The catalytically active copper-amyloid-Beta state: coordination site responsible for reactive oxygen species production

Prion Peptides Are Extremely Sensitive to Copper Induced Oxidative Stress

Copper(II) binding to prion peptides does not prevent Cu redox cycling and formation of reactive oxygen species (ROS) in the presence of reducing agents. The toxic effects of these species are exacerbated in the presence of catecholamines, indicating that dysfunction of catecholamine vesicular sequestration or recovery after synaptic release is a dangerous amplifier of Cu induced oxidative stress. Cu bound to prion peptides including the high affinity site involving histidines adjacent to the octarepeats exhibits marked catalytic activity toward dopamine and 4-methylcatechol. The resulting quinone oxidation products undergo parallel oligomerization and endogenous peptide modification yielding catechol adducts at the histidine binding ligands. These modifications add to the more common oxidation of Met and His residues produced by ROS. Derivatization of Cu-prion peptides is much faster than that undergone by Cu-β-amyloid and Cu-α-synuclein complexes in the same conditions.

Identification of key structural features of the elusive Cu-Aβ complex that generates ROS in Alzheimer's disease

Oxidative stress is linked to the etiology of Alzheimer's disease (AD), the most common cause of dementia in the elderly. Redox active metal ions such as copper catalyze the production of Reactive Oxygen Species (ROS) when bound to the amyloid-β (Aβ) peptide encountered in AD. We propose that this reaction proceeds through a low-populated Cu-Aβ state, denoted the "catalytic in-between state" (CIBS), which is in equilibrium with the resting state (RS) of both Cu(i)-Aβ and Cu(ii)-Aβ. The nature of this CIBS is investigated in the present work. We report the use of complementary spectroscopic methods (X-ray absorption spectroscopy, EPR and NMR) to characterize the binding of Cu to a wide series of modified peptides in the RS. ROS production by the resulting Cu-peptide complexes was evaluated using fluorescence and UV-vis based methods and led to the identification of the amino acid residues involved in the Cu-Aβ CIBS species. In addition, a possible mechanism by which the ROS are produced is also proposed. These two main results are expected to affect the current vision of the ROS production mechanism by Cu-Aβ but also in other diseases involving amyloidogenic peptides with weakly structured copper binding sites.

The double-edged role of copper in the fate of amyloid beta in the presence of anti-oxidants

The biological fate of amyloid beta (Aβ) species is a fundamental question in Alzheimer's disease (AD) pathogenesis. The competition between clearance and aggregation of Aβs is critical for the onset of AD. Copper has been widely considered to be an inducer of harmful crosslinking of Aβs, and an important triggering factor for the onset of AD. In this report, however, we present data to show that copper can also be an inducer of Aβ degradation in the presence of a large excess of well-known intrinsic (such as dopamine) or extrinsic (such as vitamin C) anti-oxidants. The degraded fragments were identified using SDS-Page gels, and validated via nanoLC-MS/MS. A tentative mechanism for the degradation was proposed and validated with model peptides. In addition, we performed electrophysiological analysis to investigate the synaptic functions in brain slices, and found that in the presence of a significant excess of vitamin C, Cu(ii) could prevent an Aβ-induced deficit in synaptic transmission in the hippocampus. Collectively, our evidence strongly indicated that a proper combination of copper and anti-oxidants might have a positive effect on the prevention of AD. This double-edged function of copper in AD has been largely overlooked in the past. We believe that our report is very important for fully understanding the function of copper in AD pathology.

Catalytic Amyloid Fibrils That Bind Copper to Activate Oxygen

Amyloid-like fibrils assembled from de novo designed peptides lock ligands in a conformation optimal for metal binding and catalysis in a manner similar to how metalloenzymes provide proper coordination environment through fold. These supramolecular assemblies efficiently catalyze p-nitrophenyl ester hydrolysis in the presence of zinc and phenol oxidation by dioxygen in the presence of copper. The resulting heterogeneous catalysts are inherently switchable, as addition and removal of the metal ions turns the catalytic activity on and off, respectively. The ease of peptide preparation and self-assembly makes amyloid-like fibrils an attractive platform for developing catalysts for a broad range of chemical reactions. Here, we present a detailed protocol for the preparation of copper-containing fibrils and for kinetic characterization of their abilities to oxidize phenols.

