Tau local structure shields an amyloid-forming motif and controls aggregation propensity

Tau Repeats Disassemble Amyloid-β Fibrils In Vitro by Interacting with KLVFFA and GGVVIA Domains

The interplay between amyloid beta (Aβ) and tau protein is acknowledged as a crucial factor in the progression of Alzheimer's disease (AD), yet the precise molecular mechanisms underlying their interaction remain elusive. In this study, we explore how the key regions within tau, specifically the repeat domains, modulate Aβ aggregation. Through microscale thermophoresis and peptide mapping assays, we identified that tau repeats containing the amyloid motifs VQIINK and VQIVYK directly interact with Aβ(1-42) and Aβ(1-40), targeting the hydrophobic regions of Aβ. Tau repeats were found to inhibit Aβ fibril formation and promote the dissociation of preformed fibrils in vitro. Notably, while disassembling Aβ(1-42) fibrils, tau repeats concurrently stabilized oligomeric forms. These findings provide valuable insights into the complex mechanisms by which tau influences the Aβ pathology, with potential implications for AD progression.

Water-directed pinning is key to tau prion formation

Tau forms fibrillar aggregates that are pathological hallmarks of a family of neurodegenerative diseases known as tauopathies. The synthetic replication of disease-specific fibril structures is a critical gap for developing diagnostic and therapeutic tools. This study debuts a strategy of identifying a critical and minimal folding motif in fibrils characteristic of tauopathies and generating seeding-competent fibrils from the isolated tau peptides. The 19-residue jR2R3 peptide (295 to 313) which spans the R2/R3 splice junction of tau, and includes the P301L mutation, is one such peptide that forms prion-competent fibrils. This tau fragment contains the hydrophobic VQIVYK hexapeptide that is part of the core of all known pathological tau fibril structures and an intramolecular counterstrand that stabilizes the strand-loop-strand (SLS) motif observed in 4R tauopathy fibrils. This study shows that P301L exhibits a duality of effects: it lowers the barrier for the peptide to adopt aggregation-prone conformations and enhances the local structuring of water around the mutation site to facilitate site-directed pinning and dewetting around sites 300-301 to achieve in-register stacking of tau to cross β-sheets. We solved a 3 Å cryo-EM structure of jR2R3-P301L fibrils in which each protofilament layer contains two jR2R3-P301L copies, of which one adopts a SLS fold found in 4R tauopathies and the other wraps around the SLS fold to stabilize it, reminiscent of the three- and fourfold structures observed in 4R tauopathies. These jR2R3-P301L fibrils are competent to template full-length 4R tau in a prion-like manner.

Diverse influences on tau aggregation and implications for disease progression

Tau is an intrinsically disordered protein that accumulates in fibrillar aggregates in neurodegenerative diseases. The misfolding of tau can be understood as an equilibrium between different states and their propensity to form higher-order fibers, which is affected by several factors. First, modulation of the biochemical state of tau due to ionic conditions, post-translational modifications, cofactors, and interacting molecules or assemblies can affect the formation and structure of tau fibrils. Second, cellular processes impact tau aggregation through modulating stability, clearance, disaggregation, and transport. Third, through interactions with glial cells, the neuronal microenvironment can affect intraneuronal conditions with impacts on tau fibrilization and toxicity. Importantly, tau fibrils propagate through the brain via a "prion-like" manner, contributing to disease progression. This review highlights the biochemical and cellular pathways that modulate tau aggregation and discusses implications for pathobiology and tau-directed therapeutic approaches.

Macrocyclic β-arch peptides that mimic the structure and function of disease-associated tau folds

Tauopathies are a class of neurodegenerative disorders that feature tau protein aggregates in the brain. Misfolded tau has the capacity to seed the fibrillization of soluble tau, leading to the prion-like spread of aggregates. Within these filaments, tau protomers always exhibit a cross-β amyloid structure. However, distinct cross-β amyloid folds correlate with specific diseases. An understanding of how these conformations impact seeding activity remains elusive. Identifying the minimal epitopes required for transcellular propagation of tau aggregates represents a key step towards more relevant models of disease progression. Here we implement a diversity-oriented peptide macrocyclization approach towards miniature tau, or 'mini-tau', proteomimetics that can seed the aggregation of tau in engineered cells and primary neurons. Structural elucidation of one such seed-competent macrocycle reveals remarkable conformational congruence with core folds from patient-derived extracts of tau. The ability to impart β-arch form and function through peptide stapling has broad-ranging implications for the minimization and mimicry of pathological tau and other amyloid proteins that drive neurodegeneration.

Application of modular isoxazoline-β2,2-amino acid-based peptidomimetics as chemical model systems for studying the tau misfolding

Tau is a microtubule-associated protein essential for regulating microtubule dynamics and axonal transport in neurons. In tauopathies, the transition of tau from a physiological to a pathological form remains unclear, though the hexapeptides PHF6 and PHF6∗ are key in triggering aggregation. These sequences are shielded by a β-hairpin structure in the native state but expose hydrophobic residues during misfolding, promoting self-assembly. This study employs a non-natural β2-amino acid to induce PHF6 and PHF6∗ into either extended or β-hairpin conformations. The extended form triggers tau aggregation without additives, acting as a seed-competent monomer model system. Conversely, the β-hairpin preserves tau in a soluble monomeric state. Additionally, a β-hairpin mimic inspired by Hsp90 showed potential as a chaperone mimic and inhibitor of tau aggregation, offering insights into corrective folding and aggregation modulation in neuronal environments.

Shapeshifter W-Tau Peptide Inhibits Tau Aggregation and Disintegrates Paired Helical Filaments

Tauopathies comprise a range of neurodegenerative conditions characterized by the aberrant accumulation of tau protein clumps in the brain. These aggregates are formed by different tau splicing isoforms. Here, we analyzed the role of a specific intron-derived peptide called the W-Tau peptide on the polymerization-depolymerization of tau filaments. This peptide originates from a new isoform of the tau protein, named W-Tau, which is formed due to the retention of intron 12. AlphaFold3 (AF3)-based in silico investigations suggested that the W-Tau peptide interacts with tau monomers. Our in vitro experiments confirmed these predictions and showed that the W-Tau peptide inhibited tau aggregation. In addition, the W-Tau peptide disrupted preexisting paired helical filaments (PHFs) isolated from postmortem brain samples of patients with Alzheimer's disease, thereby supporting its potential therapeutic value. The effectiveness of the W-Tau peptide was demonstrated by the decrease in tau aggregation observed after cotransfection of the W-Tau peptide and PHF seeds, as demonstrated by analysis involving a fluorescence resonance energy transfer (FRET) cell biosensor. The W-Tau peptide breaks PHFs by selectively attaching to their ends, causing the structures to unwind and convert into circle-like formations. Considering the potential neuroprotective effects against tauopathies, the W-Tau isoform and its peptide are interesting candidates for future therapeutic interventions.

