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

Fullerene inhibits beta-amyloid peptide aggregation

We report that fullerene inhibits strongly the amyloid peptide aggregation at the early stage. It specifically binds to the central hydrophobic motif, KLVFF, of A beta peptides. The IC(50) value has been measured as 9 microM for both A beta(11-25) and A beta(1-40). On the other hand, a control experiment shows melatonin rather specifically binds to the C-terminus region. The IC(50) value of fullerene appears to be at least four times larger for A beta(1-40), compared with melatonin, and 15 times larger for A beta(11-25). This work shows that fullerene can be a promising candidate in search of AD therapeutics because it has the very high IC(50) value for A beta aggregation.

Diverse fibrillar peptides directly bind the Alzheimer's amyloid precursor protein and amyloid precursor-like protein 2 resulting in cellular accumulation

The Alzheimer's disease Abeta peptide can increase the levels of cell-associated amyloid precursor protein (APP) in vitro. To determine the specificity of this response for Abeta and whether it is related to cytotoxicity, we tested a diverse range of fibrillar peptides including amyloid-beta (Abeta), the fibrillar prion peptides PrP106-126 and PrP178-193 and human islet-cell amylin. All these peptides increased the levels of APP and amyloid precursor-like protein 2 (APLP2) in primary cultures of astrocytes and neurons. Specificity was shown by a lack of change to amyloid precursor-like protein 1, tau-1 and cellular prion protein (PrP(c)) levels. APP and APLP2 levels were elevated only in cultures exposed to fibrillar peptides as assessed by electron microscopy and not in cultures treated with non-fibrillogenic peptide variants or aggregated lipoprotein. We found that PrP106-126 and the non-toxic but fibril-forming PrP178-193 increased APP levels in cultures derived from both wild-type and PrP(c)-deficient mice indicating that fibrillar peptides up-regulate APP through a non-cytotoxic mechanism and irrespective of parental protein expression. Fibrillar PrP106-126 and Abeta peptides bound recombinant APP and APLP2 suggesting the accumulation of these proteins was mediated by direct binding to the fibrillated peptide. This was supported by decreased APP accumulation following extensive washing of the cultures to remove fibrillar aggregates. Pre-incubation of fibrillar peptide with recombinant APP18-146, the putative fibril binding site, also abrogated the accumulation of APP. These findings show that diverse fibrillogenic peptides can induce accumulation of APP and APLP2 and this mechanism could contribute to pathogenesis in neurodegenerative disorders.

Circular dichroism and Fourier transform infrared spectroscopic studies on self-assembly of tetrapeptide derivative in solution and solvated film

Aggregation of the hydrophobic peptide derivative Boc-Ala-Ile-Ile-Gly-OMe (1) was examined in methanol solution and in solvated film states. Formation of the peptide by self-assembly was evidenced using fluorescence [Mg salt of 8-anilino-naphthalenesulfonic acid (ANS) as an external probe] and circular dichroism (CD) spectroscopic techniques. In solution, peptide 1 formed as a stable aggregate at a concentration around 3 x 10(-4)m. The peptide gelled into a thin film for which we carried out CD and Fourier transform infrared (FTIR) measurements. Our spectroscopic study on peptide films at differing methanol concentrations indicates that the helical content of the peptide decreases with decreasing methanol concentration in solvated films. However, by reducing the methanol concentration we were able to observe a conformational transition from a predominantly helical turn to a beta-sheet structure via a random coil conformation. Our study focused on the aggregation of the alpha-helical turn-forming peptide derivative, which shows conformational transition on changing solvent concentration in the film form.

Conformational restriction via cyclization in beta-amyloid peptide Abeta(1-28) leads to an inhibitor of Abeta(1-28) amyloidogenesis and cytotoxicity

The aggregation process of beta-amyloid peptide Abeta into amyloid is strongly associated with the pathology of Alzheimer's disease (AD). Aggregation may involve a transition of an alpha helix in Abeta(1-28) into beta sheets and interactions between residues 18-20 of the "Abeta amyloid core." We applied an i, i+4 cyclic conformational constraint to the Abeta amyloid core and devised side chain-to-side chain lactam-bridged cyclo(17, 21)-[Lys(17), Asp(21)]Abeta(1-28). In contrast to Abeta(1-28) and [Lys(17), Asp(21)]Abeta(1-28), cyclo(17, 21)-[Lys(17), Asp(21)]Abeta(1-28) was not able to form beta sheets and cytotoxic amyloid aggregates. Cyclo(17, 21)-[Lys(17), Asp(21)]Abeta(1-28) was able to interact with Abeta(1-28) and to inhibit amyloid formation and cytotoxicity. Cyclo(17, 21)-[Lys(17), Asp(21)]Abeta(1-28) also interacted with Abeta(1-40) and interfered with its amyloidogenesis. Cyclo(17, 21)-[Lys(17), Asp(21)]Abeta(1-28) or similarly constrained Abeta sequences may find therapeutic and diagnostic applications in AD.

Alternate aggregation pathways of the Alzheimer beta-amyloid peptide: Abeta association kinetics at endosomal pH

The deposition of beta-amyloid peptide (Abeta) fibrils around neurons is an invariable feature of Alzheimer's disease and there is increasing evidence that fibrillar deposits and/or prefibrillar intermediates play a central role in the observed neurodegeneration. One site of Abeta generation is the endosomes, and we have investigated the kinetics of Abeta association at endosomal pH over physiologically relevant time frames. We have identified three distinct Abeta association phases that occur at rates comparable to endosomal transit times. Rapid formation of burst phase aggregates, larger than 200nm, was observed within 15 seconds. Two slower association phases were detected by fluorescence resonance energy transfer and termed phase 1 and phase 2 aggregation reactions. At 20 microM Abeta, pH 6, the half lives of the phase 1 and phase 2 aggregation phases were 3.15 minutes and 17.66 minutes, respectively. Atomic force microscopy and dynamic light scattering studies indicate that the burst phase aggregate is large and amorphous, while phase 1 and 2 aggregates are spherical with hydrodynamic radii around 30 nm. There is an apparent equilibrium, potentially mediated through a soluble Abeta intermediate, between the large burst phase aggregates and phase 1 and 2 spherical particles. The large burst phase aggregates form quickly, however, they disappear as the equilibrium shifts toward the spherical aggregates. These aggregated species do not contain alpha-helical or beta-structure as determined by circular dichroism spectroscopy. However, after two weeks beta-structure is observed and is attributable to the insoluble portion of the sample. After two months, mature amyloid fibrils appear and the spherical aggregates are significantly diminished.

