Secondary nucleating sequences affect kinetics and thermodynamics of tau aggregation

The role of shear rates on amyloid formation from biofilm peptide phenol-soluble modulins

Biofilms, microbial communities enclosed in the self-produced extracellular matrix, have a significant impact on human health, environment, and industry. The pathogen Staphylococcus aureus (S. aureus) is recognized as one of the most frequent causes of biofilm-related infections. Phenol-soluble modulins (PSMs) serve as a crucial component, fortifying S. aureus biofilm matrix through self-assembly into amyloid fibrils, which enhances S. aureus colonization and resistance to antibiotics. However, the role of shear rate, one of the critical physiological factors within blood vessels, on the formation of PSM amyloids remains poorly understood. In this work, using a combination of thioflavin T fluorescence kinetic studies, circular dichroism spectrometry, and electron microscopy, we demonstrated that shear rates ranging from 150 to 300 s-1 accelerate fibrillation of PSMα1, α3, and α4 into amyloids, resulting in elongated amyloid structures. Furthermore, PSMα1, α3, and α4 predominantly self-assembled into amyloid fibers with a cross-α structure under shear conditions, deviating from the typical β-sheet configuration of PSM amyloids. These findings imply the role of shear rates within the bloodstream on enhancing PSM self-assembly that is associated with S. aureus biofilm formation.

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.

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.

The Role of Copper in Tau-Related Pathology in Alzheimer's Disease

All tauopathies, including Alzheimer's disease (AD), are characterized by the intracellular accumulation of abnormal forms of tau protein in neurons and glial cells, which negatively affect microtubule stability. Under physiological conditions, tubulin-associated unit (Tau) protein is intrinsically disordered, almost without secondary structure, and is not prone to aggregation. In AD, it assembles, and forms paired helical filaments (PHFs) that further build-up neurofibrillary tangles (NFTs). Aggregates are composed of hyperphosphorylated tau protein that is more prone to aggregation. The pathology of AD is also linked to disturbed copper homeostasis, which promotes oxidative stress (OS). Copper imbalance is widely observed in AD patients. Deregulated copper ions may initiate and exacerbate tau hyperphosphorylation and formation of β-sheet-rich tau fibrils that ultimately contribute to synaptic failure, neuronal death, and cognitive decline observed in AD patients. The present review summarizes factors affecting the process of tau aggregation, conformational changes of small peptide sequences in the microtubule-binding domain required for these motifs to act as seeding sites in aggregation, and the role of copper in OS induction, tau hyperphosphorylation and tau assembly. A better understanding of the various factors that affect tau aggregation under OS conditions may reveal new targets and novel pharmacological approaches for the therapy of AD.

Usnic acid derivatives as tau-aggregation and neuroinflammation inhibitors

Accumulation of tau protein aggregation plays a crucial role in neurodegenerative diseases, such as Alzheimer's disease (AD). Uncontrollable neuroinflammation and tau pathology form a vicious circle that further aggravates AD progression. Herein, we reported the synthesis of usnic acid derivatives and evaluation of their inhibitory activities against tau-aggregation and neuroinflammation. The inhibitory activity of the derivatives against the self-fibrillation of the hexapeptide AcPHF6 was initially screened by ThT fluorescence assay. Using circular dichroism and transmission electron microscopy, compound 30 showed the most potent inhibitory activity against AcPHF6 self-fibrillation. Compound 30 was further confirmed to inhibit the aggregation of full-length 2N4R tau protein by a heparin-induced mechanism. In addition, we investigated the anti-inflammatory activity of compound 30, and showed that compared with sodium usnate, it reduced NO release in LPS-stimulated mouse microglia BV2 cells. More importantly, 30 showed significant protective effects against okadaic acid-induced memory impairment in rats. Thus, 30 was a novel tau-aggregation and neuroinflammation inhibitor that represented a potential therapeutic candidate for AD.

