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

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.

Mapping neuroreceptors with metal-labeled radiopharmaceuticals

The growing epidemiological and economic burden of neurological diseases on society is tremendous. A correct and timely diagnosis can help in lowering the burden and improving the life quality of both the diseased person and the caretaker. Imaging of the brain (neuroimaging) using CT, MRI, and nuclear imaging methods can provide anatomical and functional information. Neuroreceptors are central to neurotransmission and neuromodulation in the CNS. In vivo imaging of receptors in the brain provides powerful tools for the functional study of the central nervous system (CNS) in normal or diseased states. Presently, PET imaging using non-metallic radiotracers dominates the imaging of neuroreceptors. Metal-based probes for SPECT and PET can be economical and logistically easier to use without compromising the information. This review focuses on the development of metallic radiotracers for (99mTc) SPECT and (68Ga) PET along with future directions based on the metallic probes developed for other imaging modalities namely MRI.

Luminometric Nanoparticle-Based Assay for High Sensitivity Detection of β-Amyloid Aggregation

A nanoparticle-based assay utilizing time-resolved luminescence resonance energy transfer (TR-LRET) was developed for the detection of β-amyloid aggregation. The assay is based on the competitive adsorption of the sample and the acceptor-labeled protein to donor europium(III) polystyrene nanoparticles. The performance of the assay was demonstrated by following the fibrillization of β-amyloid peptide 1-42 (Aβ₄₂) as a function of time and by comparing to the reference methods atomic force microscopy (AFM) and thioflavin T (ThT) assay. The fibrillization leads to reduced adsorption of Aβ₄₂ to the nanoparticles increasing the TR-LRET signal. The investigated methods detected fibril formation with equal sensitivities. Eight potential fibrillization inhibitor compounds reported in the literature were tested and the results obtained with each method were compared. It was shown with AFM imaging that the inhibition of fibril formation was not complete with any of the compounds. The developed TR-LRET nanoparticle assay gave corresponding results with the AFM imaging. However, the ThT assay led to contradictory results, as low fluorescence signal was measured in the presence of all tested compounds suggesting inhibition of fibrillization. Our results suggest that the developed TR-LRET nanoparticle assay can be exploited for screening of potential β-amyloid aggregation inhibitors, whereas some of the tested compounds may be measured as false positive inhibitors with the much-utilized ThT assay.

A ratiometric sensor for DNA based on a dual emission Ru(dppz) light-switch complex

Herein we describe the DNA binding properties of two new water-soluble ruthenium complexes; experimental and computational data reveal that both complexes display dual emission from MLCT and LLCT excited states.

Turn-On Colorimetric Platform for Dual Activity Detection of Acid and Alkaline Phosphatase in Human Whole Blood

The activity detection of acid phosphatase (ACP) and alkaline phosphatase (ALP) is of great importance to the diagnosis and prognosis of related diseases. In this work, we report for the first time a turn-on colorimetric platform for the activity detection of ACP and ALP, by exploiting Cu(BCDS)₂^2- (BCDS=bathocuproinedisulfonate) as the probe. The presence of ACP or ALP dephosphorylates the substrate ascorbic acid 2-phosphate to produce ascorbic acid, which then reduces Cu(BCDS)₂^2- into Cu(BCDS)₂^3- , leading to a turn-on spectral absorption at 484 nm and a dramatic color change of the solution from colorless to orange-red. The underlying metal-to-ligand charge-transfer mechanism has been demonstrated by quantum mechanical computations. This platform allows a rapid, sensitive readout of ACP and ALP activities within the dynamic range from 0 to 220 mU ml^-1 . In addition, it is highly immune to false-positive results and also highly selective. More importantly, it is applicable in the presence of human serum and even whole blood samples. These results demonstrate that our platform holds great potential in clinical practices and in the point-of-care analysis.

Protein Nanocages for Delivery and Release of Luminescent Ruthenium(II) Polypyridyl Complexes

In this report, noncovalent encapsulation of hydrophobic ruthenium(II) polyridyl complexes, Ru(bpy)2dppz(2+) and Ru(phen)2dppz(2+), into apoferritin cavity was achieved with high loading contents by effective prevention of Ru complex-induced protein aggregation, without disruption of protein native architecture. The Ru-loaded luminescent nanocomposites have demonstrated improved water solubility, easy manipulation, reduced cytotoxicity, and enhanced cellular uptake as compared to the nontreated Ru complexes.

