Role of copper chelating agents: between old applications and new perspectives in neuroscience

The role of copper element has been an increasingly relevant topic in recent years in the fields of human and animal health, for both the study of new drugs and innovative food and feed supplements. This metal plays an important role in the central nervous system, where it is associated with glutamatergic signaling, and it is widely involved in inflammatory processes. Thus, diseases involving copper (II) dyshomeostasis often have neurological symptoms, as exemplified by Alzheimer's and other diseases (such as Parkinson's and Wilson's diseases). Moreover, imbalanced copper ion concentrations have also been associated with diabetes and certain types of cancer, including glioma. In this paper, we propose a comprehensive overview of recent results that show the importance of these metal ions in several pathologies, mainly Alzheimer's disease, through the lens of the development and use of copper chelators as research compounds and potential therapeutics if included in multi-target hybrid drugs. Seeing how copper homeostasis is important for the well-being of animals as well as humans, we shortly describe the state of the art regarding the effects of copper and its chelators in agriculture, livestock rearing, and aquaculture, as ingredients for the formulation of feed supplements as well as to prevent the effects of pollution on animal productions.

Synthesis and bio-activities of bifunctional tetrahydrosalen Cu (II) chelators with potential efficacy in Alzheimer's disease therapy

The dyshomeostasis of metal ions in the brain leads to the accumulation of excess metals in extracellular and inter-neuronal locations and the Amyloid β peptide (Aβ) binds these transition metals, which ultimately cause the Aβ aggregation and severe oxidative stress in the brain. The aggregation of Aβ and oxidative stress are important factors to trigger Alzheimer's disease (AD). Metal chelation therapy is a promising approach to removing metals from Aβ-M species and relieve the oxidative stress. Therefore, 4 tetrahydrosalens containing benzothiazole moiety were designed and synthesized. Their biological activities for Alzheimer's disease therapy in vitro were determined by Turbidity assay, BCA protein assay, MTT assay and fluorescent probe of DCFH-DA. The results were comparing with that of non-specific chelator (cliquinol, CQ) and non-benzothiazole functionalized tetrahydrosalens, the results demonstrated that benzothiazole functionalized chelators had more efficient bio-activities in preventing Cu2+-induced Aβ aggregation, attenuating cytotoxicity mediated by Aβ-Cu2+ species and decrease the level of reactive oxygen species (ROS) in Cu2+-Aβ treated PC12 cells than that of cliquinol and non-benzothiazole functionalized analogues.

Role of Amyloidogenic and Non-Amyloidogenic Protein Spaces in Neurodegenerative Diseases and their Mitigation Using Theranostic Agents

Neurodegenerative diseases (NDDs) refer to a complex heterogeneous group of diseases which are associated with the accumulation of amyloid fibrils or plaques in the brain leading to progressive loss of neuronal functions. Alzheimer's disease is one of the major NDD responsible for 60-80 % of all dementia cases. Currently, there are no curative or disease-reversing/modifying molecules for many of the NDDs except a few such as donepezil, rivastigmine, galantamine, carbidopa and levodopa which treat the disease-associated symptoms. Similarly, there are very few FDA-approved tracers such as flortaucipir (Tauvid) for tau fibril imaging and florbetaben (Neuraceq), flutemetamol (Vizamyl), and florbetapir (Amyvid) for amyloid imaging available for diagnosis. Recent advances in the cryogenic electron microscopy reported distinctly different microstructures for tau fibrils associated with different tauopathies highlighting the possibility to develop tauopathy-specific imaging agents and therapeutics. In addition, it is important to identify the proteins that are associated with disease development and progression to know about their 3D structure to develop various diagnostics, therapeutics and theranostic agents. The current article discusses in detail the disease-associated amyloid and non-amyloid proteins along with their structural insights. We comprehensively discussed various novel proteins associated with NDDs and their implications in disease pathology. In addition, we document various emerging chemical compounds developed for diagnosis and therapy of different NDDs with special emphasis on theranostic agents for better management of NDDs.

