Interaction between A beta(1-42) and A beta(1-40) in Alzheimer's beta-amyloid fibril formation in vitro

We analyzed the interaction of two kinds of amyloid beta-peptides (A beta), i.e., A beta(1-42) and A beta(1-40), in the kinetics of beta-amyloid fibril (fA beta) formation in vitro, based on a nucleation-dependent polymerization model using fluorescence spectroscopy with thioflavin T. When 25 microM A beta(1-42) was incubated with increasing concentrations of amyloidogenic A beta(1-40), the time to proceed to equilibrium was extended dose-dependently. A similar inhibitory effect was observed when 45 microM A beta(1-40) was incubated with increasing concentrations of A beta(1-42). On the other hand, when 50 microM of nonamyloidogenic A beta(1-40) was incubated with A beta(1-42) at a molar ratio of 10:1 or 5:1, A beta(1-42) initiated fA beta formation from A beta(1-40). The lag time of the reaction shortened in a concentration-dependent manner, with A beta(1-42). We next examined the seeding effect of fA beta formed from A beta(1-42) (fA beta(1-42)) on nonamyloidogenic A beta(1-40). When 50 microM of nonamyloidogenic A beta(1-40) was incubated with 10 or 20 microg/mL (2.2 or 4.4 microM) of fA beta(1-42), the fluorescence showed a sigmoidal increase. The lag time of the reaction was shortened by fA beta(1-42) in a concentration-dependent manner. However, the time to proceed to equilibrium was much longer than when an equal concentration of fA beta formed from A beta(1-40) (fA beta(1-40)) was added to A beta(1-40). The fluorescence increased hyperbolically without a lag phase when 25 microM A beta(1-42) was incubated with 10 or 20 microg/mL (2.3 or 4.6 microM) of fA beta(1-40), and proceeded to equilibrium more rapidly than without fA beta(1-40). An electron microscopic study indicated that the morphology of fA beta formed is governed by the major component of fresh A beta peptides in the reaction mixture, not by the morphology of preexisting fibrils. These results may indicate the central role of A beta(1-42) for fA beta deposition in vivo, among the different coexisting A beta species.     

Association between acetylcholinesterase and beta-amyloid peptide in Alzheimer's cerebrospinal fluid

The altered expression of acetylcholinesterase (AChE) in the brains of patients with Alzheimer's disease (AD) has raised much interest of late. Despite an overall decrease in the AD brain, the activity of AChE increases around beta-amyloid plaques and indeed, the beta-amyloid peptide (Abeta) can influence AChE levels. Such evidence stimulated our interest in the possibility that the levels of AChE and amyloid might vary together in AD. We previously found that the different AChE forms present in both the brain and in the cerebrospinal fluid (CSF) of AD patients varied in conjunction with abnormal glycosylation. Thus, the alterations in glycosylation are correlated with the accumulation of a minor subspecies of AChE monomers. We also recently analysed whether long-term exposure to the cholinesterase inhibitor (ChE-I) donepezil influences the AChE species found in AD CSF. The marked increase in CSF-AChE activity in AD patients following long-term treatment with donepezil was not paralleled by a rise in this subset of light variants. Hence, the correlation with the levels of CSF-Abeta is unique to these AChE species in patients receiving such treatment. The aim of this report is to review the links between AChE and beta-amyloid, and to discuss the significance of the responses of the distinct AChE species to ChE-I during the treatment of AD.     

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.     