Distinct Conformers of Assembled Tau in Alzheimer's and Pick's Diseases

Tau filaments with distinct morphologies and/or isoform compositions underlie a large number of human neurodegenerative diseases. In conjunction with experimental studies, this has led to the suggestion that conformers of aggregated tau exist. Electron cryo-microscopy can be used to determine high-resolution structures of amyloid filaments from human brain. Paired helical and straight tau filaments of Alzheimer's disease (AD) are ultrastructural polymorphs. Each filament core is composed of two identical protofilaments extending from G273/304-E380 (in the numbering of the 441-amino acid isoform of human tau), which adopt a combined cross-β/β-helix structure. They comprise the ends of the first or second microtubule-binding repeat (R1 or R2), the whole of R3 and R4, and 12 amino acids after R4. In contrast, the core of the narrow filaments of Pick's disease (PiD) consists of a single protofilament extending from K254-F378 of 3R tau, which adopts a cross-β structure. It comprises the last 21 amino acids of R1, all of R3 and R4, and 10 amino acids after R4. Wide tau filaments of PiD, which are in the minority, consist of two narrow filaments packed against each other. The tau filament folds of AD and PiD appear to be conserved between different cases of disease. These findings show that filamentous tau adopts one fold in AD and a different fold in PiD, establishing the existence of distinct conformers.

Ordered Assembly of Tau Protein and Neurodegeneration

Tau filaments with distinct morphologies and/or isoform compositions underlie a large number of human neurodegenerative diseases. Their formation is important, because dominantly inherited mutations in MAPT, the tau gene, cause frontotemporal dementia with abundant filamentous tau inclusions. Assembly of tau may begin in a specific region of the brain, from where it spreads to other areas. It remains to be seen if the molecular species underlying tau aggregate-mediated neurodegeneration and propagation are the same or different. In the brains of mice transgenic for human mutant P301S tau, small tau filaments are the predominant seed-competent species. It has been suggested that different conformers of assembled tau may give rise to different human tauopathies, but until recently, it was not possible to study this directly. Electron cryo-microscopy can now be used to determine high-resolution structures of amyloid filaments from human brain. Paired helical and straight tau filaments of Alzheimer's disease are ultrastructural polymorphs. Each filament core is composed of two identical protofilaments extending from G273/304-E380 (in the numbering of the 441 amino acid isoform of human tau), which adopt a combined cross-β/β-helix structure. They comprise the ends of the first or second microtubule-binding repeat (R1 or R2), the whole of R3 and R4, as well as 12 amino acids after R4. By contrast, the core of the narrow filament of Pick's disease consists of a single protofilament extending from K254-F378 of 3R tau, which adopts a cross-β structure. It comprises the last 21 amino acids of R1, all of R3 and R4, as well as 10 amino acids after R4. Wide tau filaments of Pick's disease, which are in the minority, consist of two narrow filaments packed against each other. The tau filament folds of Alzheimer's and Pick's diseases appear to be conserved between different cases of disease. These findings show that filamentous tau adopts one fold in Alzheimer's disease and a different fold in Pick's disease, establishing the existence of distinct conformers.

Mapping interactions with the chaperone network reveals factors that protect against tau aggregation

A network of molecular chaperones is known to bind proteins ('clients') and balance their folding, function and turnover. However, it is often unclear which chaperones are critical for selective recognition of individual clients. It is also not clear why these key chaperones might fail in protein-aggregation diseases. Here, we utilized human microtubule-associated protein tau (MAPT or tau) as a model client to survey interactions between ~30 purified chaperones and ~20 disease-associated tau variants (~600 combinations). From this large-scale analysis, we identified human DnaJA2 as an unexpected, but potent, inhibitor of tau aggregation. DnaJA2 levels were correlated with tau pathology in human brains, supporting the idea that it is an important regulator of tau homeostasis. Of note, we found that some disease-associated tau variants were relatively immune to interactions with chaperones, suggesting a model in which avoiding physical recognition by chaperone networks may contribute to disease.

