Synthesis of a highly HOCl-selective fluorescent probe and its use for imaging HOCl in cells and organisms

Design Principles, Sensing Mechanisms, and Applications of Highly Specific Fluorescent Probes for HOCl/OCl

Hypochlorous acid/hypochlorite (HOCl/OCl^-), one of the most important reactive oxygen species (ROS), plays vital roles in various physiological and pathological processes. At normal concentrations, OCl^- acts as part of an immune defense system by destroying invasive bacteria and pathogens. However, nonproperly located or excessive amounts of OCl^- are related to many diseases, including cancers. Thus, detection of OCl^- has great importance. Owing to their high sensitivities, selectivities, fast response times, technical simplicities, and high temporal and spatial resolution, fluorescent probes are powerful tools for in vitro and in vivo sensing of target substances. This Account focuses on the development of new chemosensors for detection of OCl^-, which operate by undergoing a chemical reaction with this ROS in conjunction with a change in emission properties. As part of the presentation, we first introduce several important factors that need to be considered in the design of fluorescent chemosensors for OCl^-, including fluorophores, reaction groups, cosolvents, and buffers. Discussion here revolves around the need to select fluorophores that resist oxidation by OCl^-. As well, attention is given to the sensitivities and selectivities of groups in the sensors that react with OCl^- to trigger a fluorescence response. Moreover, well-known reaction groups, which react with highly reactive ROS (hROS), have been redesigned to be specific for OCl^-. In addition, it is pointed out that several cosolvents and buffers such as DMSO and HEPES are not suitable for use in systems for the detection of OCl^- because they are readily oxidized by this ROS. We further discuss recent investigations carried out by us and others aimed at the development of fluorescent probes for in vitro and in vivo detection of OCl^-. These efforts led to the new "dual lock" strategy for designing OCl^- chemosensors as well as several new specific reaction groups such as imidazoline-2-thiones and imidazoline-2-boranes. Probes created using this strategy and the new reacting groups have been successfully applied to imaging exogenous and endogenous OCl^- in live cells and/or tissues. The design concepts and strategies emanating from our studies of fluorescent OCl^- probes have provided insight into the general field of fluorescent probes. Despite the progress made thus far, challenges still remain in developing and applying fluorescent OCl^- probes. For example, more highly specific and sensitive fluorescent OCl^- probes are still in great demand for studies of the biological roles played by OCl^-. Thus, interdisciplinary collaborations of chemists, biologists, and medical practitioners are needed to drive future developments of OCl^- probes for disease diagnosis and drug screening.

A highly specific and sensitive turn-on fluorescence probe for hypochlorite detection and its bioimaging applications

Development of high performance fluorescent chemosensors for the detection of ClO- in vitro and in vivo is very desirable, because many human diseases are caused by ClO-. In this paper, a highly selectivity and sensitive fluorescent probe, EDPC, based on 3-acetylcoumarin, was synthesized, which could respond to ClO- and exhibit an "off-on" mode in Tris-HCl buffer (pH = 7.2, 10 mM, 50% C2H5OH) solutions. The detection limit of the EDPC probe for ClO- was as low as 1.2 × 10-8 M. Moreover, the high selectivity and high sensitivity of EDPC towards ClO- are attributed to the oxidation reaction between the C-O of the coumarin lactone and the C[double bond, length as m-dash]C formed by aldol condensation and the mechanism was further verified using ESI-MS and DFT. Additionally, the concentrations of ClO- in real water were also calculated using the EDPC probe and showed good recovery. Finally, the distribution of intracellular endogenous ClO- was gained by confocal fluorescence microscopy in living HEK293T cells.

