Structure of beta-crystallite assemblies formed by Alzheimer beta-amyloid protein analogues: analysis by x-ray diffraction

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.

Structural features of the Abeta amyloid fibril elucidated by limited proteolysis

Although the gross morphology of amyloid fibrils is fairly well understood, very little is known about how the constituent polypeptides fold within the amyloid folding motif. In the experiments reported here, we used trypsin and chymotrypsin to conduct limited proteolysis studies on synthetic amyloid fibrils composed of the Alzheimer's disease peptide Abeta(1-40). In both reactions, the extreme N-terminal proteolytic fragment is released from fibrils as rapidly as it is from the Abeta monomer, while other proteolytic fragments are generated much more slowly. Furthermore, aggregated material isolated by centrifugation of intermediate digestion time points from both proteases contains, in addition to full-length material, peptides that possess mature C-termini but truncated N-termini. These data strongly suggest that the N-terminal region of Abeta is not involved in the beta-sheet network of the amyloid fibril, while the C-terminus is essentially completely engaged in protective-presumably beta-sheet-structure. In both digests, release of the extreme N-terminal fragments of Abeta(1-40) reaches plateau values corresponding to about 80% of the total available Abeta. This suggests that there are two classes of peptides in the fibril: while the majority of Abeta molecules have an exposed N-terminus, about 20% of the peptides have an N-terminus that is protected from proteolysis within the fibril structure. The most likely cause of this heterogeneity is the lateral association of protofilaments into the fibril structure, which would be expected to generate a unique environment for those Abeta N-termini located at protofilament packing interfaces and/or in the interior core region between the packed protofilaments. This suggests that the N-terminal region of Abeta, while not directly involved in the beta-sheet network of the fibril, may contribute to fibril stability by participating in protofilament packing.

Zinc binding to Alzheimer's Abeta(1-16) peptide results in stable soluble complex

Aggregation of the human amyloid beta-peptide (Abeta) into insoluble plaques is a key event in Alzheimer's disease. Zinc sharply accelerates the Abeta aggregation in vitro, and the Abeta region 6-28 was suggested to be the obligatory zinc binding site. However, time-dependent aggregation of the zinc-bound Abeta species investigated so far prevented their structural analysis. By using CD spectroscopy, we have shown here for the first time that (i) the protected synthetic peptide spanning the fragment 1-16 of Abeta binds specifically zinc with 1:1 and 1:2 stoichiometry under physiologically relevant conditions; (ii) the peptide-zinc complex is soluble and stable for several months; (iii) zinc binding causes a conformational change of the peptide towards a more structured state. These findings suggest the region 1-16 to be the minimal autonomous zinc binding domain of Abeta.

Mechanisms of amyloid beta protein-induced modification in ion transport systems: implications for neurodegenerative diseases

1. Alzheimer's disease (AD) is a neurodegenerative disorder that affects the cognitive function of the brain. Pathological changes in AD are characterized by the formation of amyloid plaques and neurofibrillary tangles as well as extensive neuronal loss. Abnormal proteolytic processing of amyloid precursor protein (APP) is the central step that leads to formation of amyloid plaque, neurofibrillary tangles, and neuronal loss. 2. The plaques, which accumulate extracellularly in the brain, are composed of aggregates and cause direct neurotoxic effects and/or increase neuronal vulnerability to excitotoxic insults. The aggregates consist of soluble pathologic amyloid beta peptides AbetaP[1-42] and AbetaP[1-43] and soluble nonpathologic AbetaP[1-40]. Both APP and AbetaP interact with ion transport systems. AbetaP induces a wide range of effects as the result of activating a cascade of mechanisms. 3. The major mechanisms proposed for AbetaP-induced cytotoxicity involve the loss of Ca2+ homeostasis and the generation of reactive oxygen species (ROS). The changes in Ca2+ homeostasis could be the result of (1) changes in endogenous ion transport systems, e.g. Ca2+ and K+ channels and Na+/K+-ATPase, and membrane receptor proteins, such as ligand-driven ion channels and G-protein-driven releases of second messengers, and (2) formation of heterogeneous ion channels. 4. The consequences of changes in Ca2+-homeostasis-induced generation of ROS are (a) direct modification of intrinsic ion transport systems and their regulatory mechanisms, and (b) indirect effects on ion transport systems via peroxidation of phospholipids in the membrane, inhibition of phosphorylation, and reduction of ATP levels and cytoplasmic pH. 5. We propose that in AD, AbetaP with its different conformations alters cell regulation by modifying several ion transport systems and also by forming heterogeneous ion channels. The changes in membrane transport systems are proposed as early steps in impairing neuronal function preceding plaque formation. We conclude that these changes damage the membrane by compromising its integrity and increasing its ion permeability. This mechanism of membrane damage is not only central for AD but also may explain other malfunctioned protein-processing-related pathologies.

Molecular structure of a fibrillar Alzheimer's A beta fragment

Amyloid-beta (Abeta) peptide deposition as fibrillar senile plaques is a key element in the pathology of Alzheimer's disease. Here we present a high-resolution structure of an Abeta amyloid fibril using magnetically aligned preparations of a central Abeta domain which forms representative amyloid fibrils. Diffraction analysis of these samples revealed Bragg reflections on layer lines consistent with a preferred orientation, as opposed to the typical symmetry associated with fibers. These crystalline properties permitted a molecular replacement approach based upon a beta-hairpin motif resulting in a structure of the fibrillar Abeta peptide. This detailed molecular structure of Abeta in its fibrous state provides clues as to the mechanism of amyloid assembly and identifies potential targets for controlling the aggregation process.

