Aggregation and secondary structure of synthetic amyloid beta A4 peptides of Alzheimer's disease

Binding of trypsin to fibrillar amyloid beta-protein

We have recently reported that fibrillar amyloid beta-protein (Abeta) inhibits the proteolytic activity of trypsin and high molecular weight bovine brain protease. We report here that trypsin binds to fibrillar Abeta (fAbeta) and the resulting complex of trypsin/fAbeta is sodium dodecyl sulfate (SDS)-stable. Electron microscopic analysis confirmed the binding of trypsin on the fibrils of both Abeta 1-40 and Abeta 1-42. SDS-polyacrylamide gel electrophoresis (PAGE) of fAbeta sample incubated in the presence of trypsin showed that major amount of trypsin was associated with fAbeta that did not enter the gel. The presence of trypsin in this protein complex was confirmed by Western blotting after its elution from the gel. Kinetic studies showed that the binding of trypsin to fibrillar Abeta was dependent on the degree of Abeta fibrillization and on the concentration of fAbeta. However, the trypsin binding to Abeta oligomers did not affect the fibril growth. The maximum binding (B(max)) of trypsin to fAbeta 1-40 and fAbeta 1-42 was 36 pmol and 40 pmol, and dissociation constant (K(d)) was 18.31 microM and 20 microM respectively. Similar to fAbeta, trypsin could also bind to fibrillar amylin. This binding was dependent on the concentration of fibrillar amylin. Under similar conditions, bovine serum albumin did not bind to fibrillar Abeta. These results suggest that fAbeta and fibrillar amylin have strong affinities for trypsin, and chelation of proteases by abnormal aggregated proteins may be a general mechanism for inflicting pathological conditions in various diseases.

Free energy landscapes for amyloidogenic tetrapeptides dimerization

The oligomerization of four peptide sequences, KFFE, KVVE, KLLE, and KAAE is studied using replica-exchange molecular dynamics simulations with an atomically detailed peptide model. Previous experimental studies reported that of these four peptides, only those containing phenylalanine and valine residues form fibrils. We show that the fibrillogenic propensities of these peptides can be rationalized in terms of the equilibrium thermodynamics of their early oligomers. Thermodynamic stability of dimers, as measured by the temperature of monomer association, is seen to be higher for those peptides that are able to form fibrils. Although the relative high and low stabilities of the KFFE and KAAE dimers arise from their respective high and low interpeptide interaction energies, the higher stability of the KVVE dimer over the KLLE system results from the smaller loss of configurational entropy accompanying the dimerization of KVVE. Free energy landscapes for dimerization are found to be strongly sequence-dependent, with a high free energy barrier separating the monomeric and dimeric states for KVVE, KLLE, and KAAE sequences. In contrast, the most fibrillogenic peptide, KFFE, displayed downhill assembly, indicating enhanced kinetic accessibility of its dimeric states. The dimeric phase for all peptide sequences is found to be heterogeneous, containing both antiparallel beta-sheet structures that can grow into full fibrils as well as disordered dimers acting as on- or off-pathway intermediates for fibrillation.

Conformational dynamics of amyloid beta-protein assembly probed using intrinsic fluorescence

Formation of toxic oligomeric and fibrillar structures by the amyloid beta-protein (Abeta) is linked to Alzheimer's disease (AD). To facilitate the targeting and design of assembly inhibitors, intrinsic fluorescence was used to probe assembly-dependent changes in Abeta conformation. To do so, Tyr was substituted in Abeta40 or Abeta42 at position 1, 10 (wild type), 20, 30, 40, or 42. Fluorescence then was monitored periodically during peptide monomer folding and assembly. Electron microscopy revealed that all peptides assembled readily into amyloid fibrils. Conformational differences between Abeta40 and Abeta42 were observed in the central hydrophobic cluster (CHC) region, Leu17-Ala21. Tyr20 was partially quenched in unassembled Abeta40 but displayed a significant and rapid increase in intensity coincident with the maturation of an oligomeric, alpha-helix-containing intermediate into amyloid fibrils. This process was not observed during Abeta42 assembly, during which small decreases in fluorescence intensity were observed in the CHC. These data suggest that the structure of the CHC in Abeta42 is relatively constant within unassembled peptide and during the self-association process. Solvent accessibility of the Tyr ring was studied using a mixed solvent (dimethyl sulfoxide/water) system. [Tyr40]Abeta40, [Tyr30]Abeta42, and [Tyr42]Abeta42 all were relatively shielded from solvent. Analysis of the assembly dependence of the site-specific intrinsic fluorescence data suggests that the CHC is particularly important in controlling Abeta40 assembly, whereas the C-terminus plays the more significant role in Abeta42 assembly.

Structural models of amyloid-like fibrils

Amyloid fibrils are elongated, insoluble protein aggregates deposited in vivo in amyloid diseases, and amyloid-like fibrils are formed in vitro from soluble proteins. Both of these groups of fibrils, despite differences in the sequence and native structure of their component proteins, share common properties, including their core structure. Multiple models have been proposed for the common core structure, but in most cases, atomic-level structural details have yet to be determined. Here we review several structural models proposed for amyloid and amyloid-like fibrils and relate features of these models to the common fibril properties. We divide models into three classes: Refolding, Gain-of-Interaction, and Natively Disordered. The Refolding models propose structurally distinct native and fibrillar states and suggest that backbone interactions drive fibril formation. In contrast, the Gain-of-Interaction models propose a largely native-like structure for the protein in the fibril and highlight the importance of specific sequences in fibril formation. The Natively Disordered models have aspects in common with both Refolding and Gain-of-Interaction models. While each class of model suggests explanations for some of the common fibril properties, and some models, such as Gain-of-Interaction models with a cross-beta spine, fit a wider range of properties than others, no one class provides a complete explanation for all amyloid fibril behavior.

Structure of amyloid beta fragments in aqueous environments

Conformational studies on amyloid beta peptide (Abeta) in aqueous solution are complicated by its tendency to aggregate. In this study, we determined the atomic-level structure of Abeta(28-42) in an aqueous environment. We fused fragments of Abeta, residues 10-24 (Abeta(10-24)) or 28-42 (Abeta(28-42)), to three positions in the C-terminal region of ribonuclease HII from a hyperthermophile, Thermococcus kodakaraensis (Tk-RNase HII). We then examined the structural properties in an aqueous environment. The host protein, Tk-RNase HII, is highly stable and the C-terminal region has relatively little interaction with other parts. CD spectroscopy and thermal denaturation experiments demonstrated that the guest amyloidogenic sequences did not affect the overall structure of the Tk-RNase HII. Crystal structure analysis of Tk-RNase HII(1-197)-Abeta(28-42) revealed that Abeta(28-42) forms a beta conformation, whereas the original structure in Tk-RNase HII(1-213) was alpha helix, suggesting beta-structure formation of Abeta(28-42) within full-length Abeta in aqueous solution. Abeta(28-42) enhanced aggregation of the host protein more strongly than Abeta(10-24). These results and other reports suggest that after proteolytic cleavage, the C-terminal region of Abeta adopts a beta conformation in an aqueous environment and induces aggregation, and that the central region of Abeta plays a critical role in fibril formation. This study also indicates that this fusion technique is useful for obtaining structural information with atomic resolution for amyloidogenic peptides in aqueous environments.

Adsorption of Amyloid β-Peptide at Polymer Surfaces: A Neutron Reflectivity Study

The adsorption of amyloid beta-peptide at hydrophilic and hydrophobic modified silicon-liquid interfaces was characterized by neutron reflectometry. Distinct polymeric films were used to obtain noncharged (Formvar), negatively (sodium poly(styrene sulfonate)) and positively charged (poly(allylamine hydrochloride)) hydrophilic as well as hydrophobic surfaces (polystyrene and a polysiloxane-dodecanoic acid complex). Amyloid beta-peptide was found to adsorb at positively charged hydrophilic and hydrophobic surfaces, whereas no adsorbed layer was detected on hydrophilic noncharged and negatively charged films. The peptide adsorbed at the positively charged film as patches, which were dispersed on the surface, whereas a uniform layer was observed at hydrophobic surfaces. The thickness of the adsorbed peptide layer was estimated to be approximately 20 A. The peptide formed a tightly packed layer, which did not contain water. These studies provide information about the affinity of the amyloid beta-peptide to different substrates in aqueous solution and suggest that the amyloid fibril formation may be driven by interactions with surfaces.