Rational Design of a Long-Wavelength Fluorescent Probe for Highly Selective Sensing of Carboxylesterase 1 in Living Systems

Rational design of practical probes with excellent specificity and improved optical properties for a particular enzyme is always a big challenge. Herein, a practical and highly specific fluorescent probe for carboxylesterase 1 (CES1) was rationally designed using meso-carboxyl-BODIPY as the basic fluorophore based on the substrate preference and catalytic properties of CES1. Following molecular docking-based virtual screening combined with reaction phenotyping-based experimental screening, we found that MMB (probe 7) exhibited the optimal combination of sensitivity and specificity toward human CES1 in contrast to other ester derivatives. Under physiological conditions, MMB could be readily hydrolyzed by CES1 and release MCB; such biotransformation brought great changes in the electronic properties at the meso position of the fluorophore and triggered a dramatic increase in fluorescence emission around 595 nm. Moreover, MMB was cell membrane permeable and was successfully applied to monitor the real activities of CES1 in various biological samples including living cells, tissue slices, organs, and zebrafish. In summary, this study showed a good example for constructing specific fluorescent probe(s) for a target enzyme and also provided a practical and sensitive tool for real-time sensing of CES1 activities in complicated biological samples. All these findings would strongly facilitate high-throughput screening of CES1 modulators and the studies on CES1-associated physiological and pathological processes.

Synthesis of an ultrasensitive BODIPY-derived fluorescent probe for detecting HOCl in live cells

Hypochlorous acid (HOCl) is a critical member of the reactive oxygen species (ROS) produced by immune cells to fight infections. On the other hand, HOCl in homeostasis causes oxidative damage to biomolecules and is linked to many diseases, including inflammatory, neurodegenerative, and cardiovascular diseases. Herein, we detail a procedure for the preparation of a boron-dipyrromethene (BODIPY)-derived fluorescent probe for HOCl (BClO) and its application as an imaging reagent in living cells. BClO is synthesized in one pot through a four-step procedure that is nearly the same as that for conventional BODIPY dye preparation, except for the ratio of starting materials. BClO has an extremely rapid response (saturated within seconds) and is ultrasensitive to HOCl. The detection limit of BClO reaches the subnanomolar range, which is the highest HOCl sensitivity to date. Taking advantage of the ultrasensitive character of BClO, we have previously demonstrated its ability to detect endogenous HOCl generated by macrophages and shown that it can also be used to discriminate cancer cell lines (which show high HOCl production) from non-cancer cell lines (which show low HOCl production). The protocol requires ~2 d for probe synthesis and up to ~18 h for fluorescence imaging and flow cytometry assays.

Two coumarin-based turn-on fluorescent probes based on for hypochlorous acid detection and imaging in living cells

This work, two turn-on fluorescent probes (3-acetyl-2H-chromen-2-one (ACO) & (1E)-1-(1-(2-oxo-2H-chromen-3-yl)ethylidene)thiosemicarbazide (CETC)) based on coumarin have been designed and synthesized, which could selectively and sensitively recognize ClO^- with fast response time. ACO &CETC were almost non fluorescent possibly due to both the lacton form of coumarin and unbridged CN bonds which can undergo a nonradiative decay process in the excited state. Upon the addition of ClO^-, ACO &CETC were oxidized to ring - opened by cleavage the CO and CN and the fluorescence intensity were increased considerably. Fluorescence titration experiments showed that the detection limit ACO &CETC is as low as 22 nm and 51 nm respectively. In particular, some relevant reactive species, including OH, ¹O₂, H₂O₂, KO₂, some anions and cations cannot be interference with the test. In live cell experiments, ACO &CETC were successfully applied to image exogenous ClO^- in HepG2 cells. Therefore, ACO &CETC not only could image ClO^- in living cells but also proved that CO and CN can be cleavage by ClO^-.

Bioluminescent probe for detecting endogenous hypochlorite in living mice

As a kind of biologically important reactive oxygen species (ROS), hypochlorite (ClO^-) plays a crucial role in many physiological processes. As such, endogenous ClO^- is a powerful antibacterial agent during pathogen invasion. Nonetheless, excessive endogenous ClO^- could pose a health threat to mammalian animals including humans. However, the detection of endogenous ClO^- by bioluminescence probes in vivo remains a considerable challenge. Herein, based on a caged strategy, we developed a turn-on bioluminescent probe 1 for the highly selective detection of ClO^-in vitro and imaging endogenous ClO^- in a mouse inflammation model. We anticipate that such a probe could help us understand the role of endogenous ClO^- in a variety of physiological and pathological processes.

