Metal binding modes of Alzheimer's amyloid beta-peptide in insoluble aggregates and soluble complexes

Nucleoside-5'-phosphorothioate analogues are biocompatible antioxidants dissolving efficiently amyloid beta-metal ion aggregates

Amyloid beta (Aβ) peptide is known to precipitate and form aggregates with zinc and copper ions in vitro and, in vivo in Alzheimer's disease (AD) patients. Metal-ion-chelation was suggested as therapy for the metal-ion-induced Aβ aggregation, metal-ion overload, and oxidative stress. In a quest for biocompatible metal-ion chelators potentially useful for AD therapy, we tested a series of nucleoside 5'-phosphorothioate derivatives as re-solubilization agents of Cu(+)/Cu(2+)/Zn(2+)-induced Aβ-aggregates, and inhibitors of Fenton reaction in Cu(+) or Fe(2+)/H(2)O(2) system. The most promising chelator in this series was found to be APCPP-γ-S. This nucleotide was found to be more efficient than EDTA in re-solubilization of Aβ(40)-Cu(2+) aggregates as observed by the lower diameter, d(H), (86 vs. 64 nm, respectively) obtained in dynamic light scattering measurements. Likewise, APCPP-γ-S dissolved Aβ(40)-Cu(+) and Aβ(42)-Cu(2+)/Zn(2+) aggregates, as monitored by (1)H-NMR and turbidity assays, respectively. Furthermore, addition of APCPP-γ-S to nine-day old Aβ(40)-Cu(2+)/Zn(2+) aggregates, resulted in size reduction as observed by transition electron microscopy (diameter reduction from 2.5 to 0.1 μm for Aβ(40)-Cu(2+) aggregates). APCPP-γ-S proved to be more efficient than ascorbic acid and GSH in reducing OH radical production in Fe(2+)/H(2)O(2) system (IC(50) values 85, 216 and, 92 μM, respectively). Therefore, we propose APCPP-γ-S as a potential AD therapy capable of both reducing OH radical production and re-solubilization of Aβ(40/42)-M(n+) aggregates.

Environmental factors preceding aβ40 monomer to oligomers and the detection of oligomers in Alzheimer's disease patient serum

We present here environmental factors including pH shifts, temperature, and metal ions surrounding Aβ40 monomer to precede the oligomers. We also suggest a new idea to detect Aβ40 oligomers with anti-Aβ40 monoclonal antibody using enzyme-linked immunosorbent assay. This method involves the different sensitivity of the thermal shifts between Aβ40 monomer and the oligomers. The idea is useful for the diagnostics of Alzheimer's disease to detect Aβ40 oligomers in the serum from the patients.

Prolactin and growth hormone aggregates in secretory granules: the need to understand the structure of the aggregate

Prolactin and GH form reversible aggregates in the trans-Golgi lumen that become the dense cores of secretory granules. Aggregation is an economical means of sorting, because self-association removes the hormones from other possible pathways. Secretory granules containing different aggregates show different behavior, such as the reduction in stimulated release of granules containing R183H-GH compared with release of those containing wild-type hormone. Aggregates may facilitate localization of membrane proteins necessary for transport and exocytosis of secretory granules, and therefore understanding their properties is important. Three types of self-association have been characterized: dimers of human GH that form with Zn(2+), low-affinity self-association of human prolactin caused by acidic pH and Zn(2+) with macromolecular crowding, and amyloid fibers of prolactin. The best candidate for the form in most granules may be low-affinity self-association because it occurs rapidly at Zn(2+) concentrations that are likely to be in granules and reverses rapidly in neutral pH. Amyloid may form in older granules. Determining differences between aggregates of wild type and those of R183H-GH should help to understand why granules containing the mutant behave differently from those containing wild-type hormone. If reversible aggregation of other hormones, including those that are proteolytically processed, is the crucial act in forming granules, rather than use of a sorting signal, then prohormones should form reversible aggregates in solution in conditions that resemble those of the trans-Golgi lumen, including macromolecular crowding.

