Sensitive detection and estimation of cell-derived peroxynitrite fluxes using fluorescein-boronate

Phenyl Radical-Mediated Fluorogenic Cyclization for Specific Detection of Peroxynitrite

Peroxynitrite (ONOO-), a biological oxidizing and nitrating species responsible for post-translational modification of cellular proteins, has been implicated in numerous pathologies carrying an inflammatory component. Specific detection of ONOO- in biological systems remains a challenge, and boronates are regarded as the most promising class of probes for the detection and quantitation of ONOO-. Oxidation of boronate probes by ONOO- results in the formation of minor ONOO--specific products via a pathway involving a phenyl radical-type intermediate, in addition to the major phenolic product. Here, we report fluorogenic cyclization of the phenyl-type radical formed during oxidation of a boronate probe by ONOO-, with the production of a fluorescent product, and we propose a new approach for the specific detection of ONOO- based on this observation. We characterized the kinetics and stoichiometry of the reaction of benzophenone-2-boronic acid with ONOO- and identified 2-hydroxybenzophenone as the major product and fluorenone (FLN) and 2-nitrobenzophenone as the minor ONOO--specific products. Hydrogen peroxide neither alone nor in the presence of myeloperoxidase and nitrite produces FLN or 2-nitrobenzophenone. FLN can be selectively detected using fluorescence spectroscopy, providing a chemical principle for the development of next-generation probes for ONOO-, with noninvasive, fluorescence-based detection of ONOO--specific products. Fluorescence-based monitoring of FLN was successfully applied for the detection of ONOO- generated in situ from the decomposition of SIN-1, a thermal source of the superoxide radical anion and nitric oxide.

Profiling Proteins Involved in Peroxynitrite Homeostasis Using ROS/RNS Conditional Proteomics

Peroxynitrite (ONOO-), the product of the diffusion-controlled reaction of superoxide (O2•-) with nitric oxide (NO•), plays a crucial role in oxidative and nitrative stress and modulates key physiological processes such as redox signaling. While biological ONOO- is conventionally analyzed using 3-nitrotyrosine antibodies and fluorescent sensors, such probes lack specificity and sensitivity, making high-throughput and comprehensive profiling of ONOO--associated proteins challenging. In this study, we used a conditional proteomics approach to investigate ONOO- homeostasis by identifying its protein neighbors in cells. We developed Peroxynitrite-responsive protein Labeling reagents (Porp-L) and, for the first time, discovered 2,6-dichlorophenol as an ideal moiety that can be selectively and rapidly activated by ONOO- for labeling of proximal proteins. The reaction of Porp-L with ONOO- generated several short-lived reactive intermediates that can modify Tyr, His, and Lys residues on the protein surface. We have demonstrated the Porp-L-based conditional proteomics in immune-stimulated macrophages, which indeed identified proteins known to be involved in the generation and modification of ONOO- and revealed the endoplasmic reticulum (ER) as a ONOO- hot spot. Moreover, we discovered a previously unknown role for Ero1a, an ER-resident protein, in the formation of ONOO-. Overall, Porp-L represent a promising research tool for advancing our understanding of the biological roles of ONOO-.

Small molecule probes for peroxynitrite detection

Peroxynitrite (ONOO‒/ONOOH) is a short-lived but highly reactive species that is formed in the diffusion-controlled reaction between nitric oxide and the superoxide radical anion. It can oxidize certain biomolecules and has been considered as a key cellular oxidant formed under various pathophysiological conditions. It is crucial to selectively detect and quantify ONOO- to determine its role in biological processes. In this review, we discuss various approaches used to detect ONOO‒ in cell-free and cellular systems with the major emphasis on small-molecule chemical probes. We review the chemical principles and mechanisms responsible for the formation of the detectable products, and plausible limitations of the probes. We recommend the use of boronate-based chemical probes for ONOO‒, as they react directly and rapidly with ONOO-, they produce minor but ONOO‒‒specific products, and the reaction kinetics and mechanism have been rigorously characterized. Specific experimental approaches and protocols for the detection of ONOO- in cell-free, cellular, and in vivo systems using boronate-based molecular probes are provided (as shown in Boxes 1-6).

A host-directed adjuvant resuscitates and sensitizes intracellular bacterial persisters to antibiotics

There are two major problems in the field of antimicrobial chemotherapy-antibiotic resistance and antibiotic tolerance. In the case of antibiotic tolerance, antibiotics fail to kill the bacteria as their phenotypic state affords them protection from the bactericidal activity of the antibiotic. Antibiotic tolerance can affect an entire bacterial population, or a subset of cells known as persister cells. Interaction with the host induces the formation of persister cells in numerous pathogens, with reactive oxygen and nitrogen species production being heavily implicated in the collapse of bacterial energy levels and entrance into an antibiotic tolerant state. Here, we developed a high-throughput screen to identify energy modulators for intracellular Staphylococcus aureus . The identified compound, KL1 , increases intracellular bacterial energy and sensitizes the persister population to antibiotics, without causing cytotoxicity or bacterial outgrowth. We demonstrate that KL1 exhibits adjuvant activity in a murine model of S. aureus bacteremia and intracellular infection of Salmonella Typhimurium . Transcriptomic analysis and further studies on its mechanism of action reveal that KL1 modulates host immune response genes and suppresses the production of reactive species in host macrophages, alleviating one of the major stressors that induce antibiotic tolerance. Our findings highlight the potential to target intracellular persister cells by stimulating their metabolism and encourage larger efforts to address antibiotic tolerance at the host-pathogen interface, particularly within the intracellular milieu.

