Effect of the surface charge of artificial model membranes on the aggregation of amyloid β-peptide

Impact of Membrane Phospholipids and Exosomes on the Kinetics of Amyloid-β Fibril Assembly

Alzheimer's disease (AD) is linked with the self-association of the amyloid-β peptide (Aβ) into oligomers and fibrils. The brain is a lipid rich environment for Aβ to assemble, while the brain membrane composition varies in an age dependent manner, we have therefore monitored the influence of lipid bilayer composition on the kinetics of Aβ40 fibril assembly. Using global-fitting models of fibril formation kinetics, we show that the microscopic rate constant for primary nucleation is influenced by variations in phospholipid composition. Anionic phospholipids and particularly those with smaller headgroups shorten fibril formation lag-times, while zwitterionic phospholipids tend to extend them. Using a physiological vesicle model, we show cellular derived exosomes accelerate Aβ40 and Aβ42 fibril formation. Two distinct effects are observed, the presence of even small amounts of any phospholipid will impact the slope of the fibril growth curve. While subsequent additions of phospholipids only affect primary nucleation with the associated change in lag-times. Heightened anionic phospholipids and cholesterol levels are associated with aging and AD respectively, both these membrane components strongly accelerate primary nucleation during Aβ assembly, making a link between disrupted lipid metabolism and Alzheimer's disease.

Multifunctional Liposomes Targeting Amyloid-β Oligomers for Early Diagnosis and Therapy of Alzheimer's Disease

Early detection and treatment are crucial for Alzheimer's disease (AD) management. Current diagnostic and therapeutic methods focus on late-stage amyloid fibrils and plaques, overlooking toxic soluble amyloid β oligomers (AβOs) accumulating early in AD. A multifunctional liposome-based platform is designed for early diagnosis and therapy of AD, leveraging a novel self-assembled cyclic d,l-α-peptide (CP-2) that selectively targets AβOs. Biocompatible CP-2 conjugated liposomes (CP-2-LPs) effectively disrupt Aβ aggregation and mitigate Aβ-mediated toxicity in human neuroblastoma cells. In transgenic Caenorhabditis elegans AD models, CP-2-LPs significantly outperformed free CP-2 by improving cognitive and behavioral functions, extending lifespan, and reducing toxic AβO levels. Intravenous injection of fluorescently labeled CP-2-LPs reveals effective blood-brain barrier penetration, with significantly higher brain fluorescence in transgenic mice than WT, enabling precise diagnosis. These findings underscore CP-2-LPs as a valuable tool for early detection and targeted therapy in AD.

Investigating the Effects of the POPC-POPG Lipid Bilayer Composition on PAP248-286 Binding Using CG Molecular Dynamics Simulations

PAP248-286 is a fusogenic peptide derived from prostatic acid phosphatase, commonly found in human semen, and is known to mediate HIV fusion with cell membranes. In this study, we performed 120 independent coarse-grained molecular dynamics simulations to investigate the spontaneous binding of PAP248-286 monomers, considering both charged and neutral histidine (His) residues, to membrane bilayers composed of different lipid compositions: 100% POPC, 70% POPC-30% POPG, and 50% POPC-50% POPG. Our simulations revealed that PAP248-286 displayed spontaneous binding to the membrane, with increased binding observed in the presence of anionic lipid POPG. Specifically, in systems containing 30% and 50% POPG lipids, monomer residues, particularly in the systems containing charged histidine (His) residues, exhibited prolonged binding with the membrane. Furthermore, our simulations indicated that PAP248-286 adopted a parallel orientation with the membrane, exposing its positively charged residues to the lipid bilayer. Interestingly, systems containing charged His residues showed a higher lipid occupancy around the peptide. These findings are consistent with previous experimental data, suggesting that PAP248-286 binding is enhanced in membranes with charged His residues, resembling the conditions found in the acidic vaginal pH environment. The results of our study provide further insights into the molecular mechanisms underlying the membrane binding of PAP248-286, contributing to our understanding of its potential role in HIV fusion and infection.

Imaging Amyloid-β Membrane Interactions: Ion-Channel Pores and Lipid-Bilayer Permeability in Alzheimer's Disease

The accumulation of the amyloid-β peptides (Aβ) is central to the development of Alzheimer's disease. The mechanism by which Aβ triggers a cascade of events that leads to dementia is a topic of intense investigation. Aβ self-associates into a series of complex assemblies with different structural and biophysical properties. It is the interaction of these oligomeric, protofibril and fibrillar assemblies with lipid membranes, or with membrane receptors, that results in membrane permeability and loss of cellular homeostasis, a key event in Alzheimer's disease pathology. Aβ can have an array of impacts on lipid membranes, reports have included: a carpeting effect; a detergent effect; and Aβ ion-channel pore formation. Recent advances imaging these interactions are providing a clearer picture of Aβ induced membrane disruption. Understanding the relationship between different Aβ structures and membrane permeability will inform therapeutics targeting Aβ cytotoxicity.

