Cholesterol inhibits the insertion of the Alzheimer’s peptide Aβ(25–35) in lipid bilayers

Aggregation Behavior of Amyloid Beta Peptide Depends Upon the Membrane Lipid Composition

Protein aggregation plays a crucial role in the development of several neurodegenerative diseases. It is important to understand the aggregation process for the detection of the onset of these diseases. Alzheimer's Disease (AD) is one of the most prevalent neurodegenerative diseases caused by the aggregation of Aβ-40 and Aβ-42 peptides. The smaller oligomers lead to the formation of protein plaque at the neural membranes leading to memory loss and other disorders. Interestingly, aggregation takes place at the neural membranes, therefore the membrane composition seems to play an important role in the aggregation process. Despite a large number of literatures on the effect of lipid composition on protein aggregation, there are very few concise reviews that highlight the role of membrane composition in protein aggregation. In this review, we have discussed the implication of membrane composition on the aggregation of amyloid beta peptide with a special emphasis on cholesterol. We have further discussed the role of the degree of unsaturation of fatty acids and the participation of apolipoprotein E4 (ApoE4) in the onset of AD.

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.

Quantitative Measurement of the Affinity of Toxic and Nontoxic Misfolded Protein Oligomers for Lipid Bilayers and of its Modulation by Lipid Composition and Trodusquemine

Many neurodegenerative diseases are associated with the self-assembly of peptides and proteins into fibrillar aggregates. Soluble misfolded oligomers formed during the aggregation process, or released by mature fibrils, play a relevant role in neurodegenerative processes through their interactions with neuronal membranes. However, the determinants of the cytotoxicity of these oligomers are still unclear. Here we used liposomes and toxic and nontoxic oligomers formed by the same protein to measure quantitatively the affinity of the two oligomeric species for lipid membranes. To this aim, we quantified the perturbation to the lipid membranes caused by the two oligomers by using the fluorescence quenching of two probes embedded in the polar and apolar regions of the lipid membranes and a well-defined protein-oligomer binding assay using fluorescently labeled oligomers to determine the Stern-Volmer and dissociation constants, respectively. With both approaches, we found that the toxic oligomers have a membrane affinity 20-25 times higher than that of nontoxic oligomers. Circular dichroism, intrinsic fluorescence, and FRET indicated that neither oligomer type changes its structure upon membrane interaction. Using liposomes enriched with trodusquemine, a potential small molecule drug known to penetrate lipid membranes and make them refractory to toxic oligomers, we found that the membrane affinity of the oligomers was remarkably lower. At protective concentrations of the small molecule, the binding of the oligomers to the lipid membranes was fully prevented. Furthermore, the affinity of the toxic oligomers for the lipid membranes was found to increase and slightly decrease with GM1 ganglioside and cholesterol content, respectively, indicating that physicochemical properties of lipid membranes modulate their affinity for misfolded oligomeric species.

Curcumin and Homotaurine Suppress Amyloid-β25-35 Aggregation in Synthetic Brain Membranes

Amyloid-β (Aβ) peptides spontaneously aggregate into β- and cross-β-sheets in model brain membranes. These nanometer sized can fuse into larger micrometer sized clusters and become extracellular and serve as nuclei for further plaque and fibril growth. Curcumin and homotaurine represent two different types of Aβ aggregation inhibitors. While homotaurine is a peptic antiaggregant that binds to amyloid peptides, curcumin is a nonpeptic molecule that can inhibit aggregation by changing membrane properties. By using optical and fluorescent microscopy, X-ray diffraction, and UV-vis spectroscopy, we study the effect of curcumin and homotaurine on Aβ_25-35 aggregates in synthetic brain membranes. Both molecules partition spontaneously and uniformly in membranes and do not lead to observable membrane defects or disruption in our experiments. Both curcumin and homotaurine were found to significantly reduce the number of small, nanoscopic Aβ aggregates and the corresponding β- and cross-β-sheet signals. While a number of research projects focus on potential drug candidates that target Aβ peptides directly, membrane-lipid therapy explores membrane-mediated pathways to suppress peptide aggregation. Based on the results obtained, we conclude that membrane active drugs can be as efficient as peptide targeting drugs in inhibiting amyloid aggregation in vitro.

The Effects of Resveratrol, Caffeine, β-Carotene, and Epigallocatechin Gallate (EGCG) on Amyloid- β 25 -- 35 Aggregation in Synthetic Brain Membranes

SCOPE

Alzheimer's disease is a neurodegenerative condition marked by the formation and aggregation of amyloid-β (Aβ) peptides. There exists, to this day, no cure or effective prevention for the disease; however, there is evidence that a healthy diet and certain food products can slow down first occurrence and progression. To investigate if food ingredients can interact with peptide aggregates, synthetic membranes that contained aggregates consisting of cross-β sheets of the membrane active fragment A β 25 -- 35 are prepared.

