Profile of changes in lipid bilayer structure caused by beta-amyloid peptide

A unifying framework for amyloid-mediated membrane damage: The lipid-chaperone hypothesis

Over the past thirty years, researchers have highlighted the role played by a class of proteins or polypeptides that forms pathogenic amyloid aggregates in vivo, including i) the amyloid Aβ peptide, which is known to form senile plaques in Alzheimer's disease; ii) α-synuclein, responsible for Lewy body formation in Parkinson's disease and iii) IAPP, which is the protein component of type 2 diabetes-associated islet amyloids. These proteins, known as intrinsically disordered proteins (IDPs), are present as highly dynamic conformational ensembles. IDPs can partially (mis) fold into (dys) functional conformations and accumulate as amyloid aggregates upon interaction with other cytosolic partners such as proteins or lipid membranes. In addition, an increasing number of reports link the toxicity of amyloid proteins to their harmful effects on membrane integrity. Still, the molecular mechanism underlying the amyloidogenic proteins transfer from the aqueous environment to the hydrocarbon core of the membrane is poorly understood. This review starts with a historical overview of the toxicity models of amyloidogenic proteins to contextualize the more recent lipid-chaperone hypothesis. Then, we report the early molecular-level events in the aggregation and ion-channel pore formation of Aβ, IAPP, and α-synuclein interacting with model membranes, emphasizing the complexity of these processes due to their different spatial-temporal resolutions. Next, we underline the need for a combined experimental and computational approach, focusing on the strengths and weaknesses of the most commonly used techniques. Finally, the last two chapters highlight the crucial role of lipid-protein complexes as molecular switches among ion-channel-like formation, detergent-like, and fibril formation mechanisms and their implication in fighting amyloidogenic diseases.

Genotoxicity of advanced glycation end products in vitro is influenced by their preparation temperature, purification, and cell exposure time

Advanced glycation end products (AGEs) are formed via non-enzymatic reactions between amino groups of proteins and the carbonyl groups of reducing sugars. Previous studies have shown that highly glycated albumin prepared using a glucose-bovine serum albumin (Glu-BSA) model system incubated at 60°C for 6 weeks induces genotoxicity in WIL2-NS cells at 9 days of exposure measured by the cytokinesis-block micronucleus cytome (CBMNcyt) assay. However, this AGE model system is not physiologically relevant as normal body temperature is 37°C and the degree of glycation may exceed the extent of albumin modification in vivo. We hypothesised that the incubation temperature and purification method used in these studies may cause changes to the chemical profile of the glycated albumin and may influence the extent of genotoxicity observed at 3, 6 and 9 days of exposure. We prepared AGEs generated using Glu-BSA model systems incubated at 60°C or 37°C purified using trichloroacetic acid (TCA) precipitation or ultrafiltration (UF) and compared their chemical profile (glycation, oxidation, and aggregation) and genotoxicity in WIL2-NS cells using the CBMNcyt assay after 3, 6, and 9 days of exposure. The number of micronuclei (MNi) was significantly higher for cells treated with Glu-BSA incubated at 60°C and purified via TCA (12 ± 1 MNi/1000 binucleated cells) compared to Glu-BSA incubated at 37°C and purified using UF (6 ± 1 MNi/1000 binucleated cells) after 9 days (p < 0.0001). The increase in genotoxicity observed could be explained by a higher level of protein glycation, oxidation, and aggregation of the Glu-BSA model system incubated at 60°C relative to 37°C. This study highlighted that the incubation temperature, purification method and cell exposure time are important variables to consider when generating AGEs in vitro and will enable future studies to better reflect in vivo situations of albumin glycation.

Molecular rotors report on changes in live cell plasma membrane microviscosity upon interaction with beta-amyloid aggregates

Amyloid deposits of aggregated beta-amyloid Aβ(1-42) peptides are a pathological hallmark of Alzheimer's disease. Aβ(1-42) aggregates are known to induce biophysical alterations in cells, including disruption of plasma membranes. We investigated the microviscosity of plasma membranes upon interaction with oligomeric and fibrillar forms of Aβ(1-42). Viscosity-sensing fluorophores termed molecular rotors were utilised to directly measure the microviscosities of giant plasma membrane vesicles (GPMVs) and plasma membranes of live SH-SY5Y and HeLa cells. The fluorescence lifetimes of membrane-inserting BODIPY-based molecular rotors revealed a decrease in bilayer microviscosity upon incubation with Aβ(1-42) oligomers, while fibrillar Aβ(1-42) did not significantly affect the microviscosity of the bilayer. In addition, we demonstrate that the neuroprotective peptide H3 counteracts the microviscosity change induced by Aβ(1-42) oligomers, suggesting the utility of H3 as a neuroprotective therapeutic agent in neurodegenerative disorders and indicating that ligand-induced membrane stabilisation may be a possible mechanism of neuroprotection during neurodegenerative disorders such as Alzheimer's disease.

Cause and consequence of Aβ - Lipid interactions in Alzheimer disease pathogenesis

Self-templating propagation of protein aggregate conformations is increasingly becoming a significant factor in many neurological diseases. In Alzheimer disease (AD), intrinsically disordered amyloid-β (Aβ) peptides undergo aggregation that is sensitive to environmental conditions. High-molecular weight aggregates of Aβ that form insoluble fibrils are deposited as senile plaques in AD brains. However, low-molecular weight aggregates called soluble oligomers are known to be the primary toxic agents responsible for neuronal dysfunction. The aggregation process is highly stochastic involving both homotypic (Aβ-Aβ) and heterotypic (Aβ with interacting partners) interactions. Two of the important members of interacting partners are membrane lipids and surfactants, to which Aβ shows a perpetual association. Aβ-membrane interactions have been widely investigated for more than two decades, and this research has provided a wealth of information. Although this has greatly enriched our understanding, the objective of this review is to consolidate the information from the literature that collectively showcases the unique phenomenon of lipid-mediated Aβ oligomer generation, which has largely remained inconspicuous. This is especially important because Aβ aggregate "strains" are increasingly becoming relevant in light of the correlations between the structure of aggregates and AD phenotypes. Here, we will focus on aspects of Aβ-lipid interactions specifically from the context of how lipid modulation generates a wide variety of biophysically and biochemically distinct oligomer sub-types. This, we believe, will refocus our thinking on the influence of lipids and open new approaches in delineating the mechanisms of AD pathogenesis. This article is part of a Special Issue entitled: Protein Aggregation and Misfolding at the Cell Membrane Interface edited by Ayyalusamy Ramamoorthy.

