Soluble amyloid Abeta-(1-40) exists as a stable dimer at low concentrations

Permeabilization of Lipid Bilayers Is a Common Conformation-dependent Activity of Soluble Amyloid Oligomers in Protein Misfolding Diseases

Amyloid fibrillization is multistep process involving soluble oligomeric intermediates, including spherical oligomers and protofibrils. Amyloid oligomers have a common, generic structure, and they are intrinsically toxic to cells, even when formed from non-disease related proteins, which implies they also share a common mechanism of pathogenesis and toxicity. Here we report that soluble oligomers from several types of amyloids specifically increase lipid bilayer conductance regardless of the sequence, while fibrils and soluble low molecular weight species have no effect. The increase in membrane conductance occurs without any evidence of discrete channel or pore formation or ion selectivity. The conductance is dependent on the concentration of oligomers and can be reversed by anti-oligomer antibody. These results indicate that soluble oligomers from many types of amyloidogenic proteins and peptides increase membrane conductance in a conformation-specific fashion and suggest that this may represent the common primary mechanism of pathogenesis in amyloid-related degenerative diseases.

Solution NMR studies of the A beta(1-40) and A beta(1-42) peptides establish that the Met35 oxidation state affects the mechanism of amyloid formation

The pathogenesis of Alzheimer's disease is characterized by the aggregation and fibrillation of the 40-residue A beta(1-40) and 42-residue A beta(1-42) peptides into amyloid plaques. The structural changes associated with the conversion of monomeric A beta peptide building blocks into multimeric fibrillar beta-strand aggregates remain unknown. Recently, we established that oxidation of the methionine-35 side chain to the sulfoxide (Met35(red) --> Met35(ox)) significantly impedes the rate of aggregation and fibrillation of the A beta peptide. To explore this effect at greater resolution, we carefully compared the (1)H, (15)N, and (13)C NMR chemical shifts of four A beta peptides that had the Met35 reduced or oxidized (A beta(1-40)Met35(red), A beta(1-40)Met35(ox), A beta(1-42)Met35(red), and A beta(1-42)Met35(ox)). With the use of a special disaggregation protocol, the highly aggregation prone A beta peptides could be studied at higher, millimolar concentrations (as required by NMR) in aqueous solution at neutral pH, remaining largely monomeric at 5 degrees C as determined by sedimentation equilibrium studies. The NOE, amide-NH temperature coefficients, and chemical shift indices of the (1)H alpha, (13)C alpha, and (13)C beta established that the four peptides are largely random, extended chain structures, with the Met35(ox) reducing the propensity for beta-strand structure at two hydrophobic regions (Leu17-Ala21 and Ile31-Val36), and turn- or bendlike structures at Asp7-Glu11 and Phe20-Ser26. Additional NMR studies monitoring changes that occur during aging at 37 degrees C established that, along with a gradual loss of signal/noise, the Met35(ox) significantly hindered upfield chemical shift movements of the 2H NMR signals for the His6, His13, and His14 side chains. Taken together, the present NMR studies demonstrate that the Met35(red) --> Met35(ox) conversion prevents aggregation by reducing both hydrophobic and electrostatic association and that the A beta(1-40)Met35(red), A beta(1-40)Met35(ox), A beta(1-42)Met35(red), and A beta(1-42)Met35(ox) peptides may associate differently, through specific, sharp changes in structure during the initial stages of aggregation.

Unique Physicochemical Profile of β-Amyloid Peptide Variant Aβ1–40E22G Protofibrils: Conceivable Neuropathogen in Arctic Mutant Carriers

A new early-onset form of Alzheimer's disease (AD) was described recently where a point mutation was discovered in codon 693 of the beta-amyloid (Abeta) precursor protein gene, the Arctic mutation. The mutation translates into a single amino acid substitution, glutamic acid-->glycine, in position 22 of the Abeta peptide. The mutation carriers have lower plasma levels of Abeta than normal, while in vitro studies show that Abeta1-40E22G protofibril formation is significantly enhanced. We have explored the nature of the Abeta1-40E22G peptide in more detail, in particular the protofibrils. Using size-exclusion chromatography (SEC) and circular dichroism spectroscopy (CD) kinetic and secondary structural characteristics were compared with other Abeta1-40 peptides and the Abeta12-28 fragment, all having single amino acid substitutions in position 22. We have found that Abeta1-40E22G protofibrils are a group of comparatively stabile beta-sheet-containing oligomers with a heterogeneous size distribution, ranging from >100 kDa to >3000 kDa. Small Abeta1-40E22G protofibrils are generated about 400 times faster than large ones. Salt promotes their formation, which significantly exceeds all the other peptides studied here, including the Dutch mutation Abeta1-40E22Q. Position 22 substitutions had significant effects on aggregation kinetics of Abeta1-40 and in Abeta12-28, although the qualitative aspects of the effects differed between the native peptide and the fragment, as no protofibrils were formed by the fragments. The rank order of protofibril formation of Abeta1-40 and its variants was the same as the rank order of the length of the nucleation/lag phase of the Abeta12-28 fragments, E22V>E22A?E22G>E22Q?E22, and correlated with the degree of hydrophobicity of the position 22 substituent. The molecular mass of peptide monomers and protofibrils were estimated better in SEC studies using linear rather than globular calibration standards. The characteristics of the Abeta1-40E22G suggest an important role for the peptide in the neuropathogenesis in the Arctic form of AD.

Measurement of Intermolecular Distances for the Natural Agonist Peptide Docked at the Cholecystokinin Receptor Expressed in Situ Using Fluorescence Resonance Energy Transfer

Fluorescence resonance energy transfer is a powerful biophysical technique used to analyze the structure of membrane proteins. Here, we used this tool to determine the distances between a distinct position within a docked agonist and a series of distinct sites within the intramembranous confluence of helices and extracellular loops of the cholecystokinin (CCK) receptor. Pseudo-wild-type CCK receptor constructs having single reactive cysteine residues inserted into each of these sites were developed. The experimental strategy included the use of the full agonist, Alexa488-CCK, bound to these receptors as donor, with Alexa568 covalently bound to the specific sites within the CCK receptor as acceptor. Site-labeling was achieved by derivatization of intact cells with a novel fluorescent methanethiosulfonate reagent. A high degree of spectral overlap was observed between receptor-bound donor and receptor-derivatized acceptors, with no transfer observed for a series of controls representing saturation of the receptor binding site with nonfluorescent ligand and use of a null-reactive CCK receptor construct. The measured distances between the fluorophore within the docked agonist and the sites within the first (residue 102) and third (residue 341) extracellular loops of the receptor were shorter than those directed to the second loop (residue 204) or to intramembranous helix two (residue 94). These distances were accommodated well within a refined molecular model of the CCK-occupied receptor that is fully consistent with all existing structure-activity and photoaffinity-labeling studies. This approach provides the initial insights into the conformation of extracellular loop regions of this receptor and establishes clear differences from analogous loops in the rhodopsin crystal structure.