METHODS AND RESULTS

The impact of resveratrol, found in grapes, caffeine, the main active ingredient in coffee, β-carotene, found in orange fruits and vegetables, and epigallocatechin gallate (EGCG), a component of green tea, on the size and volume fraction of Aβ aggregates is studied using optical and fluorescence microscopy, X-ray diffraction, UV-vis spectroscopy, and molecular dynamics simulations. All compounds are membrane active and spontaneously partitioned in the synthetic brain membranes. While resveratrol and caffeine lead to membrane thickening and reduced membrane fluidity, β-carotene and EGCG preserve or increase fluidity.

CONCLUSION

Resveratrol and caffeine do not reduce the volume fraction of peptide aggregates while β-carotene significantly reduces plaque size. Interestingly, EGCG dissolves peptide aggregates and significantly decreases the corresponding cross-β and β-sheet signals.

Far from Inert: Membrane Lipids Possess Intrinsic Reactivity That Has Consequences for Cell Biology

In this article, it is hypothesized that a fundamental chemical reactivity exists between some non-lipid constituents of cellular membranes and ester-based lipids, the significance of which is not generally recognized. Many peptides and smaller organic molecules have now been shown to undergo lipidation reactions in model membranes in circumstances where direct reaction with the lipid is the only viable route for acyl transfer. Crucially, drugs like propranolol are lipidated in vivo with product profiles that are comparable to those produced in vitro. Some compounds have also been found to promote lipid hydrolysis. Drugs with high lytic activity in vivo tend to have higher toxicity in vitro. Deacylases and lipases are proposed as key enzymes that protect cells against the effects of intrinsic lipidation. The toxic effects of intrinsic lipidation are hypothesized to include a route by which nucleation can occur during the formation of amyloid fibrils.

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.

Do Cholesterol and Sphingomyelin Change the Mechanism of Aβ25-35 Peptide Binding to Zwitterionic Bilayer?

Using replica exchange with solute tempering all-atom molecular dynamics, we studied the equilibrium binding of Aβ_25-35 peptide to the ternary bilayer composed of an equimolar mixture of dimyristoylphosphatidylcholine (DMPC), N-palmitoylsphingomyelin (PSM), and cholesterol. Binding of the same peptide to the pure DMPC bilayer served as a control. Due to significant C-terminal hydrophobic moment, binding to the ternary and DMPC bilayers promotes helical structure in the peptide. For both bilayers a polarized binding profile is observed, in which the N-terminus anchors to the bilayer surface, whereas the C-terminus alternates between unbound and inserted states. Both ternary and DMPC bilayers feature two Aβ_25-35 bound states, surface bound, S, and inserted, I, separated by modest free energy barriers. Experimental data are in agreement with our results but indicate that cholesterol impact is Aβ fragment dependent. For Aβ_25-35, we predict that its binding mechanism is independent of the inclusion of PSM and cholesterol into the bilayer.

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.

Structure of amyloid β25-35 in lipid environment and cholesterol-dependent membrane pore formation

The amyloid β (Aβ) peptide and its shorter variants, including a highly cytotoxic Aβ_25–35 peptide, exert their neurotoxic effect during Alzheimer’s disease by various mechanisms, including cellular membrane permeabilization. The intrinsic polymorphism of Aβ has prevented the identification of the molecular basis of Aβ pore formation by direct structural methods, and computational studies have led to highly divergent pore models. Here, we have employed a set of biophysical techniques to directly monitor Ca²⁺-transporting Aβ_25–35 pores in lipid membranes, to quantitatively characterize pore formation, and to identify the key structural features of the pore. Moreover, the effect of membrane cholesterol on pore formation and the structure of Aβ_25–35 has been elucidated. The data suggest that the membrane-embedded peptide forms 6- or 8-stranded β-barrel like structures. The 8-stranded barrels may conduct Ca²⁺ ions through an inner cavity, whereas the tightly packed 6-stranded barrels need to assemble into supramolecular structures to form a central pore. Cholesterol affects Aβ_25–35 pore formation by a dual mechanism, i.e., by direct interaction with the peptide and by affecting membrane structure. Collectively, our data illuminate the molecular basis of Aβ membrane pore formation, which should advance both basic and clinical research on Alzheimer’s disease and membrane-associated pathologies in general.

Computational insights into lipid assisted peptide misfolding and aggregation in neurodegeneration

An overview of recent advances in computational investigation of peptide–lipid interactions in neurodegeneration – Alzheimer's, Parkinson's and Huntington's disease.