Tau aggregation induces cell death in iPSC-derived neurons

Abnormal accumulation of tau proteins in the brain is a hallmark of neurodegenerative diseases such as Alzheimer's disease and is closely linked with neuronal cell death. Tau accumulation is a prominent therapeutic target for Alzheimer's disease, since tau accumulation correlates well with the disease progression, and tau-targeting drugs hold potentials to halt the disease progression. Given the differential response of human and mouse neuronal cells, there is a critical need for a human cellular platform to quickly screen for tau-related neurodegenerative disease therapeutics. However, inducing rapid, tau-dependent neuronal cell death in human models remains challenging. In this study, we established a human cellular model capable of inducing tau aggregation-dependent neuronal cell death within two weeks via tau overexpression. Additionally, we demonstrated the neuroprotective efficacy of known tau-targeting compounds within this system. These findings suggest that our cellular model recapitulates the molecular pathogenesis of tau-induced neurodegeneration and could serve as a valuable platform for drug screening in tauopathies.

Inducers and modulators of protein aggregation in Alzheimer's disease - Critical tools for understanding the foundations of aggregate structures

Amyloidogenic protein aggregation is a pathological hallmark of Alzheimer's Disease (AD). As such, this critical feature of the disease has been instrumental in guiding research on the mechanistic basis of disease, diagnostic biomarkers and preventative and therapeutic treatments. Here we review identified molecular triggers and modulators of aggregation for two of the proteins associated with AD: amyloid beta and tau. We aim to provide an overview of how specific molecular factors can impact aggregation kinetics and aggregate structure to promote disease. Looking toward the future, we highlight some research areas of focus that would accelerate efforts to effectively target protein aggregation in AD.

Geometry based prediction of tau protein sites and motifs associated with misfolding and aggregation

Recent studies of tau proteins point to specific sites or motifs along the protein related to its misfolding and aggregation propensity, which is associated with neurodegenerative diseases of structure-dependent pathology. In this manuscript we employ topology and geometry to analyze the local structure of tau proteins obtained from the Protein Data Bank. Our results show that mathematical topology/geometry of cryo-EM structures alone identify the PGGG motifs, and the PHF6(*) motifs as sites of interest and reveal a geometrical hierarchy of the PGGG motifs that differs for 3R+4R and 4R tauopathies. By employing the Local Topological Free Energy (LTE), we find that progressive supranuclear palsy (PSP) and globular glial tauopathy (GGT) have the highest LTE values around residues 302-305, which are inside the jR2R3 peptide and in the vicinity of the 301 site, experimentally associated with aggregation. By extending the LTE definition to estimate a global topological free energy, we find that the jR2R3 peptide of PSP and GGT, has in fact the lowest global topological free energy among other tauopathies. These results point to a possible correlation between the global topological free energy of parts of the protein and the LTE of specific sites.

Effect of PHF-1 hyperphosphorylation on the seeding activity of C-terminal Tau fragments

Tau proteins as neurofibrillary tangles are one of the molecular hallmarks of Alzheimer's disease (AD) and play a central role in tauopathies, a group of age-related neurodegenerative disorders. The filament cores from diverse tauopathies share a common region of tau consisting of the R3-R4 microtubule-binding repeats and part of the C-terminal domain, but present a structural polymorphism. Unlike the fibril structure, the PTM signature of tau found in neuronal inclusions, more particularly hyperphosphorylation, is variable between individuals with the same tauopathy, giving rise to diverse strains with different seeding properties that could modulate the aggressiveness of tau pathology. Here, we investigate the conformation, function and seeding activity of two tau fragments and their GSK3β-phosphorylated variants. The R2Ct and R3Ct fragments encompass the aggregation-prone region of tau starting at the R2 and R3 repeats, respectively, and the full C-terminal domain including the PHF-1 epitope (S396, S400, S404), which undergoes a triple phosphorylation upon GSK3β activity. We found that the R3Ct fragment shows both a greater loss of function and pathological activity in seeding of aggregation than the R2Ct fragment which imposes a cross-seeding barrier. PHF-1 hyperphosphorylation induces a local conformational change with a propensity to adopt a β-sheet conformation in the region spanning residues 392-402, and exacerbates the seeding ability of fragments to induce aggregation by overcoming a cross-seeding barrier between tau variants.

Amyloid-motif-dependent tau self-assembly is modulated by isoform sequence context

The microtubule-associated protein tau is implicated in neurodegenerative diseases characterized by amyloid formation. Mutations associated with frontotemporal dementia increase tau aggregation propensity and disrupt its endogenous microtubule-binding activity. However, the structural relationship between aggregation propensity and biological activity remains unclear. We employed a multi-disciplinary approach, including computational modeling, NMR, cross-linking mass spectrometry, and cell models to engineer tau sequences that modulate its structural ensemble. Our findings show that substitutions near the conserved 'PGGG' β-turn motif informed by tau isoform context reduce tau aggregation in vitro and cells and can even counteract aggregation induced by turn destabilizing disease-associated proline-to-serine mutations. Engineered tau sequences maintain microtubule binding and explain why 3R isoforms exhibit reduced pathogenesis compared to 4R. We propose a simple mechanism to reduce the formation of pathogenic species while preserving biological function, thus offering insights for therapeutic strategies aimed at reducing tau protein misfolding in neurodegenerative diseases.

Abstract Figure

Pathogenic Mutation ΔK280 Promotes Hydrophobic Interactions Involving Microtubule-Binding Domain and Enhances Liquid-Liquid Phase Separation of Tau

Liquid-liquid phase separation (LLPS) of tau protein can initiate its aggregation which is associated with Alzheimer's disease. The pathogenic mutation ΔK280 can enhance the aggregation of K18, a truncated tau variant comprising the microtubule-binding domain. However, the impact of ΔK280 on K18 LLPS and underlying mechanisms are largely unexplored. Herein, the conformational ensemble and LLPS of ΔK280 K18 through multiscale molecular simulations and microscopy experiments are investigated. All-atom molecular dynamic simulations reveal that ΔK280 significantly enhances the collapse degree and β-sheet content of the K18 monomer, indicating that ΔK280 mutation may promote K18 LLPS, validated by coarse-grained phase-coexistence simulations and microscopy experiments. Importantly, ΔK280 mutation promotes β-sheet formation of six motifs (especially PHF6), increases the hydrophobic solvent exposure of PHF6* and PHF6, and enhances hydrophobic, hydrogen bonding, and cation-π interactions involving most of the motifs, thus facilitating the phase separation of K18. Notably, ΔK280 alters the interaction network among the six motifs, inducing the formation of K18 conformations with high β-sheet contents and collapse degree. Coarse-grained simulations on full-length tau reveal that ΔK280 promotes tau LLPS by enhancing the hydrophobic interactions involving the microtubule-binding domain. These findings offer detailed mechanistic insights into ΔK280-induced tau pathogenesis, providing potential targets for therapeutic intervention.

