and eliminate extracellular pathogenic tau species, which are thought to be responsible for seeding and propagation of tau pathology. Tau isoforms in misfolded states can propagate disease pathology in a template-dependent manner, proposed to be mediated by the release and internalization of extracellular tau. Monoclonal antibody DC8E8, binding four highly homologous and independent epitopes in microtubule-binding domain (MTBD) of diseased tau, inhibits tau-tau interaction, discriminates between healthy and pathologically truncated tau and reduces tau pathology in animal model in vivo. Here, we show that DC8E8 antibody acts via extracellular mechanism and does not influence viability and physiological functions of neurons. Importantly, in vitro functional assays showed that DC8E8 recognises pathogenic tau proteins of different size and origin, and potently blocks their entry into neurons. Next, we examined the mechanisms by which mouse antibody DC8E8 and its humanized version AX004 effectively block the neuronal internalization of extracellular AD tau species. We determined a novel mode of action of a therapeutic candidate antibody, which potently inhibits neuronal internalization of AD tau species by masking of epitopes present in MTBD important for interaction with neuron surface Heparan Sulfate Proteoglycans (HSPGs). We show that interference of tau-heparane sulfate interaction with DC8E8 antibody via steric hindrance represents an efficient and important therapeutic approach halting tau propagation.

Distinct Conformations, Aggregation and Cellular Internalization of Different Tau Strains

The inter-cellular propagation of tau aggregates in several neurodegenerative diseases involves, in part, recurring cycles of extracellular tau uptake, initiation of endogenous tau aggregation, and extracellular release of at least part of this protein complex. However, human brain tau extracts from diverse tauopathies exhibit variant or “strain” specificity in inducing inter-cellular propagation in both cell and animal models. It is unclear if these distinctive properties are affected by disease-specific differences in aggregated tau conformation and structure. We have used a combined structural and cell biological approach to study if two frontotemporal dementia (FTD)-associated pathologic mutations, V337M and N279K, affect the aggregation, conformation and cellular internalization of the tau four-repeat domain (K18) fragment. In both heparin-induced and native-state aggregation experiments, each FTD variant formed soluble and fibrillar aggregates with remarkable morphological and immunological distinctions from the wild type (WT) aggregates. Exogenously applied oligomers of the FTD tau-K18 variants (V337M and N279K) were significantly more efficiently taken up by SH-SY5Y neuroblastoma cells than WT tau-K18, suggesting mutation-induced changes in cellular internalization. However, shared internalization mechanisms were observed: endocytosed oligomers were distributed in the cytoplasm and nucleus of SH-SY5Y cells and the neurites and soma of human induced pluripotent stem cell-derived neurons, where they co-localized with endogenous tau and the nuclear protein nucleolin. Altogether, evidence of conformational and aggregation differences between WT and disease-mutated tau K18 is demonstrated, which may explain their distinct cellular internalization potencies. These findings may account for critical aspects of the molecular pathogenesis of tauopathies involving WT and mutated tau.

Four-repeat tauopathies

Tau is a microtubule-associated protein with versatile functions in the dynamic assembly of the neuronal cytoskeleton. Four-repeat (4R-) tauopathies are a group of neurodegenerative diseases defined by cytoplasmic inclusions predominantly composed of tau protein isoforms with four microtubule-binding domains. Progressive supranuclear palsy, corticobasal degeneration, argyrophilic grain disease or glial globular tauopathy belong to the group of 4R-tauopathies. The present review provides an introduction in the current concept of 4R-tauopathies, including an overview of the neuropathological and clinical spectrum of these diseases. It describes the genetic and environmental etiological factors, as well as the contemporary knowledge about the pathophysiological mechanisms, including post-translational modifications, aggregation and fragmentation of tau, as well as the role of protein degradation mechanisms. Furthermore, current theories about disease propagation are discussed, involving different extracellular tau species and their cellular release and uptake mechanisms. Finally, molecular diagnostic tools for 4R-tauopathies, including tau-PET and fluid biomarkers, and investigational therapeutic strategies are presented. In summary, we report on 4R-tauopathies as overarching disease concept based on a shared pathophysiological concept, and highlight the challenges and opportunities on the way towards a causal therapy.

Anti-prion systems in yeast

Yeast prions have become important models for the study of the basic mechanisms underlying human amyloid diseases. Yeast prions are pathogenic (unlike the [Het-s] prion of Podospora anserina), and most are amyloid-based with the same in-register parallel β-sheet architecture as most of the disease-causing human amyloids studied. Normal yeast cells eliminate the large majority of prion variants arising, and several anti-prion/anti-amyloid systems that eliminate them have been identified. It is likely that mammalian cells also have anti-amyloid systems, which may be useful in the same way humoral, cellular, and innate immune systems are used to treat or prevent bacterial and viral infections.

Elucidating Tau function and dysfunction in the era of cryo-EM

Tau is a microtubule-associated protein involved in the regulation of axonal microtubules in neurons. In pathological conditions, it forms fibrils that are molecular hallmarks of neurological disorders known as tauopathies. In the last 2 years, cryo-EM has given unprecedented high-resolution views of Tau in both physiological and pathological conditions. We review here these new findings and put them into the context of the knowledge about Tau before this structural breakthrough. The first structures of Tau fibrils, a molecular hallmark of Alzheimer's disease (AD), were based on fibrils from the brain of an individual with AD and, along with similar patient-derived structures, have set the gold standard for the field. Cryo-EM structures of Tau fibers in three distinct diseases, AD, Pick's disease, and chronic traumatic encephalopathy, represent the end points of Tau's molecular trajectory. We propose that the recent Tau structures may call for a re-examination of databases that link different Tau variants to various forms of dementia. We also address the question of how this structural information may link Tau's functional and pathological aspects. Because this structural information on Tau was obtained in a very short period, the new structures should be viewed in light of earlier structural observations and past and present functional data to shed additional light on Tau function and dysfunction.

Mechanistic approaches to understand the prion-like propagation of aggregates of the human tau protein

The dynamic nature of the tau protein under physiological conditions is likely to be critical for it to perform its diverse functions inside a cell. Under some conditions, this intrinsically disordered protein assembles into pathogenic aggregates that are self-perpetuating, toxic and infectious in nature. The role of liquid-liquid phase separation in the initiation of the aggregation reaction remains to be delineated. Depending on the nature of the aggregate, its structure, and its localization, neurodegenerative disorders with diverse clinical features are manifested. The prion-like mechanism by which these aggregates propagate and spread across the brain is not well understood. Various factors (PTMs, mutations) have been strongly associated with the pathological aggregates of tau. However, little is known about how these factors modulate the pathological properties linked to aggregation. This review describes the current progress towards understanding the mechanism of propagation of tau aggregates.

The elusive tau molecular structures: can we translate the recent breakthroughs into new targets for intervention?

Insights into tau molecular structures have advanced significantly in recent years. This field has been the subject of recent breakthroughs, including the first cryo-electron microscopy structures of tau filaments from Alzheimer’s and Pick’s disease inclusions, as well as the structure of the repeat regions of tau bound to microtubules. Tau structure covers various species as the tau protein itself takes many forms. We will here address a range of studies that help to define the many facets of tau protein structures and how they translate into pathogenic forms. New results shed light on previous data that need now to be revisited in order to up-date our knowledge of tau molecular structure. Finally, we explore how these data can contribute the important medical aspects of this research - diagnosis and therapeutics.