Rational design of a ratiometric two-photon fluorescent probe for real-time visualization of apoptosis

Investigating Oxidative Stress Associated with Myocardial Fibrosis by High-Fidelity Visualization and Accurate Evaluation of Mitochondrial GSH Levels

Myocardial fibrosis is frequently accompanied by elevated levels of oxidative stress. Mitochondrial glutathione (mGSH), an essential biomolecule for maintaining redox homeostasis in mitochondria, could serve as an effective indicator for investigating the oxidative stress associated with myocardial fibrosis. In this study, a ratiometric fluorescent probe named Mito-NS6, capable of being anchored in mitochondria and reversibly responding to GSH with an appropriate dissociation equilibrium constant, was rationally designed and utilized to visualize and evaluate the changes of mGSH levels caused by oxidative stress in myocardial fibrosis. Benefiting from the good performance of Mito-NS6, we successfully achieved the quantification of mGSH in cardiac fibroblasts using a confocal laser-scanning microscope, revealing that salvianolic acid B (SalB) can act as an effective drug to alleviate myocardial fibrosis through depressing oxidative stress. Moreover, we employed ratio fluorescence imaging to track the fluctuation in GSH levels within a mice model of myocardial fibrosis induced by isoproterenol and found that myocardial fibrosis caused a higher oxidative stress level in myocardial tissue as well as heart organs. These results provide a novel point of view for the diagnosis and treatment of myocardial fibrosis.

A dual-salt fluorescent probe for specific recognition of mitochondrial NADH and potential cancer diagnosis

Reduced nicotinamide adenine dinucleotide (NADH) is a kind of coenzyme and widely works as a biomarker in cancer cells. It plays a crucial role in many cellular metabolic processes, especially NADH in mitochondria is indispensable for the mitochondrial respiration chain that produces ATP. Herein, we designed a fluorescent probe Mito-FCC based on an ethylene-bridging dual-salt structure, in which benzo[e]indolium fluorophore was used as the mitochondria-targeting group and 1-methylquinolinium moiety as the NADH recognition unit. Mito-FCC exhibited high sensitivity and selectivity for NADH with a rapid "turn-on" fluorescence signal. The dual-salt structure endowed the probe with a reliable mitochondria-targeted ability even after the recognition unit was reduced by NADH. With the help of the probe, the fluctuations of endogenous NADH induced by glucose or pyruvate were imaged. Besides, Mito-FCC had a capability to make a distinction between cancer cells and normal cells due that the content of NADH in cancer cells was distinctly higher than that in normal ones. Notably, the visualization of tumor in vivo through monitoring NADH using Mito-FCC was realized successfully. These experimental results showed that Mito-FCC hold a great perspective in study of mitochondrial function and potential diagnosis of cancer diseases.

Cysteine-Activatable Near-Infrared Fluorescent Probe for Dual-Channel Tracking Lipid Droplets and Mitochondria in Epilepsy

Dual-channel fluorescent probes could respond to a specific target and emit different wavelengths of fluorescence before and after the response. Such probes could alleviate the influence caused by the variation of the probe concentration, excitation intensity, and so on. However, for most dual-channel fluorescent probes, the probe and fluorophore faced spectral overlap, which reduced sensitivity and accuracy. Herein, we introduced a cysteine (Cys)-responsive and near-infrared (NIR) emissive AIEgen (named TSQC) with good biocompatibility to dual-channel monitor Cys in mitochondria and lipid droplets (LDs) during cell apoptosis through wash-free fluorescence bio-imaging. TSQC can label mitochondria with bright fluorescence around 750 nm, and after reacting with Cys, the reaction product TSQ could spontaneously target LDs with emissions around 650 nm. Such spatially separated dual-channel fluorescence responses could significantly improve detection sensitivity and accuracy. Furthermore, the Cys-triggered dual-channel fluorescence imaging in LDs and mitochondria during apoptosis induced by UV light exposure, H₂O₂, or LPS treatment is clearly observed for the first time. Besides, we also report here that TSQC can be used to image subcellular Cys in different cell lines by measuring the fluorescence intensities of different emission channels. In particular, TSQC shows superior utility for the in vivo imaging of apoptosis in acute and chronic epilepsy mice. In brief, the newly designed NIR AIEgen TSQC can respond to Cys and separate two fluorescence signals to mitochondria and LDs, respectively, to study Cys-related apoptosis.

