Attenuation of the aggregation and neurotoxicity of amyloid-β peptides by catalytic photooxygenation

The 12th AFMC International Medicinal Chemistry Symposium (AIMECS 2019) in Istanbul, Turkey

AFMC-AIMECS meetings are internationally organized biannually by the Asian Federation for Medicinal Chemistry (AFMC) and are focused on recent studies in drug discovery and development both in academia and industry. Member organizations of the AFMC are the Pharmaceutical Society of Japan, the Chinese Pharmaceutical Association, the Royal Australian Chemical Institute, the Pharmaceutical Society of Korea, the Korean Chemical Society, the Chemical Society Located in Taipei, the Indonesian Society of Medicinal Chemistry, the Medicinal Chemistry Section of the Israel Chemical Society, and the Computer-Aided Drug Design & Development Society in Turkey. Each time, the symposium is organized within these member countries. The AIMECS 2019 symposium was held in Turkey this year, as Prof. Dr. Esin Aki-Yalcin is the current president of the AFMC (2018-2020); the next AIMECS meeting will be organized in 2021 in Tokyo, Japan. In this report, we discuss key topics at the 12^th AFMC International Medicinal Chemistry Symposium - New Avenues for Design and Development of Translational Medicine (AIMECS 2019) held in Istanbul, September 8-11, 2019.

Transition metal complexes as imaging or therapeutic agents for neurodegenerative diseases

Neurodegenerative diseases are the result of neurodegeneration, which is the process of losing neuronal functions gradually due to the irreversible damage and death of neurons. Metal complexes have attracted intense interest over recent decades as probes or inhibitors of biomolecules.

Redox active metals in neurodegenerative diseases

Copper (Cu) and iron (Fe) are redox active metals essential for the regulation of cellular pathways that are fundamental for brain function, including neurotransmitter synthesis and release, neurotransmission, and protein turnover. Cu and Fe are tightly regulated by sophisticated homeostatic systems that tune the levels and localization of these redox active metals. The regulation of Cu and Fe necessitates their coordination to small organic molecules and metal chaperone proteins that restrict their reactions to specific protein centres, where Cu and Fe cycle between reduced (Fe²⁺, Cu⁺) and oxidised states (Fe³⁺, Cu²⁺). Perturbation of this regulation is evident in the brain affected by neurodegeneration. Here we review the evidence that links Cu and Fe dyshomeostasis to neurodegeneration as well as the promising preclinical and clinical studies reporting pharmacological intervention to remedy Cu and Fe abnormalities in the treatment of Alzheimer's disease (AD), Parkinson's disease (PD) and Amyotrophic lateral sclerosis (ALS).

Ion mobility spectrometry combined with multivariate statistical analysis: revealing the effects of a drug candidate for Alzheimer's disease on Aβ1-40 peptide early assembly

Inhibition of the initial stages of amyloid-β peptide self-assembly is a key approach in drug development for Alzheimer's disease, in which soluble and highly neurotoxic low molecular weight oligomers are produced and aggregate in the brain over time. Here we report a high-throughput method based on ion mobility mass spectrometry and multivariate statistical analysis to rapidly select statistically significant early-stage species of amyloid-β1-40 whose formation is inhibited by a candidate theranostic agent. Using this method, we have confirmed the inhibition of a Zn-porphyrin-peptide conjugate in the early self-assembly of Aβ40 peptide. The MS/MS fragmentation patterns of the species detected in the samples containing the Zn-porphyrin-peptide conjugate suggested a porphyrin-catalyzed oxidation at Met-35(O) of Aβ40. We introduce ion mobility MS combined with multivariate statistics as a systematic approach to perform data analytics in drug discovery/amyloid research that aims at the evaluation of the inhibitory effect on the Aβ early assembly in vitro models at very low concentration levels of Aβ peptides.

Inactivation of myostatin by photo-oxygenation using catalyst-functionalized peptides

Inhibition of myostatin is an attractive treatment for muscular dystrophy and other amyotrophic diseases. A myostatin-binding peptide was functionalized by equipped with an on/off switchable photo-oxygenation catalyst. This peptide induces a selective oxygenation of myostatin under near-infrared light, resulting in inactivation of myostatin. This peptide shows several orders of magnitude greater inhibitory effect than the original peptide.