Supramolecular structural constraints on Alzheimer's beta-amyloid fibrils from electron microscopy and solid-state nuclear magnetic resonance

We describe electron microscopy (EM), scanning transmission electron microscopy (STEM), and solid-state nuclear magnetic resonance (NMR) measurements on amyloid fibrils formed by the 42-residue beta-amyloid peptide associated with Alzheimer's disease (Abeta(1)(-)(42)) and by residues 10-35 of the full-length peptide (Abeta(10)(-)(35)). These measurements place constraints on the supramolecular structure of the amyloid fibrils, especially the type of beta-sheets present in the characteristic amyloid cross-beta structural motif and the assembly of these beta-sheets into a fibril. EM images of negatively stained Abeta(10)(-)(35) fibrils and measurements of fibril mass per length (MPL) by STEM show a strong dependence of fibril morphology and MPL on pH. Abeta(10)(-)(35) fibrils formed at pH 3.7 are single "protofilaments" with MPL equal to twice the value expected for a single cross-beta layer. Abeta(10)(-)(35) fibrils formed at pH 7.4 are apparently pairs of protofilaments or higher order bundles. EM and STEM data for Abeta(1)(-)(42) fibrils indicate that protofilaments with MPL equal to twice the value expected for a single cross-beta layer are also formed by Abeta(1)(-)(42) and that these protofilaments exist singly and in pairs at pH 7.4. Solid-state NMR measurements of intermolecular distances in Abeta(10)(-)(35) fibrils, using multiple-quantum (13)C NMR, (13)C-(13)C dipolar recoupling, and (15)N-(13)C dipolar recoupling techniques, support the in-register parallel beta-sheet organization previously established by Lynn, Meredith, Botto, and co-workers [Benzinger et al. (1998) Proc. Natl. Acad. Sci. U.S.A. 95, 13407-13412; Benzinger et al. (2000) Biochemistry 39, 3491-3499] and show that this beta-sheet organization is present at pH 3.7 as well as pH 7.4 despite the differences in fibril morphology and MPL. Solid-state NMR measurements of intermolecular distances in Abeta(1)(-)(42) fibrils, which represent the first NMR data on Abeta(1)(-)(42) fibrils, also indicate an in-register parallel beta-sheet organization. These results, along with previously reported data on Abeta(1)(-)(40) fibrils, suggest that the supramolecular structures of Abeta(10)(-)(35), Abeta(1)(-)(40), and Abeta(1)(-)(42) fibrils are quite similar. A schematic structural model of these fibrils, consistent with known experimental EM, STEM, and solid-state NMR data, is presented.

A structural model for Alzheimer's beta -amyloid fibrils based on experimental constraints from solid state NMR

We present a structural model for amyloid fibrils formed by the 40-residue beta-amyloid peptide associated with Alzheimer's disease (Abeta(1-40)), based on a set of experimental constraints from solid state NMR spectroscopy. The model additionally incorporates the cross-beta structural motif established by x-ray fiber diffraction and satisfies constraints on Abeta(1-40) fibril dimensions and mass-per-length determined from electron microscopy. Approximately the first 10 residues of Abeta(1-40) are structurally disordered in the fibrils. Residues 12-24 and 30-40 adopt beta-strand conformations and form parallel beta-sheets through intermolecular hydrogen bonding. Residues 25-29 contain a bend of the peptide backbone that brings the two beta-sheets in contact through sidechain-sidechain interactions. A single cross-beta unit is then a double-layered beta-sheet structure with a hydrophobic core and one hydrophobic face. The only charged sidechains in the core are those of D23 and K28, which form salt bridges. Fibrils with minimum mass-per-length and diameter consist of two cross-beta units with their hydrophobic faces juxtaposed.

Metal complexing agents as therapies for Alzheimer's disease

Modern research approaches into drug development for Alzheimer's disease (AD) target beta-amyloid (Abeta) accumulation in the brain. The main approaches attempt to prevent Abeta production (secretase inhibitors) or to clear Abeta (vaccine). However, there is now compelling evidence that Abeta does not spontaneously aggregate, but that there is an age-dependent reaction with excess brain metal (copper, iron and zinc), which induces the protein to precipitate into metal-enriched masses (plaques). The abnormal combination of Abeta with Cu or Fe induces the production of hydrogen peroxide, which may mediate the conspicuous oxidative damage to the brain in AD. We have developed metal-binding compounds that inhibit the in vitro generation of hydrogen peroxide by Abeta, as well as reverse the aggregation of the peptide in vitro and from human brain post-mortem specimens. Most recently, one of the compounds, clioquinol (CQ; a USP antibiotic) was given orally for 9 weeks to amyloid-bearing transgenic mice, and succeeded in markedly inhibiting Abeta accumulation. On the basis of these results, CQ is being tested in clinical trials.

