Interactions of amyloid beta-peptide (1-40) with ganglioside-containing membranes

Transformation of β-sheet structures of the amyloid peptide induced by molecular modulators

In this work we report the controlled modulation of secondary structures of the amyloid peptide by terminus molecular modulators.

Binding, conformational transition and dimerization of amyloid-β peptide on GM1-containing ternary membrane: insights from molecular dynamics simulation

Interactions of amyloid-β (Aβ) with neuronal membrane are associated with the progression of Alzheimer's disease (AD). Ganglioside GM1 has been shown to promote the structural conversion of Aβ and increase the rate of peptide aggregation; but the exact nature of interaction driving theses processes remains to be explored. In this work, we have carried out atomistic-scale computer simulations (totaling 2.65 µs) to investigate the behavior of Aβ monomer and dimers in GM1-containing raft-like membrane. The oligosaccharide head-group of GM1 was observed to act as scaffold for Aβ-binding through sugar-specific interactions. Starting from the initial helical peptide conformation, a β-hairpin motif was formed at the C-terminus of the GM1-bound Aβ-monomer; that didn't appear in absence of GM1 (both in fluid POPC and liquid-ordered cholesterol/POPC bilayers and also in aqueous medium) within the simulation time span. For Aβ-dimers, the β-structure was further enhanced by peptide-peptide interactions, which might influence the propensity of Aβ to aggregate into higher-ordered structures. The salt-bridges and inter-peptide hydrogen bonds were found to account for dimer stability. We observed spontaneous formation of intra-peptide D(23)-K(28) salt-bridge and a turn at V(24)GSN(27) region - long been accepted as characteristic structural-motifs for amyloid self-assembly. Altogether, our results provide atomistic details of Aβ-GM1 and Aβ-Aβ interactions and demonstrate their importance in the early-stages of GM1-mediated Aβ-oligomerisation on membrane surface.

Effects of amyloid β-peptides and gangliosides on mouse neural stem cells

The interaction of amyloid β-proteins (Aβs) with membrane lipids has been postulated as an early event in Aβ fibril formation in Alzheimer's disease. We evaluated the effects of several putative bioactive Aβs and gangliosides on neural stem cells (NSCs) isolated from embryonic mouse brains or the subventricular zone of adult mouse brains. Incubation of the isolated NSCs with soluble Aβ1-40 alone did not cause any change in the number of NSCs, but soluble Aβ1-42 increased their number. Aggregated Aβ1-40 and Aβ1-42 increased the number of NSCs but soluble and aggregated Aβ25-35 decreased the number. Soluble Aβ1-40 and Aβ1-42 did not affect the number of apoptotic cells but aggregated Aβ1-40 and Aβ1-42 did. When NSCs were treated with a combination of GM1 or GD3 and soluble Aβ1-42, cell proliferation was enhanced, indicating that both GM1 and GD3 as well as Aβs are involved in promoting cell proliferation and survival of NSCs. These observations suggest the potential of beneficial effects of using gangliosides and Aβs for promoting NSC proliferation.

Curcumin and its derivatives: their application in neuropharmacology and neuroscience in the 21st century

Curcumin (diferuloylmethane), a polyphenol extracted from the plant Curcuma longa, is widely used in Southeast Asia, China and India in food preparation and for medicinal purposes. Since the second half of the last century, this traditional medicine has attracted the attention of scientists from multiple disciplines to elucidate its pharmacological properties. Of significant interest is curcumin's role to treat neurodegenerative diseases including Alzheimer's disease (AD), and Parkinson's disease (PD) and malignancy. These diseases all share an inflammatory basis, involving increased cellular reactive oxygen species (ROS) accumulation and oxidative damage to lipids, nucleic acids and proteins. The therapeutic benefits of curcumin for these neurodegenerative diseases appear multifactorial via regulation of transcription factors, cytokines and enzymes associated with (Nuclear factor kappa beta) NFκB activity. This review describes the historical use of curcumin in medicine, its chemistry, stability and biological activities, including curcumin's anti-cancer, anti-microbial, anti-oxidant, and anti-inflammatory properties. The review further discusses the pharmacology of curcumin and provides new perspectives on its therapeutic potential and limitations. Especially, the review focuses in detail on the effectiveness of curcumin and its mechanism of actions in treating neurodegenerative diseases such as Alzheimer's and Parkinson's diseases and brain malignancies.

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.

