Molecular cloning and expression of cDNAs coding for soluble guanylate cyclase from rat lung

Soluble guanylyl cyclase: A novel target for the treatment of vascular cognitive impairment?

Vascular cognitive impairment (VCI) describes neurodegenerative disorders characterized by a vascular component. Pathologically, it involves decreased cerebral blood flow (CBF), white matter lesions, endothelial dysfunction, and blood-brain barrier (BBB) impairments. Molecularly, oxidative stress and inflammation are two of the major underlying mechanisms. Nitric oxide (NO) physiologically stimulates soluble guanylate cyclase (sGC) to induce cGMP production. However, under pathological conditions, NO seems to be at the basis of oxidative stress and inflammation, leading to a decrease in sGC activity and expression. The native form of sGC needs a ferrous heme group bound in order to be sensitive to NO (Fe(II)sGC). Oxidation of sGC leads to the conversion of ferrous to ferric heme (Fe(III)sGC) and even heme-loss (apo-sGC). Both Fe(III)sGC and apo-sGC are insensitive to NO, and the enzyme is therefore inactive. sGC activity can be enhanced either by targeting the NO-sensitive native sGC (Fe(II)sGC), or the inactive, oxidized sGC (Fe(III)sGC) and the heme-free apo-sGC. For this purpose, sGC stimulators acting on Fe(II)sGC and sGC activators acting on Fe(III)sGC/apo-sGC have been developed. These sGC agonists have shown their efficacy in cardiovascular diseases by restoring the physiological and protective functions of the NO-sGC-cGMP pathway, including the reduction of oxidative stress and inflammation, and improvement of vascular functioning. Yet, only very little research has been performed within the cerebrovascular system and VCI pathology when focusing on sGC modulation and its potential protective mechanisms on vascular and neural function. Therefore, within this review, the potential of sGC as a target for treating VCI is highlighted.

IL-1β dysregulates cGMP signaling in the newborn lung

Cyclic guanosine monophosphate (cGMP) signaling is an important regulator of newborn lung function and development. Although cGMP signaling is decreased in many models of newborn lung injury, the mechanisms are poorly understood. We determined how IL-1β regulates the expression of the α1-subunit of soluble guanylate cyclase (sGCα1), a prime effector of pulmonary cGMP signaling. Physiologic levels of IL-1β were discovered to rapidly decrease sGCα1 mRNA expression in a human fetal lung fibroblast cell line (IMR-90 cells) and protein levels in primary mouse pup lung fibroblasts. This sGCα1 expression inhibition appeared to be at a transcriptional level; IL-1β treatment did not alter sGCα1 mRNA stability although it reduced sGCα1 promoter activity. TGFβ-activated kinase 1 (TAK1) was determined to be required for IL-1β's regulation of sGCα1 expression; TAK1 knockdown protected sGCα1 mRNA expression in IL-1β-treated IMR-90 cells. Moreover, heterologously expressed TAK1 was sufficient to decrease sGCα1 mRNA levels in those cells. Nuclear factor-kappaB (NF-κB) signaling played a critical role in the IL-1β-TAK1-sGCα1 regulatory pathway; chromatin immunoprecipitation studies demonstrated enhanced activated NF-kB subunit (RelA) binding to the sGCα1 promoter after IL-1β treatment unless were treated with an IκB kinase2 inhibitor. Also, this NF-kB signaling inhibition protected sGCα1 expression in IL-1β-treated fibroblasts. Lastly, using transgenic mice in which active IL-1β was conditionally expressed in lung epithelial cells, we established that IL-1β expression is sufficient to stimulate TAK1 and decrease sGCα1 protein expression in the newborn lung. Together these results detail the role and mechanisms by which IL-1β inhibits cGMP signaling in the newborn lung.

Transforming growth factor-β downregulates sGC subunit expression in pulmonary artery smooth muscle cells via MEK and ERK signaling

TGFβ activation during newborn lung injury decreases the expression of pulmonary artery smooth muscle cell (PASMC)-soluble guanylate cyclase (sGC), a critical mediator of nitric oxide signaling. Using a rat PASMC line (CS54 cells), we determined how TGFβ downregulates sGC expression. We found that TGFβ decreases sGC expression through stimulating its type I receptor; TGFβ type I receptor (TGFβR1) inhibitors prevented TGFβ-1-mediated decrease in sGCα1 subunit mRNA levels in the cells. However, TGFβR1-Smad mechanisms do not regulate sGC; effective knockdown of Smad2 and Smad3 expression and function did not protect sGCα1 mRNA levels during TGFβ-1 exposure. A targeted small-molecule kinase inhibitor screen suggested that MEK signaling regulates sGC expression in TGFβ-stimulated PASMC. TGFβ activates PASMC MEK/ERK signaling; CS54 cell treatment with TGFβ-1 increased MEK and ERK phosphorylation in a biphasic, time- and dose-dependent manner. Moreover, MEK/ERK activity appears to be required for TGFβ-mediated sGC expression inhibition in PASMC; MEK and ERK inhibitors protected sGCα1 mRNA expression in TGFβ-1-treated CS54 cells. Nuclear ERK activity is sufficient for sGC regulation; heterologous expression of a nucleus-retained, constitutively active ERK2-MEK1 fusion protein decreased CS54 cell sGCα1 mRNA levels. The in vivo relevance of this TGFβ-MEK/ERK-sGC downregulation pathway is suggested by the detection of ERK activation and sGCα1 protein expression downregulation in TGFβ-associated mouse pup hyperoxic lung injury, and the determination that ERK decreases sGCα1 protein expression in TGFβ-1-treated primary PASMC obtained from mouse pups. These studies identify MEK/ERK signaling as an important pathway by which TGFβ regulates sGC expression in PASMC.

Thiol-Based Redox Modulation of Soluble Guanylyl Cyclase, the Nitric Oxide Receptor

SIGNIFICANCE

Soluble guanylyl cyclase (sGC), which produces the second messenger cyclic guanosine 3', 5'-monophosphate (cGMP), is at the crossroads of nitric oxide (NO) signaling: sGC catalytic activity is both stimulated by NO binding to the heme and inhibited by NO modification of its cysteine (Cys) thiols (S-nitrosation). Modulation of sGC activity by thiol oxidation makes sGC a therapeutic target for pathologies originating from oxidative or nitrosative stress. sGC has an unusually high percentage of Cys for a cytosolic protein, the majority solvent exposed and therefore accessible modulatory targets for biological and pathophysiological signaling. Recent Advances: Thiol oxidation of sGC contributes to the development of cardiovascular diseases by decreasing NO-dependent cGMP production and thereby vascular reactivity. This thiol-based resistance to NO (e.g., increase in peripheral resistance) is observed in hypertension and hyperaldosteronism.

CRITICAL ISSUES

Some roles of specific Cys thiols have been identified in vitro. So far, it has not been possible to pinpoint the roles of specific Cys of sGC in vivo and to investigate the molecular mechanisms in an animal model.

FUTURE DIRECTIONS

The role of Cys as redox sensors, intermediates of activation, and mediators of change in sGC conformation, activity, and dimerization remains largely unexplored. To understand modulation of sGC activity, it is critical to investigate the roles of specific oxidative thiol modifications that are formed during these processes. Where the redox state of sGC thiols contribute to pathologies (vascular resistance and sGC desensitization by NO donors), it becomes crucial to design therapeutic strategies to restore sGC to its normal, physiological thiol redox state. Antioxid. Redox Signal. 26, 137-149.

Constitutive cyclic GMP accumulation in Arabidopsis thaliana compromises systemic acquired resistance induced by an avirulent pathogen by modulating local signals

The infection of Arabidopsis thaliana plants with avirulent pathogens causes the accumulation of cGMP with a biphasic profile downstream of nitric oxide signalling. However, plant enzymes that modulate cGMP levels have yet to be identified, so we generated transgenic A. thaliana plants expressing the rat soluble guanylate cyclase (GC) to increase genetically the level of cGMP and to study the function of cGMP in plant defence responses. Once confirmed that cGMP levels were higher in the GC transgenic lines than in wild-type controls, the GC transgenic plants were then challenged with bacterial pathogens and their defence responses were characterized. Although local resistance was similar in the GC transgenic and wild-type lines, differences in the redox state suggested potential cross-talk between cGMP and the glutathione redox system. Furthermore, large-scale transcriptomic and proteomic analysis highlighted the significant modulation of both gene expression and protein abundance at the infection site, inhibiting the establishment of systemic acquired resistance. Our data indicate that cGMP plays a key role in local responses controlling the induction of systemic acquired resistance in plants challenged with avirulent pathogens.

