Preferential accumulation of Aβ(1−42) on gel phase domains of lipid bilayers: An AFM and fluorescence study

Classification of binding property of amyloid β to lipid membranes: Membranomic research using quartz crystal microbalance combined with the immobilization of lipid planar membranes

A biomembrane-related fibrillogenesis of Amyloid β from Alzheimer' disease (Aβ) is closely related to its accumulation behavior. A binding property of Aβ peptides from Alzheimer' disease to lipid membranes was then classified by a quartz crystal microbalance (QCM) method combined with an immobilization technique using thiol self-assembled membrane. The accumulated amounts of Aβ, Δfmax, was determined from the measurement of the maximal frequency reduction using QCM. The plots of Δfmax to Aβ concentration gave the slope and saturated value of Δfmax, (Δfmax)sat that are the parameters for binding property of Aβ to lipid membranes. Therefore, the Aβ-binding property on lipid membranes was classified by the slope and (Δfmax)sat. The plural lipid system was described as X + Y where X = L1, L1/L2, and L1/L2/L3. The slope and (Δfmax)sat values plotted as a function of mixing ratio of Y to X was classified on a basis of the lever principle (LP). The LP violation observed in both parameters resulted from the formation of the crevice or pothole, as Aβ-specific binding site, generated at the boundary between ld and lo phases. The LP violation observed only in the slope resulted from glycolipid-rich domain acting as Aβ-specific binding site. Furthermore, lipid planar membranes indicating strong LP violation favored strong fibrillogenesis. Especially, lipid planar membranes indicating the LP violation only in the slope induced lateral aggregated and spherulitic fibrillar aggregates. Thus, the classification of Aβ binding property on lipid membranes appeared to be related to the fibrillogenesis with a certain morphology.

Real-time monitoring of the amyloid β1-42 monomer-to-oligomer channel transition using a lipid bilayer system

This study describes the observation of the transformation of monomeric amyloid β1-42 (Aβ42) into oligomers in a lipid membrane utilizing a lipid bilayer system for electrophysiological measurement. The relevance of oligomers and protofibrils in Alzheimer's disease (AD) is underscored given their significant neurotoxicity. By closely monitoring the shift of Aβ42 from its monomeric state to forming oligomeric channels in phospholipid membranes, we noted that this transformation transpired within a 2-h frame. We manipulated the lipid membrane's constitution with components such as glycerophospholipid, porcine brain total lipid extract, sphingomyelin (SM), and cholesterol (Chol.) to effectively imitate nerve cell membranes. Interesting findings showcased Chol.'s ability to foster stable oligomeric channel formation in the lipid membrane, with SM and GM1 lipids potentially enhancing channel formation as well. Additionally, the study identified the potential of a catechin derivative, epigallocatechin gallate (EGCG), in obstructing oligomerization. With EGCG present in the outer solution of the Aβ42-infused membrane, a noteworthy reduction in channel current was observed, suggesting the successful inhibition of oligomerization. This conclusion held true in both, prior and subsequent, stages of oligomerization. Our findings shed light on the toxicity of oligomers, promising invaluable information for future advancements in AD treatment strategies.

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.

l-Phenylalanine Partitioning Mechanisms in Model Biological Membranes

Time-resolved fluorescence spectroscopy in combination with differential scanning calorimetry (DSC) was used to study the chemical interactions that occur when l-phenylalanine is introduced to solutions containing phosphatidylcholine vesicles. Studies reported in this work address open questions about l-Phe's affinity for lipid vesicle bilayers, the effects of l-Phe partitioning on bilayer properties, l-Phe's solvation within a lipid bilayer, and the amount of l-Phe within that local solvation environment. DSC data show that l-Phe reduces the amount of heat necessary to melt saturated phosphatidylcholine bilayers from their gel to liquid-crystalline state but does not change the transition temperature (T_gel-lc). Time-resolved emission shows only a single l-Phe lifetime at low temperatures corresponding to l-Phe remaining solvated in aqueous solution. At temperatures close to T_gel-lc, a second, shorter lifetime appears that is assigned to l-Phe already embedded within the membrane that becomes hydrated as water starts to permeate the lipid bilayer. This new lifetime is attributed to a conformationally restricted rotamer in the bilayer's polar headgroup region and accounts for up to 30% of the emission amplitude. Results reported for dipalmitoylphosphatidylcholine (DPPC, 16:0) lipid vesicles prove to be general, with similar effects observed for dimyristoylphosphatidylcholine (DMPC, 14:0) and distearoylphosphatidylcholine (DSPC, 18:0) vesicles. Taken together, these results create a complete and compelling picture of how l-Phe associates with model biological membranes. Furthermore, this approach to examining amino acid partitioning into membranes and the resulting solvation forces points to new strategies for studying the structure and chemistry of membrane-soluble peptides and selected membrane proteins.

Molecular Insights for Alzheimer's Disease: An Unexplored Storyline on the Nanoscale Impact of Nascent Aβ1-42 toward the Lipid Membrane

Deciphering the mechanism of Alzheimer's disease is a key element for designing an efficient therapeutic strategy. Molecular dynamics (MD) calculations, atomic force microscopy, and infrared spectroscopy were combined to investigate β-amyloid (Aβ_1-42) peptide interactions with supported lipid bilayers (SLBs). The MD simulations showed that nascent Aβ_1-42 monomers remain anchored within a model phospholipid bilayer's hydrophobic core, which suggests their stability in their native environment. We tested this prediction experimentally by studying the behavior of Aβ_1-42 monomers and oligomers when interacting with SLBs. When Aβ_1-42 monomers and oligomers were self-assembled with a lipid bilayer and deposited as an SLB, they remain within the bilayers. Their presence in the bilayers induces destabilization of the model membranes. No specific interactions between Aβ_1-42 and the SLBs were detected when SLBs free of Aβ_1-42 were exposed to Aβ_1-42. This study suggests that Aβ can remain in the membrane after cleavage by γ-secretase and cause severe damage to the membrane.

