Amyloidogenic protein-membrane interactions: mechanistic insight from model systems

Effect of Cholesterol on C99 Dimerization: Revealed by Molecular Dynamics Simulations

C99 is the immediate precursor for amyloid beta (Aβ) and therefore is a central intermediate in the pathway that is believed to result in Alzheimer’s disease (AD). It has been suggested that cholesterol is associated with C99, but the dynamic details of how cholesterol affects C99 assembly and the Aβ formation remain unclear. To investigate this question, we employed coarse-grained and all-atom molecular dynamics simulations to study the effect of cholesterol and membrane composition on C99 dimerization. We found that although the existence of cholesterol delays C99 dimerization, there is no direct competition between C99 dimerization and cholesterol association. In contrast, the existence of cholesterol makes the C99 dimer more stable, which presents a cholesterol binding C99 dimer model. Cholesterol and membrane composition change the dimerization rate and conformation distribution of C99, which will subsequently influence the production of Aβ. Our results provide insights into the potential influence of the physiological environment on the C99 dimerization, which will help us understand Aβ formation and AD’s etiology.

Targeting the alternative oxidase (AOX) for human health and food security, a pharmaceutical and agrochemical target or a rescue mechanism?

Some of the most threatening human diseases are due to a blockage of the mitochondrial electron transport chain (ETC). In a variety of plants, fungi, and prokaryotes, there is a naturally evolved mechanism for such threats to viability, namely a bypassing of the blocked portion of the ETC by alternative enzymes of the respiratory chain. One such enzyme is the alternative oxidase (AOX). When AOX is expressed, it enables its host to survive life-threatening conditions or, as in parasites, to evade host defenses. In vertebrates, this mechanism has been lost during evolution. However, we and others have shown that transfer of AOX into the genome of the fruit fly and mouse results in a catalytically engaged AOX. This implies that not only is the AOX a promising target for combating human or agricultural pathogens but also a novel approach to elucidate disease mechanisms or, in several cases, potentially a therapeutic cure for human diseases. In this review, we highlight the varying functions of AOX in their natural hosts and upon xenotopic expression, and discuss the resulting need to develop species-specific AOX inhibitors.

Monosialotetrahexosylganglioside Promotes Early Aβ42 Oligomer Formation and Maintenance

The aggregation of the amyloid beta (Aβ) peptide is associated with Alzheimer's disease (AD) pathogenesis. Cell membrane composition, especially monosialotetrahexosylganglioside (GM1), is known to promote the formation of Aβ fibrils, yet little is known about the roles of GM1 in the early steps of Aβ oligomer formation. Here, by using GM1-contained liposomes as a mimic of the neuronal cell membrane, we demonstrate that GM1 is a critical trigger of Aβ oligomerization and aggregation. We find that GM1 not only promotes the formation of Aβ fibrils but also facilitates the maintenance of Aβ42 oligomers on liposome membranes. We structurally characterize the Aβ42 oligomers formed on the membrane and find that GM1 captures Aβ by binding to its arginine-5 residue. To interrogate the mechanism of Aβ42 oligomer toxicity, we design a new liposome-based Ca²⁺-encapsulation assay and provide new evidence for the Aβ42 ion channel hypothesis. Finally, we determine the toxicity of Aβ42 oligomers formed on membranes. Overall, by uncovering the roles of GM1 in mediating early Aβ oligomer formation and maintenance, our work provides a novel direction for pharmaceutical research for AD.

Linking hIAPP misfolding and aggregation with type 2 diabetes mellitus: a structural perspective

There are over 40 identified human disorders that involve certain proteins folding incorrectly, accumulating in the body causing damage to cells and organs and causing disease. Type 2 Diabetes Mellitus (T2DM) is one of these protein misfolding disorders (PMDs) and involves human islet amyloid polypeptide (hIAPP) misfolding and accumulating in parts of the body, primarily in the pancreas, causing damage to islet cells and affecting glucose regulation. In this review, we have summarised our current understanding of what causes hIAPP to misfold, what conformations are found in different parts of the body with a particular focus on what is known about the structure of hIAPP and how this links to T2DM. Understanding the molecular basis behind these misfolding events is essential for understanding the role of hIAPP to develop better therapeutics since type 2 diabetes currently affects over 4.9 million people in the United Kingdom alone and is predicted to increase as our population ages.

Physical Exercise, a Potential Non-Pharmacological Intervention for Attenuating Neuroinflammation and Cognitive Decline in Alzheimer's Disease Patients

This narrative review summarises the evidence for considering physical exercise (PE) as a non-pharmacological intervention for delaying cognitive decline in patients with Alzheimer’s disease (AD) not only by improving cardiovascular fitness but also by attenuating neuroinflammation. Ageing is the most important risk factor for AD. A hallmark of the ageing process is a systemic low-grade chronic inflammation that also contributes to neuroinflammation. Neuroinflammation is associated with AD, Parkinson’s disease, late-onset epilepsy, amyotrophic lateral sclerosis and anxiety disorders. Pharmacological treatment of AD is currently limited to mitigating the symptoms and attenuating progression of the disease. AD animal model studies and human studies on patients with a clinical diagnosis of different stages of AD have concluded that PE attenuates cognitive decline not only by improving cardiovascular fitness but possibly also by attenuating neuroinflammation. Therefore, low-grade chronic inflammation and neuroinflammation should be considered potential modifiable risk factors for AD that can be attenuated by PE. This opens the possibility for personalised attenuation of neuroinflammation that could also have important health benefits for patients with other inflammation associated brain disorders (i.e., Parkinson’s disease, late-onset epilepsy, amyotrophic lateral sclerosis and anxiety disorders). In summary, life-long, regular, structured PE should be considered as a supplemental intervention for attenuating the progression of AD in human. Further studies in human are necessary to develop optimal, personalised protocols, adapted to the progression of AD and the individual’s mental and physical limitations, to take full advantage of the beneficial effects of PE that include improved cardiovascular fitness, attenuated systemic inflammation and neuroinflammation, stimulated brain Aβ peptides brain catabolism and brain clearance.

