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.

Research progress of hydrogen sulfide fluorescent probes targeting organelles

Hydrogen sulfide (H2S) is implicated in numerous physiological and pathological processes in living organisms. Abnormal levels of H2S can result in various physiological disorders, highlighting the crucial need for effective identification and detection of H2S at the organellar level. Although numerous H2S fluorescent probes targeting organelles have been reported, a comprehensive review of these probes is required. This review focuses on the strategic selection of organelle-targeting groups and recognition sites for H2S fluorescent probes. This review examines H2S fluorescent probes that can specifically target lysosomes, mitochondria, endoplasmic reticulum, Golgi apparatus, and lipid droplets. These fluorescent probes have been meticulously classified and summarized based on their distinct targets, emphasizing their chemical structure, reaction mechanisms, and biological applications. We carefully designed fluorescent probes to efficiently enhance their ability to recognize target substances and exhibit significant fluorescence variations. Furthermore, we discuss the challenges inherent in the development of fluorescent probes and outline potential future directions for this exciting field.

Activity-Based Fluorescent Probes for Hydrogen Sulfide and Related Reactive Sulfur Species

Hydrogen sulfide (H2S) is not only a well-established toxic gas but also an important small molecule bioregulator in all kingdoms of life. In contemporary biology, H2S is often classified as a "gasotransmitter," meaning that it is an endogenously produced membrane permeable gas that carries out essential cellular processes. Fluorescent probes for H2S and related reactive sulfur species (RSS) detection provide an important cornerstone for investigating the multifaceted roles of these important small molecules in complex biological systems. A now common approach to develop such tools is to develop "activity-based probes" that couple a specific H2S-mediated chemical reaction to a fluorescent output. This Review covers the different types of such probes and also highlights the chemical mechanisms by which each probe type is activated by specific RSS. Common examples include reduction of oxidized nitrogen motifs, disulfide exchange, electrophilic reactions, metal precipitation, and metal coordination. In addition, we also outline complementary activity-based probes for imaging reductant-labile and sulfane sulfur species, including persulfides and polysulfides. For probes highlighted in this Review, we focus on small molecule systems with demonstrated compatibility in cellular systems or related applications. Building from breadth of reported activity-based strategies and application, we also highlight key unmet challenges and future opportunities for advancing activity-based probes for H2S and related RSS.

Mitochondria-targetable small molecule fluorescent probes for the detection of cancer-associated biomarkers: A review

Cancer represents a global threat to human health, and effective strategies for improved cancer early diagnosis and treatment are urgently needed. The detection of tumor biomarkers has been one of the important auxiliary means for tumor screening and diagnosis. Mitochondria are crucial subcellular organelles that produce most chemical energy used by cells, control metabolic processes, and maintain cell function. Evidence suggests the close involvement of mitochondria with cancer development. As a consequence, the identification of cancer-associated biomarker expression levels in mitochondria holds significant importance in the diagnosis of early-stage diseases and the monitoring of therapy efficacy. Small-molecule fluorescent probes are effective for the identification and visualization of bioactive entities within biological systems, owing to their heightened sensitivity, expeditious non-invasive analysis and real-time detection capacities. The design principles and sensing mechanisms of mitochondrial targeted fluorescent probes are summarized in this review. Additionally, the biomedical applications of these probes for detecting cancer-associated biomarkers are highlighted. The limitations and challenges of fluorescent probes in vivo are also considered and some future perspectives are provided. This review is expected to provide valuable insights for the future development of novel fluorescent probes for clinical imaging, thereby contributing to the advancement of cancer diagnosis and treatment.

