Sensing Amyloid-β Aggregation Using Luminescent Dipyridophenazine Ruthenium(II) Complexes

Detection and disaggregation of amyloid fibrils by luminescent amphiphilic platinum(II) complexes

Cyclometallated Pt(II) complexes possessing hydrophobic 2-phenylpyridine (ppy) ligands and hydrophilic acetonylacetone (acac) ligands have been investigated for their ability to detect amyloid fibrils via luminescence response. Using hen egg-white lysozyme (HEWL) as a model amyloid protein, Pt(II) complexes featuring benzanilide-substituted ppy ligands and ethylene glycol-functionalized acac ligands demonstrated enhanced luminescence in the presence of HEWL fibrils, whereas Pt(II) complexes lacking complementary hydrophobic/hydrophilic ligand sets displayed little to no emission enhancement. An amphiphilic Pt(II) complex incorporating a bis(ethylene glycol)-derivatized acac ligand was additionally found to trigger restructuring of HEWL fibrils into smaller spherical aggregates. Amphiphilic Pt(II) complexes were generally non-toxic to SH-SY5Y neuroblastoma cells, and several complexes also exhibited enhanced luminescence in the presence of Aβ42 fibrils associated with Alzheimer's disease. This study demonstrates that easily prepared and robust (ppy)PtII(acac) complexes show promising reactivity toward amyloid fibrils and represent attractive molecular scaffolds for design of small-molecule probes targeting amyloid assemblies.

Chemical sensors detect and resolve proteome aggregation in peripheral neuropathy cell model induced by chemotherapeutic agents

As a consequence of somatosensory nervous system injury or disease, neuropathic pain is commonly associated with chemotherapies, known as chemotherapy-induced peripheral neuropathy (CIPN). However, the mechanisms underlying CIPN-induced proteome aggregation in neuronal cells remain elusive due to limited detection tools. Herein, we present series sensors for fluorescence imaging (AggStain) and proteomics analysis (AggLink) to visualize and capture aggregated proteome in CIPN neuronal cell model. The environment-sensitive AggStain imaging sensor selectively binds and detects protein aggregation with 12.3 fold fluorescence enhancement. Further, the covalent AggLink proteomic sensor captures cellular aggregated proteins and profiles their composition via LC-MS/MS analysis. This integrative sensor platform reveals the presence of proteome aggregation in CIPN cell model and highlights its potential for broader applications in assessing proteome stability under various cellular stress conditions.

Advances in targeted tracking and detection of soluble amyloid-β aggregates as a biomarker of Alzheimer's disease

Misfolding and aggregation of amyloid-β (Aβ) peptides are key hallmarks of Alzheimer's disease (AD). With accumulating evidence suggesting that different Aβ species have varied neurotoxicity and implications in AD development, the discovery of affinity ligands and analytical approaches to selective distinguish, detect, and monitor Aβ becomes an active research area. Remarkable advances have been achieved, which not only promote our understanding of the biophysical chemistry of the protein aggregation during neurodegeneration, but also provide promising tools for early detection of the disease. In view of this, we summarize the recent progress in selective and sensitive approaches for tracking and detection of Aβ species. Specific attentions are given to soluble Aβ oligomers, due to their crucial roles in AD development and occurrence at early stages. The design principle, performance of targeting units, and their cooperative effects with signal reporters for Aβ analysis are discussed. The applications of the novel targeting probes and sensing systems for dynamic monitoring oligomerization, measuring Aβ in biosamples and in vivo imaging in brain are summarized. Finally, the perspective and challenges are discussed regarding the future development of Aβ-targeting analytical tools to explore the unknown field to contribute to the early diagnosis and treatment of AD.

Advanced Techniques for Detecting Protein Misfolding and Aggregation in Cellular Environments

Protein misfolding and aggregation, a key contributor to the progression of numerous neurodegenerative diseases, results in functional deficiencies and the creation of harmful intermediates. Detailed visualization of this misfolding process is of paramount importance for improving our understanding of disease mechanisms and for the development of potential therapeutic strategies. While in vitro studies using purified proteins have been instrumental in delivering significant insights into protein misfolding, the behavior of these proteins in the complex milieu of living cells often diverges significantly from such simplified environments. Biomedical imaging performed in cell provides cellular-level information with high physiological and pathological relevance, often surpassing the depth of information attainable through in vitro methods. This review highlights a variety of methodologies used to scrutinize protein misfolding within biological systems. This includes optical-based methods, strategies leaning on mass spectrometry, in-cell nuclear magnetic resonance, and cryo-electron microscopy. Recent advancements in these techniques have notably deepened our understanding of protein misfolding processes and the features of the resulting misfolded species within living cells. The progression in these fields promises to catalyze further breakthroughs in our comprehension of neurodegenerative disease mechanisms and potential therapeutic interventions.