Potential pathogenic role of beta-amyloid(1-42)-aluminum complex in Alzheimer's disease

The etiopathogenesis of Alzheimer's disease is far from being clearly understood. However, the involvement of metal ions as a potential key factor towards conformational modifications and aggregation of amyloid is widely recognized. The aim of the present study is to shed some light on the relationship between metal ions, amyloid conformation/aggregation, and their potential relationship with the conformational aspects of AD. We compare the effects of beta-amyloid(1-42) and its various metal complexes (beta-amyloid-Al, beta-amyloid-Zn, beta-amyloid-Cu, beta-amyloid-Fe) in human neuroblastoma cells in terms of cell viability, membrane structure properties, and cell morphology. No significant toxic effects were observed in neuroblastoma cells after 24h treatment both with beta-amyloid and beta-amyloid-metals (beta-amyloid-Zn, beta-amyloid-Cu, beta-amyloid-Fe); on the other hand, there was a marked reduction of cellular viability after treatment with beta-amyloid-Al complex. In addition, treatment with beta-amyloid-Al increased membrane fluidity much more than other beta-amyloid-metal complexes, whose contribution was negligible. Furthermore, the cellular morphology, as observed by electron microscopy, was deeply altered by beta-amyloid-Al. Importantly, beta-amyloid-Al toxicity is closely and significantly associated with a great difference in the structure/aggregation of this complex with respect to that of beta-amyloid alone and other beta-amyloid-metal complexes. In addition, beta-amyloid, as a consequence of Al binding, becomes strongly hydrophobic in character. These findings show a significant involvement of Al, compared to the other metal ions used in our experiments, in promoting a specific amyloid(1-42) aggregation, which is able to produce marked toxic effects on neuroblastoma cells, as clearly demonstrated for the first time in this study.     

Enhanced G-protein activation by a mixture of Abeta(25-35), Abeta(1-40/42) and zinc

Beta-amyloid peptides (Abetas) bind to several G-protein coupled receptor proteins and stimulate GTPase activity in neurons. In this study we determined the effects of Abeta(1-42), Abeta(1-40), Abeta(25-35) and their mixtures on [(35)S]GTP binding in rat brain cortical membranes in the absence and presence of zinc. We found that the Abetas alone induced a concentration-dependent activation of G-proteins (IC50 approximately 10(-6) m), while aggregated Abeta fibrils only affected GTP binding at concentrations above 10(-5) m. Mixing Abeta(25-35) with Abeta(1-42) or Abeta(1-40) induced a several-fold increase in GTP-binding. This potentiation followed a bell shaped curve with a maximum at 50 : 50 ratios. No potentiating effect could be seen by mixing Abeta(1-40) and Abeta(1-42) or highly aggregated Abetas. Zinc had no effect on Abeta(1-40/42) but strongly potentiated the Abeta(25-35) or the mixed peptides-induced GTP-binding. Changes in secondary structure accompanied the mixed peptides or the peptide/zinc complexes induced potentiation, revealing that structural alterations are behind the increased biological action. These concentration dependent potentiating effects of zinc and the peptide mixtures could be physiologically important at brain regions where peptide fragments and/or zinc are present at elevated concentrations.     

Kinetics of adsorption of beta-amyloid peptide Abeta(1-40) to lipid bilayers

The Alzheimer's disease-related peptide beta-amyloid (Abeta) is toxic to neurons. The toxicity of the peptide appears to require conversion of the monomeric form to an aggregated fibrillar species. The interaction of Abeta with cell membranes has attracted interest as one plausible mechanism by which the peptide exerts its toxic activity. We developed two methods to measure the adsorption of fresh (monomeric) and aged (aggregated) Abeta to lipid bilayers. In one method, the kinetics of Abeta adsorption and desorption to liposomes deposited onto a dextran-coated surface was measured using surface plasmon resonance. In the other method, Abeta was contacted with liposome-coated magnetic beads; adsorbed Abeta was separated from solution-phase peptide by use of a magnetic field. Monomeric Abeta adsorbed quickly but reversibly to lipid bilayers with low affinity, while aggregated Abeta adsorbed slowly but irreversibly. These two methods provide complementary means of quantifying the adsorption of aggregating proteins to membranes. The results correlate strongly with previous observations that fibrillar, but not monomeric, Abeta restricts the motion of acyl tails in phospholipid bilayers. The methods should be useful for further elucidation of the role of membrane adsorption in mediating Abeta toxicity, and in the search for inhibitors of toxicity.     

