Interaction of Aβ42 with Membranes Triggers the Self-Assembly into Oligomers

Exploring Heparan Sulfate Proteoglycans as Mediators of Human Mesenchymal Stem Cell Neurogenesis

Alzheimer's disease (AD) and traumatic brain injury (TBI) are major public health issues worldwide, with over 38 million people living with AD and approximately 48 million people (27-69 million) experiencing TBI annually. Neurodegenerative conditions are characterised by the accumulation of neurotoxic amyloid beta (Aβ) and microtubule-associated protein Tau (Tau) with current treatments focused on managing symptoms rather than addressing the underlying cause. Heparan sulfate proteoglycans (HSPGs) are a diverse family of macromolecules that interact with various proteins and ligands and promote neurogenesis, a process where new neural cells are formed from stem cells. The syndecan (SDC) and glypican (GPC) HSPGs have been implicated in AD pathogenesis, acting as drivers of disease, as well as potential therapeutic targets. Human mesenchymal stem cells (hMSCs) provide an attractive therapeutic option for studying and potentially treating neurodegenerative diseases due to their relative ease of isolation and subsequent extensive in vitro expansive potential. Understanding how HSPGs regulate protein aggregation, a key feature of neurodegenerative disorders, is essential to unravelling the underlying disease processes of AD and TBI, as well as any link between these two neurological disorders. Further research may validate HSPG, specifically SDCs or GPCs, use as neurodegenerative disease targets, either via driving hMSC stem cell therapy or direct targeting.

Binding mechanism of full-length Aβ40 peptide to a mixed lipid bilayer

The destructive effect of Aβ peptides on membranes is an important source of its cytotoxicity in the pathogenesis of Alzheimer's disease. We have investigated the binding mechanism between the Aβ42 peptide and bilayer in our former work. However, as another abundant form of Aβ peptides in the physiological environment, the binding mechanism between Aβ40 peptide and the lipid bilayer still remains ambiguous. Hence, we performed all-atom simulations on the Aβ40 peptides with the lipid bilayer herein using replica exchange with the solute tempering 2 method. We obtained four major binding models with the hydrophobic C-terminus as the most preferable binding region. Hydrophobic residues and positively charged residues are the principal residues involved in the peptide-bilayer interactions. Aβ40 peptides in our simulation mainly adopt a β-rich conformation in both bound and unbound states. Besides, we determined peptide-water interactions and found that bound peptides prefer forming hydrogen bonds with water molecules than unbound peptides. Our findings herein may provide new insights for the in-depth understanding of the membrane-destructive mechanism of Aβ peptides.

Aggregation, structure and water permeability of membrane-embedded helical Aβ oligomers

It is widely recognized that membranes can facilitate the aggregation of amyloid-β (Aβ) peptides, while Aβ can in turn cause membrane damage. Many studies focus on the peptide-membrane interactions of Aβ oligomers with β-rich structures. However, the exact aggregation and toxicity mechanism of the membrane-embedded helical Aβ oligomers remain ambiguous. Herein, the molecular dynamics simulations were performed on membrane-embedded helical Aβ42 peptides. Initiated by eight Aβ42 monomers embedded in a lipid bilayer, the monomers aggregate into oligomers with stable transmembrane helix structures. With the aggregation of peptides, the membrane perturbations caused by Aβ aggregates decrease. The molecular architectures of oligomers were characterized and a helix-rich octamer stabilized by an annular network of hydrogen bonds was observed. The oligomers demonstrate the capability to assist transmembrane water transport. Our study may provide new insights for the investigation of transmembrane Aβ oligomers.

Anti-amyloid potential of some phytochemicals against Aβ-peptide and α-synuclein, tau, prion, and Huntingtin protein

Some molecules self-assemble to create complex structures through molecular self-assembly. Hydrogel preparation, tissue repair, and therapeutic drug delivery are a few applications of molecular self-assembly. However, the self-assembly of amino acids, peptides, and proteins forms amyloid fibrils, resulting in various disorders, most notably neurodegenerative ailments. Examples include the self-assembly of phenylalanine, which causes phenylketonuria; Aβ, which causes Alzheimer's disease; the tau protein, which causes both Alzheimer's and Parkinson's diseases; and α-synuclein, which causes Parkinson's illness. This review provides information related to phytochemicals of great significance that can prevent the formation of, or destabilize, amino acid, peptide, and protein self-assemblies.