Photophysical properties and application in live cell imaging of B,B-fluoro-perfluoroalkyl BODIPYs

The photophysical properties of newly identified B,B-fluoro-perfluoroalkyl BODIPYs (2 and 3), which possess a fluoro group and a trifluoromethyl or pentafluoroethyl group at the boron center, were investigated. B,B-Fluoro-perfluoroalkyl BODIPYs 2 and 3 exhibited better photophysical/chemical properties than B,B-difluoro-BODIPY 1, as follows: (1) higher photostability both in methanol solvent and in a live cell environment, (2) higher stability against acid degradation, and (3) improved fluorescence signal-to-noise ratios in a cell system. These favorable properties of B,B-fluoro-perfluoroalkyl BODIPYs are likely due to the highly electron-withdrawing nature of the perfluoroalkyl groups on the boron atom, which reduces the reactivity to 1O2 and strengthens the complexation of the dipyrromethene ligands to the boron atom. Thus, B,B-fluoro perfluoroalkyl BODIPYs may be useful functional molecules for various applications.

Near-Infrared Activated Black Phosphorus as a Nontoxic Photo-Oxidant for Alzheimer's Amyloid-β Peptide

The inhibition of amyloid-β (Aβ) aggregation by photo-oxygenation has become an effective way of treating Alzheimer's disease (AD). New near-infrared (NIR) activated treatment agents, which not only possess high photo-oxygenation efficiency, but also show low biotoxicity, are urgently needed. Herein, for the first time, it is demonstrated that NIR activated black phosphorus (BP) could serve as an effective nontoxic photo-oxidant for amyloid-β peptide in vitro and in vivo. The nanoplatform BP@BTA (BTA: one of thioflavin-T derivatives) possesses high affinity to the Aβ peptide due to specific amyloid selectivity of BTA. Importantly, under NIR light, BP@BTA can significantly generate a high quantum yield of singlet oxygen (¹ O₂ ) to oxygenate Aβ, thereby resulting in inhibiting the aggregation and attenuating Aβ-induced cytotoxicity. In addition, BP could finally degrade into nontoxic phosphate, which guarantees the biosafety. Using transgenic Caenorhabditis elegans CL2006 as AD model, the results demonstrate that the ¹ O₂ -generation system could dramatically promote life-span extension of CL2006 strain by decreasing the neurotoxicity of Aβ.

Multifunctional carbon dots as a therapeutic nanoagent for modulating Cu(ii)-mediated β-amyloid aggregation

The abnormal self-assembly of cerebral β-amyloid (Aβ) peptides into toxic aggregates is a hallmark of Alzheimer's disease (AD). Here, we report on multifunctional carbon dots that can chelate Cu(ii) ions, suppress Aβ aggregation, and photooxygenate Aβ peptides. Copper ions have high relevance to AD pathogenesis, causing Cu(ii)-mediated Aβ aggregation and oxidative damage to neuronal cells. For effective conjugation with Cu(ii)-bound Aβ complexes, we have designed carbon dots that possess nitrogen (N)-containing polyaromatic functionalities on their surface by employing o-phenylenediamine (OPD) as a polymerization precursor. We demonstrate that the polymerized OPD (pOPD)-derived carbon dots exhibit multiple capabilities against Cu(ii)-mediated Aβ aggregation. Furthermore, the pOPD-derived carbon dots exhibited dramatically enhanced absorption and fluorescence upon coordination with Cu(ii) ions and effectively photooxygenated Aβ peptides. The photodynamically modulated Aβ residues lost the propensity to coordinate with Cu(ii) and to assemble into toxic aggregates. This work demonstrates the potential of carbon dots as a multifunctional β-sheet breaker and provides a promising anti-amyloidogenic strategy for future Aβ-targeted AD treatments.

Chemical and mechanistic analysis of photodynamic inhibition of Alzheimer's β-amyloid aggregation

Inhibition of Aβ₄₂ aggregation by photoexcited thioflavin T that generates singlet oxygen to oxidize monomeric Aβ₄₂.