Elucidation of primary structure elements controlling early amyloid beta-protein oligomerization

Assembly of monomeric amyloid beta-protein (A beta) into oligomeric structures is an important pathogenetic feature of Alzheimer's disease. The oligomer size distributions of aggregate-free, low molecular weight A beta 40 and A beta 42 can be assessed quantitatively using the technique of photo-induced cross-linking of unmodified proteins. This approach revealed that low molecular weight A beta 40 is a mixture of monomer, dimer, trimer, and tetramer, in rapid equilibrium, whereas low molecular weight A beta 42 preferentially exists as pentamer/hexamer units (paranuclei), which self-associate to form larger oligomers. Here, photo-induced cross-linking of unmodified proteins was used to evaluate systematically the oligomerization of 34 physiologically relevant A beta alloforms, including those containing familial Alzheimer's disease-linked amino acid substitutions, naturally occurring N-terminal truncations, and modifications altering the charge, the hydrophobicity, or the conformation of the peptide. The most important structural feature controlling early oligomerization was the length of the C terminus. Specifically, the side-chain of residue 41 in A beta 42 was important both for effective formation of paranuclei and for self-association of paranuclei into larger oligomers. The side-chain of residue 42, and the C-terminal carboxyl group, affected paranucleus self-association. A beta 40 oligomerization was particularly sensitive to substitutions of Glu22 or Asp23 and to truncation of the N terminus, but not to substitutions of Phe19 or Ala21. A beta 42 oligomerization, in contrast, was largely unaffected by substitutions at positions 22 or 23 or by N-terminal truncations, but was affected significantly by substitutions of Phe19 or Ala21. These results reveal how specific regions and residues control A beta oligomerization and show that these controlling elements differ between A beta 40 and A beta 42.

Alzheimer beta-amyloid homodimers facilitate A beta fibrillization and the generation of conformational antibodies

We reported previously that stabilized beta-amyloid peptide dimers were derived from mutant amyloid precursor protein with a single cysteine in the ectodomain juxtamembrane position. In vivo studies revealed that two forms of SDS-stable A beta homodimers exist, species ending at A beta 40 and A beta 42. The phenomenon of the transformation of the initially deposited 42-residue beta-amyloid peptide into the amyloid fibrils of Alzheimer's disease plaques remains to be explained in physical terms, i.e. energetically and structurally. We therefore performed spectroscopic analyses revealing that engineered dimeric peptides ending at residue 42 displayed a much more pronounced beta-structural transition than corresponding monomers. Specifically, the single chemically induced dimerization of A beta peptides significantly increased the beta-sheet content by a factor of 2. The C-terminal residues Ile-41 and Ala-42 of dimeric forms further increased the beta-sheet content by roughly one-third. In contrast to A beta 42, the beta-sheet content of the alpha- and gamma-secretase-generated p3 fragments did not necessarily correlate with the tendency to form fibrils, although p3/17-42 had a pronounced thread forming character with fibril lengths of up to 2.5 microM. Electron microscopic images show that forms of p3/17-42 generated smaller granular particles than forms ending at residue 40. We discuss these findings in terms of A beta 1-42 dimers representing paranuclei, which self-aggregate into ribbon-like ordered fibrils by elongation. Based on A beta 42 dimer-specific titers of a polyclonal antiserum we propose that the A beta homodimer represents a nidus for plaque formation and a well defined novel therapeutic target.

Beta-amyloid regulation of presynaptic nicotinic receptors in rat hippocampus and neocortex

Alteration by beta-amyloid (Abeta) of signaling via nicotinic acetylcholine receptors (nAChRs) has been implicated in the early stages of Alzheimer's disease. nAChRs function both post- and presynaptically in the nervous system; however, little is known about the functional consequence of the interaction of Abeta with these receptors, particularly those on presynaptic nerve terminals. In view of the strong correlation between loss of synaptic terminals and dementia, together with the reduction in nAChRs in Alzheimer's disease, the possibility exists that presynaptic nAChRs may be targets for Abeta. To explore this possibility, we assessed the effect of Abeta peptides on nicotine-evoked changes in presynaptic Ca2+ level via confocal imaging of isolated presynaptic nerve endings from rat hippocampus and neocortex. Abeta1-42 appeared to inhibit presynaptic nAChR activation by nicotine. Surprisingly, picomolar Abeta1-42 was found to directly evoke sustained increases in presynaptic Ca2+ via nAChRs, revealing that the apparent inhibitory action of Abeta1-42 was the result of an occlusion of nicotine to further stimulate the receptors. The direct effect of Abeta was found to be sensitive to alpha-bungarotoxin, mecamylamine, and dihydro-beta-erythroidine, indicating involvement of alpha7-containing nAChRs and non-alpha7-containing nAChRs. Prior depolarization strongly attenuated subsequent Abeta-evoked responses in a manner dependent on the amplitude of the initial presynaptic Ca2+ increase, suggesting that nerve activity or Ca2+ channel density may control the impact of Abeta on presynaptic nerve terminal function. Together, these results suggest that the sustained increases in presynaptic Ca2+ evoked by Abeta may underlie disruptions in neuronal signaling via nAChRs in the early stages of Alzheimer's disease.

Melatonin prevents free radical formation due to the interaction between beta-amyloid peptides and metal ions [Al(III), Zn(II), Cu(II), Mn(II), Fe(II)]

Alzheimer's disease, among other pathological features, is characterized by an over-accumulation of amyloid-beta peptide, metal ions, and oxidative stress proteins in the brain. Amyloid-beta aggregated peptides with bound metal ions may initiate free radical generation with consequent protein and lipid oxidation, reactive oxygen species formation and eventually neuronal death. Melatonin is able to dramatically reduce the free radical formation which follows the interaction between transition metal ions and amyloid-beta. This paper reports the scavenging effect of melatonin of reactants generated by amyloid peptides in combination with some metal ions.

Y10W beta(1-40) fluorescence reflects epitope exposure in conformers of Alzheimer's beta-peptide

The Alzheimer's beta-peptide in neutral aqueous solution is characterized variously as a random coil or a heterogeneous mixture of conformers. Under conditions of lowered pH characteristic of intracellular compartments such as endosomes or lysosomes, a different conformation is favored, which is reflected in the biophysical and biological properties of the peptide. The reactivity of the epitope of the monoclonal antibody 6F/3D, encompassing residues 9-14, is drastically reduced. The fluorescence of human sequence beta(1-40) with the tyrosine at position 10 substituted with tryptophan (Y10W beta(1-40)) is quenched nearly 50% when the peptide is shifted to pH 4.6. The exposure of the 6F/3D epitope parallels Y10W beta(1-40) fluorescence changes induced by a variety of perturbations. The linkage of the sensitivity of immunological detection with the potential for monitoring rapid changes by fluorescence offers convergence of biology and biophysics in the study of beta-amyloid peptide conformation.

