Abeta42-peptide assembly on lipid bilayers

Correlative optical and scanning probe microscopies for mapping interactions at membranes

Innovative approaches for real-time imaging on molecular-length scales are providing researchers with powerful strategies for characterizing molecular and cellular structures and dynamics. Combinatorial techniques that integrate two or more distinct imaging modalities are particularly compelling as they provide a means for overcoming the limitations of the individual modalities and, when applied simultaneously, enable the collection of rich multi-modal datasets. Almost since its inception, scanning probe microscopy has closely associated with optical microscopy. This is particularly evident in the fields of cellular and molecular biophysics where researchers are taking full advantage of these real-time, in situ, tools to acquire three-dimensional molecular-scale topographical images with nanometer resolution, while simultaneously characterizing their structure and interactions though conventional optical microscopy. The ability to apply mechanical or optical stimuli provides an additional experimental dimension that has shown tremendous promise for examining dynamic events on sub-cellular length scales. In this chapter, we describe recent efforts in developing these integrated platforms, the methodology for, and inherent challenges in, performing coupled imaging experiments, and the potential and future opportunities of these research tools for the fields of molecular and cellular biophysics with a specific emphasis on the application of these coupled approaches for the characterization of interactions occurring at membrane interfaces.

Weak glycolipid binding of a microdomain-tracer peptide correlates with aggregation and slow diffusion on cell membranes

Organized assembly or aggregation of sphingolipid-binding ligands, such as certain toxins and pathogens, has been suggested to increase binding affinity of the ligand to the cell membrane and cause membrane reorganization or distortion. Here we show that the diffusion behavior of the fluorescently tagged sphingolipid-interacting peptide probe SBD (Sphingolipid Binding Domain) is altered by modifications in the construction of the peptide sequence that both result in a reduction in binding to ganglioside-containing supported lipid membranes, and at the same time increase aggregation on the cell plasma membrane, but that do not change relative amounts of secondary structural features. We tested the effects of modifying the overall charge and construction of the SBD probe on its binding and diffusion behavior, by Surface Plasmon Resonance (SPR; Biacore) analysis on lipid surfaces, and by Fluorescence Correlation Spectroscopy (FCS) on live cells, respectively. SBD binds preferentially to membranes containing the highly sialylated gangliosides GT1b and GD1a. However, simple charge interactions of the peptide with the negative ganglioside do not appear to be a critical determinant of binding. Rather, an aggregation-suppressing amino acid composition and linker between the fluorophore and the peptide are required for optimum binding of the SBD to ganglioside-containing supported lipid bilayer surfaces, as well as for interaction with the membrane. Interestingly, the strength of interactions with ganglioside-containing artificial membranes is mirrored in the diffusion behavior by FCS on cell membranes, with stronger binders displaying similar characteristic diffusion profiles. Our findings indicate that for aggregation-prone peptides, aggregation occurs upon contact with the cell membrane, and rather than giving a stronger interaction with the membrane, aggregation is accompanied by weaker binding and complex diffusion profiles indicative of heterogeneous diffusion behavior in the probe population.

Structure, orientation, and surface interaction of Alzheimer amyloid-β peptides on the graphite

The misfolding and aggregation of amyloid-β (Aβ) peptides into amyloid fibrils in solution and on the cell membrane has been linked to the pathogenesis of Alzheimer's disease. Although it is well-known that the presence of different surfaces can accelerate the aggregation of Aβ peptides into fibrils, surface-induced conformation, orientation, aggregation, and adsorption of Aβ peptides have not been well understood at the atomic level. Here, we perform all-atom explicit-water molecular dynamics (MD) simulations to study the orientation change, conformational dynamics, surface interaction of small Aβ aggregates with different sizes (monomer to tetramer), and conformations (α-helix and β-hairpin) upon adsorption on the graphite surface, in comparison with Aβ structures in bulk solution. Simulation results show that hydrophobic graphite induces the quick adsorption of Aβ peptides regardless of their initial conformations and sizes. Upon the adsorption, Aβ prefers to adopt random structure for monomers and to remain β-rich-structure for small oligomers, but not helical structures. More importantly, due to the amphiphilic sequence of Aβ and the hydrophobic nature of graphite, hydrophobic C-terminal residues of higher-order Aβ oligomers appear to have preferential interactions with the graphite surface for facilitating Aβ fibril formation and fibril growth. In combination of atomic force microscopy (AFM) images and MD simulation results, a postulated mechanism is proposed to describe the structure and kinetics of Aβ aggregation from aqueous solution to the graphite surface, providing parallel insights into Aβ aggregation on biological cell membranes.

