Challenges and Opportunities for Small-Molecule Fluorescent Probes in Redox Biology Applications

Enhanced optical imaging and fluorescent labeling for visualizing drug molecules within living organisms

The visualization of drugs in living systems has become key techniques in modern therapeutics. Recent advancements in optical imaging technologies and molecular design strategies have revolutionized drug visualization. At the subcellular level, super-resolution microscopy has allowed exploration of the molecular landscape within individual cells and the cellular response to drugs. Moving beyond subcellular imaging, researchers have integrated multiple modes, like optical near-infrared II imaging, to study the complex spatiotemporal interactions between drugs and their surroundings. By combining these visualization approaches, researchers gain supplementary information on physiological parameters, metabolic activity, and tissue composition, leading to a comprehensive understanding of drug behavior. This review focuses on cutting-edge technologies in drug visualization, particularly fluorescence imaging, and the main types of fluorescent molecules used. Additionally, we discuss current challenges and prospects in targeted drug research, emphasizing the importance of multidisciplinary cooperation in advancing drug visualization. With the integration of advanced imaging technology and molecular design, drug visualization has the potential to redefine our understanding of pharmacology, enabling the analysis of drug micro-dynamics in subcellular environments from new perspectives and deepening pharmacological research to the levels of the cell and organelles.

Accelerated Activity-Based Sensing by Fluorogenic Reporter Engineering Enables to Rapidly Determine Unstable Analyte

Accurate detection of labile analytes through activity based fluorogenic sensing is meaningful but remains a challenge because of nonrapid reaction kinetic. Herein, we present a signaling reporter engineering strategy to accelerate azoreduction reaction by positively charged fluorophore promoted unstable anion recognition for rapidly sensing sodium dithionite (Na2S2O4), a kind of widespread used but harmful inorganic reducing agent. Its quick decomposition often impedes application reliability of traditional fluorogenic probes in real samples because of their slow responses. In this work, four azo-based probes with different charged fluorophores (positive, zwitterionic, neutral, and negative) were synthesized and compared. Among of them, with sequestration effect of positively charged anthocyanin fluorophore for dithionite anion via electrostatic attraction, the cationic probe Azo-Pos displayed ultrafast fluorogenic response (∼2 s) with the fastest response kinetic (kpos' = 0.373 s-1) that is better than other charged ones (kzwi' = 0.031 s-1, kneu' = 0.013 s-1, kneg' = 0.003 s-1). Azo-Pos was demonstrated to be capable to directly detect labile Na2S2O4 in food samples and visualize the presence of Na2S2O4 in living systems in a timely fashion. This new probe has potential as a robust tool to fluorescently monitor excessive food additives and biological invasion of harmful Na2S2O4. Moreover, our proposed accelerating strategy would be versatile to develop more activity-based sensing probes for quickly detecting other unstable analytes of interest.

FRBM Mini REVIEW: Chemogenetic approaches to probe redox dysregulation in heart failure

Chemogenetics refers to experimental methods that use novel recombinant proteins that can be dynamically and uniquely regulated by specific biochemicals. Chemogenetic approaches allow the precise manipulation of cellular signaling to delineate the molecular pathways involved in both physiological and pathological disease states. Approaches utilizing yeast d-amino acid oxidase (DAAO) enable manipulation of intracellular redox metabolism through generation of hydrogen peroxide in the presence of d-amino acids and have led to the development of new and informative animal models to characterize the impact of oxidative stress in heart failure and neurodegeneration. These chemogenetic models, in which DAAO expression is regulated by different tissue-specific promoters, have led to a range of cardiac phenotypes. This review discusses chemogenetic approaches to manipulate oxidative stress in models of heart failure. These approaches provide new insights into the relationships between redox metabolism and normal and pathologic states in the heart, as well as in other diseases characterized by oxidative stress.

Exploring the fluorescence properties of tellurium-containing molecules and their advanced applications

This review article explores the fascinating realm of fluorescence using organochalcogen molecules, with a particular emphasis on tellurium (Te). The discussion encompasses the underlying mechanisms, structural motifs influencing fluorescence, and the applications of these intriguing phenomena. This review not only elucidates the current state of knowledge but also identifies avenues for future research, thereby serving as a valuable resource for researchers and enthusiasts in the field of fluorescence chemistry with a focus on Te-based molecules. By highlighting challenges and prospects, this review sparks a conversation on the transformative potential of Te-containing compounds across different fields, ranging from environmental solutions to healthcare and materials science applications. This review aims to provide a comprehensive understanding of the distinct fluorescence behaviors exhibited by Te-containing compounds, contributing valuable insights to the evolving landscape of chalcogen-based fluorescence research.

Chronological development of functional fluorophores for bio-imaging

Functional fluorophores represent an emerging research field, distinguished by their diverse applications, especially in sensing and cellular imaging. After the discovery of quinine sulfate and subsequent elucidation of the fluorescence mechanism by Sir George Stokes, research in the field of fluorescence gained momentum. Over the past few decades, advancements in sophisticated instruments, including super-resolution microscopy, have further promoted cellular imaging using traditional fluorophores. These advancements include deciphering sensing mechanisms via photochemical reactions and scrutinizing the applications of fluorescent probes that specifically target organelles. This approach elucidates molecular interactions with biomolecules. Despite the abundance of literature illustrating different classes of probe development, a concise summary of newly developed fluorophores remains inadequate. In this review, we systematically summarize the chronological discovery of traditional fluorophores along with new fluorophores. We briefly discuss traditional fluorophores ranging from visible to near-infrared (NIR) in the context of cellular imaging and in vivo imaging. Furthermore, we explore ten new core fluorophores developed between 2007 and 2022, which exhibit advanced optical properties, providing new insights into bioimaging. We illustrate the utilization of new fluorophores in cellular imaging of biomolecules, such as reactive oxygen species (ROS), reactive nitrogen species (RNS), and proteins and microenvironments, especially pH and viscosity. Few of the fluorescent probes provided new insights into disease progression. Furthermore, we speculate on the potential prospects and significant challenges of existing fluorophores and their potential biomedical research applications. By addressing these aspects, we intend to illuminate the compelling advancements in fluorescent probe development and their potential influence across various fields.

