A Highly Sensitive Fluorescence Probe for Fast Thiol-Quantification Assay of Glutathione Reductase

STED Imaging the Dynamics of Lysosomes by Dually Fluorogenic Si-Rhodamine

Super-resolution microscopy (SRM) imaging of the finite subcellular structures and subtle bioactivities inside organelles delivers abundant cellular information with high fidelity to unravel the intricate biological processes. An ideal fluorescent probe with precise control of fluorescence is critical in SRM technique like stimulated emission depletion (STED). Si-rhodamine was decorated with both targeting group and H⁺ -receptor, affording the dually fluorogenic Si-rhodamine in which the NIR fluorescence was efficiently controlled by the coalescent of spirolactone-zwitterion equilibrium and PeT mechanism. The dually fluorogenic characters of the probe offer a perfect mutual enhancement in sensitivity, specificity and spatial resolution. Strong fluorescence only released in the existence of targeting protein at acidic lysosomal pH, ensured precisely tracking the dynamic of lysosomal structure and pH in living cells by STED.

A nanohybrid composed of MoS2 quantum dots and MnO2 nanosheets with dual-emission and peroxidase mimicking properties for use in ratiometric fluorometric detection and cellular imaging of glutathione

A nanohybrid probe was fabricated from manganese dioxide nanosheets (MnO₂ NSs), molybdenum disulfide quantum dots (MoS₂ QDs) and o-phenylenediamine (OPD) for ratiometric detection of glutathione (GSH) in aqueous solutions and living cells. The MoS₂ QDs act as the fluorescent "turn off-on" units. The MnO₂ NSs have 3 functions, viz. (a) as fluorescence quencher, (b) as fluorescence initiator for oxidized OPD (ox OPD) and (c) as selective recognizer of GSH. The quenched blue fluorescence of the MoS₂ QDs can be restored by introducing GSH that reduces the MnO₂ NSs. However, the green fluorescence of ox OPD is decreased through the loss of peroxidase activity of MnO₂ NSs in the presence of GSH. Therefore, the ratio of the fluorescence intensities at 560 and 400 nm (from ox OPD and MoS₂ QDs, respectively) linearly decreases with increasing concentrations of GSH. Under the optimal conditions, the detection limit for GSH is as low as 90 nM. The method was successfully applied to the determination of GSH in human serum samples. This nanohybrid also is shown to be membrane-permeable and to have low cytotoxicity. This paved the way to intracellular sensing of GSH in living normal HFF and cancerous HeLa cells. Additionally, by combining with logic gate, this assay was successfully applied to visually discriminate changes in the intracellular GSH. The combination of ratiometric fluorometry and peroxidase mimicking can provide a wide range of application in bioanalysis and intracellular imaging. Graphical abstract Schematic representation of the ratiometric fluorometric detection and cellular imaging of glutathione using a nanohybrid composed of MoS₂ quantum dots and MnO₂ nanosheets with dual (blue and green emission and peroxidase mimicking properties.

Emerging Applications of Fluorogenic and Non-fluorogenic Bifunctional Linkers

Homo- and hetero-bifunctional linkers play vital roles in constructing a variety of functional systems, ranging from protein bioconjugates with drugs and functional agents, to surface modification of nanoparticles and living cells, and to the cyclization/dimerization of synthetic polymers and biomolecules. Conventional approaches for assaying conjugation extents typically rely on ex situ techniques, such as mass spectrometry, gel electrophoresis, and size-exclusion chromatography. If the conjugation process involving bifunctional linkers was rendered fluorogenic, then in situ monitoring, quantification, and optical tracking/visualization of relevant processes would be achieved. In this review, conventional non-fluorogenic linkers are first discussed. Then the focus is on the evolution and emerging applications of fluorogenic bifunctional linkers, which are categorized into hetero-bifunctional single-caging fluorogenic linkers, homo-bifunctional double-caging fluorogenic linkers, and hetero-bifunctional double-caging fluorogenic linkers. In addition, stimuli-cleavable bifunctional linkers designed for both conjugation and subsequent site-specific triggered release are also summarized.