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

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

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

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

Assembling amyloid fibrils from designed structures containing a significant amyloid beta-peptide fragment

The amyloid plaque, consisting of amyloid beta-peptide (Abeta) fibrils surrounded by dystrophic neurites, is an invariable feature of Alzheimer's disease. The determination of the molecular structure of Abeta fibrils is a significant goal that may lead to the structure-based design of effective therapeutics for Alzheimer's disease. Technical challenges have thus far rendered this goal impossible. In the present study, we develop an alternative methodology. Rather than determining the structure directly, we design conformationally constrained peptides and demonstrate that only certain 'bricks' can aggregate into fibrils morphologically identical to Abeta fibrils. The designed peptides include variants of a decapeptide fragment of Abeta, previously shown to be one of the smallest peptides that (1) includes a pentapeptide sequence necessary for Abeta-Abeta binding and aggregation and (2) can form fibrils indistinguishable from those formed by full-length Abeta. The secondary structure of these bricks is monitored by CD spectroscopy, and electron microscopy is used to study the morphology of the aggregates formed. We then made various residue deletions and substitutions to determine which structural features are essential for fibril formation. From the constraints, statistical analysis of side-chain pair correlations in beta-sheets and experimental data, we deduce a detailed model of the peptide strand alignment in fibrils formed by these bricks. Our results show that the constrained decapeptide dimers rapidly form an intramolecular, antiparallel beta-sheet and polymerize into amyloid fibrils at low concentrations. We suggest that the formation of an exposed beta-sheet (e.g. an Abeta dimer formed by interaction in the decapeptide region) could be a rate-limiting step in fibril formation. A theoretical framework that explains the results is presented in parallel with the data.

Supramolecular structure in full-length Alzheimer's beta-amyloid fibrils: evidence for a parallel beta-sheet organization from solid-state nuclear magnetic resonance

We report constraints on the supramolecular structure of amyloid fibrils formed by the 40-residue beta-amyloid peptide associated with Alzheimer's disease (A beta(1-40)) obtained from solid-state nuclear magnetic resonance (NMR) measurements of intermolecular dipole-dipole couplings between (13)C labels at 11 carbon sites in residues 2 through 39. The measurements are carried out under magic-angle spinning conditions, using the constant-time finite-pulse radiofrequency-driven recoupling (fpRFDR-CT) technique. We also present one-dimensional (13)C magic-angle spinning NMR spectra of the labeled A beta(1-40) samples. The fpRFDR-CT data reveal nearest-neighbor intermolecular distances of 4.8 +/- 0.5 A for carbon sites from residues 12 through 39, indicating a parallel alignment of neighboring peptide chains in the predominantly beta-sheet structure of the amyloid fibrils. The one-dimensional NMR spectra indicate structural order at these sites. The fpRFDR-CT data and NMR spectra also indicate structural disorder in the N-terminal segment of A beta(1-40), including the first nine residues. These results place strong constraints on any molecular-level structural model for full-length beta-amyloid fibrils.

Beta-amyloid (Abeta) protein in cerebrospinal fluid as a biomarker for Alzheimer's disease

With the arrival of symptomatic treatment (acetylcholine esterase inhibitors) and the promise of drugs that may delay disease progression, development of diagnostic biomarkers for Alzheimer's disease (AD) are important. Beta-Amyloid (Abeta) protein is the main component of senile plaques. A marked reduction in cerebrospinal fluid (CSF)-Abeta42 in AD has been found in numerous studies. Importantly, reduced CSF-Abeta42 is also found very early in the disease process, before the onset of clinical symptoms. Recent studies suggest that CSF-Abeta42 have a satisfactory performance when used as a diagnostic marker for AD in clinical routine. This paper reviews CSF-Abeta42 as a biomarker for AD.

Senile plaque composition and posttranslational modification of amyloid-beta peptide and associated proteins

Amyloid deposits are primarily composed of the amyloid-beta protein, although other proteins (and metal ions) also have been colocalized to these lesions. The pattern of oxidative modifications in amyloid plaques is very different to that associated with neurofibrillary tangles and neuronal cell bodies, likely reflecting the different composition of these structures, accessibility of oxidants, the generation of oxidants in and around these structures and the intrinsic antioxidant defense systems to protect these structures. Future studies directed at understanding Abeta interactions with other amyloid components, the role of oxidative modifications in stabilizing amyloid deposits and the determination of protease cleavage sites on Abeta may provide mechanistic insights regarding both amyloid formation and removal.

Laminin affects polymerization, depolymerization and neurotoxicity of Abeta peptide

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

Mutations that reduce aggregation of the Alzheimer's Abeta42 peptide: an unbiased search for the sequence determinants of Abeta amyloidogenesis

The primary component of amyloid plaque in the brains of Alzheimer's patients is the 42 residue amyloid-beta-peptide (Abeta42). Although the amino acid residue sequence of Abeta42 is known, the molecular determinants of Abeta amyloidogenesis have not been elucidated. To facilitate an unbiased search for the sequence determinants of Abeta aggregation, we developed a genetic screen that couples a readily observable phenotype in E. coli to the ability of a mutation in Abeta42 to reduce aggregation. The screen is based on our finding that fusions of the wild-type Abeta42 sequence to green fluorescent protein (GFP) form insoluble aggregates in which GFP is inactive. Cells expressing such fusions do not fluoresce. To isolate variants of Abeta42 with reduced tendencies to aggregate, we constructed and screened libraries of Abeta42-GFP fusions in which the sequence of Abeta42 was mutated randomly. Cells expressing GFP fusions to soluble (non-aggregating) variants of Abeta42 exhibit green fluorescence. Implementation of this screen enabled the isolation of 36 variants of Abeta42 with reduced tendencies to aggregate. The sequences of most of these variants are consistent with previous models implicating hydrophobic regions as determinants of Abeta42 aggregation. Some of the variants, however, contain amino acid substitutions not implicated in pre-existing models of Abeta amyloidogenesis.