Cross-Seeding Controls Aβ Fibril Populations and Resulting Functions

Amyloid peptides nucleate from monomers to aggregate into fibrils through primary nucleation. Pre-existing fibrils can then act as seeds for additional monomers to fibrillize through secondary nucleation. Both nucleation processes occur simultaneously, yielding a distribution of fibril polymorphs that can generate a spectrum of neurodegenerative effects. Understanding the mechanisms driving polymorph structural distribution during both nucleation processes is important for uncovering fibril structure-function relationships, as well as for creating polymorph distributions in vitro that better match fibril structures found in vivo. Here, we explore how cross-seeding wild-type (WT) Aβ_1-40 with Aβ_1-40 mutants E22G (Arctic) and E22Δ (Osaka), as well as with WT Aβ_1-42, affects the distribution of fibril structural polymorphs and how changes in structural distribution impact toxicity. Transmission electron microscopy analysis revealed that fibril seeds derived from mutants of Aβ_1-40 imparted their structure to WT Aβ_1-40 monomers during secondary nucleation, but WT Aβ_1-40 fibril seeds do not affect the structure of fibrils assembled from mutant Aβ_1-40 monomers, despite the kinetic data indicating accelerated aggregation when cross-seeding of any combination of mutants. Additionally, WT Aβ_1-40 fibrils seeded with mutant fibrils produced similar structural distributions to the mutant seeds with similar cytotoxicity profiles. This indicates that mutant fibril seeds not only impart their structure to growing WT Aβ_1-40 aggregates but also impart cytotoxic properties. Our findings establish a relationship between the fibril structure and the phenotype on a polymorph population level and that these properties can be passed on through secondary nucleation to the succeeding generations of fibrils.

Amyloid β interaction with model cell membranes - What are the toxicity-defining properties of amyloid β?

Disruption of the neuronal membrane by toxic amyloid β oligomers is hypothesized to be the major event associated with Alzheimer's disease's neurotoxicity. Misfolding of amyloid β is followed by aggregation via different pathways in which structurally different amyloid β oligomers can be formed. The respective toxic actions of these structurally diverse oligomers can vary significantly. Linking a particular toxic action to a structurally unique kind of amyloid β oligomers and resolving their toxicity-determining feature remains challenging because of their transient stability and heterogeneity. Moreover, the lipids that make up the membrane affect amyloid β oligomers' behavior, thus adding to the problem's complexity. The present review compares and analyzes the latest results to improve understanding of amyloid β oligomers' interaction with lipid bilayers.

Triterpenoids impede the fibrillation and cytotoxicity of human islet amyloid polypeptide

The inhibition of human islet amyloid polypeptide (hIAPP) deposition to block its toxicity is an important strategy for the prevention and treatment of type II diabetes mellitus (T2DM).Natural compounds with pharmacological properties and low toxicity can serve as a good point to discover potential inhibitors of protein misfolding, which may be useful for the treatment of various amyloidosis-related diseases. Previous studies have reported that triterpenoids, such as maslinic acid (MA) and momordicin I (MI), can modulate glucose metabolism partially by reducing insulin resistance. However, the internal antidiabetic mechanism of these triterpenoids remains unclear. In this study, we examined the inhibition and disaggregation of MAandits isomer MI on the fibrillation of hIAPP using various experimental and computational approaches. The assembly behaviors and peptide-induced cytotoxicity of hIAPP could be effectively resisted by MA and MI. Moreover, the interaction of the two triterpenoids with hIAPP displayed a spontaneous and exothermic process. Moreover, molecular dynamics simulation results of different peptides revealed that MA and MI could bind to Asn and other non-polar residues near the core C-terminal region and reduce the oligomerization of hIAPP. The binding affinity was predominantly contributed by hydrophobic, electrostatic and hydrogen bonding interactions. The present work provides valuable data for MA and MI to treat T2DM and amyloidosis-related diseases.

The Pathological G51D Mutation in Alpha-Synuclein Oligomers Confers Distinct Structural Attributes and Cellular Toxicity

A wide variety of oligomeric structures are formed during the aggregation of proteins associated with neurodegenerative diseases. Such soluble oligomers are believed to be key toxic species in the related disorders; therefore, identification of the structural determinants of toxicity is of upmost importance. Here, we analysed toxic oligomers of α-synuclein and its pathological variants in order to identify structural features that could be related to toxicity and found a novel structural polymorphism within G51D oligomers. These G51D oligomers can adopt a variety of β-sheet-rich structures with differing degrees of α-helical content, and the helical structural content of these oligomers correlates with the level of induced cellular dysfunction in SH-SY5Y cells. This structure–function relationship observed in α-synuclein oligomers thus presents the α-helical structure as another potential structural determinant that may be linked with cellular toxicity in amyloid-related proteins.

Molecular insights into the primary nucleation of polymorphic amyloid β dimers in DOPC lipid bilayer membrane

Alzheimer's disease (AD) pathology is characterized by loss of memory cognitive and behavioral deterioration. One of the hallmarks of AD is amyloid β (Aβ) plaques in the brain that consists of Aβ oligomers and fibrils. It is accepted that oligomers, particularly dimers, are toxic species that are produced extracellularly and intracellularly in membranes. It is believed that the disruption of membranes by polymorphic Aβ oligomers is the key for the pathology of AD. This is a first study that investigate the effect of polymorphic “α‐helix/random coil” and “fibril‐like” Aβ dimers on 1,2‐dioleoyl‐sn‐glycero‐3‐phosphocholine (DOPC) membrane. It has been found that the DOPC membrane promotes Aβ_1–42 “fibril‐like” dimers and impedes Aβ_1–42 “α‐helix/random coil” dimers. The N‐termini domains within Aβ_1–42 dimers play a role in Aβ aggregation in membrane milieus. In addition, the aromatic π–π interactions (involving residues F19 and F20 in Aβ_1–42) are the driving forces for the hydrophobic interactions that initiate the primary nucleation of polymorphic Aβ_1–42 dimers within DOPC membrane. Finally, the DOPC bilayer membrane thickness is locally decreased, and it is disrupted by an embedded distinct Aβ_1–42 dimer, due to relatively large contacts between Aβ_1–42 monomers and the DOPC membrane. This study reveals insights into the molecular mechanisms by which polymorphic early‐stage Aβ_1–42 dimers have distinct impacts on DOPC membrane.

Gangliosides smelt nanostructured amyloid Aβ(1-40) fibrils in a membrane lipid environment

Gangliosides induced a smelting process in nanostructured amyloid fibril-like films throughout the surface properties contributed by glycosphingolipids when mixed with 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC)/Aβ(1-40) amyloid peptide. We observed a dynamical smelting process when pre-formed amyloid/phospholipid mixture is laterally mixed with gangliosides. This particular environment, gangliosides/phospholipid/Aβ(1-40) peptide mixed interfaces, showed complex miscibility behavior depending on gangliosides content. At 0% of ganglioside covered surface respect to POPC, Aβ(1-40) peptide forms fibril-like structure. In between 5 and 15% of gangliosides, the fibrils dissolve into irregular domains and they disappear when the proportion of gangliosides reach the 20%. The amyloid interfacial dissolving effect of gangliosides is taken place at lateral pressure equivalent to the organization of biological membranes. Domains formed at the interface are clearly evidenced by Brewster Angle Microscopy and Atomic Force Microscopy when the films are transferred onto a mica support. The domains are thioflavin T (ThT) positive when observed by fluorescence microscopy. We postulated that the smelting process of amyloids fibrils-like structure at the membrane surface provoked by gangliosides is a direct result of a new interfacial environment imposed by the complex glycosphingolipids. We add experimental evidence, for the first time, how a change in the lipid environment (increase in ganglioside proportion) induces a rapid loss of the asymmetric structure of amyloid fibrils by a simple modification of the membrane condition (a more physiological situation).