Heptamethine Cyanine-Based Molecule Release Triggered by Mitochondrial ROS

Conditionally activated molecule release in live cells would provide spatiotemporal control for the study and intervention of biological processes, e.g., bioactive molecule monitoring and controlled drug release. Mitochondria are the main sites of reactive oxygen species (ROS) production in cells. Here, we report an ROS-triggered molecule release strategy in mitochondria. A molecule IRTO with dual targeting groups was designed by covalently linking IR-780 (a mitochondrial targeted heptamethine cyanine) and 4-aminobutyl-thiazole orange (NH2-TO, a nuclear dye). IRTO diffused into live cells and first accumulated in mitochondria. As the cyanine moiety reacted with mitochondrial ROS directly or with the help of mitochondrial cytochromes, NH2-TO was released, escaped from mitochondria, and finally located in the nucleus. This process could be visualized by fluorescent imaging, i.e., red fluorescence (from the cyanine moiety of IRTO) first located in mitochondria, and green fluorescence (from NH2-TO) appeared and gradually enhanced in the nucleus with the increase of incubation time. The addition of H2O2 or lipopolysaccharide (LPS, an ROS accelerator) could accelerate the release of NH2-TO, whereas N-acetyl-l-cysteine (NAC, an ROS inhibitor) and mitoquinone mesylate (MitoQ, a mitochondrial ROS scavenger) could obviously decrease the release of NH2-TO. These results suggest that IRTO could serve as a fluorescent probe for monitoring ROS in mitochondria and that IR-780 might be a promising endogenous ROS-triggered molecule release platform.

Nucleolus imaging based on naphthalimide derivatives

Nucleolus was an important cellular organelle. The abnormal morphology and number of the nucleolus have been considered as diagnostic biomarkers for some human diseases. However, the imaging agent based on nucleolus was limited. In this manuscript, a series of nucleolar fluorescent probes based on naphthalimide derivatives (NI-1 ∼ NI-5) had been designed and synthesized. NI-1 ∼ NI-5 could penetrate cell membranes and nuclear membranes, achieve clear nucleolar staining in living cells. These results suggested that the presence of amino groups on the side chains of naphthalimide backbone could enhance the targeting to the cell nucleolus. In addition, the molecular docking results showed that NI-1 ∼ NI-5 formed hydrogen bonds and hydrophobic interactions with RNA, and exhibited enhanced fluorescence upon binding with RNA. These results will provide favorable support for the diagnosis and treatment of nucleolus-related diseases in the future.

Surface Functionalised Optical Fibre for Detection of Hydrogen Sulphide

Dysregulated production of hydrogen sulphide in the human body has been associated with various diseases including cancer, underlining the importance of accurate detection of this molecule. Here, we report the detection of hydrogen sulphide using fluorescence-emission enhancement of two 1,8-naphthalimide fluorescent probes with an azide moiety in position 4. One probe, serving as a control, featured a methoxyethyl moiety through the imide to evaluate its effectiveness for hydrogen sulphide detection, while the other probe was modified with (3-aminopropyl)triethoxysilane (APTES) to enable direct covalent attachment to an optical fibre tip. We coated the optical fibre tip relatively homogeneously with the APTES-azide fluorophore, as confirmed via x-ray photoelectron spectroscopy (XPS). The absorption and fluorescence responses of the control fluorophore free in PBS were analysed using UV-Vis and fluorescence spectrophotometry, while the fluorescence emission of the APTES-azide fluorophore-coated optical fibres was examined using a simple, low-cost optical fibre-based setup. Both fluorescent probes exhibited a significant increase (more than double the initial value) in fluorescence emission upon the addition of HS- when excited with 405 nm. However, the fluorescence enhancement of the coated optical fibres demonstrated a much faster response time of 2 min (time for the fluorescence intensity to reach 90% of its maximum value) compared to the control fluorophore in solution (30 min). Additionally, the temporal evolution of fluorescence intensity of the fluorophore coated on the optical fibre was studied at two pH values (7.4 and 6.4), demonstrating a reasonable overlap and confirming the compound pH insensitivity within this range. The promising results from this study indicate the potential for developing an optical fibre-based sensing system for HS- detection using the synthesised fluorophore, which could have significant applications in health monitoring and disease detection.

Organelle-targeting ratiometric fluorescent probes: design principles, detection mechanisms, bio-applications, and challenges

Biological species, including reactive oxygen species (ROS), reactive sulfur species (RSS), reactive nitrogen species (RNS), F-, Pd2+, Cu2+, Hg2+, and others, are crucial for the healthy functioning of cells in living organisms. However, their aberrant concentration can result in various serious diseases. Therefore, it is essential to monitor biological species in cellular organelles such as the cell membrane, mitochondria, lysosome, endoplasmic reticulum, Golgi apparatus, and nucleus. Among various fluorescent probes for species detection within the organelles, ratiometric fluorescent probes have drawn special attention as a potential way to get beyond the drawbacks of intensity-based probes. This method depends on measuring the intensity change of two emission bands (caused by an analyte), which produces an efficient internal referencing that increases the detection's sensitivity. This review article discusses the literature publications (from 2015 to 2022) on organelle-targeting ratiometric fluorescent probes, the general strategies, the detecting mechanisms, the broad scope, and the challenges currently faced by fluorescent probes.

