Role of Metal Cations of Copper, Iron, and Aluminum and Multifunctional Ligands in Alzheimer's Disease: Experimental and Computational Insights

Alzheimer's disease (AD) is the most common form of dementia, affecting millions of people around the world. Even though the causes of AD are not completely understood due to its multifactorial nature, some neuropathological hallmarks of its development have been related to the high concentration of some metal cations. These roles include the participation of these metal cations in the production of reactive oxygen species, which have been involved in neuronal damage. In order to avoid the increment in the oxidative stress, multifunctional ligands used to coordinate these metal cations have been proposed as a possible treatment to AD. In this review, we present the recent advances in experimental and computational works aiming to understand the role of two redox active and essential transition-metal cations (Cu and Fe) and one nonbiological metal (Al) and the recent proposals on the development of multifunctional ligands to stop or revert the damaging effects promoted by these metal cations.

Tuning Receptor Properties of Metal-Amyloid Beta Complexes. Studies on the Interaction between Ni(II)-Aβ5-9 and Phosphates/Nucleotides

Amyloid-beta (Aβ) peptides, potentially relevant in the pathology of Alzheimer's disease, possess distinctive coordination properties, enabling an effective binding of transition-metal ions, with a preference for Cu(II). In this work, we found that a N-truncated Aβ analogue bearing a His-2 motif, Aβ_5-9, forms a stable Ni(II) high-spin octahedral complex at a physiological pH of 7.4 with labile coordination sites and facilitates ternary interactions with phosphates and nucleotides. As the pH increased above 9, a spin transition from a high-spin to a low-spin square-planar Ni(II) complex was observed. Employing electrochemical techniques, we showed that interactions between the binary Ni(II)-Aβ_5-9 complex and phosphate species result in significant changes in the Ni(II) oxidation signal. Thus, the Ni(II)-Aβ_5-9 complex could potentially serve as a receptor in electrochemical biosensors for phosphate species. The obtained results could also be important for nickel toxicology.

Identification of ortho catechol-containing isoflavone as a privileged scaffold that directly prevents the aggregation of both amyloid β plaques and tau-mediated neurofibrillary tangles and its in vivo evaluation

In this study, polyhydroxyisoflavones that directly prevent the aggregation of both amyloid β (Aβ) and tau were expediently synthesized via divergent Pd(0)-catalyzed Suzuki-Miyaura coupling and then biologically evaluated. By preliminary structure-activity relationship studies using thioflavin T (ThT) assays, an ortho-catechol containing isoflavone scaffold was proven to be crucial for preventing both Aβ aggregation and tau-mediated neurofibrillary tangle formation. Additional TEM experiment confirmed that ortho-catechol containing isoflavone 4d significantly prevented the aggregation of both Aβ and tau. To investigate the mode of action (MOA) of 4d, which possesses an ortho-catechol moiety, ¹H-¹⁵N HSQC NMR analysis was thoroughly performed and the result indicated that 4d could directly inhibit both the formation of Aβ₄₂ fibrils and the formation of tau-derived neurofibrils, probably through the catechol-mediated nucleation of tau. Finally, 4d was demonstrated to alleviate cognitive impairment and pathologies related to Alzheimer's disease in a 5XFAD transgenic mouse model.

N-Terminal Cu-Binding Motifs (Xxx-Zzz-His, Xxx-His) and Their Derivatives: Chemistry, Biology and Medicinal Applications

Peptides and proteins with N-terminal amino acid sequences NH2 -Xxx-His (XH) and NH2 -Xxx-Zzz-His (XZH) form well-established high-affinity CuII -complexes. Key examples are Asp-Ala-His (in serum albumin) and Gly-His-Lys, the wound healing factor. This opens a straightforward way to add a high-affinity CuII -binding site to almost any peptide or protein, by chemical or recombinant approaches. Thus, these motifs, NH2 -Xxx-Zzz-His in particular, have been used to equip peptides and proteins with a multitude of functions based on the redox activity of Cu, including nuclease, protease, glycosidase, or oxygen activation properties, useful in anticancer or antimicrobial drugs. More recent research suggests novel biological functions, mainly based on the redox inertness of CuII in XZH, like PET imaging (with 64 Cu), chelation therapies (for instance in Alzheimer's disease and other types of neurodegeneration), antioxidant units, Cu transporters and activation of biological functions by strong CuII binding. This Review gives an overview of the chemical properties of Cu-XH and -XZH motifs and discusses the pros and cons of the vastly different biological applications, and how they could be improved depending on the application.

