Solvent effects on self-assembly of beta-amyloid peptide

Study of the non-covalent interaction between amyloid-beta-peptide and melatonin using electrospray ionization mass spectrometry

Oxidative stress and unregulated immune response are believed to play a key role in the processes inherent to Alzheimer's disease (AD). The fact that free radicals can result in neurodegeneration suggests that actions against reactive oxygen species may be beneficial in treating and preventing AD. In the light of the suggested link between oxidative stress and AD, it is proposed that antioxidants and, even more, endogenous antioxidants may offer a therapeutic regime for protection against the risk of this disease. For this reason, the formation of non-covalent complexes between amyloid-beta-peptide (A beta) or its oxidized forms and melatonin was studied by quadrupole and Fourier transform ion cyclotron resonance electrospray ionization mass spectrometry. The stability of the non-covalent complex was examined under several experimental conditions, such as orifice voltage, pH, presence of organic modifier, concentration and time. Two different digestion protocols combined with mass spectrometric analysis of the resulting peptide fragments were employed in order to locate the binding site of melatonin in A beta.

Structure of A beta(25-35) peptide in different environments

The fragment A beta(25-35) of the Alzheimer's amyloid beta-peptide, like its full-length peptide A beta(1-42), has shown neurotoxic activities in cultured cells. The conformational preference of this important peptide is examined here in solution, gel, and film states (obtained with organic and aqueous solvents) by vibrational circular dichroism spectroscopy for the first time. For comparative studies, vibrational absorption and electronic circular dichroism measurements were also carried out under identical conditions. The peptide was found to adopt beta-sheet and beta-turn structures, with their relative proportions changing in different environments.

The ‘O-acyl isopeptide method’ for the synthesis of difficult sequence-containing peptides: application to the synthesis of Alzheimer's disease-related amyloid β peptide (Aβ) 1-42

An efficient 'O-acyl isopeptide method' for the synthesis of difficult sequence-containing peptides was applied successfully to the synthesis of amyloid beta peptide (Abeta) 1-42 via a water-soluble O-acyl isopeptide of Abeta1-42, i.e. '26-O-acyl isoAbeta1-42' (6). This paper describes the detailed synthesis of Abeta1-42 focusing on the importance of resin selection and the analysis of side reactions in the O-acyl isopeptide method. Protected '26-O-acyl isoAbeta1-42' peptide resin was synthesized using 2-chlorotrityl chloride resin with minimum side reactions in comparison with other resins and deprotected crude 26-O-acyl isoAbeta1-42 was easily purified by HPLC due to its relatively good purity and narrow elution with reasonable water solubility. This suggests that only one insertion of the isopeptide structure into the sequence of the 42-residue peptide can suppress the unfavourable nature of its difficult sequence. The migration of O-acyl isopeptide to intact Abeta1-42 under physiological conditions (pH 7.4) via O--N intramolecular acyl migration reaction was very rapid and no other by-product formation was observed while 6 was stable under storage conditions. These results concluded that our strategy not only overcomes the solubility problem in the synthesis of Abeta1-42 and can provide intact Abeta1-42 efficiently, but is also applicable in the synthesis of large difficult sequence-containing peptides at least up to 50 amino acids. This synthesis method would provide a biological evaluation system in Alzheimer's disease research, in which 26-O-acyl isoAbeta1-42 can be stored in a solubilized form before use and then rapidly produces intact Abeta1-42 in situ during biological experiments.

Mechanism of accelerated assembly of beta-amyloid filaments into fibrils by KLVFFK(6)

Extracellular senile plaques are a central pathological feature of Alzheimer's disease. At the core of these plaques are fibrillar deposits of beta-amyloid peptide (Abeta). In vitro, Abeta spontaneously assembles into amyloid fibrils of cross-beta sheet structure. Although it was once believed that the fibrils themselves were toxic, more recent data supports the hypothesis that aggregation intermediates, rather than fully formed fibrils, are the most damaging to neuronal tissue. In previously published work, we identified several small peptides that interact with Abeta and increase its aggregation rate while decreasing its toxicity. In this work, we examined in detail the interaction between Abeta and one of these peptides. Using a mathematical model of Abeta aggregation kinetics, we show that the dominant effect of the peptide is to accelerate lateral association of Abeta filaments into fibrils.

Selective Cu2+/ascorbate-dependent oxidation of alzheimer's disease beta-amyloid peptides

This review summarizes tandem mass spectrometric investigations on the selectivity of metal-catalyzed oxidation of beta-amyloid peptide (betaAP) and related sequences. A remarkable feature of the Cu(2+)/ascorbate-dependent oxidation of these peptides is the switch from predominantly His oxidation in the neurotoxic peptide betaAP1-40 to predominantly Tyr oxidation in the nonneurotoxic reverse sequence betaAP40-1. Within betaAP1-40, His(13) and His(14) of the high-affinity Cu(2+)-binding site are most sensitive to oxidation. Eventually, the oxidation of one or both of these His residues could result in a less redox-active betaAP-Cu(2+) complex, lowering the incidence of betaAP-Cu(2+)-dependent Fenton-type reactions for the benefit of surrounding biological tissue.

The effect of cholesterol and monosialoganglioside (GM1) on the release and aggregation of amyloid beta-peptide from liposomes prepared from brain membrane-like lipids

In order to investigate the influence of cholesterol (Ch) and monosialoganglioside (GM1) on the release and subsequent deposition/aggregation of amyloid beta peptide (Abeta)-(1-40) and Abeta-(1-42), we have examined Abeta peptide model membrane interactions by circular dichroism, turbidity measurements, and transmission electron microscopy (TEM). Model liposomes containing Abeta peptide and a lipid mixture composition similar to that found in the cerebral cortex membranes (CCM-lipid) have been prepared. In all, four Abeta-containing liposomes were investigated: CCM-lipid; liposomes with no GM1 (GM1-free lipid); those with no cholesterol (Ch-free lipid); liposomes with neither cholesterol nor GM1 (Ch-GM1-free lipid). In CCM liposomes, Abeta was rapidly released from membranes to form a well defined fibril structure. However, for the GM1-free lipid, Abeta was first released to yield a fibril structure about the membrane surface, then the membrane became disrupted resulting in the formation of small vesicles. In Ch-free lipid, a fibril structure with a phospholipid membrane-like shadow formed, but this differed from the well defined fibril structure seen for CCM-lipid. In Ch-GM1-free lipid, no fibril structure formed, possibly because of membrane solubilization by Abeta. The absence of fibril structure was noted at physiological extracellular pH (7.4) and also at liposomal/endosomal pH (5.5). Our results suggest a possible role for both Ch and GM1 in the membrane release of Abeta from brain lipid bilayers.

