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.

Quantum chemical analysis of the unfolding of a penta-alanyl 3(10)-helix initiated by HO(•), HO2(•) and O2(-•)

In order to elucidate the mechanisms of radical-initiated unfolding of a helix, the thermodynamic functions of hydrogen abstraction from the C(α), C(β), and amide nitrogen of Ala(3) in a homopeptapeptide (N-Ac-AAAAA-NH(2); A5) by HO(•), HO(2)(•), and O(2)(-•) were computed using the B3LYP density functional. The thermodynamic functions, standard enthalpy (ΔH(o)), Gibbs free energy (ΔG(o)), and entropy (ΔS(o)), of the reactants and products of these reactions were computed with A5 in the 3(10)-helical (A5(Hel)) and fully extended (A5(Ext)) conformations at the B3LYP/6-31G(d) and B3LYP/6-311+G(d,p) levels of theory, both in the gas phase and using the C-PCM implicit water model. With quantum chemical calculations, we have shown that H abstraction is the most favorable at the C(α), followed by the C(β), then amide N in a model helix. The secondary structure has a strong influence on the bond dissociation energy of the H-C(α), but a negligible effect on the dissociation energy of the H-CH(2) and H-N bonds. The HO(•) radical is the strongest hydrogen abstractor, followed by HO(2)(•) and finally O(2)(-•). More importantly, secondary structure elements, such as H-bonds in the 3(10)-helix, protect the peptide from radical attack by hindering the potential electron delocalization at the C(α) when the peptide is in the extended conformation. We also show that he unfolding of the A5 peptide radicals have a significantly higher propensity to unfold than the closed shell A5 peptide and confirm that only the HO(•) can initiate the unfolding of A5(Hel) and the formation of A5(Ext)(•). By comparing the structures, energies, and thermodynamic functions of A5 and its radical derivatives, we have shown how free radicals can initiate the unfolding of helical structures to β-sheets in the cellular condition known as oxidative stress.

Luteolin isolated from the medicinal plant Elsholtzia rugulosa (Labiatae) prevents copper-mediated toxicity in β-amyloid precursor protein Swedish mutation overexpressing SH-SY5Y cells

Luteolin, a 3',4',5,7-tetrahydroxyflavone, is a plant flavonoid and pharmacologically active agent that has been isolated from several plant species. In the present study, the effects of luteolin obtained from the medicinal plant Elsholtzia rugulosa and the related mechanisms were examined in an Alzheimer's disease (AD) cell model. In this model, copper was used to exacerbate the neurotoxicity in β-amyloid precursor protein Swedish mutation stably overexpressed SH-SY5Y cells (named "APPsw cells" for short). Based on this model, we demonstrated that luteolin increased cell viability, reduced intracellular ROS generation, enhanced the activity of SOD and reversed mitochondrial membrane potential dissipation. Inhibition of caspase-related apoptosis was consistently involved in the neuroprotection afforded by luteolin. Furthermore, it down-regulated the expression of AβPP and lowered the secretion of Aβ₁₋₄₂. These results indicated that luteolin from the Elsholtzia rugulosa exerted neroprotective effects through mechanisms that decrease AβPP expression, lower Aβ secretion, regulate the redox imbalance, preserve mitochondrial function, and depress the caspase family-related apoptosis.

Metal ions, Alzheimer's disease and chelation therapy

In the last few years, various studies have been providing evidence that metal ions are critically involved in the pathogenesis of major neurological diseases (Alzheimer, Parkinson). Metal ion chelators have been suggested as potential therapies for diseases involving metal ion imbalance. Neurodegeneration is an excellent target for exploiting the metal chelator approach to therapeutics. In contrast to the direct chelation approach in metal ion overload disorders, in neurodegeneration the goal seems to be a better and subtle modulation of metal ion homeostasis, aimed at restoring ionic balance. Thus, moderate chelators able to coordinate deleterious metals without disturbing metal homeostasis are needed. To date, several chelating agents have been investigated for their potential to treat neurodegeneration, and a series of 8-hydroxyquinoline analogues showed the greatest potential for the treatment of neurodegenerative diseases.

