The double faced role of copper in Aβ homeostasis: A survey on the interrelationship between metal dyshomeostasis, UPS functioning and autophagy in neurodegeneration

Metal ion mediated aggregation of Alzheimer's disease peptides and proteins in solutions and at surfaces

Although the pathogenesis of Alzheimer's disease (AD) is still unclear, abnormally high concentrations of metal ions, like copper, iron and zinc, were found in senile plaques of AD brain, which inspires extensive studies on the fundamental molecular interactions of metal ions with the pathogenic hallmarks, amyloid-β (Aβ) peptides and tau proteins, respectively forming senile plaques and neurofibrillary tangles (NFTs) in AD brains. Early works concern the concentration effect of the metal ions on Aβ and tau aggregation. Yet, it is obvious that the surrounding environment of the metal ions must also be considered, not just the metal ions as free accessible forms in the solution phase. The most important surrounding environment in vivo is a very large surface area from cell membranes and other macromolecular surfaces. These bio-interfaces make the kinetic pathways of metal ion mediated Aβ and tau aggregation radically different from those in the solution phase. To better understand the role of metal ions in AD peptide and protein aggregation, we summarize and discuss the recent achievements in the research of metal ion mediated Aβ and tau aggregation, particularly the corresponding mechanism differences between the solution phase and the surface environment. The metal ion chelation therapy for AD is also discussed from the point of the surface pool of metal ions.

Stretching the chains: the destabilizing impact of Cu2+ and Zn2+ ions on K48-linked diubiquitin

Ubiquitin signalling and metal homeostasis play key roles in controlling several physiological cellular activities, including protein trafficking and degradation. While some relationships between these two biochemical pathways have started to surface, our knowledge of their interplay remains limited. Here, we employ a variety of techniques, such as circular dichroism, differential scanning calorimetry, pressure perturbation calorimetry, fluorescence emission, SDS-PAGE, and small-angle X-ray scattering (SAXS) to evaluate the impact of Cu2+ and Zn2+ ions on the structure and stability of K48 linked diubiquitin (K48-Ub2), a simple model for polyubiquitin chains. The SAXS analysis results show that the structure of the metal-free protein is similar to that observed when the protein is bound to the E2 conjugating enzyme, lending support to the idea that the structure of unanchored K48-linked ubiquitin chains is sufficient for identification by conjugating enzymes without the need for an induced fit mechanism. Our results indicate that K48-Ub2 can coordinate up to four metal ions with both copper and zinc ions inducing slight changes to the secondary structure of the protein. However, we noted significant distinctions in their impacts on protein stability and overall architecture. Specifically, Cu2+ ions resulted in a destabilization of the protein structure, which facilitated the formation of dimer aggregates. Next, we observed a shift in the conformational dynamics of K48-Ub2 toward less compact and more flexible states upon metal ion binding, with Zn2+ inducing a more significant effect than Cu2+ ions. Our structural modelling study demonstrates that both metal ions induced perturbations in the K48-Ub2 structure, leading to the separation of the two monomers thus inhibiting interactions with E2 enzymes. In conclusion, the findings from this study enhance our comprehension of the mechanisms underlying Ub chains recognition. Moreover, they strengthen the notion that drug discovery initiatives aimed at targeting metal-mediated disruptions in Ub signaling hold great potential for treating a wide range of diseases that stem from abnormal protein accumulation.

Aβ and Tau Interact with Metal Ions, Lipid Membranes and Peptide-Based Amyloid Inhibitors: Are These Common Features Relevant in Alzheimer’s Disease?

In the last two decades, the amyloid hypothesis, i.e., the abnormal accumulation of toxic Aβ assemblies in the brain, has been considered the mainstream concept sustaining research in Alzheimer’s Disease (AD). However, the course of cognitive decline and AD development better correlates with tau accumulation rather than amyloid peptide deposition. Moreover, all clinical trials of amyloid-targeting drug candidates have been unsuccessful, implicitly suggesting that the amyloid hypothesis needs significant amendments. Accumulating evidence supports the existence of a series of potentially dangerous relationships between Aβ oligomeric species and tau protein in AD. However, the molecular determinants underlying pathogenic Aβ/tau cross interactions are not fully understood. Here, we discuss the common features of Aβ and tau molecules, with special emphasis on: (i) the critical role played by metal dyshomeostasis in promoting both Aβ and tau aggregation and oxidative stress, in AD; (ii) the effects of lipid membranes on Aβ and tau (co)-aggregation at the membrane interface; (iii) the potential of small peptide-based inhibitors of Aβ and tau misfolding as therapeutic tools in AD. Although the molecular mechanism underlying the direct Aβ/tau interaction remains largely unknown, the arguments discussed in this review may help reinforcing the current view of a synergistic Aβ/tau molecular crosstalk in AD and stimulate further research to mechanism elucidation and next-generation AD therapeutics.