O?N intramolecular acyl migration reaction in the development of prodrugs and the synthesis of difficult sequence-containing bioactive peptides

N-Ointramolecular acyl migration in Ser- or Thr-containing peptides is a well-known side reaction in peptide chemistry. It results in the mutual conversion of ester and amide bonds. Our medicinal chemistry study focused on the fact that the O-acyl product can be readily converted to the original N-acyl form under neutral or slightly basic conditions in an aqueous buffer and the liberated ionized amino group enhances the water solubility of O-acyl products. Because of this, we have developed a novel class of "O-N intramolecular acyl migration"-type water-soluble prodrugs of HIV-1 protease inhibitors. These prodrugs released the parent drugs via a simple chemical mechanism with no side reaction. In this study, we applied this strategy to important cancer chemotherapeutic agents, paclitaxel and its derivatives, to develop water-soluble taxoid prodrugs, and found that these prodrugs, 2'-O-isoform of taxoids, showed promising results with higher water solubility and proper kinetics in their parent drug formation by a simple pH-dependent chemical mechanism with O-N intramolecular acyl migration. These results suggest that this strategy would be useful in toxicology and medical economics. After the successful application of O-N intramolecular acyl migration in medicinal chemistry, this concept was recently used in peptide chemistry for the synthesis of "difficult sequence-containing peptides." The strategy was based on hydrophilic O-acyl isopeptide synthesis followed by the O-N intramolecular acyl migration reaction, leading to the desired peptide. In a model study with small, difficult sequence-containing peptides, synthesized "O-acyl isopeptides" not only improved the solubility in various media and efficiently performed the high performance liquid chromatography purification, but also altered the nature of the difficult sequence during SPPS, resulting in the efficient synthesis of O-acyl isopeptides with no complications. The subsequent O-N intramolecular acyl migration of purified O-acyl isopeptides afforded the desired peptides as precipitates with high yield and purity. Further study of the synthesis of a larger difficult sequence-containing peptide, Alzheimer's disease-related peptide (A beta 1-42), surprisingly showed that only one insertion of the O-acyl group drastically improved the unfavorable nature of the difficult sequence in A beta 1-42, and achieved efficient synthesis of 26-O-acyl isoA beta 1-42 and subsequent complete conversion to A beta 1-42 via the O-N intramolecular acyl migration reaction of 26-O-acyl isoA beta 1-42. This suggests that our new method based on O-N intramolecular acyl migration is an important method for the synthesis of difficult sequence-containing bioactive peptides.

Contact regions in the dimer of Alzheimer beta-amyloid domain [1-28] studied by mass spectrometry

Information is provided about the amino acid residues in the [1-28] domain of the Alzheimer b- amyloid protein, which participate in interstrand pairing and initiate fibillogenesis. The study was carried out using electrospray ionization on a four sector mass spectrometer, measuring kinetic energy release for a fragmentation process, and modeling the transition state with molecular dynamics calculations. The results eliminate the sequence [11-24] proposed earlier as the central core, and are consistent with, but do not distinguish between, residues [17-28] and [17-23] proposed by others based on biochemical studies.

pH-dependent amyloid and protofibril formation by the ABri peptide of familial British dementia

The ABri is a 34 residue peptide that is the major component of amyloid deposits in familial British dementia. In the amyloid deposits, the ABri peptide adopts aggregated beta-pleated sheet structures, similar to those formed by the Abeta peptide of Alzheimer's disease and other amyloid forming proteins. As a first step toward elucidating the molecular mechanisms of the beta-amyloidosis, we explored the ability of the environmental variables (pH and peptide concentration) to promote beta-sheet fibril structures for synthetic ABri peptides. The secondary structures and fibril morphology were characterized in parallel using circular dichroism, atomic force microscopy, negative stain electron microscopy, Congo red, and thioflavin-T fluorescence spectroscopic techniques. As seen with other amyloid proteins, the ABri fibrils had characteristic binding with Congo red and thioflavin-T, and the relative amounts of beta-sheet and amyloid fibril-like structures are influenced strongly by pH. In the acidic pH range 3.1-4.3, the ABri peptide adopts almost exclusively random structure and a predominantly monomeric aggregation state, on the basis of analytical ultracentrifugation measurements. At neutral pH, 7.1-7.3, the ABri peptide had limited solubility and produced spherical and amorphous aggregates with predominantly beta-sheet secondary structure, whereas at slightly acidic pH, 4.9, spherical aggregates, intermediate-sized protofibrils, and larger-sized mature amyloid fibrils were detected by atomic force microscopy. With aging at pH 4.9, the protofibrils underwent further association and eventually formed mature fibrils. The presence of small amounts of aggregated peptide material or seeds encourage fibril formation at neutral pH, suggesting that generation of such seeds in vivo could promote amyloid formation. At slightly basic pH, 9.0, scrambling of the Cys5-Cys22 disulfide bond occurred, which could lead to the formation of covalently linked aggregates. The presence of the protofibrils and the enhanced aggregation at slightly acidic pH is consistent with the behavior of other amyloid-forming proteins, which supports the premise that a common mechanism may be involved in protein misfolding and beta-amyloidosis.

Influence of Hydrophobic Teflon Particles on the Structure of Amyloid β-Peptide

The amyloid beta-protein (Abeta) constitutes the major peptide component of the amyloid plaque deposits of Alzheimer's disease in humans. The Abeta changes from a nonpathogenic to a pathogenic conformation resulting in self-aggregation and deposition of the peptide. It has been established that denaturing factors (such as the interaction with membranes) are involved in the structural transition. This work is aimed at determining the effect of hydrophobic Teflon on the conformation of the Abeta (1-40). Prior to adsorption, the secondary structure and self-aggregation state of the Abeta in solution were established as a function of pH. Three different species coexist: unordered monomers/dimers, small oligomers in mainly a regular beta-sheet structure, and bigger aggregates having a twisted beta-sheet conformation. Transferring the Abeta from the solution to the Teflon surface strongly promotes alpha-helix formation. Furthermore, increasing the degree of coverage of the Teflon by the Alphabeta protein leads to a conformational change toward a more enriched beta-sheet structure.

Dimethyl sulfoxide delays PrP sc accumulation and disease symptoms in prion-infected hamsters

PrP(Sc), an aberrantly folded protein, is the only identified component of the prion, an agent causing fatal neurodegenerative diseases such as scrapie and bovine spongiform encephalopathy. Dimethyl sulfoxide (DMSO) has been shown to reduce the accumulation of PrP(Sc) in scrapie-infected (ScN2a) cells, and to inhibit its aggregation in vitro. In humans, DMSO was used successfully in the treatment of various peripheral amyloidotic diseases. Here we show that administration of DMSO to scrapie-infected hamsters significantly prolonged disease incubation time, as well as delayed the accumulation of PrP(Sc) in Syrian hamster brains. Interestingly, administration of DMSO to scrapie sick hamsters resulted in increased clearance of protease-resistant PrP in their urine. We conclude that although DMSO by itself may not be sufficient to cure prion diseases, it may be considered as a component in a 'cocktail' drug approach for these disorders. Also, urine PrP testing should be considered for the assessment of treatment efficacy.