Quantifying mitochondrial redox and bilirubin content in intact primary hepatocytes of obese mice using fluorescent reporters

Assessing the physiological role of H2O2 requires sensitive techniques to quantify H2O2 and antioxidants in live cells. Here, we present a protocol to assess the mitochondrial redox state and unconjugated bilirubin levels in intact live primary hepatocytes from obese mice. We described steps to quantify H2O2, GSSG/GSH, and bilirubin content in the mitochondrial matrix and the cytosol using the fluorescent reporters roGFP2-ORP1, GRX1-roGFP2, and UnaG, respectively. We detail hepatocyte isolation, plating, and transduction and live-cell imaging using a high-content imaging reader. For complete details on the use and execution of this protocol, please refer to Shum et al.1.

Quantification of glutathione with high throughput live-cell imaging

Reduction oxidation (redox) reactions are central in life and altered redox state is associated with a spectrum of human diseases. Glutathione (GSH) is the most abundant antioxidant in eukaryotic cells and plays critical roles in maintaining redox homeostasis. Thus, measuring intracellular GSH level is an important method to assess the redox state of organism. The currently available GSH probes are based on irreversible chemical reactions with glutathione and can't monitor the real-time glutathione dynamics. Our group developed the first reversible reaction based fluorescent probe for glutathione, which can measure glutathione levels at high resolution using a confocal microscope and in the bulk scale with a flow cytometry. Most importantly it can quantitatively monitor the real-time GSH dynamics in living cells. Using the 2 ^nd generation of GSH probe, RealThiol (RT), this study measured the GSH level in living Hela cells after treatment with varying concentrations of DL-Buthionine sulfoximine (BSO) which inhibits GSH synthesis, using a high throughput imaging system, Cytation™ 5 cell imaging reader. The results revealed that GSH probe RT at the concentration of 2.0 µM accurately monitored the BSO treatment effect on GSH level in the Hela cells. The present results demonstrated that the GSH probe RT is sensitive and precise in GSH measurement in living cells at a high throughput imaging platform and has the potential to be applied to any cell lines.

EvIPqPCR, Target Circulating Tumorous Extracellular Vesicles for Detection of Pancreatic Cancer

Pancreatic cancer patients predominantly present with advanced disease at diagnosis, contributing to its high mortality. A noninvasive, fast screening method to detect this disease is an unmet need. Tumor-derived extracellular vesicles (tdEVs) bearing information from parental cells have emerged as a promising cancer diagnostic biomarker. However, most tdEV-based assays have impractical sample volumes and time-consuming, complex, and costly techniques. To overcome these limitations, we developed a novel diagnostic method for pancreatic cancer screening. Our approach utilizes the mitochondrial DNA to nuclear DNA ratio of EVs as a collective cell-specific characteristic. We introduce EvIPqPCR, a fast method that combines immunoprecipitation (IP) and qPCR quantification to detect tumor-derived EVs directly from serum. Importantly, our method employs DNA isolation-free and duplexing probes for qPCR, saving at least 3 h. This technique has the potential to serve as a translational assay for cancer screening with a weak correlation to prognosis biomarkers and sufficient discriminatory power among healthy controls, pancreatitis, and pancreatic cancer cases.

A Novel Strategy Based on Zn(II) Porphyrins and Silver Nanoparticles to Photoinactivate Candida albicans

Background

Photodynamic inactivation (PDI) is an attractive alternative to treat Candida albicans infections, especially considering the spread of resistant strains. The combination of the photophysical advantages of Zn(II) porphyrins (ZnPs) and the plasmonic effect of silver nanoparticles (AgNPs) has the potential to further improve PDI. Here, we propose the novel association of polyvinylpyrrolidone (PVP) coated AgNPs with the cationic ZnPs Zn(II) meso-tetrakis(N-ethylpyridinium-2-yl)porphyrin or Zn(II) meso-tetrakis(N-n-hexylpyridinium-2-yl)porphyrin to photoinactivate C. albicans.

Methods

AgNPs stabilized with PVP were chosen to allow for (i) overlap between the NP extinction and absorption spectra of ZnPs and (ii) favor AgNPs-ZnPs interaction; prerequisites for exploring the plasmonic effect. Optical and zeta potential (ζ) characterizations were performed, and reactive oxygen species (ROS) generation was also evaluated. Yeasts were incubated with individual ZnPs or their respective AgNPs-ZnPs systems, at various ZnP concentrations and two proportions of AgNPs, then irradiated with a blue LED. Interactions between yeasts and the systems (ZnP alone or AgNPs-ZnPs) were evaluated by fluorescence microscopy.

Results

Subtle spectroscopic changes were observed for ZnPs after association with AgNPs, and the ζ analyses confirmed AgNPs-ZnPs interaction. PDI using ZnP-hexyl (0.8 µM) and ZnP-ethyl (5.0 µM) promoted a 3 and 2 log₁₀ reduction of yeasts, respectively. On the other hand, AgNPs-ZnP-hexyl (0.2 µM) and AgNPs-ZnP-ethyl (0.6 µM) systems led to complete fungal eradication under the same PDI parameters and lower porphyrin concentrations. Increased ROS levels and enhanced interaction of yeasts with AgNPs-ZnPs were observed, when compared with ZnPs alone.

Conclusion

We applied a facile synthesis of AgNPs which boosted ZnP efficiency. We hypothesize that the plasmonic effect combined with the greater interaction between cells and AgNPs-ZnPs systems resulted in an efficient and improved fungal inactivation. This study provides insight into the application of AgNPs in PDI and helps diversify our antifungal arsenal, encouraging further developments toward inactivation of resistant Candida spp.