Structure-based inhibitors of tau aggregation

Aggregated tau protein is associated with over 20 neurological disorders, which include Alzheimer's disease. Previous work has shown that tau's sequence segments VQIINK and VQIVYK drive its aggregation, but inhibitors based on the structure of the VQIVYK segment only partially inhibit full-length tau aggregation and are ineffective at inhibiting seeding by full-length fibrils. Here we show that the VQIINK segment is the more powerful driver of tau aggregation. Two structures of this segment determined by the cryo-electron microscopy method micro-electron diffraction explain its dominant influence on tau aggregation. Of practical significance, the structures lead to the design of inhibitors that not only inhibit tau aggregation but also inhibit the ability of exogenous full-length tau fibrils to seed intracellular tau in HEK293 biosensor cells into amyloid. We also raise the possibility that the two VQIINK structures represent amyloid polymorphs of tau that may account for a subset of prion-like strains of tau.

Novel cell-penetrating-amyloid peptide conjugates preferentially kill cancer cells

The goal of this study was to develop a peptide which could use the toxic effects of amyloid, a substance which is the hallmark of over 25 known human diseases, to selectively kill cancer cells. Here we demonstrate that two separate amyloid-forming hexapeptides, one from the microtubule associated protein Tau involved in formation of paired helical filaments of Alzheimer's disease, and the other an amyloid forming sequence from apolipoprotein A1, when conjugated to a cell penetrating peptide (CPP) sequence, form toxic oligomers which are stable for up to 14 h and able to enter cells by a combination of endocytosis and transduction. The amyloid peptide conjugates showed selective cytotoxicity to breast cancer, neuroblastoma and cervical cancer cells in culture compared to normal cells. Fluorescence imaging experiments showed the CPP-amyloid peptide oligomers formed intracellular fibrous amyloid, visible in the endosomes/lysosomes, cytosol and nucleus with thioflavin S (ThS) staining. Further experiments with rhodamine-conjugated Dextran, propidium iodide (PI), and acridine orange (AO) suggested the mechanism of cell death was the permeability of the lysosomal membrane brought about by the formation of amyloid pores. Cytotoxicity could be abrogated by inhibitors of lysosomal hydrolases, consistent with a model where lysosomal hydrolases leak into the cytosol and induce cytotoxicity in subsequent downstream steps. Taken together, our data suggest that CPP-amyloid peptide conjugates show potential as a new class of anti-cancer peptides (ACPs).

Novel Cell Model for Tauopathy Induced by a Cell-Permeable Tau-Related Peptide

In the present study, a cell penetrating peptide (CPP)-amyloid conjugate was prepared (T-peptide), where the amyloid-forming sequence was homologous to a nucleating sequence from human Tau protein (³⁰⁶VQIVYK³¹¹). Kinetic and biophysical studies showed the peptide formed long-lived oligomers which were taken up by endocytosis and localized in perinuclear vesicles and in the cytoplasm of murine hippocampal neuroblastoma cells and human HeLa cells. Thioflavin S (ThS) staining of amyloid colocalized with pathological phosphorylated Tau, suggesting that the peptide was able to seed endogenous wild-type Tau. Subsequent experiments showed that aggregates present in the lysosomes mediated lysosome membrane permeability (LMP). We observed a decrease in total Tau, irrespective of phosphorylation state, consistent with Tau fragmentation by lysosomal proteases. We found cytotoxicity of T-peptide could be abrogated by inhibitors of lysosomal hydrolases and caspases, consistent with a model where Tau fragments processed by the lysosome leak into the cytoplasm and induce toxicity in subsequent downstream steps. It is our hope that the T-peptide system may prove amenable to the evaluation of small molecule inhibitors of cytotoxicity, especially those which target either Tau aggregation or the lysosomal/autophagy system.