Quantitative determination of the topological propensities of amyloidogenic peptides

One of the interesting puzzles of amyloid beta-peptide of Alzheimer's disease (Abeta) is that it appears to polymerize into amyloid fibrils in a parallel beta sheet topology, while smaller subsets of the peptide produce anti-parallel beta sheets. In order to target potential weak points of amyloid fibrils in a rational drug design effort, it would be helpful to understand the forces that drive this change. We have designed two peptides CHQKLVFFAEDYNGKDEAFFVLKQHW and CHQKLVFFAEDYNGKHQKLVFFAEDW that join the significant amyloidogenic Abeta (14-23) sequence HQKLVFFAED in parallel and anti-parallel topologies, respectively. (Here, the word "parallel" refers only to residue sequence and not backbone topology). The N-termini of the hairpins were labeled with the fluorescent dye 5-((((2-iodoacetyl)amino)ethyl)amino)naphthalene-1-sulfonic acid (IAEDANS), forming a fluorescence energy transfer donor-acceptor pair with the C-terminus tryptophan. Circular dichroism results show that the anti-parallel hairpin adopts a beta-sheet conformation, while the parallel hairpin is disordered. Fluorescent Resonance Energy Transfer (FRET) results show that the distance between the donor and the acceptor is significantly shorter in the anti-parallel topology than in the parallel topology. The fluorescence intensity of anti-parallel hairpin also displays a linear concentration dependence, indicating that the FRET observed in the anti-parallel hairpin is from intra-molecular interactions. The results thus provide a quantitative estimate of the relative topological propensities of amyloidogenic peptides. Our FRET and CD results show that beta sheets involving the essential Abeta (14-23) fragment, strongly prefer the anti-parallel topology. Moreover, we provide a quantitative estimate of the relative preference for these two topologies. Such analysis can be repeated for larger subsets of Abeta to determine quantitatively the relative degree of preference for parallel/anti-parallel topologies in given fragments of Abeta.

Role of hydrophobic interactions and salt-bridges in β-hairpin folding

Beta-hairpins are the simplest form of beta-sheets which, due to the presence of long-range interactions, can be considered as tertiary structures. Molecular dynamics simulation is a powerful tool that can unravel whole pathways of protein folding/unfolding at atomic resolution. We have performed several molecular dynamics simulations, to a total of over 250 ns, of a beta-hairpin peptide in water using GROMACS. We show that hydrophobic interactions are necessary for initiating the folding of the peptide. Once formed, the peptide is stabilized by hydrogen bonds and disruption of hydrophobic interactions in the folded peptide does not denature the structure. In the absence of hydrophobic interactions, the peptide fails to fold. However, the introduction of a salt-bridge compensates for the loss of hydrophobic interactions to a certain extent.

On the nucleation of amyloid beta-protein monomer folding

Neurotoxic assemblies of the amyloid beta-protein (Abeta) have been linked strongly to the pathogenesis of Alzheimer's disease (AD). Here, we sought to monitor the earliest step in Abeta assembly, the creation of a folding nucleus, from which oligomeric and fibrillar assemblies emanate. To do so, limited proteolysis/mass spectrometry was used to identify protease-resistant segments within monomeric Abeta(1-40) and Abeta(1-42). The results revealed a 10-residue, protease-resistant segment, Ala21-Ala30, in both peptides. Remarkably, the homologous decapeptide, Abeta(21-30), displayed identical protease resistance, making it amenable to detailed structural study using solution-state NMR. Structure calculations revealed a turn formed by residues Val24-Lys28. Three factors contribute to the stability of the turn, the intrinsic propensities of the Val-Gly-Ser-Asn and Gly-Ser-Asn-Lys sequences to form a beta-turn, long-range Coulombic interactions between Lys28 and either Glu22 or Asp23, and hydrophobic interaction between the isopropyl and butyl side chains of Val24 and Lys28, respectively. We postulate that turn formation within the Val24-Lys28 region of Abeta nucleates the intramolecular folding of Abeta monomer, and from this step, subsequent assembly proceeds. This model provides a mechanistic basis for the pathologic effects of amino acid substitutions at Glu22 and Asp23 that are linked to familial forms of AD or cerebral amyloid angiopathy. Our studies also revealed that common C-terminal peptide segments within Abeta(1-40) and Abeta(1-42) have distinct structures, an observation of relevance for understanding the strong disease association of increased Abeta(1-42) production. Our results suggest that therapeutic approaches targeting the Val24-Lys28 turn or the Abeta(1-42)-specific C-terminal fold may hold promise.

Imaging amyloid β peptide oligomeric particles in solution

While all protein misfolding diseases are characterized by fibrous amyloid deposits, the favorable free energy and strongly cooperative nature of the self-assembly have complicated the development of therapeutic strategies aimed at preventing their formation. As structural models for the amyloid fibrils approach atomic resolution, increasing evidence suggests that early folding intermediates, rather than the final structure, are more strongly associated with the loss of neuronal function. For that reason we now demonstrate the use of cryo-etch high-resolution scanning electron microscopy (cryo-HRSEM) for the direct observation of pathway intermediates in amyloid assembly. A congener of the Abeta peptide of Alzheimer's disease, Abeta(13-21), samples a variety of time-dependent self-assembles in a manner similar to those seen for larger proteins. A morphological description of these intermediates is the first step towards their structural characterization and the definition of their role in both amyloid assembly and neurotoxicity.

Effect of d-amino acids at Asp23 and Ser26 residues on the conformational preference of Aβ20–29 peptides

The effects of d-amino acids at Asp(23) and Ser(26) residues on the conformational preference of beta-amyloid (Abeta) peptide fragment (Abeta(20-29)) have been studied using different spectroscopic techniques, namely vibrational circular dichroism (VCD), vibrational absorption, and electronic circular dichroism. To study the structure of the Abeta(20-29), [d-Asp(23)]Abeta(20-29), and [d-Ser(26)]Abeta(20-29) peptides under different conditions, the spectra were measured in 10mM acetate buffer (pH 3) and in 2,2,2-trifluoroethanol (TFE). The spectroscopic results indicated that at pH 3, Abeta(20-29) peptide takes random coil with beta-turn structure, while [d-Ser(26)]Abeta(20-29) peptide adopts significant amount of polyproline II (PPII) type structure along with beta-turn contribution and d-Asp-substituted peptide ([d-Asp(23)]Abeta(20-29)) adopts predominantly PPII type structure. The increased propensity for PPII conformation upon d-amino acid substitution, in acidic medium, has important biological implications. In TFE, Abeta(20-29), [d-Asp(23)]Abeta(20-29), and [d-Ser(26)]Abeta(20-29) peptides adopt 3(10)-helix, alpha-helix, and random coil with some beta-turn structures, respectively. The VCD data obtained for the Abeta peptide films suggested that the secondary structures for the peptide films are not the same as those for corresponding solution and are also different among the Abeta peptides studied here. This observation suggests that dehydration can have a significant influence on the structural preferences of these peptides.