Recent progress of small-molecule-based theranostic agents in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of neurodegenerative dementia. As a multifactorial disease, AD involves several etiopathogenic mechanisms, in which multiple pathological factors are interconnected with each other. This complicated and unclear pathogenesis makes AD lack effective diagnosis and treatment. Theranostics, exerting the synergistic effect of diagnostic and therapeutic functions, would provide a promising strategy for exploring AD pathogenesis and developing drugs for combating AD. With the efforts in small drug-like molecules for both diagnosis and treatment of AD, small-molecule-based theranostic agents have attracted significant attention owing to their facile synthesis, high biocompatibility and reproducibility, and easy clearance from the body through the excretion systems. In this review, the small-molecule-based theranostic agents reported in the literature for anti-AD are classified into four groups according to their diagnostic modalities. Their design rationales, chemical structures, and working mechanisms for theranostics are summarized. Finally, the opportunities for small-molecule-based theranostic agents in AD are also proposed.

A Nanoenzyme Constructed from Manganese and Strandberg-Type Phosphomolybdate with Versatility in Antioxidant and Modulating Conformation of Aβ Protein Misfolding Aggregates In Vitro

Amyloid β-peptide (Aβ) misfolding aggregates with β-sheet structures and surplus reactive oxygen species (ROS) are both considered to be the culprit of neuronal toxicity in Alzheimer's disease (AD). Therefore, modulating the misfolding mode of Aβ and inhibiting ROS simultaneous has become an important method for anti-AD. Herein, a nanoscale manganese-substituted polyphosphomolybdate (H₂en)₃[Mn(H₂O)₄][Mn(H₂O)₃]₂[P₂Mo₅O₂₃]₂·14.5H₂O (abbreviated as MnPM) (en = ethanediamine) was designed and synthesized by single crystal to single crystal transformation method. MnPM can modulate the β-sheet rich conformation of Aβ aggregates, and thus reduce the formation of toxic species. Moreover, MnPM also possesses the ability to eliminate the free radicals produced by Cu²⁺-Aβ aggregates. It can inhibit the cytotoxicity of β-sheet-rich species and protect synapses of PC12 cells. MnPM combines the conformation modulating ability of Aβ and anti-oxidation ability, which makes a promising multi-funcational molecular with a composite mechanism for the new conceptual designing in treatment of such protein-misfolding diseases.

A Cyclen-Functionalized Cobalt-Substituted Sandwich-Type Tungstoarsenate with Versatility in Removal of Methylene Blue and Anti-ROS-Sensitive Tumor Cells

Oxidative degradation by using reactive oxygen species (ROS) is an effective method to treat pollutants. The synthesis of artificial oxidase for the degradation of dyes is a hot spot in molecular science. In this study, a nanoscale sandwich-type polyoxometalate (POM) on the basis of a tetra-nuclear cobalt cluster and trivacant B-α-Keggin-type tungstoarsenate {[Co(C₈H₂₀N₄)]₄}{Co₄(H₂O)₂[HAsW₉O₃₄]₂}∙4H₂O (abbreviated as CAW, C₈H₂₀N₄ = cyclen) has been synthesized and structurally examined by infrared (IR) spectrum, ultraviolet–visible (UV–Vis) spectrum, X-ray photoelectron spectrum (XPS), single-crystal X-ray diffraction (SXRD), and bond valence sum (Σs) calculation. According to the structural analysis, the principal element of the CAW is derived from modifying sandwich-type polyanion {Co₄(H₂O)₂ [HAsW₉O₃₄]₂}^8– with four [Co(Cyclen)]²⁺, in which 1,4,7,10-tetraazacyclododecane (cyclen) is firstly applied to modify POM. It is also demonstrated that CAW is capable of efficiently catalyzing the production of ROS by the synergistic effects of POM fragments and Co–cyclen complexes. Moreover, CAW can interfere with the morphology and proliferation of sensitive cells by producing ROS and exhibits ability in specifically eliminating methylene blue (MB) dyes from the solution system by both adsorption and catalytic oxidation.