The Amazonian herbal Marapuama attenuates cognitive impairment and neuroglial degeneration in a mouse Alzheimer model

Alzheimer's disease (AD) is expected to affect more than 22 million people worldwide by 2025, causing devastating suffering and enormous costs to families and society. AD is a multifactorial disease, with a complex pathological mosaic. In rodents, AD-like dementia can be induced by cerebral microinjection of Aβ peptide, leading to amyloid deposits, amnesia and various features of neurodegeneration. Marapuama (Ptychopetalum olacoides) is regarded as a "brain tonic" in the Amazon region and shows a nootropic profile in rodents.

AIM OF THE STUDY

Because a specific extract (POEE) of Marapuama was shown to possess promnesic and anti-amnesic properties, the aim of this study was to verify if POEE is also effective against Aβ(1-42)-induced cognitive deficit in mice. Additionally, Aβ deposits (Congo red), GFAP immunoreactivity (immunohistochemistry), and neurodegenerative changes in the hippocampal pyramidal layer (Nissl) were examined as measures of Aβ(1-42)-induced neurodegeneration.

MATERIALS AND METHODS

CF1 mice were subjected to the experimental Alzheimer model with the Aβ(1-42) i.c.v. administration. The effects of POEE 800 mg/kg were evaluated over 14 consecutive days of treatment.

RESULTS

The data show that 14 days of oral treatment with POEE (800 mg/kg) was effective in preventing Aβ-induced cognitive impairment, without altering the levels of BDNF and with parallel reductions in Aβ deposits and astrogliosis. CA1 hippocampus loss induced by Aβ(1-42) was also diminished in POEE-treated mice.

CONCLUSION

This study offers evidence of functional and neuroprotective effects of two weeks treatment with a Ptychopetalum olacoides extract against Aβ peptide-induced neurotoxicity in mice. Given the multifactorial nature of neurodegeneration, the considerable potential for an AChE inhibitor displaying associated neuroprotective properties such as here reported warrants further clinic evaluation.

Novel drug targets based on metallobiology of Alzheimer's disease

IMPORTANCE OF THE FIELD

Increased localization of Zn, Fe, Cu and Al within the senile plaques (SP) exacerbates amyloid beta (Aβ)-mediated oxidative damage, and acts as catalyst for Aβ aggregation in Alzheimer's disease (AD). Thus, disruption of aberrant metal-peptide interactions via chelation therapy holds considerable promise as a rational therapeutic strategy against Alzheimer's amyloid pathogenesis.

AREAS COVERED IN THIS REVIEW

The complexities of metal-induced genesis of SP are reviewed. The recent advances in the molecular mechanism of action of metal chelating agents are discussed with critical assessment of their potential to become drugs.

WHAT THE READER WILL GAIN

Taking into consideration the interaction of metals with the metal-responsive elements on the Alzheimer's amyloid precursor protein (APP), readers will gain understanding of several points to bear in mind when developing a screening campaign for AD-therapeutics.

TAKE HOME MESSAGE

A functional iron-responsive element (IRE) RNA stem loop in the 5' untranslated region (UTR) of the APP transcript regulates neural APP translation. Desferrioxamine, clioquinol, tetrathiolmolybdate, dimercaptopropanol, VK-28, and natural antioxidants, such as curcumin and ginko biloba need critical evaluation as AD therapeutics. There is a necessity for novel screens (related to metallobiology) to identify therapeutics effective in AD.

Can molecular dynamics simulations assist in design of specific inhibitors and imaging agents of amyloid aggregation? Structure, stability and free energy predictions for amyloid oligomers of VQIVYK, MVGGVV and LYQLEN

