Targeting Biometals in Alzheimer's Disease with Metal Chelating Agents Including Coumarin Derivatives

Numerous physiological processes happening in the human body, including cerebral development and function, require the participation of biometal ions such as iron, copper, and zinc. Their dyshomeostasis may, however, contribute to the onset of Alzheimer's disease (AD) and potentially other neurodegenerative diseases. Chelation of biometal ions is therefore a therapeutic strategy against AD. This review provides a survey of natural and synthetic chelating agents that are or could potentially be used to target the metal hypothesis of AD. Since metal dyshomeostasis is not the only pathological aspect of AD, and the nature of this disorder is very complex and multifactiorial, the most efficient therapeutics should target as many neurotoxic factors as possible. Various coumarin derivatives match this description and apart from being able to chelate metal ions, they exhibit the capacity to inhibit cholinesterases (ChEs) and monoamine oxidase B (MAO-B) while also possessing antioxidant, anti-inflammatory, and numerous other beneficial effects. Compounds based on the coumarin scaffold therefore represent a desirable class of anti-AD therapeutics.

Targeting misfolding and aggregation of the amyloid-β peptide and mutant p53 protein using multifunctional molecules

Biomolecule misfolding and aggregation play a major role in human disease, spanning from neurodegeneration to cancer. Inhibition of these processes is of considerable interest, and due to the multifactorial nature of these diseases, the development of drugs that act on multiple pathways simultaneously is a promising approach. This Feature Article focuses on the development of multifunctional molecules designed to inhibit the misfolding and aggregation of the amyloid-β (Aβ) peptide in Alzheimer's disease (AD), and the mutant p53 protein in cancer. While for the former, the goal is to accelerate the removal of the Aβ peptide and associated aggregates, for the latter, the goal is reactivation via stabilization of the active folded form of mutant p53 protein and/or aggregation inhibition. Due to the similar aggregation pathway of the Aβ peptide and mutant p53 protein, a common therapeutic approach may be applicable.

Alzheimer's Drug PBT2 Interacts with the Amyloid β 1-42 Peptide Differently than Other 8-Hydroxyquinoline Chelating Drugs

Although Alzheimer's disease (AD) was first described over a century ago, it remains the leading cause of age-related dementia. Innumerable changes have been linked to the pathology of AD; however, there remains much discord regarding which might be the initial cause of the disease. The "amyloid cascade hypothesis" proposes that the amyloid β (Aβ) peptide is central to disease pathology, which is supported by elevated Aβ levels in the brain before the development of symptoms and correlations of amyloid burden with cognitive impairment. The "metals hypothesis" proposes a role for metal ions such as iron, copper, and zinc in the pathology of AD, which is supported by the accumulation of these metals within amyloid plaques in the brain. Metals have been shown to induce aggregation of Aβ, and metal ion chelators have been shown to reverse this reaction in vitro. 8-Hydroxyquinoline-based chelators showed early promise as anti-Alzheimer's drugs. Both 5-chloro-7-iodo-8-hydroxyquinoline (CQ) and 5,7-dichloro-2-[(dimethylamino)methyl]-8-hydroxyquinoline (PBT2) underwent unsuccessful clinical trials for the treatment of AD. To gain insight into the mechanism of action of 8HQs, we have investigated the potential interaction of CQ, PBT2, and 5,7-dibromo-8-hydroxyquinoline (B2Q) with Cu(II)-bound Aβ(1-42) using X-ray absorption spectroscopy (XAS), high energy resolution fluorescence detected (HERFD) XAS, and electron paramagnetic resonance (EPR). By XAS, we found CQ and B2Q sequestered ∼83% of the Cu(II) from Aβ(1-42), whereas PBT2 sequestered only ∼59% of the Cu(II) from Aβ(1-42), suggesting that CQ and B2Q have a higher relative Cu(II) affinity than PBT2. From our EPR, it became clear that PBT2 sequestered Cu(II) from a heterogeneous mixture of Cu(II)Aβ(1-42) species in solution, leaving a single Cu(II)Aβ(1-42) species. It follows that the Cu(II) site in this Cu(II)Aβ(1-42) species is inaccessible to PBT2 and may be less solvent-exposed than in other Cu(II)Aβ(1-42) species. We found no evidence to suggest that these 8HQs form ternary complexes with Cu(II)Aβ(1-42).