The influence of phospholipid membranes on bovine calcitonin peptide's secondary structure and induced neurotoxic effects

The peptide hormone, calcitonin, which is associated with medullary carcinoma of the thyroid, has a marked tendency to form amyloid fibrils and may be a useful model in probing the role of peptide-membrane interactions in beta-sheet and amyloid formation and amyloid neurotoxicity. Using bovine calcitonin, we found that, like other amyloids, the peptide was toxic only when in a beta-sheet-rich, amyloid form, but was non-toxic, when it lacked an amyloid structure. We found that the peptide bound with significant affinity to membranes that contained either cholesterol and gangliosides. In addition, incubation of calcitonin with cholesterol-rich and ganglioside-containing membranes resulted in significant changes in peptide structure yielding a peptide enriched in beta-sheet and amyloid content. Because the cholesterol- and ganglioside-rich phospholipid systems enhanced the calcitonin beta-sheet and amyloid contents, and peptide amyloid content was associated with neurotoxicity, we then investigated whether depleting cellular cholesterol and gangliosides affected calcitonin neurotoxicity. We found that cholesterol and ganglioside removal significantly reduced the calcitonin-induced PC12 cell neurotoxicity. Similar results have been observed with other amyloid-forming peptides such as beta-amyloid (A beta) of Alzheimer's disease and suggest that modulation of membrane composition and peptide-membrane interactions may prove useful in the control of amyloid formation and amyloid neurotoxicity.

The HIV Fusion Peptide Adopts Intermolecular Parallel β-Sheet Structure in Membranes when Stabilized by the Adjacent N-Terminal Heptad Repeat: A 13C FTIR Study

The HIV gp41 protein mediates fusion with target host cells. The region primarily involved in directing fusion, the fusion peptide (FP), is poorly understood at the level of structure and function due to its toxic effect in expression systems. To overcome this, we used a synthetic approach to generate the N70 construct, whereby the FP is stabilized in context of the adjacent auto oligomerization domain. The amide I profile of unlabeled N70 in membranes reveals prominent alpha-helical contribution, along with significant beta-structure. By truncating the N terminus (FP region) of N70, beta-structure is eliminated, suggesting that the FP adopts a beta-structure in membranes. To assess this directly, (13)C Fourier-transformed infra-red analysis was carried out to map secondary structure of the 16 N-terminal hydrophobic residues of the fusion peptide (FP16). The (13)C isotope shifted absorbance of the FP was filtered from the global secondary structure of the 70 residue construct (N70). On the basis of the peak shift induced by the (13)C-labeled residues of FP16, we directly assign beta-sheet structure in ordered membranes. A differential labeling scheme in FP16 allows us to distinguish the type of beta-sheet structure as parallel. Dilution of each FP16-labeled N70 peptide, by mixing with unlabeled N70, shows directly that the FP16 beta-strand region self-assembles. We discuss our structural findings in the context of the prevailing gp41 fusion paradigm. Specifically, we address the role of the FP region in organizing supramolecular gp41 assembly, and we also discuss the mechanism by which exogenous, free FP constructs inhibit gp41-induced fusion.

Characterization of chemical exchange between soluble and aggregated states of beta-amyloid by solution-state NMR upon variation of salt conditions

Alzheimer's disease (AD) is characterized by the accumulation of insoluble fibrillar aggregates of beta-amyloid peptides (Abeta), a 39-42 residue peptide, in the brain of AD patients. It is hypothesized that the disease causing form is not the fibrillar species but an oligomeric Abeta molecule, which is often referred to as the "critical oligomer" of Abeta. We show in this paper that Abeta(1-40) undergoes chemical exchange between a monomeric, soluble state and an oligomeric, aggregated state under physiological conditions. In circular dichroism spectroscopy, we observe for this intermediate an alpha-helical structure. The oligomer is assigned a molecular weight of >100 kDa by diffusion-ordered spectroscopy-solution-state NMR spectroscopy (NMR). We can show by saturation transfer difference NMR experiments that the oligomer is related to monomeric Abeta. This experiment also allows us to identify the chemical groups that are involved in interactions between mono- and oligomeric Abeta molecules. Variation of the anionic strength in the buffer induces a shift of equilibrium between mono- and oligomeric states and possibly allows for the stabilization of these intermediate structures.

Abeta40-Lactam(D23/K28) models a conformation highly favorable for nucleation of amyloid

Recent solid-state NMR data (1) demonstrate that Abeta(1)(-)(40) adopts a conformation in amyloid fibrils with two in-register, parallel beta-sheets, connected by a bend structure encompassing residues D(23)VGSNKG(29), with a close contact between the side chains of Asp23 and Lys28. We hypothesized that forming this bend structure might be rate-limiting in fibril formation, as indicated by the lag period typically observed in the kinetics of Abeta(1)(-)(40) fibrillogenesis. We synthesized Abeta(1)(-)(40)-Lactam(D23/K28), a congener Abeta(1)(-)(40) peptide that contains a lactam bridge between the side chains of Asp23 and Lys28. Abeta(1)(-)(40)-Lactam(D23/K28) forms fibrils similar to those formed by Abeta(1)(-)(40). The kinetics of fibrillogenesis, however, occur without the typical lag period, and at a rate approximately 1000-fold greater than is seen with Abeta(1)(-)(40) fibrillogenesis. The strong tendency toward self-association is also shown by size exclusion chromatography in which Abeta(1)(-)(40)-Lactam(D23/K28) forms oligomers even at concentrations of approximately 1-5 microM. Under the same conditions, Abeta(1)(-)(40) shows no detectable oligomers by size exclusion chromatography. Our data suggest that Abeta(1)(-)(40)-Lactam(D23/K28) could bypass an unfavorable folding step in fibrillogenesis, because the lactam linkage "preforms" a bendlike structure in the peptide. Consistent with this view Abeta(1)(-)(40) growth is efficiently nucleated by Abeta(1)(-)(40)-Lactam(D23/K28) fibril seeds.

Structure of β-amyloid fibrils and its relevance to their neurotoxicity: Implications for the pathogenesis of Alzheimer’s disease

Alzheimer's disease and cerebral amyloid angiopathy are characterized by the deposition of beta-amyloid fibrils consisting of 40- and 42-mer peptides (A beta 40 and A beta 42). Since the aggregation (fibrilization) of these peptides is closely related to the pathogenesis of these diseases, numerous structural analyses of A beta 40 and A beta 42 fibrils have been carried out. A beta 42 plays a more important role in the pathogenesis of these diseases since its aggregative ability and neurotoxicity are considerably greater than those of A beta 40. This review summarizes mainly our own recent findings from the structural analysis of A beta 42 fibrils and discusses its relevance to their neurotoxicity in vitro.

RAGE and amyloid beta interactions: atomic force microscopy and molecular modeling

In the AD brain, there are elevated amounts of soluble and insoluble Abeta peptides which enhance the expression of membrane bound and soluble receptor for advanced glycation end products (RAGE). The binding of soluble Abeta to soluble RAGE inhibits further aggregation of Abeta peptides, while membrane bound RAGE-Abeta interactions elicit activation of the NF-kappaB transcription factor promoting sustained chronic neuroinflammation. Atomic force microscopy observations demonstrated that the N-terminal domain of RAGE, by interacting with Abeta, is a powerful inhibitor of Abeta polymerization even at prolonged periods of incubation. Hence, the potential RAGE-Abeta structural interactions were further explored utilizing a series of computational chemistry algorithms. Our modeling suggests that a soluble dimeric RAGE assembly creates a positively charged well into which the negative charges of the N-terminal domain of dimeric Abeta dock.

β Sheet Structure in Amyloid β Fibrils and Vibrational Dipolar Coupling

Fibrils formed by amyloid beta proteins were labeled with 13C at various positions and examined by infrared spectroscopy to detect vibrational dipolar coupling, implying close physical proximity. The results support key features of several recently proposed models for amyloid fibril structure, but they also add some important caveats. For instance, they support the conclusion that the beta structure is parallel; however, the coupling is not as strong as expected when residues are in register. This may be explained by out-of-register alignment of adjacent strands, or nonstandard parallel sheet structure that yields suboptimal alignment of labeled dipole moments. The data also point to a significant structural difference between fibrils formed by the 40-residue amyloid beta protein and fibrils formed by residues 10-35.