Interaction of the β amyloid - Aβ(25-35) - peptide with zwitterionic and negatively charged vesicles with and without cholesterol

The interactions of the Alzheimer's β-amyloid peptide, Aβ(25-35), with 18:1 (Δ9-Cis) PC 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), L-α-phosphatidylcholine (EPC), 1,2-dioleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (sodium salt) (DOPG), and L-α-phosphatidylglycerol (EPG) phospholipid vesicles with and without cholesterol (Ch) are studied by the nitroxide spin probe electron paramagnetic resonance (EPR) method. Two nitroxide spin probes, 2,2,6,6-tetramethyl-piperidin-1-oxyl-4-yl hexadecanoate (TP, TEMPO-Palmitate) and 2-Ethyl-2-(15-methoxy-15-oxopentadecyl)-4,4-dimethyl-3-oxazolidinyloxy (16-DSE), are utilized in the study. TEMPO-Palmitate has the reporting EPR moiety located at the top of this spin probe, while 16-DSE has the reporting EPR moiety located at the tail of the spin probe. These two probes enable us to sample the surface and the middle of the phospholipid bilayer, respectively. All EPR measurements are done above the melting points of all four phospholipids when the bilayer is in the liquid crystal phase, the physiologically relevant phase. Due to non-linear spectral line fitting, the EPR spectral parameters are extracted with high precision. The results show that there are two populations of Aβ(25-35) and that one of them is located in the hydrophobic phospholipid layer below the hydrophilic headgroup region. The second population appears to be weakly coupled to the surface of the bilayer. Both hydrophobic and electrostatic interactions affect the insertion of Aβ(25-35) in the bilayer. Also, there is strong evidence for an interaction between cholesterol and Aβ(25-35), which affects the dielectric and dynamic properties of the bilayer.

Membrane-Modulating Drugs can Affect the Size of Amyloid-β25-35 Aggregates in Anionic Membranes

The formation of amyloid-β plaques is one of the hallmarks of Alzheimer's disease. The presence of an amphiphatic cell membrane can accelerate the formation of amyloid-β aggregates, making it a potential druggable target to delay the progression of Alzheimer's disease. We have prepared unsaturated anionic membranes made of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1,2-dimyristoyl-sn-glycero-3-phospho-L-serine (DMPS) and added the trans-membrane segment Aβ_25-35. Peptide plaques spontaneously form in these membranes at high peptide concentrations of 20 mol%, which show the characteristic cross-β motif (concentrations are relative to the number of membrane lipids and indicate the peptide-to-lipid ratio). We used atomic force microscopy, fluorescence microscopy, x-ray microscopy, x-ray diffraction, UV-vis spectroscopy and Molecular Dynamics (MD) simulations to study three membrane-active molecules which have been speculated to have an effect in Alzheimer's disease: melatonin, acetylsalicyclic acid (ASA) and curcumin at concentrations of 5 mol% (drug-to-peptide ratio). Melatonin did not change the structural parameters of the membranes and did not impact the size or extent of peptide clusters. While ASA led to a membrane thickening and stiffening, curcumin made membranes softer and thinner. As a result, ASA was found to lead to the formation of larger peptide aggregates, whereas curcumin reduced the volume fraction of cross-β sheets by ~70%. We speculate that the interface between membrane and peptide cluster becomes less favorable in thick and stiff membranes, which favors the formation of larger aggregates, while the corresponding energy mismatch is reduced in soft and thin membranes. Our results present evidence that cross-β sheets of Aβ_25-35 in anionic unsaturated lipid membranes can be re-dissolved by changing membrane properties to reduce domain mismatch.

Role of the cell membrane interface in modulating production and uptake of Alzheimer's beta amyloid protein

The beta amyloid protein (Aβ) plays a central role in Alzheimer's disease (AD) pathogenesis and its interaction with cell membranes in known to promote mutually disruptive structural perturbations that contribute to amyloid deposition and neurodegeneration in the brain. In addition to protein aggregation at the membrane interface and disruption of membrane integrity, growing reports demonstrate an important role for the membrane in modulating Aβ production and uptake into cells. The aim of this review is to highlight and summarize recent literature that have contributed insight into the implications of altered membrane composition on amyloid precursor protein (APP) proteolysis, production of Aβ, its internalization in to cells via permeabilization and receptor mediated uptake. Here, we also review the various membrane model systems and experimental tools used for probing Aβ-membrane interactions to investigate the key mechanistic aspects underlying the accumulation and toxicity of Aβ in AD.

