A metabolic enzyme for S-nitrosothiol conserved from bacteria to humans

S-Nitrosoglutathione Reductase-The Master Regulator of Protein S-Nitrosation in Plant NO Signaling

S-nitrosation has been recognized as an important mechanism of protein posttranslational regulations, based on the attachment of a nitroso group to cysteine thiols. Reversible S-nitrosation, similarly to other redox-base modifications of protein thiols, has a profound effect on protein structure and activity and is considered as a convergence of signaling pathways of reactive nitrogen and oxygen species. In plant, S-nitrosation is involved in a wide array of cellular processes during normal development and stress responses. This review summarizes current knowledge on S-nitrosoglutathione reductase (GSNOR), a key enzyme which regulates intracellular levels of S-nitrosoglutathione (GSNO) and indirectly also of protein S-nitrosothiols. GSNOR functions are mediated by its enzymatic activity, which catalyzes irreversible GSNO conversion to oxidized glutathione within the cellular catabolism of nitric oxide. GSNOR is involved in the maintenance of balanced levels of reactive nitrogen species and in the control of cellular redox state. Multiple functions of GSNOR in plant development via NO-dependent and -independent signaling mechanisms and in plant defense responses to abiotic and biotic stress conditions have been uncovered. Extensive studies of plants with down- and upregulated GSNOR, together with application of transcriptomics and proteomics approaches, seem promising for new insights into plant S-nitrosothiol metabolism and its regulation.

S-Nitrosoglutathione Reductase-The Master Regulator of Protein S-Nitrosation in Plant NO Signaling

S-nitrosation has been recognized as an important mechanism of protein posttranslational regulations, based on the attachment of a nitroso group to cysteine thiols. Reversible S-nitrosation, similarly to other redox-base modifications of protein thiols, has a profound effect on protein structure and activity and is considered as a convergence of signaling pathways of reactive nitrogen and oxygen species. In plant, S-nitrosation is involved in a wide array of cellular processes during normal development and stress responses. This review summarizes current knowledge on S-nitrosoglutathione reductase (GSNOR), a key enzyme which regulates intracellular levels of S-nitrosoglutathione (GSNO) and indirectly also of protein S-nitrosothiols. GSNOR functions are mediated by its enzymatic activity, which catalyzes irreversible GSNO conversion to oxidized glutathione within the cellular catabolism of nitric oxide. GSNOR is involved in the maintenance of balanced levels of reactive nitrogen species and in the control of cellular redox state. Multiple functions of GSNOR in plant development via NO-dependent and -independent signaling mechanisms and in plant defense responses to abiotic and biotic stress conditions have been uncovered. Extensive studies of plants with down- and upregulated GSNOR, together with application of transcriptomics and proteomics approaches, seem promising for new insights into plant S-nitrosothiol metabolism and its regulation.

Signaling mechanisms underlying systemic acquired resistance to microbial pathogens

Plants respond to biotic stress by inducing a variety of responses, which not only protect against the immediate diseases but also provide immunity from future infections. One example is systemic acquired resistance (SAR), which provides long-lasting and broad-spectrum protection at the whole plant level. The induction of SAR prepares the plant for a more robust response to subsequent infections from related and unrelated pathogens. SAR involves the rapid generation of signals at the primary site of infection, which are transported to the systemic parts of the plant presumably via the phloem. SAR signal generation and perception requires an intact cuticle, a waxy layer covering all aerial parts of the plant. A chemically diverse set of SAR inducers has already been identified, including hormones (salicylic acid, methyl salicylate), primary/secondary metabolites (nitric oxide, reactive oxygen species, glycerol-3-phosphate, azelaic acid, pipecolic acid, dihyroabetinal), fatty acid/lipid derivatives (18 carbon unsaturated fatty acids, galactolipids), and proteins (DIR1-Defective in Induced Resistance 1, AZI1-Azelaic acid Induced 1). Some of these are demonstrably mobile and the phloem loading routes for three of these SAR inducers is known. Here we discuss the recent findings related to synthesis, transport, and the relationship between these various SAR inducers.

The protein S-nitrosylation of splicing and translational machinery in vascular endothelial cells is susceptible to oxidative stress induced by oxidized low-density lipoprotein

Oxidized low-density lipoprotein (ox-LDL) can impair endothelial function and lead to the atherosclerosis development. Protein S-nitrosylation is sensitive to cellular redox state and acts as a crucial regulator and executor of nitric oxide (NO) signaling pathways. Aberrant S-nitrosylation contributes to the pathogenesis of cardiovascular and cerebrovascular diseases. However, the effect of ox-LDL on S-nitrosylation and its significance for endothelial dysfunction have not been studied at proteome level. Herein, the combined quantitative analysis of proteome and S-nitrosoproteome was performed using an integrated biotin switch and iTRAQ labeling approach in EA.hy926 cell line derived from human umbilical vein endothelial cell (HUVEC) treated with ox-LDL. A total of 2204 S-nitrosylated (SNO) peptides of 1318 SNO-proteins were quantified. Notably, 352 SNO-peptides of 262 SNO-proteins were significantly regulated after excluding S-nitrosylation changes caused by protein expression alterations. Many of them belonged to mRNA splicing, ribosomal structure and translational regulatory proteins, covering the entire translation process. The results indicated that S-nitrosylation of the splicing and translational machinery in vascular endothelial cells was susceptible to ox-LDL. Abnormal protein S-nitrosylation may be one pivotal mechanism underlying endothelial dysfunction induced by ox-LDL. This study potentially enriches the present understanding of pro-atherogenic effect of ox-LDL from the perspective of S-nitrosylation. SIGNIFICANCE: The role of ox-LDL in endothelial dysfunction and atherosclerosis development has been recognized from the aspect of impaired NO production. However, its effect on S-nitrosylation, which is directly related to NO signaling pathway, still remains largely unexplored. Our work initially provided a systematic characterization of S-nitrosoproteome in ox-LDL-treated endothelial cells after ruling out the changes of S-nitrosylation modification caused by protein expression alone. MS-based approach coupled with iTRAQ technique indicated 262 SNO-proteins were significantly regulated. Functional enrichment and interaction network analysis revealed that proteins involved in mRNA splicing and translational machinery were susceptible to abnormal S-nitrosylation under ox-LDL treatment. This achievement suggested one potential mechanism underlying endothelial dysfunction induced by ox-LDL from the perspective of S-nitrosoproteome.

Elevated CO2-induced production of nitric oxide differentially modulates nitrate assimilation and root growth of wheat seedlings in a nitrate dose-dependent manner

Wheat is a major staple food crop worldwide contributing approximately 20% of total protein consumed by mankind. The nitrogen and protein concentration of wheat crop and grain often decline as a result of exposure of the crop to elevated CO₂ (EC). The changes in nitrogen (N) assimilation, root system architecture, and nitric oxide (NO)-mediated N signaling and expression of genes involved in N assimilation and high affinity nitrate uptake were examined in response to different nitrate levels and EC in wheat. Activity of enzyme nitrate reductase (NRA) was downregulated under EC both in leaf and root tissues. Plants grown under EC displayed enhanced production of NO and more so when nitrate supply was high. Based on exogenous supply of NO, inhibitors of NO production, and NO scavenger, regulatory role of NO on EC mediated changes in root morphology and NRA was revealed. The enhanced NO production under EC and high N levels negatively regulated the transcript abundance of NR and high affinity nitrate transporters (HATS).