Inhibitors of Acetylcholinesterase Derived from 7-Methoxytacrine and Their Effects on the Choline Transporter CHT1

BACKGROUND

Reversible acetylcholinesterase inhibitors are used in Alzheimer disease therapy. However, tacrine and its derivatives have severe side effects. Derivatives of the tacrine analogue 7-methoxytacrine (MEOTA) are less toxic.

METHODS

We evaluated new derivatives of 7-MEOTA (2 homodimers linked by 2 C4-C5 chains and 5 N-alkylated C4-C8 side chain derivatives) in vitro, using the rat hippocampal choline transporter CHT1.

RESULTS

Some derivatives were effective inhibitors of rat acetylcholinesterase and comparable with 7-MEOTA. All derivatives were able to inhibit CHT1, probably via quaternary ammonium, and this interaction could be involved in the enhancement of their detrimental side effects and/or in the attenuation of their promising effects. Under conditions of disrupted lipid rafts, the unfavorable effects of some derivatives were weakened. Only tacrine was probably able to stereospecifically interact with the naturally occurring amyloid-β isoform and to simultaneously stimulate CHT1. Some derivatives, when coincubated with amyloid β, did not influence CHT1. All derivatives also increased the fluidity of the cortical membranes.

CONCLUSION

The N-alkylated derivative of 7-MEOTA bearing from C4 side chains appears to be the most promising compound and should be evaluated in future in vivo research.

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.

Amyloid Beta Peptide Folding in Reverse Micelles

Previously published experimental studies have suggested that when the 40-residue amyloid beta peptide is encapsulated in a reverse micelle, it folds into a structure that may nucleate amyloid fibril formation (Yeung, P. S.-W.; Axelsen, P. H. J. Am. Chem. Soc. 2012, 134, 6061 ). The factors that induce the formation of this structure have now been identified in a multi-microsecond simulation of the same reverse micelle system that was studied experimentally. Key features of the polypeptide-micelle interaction include the anchoring of a hydrophobic residue cluster into gaps in the reverse micelle surface, the formation of a beta turn at the anchor point that brings N- and C-terminal segments of the polypeptide into proximity, high ionic strength that promotes intramolecular hydrogen bond formation, and deformation of the reverse micelle surface to facilitate interactions with the surface along the entire length of the polypeptide. Together, these features cause the simulation-derived vibrational spectrum to red shift in a manner that reproduces the red-shift previously reported experimentally. On the basis of these findings, a new mechanism is proposed whereby membranes nucleate fibril formation and facilitate the in-register alignment of polypeptide strands that is characteristic of amyloid fibrils.

Disruption of endocytic transport by transthyretin aggregates

The cytotoxicity of amyloidogenic proteins such as transthyretin (TTR) has implications for neurodegeneration and other pathologies, but is not well understood. In the current study, potential effects of misfolded, aggregated TTRs (agTTR) upon a major cell membrane function-endocytosis-were assessed. Internalization of transferrin (Tf), a ligand whose receptor-mediated endocytosis is well characterized, was analyzed in different cell types after treatment with agTTR. The results indicate disruption of Tf endocytosis: 20-25% inhibition by agTTR relative to the same concentrations of normal soluble TTR, or relative to another control protein, albumin (p<0.05 for agTTR relative to controls). No statistically significant difference was observed for cell surface Tf binding between agTTR-treated and control cells. This is the first evidence for endocytic disruption by agTTR, and presents a novel cytotoxicity mechanism that may account for previously reported inhibitory effects of amyloidogenic TTR on neuronal growth.

Replica exchange molecular dynamics study of the amyloid beta (11–40) trimer penetrating a membrane

The transmembrane Aβ_11–40 trimer is investigated for the first time using REMD and FEP.

Sex-Dependent Changes in Striatal Dopamine Transport in Preadolescent Rats Exposed Prenatally and/or Postnatally to Methamphetamine

Methamphetamine (MA) is the most commonly used psychostimulant drug, the chronic abuse of which leads to neurodegenerative changes in the brain. The global use of MA is increasing, including in pregnant women. Since MA can cross both placental and haematoencephalic barriers and is also present in maternal milk, children of chronically abused mothers are exposed prenatally as well as postnatally. Women seem to be more vulnerable to some aspects of MA abuse than men. MA is thought to exert its effects among others via direct interactions with dopamine transporters (DATs) in the brain tissue. Sexual dimorphism of the DAT system could be a base of sex-dependent actions of MA observed in behavioural and neurochemical studies. Possible sex differences in the DATs of preadolescent offspring exposed to MA prenatally and/or postnatally have not yet been evaluated. We examined the striatal synaptosomal DATs (the activity and density of surface expressed DATs and total DAT expression) in preadolescent male and female Wistar rats (31-35-day old animals) exposed prenatally and/or postnatally to MA (daily 5 mg/kg, s.c. to mothers during pregnancy and lactation). To distinguish between specific and nonspecific effects of MA on DATs, we also evaluated the in vitro effects of lipophilic MA on the fluidity of striatal membranes isolated from preadolescent and young adult rats of both sexes. We observed similar changes in the DATs of preadolescent rats exposed prenatally or postnatally (MA-mediated drop in the reserve pool but no alterations in surface-expressed DATs). However, prenatal exposure evoked significant changes in males and postnatal exposure in females. A significant decrease in the activity of surface-expressed DATs was found only in postnatally exposed females sensitized to MA via prenatal exposure. MA applied in vitro increased the fluidity of striatal membranes of preadolescent female but not male rats. In summary, DATs of preadolescent males are more sensitive to prenatal MA exposure via changes in the reserve pool and those of preadolescent females to postnatal MA exposure via the same mechanism. The combination of prenatal and postnatal MA exposure increases the risk of dopaminergic deficits via alterations in the activity of surface-expressed DATs especially in preadolescent females. MA-mediated changes in DATs of preadolescent females could be still enhanced via nonspecific disordering actions of MA on striatal membranes.

