Membrane disruption by Alzheimer beta-amyloid peptides mediated through specific binding to either phospholipids or gangliosides. Implications for neurotoxicity

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.

Membrane Incorporation, Channel Formation, and Disruption of Calcium Homeostasis by Alzheimer's β-Amyloid Protein

Oligomerization, conformational changes, and the consequent neurodegeneration of Alzheimer's β-amyloid protein (AβP) play crucial roles in the pathogenesis of Alzheimer's disease (AD). Mounting evidence suggests that oligomeric AβPs cause the disruption of calcium homeostasis, eventually leading to neuronal death. We have demonstrated that oligomeric AβPs directly incorporate into neuronal membranes, form cation-sensitive ion channels (“amyloid channels”), and cause the disruption of calcium homeostasis via the amyloid channels. Other disease-related amyloidogenic proteins, such as prion protein in prion diseases or α-synuclein in dementia with Lewy bodies, exhibit similarities in the incorporation into membranes and the formation of calcium-permeable channels. Here, based on our experimental results and those of numerous other studies, we review the current understanding of the direct binding of AβP into membrane surfaces and the formation of calcium-permeable channels. The implication of composition of membrane lipids and the possible development of new drugs by influencing membrane properties and attenuating amyloid channels for the treatment and prevention of AD is also discussed.

Site-directed mutations of thermostable direct hemolysin from Grimontia hollisae alter its arrhenius effect and biophysical properties

Recombinant thermostable direct hemolysin from Grimontia hollisae (Gh-rTDH) exhibits paradoxical Arrhenius effect, where the hemolytic activity is inactivated by heating at 60 °C but is reactivated by additional heating above 80 °C. This study investigated individual or collective mutational effect of Tyr53, Thr59, and Ser63 positions of Gh-rTDH on hemolytic activity, Arrhenius effect, and biophysical properties. In contrast to the Gh-rTDH wild-type (Gh-rTDH(WT)) protein, a 2-fold decrease of hemolytic activity and alteration of Arrhenius effect could be detected from the Gh-rTDH(Y53H/T59I) and Gh-rTDH(T59I/S63T) double-mutants and the Gh-rTDH(Y53H/T59I/S63T) triple-mutant. Differential scanning calorimetry results showed that the Arrhenius effect-loss and -retaining mutants consistently exhibited higher and lower endothermic transition temperatures, respectively, than that of the Gh-rTDH(WT). Circular dichroism measurements of Gh-rTDH(WT) and Gh-rTDH(mut) showed a conspicuous change from a β-sheet to α-helix structure around the endothermic transition temperature. Consistent with the observation is the conformational change of the proteins from native globular form into fibrillar form, as determined by Congo red experiments and transmission electron microscopy.

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.

Lysophosphatidylcholine modulates fibril formation of amyloid beta peptide

Phospholipids are known to influence fibril formation of amyloid beta (Aβ) peptide. Here, we show that lysophosphatidylcholine (LPC), a polar phospholipid, enhances Aβ(1-42) fibril formation, by decreasing the lag time and the critical peptide concentration required for fibril formation, and increasing the fibril elongation rate. Conversely, LPC did not have an enhancing effect on Aβ(1-40) fibril formation, and appeared to be inhibitory. Tyrosine fluorescence spectroscopy showed that LPC altered the fluorescence spectra of Aβ(1-40) and Aβ(1-42) in opposite ways. Further, 8-anilino-1-naphthalene sulfonic acid fluorescence spectroscopy showed that LPC significantly increased the hydrophobicity of Aβ(1-42), but not of Aβ(1-40). Tris-tricine gradient SDS/PAGE revealed that LPC increased the formation of higher-molecular-weight species of Aβ(1-42), including trimers and tetramers. LPC had no such effect on Aβ(1-40), and thus may specifically influence the oligomerization and nucleation processes of Aβ(1-42) in a manner dependent on its native structure. Dot-blot assays confirmed that LPC induced Aβ(1-42) oligomer formation at an early time point. Thus our results indicate that LPC specifically enhances the formation of Aβ(1-42) fibrils, the main component of senile plaques in Alzheimer's disease patients, and may be involved in Alzheimer's disease pathology.

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.

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.

