Fluorescent probes for real-time measurement of nitric oxide in living cells

Products of oxidative and non-oxidative metabolism of L-arginine as potential regulators of Ca2+ transport in mitochondria of uterine smooth muscle

Mitochondria play a crucial role in maintaining Ca2+ homeostasis in cells. Due to the critical regulatory role of the products of oxidative and non-oxidative metabolism of L-arginine, it is essential to clarify their effect on Ca2+ transport in smooth muscle mitochondria. Experiments were performed on the uterine myocytes of rats and isolated mitochondria. The possibility of NO synthesis by mitochondria was demonstrated by confocal microscopy and spectrofluorimetry methods using the NO-sensitive fluorescent probe DAF-FM and Mitotracker Orange CM-H2TMRos. It was shown that 50 μM L-arginine stimulates the energy-dependent accumulation of Ca2+ in mitochondria using the fluorescent probe Fluo-4 AM. A similar effect occurred when using nitric oxide donors 100 μM SNP, SNAP, and sodium nitrite (SN) directly. The stimulating effect was eliminated in the presence of the NO scavenger C-PTIO. Nitric oxide reduces the electrical potential in mitochondria without causing them to swell. The stimulatory effect of spermine on the accumulation of Ca2+ by mitochondria is attributed to the enhancement of NO synthesis, which was demonstrated with the use of C-PTIO, NO-synthase inhibitors (100 μM NA and L-NAME), as well as by direct monitoring of NO synthesis fluorescent probe DAF-FM. A conclusion was drawn about the potential regulatory effect of the product of the oxidative metabolism of L-arginine - NO on the transport of Ca2+ in the mitochondria of the myometrium, as well as the corresponding effect of the product of non-oxidative metabolism -spermine by increasing the synthesis of NO in these subcellular structures.

A fluorescent probe with an ultra-rapid response to nitric oxide

Nitric oxide (NO) is a diatomic inorganic free radical ubiquitous in mammalian tissues and cells that plays a multifaceted role in a variety of physiological and pathophysiological processes. The strict dependence of the biological effects of NO on its concentration makes its real-time monitoring crucial. In view of the reactivity of NO with multiple bio-targets, the development of NO sensors that associate a fast response rate with selectivity and sensitivity is very challenging. Herein we report a fluorescent NO probe based on a BODIPY fluorogenic unit covalently linked to a trimethoxy aniline derivative through a flexible spacer. NO leads to effective nitrosation of the highly electron-rich amino active site of the probe through the secondary oxide N2O3, resulting in an increase of BODIPY fluorescence quantum yield from Φf = 0.06 to Φf = 0.55, accompanied by significant changes in the relative amplitude of the fluorescence lifetimes. In situ generation of NO, achieved by a tailored light-activatable NO releaser, allows the real-time detection of NO as a function of its concentration and permits demonstrating that the probe exhibits a very fast response time, being ≤0.1 s. This remarkable data combines with the high sensitivity of the probe to NO (LOD = 35 nM), responsiveness also to ONOO-, the other important secondary oxide of NO, independence from the fluorescence response within a wide pH range, good selectivity towards different analytes and small interference by typical physiological concentrations of glutathione. Validation of this probe in melanoma cell lines is also reported.

Development of an innovative turn-on fluorescent probe for targeted in-vivo detection of nitric oxide in rat brain extracts as a biomarker for migraine disease

Nitric oxide (NO) is one of the reactive nitrogen species (RNS) that has been proposed to be a key signaling molecule in migraine. Migraine is a neurological disorder that is linked to irregular NO levels, which necessitates precise NO quantification for effective diagnosis and treatment. This work introduces a novel fluorescent probe, 2,3-diaminonaphthelene-1,4-dione (DAND), which was designed and synthesized to selectively detect NO in-vitro and in-vivo as a migraine biomarker. DAND boasts high aqueous solubility, biocompatibility, and facile synthesis, which enable highly selective and sensitive detection of NO under physiological conditions. NO reacts with diamine moieties (recognition sites) of DAND, results in the formation of a highly fluorescent product (DAND-NO) known as 1H-naphtho[2,3-d][1,2,3]triazole-4,9-dione at λem 450 nm. The fluorescence turn-on sensing mechanism operates through an intramolecular charge transfer (ICT) mechanism. To maximize fluorescence signal intensity, parameters including DAND concentration, reaction temperature, reaction time and pH were systematically optimized for sensitive and precise NO determination. The enhanced detection capability (LOD = 0.08 μmol L-1) and high selectivity of the probe make it a promising tool for NO detection in brain tissue homogenates. This demonstrates the potential diagnostic value of the probe for individuals suffering from migraine. Furthermore, this study sheds light on the potential role of zolmitriptan (ZOLM), an antimigraine medication, in modulating NO levels in the brain of rats with nitroglycerin-induced migraine, emphasizing its significant impact on reducing NO levels. The obtained results could have significant implications for understanding how ZOLM affects NO levels and may aid in the development of more targeted and effective migraine treatment strategies.

Unique Synthetic Strategy for Probing in Situ Lysosomal NO for Screening Neuroinflammatory Phenotypes against SARS-CoV-2 RNA in Phagocytotic Microglia

In the pathogenesis of microglia, brain immune cells promote nitrergic stress by overproducing nitric oxide (NO), leading to neuroinflammation. Furthermore, NO has been linked to COVID-19 progression, which has caused significant morbidity and mortality. SARS-CoV-2 infection activates inflammation by releasing excess NO and causing cell death in human microglial clone 3 (HMC3). In addition, NO regulates lysosomal functions and complex machinery to neutralize pathogens through phagocytosis. Therefore, developing lysosome-specific NO probes to monitor phagocytosis in microglia during the COVID-19 infection would be a significant study. Herein, a unique synthetic strategy was adopted to develop a NO selective fluorescent probe, PDM-NO, which can discriminate activated microglia from their resting state. The nonfluorescent PDM-NO exhibits a turn-on response toward NO only at lysosomal pH (4.5-5.5). Quantum chemical calculations (DFT/TD-DFT/PCM) and photophysical study revealed that the photoinduced electron transfer (PET) process is pivotal in tuning optical properties. PDM-NO demonstrated good biocompatibility and lysosomal specificity in activated HMC3 cells. Moreover, it can effectively map the dynamics of lysosomal NO against SARS-CoV-2 RNA-induced neuroinflammation in HMC3. Thus, PDM-NO is a potential fluorescent marker for detecting RNA virus infection and monitoring phagocytosis in HMC3.