Tri- and tetranuclear RuII–GdIII2 and RuII–GdIII3 d–f heterometallic complexes as potential bimodal imaging probes for MRI and optical imaging

Tri- and tetranuclear Ru^II–GdII2 and Ru^II–GdIII3 d–f heterometallic complexes, which function as contrast agents for MRI and as optical probes for fluorescence imaging, are reported. In vitro studies using the HeLa cell lines show that these complexes exhibit anticancer activity.

Roles of DMSO-type ruthenium complexes in disaggregation of prion neuropeptide PrP106–126

DMSO-type ruthenium complexes with aromatic ligands disaggregate the mature PrP106–126 fibrilsviametal coordination and hydrophobic interaction.

Ruthenium(ii) complexes with dppz: from molecular photoswitch to biological applications

The present article describes the recent advances in biological applications of the Ru-dppz systems in DNA binding, cellular imaging, anticancer drugs, phototherapy, protein aggregation detecting and chemosensors.

Spirally-patterned pinhole arrays for long-term fluorescence cell imaging

Fluorescence cell imaging using a fluorescence microscope is an extensively used technique to examine the cell nucleus, internal structures, and other cellular molecules with fluorescence response time and intensity. However, it is difficult to perform high resolution cell imaging for a long period of time with this technique due to necrosis and apoptosis depending on the type and subcellular location of the damage caused by phototoxicity. A large number of studies have been performed to resolve this problem, but researchers have struggled to meet the challenge between cellular viability and image resolution. In this study, we employ a specially designed disc to reduce cell damage by controlling total fluorescence exposure time without deterioration of the image resolution. This approach has many advantages such as, the apparatus is simple, cost-effective, and easily integrated into the optical pathway through a conventional fluorescence microscope.

Inhibition of Beta-Amyloid Fibrillation by Luminescent Iridium(III) Complex Probes

We report herein the application of kinetically inert luminescent iridium(III) complexes as dual inhibitors and probes of beta-amyloid fibrillogenesis. These iridium(III) complexes inhibited Aβ_1–40 peptide aggregation in vitro, and protected against Aβ-induced cytotoxicity in neuronal cells. Furthermore, the complexes differentiated between the aggregated and unaggregated forms of Aβ_1–40 peptide on the basis of their emission response.

Mixed-ligand ruthenium polypyridyl complexes as apoptosis inducers in cancer cells, the cellular translocation and the important role of ROS-mediated signaling

Ruthenium (Ru) polypyridyl complexes have emerged as leading players among the potential metal-based candidates for cancer treatment. However, the roles of cellular translocation in their action mechanisms remain elusive. Herein we present the synthesis and characterization of a series of ruthenium (Ru) complexes containing phenanthroline derivatives with varying lipophilicities, and examine their mechanism of anticancer action. Results showed that increasing the lipophilicity of complexes can enhance the rates of cellular uptake. The in vitro anticancer efficacy of these complexes depended on the levels of ROS overproduction, rather than on cellular Ru uptake levels. The introduction of a phenolic group on the ligand effectively enhanced their intracellular ROS generation and anticancer activities. In particular, complex 4, with an ortho-phenolic group on the ligand, exhibited better selectivity between cancer and normal cells in comparison with cisplatin. Notably, complex 4 entered the cancer cells partially through transferrin receptor-mediated endocytosis, and then it translocated from lysosomes to the mitochondria, where it activated mitochondrial dysfunction by regulation of Bcl-2 family proteins, thus leading to intracellular ROS overproduction. Excess ROS amplified apoptotic signals by activating many downstream pathways such as p53 and MAPK pathways to promote cell apoptosis. Overall, this study provides a drug design strategy for discovery of Ru-based apoptosis inducers, and elucidates the intracellular translocation of these complexes.

Simple Colorimetric Detection of Amyloid β-peptide (1-40) based on Aggregation of Gold Nanoparticles in the Presence of Copper Ions

A simple method for specific colorimetric sensing of Alzheimer's disease related amyloid-β peptide (Aβ) is developed based on the aggregation of gold nanoparticles in the presence of copper ion. The detection of limit for Aβ(1-40) is 0.6 nM and the promising results from practical samples (human serum) indicate the great potential for the routine detection.

Synthesis and evaluation of a glucose attached pyrene, as a fluorescent molecular probe in sugar and non-sugar based micro-heterogeneous media

A new fluorescent pyrene–glucose conjugate (pyd-glc), 1-(4,6-O-butylidene-β-d-glucopyranosyl)-4-(1-pyrene)-butan-2-one, has been synthesized by attaching a pyrene molecule to acetal (butylidene) protected glucose via a butane-2-one linker.