Shared pathological pathways of Alzheimer's disease with specific comorbidities: current perspectives and interventions

Alzheimer's disease (AD) belongs to one of the most multifactorial, complex and heterogeneous morbidity-leading disorders. Despite the extensive research in the field, AD pathogenesis is still at some extend obscure. Mechanisms linking AD with certain comorbidities, namely diabetes mellitus, obesity and dyslipidemia, are increasingly gaining importance, mainly because of their potential role in promoting AD development and exacerbation. Their exact cognitive impairment trajectories, however, remain to be fully elucidated. The current review aims to offer a clear and comprehensive description of the state-of-the-art approaches focused on generating in-depth knowledge regarding the overlapping pathology of AD and its concomitant ailments. Thorough understanding of associated alterations on a number of molecular, metabolic and hormonal pathways, will contribute to the further development of novel and integrated theranostics, as well as targeted interventions that may be beneficial for individuals with age-related cognitive decline.

Membrane Binding and Pore Formation by a Cytotoxic Fragment of Amyloid β Peptide

Amyloid β (Aβ) peptide contributes to Alzheimer's disease by a yet unidentified mechanism. In the brain tissue, Aβ occurs in various forms, including an undecapeptide Aβ_25-35, which exerts a neurotoxic effect through the mitochondrial dysfunction and/or Ca²⁺-permeable pore formation in cell membranes. This work was aimed at the biophysical characterization of membrane binding and pore formation by Aβ_25-35. Interaction of Aβ_25-35 with anionic and zwitterionic membranes was analyzed by microelectrophoresis. In pore formation experiments, Aβ_25-35 was incubated in aqueous buffer to form oligomers and added to Quin-2-loaded vesicles. Gradual increase in Quin-2 fluorescence was interpreted in terms of membrane pore formation by the peptide, Ca²⁺ influx, and binding to intravesicular Quin-2. The kinetics and magnitude of this process were used to evaluate the rate constant of pore formation, peptide-peptide association constants, and the oligomeric state of the pores. Decrease in membrane anionic charge and high ionic strength conditions significantly suppressed membrane binding and pore formation, indicating the importance of electrostatic interactions in these events. Circular dichroism spectroscopy showed that Aβ_25-35 forms the most efficient pores in β-sheet conformation. The data are consistent with an oligo-oligomeric pore model composed of up to eight peptide units, each containing 6-8 monomers.

Cholesterol Changes the Mechanisms of Aβ Peptide Binding to the DMPC Bilayer

Using isobaric-isothermal all-atom replica-exchange molecular dynamics (REMD) simulations, we investigated the equilibrium binding of Aβ10-40 monomers to the zwitterionic dimyristoylphosphatidylcholine (DMPC) bilayer containing cholesterol. Our previous REMD simulations, which studied binding of the same peptide to the cholesterol-free DMPC bilayer, served as a control, against which we measured the impact of cholesterol. Our findings are as follows. First, addition of cholesterol to the DMPC bilayer partially expels the Aβ peptide from the hydrophobic core and promotes its binding to bilayer polar headgroups. Using thermodynamic and energetics analyses, we argued that Aβ partial expulsion is not related to cholesterol-induced changes in lateral pressure within the bilayer but is caused by binding energetics, which favors Aβ binding to the surface of the densely packed cholesterol-rich bilayer. Second, cholesterol has a protective effect on the DMPC bilayer structure against perturbations caused by Aβ binding. More specifically, cholesterol reduces bilayer thinning and overall depletion of bilayer density beneath the Aβ binding footprint. Third, we found that the Aβ peptide contains a single cholesterol binding site, which involves hydrophobic C-terminal amino acids (Ile31-Val36), Phe19, and Phe20 from the central hydrophobic cluster, and cationic Lys28 from the turn region. This binding site accounts for about 76% of all Aβ-cholesterol interactions. Because cholesterol binding site in the Aβ10-40 peptide does not contain the GXXXG motif featured in cholesterol interactions with the transmembrane domain C99 of the β-amyloid precursor protein, we argued that the binding mechanisms for Aβ and C99 are distinct reflecting their different conformations and positions in the lipid bilayer. Fourth, cholesterol sharply reduces the helical propensity in the bound Aβ peptide. As a result, cholesterol largely eliminates the emergence of helical structure observed upon Aβ transition from a water environment to the cholesterol-free DMPC bilayer. We explain this effect by the formation of hydrogen bonds between cholesterol and the Aβ backbone, which prevent helix formation. Taken together, we expect that our simulations will advance understanding of a molecular-level mechanism behind the role of cholesterol in Alzheimer's disease.

