Prion Peptides Are Extremely Sensitive to Copper Induced Oxidative Stress

A Focus on the Link Between Metal Dyshomeostasis, Norepinephrine, and Protein Aggregation

Neurodegenerative disorders are one of the main public health problems worldwide and, for this reason, they have attracted the attention of several researchers who aim to better understand the molecular processes linked to the etiology of these disorders, including Alzheimer's and Parkinson's diseases. In this review, we describe both the beneficial and toxic effect of norepinephrine (NE) and its connected ROS/metal-mediated pathways, which end in neuromelanin (NM) formation and protein aggregation. In particular, we emphasize the importance of stabilizing the delicate homeostatic balance that regulates (i) the metal/ROS-promoted oxidation of catecholamines, as NE, and (ii) the generation of oxidative by-products capable of covalently and non-covalently modifying neuroproteins, thus altering their stability and their oligomerization; these processes may end in (iii) the incorporation of protein conjugates into vesicles, which then evolve into neuromelanin (NM) organelles. In general, we aim to provide an up-to-date overview of the challenges and controversies emerging from the current literature to delineate a direction for future research.

Active Site Environment and Reactivity of Copper-Aβ in Membrane Mimetic SDS Micellar Environment

Alzheimer's disease (AD) is characterized by the abnormal aggregation of amyloid β (Aβ) peptide in extracellular deposits generated upon proteolysis of Amyloid Precursor Protein (APP). While copper (Cu(II)) binds to Aβ in soluble oligomeric and aggregated forms, its interaction with membrane-bound Aβ remains elusive. Investigating these interactions is crucial for understanding AD pathogenesis. Here, utilizing SDS micelles as a simplified membrane mimic, we focus on elucidating the interplay between membrane-anchored Aβ and copper, given their pivotal roles in AD. We employed spectroscopic techniques including UV, CD, and EPR to characterize the active site of Cu-Aβ complexes. Our findings demonstrate that copper interacts with Aβ peptides in membrane-mimicking micellar environments similarly to aqueous buffer solutions. Cu-Aβ complexes in this medium also induce higher hydrogen peroxide (H2O2) production, potentially contributing to AD-related oxidative stress. Moreover, we observe an increased oxidation rate of neurotransmitter such as dopamine by Cu-Aβ complexes. These results enhance our understanding of Cu-Aβ interactions in AD pathology and offer insights into potential therapeutic interventions targeting this interaction.

Copper Binding and Redox Activity of α-Synuclein in Membrane-Like Environment

α-Synuclein (αSyn) constitutes the main protein component of Lewy bodies, which are the pathologic hallmark in Parkinson's disease. αSyn is unstructured in solution but the interaction of αSyn with lipid membrane modulates its conformation by inducing an α-helical structure of the N-terminal region. In addition, the interaction with metal ions can trigger αSyn conformation upon binding and/or through the metal-promoted generation of reactive oxygen species which lead to a cascade of structural alterations. For these reasons, the ternary interaction between αSyn, copper, and membranes needs to be elucidated in detail. Here, we investigated the structural properties of copper-αSyn binding through NMR, EPR, and XAS analyses, with particular emphasis on copper(I) coordination since the reduced state is particularly relevant for oxygen activation chemistry. The analysis was performed in different membrane model systems, such as micellar sodium dodecyl sulfate (SDS) and unilamellar vesicles, comparing the binding of full-length αSyn and N-terminal peptide fragments. The presence of membrane-like environments induced the formation of a copper:αSyn = 1:2 complex where Cu⁺ was bound to the Met1 and Met5 residues of two helical peptide chains. In this coordination, Cu⁺ is stabilized and is unreactive in the presence of O₂ in catechol substrate oxidation.

