The Nucleation of Protein Aggregates - From Crystals to Amyloid Fibrils

Insights into the Assembly of Peptides Catalyzed by Polysaccharides

Nucleation is a critical step that determines the assembly pathway and the structure and functions of the peptide assemblies. However, the dynamic evolution of interactions between nucleating agents and peptides, as well as between peptides themselves during the nucleation process, remains elusive. Herein, we show that the heterogeneous nucleating agent carboxymethylcellulose (CMC) can promote the nucleation of Aβ16-20 (KF) peptide. The Förster resonance energy transfer (FRET) technology was used to unveil the interaction dynamics between the CMC and KF peptide. Initially, CMC enriches KF monomers through weak nondirectional electrostatic interactions. The electrostatic screening reduces the electrostatic repulsion between KF molecules. Subsequently, KF-KF interactions become dominant, leading to the dissociation of KF from the CMC and nucleation. By adjustment of the adding time, dosage, size, and active sites of CMC, the assembly kinetics of KF can be effectively controlled. This study helps gain a deep understanding of the early heterogeneous nucleation process of peptide assembly and provides valuable guidance for the rational design of efficient nucleating agents for peptide assembly toward functional materials.

A tool to dissect heterotypic determinants of homotypic protein phase behavior

Proteins commonly self-assemble to create liquid or solid condensates with diverse biological activities. The mechanisms of assembly are determined by each protein's sequence and cellular context. We previously developed distributed amphifluoric FRET (DAmFRET) to analyze sequence determinants of self-assembly in cells. Here, we extend DAmFRET by creating a nanobody (mEosNb) against the fluorescent protein mEos3 to physically tether and thereby recruit candidate modifier proteins to mEos3-fused query proteins. This accessorization allows us to rapidly screen for effects on the phase behavior of query proteins by modulating the expression level and valency of mEosNb-fused modifiers. We show that our system recapitulates known effects of multivalency on liquid-liquid phase separation and can discriminate between nucleation mechanisms of amyloid and amyloid-like assemblies. Our approach adds a new experimental dimension for interrogating the mechanisms of intracellular phase transitions.

Massively parallel genetic perturbation reveals the energetic architecture of an amyloid beta nucleation reaction

Amyloid protein aggregates are pathological hallmarks of more than fifty human diseases but how soluble proteins nucleate to form amyloids is poorly understood. Here we use combinatorial mutagenesis, a kinetic selection assay, and machine learning to massively perturb the energetics of the nucleation reaction of amyloid beta (Aβ42), the protein that aggregates in Alzheimer's disease. In total, we quantify the nucleation rates of >140,000 variants of Aβ42. This allows us to accurately quantify the changes in reaction activation energy for all possible amino acid substitutions in a protein for the first time and, in addition, to quantify >600 energetic interactions between mutations. The data reveal the simple and interpretable genetic architecture of an amyloid nucleation reaction. Strikingly, strong energetic couplings are rare and identify a subset of structural contacts in mature fibrils. Together with the activation energy changes, this strongly suggests that the Aβ42 nucleation reaction transition state is structured in a short C-terminal region, providing a structural model for the reaction that may initiate Alzheimer's disease. We believe this approach can be widely applied to probe the energetics and transition state structures of protein reactions.

Helicobacter pylori outer membrane vesicles directly promote Aβ aggregation and enhance Aβ toxicity in APP/PS1 mice

Helicobacter pylori (H. pylori) infection has been found associated with Alzheimer's disease (AD) with unclear mechanisms. Outer Membrane Vesicles (OMVs) are spherical particles secreted by Gram-negative bacteria. Here we explore the effect of H. pylori OMVs on Aβ aggregation and toxicity. We show intraperitoneally-injected H. pylori OMVs enter the brain and co-localize with Aβ plaques in APP/PS1 mice, accompanied by aggravated Aβ pathology, exacerbated cognitive deficits and synaptic impairment, indicating that H. pylori OMVs promote β-amyloidosis and AD development. The in vitro results further identify that H. pylori OMVs significantly accelerate Aβ aggregation and increase Aβ-induced neurotoxicity. Through lipidomic analysis, we reveal that lipid components, particularly LPC 18:0 in H. pylori OMVs accelerate Aβ aggregation and enhance Aβ neurotoxicity. Moreover, H. pylori OMVs-enhanced Aβ neurotoxicity is mediated by Ca2+. These findings reveal a mechanism of H. pylori OMVs in accelerating AD development in which the bacterial OMVs-originated lipid components play a key role in promoting Aβ aggregation and neurotoxicity.

From adsorption to crystallization of proteins: Evidence for interface-assisted nucleation

Protein crystallization is among the key processes in biomolecular research, but the underlying mechanisms are still elusive. Here, we address the role of inevitable interfaces for the nucleation process. Quartz crystal microbalance with dissipation monitoring (QCM-D) with simultaneously optical microscopy, confocal microscopy, and grazing-incidence small angle X-rays scattering (GISAXS) were employed to investigate the temporal behavior from the initial stage of protein adsorption to crystallization. Here we studied the crystallization of the Human Serum Albumin (HSA), the most abundant blood protein, in the presence of a charged surface and a trivalent salt. We found evidence for interface-assisted nucleation of crystals. The kinetic stages involved are initial adsorption followed by enhanced adsorption after longer times, subsequent nucleation, and finally crystal growth. The results highlight the importance of interfaces for protein phase behavior and in particular for nucleation.