Growth of beta-amyloid(1-40) protofibrils by monomer elongation and lateral association. Characterization of distinct products by light scattering and atomic force microscopy

Amyloid plaques in brain tissue are a hallmark of Alzheimer's disease. Primary components of these plaques are 40- and 42-residue peptides, denoted A beta(1-40) and A beta(1-42), that are derived by proteolysis of cellular amyloid precursor protein. Synthetic A beta(1-40) and A beta(1-42) form amyloid fibrils in vitro that share many features with the amyloid in plaques. Soluble intermediates in A beta fibrillogenesis, termed protofibrils, have been identified previously, and here we describe the in vitro formation and isolation of A beta(1-40) protofibrils by size exclusion chromatography. In some experiments, the A beta(1-40) was radiomethylated to better quantify various A beta species. Mechanistic studies clarified two separate modes of protofibril growth, elongation by monomer deposition and protofibril-protofibril association, that could be resolved by varying the NaCl concentration. Small isolated protofibrils in dilute Tris-HCl buffers were directed along the elongation pathway by addition of A beta(1-40) monomer or along the association pathway by addition of NaCl. Multi-angle light scattering analysis revealed that protofibrils with initial molecular masses M(w) of (7-30) x 10(3) kDa grew to M(w) values of up to 250 x 10(3) kDa by these two growth processes. However, the mass per unit length of the associated protofibrils was about 2-3 times that of the elongated protofibrils. Rate constants for further elongation by monomer deposition with the elongated, associated, and initial protofibril pools were identical when equal number concentrations of original protofibrils were compared, indicating that the original number of protofibril ends had not been altered by the elongation or association processes. Atomic force microscopy revealed heterogeneous initial protofibrils that became more rodlike following the elongation reaction. Our data indicate that protofibril elongation in the absence of NaCl results from monomer deposition only at the ends of protofibrils and proceeds without an increase in protofibril diameter. In contrast, protofibril association occurs in the absence of monomer when NaCl is introduced, but this association involves lateral interactions that result in a relatively disordered fibril structure.

Highly conserved and disease-specific patterns of carboxyterminally truncated Abeta peptides 1-37/38/39 in addition to 1-40/42 in Alzheimer's disease and in patients with chronic neuroinflammation

Human lumbar CSF patterns of Abeta peptides were analysed by urea-based beta-amyloid sodium dodecyl sulphate polyacrylamide gel electrophoresis with western immunoblot (Abeta-SDS-PAGE/immunoblot). A highly conserved pattern of carboxyterminally truncated Abeta1-37/38/39 was found in addition to Abeta1-40 and Abeta1-42. Remarkably, Abeta1-38 was present at a higher concentration than Abeta1-42, being the second prominent Abeta peptide species in CSF. Patients with Alzheimer's disease (AD, n = 12) and patients with chronic inflammatory CNS disease (CID, n = 10) were differentiated by unique CSF Abeta peptide patterns from patients with other neuropsychiatric diseases (OND, n = 37). This became evident only when we investigated the amount of Abeta peptides relative to their total Abeta peptide concentration (Abeta1-x%, fractional Abeta peptide pattern), which may reflect disease-specific gamma-secretase activities. Remarkably, patients with AD and CID shared elevated Abeta1-38% values, whereas otherwise the patterns were distinct, allowing separation of AD from CID or OND patients without overlap. The presence of one or two ApoE epsilon4 alleles resulted in an overall reduction of CSF Abeta peptides, which was pronounced for Abeta1-42. The severity of dementia was significantly correlated to the fractional Abeta peptide pattern but not to the absolute Abeta peptide concentrations.

Pentapeptide amides interfere with the aggregation of beta-amyloid peptide of Alzheimer's disease

Amyloid peptides (Abeta) play a central role in the pathogenesis of Alzheimer's disease (AD). The aggregation of Abeta molecules leads to fibril and plaque formation. Fibrillogenesis is at the same time a marker and an indirect cause of AD. Inhibition of the aggregation of Abeta could be a realistic therapy for the illness. Beta sheet breakers (BSBs) are one type of fibrillogenesis inhibitors. The first BSB peptides were designed by Tjernberg et al. (1996) and Soto et al. (1998). These pentapeptides have proved their efficiency in vitro and in vivo. In the present study, the effects of two pentapeptide amides are reported. These compounds were designed by using the C-terminal sequence of the amyloid peptide as a template. Biological assays were applied to demonstrate efficiency. Modes of action were studied by FT-IR spectroscopy and molecular modeling methods.

Disassembly of amyloid beta-protein fibril by basement membrane components

Amyloid beta-protein (A3) fibril in senile plaque may be related to the pathogenesis of Alzheimer's disease (AD). Basement membrane (BM) components are associated with the plaques in AD brain. It suggests that the BM components may play an important role in the deposition of the plaque. We investigated the potential of BM components, such as type IV collagen (collagen IV) and entactin, to induce disassembly of preformed Abeta1-42 (Abeta42) fibrils in direct comparison to laminin. Thioflavin T assays revealed that these BM components disrupted preformed Abeta42 fibrils in a dose-dependent manner. The high concentration of BM components, 100 microg/mL laminin, 50 microg/mL collagen IV and 50 microg/mL entactin, had most effect on disassembly of preformed Abeta42 fibrils (Molar ratio; Abeta42:laminin = 90:1, Abeta42:collagen IV = 34:1, Abeta42:entactin = 20:1). Circular dichroism spectroscopy data indicated that the high concentration of BM components induced structural transition in Abeta42 from beta-sheet to random structures. These results suggest that collagen IV and entactin, as well as laminin, are effective inducers of disassembly of Abeta42 fibrils. The ability of these BM components to induce random structures may be linked to the disassembly of preformed Abeta42 fibrils.