Effective lipid-detergent system for study of membrane active peptides in fluid liposomes

The structure of peptide antibiotic gramicidin A (gA) was studied in phosphatidylcholin liposomes modified by nonionic detergent Triton X-100. First, the detergent : lipid ratio at which the saturation of lipid membrane by Triton X-100 occurs (Re (sat)), was determined by light scattering. Measurements of steady-state fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene at sublytic concentrations of detergent showed that after saturation of the membrane by Triton X-100 microviscosity of lipid bilayer is reduced by 20%. The equilibrium conformational state of gA in phosphatidylcholine liposomes at Re (sat) was studied by CD spectroscopy. It was found that the conformational state of this channel-forming peptide changed crucially when Triton X-100 induced transition to more fluid membranes. The gA single-channel measurements were made with Triton X-100 containing bilayers. Tentative assignment of the channel type and gA structures was made by correlation of CD data with conductance histograms. Lipid-detergent system with variable viscosity developed in this work can be used to study the structure and folding of other membrane-active peptides.

Variation in lipid and fatty acid uptake among nematode and cestode parasites and their host, domestic fowl: host-parasite interaction

Lipid synthesis is an important process in most organisms as well as in helminths. The present observation shows the variation of lipid and fatty acid uptake among cestode, Raillietina (Fuhrmannetta) echinobothrida; nematode, Ascaridia galli and their host, Gallus domesticus, the common country fowl. Total lipid (TL), neutral lipid (NL), glycolipid (GL), phospholipid (PL) and their fatty acid of cestode, nematode and liver and intestinal fluid of the host were analyzed by thin layer chromatography and gas liquid chromatography respectively. The result shows that liver take more TL, PL and GL except NL. Utilization of lipid from intestinal fluid when compare between the parasites, it is found that TL and PL content of cestode are higher than nematode, whereas, nematode absorbs more NL and GL than cestode. The percent of cholesterol is more in cestode than nematode. Palmitic, stearic, oleic and linoleic are the predominant fatty acids among all the samples. The present study reveals that the cestode having large surface area is more opportunistic in the resource utilization over the nematode as well as the host.

FRET evidence for untwisting of amyloid fibrils on the surface of model membranes

Apolipoprotein A-I (apoA-I) is an amyloid-forming protein whose amyloidogenic properties are attributed mainly to its N-terminal fragment. Cell membranes are thought to be the primary target for the toxic amyloid aggregates. In the present study Förster resonance energy transfer (FRET) between the membrane fluorescent probe Laurdan as a donor and amyloid-specific dye Thioflavin T (ThT) as an acceptor was employed to explore the interactions of amyloid fibrils from apoA-I variants 1-83/G26R and 1-83/G26R/W@8 with the model membranes composed of phosphatidylcholine and its mixture with cholesterol. The changes in FRET efficiency upon fibril-lipid binding were found to correlate with the extent of protein fibrillization. AFM imaging revealed the presence of two polymorphic states of fibrillar 1-83/G26R/W@8 with the helical and twisted ribbon morphologies. The simulation-based analysis of the experimental FRET profiles provided the arguments in favor of untwisting of fibrillar assemblies upon their interaction with the model membranes. Evidence for the face-on orientation and superficial bilayer location of the membrane-bound fragments of 1-83/G26R/W@8 fibrils was obtained.

Interactions of Lipid Membranes with Fibrillar Protein Aggregates

Amyloid fibrils are an intriguing class of protein aggregates with distinct physicochemical, structural and morphological properties. They display peculiar membrane-binding behavior, thus adding complexity to the problem of protein-lipid interactions. The consensus that emerged during the past decade is that amyloid cytotoxicity arises from a continuum of cross-β-sheet assemblies including mature fibrils. Based on literature survey and our own data, in this chapter we address several aspects of fibril-lipid interactions, including (i) the effects of amyloid assemblies on molecular organization of lipid bilayer; (ii) competition between fibrillar and monomeric membrane-associating proteins for binding to the lipid surface; and (iii) the effects of lipids on the structural morphology of fibrillar aggregates. To illustrate some of the processes occurring in fibril-lipid systems, we present and analyze fluorescence data reporting on lipid bilayer interactions with fibrillar lysozyme and with the N-terminal 83-residue fragment of amyloidogenic mutant apolipoprotein A-I, 1-83/G26R/W@8. The results help understand possible mechanisms of interaction and mutual remodeling of amyloid fibers and lipid membranes, which may contribute to amyloid cytotoxicity.

Lipids in Amyloid-β Processing, Aggregation, and Toxicity

Aggregation of amyloid-beta (Aβ) peptide is the major event underlying neuronal damage in Alzheimer's disease (AD). Specific lipids and their homeostasis play important roles in this and other neurodegenerative disorders. The complex interplay between the lipids and the generation, clearance or deposition of Aβ has been intensively investigated and is reviewed in this chapter. Membrane lipids can have an important influence on the biogenesis of Aβ from its precursor protein. In particular, increased cholesterol in the plasma membrane augments Aβ generation and shows a strong positive correlation with AD progression. Furthermore, apolipoprotein E, which transports cholesterol in the cerebrospinal fluid and is known to interact with Aβ or compete with it for the lipoprotein receptor binding, significantly influences Aβ clearance in an isoform-specific manner and is the major genetic risk factor for AD. Aβ is an amphiphilic peptide that interacts with various lipids, proteins and their assemblies, which can lead to variation in Aβ aggregation in vitro and in vivo. Upon interaction with the lipid raft components, such as cholesterol, gangliosides and phospholipids, Aβ can aggregate on the cell membrane and thereby disrupt it, perhaps by forming channel-like pores. This leads to perturbed cellular calcium homeostasis, suggesting that Aβ-lipid interactions at the cell membrane probably trigger the neurotoxic cascade in AD. Here, we overview the roles of specific lipids, lipid assemblies and apolipoprotein E in Aβ processing, clearance and aggregation, and discuss the contribution of these factors to the neurotoxicity in AD.