Thermodynamic surprises of Cu(II)-amylin analogue complexes in membrane mimicking solutions

Membrane environment often has an important effect on the structure, and therefore also on the coordination mode of biologically relevant metal ions. This is also true in the case of Cu(II) coordination to amylin analogues—rat amylin, amylin_1–19, pramlintide and Ac-pramlintide, which offer N-terminal amine groups and/or histidine imidazoles as copper(II) anchoring sites. Complex stabilities are comparable, with the exception of the very stable Cu(II)–amylin_1–19, which proves that the presence of the amylin C-terminus lowers its affinity for copper(II); although not directly involved, its appropriate arrangement sterically prevents early metal binding. Most interestingly, in membrane-mimicking solution, the Cu(II) affinities of amylin analogues are lower than the ones in water, probably due to the crowding effect of the membrane solution and the fact that amide coordination occurs at higher pH, which happens most likely because the α-helical structure, imposed by the membrane-mimicking solvent, prevents the amides from binding at lower pH, requiring a local unwinding of the α-helix.

Fӧrster resonance energy transfer analysis of amyloid state of proteins

The Förster resonance energy transfer (FRET) is a well-established and versatile spectroscopic technique extensively used for exploring a variety of biomolecular interactions and processes. The present review is intended to cover the main results of our FRET studies focused on amyloid fibrils, a particular type of disease-associated protein aggregates. Based on the examples of several fibril-forming proteins including insulin, lysozyme and amyloidogenic variants of N-terminal fragment of apolipoprotein A-I, it was demonstrated that: (i) the two- and three-step FRET with the classical amyloid marker Thioflavin T as an input donor has a high amyloid-sensing potential and can be used to refine the amyloid detection assays; (ii) the intermolecular time-resolved and single-molecule pulse interleaved excitation FRET can give quantitative information on the nucleation of amyloid fibrils; (iii) FRET between the membrane fluorescent probes and protein-associated intrinsic or extrinsic fluorophores is suitable for monitoring the membrane binding of fibrillar proteins, exploring their location relative to lipid-water interface and restructuring on a lipid matrix; (iv) the FRET-based distance estimation between fibril-bound donor and acceptor fluorophores can serve as one of the verification criteria upon structural modeling of amyloid fibrils.

Interaction of membrane vesicles with the Pseudomonas functional amyloid protein FapC facilitates amyloid formation

Functional amyloids (FA) are proteins which are evolutionarily optimized to form highly stable fibrillar structures that strengthen the bacterial biofilm matrix. FA such as CsgA (E. coli) and FapC (Pseudomonas) are secreted to the bacterial surface where they integrate into growing fibril structures projecting from the outer membrane. FA are exposed to membrane surfaces in this process, but it remains unclear how membranes can interact with FA and potentially affect the self-assembly. Here we report the effect of different vesicles (DOPG, DMPG, DOPS, DOPC and DMPC) on the kinetics and structural endpoints of FapC fibrillation using various biophysical techniques. Particularly anionic lipids such as DMPG trigger FapC fibrillation, and the protein's second repeat sequence (R2) appears to be important for this interaction. Vesicles formed from phospholipids extracted from three different Pseudomonas strains (Δfap, ΔFapC and pfap) induce FapC fibrillation by accelerating nucleation. The general aggregation inhibitor epigallocatechin gallate (EGCG) inhibits FapC fibrillation by blocking interactions between FapC and vesicles and redirecting FapC monomers to oligomer structures. Our work indicates that biological membranes can contribute significantly to the fibrillation of functional amyloid.

Analyzing Morphological Properties of Early-Stage Toxic Amyloid β Oligomers by Atomic Force Microscopy

Protein misfolding diseases, like Alzheimer's, Parkinson's, and Huntington's disease, are associated with misfolded protein aggregation. Alzheimer's disease is related to a progressive neuronal death induced by small amyloid β oligomers. Here, we describe the procedure to prepare and identify different types of small toxic amyloid β oligomers by atomic force microscopy (AFM).

Graphene quantum dots obstruct the membrane axis of Alzheimer's amyloid beta

Alzheimer's disease (AD) is a primary form of dementia with debilitating consequences, but no effective cure is available. While the pathophysiology of AD remains multifactorial, the aggregation of amyloid beta (Aβ) mediated by the cell membrane is known to be the cause for the neurodegeneration associated with AD. Here we examined the effects of graphene quantum dots (GQDs) on the obstruction of the membrane axis of Aβ in its three representative forms of monomers (Aβ-m), oligomers (Aβ-o), and amyloid fibrils (Aβ-f). Specifically, we determined the membrane fluidity of neuroblastoma SH-SY5Y cells perturbed by the Aβ species, especially by the most toxic Aβ-o, and demonstrated their recovery by GQDs using confocal fluorescence microscopy. Our computational data through discrete molecular dynamics simulations further revealed energetically favorable association of the Aβ species with the GQDs in overcoming peptide-peptide aggregation. Overall, this study positively implicated GQDs as an effective agent in breaking down the membrane axis of Aβ, thereby circumventing adverse downstream events and offering a potential therapeutic solution for AD.

How does excess phenylalanine affect the packing density and fluidity of a lipid membrane?

Phenylketonuria (PKU) is an autosomal recessive error of phenylalanine (Phe) metabolism, where untreated Phe becomes cytotoxic. Previous experiments found that excess Phe decreases the packing density and increases the fluidity and permeability of a lipid membrane. It was proposed that Phe forms cytotoxic nanoscopic amyloid-like fibrils. In another study, the Phe fibrils were not visible near the lipid membrane. So, what leads to the deleterious effect of Phe on the lipid membrane? We put forward a molecular mechanism for the observed effect of excess Phe on the lipid membrane using all-atom molecular dynamics simulation. This study suggests that Phe monomers spontaneously intercalate into the membrane and form small hydrogen-bonded clusters, some of which locally perturb the membrane. These local effects result in an overall reduction in the membrane packing density, enhancement of membrane fluidity, and an increase of water permeability, observed in experiments. The present study does not observe any effect of the nanoscopic fibrillar structure of Phe on the membrane. This study, therefore, provides alternative insights into the excess Phe cytotoxicity in PKU disease.