“Interplay of lipid-head group and packing defects in driving Amyloid-beta mediated myelin-like model membrane deformation”

Accumulating evidence suggests that amyloid plaque associated myelin lipid loss as a result of elevated amyloid burden might also contribute to Alzheimer's disease. The amyloid fibrils though closely associated with lipids under physiological conditions, however, the progression of membrane remodeling events leading to lipid-fibril assembly remains unknown. Here we first reconstitute the interaction of Aβ-40 with myelin-like model membrane and show that the binding of Aβ-40 induces extensive tubulation. To look into the mechanism of membrane tubulation we chose a set of membrane conditions varying in lipid packing density and net charge that allows us to dissect the contribution of lipid specificity of Aβ-40 binding, aggregation kinetics, and subsequent changes in membrane parameters such as fluidity, diffusion, and compressibility modulus. We show that the binding of Aβ-40 depends predominantly on the lipid packing defect densities and electrostatic interactions and results in rigidification of the myelin-like model membrane during the early phase of amyloid aggregation. Furthermore, elongation of Aβ-40 into higher oligomeric and fibrillar species leads to eventual fluidization of the model membrane followed by extensive lipid membrane tubulation observed in the late phase. Taken together, our results capture mechanistic insights into snapshots of temporal dynamics of Aβ-40 - myelin-like model membrane interaction and demonstrate how short timescale, local phenomena of binding, and fibril-mediated load generation results in the consequent association of lipids with growing amyloid fibrils.

Probing the interactions between amyloidogenic proteins and bio-membranes

Protein misfolding diseases (PMDs) in humans are characterized by the deposition of protein aggregates in tissues, including Alzheimer's disease, Parkinson's disease, type 2 diabetes, and amyotrophic lateral sclerosis. Misfolding and aggregation of amyloidogenic proteins play a central role in the onset and progression of PMDs, and these processes are regulated by multiple factors, especially the interaction between proteins and bio-membranes. Bio-membranes induce conformational changes in amyloidogenic proteins and affect their aggregation; on the other hand, the aggregates of amyloidogenic proteins may cause membrane damage or dysfunction leading to cytotoxicity. In this review, we summarize the factors that affect the binding of amyloidogenic proteins and membranes, the effects of bio-membranes on the aggregation of amyloidogenic proteins, mechanisms of membrane disruption by amyloidogenic aggregates, technical approaches for detecting these interactions, and finally therapeutic strategies targeting membrane damage caused by amyloidogenic proteins.

Six-Dimensional Single-Molecule Imaging with Isotropic Resolution using a Multi-View Reflector Microscope

Imaging both the positions and orientations of single fluorophores, termed single-molecule orientation-localisation microscopy, is a powerful tool to study biochemical processes. However, the limited photon budget associated with single-molecule fluorescence makes high-dimensional imaging with isotropic, nanoscale spatial resolution a formidable challenge. Here, we realise a radially and azimuthally polarized multi-view reflector (raMVR) microscope for the imaging of the 3D positions and 3D orientations of single molecules, with precision of 10.9 nm and 2.0° over a 1.5 μm depth range. The raMVR microscope achieves 6D super-resolution imaging of Nile red (NR) molecules transiently bound to lipid-coated spheres, accurately resolving their spherical morphology despite refractive-index mismatch. By observing the rotational dynamics of NR, raMVR images also resolve the infiltration of lipid membranes by amyloid-beta oligomers without covalent labelling. Finally, we demonstrate 6D imaging of cell membranes, where the orientations of specific fluorophores reveal heterogeneity in membrane fluidity. With its nearly isotropic 3D spatial resolution and orientation measurement precision, we expect the raMVR microscope to enable 6D imaging of molecular dynamics within biological and chemical systems with exceptional detail.

A Kinetic Map of the Influence of Biomimetic Lipid Model Membranes on Aβ42 Aggregation

The aggregation of the amyloid β (Aβ) peptide is one of the molecular hallmarks of Alzheimer's disease (AD). Although Aβ deposits have mostly been observed extracellularly, various studies have also reported the presence of intracellular Aβ assemblies. Because these intracellular Aβ aggregates might play a role in the onset and progression of AD, it is important to investigate their possible origins at different locations of the cell along the secretory pathway of the amyloid precursor protein, from which Aβ is derived by proteolytic cleavage. Senile plaques found in AD are largely composed of the 42-residue form of Aβ (Aβ₄₂). Intracellularly, Aβ₄₂ is produced in the endoplasmatic reticulum (ER) and Golgi apparatus. Since lipid bilayers have been shown to promote the aggregation of Aβ, in this study, we measure the effects of the lipid membrane composition on the in vitro aggregation kinetics of Aβ₄₂. By using large unilamellar vesicles to model cellular membranes at different locations, including the inner and outer leaflets of the plasma membrane, late endosomes, the ER, and the Golgi apparatus, we show that Aβ₄₂ aggregation is inhibited by the ER and Golgi model membranes. These results provide a preliminary map of the possible effects of the membrane composition in different cellular locations on Aβ aggregation and suggest the presence of an evolutionary optimization of the lipid composition to prevent the intracellular aggregation of Aβ.

