Recent advances in fluorescent probes for the detection of reactive oxygen species

Fluorescent probes for bioimaging applications

Fluorescent probes based on small organic molecules have become indispensable tools in modern biology because they provide dynamic information concerning the localization and quantity of the molecules of interest, without the need of genetic engineering of the sample. In this review, following a brief outline of the principle of fluorescence imaging, we recount some recent achievements in the field of small-molecular fluorescent probes. First, probes for metal cations, including those suitable for two-photon imaging, are introduced. Next, methodologies to visualize proteases are discussed, with special emphasis on activity-based probes for use in vivo. All these probes have been confirmed to be applicable to cellular or in vivo imaging.

Oxidative stress as a mediator of life history trade-offs: mechanisms, measurements and interpretation

The concept of trade-offs is central to our understanding of life-history evolution. The underlying mechanisms, however, have been little studied. Oxidative stress results from a mismatch between the production of damaging reactive oxygen species (ROS) and the organism's capacity to mitigate their damaging effects. Managing oxidative stress is likely to be a major determinant of life histories, as virtually all activities generate ROS. There is a recent burgeoning of interest in how oxidative stress is related to different components of animal performance. The emphasis to date has been on immediate or short-term effects, but there is an increasing realization that oxidative stress will influence life histories over longer time scales. The concept of oxidative stress is currently used somewhat loosely by many ecologists, and the erroneous assumption often made that dietary antioxidants are necessarily the major line of defence against ROS-induced damage. We summarize current knowledge on how oxidative stress occurs and the different methods for measuring it, and highlight where ecologists can be too simplistic in their approach. We critically review the potential role of oxidative stress in mediating life-history trade-offs, and present a framework for formulating appropriate hypotheses and guiding experimental design. We indicate throughout potentially fruitful areas for further research.

Biomarkers of oxidative stress and damage in human populations exposed to arsenic

Arsenic (As) is an ubiquitous element in the environment for which the main route of human exposure is through consumption of drinking water. Reactive oxygen species generation (ROS) associated with As exposure is known to play a fundamental role in the induction of adverse health effects and disease (cancer, diabetes, hypertension, and cardiovascular and neurological diseases). However, the precise mechanisms of oxidative stress and damage from As exposure are not fully understood and moreover the use of non-invasive methods of measuring ROS generation and oxidative damage footprints in humans is no easy task. Although As induces adverse health effects not all exposed individuals develop degenerative chronic diseases or even manifest adverse effects or symptoms, suggesting that genetic susceptibility is an important factor involved in the human response to As exposure. This mini-review summarizes the literature describing the molecular mechanisms affected by As, as well as the most used biomarkers of oxidative stress and damage in human populations. The most reported biomarkers of oxidative DNA damage are the urinary excretion of 8-OHdG and the comet assay in lymphocytes, and more recently DNA repair mechanism markers from the base and nuclear excision repair pathways (BER and NER). Genetic heterogeneity in the oxidative stress pathways involved in As metabolism are important causative factors of disease. Thus further refinement of human exposure assessment is needed to reinforce study design to evaluate exposure-response relationships and study gene-environment interactions. The use of microarray-based gene expression analysis can provide better insights of the underlying mechanisms involved in As-induced diseases and could help to identify target genes that can be modulated to prevent disease.

A red-emitting naphthofluorescein-based fluorescent probe for selective detection of hydrogen peroxide in living cells

We report the synthesis, properties, and cellular application of Naphtho-Peroxyfluor-1 (NPF1), a new fluorescent indicator for hydrogen peroxide based on a red-emitting naphthofluorescein platform. Owing to its boronate cages, NPF1 features high selectivity for hydrogen peroxide over a panel of biologically relevant reactive oxygen species (ROS), including superoxide and nitric oxide, as well as excitation and emission profiles in the far-red region of the visible spectrum (>600nm). Flow cytometry experiments in RAW264.7 macrophages establish that NPF1 can report changes in peroxide levels in living cells.

Rational Design of a Fluorescent Hydrogen Peroxide Probe Based on the Umbelliferone Fluorophore

In this study, we report a novel water-soluble umbelliferone-based fluorescent probe for hydrogen peroxide. This probe shows very large increases (up to 100 fold) in fluorescent intensity upon reaction with hydrogen peroxide, and good selectivity over other reactive oxygen species (ROS).