Expression and purification of a recombinant peptide from the Alzheimer's beta-amyloid protein for solid-state NMR

Fibrillar protein aggregates contribute to the pathology of a number of disease states. To facilitate structural studies of these amyloid fibrils by solid-state NMR, efficient methods for the production of milligram quantities of isotopically labeled peptide are necessary. Bacterial expression of recombinant amyloid proteins and peptides allows uniform isotopic labeling, as well as other patterns of isotope incorporation. However, large-scale production of recombinant amyloidogenic peptides has proven particularly difficult, due to their inherent propensity for aggregation and the associated toxicity of fibrillar material. Yields of recombinant protein are further reduced by the small molecular weights of short amyloidogenic fragments. Here, we report high-yield expression and purification of a peptide comprising residues 11-26 of the Alzheimer's beta-amyloid protein (Abeta(11-26)), with homoserine lactone replacing serine at residue 26. Expression in inclusion bodies as a ketosteroid isomerase fusion protein and subsequent purification under denaturing conditions allows production of milligram quantities of uniformly labeled (13)C- and (15)N-labeled peptide, which forms amyloid fibrils suitable for solid-state NMR spectroscopy. Initial structural data obtained by atomic force microscopy, electron microscopy, and solid-state NMR measurements of Abeta(11-26) fibrils are also presented.

N-Terminal Deletions Modify the Cu2+ Binding Site in Amyloid-β

Copper is implicated in the in vitro formation and toxicity of Alzheimer's disease amyloid plaques containing the beta-amyloid (Abeta) peptide (Bush, A. I., et al. (2003) Proc. Natl. Acad. Sci. U.S.A. 100, 11934). By low temperature electron paramagnetic resonance (EPR) spectroscopy, the importance of the N-terminus in creating the Cu(2+) binding site in native Abeta has been examined. Peptides that contain the proposed binding site for Cu(2+)-three histidines (H6, H13, and H14) and a tyrosine (Y10)-but lack one to three N-terminal amino acids, do not bind Cu(2+) in the same coordination environment as the native peptide. EPR spectra of soluble Abeta with stoichiometric amounts of Cu(2+) show type 2 Cu(2+) EPR spectra for all peptides. The ligand donor atoms to Cu(2+) are 3N1O when Cu(2+) is bound to any of the Abetapeptides (Abeta16, Abeta28, Abeta40, and Abeta42) that contain the first 16 amino acids of full-length Abeta. When a Y10F mutant of Abeta is used, the coordination environment for Cu(2+) remains 3N1O and Cu(2+) EPR spectra of this mutant are identical to the wild-type spectra. Isotopic labeling experiments show that water is not the O-atom donor to Cu(2+) in Abeta fibrils or in the Y10F mutant. Further, we find that Cu(2+) cannot be removed from Cu(2+)-containing fibrils by washing with buffer, but that Cu(2+) binds to fibrils initially assembled without Cu(2+) in the same coordination environment as in fibrils assembled with Cu(2+). Together, these results indicate (1) that the O-atom donor ligand to Cu(2+) in Abeta is not tyrosine, (2) that the native Cu(2+) binding site in Abeta is sensitive to small changes at the N-terminus, and (3) that Cu(2+) binds to Abetafibrils in a manner that permits exchange of Cu(2+) into and out of the fibrillar architecture.

The N-terminal active centre of human angiotensin-converting enzyme degrades Alzheimer amyloid β-peptide

We reported recently that angiotensin-converting enzyme (ACE) significantly degraded amyloid beta-peptide (A beta) to inhibit aggregation and cytotoxicity of A beta in PC12h cells in vitro. On the other hand, others reported that ACE had two domains with highly homologous active centres, the N-domain and C-domain, but that they differed in their characteristics such as optimum chloride ion concentration, inhibition kinetics for various ACE inhibitors and rate of hydrolysis for many substrates. The aim of this study was to determine the specific ACE domain primarily responsible for degradation of A beta. For this purpose, a series of ACE recombinant proteins, each containing only one intact domain, was constructed and expressed in COS7. Our results showed that all ACE recombinant proteins obtained were enzymatically active in terms of angiotensin I cleavage. However, inhibition of A beta aggregation and cytotoxicity of the N-domain were higher than those of the C-domain. Reverse-phase high-performance liquid chromatography analyses confirmed that the N domain degraded A beta. Our results indicate that the N domain of ACE is primarily responsible for the degradation of A beta.

An intersheet packing interaction in A beta fibrils mapped by disulfide cross-linking

Most models for the central cross-beta folding unit in amyloid fibrils of the Alzheimer's plaque protein Abeta align the peptides in register in H-bonded, parallel beta-sheet structure. Some models require the Abeta peptide to undergo a chain reversal when folding into the amyloid core, while other models feature very long extended chains, or zigzag chains, traversing the protofilament. In this paper we introduce the use of disulfide bond cross-linking to probe the fold within the core and the packing interactions between beta-sheets. In one approach, amyloid fibrils grown under reducing conditions from each of three double cysteine mutants (17/34, 17/35, and 17/36) of the Abeta(1-40) sequence were subjected to oxidizing conditions. Of these three mutants, only the Leu17Cys/Leu34Cys peptide could be cross-linked efficiently while resident in fibrils. In another approach, double Cys mutants were cross-linked as monomers before aggregation, and the resulting fibrils were assessed for stability, antibody binding, dye binding, and cross-seeding efficiency. Here too, fibrils from the 17/34 double Cys mutant most closely resemble wild-type Abeta(1-40) fibrils. These data support models of the Abeta fibril in which the Leu17 and Leu34 side chains of the same peptide pack against each other at the beta-sheet interface within the amyloid core. Related cross-linking strategies may reveal longer range spatial relationships. The ability of the cross-linked 17/35 double Cys mutant Abeta to also make amyloid fibrils illustrates a remarkable plasticity of the amyloid structure and suggests a structural mechanism for the generation of conformational variants of amyloid.

Eight-residue Abeta peptides inhibit the aggregation and enzymatic activity of Abeta42

Insoluble Abeta1-42 is the main component of the amyloid plaque. We have previously demonstrated that exposure to low pH can confer the molten globule state on soluble Abeta1-42 in vitro [Biochem. J. 361 (2000) 547] and unfolding experiments with guadinine hydrochloride (GdnHCl) have now confirmed this observation. The molten globule state of the protein has many biological properties and understanding the mechanisms of its formation is an important step in devising a therapeutic strategy for Alzheimer's disease (AD). We therefore investigated the ability of a series of synthetic eight-residue peptides derived from Abeta1-42 to inhibit the acid-induced aggregation of Abeta1-42 and identified the potent peptides to be Abeta15-22, Abeta16-23 and Abeta17-24. A1-antichymotrypsin, a member of the serine proteinase inhibitor (serpin) family is another major component of the amyloid plaque. In the present study, we investigated the proteolytic activity of Abeta1-42 against casein at different pHs. Chemical modification of amino acid residues in Abeta1-42 indicated that serine and histidine residues, but not aspartic acid, are necessary for enzymatic activity, suggesting that it is a serine proteinase. Amino acid substitution studies indicate that glutamic acids at positions 11 and 22 participate indirectly in proteolysis and we surmise that amino acid residues 29-42 are required to stabilize the conformer. A study of metal ions suggested that Cu2+ affected the enzymatic activity, but Zn2+ and Fe2+ did not. Interestingly, Abeta14-21 and Abeta15-22 were the only peptides that inhibited the proteolytic activity of Abeta42. Therefore, Abeta15-22 may control both aggregation of Abeta1-42 at acidic pH and its proteolytic activity at neutral pH. Consequently, we suggest that it may be of use in the therapy of Alzheimer's disease.