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

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

Mitochondrial Ca2+-coupled generation of reactive oxygen species, peroxynitrite formation, and endothelial dysfunction in Cantú syndrome

Cantú syndrome is a multisystem disorder caused by gain-of-function (GOF) mutations in KCNJ8 and ABCC9, the genes encoding the pore-forming inward rectifier Kir6.1 and regulatory sulfonylurea receptor SUR2B subunits, respectively, of vascular ATP-sensitive K+ (KATP) channels. In this study, we investigated changes in the vascular endothelium in mice in which Cantú syndrome-associated Kcnj8 or Abcc9 mutations were knocked in to the endogenous loci. We found that endothelium-dependent dilation was impaired in small mesenteric arteries from Cantú mice. Loss of endothelium-dependent vasodilation led to increased vasoconstriction in response to intraluminal pressure or treatment with the adrenergic receptor agonist phenylephrine. We also found that either KATP GOF or acute activation of KATP channels with pinacidil increased the amplitude and frequency of wave-like Ca2+ events generated in the endothelium in response to the vasodilator agonist carbachol. Increased cytosolic Ca2+ signaling activity in arterial endothelial cells from Cantú mice was associated with elevated mitochondrial [Ca2+] and enhanced reactive oxygen species (ROS) and peroxynitrite levels. Scavenging intracellular or mitochondrial ROS restored endothelium-dependent vasodilation in the arteries of mice with KATP GOF mutations. We conclude that mitochondrial Ca2+ overload and ROS generation, which subsequently leads to nitric oxide consumption and peroxynitrite formation, cause endothelial dysfunction in mice with Cantú syndrome.

Production of Peroxymonocarbonate by Steady-State Micromolar H2O2 and Activated Macrophages in the Presence of CO2/HCO3- Evidenced by Boronate Probes

Peroxymonocarbonate (HCO4-/HOOCO2-) is produced by the reversible reaction of CO2/HCO3- with H2O2 (K = 0.33 M-1, pH 7.0). Although produced in low yields at physiological pHs and H2O2 and CO2/HCO3- concentrations, HCO4- oxidizes most nucleophiles with rate constants 10 to 100 times higher than those of H2O2. Boronate probes are known examples because HCO4- reacts with coumarin-7-boronic acid pinacolate ester (CBE) with a rate constant that is approximately 100 times higher than that of H2O2 and the same holds for fluorescein-boronate (Fl-B) as reported here. Therefore, we tested whether boronate probes could provide evidence for HCO4- formation under biologically relevant conditions. Glucose/glucose oxidase/catalase were adjusted to produce low steady-state H2O2 concentrations (2-18 μM) in Pi buffer at pH 7.4 and 37 °C. Then, CBE (100 μM) was added and fluorescence increase was monitored with time. The results showed that each steady-state H2O2 concentration reacted more rapidly (∼30%) in the presence of CO2/HCO3- (25 mM) than in its absence, and the data permitted the calculation of consistent rate constants. Also, RAW 264.7 macrophages were activated with phorbol 12-myristate 13-acetate (PMA) (1 μg/mL) at pH 7.4 and 37 °C to produce a time-dependent H2O2 concentration (8.0 ± 2.5 μM after 60 min). The media contained 0, 21.6, or 42.2 mM HCO3- equilibrated with 0, 5, or 10% CO2, respectively. In the presence of CBE or Fl-B (30 μM), a time-dependent increase in the fluorescence of the bulk solution was observed, which was higher in the presence of CO2/HCO3- in a concentration-dependent manner. The Fl-B samples were also examined by fluorescence microscopy. Our results demonstrated that mammalian cells produce HCO4- and boronate probes can evidence and distinguish it from H2O2 under biologically relevant concentrations of H2O2 and CO2/HCO3-.

An AIE-based fluorescent probe to detect peroxynitrite levels in human serum and its cellular imaging

An AIE-based fluorescent probe was designed to evaluate peroxynitrite levels in complex biological samples. The newly synthesized hydrazone-conjugated probe fluoresces strongly in the presence of peroxynitrite. Clinically, the peroxynitrite levels can be measured in human serum and cellular mitochondria with an LOD of 6.5 nM by fluorescence imaging in vitro.

Evaluation of a novel pyridinium cation-linked styryl-based boronate probe for the detection of selected inflammation-related oxidants

Reactive oxygen and nitrogen species (RONS) are a range of chemical individuals produced by living cells that contribute to the proper functioning of organisms. Cells under oxidative and nitrative stress show excessive production of RONS (including hydrogen peroxide, H2O2, hypochlorous acid, HOCl, and peroxynitrite, ONOO-) which may result in a damage proteins, lipids, and genetic material. Thus, the development of probes for in vivo detection of such oxidants is an active area of research, focusing on molecular redox sensors, including boronate-caged fluorophores. Here, we report a boronate-based styryl probe with a cationic pyridinium moiety (BANEP+) for the fluorescent detection of selected biological oxidants in vitro and in vivo. We compare the chemical reactivity of the BANEP+ probe toward H2O2, HOCl, and ONOO- and examine the influence of the major intracellular non-enzymatic antioxidant molecule, glutathione (GSH). We demonstrate that, at the physiologically relevant GSH concentration, the BANEP+ probe is efficiently oxidized by peroxynitrite, forming its phenolic derivative HNEP+. GSH does not affect the fluorescence properties of the BANEP+ and HNEP+ dyes. Finally, we report the identification of a novel type of molecular marker, with the boronate moiety replaced by the iodine atom, formed from the probe in the presence of HOCl and iodide anion. We conclude that the reported chemical reactivity and structural features of the BANEP+ probe may be a basis for the development of new red fluorescent probes for in vitro and in vivo detection of ONOO-.