Modulation of the 20S Proteasome Activity by Porphyrin Derivatives Is Steered through Their Charge Distribution

Cationic porphyrins exhibit an amazing variety of binding modes and inhibition mechanisms of 20S proteasome. Depending on the spatial distribution of their electrostatic charges, they can occupy different sites on α rings of 20S proteasome by exploiting the structural code responsible for the interaction with regulatory proteins. Indeed, they can act as competitive or allosteric inhibitors by binding at the substrate gate or at the grooves between the α subunits, respectively. Moreover, the substitution of a charged moiety in the peripheral arm with a hydrophobic moiety revealed a “new” 20S functional state with higher substrate affinity and catalytic efficiency. In the present study, we expand our structure–activity relationship (SAR) analysis in order to further explore the potential of this versatile class of 20S modulators. Therefore, we have extended the study to additional macrocyclic compounds, displaying different structural features, comparing their interaction behavior on the 20S proteasome with previously investigated compounds. In particular, in order to evaluate how the introduction of a peptidic chain can affect the affinity and the interacting mechanism of porphyrins, we investigate the MTPyApi, a porphyrin derivatized with an Arg–Pro-rich antimicrobial peptide. Moreover, to unveil the role played by the porphyrin core, this was replaced with a corrole scaffold, a “contracted” version of the tetrapyrrolic ring due to the lack of a methine bridge. The analysis has been undertaken by means of integrated kinetic, Nuclear Magnetic Resonance, and computational studies. Finally, in order to assess a potential pharmacological significance of this type of investigation, a preliminary attempt has been performed to evaluate the biological effect of these molecules on MCF7 breast cancer cells in dark conditions, envisaging that porphyrins may indeed represent a powerful tool for the modulation of cellular proteostasis.

Second Sphere Interactions in Amyloidogenic Diseases

Amyloids are protein aggregates bearing a highly ordered cross β structural motif, which may be functional but are mostly pathogenic. Their formation, deposition in tissues and consequent organ dysfunction is the central event in amyloidogenic diseases. Such protein aggregation may be brought about by conformational changes, and much attention has been directed toward factors like metal binding, post-translational modifications, mutations of protein etc., which eventually affect the reactivity and cytotoxicity of the associated proteins. Over the past decade, a global effort from different groups working on these misfolded/unfolded proteins/peptides has revealed that the amino acid residues in the second coordination sphere of the active sites of amyloidogenic proteins/peptides cause changes in H-bonding pattern or protein-protein interactions, which dramatically alter the structure and reactivity of these proteins/peptides. These second sphere effects not only determine the binding of transition metals and cofactors, which define the pathology of some of these diseases, but also change the mechanism of redox reactions catalyzed by these proteins/peptides and form the basis of oxidative damage associated with these amyloidogenic diseases. The present review seeks to discuss such second sphere modifications and their ramifications in the etiopathology of some representative amyloidogenic diseases like Alzheimer's disease (AD), type 2 diabetes mellitus (T2Dm), Parkinson's disease (PD), Huntington's disease (HD), and prion diseases.

Metals and Metal-Nanoparticles in Human Pathologies: From Exposure to Therapy

An increasing number of pathologies correlates with both toxic and essential metal ions dyshomeostasis. Next to known genetic disorders (e.g., Wilson's Disease and β-Thalassemia) other pathological states such as neurodegeneration and diabetes are characterized by an imbalance of essential metal ions. Metal ions can enter the human body from the surrounding environment in the form of free metal ions or metal-nanoparticles, and successively translocate to different tissues, where they are accumulated and develop distinct pathologies. There are no characteristic symptoms of metal intoxication, and the exact diagnosis is still difficult. In this review, we present metal-related pathologies with the most common onsets, biomarkers of metal intoxication, and proper techniques of metal qualitative and quantitative analysis. We discuss the possible role of drugs with metal-chelating ability in metal dyshomeostasis, and present recent advances in therapies of metal-related diseases.

Living with the enemy: from protein-misfolding pathologies we know, to those we want to know

Conformational diseases are caused by the aggregation of misfolded proteins. The risk for such pathologies develops years before clinical symptoms appear, and is higher in people with alpha-1 antitrypsin (AAT) polymorphisms. Thousands of people with alpha-1 antitrypsin deficiency (AATD) are underdiagnosed. Enemy-aggregating proteins may reside in these underdiagnosed AATD patients for many years before a pathology for AATD fully develops. In this perspective review, we hypothesize that the AAT protein could exert a new and previously unconsidered biological effect as an endogenous metal ion chelator that plays a significant role in essential metal ion homeostasis. In this respect, AAT polymorphism may cause an imbalance of metal ions, which could be correlated with the aggregation of amylin, tau, amyloid beta, and alpha synuclein proteins in type 2 diabetes mellitus (T2DM), Alzheimer's and Parkinson's diseases, respectively.

Metallobiology and therapeutic chelation of biometals (copper, zinc and iron) in Alzheimer's disease: Limitations, and current and future perspectives

BACKGROUND

Alzheimer's disease (AD) is the most prevalent cause of cognitive impairment and dementia worldwide. The pathobiology of the disease has been studied in the form of several hypotheses, ranging from oxidative stress, amyloid-beta (Aβ) aggregation, accumulation of tau forming neurofibrillary tangles (NFT) through metal dysregulation and homeostasis, dysfunction of the cholinergic system, and to inflammatory and autophagic mechanism. However, none of these hypotheses has led to confirmed diagnostics or approved cure for the disease.