Histidine residues underlie Congo red binding to A beta analogs

The binding mechanism of Congo red (CR) to Alzheimer's disease (AD) amyloid fibrils (A beta) in terms of binding affinity and number of sites was quantitated from absorption spectroscopy (at 200-700 nm) by measuring the concentration of CR bound (CR-B) to AD A beta assemblies as a function of CR concentration and pH in 80% ethanol. The rationale for the use of this high concentration of ethanol derives from its use in histological screens for amyloid in tissue sections. Moreover, free CR can be separated from bound CR by filtration in ethanolic but not aqueous medium. The A beta analogs studied here included: (1) peptides having different lengths: A beta1-40, A beta11-28, A beta13-28, A beta19-28, A beta11-25; (2) wildtype, control sequences of A beta1-40 and sequences having different natural amino acid substitutions: primate Pr1-40, rodent Ro1-40, hereditary cerebral haemorrhage with amyloidosis, Dutch type (HCHWA-D) Du1-40, primate reverse sequence Pr40-1; and (3) A beta11-25 sequences having different substitutions: H13D, H14D, and D23K. Negative-staining showed that A beta1-40 fibrils in buffer were indistinguishable from those in buffered ethanolic medium. For all amyloid analogs except A beta19-28, which has no histidine residues and showed no CR binding over the entire pH range 4.0-9.5, CR-B decreased as a function of increasing pH. The decrease was steepest at about pH 5 and became zero above pH 7. For analogs having the same number of histidines, CR-B fell on the same binding curve, indicating that histidine residues are the likely binding sites for CR in this medium. The pH titration of the binding was parameterized by the stoichiometry of dye to the sites, the number of histidines per molecule, the binding dissociation constant Kd, and the apparent proton dissociation constant pK of the histidine; and the calculated pH-titration curves were found to fit the observed ones. For the peptides having 1-3 histidines the average pK was 5.0-5.5, which was similar to the expected pK of histidine in low dielectric medium (80% ethanol), and the Kd's were 2.8-5.9 microM. That histidine residues underlie CR binding in A beta amyloid is consistent with previous findings that A beta peptides sediment as fibrillar assemblies at pH-3-7 and bind Congo red over the same pH range in aqueous medium. Further, the conformation near the binding motif His13-His14-Gln15-Lys16 in A beta assemblies is not greatly altered in 80% ethanol.

Structural changes in a hydrophobic domain of the prion protein induced by hydration and by ala-->Val and pro-->Leu substitutions

X-ray diffraction was used to study the structure of assemblies formed by synthetic peptide fragments of the prion protein (PrP) that include the hydrophobic domain implicated in the Gerstmann-Sträussler-Scheinker (GSS) mutation (P102L). The effects of hydration on polypeptide assembly and of Ala-->Val substitutions in the hydrophobic domain were characterized. Synthetic peptides included: (i) Syrian hamster (SHa) hydrophobic core, SHa106-122 (KTNMKHMAGAAAAGAVV); (ii) SHa104-122(3A-V), with A-->V mutations at 113, 115 and 118 (KPKTNMKHMVGVAAVGAVV); (iii) mouse (Mo) wild-type sequence of the N-terminal hydrophobic domain, Mo89-143WT; and (iv) the same mouse sequence with leucine substitution for proline at residue number 101, Mo89-143(P101L). Samples of SHa106-122 that formed assemblies while drying under ambient conditions showed X-ray patterns indicative of 33 A thick slab-like structures having extensive H-bonding and intersheet stacking. By contrast, lyophilized peptide that was equilibrated against 100 % relative humidity showed assemblies with only a few layers of beta-sheets. The Ala-->Val substitutions in SHa104-122 and Mo89-143(P101L) resulted in the formation of 40 A wide, cross-beta fibrils. Observation of similar size beta-sheet fibrils formed by peptides SHa104-122(3A-V) and the longer Mo89-143(P101L) supports the notion that the hydrophobic sequence forms a template or core that promotes the beta-folding of the longer peptide. The substitution of amino acids in the mutants, e.g. 3A-->V and P101L, enhances the folding of the peptide into compact structural units, significantly enhancing the formation of the extensive beta-sheet fibrils.

The protofilament substructure of amyloid fibrils

Tissue deposition of normally soluble proteins, or their fragments, as insoluble amyloid fibrils causes the usually fatal, acquired and hereditary systemic amyloidoses and is associated with the pathology of Alzheimer's disease, type 2 diabetes and the transmissible spongiform encephalopathies. Although each type of amyloidosis is characterised by a specific amyloid fibril protein, the deposits share pathognomonic histochemical properties and the structural morphology of all amyloid fibrils is very similar. We have previously demonstrated that transthyretin amyloid fibrils contain four constituent protofilaments packed in a square array. Here, we have used cross-correlation techniques to average electron microscopy images of multiple cross-sections in order to reconstruct the sub-structure of ex vivo amyloid fibrils composed of amyloid A protein, monoclonal immunoglobulin lambda light chain, Leu60Arg variant apolipoprotein AI, and Asp67His variant lysozyme, as well as synthetic fibrils derived from a ten-residue peptide corresponding to the A-strand of transthyretin. All the fibrils had an electron-lucent core but the packing arrangement comprised five or six protofilaments rather than four. The structural similarity that defines amyloid fibres thus exists principally at the level of beta-sheet folding of the polypeptides within the protofilament, while the different types vary in the supramolecular assembly of their protofilaments.

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.

Approaches to discovery and characterization of inhibitors of amyloid beta-peptide polymerization

Polymerization of the amyloid beta-peptide (Abeta) has been identified as a major feature of the pathogenesis of Alzheimer's disease (AD). Inhibition of the formation of these toxic polymers of Abeta has thus emerged as an approach to developing therapeutics for AD. Techniques for studying Abeta polymerization include the use of fibril nucleation and extension assays in a variety of formats. Detection of polymeric forms of Abeta has been achieved using turbidity, dye binding, light scattering and toxicity among other methods. Direct and indirect methods have been described for the measurement of binding affinities for Abeta fibrils. Imaging techniques include electron microscopy, X-ray diffraction and atomic force microscopy. These techniques have been used to characterize different classes of compounds that inhibit the formation of Abeta polymers. These compounds include dyes such as Congo Red, the antibiotic rifampicin, the anthracycline 4'-iodo-4'-deoxydoxorubicin, and a large variety of Abeta-derived peptides and modified peptides, among other reported inhibitors.

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.

Activation barriers to structural transition determine deposition rates of Alzheimer's disease a beta amyloid

Brain amyloid composed of the approximately 40-amino-acid human beta-amyloid peptide A beta is integral to Alzheimer's disease pathology. To probe the importance of a conformational transition in Abeta during amyloid growth, we synthesized and examined the solution conformation and amyloid deposition activity of A beta congeners designed to have similar solution structures but to vary substantially in their barriers to conformational transition. Although all these peptides adopt similar solution conformations, a covalently restricted Abeta congener designed to have a very high barrier to conformational rearrangement was inactive, while a peptide designed to have a reduced barrier to conformational transition displayed an enhanced deposition rate relative to wild-type A beta. The hyperactive peptide, which is linked to a heritable A beta amyloidosis characterized by massive amyloid deposition at an early age, displayed a reduced activation barrier to deposition consistent with a larger difference in activation entropy than in activation enthalpy relative to wild-type A beta. These results suggest that in Alzheimer's disease, as in the prion diseases, a conformational transition in the depositing peptide is essential for the conversion of soluble monomer to insoluble amyloid, and alterations in the activation barrier to this transition affect amyloidogenicity and directly contribute to human disease.