Defining the limits: Protein aggregation and toxicity in vivo

Abstract others complementary, to resolve mis-folded proteins when they arise, ranging from refolding through the action of molecular chaperones to elimination through regulated proteolytic mechanisms. These protein quality control pathways are sufficient, under normal conditions, to maintain a functioning proteome, but in response to diverse environmental, genetic and/or stochastic events, protein mis-folding exceeds the corrective capacity of these pathways, leading to the accumulation of aggregates and ultimately toxicity. Particularly devastating examples of these effects include certain neurodegenerative diseases, such as Huntington's Disease, which are associated with the expansion of polyglutamine tracks in proteins. In these cases, protein mis-folding and aggregation are clear contributors to pathogenesis, but uncovering the precise mechanistic links between the two events remains an area of active research. Studies in the yeast Saccharomyces cerevisiae and other model systems have uncovered previously unanticipated complexity in aggregation pathways, the contributions of protein quality control processes to them and the cellular perturbations that result from them. Together these studies suggest that aggregate interactions and localization, rather than their size, are the crucial considerations in understanding the molecular basis of toxicity.

Change of dynamics of raft-model membrane induced by amyloid-β protein binding

While the steady-state existence in the size and shape of liquid-ordered microdomains in cell membranes, the so-called "lipid rafts", still remain the subject of debate, glycosphingolipid-cholesterol rich regions in plasma membranes have been considered to have a function as platforms for signaling and sorting. In addition, recent spectroscopic studies show that the interaction between monosialoganglioside and amyloid beta (Aβ protein promotes the transition of Aβ from the native structure to the cross-beta fold in amyloid aggregates. However, there is few evidence on the dynamics of "lipid rafts" membranes. As the neutron spin-echo (NSE) technique is well known to detect directly slow dynamics of membrane systems in situ, by the combination of NSE and small-angle X-ray scattering we have studied the effect of the interaction between raft-model membrane and amyloid Aβ proteins on the structure and dynamics of a large uni-lamellar vesicle (LUV) consisting of monosialoganglioside-cholesterol-phospholipid ternary mixtures as a model of lipid-raft membrane. We have found that the interaction between the Aβ proteins and the model membrane at the liquid crystal phase significantly suppresses a bending-diffusion motion with a minor effect on the LUV structure. The present results would suggest a possibility of non-receptor-mediated disorder in signaling through a modulation of a membrane dynamics induced by the association of amyloidogenic peptides on a plasma membrane.

β-Amyloid (1-40) peptide interactions with supported phospholipid membranes: a single-molecule study

Recent evidence supports the hypothesis that the oligomers formed by the β-amyloid peptide early in its aggregation process are neurotoxic and may feature in Alzheimer's disease. Although the mechanism underlying this neurotoxicity remains unclear, interactions of these oligomers with neuronal membranes are believed to be involved. Identifying the neurotoxic species is challenging because β-amyloid peptides form oligomers at very low physiological concentrations (nM), and these oligomers are highly heterogeneous and metastable. Here, we report the use of single-molecule imaging techniques to study the interactions between β-amyloid (1-40) peptides and supported synthetic model anionic lipid membranes. The evolution of the β-amyloid species on the membranes was monitored for up to several days, and the results indicate an initial tight, uniform, binding of β-amyloid (1-40) peptides to the lipid membranes, followed by oligomer formation in the membrane. At these low concentrations, the behavior at early times during the formation of small oligomers is interpreted qualitatively in terms of the two-state model proposed by H. W. Huang for the interaction between amphipathic peptides and membranes. However, the rate of oligomer formation in the membrane and their size are highly dependent on the concentrations of β-amyloid (1-40) peptides in aqueous solution, suggesting two different pathways of oligomer formation, which lead to drastically different species in the membrane and a departure from the two-state model as the concentration increases.

Aβ42 oligomers, but not fibrils, simultaneously bind to and cause damage to ganglioside-containing lipid membranes

Aβ (amyloid-β peptide) assembles to form amyloid fibres that accumulate in senile plaques associated with AD (Alzheimer's disease). The major constituent, a 42-residue Aβ, has the propensity to assemble and form soluble and potentially cytotoxic oligomers, as well as ordered stable amyloid fibres. It is widely believed that the cytotoxicity is a result of the formation of transient soluble oligomers. This observed toxicity may be associated with the ability of oligomers to associate with and cause permeation of lipid membranes. In the present study, we have investigated the ability of oligomeric and fibrillar Aβ42 to simultaneously associate with and affect the integrity of biomimetic membranes in vitro. Surface plasmon field-enhanced fluorescence spectroscopy reveals that the binding of the freshly dissolved oligomeric 42-residue peptide binds with a two-step association with the lipid bilayer, and causes disruption of the membrane resulting in leakage from vesicles. In contrast, fibrils bind with a 2-fold reduced avidity, and their addition results in approximately 2-fold less fluorophore leakage compared with oligomeric Aβ. Binding of the oligomers may be, in part, mediated by the GM1 ganglioside receptors as there is a 1.8-fold increase in oligomeric Aβ binding and a 2-fold increase in permeation compared with when GM1 is not present. Atomic force microscopy reveals the formation of defects and holes in response to oligomeric Aβ, but not preformed fibrillar Aβ. The results of the present study indicate that significant membrane disruption arises from association of low-molecular-mass Aβ and this may be mediated by mechanical damage to the membranes by Aβ aggregation. This membrane disruption may play a key role in the mechanism of Aβ-related cell toxicity in AD.