Phenylephrine activates eNOS Ser 1177 phosphorylation and nitric oxide signaling in renal hypertensive rat aorta

The endothelial nitric oxide synthase (eNOS) plays an important role in the control of the vascular tone. This work aimed to evaluate the role of an α1-adrenoceptor agonist phenylephrine (PE) on eNOS activity and downstream signaling pathway activation in normotensive (2K) and renal hypertensive (2K-1C) intact-endothelium rat aortas. Concentration-effect curves were performed for PE in intact-endothelium aortas from 2K and 2K-1C rats, in the absence of or in the presence of NOS or soluble guanylyl cyclase (sGC) inhibitor. Intact endothelium aortas were stimulated with PE in organ chambers and eNOS Ser(1177)/Thr(495) phosphorylation expression was evaluated by western blot. Nitric Oxide (NO) production was evaluated in isolated endothelial cells from 2K and 2K-1C rat aortas by flow-cytometry using NO selective fluorescent probe, DAF-2DA. The sGC activity/expression was also evaluated. PE-induced contractile response is lower in 2K-1C than in 2K intact-endothelium rat aorta. This is due to higher eNOS Ser(1177) phosphorylation in 2K-1C, which induces the eNOS overactivation. It was abolished by NOS or sGC inhibition. Phenylephrine reduces NO production in 2K as compared to the basal level, but it is not modified in 2K-1C. In PE-stimulated endothelial cells, the NO production is higher in 2K-1C than in 2K. Phenylephrine induces higher cGMP production in 2K-1C than in 2K, despite the lower expression of sGC in 2K-1C. Our results suggest that alpha1-adrenoceptor activation contributes to the increased activity of the enzyme eNOS by Ser(1177) phosphorylation in 2K-1C intact-endothelium aorta, which consequently decreases PE-induced contractile response.

Pulmonary and systemic vasodilator responses to the soluble guanylyl cyclase stimulator, BAY 41-8543, are modulated by nitric oxide

BAY 41-8543 is a nitric oxide (NO)-independent stimulator of soluble guanylyl cyclase (sGC). Responses to intravenous injections of BAY 41-8543 were investigated under baseline and elevated tone conditions and when NO synthase (NOS) was inhibited with N(ω)-nitro-L-arginine methyl ester (L-NAME). Under baseline conditions, intravenous injections of BAY 41-8543 caused small decreases in pulmonary arterial pressure, larger decreases in systemic arterial pressure, and increases in cardiac output. When pulmonary arterial pressure was increased to ∼30 mmHg with an intravenous infusion of U-46619, intravenous injections of BAY 41-8543 produced larger dose-dependent decreases in pulmonary arterial pressure, and the relative decreases in pulmonary and systemic arterial pressure in response to the sGC stimulator were similar. Treatment with L-NAME markedly decreased responses to BAY 41-8543 when pulmonary arterial pressure was increased to similar values (∼30 mmHg) in U-46619-infused and in U-46619-infused plus L-NAME-treated animals. The intravenous injection of a small dose of sodium nitroprusside (SNP) when combined with BAY 41-8543 enhanced pulmonary and systemic vasodilator responses to the sGC stimulator in L-NAME-treated animals. The present results indicate that BAY 41-8543 has similar vasodilator activity in the systemic and pulmonary vascular beds when pulmonary vasoconstrictor tone is increased with U-46619. These results demonstrate that pulmonary and systemic vasodilator responses to BAY 41-8543 are significantly attenuated when NOS is inhibited by L-NAME and show that vasodilator responses to BAY 41-8543 are enhanced when combined with a small dose of SNP in L-NAME-treated animals. The present results are consistent with the concept that pulmonary and systemic vasodilator responses to the sGC stimulator are NO-independent; however, the vasodilator activity of the compound is greatly diminished when endogenous NO production is inhibited with L-NAME. These data show that BAY 41-8543 has similar vasodilator activity in the pulmonary and systemic vascular beds in the rat.

The function of NO-sensitive guanylyl cyclase: what we can learn from genetic mouse models

The signaling molecule nitric oxide (NO) acts as physiological activator of NO-sensitive guanylyl cyclase (NO-GC) in the cardiovascular, gastrointestinal and nervous systems. Two isoforms of NO-GC are known to exist on the protein level. The enzyme is a heterodimer consisting of an alpha (alpha(1) or alpha(2)) and a beta subunit (beta(1)). Strategies for the genomic deletion of either subunit have been developed in the recent years. Removal of one of the two isoforms by deletion of one of the alpha subunits allowed the investigation of the specific functions of the respective isoform. The deletion of the beta(1) subunit led to complete knock-out thus completely disrupting the NO/cGMP signaling cascade. The phenotypes of these KO mice have corroborated the already known physiological importance of the NO/cGMP cascade e.g. in the regulation of blood pressure, platelet inhibition, interneuronal communication; yet, they have also given hints to novel functions and mechanisms. In addition, mice lacking both NO-GC isoforms permitted the investigation of possible cGMP-independent signaling pathways of NO. As cell- and tissue-specific knock-out models are beginning to emerge, a more detailed analysis of the importance of the NO receptor in specific tissues will become possible.

A short history of cGMP, guanylyl cyclases, and cGMP-dependent protein kinases

Here, we review the early studies on cGMP, guanylyl cyclases, and cGMP-dependent protein kinases to facilitate understanding of development of this exciting but complex field of research encompassing pharmacology, biochemistry, physiology, and molecular biology of these important regulatory molecules.

Late, but not early, inhibition of soluble guanylate cyclase decreases mortality in a rat sepsis model

Overproduction of nitric oxide and activation of soluble guanylate cyclase (sGC) are important in sepsis-induced hypotension and hyporesponsiveness to vasoconstrictors. A time course of the expression and activity of sGC in a sepsis model [cecal ligation and puncture (CLP)] was evaluated in rats. Soluble GC alpha-1 and beta-1 subunit mRNA levels increased in the lungs, but not in the aorta. However, in both tissues, the protein levels increased 24 h after sepsis and remained high for up to 48 h. Sodium nitroprusside-stimulated cGMP accumulation was higher 48 h after CLP in the lung and aorta. NOS-2 protein expression peaked 24 h after CLP, decreasing thereafter. The impact of inhibiting the expression of sGC early (8 h) or late (20 h) on vascular reactivity and the indexes of organ damage and mortality were also studied. Late administration of methylene blue (MB) or ODQ (1H-[1,2,4]-oxadiazole[4,3-a]quinoxalin-1-one) restored the blood pressure and vascular responsiveness to vasoconstrictors to normal levels but was ineffective in early sepsis. Late MB injection reduced the plasma levels of urea, creatinine, and lactate. MB improved the survival if administered late, but it increased the mortality when administrated early after sepsis onset. The increased sGC expression/activity may be relevant for the late hypotension and hyporesponsiveness to vasoconstrictors in sepsis. In accordance, MB increased survival if administered in late sepsis, but not in early sepsis. Therefore, differential responsiveness to sGC during the course of sepsis may determine the success or failure of treatment with sGC inhibitors.

The role of nitric oxide in female reproduction

The cardiovascular system undergoes profound changes during pregnancy. Maternal intravascular volume begins to increase in the first trimester rising an average of 45% by term.1Cardiac output increases similarly2and is redistributed to organs whose functions are crucial for a successful pregnancy. In the guinea pig, uterine artery (UA) blood flow increases 3500%, while mesenteric and renal artery blood flows increase only 90% and 10% respectively.3Blood flow to the trunk actually diminishes. The mechanism underlying this redistribution is unknown. Coupled with the rise in cardiac output is a decrease in the systemic pressor response to angiotensin II (AII), norepinephrine(NE), and epinephrine.4–8There is also a decrease in the contraction response among some but not all vascular beds. For example, contraction of UA to NE and thromboxane is characteristically reduced by pregnancy, whereas the response of the carotid artery is unaltered8–10Since pregnancy does not alter neuroeffector mechanisms of NE such as release, receptor sensitivity, and accumulation11, changes in sympathetic control during pregnancy must be dependent on alterations at sites other than the neuroeffector junction. We have hypothesized that the mechanisms which alter vascular reactivity during pregnancy also mediate the redistribution of maternal cardiac output.9We have further hypothesized that many of these mechanisms involve endothelium-dependent factors which are modulated by sex hormones.