Exploiting amyloid fibril lamination for nanotube self-assembly

Fundamental questions about the relative arrangement of the beta-sheet arrays within amyloid fibrils remain central to both its structure and the mechanism of self-assembly. Recent computational analyses suggested that sheet-to-sheet lamination was limited by the length of the strand. On the basis of this hypothesis, a short seven-residue segment of the Alzheimer's disease-related Abeta peptide, Abeta(16-22), was allowed to self-assemble under conditions that maintained the basic amphiphilic character of Abeta. Indeed, the number increased over 20-fold to 130 laminates, giving homogeneous bilayer structures that supercoil into long robust nanotubes. Small-angle neutron scattering and X-ray scattering defined the outer and inner radii of the nanotubes in solution to contain a 44-nm inner cavity with 4-nm-thick walls. Atomic force microscopy and transmission electron microscopy images further confirmed these homogeneous arrays of solvent-filled nanotubes arising from a flat rectangular bilayer, 130 nm wide x 4 nm thick, with each bilayer leaflet composed of laminated beta-sheets. The corresponding backbone H-bonds are along the long axis, and beta-sheet lamination defines the 130-nm bilayer width. This bilayer coils to give the final nanotube. Such robust and persistent self-assembling nanotubes with positively charged surfaces of very different inner and outer curvature now offer a unique, robust, and easily accessible scaffold for nanotechnology.

Spherical aggregates of beta-amyloid (amylospheroid) show high neurotoxicity and activate tau protein kinase I/glycogen synthase kinase-3beta

beta-Amyloid (Abeta) acquires toxicity by self-aggregation. To identify and characterize the toxic form(s) of Abeta aggregates, we examined in vitro aggregation conditions by using large quantities of homogenous, chemically synthesized Abeta1-40 peptide. We found that slow rotation of Abeta1-40 solution reproducibly gave self-aggregated Abeta1-40 containing a stable and highly toxic moiety. Examination of the aggregates purified by glycerol-gradient centrifugation by atomic force microscopy and transmission electron microscopy revealed that the toxic moiety is a perfect sphere, which we call amylospheroid (ASPD). Other Abeta1-40 aggregates, including fibrils, were nontoxic. Correlation studies between toxicity and sphere size indicate that 10- to 15-nm ASPD was highly toxic, whereas ASPD <10 nm was nontoxic. A positive correlation between the toxicity and ASPD >10 nm also appeared to exist when Abeta1-42 formed ASPD by slow rotation. However, Abeta1-42-ASPD formed more rapidly, killed neurons at lower concentrations, and showed approximately 100-fold-higher toxicity than Abeta1-40-ASPD. The toxic ASPD was associated with SDS-resistant oligomeric bands in immunoblotting, which were absent in nontoxic ASPD. Because the formation of ASPD was not disturbed by pentapeptides that break beta-sheet interactions, Abeta may form ASPD through a pathway that is at least partly distinct from that of fibril formation. Inhibition experiments with lithium suggest the involvement of tau protein kinase I/glycogen synthase kinase-3beta in the early stages of ASPD-induced neurodegeneration. Here we describe the identification and characterization of ASPD and discuss its possible role in the neurodegeneration in Alzheimer's disease.

Micelle formation by a fragment of human islet amyloid polypeptide

Human islet amyloid polypeptide (hIAPP) is the major component of amyloid plaques found in the pancreatic islets of persons with type 2 diabetes mellitus. HIAPP belongs to the group of amyloidogenic proteins, characterized by their aggregation and deposition as fibrillar amyloid in various body tissues. The aggregation of amyloidogenic proteins is thought to occur via a common pathway, but currently no unifying kinetic model exists. In previous work, we presented a model of amyloid fibril formation formulated from our observations of the aggregation of an amyloidogenic fragment of hIAPP, amino acids 20-29. Our model is based on nucleation-dependent aggregation, modified by the formation of off-pathway hIAPP micelles. In the present study we confirm the presence of peptide micelles, and experimentally determine the critical micelle concentration in solutions of hIAPP fragments using three different techniques: conductivity, pH, and fluorescence. All three techniques yield a critical micelle concentration of 3-3.5 micro M peptide. Furthermore, based on changes in the fluorescence intensity of a labeled peptide fragment as well as a decrease in solution pH as a result of deprotonation of the amino terminus, we conclude that the amino terminus of the fragment undergoes a significant change of environment upon micellization.

Assemblies of Alzheimer's peptides A beta 25-35 and A beta 31-35: reverse-turn conformation and side-chain interactions revealed by X-ray diffraction

Alzheimer's beta amyloid protein (A beta) is a 39 to 43 amino acid peptide that is a major component in the neuritic plaques of Alzheimer's disease (AD). The assemblies constituted from residues 25-35 (A beta 25-35), which is a sequence homologous to the tachykinin or neurokinin class of neuropeptides, are neurotoxic. We used X-ray diffraction and electron microscopy to investigate the structure of the assemblies formed by A beta 25-35 peptides and of various length sequences therein, and of tachykinin-like analogues. Most solubilized peptides after subsequent drying produced diffraction patterns characteristic of beta-sheet structure. Moreover, the peptides A beta 31-35 (Ile-Ile-Gly-Leu-Met) and tachykinin analogue A beta(Phe(31))31-35 (Phe-Ile-Gly-Leu-Met) gave powder diffraction patterns to 2.8A Bragg spacing. The observed reflections were indexed by an orthogonal unit cell having dimensions of a=9.36 A, b=15.83 A, and c=20.10 A for the native A beta 31-35 peptide, and a=9.46 A, b=16.22 A, and c=11.06 A for the peptide having the Ile31Phe substitution. The initial model was a beta strand where the hydrogen bonding, chain, and intersheet directions were placed along the a, b, and c axes. An atomic model was fit to the electron density distribution, and subsequent refinement resulted in R factors of 0.27 and 0.26, respectively. Both peptides showed a reverse turn at Gly33 which results in intramolecular hydrogen bonding between the antiparallel chains. Based on previous reports that antagonists for the tachykinin substance P require a reverse turn, and that A beta is cytotoxic when it is oligomeric or fibrillar, we propose that the tachykinin-like A beta 31-35 domain is a turn exposed at the A beta oligomer surface where it could interact with the ligand-binding site of the tachykinin G-protein-coupled receptor.

Structural and dynamic features of Alzheimer's Abeta peptide in amyloid fibrils studied by site-directed spin labeling

Electron paramagnetic resonance spectroscopy analysis of 19 spin-labeled derivatives of the Alzheimer's amyloid beta (Abeta) peptide was used to reveal structural features of amyloid fibril formation. In the fibril, extensive regions of the peptide show an in-register, parallel arrangement. Based on the parallel arrangement and side chain mobility analysis we find the amyloid structure to be mostly ordered and specific, but we also identify more dynamic regions (N and C termini) and likely turn or bend regions (around residues 23-26). Despite their different aggregation properties and roles in disease, the two peptides, Abeta40 and Abeta42, homogeneously co-mix in amyloid fibrils suggesting that they possess the same structural architecture.