The heterogeneous nature of Cu2+ interactions with Alzheimer's amyloid-β peptide

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by progressive cognitive and memory impairment. Within the brain, senile plaques, which comprise extracellular deposits of the amyloid-β peptide (Aβ), are the most common pathological feature of AD. A high concentration of Cu(2+) is found within these plaques, which are also areas under oxidative stress. Laboratory work has shown that in vitro Aβ will react with Cu(2+) to induce peptide aggregation and the production of reactive oxygen species. As such, this interaction offers a possible explanation for two of the defining pathological features observed in the AD brain: the presence of amyloid plaques, which consist largely of insoluble Aβ aggregates, and the abundant oxidative stress therein. Researchers have accordingly put forth the "metals hypothesis" of AD, which postulates that compounds designed to inhibit Cu(2+)/Aβ interactions and redistribute Cu(2+) may offer therapeutic potential for treating AD. Characterization of the pH-dependent Cu(2+) coordination of Aβ is fundamental to understanding the neurological relevance of Cu(2+)/Aβ interactions and aiding the design of new therapeutic agents. In an effort to shed light on the problem, many experimental and theoretical techniques, using a variety of model systems, have been undertaken. The preceding decade has seen numerous conflicting spectroscopic reports concerning the nature of the Cu(2+)/Aβ coordination. As the number of studies has grown, the nature of the pH-dependent ligand environment surrounding the Cu(2+) cation has remained a point of contention. In large part, the difficulties can be attributed to inappropriate choices of the model system or to methods that are incapable of quantitatively delineating the presence and identity of multiple Cu(2+) coordination modes. Electron paramagnetic resonance (EPR) is the method of choice for studying paramagnetic metal-protein interactions. With the introduction of site-specific (15)N, (17)O, and (13)C isotopic labels and the application of advanced techniques, EPR is capable of eliminating much of the ambiguity. Recent EPR studies have produced the most definitive picture of the pH-dependent Cu(2+) coordination modes of Aβ and enabled researchers to address the inconsistencies present in the literature. In this Account, we begin by briefly introducing the evidence for a role of Cu(2+) in AD as well as the potential physiological and therapeutic implications of that role. We then outline the EPR methodology used to resolve the molecular details of the Cu(2+)/Aβ interactions. No drugs are currently available for altering the course of AD, and existing therapies only offer short-term symptomatic relief. This focused picture of the role of Cu(2+) in AD-related plaques offers welcome potential for the development of new methods to combat this devastating disease.

Identification of [PtCl2(phen)] binding modes in amyloid-β peptide and the mechanism of aggregation inhibition

Platinum phenanthroline complexes inhibit amyloid-β (Aβ) aggregation and reduce Aβ-caused neurotoxicity [Proc. Natl. Acad. Sci., 2008, 105, 6813-6818]. In this study, we investigated the interactions of Aβ(1-16) with [PtCl(2)(phen)] (phen=1,10-phenanthroline) using HPLC, ESI-MS, and NMR spectroscopy , and characterized the identity of products using tandem mass spectrometry. Results indicated that the phenanthroline ligand could induce noncovalent interactions between Aβ peptide and platinum complexes, leading to rapid Aβ platination. Multiple products were generated in the reaction, in which His6/His14 chelation was preferentially formed. Coordination of Asp7, His13, and Lys16 was also detected in other products. The majority of products were monoplatinated adducts with binding of the {Pt(phen)} scaffold, which impeded intermolecular interactions between Aβ peptides. Moreover, noncovalent interactions were confirmed by the interaction between Aβ peptide and [Pt(phen)(2)]Cl(2). The synergistic roles of the phen ligand and platinum(II) atom in the inhibition of Aβ aggregation are discussed.

Vitamin B6s inhibit oxidative stress caused by Alzheimer's disease-related Cu(II)-β-amyloid complexes-cooperative action of phospho-moiety

Cu(II) complexes of Alzheimer's disease-related β-amyloid (Aβ) peptides exhibit metal-centered oxidation chemistry. The metallo-Aβ complexes are the hallmark of the disease and have been attributed to the generation of reactive oxygen species (ROS), causing oxidative stress. In this communication, the inhibitions of the oxidative activity of Cu(II)-Aβ by vitamin B6 compounds pyridoxamine (PM), pyridoxine (PN), pyridoxal (PL), and pyridoxal-5'-phosphate (PLP) are presented. These B6's are competitive inhibitors toward dopamine oxidation by Cu(II)-Aβ(1-20), with K(i) values of 1.4, 8.3, 1.2, and 0.2mM, respectively. The phospho-moiety in PLP seems to exhibit cooperative inhibition, affording a clue for future design of inhibitors.