Site-Specific Incorporation of Fluorinated Prolines into Proteins and Their Impact on Neighbouring Residues

The incorporation of fluorinated amino acids into proteins provides new opportunities to study biomolecular structure-function relationships in an elegant manner. The available strategies to incorporate the majority of fluorinated amino acids are not site-specific or imply important structural modifications. Here, we present a chemical biology approach for the site-specific incorporation of three commercially available Cγ-modified fluoroprolines that has been validated using a non-pathogenic version of huntingtin exon-1 (HttExon-1). 19F, 1H and 15N NMR chemical shifts measured for multiple variants of HttExon-1 indicated that the trans/cis ratio was strongly dependent on the fluoroproline variant and the sequence context. By isotopically labelling the rest of the protein, we have shown that the extent of spectroscopic perturbations to the neighbouring residues depends on the number of fluorine atoms and the stereochemistry at Cγ, as well as the isomeric form of the fluoroproline. We have rationalized these observations by means of extensive molecular dynamics simulations, indicating that the observed atomic chemical shift perturbations correlate with the distance to fluorine atoms and that the effect remains very local. These results validate the site-specific incorporation of fluoroprolines as an excellent strategy to monitor intra- and intermolecular interactions in disordered proline-rich proteins.

Drug Discovery and Screening Tool Development for Tauopathies by Focusing on Pathogenic Tau Repeat 3 Oligomers

Comprehending early amyloidogenesis is essential for the development of effective therapeutic strategies. In tauopathies like Alzheimer's disease (AD), the abnormal accumulation of tau protein is initiated by pathological tau seeds. Mounting evidence implies that the microtubule binding domain, consisting of three to four repeats, plays a pivotal role in this process, yet the exact region driving the formation of pathogenic species needs to be further scrutinized. Here, we chemically synthesized individual tau repeats to identify those exhibiting pathogenic prion-like characteristics. Notably, repeat 3 (R3) displayed a remarkable propensity to polymerize, form toxic filaments, and induce cognitive impairment, even in the absence of an aggregation-promoting inducer, highlighting its physiological relevance. Additionally, oligomeric R3 was identified as a particularly pathological form, prompting the establishment of a screening platform. Through screening, tolcapone was found to possess therapeutic efficacy against pathological tau aggregates in PS19 transgenic mice. This screening platform provides a valuable avenue for identifying compounds that selectively interact with peptides implicated in the progression of tauopathies.

Effects of All-Atom and Coarse-Grained Molecular Mechanics Force Fields on Amyloid Peptide Assembly: The Case of a Tau K18 Monomer

To propose new mechanism-based therapeutics for Alzheimer's disease (AD), it is crucial to study the kinetics and oligomerization/aggregation mechanisms of the hallmark tau proteins, which have various isoforms and are intrinsically disordered. In this study, multiple all-atom (AA) and coarse-grained (CG) force fields (FFs) have been benchmarked on molecular dynamics (MD) simulations of K18 tau (M243-E372), which is a truncated form (130 residues) of full-length tau (441 residues). FF19SB is first excluded because the dynamics are too slow, and the conformations are too stable. All other benchmarked AAFFs (Charmm36m, FF14SB, Gromos54A7, and OPLS-AA) and CGFFs (Martini3 and Sirah2.0) exhibit a trend of shrinking K18 tau into compact structures with the radius of gyration (ROG) around 2.0 nm, which is much smaller than the experimental value of 3.8 nm, within 200 ns of AA-MD or 2000 ns of CG-MD. Gromos54A7, OPLS-AA, and Martini3 shrink much faster than the other FFs. To perform meaningful postanalysis of various properties, we propose a strategy of selecting snapshots with 2.5 < ROG < 4.5 nm, instead of using all sampled snapshots. The calculated chemical shifts of all C, CA, and CB atoms have very good and close root-mean-square error (RMSE) values, while Charmm36m and Sirah2.0 exhibit better chemical shifts of N than other FFs. Comparing the calculated distributions of the distance between the CA atoms of CYS291 and CYS322 with the results of the FRET experiment demonstrates that Charmm36m is a perfect match with the experiment while other FFs exhibit limitations. In summary, Charmm36m is recommended as the best AAFF, and Sirah2.0 is recommended as an excellent CGFF for simulating tau K18.

Intracellular tau fragment droplets serve as seeds for tau fibrils

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

Interaction of full-length Tau with negatively charged lipid membranes leads to polymorphic aggregates

The Tau protein is implicated in various diseases collectively known as tauopathies, including Alzheimer's disease and frontotemporal dementia. The precise mechanism underlying Tau pathogenicity remains elusive. Recently, the role of lipids has garnered interest due to their implications in Tau aggregation, secretion, uptake, and pathogenic dysregulation. Previous investigations have highlighted critical aspects: (i) Tau's tendency to aggregate into fibers when interacting with negatively charged lipids, (ii) its ability to form structured species upon contact with anionic membranes, and (iii) the potential disruption of the membrane upon Tau binding. In this study, we examine the disease-associated P301L mutation of the 2N4R isoform of Tau and its effects on membranes composed on phosphatidylserine (PS) lipids. Aggregation studies and liposome leakage assays demonstrate Tau's ability to bind to anionic lipid vesicles, leading to membrane disruption. Attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) reveals the accumulation of Tau on the membrane surface without protein insertion, structuration, or lipid removal. Plasmon waveguide resonance (PWR) demonstrates a strong binding of Tau on PS bilayers with an apparent Kd in the micromolar range, indicating the deposition of a thick protein layer. Atomic force microscopy (AFM) real-time imaging allows the observation of partial lipid solubilization and the deposition of polymorphic aggregates in the form of thick patches and fibrillary structures resembling amyloid fibers, which could grow from a combination of extracted anionic phospholipids from the membrane and Tau protein. This study deepens our understanding of full-length Tau's multifaceted interactions with lipids, shedding light on potential mechanisms leading to the formation of pathogenic Tau assemblies.

Unraveling the Structure and Dynamics of Ac-PHF6-NH2 Tau Segment Oligomers

The aggregation of the proteins tau and amyloid-β is a salient feature of Alzheimer's disease, the most common form of neurodegenerative disorders. Upon aggregation, proteins transition from their soluble, monomeric, and functional state into insoluble, fibrillar deposits through a complex process involving a variety of intermediate species of different morphologies, including monomers, toxic oligomers, and insoluble fibrils. To control and direct peptide aggregation, a complete characterization of all species present and an understanding of the molecular processes along the aggregation pathway are essential. However, this is extremely challenging due to the transient nature of oligomers and the complexity of the reaction networks. Therefore, we have employed a combined approach that allows us to probe the structure and kinetics of oligomeric species, following them over time as they form fibrillar structures. Targeting the tau protein peptide segment Ac-PHF6-NH2, which is crucial for the aggregation of the full protein, soft nano-electrospray ionization combined with ion mobility mass spectrometry has been employed to study the kinetics of heparin-induced intact oligomer formation. The oligomers are identified and characterized using high-resolution ion mobility mass spectrometry, demonstrating that the addition of heparin does not alter the structure of the oligomeric species. The kinetics of fibril formation is monitored through a Thioflavin T fluorescence assay. Global fitting of the kinetic data indicates that secondary nucleation plays a key role in the aggregation of the Ac-PHF6-NH2 tau segment, while the primary nucleation rate is greatly accelerated by heparin.