OBJECTIVE

This review is aimed as a basic and an encyclopedic short course into metals in AD and discusses the advances in chelation strategies and developments adopted in the treatment of the disease. Since there is accumulating evidence of the role of both biometal dyshomeostasis (iron (Fe), copper (Cu), and zinc (Zn)) and metal-amyloid interactions that lead to the pathogenesis of AD, this review focuses on unraveling therapeutic chelation strategies that have been considered in the treatment of the disease, aiming to sequester free and protein-bound metal ions and reducing cerebral metal burden. Promising compounds possessing chemically modified moieties evolving as multi-target ligands used as anti-AD drug candidates are also covered.

RESULTS AND CONCLUSION

Several multidirectional and multifaceted studies on metal chelation therapeutics show the need for improved synthesis, screening, and analysis of compounds to be able to effectively present chelating anti-AD drugs. Most drug candidates studied have limitations in their physicochemical properties; some enhance redistribution of metal ions, while others indirectly activate signaling pathways in AD. The metal chelation process in vivo still needs to be established and the design of potential anti-AD compounds that bi-functionally sequester metal ions as well as inhibit the Aβ aggregation by competing with the metal ions and reducing metal-induced oxidative damage and neurotoxicity may signal a bright end in chelation-based therapeutics of AD.

Tau/Aβ chimera peptides: A Thioflavin-T and MALDI-TOF study of Aβ amyloidosis in the presence of Cu(II) or Zn(II) ions and total lipid brain extract (TLBE) vesicles

Currently, Alzheimer's Disease (AD) is a complex neurodegenerative condition, with limited therapeutic options. Several factors, like Amyloid β (Aβ) aggregation, tau protein hyperphosphorylation, bio-metals dyshomeostasis and oxidative stress contribute to AD pathogenesis. These pathogenic processes might occur in the aqueous phase but also on neuronal membranes. Thus, investigating the connection between Aβ and biomembranes, becomes important for unveiling the molecular mechanism underlying Aβ amyloidosis as a critical event in AD pathology. In this work, the interaction of two peptides, made up with hybrid sequences from Tau protein 9-16 (EVMEDHAG) or 26-33 (QGGYTMHQ) N-terminal domain and Aβ_16-20 (KLVFF) hydrophobic region, with full length Aβ40 or Aβ42 peptides is reported. The studied "chimera" peptides Ac-EVMEDHAGKLVFF-NH₂ (τ_9-16-KL) and Ac-QGGYTMHQKLVFF-NH₂ (τ_26-33-KL) are endowed with Aβ recognition and metal ion interaction capabilities provided by the tau or Aβ sequences, respectively. These peptides were characterized in previous study along with their metal dependent interaction and amyloidogenesis, either in the presence or absence of metal ion and artificial membranes made up with Total Lipid Brain Extract (TLBE) components, (Sciacca et al., 2020). In the present paper, the ability of the two peptides to inhibit Aβ aggregation is studied using composite experimental conditions including aqueous solution, the presence of metal ions (Cu or Zn), the presence of lipid vesicles mimicking neuronal membranes as well as the co-presence of metals and TLBE artificial membranes. We used Thioflavine-T (ThT) fluorescence or MALDI-TOF spectrometry analysis of Aβ limited proteolysis to respectively monitor the Aβ aggregation kinetic or validation of the Aβ interacting regions. We demonstrate that τ_9-16-KL and τ_26-33-KL peptides differently affect Aβ aggregation kinetics, with the tau sequence playing a crucial role. The results are discussed in terms of chimera's peptides hydrophobicity and electrostatic driven interactions at the aqueous/membrane interface.

The interplay between lipid and Aβ amyloid homeostasis in Alzheimer's Disease: risk factors and therapeutic opportunities

Alzheimer's Diseases (AD) is characterized by the accumulation of amyloid deposits of Aβ peptide in the brain. Besides genetic background, the presence of other diseases and an unhealthy lifestyle are known risk factors for AD development. Albeit accumulating clinical evidence suggests that an impaired lipid metabolism is related to Aβ deposition, mechanistic insights on the link between amyloid fibril formation/clearance and aberrant lipid interactions are still unavailable. Recently, many studies have described the key role played by membrane bound Aβ assemblies in neurotoxicity. Moreover, it has been suggested that a derangement of the ubiquitin proteasome pathway and autophagy is significantly correlated with toxic Aβ aggregation and dysregulation of lipid levels. Thus, studies focusing on the role played by lipids in Aβ aggregation and proteostasis could represent a promising area of investigation for the design of valuable treatments. In this review we examine current knowledge concerning the effects of lipids in Aβ aggregation and degradation processes, focusing on the therapeutic opportunities that a comprehensive understanding of all biophysical, biochemical, and biological processes involved may disclose.