Bismuth Oxide (Bi2O3) Nanoparticles Cause Selective Toxicity in a Human Endothelial (HUVE) Cell Line Compared to Epithelial Cells

A review of recent literature suggests that bismuth oxide (Bi₂O₃, referred to as B in this article) nanoparticles (NPs) elicit an appreciable response only after a concentration above 40-50 µg/mL in different cells all having an epithelial origin, to the best of our knowledge. Here, we report the toxicological profile of Bi₂O₃ NPs (or BNPs) (71 ± 20 nm) in a human endothelial cell (HUVE cell line) in which BNPs exerted much steeper cytotoxicity. In contrast to a high concentration of BNPs (40-50 µg/mL) required to stimulate an appreciable toxicity in epithelial cells, BNPs induced 50% cytotoxicity in HUVE cells at a very low concentration (6.7 µg/mL) when treated for 24 h. BNPs induced reactive oxygen species (ROS), lipid peroxidation (LPO), and depletion of the intracellular antioxidant glutathione (GSH). BNPs also induced nitric oxide (NO,) which can result in the formation of more harmful species in a fast reaction that occurs with superoxide (O₂^•-). Exogenously applied antioxidants revealed that NAC (intracellular GSH precursor) was more effective than Tiron (a preferential scavenger of mitochondrial O₂^•-) in preventing the toxicity, indicating ROS production is extra-mitochondrial. Mitochondrial membrane potential (MMP) loss mediated by BNPs was significantly less than that of exogenously applied oxidant H₂O₂, and MMP loss was not as intensely reduced by either of the antioxidants (NAC and Tiron), again suggesting BNP-mediated toxicity in HUVE cells is extra-mitochondrial. When we compared the inhibitory capacities of the two antioxidants on different parameters of this study, ROS, LPO, and GSH were among the strongly inhibited biomarkers, whereas MMP and NO were the least inhibited group. This study warrants further research regarding BNPs, which may have promising potential in cancer therapy, especially via angiogenesis modulation.

Aggregation-Induced Electrochemiluminescence and Nitric Oxide Recognition by Halogen Bonding with a Ruthenium(II) Complex

In this study, a new strategy for NO detection based on the aggregation-induced electrochemical luminescence (AIECL) of a ruthenium-based complex and the halogen bonding effect was developed. First, [Ru(phen)₂ (phen-Br₂ )]²⁺ (phen : 1,10-phenanthroline, phen-Br₂  : 3,8-dibromo-1,10-phenanthroline) was synthesized and exhibited aggregation-induced emission (AIE) and AIECL properties in a poor solvent (H₂ O). [Ru(phen)₂ (phen-Br₂ )]²⁺ exhibited greatly enhanced AIECL properties compared to its AIE intensity. When the volume fraction of water (f_w , v %) in the H₂ O-acetonitrile (MeCN) system was increased from 30 to 90 %, the photoluminescence and electrochemiluminescence (ECL) intensities were three- and 800-fold that of the pure MeCN system, respectively. Dynamic light scattering and scanning electron microscopy results indicated that [Ru(phen)₂ (phen-Br₂ )]²⁺ aggregated into nanoparticles. AIECL is sensitive to NO because of its halogen bonding effect. The C-Br⋅⋅⋅N bond between [Ru(phen)₂ (phen-Br₂ )]²⁺ and NO increased the distance of complex molecules, resulting in ECL quenching. A detection limit of 2 nM was obtained with 5 orders of magnitude linear range. The combination of the AIECL system and the halogen bond effect expands the theoretical research and applications in biomolecular detection, molecular sensors, and stages of medical diagnosis.

Recent Advances in Imaging Agents Anchored with pH (Low) Insertion Peptides for Cancer Theranostics

The acidic extracellular microenvironment has become an effective target for diagnosing and treating tumors. A pH (low) insertion peptide (pHLIP) is a kind of peptide that can spontaneously fold into a transmembrane helix in an acidic microenvironment, and then insert into and cross the cell membrane for material transfer. The characteristics of the acidic tumor microenvironment provide a new method for pH-targeted molecular imaging and tumor-targeted therapy. As research has increased, the role of pHLIP as an imaging agent carrier in the field of tumor theranostics has become increasingly prominent. In this paper, we describe the current applications of pHLIP-anchored imaging agents for tumor diagnosis and treatment in terms of different molecular imaging methods, including magnetic resonance T1 imaging, magnetic resonance T2 imaging, SPECT/PET, fluorescence imaging, and photoacoustic imaging. Additionally, we discuss relevant challenges and future development prospects.