Kinetic studies of amyloid beta-protein fibril assembly. Differential effects of alpha-helix stabilization

Amyloid beta-protein (Abeta) fibril assembly is a defining characteristic of Alzheimer's disease. Fibril formation is a complex nucleation-dependent polymerization process characterized in vitro by an initial lag phase. To a significant degree, this phase is a consequence of the energy barrier that must be overcome in order for Abeta monomers to fold and oligomerize into fibril nuclei. Here we show that low concentrations of 2,2,2-trifluoroethanol (TFE) convert predominately unstructured Abeta monomers into partially ordered, quasistable conformers. Surprisingly, this results in a temporal decrease in the lag phase for fibril formation and a significant increase in the rate of fibril elongation. The TFE effect is concentration dependent and is maximal at approximately 20% (v/v). In the presence of low concentrations of TFE, fibril formation is observed in Abeta samples at nanomolar concentration, well below the critical concentration for Abeta fibril formation in the absence of TFE. As the amount of TFE is increased above 20%, helix content progressively rises to approximately 80%, a change paralleled first by a decrease in elongation rate and then by a complete cessation of fibril growth. These findings are consistent with the hypothesis that a partially folded helix-containing conformer is an intermediate in Abeta fibril assembly. The requirement that Abeta partially folds in order to assemble into fibrils contrasts with the mechanism of amyloidogenesis of natively folded proteins such as transthyretin and lysozyme, in which partial unfolding is a prerequisite. Our results suggest that in vivo, factors that affect helix formation and stability will have significant effects on the kinetics of Abeta fibril formation.

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.

Inhibition by naloxone stereoisomers of beta-amyloid peptide (1-42)-induced superoxide production in microglia and degeneration of cortical and mesencephalic neurons

Previously we reported that naloxone stereoisomers, in an opioid receptor-independent manner, attenuated the inflammation-mediated degeneration of dopaminergic neurons by inhibition of the activation of microglia, the resident immune cells in the brain. Recently we discovered that beta-amyloid peptide Abeta (1-42) exhibited enhanced neurotoxicity toward both cortical and mesencephalic neurons through the activation of microglia and production of superoxide. The purpose of this study was to determine whether naloxone isomers had any effect on Abeta (1-42)-induced neurodegeneration. Pretreatment of either cortical or mesencephalic neuron-glia cultures with 1 to 10 microM (-)-naloxone, prior to treatment for up to 11 days with 0.1 to 3 microM Abeta (1-42), afforded significant neuroprotection as judged by neurotransmitter uptake, immunocytochemical analysis, and cell counting. More importantly, (+)-naloxone, the ineffective enantiomer of (-)-naloxone in binding opioid receptors, was equally effective in affording neuroprotection. Mechanistically, inhibition of Abeta (1-42)-induced production of superoxide in microglia underlay the neuroprotective effect of naloxone stereoisomers. Moreover, neuroprotection and inhibition of Abeta (1-42)-induced superoxide production was also achieved with naloxone methiodide, a charged analog with quaternary amine, suggesting that the site of action for naloxone isomers is at the cell surface of microglia. These results demonstrated that naloxone isomers, through mechanisms unrelated to the opioid receptors, were capable of inhibiting Abeta (1-42)-induced microglial activation and degeneration of both cortical and mesencephalic neurons. Combined with our previous observations with inflammagen-induced neurodegeneration, naloxone analogs, especially (+)-naloxone, may have potential therapeutic efficacy for the treatment of Alzheimer's and Parkinson's disease.

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.

beta -Amyloid peptide activates alpha 7 nicotinic acetylcholine receptors expressed in Xenopus oocytes

The alpha7 nicotinic acetylcholine receptor is highly expressed in hippocampus and in cholinergic projection neurons from the basal forebrain, structures that are particularly vulnerable to the ravages of Alzheimer's disease. Previous work suggests that beta-amyloid peptide can interact with alpha7 nicotinic acetylcholine receptors, although the nature of this interaction has not been well characterized. To test whether beta-amyloid peptide can activate alpha7 nicotinic acetylcholine receptors, we expressed these receptors in Xenopus oocytes and performed two-electrode voltage clamp recordings, characterizing the response to beta-amyloid peptide 1-42 applied at concentrations ranging from 1 pm to 100 nm. In alpha7-expressing oocytes, beta-amyloid peptide 1-42 elicits inward currents at low concentrations (1-100 pm), whereas at higher concentrations (nm), less effective receptor activation is observed, indicative of receptor desensitization. Preincubation with the alpha7-selective agents, the antagonist methyllycaconatine, and the agonist 4-OH-GTS-21 blocked beta-amyloid peptide-induced receptor activation. beta-amyloid peptide 1-42 at low concentrations was able to activate the L250T mutant alpha7 receptor. The endogenous Ca(2+)-activated chloride current in Xenopus oocytes is recruited upon receptor activation since replacing Ca(2+) with Ba(2+) in the recording solution reduced current amplitude. Thus, when beta-amyloid peptide activation of alpha7 receptors occurs, these currents are comprised, at least in part, of Ca(2+).

Characterization of cholyl-leu-val-phe-phe-ala-OH as an inhibitor of amyloid beta-peptide polymerization

Cholyl-LVFFA-OH (1, PPI-368) is an organic-modified peptide based on the sequence of amyloid beta-peptide (A beta). It is a potent and selective inhibitor of A beta polymerization that blocks the formation of neurotoxic species of A beta. In a nucleation-dependent polymerization assay of 50 microM A beta(1-40), equimolar concentrations of PPI-368 block polymerization based on turbidity and electron microscopy. Monomeric A beta(1-40) and A beta(1-42) are non-toxic when incubated with neuronal cell lines, but become toxic during polymerization. PPI-368 coordinately delays the onset of polymerization and the formation of neurotoxic A beta species for both peptides. In a polymerization extension assay seeded with pre-formed A beta polymer, similar inhibition and dose-dependency phenomena are observed with PPI-368. Radiolabeled PPI-368 is incorporated into fibrils during polymerization demonstrating binding to A beta peptide within afibrillar structure. Gel-filtration studies show progressive disappearance of A beta monomer and concomitant appearance of soluble higher molecular weight oligomers. In the presence of submolar concentrations of PPI-368, monomeric A beta is still present and oligomers are not observed PPI-368 does not inhibit the polymerization of other amyloidogenic proteins such as transthyretin (TTR) or islet amyloid polypeptide (IAPP(20-29).

Amyloid beta protein forms ion channels: implications for Alzheimer's disease pathophysiology

Amyloid beta protein (AbetaP) is the major constituent of senile plaques associated with Alzheimer's disease (AD). However, its mechanistic role in AD pathogenesis is poorly understood. Globular and nonfibrillar AbetaPs are continuously released during normal metabolism. Using techniques of atomic force microscopy, laser confocal microscopy, electrical recording, and biochemical assays, we have examined the molecular conformations of reconstituted globular AbetaPs as well as their real-time and acute effects on neuritic degeneration. Atomic force microscopy (AFM) of AbetaP1-42 shows globular structures that do not form fibers in physiological-buffered solution for up to 8 h of continuous imaging. AFM of AbetaP1-42 reconstituted in a planar lipid bilayer reveals multimeric channel-like structures. Consistent with these AFM resolved channel-like structures, biochemical analysis demonstrates that predominantly monomeric AbetaPs in solution form stable tetramers and hexamers after incorporation into lipid membranes. Electrophysiological recordings demonstrate the presence of multiple single channel currents of different sizes. At the cellular level, AbetaP1-42 allows calcium uptake and induces neuritic abnormality in a dose- and time-dependent fashion. At physiological nanomolar concentrations, rapid neuritic degeneration was observed within minutes; at micromolar concentrations, neuronal death was observed within 3-4 h. These effects are prevented by zinc (an AbetaP channel blocker) and by the removal of extracellular calcium, but are not prevented by antagonists of putative AbetaP cell surface receptors. Thus, AbetaP channels may provide a direct pathway for calcium-dependent AbetaP toxicity in AD.