Imaging Amyloid-β Membrane Interactions: Ion-Channel Pores and Lipid-Bilayer Permeability in Alzheimer's Disease

The accumulation of the amyloid-β peptides (Aβ) is central to the development of Alzheimer's disease. The mechanism by which Aβ triggers a cascade of events that leads to dementia is a topic of intense investigation. Aβ self-associates into a series of complex assemblies with different structural and biophysical properties. It is the interaction of these oligomeric, protofibril and fibrillar assemblies with lipid membranes, or with membrane receptors, that results in membrane permeability and loss of cellular homeostasis, a key event in Alzheimer's disease pathology. Aβ can have an array of impacts on lipid membranes, reports have included: a carpeting effect; a detergent effect; and Aβ ion-channel pore formation. Recent advances imaging these interactions are providing a clearer picture of Aβ induced membrane disruption. Understanding the relationship between different Aβ structures and membrane permeability will inform therapeutics targeting Aβ cytotoxicity.

Tuning the surface charge of phospholipid bilayers inhibits insulin fibrilization

The interactions between proteins and materials, in particular lipid bilayers, have been studied extensively for their relevance in diseases and for the formulation of protein-based therapeutics and vaccines. However, the precise rules by which material properties induce favorable or unfavorable structural states in biomolecules are incompletely understood, and as a result, the rational design of materials remains challenging. Here, we investigated the influence of lipid bilayers (in the form of small unilamellar vesicles) on the formation of insulin amyloid fibrils using a fibril-specific assay (thioflavin T), polyacrylamide gel electrophoresis, and circular dichroism spectroscopy. Lipid bilayers composed of equal mixtures of cationic and anionic lipids effectively inhibited fibril formation and stabilized insulin in its native conformation. However, other lipid bilayer compositions failed to inhibit fibril formation or even destabilized insulin, exacerbating fibrilization and/or non-amyloid aggregation. Our findings suggest that electrostatic interactions with lipid bilayers can play a critical role in stabilizing or destabilizing insulin, and preventing the conversion of insulin to its amyloidogenic, disease-associated state.

Ganglioside-Enriched Phospholipid Vesicles Induce Cooperative Aβ Oligomerization and Membrane Disruption

A major hallmark of Alzheimer's disease (AD) is the accumulation of extracellular aggregates of amyloid-β (Aβ). Structural polymorphism observed among Aβ fibrils in AD brains seem to correlate with the clinical subtypes suggesting a link between fibril polymorphism and pathology. Since fibrils emerge from a templated growth of low-molecular-weight oligomers, understanding the factors affecting oligomer generation is important. Membrane lipids are key factors to influence early stages of Aβ aggregation and oligomer generation, which cause membrane disruption. We have previously demonstrated that conformationally discrete Aβ oligomers can be generated by modulating the charge, composition, and chain length of lipids and surfactants. Here, we extend our studies into liposomal models by investigating Aβ oligomerization on large unilamellar vesicles (LUVs) of total brain extracts (TBE), reconstituted lipid rafts (LRs), or 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). Varying the vesicle composition by specifically increasing the amount of GM1 gangliosides as a constituent, we found that only GM1-enriched liposomes induce the formation of toxic, low-molecular-weight oligomers. Furthermore, we found that the aggregation on liposome surface and membrane disruption are highly cooperative and sensitive to membrane surface characteristics. Numerical simulations confirm such a cooperativity and reveal that GM1-enriched liposomes form twice as many pores as those formed in the absence GM1. Overall, this study uncovers mechanisms of cooperativity between oligomerization and membrane disruption under controlled lipid compositional bias, and refocuses the significance of the early stages of Aβ aggregation in polymorphism, propagation, and toxicity in AD.

Macrocyclic Peptides Derived from Familial Alzheimer's Disease Mutants Show Charge-Dependent Oligomeric Assembly and Toxicity

This work probes the role of charge in the oligomeric assembly, toxicity, and membrane destabilization of a series of peptides derived from Aβ and the E22Q and E22K familial mutants. In the mutant Aβ peptides, an acidic residue (E) is replaced with either a neutral or basic residue (Q or K), thus altering the net charge of the peptide. Acetylation at peripheral positions permits modulation of charge of the peptides and allows investigation of the role of charge in their oligomeric assembly, cytotoxicity, and membrane disruption. Peptides with the same net charge generally behave similarly even if the amino acid residue at position 22 differs. As the net charge of the peptide decreases, so does the extent of assembly, cytotoxicity, and membrane destabilization, which were determined using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, lactate dehydrogenase (LDH)-release assays with SH-SY5Y cells, and dye leakage assays using liposomes. These findings suggest that the charge of the amino acid side chain, rather than its size or hydrophobicity, accounts for the differences in the oligomeric assembly and toxicity of the E22 familial mutants of Aβ.