Change in the Kinetic Regime of Aggregation of Yeast Alcohol Dehydrogenase in the Presence of 2-Hydroxypropyl-β-cyclodextrin

Chemical chaperones are low-molecular-weight compounds that suppress protein aggregation. They can influence different stages of the aggregation process-the stage of protein denaturation, the nucleation stage and the stage of aggregate growth-and this may lead to a change in the aggregation kinetic regime. Here, the possibility of changing the kinetic regime in the presence of a chemical chaperone 2-hydroxypropyl-β-cyclodextrin (2-HP-β-CD) was investigated for a test system based on the thermally induced aggregation of yeast alcohol dehydrogenase (yADH) at 56 °C. According to differential scanning calorimetry data, 2-HP-β-CD did not affect the stage of the protein molecule unfolding. Dynamic light scattering data indicated changes in the aggregation kinetics of yADH during the nucleation and aggregate growth stages in the presence of the chaperone. The analysis of kinetic curves showed that the order of aggregation with respect to protein (nc), calculated for the stage of aggregate growth, changed from nc = 1 to nc = 2 with the addition of 100 mM 2-HP-β-CD. The mechanism of 2-HP-β-CD action on the yADH thermal aggregation leading to a change in its kinetic regime of aggregation is discussed.

Nucleation of α-Synuclein Amyloid Fibrils Induced by Cross-Interaction with β-Hairpin Peptides Derived from Immunoglobulin Light Chains

Heterologous interactions between different amyloid-forming proteins, also called cross-interactions, may have a critical impact on disease-related amyloid formation. β-hairpin conformers of amyloid-forming proteins have been shown to affect homologous interactions in the amyloid self-assembly process. Here, we applied two β-hairpin-forming peptides derived from immunoglobulin light chains as models to test how heterologous β-hairpins modulate the fibril formation of Parkinson's disease-associated protein α-synuclein (αSyn). The peptides SMAhp and LENhp comprise β-strands C and C' of the κ4 antibodies SMA and LEN, which are associated with light chain amyloidosis and multiple myeloma, respectively. SMAhp and LENhp bind with high affinity to the β-hairpin-binding protein β-wrapin AS10 according to isothermal titration calorimetry and NMR spectroscopy. The addition of SMAhp and LENhp affects the kinetics of αSyn aggregation monitored by Thioflavin T (ThT) fluorescence, with the effect depending on assay conditions, salt concentration, and the applied β-hairpin peptide. In the absence of agitation, substoichiometric concentrations of the hairpin peptides strongly reduce the lag time of αSyn aggregation, suggesting that they support the nucleation of αSyn amyloid fibrils. The effect is also observed for the aggregation of αSyn fragments lacking the N-terminus or the C-terminus, indicating that the promotion of nucleation involves the interaction of hairpin peptides with the hydrophobic non-amyloid-β component (NAC) region.

Pathologic polyglutamine aggregation begins with a self-poisoning polymer crystal

A long-standing goal of amyloid research has been to characterize the structural basis of the rate-determining nucleating event. However, the ephemeral nature of nucleation has made this goal unachievable with existing biochemistry, structural biology, and computational approaches. Here, we addressed that limitation for polyglutamine (polyQ), a polypeptide sequence that causes Huntington's and other amyloid-associated neurodegenerative diseases when its length exceeds a characteristic threshold. To identify essential features of the polyQ amyloid nucleus, we used a direct intracellular reporter of self-association to quantify frequencies of amyloid appearance as a function of concentration, conformational templates, and rational polyQ sequence permutations. We found that nucleation of pathologically expanded polyQ involves segments of three glutamine (Q) residues at every other position. We demonstrate using molecular simulations that this pattern encodes a four-stranded steric zipper with interdigitated Q side chains. Once formed, the zipper poisoned its own growth by engaging naive polypeptides on orthogonal faces, in a fashion characteristic of polymer crystals with intramolecular nuclei. We further show that self-poisoning can be exploited to block amyloid formation, by genetically oligomerizing polyQ prior to nucleation. By uncovering the physical nature of the rate-limiting event for polyQ aggregation in cells, our findings elucidate the molecular etiology of polyQ diseases.

Pathologic polyglutamine aggregation begins with a self-poisoning polymer crystal

A long-standing goal of amyloid research has been to characterize the structural basis of the rate-determining nucleating event. However, the ephemeral nature of nucleation has made this goal unachievable with existing biochemistry, structural biology, and computational approaches. Here, we addressed that limitation for polyglutamine (polyQ), a polypeptide sequence that causes Huntington's and other amyloid-associated neurodegenerative diseases when its length exceeds a characteristic threshold. To identify essential features of the polyQ amyloid nucleus, we used a direct intracellular reporter of self-association to quantify frequencies of amyloid appearance as a function of concentration, conformational templates, and rational polyQ sequence permutations. We found that nucleation of pathologically expanded polyQ involves segments of three glutamine (Q) residues at every other position. We demonstrate using molecular simulations that this pattern encodes a four-stranded steric zipper with interdigitated Q side chains. Once formed, the zipper poisoned its own growth by engaging naive polypeptides on orthogonal faces, in a fashion characteristic of polymer crystals with intramolecular nuclei. We further show that self-poisoning can be exploited to block amyloid formation, by genetically oligomerizing polyQ prior to nucleation. By uncovering the physical nature of the rate-limiting event for polyQ aggregation in cells, our findings elucidate the molecular etiology of polyQ diseases.