Role of the Cysteine in R3 Tau Peptide in Copper Binding and Reactivity

Tau is a widespread neuroprotein that regulates the cytoskeleton assembly. In some neurological disorders, known as tauopathies, tau is dissociated from the microtubule and forms insoluble neurofibrillary tangles. Tau comprises four pseudorepeats (R1-R4), containing one (R1, R2, R4) or two (R3) histidines, that potentially act as metal binding sites. Moreover, Cys291 and Cys322 in R2 and R3, respectively, might have an important role in protein aggregation, through possible disulfide bond formation, and/or affecting the binding and reactivity of redox-active metal ions, as copper. We, therefore, compare the interaction of copper with octadeca-R3-peptide (R3C) and with the mutant containing an alanine residue (R3A) to assess the role of thiol group. Spectrophotometric titrations allow to calculate the formation constant of the copper(I) complexes, showing a remarkable stronger interaction in the case of R3C (l log Kf = 13.4 and 10.5 for copper(I)-R3C and copper(I)-R3A, respectively). We also evaluate the oxidative reactivity associated to these copper complexes in the presence of dopamine and ascorbate. Both R3A and R3C peptides increase the capability of copper to oxidize catechols, but copper-R3C displays a peculiar mechanism due to the presence of cysteine. HPLC-MS analysis shows that cysteine can form disulfide bonds and dopamine-Cys covalent adducts, with potential implication in tau aggregation process.

Oxidase Reactivity of CuII Bound to N-Truncated Aβ Peptides Promoted by Dopamine

The redox chemistry of copper(II) is strongly modulated by the coordination to amyloid-β peptides and by the stability of the resulting complexes. Amino-terminal copper and nickel binding motifs (ATCUN) identified in truncated Aβ sequences starting with Phe4 show very high affinity for copper(II) ions. Herein, we study the oxidase activity of [Cu–Aβ_4−x] and [Cu–Aβ_1−x] complexes toward dopamine and other catechols. The results show that the Cu^II–ATCUN site is not redox-inert; the reduction of the metal is induced by coordination of catechol to the metal and occurs through an inner sphere reaction. The generation of a ternary [Cu^II–Aβ–catechol] species determines the efficiency of the oxidation, although the reaction rate is ruled by reoxidation of the Cu^I complex. In addition to the N-terminal coordination site, the two vicinal histidines, His13 and His14, provide a second Cu-binding motif. Catechol oxidation studies together with structural insight from the mixed dinuclear complexes Ni/Cu–Aβ_4−x reveal that the His-tandem is able to bind Cu^II ions independently of the ATCUN site, but the N-terminal metal complexation reduces the conformational mobility of the peptide chain, preventing the binding and oxidative reactivity toward catechol of Cu^II bound to the secondary site.

A Cu-bis(imidazole) Substrate Intermediate Is the Catalytically Competent Center for Catechol Oxidase Activity of Copper Amyloid-β

Interaction of copper ions with Aβ peptides alters the redox activity of the metal ion and can be associated with neurodegeneration. Many studies deal with the characterization of the copper binding mode responsible for the reactivity. Oxidation experiments of dopamine and related catechols by copper(II) complexes with the N-terminal amyloid-β peptides Aβ₁₆ and Aβ₉, and the Aβ₁₆[H6A] and Aβ₁₆[H13A] mutant forms, both in their free amine and N-acetylated forms show that efficient reactivity requires the oxygenation of a Cu^I-bis(imidazole) complex with a bound substrate. Therefore, the active intermediate for catechol oxidation differs from the proposed “in-between state” for the catalytic oxidation of ascorbate. During the catechol oxidation process, hydrogen peroxide and superoxide anion are formed but give only a minor contribution to the reaction.

The redox cycling of copper bound to amyloid-β peptide requires the generation of a Cu(I)-Aβ-catecholate complex. When copper(II) is confined in the N-terminal portion, its reduction is slow and causes a shift toward a bis-His coordination environment. The addition of catechol to the Cu(I)-bis(imidazole) complex results in a faster reaction with dioxygen. The reactive species for catechol oxidation does not correspond to the proposed “in-between state” for ascorbate oxidation.