Membrane-Accelerated Amyloid-β Aggregation and Formation of Cross-β Sheets

Amyloid- β aggregates play a causative role in Alzheimer's disease. These aggregates are a product of the physical environment provided by the basic neuronal membrane, composed of a lipid bilayer. The intrinsic properties of the lipid bilayer allow amyloid- β peptides to nucleate and form well-ordered cross- β sheets within the membrane. Here, we correlate the aggregation of the hydrophobic fragment of the amyloid- β protein, A β 25 - 35 , with the hydrophobicity, fluidity, and charge density of a lipid bilayer. We summarize recent biophysical studies of model membranes and relate these to the process of aggregation in physiological systems.

Amyloid and membrane complexity: The toxic interplay revealed by AFM

Lipid membranes play a fundamental role in the pathological development of protein misfolding diseases. Several pieces of evidence suggest that the lipid membrane could act as a catalytic surface for protein aggregation. Furthermore, a leading theory indicates the interaction between the cell membrane and misfolded oligomer species as the responsible for cytotoxicity, hence, for neurodegeneration in disorders such as Alzheimer's and Parkinson's disease. The definition of the mechanisms that drive the interaction between pathological protein aggregates and plasma membrane is fundamental for the development of effective therapies for a large class of diseases. Atomic force microscopy (AFM) has been employed to study how amyloid aggregates affect the cell physiological properties. Considerable efforts were spent to characterize the interaction with model systems, i.e., planar supported lipid bilayers, but some works also addressed the problem directly on living cells. Here, an overview of the main works involving the use of the AFM on both model system and living cells will be provided. Different kind of approaches will be presented, as well as the main results derived from the AFM analysis.

BioRef II-Neutron reflectometry with relaxed resolution for fast, kinetic measurements at HZB

We present an upgrade to the time-of-flight neutron reflectometer BioRef at the research reactor BER II of the Helmholtz-Zentrum Berlin für Materialien und Energie (HZB). Through the integration of an additional chopper into the existing setup, the available wavelength resolution is significantly extended. Now two distinct operation modes can be used: a high resolution mode with Δλ/λ ranging from 1% to 5%, which allows for the investigation of thick films up to 4000 Å, and a high flux mode with Δλ/λ = 7%-11%. In the high flux mode, reflectivity curves from 0.007 Å^-1 to 0.2 Å^-1 with three angular settings can be recorded in 7 min. For a single angular setting and its respective window in Q-space, a time resolution of even less than 4 min is reached. The different configurations are documented by respective measurements (a) on a Ni-Ti multilayer and (b) the swelling kinetics of a solid-supported phospholipid coating upon incubation in a polyelectrolyte solution.

Amyloid-β(25-35) peptides aggregate into cross-β sheets in unsaturated anionic lipid membranes at high peptide concentrations

One of the hallmarks of Alzheimer's disease is the formation of protein plaques in the brain, which mainly consist of amyloid-β peptides of different lengths. While the role of these plaques in the pathology of the disease is not clear, the mechanism behind peptide aggregation is a topic of intense research and discussion. Because of their simplicity, synthetic membranes are promising model systems to identify the elementary processes involved. We prepared unsaturated zwitterionic/anionic lipid membranes made of 1-palmitoyl-2-oleoyl-sn-glycero-phosphocholine (POPC) and 1,2-dimyristoyl-sn-glycero-3-phospho-l-serine (DMPS) at concentrations of POPC/3 mol% DMPS containing 0 mol%, 3 mol%, 10 mol%, and 20 mol% amyloid-β25-35 peptides. Membrane-embedded peptide clusters were observed at peptide concentrations of 10 and 20 mol% with a typical cluster size of ∼11 μm. Cluster density increased with peptide concentration from 59 (±3) clusters per mm(2) to 920 (±64) clusters per mm(2), respectively. While monomeric peptides take an α-helical state when embedded in lipid bilayers at low peptide concentrations, the peptides in peptide clusters were found to form cross-β sheets and showed the characteristic pattern in X-ray experiments. The presence of the peptides was accompanied by an elastic distortion of the bilayers, which can induce a long range interaction between the peptides. The experimentally observed cluster patterns agree well with Monte Carlo simulations of long-range interacting peptides. This interaction may be the fundamental process behind cross-β sheet formation in membranes and these sheets may serve as seeds for further growth into amyloid fibrils.

The Position of Aβ22-40 and Aβ1-42 in Anionic Lipid Membranes Containing Cholesterol

Amyloid-β peptides interact with cell membranes in the human brain and are associated with neurodegenerative diseases, such as Alzheimer's disease. An emerging explanation of the molecular mechanism, which results in neurodegeneration, places the cause of neurotoxicity of the amyloid- peptides on their potentially negative interaction with neuronal membranes. It is known that amyloid-β peptides interact with the membrane, modifying the membrane's structural and dynamic properties. We present a series of X-ray diffraction experiments on anionic model lipid membranes containing various amounts of cholesterol. These experiments provide experimental evidence for an interaction of both the full length amyloid-β1-42 peptide, and the peptide fragment amyloid-β22-40 with anionic bilayer containing cholesterol. The location of the amyloid-β peptides was determined from these experiments, with the full length peptide embedding into the membrane, and the peptide fragment occupying 2 positions-on the membrane surface and embedded into the membrane core.