Proteins Do Not Replicate, They Precipitate: Phase Transition and Loss of Function Toxicity in Amyloid Pathologies

Protein aggregation into amyloid fibrils affects many proteins in a variety of diseases, including neurodegenerative disorders, diabetes, and cancer. Physicochemically, amyloid formation is a phase transition process, where soluble proteins are transformed into solid fibrils with the characteristic cross-β conformation responsible for their fibrillar morphology. This phase transition proceeds via an initial, rate-limiting nucleation step followed by rapid growth. Several well-defined nucleation pathways exist, including homogenous nucleation (HON), which proceeds spontaneously; heterogeneous nucleation (HEN), which is catalyzed by surfaces; and seeding via preformed nuclei. It has been hypothesized that amyloid aggregation represents a protein-only (nucleic-acid free) replication mechanism that involves transmission of structural information via conformational templating (the prion hypothesis). While the prion hypothesis still lacks mechanistic support, it is also incompatible with the fact that proteins can be induced to form amyloids in the absence of a proteinaceous species acting as a conformational template as in the case of HEN, which can be induced by lipid membranes (including viral envelopes) or polysaccharides. Additionally, while amyloids can be formed from any protein sequence and via different nucleation pathways, they invariably adopt the universal cross-β conformation; suggesting that such conformational change is a spontaneous folding event that is thermodynamically favorable under the conditions of supersaturation and phase transition and not a templated replication process. Finally, as the high stability of amyloids renders them relatively inert, toxicity in some amyloid pathologies might be more dependent on the loss of function from protein sequestration in the amyloid state rather than direct toxicity from the amyloid plaques themselves.

Electric-field induced modulation of amorphous protein aggregates: polarization, deformation, and reorientation

Proteins in their native state are only marginally stable and tend to aggregate. However, protein misfolding and condensation are often associated with undesired processes, such as pathogenesis, or unwanted properties, such as reduced biological activity, immunogenicity, or uncontrolled materials properties. Therefore, controlling protein aggregation is very important, but still a major challenge in various fields, including medicine, pharmacology, food processing, and materials science. Here, flexible, amorphous, micron-sized protein aggregates composed of lysozyme molecules reduced by dithiothreitol are used as a model system. The preformed amorphous protein aggregates are exposed to a weak alternating current electric field. Their field response is followed in situ by time-resolved polarized optical microscopy, revealing field-induced deformation, reorientation and enhanced polarization as well as the disintegration of large clusters of aggregates. Small-angle dynamic light scattering was applied to probe the collective microscopic dynamics of amorphous aggregate suspensions. Field-enhanced local oscillations of the intensity auto-correlation function are observed and related to two distinguishable elastic moduli. Our results validate the prospects of electric fields for controlling protein aggregation processes.

Strong inhibition of peptide amyloid formation by a fatty acid

The aggregation of peptides into amyloid fibrils is associated with several diseases, including Alzheimer’s and Parkinson’s disease. Because hydrophobic interactions often play an important role in amyloid formation, the presence of various hydrophobic or amphiphilic molecules, such as lipids, may influence the aggregation process. We have studied the effect of a fatty acid, linoleic acid, on the fibrillation process of the amyloid-forming model peptide NACore (GAVVTGVTAVA). NACore is a peptide fragment spanning residue 68–78 of the protein α-synuclein involved in Parkinson’s disease. Based primarily on circular dichroism measurements, we found that even a very small amount of linoleic acid can substantially inhibit the fibrillation of NACore. This inhibitory effect manifests itself through a prolongation of the lag phase of the peptide fibrillation. The effect is greatest when the fatty acid is present from the beginning of the process together with the monomeric peptide. Cryogenic transmission electron microscopy revealed the presence of nonfibrillar clusters among NACore fibrils formed in the presence of linoleic acid. We argue that the observed inhibitory effect on fibrillation is due to co-association of peptide oligomers and fatty acid aggregates at the early stage of the process. An important aspect of this mechanism is that it is nonmonomeric peptide structures that associate with the fatty acid aggregates. Similar mechanisms of action could be relevant in amyloid formation occurring in vivo, where the aggregation takes place in a lipid-rich environment.

Aggregation and structure of amyloid β-protein

Alzheimer's disease (AD) is the most common age-related neurodegenerative disorder and is characterized by major pathological hallmarks in the brain, including plaques composed of amyloid β-protein (Aβ) and neurofibrillary tangles of tau protein. Genetic studies, biochemical data, and animal models have suggested that Aβ is a critical species in the pathogenesis of AD. Aβ molecules aggregate to form oligomers, protofibrils (PFs), and mature fibrils. Because of their instability and structural heterogeneity, the misfolding and aggregation of Aβ is a highly complex process, leading to a variety of aggregates with different structures and morphologies. However, the elucidation of Aβ molecules is essential because they are believed to play an important role in AD pathogenesis. Recent combination studies using nuclear magnetic resonance (NMR) and cryo-electron microscopy (cryo-EM) have primarily revealed more detailed information about their aggregation process, including fibril extension and secondary nucleation, and the structural polymorphism of the fibrils under a variety of some conditions, including the actual brain. This review attempts to summarize the existing information on the major properties of the structure and aggregation of Aβ.