Pressure-induced transformation of alpha-helix to beta-sheet in the secondary structures of amyloid beta (1-40) peptide exacerbated by temperature

The effect of pressure on the conformational structure of amyloid beta (1-40) peptide (A beta(1-40)), exacerbated with or without temperature, was determined by Fourier transform infrared (FT-IR) microspectroscopy. The result indicates the shift of the maximum peak of amide I band of intact solid A beta(1-40) from 1655 cm(-1) (alpha-helix) to 1647-1643 cm(-1) (random coil) with the increase of the mechanical pressure. A new peak at 1634 cm(-1) assigned to beta-antiparallel sheet structure was also evident. Furthermore, the peak at 1540 cm(- 1) also shifted to 1527 (1529) cm(-1) in amide II band. The former was assigned to the combination of alpha-helix and random coil structures, and the latter was due to beta-sheet structure. Changes in the composition of each component in the deconvoluted and curve-fitted amide I band of the compressed A beta(1-40) samples were obtained from 33% to 22% for alpha-helix/random coil structures and from 47% to 57% for beta-sheet structure with the increase of pressure, respectively. This demonstrates that pressure might induce the conformational transition from alpha-helix to random coil and to beta- sheet structure. The structural transformation of the compressed A beta(1-40) samples was synergistically influenced by the combined effects of pressure and temperature. The thermal-induced formation of beta-sheet structure was significantly dependent on the pressures applied. The smaller the pressure applied the faster the beta-sheet structure transformed. The thermal-dependent transition temperatures of solid A beta(1-40) prepared by different pressures were near 55-60 degrees C.

Secretases as targets for drug design in Alzheimer's disease

Alzheimer's disease accounts for the majority of dementia in the elderly. Worldwide, approximately 20 million people are suffering from this devastating disease, with no effective treatment currently available. For efficient drug design, it is important to identify the molecular mechanisms underlying the pathology of the disease. An invariant feature in the pathology of Alzheimer's disease is the amyloid-beta peptide. Amyloid-beta is produced by endoproteolytic cleavage of the amyloid precursor protein by beta- and gamma-secretase. In the past 2 years, the protein responsible for beta-secretase activity has been isolated and researchers are close to identifying gamma-secretase. These recent achievements in Alzheimer's disease research have provided helpful tools for the development of therapeutics.

Angiotensin-converting enzyme degrades Alzheimer amyloid beta-peptide (A beta ); retards A beta aggregation, deposition, fibril formation; and inhibits cytotoxicity

We have demonstrated that the angiotensin-converting enzyme (ACE) genotype is associated with Alzheimer's disease (AD) in the Japanese population (). To determine why ACE affects susceptibility to AD, we examined the effect of purified ACE on aggregation of the amyloid beta-peptide (A beta) in vitro. Surprisingly, ACE was found to significantly inhibit A beta aggregation in a dose response manner. The inhibition of aggregation was specifically blocked by preincubation of ACE with an ACE inhibitor, lisinopril. ACE was confirmed to retard A beta fibril formation with electron microscopy. ACE inhibited A beta deposits on a synthaloid plate, which was used to monitor A beta deposition on autopsied brain tissue. ACE also significantly inhibited A beta cytotoxicity on PC12 h. The most striking fact was that ACE degraded A beta by cleaving A beta-(1-40) at the site Asp(7)-Ser(8). This was proven with reverse-phase HPLC, amino acid sequence analysis, and MALDI-TOF/MS. Compared with A beta-(1-40), aggregation and cytotoxic effects of the degradation products A beta-(1-7) and A beta-(8-40) peptides were reduced or virtually absent. These findings led to the hypothesis that ACE may affect susceptibility to AD by degrading A beta and preventing the accumulation of amyloid plaques in vivo.

Melatonin reverses the profibrillogenic activity of apolipoprotein E4 on the Alzheimer amyloid Abeta peptide

Inheritance of apoE4 is a strong risk factor for the development of late-onset sporadic Alzheimer's disease (AD). Several lines of evidence suggest that apoE4 binds to the Alzheimer Abeta protein and, under certain experimental conditions, promotes formation of beta-sheet structures and amyloid fibrils. Deposition of amyloid fibrils is a critical step in the development of AD. We report here that addition of melatonin to Abeta in the presence of apoE resulted in a potent isoform-specific inhibition of fibril formation, the extent of which was far greater than that of the inhibition produced by melatonin alone. This effect was structure-dependent and unrelated to the antioxidant properties of melatonin, since it could be reproduced neither with the structurally related indole N-acetyl-5-hydroxytryptamine nor with the antioxidants ascorbate, alpha-tocophenol, and PBN. The enhanced inhibitory effects of melatonin and apoE were lost when bovine serum albumin was substituted for apoE. In addition, Abeta in combination with apoE was highly neurotoxic (apoE4 > apoE3) to neuronal cells in culture, and this activity was also prevented by melatonin. These findings suggest that reductions in brain melatonin, which occur during aging, may contribute to a proamyloidogenic microenvironment in the aging brain.

Beta-amyloid fibrils activate the C1 complex of complement under physiological conditions: evidence for a binding site for A beta on the C1q globular regions

Previous studies based on the use of serum as a source of C have shown that fibrils of beta-amyloid peptides that accumulate in the brain of patients with Alzheimer's disease have the ability to bind C1q and activate the classical C pathway. The objective of the present work was to test the ability of fibrils of peptide Abeta1-42 to trigger direct activation of the C1 complex and to carry out further investigations on the site(s) of C1q involved in the interaction with Abeta1-42. Using C1 reconstituted from purified C1q, C1r, and C1s, it was shown that Abeta1-42 fibrils trigger direct C1 activation both in the absence of C1 inhibitor and at C1 inhibitor:C1 ratios up to 8:0, i.e., under conditions consistent with the physiological context in serum. The truncated peptide Abeta12-42 and the double mutant (D7N, E11Q) of Abeta1-42 did not yield C1 activation, providing further evidence that the C1 binding site of beta-amyloid fibrils is located in the acidic N-terminal 1-11 region of the Abeta1-42 peptide. Binding studies performed using a solid phase assay provided strong evidence that C1q interacts with Abeta1-42 fibrils through its C-terminal globular regions. In contrast to previous studies based on a different experimental design, no significant involvement of the C1q collagen-like domain was detected. These findings were confirmed by additional experiments based on C1 activation and C4 consumption assays. These observations provide direct evidence of the ability of beta-amyloid fibrils to trigger activation of the classical C pathway and further support the hypothesis that C activation may be a component of the pathogenesis of Alzheimer's disease.