High resolution scanning tunnelling microscopy of the beta-amyloid protein (Abeta1-40) of Alzheimer's disease suggests a novel mechanism of oligomer assembly

The aggregation of the beta-amyloid protein (Abeta) is an important step in the pathogenesis of Alzheimer's disease. There is increasing evidence that lower molecular weight oligomeric forms of Abeta may be the most toxic species in vivo. However, little is known about the structure of Abeta oligomers. In this study, scanning tunnelling microscopy (STM) was used to examine the structure of Abeta monomers, dimers and oligomers. Abeta1-40 was visualised by STM on a surface of atomically flat gold. At low concentrations (0.5 microM) small globular structures were observed. High resolution STM of these structures revealed them to be monomers of Abeta. The monomers measured approximately 3-4 nm in diameter. Internal structure was seen in many of the monomers consistent with a conformation in which the polypeptide chain is folded into 3 or 4 domains. Oligomers were seen after ageing the Abeta solution for 24 h. The oligomers were also 3-4 nm in width and appeared to be formed by the end-to-end association of monomers with the polypeptide chain oriented at 90 degrees to the axis of the oligomer. The results suggest that the oligomer formation can proceed through a mechanism involving the linear association of monomers.     

Characterizations of distinct amyloidogenic conformations of the Abeta (1-40) and (1-42) peptides

Major constituents of the amyloid plaques found in the brain of Alzheimer's patients are the 39-43 residue beta-amyloid (Abeta) peptides. Extensive in vitro as well as in vivo biochemical studies have shown that the 40- and 42-residue Abeta peptides play major roles in the neurodegenerative pathology of Alzheimer's disease. Although the two Abeta peptides share common aggregation properties, the 42-residue peptide is more amyloidogenic and more strongly associated with amyloid pathology. Thus, characterizations of the two Abeta peptides are of critical importance in understanding the molecular mechanism of Abeta amyloid formation. In this report, we present combined CD and NMR studies of the monomeric states of the two peptides under both non-amyloidogenic (<5 degrees C) and amyloid-forming conditions (>5 degrees C) at physiological pH. Our CD studies of the Abeta peptides showed that initially unfolded Abeta peptides at low temperature (<5 degrees C) gradually underwent conformational changes to more beta-sheet-like monomeric intermediate states at stronger amyloidogenic conditions (higher temperatures). Detailed residue-specific information on the structural transition was obtained by using NMR spectroscopy. Residues in the N-terminal (3-12) and 20-22 regions underwent conformational changes to more extended structures at the stronger amyloidogenic conditions. Almost identical structural transitions of those residues were observed in the two Abeta peptides, suggesting a similar amyloidogenic intermediate for the two peptides. The 42-residue Abeta (1-42) peptide was, however, more significantly structured at the C-terminal region (39-42), which may lead to the different aggregation propensity of the two peptides.     

Protective effect of quercetin in primary neurons against Abeta(1-42): relevance to Alzheimer's disease

Quercetin, a flavonoid found in various foodstuffs, has antioxidant properties and increases glutathione (GSH) levels and antioxidant enzyme function. Considerable attention has been focused on increasing the intracellular GSH levels in many diseases, including Alzheimer's disease (AD). Amyloid beta-peptide [Abeta(1-42)], elevated in AD brain, is associated with oxidative stress and neurotoxicity. We aimed to investigate the protective effects of quercetin on Abeta(1-42)-induced oxidative cell toxicity in cultured neurons in the present study. Decreased cell survival in neuronal cultures treated with Abeta(1-42) correlated with increased free radical production measured by dichlorofluorescein fluorescence and an increase in protein oxidation (protein carbonyl, 3-nitrotyrosine) and lipid peroxidation (protein-bound 4-hydroxy-2-nonenal). Pretreatment of primary hippocampal cultures with quercetin significantly attenuated Abeta(1-42)-induced cytotoxicity, protein oxidation, lipid peroxidation and apoptosis. A dose-response study suggested that quercetin showed protective effects against Abeta(1-42) toxicity by modulating oxidative stress at lower doses, but higher doses were not only non-neuroprotective but also toxic. These findings provide motivation to test the hypothesis that quercetin may provide a promising approach for the treatment of AD and other oxidative-stress-related neurodegenerative diseases.     