Amyloid-β peptide 37, 38 and 40 individually and cooperatively inhibit amyloid-β 42 aggregation

The pathology of Alzheimer's disease is connected to the aggregation of β-amyloid (Aβ) peptide, which in vivo exists as a number of length-variants. Truncations and extensions are found at both the N- and C-termini, relative to the most commonly studied 40- and 42-residue alloforms. Here, we investigate the aggregation of two physiologically abundant alloforms, Aβ₃₇ and Aβ₃₈, as pure peptides and in mixtures with Aβ₄₀ and Aβ₄₂. A variety of molar ratios were applied in quaternary mixtures to investigate whether a certain ratio is maximally inhibiting of the more toxic alloform Aβ₄₂. Through kinetic analysis, we show that both Aβ₃₇ and Aβ₃₈ self-assemble through an autocatalytic secondary nucleation reaction to form fibrillar β-sheet-rich aggregates, albeit on a longer timescale than Aβ₄₀ or Aβ₄₂. Additionally, we show that the shorter alloforms co-aggregate with Aβ₄₀, affecting both the kinetics of aggregation and the resulting fibrillar ultrastructure. In contrast, neither Aβ₃₇ nor Aβ₃₈ forms co-aggregates with Aβ₄₂; however, both short alloforms reduce the rate of Aβ₄₂ aggregation in a concentration-dependent manner. Finally, we show that the aggregation of Aβ₄₂ is more significantly impeded by a combination of Aβ₃₇, Aβ₃₈, and Aβ₄₀ than by any of these alloforms independently. These results demonstrate that the aggregation of any given Aβ alloform is significantly perturbed by the presence of other alloforms, particularly in heterogeneous mixtures, such as is found in the extracellular fluid of the brain.

The role of membranes in function and dysfunction of intrinsically disordered amyloidogenic proteins

Membrane-protein interactions play a major role in human physiology as well as in diseases pathology. Interaction of a protein with the membrane was previously thought to be dependent on well-defined three-dimensional structure of the protein. In recent decades, however, it has become evident that a large fraction of the proteome, particularly in eukaryotes, stays disordered in solution and these proteins are termed as intrinsically disordered proteins (IDPs). Also, a vast majority of human proteomes have been reported to contain substantially long disordered regions, called intrinsically disordered regions (IDRs), in addition to the structurally ordered regions. IDPs exist in an ensemble of conformations and the conformational flexibility enables IDPs to achieve functional diversity. IDPs (and IDRs) are found to be important players in cell signaling, where biological membranes act as anchors for signaling cascades. Therefore, IDPs modulate the membrane architectures, at the same time membrane composition also affects the binding of IDPs. Because of intrinsic disorders, misfolding of IDPs often leads to formation of oligomers, protofibrils and mature fibrils through progressive self-association. Accumulation of amyloid-like aggregates of some of the IDPs is a known causative agent for numerous diseases. In this chapter we highlight recent advances in understanding membrane interactions of some of the intrinsically disordered proteins involved in the pathogenesis of human diseases.

Caffeic acid for the prevention and treatment of Alzheimer's disease: The effect of lipid membranes on the inhibition of aggregation and disruption of Aβ fibrils

The onset of Alzheimer's disease (AD) is triggered by the aggregation of amyloid β (Aβ) peptides which leads to the formation of fibrils. Molecules that are able to inhibit fibrillation and/or disrupt fibrils have aroused interest for AD therapy. Fibrillation is a complex process highly dependent on the surrounding environment. One of the most relevant factors affecting Aβ aggregation is the presence of cellular membranes. Here, the ability of caffeic acid (CA) in preventing the Aβ_1-42 aggregation and disaggregating mature fibrils was evaluated in a membrane-like environment and in a bulk solution for comparison. To this end, liposomes were used as in vitro models of neuronal membranes. CA exhibited strong activity in inhibiting the fibrillation of Aβ_1-42 in the aqueous medium, which remained in the presence of liposomes. Furthermore, CA disrupted instantly preformed fibrils in the aqueous medium. However, the CA's disaggregating activity was disturbed by the presence of lipid membranes. Instead of being immediate, the CA's disaggregating activity increased over time. The moderate affinity of CA for the lipid bilayer may explain the distinct fibrils disaggregation profiles. These findings emphasize the therapeutic potential of CA in preventing and treating AD, thus justifying further investigations in animal models.