Toward the equilibrium and kinetics of amyloid peptide self-assembly

Several devastating human diseases are linked to peptide self-assembly, but our understanding their onset and progression is not settled. This is a sign of the complexity of the aggregation process, which is prevented, catalyzed, or retarded by numerous factors in body fluids and cells, varying in time and space. Biophysical studies of pure peptide solutions contribute insights into the underlying steps in the process and quantitative parameters relating to rate constants (energy barriers) and equilibrium constants (population distributions). This requires methods to quantify the concentration of at least one species in the process. Translation to an in vivo situation poses an enormous challenge, and the effects of selected components (bottom up) or entire body fluids (top down) need to be quantified.

Protein nanofibril design via manipulation of hydrogen bonds

The process of amyloid nanofibril formation has broad implications including the generation of the strongest natural materials, namely silk fibers, and their major contribution to the progression of many degenerative diseases. The key question that remains unanswered is whether the amyloidogenic nature, which includes the characteristic H-bonded β-sheet structure and physical characteristics of protein assemblies, can be modified via controlled intervention of the molecular interactions. Here we show that tailored changes in molecular interactions, specifically in the H-bonded network, do not affect the nature of amyloidogenic fibrillation, and even have minimal effect on the initial nucleation events of self-assembly. However, they do trigger changes in networks at a higher hierarchical level, namely enhanced 2D packaging which is rationalized by the 3D hierarchy of β-sheet assembly, leading to variations in fibril morphology, structural composition and, remarkably, nanomechanical properties. These results pave the way to a better understanding of the role of molecular interactions in sculpting the structural and physical properties of protein supramolecular constructs.

Secondary Nucleation and the Conservation of Structural Characteristics of Amyloid Fibril Strains

Amyloid fibrils are ordered protein aggregates and a hallmark of many severe neurodegenerative diseases. Amyloid fibrils form through primary nucleation from monomeric protein, grow through monomer addition and proliferate through fragmentation or through the nucleation of new fibrils on the surface of existing fibrils (secondary nucleation). It is currently still unclear how amyloid fibrils initially form in the brain of affected individuals and how they are amplified. A given amyloid protein can sometimes form fibrils of different structure under different solution conditions in vitro, but often fibrils found in patients are highly homogeneous. These findings suggest that the processes that amplify amyloid fibrils in vivo can in some cases preserve the structural characteristics of the initial seed fibrils. It has been known for many years that fibril growth by monomer addition maintains the structure of the seed fibril, as the latter acts as a template that imposes its fold on the newly added monomer. However, for fibrils that are formed through secondary nucleation it was, until recently, not clear whether the structure of the seed fibril is preserved. Here we review the experimental evidence on this question that has emerged over the last years. The overall picture is that the fibril strain that forms through secondary nucleation is mostly defined by the solution conditions and intrinsic structural preferences, and not by the seed fibril strain.

Nonclassical Nucleation—Role of Metastable Intermediate Phase in Crystal Nucleation: An Editorial Prefix

Classical nucleation theory (CNT), which was established about 90 years ago, represents the most commonly used theory in describing nucleation processes. For a fluid-to-solid phase transition, CNT states that the solutes in a supersaturated solution reversibly form small clusters. Once a cluster reaches its critical size, it becomes thermodynamically stable and is favored for further growth. One of the most important assumptions of CNT is that the nucleation process is described by one reaction coordinate and all order parameters proceed simultaneously. Recent studies in experiments, computer simulations, and theory have revealed nonclassical features in the early stage of nucleation. In particular, the decoupling of order parameters involved during a fluid-to-solid transition leads to the so-called two-step nucleation mechanism, in which a metastable intermediate phase (MIP) exists in parallel to the initial supersaturated solution and the final crystals. These MIPs can be high-density liquid phases, mesoscopic clusters, or preordered states. In this Special Issue, we focus on the role of the various MIPs in the early stage of crystal nucleation of organic materials, metals and alloys, aqueous solutions, minerals, colloids, and proteins, and thus on various scenarios of nonclassical pathways of crystallization.

The role of surfaces on amyloid formation

Interfaces can strongly accelerate or inhibit protein aggregation, destabilizing proteins that are stable in solution or, conversely, stabilizing proteins that are aggregation-prone. Although this behaviour is well-known, our understanding of the molecular mechanisms underlying surface-induced protein aggregation is still largely incomplete. A major challenge is represented by the high number of physico-chemical parameters involved, which are highly specific to the considered combination of protein, surface properties, and solution conditions. The key aspect determining the role of interfaces is the relative propensity of the protein to aggregate at the surface with respect to bulk. In this review, we discuss the multiple molecular determinants that regulate this balance. We summarize current experimental techniques aimed at characterizing protein aggregation at interfaces, and highlight the need to complement experimental analysis with theoretical modelling. In particular, we illustrate how chemical kinetic analysis can be combined with experimental methods to provide insights into the molecular mechanisms underlying surface-induced protein aggregation, under both stagnant and agitation conditions. We summarize recent progress in the study of important amyloids systems, focusing on selected relevant interfaces.