Recent Computational Advances Regarding Amyloid-β and Tau Membrane Interactions in Alzheimer's Disease

The interactions of amyloid proteins with membranes have been subject to many experimental and computational studies, as these interactions contribute in part to neurodegenerative diseases. In this review, we report on recent simulations that have focused on the adsorption and insertion modes of amyloid-β and tau proteins in membranes. The atomistic-resolution characterization of the conformational changes of these amyloid proteins upon lipid cell membrane and free lipid interactions is of interest to rationally design drugs targeting transient oligomers in Alzheimer's disease.

Morphometric and Nanomechanical Screening of Peripheral Blood Cells with Atomic Force Microscopy for Label-Free Assessment of Alzheimer's Disease, Parkinson's Disease, and Amyotrophic Lateral Sclerosis

Neurodegenerative disorders (NDDs) are complex, multifactorial disorders with significant social and economic impact in today's society. NDDs are predicted to become the second-most common cause of death in the next few decades due to an increase in life expectancy but also to a lack of early diagnosis and mainly symptomatic treatment. Despite recent advances in diagnostic and therapeutic methods, there are yet no reliable biomarkers identifying the complex pathways contributing to these pathologies. The development of new approaches for early diagnosis and new therapies, together with the identification of non-invasive and more cost-effective diagnostic biomarkers, is one of the main trends in NDD biomedical research. Here we summarize data on peripheral biomarkers, biofluids (cerebrospinal fluid and blood plasma), and peripheral blood cells (platelets (PLTs) and red blood cells (RBCs)), reported so far for the three most common NDDs-Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS). PLTs and RBCs, beyond their primary physiological functions, are increasingly recognized as valuable sources of biomarkers for NDDs. Special attention is given to the morphological and nanomechanical signatures of PLTs and RBCs as biophysical markers for the three pathologies. Modifications of the surface nanostructure and morphometric and nanomechanical signatures of PLTs and RBCs from patients with AD, PD, and ALS have been revealed by atomic force microscopy (AFM). AFM is currently experiencing rapid and widespread adoption in biomedicine and clinical medicine, in particular for early diagnostics of various medical conditions. AFM is a unique instrument without an analog, allowing the generation of three-dimensional cell images with extremely high spatial resolution at near-atomic scale, which are complemented by insights into the mechanical properties of cells and subcellular structures. Data demonstrate that AFM can distinguish between the three pathologies and the normal, healthy state. The specific PLT and RBC signatures can serve as biomarkers in combination with the currently used diagnostic tools. We highlight the strong correlation of the morphological and nanomechanical signatures between RBCs and PLTs in PD, ALS, and AD.

Lipid rafts and human diseases: why we need to target gangliosides

Gangliosides are functional components of membrane lipid rafts that control critical functions in cell communication. Many pathologies involve raft gangliosides, which therefore represent an approach of choice for developing innovative therapeutic strategies. Beginning with a discussion of what a disease is (and is not), this review lists the major human pathologies that involve gangliosides, which includes cancer, diabetes, and infectious and neurodegenerative diseases. In most cases, the problem is due to a protein whose binding to gangliosides either creates a pathological condition or impairs a physiological function. Then, I draw up an inventory of the different molecular mechanisms of protein-ganglioside interactions. I propose to classify the ganglioside-binding domains of proteins into four categories, which I name GBD-1, GBD-2, GBD-3, and GBD-4. This structural and functional classification could help to rationalize the design of innovative molecules capable of disrupting the binding of selected proteins to gangliosides without generating undesirable effects. The biochemical specificities of gangliosides expressed in the human brain must also be taken into account to improve the reliability of animal models (or any animal-free alternative) of Alzheimer's and Parkinson's diseases.