Three dimensional models of Cu(2+)-Aβ(1-16) complexes from computational approaches

Elucidation of the coordination of metal ions to Aβ is essential to understand their role in its aggregation and to rationally design new chelators with potential therapeutic applications in Alzheimer disease. Because of that, in the last 10 years several studies have focused their attention in determining the coordination properties of Cu(2+) interacting with Aβ. However, more important than characterizing the first coordination sphere of the metal is the determination of the whole Cu(2+)-Aβ structure. In this study, we combine homology modeling (HM) techniques with quantum mechanics based approaches (QM) to determine plausible three-dimensional models for Cu(2+)-Aβ(1-16) with three histidines in their coordination sphere. We considered both ε and δ coordination of histidines 6, 13, and 14 as well as the coordination of different possible candidates containing oxygen as fourth ligand (Asp1, Glu3, Asp7, Glu11, and CO(Ala2)). Among the 32 models that enclose COO(-), the lowest energy structures correspond to [O(E3),N(δ)(H6),N(ε)(H13),N(ε)(H14)] (1), [O(E3),N(δ)(H6),N(δ)(H13),N(δ)(H14)] (2), and [O(D7),N(ε)(H6),N(δ)(H13),N(δ)(H14)] (3). The most stable model containing CO(Ala2) as fourth ligand in the Cu(2+) coordination sphere is [O(c)(A2),N(ε)(H6),N(δ)(H13),N(ε)(H14)] (4). An estimation of the relative stability between Glu3 (1) and CO(Ala2) (4) coordinated complexes seems to indicate that the preference for the latter coordination may be due to solvent effects. The present results also show the relationship between the peptidic and metallic moieties in defining the overall geometry of the complex and illustrate that the final stability of the complexes results from a balance between the metal coordination site and amyloid folding upon complexation.

The electrostatic probe: a tool for the investigation of the Aβ(1-16) peptide deformations using the static modes

We investigate the conformational changes of the Amyloid β(1-16) peptide induced by moving Zn(2+) ions in the solvent, which we call the electrostatic probe. We use our recently developed approach of static modes which allows treating the flexibility of biological molecules at the atomic scale. Starting from an experimental apostructure, we find that several ion impacts allow the transition of the peptide toward its folded conformation, observed experimentally in the presence of Zn(2+) ions. This result shows the ability of our model and its associated software tool to describe properly the conformational changes and opens a new path toward the molecule/molecule docking problem.

Cu(II) mediates kinetically distinct, non-amyloidogenic aggregation of amyloid-beta peptides

Cu(II) ions are implicated in the pathogenesis of Alzheimer disease by influencing the aggregation of the amyloid-β (Aβ) peptide. Elucidating the underlying Cu(II)-induced Aβ aggregation is paramount for understanding the role of Cu(II) in the pathology of Alzheimer disease. The aim of this study was to characterize the qualitative and quantitative influence of Cu(II) on the extracellular aggregation mechanism and aggregate morphology of Aβ(1-40) using spectroscopic, microelectrophoretic, mass spectrometric, and ultrastructural techniques. We found that the Cu(II):Aβ ratio in solution has a major influence on (i) the aggregation kinetics/mechanism of Aβ, because three different kinetic scenarios were observed depending on the Cu(II):Aβ ratio, (ii) the metal:peptide stoichiometry in the aggregates, which increased to 1.4 at supra-equimolar Cu(II):Aβ ratio; and (iii) the morphology of the aggregates, which shifted from fibrillar to non-fibrillar at increasing Cu(II):Aβ ratios. We observed dynamic morphological changes of the aggregates, and that the formation of spherical aggregates appeared to be a common morphological end point independent on the Cu(II) concentration. Experiments with Aβ(1-42) were compatible with the conclusions for Aβ(1-40) even though the low solubility of Aβ(1-42) precluded examination under the same conditions as for the Aβ(1-40). Experiments with Aβ(1-16) and Aβ(1-28) showed that other parts than the Cu(II)-binding His residues were important for Cu(II)-induced Aβ aggregation. Based on this study we propose three mechanistic models for the Cu(II)-induced aggregation of Aβ(1-40) depending on the Cu(II):Aβ ratio, and identify key reaction steps that may be feasible targets for preventing Cu(II)-associated aggregation or toxicity in Alzheimer disease.

Computational insights into the development of novel therapeutic strategies for Alzheimer's disease

BACKGROUND

β-amyloidosis and oxidative stress have been implicated as root causes of Alzheimer's disease (AD). Current potential therapeutic strategies for the treatment of AD include inhibition of amyloid β (Aβ) production, stimulation of Aβ degradation and prevention of Aβ oligomerization. However, efforts in this direction are hindered by the lack of understanding of the biochemical processes occurring at the atomic level in AD.

DISCUSSION

A radically different approach to achieve this goal would be the application of comprehensive theoretical and computational techniques such as molecular dynamics, quantum mechanics, hybrid quantum mechanics/molecular mechanics, bioinformatics and rotational spectroscopy to investigate complex chemical and physical processes in β-amyloidosis and the oxidative stress mechanism.