A mechanistic survey of Alzheimer's disease

Alzheimer's disease (AD) is the most common, age-dependent neurodegenerative disorder. While AD has been intensively studied from different aspects, there is no effective cure for AD, largely due to a lack of a clear mechanistic understanding of AD. In this mini-review, we mainly focus on the discussion and summary of mechanistic causes of Alzheimer's disease (AD). While different AD mechanisms illustrate different molecular and cellular pathways in AD pathogenesis, they do not necessarily exclude each other. Instead, some of them could work together to initiate, trigger, and promote the onset and development of AD. In a broader viewpoint, some AD mechanisms (e.g., amyloid aggregation mechanism, microbial infection/neuroinflammation mechanism, and amyloid cross-seeding mechanism) could also be applicable to other amyloid diseases including type II diabetes, Parkinson's disease, and prion disease. Such common mechanisms for AD and other amyloid diseases explain not only the pathogenesis of individual amyloid diseases, but also the spreading of pathologies between these diseases, which will inspire new strategies for therapeutic intervention and prevention for AD.

Systematic Exploration in Tissue-Pathway Associations of Complex Traits Using Comprehensive eQTLs Catalog

The collection of expression quantitative trait loci (eQTLs) is an important resource to study complex traits through understanding where and how transcriptional regulations are controlled by genetic variations in the non-coding regions of the genome. Previous studies have focused on associating eQTLs with traits to identify the roles of trait-related eQTLs and their corresponding target genes involved in trait determination. Since most genes function as a part of pathways in a systematic manner, it is crucial to explore the pathways’ involvements in complex traits to test potentially novel hypotheses and to reveal underlying mechanisms of disease pathogenesis. In this study, we expanded and applied loci2path software to perform large-scale eQTLs enrichment [i.e., eQTLs’ target genes (eGenes) enrichment] analysis at pathway level to identify the tissue-specific enriched pathways within trait-related genomic intervals. By utilizing 13,791,909 eQTLs cataloged in the Genotype-Tissue Expression (GTEx) V8 data for 49 tissue types, 2,893 pathway sets reported from MSigDB, and query regions derived from the Phenotype-Genotype Integrator (PheGenI) catalog, we identified intriguing biological pathways that are likely to be involved in ten traits [Alzheimer’s disease (AD), body mass index, Parkinson’s disease (PD), schizophrenia, amyotrophic lateral sclerosis, non-small cell lung cancer (NSCLC), stroke, blood pressure, autism spectrum disorder, and myocardial infarction]. Furthermore, we extracted the most significant pathways for AD, such as BioCarta D4-GDI pathway and WikiPathways sulfation biotransformation reaction and viral acute myocarditis pathways, to study specific genes within pathways. Our data presented new hypotheses in AD pathogenesis supported by previous studies, like the increased level of caspase-3 in the amygdala that cleaves GDP dissociation inhibitor and binds to beta-amyloid, leading to increased apoptosis and neuronal loss. Our findings also revealed potential pathogenesis mechanisms for PD, schizophrenia, NSCLC, blood pressure, autism spectrum disorder, and myocardial infarction, which were consistent with past studies. Our results indicated that loci2path′s eQTLs enrichment test was valuable in unveiling novel biological mechanisms of complex traits. The discovered mechanisms of disease pathogenesis and traits require further in-depth analysis and experimental validation.

Analysis of PDA Dose Curves for the Extraction of Antimicrobial Peptide Properties

A mechanochromic polymer, polydiacetylene, changes color upon ligand binding, being a popular material in biosensing. However, whether it can also detect ligand functions in addition to binding is left understudied. In this work, we report that the polydiacetylene can be used to determine the net charges and the mode of actions (carpet model, toroidal pore model, etc.) of antimicrobial peptides and detergents via EC₅₀ and Hill coefficients from the colorimetric response-dose curves. This opens a potential for high-throughput peptide screening by functions, which is difficult with the conventional methods.

Delineation of Neuroprotective Effects and Possible Benefits of AntioxidantsTherapy for the Treatment of Alzheimer's Diseases by Targeting Mitochondrial-Derived Reactive Oxygen Species: Bench to Bedside

Alzheimer's disease (AD) is considered the sixth leading cause of death in elderly patients and is characterized by progressive neuronal degeneration and impairment in memory, language, etc. AD is characterized by the deposition of senile plaque, accumulation of fibrils, and neurofibrillary tangles (NFTs) which are responsible for neuronal degeneration. Amyloid-β (Aβ) plays a key role in the process of neuronal degeneration in the case of AD. It has been reported that Aβ is responsible for the production of reactive oxygen species (ROS), depletion of endogenous antioxidants, increase in intracellular Ca²⁺ which further increases mitochondria dysfunctions, oxidative stress, release of pro-apoptotic factors, neuronal apoptosis, etc. Thus, oxidative stress plays a key role in the pathogenesis of AD. Antioxidants are compounds that have the ability to counteract the oxidative damage conferred by ROS. Therefore, the antioxidant therapy may provide benefits and halt the progress of AD to advance stages by counteracting neuronal degeneration. However, despite the beneficial effects imposed by the antioxidants, the findings from the clinical studies suggested inconsistent results which might be due to poor study design, selection of the wrong antioxidant, inability of the molecule to cross the blood-brain barrier (BBB), treatment in the advanced state of disease, etc. The present review insights into the neuroprotective effects and limitations of the antioxidant therapy for the treatment of AD by targeting mitochondrial-derived ROS. This particular article will certainly help the researchers to search new avenues for the treatment of AD by utilizing mitochondrial-derived ROS-targeted antioxidant therapies.

Peptide-Membrane Interactions Monitored by Fluorescence Lifetime Imaging: A Study Case of Transportan 10

The interest on detailed analysis of peptide-membrane interactions is of great interest in both fundamental and applied sciences as these may relate to both functional and pathogenic events. Such interactions are highly dynamic and spatially heterogeneous, making the investigation of the associated phenomena highly complex. The specific properties of membranes and peptide structural details, together with environmental conditions, may determine different events at the membrane interface, which will drive the fate of the peptide-membrane system. Here, we use an experimental approach based on the combination of spectroscopy and fluorescence microscopy methods to characterize the interactions of the multifunctional amphiphilic peptide transportan 10 with model membranes. Our approach, based on the use of suitable fluorescence reporters, exploits the advantages of phasor plot analysis of fluorescence lifetime imaging microscopy measurements to highlight the molecular details of occurring membrane alterations in terms of rigidity and hydration. Simultaneously, it allows following dynamic events in real time without sample manipulation distinguishing, with high spatial resolution, whether the peptide is adsorbed to or inserted in the membrane.