A common landscape for membrane-active peptides

Three families of membrane-active peptides are commonly found in nature and are classified according to their initial apparent activity. Antimicrobial peptides are ancient components of the innate immune system and typically act by disruption of microbial membranes leading to cell death. Amyloid peptides contribute to the pathology of diverse diseases from Alzheimer's to type II diabetes. Preamyloid states of these peptides can act as toxins by binding to and permeabilizing cellular membranes. Cell-penetrating peptides are natural or engineered short sequences that can spontaneously translocate across a membrane. Despite these differences in classification, many similarities in sequence, structure, and activity suggest that peptides from all three classes act through a small, common set of physical principles. Namely, these peptides alter the Brownian properties of phospholipid bilayers, enhancing the sampling of intrinsic fluctuations that include membrane defects. A complete energy landscape for such systems can be described by the innate membrane properties, differential partition, and the associated kinetics of peptides dividing between surface and defect regions of the bilayer. The goal of this review is to argue that the activities of these membrane-active families of peptides simply represent different facets of what is a shared energy landscape.

Induction of highly curved structures in relation to membrane permeabilization and budding by the triterpenoid saponins, α- and δ-Hederin

The interactions of triterpenoid monodesmosidic saponins, α-hederin and δ-hederin, with lipid membranes are involved in their permeabilizing effect. Unfortunately, the interactions of these saponins with lipid membranes are largely unknown, as are the roles of cholesterol or the branched sugar moieties (two for α-hederin and one for δ-hederin) on the aglycone backbone, hederagenin. The differences in sugar moieties are responsible for differences in the molecular shape of the saponins and the effects on membrane curvature that should be the most positive for α-hederin in a transbilayer direction. In large unilamellar vesicles and monocyte cells, we showed that membrane permeabilization was dependent on the presence of membrane cholesterol and saponin sugar chains, being largest for α-hederin and smallest for hederagenin. In the presence of cholesterol, α-hederin induced the formation of nonbilayer phases with a higher rate of Brownian tumbling or lateral diffusion. A reduction of Laurdan's generalized polarization in relation to change in order of the polar heads of phospholipids was observed. Using giant unilamellar vesicles, we visualized the formation of wrinkled borders, the decrease in liposome size, budding, and the formation of macroscopic pores. All these processes are highly dependent on the sugars linked to the aglycone, with α-hederin showing a greater ability to induce pore formation and δ-hederin being more efficient in inducing budding. Hederagenin induced intravesicular budding but no pore formation. Based on these results, a curvature-driven permeabilization mechanism dependent on the interaction between saponin and sterols and on the molecular shape of the saponin and its ability to induce local spontaneous curvature is proposed.

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

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

Altered distribution of the gangliosides GM1 and GM2 in Alzheimer's disease

BACKGROUND

Alzheimer's disease (AD) is a neurodegenerative disorder where β-amyloid tends to aggregate and form plaques. Lipid raft-associated ganglioside GM1 has been suggested to facilitate β-amyloid aggregation; furthermore, GM1 and GM2 are increased in lipid rafts isolated from cerebral cortex of AD cases.

AIM/METHOD

The distribution of GM1 and GM2 was studied by immunohistochemistry in the frontal and temporal cortex of AD cases. Frontotemporal dementia (FTD) was included as a contrast group.

RESULTS

The distribution of GM1 and GM2 changes during the process of AD (n = 5) and FTD (n = 3) compared to controls (n = 5). Altered location of the GM1-positive small circular structures seems to be associated with myelin degradation. In the grey matter, the staining of GM1-positive plasma membranes might reflect neuronal loss in the AD/FTD tissue. The GM1-positive compact bundles were only visible in cells located in the AD frontal grey matter, possibly reflecting raft formation of GM1 and thus a pathological connection. Furthermore, our results suggest GM2 to be enriched within vesicles of pyramidal neurons of the AD/FTD brain.

CONCLUSION

Our study supports the biochemical finding of ganglioside accumulation in cellular membranes of AD patients and shows a redistribution of these molecules.

GM1 cluster mediates formation of toxic Aβ fibrils by providing hydrophobic environments