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

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

Interaction between synthetic amyloid-beta-peptide (1-40) and its aggregation inhibitors studied by electrospray ionization mass spectrometry

It is generally postulated that amyloid-beta-peptides play a central role in the progressive neurodegeneration observed in Alzheimer's disease. Important pathological properties of these peptides, such as neurotoxicity and resistance to proteolytic degradation, depend on the ability of amyloid-beta-peptides to form beta-sheet structures and/or amyloid fibrils. Amyloid-beta-peptides are known to aggregate spontaneously in vitro with the formation of amyloid fibrils. The intervention on the amyloid-beta-peptides aggregation process can be envisaged as an approach to stopping or slowing the progression of Alzheimer's disease. In the last few years a number of small molecules have been reported to interfere with the in vitro aggregation of amyloid-beta-peptides. Melatonin, a hormone recently found to protect neurons against amyloid-beta-peptide toxicity, interacts with amyloid-beta-peptide (1-40) and amyloid-beta-peptide (1-42) and inhibits the progressive formation of beta-sheet and/or amyloid fibrils. These interactions between melatonin and the amyloid peptides have been demonstrated by circular dichroism (CD) and electron microscopy for amyloid-beta-peptide (1-40) and amyloid-beta-peptide (1-42) and by nuclear magnetic resonance (NMR) spectroscopy for amyloid-beta-peptide (1-40). Our electrospray ionization mass spectrometric (ESI-MS) studies also proved that there is a hydrophobic interaction between amyloid-beta-peptide (1-40) and melatonin and the proteolytic investigations suggested that the interaction took place on the 29-40 amyloid-beta-peptide segment. The wide-ranging application of these results would provide further information and help in biological research.

Photoaffinity cross-linking of Alzheimer's disease amyloid fibrils reveals interstrand contact regions between assembled beta-amyloid peptide subunits

The assembly of the beta-amyloid peptide (Abeta) into amyloid fibrils is essential to the pathogenesis of Alzheimer's disease. Detailed structural information about fibrillogenesis has remained elusive due to the highly insoluble, noncrystalline nature of the assembled peptide. X-ray fiber diffraction, infrared spectroscopy, and solid-state NMR studies performed on fibrils composed of Abeta peptides have led to conflicting models of the intermolecular alignment of beta-strands. We demonstrate here the use of photoaffinity cross-linking to determine high-resolution structural constraints on Abeta monomers within amyloid fibrils. A photoreactive Abeta(1-40) ligand was synthesized by substituting L-p-benzoylphenylalanine (Bpa) for phenylalanine at position 4 (Abeta(1-40) F4Bpa). This peptide was incorporated into synthetic amyloid fibrils and irradiated with near-UV light. SDS-PAGE of dissolved fibrils revealed the light-dependent formation of a covalent Abeta dimer. Enzymatic cleavage followed by mass spectrometric analysis demonstrated the presence of a dimer-specific ion at MH(+) = 1825.9, the predicted mass of a fragment composed of the N-terminal Abeta(1-5) F4Bpa tryptic peptide covalently attached to the C-terminal Abeta(29-40) tryptic peptide. MS/MS experiments and further chemical modifications of the cross-linked dimer led to the localization of the photo-cross-link between the ketone of the Bpa4 side chain and the delta-methyl group of the Met35 side chain. The Bpa4-Met35 intermolecular cross-link is consistent with an antiparallel alignment of Abeta peptides within amyloid fibrils.

Intracellular mechanisms of amyloid accumulation and pathogenesis in Alzheimer's disease

Cell-culture studies have revealed some of the fundamental features of the interaction of amyloid Abeta with cells and the mechanism of amyloid accumulation and pathogenesis in vitro. A(beta)1-42, the longer isoform of amyloid that is preferentially concentrated in senile plaque (SP) amyloid deposits in Alzheimer's disease (AD), is resistant to degradation and accumulates as insoluble aggregates in late endosomes or lysosomes. Once these aggregates have nucleated inside the cell, they grow by the addition of aberrantly folded APP and amyloidgenic fragments of APP, that would otherwise be degraded, onto the amyloid lattice in a fashion analogous to prion replication. This accumulation of heterogeneous aggregated APP fragments and Abeta appears to mimic the pathophysiologyof dystrophic neurites, where the same spectrum of components has been identified by immunohistochemistry. In the brain, this residue appears to be released into the extracellular space, possibly by a partially apoptotic mechanism that is restricted to the distal compartments of the neuron. Ultimately, this insoluble residue may be further digested to the protease-resistant A(beta)n-42 core, perhaps by microglia, where it accumulates as senile plaques. Thus, the dystrophic neurites are likely to be the source of the immediate precursors of amyloid in the senile plaques. This is the opposite of the commonly held view that extracellular accumulation of amyloid induces dystrophic neurites. Many of the key pathological events of AD may also be directly related to the intracellular accumulation of this insoluble amyloid. The aggregated, intracellular amyloid induces the production of reactive oxygen species (ROS) and lipid peroxidation products and ultimately results in the leakage of the lysosomal membrane. The breakdown of the lysosomal membrane may be a key pathogenic event, leading to the release of heparan sulfate and lysosomal hydrolases into the cytosol. Together, these observations provide the novel view that amyloid deposits and some of the early events of amyloid pathogenesis initiate randomly within single cells in AD. This pathogenic mechanism can explain some of the more enigmatic features of Alzheimer's pathogenesis, like the focal nature of amyloid plaques, the relationship between amyloid, dystrophic neurites and neurofibrillary-tangle pathology, and the miscompartmentalization of extracellular and cytosolic components observed in AD brain.