Photophysical and photochemical properties of a family of isoelectronic tris chelated ruthenium(ii) aza-/azo-aromatic complexes

We have investigated the electrochemical, spectroscopic and electroluminescent properties of a family of aza-aromatic complexes of ruthenium of type [Ru^II(bpy/phen)₂(L)]²⁺ (4d⁶) with three isomeric L ligands.

Molecular “light switch” [Ru(phen)2dppzidzo]2+monitoring the aggregation of tau

The luminescence of [Ru(phen)₂dppzidzo]²⁺has a linear response to the amounts of tau filaments. It may have a possible binding mode as depicted in the right diagram.

Impacts of terminal modification of [Ru(phen)2dppz]2+on the luminescence properties: a theoretical study

Lowest singlet transitions were found to be related to the intriguing luminescence properties of three different dppz-like ruthenium(ii) complexes through a theoretical study.

Sensing and inhibition of amyloid-β based on the simple luminescent aptamer-ruthenium complex system

Aggregation of amyloid-β (Aβ) peptide has been known to be pathologically associated with Alzheimer and dementia diseases. Amyloid-β fibrils serve as an important target for the drugs development and diagnosis of neurodegenerative diseases. Herein, we report a new [Ru(dmbpy)(dcbpy)dppz)] complex (dmbpy; 4,4'-dimethyl-2,2'-bipyridine, dcbpy; 4,4'-dicorboxy-2,2'-bipyridine, dppz; dipyridophenazine) intercalated aptamer based recognition of amyloid-β. Interestingly, aforementioned Ru(II) complex shows weak luminescence intensity in the aqueous medium but it shows strong luminescence intensity in the presence of RNA aptamer. Upon addition of amyloid-β monomers, the luminescence intensity of Ru(II) complex is reduced due to the strong interaction of aptamer with amyloid-β monomer/small oligomers. Furthermore, present study implies that our system has ability to inhibit the formation of amyloid-β fibrils, which is confirmed from the AFM morphological structures in the absence and presence of aptamer. This work may contribute the rapid diagnosis and inhibition of amyloid-β aggregation in the clinical applications.

Labeling phospholipid membranes with lipid mimetic luminescent metal complexes

Lipid-mimetic metallosurfactant based luminophores are promising candidates for labeling phospholipid membranes without altering their biophysical characteristics. The metallosurfactants studied exhibit high structural and physicochemical similarity to phospholipid molecules, designed to incorporate into the membrane structure without the need for covalent attachment to a lipid molecule. In this work, two lipid-mimetic phosphorescent metal complexes are described: [Ru(bpy)2(dn-bpy)](2+) and [Ir(ppy)2(dn-bpy)](+) where bpy is 2,2'-bipyridine, dn-bpy is 4,4'-dinonyl-2,2'-bipyridine and ppy is 2-phenylpyridine. Apart from being lipid-mimetic in size, shape and physical properties, both complexes exhibit intense photoluminescence and enhanced photostability compared with conventional organic fluorophores, allowing for prolonged observation. Moreover, the large Stokes shift and long luminescence lifetime associated with these complexes make them more suitable for spectroscopic studies. The complexes are easily incorporated into dimyristoil-phosphatidyl-choline (DMPC) liposomes by mixing in the organic solvent phase. DLS reveals the labeled membranes form liposomes of similar size to that of neat DMPC membrane. Synchrotron Small-Angle X-ray Scattering (SAXS) measurements confirmed that up to 5% of either complex could be incorporated into DMPC membranes without producing any structural changes in the membrane. Fluorescence microscopy reveals that 0.5% label content is sufficient for imaging. Atomic Force Microscopic imaging confirms that liposomes of the labeled bilayers on a mica surface can fuse into a flat lamellar membrane that is morphologically identical to neat lipid membranes. These results demonstrate the potential of such lipid-mimetic luminescent metal complexes as a new class of labels for imaging lipid membranes.