The conversion of soluble, nontoxic amyloid β-proteins (Aβ) to aggregated, toxic forms rich in β-sheets is considered to be a key step in the development of Alzheimer's disease. Accumulating evidence suggests that lipid-protein interactions play a crucial role in the aggregation of amyloidogenic proteins like Aβ. Our group has previously reported that amyloid fibrils of Aβ formed on membranes containing clusters of GM1 ganglioside (M-fibrils) exhibit greater cytotoxicity than fibrils formed in aqueous solution (W-fibrils) [ Okada ( 2008 ) J. Mol. Biol. 382 , 1066 - 1074 ]. W-fibrils are considered to consist of in-register parallel β-sheets. However, the precise molecular structure of M-fibrils and force driving the formation of toxic fibrils remain unclear. In this study, we hypothesized that low-polarity environments provided by GM1 clusters drive the formation of toxic fibrils and compared the structure and cytotoxicity of W-fibrils, M-fibrils, and aggregates formed in a low-polarity solution mimicking membrane environments. First, we determined solvent conditions which mimic the polarity of raftlike membranes using Aβ-(1-40) labeled with the 7-diethylaminocoumarin-3-carbonyl dye. The polarity of a mixture of 80% 1,4-dioxane and 20% water (v/v) was found to be close to that of raftlike membranes. Aβ-(1-40) formed amyloid fibrils within several hours in 80% dioxane (D-fibrils) or in the presence of raftlike membranes, whereas a much longer incubation time was required for fibril formation in a conventional buffer. D-fibrils were morphologically similar to M-fibrils. Fourier-transform infrared spectroscopy suggested that M-fibrils and D-fibrils contained antiparallel β-sheets. These fibrils had greater surface hydrophobicity and exhibited significant toxicity against human neuroblastoma SH-SY5Y cells, whereas W-fibrils with less surface hydrophobicity were not cytotoxic. We concluded that ganglioside clusters mediate the formation of toxic amyloid fibrils of Aβ with an antiparallel β-sheet structure by providing less polar environments.

Formation of GM1 ganglioside clusters on the lipid membrane containing sphingomyeline and cholesterol

GM1 gangliosides form a microdomain with sphingomyeline (SM) and cholesterol (Chol) and are deeply involved in the aggregation of amyloid beta (Aβ) peptides on neural membranes. We performed molecular dynamics simulations on two kinds of lipid bilayers containing GM1 ganglioside: GM1/SM/Chol and GM1/POPC. Both 10 and 100 ns simulations and another set of 10 ns simulations with different initial lipid arrangement essentially showed the same computational results. GM1 molecules in the GM1/SM/Chol membrane were condensed, whereas those in GM1/POPC membrane scattered. That is, the formation of GM1 cluster was observed only on the GM1/SM/Chol mixed membrane. There appeared numerous hydrogen bonds among glycan portions of the GM1 clusters due to the condensation. A comparison in distribution of lipid molecules between the two kinds of membranes suggested that cholesterol had important roles to prevent the membrane from interdigitation and to stabilize other lipids for interacting with each other. This property of cholesterol promotes the formation of GM1 clusters.

Brain insulin resistance accelerates Aβ fibrillogenesis by inducing GM1 ganglioside clustering in the presynaptic membranes

Type 2 diabetes mellitus is thought to be a significant risk factor for Alzheimer's disease. Insulin resistance also affects the central nervous system by regulating key processes, such as neuronal survival and longevity, learning and memory. However, the mechanisms underlying these effects remain uncertain. To investigate whether insulin resistance is associated with the assembly of amyloid β-protein (Aβ) at the cell surface of neurons, we inhibited insulin-signalling pathways of primary neurons. The treatments of insulin receptor (IR)-knockdown and a phosphatidylinositol 3-kinase inhibitor (LY294002), but not an extracellular signal-regulated kinase inhibitor, induced an increase in GM1 ganglioside (GM1) levels in detergent-resistant membrane microdomains of the neurons. The aged db/db mouse brain exhibited reduction in IR expression and phosphorylation of Akt, which later induced an increase in the high-density GM1-clusters on synaptosomes. Neurons treated with IR knockdown or LY294002, and synaptosomes of the aged db/db mouse brains markedly accelerated an assembly of Aβs. These results suggest that ageing and peripheral insulin resistance induce brain insulin resistance, which accelerates the assembly of Aβs by increasing and clustering of GM1 in detergent-resistant membrane microdomains of neuronal membranes.

Lipid composition influences the release of Alzheimer's amyloid β-peptide from membranes

The behavior of the amyloid β-peptide (Aβ) within a membrane environment is integral to its toxicity and the progression of Alzheimer's disease. Ganglioside GM1 has been shown to enhance the aggregation of Aβ, but the underlying mechanism is unknown. Using atomistic molecular dynamics simulations, we explored the interactions between the 40-residue alloform of Aβ (Aβ(40) ) and several model membranes, including pure palmitoyloleoylphosphatidylcholine (POPC) and palmitoyloleoylphosphatidylserine (POPS), an equimolar mixture of POPC and palmitoyloleoylphosphatidylethanolamine (POPE), and lipid rafts, both with and without GM1, to understand the behavior of Aβ(40) in various membrane microenvironments. Aβ(40) remained inserted in POPC, POPS, POPC/POPE, and raft membranes, but in several instances exited the raft containing GM1. Aβ(40) interacted with GM1 largely through hydrogen bonding, producing configurations containing β-strands with C-termini that, in some cases, exited the membrane and became exposed to solvent. These observations provide insight into the release of Aβ from the membrane, a previously uncharacterized process of the Aβ aggregation pathway.