A novel porphyrin-based near-infrared fluorescent probe for hypochlorite detection and its application in vitro and in vivo

Reactive oxygen species (ROS), especially HOCl/ClO-, have been demonstrated to play essential roles in both physiological and pathological processes, and an abnormal level of HOCl/ClO- is related to some diseases. In this work, a very fast responsive (within 30 seconds) porphyrin-based fluorescent probe, TPP-TCF, for ClO- with a NIR emissive wavelength was prepared. This probe exhibited excellent selectivity towards ClO- and would not be interfered with by other ROS and typical nucleophiles. The limit of detection (LOD) for ClO- was evaluated to be 0.29 μM, indicating high sensitivity towards ClO-. In further bioimaging experiments, TPP-TCF displayed low-cytotoxicity and good cell penetrability for recognizing exogenous ClO- in HeLa cells. Moreover, this probe was successfully applied in imaging endogenous ClO- in living animals.

A highly sensitive and selective two-photon fluorescent probe for glutathione S-transferase detection and imaging in living cells and tissues

Design and development of a two-photon fluorescent probe for GST detection and imaging in living cells and deep tissues.

Enhanced fluorescence sensing of hypochlorous acid using serum albumin as a signal amplifier

A mitochondria-targeting fluorescent probe for ClO⁻ was developed and a signal amplifier BSA was utilized to promote the fluorescent signal.

Novel fluorescent probe with a bridged Si–O–Si bond for the reversible detection of hypochlorous acid and biothiol amino acids in live cells and zebrafish

Herein, we report the design of a novel fluorescent probe consisting of a naphthalimide fluorophore and a silicone small molecule for the reversible detection of hypochlorous acid and biothiol amino acids.

Responsive Upconversion Nanoprobe for Background-Free Hypochlorous Acid Detection and Bioimaging

Responsive nanoprobes play an important role in bioassay and bioimaging, early diagnosis of diseases and treatment monitoring. Herein, a upconversional nanoparticle (UCNP)-based nanoprobe, Ru@UCNPs, for specific sensing and imaging of hypochlorous acid (HOCl) is reported. This Ru@UCNP nanoprobe consists of two functional components,, i.e., NaYF₄ :Yb, Tm UCNPs that can convert near infrared light-to-visible light as the energy donor, and a HOCl-responsive ruthenium(II) complex [Ru(bpy)₂ (DNCH-bpy)](PF₆ )₂ (Ru-DNPH) as the energy acceptor and also the upconversion luminescence (UCL) quencher. Within this luminescence resonance energy transfer nanoprobe system, the UCL OFF-ON emission is triggered specifically by HOCl. This triggering reaction enables the detection of HOCl in aqueous solution and biological systems. As an example of applications, the Ru@UCNPs nanoprobe is loaded onto test papers for semiquantitative HOCl detection without any interference from the background fluorescence. The application of Ru@UCNPs for background-free detection and visualization of HOCl in cells and mice is successfully demonstrated. This research has thus shown that Ru@UCNPs is a selective HOCl-responsive nanoprobe, providing a new way to detect HOCl and a new strategy to develop novel nanoprobes for in situ detection of various biomarkers in cells and early disgnosis of animal diseases.

Fluorescent Chemosensors for Various Analytes Including Reactive Oxygen Species, Biothiol, Metal Ions, and Toxic Gases

The development of fluorescent chemosensors for various analytes has been actively pursued by chemists. Since their inception, these efforts have led to many new sensors that have found wide applications in the fields of chemistry, biology, environmental science, and physiology. The search for fluorescent chemosensors was initiated by a few pioneering groups in the late 1970s and 1980s and blossomed during the last two decades to include more than hundreds of research groups around the world. The targets for these sensors vary from metal ions, anions, reactive oxygen/nitrogen species, biothiols, and toxic gases. Our group has made contributions to this area in last 18 years. In this perspective, we briefly introduce the history of chemosensors and review studies that we have carried out.