Inhibition of amyloid β1-42 peptide aggregation by newly designed cyclometallated palladium complexes

Uncontrolled amyloid aggregation is a frequent cause of neurodegenerative disorders such as prions and Alzheimer's disease (AD). As a result, many drug development approaches focus on evaluating novel molecules that can alter self-recognition pathways. Herein, we designed and synthesized the cyclometallated pyrene (Pd-1 and Pd-3) and anthracene (Pd-2) based palladium complexes ([Pd((L1)Cl] Pd-1, [Pd(L2)Cl](Pd-2), and [Pd(L3)Cl] (Pd-3)). This study explores the effect of these complexes on the aggregation, fibrillation, and amyloid formation of bovine serum albumin (BSA) and Aβ1-42 peptide. Several spectroscopic methods were used to characterize all the Pd-complexes, and the molecular structure of Pd-3 was determined by X-ray crystallography. The secondary structures were studied using circular dichroism (CD) and transmission electron microscopy (TEM), while amyloid aggregation and inhibitory activities were investigated using the Thioflavin-T (ThT) fluorescence assay. Molecular docking of the Pd-complex (Pd-3) was done using fibril (PDB: 2BEG) and monomeric (PDB: 1IYT) peptides using Auto-dock Vina. As a result, the hydrogen bonding and hydrophobic interaction between the aromatic rings of the Pd-complexes and the amino acids of amyloid-β peptides significantly reduced the production of ordered β-sheets of amyloid fibrils and protein aggregation in the presence of Pd-2 and Pd-3 complexes.

Deconvoluting binding sites in amyloid nanofibrils using time-resolved spectroscopy

Steady-state fluorescence spectroscopy has a central role not only for sensing applications, but also in biophysics and imaging. Light switching probes, such as ruthenium dipyridophenazine complexes, have been used to study complex systems such as DNA, RNA, and amyloid fibrils. Nonetheless, steady-state spectroscopy is limited in the kind of information it can provide. In this paper, we use time-resolved spectroscopy for studying binding interactions between amyloid-β fibrillar structures and photoluminescent ligands. Using time-resolved spectroscopy, we demonstrate that ruthenium complexes with a pyrazino phenanthroline derivative can bind to two distinct binding sites on the surface of fibrillar amyloid-β, in contrast with previous studies using steady-state photoluminescence spectroscopy, which only identified one binding site for similar compounds. The second elusive binding site is revealed when deconvoluting the signals from the time-resolved decay traces, allowing the determination of dissociation constants of 3 and 2.2 μM. Molecular dynamic simulations agree with two binding sites on the surface of amyloid-β fibrils. Time-resolved spectroscopy was also used to monitor the aggregation of amyloid-β in real-time. In addition, we show that common polypyridine complexes can bind to amyloid-β also at two different binding sites. Information on how molecules bind to amyloid proteins is important to understand their toxicity and to design potential drugs that bind and quench their deleterious effects. The additional information contained in time-resolved spectroscopy provides a powerful tool not only for studying excited state dynamics but also for sensing and revealing important information about the system including hidden binding sites.

Selectively inhibiting malignant melanoma migration and invasion in an engineered skin model using actin-targeting dinuclear RuII-complexes

Due to the poor prognosis of metastatic cancers, there is a clinical need for agents with anti-metastatic activity. Here we report on the anti-metastatic effect of a previously reported Ru(ii) complex [{(phen)₂Ru}₂(tpphz)]⁴⁺, 1⁴⁺, that has recently been shown to disrupt actin fiber assembly. In this study, we investigated the anti-migratory effect of ⁺1⁴⁺ and a close structural analogue⁺, 2⁴⁺, on two highly invasive, metastatic human melanoma cell lines. Laser scanning confocal imaging was used to investigate the structure of actin filament and adhesion molecule vinculin and results show disassembly of central actin filaments and focal adhesions. The effect of both compounds on actin filaments was also found to be reversible. As these results revealed that the complexes were cytostatic and produced a significant inhibitory effect on the migration of both melanoma cell lines but not human dermal fibroblasts their effect on 3D-spheroids and a tissue-engineered living skin model were also investigated. These experiments demonstrated that the compounds inhibited the growth and invasiveness of the melanoma-based spheroidal tumor model and both complexes were found to penetrate the epidermis of the skin tissue model and inhibit the invasion of melanoma cells. Taken together, the cytostatic and antimigratory effects of the complexes results in an antimetastatic effect that totally prevent invasion of malignant melanoma into skin tissue.

Rational design of photoactivatable metal complexes to target and modulate amyloid-β peptides

The accumulation of amyloid-β (Aβ) aggregates is found in the brains of Alzheimer's disease patients. Thus, numerous efforts have been made to develop chemical reagents capable of targeting Aβ peptides and controlling their aggregation. In particular, tunable coordination and photophysical properties of transition metal complexes, with variable oxidation and spin states on the metal centers, can be utilized to probe Aβ aggregates and alter their aggregation profiles. In this review, we illustrate some rational strategies for designing photoactivatable metal complexes as chemical sensors for Aβ peptides or modulators against their aggregation pathways, with some examples.