CONCLUSION

Results obtained from these studies will provide an atomic level understanding of biochemical processes occurring in AD and advance efforts to develop effective therapeutic strategies for this disease.

Ab initio design of chelating ligands relevant to Alzheimer's disease: influence of metalloaromaticity

Evidence supporting the role of metal ions in Alzheimer's disease (AD) has rendered metal ion chelation as a promising therapeutic treatment. The rational design of efficient chelating ligands requires, however, a good knowledge of the electronic and molecular structure of the complexes formed. In the present work, the coordinative properties of a set of chelating ligands toward Cu(II) have been analyzed by means of DFT(B3LYP) calculations. Special attention has been paid to the aromatic behavior of the metalated rings of the complex and its influence on the chelating ability of the ligand. Ligands considered have identical metal binding sites (through N/O coordination) and only differ on the kind and size of the aromatic moieties. Results indicate that there is a good correlation between the stability constants (log β(2)) and the degree of metalloaromaticity determined through the I(NG) and HOMA indices; that is, the higher the metalloaromaticity, the larger the log β(2) value. MOs and aromaticity descriptors confirm that present complexes exhibit Möbius metalloaromaticity. Detailed analysis of the nature of the Cu(II)-ligand bonding, performed through an energy decomposition analysis, indicates that ligands with less aromatic moieties have the negative charge more localized in the metalated ring, thus increasing their σ-donor character and the metalloaromaticity of the complexes they form.

Molecular-level examination of Cu2+ binding structure for amyloid fibrils of 40-residue Alzheimer's β by solid-state NMR spectroscopy

Cu(2+) binding to Alzheimer's β (Aβ) peptides in amyloid fibrils has attracted broad attention, as it was shown that Cu ion concentration elevates in Alzheimer's senile plaque and such association of Aβ with Cu(2+) triggers the production of neurotoxic reactive oxygen species (ROS) such as H(2)O(2). However, detailed binding sites and binding structures of Cu(2+) to Aβ are still largely unknown for Aβ fibrils or other aggregates of Aβ. In this work, we examined molecular details of Cu(2+) binding to amyloid fibrils by detecting paramagnetic signal quenching in 1D and 2D high-resolution (13)C solid-state NMR (SSNMR) for full-length 40-residue Aβ(1-40). Selective quenching observed in (13)C SSNMR of Cu(2+)-bound Aβ(1-40) suggested that primary Cu(2+) binding sites in Aβ(1-40) fibrils include N(ε) in His-13 and His-14 and carboxyl groups in Val-40 as well as in Glu sidechains (Glu-3, Glu-11, and/or Glu-22). (13)C chemical shift analysis demonstrated no major structural changes upon Cu(2+) binding in the hydrophobic core regions (residues 18-25 and 30-36). Although the ROS production via oxidization of Met-35 in the presence of Cu(2+) has been long suspected, our SSNMR analysis of (13)C(ε)H(3)-S- in M35 showed little changes after Cu(2+) binding, excluding the possibility of Met-35 oxidization by Cu(2+) alone. Preliminary molecular dynamics (MD) simulations on Cu(2+)-Aβ complex in amyloid fibrils confirmed binding sites suggested by the SSNMR results and the stabilities of such bindings. The MD simulations also indicate the coexistence of a variety of Cu(2+)-binding modes unique in Aβ fibril, which are realized by both intra- and intermolecular contacts and highly concentrated coordination sites due to the in-register parallel β-sheet arrangements.

Gene-environment interaction research and transgenic mouse models of Alzheimer's disease

The etiology of the sporadic form of Alzheimer's disease (AD) remains largely unknown. Recent evidence has suggested that gene-environment interactions (GxE) may play a crucial role in its development and progression. Whereas various susceptibility loci have been identified, like the apolipoprotein E4 allele, these cannot fully explain the increasing prevalence of AD observed with aging. In addition to such genetic risk factors, various environmental factors have been proposed to alter the risk of developing AD as well as to affect the rate of cognitive decline in AD patients. Nevertheless, aside from the independent effects of genetic and environmental risk factors, their synergistic participation in increasing the risk of developing AD has been sparsely investigated, even though evidence points towards such a direction. Advances in the genetic manipulation of mice, modeling various aspects of the AD pathology, have provided an excellent tool to dissect the effects of genes, environment, and their interactions. In this paper we present several environmental factors implicated in the etiology of AD that have been tested in transgenic animal models of the disease. The focus lies on the concept of GxE and its importance in a multifactorial disease like AD. Additionally, possible mediating mechanisms and future challenges are discussed.