Superoxide, nitric oxide and peroxynitrite production by macrophages under different physiological oxygen tensions

Macrophages count on two O2-consuming enzymes to form reactive radical species: NAPDH oxidase 2 (Nox2) and nitric oxide synthase 2 (inducible isoform, iNOS) that produce superoxide radical (O2•-) and nitric oxide (•NO), respectively. If formed simultaneously, the diffusion-controlled reaction of O2•- and •NO yields peroxynitrite, a potent cytotoxic oxidant. In human tissues and cells, the oxygen partial pressure (pO2) normally ranges within 2-14 %, with a typical average pO2 value for most tissues ca. 5 %. Given that O2 is a substrate for both Nox2 and iNOS, its tissue and cellular concentration can affect O2•- and •NO production. Also, O2 is a modulator of the macrophage adaptative response and may influence iNOS expression in a hypoxia inducible factor 1-α (HIF1α-)-dependent manner. However, most of the reported experiments in cellula, analyzing the formation and effects of O2•- and •NO during macrophage activation and cytotoxicity towards pathogens, have been performed in cells exposed to atmospheric air supplemented with 5 % CO2; under these conditions, most cells are exposed to supraphysiologic oxygen tensions (ca. 20 % O2) which are far from the physiological pO2. Here, the role of O2 as substrate in the oxidative response of J774A.1 macrophages was explored upon exposure to different pO2 and O2•- and •NO formation rates were measured, obtaining a KM of 26 and 42 μM O2 for Nox2 and iNOS, respectively. Consequently, peroxynitrite formation was influenced by pO2, reaching a maximum at ≥ 10 % O2, but even at levels as low as 2 % O2, a substantial formation rate of this oxidant was detected. Indeed, the cytotoxic capacity of immunostimulated macrophages against the intracellular parasite T. cruzi was significant, even at low pO2 values, confirming the role of peroxynitrite as a potent oxidizing cytotoxin within a wide range of physiological oxygen tensions.

Inflammasome-mediated glucose limitation induces antibiotic tolerance in Staphylococcus aureus

Staphylococcus aureus is a leading human pathogen that frequently causes relapsing infections. The failure of antibiotics to eradicate infection contributes to infection relapse. Host-pathogen interactions have a substantial impact on antibiotic susceptibility and the formation of antibiotic tolerant cells. In this study, we interrogate how a major S. aureus virulence factor, α-toxin, interacts with macrophages to alter the microenvironment of the pathogen, thereby influencing its susceptibility to antibiotics. We find α-toxin-mediated activation of the NLRP3 inflammasome induces antibiotic tolerance. Induction of tolerance is driven by increased glycolysis in the host cells, resulting in glucose limitation and ATP depletion in S. aureus. Additionally, inhibition of NLRP3 activation improves antibiotic efficacy in vitro and in vivo, suggesting that this strategy has potential as a host-directed therapeutic to improve outcomes. Our findings identify interactions between S. aureus and the host that result in metabolic crosstalk that can determine the outcome of antimicrobial therapy.

Regulating the activity of boronate moiety to construct fluorescent probes for the detection of ONOO-in vitro and in vivo

Abnormal intracellular peroxynitrite (ONOO^-) concentration is related to oxidative damage, which is correlated with many pathological consequences, such as local inflammation and other diseases. In this work, a series of resorufin benzyl ether-based fluorescent probes were designed using boronate as a recognizing moiety installed on a phenyl moiety for ONOO^- detection via a self-immolation mechanism. The location of the boronate as well as the substitution patterns on the phenyl moiety were investigated and the responding behaviors of the designed probes to ONOO^-, other reactive oxygen species, and biothiols were examined. It was found that all the immolative probes were inevitably dominated by ONOO^-. Compared with other probes, p-Borate possessed favorable selectivity and high sensitivity to ONOO^-. Moreover, p-Borate was successfully used to detect ONOO^- in cells and inflamed mice.

The Chemistry of HNO: Mechanisms and Reaction Kinetics

Azanone (HNO, also known as nitroxyl) is the protonated form of the product of one-electron reduction of nitric oxide (^•NO), and an elusive electrophilic reactive nitrogen species of increasing pharmacological significance. Over the past 20 years, the interest in the biological chemistry of HNO has increased significantly due to the numerous beneficial pharmacological effects of its donors. Increased availability of various HNO donors was accompanied by great progress in the understanding of HNO chemistry and chemical biology. This review is focused on the chemistry of HNO, with emphasis on reaction kinetics and mechanisms in aqueous solutions.

Fluorescent probes for monitoring myeloperoxidase-derived hypochlorous acid: a comparative study

MPO-derived oxidants including HOCl contribute to tissue damage and the initiation and propagation of inflammatory diseases. The search for small molecule inhibitors of myeloperoxidase, as molecular tools and potential drugs, requires the application of high throughput screening assays based on monitoring the activity of myeloperoxidase. In this study, we have compared three classes of fluorescent probes for monitoring myeloperoxidase-derived hypochlorous acid, including boronate-, aminophenyl- and thiol-based fluorogenic probes and we show that all three classes of probes are suitable for this purpose. However, probes based on the coumarin fluorophore turned out to be not reliable indicators of the inhibitors’ potency. We have also determined the rate constants of the reaction between HOCl and the probes and they are equal to 1.8 × 10⁴ M⁻¹s⁻¹ for coumarin boronic acid (CBA), 1.1 × 10⁴ M⁻¹s⁻¹ for fluorescein based boronic acid (FLBA), 3.1 × 10⁴ M⁻¹s⁻¹ for 7-(p-aminophenyl)-coumarin (APC), 1.6 × 10⁴ M⁻¹s⁻¹ for 3’-(p-aminophenyl)-fluorescein (APF), and 1 × 10⁷ M⁻¹s⁻¹ for 4-thiomorpholino-7-nitrobenz-2-oxa-1,3-diazole (NBD-TM). The high reaction rate constant of NBD-TM with HOCl makes this probe the most reliable tool to monitor HOCl formation in the presence of compounds showing HOCl-scavenging activity.