Amyloid Imaging: From Benchtop to Bedside

Tremendous efforts have been made in the search for a cure or effective treatment of Alzheimer's disease (AD) to develop therapies aimed at halting or reversing amyloid plaque deposition in the brain. This necessitates in vivo detection and quantification of amyloid plaques in the brain for efficacy evaluation of anti-amyloid therapies. For this purpose, a wide array of amyloid-imaging probes has been developed, mainly for in vivo studies based on positron emission tomography and single photon emission computed tomography. This review provides a full account of the development of amyloid-imaging agents. The in vitro binding properties and in vivo pharmacokinetic profiles of all amyloid-imaging agents so far reported are comprehensively and uniquely surveyed. Emphasis is placed on the development of small-molecule probes based on amyloid dyes, such as Congo red and thioflavin T. Compared to large biomolecules, these small-molecule probes have been systematically investigated through extensive structure activity relationship studies. Many of the probes show favorable properties for in vivo studies. As a result, three lead compounds, termed PIB (Pittsburgh-Compound B, [(11)C]6-OH-BTA-1), FDDNP (2-(1-[6-[(2-[(18)F]fluoroethyl)(methyl)amino]-2-naphthyl]ethylidene)malononitrile), and SB-13 (4-N-methylamino-4'-hydroxystilbene), have been identified and evaluated in human subjects. Preliminary studies have indicated that these lead compounds exhibit a characteristic retention in AD subjects that is consistent with the AD pathology, thus proving the concept that amyloid deposits in the brain can be readily detected and quantified in vivo. The progress to date paves the way for further investigation in various aspects of AD research. Once developed, these amyloid-imaging agents could be used as biomarkers to aid in early and definitive diagnosis of AD, facilitate drug discovery and development, and allow pathophysiological studies of the disease mechanism. Furthermore, the success in the development of amyloid-imaging agents helps with the development of imaging agents for in vivo studies of other AD pathologies in particular and of neurodegenerative disorders in general.

Alzheimer's Disease Aβ Peptide Fragment 10–30 Forms a Spectrum of Metastable Oligomers with Marked Preference for N to N and C to C Monomer Termini Proximity

Oligomers of Abeta peptide have been indicated recently as a possible main causative agent of Alzheimer's disease. However, information concerning their structural properties is very limited. Here Abeta oligomers are studied by non-covalent complexes mass spectrometry and disulfide rearrangement. As a model molecule, an Abeta fragment spanning residues 10-30 (Abeta10-30) has been used. This model peptide is known to contain the core region responsible for Abeta aggregation to fibrils. Non-covalent complexes mass spectrometry indicates that, at neutral pH, monomers are accompanied by oligomers up to hexamers of gradually decreasing population. H-2H exchange studies and direct monomer exchange rate measurements with the use of 15N labeled peptides and mass spectrometry show a fast exchange of monomeric units between oligomers. Disulfide exchange studies of cysteine tagged Abeta10-30 and its mutant show proximity of N-N and C-C termini of monomers in oligomers. The presented data underscore a dynamic character for pre-nucleation forms of Abeta, however, with a marked tendency for parallel strand orientation in oligomers.

Structure and function of amyloid in Alzheimer's disease

This review is focused on the structure and function of Alzheimer's amyloid deposits. Amyloid formation is a process in which normal well-folded cellular proteins undergo a self-assembly process that leads to the formation of large and ordered protein structures. Amyloid deposition, oligomerization, and higher order polymerization, and the structure adopted by these assemblies, as well as their functional relationship with cell biology are underscored. Numerous efforts have been directed to elucidate these issues and their relation with senile dementia. Significant advances made in the last decade in amyloid structure, dynamics and cell biology are summarized and discussed. The mechanism of amyloid neurotoxicity is discussed with emphasis on the Wnt signaling pathway. This review is focused on Alzheimer's amyloid fibrils in general and has been divided into two parts dealing with the structure and function of amyloid.

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

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

Trace metal contamination initiates the apparent auto-aggregation, amyloidosis, and oligomerization of Alzheimer's Abeta peptides

Nucleation-dependent protein aggregation ("seeding") and amyloid fibril-free formation of soluble SDS-resistant oligomers ("oligomerization") by hydrophobic interaction is an in vitro model thought to propagate beta-amyloid (Abeta) deposition, accumulation, and incur neurotoxicity and synaptotoxicity in Alzheimer's disease (AD), and other amyloid-associated neurodegenerative diseases. However, Abeta is a high-affinity metalloprotein that aggregates in the presence of biometals (zinc, copper, and iron), and neocortical Abeta deposition is abolished by genetic ablation of synaptic zinc in transgenic mice. We now present in vitro evidence that trace (<or=0.8 microM) levels of zinc, copper, and iron, present as common contaminants of laboratory buffers and culture media, are the actual initiators of the classic Abeta1-42-mediated seeding process and Abeta oligomerization. Replicating the experimental conditions of earlier workers, we found that the in vitro precipitation and amyloidosis of Abeta1-40 (20 microM) initiated by Abeta1-42 (2 microM) were abolished by chelation of trace metal contaminants. Further, metal chelation attenuated formation of soluble Abeta oligomers from a cell-free culture medium. These data suggest that protein self-assembly and oligomerization are not spontaneous in this system as previously thought, and that there may be an obligatory role for metal ions in initiating Abeta amyloidosis and oligomerization.

Electrophoretic mobility of Alzheimer's amyloid-beta peptides in urea-sodium dodecyl sulfate-polyacrylamide gel electrophoresis

The 43-amino acid Alzheimer's amyloid-beta peptide (Abeta peptide) retains a predominantly alpha-helix and beta-strand structure in sodium dodecyl sulfate (SDS) solution. This conformer has a high tendency to aggregate during conventional SDS-polyacrylamide gel electrophoresis (PAGE). Both the secondary structure and the proclivity for aggregation are obviated by the use of urea-SDS-PAGE: In 8M urea-with or without SDS-the Abeta peptide becomes 100% random coil and remains monomeric. However, during electrophoresis in this medium, the peptide and its truncated variants do not obey the law of mass/mobility relationship that most proteins-including Abeta peptides-follow in conventional SDS-PAGE. Rather, the smaller carboxy-terminally truncated peptides migrate slower than the larger full-length peptide, while the amino terminally truncated peptide does migrate faster than the full-length Abeta peptide. Thus, despite their small size (2-4kDa) and minor differences between their lengths, the Abeta peptides display a wide separation in this low-porosity (12% acrylamide) gel. We found that this unusual electrophoretic mobility in 8M urea is due to the fact that the quantity of [35S]SDS bound to the Abeta peptides, instead of being proportional to the total number of amino acids, is rather proportional to the sum of the hydrophobicity consensus indices of the constituent amino acids. It is then their hydrophobicity and, hence, the net negative charges contributed by the peptide-bound SDS that plays a major role in determining the mobility of Abeta peptides in 8M urea-SDS-PAGE. The high selectivity of the 8M urea-SDS-PAGE method allowed us to detect the presence of hitherto unknown Abeta peptide variants that were secreted in the conditioned medium by cultured HeLa cells.

Amyloid-like formation by self-assembly of peptidolipids in two dimensions

The accumulation of beta-amyloid peptide (Abeta) in the human brain is known to be the major cause that drives Alzheimer's disease pathogenesis. Abeta, a 39-42 amino acid peptide, is the cleavage product of amyloid precursor protein in the hydrophobic transmembrane region. The present study employs a two-dimensional (2D) approach. Two synthetic peptidolipids, C18-IIGLM-OH and C18-IIGLM-NH2, are selected based on the fragment 31-35 of Abeta which is recognized as one of the determining segments that induces formation of amyloid fibril plaques. The aliphatic hydrocarbon chain C18 is attached to the N-terminal of the fragment 31-35 to facilitate the 2D study at the air-water interface. The aggregation process is observed by two measurements: (1) surface pressure-area and surface dipole moment-area isotherms and (2) epifluorescence microscopy of the Langmuir films to investigate the topography of the amyloid-like formation.