Quantification of the binding properties of Cu2+ to the amyloid beta peptide: coordination spheres for human and rat peptides and implication on Cu2+-induced aggregation

There is no consensus on the coordinating ligands for Cu(2+) by Abeta. However, the differences in peptide sequence between human and rat have been hypothesized to alter metal ion binding in a manner that alters Cu(2+)-induced aggregation of Abeta. Herein, we employ isothermal titration calorimetry (ITC), circular dichroism (CD), and electron paramagnetic resonance (EPR) spectroscopy to examine the Cu(2+) coordination spheres to human and rat Abeta and an extensive set of Abeta(16) mutants. EPR of the mutant peptides is consistent with a 3N1O binding geometry, like the native human peptide at pH 7.4. The thermodynamic data reveal an equilibrium between three coordination spheres, {NH(2), O, N(Im)(His6), N(-)}, {NH(2), O, N(Im)(His6), N(Im)(His13)}, and {NH(2), O, N(Im)(His6), N(Im)(His14)}, for human Abeta(16) but one dominant coordination for rat Abeta(16), {NH(2), O, N(Im)(His6), N(-)}, at pH 7.4-6.5. ITC and CD data establish that the mutation R5G is sufficient for reproducing this difference in Cu(2+) binding properties at pH 7.4. The substitution of bulky and positively charged Arg by Gly is proposed to stabilize the coordination {NH(2), O-, N(Im)(His6), N(-)} that then results in one dominating coordination sphere for the case of the rat peptide. The differences in the coordination geometries for Cu(2+) by the human and rat Abeta are proposed to contribute to the variation in the ability of Cu(2+) to induce aggregation of Abeta peptides.

Apolipoprotein E ablation decreases synaptic vesicular zinc in the brain

Both apolipoprotein E (apoE) and zinc are involved in amyloid β (Aβ) aggregation and deposition, in the hallmark neuropathology of Alzheimer's disease (AD). Recent studies have suggested that interaction of apoE with metal ions may accelerate amyloidogenesis in the brain. Here we examined the impact of apoE deficiency on the histochemically reactive zinc pool in the brains of apoE knockout mice. While there was no change in total contents of metals (zinc, copper, and iron), the level of histochemically reactive zinc (principally synaptic zinc) was significantly reduced in the apoE-deficient brain compared to wild-type. This reduction was accompanied by reduced expressions of the presynaptic zinc transporter, ZnT3, as well as of the δ-subunit of the adaptor protein complex-3 (AP3δ), which is responsible for post-translational stability and activity of ZnT3. In addition, the level of histochemically reactive zinc was also decreased in the cerebrovascular micro-vessels of apoE-deficient mice, the site of cerebral amyloid angiopathy in AD. These results suggest that apoE may affect the cerebral free zinc pool that contributes to AD pathology.

Cu(II) interaction with amyloid-beta peptide: a review of neuroactive mechanisms in AD brains

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by the dys-homeostasis of biometal metabolism, the extracellular accumulation of neurotoxic amyloid-beta (Abeta) peptide, the intracellular accumulation of hyperphosphorylated tau and the loss of synapses. Copper plays a key role in AD development. The Abeta peptide and amyloid precursor protein (APP), the parental molecule of Abeta, are modulated by copper in the brain. Increased copper concentration has been found in the AD brain that implies that copper may participate in the pathophysiology of AD. Copper can bind to APP and Abeta, then affects the structure and toxic of APP and Abeta. Some researchers have reported that copper could affect the formation of beta-sheet structure that is widely accepted as toxic secondary structure of Abeta. This review explores the role of copper on the conformation and aggregation of Abeta, and the copper-induced neuroactive mechanisms. Copper may be involved in the following pathways to affect the neuroactivation of Abeta: (1) change of the secondary structure of Abeta; (2) induction of oxidative stress in AD brains, and (3) regulation of cellular signal pathway. Thus, correcting brain copper imbalance may represent a relevant therapeutic target for Alzheimer's disease.

Zinc ions promote Alzheimer Abeta aggregation via population shift of polymorphic states

Although a key factor in Alzheimer's disease etiology is enrichment of Zn(2+) in aggregates, and there are data suggesting that zinc promotes aggregation, how Zn(2+)-Abeta coordination promotes aggregation is elusive. Here we probe the structures and mechanisms through which Zn(2+) can affect amyloidosis. By covalently linking fragments (that have experiment-based coordinates) we observed that, in oligomeric Zn(2+)-Abeta(42), Zn(2+) can simultaneously coordinate intra- and intermolecularly, bridging two peptides. Zinc coordination significantly decreases the solvation energy for large Zn(2+)-Abeta(42) oligomers and thus enhances their aggregation tendency. Zn(2+) binding does not change the beta-sheet association around the C-terminal hydrophobic region; however, it shifts the relative population of the preexisting amyloid polymorphic ensembles. As a result, although a parallel beta-sheet arrangement is still preferred, antiparallel and other less structured assemblies are stabilized, also becoming major species. Overall, Zn(2+) coordination promotes Abeta(42) aggregation leading to less uniform structures. Our replica exchange molecular dynamics simulations further reproduced an experimental observation that the increasing Zn(2+) concentration could slow down the aggregation rate, even though the aggregation rates are still much higher than in Zn(2+)-free solution.