Redox Monitoring in Nuclear Medical Imaging

Significance: The imbalance in redox homeostasis is known as oxidative stress, which is relevant to many diseases such as cancer, arteriosclerosis, and neurodegenerative disorders. Overproduction of reactive oxygen species (ROS) is one of the factors that trigger the redox state imbalance in vivo. The ROS have high reactivity and impair biomolecules, whereas antioxidants and antioxidant enzymes, such as ascorbate and glutathione, reduce the overproduction of ROS to rectify the redox imbalance. Owing to this, redox monitoring tools have been developed to understand the redox fluctuations in oxidative stress-related diseases. Recent Advances: In an attempt to monitor redox substances, including ROS and radical species, versatile modalities have been developed, such as electron spin resonance, chemiluminescence, and fluorescence. In particular, many fluorescent probes have been developed that are selective for ROS. This has significantly contributed to understanding the relevance of ROS in disease onset and progression. Critical Issues: To date, the dynamics of ROS and radical fluctuation in in vivo redox states remain unclear, and there are a few methods for the in vivo detection of redox fluctuations. Future Directions: In this review, we summarize the development of radiolabeled probes for monitoring redox-relevant species by nuclear medical imaging that is applicable in vivo. In the future, translational research is likely to be advanced through the development of highly sensitive and in vivo applicable detection methods, such as nuclear medical imaging, to clarify the underlying dynamics of ROS, radicals, and redox substances in many diseases. Antioxid. Redox Signal. 36, 797-810.

Water-soluble cationic boronate probe based on coumarin imidazolium scaffold: Synthesis, characterization, and application to cellular peroxynitrite detection

Peroxynitrite (ONOO^-) has been implicated in numerous pathologies associated with an inflammatory component, but its selective and sensitive detection in biological settings remains a challenge. Here, the development of a new water-soluble and cationic boronate probe based on a coumarin-imidazolium scaffold (CI-Bz-BA) for the fluorescent detection of ONOO^- in cells is reported. The chemical reactivity of the CI-Bz-BA probe toward selected oxidants known to react with the boronate moiety was characterized, and the suitability of the probe for the direct detection of ONOO^- in cell-free and cellular system is reported. Oxidation of the probe results in the formation of the primary hydroxybenzyl product (CI-Bz-OH), followed by the spontaneous elimination of the quinone methide moiety to produce the secondary phenol (CI-OH), which is accompanied by a red shift in the fluorescence emission band from 405 nm to 481 nm. CI-Bz-BA reacts with ONOO^- stoichiometrically with a rate constant of ∼1 × 10⁶ M^-1s^-1 to form, in addition to the major phenolic product CI-OH, the minor nitrated product CI-Bz-NO₂, which is not formed by other oxidants tested or via myeloperoxidase-catalyzed oxidation/nitration. Both CI-OH and CI-Bz-NO₂ products were also formed in the presence of cogenerated fluxes of nitric oxide and superoxide radical anion produced during decomposition of a SIN-1 donor. Using RAW 264.7 cells, we demonstrate the ability of the probe to report endogenously produced ONOO^-via fluorescence measurements, including plate reader real time monitoring and two-photon fluorescence imaging. Liquid chromatography/mass spectrometry analyses of cell extracts and media confirmed the formation of both CI-OH and CI-Bz-NO₂ in macrophages activated to produce ONOO^-. We propose the use of a combination of real-time monitoring of probe oxidation using fluorimetry and fluorescence microscopy with liquid chromatography/mass spectrometry-based product identification for rigorous detection and quantitative analyses of ONOO^- in biological systems.

A Fluorescent Probe Strategy for the Detection and Discrimination of Hydrogen Peroxide and Peroxynitrite in Cells

Aryl boronate fluorescent probes allow the non-invasive study of dynamic cellular processes involving the reactive species, hydrogen peroxide (H2O2) and peroxynitrite (ONOO-). However, the ability of these probes to differentiate...

Evaluation of borinic acids as new, fast hydrogen peroxide-responsive triggers

Hydrogen peroxide (H2O2) is responsible for numerous damages when overproduced, and its detection is crucial for a better understanding of H2O2-mediated signaling in physiological and pathological processes. For this purpose, various "off-on" small fluorescent probes relying on a boronate trigger have been prepared, and this design has also been involved in the development of H2O2-activated prodrugs or theranostic tools. However, this design suffers from slow kinetics, preventing activation by H2O2 with a short response time. Therefore, faster H2O2-reactive groups are awaited. To address this issue, we have successfully developed and characterized a prototypic borinic-based fluorescent probe containing a coumarin scaffold. We determined its in vitro kinetic constants toward H2O2-promoted oxidation. We measured 1.9 × 104 m-1⋅s-1 as a second-order rate constant, which is 10,000-fold faster than its well-established boronic counterpart (1.8 m-1⋅s-1). This improved reactivity was also effective in a cellular context, rendering borinic acids an advantageous trigger for H2O2-mediated release of effectors such as fluorescent moieties.

Kinetics of Azanone (HNO) Reactions with Thiols: Effect of pH

HNO (nitroxyl, IUPAC name azanone) is an electrophilic reactive nitrogen species of growing pharmacological and biological significance. Here, we present data on the pH-dependent kinetics of azanone reactions with the low molecular thiols glutathione and N-acetylcysteine, as well as with important serum proteins: bovine serum albumin and human serum albumin. The competition kinetics method used is based on two parallel HNO reactions: with RSH/RS- or with O2. The results provide evidence that the reaction of azanone with the anionic form of thiols (RS-) is favored over reactions with the protonated form (RSH). The data are supported with quantum mechanical calculations. A comprehensive discussion of the HNO reaction with thiolates is provided.

Kinetic Study on the Reactivity of Azanone (HNO) toward Cyclic C-Nucleophiles

Azanone (HNO) is an elusive electrophilic reactive nitrogen species of growing pharmacological and biological significance. Here, we present a comparative kinetic study of HNO reactivity toward selected cyclic C-nucleophiles under aqueous conditions at pH 7.4. We applied the competition kinetics method, which is based on the use of a fluorescein-derived boronate probe FlBA and two parallel HNO reactions: with the studied scavenger or with O₂ (k = 1.8 × 10⁴ M⁻¹s⁻¹). We determined the second-order rate constants of HNO reactions with 13 structurally diverse C-nucleophiles (k = 33–20,000 M⁻¹s⁻¹). The results show that the reactivity of HNO toward C-nucleophiles depends strongly on the structure of the scavenger. The data are supported with quantum mechanical calculations. A comprehensive discussion of the HNO reaction with C-nucleophiles is provided.