Looking Beyond the Core: The Role of Flanking Regions in the Aggregation of Amyloidogenic Peptides and Proteins

Amyloid proteins are involved in many neurodegenerative disorders such as Alzheimer’s disease [Tau, Amyloid β (Aβ)], Parkinson’s disease [alpha-synuclein (αSyn)], and amyotrophic lateral sclerosis (TDP-43). Driven by the early observation of the presence of ordered structure within amyloid fibrils and the potential to develop inhibitors of their formation, a major goal of the amyloid field has been to elucidate the structure of the amyloid fold at atomic resolution. This has now been achieved for a wide variety of sequences using solid-state NMR, microcrystallography, X-ray fiber diffraction and cryo-electron microscopy. These studies, together with in silico methods able to predict aggregation-prone regions (APRs) in protein sequences, have provided a wealth of information about the ordered fibril cores that comprise the amyloid fold. Structural and kinetic analyses have also shown that amyloidogenic proteins often contain less well-ordered sequences outside of the amyloid core (termed here as flanking regions) that modulate function, toxicity and/or aggregation rates. These flanking regions, which often form a dynamically disordered “fuzzy coat” around the fibril core, have been shown to play key parts in the physiological roles of functional amyloids, including the binding of RNA and in phase separation. They are also the mediators of chaperone binding and membrane binding/disruption in toxic amyloid assemblies. Here, we review the role of flanking regions in different proteins spanning both functional amyloid and amyloid in disease, in the context of their role in aggregation, toxicity and cellular (dys)function. Understanding the properties of these regions could provide new opportunities to target disease-related aggregation without disturbing critical biological functions.

On the Mechanism of Self-Assembly by a Hydrogel-Forming Peptide

Self-assembling peptide-based hydrogels are a class of tunable soft materials that have been shown to be highly useful for a number of biomedical applications. The dynamic formation of the supramolecular fibrils that compose these materials has heretofore remained poorly characterized. A better understanding of this process would provide important insights into the behavior of these systems and could aid in the rational design of new peptide hydrogels. Here, we report the determination of the microscopic steps that underpin the self-assembly of a hydrogel-forming peptide, SgI_37-49. Using theoretical models of linear polymerization to analyze the kinetic self-assembly data, we show that SgI_37-49 fibril formation is driven by fibril-catalyzed secondary nucleation and that all the microscopic processes involved in SgI_37-49 self-assembly display an enzyme-like saturation behavior. Moreover, this analysis allows us to quantify the rates of the underlying processes at different peptide concentrations and to calculate the time evolution of these reaction rates over the time course of self-assembly. We demonstrate here a new mechanistic approach for the study of self-assembling hydrogel-forming peptides, which is complementary to commonly used materials science characterization techniques.

Self-Replication of Prion Protein Fragment 89-230 Amyloid Fibrils Accelerated by Prion Protein Fragment 107-143 Aggregates

Prion protein amyloid aggregates are associated with infectious neurodegenerative diseases, known as transmissible spongiform encephalopathies. Self-replication of amyloid structures by refolding of native protein molecules is the probable mechanism of disease transmission. Amyloid fibril formation and self-replication can be affected by many different factors, including other amyloid proteins and peptides. Mouse prion protein fragments 107-143 (PrP(107-143)) and 89-230 (PrP(89-230)) can form amyloid fibrils. β-sheet core in PrP(89-230) amyloid fibrils is limited to residues ∼160-220 with unstructured N-terminus. We employed chemical kinetics tools, atomic force microscopy and Fourier-transform infrared spectroscopy, to investigate the effects of mouse prion protein fragment 107-143 fibrils on the aggregation of PrP(89-230). The data suggest that amyloid aggregates of a short prion-derived peptide are not able to seed PrP(89-230) aggregation; however, they accelerate the self-replication of PrP(89-230) amyloid fibrils. We conclude that PrP(107-143) fibrils could facilitate the self-replication of PrP(89-230) amyloid fibrils in several possible ways, and that this process deserves more attention as it may play an important role in amyloid propagation.

Thermodynamic and kinetics investigation of homogeneous and heterogeneous nucleation

Nucleation is a fundamental process widely studied in different areas of industry and biology. This review paper comprehensively discussed the principles of classical nucleation theory (primary homogeneous), and heterogeneous nucleation. In the homogeneous part, the nucleation rate in the transient and intransient state is monitored and also heterogeneous nucleation is covered. Finally, conclusions have been deduced from the collected works studied here, and offers for future studies are proposed.

Disentangling the Amyloid Pathways: A Mechanistic Approach to Etiology

Amyloids are fibrillar protein aggregates associated with diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), type II diabetes and Creutzfeldt-Jakob disease. The process of amyloid polymerization involves three pathological protein transformations; from natively folded conformation to the cross-β conformation, from biophysically soluble to insoluble, and from biologically functional to non-functional. While amyloids share a similar cross-β conformation, the biophysical transformation can either take place spontaneously via a homogeneous nucleation mechanism (HON) or catalytically on an exogenous surface via a heterogeneous nucleation mechanism (HEN). Here, we postulate that the different nucleation pathways can serve as a mechanistic basis for an etiological classification of amyloidopathies, where hereditary forms generally follow the HON pathway, while sporadic forms follow seed-induced (prions) or surface-induced (including microbially induced) HEN pathways. Critically, the conformational and biophysical amyloid transformation results in loss-of-function (LOF) of the original natively folded and soluble protein. This LOF can, at least initially, be the mechanism of amyloid toxicity even before amyloid accumulation reaches toxic levels. By highlighting the important role of non-protein species in amyloid formation and LOF mechanisms of toxicity, we propose a generalized mechanistic framework that could help better understand the diverse etiology of amyloid diseases and offer new opportunities for therapeutic interventions, including replacement therapies.