[Conformational regulation of amyloid beta-peptide by lipid membranes and metal ions]

Conformational transition of monomeric amyloid beta-peptide (Abeta) to a self-associated beta-sheet structure is considered to be an initial step in the development of Alzheimer's disease. Several lines of evidence suggest that physiologically abundant lipid membranes and metal ions are involved in this step. We have demonstrated that Abeta binds to the phosphatidylcholine membrane in the lamellar gel phase but not in the liquid crystalline phase by using fluorescence and circular dichroism spectroscopy. The membrane-bound Abeta molecule takes alpha-helical or beta-sheet structure depending on the temperature. Tightly packed phosphatidylcholine membranes appear to serve as a platform for non-electrostatic binding and self-association of Abeta. We have also examined Zn(II) and Cu(II) binding modes of Abeta by Raman spectroscopy. The Raman spectra demonstrate that three histidine residues in the N-terminal region of Abeta provide primary metal binding sites. Zn(II) binds to the N(tau) atom of histidine and the peptide aggregates through intermolecular His-Zn-His bridges. In contrast, Cu(II) ion is chelated by the N(pi) atom of histidine and deprotonated main-chain amide nitrogens to form a soluble complex. Our findings on the conformational regulation of Abeta may help in better understanding the molecular basis for the development of Alzheimer's disease.

Probing Alzheimer amyloid peptide aggregation using a cell-free fluorescent protein refolding method

Fibrillation of the Alzheimer beta-amyloid peptide (Abeta) and (or) formation of toxic oligomers are key pathological events in Alzheimer's disease. Several strategies have been introduced to identify small molecule aggregation inhibitors, and based on these methods, a number of aggregation inhibitors have been identified. However, most of these methods use chemically synthesized Abeta42 peptides, which are difficult to maintain in a monomeric state at neutral pH where anti-aggregation screening is usually carried out. We have developed a cell-free Abeta42 aggregation assay based on fluorescence protein refolding. This assay utilizes nanomolar concentrations of protein. We genetically fused Abeta42 to the N-terminus of vYFP, a variant of of GFP, and expressed and purified the folded fusion protein. The refolding efficiency of Abeta42-vYFP fusion was inversely correlated with the solubility of Abeta42. Using fluorescence to monitor refolding of Abeta42-vYFP, we confirmed that Zn2+ binds to Abeta42 and increases its aggregation. The IC50 value estimated for Zn binding is 3.03 +/- 0.65 micromol/L. We also show that this technique is capable of monitoring the aggregation of chemically synthesized Abeta42.

NMR studies of zinc binding in a multi-histidinic peptide fragment

A multi-histidinic peptide and its minimal models have been investigated for Zn(ii) binding. We have used NMR spectroscopy to probe the binding of zinc to the three repeats (T(1)R(2)S(3)R(4)S(5)H(6)T(7)S(8)E(9)G(10))(3) and to its mono-histidinic minimal models, the 9- and 10-aminoacid fragment. (1)H-(1)H TOCSY, (1)H-(13)C HSQC, (1)H-(1)H NOESY and (1)H-(1)H ROESY multidimensional NMR techniques were performed to understand the details of metal binding sites and the conformational behaviour of the peptides at different pH values and at different ligand to metal molar ratios. Zinc coordination involves imidazole N(delta) of His6 and carboxyl gamma-O of Glu9 residues; interaction with peptide oxygens of the His6-Thr7 or Thr7-Ser8 bonds in a tetrahedral arrangement with the minimal model peptides, cannot be excluded. Zinc coordination involves, at physiologic pH, all the three imidazole N(delta) donors of His6, His16 and His26 as well as carboxyl gamma-O of Glu residues in a tetra, penta or octahedral arrangement with the three repeats, the 30-aminoacid fragment. Zinc complexation induces important structural changes with the C-terminal portion of the ligand, constraining it to leave its disordered conformation. Our results give rise to a model of the induced structure of the peptides when bound to zinc. At high pH, amide deprotonation does not take place and hydroxo or high molecular weight polymeric species may be formed.