Small Molecules: Therapeutic Application in Neuropsychiatric and Neurodegenerative Disorders

In recent years, an increasing number of studies have been published, focusing on the potential therapeutic use of small catalytic agents with strong biological properties. So far, most of these works have only regarded specific clinical fields, such as oncology, infectivology and general pathology, in particular with respect to the treatment of significant inflammatory processes. However, interesting data on possible therapeutic applications of small molecules for the treatment of neuropsychiatric and neurodegenerative illnesses are emerging, especially with respect to the possibility to modulate the cellular redox state. Indeed, a crucial role of redox dysregulation in the pathogenesis of these disorders has been widely demonstrated by both pre-clinical and clinical studies, being the reduction of the total amount of free radicals a promising novel therapeutic approach for these diseases. In this review, we focused our interest on studies published during the last ten years reporting therapeutic potential of small molecules for the treatment of neuropsychiatric and neurodegenerative disorders, also based on the biological efficiency of these compounds in detecting intracellular disturbances induced by increased production of reactive oxygen species.

Bamboo leaf extract improves spatial learning ability in a rat model with senile dementia

Senile dementia (SD) is a syndrome characterized by progressive neurological deterioration. Treatment for the disease is still under investigation. Bamboo leaf extract (B-extract) has been known for its biological efficacy in anti-oxidant and anti-cancer activities. However, study on B-extract for its protection against dementia is very limited. The effect of B-extract on a rat model with SD was examined. B-extract improved spatial learning ability of the dementia rats. The hippocampus of dementia model rats showed reduced levels of acetylcholine (ACh), epinephrine (E), norepinephrine (NE), and dopamine (DA), and increased activities of acetylcholine esterase (AChE) and monoamine oxidase (MAO). Treatment with B-extract 20 mg/(kg·d) for 7 weeks significantly inhibited the enzyme activity compared with untreated dementia rats, and raised the levels of ACh, E, and DA in the hippocampus. In addition, treatment with B-extract elevated the level of γ-aminobutyric acid (GABA), but reduced the level of glutamate (Glu) in the brain. These data suggest that B-extract might be a potential drug in treating impairment of spatial memory in dementia rats by regulating the central neurotransmitter function.

New Hydroxyquinoline-Based Derivatives as Potent Modulators of Amyloid-β Aggregations

Copper and zinc have been found to contribute to the burden of amyloid-β (Aβ) aggregations in neurodegenerative Alzheimer's disease (AD). Dysregulation of these metals leads to the generation of reactive oxygen species (ROS) and eventually results in oxidative damage and accumulation of the Aβ peptide, which are the key elements of the disease. Aiming to pursue the discovery of new modulators for the disease, we here rationally focused on conjugating the core hydroxyquinoline of the metal-protein attenuating compound PBT2 and the N-methylanilide analogous moiety of the Aβ imaging agent to build a new type of multi-target modulators of Aβ aggregations. We found that the N,N-dimethylanilinyl imines 7a, 8a, and the corresponding amines 7b, 8b exerted efficient inhibition of Cu(2+) - or Zn(2+) -induced Aβ aggregations and significant disassembly of metal-mediated Aβ aggregated fibrils. Further, 7a and 7b also exhibited significant ROC scavenging effects compared to PBT2. The results suggested that 7a and 7b are promising lead compounds for the development of a new therapy for AD.

Overview of Alzheimer's Disease and Some Therapeutic Approaches Targeting Aβ by Using Several Synthetic and Herbal Compounds

Alzheimer's disease (AD) is a complex age-related neurodegenerative disease. In this review, we carefully detail amyloid-β metabolism and its role in AD. We also consider the various genetic animal models used to evaluate therapeutics. Finally, we consider the role of synthetic and plant-based compounds in therapeutics.