In-Cell Generation of Anticancer Phenanthridine Through Bioorthogonal Cyclization in Antitumor Prodrug Development

Pharmacological inactivation of antitumor drugs toward healthy cells is a critical factor in prodrug development. Typically, pharmaceutical chemists graft temporary moieties to existing antitumor drugs to reduce their pharmacological activity. Here, we report a platform able to generate the cytotoxic agent by intramolecular cyclization. Using phenanthridines as cytotoxic model compounds, we designed ring-opened biaryl precursors that generated the phenanthridines through bioorthogonal irreversible imination. This reaction was triggered by reactive oxygen species, commonly overproduced in cancer cells, able to convert a vinyl boronate ester function into a ketone that subsequently reacted with a pendant aniline. An inactive precursor was shown to engender a cytotoxic phenanthridine against KB cancer cells. Moreover, the kinetic of cyclization of this prodrug was extremely rapid inside living cells of KB cancer spheroids so as to circumvent drug action.

The Labile Iron Pool Reacts Rapidly and Catalytically with Peroxynitrite

While investigating peroxynitrite-dependent oxidation in murine RAW 264.7 macrophage cells, we observed that removal of the Labile Iron Pool (LIP) by chelation increases the intracellular oxidation of the fluorescent indicator H2DCF, so we concluded that the LIP reacts with peroxynitrite and decreases the yield of peroxynitrite-derived oxidants. This was a paradigm-shifting finding in LIP biochemistry and raised many questions. In this follow-up study, we address fundamental properties of the interaction between the LIP and peroxynitrite by using the same cellular model and fluorescence methodology. We have identified that the reaction between the LIP and peroxynitrite has catalytic characteristics, and we have estimated that the rate constant of the reaction is in the range of 106 to 107 M-1s-1. Together, these observations suggest that the LIP represents a constitutive peroxynitrite reductase system in RAW 264.7 cells.

Nox2-derived superoxide radical is crucial to control acute Trypanosoma cruzi infection

Trypanosoma cruzi is a flagellated protozoan that undergoes a complex life cycle between hematophagous insects and mammals. In humans, this parasite causes Chagas disease, which in thirty percent of those infected, would result in serious chronic pathologies and even death. Macrophages participate in the first stages of infection, mounting a cytotoxic response which promotes massive oxidative damage to the parasite. On the other hand, T. cruzi is equipped with a robust antioxidant system to repeal the oxidative attack from macrophages. This work was conceived to explicitly assess the role of mammalian cell-derived superoxide radical in a murine model of acute infection by T. cruzi. Macrophages derived from Nox2-deficient (gp91^phox-/-) mice produced marginal amounts of superoxide radical and were more susceptible to parasite infection than those derived from wild type (wt) animals. Also, the lack of superoxide radical led to an impairment of parasite differentiation inside gp91^phox-/- macrophages. Biochemical or genetic reconstitution of intraphagosomal superoxide radical formation in gp91^phox-/- macrophages reverted the lack of control of infection. Along the same line, gp91^phox-/- infected mice died shortly after infection. In spite of the higher lethality, parasitemia did not differ between gp91^phox-/- and wt animals, recapitulating an observation that has led to conflicting interpretations about the importance of the mammalian oxidative response against T. cruzi. Importantly, gp91^phox-/- mice presented higher and disseminated tissue parasitism, as evaluated by both qPCR- and bioimaging-based methodologies. Thus, this work supports that Nox2-derived superoxide radical plays a crucial role to control T. cruzi infection in the early phase of a murine model of Chagas disease.

∙

Nox2 derived-superoxide radical is required to control Trypanosoma cruzi infection in macrophages

∙Nox2-deficient mice (gp91^phox-/-) are highly susceptible to Trypanosoma cruzi infection

∙Parasitemia does not reflect the level of organ infection observed in wt and gp91^phox-/- mice.

∙gp91^phox-/- mice collapse to infection due to uncontrolled parasite proliferation in tissues

The mitochondrial thioredoxin reductase system (TrxR2) in vascular endothelium controls peroxynitrite levels and tissue integrity

The mitochondrial thioredoxin/peroxiredoxin system encompasses NADPH, thioredoxin reductase 2 (TrxR2), thioredoxin 2, and peroxiredoxins 3 and 5 (Prx3 and Prx5) and is crucial to regulate cell redox homeostasis via the efficient catabolism of peroxides (TrxR2 and Trxrd2 refer to the mitochondrial thioredoxin reductase protein and gene, respectively). Here, we report that endothelial TrxR2 controls both the steady-state concentration of peroxynitrite, the product of the reaction of superoxide radical and nitric oxide, and the integrity of the vascular system. Mice with endothelial deletion of the Trxrd2 gene develop increased vascular stiffness and hypertrophy of the vascular wall. Furthermore, they suffer from renal abnormalities, including thickening of the Bowman's capsule, glomerulosclerosis, and functional alterations. Mechanistically, we show that loss of Trxrd2 results in enhanced peroxynitrite steady-state levels in both vascular endothelial cells and vessels by using a highly sensitive redox probe, fluorescein-boronate. High steady-state peroxynitrite levels were further found to coincide with elevated protein tyrosine nitration in renal tissue and a substantial change of the redox state of Prx3 toward the oxidized protein, even though glutaredoxin 2 (Grx2) expression increased in parallel. Additional studies using a mitochondria-specific fluorescence probe (MitoPY1) in vessels revealed that enhanced peroxynitrite levels are indeed generated in mitochondria. Treatment with Mn(III)tetrakis(1-methyl-4-pyridyl)porphyrin [Mn(III)TMPyP], a peroxynitrite-decomposition catalyst, blunted intravascular formation of peroxynitrite. Our data provide compelling evidence for a yet-unrecognized role of TrxR2 in balancing the nitric oxide/peroxynitrite ratio in endothelial cells in vivo and thus establish a link between enhanced mitochondrial peroxynitrite and disruption of vascular integrity.