Beam pen lithography as a new tool for spatially controlled photochemistry, and its utilization in the synthesis of multivalent glycan arrays

Herein, we describe how cantilever-free scanning probes can be used to deposit precursor material and subsequently irradiate the precursor to initiate polymerization, resulting in a 3D lithographic method wherein the position, height and diameter of each feature can be tuned independently. Specifically, acrylate and methacrylate monomers were patterned onto thiol terminated glass and subsequently exposed to UV light produced brush polymers by a photoinduced radical acrylate polymerization reaction. Here, we report the first examples of glycan arrays, comprised of methacrylate brush polymers that are side-chain functionalized with α-glucose, by this new lithographic approach. Their binding with fluorophore labeled concanavalin A (ConA) was assayed by fluorescence microscopy. The fluorescence of these brush polymers was compared to glycan arrays composed of monolayers of α-mannosides and α-glucosides prepared by combining polymer pen lithography (PPL) with the thiol-ene photochemical reaction or the copper-catalyzed azide-alkyne cycloaddition. At high ConA concentration, the fluorescence signal of the brush polymer was nearly 20 times greater than that of the glycan monolayers, and the brush polymer arrays had a detection limit nearly two orders of magnitude better than their monolayer counterparts. Because of the ability of this method to control precisely the polymer length, the relationship between limit of detection and multivalency could be explored, and it was found that the longer polymers (136 nm) are an order of magnitude more sensitive towards ConA binding than the shorter polymers (8 nm) and that binding affinity decreased systematically with length. These glycan arrays are a new tool to study the role of multivalency on carbohydrate recognition, and the photopolymerization route towards forming multivalent glycan scaffolds described herein, is a promising route to create multiplexed glycan arrays with nanoscale feature dimensions.

Metal complexes designed to bind to amyloid-β for the diagnosis and treatment of Alzheimer's disease

Alzheimer's disease is the most common form of age-related neurodegenerative dementia. The disease is characterised by the presence of plaques in the cerebral cortex. The major constituent of these plaques is aggregated amyloid-β peptide. This review focuses on the molecular aspects of metal complexes designed to bind to amyloid-β. The development of radioactive metal-based complexes of copper and technetium designed as diagnostic imaging agents to detect amyloid burden in the brain is discussed. Separate sections of the review discuss the use of luminescent metal complexes to act as non-conventional probes of amyloid formation and recent research into the use of metal complexes as inhibitors of amyloid formation and toxicity.

Dinuclear ruthenium(II) complexes as two-photon, time-resolved emission microscopy probes for cellular DNA

The first transition-metal complex-based two-photon absorbing luminescence lifetime probes for cellular DNA are presented. This allows cell imaging of DNA free from endogenous fluorophores and potentially facilitates deep tissue imaging. In this initial study, ruthenium(II) luminophores are used as phosphorescent lifetime imaging microscopy (PLIM) probes for nuclear DNA in both live and fixed cells. The DNA-bound probes display characteristic emission lifetimes of more than 160 ns, while shorter-lived cytoplasmic emission is also observed. These timescales are orders of magnitude longer than conventional FLIM, leading to previously unattainable levels of sensitivity, and autofluorescence-free imaging.

Histone-deacetylase-targeted fluorescent ruthenium(II) polypyridyl complexes as potent anticancer agents

Histone deacetylases inhibitors (HDACis) have gained much attention as a new class of anticancer agents in recent years. Herein, we report a series of fluorescent ruthenium(II) complexes containing N(1)-hydroxy-N(8)-(1,10-phenanthrolin-5-yl)octanediamide (L), a suberoylanilide hydroxamic acid (SAHA) derivative, as a ligand. As expected, these complexes show interesting chemiphysical properties, including relatively high quantum yields, large Stokes shifts, and long emission lifetimes. The in vitro inhibitory effect of the most effective drug, [Ru(DIP)2L](PF6)2 (3; DIP: 4,7-diphenyl-1,10-phenanthroline), on histone deacetylases (HDACs) is approximately equivalent in activity to that of SAHA, and treatment with complex 3 results in increased levels of the acetylated histone H3. Complex 3 is highly active against a panel of human cancer cell lines, whereas it shows relatively much lower toxicity to normal cells. Further mechanism studies show that complex 3 can elicit cell cycle arrest and induce apoptosis through mitochondria-related pathways and the production of reactive oxygen species. These data suggest that these fluorescent ruthenium(II)-HDACi conjugates may represent a promising class of anticancer agents for potential dual imaging and therapeutic applications targeting HDACs.