Sources of Vascular Nitric Oxide and Reactive Oxygen Species and Their Regulation

Nitric oxide (NO) is a small free radical with critical signaling roles in physiology and pathophysiology. The generation of sufficient NO levels to regulate the resistance of the blood vessels and hence the maintenance of adequate blood flow is critical to the healthy performance of the vasculature. A novel paradigm indicates that classical NO synthesis by dedicated NO synthases is supplemented by nitrite reduction pathways under hypoxia. At the same time, reactive oxygen species (ROS), which include superoxide and hydrogen peroxide, are produced in the vascular system for signaling purposes, as effectors of the immune response, or as byproducts of cellular metabolism. NO and ROS can be generated by distinct enzymes or by the same enzyme through alternate reduction and oxidation processes. The latter oxidoreductase systems include NO synthases, molybdopterin enzymes, and hemoglobins, which can form superoxide by reduction of molecular oxygen or NO by reduction of inorganic nitrite. Enzymatic uncoupling, changes in oxygen tension, and the concentration of coenzymes and reductants can modulate the NO/ROS production from these oxidoreductases and determine the redox balance in health and disease. The dysregulation of the mechanisms involved in the generation of NO and ROS is an important cause of cardiovascular disease and target for therapy. In this review we will present the biology of NO and ROS in the cardiovascular system, with special emphasis on their routes of formation and regulation, as well as the therapeutic challenges and opportunities for the management of NO and ROS in cardiovascular disease.

Discovery of 5-(2-chloro-4'-(1H-imidazol-1-yl)-[1,1'-biphenyl]-4-yl)-1H-tetrazole as potent and orally efficacious S-nitrosoglutathione reductase (GSNOR) inhibitors for the potential treatment of COPD

Endogenous nitrosothiols (SNOs) including S-nitrosoglutathione (GSNO) serve as reservoir for bioavailable nitric oxide (NO) and mediate NO-based signaling, inflammatory status and smooth muscle function in the lung. GSNOR inhibition increases pulmonary GSNO and induces bronchodilation while reducing inflammation in lung diseases. In this letter, design, synthesis and structure-activity relationships (SAR) of novel imidazole-biaryl-tetrazole based GSNOR inhibitors are described. Many potent inhibitors (30, 39, 41, 42, 44, 45 and 58) were identified with low nanomolar activity (IC₅₀s: <15 nM) along with adequate metabolic stability. Lead compounds 30 and 58 exhibited good exposure and oral bioavailability in mouse pharmacokinetic (PK) study. Compound 30 was selected for further profiling and revealed comparable mouse and rat GSNOR potency, high selectivity against alcohol dehydrogenase (ADH) and carbonyl reductase (CBR1) family of enzymes, low efflux ratio and permeability in PAMPA, a high permeability in CALU-3 assay, significantly low hERG activity and minimal off-target activity. Further, an in vivo efficacy of compound 30 is disclosed in cigarette smoke (CS) induced mouse model for COPD.

Molecular recognition of S-nitrosothiol substrate by its cognate protein denitrosylase

Protein S-nitrosylation mediates a large part of nitric oxide's influence on cellular function by providing a fundamental mechanism to control protein function across different species and cell types. At steady state, cellular S-nitrosylation reflects dynamic equilibria between S-nitrosothiols (SNOs) in proteins and small molecules (low-molecular-weight SNOs) whose levels are regulated by dedicated S-nitrosylases and denitrosylases. S-Nitroso-CoA (SNO-CoA) and its cognate denitrosylases, SNO-CoA reductases (SCoRs), are newly identified determinants of protein S-nitrosylation in both yeast and mammals. Because SNO-CoA is a minority species among potentially thousands of cellular SNOs, SCoRs must preferentially recognize this SNO substrate. However, little is known about the molecular mechanism by which cellular SNOs are recognized by their cognate enzymes. Using mammalian cells, molecular modeling, substrate-capture assays, and mutagenic analyses, we identified a single conserved surface Lys (Lys-127) residue as well as active-site interactions of the SNO group that mediate recognition of SNO-CoA by SCoR. Comparing SCoR^K127A versus SCoR^WT HEK293 cells, we identified a SNO-CoA-dependent nitrosoproteome, including numerous metabolic protein substrates. Finally, we discovered that the SNO-CoA/SCoR system has a role in mitochondrial metabolism. Collectively, our findings provide molecular insights into the basis of specificity in SNO-CoA-mediated metabolic signaling and suggest a role for SCoR-regulated S-nitrosylation in multiple metabolic processes.

Thioredoxin and glutaredoxin regulate metabolism through different multiplex thiol switches

The aim of the present study was to define the role of Trx and Grx on metabolic thiol redox regulation and identify their protein and metabolite targets. The hepatocarcinoma-derived HepG2 cell line under both normal and oxidative/nitrosative conditions by overexpression of NO synthase (NOS3) was used as experimental model. Grx1 or Trx1 silencing caused conspicuous changes in the redox proteome reflected by significant changes in the reduced/oxidized ratios of specific Cys's including several glycolytic enzymes. Cys⁹¹ of peroxiredoxin-6 (PRDX6) and Cys¹⁵³ of phosphoglycerate mutase-1 (PGAM1), that are known to be involved in progression of tumor growth, are reported here for the first time as specific targets of Grx1. A group of proteins increased their Cys_RED/Cys_OX ratio upon Trx1 and/or Grx1 silencing, including caspase-3 Cys¹⁶³, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) Cys²⁴⁷ and triose-phosphate isomerase (TPI) Cys²⁵⁵ likely by enhancement of NOS3 auto-oxidation. The activities of several glycolytic enzymes were also significantly affected. Glycolysis metabolic flux increased upon Trx1 silencing, whereas silencing of Grx1 had the opposite effect. Diversion of metabolic fluxes toward synthesis of fatty acids and phospholipids was observed in siRNA-Grx1 treated cells, while siRNA-Trx1 treated cells showed elevated levels of various sphingomyelins and ceramides and signs of increased protein degradation. Glutathione synthesis was stimulated by both treatments. These data indicate that Trx and Grx have both, common and specific protein Cys redox targets and that down regulation of either redoxin has markedly different metabolic outcomes. They reflect the delicate sensitivity of redox equilibrium to changes in any of the elements involved and the difficulty of forecasting metabolic responses to redox environmental changes.

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Trx1 and Grx1 Cys redox targets are abundant among Glycolytic enzymes.

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PRDX6-Cys⁹¹ and PGAM-Cys¹⁵³ are specific targets of Grx1.

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Down regulation of thioredoxin and glutaredoxin have different metabolic outcomes.

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Glutathione synthesis and membrane lipid composition are sensitive to Trx1 and Grx1 down regulation.

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Redoxins down regulation also induce target Cys reductive changes under NOS3 overexpression.

Pathophysiological Role of S-Nitrosylation and Transnitrosylation Depending on S-Nitrosoglutathione Levels Regulated by S-Nitrosoglutathione Reductase

Nitric oxide (NO) mediates various physiological and pathological processes, including cell proliferation, differentiation, and inflammation. Protein S-nitrosylation (SNO), a NO-mediated reversible protein modification, leads to changes in the activity and function of target proteins. Recent findings on protein-protein transnitrosylation reactions (transfer of an NO group from one protein to another) have unveiled the mechanism of NO modulation of specific signaling pathways. The intracellular level of S-nitrosoglutathione (GSNO), a major reactive NO species, is controlled by GSNO reductase (GSNOR), a major regulator of NO/SNO signaling. Increasing number of GSNOR-related studies have shown the important role that denitrosylation plays in cellular NO/SNO homeostasis and human pathophysiology. This review introduces recent evidence of GSNO-mediated NO/SNO signaling depending on GSNOR expression or activity. In addition, the applicability of GSNOR as a target for drug therapy will be discussed in this review.