Ir(III)-Based Agents for Monitoring the Cytochrome P450 3A4 Active Site Occupancy

Cytochromes P450 (CYPs) are a superfamily of enzymes responsible for biosynthesis and drug metabolism. Monitoring the activity of CYP3A4, the major human drug-metabolizing enzyme, is vital for assessing the metabolism of pharmaceuticals and identifying harmful drug-drug interactions. Existing probes for CYP3A4 are irreversible turn-on substrates that monitor activity at specific time points in end-point assays. To provide a more dynamic approach, we designed, synthesized, and characterized emissive Ir(III) and Ru(II) complexes that allow monitoring of the CYP3A4 active-site occupancy in real time. In the bound state, probe emission is quenched by the active-site heme. Upon displacement from the active site by CYP3A4-specific inhibitors or substrates, these probes show high emission turn-on. Direct probe binding to the CYP3A4 active site was confirmed by X-ray crystallography. The lead Ir(III)-based probe has nanomolar Kd and high selectivity for CYP3A4, efficient cellular uptake, and low toxicity in CYP3A4-overexpressing HepG2 cells.

A Biophysical Study of Ru(II) Polypyridyl Complex, Properties and its Interaction with DNA

Mononuclear Ru(II)Polypyridyl complexes of type [Ru(A)₂BPIIP] (ClO₄)₂.2H₂O, where BPIIP = 2-(3-(4-bromophenyl)isoxazole-5-yl)-1 H-imidazo [4,5-f] [1, 10] phenanthroline and A = bpy = bipyridyl (1), phen = 1,10 Phenanthroline (2), dmb = 4, 4' -dimethyl 2, 2'- bipyridine (3) & dmp = 4,4'-dimethyl-1,10 -Ortho Phenanthroline (4), were synthesized and their antibacterial activity were examined. The synthesized complexes were characterized and their interaction with DNA was studied using Computational and Biophysical methods (Absorption, emission methods, and viscosity). Molecular modelling studies were carried out for molecular geometry and electronic properties (Frontier molecular orbital HOMO-LUMO). The electrostatic potential surface contours for the complexes were analysed to give their nucleophilic level of sensitivity. The study reveals that the Ru(II) Polypyridyl complexes bind to DNA preponderantly by intercalation. The results recommend that the phen and dmp complex have more effective binding ability than the bpy and dmb, indicating the role of the ancillary ligand in determining their specificity for DNA binding. Further molecular docking studies suggested an octahedral geometry and bind to DNA by preferential binding to Guanine. The docking study additionally sustains the binding constant data acquired with the absorption and emission techniques.The results reveal that the nature of the ancillary Ligand plays a considerable role for the intercalation of the Ru(II) polypyridyl complex to DNA, which subsequently influences the antibacterial activity. Biological studies conducted on Gram-Negative (E.coli and K.pneumonia) and Gram-Positive (S. aureus and E. faecalis) bacteria establish that complex 1 and 2 were considerably active against S. aureus and E. coli.

Ruthenium(II)–arene complexes with chelating quinoline ligands as anti-amyloid agents

Recent recognition of the soluble form of the amyloid-beta (Aβ) peptide as a neurotoxic agent in Alzheimer’s disease (AD) has spurred the development of agents to target this species. Because Aβ is known to chelate metal ions in solution, metal-based therapeutics are uniquely suited to exploit this affinity, where coordination to Aβ has been shown to impact the neurotoxicity of the peptide. Ruthenium(II)–arene complexes are unique candidates for evaluation, as one face of the molecule is blocked by the hydrophobic arene ring, while coordination to the Aβ peptide can occur on the other side of the molecule. We have prepared and evaluated two Ru(II)–arene complexes with chelating quinoline-based ligands, Ru1 and Ru2, for their respective anti-amyloid abilities. Although both complexes decreased the aggregation of soluble Aβ, Ru1 displayed promise in disrupting formed aggregates of the peptide. These findings represent an exciting new avenue for therapeutic development in AD, where both sides of the aggregation equilibrium are affected.

A Comparative Study of High-Contrast Fluorescence Lifetime Probes for Imaging Amyloid in Tissue

Optical imaging of protein aggregates in living and post-mortem tissue can often be impeded by unwanted fluorescence, prompting the need for novel methods to extract meaningful signal in complex biological environments. Historically, benzothiazolium derivatives, prominently Thioflavin T, have been the state-of-the-art fluorescent probes for amyloid aggregates, but their optical, structural, and binding properties typically limit them to in vitro applications. This study compares the use of novel uncharged derivative, PAP_1, with parent Thioflavin T as a fluorescence lifetime imaging probe. This is applied specifically to imaging recombinant α-synuclein aggregates doped into brain tissue. Despite the 100-fold lower brightness of PAP_1 compared to that of Thioflavin T, PAP_1 binds to α-synuclein aggregates with an affinity several orders of magnitude greater than Thioflavin T; thus, we observe a specific decrease in the fluorescence lifetime of PAP_1 bound to α-synuclein aggregates, resulting in a separation of >1.4 standard deviations between PAP_1-stained brain tissue background and α-synuclein aggregates that is not observed with Thioflavin T. This enables contrast between highly fluorescent background tissue and amyloid fibrils that is attributed to the greater affinity of PAP_1 for α-synuclein aggregates, avoiding the substantial off-target staining observed with Thioflavin T.

