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

Current biomarkers and treatment strategies in Alzheimer disease: An overview and future perspectives

Alzheimer's disease (AD), a progressive degenerative disorder first identified by Alois Alzheimer in 1907, poses a significant public health challenge. Despite its prevalence and impact, there is currently no definitive ante mortem diagnosis for AD pathogenesis. By 2050, the United States may face a staggering 13.8 million AD patients. This review provides a concise summary of current AD biomarkers, available treatments, and potential future therapeutic approaches. The review begins by outlining existing drug targets and mechanisms in AD, along with a discussion of current treatment options. We explore various approaches targeting Amyloid β (Aβ), Tau Protein aggregation, Tau Kinases, Glycogen Synthase kinase-3β, CDK-5 inhibitors, Heat Shock Proteins (HSP), oxidative stress, inflammation, metals, Apolipoprotein E (ApoE) modulators, and Notch signaling. Additionally, we examine the historical use of Estradiol (E2) as an AD therapy, as well as the outcomes of Randomized Controlled Trials (RCTs) that evaluated antioxidants (e.g., vitamin E) and omega-3 polyunsaturated fatty acids as alternative treatment options. Notably, positive effects of docosahexaenoic acid nutriment in older adults with cognitive impairment or AD are highlighted. Furthermore, this review offers insights into ongoing clinical trials and potential therapies, shedding light on the dynamic research landscape in AD treatment.

Electrochemical characterization of Cu(II) complexes of brain-related tau peptides

Metal ion dyshomeostasis plays an important role in diseases, including neurodegeneration. Tau protein is a known neurodegeneration biomarker, but its interactions with biologically relevant metal ions, such as Cu(II), are not fully understood. Herein, the Cu(II) complexes of four tau R peptides, based on the tau repeat domains, R1, R2, R3, and R4, were characterized by electrochemical methods, including cyclic voltammetry, square-wave voltammetry, and differential pulse voltammetry in solution under aerobic conditions. The current and potential associated with Cu(II)/(I) redox couple was modulated as a function of R peptide sequence and concentration. All R peptides coordinated Cu(II) resulting in a dramatic decrease in the current associated with free Cu(II), and the appearance of a new redox couple due to metallo–peptide complex. The metallo–peptide complexes were characterized by the irreversible redox couple at more positive potentials and slower electron-transfer rates compared with the free Cu(II). The competition binding studies between R peptides with Cu(II) indicated that the strongest binding affinity was observed for the R3 peptide, which contained 2 His and 1 Cys residues. The formation of complexes was also evaluated as a function of peptide concentration and in the presence of competing Zn(II) ions. Data indicate that all metallo–peptides remain redox active pointing to the potential importance of the interactions between tau protein with metal ions in a biological setting.

Redox-Dependent Copper Ion Modulation of Amyloid-β (1-42) Aggregation In Vitro

Plaque deposits composed of amyloid-β (Aβ) fibrils are pathological hallmarks of Alzheimer's disease (AD). Although copper ion dyshomeostasis is apparent in AD brains and copper ions are found co-deposited with Aβ peptides in patients' plaques, the molecular effects of copper ion interactions and redox-state dependence on Aβ aggregation remain elusive. By combining biophysical and theoretical approaches, we here show that Cu2+ (oxidized) and Cu+ (reduced) ions have opposite effects on the assembly kinetics of recombinant Aβ(1-42) into amyloid fibrils in vitro. Cu2+ inhibits both the unseeded and seeded aggregation of Aβ(1-42) at pH 8.0. Using mathematical models to fit the kinetic data, we find that Cu2+ prevents fibril elongation. The Cu2+-mediated inhibition of Aβ aggregation shows the largest effect around pH 6.0 but is lost at pH 5.0, which corresponds to the pH in lysosomes. In contrast to Cu2+, Cu+ ion binding mildly catalyzes the Aβ(1-42) aggregation via a mechanism that accelerates primary nucleation, possibly via the formation of Cu+-bridged Aβ(1-42) dimers. Taken together, our study emphasizes redox-dependent copper ion effects on Aβ(1-42) aggregation and thereby provides further knowledge of putative copper-dependent mechanisms resulting in AD.