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.

Non-esterified fatty acids generate distinct low-molecular weight amyloid-β (Aβ42) oligomers along pathway different from fibril formation

Amyloid-β (Aβ) peptide aggregation is known to play a central role in the etiology of Alzheimer’s disease (AD). Among various aggregates, low-molecular weight soluble oligomers of Aβ are increasingly believed to be the primary neurotoxic agents responsible for memory impairment. Anionic interfaces are known to influence the Aβ aggregation process significantly. Here, we report the effects of interfaces formed by medium-chain (C9–C12), saturated non-esterified fatty acids (NEFAs) on Aβ42 aggregation. NEFAs uniquely affected Aβ42 aggregation rates that depended on both the ratio of Aβ:NEFA as well the critical micelle concentration (CMC) of the NEFAs. More importantly, irrespective of the kind of NEFA used, we observed that two distinct oligomers, 12–18 mers and 4–5 mers were formed via different pathway of aggregation under specific experimental conditions: (i) 12–18 mers were generated near the CMC in which NEFAs augment the rate of Aβ42 aggregation towards fibril formation, and, (ii) 4–5 mers were formed above the CMC, where NEFAs inhibit fibril formation. The data indicated that both 12–18 mers and 4–5 mers are formed along an alternate pathway called ‘off-pathway’ that did not result in fibril formation and yet have subtle structural and morphological differences that distinguish their bulk molecular behavior. These observations, (i) reflect the possible mechanism of Aβ aggregation in physiological lipid-rich environments, and (ii) reiterate the fact that all oligomeric forms of Aβ need not be obligatory intermediates of the fibril formation pathway.

Impacts of membrane biophysics in Alzheimer's disease: from amyloid precursor protein processing to aβ Peptide-induced membrane changes

An increasing amount of evidence supports the notion that cytotoxic effects of amyloid-β peptide (Aβ), the main constituent of senile plaques in Alzheimer's disease (AD), are strongly associated with its ability to interact with membranes of neurons and other cerebral cells. Aβ is derived from amyloidogenic cleavage of amyloid precursor protein (AβPP) by β- and γ-secretase. In the nonamyloidogenic pathway, AβPP is cleaved by α-secretases. These two pathways compete with each other, and enhancing the non-amyloidogenic pathway has been suggested as a potential pharmacological approach for the treatment of AD. Since AβPP, α-, β-, and γ-secretases are membrane-associated proteins, AβPP processing and Aβ production can be affected by the membrane composition and properties. There is evidence that membrane composition and properties, in turn, play a critical role in Aβ cytotoxicity associated with its conformational changes and aggregation into oligomers and fibrils. Understanding the mechanisms leading to changes in a membrane's biophysical properties and how they affect AβPP processing and Aβ toxicity should prove to provide new therapeutic strategies for prevention and treatment of AD.

Formation of Toxic Amyloid Fibrils by Amyloid β-Protein on Ganglioside Clusters

It is widely accepted that the conversion of the soluble, nontoxic amyloid β-protein (Aβ) monomer to aggregated toxic Aβ rich in β-sheet structures is central to the development of Alzheimer's disease. However, the mechanism of the abnormal aggregation of Aβ in vivo is not well understood. Accumulating evidence suggests that lipid rafts (microdomains) in membranes mainly composed of sphingolipids (gangliosides and sphingomyelin) and cholesterol play a pivotal role in this process. This paper summarizes the molecular mechanisms by which Aβ aggregates on membranes containing ganglioside clusters, forming amyloid fibrils. Notably, the toxicity and physicochemical properties of the fibrils are different from those of Aβ amyloids formed in solution. Furthermore, differences between Aβ-(1–40) and Aβ-(1–42) in membrane interaction and amyloidogenesis are also emphasized.

The pathological roles of ganglioside metabolism in Alzheimer's disease: effects of gangliosides on neurogenesis

Conversion of the soluble, nontoxic amyloid β-protein (Aβ) into an aggregated, toxic form rich in β-sheets is a key step in the onset of Alzheimer's disease (AD). It has been suggested that Aβ induces changes in neuronal membrane fluidity as a result of its interactions with membrane components such as cholesterol, phospholipids, and gangliosides. Gangliosides are known to bind Aβ. A complex of GM1 and Aβ, termed “GAβ”, has been identified in AD brains. Abnormal ganglioside metabolism also may occur in AD brains. We have reported an increase of Chol-1α antigens, GQ1bα and GT1aα, in the brain of transgenic mouse AD model. GQ1bα and GT1aα exhibit high affinities to Aβs. The presence of Chol-1α gangliosides represents evidence for genesis of cholinergic neurons in AD brains. We evaluated the effects of GM1 and Aβ1–40 on mouse neuroepithelial cells. Treatment of these cells simultaneously with GM1 and Aβ1–40 caused a significant reduction of cell number, suggesting that Aβ1–40 and GM1 cooperatively exert a cytotoxic effect on neuroepithelial cells. An understanding of the mechanism on the interaction of GM1 and Aβs in AD may contribute to the development of new neuroregenerative therapies for this disorder.