Regulation of Fluorescence Solvatochromism To Resolve Cellular Polarity upon Protein Aggregation

Common solvatochromic fluorophores exhibit a bathochromic fluorescence emission wavelength shift accompanied by intensity attenuation due to the presence of nonradiative decay pathways at the excited state. Such intrinsic but inevitable fluorescence quenching of solvatochromism impedes its applications to faithfully quantify local polarity, especially in a polar environment. Herein, we report a new donor-π-acceptor (D-π-A) type solvatochromic fluorophore scaffold containing a perfluorophenyl group that exhibits both a solvatochromic emission wavelength shift and a controllable emission intensity upon polarity fluctuation. The regulation of fluorescence solvatochromism and colors was achieved by tuning the aryl donors. We exploited such desired solvatochromism of these probes to monitor protein misfolding and aggregation via wavelength shift. Finally, the polarity of pathogenic aggregated proteins was quantified by HaloTag bioorthogonal labeling technology in live cells. While much effort has been devoted to resolving the morphology of pathogenic aggregated proteins, this work provides quantitative hints regarding the chemical information at this disease-related protein interphase.

The Role of Ruthenium Compounds in Neurological Diseases: A Minireview

Ruthenium (Ru) compounds, nitric oxide donors in biological systems, have emerged as a promising therapeutical alternative to conventional drugs in anticancer chemotherapy and as a potential neuroprotective agent, with less cytotoxic effects. This minireview summarizes promising studies with ruthenium complexes and their roles in cancer, neuroinflammation, neurovascular, and neurodegenerative diseases. The up-to-date evidence supports that ruthenium-based compounds have beneficial effects against gliomas, and other types of brain cancers, reduce motor symptoms in models of cerebral ischemia-reperfusion, and may act in the control of nociceptive and inflammatory events, such as seen in early Alzheimer's disease. More studies are needed to fill many current knowledge gaps about the intricate and complex ruthenium biological effects and therapeutic-related mechanisms, stimulating further research. Significance Statement In our minireview, we summarize interesting studies addressing the role of ruthenium compounds on neurological illnesses, focusing on brain cancer, neurovascular and neurodegenerative diseases. No such review is available in the literature.

Gallium nanoparticles as novel inhibitors of Aβ40 aggregation

Alzheimer's disease (AD) has been consistently related to the formation of senile amyloid plaques mainly composed of amyloid β (Aβ) peptides. The toxicity of Aβ aggregates has been indicated to be responsible for AD pathology. One scenario to decrease Aβ toxicity is the development of effective inhibitors against Aβ amyloid formation. In this study, we investigate the effect of gallium nitride nanoparticles (GaN NPs) as inhibitors of Aβ40 amyloid formation using a combination of biophysical approaches. Our results show that the lag phase of Aβ40 aggregation kinetics is significantly retarded by GaN NPs in a concentration dependent manner, implying the activity of GaN NPs in interfering with the formation of the crucial nucleus during Aβ aggregation. Our results also show that GaN NPs can reduce the amyloid fibril elongation rate in the course of the aggregation kinetics. It is speculated that the high polarization characteristics of GaN NPs may provoke a strong interaction between the particles and Aβ40 peptide and in this way decrease self-association of the peptide monomers to form amyloids.

Rational Design of Crystallization-Induced-Emission Probes To Detect Amorphous Protein Aggregation in Live Cells

Unlike amyloid aggregates, amorphous protein aggregates with no defined structures have been challenging to target and detect in a complex cellular milieu. In this study, we rationally designed sensors of amorphous protein aggregation from aggregation-induced-emission probes (AIEgens). Utilizing dicyanoisophorone as a model AIEgen scaffold, we first sensitized the fluorescence of AIEgens to a nonpolar and viscous environment mimicking the interior of amorphous aggregated proteins. We identified a generally applicable moiety (dimethylaminophenylene) for selective binding and fluorescence enhancement. Regulation of the electron-withdrawing groups tuned the emission wavelength while retaining selective detection. Finally, we utilized the optimized probe to systematically image aggregated proteome upon proteostasis network regulation. Overall, we present a rational approach to develop amorphous protein aggregation sensors from AIEgens with controllable sensitivity, spectral coverage, and cellular performance.