Determination of immunomodulatory effects: focus on functional analysis of phagocytes as representatives of the innate immune system

The evaluation of the effects of drugs or chemicals on the functions of the immune system is an increasingly important task. Due to the accessibility of primary cells and cell lines, in vitro cellular functional tests are frequently being performed with cells representing the innate immune system, in particular those with phagocytotic activities, such as neutrophils and macrophages. Suitable functional parameters are the efficiency of phagocytosis, the efficiency with which viable pathogens are killed, the production of reactive oxygen and nitrogen species (ROS and RNS) and that of cytokines. Corresponding analytical procedures are available, but standardization is required, as varying the procedure may influence the outcomes of the assays.

Singlet oxygen reacts with 2',7'-dichlorodihydrofluorescein and contributes to the formation of 2',7'-dichlorofluorescein

There are controversial reports in the literature concerning the reactivity of singlet oxygen ((1)O(2)) with the redox probe 2',7'-dichlorodihydrofluorescein (DCFH). By carefully preparing solutions in which (1)O(2) is quantitatively generated in the presence of DCFH, we were able to show that the formation rate of the fluorescent molecule derived from DCFH oxidation, which is 2',7'-dichlorofluorescein (DCF), increases in D(2)O and decreases in sodium azide, proving the direct role of (1)O(2) in this process. We have also prepared solutions in which either (1)O(2) or dication (MB(2+)) and semi-reduced (MB) radicals of the sensitizer and subsequently super-oxide radical (O(2)(-)) are generated. The absence of any effect of SOD and catalase ruled out the DCFH oxidation by O(2)(-), indicating that both (1)O(2) and MB(2+) react with DCFH. Although the formation of DCF was 1 order of magnitude larger in the presence of MB(2+) than in the presence of (1)O(2), considering the rate of spontaneous decays of these species in aqueous solution, we were able to conclude that the reactivity of (1)O(2) with DCFH is actually larger than that of MB(2+). We conclude that DCFH can continue to be used as a probe to monitor general redox misbalance induced in biologic systems by oxidizing radicals and (1)O(2).

Comparison of fluorescence-based techniques for the quantification of particle-induced hydroxyl radicals

BACKGROUND

Reactive oxygen species including hydroxyl radicals can cause oxidative stress and mutations. Inhaled particulate matter can trigger formation of hydroxyl radicals, which have been implicated as one of the causes of particulate-induced lung disease. The extreme reactivity of hydroxyl radicals presents challenges to their detection and quantification. Here, three fluorescein derivatives [aminophenyl fluorescamine (APF), amplex ultrared, and dichlorofluorescein (DCFH)] and two radical species, proxyl fluorescamine and tempo-9-ac have been compared for their usefulness to measure hydroxyl radicals generated in two different systems: a solution containing ferrous iron and a suspension of pyrite particles.

RESULTS

APF, amplex ultrared, and DCFH react similarly to the presence of hydroxyl radicals. Proxyl fluorescamine and tempo-9-ac do not react with hydroxyl radicals directly, which reduces their sensitivity. Since both DCFH and amplex ultrared will react with reactive oxygen species other than hydroxyl radicals and another highly reactive species, peroxynitite, they lack specificity.

CONCLUSION

The most useful probe evaluated here for hydroxyl radicals formed from cell-free particle suspensions is APF due to its sensitivity and selectivity.

Mitochondrial production of reactive oxygen species: role of complex I and quinone analogues

Mitochondrial reactive oxygen species (ROS) are mainly produced by the respiratory chain enzymes. The sites for ROS production in mitochondrial respiratory chain are normally ascribed to the activity of Complex I and III. The presence of specific inhibitors modulates reactive oxygen species production in Complex I: inhibitors such as rotenone induce a strong ROS increase, while inhibitors such as stigmatellin prevent it. We have investigated the effect of hydrophilic quinones on Complex I ROS production in presence of different inhibitors. Some short chain quinones are Complex I inhibitors (CoQ2, idebenone and its derivatives), while CoQ1, decylubiquinone~ (DB) and duroquinone (DQ) are good electron acceptors from Complex I. Our results show that the ability of short chain quinones to induce an oxidative stress depends on the site of interaction with Complex I and on their physical-chemical characteristics. We can conclude that hydrophilic quinones may enhance oxidative stress by interaction with the electron escape sites on Complex I while more hydrophobic quinones can be reduced only at the physiological quinone reducing site without reacting with molecular oxygen.