Inhibitory phosphorylation of soluble guanylyl cyclase by muscarinic m2 receptors via Gbetagamma-dependent activation of c-Src kinase

In gastrointestinal smooth muscle, cGMP levels in response to relaxant agonists are regulated by activation of phosphodiesterase 5 and inhibition of soluble guanylyl cyclase (sGC) in a feedback mechanism via cGMP-dependent protein kinase. The aim of the present study was to determine whether contractile agonists modulate cGMP levels by cross-regulating sGC activity. In gastric muscle cells, acetylcholine (ACh) stimulated Src activity and induced sGC phosphorylation. Concurrent stimulation of cells with ACh attenuated sGC activity and cGMP formation in response to the nitric oxide (NO) donor, S-nitrosoglutathione (GSNO). The effect of ACh on Src activity, sGC phosphorylation, and on GSNO-stimulated sGC activity and cGMP formation were blocked by the m2 receptor antagonist (methoctramine), pertussis toxin, and by inhibitors of phosphatidylinositol 3 kinase, LY294002 [2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride], or Src kinase, 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine, in dispersed muscle cells and in cells expressing Galpha(i) minigene or Gbetagamma-scavenging peptide, whereas the m3 receptor antagonist, N-(2-chloroethyl)-4-piperidinyl diphenylacetate, or expression of the Galpha(q) minigene had no effect. ACh also attenuated sGC activity and cGMP formation in response to the NO-independent activator, YC-1 [3-(5'-hydroxymethyl-2'furyl)-1-benzylindazole]. The pattern implied that phosphorylation of sGC by c-Src kinase inhibits NO-sensitive sGC activity, and the inhibition was not due to a decrease in the binding of NO but probably due to decrease in catalytic activity. We conclude that cGMP levels are cross-regulated by contractile agonists via a mechanism that involves c-Src-dependent phosphorylation of sGC, leading to inhibition of sGC activity and cGMP formation. The finding highlights a novel mechanism for attenuation of the NO/sGC/cGMP signal by G(i)-coupled contractile agonists, in addition to their inhibitory effect on adenylyl cyclase and cAMP formation.

cGMP-enhancing- and alpha1A/alpha1D-adrenoceptor blockade-derived inhibition of Rho-kinase by KMUP-1 provides optimal prostate relaxation and epithelial cell anti-proliferation efficacy

BACKGROUND

Soluble guanylyl cyclase (sGC)/cyclic guanosine monophosphate (cGMP)/protein kinase G (PKG) and Rho kinase (ROCK2) pathways are important in the regulation of prostate smooth muscle tone. This study is aimed to examine the relaxation activities of a sGC activator and PDE5A/ROCK2 inhibitor KMUP-1 in rat prostate and associated anti-proliferation activity in human prostatic epithelial cells.

METHODS

The action characteristics of KMUP-1 were identified by isometric tension measurement, receptor binding assay, Western blotting and radioimmunoassay in rat prostate. Anti-proliferation activity of KMUP-1 in human prostatic epithelial PZ-HPV-7 cells was identified using flow cytometry and real time QRT-PCR.

RESULTS

KMUP-1 inhibited phenylephrine-induced contractility in a concentration-dependent manner. KMUP-1 possessed potent alpha(1A/)alpha(1D)-adrenoceptor binding inhibition activity, increased cAMP/cGMP levels and increased the expression of sGC, PKG, and PKA protein in rat prostate. Moreover, KMUP-1 inhibited phenylephrine-induced ROCK2 expression. KMUP-1 inhibited cell growth, arrested the cell cycle at G(0)/G(1) phase and increased the expression of p21 in PZ-HPV-7 cells.

CONCLUSIONS

These results broaden our knowledge of sGC/cGMP/PKG and ROCK2 regulation on the relaxation and proliferation of prostate, which may help in the design of benign prostate hyperplasia (BPH) therapies that target these signaling pathways. KMUP-1 possesses the potential benefit in the treatment of BPH by its alpha(1A/)alpha(1D)-adrenoceptor blockade, sGC activation, inhibition of PDE5A and ROCK2 and p21 protein enhancement, leading to attenuation of the smooth muscle tone and the proliferation of epithelial PZ-HPV-7 cells. The synergistic contribution of these pathways by KMUP-1 may benefit BPH patients with lower urinary tract symptoms.

Acrylamide analog as a novel nitric oxide-independent soluble guanylyl cyclase activator

Soluble guanylyl cyclase (sGC) is a target enzyme for endogenous nitric oxide (NO), and it converts GTP to cyclic GMP (guanosine 3',5'-cyclic monophosphate) as part of a cascade that results in physiological processes such as smooth muscle relaxation, neurotransmission, and inhibition of platelet aggregation. Here we examine a representative of the novel class sCG activators, A-778935 ((+/-)-cis-3-[2-(2,2-dimethyl-propylsulfanyl)-pyridin-3-yl]-N-(3-hydroxy-cyclohexyl)-acrylamide). A-778935 activated sGC synergistically with sodium nitroprusside (SNP) over a wide range of concentration, inducing up to 420-fold activation. A specific inhibitor of sGC, ODQ (1H-[1,2,4]-oxadiazolo[4,3-alpha]quinoxalin-1-one), did not block basal sGC activity, but competitively inhibited the activation by A-778935. A-778935, with or without SNP, did not activate heme-deficient sGC, indicating that the activation of sGC by A-778935 is fully heme-dependent. A-778935 increased intracellular cGMP level dose-dependently in smooth muscle cells. In the presence of 1 microM SNP, a lower concentration of A-778935 increased cGMP than A-778935 alone, and the cGMP concentration reached the same level at 100 microM of A-778935. A-778935 relaxed cavernosum tissue strips in a dose-dependent manner; and in the presence of 1 microM SNP, A-778935 relaxed the strips more potently, shifting the dose-response curve to the left. This novel activator of sGC may have potential efficacy for the treatment of a variety of disorders associated with reduced NO signaling.

Adjudin-mediated junction restructuring in the seminiferous epithelium leads to displacement of soluble guanylate cyclase from adherens junctions

A plethora of evidence supports the role of cyclic nucleotides in junction restructuring. For instance, studies have shown cGMP to be a key regulator of junction assembly and disassembly in different in vitro and in vivo systems. In this study, we examine the role of soluble guanylate cyclase (sGC) in junction restructuring in the seminiferous epithelium of the rat testis. First, the interaction of soluble guanylate cyclase beta1 (sGCbeta1; sGC is a heterodimer comprised of an alpha and a beta subunit) with proteins that constitute adherens and tight junctions in the testis was demonstrated. By immunoprecipitation, sGCbeta1 was found to associate with occludin, JAM-A, and ZO-1, as well as with cadherin, catenin, nectin, afadin, ponsin, and espin, suggestive of its role in cell junction dynamics. These results were corroborated in part by immunohistochemistry experiments, which revealed that the localization of sGCbeta1 was largely restricted to the site of the apical and basal ectoplasmic specialization. Next, the role of sGC in junction dynamics was addressed by using an in vivo model of junction restructuring. Administration of Adjudin--a chemical entity known to specifically perturb adhesion between Sertoli and germ cells (i.e., round and elongate(ing) spermatids and most spermatocytes)--resulted in a approximately 1.5-fold increase in sGCbeta1, coinciding with the loss of germ cells from the epithelium. More importantly, the ability of sGCbeta1 to associate with cadherin increased approximately three-fold during Adjudin-mediated restructuring of Sertoli-germ cell junctions, whereas its interaction with tight junction proteins (i.e., occludin and ZO-1) decreased. Taken collectively, these results suggest that sGC participates in the remodeling of cell junctions during spermatogenesis.

Functional characterization of two nucleotide-binding sites in soluble guanylate cyclase

Soluble guanylate cyclase is a heterodimeric hemoprotein composed of alpha- and beta-subunits with a homologous motif to the nucleotide-binding sites of adenylate cyclases. Homology modeling of guanylate cyclase, based on the crystal structure of adenylate cyclase, reveals a single GTP-binding site and a putative second site pseudosymmetric to the GTP-binding site. However, the role of this pseudosymmetric site has remained unclear. Using equilibrium dialysis, we identified two nucleotide-binding sites with high and low affinity for alpha,beta-methylene guanosine 5'-triphosphate (GMP-CPP). In contrast, 2'-dADP occupied both sites with equivalent affinities. Adenosine-5'-beta,gamma-imido triphosphate (AMP-PNP), which competitively inhibited the cyclase reaction, bound solely to the high affinity site, indicating the role of this site as the catalytic site. The function of the low affinity site was examined using allosteric activators YC-1 and BAY 41-2272. YC-1 significantly reduced the affinity of 2'-dADP, probably by competing for the same site as 2'-dADP. BAY 41-2272 totally inhibited the specific binding of one molecule of 2'-dADP as well as GMP-CPP. This suggests that the activators compete with these nucleotides for the low affinity site. Infrared and EPR analyses of the enzymic CO- and NO-hemes also supported the suggested role of the low affinity site as a target for the activators. Our results imply that the low affinity site is the pseudosymmetric site, which binds YC-1 or BAY 41-2272.