The Positive Side of the Alzheimer's Disease Amyloid Cross-Interactions: The Case of the Aβ 1-42 Peptide with Tau, TTR, CysC, and ApoA1

Alzheimer's disease (AD) represents a progressive amyloidogenic disorder whose advancement is widely recognized to be connected to amyloid-β peptides and Tau aggregation. However, several other processes likely contribute to the development of AD and some of them might be related to protein-protein interactions. Amyloid aggregates usually contain not only single type of amyloid protein, but also other type of proteins and this phenomenon can be rationally explained by the process of protein cross-seeding and co-assembly. Amyloid cross-interaction is ubiquitous in amyloid fibril formation and so a better knowledge of the amyloid interactome could help to further understand the mechanisms of amyloid related diseases. In this review, we discuss about the cross-interactions of amyloid-β peptides, and in particular Aβ1-42, with other amyloids, which have been presented either as integrated part of Aβ neurotoxicity process (such as Tau) or conversely with a preventive role in AD pathogenesis by directly binding to Aβ (such as transthyretin, cystatin C and apolipoprotein A1). Particularly, we will focus on all the possible therapeutic strategies aiming to rescue the Aβ toxicity by taking inspiration from these protein-protein interactions.

Copper mediated amyloid-β binding to Transthyretin

Transthyretin (TTR), a homotetrameric protein that transports thyroxine and retinol both in plasma and in cerebrospinal (CSF) fluid provides a natural protective response against Alzheimer's disease (AD), modulates amyloid-β (Aβ) deposition by direct interaction and co-localizes with Aβ in plaques. TTR levels are lower in the CSF of AD patients. Zn²⁺, Mn²⁺ and Fe²⁺ transform TTR into a protease able to cleave Aβ. To explain these activities, monomer dissociation or conformational changes have been suggested. Here, we report that when TTR crystals are exposed to copper or iron salts, the tetramer undergoes a significant conformational change that alters the dimer-dimer interface and rearranges residues implicated in TTR's ability to neutralize Aβ. We also describe the conformational changes in TTR upon the binding of the various metal ions. Furthermore, using bio-layer interferometry (BLI) with immobilized Aβ(1-28), we observe the binding of TTR only in the presence of copper. Such Cu²⁺-dependent binding suggests a recognition mechanism whereby Cu²⁺ modulates both the TTR conformation, induces a complementary Aβ structure and may participate in the interaction. Cu²⁺-soaked TTR crystals show a conformation different from that induced by Fe²⁺, and intriguingly, TTR crystals grown in presence of Aβ(1-28) show different positions for the copper sites from those grown its absence.

Cu and Zn coordination to amyloid peptides: From fascinating chemistry to debated pathological relevance

Several diseases share misfolding of different peptides and proteins as a key feature for their development. This is the case of important neurodegenerative diseases such as Alzheimer's and Parkinson's diseases and type II diabetes mellitus. Even more, metal ions such as copper and zinc might play an important role upon interaction with amyloidogenic peptides and proteins, which could impact their aggregation and toxicity abilities. In this review, the different coordination modes proposed for copper and zinc with amyloid-β, α-synuclein and IAPP will be reviewed as well as their impact on the aggregation, and ROS production in the case of copper. In addition, a special focus will be given to the mutations that affect metal binding and lead to familial cases of the diseases. Different modifications of the peptides that have been observed in vivo and could be relevant for the coordination of metal ions are also described.