Receptor-mediated toxicity of human amylin fragment aggregated by short- and long-term incubations with copper ions

Human amylin (hA1-37) is a polypeptide hormone secreted in conjunction with insulin from the pancreatic β-cells involved in the pathogenesis of type 2 diabetes mellitus (T2DM). The shorter fragment hA17-29 than full-length peptide is capable to form amyloids "in vitro". Here, we monitored the time course of hA17-29 β-amyloid fibril and oligomer formation [without and with copper(II)], cellular toxicity of different amyloid aggregates, and involvement of specific receptors (receptor for advanced glycation end-products, RAGE; low-affinity nerve growth factor receptor, p75-NGFR) in aggregate toxicity. Fibril and oligomer formation of hA17-29 incubated at 37 °C for 0, 48, and 120 h, without or with copper(II), were measured by the thioflavin T fluorescence assay and ELISA, respectively. Toxicity of hA17-29 aggregates and effects of anti-RAGE and anti-p75-NGFR antibodies were evaluated on neuroblastoma SH-SY5Y viability. Fluorescence assay of hA17-29 indicates an initial slow rate of soluble fibril formation (48 h), followed by a slower rate of insoluble aggregate formation (120 h). The highest quantity of oligomers was recorded when hA17-29 was pre-aggregated for 48 h in the presence of copper(II) showing also the maximal cell toxicity (-44% of cell viability, p < 0.01 compared to controls). Anti-RAGE or anti-p75-NGFR antibodies almost abolished cell toxicity of hA17-29 aggregates. These results indicate that copper(II) influences the aggregation process and hA17-29 toxicities are especially attributable to oligomeric aggregates. hA17-29 aggregate toxicity seems to be mediated by RAGE and p75-NGFR receptors which might be potential targets for new drugs in T2DM treatment.

Co-assembly of polyoxometalates and peptides towards biological applications

The synergistic self-assembly of biomolecules with polyoxometalates (POMs) has recently been considered as an effective approach to construct nano-biomaterials with diverse structures and morphologies towards applications in drug delivery, controlled release, tissue engineering scaffolds, and biomineralization, due to the unique features of the clusters in addition to many well-known inorganic nanoparticles. This review presents an overview of recent work focusing on the noncovalent co-assembly of peptides and POMs as well as their biological applications. In the co-assemblies triggered by the interaction between the components significant advantages are observed that POMs or peptides alone do not possess; examples include chiral recognition of hybrid metal oxides, the quick hydrolysis of peptides, and enhanced inhibition of Aβ aggregation. Finally, we outline a brief perspective on possible unresolved issues and future opportunities in this field.

Copper-Aβ Peptides and Oxidation of Catecholic Substrates: Reactivity and Endogenous Peptide Damage

The oxidative reactivity of copper complexes with Aβ peptides 1-16 and 1-28 (Aβ16 and Aβ28) against dopamine and related catechols under physiological conditions has been investigated in parallel with the competitive oxidative modification undergone by the peptides. It was found that both Aβ16 and Aβ28 markedly increase the oxidative reactivity of copper(II) towards the catechol compounds, up to a molar ratio of about 4:1 of peptide/copper(II). Copper redox cycling during the catalytic activity induces the competitive modification of the peptide at selected amino acid residues. The main modifications consist of oxidation of His13/14 to 2-oxohistidine and Phe19/20 to ortho-tyrosine, and the formation of a covalent His6-catechol adduct. Competition by the endogenous peptide is rather efficient, as approximately one peptide molecule is oxidized every 10 molecules of 4-methylcatechol.