Tuning the conformation of G-quadruplexes by sodium and potassium ions: application to photometric and fluorometric determination of amyloid β(1-40)

A dual channel method is described for the determination of the amyloid-β peptide Aβ(1-40) that is associated with Alzheimer's disease. The method exploits (a) conformational changes of a G-quadruplex that are triggered by Na⁺ and K⁺ ions and (b) the strong affinity between Aβ(1-40) and Cu²⁺. A G-quadruplex DNA forms an antiparallel structure in the presence of Na⁺ and can catalyze the oxidation of tetramethylbenzidine by H₂O₂ system in the presence of Cu²⁺ to form a visible blue color. If, however, Cu²⁺ binds to Aβ(1-40), the blue color is no longer formed. Measuring the absorption decrease at 452 nm, the determination of Aβ(1-40) is realized. If K⁺ is added to the Na⁺-containing buffer, the antiparallel G-quadruplex DNA is transformed to parallel. This leads to the insertion of protoporphyrin IX (PPIX) into the G-quadruplex and generates enhanced fluorescent signal, with excitation/emission wavelength at 410/630 nm. The G-quadruplex then catalyzes the metalation of PPIX by Cu²⁺, and the fluorescence intensity decreases. In the presence of Aβ(1-40), the formation of Aβ(1-40)-Cu²⁺ triggers the recovery of the fluorescence. The Na⁺/K⁺-induced tuning of the conformation of the G-quadruplex with the same sequence enables dual (colorimetric and fluorometric) determination of Aβ(1-40), with detection limits of 4.9 pM and 2.3 pM, respectively. The cost is quite low since the developed strategy is label free and enzyme free by using low-cost DNA and Cu²⁺. More importantly, the dual channel determination operation is very simple without any further modification process. Tuning the conformation of G-quadruplexes by sodium(I) and potassium(I): application to photometric and fluorometric determination of amyloid β(1-40).

Fluorescent probes for bioimaging of potential biomarkers in Parkinson's disease

This review comprehensively summarizes various types of fluorescent probes for PD and their applications for detection of various PD biomarkers.

Photophysical studies on lanthanide(III) chelates conjugated to Pittsburgh compound B as luminescent probes targeted to Aβ amyloid aggregates

The photophysical properties of Eu3+ and Tb3+ complexes of DOTAGA and DO3A-monoamide conjugates of the Pittsburgh compound B (PiB) chromophore, prepared using linkers of different lengths and flexibilities, and which form stable negatively charged (LnL1), and uncharged (LnL2) complexes, respectively, were studied as potential probes for optical detection of amyloid aggregates. The phenylbenzothiazole (PiB) moiety absorbs light at wavelengths longer than 330 nm with a high molar absorption coefficient in both probes, and acts as an antenna in these systems. The presence of the luminescent Ln3+ ion quenches the excited states of PiB through an energy transfer process from the triplet state of PiB to the metal centre, and structured emission is seen from Eu3+ and Tb3+. The luminescence study indicates the presence of a 5D4 → T1 back transfer process in the Tb3+ complexes. It also provides insights on structural properties of the Eu3+ complexes, such as the high symmetry environment of the Eu3+ ion in a single macrocyclic conformation and the presence of one water molecule in its inner coordination sphere. The overall quantum yield of luminescence of EuL1 is higher than for EuL2. However, their low values reflect the low overall sensitization efficiency of the energy transfer process, which is a consequence of the large distances between the metal center and the antenna, especially in the EuL2 complex. DFT calculations confirmed that the most stable conformation of the Eu3+ complexes involves a combination of a square antiprismatic (SAP) geometry of the chelate and an extended conformation of the linker. The large calculated average distances between the metal center and the antenna point to the predominance of the Förster energy transfer mechanism, especially for EuL2. This study provides insights into the behavior of amyloid-targeted Ln3+ complexes as optical probes, and contributes towards their rational design.

Modulation of Amyloidogenic Peptide Aggregation by Photoactivatable CO-Releasing Ruthenium(II) Complexes

Three Ru(II)-based CO-releasing molecules featuring bidentate benzimidazole and terpyridine derivatives as ligands were investigated for their ability to modulate the aggregation process of the second helix of the C-terminal domain of nucleophosmin 1, namely nucleophosmin 1 (NPM1)_264-277, a model amyloidogenic system, before and after irradiation at 365 nm. Thioflavin T (ThT) binding assays and UV/Vis absorption spectra indicate that binding of the compounds to the peptide inhibits its aggregation and that the inhibitory effect increases upon irradiation (half maximal effective concentration (EC₅₀) values in the high micromolar range). Electrospray ionization mass spectrometry data of the peptide in the presence of one of these compounds confirm that the modulation of amyloid aggregation relies on the formation of adducts obtained when the Ru compounds react with the peptide upon releasing of labile ligands, like chloride and carbon monoxide. This mechanism of action explains the subtle different behavior of the three compounds observed in ThT experiments. Overall, data support the hypothesis that metal-based CO releasing molecules can be used to develop metal-based drugs with potential application as anti-amyloidogenic agents.