Synthesis, characterization and pharmacological evaluation of quinoline derivatives and their complexes with copper(ΙΙ) in in vitro cell models of Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder of the central nervous system. The main pathophysiological mechanisms involve cholinergic neurotransmission, beta-amyloid (Αβ) and Tau proteins, several metal ions and oxidative stress, among others. Current drugs offer only relief of symptoms and not a cure of AD. Accumulating evidence suggests that multifunctional compounds, targeting multiple pathophysiological mechanisms, may have a great potential for the treatment of AD. In this study, we report on the synthesis and physicochemical characterization of four quinoline-based metal chelators and their respective copper(II) complexes. Most compounds were non-toxic at concentrations ≤5 μM. In neuroprotection studies employing undifferentiated and differentiated SH-SY5Y cells, the metal chelator N²,N⁶-di(quinolin-8-yl)pyridine-2,6-dicarboxamide (H₂dqpyca) appeared to exert significant neuroprotection against both, Aβ peptide- and H₂O₂-induced toxicities. The copper(II) complex [Cu^II(H₂bqch)Cl₂]^.3H₂O (H₂bqch = N,N'-Bis(8-quinolyl)cyclohexane-1,2-diamine) also protected against H₂O₂-induced toxicity, with a half-maximal effective concentration of 80 nM. Molecular docking simulations, using the crystal structure of the acetylcholinesterase (AChE)-rivastigmine complex as a template, indicated a strong interaction of the metal chelator H₂dqpyca, followed by H₂bqch, with both the peripheral anionic site and the catalytic active site of AChE. In conclusion, the sufficient neuroprotection provided by the metal chelator H₂dqpyca and the copper(II) complex [Cu^II(H₂bqch)Cl₂]^.3H₂O along with the evidence for interaction between H₂dqpyca and AChE, indicate that these compounds have the potential and should be further investigated in the framework of preclinical studies employing animal models of AD as candidate multifunctional lead compounds for the treatment of the disease.

Neurotoxic Effect of Flavonol Myricetin in the Presence of Excess Copper

Oxidative stress (OS) induced by the disturbed homeostasis of metal ions is one of the pivotal factors contributing to neurodegeneration. The aim of the present study was to investigate the effects of flavonoid myricetin on copper-induced toxicity in neuroblastoma SH-SY5Y cells. As determined by the MTT method, trypan blue exclusion assay and measurement of ATP production, myricetin heightened the toxic effects of copper and exacerbated cell death. It also increased copper-induced generation of reactive oxygen species, indicating the prooxidative nature of its action. Furthermore, myricetin provoked chromatin condensation and loss of membrane integrity without caspase-3 activation, suggesting the activation of both caspase-independent programmed cell death and necrosis. At the protein level, myricetin-induced upregulation of PARP-1 and decreased expression of Bcl-2, whereas copper-induced changes in the expression of p53, p73, Bax and NME1 were not further affected by myricetin. Inhibitors of ERK1/2 and JNK kinases, protein kinase A and L-type calcium channels exacerbated the toxic effects of myricetin, indicating the involvement of intracellular signaling pathways in cell death. We also employed atomic force microscopy (AFM) to evaluate the morphological and mechanical properties of SH-SY5Y cells at the nanoscale. Consistent with the cellular and molecular methods, this biophysical approach also revealed a myricetin-induced increase in cell surface roughness and reduced elasticity. Taken together, we demonstrated the adverse effects of myricetin, pointing out that caution is required when considering powerful antioxidants for adjuvant therapy in copper-related neurodegeneration.

Copper (II) binding properties of an octapeptide fragment from the R3 region of tau protein: A combined potentiometric, spectroscopic and mass spectrometric study

The copper(II) complexes of a peptide fragment of the R3 domain of tau protein (tau(326-333) Ac-GNIHHKPG-NH₂) and its mutants (Ac-GNGHHKPG-NH₂, Ac-GNIHHKAG-NH₂, Ac-GNGAHKPG-NH₂ and Ac-GNGHAKPG-NH₂) have been studied by potentiometric and spectroscopic (UV-Vis, CD) methods. ESR spectroscopy and mass spectrometry were also used to prove the coordination mode of the mononuclear complexes and the formation of dinuclear species, respectively. It has been demonstrated that the (326-333) fragment of tau protein is a versatile and effective ligand for copper(II) coordination. The versatility of copper(II) binding is related to the presence of two adjacent histidyl residues in the sequence, which results in the coexistence of mononuclear, bis(ligand) and dinuclear complexes at different metal to ligand ratios. The 1:1 mononuclear complexes are, however, the dominant species with all peptides and the imidazole-N and one to three deprotonated amide nitrogen atoms towards the N-terminal side of the histidyl residue have been suggested as metal binding sites. This binding mode allows the formation of coordination isomers because any of the two histidine moieties can be the primary anchoring site. It is evident from the CD spectroscopic measurements that the isomers are present in almost equal concentration. The copper(II) binding affinity of the native fragment of tau protein is comparable to that of a similar 2-histidine fragment of amyloid-β mutant, Ac-SGAEGHHQK-NH₂ but the comparison with an independent histidyl residue (H32) from the N-terminal region of the protein reveals the predominance of H32 over the histidines in the R3 domain.