The role of amyloids in Alzheimer's and Parkinson's diseases

With varying clinical symptoms, most neurodegenerative diseases are associated with abnormal loss of neurons. They share the same common pathogenic mechanisms involving misfolding and aggregation, and these visible aggregates of proteins are deposited in the central nervous system. Amyloid formation is thought to arise from partial unfolding of misfolded proteins leading to the exposure of hydrophobic surfaces, which interact with other similar structures and give rise to form dimers, oligomers, protofibrils, and eventually mature fibril aggregates. Accumulating evidence indicates that amyloid oligomers, not amyloid fibrils, are the most toxic species that causes Alzheimer's disease (AD) and Parkinson's disease (PD). AD has recently been recognized as the 'twenty-first century plague', with an incident rate of 1% at 60 years of age, which then doubles every fifth year. Currently, 5.3 million people in the US are afflicted with this disease, and the number of cases is expected to rise to 13.5 million by 2050. PD, a disorder of the brain, is the second most common form of dementia, characterized by difficulty in walking and movement. Keeping the above views in mind, in this review we have focused on the roles of amyloid in neurodegenerative diseases including AD and PD, the involvement of amyloid in mitochondrial dysfunction leading to neurodegeneration, are also considered in the review.

The effect of charged residue substitutions on the thermodynamics of protein-surface interactions

The interactions of proteins with surfaces are important in both biological processes and biotechnologies. In contrast to decades of study regarding the biophysics of proteins in bulk solution, however, our mechanistic understanding of the biophysics of proteins interacting with surfaces remains largely qualitative. In response, we have set to explore quantitatively the thermodynamics of protein-surface interactions. In this work, we explore systematically the role of electrostatics in modulating the interaction between proteins and charged surfaces. In particular, we use electrochemistry to explore the extent to which a macroscopic, hydroxyl-coated surface held at a slightly negative potential affects the folding thermodynamics of surface-attached protein variants with different composition of charged amino acids. Doing so, we find that attachment to the surface generally leads to a net stabilization, presumably due to excluded volume effects that reduce the entropy of the unfolded state. The magnitude of this stabilization, however, is strongly correlated with the charged-residue content of the protein. In particular, we find statistically significant correlations with both the net charge of the protein, with greater negative charge leading to less stabilization by the surface, and with the number of arginines, with more arginines leading to greater stabilization. Such findings refine our understanding of protein-surface interactions, providing in turn a guiding rationale to achieve the functional deposition of proteins on artificial surfaces for implementation in, for example, protein-based biotechnologies.

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.

The amyloid-inhibiting NCAM-PrP peptide targets Aβ peptide aggregation in membrane-mimetic environments

Substantial research efforts have gone into elucidating the role of protein misfolding and self-assembly in the onset and progression of Alzheimer's disease (AD). Aggregation of the Amyloid-β (Aβ) peptide into insoluble fibrils is closely associated with AD. Here, we use biophysical techniques to study a peptide-based approach to target Aβ amyloid aggregation. A peptide construct, NCAM-PrP, consists of a largely hydrophobic signal sequence linked to a positively charged hexapeptide. The NCAM-PrP peptide inhibits Aβ amyloid formation by forming aggregates which are unavailable for further amyloid aggregation. In a membrane-mimetic environment, Aβ and NCAM-PrP form specific heterooligomeric complexes, which are of lower aggregation states compared to Aβ homooligomers. The Aβ:NCAM-PrP interaction appears to take place on different aggregation states depending on the absence or presence of a membrane-mimicking environment. These insights can be useful for the development of potential future therapeutic strategies targeting Aβ at several aggregation states.

Cryo-electron Microscopy Imaging of Alzheimer's Amyloid-beta 42 Oligomer Displayed on a Functionally and Structurally Relevant Scaffold

Amyloid-β peptide (Aβ) oligomers are pathogenic species of amyloid aggregates in Alzheimer's disease. Like certain protein toxins, Aβ oligomers permeabilize cellular membranes, presumably through a pore formation mechanism. Owing to their structural and stoichiometric heterogeneity, the structure of these pores remains to be characterized. We studied a functional Aβ42-pore equivalent, created by fusing Aβ42 to the oligomerizing, soluble domain of the α-hemolysin (αHL) toxin. Our data reveal Aβ42-αHL oligomers to share major structural, functional, and biological properties with wild-type Aβ42-pores. Single-particle cryo-EM analysis of Aβ42-αHL oligomers (with an overall 3.3 Å resolution) reveals the Aβ42-pore region to be intrinsically flexible. The Aβ42-αHL oligomers will allow many of the features of the wild-type amyloid oligomers to be studied that cannot be otherwise, and may be a highly specific antigen for the development of immuno-base diagnostics and therapies.

Conformation-specific perturbation of membrane dynamics by structurally distinct oligomers of Alzheimer's amyloid-β peptide

The accumulation of toxic soluble oligomers of the amyloid-β peptide (Aβ) is a key step in the pathogenesis of Alzheimer's disease. There are mainly two conformationally distinct oligomers, namely, prefibrillar and fibrillar oligomers, that are recognized by conformation-specific antibodies, anti-amyloid oligomer antibody (A11) and anti-amyloid fibrillar antibody (OC), respectively. Previous studies have shown that the interaction of Aβ oligomers with the lipid membrane is one of the key mechanisms of toxicity produced by Aβ oligomers. However, the mechanism by which structurally distinct Aβ oligomers interact with the lipid membrane remains elusive. In this work, we dissect the molecular mechanism underlying the interaction of structurally distinct Aβ42 oligomers with the lipid membrane derived from the brain total lipid extract. Using picosecond time-resolved fluorescence spectroscopy, we show that the A11-positive Aβ42 oligomers undergo a membrane-induced conformational change that promotes the deeper immersion of these oligomers into the lipid hydrocarbon region and results in an increase in the membrane micro-viscosity. In sharp contrast, OC-positive Aβ42 oligomers interact with the lipid membrane via electrostatic interactions between the negatively-charged lipid headgroup and positively-charged residues of Aβ42 without perturbing the membrane dynamics. We show that the two structurally distinct Aβ42 oligomers demonstrating different interaction mechanisms with the lipid membrane eventually lead to the formation of typical amyloid fibrils. Our findings provide the mechanistic underpinning of the perturbation of lipid membranes by two conformationally distinct Aβ42 oligomers and can be of prime importance in designing anti-Alzheimer's therapeutic agents targeting Aβ-membrane interactions.