Dual-Aptamer-Conjugated Molecular Modulator for Detecting Bioactive Metal Ions and Inhibiting Metal-Mediated Protein Aggregation

Bioactive metal ions play important roles in both physiological and pathological processes. Developing biosensing probes for bioactive metal ion detection can contribute to fields including disease diagnosis and therapy and studying the mechanisms of biological activities. In this work, we designed a dual-aptamer-conjugated molecular modulator that can detect Zn²⁺ and further inhibit Zn²⁺-induced amyloid β (Aβ) aggregation. The molecular modulator is able to selectively target Aβ species and block Zn²⁺ due to the specific recognition capability of aptamers. With the binding of Zn²⁺, the fluorescence signal of this molecular modulator is restored, thus allowing for Zn²⁺ detection. More importantly, this molecular modulator can inhibit the generation of Zn²⁺-triggered Aβ aggregates due to the trapping of Zn²⁺ around Aβ species. Circular dichroism measurements reveal that the dual-aptamer-conjugated molecular modulator prevents the conformational transition of the Aβ monomer from a random coil to a β-sheet. Furthermore, after treating with the molecular modulator, no Aβ aggregate is observed in the Aβ solution with added Zn²⁺, demonstrating that Aβ aggregation is successfully inhibited by this molecular modulator. Our approach provides a promising tool for detecting bioactive metal ions and studying the molecular mechanisms behind life activities.

Porphyrinic Metal-Organic Framework PCN-224 Nanoparticles for Near-Infrared-Induced Attenuation of Aggregation and Neurotoxicity of Alzheimer's Amyloid-β Peptide

The aberrant aggregation of amyloid-β peptide (Aβ) in the brain has been considered as the major pathological hallmark of Alzheimer's diseases (AD). Inhibition of Aβ aggregation is considered as an attractive therapeutic intervention for alleviating amyloid-associated neurotoxicity. Here, we report the near-infrared light (NIR)-induced suppression of Aβ aggregation and reduction of Aβ-induced cytotoxicity via porphyrinic metal-organic framework (MOF) PCN-224 nanoparticles. PCN-224 nanoparticles are hydrothermally synthesized by coordinating tetra-kis(4-carboxyphenyl)porphyrin (TCPP) ligands with zirconium. The PCN-224 nanoparticles show high photo-oxygenation efficiency, good biocompatibility, and high stability. The study reveals that the porphyrinic MOF-based nanoprobe activated by NIR light could successfully inhibit self-assembly of monomeric Aβ into a β-sheet-rich structure. Furthermore, photoexcited PCN-224 nanoparticles also significantly reduce Aβ-induced cytotoxicity under NIR irradiation.

Photoactive chlorin e6 is a multifunctional modulator of amyloid-β aggregation and toxicity via specific interactions with its histidine residues

Photoactive chlorin e6 selectively damage the histidine residues of amyloid-β and reduce its aggregation and toxicity even in the presence of Cu ions.

The self-assembly of Aβ to β-sheet-rich neurotoxic oligomers is a main pathological event leading to Alzheimer's disease (AD). Selective targeting of Aβ oligomers without affecting other functional proteins is therefore an attractive approach to prevent the disease and its progression. In this study, we report that photodynamic treatment of Aβ in the presence of catalytic amounts of chlorin e6 can selectively damage Aβ and inhibit its aggregation and toxicity. Chlorin e6 also reversed the amyloid aggregation process in the dark by binding its soluble and low molecular weight oligomers, as shown by thioflavin T (ThT) fluorescence and photoinduced cross-linking of unmodified protein (PICUP) methods. Using HSQC NMR spectroscopy, ThT assays, amino acid analysis, SDS/PAGE, and EPR spectroscopy, we show that catalytic amounts of photoexcited chlorin e6 selectively damage the Aβ histidine residues H6, H13, and H14, and induce Aβ cross-linking by generating singlet oxygen. In contrast, photoexcited chlorin e6 was unable to cross-link ubiquitin and α-synuclein, demonstrating its high selectivity for Aβ. By binding to the Aβ histidine residues, catalytic amounts of chlorin e6 can also inhibit the Cu²⁺-induced aggregation and toxicity in darkness, while at stoichiometric amounts it acts as a chelator to reduce the amount of free Cu²⁺. This study demonstrates the great potential of chlorin e6 as a multifunctional agent for treatment of AD, and shows that the three N-terminal Aβ histidine residues are a suitable target for Aβ-specific drugs.