Concentration effect on the aggregation of a self-assembling oligopeptide

Concentration is a key parameter in controlling the aggregation of self-assembling oligopeptides. By investigating the concentration effects, an aggregation mechanism of EAK16-II is proposed. Depending on the critical aggregation concentration (CAC) of EAK16-II, the oligopeptide aggregates into protofibrils through seeding and/or a nucleation process. Protofibrils then associate with each other to form fibrils. The CAC was found to be approximately 0.1 mg/ml by surface tension measurements. The nanostructures of aggregates were imaged and analyzed by atomic force microscopy. Globular and fibrillar aggregates were observed, and their dimensions were further quantified. To ensure that the aggregates were formed in bulk solution, light scattering (LS) measurements were conducted to monitor the fibril formation with time. The LS profile showed two different rates of aggregation depending on whether the peptide concentration was above or below the CAC. At high concentrations, the LS intensity increased strongly at early times. At low concentrations, the LS intensity increased only slightly. Our study provides information about the nature of the oligopeptide self-assembly, which is important to the understanding of the fibrillogenesis occurring in conformational diseases and to many biomedical engineering applications.

In vitro characterization of conditions for amyloid-beta peptide oligomerization and fibrillogenesis

Extensive research causally links amyloid-beta peptide (A beta) to Alzheimer's disease, although the pathologically relevant A beta conformation remains unclear. A beta spontaneously aggregates into the fibrils that deposit in senile plaques. However, recent in vivo and in vitro reports describe a potent biological activity for oligomeric assemblies of A beta. To consistently prepare in vitro oligomeric and fibrillar forms of A beta 1-42, a detailed knowledge of how solution parameters influence structure is required. This manuscript represents the first study using a single chemically and structurally homogeneous unaggregated starting material to demonstrate that the formation of oligomers, fibrils, and fibrillar aggregates is determined by time, concentration, temperature, pH, ionic strength, and A beta species. We recently reported that oligomers inhibit neuronal viability 10-fold more than fibrils and approximately 40-fold more than unaggregated peptide, with oligomeric A beta 1-42-induced neurotoxicity significant at 10 nm. In addition, we were able to differentiate by structure and neurotoxic activity wild-type A beta1-42 from isoforms containing familial mutations (Dahlgren, K. N., Manelli, A. M., Stine, W. B., Jr., Baker, L. K., Krafft, G. A., and LaDu, M. J. (2002) J. Biol. Chem. 277, 32046-32053). Understanding the biological role of specific A beta conformations may define the link between A beta and Alzheimer's disease, re-focusing therapeutic approaches by identifying the pernicious species of A beta ultimately responsible for the cognitive dysfunction that defines the disease.

Methionine 35 oxidation reduces fibril assembly of the amyloid abeta-(1-42) peptide of Alzheimer's disease

The major component of amyloid plaques in Alzheimer's disease (AD) is Abeta, a small peptide that has high propensity to assemble as aggregated beta-sheet structures. Using three well established techniques for studying amyloid structure, namely circular dichroism, thioflavin-T fluorescence, and atomic force microscopy, we demonstrate that oxidation of the Met-35 side chain to a methionine sulfoxide (Met-35(ox)) significantly hinders the rate of fibril formation for the 42-residue Abeta-(1-42) at physiological pH. Met-35(ox) also alters the characteristic Abeta fibril morphology and prevents formation of the protofibril, which is a key intermediate in beta-amyloidosis and the associated neurotoxicity. The implications of these results for the biological function and role of Abeta with oxidative stress in AD are discussed.

Effect of secondary structure on the potential of mean force for poly-L-lysine in the alpha-helix and beta-sheet conformations

Because poly-L-lysine (PLL) can exist in the alpha-helix or beta-sheet conformation depending on solution preparation and solution conditions, PLL is a suitable candidate to probe the dependence of protein interactions on secondary structure. The osmotic second virial coefficient and weight-average molecular weight are reported from low-angle laser-light scattering measurements for PLL as a function of NaCl concentration, pH, and alpha-helix or beta-sheet content. Interactions between PLL molecules become more attractive as salt concentration increases due to screening of PLL charge by salt ions and at low salt concentration become more attractive as pH increases due to decreased net charge on PLL. The experimental results show that interactions are stronger for the beta-sheet conformation than for the alpha-helix conformation. A spherically-symmetric model for the potential of mean force is used to account for specific interactions not described by DLVO theory and to show how differences in secondary structure affect PLL interactions.

Development of a novel diffusion-based method to estimate the size of the aggregated Abeta species responsible for neurotoxicity

beta-Amyloid peptide (Abeta) is the primary protein component of senile plaques in Alzheimer's disease and is believed to be responsible for the neurodegeneration associated with the disease. Abeta is toxic only when aggregated, however, the size and structure of the aggregated species associated with toxicity is unknown. In the present study, we developed a diffusion-based method to simultaneously separate and detect the biological activity of toxic Abeta oligomers and used the method to examine the relationship between size of aggregated protein and toxicity to SH-SY5Y cells. From these measurements, the effective diffusivity and hydrodynamic radius of the toxic oligomeric species of Abeta could be determined. A sensitivity analysis was performed to examine the effects of model assumptions used in data analysis on the effective diffusivity calculated. The method provides a new estimate of the size of small toxic Abeta species associated with fibril formation. This work contributes to our understanding of the relationship between Abeta structure and toxicity and with further refinements may aid in our ability to design agents which alter the Abeta aggregation/dissociation processes associated with neurotoxicity.

Structural, kinetic and cytotoxicity aspects of 12-28 beta-amyloid protein fragment: a reappraisal

A chemical, structural and biological study on the beta-amyloid peptide beta12-28 is reported which was carried out in order to assess the feasibility using this peptide fragment as a model of the natural beta-amyloid protein. The aggregation properties of beta12-28 have been investigated by pulse field-gradient NMR spectroscopy, Fourier transform infrared spectroscopy and transmission electron microscopy. The results obtained suggest that beta12-28 behaviour is comparable to that of the natural beta-amyloid protein although kinetically slower. Translational diffusion coefficients obtained by NMR on an aged beta12-28 solution suggest that the soluble peptide fraction is composed of oligomeric intermediates adopting an extended ellipsoidal assembly rather than a spherical one. The beta12-28 peptide proved to be cytotoxic in PC12 cell cultures as monitored by the MTT assay, although a lack of reproducibility was observed in the dose-response experiments.

4,4(')-Dianilino-1,1(')-binaphthyl-5,5(')-disulfonate: report on non-beta-sheet conformers of Alzheimer's peptide beta(1-40)

The venerable fluorescent probe of protein hydrophobic regions, 4,4(')-dianilino-1,1(')-binaphthyl-5,5(')-disulfonate (bis-ANS), unexpectedly increases in fluorescence with soluble beta(1-40) in acidic buffer solutions but reacts weakly with amyloid fibrils while other hydrophobic probes react with the fibrils. CD analysis correlates reaction with the probe with random coil/mixed conformations and alpha-helical forms of beta(1-40) in buffer solutions but less so with soluble beta-sheet forms or amyloid fibrils. The kinetics of the fluoroalcohol-induced interconversion of conformers can be followed by changes in bis-ANS fluorescence. Formation of the beta-sheet form in aqueous buffer is limited by a slow component (minutes) while fluoroalcohol-promoted changes between beta-sheet and alpha-helix occur over seconds. Variants of beta(1-40) such as beta(1-42) or the Dutch E22Q mutation of beta(1-40) and fragments beta(1-28), beta(12-28), beta(10-20 amide), and beta(10-35 amide) react with bis-ANS under conditions that do not support fibril formation. Primary amino acid sequence is important as beta(1-11) does not cause bis-ANS fluorescence while beta(1-16) does, but hydrophobicity is not as beta(25-35) and beta(15-20 amide) are unreactive. bis-ANS is a useful biophysical tool for characterizing particular, but not all, soluble Abeta conformations distinct from the fibrillar form of amyloid peptides detected by Thioflavin T.