Recent progress in the development of fluorescent probes for imaging pathological oxidative stress

Oxidative stress is closely related to the physiopathology of numerous diseases. Reactive oxygen species (ROS), reactive nitrogen species (RNS), and reactive sulfur species (RSS) are direct participants and important biomarkers of oxidative stress. A comprehensive understanding of their changes can help us evaluate disease pathogenesis and progression and facilitate early diagnosis and drug development. In recent years, fluorescent probes have been developed for real-time monitoring of ROS, RNS and RSS levels in vitro and in vivo. In this review, conventional design strategies of fluorescent probes for ROS, RNS, and RSS detection are discussed from three aspects: fluorophores, linkers, and recognition groups. We introduce representative fluorescent probes for ROS, RNS, and RSS detection in cells, physiological/pathological processes (e.g., Inflammation, Drug Induced Organ Injury and Ischemia/Reperfusion Injury etc.), and specific diseases (e.g., neurodegenerative diseases, epilepsy, depression, diabetes and cancer, etc.). We then highlight the achievements, current challenges, and prospects for fluorescent probes in the pathophysiology of oxidative stress-related diseases.

Enlightening the Path to Protein Engineering: Chemoselective Turn-On Probes for High-Throughput Screening of Enzymatic Activity

Many successful stories in enzyme engineering are based on the creation of randomized diversity in large mutant libraries, containing millions to billions of enzyme variants. Methods that enabled their evaluation with high throughput are dominated by spectroscopic techniques due to their high speed and sensitivity. A large proportion of studies relies on fluorogenic substrates that mimic the chemical properties of the target or coupled enzymatic assays with an optical read-out that assesses the desired catalytic efficiency indirectly. The most reliable hits, however, are achieved by screening for conversions of the starting material to the desired product. For this purpose, functional group assays offer a general approach to achieve a fast, optical read-out. They use the chemoselectivity, differences in electronic and steric properties of various functional groups, to reduce the number of false-positive results and the analytical noise stemming from enzymatic background activities. This review summarizes the developments and use of functional group probes for chemoselective derivatizations, with a clear focus on screening for enzymatic activity in protein engineering.

Defining the nuanced nature of redox biology in post-traumatic stress disorder

Post-traumatic stress disorder (PTSD) is a mental health disorder that arises after experiencing or witnessing a traumatic event. Despite affecting around 7% of the population, there are currently no definitive biological signatures or biomarkers used in the diagnosis of PTSD. Thus, the search for clinically relevant and reproducible biomarkers has been a major focus of the field. With significant advances of large-scale multi-omic studies that include genomic, proteomic, and metabolomic data, promising findings have been made, but the field still has fallen short. Amongst the possible biomarkers examined, one area is often overlooked, understudied, or inappropriately investigated: the field of redox biology. Redox molecules are free radical and/or reactive species that are generated as a consequence of the necessity of electron movement for life. These reactive molecules, too, are essential for life, but in excess are denoted as "oxidative stress" and often associated with many diseases. The few studies that have examined redox biology parameters have often utilized outdated and nonspecific methods, as well as have reported confounding results, which has made it difficult to conclude the role for redox in PTSD. Herein, we provide a foundation of how redox biology may underlie diseases like PTSD, critically examine redox studies of PTSD, and provide future directions the field can implement to enhance standardization, reproducibility, and accuracy of redox assessments for the use of diagnosis, prognosis, and therapy of this debilitating mental health disorder.

Fluorescent Organic Small Molecule Probes for Bioimaging and Detection Applications

The activity levels of key substances (metal ions, reactive oxygen species, reactive nitrogen, biological small molecules, etc.) in organisms are closely related to intracellular redox reactions, disease occurrence and treatment, as well as drug absorption and distribution. Fluorescence imaging technology provides a visual tool for medicine, showing great potential in the fields of molecular biology, cellular immunology and oncology. In recent years, organic fluorescent probes have attracted much attention in the bioanalytical field. Among various organic fluorescent probes, fluorescent organic small molecule probes (FOSMPs) have become a research hotspot due to their excellent physicochemical properties, such as good photostability, high spatial and temporal resolution, as well as excellent biocompatibility. FOSMPs have proved to be suitable for in vivo bioimaging and detection. On the basis of the introduction of several primary fluorescence mechanisms, the latest progress of FOSMPs in the applications of bioimaging and detection is comprehensively reviewed. Following this, the preparation and application of fluorescent organic nanoparticles (FONPs) that are designed with FOSMPs as fluorophores are overviewed. Additionally, the prospects of FOSMPs in bioimaging and detection are discussed.

The potential of gas plasma technology for targeting breast cancer

Despite therapeutic improvements in recent years, breast cancer remains an often fatal disease. In addition, breast cancer ulceration may occur during late stages, further complicating therapeutic or palliative interventions. In the past decade, a novel technology received significant attention in the medical field: gas plasma. This topical treatment relies on the partial ionization of gases that simultaneously produce a plethora of reactive oxygen and nitrogen species (ROS/RNS). Such local ROS/RNS overload inactivates tumour cells in a non-necrotic manner and was recently identified to induce immunogenic cancer cell death (ICD). ICD promotes dendritic cell maturation and amplifies antitumour immunity capable of targeting breast cancer metastases. Gas plasma technology was also shown to provide additive toxicity in combination with radio and chemotherapy and re-sensitized drug-resistant breast cancer cells. This work outlines the assets of gas plasma technology as a novel tool for targeting breast cancer by summarizing the action of plasma devices, the roles of ROS, signalling pathways, modes of cell death, combination therapies and immunological consequences of gas plasma exposure in breast cancer cells in vitro, in vivo, and in patient-derived microtissues ex vivo.

Determination and Imaging of Small Biomolecules and Ions Using Ruthenium(II) Complex-Based Chemosensors

Luminescence chemosensors are one of the most useful tools for the determination and imaging of small biomolecules and ions in situ in real time. Based on the unique photo-physical/-chemical properties of ruthenium(II) (Ru(II)) complexes, the development of Ru(II) complex-based chemosensors has attracted increasing attention in recent years, and thus many Ru(II) complexes have been designed and synthesized for the detection of ions and small biomolecules in biological and environmental samples. In this work, we summarize the research advances in the development of Ru(II) complex-based chemosensors for the determination of ions and small biomolecules, including anions, metal ions, reactive biomolecules and amino acids, with a particular focus on binding/reaction-based chemosensors for the investigation of intracellular analytes’ evolution through luminescence analysis and imaging. The advances, challenges and future research directions in the development of Ru(II) complex-based chemosensors are also discussed.