The aggregation modes of hexapeptide fragments of Tau, Insulin and Aβ peptide (VQIVYK, MVGGVV and LYQLEN) were found from their microcrystalline structures that had been recently resolved by X-ray analysis. The atomic structures reveal a dry self-complementary interface between the neighboring β-sheet layers, termed "steric zipper". In this study we perform several all-atom molecular dynamics simulations with explicit water to analyze stability of the crystalline fragments of 2-10 hexapeptides each and their analogs with single glycine replacement mutations to investigate the structural stability, aggregation behavior and thermodynamic of the amyloid oligomers. Upon comparing single and double layer models, our results reveal that additional strands contribute significantly to the structural stability of the peptide oligomers for double layer model, while in the case of single layer model the stability decreases (or remains the same in the case of LYQLEN). This is in agreement with the previous studies performed on different types of amyloid models. We also replaced the side-chains participating in the steric zipper interfaces with glycine. None of the mutants were structurally stable compared to the respective wild type model, except for mutants V2G and V6G in MVGGVV2 case. The exception can be explained by structural features of this particular polymorph. The double layer decamer and dodecamer aggregates of the wild type hexapeptides appear to be stable at 300K, which is confirmed by the conservation of high anti-parallel β-sheet content throughout the whole simulation time. Deletions of the side chains resulted in decline of secondary structure content compared to corresponding wild type indicating that the role of the replaced amino acid in stabilizing the structure. Detailed analysis of the binding energy reveals that stability of these peptide aggregates is determined mainly by the van der Waals and hydrophobic forces that can serve as quantitative measure of shape complementarities between the side chains. This observation implies that interactions among side chains forming the dehydrated steric zipper, rather than among those exposed to water, are the major structural determinant. The electrostatic repulsion destabilizes the studied double layer aggregates in two cases, while stabilizes the other two. Negative total binding free energy indicates that both wild type and mutants complex formation is favorable. However, the mutants complexation is less favorable than the wild type's. The present study provides the atomic level understanding of the aggregation behavior and the driving force for the amyloid aggregates, and could be useful for rational design of amyloid inhibitors and amyloid-specific biomarkers for diagnostic purposes.

The structures and stabilities of the complexes of biologically available ligands with Fe(III)-porphine: an ab initio study

Fe(III) porphine complexes and a limited number of Fe(II) porphine complexes were investigated at the 'MP2/LB'//B3LYP/SB level of theory, where SB and LB represent small and large basis sets, 6-31+G(d) and 6-311+G(2df,2p), respectively. Solvation effects were incorporated by the IEFPCM procedure. Most of the ligands whereby the heme prosthetic group is bound in biological systems were modeled in the study. These include H(2)O, Im (imidazole), CH(3)NH(2), CH(3)CO(2)(-), CH(3)S(-), CH(3)PhO(-), OH(-), and Cl(-). Fe(III) porphine, 2(+), and the pentacoordinated complexes, 2(+)(Im), 2(+)(CH(3)NH(2)), and 2(+)(H(2)O), have quartet ground states. The pentacoordinated complexes with negatively charged ligands all have high spin hextet ground states. All of the hexacoordinated complexes have low spin doublet ground states, with the exception of 2(+)(H(2)O)(2) and 2(+)(H(2)O)(Im) which have intermediate spin quartet ground states. None of the pentacoordinated complexes, 2(+)(OH(-)), 2(+)(CH(3)PhO(-)), and 2(+)(CH(3)S(-)), are predicted to form stable hexacoordinated complexes in water with any of the ligands of the present study. The most stable species in water is 2(+)(OH(-)). The hydroxide may be displaced by CH(3)PhOH and CH(3)SH at physiological pH, and by Cl(-), CH(3)CO(2)(-), and Im under acidic conditions, but not by CH(3)NH(3)(+). The relevance of the present results for the pH-dependent transitions of cytochrome c and the fragments, NAcMP8, and NAcMP11, the resting state of cytochrome P450, and the bonding interactions between heme and Aβ, is discussed.

Recent Development of Bifunctional Small Molecules to Study Metal-Amyloid-β Species in Alzheimer's Disease

Alzheimer's disease (AD) is a multifactorial neurodegenerative disease related to the deposition of aggregated amyloid-β (Aβ) peptides in the brain. It has been proposed that metal ion dyshomeostasis and miscompartmentalization contribute to AD progression, especially as metal ions (e.g., Cu(II) and Zn(II)) found in Aβ plaques of the diseased brain can bind to Aβ and be linked to aggregation and neurotoxicity. The role of metal ions in AD pathogenesis, however, is uncertain. To accelerate understanding in this area and contribute to therapeutic development, recent efforts to devise suitable chemical reagents that can target metal ions associated with Aβ have been made using rational structure-based design that combines two functions (metal chelation and Aβ interaction) in the same molecule. This paper presents bifunctional compounds developed by two different design strategies (linkage or incorporation) and discusses progress in their applications as chemical tools and/or potential therapeutics.