The Progress of Label-Free Optical Imaging in Alzheimer's Disease Screening and Diagnosis

As the major neurodegenerative disease of dementia, Alzheimer's disease (AD) has caused an enormous social and economic burden on society. Currently, AD has neither clear pathogenesis nor effective treatments. Positron emission tomography (PET) and magnetic resonance imaging (MRI) have been verified as potential tools for diagnosing and monitoring Alzheimer's disease. However, the high costs, low spatial resolution, and long acquisition time limit their broad clinical utilization. The gold standard of AD diagnosis routinely used in research is imaging AD biomarkers with dyes or other reagents, which are unsuitable for in vivo studies owing to their potential toxicity and prolonged and costly process of the U.S. Food and Drug Administration (FDA) approval for human use. Furthermore, these exogenous reagents might bring unwarranted interference to mechanistic studies, causing unreliable results. Several label-free optical imaging techniques, such as infrared spectroscopic imaging (IRSI), Raman spectroscopic imaging (RSI), optical coherence tomography (OCT), autofluorescence imaging (AFI), optical harmonic generation imaging (OHGI), etc., have been developed to circumvent this issue and made it possible to offer an accurate and detailed analysis of AD biomarkers. In this review, we present the emerging label-free optical imaging techniques and their applications in AD, along with their potential and challenges in AD diagnosis.

Dynamic and Reversible Aggregation of the Human CAP Superfamily Member GAPR-1 in Protein Inclusions in Saccharomyces cerevisiae

Many proteins that can assemble into higher order structures termed amyloids can also concentrate into cytoplasmic inclusions via liquid-liquid phase separation. Here, we study the assembly of human Golgi-Associated plant Pathogenesis Related protein 1 (GAPR-1), an amyloidogenic protein of the Cysteine-rich secretory proteins, Antigen 5, and Pathogenesis-related 1 proteins (CAP) protein superfamily, into cytosolic inclusions in Saccharomyces cerevisiae. Overexpression of GAPR-1-GFP results in the formation GAPR-1 oligomers and fluorescent inclusions in yeast cytosol. These cytosolic inclusions are dynamic and reversible organelles that gradually increase during time of overexpression and decrease after promoter shut-off. Inclusion formation is, however, a regulated process that is influenced by factors other than protein expression levels. We identified N-myristoylation of GAPR-1 as an important determinant at early stages of inclusion formation. In addition, mutations in the conserved metal-binding site (His54 and His103) enhanced inclusion formation, suggesting that these residues prevent uncontrolled protein sequestration. In agreement with this, we find that addition of Zn²⁺ metal ions enhances inclusion formation. Furthermore, Zn²⁺ reduces GAPR-1 protein degradation, which indicates stabilization of GAPR-1 in inclusions. We propose that the properties underlying both the amyloidogenic properties and the reversible sequestration of GAPR-1 into inclusions play a role in the biological function of GAPR-1 and other CAP family members.

Mechanisms of amyloid proteins aggregation and their inhibition by antibodies, small molecule inhibitors, nano-particles and nano-bodies

Protein misfolding and aggregation can be induced by a wide variety of factors, such as dominant disease-associated mutations, changes in the environmental conditions (pH, temperature, ionic strength, protein concentration, exposure to transition metal ions, exposure to toxins, posttranslational modifications including glycation, phosphorylation, and sulfation). Misfolded intermediates interact with similar intermediates and progressively form dimers, oligomers, protofibrils, and fibrils. In amyloidoses, fibrillar aggregates are deposited in the tissues either as intracellular inclusion or extracellular plaques (amyloid). When such proteinaceous deposit occurs in the neuronal cells, it initiates degeneration of neurons and consequently resulting in the manifestation of various neurodegenerative diseases. Several different types of molecules have been designed and tested both in vitro and in vivo to evaluate their anti-amyloidogenic efficacies. For instance, the native structure of a protein associated with amyloidosis could be stabilized by ligands, antibodies could be used to remove plaques, oligomer-specific antibody A11 could be used to remove oligomers, or prefibrillar aggregates could be removed by affibodies. Keeping the above views in mind, in this review we have discussed protein misfolding and aggregation, mechanisms of protein aggregation, factors responsible for aggregations, and strategies for aggregation inhibition.

Effects of the Hydrophilic N-Terminal Region on Aβ-Mediated Membrane Disruption

Amyloidogenesis of amyloid-β (Aβ) peptides is intimately related to pathological neurodegeneration in Alzheimer's disease. Here, we investigated the membrane damage activity of Aβ40 and its derivatives that contain mutation at the N-terminal charged residues using a membrane leakage assay. A model 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) phospholipid vesicle encapsulating the fluorescent dye carboxyfluorescein was used in the study. Our results show that the mutations of the N-terminal charged residues of Aβ40 significantly affect the peptide-induced membrane leakage. The results suggest that favorable electrostatic interactions of the N-terminal charged residues and the phosphatidylcholine membrane surface are crucial in Aβ-mediated membrane permeation. The flexible and charge-rich N-terminal region may play a critical role in directing Aβ self-association on the membrane surface and in anchoring and stabilizing the peptide aggregates inserted in the phospholipid membrane, which are closely related with membrane disruption activity of Aβ. The results provide new mechanistic insight into the Aβ-mediated membrane damage process, which may be critical for understanding the mechanism of Aβ neurotoxicity in Alzheimer's disease.

Antimicrobial α-defensins as multi-target inhibitors against amyloid formation and microbial infection

Amyloid aggregation and microbial infection are considered as pathological risk factors for developing amyloid diseases, including Alzheimer's disease (AD), type II diabetes (T2D), Parkinson's disease (PD), and medullary thyroid carcinoma (MTC). Due to the multifactorial nature of amyloid diseases, single-target drugs and treatments have mostly failed to inhibit amyloid aggregation and microbial infection simultaneously, thus leading to marginal benefits for amyloid inhibition and medical treatments. Herein, we proposed and demonstrated a new "anti-amyloid and antimicrobial hypothesis" to discover two host-defense antimicrobial peptides of α-defensins containing β-rich structures (human neutrophil peptide of HNP-1 and rabbit neutrophil peptide of NP-3A), which have demonstrated multi-target, sequence-independent functions to (i) prevent the aggregation and misfolding of different amyloid proteins of amyloid-β (Aβ, associated with AD), human islet amyloid polypeptide (hIAPP, associated with T2D), and human calcitonin (hCT, associated with MTC) at sub-stoichiometric concentrations, (ii) reduce amyloid-induced cell toxicity, and (iii) retain their original antimicrobial activity upon the formation of complexes with amyloid peptides. Further structural analysis showed that the sequence-independent amyloid inhibition function of α-defensins mainly stems from their cross-interactions with amyloid proteins via β-structure interactions. The discovery of antimicrobial peptides containing β-structures to inhibit both microbial infection and amyloid aggregation greatly expands the new therapeutic potential of antimicrobial peptides as multi-target amyloid inhibitors for better understanding pathological causes and treatments of amyloid diseases.