Amyloids on Membrane Interfaces: Implications for Neurodegeneration

Membrane interfaces are vital for various cellular processes, and their involvement in neurodegenerative disorders such as Alzheimer's and Parkinson's disease has taken precedence in recent years. The amyloidogenic proteins associated with neurodegenerative diseases interact with the neuronal membrane through various means, which has implications for both the onset and progression of the disease. The parameters that regulate the interaction between the membrane and the amyloids remain poorly understood. The review focuses on the various aspects of membrane interactions of amyloids, particularly amyloid-β (Aβ) peptides and Tau involved in Alzheimer's and α-synuclein involved in Parkinson's disease. The genetic, cell biological, biochemical, and biophysical studies that form the basis for our current understanding of the membrane interactions of Aβ peptides, Tau, and α-synuclein are discussed.

Cholesterol as a key player in amyloid β-mediated toxicity in Alzheimer’s disease

Alzheimer’s disease (AD) is a neurodegenerative disorder that is one of the most devastating and widespread diseases worldwide, mainly affecting the aging population. One of the key factors contributing to AD-related neurotoxicity is the production and aggregation of amyloid β (Aβ). Many studies have shown the ability of Aβ to bind to the cell membrane and disrupt its structure, leading to cell death. Because amyloid damage affects different parts of the brain differently, it seems likely that not only Aβ but also the nature of the membrane interface with which the amyloid interacts, helps determine the final neurotoxic effect. Because cholesterol is the dominant component of the plasma membrane, it plays an important role in Aβ-induced toxicity. Elevated cholesterol levels and their regulation by statins have been shown to be important factors influencing the progression of neurodegeneration. However, data from many studies have shown that cholesterol has both neuroprotective and aggravating effects in relation to the development of AD. In this review, we attempt to summarize recent findings on the role of cholesterol in Aβ toxicity mediated by membrane binding in the pathogenesis of AD and to consider it in the broader context of the lipid composition of cell membranes.

Amyloid-β aggregates induced by β-cholesteryl glucose-embedded liposomes

Senile plaques that is characterized as an amyloid deposition found in Alzheimer's disease are composed primarily of fibrils of an aggregated peptide, amyloid β (Aβ). The ability to monitor senile plaque formation on a neuronal membrane under physiological conditions provides an attractive model. In this study, the growth behavior of amyloid Aβ fibrils in the presence of liposomes incorporating β-cholesteryl-D-glucose (β-CG) was examined using total internal reflection fluorescence microscopy, transmittance electron microscopy, and other spectroscopic methods. We found that β-CG on the liposome membrane induced the spontaneous formation of spherulitic Aβ fibrillar aggregates. The β-CG cluster formed on liposome membranes appeared to induce the accumulation of Aβ, followed by the growth of the spherulitic Aβ aggregates. In contrast, DMPC and DMPC incorporated cholesterol-induced fibrils that are laterally associated with each other. A comparison study using three types of liposomes implied that the induction of glucose contributed to the agglomeration of Aβ fibrils and liposomes. This agglomeration required the spontaneous formation of spherulitic Aβ fibrillary aggregates. This action can be regarded as a counterbalance to the growth of fibrils and their toxicity, which has great potential in the study of amyloidopathies.

Atomic Force Microscopy to Characterize Antimicrobial Peptide-Induced Defects in Model Supported Lipid Bilayers

Antimicrobial peptides (AMPs) interact with bacterial cell membranes through a variety of mechanisms, causing changes extending from nanopore formation to microscale membrane lysis, eventually leading to cell death. Several AMPs also disrupt mammalian cell membranes, despite their significantly different lipid composition and such collateral hemolytic damage hinders the potential therapeutic applicability of the AMP as an anti-microbial. Elucidating the mechanisms underlying the AMP-membrane interactions is challenging due to the variations in the chemical and structural features of the AMPs, the complex compositional variations of cell membranes and the inadequacy of any single experimental technique to comprehensively probe them. (1) Background: Atomic Force Microscopy (AFM) imaging can be used in combination with other techniques to help understand how AMPs alter the orientation and structural organization of the molecules within cell membranes exposed to AMPs. The structure, size, net charge, hydrophobicity and amphipathicity of the AMPs affect how they interact with cell membranes of differing lipid compositions. (2) Methods: Our study examined two different types of AMPs, a 20-amino acid, neutral, α-helical (amphipathic) peptide, alamethicin, and a 13-amino acid, non-α-helical cationic peptide, indolicidin (which intramolecularly folds, creating a hydrophobic core), for their interactions with supported lipid bilayers (SLBs). Robust SLB model membranes on quartz supports, incorporating predominantly anionic lipids representative of bacterial cells, are currently not available and remain to be developed. Therefore, the SLBs of zwitterionic egg phosphatidylcholine (PC), which represents the composition of a mammalian cell membrane, was utilized as the model membrane. This also allows for a comparison with the results obtained from the Quartz Crystal Microbalance with Dissipation (QCM-D) experiments conducted for these peptides interacting with the same zwitterionic SLBs. Further, in the case of alamethicin, because of its neutrality, the lipid charge may be less relevant for understanding its membrane interactions. (3) Results: Using AFM imaging and roughness analysis, we found that alamethicin produced large, unstable defects in the membrane at 5 µM concentrations, and completely removed the bilayer at 10 µM. Indolicidin produced smaller holes in the bilayer at 5 and 10 µM, although they were able to fill in over time. The root-mean-square (RMS) roughness values for the images showed that the surface roughness caused by visible defects peaked after peptide injection and gradually decreased over time. (4) Conclusions: AFM is useful for helping to uncover the dynamic interactions between different AMPs and cell membranes, which can facilitate the selection and design of more efficient AMPs for use in therapeutics and antimicrobial applications.