Oxidative stress and cell membranes in the pathogenesis of Alzheimer's disease

Amyloid β proteins and oxidative stress are believed to have central roles in the development of Alzheimer's disease. Lipid membranes are among the most vulnerable cellular components to oxidative stress, and membranes in susceptible regions of the brain are compositionally distinct from those in other tissues. This review considers the evidence that membranes are either a source of neurotoxic lipid oxidation products or the target of pathogenic processes involving amyloid β proteins that cause permeability changes or ion channel formation. Progress toward a comprehensive theory of Alzheimer's disease pathogenesis is discussed in which lipid membranes assume both roles and promote the conversion of monomeric amyloid β proteins into fibrils, the pathognomonic histopathological lesion of the disease.

Physiological cholesterol concentration is a neuroprotective factor against β-amyloid and β-amyloid-metal complexes toxicity

Alzheimer's disease is one of the most common forms of dementia in the elderly. One of its hallmarks is the abnormal aggregation and deposition of β-amyloid (Aβ). Endogenous and exogenous metal ions seem to influence β-amyloid folding process, aggregation and deposition. Besides these variables other elements appear to affect β-amyloid behavior, such as cholesterol. The physiological concentration of cholesterol in the cerebrospinal fluid (CSF) was used in order to determine the extent in which Aβ and Aβ-metal complexes in vitro aggregation and their toxicity on human neuroblastoma cell cultures is affected. Cholesterol did not appear to influence Aβ and Aβ-metal complexes aggregation, but it was effective in protecting neuroblastoma cells against Aβ complexes' toxicity. The Aβ-Al complex seemed to be the most effective in disrupting and damaging membrane external layer, and simultaneously it appears to increase its toxicity on cell cultures; both of these effects are preventable by cholesterol. The presence in physiological concentrations of cholesterol seemed to compensate membrane damage that occurred to neuroblastoma cells. These findings appear to contradict some data reported in literature. We believe that our results might shed some light on the role played by cholesterol at physiological concentrations in both cellular balance and membrane protection.

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.

Investigating the effects of sodium dodecyl sulfate on the aggregative behavior of hen egg-white lysozyme at acidic pH

The research presented here is aimed at examining the effects of sodium dodecyl sulfate on the aggregative behavior of hen egg-white lysozyme at pH 2.0. Through various spectroscopic techniques, dynamic light scattering, and electron microscopy, we first demonstrated that SDS exhibited a biphasic effect on lysozyme fibrillation. The presence of SDS at higher concentrations (e.g., 0.25, 5.00, or 20.00 mM SDS) was found to suppress fibril formation of lysozyme whereas fibrillogenic lysozyme-SDS ensemble containing beta-sheet-rich conformation was observed upon the addition of lower concentrations of SDS (e.g., 0.00, 0.06, or 0.1mM SDS). Next, our equilibrium urea-unfolding data revealed that lysozyme samples with higher SDS concentrations showed superior thermodynamic stabilities over the ones with no or lower levels of SDS. Finally, the correlation between SDS concentration and lysozyme aggregative/fibrillogenic propensity and the underlying interacting mechanism were further explored using surface tensiometry and isothermal titration calorimetry. We believe the outcome from this work may not only help decipher the molecular mechanism of amyloid fibrillation, but also shed light on a rational design of potential therapeutic strategies for amyloid pathology.

Interactions of oritavancin, a new semi-synthetic lipoglycopeptide, with lipids extracted from Staphylococcus aureus

Oritavancin, a lipoglycopeptide with marked bactericidal activity against vancomycin-resistant Staphylococcus aureus and enterococci, induces calcein release from CL:POPE and POPG:POPE liposomes, an effect enhanced by an increase in POPG:POPE ratio, and decreased when replacing POPG by DPPG (Domenech et al., Biochim Biophys Acta 2009; 1788:1832-40). Using vesicles prepared from lipids extracted from S. aureus, we showed that oritavancin induces holes, erosion of the edges, and decrease of the thickness of the supported lipid bilayers (atomic force microscopy; AFM). Oritavancin also induced an increase of membrane permeability (calcein release) on a time- and dose-dependent manner. These effects were probably related to the ability of the drug to bind to lipid bilayers as shown by 8-anilino-1- naphthalene sulfonic acid (ANS) assay. Interaction of oritavancin with phospholipids at the level of their glycerol backbone and hydrophobic domain was studied by monitoring changes of Laurdan excitation generalized polarization (GP(ex)) and 1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescence anisotropy upon temperature increase. Oritavancin increased GP(ex) values and the transition temperature, indicating a more ordered structure at the level of the glycerol backbone. Oritavancin slightly decreased DPH fluorescence depolarization intensities, suggesting an increase in fluidity at the level of acyl chains. Together, our data confirm the interaction of oritavancin with lipids and the potential role of a rigidifying effect at the level of glycerol backbone for membrane permeabilization. This work shows how AFM and biophysical methods may help in characterizing drug-membrane interactions, and sheds further light on the mode of action of oritavancin.

Interplay between protein homeostasis networks in protein aggregation and proteotoxicity

The misfolding and aggregation of disease proteins is characteristic of numerous neurodegenerative diseases. Particular neuronal populations are more vulnerable to proteotoxicity while others are more apt to tolerate the misfolding and aggregation of disease proteins. Thus, the cellular environment must play a significant role in determining whether disease proteins are converted into toxic or benign forms. The endomembrane network of eukaryotes divides the cell into different subcellular compartments that possess distinct sets of molecular chaperones and protein interaction networks. Chaperones act as agonists and antagonists of disease protein aggregation to prevent the accumulation of toxic intermediates in the aggregation pathway. Interacting partners can also modulate the conformation and localization of disease proteins and thereby influence proteotoxicity. Thus, interplay between these protein homeostasis network components can modulate the self-association of disease proteins and determine whether they elicit a toxic or benign outcome.