Influence of in vitro neuronal membranes on the anti-amyloidogenic activity of gallic acid: Implication for the therapy of Alzheimer's disease

Molecules inhibiting the amyloid beta (Aβ) peptide aggregation and/or disaggregating mature fibrils are a promising approach for the Alzheimer's disease (AD) therapy, as the Aβ fibrillation is one of the key triggers of the disease. Gallic acid (GA) is a phenolic acid with anti-amyloidogenic activity against Aβ in buffered solutions. However, there is still no evidence of these properties in vivo. Given the rate of failures of AD drug development, there is a huge demand of replicating the in vivo environment in in vitro studies, thus allowing to stop earlier the study of molecules with no effect in vivo. Thus, this study aims to evaluate the effect of in vitro neuronal membranes on the GA's ability in preventing Aβ_1-42 aggregation and disrupting preformed fibrils. To this end, liposomes were employed to mimic the cell membrane environment. The results reveal that the lipid membranes did not affect the GA's ability in inhibiting Aβ_1-42 fibrillation. However, in vitro neuronal membranes modulate the GA-induced Aβ fibrils disaggregation, which may be related with the moderate affinity of the compound for the lipid membrane. Even so, GA presented strong anti-amyloidogenic properties in the cell membrane-like environment. This work highlights the promising value of GA on preventing and treating AD, thus justifying its study in animal models.

Vitamin B12 Inhibits Aβ Fibrillation and Disaggregates Preformed Fibrils in the Presence of Synthetic Neuronal Membranes

The aggregation of amyloid β (Aβ) peptide with subsequent formation of fibrils which deposit in senile plaques is considered one of the key triggers of Alzheimer's disease (AD). Molecules targeting the inhibition of Aβ fibrillation and/or the disruption of Aβ fibrils are thus promising approaches for the medical prevention and treatment of AD. However, amyloid formation is a complex process strongly influenced by the cellular environment, such as cell membranes, which may affect the effectiveness of therapeutic molecules. In this study, the effect of the vitamin B12 (VB12) on the formation and disaggregation of Aβ_1-42 fibrils was investigated in the presence of artificial neuronal membranes mimicked by liposomes. Evidence showed that VB12 slows down the Aβ fibrillization and reduces the content of fibrils in aqueous solution. Moreover, the vitamin exhibited a strong ability to disrupt preformed fibrils. However, the presence of lipid vesicles compromised the VB12's antiamyloidogenic properties due to the competitive interaction of the vitamin with the lipid membrane and the Aβ peptide. Even so, VB12 was effective in inhibiting the fibril formation and disaggregating fibrils in the lipid membrane environment. Thereby, these results indicate that VB12 could be a promising molecule both for the prevention and cure of AD, thus warranting its study in animal models.

Utilizing magnetic resonance techniques to study membrane interactions of amyloid peptides

Alzheimer's disease (AD) is a common neurodegenerative condition that involves the extracellular accumulation of amyloid plaques predominantly consisting of Aβ peptide aggregates. The amyloid plaques and soluble oligomeric species of Aβ are believed to be the major cause of synaptic dysfunction in AD brain and their cytotoxic mechanisms have been proposed to involve interactions with cell membranes. In this review, we discuss our solid-state nuclear magnetic resonance (ssNMR) studies of Aβ interactions with model membranes.

The Role of Amyloid β-Biomembrane Interactions in the Pathogenesis of Alzheimer's Disease: Insights from Liposomes as Membrane Models

The aggregation and deposition of amyloid β (Aβ) peptide onto neuronal cells, with consequent cellular membrane perturbation, are central to the pathogenesis of Alzheimer's disease (AD). Substantial evidence reveals that biological membranes play a key role in this process. Thus, elucidating the mechanisms by which Aβ interacts with biomembranes and becomes neurotoxic is fundamental to developing effective therapies for this devastating progressive disease. However, the structural basis behind such interactions is not fully understood, largely due to the complexity of natural membranes. In this context, lipid biomembrane models provide a simplified way to mimic the characteristics and composition of membranes. Aβ-biomembrane interactions have been extensively investigated applying artificial membrane models to elucidate the molecular mechanisms underlying the AD pathogenesis. This review summarizes the latest findings on this field using liposomes as biomembrane model, as they are considered the most promising 3D model. The current challenges and future directions are discussed.

Lipid membranes induce structural conversion from amyloid oligomers to fibrils

Formation of amyloid oligomers and fibrils underlies the pathogenesis of a number of neurodegenerative diseases such as Alzheimer's. One mechanism of action by which Aβ aggregates cause neuronal toxicity is through interactions with cellular membranes. Aβ aggregates have been shown to disrupt membrane integrity via pore formation, membrane thinning, or lipid extraction. At the same time, lipid membranes also affect the rate of Aβ aggregation and remodel pre-formed Aβ fibrils. Here we show that Aβ42 globulomers, a type of well-characterized and stable Aβ oligomers, convert to amyloid fibrils in the presence of DOPC liposomes. Electron paramagnetic resonance studies show that the fibrils converted from Aβ42 globulomers adopt the same structure as fibrils formed directly from monomers. Our results suggest that the interactions between Aβ oligomers and cellular membranes are dynamic. By converting Aβ oligomers to fibrils, the lipid membrane can reduce the membrane-disrupting activities caused by these oligomers. Modulation of Aβ-membrane interactions as a therapeutic strategy should take into account the dynamic nature of these interactions.