Synthesis of New Tyrosol-Based Phosphodiester Derivatives: Effect on Amyloid β Aggregation and Metal Chelation Ability

Alzheimer's disease (AD) is a multifactorial pathology that requires multifaceted agents able to address its peculiar nature. Increasing evidence has shown that aggregation of amyloid β (Aβ) and oxidative stress are strictly interconnected, and their modulation might have a positive and synergic effect in contrasting AD-related impairments. Herein, a new and efficient fragment-based approach towards tyrosol phosphodiester derivatives (TPDs) has been developed starting from suitable tyrosol building blocks and exploiting the well-established phosphoramidite chemistry. The antioxidant activity of new TPDs has been tested as well as their ability to inhibit Aβ protein aggregation. In addition, their metal chelating ability has been evaluated as a possible strategy to develop new natural-based entities for the prevention or therapy of AD. Interestingly, TPDs containing a catechol moiety have demonstrated highly promising activity in inhibiting the aggregation of Aβ₄₀ and a strong ability to chelate biometals such as Cu^II and Zn^II .

Neuroprotective Effects of Baicalein, Wogonin, and Oroxylin A on Amyloid Beta-Induced Toxicity via NF-κB/MAPK Pathway Modulation

Amyloid beta (Aβ) peptide, one of the most important pathogenic traits of Alzheimer’s disease (AD), invokes a cascade of oxidative damage and eventually leads to neuronal death. In the present study, baicalein, wogonin, and oroxylin A, main active flavones in Scutellaria baicalensis, were evaluated for their neuroprotective effects against Aβ_25–35-stimulated damage. All tested compounds decreased Aβ_25–35-induced ROS generation and cell cycle arrest. In particular, baicalein exhibited the strongest antioxidant activity. In addition, these compounds suppressed apoptosis by attenuating mitochondrial dysfunction such as loss of membrane potential, Ca²⁺ accumulation and Bax/Bcl-2 ratio. Furthermore, all tested flavones inhibited the expression of iNOS and COX-2, which resulted in suppressing inflammatory cytokines including TNF-α, NO, and PGE₂. Noticeably, all compounds exhibited the anti-inflammatory effects through downregulating NF-κB/MAPK pathway. Especially, oroxylin A was effective against both p65 and IκBα, while wogonin and baicalein were suppressed phospho-p65 and phospho-IκBα, respectively. Taken together, baicalein, wogonin, and oroxylin A can effectively relieve Aβ_25–35-stimulated neuronal apoptosis and inflammation in PC12 cells via downregulating NF-κB/MAPK signaling pathway.

The proteasome as a druggable target with multiple therapeutic potentialities: Cutting and non-cutting edges

Ubiquitin Proteasome System (UPS) is an adaptable and finely tuned system that sustains proteostasis network under a large variety of physiopathological conditions. Its dysregulation is often associated with the onset and progression of human diseases; hence, UPS modulation has emerged as a promising new avenue for the development of treatments of several relevant pathologies, such as cancer and neurodegeneration. The clinical interest in proteasome inhibition has considerably increased after the FDA approval in 2003 of bortezomib for relapsed/refractory multiple myeloma, which is now used in the front-line setting. Thereafter, two other proteasome inhibitors (carfilzomib and ixazomib), designed to overcome resistance to bortezomib, have been approved for treatment-experienced patients, and a variety of novel inhibitors are currently under preclinical and clinical investigation not only for haematological malignancies but also for solid tumours. However, since UPS collapse leads to toxic misfolded proteins accumulation, proteasome is attracting even more interest as a target for the care of neurodegenerative diseases, which are sustained by UPS impairment. Thus, conceptually, proteasome activation represents an innovative and largely unexplored target for drug development. According to a multidisciplinary approach, spanning from chemistry, biochemistry, molecular biology to pharmacology, this review will summarize the most recent available literature regarding different aspects of proteasome biology, focusing on structure, function and regulation of proteasome in physiological and pathological processes, mostly cancer and neurodegenerative diseases, connecting biochemical features and clinical studies of proteasome targeting drugs.

PI3K/Akt and ERK1/2 Signalling Are Involved in Quercetin-Mediated Neuroprotection against Copper-Induced Injury

Copper, a transition metal with essential cellular functions, exerts neurotoxic effects when present in excess by promoting production of reactive oxygen species (ROS). The aim of the present study was to investigate potential benefits of flavonoid quercetin against copper-induced toxicity. Results obtained with MTT assay indicate that the effects of quercetin are determined by the severity of the toxic insult. In moderately injured P19 neuronal cells, concomitant treatment with 150 μM quercetin improved viability by preventing ROS formation, caspase-3 activation, and chromatin condensation. Western blot analysis revealed that quercetin reduced copper-induced increase in p53 upregulated modulator of apoptosis (PUMA) expression and promoted upregulation of nucleoside diphosphate kinase NME1. Levels of p53 and Bax proteins were not affected by both copper and quercetin. UO126 and wortmannin, inhibitors of ERK1/2 and PI3K/Akt signalling pathways, respectively, prevented neuroprotective effects of quercetin. In severely injured neurons, 30 μM quercetin exerted strong prooxidative action and exacerbated cytotoxic effects of copper, whereas 150 μM quercetin failed to affect neuronal survival. These results demonstrate the dual nature of quercetin action in copper-related neurodegeneration. Hence, they are relevant in the context of considering quercetin as a possible therapeutic for neuroprotection and imply that detailed pharmacological and toxicological studies must be carried out for natural compounds capable of acting both as antioxidants and prooxidants.