Transient Aggregation and Stable Dimerization Induced by Introducing an Alzheimer Sequence into a Water-Soluble Protein

Transient contacts between denatured polypeptide chains are likely to play an important part in the initial stages of protein aggregation and fibrillation. To analyze the nature of such contacts, we have carried out a protein engineering study of the 102-residue protein U1A, which aggregates transiently in the wild-type form during refolding from the guanidinium chloride-denatured state. We have prepared a series of mutants with increased aggregation tendencies by increasing the homology between two beta-strands of U1A and the Alzheimer peptide (beta-AP). These mutants undergo transient aggregation during refolding, as measured by concentration dependence, double-jump experiments, and binding of ANS, a probe for exposed hydrophobic patches on protein surfaces. The propensity to aggregate increases with increasing homology to beta-AP. Further, the degree of transient ANS binding correlates reasonably well with the structural parameters recently shown to play a role in the fibrillation of natively unfolded proteins. Two mutants highly prone to transient aggregation, U1A-J and U1A-G, were also studied by NMR. Secondary structural elements of the U1A-J construct (with lower beta-AP homology) are very similar to those observed in U1A-wt. In contrast, the high-homology construct U1A-G exhibits local unfolding of the C-terminal helix, which packs against the beta-sheet in the wild-type protein. U1A-G is mainly dimeric according to (15)N spin relaxation data, and the dimer interface most likely involves the beta-sheet. Our data suggest that the transient aggregate relies on specific intermolecular interactions mediated by structurally flexible regions and that contacts may be formed in different beta-strand registers.

Abeta(1-28) fragment of the amyloid peptide predominantly adopts a polyproline II conformation in an acidic solution

To structurally characterize the nonaggregated state of the amyloid beta peptide, which assembles into the hallmark fibrils of Alzheimer disease, we investigated the conformation of the N-terminal extracellular peptide fragment Abeta(1-28) in D(2)O at acidic pD by utilizing combined FTIR and isotropic and anisotropic Raman spectra measured between 1550 and 1750 cm(-1). Peptide aggregation is avoided under the conditions chosen. The amide I' band was found to exhibit a significant noncoincidence effect in that the first moment of the anisotropic Raman and of the IR band profile appears red-shifted from that of the isotropic Raman scattering. A simulation based on a coupled oscillator model involving all 27 amide I' modes of the peptide reveals that the peptide adopts a predominantly polyproline II conformation. Our results are inconsistent with the notion that the monomeric form of Abeta(1-28) is a totally disordered, random-coil structure. Generally, they underscore the notion that polyproline II is a characteristic motif of the unfolded state of proteins and peptides.

Modulation of S6 Fibrillation by Unfolding Rates and Gatekeeper Residues

We present a protein engineering analysis of the fibrillation of a protein from a thermophilic organism, the 101 residue S6 from Thermus thermophilus. When agitated, S6 fibrillates at pH 2.0 in 0.4 M NaCl. Under these solvent conditions, S6 has native-like secondary structure and also unfolds and refolds cooperatively. However, its tertiary structure appears to be more plastic than at neutral pH, and some regions of the protein may be partially unstructured. At 42 degrees C, there is a lag phase of several days after which fibrillation takes place over several hours. Data from the fibrillation behaviour of a comprehensive series of single and double mutants of S6 suggests that several factors control the onset of fibrillation. Firstly, there appears to be a contiguous region of "gatekeeper" residues that inhibit fibrillation, since their truncation significantly reduces the duration of the lag phase. This region overlaps extensively with the partially unstructured region of the protein, suggesting that residues with enhanced flexibility and solvent-accessibility are important for the initiation of fibrillation. Secondly, longer lag phases correlate with faster rates of unfolding. We interpret this to mean that kinetic stability also controls fibrillation but in the sense that the quasi-native state, rather than the denatured state, is the species that participates in nucleation. This implies that fibrillation can also occur from a quasi-native state as opposed to an ensemble of highly fluctuating structures, and highlights the delicate balance between flexibility and structure required to form organized assemblies of polypeptide chains.

Copper mediates dityrosine cross-linking of Alzheimer's amyloid-beta

We have previously reported that amyloid Abeta, the major component of senile plaques in Alzheimer's disease (AD), binds Cu with high affinity via histidine and tyrosine residues [Atwood, C. S., et al. (1998) J. Biol. Chem. 273, 12817-12826; Atwood, C. S., et al. (2000) J. Neurochem. 75, 1219-1233] and produces H(2)O(2) by catalyzing the reduction of Cu(II) or Fe(III) [Huang, X., et al. (1999) Biochemistry 38, 7609-7616; Huang, X., et al. (1999) J. Biol. Chem. 274, 37111-37116]. Incubation with Cu induces the SDS-resistant oligomerization of Abeta [Atwood, C. S., et al. (2000) J. Neurochem. 75, 1219-1233], a feature characteristic of neurotoxic soluble Abeta extracted from the AD brain. Since residues coordinating Cu are most vulnerable to oxidation, we investigated whether modifications of these residues were responsible for Abeta cross-linking. SDS-resistant oligomerization of Abeta caused by incubation with Cu was found to induce a fluorescence signal characteristic of tyrosine cross-linking. Using ESI-MS and a dityrosine specific antibody, we confirmed that Cu(II) (at concentrations lower than that associated with amyloid plaques) induces the generation of dityrosine-cross-linked, SDS-resistant oligomers of human, but not rat, Abeta peptides. The addition of H2O2 strongly promoted Cu-induced dityrosine cross-linking of Abeta1-28, Abeta1-40, and Abeta1-42, suggesting that the oxidative coupling is initiated by interaction of H2O2 with a Cu(II) tyrosinate. The dityrosine modification is significant since it is highly resistant to proteolysis and is known to play a role in increasing structural strength. Given the elevated concentration of Cu in senile plaques, our results suggest that Cu interactions with Abeta could be responsible for causing the covalent cross-linking of Abeta in these structures.

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.

Copper selectively triggers beta-sheet assembly of an N-terminally truncated amyloid beta-peptide beginning with Glu3

Metal ions have been suggested to induce aggregation of amyloid beta-peptide (Abeta), which is a key event in Alzheimer's disease. However, direct evidence that specific metal-peptide interactions are responsible for the amyloid formation has not previously been provided. Here we present the first example of the metal-induced amyloid formation by an Abeta fragment, which exhibits a clear-cut dependence on the amino acid sequence. A heptapeptide, EFRHDSG, corresponding to the amino acid residues 3-9 of Abeta (Abeta(3-9)) undergoes a conformational transition from irregular to beta-sheet and self-associates into insoluble aggregates upon Cu(II) binding. A Raman spectrum analysis of the Cu(II)-Abeta(3-9) complex and aggregation assays of mutated Abeta(3-9) peptides demonstrated that a concerted Cu(II) coordination of the imidazole side chain of His6, the carboxyl groups of Glu3 and Asp7, and the amino group at the N-terminus is essential for the amyloid formation. Although Abeta(1-9) and Abeta(2-9) also contain the metal binding sites, neither of these peptides forms amyloid depositions in the presence of Cu(II). The results of this study may not only provide new insight into the mechanism of amyloid formation, but also be important as a step toward the construction of proteinaceous materials with a specific function under the control of Cu(II).