A mitochondria-targeted near-infrared fluorescent probe for detection and imaging of HSO3- in living cells

Sulfur dioxide, an essential gas signaling molecule mainly produced in mitochondria, plays important roles in many physiological and pathological processes. Herein, a near-infrared fluorescent probe, A1, with good mitochondria targeting ability was developed for colorimetric and fluorescence detection of HSO₃^-. Probe A1 has a conjugated cyanine structure that can selectively react with HSO₃^- through the nucleophilic addition. The reaction with HSO₃^- destroys the conjugated structure of probe A1, resulting in fluorescence quenching, and accompaniedby color change of probe A1 solution from purple-red to colorless. Probe A1 showed high selectivity and good sensitivity to HSO₃^- in PBS. And the limit of detection was calculated to be 1.28 and 0.037 μM for colorimetry and fluorescence spectrophotometry respectively. In addition, probe A1 mainly entered the mitochondria in living cells, and was successfully used for imaging the exogenous/endogenous HSO₃^- in cells. These results suggest the potential applications of probe A1 in biological systems.

Near-infrared fluorescent probes based on a quinoxaline skeleton for imaging nucleic acids in mitochondria

In this paper, two cationic probes 1a and 1b and a neutral dye 1c were successfully designed and synthesized according to the Knoevenagel condensation reaction, which combines the good optical properties of hemocyanine and the biocompatibility of nitrogen-containing heterocyclic rings based on a quinoxaline skeleton. Probes 1a and 1b showed an OFF-ON fluorescence response to nucleic acids with excellent selectivity. Specifically, the fluorescence intensity of probe 1a was enhanced by 18 and 133 times, respectively, along with the increase of DNA or RNA concentrations (0-600 μg mL^-1). Furthermore, a good linear correlation between the fluorescence intensity of probes 1a and 1b and the concentrations of DNA or RNA (0-350 μg mL^-1) was obtained. In particular, the maximum emission wavelengths of probes 1a and 1b reached the near-infrared region (660-664 nm) when DNA or RNA was detected, which might reduce the light damage to cells and facilitate cell experiments. Fluorescence imaging revealed that all three dyes could be localized in the mitochondria of HeLa cells. The difference was that probes 1a and 1b could stain the nucleic acid in the mitochondria, while dye 1c was only a neutral mitochondrial biomarker. The results indicated that probes 1a and 1b are promising in the development of low toxicity mitochondrial nucleic acid probes and are expected to be used in monitoring the normal state of mitochondrial nucleic acids for living cells, which will help improve the situation in that currently reported studies of fluorescent probes are mainly focused on the nucleic acids in the nucleus, but less so on DNA in the mitochondria.

Naphthalimide derivatives as fluorescent probes for imaging endogenous gasotransmitters

Gasotransmitters have gained significant recognition attributed to their evident biological impacts, and is accepted as a promising and less-explored area with immense research scope. The three-member family comprising of nitric oxide, carbon monoxide and hydrogen sulphide as endogenous gaseous signaling molecules have been found to elicit a plethora of crucial biological functions, spawning a new research area. The sensing of these small molecules is vital to gain deeper insights into their functions, as they can act both as a friend or a foe in mammalian systems. The initial sections of the review present the physiological and pathophysiological roles of these endogenous gas transmitters and their synergistic interactions. Further, various detection approaches, especially the usage of fascinating features of 1,8-naphthalimide as fluorescent probe in the detection and monitoring of these small signaling molecules are highlighted. The current limitations and the future scope of improving the sensing of the three gasotransmitters are also discussed.