Measurement of the attachment and assembly of small amyloid-β oligomers on live cell membranes at physiological concentrations using single-molecule tools

It is thought that the pathological cascade in Alzheimer's disease is initiated by the formation of amyloid-β (Aβ) peptide complexes on cell membranes. However, there is considerable debate about the nature of these complexes and the type of solution-phase Aβ aggregates that may contribute to their formation. Also, it is yet to be shown that Aβ attaches strongly to living cell membranes, and that this can happen at low, physiologically relevant Aβ concentrations. Here, we simultaneously measure the aggregate size and fluorescence lifetime of fluorescently labeled Aβ(1-40) on and above the membrane of cultured PC12 cells at near-physiological concentrations. We find that at 350 nM Aβ concentration, large (>>10 nm average hydrodynamic radius) assemblies of codiffusing, membrane-attached Aβ molecules appear on the cell membrane together with a near-monomeric species. When the extracellular concentration is 150 nM, the membrane contains only the smaller species, but with a similar degree of attachment. At both concentrations, the extracellular solution contains only small (∼2.3 nm average hydrodynamic radius) Aβ oligomers or monomers. We conclude that at near-physiological concentrations only the small oligomeric Aβ species are relevant, they are capable of attaching to the cell membrane, and they assemble in situ to form much larger complexes.

Spectroscopic Characterization of Intermolecular Interaction of Amyloid β Promoted on GM1 Micelles

Clusters of GM1 gangliosides act as platforms for conformational transition of monomeric, unstructured amyloid β (Aβ) to its toxic β-structured aggregates. We have previously shown that Aβ(1–40) accommodated on the hydrophobic/hydrophilic interface of lyso-GM1 or GM1 micelles assumes α-helical structures under ganglioside-excess conditions. For better understanding of the mechanisms underlying the α-to-β conformational transition of Aβ on GM1 clusters, we performed spectroscopic characterization of Aβ(1–40) titrated with GM1. It was revealed that the thioflavin T- (ThT-) reactive β-structure is more populated in Aβ(1–40) under conditions where the Aβ(1–40) density on GM1 micelles is high. Under this circumstance, the C-terminal hydrophobic anchor Val³⁹-Val⁴⁰ shows two distinct conformational states that are reactive with ThT, while such Aβ species were not generated by smaller lyso-GM1 micelles. These findings suggest that GM1 clusters promote specific Aβ-Aβ interactions through their C-termini coupled with formation of the ThT-reactive β-structure depending on sizes and curvatures of the clusters.

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.

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.

Structural properties and dynamic behavior of nonfibrillar oligomers formed by PrP(106-126)

The formation of nonfibrillar oligomers has been proposed as a common element of the aggregation pathway of proteins and peptides associated with neurodegenerative diseases such as Alzheimer's and Creutzfeldt-Jakob disease. While fibrillar structures have long been considered indicators of diseases linked with the accumulation of amyloid plaques, it has more recently been proposed that amyloid oligomers are in fact the cytotoxic form. Here we describe the local structure and dynamics of stable oligomers formed by a peptide comprising residues 106-126 of the human prion protein (PrP). Structural constraints from solid-state NMR reveal quaternary packing interactions within the hydrophobic core, similar to those previously reported for amyloid fibrils formed by this peptide, and consistent with structural studies of oligomers formed by the Alzheimer's beta-amyloid peptide. However, a hydration-dependent increase in disorder is observed for nonfibrillar oligomers of PrP(106-126). In solution NMR spectra we observe narrow (1)H and (13)C resonances corresponding to a monomer in exchange with the approximately 30 nm diameter nonfibrillar oligomers, giving additional information on the molecular structure of these species. Taken together, our data support a model in which the local structure of the oligomers contains the basic elements of amyloid fibrils, but with long-range disorder and local mobility that distinguishes these assemblies from the fibrillar form of PrP(106-126). These characteristics may provide a basis for the differing biological activities of amyloid fibrils and oligomers.