Amyloid beta-protein oligomerization: prenucleation interactions revealed by photo-induced cross-linking of unmodified proteins

Assembly of the amyloid beta-protein (Abeta) into neurotoxic oligomers and fibrils is a seminal event in Alzheimer's disease. Understanding the earliest phases of Abeta assembly, including prenucleation and nucleation, is essential for the development of rational therapeutic strategies. We have applied a powerful new method, photoinduced cross-linking of unmodified proteins (PICUP), to the study of Abeta oligomerization. Significant advantages of this method include an extremely short reaction time, enabling the identification and quantification of short lived metastable assemblies, and the fact that no pre facto structural modification of the native peptide is required. Using PICUP, the distribution of Abeta oligomers existing prior to assembly was defined. A rapid equilibrium was observed involving monomer, dimer, trimer, and tetramer. A similar distribution was seen in studies of an unrelated amyloidogenic peptide, whereas nonamyloidogenic peptides yielded distributions indicative of a lack of monomer preassociation. These results suggest that simple nucleation-dependent polymerization models are insufficient to describe the dynamic equilibria associated with prenucleation phases of Abeta assembly.

Homodimerization of amyloid precursor protein and its implication in the amyloidogenic pathway of Alzheimer's disease

We reported previously that the carbohydrate domain of the amyloid precursor protein is involved in amyloid precursor protein (APP)-APP interactions. Functional in vitro studies suggested that this interaction occurs through the collagen binding site of APP. The physiological significance remained unknown, because it is not understood whether and how APP dimerization occurs in vivo. Here we report that cellular APP exists as homodimers matching best with a two-site model. Consistent with our published crystallographic data, we show that a deletion of the entire sequence after the kunitz protease inhibitor domain did not abolish APP homodimerization, suggesting that two domains are critically involved but that neither is essential for homodimerization. Finally, we generated stabilized dimers by expressing mutant APP with a single cysteine in the ectodomain juxtamembrane region. Mutation of Lys(624) to cysteine produced approximately 6-8-fold more A beta than cells expressing normal APP. Our results suggest that amyloid A beta production can in principle be positively regulated by dimerization in vivo. We suggest that dimerization could be a physiologically important mechanism for regulating the proposed signal activity of APP.

All or none fibrillogenesis of a prion peptide

Amyloid proteins and peptides comprise a diverse group of molecules that vary both in size and amino-acid sequence, yet assemble into amyloid fibrils that have a common core structure. Kinetic studies of amyloid fibrillogenesis have revealed that certain amyloid proteins form oligomeric intermediates prior to fibril formation. We have investigated fibril formation with a peptide corresponding to residues 195-213 of the human prion protein. Through a combination of kinetic and equilibrium studies, we have found that the fibrillogenesis of this peptide proceeds as an all-or-none reaction where oligomeric intermediates are not stably populated. This variation in whether oligomeric intermediates are stably populated during fibril formation indicates that amyloid proteins assemble into a common fibrillar structure; however, they do so through different pathways.

Identification of a subunit interface in transthyretin amyloid fibrils: evidence for self-assembly from oligomeric building blocks

Amyloid and prion diseases appear to stem from the conversion of normally folded proteins into insoluble, fiber-like assemblies. Despite numerous structural studies, a detailed molecular characterization of amyloid fibrils remains elusive. In particular, models of amyloid fibrils proposed thus far have not adequately defined the constituent protein subunit interactions. To further our understanding of amyloid structure, we employed thiol-specific cross-linking and site-directed spin labeling to identify specific protein-protein associations in transthyretin (TTR) amyloid fibrils. We find that certain cysteine mutants of TTR, when dimerized by chemical cross-linkers, still form fibers under typical in vitro fibrillogenic conditions. In addition, site-directed spin labeling of many residues at the natural dimer interface reveals that their spatial proximity is preserved in the fibrillar state even in the absence of cross-linking constraints. Here, we present the first view of a subunit interface in TTR fibers and show that it is very similar to one of the natural dimeric interchain associations evident in the structure of soluble TTR. The results clarify varied models of amyloidogenesis by demonstrating that transthyretin amyloid fibrils may assemble from oligomeric protein building blocks rather than structurally rearranged monomers.

Beta-amyloid activates the mitogen-activated protein kinase cascade via hippocampal alpha7 nicotinic acetylcholine receptors: In vitro and in vivo mechanisms related to Alzheimer's disease

Alzheimer's Disease (AD) is the most common of the senile dementias, the prevalence of which is increasing rapidly, with a projected 14 million affected worldwide by 2025. The signal transduction mechanisms that underlie the learning and memory derangements in AD are poorly understood. beta-Amyloid (Abeta) peptides are elevated in brain tissue of AD patients and are the principal component of amyloid plaques, a major criterion for postmortem diagnosis of the disease. Using acute and organotypic hippocampal slice preparations, we demonstrate that Abeta peptide 1-42 (Abeta42) couples to the mitogen-activated protein kinase (MAPK) cascade via alpha7 nicotinic acetylcholine receptors (nAChRs). In vivo elevation of Abeta, such as that exhibited in an animal model for AD, leads to the upregulation of alpha7 nAChR protein. alpha7 nAChR upregulation occurs concomitantly with the downregulation of the 42 kDa isoform of extracellular signal-regulated kinase (ERK2) MAPK in hippocampi of aged animals. The phosphorylation state of a transcriptional mediator of long-term potentiation and a downstream target of the ERK MAPK cascade, the cAMP-regulatory element binding (CREB) protein, were affected also. These findings support the model that derangement of hippocampus signal transduction cascades in AD arises as a consequence of increased Abeta burden and chronic activation of the ERK MAPK cascade in an alpha7 nAChR-dependent manner that eventually leads to the downregulation of ERK2 MAPK and decreased phosphorylation of CREB protein.

Fluorescence anisotropy: a method for early detection of Alzheimer beta-peptide (Abeta) aggregation

Time-resolved anisotropy measurements (TRAMS) have been used to study the aggregation of the beta-amyloid (Abeta) peptide which is suspected of playing a central role in the pathogenesis of Alzheimer's Disease (AD). The experiments, which employ small quantities of fluorescently-labelled Abeta, in addition to the untagged peptide, have shown that the sensitive TRAMS technique detects the presence of preformed "seed" particles in freshly prepared solutions of Abeta. More importantly, as 100 microM solutions of Abeta containing tagged Abeta at a concentration level of either 0.5 or 1 microM are incubated, the TRAMS prove capable of detection of the peptide aggregation process through the appearance of a continuously increasing "residual anisotropy" within the time-resolved fluorescence data. The method detects Abeta aggregation in its earliest stages, well before complexation becomes apparent in more conventional methods such as the thioflavin T fluorescence assay. The TRAMS approach promises to provide a most attractive route for establishment of a high-throughput procedure for the early detection of the presence of amyloid aggregates in the screening of biological samples.