Mixed Phospholipid Vesicles Catalytically Inhibit and Reverse Amyloid Fibril Formation

While many approaches to reduce fibrillation of amyloid-β (Aβ) have been aimed at slowing fibril formation, the degradation of fibrils remains challenging. We provide insight into fibril degradation as well as the inhibition of fiber formation by lipid vesicles composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine and 1,2-dioleoyl-sn-glycero-3-phospho-(1'-rac-glycerol). In the presence of vesicles with the optimal lipid composition, fibril formation was inhibited up to 76%. Additionally, by tuning the lipid composition, mature fibril content decreased up to 74% and the β-sheet content of Aβ was significantly reduced. The reduction in fibril and β-sheet content was consistent with a decrease in fibril diameter and could be attributed to the chaperone-like activity of the mixed vesicles. While demonstrating this remarkable activity, our findings present new evidence that lipid composition has a significant effect on the strength of the interaction between lipid bilayers and Aβ peptides/fibrils. This understanding has intriguing therapeutic implications in treating protein misfolding diseases.

Molecular chirality mediated amyloid formation on phospholipid surfaces

One of the neuropathological features of Alzheimer's disease (AD) is the misfolding of amyloid-β to form amyloid aggregates, a process highly associated with biological membranes. However, how molecular chirality affects the amyloid formation on phospholipid surfaces has seldom been reported. Here, l- and d-aspartic acid-modified 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (l-/d-Asp–DPPE) is synthesized to construct chiral phospholipid bilayers. We discover that the l-Asp–DPPE liposomes slightly inhibit the Aβ(1–40) nucleation process but cannot affect the oligomer elongation process. By contrast, the d-Asp–DPPE liposomes strongly inhibit both nucleation and elongation of the peptide. Notably, l- and d-Asp–DPPE liposomes not only have good biocompatibility but can also rescue Aβ(1–40)-aggregation induced cytotoxicity with significant chiral discrimination, in which the cell viability is higher in the presence of d-Asp–DPPE liposomes. Mechanism analysis and molecular dynamics simulation clearly demonstrate that differential electrostatic interactions of Lys16 in Aβ(1–40) with l- or d-Asp on the phospholipid contribute to the remarkable chiral discrimination. This study provides a deeper understanding of the crucial amyloidosis process from the perspective of the chiral interface and reveals that the convergence of d-amino acids with the liposomes might be a feasible route for AD prevention.

A remarkable inhibition effect and chiral discrimination are observed when the amyloid peptide aggregates on chiral phospholipid surfaces.

Acetylation of Aβ40 Alters Aggregation in the Presence and Absence of Lipid Membranes

A hallmark of Alzheimer's disease (AD) is the formation of senile plaques comprised of the β-amyloid (Aβ) peptide. Aβ fibrillization is a complex nucleation-dependent process involving a variety of metastable intermediate aggregates and features the formation of inter- and intramolecular salt bridges involving lysine residues, K16 and K28. Cationic lysine residues also mediate protein-lipid interactions via association with anionic lipid headgroups. As several toxic mechanisms attributed to Aβ involve membrane interactions, the impact of acetylation on Aβ₄₀ aggregation in the presence and absence of membranes was determined. Using chemical acetylation, varying mixtures of acetylated and nonacetylated Aβ₄₀ were produced. With increasing acetylation, fibril and oligomer formation decreased, eventually completely arresting fibrillization. In the presence of total brain lipid extract (TBLE) vesicles, acetylation reduced the interaction of Aβ₄₀ with membranes; however, fibrils still formed at near complete levels of acetylation. Additionally, the combination of TBLE and acetylated Aβ promoted annular aggregates. Finally, toxicity associated with Aβ₄₀ was reduced with increasing acetylation in a cell culture assay. These results suggest that in the absence of membranes that the cationic character of lysine plays a major role in fibril formation. However, acetylation promotes unique aggregation pathways in the presence of lipid membranes.