Alzheimer Abeta(1-42) monomer adsorbed on the self-assembled monolayers

Amyloid-beta (Abeta) peptide aggregation on the cell membranes is a key pathological event responsible for neuron cell death in Alzheimer's disease (AD). We present a collection of molecular docking and molecular dynamics simulations to study the conformational dynamics and adsorption behavior of Abeta monomer on the self-assembled monolayer (SAM), in comparison to Abeta structure in bulk solution. Two distinct Abeta conformations (i.e., alpha-helix and beta-hairpin) are selected as initial structures to mimic different adsorption states, whereas four SAM surfaces with different end groups in hydrophobicity and charge distribution are used to examine the effect of surface chemistry on Abeta structure and adsorption. Simulation results show that alpha-helical monomer displays higher structural stability than beta-hairpin monomer on all SAMs, suggesting that the preferential conformation of Abeta monomer could be alpha-helical or random structure when bound to surfaces. Structural stability and adsorption behavior of Abeta monomer on the SAMs originates from competitive interactions between Abeta and SAM and between SAM and interfacial water, which involve the conformation of Abeta, the surface chemistry of SAM, and the structure and dynamics of interfacial waters. The relative net binding affinity of Abeta with the SAMs is in the favorable order of COOH-SAM > NH(2)-SAM > CH(3)-SAM > OH-SAM, highlighting the importance of electrostatic and hydrophobic interactions for driving Abeta adsorption at the SAMs, but both interactions contribute differently to each Abeta-SAM complex. This work provides parallel insights into the understanding of Abeta structure and aggregation on cell membrane.

Phospholipase D in brain function and Alzheimer's disease

Alzheimer's disease is the most common neurodegenerative disorder. Although lipids are major constituents of brain, their role in Alzheimer's disease pathogenesis is poorly understood. Much attention has been given to cholesterol, but growing evidence suggests that other lipids, such as phospholipids, might play an important role in this disorder. In this review, we will summarize the evidence linking phospholipase D, a phosphatidic acid-synthesizing enzyme, to multiple aspects of normal brain function and to Alzheimer's disease. The role of phospholipase D in signaling mechanisms downstream of beta-amyloid as well as in the trafficking and processing of amyloid precursor protein will be emphasized.

Calcium signaling and amyloid toxicity in Alzheimer disease

Intracellular Ca(2+) signaling is fundamental to neuronal physiology and viability. Because of its ubiquitous roles, disruptions in Ca(2+) homeostasis are implicated in diverse disease processes and have become a major focus of study in multifactorial neurodegenerative diseases such as Alzheimer disease (AD). A hallmark of AD is the excessive production of beta-amyloid (Abeta) and its massive accumulation in amyloid plaques. In this minireview, we highlight the pathogenic interactions between altered cellular Ca(2+) signaling and Abeta in its different aggregation states and how these elements coalesce to alter the course of the neurodegenerative disease. Ca(2+) and Abeta intersect at several functional levels and temporal stages of AD, thereby altering neurotransmitter receptor properties, disrupting membrane integrity, and initiating apoptotic signaling cascades. Notably, there are reciprocal interactions between Ca(2+) pathways and amyloid pathology; altered Ca(2+) signaling accelerates Abeta formation, whereas Abeta peptides, particularly in soluble oligomeric forms, induce Ca(2+) disruptions. A degenerative feed-forward cycle of toxic Abeta generation and Ca(2+) perturbations results, which in turn can spin off to accelerate more global neuropathological cascades, ultimately leading to synaptic breakdown, cell death, and devastating memory loss. Although no cause or cure is currently known, targeting Ca(2+) dyshomeostasis as an underlying and integral component of AD pathology may result in novel and effective treatments for AD.

Up-and-down topological mode of amyloid beta-peptide lying on hydrophilic/hydrophobic interface of ganglioside clusters

Growing evidence has indicated that GM1 ganglioside specifically interacts with Amyloid beta-peptide (Abeta) and thereby promotes Alzheimer's disease-associated Abeta assembly. To characterize the conformation of Abeta bound to the ganglioside, we performed 920 MHz ultra-high field NMR analyses using isotopically labeled Abeta(1-40) in association with GM1 and lyso-GM1 micelles. Our NMR data revealed that (1) Abeta(1-40) forms discontinuous alpha-helices at the segments His(14)-Val(24) and Ile(31)-Val(36) upon binding to the gangliosidic micelles, leaving the remaining regions disordered, and (2) Abeta(1-40) lies on hydrophobic/hydrophilic interface of the ganglioside cluster exhibiting an up-and-down topological mode in which the two alpha-helices and the C-terminal dipeptide segment are in contact with the hydrophobic interior, whereas the remaining regions are exposed to the aqueous environment. These findings suggest that the ganglioside clusters serve as a unique platform for binding coupled with conformational transition of Abeta molecules, rendering their spatial rearrangements restricted to promote specific intermolecular interactions.