On the Structural Diversity and Individuality of Polymorphic Amyloid Protein Assemblies

The prediction of highly ordered three-dimensional structures of amyloid protein fibrils from the amino acid sequences of their monomeric self-assembly precursors constitutes a challenging and unresolved aspect of the classical protein folding problem. Because of the polymorphic nature of amyloid assembly whereby polypeptide chains of identical amino acid sequences under identical conditions are capable of self-assembly into a spectrum of different fibril structures, the prediction of amyloid structures from an amino acid sequence requires a detailed and holistic understanding of its assembly free energy landscape. The full extent of the structure space accessible to the cross-β molecular architecture of amyloid must also be resolved. Here, we review the current understanding of the diversity and the individuality of amyloid structures, and how the polymorphic landscape of amyloid links to biology and disease phenotypes. We present a comprehensive review of structural models of amyloid fibrils derived by cryo-EM, ssNMR and AFM to date, and discuss the challenges ahead for resolving the structural basis and the biological consequences of polymorphic amyloid assemblies.

Ultrasmall Molybdenum Disulfide Quantum Dots Cage Alzheimer's Amyloid Beta to Restore Membrane Fluidity

Alzheimer's disease (AD) is a major cause of dementia characterized by the overexpression of transmembrane amyloid precursor protein and its neurotoxic byproduct amyloid beta (Aβ). A small peptide of considerable hydrophobicity, Aβ is aggregation prone catalyzed by the presence of cell membranes, among other environmental factors. Accordingly, current AD mitigation strategies often aim at breaking down the Aβ-membrane communication, yet no data is available concerning the cohesive interplay of the three key entities of the cell membrane, Aβ, and its inhibitor. Using a lipophilic Laurdan dye and confocal fluorescence microscopy, we observed cell membrane perturbation and actin reorganization induced by Aβ oligomers but not by Aβ monomers or amyloid fibrils. We further revealed recovery of membrane fluidity by ultrasmall MoS2 quantum dots, also shown in this study as a potent inhibitor of Aβ amyloid aggregation. Using discrete molecular dynamics simulations, we uncovered the binding of MoS2 and Aβ monomers as mediated by hydrophilic interactions between the quantum dots and the peptide N-terminus. In contrast, Aβ oligomers and fibrils were surface-coated by the ultrasmall quantum dots in distinct testudo-like, reverse protein-corona formations to prevent their further association with the cell membrane and adverse effects downstream. This study offers a crucial new insight and a viable strategy for regulating the amyloid aggregation and membrane-axis of AD pathology with multifunctional nanomedicine.

Neuroprotective Effect of Terpenoids Recovered from Olive Oil By-Products

The neuroprotective potential of 32 natural extracts obtained from olive oil by-products was investigated. The online coupling of supercritical fluid extraction (SFE) and dynamic adsorption/desorption allowed the selective enrichment of olive leaves extracts in different terpenoids’ families. Seven commercial adsorbents based on silica gel, zeolite, aluminum oxide, and sea sand were used with SFE at three different extraction times to evaluate their selectivity towards different terpene families. Collected fractions were analyzed by gas chromatography coupled to quadrupole-time-of-flight mass spectrometry (GC-QTOF-MS) to quantify the recoveries of monoterpenes (C10), sesquiterpenes (C15), diterpenes (C20), and triterpenes (C30). A systematic analysis of the neuroprotective activity of the natural extracts was then carried out. Thus, a set of in vitro bioactivity assays including enzymatic (acetylcholinesterase (AChE), butyrylcholinesterase (BChE)), and anti-inflammatory (lipoxidase (LOX)), as well as antioxidant (ABTS), and reactive oxygen and nitrogen species (ROS and RNS, respectively) activity tests were applied to screen for the neuroprotective potential of these extracts. Statistical analysis showed that olive leaves adsorbates from SS exhibited the highest biological activity potential in terms of neuroprotective effect. Blood–brain barrier permeation and cytotoxicity in HK-2 cells and human THP-1 monocytes were studied for the selected olive leaves fraction corroborating its potential.

Self-Assembled Nanochaperones Inhibit the Aggregation of Human Islet Amyloid Polypeptide Associated with Type 2 Diabetes

Human islet amyloid polypeptide (hIAPP) aggregation is closely associated with dysfunction and apoptosis of pancreatic β-cells in type 2 diabetes (T2D). Accordingly, hIAPP amyloid inhibitors have shown promise against T2D. Here, by mimicking the function of natural molecular chaperones, nanochaperones (nChaps) based on self-assembled polymeric micelles with tunable surface microdomains for T2D treatment are reported. By capturing the aggregation-prone species of hIAPP onto the hydrophobic microdomains and segregating them by hydrophilic PEG chains, this kind of nChaps could effectively prevent hIAPP aggregation, block cell adhesion of hIAPP, facilitate hIAPP aggregates degradation and reduce hIAPP-related cytotoxicity. Therefore, our work will provide useful insights to develop a biomimetic strategy for the treatment of T2D.

Investigating the Disordered and Membrane-Active Peptide A-Cage-C Using Conformational Ensembles

The driving forces and conformational pathways leading to amphitropic protein-membrane binding and in some cases also to protein misfolding and aggregation is the subject of intensive research. In this study, a chimeric polypeptide, A-Cage-C, derived from α-Lactalbumin is investigated with the aim of elucidating conformational changes promoting interaction with bilayers. From previous studies, it is known that A-Cage-C causes membrane leakages associated with the sporadic formation of amorphous aggregates on solid-supported bilayers. Here we express and purify double-labelled A-Cage-C and prepare partially deuterated bicelles as a membrane mimicking system. We investigate A-Cage-C in the presence and absence of these bicelles at non-binding (pH 7.0) and binding (pH 4.5) conditions. Using in silico analyses, NMR, conformational clustering, and Molecular Dynamics, we provide tentative insights into the conformations of bound and unbound A-Cage-C. The conformation of each state is dynamic and samples a large amount of overlapping conformational space. We identify one of the clusters as likely representing the binding conformation and conclude tentatively that the unfolding around the central W23 segment and its reorientation may be necessary for full intercalation at binding conditions (pH 4.5). We also see evidence for an overall elongation of A-Cage-C in the presence of model bilayers.