Copper Toxicity Links to Pathogenesis of Alzheimer's Disease and Therapeutics Approaches

Alzheimer's disease (AD) is an irreversible, age-related progressive neurological disorder, and the most common type of dementia in aged people. Neuropathological lesions of AD are neurofibrillary tangles (NFTs), and senile plaques comprise the accumulated amyloid-beta (Aβ), loaded with metal ions including Cu, Fe, or Zn. Some reports have identified metal dyshomeostasis as a neurotoxic factor of AD, among which Cu ions seem to be a central cationic metal in the formation of plaque and soluble oligomers, and have an essential role in the AD pathology. Cu-Aβ complex catalyzes the generation of reactive oxygen species (ROS) and results in oxidative damage. Several studies have indicated that oxidative stress plays a crucial role in the pathogenesis of AD. The connection of copper levels in AD is still ambiguous, as some researches indicate a Cu deficiency, while others show its higher content in AD, and therefore there is a need to increase and decrease its levels in animal models, respectively, to study which one is the cause. For more than twenty years, many in vitro studies have been devoted to identifying metals' roles in Aβ accumulation, oxidative damage, and neurotoxicity. Towards the end, a short review of the modern therapeutic approach in chelation therapy, with the main focus on Cu ions, is discussed. Despite the lack of strong proofs of clinical advantage so far, the conjecture that using a therapeutic metal chelator is an effective strategy for AD remains popular. However, some recent reports of genetic-regulating copper transporters in AD models have shed light on treating this refractory disease. This review aims to succinctly present a better understanding of Cu ions' current status in several AD features, and some conflicting reports are present herein.

Cu and Zn interactions with Aβ peptides: consequence of coordination on aggregation and formation of neurotoxic soluble Aβ oligomers

The coordination chemistry of transition metal ions (Fe, Cu, Zn) with the amyloid-β (Aβ) peptides has attracted a lot of attention in recent years due to its repercussions in Alzheimer's disease (AD).

Azo-dyes based small bifunctional molecules for metal chelation and controlling amyloid formation

Chemical tools are needed to discover new effective drugs for tackling multifaceted complex neurodegenerative diseases like Alzheimer's disease (AD). Multifunctional nature of two compounds, 5-((4-nitro-phenyl)diazenyl)quinolin-8-ol (HL1) and 4-((4-nitrophenyl)diazenyl)benzene-1,3-diol (HL2) is reported w.r.t. their ability to bind Cu2+ ions and amyloid aggregates related to AD. HL1 and HL2 have half congo-red type azo-stilbene structural framework incorporated with metal chelating groups, designed to chelate metal ions from metal-amyloid species. Metal binding studies of HL1 and HL2 are established by the methods of Job's Plot, UV-vis spectra with metal ions and stability constant determination. In addition, their metal complexes are isolated, purity checked by elemental analysis, spectroscopically characterized and their structural analyses were obtained from DFT based calculations including binding energy determination. Chicken egg white Lysozyme (CEWL) was used as a model peptide for fibrillation studies. HL1 is found as an excellent colorimetric sensor for amyloid fibrils. Inhibitory effect of HL1 and HL2 and their isolated metal complexes L1-Cu and L2-Cu on CEWL fibrillation was studied using ThT and ANS fluorescence assay along with TEM imaging. In addition, the cell toxicity studies on these compounds suggest that although azo dyes may be non-toxic but having a nitro-substitution lead to significant cell toxicity. Overall, these results suggest that this new class of multifunctional small molecules can interact with amyloids as well as metal ions and could be potential anti-aggregation metal chelating agents.

Effects of hydroxyl group variations on a flavonoid backbone toward modulation of metal-free and metal-induced amyloid-β aggregation

Structural variations of a flavonoid framework noticeably tune the interaction and reactivity of flavonoids with metals, Aβ, and metal–Aβ.

Synthesis, Spectral Characterization, and Biochemical Evaluation of Antidiabetic Properties of a New Zinc-Diosmin Complex Studied in High Fat Diet Fed-Low Dose Streptozotocin Induced Experimental Type 2 Diabetes in Rats