Thermodynamics of amyloid fibril formation from chemical depolymerization

Amyloid fibrils are homo-molecular protein polymers that play an important role in disease and biological function. While much is known about their kinetics and mechanisms of formation, the origin and magnitude of their thermodynamic stability has received significantly less attention. This is despite the fact that the thermodynamic stability of amyloid fibrils is an important determinant of their lifetimes and processing in vivo. Here we use depolymerization by chemical denaturants of amyloid fibrils of two different proteins (PI3K-SH3 and glucagon) at different concentrations and show that the previously applied isodesmic linear polymerization model is an oversimplification that does not capture the concentration dependence of chemical depolymerization of amyloid fibrils. We show that cooperative polymerization, which is compatible with the picture of amyloid formation as a nucleated polymerization process, is able to quantitatively describe the thermodynamic data. We use this combined experimental and conceptual framework in order to probe the ionic strength dependence of amyloid fibril stability. In combination with previously published data on the ionic strength dependence of amyloid fibril growth kinetics, our results provide strong evidence for the product-like nature of the transition state of amyloid fibril growth.

Structural conformation and self-assembly process of p31-43 gliadin peptide in aqueous solution. Implications for celiac disease

Celiac disease (CeD) is a highly prevalent chronic immune-mediated enteropathy developed in genetically predisposed individuals after ingestion of a group of wheat proteins (called gliadins and glutenins). The 13mer α-gliadin peptide, p31-43, induces proinflammatory responses, observed by in vitro assays and animal models, that may contribute to innate immune mechanisms of CeD pathogenesis. Since a cellular receptor for p31-43 has not been identified, this raises the question of whether this peptide could mediate different biological effects. In this work, we aimed to characterize the p31-43 secondary structure by different biophysical and in silico techniques. By dynamic light scattering and using an oligomer/fibril-sensitive fluorescent probe, we showed the presence of oligomers of this peptide in solution. Furthermore, atomic force microscopy analysis showed p31-43 oligomers with different height distribution. Also, peptide concentration had a very strong influence on peptide self-organization process. Oligomers gradually increased their size at lower concentration. Whereas, at higher ones, oligomers increased their complexity, forming branched structures. By CD, we observed that p31-43 self-organized in a polyproline II conformation in equilibrium with β-sheets-like structures, whose pH remained stable in the range of 3-8. In addition, these findings were supported by molecular dynamics simulation. The formation of p31-43 nanostructures with increased β-sheet structure may help to explain the molecular etiopathogenesis in the induction of proinflammatory effects and subsequent damage at the intestinal mucosa in CeD.

The growth of amyloid fibrils: rates and mechanisms

Amyloid fibrils are β-sheet-rich linear protein polymers that can be formed by a large variety of different proteins. These assemblies have received much interest in recent decades, due to their role in a range of human disorders. However, amyloid fibrils are also found in a functional context, whereby their structural, mechanical and thermodynamic properties are exploited by biological systems. Amyloid fibrils form through a nucleated polymerisation mechanism with secondary processes acting in many cases to amplify the number of fibrils. The filamentous nature of amyloid fibrils implies that the fibril growth rate is, by several orders of magnitude, the fastest step of the overall aggregation reaction. This article focusses specifically on in vitro experimental studies of the process of amyloid fibril growth, or elongation, and summarises the state of knowledge of its kinetics and mechanisms. This work attempts to provide the most comprehensive summary, to date, of the available experimental data on amyloid fibril elongation rate constants and the temperature and concentration dependence of amyloid fibril elongation rates. These data are compared with those from other types of protein polymers. This comparison with data from other polymerising proteins is interesting and relevant because many of the basic ideas and concepts discussed here were first introduced for non-amyloid protein polymers, most notably by the Japanese school of Oosawa and co-workers for cytoskeletal filaments.

Kinetic regime of Ca2+ and Mg2+-induced aggregation of phosphorylase kinase at 40 °C

Many functions of phosphorylase kinase (PhK) are regulated by Ca²⁺ and Mg²⁺ ions. Ca²⁺ and Mg²⁺ ions stimulate activity of PhK, induce the changes in the tertiary and quaternary structure of the hexadecameric enzyme molecule, provoke association/aggregation of PhK molecules, enhance PhK binding to glycogen. To establish the kinetic regime of Ca²⁺ and Mg²⁺-induced aggregation of PhK from rabbit skeletal muscles at 40 °C, in the present work the kinetics of aggregation was studied at various protein concentrations using the dynamic light scattering. The proposed mechanism of aggregation involves the stage of unfolding of the protein molecule with retention of the integrity of its oligomeric structure, the nucleation stage and stages of the growth of protein aggregates. The initial rate of the aggregation process at the stage of aggregate growth depends linearly on the protein concentration. This means that the order of aggregation with respect to the protein is equal to unity and the aggregation rate is limited by the rate of protein unfolding. The rate constant of the first order characterizing the stage of protein unfolding was found to be equal to 0.071 min^-1 (40 mM Hepes, pH 6.8, 100 mM NaCl, 0.1 mM Ca²⁺, 10 mM Mg²⁺).