Hiding in Plain Sight: Nebivolol Exhibits Compelling Tocolytic Properties

Preterm birth before 37 weeks of completed gestation results in numerous health consequences for the foetus. Preterm labour leads to preterm birth in over 50% of cases, and no FDA‐approved treatment can prevent labour or help a foetus remain in the womb until term. Examination of nitric oxide mediated relaxation signaling in the uterine smooth muscle reveals a role for protein S‐nitrosation. The recent discovery of upregulated S‐nitrosoglutathione reductase (GSNOR) in spontaneously preterm labouring women has emphasized the need to explore the function of S‐nitrosation regulation in the maintenance of uterine quiescence. Here we have examined the ability of nebivolol to relax uterine smooth muscle and tested recent claims that nebivolol is a GSNOR inhibitor. In uterine smooth muscle strips from both mouse and human, nebivolol relaxes oxytocin‐induced contractions in a dose dependent manner. Our data indicates that nebivolol has no effect on GSNOR activity, nor does nebivolol inhibit thioredoxin reductase, two of the major protein denitrosylases. The ability of nebivolol to relax uterine smooth muscle is likely the combined effects of increased nitric oxide synthase activity and β3‐adregnegic stimulation.

S-nitrosylation of endothelial nitric oxide synthase impacts erectile function

Neuronal and endothelial nitric oxide synthases (nNOS and eNOS respectively) play major roles in generating the nitric oxide bioactivity necessary for erectile function. S-nitrosylation has been shown to regulate NOS activity. The presence of S-nitrosylated NOS in the penis and the impact of NOS S-nitrosylation/denitrosylation on erectile function were examined. S-nitrosylated forms of NOS were identified by biotin-switch assay followed by western blot analysis. Erectile function in S-nitrosoglutathione reductase deficient (GSNO^+/-) and null (GSNO^-/-) mice were assessed by continuous cavernous nerve electrical stimulation (CCNES). Glutathione ethyl ester (GSHee) was used to manipulate S-nitrosylated NOS levels. Immunohistological and immunofluorescence analyses were used to identify the location of eNOS and GSNO-R in corporal tissue. eNOS and nNOS were S-nitrosylated in unstimulated penises of the mice. CCNES resulted in a time-dependent increase in eNOS S-nitrosylation with peak eNOS S-nitrosylation observed during detumescence. S-nitrosylated nNOS levels were unchanged. Intracorporal injection of GSHee reduced S-nitrosylated eNOS levels, enhancing time to maximum intracorporal pressure (ICP). eNOS and GSNO-R co-localize to the endothelium of the corpus cavernosum in the mouse and the human. ICP measurements obtained during CCNES demonstrate GSNO-R^+/- and GSNO-R^-/- animals cannot maintain an elevated ICP. Results suggest eNOS S-nitrosylation/denitrosylation is an important mechanism regulating eNOS activity during erectile function. GSNO-R is a key enzyme involved in the eNOS denitrosylation. The increase in eNOS S-nitrosylation (inactivation) observed with tumescence may begin a cycle leading to detumescence. Clinically this may indicate that alterations in the balance of S-nitrosylation/denitrosylation either directly or indirectly contribute to erectile dysfunction.

Pyrrole: An insight into recent pharmacological advances with structure activity relationship

Pyrrole is a heterocyclic ring template with multiple pharmacophores that provides a way for the generation of library of enormous lead molecules. Owing to its vast pharmacological profile, pyrrole and its analogues have drawn much attention of the researchers/chemists round the globe to be explored exhaustively for the benefit of mankind. This review focusses on recent advancements; pertaining to pyrrole scaffold, discussing various aspects of structure activity relationship and its bioactivities.

Regulation of SCFTIR1/AFBs E3 ligase assembly by S-nitrosylation of Arabidopsis SKP1-like1 impacts on auxin signaling

The F-box proteins (FBPs) TIR1/AFBs are the substrate recognition subunits of SKP1–cullin–F-box (SCF) ubiquitin ligase complexes and together with Aux/IAAs form the auxin co-receptor. Although tremendous knowledge on auxin perception and signaling has been gained in the last years, SCF^TIR1/AFBs complex assembly and stabilization are emerging as new layers of regulation. Here, we investigated how nitric oxide (NO), through S-nitrosylation of ASK1 is involved in SCF^TIR1/AFBs assembly. We demonstrate that ASK1 is S-nitrosylated and S-glutathionylated in cysteine (Cys) 37 and Cys118 residues in vitro. Both, in vitro and in vivo protein-protein interaction assays show that NO enhances ASK1 binding to CUL1 and TIR1/AFB2, required for SCF^TIR1/AFB2 assembly. In addition, we demonstrate that Cys37 and Cys118 are essential residues for proper activation of auxin signaling pathway in planta. Phylogenetic analysis revealed that Cys37 residue is only conserved in SKP proteins in Angiosperms, suggesting that S-nitrosylation on Cys37 could represent an evolutionary adaption for SKP1 function in flowering plants. Collectively, these findings indicate that multiple events of redox modifications might be part of a fine-tuning regulation of SCF^TIR1/AFBs for proper auxin signal transduction.

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ASK1 adaptor protein of the SCF^TIR1/AFB E3 ligase complex is redox regulated.

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NO regulates ASK1 function by S-nitrosylation in Cys37 and Cys118 residues.

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NO enhances ASK1-CUL1 and ASK1-TIR1/AFB2 protein-protein interactions required for SCF^TIR1/AFB2 assembly in vitro and in vivo.

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S-nitrosylated residues in ASK1 are essential for activation of auxin signaling pathway in plants.

Crosstalk between reactive oxygen species and nitric oxide in plants: Key role of S-nitrosoglutathione reductase

Nitric oxide (.NO) acts as signaling molecule in plants being involved in diverse physiological processes such as germination, root growth, stomata closing and response to biotic and abiotic stress. S-Nitrosoglutathione (GSNO) is the storage and transport form of.NO and has a very important function in.NO signaling since it can transfer its.NO moiety to other proteins (trans-nitrosylation). The level of GSNO and thus the level of S-nitrosylated proteins are regulated by GSNO-reductase (GSNOR). In this way, this enzyme regulates the S-nitrosothiol levels and plays a balancing role in fine-tuning.NO signaling. Interestingly, oxidative post-translationally modification of GSNOR inhibited the activity of this enzyme suggesting a direct crosstalk between ROS- and RNS-signaling. In this review article the regulatory effects of ROS on GSNOR are highlighted and their physiological function in context of crosstalk between ROS and.NO and species in plants are discussed.

S-Nitrosylation Targets GSNO Reductase for Selective Autophagy during Hypoxia Responses in Plants

Nitric oxide (NO) regulates diverse cellular signaling through S-nitrosylation of specific Cys residues of target proteins. The intracellular level of S-nitrosoglutathione (GSNO), a major bioactive NO species, is regulated by GSNO reductase (GSNOR), a highly conserved master regulator of NO signaling. However, little is known about how the activity of GSNOR is regulated. Here, we show that S-nitrosylation induces selective autophagy of Arabidopsis GSNOR1 during hypoxia responses. S-nitrosylation of GSNOR1 at Cys-10 induces conformational changes, exposing its AUTOPHAGY-RELATED8 (ATG8)-interacting motif (AIM) accessible by autophagy machinery. Upon binding by ATG8, GSNOR1 is recruited into the autophagosome and degraded in an AIM-dependent manner. Physiologically, the S-nitrosylation-induced selective autophagy of GSNOR1 is relevant to hypoxia responses. Our discovery reveals a unique mechanism by which S-nitrosylation mediates selective autophagy of GSNOR1, thereby establishing a molecular link between NO signaling and autophagy.