Discovery of the cysteine dynamics during the development and treatment of diabetic process by fluorescent imaging

Herein, a novel fluorescent probe RhoDCM was developed for monitoring the cysteine (Cys) dynamics. For the first time, the Cys-triggered implement was applied in relatively complete diabetic mice models. The response of RhoDCM towards Cys suggested advantages including practical sensitivity, high selectivity, rapid reaction, and steadiness in various pH and temperature conditions. RhoDCM could basically monitor the intracellular Cys level, both exogenous and endogenous. It could further monitor the glucose level via detecting consumed Cys. Furthermore, the diabetic mice models including the no diabetic control group, the induced model groups by streptozocin (STZ) or alloxan, and the treatment groups induced by STZ and treated with vildagliptin (Vil), dapagliflozin (DA), or metformin (Metf) were constructed. The models were checked by oral glucose tolerance test and significant liver-related serum indexes. Based on the models, the in vivo imaging and penetrating depth fluorescence imaging both indicated that RhoDCM could characterize the status of the development and treatment in the diabetic process via monitoring the Cys dynamics. Consequently, RhoDCM seemed beneficial for inferring the order of severity in the diabetic process and evaluating the potency of therapeutic schedules, which might be informatic for correlated investigations.

Accurate Monitoring and Multiple Evaluations of Mitophagy by a Versatile Two-Photon Fluorescent Probe

Mitophagy plays a critical role in regulating and maintaining cellular functions, particularly regulating the quantity and quality of mitochondria. In this research, a multifunctional two-photon fluorescent probe Mito-PV with improved mitochondria-anchored ability was designed. The proposed probe can track the fluctuation of polarity and viscosity in mitochondria simultaneously with two well-distinguished emissions. It can also precisely visualize the change in mitochondrial morphology (including mitochondrial form factor and length). The real-time and accurate monitoring of mitophagy under two-photon excitation was successfully achieved by utilizing probe Mito-PV through supervising the alterations of diverse mitophagy-related parameters (including colocalization coefficient, polarity, viscosity, and mitochondrial morphology). In addition, probe Mito-PV can be applied to evaluate drug bpV(phen) as an effective mitophagy inhibitor. Therefore, our work may provide a more efficient and reliable method for precisely monitoring mitophagy from multiple evaluations.

Catalytic route electrochemiluminescence microscopy of cell membranes with nitrogen-doped carbon dots as nano-coreactants

Catalytic route electrochemiluminescence microscopy enables us to image upper cell membranes with a vertical resolution mode by using nitrogen-doped carbon dots as nano-coreactants and labels.

A fluorescent probe for specific detection of cysteine in lysosomes via dual-color mode imaging

Biothiols, as part of the reactive sulfur species (RSS), are a class of bioactive molecules that play important physiological roles in human body. However, due to the similarity in structure and reaction sites of biothiols, it is difficult to differentiated detection them at the same time. In this work, a fluorescent probe CM-NBD combined coumarin derivative and 7-nitrobenzofurazan has been developed, which can effectively detect biothiols through simple ether cleavage. Because of a specific location group, CM-NBD can well localize in lysosomes with a high co-localization coefficient. Interesting, due to the weakly acidic environment of lysosomes, Cys can be distinguished from Hcy/GSH and H₂S via dual-color mode. The probe is able not only to image exogenous biothiols but also to discriminate Cys from Hcy/GSH and H₂S in cells and zebrafish model.

Intelligent Nanoprobe: Acid-Responsive Drug Release and In Situ Evaluation of Its Own Therapeutic Effect

The design of an intelligent nanoprobe capable of intracellular controlled release of apoptosis inducers and subsequent high-fidelity imaging of the drug-induced apoptosis is highly desirable for precise cancer treatment. Herein, we report an intelligent nanoprobe that combined therapeutic and imaging functions in one agent. Briefly, a gold nanoparticle is designed to be conjugated with acid-responsive DNA duplexes (Dox intercalates in this region) and caspase-3-specific cleavable peptides (labeled with fluorophore). We demonstrated that the nanoprobe could efficiently deliver an anticancer drug (Dox) into cancer cells and achieve acid-responsive drug release. Furthermore, the apoptotic process was in situ-monitored by detection of fluorescence through the cleavage of the peptide linker by caspase-3, which is one of the executioner caspases involved in apoptosis. This newly developed nanoprobe could serve as a theranostic agent for targeted responsive chemotherapy and also provide feedback apoptosis imaging of the self-therapeutic effect.