Oxidative stress monitoring in iPSC-derived motor neurons using genetically encoded biosensors of H2O2

Oxidative stress plays an important role in the development of neurodegenerative diseases, being either the initiator or part of a pathological cascade that leads to the neuron’s death. Genetically encoded biosensors of oxidative stress demonstrated their general functionality and overall safety in various systems. However, there is still insufficient data regarding their use in the research of disease-related phenotypes in relevant model systems, such as human cells. Here, we establish an approach for monitoring the redox state of live motor neurons with SOD1 mutations associated with amyotrophic lateral sclerosis. Using CRISPR/Cas9, we insert genetically encoded biosensors of cytoplasmic and mitochondrial H₂O₂ in the genome of induced pluripotent stem cell (iPSC) lines. We demonstrate that the biosensors remain functional in motor neurons derived from these iPSCs and reflect the differences in the stationary redox state of the neurons with different genotypes. Moreover, we show that the biosensors respond to alterations in motor neuron oxidation caused by either environmental changes or cellular stress. Thus, the obtained platform is suitable for cell-based research of neurodegenerative mechanisms.

Ratiometric Imaging of Mitochondrial Hydrogen Peroxide in Aβ42-Mediated Neurotoxicity

Oxidative stress is important for the etiology and pathogenesis of Alzheimer's disease (AD). Research tools that can conveniently evaluate oxidative stress in AD models are expected to catalyze and accelerate research on AD. This study explored the use of genetically encoded fluorescent indicators (GEFIs) to detect mitochondrial oxidative stress in organotypic brain slices and AD mouse models. To enable ratiometric normalization and avoid tissue autofluorescence, we genetically fused a green fluorescent hydrogen peroxide (H₂O₂) indicator, HyPer7, with each of two selected, bright red fluorescent proteins (RFPs), mScarlet-I and tdTomato. The resultant indicators, namely, HyPerGR_S and HyPerGR_T, were tagged with mitochondrial targeting sequences and examined for localization and function in cultured HeLa cells and primary mouse neurons. We further utilized HyPerGR_T, which is a genetic fusion of HyPer7 with tdTomato, to monitor mitochondrial H₂O₂ in response to the human β-amyloid 1-42 isoform (Aβ₄₂) in cultured brain slices and an AD mouse model. Owing to the high sensitivity and low autofluorescence interference resulting from HyPerGR_T, we successfully detected Aβ₄₂-mediated mitochondrial H₂O₂ in these AD models. The results suggest that HyPerGR_T is a valuable tool for studying mitochondrial oxidative stress in tissues and animals.

Evaluate the bisphenol A-induced redox state in cells, zebrafish and in vivo with a hydrogen peroxide turn-on fluorescent probe

Hydrogen peroxide (H₂O₂) is an important active oxygen species that plays a major role in redox balance and in physiological and pathological processes of various diseases of biological systems. As H₂O₂ is an endogenous active molecule, fluctuations in H₂O₂ content are not only affected by the state of biological system itself but also easily affected by Bisphenol A (BPA, a typical estrogenic environmental pollutant) in the external environment. Here, the near-infrared fluorescent probe Cy-NOH2 (λ_em = 750 nm) as a tool was synthesized to detect fluctuations in H₂O₂ content in cells and organisms induced by BPA. High sensitivity and excellent selectivity were found when the probe Cy-NOH2 was used to monitor endogenous H₂O₂ in vitro. In addition, the expression of H₂O₂ induced by different concentrations of BPA was able to be detected by the probe. Zebrafish and mice models were induced with different concentrations of BPA, and the H₂O₂ content showed significant increasing trends in zebrafish and livers of mice with increasing BPA concentrations. This study reveals that the probe Cy-NOH2 can be used as an effective tool to monitor the redox state in vivo under the influence of BPA, which provides a basis for clarifying the mechanisms of BPA in a variety of physiological and pathological processes.

Chemogenetic Approaches to Probe Redox Pathways: Implications for Cardiovascular Pharmacology and Toxicology

Chemogenetics refers to experimental systems that dynamically regulate the activity of a recombinant protein by providing or withholding the protein's specific biochemical stimulus. Chemogenetic tools permit precise dynamic control of specific signaling molecules to delineate the roles of those molecules in physiology and disease. Yeast d-amino acid oxidase (DAAO) enables chemogenetic manipulation of intracellular redox balance by generating hydrogen peroxide only in the presence of d-amino acids. Advances in biosensors have allowed the precise quantitation of these signaling molecules. The combination of chemogenetic approaches with biosensor methodologies has opened up new lines of investigation, allowing the analysis of intracellular redox pathways that modulate physiological and pathological cell responses. We anticipate that newly developed transgenic chemogenetic models will permit dynamic modulation of cellularredox balance in diverse cells and tissues and will facilitate the identification and validation of novel therapeutic targets involved in both physiological redox pathways and pathological oxidative stress.

Michael addition–elimination–cyclization based turn-on fluorescence (MADELCY TOF) probes for cellular cysteine imaging and estimation of blood serum cysteine and aminoacylase-1

Michael addition–elimination–cyclization based turn-on fluorescence (MADELCY TOF) probes for the highly sensitive estimation of Cys and aminoacylase-1 (ACY-1).

Recent research progress in galactose-based fluorescent probes for detection of biomarkers of liver diseases

This highlight illustrates the challenges and latest progress in galactose-based fluorescent probes for early diagnosis of liver diseases.