Nitric oxide buffering and conditional nitric oxide release in stress response

Nitric oxide (NO) has emerged as an essential biological messenger in plant biology that usually transmits its bioactivity by post-translational modifications such as S-nitrosylation, the reversible addition of an NO group to a protein cysteine residue leading to S-nitrosothiols (SNOs). In recent years, SNOs have risen as key signalling molecules mainly involved in plant response to stress. Chief among SNOs is S-nitrosoglutathione (GSNO), generated by S-nitrosylation of the key antioxidant glutathione (GSH). GSNO is considered the major NO reservoir and a phloem mobile signal that confers to NO the capacity to be a long-distance signalling molecule. GSNO is able to regulate protein function and gene expression, resulting in a key role for GSNO in fundamental processes in plants, such as development and response to a wide range of environmental stresses. In addition, GSNO is also able to regulate the total SNO pool and, consequently, it could be considered the storage of NO in cells that may control NO signalling under basal and stress-related responses. Thus, GSNO function could be crucial during plant response to environmental stresses. Besides the importance of GSNO in plant biology, its mode of action has not been widely discussed in the literature. In this review, we will first discuss the GSNO turnover in cells and secondly the role of GSNO as a mediator of physiological and stress-related processes in plants, highlighting those aspects for which there is still some controversy.

Specificity in nitric oxide signalling

Reactive nitrogen species (RNS) and their cognate redox signalling networks pervade almost all facets of plant growth, development, immunity, and environmental interactions. The emerging evidence implies that specificity in redox signalling is achieved by a multilayered molecular framework. This encompasses the production of redox cues in the locale of the given protein target and protein tertiary structures that convey the appropriate local chemical environment to support redox-based, post-translational modifications (PTMs). Nascent nitrosylases have also recently emerged that mediate the formation of redox-based PTMs. Reversal of these redox-based PTMs, rather than their formation, is also a major contributor of signalling specificity. In this context, the activities of S-nitrosoglutathione (GSNO) reductase and thioredoxin h5 (Trxh5) are a key feature. Redox signalling specificity is also conveyed by the unique chemistries of individual RNS which is overlaid on the structural constraints imposed by tertiary protein structure in gating access to given redox switches. Finally, the interactions between RNS and ROS (reactive oxygen species) can also indirectly establish signalling specificity through shaping the formation of appropriate redox cues. It is anticipated that some of these insights might function as primers to initiate their future translation into agricultural, horticultural, and industrial biological applications.

Nitric oxide induced S-nitrosation causes base excision repair imbalance

It is well established that inflammation leads to the creation of potent DNA damaging chemicals, including reactive oxygen and nitrogen species. Nitric oxide can react with glutathione to create S-nitrosoglutathione (GSNO), which can in turn lead to S-nitrosated proteins. Of particular interest is the impact of GSNO on the function of DNA repair enzymes. The base excision repair (BER) pathway can be initiated by the alkyl-adenine DNA glycosylase (AAG), a monofunctional glycosylase that removes methylated bases. After base removal, an abasic site is formed, which then gets cleaved by AP endonuclease and processed by downstream BER enzymes. Interestingly, using the Fluorescence-based Multiplexed Host Cell Reactivation Assay (FM-HCR), we show that GSNO actually enhances AAG activity, which is consistent with the literature. This raised the possibility that there might be imbalanced BER when cells are challenged with a methylating agent. To further explore this possibility, we confirmed that GSNO can cause AP endonuclease to translocate from the nucleus to the cytoplasm, which might further exacerbate imbalanced BER by increasing the levels of AP sites. Analysis of abasic sites indeed shows GSNO induces an increase in the level of AP sites. Furthermore, analysis of DNA damage using the CometChip (a higher throughput version of the comet assay) shows an increase in the levels of BER intermediates. Finally, we found that GSNO exposure is associated with an increase in methylation-induced cytotoxicity. Taken together, these studies support a model wherein GSNO increases BER initiation while processing of AP sites is decreased, leading to a toxic increase in BER intermediates. This model is also supported by additional studies performed in our laboratory showing that inflammation in vivo leads to increased large-scale sequence rearrangements. Taken together, this work provides new evidence that inflammatory chemicals can drive cytotoxicity and mutagenesis via BER imbalance.

GSNOR modulates hyperhomocysteinemia-induced T cell activation and atherosclerosis by switching Akt S-nitrosylation to phosphorylation

The adaptive immune system plays a critical role in hyperhomocysteinemia (HHcy)-accelerated atherosclerosis. Recent studies suggest that HHcy aggravates atherosclerosis with elevated oxidative stress and reduced S-nitrosylation level of redox-sensitive protein residues in the vasculature. However, whether and how S-nitrosylation contributes to T-cell-driven atherosclerosis remain unclear. In the present study, we report that HHcy reduced the level of protein S-nitrosylation in T cells by inducing S-nitrosoglutathione reductase (GSNOR), the key denitrosylase that catalyzes S-nitrosoglutathione (GSNO), which is the main restored form of nitric oxide in vivo. Consequently, secretion of inflammatory cytokines [interferon-γ (IFN-γ) and interleukin-2] and proliferation of T cells were increased. GSNOR knockout or GSNO stimulation rectified HHcy-induced inflammatory cytokine secretion and T-cell proliferation. Site-directed mutagenesis of Akt at Cys224 revealed that S-nitrosylation at this site was pivotal for the reduced phosphorylation at Akt Ser473, which led to impaired Akt signaling. Furthermore, on HHcy challenge, as compared with GSNOR^+/+ApoE^-/- littermate controls, GSNOR^-/-ApoE^-/- double knockout mice showed reduced T-cell activation with concurrent reduction of atherosclerosis. Adoptive transfer of GSNOR^-/- T cells to ApoE^-/- mice fed homocysteine (Hcy) decreased atherosclerosis, with fewer infiltrated T cells and macrophages in plaques. In patients with HHcy and coronary artery disease, the level of plasma Hcy was positively correlated with Gsnor expression in peripheral blood mononuclear cells and IFN-γ⁺ T cells but inversely correlated with the S-nitrosylation level in T cells. These data reveal that T cells are activated, in part via GSNOR-dependent Akt denitrosylation during HHcy-induced atherosclerosis. Thus, suppression of GSNOR in T cells may reduce the risk of atherosclerosis.