Natural Tripeptide-Based Inhibitor of Multifaceted Amyloid β Toxicity

Accumulation of amyloid beta (Aβ) peptide and its aggregates in the human brain is considered as one of the hallmarks of Alzheimer's disease (AD). The polymorphic oligomers and fully grown fibrillar aggregates of Aβ exhibit different levels of neuronal toxicity. Moreover, aggregation of Aβ in the presence of redox-active metal ions like Cu(2+) is responsible for the additional trait of cellular toxicity induced by the generation of reactive oxygen species (ROS). Herein, a multifunctional peptidomimetic inhibitor (P6) has been presented, based on a naturally occurring metal chelating tripeptide (GHK) and the inhibitor of Aβ aggregation. It was shown by employing various biophysical studies that P6 interact with Aβ and prevent the formation of toxic Aβ forms like oligomeric species and fibrillar aggregates. Further, P6 successfully sequestered Cu(2+) from the Aβ-Cu(2+) complex and maintained it in a redox-dormant state to prevent the generation of ROS. P6 inhibited membrane disruption by Aβ oligomers and efficiently prevented DNA damage caused by the Aβ-Cu(2+) complex. PC12 cells were rescued from multifaceted Aβ toxicity when treated with P6, and the amount of ROS generated in cells was reduced. These attributes make P6 a potential therapeutic candidate to ameliorate the multifaceted Aβ toxicity in AD.

Short Self-Assembling Peptides Are Able to Bind to Copper and Activate Oxygen

We have shown that de novo designed peptides self-assemble in the presence of copper to create supramolecular assemblies capable of carrying out the oxidation of dimethoxyphenol in the presence of dioxygen. Formation of the supramolecular assembly, which is akin to a protein fold, is critical for productive catalysis since peptides possessing the same functional groups but lacking the ability to self-assemble do not catalyze substrate oxidation. The ease with which we have discovered robust and productive oxygen activation catalysts suggests that these prion-like assemblies might have served as intermediates in the evolution of enzymatic function and opens the path for the development of new catalyst nanomaterials.

Methionine oxidation reduces lag-times for amyloid-β(1-40) fiber formation but generates highly fragmented fibers

Oxidative stress and the formation of amyloid plaques containing amyloid-β (Aβ) peptides are two key hallmarks of Alzheimer's disease. A proportion of methionine (Met) at position 35 within Aβ is oxidized to methionine sulphoxide (Met(OX)) within the Alzheimer's plaques. These oxidative processes may be the key to understanding the early stages of Alzheimer's disease. In vitro oxidation of Aβ, by the physiological oxidant H2O2, was monitored using (1)H NMR and mass spectrometry. Here we investigate the effect of Aβ methionine oxidation on fiber formation kinetics and morphology using the amyloid specific fluorescence dye Thioflavin T (ThT) and Transmission Electron Microscopy (TEM). Methionine oxidation reduces the total amount of fibers generated for both dominant forms of Aβ, however there are marked differences in the effect of Met(OX) between Aβ(1-40) and Aβ(1-42). Surprisingly the presence of Met(OX) reduces lag-times for Aβ(1-40) fiber formation but extends lag-times for Aβ(1-42). TEM indicates a change in fiber morphology with a pronounced reduction in fiber length for both methionine oxidized Aβ(1-40) and Aβ(1-42). In contrast, the morphology of preformed amyloid fibers is largely unaffected by the presence of H2O2. Our studies suggest that methionine oxidation promotes highly fragmented fiber assemblies of Aβ. Oxidative stress associated with Alzheimer's disease can cause oxidation of methionine within Aβ and this in turn will influence the complex assembly of Aβ monomer into amyloid fibers, which is likely to impact Aβ toxicity.

Amyloid-β and α-Synuclein Decrease the Level of Metal-Catalyzed Reactive Oxygen Species by Radical Scavenging and Redox Silencing

The formation of reactive oxygen species (ROS) is linked to the pathogenesis of neurodegenerative diseases. Here we have investigated the effect of soluble and aggregated amyloid-β (Aβ) and α-synuclein (αS), associated with Alzheimer’s and Parkinson’s diseases, respectively, on the Cu²⁺-catalyzed formation of ROS in vitro in the presence of a biological reductant. We find that the levels of ROS, and the rate by which ROS is generated, are significantly reduced when Cu²⁺ is bound to Aβ or αS, particularly when they are in their oligomeric or fibrillar forms. This effect is attributed to a combination of radical scavenging and redox silencing mechanisms. Our findings suggest that the increase in ROS associated with the accumulation of aggregated Aβ or αS does not result from a particularly ROS-active form of these peptides, but rather from either a local increase of Cu²⁺ and other ROS-active metal ions in the aggregates or as a downstream consequence of the formation of the pathological amyloid structures.