Novel tau filament folds in individuals with MAPT mutations P301L and P301T

Mutations in MAPT, the microtubule-associated protein tau gene, give rise to cases of frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) with abundant filamentous tau inclusions in brain cells. Individuals with pathological MAPT variants exhibit behavioural changes, cognitive impairment and signs of parkinsonism. Missense mutations of residue P301, which are the most common MAPT mutations associated with FTDP-17, give rise to the assembly of mutant four-repeat tau into filamentous inclusions, in the absence of extracellular deposits. Here we report the cryo-EM structures of tau filaments from five individuals belonging to three unrelated families with mutation P301L and from one individual belonging to a family with mutation P301T. A novel three-lobed tau fold resembling the two-layered tau fold of Pick's disease was present in all cases with the P301L tau mutation. Two different tau folds were found in the case with mutation P301T, the less abundant of which was a variant of the three-lobed fold. The major P301T tau fold was V-shaped, with partial similarity to the four-layered tau folds of corticobasal degeneration and argyrophilic grain disease. These findings suggest that FTDP-17 with mutations in P301 should be considered distinct inherited tauopathies and that model systems with these mutations should be used with caution in the study of sporadic tauopathies.

Tau in neurodegenerative diseases: molecular mechanisms, biomarkers, and therapeutic strategies

The deposition of abnormal tau protein is characteristic of Alzheimer's disease (AD) and a class of neurodegenerative diseases called tauopathies. Physiologically, tau maintains an intrinsically disordered structure and plays diverse roles in neurons. Pathologically, tau undergoes abnormal post-translational modifications and forms oligomers or fibrous aggregates in tauopathies. In this review, we briefly introduce several tauopathies and discuss the mechanisms mediating tau aggregation and propagation. We also describe the toxicity of tau pathology. Finally, we explore the early diagnostic biomarkers and treatments targeting tau. Although some encouraging results have been achieved in animal experiments and preclinical studies, there is still no cure for tauopathies. More in-depth basic and clinical research on the pathogenesis of tauopathies is necessary.

moPPIt: De Novo Generation of Motif-Specific Binders with Protein Language Models

The ability to precisely target specific motifs on disease-related proteins, whether conserved epitopes on viral proteins, intrinsically disordered regions within transcription factors, or breakpoint junctions in fusion oncoproteins, is essential for modulating their function while minimizing off-target effects. Current methods struggle to achieve this specificity without reliable structural information. In this work, we introduce a motif-specific PPI targeting algorithm, moPPIt, for de novo generation of motif-specific peptide binders from the target protein sequence alone. At the core of moPPIt is BindEvaluator, a transformer-based model that interpolates protein language model embeddings of two proteins via a series of multi-headed self-attention blocks, with a key focus on local motif features. Trained on over 510,000 annotated PPIs, BindEvaluator accurately predicts target binding sites given protein-protein sequence pairs with a test AUC > 0.94, improving to AUC > 0.96 when fine-tuned on peptide-protein pairs. By combining BindEvaluator with our PepMLM peptide generator and genetic algorithm-based optimization, moPPIt generates peptides that bind specifically to user-defined residues on target proteins. We demonstrate moPPIt's efficacy in computationally designing binders to specific motifs, first on targets with known binding peptides and then extending to structured and disordered targets with no known binders. In total, moPPIt serves as a powerful tool for developing highly specific peptide therapeutics without relying on target structure or structure-dependent latent spaces.

DNAJB8 oligomerization is mediated by an aromatic-rich motif that is dispensable for substrate activity

J-domain protein (JDP) molecular chaperones have emerged as central players that maintain a healthy proteome. The diverse members of the JDP family function as monomers/dimers and a small subset assemble into micron-sized oligomers. The oligomeric JDP members have eluded structural characterization due to their low-complexity, intrinsically disordered middle domains. This in turn, obscures the biological significance of these larger oligomers in protein folding processes. Here, we identified a short, aromatic motif within DNAJB8 that drives self-assembly through π-π stacking and determined its X-ray structure. We show that mutations in the motif disrupt DNAJB8 oligomerization in vitro and in cells. DNAJB8 variants that are unable to assemble bind to misfolded tau seeds more specifically and retain capacity to reduce protein aggregation in vitro and in cells. We propose a new model for DNAJB8 function in which the sequences in the low-complexity domains play distinct roles in assembly and substrate activity.

Role of G326 in Determining the Aggregation Propensity of R3 Tau Repeat: Insights from Studies on R1R3 Tau Construct

The Microtubule-binding repeat region (MTBR) of Tau has been studied extensively due to its pathological implications in neurodegenerative diseases like Alzheimer's disease. The pathological property of MTBR is mainly due to the R3 repeat's high propensity for self-aggregation, highlighting the critical molecular grammar of the repeat. Utilizing the R1R3 construct (WT) and its G326E mutant (EE), we determine the distinct characteristics of various peptide segments that modulate the aggregation propensity of the R3 repeat using NMR spectroscopy. Through time-dependent experiments, we have identified 317KVTSKCGS324 in R3 repeat as the aggregation initiating motif (AIM) due to its role at the initial stages of aggregation. The G326E mutation induces changes in conformation and dynamics at the AIM, thereby effectively abrogating the aggregation propensity of the R1R3 construct. We further corroborate our findings through MD simulations and propose that AIM is a robust site of interest for tauopathy drug design.

Precision proteoform design for 4R tau isoform selective templated aggregation

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

Multivalent coiled-coil interactions enable full-scale centrosome assembly and strength

The outermost layer of centrosomes, called pericentriolar material (PCM), organizes microtubules for mitotic spindle assembly. The molecular interactions that enable PCM to assemble and resist external forces are poorly understood. Here, we use crosslinking mass spectrometry (XL-MS) to analyze PLK-1-potentiated multimerization of SPD-5, the main PCM scaffold protein in C. elegans. In the unassembled state, SPD-5 exhibits numerous intramolecular crosslinks that are eliminated after phosphorylation by PLK-1. Thus, phosphorylation induces a structural opening of SPD-5 that primes it for assembly. Multimerization of SPD-5 is driven by interactions between multiple dispersed coiled-coil domains. Structural analyses of a phosphorylated region (PReM) in SPD-5 revealed a helical hairpin that dimerizes to form a tetrameric coiled-coil. Mutations within this structure and other interacting regions cause PCM assembly defects that are partly rescued by eliminating microtubule-mediated forces, revealing that PCM assembly and strength are interdependent. We propose that PCM size and strength emerge from specific, multivalent coiled-coil interactions between SPD-5 proteins.