Site directed mutagenesis of insulin-degrading enzyme allows singling out the molecular basis of peptidase versus E1-like activity: the role of metal ions

Four specifically designed IDE mutants have been used to unveil the molecular basis of peptidase versus E1-like activity of the enzyme. We have found that physiological concentrations of copper(ii) ions inhibit the proteolytic activity of the enzyme towards small and large substrates but have no effect on the E1-like activity of the enzyme.

Dual Function of USP14 Deubiquitinase in Cellular Proteasomal Activity and Autophagic Flux

The ubiquitin-proteasome system and the autophagy-lysosome system are two major intracellular proteolytic pathways in eukaryotes. Although several biochemical mechanisms underlying the crosstalk between them have been suggested, little is known about the effect of enhanced proteasome activity on autophagic flux. Here, we found that upregulation of proteasome activity, which was achieved through the inhibition of USP14, significantly impaired cellular autophagic flux, especially at the autophagosome-lysosome fusion step. UVRAG appeared to function as a crucial checkpoint for the proper progression of autophagic flux. Although proteasome activation through USP14 inhibition facilitated the clearance of microtubule-associated protein tau (MAPT) and reduced the amount of its oligomeric forms, the same conditions increased the formation of inclusion bodies from nonproteasomal substrates such as huntingtin with long polyglutamine repeats. Our results collectively indicate that USP14 may function as a common denominator in the compensatory negative feedback between the two major proteolytic processes in the cell.

Design, Synthesis and Biological Evaluation of Diosgenin-Amino Acid Derivatives with Dual Functions of Neuroprotection and Angiogenesis

Diosgenin, a natural product with steroidal structure, has a wide range of clinical applications in China. It also shows great potential in the treatment of blood clots and nerve damage. To enhance the bioavailability as well as efficacy of diosgenin, eighteen diosgenin-amino acid derivatives were designed and synthesized. The neuroprotective effects of these compounds were evaluated by SH-SY5Y cell line and the biosafety was evaluated by H9c2 cell line. The results displayed that part of the derivatives' activities (EC₅₀ < 20 μM) were higher than positive control edaravone (EC₅₀ = 21.60 ± 3.04 μM), among which, DG-15 (EC₅₀ = 6.86 ± 0.69 μM) exhibited the best neuroprotection. Meanwhile, biosafety evaluation showed that DG-15 had no cytotoxicity on H9c2 cell lines. Interestingly, combined neuroprotective and cytotoxic results, part of the derivatives without their protecting group were superior to compounds with protecting group. Subsequently, Giemsa staining and DAPI (4',6-diamidino-2-phenylindole) staining indicated that DG-15 had a protective effect on damaged SH-SY5Y cells by reducing apoptosis. Moreover, DG-15 showed a higher role in promoting angiogenesis at high concentrations (4 mg/mL) on the chorioallantoic membrane model. This finding displayed that DG-15 had dual functions of neuroprotection and angiogenesis, which provided further insight into designing agent for the application in treatment of ischemic stroke.

Rebalancing metal dyshomeostasis for Alzheimer's disease therapy

Alzheimer's disease (AD) is a type of neurodegenerative malady that is associated with the accumulation of amyloid plaques. Metal ions are critical for the development and upkeep of brain activity, but metal dyshomeostasis can contribute to the development of neurodegenerative diseases, including AD. This review highlights the association between metal dyshomeostasis and AD pathology, the feasibility of rebalancing metal homeostasis as a therapeutic strategy for AD, and a survey of current drugs that action via rebalancing metal homeostasis. Finally, we discuss the challenges that should be overcome by researchers in the future to enable the practical use of metal homeostasis rebalancing agents for clinical application.

Ginsenoside Re Inhibits ROS/ASK-1 Dependent Mitochondrial Apoptosis Pathway and Activation of Nrf2-Antioxidant Response in Beta-Amyloid-Challenged SH-SY5Y Cells

Accumulation of amyloid-β (Aβ), which results in the formation of senile plaques that cause oxidative damage and neuronal cell death, has been accepted as the major pathological mechanism of Alzheimer’s disease (AD). Hence, inhibition of Aβ-induced oxidative damage and neuronal cell apoptosis represents the effective strategies in combating AD. Ginsenoside Re (Re) has pharmacological effects against Aβ-induced neurotoxicity. However, its molecular mechanism remains elusive. The present study evaluated the effect of Re against Aβ-induced cytotoxicity and apoptosis in SH-SY5Y cells, and investigated the underlying mechanism. We demonstrate that Re inhibits the Aβ-triggered mitochondrial apoptotic pathway, as indicated by maintenance of mitochondrial functional, elevated Bcl-2/Bax ratio, reduced cytochrome c release, and inactivation of caspase-3/9. Re attenuated Aβ-evoked reactive oxygen species (ROS) production, apoptosis signal-regulating kinase 1 (ASK1) phosphorylation, and JNK activation. ROS-scavenging abrogated the ability of Re to alter ASK-1 activation. Simultaneously, inhibition of JNK abolished Re-induced Bax downregulation in Aβ-challenged SH-SY5Y cells. In addition, Re enhanced activation of the nuclear factor-E2-related factor 2 (Nrf2) in Aβ-induced SH-SY5Y cells. Knockdown of Nrf2 by small interfering RNA targeting Nrf2 abolished the protective effect of Re. Our findings indicate that Re could be a potential therapeutic approach for the treatment of AD.