Physiological roles of hydrogen sulfide in mammalian cells, tissues and organs

H2S belongs to the class of molecules known as gasotransmitters, which also includes nitric oxide (NO) and carbon monoxide (CO). Three enzymes are recognized as endogenous sources of H2S in various cells and tissues: cystathionine g-lyase (CSE), cystathionine β-synthase (CBS) and 3-mercaptopyruvate sulfurtransferase (3-MST). The current article reviews the regulation of these enzymes as well as the pathways of their enzymatic and non-enzymatic degradation and elimination. The multiple interactions of H2S with other labile endogenous molecules (e.g. NO) and reactive oxygen species are also outlined. The various biological targets and signaling pathways are discussed, with special reference to H2S and oxidative posttranscriptional modification of proteins, the effect of H2S on channels and intracellular second messenger pathways, the regulation of gene transcription and translation and the regulation of cellular bioenergetics and metabolism. The pharmacological and molecular tools currently available to study H2S physiology are also reviewed, including their utility and limitations. In subsequent sections, the role of H2S in the regulation of various physiological and cellular functions is reviewed. The physiological role of H2S in various cell types and organ systems are overviewed. Finally, the role of H2S in the regulation of various organ functions is discussed as well as the characteristic bell-shaped biphasic effects of H2S. In addition, key pathophysiological aspects, debated areas, and future research and translational areas are identified A wide array of significant roles of H2S in the physiological regulation of all organ functions emerges from this review.

A hydrostable CuII coordination network prepared hydrothermally as a "turn-on" fluorescent sensor for S2- and a selective adsorbent for methylene blue

The effective monitoring of water pollution and further purification are pressing yet challenging issues for guaranteeing the health of human beings and the stabilization of ecological systems. For this purpose, the development of efficient sensing and adsorption materials as a result of supramolecular interactions, including coordination and H-bonding etc., have been attracting increasing attention. With the aid of a coordination-driven self-assembly strategy, a new nonporous 2D CuII coordination network, [Cu2L(H2O)2]n (donated as CuCP), based on H4L, where H4L = 4-(4-(3,5-di-carboxy-pyridin-4-yl)phenyl)pyridine-2,6-dicarboxylic acid, was afforded hydrothermally. Structural analysis indicated that CuCP featured a wrinkled network similar to the ancient Chinese folding screens and constructed by the fully deprotonated ligand L4- with the coordination mode of bis(μ2-η1:η1:η2) and penta-coordinated Cu2+, which could be further upgraded to a supramolecular 3D framework as a result of the synergism of multiple C-H⋯O hydrogen bonds. The hydrostability of CuCP could be maintained within a wide pH range from 2 to 12 as verified by PXRD determination, endowing it with potential environmental applications. Thanks to the combination of the soft Lewis acidity of Cu2+ and its large conjugated structure, CuCP could be used as a turn-on fluorescence sensor for S2- and exhibited a different fluorescence response when Na2S, (NH4)2S or H2S were incorporated, even in actual water samples. The sensing mechanisms were disclosed in detail by the combination of experiments and density functional theory (DFT) calculations. Furthermore, CuCP was shown to be a selective and recoverable adsorbent with a maximum adsorption capacity of 379 mg g-1 in 60 minutes for methylene blue (MB). The adsorption mechanism could be a combination of π⋯π stacking, n⋯π interaction, aggregation effects and Soft and Hard Acid-Base theory (HSAB). The results presented herein open up new perspectives for CuII species in environmental applications.

p-Aminostyryl thiazole orange derivatives for monitoring mitochondrial viscosity in live cells

Viscosity of cell microenvironment plays a significant role in maintaining the normal life activities of cells. Particularly, the abnormal viscosity in mitochondria is closely associated with lots of diseases and cellular dysfunctions. Herein, we developed a group of p-aminostyryl thiazole orange derivatives with different amino side chains. These probes showed good fluorescence response to viscosity with twisted intramolecular charge transfer mechanism, among them, the probes with diethylamino (TOB), dibutylamino (TOC) and pyrrolidin (TOE) side chains showed better response to the viscosity with 78-fold, 55-fold, and 88-fold fluorescence enhancement in 95% glycerol solution respectively. TOB, TOC, and TOE could enter live cells and mainly located in mitochondria. Treatment HeLa cells with nystatin, lipopolysaccharide or oleic acid caused significant fluorescence enhancement of these probes, suggesting the good potential for monitoring the variation of mitochondrial viscosity, as well as for investigating the related physiological process of inflammation and lipid metabolism.