Comparative molecular dynamics study of Abeta adsorption on the self-assembled monolayers

The adsorption and aggregation of the amyloid-beta (Abeta) peptides on the cell membrane plays a causal role in the pathogenesis of Alzheimer's disease. Here, we report all-atom molecular dynamics (MD) simulations to study the interactions of Abeta oligomer with self-assembled monolayers (SAMs) terminated with hydrophobic CH(3) and hydrophilic OH functional groups, with particular interests in how surface chemistry and Abeta orientation affect the adsorption behavior of Abeta. Simulation results show that the CH(3)-SAM has a stronger binding affinity to Abeta than the OH-SAM does, although both surfaces can induce Abeta adsorption. Regardless of the characteristics of the surface, the hydrophobic C-terminal region is more likely to be adsorbed on the SAMs, indicating a preferential orientation and interface for Abeta adsorption. Structural and energetic comparison among six Abeta-SAM systems further reveals that Abeta orientation, SAM surface hydrophobicity, and interfacial waters all determine Abeta adsorption behavior on the surface, highlighting the importance of hydrophobic interactions at the interface. This work may provide parallel insights into the interactions of Abeta with lipid bilayers.

The second Cu(II)-binding site in a proton-rich environment interferes with the aggregation of amyloid-beta(1-40) into amyloid fibrils

The overall morphology and Cu(II) ion coordination for the aggregated amyloid-beta(1-40) [Abeta(1-40)] in N-ethylmorpholine (NEM) buffer are affected by Cu(II) ion concentration. This effect is investigated by transmission electron microscopy (TEM), atomic force microscopy (AFM), and electron spin echo envelope modulation (ESEEM) spectroscopy. At lower than equimolar concentrations of Cu(II) ions, fibrillar aggregates of Abeta(1-40) are observed. At these concentrations of Cu(II), the monomeric and fibrillar Abeta(1-40) ESEEM data indicate that the Cu(II) ion is coordinated by histidine residues. For aggregated Abeta(1-40) at a Cu(II):Abeta molar ratio of 2:1, TEM and AFM images show both linear fibrils and granular amorphous aggregates. The ESEEM spectra show that the multi-histidine coordination for Cu(II) ion partially breaks up and becomes exposed to water or exchangeable protons of the peptide at a higher Cu(II) concentration. Since the continuous-wave electron spin resonance results also suggest two copper-binding sites in Abeta(1-40), the proton ESEEM peak may arise from the second copper-binding site, which may be significantly involved in the formation of granular amorphous aggregates. Thioflavin T fluorescence and circular dichroism experiments also show that Cu(II) inhibits the formation of fibrils and induces a nonfibrillar beta-sheet conformation. Therefore, we propose that Abeta(1-40) has a second copper-binding site in a proton-rich environment and the second binding Cu(II) ion interferes with a conformational transition into amyloid fibrils, inducing the formation of granular amorphous aggregates.

Two disaccharides and trimethylamine N-oxide affect Abeta aggregation differently, but all attenuate oligomer-induced membrane permeability

Interaction between aggregates of amyloid beta protein (Abeta) and membranes has been hypothesized by many to be a key event in the mechanism of neurotoxicity associated with Alzheimer's disease (AD). Proposed membrane-related mechanisms of neurotoxicity include ion channel formation, membrane disruption, changes in membrane capacitance, and lipid membrane oxidation. Recently, osmolytes such as trehalose have been found to delay Abeta aggregation in vitro and reduce neurotoxicity. However, no direct measurements have separated the effects of osmolytes on Abeta aggregation versus membrane interactions. In this article, we tested the influence of trehalose, sucrose and trimethylamine-N-oxide (TMAO) on Abeta aggregation and fluorescent dye leakage induced by Abeta aggregates from liposomes. In the absence of lipid vesicles, trehalose and sucrose, but not TMAO, were found to delay Abeta aggregation. In contrast, all of the osmolytes significantly attenuated dye leakage. Dissolution of preformed Abeta aggregates was excluded as a possible mechanism of dye leakage attenuation by measurements of Congo red binding as well as hydrogen-deuterium exchange detected by mass spectrometry (HX-MS). However, the accelerated conversion of high order oligomers to fibril caused by vesicles did not take place if any of the three osmolytes presented. Instead, in the case of disaccharide, osmolytes were found to form adducts with Abeta, and change the dissociation dynamics of soluble oligomeric species. Both effects may have contributed to the observed osmolyte attenuation of dye leakage. These results suggest that disaccharides and TMAO may have very different effects on Abeta aggregation because of the different tendencies of the osmolytes to interact with the peptide backbone. However, the effects on Abeta membrane interaction may be due to much more general phenomena associated with osmolyte enhancement of Abeta oligomer stability and/or direct interaction of osmolyte with the membrane surface.