CeO2-Zn Nanocomposite Induced Superoxide, Autophagy and a Non-Apoptotic Mode of Cell Death in Human Umbilical-Vein-Derived Endothelial (HUVE) Cells

In this study, a nanocomposite of cerium oxide-zinc (CeO₂-Zn; 26 ± 11 nm) based on the antioxidant rare-earth cerium oxide (CeO₂) nanoparticles (NPs) with the modifier zinc (Zn) was synthesized by sintering method and characterized. Its bio-response was examined in human umbilical-vein-derived endothelial (HUVE) cells to get insight into the components of vascular system. While NPs of CeO₂ did not significantly alter cell viability up to a concentration of 200 µg/mL for a 24 h exposure, 154 ± 6 µg/mL of nanocomposite CeO₂-Zn induced 50% cytotoxicity. Mechanism of cytotoxicity occurring due to nanocomposite by its Zn content was compared by choosing NPs of ZnO, possibly the closest nanoparticulate form of Zn. ZnO NPs lead to the induction of higher reactive oxygen species (ROS) (DCF-fluorescence), steeper depletion in antioxidant glutathione (GSH) and a greater loss of mitochondrial membrane potential (MMP) as compared to that induced by CeO₂-Zn nanocomposite. Nanocomposite of CeO₂-Zn, on the other hand, lead to significant higher induction of superoxide radical (O₂^•−, DHE fluorescence), nitric oxide (NO, determined by DAR-2 imaging and Griess reagent) and autophagic vesicles (determined by Lysotracker and monodansylcadeverine probes) as compared to that caused by ZnO NP treatment. Moreover, analysis after triple staining (by annexin V-FITC, PI, and Hoechst) conducted at their respective IC50s revealed an apoptosis mode of cell death due to ZnO NPs, whereas CeO₂-Zn nanocomposite induced a mechanism of cell death that was significantly different from apoptosis. Our findings on advanced biomarkers such as autophagy and mode of cell death suggested the CeO₂-Zn nanocomposite might behave as independent nanostructure from its constituent ones. Since nanocomposites can behave independently of their constituent NPs/elements, by creating nanocomposites, NP versatility can be increased manifold by just manipulating existing NPs. Moreover, data in this study can furnish early mechanistic insight about the potential damage that could occur in the integrity of vascular systems.

Simultaneous imaging of hypochlorous acid and nitric oxide in live cells based on a dual-channel fluorescent probe

Both reactive oxygen species (ROS) and reactive nitrogen species (RNS) are inevitably produced during normal human metabolism. Various ROS and RNS together form tangled networks that play important roles in many physiological and pathological processes. Here we used 1,8-naphthalene diamine as a reactive group to develop a fluorescent probe, N-[2-(6-phenylethynyl)quinolinylmethyl]-1,8-diamino naphthalene (QBN), for HOCl and NO. QBN showed a "turn-on" fluorescent response at 464 nm to HOCl in the range of 0-75 μM with rapid responding time (10 s) and detection limit (0.11 ± 0.03 μM). Furthermore, a "turn-on" fluorescent responses at 512 nm to NO in the range of 0-40 μM with responding time (20 s) and detection limit (25.7 ± 3.4 nM) was found. The response mechanisms of QBN to HOCl and NO were discussed based on mass analysis of the different products. The dual-channel probe was then successfully applied for simultaneous imaging of both exogenous and endogenous HOCl and NO in live cells.

Anti-Inflammatory CeO2 Nanoparticles Prevented Cytotoxicity Due to Exogenous Nitric Oxide Donors via Induction Rather Than Inhibition of Superoxide/Nitric Oxide in HUVE Cells

The mechanism behind the cytoprotective potential of cerium oxide nanoparticles (CeO2 NPs) against cytotoxic nitric oxide (NO) donors and H2O2 is still not clear. Synthesized and characterized CeO2 NPs significantly ameliorated the lipopolysaccharide (LPS)-induced cytokines IL-1β and TNF-α. The main goal of this study was to determine the capacities of NPs regarding signaling effects that could have occurred due to reactive oxygen species (ROS) and/or NO, since NP-induced ROS/NO did not lead to toxicity in HUVE cells. Concentrations that induced 50% cell death (i.e., IC50s) of two NO donors (DETA-NO; 1250 ± 110 µM and sodium nitroprusside (SNP); 950 ± 89 µM) along with the IC50 of H2O2 (120 ± 7 µM) were utilized to evaluate cytoprotective potential and its underlying mechanism. We determined total ROS (as a collective marker of hydrogen peroxide, superoxide radical (O2•-), hydroxyl radical, etc.) by DCFH-DA and used a O2•- specific probe DHE to decipher prominent ROS. The findings revealed that signaling effects mediated mainly by O2•- and/or NO are responsible for the amelioration of toxicity by CeO2 NPs at 100 µg/mL. The unaltered effect on mitochondrial membrane potential (MMP) due to NP exposure and, again, CeO2 NPs-mediated recovery in the loss of MMP due to exogenous NO donors and H2O2 suggested that NP-mediated O2•- production might be extra-mitochondrial. Data on activated glutathione reductase (GR) and unaffected glutathione peroxidase (GPx) activities partially explain the mechanism behind the NP-induced gain in GSH and persistent cytoplasmic ROS. The promoted antioxidant capacity due to non-cytotoxic ROS and/or NO production, rather than inhibition, by CeO2 NP treatment may allow cells to develop the capacity to tolerate exogenously induced toxicity.

In vitro cytoprotective activity of cyanidin 3-glucoside extracts from Haematocarpus validus pomace on streptozotocin induced oxidative damage in pancreatic β-cells

Cyanidin-3-glucoside (C3Ghv) compounds were purified and isolated from the anthocyanins extract of Haematocarpus validus. C3Ghv were studied for antioxidant and cytoprotective properties on pancreatic β-cells of rat insulinoma cells (RINm5F) against the oxidative stress induced by streptozotocin (STZ). The exposure of RINm5F cells to C3Ghv at concentration of 100 and 200 μg/mL for 24 h reduced 10% and 23% cell viability, respectively, as compared to control cells. The pre-treatment of RINm5F cells with C3Ghv (50 µg/mL) increased the cell viability by 29% as compared to control, on being treated with STZ (10 mM) for 24 h. The pre-treatment of RINm5F cells with C3Ghv (50 µg/mL) for 24 h followed by exposure to STZ (10 mM) for 1 h decreased the generation of reactive oxygen species (ROS) by 57%, generation of nitric oxide by 22.8%, generation of malondialdehyde (MDA) by 32%, the production of p-ERK ½ by 83%, p-JNK by 82.6%, p-MEK by 57%, and p-p38 MAPK by 64%. The C3Ghv treatment also decreased the ratio of apoptotic proteins Bax to Bcl-2 by 61%, and improved the M2 phase of cell cycle by 75% as compared to STZ treated cells. The overall results suggest that C3Ghv protects pancreatic β-cells against oxidative stress-induced apoptosis, thereby implicating the significant role of C3Ghv as an antidiabetic agent.