Effect of the Lipid Landscape on the Efficacy of Cell-Penetrating Peptides

Every cell biological textbook teaches us that the main role of the plasma membrane is to separate cells from their neighborhood to allow for a controlled composition of the intracellular space. The mostly hydrophobic nature of the cell membrane presents an impenetrable barrier for most hydrophilic molecules larger than 1 kDa. On the other hand, cell-penetrating peptides (CPPs) are capable of traversing this barrier without compromising membrane integrity, and they can do so on their own or coupled to cargos. Coupling biologically and medically relevant cargos to CPPs holds great promise of delivering membrane-impermeable drugs into cells. If the cargo is able to interact with certain cell types, uptake of the CPP-drug complex can be tailored to be cell-type-specific. Besides outlining the major membrane penetration pathways of CPPs, this review is aimed at deciphering how properties of the membrane influence the uptake mechanisms of CPPs. By summarizing an extensive body of experimental evidence, we argue that a more ordered, less flexible membrane structure, often present in the very diseases planned to be treated with CPPs, decreases their cellular uptake. These correlations are not only relevant for understanding the cellular biology of CPPs, but also for rationally improving their value in translational or clinical applications.

Role of phosphatidylserine in amyloid-beta oligomerization at asymmetric phospholipid bilayers

Amyloid-beta (Aβ_1-42) aggregation triggers neurotoxicity and is linked to Alzheimer's disease. Aβ_1-42 oligomers, rather than extended fibrils, adhere to the cell membrane, causing cell death. Phosphatidylserine (PS), an anionic phospholipid, is prevalent in neuronal membranes (< 20 molar percentage) and, while isolated to the cytoplasmic leaflet of the membrane in healthy cells, its exposure in apoptotic cells and migration to exoplasmic leaflet is triggered by oxidative damage to the membrane. It is widely believed that PS plays a crucial role in the Aβ peptide interaction in the membranes of neuronal cells. However, due to the complexity of the cell membrane, it can be challenging to address molecular level understanding of the PS-Aβ binding and oligomerization processes. Herein, we use microcavity supported lipid bilayers (MSLBs) to analyse PS and Aβ_1-42 binding, oligomer formation, and membrane damage. MSLBs are a useful model to evaluate protein-membrane interactions because of their cell-like dual aspect fluidity, their addressability and compositional versatility. We used electrochemical impedance spectroscopy (EIS) and confocal fluorescence microscopy to compare the impact of Aβ_1-42 on simple zwitterioinic membrane, dioleoylphosphatidylcholine (DOPC), with MSLBs comprised of transversally asymmetric binary DOPC and dioleoylphosphatidylserine (DOPS). Monomeric Aβ_1-42 adsorbs weakly to the pristine zwitterionic DOPC membrane without aggregation. Using a membrane integrity test, with pyranine trapped within the cavities beneath the membrane, Aβ_1-42 exposure did not result in pyranine leakage, indicating that DOPC membranes were intact. When 10 mol% DOPS was doped asymmetrically into the membrane's outer leaflet, oligomerization of Aβ_1-42 monomer was evident in EIS and atomic force microscopy (AFM), and confocal imaging revealed that membrane damage, resulted in extensive pyranine leakage from the pores. The effects were time, and DOPS and Aβ_1-42 concentration-dependent. Membrane pore formation was visible within 30 minutes, and oligomerization, membrane-oligomer multilayer, and Aβ_1-42 fibril formation evident over 3 to 18 hours. In asymmetric membranes with DOPS localized to the lower leaflet, optothermally (laser induced) damage increased local DOPS concentrations at the distal leaflet, promoting Aβ_1-42 aggregation.

Protein Self-Assembly at the Liquid-Surface Interface. Surface-Mediated Aggregation Catalysis

Protein self-assembly into aggregates of various morphologies is a ubiquitous phenomenon in physical chemistry and biophysics. The critical role of amyloid assemblies in the development of diseases, neurodegenerative diseases especially, highlights the importance of understanding the mechanistic picture of the self-assembly process. The translation of this knowledge to the development of efficient preventions and treatments for diseases requires designing experiments at conditions mimicking those in vivo. This Perspective reviews data satisfying two major requirements: membrane environment and physiologically low concentrations of proteins. Recent progress in experiments and computational modeling resulted in a novel model for the amyloid aggregation process at the membrane-liquid interface. The self-assembly under such conditions has a number of critical features, further understanding of which can lead to the development of efficient preventive means and treatments for Alzheimer's and other devastating neurodegenerative disorders.