Ganglioside-mediated aggregation of amyloid β-proteins (Aβ): comparison between Aβ-(1-42) and Aβ-(1-40)

Conversion of the soluble, non-toxic amyloid β-protein (Aβ) into an aggregated, toxic form rich in β-sheets is considered a key step in the development of Alzheimer's disease. Accumulating evidence suggests that lipid rafts in membranes play a pivotal role in this process. We have proposed that Aβ-(1-40) specifically bound to a ganglioside cluster forms cytotoxic fibrils via a conformational transition from an α-helix-rich structure to a β-sheet-rich one. In the present study, we compared the interaction of Aβ-(1-40) and Aβ-(1-42) with both model and living cell membranes. Aβ-(1-42) exhibited lipid specificity and affinity similar to Aβ-(1-40), though its amyloidogenic activity was more than 10-fold that of Aβ-(1-40). Antibody staining experiments, using the A11 antibody specific to Aβ oligomers, demonstrated that oligomers were not detected during the aggregation process, and cell death was observed only after significant accumulation of the proteins, suggesting that the fibril-induced disruption of cell membranes leads to the cytotoxicity. Furthermore, we succeeded in visualizing fibrils formed on cell membranes using total internal reflection fluorescence microscopy. Aβ-(1-40) formed long fibrils extruding to the aqueous phase, whereas Aβ-(1-42) fibrils appeared to be laterally co-assembled and short.

Pathological significance of ganglioside clusters in Alzheimer's disease

One of the key questions regarding the pathogenesis of Alzheimer's disease (AD) is how amyloid β-protein (Aβ), a proteinaceous component of senile plaques, starts to assemble into amyloid fibrils in the brain. A body of evidence is growing to suggest that Aβ binds to ganglioside on neuronal membranes, and then, is converted to an endogenous seed with an altered conformation (ganglioside-bound Aβ, GAβ) for amyloid fibril formation in the brain. Notably, the risk factors for the development of AD, including aging and apolipoprotein E4, likely facilitate the formation of ganglioside clusters in lipid raft-like membrane microdomains at pre-synaptic terminals, which provide a favorable milieu for the GAβ generation. Furthermore, it has also been suggested that endocytic pathway abnormality of neurons is involved in the formation of the ganglioside clusters. In this review, the nature of the ganglioside clusters and how gangliosides behave in the clusters leading to the GAβ generation are discussed.

Surface plasmon resonance spectroscopy: a new lead in studying the membrane binding of amyloidogenic transthyretin

Surface plasmon resonance (SPR) employs the optical principle of SPR to measure changes in mass on a sensor chip surface in real time. Surface chemistry has been developed which enables the immoblization of lipid bilayers and determination of protein-membrane interactions in real time. In the last decade, the plasma membrane has been demonstrated to play an important role in amyloidogenesis and cytotoxicity induced by amyloidogenic proteins. SPR provides an ideal way to study the membrane binding of amyloidogenic proteins. In this chapter, we describe the application of SPR to the study of amyloidogenic transthyretin binding to the plasma membrane and artificial lipid bilayers.

Mechanisms of amyloid-Beta Peptide uptake by neurons: the role of lipid rafts and lipid raft-associated proteins

A hallmark pathological feature of Alzheimer's disease (AD) is the accumulation of extracellular plaques composed of the amyloid-beta (Aβ) peptide. Thus, classically experiments were designed to examine Aβ toxicities within the central nervous system (CNS) from the extracellular space. However, a significant amount of evidence now suggests that intraneuronal accumulation of Aβ is neurotoxic and may play an important role in the disease progression of AD. One of the means by which neurons accumulate intracellular Aβ is through uptake of extracellular Aβ peptides, and this process may be a potential link between Aβ generation, synaptic dysfunction, and AD pathology. Recent studies have found that neuronal internalization of Aβ involves lipid rafts and various lipid raft-associated receptor proteins. Uptake mechanisms independent of lipid rafts have also been implicated. The aim of this paper is to summarize these findings and discuss their significance in the pathogenesis of AD.