Amyloid beta-protein (Abeta)1-40 protects neurons from damage induced by Abeta1-42 in culture and in rat brain

Previously, we found that amyloid beta-protein (Abeta)1-42 exhibits neurotoxicity, while Abeta1-40 serves as an antioxidant molecule by quenching metal ions and inhibiting metal-mediated oxygen radical generation. Here, we show another neuroprotective action of nonamyloidogenic Abeta1-40 against Abeta1-42-induced neurotoxicity in culture and in vivo. Neuronal death was induced by Abeta1-42 at concentrations higher than 2 microm, which was prevented by concurrent treatment with Abeta1-40 in a dose-dependent manner. However, metal chelators did not prevent Abeta1-42-induced neuronal death. Circular dichroism spectroscopy showed that Abeta1-40 inhibited the beta-sheet transformation of Abeta1-42. Thioflavin-T assay and electron microscopy analysis revealed that Abeta1-40 inhibited the fibril formation of Abeta1-42. In contrast, Abeta1-16, Abeta25-35, and Abeta40-1 did not inhibit the fibril formation of Abeta1-42 nor prevent Abeta1-42-induced neuronal death. Abeta1-42 injection into the rat entorhinal cortex (EC) caused the hyperphosphorylation of tau on both sides of EC and hippocampus and increased the number of glial fibrillary acidic protein (GFAP)-positive astrocytes in the ipsilateral EC, which were prevented by the concurrent injection of Abeta1-40. These results indicate that Abeta1-40 protects neurons from Abeta1-42-induced neuronal damage in vitro and in vivo, not by sequestrating metals, but by inhibiting the beta-sheet transformation and fibril formation of Abeta1-42. Our data suggest a mechanism by which elevated Abeta1-42/Abeta1-40 ratio accelerates the development of Alzheimer's disease (AD) in familial AD.     

Secondary conformations and temperature effect on structural transformation of amyloid beta (1-28), (1-40) and (1-42) peptides

Secondary structure of three amyloid b-peptides [A beta(1-28), A beta(1-40) and A beta(1-42)] in the solid state was respectively determined by Fourier transform infrared (FT-IR) microspectroscopy. Their thermal-dependent structural transformation were also investigated by FT-IR microspectroscopy equipped with a thermal analyzer. The present result demonstrates that the solid-state A beta(1-28), A beta(1-40) and A beta(1-42) peptides showed a significant IR spectral difference in the amide I and II bands. The secondary conformation of A beta(1-28) peptide was the combination of major beta-sheet and minor alpha-helix with little random coil structures, but A beta(1-40) peptide showed the co-existence of major beta-sheet and minor random coil with little alpha-helix structures. A beta(1-42) peptide mainly consisted of the predominant b-sheet structure. Although the intact A beta(1-28), A beta(1-40) or A beta(1-42) peptide exhibits a different secondary structure, a similar beta-conformation may form after thermal treatment. A thermal-dependent transition was found for solid A beta(1-28) and A beta(1-40) peptides near 40 degrees C and 45 degrees C, respectively. There was no transition temperature for solid A beta(1-42) peptide, however, due to only a very little level of alpha-helix and random coil structure containing in the solid A beta(1-42) peptide. The thermal denaturation plays an important role in the structural transformation from alpha-helix/random coil to beta-sheet.     