Two-photon fluorescent probe for cellular peroxynitrite: Fluorescence detection, imaging, and identification of peroxynitrite-specific products

A new naphthalene-based boronate probe, NAB-BE, for the fluorescence-based detection of inflammatory oxidants, including peroxynitrite, hypochlorous acid, and hydrogen peroxide, is reported. The chemical reactivity and fluorescence properties of the probe and the products are described. The major, phenolic oxidation product, NAB-OH, is formed in case of all three oxidants tested. This product shows green fluorescence, with a maximum at 512 nm, and can be excited either at 340 nm or in the near infrared region (745 nm) for two-photon fluorescence imaging. Peroxynitrite is the fastest of the oxidants tested and, in addition to the phenolic product, leads to the formation of a nitrated product, NAB-NO₂, which can serve as a fingerprint for peroxynitrite. The probe was applied to detect peroxynitrite in activated macrophages using fluorimetry and two-photon fluorescence microscopy, and both NAB-OH and NAB-NO₂ products were detected in cell extracts by liquid chromatography-mass spectrometry. The combined use of fluorometric high-throughput analyses, fluorescence imaging, and liquid chromatography-mass spectrometry-based product identification and quantitation is proposed for most comprehensive and rigorous characterization of oxidants in biological systems.

Carnosine Protects Macrophages against the Toxicity of Aβ1-42 Oligomers by Decreasing Oxidative Stress

Carnosine (β-alanyl-L-histidine) is a naturally occurring endogenous peptide widely distributed in excitable tissues such as the brain. This dipeptide has well-known antioxidant, anti-inflammatory, and anti-aggregation activities, and it may be useful for treatment of neurodegenerative disorders such as Alzheimer’s disease (AD). In this disease, peripheral infiltrating macrophages play a substantial role in the clearance of amyloid beta (Aβ) peptides from the brain. Correspondingly, in patients suffering from AD, defects in the capacity of peripheral macrophages to engulf Aβ have been reported. The effects of carnosine on macrophages and oxidative stress associated with AD are consequently of substantial interest for drug discovery in this field. In the present work, a model of stress induced by Aβ1-42 oligomers was investigated using a combination of methods including trypan blue exclusion, microchip electrophoresis with laser-induced fluorescence, flow cytometry, fluorescence microscopy, and high-throughput quantitative real-time PCR. These assays were used to assess the ability of carnosine to protect macrophage cells, modulate oxidative stress, and profile the expression of genes related to inflammation and pro- and antioxidant systems. We found that pre-treatment of RAW 264.7 macrophages with carnosine counteracted cell death and apoptosis induced by Aβ1-42 oligomers by decreasing oxidative stress as measured by levels of intracellular nitric oxide (NO)/reactive oxygen species (ROS) and production of peroxynitrite. This protective activity of carnosine was not mediated by modulation of the canonical inflammatory pathway but instead can be explained by the well-known antioxidant and free-radical scavenging activities of carnosine, enhanced macrophage phagocytic activity, and the rescue of fractalkine receptor CX3CR1. These new findings obtained with macrophages challenged with Aβ1-42 oligomers, along with the well-known multimodal mechanism of action of carnosine in vitro and in vivo, substantiate the therapeutic potential of this dipeptide in the context of AD pathology.

A generator of peroxynitrite activatable with red light

The generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS) as “unconventional” therapeutics with precise spatiotemporal control by using light stimuli may open entirely new horizons for innovative therapeutic modalities. Among ROS and RNS, peroxynitrite (ONOO⁻) plays a dominant role in chemistry and biology in view of its potent oxidizing power and cytotoxic action. We have designed and synthesized a molecular hybrid based on benzophenothiazine as a red light-harvesting antenna joined to an N-nitroso appendage through a flexible spacer. Single photon red light excitation of this molecular construct triggers the release of nitric oxide (˙NO) and simultaneously produces superoxide anions (O₂˙⁻). The diffusion-controlled reaction between these two radical species generates ONOO⁻, as confirmed by the use of fluorescein-boronate as a highly selective chemical probe. Besides, the red fluorescence of the hybrid allows its tracking in different types of cancer cells where it is well-tolerated in the dark but induces remarkable cell mortality under irradiation with red light in a very low concentration range, with very low light doses (ca. 1 J cm⁻²). This ONOO⁻ generator activatable by highly biocompatible and tissue penetrating single photon red light can open up intriguing prospects in biomedical research, where precise and spatiotemporally controlled concentrations of ONOO⁻ are required.

Excitation of a molecular hybrid with highly biocompatible red light generates cytotoxic peroxynitrite, produces red fluorescence useful for cell tracking and induces remarkable cancer cell death at very low concentrations and very low light doses.

The recent development of fluorescent probes for the detection of NADH and NADPH in living cells and in vivo

Reduced nicotinamide adenine dinucleotide (NADH) and its phosphate ester (NADPH) participate in numerous metabolic processes in living cells as electron carriers. The levels of NADH and NADPH in a cell are closely related to its metabolic and pathological state. It is important to monitor the levels of NADH and NADPH in living cells and in vivo in real-time. This review mainly focuses on fluorescent probes developed for monitoring NADH and NADPH in living cells and in vivo, and classifies them according to the recognition units. These fluorescence probes can rapidly respond to changes in NADH and NADPH levels without interference from other biomolecules, both in cell culture and in vivo. These probes have been employed to monitor NADH and NADPH levels in living cells, tumor spheroids, and in vivo; moreover, some of them can be used to discriminate normal cells from cancer cells, and detect cancer cell death due to reductive stress induced by natural antioxidants. This review is expected to inspire the generation of novel fluorescent probes for the detection of NADH and NADPH, and stimulate more attention in the development of fluorescent probes based on carbon dots and nanoparticles, as well as metal complex-based, time-gated luminescent probes for monitoring NADH and NADPH in both living cells and in vivo.