In view of the established antidiabetic properties of zinc, the present study was aimed at evaluating the hypoglycemic properties of a new zinc-diosmin complex in high fat diet fed-low dose streptozotocin induced experimental type 2 diabetes in rats. Zinc-diosmin complex was synthesized and characterized by various spectral studies. The complexation between zinc ions and diosmin was further evidenced by pH-potentiometric titrations and Job's plot. Diabetic rats were orally treated with zinc-diosmin complex at a concentration of 20 mg/kg b.w./rat/day for 30 days. At the end of the experimental period, the rats were subjected to oral glucose tolerance test. In addition, HOMA-IR and various biochemical parameters related to glucose homeostasis were analyzed. Treatment with zinc-diosmin complex significantly improved the glucose homeostasis in diabetic rats. Treatment with zinc-diosmin complex significantly improved insulin sensitivity, at least in part, through enhancing protein metabolism and alteration in the levels of muscle and liver glycogen. The assay of clinical marker enzymes revealed the nontoxic nature of the complex. Determination of renal tissue markers such as blood urea and serum creatinine indicates the renoprotective nature of the complex. These findings suggest that zinc-diosmin complex is nontoxic and has complimentary potential to develop as an antihyperglycemic agent for the treatment of diabetes mellitus.

Insight into inhibition of the human amyloid beta protein precursor (APP: PDB ID ) using (E)-N-(pyridin-2-ylmethylene)arylamine (LR) models: structure elucidation of a family of ZnX2-LR complexes

The amyloid beta precursor protein (APP) and its neurotoxic cleavage product amyloid beta (Aβ) are a cause of Alzheimer's disease and appear essential for neuronal development and cell homeostasis. Proteolytic processing of APP is influenced by metal ions and protein ligands, however the structural and functional mechanism of APP regulation is not known so far. In this context, molecular modeling studies were performed to understand the molecular behavior of (E)-N-(pyridin-2-ylmethylene)arylamines (LR) with an E2 domain of the APP in its complex with zinc (APP; PDB ID: ). Docking results indeed confirmed that the LR interacts with Zn in the binding site of the protein between two α-helical chains. In view of these findings, LR was further investigated for complexation reactions with Zn(2+) in order to establish the structural models in solution and in the solid state. Five new Zn(2+) complexes of compositions viz. [Zn(Br)2(L2-Me)] (), [Zn(Br)2(L2-OMe)] (), [Zn(i)2(L2-OMe)] (), [Zn(NO3)2(L2-OMe)(H2O)] () and [Zn(L4-Me)2(H2O)2](NO3)2 () were synthesized and their structures were ascertained by microanalysis, IR and (1)H NMR spectroscopy, and single-crystal X-ray diffraction. The zinc atom in complex exhibits a distorted tetrahedral geometry while the crystal structures of complexes and show distorted square pyramidal geometries. The zinc cation in and has an octahedral coordination environment, but in the zinc coordination geometry is less distorted. The Zn(ii) cations take part in one ( and ) or two () 5-membered metallacycles imposed by the NN or NNO chelation modes of LR. The significant intermolecular ππ interactions are also discussed.

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.

The ongoing search for small molecules to study metal-associated amyloid-β species in Alzheimer's disease

The development of a cure for Alzheimer's disease (AD) has been impeded by an inability to pinpoint the root cause of this disorder. Although numerous potential pathological factors have been indicated, acting either individually or mutually, the molecular mechanisms leading to disease onset and progression have not been clear. Amyloid-β (Aβ), generated from proteolytic processing of the amyloid precursor protein (APP), and its aggregated forms, particularly oligomers, are suggested as key pathological features in AD-affected brains. Historically, highly concentrated metals are found colocalized within Aβ plaques. Metal binding to Aβ (metal-Aβ) generates/stabilizes potentially toxic Aβ oligomers, and produces reactive oxygen species (ROS) in vitro (redox active metal ions; plausible contribution to oxidative stress). Consequently, clarification of the relationship between Aβ, metal ions, and toxicity, including oxidative stress via metal-Aβ, can lead to a deeper understanding of AD development. To probe the involvement of metal-Aβ in AD pathogenesis, rationally designed and naturally occurring molecules have been examined as chemical tools to target metal-Aβ species, modulate the interaction between the metal and Aβ, and subsequently redirect their aggregation into nontoxic, off-pathway unstructured aggregates. These ligands are also capable of attenuating the generation of redox active metal-Aβ-induced ROS to mitigate oxidative stress. One rational design concept, the incorporation approach, installs a metal binding site into a framework known to interact with Aβ. This approach affords compounds with the simultaneous ability to chelate metal ions and interact with Aβ. Natural products capable of Aβ interaction have been investigated for their influence on metal-induced Aβ aggregation and have inspired the construction of synthetic analogues. Systematic studies of these synthetic or natural molecules could uncover relationships between chemical structures, metal/Aβ/metal-Aβ interactions, and inhibition of Aβ/metal-Aβ reactivity (i.e., aggregation modes of Aβ/metal-Aβ; associated ROS production), suggesting mechanisms to refine the design strategy. Interdisciplinary investigations have demonstrated that the designed molecules and natural products control the aggregation pathways of metal-Aβ species transforming their size/conformation distribution. The aptitude of these molecules to impact metal-Aβ aggregation pathways, either via inhibition of Aβ aggregate formation, most importantly of oligomers, or disaggregation of preformed fibrils, could originate from their formation of complexes with metal-Aβ. Potentially, these molecules could direct metal-Aβ size/conformational states into alternative nontoxic unstructured oligomers, and control the geometry at the Aβ-ligated metal center for limited ROS formation to lessen the overall toxicity induced by metal-Aβ. Complexation between small molecules and Aβ/metal-Aβ has been observed by nuclear magnetic resonance spectroscopy (NMR) and ion mobility-mass spectrometry (IM-MS) pointing to molecular level interactions, validating the design strategy. In addition, these molecules exhibit other attractive properties, such as antioxidant capacity, prevention of ROS production, potential blood-brain barrier (BBB) permeability, and reduction of Aβ-/metal-Aβ-induced cytotoxicity, making them desirable tools for unraveling AD complexity. In this Account, we summarize the recent development of small molecules, via both rational design and the selection and modification of natural products, as tools for investigating metal-Aβ complexes, to advance our understanding of their relation to AD pathology.