The second Cu(II)-binding site in a proton-rich environment interferes with the aggregation of amyloid-beta(1-40) into amyloid fibrils

The overall morphology and Cu(II) ion coordination for the aggregated amyloid-beta(1-40) [Abeta(1-40)] in N-ethylmorpholine (NEM) buffer are affected by Cu(II) ion concentration. This effect is investigated by transmission electron microscopy (TEM), atomic force microscopy (AFM), and electron spin echo envelope modulation (ESEEM) spectroscopy. At lower than equimolar concentrations of Cu(II) ions, fibrillar aggregates of Abeta(1-40) are observed. At these concentrations of Cu(II), the monomeric and fibrillar Abeta(1-40) ESEEM data indicate that the Cu(II) ion is coordinated by histidine residues. For aggregated Abeta(1-40) at a Cu(II):Abeta molar ratio of 2:1, TEM and AFM images show both linear fibrils and granular amorphous aggregates. The ESEEM spectra show that the multi-histidine coordination for Cu(II) ion partially breaks up and becomes exposed to water or exchangeable protons of the peptide at a higher Cu(II) concentration. Since the continuous-wave electron spin resonance results also suggest two copper-binding sites in Abeta(1-40), the proton ESEEM peak may arise from the second copper-binding site, which may be significantly involved in the formation of granular amorphous aggregates. Thioflavin T fluorescence and circular dichroism experiments also show that Cu(II) inhibits the formation of fibrils and induces a nonfibrillar beta-sheet conformation. Therefore, we propose that Abeta(1-40) has a second copper-binding site in a proton-rich environment and the second binding Cu(II) ion interferes with a conformational transition into amyloid fibrils, inducing the formation of granular amorphous aggregates.

Amyloid plaques in PSAPP mice bind less metal than plaques in human Alzheimer's disease

Amyloid beta (Abeta) is the primary component of Alzheimer's disease (AD) plaques, a key pathological feature of the disease. Metal ions of zinc (Zn), copper (Cu), iron (Fe), and calcium (Ca) are elevated in human amyloid plaques and are thought to be involved in neurodegeneration. Transgenic mouse models of AD also exhibit amyloid plaques, but fail to exhibit the high degree of neurodegeneration observed in humans. In this study, we imaged the Zn, Cu, Fe, and Ca ion distribution in the PSAPP transgenic mouse model representing end-stage AD (N=6) using synchrotron X-ray fluorescence (XRF) microprobe. In order to account for differences in density in the plaques, the relative protein content was imaged with synchrotron Fourier transform infrared microspectroscopy (FTIRM) on the same samples. FTIRM results revealed a 61% increase in protein content in the plaques compared to the surrounding tissue. After normalizing to protein density, we found that the PSAPP plaques contained only a 29% increase in Zn and there was actually less Cu, Fe, and Ca in the plaque compared to the surrounding tissue. Since metal binding to Abeta is thought to induce redox chemistry that is toxic to neurons, the reduced metal binding in PSAPP mice is consistent with the lack of neurodegeneration in these animals. These findings were in stark contrast to the high metal ion content observed in human AD plaques, further implicating the role of metal ions in human AD pathology.

Chronic exposure to high levels of zinc or copper has little effect on brain metal homeostasis or Abeta accumulation in transgenic APP-C100 mice

Aberrant metal homeostasis may enhance the formation of reactive oxygen species and Abeta oligomerization and may therefore be a contributing factor in Alzheimer's disease. This study investigated the effect of chronic high intake of dietary Zn or Cu on brain metal levels and the accumulation and solubility of Abeta in vivo, using a transgenic mouse model that over expresses the C-terminal containing Abeta fragment of human amyloid precursor protein but does not develop amyloid deposits. Exposure to chronic high Zn or Cu in the drinking water resulted in only slight elevations of the respective metals in the brain. Total Abeta levels were unchanged although soluble Abeta levels were slightly decreased, without visible plaque formation, enhanced gliosis, antioxidant upregulation or neuronal loss. This study indicates that brain metal levels are only marginally altered by long term oral exposure to extremely high Cu or Zn levels, and that this does not induce Abeta-amyloid formation in human Abeta expressing, amyloid-free mice, although this is sufficient to modulate Abeta solubility in vivo.

