A 1,8-naphthalimide-based turn-on fluorescent probe for imaging mitochondrial hydrogen peroxide in living cells

Fluorescent probes for detecting and imaging mitochondrial hydrogen sulfide

Hydrogen sulfide (H2S) is a potent redox-active signaling molecule commonly dysregulated in disease states. The production of H2S and its involvement in various pathological conditions associated with mitochondrial dysfunction have extensively documented. During stress, cystathionine gamma-lyase and cystathionine beta-synthase in cytosol are copiously translocated into the mitochondria to boost H2S production, confirming its pivotal role in mitochondrial activities. However, little study has been done on H2S levels in tissues, cells and organelles, mainly due to the absence of precise and accurate detection tools. Thus, there is an urgent need to determine and monitor the levels of H2S in these important organelles. Fluorescent probes are efficient tools for detecting and monitoring various important biomolecules including biological thiols. The development of fluorescent probes is a multi-pronged approach which involves coupling fluorophores with responsive sites. The use of fluorescent probes for monitoring mitochondrial H2S levels has recently received widespread attention, resulting in numerous publications depicting their synthesis, mechanism of action, application, and potential challenges. Fluorescent probes offer precise and timely results, high sensitivity and selectivity, low biotoxicity, and minimal background interference. In this review, we aim to report designs of such probes, reaction mechanisms and their application in detecting mitochondrial H2S levels. Fluorescent probes can help uncover physio/pathological levels of H2S in essential organelles, its interactions with various biomarkers and associated consequences in biological systems.

Rational design and syntheses of naphthalimide-based fluorescent probes for targeted detection of diabetes biomarkers

Diabetes poses serious health risks, leading to complications such as liver damage, renal issues, and heart inflammation. Diagnosis typically relies on blood sugar level testing, but qualitative markers like obesity and fatigue often manifest only after prolonged illness. To address the delay in diagnosis, the development of fluorescent probes has drawn the key attention. This review examines the recent advancements especially on Naphthalimide (NM) based fluorescent construct for detecting biomolecular changes related to diabetes and its complications. For the first time this review discusses the synthetic methods and design principles for these probes, providing valuable insights for researchers focused on diabetes treatment and probe development, and laying the groundwork for future clinical applications of these probes in early diabetes diagnosis and intervention.

Ex Tenebris Lux: Illuminating Reactive Oxygen and Nitrogen Species with Small Molecule Probes

Reactive oxygen and nitrogen species are small reactive molecules derived from elements in the air─oxygen and nitrogen. They are produced in biological systems to mediate fundamental aspects of cellular signaling but must be very tightly balanced to prevent indiscriminate damage to biological molecules. Small molecule probes can transmute the specific nature of each reactive oxygen and nitrogen species into an observable luminescent signal (or even an acoustic wave) to offer sensitive and selective imaging in living cells and whole animals. This review focuses specifically on small molecule probes for superoxide, hydrogen peroxide, hypochlorite, nitric oxide, and peroxynitrite that provide a luminescent or photoacoustic signal. Important background information on general photophysical phenomena, common probe designs, mechanisms, and imaging modalities will be provided, and then, probes for each analyte will be thoroughly evaluated. A discussion of the successes of the field will be presented, followed by recommendations for improvement and a future outlook of emerging trends. Our objectives are to provide an informative, useful, and thorough field guide to small molecule probes for reactive oxygen and nitrogen species as well as important context to compare the ecosystem of chemistries and molecular scaffolds that has manifested within the field.

A Novel Fluorescent Probe for the Detection of Hydrogen Peroxide

Hydrogen peroxide (H₂O₂) is one of the important reactive oxygen species (ROS), which is closely related to many pathological and physiological processes in living organisms. Excessive H₂O₂ can lead to cancer, diabetes, cardiovascular diseases, and other diseases, so it is necessary to detect H₂O₂ in living cells. Since this work designed a novel fluorescent probe to detect the concentration of H₂O₂, the H₂O₂ reaction group arylboric acid was attached to the fluorescein 3-Acetyl-7-hydroxycoumarin as a specific recognition group for the selective detection of hydrogen peroxide. The experimental results show that the probe can effectively detect H₂O₂ with high selectivity and measure cellular ROS levels. Therefore, this novel fluorescent probe provides a potential monitoring tool for a variety of diseases caused by H₂O₂ excess.

A Two-Photon Fluorescent Probe for the Visual Detection of Peroxynitrite in Living Cells and Zebrafish

Peroxynitrite (ONOO−), as an important reactive oxygen species (ROS), holds great potential to react with a variety of biologically active substances, leading to the occurrence of various diseases such as cancer and neurodegenerative diseases. In this work, we developed a novel mitochondria-localized fluorescent probe, HDBT-ONOO−, which was designed as a mitochondria-targeting two-photon fluorescence probe based on 1,8-naphthylimide fluorophore and the reactive group of 4-(bromomethyl)-benzene boronic acid pinacol ester. More importantly, the probe exhibited good biocompatibility, sensitivity, and selectivity, enabling its successful application in imaging the generation of intracellular and extracellular ONOO−. Furthermore, exogenous and endogenous ONOO− products in live zebrafish were visualized. It is greatly expected that the designed probe can serve as a useful imaging tool for clarifying the distribution and pathophysiological functions of ONOO− in cells and zebrafish.