Binding Models of Aβ42 Peptide with Membranes Explored by Molecular Simulations

One of the factors contributing to the toxicity of amyloid-β (Aβ) peptides is the destruction of membrane integrity through Aβ peptide-membrane interactions. The binding of Aβ peptides to membranes has been studied by experiments and theoretical simulations extensively. The exact binding mechanism, however, still remains elusive. In the present study, the molecular basis of the peptide-bilayer binding mechanism of the full-length Aβ42 monomer with POPC/POPS/CHOL bilayers is investigated by all-atom (AA) simulations. Three main binding models in coil, bend, and turn structures are obtained. Model 1 of the three models with the central hydrophobic core (CHC) buried inside the membrane is the dominant binding model. The structural features of the peptide, the peptide-bilayer interacting regions, the intrapeptide interactions, and peptide-water interactions are studied. The binding of the Aβ42 monomer to the POPC/POPS/CHOL bilayer is also explored by coarse-grained (CG) simulations as a complement. Both the AA and CG simulations show that residues in CHC prefer forming interactions with the bilayer, indicating the crucial role of CHC in peptide-bilayer binding. Our results can provide new insights for the investigation of the peptide-bilayer binding mechanism of the Aβ peptide.

Amyloid-β: Structure, Function, and Pathophysiological Significance in Neurodegenerative Diseases

The rate of dementia continues to increase worldwide; however, there currently exist no therapeutic strategies for this condition [...]

Unraveling Membrane Perturbations Caused by the Bacterial Riboregulator Hfq

Hfq is a pleiotropic regulator that mediates several aspects of bacterial RNA metabolism. The protein notably regulates translation efficiency and RNA decay in Gram-negative bacteria, usually via its interaction with small regulatory RNAs. Previously, we showed that the Hfq C-terminal region forms an amyloid-like structure and that these fibrils interact with membranes. The immediate consequence of this interaction is a disruption of the membrane, but the effect on Hfq structure was unknown. To investigate details of the mechanism of interaction, the present work uses different in vitro biophysical approaches. We show that the Hfq C-terminal region influences membrane integrity and, conversely, that the membrane specifically affects the amyloid assembly. The reported effect of this bacterial master regulator on membrane integrity is discussed in light of the possible consequence on small regulatory RNA-based regulation.

Free Cholesterol Accelerates Aβ Self-Assembly on Membranes at Physiological Concentration

The effects of membranes on the early-stage aggregation of amyloid β (Aβ) have come to light as potential mechanisms by which neurotoxic species are formed in Alzheimer’s disease. We have shown that direct Aβ-membrane interactions dramatically enhance the Aβ aggregation, allowing for oligomer assembly at physiologically low concentrations of the monomer. Membrane composition is also a crucial factor in this process. Our results showed that apart from phospholipids composition, cholesterol in membranes significantly enhances the aggregation kinetics. It has been reported that free cholesterol is present in plaques. Here we report that free cholesterol, along with its presence inside the membrane, further accelerate the aggregation process by producing aggregates more rapidly and of significantly larger sizes. These aggregates, which are formed on the lipid bilayer, are able to dissociate from the surface and accumulate in the bulk solution; the presence of free cholesterol accelerates this dissociation as well. All-atom molecular dynamics simulations show that cholesterol binds Aβ monomers and significantly changes the conformational sampling of Aβ monomer; more than doubling the fraction of low-energy conformations compared to those in the absence of cholesterol, which can contribute to the aggregation process. The results indicate that Aβ-lipid interaction is an important factor in the disease prone amyloid assembly process.