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.

Calcium dyshomeostasis and neurotoxicity of Alzheimer's beta-amyloid protein

Neurotoxicity of Alzheimer's beta-amyloid protein (AbetaP) is central to the pathogenesis of Alzheimer's disease (AD). Recent approaches have emphasized the importance of AbetaP oligomerization, which causes synaptic degeneration and neuronal loss, finally leading to the pathogenesis of AD. Although the precise molecular mechanism of AbetaP neurotoxicity remains elusive, our and other numerous findings have demonstrated that AbetaP directly incorporated into neuronal membranes formed calcium-permeable ion channels (amyloid channels) and resulted in an abnormal elevation of the intracellular calcium levels. The formation of amyloid channels and the abnormal increase of intracellular Ca(2+) have also been commonly observed in other neurodegenerative diseases, including conformational diseases such as prion disease or dementia with Lewy bodies. This article reviews the current understanding of the pathology of AD based on the hypothesis that the disruption of calcium homeostasis through amyloid channels may be the molecular basis of AbetaP neurotoxicity. The potential development of preventive agents is also discussed.

The comorbidity of HIV-associated neurocognitive disorders and Alzheimer's disease: a foreseeable medical challenge in post-HAART era

Although the introduction of highly active antiretroviral therapy (HAART) has led to a strong reduction of HIV-associated dementia (HAD) incidence, the prevalence of minor HIV-1-associated neurocognitive disorder (HAND) is rising among AIDS patients. HAART medication has shifted neuropathology from a subacute encephalitic condition to a subtle neurodegenerative process involving synaptic and dendritic degeneration, particularly of hippocampal neurons that are spared prior to HAART medication. Considerable neuroinflammation coupled with mononuclear phagocyte activation is present in HAART-medicated brains, particularly in the hippocampus. Accumulating evidence suggests that the resultant elevated secretion of pro-inflammatory cytokines such as interferon-gamma, tumor necrosis factor-alpha, and interleukin-1beta can increase amyloid-beta peptide (Abeta) generation and reduce Abeta clearance. Recent advancements in Alzheimer's disease (AD) research identified Abeta biogenesis and clearance venues that are potentially influenced by HIV viral infection, providing new insights into beta-amyloidosis segregation in HIV patients. Our study suggests enhanced beta-amyloidosis in ART-treated HAD and HIV-associated encephalitis brains and suppression of Abeta clearance by viral infection of human primary macrophages. A growing awareness of potential convergent mechanisms leading to neurodegeneration shared by HIV and Abeta points to a significant chance of comorbidity of AD and HAND in senile HIV patients, which calls for a need of basic studies.

Amyloid beta-protein assembly as a therapeutic target of Alzheimer's disease

Alzheimer's disease (AD), the most common neurodegenerative disorder in the aged, is characterized by the cerebral deposition of fibrils formed by the amyloid beta-protein (Abeta), a 40-42 amino acid peptide. The folding of Abeta into neurotoxic oligomeric, protofibrillar, and fibrillar assemblies is hypothesized to be the key pathologic event in AD. Abeta is formed through cleavage of the Abeta precursor protein by two endoproteinases, beta-secretase and gamma-secretase, that cleave the Abeta N-terminus and C-terminus, respectively. These facts support the relevance of therapeutic strategies targeting Abeta production, assembly, clearance, and neurotoxicity. Currently, no disease-modifying therapeutic agents are available for AD patients. Instead, existing therapeutics provide only modest symptomatic benefits for a limited time. We summarize here recent efforts to produce therapeutic drugs targeting Abeta assembly. A number of approaches are being used in these efforts, including immunological, nutraceutical, and more classical medicinal chemical (peptidic inhibitors, carbohydrate-containing compounds, polyamines, "drug-like" compounds, chaperones, metal chelators, and osmolytes), and many of these have progressed to phase III clinical trails. We also discuss briefly a number of less mature, but intriguing, strategies that have therapeutic potential. Although initial trials of some disease-modifying agents have failed, we argue that substantial cause for optimism exists.