Amyloid-β Peptide Triggers Membrane Remodeling in Supported Lipid Bilayers Depending on Their Hydrophobic Thickness

Amyloid-β (Aβ) peptide has been implicated in Alzheimer's disease, which is a leading cause of death worldwide. The interaction of Aβ peptides with the lipid bilayers of neuronal cells is a critical step in disease pathogenesis. Recent evidence indicates that lipid bilayer thickness influences Aβ membrane-associated aggregation, while understanding how Aβ interacts with lipid bilayers remains elusive. To address this question, we employed supported lipid bilayer (SLB) platforms composed of different-length phosphatidylcholine (PC) lipids (C12:0 DLPC, C18:1 DOPC, C18:1-C16:0 POPC), and characterized the resulting interactions with soluble Aβ monomers. Quartz crystal microbalance-dissipation (QCM-D) experiments identified concentration-dependent Aβ peptide adsorption onto all tested SLBs, which was corroborated by fluorescence recovery after photobleaching (FRAP) experiments indicating that higher Aβ concentrations led to decreased membrane fluidity. These commonalities pointed to strong Aβ peptide-membrane interactions in all cases. Notably, time-lapsed fluorescence microscopy revealed major differences in Aβ-induced membrane morphological responses depending on SLB hydrophobic thickness. For thicker DOPC and POPC SLBs, membrane remodeling involved the formation of elongated tubule and globular structures as a passive means to regulate membrane stress depending on Aβ concentration. In marked contrast, thin DLPC SLBs were not able to accommodate extensive membrane remodeling. Taken together, our findings reveal that membrane thickness influences the membrane morphological response triggered upon Aβ adsorption.

Interactions between amyloid β peptide and lipid membranes

The presence of amyloid plaques in the brain is a typical characteristic of Alzheimer's disease (AD). Amyloid plaques are formed from the deposits of aggregated amyloid β peptide (Aβ). The toxicity induced by Aβ aggregates is correlated with Aβ-membrane interactions. The mutual influences between aggregation and membranes are complicated and unclear. In recent years advanced experiments and findings are emerging to give us more detailed information on Aβ-membrane interactions. In this review, we mainly focus on the Aβ-membrane interactions and membrane-induced Aβ structures. The mechanism of Aβ-membrane interactions is also summarized, which provides insights into the prevention and treatment of AD.

Binding of protofibrillar Aβ trimers to lipid bilayer surface enhances Aβ structural stability and causes membrane thinning

Alzheimer's disease, a common neurodegenerative disease, is characterized by the aggregation of amyloid-β (Aβ) peptides. The interactions of Aβ with membranes cause changes in membrane morphology and ion permeation, which are responsible for its neurotoxicity and can accelerate fibril growth. However, the Aβ-lipid interactions and how these induce membrane perturbation and disruption at the atomic level and the consequences for the Aβ organization are not entirely understood. Here, we perform multiple atomistic molecular dynamics simulations on three protofibrillar Aβ9-40 trimers. Our simulations show that, regardless of the morphologies and the initial orientations of the three different protofibrillar Aβ9-40 trimers, the N-terminal β-sheet of all trimers preferentially binds to the membrane surface. The POPG lipid bilayers enhance the structural stability of protofibrillar Aβ trimers by stabilizing inter-peptide β-sheets and D23-K28 salt-bridges. The interaction causes local membrane thinning. We found that the trimer structure related to Alzheimer's disease brain tissue () is the most stable both in water solution and at membrane surface, and displays slightly stronger membrane perturbation capability. These results provide mechanistic insights into the membrane-enhanced structural stability of protofibrillar Aβ oligomers and the first step of Aβ-induced membrane disruption at the atomic level.

The molecular behavior of a single β-amyloid inside a dipalmitoylphosphatidylcholine bilayer at three different temperatures: An atomistic simulation study: Aβ interaction with DPPC: Atomistic simulation

The behavior of a single Aβ40 molecule within a dipalmitoylphosphatidylcholine (DPPC) bilayer was studied by all-atom molecular dynamics simulations. The effect of membrane structure was investigated on Aβ40 behavior, secondary structure, and insertion depth. Simulations were performed at three temperatures (323, 310, and 300 K) to probe three different bilayer fluidities. Results show that at all above temperatures, the peptide contains two short helices, coil, bend, and turn structures. At 300 K, the peptide contains a region with β structure in C-terminal region. Our results also show that Aβ decreases the bilayer thickness and the order of lipids in its vicinity which leads to water insertion into the bilayer and concomitant increase in the local fluidity. The peptide remains embedded in the bilayer at all temperatures, and become inserted into the bilayer up to several residues at 323 and 310 K. At 310 and 300 K, the dominant interaction energy between Aβ and bilayer changes from electrostatic to van der Waals. It can be proposed that at higher temperatures (e.g., 323 K), Lys28 and the C-terminal region of the peptide play the role of two anchors that keep Aβ inside the top leaflet. This study demonstrates that Aβ molecule can perturb the integrity of cellular membranes. Proteins 2017; 85:1298-1310. © 2017 Wiley Periodicals, Inc.

