A ratiometric fluorescent sensor for the mitochondrial copper pool

Small-Molecule Fluorescent Probes for Binding- and Activity-Based Sensing of Redox-Active Biological Metals

Although transition metals constitute less than 0.1% of the total mass within a human body, they have a substantial impact on fundamental biological processes across all kingdoms of life. Indeed, these nutrients play crucial roles in the physiological functions of enzymes, with the redox properties of many of these metals being essential to their activity. At the same time, imbalances in transition metal pools can be detrimental to health. Modern analytical techniques are helping to illuminate the workings of metal homeostasis at a molecular and atomic level, their spatial localization in real time, and the implications of metal dysregulation in disease pathogenesis. Fluorescence microscopy has proven to be one of the most promising non-invasive methods for studying metal pools in biological samples. The accuracy and sensitivity of bioimaging experiments are predominantly determined by the fluorescent metal-responsive sensor, highlighting the importance of rational probe design for such measurements. This review covers activity- and binding-based fluorescent metal sensors that have been applied to cellular studies. We focus on the essential redox-active metals: iron, copper, manganese, cobalt, chromium, and nickel. We aim to encourage further targeted efforts in developing innovative approaches to understanding the biological chemistry of redox-active metals.

Tracking mitochondrial Cu(I) fluctuations through a ratiometric fluorescent probe in AD model cells: Towards understanding how AβOs induce mitochondrial Cu(I) dyshomeostasis

Mitochondrial copper signaling pathway plays a role in Alzheimer's disease (AD), especially in relevant Amyloid-β oligomers (AβOs) neurotoxicity and mitochondrial dysfunction. Clarifying the relationship between mitochondrial copper homeostasis and both of mitochondrial dysfunction and AβOs neurotoxicity is important for understanding AD pathogenesis. Herein, we designed and synthesized a ratiometric fluorescent probe CHC-NS4 for Cu(I). CHC-NS4 possesses excellent ratiometric response, high selectivity to Cu(I) and specific ability to target mitochondria. Under mitochondrial dysfunction induced by oligomycin, mitochondrial Cu(I) levels gradually increased, which may be related to inhibition of ATP7A-mediated Cu(I) exportation and/or high expression of COX. On this basis, CHC-NS4 was further utilized to visualize the fluctuations of mitochondrial Cu(I) levels during progression of AD model cells induced by AβOs. It was found that mitochondrial Cu(I) levels were gradually elevated during the AD progression, which depended on not only AβOs concentration but also incubation time. Moreover, endocytosis maybe served as a prime pathway mode for mitochondrial Cu(I) dyshomeostasis induced by AβOs during AD progression. These results have provided a novel inspiration into mitochondrial copper biology in AD pathogenesis.

Unravelling the Mystery of COVID-19 Pathogenesis: Spike Protein and Cu Can Synergize to Trigger ROS Production

The COVID-19 pandemic has had a devastating impact on global health, highlighting the need to understand how the SARS-CoV-2 virus damages the lungs in order to develop effective treatments. Recent research has shown that patients with COVID-19 experience severe oxidative damage to various biomolecules. We propose that the overproduction of reactive oxygen species (ROS) in SARS-CoV-2 infection involves an interaction between copper ions and the virus's spike protein. We tested two peptide fragments, Ac-ELDKYFKNH-NH2 (L1) and Ac-WSHPQFEK-NH2 (L2), derived from the spike protein of the Wuhan strain and the β variant, respectively, and found that they bind Cu(II) ions and form a three-nitrogen complexes at lung pH. Our research demonstrates that these complexes trigger the overproduction of ROS, which can break both DNA strands and transform DNA into its linear form. Using A549 cells, we demonstrated that ROS overproduction occurs in the mitochondria, not in the cytoplasm. Our findings highlight the importance of the interaction between copper ions and the virus's spike protein in the development of lung damage and may aid in the development of therapeutic procedures.