Hybrid resolution molecular dynamics simulations of amyloid proteins interacting with membranes

A broad range of human diseases, including Alzheimer's and Parkinson's diseases, arise from or have as key players intrinsically disordered proteins. The aggregation of these amyloid proteins into fibrillar aggregates are the key events of such diseases. Characterizing the conformation dynamics of the proteins involved is crucial for understanding the molecular mechanisms of aggregation, which in turn is important for drug development efforts against these diseases. Computational approaches have provided extensive detail about some steps of the aggregation process, however the biologically relevant elements responsible for the aggregation and or aggregation propagation have not been fully characterized. Here we describe a hybrid resolution molecular dynamics simulation method that can be employed to investigate the interaction of amyloid proteins with lipid membranes, shown to dramatically accelerate the aggregation propensity of amyloid proteins. The hybrid resolution method enables routine and accurate simulation of multi-protein and complex membrane systems, mimicking biologically relevant lipid membranes, on microsecond time scales. The hybrid resolution method was applied to computer modeling of the interactions of α -synuclein protein with a mixed lipid bilayer.

Topographically smooth and stable supported lipid bilayer for high-resolution AFM studies

The cellular membrane has been identified to play a critical role in various biological processes including the assembly of biological systems. Membranes are complex, primarily two-dimensional assemblies with varied lipid compositions depending on the particular region of the cell. Supported lipid bilayers are considered as appropriate models for physio-chemical studies of membranes including numerous single molecule techniques. Atomic force microscopy (AFM) as a topographic technique is a fully appropriate single molecule technique capable of direct observation of molecular processes on membranes. However, reliable experimental AFM studies require the preparation of the bilayer with a sub-nanometer smooth morphology, which remains stable over long-time observation. Here we present the methodology, which allows one to prepare a smooth, stable, structurally homogeneous lipid bilayer without the presence of any trapped vesicles. We described the application of such lipid bilayers to probe time-dependent early stages of aggregation of monomeric amyloid proteins. Importantly, the proposed methodology can be extended to bilayers with various compositions, by incorporating different lipids for on-membrane aggregation study including cholesterol. Furthermore, this methodology development allowed us to monitor the aggregation of amyloid protein at its physiologically relevant low protein concentration. The flexibility of altering the membrane composition allows to identify the specific role of a particular lipid towards the aggregation kinetics, revealing the plausible mechanism of disease development.

Biological and methodological complexities of beta-amyloid peptide: Implications for Alzheimer's disease research

Although controversial, the amyloid cascade hypothesis remains central to the Alzheimer's disease (AD) field and posits amyloid-beta (Aβ) as the central factor initiating disease onset. In recent years, there has been an increase in emphasis on studying the role of low molecular weight aggregates, such as oligomers, which are suggested to be more neurotoxic than fibrillary Aβ. Other Aβ isoforms, such as truncated Aβ, have also been implicated in disease. However, developing a clear understanding of AD pathogenesis has been hampered by the complexity of Aβ biochemistry in vitro and in vivo. This review explores factors contributing to the lack of consistency in experimental approaches taken to model Aβ aggregation and toxicity and provides an overview of the different techniques available to analyse Aβ, such as electron and atomic force microscopy, nuclear magnetic resonance spectroscopy, dye-based assays, size exclusion chromatography, mass spectrometry and SDS-PAGE. The review also explores how different types of Aβ can influence Aβ aggregation and toxicity, leading to variation in experimental outcomes, further highlighting the need for standardisation in Aβ preparations and methods used in current research.

Frustrated peptide chains at the fibril tip control the kinetics of growth of amyloid-β fibrils

Amyloid fibrillization is an exceedingly complex process in which incoming peptide chains bind to the fibril while concertedly folding. The coupling between folding and binding is not fully understood. We explore the molecular pathways of association of Aβ40 monomers to fibril tips by combining time-resolved in situ scanning probe microscopy with molecular modeling. The comparison between experimental and simulation results shows that a complex supported by nonnative contacts is present in the equilibrium structure of the fibril tip and impedes fibril growth in a supersaturated solution. The unraveling of this frustrated state determines the rate of fibril growth. The kinetics of growth of freshly cut fibrils, in which the bulk fibril structure persists at the tip, complemented by molecular simulations, indicate that this frustrated complex comprises three or four monomers in nonnative conformations and likely is contained on the top of a single stack of peptide chains in the fibril structure. This pathway of fibril growth strongly deviates from the common view that the conformational transformation of each captured peptide chain is templated by the previously arrived peptide. The insights into the ensemble structure of the frustrated complex may guide the search for suppressors of Aβ fibrillization. The uncovered dynamics of coupled structuring and assembly during fibril growth are more complex than during the folding of most globular proteins, as they involve the collective motions of several peptide chains that are not guided by a funneled energy landscape.