Design and Synthesis of New Acridone-Based Nitric Oxide Fluorescent Probe

Nitric oxide (NO) is an important signaling molecule involved in a wide range of physiological and pathological processes. Fluorescent imaging is a useful tool for monitoring NO concentration, which could be essential in various biological and biochemical studies. Here, we report the design of a novel small-molecule fluorescent probe based on 9(10H)acridone moiety for nitric oxide sensing. 7,8-Diamino-4-carboxy-10-methyl-9(10H)acridone reacts with NO in aqueous media in the presence of O₂, yielding a corresponding triazole derivative with fivefold increased fluorescence intensity. The probe was shown to be capable of nitric oxide sensing in living Jurkat cells.

Chitosan based macromolecular probes for the selective detection and removal of Fe3+ ion

Chitosan has been widely used due to its biodegradable, cost-effective and environmentally friendly properties. Modification of chitosan attracts much attention as promising methods to detect and remove organic and inorganic pollutants. In this work, chitosan-based macromolecular probes were designed and synthesized. The probes can detect Fe³⁺ in the presence of other metal ions. The detection mechanism is investigated as well. The probe's fluorescence quenching upon the addition of Fe³⁺ ion could be ascribed to the complexation between the electron-deficient ion Fe³⁺ and "C=N" (electron-rich group) of fluorescent chitosan probes. What's more, the obtained fluorescent macromolecular probes can be used for the removal of Fe³⁺ in solution. The probes could adsorb the Fe³⁺ in solution and the removal efficiency can reach as high as 62.0% while the removal efficiency of original chitosan is only 16.0%. The probes have good selective detection for Fe³⁺ and the detection limit reaches 1.2 μM.

Pt-Coated Au Nanoparticle Toxicity Is Preferentially Triggered Via Mitochondrial Nitric Oxide/Reactive Oxygen Species in Human Liver Cancer (HepG2) Cells

Reactive nitrogen species (RNS) that are formed from the reaction of versatile nitric oxide (NO) with reactive oxygen species (ROS) have been less explored in potential cancer therapy. This may be partly due to the fewer available agents that could induce NO in cells. Here, we report platinum-coated gold nanoparticles (Pt-coated Au NPs; 27 ± 20 nm) as a strong inducer of NO (assessed by live-cell imaging under NO-specific DAR-1 probe labeling and indirectly using a Griess reagent) in human liver carcinoma (HepG2) cells. In addition to NO, this study found a critical role of ROS from mitochondrial sources in the mechanism of toxicity caused by Pt-coated Au NPs. Cotreatment with a thiol-replenishing general antioxidant NAC (N-acetyl cysteine) led to significant amelioration of oxidative stress against NP-induced toxicity. However, NAC did not exhibit as much ameliorative potential against NP-induced oxidative stress as the superoxide radical (O2•-)-scavenging mitochondrial specific antioxidant mito-TEMPO did. The higher protective potential of mito-TEMPO in comparison to NAC reveals mitochondrial ROS as an active mediator of NP-induced toxicity in HepG2 cells. Moreover, the relatively unaltered NP-induced NO concentration under cotreatment of GSH modulators NAC and buthionine sulfoximine (BSO) suggested that NO production due to NP treatment is rather independent of the cellular thiols at least in HepG2 cells. Moreover, toxicity potentiation by exogenous H2O2 again suggested a more direct involvement of ROS/RNS in comparison to the less potentiation of toxicity due to GSH-exhausting BSO. A steeper amelioration in NP-induced NO and ROS and, consequently, cytotoxicity by mito-TEMPO in comparison to NAC reveal a pronounced role of NO and ROS via the mitochondrial pathway in the toxicity of Pt-coated Au NPs in HepG2 cells.

Measurement of Intracellular Nitric Oxide with a Quantitative Mass Spectrometry Probe Approach

Nitric oxide (NO) is a molecule of physiological importance, and the function of NO depends on its concentration in biological systems, particularly in cells. Concentration-based analysis of intracellular NO can provide insight into its precise role in health and disease. However, current methods for detecting intracellular NO are still inadequate for quantitative analysis. In this study, we report a quantitative mass spectrometry probe approach to measure NO levels in cells. The probe, Amlodipine (AML), comprises a Hantzsch ester group that reacts with NO to form a pyridine, Dehydro Amlodipine (DAM). Quantification of DAM by ultraperformance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) allows specific measurement of intracellular NO levels. Notably, the AML/NO reaction proceeds rapidly (within 1 s), which is favorable for NO detection considering its large diffusivity and short half-life. Meanwhile, studies under simulated physiological conditions revealed that the AML response to NO is proportional and selective. The presented UPLC-MS/MS method showed high sensitivity (LLOQ = 0.24 nM) and low matrix interference (less than 15%) in DAM quantification. Furthermore, the mass spectrometry probe approach was demonstrated by enabling the measurement of endogenous and exogenous NO in cells. Hence, the quantitative UPLC-MS/MS method developed using AML as a probe is expected to be a new method for intracellular NO analysis.