Role of the helical structure of the N-terminal region of Plasmodium falciparum merozoite surface protein 2 in fibril formation and membrane interaction

Merozoite surface protein 2 (MSP2), an abundant glycosylphosphatidylinositol-anchored protein on the surface of Plasmodium falciparum merozoites, is a promising malaria vaccine candidate. MSP2 is intrinsically disordered and forms amyloid-like fibrils in solution under physiological conditions. The 25 N-terminal residues (MSP2(1-25)) play an important role in both fibril formation and membrane binding of the full-length protein. In this study, the fibril formation and solution structure of MSP2(1-25) in the membrane mimetic solvents sodium dodecyl sulfate (SDS), dodecylphosphocholine (DPC), and trifluoroethanol (TFE) have been investigated by transmission electronic microscopy, turbidity, thioflavin T fluorescence, circular dichroism (CD), and nuclear magnetic resonance (NMR) spectroscopy. Turbidity data showed that the aggregation of MSP2(1-25) was suppressed in the presence of membrane mimetic solvents. CD spectra indicated that helical structure in MSP2(1-25) was stabilized in SDS and DPC micelles and in high concentrations of TFE. The structure of MSP2(1-25) in 50% aqueous TFE, determined using NMR, showed that the peptide formed an amphipathic helix encompassing residues 10-24. Low concentrations of TFE favored partially folded helical conformations, as demonstrated by CD and NMR, and promoted MSP2(1-25) fibril formation. Our data suggest that partially folded helical conformations of the N-terminal region of MSP2 are on the pathway to amyloid fibril formation, while higher degrees of helical structure stabilized by high concentrations of TFE or membrane mimetics suppress self-association and thus inhibit fibril formation. The roles of the induced helical conformations in membrane interactions are also discussed.

Molecular interactions of Alzheimer's Aβ protofilaments with lipid membranes

Amyloid fibrils and peptide oligomers play central roles in the pathology of Alzheimer's disease, type 2 diabetes, Parkinson's disease, Huntington's disease, and prion-related disease. Here, we investigate the molecular interactions between preformed amyloid β (Aβ) molecular protofilaments and lipid bilayer membranes, in the presence of explicit water molecules, using computational models and all-atom molecular dynamics. These interactions play an important role in the stability and function of both Aβ fibrils and the adjacent cellular membrane. Taking advantage of the symmetry-related and directional properties of the protofilaments, we build models that cover several relative protofilament-membrane orientations. Our molecular dynamics simulations reveal the relative contributions of different structural elements to the dynamics and stability of Aβ protofilament segments near membranes, and the first steps in the mechanism of fibril-membrane interactions. During this process, we observe a significant alteration of the side-chain contact pattern in protofilaments, although a fraction of the characteristic β-sheet content is preserved. As a major driving force, we identify the electrostatic interactions between Aβ charged side chains, including E22, D23, and K28, and lipid headgroups. Together with hydrogen bonding with atoms from lipid headgroups, these interactions can facilitate the penetration of hydrophobic C-terminal amino acids through the lipid headgroup region, which can finally lead both to further loss of the initial fibril structure and to local membrane-thinning effects. Our results may guide new experiments that could test the extent to which the structural features of water-formed amyloid fibrils are preserved, lost, or reshaped by membrane-mediated interactions.