Effect of packing density of lipid vesicles on the Aβ42 fibril polymorphism

The aggregation of amyloid-β 1-42 (Aβ42) on lipid membranes is closely related to the pathology of Alzheimer's disease (AD). Herein, we demonstrated the effect of the packing density of lipid vesicles on the Aβ42 fibrillation kinetics and fibril morphology. We used three distinct phosphatidylcholine (PC) lipids, containing different numbers of cis-double bonds in acyl chains, and therefore, a different packing density in the lipid vesicles. Our results showed that the fibrillation of Aβ42 was greatly enhanced and the formed fibrils became shorter as the number of double bonds in lipids increased. Due to the low-density characteristics of dioleoyl phosphatidylcholine (DOPC), Aβ42 monomers were able to interact with the hydrophobic acyl chain of lipids exposed to the aqueous phase, thereby inducing rapid fibrillation and short fibril morphologies. Furthermore, the effects of the anionic lipids dioleoyl phosphatidylserine (DOPS) and dioleoyl phosphatidylglycerol (DOPG), and mixed vesicles of DOPC/DOPS and DOPC/DOPG on Aβ42 fibrillations were investigated. The tight binding of Aβ42 to the lipid head groups via electrostatic interactions was able to suppress the modulation of Aβ42 fibrillations compared to accelerated fibrillations on loosely packed membranes. Our proposed mechanism regarding the influence of lipid packing density on Aβ42 fibrillations provides an advanced understanding of lipid-associated amyloid fibrillations.

Physical mechanisms of amyloid nucleation on fluid membranes

Biological membranes can dramatically accelerate the aggregation of normally soluble protein molecules into amyloid fibrils and alter the fibril morphologies, yet the molecular mechanisms through which this accelerated nucleation takes place are not yet understood. Here, we develop a coarse-grained model to systematically explore the effect that the structural properties of the lipid membrane and the nature of protein-membrane interactions have on the nucleation rates of amyloid fibrils. We identify two physically distinct nucleation pathways-protein-rich and lipid-rich-and quantify how the membrane fluidity and protein-membrane affinity control the relative importance of those molecular pathways. We find that the membrane's susceptibility to reshaping and being incorporated into the fibrillar aggregates is a key determinant of its ability to promote protein aggregation. We then characterize the rates and the free-energy profile associated with this heterogeneous nucleation process, in which the surface itself participates in the aggregate structure. Finally, we compare quantitatively our data to experiments on membrane-catalyzed amyloid aggregation of α-synuclein, a protein implicated in Parkinson's disease that predominately nucleates on membranes. More generally, our results provide a framework for understanding macromolecular aggregation on lipid membranes in a broad biological and biotechnological context.

Melatonin Alters Fluid Phase Coexistence in POPC/DPPC/Cholesterol Membranes

The structure and biophysical properties of lipid membranes are important for cellular functions in health and disease. In Alzheimer's disease, the neuronal membrane is a target for toxic amyloid-β (Aβ). Melatonin is an important pineal gland hormone that has been shown to protect against Aβ toxicity in cellular and animal studies, but the molecular mechanism of this protection is not fully understood. Melatonin is a small membrane-active molecule that has been shown to interact with model lipid membranes and alter the membrane biophysical properties, such as membrane molecular order and dynamics. This effect of melatonin has been previously studied in simple model bilayers with one or two lipid components. To make it more relevant to neuronal membranes, we used a more complex ternary lipid mixture as our membrane model. In this study, we used ²H-NMR to investigate the effect of melatonin on the phase behavior of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), and cholesterol lipid membranes. We used deuterium-labeled POPC-d₃₁ and DPPC-d₆₂,separately to probe the changes in hydrocarbon chain order as a function of temperature and melatonin concentration. We find that POPC/DPPC/cholesterol at molar proportions of 3:3:2 is close to liquid-disordered/liquid-ordered phase separation and that melatonin can induce phase separation in these ternary mixtures by preferentially incorporating into the disordered phase and increasing its level of disorder. At 5 mol% melatonin, we observed phase separation in samples with POPC-d₃₁, but not with DPPC-d₆₂, whereas at 10 mol% melatonin, phase separation was observed in both samples with either POPC-d₃₁ or DPPC-d₆₂. These results indicate that melatonin can have a strong effect on membrane structure and physical properties, which may provide some clues to understanding how melatonin protects against Aβ, and that choice of chain perdeuteration is an important consideration from a technical point of view.

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

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

Modelling of interactions between Aβ(25-35) peptide and phospholipid bilayers: effects of cholesterol and lipid saturation

Aggregation of amyloid beta (Aβ) peptides in neuronal membranes is a known promoter of Alzheimer’s disease. To gain insight into the molecular details of Aβ peptide aggregation and its effect on model neuronal membranes, we carried out molecular dynamics simulations of the Aβ(25–35) fragment of the amyloid precursor protein in phospholipid bilayers composed of either fully saturated or highly unsaturated lipids, in the presence or absence of cholesterol. It was found that the peptide does not penetrate through any of the considered membranes, but can reside in the headgroup region and upper part of the lipid tails showing a clear preference to a polyunsaturated cholesterol-free membrane. Due to the ordering and condensing effect upon addition of cholesterol, membranes become more rigid facilitating peptide aggregation on the surface. Except for the case of the cholesterol-free saturated lipid bilayer, the peptides have a small effect on the membrane structure and ordering. It was also found that the most “active” amino-acid for peptide–lipid and peptide–cholesterol interaction is methionine-35, followed by asparagine-27 and serine-26, which form hydrogen bonds between peptides and polar atoms of lipid headgroups. These amino acids are also primarily responsible for peptide aggregation. This work will be relevant for designing strategies to develop drugs to combat Alzheimer’s disease.