A single benzene fluorescent probe for efficient formaldehyde sensing in living cells using glutathione as an amplifier

Formaldehyde (FA), a simple reactive carbonyl molecule, is endogenously produced in the cell at various physiological condition. At elevated level, FA causes severe cell toxicity as well as damage in macromolecules such proteins and DNA. For detecting FA in living cell, we identify a small but effective fluorescent turn on probe comprising single benzene-based orothophenylenediamine compound. Further study reveals that carboxylic group in orothophenylenediamine plays the important role in enhancing fluorescent signal than another electron withdrawing group. It is even interesting to observe the occurrence of fluorescent enhancement in glutathione (GSH) environment which is generally abundant in every cell. Our probe enables to detect FA over other bio-analytes efficiently with limit of detection of 123 nM and 355-fold of enhancement in cellular mimicking conditions. Moreover, this probe could be useful in discriminating cell that has high concentration of FA as well as GSH.

Diaminorhodamine and Light-Activatable NO Donors: Photorelease Quantification and Potential Pitfalls

Light-activatable nitric oxide (NO) donors have become of interest in the recent years. They produce NO when illuminated by light, which enables the control of its local concentration and is promising for biomedical applications. Several successful prototypes of photodonors have been published, but further research is needed to improve their properties such as water-solubility, activation wavelength, biocompatibility etc. One of major challenges on this way is to evaluate the efficiency of NO generation. Several methods may be used to track NO, including spin traps, specific electrodes and fluorescence-based probes. We have studied the applicability of well-known fluorescent reporter, diaminorhodamine (DAR-2), for the evaluation of NO production by photodonors. Our results indicate that DAR-2 can be used for the quantification of NO photorelease if this process is not accompanied by the singlet oxygen formation. Otherwise the oxidation of probe results in huge fluorescence increase, which interferes with signal due to reaction with NO. This issue should be taken into account when studying hybrids releasing both NO and ¹O₂, which are promising for photodynamic therapy.

Recent progress on the organic and metal complex-based fluorescent probes for monitoring nitric oxide in living biological systems

Nitric oxide (NO) is an important gaseous signaling molecule related to various human diseases. To investigate the biological functions of NO, many strategies have been developed for real-time monitoring the NO levels in biological systems. Among these strategies, fluorescent probes are considered to be one of the most efficient and applicable methods owing to their excellent sensitivity and selectivity, high spatiotemporal resolution, noninvasiveness, and experimental convenience. Therefore, great efforts have been paid to the design, synthesis, and fluorescence investigation of novel NO fluorescent probes in the past several years. However, few of them exhibit practical applications owing to the low concentration, short half-life, and rapid diffusion characteristics of NO in biological systems. Rational design of NO fluorescent probes with excellent selectivity and sensitivity, low cytotoxicity, long-lived fluorescent emission, and low background interference is still a challenge for scientists all over the word. To provide spatial-temporal information, this article focuses on the progress made in the organic and metal complex-based NO fluorescent probes during the past five years. The key structural elements and sensing mechanisms of NO fluorescent probes are discussed. Some novel ratiometric, luminescence, and photoacoustic probes with low background interference and deep tissue penetrating ability are mentioned. All these probes have been used for imaging exogenous and endogenous NO in cells and animal models. More importantly, this article also describes the development of multi-functional NO fluorescent probes, such as organelle targeting probes, dual-analysis probes, and probe-drug conjugates, which will inspire the design of various functional fluorescent probes.

Membrane Inlet Mass Spectrometry: A Powerful Tool for Algal Research

Since the first great oxygenation event, photosynthetic microorganisms have continuously shaped the Earth's atmosphere. Studying biological mechanisms involved in the interaction between microalgae and cyanobacteria with the Earth's atmosphere requires the monitoring of gas exchange. Membrane inlet mass spectrometry (MIMS) has been developed in the early 1960s to study gas exchange mechanisms of photosynthetic cells. It has since played an important role in investigating various cellular processes that involve gaseous compounds (O2, CO2, NO, or H2) and in characterizing enzymatic activities in vitro or in vivo. With the development of affordable mass spectrometers, MIMS is gaining wide popularity and is now used by an increasing number of laboratories. However, it still requires an important theory and practical considerations to be used. Here, we provide a practical guide describing the current technical basis of a MIMS setup and the general principles of data processing. We further review how MIMS can be used to study various aspects of algal research and discuss how MIMS will be useful in addressing future scientific challenges.

New directions of activity-based sensing for in vivo NIR imaging

In vivo imaging is a powerful approach to study biological processes. Beyond cellular methods, in vivo studies allow for biological stimuli (small molecules or proteins) to be studied in their native environment. This has the potential to aid in the discovery of new biology and guide the development of diagnostics and therapies for diseases. To ensure selectivity and an observable readout, the probe development field is shifting towards activity-based sensing (ABS) approaches and near-infrared (NIR) imaging modalities. This perspective will highlight recent in vivo ABS applications that utilize NIR imaging platforms.

Dynamic Quantum Sensing of Paramagnetic Species Using Nitrogen-Vacancy Centers in Diamond

Naturally occurring paramagnetic species (PS), such as free radicals and paramagnetic metalloproteins, play an essential role in a multitude of critical physiological processes including metabolism, cell signaling, and immune response. These highly dynamic species can also act as intrinsic biomarkers for a variety of disease states, while synthetic paramagnetic probes targeted to specific sites on biomolecules enable the study of functional information such as tissue oxygenation and redox status in living systems. The work presented herein describes a new sensing method that exploits the spin-dependent emission of photoluminescence (PL) from an ensemble of nitrogen-vacancy centers in diamond for rapid, nondestructive detection of PS in living systems. Uniquely this approach involves simple measurement protocols that assess PL contrast with and without the application of microwaves. The method is demonstrated to detect concentrations of paramagnetic salts in solution and the widely used magnetic resonance imaging contrast agent gadobutrol with a limit of detection of less than 10 attomol over a 100 μm × 100 μm field of view. Real-time monitoring of changes in the concentration of paramagnetic salts is demonstrated with image exposure times of 20 ms. Further, dynamic tracking of chemical reactions is demonstrated via the conversion of low-spin cyanide-coordinated Fe3+ to hexaaqua Fe3+ under acidic conditions. Finally, the capability to map paramagnetic species in model cells with subcellular resolution is demonstrated using lipid membranes containing gadolinium-labeled phospholipids under ambient conditions in the order of minutes. Overall, this work introduces a new sensing approach for the realization of fast, sensitive imaging of PS in a widefield format that is readily deployable in biomedical settings. Ultimately, this new approach to nitrogen vacancy-based quantum sensing paves the way toward minimally invasive real-time mapping and observation of free radicals in in vitro cellular environments.