Designed fluorescent probes reveal interactions between amyloid-beta(1-40) peptides and GM1 gangliosides in micelles and lipid vesicles

A hallmark of the common Alzheimer's disease (AD) is the pathological conversion of its amphiphatic amyloid-beta (Abeta) peptide into neurotoxic aggregates. In AD patients, these aggregates are often found to be tightly associated with neuronal G(M1) ganglioside lipids, suggesting an involvement of G(M1) not only in aggregate formation but also in neurotoxic events. Significant interactions were found between micelles made of newly synthesized fluorescent G(M1) gangliosides labeled in the polar headgroup or the hydrophobic chain and Abeta(1-40) peptide labeled with a BODIPY-FL-C1 fluorophore at positions 12 and 26, respectively. From an analysis of energy transfer between the different fluorescence labels and their location in the molecules, we were able to place the Abeta peptide inside G(M1) micelles, close to the hydrophobic-hydrophilic interface. Large unilamellar vesicles composed of a raftlike G(M1)/bSM/cholesterol lipid composition doped with labeled G(M1) at various positions also interact with labeled Abeta peptide tagged to amino acids 2 or 26. A faster energy transfer was observed from the Abeta peptide to bilayers doped with 581/591-BODIPY-C(11)-G(M1) in the nonpolar part of the lipid compared with 581/591-BODIPY-C(5)-G(M1) residing in the polar headgroup. These data are compatible with a clustering process of G(M1) molecules, an effect that not only increases the Abeta peptide affinity, but also causes a pronounced Abeta peptide penetration deeper into the lipid membrane; all these factors are potentially involved in Abeta peptide aggregate formation due to an altered ganglioside metabolism found in AD patients.

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.

Duramycin-induced destabilization of a phosphatidylethanolamine monolayer at the air-water interface observed by vibrational sum-frequency generation spectroscopy

Duramycin is a small tetracyclic peptide which binds specifically to ethanolamine phospholipids (PE). In this study, we used lipid monolayers consisting of 1-palmitoyl-2-oleoyl phosphatidylethanolamine (POPE) and various phosphatidylcholines (PC) to investigate the effect of duramycin on the organization of lipids and its influence on surrounding water molecules, using vibrational sum-frequency generation spectroscopy in conjunction with surface pressure measurements and fluorescence microscopy. The results show that while duramycin has no effect on the PC lipid monolayers, it induces significant disorder of PE molecules and causes an increase of the PE monolayer surface pressure. Duramycin adopts a β-sheet conformation and is well-ordered at the air-water interface as well as after binding to PE. Our results are consistent with duramycin inserting into the PE monolayer via its hydrophobic end, exposing phenylalanine residues to the lipid. Binding of duramycin to PE broadens the hydrogen-bond distribution of lipid-bound water molecules, notably increasing the fraction of the less strongly hydrogen-bonded, possibly undercoordinated, water molecules. Fluorescence microscopy reveals that the interaction of duramycin with PE causes a change in the shape of the liquid-condensed domains of the PE monolayer from circular to horseshoe-like, indicating a reduction of line tension at the boundary of the two lipid phases. These results reveal that the first steps in the disruption of membrane integrity by duramycin consist of a reduction of the line tension, a decrease in the lipid order, and a weakening of the hydrogen bonding network of water around PE.

Models of membrane-bound Alzheimer's Abeta peptide assemblies

Although it is clear that amyloid beta (Aβ) peptides play a pivotal role in the development of Alzheimer's disease, the precise molecular model of action remains unclear. Aβ peptide forms assemble both in aqueous solution and in lipid membranes. It has been proposed that deleterious effects occur when the peptides interact with membranes, possibly by forming Ca(2+) permeant ion channels. In the accompanying manuscript, we propose models in which the C-terminus third of six Aβ42 peptides forms a six-stranded β-barrel in highly toxic soluble oligomers. Here we extend this hypothesis to membrane-bound assemblies. In these Aβ models, the hydrophobic β-barrel of a hexamer may either reside on the surface of the bilayer, or span the bilayer. Transmembrane pores are proposed to form between several hexamers. Once the β-barrels of six hexamers have spanned the bilayer, they may merge to form a more stable 36-stranded β-barrel. We favor models in which parallel β-barrels formed by N-terminus segments comprise the lining of the pores. These types of models explain why the channels are selective for cations and how metal ions, such as Zn(2+) , synthetic peptides that contain histidines, and some small organic cations may block channels or inhibit formation of channels. Our models were developed to be consistent with microscopy studies of Aβ assemblies in membranes, one of which is presented here for the first time.

Chaperon-mediated single molecular approach toward modulating Abeta peptide aggregation

We report here a single molecular approach using chaperone-like molecular modulators for modulating the aggregation behavior of a vital analogue of beta-amyloid peptide (Abeta) by using scanning tunneling microscopy. The molecular structures of the beta-sheets for Abeta33-42 peptide are revealed, which are keen to the aggregation of Abeta42 relating to Alzheimer's disease. It was identified that the introduction of chaperone-like modulators could regulate the assembling behavior of the peptide at molecular level. Furthermore, the modulators could also significantly accelerate the aggregation of the peptide in aqueous solution as revealed by light scattering studies. These observations of the molecular modulator effect in peptide assemblies could provide a novel approach toward modulating Abeta peptide aggregations.