Ultrasound-Excited Protoporphyrin IX-Modified Multifunctional Nanoparticles as a Strong Inhibitor of Tau Phosphorylation and β-Amyloid Aggregation

Alzheimer's disease (AD) has become one of the most serious societal problems globally, with no effective treatments. Parenchymal accumulation of amyloid beta (Aβ) plaques and the formation of neurofibrillary tangles are the hallmarks of AD. Their possible interactions and synergistic effects in AD have been gradually elucidated. The failure of many clinical trials suggests that it is difficult to treat AD with a focus on a single target. Instead, multiple targets may be an important direction for AD drug research. In this study, we used protoporphyrin IX (PX)-modified oxidized mesoporous carbon nanospheres (OMCN) (PX@OMCN@PEG(OP)@RVGs) as a novel AD multifunctional nanodrug having multiple targets. The nanodrug efficiently inhibits tau phosphorylation. In addition, the use of PX with focused ultrasound triggered the production of reactive oxygen species that significantly inhibited Aβ aggregation. Both approaches notably increased the cognitive level of APP/PS1 transgenic (Tg) mice and ultimately achieved dual-target inhibition of AD. Furthermore, the safe and effective delivery of PX across the blood-brain barrier (BBB) due to modification of the RVG peptide was demonstrated in vivo and in vitro. The favorable photothermal effect of the nanoparticles improved the BBB permeability of PX@OP@RVGs under near-infrared irradiation. The results demonstrated that the novel PX@OP@RVG multifunctional nanomedicine has a dual-target treatment capability for AD and can traverse the BBB, indicating the potential for the effective treatment of AD.

Near-Infrared Switchable Fullerene-Based Synergy Therapy for Alzheimer's Disease

C60 has a special dual function; it can act as both a powerful reactive oxygen species (ROS) producer under UV or visible light and an ROS scavenger in the dark. However, ROS has double-edged effects in living systems. It is still a great challenge for biomedical application to switch and adjust the two opposite properties of C60 in one system. Herein, UCNP@C60 -pep (UCNP: upconversion nanoparticle, pep: Aβ-target peptide KLVFF) is designed as a near-infrared-switchable nanoplatform for synergy therapy of Alzheimer's disease (AD). Under near-infrared (NIR) light, the Aβ-targeting hybrid nanoparticles produce ROS and result in Aβ photooxygenation, which can hinder Aβ aggregation and mitigate the attendant cytotoxicity. In the dark, UCNP@C60 -pep shows protective effects against the increased oxidative stress. The ROS-generating and ROS-quenching abilities of UCNP@C60 -pep are both beneficial for decreasing Aβ-induced neurotoxicity and extending the longevity of the commonly used transgenic AD model Caenorhabditis elegans CL2006. Moreover, UCNP@C60-pep can also be used for upconversion luminescence (UCL) and magnetic resonance imaging (MRI), which has benefits for "image-guided therapy." This study may offer a new perspective for the biological applications of C60 .

Preparation and photo-induced activities of water-soluble amyloid β-C60 complexes

We have shown that fullerene (C60) becomes soluble in water by mixing fullerene and amyloid β peptide (Aβ40) whose fibril structures are considered to be associated with Alzheimer's disease. The water-solubility of fullerene arises from the generation of a nanosized complex between fullerene and the monomer species of Aβ40 (Aβ40-C60). The prepared Aβ40-C60 exhibits photo-induced activity with visible light to induce the inhibition of Aβ40 fibrillation and the cytotoxicity for cultured HeLa cells. The observed photo-induced phenomena result from the generation of singlet oxygen via photoexcitation, inducing oxidative damage to Aβ40 and HeLa cells. The oxidized Aβ40 following photoexcitation of Aβ40-C60 was confirmed by mass spectrometry.