Making the Right Link to Theranostics: The Photophysical and Biological Properties of Dinuclear RuII-ReI dppz Complexes Depend on Their Tether

The synthesis of new dinuclear complexes containing linked Ru^II(dppz) and Re^I(dppz) moieties is reported. The photophysical and biological properties of the new complex, which incorporates a N,N'-bis(4-pyridylmethyl)-1,6-hexanediamine tether ligand, are compared to a previously reported Ru^II/Re^I complex linked by a simple dipyridyl alkane ligand. Although both complexes bind to DNA with similar affinities, steady-state and time-resolved photophysical studies reveal that the nature of the linker affects the excited state dynamics of the complexes and their DNA photocleavage properties. Quantum-based DFT calculations on these systems offer insights into these effects. While both complexes are live cells permeant, their intracellular localizations are significantly affected by the nature of the linker. Notably, one of the complexes displayed concentration-dependent localization and possesses photophysical properties that are compatible with SIM and STED nanoscopy. This allowed the dynamics of its intracellular localization to be tracked at super resolutions.

Photoactive metal complexes that bind DNA and other biomolecules as cell probes, therapeutics, and theranostics

Using selected transition metal centres and linking ligand “building blocks” a modular approach to the development of cellular imaging agents and therapeutics is discussed and illustrated with examples from research by the Thomas group.

Ratiometric detection of amyloid-β aggregation by a dual-emissive tris-heteroleptic ruthenium complex

A dual-emissive tris-heteroleptic ruthenium complex is reported for the ratiometric photoluminescent detection of amyloid-β aggregation in steady and transient states.

Metal-based imaging agents: progress towards interrogating neurodegenerative disease

Transition metals and lanthanide ions display unique properties that enable the development of non-invasive diagnostic tools for imaging. In this review, we highlight various metal-based imaging strategies used to interrogate neurodegeneration.

Monitoring the Formation of Amyloid Oligomers Using Photoluminescence Anisotropy

The formation of oligomeric soluble aggregates is related to the toxicity of amyloid peptides and proteins. In this manuscript, we report the use of a ruthenium polypyridyl complex ([Ru(bpy)₂(dpqp)]²⁺) to track the formation of amyloid oligomers at different times using photoluminescence anisotropy. This technique is sensitive to the rotational correlation time of the molecule under study, which is consequently related to the size of the molecule. [Ru(bpy)₂(dpqp)]²⁺ presents anisotropy values of zero when free in solution (due to its rapid rotation and long lifetime) but larger values as the size and concentration of amyloid-β (Aβ) oligomers increase. Our assays show that Aβ forms oligomers immediately after the assay is started, reaching a steady state at ∼48 h. SDS-PAGE, DLS, and TEM were used to confirm and characterize the formation of oligomers. Our experiments show that the rate of formation for Aβ oligomers is temperature dependent, with faster rates as the temperature of the assay is increased. The probe was also effective in monitoring the formation of α-synuclein oligomers at different times.

What Are the Major Physicochemical Factors in Determining the Preferential Nuclear Uptake of the DNA "Light-Switching" Ru(II)-Polypyridyl Complex in Live Cells via Ion-Pairing with Chlorophenolate Counter-Anions?

Delivering potential theranostic metal complexes into preferential cellular targets is becoming of increasing interest. Here we report that nuclear uptake of a cell-impermeable DNA "light-switching" Ru(II)-polypyridyl complex can be significantly facilitated by chlorophenolate counter-anions, which was found, unexpectedly, to be correlated positively with the binding stability but inversely with the lipophilicity of the formed ion pairs.

Amyloidosis and Longevity: A Lesson from Plants

The variety of lifespans of different organisms in nature is amazing. Although it is acknowledged that the longevity is determined by a complex interaction between hereditary and environmental factors, many questions about factors defining lifespan remain open. One of them concerns a wide range of lifespans of different organisms. The reason for the longevity of certain trees, which reaches a thousand years and exceeds the lifespan of most long living vertebrates by a huge margin is also not completely understood. Here we have discussed some distinguishing characteristics of plants, which may explain their remarkable longevity. Among them are the absence (or very low abundance) of intracellular inclusions composed of amyloidogenic proteins, the lack of certain groups of proteins prone to aggregate and form amyloids in animals, and the high level of compounds which inhibit protein aggregation and possess antiaging properties.

Photocrosslinking of collagen using Ru(II)-polypyridyl complex functionalized gold nanoparticles

Collagen, an extracellular matrix protein, has been used for diverse biological applications due to its clinically safe in nature and for the development of various biomedical devices. As the ECM protein is prone to degradation process, it is necessary to stabilize the collagen. In the present study, we have carried out the stabilization of collagen using newly synthesized gold nanoparticles conjugated with Ru(II) complexes (NCs) possessing different ligand environment. From the DLS measurements, the size of the nanoparticles varies from 20 ± 6 nm. Fibrillation assay studies show that the NCs in the presence of photo-irradiation delays the fibrillation process significantly, while in the presence of persulfate, the acceleration in fibrillation process occurs. Circular dichroic and infra-red spectroscopic studies reveal that no alteration in triple helical structure observed for the photo-irradiated samples. SDS-PAGE analysis data reveal that the NCs facilitate the collagen crosslinks and hinders the enzymatic digestion, while neither Au-NPs nor Ru(II) complexes alone did not impart any stability to the collagen. The results from this study help us to understand the photochemical reaction of nanoparticle conjugate on collagen crosslinking and might be helpful in developing new photocatalyst for corneal application.