The first application of terephthalate fluorescence for highly selective detection of hydroxyl radicals in thylakoid membranes

Possibilities and limitations of the detection of hydroxyl radicals via the conversion of terephthalate (TPA) into the strongly fluorescent hydroxyterephthalate were investigated in order to adapt this method for chlorophyll-containing samples. Using model chemical sources of various reactive oxygen species, we confirmed that TPA detects hydroxyl radicals very sensitively, but is not reactive to either hydrogen peroxide or superoxide radicals. As a new result, we showed that the conversion of TPA to hydroxyterephthalate cannot be induced by singlet oxygen, which may be produced in photosynthetic systems under stress. Until now, the TPA method has not been used in photosynthesis research, so necessary adaptations to minimise the effects of chlorophyll and buffering sugars on hydroxyl radical detection were also explored and optimal conditions for using the method in thylakoid preparations are suggested. Anticipating further plant physiology applications, usefulness of the TPA method was tested in a wider range of pH than reported earlier. To demonstrate that this simple and highly specific method can be used as an alternative approach for the detection of hydroxyl radicals in plant samples, we measured these radicals in isolated thylakoid membranes exposed to 312 nm ultraviolet radiation.

Simultaneous detection of apoptosis and mitochondrial superoxide production in live cells by flow cytometry and confocal microscopy

Annexin V and Sytox Green are widely used markers to evaluate apoptosis in various cell types using flow cytometry and fluorescent microscopy. Recently, a novel fluoroprobe MitoSOX Red was introduced for selective detection of superoxide in the mitochondria of live cells and was validated for confocal microscopy and flow cytometry. This protocol describes simultaneous measurements of mitochondrial superoxide generation with apoptotic markers (Annexin V and Sytox Green) by both flow cytometry and confocal microscopy in endothelial cell lines. The advantages of the described flow cytometry method over other cell-based techniques are the tremendous speed (1-2 h), exquisite precision and the possibility of simultaneous quantitative measurements of mitochondrial superoxide generation and apoptotic (and other) markers, with maximal preservation of cellular functions. This method combined with fluorescent microscopy may be very useful to reveal important spatial-temporal changes in mitochondrial superoxide production and execution of programmed cell death in virtually any cell type.

Fluorescent probes for nitric oxide and hydrogen peroxide in cell signaling

Nitric oxide (NO) and hydrogen peroxide (H(2)O(2)) have emerged as essential small molecules for cellular signal transduction owing largely to their ability to mediate oxidative posttranslational modifications (PTMs). Inventing new ways to track these small, diffusible, and reactive species with spatial and temporal resolution is a key challenge in elucidating their chemistry in living systems. Recent progress in the development of fluorescent probes that respond selectively to NO and H(2)O(2) produced at cell signaling levels offers a promising approach to interrogating their physiological production, accumulation, trafficking, and function.

Swallow-tailed perylene derivative: a new tool for fluorescent imaging of lipid hydroperoxides

A swallow-tailed perylene derivative including a triphenylphosphine moiety was synthesized and applied to the detection and the live-cell imaging of lipid hydroperoxides. The novel probe, named Spy-LHP, reacted rapidly and quantitatively with lipid hydroperoxides to form the corresponding oxide, Spy-LHPOx, which emits extremely strong fluorescence (Phi approximately 1) in the visible range (lambda(em) = 535 nm, 574 nm). Spy-LHP was highly selective for lipid hydroperoxides, and the addition of other reactive oxygen species (ROS) including hydrogen peroxides, hydroxyl radical, superoxide anion, nitric oxide, peroxynitrite, and alkylperoxyl radical, caused no significant increase in the fluorescence intensity. The probe exhibited good localization to cellular membranes and was successfully applied to the confocal laser scanning microscopy (CLSM) imaging of lipid hydroperoxides in live J774A.1 cells, in which lipid peroxidation was proceeded by the stimulation of 2,2-azobis(2-amidinopropane)dihydrochloride (AAPH). These findings establish Spy-LHP as a promising new tool for investigating the physiology of lipid hydroperoxides.