Ultraviolet irradiation-mediated formation of Aβ42oligomers and reactive oxygen species in Zn2+-bound Aβ42aggregates irrespective of the removal of Zn2+

The controlled UV light exposure converts redox-inert Zn²⁺-bound Aβ₄₂aggregates into cytotoxic Aβ₄₂oligomers and reactive oxygen species.

On the generation of OH(·) radical species from H2O2 by Cu(I) amyloid beta peptide model complexes: a DFT investigation

According to different studies, the interaction between amyloid β-peptide (Aβ) and copper ions could yield radical oxygen species production, in particular the highly toxic hydroxyl radical OH(·) that is suspected to contribute to Alzheimer's disease pathogenesis. Despite intensive experimental and computational studies, the nature of the interaction between copper and Aβ peptide, as well as the redox reactivity of the system, are still matter of debate. It was proposed that in Cu(II) → Cu(I) reduction the complex Cu(II)-Aβ could follow a multi-step conformational change with redox active intermediates that may be responsible for OH(·) radical production from H2O2 through a Fenton-like process. The purpose of this work is to evaluate, using ab initio Density Functional Theory computations, the reactivity of different Cu(I)-Aβ coordination modes proposed in the literature, in terms of OH(·) production. For each coordination model, we considered the corresponding H2O2 adduct and performed a potential energy surface scan along the reaction coordinate of O-O bond dissociation of the peroxide, resulting in the production of OH(·) radical, obtaining reaction profiles for the evaluation of the energetic of the process. This procedure allowed us to confirm the hypothesis according to which the most populated Cu(I)-Aβ two-histidine coordination is not able to perform efficiently H2O2 reduction, while a less populated three-coordinated form would be responsible for the OH(·) production. We show that coordination modes featuring a third nitrogen containing electron-donor ligand (an imidazole ring of an histidine residue is slightly favored over the N-terminal amine group) are more active towards H2O2 reduction.

Free Superoxide is an Intermediate in the Production of H2O2 by Copper(I)-Aβ Peptide and O2

Oxidative stress is considered as an important factor and an early event in the etiology of Alzheimer's disease (AD). Cu bound to the peptide amyloid-β (Aβ) is found in AD brains, and Cu-Aβ could contribute to this oxidative stress, as it is able to produce in vitro H2O2 and HO˙ in the presence of oxygen and biological reducing agents such as ascorbate. The mechanism of Cu-Aβ-catalyzed H2O2 production is however not known, although it was proposed that H2O2 is directly formed from O2 via a 2-electron process. Here, we implement an electrochemical setup and use the specificity of superoxide dismutase-1 (SOD1) to show, for the first time, that H2O2 production by Cu-Aβ in the presence of ascorbate occurs mainly via a free O2˙(-) intermediate. This finding radically changes the view on the catalytic mechanism of H2O2 production by Cu-Aβ, and opens the possibility that Cu-Aβ-catalyzed O2˙(-) contributes to oxidative stress in AD, and hence may be of interest.

Copper(I/II), α/β-Synuclein and Amyloid-β: Menage à Trois?

Copper binding to α-synuclein (aS) and to amyloid-β (Ab) has been connected to Parkinson's and Alzheimer's disease (AD), respectively, because Cu ions can modulate the peptide aggregation, and these Cu ⋅ peptide complexes can catalyse the production of reactive oxygen species (ROS). In a significant proportion of AD brains, aggregation of aS and Ab has been detected, and it was proposed that Ab and aS interact with each other. Thus, we investigated the potential interactions of Ab and aS through their binding of copper(I) and copper(II). Additionally, β-synuclein (bS) was investigated, due to its additional methionine residue, a potential Cu(I) ligand. We found that: 1) the peptides containing the Cu-binding domains Ab1-16, aS1-15 and bS1-15 have similar affinities towards Cu(II) and towards Cu(I), with Ab1-16 being slightly stronger, 2) in the case of Cu(I), the additional Met residue in bS1-15 increased the affinity slightly, 3) the exchange of Cu(I/II) between the two peptides is rapid (≤ ms), 4) a/bS1-15 and Ab1-16 form a heterodimeric complex with Cu(II), 5) Cu(I) probably promotes a transient ternary complex, 6) the different Cu(I/II) coordination of Ab1-16, aS1-15 and bS1-15 impacts the capacity to produce ROS and to oxidise catechol, and 7) when Ab1-16, aS1-15 and Cu are present, the ROS production more closely resembles that by Ab1-16. The work gives insights into the coordination chemistry of these related peptides, and the relevance of coordination differences, the ternary complex and ROS production are discussed.