Dissecting in vivo and in vitro redox responses using chemogenetics

Hydrogen peroxide (H2O2) is the most abundant reactive oxygen species (ROS) within mammalian cells. At low concentrations, H2O2 serves as a versatile cell signaling molecule that mediates vital physiological functions. Yet at higher concentrations, H2O2 can be a toxic molecule by promoting pathological oxidative stress in cells and tissues. Within normal cells, H2O2 is differentially distributed in a variety of subcellular locales. Moreover, many redox-active enzymes and their substrates are themselves differentially distributed within cells. Numerous reports have described the biological and biochemical consequences of adding exogenous H2O2 to cultured cells and tissues, but many of these observations are difficult to interpret: the effects of exogenous H2O2 do not necessarily replicate the cellular responses to endogenous H2O2. In recent years, chemogenetic approaches have been developed to dynamically regulate the abundance of H2O2 in specific subcellular locales. Chemogenetic approaches have been applied in multiple experimental systems, ranging from in vitro studies on the intracellular transport and metabolism of H2O2, all the way to in vivo studies that generate oxidative stress in specific organs in living animals. These chemogenetic approaches have exploited a yeast-derived d-amino acid oxidase (DAAO) that synthesizes H2O2 only in the presence of its d-amino acid substrate. DAAO can be targeted to various subcellular locales, and can be dynamically activated by the addition or withdrawal of its d-amino acid substrate. In addition, recent advances in the development of highly sensitive genetically encoded H2O2 biosensors are providing a better understanding of both physiological and pathological oxidative pathways. This review highlights several applications of DAAO as a chemogenetic tool across a wide range of biological systems, from analyses of subcellular H2O2 metabolism in cells to the development of new disease models caused by oxidative stress in vivo.

Translational Application of Fluorescent Molecular Probes for the Detection of Reactive Oxygen and Nitrogen Species Associated with Intestinal Reperfusion Injury

Acute mesenteric ischemia, caused by an abrupt interruption of blood flow in the mesenteric vessels, is associated with high mortality. When treated with surgical interventions or drugs to re-open the vascular lumen, the reperfusion process itself can inflict damage to the intestinal wall. Ischemia and reperfusion injury comprise complex mechanisms involving disarrangement of the splanchnic microcirculatory flow and impairment of the mitochondrial respiratory chain due to initial hypoxemia and subsequent oxidative stress during the reperfusion phase. This pathophysiologic process results in the production of large amounts of reactive oxygen (ROS) and nitrogen (RNS) species, which damage deoxyribonucleic acid, protein, lipids, and carbohydrates by autophagy, mitoptosis, necrosis, necroptosis, and apoptosis. Fluorescence-based systems using molecular probes have emerged as highly effective tools to monitor the concentrations and locations of these often short-lived ROS and RNS. The timely and accurate detection of both ROS and RNS by such an approach would help to identify early injury events associated with ischemia and reperfusion and increase overall clinical diagnostic sensitivity. This abstract describes the pathophysiology of intestinal ischemia and reperfusion and the early biological laboratory diagnosis using fluorescent molecular probes anticipating clinical decisions in the face of an extremely morbid disease.

A phase-transfer catalyst-based nanoreactor for accelerated hydrogen sulfide bio-imaging

Hydrogen sulfide (H₂S) is an important signaling molecule in various biological processes; however, its real-time monitoring in living cells is hampered by long detection time for current fluorescent probes. To overcome this challenge, we designed a phase-transfer catalyst (PTC) approach to accelerate the reaction between the probe and the analyte by conjugating common fluorescent probes - mostly hydrophobic small molecules - with an amphiphilic PEG-PPG-PEG polymer, enabling the controllable assembly of H₂S nanoprobes in an aqueous solution. The PEG block helps to establish a PTC microenvironment that endows the assembled nanoprobes with a significantly reduced detection time (3-10 min; versus 20-60 min for small-molecule probes). Based on this approach, we synthesised two nanoprobes of different wavelengths, DS-Blue-nano and DN-Green-nano, which can sensitively detect H₂S in living macrophage cells with bright fluorescence starting at as early as 7 min and reaching stability at 15 min. These data suggest PTC-based nanoprobes as a new and generic approach for constructing sensitive fluorescent probes for the real-time imaging of H₂S, and perhaps other molecules in future, under biological conditions.

Lessons in Organic Fluorescent Probe Discovery

Fluorescent probes have gained profound use in biotechnology, drug discovery, medical diagnostics, molecular and cell biology. The development of methods for the translation of fluorophores into fluorescent probes continues to be a robust field for medicinal chemists and chemical biologists, alike. Access to new experimental designs has enabled molecular diversification and led to the identification of new approaches to probe discovery. This review provides a synopsis of the recent lessons in modern fluorescent probe discovery.

1,2,4-Triaminobenzene as a Fluorescent Probe for Intracellular pH Imaging and Point-of-Care Ammonia Sensing

As one of the health indicators, intracellular pH plays important roles in many processes of cell functions. Abnormal pH changes would result in the occurrence of inflammation, cancer, and other diseases. Thus, it is of significant importance to develop effective techniques for sensitive detection of pH changes for the clinical diagnosis of various diseases related to cells. In this paper, 1,2,4-triaminobenzene hydrochloride was explored as an organic molecular fluorescent probe for sensitive and selective detection of intracellular pH changes for the first time. Due to the protonation and deprotonation of amino groups of the probe, its fluorescent intensity at 599 nm or the ratio of absorbance at 505 and 442 nm has a good linear relationship with pH values in the range of 5.0-7.0. Benefiting from the excellent physical and chemical properties of 1,2,4-triaminobenzene hydrochloride, the fluorescent probe has good water solubility, low toxicity, high photostability, great reversibility, good cell penetration, fast response speed, and so on. As a proof-of-concept demonstration, the proposed probe is employed for the fluorescence imaging of cells and mouse tissue sections with satisfactory performance in pH differentiation. Additionally, the probe was successfully employed to prepare test strips as a kind of point-of-care testing device to detect ammonia, which showed great potential in practical applications.