A highly selective fluorescent probe for the detection of hypochlorous acid in tap water and living cells

A turn-on fluorescent probe (DAME) for sensing hypochlorous acid (HClO) with excellent selectivity was presented. The fluorescent probe was composed of coumarin derivative as the fluorophore and dimethylcarbamothioic chloride group with a sulfide moiety as modulator. Additionally, the sulfide moiety would be oxidized by HClO, and then free dye of coumarin derivate was released and exhibited significant fluorescence. In addition, the probe could respond to HClO in solutions within 60 s and the limit of detection was down to 34.75 nM. Moreover, the probe was used for the detection of HClO in tap water through the home-made test paper. And confocal images confirmed that probe DAME could be used for recognizing HClO in living cells.

Two-Photon Excited FRET Dyads for Lysosome-Targeted Imaging and Photodynamic Therapy

Two-photon excitable fluorescent dyes with integrated functions of targeted imaging and photodynamic therapy (PDT) are highly desired for the development of cancer theranostic agents. Herein, fluorescence resonance energy transfer (FRET) dyads, AceDAN-H₂Por-Lyso (1a) and AceDAN-ZnPor-Lyso (1b), were developed for two-photon excited (TPE) lysosome-targeted fluorescence imaging and PDT of cancer cells. Under one-photon or two-photon excitation, the AceDAN donor can effectively transfer the excited state energy to the porphyrin acceptor via high efficient FRET, leading to the generation of deep-red fluorescence and singlet oxygen for cell imaging and PDT, respectively. 1a and 1b exhibit high photocytotoxicity and low dark cytotoxicity, in addition to strong lysosomal targeting capability in living cells. By taking the advantages of the two-photon absorption properties of the AceDAN donor and the properly distributed S₁ and T₁ states of the porphyrin acceptor, the AceDAN-porphyrin dyads 1a and 1b have been successfully applied to TPE-fluorescence imaging for tracking the significant morphology changes of cancer cells under two-photon laser irradiation.

Heterogeneity of hypochlorous acid production in individual neutrophil phagosomes revealed by a rhodamine-based probe

The rhodamine-based probe R19-S has been shown to react with hypochlorous acid (HOCl) to yield fluorescent R19, but not with some other oxidants including hydrogen peroxide. Here, we further examined the specificity of R19-S and used it for real-time monitoring of HOCl production in neutrophil phagosomes. We show that it also reacts rapidly with hypobromous acid, bromamines, and hypoiodous acid, indicating that R19-S responds to these reactive halogen species as well as HOCl. Hypothiocyanous acid and taurine chloramine were unreactive, however, and ammonia chloramine and dichloramine reacted only very slowly. MS analyses revealed additional products from the reaction of HOCl with R19-S, including a chlorinated species as a minor product. Of note, phagocytosis of opsonized zymosan or Staphylococcus aureus by neutrophils was accompanied by an increase in R19 fluorescence. This increase depended on NADPH oxidase and myeloperoxidase activities, and detection of chlorinated R19-S confirmed its specificity for HOCl. Using live-cell imaging to track individual phagosomes in single neutrophils, we observed considerable heterogeneity among the phagosomes in the time from ingestion of a zymosan particle to when fluorescence was first detected, ranging from 1 to >30 min. However, once initiated, the subsequent fluorescence increase was uniform, reaching a similar maximum in ∼10 min. Our results confirm the utility of R19-S for detecting HOCl in real-time and provide definitive evidence that isolated neutrophils produce HOCl in phagosomes. The intriguing variability in the onset of HOCl production among phagosomes identified here could influence the way they kill ingested bacteria.