Ultraviolet light triggers the conversion of Cu2+-bound Aβ42 aggregates into cytotoxic species in a copper chelation-independent manner

Increasing evidence indicates that abnormal Cu2+ binding to Aβ peptides are responsible for the formation of soluble Aβ oligomers and ROS that play essential roles in AD pathogenesis. During studying the Cu2+-chelating treatment of Cu2+-bound Aβ42 aggregates, we found that UV light exposure pronouncedly enhances cytotoxicity of the chelator-treated and -untreated Cu2+-bound Aβ42 aggregates. This stimulated us to thoroughly investigate (1) either the chelation treatment or UV light exposure leads to the increased cytotoxicity of the aggregates, and (2) why the chelator-treated and -untreated Cu2+-bound Aβ42 aggregates exhibit the increased cytotoxicity following UV light exposure if the latter is the case. The data indicated that the controlled UV exposure induced the dissociation of Cu2+-free and -bound Aβ42 aggregates into SDS-stable soluble oligomers and the production of ROS including H2O2 in an UV light intensity- and time-dependent, but Cu2+ chelation-independent manner. Although we can't fully understand the meaning of this finding at the current stage, the fact that the UV illuminated Aβ42 aggregates can efficiently kill HeLa cells implies that the aggregates after UV light exposure could be used to decrease the viability of skin cancer cells through skin administration.

Effect of methionine-35 oxidation on the aggregation of amyloid-β peptide

Aggregation of Aβ peptides into amyloid plaques is considered to trigger the Alzheimer’s disease (AD), however the mechanism behind the AD onset has remained elusive. It is assumed that the insoluble Aβ aggregates enhance oxidative stress (OS) by generating free radicals with the assistance of bound copper ions. The aim of our study was to establish the role of Met35 residue in the oxidation and peptide aggregation processes. Met35 can be readily oxidized by H₂O₂. The fibrillization of Aβ with Met35 oxidized to sulfoxide was three times slower compared to that of the regular peptide. The fibrils of regular and oxidized peptides looked similar under transmission electron microscopy. The relatively small inhibitory effect of methionine oxidation on the fibrillization suggests that the possible variation in the Met oxidation state should not affect the in vivo plaque formation. The peptide oxidation pattern was more complex when copper ions were present: addition of one oxygen atom was still the fastest process, however, it was accompanied by multiple unspecific modifications of peptide residues. Addition of copper ions to the Aβ with oxidized Met35 in the presence of H₂O₂, resulted a similar pattern of nonspecific modifications, suggesting that the one-electron oxidation processes in the peptide molecule do not depend on the oxidation state of Met35 residue. Thus, it can be concluded that Met35 residue is not a part of the radical generating mechanism of Aβ–Cu(II) complex.

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Aβ peptides with oxidized Met35 residue fibrillize three times slower than the reduced peptide.

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Met35 is the only residue in Aβ peptide that is oxidized by H₂O₂ in the absent of copper ions.

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In the presence of copper ions as catalyst multiple unspecific oxidative processes occur in Aβ.

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Previous oxidation of Met35 does not affect the unspecific oxidation in the presence of copper ions.

Few-layer bismuth selenides exfoliated by hemin inhibit amyloid-β1-42 fibril formation

Inhibiting amyloid-β (Aβ) fibril formation is the primary therapeutic strategy for Alzheimer's disease. Several small molecules and nanomaterials have been used to inhibit Aβ fibril formation. However, insufficient inhibition efficiency or poor metabolization limits their further applications. Here, we used hemin to exfoliate few-layer Bi(2)Se(3) in aqueous solution. Then we separated few-layer Bi(2)Se(3) with different sizes and thicknesses by fractional centrifugation, and used them to attempt to inhibit Aβ(1-42) aggregation. The results show that smaller and thinner few-layer Bi(2)Se(3) had the highest inhibition efficiency. We further investigated the interaction between few-layer Bi(2)Se(3) and Aβ(1-42) monomers. The results indicate that the inhibition effect may be due to the high adsorption capacity of few-layer Bi(2)Se(3) for Aβ(1-42) monomers. Few-layer Bi(2)Se(3) also decreased Aβ-mediated peroxidase-like activity and cytotoxicity according to in vitro neurotoxicity studies under physiological conditions. Therefore, our work shows the potential for applications of few-layer Bi(2)Se(3) in the biomedical field.