Searching for conditions of protein self-assembly by protein crystallization screening method

The self-assembly of biomacromolecules is an extremely important process. It is potentially useful in the fields of life science and materials science. To carry out the study on the self-assembly of proteins, it is necessary to find out the suitable self-assembly conditions, which have always been a challenging task in practice. Inspired by the screening technique in the field of protein crystallization, we proposed using the same screening technique for seeking suitable protein self-assembly conditions. Based on this consideration, we selected 5 proteins (β-lactoglobulin, hemoglobin, pepsin, lysozyme, α-chymotrypsinogen (II) A) together with 5 screening kits (Index^TM, BML, Morpheus, JCSG, PEG/Ion Screen^TM) to investigate the performance of these crystallization screening techniques in order to discover new optimized conditions of protein self-assembly. The screens were all kept at 293 K for certain days, and were analyzed using optical microscope, scanning electron microscope, transmission electron microscope, atomic force microscope, fluorescence microscope, and atomic absorption spectroscope. The results demonstrated that the method of protein crystallization screening can be successfully applied in the screening of self-assembly conditions. This method is fast, high throughput, and easily implemented in an automated system, with a low protein consumption feature. These results suggested that such strategy can be applied to finding new conditions or forms in routine research of protein self-assembly. KEY POINTS: • Protein crystallization screening method is successfully applied in the screening of self-assembly conditions. • This screening method can be applied on various kinds of proteins and possess a feature of low protein consumption. • This screening method is fast, high throughput, and easily implemented in an automated system.

Cholesterol in Membranes Facilitates Aggregation of Amyloid β Protein at Physiologically Relevant Concentrations

The formation of amyloid β (1-42) (Aβ42) oligomers is considered to be a critical step in the development of Alzheimer's disease (AD). However, the mechanism underlying this process at physiologically low concentrations of Aβ42 remains unclear. We have previously shown that oligomers assemble at such low Aβ42 monomer concentrations in vitro on phospholipid membranes. We hypothesized that membrane composition is the factor controlling the aggregation process. Accumulation of cholesterol in membranes is associated with AD development, suggesting that insertion of cholesterol into membranes may initiate the Aβ42 aggregation, regardless of a low monomer concentration. We used atomic force microscopy (AFM) to test the hypothesis and directly visualize the aggregation process of Aβ42 on the surface of a lipid bilayer depending on the cholesterol presence. Time-lapse AFM imaging unambiguously demonstrates that cholesterol in the lipid bilayer significantly enhances the aggregation process of Aβ42 at nanomolar monomer concentration. Quantitative analysis of the AFM data shows that both the number of Aβ42 oligomers and their sizes grow when cholesterol is present. Importantly, the aggregation process is dynamic, so the aggregates assembled on the membrane can dissociate from the bilayer surface into the bulk solution. Computational modeling demonstrated that the lipid bilayer containing cholesterol had an elevated affinity to Aβ42. Moreover, monomers adopted the aggregation-prone conformations present in amyloid fibrils. The results lead to the model for the on-surface aggregation process in which the self-assembly of Aβ oligomers is controlled by the lipid composition of cellular membranes.