Role of membrane biophysics in Alzheimer's-related cell pathways

Cellular membrane alterations are commonly observed in many diseases, including Alzheimer's disease (AD). Membrane biophysical properties, such as membrane molecular order, membrane fluidity, organization of lipid rafts, and adhesion between membrane and cytoskeleton, play an important role in various cellular activities and functions. While membrane biophysics impacts a broad range of cellular pathways, this review addresses the role of membrane biophysics in amyloid-β peptide aggregation, Aβ-induced oxidative pathways, amyloid precursor protein processing, and cerebral endothelial functions in AD. Understanding the mechanism(s) underlying the effects of cell membrane properties on cellular processes should shed light on the development of new preventive and therapeutic strategies for this devastating disease.

Could α-synuclein amyloid-like aggregates trigger a prionic neuronal invasion?

Parkinson's disease (PD), a progressive neurodegenerative disease primarily affecting voluntary and controlled movement, is characterized by abnormal accumulations of α-synuclein (α-syn) in intraneuronal Lewy bodies. In the last years, the increased number of evidences from both the in vitro and in vivo studies has shown the ability of α-syn to misfold in amyloid conformations and to spread via neuron-to-neuron transmission, suggesting a prion-like behaviour. However, in contrast to prion protein (PrP), α-syn transmission is far from neuronal invasion. The high neuronal toxicity of both mature fibres and oligomeric species, as well as the intracellular localization of the protein and the difficulty to be secreted, could be key factors impeding the prion ability of α-syn aggregates.

The Arctic mutation accelerates Aβ aggregation in SDS through reducing the helical propensity of residues 15-25

Mutations within the β-amyloid peptide (Aβ) sequence that cause early onset familial Alzheimer's disease (FAD) have been shown to promote Aβ aggregation. How these FAD-related mutants increase the aggregative ability of Aβ is not fully understood. Here, we characterized the effect of the Arctic variant (E22G) on the conformational stability of Aβ using various forms of spectroscopy and kinetic analyses, including nuclear magnetic resonance (NMR), circular dichroism (CD) spectroscopy, Fourier-transform infrared (FT-IR) spectroscopy and transmission electron microscopy (TEM). The E22G mutation in the Arctic variant reduced the α-helical propensity and conformational stability of Aβ on residues 15-25. This mutation also caused an increase in both α-helix-to-β-strand conversion and fibril nucleation rates. Our results suggest that the α-helical propensity of residues 15-25 may play a determinant role in the aggregative ability of Aβ. This may provide a structural basis for understanding the molecular mechanism of Aβ aggregation.

Amyloid formation by human carboxypeptidase D transthyretin-like domain under physiological conditions

Protein aggregation is linked to a growing list of diseases, but it is also an intrinsic property of polypeptides, because the formation of functional globular proteins comes at the expense of an inherent aggregation propensity. Certain proteins can access aggregation-prone states from native-like conformations without the need to cross the energy barrier for unfolding. This is the case of transthyretin (TTR), a homotetrameric protein whose dissociation into its monomers initiates the aggregation cascade. Domains with structural homology to TTR exist in a number of proteins, including the M14B subfamily carboxypeptidases. We show here that the monomeric transthyretin-like domain of human carboxypeptidase D aggregates under close to physiological conditions into amyloid structures, with the population of folded but aggregation-prone states being controlled by the conformational stability of the domain. We thus confirm that the TTR fold keeps a generic residual aggregation propensity upon folding, resulting from the presence of preformed amyloidogenic β-strands in the native state. These structural elements should serve for functional/structural purposes, because they have not been purged out by evolution, but at the same time they put proteins like carboxypeptidase D at risk of aggregation in biological environments and thus can potentially lead to deposition diseases.

When amyloids become prions

The conformational diseases, linked to protein aggregation into amyloid conformations, range from non-infectious neurodegenerative disorders, such as Alzheimer disease (AD), to highly infectious ones, such as human transmissible spongiform encephalopathies (TSEs). They are commonly known as prion diseases. However, since all amyloids could be considered prions (from those involved in cell-to-cell transmission to those responsible for real neuronal invasion), it is necessary to find an underlying cause of the different capacity to infect that each of the proteins prone to form amyloids has. As proposed here, both the intrinsic cytotoxicity and the number of nuclei of aggregation per cell could be key factors in this transmission capacity of each amyloid.

The involvement of lipids in Alzheimer's disease

It has been estimated that Alzheimer's disease (AD), the most common form of dementia, will affect approximately 81 million individuals by 2040. To date, the actual cause and cascade of events in the progression of this disease have not been fully determined. Furthermore, there is currently no definitive blood test or simple diagnostic method for AD. Considerable efforts have been put into proteomic approaches to develop a diagnostic blood test, but to date these efforts have not been successful. More recently, there has been a stronger focus on lipidomic studies in the hope of increasing our understanding of the underlying mechanisms leading to AD and developing an AD blood test. It is well known that the strongest genetic risk factor for AD is the ε4 variant of apolipoprotein E (APOE). Evidence suggests that the ApoE protein, a major lipid transporter, plays a key role in the pathogenesis of AD, and its role in both normal and aberrant lipid metabolism warrants further extensive investigation. Here, we review ApoE-lipid interactions, as well as the roles that lipids may play in the pathogenesis of AD.

