This 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 used to highlight some of the primary considerations for studying misfolded proteins associated with human diseases. 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 studying the complex nature of misfolded proteins at the molecular level.

Despite the significance of Alzheimer's disease, the link between metal-associated amyloid-β (metal-Aβ) and disease etiology remains unclear. To elucidate this relationship, chemical tools capable of specifically targeting and modulating metal-Aβ species are necessary, along with a fundamental understanding of their mechanism at the molecular level. Herein, we investigated and compared the interactions and reactivities of the green tea extract, (−)-epigallocatechin-3-gallate [(2R,3R)-5,7-dihydroxy-2-(3,4,5-trihydroxyphenyl)-3,4-dihydro-2H-1-benzopyran-3-yl 3,4,5-trihydroxybenzoate; EGCG], with metal [Cu(II) and Zn(II)]-Aβ and metal-free Aβ species. We found that EGCG interacted with metal-Aβ species and formed small, unstructured Aβ aggregates more noticeably than in metal-free conditions in vitro. In addition, upon incubation with EGCG, the toxicity presented by metalfree Aβ and metal-Aβ was mitigated in living cells. To understand this reactivity at the molecular level, structural insights were obtained by ion mobility-mass spectrometry (IM-MS), 2D NMR spectroscopy, and computational methods. These studies indicated that (i) EGCG was bound to Aβ monomers and dimers, generating more compact peptide conformations than those from EGCGuntreated Aβ species; and (ii) ternary EGCG-metal-Aβ complexes were produced. Thus, we demonstrate the distinct antiamyloidogenic reactivity of EGCG toward metal-Aβ species with a structurebased mechanism.amyloid-β peptide | metal ions | natural products | amyloidogenesis T he brain of individuals with Alzheimer's disease (AD) has protein aggregates composed of misfolded amyloid-β (Aβ) peptides (1-4). The Aβ peptides are produced endogenously through enzymatic cleavage of amyloid precursor protein. Aβ monomers can misfold and oligomerize into various intermediates before the formation and elongation of fibrils that exhibit a characteristic cross-β-sheet structure (1-4). The accumulation of aggregated Aβ species has been a key feature of the amyloid cascade hypothesis, which cites that these aggregates are possible causative agents in AD. In addition, transition metals, such as Cu and Zn, whose misregulation leads to aberrant neuronal function, have a suggested link to AD pathology (1,(3)(4)(5)(6)(7)(8). In vitro and in vivo studies have provided evidence for the direct interactions of metal ions with Aβ and their presence within Aβ plaques, indicating the formation of metal-associated Aβ (metal-Aβ) species. These metal-Aβ species have been implicated in processes that could lead to neurotoxicity (e.g., metal-induced Aβ aggregation and metal-Aβ-mediated reactive oxygen species generation) (1, 3-8). The involvement of metal-Aβ species in AD pathogenesis, however, has not been clearly elucidated. To advance our understanding of the potential neurotoxicity of metal-Aβ species, efforts to develop chemical tools capable of interacting directly with metal-Aβ species and modulating their reactivity in vitro and in biological systems are under way (1,(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)...

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...

One of the most severe and incurable forms of neurodegeneration, Alzheimer's disease (AD), is characterized in the brain by the accumulation of aggregated amyloid-b (Ab) peptides. [1][2][3] In the diseased brain, elevated concentrations of metals, such as Fe, Cu, and Zn, are found in Ab plaques. [1][2][3][4] It has been proposed that metal ions, such as Cu II and Zn II, can bind to Ab; this causes enhanced peptide aggregation, and in the case of redox active metal ions (e.g., Cu), the generation of reactive oxygen species (ROS) leading to oxidative stress and neuronal death. [1][2][3][4][5][6][7][8][9] While peptide aggregation and oxidative stress have been implicated in AD progression, the role of metal ions associated with Ab species in the development of this disease remains unclear.To clarify the function of metal ions in Ab-related pathological events, small molecule-based tools that contain bifunctionality for probing both metal ions and Ab have been sought. [10][11][12][13][14] Several small molecules have been fashioned according to a rational structure-based design strategy to target metal-associated Ab species (metal-Ab species) and to interrogate metal-induced Ab aggregation and neurotoxicity. [3,[9][10][11][12][13][14] Due to the range of possible conformations of metal-Ab that could be involved in AD neuropathogenesis, [2][3][4]7] discovery of novel structural frameworks that can target these species might advance progress for this design strategy. One tactic to identify new classes of basic structural scaffolds is through screening of naturally occurring compounds, such as flavonoids.Flavonoids are a class of polyphenolic compounds that are abundant in natural products, such as berries, fruits, and vegetables, and have been investigated as potential therapeutic agents in human diseases including cancer, cardiovascular disease, and AD. [15][16][17][18][19][20][21][22] These naturally occurring compounds have been shown independently to chelate metal ions and to interact with Ab, suggesting their potential bifunctionality toward metal-Ab species. [19][20][21][22][23][24][25] One of the flavonoid compounds, myricetin (Scheme 1), previously demonstrated an anti-amyloidogenic effect through its reversible binding to fibrillar Ab but not to the monomeric species.[21] In the case of its metal binding property, prior studies have shown that myricetin has multiple potential sites for metal chelation including positions between the 4-oxo and the 3-or 5-OH groups (Scheme 1) that can form complexes with a binding stoichiometry of 1:1 or 1:2, metal/myricetin. [24,25] Despite the known interactions of myricetin or other members of the flavonoid family with metal ions and Ab, their influence on metal-induced Ab aggregation pathways and neurotoxicity has not been investigated. Herein, we report that myricetin, exhibiting bifunctionality (metal chelation and Ab interaction), was capable of modulating Cu II -and Zn II -induced Ab aggregation and neurotoxicity in vitro and in human neuroblastoma cells. To the best of...

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, 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.