Hairpin trimer transition state of amyloid fibril

Protein fibril self-assembly is a universal transition implicated in neurodegenerative diseases. Although fibril structure/growth are well characterized, fibril nucleation is poorly understood. Here, we use a computational-experimental approach to resolve fibril nucleation. We show that monomer hairpin content quantified from molecular dynamics simulations is predictive of experimental fibril formation kinetics across a tau motif mutant library. Hairpin trimers are predicted to be fibril transition states; one hairpin spontaneously converts into the cross-beta conformation, templating subsequent fibril growth. We designed a disulfide-linked dimer mimicking the transition state that catalyzes fibril formation, measured by ThT fluorescence and TEM, of wild-type motif - which does not normally fibrillize. A dimer compatible with extended conformations but not the transition-state fails to nucleate fibril at any concentration. Tau repeat domain simulations show how long-range interactions sequester this motif in a mutation-dependent manner. This work implies that different fibril morphologies could arise from disease-dependent hairpin seeding from different loci.

Disentangling tau: One protein, many therapeutic approaches

The tauopathies encompass over 20 adult neurodegenerative diseases and are characterized by the dysfunction and accumulation of insoluble tau protein. Among them, Alzheimer's disease, frontotemporal dementia, and progressive supranuclear palsy collectively impact millions of patients and their families worldwide. Despite years of drug development using a variety of mechanisms of action, no therapeutic directed against tau has been approved for clinical use. This raises important questions about our current model of tau pathology and invites thoughtful consideration of our approach to nonclinical models and clinical trial design. In this article, we review what is known about the biology and genetics of tau, placing it in the context of current and failed clinical trials. We highlight potential reasons for the lack of success to date and offer suggestions for new pathways in therapeutic development. Overall, our viewpoint to the future is optimistic for this important group of neurodegenerative diseases.

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.

Local structural preferences in shaping tau amyloid polymorphism

Tauopathies encompass a group of neurodegenerative disorders characterised by diverse tau amyloid fibril structures. The persistence of polymorphism across tauopathies suggests that distinct pathological conditions dictate the adopted polymorph for each disease. However, the extent to which intrinsic structural tendencies of tau amyloid cores contribute to fibril polymorphism remains uncertain. Using a combination of experimental approaches, we here identify a new amyloidogenic motif, PAM4 (Polymorphic Amyloid Motif of Repeat 4), as a significant contributor to tau polymorphism. Calculation of per-residue contributions to the stability of the fibril cores of different pathologic tau structures suggests that PAM4 plays a central role in preserving structural integrity across amyloid polymorphs. Consistent with this, cryo-EM structural analysis of fibrils formed from a synthetic PAM4 peptide shows that the sequence adopts alternative structures that closely correspond to distinct disease-associated tau strains. Furthermore, in-cell experiments revealed that PAM4 deletion hampers the cellular seeding efficiency of tau aggregates extracted from Alzheimer's disease, corticobasal degeneration, and progressive supranuclear palsy patients, underscoring PAM4's pivotal role in these tauopathies. Together, our results highlight the importance of the intrinsic structural propensity of amyloid core segments to determine the structure of tau in cells, and in propagating amyloid structures in disease.

Magnetic resonance investigation of conformational responses of tau protein to specific phosphorylation

Intrinsically disordered proteins (IDPs) are known to adopt many rapidly interconverting structures, making it difficult to pinpoint the specific conformational states that are relevant for their function. Tau is an important IDP, and its conformation is known to be affected by post-translational modifications (PTMs), such as phosphorylation. To investigate the effect of specific phosphorylation on full-length Tau's dynamic global conformation, we employed a combination of nuclear magnetic resonance-based paramagnetic relaxation interference methods and electron paramagnetic resonance spectroscopy. By reproducing the AT8 epitope, comprising exclusive phosphorylation at residues S202 and T205, we were able to identify conformations specific to phosphorylated Tau, which exhibited a tendency towards less compact states. These mechanistic details are of significance to understand the path leading from soluble Tau to the ordered structure of Tau fibers. This approach proved to be successful for studying the conformational changes of (phosphorylated) full-length Tau and can potentially be extended to the study of other IDPs that undergo various PTMs.

Martini on the Rocks: Can a Coarse-Grained Force Field Model Crystals?

Computational chemistry is an important tool in numerous scientific disciplines, including drug discovery and structural biology. Coarse-grained models offer simple representations of molecular systems that enable simulations of large-scale systems. Because there has been an increase in the adoption of such models for simulations of biomolecular systems, critical evaluation is warranted. Here, the stability of the amyloid peptide and organic crystals is evaluated using the Martini 3 coarse-grained force field. The crystals change shape drastically during the simulations. Radial distribution functions show that the distance between backbone beads in β-sheets increases by ∼1 Å, breaking the crystals. The melting points of organic compounds are much too low in the Martini force field. This suggests that Martini 3 lacks the specific interactions needed to accurately simulate peptides or organic crystals without imposing artificial restraints. The problems may be exacerbated by the use of the 12-6 potential, suggesting that a softer potential could improve this model for crystal simulations.

DNAJB8 oligomerization is mediated by an aromatic-rich motif that is dispensable for substrate activity

J-domain protein (JDP) molecular chaperones have emerged as central players that maintain a healthy proteome. The diverse members of the JDP family function as monomers/dimers and a small subset assemble into micron-sized oligomers. The oligomeric JDP members have eluded structural characterization due to their low-complexity, intrinsically disordered middle domains. This in turn, obscures the biological significance of these larger oligomers in protein folding processes. Here, we identified a short, aromatic motif within DNAJB8, that drives self-assembly through pi-pi stacking and determined its X-ray structure. We show that mutations in the motif disrupt DNAJB8 oligomerization in vitro and in cells. DNAJB8 variants that are unable to assemble bind to misfolded tau seeds more specifically and retain capacity to reduce protein aggregation in vitro and in cells. We propose a new model for DNAJB8 function in which the sequences in the low-complexity domains play distinct roles in assembly and substrate activity.