A beta fibrillogenesis: kinetic parameters for fibril formation from congo red binding

Using Scatchard analysis, we have formulated as a function of time and pH the relationship between the binding of Congo red to Alzheimer's beta-amyloid and the aggregation number (i.e., monomer concentration within fibrils) as defined by nucleation-dependent self-assembly. This provides a basis on which to determine the kinetic parameters for fibril formation from the observed concentration of bound Congo red.

The Alzheimer's peptide a beta adopts a collapsed coil structure in water

The self-assembly of the soluble peptide Abeta into Alzheimer's disease amyloid is believed to involve a conformational change. Hence the solution conformation of Abeta is of significant interest. In contrast to studies in other solvents, in water Abeta is collapsed into a compact series of loops, strands, and turns and has no alpha-helical or beta-sheet structure. Conformational stabilization is primarily attributed to van der Waals and electrostatic forces. A large conspicuous uninterrupted hydrophobic patch covers approximately 25% of the surface. The compact coil structure appears meta-stable, and because fibrillization leads to formation of intermolecular beta-sheet secondary structure, a global conformational rearrangement is highly likely. A molecular hypothesis for amyloidosis includes at least two primary driving forces, changes in solvation thermodynamics during formation of amyloid deposits and relief of internal conformational stress within the soluble precursor during formation of lower-energy amyloid fibrils.

Review: modulating factors in amyloid-beta fibril formation

Amyloid formation is a key pathological feature of Alzheimer's disease and is considered to be a major contributing factor to neurodegeneration and clinical dementia. Amyloid is found as both diffuse and senile plaques in the parenchyma of the brain and is composed primarily of the 40- to 42-residue amyloid-beta (Abeta) peptides. The characteristic amyloid fiber exhibits a high beta-sheet content and may be generated in vitro by the nucleation-dependent self-association of the Abeta peptide and an associated conformational transition from random to beta-conformation. Growth of the fibrils occurs by assembly of the Abeta seeds into intermediate protofibrils, which in turn self-associate to form mature fibers. This multistep process may be influenced at various stages by factors that either promote or inhibit Abeta fiber formation and aggregation. Identification of these factors and understanding the driving forces behind these interactions as well as the structural motifs necessary for these interactions will help to elucidate potential sites that may be targeted to prevent amyloid formation and its associated toxicity. This review will discuss some of the modulating factors that have been identified to date and their role in fibrillogenesis.

Betabellins 15D and 16D, de Novo designed beta-sandwich proteins that have amyloidogenic properties

The betabellin structure is a de novo designed beta-sandwich protein consisting of two 32-residue beta-sheets packed against one another by hydrophobic interactions. d-Amino acid residues are used to energetically favor formation of type-I' beta turns. Air oxidation of betabellin 15S (B15S) (HSLTAKIpkLTFSIAphTYTCAVpkYTAKVSH, where p denotes d-Pro, h denotes d-His, and k denotes d-Lys) yields betabellin 15D (B15D), a 64-residue disulfide-bridged protein. The amino acid sequence of B15D contains a conformationally constrained d-Pro residue at the i + 1 position of each type-I' beta turn. To test whether d-Pro residues are necessary for folding at these positions, the six d-Pro residues of B15D are replaced by d-Ala residues in betabellin 16D (B16D). Previously, transmission electron microscopy showed that B15D forms unbranched, 35-A wide fibrils that associate into bundles in 5.0 mM 3-(N-morpholino)propanesulfonate and 250 mM NaCl at pH 7; under these conditions, B16D forms ribbon-like assemblies. The B15D fibrils resemble the protofilaments that constitute amyloid fibrils. The present studies show that both B15D and B16D have characteristics of amyloidogenic proteins: the unbranched fibrils and ribbons stained with Congo red and displayed a green birefringence, exhibited a cross-beta structure, and bound 1-anilino-8-naphthalenesulfonate. Thus, these de novo designed beta-sandwich proteins should provide useful models for studying the mechanism of amyloid protofilament formation and assembly into amyloid fibrils and for designing potential inhibitors of amyloidogenesis.

Self-assembly of beta-amyloid 42 is retarded by small molecular ligands at the stage of structural intermediates

Assemblyof the amyloid-beta peptide (Abeta) into fibrils and its deposition in distinct brain areas is considered responsible for the pathogenesis of Alzheimer's disease (AD). Thus, inhibition of fibril assembly is a potential strategy for therapeutic intervention. Electron cryomicroscopy was used to monitor the initial, native assembly structure of Abeta42. In addition to the known fibrillar intermediates, a nonfibrillar, polymeric sheet-like structure was identified. A temporary sequence of supramolecular structures was revealed with (i) polymeric Abeta42 sheets during the onset of assembly, inversely related to the appearance of (ii) fibril intermediates, which again are time-dependently replaced by (iii) mature fibrils. A cell-based primary screening assay was used to identify compounds that decrease Abeta42-induced toxicity. Hit compounds were further assayed for binding to Abeta42, radical scavenger activity, and their influence on the assembly structure of Abeta42. One compound, Ro 90-7501, was found to efficiently retard mature fibril formation, while extended polymeric Abeta42 sheets and fibrillar intermediates are accumulated. Ro 90-7501 may serve as a prototypic inhibitor for Abeta42 fibril formation and as a tool for studying the molecular mechanism of fibril assembly.

Direct visualisation of the beta-sheet structure of synthetic Alzheimer's amyloid

Amyloid fibrils are a major pathological feature of Alzheimer's disease as well as other amyloidoses including the prion diseases. They are an unusual phenomenon, being made up of different, normally soluble proteins which undergo a profound conformational change and assemble to form very stable, insoluble fibrils which accumulate in the extracellular spaces. In Alzheimer's disease the amyloid fibrils are composed of the A beta protein. Knowledge of the structure of amyloid is essential for understanding the abnormal assembly and deposition of these fibrils and could lead to the rational design of therapeutic agents for their prevention or disaggregation. Here we reveal the core structure of an Alzheimer's amyloid fibril by direct visualisation using cryo-electron microscopy. Synthetic amyloid fibrils composed of A beta residues 11 to 25 and 1 to 42 were examined. The A beta (11-25) fibrils are clearly composed of beta-sheet structure that is observable as striations across the fibres. The beta-strands run perpendicular to the fibre axis and the projections show that the fibres are composed of beta-sheets with the strands in direct register. This observation has implications not only for the further understanding of amyloid, but also for the development of cryo-electron microscopy for direct visualisation of secondary structure.

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.

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.