NO responsiveness in pulmonary artery and airway smooth muscle: the role of cGMP regulation

The purpose of this study was to assess intrinsic smooth muscle mechanisms contributing to greater nitric oxide (NO) responsiveness in pulmonary vascular vs. airway smooth muscle. Canine pulmonary artery smooth muscle (PASM) and tracheal smooth muscle (TSM) strips were used to perform concentration response studies to an NO donor, (Z)-1-[N-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (DETA-NO). PASM exhibited a greater NO responsiveness whether PASM and TSM were contracted with receptor agonists, phenylephrine and acetylcholine, respectively, or with KCl. The >10-fold difference in NO sensitivity in PASM was observed with both submaximal and maximal contractions. This difference in NO responsiveness was not due to differences in endothelial or epithelial barriers, since these were removed, nor was it due to the presence of cGMP-independent NO-mediated relaxation in either tissue. At equal concentrations of NO, the intracellular cGMP concentration ([cGMP]i) was also greater in PASM than in TSM. Phosphodiesterase (PDE) inhibition using isobutylmethylxanthine indicated that the greater [cGMP]i in PASM was not due to greater PDE activity in TSM. Expression of soluble guanylate cyclase (sGC) subunit mRNA (2 +/- 0.2 and 1.3 +/- 0.2 attomol/microg total RNA, respectively) and protein (47.4 +/- 2 and 27.8 +/- 3.9 ng/mg soluble homogenate protein, respectively) was greater in PASM than in TSM. sGCalpha1 and sGCbeta1 mRNA expression was equal in PASM but was significantly different in TSM, suggesting independent regulation of their expression. An intrinsic smooth muscle mechanism accounting for greater NO responsiveness in PASM vs. TSM is greater sGC activity.

Diverse Forms of Guanylyl Cyclases in Medaka Fish – Their Genomic Structure and Phylogenetic Relationships to those in Vertebrates and Invertebrates

Fish species such as medaka fish, fugu, and zebrafish contain more guanylyl cyclases (GCs) than do mammals. These GCs can be divided into two types: soluble GCs and membrane GCs. The latter are further divided into four subfamilies: (i) natriuretic peptide receptors, (ii) STa/guanylin receptors, (iii) sensory-organ-specific membrane GCs, and (iv) orphan receptors. Phylogenetic analyses of medaka fish GCs, along with those of fugu and zebrafish, suggest that medaka fish is a much closer relative to fugu than to zebrafish. Analyses of nucleotide data available on a web site (http://www.ncbi. nlm.nih.gov/) of GCs from a range of organisms from bacteria to vertebrates suggest that gene duplication, and possibly chromosomal duplication, play important roles in the divergence of GCs. In particular, the membrane GC genes were generated by chromosomal duplication before the divergence of tetrapods and teleosts.

Soluble guanylyl cyclase: more secrets revealed

Guanylyl cyclases (GCs) are enzymes that convert guanosine-5'-triphosphate (GTP) to cyclic guanosine-3',5'-monophosphate (cGMP). The second messenger cGMP participates in signaling by (1) stimulating the activity of kinases that belong to the protein kinase G family, (2) altering the conductance of cGMP-gated ion channels and (3) changing the activity of cGMP-regulated phosphodiesterases. In contrast to adenylyl cyclases which exist as membrane-bound molecules, guanylyl cyclases (GC) occur in both membrane-bound and cytosolic forms. The particulate GC (pGC) isoforms serve as receptors for natriuretic peptides, while soluble GC (sGC) is the "receptor" for nitric oxide (NO). In addition to the difference in ligands and subcellular organization, the two forms of GC also differ in that pGC exists in homodimeric form, while typically sGC occurs as a heterodimer. Herein, we will review the literature on sGC subunit structure and discuss the regulation of the enzyme at the transcriptional and post-translational level.

Soluble guanylate cyclase and neuronal nitric oxide synthase colocalize in rat nucleus tractus solitarii

Nitric oxide has been implicated in transmission of cardiovascular signals in the nucleus tractus solitarii (NTS). Pharmacological studies suggest that activation of neurons by nitric oxide in the NTS may involve soluble guanylate cyclase (sGC). However, anatomical data supporting this suggestion have not been available. In this study, we tested the hypothesis that neurons and fibers containing neuronal nitric oxide synthase (nNOS) lie in close proximity to those containing sGC and the two enzymes colocalize in some neurons and fibers in the NTS. We perfused six rats and obtained brain stem sections for double immunofluorescent staining utilizing antibodies selective for sGC and for nNOS combined with confocal microscopy. The distribution and staining intensity of nNOS-immunoreactivity (IR) was similar to our earlier reports. IR of sGC was present in cell bodies, proximal dendrites and fibers of many brain stem regions. Strong sGC-IR was noted in the hypoglossal, dorsal motor nucleus of vagus and gracilis nuclei. The NTS exhibited moderate sGC-IR. Superimposed images showed that many NTS neurons contained both nNOS-IR and sGC-IR. The percentage of sGC-IR positive cells that were also nNOS-IR positive differed among NTS subnuclei. Similarly, the percentage of nNOS-IR positive cells that were also sGC positive differed among NTS subnuclei. Fibers stained for both nNOS-IR and sGC-IR were also present in NTS subnuclei. In addition, we identified fibers that were stained for nNOS-IR or sGC-IR alone and often found such singly labeled fibers apposed to each other. These data support our hypothesis and provide anatomical support for the suggestion that nitroxidergic activation of the NTS involves sGC.

Ligand specificity of H-NOX domains: from sGC to bacterial NO sensors

Soluble guanylate cyclase (sGC) is a nitric oxide (NO) sensing hemoprotein that has been found in eukaryotes from Drosophila to humans. Prokaryotic proteins with significant homology to the heme domain of sGC have recently been identified through genomic analysis. This family of heme proteins has been named the H-NOX domain, for Heme-Nitric oxide/OXygen binding domain. The key observation from initial studies in this family is that some members, those proteins from most eukaryotes and facultative aerobic prokaryotes, bind NO in a five-coordinate heme complex, but do not bind oxygen (O(2)), the same ligand binding characteristics as sGC. H-NOX family members from obligate aerobic prokaryotes bind O(2) and NO in six-coordinate complexes, similar to the globins and other O(2)-sensing heme proteins. The molecular factors that contribute to these differences in ligand specificity, within a family of sequence related proteins, are the subject of this review.

Activation of soluble guanylyl cyclase inhibits estradiol production and cyclic AMP accumulation from cultured rat granulosa cells

OBJECTIVE

To demonstrate the expression of soluble guanylyl cyclase (sGC) alpha and beta subunits in rat granulosa cells and determine the effects sGC activation on levels of cyclic GMP (cGMP), E2, and cAMP.

DESIGN

Basic research study.

SETTING

University research laboratory.

ANIMAL(S)

Estrogen-treated immature Sprague-Dawley female rats from which primary cell culture of granulosa cells was obtained.

INTERVENTION(S)

Functionally immature rat granulosa cells were incubated for 48 hours with media alone, FSH, or FSH plus YC-1, a specific activator of sGC.

MAIN OUTCOME MEASURE(S)

Expression of sGC alpha and beta subunits was determined by immunoblot analysis. Media concentrations of E2, cAMP, and cGMP were measured by radioimmunoassays.

RESULT(S)

Immunoblot analysis of granulosa cells revealed the expression of sGC alpha and beta subunits. While cGMP accumulation was low in cells incubated with media alone or with FSH, cotreatment with FSH plus YC-1 increased cGMP levels approximately five-fold. Incubation of cells with FSH stimulated E2 production in a dose-dependent manner. However, cotreatment of cells with FSH plus YC-1 significantly decreased E2 concentrations at all doses of FSH tested. Similarly, while FSH increased cAMP accumulation from granulosa cells, cotreatment with YC-1 markedly inhibited FSH-stimulated cAMP levels.

CONCLUSION(S)

These findings demonstrate the expression of sGC subunits in rat granulosa cells and indicate that activation of sGC increases cGMP levels, which are associated with inhibition of FSH-stimulated E2 production and cAMP accumulation.