Activatable fluorescence molecular imaging and anti-tumor effects investigation of GSH-sensitive BRD4 ligands

Overexpression of bromodomain 4 (BRD4) is closely correlated with a variety of human cancers by regulating the histone post-translational modifications, which renders BRD4 a promising target for pharmacological discoveries of novel therapeutic agents for cancer therapy. We herein present the design, chemical synthesis, cellular imaging and biological assessment of a novel tumor-sensitive BRD4 ligand (compound 4) by introducing anticancer BRD4 inhibitor into naphthalimide moiety (fluorescent reporter) via a sulfonamide unit as glutathione (GSH)-specific cleavable linker. Upon reaction with abundant intramolecular GSH in cancer cells or free GSH in aqueous solution (pH = 7.4), sulfonamide cleavage of 4 occurs, leading to the release of BRD4 inhibitor and concomitant fluorescence-on. This activatable fluorescence molecular imaging was demonstrated to preferentially occur in tumor cells. Moreover, towards cancer cell lines MGC-803 cells and THP-1, compound 4 was identified to show better antitumor efficacy than net BRD4 inhibitor. Collectively, this study presents a drug delivery strategy, wherein the drug release can be directly monitored in the cellular content by fluorescence imaging, and provides a valuable compound 4 as a potential antitumor agent. Compound 4 may represent a useful tool for explorative studies of BRD4 inhibition, such as an improved understanding of BRD4 inhibitor release-related information.

Rational design of a fluorescent probe and its applications of imaging and distinguishing between exogenous and endogenous H2S in living cells

Hydrogen sulfide (H₂S), a recognized environmental pollutant, comes from a wide range of sources. For example, H₂S will be produced in the process of plant protein corruption, the decomposition of domestic sewage and garbage, food processing (wine brewing), etc. and once the concentration is too high, it will cause significant damage of environment and human body. Besides H₂S is an important gas signal molecule in vivo, which can be transferred through lipid membrane. Its existence level is closely related to many diseases. If we can "visually" trace the transmembrane transmission of hydrogen sulfide, it will be very helpful for the study of oxidative stress processes, cell protection, signal transduction and related diseases closely related to H₂S. Although some probes can detect H₂S in environment, cytoplasm and organelles, there are few reports on the release and internalization of H₂S. In this work, we report a H₂S fluorescence probe that can retain on the cell membrane, named PCM. The probe PCM can not only detect endogenous and exogenous H₂S, but also distinguish them, this provides a general strategy for the construction of probes to detect other biomarkers. In addition, PCM has been successfully applied to the detection of endogenous and exogenous H₂S in zebrafish, which has the potential to become a new chemical tool and provide help for the research of H₂S-related diseases.

New horizons in the identification of circulating tumor cells (CTCs): An emerging paradigm shift in cytosensors

Circulating tumor cells (CTCs) are cancer cells that are shed from a primary tumor into the bloodstream and function as seeds for cancer metastasis at distant locations. Enrichment and identification methods of CTCs in the blood of patients plays an important role in diagnostic assessments and personalized treatments of cancer. However, the current traditional identification methods not only impact the viability of cells, but also cannot determine the type of cancer cells when the disease is unknown. Hence, new methods to identify CTCs are urgently needed. In this context, many advanced and safe technologies have emerged to distinguish between cancer cells and blood cells, and to distinguish specific types of cancer cells. In this review, at first we have briefly discussed recent advances in technologies related to the enrichment of CTCs, which lay a good foundation for the identification of CTCs. Next, we have summarized state-of-the-art technologies to confirm whether a given cell is indeed a tumor cell and determine the type of tumor cell. Finally, the challenges for application and potential directions of the current identification methods in clinical analysis of CTCs have been discussed.

Irreversible coumarin based fluorescent probe for selective detection of Cu2+ in living cells

Copper ion (Cu²⁺) is an essential part of the living organisms. Cu²⁺ ions play a vital role in many biotic processes. An abnormal amount of Cu²⁺ ions may result in serious diseases. Herein, a novel "fluorescent ON" probe NC-Cu to trace minute levels of Cu²⁺ ions in presence of various biological active species has been developed. Lysosomal cells targeting group (Morpholine) was added to the probe. The spectral properties of probe NC-Cu were recorded in HEPES buffer (0.01 M, pH = 7.4, comprising 50% CH₃CN, λ_ex = 430 nm, slit: 5 nm). The synthesized probe NC-Cu work based on copper promoted catalytic hydrolysis of hydrazone and shows remarkable fluorescence enhancement. The reaction of the probe with Cu²⁺ ions was completed within 20 min. An excellent linear relationship (R² = 0.9952) was found and the limit of detection (LOD, according to the 3σ/slope) for Cu²⁺ ions was calculated to be 5.8 µM. Furthermore, NC-Cu was effectively functional in the living cells (KYSE30 cells) to trace Cu²⁺ ions.