Molecular dynamics simulations of Aβ(25–35) peptides in phospholipid bilayers are carried out to investigate the effect of polyunsaturated lipids and cholesterol on aggregation of the peptides.

Membrane domain modulation of Aβ1-42 oligomer interactions with supported lipid bilayers: an atomic force microscopy investigation

Alzheimer's disease is a devastating pathology affecting an increasing number of individuals following the general rise in life expectancy. Amyloid peptide Aβ_1-42 has been identified as one of the main culprits of the disease. The peptide has been shown to have major effects on lipid membranes, including membrane fragmentation. The membrane composition has been identified as a factor that plays a pivotal role in regulating peptide/membrane interactions and several results suggest that lipid domains, or rafts, can promote peptide-induced membrane damage. In this work, we examined the effects of lipid segregation on the membrane-perturbing ability of Aβ_1-42 and an oligomeric mutant (G37C), a peptide that shares common features with the suspected toxic intermediates involved in the neurodegeneration process. Atomic force microscopy (AFM) was used to determine the impact of these peptides on the supported lipid bilayers of various compositions. In 1,2-dioleoyl-sn-glycero-3-phosphocholine/1,2-dipalmitoyl-sn-glycero-3-phosphocholine/cholesterol (DOPC/DPPC/cholesterol) and DOPC/sphingomyelin/cholesterol ternary mixtures, two systems exhibiting liquid-liquid phase separations, it was shown that Aβ_1-42 and G37C exclusively aggregated on liquid-disordered-phase domains, creating large deposits and even causing membrane fragmentation for the latter composition. Cholesterol and ganglioside GM1, the two most documented lipids in the context of Alzheimer's disease, are also considered to play a crucial role in promoting detrimental interactions with amyloid peptides. We show that, in model 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membranes, the presence of either cholesterol or GM1 in a proportion of 10 mol%, a content supposed to lead to domain formation, favoured the association of both Aβ_1-42 and G37C, leading to a harmful membrane fragmentation. The AFM results established that the presence of domains favoured membrane perturbations induced by the amyloid peptides. It is proposed that lipid packing defects at the domain interface could act as adsorption and nucleation sites for the amyloid peptides. The more extensive bilayer perturbations induced by G37C compared to Aβ_1-42 supported this hypothesis, indicating that oligomers that cannot mature to the fibril state can present considerable toxicity.

High speed atomic force microscopy to investigate the interactions between toxic Aβ1-42 peptides and model membranes in real time: impact of the membrane composition

Due to an aging population, neurodegenerative diseases have become a major health issue, the most common being Alzheimer's disease. The mechanisms leading to neuronal loss still remain unclear but recent studies suggest that soluble Aβ oligomers have deleterious effects on neuronal membranes. Here, high-speed atomic force microscopy was used to assess the effect of oligomeric species of a variant of Aβ1-42 amyloid peptide on model membranes with various lipid compositions. Results showed that the peptide does not interact with membranes composed of phosphatidylcholine and sphingomyelin. Ganglioside GM1, but not cholesterol, is required for the peptide to interact with the membrane. Interestingly, when they are both present, a fast disruption of the membrane was observed. It suggests that the presence of ganglioside GM1 and cholesterol in membranes promotes the interaction of the oligomeric Aβ1-42 peptide with the membrane. This interaction leads to the membrane's destruction in a few seconds. This study highlights the power of high-speed atomic force microscopy to explore lipid-protein interactions with high spatio-temporal resolution.

Amyloid-β oligomers have a profound detergent-like effect on lipid membrane bilayers, imaged by atomic force and electron microscopy

The ability of amyloid-β peptide (Aβ) to disrupt membrane integrity and cellular homeostasis is believed to be central to Alzheimer's disease pathology. Aβ is reported to have various impacts on the lipid bilayer, but a clearer picture of Aβ influence on membranes is required. Here, we use atomic force and transmission electron microscopies to image the impact of different isolated Aβ assembly types on lipid bilayers. We show that only oligomeric Aβ can profoundly disrupt the bilayer, visualized as widespread lipid extraction and subsequent deposition, which can be likened to an effect expected from the action of a detergent. We further show that Aβ oligomers cause widespread curvature and discontinuities within lipid vesicle membranes. In contrast, this detergent-like effect was not observed for Aβ monomers and fibers, although Aβ fibers did laterally associate and embed into the upper leaflet of the bilayer. The marked impact of Aβ oligomers on membrane integrity identified here reveals a mechanism by which these oligomers may be cytotoxic.