Development of a synthetic cyclized peptide derived from alpha-fetoprotein that prevents the growth of human breast cancer

The peptide, EMTPVNPG, derived from alpha-fetoprotein, inhibits estrogen-stimulated growth of immature mouse uterus and estrogen-dependent proliferation of human breast cancer cells. However, the biological activities of the peptide diminish over time in storage, even when in the lyophilized state, probably because of peptide aggregation through hydrophobic interaction among monomers. Two analogs of EMTPVNPG were designed with the intent of minimizing aggregation and retaining biological activity during prolonged storage. EMTOVNOG, where O is 4-hydroxyproline, is a linear peptide generated by substituting 4-hydroxyproline for the two prolines, thereby increasing peptide hydrophilicity. This analog exhibited a dose-dependent inhibition of estrogen-stimulated growth of immature mouse uterus similar to that of EMTPVNPG (maximal activity at 1 microg/mouse). A second analog, cyclo-(EMTOVNOGQ), a hydrophilic, cyclic analog with increased conformational constraint, was as potent as the other peptides in its inhibition of estrogen-dependent growth of immature mouse uterus, and had an expanded effective dose range. Both linear and cyclized hydroxyproline-substituted analogs exhibited indefinite shelf-life. Furthermore, both analogs inhibited the estrogen-dependent growth of MCF-7 human breast cancer growing as a xenograft in SCID mice. These analogs may become significant, novel agents for the treatment of breast cancer.

A portrait of Alzheimer secretases--new features and familiar faces

The amyloid beta-peptide (Abeta) is a principal component of the cerebral plaques found in the brains of patients with Alzeheimer's disease (AD). This insoluble 40- to 42-amino acid peptide is formed by the cleavage of the Abeta precursor protein (APP). The three proteases that cleave APP, alpha-, beta-, and gamma-secretases, have been implicated in the etiology of AD. beta-Secretase is a membrane-anchored protein with clear homology to soluble aspartyl proteases, and alpha-secretase displays characteristics of certain membrane-tethered metalloproteases. gamma-Secretase is apparently an oligomeric complex that includes the presenilins, which may be the catalytic component of this protease. Identification of the alpha-, beta-, and gamma-secretases provides potential targets for designing new drugs to treat AD.

Distinguishable effects of presenilin-1 and APP717 mutations on amyloid plaque deposition

Both APP and PS-1 are causal genes for early-onset familial Alzheimer's disease (AD) and their mutation effects on cerebral Abeta deposition in the senile plaques were examined in human brains of 29 familial AD (23 PS-1, 6 APP) cases and 14 sporadic AD cases in terms of Abeta40 and Abeta42. Abeta isoform data were evaluated using repeated measures analysis of variance which adjusted for within-subject measurement variation and confounding effects of individual APP and PS-1 mutations, age at onset, duration of illness and APOE genotype. We observed that mutations in both APP and PS-1 were associated with a significant increase of Abeta42 in plaques as been documented previously. In comparison to sporadic AD cases, both APP717 and PS-1 mutation cases had an increased density (measured as the number of plaques/mm(2)) and area (%) of Abeta42 plaques. However, we found an unexpected differential effect of PS-1 but not APP717 mutation cases. At least some of PS-1 but not APP717 mutation cases had the significant increase of density and area of Abeta40-plaques as compared to sporadic AD independently of APOE genotype. Our results suggest that PS-1 mutations affect cerebral accumulation of Abeta burden in a different fashion from APP717 mutations in their familial AD brains.

Biomolecular solid state NMR: advances in structural methodology and applications to peptide and protein fibrils

Solid state nuclear magnetic resonance (NMR) methods can provide atomic-level structural constraints on peptides and proteins in forms that are not amenable to characterization by other high-resolution structural techniques, owing to insolubility, high molecular weight, noncrystallinity, or other characteristics. Important examples include peptide and protein fibrils and membrane-bound peptides and proteins. Recent advances in solid state NMR methodology aimed at structural problems in biological systems are reviewed. The power of these methods is illustrated by experimental results on amyloid fibrils and other protein fibrils.

Cholesterol-dependent formation of GM1 ganglioside-bound amyloid beta-protein, an endogenous seed for Alzheimer amyloid

GM1 ganglioside-bound amyloid beta-protein (GM1/Abeta), found in brains exhibiting early pathological changes of Alzheimer's disease (AD) including diffuse plaques, has been suggested to be involved in the initiation of amyloid fibril formation in vivo by acting as a seed. To elucidate the molecular mechanism underlying GM1/Abeta formation, the effects of lipid composition on the binding of Abeta to GM1-containing lipid bilayers were examined in detail using fluorescent dye-labeled human Abeta-(1-40). Increases in not only GM1 but also cholesterol contents in the lipid bilayers facilitated the binding of Abeta to the membranes by altering the binding capacity but not the binding affinity. An increase in membrane-bound Abeta concentration triggered its conformational transition from helix-rich to beta-sheet-rich structures. Excimer formation of fluorescent dye-labeled GM1 suggested that Abeta recognizes a GM1 "cluster" in membranes, the formation of which is facilitated by cholesterol. The results of the present study strongly suggested that increases in intramembrane cholesterol content, which are likely to occur during aging, appear to be a risk factor for amyloid fibril formation.

The functions of the amyloid precursor protein gene

The amyloid precursor protein (APP) gene and its protein products have multiple functions in the central nervous system and fulfil criteria as neuractive peptides: presence, release and identity of action. There is increased understanding of the role of secretases (proteases) in the metabolism of APP and the production of its peptide fragments. The APP gene and its products have physiological roles in synaptic action, development of the brain, and in the response to stress and injury. These functions reveal the strategic importance of APP in the workings of the brain and point to its evolutionary significance.