Characterization of the interactions between β-amyloid peptide and the membranes of human SK-N-SH cells

Interaction of β-amyloid peptide (Aβ) with cell membranes is thought to be an initial step in Alzheimer's disease (AD). However, some data are controversial and the underlying mechanisms remain unclear. In this report, two populations of Aβ were found in human SK-N-SH neuroblastoma cells. Notably, one of the Aβ populations was tightly inserted into the plasma membrane whilst the other was only peripherally associated with it. Here we show that reducing membrane cholesterol decreased the number of membrane-embedded Aβs and increased the number of membrane-attached Aβs. We also found that cholesterol depletion inhibited Aβ degradation and exacerbated Aβ-mediated cytotoxicity. Our detailed and direct observations provide specific insights into the mechanism of Aβ membrane-associated toxicity.

Cholesterol modulates the interaction of the islet amyloid polypeptide with membranes

The deposition of insoluble amyloid fibrils resulting from the aggregation of the human islet amyloid polypeptide (hIAPP) within the islet of Langerhans is a pathological feature of type 2 diabetes mellitus (T2DM). Increasing evidence indicates that biological membranes play a key role in amyloid aggregation, modulating among others the kinetics of amyloid formation, and being the target of toxic species generated during amyloid formation. In T2DM patients, elevated levels of cholesterol, an important determinant of the physical state of biological membranes, are observed in β-cells and are thought to directly impair β-cell function and insulin secretion. However, it is not known whether cholesterol enhances membrane-interaction or membrane-insertion of hIAPP. In this study, we investigated the effect of cholesterol incorporated in zwitterionic and anionic membranes. Our circular dichroism and liquid state NMR data reveal that 10-30% of cholesterol slightly affects the aggregational and conformational behaviour of hIAPP. Additional fluorescence results indicate that 10 and 20% of cholesterol slightly slow down the kinetics of oligomer and fibril formation while anionic lipids accelerate this kinetics. This behavior might be caused by differences in membrane insertion and therefore in membrane binding of hIAPP. The membrane binding affinity was evaluated using (1)H NMR experiments and our results show that the affinity of hIAPP for membranes containing cholesterol is significantly smaller than that for membranes containing anionic lipids. Furthermore, we found that hIAPP-induced membrane damage is synchronized to fibril formation in the absence and in the presence of cholesterol.

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.

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.

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.

Structure and membrane interactions of the β-amyloid fragment 25-35 as viewed using spectroscopic approaches

The β-amyloid fragment peptide 25-35 (Aβ(25-35)) is recognized as the cytotoxic sequence of the parent peptide Aβ. However, it remains unclear whether its neurotoxicity originates from its fibrillar form, how it interacts with lipid membranes, and whether cholesterol modulates these interactions. These questions have been addressed at a molecular level using various microscopic and spectroscopic techniques. The data show that Aβ(25-35) forms protofilaments at pH 7.4 at a concentration of 5 mM in the absence and presence of DMPC/DMPG model membranes. The peptide adopts a predominant aggregated β-sheet conformation under these conditions. However, as the peptide concentration decreases, the β-sheet structure tends to disappear for the benefit of β-turns, suggesting that the peptide association is reversible. The β-sheet structure formed by Aβ(25-35) appears to be atypical and characterized by the absence of intermolecular dipolar coupling and by a parallel strand configuration. The data show that Aβ(25-35)-phospholipid interactions are characterized by an increase in the conformational order of the lipid acyl chains and a change in the fluidity/elasticity of the bilayers. Concomitantly, the peptide seems to lose a few β-sheet structures, which suggests that the interactions between Aβ(25-35) and DMPC/DMPG membranes are partly driven by peptide concentration. Interactions indeed seem to occur when part of the peptides is not involved in protofilaments and increase as the proportion of the free peptide species increases. The interactions are very similar in the presence of cholesterol, except that the concentration effect of Aβ(25-35) is cancelled, suggesting that Chol limits the penetration of the peptide inside the bilayers.