CuII Binding Properties of N-Truncated Aβ Peptides: In Search of Biological Function

As life expectancy increases, the number of people affected by progressive and irreversible dementia, Alzheimer's Disease (AD), is predicted to grow. No drug designs seem to be working in humans, apparently because the origins of AD have not been identified. Invoking amyloid cascade, metal ions, and ROS production hypothesis of AD, herein we share our point of view on Cu(II) binding properties of Aβ_4-x, the most prevalent N-truncated Aβ peptide, currently known as the main constituent of amyloid plaques. The capability of Aβ_4-x to rapidly take over copper from previously tested Aβ_1-x peptides and form highly stable complexes, redox unreactive and resistant to copper exchange reactions, prompted us to propose physiological roles for these peptides. We discuss the new findings on the reactivity of Cu(II)Aβ_4-x with coexisting biomolecules in the context of synaptic cleft; we suggest that the role of Aβ_4-x peptides is to quench Cu(II) toxicity in the brain and maintain neurotransmission.

Recent Developments in Metal-Based Drugs and Chelating Agents for Neurodegenerative Diseases Treatments

The brain has a unique biological complexity and is responsible for important functions in the human body, such as the command of cognitive and motor functions. Disruptive disorders that affect this organ, e.g. neurodegenerative diseases (NDDs), can lead to permanent damage, impairing the patients' quality of life and even causing death. In spite of their clinical diversity, these NDDs share common characteristics, such as the accumulation of specific proteins in the cells, the compromise of the metal ion homeostasis in the brain, among others. Despite considerable advances in understanding the mechanisms of these diseases and advances in the development of treatments, these disorders remain uncured. Considering the diversity of mechanisms that act in NDDs, a wide range of compounds have been developed to act by different means. Thus, promising compounds with contrasting properties, such as chelating agents and metal-based drugs have been proposed to act on different molecular targets as well as to contribute to the same goal, which is the treatment of NDDs. This review seeks to discuss the different roles and recent developments of metal-based drugs, such as metal complexes and metal chelating agents as a proposal for the treatment of NDDs.

Ubiquitin binds the amyloid β peptide and interferes with its clearance pathways

Several lines of evidence point to a compromised proteostasis associated with a reduction of the Ubiquitin Proteasome System (UPS) activity in patients affected by Alzheimer's Disease (AD) and suggest that the amyloid β peptide (Aβ) is an important player in the game. Inspired also by many reports, underlining the presence of ubiquitin (Ub) in the amyloid plaques of AD brains, here we set out to test whether Ub may bind the Aβ peptide and have any effect on its clearance pathways. By using an integrated array of MALDI-TOF/UPLC-HRMS, fluorescence, NMR, SPR, Microscale Thermophoresis (MST) and molecular dynamics studies, we consistently demonstrated that Aβ40 binds Ub with a 1 : 1 stoichiometry and K d in the high micromolar range. In particular, we show that the N-terminal domain of the Aβ peptide (through residues D1, E3 and R5) interacts with the C-terminal tail of Ub (involving residues K63 and E64), inducing the central region of Aβ (14HQKLVFFAEDVGSNK28) to adopt a mixed α-helix/β-turn structure. ELISA assays, carried out in neuroblastoma cell lysates, suggest that Aβ competitively binds Ub also in the presence of the entire pool of cytosolic Ub binding proteins. Ub-bound Aβ has a lower tendency to aggregate into amyloid-like fibrils and is more slowly degraded by the Insulin Degrading Enzyme (IDE). Finally, we observe that the water soluble fragment Aβ1-16 significantly inhibits Ub chain growth reactions. These results evidence how the non-covalent interaction between Aβ peptides and Ub may have relevant effects on the regulation of the upstream events of the UPS and pave the way to future in vivo studies addressing the role played by Aβ peptide in the malfunction of proteome maintenance occurring in AD.

Mass spectrometry is a multifaceted weapon to be used in the battle against Alzheimer's disease: Amyloid beta peptides and beyond

Amyloid-β peptide (Aβ) accumulation and aggregation have been considered for many years the main cause of Alzheimer's disease (AD), and therefore have been the principal target of investigation as well as of the proposed therapeutic approaches (Grasso [2011] Mass Spectrom Rev. 30: 347-365). However, the amyloid cascade hypothesis, which considers Aβ accumulation the only causative agent of the disease, has proven to be incomplete if not wrong. In recent years, actors such as metal ions, oxidative stress, and other cofactors have been proposed as possible co-agents or, in some cases, main causative factors of AD. In this scenario, MS investigation has proven to be fundamental to design possible diagnostic strategies of this elusive disease, as well as to understand the biomolecular mechanisms involved, in the attempt to find a possible therapeutic solution. We review the current applications of MS in the search for possible Aβ biomarkers of AD to help the diagnosis of the disease. Recent examples of the important contributions that MS has given to prove or build theories on the molecular pathways involved with such terrible disease are also reviewed.