Sialic Acid Hydroxamate: A Potential Antioxidant and Inhibitor of Metal-Induced β-Amyloid Aggregates

Current methods for Alzheimer's treatment require a three-component system: metal chelators, antioxidants, and amyloid β (Aβ)-peptide-binding scaffolds. We report sialic acid (Sia) hydroxamate as a potential radical scavenger and metal chelator to inhibit Aβ aggregation. A cell viability assay revealed that Sia hydroxamate can protect HeLa and glioblastoma (LN229) cells from oxidative damage induced by the Fenton reaction. Sedimentation and turbidity assays showed profound protection of neuroblastoma SH-SY5Y cells from metal-induced Aβ aggregation and neural toxicity.

Computational evidence to design an appropriate candidate for the treatment of Alzheimer's disease through replacement of the heptamethylene linker of bis(7)tacrine with S-allylcysteine

Due to the multiple pathogens of Alzheimer's disease, multitarget-directed ligand (MTDL) design has been highly regarded in recent years.

Synthetic Flavonoids, Aminoisoflavones: Interaction and Reactivity with Metal-Free and Metal-Associated Amyloid-β Species

Metal ion homeostasis in conjunction with amyloid-β (Aβ) aggregation in the brain has been implicated in Alzheimer's disease (AD) pathogenesis. To uncover the interplay between metal ions and Aβ peptides, synthetic, multifunctional small molecules have been employed to modulate Aβ aggregation in vitro. Naturally occurring flavonoids have emerged as a valuable class of compounds for this purpose due to their ability to modulate both metal-free and metal-induced Aβ aggregation. Although, flavonoids have shown anti-amyloidogenic effects, the structural moieties of flavonoids responsible for such reactivity have not been fully identified. In order to understand the structure-interaction-reactivity relationship within the flavonoid family for metal-free and metal-associated Aβ, we designed, synthesized, and characterized a set of isoflavone derivatives, aminoisoflavones (1-4), that displayed reactivity (i.e., modulation of Aβ aggregation) in vitro. NMR studies revealed a potential binding site for aminoisoflavones between the N-terminal loop and central helix on prefibrillar Aβ different from the non-specific binding observed for other flavonoids. The absence or presence of the catechol group differentiated the binding affinities and enthalpy/entropy balance between aminoisoflavones and Aβ. Furthermore, having a catechol group influenced the binding mode with fibrillar Aβ. Inclusion of additional substituents moderately tuned the impact of aminoisoflavones on Aβ aggregation. Overall, through these studies, we obtained valuable insights on the requirements for parity among metal chelation, intermolecular interactions, and substituent variation for Aβ interaction.

Hippocampal neuronal metal ion imbalance related oxidative stress in a rat model of chronic aluminum exposure and neuroprotection of meloxicam

Neurodegenerative diseases remain a significant unresolved societal burden afflicting millions of people worldwide. Neurons in the brain are highly sensitive to oxidative stress, which can be induced by metal toxicity. In this paper, a chronic aluminum overload-induced model of neurodegeneration was used to investigate whether metal ions (Al, Fe, Mn, Cu and Zn)-related oxidative stress was involved in neurodegenerative mechanism and to identify the protective action of meloxicam against rat hippocampal neuronal injury. The metal ion contents, activity of superoxide dismutase (SOD), and content of malondialdehyde (MDA) were detected. The results showed that the spatial learning and memory (SLM) function was significantly impaired in chronic aluminum overload rats. Considerable karyopycnosis was observed in hippocampal neurons. The SOD activity was weakened and the MDA content increased both significantly. In the hippocampus, Al, Fe, Mn, Cu, and Zn contents increased by 184.1%, 186.1%, 884.2%, 199.4% and 149.2%, respectively. Meloxicam administration (without Al) had no effect compared with the control group, while meloxicam treatment with aluminum exposure significantly protected rats from SLM function impairment, neuron death, lower SOD activity, higher MDA content and brain metal ion imbalance. Our findings suggest that the cerebral metal ion imbalance-related oxidative stress is involved in mechanism of cerebral injury and neurodegeneration induced by chronic Al overload in rats, and that meloxicam protects neurons by reducing metal ion imbalance-related oxidative stress.