Embedding Aβ42 in heterogeneous membranes depends on cholesterol asymmetries

Using a coarse-grained lipid and peptide model, we show that the free energy stabilization of amyloid-β in heterogeneous lipid membranes is predicted to have a dependence on asymmetric distributions of cholesterol compositions across the membrane leaflets. We find that a highly asymmetric cholesterol distribution that is depleted on the exofacial leaflet but enhanced on the cytofacial leaflet of the model lipid membrane thermodynamically favors membrane retention of a fully embedded Aβ peptide. However, in the case of cholesterol redistribution that increases concentration of cholesterol on the exofacial layer, typical of aging or Alzheimer's disease, the free energy favors peptide extrusion of the highly reactive N-terminus into the extracellular space that may be vulnerable to aggregation, oligomerization, or deleterious oxidative reactivity.

Copper Modulation of Amyloid Beta 42 Interactions with Model Membranes

Growing evidence supports that interactions of the amyloid-β peptide Aβ(1–42) with neuronal cell membranes and copper are involved in Alzheimer’s disease pathogenesis. We report using solid-state NMR that the peptide significantly perturbed the phosphate and upper acyl chain region of bilayers comprising brain total lipid extract to cause domain segregation. Deep headgroup perturbations were also realized for palmitoyloleoylphospatidylcholine–cholesterol model systems; however, incorporating 10 % palmitoyloleoylphosphatidylserine or the ganglioside GM1 resulted in a more peripheral interaction. Cu2+ at a 1 : 7 molar ratio to peptide caused deeper penetration into model systems, but partially attenuated interactions with brain total lipid extract. Thioflavin T assay showed that bilayers affected amyloid formation in a mode dependant on lipid content, and was further modulated by addition of Cu2+. Our data support that ternary interactions between Cu2+, lipids and Aβ(1–42) may have significance in Alzheimer’s disease, and challenge the validity of model bilayers as substitutes for natural systems.

Membrane cholesterol modulates {beta}-amyloid-dependent tau cleavage by inducing changes in the membrane content and localization of N-methyl-D-aspartic acid receptors

We have previously shown that β-amyloid (Aβ) treatment resulted in an age-dependent calpain activation leading to Tau cleavage into a neurotoxic 17-kDa fragment in a cellular model of Alzheimer disease. This detrimental cellular response was mediated by a developmentally regulated increase in membrane cholesterol levels. In this study, we assessed the molecular mechanisms by which cholesterol modulated Aβ-induced Tau cleavage in cultured hippocampal neurons. Our results indicated that these mechanisms did not involve the regulation of the binding of Aβ aggregates to the plasma membrane. On the other hand, experiments using N-methyl-d-aspartic acid receptor inhibitors suggested that these receptors played an essential role in cholesterol-mediated Aβ-dependent calpain activity and 17-kDa Tau production. Biochemical and immunocytochemical analyses demonstrated that decreasing membrane cholesterol levels in mature neurons resulted in a significant reduction of the NR1 subunit at the membrane as well as an increase in the number of large NR1, NR2A, and NR2B subunit clusters. Moreover, the majority of these larger N-methyl-d-aspartic acid receptor subunit immunoreactive spots was not juxtaposed to presynaptic sites in cholesterol-reduced neurons. These data suggested that changes at the synaptic level underlie the mechanism by which membrane cholesterol modulates developmental changes in the susceptibility of hippocampal neurons to Aβ-induced toxicity.

Retardation of Abeta fibril formation by phospholipid vesicles depends on membrane phase behavior

An increasing amount of evidence suggests that in several amyloid diseases, the fibril formation in vivo and the mechanism of toxicity both involve membrane interactions. We have studied Alzheimer's disease related amyloid beta peptide (Abeta). Recombinant Abeta(M1-40) and Abeta(M1-42) produced in Escherichia coli, allows us to carry out large scale kinetics assays with good statistics. The amyloid formation process is followed in means of thioflavin T fluorescence at relatively low (down to 380 nM) peptide concentration approaching the physiological range. The lipid membranes are introduced in the system as large and small unilamellar vesicles. The aggregation lagtime increases in the presence of lipid vesicles for all situations investigated and the phase behavior of the membrane in the vesicles has a large effect on the aggregation kinetics. By comparing vesicles with different membrane phase behavior we see that the solid gel phase dipalmitoylphosphatidylcholine bilayers cause the largest retardation of Abeta fibril formation. The membrane-induced retardation reaches saturation and is present when the vesicles are added during the lag time up to the nucleation point. No significant difference is detected in lag time when increasing amount of negative charge is incorporated into the membrane.