Structural evaluations of tau protein conformation: methodologies and approaches

Protein-misfolding diseases are based on a common principle of aggregation initiated by intra- and inter-molecular contacts. The structural and conformational changes induced by biochemical transformations such as post-translational modifications (PTMs), often lead to protein unfolding and misfolding. Thus, these order-to-disorder or disorder-to-order transitions may regulate cellular function. Tau, a neuronal protein, regulates microtubule (MT) structure and overall cellular integrity. However, misfolded tau modified by PTMs results in MT destabilization, toxic tau aggregate formation, and ultimately cell death, leading to neurodegeneration. Currently, the lack of structural information surrounding tau severely limits understanding of neurodegeneration. This minireview focuses on the current methodologies and approaches aimed at probing tau conformation and the role of conformation in various aspects of tau biochemistry. The recent applications of nuclear magnetic resonance, mass spectrometry, Förster resonance electron transfer, and molecular dynamics simulations toward structural analysis of conformational landscapes of tau will be described. The strategies developed for structural evaluation of tau may significantly improve our understanding of misfolding diseases.

Naphthoquinone-Tryptophan Hybrid Inhibits Aggregation of the Tau-Derived Peptide PHF6 and Reduces Neurotoxicity

Tauopathies, such as Alzheimer's disease (AD), are a group of disorders characterized neuropathologically by intracellular toxic accumulations of abnormal protein aggregates formed by misfolding of the microtubule-associated protein tau. Since protein self-assembly appears to be an initial key step in the pathology of this group of diseases, intervening in this process can be both a prophylactic measure and a means for modifying the course of the disease for therapeutic purposes. We and others have shown that aromatic small molecules can be effective inhibitors of aggregation of various protein assemblies, by binding to the aromatic core in aggregation-prone motifs and preventing their self-assembly. Specifically, we have designed a series of small aromatic naphthoquinone-tryptophan hybrid molecules as candidate aggregation inhibitors of β -sheet based assembly and demonstrated their efficacy toward inhibiting aggregation of the amyloid-β peptide, another culprit of AD, as well as of various other aggregative proteins involved in other protein misfolding diseases. Here we tested whether a leading naphthoquinone-tryptophan hybrid molecule, namely NQTrp, can be repurposed as an inhibitor of the aggregation of the tau protein in vitro and in vivo. We show that the molecule inhibits the in vitro assembly of PHF6, the aggregation-prone fragment of tau protein, reduces hyperphosphorylated tau deposits and ameliorates tauopathy-related behavioral defect in an established transgenic Drosophila model expressing human tau. We suggest that NQTrp, or optimized versions of it, could act as novel disease modifying drugs for AD and other tauopathies.

How Does Hyperphopsphorylation Promote Tau Aggregation and Modulate Filament Structure and Stability?

Tau proteins are hyperphosphorylated at common sites in their N- and C-terminal domains in at least three neurodegenerative diseases, Parkinson, dementia with Lewy bodies, and Alzheimer's, suggesting specific pathology but general mechanism. Full-length human tau filament comprises a rigid core and a two-layered fuzzy coat. Tau is categorized into two groups of isoforms, with either four repeats (R1-R4) or three repeats (R1, R3, and R4); their truncated constructs are respectively called K18 and K19. Using multiscale molecular dynamics simulations, we explored the conformational consequences of hyperhposphorylation on tau's repeats. Our lower conformational energy filament models suggest a rigid filament core with a radius of ∼30 to 40 Å and an outer layer with a thickness of ∼140 Å consisting of a double-layered polyelectrolyte. The presence of the phosphorylated terminal domains alters the relative stabilities in the K18 ensemble, thus shifting the populations of the full-length filaments. However, the structure with the straight repeats in the core region is still the most stable, similar to the truncated K18 peptide species without the N- and C-terminus. Our simulations across different scales of resolution consistently reveal that hyperphosphorylation of the two terminal domains decreases the attractive interactions among the N- and C-terminus and repeat domain. To date, the relationship on the conformational level between phosphorylation and aggregation has not been understood. Our results suggest that the exposure of the repeat domain upon hyperphosphorylation could enhance tau filament aggregation. Thus, we discovered that even though these neurodegenerative diseases vary and their associated tau filaments are phosphorylated to different extents, remarkably, the three pathologies appear to share a common tau aggregation mechanism.