An update on methods and approaches for interrogating mitochondrial reactive oxygen species production

The chief ROS formed by mitochondria are superoxide (O2·−) and hydrogen peroxide (H₂O₂). Superoxide is converted rapidly to H₂O₂ and therefore the latter is the chief ROS emitted by mitochondria into the cell. Once considered an unavoidable by-product of aerobic respiration, H₂O₂ is now regarded as a central mitokine used in mitochondrial redox signaling. However, it has been postulated that O2·− can also serve as a signal in mammalian cells. Progress in understanding the role of mitochondrial H₂O₂ in signaling is due to significant advances in the development of methods and technologies for its detection. Unfortunately, the development of techniques to selectively measure basal O2·− changes has been met with more significant hurdles due to its short half-life and the lack of specific probes. The development of sensitive techniques for the selective and real time measure of O2·− and H₂O₂ has come on two fronts: development of genetically encoded fluorescent proteins and small molecule reporters. In 2015, I published a detailed comprehensive review on the state of knowledge for mitochondrial ROS production and how it is controlled, which included an in-depth discussion of the up-to-date methods utilized for the detection of both superoxide (O2·−) and H₂O₂. In the article, I presented the challenges associated with utilizing these probes and their significance in advancing our collective understanding of ROS signaling. Since then, many other authors in the field of Redox Biology have published articles on the challenges and developments detecting O2·− and H₂O₂ in various organisms [[1], [2], [3]]. There has been significant advances in this state of knowledge, including the development of novel genetically encoded fluorescent H₂O₂ probes, several O2·− sensors, and the establishment of a toolkit of inhibitors and substrates for the interrogation of mitochondrial H₂O₂ production and the antioxidant defenses utilized to maintain the cellular H₂O₂ steady-state. Here, I provide an update on these methods and their implementation in furthering our understanding of how mitochondria serve as cell ROS stabilizing devices for H₂O₂ signaling.

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Details on the toolkit for interrogating the 12 sites for mitochondrial ROS production.

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Approaches to assess mitochondrial ROS clearance.

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Novel genetically encoded H₂O₂ sensors.

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Small chemical probes for sensitive detection of O2·−.

Recent progress in developing fluorescent probes for imaging cell metabolites

Cellular metabolites play a crucial role in promoting and regulating cellular activities, but it has been difficult to monitor these cellular metabolites in living cells and in real time. Over the past decades, iterative development and improvements of fluorescent probes have been made, resulting in the effective monitoring of metabolites. In this review, we highlight recent progress in the use of fluorescent probes for tracking some key metabolites, such as adenosine triphosphate, cyclic adenosine monophosphate, cyclic guanosine 5'-monophosphate, Nicotinamide adenine dinucleotide (NADH), reactive oxygen species, sugar, carbon monoxide, and nitric oxide for both whole cell and subcellular imaging.

Imaging Biomarkers for Monitoring the Inflammatory Redox Landscape in the Brain

Inflammation is one key process in driving cellular redox homeostasis toward oxidative stress, which perpetuates inflammation. In the brain, this interplay results in a vicious cycle of cell death, the loss of neurons, and leakage of the blood–brain barrier. Hence, the neuroinflammatory response fuels the development of acute and chronic inflammatory diseases. Interrogation of the interplay between inflammation, oxidative stress, and cell death in neurological tissue in vivo is very challenging. The complexity of the underlying biological process and the fragility of the brain limit our understanding of the cause and the adequate diagnostics of neuroinflammatory diseases. In recent years, advancements in the development of molecular imaging agents addressed this limitation and enabled imaging of biomarkers of neuroinflammation in the brain. Notable redox biomarkers for imaging with positron emission tomography (PET) tracers are the 18 kDa translocator protein (TSPO) and monoamine oxygenase B (MAO–B). These findings and achievements offer the opportunity for novel diagnostic applications and therapeutic strategies. This review summarizes experimental as well as established pharmaceutical and biotechnological tools for imaging the inflammatory redox landscape in the brain, and provides a glimpse into future applications.

Fluorescent small organic probes for biosensing

Small-molecule based fluorescent probes are increasingly important for the detection and imaging of biological signaling molecules due to their simplicity, high selectivity and sensitivity, whilst being non-invasive, and suitable for real-time analysis of living systems. With this perspective we highlight sensing mechanisms including Förster resonance energy transfer (FRET), intramolecular charge transfer (ICT), photoinduced electron transfer (PeT), excited state intramolecular proton transfer (ESIPT), aggregation induced emission (AIE) and multiple modality fluorescence approaches including dual/triple sensing mechanisms (DSM or TSM). Throughout the perspective we highlight the remaining challenges and suggest potential directions for development towards improved small-molecule fluorescent probes suitable for biosensing.

Structural Similarities between Some Common Fluorophores Used in Biology, Marketed Drugs, Endogenous Metabolites, and Natural Products

It is known that at least some fluorophores can act as 'surrogate' substrates for solute carriers (SLCs) involved in pharmaceutical drug uptake, and this promiscuity is taken to reflect at least a certain structural similarity. As part of a comprehensive study seeking the 'natural' substrates of 'orphan' transporters that also serve to take up pharmaceutical drugs into cells, we have noted that many drugs bear structural similarities to natural products. A cursory inspection of common fluorophores indicates that they too are surprisingly 'drug-like', and they also enter at least some cells. Some are also known to be substrates of efflux transporters. Consequently, we sought to assess the structural similarity of common fluorophores to marketed drugs, endogenous mammalian metabolites, and natural products. We used a set of some 150 fluorophores along with standard fingerprinting methods and the Tanimoto similarity metric. Results: The great majority of fluorophores tested exhibited significant similarity (Tanimoto similarity > 0.75) to at least one drug, as judged via descriptor properties (especially their aromaticity, for identifiable reasons that we explain), by molecular fingerprints, by visual inspection, and via the "quantitative estimate of drug likeness" technique. It is concluded that this set of fluorophores does overlap with a significant part of both the drug space and natural products space. Consequently, fluorophores do indeed offer a much wider opportunity than had possibly been realised to be used as surrogate uptake molecules in the competitive or trans-stimulation assay of membrane transporter activities.

Biocompatible fluorescent probe for detecting mitochondrial alkaline phosphatase activity in live cells

Alkaline phosphatase (ALP) is an enzyme that actively plays a significant role in the various metabolic processes by transferring a phosphate group to the protein, nucleic acid, etc. The elevated level of ALP in blood plasma is the hallmark of inflammation/cancer. The hyperactive mitochondria in cancer cells produce an excess of ATP to fulfill the high energy demand. Thus, we have developed a fluorescent probe Mito-Phos for ALP, which can detect phosphatase expression in mitochondria in live cells. The probe Mito-Phos has shown ~15-fold fluorescence intensity increments at 450 nm in the presence of 500 ng/mL of ALP. It takes about 60 min to consume the whole amount of ALP (500 ng/mL) in physiological buffer saline. It can selectively react with ALP even in the presence of other probable cellular reactive components. It is highly biocompatible and nontoxic to the live cells. It has shown ALP expression in a dose-dependent manner by providing concomitant fluorescence images in the blue-channel region. It has localized exclusively in the mitochondria in live cells. The probe Mito-Phos is highly biocompatible with the ability to assess ALP expression in mitochondria in live cells.