Antibody-mediated phagocytosis of the amyloid beta-peptide in microglia is differentially modulated by C1q

Microglial ingestion of the amyloid beta-peptide (Abeta) has been viewed as a therapeutic target in Alzheimer's disease, in that approaches that enhance clearance of Abeta relative to its production are predicted to result in decreased senile plaque formation, a proposed contributor to neuropathology. In vitro, scavenger receptors mediate ingestion of fibrillar Abeta (fAbeta) by microglia. However, the finding that cerebral amyloid deposition in a transgenic mouse model of Alzheimer's disease was diminished by inoculation with synthetic Abeta has suggested a possible therapeutic role for anti-Abeta Ab-mediated phagocytosis. Microglia also express C1qR(P), a receptor for complement protein C1q, ligation of which in vitro enhances phagocytosis of immune complexes formed with IgG levels below that required for optimal FcR-mediated phagocytosis. The data presented here demonstrate FcR-dependent ingestion of Abeta-anti-Abeta complexes (IgG-fAbeta) by microglia that is a function of the amount of Ab used to form immune complexes. In addition, C1q incorporated into IgG-fAbeta enhanced microglial uptake of these complexes when they contained suboptimal levels of anti-Abeta Ab. Mannose binding lectin and lung surfactant protein A, other ligands of C1qR(P), also enhanced ingestion of suboptimally opsonized IgG-fAbeta, whereas control proteins did not. Our data suggest that C1qR(P)-mediated events may promote efficient ingestion of Abeta at low Ab titers, and this may be beneficial in paradigms that seek to clear amyloid via FcR-mediated mechanisms by minimizing the potential for destructive Ab-induced complement-mediated processes.

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.

Polylysine as a vehicle for extracellular matrix-targeted local drug delivery, providing high accumulation and long-term retention within the vascular wall

We present the first steps in the elaboration of an approach of extracellular matrix-targeted local drug delivery (ECM-LDD), designed to provide a high concentration, ubiquitous distribution, and long-term retention of a drug within the vessel wall after local intravascular delivery. The approach is based on the concept of a bifunctional drug comprising a "therapeutic effector" and an "affinity vehicle," which should bind to an abundant component of the vessel wall. The aim of the present study was to select molecules suitable for the role of affinity vehicles for ECM-LDD and to study their intravascular delivery and retention ex vivo and in an animal model. By use of fluorescence microscopy, the following molecules were selected on the basis of strong binding to cross sections of human vessels: protamine, polylysine, polyarginine, a glycosaminoglycan-binding peptide from vitronectin, and a synthetic dendrimer. With polylysine as a prototypic affinity vehicle, we showed that after intravascular delivery, polylysine was concentrated throughout a luminal layer of the vascular wall to an extremely high concentration of 20 g/L and was retained therein for at least 72 hours of perfusion without noticeable losses. Low molecular weight (fluorescein) and high molecular weight (hirudin) compounds could be chemically conjugated to polylysine and were retained in the vessel wall after intravascular delivery of the conjugates. In conclusion, by use of the ECM-LDD method, an extremely high concentration and long-term retention of locally delivered drug can be reached. Polycationic molecules can be considered as potential affinity vehicles for ECM-LDD.

Lysosomal membrane damage in soluble Abeta-mediated cell death in Alzheimer's disease

Our previous studies suggest that a failure to degrade aggregated Abeta1-42 in late endosomes or secondary lysosomes is a mechanism that contributes to intracellular accumulation in Alzheimer's disease. In this study, we demonstrate that cultured primary neurons are able to internalize soluble Abeta1-42 from the culture medium and accumulate inside the endosomal/lysosomal system. The intracellular Abeta1-42 is resistant to protease degradation and stable for at least 48 h within the cultured neurons. Incubation of cultured neurons with a cytotoxic concentration of soluble Abeta1-42 invokes the rapid free radical generation within lysosomes and disruption of lysosomal membrane proton gradient which precedes cell death. The loss of lysosomal membrane impermeability is only specific to the Abeta1-42 isoform since incubation of cells with high concentrations of Abeta1-40 has no effect on lysosomal hydrolase release. To further support the role of lysosomal membrane damage in Abeta-mediated cell death, we demonstrate that photodisruption of acridine orange (AO)-loaded lysosomes with intense blue light induces a relatively rapid synchronous lysosomal membrane damage and neuronal death similar to that observed as a result of Abeta exposure. AO leaks quickly from late endosomes and lysosomes and partially shifts the fluorescence from an orange fluorescence to a diffuse, green cytoplasmic fluorescence. Such AO relocalization is due to an initial disruption of the lysosomal proton gradient, followed by the release of lysosomal hydrolases into the cytoplasmic compartment. Treatment of cells with either the antioxidant n-propyl gallate or lysosomotropic amine (methylamine) partially blocks the release of lysosomal contents suggesting that this AO relocalization is due to lysosomal membrane oxidation. Based on these findings, we propose that the cell death mediated by the soluble Abeta may be fundamentally different from the cell loss observed following extracellular Abeta deposition.

Nicotine and amyloid formation

The major protein constituents of amyloid deposits in Alzheimer's disease (AD) are the 40-residue beta-amyloid (Abeta) (1-40) peptide and the 42-residue Abeta(1-42) peptide. The Abeta(1-42) is more pathogenic and produced in greater quantities in familial forms of AD. A major goal of research is to uncover a suitable inhibitor that either slows down or inhibits Abeta formation (beta-amyloidosis). During beta-amyloidosis, structural changes associated with the conversion of monomeric Abeta peptide building blocks into the aggregated fibrillar beta-sheet structures occur (alpha-helix-->beta-sheet or random, extended chain-->beta-sheet). In previous work, we and others established that nicotine, a major component of cigarette smoke, inhibits beta-amyloidosis of the Abeta(1-42), which may result from nicotine binding to the alpha-helical structure. These conclusions were based on solution nuclear magnetic resonance (NMR) spectroscopic studies with the nonnative 28-residue Abeta(1-28). This information suggests that, when administered therapeutically to AD patients, nicotine may not only affect cholinergic activation, but could also conceivably alter amyloid deposition. In this report, NMR studies were augmented with the naturally occurring Abeta(1-42), under conditions where the peptide folds into a predominantly alpha-helical or random, extended chain structure. The major result is that nicotine shows only modest binding to these conformations, indicating that the nicotine inhibition to beta-amyloidosis probably results from binding to a small, soluble beta-sheet aggregate that is NMR invisible.

Characterization of copper interactions with alzheimer amyloid beta peptides: identification of an attomolar-affinity copper binding site on amyloid beta1-42

Cu and Zn have been shown to accumulate in the brains of Alzheimer's disease patients. We have previously reported that Cu(2+) and Zn(2+) bind amyloid beta (Abeta), explaining their enrichment in plaque pathology. Here we detail the stoichiometries and binding affinities of multiple cooperative Cu(2+)-binding sites on synthetic Abeta1-40 and Abeta1-42. We have developed a ligand displacement technique (competitive metal capture analysis) that uses metal-chelator complexes to evaluate metal ion binding to Abeta, a notoriously self-aggregating peptide. This analysis indicated that there is a very-high-affinity Cu(2+)-binding site on Abeta1-42 (log K(app) = 17.2) that mediates peptide precipitation and that the tendency of this peptide to self-aggregate in aqueous solutions is due to the presence of trace Cu(2+) contamination (customarily approximately 0.1 microM). In contrast, Abeta1-40 has much lower affinity for Cu(2+) at this site (estimated log K(app) = 10.3), explaining why this peptide is less self-aggregating. The greater Cu(2+)-binding affinity of Abeta1-42 compared with Abeta1-40 is associated with significantly diminished negative cooperativity. The role of trace metal contamination in inducing Abeta precipitation was confirmed by the demonstration that Abeta peptide (10 microM) remained soluble for 5 days only in the presence of high-affinity Cu(2+)-selective chelators.

