2-(4-Carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potentiates nitrosation of a heterocyclic amine carcinogen by nitric oxide

Using diaminofluoresceins (DAFs) in nitric oxide research

Diaminofluoresceins (DAFs) are fluorescent probes widely applied to measure nitric oxide (NO) formation in cells and tissues. The main advantages of these compounds are their availability and low cost, and the general availability of instruments able to detect green fluorescence in all laboratories; these include fluorimeters, flow cytometers, and fluorescent microscopes. What made these molecules particularly interesting for many scientists approaching the NO field is that they are apparently very easy to use, as compared with other techniques requiring specific instrumentation and knowledge like chemiluminescence and electron paramagnetic resonance. However, the reactivity and biological chemistry of these probes in the cellular environment is rather complex and still not fully understood. Moreover, secondary reactions with ascorbate, or interference with thiols occur in cells. Therefore, the use of DAFs requires specific experimental planning and a careful interpretation of the results obtained. In this methodological review, we described in detail what is known about the reactivity of DAFs, their application in biological assays, list some principles to help experimental planning, including the necessary controls, and list the caveats concerning result interpretation. These guiding principles will help to understand the "Method behind our DAF-madness".

Cryptochrome in sponges: a key molecule linking photoreception with phototransduction

Sponges (phylum: Porifera) react to external light or mechanical signals with contractile or metabolic reactions and are devoid of any nervous or muscular system. Furthermore, elements of a photoreception/phototransduction system exist in those animals. Recently, a cryptochrome-based photoreceptor system has been discovered in the demosponge. The assumption that in sponges the siliceous skeleton acts as a substitution for the lack of a nervous system and allows light signals to be transmitted through its glass fiber network is supported by the findings that the first spicules are efficient light waveguides and the second sponges have the enzymatic machinery for the generation of light. Now, we have identified/cloned in Suberites domuncula two additional potential molecules of the sponge cryptochrome photoreception system, the guanine nucleotide-binding protein β subunit, related to β-transducin, and the nitric oxide synthase (NOS)-interacting protein. Cryptochrome and NOSIP are light-inducible genes. The studies show that the NOS inhibitor L-NMMA impairs both morphogenesis and motility of the cells. Finally, we report that the function of primmorphs to produce reactive nitrogen species can be abolished by a NOS inhibitor. We propose that the sponge cryptochrome-based photoreception system, through which photon signals are converted into radicals, is coupled to the NOS apparatus.

RBC-NOS-dependent S-nitrosylation of cytoskeletal proteins improves RBC deformability

Background

Red blood cells (RBC) possess a nitric oxide synthase (RBC-NOS) whose activation depends on the PI3-kinase/Akt kinase pathway. RBC-NOS-produced NO exhibits important biological functions like maintaining RBC deformability. Until now, the cellular target structure for NO, to exert its influence on RBC deformability, remains unknown. In the present study we analyzed the modification of RBC-NOS activity by pharmacological treatments, the resulting influence on RBC deformability and provide first evidence for possible target proteins of RBC-NOS-produced NO in the RBC cytoskeletal scaffold.

Methods/Findings

Blood from fifteen male subjects was incubated with the NOS substrate L-arginine to directly stimulate enzyme activity. Direct inhibition of enzyme activity was induced by L-N5-(1-Iminoethyl)-ornithin (L-NIO). Indirect stimulation and inhibition of RBC-NOS were achieved by applying insulin and wortmannin, respectively, substances known to affect PI3-kinase/Akt kinase pathway. The NO donor sodium nitroprusside (SNP) and the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) were additionally applied as NO positive and negative controls, respectively. Immunohistochemical staining was used to determine phosphorylation and thus activation of RBC-NOS. As a marker for NO synthesis nitrite was measured in plasma and RBCs using chemiluminescence detection. S-nitrosylation of erythrocyte proteins was determined by biotin switch assay and modified proteins were identified using LC-MS. RBC deformability was determined by ektacytometry. The data reveal that activated RBC-NOS leads to increased NO production, S-nitrosylation of RBC proteins and RBC deformability, whereas RBC-NOS inhibition resulted in contrary effects.

Conclusion/Significance

This study first-time provides strong evidence that RBC-NOS-produced NO modifies RBC deformability through direct S-nitrosylation of cytoskeleton proteins, most likely α- and β-spectrins. Our data, therefore, gain novel insights into biological functions of RBC-NOS by connecting impaired RBC deformability abilities to specific posttranslational modifications of RBC proteins. By identifying likely NO-target proteins in RBC, our results will stimulate new therapeutic approaches for patients with microvascular disorders.

A multilevel analytical approach for detection and visualization of intracellular NO production and nitrosation events using diaminofluoresceins

Diaminofluoresceins are widely used probes for detection and intracellular localization of NO formation in cultured/isolated cells and intact tissues. The fluorinated derivative 4-amino-5-methylamino-2',7'-difluorofluorescein (DAF-FM) has gained increasing popularity in recent years because of its improved NO sensitivity, pH stability, and resistance to photobleaching compared to the first-generation compound, DAF-2. Detection of NO production by either reagent relies on conversion of the parent compound into a fluorescent triazole, DAF-FM-T and DAF-2-T, respectively. Although this reaction is specific for NO and/or reactive nitrosating species, it is also affected by the presence of oxidants/antioxidants. Moreover, the reaction with other molecules can lead to the formation of fluorescent products other than the expected triazole. Thus additional controls and structural confirmation of the reaction products are essential. Using human red blood cells as an exemplary cellular system we here describe robust protocols for the analysis of intracellular DAF-FM-T formation using an array of fluorescence-based methods (laser-scanning fluorescence microscopy, flow cytometry, and fluorimetry) and analytical separation techniques (reversed-phase HPLC and LC-MS/MS). When used in combination, these assays afford unequivocal identification of the fluorescent signal as being derived from NO and are applicable to most other cellular systems without or with only minor modifications.

NO release by nitric oxide donors in vitro and in planta

Artificial nitric oxide (NO) donors are widely used as tools to study the role of NO in plants. However, reliable and reproducible characterisation of metabolic responses induced by different NO donors is complicated by the variability of their NO release characteristics. The latter are affected by different physical and biological factors including temperature and light. Here we critically evaluate NO release characteristics of the donors sodium nitroprusside (SNP), S-nitrosoglutathione (GSNO) and nitric oxide synthase (NOS), both in vitro and in planta (Nicotiana tabacum L. cv. BelW3) and assess their effects on NO dependent processes such as the transcriptional regulation of the mitochondrial alternative oxidase gene (AOX1a), accumulation of H(2)O(2) and induction of cell death. We demonstrate that, contrary to NOS and SNP, GSNO is not an efficient NO generator in leaf tissue. Furthermore, spectrophotometric measurement of NO with a haemoglobin assay, rather than diaminofluorescein (DAF-FM) based detection, is best suited for the quantification of tissue NO. In spite of the different NO release signatures by SNP and NOS in tissue, the NO dependent responses examined were similar, suggesting that there is a critical threshold for the NO response.