Detection and quantification through a lipid membrane using the molecularly controlled semiconductor resistor

The detection of covalent and noncovalent binding events between molecules and biomembranes is a fundamental goal of contemporary biochemistry and analytical chemistry. Currently, such studies are performed routinely using fluorescence methods, surface-plasmon resonance spectroscopy, and electrochemical methods. However, there is still a need for novel sensitive miniaturizable detection methods where the sample does not have to be transferred to the sensor, but the sensor can be brought into contact with the sample studied. We present a novel approach for detection and quantification of processes occurring on the surface of a lipid bilayer membrane, by monitoring the current change through the n-type GaAs-based molecularly controlled semiconductor resistor (MOCSER), on which the membrane is adsorbed. Since GaAs is susceptible to etching in an aqueous environment, a protective thin film of methoxysilane was deposited on the device. The system was found to be sensitive enough to allow monitoring changes in pH and in the concentration of amino acids in aqueous solution on top of the membrane. When biotinylated lipids were incorporated into the membrane, it was possible to monitor the binding of streptavidin or avidin. The device modified with biotin-streptavidin complex was capable of detecting the binding of streptavidin antibodies to immobilized streptavidin with high sensitivity and selectivity. The response depends on the charge on the analyte. These results open the way to facile electrical detection of protein-membrane interactions.

Nucleation and growth of elastin-like peptide fibril multilayers: an in situ atomic force microscopy study

Controlling how molecules assemble into complex supramolecular architectures requires careful consideration of the subtle inter- and intra-molecular interactions that control their association. This is particularly crucial in the context of assembly at interfaces, where both surface chemistry and structure can play a role in directing structure formation. We report here the results of a study into the self-assembly of the elastin-like peptide EP I on structurally modified highly ordered pyrolytic graphite, including the role of spatial confinement on fibril nucleation and the growth of oriented fibril multilayers. In situ atomic force microscopy performed in fluid and at elevated temperature provided direct evidence of frustrated fibril nuclei and oriented growth of independent fibril domains. These results portend the application of this in situ strategy for studies of the nucleation and growth mechanisms of other fibril- and amyloid-forming proteins.

The modulating effect of mechanical changes in lipid bilayers caused by apoE-containing lipoproteins on Aβ induced membrane disruption

A major feature of Alzheimer's disease (AD), a late-onset neurodegenerative disorder, is the ordered aggregation of the β-amyloid peptide (Aβ) into fibrils that comprise extracellular neuritic plaques found in the disease brain. One of many potential pathways for Aβ toxicity may be modulation of lipid membrane function. Here, we show by in situ atomic force microscopy (AFM) that astrocyte secreted lipoprotein particles (ASLPs) containing different isoforms of apolipoprotein E (apoE), of which the apoE4 allele is a major risk factor for the development of AD, can protect total brain lipid extract bilayers from Aβ(1-40) induced disruption. The apoE4 allele was less effective in protecting lipid bilayers from disruption compared with apoE3. Size analysis of apoE-containing ASLPs and mechanical studies of bilayer properties revealed that apoE-containing ASLPs modulate the mechanical properties of bilayers by acquiring some bilayer components (most likely cholesterol and/or oxidatively damaged lipids). Measurement of bilayer mechanical properties was accomplished with scanning probe acceleration microscopy (SPAM). These measurements demonstrated that apoE4 was also less effective in modulating mechanical properties of bilayers in comparison with apoE3. This ability of apoE to alter the mechanical properties of lipid membranes may represent a potential mechanism for the suppression of Aβ(1-40) induced bilayer disruption.

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

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

Structural, morphological, and kinetic studies of β-amyloid peptide aggregation on self-assembled monolayers

The misfolding and aggregation of β-amyloid peptides (Aβ) into amyloid fibrils, a process that has been pathologically linked to the onset of Alzheimer's disease, is dependent on the presence of a heterogeneous surface (e.g., cell membrane). Understanding of the kinetics of amyloid fibril formation and associated structural transition from monomers to intermediates and eventually to fibrils is critical for the development of viable therapeutic agents. In this work, using circular dichroism (CD), atomic force microscopy (AFM), surface plasmon resonance (SPR), and molecular dynamics (MD) simulations, we studied the adsorption, aggregation, and conformational changes of Aβ(1-42) from fresh monomers to fully grown fibrils on four model self-assembled monolayers (SAMs): hydrophobic CH(3)-terminated SAM, hydrophilic OH-terminated SAM, negatively charged COOH-terminated SAMs, and positively charged NH(2)-terminated SAM. The seeding effect of Aβ(1-42) on the kinetics of Aβ aggregation on different SAMs is also examined. The CD, AFM, and SPR data show that all of these SAMs greatly accelerate the formation of β-sheets and amyloid fibrils through surface-enhanced interactions, but Aβ(1-42) peptides preferentially adsorb on a hydrophobic CH(3)-SAM and a positively charged NH(2)-SAM with much stronger interactions than on a hydrophilic OH-SAM and a negatively charged COOH-SAM. MD simulations further reveal that hydrophobic interactions present a general driving force for Aβ adsorption on all SAMs. As Aβ aggregates grow into larger species by packing hydrophobic C-terminals to form a hydrophobic core while exposing hydrophilic and negatively charged N-terminals to solution, electrostatic interactions become more strengthened when they interact with the SAMs especially for the COOH-SAM and the NH(2)-SAM. Thus, both hydrophobic and electrostatic interactions contribute differently to different Aβ-SAM systems and to different aggregation stages. A postulated mechanism is proposed to describe the structure and kinetics of Aβ aggregation from aqueous solution to the SAMs, providing valuable insights into Aβ aggregation on biological cell membranes.