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.

Ganglioside-induced amyloid formation by human islet amyloid polypeptide in lipid rafts

Human islet amyloid polypeptide (hIAPP) is the primary component of the amyloid deposits found in the pancreatic islets of patients with type 2 diabetes mellitus. However, it is unknown how amyloid fibrils are formed in vivo. In this study, we demonstrate that gangliosides play an essential role in the formation of amyloid deposits by hIAPP on plasma membranes. Amyloid fibrils accumulated in ganglioside- and cholesterol-rich microscopic domains ('lipid rafts'). The depletion of gangliosides or cholesterol significantly reduced the amount of amyloid deposited. These results clearly showed that the formation of amyloid fibrils was mediated by gangliosides in lipid rafts.

Aggregation of amyloidogenic peptides near hydrophobic and hydrophilic surfaces

The general effect of surface hydrophobicity/hydrophilicity on the aggregation of peptides is studied by simulations of oversaturated aqueous solutions of hydrophobic and hydrophilic amyloidogenic peptides. Peptide aggregation was studied in bulk solution, in solutions confined between hydrophobic boundaries (smooth planar paraffin-like surfaces and liquid-vapor interfaces) and in solutions confined between hydrophilic surfaces (smooth planar silica-like surfaces). Aggregation of hydrophobic peptides strongly enhances due to the confinement between hydrophobic surfaces with all peptides adsorbed at the boundaries and aligned predominantly parallel to them. In the other three cases considered, the peptides are repelled from the walls and do not reveal orientational ordering with respect to the surface. The degree of peptide aggregation in these cases is only slightly affected by the confinement (it is enhanced for hydrophobic peptides and decreased for hydrophilic peptides). Our results show that even a single environmental factor such as water-mediated peptide-surface interaction has a drastic effect on the degree and character of peptide aggregation. A wide diversity of possible scenarios can be expected when specific peptide-surface interactions are additionally taken into account.

Sugar microarray via click chemistry: molecular recognition with lectins and amyloid β (1-42)

Sugar microarrays were fabricated on various substrates via click chemistry. Acetylene-terminated substrates were prepared by forming self-assembled monolayers (SAMs) on a gold substrate with alkyl-disulfide and on silicon, quartz and glass substrates with a silane-coupling reagent. The gold substrates were subjected to surface plasmon resonance measurements, and the quartz and glass substrates were subjected to spectroscopy measurements and optical microscopy observation. The saccharide-immobilized substrate on the gold substrate showed specific interaction with the corresponding lectin, and the saccharides showed inert surface properties to other proteins with a high signal-to-noise ratio. We also focused on the saccharide-protein interaction on protein amyloidosis of Alzheimer amyloid β. Amyloid β peptide showed conformation transition on the saccharide-immobilization substrate into a β-sheet, and fibril formation and amyloid aggregates were found on the specific saccharides.

Membrane-targeted strategies for modulating APP and Abeta-mediated toxicity

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by numerous pathological features including the accumulation of neurotoxic amyloid-β (Aβ) peptide. There is currently no effective therapy for AD, but the development of therapeutic strategies that target the cell membrane is gaining increased interest. The amyloid precursor protein (APP) from which Aβ is formed is a membrane-bound protein, and Aβ production and toxicity are both membrane mediated events. This review describes the critical role of cell membranes in AD with particular emphasis on how the composition and structure of the membrane and its specialized regions may influence toxic or benign Aβ/APP pathways in AD. The putative role of copper (Cu) in AD is also discussed, and we highlight how targeting the cell membrane with Cu complexes has therapeutic potential in AD.

Induction of negative curvature as a mechanism of cell toxicity by amyloidogenic peptides: the case of islet amyloid polypeptide