Ruthenium red colorimetric and birefringent staining of amyloid-β aggregates in vitro and in Tg2576 mice

Alzheimer's disease (AD) is a devastating neurodegenerative disease most notably characterized by the misfolding of amyloid-β (Aβ) into fibrils and its accumulation into plaques. In this Article, we utilize the affinity of Aβ fibrils to bind metal cations and subsequently imprint their chirality to bound molecules to develop novel imaging compounds for staining Aβ aggregates. Here, we investigate the cationic dye ruthenium red (ammoniated ruthenium oxychloride) that binds calcium-binding proteins, as a labeling agent for Aβ deposits. Ruthenium red stained amyloid plaques red under light microscopy, and exhibited birefringence under crossed polarizers when bound to Aβ plaques in brain tissue sections from the Tg2576 mouse model of AD. Staining of Aβ plaques was confirmed via staining of the same sections with the fluorescent amyloid binding dye Thioflavin S. In addition, it was confirmed that divalent cations such as calcium displace ruthenium red, consistent with a mechanism of binding by electrostatic interaction. We further characterized the interaction of ruthenium red with synthetic Aβ fibrils using independent biophysical techniques. Ruthenium red exhibited birefringence and induced circular dichroic bands at 540 nm upon binding to Aβ fibrils due to induced chirality. Thus, the chirality and cation binding properties of Aβ aggregates could be capitalized for the development of novel amyloid labeling methods, adding to the arsenal of AD imaging techniques and diagnostic tools.

Interaction of manganese(II) complex with apotransferrin and the apotransferrin enhanced anticancer activities

Apotransferrin could bind a number of metal ions besides Fe, which makes it an attractive delivery vehicle for metal-based medicines. In order to evaluate whether anticancer Mn(II) complex of [(Adpa)Mn(Cl)(H(2)O)] Adpa=bis(2-pyridylmethyl)amino-2-propionic acid) (AdpaMn) could be transported by apotransferrin, we investigated its interaction with human apotransferrin by fluorescence and circular dichroism spectroscopy (CD). The association dynamics show that AdpaMn could bind to apotransferrin spontaneously in Hepes buffer. Synchronous fluorescence spectroscopy and CD spectroscopy show that the conjugation of AdpaMn and apotransferrin by hydrophobic interactions induces the change of the microenvironment and conformation of apotransferrin. The reversible binding and release of AdpaMn was studied with fluorescence titration method. The AdpaMn complex can be released from the AdpaMn-apotransferrin entity in weak acid environments. MTT assay in vitro confirms that apotransferrin can enhance the inhibition rate of AdpaMn on the proliferation of HepG-2 cells, so we deduce that AdpaMn could be transported by apotransferrin in vivo.

In situ monitoring Alzheimer's disease β-amyloid aggregation and screening of Aβ inhibitors using a perylene probe

A cationic perylene tetracarboxylic acid diimide derivative (1) is employed as a probe for in situ monitoring of Aβ aggregation and screening Aβ inhibitors. The assay is based on the fluorescence change through the aggregation of compound 1 following Aβ assembly. Importantly, this probe, compared with the well known amyloid-staining compound thioflavin T (ThT), is more sensitive to Aβ oligomer, which is highly toxic and plays a crucial role in the early stages of Alzheimer's disease.

Detection of α-synuclein amyloidogenic aggregates in vitro and in cells using light-switching dipyridophenazine ruthenium(II) complexes

Protein aggregation is the hallmark of a number of neurodegenerative diseases including Parkinson's and Huntington's diseases. There is a significant interest in understanding the molecular mechanisms involved in the self-association and fibrillization of monomeric soluble proteins into insoluble deposits in vivo and in vitro. Probes with novel properties, such as red-shifted emission, large Stokes shifts, and high photostability, are desirable for a variety of protein aggregation studies. To respond to the increasing need for aggregation-responsive compounds suitable to cellular studies, we present a ruthenium(II) dipyridophenazine derivative, [Ru(phen)(2)dppz](2+) (phen =1,10-phenanthroline, dppz = dipyrido[3,2-a:2'.3'-c]phenazine), to study aggregation of α-synuclein (αS), which is associated with the development of Parkinson's disease. We demonstrated the use of [Ru(phen)(2)dppz](2+) to monitor αS fibril formation in real-time and to detect and quantify αS aggregates in neuroglioma cells, thereby providing a novel molecular tool to study protein deposition diseases in vitro and in vivo.

Facile methodology for monitoring amyloid-β fibrillization

Amyloid-β (Aβ) is a peptide fragment that is prone to aggregate into large fibrils under physiological conditions. Many techniques have been developed to quickly monitor the transition from a primarily monomeric peptide into fibrils. Here we propose a novel method for both incubating and monitoring changes in Aβ aggregation by using modified NMR tubes, a microtube thermoshaker, and a fluorescence or UV-vis spectrometer. These NMR tubes are thin and cylindrical, which allows efficient heat transfer and orbital shaking. Our results demonstrate that our technique is both reliable and expedient when tracking Aβ fibrillization using fluorescence or turbidity assays, which presents an alternative for laboratories without specialized equipment for incubating peptide.