Oxidative stress and the amyloid beta peptide in Alzheimer's disease

Oxidative stress is known to play an important role in the pathogenesis of a number of diseases. In particular, it is linked to the etiology of Alzheimer's disease (AD), an age-related neurodegenerative disease and the most common cause of dementia in the elderly. Histopathological hallmarks of AD are intracellular neurofibrillary tangles and extracellular formation of senile plaques composed of the amyloid-beta peptide (Aβ) in aggregated form along with metal-ions such as copper, iron or zinc. Redox active metal ions, as for example copper, can catalyze the production of Reactive Oxygen Species (ROS) when bound to the amyloid-β (Aβ). The ROS thus produced, in particular the hydroxyl radical which is the most reactive one, may contribute to oxidative damage on both the Aβ peptide itself and on surrounding molecule (proteins, lipids, …). This review highlights the existing link between oxidative stress and AD, and the consequences towards the Aβ peptide and surrounding molecules in terms of oxidative damage. In addition, the implication of metal ions in AD, their interaction with the Aβ peptide and redox properties leading to ROS production are discussed, along with both in vitro and in vivo oxidation of the Aβ peptide, at the molecular level.

[Amyloid-selective Photooxygenation toward Treatment for Amyloid Diseases]

 Amyloid proteins and peptides form aggregates which lead to amyloid diseases. For example, Alzheimer's disease-related amyloid β (Aβ) forms oligomers, protofibrils, and amyloid fibrils, which exhibit neurotoxicity. Controlling the aggregation and toxicity of Aβ would be a therapeutic strategy for the treatment of Alzheimer's disease. Recently, we have investigated an artificial oxygenative modification (chemical introduction of oxygen atoms) of amyloid proteins using a photocatalyst, which attenuated the aggregation potency and toxicity of these proteins. The oxygenation of Aβ1-42 was efficiently induced using a riboflavin catalyst (1). The oxygenated Aβ was less aggregative and cytotoxic than native Aβ. The oxygenated Aβ also showed inhibitory activity against aggregation and the onset of toxicity of native Aβ. Flavin catalyst 2, bearing an Aβ-binding peptide, allowed the selective oxygenation of Aβ even in the presence of living cells, due to its Aβ-affinity. Furthermore, "On/Off" switchable photooxygenation catalysts 3 and 4, which can sense a higher-order amyloid structure (i.e., cross-β-sheet structure), were developed based on the amyloid fluorescence probe thioflavin-T. The photo-excited catalysts generated singlet oxygens to induce oxygenation when binding to the amyloid structure ("On"). In contrast, the free catalysts, without binding to the amyloid structure, produced no singlet oxygen, even if photo-excited ("Off"). This "On/Off" switchable function enabled highly Aβ-selective oxygenation. Catalyst 3 was successfully used for the selective oxygenation of other amyloid proteins and peptides. These findings suggest that amyloid-selective oxygenation could provide a versatile system in developing effective new treatments for amyloid diseases.

Target-protein-selective inactivation and labelling using an oxidative catalyst

Reactive oxygen species (ROS) and radical species generated by oxidative single-electron transfer (SET) catalysts induce local environmental oxidative reactions, resulting in protein inactivation and labelling in proximity to the catalysts.

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.

Shedding Light on Alzheimer's β-Amyloidosis: Photosensitized Methylene Blue Inhibits Self-Assembly of β-Amyloid Peptides and Disintegrates Their Aggregates

Abnormal aggregation of β-amyloid (Aβ) peptides is a major hallmark of Alzheimer’s disease (AD). In spite of numerous attempts to prevent the β-amyloidosis, no effective drugs for treating AD have been developed to date. Among many candidate chemicals, methylene blue (MB) has proved its therapeutic potential for AD in a number of in vitro and in vivo studies; but the result of recent clinical trials performed with MB and its derivative was negative. Here, with the aid of multiple photochemical analyses, we first report that photoexcited MB molecules can block Aβ₄₂ aggregation in vitro. Furthermore, our in vivo study using Drosophila AD model demonstrates that photoexcited MB is highly effective in suppressing synaptic toxicity, resulting in a reduced damage to the neuromuscular junction (NMJ), an enhanced locomotion, and decreased vacuole in the brain. The hindrance effect is attributed to Aβ₄₂ oxidation by singlet oxygen (¹O₂) generated from photoexcited MB. Finally, we show that photoexcited MB possess a capability to disaggregate the pre-existing Aβ₄₂ aggregates and reduce Aβ-induced cytotoxicity. Our work suggests that light illumination can provide an opportunity to boost the efficacies of MB toward photodynamic therapy of AD in future.