A conformation change in the carboxyl terminus of Alzheimer's Abeta (1-40) accompanies the transition from dimer to fibril as revealed by fluorescence quenching analysis

Alzheimer's disease is characterized by the presence of insoluble, fibrous deposits composed principally of amyloid beta (Abeta) peptide. A number of studies have provided information on the fibril structure and on the factors affecting fiber formation, but the details of the fibril structure are not known. We used fluorescence quenching to investigate the solvent accessibility and surface charge of the soluble Abeta(1-40) dimer and amyloid fibrils. Analogs of Abeta(1-40) containing a single tryptophan were synthesized by substituting residues at positions 4, 10, 34, and 40 with tryptophan. Quenching measurements in the dimeric state indicate that the amino-terminal analogs (AbetaF4W and AbetaY10W) are accessible to polar quenchers, and the more carboxyl-terminal analog AbetaV34W is less accessible. AbetaV40W, on the other hand, exhibits a low degree of quenching, indicating that this residue is highly shielded from the solvent in the dimeric state. Correcting for the effect of reduced translational and rotational diffusion, fibril formation was associated with a selective increase in solvent exposure of residues 34 and 40, suggesting that a conformation change may take place in the carboxyl-terminal region coincident with the dimer to fibril transition.

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.

Oligomerizaiton and fibril asssembly of the amyloid-beta protein

In this chapter, we attempt to analyze the evolution of the amyloid-beta (Abeta) molecular structure from its inception as part of the Abeta precursor protein to its release by the secretases and its extrusion from membrane into an aqueous environment. Biophysical studies suggest that the Abeta peptide sustains a series of transitions from a molecule rich in alpha-helix to a molecule in which beta-strands prevail. It is proposed that initially the extended C-termini of two opposing Abeta dimers form an antiparallel beta-sheet and that the subsequent addition of dimers generates a helical Abeta protofilament. Two or more protofilaments create a strand in which the hydrophobic core of the beta-sheets is shielded from the aqueous environment by the N-terminal polar domains of the Abeta dimers. Once the nucleation has occurred, the Abeta filament grows in length by the addition of dimers or tetramers.

Oligomerization of beta-amyloid of the Alzheimer's and the Dutch-cerebral-haemorrhage types

A novel ELISA has been developed which detects oligomerization of beta-amyloid (A beta). Oligomerization, fibrillization and neurotoxicity of native A beta associated with Alzheimer's disease (AD) type has been compared with E22Q A beta (amyloid beta-protein containing residues 1--40 with the native Glu at residue 22 changed to Gln) implicated in Dutch cerebral haemorrhage disease. Solutions of A beta rapidly yield soluble oligomers in a concentration-dependent manner, which are detected by the ELISA, and by size-exclusion gel chromatography. Conformational changes from disordered to beta-sheet occur more slowly than oligomerization, and fibrils are produced after prolonged incubation. The E22Q A beta oligomerizes, changes conformation and fibrillizes more rapidly than the native form and produces shorter stubbier fibrils. Aged fibrillar preparations of E22Q A beta are more potent than aged fibrils of native A beta in inducing apoptotic changes and toxic responses in human neuroblastoma cell lines, whereas low-molecular-mass oligomers in briefly incubated solutions are much less potent. The differences in the rates of oligomerization of the two A beta forms, their conformational behaviour over a range of pH values, and NMR data reported elsewhere, are consistent with a molecular model of oligomerization in which strands of A beta monomers initially overcome charge repulsion to form dimers in parallel beta-sheet arrangement, stabilized by intramolecular hydrophobic interactions, with amino acids of adjacent chains in register.

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.

Review: model peptides and the physicochemical approach to beta-amyloids

beta-Amyloid peptides are the main protein components of neuritic plaques and may be important in the pathogenesis of Alzheimer's Disease. The determination of the structure of beta-amyloid fibrils poses a challenge because of the limited solubility of beta-amyloid peptides and the noncrystalline nature of fibrils formed from these peptides. In this paper, we describe several physicochemical approaches which have been used to examine fibrils and the fibrillogenesis of peptide models of beta-amyloid. Recent advances in solid state NMR, such as the DRAWS pulse sequence, have made this approach a particularly attractive one for peptides such as beta-amyloid, which are not yet amenable to high-resolution solution phase NMR and crystallography. The application of solid state NMR techniques has yielded information on a model peptide comprising residues 10-35 of human beta-amyloid and indicates that in fibrils, this peptide assumes a parallel beta-strand conformation, with all residues in exact register. In addition, we discuss the use of block copolymers of Abeta peptides and polyethylene glycol as probes for the pathways of fibrillogenesis. These methods can be combined with other new methods, such as high-resolution synchrotron X-ray diffraction and small angle neutron and X-ray scattering, to yield structural data of relevance not only to disease, but to the broader question of protein folding and self-assembly.

Fresh and nonfibrillar amyloid beta protein(1-40) induces rapid cellular degeneration in aged human fibroblasts: evidence for AbetaP-channel-mediated cellular toxicity

Alzheimer's disease (AD) is primarily nonfamilial or sporadic (SAD) in origin, although several genetic linkages are reported. Tissues from AD patients contain fibrillar plaques made of 39 to 43 amino acid-long amyloid beta peptide (AbetaP), although the mechanisms of AbetaP toxicity are poorly understood. AbetaP(1-40) is the most prevalent AbetaP present in the neuronal and non-neuronal tissues from SAD patients. AbetaP(1-40) toxicity has been examined mainly after prolonged incubation and correlates with the age and fibrillar morphology of AbetaP(1-40). Globular and nonfibrillar AbetaPs are released continually during normal cellular metabolism; they elevate cellular Ca(2+) and form cation-permeable channels. However, their role in cellular toxicity is poorly understood. We have used an integrated atomic force and light fluorescence microscopy (AFM-LFM), laser confocal microscopy, and calcium imaging to examine real-time and acute effect of fresh and globular AbetaP(1-40) on cultured, aged human, AD-free fibroblasts. AFM images show that freshly prepared AbetaP(1-40) in phosphate-buffered saline (PBS) are globular and do not form fiber for an extended time period. AbetaP(1-40) induced rapid structural modifications, including cytoskeletal reorganization, retraction of cellular processes, and loss of cell-cell contacts, within minutes of incubation. This led to eventual cellular degeneration. AbetaP(1-40)-induced degeneration was prevented by anti-AbetaP antibody, zinc, and Tris, but not by tachykinin neuropeptides. In Ca(2+)-free extracellular medium, AbetaP(1-40) did not induce cellular degeneration. In the presence of extracellular Ca(2+), AbetaP(1-40) induced a sustained increase in the cellular Ca(2+). Thus, short-term and acute AbetaP(1-40) toxicity is mediated by Ca(2+) uptake, most likely via calcium-permeable AbetaP pores. Such rapid degeneration does not require fibrillar plaques, suggesting that the plaques may not have any causative role.