Rational design of a structural framework with potential use to develop chemical reagents that target and modulate multiple facets of Alzheimer's disease

Alzheimer's disease (AD) is characterized by multiple, intertwined pathological features, including amyloid-β (Aβ) aggregation, metal ion dyshomeostasis, and oxidative stress. We report a novel compound (ML) prototype of a rationally designed molecule obtained by integrating structural elements for Aβ aggregation control, metal chelation, reactive oxygen species (ROS) regulation, and antioxidant activity within a single molecule. Chemical, biochemical, ion mobility mass spectrometric, and NMR studies indicate that the compound ML targets metal-free and metal-bound Aβ (metal-Aβ) species, suppresses Aβ aggregation in vitro, and diminishes toxicity induced by Aβ and metal-treated Aβ in living cells. Comparison of ML to its structural moieties (i.e., 4-(dimethylamino)phenol (DAP) and (8-aminoquinolin-2-yl)methanol (1)) for reactivity with Aβ and metal-Aβ suggests the synergy of incorporating structural components for both metal chelation and Aβ interaction. Moreover, ML is water-soluble and potentially brain permeable, as well as regulates the formation and presence of free radicals. Overall, we demonstrate that a rational structure-based design strategy can generate a small molecule that can target and modulate multiple factors, providing a new tool to uncover and address AD complexity.

Amyloid-β-neuropeptide interactions assessed by ion mobility-mass spectrometry

Recently, small peptides have been shown to modulate aggregation and toxicity of the amyloid-β protein (Aβ). As such, these new scaffolds may help discover a new class of biotherapeutics useful in the treatment of Alzheimer's disease. Many of these inhibitory peptide sequences have been derived from natural sources or from Aβ itself (e.g., C-terminal Aβ fragments). In addition, much earlier work indicates that tachykinins, a broad class of neuropeptides, display neurotrophic properties, presumably through direct interactions with either Aβ or its receptors. Based on this work, we undertook a limited screen of neuropeptides using ion mobility-mass spectrometry to search for similar such peptides with direct Aβ binding properties. Our results reveal that the neuropeptides leucine enkephalin (LE) and galanin interact with both the monomeric and small oligomeric forms of Aβ(1-40) to create a range of complexes having diverse stoichiometries, while some tachyknins (i.e., substance P) do not. LE interacts with Aβ more strongly than galanin, and we utilized ion mobility-mass spectrometry, molecular dynamics simulations, gel electrophoresis/Western blot, and transmission electron microscopy to study the influence of this peptide on the structure of Aβ monomer, small Aβ oligomers, as well as the eventual formation of Aβ fibrils. We find that LE binds selectively within a region of Aβ between its N-terminal tail and hydrophobic core. Furthermore, our data indicate that LE modulates fibril generation, producing shorter fibrillar aggregates when added in stoichiometric excess relative to Aβ.