Laminin affects polymerization, depolymerization and neurotoxicity of Abeta peptide

Amyloid deposition in Alzheimer fibrils forms neurotoxic senile plaques in a process that may be modulated by associated proteins. In this work we demonstrate the ability of laminin-1 and laminin-2 to inhibit fibril formation and toxicity on cultured rat hippocampal neurons. We confirm that the laminin-1-derived peptide YFQRYLI inhibits efficiently both fibril formation and neurotoxicity and show that the IKVAV peptide inhibits amyloid neurotoxicity despite its slight inhibition of fibril formation. On other hand, laminin-1 induces disaggregation of preformed fibrils in vitro, characterized as a progressive disassembly of fibrils into protofibrils and further clearance of these latter species, leading to a continual inhibition of amyloid neurotoxicity.

Oxidation of methionine 35 attenuates formation of amyloid beta -peptide 1-40 oligomers

Amyloid plaques formed by aggregation of the amyloid beta-peptide (Abeta) are an intrinsic component of Alzheimer disease pathogenesis. It has been suggested that oxidation of methionine 35 in Abeta has implications for Alzheimer disease, and it has been shown that oxidation of Met-35 significantly inhibits aggregation in vitro. In this study, the aggregational properties of Abeta-(1-40) before and after Met-35 oxidation were investigated using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. The results show that Abeta-(1-40)Met-35(O) trimer and tetramer formation is significantly attenuated as compared with Abeta-(1-40). This suggests that oxidation of Met-35 inhibits a conformational switch in Abeta-(1-40) necessary for trimer but not dimer formation. Random incorporation of Abeta-(1-40) and Abeta-(1-40)Met-35(O) in homo- and heterooligomers could also be observed. This is the first report of an early rate-limiting step in Abeta-(1-40) aggregation. Slowing of the fibrillization process at this early step is likely to support prolonged solubility and clearance of Abeta from brain and may reduce disease progression.

Islet amyloid and type 2 diabetes: from molecular misfolding to islet pathophysiology

Islet amyloid polypeptide (IAPP, amylin) is secreted from pancreatic islet beta-cells and converted to amyloid deposits in type 2 diabetes. Conversion from soluble monomer, IAPP 1-37, to beta-sheet fibrils involves changes in the molecular conformation, cellular biochemistry and diabetes-related factors. In addition to the recognised amyloidogenic region, human IAPP (hIAPP) 20-29, the peptides human or rat IAPP 30-37 and 8-20, assume beta-conformation and form fibrils. These three amyloidogenic regions of hIAPP can be modelled as a folding intermediate with an intramolecular beta-sheet. A hypothesis is proposed for co-secretion of proIAPP with proinsulin in diabetes and formation of a 'nidus' adjacent to islet capillaries for subsequent accumulation of secreted IAPP to form the deposit. Although intracellular fibrils have been identified in experimental systems, extracellular deposition predominates in animal models and man. Extensive fibril accumulations replace islet cells. The molecular species of IAPP that is cytotoxic remains controversial. However, since fibrils form invaginations in cell membranes, small non-toxic IAPP fibrillar or amorphous accumulations could affect beta-cell stimulus-secretion coupling. The level of production of hIAPP is important but not a primary factor in islet amyloidosis; there is little evidence for inappropriate IAPP hypersecretion in type 2 diabetes and amyloid formation is generated in transgenic mice overexpressing the gene for human IAPP only against a background of obesity. Animal models of islet amyloidosis suggest that diabetes is induced by the deposits whereas in man, fibril formation appears to result from diabetes-associated islet dysfunction. Islet secretory failure results from progressive amyloidosis which provides a target for new therapeutic interventions.

Reduction in cholesterol and sialic acid content protects cells from the toxic effects of beta-amyloid peptides

beta-Amyloid (Abeta) is the primary protein component of senile plaques associated with Alzheimer's disease and has been implicated in the neurotoxicity associated with the disease. A variety of evidence points to the importance of Abeta-membrane interactions in the mechanism of Abeta neurotoxicity and indicates that cholesterol and gangliosides are particularly important for Abeta aggregation and binding to membranes. We investigated the effects of cholesterol and sialic acid depletion on Abeta-induced GTPase activity in cells, a step implicated in the mechanism of Abeta toxicity, and Abeta-induced cell toxicity. Cholesterol reduction and depletion of membrane-associated sialic acid residues both significantly reduced the Abeta-induced GTPase activity. In addition, cholesterol and membrane-associated sialic acid residue depletion or inhibition of cholesterol and ganglioside synthesis protected PC12 cells from Abeta-induced toxicity. These results indicate the importance of Abeta-membrane interactions in the mechanism of Abeta toxicity. In addition, these results suggest that control of cellular cholesterol and/or ganglioside content may prove useful in the prevention or treatment of Alzheimer's disease.

Aspirin and non-steroidal anti-inflammatory drugs inhibit amyloid-beta aggregation

The neurotoxic and proinflammatory actions of the Alzheimer peptide amyloid-beta (Abeta) are dependent on its aggregation and beta-sheet conformation. Chronic use of non-steroidal anti-inflammatory drugs (NSAIDs) such as aspirin for arthritis decreases the risk of developing Alzheimer's disease (AD) by unknown mechanisms. We report that these drugs inhibit human Abeta aggregation in vitro and reverse the beta-sheet conformation of preformed fibrils at clinically relevant doses. Aspirin prevented enhanced Abeta aggregation by aluminum, an environmental risk factor for AD. This anti-aggregatory effect was restricted to NSAIDs and was not exhibited by other drugs used in AD therapy. NSAIDS may have a role in the prevention and treatment of AD, in addition to a number of age-related disorders such as arthritis, cardiovascular disease and cancer.

A mathematical model of the kinetics of beta-amyloid fibril growth from the denatured state

Spontaneous conversion of beta-amyloid peptide (Abeta) from soluble monomer to insoluble fibril may underlie the neurodegeneration associated with Alzheimer's disease. A complete description of Abeta self-association kinetics requires identification of the oligomeric species present and the pathway of association, as well as quantitation of rate constants and reaction order. Abeta was rendered monomeric and denatured by dissolution in 8 M urea, pH 10. "Refolding" and fibrillization were initiated by rapid dilution into phosphate-buffered saline, pH 7.4. The kinetics of growth were followed at three different concentrations, using size exclusion chromatography, dynamic light scattering, and static light scattering. A multi-step pathway for fibril formation and growth was postulated. This pathway included 1) rapid commitment to either stable monomer/dimer or unstable intermediate, 2) cooperative association of intermediate into a multimeric "nucleus," 3) elongation of the "nucleus" into filaments via addition of intermediate, 4) lateral aggregation of filaments into fibrils, and 5) fibril elongation via end-to-end association. Differential and algebraic equations describing this kinetic pathway were derived, and model parameters were determined by fitting the data. The utility of the model for identifying toxic Abeta oligomeric specie(s) is demonstrated. The model should prove useful for designing compounds that inhibit Abeta aggregation and/or toxicity.