Growth arrest of individual senile plaques in a model of Alzheimer's disease observed by in vivo multiphoton microscopy

In Alzheimer's disease, amyloid-beta peptide aggregates in the extracellular space to form senile plaques. The process of plaque deposition and growth has been modeled on the basis of in vitro experiments in ways that lead to divergent predictions: either a diffusion-limited growth model in which plaques grow by first-order kinetics, or a dynamic model of continual deposition and asymmetrical clearance in which plaques reach a stable size and stop growing but evolve morphologically over time. The models have not been tested in vivo because plaques are too small (by several orders of magnitude) for conventional imaging modalities. We now report in vivo multiphoton laser scanning imaging of thioflavine S-stained senile plaques in the Tg2576 transgenic mouse model of Alzheimer's disease to test these biophysical models and show that there is no detectable change in plaque size over extended periods of time. Qualitatively, geometric features remain unchanged over time in the vast majority of the 349 plaques imaged and re-imaged. Intervals as long as 5 months were obtained. Nonetheless, rare examples of growth or shrinkage of individual plaques do occur, and new plaques appear between imaging sessions. These results indicate that thioflavine S-positive plaques appear and then are stable, supporting a dynamic feedback model of plaque growth.

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.

Is aggregation of beta-amyloid peptides a mis-functioning of a current interaction process?

In a previous study, Hughes et al. [Proc. Natl. Acad. Sci. USA 93 (1996) 2065-2070] demonstrated that the amyloid peptide is able to interact with itself in a two-hybrid system and that interaction is specific. They further supported that the method could be used to define the sequences that might be important in nucleation-dependent aggregation. The sequence of the amyloid peptide can be split into four clusters, two hydrophilic (1-16 and 22-28) and two hydrophobic (17-21 and 29-42). We designed by molecular modeling and tested by the two-hybrid approach, series of mutations spread all over the sequence and changing the distribution of hydrophobicity and/or the spatial hindrance. In the two-hybrid assay, interaction of native Abeta is reproduced. Screening of mutations demonstrates that the C-domain (residues 29-40 (42)), the median domain (residues 17-22) and the N-domain (1-16) are all crucial for interaction. This demonstrates that almost all fragments of the amyloid peptide but a loop (residues 23-28) and the C-term amino acid are important for the native interaction. We support that the folded three-dimensional (3D) structure is the Abeta-Abeta interacting species, that the whole sequence is involved in that 3D fold which has a low secondary structure propensity and a high susceptibility to mutations and thus should have a low stability. The native fold of Abeta could be stabilized in Abeta-Abeta complexes which could in other circumstances facilitate the nucleation event of aggregation that leads to the formation of stable senile plaques.

Characterization of High-Affinity Binding between Gangliosides and Amyloid β-Protein

The binding specificities of amyloid beta-protein (A beta) such as A beta 1-40, A beta 1-42, A beta 40-1, A beta 1-38, A beta 25-35, and amyloid beta precursor protein (beta-APP) analogues for different glycosphingolipids were determined by surface plasmon resonance (SPR) using a liposome capture method. A beta 1-42, A beta 1-40, A beta 40-1, and A beta 1-38, but not A beta 25-35, bound to GM1 ganglioside in the following rank order: A beta 1-42 > A beta 40-1 > A beta 1-40 > A beta 1-38. The beta-APP analogues bound to GM1 ganglioside with a relatively lower affinity. Aged derivatives of A beta were found to have higher affinity to GM1 ganglioside than fresh or soluble derivatives. A beta 1-40 bound to a number of gangliosides with the following order of binding strength: GQ1b alpha > GT1a alpha > GQ1b > GT1b > GD3 > GD1a = GD1b > LM1 > GM1 > GM2 = GM3 > GM4. Neutral glycosphingolipids had a lower affinity for A beta 1-40 than gangliosides with the following order of binding strength: Gb4 > asialo-GM1 (GA1) > Gb3 > asialo-GM2 (GA2) = LacCer. The results seem to indicate that an alpha2,3NeuAc residue on the neutral oligosaccharide core is required for binding. In addition, the alpha2-6NeuAc residue linked to GalNAc contributes significantly to binding affinity for A beta.

Beta-amyloid therapies in Alzheimer's disease

Neurones in the brain produce beta-amyloid fragments from a larger precursor molecule termed the amyloid precursor protein (APP). When released from the cell, these protein fragments may accumulate in extracellular amyloid plaques and consequently hasten the onset and progression of Alzheimer's disease (AD). A beta fragments are generated through the action of specific proteases within the cell. Two of these enzymes, beta- and gamma-secretase, are particularly important in the formation of A beta as they cleave within the APP protein to give rise to the N-terminal and C-terminal ends of the A beta fragment, respectively. Consequently, many researchers are investigating therapeutic approaches that inhibit either beta- or gamma-secretase activity, with the ultimate goal of limiting A beta; production. An alternative AD therapeutic approach that is being investigated is to employ anti-A beta antibodies to dissolve plaques that have already formed. Both of these approaches focus on the possibility that accrual of A beta leads to neuronal degeneration and cognitive impairment characterised by AD and test the hypothesis that limiting A beta deposition in neuritic plaques may be an effective treatment for AD.

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

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

Fibrillar amyloid beta-protein forms a membrane-like hydrophobic domain

Microviscosity of the biological membranes is determined by measuring the fluorescence polarization of diphenylhexatriene (DPH). DPH, a hydrophobic probe, has negligible fluorescence in the solution. When DPH is incorporated into the membrane, it is localized in the membrane hydrophobic core and fluoresces strongly. We report here that DPH also fluoresces in the presence of fibrillar Abeta (fAbeta). However, it does not fluoresce when it is added to the soluble Abeta (sAbeta). DPH inserts into Abeta fibrils in a time-dependent manner, and upon centrifugation, it is sedimented along with fibrils. The steady state fluorescence polarization of DPH with fAbeta1-40 and fAbeta 1-42 was 0.4592 and 0.4898 respectively. These results suggest that fAbeta (but not sAbeta) forms a hydrophobic domain similar to that of membrane.

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

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