Electrochemical and conformational consequences of copper (Cu(I) and Cu(II)) binding to beta-amyloid(1-40)

Copper-induced structural rearrangements of Abeta40 structure and its redox properties are described in this study. Electrochemical and fluorescent methods are used to characterise the behaviour of Abeta-Cu species. The data suggest that time-dependent folding of Abeta-Cu species may cause changes in the redox potentials.Extracellular deposits of beta-amyloid (Abeta) into senile plaques are the major features observed in brains of Alzheimer's disease (AD) patients. A high concentration of copper has been associated with insoluble amyloid plaques. It is known that Abeta(1-40) can bind copper with high affinity, but electrochemical properties of Abeta(1-40)-Cu complexes are not well-characterised. In this study we demonstrate that complexation of copper (both as Cu(I) and Cu(II)) by Abeta(1-40) reduces the metal electrochemical activity. Formation of copper-Abeta(1-40) complexes is associated with alteration of the redox potential. The data reveal significant redox activity of fresh Abeta-copper solutions. However, copper-induced structural rearrangements of the peptide, documented by CD, correspond with time-dependent changes of formal reduction potentials (E(0')) of the complex. Fluorescent and electrochemical (cyclic voltammetry and differential pulse voltammetry) techniques suggest that reduction of the redox activity by Abeta-Cu complexes could be attributed to conformational changes that diminished copper accessibility to the external environment. According to our evidence, conformational rearrangements, induced by copper binding to amyloid, elongate the time necessary to attain the same beta-sheet content as for the metal-free peptide. Although the redox activity of Abeta-Cu complexes diminishes in a time-dependent manner, they are not completely devoid of toxicity as they destabilize red blood cells osmotic fragility, even after prolonged incubation.

Copper(II) binding to amyloid-beta fibrils of Alzheimer's disease reveals a picomolar affinity: stoichiometry and coordination geometry are independent of Abeta oligomeric form

Cu(2+) ions are found concentrated within senile plaques of Alzheimer's disease patients directly bound to amyloid-beta peptide (Abeta) and are linked to the neurotoxicity and self-association of Abeta. The affinity of Cu(2+) for monomeric Abeta is highly disputed, and there have been no reports of affinity of Cu(2+) for fibrillar Abeta. We therefore measured the affinity of Cu(2+) for both monomeric and fibrillar Abeta(1-42) using two independent methods: fluorescence quenching and circular dichroism. The binding curves were almost identical for both fibrillar and monomeric forms. Competition studies with free glycine, l-histidine, and nitrilotriacetic acid (NTA) indicate an apparent (conditional) dissociation constant of 10(-11) M, at pH 7.4. Previous studies of Cu-Abeta have typically found the affinity 2 or more orders of magnitude weaker, largely because the affinity of competing ligands or buffers has been underestimated. Abeta fibers are able to bind a full stoichiometric complement of Cu(2+) ions with little change in their secondary structure and have coordination geometry identical to that of monomeric Abeta. Electron paramagnetic resonance studies (EPR) with Abeta His/Ala analogues suggest a dynamic view of the tetragonal Cu(2+) complex, with axial as well as equatorial coordination of imidazole nitrogens creating an ensemble of coordination geometries in exchange between each other. Furthermore, the N-terminal amino group is essential for the formation of high-pH complex II. The Abeta(1-28) fragment binds an additional Cu(2+) ion compared to full-length Abeta, with appreciable affinity. This second binding site is revealed in Abeta(1-42) upon addition of methanol, indicating hydrophobic interactions block the formation of this weaker carboxylate-rich complex. A Cu(2+) affinity for Abeta of 10(11) M(-1) supports a modified amyloid cascade hypothesis in which Cu(2+) is central to Abeta neurotoxicity.

The effect of cell fixation on the discrimination of normal and leukemia cells with laser tweezers Raman spectroscopy

Laser tweezers Raman spectroscopy (LTRS) was used to characterize the effect of different chemical fixation procedures on the Raman spectra of normal and leukemia cells. Individual unfixed, paraformaldehyde-fixed, and methanol-fixed normal and transformed lymphocytes from three different cell lines were analyzed with LTRS. When compared to the spectra of unfixed cells, the fixed cell spectra show clear, reproducible changes in the intensity of specific Raman markers commonly assigned to DNA, RNA, protein, and lipid vibrations (e.g. 785, 1230, 1305, 1660 cm(-1)) in mammalian cells, many of which are important markers that have been used to discriminate between normal and cancer lymphocytes. Statistical analyses of the Raman data and classification using principal component analysis and linear discriminant analysis indicate that methanol fixation induces a greater change in the Raman spectra than paraformaldehyde. In addition, we demonstrate that the spectral changes as a result of the fixation process have an adverse effect on the accurate Raman discrimination of the normal and cancer cells. The spectral artifacts created by the use of fixatives indicate that the method of cell preparation is an important parameter to consider when applying Raman spectroscopy to characterize, image, or differentiate between different fixed cell samples to avoid potential misinterpretation of the data.