PrP (58-93) peptide from unstructured N-terminal domain of human prion protein forms amyloid-like fibrillar structures in the presence of Zn2+ ions

Many transition metal ions modulate the aggregation of different amyloid peptides. Substoichiometric zinc concentrations can inhibit aggregation, while an excess of zinc can accelerate the formation of cytotoxic fibrils. In this study, we report the fibrillization of the octarepeat domain to amyloid-like structures. Interestingly, this self-assembling process occurred only in the presence of Zn(ii) ions. The formed peptide aggregates are able to bind amyloid specific dyes thioflavin T and Congo red. Atomic force microscopy and transmission electron microscopy revealed the formation of long, fibrillar structures. X-ray diffraction and Fourier transform infrared spectroscopy studies of the formed assemblies confirmed the presence of cross-β structure. Two-component analysis of synchrotron radiation SAXS data provided the evidence for a direct decrease in monomeric peptide species content and an increase in the fraction of aggregates as a function of Zn(ii) concentration. These results could shed light on Zn(ii) as a toxic agent and on the metal ion induced protein misfolding in prion diseases.

The viral protein corona directs viral pathogenesis and amyloid aggregation

Artificial nanoparticles accumulate a protein corona layer in biological fluids, which significantly influences their bioactivity. As nanosized obligate intracellular parasites, viruses share many biophysical properties with artificial nanoparticles in extracellular environments and here we show that respiratory syncytial virus (RSV) and herpes simplex virus type 1 (HSV-1) accumulate a rich and distinctive protein corona in different biological fluids. Moreover, we show that corona pre-coating differentially affects viral infectivity and immune cell activation. In addition, we demonstrate that viruses bind amyloidogenic peptides in their corona and catalyze amyloid formation via surface-assisted heterogeneous nucleation. Importantly, we show that HSV-1 catalyzes the aggregation of the amyloid β-peptide (Aβ42), a major constituent of amyloid plaques in Alzheimer's disease, in vitro and in animal models. Our results highlight the viral protein corona as an acquired structural layer that is critical for viral-host interactions and illustrate a mechanistic convergence between viral and amyloid pathologies.

The Kinetics, Thermodynamics and Mechanisms of Short Aromatic Peptide Self-Assembly

The self-assembly of short aromatic peptides and peptide derivatives into a variety of different nano- and microstructures (fibrillar gels, crystals, spheres, plates) is a promising route toward the creation of bio-compatible materials with often unexpected and useful properties. Furthermore, such simple self-assembling systems have been proposed as model systems for the self-assembly of longer peptides, a process that can be linked to biological function and malfunction. Much effort has been made in the last 15 years to explore the space of peptide sequences, chemical modifications and solvent conditions in order to maximise the diversity of assembly morphologies and properties. However, quantitative studies of the corresponding mechanisms of, and driving forces for, peptide self-assembly have remained relatively scarce until recently. In this chapter we review the current state of understanding of the thermodynamic driving forces and self-assembly mechanisms of short aromatic peptides into supramolecular structures. We will focus on experimental studies of the assembly process and our perspective will be centered around diphenylalanine (FF), a key motif of the amyloid β sequence and a paradigmatic self-assembly building block. Our main focus is the basic physical chemistry and key structural aspects of such systems, and we will also compare the mechanism of dipeptide aggregation with that of longer peptide sequences into amyloid fibrils, with discussion on how these mechanisms may be revealed through detailed analysis of growth kinetics, thermodynamics and other fundamental properties of the aggregation process.

Biomolecular Assemblies: Moving from Observation to Predictive Design

Biomolecular assembly is a key driving force in nearly all life processes, providing structure, information storage, and communication within cells and at the whole organism level. These assembly processes rely on precise interactions between functional groups on nucleic acids, proteins, carbohydrates, and small molecules, and can be fine-tuned to span a range of time, length, and complexity scales. Recognizing the power of these motifs, researchers have sought to emulate and engineer biomolecular assemblies in the laboratory, with goals ranging from modulating cellular function to the creation of new polymeric materials. In most cases, engineering efforts are inspired or informed by understanding the structure and properties of naturally occurring assemblies, which has in turn fueled the development of predictive models that enable computational design of novel assemblies. This Review will focus on selected examples of protein assemblies, highlighting the story arc from initial discovery of an assembly, through initial engineering attempts, toward the ultimate goal of predictive design. The aim of this Review is to highlight areas where significant progress has been made, as well as to outline remaining challenges, as solving these challenges will be the key that unlocks the full power of biomolecules for advances in technology and medicine.

On the Applicability of Force Fields To Study the Aggregation of Amyloidogenic Peptides Using Molecular Dynamics Simulations

Molecular dynamics simulations play an essential role in understanding biomolecular processes such as protein aggregation at temporal and spatial resolutions which are not attainable by experimental methods. For a correct modeling of protein aggregation, force fields must accurately represent molecular interactions. Here, we study the effect of five different force fields on the oligomer formation of Alzheimer's Aβ_16-22 peptide and two of its mutants: Aβ_16-22(F19V,F20V), which does not form fibrils, and Aβ_16-22(F19L) which forms fibrils faster than the wild type. We observe that while oligomer formation kinetics depends strongly on the force field, structural properties, such as the most relevant protein-protein contacts, are similar between them. The oligomer formation kinetics obtained with different force fields differ more from each other than the kinetics between aggregating and nonaggregating peptides simulated with a single force field. We discuss the difficulties in comparing atomistic simulations of amyloid oligomer formation with experimental observables.