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.

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.

Point mutations in Aβ result in the formation of distinct polymorphic aggregates in the presence of lipid bilayers

A hallmark of Alzheimer's disease (AD) is the rearrangement of the β-amyloid (Aβ) peptide to a non-native conformation that promotes the formation of toxic, nanoscale aggregates. Recent studies have pointed to the role of sample preparation in creating polymorphic fibrillar species. One of many potential pathways for Aβ toxicity may be modulation of lipid membrane function on cellular surfaces. There are several mutations clustered around the central hydrophobic core of Aβ near the α-secretase cleavage site (E22G Arctic mutation, E22K Italian mutation, D23N Iowa mutation, and A21G Flemish mutation). These point mutations are associated with hereditary diseases ranging from almost pure cerebral amyloid angiopathy (CAA) to typical Alzheimer's disease pathology with plaques and tangles. We investigated how these point mutations alter Aβ aggregation in the presence of supported lipid membranes comprised of total brain lipid extract. Brain lipid extract bilayers were used as a physiologically relevant model of a neuronal cell surface. Intact lipid bilayers were exposed to predominantly monomeric preparations of Wild Type or different mutant forms of Aβ, and atomic force microscopy was used to monitor aggregate formation and morphology as well as bilayer integrity over a 12 hour period. The goal of this study was to determine how point mutations in Aβ, which alter peptide charge and hydrophobic character, influence interactions between Aβ and the lipid surface. While fibril morphology did not appear to be significantly altered when mutants were prepped similarly and incubated under free solution conditions, aggregation in the lipid membranes resulted in a variety of polymorphic aggregates in a mutation dependent manner. The mutant peptides also had a variable ability to disrupt bilayer integrity.

Assessing Aβ aggregation state by atomic force microscopy

There has been a growing recognition of a wide variety of diseases commonly referred to as conformational diseases, which share the feature of specific disease-related proteins adopting nonnative conformation that promote their ordered aggregation and deposition on surfaces. Due to the nanoscale dimensions and the varied morphology of such aggregates, atomic force microscopy (AFM) has emerged as an ideal tool for distinguishing structural features of the numerous potential aggregate forms, ranging from small globular oligomers to large mature amyloid fibrils. Beyond the ability to morphologically distinguish aggregate forms, AFM also can dynamically track the aggregation process due to its unique ability to be operated not only in air (ex situ), but also in solution (in situ). This feature provides for tracking the fate of individual aggregates over time. This chapter describes the use of AFM in characterizing the aggregation of the amyloid-β peptide (Aβ), which is hypothesized to play a key role in Alzheimer's disease (AD), a late-onset neurodegenerative conformational disease.

Assessing mutant huntingtin fragment and polyglutamine aggregation by atomic force microscopy

Huntington disease (HD), a neurodegenerative disorder, is caused by an expansion of more than 35-40 polyglutamine (polyQ) repeats located near the N-terminus of the huntingtin (htt) protein. The expansion of the polyQ domain results in the ordered assembly of htt fragments into fibrillar aggregates that are the main constituents of inclusion bodies, which are a hallmark of the disease. This paper describes protocols for studying the aggregation of mutant htt fragments and synthetic polyQ peptides with atomic force microscopy (AFM). Ex situ AFM is used to characterize aggregate formation in protein incubation as a function of time. Methods to quickly and unambiguously distinguish specific aggregate species from complex, heterogeneous aggregation reactions based on simple morphological features are presented. Finally, the application of time lapse atomic force microscopy in solution is presented for studying synthetic model polyQ peptides, which allows for tracking the formation and fate of individual aggregates on surfaces over time. This ability allows for dynamic studies of the aggregation process and direct observation of the interplay between different types of aggregates.