Manipulating the amyloid-beta aggregation pathway with chemical chaperones

Amyloid-beta (Abeta) assembly into fibrillar structures is a defining characteristic of Alzheimer's disease that is initiated by a conformational transition from random coil to beta-sheet and a nucleation-dependent aggregation process. We have investigated the role of organic osmolytes as chemical chaperones in the amyloid pathway using glycerol to mimic the effects of naturally occurring molecules. Osmolytes such as the naturally occurring trimethylamine N-oxide and glycerol correct folding defects by preferentially hydrating partially denatured proteins and entropically stabilize native conformations and polymeric states. Trimethylamine N-oxide and glycerol were found to rapidly accelerate the Abeta random coil-to-beta-sheet conformational change necessary for fiber formation. This was accompanied by an immediate conversion of amorphous unstructured aggregates into uniform globular and possibly nucleating structures. Osmolyte-facilitated changes in Abeta hydration also affected the final stages of amyloid formation and mediated transition from the protofibrils to mature fibers that are observed in vivo. These findings suggest that hydration forces can be used to control fibril assembly and may have implications for the accumulation of Abeta within intracellular compartments such as the endoplasmic reticulum and in vitro modeling of the amyloid pathway.

Monitoring the assembly of Ig light-chain amyloid fibrils by atomic force microscopy

Aggregation of Ig light chains to form amyloid fibrils is a characteristic feature of light-chain amyloidosis, a light-chain deposition disease. A recombinant variable domain of the light chain SMA was used to form amyloid fibrils in vitro. Fibril formation was monitored by atomic force microscopy imaging. Single filaments 2.4 nm in diameter were predominant at early times; protofibrils 4.0 nm in diameter were predominant at intermediate times; type I and type II fibrils 8.0 nm and 6.0 nm in diameter, respectively, were predominant at the endpoints. The increase in number of fibrils correlated with increased binding of the fluorescent dye thioflavin T. The fibrils and protofibrils showed a braided structure, suggesting that their formation involves the winding of protofibrils and filaments, respectively. These observations support a model in which two filaments combine to form a protofibril, two protofibrils intertwine to form a type I fibril, and three filaments form a type II fibril.

Amyloid beta-protein fibrillogenesis. Structure and biological activity of protofibrillar intermediates

Alzheimer's disease is characterized by extensive cerebral amyloid deposition. Amyloid deposits associated with damaged neuropil and blood vessels contain abundant fibrils formed by the amyloid beta-protein (Abeta). Fibrils, both in vitro and in vivo, are neurotoxic. For this reason, substantial effort has been expended to develop therapeutic approaches to control Abeta production and amyloidogenesis. Achievement of the latter goal is facilitated by a rigorous mechanistic understanding of the fibrillogenesis process. Recently, we discovered a novel intermediate in the pathway of Abeta fibril formation, the amyloid protofibril (Walsh, D. M., Lomakin, A., Benedek, G. B., Condron, M. M., and Teplow, D. B. (1997) J. Biol. Chem. 272, 22364-22372). We report here results of studies of the assembly, structure, and biological activity of these polymers. We find that protofibrils: 1) are in equilibrium with low molecular weight Abeta (monomeric or dimeric); 2) have a secondary structure characteristic of amyloid fibrils; 3) appear as beaded chains in rotary shadowed preparations examined electron microscopically; 4) give rise to mature amyloid-like fibrils; and 5) affect the normal metabolism of cultured neurons. The implications of these results for the development of therapies for Alzheimer's disease and for our understanding of fibril assembly are discussed.

An atomic model for the pleated beta-sheet structure of Abeta amyloid protofilaments

Synchrotron x-ray studies on amyloid fibrils have suggested that the stacked pleated beta-sheets are twisted so that a repeating unit of 24 beta-strands forms a helical turn around the fibril axis (. J. Mol. Biol. 273:729-739). Based on this morphological study, we have constructed an atomic model for the twisted pleated beta-sheet of human Abeta amyloid protofilament. In the model, 48 monomers of Abeta 12-42 stack (four per layer) to form a helical turn of beta-sheet. Each monomer is in an antiparallel beta-sheet conformation with a turn located at residues 25-28. Residues 17-21 and 31-36 form a hydrophobic core along the fibril axis. The hydrophobic core should play a critical role in initializing Abeta aggregation and in stabilizing the aggregates. The model was tested using molecular dynamics simulations in explicit aqueous solution, with the particle mesh Ewald (PME) method employed to accommodate long-range electrostatic forces. Based on the molecular dynamics simulations, we hypothesize that an isolated protofilament, if it exists, may not be twisted, as it appears to be when in the fibril environment. The twisted nature of the protofilaments in amyloid fibrils is likely the result of stabilizing packing interactions of the protofilaments. The model also provides a binding mode for Congo red on Abeta amyloid fibrils. The model may be useful for the design of Abeta aggregation inhibitors.

Basement membranes, microfibrils and beta amyloid fibrillogenesis in Alzheimer's disease: high resolution ultrastructural findings

It is known that beta amyloid fibrils are deposited at the basement membrane of the cerebromicrovasculature in the brains of patients with Alzheimer's disease, and the assembly of the fibrils may be in continuation with the core of senile plaques. The fibrils accumulate in a manner similar to that in which microfibrils accumulate in the glomerular basement membrane of the rat kidney during long-term experimental diabetes, and in the alveolar-capillary basement membrane of the normal lung. beta amyloid fibrils in-situ are known to be about 10 nm wide tubular structures and they closely resemble connective tissue microfibrils. Our recent high resolution ultrastructural studies combined with immunogold labeling demonstrated that beta amyloid fibrils in-situ are indeed microfibril-like structures, and the beta protein is associated with their surface in the form of loose assemblies of 1 nm wide flexible filaments. Thus, the result of this study indicates that in-situ a major component of the beta amyloid deposit is the microfibril-like structure. The elucidation of the mechanism of cerebral beta amyloid fibrillogenesis in Alzheimer's disease may therefore require understanding the mechanism of 'normal' microfibrils biogenesis.