Oxidation state-specific fluorescent copper sensors reveal oncogene-driven redox changes that regulate labile copper(II) pools

Copper is an essential metal nutrient for life that often relies on redox cycling between Cu(I) and Cu(II) oxidation states to fulfill its physiological roles, but alterations in cellular redox status can lead to imbalances in copper homeostasis that contribute to cancer and other metalloplasias with metal-dependent disease vulnerabilities. Copper-responsive fluorescent probes offer powerful tools to study labile copper pools, but most of these reagents target Cu(I), with limited methods for monitoring Cu(II) owing to its potent fluorescence quenching properties. Here, we report an activity-based sensing strategy for turn-on, oxidation state-specific detection of Cu(II) through metal-directed acyl imidazole chemistry. Cu(II) binding to a metal and oxidation state-specific receptor that accommodates the harder Lewis acidity of Cu(II) relative to Cu(I) activates the pendant dye for reaction with proximal biological nucleophiles and concomitant metal ion release, thus avoiding fluorescence quenching. Copper-directed acyl imidazole 649 for Cu(II) (CD649.2) provides foundational information on the existence and regulation of labile Cu(II) pools, including identifying divalent metal transporter 1 (DMT1) as a Cu(II) importer, labile Cu(II) increases in response to oxidative stress induced by depleting total glutathione levels, and reciprocal increases in labile Cu(II) accompanied by decreases in labile Cu(I) induced by oncogenic mutations that promote oxidative stress.

A Fluorescent Sensor for Quantitative Super-resolution Imaging of Amyloid Fibril Assembly

Many soluble proteins can self-assemble into macromolecular structures called amyloids, a subset of which are implicated in a range of neurodegenerative disorders. The nanoscale size and structural heterogeneity of prefibrillar and early aggregates, as well as mature amyloid fibrils, pose significant challenges for the quantification of amyloid species, identification of their cellular interaction partners and for elucidation of the molecular basis for cytotoxicity. We report a fluorescent amyloid sensor AmyBlink-1 and its application in super-resolution imaging of amyloid structures. AmyBlink-1 exhibits a 5-fold increase in ratio of the green (thioflavin T) to red (Alexa Fluor 647) emission intensities upon interaction with amyloid fibrils. Using AmyBlink-1 , we performed nanoscale imaging of four different types of amyloid fibrils, achieving a resolution of ~30 nm. AmyBlink-1 enables nanoscale visualization and subsequent quantification of morphological features, such as the length and skew of individual amyloid aggregates formed at different times along the amyloid assembly pathway.

Tracking Labile Copper Fluctuation In Vivo/Ex Vivo: Design and Application of a Ratiometric Near-Infrared Fluorophore Derived from 4-Aminostyrene-Conjugated Boron Dipyrromethene

Specimen differences, tissue-dependent background fluorescence and scattering, and deviated specimen position and sensor concentration make optical imaging for labile copper fluctuation in animals questionable, and a signal comparison between specimens is infeasible. We proposed ratiometric optical imaging as an alternative to overcome these disadvantages, and a near-infrared (NIR) ratiometric sensor, BDPS1, was devised therefore by conjugating boron dipyrromethene (BODIPY) with 4-aminostyrene and modifying the 4-amino group as a Cu⁺ chelator. BDPS1 possessed an excitation ratiometric copper-sensing ability to show the ratio of NIR emission (710 nm) upon excitation at 600 nm to that at 660 nm, F_ex600/F_ex660, increasing from 2.8 to 10.7. This sensor displayed still the opposite copper response of its internal charge transfer (ICT; 670 nm) and local (581 nm) emission bands. Ratiometric imaging with this sensor disclosed a higher labile copper region near the nucleus apparatus, and HEK-293T cells were more sensitive to copper incubation than MCF-7 cells. Dual excitation ratiometric imaging with this sensor realized tracking of labile copper fluctuation in mice, and the whole-body imaging found that tail intravenous injection of CUTX-101, a therapeutical agent for Menkes disease, led to a distinct labile copper increase in the upper belly. The ex vivo imaging of the resected viscera of mice revealed that CUTX-101 injection enhanced the labile copper level in the liver, intestine, lung, and gall bladder in sequence, yet the kidney, heart, and spleen showed almost no response. This study indicated that modifying BODIPY as an extended ICT fluorophore, with its electron-donating group being derived as a metal chelator, is an effective design rationale of NIR ratiometric sensors for copper tracking in vivo/ex vivo.