An evolutionarily conserved mechanism for sensitization of soluble guanylyl cyclase reveals extensive nitric oxide-mediated upregulation of cyclic GMP in insect brain

Soluble guanylyl cyclase (SGC) is the main receptor for the gaseous signalling molecule nitric oxide (NO) in vertebrates and invertebrates. Recently, a novel class of drugs that regulate mammalian SGC by NO-independent allosteric mechanisms has been identified [e.g. 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole, YC-1]. To assess the evolutionary conservation and hence the potential physiological relevance of these mechanisms, we have tested YC-1 on the brains of two model insects, the cockroach Periplaneta americana and the locust Schistocerca gregaria. YC-1 strongly potentiated the NO-induced elevation of total cyclic 3',5'-guanosine monophosphate (cGMP) and amplified the intensity and consistency of NO-induced cGMP-immunoreactivity in the brain. Our data indicate that the effect of YC-1 was independent of phosphodiesterase inhibition and thus mediated by direct sensitization of SGC. Immunohistopharmacology and co-labelling with antibodies against the SGC alpha-subunit confirmed that cGMP induced by co-application of NO and YC-1 is predominantly attributable to SGC. The staggering number of NO-responsive neurons revealed by YC-1 suggests that previous studies may have considerably underestimated the number of cellular targets for NO in the insect brain. Moreover, a subset of these targets exhibited cGMP-immunoreactivity without application of exogenous NO, demonstrating that YC-1 can be exploited for visualization of physiological cGMP signals in response to endogenous NO production. In conclusion, our discovery that YC-1 is a potent sensitizer of insect SGC indicates that a NO-independent regulatory site is an evolutionarily conserved feature of SGC. Our findings add considerable momentum to the concept of an as yet unidentified endogenous ligand that regulates the gain of the NO-cGMP signalling pathway.

Molecular cloning of a novel variant of the rat soluble guanylate cyclase beta2 subunit

Soluble guanylate cyclases (sGCs) are heterodimeric enzymes consisting of alpha and beta subunits and catalyze the formation of cGMP from GTP. The beta1 subunit has been characterized in detail, whereas the function and physiological role of the beta2 subunit are poorly understood. In this study, I isolated two distinct cDNA fragments for the beta2 subunit of sGC (beta2a and beta2b) from a rat brain cDNA library by 3' rapid amplification of cDNA ends using degenerate sense primers based on amino acid sequences conserved among membrane-bound guanylate cyclases. The deduced amino acid sequence of beta2a is identical with the corresponding sequence of the previously described beta2 subunit, whereas that of beta2b is C-terminally shorter by 46 amino acids and thus does not contain a consensus sequence for isoprenylation/carboxymethylation. Reverse transcription-polymerase chain reaction (RT-PCR) analysis demonstrated that both variants are expressed in various tissues, including kidney, liver, and brain. Although the functional significance of the C-terminal region containing the consensus sequence for isoprenylation/carboxymethylation of beta2a remains unclear yet, it is likely that these beta2 subunits play some physiological or pathophysiological role in various tissues.

Nitric oxide-sensitive guanylyl cyclase: structure and regulation

By the formation of the second messenger cGMP, NO-sensitive guanylyl cyclase (GC) plays a key role within the NO/cGMP signaling cascade which participates in vascular regulation and neurotransmission. The enzyme contains a prosthetic heme group that acts as the acceptor site for NO. High affinity binding of NO to the heme moiety leads to an up to 200-fold activation of the enzyme. Unexpectedly, NO dissociates with a half-life of a few seconds which appears fast enough to account for the deactivation of the enzyme in biological systems. YC-1 and its analogs act as NO sensitizers and led to the discovery of a novel pharmacologically and conceivably physiologically relevant regulatory principle of the enzyme. The two isoforms of the heterodimeric enzyme (alpha1beta1, alpha2beta1) are known that are functionally indistinguishable. The alpha2beta1-isoform mainly occurs in brain whereas the alpha1beta1-enzyme shows a broader distribution and represents the predominantly expressed form of NO-sensitive GC. Until recently, the enzyme has been thought to occur in the cytosol. However, latest evidence suggests that the alpha2-subunit mediates the membrane association of the alpha2beta1-isoform via interaction with a PDZ domain of the post-synaptic scaffold protein PSD-95. Binding to PSD-95 locates this isoform in close proximity to the NO-generating synthases thereby enabling the NO sensor to respond to locally elevated NO concentrations. In sum, the two known isoforms may stand for the neuronal and vascular form of NO-sensitive GC reflecting a possible association to the neuronal and endothelial NO-synthase, respectively.

Expression of inducible nitric oxide synthase messenger RNA, but not guanylate cyclase messenger RNA, depends on gestational age in rat myometrium

We studied the expression of inducible nitric oxide (NO) synthase (iNOS) and soluble guanylate cyclase (sGC) mRNAs in pregnant rat myometrium. Expression of iNOS and sGC alpha1, beta1 and beta2 mRNA was analyzed in non-pregnant and pregnant (days 10, 14, 17 and 21) Wistar rats by reverse transcription-polymerase chain reaction. Expression of iNOS mRNA increased during pregnancy but decreased on day 21 of gestation. Expression of GC alpha1 mRNA was greater than GC beta1 mRNA at all time points. Expression of uterine GC alpha1 and GC beta1 mRNA did not change significantly during pregnancy and did not differ significantly from non-pregnant levels. The values of sGC beta2 mRNA were below the limit of detection. In conclusion, the expression of iNOS mRNA increased during pregnancy in the myometrium and decreased at term, while the expression of sGC mRNA was not affected by pregnancy. Thus, it is the changes in NO production, rather than changes in its target, that are responsible for uterine quiescence during pregnancy and initiation of labor.

Hypoxia Decreases Expression of Soluble Guanylate Cyclase in Cultured Rat Pulmonary Artery Smooth Muscle Cells

Nitric oxide (NO) has an important role in modulating the pulmonary vascular tone. NO acts, in part, by stimulating soluble guanylate cyclase (sGC) to synthesize the intracellular second messenger cyclic GMP. In vascular smooth muscle cells, sGC is a heterodimer composed of alpha1 and beta1 subunits. The objective of this study was to test whether oxygen concentration regulates sGC expression in cultured rat pulmonary artery smooth muscle cells (rPaSMC). rPaSMC were exposed to 0, 3, and 20% oxygen for 1-48 h, and sGC subunit mRNA levels were measured. Compared with rPaSMC exposed to 20% oxygen, sGC alpha1 and beta1 subunit mRNA levels were markedly decreased in rPaSMC exposed to 0% and 3% oxygen. The decrease in sGC subunit mRNA levels in hypoxic rPaSMC was detected as early as 6 h of exposure. Compared with rPaSMC exposed to 20% oxygen, exposure of rPaSMC to 3% oxygen progressively decreased sGC subunit protein levels at 24 and 48 h. There was also a 30% and 50% decrease in sGC enzyme activity in cells exposed to hypoxia for 24 and 48 h (P < 0.05 and P < 0.001, respectively, as compared with cells maintained in normoxia). These results demonstrate that hypoxia decreases sGC expression in cultured pulmonary artery smooth muscle cells and suggest that, in hypoxic vascular smooth muscle, decreased cyclic GMP synthesis may limit the vasodilator response to NO.

Regulation of nitric oxide and soluble guanylyl cyclase

Since the discoveries that have verified nitric oxide (NO) as an endogenously produced cell signaling molecule, research surrounding its production and mechanisms of action have been studied at an exponentially increasing rate. NO is produced by a family of enzymes termed the NO synthases (NOS), which are regulated independently by various stimuli. Once produced, NO can solicit numerous biological events by reacting with various metals, thiols, and oxygen species to modify proteins, DNA and lipids. One of the most biologically relevant actions of NO is its binding to the heme moiety in the heterodimeric enzyme, soluble guanylyl cyclase (sGC). Activation of sGC by NO results in the production of the second messenger molecule, 3',5'-cyclic guanosine monophosphate (cGMP), which can regulate numerous physiological events such as vasodilatation and neurotransmission. Here we will review the synthesis and fate of NO, and discuss the activation and regulation of the NO receptor, sGC.