Copper Binding Induces Nitration of NPY under Nitrative Stress: Complicating the Role of NPY in Alzheimer's Disease

Neuropeptide Y (NPY) is a 36 amino acid peptide that regulates a multitude of physiological functions in the central nervous system and has been shown to be involved in Alzheimer's disease (AD). A change in copper homeostasis is a remarkable feature of AD, and the dysregulation may contribute to toxicity in neural cells. Moreover, it has been shown that copper could interact with many neuropeptides and result in catalyzing the production of reactive oxygen species, which may lead to peptide oxidation. Besides, copper could also catalyze protein tyrosine nitration under oxidative stress, and there are two tyrosine residues playing an important role in NPY. Therefore, it is also likely that copper has an action on NPY and potentially influences its functions through tyrosine nitration. In this paper, the studies of the interaction of copper with NPY and the copper-catalyzed NPY nitration were performed. The electrochemical techniques, UV-vis spectroscopy, mass spectrometry, and fluorescence titration, have been applied to show that copper can interact with NPY to form a Cu-NPY complex with a conditional dissociation constant of 0.021 μmol/L, and the binding promotes the generation of •OH. Dot blotting results reveal that NPY can be nitrated upon binding with copper under nitrative stress. Furthermore, liquid chromatography-mass spectrometry (LC-MS) identify that the tyrosine residues in NPY are all nitrated during the nitration process, which will cause the inactivation of NPY shown by our previous study. This study supports the hypothesis that copper has a close correlation with NPY and implicates the peptide in AD. These data may provide a new insight into understanding the pathology and pathogenesis of AD.

Effect of ferric citrate on amyloid-beta peptides behavior

Amyloid beta (Aβ) aggregation and oxidative stress are two of the central events in Alzheimer's Disease (AD). Both these phenomena can be caused by the interaction of Aβ with metal ions. In the last years the interaction between Zn^II , Cu^II , and Aβ was much studied, but between iron and Aβ it is still little known. In this work we determine how three Aβ peptides, present in AD, interact with Fe^III -citrate. The three Aβ peptides are: full length Aβ1-42, an isoform truncated at Glutamic acid in position three, Aβ3-42, and its pyroglutamated form AβpE3-42. Conformation and morphology of the three peptides, aggregated with and without Fe^III -citrate were studied. Besides, we have determined the strength of the interactions Aβ/Fe^III -citrate studying the effect of ethylenediaminetetraacetic acid as chelator. Results reported here demonstrate that Fe^III -citrate promotes the aggregation in all the three peptides. Moreover, Aspartic acid 1, Glutamic acid 3, and Tyrosine 10 have an important role in the coordination with iron, generating a more stable complex for Aβ1-42 compared to that for the truncated peptides.

Key roles of Tyr 10 in Cu bound Aβ complexes and its relevance to Alzheimer's disease

Recent studies show that the accumulation of redox-active Cu mediates the aggregation of amyloid β-peptide (Aβ) and conspicuous oxidative damage to the brain in Alzheimer's disease (AD). However, the key roles for Tyr 10 in Aβ-Cu(II) complex and its potential biological relevance to AD etiology under oxidative stress, were not stressed enough. Interestingly, our results indicated that Aβ40 (not Aβ16)-Cu(II) complex showed obviously enhanced peroxidase activity than free Cu(II). Although Tyr 10 was not the residue binding Cu(II), the mutation of Tyr 10 residue in Aβ40 decreased the peroxidase activity of Aβ40-Cu(II) complex, and the mutation of Tyr 10 could inhibit Aβ40 aggregation. Under oxidative and nitrative stress conditions, the Aβ-Cu(II) complex caused oxidation and nitration of the Aβ Tyr 10 residue through peroxidase-like reactions, where the formation of Cu(I) and hydroxyl radical (OH) was proposed as a chemical mechanism. We also showed that, when Aβ40 aggregates were bound to Cu(II), they retained peroxidase-like activity. Therefore, Tyr 10 residue is pivotal in Aβ-Cu(II) complex and shows important relevance to oxidative stress, implicating the novel significance of Tyr 10 residue as well as Aβ-Cu(II) complex in the pathology of AD.