AbsoluRATE: An in-silico method to predict the aggregation kinetics of native proteins

Protein aggregation has two aspects, namely, mechanistic and kinetics. Understanding protein aggregation kinetics is critical for prediction of progression of diseases caused by amyloidosis, accumulation of aggregates in biotherapeutics during storage and engineering commercial nano-biomaterials. In this work, we have collected experimentally determined absolute protein aggregation rates and developed an SVM based regression model to predict absolute rates of protein and peptide aggregation near-physiological conditions. The regression model achieved a correlation coefficient of 0.72 with MAE of 0.91 (natural log of k_app, where k_app is in hour^-1) using leave-one-out cross-validation on a dataset of 82 non-redundant proteins/peptides. The model accounts for the experimental conditions (such as temperature, pH, ionic and protein concentration) and sequence-based properties. The amino acid sequence features revealed by this model as being important for aggregation kinetics, are also associated with the aggregation mechanism. In particular, inherent aggregation propensity of the protein/peptide sequence and number of aggregation prone regions (APRs) unpunctuated by the gatekeeping residues, were found to play important roles in the prediction of the absolute aggregation rates. This analysis shows that mechanism and kinetics of protein aggregation are coupled via common sequence attributes. The aggregation kinetic prediction method developed in this work is available at https://web.iitm.ac.in/bioinfo2/absolurate-pred/index.html.

An Unbalanced Synaptic Transmission: Cause or Consequence of the Amyloid Oligomers Neurotoxicity?

Amyloid-β (Aβ) 1-40 and 1-42 peptides are key mediators of synaptic and cognitive dysfunction in Alzheimer's disease (AD). Whereas in AD, Aβ is found to act as a pro-epileptogenic factor even before plaque formation, amyloid pathology has been detected among patients with epilepsy with increased risk of developing AD. Among Aβ aggregated species, soluble oligomers are suggested to be responsible for most of Aβ's toxic effects. Aβ oligomers exert extracellular and intracellular toxicity through different mechanisms, including interaction with membrane receptors and the formation of ion-permeable channels in cellular membranes. These damages, linked to an unbalance between excitatory and inhibitory neurotransmission, often result in neuronal hyperexcitability and neural circuit dysfunction, which in turn increase Aβ deposition and facilitate neurodegeneration, resulting in an Aβ-driven vicious loop. In this review, we summarize the most representative literature on the effects that oligomeric Aβ induces on synaptic dysfunction and network disorganization.

Effect of Methionine Sulfoxide on the Synthesis and Purification of Aggregation-Prone Peptides

A two-step synthesis for methionine-containing hydrophobic and/or aggregation-prone peptides is presented that takes advantage of the reversibility of methionine oxidation. The use of polar methionine sulfoxide as a building block in solid-phase peptide synthesis improves the synthesis quality and yields the crude peptide, with significantly improved solubility compared to the reduced species. This facilitates the otherwise often laborious peptide purification by high-performance liquid chromatography. The subsequent reduction proceeds quantitatively. This approach has been optimised with the methionine-rich Tar-DNA-binding protein 43 (307-347), but is also more generally applicable, as demonstrated by the syntheses of human calcitonin and two aggregation-prone peptides from the human prion protein.

Promoted Aggregation of Aβ on Lipid Bilayers in an Open Flowing System

Self-assembly of amyloid-β (Aβ) peptides in nonequilibrium, flowing conditions is associated with pathogenesis of Alzheimer's disease. We examined the role of biologically relevant, nonequilibrium, flowing conditions in the desorption, diffusion, and integration of Aβ-lipid assemblies at the membrane surface using a microchannel connected with microsyringes. A 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayer was formed on a glass substrate and incubated in Aβ solution under either a quiescent condition (no flow) or flowing condition for 24 h. Although dot-like aggregates (<1 μm) comprising Aβ fibrils formed on the DMPC membrane under the quiescent condition, larger plaque-like aggregates formed under the flowing condition, suggesting that nonequilibrium continuous flow governs the cytotoxicity of Aβ species. We propose that Aβ adsorption on the membrane surface involves spontaneous desorption of Aβ-lipid to form self-assembling aggregates, with this accelerated by surface shear forces. These findings suggest that nonequilibrium, flowing conditions influence inter/intra-molecular Aβ-fibril formation to trigger formation of amyloid plaques.

Effect of Surface Roughness on Aggregation of Polypeptide Chains: A Monte Carlo Study

The self-assembly of amyloidogenic peptides and proteins into fibrillar structures has been intensively studied for several decades, because it seems to be associated with a number of neurodegenerative diseases, such as Alzheimer’s and Parkinson’s disease. Therefore, understanding the molecular mechanisms of this phenomenon is important for identifying an effective therapy for the corresponding diseases. Protein aggregation in living organisms very often takes place on surfaces like membranes and the impact of a surface on this process depends not only on the surface chemistry but also on its topology. Our goal was to develop a simple lattice model for studying the role of surface roughness in the aggregation kinetics of polypeptide chains and the morphology of aggregates. We showed that, consistent with the experiment, an increase in roughness slows down the fibril formation, and this process becomes inhibited at a very highly level of roughness. We predicted a subtle catalytic effect that a slightly rough surface promotes the self-assembly of polypeptide chains but does not delay it. This effect occurs when the interaction between the surface and polypeptide chains is moderate and can be explained by taking into account the competition between energy and entropy factors.

3D-visualization of amyloid-β oligomer interactions with lipid membranes by cryo-electron tomography

Amyloid-β (Aβ) assemblies have been shown to bind to lipid bilayers. This can disrupt membrane integrity and cause a loss of cellular homeostasis, that triggers a cascade of events leading to Alzheimer's disease. However, molecular mechanisms of Aβ cytotoxicity and how the different assembly forms interact with the membrane remain enigmatic. Here we use cryo-electron tomography (cryoET) to obtain three-dimensional nano-scale images of various Aβ assembly types and their interaction with liposomes. Aβ oligomers and curvilinear protofibrils bind extensively to the lipid vesicles, inserting and carpeting the upper-leaflet of the bilayer. Aβ oligomers concentrate at the interface of vesicles and form a network of Aβ-linked liposomes, while crucially, monomeric and fibrillar Aβ have relatively little impact on the membrane. Changes to lipid membrane composition highlight a significant role for GM1-ganglioside in promoting Aβ-membrane interactions. The different effects of Aβ assembly forms observed align with the highlighted cytotoxicity reported for Aβ oligomers. The wide-scale incorporation of Aβ oligomers and curvilinear protofibrils into the lipid bilayer suggests a mechanism by which membrane integrity is lost.

Cryo-electron tomography 3D imaging of amyloid-β oligomers carpeting the surface of lipid bilayers in near native conditions.