Establishment of a ΔF508-CF promyelocytic cell line for cystic fibrosis research and drug screening

Cystic fibrosis (CF), one of the most common genetic disorders, is caused by mutations in the CF transmembrane conductance regulator (CFTR) gene. In spite of significant improvement in patient life expectancy, the disease remains lethal and incurable. Clinically, CF lung disease claims the most morbidity and mortality, characterized by chronic bacterial infection, persistent neutrophilic inflammation, and purulent small airway obstruction. Although all these manifestations are highly associated with neutrophils, the actual role of this phagocyte in the disease pathogenesis has not been fully appreciated. One of the major obstacles impeding such progress is the lack of CF neutrophil cell lines. Taking advantage of the new CRISPR/Cas9 gene-editing technology, we have generated a homozygous ΔF508-CF promyelocytic cell line from HL-60 cells, from which unlimited CF neutrophil cells can be differentiated. The derived cells showed defective CFTR presentation, deficient phagosomal hypochlorous acid (HOCl) production, and compromised microbial killing. Such a phenotype recapitulates that of primary neutrophils from CF patients. Thus, the established human CF promyelocytic cell line should be a useful tool for future CF basic research and drug screening.

Deformylation reaction-based probe for in vivo imaging of HOCl

The detection of hypochlorous acid (HOCl) in vivo is vitally important because the local concentration of HOCl is highly correlated with some diseases such as atherosclerosis and rheumatoid arthritis. However, in vivo detection of HOCl remains a challenge due to the lack of a suitable probe. We report here a near-infrared (NIR) emissive "turn-on" probe (FDOCl-1) based on a methylene blue derivative, which can quickly detect HOCl via a newly found deformylation mechanism. FDOCl-1 displays remarkable selectivity and sensitivity towards HOCl. The dramatic changes in colour and NIR emission were used to detect HOCl in vitro and in vivo in a mouse arthritis model.

A highly sensitive and selective fluorescent probe for fast sensing of endogenous HClO in living cells

A highly sensitive and fast-response fluorescent probe for HClO detection was developed and employed to reveal the H₂S-induced HClO biogenesis in living cells.

Superoxide Anion Biosensor Based on Bionic-Enzyme Hyperbranched Polyester Particles

Self-assembly techniques have been demonstrated to be a useful approach to developing new functional nanomaterials. In this study, a novel method to fabricate a manganese phosphate self-assembly monolayer (SAM) on a hyperbranched polyester (HBPE-OH) nanoparticle surface is described. First, the second-generation aliphatic HBPE-OH was carboxy-terminated, phosphorylated, and then ionized with manganese by a three-step modification process. The final product of HBPE-AMPA-Mn2+ particles was obtained and characterised by FT-IR spectroscopy, 1H NMR spectroscopy, transmission electron microscopy (TEM), Zeta potential, and energy dispersive spectroscopy (EDS). Moreover, the HBPE-AMPA-Mn2+ particles were used to construct a novel biosensor for detection of superoxide anions (O2•−) released from HeLa cells. Results showed that the response currents of this biosensor were proportional to the O2•− concentration ranging from 0.79 to 16.6 μM, and provided an extremely low detection limit of 0.026 μM (S/N = 3). The results indicate that the particle-decorated electrode surface, which involved a hyperbranched structure and a surface self-assembly technology, proposed here will offer the ideal catalytic system for electrochemical enzymatic sensors.

Tracking HOCl concentrations across cellular organelles in real time using a super resolution microscopy probe

A probe that specifically images HOCl in Golgi apparatus and lysosomes that is compatible wih the super resolution microscopy technique, SIM, is reported.