Lipid phase influences the binding of Bacillus thuringiensis Cyt2Aa2 toxin on model lipid membranes

Bacillus thuringiensis is a bacterium that produces many insecticidal proteins including cytolytic proteins or Cyt toxins. Although the Cyt toxin shows specific toxicity against Dipteran insect species, the toxin binds directly to the lipid membrane without a specific protein receptor requirement. In this work, we have investigated the interaction of Cyt2Aa2 toxin with lipid bilayers composed of different lipid phases. By means of atomic force microscopy (AFM), lipid phase separation was observed for 1:1 and 4:1 M mixtures of DPPC/POPC bilayers. The exposure of Cyt2Aa2 to these lipid bilayers revealed that the toxin selectively bound to L_d lipid bilayer (corresponding to POPC). In turn, it did not bind to the L_o and S_o phases (corresponding to DPPC). Interestingly, for the bilayer of 4:1 DPPC/POPC mixture, the binding of Cyt2Aa2 was localized at the lipid phase boundary instead of L_d domain as occurred for the 1:1 DPPC/POPC bilayer. In addition, quartz crystal microbalance with dissipation experiments confirmed AFM results. In particular, the measurements showed that amount of protein bound to 1:1 DPPC/POPC (with phase separation) was half of the binding quantified for the L_d phase lipid bilayer (pure POPC and 1:4 DPPC/POPC mixture). These results indicate that the lipid phase (lipid acyl chain) influences the Cyt2Aa2-lipid interaction.

Membrane Lipid Nanodomains

Lipid membranes can spontaneously organize their components into domains of different sizes and properties. The organization of membrane lipids into nanodomains might potentially play a role in vital functions of cells and organisms. Model membranes represent attractive systems to study lipid nanodomains, which cannot be directly addressed in living cells with the currently available methods. This review summarizes the knowledge on lipid nanodomains in model membranes and exposes how their specific character contrasts with large-scale phase separation. The overview on lipid nanodomains in membranes composed of diverse lipids (e.g., zwitterionic and anionic glycerophospholipids, ceramides, glycosphingolipids) and cholesterol aims to evidence the impact of chemical, electrostatic, and geometric properties of lipids on nanodomain formation. Furthermore, the effects of curvature, asymmetry, and ions on membrane nanodomains are shown to be highly relevant aspects that may also modulate lipid nanodomains in cellular membranes. Potential mechanisms responsible for the formation and dynamics of nanodomains are discussed with support from available theories and computational studies. A brief description of current fluorescence techniques and analytical tools that enabled progress in lipid nanodomain studies is also included. Further directions are proposed to successfully extend this research to cells.

Effects of Passive Phospholipid Flip-Flop and Asymmetric External Fields on Bilayer Phase Equilibria

Compositional asymmetry between the leaflets of bilayer membranes modifies their phase behavior and is thought to influence other important features such as mechanical properties and protein activity. We address here how phase behavior is affected by passive phospholipid flip-flop, such that the compositional asymmetry is not fixed. We predict transitions from "pre-flip-flop" behavior to a restricted set of phase equilibria that can persist in the presence of passive flip-flop. Surprisingly, such states are not necessarily symmetric. We further account for external symmetry breaking, such as a preferential substrate interaction, and show how this can stabilize strongly asymmetric equilibrium states. Our theory explains several experimental observations of flip-flop-mediated changes in phase behavior and shows how domain formation and compositional asymmetry can be controlled in concert, by manipulating passive flip-flop rates and applying external fields.

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.

Gel Phase Membrane Retards Amyloid β-Peptide (1-42) Fibrillation by Restricting Slaved Diffusion of Peptides on Lipid Bilayers

Plasma membranes in the human brain can interact with amyloid β-peptide (1-42; Aβ₄₂) and induce Aβ₄₂ fibrillation, which is considered to be a crucial process underlying the neurotoxicity of Aβ₄₂ and the pathogenesis of Alzheimer's disease (AD). However, the mechanism of membrane-mediated Aβ₄₂ fibrillation at the molecular level remains elusive. Here we study the role of adsorbed Aβ₄₂ peptides on membrane-mediated fibrillation using supported lipid bilayers of varying phase structures (gel and fluid). Using total internal reflection fluorescence microscopy and interfacial specific second-order nonlinear optical spectroscopy, we show that the dynamics of 2D-mobile Aβ₄₂ molecules, facilitated by the highly mobile lipids underneath the peptides, are critical to Aβ₄₂ fibrillation on liquid phase membranes. This growth mechanism is retarded on gel phase membranes where the dynamics of 2D-mobile peptides are restricted by the "frozen" lipids with less mobility.

Changes in lipid membranes may trigger amyloid toxicity in Alzheimer's disease

Amyloid-beta peptides (Aβ), implicated in Alzheimer’s disease (AD), interact with the cellular membrane and induce amyloid toxicity. The composition of cellular membranes changes in aging and AD. We designed multi-component lipid models to mimic healthy and diseased states of the neuronal membrane. Using atomic force microscopy (AFM), Kelvin probe force microscopy (KPFM) and black lipid membrane (BLM) techniques, we demonstrated that these model membranes differ in their nanoscale structure and physical properties, and interact differently with Aβ_1–42. Based on our data, we propose a new hypothesis that changes in lipid membrane due to aging and AD may trigger amyloid toxicity through electrostatic mechanisms, similar to the accepted mechanism of antimicrobial peptide action. Understanding the role of the membrane changes as a key activating amyloid toxicity may aid in the development of a new avenue for the prevention and treatment of AD.

Experimental Parameters Leading to Optimal Bilayers for Total Internal Reflection Fluorescence Microscopy Visualization

Supported lipid bilayer systems were evaluated following various experimental procedures in an effort to determine their appropriateness for visualization using total internal reflection fluorescence (TIRF) microscopy. The incorporation and distribution of Texas Red® 1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (TR-DHPE) was studied when incorporated into bilayers of variable lipid composition using different forms of mechanical shearing. Results showed that 0.8 mol% TR-DHPE provides the most optimum TIRF images. At this concentration, a sufficient level of photostability can be achieved without an undesirable increase in TR-DHPE aggregates caused by excess probe molecules. Solutions composed of a 3:1 molar ratio of DOPC:DPPC with 0.8 mol% TR-DHPE produce bilayers that consistently display clear, distinct, rounded domains, whereas other lipid compositions did not. This optimum phase separation appears to be influenced by an increase in mechanical shearing during the vesicle formation process, when the lipid solutions were exposed to sonication and extrusion processes. The combination of a sonication and extrusion process also helped with eliminating the presence of TR-DHPE aggregates within the model membranes. It was also shown that bilayers formed on conditioned glass, placed on a slide, produced more highly detailed bilayers in which distinct lipid phase separation could be optimally visualized using TIRF microscopy.