In situ atomic force microscopy study of Alzheimer's beta-amyloid peptide on different substrates: new insights into mechanism of beta-sheet formation

We have applied in situ atomic force microscopy to directly observe the aggregation of Alzheimer's beta-amyloid peptide (Abeta) in contact with two model solid surfaces: hydrophilic mica and hydrophobic graphite. The time course of aggregation was followed by continuous imaging of surfaces remaining in contact with 10-500 microM solutions of Abeta in PBS (pH 7.4). Visualization of fragile nanoscale aggregates of Abeta was made possible by the application of a tapping mode of imaging, which minimizes the lateral forces between the probe tip and the sample. The size and the shape of Abeta aggregates, as well as the kinetics of their formation, exhibited pronounced dependence on the physicochemical nature of the surface. On hydrophilic mica, Abeta formed particulate, pseudomicellar aggregates, which at higher Abeta concentration had the tendency to form linear assemblies, reminiscent of protofibrillar species described recently in the literature. In contrast, on hydrophobic graphite Abeta formed uniform, elongated sheets. The dimensions of those sheets were consistent with the dimensions of beta-sheets with extended peptide chains perpendicular to the long axis of the aggregate. The sheets of Abeta were oriented along three directions at 120 degrees to each other, resembling the crystallographic symmetry of a graphite surface. Such substrate-templated self-assembly may be the distinguishing feature of beta-sheets in comparison with alpha-helices. These studies show that in situ atomic force microscopy enables direct assessment of amyloid aggregation in physiological fluids and suggest that Abeta fibril formation may be driven by interactions at the interface of aqueous solutions and hydrophobic substrates, as occurs in membranes and lipoprotein particles in vivo.

Tetrameric assembly of full-sequence protein zero myelin glycoprotein by synchrotron x-ray scattering

Highly purified myelin P0 glycoprotein was solubilized to 1-8 mg/ml in 0.1% sodium dodecyl sulfate (SDS), and the solution structure of the P0 assembly was studied using synchrotron x-ray scattering. The full-length P0, which was isolated from bovine intradural roots, included both the extracellular and cytoplasmic domains of the molecule. At the higher concentrations (4, 6, and 8 mg/ml, respectively), an x-ray intensity maximum was observed at 316 A, 245 A, and 240 A Bragg spacing. Because the position of this intensity depended on P0 concentration, it is most likely due to interparticle interference. By contrast, the position of a second intensity maximum, which was at approximately 40 A Bragg spacing, was invariant with P0 concentration. This latter intensity was accounted for by monodispersed, 80 A-diameter particles that are composed of eight, approximately 30 A-diameter spheres. Chemical parameters suggest that the 80 A particles correspond to the size of a tetramer of P0 molecules. Therefore, the approximately 30 A spheres would correspond to the sizes of the extracellular and cytoplasmic domains for each of the P0 monomers. The invariance of the second intensity maximum with P0 concentration indicates that the structure of the 80 A-diameter, tetrameric particles is unaltered. According to the liquid model for interparticle interference from charged spheres, the 80 A-diameter particle has 10 negative surface charges which likely arise from negatively charged SDS molecules bound to the transmembrane domain of P0. This binding, however, apparently does not alter the tetrameric assembly of P0, suggesting that intermolecular interactions involving extracellular domains and cytoplasmic domains likely stabilize this assembly. Some of our results have been published in abstract form (Inouye, H., H. Tsuruta, D. A. Kirschner, J. Sedzik, and K. Uyemura. Abstracts of the 4th International School and Symposium on Synchrotron Radiation in Natural Science, June 15-20, 1998. Ustron-Jaszowiec, Poland. p. 31).

In vitro amyloid fibril formation by synthetic peptides corresponding to the amino terminus of apoSAA isoforms from amyloid-susceptible and amyloid-resistant mice

Specific proteins of the apolipoprotein serum amyloid (apoSAA) family that are synthesized in large quantities during the acute, early phase of inflammation can serve as the proteinaceous precursors for amyloid fibrils. To model fibrillogenesis in such inflammatory diseases, we have used electron microscopy and X-ray diffraction to examine the structures formed by synthetic peptides corresponding in sequence to the 11 amino-terminal amino acids of murine apoSAA1, apoSAAcej, and apoSAA2 and to the 15 amino-terminal amino acids of apoSAA2. This region is reported to be the major fibrillogenic determinant of apoSAA isoforms. Both in 1 mM Tris buffer and in 35% acetonitrile, 0.1% trifluoracetic acid (ACN/TFA), all of the peptides formed macromolecular assemblies consisting of twisted, approximately 40- to 60-A-thick ribbons, which varied in width from around 40-70 A (for 11-mer apoSAA2 in Tris) up to 900 A (for the other peptides). X-ray diffraction patterns recorded from lyophilized peptides, vapor-hydrated samples, and solubilized/dried samples showed hydrogen bonding and intersheet reflections typical of a beta-pleated sheet conformation. The coherent lengths measured from the breadths of the X-ray reflections indicated that with hydration the growth of the assemblies in the intersheet stacking direction was comparable to that in the hydrogen-bonding direction, and analysis of oriented samples showed that the beta-strands were oriented perpendicular to both the long axis and the face of the assemblies. These X-ray results are consistent with the ribbon- or plate-like morphology of the individual aggregates and emphasize the polymorphic nature of amyloidogenic peptides. Our findings demonstrate that X-ray diffraction measurements on vapor-hydrated or solubilized/dried versus lyophilized, amyloidogenic peptides are a good indicator of their fibrillogenic potential. For example, from the highest to the lowest potential, the peptides examined here were ranked as: Abeta1-28 > Abeta1-40 > apoSAA1 approximately apoSAAcej > apoSAA2 > Abeta17-42. Experiments in which the three different 11-mer apoSAA isoforms were solubilized in ACN/TFA and then combined as binary mixtures showed that the ribbon morphology was not affected but that the extent of hydrogen bonding in the assemblies was substantially reduced. Our observations on the in vitro assembly of apoSAA analogs emphasize that amyloid fibril formation and morphology depend on primary sequence, length of polypeptide chain, the presence of additional fibrillogenic polypeptides, and solvent conditions.

Polypeptide chain folding in the hydrophobic core of hamster scrapie prion: analysis by X-ray diffraction

Conversion of the noninfectious, cellular form of the scrapie prion (PrPC) to the infectious form (PrPSc) is thought to be driven by an alpha-helical to beta-sheet conformational transition. The N-truncated polypeptide PrP27-30, which encompasses residues 90-231 of PrPSc and from which the truncated peptide is derived by limited proteolysis, assembles into amyloid rods that are rich in the beta-sheet conformation. The N-terminal half of PrP27-30, which includes residues 90-145 of PrP (SHa90-145) and contains the two putative alpha-helical domains H1 (PrP109-122) and H2 (PrP129-141), appears to be particularly crucial in the alpha --> beta conversion. To assess their role in this conformational transition, we have analyzed in detail X-ray diffraction patterns from the prion-related peptides A8A (PrP113-120), H1, and SHa90-145. We used iterative Fourier synthesis with beta-silk as an initial model for assigning phases. For H1, the lyophilized and acetonitrile-solubilized/dehydrated specimens gave two different electron density maps. The former showed that the beta-sheets were composed of small side chains as in A8A. The latter showed two types of beta-sheets having smaller and larger side chains, suggesting a turn. Such a turn was not observed in the lyophilized H1, indicating that the internal turn in H1 depends on the physical-chemical environment. In SHa90-145, the beta-chains are assembled in approximately 40 A-wide crystal domains (termed beta-crystallites), and the electron density maps of these crystallites showed evidence for turns within both the H1 and H2 domains. The molecular folding of H1-H2 is compared here with the recent NMR solution structure of recombinant hamster prion, and the effect of pH on the conformational change is discussed. The most compact structure based on the X-ray diffraction analysis showed that the N-terminal, smaller residues of H2 fold back and are hydrogen-bonded with the C-terminal, smaller residues of H1. Similar folding is observed in the NMR solution structure. Comparison of the NMR structures at different pH with the X-ray diffraction results suggests that histidine and lysine residues in the N-terminal sequence of PrP may figure in the alpha --> beta structure transition of PrP.