Therapeutic Strategies to Reduce the Toxicity of Misfolded Protein Oligomers

The aberrant aggregation of proteins is implicated in the onset and pathogenesis of a wide range of neurodegenerative disorders, including Alzheimer’s and Parkinson’s diseases. Mounting evidence indicates that misfolded protein oligomers produced as intermediates in the aggregation process are potent neurotoxic agents in these diseases. Because of the transient and heterogeneous nature of these elusive aggregates, however, it has proven challenging to develop therapeutics that can effectively target them. Here, we review approaches aimed at reducing oligomer toxicity, including (1) modulating the oligomer populations (e.g., by altering the kinetics of aggregation by inhibiting, enhancing, or redirecting the process), (2) modulating the oligomer properties (e.g., through the size–hydrophobicity–toxicity relationship), (3) modulating the oligomer interactions (e.g., by protecting cell membranes by displacing oligomers), and (4) reducing oligomer toxicity by potentiating the protein homeostasis system. We analyze examples of these complementary approaches, which may lead to the development of compounds capable of preventing or treating neurodegenerative disorders associated with protein aggregation.

Amyloid B-Protein Aggregation at Physiologically Relevant Concentrations. A Critical Role of Membranes

BACKGROUND

The aggregation of amyloid beta (Aβ) is a self-assembly process that results in the production of fibrillar structures along with neurotoxic aggregates. However, in the vast majority studies in vitro the required Ab concentrations is several orders higher of the physiological relevant concentrations of Aβ; no aggregation is observed at physiological low nanomolar range of Aβ. This suggests that the assembly of Aβ in aggregates in vivo utilizes pathways different from those used in experiments in vitro.

RESULTS

The spontaneous assembly of Aβ oligomers within the physiologically relevant concentration range can occur, but it is the on-surface aggregation mechanism, in which the surface pays a role of the catalyst of the aggregation process. The model for the on-surface aggregation process suggests that the self-assembly of Aβ oligomers is initiated by the interaction of amyloid proteins with the cellular membrane. The membrane catalyzes amyloid aggregation by stabilizing an aggregation-prone conformation of amyloids. The lipid composition contributes to the membrane-mediated misfolding and aggregation of Aβ monomers.

CONCLUSION

Membrane-mediated aggregation catalysis explains a number of observations associated with the development of AD. The affinity of Aβ monomers to the membrane surface is the major factor defining the aggregation process rather than Aβ concentration. According to the model, the development of potential preventions for the interaction of monomeric amyloids with membrane can help control the aggregation process. This is a paradigm change for the development of efficient treatments, early diagnostics, and preventions for Alzheimer's disease.

Meeting report - NSF-sponsored workshop 'Progress and Prospects of Single-Molecule Force Spectroscopy in Biological and Chemical Sciences'

The goals of the workshop organized by Piotr Marszalek and Andres Oberhauser that took place between 29 August and 1 September 2019 at Duke University were to bring together leading experts and junior researchers to review past accomplishments, recent advances and limitations in the single-molecule force spectroscopy field, which examines nanomechanical forces in diverse biological processes and pathologies. Talks were organized into four sessions, and two in-depth roundtable discussion sessions were held.

Seeding and Growth of β-Amyloid Aggregates upon Interaction with Neuronal Cell Membranes

In recent years, the prevalence of amyloid neurodegenerative diseases such as Alzheimer's disease (AD) has significantly increased in developed countries due to increased life expectancy. This amyloid disease is characterized by the presence of accumulations and deposits of β-amyloid peptide (Aβ) in neuronal tissue, leading to the formation of oligomers, fibers, and plaques. First, oligomeric intermediates that arise during the aggregation process are currently thought to be primarily responsible for cytotoxicity in cells. This work aims to provide further insights into the mechanisms of cytotoxicity by studying the interaction of Aβ aggregates with Neuro-2a (N2a) neuronal cells and the effects caused by this interaction. For this purpose, we have exploited the advantages of advanced, multidimensional fluorescence microscopy techniques to determine whether different types of Aβ are involved in higher rates of cellular toxicity, and we measured the cellular stress caused by such aggregates by using a fluorogenic intracellular biothiol sensor. Stress provoked by the peptide is evident by N2a cells generating high levels of biothiols as a defense mechanism. In our study, we demonstrate that Aβ aggregates act as seeds for aggregate growth upon interacting with the cellular membrane, which results in cell permeability and damage and induces lysis. In parallel, these damaged cells undergo a significant increase in intracellular biothiol levels.