From protons to OXPHOS supercomplexes and Alzheimer's disease: structure-dynamics-function relationships of energy-transducing membranes

By the elucidation of high-resolution structures the view of the bioenergetic processes has become more precise. But in the face of these fundamental advances, many problems are still unresolved. We have examined a variety of aspects of energy-transducing membranes from large protein complexes down to the level of protons and functional relevant picosecond protein dynamics. Based on the central role of the ATP synthase for supplying the biological fuel ATP, one main emphasis was put on this protein complex from both chloroplast and mitochondria. In particular the stoichiometry of protons required for the synthesis of one ATP molecule and the supramolecular organisation of ATP synthases were examined. Since formation of supercomplexes also concerns other complexes of the respiratory chain, our work was directed to unravel this kind of organisation, e.g. of the OXPHOS supercomplex I(1)III(2)IV(1), in terms of structure and function. Not only the large protein complexes or supercomplexes work as key players for biological energy conversion, but also small components as quinones which facilitate the transfer of electrons and protons. Therefore, their location in the membrane profile was determined by neutron diffraction. Physico-chemical features of the path of protons from the generators of the electrochemical gradient to the ATP synthase, as well as of their interaction with the membrane surface, could be elucidated by time-resolved absorption spectroscopy in combination with optical pH indicators. Diseases such as Alzheimer's dementia (AD) are triggered by perturbation of membranes and bioenergetics as demonstrated by our neutron scattering studies.

Abeta(25-35) attenuated SREBP level in nuclear extracts of serum-deprived human neuroblastoma cells

Disturbance in cholesterol homeostasis appears to be an important factor in the pathogenesis of neurodegenerative disorders. The aim of the present study was to investigate sterol regulatory element binding protein (SREBP) levels in the nuclear extracts of human neuroblastoma cells and the possible interaction of beta-amyloid peptide (Abeta) and cholesterol with this transcription factor. In this study, cultured human neuroblastoma cells (SHSY-5Y) were incubated in serum-deprived media in the presence or absence of Abeta((25-35)) (1 microM) or cholesterol (300 microM) for 24 h. Nuclear extracts were subjected to SDS-PAGE, and SREBP cleavage product (68 kDa) was detected by immunoblotting. SREBP levels were elevated in the cells incubated 24 h in serum-deprived experimental media and were attenuated by Abeta or cholesterol-supplementation. It is likely that the ability of Abeta to release cholesterol into the medium and downregulate SREBP is due to a feedback mechanism.

Membrane interactions and the effect of metal ions of the amyloidogenic fragment Abeta(25-35) in comparison to Abeta(1-42)

Abeta(1-42) peptide, found as aggregated species in Alzheimer's disease brain, is linked to the onset of Alzheimer's disease. Many reports have linked metals to inducing Abeta aggregation and amyloid plaque formation. Abeta(25-35), a fragment from the C-terminal end of Abeta(1-42), lacks the metal coordinating sites found in the full-length peptide and is neurotoxic to cortical cortex cell cultures. We report solid-state NMR studies of Abeta(25-35) in model lipid membrane systems of anionic phospholipids and cholesterol, and compare structural changes to those of Abeta(1-42). When added after vesicle formation, Abeta(25-35) was found to interact with the lipid headgroups and slightly perturb the lipid acyl-chain region; when Abeta(25-35) was included during vesicle formation, it inserted deeper into the bilayer. While Abeta(25-35) retained the same beta-sheet structure irrespective of the mode of addition, the longer Abeta(1-42) appeared to have an increase in beta-sheet structure at the C-terminus when added to phospholipid liposomes after vesicle formation. Since the Abeta(25-35) fragment is also neurotoxic, the full-length peptide may have more than one pathway for toxicity.

Exploring interaction of beta-amyloid segment (25-35) with membrane models through paramagnetic probes

The accumulation of beta-amyloid peptides into senile plaques is one of the hallmarks of Alzheimer's disease (AD). There is mounting evidence that the lipid matrix of neuronal cell membranes plays an important role in the beta-sheet oligomerization process of beta-amyloid. Abeta(25-35), the sequence of which is GSNKGAIIGLM, is a highly toxic segment of amyloid beta (Abeta)-peptides, which forms fibrillary aggregates. In the present work, two spin-labelled Abeta(25-35) analogues containing the nitroxide group of the amino acid TOAC (2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid) as a paramagnetic probe at the N- or the C-terminus of the peptide sequence, respectively, were synthesized in order to investigate the peptide-membrane interaction. The orientation and associated changes of the peptide conformation in the presence of different artificial membrane models (micelles, liposomes) were evaluated by electron paramagnetic resonance and circular dichroism techniques. The results of this study allowed us to propose a model in which the C-terminal portion of the peptide is highly associated to the membrane, while the N-terminal part extends into the aqueous phase with occasional contacts with the lipid head-group region. Interestingly, the interaction of the C-terminal portion of the peptide is particularly enhanced in the presence of sodium dodecyl sulfate (SDS) molecules.