Studies of the growth, evolution, and self-aggregation of β-amyloid fibrils using tapping-mode atomic force microscopy

Amyloid peptide (Aβ) is the major protein component of plaques found in Alzheimer's disease, and the aggregation of Aβ into oligomeric and fibrillic assemblies has been shown to be an early event of the disease pathway. Visualization of the progressive evolution of nanoscale changes in the morphology of Aβ oligomeric assemblies and amyloid fibrils has been accomplished ex situ using atomic force microscopy (AFM) in ambient conditions. In this report, the size and the shape of amyloid β(1-40) fibrils, as well as the secondary organization into aggregate structures were monitored at different intervals over a period of 5 months. Characterizations with tapping-mode AFM serve to minimize the strong adhesive forces between the probe and the sample to prevent damage or displacement of fragile fibrils. The early stages of Aβ growth showed a predominance of spherical seed structures, oligomeric assemblies, and protofibrils; however the size and density of fibrils progressively increased with time. Within a few days of incubation, linear assemblies and fibrils became apparent. Over extended time scales of up to 5 months, the fibrils formed dense ensembles spanning lengths of several microns, which exhibit interesting changes due to self-organization of the fibrils into bundles or tangles. Detailed characterization of the Aβ assembly process at the nanoscale will help elucidate the role of Aβ in the pathology of Alzheimer's disease.

A molecular dynamics study of the early stages of amyloid-beta(1-42) oligomerization: the role of lipid membranes

As research progresses toward understanding the role of the amyloid-beta (Abeta) peptide in Alzheimer's disease, certain aspects of the aggregation process for Abeta are still not clear. In particular, the accepted constitution of toxic aggregates in neurons has shifted toward small oligomers. However, the process of forming these oligomers in cells is also not full clear. Even more interestingly, it has been implied that cell membranes, and, in particular, anionic lipids within those membranes, play a key role in the progression of Abeta aggregation, but the exact nature of the Abeta-membrane interaction in this process is unknown. In this work, we use a thermodynamic cycle and umbrella sampling molecular dynamics to investigate dimerization of the 42-residue Abeta peptide on model zwitterionic dipalmitoylphosphatidylcholine (DPPC) or model anionic dioleoylphosphatidylserine (DOPS) bilayer surfaces. We determined that Abeta dimerization was strongly favored through interactions with the DOPS bilayer. Further, our calculations showed that the DOPS bilayer promoted strong protein-protein interactions within the Abeta dimer, whereas DPPC favored strong protein-lipid interactions. By promoting dimer formation and subsequent dimer release into the solvent, the DOPS bilayer acts as a catalyst in Abeta aggregation.

Interaction of the prion protein fragment PrP 185-206 with biological membranes: effect on membrane permeability

Amyloids are proteinaceous aggregates related to the so-called conformational diseases, such as Alzheimer's and prion diseases. The cytotoxicity of amyloids may be related to the interaction of the amiloidogenic peptides or proteins with the cell membrane. In order to gain information on the physico-chemical effects of amyloids on membranes, we have studied the interaction of the human prion amyloidogenic fragment PrP 185-206 with negatively charged model membranes. The results show that the peptide causes the destabilization of the membrane, making it permeable to potassium ions and to charged organic compounds. This effect correlates with the interaction of the peptide with the membrane, causing a variation in the magnitude of the electrostatic surface and dipole membrane potentials. This effect on the electrostatic properties of the membranes may help explaining the observed permeability: a neutralization of the surface negative charge and a decrease of the inside-positive dipole potential would facilitate the translocation of positive ions. The structural analysis of the peptide in the presence of model membranes reveals that it adopts a predominantly unordered structure without any signs of amyloid formation. The results may be relevant in relation to the recently described cell toxic capacity of the peptide.