Function and toxicity of amyloid beta and recent therapeutic interventions targeting amyloid beta in Alzheimer's disease

Our Feature Article details the physiological role of amyloid beta (Aβ), elaborates its toxic effects and outlines therapeutic molecules designed in the last two years targeting different aspects of Aβ for preventing AD.

Microspectroscopy (μFTIR) reveals co-localization of lipid oxidation and amyloid plaques in human Alzheimer disease brains

Amyloid peptides are the main component of one of the characteristic pathological hallmarks of Alzheimer's disease (AD): senile plaques. According to the amyloid cascade hypothesis, amyloid peptides may play a central role in the sequence of events that leads to neurodegeneration. However, there are other factors, such as oxidative stress, that may be crucial for the development of the disease. In the present paper, we show that it is possible, by using Fourier tranform infrared (FTIR) microscopy, to co-localize amyloid deposits and lipid peroxidation in tissue slides from patients affected by Alzheimer's disease. Plaques and lipids can be analyzed in the same sample, making use of the characteristic infrared bands for peptide aggregation and lipid oxidation. The results show that, in samples from patients diagnosed with AD, the plaques and their immediate surroundings are always characterized by the presence of oxidized lipids. As for samples from non-AD individuals, those without amyloid plaques show a lower level of lipid oxidation than AD individuals. However, it is known that plaques can be detected in the brains of some non-AD individuals. Our results show that, in such cases, the lipid in the plaques and their surroundings display oxidation levels that are similar to those of tissues with no plaques. These results point to lipid oxidation as a possible key factor in the path that goes from showing the typical neurophatological hallmarks to suffering from dementia. In this process, the oxidative power of the amyloid peptide, possibly in the form of nonfibrillar aggregates, could play a central role.

The Necessity of Having a Tetradentate Ligand to Extract Copper(II) Ions from Amyloids

The accumulation of redox-active metal ions, in particular copper, in amyloid plaques is considered to the cause of the intensive oxidation damage to the brain of patients with Alzheimers disease (AD). Drug candidates based on a bis(8-aminoquinoline) tetradentate ligand are able to efficiently extract Cu(2+) from copper-loaded amyloids (Cu-Aβ). Contrarily, in the presence of a bidentate hydroxyquinoline, such as clioquinol, the copper is not released from Aβ, but remains sequestrated within a Aβ-Cu-clioquinol ternary complex that has been characterized by mass spectrometry. Facile extraction of copper(II) at a low amyloid/ligand ratio is essential for the re-introduction of copper in regular metal circulation in the brain. As, upon reduction, the Cu(+) is easily released from the bis(8-aminoquinoline) ligand unable to accommodate Cu(I), it should be taken by proteins with an affinity for copper. So, the tetradentate bis(8-aminoquinoline) described here might act as a regulator of copper homeostasis.

Bioinorganic chemistry of synucleinopathies: deciphering the binding features of Met motifs and His-50 in AS-Cu(I) interactions

The aggregation of alpha-synuclein (AS) is a critical step in the etiology of Parkinson's disease (PD) and other neurodegenerative synucleinopathies. This process is selectively enhanced by copper in vitro and the interaction is proposed to play a potential role in vivo. Presently, the identity of the Cu(I) binding sites in AS and their relative affinities are under debate. In this work we have addressed unresolved details related to the structural binding specificity and affinity of Cu(I) to full-length AS. We demonstrated conclusively that: (i) the binding preferences of Cu(I) for the Met-binding sites at the N- (Kd=20 μM) and C-terminus (Kd=270 μM) of AS are widely different: (ii) the imidazole ring of His-50 acts as an effective anchoring residue (Kd=50 μM) for Cu(I) binding to AS; and (iii) no major structural rearrangements occur in the protein upon Cu(I) binding. Overall, our work shows that Cu(I) binding to the N- and C-terminal regions of AS are two independent events, with substantial differences in their affinities, and suggest that protein oxidative damage derived from a misbalance in cellular copper homeostasis would target preferentially the N-terminal region of AS. This knowledge is key to understanding the structural-aggregation basis of the copper catalyzed oxidation of AS.