Structural and functional characterization of the dimerization region of soluble guanylyl cyclase

Soluble guanylyl cyclase (sGC) is a ubiquitous enzyme that functions as a receptor for nitric oxide. Despite the obligate heterodimeric nature of sGC, the sequence segments mediating subunit association have remained elusive. Our initial screening for relevant interaction site(s) in the most common sGC isoenzyme, alpha(1) beta(1), identified two regions in each subunit, i.e. the regulatory domains and the central regions, contributing to heterodimer formation. To map the relevant segments in the beta(1) subunit precisely, we constructed multiple N- and C-terminal deletion variants and cotransfected them with full-length alpha(1) in COS cells. Immunoprecipitation revealed that a sequence segment spanning positions 204-408 mediates binding of beta(1) to alpha(1) The same region of beta(1)[204-408] was found to promote beta /beta(1) homodimerization. Fusion of [204 beta(1)-408] to enhanced green fluorescent protein conferred binding activity to the recipient protein. Coexpression of beta(1)[204-408] with alpha(1) or beta(1) targeted the sGC subunits for proteasomal degradation, suggesting that beta(1)[204-408] forms structurally deficient complexes with alpha(1) and beta(1). Analysis of deletion constructs lacking portions of the beta(1) dimerization region identified two distinct segments contributing to alpha(1) binding, i.e. an N-terminal site covering positions 204-244 and a C-terminal site at 379-408. Both sites are crucial for sGC function because deletion of either site rendered sGC dimerization-deficient and thus functionally inactive. We conclude that the dimerization region of beta(1) extends over 205 residues of its regulatory and central domains and that two discontinuous sites of 41 and 30 residues, respectively, facilitate binding of beta(1) to the alpha(1) subunit of sGC.

Guanylyl cyclases, nitric oxide, natriuretic peptides, and airway smooth muscle function

Airway smooth muscle (ASM) plays an important role in asthma pathophysiology through its contractile and proliferative functions. The cyclic nucleotides adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP) are second messengers capable of mediating the effects of a variety of drugs and hormones. There is a large body of evidence to support the hypothesis that cAMP is a mediator of the ASM's relaxant effects of drugs, such as beta2-adrenoceptor agonists, in human airways. Although most attention has been paid to this second messenger and the signal transduction pathways it activates, recent evidence suggests that cGMP is also an important second messenger in ASM with important relaxant and antiproliferative effects. Here, we review the regulation and function of cGMP in ASM and discuss the implications for asthma pathophysiology and therapeutics. Recent studies suggest that activators of soluble and particulate guanylyl cyclases, such as nitric oxide donors and natriuretic peptides, have both relaxant and antiproliferative effects that are mediated through cGMP-dependent and cGMP-independent pathways. Abnormalities in these pathways may contribute to asthma pathophysiology, and therapeutic manipulation may complement the effects of beta2-adrenoceptor agonists.

Differential expression of NO-sensitive guanylyl cyclase subunits during the development of rat cerebellar granule cells: regulation via N-methyl-D-aspartate receptors

In primary cultures of rat cerebellar granule cells with a functional network of glutamatergic neurons, the expression pattern of the different subunits of nitric-oxide (NO)-sensitive guanylyl cyclase changes during cell differentiation. These cells express the alpha1, alpha2 and beta1 subunits of NO-sensitive guanylyl cyclase and synthesize cyclic guanosine monophosphate (cGMP) in response to exogenous or endogenous nitric oxide. In this study, we determined the protein content of the alpha1 and beta1 subunits and quantified alpha1, alpha2 and beta1 mRNA by reverse transcription coupled to a polymerase chain reaction (RT-PCR). Expression of the beta1 subunit increased with the degree of cell differentiation, although most marked changes occurred at the alpha subunit level. In cells freshly isolated from rat pups on postnatal day 7 (P7) the most abundant alpha subunit was alpha1, while alpha2 appeared as the predominant subunit of this type in cultured cells. N-methyl-D-aspartate (NMDA) receptor stimulation in 7- or 14-day-cultured cells led to the upregulation of guanylyl cyclase subunit mRNAs; alpha2 mRNA levels undergoing most significant change. This enhanced subunit expression was accompanied by an increase in the amount of cGMP synthesized in response to NO. Thus, it seems that alpha2 subunits are increasingly expressed as granule cells mature. The presence of this subunit in the guanylyl cyclase heterodimer facilitates its localization at synaptic membranes, where the enzyme acts as a sensor for NO formed by the postsynaptic protein 95 (PSD-95)-associated neuronal NO synthase.

Regulation of nitric oxide-sensitive guanylyl cyclase

In this review, we outline the current knowledge on the regulation of nitric oxide (NO)-sensitive guanylyl cyclase (GC). Besides NO, the physiological activator that binds to the prosthetic heme group of the enzyme, two novel classes of GC activators have been identified that may have broad pharmacological implications. YC-1 and YC-1-like substances act as NO sensitizers, whereas the substance BAY 58-2667 stimulates NO-sensitive GC NO-independently and preferentially activates the heme-free form of the enzyme. Sensitization and desensitization of NO/cGMP signaling have been reported to occur on the level of NO-sensitive GC; in the present study, an alternative mechanism is introduced explaining the adaptation of the NO-induced cGMP response by a long-term activation of the cGMP-degrading phosphodiesterase 5 (PDE5). Finally, regulation of GC expression and a possible modulation of GC activity by other factors are discussed.

Pulmonary vascular dysfunction in preterm lambs with chronic lung disease

Chronic lung injury from prolonged mechanical ventilation after premature birth inhibits the normal postnatal decrease in pulmonary vascular resistance (PVR) and leads to structural abnormalities of the lung circulation in newborn sheep. Compared with normal lambs born at term, chronically ventilated preterm lambs have increased pulmonary arterial smooth muscle and elastin, fewer lung microvessels, and reduced abundance of endothelial nitric oxide synthase. These abnormalities may contribute to impaired respiratory gas exchange that often exists in infants with chronic lung disease (CLD). Nitric oxide inhalation (iNO) reduces PVR in human infants and lambs with persistent pulmonary hypertension. We wondered whether iNO might have a similar effect in lambs with CLD. We therefore studied the effect of iNO on PVR in lambs that were delivered prematurely at approximately 125 days of gestation (term = 147 days) and mechanically ventilated for 3 wk. All of the lambs had chronically implanted catheters for measurement of pulmonary vascular pressures and blood flow. During week 2 of mechanical ventilation, iNO at 15 parts/million for 1 h decreased PVR by approximately 20% in 12 lambs with evolving CLD. When the same study was repeated in eight lambs at the end of week 3, iNO had no significant effect on PVR. To see whether this loss of iNO effect on PVR might reflect dysfunction of lung vascular smooth muscle, we infused 8-bromo-guanosine 3',5'-cyclic monophosphate (cGMP; 150 microg. kg(-1). min(-1) iv) for 15-30 min in four of these lambs at the end of week 3. PVR consistently decreased by 30-35%. Lung immunohistochemistry and immunoblot analysis of excised pulmonary arteries from lambs with CLD, compared with control term lambs, showed decreased soluble guanylate cyclase (sGC). These results suggest that loss of pulmonary vascular responsiveness to iNO in preterm lambs with CLD results from impaired signaling, possibly related to deficient or defective activation of sGC, the intermediary enzyme through which iNO induces increased vascular smooth muscle cell cGMP and resultant vasodilation.

Soluble guanylyl cyclase: physiological role as an NO receptor and the potential molecular target for therapeutic application

Nitric oxide (NO) activates soluble guanylyl cyclase, which results in an increased synthesis of cyclic guanosine 3',5'-cyclic monophosphate (cGMP), smooth muscle relaxation and vasodilation. The heme group in soluble guanylyl cyclase binds NO and allosterically activates the catalytic site. In addition, a second allosteric site that synergistically activates the enzyme has been reported. BAY 41-2272 was reported as an NO-independent activator of soluble guanylyl cyclase. Treatment with this compound results in anti-platelet activity, a decrease in blood pressure and an increase in survival, indicating a potential for treating cardiovascular diseases. YC-1, another NO-independent activator, activates soluble guanylyl cyclase and the activity is enhanced in the presence of NO. YC-1 relaxed tissue strips in organ bath. Consistent with its biochemical activity, YC-1 induced penile erection in a conscious rat model. Recently, we found a novel series of soluble guanylyl cyclase activators that also NO-independently activate soluble guanylyl cyclase and cause penile erection, suggesting a synergy with the endogenous NO production in vivo. Here I review the NO/cGMP signal transduction pathway and define soluble guanylyl cyclase modulators as a novel approach for the treatment of cardiovascular diseases and erectile dysfunction.