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.

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

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

Membrane curvature induction and tubulation are common features of synucleins and apolipoproteins

Synucleins and apolipoproteins have been implicated in a number of membrane and lipid trafficking events. Lipid interaction for both types of proteins is mediated by 11 amino acid repeats that form amphipathic helices. This similarity suggests that synucleins and apolipoproteins might have comparable effects on lipid membranes, but this has not been shown directly. Here, we find that α-synuclein, β-synuclein, and apolipoprotein A-1 have the conserved functional ability to induce membrane curvature and to convert large vesicles into highly curved membrane tubules and vesicles. The resulting structures are morphologically similar to those generated by amphiphysin, a curvature-inducing protein involved in endocytosis. Unlike amphiphysin, however, synucleins and apolipoproteins do not require any scaffolding domains and curvature induction is mediated by the membrane insertion and wedging of amphipathic helices alone. Moreover, we frequently observed that α-synuclein caused membrane structures that had the appearance of nascent budding vesicles. The ability to function as a minimal machinery for vesicle budding agrees well with recent findings that α-synuclein plays a role in vesicle trafficking and enhances endocytosis. Induction of membrane curvature must be under strict regulation in vivo; however, as we find it can also cause disruption of membrane integrity. Because the degree of membrane curvature induction depends on the concerted action of multiple proteins, controlling the local protein density of tubulating proteins may be important. How cellular safeguarding mechanisms prevent such potentially toxic events and whether they go awry in disease remains to be determined.

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.

Surface plasmon resonance spectroscopy in determination of the interactions between amyloid beta proteins (Abeta) and lipid membranes

Surface plasmon resonance (SPR) spectroscopy is emerging as a useful tool for determination of molecular interactions in real time. Studies on the molecular pathogenesis of amyloidoses have shown that the plasma membrane plays an important role in amyloidogenesis and cytotoxicity induced by amyloidogenic proteins. By immobilizing lipid bilayers on a sensor chip surface, SPR spectroscopy has been employed to examine the binding of amyloidogenic proteins, such as amyloid beta protein (Abeta), to a variety of lipid membranes, and it provided new insights into the molecular interactions between these amyloidogenic proteins and membranes. In this chapter, we describe the application of SPR spectroscopy to the determination of the binding of Abeta to lipid membranes.

Preparation of fluorescently-labeled amyloid-beta peptide assemblies: the effect of fluorophore conjugation on structure and function

Recent research has focused on soluble oligomeric assemblies of the 42 amino acid isoform of the amyloid-beta peptide (A beta 42) as the proximal cause of neuronal injury, synaptic loss, and the eventual dementia associated with Alzheimer's disease (AD). While neurotoxicity, neuroinflammation, and deficits in behavior and memory have all been attributed to oligomeric A beta 42, the specific roles for this assembly in the cellular neuropathology of AD remain poorly understood. In particular, lack of reliable and well-characterized forms of easily detectable A beta 42 oligomers has hindered study of the cellular trafficking of exogenous A beta 42 by neurons in vitro and in vivo. Therefore, the objective of this study is to fluorescently label soluble oligomeric A beta 42 without altering the structure or function of this assembly. Previous studies have demonstrated the advantages of using tapping mode atomic force microscopy (AFM) to characterize the structural assemblies formed by synthetic A beta 42 under specific solution conditions (e.g., oligomers, protofibrils, and fibrils). Here, we extend these methods to establish a strategy for fluorescent labeling of oligomeric A beta 42 assemblies that are structurally comparable to unlabeled oligomeric A beta 42. To compare function, we demonstrate that the uptake of labeled and unlabeled oligomeric A beta 42 by neurons in vitro is similar. AFM-characterized fluorophore-A beta 42 oligomers are an exciting new reagent for use in a variety of studies designed to elucidate critical cellular and molecular mechanisms underlying the functions of this A beta 42 assembly form in AD.