Metal ions and intrinsically disordered proteins and peptides: from Cu/Zn amyloid-β to general principles

The interaction of d-block metal ions (Cu, Zn, Fe, etc.) with intrinsically disordered proteins (IDPs) has gained interest, partly due to their proposed roles in several diseases, mainly neurodegenerative. A prominent member of IDPs is the peptide amyloid-β (Aβ) that aggregates into metal-enriched amyloid plaques, a hallmark of Alzheimer's disease, in which Cu and Zn are bound to Aβ. IDPs are a class of proteins and peptides that lack a unique 3D structure when the protein is isolated. This disordered structure impacts their interaction with metal ions compared with structured metalloproteins. Metalloproteins either have a preorganized metal binding site or fold upon metal binding, resulting in defined 3D structure with a well-defined metal site. In contrast, for Aβ and likely most of the other IDPs, the affinity for Cu(I/II) and Zn(II) is weaker and the interaction is flexible with different coordination sites present. Coordination of Cu(I/II) with Aβ is very dynamic including fast Cu-exchange reactions (milliseconds or less) that are intrapeptidic between different sites as well as interpeptidic. This highly dynamic metal-IDP interaction has a strong impact on reactivity and potential biological role: (i) Due to the low affinity compared with classical metalloproteins, IDPs likely bind metals only at special places or under special conditions. For Aβ, this is likely in the neurons that expel Zn or Cu into the synapse and upon metal dysregulation occurring in Alzheimer's disease. (ii) Amino acid substitutions (mutations) on noncoordinating residues can change drastically the coordination sphere. (iii) Considering the Cu/Zn-Aβ aberrant interaction, therapeutic strategies can be based on removal of Cu/Zn or precluding their binding to the peptide. The latter is very difficult due to the multitude of metal-binding sites, but the fast koff facilitates removal. (iv) The high flexibility of the Cu-Aβ complex results in different conformations with different redox activity. Only some conformations are able to produce reactive oxygen species. (v) Other, more specific catalysis (like enzymes) is very unlikely for Cu/Zn-Aβ. (vi) The Cu/Zn exchange reactions with Aβ are faster than the aggregation process and can hence have a strong impact on this process. In conclusion, the coordination chemistry is fundamentally different for most of IDPs compared with the classical, structured metalloproteins or with (bio)-inorganic complexes. The dynamics is a key parameter to understand this interaction and its potential biological impact.

Chiral metallohelical complexes enantioselectively target amyloid β for treating Alzheimer's disease

Stereochemistry is a very important issue for the pharmaceutical industry and can determine drug efficacy. The design and synthesis of small molecules, especially chiral molecules, which selectively target and inhibit amyloid-β (Aβ) aggregation, represent valid therapeutic strategies for treatment of Alzheimer's disease (AD). Herein we report that two triple-helical dinuclear metallosupramolecular complexes can act as a novel class of chiral amyloid-β inhibitors. Through targeting α/β-discordant stretches at the early steps of aggregation, these metal complexes can enantioselectively inhibit Aβ aggregation, which is demonstrated using fluorescent living cell-based screening and multiple biophysical and biochemical approaches. To the best of our knowledge, this is the first report of enantioselective inhibition of Aβ aggregation. Intriguingly, as a promising candidate for AD treatment, the chiral metal complex can cross the blood-brain barrier and have superoxide dismutase activity. It is well-known that chiral discrimination is important for understanding chiral drug action. Generally, one enantiomer is pharmaceutically active while the other is inactive or exerts severe side effects. Chiral discrimination should be important for AD treatment. Our work provides new insights into chiral inhibition of Aβ aggregation and opens a new avenue for design and screening of chiral agents as Aβ inhibitors against AD.