Recent advances in hybrid measurement methods based on atomic force microscopy and surface sensitive measurement techniques

This review summaries the recent progress of the combination of optical and non-optical surface sensitive techniques with the atomic force microscopy.

Oriented Membrane Protein Reconstitution into Tethered Lipid Membranes for AFM Force Spectroscopy

Membrane proteins act as a central interface between the extracellular environment and the intracellular response and as such represent one of the most important classes of drug targets. The characterization of the molecular properties of integral membrane proteins, such as topology and interdomain interaction, is key to a fundamental understanding of their function. Atomic force microscopy (AFM) and force spectroscopy have the intrinsic capabilities of investigating these properties in a near-native setting. However, atomic force spectroscopy of membrane proteins is traditionally carried out in a crystalline setup. Alternatively, model membrane systems, such as tethered bilayer membranes, have been developed for surface-dependent techniques. While these setups can provide a more native environment, data analysis may be complicated by the normally found statistical orientation of the reconstituted protein in the model membrane. We have developed a model membrane system that enables the study of membrane proteins in a defined orientation by single-molecule force spectroscopy. Our approach is demonstrated using cell-free expressed bacteriorhodopsin coupled to a quartz glass surface in a defined orientation through a protein anchor and reconstituted inside an artificial membrane system. This approach offers an effective way to study membrane proteins in a planar lipid bilayer. It can be easily transferred to all membrane proteins that possess a suitable tag and can be reconstituted into a lipid bilayer. In this respect, we anticipate that this technique may contribute important information on structure, topology, and intra- and intermolecular interactions of other seven-transmembrane helical receptors.

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.

The organization of melatonin in lipid membranes

Melatonin is a hormone that has been shown to have protective effects in several diseases that are associated with cholesterol dysregulation, including cardiovascular disease, Alzheimer's disease, and certain types of cancers. We studied the interaction of melatonin with model membranes made of dimyristoylphosphatidylcholine (DMPC) at melatonin concentrations ranging from 0.5mol% to 30mol%. From 2-dimensional X-ray diffraction measurements, we find that melatonin induces a re-ordering of the lipid membrane that is strongly dependent on the melatonin concentration. At low melatonin concentrations, we observe the presence of melatonin-enriched patches in the membrane, which are significantly thinner than the lipid bilayer. The melatonin molecules were found to align parallel to the lipid tails in these patches. At high melatonin concentrations of 30mol%, we observe a highly ordered melatonin structure that is uniform throughout the membrane, where the melatonin molecules align parallel to the bilayers and one melatonin molecule associates with 2 lipid molecules. Understanding the organization and interactions of melatonin in membranes, and how these are dependent on the concentration, may shed light into its anti-amyloidogenic, antioxidative and photoprotective properties and help develop a structural basis for these properties.

Cholesterol drives aβ(1-42) interaction with lipid rafts in model membranes

The molecular mechanism at the basis of the neurodegenerative process related to Alzheimer's disease (AD) is triggered by the local composition of the neural plasma membrane. The role of cholesterol is controversial. In this investigation the interaction of the AD peptide amyloid-beta (1-42) with model membranes containing lipid rafts has been investigated by atomic force microscopy techniques. Supported lipid membranes made of phospholipids/sphingomyelin/cholesterol have been investigated as a function of the molar content of cholesterol, in a range spanning the phase diagram of the lipid system. The administration of amyloid-beta induced a phase reorganization of the lipid domains, when the cholesterol molar fraction was below 5%. At the same time, a mechanical destabilization and an appreciable thinning of the membrane induced by the peptide were detected. The major interaction was observed in the presence of the gel phase Lβ, and was enhanced by a low cholesterol amount. With the appearance of the liquid ordered phase Lo, the effect was hindered. At high cholesterol content (20% mol), no detectable effects in the bilayer morphology or in its mechanical stability were recorded. These findings give new insights on the molecular mechanism of the amyloid/membrane interaction, highlighting the peculiar role of cholesterol.

Morphological and physical analysis of natural phospholipids-based biomembranes

Background

Liposomes are currently an important part of biological, pharmaceutical, medical and nutritional research, as they are considered to be among the most effective carriers for the introduction of various types of bioactive agents into target cells.

Scope of Review

In this work, we study the lipid organization and mechanical properties of biomembranes made of marine and plant phospholipids. Membranes based on phospholipids extracted from rapeseed and salmon are studied in the form of liposome and as supported lipid bilayer. Dioleylphosphatidylcholine (DOPC) and dipalmitoylphosphatidylcholine (DPPC) are used as references to determine the lipid organization of marine and plant phospholipid based membranes. Atomic force microscopy (AFM) imaging and force spectroscopy measurements are performed to investigate the membranes' topography at the micrometer scale and to determine their mechanical properties.

Major Conclusions

The mechanical properties of the membranes are correlated to the fatty acid composition, the morphology, the electrophoretic mobility and the membrane fluidity. Thus, soft and homogeneous mechanical properties are evidenced for salmon phospholipids membrane containing various polyunsaturated fatty acids. Besides, phase segregation in rapeseed membrane and more important mechanical properties were emphasized for this type of membranes by contrast to the marine phospholipids based membranes.