A naphthalimide-based peptide conjugate for concurrent imaging and apoptosis induction in cancer cells by utilizing endogenous hydrogen sulfide

The excessive production of endogenous hydrogen sulfide (H2S) in cancer cells leads to enhanced tumor growth and metastasis. On the other hand, decreased endogenous H2S suppresses tumor growth. The reported approaches for inhibiting tumor growth are selective silencing of the tumor-promoting genes and pharmacological inhibition of these proteins. To enhance the antitumor efficacy of frontline chemotherapeutic agents, herein, we synthesized a highly sensitive endogenous H2S responsive fluorescent probe, i.e., a hydrogen sulfide-sensing naphthalimide-based peptide conjugate (HSNPc), which showed selective inhibition of proliferation of cancer cells due to apoptosis induction. Furthermore, HSNPc suppressed the glycolytic reserve, a critical energy source for the proliferation of cancer cells. HSNPc also decreased the Young's modulus of HeLa cells compared to the control cells, which demonstrated a direct relation between cell apoptosis and cell stiffness. Taken together, we demonstrated the dual function of detection and killing of cancer cells by HSNPc that can be likened to a theranostic role.

A phenothiazine-based turn-on fluorescent probe for the selective detection of hydrogen sulfide in food, live cells and animals

In this work, a phenothiazine-based fluorescent probe (PR) has been developed for the selective detection of hydrogen sulfide (H₂S) in biosystems and monitoring H₂S produced in the food spoilage process. The nucleophilic attack of H₂S on the CC double bond of PRvia a Michael addition interdicted the ICT process to trigger 34-fold enhancement of the fluorescence emission. PR featured high selectivity and sensitivity (1.8 μM), low cytotoxicity and reliability at physiological pH. "Naked-eye" monitoring of H₂S produced in the food spoilage process using PR was successfully accomplished. The preliminary fluorescence imaging studies showed that PR is suitable for the visualization of exogenous and endogenous H₂S in living cells and live animals. Moreover, PR has been successfully applied to the visualization of H₂S generation in an inflammation model. The results indicated that PR is an effective tool to monitor H₂S production in the fields of biomedicine and food safety.

Progress on the reaction-based methods for detection of endogenous hydrogen sulfide

Hydrogen sulfide (H₂S) is a biologically signaling molecule that mediates a wide range of physiological functions, which is frequently misregulated in numerous pathological processes. As such, measurement of H₂S holds great attention due to its unique physiological and pathophysiological roles. Currently, a variety of methods based on the H₂S-involved reactions have been reported for detection of endogenous H₂S, bearing the advantages of good specificity and high sensitivity. This review describes in detail the types of reactions, their mechanisms, and their applications in biological research, thus hopefully providing some guidelines to the researchers in this field for further investigation.

A 4-aminonaphthalimide-based fluorescent traceable prodrug with excellent photoinduced cytotoxicity

A blue light activated anti-cancer prodrug, NST, was designed based on a photoactive 4-aminonaphthalimide derivative and an anticancer drug, 10-hydroxycamptothecin. NST was hard to be taken up by living cells and showed negligible dark cytotoxicity. The irradiation caused photocleavage of NST and resulted in high cytotoxicity.

A mitochondria-targeted fluorescent dye naphthalimide-thioether-cyanine for NIR-activated photodynamic treatment of cancer cells

In this work, an NIR-activated fluorescent dye naphthalimide-thioether-cyanine (NPSCY) was developed for the photodynamic treatment of cancer cells. In this dye, naphthalimide and cyanine were selected as the two fluorophores, which were linked by the thioether group. Under 660 nm irradiation, NPSCY could produce 1O2 rapidly, suggesting the potential for photodynamic therapy. Cys can be considered as one of the markers of cancer cells and NPSCY could distinguish Cys from three channels (433 nm, 475 nm, 733 nm) due to the bilateral recognition of the thioether group, which was helpful for accurately locating cancer cells. Fortunately, NPSCY could also produce 1O2 after being reacted with the intracellular biological thiols, which also avoided the inactivation of the photosensitizer in cancer cells. The co-localization coefficient of 0.873 indicated that the cyanine group promoted the aggregation of NPSCY in mitochondria. This photosensitizer showed low dark toxicity and high phototoxicity. Meanwhile, the half-maximal inhibitory concentration (IC50) was calculated to be 3.7 μM. NPSCY could inhibit cell migration after irradiation at 660 nm.