Oxidative and hydrolytic properties of beta-amyloid

beta-Amyloid protein is the major component of senile plaques found in the brains of Alzheimer's patients. Previously, a new biochemical property of amyloid, its ability to disrupt ester and peptide bonds, was described [Elbaum, D., Brzyska, M., Bacia, A. & Alkon, D. (2000) Biochem. Biophys. Res. Commun. 267, 733-738]. In the present work we compare the ability of beta-amyloid to hydrolyse and oxidize model fluorescent derivatives of dichlorofluorescein [dichlorodihydrofluorescein (H2DCF) or dichlorofluorescein diacetate (DCF-DA), respectively] to the same final product (dichlorofluorescein). Although there is accumulating evidence of oxidative properties of beta-amyloid, little is known about its hydrolytic abilities. Chemical modification studies revealed that hydrolytic properties are related to a His, Ser and Asp/Glu triad, while residues of His, Tyr and Met are involved in the oxidative activity of amyloid. Studies with the rat homologue of human beta-amyloid (1-40), containing three amino-acid substitutions (Arg5-->Gly, Tyr10-->Phe and His13-->Arg) confirmed a role of His in the studied processes. Reduction of the hydrolysis product caused by inhibitors of Ser esterases (phenylmethylsulphonyl fluoride and eserine) suggests that beta-amyloid-mediated hydrolysis is Ser sensitive. Antioxidants and metal chelators that reduced H2DCF oxidation did not change or increase DCF-DA hydrolysis. Solvent isotope effects suggest the involvement of hydrogen bonds in the hydrolysis reaction. Hydrolysis was inhibited by redox-active metal ions and was practically oxygen independent while the oxidation process was redox-active-metal enhanced [Cu(II) and Fe(II) primarily], and oxygen dependent. Product formation was significantly inhibited by catalase and superoxide dismutase as well as benzoquinone, a specific superoxide anion radical scavenger. Increase of fluorescence by oxidation was strongly inhibited by azide and His and enhanced in samples prepared with deuterated phosphate buffer, suggesting singlet oxygen intermediacy. Our data are consistent with superoxide-mediated singlet oxygen intermediate in this Fenton mechanism-driven reaction. These results indicate that hydrolytic and oxidative properties of beta-amyloid are distinct features of this peptide and probably require different mechanisms to occur, but both of them may contribute to beta-amyloid toxicity.

Models of amyloid seeding in Alzheimer's disease and scrapie: mechanistic truths and physiological consequences of the time-dependent solubility of amyloid proteins

Ordered protein aggregation in the brain is a hallmark of Alzheimer's disease and scrapie. The disease-specific amyloid fibrils comprise primarily a single protein, amyloid beta, in Alzheimer's disease, and the prion protein in scrapie. These proteins can be induced to form aggregates in vitro that are indistinguishable from brain-derived fibrils. Consequently, much effort has been invested in the development of in vitro model systems to study the details of the aggregation processes and the effects of endogenous molecules that have been implicated in disease. Selected studies of this type are reviewed herein. A simple mechanistic model has emerged for both processes that involves a nucleation-dependent polymerization. This mechanism dictates that aggregation is dependent on protein concentration and time. Furthermore, amyloid formation can be seeded by a preformed fibril. The physiological consequences of this mechanism are discussed.

Temperature-induced conformational changes in amyloid beta(1-40) peptide investigated by simultaneous FT-IR microspectroscopy with thermal system

Temperature-dependent secondary structures of the amyloid beta(1-40) peptide in the solid state were studied by simultaneous Fourier transform infrared/differential scanning calorimetry (FT-IR/DSC) microspectroscopic system with the heating-cooling-reheating cycle. The result indicates that a thermal transition temperature at 45 degrees C was easily obtained from the three-dimensional plot of the transmission FT-IR spectra as a function of temperature. Furthermore, the thermal-dependent conformational transformations, due to denaturation and aggregation, of solid amyloid beta(1-40) were mainly evidenced by reducing the compositions from 37 to 20-24% for alpha-helical and random coil structures but increasing the components from 27 to 45% for intermolecular beta-sheet structures. Thermal-irreversible behavior and a poor thermal stability of solid amyloid beta(1-40) were also observed from the poor restoration of the secondary conformational changes in the heated sample.

Human platelet calcium mobilisation in response to beta-amyloid (25-35): buffer dependency and unchanged response in Alzheimer's disease

In the present study, the effects of beta-amyloid (25-35) (Abeta (25-35)) upon calcium signalling by the human platelet has been investigated. When assays were conducted using HEPES buffers, Abeta (25-35), but not the inactive peptide Abeta (35-25), produced a robust increase in intracellular calcium that remained after removal of extracellular calcium but was abolished by the phospholipase C inhibitor U-73122. There was no significant difference between the calcium response to Abeta (25-35) in platelets from patients with Alzheimer's disease and from age-matched controls. In contrast to the robust effects on calcium mobilisation in HEPES buffers, very little calcium response to Abeta (25-35) was seen when Krebs (pH 7.8) buffer was used.

The role of G protein activation in the toxicity of amyloidogenic Abeta-(1-40), Abeta-(25-35), and bovine calcitonin

More than 16 different proteins have been identified as amyloid in clinical diseases; among these, beta-amyloid (Abeta) of Alzheimer's disease is the best characterized. In the present study, we performed experiments with Abeta and calcitonin, another amyloid-forming peptide, to examine the role of G protein activation in amyloid toxicity. We demonstrated that the peptides, when prepared under conditions that promoted beta-sheet and amyloid fibril (or protofibril) formation, increased high affinity GTPase activity, but the nonamyloidogenic peptides had no discernible effects on GTP hydrolysis. These increases in GTPase activity were correlated to toxicity. In addition, G protein inhibitors significantly reduced the toxic effects of the amyloidogenic Abeta and calcitonin peptides. Our results further indicated that the amyloidogenic peptides significantly increased GTPase activity of purified Galpha(o) and Galpha(i) subunits and that the effect was not receptor-mediated. Collectively, these results imply that the amyloidogenic structure, regardless of the actual peptide or protein sequence, may be sufficient to cause toxicity and that toxicity is mediated, at least partially, through G protein activation. Our abilities to manipulate G protein activity may lead to novel treatments for Alzheimer's disease and the other amyloidoses.