Near-Infrared Fluorescent Probes as Imaging and Theranostic Modalities for Amyloid-Beta and Tau Aggregates in Alzheimer's Disease

A person suspected of having Alzheimer's disease (AD) is clinically diagnosed for the presence of principal biomarkers, especially misfolded amyloid-beta (Aβ) and tau proteins in the brain regions. Existing radiotracer diagnostic tools, such as PET imaging, are expensive and have limited availability for primary patient screening and pre-clinical animal studies. To change the status quo, small-molecular near-infrared (NIR) probes have been rapidly developed, which may serve as an inexpensive, handy imaging tool to comprehend the dynamics of pathogenic progression in AD and assess therapeutic efficacy in vivo. This Perspective summarizes the biochemistry of Aβ and tau proteins and then focuses on structurally diverse NIR probes with coverages of their spectroscopic properties, binding affinity toward Aβ and tau species, and theranostic effectiveness. With the summarized information and perspective discussions, we hope that this paper may serve as a guiding tool for designing novel in vivo imaging fluoroprobes with theranostic capabilities in the future.

Advances in fluorescent probes for detection and imaging of amyloid-β peptides in Alzheimer's disease

Amyloid plaques generated from the accumulation of amyloid-β peptides (Aβ) fibrils in the brain is one of the main hallmarks of Alzheimer's disease (AD), a most common neurodegenerative disorder. Aβ aggregation can produce neurotoxic oligomers and fibrils, which has been widely accepted as the causative factor in AD pathogenesis. Accordingly, both soluble oligomers and insoluble fibrils have been considered as diagnostic biomarkers for AD. Among the existing analytical methods, fluorometry using fluorescent probes has exhibited promising potential in quantitative detection and imaging of both soluble and insoluble Aβ species, providing a valuable approach for the diagnosis and drug development of AD. In this review, the most recent advances in the fluorescent probes for soluble or insoluble Aβ aggregates are discussed in terms of design strategy, probing mechanism, and potential applications. In the end, future research directions of fluorescent probes for Aβ species are also proposed.

Enzyme mimics based membrane reactor for di(2-ethylhexyl) phthalate degradation

Di(2-ethylhexyl) phthalate (DEHP), the most abundantly used plasticizer, was considered to be a hazardous chemical that was difficult to be degraded naturally. In this study, inspired by the "catalytic triad'' in serine proteases, an enzyme mimic material was developed by combining the proteases's active sites of serine, histidine and aspartate (S-H-D) with the self-assembling sequence of LKLKLKL and the aromatic group of fluorenylmethyloxycarbonyl (Fmoc). By mixing the monomer of peptides containing separate S, H and D residues with a ratio of 2:1:1, the enzyme mimics were found to co- assemble into nanofibers (Co-HSD) and showed the highest activity towards DEHP degradation because of the synergistic effects of active sites, orderly secondary structure and stable molecular conformation. To further improve ability and applicability, the high active mimetic enzyme was immobilized onto regenerated cellulose (RC) membranes for DEHP degradation in a continuous recycling mode. The RC membranes were first functionalized by the NaIO₄ oxidation method to form aldehyde groups and then conjugated with the enzyme mimics via Schiff-base reaction. As a biocatalytic membrane, this membrane could not only effectively degrade DEHP, but also showed good stability, thus establishing a promising biomaterial for large scale biodegradation of DEHP in water decontamination and liquid food depollution.

Small molecule-mediated co-assembly of amyloid-β oligomers reduces neurotoxicity through promoting non-fibrillar aggregation

Amyloid-β (Aβ) oligomers, particularly low molecular weight (LMW) oligomers, rather than fibrils, contribute very significantly to the onset and progression of Alzheimer's Disease (AD). However, due to the inherent heterogeneity and metastability of oligomers, most of the conventional anti-oligomer therapies have indirectly modulated oligomers' toxicity through manipulating Aβ self-assembly to reduce oligomer levels, which are prone to suffering from the risk of regenerating toxic oligomers from the products of modulation. To circumvent this disadvantage, we demonstrate, for the first time, rational design of rigid pincer-like scaffold-based small molecules with blood–brain barrier permeability that specifically co-assemble with LMW Aβ oligomers through directly binding to the exposed hydrophobic regions of oligomers to form non-fibrillar, degradable, non-toxic co-aggregates. As a proof of concept, treatment with a europium complex (EC) in such a structural mode can rescue Aβ-mediated dysfunction in C. elegans models of AD in vivo. This small molecule-mediated oligomer co-assembly strategy offers an efficient approach for AD treatment.