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

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

Fluorescent detection of peroxynitrite during antibody-dependent cellular phagocytosis

Peroxynitrite (PNT) is a highly reactive oxidant that plays a key role in the destruction of foreign pathogens by specific phagocytic immune cells such as macrophages. However, when its production is dysregulated, this oxidant can contribute to cardiovascular disease, neurological diseases, and cancer. To facilitate the detection of PNT in living cells, we designed and synthesized a fluorescent sensor termed PS3 that accumulates in membranes of the endoplasmic reticulum (ER). This subcellular targeting enhances the proximity of PS3 to the phagosome of macrophages where PNT is generated. When PS3-treated macrophages are stimulated with 10 µm opsonized tentagel microspheres, antibody-dependent cellular phagocytosis (ADCP) of these particles results in production of endogenous PNT, oxidative cleavage of the fluorescence-quenching phenolic side chain of PS3, and increased fluorescence that can be detected by confocal laser scanning microscopy, flow cytometry, and other assays. We describe methods for the synthesis of PS3 and evaluation of its photophysical properties, selectivity, and reactivity. We further report differential production of PNT during ADCP by the phagocytic cell lines RAW 264.7, J774A.1, and THP-1, as detected by confocal microscopy and changes in fluorescence intensity on 96-well plates. This approach may be useful for identification of modulators of PNT and related studies of ADCP.

Tracking isotopically labeled oxidants using boronate-based redox probes

Reactive oxygen and nitrogen species have been implicated in many biological processes and diseases, including immune responses, cardiovascular dysfunction, neurodegeneration, and cancer. These chemical species are short-lived in biological settings, and detecting them in these conditions and diseases requires the use of molecular probes that form stable, easily detectable, products. The chemical mechanisms and limitations of many of the currently used probes are not well-understood, hampering their effective applications. Boronates have emerged as a class of probes for the detection of nucleophilic two-electron oxidants. Here, we report the results of an oxygen-18-labeling MS study to identify the origin of oxygen atoms in the oxidation products of phenylboronate targeted to mitochondria. We demonstrate that boronate oxidation by hydrogen peroxide, peroxymonocarbonate, hypochlorite, or peroxynitrite involves the incorporation of oxygen atoms from these oxidants. We therefore conclude that boronates can be used as probes to track isotopically labeled oxidants. This suggests that the detection of specific products formed from these redox probes could enable precise identification of oxidants formed in biological systems. We discuss the implications of these results for understanding the mechanism of conversion of the boronate-based redox probes to oxidant-specific products.

Hydrogen sulfide mediated alleviation of cadmium toxicity in Phlox paniculata L. and establishment of a comprehensive evaluation model for corresponding strategy

A laboratory experiment was performed to evaluate the potential role of H₂S on cadmium (Cd) toxicity in Phlox paniculata L. Seeds pretreated with 0.3, 0.6, 0.9, and 1.2 mM NaHS as a donor of H₂S for 24 h and subsequently exposed to 100, 200, and 300 μM CdCl₂ for 26 days had significantly higher germination rate compared with Cd alone. Meanwhile, 2-year-old seedlings sprayed with 0.3, 0.6, and 0.9 μM NaHS were grown in soil with 0.3, 0.6, and 1.2 mg/kg CdCl₂, respectively. We observed that H₂S decreased Cd accumulation in leaves and elevated Cd concentration in roots. Cd toxicity in seedlings resulted in a substantial increase in Cd-induced overproduction of malondialdehyde (MDA), Cd accumulation, and electrolyte leakage. Meanwhile, addition of NaHS increased photosynthetic performance compared with Cd alone. Exogenous H₂S significantly elevated biomass, improved antioxidant enzyme activities, and reduced ABA content compared with Cd alone. H₂S also plays an important role in the ABA signaling pathway during stress. Notably, NaHS promoted Cd uptake by Phlox paniculate L. from soil. The prediction model of H₂S for increasing plant resistance and reducing soil Cd pollution was established by factor analysis method based on comprehensive evaluation of plant stress physiology.

Fatty acid nitration in human low-density lipoprotein

Lipid nitration occurs during physiological and pathophysiological conditions, generating a variety of biomolecules capable to modulate inflammatory cell responses. Low-density lipoprotein (LDL) oxidation has been extensively related to atherosclerotic lesion development while oxidative modifications confer the particle pro-atherogenic features. Herein, we reviewed the oxidation versus nitration of human LDL protein and lipid fractions. We propose that unsaturated fatty acids present in LDL can be nitrated under mild nitration conditions, suggesting an anti-atherogenic role for LDL carrying nitro-fatty acids (NFA).

Detection and quantification of nitric oxide-derived oxidants in biological systems

The free radical nitric oxide (NO•) exerts biological effects through the direct and reversible interaction with specific targets (e.g. soluble guanylate cyclase) or through the generation of secondary species, many of which can oxidize, nitrosate or nitrate biomolecules. The NO•-derived reactive species are typically short-lived, and their preferential fates depend on kinetic and compartmentalization aspects. Their detection and quantification are technically challenging. In general, the strategies employed are based either on the detection of relatively stable end products or on the use of synthetic probes, and they are not always selective for a particular species. In this study, we describe the biologically relevant characteristics of the reactive species formed downstream from NO•, and we discuss the approaches currently available for the analysis of NO•, nitrogen dioxide (NO2•), dinitrogen trioxide (N2O3), nitroxyl (HNO), and peroxynitrite (ONOO-/ONOOH), as well as peroxynitrite-derived hydroxyl (HO•) and carbonate anion (CO3•-) radicals. We also discuss the biological origins of and analytical tools for detecting nitrite (NO2-), nitrate (NO3-), nitrosyl-metal complexes, S-nitrosothiols, and 3-nitrotyrosine. Moreover, we highlight state-of-the-art methods, alert readers to caveats of widely used techniques, and encourage retirement of approaches that have been supplanted by more reliable and selective tools for detecting and measuring NO•-derived oxidants. We emphasize that the use of appropriate analytical methods needs to be strongly grounded in a chemical and biochemical understanding of the species and mechanistic pathways involved.