Ratiometric Luminescence Detection of Copper(I) by a Resonant System Comprising Two Antenna/Lanthanide Pairs

Selective and sensitive detection of Cu(I) is an ongoing challenge due to its important role in biological systems, for example. Herein, we describe a photoluminescent molecular chemosensor integrating two lanthanide ions (Tb³⁺ and Eu³⁺) and respective tryptophan and naphthalene antennas onto a polypeptide backbone. The latter was structurally inspired from copper-regulating biomacromolecules in Gram-negative bacteria and was found to bind Cu⁺ effectively under pseudobiological conditions (log K_Cu⁺ = 9.7 ± 0.2). Ion regulated modulation of lanthanide luminescence in terms of intensity and long, millisecond lifetime offers perspectives in terms of ratiometric and time-gated detection of Cu⁺. The role of the bound ion in determining the photophysical properties is discussed with the aid of additional model compounds.

Getting out what you put in: Copper in mitochondria and its impacts on human disease

Mitochondria accumulate copper in their matrix for the eventual maturation of the cuproenzymes cytochrome c oxidase and superoxide dismutase. Transport into the matrix is achieved by mitochondrial carrier family (MCF) proteins. The major copper transporting MCF described to date in yeast is Pic2, which imports the metal ion into the matrix. Pic2 is one of ~30 MCFs that move numerous metabolites, nucleotides and co-factors across the inner membrane for use in the matrix. Genetic and biochemical experiments showed that Pic2 is required for cytochrome c oxidase activity under copper stress, and that it is capable of transporting ionic and complexed forms of copper. The Pic2 ortholog SLC25A3, one of 53 mammalian MCFs, functions as both a copper and a phosphate transporter. Depletion of SLC25A3 results in decreased accumulation of copper in the matrix, a cytochrome c oxidase defect and a modulation of cytosolic superoxide dismutase abundance. The regulatory roles for copper and cuproproteins resident to the mitochondrion continue to expand beyond the organelle. Mitochondrial copper chaperones have been linked to the modulation of cellular copper uptake and export and the facilitation of inter-organ communication. Recently, a role for matrix copper has also been proposed in a novel cell death pathway termed cuproptosis. This review will detail our understanding of the maturation of mitochondrial copper enzymes, the roles of mitochondrial signals in regulating cellular copper content, the proposed mechanisms of copper transport into the organelle and explore the evolutionary origins of copper homeostasis pathways.

Dual-Functionalisation of Fluorophores for the Preparation of Targeted and Selective Probes

A key current challenge in biological research is the elucidation of the that roles chemicals and chemical reactions play in cellular function and dysfunction. Of the available cellular imaging techniques, fluorescence imaging offers a balance between sensitivity and resolution, enabling the cost-effective and rapid visualisation of model biological systems. Importantly, the use of responsive fluorescent probes in conjunction with ever-advancing microscopy and flow cytometry techniques enables the visualisation, with high spatiotemporal resolution, of both specific chemical species and chemical reactions in living cells. Ideal responsive fluorescent probes are those that contain a fluorophore tethered to both a sensing unit, to ensure selectivity of response, and a targeting group, to control the sub-cellular localisation of the probe. To date, probes that are both targeted and selective are relatively rare and most localised probes are discovered serendipitously rather than by design. A challenge in this field is therefore the identification of suitable fluorophore scaffolds that can be readily attached to both sensing and targeting groups. Here we review current strategies for dual-functionalisation of fluorophores, highlighting key examples of targeted, responsive probes.

Activity-Based Sensing with a Metal-Directed Acyl Imidazole Strategy Reveals Cell Type-Dependent Pools of Labile Brain Copper

Copper is a required nutrient for life and particularly important to the brain and central nervous system. Indeed, copper redox activity is essential to maintaining normal physiological responses spanning neural signaling to metabolism, but at the same time copper misregulation is associated with inflammation and neurodegeneration. As such, chemical probes that can track dynamic changes in copper with spatial resolution, especially in loosely bound, labile forms, are valuable tools to identify and characterize its contributions to healthy and disease states. In this report, we present an activity-based sensing (ABS) strategy for copper detection in live cells that preserves spatial information by a copper-dependent bioconjugation reaction. Specifically, we designed copper-directed acyl imidazole dyes that operate through copper-mediated activation of acyl imidazole electrophiles for subsequent labeling of proximal proteins at sites of elevated labile copper to provide a permanent stain that resists washing and fixation. To showcase the utility of this new ABS platform, we sought to characterize labile copper pools in the three main cell types in the brain: neurons, astrocytes, and microglia. Exposure of each of these cell types to physiologically relevant stimuli shows distinct changes in labile copper pools. Neurons display translocation of labile copper from somatic cell bodies to peripheral processes upon activation, whereas astrocytes and microglia exhibit global decreases and increases in intracellular labile copper pools, respectively, after exposure to inflammatory stimuli. This work provides foundational information on cell type-dependent homeostasis of copper, an essential metal in the brain, as well as a starting point for the design of new activity-based probes for metals and other dynamic signaling and stress analytes in biology.