Neuroprotective peptide–macrocycle conjugates reveal complex structure–activity relationships in their interactions with amyloid β

Novel neuroprotective peptide–macrocycle conjugates exhibit complex, multifaceted structure–activity relationships in their interactions with amyloid β.

Neuroprotective Role of a Novel Copper Chelator against Aβ 42 Induced Neurotoxicity

Alzheimer's disease (AD) is a progressive neurodegenerative disease and associated with the extracellular deposits of amyloid- β peptide in hippocampus region. Metal ions like Cu, Fe and Zn are known to associate with the amyloid beta (A β ) at high concentration and interaction of these ions with soluble and aggregated forms of A β peptide help in development of AD. Here we showed Cu mediated neurotoxicity in the eye tissues of transgenic Drosophila expressing human amyloid β and its rescue through a novel Cu chelator. In this context, we have synthesised and characterized the compound L 2,6-Pyridinedicarboxylic acid, 2,6-bis[2-[(4-carboxyphenyl) methylene] hydrazide] by Mass spectra (MS) and Elemental analysis (EA). The Cu chelation potential of the compound L is tested in vivo in Drosophila. Oral administration of Copper to the transgenic larvae resulted in severe degeneration in eye tissues, which was rescued by the supplementation of compound L. The levels of anti-oxidant markers like SOD and MDA were measured in compound L treated flies and found a significant rescue (P < 0.001). Further rescue of the eye degeneration phenotypes as revealed by SEM affirm the role of copper in A β toxicity. Hence, use of compound L, an amidoamine derivative, could be a possible therapeutic measure for A β induced neurotoxicity.

A bioinorganic view of Alzheimer's disease: when misplaced metal ions (re)direct the electrons to the wrong target

Metal ions Cu, Zn and Fe, seem to play a pivotal role in Alzheimer's disease and other neurodegenerative diseases. In order to understand this in a broader sense, one has to considerer the peculiarities of metal metabolism in the brain compared to most other tissues, as well as the importance of the redox active metal ions, Fe and Cu, in oxygen metabolism and the connected oxidative stress.

Reactivity of diphenylpropynone derivatives toward metal-associated amyloid-β species

In Alzheimer's disease (AD), metal-associated amyloid-β (metal-Aβ) species have been suggested to be involved in neurotoxicity; however, their role in disease development is still unclear. To elucidate this aspect, chemical reagents have been developed as valuable tools for targeting metal-Aβ species, modulating the interaction between the metal and Aβ, and subsequently altering metal-Aβ reactivity. Herein, we report the design, preparation, characterization, and reactivity of two diphenylpropynone derivatives (DPP1 and DPP2) composed of structural moieties for metal chelation and Aβ interaction (bifunctionality). The interactions of these compounds with metal ions and Aβ species were confirmed by UV-vis, NMR, mass spectrometry, and docking studies. The effects of these bifunctional molecules on the control of in vitro metal-free and metal-induced Aβ aggregation were investigated and monitored by gel electrophoresis and transmission electron microscopy (TEM). Both DPP1 and DPP2 showed reactivity toward metal-Aβ species over metal-free Aβ species to different extents. In particular, DPP2, which contains a dimethylamino group, exhibited greater reactivity with metal-Aβ species than DPP1, suggesting a structure-reactivity relationship. Overall, our studies present a new bifunctional scaffold that could be utilized to develop chemical reagents for investigating metal-Aβ species in AD.

A new water-soluble Cu(II) chelator that retrieves Cu from Cu(amyloid-β) species, stops associated ROS production and prevents Cu(II)-induced Aβ aggregation

The synthesis of the H(2)L(2-) ligand (N,N'-Bis[(5-sulfonato-2-hydroxy)benzyl]-N,N'-dimethyl-ethane-1,2-diamine) and characterizations of the corresponding Cu(II) complex [Cu(L)(H(2)O)](2-) (1) by X-ray diffraction, EPR, UV-Visible and potentiometry are described. At pH 7.4, the affinity of Cu(II) for this ligand is approximately 4 × 10(14)M(-1). Coordination of redox active metal ions such as copper or iron to the amyloid-β (Aβ) peptide has been linked to deleterious processes encountered in the etiology of Alzheimer disease (AD), such as Aβ aggregation and reactive oxygen species (ROS) production. In this context, the ability of the H(2)L(2-) to extract Cu(II) from Cu(Aβ) species where Aβ is the peptide involved in AD, is reported as well as its capacity to redox silence the Cu(Aβ) induced ROS formation and to prevent Cu(II)-induced Aβ aggregation. Such water soluble sulfonato-derivatives of Cu(II) chelators are very interesting counterparts for in vitro study of chelators' properties required to attend further biological applications as therapeutic tools against AD.