The interaction of amyloid Abeta(1-40) with lipid bilayers and ganglioside as studied by 31P solid-state NMR

Amyloid beta-peptide (Abeta) is a major component of plaques in Alzheimer's disease, and formation of senile plaques has been suggested to originate from regions of neuronal membrane rich in gangliosides. We analyzed the mode of interaction of Abeta with lipid bilayers by multinuclear NMR using (31)P nuclei. We found that Abeta (1-40) strongly perturbed the bilayer structure of dimyristoylphosphatidylcholine (DMPC), to form a non-lamellar phase (most likely micellar). The ganglioside GM1 potentiated the effect of Abeta (1-40), as viewed from (31)P NMR. The difference of the isotropic peak intensity between DMPC/Abeta and DMPC/GM1/Abeta suggests a specific interaction between Abeta and GM1. We show that in the DMPC/GM1/Abeta system there are three lipid phases, namely a lamellar phase, a hexagonal phase and non-oriented lipids. The latter two phases are induced by the presence of the Abeta peptide, and facilitated by GM1.

Amyloid-beta membrane binding and permeabilization are distinct processes influenced separately by membrane charge and fluidity

The 40 and 42 residue amyloid-beta (Abeta) peptides are major components of the proteinaceous plaques prevalent in the Alzheimer's disease-afflicted brain and have been shown to have an important role in instigating neuronal degeneration. Whereas it was previously thought that Abeta becomes cytotoxic upon forming large fibrillar aggregates, recent studies suggest that soluble intermediate-sized oligomeric species cause cell death through membrane permeabilization. The present study examines the interactions between Abeta40 and lipid membranes using liposomes as a model system to determine how changes in membrane composition influence the conversion of Abeta into these toxic species. Abeta40 membrane binding was monitored using fluorescence-based assays with a tryptophan-substituted peptide (Abeta40 [Y10W]). We extend previous observations that Abeta40 interacts preferentially with negatively charged membranes, and show that binding of nonfibrillar, low molecular mass oligomers of Abeta40 to anionic, but not neutral, membranes involves insertion of the peptide into the bilayer, as well as sequential conformational changes corresponding to the degree of oligomerization induced. Significantly, while anionic membranes in the gel, liquid crystalline, and liquid ordered phases induce these conformational changes equally, membrane permeabilization is reduced dramatically as the fluidity of the membrane is decreased. These findings demonstrate that binding alone is not sufficient for membrane permeabilization, and that the latter is also highly dependent on the fluidity and phase of the membrane. We conclude that binding and pore formation are two distinct steps. The differences in Abeta behavior induced by membrane composition may have significant implications on the development and progression of AD as neuronal membrane composition is altered with age.

Amyloid-beta peptide induces temporal membrane biphasic changes in astrocytes through cytosolic phospholipase A2

Oligomeric amyloid-beta peptide (Abeta) is known to induce cytotoxic effects and to damage cell functions in Alzheimer's disease. However, mechanisms underlying the effects of Abeta on cell membranes have yet to be fully elucidated. In this study, Abeta 1-42 (Abeta(42)) was shown to cause a temporal biphasic change in membranes of astrocytic DITNC cells using fluorescence microscopy of Laurdan. Abeta(42) made astrocyte membranes became more molecularly-disordered within the first 30 min to 1 h, but gradually changed to more molecularly-ordered after 3 h. However, Abeta(42) caused artificial membranes of vesicles made of rat whole brain lipid extract to become more disordered only. The trend for more molecularly-ordered membranes in astrocytes induced by Abeta(42) was abrogated by either an NADPH oxidase inhibitor, apocynin, or an inhibitor of cytosolic phospholipase A(2) (cPLA(2)), but not by an inhibitor of calcium-independent PLA(2) (iPLA(2)). Apocynin also suppressed the increased production of superoxide anions (O(2)(-)) and phosphorylation of cPLA(2) induced by Abeta(42). In addition, hydrolyzed products of cPLA(2), arachidonic acid (AA), but not lysophosphatidylcholine (LPC) caused astrocyte membranes to become more molecularly-ordered. These results suggest (1) a direct interaction of Abeta(42) with cell membranes making them more molecularly-disordered, and (2) Abeta(42) also indirectly makes membranes become more molecularly-ordered by triggering the signaling pathway involving NADPH oxidase and cPLA(2) in astrocytes.

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.