Form Follows Function: Designer Chemistry at the 52nd Bürgenstock Conference

The 52nd Bürgenstock Conference on Stereochemistry took place from April 30-May 4, 2017, and showed how chemistry and design go hand-in-hand (as reflected in the image of the Bauhausarchiv in Berlin). In this Conference Report, Philipp Heretsch outlines the program.

Hematite-Based Photoelectrode Materials for Photoelectrocatalytic Inhibition of Alzheimer's β-Amyloid Self-Assembly

A visible light-active, hematite-based photoelectrode platform for suppressing β-amyloid (Aβ) self-assembly in vitro is reported. Upon illumination of a light-emitting diode with an anodic bias, the hematite photoanode generates reactive radical species, such as superoxide ions and hydroxyl radicals, via photoelectrocatalytic process. According to our analyses, the hematite photoanode exhibited a strong inhibitory effect on Aβ aggregation under visible light illumination and anodic bias. We found that hole-derived radicals played a significant role of oxidizing Aβ peptides, which effectively blocked further aggregation. The efficacy of photoelectrocatalytic inhibition on Aβ aggregation was enhanced by introducing cobalt phosphate (Co-Pi) as a co-catalyst on the hematite photoanode, which facilitated the separation of electron-hole pairs. We verified that both bare and Co-Pi@hematite photoanodes are biocompatible and effective in reducing Aβ aggregation-induced cytotoxicity.

Rattle-Structured Upconversion Nanoparticles for Near-IR-Induced Suppression of Alzheimer's β-Amyloid Aggregation

Rose bengal (RB)-loaded upconverting nanocomposites are synthesized as a near-infrared (NIR)-responsive inhibitor of Aβ aggregation. Rattle-structured, organosilica shell (ROS) is deposited on NaYF₄ :Yb,Er nanocrystals (UCNPs) for high loading efficiency and disaggregation of RB. RB/UCNP@ROS successfully inhibits Aβ self-assembly under NIR irradiation by generating ¹ O₂ . Furthermore, photoexcited RB/UCNP@ROS is effective in suppressing Aβ-induced cytotoxicity.

An Iridium(III) Complex as a Photoactivatable Tool for Oxidation of Amyloidogenic Peptides with Subsequent Modulation of Peptide Aggregation

Aggregates of amyloidogenic peptides are involved in the pathogenesis of several degenerative disorders. Herein, an iridium(III) complex, Ir-1, is reported as a chemical tool for oxidizing amyloidogenic peptides upon photoactivation and subsequently modulating their aggregation pathways. Ir-1 was rationally designed based on multiple characteristics, including 1) photoproperties leading to excitation by low-energy radiation; 2) generation of reactive oxygen species responsible for peptide oxidation upon photoactivation under mild conditions; and 3) relatively easy incorporation of a ligand on the Ir^III center for specific interactions with amyloidogenic peptides. Biochemical and biophysical investigations illuminate that the oxidation of representative amyloidogenic peptides (i.e., amyloid-β, α-synuclein, and human islet amyloid polypeptide) is promoted by light-activated Ir-1, which alters the conformations and aggregation pathways of the peptides. Additionally, their potential oxidation sites are identified as methionine, histidine, or tyrosine residues. Overall, our studies on Ir-1 demonstrate the feasibility of devising metal complexes as chemical tools suitable for elucidating the nature of amyloidogenic peptides at the molecular level, as well as controlling their aggregation.