Diagnostic utility of APOE, soluble CD40, CD40L, and Abeta1-40 levels in plasma in Alzheimer's disease

A continuous inflammatory state is associated with Alzheimer's disease (AD) evidenced by an increase in proinflammatory cytokines around beta-amyloid (Abeta) deposits. In addition, functional loss of CD40L is shown to result in diminished Amyloid precursor proton (APP) processing and microglial activation, supporting a prominent role of CD40-CD40L in AD etiology. We therefore hypothesize that a peripheral increase in Abeta may result in corresponding increase of sCD40 and sCD40L further contributing to AD pathogenesis. We measured plasma Abeta, sCD40 and sCD40L levels in 73 AD patients and compared to 102 controls matched on general demographics. We demonstrated that Abeta(1-40), levels of sCD40 and sCD40L are increased in AD and declining MMSE scores correlated with increasing sCD40L, which in turn, correlated positively with Abeta(1-42). We then combined sCD40, sCD40L, Abeta and APOE and found that this biomarker panel has high sensitivity and specificity (>90%) as a predictor of clinical AD diagnosis. Given the imminent availability of potentially disease modifying therapies for AD, a great need exists for peripheral diagnostic markers of AD. Thus, we present preliminary evidence for potential usefulness for combination of plasma sCD40, sCD40L along with Abeta(1-40) and APOE epsilon4 in improving the clinical diagnosis of AD.     

Apolipoprotein E and beta-amyloid (1-42) regulation of glycogen synthase kinase-3beta

Glycogen synthase kinase-3beta (GSK-3beta) is implicated in regulating apoptosis and tau protein hyperphosphorylation in Alzheimer's disease (AD). We investigated the effects of two key AD molecules, namely apoE (E3 and E4 isoforms) and beta-amyloid (Abeta) 1-42 on GSK-3beta and its major upstream regulators, intracellular calcium and protein kinases C and B (PKC and PKB) in human SH-SY5Y neuroblastoma cells. ApoE3 induced a mild, transient, Ca2+-independent and early activation of GSK-3beta. ApoE4 effects were biphasic, with an early strong GSK-3beta activation that was partially dependent on extracellular Ca2+, followed by a GSK-3beta inactivation. ApoE4 also activated PKC-alpha and PKB possibly giving the subsequent GSK-3beta inhibition. Abeta(1-42) effects were also biphasic with a strong activation dependent partially on extracellular Ca2+ followed by an inactivation. Abeta(1-42) induced an early and potent activation of PKC-alpha and a late decrease of PKB activity. ApoE4 and Abeta(1-42) were more toxic than apoE3 as shown by MTT reduction assays and generation of activated caspase-3. ApoE4 and Abeta(1-42)-induced early activation of GSK-3beta could lead to apoptosis and tau hyperphosphorylation. A late inhibition of GSK-3beta through activation of upstream kinases likely compensates the effects of apoE4 and Abeta(1-42) on GSK-3beta, the unbalanced regulation of which may contribute to AD pathology.     

Human cerebrospinal fluid apolipoprotein E isoforms are apparently inefficient at complexing with synthetic Alzheimer's amyloid-[beta] peptide (A[beta] 1-40 ) in vitro