A ratiometric fluorescent probe for simultaneous detection of Cys/Hcy and GSH

GSH, Cys and Hcy are the main intracellular thiols to play crucial roles in human pathologies. It is a great challenge to differentiate these three biothiols using single molecular fluorescent probes due to their close similarities in chemical structure and reactivity. In this work, based on the fluorescence resonance energy transfer (FRET) mechanism, a fluorescent probe CPR was constructed to simultaneously distinguish GSH and Cys/Hcy by means of ratiometric fluorescence changes: from red (584 nm) to green (542 nm) for GSH and from red (584 nm) to blue (472 nm) for Cys/Hcy. This probe showed high sensitivity and selectivity with low limits of detection (LOD = 12 nm, 13 nm and 30 nm for Cys, Hcy and GSH, respectively) and was capable of imaging GSH and Cys/Hcy in cells and zebrafish in a ratiometric manner with low toxicity.

A lysosome-targetable fluorescent probe for real-time imaging cysteine under oxidative stress in living cells

As an effective lysosomal biomarker for oxidative stress status, cysteine (Cys) plays an important role in lysosomal proteolysis. Herein, we report the first lysosome-targetable fluorescence probe (MCAB) for Cys-selective detection based on nucleophilic addition reaction of sulfhydryl toward a α, β-unsaturated ketone and demonstrate its application to lysosomal-targetable imaging. MCAB is designed based on a α, β-unsaturated ethanoylcarbazole as the fluorophore and the thiols reaction site, and a methylcarbitol unit as a lysosome-targetable group. Upon reacting with Cys, this probe turns on highly specific fluorescence signals linearly proportional to Cys concentrations over the range of 0-300 μM. MCAB detects Cys with a rapid response time (within 12 min) and low limit of detection (0.38 μM). MCAB is highly selective to Cys over other similar biothiols including homocysteine (Hcy) and glutathione (GSH). Moreover, it also exhibits significant lysosomal-targetable ability, which is ideal for lysosomal Cys-selective imaging. Using MCAB, we have successfully visualized the fluctuation endogenous Cys in lysosomes under oxidative stress status in real-time.

Targeting and Imaging of Mitochondria Using Near-Infrared Cyanine Dye and Its Application to Multicolor Imaging

Herein, we report water-soluble mitochondria-selective molecules that consist of a target-specific moiety conjugated with a near-infrared (NIR) imaging agent through variable spacer length. The presented NIR fluorescent cyanine-5 (Cy-5) chromophore exhibits excellent photostability, narrow NIR absorption and emission bands, high molar extinction coefficient, high fluorescence quantum yield, and long fluorescence lifetime. The biological compatibility and negligible cytotoxicity further make the dye an attractive choice for biological applications. Confocal fluorescence microscopic studies in the fixed human lung carcinoma cell line (A549) stained with the targeting NIR Cy-5 dyes (Cy-5a and Cy-5b) at 700 nM concentration show their cellular uptake and localization, which is compared with the nontargeting Cy-5c. Mitochondrial target specificity is demonstrated by colocalization experiments using the mitochondrion-tracking probe, MitoTracker Red and lysosome-tracking probe, LysoTracker Green. Multicolor imaging of cellular organelles in A549 cells is achieved in combination with suitable target-specific dyes with distinct excitation and emission, such as green emitting FM 1-43FX to selectively image the plasma membrane, blue-fluorescent DAPI to stain the nucleus, and the synthesized NIR Cy-5 to image the mitochondria. Higher accumulation of the dye inside the cancer cell mitochondria compared to the noncancerous cell is also demonstrated.

A FRET-Based Near-Infrared Fluorescent Probe for Ratiometric Detection of Cysteine in Mitochondria

We report a near-infrared fluorescent probe A for the ratiometric detection of cysteine based on FRET from a coumarin donor to a near-infrared rhodamine acceptor. Upon addition of cysteine, the coumarin fluorescence increased dramatically up to 18-fold and the fluorescence of the rhodamine acceptor decreased moderately by 45 % under excitation of the coumarin unit. Probe A has been used to detect cysteine concentration changes in live cells ratiometrically and to visualize fluctuations in cysteine concentrations induced by oxidation stress through treatment with hydrogen peroxide or lipopolysaccharide (LPS). Finally, probe A was successfully applied for the in vivo imaging of Drosophila melanogaster larvae to measure cysteine concentration changes.

Advances in the design of cell-permeable fluorescent probes for applications in live cell imaging

Advances in the design strategy of cell-permeable small fluorescent probes are discussed. Their applications in imaging specific cell types and intracellular bioanalytes, as well as the cellular environment in live conditions, are presented.