The consequences of mitochondrial amyloid beta-peptide in Alzheimer's disease

The Abeta (amyloid-beta peptide) has long been associated with Alzheimer's disease, originally in the form of extracellular plaques. However, in the present paper we review the growing evidence for the role of soluble intracellular Abeta in the disease progression, with particular reference to Abeta found within the mitochondria. Once inside the cell, Abeta is able to interact with a number of targets, including the mitochondrial proteins ABAD (amyloid-binding alcohol dehydrogenase) and CypD (cyclophilin D), which is a component of the mitochondrial permeability transition pore. Interference with the normal functions of these proteins results in disruption of cell homoeostasis and ultimately cell death. The present review explores the possible mechanisms by which cell death occurs, considering the evidence presented on a molecular, cellular and in vivo level.

Impact of phospholipid bilayer saturation on amyloid-beta protein aggregation intermediate growth: a quartz crystal microbalance analysis

Evidence that membrane-associated amyloid aggregate growth can impart membrane damage represents one possible mechanism for the neurodegeneration associated with deposited amyloid-beta protein (Abeta) aggregates in the brains of Alzheimer's disease (AD) patients. This potential pathogenic event necessitates an understanding of the impact that cellular membrane composition may have on Abeta aggregate growth. In the current study, a quartz crystal microbalance (QCM) was employed to examine the growth of Abeta(1-40) aggregation intermediates on supported phospholipid bilayers (SPBs) assembled at the crystal surface. These surface-specific measurements illustrate that zwitterionic SPBs selectively bind aggregated but not monomeric protein, and these bound aggregates are capable of supporting nonsaturable reversible growth via monomer addition. Growth-capable Abeta(1-40) aggregation intermediates more readily bind SPBs composed of phospholipids with a greater degree of carbon saturation. Furthermore, kinetic analysis afforded by the quantitative real-time QCM measurements reveals that SPBs with greater saturation also better support the growth of bound Abeta(1-40) aggregation intermediates as a result of the slower dissociation of bound monomer rather than more efficient recognition between aggregate and monomeric protein. These findings correlate with epidemiological and experimental evidence that links increased dietary intake of polyunsaturated fatty acids to a reduced risk of AD.

Amyloid-beta fibrillogenesis: structural insight and therapeutic intervention

Structural insight into the conformational changes associated with aggregation and assembly of fibrils has provided a number of targets for therapeutic intervention. Solid-state NMR, hydrogen/deuterium exchange and mutagenesis strategies have been used to probe the secondary and tertiary structure of amyloid fibrils and key intermediates. Rational design of peptide inhibitors directed against key residues important for aggregation and stabilization of fibrils has demonstrated effectiveness at inhibiting fibrillogenesis. Studies on the interaction between Abeta and cell membranes led to the discovery that inositol, the head group of phosphatidylinositol, inhibits fibrillogenesis. As a result, scyllo-inositol is currently in clinical trials for the treatment of AD. Additional small-molecule inhibitors, including polyphenolic compounds such as curcumin, (-)-epigallocatechin gallate (EGCG), and grape seed extract have been shown to attenuate Abeta aggregation through distinct mechanisms, and have shown effectiveness at reducing amyloid levels when administered to transgenic mouse models of AD. Although the results of ongoing clinical trials remain to be seen, these compounds represent the first generation of amyloid-based therapeutics, with the potential to alter the progression of AD and, when used prophylactically, alleviate the deposition of Abeta.

Cholesterol modulates the interaction of beta-amyloid peptide with lipid bilayers

The interaction of an amphiphilic, 40-amino acid beta-amyloid (Abeta) peptide with liposomal membranes as a function of sterol mole fraction (X(sterol)) was studied based on the fluorescence anisotropy of a site-specific membrane sterol probe, dehydroergosterol (DHE), and fluorescence resonance energy transfer (FRET) from the native Tyr-10 residue of Abeta to DHE. Without Abeta, peaks or kinks in the DHE anisotropy versus X(sterol) plot were detected at X(sterol) approximately 0.25, 0.33, and 0.53. Monomeric Abeta preserved these peaks/kinks, but oligomeric Abeta suppressed them and created a new DHE anisotropy peak at X(sterol) approximately 0.38. The above critical X(sterol) values coincide favorably with the superlattice compositions predicted by the cholesterol superlattice model, suggesting that membrane cholesterol tends to adopt a regular lateral arrangement, or domain formation, in the lipid bilayers. For FRET, a peak was also detected at X(sterol) approximately 0.38 for both monomeric and oligomeric Abeta, implying increased penetration of Abeta into the lipid bilayer at this sterol mole fraction. We conclude that the interaction of Abeta with membranes is affected by the lateral organization of cholesterol, and hypothesize that the formation of an oligomeric Abeta/cholesterol domain complex may be linked to the toxicity of Abeta in neuronal membranes.

Electrochemical and conformational consequences of copper (Cu(I) and Cu(II)) binding to beta-amyloid(1-40)

Copper-induced structural rearrangements of Abeta40 structure and its redox properties are described in this study. Electrochemical and fluorescent methods are used to characterise the behaviour of Abeta-Cu species. The data suggest that time-dependent folding of Abeta-Cu species may cause changes in the redox potentials.Extracellular deposits of beta-amyloid (Abeta) into senile plaques are the major features observed in brains of Alzheimer's disease (AD) patients. A high concentration of copper has been associated with insoluble amyloid plaques. It is known that Abeta(1-40) can bind copper with high affinity, but electrochemical properties of Abeta(1-40)-Cu complexes are not well-characterised. In this study we demonstrate that complexation of copper (both as Cu(I) and Cu(II)) by Abeta(1-40) reduces the metal electrochemical activity. Formation of copper-Abeta(1-40) complexes is associated with alteration of the redox potential. The data reveal significant redox activity of fresh Abeta-copper solutions. However, copper-induced structural rearrangements of the peptide, documented by CD, correspond with time-dependent changes of formal reduction potentials (E(0')) of the complex. Fluorescent and electrochemical (cyclic voltammetry and differential pulse voltammetry) techniques suggest that reduction of the redox activity by Abeta-Cu complexes could be attributed to conformational changes that diminished copper accessibility to the external environment. According to our evidence, conformational rearrangements, induced by copper binding to amyloid, elongate the time necessary to attain the same beta-sheet content as for the metal-free peptide. Although the redox activity of Abeta-Cu complexes diminishes in a time-dependent manner, they are not completely devoid of toxicity as they destabilize red blood cells osmotic fragility, even after prolonged incubation.