Hybrid antioxidants

The article is a review of the data on synthesis and physiological activity of hybrid antioxidants. The introduction offers an explanation to the fact why, in some cases, it is necessary to add drug molecules with fragments responsible for various properties and aimed at various targets. A large group of hybrid antioxidants comprise stable nitroxyl radicals that behave as antioxidants in free-radical reactions of oxidation. Compounds of this type were synthesized extensively to form a group of antitumor agents. As a rule, the specific (antitumor) activity retained or even increased as compared with the initial compounds (without nitroxyl radicals); the toxicity decreased 5 to 10 times, which made it possible to apply the drug in considerably higher concentrations. There are reported data on nitroxyl derivatives of anthracycline antibiotics, antimetabolites, alkyl ting agents, and the recent results on platinum complexes with nitroxyl fragments. Much attention is given to hindered phenols with “buoyancy” properties, particularly, to biochemical effects, making them promising agents to treat Alzheimer’s disease.

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.

Abeta peptide interactions with isoflurane, propofol, thiopental and combined thiopental with halothane: a NMR study

Abeta peptide is the major component of senile plaques (SP) which accumulates in AD (Alzheimer's disease) brain. Reports from different laboratories indicate that anesthetics interact with Abeta peptide and induce Abeta oligomerization. The molecular mechanism of Abeta peptide interactions with these anesthetics was not determined. We report molecular details for the interactions of uniformly (15)N labeled Abeta40 with different anesthetics using 2D nuclear magnetic resonance (NMR) experiments. At high concentrations both isoflurane and propofol perturb critical amino acid residues (G29, A30 and I31) of Abeta peptide located in the hinge region leading to Abeta oligomerization. In contrast, these three specific residues do not interact with thiopental and subsequently no Abeta oligomerization was observed. However, studies with combined anesthetics (thiopental and halothane), showed perturbation of these residues (G29, A30 and I31) and subsequently Abeta oligomerization was found. Perturbation of these specific Abeta residues (G29, A30 and I31) by different anesthetics could play an important role to induce Abeta oligomerization.

Lipid-induced conformational transition of the amyloid core fragment Abeta(28-35) and its A30G and A30I mutants

The interaction of the beta-amyloid peptide (Abeta) with neuronal membranes could play a key role in the pathogenesis of Alzheimer's disease. Recent studies have focused on the interactions of Abeta oligomers to explain the neuronal toxicity accompanying Alzheimer's disease. In our study, we have investigated the role of lipid interactions with soluble Abeta(28-35) (wild-type) and its mutants A30G and A30I in their aggregation and conformational preferences. CD and Trp fluorescence spectroscopic studies indicated that, immediately on dissolution, these peptides adopted a random coil structure. Upon addition of negatively charged 1,2-dipalmitoyl-syn-glycero-3-phospho-rac-(glycerol) sodium salt (PG) lipid, the wild-type and A30I mutant underwent reorganization into a predominant beta-sheet structure. However, no conformational changes were observed in the A30G mutant on interaction with PG. In contrast, the presence of zwitterionic 1,2-dipalmitoyl-syn-glycero-3-phosphatidylcholine (PC) lipid had no effect on the conformation of these three peptides. These observations were also confirmed with atomic force microscopy and the thioflavin-T assay. In the presence of PG vesicles, both the wild-type and A30I mutant formed fibrillar structures within 2 days of incubation in NaCl/P(i), but not in their absence. Again, no oligomerization was observed with PC vesicles. The Trp studies also revealed that both ends of the three peptides are not buried deep in the vesicle membrane. Furthermore, fluorescence spectroscopy using the environment-sensitive probe 1,6-diphenyl-1,3,5-hexatriene showed an increase in the membrane fluidity upon exposure of the vesicles to the peptides. The latter effect may result from the lipid head group interactions with the peptides. Fluorescence resonance energy transfer experiments revealed that these peptides undergo a random coil-to-sheet conversion in solution on aging and that this process is accelerated by negatively charged lipid vesicles. These results indicate that aggregation depends on hydrophobicity and propensity to form beta-sheets of the amyloid peptide, and thus offer new insights into the mechanism of amyloid neurodegenerative disease.

Membrane damage by human islet amyloid polypeptide through fibril growth at the membrane