Bifunctional compounds for controlling metal-mediated aggregation of the aβ42 peptide

Abnormal interactions of Cu and Zn ions with the amyloid β (Aβ) peptide are proposed to play an important role in the pathogenesis of Alzheimer's disease (AD). Disruption of these metal-peptide interactions using chemical agents holds considerable promise as a therapeutic strategy to combat this incurable disease. Reported herein are two bifunctional compounds (BFCs) L1 and L2 that contain both amyloid-binding and metal-chelating molecular motifs. Both L1 and L2 exhibit high stability constants for Cu(2+) and Zn(2+) and thus are good chelators for these metal ions. In addition, L1 and L2 show strong affinity toward Aβ species. Both compounds are efficient inhibitors of the metal-mediated aggregation of the Aβ(42) peptide and promote disaggregation of amyloid fibrils, as observed by ThT fluorescence, native gel electrophoresis/Western blotting, and transmission electron microscopy (TEM). Interestingly, the formation of soluble Aβ(42) oligomers in the presence of metal ions and BFCs leads to an increased cellular toxicity. These results suggest that for the Aβ(42) peptide-in contrast to the Aβ(40) peptide-the previously employed strategy of inhibiting Aβ aggregation and promoting amyloid fibril dissagregation may not be optimal for the development of potential AD therapeutics, due to formation of neurotoxic soluble Aβ(42) oligomers.

Misfolded proteins in Alzheimer's disease and type II diabetes

This tutorial review presents descriptions of two amyloidogenic proteins, amyloid-β (Aβ) peptides and islet amyloid polypeptide (IAPP), whose misfolding propensities are implicated in Alzheimer's disease (AD) and type II diabetes, respectively. Protein misfolding diseases share similarities, as well as some unique protein-specific traits, that could contribute to the initiation and/or development of their associated conditions. Aβ and IAPP are representative amyloidoses and are used to highlight some of the primary considerations for studying misfolded proteins associated with human diseases in this review. Among these factors, their physiological formation, aggregation, interactions with metal ions and other protein partners, and toxicity are presented. Small molecules that target and modulate the metal-Aβ interaction and neurotoxicity are included to illustrate one of the current approaches for uncovering the complexities of protein misfolding at the molecular level.

Development of bifunctional stilbene derivatives for targeting and modulating metal-amyloid-β species

Amyloid-β (Aβ) peptides and their metal-associated aggregated states have been implicated in the pathogenesis of Alzheimer's disease (AD). Although the etiology of AD remains uncertain, understanding the role of metal-Aβ species could provide insights into the onset and development of the disease. To unravel this, bifunctional small molecules that can specifically target and modulate metal-Aβ species have been developed, which could serve as suitable chemical tools for investigating metal-Aβ-associated events in AD. Through a rational structure-based design principle involving the incorporation of a metal binding site into the structure of an Aβ interacting molecule, we devised stilbene derivatives (L1-a and L1-b) and demonstrated their reactivity toward metal-Aβ species. In particular, the dual functions of compounds with different structural features (e.g., with or without a dimethylamino group) were explored by UV-vis, X-ray crystallography, high-resolution 2D NMR, and docking studies. Enhanced bifunctionality of compounds provided greater effects on metal-induced Aβ aggregation and neurotoxicity in vitro and in living cells. Mechanistic investigations of the reaction of L1-a and L1-b with Zn(2+)-Aβ species by UV-vis and 2D NMR suggest that metal chelation with ligand and/or metal-ligand interaction with the Aβ peptide may be driving factors for the observed modulation of metal-Aβ aggregation pathways. Overall, the studies presented herein demonstrate the importance of a structure-interaction-reactivity relationship for designing small molecules to target metal-Aβ species allowing for the modulation of metal-induced Aβ reactivity and neurotoxicity.