Ultrastructural organization of amyloid fibrils by atomic force microscopy

Atomic force microscopy has been employed to investigate the structural organization of amyloid fibrils produced in vitro from three very different polypeptide sequences. The systems investigated are a 10-residue peptide derived from the sequence of transthyretin, the 90-residue SH3 domain of bovine phosphatidylinositol-3'-kinase, and human wild-type lysozyme, a 130-residue protein containing four disulfide bridges. The results demonstrate distinct similarities between the structures formed by the different classes of fibrils despite the contrasting nature of the polypeptide species involved. SH3 and lysozyme fibrils consist typically of four protofilaments, exhibiting a left-handed twist along the fibril axis. The substructure of TTR(10-19) fibrils is not resolved by atomic force microscopy and their uniform appearance is suggestive of a regular self-association of very thin filaments. We propose that the exact number and orientation of protofilaments within amyloid fibrils is dictated by packing of the regions of the polypeptide chains that are not directly involved in formation of the cross-beta core of the fibrils. The results obtained for these proteins, none of which is directly associated with any human disease, are closely similar to those of disease-related amyloid fibrils, supporting the concept that amyloid is a generic structure of polypeptide chains. The detailed architecture of an individual fibril, however, depends on the manner in which the protofilaments assemble into the fibrillar structure, which in turn is dependent on the sequence of the polypeptide and the conditions under which the fibril is formed.

The role of prion peptide structure and aggregation in toxicity and membrane binding

Prion diseases are neurodegenerative disorders associated with a conformational change in the normal cellular isoform of the prion protein, PrP(C), to an abnormal scrapie isoform, PrP(SC). Unlike the alpha-helical PrP(C), the protease-resistant core of PrP(SC) is predominantly beta-sheet and possesses a tendency to polymerize into amyloid fibrils. We performed experiments with two synthetic human prion peptides, PrP(106-126) and PrP(127-147), to determine how peptide structure affects neurotoxicity and protein-membrane interactions. Peptide solutions possessing beta-sheet and amyloid structures were neurotoxic to PC12 cells in vitro and bound with measurable affinities to cholesterol-rich phospholipid membranes at ambient conditions, but peptide solutions lacking stable beta-sheet structures and amyloid content were nontoxic and possessed less than one tenth of the binding affinities of the amyloid-containing peptides. Regardless of structure, the peptide binding affinities to cholesterol-depleted membranes were greatly reduced. These results suggest that the beta-sheet and amyloid structures of the prion peptides give rise to their toxicity and membrane binding affinities and that membrane binding affinity, especially in cholesterol-rich environments, may be related to toxicity. Our results may have significance in understanding the role of the fibrillogenic cerebral deposits associated with some of the prion diseases in neurodegeneration and may have implications for other amyloidoses.

An improved method of preparing the amyloid beta-protein for fibrillogenesis and neurotoxicity experiments

Synthetic amyloid beta-protein (A beta) is used widely to study fibril formation and the physiologic effects of low molecular weight and fibrillar forms of the peptide on cells in culture or in experimental animals. Not infrequently, conflicting results have arisen in these studies, in part due to variation in the starting conformation and assembly state of A beta. To avoid these problems, we sought a simple, reliable means of preparing A beta for experimental use. We found that solvation of synthetic peptide with sodium hydroxide (A beta x NaOH), followed by lyophilization, produced stocks with superior solubility and fibrillogenesis characteristics. Solubilization of the pretreated material with neutral buffers resulted in a pH transition from approximately 10.5 to neutral, avoiding the isoelectric point of A beta (pI approximately 5.5), at which A beta precipitation and aggregation propensity are maximal. Relative to trifluoroacetate (A beta x TFA) or hydrochloric acid (A beta x HCl) salts of A beta, yields of "low molecular weight A beta" (monomers and/or dimers) were improved significantly by NaOH pretreatment. Time-dependent changes in circular dichroism spectra and Congo red dye-binding showed that A beta x NaOH formed fibrils more readily than did the other A beta preparations and that these fibrils were equally neurotoxic. NaOH pretreatment thus offers advantages for the preparation of A beta for biophysical and physiologic studies.

Alzheimer's amyloid fibrils: structure and assembly

Structural studies of Alzheimer's amyloid fibrils have revealed information about the structure at different levels. The amyloid-beta peptide has been examined in various solvents and conditions and this has led to a model by which a conformational switching occurs from alpha-helix or random coil, to a beta-sheet structure. Amyloid fibril assembly proceeds by a nucleation dependent pathway leading to elongation of the fibrils. Along this pathway small oligomeric intermediates and short fibrillar structures (protofibrils) have been observed. In cross-section the fibril appears to be composed of several subfibrils or protofilaments. Each of these protofilaments is composed of beta-sheet structure in which hydrogen bonding occurs along the length of the fibre and the beta-strands run perpendicular to the fibre axis. This hierarchy of structure is discussed in this review.

Design of peptides undergoing self-catalytic alpha-to-beta transition and amyloidogenesis

Improved understanding of amyloidogenic peptides and proteins such as prion proteins and Alzheimer's beta peptides has attracted much attention to the elucidation of the molecular mechanisms of such amyloidogenesis. As a representative, in the prion protein, the conformational transitions from alpha-helix to beta-structure undergo along with the amyloidogenesis in a self-catalytic manner. Moreover, recent studies by the de novo design of peptides and proteins as well as the amyloidogenesis of peptides and proteins including pathogenic protein mutants have provided insight into the conformational changes essential to amyloidogenesis and correct folding.

Glutamine synthetase, hemoglobin alpha-chain, and macrophage migration inhibitory factor binding to amyloid beta-protein: their identification in rat brain by a novel affinity chromatography and in Alzheimer's disease brain by immunoprecipitation

Proteins binding to amyloid beta-protein (Abeta) may modulate the accumulation of Abeta in Alzheimer's disease (AD) brain. We developed a monomeric Abeta column for isolation of the proteins binding to Abeta from rat brain. By amino acid sequence analysis and immunoreactivity with specific antibodies, we identified three new Abeta-binding proteins, glutamine synthetase, hemoglobin alpha-chain, and macrophage migration inhibitory factor as well as serum albumin, beta-tubulin, and glyceraldehyde-3-phosphate dehydrogenase already identified as proteins bound to amyloid beta-protein precursor. In addition, the retained fraction contained both apolipoprotein E and alpha(1)-antichymotrypsin already known as Abeta binding proteins. Furthermore, we detected the complexes of these new binding proteins with Abeta in a soluble fraction of the cerebral cortex of AD brain by immunoprecipitation. Our results suggest that these binding proteins also associate with Abeta, leading to the clearance or the accumulation of Abeta and the neuronal cell damage in human brain.