A kinetic model for beta-amyloid adsorption at the air/solution interface and its implication to the beta-amyloid aggregation process

At the air/buffer solution interface the kinetics of adsorption of amyloid beta peptide, Abeta(1-42), whose bulk concentration (submicromolar) is more than 2 orders of magnitude lower than that typically used in other in vitro aggregation studies, has been studied using a Langmuir-Blodgett trough. The pressure-time curves exhibit a lag phase, wherein the surface pressure essentially remains at zero, and a rising phase, corresponding to the Abeta adsorption at the interface. The duration of the lag phase was found to be highly dependent on both the Abeta bulk concentration and the solution temperature. A large activation energy (62.2 +/- 4.1 KJ/mol) was determined and the apparent adsorption rate constant was found to be linearly dependent on the Abeta bulk concentration. Attenuated total reflection-IR spectra of the adsorbed Abeta transferred to a solid substrate and circular dichroism measurements of Abeta in the solution layer near the interface reveal that the natively unstructured Abeta in the bulk undergo a conformation change (folding) to mainly the alpha-helical structure. The results suggest that, prior to the adsorption step, an equilibrium between Abeta conformations is established within the subsurface. The kinetic equation derived from this model confirms that the overall Abeta adsorption is kinetically controlled and the apparent rate constant is proportional to the Abeta bulk concentration. This model also indicates that interfaces such as cell membranes and lipid bilayers may facilitate Abeta aggregation/ fibrillation by providing a thin hydrophobic layer adjacent to the interface for the initial A/beta conformation change (misfolding) and accumulation. Such a preconcentration effect offers a plausible explanation of the fact that Abeta fibrillation occurs in vivo at nanomolar concentrations. Another important biological implication from our work is that Abeta misfolding may occur before its adsorption onto a cell membrane. This general kinetic model should also find applications in adsorption studies of other types of biomolecules whose overall kinetics exhibits a lag phase that is dependent on the bulk concentration of the adsorbate.

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.

Effects of lipid composition and phase on the membrane interaction of the prion peptide 106-126 amide

Lipid rafts are specialized liquid-ordered (L(o)) phases of the cell membrane that are enriched in sphingolipids and cholesterol (Chl), and surrounded by a liquid-disordered (L(d)) phase enriched in glycerophospholipids. Lipid rafts are involved in the generation of pathological forms of proteins that are associated with neurodegenerative diseases. To investigate the effects of lipid composition and phase on the generation of pathological forms of proteins, we constructed an L(d)-gel phase-separated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/sphingomyelin (from bovine brain (BSM))-supported lipid bilayer (SLB) and an L(d)-L(o) phase-separated POPC/BSM/Chl SLB. We used in situ time-lapse atomic force microscopy to study the interactions between these SLBs and the prion peptide K(106)TNMKHMAGAAAAGAVVGGLG(126) (PrP106-126) amide, numbered according to the human prion-peptide sequence. Our results show that: 1), with the presence of BSM in the L(d) phase, the PrP106-126 amide induces fully penetrated porations in the L(d) phase of POPC/BSM SLB and POPC/BSM/Chl SLB; 2), with the presence of both BSM and Chl in the L(d) phase, the PrP106-126 amide induces the disintegration of the L(d) phase of POPC/BSM/Chl SLB; and 3), with the presence of both BSM and Chl in the L(o) phase, PrP106-126 amide induces membrane thinning in the L(o) phase of POPC/BSM/Chl SLB. These results provide comprehensive insight into the process by which the PrP106-126 amide interacts with lipid membranes.

The interplay of catalysis and toxicity by amyloid intermediates on lipid bilayers: insights from type II diabetes

The dynamics, energies, and structures governing protein folding are critical to biological function. Amyloidoses are a class of disease defined, in part, by the misfolding and aggregation of functional protein precursors into fibrillar states. Amyloid fibers contribute to the pathology of many diseases, including type II diabetes, Alzheimer's, and Parkinson's. In these disorders, amyloid fibers are present in affected tissues. However, it has become clear that intermediate states, rather than mature fibers, represent the cytotoxic species. In this review, we focus particularly on lipid bilayer-bound intermediates. Remarkably, the precursors of these fibers are intrinsically disordered, and yet catalysis of beta-sheet formation appears to be mediated by the stabilization of alpha-helical states. On the lipid bilayer, these intermediate species have been implicated as cytotoxic through elimination of ionic homeostasis. Recent advances are enabling insights at a molecular level that promise to provide meaningful targets for the development of therapeutics.

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.