Three dimensional models of Cu(2+)-Aβ(1-16) complexes from computational approaches

Elucidation of the coordination of metal ions to Aβ is essential to understand their role in its aggregation and to rationally design new chelators with potential therapeutic applications in Alzheimer disease. Because of that, in the last 10 years several studies have focused their attention in determining the coordination properties of Cu(2+) interacting with Aβ. However, more important than characterizing the first coordination sphere of the metal is the determination of the whole Cu(2+)-Aβ structure. In this study, we combine homology modeling (HM) techniques with quantum mechanics based approaches (QM) to determine plausible three-dimensional models for Cu(2+)-Aβ(1-16) with three histidines in their coordination sphere. We considered both ε and δ coordination of histidines 6, 13, and 14 as well as the coordination of different possible candidates containing oxygen as fourth ligand (Asp1, Glu3, Asp7, Glu11, and CO(Ala2)). Among the 32 models that enclose COO(-), the lowest energy structures correspond to [O(E3),N(δ)(H6),N(ε)(H13),N(ε)(H14)] (1), [O(E3),N(δ)(H6),N(δ)(H13),N(δ)(H14)] (2), and [O(D7),N(ε)(H6),N(δ)(H13),N(δ)(H14)] (3). The most stable model containing CO(Ala2) as fourth ligand in the Cu(2+) coordination sphere is [O(c)(A2),N(ε)(H6),N(δ)(H13),N(ε)(H14)] (4). An estimation of the relative stability between Glu3 (1) and CO(Ala2) (4) coordinated complexes seems to indicate that the preference for the latter coordination may be due to solvent effects. The present results also show the relationship between the peptidic and metallic moieties in defining the overall geometry of the complex and illustrate that the final stability of the complexes results from a balance between the metal coordination site and amyloid folding upon complexation.

Recent Development of Bifunctional Small Molecules to Study Metal-Amyloid-β Species in Alzheimer's Disease

Alzheimer's disease (AD) is a multifactorial neurodegenerative disease related to the deposition of aggregated amyloid-β (Aβ) peptides in the brain. It has been proposed that metal ion dyshomeostasis and miscompartmentalization contribute to AD progression, especially as metal ions (e.g., Cu(II) and Zn(II)) found in Aβ plaques of the diseased brain can bind to Aβ and be linked to aggregation and neurotoxicity. The role of metal ions in AD pathogenesis, however, is uncertain. To accelerate understanding in this area and contribute to therapeutic development, recent efforts to devise suitable chemical reagents that can target metal ions associated with Aβ have been made using rational structure-based design that combines two functions (metal chelation and Aβ interaction) in the same molecule. This paper presents bifunctional compounds developed by two different design strategies (linkage or incorporation) and discusses progress in their applications as chemical tools and/or potential therapeutics.

Design of small molecules that target metal-A{beta} species and regulate metal-induced A{beta} aggregation and neurotoxicity

The accumulation of metal ions and amyloid-β (Aβ) aggregates found in the brain of patients with Alzheimer's disease (AD) has been suggested to be involved in AD pathogenesis. To investigate metal-Aβ-associated pathways in AD, development of chemical tools to target metal-Aβ species is desired. Only a few efforts, however, have been reported. Here, we report bifunctional small molecules, N-(pyridin-2-ylmethyl)aniline (L2-a) and N(1),N(1)-dimethyl-N(4)-(pyridin-2-ylmethyl)benzene-1,4-diamine (L2-b) that can interact with both metal ions and Aβ species, as determined by spectroscopic methods including high-resolution NMR spectroscopy. Using the bifunctional compound L2-b, metal-induced Aβ aggregation and neurotoxicity were modulated in vitro as well as in human neuroblastoma cells. Furthermore, treatment of human AD brain tissue homogenates containing metal ions and Aβ species with L2-b showed disassembly of Aβ aggregates. Therefore, our studies presented herein demonstrate the value of bifunctional compounds as chemical tools for investigating metal-Aβ-associated events and their mechanisms in the development and pathogenesis of AD and as potential therapeutics.