Mitochondrial GSH Systems in CA1 Pyramidal Cells and Astrocytes React Differently during Oxygen-Glucose Deprivation and Reperfusion

Pyramidal cells and astrocytes have differential susceptibility to oxygen-glucose deprivation and reperfusion (OGD-RP). It is known that excessive reactive oxygen species (ROS) in mitochondria initiates cell death, while glutathione (GSH) is one of the major defenses against ROS. Although it is known that astrocytes contain a higher concentration of GSH than neurons, and that astrocytes can provide neurons with GSH, we are unaware of a detailed and quantitative examination of the dynamic changes in the mitochondrial GSH system in the two cell types during OGD-RP. Here, we determined mitochondrial membrane potential and the degrees of oxidation of the mitochondrially targeted roGFP-based sensors for hydrogen peroxide (OxD_P) and GSH (OxD_G). We also developed a method to estimate the mitochondrial GSH (mGSH) concentration in single cells in the CA1 region of organotypic hippocampal slice cultures at several time-points during OGD-RP. We find that mitochondrial membrane potential drops in pyramidal cells during OGD while it is relatively stable in astrocytes. In both types of cell, the mitochondrial membrane potential decreases during RP. During OGD-RP, mitochondrial peroxide levels are the same. Astrocytic mGSH is more than four times higher than pyramidal cell mGSH (3.2 vs 0.7 mM). Astrocytic mGSH is drained from mitochondria during OGD, whereas in pyramidal cells it remains fairly constant. OxD_GSH prior to and during OGD is lower (less oxidized) in pyramidal cells than in astrocytes, but the two nearly converge during RP. The larger changes of redox status in the GSH system in pyramidal cells than astrocytes is an upstream sign of the higher mortality of the pyramidal cells after facing an insult. The pattern of [mGSH] changes in the two cell types could be recognized as another mechanism by which astrocytes protect neurons from transient, extreme conditions.

A Vinyl Sulfone-Based Fluorogenic Probe Capable of Selective Labeling of PHGDH in Live Mammalian Cells

Chemical probes are powerful tools for interrogating small molecule-target interactions. With additional fluorescence Turn-ON functionality, such probes might enable direct measurements of target engagement in live mammalian cells. DNS-pE (and its terminal alkyne-containing version DNS-pE2) is the first small molecule that can selectively label endogenous 3-phosphoglycerate dehydrogenase (PHGDH) from various mammalian cells. Endowed with an electrophilic vinyl sulfone moiety that possesses fluorescence-quenching properties, DNS-pE/DNS-pE2 became highly fluorescent only upon irreversible covalent modification of PHGDH. With an inhibitory property (in vitro K_i =7.4 μm) comparable to that of known PHGDH inhibitors, our probes thus offer a promising approach to simultaneously image endogenous PHGDH activities and study its target engagement in live-cell settings.

Doubly Caged Linker for AND-Type Fluorogenic Construction of Protein/Antibody Bioconjugates and In Situ Quantification

In situ quantification of the conjugation efficiency of azide-terminated synthetic polymers/imaging probes and thiol-functionalized antibodies/proteins/peptides was enabled by a doubly caged profluorescent and heterodifunctional core molecule C1 as a self-sorting bridging unit. Orthogonal dual "click" coupling of C1 with azide- and thiol-functionalized precursors led to highly fluorescent bioconjugates, whereas single-click products remained essentially nonfluorescent. Integration with FRET processes was also possible. For the construction of antibody-probe conjugates from an anti-carcinoembryonic antigen and a quinone-caged profluorescent naphthalimide derivative, the dual "click" coupling process with C1 was monitored on the basis of the emission turn-on of C1, whereas prominent changes in FRET ratios occurred for antibody-imaging-probe conjugates when specifically triggered by quinone oxidoreductase (NQO1), which is overexpressed in various types of cancer cells.