Critical role of acetylation in tau-mediated neurodegeneration and cognitive deficits

Tauopathies, including frontotemporal dementia (FTD) and Alzheimer's disease (AD), are neurodegenerative diseases in which tau fibrils accumulate. Recent evidence supports soluble tau species as the major toxic species. How soluble tau accumulates and causes neurodegeneration remains unclear. Here we identify tau acetylation at Lys174 (K174) as an early change in AD brains and a critical determinant in tau homeostasis and toxicity in mice. The acetyl-mimicking mutant K174Q slows tau turnover and induces cognitive deficits in vivo. Acetyltransferase p300-induced tau acetylation is inhibited by salsalate and salicylate, which enhance tau turnover and reduce tau levels. In the PS19 transgenic mouse model of FTD, administration of salsalate after disease onset inhibited p300 activity, lowered levels of total tau and tau acetylated at K174, rescued tau-induced memory deficits and prevented hippocampal atrophy. The tau-lowering and protective effects of salsalate were diminished in neurons expressing K174Q tau. Targeting tau acetylation could be a new therapeutic strategy against human tauopathies.

Tau assembly: the dominant role of PHF6 (VQIVYK) in microtubule binding region repeat R3

Self-aggregation of the microtubule-binding protein Tau reduces its functionality and is tightly associated with Tau-related diseases, termed tauopathies. Tau aggregation is also strongly associated with two nucleating six-residue segments, namely PHF6 (VQIVYK) and PHF6* (VQIINK). In this paper, using experiments and computational modeling, we study the self-assembly of individual and binary mixtures of Tau fragments containing PHF6* (R2/wt; (273)GKVQIINKKLDL(284)) and PHF6 (R3/wt; (306)VQIVYKPVDLSK(317)) and a mutant R2/ΔK280 associated with a neurodegenerative tauopathy. The initial stage of aggregation is probed by ion-mobility mass spectrometry, the kinetics of aggregation monitored with Thioflavin T assays, and the morphology of aggregates visualized by transmission electron microscopy. Insights into the structure of early aggregates and the factors stabilizing the aggregates are obtained from replica exchange molecular dynamics simulations. Our data suggest that R3/wt has a much stronger aggregation propensity than either R2/wt or R2/ΔK280. Heterodimers containing R3/wt are less stable than R3/wt homodimers but much more stable than homodimers of R2/wt and R2/ΔK280, suggesting a possible role of PHF6*-PHF6 interactions in initiating the aggregation of full-length Tau. Lastly, R2/ΔK280 binds more strongly to R3/wt than R2/wt, suggesting a possible mechanism for a pathological loss of normal Tau function.

Resonance Raman spectroscopic measurements delineate the structural changes that occur during tau fibril formation

The aggregation of the microtubule-associated protein, tau, into amyloid fibrils is a hallmark of neurodegenerative diseases such as the tauopathies and Alzheimer's disease. Since monomeric tau is an intrinsically disordered protein and the polymeric fibrils possess an ordered cross-β core, the aggregation process is known to involve substantial conformational conversion besides growth. The aggregation mechanism of tau in the presence of inducers such as heparin, deciphered using probes such as thioflavin T/S fluorescence, light scattering, and electron microscopy assays, has been shown to conform to ligand-induced nucleation-dependent polymerization. These probes do not, however, distinguish between the processes of conformational conversion and fibril growth. In this study, UV resonance Raman spectroscopy is employed to look directly at signatures of changes in secondary structure and side-chain packing during fibril formation by the four repeat functional domain of tau in the presence of the inducer heparin, at pH 7 and at 37 °C. Changes in the positions and intensities of the amide Raman bands are shown to occur in two distinct stages during the fibril formation process. The first stage of UVRR spectral changes corresponds to the transformation of monomer into early fibrillar aggregates. The second stage corresponds to the transformation of these early fibrillar aggregates into the final, ordered, mature fibrils and during this stage; the processes of conformational conversion and the consolidation of the fibril core occur simultaneously. Delineation of these secondary structural changes accompanying the formation of tau fibrils holds significance for the understanding of generic and tau-specific principles of amyloid assembly.