BACKGROUND: Variation at the apolipoprotein E locus on chromosome 19 plays a role in more cases of Alzheimer's disease than does any other identified genetic determinant. We have previously reported the isoform-specific interaction of native human apolipoprotein E (APOE, gene; apoE, protein) epsilon 3 with the amyloid-ss peptide, Ass(1-40), the major component of the cerebral amyloid deposits that appear to cause Alzheimer's disease. MATERIALS AND METHODS: In order to investigate the apoE: A beta interaction further, a modified assay was developed based on co-immunoprecipitation of the complex using an anti-apoE antibody (anti-apoE IP assay). RESULTS: Application of this assay demonstrated that the interaction of Ass(1-40) and apoE can be distinguished into two types: sodium dodecyl sulfate (SDS) -resistant and SDS-releasable. The SDS-resistant interaction between epsilon;3 and Ass(1-40) is apparently maximal at an Ass(1-40) concentration of approximately 75 micro M, and an Ass(1-40) /epsilon 3 molar ratio of about 250:1. The major apoE-isoform-specific difference in interaction with Ass(1-40) is the ability of Ass(1-40) to form SDS- resistant complexes with epsilon 3 but not with epsilon 4. Using the anti-apoE co-IP assay, we found that human cerebrospinal fluid (CSF) epsilon 3 can also form an SDS-resistant complex with Ass(1-40) but human CSF epsilon;4 cannot. However, when compared with apoE epsilon;3 collected from the conditioned medium of APOE epsilon 3-transfected cells, the competence of equal concentrations of CSF apoE epsilon 3 to form SDS-resistant complexes with Ass(1-40) is apparently diminished. A 1:1 mixture of CSF plus apoE epsilon 3-containing conditioned medium is associated with diminished Ass(1-40) /epsilon;3 complex formation to a greater extent than that observed when an identical volume of phosphate-buffered saline is added to apoE epsilon;3 medium. CONCLUSIONS: These results suggest the presence in CSF of factors that interfere with the formation of complexes between synthetic Ass(1-40) and apoE epsilon 3.     

Expression of beta-amyloid precursor protein-CD3gamma chimeras to demonstrate the selective generation of amyloid beta(1-40) and amyloid beta(1-42) peptides within secretory and endocytic compartments.

Amyloid beta-protein (Abeta) is the main constituent of amyloid fibrils found in senile plaques and cerebral vessels in Alzheimer's disease (AD) and is derived by proteolysis from the beta-amyloid precursor protein (APP). We have analyzed the amyloidogenic processing of APP using chimeric proteins stably transfected in Chinese hamster ovary cells. The extracellular and transmembrane domains of APP were fused to the cytoplasmic region derived from the CD3 gamma chain of the T cell antigen receptor (CD3gamma). CD3gamma contains an endoplasmic reticulum (ER) retention motif (RKK), in the absence of which the protein is targeted to lysosomes without going through the cell surface (Letourneur, F., and Klausner, R.D. (1992) Cell 69, 1143-1157). We used the wild-type sequence of CD3gamma to create an APP chimera predicted to remain in the ER (gammaAPP(ER)). Deletion of the RKK motif at the C terminus directed the protein directly to the lysosomes (gammaAPP(LYS)). A third chimera was created by removing both lysosomal targeting signals in addition to RKK (gammaAPP(DeltaDelta)). This last construct does not contain known targeting signals and consequently accumulates at the cell surface. We show by immunofluorescence and by biochemical methods that all three APP chimeras localize to the predicted compartments within the cell, thus providing a useful model to study the processing of APP. We found that Abeta(1-40) is generated in the early secretory and endocytic pathways, whereas Abeta(1-42) is made mainly in the secretory pathway. More importantly, we provide evidence that, unlike in neuronal models, both ER/intermediate compartment- and endocytic-derived Abeta forms can enter the secretable pool. Finally, we directly demonstrate that lysosomal processing is not involved in the generation or secretion of either Abeta(1-40) or Abeta(1-42).     

Secondary structure conversions of Alzheimer's Abeta(1-40) peptide induced by membrane-mimicking detergents