Analysis of x-ray diffraction patterns from amyloid of biopsied vitreous humor and kidney of transthyretin (TTR) Met30 familial amyloidotic polyneuropathy (FAP) patients: axially arrayed TTR monomers constitute the protofilament

Familial amyloidotic polyneuropathy (FAP) is characterized by deposits of amyloid fibers in which the major protein component is transthyretin (TTR). Nearly fifty mutations have been reported for the TTR in hereditary FAP. Protein crystallography of mutant TTRs has shown that the molecular structures of the variant molecules are similar to those found in the wild type. On this basis, the FAP fibers were initially proposed to consist of native-like TTR tetramers. In the current paper, we used x-ray fiber diffraction to study the structure of FAP fibers from biopsy samples of vitreous humor and kidney. The reflections of the vitreous sample showed a cross-beta diffraction pattern. All the meridional reflections were indexed by a one-dimensional, 29 A-period lattice, and the equatorial reflections were indexed by an apparent one-dimensional 67 A-period lattice. The x-ray intensity distribution indicated that the unit structure, which is similar to a TTR monomer, is composed of a pair of beta-sheets consisting of four hydrogen-bonded beta-chains per sheet, with the beta-chains oriented approximately normal to the fiber axis. The axial disposition of these units, with a 29 A-period, constitutes the protofilament; and a tetrameric lateral assembly of the protofilaments containing the core domain of the approximately 20 A-wide beta-sheet structure constitutes the FAP amyloid fiber. An inter-fiber separation of 75 A in these concentrated samples accounts for the apparent one-dimensional lattice perpendicular to the fiber axis. In the delipidated kidney FAP sample, the diffraction pattern indicated a pair of beta-sheets, suggesting that the protofilament structure in kidney is similar to that in vitreous humor. In the non-delipidated sample the successive sharp reflections indexed to a one-dimensional, 48.9 A-lattice, and the electron density projection showed a density elevation at the center of a lipid bilayer. This suggests that lipid may be associated with the monomeric TTR in the kidney FAP protofilament.

Rapid assembly of Alzheimer-like paired helical filaments from microtubule-associated protein tau monitored by fluorescence in solution

Alzheimer's disease is characterized by the progressive deposition of two types of fibers in the affected brains, the amyloid fibers (consisting of the Abeta peptide, generating the amyloid plaques) and paired helical filaments (PHFs, made up of tau protein, forming the neurofibrillary tangles). While the principles of amyloid aggregation are known in some detail, the investigation of PHF assembly has been hampered by the low efficiency of tau aggregation, the requirement of high protein concentrations, and the lack of suitable detection methods. Here we report a quantitative assay system that permits monitoring of the assembly of PHFs in real time by the fluorescence of dyes such as thioflavine S or T. Using this assay, we evaluated parameters that influence the efficiency of filament formation. Disulfide-linked dimers of tau constructs representing the repeat domain assemble into PHFs most efficiently, but other tau isoforms or constructs form bona fide PHFs as well. The rate of assembly is greatly enhanced by polyanions such as RNA, heparin, and notably polyglutamate which resembles the acidic tail of tubulin. The assembly is optimal at pH approximately 6 and low ionic strengths (<50 mM) and increases steeply with temperatures above 30 degreesC, indicating that it is an entropy-driven process.

Structural and kinetic features of amyloid beta-protein fibrillogenesis

Alzheimer's disease (AD) is an archetype of a class of diseases characterized by abnormal protein deposition. In each case, deposition manifests itself in the form of amyloid deposits composed of fibrils of otherwise normal, soluble proteins or peptides. An ever-increasing body of genetic, physiologic, and biochemical data supports the hypothesis that fibrillogenesis of the amyloid beta-protein is a seminal event in Alzheimer's disease. Inhibiting A beta fibrillogenesis is thus an important strategy for AD therapy. However, before this strategy can be implemented, a mechanistic understanding of the fibrillogenesis process must be achieved and appropriate steps selected as therapeutic targets. Following a brief introduction to AD, I review here the current state of knowledge of A beta fibrillogenesis. Special emphasis is placed on the morphologic, structural, and kinetic aspects of this complex process.

Protein aggregation: folding aggregates, inclusion bodies and amyloid

Aggregation results in the formation of inclusion bodies, amyloid fibrils and folding aggregates. Substantial data support the hypothesis that partially folded intermediates are key precursors to aggregates, that aggregation involves specific intermolecular interactions and that most aggregates involve beta sheets.

A beta amyloidogenesis: unique, or variation on a systemic theme?

For more than a century amyloid was considered to be an interesting, unique, but inconsequential pathologic entity that rarely caused significant clinical problems. We now recognize that amyloid is not one entity. In vivo it is a uniform organization of a disease, or process, specific protein co-deposited with a set of common structural components. Amyloid has been implicated in the pathogenesis of diseases affecting millions of patients. These range from Alzheimer's disease, adult-onset diabetes, consequences of prolonged renal dialysis, to the historically recognized systemic forms associated with inflammation and plasma cell disturbances. Strong evidence is emerging that even when deposited in local organ sites significant physiologic effects may ensue. With emphasis on A beta amyloid, we review the present definition, classification, and general in vivo pathogenetic events believed to be involved in the deposition of amyloids. This encompasses the need for an adequate amyloid precursor protein pool, whether precursor proteolysis is required prior to deposition, amyloidogenic amino acid sequences, fibrillogenic nucleating particles, and an in vivo microenvironment conducive to fibrillogenesis. The latter includes several components that seem to be part of all amyloids. The role these common components may play in amyloid accumulation, why amyloids tend to be associated with basement membranes, and how one may use these findings for anti-amyloid therapeutic strategies is also examined.