Dual-function triazole-pyridine derivatives as inhibitors of metal-induced amyloid-β aggregation

Dysregulated metal ions are hypothesized to play a role in the aggregation of the amyloid-β (Aβ) peptide, leading to Alzheimer's disease (AD) pathology. In addition to direct effects on Aβ aggregation, both Cu and Fe can catalyze the generation of reactive oxygen species (ROS), possibly contributing to significant neuronal toxicity. Therefore, disruption of metal-Aβ interactions has become a viable strategy for AD therapeutic development. Herein, we report a new series of dual-function triazole-pyridine ligands [4-(2-(4-(pyridin-2-yl)-1H-1,2,3-triazol-1-yl)ethyl)morpholine (L1), 3-(4-(pyridin-2-yl)-1H-1,2,3-triazol-1-yl)propan-1-ol (L2), 2-(4-(pyridin-2-yl)-1H-1,2,3-triazol-1-yl)acetic acid (L3), and 5-(4-(pyridin-2-yl)-1H-1,2,3-triazol-1-yl)pentan-1-amine (L4)] that interact with the Aβ peptide and modulate its aggregation in vitro. Metal chelation and Aβ interaction properties of these molecules were studied by UV-vis, NMR spectroscopy and X-ray crystallography. In addition, turbidity and transmission electron microscopy (TEM) were employed to determine the anti-aggregation properties of L1-L4. All compounds demonstrated an ability to limit metal-induced Aβ aggregation. Overall, our studies suggest the utility of the triazole-pyridine framework in the development of chemical reagents toward inhibitors for metal-triggered Aβ aggregation.

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.

Exploring the reactivity of flavonoid compounds with metal-associated amyloid-β species

Metal ions associated with amyloid-β (Aβ) peptides have been suggested to be involved in the development of Alzheimer's disease (AD), but this remains unclear and controversial. Some attempts to rationally design or select small molecules with structural moieties for metal chelation and Aβ interaction (i.e., bifunctionality) have been made to gain a better understanding of the hypothesis. In order to contribute to these efforts, four synthetic flavonoid derivatives FL1-FL4 were rationally selected according to the principles of bifunctionality and their abilities to chelate metal ions, interact with Aβ, inhibit metal-induced Aβ aggregation, scavenge radicals, and regulate the formation of reactive oxygen species (ROS) were studied using physical methods and biological assays. The compounds FL1-FL3 were able to chelate metal ions, but showed limited solubility in aqueous buffered solutions. In the case of FL4, which was most compatible with aqueous conditions, its binding affinities for Cu(2+) and Zn(2+) (nM and μM, respectively) were obtained through solution speciation studies. The direct interaction between FL4 and Aβ monomer was weak, which was monitored by NMR spectroscopy and mass spectrometry. Employing FL1-FL4, no noticeable inhibitory effect on metal-mediated Aβ aggregation was observed. Among FL1-FL4, FL3, having 3-OH, 4-oxo, and 4'-N(CH(3))(2) groups, exhibited similar antioxidant activity to the vitamin E analogue, Trolox, and ca. 60% reduction in the amount of hydrogen peroxide (H(2)O(2)) generated by Cu(2+)-Aβ in the presence of dioxygen (O(2)) and a reducing agent. Overall, the studies here suggest that although four flavonoid molecules were selected based on expected bifunctionality, their properties and metal-Aβ reactivity were varied depending on the structure differences, demonstrating that bifunctionality must be well tuned to afford desirable reactivity.

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.

Histidine-rich branched peptides as Cu(II) and Zn(II) chelators with potential therapeutic application in Alzheimer's disease

Two histidine-rich branched peptides with one lysine as a branching unit have been designed and synthesized by solid-phase peptide synthesis. Their complex formation with Cu(II) and Zn(II) as well as their ability to attenuate the metal-ion induced amyloid aggregation has been characterized. Both peptides can keep Cu(II) and Zn(II) in complexed forms at pH 7.4 and can bind two equivalents of metal ions in solutions with excess metal. The stoichiometry, stability and structure of the complexes formed have been determined by pH potentiometry, UV-Vis spectrophotometry, circular dichroism, EPR and NMR spectroscopy and ESI-MS. Both mono- and bimetallic species have been detected over the whole pH range studied. The basic binding mode is either a tridentate {N(amino), N(amide), N(im)} or a histamine-type of coordination which is complemented by the binding of far imidazole or amino groups leading to macrochelate formation. The peptides were able to prevent Cu(II)-induced Aβ(1-40) aggregation but could not effectively compete for Zn(II) in vitro. Our results suggest that branched peptides containing potential metal-binding sites may be suitable metal chelators for reducing the risk of amyloid plaque formation in Alzheimer's disease.

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.