The amyloid beta peptide (Abeta) with 39-42 residues is the major component of amyloid plaques found in brains of Alzheimer's disease patients, and soluble oligomeric peptide aggregates mediate toxic effects on neurons. The Abeta aggregation involves a conformational change of the peptide structure to beta-sheet. In the present study, we report on the effect of detergents on the structure transitions of Abeta, to mimic the effects that biomembranes may have. In vitro, monomeric Abeta(1-40) in a dilute aqueous solution is weakly structured. By gradually adding small amounts of sodium dodecyl sulfate (SDS) or lithium dodecyl sulfate to a dilute aqueous solution, Abeta(1-40) is converted to beta-sheet, as observed by CD at 3 degrees C and 20 degrees C. The transition is mainly a two-state process, as revealed by approximately isodichroic points in the titrations. Abeta(1-40) loses almost all NMR signals at dodecyl sulfate concentrations giving rise to the optimal beta-sheet content (approximate detergent/peptide ratio = 20). Under these conditions, thioflavin T fluorescence measurements indicate a maximum of aggregated amyloid-like structures. The loss of NMR signals suggests that these are also involved in intermediate chemical exchange. Transverse relaxation optimized spectroscopy NMR spectra indicate that the C-terminal residues are more dynamic than the others. By further addition of SDS or lithium dodecyl sulfate reaching concentrations close to the critical micellar concentration, CD, NMR and FTIR spectra show that the peptide rearranges to form a micelle-bound structure with alpha-helical segments, similar to the secondary structures formed when a high concentration of detergent is added directly to the peptide solution.     

Increase of total homocysteine concentration in cerebrospinal fluid in patients with Alzheimer's disease and Parkinson's disease

We determined the concentrations of free homocysteine (HC) and total HC in the cerebrospinal fluid (CSF) of patients with Alzheimer's disease (AD) or Parkinson's disease (PD) in order to elucidate whether HC is related to the pathogenesis of these neurodegenerative diseases. The concentration of free HC did not differ significantly from that of the normal controls, while the concentration of total HC was significantly higher in the AD and PD patients (+31% in AD,+31% in PD; p<0.05). These findings suggest that an increase of total HC concentration in the brain is commonly seen in patients with AD and PD, and this may be related to the pathogenesis of these two diseases.     

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

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

CU(II)-catalyzed oxidation of Alzheimer's disease beta-amyloid peptide and related sequences: remarkably different selectivities of neurotoxic betaAP1-40 and non-toxic betaAP40-1.

We investigated the CuII-catalyzed oxidation of beta-amyloid peptides betaAP10-20 and betaAP40-1 by tandem mass spectrometry and compared oxidation yields and selectivities to those for betaAP1-16, betaAP1-28 and betaAP1-40, which were obtained earlier (26). While betaAP1-16, betaAP1-28 and betaAP1-40 showed an almost exclusive oxidation of His residues to 2-oxo-histidine, the selectivity pattern is changed for betaAP10-20,which shows oxidation of His but also hydroxylation of Tyr and Phe. In contrast to betaAP1-40, the reverse sequence betaAP40-1 shows a strong selectivity for the hydroxylation of Tyr31 while only negligible His oxidation is observed at early time points. These selectivity patterns show the importance of the geometry of the metal-binding site for peptide/protein oxidation. The significantly different characteristic of betaAP1-40 and betaAP40-1 with regard to metal catalyzed processes may be related to the differences in the neurotoxic properties of these sequences.     

Abeta(25-35) and Abeta(1-40) act on different calcium channels in CA1 hippocampal neurons

The acute effects of beta-amyloid (25-35) and (1-40) on high voltage activated calcium channels were compared in CA1 pyramidal cells of adult mouse hippocampal slices using the whole-cell patch-clamp recording. Bath application of oligomeric beta-amyloid (25-35) reversibly increased the barium current (I(Ba)) to 1.61 (normalized amplitude), while oligomeric beta-amyloid (1-40) reversibly enhanced the I(Ba) to 1.74. Reverse-sequence beta-amyloid [(35-25) and (40-1)] had no effect. The effect of beta-amyloid (25-35) was blocked by nifedipine, a selective antagonist of L-type calcium channels. In contrast, the effect of beta-amyloid (1-40) was not blocked by nifedipine and I(Ba) was enhanced to 4.96. It is concluded that these oligomeric peptides may act through different types of calcium channels and/or receptors. The toxicity of Abeta(25-35) implicates a potentiation of L-type calcium channels while the one of Abeta(1-40) is related to an increase of non-L-type calcium channels, which may involve an increase in transmitter release.