Involvement of S-nitrosothiols modulation by S-nitrosoglutathione reductase in defence responses of lettuce and wild Lactuca spp. to biotrophic mildews

MAIN CONCLUSION

Resistant Lactuca spp. genotypes can efficiently modulate levels of S-nitrosothiols as reactive nitrogen species derived from nitric oxide in their defence mechanism against invading biotrophic pathogens including lettuce downy mildew. S-Nitrosylation belongs to principal signalling pathways of nitric oxide in plant development and stress responses. Protein S-nitrosylation is regulated by S-nitrosoglutathione reductase (GSNOR) as a key catabolic enzyme of S-nitrosoglutathione (GSNO), the major intracellular S-nitrosothiol. GSNOR expression, level and activity were studied in leaves of selected genotypes of lettuce (Lactuca sativa) and wild Lactuca spp. during interactions with biotrophic mildews, Bremia lactucae (lettuce downy mildew), Golovinomyces cichoracearum (lettuce powdery mildew) and non-pathogen Pseudoidium neolycopersici (tomato powdery mildew) during 168 h post inoculation (hpi). GSNOR expression was increased in all genotypes both in the early phase at 6 hpi and later phase at 72 hpi, with a high increase observed in L. sativa UCDM2 responses to all three pathogens. GSNOR protein also showed two-phase increase, with highest changes in L. virosa-B. lactucae and L. sativa cv. UCDM2-G. cichoracearum pathosystems, whereas P. neolycopersici induced GSNOR protein at 72 hpi in all genotypes. Similarly, a general pattern of modulated GSNOR activities in response to biotrophic mildews involves a two-phase increase at 6 and 72 hpi. Lettuce downy mildew infection caused GSNOR activity slightly increased only in resistant L. saligna and L. virosa genotypes; however, all genotypes showed increased GSNOR activity both at 6 and 72 hpi by lettuce powdery mildew. We observed GSNOR-mediated decrease of S-nitrosothiols as a general feature of Lactuca spp. response to mildew infection, which was also confirmed by immunohistochemical detection of GSNOR and GSNO in infected plant tissues. Our results demonstrate that GSNOR is differentially modulated in interactions of susceptible and resistant Lactuca spp. genotypes with fungal mildews and uncover the role of S-nitrosylation in molecular mechanisms of plant responses to biotrophic pathogens.

ROS and RNS signalling: adaptive redox switches through oxidative/nitrosative protein modifications

Over the last decade, a dual character of cell response to oxidative stress, eustress versus distress, has become increasingly recognized. A growing body of evidence indicates that under physiological conditions, low concentrations of reactive oxygen and nitrogen species (RONS) maintained by the activity of endogenous antioxidant system (AOS) allow reversible oxidative/nitrosative modifications of key redox-sensitive residues in regulatory proteins. The reversibility of redox modifications such as Cys S-sulphenylation/S-glutathionylation/S-nitrosylation/S-persulphidation and disulphide bond formation, or Tyr nitration, which occur through electrophilic attack of RONS to nucleophilic groups in amino acid residues provides redox switches in the activities of signalling proteins. Key requirement for the involvement of the redox modifications in RONS signalling including ROS-MAPK, ROS-PI3K/Akt, and RNS-TNF-α/NF-kB signalling is their specificity provided by a residue microenvironment and reaction kinetics. Glutathione, glutathione peroxidases, peroxiredoxins, thioredoxin, glutathione reductases, and glutaredoxins modulate RONS level and cell signalling, while some of the modulators (glutathione, glutathione peroxidases and peroxiredoxins) are themselves targets for redox modifications. Additionally, gene expression, activities of transcription factors, and epigenetic pathways are also under redox regulation. The present review focuses on RONS sources (NADPH-oxidases, mitochondrial electron-transportation chain (ETC), nitric oxide synthase (NOS), etc.), and their cross-talks, which influence reversible redox modifications of proteins as physiological phenomenon attained by living cells during the evolution to control cell signalling in the oxygen-enriched environment. We discussed recent advances in investigation of mechanisms of protein redox modifications and adaptive redox switches such as MAPK/PI3K/PTEN, Nrf2/Keap1, and NF-κB/IκB, powerful regulators of numerous physiological processes, also implicated in various diseases.

S-Nitrosoglutathione Reductase Underlies the Dysfunctional Relaxation to Nitric Oxide in Preterm Labor

Tocolytics show limited efficacy to prevent preterm delivery. In uterine smooth muscle cGMP accumulation following addition of nitric oxide (NO) has little effect on relaxation suggesting a role for protein S-nitrosation. In human myometrial tissues from women in labor at term (TL), or spontaneously in labor preterm (sPTL), direct stimulation of soluble guanylyl cyclase (sGC) fails to relax myometrium, while the same treatment relaxes vascular smooth muscle completely. Unlike term myometrium, effects of NO are not only blunted in sPTL, but global protein S-nitrosation is also diminished, suggesting a dysfunctional response to NO-mediated protein S-nitrosation. Examination of the enzymatic regulator of endogenous S-nitrosoglutathione availability, S-nitrosoglutathione reductase, reveals increased expression of the reductase in preterm myometrium associated with decreased total protein S-nitrosation. Blockade of S-nitrosoglutathione reductase relaxes sPTL tissue. Addition of NO donor to the actin motility assay attenuates force. Failure of sGC activation to mediate relaxation in sPTL tissues, together with the ability of NO to relax TL, but not sPTL myometrium, suggests a unique pathway for NO-mediated relaxation in myometrium. Our results suggest that examining the action of S-nitrosation on critical contraction associated proteins central to the regulation of uterine smooth muscle contraction can reveal new tocolytic targets.

Evidence that thiol-based redox state is critical for xylem vessel cell differentiation

Nitric oxide (NO), which plays essential roles in a variety of cell signaling processes, is the precursor of a family of NO-derived molecules, including toxic reactive nitrogen species. The NO-based regulation of cellular activity is mediated by the reversible modification of cysteine thiol groups in redox-sensitive proteins. One such modification is protein S-nitrosylation, i.e., the addition of an NO moiety to a cysteine thiol, and this S-nitrosylation is regulated by enzymes such as S-nitrosoglutathione reductase (GSNOR). Recently, we reported a novel loss-of-function allele of gsnor1, named suppressor of ectopic vessel cell differentiation induced by VND7-1 (seiv1), based on the VND7-inducible system, in which almost all cell types are transdifferentiated into xylem vessel cells upon activation of the NAC transcription factor VND7. We also found that VND7 can be S-nitrosylated and that the target cysteine residues for S-nitrosylation are critical for VND7 transactivation activity. Here, we further discuss roles for GSNOR1 in xylem vessel cell differentiation, and provide additional data on the effects of cellular NO level on VND7 activity.

Immunodetection of S-Nitrosoglutathione Reductase Protein in Plant Samples

S-nitrosation, the attachment of a nitroso group to cysteine thiols, has been recognized as an important posttranslational modification of proteins by nitric oxide and related reactive nitrogen species. Mechanisms and significance of S-nitrosation in the regulation of the structure and activity of proteins have been extensively studied in animal and plant systems. In plants, protein S-nitrosation is involved in signaling pathways of plant hormones and regulators during plant growth and development and in responses to abiotic and biotic stress stimuli. S-nitrosoglutathione reductase (GSNOR) has been identified as a key enzyme controlling the intracellular level of S-nitrosothiols. GSNOR irreversibly degrades S-nitrosoglutathione (GSNO), the major low molecular weight S-nitrosothiol involved in the formation of protein S-nitrosothiols through transnitrosylation. GSNOR level and activity in plant cells are modulated during plant development and in response to external stimuli such as pathogen infection. In this chapter, we give a detailed description of the immunochemical detection of the GSNOR protein in plant samples.

S-nitrosylation drives cell senescence and aging in mammals by controlling mitochondrial dynamics and mitophagy

S-nitrosylation, a prototypic redox-based posttranslational modification, is frequently dysregulated in disease. S-nitrosoglutathione reductase (GSNOR) regulates protein S-nitrosylation by functioning as a protein denitrosylase. Deficiency of GSNOR results in tumorigenesis and disrupts cellular homeostasis broadly, including metabolic, cardiovascular, and immune function. Here, we demonstrate that GSNOR expression decreases in primary cells undergoing senescence, as well as in mice and humans during their life span. In stark contrast, exceptionally long-lived individuals maintain GSNOR levels. We also show that GSNOR deficiency promotes mitochondrial nitrosative stress, including excessive S-nitrosylation of Drp1 and Parkin, thereby impairing mitochondrial dynamics and mitophagy. Our findings implicate GSNOR in mammalian longevity, suggest a molecular link between protein S-nitrosylation and mitochondria quality control in aging, and provide a redox-based perspective on aging with direct therapeutic implications.