Mutations in Tau Protein Promote Aggregation by Favoring Extended Conformations

Amyloid aggregation of the intrinsically disordered protein (IDP) tau is involved in several diseases, called tauopathies. Some tauopathies can be inherited due to mutations in the gene encoding tau, which might favor the formation of tau amyloid fibrils. This work aims at deciphering the mechanisms through which the disease-associated single-point mutations promote amyloid formation. We combined biochemical and biophysical characterization, notably, small-angle X-ray scattering (SAXS), to study six different FTDP-17 derived mutations. We found that the mutations promote aggregation to different degrees and can modulate tau conformational ensembles, intermolecular interactions, and liquid-liquid phase separation propensity. In particular, we found a good correlation between the aggregation lag time of the mutants and their radii of gyration. We show that mutations disfavor intramolecular protein interactions, which in turn favor extended conformations and promote amyloid aggregation. This work proposes a new connection between the structural features of tau monomers and their propensity to aggregate, providing a novel assay to evaluate the aggregation propensity of IDPs.

Ensemble-based design of tau to inhibit aggregation while preserving biological activity

The microtubule-associated protein tau is implicated in neurodegenerative diseases characterized by amyloid formation. Mutations associated with frontotemporal dementia increase tau aggregation propensity and disrupt its endogenous microtubule-binding activity. The structural relationship between aggregation propensity and biological activity remains unclear. We employed a multi-disciplinary approach, including computational modeling, NMR, cross-linking mass spectrometry, and cell models to design tau sequences that stabilize its structural ensemble. Our findings reveal that substitutions near the conserved 'PGGG' beta-turn motif can modulate local conformation, more stably engaging in interactions with the 306VQIVYK311 amyloid motif to decrease aggregation in vitro and in cells. Designed tau sequences maintain microtubule binding and explain why 3R isoforms of tau exhibit reduced pathogenesis over 4R isoforms. We propose a simple mechanism to reduce the formation of pathogenic species while preserving biological function, offering insights for therapeutic strategies aimed at reducing protein misfolding in neurodegenerative diseases.

Travels with tau prions

Tau was originally identified as a microtubule associated protein, and subsequently recognized to constitute the fibrillar assemblies found in Alzheimer disease and related neurodegenerative tauopathies. Point mutations in the microtubule associated protein tau (MAPT) gene cause dominantly inherited tauopathies, and most predispose it to aggregate. This indicates tau aggregation underlies pathogenesis of tauopathies. Our work has suggested that tau functions as a prion, forming unique intracellular pathological assemblies that subsequently move to other cells, inducing further aggregation that underlies disease progression. Remarkably, in simple cells tau forms stably propagating aggregates of distinct conformation, termed strains. Each strain induces a unique and, in some cases, transmissible, neuropathological phenotype upon inoculation into a mouse model. After binding heparan sulfate proteoglycans on the plasma membrane, tau assemblies enter cells via macropinocytosis. From within a vesicle, if not trafficked to the endolysosomal system, tau subsequently enters the cytoplasm, where it becomes a template for its own replication, apparently after processing by valosin containing protein. The smallest seed unit is a stable monomer, which suggests that initial folding events in tau presage subsequent pathological aggregation. The study of tau prions has raised important questions about basic cell biological processes that underlie their replication and propagation, with implications for therapy of tauopathies.

Molecular Mechanism of Tau Misfolding and Aggregation: Insights from Molecular Dynamics Simulation

Tau dysfunction has a close association with many neurodegenerative diseases, which are collectively referred to as tauopathies. Neurofibrillary tangles (NFTs) formed by misfolding and aggregation of tau are the main pathological process of tauopathy. Therefore, uncovering the misfolding and aggregation mechanism of tau protein will help to reveal the pathogenic mechanism of tauopathies. Molecular dynamics (MD) simulation is well suited for studying the dynamic process of protein structure changes. It provides detailed information on protein structure changes over time at the atomic resolution. At the same time, MD simulation can also simulate various conditions conveniently. Based on these advantages, MD simulations are widely used to study conformational transition problems such as protein misfolding and aggregation. Here, we summarized the structural features of tau, the factors affecting its misfolding and aggregation, and the applications of MD simulations in the study of tau misfolding and aggregation.

DnaJs are enriched in tau regulators

The aberrant accumulation of tau protein is implicated as a pathogenic factor in many neurodegenerative diseases. Tau seeding may underlie its predictable spread in these diseases. Molecular chaperones can modulate tau pathology, but their effects have mainly been studied in isolation. This study employed a semi-high throughput assay to identify molecular chaperones influencing tau seeding using Tau RD P301S FRET Biosensor cells, which express a portion of tau containing the frontotemporal dementia-related P301S tau mutation fused to a FRET biosensor. Approximately fifty chaperones from five major families were screened using live cell imaging to monitor FRET-positive tau seeding. Among the tested chaperones, five exhibited significant effects on tau in the primary screen. Notably, three of these were from the DnaJ family. In subsequent studies, overexpression of DnaJA2, DnaJB1, and DnaJB6b resulted in significant reductions in tau levels. Knockdown experiments by shRNA revealed an inverse correlation between DnaJB1 and DnaJB6b with tau levels. DnaJB6b overexpression, specifically, reduced total tau levels in a cellular model with a pre-existing pool of tau, partially through enhanced proteasomal degradation. Further, DnaJB6b interacted with tau complexes. These findings highlight the potent chaperone activity within the DnaJ family, particularly DnaJB6b, towards tau.

Disease-Associated Mutations in Tau Encode for Changes in Aggregate Structure Conformation

The accumulation of tau fibrils is associated with neurodegenerative diseases, which are collectively termed tauopathies. Cryo-EM studies have shown that the packed fibril core of tau adopts distinct structures in different tauopathies, such as Alzheimer's disease, corticobasal degeneration, and progressive supranuclear palsy. A subset of tauopathies are linked to missense mutations in the tau protein, but it is not clear whether these mutations impact the structure of tau fibrils. To answer this question, we developed a high-throughput protein purification platform and purified a panel of 37 tau variants using the full-length 0N4R splice isoform. Each of these variants was used to create fibrils in vitro, and their relative structures were studied using a high-throughput protease sensitivity platform. We find that a subset of the disease-associated mutations form fibrils that resemble wild-type tau, while others are strikingly different. The impact of mutations on tau structure was not clearly associated with either the location of the mutation or the relative kinetics of fibril assembly, suggesting that tau mutations alter the packed core structures through a complex molecular mechanism. Together, these studies show that single-point mutations can impact the assembly of tau into fibrils, providing insight into its association with pathology and disease.

Mechanisms and pathology of protein misfolding and aggregation

Despite advances in machine learning-based protein structure prediction, we are still far from fully understanding how proteins fold into their native conformation. The conventional notion that polypeptides fold spontaneously to their biologically active states has gradually been replaced by our understanding that cellular protein folding often requires context-dependent guidance from molecular chaperones in order to avoid misfolding. Misfolded proteins can aggregate into larger structures, such as amyloid fibrils, which perpetuate the misfolding process, creating a self-reinforcing cascade. A surge in amyloid fibril structures has deepened our comprehension of how a single polypeptide sequence can exhibit multiple amyloid conformations, known as polymorphism. The assembly of these polymorphs is not a random process but is influenced by the specific conditions and tissues in which they originate. This observation suggests that, similar to the folding of native proteins, the kinetics of pathological amyloid assembly are modulated by interactions specific to cells and tissues. Here, we review the current understanding of how intrinsic protein conformational propensities are modulated by physiological and pathological interactions in the cell to shape protein misfolding and aggregation pathology.