Interaction between Hemin and Prion Peptides: Binding, Oxidative Reactivity and Aggregation

We investigate the interaction of hemin with four fragments of prion protein (PrP) containing from one to four histidines (PrP_106-114, PrP_95-114, PrP_84-114, PrP_76-114) for its potential relevance to prion diseases and possibly traumatic brain injury. The binding properties of hemin-PrP complexes have been evaluated by UV-visible spectrophotometric titration. PrP peptides form a 1:1 adduct with hemin with affinity that increases with the number of histidines and length of the peptide; the following log K₁ binding constants have been calculated: 6.48 for PrP_76-114, 6.1 for PrP_84-114, 4.80 for PrP_95-114, whereas for PrP_106-114, the interaction is too weak to allow a reliable binding constant calculation. These constants are similar to that of amyloid-β (Aβ) for hemin, and similarly to hemin-Aβ, PrP peptides tend to form a six-coordinated low-spin complex. However, the concomitant aggregation of PrP induced by hemin prevents calculation of the K₂ binding constant. The turbidimetry analysis of [hemin-PrP_76-114] shows that, once aggregated, this complex is scarcely soluble and undergoes precipitation. Finally, a detailed study of the peroxidase-like activity of [hemin-(PrP)] shows a moderate increase of the reactivity with respect to free hemin, but considering the activity over long time, as for neurodegenerative pathologies, it might contribute to neuronal oxidative stress.

The "Sick-but-not-Dead" Phenomenon Applied to Catecholamine Deficiency in Neurodegenerative Diseases

The catecholamines dopamine and norepinephrine are key central neurotransmitters that participate in many neurobehavioral processes and disease states. Norepinephrine is also the main neurotransmitter mediating regulation of the circulation by the sympathetic nervous system. Several neurodegenerative disorders feature catecholamine deficiency. The most common is Parkinson's disease (PD), in which putamen dopamine content is drastically reduced. PD also entails severely decreased myocardial norepinephrine content, a feature that characterizes two other Lewy body diseases-pure autonomic failure and dementia with Lewy bodies. It is widely presumed that tissue catecholamine depletion in these conditions results directly from loss of catecholaminergic neurons; however, as highlighted in this review, there are also important functional abnormalities in extant residual catecholaminergic neurons. We refer to this as the "sick-but-not-dead" phenomenon. The malfunctions include diminished dopamine biosynthesis via tyrosine hydroxylase (TH) and L-aromatic-amino-acid decarboxylase (LAAAD), inefficient vesicular sequestration of cytoplasmic catecholamines, and attenuated neuronal reuptake via cell membrane catecholamine transporters. A unifying explanation for catecholaminergic neurodegeneration is autotoxicity exerted by 3,4-dihydroxyphenylacetaldehyde (DOPAL), an obligate intermediate in cytoplasmic dopamine metabolism. In PD, putamen DOPAL is built up with respect to dopamine, associated with a vesicular storage defect and decreased aldehyde dehydrogenase activity. Probably via spontaneous oxidation, DOPAL potently oligomerizes and forms quinone-protein adducts with ("quinonizes") α-synuclein (AS), a major constituent in Lewy bodies, and DOPAL-induced AS oligomers impede vesicular storage. DOPAL also quinonizes numerous intracellular proteins and inhibits enzymatic activities of TH and LAAAD. Treatments targeting DOPAL formation and oxidation therefore might rescue sick-but-not-dead catecholaminergic neurons in Lewy body diseases.