Anaerobic Bacterial Response to Nitrosative Stress

This chapter provides an overview of current knowledge of how anaerobic bacteria protect themselves against nitrosative stress. Nitric oxide (NO) is the primary source of this stress. Aerobically its removal is an oxidative process, whereas reduction is required anaerobically. Mechanisms required to protect aerobic and anaerobic bacteria are therefore different. Several themes recur in the review. First, how gene expression is regulated often provides clues to the physiological function of the gene products. Second, the physiological significance of reports based upon experiments under extreme conditions that bacteria do not encounter in their natural environment requires reassessment. Third, responses to the primary source of stress need to be distinguished from secondary consequences of chemical damage due to failure of repair mechanisms to cope with extreme conditions. NO is generated by many mechanisms, some of which remain undefined. An example is the recent demonstration that the hybrid cluster protein combines with YtfE (or RIC protein, for repair of iron centres damaged by nitrosative stress) in a new pathway to repair key iron-sulphur proteins damaged by nitrosative stress. The functions of many genes expressed in response to nitrosative stress remain either controversial or are completely unknown. The concentration of NO that accumulates in the bacterial cytoplasm is essentially unknown, so dogmatic statements cannot be made that damage to transcription factors (Fur, FNR, SoxRS, MelR, OxyR) occurs naturally as part of a physiologically relevant signalling mechanism. Such doubts can be resolved by simple experiments to meet six proposed criteria.

Natural Products Containing a Nitrogen-Sulfur Bond

Only about 100 natural products are known to contain a nitrogen-sulfur (N-S) bond. This review thoroughly categorizes N-S bond-containing compounds by structural class. Information on biological source, biological activity, and biosynthesis is included, if known. We also review the role of N-S bond functional groups as post-translational modifications of amino acids in proteins and peptides, emphasizing their role in the metabolism of the cell.

Nitric oxide signalling in cardiovascular health and disease

Nitric oxide (NO) signalling has pleiotropic roles in biology and a crucial function in cardiovascular homeostasis. Tremendous knowledge has been accumulated on the mechanisms of the nitric oxide synthase (NOS)-NO pathway, but how this highly reactive, free radical gas signals to specific targets for precise regulation of cardiovascular function remains the focus of much intense research. In this Review, we summarize the updated paradigms on NOS regulation, NO interaction with reactive oxidant species in specific subcellular compartments, and downstream effects of NO in target cardiovascular tissues, while emphasizing the latest developments of molecular tools and biomarkers to modulate and monitor NO production and bioavailability.

S-Nitrosoglutathione Reductase Dysfunction Contributes to Obesity-Associated Hepatic Insulin Resistance via Regulating Autophagy

Obesity is associated with elevated intracellular nitric oxide (NO) production, which promotes nitrosative stress in metabolic tissues such as liver and skeletal muscle, contributing to insulin resistance. The onset of obesity-associated insulin resistance is due, in part, to the compromise of hepatic autophagy, a process that leads to lysosomal degradation of cellular components. However, it is not known how NO bioactivity might impact autophagy in obesity. Here, we establish that S-nitrosoglutathione reductase (GSNOR), a major protein denitrosylase, provides a key regulatory link between inflammation and autophagy, which is disrupted in obesity and diabetes. We demonstrate that obesity promotes S-nitrosylation of lysosomal proteins in the liver, thereby impairing lysosomal enzyme activities. Moreover, in mice and humans, obesity and diabetes are accompanied by decreases in GSNOR activity, engendering nitrosative stress. In mice with a GSNOR deletion, diet-induced obesity increases lysosomal nitrosative stress and impairs autophagy in the liver, leading to hepatic insulin resistance. Conversely, liver-specific overexpression of GSNOR in obese mice markedly enhances lysosomal function and autophagy and, remarkably, improves insulin action and glucose homeostasis. Furthermore, overexpression of S-nitrosylation-resistant variants of lysosomal enzymes enhances autophagy, and pharmacologically and genetically enhancing autophagy improves hepatic insulin sensitivity in GSNOR-deficient hepatocytes. Taken together, our data indicate that obesity-induced protein S-nitrosylation is a key mechanism compromising the hepatic autophagy, contributing to hepatic insulin resistance.

Nitric Oxide Signalling in Yeast

Nitric oxide (NO) is a cellular signalling molecule widely conserved among organisms, including microorganisms such as bacteria, yeasts, and fungi, and higher eukaryotes such as plants and mammals. NO is mainly produced by the activities of NO synthase (NOS) or nitrite reductase (NIR). There are several NO detoxification systems, including NO dioxygenase (NOD) and S-nitrosoglutathione reductase (GSNOR). NO homeostasis, based on the balance between NO synthesis and degradation, is important for regulating its physiological functions, since an excess of NO causes nitrosative stress due to the high reactivity of NO and NO-derived compounds. In yeast, NO may be involved in stress responses, but the role of NO and the mechanism underlying NO signalling are poorly understood due to the lack of mammalian NOS orthologs in the yeast genome. NOS and NIR activities have been observed in yeast cells, but the gene-encoding NOS and the mechanism by which NO production is catalysed by NIR remain unclear. On the other hand, yeast cells employ NOD and GSNOR to maintain intracellular redox balance following endogenous NO production, treatment with exogenous NO, or exposure to environmental stresses. This article reviews NO metabolism (synthesis, degradation) and its regulation in yeast. The physiological roles of NO in yeast, including the oxidative stress response, are also discussed. Such investigations into NO signalling are essential for understanding how NO modulates the genetics and physiology of yeast. In addition to being responsible for the pathology and pharmacology of various degenerative diseases, NO signalling may be a potential target for the construction and engineering of industrial yeast strains.

Protein S-Nitrosylation Regulates Xylem Vessel Cell Differentiation in Arabidopsis

Post-translational modifications of proteins have important roles in the regulation of protein activity. One such modification, S-nitrosylation, involves the covalent binding of nitric oxide (NO)-related species to a cysteine residue. Recent work showed that protein S-nitrosylation has crucial functions in plant development and environmental responses. In the present study, we investigated the importance of protein S-nitrosylation for xylem vessel cell differentiation using a forward genetics approach. We performed ethyl methanesulfonate mutagenesis of a transgenic Arabidopsis 35S::VND7-VP16-GR line in which the activity of VASCULAR-RELATED NAC-DOMAIN7 (VND7), a key transcription factor involved in xylem vessel cell differentiation, can be induced post-translationally by glucocorticoid treatment, with the goal of obtaining suppressor mutants that failed to differentiate ectopic xylem vessel cells; we named these mutants suppressor of ectopic vessel cell differentiation induced by VND7 (seiv) mutants. We found the seiv1 mutant to be a recessive mutant in which ectopic xylem cell differentiation was inhibited, especially in aboveground organs. In seiv1 mutants, a single nucleic acid substitution (G to A) leading to an amino acid substitution (E36K) was present in the gene encoding S-NITROSOGLUTATHIONE REDUCTASE 1 (GSNOR1), which regulates the turnover of the natural NO donor, S-nitrosoglutathione. An in vitro S-nitrosylation assay revealed that VND7 can be S-nitrosylated at Cys264 and Cys320 located near the transactivation activity-related domains, which were shown to be important for transactivation activity of VND7 by transient reporter assay. Our results suggest crucial roles for GSNOR1-regulated protein S-nitrosylation in xylem vessel cell differentiation, partly through the post-translational modification of VND7.