The design, synthesis and cellular imaging of a tumor-anchored, potent and cell-permeable BRD4-targeted fluorescent ligands

Bromodomain 4 (BRD4) proteins play an important role in histone post-translational modifications and facilitate several important physiological and pathological processes, including cancers. The inhibition of BRD4 by small molecule inhibitors shows promise as a therapeutic strategy for cancer treatment. However, their clinical applications were limited, which is largely hampered by off-target effects-induced toxicity. We herein report the design, synthesis, and cellular imaging of a set of tumor-anchored and BRD4-targeted fluorescent ligands by introducing selective and potent BRD4 inhibitor into different fluorophores via variable linkers. One of the fluorescent conjugates (compound 6) was demonstrated to be cell-permeable and low cytotoxic, preferentially accumulated in cancer cells, and display pronounced fluorescent signal. More importantly, 6 was identified to show specific BRD4 engagement in the cellular content. Collectively, this study provides a pathway for developing labeled BRD4 ligands and highlights that compound 6 may represent a valuable tool for explorative learning and target delivery study of BRD4.

Designing an ESIPT-based fluorescent probe for imaging of hydrogen peroxide during the ferroptosis process

Hydrogen peroxide (H₂O₂), depending on its levels, plays a crucial role in either modulating various biological processes as a signal molecule, or mediating oxidative damage as a toxin. Therefore, monitoring intracellular H₂O₂ levels is pivotal for exploring its physiological and pathological roles. Using a modified 2-(2'-hydroxyphenyl) benzothiazole (HBT) as the fluorophore, and a pinacol phenylborate ester as the responsive group, herein we developed an excited-state intramolecular proton transfer (ESIPT)-based probe BTFMB. The probe exhibited turn-on fluorescence response, large Stokes shift (162 nm) and low detection limit (109 nM) toward H₂O₂, and was successfully applied for monitoring exogenous and endogenous production of H₂O₂, and identifying accumulation of H₂O₂ during the ferroptosis process.

A novel ratiometric fluorescent probe for the detection of mitochondrial pH dynamics during cell damage

A sensitive fluorescent probe (E)-4-(3-(benzo[d]thiazol-2-yl)-4-hydroxy-5-methylstyryl)-1-methylpyridin-1-ium iodide (HBTMP) for the monitoring of pH in mitochondria was rationally exploited.

Boronate-Based Probes for Biological Oxidants: A Novel Class of Molecular Tools for Redox Biology

Boronate-based molecular probes are emerging as one of the most effective tools for detection and quantitation of peroxynitrite and hydroperoxides. This review discusses the chemical reactivity of boronate compounds in the context of their use for detection of biological oxidants, and presents examples of the practical use of those probes in selected chemical, enzymatic, and biological systems. The particular reactivity of boronates toward nucleophilic oxidants makes them a distinct class of probes for redox biology studies. We focus on the recent progress in the design and application of boronate-based probes in redox studies and perspectives for further developments.

1,8-Naphthalimide-Based Multifunctional Compounds as Cu2+ Probes, Lysosome Staining Agents, and Non-viral Vectors

A series of multifunctional compounds (MFCs) 1a-1d based on 1,8-naphthalimide moiety were designed and synthesized. Due to the good fluorescence property and nucleic acid binding ability of 1,8-naphthalimide, these MFCs were applied in Cu2+ ion recognition, lysosome staining as well as RNA delivery. It was found that these MFCs exhibited highly selective fluorescence turn-off for Cu2+ in aqueous solution. The fluorescence emission of 1a-1d was quenched by a factor of 116-, 20-, 12-, and 14-fold in the presence of Cu2+ ions, respectively. Most importantly, 1a-Cu and 1b-Cu could be used as imaging reagents for detection of lysosome in live human cervical cancer cells (HeLa) using fluorescence microscopy. Furthermore, in order to evaluate the RNA delivery ability of 1a-1d, cellular uptake experiments were performed in HeLa, HepG2, U2Os, and MC3T3-E1 cell lines. The results showed that all the materials could deliver Cy5-labled RNA into the targeted cells. Among them, compound 1d modified with long hydrophobic chain exhibited the best RNA delivery efficiency in the four tested cell lines, and the performance was far better than lipofectamine 2000 and 25 kDa PEI, indicating the potential application in non-viral vectors.

Mitochondria-Targeted New Blue Light-Emitting Fluorescent Molecular Probe

Discovery of a nontoxic fluorescent molecular probe to "light up" specific cellular organelles is extremely essential to understand dynamics of intracellular components. Here, we report a new nontoxic mitochondria-targeted linear bithiazole compound, containing trifluoroacetyl terminal groups, which emits intense blue fluorescence and stained mitochondria of various cells. Interestingly, the power of fluorescence is completely off when the bithiazole unit is stapled by a carbonyl bridge.