The death of insulin-producing beta-cells is a key step in the pathogenesis of type 2 diabetes. The amyloidogenic peptide Islet Amyloid Polypeptide (IAPP, also known as amylin) has been shown to disrupt beta-cell membranes leading to beta-cell death. Despite the strong evidence linking IAPP to the destruction of beta-cell membrane integrity and cell death, the mechanism of IAPP toxicity is poorly understood. In particular, the effect of IAPP on the bilayer structure has largely been uncharacterized. In this study, we have determined the effect of the amyloidogenic and toxic hIAPP(1-37) peptide and the nontoxic and nonamyloidogenic rIAPP(1-37) peptide on membranes by a combination of DSC and solid-state NMR spectroscopy. We also characterized the toxic but largely nonamyloidogenic rIAPP(1-19) and hIAPP(1-19) fragments. DSC shows that both amyloidogenic (hIAPP(1-37)) and largely nonamyloidogenic (hIAPP(1-19) and rIAPP(1-19)) toxic versions of the peptide strongly favor the formation of negative curvature in lipid bilayers, while the nontoxic full-length rat IAPP(1-37) peptide does not. This result was confirmed by solid-state NMR spectroscopy which shows that in bicelles composed of regions of high curvature and low curvature, nontoxic rIAPP(1-37) binds to the regions of low curvature while toxic rIAPP(1-19) binds to regions of high curvature. Similarly, solid-state NMR spectroscopy shows that the toxic rIAPP(1-19) peptide significantly disrupts the lipid bilayer structure, whereas the nontoxic rIAPP(1-37) does not have a significant effect. These results indicate IAPP may induce the formation of pores by the induction of excess membrane curvature and can be used to guide the design of compounds that can prevent the cell-toxicity of IAPP. This mechanism may be important to understand the toxicity of other amyloidogenic proteins. Our solid-state NMR results also demonstrate the possibility of using bicelles to measure the affinity of biomolecules for negatively or positively curved regions of the membrane, which we believe will be useful in a variety of biochemical and biophysical investigations related to the cell membrane.

Exploring the mechanism of beta-amyloid toxicity attenuation by multivalent sialic acid polymers through the use of mathematical models

beta-Amyloid peptide (A beta), the primary protein component in senile plaques associated with Alzheimer's disease (AD), has been implicated in neurotoxicity associated with AD. Previous studies have shown that the A beta-neuronal membrane interaction plays a role in the mechanism of A beta toxicity. More specifically, it is thought that A beta interacts with ganglioside rich and sialic acid rich regions of cell surfaces. In light of such evidence, we have used a number of different sialic acid compounds of different valency or number of sialic acid moieties per molecule to attenuate A beta toxicity in a cell culture model. In this work, we proposed various mathematical models of A beta interaction with both the cell membrane and with the multivalent sialic acid compounds, designed to act as membrane mimics. These models allow us to explore the mechanism of action of this class of sialic acid membrane mimics in attenuating the toxicity of A beta. The mathematical models, when compared with experimental data, facilitate the discrimination between different modes of action of these materials. Understanding the mechanism of action of A beta toxicity inhibitors should provide insight into the design of the next generation of molecules that could be used to prevent A beta toxicity associated with AD.

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.

Age-dependent high-density clustering of GM1 ganglioside at presynaptic neuritic terminals promotes amyloid beta-protein fibrillogenesis

The deposition of amyloid beta-protein (Abeta) is an invariable feature of Alzheimer's disease (AD); however, the biological mechanism underlying Abeta assembly into fibrils in the brain remains unclear. Here, we show that a high-density cluster of GM1 ganglioside (GM1), which was detected by the specific binding of a novel peptide (p3), appeared selectively on synaptosomes prepared from aged mouse brains. Notably, the synaptosomes bearing the high-density GM1 cluster showed extraordinary potency to induce Abeta assembly, which was suppressed by an antibody specific to GM1-bound Abeta, an endogenous seed for AD amyloid. Together with evidence that Abeta deposition starts at presynaptic terminals in the AD brain and that GM1 levels significantly increase in amyloid-positive synaptosomes prepared from the AD brain, our results suggest that the age-dependent high-density GM1 clustering at presynaptic neuritic terminals is a critical step for Abeta deposition in AD.

Calcium-activated membrane interaction of the islet amyloid polypeptide: implications in the pathogenesis of type II diabetes mellitus

The role played by Ca(2+) ions in the interaction of the human islet amyloid polypeptide (hIAPP) with model membranes has been investigated by differential scanning calorimetry (DSC) and circular dichroism (CD) experiments. In particular, the interaction of hIAPP and its rat isoform (rIAPP) with zwitterionic dipalmitoyl-phosphatidylcholine (DPPC), negatively charged dipalmitoyl-phosphatidylserine (DPPS) vesicles and with a 3:1 mixtures of them, has been studied in the presence of Ca(2+) ions. The experiments have evidenced that amorphous, soluble hIAPP assemblies interact with the hydrophobic core of DPPC bilayers. Conversely, the presence of Ca(2+) ions is necessary to activate a preferential interaction of hIAPP with the hydrophobic core of DPPS membranes. These findings support the hypothesis that an impaired cellular homeostasis of Ca(2+) ions may promote the insertion of hIAPP into the hydrophobic core of carrier vesicles which is thought to contribute to an eventual intracellular accumulation of beta-sheet rich hIAPP aggregates.