3,4-Dihydroxyphenylacetaldehyde Is More Efficient than Dopamine in Oligomerizing and Quinonizing α-Synuclein

Lewy body diseases such as Parkinson's disease involve intraneuronal deposition of the protein α-synuclein (AS) and depletion of nigrostriatal dopamine (DA). Interactions of AS with DA oxidation products may link these neurohistopathologic and neurochemical abnormalities via two potential pathways: spontaneous oxidation of DA to dopamine-quinone and enzymatic oxidation of DA catalyzed by monoamine oxidase to form 3,4-dihydroxyphenylacetaldehyde (DOPAL), which is then oxidized to DOPAL-Q. We compared these two pathways in terms of the ability of DA and DOPAL to modify AS. DOPAL was far more potent than DA both in oligomerizing and forming quinone-protein adducts with (quinonizing) AS. The DOPAL-induced protein modifications were enhanced similarly by pro-oxidation with Cu(II) or tyrosinase and inhibited similarly by antioxidation with N-acetylcysteine. Dopamine oxidation evoked by Cu(II) or tyrosinase did not quinonize AS. In cultured MO3.13 human oligodendrocytes DOPAL resulted in the formation of numerous intracellular quinoproteins that were visualized by near-infrared spectroscopy. We conclude that of the two routes by which oxidation of DA modifies AS and other proteins the route via DOPAL is more prominent. The results support developing experimental therapeutic strategies that might mitigate deleterious modifications of proteins such as AS in Lewy body diseases by targeting DOPAL formation and oxidation. SIGNIFICANCE STATEMENT: Interactions of the protein α-synuclein with products of dopamine oxidation in the neuronal cytoplasm may link two hallmark abnormalities of Parkinson disease: Lewy bodies (which contain abundant AS) and nigrostriatal DA depletion (which produces the characteristic movement disorder). Of the two potential routes by which DA oxidation may alter AS and other proteins, the route via the autotoxic catecholaldehyde 3,4-dihydroxyphenylacetaldehyde is more prominent; the results support experimental therapeutic strategies targeting DOPAL formation and DOPAL-induced protein modifications.

Membrane Binding Strongly Affecting the Dopamine Reactivity Induced by Copper Prion and Copper/Amyloid-β (Aβ) Peptides. A Ternary Copper/Aβ/Prion Peptide Complex Stabilized and Solubilized in Sodium Dodecyl Sulfate Micelles

The combination between dyshomeostatic levels of catecholamine neurotransmitters and redox-active metals such as copper and iron exacerbates the oxidative stress condition that typically affects neurodegenerative diseases. We report a comparative study of the oxidative reactivity of copper complexes with amyloid-β (Aβ₄₀) and the prion peptide fragment 76-114 (PrP_76-114), containing the high-affinity binding site, toward dopamine and 4-methylcatechol, in aqueous buffer and in sodium dodecyl sulfate micelles, as a model membrane environment. The competitive oxidative and covalent modifications undergone by the peptides were also evaluated. The high binding affinity of Cu/peptide to micelles and lipid membranes leads to a strong reduction (Aβ₄₀) and quenching (PrP_76-114) of the oxidative efficiency of the binary complexes and to a stabilization and redox silencing of the ternary complex Cu^II/Aβ₄₀/PrP_76-114, which is highly reactive in solution. The results improve our understanding of the pathological and protective effects associated with these complexes, depending on the physiological environment.

Binding and Reactivity of Copper to R1 and R3 Fragments of tau Protein

Tau protein is present in significant amounts in neurons, where it contributes to the stabilization of microtubules. Insoluble neurofibrillary tangles of tau are associated with several neurological disorders known as tauopathies, among which is Alzheimer's disease. In neurons, tau binds tubulin through its microtubule binding domain which comprises four imperfect repeats (R₁-R₄). The histidine residues contained in these fragments are potential binding sites for metal ions and are located close to the regions that drive the formation of amyloid aggregates of tau. In this study, we present a detailed characterization through potentiometric and spectroscopic methods of the binding of copper in both oxidation states to R₁ and R₃ peptides, which contain one and two histidine residues, respectively. We also evaluate how the redox cycling of copper bound to tau peptides can mediate oxidation that can potentially target exogenous substrates such as neuronal catecholamines. The resulting quinone oxidation products undergo oligomerization and can competitively give post-translational peptide modifications yielding catechol adducts at amino acid residues. The presence of His-His tandem in the R₃ peptide strongly influences both the binding of copper and the reactivity of the resulting copper complex. In particular, the presence of the two adjacent histidines makes the copper(I) binding to R₃ much stronger than in R₁. The copper-R₃ complex is also much more active than the copper-R₁ complex in promoting oxidative reactions, indicating that the two neighboring histidines activate copper as a catalyst in molecular oxygen activation reactions.