Molecular chaperones antagonize proteotoxicity by differentially modulating protein aggregation pathways

The self-association of misfolded or damaged proteins into ordered amyloid-like aggregates characterizes numerous neurodegenerative disorders. Insoluble amyloid plaques are diagnostic of many disease states. Yet soluble, oligomeric intermediates in the aggregation pathway appear to represent the toxic culprit. Molecular chaperones regulate the fate of misfolded proteins and thereby influence their aggregation state. Chaperones conventionally antagonize aggregation of misfolded, disease proteins and assist in refolding or degradation pathways. Recent work suggests that chaperones may also suppress neurotoxicity by converting toxic, soluble oligomers into benign aggregates. Chaperones can therefore suppress or promote aggregation of disease proteins to ameliorate the proteotoxic accumulation of soluble, assembly intermediates.

Amyloid aggregation on lipid bilayers and its impact on membrane permeability

Fibrillar protein aggregates (amyloids) are involved in several common pathologies, e.g., Alzheimer's disease and type II diabetes. Accumulating evidence suggests that toxicity in amyloid-related diseases originates from the deposition of protein aggregates on the cell membrane, which results in bilayer disruption and cell leakage. The molecular mechanism of damage to the membrane, however, is still obscure. To shed light on it we have performed coarse-grained molecular dynamics simulations of fibril-forming amphipathic peptides in the presence of lipid vesicles. The simulation results show that highly amyloidogenic peptides fibrillate on the surface of the vesicle, damaging the bilayer and promoting leakage. In contrast, the ordered aggregation of peptides with low amyloidogenicity is hindered by the vesicles. Remarkably, leakage from the vesicle is caused by growing aggregates, but not mature fibrils. The simulation results provide a basis for understanding the range of aggregation behavior that is observed in experiments with fibril-forming (poly)peptides.

Disordered proteins: biological membranes as two-dimensional aggregation matrices

Aberrant folded proteins and peptides are hallmarks of amyloidogenic diseases. However, the molecular processes that cause these proteins to adopt non-native structures in vivo and become cytotoxic are still largely unknown, despite intense efforts to establish a general molecular description of their behavior. Clearly, the fate of these proteins is ultimately linked to their immediate biochemical environment in vivo. In this review, we focus on the role of biological membranes, reactive interfaces that not only affect the conformational stability of amyloidogenic proteins, but also their aggregation rates and, probably, their toxicity. We first provide an overview of recent work, starting with findings regarding the amphiphatic amyloid-beta protein (Abeta), which give evidence that membranes can directly promote aggregation, and that the effectiveness in this process can be related to the presence of specific neuronal ganglioside lipids. In addition, we discuss the implications of recent research (medin as an detailed example) regarding putative roles of membranes in the misfolding behavior of soluble, non-amphiphatic proteins, which are attracting increasing interest. The potential role of membranes in exerting the toxic action of misfolded proteins will also be highlighted in a molecular context. In this review, we discuss novel NMR-based approaches for exploring membrane-protein interactions, and findings obtained using them, which we use to develop a molecular concept to describe membrane-mediated protein misfolding as a quasi-two-dimensional process rather than a three-dimensional event in a biochemical environment. The aim of the review is to provide researchers with a general understanding of the involvement of membranes in folding/misfolding processes in vivo, which might be quite universal and important for future research concerning amyloidogenic and misfolding proteins, and possible ways to prevent their toxic actions.

Effects of amyloid beta-peptides on the lysis tension of lipid bilayer vesicles containing oxysterols

Amyloid beta-peptides (Abeta) applied directly from solution to model lipid membranes produced dramatic changes in the material properties of the bilayer when certain oxysterols were present in the bilayer. These effects were dependent on both lipid and peptide composition, and occurred at peptide concentrations as low as 100 nM. Using micropipette manipulation of giant unilamellar vesicles, we directly measured the lysis tension of lipid bilayers of various compositions. The glycerophospholipid 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC) constituted the main lipid component at 70 mol %. The remaining 30 mol % was composed of the following pure or mixed sterols: cholesterol (CHOL), 7-ketocholesterol (KETO), or 7beta-hydroxycholesterol (OHCHOL). SOPC/CHOL bilayers did not exhibit significant changes in mechanical properties after exposure to either Abeta(1-42) or Abeta(1-40). Partial substitution of CHOL with KETO (5 mol %), however, caused a drastic reduction of the lysis tension after exposure to Abeta(1-42) but not to Abeta(1-40). Partial substitution of CHOL with OHCHOL (5 mol %) caused a drastic reduction of the lysis tension after exposure to Abeta(1-40) and to Abeta(1-42). We attribute these effects to the reduction in intermolecular cohesive interactions caused by the presence of the second dipole of oxysterols, which reduces the energetic barrier for Abeta insertion into the bilayer.