Probing the kinetics of beta-amyloid self-association

Spontaneous conversion of beta-amyloid peptide (Abeta) from soluble monomer to insoluble fibrillar precipitate may underlie the neurodegeneration associated with Alzheimer's disease. A complete description of Abeta self-association kinetics requires identification of the oligomeric species present and the pathway of association, as well as quantitation of rate constants and reaction order. Previously published work delineating progress in developing quantitative kinetic models is reviewed briefly. The use of light scattering to probe Abeta fibril size and shape as a function of time and concentration is described. Data describing Abeta self-association at physiological pH starting from the urea-denatured state are presented. High-molecular-weight species formed rapidly, within minutes of dilution. At 70 microM, the size of the major fibrillar species was virtually constant with time, whereas at 280 microM two populations were detected, each of which increased in size with time. Incipient precipitation, prior to the appearance of a visible solid phase, was detected reliably by dynamic light scattering. Surprisingly, precipitates appeared earliest at the lowest concentration, and the physical state of the precipitate changed markedly with concentration. A gel diffusion assay showed that monomer Abeta dissociates from preformed fibrils, indicating that reversible heterogeneous association must be incorporated into any realistic model of Abeta association.

beta-Amyloid(25-35) inhibits the activity of inositol(1,4,5)-trisphosphate-5-phosphatase

beta-amyloid (A beta) peptides are known to disrupt calcium homeostasis in cells. In the present study, the effects of A beta(25-35) upon the activity of soluble Ins(1,4,5)P3-5-phosphatase have been investigated. A beta(25-35) inhibited, and dithiothreitol (DTT) increased the activity of soluble rat cerebellar Ins(1,4,5)P3-5-phosphatase. The change in activity was not accompanied by an obvious change in the sensitivity of the Ins(1,4,5)P3-5-phosphatase to inhibition by glucose-6-phosphate or phytic acid. A beta(35-25) also inhibited soluble Ins(1,4,5)P3-5-phosphatase activity, but at a lower potency than A beta(25-35). It is concluded that A beta(25-35) affects the metabolism of Ins(1,4,5)P3 although the potency is not sufficiently high to contribute to any significant extent to the effects of this peptide upon calcium homeostasis.

Studies on the in vitro assembly of a beta 1-40: implications for the search for a beta fibril formation inhibitors

The progressive deposition of the amyloid beta peptide (Abeta) in fibrillar form is a key feature in the development of the pathology in Alzheimer's disease (AD). We have characterized the time course of Abeta fibril formation using a variety of assays and under different experimental conditions. We describe in detail the morphological development of the Abeta polymerization process from pseudo-spherical structures and protofibrils to mature thioflavin-T-positive/Congo red-positive amyloid fibrils. Moreover, we structurally characterize the various polymorphic fibrillar assemblies using transmission electron microscopy and determine their mass using scanning transmission electron microscopy. These results provide the framework for future investigations into how target compounds may interfere with the polymerization process. Such substances might have a therapeutic potential in AD.

Inhibition of beta-amyloid-induced neurotoxicity by imidazopyridoindoles derived from a synthetic combinatorial library

Alzheimer's disease is a progressive neurodegenerative disorder characterized by the deposit of amyloid fibrils in the brain that result from the self-aggregative polymerization of the beta-amyloid peptide (Abeta). Evidence of a direct correlation between the ability of Abeta to form stable aggregates in aqueous solution and its neurotoxicity has been reported. The cytotoxic effects of Abeta have been attributed to the aggregation properties of a domain corresponding to the peptide fragment Abeta25-35. In an effort to generate novel inhibitors of Abeta neurotoxicity and/or aggregation, a mixture-based synthetic combinatorial library composed of 23 375 imidazopyridoindoles was generated and screened for inhibition of Abeta25-35 neurotoxicity toward the rat pheochromocytoma PC-12 cell line. The effect of the identified lead compounds on Abeta25-35 aggregation was then evaluated by means of circular dichroism (CD) and thioflavin-T fluorescence spectroscopy. Their activity against Abeta1-42 neurotoxicity toward the PC-12 cell line was also determined. The most active imidazopyridoindoles inhibited both Abeta25-35 and Abeta1-42 neurotoxicity in the low- to mid-micromolar range. Furthermore, inhibition of the random coil to beta-sheet transition and self-aggregation of Abeta25-35 was observed by CD and fluorescence spectroscopy, supporting the relationship between inhibition of the Abeta aggregation process and neurotoxicity.

Interactions of proteins in aqueous electrolyte solutions from fluorescence anisotropy and circular-dichroism measurements

Understanding aqueous protein-protein interactions is crucial for the development of a molecular-thermodynamic model for salt-induced protein precipitation. In addition, protein interactions are important in many disease states, including cataract formation and alpha-amyloid diseases. Fluorescence anisotropy provides a means to measure intermolecular interactions. In this work, monomer-dimer equilibrium of the peptide T4 LYS(11-36) was studied by fluorescence anisotropy over the pH range 4-7 and the NaCl concentration range 0.0-1.0 M, in a 25 mM sodium phosphate buffer. This 26 amino-acid peptide is derived from the beta-sheet region of the T4 lysozyme molecule and has the potential to form amyloid fibrils. The association constant for dimerization increases with rising pH and ionic strength. The potential of mean force for peptide-peptide interactions was calculated from these association constants. Circular-dichroism measurements show that the peptide becomes more structured as the pH rises, possibly contributing to increased association.

Identification of a penta- and hexapeptide of islet amyloid polypeptide (IAPP) with amyloidogenic and cytotoxic properties

Pancreatic amyloid is found in more than 95 % of type II diabetes patients. Pancreatic amyloid is formed by the aggregation of islet amyloid polypeptide (hIAPP or amylin), which is a 37-residue peptide. Because pancreatic amyloid is cytotoxic, it is believed that its formation is directly associated with the development of the disease. We recently showed that hIAPP amyloid formation follows the nucleation-dependent polymerization mechanism and proceeds via a conformational transition of soluble hIAPP into aggregated beta-sheets. Here, we report that the penta- and hexapeptide sequences, hIAPP(23-27) (FGAIL) and hIAPP(22-27) (NFGAIL) of hIAPP are sufficient for the formation of beta-sheet-containing amyloid fibrils. Although these two peptides differ by only one amino acid residue, they aggregate into completely different fibrillar assemblies. hIAPP(23-27) (FGAIL) fibrils self-assemble laterally into unusually broad ribbons, whereas hIAPP(22-27) (NFGAIL) fibrils coil around each other in a typical amyloid fibril morphology. hIAPP(20-27) (SNNFGAIL) also aggregates into beta-sheet-containing fibrils, whereas no amyloidogenicity is found for hIAPP(24-27) (GAIL), indicating that hIAPP(23-27) (FGAIL) is the shortest fibrillogenic sequence of hIAPP. Insoluble amyloid formation by the partial hIAPP sequences followed kinetics that were consistent with a nucleation-dependent polymerization mechanism. hIAPP(22-27) (NFGAIL), hIAPP(20-27) (SNNFGAIL), and also the known fibrillogenic sequence, hIAPP(20-29) (SNNFGAILSS) exhibited significantly lower kinetic and thermodynamic solubilities than the pentapeptide hIAPP(23-27) (FGAIL). Fibrils formed by all short peptide sequences and also by hIAPP(20-29) were cytotoxic towards the pancreatic cell line RIN5fm, whereas no cytotoxicity was observed for the soluble form of the peptides, a notion that is consistent with hIAPP cytotoxicity. Our results suggest that a penta- and hexapeptide sequence of an appropriate amino acid composition can be sufficient for beta-sheet and amyloid fibril formation and cytotoxicity and may assist in the rational design of inhibitors of pancreatic amyloid formation or other amyloidosis-related diseases.