Design of small molecules that target metal-A{beta} species and regulate metal-induced A{beta} aggregation and neurotoxicity

The accumulation of metal ions and amyloid-β (Aβ) aggregates found in the brain of patients with Alzheimer's disease (AD) has been suggested to be involved in AD pathogenesis. To investigate metal-Aβ-associated pathways in AD, development of chemical tools to target metal-Aβ species is desired. Only a few efforts, however, have been reported. Here, we report bifunctional small molecules, N-(pyridin-2-ylmethyl)aniline (L2-a) and N(1),N(1)-dimethyl-N(4)-(pyridin-2-ylmethyl)benzene-1,4-diamine (L2-b) that can interact with both metal ions and Aβ species, as determined by spectroscopic methods including high-resolution NMR spectroscopy. Using the bifunctional compound L2-b, metal-induced Aβ aggregation and neurotoxicity were modulated in vitro as well as in human neuroblastoma cells. Furthermore, treatment of human AD brain tissue homogenates containing metal ions and Aβ species with L2-b showed disassembly of Aβ aggregates. Therefore, our studies presented herein demonstrate the value of bifunctional compounds as chemical tools for investigating metal-Aβ-associated events and their mechanisms in the development and pathogenesis of AD and as potential therapeutics.

Hypothesis: soluble aβ oligomers in association with redox-active metal ions are the optimal generators of reactive oxygen species in Alzheimer's disease

Considerable evidence points to oxidative stress in the brain as an important event in the early stages of Alzheimer's disease (AD). The transition metal ions of Cu, Fe, and Zn are all enriched in the amyloid cores of senile plaques in AD. Those of Cu and Fe are redox active and bind to Aβ in vitro. When bound, they can facilitate the reduction of oxygen to hydrogen peroxide, and of the latter to the hydroxyl radical. This radical is very aggressive and can cause considerable oxidative damage. Recent research favours the involvement of small, soluble oligomers as the aggregating species responsible for Aβ neurotoxicity. We propose that the generation of reactive oxygen species (i.e., hydrogen peroxide and hydroxyl radicals) by these oligomers, in association with redox-active metal ions, is a key molecular mechanism underlying the pathogenesis of AD and some other neurodegenerative disorders.

Atomic-level characterization of the ensemble of the Aβ(1-42) monomer in water using unbiased molecular dynamics simulations and spectral algorithms

Aβ(1-42) is the highly pathologic isoform of amyloid-β, the peptide constituent of fibrils and neurotoxic oligomers involved in Alzheimer's disease. Recent studies on the structural features of Aβ in water have suggested that the system can be described as an ensemble of distinct conformational species in fast exchange. Here, we use replica exchange molecular dynamics (REMD) simulations to characterize the conformations accessible to Aβ42 in explicit water solvent, under the ff99SB force field. Monitoring the correlation between J-coupling((3)J(H(N))(H(α))) and residual dipolar coupling (RDC) data calculated from the REMD trajectories to their experimental values, as determined by NMR, indicates that the simulations converge towards sampling an ensemble that is representative of the experimental data after 60 ns/replica of simulation time. We further validate the converged MD-derived ensemble through direct comparison with (3)J(H(N))(H(α)) and RDC experimental data. Our analysis indicates that the ff99SB-derived REMD ensemble can reproduce the experimental J-coupling values with high accuracy and further provide good agreement with the RDC data. Our results indicate that the peptide is sampling a highly diverse range of conformations: by implementing statistical learning techniques (Laplacian eigenmaps, spectral clustering, and Laplacian scores), we are able to obtain an otherwise hidden structure in the complex conformational space of the peptide. Using these methods, we characterize the peptide conformations and extract their intrinsic characteristics, identify a small number of different conformations that characterize the whole ensemble, and identify a small number of protein interactions (such as contacts between the peptide termini) that are the most discriminative of the different conformations and thus can be used in designing experimental probes of transitions between such molecular states. This is a study of an important intrinsically disordered peptide system that provides an atomic-level description of structural features and interactions that are relevant during the early stages of the oligomerization and fibril nucleation pathways.