Medicinal Chemistry Focusing on Aggregation of Amyloid-β

The aggregation of peptides/proteins is intimately related to a number of human diseases. More than 20 have been identified which aggregate into fibrils containing extensive β-sheet structures, and species generated in the aggregation processes (i.e., oligomers and fibrils) contribute to disease development. Amyloid-β peptide (designated Aβ), related to Alzheimer's disease (AD), is the representative example. The intensive aggregation property of Aβ also leads to difficulty in its synthesis. To improve the synthetic problem, we developed an O-acyl isopeptide of Aβ1-42, in which the N-acyl linkage (amide bond) of Ser(26) was replaced with an O-acyl linkage (ester bond) at the side chain. The O-acyl isopeptide demonstrated markedly higher water-solubility than that of Aβ1-42, while it quickly converted to intact monomer Aβ1-42 via an O-to-N acyl rearrangement under physiological conditions. Inhibition of the pathogenic aggregation of Aβ1-42 might be a therapeutic strategy for curing AD. We succeeded in the rational design and identification of a small molecule aggregation inhibitor based on a pharmacophore motif obtained from cyclo[-Lys-Leu-Val-Phe-Phe-]. Moreover, the inhibition of Aβ aggregation was achieved via oxygenation (i.e., incorporation of oxygen atoms to Aβ) using an artificial catalyst. We identified a selective, cell-compatible photo-oxygenation catalyst of Aβ, a flavin catalyst attached to an Aβ-binding peptide, which markedly decreased the aggregation potency and neurotoxicity of Aβ.

Switchable photooxygenation catalysts that sense higher-order amyloid structures

Proteins can misfold into amyloid structures that are associated with diseases; however, the same proteins often have important biological roles. To degrade selectively the amyloid form without affecting the fraction of functional protein is, therefore, an attractive goal. Here we report target-state-dependent photooxygenation catalysts that are active only when bound to the cross-β-sheet structure that is characteristic of pathogenic aggregated amyloid proteins. We show these catalysts can selectively oxygenate the amyloid form of amyloid β-protein (Aβ) 1-42 in the presence of non-amyloid off-target substrates. Furthermore, photooxygenation with a catalyst that bears an Aβ-binding peptide attenuated the Aβ pathogenicity in the presence of cells. We also show that selective photooxygenation is generally applicable to other amyloidogenic proteins (amylin, insulin, β2-microglobulin, transthyretin and α-synuclein) and does not affect the physiologically functional non-aggregate states of these proteins. This is the first report of an artificial catalyst that can be selectively and reversibly turned on and off depending on the structure and aggregation state of the substrate protein.

Ceria/POMs hybrid nanoparticles as a mimicking metallopeptidase for treatment of neurotoxicity of amyloid-β peptide

Protein misfolding to amyloid aggregates is the hallmark for neurodegenerative disease. While much attention has been paid to screen natural proteases that can degrade amyloid-β peptides (Aβ), it is difficult to apply them in the clinics with the intractable problem of immunogenicity in living organisms. Herein, we rationally designed an artificial nanozyme, Ceria/Polyoxometalates hybrid (CeONP@POMs) with both proteolytic and superoxide dismutase (SOD) activities. Our results indicated that CeONP@POMs could efficiently degrade Aβ aggregates and reduce intracellular reactive oxygen species (ROS). More importantly, CeONP@POMD could not only promote PC12 cell proliferation and can cross blood-brain barrier (BBB), but also inhibit Aβ-induced BV2 microglial cell activation which was demonstrated by immunoluorescence assay and flow cytometry measurements. In vivo studies further indicated that CeONP@POMD as nanozyme possessed good biocompatibility, evidenced by a detailed study of their biodistribution, body weight change, and in vivo toxicology. Therefore, our results pave the way for design of multifunctional artificial nanozyme for treatment of neurotoxicity of amyloid-β peptide.

Ultraviolet irradiation-mediated formation of Aβ42 oligomers and reactive oxygen species in Zn2+-bound Aβ42 aggregates irrespective of the removal of Zn2+

The controlled UV light exposure converts redox-inert Zn²⁺-bound Aβ₄₂ aggregates into cytotoxic Aβ₄₂ oligomers and reactive oxygen species.