Aminomethylene-Phosphonate Analogue as a Cu(II) Chelator: Characterization and Application as an Inhibitor of Oxidation Induced by the Cu(II)-Prion Peptide Complex

Recently, we reported on a series of aminomethylene-phosphonate (AMP) analogues, bearing one or two heterocyclic groups on the aminomethylene moiety, as promising Zn(II) chelators. Given the strong Zn(II) binding properties of these compounds, they may find useful applications in metal chelation therapy. With a goal of inhibiting the devastating oxidative damage caused by prion protein in prion diseases, we explored the most promising ligand, {bis[(1H-imidazol-4-yl)methyl]amino}methylphosphonic acid, AMP-(Im)₂, 4, as an inhibitor of the oxidative reactivity associated with the Cu(II) complex of prion peptide fragment 84-114. Specifically, we first characterized the Cu(II) complex with AMP-(Im)₂ by ultraviolet-visible spectroscopy and electrochemical measurements that indicated the high chemical and electrochemical stability of the complex. Potentiometric pH titration provided evidence of the formation of a stable 1:1 [Cu(II)-AMP-(Im)₂]⁺ complex (ML), with successive binding of a second AMP-(Im)₂ molecule yielding ML₂ complex [Cu(II)-(AMP-(Im)₂)₂]⁺ (log K' = 15.55), and log β' = 19.84 for ML₂ complex. The CuN3O1 ML complex was demonstrated by X-ray crystallography, indicating the thermodynamically stable square pyramidal complex. Chelation of Cu(II) by 4 significantly reduced the oxidation potential of the former. CuCl₂ and the 1:2 Cu:AMP-(Im)₂ complex showed one-electron redox of Cu(II)/Cu(I) at 0.13 and -0.35 V, respectively. Indeed, 4 was found to be a potent antioxidant that at a 1:1:1 AMP-(Im)₂:Cu(II)-PrP_84-114 molar ratio almost totally inhibited the oxidation reaction of 4-methylcatechol. Circular dichroism data suggest that this antioxidant activity is due to formation of a ternary, redox inactive Cu(II)-Prp_84-114-[AMP-(Im)₂] complex. Future studies in prion disease animal models are warranted to assess the potential of 4 to inhibit the devastating oxidative damage caused by PrP.

Dopamine, Oxidative Stress and Protein-Quinone Modifications in Parkinson's and Other Neurodegenerative Diseases

Dopamine (DA) is the most important catecholamine in the brain, as it is the most abundant and the precursor of other neurotransmitters. Degeneration of nigrostriatal neurons of substantia nigra pars compacta in Parkinson's disease represents the best-studied link between DA neurotransmission and neuropathology. Catecholamines are reactive molecules that are handled through complex control and transport systems. Under normal conditions, small amounts of cytosolic DA are converted to neuromelanin in a stepwise process involving melanization of peptides and proteins. However, excessive cytosolic or extraneuronal DA can give rise to nonselective protein modifications. These reactions involve DA oxidation to quinone species and depend on the presence of redox-active transition metal ions such as iron and copper. Other oxidized DA metabolites likely participate in post-translational protein modification. Thus, protein-quinone modification is a heterogeneous process involving multiple DA-derived residues that produce structural and conformational changes of proteins and can lead to aggregation and inactivation of the modified proteins.