Hydroxyquinoline based binders: promising ligands for chelatotherapy?

We report here a thorough physico-chemical study of the coordination properties of clioquinol, an oxine-type active neurological drug in Alzheimer's disease, toward biologically relevant divalent metal ions (Cu, Zn, Ni, Co and Mn). Using a fruitful combination of electrospray mass spectrometry, absorption spectrophotometry and potentiometry, we have characterized the mono- and bis-chelated metal ion species. The determination of the stability constants showed a classical thermodynamic behavior along the studied series with the cupric complexes being by far the most stable species. Our data are discussed within the scope of Alzheimer's disease.

Electrochemical oxidation of benzothiazole dyes for monitoring amyloid formation related to the Alzheimer's disease

Alzheimer's disease (AD) is associated with the formation and deposition of amyloid fibrils. A better understanding of the oligomeric intermediates on the pathway to fibrilization is highly desired, but efficient methods for their detection are lacking. We have studied the interfacial properties of amyloid peptides (Abeta-40 and Abeta-42) and the course of their aggregation in vitro in the presence of the benzothiazole dyes Thioflavin T (4-(3,6-dimethyl-1,3-benzothiazol-3-ium-2-yl)-N,N-dimethylaniline) chloride, ThT) and BTA-1 ([2-(4'-(methylamino)phenyl) benzothiazole]) using electrochemical techniques. The intercalative properties of these dyes between the beta-sheets of amyloids have been well-documented using fluorescence-based systems, but their electrochemistry is reported here for the first time. ThT is positively charged and water-soluble, whereas BTA-1 is neutral and hydrophobic. Immediate and significantly different electrochemical characteristics of these dyes were observed in the presence of amyloid peptides. A decrease of the BTA-1 oxidation signal was observed upon incubation with Abeta-40. Incubation of BTA-1 with Abeta-42 results in an increased rate of exponential decay, which was in agreement with the known rapid aggregation properties of Abeta-42. The aggregation of amyloid peptides with ThT resulted in an unexpected increase in signal after 24 h of incubation, consistent for both peptides. The results of the electrochemical trials were confirmed using simultaneous fluorescence analysis of the same incubated amyloid samples. The very early changes in the interfacial behavior of the amyloid peptides after the first few minutes of incubation were attributed to the fast oligomerization of the peptides with the disruption of the intercalative properties of the benzothiazole dyes between the beta-sheets. The subsequent changes in the electrochemical signals can be related to the onset of intercalation between the fibrils. Our results demonstrate the utility of electrochemical oxidation signals of the benzothiazole dyes as a new and simple tool for the investigation of amyloid formation related to the AD.

Small molecule modulators of copper-induced Abeta aggregation

Our design of bifunctional metal chelators as chemical probes and potential therapeutics for Alzheimer's disease (AD) is based on the incorporation of a metal binding moiety into structural frameworks of Abeta aggregate-imaging agents. Using this strategy, two compounds 2-[4-(dimethylamino)phenyl]imidazo[1,2-a]pyridine-8-ol (1) and N(1),N(1)-dimethyl-N(4)-(pyridin-2-ylmethylene)benzene-1,4-diamine (2) were prepared and characterized. The bifunctionality for metal chelation and Abeta interaction of 1 and 2 was verified by spectroscopic methods. Furthermore, the reactivity of 1 and 2 with Cu(II)-associated Abeta aggregates was investigated. The modulation of Cu(II)-triggered Abeta aggregation by 1 and 2 was found to be more effective than that by the known metal chelating agents CQ, EDTA, and phen. These studies suggest a new class of multifunctional molecules for the development of chemical tools to unravel metal-associated events in AD and potential therapeutic agents for metal-ion chelation therapy.