Ultraviolet light triggers the conversion of Cu2+-bound Aβ42 aggregates into cytotoxic species in a copper chelation-independent manner

Increasing evidence indicates that abnormal Cu2+ binding to Aβ peptides are responsible for the formation of soluble Aβ oligomers and ROS that play essential roles in AD pathogenesis. During studying the Cu2+-chelating treatment of Cu2+-bound Aβ42 aggregates, we found that UV light exposure pronouncedly enhances cytotoxicity of the chelator-treated and -untreated Cu2+-bound Aβ42 aggregates. This stimulated us to thoroughly investigate (1) either the chelation treatment or UV light exposure leads to the increased cytotoxicity of the aggregates, and (2) why the chelator-treated and -untreated Cu2+-bound Aβ42 aggregates exhibit the increased cytotoxicity following UV light exposure if the latter is the case. The data indicated that the controlled UV exposure induced the dissociation of Cu2+-free and -bound Aβ42 aggregates into SDS-stable soluble oligomers and the production of ROS including H2O2 in an UV light intensity- and time-dependent, but Cu2+ chelation-independent manner. Although we can't fully understand the meaning of this finding at the current stage, the fact that the UV illuminated Aβ42 aggregates can efficiently kill HeLa cells implies that the aggregates after UV light exposure could be used to decrease the viability of skin cancer cells through skin administration.

Covalent modifier-type aggregation inhibitor of amyloid-β based on a cyclo-KLVFF motif

Inhibition of amyloid-β (Aβ) aggregation could be a drug development target for treating Alzheimer disease. Insufficient activity to inhibit aggregation, however, remains a key issue. Here, we report a covalent modifier-type aggregation inhibitor of Aβ, diazirine-equipped cyclo-KLVF(β-Ph)F (2). Due to the affinity of the cyclo-KLVFF motif for Aβ, 2 selectively reacted with Aβ1-42 under UV-light irradiation to form an irreversible covalent bond. The Tyr-10 residue of Aβ1-42 was identified as the covalent modification site with 2. The extent of cross-β-sheet structure, characteristics of amyloid aggregation, and toxicity of Aβ1-42 were strongly attenuated by this chemical modification.

Synthesis of peptides containing 2-oxohistidine residues and their characterization by liquid chromatography-tandem mass spectrometry

Protein oxidation by reactive oxygen species has been associated with aging and neurodegenerative disorders, and histidine is one of the major oxidation targets due to its metal-chelating property and susceptibility to metal-catalyzed oxidation. 2-Oxohistidine, the major product of histidine oxidation, has been recently identified as a stable marker of oxidative damage in biological systems, but its biophysical and biochemical properties are understudied, partly because of difficulties in its chemical synthesis. We developed an efficient method to generate a 2-oxohistidine side chain using metal-catalyzed oxidation, applicable to both monomers and peptides. By optimizing reagent ratios and pH buffering in Cu(2+) /ascorbate/O2 reaction system, we improved the yield more than tenfold compared to reported conditions, which allowed us to obtain homogeneously modified 2-oxohisidine peptides for further studies. Analysis of 2-oxohistidine-containing model peptides by liquid chromatography-tandem mass spectrometry demonstrated increased retention time in reverse-phase chromatography and general stability of 2-oxohistidine under electrospray ionization and collision-induced dissociation. Thus, large-scale analysis of 2-oxohistidine-modified proteome should be feasible using shotgun protein mass spectrometry, and we were able to observe such peptides in proteomics datasets. The feasibility of acquiring purified peptide probes and peptide antigens containing 2-oxohistidine will help advance the study of this non-enzymatic posttranslational modification.

Non-pretreated O-acyl isopeptide of amyloid β peptide 1-42 is monomeric with a random coil structure but starts to aggregate in a concentration-dependent manner

An isopeptide of amyloid β peptide 1-42 (isoAβ42) was considered as a non-aggregative precursor molecule for the highly aggregative Aβ42. It has been applied to biological studies after several pretreatments. Here we report that isoAβ42 is monomeric with a random coil structure at 40 μM without any pretreatment. But we also found that isoAβ42 retains a slight aggregative nature, which is significantly weaker than that of the native Aβ42.

Formation of a flavin-linked peptide

In a previous study, we showed that formylmethylflavin (FMF) can bind to cysteine. In this study, FMF was reacted with native peptides (CG and CKLVFF) containing an N-terminal cysteine. The formation of flavin-CG and flavin-CKLVFF was confirmed using HPLC and ESI-MS. Storage of flavin-CKLVFF in DMSO at −30 °C for 7 days resulted in no detectable deposition. In contrast, flavin-CKLVFF formed deposits when stored in water at −30 °C for 1 day, but no deposit was observed in the aqueous solution of flavin-CKLVFF after 7 days storage in the presence of 0.1% Triton X-100.