Atomic force microscopy to study molecular mechanisms of amyloid fibril formation and toxicity in Alzheimer's disease

Alzheimer's disease (AD) is a devastating neurodegenerative disease characterized by dementia and memory loss for which no cure or effective prevention is currently available. Neurodegeneration in AD is linked to formation of amyloid plaques found in brain tissues of Alzheimer's patients during post-mortem examination. Amyloid plaques are composed of amyloid fibrils and small oligomers - insoluble protein aggregates. Although amyloid plaques are found on the neuronal cell surfaces, the mechanism of amyloid toxicity is still not well understood. Currently, it is believed that the cytotoxicity is a result of the nonspecific interaction of small soluble amyloid oligomers (rather than longer fibrils) with the plasma membrane. In recent years, nanotechnology has contributed significantly to understanding the structure and function of lipid membranes and to the study of the molecular mechanisms of membrane-associated diseases. We review the current state of research, including applications of the latest nanotechnology approaches, on the interaction of lipid membranes with the amyloid-β (Aβ) peptide in relation to amyloid toxicity. We discuss the interactions of Aβ with model lipid membranes with a focus to demonstrate that composition, charge and phase of the lipid membrane, as well as lipid domains and rafts, affect the binding of Aβ to the membrane and contribute to toxicity. Understanding the role of the lipid membrane in AD at the nanoscale and molecular level will contribute to the understanding of the molecular mechanism of amyloid toxicity and may aid into the development of novel preventive strategies to combat AD.

Specific domains of Aβ facilitate aggregation on and association with lipid bilayers

A hallmark of Alzheimer's disease, a late-onset neurodegenerative disease, is the deposition of neuritic amyloid plaques composed of aggregated forms of the β-amyloid peptide (Aβ). Aβ forms a variety of nanoscale, toxic aggregate species ranging from small oligomers to fibrils. Aβ and many of its aggregate forms strongly interact with lipid membranes, which may represent an important step in several toxic mechanisms. Understanding the role that specific regions of Aβ play in regulating its aggregation and interaction with lipid membranes may provide insights into the fundamental interaction between Aβ and cellular surfaces. We investigated the interaction and aggregation of several Aβ fragments (Aβ1-11, Aβ1-28, Aβ10-26, Aβ12-24, Aβ16-22, Aβ22-35, and Aβ1-40) in the presence of supported model total brain lipid extract (TBLE) bilayers. These fragments represent a variety of chemically unique domains within Aβ, that is, the extracellular domain, the central hydrophobic core, and the transmembrane domain. Using scanning probe techniques, we elucidated aggregate morphologies for these different Aβ fragments in free solution and in the presence of TBLE bilayers. These fragments formed a variety of oligomeric and fibrillar aggregates under free solution conditions. Exposure to TBLE bilayers resulted in distinct aggregate morphologies compared to free solution and changes in bilayer stability dependent on the Aβ sequence. Aβ10-26, Aβ16-22, Aβ22-35, and Aβ1-40 aggregated into a variety of distinct fibrillar aggregates and disrupted the bilayer structure, resulting in altered mechanical properties of the bilayer. Aβ1-11, Aβ1-28, and Aβ12-24 had minimal interaction with lipid membranes, forming only sparse oligomers.

Biophysical insights into how surfaces, including lipid membranes, modulate protein aggregation related to neurodegeneration

There are a vast number of neurodegenerative diseases, including Alzheimer’s disease (AD), Parkinson’s disease (PD), and Huntington’s disease (HD), associated with the rearrangement of specific proteins to non-native conformations that promotes aggregation and deposition within tissues and/or cellular compartments. These diseases are commonly classified as protein-misfolding or amyloid diseases. The interaction of these proteins with liquid/surface interfaces is a fundamental phenomenon with potential implications for protein-misfolding diseases. Kinetic and thermodynamic studies indicate that significant conformational changes can be induced in proteins encountering surfaces, which can play a critical role in nucleating aggregate formation or stabilizing specific aggregation states. Surfaces of particular interest in neurodegenerative diseases are cellular and subcellular membranes that are predominately comprised of lipid components. The two-dimensional liquid environments provided by lipid bilayers can profoundly alter protein structure and dynamics by both specific and non-specific interactions. Importantly for misfolding diseases, these bilayer properties can not only modulate protein conformation, but also exert influence on aggregation state. A detailed understanding of the influence of (sub)cellular surfaces in driving protein aggregation and/or stabilizing specific aggregate forms could provide new insights into toxic mechanisms associated with these diseases. Here, we review the influence of surfaces in driving and stabilizing protein aggregation with a specific emphasis on lipid membranes.