Tau P301L mutation promotes core 4R tauopathy fibril fold through near-surface water structuring and conformational rearrangement

Tau forms toxic fibrillar aggregates in a family of neurodegenerative diseases known as tauopathies. The faithful replication of tauopathy-specific fibril structures is a critical gap for developing diagnostic and therapeutic tools. This study debuts a strategy of identifying a critical segment of tau that forms a folding motif that is characteristic of a family of tauopathies and isolating it as a standalone peptide that form seeding-competent fibrils. The 19-residue jR2R3 peptide (295-313) spanning the R2/R3 splice junction of tau, in the presence of P301L, forms seeding-competent amyloid fibrils. This tau fragment contains the hydrophobic VQIVYK hexapeptide that is part of the core of every pathological tau fibril structure solved to-date and an intramolecular counter-strand that stabilizes the strand-loop-strand (SLS) motif observed in 4R tauopathy fibrils. This study shows that P301L exhibits a duality of effects: it lowers the barrier for the peptide to adopt aggregation-prone conformations and enhances the local structuring of water around the mutation site that facilitates site-specific dewetting and in-register stacking of tau to form cross β-sheets. We solve a 3 Å cryo-EM structure of jR2R3-P301L fibrils with a pseudo 2 1 screw symmetry in which each half of the fibril's cross-section contains two jR2R3-P301L peptides. One chain adopts a SLS fold found in 4R tauopathies that is stabilized by a second chain wrapping around the SLS fold, reminiscent of the 3-fold and 4-fold structures observed in 4R tauopathies. These jR2R3-P301L fibrils are able to template full length tau in a prion-like fashion.

SIGNIFICANCE STATEMENT

This study presents a first step towards designing a tauopathy specific aggregation pathway by engineering a minimal tau prion building block, jR2R3, that can template and propagate distinct disease folds. We present the discovery that P301L-among the widest used mutations in cell and animal models of Alzheimer's Disease-destabilizes an aggregation-prohibiting internal hairpin and enhances the local surface water structure that serves as an entropic hotspot to exert a hyper-localized effect in jR2R3. Our study suggests that P301L may be a more suitable mutation to include in modeling 4R tauopathies than for modelling Alzheimer's Disease, and that mutations are powerful tools for the purpose of designing of tau prion models as therapeutic tools.

Precision Proteoform Design for 4R Tau Isoform Selective Templated Aggregation

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

Advanced Techniques for Detecting Protein Misfolding and Aggregation in Cellular Environments

Protein misfolding and aggregation, a key contributor to the progression of numerous neurodegenerative diseases, results in functional deficiencies and the creation of harmful intermediates. Detailed visualization of this misfolding process is of paramount importance for improving our understanding of disease mechanisms and for the development of potential therapeutic strategies. While in vitro studies using purified proteins have been instrumental in delivering significant insights into protein misfolding, the behavior of these proteins in the complex milieu of living cells often diverges significantly from such simplified environments. Biomedical imaging performed in cell provides cellular-level information with high physiological and pathological relevance, often surpassing the depth of information attainable through in vitro methods. This review highlights a variety of methodologies used to scrutinize protein misfolding within biological systems. This includes optical-based methods, strategies leaning on mass spectrometry, in-cell nuclear magnetic resonance, and cryo-electron microscopy. Recent advancements in these techniques have notably deepened our understanding of protein misfolding processes and the features of the resulting misfolded species within living cells. The progression in these fields promises to catalyze further breakthroughs in our comprehension of neurodegenerative disease mechanisms and potential therapeutic interventions.

Untangling Tau: Molecular Insights into Neuroinflammation, Pathophysiology, and Emerging Immunotherapies

Neuroinflammation, a core pathological feature observed in several neurodegenerative diseases, including Alzheimer's disease (AD), is rapidly gaining attention as a target in understanding the molecular underpinnings of these disorders. Glial cells, endothelial cells, peripheral immune cells, and astrocytes produce a variety of pro-inflammatory mediators that exacerbate the disease progression. Additionally, microglial cells play a complex role in AD, facilitating the clearance of pathological amyloid-beta peptide (Aβ) plaques and aggregates of the tau protein. Tau proteins, traditionally associated with microtubule stabilization, have come under intense scrutiny for their perturbed roles in neurodegenerative conditions. In this narrative review, we focus on recent advances from molecular insights that have revealed aberrant tau post-translational modifications, such as phosphorylation and acetylation, serving as pathological hallmarks. These modifications also trigger the activation of CNS-resident immune cells, such as microglia and astrocytes substantially contributing to neuroinflammation. This intricate relationship between tau pathologies and neuroinflammation fosters a cascading impact on neural pathophysiology. Furthermore, understanding the molecular mechanisms underpinning tau's influence on neuroinflammation presents a frontier for the development of innovative immunotherapies. Neurodegenerative diseases have been relatively intractable to conventional pharmacology using small molecules. We further comprehensively document the many alternative approaches using immunotherapy targeting tau pathological epitopes and structures with a wide array of antibodies. Clinical trials are discussed using these therapeutic approaches, which have both promising and disappointing outcomes. Future directions for tau immunotherapies may include combining treatments with Aβ immunotherapy, which may result in more significant clinical outcomes for neurodegenerative diseases.

Anti-tau antibodies targeting a conformation-dependent epitope selectively bind seeds

Neurodegenerative tauopathies are caused by the transition of tau protein from a monomer to a toxic aggregate. They include Alzheimer disease (AD), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), and Pick disease (PiD). We have previously proposed that tau monomer exists in two conformational ensembles: an inert form (Mi), which does not self-assemble, and seed-competent form (Ms), which self-assembles and templates ordered assembly growth. We proposed that cis/trans isomerization of tau at P301, the site of dominant disease-associated S/L missense mutations, might underlie the transition of wild-type tau to a seed-competent state. Consequently, we created monoclonal antibodies using non-natural antigens consisting of fluorinated proline (P∗) at the analogous P270 in repeat 1 (R1), biased toward the trans-configuration at either the R1/R2 (TENLKHQP∗GGGKVQIINKK) or the R1/R3 (TENLKHQP∗GGGKVQIVYK) interfaces. Two antibodies, MD2.2 and MD3.1, efficiently immunoprecipitated soluble seeds from AD and PSP but not CBD or PiD brain samples. The antibodies efficiently stained brain samples of AD, PSP, and PiD, but not CBD. They did not immunoprecipitate or immunostain tau from the control brain. Creation of potent anti-seed antibodies based on the trans-proline epitope implicates local unfolding around P301 in pathogenesis. MD2.2 and MD3.1 may also be useful for therapy and diagnosis.