A rational design of pincer-like scaffold-based small molecule with blood-brain barrier permeability that can specifically co-assemble with low molecular weight Aβ oligomers to form non-fibrillar, degradable, non-toxic co-aggregates.

Determination of the Aggregate Binding Site of Amyloid Protofibrils Using Electron Capture Dissociation Tandem Mass Spectrometry

Amyloid fibril formation is a hallmark in a range of human diseases. Analysis of the molecular details of amyloid aggregation, however, is limited by the difficulties in solubilizing, separating, and identifying the aggregated biomolecules. Additional labeling or protein modification is required in many current analytical techniques in order to provide molecular details of amyloid protein aggregation, but these modifications may result in protein structure disruption. Herein, ultrahigh resolution mass spectrometry (MS) with electron capture dissociation tandem MS (ECD MS/MS) has been applied to monitor the formation of early oligomers of human islet amyloid polypeptide (hIAPP), which aggregate rapidly in the pancreas of type II diabetes (T2D) patients. ECD MS/MS results show the aggregation region of the early oligomers is at the Ser-28/Ser-29 residue of a hIAPP unit and at the Asn-35 residue of another hIAPP unit near the C-terminus in the gas phase. These data contribute to the understanding of the binding site between hIAPP units which may help for specific target region therapeutic development in the future. Furthermore, MS has also been applied to quantify the amount of soluble amyloid protein remaining in the incubated solutions, which can be used to estimate the aggregation rate of amyloid protein during incubation (28 days). These data are further correlated with the results obtained using fluorescence spectroscopy and transmission electron microscopy (TEM) to generate a general overview of amyloid protein aggregation. The methods demonstrated in this article not only explore the aggregation site of hIAPP down to an amino acid residue level, but are also applicable to many amyloid protein aggregation studies.

Multifunctional nano-enabled delivery systems in Alzheimer's disease management

This review discusses the recent advances in multifunctional nano-enabled delivery systems (NDS) for Alzheimer's disease management, including multitherapeutics, multimodal imaging-guided diagnostics, and theranostics.

Modulating Conformation of Aβ-Peptide: An Effective Way to Prevent Protein-Misfolding Disease

Alzheimer's disease (AD) is a typical protein-misfolding disease. Aggregation of amyloid β-peptide (Aβ) plays a key role in the etiology of AD. The misfolding of Aβ results in the formation of β-sheet-rich aggregates and damages the function of neurons. A modified polyoxometalate (POM), [CoL(H₂O)]₂[CoL]₂[HAs^VMo^V₆Mo^VI₆O₄₀] [CAM, L = 2-(1 H-pyrazol-3-yl)pyridine], was designed to disaggregate the Aβ aggregates, where L acts as an Aβ-targeting group and POM as a conformational modulator. X-ray crystallography shows that CAM is composed of a ε-Keggin unit and four coordination units. CAM can disaggregate the β-sheet-rich fibrils and metal-induced or self-aggregated Aβ aggregates, and it further inhibits the production of ROS; as a result, it can protect the neurons from synaptic toxicity induced by Zn²⁺- or Cu²⁺-Aβ aggregates or Aβ self-aggregation. The mechanism of disaggregation involves a transformation of Aβ conformation from β-sheet to other conformers. The nature of the process is an interference of the β-sheet conformation by CAM via hydrogen bonding. CAM specifically interacts with Aβ aggregates but does not disturb the cerebral metal homeostasis and enzymatic systems. Molecular simulation suggests that the appropriate size of CAM and the cavity of β-sheets facilitate the interaction between CAM and Aβ aggregates; additionally, the H-bonding-favored amino acid residues in the cavity provide a precondition for the interaction. Moreover, CAM is lipophilic and capable of penetrating the blood-brain barrier, and it is metabolizable without causing an untoward effect to mice at high dosages. In view of the significant inhibitory effect on the Aβ aggregation and related neurotoxicity, CAM represents a new type of leading compounds with a distinctive mechanism of action for the treatment of Alzheimer' disease. The conception of this study may be applied to other protein-misfolding diseases caused by conformational changes.