The Rich Electrochemistry and Redox Reactions of the Copper Sites in the Cellular Prion Protein

This paper reviews recent electrochemical studies of the copper complexes of prion protein (PrP) and its related peptides, and correlates their redox behavior to chemical and biologically relevant reactions. Particular emphasis is placed on the difference in redox properties between copper in the octarepeat (OR) and the non-OR domains of PrP, as well as differences between the high and low copper occupancy states in the OR domain. Several discrepancies in literature concerning these differences are discussed and reconciled. The PrP copper complexes, in comparison to copper complexes of other amyloidogenic proteins/peptides, display a more diverse and richer redox chemistry. The specific protocols and caveats that need to be considered in studying the electrochemistry and redox reactions of copper complexes of PrP, PrP-derived peptides, and other related amyloidogenic proteins are summarized.

Modeling copper binding to the amyloid-β peptide at different pH: toward a molecular mechanism for Cu reduction

Oxidative stress, including the production of reactive oxygen species (ROS), has been reported to be a key event in the etiology of Alzheimer's disease (AD). Cu has been found in high concentrations in amyloid plaques, a hallmark of AD, where it is bound to the main constituent amyloid-β (Aβ) peptide. Whereas it has been proposed that Cu-Aβ complexes catalyze the production of ROS via redox-cycling between the Cu(I) and Cu(II) state, the redox chemistry of Cu-Aβ and the precise mechanism of redox reactions are still unclear. Because experiments indicate different coordination environments for Cu(II) and Cu(I), it is expected that the electron is not transferred between Cu-Aβ and reactants in a straightforward manner but involves structural rearrangement. In this work the structures indicated by experimental data are modeled at the level of modern density-functional theory approximations. Possible pathways for Cu(II) reduction in different coordination sites are investigated by means of first-principles molecular dynamics simulations in the water solvent and at room temperature. The models of the ligand reorganization around Cu allow the proposal of a preferential mechanism for Cu-Aβ complex reduction at physiological pH. Models reveal that for efficient reduction the deprotonated amide N in the Ala 2-Glu 3 peptide bond has to be protonated and that interactions in the second coordination sphere make important contributions to the reductive pathway, in particular the interaction between COO(-) and NH(2) groups of Asp 1. The proposed mechanism is an important step forward to a clear understanding of the redox chemistry of Cu-Aβ, a difficult task for spectroscopic approaches as the Cu-peptide interactions are weak and dynamical in nature.

Biochemistry of amyloid β-protein and amyloid deposits in Alzheimer disease

Progressive cerebral deposition of the amyloid β-protein (Aβ) in brain regions serving memory and cognition is an invariant and defining feature of Alzheimer disease. A highly similar but less robust process accompanies brain aging in many nondemented humans, lower primates, and some other mammals. The discovery of Aβ as the subunit of the amyloid fibrils in meningocerebral blood vessels and parenchymal plaques has led to innumerable studies of its biochemistry and potential cytotoxic properties. Here we will review the discovery of Aβ, numerous aspects of its complex biochemistry, and current attempts to understand how a range of Aβ assemblies, including soluble oligomers and insoluble fibrils, may precipitate and promote neuronal and glial alterations that underlie the development of dementia. Although the role of Aβ as a key molecular factor in the etiology of Alzheimer disease remains controversial, clinical trials of amyloid-lowering agents, reviewed elsewhere in this book, are poised to resolve the question of its pathogenic primacy.