Recommendations on terminology and experimental best practice associated with plant nitric oxide research

Nitric oxide (NO) emerged as a key signal molecule in plants. During the last two decades impressive progress has been made in plant NO research. This small, redox-active molecule is now known to play an important role in plant immunity, stress responses, environmental interactions, plant growth and development. To more accurately and robustly establish the full spectrum of NO bioactivity in plants, it will be essential to apply methodological best practice. In addition, there are some instances of conflicting nomenclature within the field, which would benefit from standardization. In this context, we attempt to provide some helpful guidance for best practice associated with NO research and also suggestions for the cognate terminology.

3, 3'-Diaminobenzidine with dual o-phenylenediamine groups: two in one enables visual colorimetric detection of nitric oxide

Nitric oxide (NO) plays an important role in the generation of smog and ozone. Although great efforts have been made to determine NO by using o-phenylenediamine (OPD)-based fluorescent probes, more simple and reliable colorimetric assays for detection of NO are extremely scarce because a single OPD structure cannot produce enough optical absorption for chromogenesis. In this study, we report an innovative two-in-one visual colorimetric methodology. Commercially available 3,3'-diaminobenzidine (DAB) with two OPD structures in a single molecule is selected as the colorimetric probe, and it reacts with NO via diazo-coupling reaction to generate 1H,3'H-[5,5']bibenzotriazolyl because of the increase of conjugated double bonds, accompanying a distinct color change from colorless to brownish yellow. This two-in-one colorimetric assay can determine NO at a concentration as low as 3 ppm by the naked eye and 40 ppb by UV-vis spectrometry, which is the lowest limit of detection (LOD) among reported colorimetric assays for NO. Moreover, the present two-in-one visual colorimetric assay also has good selectivity toward NO over other common potential gas interferents such as CO₂, NO₂, NH₃, N₂, O₂, and SO₂. This present study provides a new insight for the design and development of assays for NO. Graphical abstract.

2-Amino-3'-dialkylaminobiphenyl-based fluorescent intracellular probes for nitric oxide surrogate N2O3

Fluorescent probes for nitric oxide (NO), or more frequently for its oxidized surrogate dinitrogen trioxide (N2O3), have enabled scientists to study the contributions of this signaling molecule to many physiological processes. Seeking to improve upon limitations of other probes, we have developed a family of fluorescent probes based on a 2-amino-3'-dialkylaminobiphenyl core. This core condenses with N2O3 to form benzo[c]cinnoline structures, incorporating the analyte into the newly formed fluorophore, which results in product fluorescence with virtually no background contribution from the initial probe. We varied the substituents in the core in order to optimize both the reactivity of the probes with N2O3 and their cinnoline products' fluorescence wavelengths and brightness. The top candidates were then applied to cultured cells to verify that they could respond to NO within cellular milieus, and the top performer, NO530, was compared with a "gold standard" commercial probe, DAF-FM, in a macrophage-derived cell line, RAW 264.7, stimulated to produce NO. NO530 demonstrated similar or better sensitivity and higher selectivity for NO than DAF, making it an attractive potential alternative for NO tracking in various applications.

Rapid and sensitive detection of nitric oxide by a BODIPY-based fluorescent probe in live cells: glutathione effects

Glutathione effects on the sensing reaction toward nitric oxide in live cells.

Mitochondrial dysfunction, autophagy stimulation and non-apoptotic cell death caused by nitric oxide-inducing Pt-coated Au nanoparticle in human lung carcinoma cells

BACKGROUND

Reactive oxygen species (ROS)-mediated cancer therapeutic has been at higher appreciation than those mediated by reactive nitrogen species. Cytotoxic mechanism of a novel nitric oxide (NO) inducing-Pt coated Au nanoparticle (NP) has been comparatively studied with the well-established ROS inducing Pt-based anticancer drug cisplatin in human lung A549 carcinoma cells.

METHODS

Cytotoxicity was evaluated by MTT assay, lactate dehydrogenase (LDH) release, thiobarbituric acid substances (TBARS) and C11-Boron dipyrromethene (BODIPY). ROS (O^2·- and H₂O₂) was measured with dihydroethidium (DHE) and H₂O₂-specific sensor. Nitric oxide (NO) and mitochondrial dysfunction were evaluated respectively by NO-specific probe DAR-1 and JC-1. Autophagy was determined by lysotracker (LTR) and monodansylcadaverine (MDC) applied tandemly whereas apoptosis/necrosis by Hoechst/PI and caspase 3 activity.

RESULTS

IC50 (concentration that inhibited cell viability by 50%) of Pt coated Au NP came to be 0.413 μM whereas IC50 of cisplatin came out to 86.5 μM in A549 cells treated for 24 h meaning NPs toxicity was over 200 times higher than cisplatin. However, no significant stimulation of intracellular ROS was observed at the IC50 of Pt coated Au NPs in A549 cells. However, markers like LDH release, TBARS, BODIPY and ROS were significantly higher due to cisplatin in comparison to Pt coated Au NP.

CONCLUSIONS

Pt coated Au NP caused NO-dependent mitochondrial dysfunction and autophagy. Mode of cell death due to NP was much different from ROS-inducing cisplatin.

GENERAL SIGNIFICANCE

Pt coated Au NP offer promising opportunity in cancer therapeutic and warrants advanced study in vivo models of cancer.