A ratiometric iron probe enables investigation of iron distribution within tumour spheroids

Iron dysregulation is implicated in numerous diseases, and iron homeostasis is profoundly influenced by the labile iron pool (LIP). Tools to easily observe changes in the LIP are limited, with calcein AM-based assays most widely used. We describe here FlCFe1, a ratiometric analogue of calcein AM, which also provides the capacity for imaging iron in 3D cell models.

Activity-based ratiometric FRET probe reveals oncogene-driven changes in labile copper pools induced by altered glutathione metabolism

Copper is essential for life, and beyond its well-established ability to serve as a tightly bound, redox-active active site cofactor for enzyme function, emerging data suggest that cellular copper also exists in labile pools, defined as loosely bound to low-molecular-weight ligands, which can regulate diverse transition metal signaling processes spanning neural communication and olfaction, lipolysis, rest-activity cycles, and kinase pathways critical for oncogenic signaling. To help decipher this growing biology, we report a first-generation ratiometric fluorescence resonance energy transfer (FRET) copper probe, FCP-1, for activity-based sensing of labile Cu(I) pools in live cells. FCP-1 links fluorescein and rhodamine dyes through a Tris[(2-pyridyl)methyl]amine bridge. Bioinspired Cu(I)-induced oxidative cleavage decreases FRET between fluorescein donor and rhodamine acceptor. FCP-1 responds to Cu(I) with high metal selectivity and oxidation-state specificity and facilitates ratiometric measurements that minimize potential interferences arising from variations in sample thickness, dye concentration, and light intensity. FCP-1 enables imaging of dynamic changes in labile Cu(I) pools in live cells in response to copper supplementation/depletion, differential expression of the copper importer CTR1, and redox stress induced by manipulating intracellular glutathione levels and reduced/oxidized glutathione (GSH/GSSG) ratios. FCP-1 imaging reveals a labile Cu(I) deficiency induced by oncogene-driven cellular transformation that promotes fluctuations in glutathione metabolism, where lower GSH/GSSG ratios decrease labile Cu(I) availability without affecting total copper levels. By connecting copper dysregulation and glutathione stress in cancer, this work provides a valuable starting point to study broader cross-talk between metal and redox pathways in health and disease with activity-based probes.

Coumarin-Based Small-Molecule Fluorescent Chemosensors

Coumarins are a very large family of compounds containing the unique 2H-chromen-2-one motif, as it is known according to IUPAC nomenclature. Coumarin derivatives are widely found in nature, especially in plants and are constituents of several essential oils. Up to now, thousands of coumarin derivatives have been isolated from nature or produced by chemists. More recently, the coumarin platform has been widely adopted in the design of small-molecule fluorescent chemosensors because of its excellent biocompatibility, strong and stable fluorescence emission, and good structural flexibility. This scaffold has found wide applications in the development of fluorescent chemosensors in the fields of molecular recognition, molecular imaging, bioorganic chemistry, analytical chemistry, materials chemistry, as well as in the biology and medical science communities. This review focuses on the important progress of coumarin-based small-molecule fluorescent chemosensors during the period of 2012-2018. This comprehensive and critical review may facilitate the development of more powerful fluorescent chemosensors for broad and exciting applications in the future.

Porphyrinic Probe for Fluorescence "Turn-On" Monitoring of Cu+ in Aqueous Buffer and Mitochondria

A zinc(II) porphyrin derivative (ZPSN) was designed and synthesized, and this probe exhibited rapid, selective and reversible binding to Cu⁺ for fluorescence monitoring in pure aqueous buffer. The detection mechanism is based on Cu⁺-activated disruption of axial coordination between the pyridyl ligand and the zinc center, which changes the molecular geometry and inhibits intramolecular electron transfer (ET), leading to fluorescence enhancement of the probe. The proposed sensing mechanism was supported by UV-vis spectroscopy/fluorescence spectral titration, NMR spectroscopy, mass spectrometry, and time-resolved fluorescence decay studies. The dissociation constant was calculated to be 6.53 × 10^-11 M. CLSM analysis strongly suggested that ZPSN could penetrate live cells and successfully visualize Cu⁺ in mitochondria. This strategy may establish a design and offer a potential building block for construction of other metal sensors based on a similar mechanism.