Small-Molecule-Based Fluorescent Sensors for Selective Detection of Reactive Oxygen Species in Biological Systems

Reactive oxygen species (ROS) encompass a collection of intricately linked chemical entities characterized by individually distinct physicochemical properties and biological reactivities. Although excessive ROS generation is well known to underpin disease development, it has become increasingly evident that ROS also play central roles in redox regulation and normal physiology. A major challenge in uncovering the relevant biological mechanisms and deconvoluting the apparently paradoxical roles of distinct ROS in human health and disease lies in the selective and sensitive detection of these transient species in the complex biological milieu. Small-molecule-based fluorescent sensors enable molecular imaging of ROS with great spatial and temporal resolution and have thus been appreciated as excellent tools for aiding discoveries in modern redox biology. We review a selection of state-of-the-art sensors with demonstrated utility in biological systems. By providing a systematic overview based on underlying chemical sensing mechanisms, we wish to highlight the strengths and weaknesses in prior sensor works and propose some guiding principles for the development of future probes.

Cytosolic Fe-superoxide dismutase safeguards Trypanosoma cruzi from macrophage-derived superoxide radical

Trypanosoma cruzi, the causative agent of Chagas disease (CD), contains exclusively Fe-dependent superoxide dismutases (Fe-SODs). During T. cruzi invasion to macrophages, superoxide radical (O₂ ^•-) is produced at the phagosomal compartment toward the internalized parasite via NOX-2 (gp91-phox) activation. In this work, T. cruzi cytosolic Fe-SODB overexpressers (pRIBOTEX-Fe-SODB) exhibited higher resistance to macrophage-dependent killing and enhanced intracellular proliferation compared with wild-type (WT) parasites. The higher infectivity of Fe-SODB overexpressers compared with WT parasites was lost in gp91-phox ^-/- macrophages, underscoring the role of O₂ ^•- in parasite killing. Herein, we studied the entrance of O₂ ^•- and its protonated form, perhydroxyl radical [(HO₂ ^•); pK_a = 4.8], to T. cruzi at the phagosome compartment. At the acidic pH values of the phagosome lumen (pH 5.3 ± 0.1), high steady-state concentrations of O₂ ^•- and HO₂ ^• were estimated (∼28 and 8 µM, respectively). Phagosomal acidification was crucial for O₂ ^•- permeation, because inhibition of the macrophage H⁺-ATPase proton pump significantly decreased O₂ ^•- detection in the internalized parasite. Importantly, O₂ ^•- detection, aconitase inactivation, and peroxynitrite generation were lower in Fe-SODB than in WT parasites exposed to external fluxes of O₂ ^•- or during macrophage infections. Other mechanisms of O₂ ^•- entrance participate at neutral pH values, because the anion channel inhibitor 5-nitro-2-(3-phenylpropylamino) benzoic acid decreased O₂ ^•- detection. Finally, parasitemia and tissue parasite burden in mice were higher in Fe-SODB-overexpressing parasites, supporting the role of the cytosolic O₂ ^•--catabolizing enzyme as a virulence factor for CD.

Reactive species and pathogen antioxidant networks during phagocytosis

The generation of phagosomal cytotoxic reactive species (i.e., free radicals and oxidants) by activated macrophages and neutrophils is a crucial process for the control of intracellular pathogens. The chemical nature of these species, the reactions they are involved in, and the subsequent effects are multifaceted and depend on several host- and pathogen-derived factors that influence their production rates and catabolism inside the phagosome. Pathogens rely on an intricate and synergistic antioxidant armamentarium that ensures their own survival by detoxifying reactive species. In this review, we discuss the generation, kinetics, and toxicity of reactive species generated in phagocytes, with a focus on the response of macrophages to internalized pathogens and concentrating on Mycobacterium tuberculosis and Trypanosoma cruzi as examples of bacterial and parasitic infection, respectively. The ability of pathogens to deal with host-derived reactive species largely depends on the competence of their antioxidant networks at the onset of invasion, which in turn can tilt the balance toward pathogen survival, proliferation, and virulence over redox-dependent control of infection.

Fluorescence and chemiluminescence approaches for peroxynitrite detection

In the last two decades, there has been a significant advance in understanding the biochemistry of peroxynitrite, an endogenously-produced oxidant and nucleophile. Its relevance as a mediator in several pathologic states and the aging process together with its transient character and low steady-state concentration, motivated the development of a variety of techniques for its unambiguous detection and estimation. Among these, fluorescence and chemiluminescence approaches have represented important tools with enhanced sensitivity but usual limited specificity. In this review, we analyze selected examples of molecular probes that permit the detection of peroxynitrite by fluorescence and chemiluminescence, disclosing their mechanism of reaction with either peroxynitrite or peroxynitrite-derived radicals. Indeed, probes have been divided into 1) redox probes that yield products by a free radical mechanism, and 2) electrophilic probes that evolve to products secondary to the nucleophilic attack by peroxynitrite. Overall, boronate-based compounds are emerging as preferred probes for the sensitive and specific detection and quantitation. Moreover, novel strategies involving genetically-modified fluorescent proteins with the incorporation of unnatural amino acids have been recently described as peroxynitrite sensors. This review analyzes the most commonly used fluorescence and chemiluminescence approaches for peroxynitrite detection and provides some guidelines for appropriate experimental design and data interpretation, including how to estimate peroxynitrite formation rates in cells.

Small-molecule luminescent probes for the detection of cellular oxidizing and nitrating species

Reactive oxygen species (ROS) have been implicated in both pathogenic cellular damage events and physiological cellular redox signaling and regulation. To unravel the biological role of ROS, it is very important to be able to detect and identify the species involved. In this review, we introduce the reader to the methods of detection of ROS using luminescent (fluorescent, chemiluminescent, and bioluminescent) probes and discuss typical limitations of those probes. We review the most widely used probes, state-of-the-art assays, and the new, promising approaches for rigorous detection and identification of superoxide radical anion, hydrogen peroxide, and peroxynitrite. The combination of real-time monitoring of the dynamics of ROS in cells and the identification of the specific products formed from the probes will reveal the role of specific types of ROS in cellular function and dysfunction. Understanding the molecular mechanisms involving ROS may help with the development of new therapeutics for several diseases involving dysregulated cellular redox status.