A review of the actions of Nitric Oxide in development and neuronal function in major invertebrate model systems

Ever since the late-eighties when endothelium-derived relaxing factor was found to be the gas nitric oxide, endogenous nitric oxide production has been observed in virtually all animal groups tested and additionally in plants, diatoms, slime molds and bacteria. The fact that this new messenger was actually a gas and therefore didn't obey the established rules of neurotransmission made it even more intriguing. In just 30 years there is now too much information for useful comprehensive reviews even if limited to animals alone. Therefore this review attempts to survey the actions of nitric oxide on development and neuronal function in selected major invertebrate models only so allowing some detailed discussion but still covering most of the primary references. Invertebrate model systems have some very useful advantages over more expensive and demanding animal models such as large, easily identifiable neurons and simple circuits in tissues that are typically far easier to keep viable. A table summarizing this information along with the major relevant references has been included for convenience.

Nitric Oxide and Hydrogen Sulfide Regulation of Ischemic Vascular Growth and Remodeling

Ischemic vascular remodeling occurs in response to stenosis or arterial occlusion leading to a change in blood flow and tissue perfusion. Altered blood flow elicits a cascade of molecular and cellular physiological responses leading to vascular remodeling of the macro- and micro-circulation. Although cellular mechanisms of vascular remodeling such as arteriogenesis and angiogenesis have been studied, therapeutic approaches in these areas have had limited success due to the complexity and heterogeneous constellation of molecular signaling events regulating these processes. Understanding central molecular players of vascular remodeling should lead to a deeper understanding of this response and aid in the development of novel therapeutic strategies. Hydrogen sulfide (H₂ S) and nitric oxide (NO) are gaseous signaling molecules that are critically involved in regulating fundamental biochemical and molecular responses necessary for vascular growth and remodeling. This review examines how NO and H₂ S regulate pathophysiological mechanisms of angiogenesis and arteriogenesis, along with important chemical and experimental considerations revealed thus far. The importance of NO and H₂ S bioavailability, their synthesis enzymes and cofactors, and genetic variations associated with cardiovascular risk factors suggest that they serve as pivotal regulators of vascular remodeling responses. © 2019 American Physiological Society. Compr Physiol 9:1213-1247, 2019.

State-of-the-art: functional fluorescent probes for bioimaging and pharmacological research

Cardiovascular diseases, neuropsychiatric disorders, and cancers seriously endanger human health. Mechanistic and pharmacological mechanisms of candidate drugs are central to the translational paradigm. Since many signal transduction and molecular events are implicated in these diseases, a novel method to interrogate the key pharmacological mechanisms is required to accelerate innovative drug discovery. Much attention now focuses on the real-time visualization of molecular disease events to yield new insights to the pathogenesis of the diseases. This review focuses on recent advances in the development of chemical probes for imaging pathological events to facilitate the study of the underlying pharmacodynamics and toxicity involved. As reviewed here, optical imaging is now frequently viewed as an indispensable technique in the field of biological research. Promoting interdisciplinary collaboration among chemistry, biology and medicine, is necessary to further refine functional fluorescent probes for diagnostic and therapeutic applications.

Real-Time Imaging of Nitric Oxide Signals in Individual Cells Using geNOps

Nitric oxide (NO^•) is a versatile signaling molecule which regulates fundamental cellular processes in all domains of life. However, due to the radical nature of NO^• it has a very short half-life that makes it challenging to trace its formation, diffusion, and degradation on the level of individual cells. Very recently, we expanded the family of genetically encoded sensors by introducing a novel class of single fluorescent protein-based NO^• probes-the geNOps. Once expressed in cells of interest, geNOps selectively respond to NO^• by fluorescence quench, which enables real-time monitoring of cellular NO^• signals. Here, we describe detailed methods suitable for imaging of NO^• signals in mammalian cells. This novel approach may facilitate a broad range of studies to (re)investigate the complex NO^• biochemistry in living cells.

BODIPY-based hydrazine as a fluorescent probe for sensitive and selective detection of nitric oxide: a new strategy

A novel fluorescent probe 8-HB was developed with a BODIPY as a fluorophore and 8-substituted hydrazine as a reactive site for sensitive and selective detection of nitric oxide (NO), generating major fluorescent dehydrazinated BODIPY and minor non-fluorescent azide BODIPY.

A high-sensitivity fluorescent probe with a self-immolative spacer for real-time ratiometric detection and imaging of alkaline phosphatase activity

The probe APW uses a self-immolative mechanism to achieve a ratio response to ALP, which has the following advantages: fast response (in less than 15 min), high quantum yield (Φ = 0.6), low detection limit (0.46 U L⁻¹) and excellent selectivity.

Self-Assembly Controls Reactivity with Nitric Oxide: Implications for Fluorescence Sensing

Three molecules containing the fluorophore 4-amino-1,8-naphthalimide (ANI) and showing different tendencies to self-assembly in aqueous environment have been prepared and fully characterized. The fluorescence emissions of two of these compounds in aqueous solutions are efficiently quenched in the presence of nitric oxide (NO) in aerated medium. Nuclear magnetic resonance and mass spectrometry techniques indicate that NO/O2 induces deamination of the ANI fluorophore, resulting in nonemissive 1,8-naphtalimide derivatives. It is found that the reactivity toward NO/O2 is regulated by the different aggregation modes presented by the molecules in aqueous medium. In this way, the molecules displaying fluorescence response toward NO/O2 are those with weak self-association properties whereas the compound with a high hydrophobic character (self-assembling into large nanoparticles) is insensitive to this species. Ultimately, the results described here could not only set the basis for the design of fluorescent bioprobes for NO/O2 based on ANI derivatives or other monoamino compounds but also could raise awareness about the importance of supramolecular interactions for the design of chemosensors.