Amyloid β-targeted metal complexes for potential applications in Alzheimer's disease

Alzheimer's disease (AD) is currently an incurable neurodegenerative disorder that affects millions of people around the world. The aggregation of amyloid-β peptides (Aβ), one of the primary pathological hallmarks of AD, plays a key role in the AD pathogenesis. In this regard, Aβ aggregates have been considered as both biomarkers and drug targets for the diagnosis and therapy of AD. Various Aβ-targeted metal complexes have exhibited promising potential as anti-AD agents due to their fascinating physicochemical properties over the past two decades. This review classifies the complexes into three groups based on their potential applications in AD including therapy, diagnosis and theranosis. The recent representative examples are highlighted in terms of design rationale, working mechanism and potential applications.

Carbon Nanodot-Sensitized Modulation of Alzheimer's β-Amyloid Self-Assembly, Disassembly, and Toxicity

The self-assembly of amyloidogenic peptides into β-sheet-rich aggregates is a general feature of many neurodegenerative diseases, including Alzheimer's disease, which signifies the need for the effective attenuation of amyloid aggregation toward alleviating amyloid-associated neurotoxicity. This study reports that photoluminescent carbon nanodots (CDs) can effectively suppress Alzheimer's β-amyloid (Aβ) self-assembly and function as a β-sheet breaker disintegrating preformed Aβ aggregates. This study synthesizes CDs using ammonium citrate through one-pot hydrothermal treatment and passivates their surface with branched polyethylenimine (bPEI). The bPEI-coated CDs (bPEI@CDs) exhibit hydrophilic and cationic surface characteristics, which interact with the negatively charged residues of Aβ peptides, suppressing the aggregation of Aβ peptides. Under light illumination, bPEI@CDs display a more pronounced effect on Aβ aggregation and on the dissociation of β-sheet-rich assemblies through the generation of reactive oxygen species from photoactivated bPEI@CDs. The light-triggered attenuation effect of Aβ aggregation using a series of experiments, including photochemical and microscopic analysis, is verified. Furthermore, the cell viability test confirms the ability of photoactivated bPEI@CDs for the suppression of Aβ-mediated cytotoxicity, indicating bPEI@CDs' potency as an effective anti-Aβ neurotoxin agent.

Probing the structure-activity relationship of a novel artificial cellobiose hydrolase

The remarkable catalytic properties of enzymes contribute to their unique 3D structures and arrangement of amino acid residues, which provide a blueprint for the design of artificial enzymes. Here, a series of peptide catalysts (PCs) that mimic the unique orientation and function of β-glycosyl hydrolases were designed. Transmission electron microscopy (TEM), fluorescence analysis, circular dichroism spectroscopy, X-ray diffraction and computational modeling were used to investigate and compare the relationship of the fibrinous structure of PCs with its glycoside hydrolysis activity. These results indicated that the catalytic activity of PCs was not only related to their amyloid-like structures, but it can also be influenced by the site, species, molecular arrangement and steric hindrance of the amino acid sequence. What's more, this is the first report on peptide-inspired catalysts that mimic the natural cellobiose hydrolases. All this provided insights into the potential use of peptide nanoenzymes in the generation of efficient artificial enzymes.

A copper–amyloid-β targeted fluorescent chelator as a potential theranostic agent for Alzheimer's disease

A fluorescent chelator is able to specifically target and attenuate Cu²⁺–Aβ aggregates in the brain of mice with Alzheimer's disease, which can be visualized by fluorescence imaging of the chelator.