DNA binding, photocleavage, antimicrobial and cytotoxic properties of Ru(II) polypyridyl complexes containing BOPIP ligand, (BOPIP = {2-(4-(benzyloxy) phenyl)-1H-imidazo [4,5-f] [1,2]phenanthroline})

A novel ligand BOPIP (BOPIP = {2-(4-(benzyloxy)phenyl)-1H-imidazo[4,5-f][1,10]phenanthroline}) and its mononuclear Ru(II) polypyridyl complexes [Ru(phen)₂ BOPIP]²⁺(1) (phen = 1,10-Phenanthrolene), [Ru(bpy)₂ BOPIP]²⁺(2) (bpy = 2,2' bipyridyl), [Ru(dmb)₂ BOPIP]²⁺(3) (dmb = 4, 4' -dimethyl 2, 2' -bipyridine), [Ru(Hdpa)₂ BOPIP]²⁺(4) (Hdpa = 2,2'dipyridylamine) have been synthesized successfully and characterized by elemental analysis, UV-vis, IR, ¹H, ¹³ C-NMR, and ESI-MS Spectroscopy. The interaction of these complexes with CT-DNA was studied using absorption, emission techniques, viscosity measurements and molecular docking studies. The docking study also supports the binding ability of complexes obtained through the absorption and emission techniques. These studies reveal that the Four Ru(II) polypyridyl complexes bind to DNA predominantly by intercalation. The Antimicrobial activity and cytotoxicity of these complexes are also reported.

Recent Developments in the Interactions of Classic Intercalated Ruthenium Compounds: [Ru(bpy)₂dppz]2+ and [Ru(phen)₂dppz]2+ with a DNA Molecule

[Ru(bpy)₂dppz]²⁺ and [Ru(phen)₂dppz]²⁺ as the light switches of the deoxyribose nucleic acid (DNA) molecule have attracted much attention and have become a powerful tool for exploring the structure of the DNA helix. Their interactions have been intensively studied because of the excellent photophysical and photochemical properties of ruthenium compounds. In this perspective, this review describes the recent developments in the interactions of these two classic intercalated compounds with a DNA helix. The mechanism of the molecular light switch effect and the selectivity of these two compounds to different forms of a DNA helix has been discussed. In addition, the specific binding modes between them have been discussed in detail, for a better understanding the mechanism of the light switch and the luminescence difference. Finally, recent studies of single molecule force spectroscopy have also been included so as to precisely interpret the kinetics, equilibrium constants, and the energy landscape during the process of the dynamic assembly of ligands into a single DNA helix.

Tailoring Hydrophobic Interactions between Probes and Amyloid-β Peptides for Fluorescent Monitoring of Amyloid-β Aggregation

Despite their unique advantages, the full potential of molecular probes for fluorescent monitoring of amyloid-β (Aβ) aggregates has not been fully exploited. This limited utility stems from the lack of knowledge about the hydrophobic interactions between the molecules of Aβ probes, as well as those between the probe and the Aβ aggregate. Herein, we report the first mechanistic study, which firmly establishes a structure-signaling relationship of fluorescent Aβ probes. We synthesized a series of five fluorescent Aβ probes based on an archetypal donor-acceptor-donor scaffold (denoted as SN1-SN5). The arylamino donor moieties were systematically varied to identify molecular factors that could influence the interactions between molecules of each probe and that could influence their fluorescence outcomes in conditions mimicking the biological milieu. Our probes displayed different responses to aggregates of Aβ, Aβ₄₀ and Aβ₄₂, two major isoforms found in Alzheimer's disease: SN2, having pyrrolidine donors, showed noticeable ratiometric fluorescence responses (Δν = 797 cm^-1) to the Aβ₄₀ and Aβ₄₂ samples that contained oligomeric species, whereas SN4, having N-methylpiperazine donors, produced significant fluorescence turn-on signaling in response to Aβ aggregates, including oligomers, protofibrils, and fibrils (with turn-on ratios of 14 and 10 for Aβ₄₂ and Aβ₄₀, respectively). Mechanistic investigations were carried out by performing field-emission scanning electron microscopy, X-ray crystallography, UV-vis absorption spectroscopy, and steady-state and transient photoluminescence spectroscopy experiments. The studies revealed that the SN probes underwent preassembly prior to interacting with the Aβ species and that the preassembled structures depended profoundly on the subtle differences between the amino moieties of the different probes. Importantly, the studies demonstrated that the mode of fluorescence signaling (i.e., ratiometric response versus turn-on response) was primarily governed by stacking geometries within the probe preassemblies. Specifically, ratiometric fluorescence responses were observed for probes capable of forming J-assembly, whereas fluorescence turn-on responses were obtained for probes incapable of forming J-aggregates. This finding provides an important guideline to follow in future efforts at developing fluorescent probes for Aβ aggregation. We also conclude, on the basis of our study, that the rational design of such fluorescent probes should consider interactions between the probe molecules, as well as those between Aβ peptides and the probe molecule.