The pharmacology of nitric oxide in the peripheral nervous system of blood vessels

Unanticipated, novel hypothesis on nitric oxide (NO) radical, an inorganic, labile, gaseous molecule, as a neurotransmitter first appeared in late 1989 and into the early 1990s, and solid evidences supporting this idea have been accumulated during the last decade of the 20th century. The discovery of nitrergic innervation of vascular smooth muscle has led to a new understanding of the neurogenic control of vascular function. Physiological roles of the nitrergic nerve in vascular smooth muscle include the dominant vasodilator control of cerebral and ocular arteries, the reciprocal regulation with the adrenergic vasoconstrictor nerve in other arteries and veins, and in the initiation and maintenance of penile erection in association with smooth muscle relaxation of the corpus cavernosum. The discovery of autonomic efferent nerves in which NO plays key roles as a neurotransmitter in blood vessels, the physiological roles of this nerve in the control of smooth muscle tone of the artery, vein, and corpus cavernosum, and pharmacological and pathological implications of neurogenic NO have been reviewed. This nerve is a postganglionic parasympathetic nerve. Mechanical responses to stimulation of the nerve, mainly mediated by NO, clearly differ from those to cholinergic nerve stimulation. The naming "nitrergic or nitroxidergic" is therefore proposed to avoid confusion of the term "cholinergic nerve", from which acetylcholine is released as a major neurotransmitter. By establishing functional roles of nitrergic, cholinergic, adrenergic, and other autonomic efferent nerves in the regulation of vascular tone and the interactions of these nerves in vivo, especially in humans, progress in the understanding of cardiovascular dysfunctions and the development of pharmacotherapeutic strategies would be expected in the future.

A functional domain of the alpha1 subunit of soluble guanylyl cyclase is necessary for activation of the enzyme by nitric oxide and YC-1 but is not involved in heme binding

Soluble guanylyl cyclase is a heterodimeric enzyme consisting of an alpha(1) and a beta(1) subunit and is an important target for endogenous nitric oxide and the guanylyl cyclase modulator YC-1. The activation of the enzyme by both substances is dependent on the presence of a prosthetic heme group. It has been unclear whether this prosthetic heme group is sandwiched between the alpha(1) and beta(1) subunits or whether it exclusively binds to the beta(1) subunit. Here we analyze progressive amino-terminal deletion mutants of the human alpha(1) subunit after co-expression with the human beta(1) subunit in the baculovirus/Sf9 system. Spectral, biochemical, and pharmacological analysis shows that the first 259 amino acids of the alpha(1) subunit can be deleted without loss of sensitivity to nitric oxide (NO) or YC-1 or loss of heme binding of the respective enzyme complex with the beta(1) subunit. This is in contrast to previous data indicating that NO sensitivity and a functional heme binding site requires full-length amino termini of bovine alpha(1) and beta(1) subunits. Further deletion of the first 364 amino acids of the alpha(1) subunit leads to an enzyme complex with preserved heme binding but loss of sensitivity to NO or YC-1 despite induction of the typical spectral shift by NO binding to the prosthetic heme group. We conclude that 1) the amino-terminal part of the alpha(1) subunit is not involved in heme binding and 2) amino acids 259-364 of the alpha(1) subunit represent an important functional domain for the transduction of the NO activation signal and likely represent the target for NO-sensitizing substances like YC-1.

A-350619: a novel activator of soluble guanylyl cyclase

Nitric oxide (NO) is a key mediator in many physiological processes and one of the major receptors through which NO exerts its effects is soluble guanylyl cyclase. Guanylyl cyclase converts GTP to cyclic GMP as part of the cascade that results in physiological processes such as smooth muscle relaxation, neurotransmission, inhibition of platelet aggregation and immune response. The properties of A-350619, a novel soluble guanylyl cyclase activator, were examined to determine the modulatory effect on the catalytic properties of soluble guanylyl cyclase. A-350619 increased V(max) from 0.1 to 14.5 micromol/min/mg (145 fold increase), and lowered K(m) from 300 to 50 microM (6 fold decrease). When YC-1 (another sGC activator) and A-350619 were combined, a 156 fold increase in V(max) and a 5 fold decrease in Km were observed, indicating that the modulation of the enzyme brought about by YC-1 and A-350619 are not additive, suggesting a common binding site. Activation of soluble guanylyl cyclase by A-350619 was partially inhibited by ODQ, a specific inhibitor of soluble guanylyl cyclase by oxidation of the enzyme heme. YC-1 and A-350619 after pre-treatment with N-omega-nitro-L-arginine, an NO-synthase inhibitor, relaxed cavernosum tissue strips in a dose-dependent manner with EC(50) of 50 microM and 80 microM, respectively. Addition of SNP potentiated the relaxation effect of YC-1 and A-350619, shifting the dose-response curve to the left to 3 microM and 10 microM, respectively. Consistent with its biochemical activity, A-350619 (1 micromol/kg) alone induced penile erection in a conscious rat model. Activation of soluble guanylyl cyclase in cavernosum tissue as an alternate method of enhancing the effect of NO may provide a novel treatment of sexual dysfunction.

Activation of soluble guanylate cyclase causes relaxation of corpus cavernosum tissue: synergism of nitric oxide and YC-1

Nitric oxide (NO) activates corpus cavernosum smooth muscle soluble guanylate cyclase (sGC) and increases the synthesis of cGMP that results in smooth muscle relaxation and ultimately, penile erection. To characterize sGC and define the potential synergy between NO and the allosteric activator YC-1 in corpus cavernosum, rat sGC was activated by either sodium nitroprusside (SNP) or YC-1, and YC-1 potentiated the effects of SNP with a 200-fold activation of sGC. Both SNP and YC-1 decreased the Km and increased the Vmax. ODQ significantly inhibited sGC activated by SNP with IC50 of 0.5 nM, but did not affect the sGC activated by YC-1 as well as basal sGC activity. SNP and YC-1 synergistically increased intracellular cGMP levels in rabbit corpus cavernosum smooth muscle cell cultures. YC-1 significantly relaxed rabbit cavernosum tissue strips in organ baths with an EC50 of 8.4 microM. In the presence of L-nitroarginine methyl ester to block endogenous NO production, co-administration of SNP shifted the dose response of YC-1 to the left, showing the synergism of SNP and YC-1 in tissue strips. In view of the clinical efficacy of phosphodiesterase-5 inhibitors, activation of sGC may provide an alternative means for enhancing the activity of neurally derived NO during sexual stimulation in the corpus cavernosum, representing a novel approach for the treatment of erectile dysfunction.

Activators of soluble guanylate cyclase for the treatment of male erectile dysfunction

Soluble guanylate cyclase (sGC) is an important enzyme in corpus cavernosum smooth muscle cells as it is one of the regulators of the synthesis of cGMP. The efficacy of sildenafil (Viagra) in the treatment of male erectile dysfunction indicates the importance of the cGMP system in the erectile response as the increased levels of cGMP induce relaxation of the corpus cavernosum. sGC is physiologically activated by nitric oxide (NO) during sexual stimulation, and its activity can be pharmacologically enhanced by several NO-donors. Agents like YC-1 can also activate sGC after binding to a novel allosteric site in the enzyme, a site different from the NO binding site. YC-1 can relax rabbit cavernosal tissue and it facilitates penile erection in vivo. This review summarizes the enzymology, biochemistry and pharmacology of this novel allosteric site and its relevance for the regulation of penile function. This type of sGC activators represent a new class of compounds with a different pharmacological profile in comparison to the classical NO-donors and they could be beneficial for the treatment of male erectile dysfunction.

Cellular distribution and function of soluble guanylyl cyclase in rat kidney and liver

Soluble guanylyl cyclase (sGC) catalyzes the biosynthesis of cGMP in response to binding of L-arginine-derived nitric oxide (NO). Functionally, the NO-sGC-cGMP signaling pathway in kidney and liver has been associated with regional hemodynamics and the regulation of glomerular parameters. The distribution of the ubiquitous sGC isoform alpha 1 beta 1 sGC was studied with a novel, highly specific antibody against the beta 1 subunit. In parallel, the presence of mRNA encoding both subunits was investigated by using in situ hybridization and reverse transcription-PCR assays. The NO-induced, sGC-dependent accumulation of cGMP in cytosolic extracts of tissues and cells was measured in vitro. Renal glomerular arterioles, including the renin-producing granular cells, mesangium, and descending vasa recta, as well as cortical and medullary interstitial fibroblasts, expressed sGC. Stimulation of isolated mesangial cells, renal fibroblasts, and hepatic Ito cells with a NO donor resulted in markedly increased cytosolic cGMP levels. This assessment of sGC expression and activity in vascular and interstitial cells of kidney and liver may have implications for understanding the role of local cGMP signaling cascades.