Reactive nitrogen species (RNS)-resistant microbes: adaptation and medical implications

Nitrosative stress results from an increase in reactive nitrogen species (RNS) within the cell. Though the RNS - nitric oxide (·NO) and peroxynitrite (ONOO-) - play pivotal physiological roles, at elevated concentrations, these moieties can be poisonous to both prokaryotic and eukaryotic cells alike due to their capacity to disrupt a variety of essential biological processes. Numerous microbes are known to adapt to nitrosative stress by elaborating intricate strategies aimed at neutralizing RNS. In this review, we will discuss both the enzymatic systems dedicated to the elimination of RNS as well as the metabolic networks that are tailored to generate RNS-detoxifying metabolites - α-keto-acids. The latter has been demonstrated to nullify RNS via non-enzymatic decarboxylation resulting in the production of a carboxylic acid, many of which are potent signaling molecules. Furthermore, as aerobic energy production is severely impeded during nitrosative stress, alternative ATP-generating modules will be explored. To that end, a holistic understanding of the molecular adaptation to nitrosative stress, reinforces the notion that neutralization of toxicants necessitates significant metabolic reconfiguration to facilitate cell survival. As the alarming rise in antimicrobial resistant pathogens continues unabated, this review will also discuss the potential for developing therapies that target the alternative ATP-generating machinery of bacteria.

S-Nitrosylation inhibits the kinase activity of tomato phosphoinositide-dependent kinase 1 (PDK1)

It is well known that the reactive oxygen species NO can trigger cell death in plants and other organisms, but the underlying molecular mechanisms are not well understood. Here we provide evidence that NO may trigger cell death in tomato (Solanum lycopersicum) by inhibiting the activity of phosphoinositide-dependent kinase 1 (SlPDK1), a conserved negative regulator of cell death in yeasts, mammals, and plants, via S-nitrosylation. Biotin-switch assays indicated that SlPDK1 is a target of S-nitrosylation. Moreover, the kinase activity of SlPDK1 was inhibited by S-nitrosoglutathione in a concentration-dependent manner, indicating that SlPDK1 activity is abrogated by S-nitrosylation. The S-nitrosoglutathione-induced inhibition was reversible in the presence of a reducing agent but additively enhanced by hydrogen peroxide (H₂O₂). Our LC-MS/MS analyses further indicated that SlPDK1 is primarily S-nitrosylated on a cysteine residue at position 128 (Cys¹²⁸), and substitution of Cys¹²⁸ with serine completely abolished SlPDK1 kinase activity, suggesting that S-nitrosylation of Cys¹²⁸ is responsible for SlPDK1 inhibition. In summary, our results establish a potential link between NO-triggered cell death and inhibition of the kinase activity of tomato PDK1.

Nitric Oxide (NO) in Plant Heat Stress Tolerance: Current Knowledge and Perspectives

High temperature is one of the biggest abiotic stress challenges for agriculture. While, Nitric oxide (NO) is gaining increasing attention from plant science community due to its involvement in resistance to various plant stress conditions, its implications on heat stress tolerance is still unclear. Several lines of evidence indicate NO as a key signaling molecule in mediating various plant responses such as photosynthesis, oxidative defense, osmolyte accumulation, gene expression, and protein modifications under heat stress. Furthermore, the interactions of NO with other signaling molecules and phytohormones to attain heat tolerance have also been building up in recent years. Nevertheless, deep insights into the functional intermediaries or signal transduction components associated with NO-mediated heat stress signaling are imperative to uncover their involvement in plant hormone induced feed-back regulations, ROS/NO balance, and stress induced gene transcription. Although, progress is underway, much work remains to define the functional relevance of this molecule in plant heat tolerance. This review provides an overview on current status and discuss knowledge gaps in exploiting NO, thereby enhancing our understanding of the role of NO in plant heat tolerance.

Increased GSNOR Expression during Aging Impairs Cognitive Function and Decreases S-Nitrosation of CaMKIIα

As the population ages, an increasing number of people suffer from age-related cognitive impairment. However, the mechanisms underlying this process remain unclear. Here, we found that S-nitrosoglutathione reductase (GSNOR), the key enzyme that metabolizes intracellular nitric oxide (NO) and regulates S-nitrosation, was significantly increased in the hippocampus of both aging humans and mice. Transgenic mice overexpressing GSNOR exclusively in neurons showed cognitive impairment in behavioral tests, including the Morris water maze, fear conditioning, and the Y-maze test. We also found that GSNOR transgenic mice have LTP defects and lower dendrite spine density, whereas GSNOR knock-out mice rescued the age-related cognitive impairment. Analysis of S-nitrosation showed significantly decreased hippocampal CaMKIIα S-nitrosation in naturally aged mice and GSNOR transgenic mice. Consistent with the change in CaMKIIα S-nitrosation, the accumulation of CaMKIIα in the hippocampal synaptosomal fraction, as well as its downstream signaling targets p(S831)-GLUR1, was also significantly decreased. All these effects could be rescued in the GSNOR knock-out mice. We further verified that the S-nitrosation of CaMKIIα was responsible for the CaMKIIα synaptosomal accumulation by mutating CaMKIIα S-nitrosated sites (C280/C289). Upregulation of the NO signaling pathway rescued the cognitive impairment in GSNOR transgenic mice. In summary, our research demonstrates that GSNOR impairs cognitive function in aging and it could serve as a new potential target for the treatment of age-related cognitive impairment. In contrast to the free radical theory of aging, NO signaling deficiency may be the main mediator of age-related cognitive impairment.SIGNIFICANCE STATEMENT This study indicated that S-nitrosoglutathione reductase (GSNOR), a key protein S-nitrosation metabolic enzyme, is a new potential target in age-related cognitive impairment; and in contrast to the free radical theory of aging, NO signaling deficiency may be the main cause of this process. In addition, increased GSNOR expression during aging decreases S-nitrosation of CaMKIIα and reduces CaMKIIα synaptosomal accumulation. To our knowledge, it is for the first time to show the cellular function regulation of CaMKIIα by GSNOR-dependent S-nitrosation as a new post-translational modification after its phosphorylation was explored. These findings elucidate a novel mechanism of age-related cognitive impairment and may provide a new potential target and strategy for slowing down this process.

Chronicles of a reductase: Biochemistry, genetics and physio-pathological role of GSNOR

S-nitrosylation is a major redox posttranslational modification involved in cell signaling. The steady state concentration of S-nitrosylated proteins depends on the balance between the relative ability to generate nitric oxide (NO) via NO synthase and to reduce nitrosothiols by denitrosylases. Numerous works have been published in last decades regarding the role of NO and S-nitrosylation in the regulation of protein structure and function, and in driving cellular activities in vertebrates. Notwithstanding an increasing number of observations indicates that impairment of denitrosylation equally affects cellular homeostasis, there is still no report providing comprehensive knowledge on the impact that denitrosylation has on maintaining correct physiological processes and organ activities. Among denitrosylases, S-nitrosoglutathione reductase (GSNOR) represents the prototype enzyme to disclose how denitrosylation plays a crucial role in tuning NO-bioactivity and how much it deeply impacts on cell homeostasis and human patho-physiology. In this review we attempt to illustrate the history of GSNOR discovery and provide the evidence so far reported in support of GSNOR implications in development and human disease.