3D structures and redox potentials of Cu2+-Aβ(1-16) complexes at different pH: a computational study

Oxidative stress induced by redox-active metal cations such as Cu(2+) is a key event in the development of Alzheimer's disease. A detailed knowledge of the structure of Cu(2+)-Aβ complex is thus important to get a better understanding of this critical process. In the present study, we use a computational approach that combines homology modeling with quantum-mechanics-based methods to determine plausible 3D structures of Cu(2+)-Aβ(1-16) complexes that enclose the different metal coordination spheres proposed experimentally at different pH values. With these models in hand, we determine their standard reduction potential (SRP) with the aim of getting new insights into the relation between the structure of these complexes and their redox behavior. Results show that in all cases copper reduction induces CObackbone decoordination, which, for distorted square planar structures in the oxidized state (Ia_δδ, IIa_εδε, IIa_εεε, and IIc_ε), leads to tricoordinated species. For the pentacoordinated structural candidate Ib_δε with Glu11 at the apical position, the reduction leads to a distorted tetrahedral structure. The present results highlight the importance of the nature of the ligands on the SRP. The computed values (with respect to the standard hydrogen electrode) for complexes enclosing negatively charged ligands in the coordination sphere (from -0.81 to -0.12 V) are significantly lower than those computed for models involving neutral ligands (from 0.19 to 0.28 V). Major geometry changes induced by reduction, on both the metal site and the peptide configuration, are discussed as well as their possible influence in the formation of reactive oxygen species.

Short peptides self-assemble to produce catalytic amyloids

Enzymes fold into unique three-dimensional structures, which underlie their remarkable catalytic properties. The requirement to adopt a stable, folded conformation is likely to contribute to their relatively large size (>10,000 Da). However, much shorter peptides can achieve well-defined conformations through the formation of amyloid fibrils. To test whether short amyloid-forming peptides might in fact be capable of enzyme-like catalysis, we designed a series of seven-residue peptides that act as Zn(2+)-dependent esterases. Zn(2+) helps stabilize the fibril formation, while also acting as a cofactor to catalyse acyl ester hydrolysis. These results indicate that prion-like fibrils are able to not only catalyse their own formation, but they can also catalyse chemical reactions. Thus, they might have served as intermediates in the evolution of modern-day enzymes. These results also have implications for the design of self-assembling nanostructured catalysts including ones containing a variety of biological and non-biological metal ions.

Biological metals and metal-targeting compounds in major neurodegenerative diseases

Metals are functionally essential, but redistribute in neurodegenerative disease where they induce protein aggregates, catalyze radical formation, and lose bioavailability.

Identifying, by first-principles simulations, Cu[amyloid-β] species making Fenton-type reactions in Alzheimer's disease

According to the amyloid cascade hypothesis, amyloid-β peptides (Aβ) play a causative role in Alzheimer's disease (AD), of which oligomeric forms are proposed to be the most neurotoxic by provoking oxidative stress. Copper ions seem to play an important role as they are bound to Aβ in amyloid plaques, a hallmark of AD. Moreover, Cu-Aβ complexes are able to catalyze the production of hydrogen peroxide and hydroxyl radicals, and oligomeric Cu-Aβ was reported to be more reactive. The flexibility of the unstructured Aβ peptide leads to the formation of a multitude of different forms of both Cu(I) and Cu(II) complexes. This raised the question of the structure-function relationship. We address this question for the biologically relevant Fenton-type reaction. Computational models for the Cu-Aβ complex in monomeric and dimeric forms were built, and their redox behavior was analyzed together with their reactivity with peroxide. A set of 16 configurations of Cu-Aβ was studied and the configurations were classified into 3 groups: (A) configurations that evolve into a linearly bound and nonreactive Cu(I) coordination; (B) reactive configurations without large reorganization between the two Cu redox states; and (C) reactive configurations with an open structure in the Cu(I)-Aβ coordination, which have high water accessibility to Cu. All the structures that showed high reactivity with H2O2 (to form HO(•)) fall into class C. This means that within all the possible configurations, only some pools are able to produce efficiently the deleterious HO(•), while the other pools are more inert. The characteristics of highly reactive configurations consist of a N-Cu(I)-N coordination with an angle far from 180° and high water crowding at the open side. This allows the side-on entrance of H2O2 and its cleavage to form a hydroxyl radical. Interestingly, the reactive Cu(I)-Aβ states originated mostly from the dimeric starting models, in agreement with the higher reactivity of oligomers. Our study gives a rationale for the Fenton-type reactivity of Cu-Aβ and how dimeric Cu-Aβ could lead to a higher reactivity. This opens a new therapeutic angle of attack against Cu-Aβ-based reactive oxygen species production.