Redox-Active Metals, Oxidative Stress, and Alzheimer's Disease Pathology

Considerable evidence is mounting that dyshomeostasis of the redox-active biometals, Cu and Fe, and oxidative stress contribute to the neuropathology of Alzheimer's disease (AD). Present data suggest that metals can interact directly with Abeta peptide, the principal component of beta-amyloid that is one of the primary lesions in AD. The binding of metals to Abeta modulates several physiochemical properties of Abeta that are thought to be central to the pathogenicity of the peptide. First, we and others have shown that metals can promote the in vitro aggregation into tinctorial Abeta amyloid. Studies have confirmed that insoluble amyloid plaques in postmortem AD brain are abnormally enriched in Cu, Fe, and Zn. Conversely, metal chelators dissolve these proteinaceous deposits from postmortem AD brain tissue and attenuate cerebral Abeta amyloid burden in APP transgenic mouse models of AD. Second, we have demonstrated that redox-active Cu(II) and, to a lesser extent, Fe(III) are reduced in the presence of Abeta with concomitant production of reactive oxygen species (ROS), hydrogen peroxide (H(2)O(2)) and hydroxyl radical (OH*). These Abeta/metal redox reactions, which are silenced by redox-inert Zn(II), but exacerbated by biological reducing agents, may lead directly to the widespread oxidation damages observed in AD brains. Moreover, studies have also shown that H(2)O(2) mediates Abeta cellular toxicity and increases the production of both Abeta and amyloid precursor protein (APP). Third, the 5' untranslated region (5'UTR) of APP mRNA has a functional iron-response element (IRE), which is consistent with biochemical evidence that APP is a redox-active metalloprotein. Hence, the redox interactions between Abeta, APP, and metals may be at the heart of a pathological positive feedback system wherein Abeta amyloidosis and oxidative stress promote each other. The emergence of redox-active metals as key players in AD pathogenesis strongly argues that amyloid-specific metal-complexing agents and antioxidants be investigated as possible disease-modifying agents for treating this horrible disease.

Amyloidosis of Alzheimer's Abeta peptides: solid-state nuclear magnetic resonance, electron paramagnetic resonance, transmission electron microscopy, scanning transmission electron microscopy and atomic force microscopy studies

Aggregation cascade for Alzheimer's amyloid-beta peptides, its relevance to neurotoxicity in the course of Alzheimer's disease and experimental methods useful for these studies are discussed. Details of the solid-phase peptide synthesis and sample preparation procedures for Alzheimer's beta-amyloid fibrils are given. Recent progress in obtaining structural constraints on Abeta-fibrils from solid-state NMR and scanning transmission electron microscopy (STEM) data is discussed. Polymorphism of amyloid fibrils and oligomers of the 'Arctic' mutant of Abeta(1-40) was studied by (1)H,(13)C solid-state NMR, transmission electron microscopy (TEM) and atomic force microscopy (AFM), and a real-time aggregation of different polymorphs of the peptide was observed with the aid of in situ AFM. Recent results on binding of Cu(II) ions and Al-citrate and Al-ATP complexes to amyloid fibrils, as studied by electron paramagnetic resonance (EPR) and solid-state (27)Al NMR techniques, are also presented.

Increased risk of type 2 diabetes in Alzheimer disease

Alzheimer disease and type 2 diabetes are characterized by increased prevalence with aging, a genetic predisposition, and comparable pathological features in the islet and brain (amyloid derived from amyloid beta protein in the brain in Alzheimer disease and islet amyloid derived from islet amyloid polypeptide in the pancreas in type 2 diabetes). Evidence is growing to link precursors of amyloid deposition in the brain and pancreas with the pathogenesis of Alzheimer disease and type 2 diabetes, respectively. Given these similarities, we questioned whether there may be a common underlying mechanism predisposing to islet and cerebral amyloid. To address this, we first examined the prevalence of type 2 diabetes in a community-based controlled study, the Mayo Clinic Alzheimer Disease Patient Registry (ADPR), which follows patients with Alzheimer disease versus control subjects without Alzheimer disease. In addition to this clinical study, we performed a pathological study of autopsy cases from this same community to determine whether there is an increased prevalence of islet amyloid in patients with Alzheimer disease and increased prevalence of cerebral amyloid in patients with type 2 diabetes. Patients who were enrolled in the ADPR (Alzheimer disease n = 100, non-Alzheimer disease control subjects n = 138) were classified according to fasting glucose concentration (FPG) as nondiabetic (FPG <110 mg/dl), impaired fasting glucose (IFG, FPG 110-125 mg/dl), and type 2 diabetes (FPG >126 mg/dl). The mean slope of FPG over 10 years in each case was also compared between Alzheimer disease and non-Alzheimer disease control subjects. Pancreas and brain were examined from autopsy specimens obtained from 105 humans (first, 28 cases of Alzheimer disease disease vs. 21 non-Alzheimer disease control subjects and, second, 35 subjects with type 2 diabetes vs. 21 non-type 2 diabetes control subjects) for the presence of islet and brain amyloid. Both type 2 diabetes (35% vs. 18%; P < 0.05) and IFG (46% vs. 24%; P < 0.01) were more prevalent in Alzheimer disease versus non-Alzheimer disease control subjects, so 81% of cases of Alzheimer disease had either type 2 diabetes or IFG. The slope of increase of FPG with age over 10 years was also greater in Alzheimer disease than non-Alzheimer disease control subjects (P < 0.01). Islet amyloid was more frequent (P < 0.05) and extensive (P < 0.05) in patients with Alzheimer disease than in non-Alzheimer disease control subjects. However, diffuse and neuritic plaques were not more common in type 2 diabetes than in control subjects. In cases of type 2 diabetes when they were present, the duration of type 2 diabetes correlated with the density of diffuse (P < 0.001) and neuritic plaques (P < 0.01). In this community cohort from southeast Minnesota, type 2 diabetes and IFG are more common in patients with Alzheimer disease than in control subjects, as is the pathological hallmark of type 2 diabetes, islet amyloid. However, there was no increase in brain plaque formation in cases of type 2 diabetes, although when it was present, it correlated in extent with duration of diabetes. These data support the hypothesis that patients with Alzheimer disease are more vulnerable to type 2 diabetes and the possibility of linkage between the processes responsible for loss of brain cells and beta-cells in these diseases.

Residues 17-20 and 30-35 of beta-amyloid play critical roles in aggregation

We examined the effects of co-incubating nine different Abeta peptide fragments with full-length Abeta1-40 (Abeta40) on protein aggregation. Six fragments enhanced aggregation of Abeta40 (Abeta1-28, 12-28, 17-28, 10-20, 25-35 and 17-40), while three others did not (Abeta1-11, 1-16, and 20-29). All of the peptides that enhanced aggregation contained either residues 17-20 or 30-35, indicating the importance of these regions for promoting aggregation of full-length Abeta. Abeta25-35 in particular increased both the rate and extent of aggregation of Abeta40 considerably as indicated by fluorescence staining. Atomic force microscope imaging (AFM) indicates the increase in fluorescence staining with Abeta25-35 is primarily due to increased formation of oligomers and protofibrils rather than formation of large amyloid fibrils. AFM images of Abeta25-35 when incubated alone also indicate formation of aggregates and long thin filaments. The increase in formation of the small toxic oligomeric morphology of Abeta40, along with formation of Abeta25-35 oligomers and thin filaments, represent two different potential pathways for Abeta25-35 toxicity. The critical roles of residues 17-20 and 30-35 of Abeta provide further insight into mechanism that underlie the formation of toxic aggregates in Alzheimer Disease (AD) and suggest targets for the design of beta-sheet breakers to modulate the aggregation and inhibit toxicity of full-length Abeta.

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

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

Both the d-(+) and l-(−) Enantiomers of Nicotine Inhibit Aβ Aggregation and Cytotoxicity

The underlying cause of Alzheimer's disease is thought to be the aggregation of monomeric beta-amyloid (Abeta), through a series of toxic oligomers, which forms the mature amyloid fibrils that accumulate at the center of senile plaques. It has been reported that L-(-)-nicotine prevents Abeta aggregation and toxicity, and inhibits senile plaque formation. Previous NMR studies have suggested that this could be due to the specific binding of L-(-)-nicotine to histidine residues (His6, His13, and His14) in the peptide. Here, we have looked at the effects of both of the L-(-) and D-(+) optical enantiomers of nicotine on the aggregation and cytotoxicity of Abeta(1-40). Surprisingly, both enantiomers inhibited aggregation of the peptide and reduced the toxic effects of the peptide on cells. In NMR studies with Abeta(1-40), both enantiomers of nicotine were seen to interact with the three histidine residues. Overall, our data indicate that nicotine can delay Abeta fibril formation and maintain a population of less toxic Abeta species. This effect cannot be due to a highly specific binding interaction between nicotine and Abeta, as previously thought, but could be due instead to weaker, relatively nonspecific binding, or to the antioxidant or metal chelating properties of nicotine. D-(+)-nicotine, being biologically much less active than L-(-)-nicotine, might be a useful therapeutic agent.