Near-Infrared Photoactivatable Nitric Oxide Donors with Integrated Photoacoustic Monitoring

Photoacoustic (PA) tomography is a noninvasive technology that utilizes near-infrared (NIR) excitation and ultrasonic detection to image biological tissue at centimeter depths. While several activatable small-molecule PA sensors have been developed for various analytes, the use of PA molecules for deep-tissue analyte delivery and monitoring remains an underexplored area of research. Herein, we describe the synthesis, characterization, and in vivo validation of photoNOD-1 and photoNOD-2, the first organic, NIR-photocontrolled nitric oxide (NO) donors that incorporate a PA readout of analyte release. These molecules consist of an aza-BODIPY dye appended with an aryl N-nitrosamine NO-donating moiety. The photoNODs exhibit chemostability to various biological stimuli, including redox-active metals and CYP450 enzymes, and demonstrate negligible cytotoxicity in the absence of irradiation. Upon single-photon NIR irradiation, photoNOD-1 and photoNOD-2 release NO as well as rNOD-1 or rNOD-2, PA-active products that enable ratiometric monitoring of NO release. Our in vitro studies show that, upon irradiation, photoNOD-1 and photoNOD-2 exhibit 46.6-fold and 21.5-fold ratiometric turn-ons, respectively. Moreover, unlike existing NIR NO donors, the photoNODs do not require encapsulation or multiphoton activation for use in live animals. In this study, we use PA tomography to monitor the local, irradiation-dependent release of NO from photoNOD-1 and photoNOD-2 in mice after subcutaneous treatment. In addition, we use a murine model for breast cancer to show that photoNOD-1 can selectively affect tumor growth rates in the presence of NIR light stimulation following systemic administration.

Iridium-based probe for luminescent nitric oxide monitoring in live cells

Nitric oxide (NO) is an intra- and extracellular messenger with important functions during human physiology process. A long-lived luminescent iridium(III) complex probe 1 has been designed and synthesized for the monitoring of NO controllably released from sodium nitroprusside (SNP). Probe 1 displayed a 15-fold switch-on luminescence in the presence of SNP at 580 nm. The probe exhibited a linear response towards SNP between 5 to 25 μM with detection limit at 0.18 μM. Importantly, the luminescent switch-on detection of NO in HeLa cells was demonstrated. Overall, complex 1 has the potential to be applied for NO tracing in complicated cellular environment.

A Ratiometric Acoustogenic Probe for in Vivo Imaging of Endogenous Nitric Oxide

Photoacoustic (PA) imaging is an emerging imaging modality that utilizes optical excitation and acoustic detection to enable high resolution at centimeter depths. The development of activatable PA probes can expand the utility of this technology to allow for detection of specific stimuli within live-animal models. Herein, we report the design, development, and evaluation of a series of Acoustogenic Probe(s) for Nitric Oxide (APNO) for the ratiometric, analyte-specific detection of nitric oxide (NO) in vivo. The best probe in the series, APNO-5, rapidly responds to NO to form an N-nitroso product with a concomitant 91 nm hypsochromic shift. This property enables ratiometric PA imaging upon selective irradiation of APNO-5 and the corresponding product, tAPNO-5. Moreover, APNO-5 displays the requisite photophysical characteristics for in vivo PA imaging (e.g., high absorptivity, low quantum yield) as well as high biocompatibility, stability, and selectivity for NO over a variety of biologically relevant analytes. APNO-5 was successfully applied to the detection of endogenous NO in a murine lipopolysaccharide-induced inflammation model. Our studies show a 1.9-fold increase in PA signal at 680 nm and a 1.3-fold ratiometric turn-on relative to a saline control.

A rhodamine-based turn-on nitric oxide sensor in aqueous medium with endogenous cell imaging: an unusual formation of nitrosohydroxylamine

The sensor L3 selectively recognizes NO in purely aqueous medium with an unusual formation of nitrosohydroxylamine with a turn-on fluorescence response which might be suitable for in vivo application.

A rhodamine-based fast and selective fluorescent probe for monitoring exogenous and endogenous nitric oxide in live cells

A sensitive and selective fluorescent probe for fast detection of nitric oxide was synthesized by grafting a NO-trapper o-phenylenediamine onto a rhodamine fluorophore.

AMPK-dependent nitric oxide release provides contractile support during hyperosmotic stress

In different pathological situations, cardiac cells undergo hyperosmotic stress (HS) and cell shrinkage. This change in cellular volume has been associated with contractile dysfunction and cell death. Given that nitric oxide (NO) is a well-recognized modulator of cardiac contractility and cell survival, we evaluated whether HS increases NO production and its impact on the negative inotropic effect observed during this type of stress. Superfusing cardiac myocytes with a hypertonic solution (HS: 440 mOsm) decreased cell volume and increased NO-sensitive DAF-FM fluorescence compared with myocytes superfused with an isotonic solution (IS: 309 mOsm). When cells were exposed to HS in addition to different inhibitors: L-NAME (NO synthase inhibitor), nitroguanidine (nNOS inhibitor), and Wortmannin (eNOS inhibitor) cell shrinkage occurred in the absence of NO release, suggesting that HS activates nNOS and eNOS. Consistently, western blot analysis demonstrated that maintaining cardiac myocytes in HS promotes phosphorylation and thus, activation of nNOS and eNOS compared to myocytes maintained in IS. HS-induced nNOS and eNOS activation and NO production were also prevented by AMPK inhibition with Dorsomorphin (DORSO). In addition, the HS-induced negative inotropic effect was exacerbated in the presence of either L-NAME, DORSO, ODQ (guanylate cyclase inhibitor), or KT5823 (PKG inhibitor), suggesting that NO provides contractile support via a cGMP/PKG-dependent mechanism. Our findings suggest a novel mechanism of AMPK-dependent NO release in cardiac myocytes with putative pathophysiological relevance determined, at least in part, by its capability to reduce the extent of contractile dysfunction associated with hyperosmotic stress.