Fibrous Protein Self-Assembly in Biomimetic Materials

Protein self-assembly processes, by which polypeptides interact and independently form multimeric structures, lead to a wide array of different endpoints. Structures formed range from highly ordered molecular crystals to amorphous aggregates. Order arises in the system from a balance between many low-energy processes occurring due to a set of interactions between residues in a chain, between residues in different chains, and between solute and solvent. In Nature, self-assembling protein systems have evolved over millions of years to organize into supramolecular structures, optimized for specific functions, with this propensity determined by the sequence of their constituent amino acids, of which only 20 are encoded in DNA. The structural materials that arise from biological self-assembly can display remarkable mechanical properties, often as a result of hierarchical structure on the nano- and microscales, and much research has been devoted to mimicking and exploiting these properties for a variety of end uses. This work presents a review of a range of studies in which biological functions are effectively reproduced through the design of self-assembling fibrous protein systems.

Key Residues for the Formation of Aβ42 Amyloid Fibrils

Formation of amyloid fibrils by Aβ42 protein is a pathological hallmark of Alzheimer's disease. Aβ42 fibrillization is a nucleation-dependent polymerization process, in which nucleation is the rate-limiting step. Structural knowledge of the fibril nucleus is important to understand the molecular mechanism of Aβ aggregation and is also critical for successful modulation of the fibrillization process. Here, we used a scanning mutagenesis approach to study the role of each residue position in Aβ42 fibrillization kinetics. The side chain we used to replace the native residue is a nitroxide spin label called R1, which was introduced using site-directed spin labeling. In this systematic study, all residue positions of Aβ42 sequence were studied, and we identified six key residues for the Aβ42 fibril formation: H14, E22, D23, G33, G37, and G38. Our results suggest that charges at positions 22 and 23 and backbone flexibilities at positions 33, 37, and 38 play key roles in Aβ42 fibrillization kinetics. Our results also suggest that the formation of a β-strand at residues 15-21 is an important feature in Aβ42 fibril nucleus. In overall evaluation of all of the mutational effects on fibrillization kinetics, we found that the thioflavin T fluorescence at the aggregation plateau is a poor indicator of aggregation rates.

Insights into the Molecular Mechanisms of Alzheimer's and Parkinson's Diseases with Molecular Simulations: Understanding the Roles of Artificial and Pathological Missense Mutations in Intrinsically Disordered Proteins Related to Pathology

Amyloid-β and α-synuclein are intrinsically disordered proteins (IDPs), which are at the center of Alzheimer's and Parkinson's disease pathologies, respectively. These IDPs are extremely flexible and do not adopt stable structures. Furthermore, both amyloid-β and α-synuclein can form toxic oligomers, amyloid fibrils and other type of aggregates in Alzheimer's and Parkinson's diseases. Experimentalists face challenges in investigating the structures and thermodynamic properties of these IDPs in their monomeric and oligomeric forms due to the rapid conformational changes, fast aggregation processes and strong solvent effects. Classical molecular dynamics simulations complement experiments and provide structural information at the atomic level with dynamics without facing the same experimental limitations. Artificial missense mutations are employed experimentally and computationally for providing insights into the structure-function relationships of amyloid-β and α-synuclein in relation to the pathologies of Alzheimer's and Parkinson's diseases. Furthermore, there are several natural genetic variations that play a role in the pathogenesis of familial cases of Alzheimer's and Parkinson's diseases, which are related to specific genetic defects inherited in dominant or recessive patterns. The present review summarizes the current understanding of monomeric and oligomeric forms of amyloid-β and α-synuclein, as well as the impacts of artificial and pathological missense mutations on the structural ensembles of these IDPs using molecular dynamics simulations. We also emphasize the recent investigations on residual secondary structure formation in dynamic conformational ensembles of amyloid-β and α-synuclein, such as β-structure linked to the oligomerization and fibrillation mechanisms related to the pathologies of Alzheimer's and Parkinson's diseases. This information represents an important foundation for the successful and efficient drug design studies.

Multistep Conformation Selection in Amyloid Assembly

Defining pathways for amyloid assembly could impact therapeutic strategies for as many as 50 disease states. Here we show that amyloid assembly is subject to different forces regulating nucleation and propagation steps and provide evidence that the more global β-sheet/β-sheet facial complementarity is a critical determinant for amyloid nucleation and structural selection.

Femtosecond-Laser-Enhanced Amyloid Fibril Formation of Insulin

Femtosecond (fs)-laser-induced crystallization as a novel crystallization technique was proposed for the first time by our group, where the crystallization time can be significantly shortened under fs laser irradiation. Similarly, we have further extended our investigation to amyloid fibril formation, also known as a nucleation-dependence process. Here we demonstrate that the necessary time for amyloid fibril formation can be significantly shortened by fs laser irradiation, leading to favorable enhancement. The enhancement was confirmed by both spectral measurements and direct observations of amyloid fibrils. The thioflavin T fluorescence intensity of laser-irradiated solution increased earlier than that of the control solution, and such a difference was simultaneously revealed by ellipticity changes. At the same time before intensity saturation in fluorescence, the number of amyloid fibrils obtained under laser irradiation was generally more than that in the control solution. Besides, such an enhancement is correlated to the laser power threshold of cavitation bubbling. Possible mechanisms are proposed by referring to fs-laser-induced crystallization and ultrasonication-induced amyloid fibril formation.