PiQSARS: A pipeline for quantitative and statistical analyses of ratiometric fluorescent biosensors

Genetically encoded ratiometric fluorescent probes are cutting-edge tools in biology. They allow precise and dynamic measurement of various physiological parameters within cell compartments. Because data extraction and analysis are time consuming and may lead to inconsistencies between results, we describe here a standardized pipeline for•Semi-automated treatment of time-lapse fluorescence microscopy images.•Quantification of individual cell signal.•Statistical analysis of the data.First, a dedicated macro was developed using the FIJI software to reproducibly quantify the fluorescence ratio as a function of time. Raw data are then exported and analyzed using R and MATLAB softwares. Calculation and statistical analysis of selected graphic parameters are performed. In addition, a functional principal component analysis allows summarizing the dataset. Finally, a principal component analysis is performed to check consistency and final analysis is presented as a visual diagram. The method is adapted to any ratiometric fluorescent probe. As an example, the analysis of the cytoplasmic HyPer probe in response to an acute cell treatment with increasing amounts of hydrogen peroxide is shown. In conclusion, the pipeline allows to save time and analyze a larger amount of samples while reducing manual interventions and consequently increasing the robustness of the analysis.

Investigation of a Sensing Strategy Based on a Nucleophilic Addition Reaction for Quantitative Detection of Bisulfite (HSO3-)

A reaction-based sensor (NAS-1) showed a high affinity and sensitivity to HSO3- via a nucleophilic addition reaction in the aqueous media, giving dual signals from absorption and emission spectra. NAS-1 was successfully applied in RK13 epithelial cells to detect HSO3- in a cellular environment.

Establishment of a Customizable Fluorescent Probe Platform for the Organelle-Targeted Bioactive Species Detection

A customizable fluorescent probe platform that can be used to detect various bioactive analytes offers significant potential for engineering a wide range of bioprobes with diverse sensing and imaging functions. Here, we show a facile and innovative strategy for introducing cis-amino-proline as a carrier scaffold, which is appended with three specific functional groups: a target group, a water-soluble group, and fluorophores with triggers. The potency of the designed strategy could be customized to generate variable multifunctional fluorescent probes for detecting bioactive species of interest, including reactive oxygen species (ROS), reactive nitrogen species (RNS), reactive sulfur species (RSS), ROS/RSS, and even enzymes. We designed and synthesized five representative water-soluble and organelle-targeted compounds, PMB, PMN, PMD, PRB, and PME, with emission wavelengths of these fluorescent probes varying from blue to red (465, 480, 535, 550, 565, and 640 nm). This strategy could be exemplified by its application to develop a mitochondria-/lysosome-targeting multifunctional fluorescent probe capable of imaging bioactive species of interest in live cells and nude mice.

Hypoxia acclimation alters reactive oxygen species homeostasis and oxidative status in estuarine killifish (Fundulus heteroclitus)

Hypoxia is common in aquatic environments, and exposure to hypoxia followed by re-oxygenation is often believed to induce oxidative stress. However, there have been relatively few studies of reactive oxygen species (ROS) homeostasis and oxidative status in fish that experience natural hypoxia-re-oxygenation cycles. We examined how exposure to acute hypoxia (2 kPa O2) and subsequent re-oxygenation (to 20 kPa O2) affects redox status, oxidative damage and anti-oxidant defenses in estuarine killifish (Fundulus heteroclitus), and whether these effects were ameliorated or potentiated by prolonged (28 days) acclimation to either constant hypoxia or intermittent cycles of nocturnal hypoxia (12 h:12 h normoxia:hypoxia). Acute hypoxia and re-oxygenation led to some modest and transient changes in redox status, increases in oxidized glutathione, depletion of scavenging capacity and oxidative damage to lipids in skeletal muscle. The liver had greater scavenging capacity, total glutathione concentrations and activities of anti-oxidant enzymes (catalase, glutathione peroxidase) than muscle, and generally experienced less variation in glutathiones and lipid peroxidation. Unexpectedly, acclimation to constant hypoxia or intermittent hypoxia led to a more oxidizing redox status (muscle and liver) and it increased oxidized glutathione (muscle). However, hypoxia-acclimated fish exhibited little to no oxidative damage (as reflected by lipid peroxidation and aconitase activity), in association with improvements in scavenging capacity and catalase activity in muscle. We conclude that hypoxia acclimation leads to adjustments in ROS homeostasis and oxidative status that do not reflect oxidative stress, but may instead be part of the suite of responses that killifish use to cope with chronic hypoxia.

Selective chemiluminescent TURN-ON quantitative bioassay and imaging of intracellular hydrogen peroxide in human living cells

Hydrogen peroxide is an unavoidable by-product of cell metabolism, but when it is not properly managed by the body it can lead to several pathologies (e.g., premature aging, cardiovascular and neurodegenerative diseases, cancer). Several methods have been proposed for the measurement of intracellular H₂O₂ but none of them has proven to be selective. We developed a rapid all-in-one chemiluminescent bioassay for the quantification of H₂O₂ in living cells with a low limit of detection (0.15 μM). The method relies on an adamantylidene-1,2-dioxetane lipophilic probe containing an arylboronate moiety; upon reaction with H₂O₂ the arylboronate moiety is converted to the correspondent phenol and the molecule decomposes leading to an excited-state fragment that emits light. The probe has been successfully employed for quantifying intracellular H₂O₂ in living human endothelial, colon and keratinocyte cells exposed to different pro-oxidant stimuli (i.e., menadione, phorbol myristate acetate and lipopolysaccharide). Imaging experiments clearly localize the chemiluminescence emission inside the cells. Treatment of cells with antioxidant molecules leads to a dose-dependent decrease of intracellular H₂O₂ levels. As a proof of concept, the bioassay has been used to measure the antioxidant activity of extracts from Brassica juncea wastes, which contain glucosinolates, isothiocyanates and other antioxidant molecules.