Mitophagy: a balance regulator of NLRP3 inflammasome activation

The NLRP3 inflammasome is activated by a variety of external or host-derived stimuli and its activation initiates an inflammatory response through caspase-1 activation, resulting in inflammatory cytokine IL-1β maturation and secretion. The NLRP3 inflammasome activation is a kind of innate immune response, most likely mediated by myeloid cells acting as a host defense mechanism. However, if this activation is not properly regulated, excessive inflammation induced by overactivated NLRP3 inflammasome can be detrimental to the host, causing tissue damage and organ dysfunction, eventually causing several diseases. Previous studies have suggested that mitochondrial damage may be a cause of NLRP3 inflammasome activation and autophagy, which is a conserved self-degradation process that negatively regulates NLRP3 inflammasome activation. Recently, mitochondria-selective autophagy, termed mitophagy, has emerged as a central player for maintaining mitochondrial homeostasis through the elimination of damaged mitochondria, leading to the prevention of hyperinflammation triggered by NLRP3 inflammasome activation. In this review, we will first focus on the molecular mechanisms of NLRP3 inflammasome activation and NLRP3 inflammasome-related diseases. We will then discuss autophagy, especially mitophagy, as a negative regulator of NLPP3 inflammasome activation by examining recent advances in research. [BMB Reports 2016; 49(10): 529-535]

Fluorescent chemical probes for accurate tumor diagnosis and targeting therapy

This review focuses on small molecular ligand-targeted fluorescent imaging probes and fluorescent theranostics, including their design strategies and applications in clinical tumor treatment.

An efficient two-photon fluorescent probe for human NAD(P)H:quinone oxidoreductase (hNQO1) detection and imaging in tumor cells

The probe TPQ, which displays high selectivity and anti-interference ability, was successfully applied to endogenous hNQO1 imaging and for the identification of different cancer cells.

In situ quantification and evaluation of ClO−/H2S homeostasis in inflammatory gastric tissue by applying a rationally designed dual-response fluorescence probe featuring a novel H+-activated mechanism

A new dual-response probe for in situ quantification of ClO⁻/H₂S homeostasis in inflammatory gastric tissue.

A NIR fluorescent probe: imaging endogenous hydrogen peroxide during an autophagy process induced by rapamycin

In this work, a near-infrared probe (Cy-B) with high sensitivity, and good specificity as well as photo-stability has been developed for monitoring both exogenous and endogenous H₂O₂ in living cells. To the best of our knowledge, it was applied successfully for the first time to monitor spontaneous hydrogen peroxide in an autophagy process induced by the stimulation of rapamycin. The mice imaging experiments indicate that the probe has a good potential to be employed in the imaging of living biological systems.

Effective Strategy for Colorimetric and Fluorescence Sensing of Phosgene Based on Small Organic Dyes and Nanofiber Platforms

Three o-phenylendiamine (OPD) derivatives, containing 4-chloro-7-nitrobenzo[c][1,2,5]oxadiazole (NBD-OPD), rhodamine (RB-OPD), and 1,8-naphthalimide (NAP-OPD) moieties, were prepared and tested as phosgene chemosensors. Unlike previously described methods to sense this toxic agent, which rely on chemical processes that transform alcohols and amines to respective phosphate esters and phosphoramides, the new sensors operate through a benzimidazolone-forming reaction between their OPD groups and phosgene. These processes promote either naked eye visible color changes and/or fluorescence intensity enhancements in conjunction with detection limits that range from 0.7 to 2.8 ppb. NBD-OPD and RB-OPD-embedded polymer fibers, prepared using the electrospinning technique, display distinct color and fluorescence changes upon exposure to phosgene even in the solid state.

A novel method for the synthesis of 1,2-benzisoxazoline-3-one and its application to hypochlorite recognition

The reaction of salicylhydroxamic acid with hypochlorite produces 1,2-benzisoxazoline-3-one, a heterocycle that contains a fluorophore.

A novel rhodamine-based fluorescent pH probe for high-throughput screening of high-yield polymalic acid strains from random mutant libraries

Polymalic acid (PMA) is produced from the yeast-like fungus Auerobasidium pullulans, and is a water-soluble biopolymer with many useful properties for pharmaceutical applications.