The role of hypernitrosylation in the pathogenesis and pathophysiology of neuroprogressive diseases

There is a wealth of data indicating that de novo protein S-nitrosylation in general and protein transnitrosylation in particular mediates the bulk of nitric oxide signalling. These processes enable redox sensing and facilitate homeostatic regulation of redox dependent protein signalling, function, stability and trafficking. Increased S-nitrosylation in an environment of increasing oxidative and nitrosative stress (O&NS) is initially a protective mechanism aimed at maintaining protein structure and function. When O&NS becomes severe, mechanisms governing denitrosylation and transnitrosylation break down leading to the pathological state referred to as hypernitrosylation (HN). Such a state has been implicated in the pathogenesis and pathophysiology of several neuropsychiatric and neurodegenerative diseases and we investigate its potential role in the development and maintenance of neuroprogressive disorders. In this paper, we propose a model whereby the hypernitrosylation of a range of functional proteins and enzymes lead to changes in activity which conspire to produce at least some of the core abnormalities contributing to the development and maintenance of pathology in these illnesses.

How widespread is stable protein S-nitrosylation as an end-effector of protein regulation?

Over the last 25 years protein S-nitrosylation, also known more correctly as S-nitrosation, has been progressively implicated in virtually every nitric oxide-regulated process within the cardiovascular system. The current, widely-held paradigm is that S-nitrosylation plays an equivalent role as phosphorylation, providing a stable and controllable post-translational modification that directly regulates end-effector target proteins to elicit biological responses. However, this concept largely ignores the intrinsic instability of the nitrosothiol bond, which rapidly reacts with typically abundant thiol-containing molecules to generate more stable disulfide bonds. These protein disulfides, formed via a nitrosothiol intermediate redox state, are rationally anticipated to be the predominant end-effector modification that mediates functional alterations when cells encounter nitrosative stimuli. In this review we present evidence and explain our reasoning for arriving at this conclusion that may be controversial to some researchers in the field.

Characterization of S-nitrosoglutathione reductase from Brassica and Lactuca spp. and its modulation during plant development

Cellular homeostasis of S-nitrosoglutathione (GSNO), a major cache of nitric oxide bioactivity in plants, is controlled by the NADH-dependent S-nitrosoglutathione reductase (GSNOR) belonging to the family of class III alcohol dehydrogenases (EC 1.1.1.1). GSNOR is a key regulator of S-nitrosothiol metabolism and is involved in plant responses to abiotic and biotic stresses. This study was focused on GSNOR from two important crop plants, cauliflower (Brassica oleracea var. botrytis, BoGSNOR) and lettuce (Lactuca sativa, LsGSNOR). Both purified recombinant GSNORs were characterized in vitro and found to exists as dimers, exhibit high thermal stability and substrate preference towards GSNO, although both enzymes have dehydrogenase activity with a broad range of long-chain alcohols and ω-hydroxy fatty acids in presence of NAD+. Data on enzyme affinities to their cofactors NADH and NAD+ obtained by isothermal titration calorimetry suggest the high affinity to NADH might underline the GSNOR capacity to function in the intracellular environment. GSNOR activity and gene expression peak during early developmental stages of lettuce and cauliflower at 20 and 30 days after germination, respectively. GSNOR activity was also measured in four other Lactuca spp. genotypes with different degree of resistance to biotrophic pathogen Bremia lactucae. Higher GSNOR activities were found in non-infected plants of susceptible genotypes L. sativa UCDM2 and L. serriola as compared to resistant genotypes. GSNOR and GSNO were localized by confocal laser scanning microscopy in vascular bundles and in epidermal and parenchymal cells of leaf cross-sections. The presented results bring new insight in the role of GSNOR in the regulation of S-nitrosothiol levels in plant growth and development.

Nitric Oxide Regulates Protein Methylation during Stress Responses in Plants

Methylation and nitric oxide (NO)-based S-nitrosylation are highly conserved protein posttranslational modifications that regulate diverse biological processes. In higher eukaryotes, PRMT5 catalyzes Arg symmetric dimethylation, including key components of the spliceosome. The Arabidopsis prmt5 mutant shows severe developmental defects and impaired stress responses. However, little is known about the mechanisms regulating the PRMT5 activity. Here, we report that NO positively regulates the PRMT5 activity through S-nitrosylation at Cys-125 during stress responses. In prmt5-1 plants, a PRMT5^C125S transgene, carrying a non-nitrosylatable mutation at Cys-125, fully rescues the developmental defects, but not the stress hypersensitive phenotype and the responsiveness to NO during stress responses. Moreover, the salt-induced Arg symmetric dimethylation is abolished in PRMT5^C125S/prmt5-1 plants, correlated to aberrant splicing of pre-mRNA derived from a stress-related gene. These findings define a mechanism by which plants transduce stress-triggered NO signal to protein methylation machinery through S-nitrosylation of PRMT5 in response to environmental alterations.

O-Aminobenzoyl-S-nitrosoglutathione: A fluorogenic, cell permeable, pseudo-substrate for S-nitrosoglutathione reductase

S-nitrosoglutathione reductase (GSNOR) is a multifunctional enzyme. It can catalyze NADH-dependent reduction of S-nitrosoglutathione (GSNO); as well as NAD⁺-dependent oxidation of hydroxymethylglutathione (HMGSH; an adduct formed by the spontaneous reaction between formaldehyde and glutathione). While initially recognized as the enzyme that is involved in formaldehyde detoxification, increasing amount of evidence has shown that GSNOR also plays a significant role in nitric oxide mediated signaling through its modulation of protein S-nitrosothiol signaling. In humans, GSNOR/S-nitrosothiols have been implicated in the etiology of several diseases including lung cancer, cystic fibrosis, asthma, pulmonary hypertension, and neuronal dysfunction. Currently, it is not possible to monitor the activity of GSNOR in live cells. In this article, we present a new compound, O-aminobenzoyl-S-nitrosoglutathione (OAbz-GSNO), which acts as a fluorogenic pseudo-substrate for GSNOR with an estimated K_m value of 320µM. The weak OAbz-GSNO fluorescence increases by approximately 14 fold upon reduction of its S-NO moiety. In live cell imaging studies, OAbz-GSNO is readily taken up by primary pulmonary endothelial cells and localizes to the same perinuclear region as GSNOR. The perinuclear OAbz-GSNO fluorescence increases in a time dependent manner and this increase in fluorescence is abolished by siRNA knockdown of GSNOR or by treatment with GSNOR-specific inhibitors N6022 and C3. Taken together, these data demonstrate that OAbz-GSNO can be used as a tool to monitor the activity of GSNOR in live cells.

Stress Adaptation

Fungal species display an extraordinarily diverse range of lifestyles. Nevertheless, the survival of each species depends on its ability to sense and respond to changes in its natural environment. Environmental changes such as fluctuations in temperature, water balance or pH, or exposure to chemical insults such as reactive oxygen and nitrogen species exert stresses that perturb cellular homeostasis and cause molecular damage to the fungal cell. Consequently, fungi have evolved mechanisms to repair this damage, detoxify chemical insults, and restore cellular homeostasis. Most stresses are fundamental in nature, and consequently, there has been significant evolutionary conservation in the nature of the resultant responses across the fungal kingdom and beyond. For example, heat shock generally induces the synthesis of chaperones that promote protein refolding, antioxidants are generally synthesized in response to an oxidative stress, and osmolyte levels are generally increased following a hyperosmotic shock. In this article we summarize the current understanding of these and other stress responses as well as the signaling pathways that regulate them in the fungi. Model yeasts such as Saccharomyces cerevisiae are compared with filamentous fungi, as well as with pathogens of plants and humans. We also discuss current challenges associated with defining the dynamics of stress responses and with the elaboration of fungal stress adaptation under conditions that reflect natural environments in which fungal cells may be exposed to different types of stresses, either sequentially or simultaneously.