Complex of Amyloid β Peptides with 24-Hydroxycholesterol and Its Effect on Hemicholinium-3 Sensitive Carriers

Brains of Alzheimer disease patients in early stages of dementia contain an increased 24(S)-hydroxycholesterol (cerebrosterol)/cholesterol ratio when compared to controls. In this study, effects of amyloid beta peptides and of racemic 24-hydroxycholesterol were evaluated in vitro on undepleted or cholesterol-depleted hippocampal synaptosomes of young and old rats via a high-affinity choline transport and membrane anisotropy measurements. Depletion of membrane cholesterol decreased the transport of [3H]choline, increased the specific binding of [3H]hemicholinium-3 and decreased membrane anisotropy. However, less alterations were found in old when compared to young brains. 500 nM nonaggregated peptides were ineffective but aggregated fragment 1-42 evoked marked drops in the transport and anisotropy values on depleted synaptosomes. 50 microM 24-hydroxycholesterol inhibited choline transport on depleted synaptosomes but it did not influence membrane anisotropy. Peptides eliminated the actions of oxysterol on choline carriers in young but not in old rats. On the other hand, oxysterol eliminated the effects of peptides on membrane anisotropy. Our study suggests a possible role of membrane cholesterol in the regulation of choline carriers and supports data reporting a protective role of membrane cholesterol against toxic effects of amyloid beta peptides. Moreover, via Raman spectroscopy we demonstrate for the first time that peptides form a complex with 24-hydroxycholesterol.

Fluorescence as a method to reveal structures and membrane-interactions of amyloidogenic proteins

Amyloidogenesis is a characteristic feature of the 40 or so known protein deposition diseases, and accumulating evidence strongly suggests that self-association of misfolded proteins into either fibrils, protofibrils, or soluble oligomeric species is cytotoxic. The most likely mechanism for toxicity is through perturbation of membrane structure, leading to increased membrane permeability and eventual cell death. There have been a rather limited number of investigations of the interactions of amyloidogenic polypeptides and their aggregated states with membranes; these are briefly reviewed here. Amyloidogenic proteins discussed include A-beta from Alzheimer's disease, the prion protein, alpha-synuclein from Parkinson's disease, transthyretin (FAP, SSA amyloidosis), immunoglobulin light chains (primary (AL) amyloidosis), serum amyloid A (secondary (AA) amyloidosis), amylin or IAPP (Type 2 diabetes) and apolipoproteins. This review highlights the significant role played by fluorescence techniques in unraveling the nature of amyloid fibrils and their interactions and effects on membranes. Fluorescence spectroscopy is a valuable and versatile method for studying the complex mechanisms of protein aggregation, amyloid fibril formation and the interactions of amyloidogenic proteins with membranes. Commonly used fluorescent techniques include intrinsic and extrinsic fluorophores, fluorescent probes incorporated in the membrane, steady-state and lifetime measurements of fluorescence emission, fluorescence correlation spectroscopy, fluorescence anisotropy and polarization, fluorescence resonance energy transfer (FRET), fluorescence quenching, and fluorescence microscopy.

Aggregation and fibrillation of bovine serum albumin

The all-alpha helix multi-domain protein bovine serum albumin (BSA) aggregates at elevated temperatures. Here we show that these thermal aggregates have amyloid properties. They bind the fibril-specific dyes Thioflavin T and Congo Red, show elongated although somewhat worm-like morphology and characteristic amyloid X-ray fiber diffraction peaks. Fibrillation occurs over minutes to hours without a lag phase, is independent of seeding and shows only moderate concentration dependence, suggesting intramolecular aggregation nuclei. Nevertheless, multi-exponential increases in dye-binding signal and changes in morphology suggest the existence of different aggregate species. Although beta-sheet content increases from 0 to ca. 40% upon aggregation, the aggregates retain significant amounts of alpha-helix structure, and lack a protease-resistant core. Thus BSA is able to form well-ordered beta-sheet rich aggregates which nevertheless do not possess the same structural rigidity as classical fibrils. The aggregates do not permeabilize synthetic membranes and are not cytotoxic. The ease with which a multidomain all-alpha helix protein can form higher-order beta-sheet structure, while retaining significant amounts of alpha-helix, highlights the universality of the fibrillation mechanism. However, the presence of non-beta-sheet structure may influence the final fibrillar structure and could be a key component in aggregated BSA's lack of cytotoxicity.

Aniracetam restores the effects of amyloid-beta protein or ageing on membrane fluidity and intracellular calcium concentration in mice synaptosomes

In the present study, we observed the in vitro effect of aniracetam on membrane fluidity and free calcium concentrations ([Ca(2+)]i) of frontal cortical (FC) and hippocampal (HP) synaptosomes of aged mice and young mice treated with amyloid-beta protein (Abeta) Membrane fluidity was measured by using fluorescence anisotropy of the lipophilic probe, 1,6-diphenyl-1,3,5-hexatriene (DPH). [Ca(2+)]i was measured by using Fura 2-AM fluorescent spectrophotometry. We found that membrane fluidity of the FC and HP synaptosomes was decreased in 14 months old mice compared with that in 3 months old mice. Similarly, Abeta25-35 (1 microM) decreased the membrane fluidity in 3 months old mice. These effects of ageing and Abeta25-35 on membrane fluidity were restored by aniracetam in a concentration-dependent manner. Furthermore, Abeta25-35 (1 microM) largely increased [Ca(2+)]i in FC and HP synaptosomes in 3 months old mice, but this effect on HP synaptosomes was effectively reversed by aniracetam (1-4 mM). The present findings suggest that aniracetam restores age- and Abeta-induced alterations in membrane fluidity or Abeta-induced increase in [Ca(2+)]i, demonstrating a possible beneficial role of aniracetam in the clinic treatment for senile dementia or Alzheimer's disease.

Physicochemical interactions of amyloid beta-peptide with lipid bilayers

The aggregation and deposition onto neuronal cells of amyloid beta-peptide (Abeta) is central to the pathogenesis of Alzheimer's disease. Accumulating evidence suggests that membranes play a catalytic role in the aggregation of Abeta. This article summarizes the structures and properties of Abeta in solution and the physicochemical interaction of Abeta with lipid bilayers of various compositions. Reasons for discrepancies between results by different research groups are discussed. The importance of ganglioside clusters in the aggregation of Abeta is emphasized. Finally, a hypothetical physicochemical cascade in the pathogenesis of the disease is proposed.