Structural analysis of Alzheimer's beta(1-40) amyloid: protofilament assembly of tubular fibrils

Detailed structural studies of amyloid fibrils can elucidate the way in which their constituent polypeptides are folded and self-assemble, and exert their neurotoxic effects in Alzheimer's disease (AD). We have previously reported that when aqueous solutions of the N-terminal hydrophilic peptides of AD beta-amyloid (A beta) are gradually dried in a 2-Tesla magnetic field, they form highly oriented fibrils that are well suited to x-ray fiber diffraction. The longer, more physiologically relevant sequences such as A beta(1-40) have not been amenable to such analysis, owing to their strong propensity to polymerize and aggregate before orientation is achieved. In seeking an efficient and inexpensive method for rapid screening of conditions that could lead to improved orientation of fibrils assembled from the longer peptides, we report here that the birefringence of a small drop of peptide solution can supply information related to the cooperative packing of amyloid fibers and their capacity for magnetic orientation. The samples were examined by electron microscopy (negative and positive staining) and x-ray diffraction. Negative staining showed a mixture of straight and twisted fibers. The average width of both types was approximately 70 A, and the helical pitch of the latter was approximately 460 A. Cross sections of plastic-embedded samples showed a approximately 60-A-wide tubular structure. X-ray diffraction from these samples indicated a cross-beta fiber pattern, characterized by a strong meridional reflection at 4.74 A and a broad equatorial reflection at 8.9 A. Modeling studies suggested that tilted arrays of beta-strands constitute tubular, 30-A-diameter protofilaments, and that three to five of these protofilaments constitute the A beta fiber. This type of structure--a multimeric array of protofilaments organized as a tubular fibril--resembles that formed by the shorter A beta fragments (e.g., A beta(6-25), A beta(11-25), A beta(1-28)), suggesting a common structural motif in AD amyloid fibril organization.

Interaction of Alzheimer beta-amyloid peptide(1-40) with lipid membranes

The beta-amyloid peptide beta AP(1-40), a 40-amino acid residues peptide, is one of the major components of Alzheimer's amyloid deposits. beta AP(1-40) exhibits only a limited solubility in aqueous solution and undergoes a concentration-dependent, cooperative random coil reversible beta-structure transition for Cpep > 10 microM [Terzi, E., Hölzemann, G., and Seelig, J. (1995) J. Mol. Biol. 252, 633-642]. In the presence of acidic lipid, the equilibrium is shifted further toward beta-structured aggregates. We have now characterized the lipid-peptide interaction using circular dichroism (CD) spectroscopy, lipid monolayers, and deuterium and phosphorus-31 solid-state nuclear magnetic resonance (NMR). CD spectroscopy revealed a distinct interaction between beta AP(1-40) and negatively charged unilamellar vesicles. In addition to the random coil reversible beta-structured aggregate equilibrium at low lipid-to-peptide (L/P) ratios, a beta-structure -->alpha-helix transition was observed at L/P > 55. beta AP(1-40) was found to insert into acidic monolayers provided the lateral pressure was low (20 mN/m). The extent of incorporation increased distinctly with the content of acidic lipid in the monolayer. However, at a lipid packing density equivalent to that of a bilayer (lateral pressure > or = 32 mN/m), no insertion of beta AP(1-40) was observed. The lipid molecular structure in the presence of beta AP(1-40) was studied with NMR. Phosphatidylcholine (PC) was selectively deuterated at the choline headgroup and at the cis-double bond of the oleic acyl chain and mixed with phosphatidylglycerol (PG). Phosphorus-31 NMR showed that the lipid phase retained the bilayer structure at all lipid-to-protein ratios. Deuterium NMR revealed no change in the headgroup conformation of the choline moiety or in the flexibility and ordering of the hydrocarbon chains upon the addition of beta AP-(1-40). It can be concluded that beta AP(1-40) binds electrostatically to the outer envelope of the polar headgroup region without penetrating between the polar groups. The data suggest a new mechanism of helix formation induced by the proper alignment of five positive charges of beta AP(1-40) on the negatively charged membrane template.

Common core structure of amyloid fibrils by synchrotron X-ray diffraction

Tissue deposition of normally soluble proteins as insoluble amyloid fibrils is associated with serious diseases including the systemic amyloidoses, maturity onset diabetes, Alzheimer's disease and transmissible spongiform encephalopathy. Although the precursor proteins in different diseases do not share sequence homology or related native structure, the morphology and properties of all amyloid fibrils are remarkably similar. Using intense synchrotron sources we observed that six different ex vivo amyloid fibrils and two synthetic fibril preparations all gave similar high-resolution X-ray fibre diffraction patterns, consistent with a helical array of beta-sheets parallel to the fibre long axis, with the strands perpendicular to this axis. This confirms that amyloid fibrils comprise a structural superfamily and share a common protofilament substructure, irrespective of the nature of their precursor proteins.

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

Recent studies have implicated the amyloid Abeta peptide and its ability to self-assemble as key factors in the pathogenesis of Alzheimer's disease. Relatively little is known about the structure of soluble Abeta or its oligomeric state, and the existing data are often contradictory. In this study, we used intrinsic fluorescence of wild type Abeta-(1-40), fluorescence resonance energy transfer (FRET), and gel filtration chromatography to examine the structure of Abeta-(1-40) in solution. We synthesized a series of mono-substituted fluorescent Abeta-(1-40) derivatives to use as donors and acceptors in FRET experiments. We selected fluorescent peptides that exhibit aggregation properties comparable to wild type Abeta for analysis in donor-acceptor pairs; two labeled with 5-(2-((iodoacetyl)amino)ethyl)aminonaphthylene-1-sulfonic acid at Cys-25 or Cys-34 and fluorescein maleimide at Cys-4 or Cys-7. Another peptide containing a Trp substitution at position 10 was used as an acceptor for the intrinsic Tyr fluorescence of wild type Abeta-(1-40). Equilibrium studies of the denaturation of Abeta-(1-40) by increasing concentrations of dimethyl sulfoxide (Me2SO) were conducted by monitoring fluorescence, with a midpoint value for the unfolding transition of both the substituted and wild type peptides at among 40 and 50% Me2SO. Abeta-(1-40) is well solvated and largely monomeric in Me2SO as evidenced by a lack of FRET. When donor and acceptor Abeta derivatives are mixed together in Me2SO and then diluted 10-fold into aqueous Tris-HCl buffer at pH 7.4, efficient FRET is observed immediately for all pairs of fluorescent peptides, indicating that donor-acceptor dimers exist in solution. FRET is abolished by the addition of an excess of unlabeled Abeta-(1-40), demonstrating that the fluorescent peptides interact with wild type Abeta-(1-40) to form heterodimers that do not exhibit FRET. The Abeta-(1-40) dimers appear to be very stable, because no subunit exchange is observed after 24 h between fluorescent homodimers. Gel filtration confirms that nanomolar concentrations of 14C-labeled Abeta-(1-40) and fluorescein-labeled Abeta-(1-40) elute at the same dimeric position as wild type Abeta-(1-40), suggesting that soluble Abeta-(1-40) is also dimeric at more physiologically plausible concentrations.