General Significance

This paper provides new information on the nanomechanical and morphological properties of membrane in form of liposome by AFM. The originality of this work is to characterize the physico-chemical properties of the nanoliposome from the natural sources containing various fatty acids and polar head.

The interaction between amyloid-β peptides and anionic lipid membranes containing cholesterol and melatonin

One of the hallmarks of Alzheimer's disease is the formation of senile plaques, primarily consisting of amyloid- (A) peptides. Peptide-membrane and peptide-lipid interactions are thought to be crucial in this process. We studied the interaction of A and A peptides with anionic lipid membranes made of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphoserine (DMPS) using X-ray diffraction. We compare the experimentally determined electron densities in the gel state of the membranes with density calculations from peptide structures reported in the Protein Data Bank in order to determine the position of the peptide in the bilayers. The full length peptide A was found to embed in the hydrocarbon core of the anionic lipid bilayers. Two populations were found for the A peptide: (1) membrane-bound states in the hydrophilic head group region of the bilayers, where the peptides align parallel to the membranes, and (2) an embedded state in the bilayer center. Aging plays an important role in the development of Alzheimer's, in particular with respect to changes in cholesterol and melatonin levels in the brain tissue. Immiscible cholesterol plaques were created by addition of 30 mol% cholesterol to the anionic membranes. The A peptides were found to strongly interact with the lipid bilayers, displacing further cholesterol molecules into the plaques, effectively lowering the cholesterol concentration in the membranes and increasing the total fraction of cholesterol plaques. Addition of 30 mol% melatonin molecules to the anionic membranes drastically reduced the population of the membrane-embedded A state. These results present experimental evidence for an interaction between A peptides, melatonin and cholesterol in lipid membranes.

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.

Preparation of DOPC and DPPC Supported Planar Lipid Bilayers for Atomic Force Microscopy and Atomic Force Spectroscopy

Cell membranes are typically very complex, consisting of a multitude of different lipids and proteins. Supported lipid bilayers are widely used as model systems to study biological membranes. Atomic force microscopy and force spectroscopy techniques are nanoscale methods that are successfully used to study supported lipid bilayers. These methods, especially force spectroscopy, require the reliable preparation of supported lipid bilayers with extended coverage. The unreliability and a lack of a complete understanding of the vesicle fusion process though have held back progress in this promising field. We document here robust protocols for the formation of fluid phase DOPC and gel phase DPPC bilayers on mica. Insights into the most crucial experimental parameters and a comparison between DOPC and DPPC preparation are presented. Finally, we demonstrate force spectroscopy measurements on DOPC surfaces and measure rupture forces and bilayer depths that agree well with X-ray diffraction data. We also believe our approach to decomposing the force-distance curves into depth sub-components provides a more reliable method for characterising the depth of fluid phase lipid bilayers, particularly in comparison with typical image analysis approaches.

Nanoscale electrostatic domains in cholesterol-laden lipid membranes create a target for amyloid binding

Amyloid fibrils are associated with multiple neurodegenerative disorders, such as Alzheimer's disease. Although biological membranes are involved in fibril plaque formation, the role of lipid membrane composition in fibril formation and toxicity is not well understood. We investigated the effect of cholesterol on the interaction of model lipid membranes with amyloid-β peptide (Aβ). With atomic force microscopy we demonstrated that binding of Aβ (1-42) to DOPC bilayer, enriched with 20% cholesterol, resulted in an intriguing formation of small nonuniform islands loaded with Aβ. We attribute this effect to the presence of nanoscale electrostatic domains induced by cholesterol in DOPC bilayers. Using frequency-modulated Kelvin probe force microscopy we were able to resolve these nanoscale electrostatic domains in DOPC monolayers. These findings directly affect our understanding of how the presence of cholesterol may induce targeted binding of amyloid deposits to biomembranes. We postulate that this nonhomogeneous electrostatic effect of cholesterol has a fundamental nature and may be present in other lipid membranes and monolayers.

Model membrane systems

The context of the membrane is crucial for the interaction of many membrane proteins with their ligands. However, many detailed studies cannot be carried out in living cells. Therefore, studying these interactions requires model membrane systems that are compatible with the used analytical method. A big variety of these methods is available, each of which has its advantages and disadvantages. This chapter gives an overview over the existing techniques, a basic introduction into work with lipids, and detailed protocols for selected methods.

Atomic force microscopy: a versatile tool to probe the physical and chemical properties of supported membranes at the nanoscale

Atomic force microscopy (AFM) was developed in the 1980s following the invention of its precursor, scanning tunneling microscopy (STM), earlier in the decade. Several modes of operation have evolved, demonstrating the extreme versatility of this method for measuring the physicochemical properties of samples at the nanoscopic scale. AFM has proved an invaluable technique for visualizing the topographic characteristics of phospholipid monolayers and bilayers, such as roughness, height or laterally segregated domains. Implemented modes such as phase imaging have also provided criteria for discriminating the viscoelastic properties of different supported lipid bilayer (SLB) regions. In this review, we focus on the AFM force spectroscopy (FS) mode, which enables determination of the nanomechanical properties of membrane models. The interpretation of force curves is presented, together with newly emerging techniques that provide complementary information on physicochemical properties that may contribute to our understanding of the structure and function of biomembranes. Since AFM is an imaging technique, some basic indications on how real-time AFM imaging is evolving are also presented at the end of this paper.