Structural organization of alpha-synuclein fibrils studied by site-directed spin labeling

Despite its importance in Parkinson's disease, a detailed understanding of the structure and mechanism of alpha-synuclein fibril formation remains elusive. In this study, we used site-directed spin labeling and electron paramagnetic resonance spectroscopy to study the structural features of monomeric and fibrillar alpha-synuclein. Our results indicate that monomeric alpha-synuclein, in solution, has a highly dynamic structure, in agreement with the notion that alpha-synuclein is a natively unfolded protein. In contrast, fibrillar aggregates of alpha-synuclein exhibit a distinct domain organization. Our data identify a highly ordered and specifically folded central core region of approximately 70 amino acids, whereas the N terminus is structurally more heterogeneous and the C terminus ( approximately 40 amino acids) is completely unfolded. Interestingly, the central core region of alpha-synuclein exhibits several features reminiscent of those observed in the core region of fibrillar Alzheimer's amyloid beta peptide, including an in-register parallel structure. Although the lengths of the respective core regions differ, fibrils from different amyloid proteins nevertheless appear to be able to take up highly similar, and possibly conserved, structures.

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.

Hydrogen exchange-mass spectrometry analysis of beta-amyloid peptide structure

beta-Amyloid peptide (A beta) is the primary protein component of senile plaques in Alzheimer's disease and is believed to be responsible for the neurodegeneration associated with the disease. A beta has proven to be toxic only when aggregated; however, the structure of the aggregated species associated with toxicity is unknown. In the present study, we use hydrogen-deuterium isotope exchange (HX)-electrospray ionization mass spectrometry (MS) along with enzymatic digestion as a tool to examine at near residue level, the changes in A beta structure associated with aggregation to a fibril form. Our results show that the structure of A beta intermediate species formed early in the course of fibrillogenesis is dependent upon solvent conditions. Additionally, the HX-MS data of peptic A beta fragments suggest that the C-terminal segment of the peptide is approximately 35% protected from exchange in fibril-containing samples, relative to monomeric A beta species prepared in DMSO/H(2)O. The N-terminus (residues 1-4) is completely unprotected from exchange, and the fragment containing residues 5-19 is over 50% protected from exchange in the fibril-containing samples. This work contributes to our understanding of A beta structure associated with aggregation and toxicity and further application of this approach may aid in the design of agents that intervene in the A beta aggregation processes associated with neurotoxicity.

Glycation of the amyloid beta-protein by a nicotine metabolite: a fortuitous chemical dynamic between smoking and Alzheimer's disease

The origin of Alzheimer's disease (AD) has been subjected to an intense amount of examination; however, a clear conclusion as to the nature of this crippling disease has yet to be identified. What is readily accepted is that a definitive marker of this disease is the aggregation of the amyloid beta-peptide (A beta) into neuritic plaques. The recent observation that nicotine exposure leads to delayed onset of AD has stimulated a flurry of research into the nature of this neuroprotective effect. This phenomenon has been debated, but no consensus has been reached, and although these studies have targeted nicotine, the primary alkaloid in tobacco, few studies have considered the physiological role of nicotine metabolites in disease states. Nornicotine is a major nicotine metabolite in the CNS and has been shown to participate in the aberrant glycation of proteins in vivo in a process termed nornicotine-based glycation. Herein is detailed a potentially fortuitous role of nornicotine-based glycation in relation to the pathology of AD. Specifically, nornicotine was found to covalently alter A beta, leading to reduced peptide aggregation. Potential consequences of this reaction cascade include reduced plaque formation and/or altered clearance of the peptide, as well as attenuated toxicity of soluble A beta aggregates. The findings described provide an alternative mechanism for nicotine neuroprotection in AD and a means for the alteration of amyloid folding based on a covalent chemical event.

Potential link between amyloid beta-protein 42 and C-terminal fragment gamma 49-99 of beta-amyloid precursor protein

A novel cleavage of beta-amyloid precursor protein (APP), referred to as epsilon-cleavage, occurs downstream of the gamma-cleavage and generates predominantly a C-terminal fragment (CTFgamma) that begins at Val-50, according to amyloid beta-protein (Abeta) numbering. Whether this cleavage occurs independently of, or is coordinated with, gamma-cleavage is unknown. Using a cell-free system, we show here that, although Abeta40 and CTFgamma 50-99 were the predominant species produced by membranes prepared from cells overexpressing wild-type (wt) APP and wt presenilin (PS) 1 or 2, the production of CTFgamma 49-99, which begins at Leu-49, was remarkably enhanced in membranes from cells overexpressing mutant (mt) APP or mtPS1/2 that increases the production of Abeta42. Furthermore, a gamma-secretase inhibitor, which suppresses Abeta40 production and paradoxically enhances Abeta42 production at low concentrations, caused the proportion of CTFgamma 50-99 to decrease and that of CTFgamma 49-99 to increase significantly. These results strongly suggest a link between the production of Abeta42 and CTFgamma 49-99 and provide an important insight into the mechanisms of altered gamma-cleavage caused by mtAPP and mtPS1/2.

Coordination abilities of the 1-16 and 1-28 fragments of beta-amyloid peptide towards copper(II) ions: a combined potentiometric and spectroscopic study

Stoichiometry, stability constants and solution structures of the copper(II) complexes of the (1-16H), (1-28H), (1-16M), (1-28M), (Ac-1-16H) and (Ac-1-16M) fragments of human (H) and mouse (M) beta-amyloid peptide were determined in aqueous solution in the pH range 2.5-10.5. The potentiometric and spectroscopic data (UV-Vis, CD, EPR) show that acetylation of the amino terminal group induces significant changes in the coordination properties of the (Ac-1-16H) and (Ac-1-16M) peptides compared to the (1-16H) and (1-16M) fragments, respectively. The (Ac-1-16H) peptide forms the 3N [N(Im)(6), N(Im)(13), N(Im)(14)] complex in a wide pH range (5-8), while for the (Ac-1-16M) fragment the 2N [N(Im)(6), N(Im)(14)] complex in the pH range 5-7 is suggested. At higher pH values sequential amide nitrogens are deprotonated and coordinated to copper(II) ions. The N-terminal amino group of the (1-16) and (1-28) fragments of human and mouse beta-amyloid peptide takes part in the coordination of the metal ion, although, at pH above 9 the complexes with the 4N [N(Im), 3N(-)] coordination mode are formed. The phenolate -OH group of the Tyr(10) residue of the human fragments does not coordinate to the metal ion.

Site-specific identification of non-beta-strand conformations in Alzheimer's beta-amyloid fibrils by solid-state NMR

The most well-established structural feature of amyloid fibrils is the cross-beta motif, an extended beta-sheet structure formed by beta-strands oriented perpendicular to the long fibril axis. Direct experimental identification of non-beta-strand conformations in amyloid fibrils has not been reported previously. Here we report the results of solid-state NMR measurements on amyloid fibrils formed by the 40-residue beta-amyloid peptide associated with Alzheimer's disease (Abeta(1-40)), prepared synthetically with pairs of (13)C labels at consecutive backbone carbonyl sites. The measurements probe the peptide backbone conformation in residues 24-30, a segment where a non-beta-strand conformation has been suggested by earlier sequence analysis, cross-linking experiments, and molecular modeling. Data obtained with the fpRFDR-CT, DQCSA, and 2D MAS exchange solid-state NMR techniques, which provide independent constraints on the phi and psi backbone torsion angles between the labeled carbonyl sites, indicate non-beta-strand conformations at G25, S26, and G29. These results represent the first site-specific identification and characterization of non-beta-strand peptide conformations in an amyloid fibril.