A Ratiometric Near-Infrared Fluorogen for the Real Time Visualization of Intracellular Redox Status during Apoptosis

Direct monitoring of apoptotic progression is a major step forward for the early assessment of therapeutic efficacy of certain treatments and the accurate evaluation of the spread of a disease. Here, the regulatory role of glutathione (GSH) is explored as a potential biomarker for tracking apoptosis. For this purpose, a near- infrared (NIR) squaraine dye is introduced that is capable of sensing GSH in a ratiometric manner by switching its emission from NIR (690 nm) to visible region (560 nm). The favorable biocompatible attributes of the probe facilitated the real-time monitoring of apoptotic process in line with the conventional apoptotic assay. Furthermore, the robust nature of the probe was utilized for the quantitative estimation of GSH during different stages of apoptosis. Through this study, an easy and reliable method of assaying apoptosis is demonstrated, which can provide valuable insights in translational clinical research.

A Comparative Analysis of Acyl-Homoserine Lactone Synthase Assays

Quorum sensing is cell-to-cell communication that allows bacteria to coordinate attacks on their hosts by inducing virulent gene expression, biofilm production, and other cellular functions, including antibiotic resistance. AHL synthase enzymes synthesize N-acyl-l-homoserine lactones, commonly referred to as autoinducers, to facilitate quorum sensing in Gram-negative bacteria. Studying the synthases, however, has proven to be a difficult road. Two assays, including a radiolabeled assay and a colorimetric (DCPIP) assay are well-documented in literature to study AHL synthases. In this paper, we describe additional methods that include an HPLC-based, C-S bond cleavage and coupled assays to investigate this class of enzymes. In addition, we compare and contrast each assay for both acyl-CoA- and acyl-ACP-utilizing synthases. The expanded toolkit described in this study should facilitate mechanistic studies on quorum sensing signal synthases and expedite discovery of antivirulent compounds.

Thioester derivatives of the natural product psammaplin A as potent histone deacetylase inhibitors

There has been significant interest in the bioactivity of the natural product psammaplin A, most recently as a potent and isoform selective HDAC inhibitor. Here we report our preliminary studies on thioester HDAC inhibitors derived from the active monomeric (thiol) form of psammaplin A, as a means to improve compound delivery into cells. We have discovered that such compounds exhibit both potent cytotoxicity and enzymatic inhibitory activity against recombinant HDAC1. The latter effect is surprising since previous SAR suggested that modification of the thiol functionality should detrimentally affect HDAC potency. We therefore also report our preliminary studies on the mechanism of action of this observed effect.

New chemodosimetric reagents as ratiometric probes for cysteine and homocysteine and possible detection in living cells and in blood plasma

In this work, we have rationally designed and synthesized two new reagents (L(1) and L(2)), each bearing a pendant aldehyde functionality. This aldehyde group can take part in cyclization reactions with β- or γ-amino thiols to yield the corresponding thiazolidine and thiazinane derivatives, respectively. The intramolecular charge-transfer (ICT) bands of these thiazolidine and thiazinane derivatives are distinctly different from those of the molecular probes (L(1) and L(2)). Such changes could serve as a potential platform for using L(1) and L(2) as new colorimetric/fluorogenic as well as ratiometric sensors for cysteine (Cys) and homocysteine (Hcy) under physiological conditions. Both reagents proved to be specific towards Cys and Hcy even in the presence of various amino acids, glucose, and DNA. Importantly, these two chemodosimetric reagents could be used for the quantitative detection of Cys present in blood plasma by using a pre-column HPLC technique. Such examples are not common in contemporary literature. MTT assay studies have revealed that these probes have low cytotoxicity. Confocal laser scanning micrographs of cells demonstrated that these probes could penetrate cell membranes and could be used to detect intracellular Cys/Hcy present within living cells. Thus, the results presented in this article not only demonstrate the efficiency and specificity of two ratiometric chemodosimeter molecules for the quantitative detection of Cys and Hcy, but also provide a strategy for developing reagents for analysis of these vital amino acids in biological samples.