Nitric oxide attenuates the expression of transforming growth factor-beta(3) mRNA in rat cardiac fibroblasts via destabilization

Transforming growth factor-beta (TGF-beta) has been implicated in the development of interstitial fibrosis in cardiac hypertrophy. NO has been regarded as a potent inhibitor of cardiac fibroblast growth, albeit the modulation of cellular events associated with interstitial fibrosis remains undefined. In this regard, the regulation of TGF-beta mRNA expression by the NO donor S-nitroso-N-acetyl-penicillamine (SNAP) was examined in neonatal rat cardiac fibroblasts. SNAP treatment for 4 hours decreased TGF-beta(3) mRNA levels, an effect mimicked by 8-bromo-cGMP. TGF-beta(3) mRNA, however, had returned to levels observed in the untreated cells after a 24-hour exposure to SNAP, whereas a decreased expression persisted with 8-bromo-cGMP. In contrast to TGF-beta(3), TGF-beta(1) mRNA levels were modestly increased in response to cGMP-generating molecules. The treatment with actinomycin D for at least 8 hours did not appreciably alter TGF-beta(3) mRNA levels. By contrast, SNAP treatment caused a rapid decrease of TGF-beta(3) mRNA with a half-life of 3.3+/-0.2 hours, thereby supporting a mechanism of destabilization. The pretreatment with SNAP inhibited angiotensin II-stimulated protein synthesis and the concomitant expression of TGF-beta(3) mRNA. These data reveal a disparate pattern of TGF-beta(1) and TGF-beta(3) mRNA regulation by NO and highlight a novel mechanism of destabilization contributing to the decreased expression of TGF-beta(3) mRNA. The modulation of both basal and angiotensin II-stimulated TGF-beta(3) mRNA expression provides a mechanism by which NO may influence the progression of interstitial fibrosis.

Human Soluble Guanylate Cyclase: Functional Expression, Purification and Structural Characterization

Soluble guanylate cyclase is an enzyme that catalyzes formation of cGMP from GTP and is a member of the nucleotide cyclase family of enzymes. sGC is a receptor for endogenous and exogenous nitric oxide and is activated several-fold upon its binding, constituting a core enzyme in the nitric oxide signal transduction pathway. cGMP generated by sGC is an important second messenger that regulates activity of several enzymes triggering such important physiologic reactions as vasodilation, smooth muscle relaxation and platelet aggregation. We report here the functional expression of the human isoform of soluble guanylate cyclase in HighFive insect cells using a baculovirus expression system. Highly active recombinant protein was obtained without heme reconstitution or supplementation of the cell growth medium and the level of protein expression was found to be heavily affected by the composition of the growth medium. We have successfully purified highly active sGC (sp act up to 940 nmol/min/mg) from adherent cultures using a three-column, 1-day procedure. The UV-Vis spectrum of the isolated protein shows a Soret band at 431 nm, consistent with a histidine-ligated, 5-coordinate heme as previously reported. Far UV CD spectroscopy, intrinsic tryptophan fluorescence, fluorescence of the hydrophobic dye bis-ANS, size-exclusion chromatography, and small angle X-ray scattering (SAXS) were used to characterize the structural properties of the purified sGC. We used two hierarchical neural network methods to predict the secondary structure of sGC and found it to be consistent with the observed CD spectrum of sGC.

NO-independent regulatory site on soluble guanylate cyclase

Nitric oxide (NO) is a widespread, potent, biological mediator that has many physiological and pathophysiological roles. Research in the field of NO appears to have followed a straightforward path, and the findings have been progressive: NO and cyclic GMP are involved in vasodilatation; glycerol trinitrate relaxes vascular smooth muscles by bioconversion to NO; mammalian cells synthesize NO; and last, NO mediates vasodilatation by stimulating the soluble guanylate cyclase (sGC), a heterodimeric (alpha/beta) haem protein that converts GTP to cGMP2-4. Here we report the discovery of a regulatory site on sGC. Using photoaffinity labelling, we have identified the cysteine 238 and cysteine 243 region in the alpha1-subunit of sGC as the target for a new type of sGC stimulator. Moreover, we present a pyrazolopyridine, BAY 41-2272, that potently stimulates sGC through this site by a mechanism that is independent of NO. This results in antiplatelet activity, a strong decrease in blood pressure and an increase in survival in a low-NO rat model of hypertension, and as such may offer an approach for treating cardiovascular diseases.

Mechanism for fetal globin gene expression: Role of the soluble guanylate cyclase-cGMP-dependent protein kinase pathway

Despite considerable concerns with pharmacological stimulation of fetal hemoglobin (Hb F) as a therapeutic option for the beta-globin disorders, the molecular basis of action of Hb F-inducing agents remains unclear. Here we show that an intracellular pathway including soluble guanylate cyclase (sGC) and cGMP-dependent protein kinase (PKG) plays a role in induced expression of the gamma-globin gene. sGC, an obligate heterodimer of alpha- and beta-subunits, participates in a variety of physiological processes by converting GTP to cGMP. Northern blot analyses with erythroid cell lines expressing different beta-like globin genes showed that, whereas the beta-subunit is expressed at similar levels, high-level expression of the alpha-subunit is preferentially observed in erythroid cells expressing gamma-globin but not those expressing beta-globin. Also, the levels of expression of the gamma-globin gene correlate to those of the alpha-subunit. sGC activators or cGMP analogs increased expression of the gamma-globin gene in erythroleukemic cells as well as in primary erythroblasts from normal subjects and patients with beta-thalassemia. Nuclear run-off assays showed that the sGC activator protoporphyrin IX stimulates transcription of the gamma-globin gene. Furthermore, increased expression of the gamma-globin gene by well known Hb F-inducers such as hemin and butyrate was abolished by inhibiting sGC or PKG activity. Taken together, these results strongly suggest that the sGC-PKG pathway constitutes a mechanism that regulates expression of the gamma-globin gene. Further characterization of this pathway should permit us to develop new therapeutics for the beta-globin disorders.

Subunits of the nitric oxide receptor, soluble guanylyl cyclase, expressed in rat brain

Despite the widespread use of nitric oxide as a signalling molecule in the central nervous system, the molecular makeup of its receptor, soluble guanylyl cyclase (sGC), therein is poorly understood. Accordingly, RT-PCR and in situ hybridization were used to identify sGC subunits expressed in rat brain. In addition to the expected mRNA for alpha 1 and beta1 subunits, message for the beta 2 subunit was detected in the cerebellum at all developmental stages investigated (1--150 days postnatum). The use of degenerate primers allowed the identification of mRNA coding for the rat alpha 2 subunit, which was also expressed at every age studied. All but beta 2 were detected by in situ hybridization in the brains of both 8-day-old and adult rats. The distribution patterns indicated that in some areas, e.g. caudate-putamen and nucleus accumbens, sGC probably exists mainly as the alpha 1 beta 1 heterodimer. In others, e.g. hippocampus and olfactory bulb, alpha 2 beta 1 is likely to be dominant. In the cerebellum, alpha 1 and beta 1 message was strong in the Purkinje cell layer but was not confined to Purkinje cells: smaller cells, presumed to be the Bergmann glia, were also labelled. In contrast, alpha 2 mRNA was concentrated in cerebellar granule cells. Western blotting indicated an excess of alpha 1 over beta 1 protein in the cerebellum, the reverse of what was found in the lung. It is concluded that, in molecular terms, sGC is likely to be more complex and exhibit more regional variation in the brain than previously thought. The functional consequences of this heterogeneity require investigation.

Cytokines decrease sGC in pulmonary artery smooth muscle cells via NO-dependent and NO-independent mechanisms

Exposure of rat pulmonary artery smooth muscle cells (rPASMC) to cytokines leads to nitric oxide (NO) production by NO synthase 2 (NOS2). NO stimulates cGMP synthesis by soluble guanylate cyclase (sGC), a heterodimer composed of alpha(1)- and beta(1)-subunits. Prolonged exposure of rPASMC to NO decreases sGC subunit mRNA and protein levels. The objective of this study was to determine whether levels of NO produced endogenously by NOS2 are sufficient to decrease sGC expression in rPASMC. Interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha) increased NOS2 mRNA levels and decreased sGC subunit mRNA levels. Exposure of rPASMC to IL-1beta and TNF-alpha for 24 h decreased sGC subunit protein levels and NO-stimulated sGC enzyme activity. L-N(6)-(1-iminoethyl)lysine (NOS2 inhibitor) or 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (sGC inhibitor) partially prevented the cytokine-mediated decrease in sGC subunit mRNA levels. However, cytokines also decreased sGC subunit mRNA levels in PASMC derived from NOS2-deficient mice. These results demonstrate that levels of NO and cGMP produced in cytokine-exposed PASMC are sufficient to decrease sGC subunit mRNA levels. In addition, cytokines can decrease sGC subunit mRNA levels via NO-independent mechanisms.