HOCl Responsive Lanthanide Complexes Using Hydroquinone Caging Units

Redox biology is still looking for tools to monitor redox potential in cellular biology and, despite a large and sustained effort, reliable molecular probes have yet to emerge. In contrast, molecular probes for reactive oxygen and nitrogen have been widely explored. In this manuscript, three kinetically inert lanthanide complexes that selectively react with hypochlorous acid are prepared and characterized. The design is based on 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (DO3A) and 1,4,7,10-tetraazacyclododecane-1,7-diacetic acid (DO2A) ligands appended with one or two redox active hydroquinone derived arms, thereby forming octadentate ligands ideally suited to complex trivalent lanthanide ions. The three complexes are found to react selectively with hypochlorous acid to form highly symmetric lanthanide(III) 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacedic acid (DOTA) complexes. The conversion of the probe to [Ln.DOTA]- is followed by luminescence, absorption, and NMR spectroscopy in a model system comprised of a Triton-X modified HEPES buffer. It was concluded that the design principle works, and that simple caging units like hydroquinones can work well in conjugation with lanthanide(III) complexes.

In vivo monitoring of tissue regeneration using a ratiometric lysosomal AIE probe

Tissue regeneration is a crucial self-renewal capability involving many complex biological processes. Although transgenic techniques and fluorescence immunohistochemical staining have promoted our understanding of tissue regeneration, simultaneous quantification and visualization of tissue regeneration processes is not easy to achieve. Herein, we developed a simple and quantitative method for the real-time and non-invasive observation of the process of tissue regeneration. The synthesized ratiometric aggregation-induced-emission (AIE) probe exhibits high selectivity and reversibility for pH responses, good ability to map lysosomal pH both in vitro and in vivo, good biocompatibility and excellent photostability. The caudal fin regeneration of a fish model (medaka larvae) was monitored by tracking the lysosomal pH change. It was found that the mean lysosomal pH is reduced during 24-48 hpa to promote the autophagic activity for cell debris degradation. Our research can quantify the changes in mean lysosomal pH and also exhibit its distribution during the caudal fin regeneration. We believe that the AIE-active lysosomal pH probe can also be potentially used for long-term tracking of various lysosome-involved biological processes, such as tracking the stress responses of tissue, tracking the inflammatory responses, and so on.

An Activatable Cancer-Targeted Hydrogen Peroxide Probe for Photoacoustic and Fluorescence Imaging

Reactive oxygen species play an important role in cancer, however, their promiscuous reactivity, low abundance, and short-lived nature limit our ability to study them in real time in living subjects with conventional noninvasive imaging methods. Photoacoustic imaging is an emerging modality for in vivo visualization of molecular processes with deep tissue penetration and high spatiotemporal resolution. Here, we describe the design and synthesis of a targeted, activatable probe for photoacoustic imaging, which is responsive to one of the major and abundant reactive oxygen species, hydrogen peroxide (H2O2). This bifunctional probe, which is also detectable with fluorescence imaging, is composed of a heptamethine carbocyanine dye scaffold for signal generation, a 2-deoxyglucose cancer localization moiety, and a boronic ester functionality that specifically detects and reacts to H2O2. The optical properties of the probe were characterized using absorption, fluorescence, and photoacoustic measurements; upon addition of pathophysiologic H2O2 concentrations, a clear increase in fluorescence and red-shift of the absorption and photoacoustic spectra were observed. Studies performed in vitro showed no significant toxicity and specific uptake of the probe into the cytosol in breast cancer cell lines. Importantly, intravenous injection of the probe led to targeted uptake and accumulation in solid tumors, which enabled noninvasive photoacoustic and fluorescence imaging of H2O2. In conclusion, the reported probe shows promise for the in vivo visualization of hydrogen peroxide. SIGNIFICANCE: This study presents the first activatable and cancer-targeted hydrogen peroxide probe for photoacoustic molecular imaging, paving the way for visualization of hydrogen peroxide at high spatiotemporal resolution in living subjects.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/79/20/5407/F1.large.jpg.

Fluorescent probes for organelle-targeted bioactive species imaging

The dynamic fluctuations of bioactive species in living cells are associated with numerous physiological and pathological phenomena. The emergence of organelle-targeted fluorescent probes has significantly facilitated our understanding on the biological functions of these species. This review describes the design, applications, challenges and potential directions of organelle-targeted bioactive species probes.

Bioactive species, including reactive oxygen species (ROS, including O₂˙^–, H₂O₂, HOCl, ¹O₂, ˙OH, HOBr, etc.), reactive nitrogen species (RNS, including ONOO^–, NO, NO₂, HNO, etc.), reactive sulfur species (RSS, including GSH, Hcy, Cys, H₂S, H₂S_n, SO₂ derivatives, etc.), ATP, HCHO, CO and so on, are a highly important category of molecules in living cells. The dynamic fluctuations of these molecules in subcellular microenvironments determine cellular homeostasis, signal conduction, immunity and metabolism. However, their abnormal expressions can cause disorders which are associated with diverse major diseases. Monitoring bioactive molecules in subcellular structures is therefore critical for bioanalysis and related drug discovery. With the emergence of organelle-targeted fluorescent probes, significant progress has been made in subcellular imaging. Among the developed subcellular localization fluorescent tools, ROS, RNS and RSS (RONSS) probes are highly attractive, owing to their potential for revealing the physiological and pathological functions of these highly reactive, interactive and interconvertible molecules during diverse biological events, which are rather significant for advancing our understanding of different life phenomena and exploring new technologies for life regulation. This review mainly illustrates the design principles, detection mechanisms, current challenges, and potential future directions of organelle-targeted fluorescent probes toward RONSS.

A reversible and turn-on type fluorescence behaviour of hydrogen sulfide via a redox cycle between selenoxide and selenide

Upon treatment with H₂S in MeCN–PBS, the fluorescence dormant selenoxides of dibenzobarrelene- and benzobarrelene-based 1-seleno-1,3-butadiene derivatives are rapidly converted to strongly fluorescent selenides.