In vivo imaging of hydrogen peroxide with chemiluminescent nanoparticles

The overproduction of hydrogen peroxide is implicated in the development of numerous diseases and there is currently great interest in developing contrast agents that can image hydrogen peroxide in vivo. In this report, we demonstrate that nanoparticles formulated from peroxalate esters and fluorescent dyes can image hydrogen peroxide in vivo with high specificity and sensitivity. The peroxalate nanoparticles image hydrogen peroxide by undergoing a three-component chemiluminescent reaction between hydrogen peroxide, peroxalate esters and fluorescent dyes. The peroxalate nanoparticles have several attractive properties for in vivo imaging, such as tunable wavelength emission (460-630 nm), nanomolar sensitivity for hydrogen peroxide and excellent specificity for hydrogen peroxide over other reactive oxygen species. The peroxalate nanoparticles were capable of imaging hydrogen peroxide in the peritoneal cavity of mice during a lipopolysaccharide-induced inflammatory response. We anticipate numerous applications of peroxalate nanoparticles for in vivo imaging of hydrogen peroxide, given their high specificity and sensitivity and deep-tissue-imaging capability.

Preparation of a copper-based fluorescent probe for nitric oxide and its use in mammalian cultured cells

A procedure for the preparation of a copper(II) complex (CuFL) as a fluorescent nitric oxide (NO) detector is described. The fluorescein-based ligand FL can be synthesized in seven reaction steps (overall yield approximately 20%), typically requiring a total time of 9 days. The CuFL probe allows for the detection of NO produced in mammalian cultured cells. The detailed protocol for the use of CuFL for imaging NO in human neuroblastoma SK-N-SH cells takes a total time of approximately 26 h. This includes plating cells on six-well tissue culture plates or imaging dishes, treatment with CuFL, stimulation of NO synthases and imaging by fluorescence microscopy.

Simple quantitative detection of mitochondrial superoxide production in live cells

Experiments with isolated mitochondria have established that these organelles are pivotal intracellular sources of superoxide in a variety of pathophysiological conditions. Recently, a novel fluoroprobe MitoSOX Red was introduced for selective detection of superoxide in the mitochondria of live cells and was validated with confocal microscopy. Here we show approximately 3-7 fold dose- and time-dependent increase in mitochondrial superoxide production (measured by MitoSOX using flow cytometry and confocal microscopy) in rat cardiac derived H9c2 myocytes and/or in human coronary artery endothelial cells triggered by Antimycin A, Paraquat, Doxorubicin or high glucose. These results establish a novel, quantitative method for simple detection of mitochondrial superoxide generation simultaneously in a large population of live cells by flow cytometry. This method can also be adapted for immune cell studies with mixed population of T or B cells or their subsets to analyze mitochondrial superoxide levels using multiple labeled surface markers in individual populations.

Molecular imaging of hydrogen peroxide produced for cell signaling

Hydrogen peroxide (H2O2) is emerging as a newly recognized messenger in cellular signal transduction. However, a substantial challenge in elucidating its diverse roles in complex biological environments is the lack of methods for probing this reactive oxygen metabolite in living systems with molecular specificity. Here we report the synthesis and application of Peroxy Green 1 (PG1) and Peroxy Crimson 1 (PC1), two new fluorescent probes that show high selectivity for H2O2 and are capable of visualizing endogenous H2O2 produced in living cells by growth factor stimulation, including the first direct imaging of peroxide produced for brain cell signaling. The combined features of reactive oxygen species selectivity, sensitivity to signaling levels of H2O2, and live-cell compatibility presage many new opportunities for PG1, PC1 and related synthetic reagents for exploring the physiological roles of H2O2 in living systems with molecular imaging.

Bioenergetics and the formation of mitochondrial reactive oxygen species

The contribution of mitochondria to the manifestation of disease is ascribed largely to the production of reactive oxygen species (ROS), which are obligatory by-products of aerobiosis. Studies using isolated mitochondria have revealed multiple potential sites and circumstances of ROS production but the relevance of these to in situ conditions is limited. In this article, we focus on bioenergetic factors that promote ROS generation at physiologically relevant sites in mitochondria. Emphasis is given to ROS generation by complex I--the first component of the respiratory chain--and to how the NADH:NAD+ ratio regulates ROS formation. Complex I is a physiologically and pathologically relevant ROS-forming site that is important not only in normal mitochondrial energy production but also in the pathogenesis of Parkinson's disease, which is the second most common neurodegenerative disease.