Interactions between amyloid β peptide and lipid membranes

The presence of amyloid plaques in the brain is a typical characteristic of Alzheimer's disease (AD). Amyloid plaques are formed from the deposits of aggregated amyloid β peptide (Aβ). The toxicity induced by Aβ aggregates is correlated with Aβ-membrane interactions. The mutual influences between aggregation and membranes are complicated and unclear. In recent years advanced experiments and findings are emerging to give us more detailed information on Aβ-membrane interactions. In this review, we mainly focus on the Aβ-membrane interactions and membrane-induced Aβ structures. The mechanism of Aβ-membrane interactions is also summarized, which provides insights into the prevention and treatment of AD.

Sulfur Nanoparticles with Novel Morphologies Coupled with Brain-Targeting Peptides RVG as a New Type of Inhibitor Against Metal-Induced Aβ Aggregation

Functionalized nanomaterials, which have been applied widely to inhibit amyloid-β protein (Aβ) aggregation, show enormous potential in the field of prevention and treatment of Alzheimer's disease (AD). A significant body of data has demonstrated that the morphology and size of nanomaterials have remarkable effects on their biological behaviors. In this work, we proposed and designed three kinds of brain-targeting sulfur nanoparticles (RVG@Met@SNPs) with novel morphologies (volute-like, tadpole-like, and sphere-like) and investigated the effect of different RVG@Met@SNPs on Aβ-Cu²⁺ complex aggregation and their corresponding neurotoxicity. Among them, the sphere-like nanoparticles (RVG@Met@SS) exhibited the most effective inhibitory activity, due to their unique mini size effect, and they reduced 61.6% the Aβ-Cu²⁺ complex aggregation and increased 92.4% SH-SY5Y cell viability in a dose of 10 μg/mL. In vitro and in vivo, the abilities of different morphologies of RVG@Met@SNPs to cross the blood-brain barrier (BBB) and target brain parenchymal cells were significantly different. Moreover, improvements in learning disability and cognitive loss were shown in the transgenic AD mice model using the Morris water maze test after multiple doses of RVG@Met@SNPs treatment. In general, the purpose of this research is to develop a biological application of sulfur nanoparticles and to provide a novel functionalized nanomaterial to treat AD.

Amyloid growth and membrane damage: Current themes and emerging perspectives from theory and experiments on Aβ and hIAPP

Alzheimer's Disease (AD) and Type 2 diabetes mellitus (T2DM) are two incurable diseases both hallmarked by an abnormal deposition of the amyloidogenic peptides Aβ and Islet Amyloid Polypeptide (IAPP) in affected tissues. Epidemiological data demonstrate that patients suffering from diabetes are at high risk of developing AD, thus making the search for factors common to the two pathologies of special interest for the design of new therapies. Accumulating evidence suggests that the toxic properties of both Aβ or IAPP are ascribable to their ability to damage the cell membrane. However, the molecular details describing Aβ or IAPP interaction with membranes are poorly understood. This review focuses on biophysical and in silico studies addressing these topics. Effects of calcium, cholesterol and membrane lipid composition in driving aberrant Aβ or IAPP interaction with the membrane will be specifically considered. The cross correlation of all these factors appears to be a key issue not only to shed light in the countless and often controversial reports relative to this area but also to gain valuable insights into the central events leading to membrane damage caused by amyloidogenic peptides. This article is part of a Special Issue entitled: Protein Aggregation and Misfolding at the Cell Membrane Interface edited by Ayyalusamy Ramamoorthy.

Strategy to discover full-length amyloid-beta peptide ligands using high-efficiency microarray technology

Although the formation of β-amyloid (Aβ) fibrils in neuronal tissues is a hallmark of Alzheimer disease (AD), small-sized Aβ oligomers rather than mature fibrils have been identified as the most neurotoxic species. Therefore, the design of new inhibitors, able to prevent the aggregation of Aβ, is believed to be a promising therapeutic approach to AD. Unfortunately, the short-lived intermediate structures that occur in a solution along the Aβ aggregation pathway escape conventional experimental investigations and there is urgent need of new tools aimed at the discovery of agents targeting monomeric Aβ and blocking the early steps of amyloid aggregation. Here, we show the combination of high-efficiency slides (HESs) with peptide microarrays as a promising tool for identifying small peptides that bind Aβ monomers. To this aim, HESs with two immobilized reference peptides, (i.e., KLVFF and Semax) with opposite behavior, were investigated for binding to fluorescently labeled Aβ peptide. Transmission electron microscopy was used to demonstrate Aβ fibrillar aggregates missing. The use of HESs was critical to ensure convenient output of the fluorescent microarrays. The resulting sensitivity, as well as the low sample consumption and the high potential for miniaturization, suggests that the proposed combination of peptide microarrays and highly efficient slides would be a very effective technology for molecule profiling in AD drug discovery.