Interactions between amyloid-β and Tau fragments promote aberrant aggregates: implications for amyloid toxicity

We have investigated at the oligomeric level interactions between Aβ(25–35) and Tau(273–284), two important fragments of the amyloid-β and Tau proteins, implicated in Alzheimer’s disease. We are able to directly observe the coaggregation of these two peptides by probing the conformations of early heteroligomers and the macroscopic morphologies of the aggregates. Ion-mobility experiment and theoretical modeling indicate that the interactions of the two fragments affect the self-assembly processes of both peptides. Tau(273–284) shows a high affinity to form heteroligomers with existing Aβ(25–35) monomer and oligomers in solution. The configurations and characteristics of the heteroligomers are determined by whether the population of Aβ(25–35) or Tau(273–284) is dominant. As a result, two types of aggregates are observed in the mixture with distinct morphologies and dimensions from those of pure Aβ(25–35) fibrils. The incorporation of some Tau into β-rich Aβ(25–35) oligomers reduces the aggregation propensity of Aβ(25–35) but does not fully abolish fibril formation. On the other hand, by forming complexes with Aβ(25–35), Tau monomers and dimers can advance to larger oligomers and form granular aggregates. These heteroligomers may contribute to toxicity through loss of normal function of Tau or inherent toxicity of the aggregates themselves.

Factors that drive peptide assembly and fibril formation: experimental and theoretical analysis of Sup35 NNQQNY mutants

Residue mutations have substantial effects on aggregation kinetics and propensities of amyloid peptides and their aggregate morphologies. Such effects are attributed to conformational transitions accessed by various types of oligomers such as steric zipper or single β-sheet. We have studied the aggregation propensities of six NNQQNY mutants: NVVVVY, NNVVNV, NNVVNY, VIQVVY, NVVQIY, and NVQVVY in water using a combination of ion-mobility mass spectrometry, transmission electron microscopy, atomic force microscopy, and all-atom molecular dynamics simulations. Our data show a strong correlation between the tendency to form early β-sheet oligomers and the subsequent aggregation propensity. Our molecular dynamics simulations indicate that the stability of a steric zipper structure can enhance the propensity for fibril formation. Such stability can be attained by either hydrophobic interactions in the mutant peptide or polar side-chain interdigitations in the wild-type peptide. The overall results display only modest agreement with the aggregation propensity prediction methods such as PASTA, Zyggregator, and RosettaProfile, suggesting the need for better parametrization and model peptides for these algorithms.

Mechanistic studies unravel the complexity inherent in tau aggregation leading to Alzheimer's disease and the tauopathies

The aggregation of the protein tau into amyloid fibrils is known to be involved in the causation of the neurodegenerative tauopathies and the progression of cognitive decline in Alzheimer's disease. This review surveys the mechanism of tau aggregation with special emphasis on the information obtained from biochemical and biophysical studies. First, tau is described from a structure-function perspective. Subsequently, the connection of tau to neurodegeneration is explained, and a description of the tau amyloid fibril is provided. Lastly, studies of the mechanism of tau fibril formation are reviewed, and the physiological significance of these studies with reference to how they can clarify many aspects of disease progression is described. The aim of this review is to underscore how mechanistic studies reveal the complexity of the tau fibril formation pathway and the plethora of species populated on or off the pathway of aggregation, and how this information can be beneficial in the design of inhibitors or drugs that ameliorate neurodegeneration.