Saturation mutagenesis of α-synuclein reveals monomer fold that modulates aggregation

α-Synuclein (aSyn) aggregation underlies neurodegenerative synucleinopathies. aSyn seeds are proposed to replicate and propagate neuronal pathology like prions. Seeding of aSyn can be recapitulated in cellular systems of aSyn aggregation; however, the mechanism of aSyn seeding and its regulation are not well understood. We developed an mEos-based aSyn seeding assay and performed saturation mutagenesis to identify with single-residue resolution positive and negative regulators of aSyn aggregation. We not only found the core regions that govern aSyn aggregation but also identified mutants outside of the core that enhance aggregation. We identified local structure within the N terminus of aSyn that hinders the fibrillization propensity of its aggregation-prone core. Based on the screen, we designed a minimal aSyn fragment that shows a ~4-fold enhancement in seeding activity and enabled discrimination of synucleinopathies. Our study expands the basic knowledge of aSyn aggregation and advances the design of cellular systems of aSyn aggregation to diagnose synucleinopathies based on protein conformation.

Aggregation, Transmission, and Toxicity of the Microtubule-Associated Protein Tau: A Complex Comprehension

The microtubule-associated protein tau is an intrinsically disordered protein containing a few short and transient secondary structures. Tau physiologically associates with microtubules (MTs) for its stabilization and detaches from MTs to regulate its dynamics. Under pathological conditions, tau is abnormally modified, detaches from MTs, and forms protein aggregates in neuronal and glial cells. Tau protein aggregates can be found in a number of devastating neurodegenerative diseases known as "tauopathies", such as Alzheimer's disease (AD), frontotemporal dementia (FTD), corticobasal degeneration (CBD), etc. However, it is still unclear how the tau protein is compacted into ordered protein aggregates, and the toxicity of the aggregates is still debated. Fortunately, there has been considerable progress in the study of tau in recent years, particularly in the understanding of the intercellular transmission of pathological tau species, the structure of tau aggregates, and the conformational change events in the tau polymerization process. In this review, we summarize the concepts of tau protein aggregation and discuss the views on tau protein transmission and toxicity.

Disease-associated patterns of acetylation stabilize tau fibril formation

Assembly of tau into beta-sheet-rich amyloids dictates the pathology of a diversity of diseases. Lysine acetylation has been proposed to drive tau amyloid assembly, but no direct mechanism has emerged. Using tau fragments, we identify patterns of acetylation that flank amyloidogenic motifs on the tau fragments that promote rapid fibril assembly. We determined a 3.9 Å cryo-EM amyloid fibril structure assembled from an acetylated tau fragment uncovering how lysine acetylation can mediate gain-of-function interactions. Comparison of the structure to an ex vivo tauopathy fibril reveals regions of structural similarity. Finally, we show that fibrils encoding disease-associated patterns of acetylation are active in cell-based tau aggregation assays. Our data uncover the dual role of lysine residues in limiting tau aggregation while their acetylation leads to stabilizing pro-aggregation interactions. Design of tau sequence with specific acetylation patterns may lead to controllable tau aggregation to direct folding of tau into distinct amyloid folds.

Blocking tau transmission by biomimetic graphene nanoparticles

Tauopathies are a class of neurodegenerative diseases resulting in cognitive dysfunction, executive dysfunction, and motor disturbance. The primary pathological feature of tauopathies is the presence of neurofibrillary tangles in the brain composed of tau protein aggregates. Moreover, tau aggregates can spread from neuron to neuron and lead to the propagation of tau pathology. Although numerous small molecules are known to inhibit tau aggregation and block tau cell-to-cell transmission, it is still challenging to use them for therapeutic applications due to poor specificity and low blood-brain barrier (BBB) penetration. Graphene nanoparticles were previously demonstrated to penetrate the BBB and are amenable to functionalization for targeted delivery. Moreover, these nanoscale biomimetic particles can self-assemble or assemble with various biomolecules including proteins. In this paper, we show that graphene quantum dots (GQDs), as graphene nanoparticles, block the seeding activity of tau fibrils by inhibiting the fibrillization of monomeric tau and triggering the disaggregation of tau filaments. This behavior is attributed to electrostatic and π-π stacking interactions of GQDs with tau. Overall, our studies indicate that GQDs with biomimetic properties can efficiently inhibit and disassemble pathological tau aggregates, and thus block tau transmission, which supports their future developments as a potential treatment for tauopathies.

Specific detection of tau seeding activity in Alzheimer's disease using rationally designed biosensor cells

BACKGROUND

The prion-like propagation of tau in neurodegenerative disorders implies that misfolded pathological tau can recruit the normal protein and template its aggregation. Here, we report the methods for the development of sensitive biosensor cell lines for the detection of tau seeding activity.

RESULTS

We performed the rational design of novel tau probes based on the current structural knowledge of pathological tau aggregates in Alzheimer's disease. We generated Förster resonance energy transfer (FRET)-based biosensor stable cell lines and characterized their sensitivity, specificity, and overall ability to detect bioactive tau in human samples. As compared to the reference biosensor line, the optimized probe design resulted in an increased efficiency in the detection of tau seeding. The increased sensitivity allowed for the detection of lower amount of tau seeding competency in human brain samples, while preserving specificity for tau seeds found in Alzheimer's disease.

CONCLUSIONS

This next generation of FRET-based biosensor cells is a novel tool to study tau seeding activity in Alzheimer's disease human samples, especially in samples with low levels of seeding activity, which may help studying early tau-related pathological events.

Effect of Cosolutes on the Aggregation of a Tau Fragment: A Combined Experimental and Simulation Approach

The intrinsically disordered protein Tau represents the main component of neurofibrillary tangles that are a hallmark of Alzheimer's disease. A small fragment of Tau, known as paired helical filament 6 (PHF6), is considered to be important for the formation of the β-structure core of the fibrils. Here we study the aggregation of this fragment in the presence of different cosolutes, including urea, TMAO, sucrose and 2-hydroxypropyl-β-cyclodextrin (2-HPβCD), using both experiments and molecular dynamics simulations. A novel implicit solvation approach (MIST - Model with Implicit Solvation Thermodynamics) is used, where an energetic contribution based on the concept of transfer free energies describes the effect of the cosolutes. The simulation predictions are compared to thioflavin-T and atomic force microscopy results, and the good agreement observed confirms the predictive ability of the computational approach herein proposed. Both simulations and experiments indicate that PHF6 aggregation is inhibited in the presence of urea and 2-HPβCD, while TMAO and sucrose stabilize associated conformations. The remarkable ability of HPβCD to inhibit aggregation represents an extremely promising result for future applications, especially considering the widespread use of this molecule as a drug carrier to the brain and as a solubilizer/excipient in pharmaceutical formulations.