Protein aging hypothesis of Alzheimer disease

Alzheimer disease (AD), the most common form of aging-related neurodegenerative disorders, is associated with formation of fibrillar deposits of amyloid beta-protein (Abeta). While the direct involvement of Abeta in AD has been well documented, the relations between Abeta production, amyloid formation, and neurodegeneration remain unknown. We propose that AD is initiated by a protein aging-related structural transformation in soluble Abeta. We hypothesize that spontaneous chemical modification of aspartyl residues in Abeta to transient succinimide induces a non-native conformation in a fraction of soluble Abeta, rendering it amyloidogenic and neurotoxic. Conformationally altered Abeta is characterized by increased stability in solution and the presence of a non-native beta-turn that determines folding of Abeta in solution and the structure of Abeta subunits incorporated into amyloid fibrils. While the soluble 'non-native' Abeta is both the factor triggering the neurodegenerative cascade and the precursor of amyloid plaques, these two events result from interaction of Abeta with different sets of cellular components and need not coincide in space and time. Extensive literature data and experimental evidence are provided in support of this hypothesis.

Alzheimer's disease, beta-amyloid protein and zinc

Alzheimer's disease (AD) is characterized by amyloid deposits within the neocortical parenchyma and the cerebrovasculature. The main component of these predominantly extracellular collections, Abeta, which is normally a soluble component of all biological fluids, is cleaved out of a ubiquitously expressed parent protein, the amyloid protein precursor (APP), one of the type 1 integral membrane glycoproteins. Considerable evidence has indicated that there is zinc dyshomeostasis and abnormal cellular zinc mobilization in AD. We have characterized both APP and Abeta as copper/zinc metalloproteins. Zinc, copper and iron have recently been reported to be concentrated to 0.5 to 1 mmol/L in amyloid plaque. In vitro, rapid Abeta aggregation is mediated by Zn(II), promoted by the alpha-helical structure of Abeta, and is reversible with chelation. In addition, Abeta produces hydrogen peroxide in a Cu(II)/Fe(III)-dependent manner, and the hydrogen peroxide formation is quenched by Zn(II). Moreover, zinc preserves the nontoxic properties of Abeta. Although the zinc-binding proteins apolipoprotein E epsilon4 allele and alpha(2)-macroglobulin have been characterized as two genetic risk factors for AD, zinc exposure as a risk factor for AD has not been rigorously studied. Based on our findings, we envisage that zinc may serve twin roles by both initiating amyloid deposition and then being involved in mechanisms attempting to quench oxidative stress and neurotoxicity derived from the amyloid mass. Hence, it remains debatable whether zinc supplementation is beneficial or deleterious for AD until additional studies clarify the issue.

Two-dimensional structure of beta-amyloid(10-35) fibrils

Beta-amyloid (Abeta) peptides are the main protein component of the pathognomonic plaques found in the brains of patients with Alzheimer's disease. These heterogeneous peptides adopt a highly organized fibril structure both in vivo and in vitro. Here we use solid-state NMR on stable, homogeneous fibrils of Abeta(10-35). Specific interpeptide distance constraints are determined with dipolar recoupling NMR on fibrils prepared from a series of singly labeled peptides containing (13)C-carbonyl-enriched amino acids, and skipping no more that three residues in the sequence. From these studies, we demonstrate that the peptide adopts the structure of an extended parallel beta-sheet in-register at pH 7.4. Analysis of DRAWS data indicates interstrand distances of 5.3 +/- 0.3 A (mean +/- standard deviation) throughout the entire length of the peptide, which is compatible only with a parallel beta-strand in-register. Intrastrand NMR constraints, obtained from peptides containing labels at two adjacent amino acids, confirm the secondary structural findings obtained using DRAWS. Using peptides with (13)C incorporated at the carbonyl position of adjacent amino acids, structural transitions from alpha-helix to beta-sheet were observed at residues 19 and 20, but using similar techniques, no evidence for a turn could be found in the putative turn region comprising residues 25-29. Implications of this extended parallel organization for Abeta(10-35) for overall fibril formation, stability, and morphology based upon specific amino acid contacts are discussed.

Structural studies of soluble oligomers of the Alzheimer beta-amyloid peptide

Recent studies have suggested that non-fibrillar soluble forms of Abeta peptides possess neurotoxic properties and may therefore play a role in the molecular pathogenesis of Alzheimer's disease. We have identified solution conditions under which two types of soluble oligomers of Abeta40 could be trapped and stabilized for an extended period of time. The first type of oligomers comprises a mixture of dimers/tetramers which are stable at neutral pH and low micromolar concentration, for a period of at least four weeks. The second type of oligomer comprises a narrow distribution of particles that are spherical when examined by electron microscopy and atomic force microscopy. The number average molecular mass of this distribution of particles is 0.94 MDa, and they are are stable at pH 3 for at least four weeks. Circular dichroism studies indicate that the dimers/tetramers possess irregular secondary structure that is not alpha-helix or beta-structure, while the 0.94 MDa particles contain beta-structure. Fluorescence resonance energy transfer experiments indicate that Abeta40 moieties in amyloid fibrils or protofibrils are more similar in structure to those in the 0.94 MDa particles than those in the dimers/tetramers. These findings indicate that soluble oligomeric forms of Abeta peptides can be trapped for extended periods of time, enabling their study by high resolution techniques that would not otherwise be possible.

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.

Molecular assembly of endogenous and synthetic big atrial natriuretic peptide (ANP) and its amyloidogenic implications

The aggregation process of alpha-hANP has been investigated in vitro at physiological concentrations by gel chromatographic procedures using a radiolabeled tracer incubated in PBS and in plasma. In PBS big forms of ANP are organized as a peak eluting from both Sephacryl S-100 and S-300 HR in the void volume of the columns; in plasma, besides this major peak, a second radioactive peak is evident, eluting from Sephacryl S-100 HR around the HSA region. After gel chromatography on Sephacryl S-300 HR the major peak appears to consist of three components of different molecular size. Some information about the nature of these peak materials comes from the result of parallel incubations of partially aggregated (seed or nucleus) and aggregate depleted tracer. The comparison between the two time courses of big ANP formation indicates that: (a) ANP aggregation is a nucleation-dependent process, with a lag time longer than 8 days, at picogram peptide levels and (b) the aggregated forms of peptide are those eluting in the void volume, the other plasma peaks being probably expression of a binding, neither saturable or reversible, to some plasma components. The principle of seeded polymerization, used to detect ANP aggregates present in the plasma, indicates that: (a) the endogenous big ANP cannot act as a nucleus for polymerization and it likely consists of non-fibrillar ANP aggregates and/or bound ANP, and (b) this experimental approach can be suitable to evidence ANP binding plasma factors for further characterization studies.