On the involvement of copper binding to the N-terminus of the amyloid Beta Peptide of Alzheimer's disease: a computational study on model systems

Density functional and second order Moller-Plesset perturbation theoretical methods, coupled with a polarizable continuum model of water, were applied to determine the structures, binding affinities, and reduction potentials of Cu(II) and Cu(I) bound to models of the Asp1, Ala2, His6, and His13His14 regions of the amyloid beta peptide of Alzheimer's disease. The results indicate that the N-terminal Asp binds to Cu(II) together with His6 and either His13 or His14 to form the lower pH Component I of Aβ. Component II of Aβ is the complex between Cu(II) and His6, His13, and His14, to which an amide O (of Ala2) is also coordinated. Asp1 does not bind to Cu(II) if three His residues are attached nor to any Cu(I) species to which one or more His residues are bound. The most stable Cu(I) species is one in which Cu(I) bridges the N_δ of His13 and His14 in a linear fashion. Cu(I) binds more strongly to Aβ than does Cu(II). The computed reduction potential that closely matches the experimental value for Cu(II)/Aβ corresponds to reduction of Component II (without Ala2) to the Cu(I) complex after endergonic attachment of His6.

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.

Electrochemical and homogeneous electron transfers to the Alzheimer amyloid-beta copper complex follow a preorganization mechanism

Deciphering the electron transfer reactivity characteristics of amyloid β-peptide copper complexes is an important task in connection with the role they are assumed to play in Alzheimer's disease. A systematic analysis of this question with the example of the amyloid β-peptide copper complex by means of its electrochemical current-potential responses and of its homogenous reactions with electrogenerated fast electron exchanging osmium complexes revealed a quite peculiar mechanism: The reaction proceeds through a small fraction of the complex molecules in which the peptide complex is "preorganized" so as the distances and angles in the coordination sphere to vary minimally upon electron transfer, thus involving a remarkably small reorganization energy (0.3 eV). This preorganization mechanism and its consequences on the reactivity should be taken into account for reactions involving dioxygen and hydrogen peroxide that are considered to be important in Alzheimer's disease through the production of harmful reactive oxygen species.

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.

Copper(I) and copper(II) binding to β-amyloid 16 (Aβ16) studied by electrospray ionization mass spectrometry

Copper-β-amyloid 16 (Aβ16) complexes were investigated by electrospray ionization mass spectrometry (ESI-MS). Copper(i) and (ii) complexes were formed on-line in a microchip electrospray emitter by using a sacrificial copper electrode as the anode in positive ionization mode. In the presence of ascorbic acid in the peptide solution, the amount of Cu(i)-Aβ16 generated electrochemically was even higher. A kinetic model is proposed to account for the generation of copper complexes. The structure of Cu(i)-Aβ16 was investigated by tandem mass spectrometry (MS/MS), and the binding site of Cu(i) to Aβ16 was identified at the His13, His14 residues. Cu(ii)-Aβ16 was also investigated by MS/MS and, based on the unusual observations of a-ions, the two binding residues of His13 and His14 of Aβ16 to Cu(ii) were also confirmed. This approach provides direct information on Cu(i)-Aβ16 complexes generated in solution from metallic copper and gives evidence that both His13 and His14 are involved in the coordination of both Cu(i)- and Cu(ii)-Aβ16 complexes.

Importance of dynamical processes in the coordination chemistry and redox conversion of copper amyloid-beta complexes

Interaction of Cu ions with the amyloid-beta (Abeta) peptide is linked to the development of Alzheimer's disease; hence, determining the coordination of Cu(I) and Cu(II) ions to Abeta and the pathway of the Cu(I)(Abeta)/Cu(II)(Abeta) redox conversion is of great interest. In the present report, we use the room temperature X-ray absorption near edge structure to show that the binding sites of the Cu(I) and Cu(II) complexes are similar to those previously determined from frozen-solution studies. More precisely, the Cu(I) is coordinated by the imidazole groups of two histidine residues in a linear fashion. However, an NMR study unravels the involvement of all three histidine residues in the Cu(I) binding due to dynamical exchange between several set of ligands. The presence of an equilibrium is also responsible for the complex redox process observed by cyclic voltammetry and evidenced by a concentration-dependent electrochemical response.