X-ray diffraction and far-UV CD studies of filaments formed by a leucine-rich repeat peptide: structural similarity to the amyloid fibrils of prions and Alzheimer's disease beta-protein

The development of neuro-degenerative diseases often involves amyloidosis, that is the formation of polymeric fibrillar structures from normal cellular proteins or peptides. For example, in Alzheimer's disease, a 42 amino acid peptide processed from the amyloid precursor protein forms filaments with a beta-sheet structure. Because of this, the structure and dynamics of polymeric peptide filaments is of considerable interest. We showed previously that a 23 amino acid peptide constituting a single leucine-rich repeat (LRRN) polymerises spontaneously in solution to form long filaments of a beta-sheet structure, a property similar to that of Alzheimer's beta-amyloid and prion peptides. Here we report that a variant of LRRN in which a highly conserved asparagine residue is replaced by aspartic acid does not form either filaments or beta structure. By contrast, a variant which replaces this asparagine residue with glutamine forms filaments ultrastructurally indistinguishable from those of LRRN. Electron micrographs of LRRN filaments show that many consist of two interleaved strands which appear to have a ribbon-like morphology. X-ray diffraction patterns from oriented LRRN fibres reveal that they are composed of long beta-sheet arrays, with the interstrand hydrogen bonding parallel to the filament axis. This 'cross-beta' structure is similar to that adopted by beta-amyloid and prion derived fibres. Taken together, these results indicate that the LRR filaments are stabilised by inter- or intra-strand hydrogen bonded interactions comparable to the asparagine ladders of beta-helix proteins or the 'glutamine zippers' of poly-glutamine peptides. We propose that similar stabilising interactions may underlie a number of characterised predispositions to neuro-degenerative diseases that are caused by mutations to amide residues. Our finding that amyloid-like filaments can form from a peptide motif not at present correlated with degenerative disease suggests that a propensity for beta-filament formation is a common feature of protein sub-domains.

Amyloid fibril formation and protein misassembly: a structural quest for insights into amyloid and prion diseases

The assembly and misassembly of normally soluble proteins into fibrilar structures is thought to be a causative agent in a variety of human amyloid and prion diseases. Structural and mechanistic studies of this process are beginning to elucidate the conformational changes required for the conversion of a normally soluble and functional protein into a defined quaternary structure.

X-ray diffraction analysis of scrapie prion: intermediate and folded structures in a peptide containing two putative alpha-helices

Small proteinaceous infectious particles called prions cause certain neurodegenerative diseases in human and animals. Limited proteolysis of infectious scrapie prions PrP(Sc) yields an N-truncated polypeptide termed PrP 27-30, which encompasses residues 90 to 231 of PrP(Sc) and which assembles into 100 to 200 A wide amyloid rods. It has been hypothesized that the infectious prion is converted from its non-infectious cellular form (PrP(C)) by means of an alpha-helical to beta-sheet conformational change. Secondary structure analysis, computer modeling, and structural biophysics methods support this hypothesis. Residues 90 to 145 of PrP, which contain two putative alpha-helical domains H1 and H2, may be of particular relevance to the disease pathogenesis, as C-terminal truncation at residue 145 was found in a patient with an inherited prion disease. Moreover, our recent X-ray diffraction analysis suggests that the peptide consisting of these residues (designated SHa 90-145) closely models the amyloidogenic beta-sheet core of PrP. In the current study, we have analyzed in detail the X-ray diffraction patterns of SHa 90-145. Two samples were examined: one that was dehydrated under ambient conditions whilst in an external magnetic field (to induce fibril orientation), and another that was sealed after partial drying. The dried, magnetically oriented sample showed a cross-beta diffraction pattern in which the fiber axis (rotation axis) was parallel to the H-bonding direction of the beta-sheets. The major wide-angle peaks indicate the presence of approximately 40 A wide beta-crystallites, which constitute the protofilament. Each crystallite is composed of several orthogonal unit cells, normal to the fiber (a-axis) direction, having lattice constants a = 9.69 A, b = 6.54 A, and c = 18.06 A. Electron density maps were calculated by iterative Fourier synthesis using beta-silk as an initial phase model. The distribution of density indicated that there were two types of beta-sheet, suggesting that larger and smaller side-chains localized to different sheets. This would arise from folding of the polypeptide in which there are turns in the middle of both the H1 and H2 domains. A monoclinic macrolattice, with a = 9.61 A, b = c = 52.99 A and alpha = 114.6 degrees, was found to index all the reflections, including those in the low-angle region. This suggests that the beta-crystallites are nearly hexagonally packed. To account for the approximately 100 A wide fibers visualized by negative staining in the electron microscope, the beta-crystallites would be arranged in 4-mers. The partially dried sample showed a sharp 4.7 A reflection (from H-bonding) and five broad peaks superimposed on monotonically decreasing diffuse scattering. This solution-like scattering was modeled by an anisometric rectangle with a thickness comparable to a singe beta-chain. The structure, which occurred during dehydration, could be a transient in the alpha-helical to beta-sheet conversion, suggesting that formation of hydrogen bonding precedes the inter-sheet interaction and assembly into the amyloid of scrapie prion.

Disaggregation of Alzheimer beta-amyloid by site-directed mAb

In Alzheimer disease, beta-amyloid peptide accumulates in the brain as insoluble amyloid plaques. Amyloid filaments, similar to those found in amyloid plaques, can be assembled in vitro from chemically synthesized beta-peptides. In this study, we report that antibodies raised against the N-terminal region (1-28) of the beta-amyloid peptide bind to the in vitro-formed beta-amyloid assemblies, leading to disaggregation of the fibrils and partial restoration of the peptide's solubility. The concomitant addition of fibrillar beta-amyloid with these antibodies to PC 12 cells leads to the inhibition of the neurotoxic effects of beta-amyloid. Some of the mAbs raised against soluble beta-peptide (1-28) have been found to prevent in vitro fibrillar aggregation of beta-amyloid peptide. These experimental data suggest that site-directed mAbs interfere with the aggregation of beta-amyloid and trigger reversal to its nontoxic, normal components. The above findings give hints on how to convert in vivo senile plaques into nontoxic, diffuse components and may have therapeutic interest for those studying Alzheimer disease and other human diseases related to amyloidogenic properties of physiological peptides and proteins.