A reversible fluorescent probe for monitoring Ag(I) ions

Silver-containing nanomaterials are of interest for their antibiotic properties, for a wide range of applications from medicine to consumer products. However, much remains to be learnt about the degradation of such materials and their effects on human health. While most analyses involve measurement of total silver levels, it is important also to be able to measure concentrations of active free Ag(I) ions. We report here the preparation of a coumarin-based probe, thiocoumarin silver sensor 1 (TcAg1), that responds reversibly to the addition of silver ions through the appearance of a new fluorescence emission peak at 565 nm. Importantly, this peak is not observed in the presence of Hg(II), a common interferent in Ag(I) sensing. To establish the utility of this sensor, we prepared silver-doped phosphate glasses with demonstrated bactericidal properties, and observed the Ag(I) release from these glasses in solutions of different ionic strength. TcAg1 is therefore a useful tool for the study of the environmental and medical effects of silver-containing materials.

Promises and Pitfalls of Metal Imaging in Biology

A picture may speak a thousand words, but if those words fail to form a coherent sentence there is little to be learned. As cutting-edge imaging technology now provides us the tools to decipher the multitude of roles played by metals and metalloids in molecular, cellular, and developmental biology, as well as health and disease, it is time to reflect on the advances made in imaging, the limitations discovered, and the future of a burgeoning field. In this Perspective, the current state of the art is discussed from a self-imposed contrarian position, as we not only highlight the major advances made over the years but use them as teachable moments to zoom in on challenges that remain to be overcome. We also describe the steps being taken toward being able to paint a completely undisturbed picture of cellular metal metabolism, which is, metaphorically speaking, the Holy Grail of the discipline.

Link of impaired metal ion homeostasis to mitochondrial dysfunction in neurons

Manganese, iron, copper, and zinc are observed to play essential roles in mitochondria. The overload and depletion of metal ions in mitochondria under pathological conditions, however, could disturb mitochondrial compartments and functions leading to cell death. In this review, we mainly summarize how impaired metal ion homeostasis affects mitochondrial systems, such as membrane potentials, the tricarboxylic acid cycle, oxidative phosphorylation, and glutathione metabolism. In addition, based on current findings, we briefly describe a recent understanding of the relationship among metal ion dysregulation, mitochondrial dysfunction, and the pathogeneses of neurodegenerative diseases.

Interactions of cisplatin and the copper transporter CTR1 in human colon cancer cells

There is much interest in understanding the mechanisms by which platinum-based anticancer agents enter cells, and the copper transporter CTR1 has been the focus of many recent studies. While there is a clinical correlation between CTR1 levels and platinum efficacy, cellular studies have provided conflicting evidence relating to the relationship between cisplatin and CTR1. We report here our studies of the relationship between cisplatin and copper homeostasis in human colon cancer cells. While the accumulation of copper and platinum do not appear to compete with each other, we did observe that cisplatin perturbs CTR1 distribution within 10 min, a far shorter incubation time than commonly employed in cellular studies of cisplatin. Furthermore, on these short time-scales, cisplatin caused an increase in the cytoplasmic labile copper pool. While the predominant focus of studies to date has been on CTR1, these studies highlight the importance of investigating the interaction of cisplatin with other copper proteins.

Recent Advances in Macrocyclic Fluorescent Probes for Ion Sensing

Small-molecule fluorescent probes play a myriad of important roles in chemical sensing. Many such systems incorporating a receptor component designed to recognise and bind a specific analyte, and a reporter or transducer component which signals the binding event with a change in fluorescence output have been developed. Fluorescent probes use a variety of mechanisms to transmit the binding event to the reporter unit, including photoinduced electron transfer (PET), charge transfer (CT), Förster resonance energy transfer (FRET), excimer formation, and aggregation induced emission (AIE) or aggregation caused quenching (ACQ). These systems respond to a wide array of potential analytes including protons, metal cations, anions, carbohydrates, and other biomolecules. This review surveys important new fluorescence-based probes for these and other analytes that have been reported over the past five years, focusing on the most widely exploited macrocyclic recognition components, those based on cyclam, calixarenes, cyclodextrins and crown ethers; other macrocyclic and non-macrocyclic receptors are also discussed.