The development of ruthenium(ii) polypyridyl complexes and conjugates for in vitro cellular and in vivo applications

Ruthenium(ii) [Ru(ii)] polypyridyl complexes have been the focus of intense investigations since work began exploring their supramolecular interactions with DNA. In recent years, there have been considerable efforts to translate this solution-based research into a biological environment with the intention of developing new classes of probes, luminescent imaging agents, therapeutics and theranostics. In only 10 years the field has expanded with diverse applications for these complexes as imaging agents and promising candidates for therapeutics. In light of these efforts this review exclusively focuses on the developments of these complexes in biological systems, both in cells and in vivo, and hopes to communicate to readers the diversity of applications within which these complexes have found use, as well as new insights gained along the way and challenges that researchers in this field still face.

Paper-based electrochemiluminescence sensor for highly sensitive detection of amyloid-β oligomerization: Toward potential diagnosis of Alzheimer's disease

Development of a rapid and sensitive method for Aβ(1-42) aggregation detection is of great importance to overcome the limitations of conventional techniques. In this study, we developed a label-free paper-based electrochemiluminescence sensor for amyloid-β aggregation detection toward potential diagnosis of Alzheimer's disease (AD). The paper-based chip used in the system serves as a low-cost and disposable detection method. In this detection platform, the bonding of [Ru(phen)2dppz]2+ to Aβ(1-42) aggregates results in enhanced electrochemiluminescence due to the change in the polarity of the microenvironment when [Ru(phen)2dppz]2+ intercalated into the β-sheets during oligomerization. The oligomerization process of Aβ(1-42) can be monitored in real time by the novel method, and as low as 100 pM equivalent monomer concentration of Aβ(1-42) could be detected simultaneously. In addition, the cerebrospinal fluid of transgenic AD model mice was tested by this method, which is highly consistent with genetic identification. In addition, we demonstrated that this detection platform could be a potential new method for the screening of Aβ(1-42) aggregation inhibitors, highlighting the practical application capacity of this platform. The platform is label free, low cost and sensitive. Therefore, the proposed platform holds great promise for the diagnosis of AD.

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

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

Probing the molecular mechanism of cerium oxide nanoparticles in protecting against the neuronal cytotoxicity of Aβ1-42 with copper ions

The molecular mechanism of CeONP in protecting against neuronal cytotoxicity from amyloid peptides and copper ions was investigated systematically by photoluminescence of [Ru(phen)2dppz](2+), morphology of TEM, mass spectroscopy, cell viability assay, ROS fluorescence assay, and EPR. The results revealed that CeONPs reduced Aβ1-42 aggregation, protected from neurotoxicity of ROS induced by Cu(2+) + Aβ1-42via blocking the production of free radicals and scavenging the radicals with Ce(3+)/Ce(4+) catalytic cycles, which provides a valuable insight into CeONPs as a therapeutic intervention in oxidative damage in Alzheimer's disease.

Recent progress in the development of metal complexes as β-amyloid imaging probes in the brain

In this review, we have focused on the recent progress in metal complexes that are able to bind to β-amyloid (Aβ) species. We have discussed various radioactive complexes of 99mTc, 68Ga, 64Cu, 89Zr, and 111In, which were designed as Aβ imaging agents for positron emission tomography (PET) and single photon emission computed tomography (SPECT) imaging, non-radioactive Re and Ru complexes as Aβ sensors using luminescence methods, and Gd3+ complexes as contrast agents for magnetic resonance imaging (MRI).

N-Methyl Mesoporphyrin IX as an Effective Probe for Monitoring Alzheimer's Disease β-Amyloid Aggregation in Living Cells

Formation of amyloid fibrils by amyloid-β peptide (Aβ) is an important step in Alzheimer's disease (AD) progression. Screening and designing of new molecules which can monitor the amyloidosis process especially in cells are diagnostically and therapeutically important. Utilizing Thioflavin T (ThT), the commonly used amyloid dye, is the most standardized way to monitor amyloid. However, with the green fluorescence emission and small Stokes shift, the fluorescence of ThT can overlap with that arising from other intrinsic fluorescent components in the cells, making it not suitable for detection of protein aggregates in vivo. Therefore, it is urgent for developing amyloid probes with large Stokes shifts and red-shifted fluorescence emission to detect Aβ aggregates in cells. In this report, we found that N-methyl mesoporphyrin IX (NMM), a widely used G-quadruplex DNA specific fluorescent binder, can be an efficient probe for monitoring Aβ fibrillation in living cells. NMM is nonfluorescent in aqueous solution or monomeric Aβ environments. However, through stacking with the Aβ assemblies, NMM emits strong fluorescence. Furthermore, the large Stokes shift and stable photoluminescence make it an ideal probe for detecting Aβ aggregates in highly fluorescent environments and cell culture. Our results provide a new sight to design and screen new reagents for monitoring the diseases associated with protein conformational disorders.