Fibrillar protein deposits (amyloid) in the pancreatic islets of Langerhans are thought to be involved in death of the insulin-producing islet beta cells in type 2 diabetes mellitus. It has been suggested that the mechanism of this beta cell death involves membrane disruption by human islet amyloid polypeptide (hIAPP), the major constituent of islet amyloid. However, the molecular mechanism of hIAPP-induced membrane disruption is not known. Here, we propose a hypothesis that growth of hIAPP fibrils at the membrane causes membrane damage. We studied the kinetics of hIAPP-induced membrane damage in relation to hIAPP fibril growth and found that the kinetic profile of hIAPP-induced membrane damage is characterized by a lag phase and a sigmoidal transition, which matches the kinetic profile of hIAPP fibril growth. The observation that seeding accelerates membrane damage supports the hypothesis. In addition, variables that are well known to affect hIAPP fibril formation, i.e., the presence of a fibril formation inhibitor, hIAPP concentration, and lipid composition, were found to have the same effect on hIAPP-induced membrane damage. Furthermore, electron microscopy analysis showed that hIAPP fibrils line the surface of distorted phospholipid vesicles, in agreement with the notion that hIAPP fibril growth at the membrane and membrane damage are physically connected. Together, these observations point toward a mechanism in which growth of hIAPP fibrils, rather than a particular hIAPP species, is responsible for the observed membrane damage. This hypothesis provides an additional mechanism next to the previously proposed role of oligomers as the main cytotoxic species of amyloidogenic proteins.

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.

Role of ganglioside metabolism in the pathogenesis of Alzheimer's disease--a review

Gangliosides are expressed in the outer leaflet of the plasma membrane of the cells of all vertebrates and are particularly abundant in the nervous system. Ganglioside metabolism is closely associated with the pathology of Alzheimer's disease (AD). AD, the most common form of dementia, is a progressive degenerative disease of the brain characterized clinically by progressive loss of memory and cognitive function and eventually death. Neuropathologically, AD is characterized by amyloid deposits or "senile plaques," which consist mainly of aggregated variants of amyloid beta-protein (Abeta). Abeta undergoes a conformational transition from random coil to ordered structure rich in beta-sheets, especially after addition of lipid vesicles containing GM1 ganglioside. In AD brain, a complex of GM1 and Abeta, termed "GAbeta," has been found to accumulate. In recent years, Abeta and GM1 have been identified in microdomains or lipid rafts. The functional roles of these microdomains in cellular processes are now beginning to unfold. Several articles also have documented the involvement of these microdomains in the pathogenesis of certain neurodegenerative diseases, such as AD. A pivotal neuroprotective role of gangliosides has been reported in in vivo and in vitro models of neuronal injury, Parkinsonism, and related diseases. Here we describe the possible involvement of gangliosides in the development of AD and the therapeutic potentials of gangliosides in this disorder.

Alzheimer's disease protein Abeta1-42 does not disrupt isolated synaptic vesicles

Synaptic vesicles are central to neurotransmission and cognition. Studies of the Alzheimer's disease (AD) associated peptide, amyloid beta (Abeta), suggest that it has the potential to non-specifically solubilize or permeabilize membranes and that it has detergent and pore-forming properties. Damage to the membrane or integrity of synaptic vesicles could compromise its function. We test the hypothesis that the intact synaptic vesicle is a direct site of attack by Abeta1-42 in AD pathology by examining the properties of individual isolated vesicles exposed to Abeta1-42. In particular, we compared the rate of leakage of dye molecules from synaptic vesicles, the rate of proton permeation across the membrane of the vesicle, and the rate of active proton transport into the vesicle interior in the presence and absence of Abeta1-42. From these experiments, we conclude that isolated synaptic vesicles are not disrupted by Abeta1-42.

Dimerization of the transmembrane domain of amyloid precursor proteins and familial Alzheimer's disease mutants

BACKGROUND

Amyloid precursor protein (APP) is enzymatically cleaved by gamma-secretase to form two peptide products, either Abeta40 or the more neurotoxic Abeta42. The Abeta42/40 ratio is increased in many cases of familial Alzheimer's disease (FAD). The transmembrane domain (TM) of APP contains the known dimerization motif GXXXA. We have investigated the dimerization of both wild type and FAD mutant APP transmembrane domains.

RESULTS

Using synthetic peptides derived from the APP-TM domain, we show that this segment is capable of forming stable transmembrane dimers. A model of a dimeric APP-TM domain reveals a putative dimerization interface, and interestingly, majority of FAD mutations in APP are localized to this interface region. We find that FAD-APP mutations destabilize the APP-TM dimer and increase the population of APP peptide monomers.

CONCLUSION

The dissociation constants are correlated to both the Abeta42/Abeta40 ratio and the mean age of disease onset in AD patients. We also show that these TM-peptides reduce Abeta production and Abeta42/Abeta40 ratios when added to HEK293 cells overexpressing the Swedish FAD mutation and gamma-secretase components, potentially revealing a new class of gamma-secretase inhibitors.