Amyloid-β (Aβ42) Peptide Aggregation Rate and Mechanism on Surfaces with Widely Varied Properties: Insights from Brownian Dynamics Simulations

Amyloid-β (Aβ) plaques, which form by aggregation of harmless Aβ peptide monomers into larger fibrils, are characteristic of neurodegenerative disorders such as Alzheimer's disease. Efforts to treat Alzheimer's disease focus on stopping or reversing the aggregation process that leads to fibril formation. However, effective treatments are elusive due to certain unknown aspects of the process. Many hypotheses point to disruption of cell membranes by adsorbed Aβ monomers or oligomers, but how Aβ behaves and aggregates on surfaces of widely varying properties, such as those present in a cell, is unclear. Elucidating the effects of various surfaces on the dynamics of Aβ and the kinetics of the aggregation process from bulk solution to a surface-adsorbed multimer can help identify what drives aggregation, leading to new methods of intervention by inhibitory drugs or other means. In this work, we used all-atom Brownian dynamics simulations to study the association of two distinct Aβ42 monomer conformations with a surface-adsorbed or free-floating Aβ42 dimer. We calculated the association time, surface interaction energy, surface diffusion coefficient, surface residence time, and the mechanism of association on four different surfaces and two different bulk solution scenarios. In the presence of a surface, the majority of monomers underwent a two-dimensional surface-mediated association that depended primarily on an Aβ42 electrostatic interaction with the self-assembled monolayer (SAM) surfaces. Moreover, aggregation could be inhibited greatly by surfaces with high affinity for Aβ42 and heterogeneous charge distribution. Our results can be used to identify new opportunities for disrupting or reversing the Aβ42 aggregation process.

High-Speed Atomic Force Microscopy Reveals the Structural Dynamics of the Amyloid-β and Amylin Aggregation Pathways

Individual Alzheimer's disease (AD) patients have been shown to have structurally distinct amyloid-β (Aβ) aggregates, including fibrils, in their brain. These findings suggest the possibility of a relationship between AD progression and Aβ fibril structures. Thus, the characterization of the structural dynamics of Aβ could aid the development of novel therapeutic strategies and diagnosis. Protein structure and dynamics have typically been studied separately. Most of the commonly used biophysical approaches are limited in providing substantial details regarding the combination of both structure and dynamics. On the other hand, high-speed atomic force microscopy (HS-AFM), which simultaneously visualizes an individual protein structure and its dynamics in liquid in real time, can uniquely link the structure and the kinetic details, and it can also unveil novel insights. Although amyloidogenic proteins generate heterogeneously aggregated species, including transient unstable states during the aggregation process, HS-AFM elucidated the structural dynamics of individual aggregates in real time in liquid without purification and isolation. Here, we review and discuss the HS-AFM imaging of amyloid aggregation and strategies to optimize the experiments showing findings from Aβ and amylin, which is associated with type II diabetes, shares some common biological features with Aβ, and is reported to be involved in AD.

Potential Therapeutic Approaches for Cerebral Amyloid Angiopathy and Alzheimer's Disease

Cerebral amyloid angiopathy (CAA) is a cerebrovascular disease directly implicated in Alzheimer’s disease (AD) pathogenesis through amyloid-β (Aβ) deposition, which may cause the development and progression of dementia. Despite extensive studies to explore drugs targeting Aβ, clinical benefits have not been reported in large clinical trials in AD patients or presymptomatic individuals at a risk for AD. However, recent studies on CAA and AD have provided novel insights regarding CAA- and AD-related pathogenesis. This work has revealed potential therapeutic targets, including Aβ drainage pathways, Aβ aggregation, oxidative stress, and neuroinflammation. The functional significance and therapeutic potential of bioactive molecules such as cilostazol and taxifolin have also become increasingly evident. Furthermore, recent epidemiological studies have demonstrated that serum levels of a soluble form of triggering receptor expressed on myeloid cells 2 (TREM2) may have clinical significance as a potential novel predictive biomarker for dementia incidence. This review summarizes recent advances in CAA and AD research with a focus on discussing future research directions regarding novel therapeutic approaches and predictive biomarkers for CAA and AD.