The effect of zinc ion on the absorption and emission spectra of glutathione derivative: predication by ab initio and DFT methods

Relying on the reaction of o-phthalaldehyde (OPA) with glutathione (GSH) to form a highly fluorescence derivative GSH-OPA has been widely used to measure reduced glutathione. In order to better understand spectra property of the GSH-OPA and the effect of zinc ion on it, the ground and the lowest singlet excited state properties, the electronic absorption and emission spectra are predicted by ab initio and DFT methods. The absorption spectra are simulated using time dependent DFT method (TD-DFT) whereas the emission spectra are approximated by optimizing the lowest singlet excited state by HF/CI-Singles and then subsequently using this geometry for the TD-DFT calculations. The solvent effects on transition energies have been described within the conductor-like polarizable continuum model (CPCM). The calculated transition energies (absorption and emission) are in agreement with available experimental information.

Metal-organic cyclohelicates as optical receptors for glutathione: syntheses, structures, and host-guest behaviors

Two trinuclear zinc-based cyclohelicates, Zn-PDB (PDB = [5-(dibenzylamino)-N1',N3'-bis(pyridin-2-ylmethylene)isophthalohydrazide]) and Zn-PMB (PMB = [5-(bodipy-oxy)-N1',N3'-bis(pyridin-2-ylmethylene)isophthalohydrazide]) containing dibenzylamino and BODIPY groups, respectively, were generated by incorporating two amide-containing tridentate chelators into meta-positions of a substituted phenyl ring. Single-crystal structure analysis and related spectroscopic characterizations demonstrated the formation of macrocyclic helicals both in the solid state and in solution. The host-guest behavior of the cyclohelical hosts towards γ-glutamyl-cysteinyl-glycine (GSH) and its component amino acids was investigated by spectroscopic titrations. UV/Vis absorption titration and NMR titrations of Zn-PDB and Zn-PMB upon addition of the above-mentioned guests suggested that the Glu residue of GSH was positioned within the cavity. The COO(-) groups interacted with metal ions through static interactions. The Cys moiety of GSH interacted with the amide groups sited in host molecules through hydrogen-bonding interactions to produce measurable spectral changes. Fluorescent titrations of Zn-PMB upon the addition of GSH and ESI-MS investigations of the titration solutions confirmed the host-guest interaction modes and revealed the possible 1:1 complexation stoichiometry. These results showed that the recognition of a substrate within the cavity of functionalized metal-organic cage-like receptors could be a useful method to produce supramolecular sensors for biomolecules.

Simple biosensor with high selectivity and sensitivity: thiol-specific biomolecular probing and intracellular imaging by AIE fluorogen on a TLC plate through a thiol-ene click mechanism

A handy, specific, sensitive bioprobe has been developed. Tetraphenylethene (TPE) was functionalized by a maleimide (MI) group, giving a TPE-MI adduct that was nonemissive in both solution and the solid state. It was readily transformed into a fluorogen showing an aggregation-induced emission (AIE) property by the click addition of thiol to its MI pendant. The click reaction and the AIE effect enabled TPE-MI to function as a thiol-specific bioprobe in the solid state. Thus, the spot of TPE-MI on a TLC plate became emissive when it had been exposed to L-cysteine, an amino acid containing a thiol group, but remained nonemissive when exposed to other amino acids that lack free thiol units. The thiol-activated emission was rapid and strong, readily detected by the naked eye at an analyte concentration as low as approximately 1 ppb, thanks to the "lighting up" nature of the bioprobing process. Similarly, the emission of TPE-MI was turned on only by the proteins containing free thiol units, such as glutathione. Clear fluorescence images were taken when living cells were stained by using TPE-MI as a visualization agent, affording a facile fluorescent maker for mapping the distribution of thiol species in cellular systems.