Implication of novel biochemical property of beta-amyloid

Alzheimer disease (AD) is a heterogeneous disorder with a variety of molecular pathologies converging predominantly on abnormal amyloid deposition particularly in the brain. beta-Amyloid aggregation into senile plaques is one of the pathological hallmarks of AD. beta-Amyloid is generated by a proteolytic cleavage of a large membrane protein, amyloid precursor protein (APP). We have observed a new property of beta-amyloid. The amyloid 1-42 beta fragment, when aggregated, possesses proteolytic and esterase-like activity, in vitro. Three independent methods were used to test the new property of beta-amyloid. While esterase activity involves imidazole catalysis, proteolytic activity is consistent with participation of a serine peptidase triad: catalytic Ser, His and Glu (or Asp). Although the amino acid triad is a necessary requirement for the protease reactivity, it is not sufficient since the secondary structure of the protein significantly contributes to the proteolytic activity. The ability of beta-amyloid to cleave peptide or ester bonds could be thus responsible for either inactivation of other proteins and/or APP proteolysis itself. This property may be responsible for early pathogenesis of AD since there is emerging evidence that non-plaque amyloid is elevated in Alzheimer patients.

Temperature-dependent beta-sheet formation in beta-amyloid Abeta(1-40) peptide in water: uncoupling beta-structure folding from aggregation

To probe the role of temperature in the conversion of soluble Alzheimer's beta-amyloid peptide (Abeta) to insoluble beta-sheet rich aggregates, we analyzed the solution conformation of Abeta(1-40) from 0 to 98 degrees C by far-UV circular dichroism (CD) and native gel electrophoresis. The CD spectra of 15-300 microg/ml Abeta(1-40) in aqueous solution (pH approximately 4.6) at 0 degrees C are concentration-independent and suggest a substantially unfolded and/or unusually folded conformation characteristic of Abeta monomer or dimer. Heating from 0 to 37 degrees C induces a rapid reversible coil to beta-strand transition that is independent of the peptide concentration and thus is not linked to oligomerization. Consequently, this transition may occur within the Abeta(1-40) monomer or dimer. Incubation at 37 degrees C leads to slow reversible concentration-dependent beta-sheet accumulation; heating to 85 degrees C induces further beta-sheet folding and oligomerization. Our results demonstrate the importance of temperature and thermal history for the conformation of Abeta.

An FT-IR study of the beta-amyloid conformation: standardization of aggregation grade

The aggregation of beta-amyloid peptides is very important for their neurotoxic effect; standardization of the aggregation grade is necessary for biological experiments. Measurement of aggregation with physicochemical methods is a difficult task. The present work revealed that FT-IR can be used for studying the aggregation properties of beta-amyloid peptides and the effects of environmental variables (solvent, pH, ions, and temperature) on aggregation. In dimethyl sulfoxide or hexafluoroisopropanol, amyloid peptides are in a monomeric state; on dilution with phosphate buffer just before measurement is made, aggregation begins. A detailed two-dimensional FT-IR correlation spectroscopic study was made of the conformational transitions that occur during the aggregation of beta-amyloid peptides. Two processes (random/helix-to-beta-sheet and aggregation of beta-sheets) and multiple conformational states were observed before the most stable form was attained. beta-Amyloid peptides undergo decomposition in basic buffers containing Ca(2+); this process should be avoided during aging experiments.

Monitoring protein assembly using quasielastic light scattering spectroscopy

This article discussed the principles and practice of QLS with respect to protein assembly reactions. Particles undergoing Brownian motion in solution produce fluctuations in scattered light intensity. We have described how the temporal correlation function of these fluctuations can be measured and how mathematical analysis of the correlation function provides information about the distribution of diffusion coefficients of the particles. We have explained that deconvolution of the correlation function is an "ill-posed" problem and therefore that careful attention must be paid to the assumptions incorporated into data analysis procedures. We have shown how the Stokes-Einstein relationship can be used to convert distributions of diffusion coefficients into distributions of particle size. In the case of fibrillar polymers, this process allows direct determination of fibril length, enabling nucleation and elongation rates to be calculated. Finally, we have used examples from studies of A beta fibrillogenesis to illustrate the power these quantitative capabilities provide for understanding the molecular mechanisms of the fibrillogenesis reaction and for guiding the development of fibrillogenesis inhibitors.

Structure of the Alzheimer beta-amyloid peptide (25-35) and its interaction with negatively charged phospholipid vesicles

The secondary structure of amyloid betaAP(25-35) peptide was studied in pure form and in the presence of different phospholipid vesicles, by using Fourier transform infrared spectroscopy (FT-IR). Pure peptide aggregated with time, forming fibrils with beta-structure. Phospholipid vesicles formed by negatively charged phospholipids such as 1,2-dimyristoyl-sn-glycerol-3-phospho-L-serine (Myr2PtdSer), 1,2-dimyristoyl-sn-glycerol-3-phospho-rac-1-glycerol (Myr2PtdGro) and 1,2-dimyristoyl-sn-glycerol 3-phosphate (Myr2PtdH), greatly accelerated the aggregation of the peptide. However, the presence of vesicles formed by the zwitterionic phospholipid, 1, 2-dimyristoyl-sn-glycerol-3-phosphocholine (Myr2PtdCho), slowed down the aggregation process. Differential scanning calorimetry (DSC) measurements showed that the effect of betaAP(25-35) on the gel to crystal liquid phase transition was small at neutral pH for negatively charged phospholipids and practically nil for Myr2PtdCho. In the case of Myr2PtdSer the effect was also zero at pH 9 but the effect was large at pH 3. The effect on Myr2PtdH was not, however, very dependent on pH. These results were fully confirmed by the observation through FT-IR of the change with temperature of the CH2 antisymmetric stretching vibration. The case of Myr2PtdGro was special as this phospholipid presents polymorphism giving solid quasicrystalline phases when it is not sufficiently hydrated, and it is remarkable that betaAP(25-35) was able to induce the formation of crystalline phases in samples prepared through a method which ensure a good hydration of phospholipid. These results show that the interaction of amyloid betaAP(25-35) peptide with phospholipids is based on electrostatic interactions, that these interactions favour the aggregation of the peptides, and that the presence of the aggregates may disturb the lipid-water interphase of the membrane.

Morphological development of beta(1-40) amyloid fibrils

The Alzheimer's disease-related peptide beta(1-40) amyloid self-associates to form fibrils exhibiting a morphology characteristic of amyloidogenic proteins. The mechanism of this fibrillization process has yet to be fully elucidated. In this study we have immobilized the beta(1-40) amyloid to flat gold surfaces using thiol-based self-assembled monolayers. Atomic force microscopy reveals the presence of spherical units of beta(1-40) amyloid immediately following the initiation of fibrillization. Short fibrillar structures, termed nascent fibrils, which appear to be formed by the association of these units are also present at this time point. At later time points extended, branching networks of fibrils are observed. Some fibrils exhibit a more beaded appearance and greater axial periodicity than others. No nascent fibrils are seen to be present. We believe that these data identify an early fibril structure which could act as an intermediate in beta-amyloid fibrillization. The oligomeric units of which these nascent fibrils are comprised are also determined.