Deconstructing endothelial dysfunction: soluble guanylyl cyclase oxidation and the NO resistance syndrome

In this issue of the JCI, Stasch and colleagues suggest that a novel drug, BAY 58-2667, potently activates a pool of oxidized and heme-free soluble guanylyl cyclase (sGC; see the related article beginning on page 2552). The increased vasodilatory potency of BAY 58-2667 the authors found in a number of animal models of endothelial dysfunction and in human blood vessels from patients with diabetes suggests that there exists a subphenotype of endothelial dysfunction characterized by receptor-level NO resistance. Diseases associated with NO resistance would appear to be ideally suited for therapies directed at restoring redox homeostasis, sGC activity, and NO sensitivity.

Targeting the heme-oxidized nitric oxide receptor for selective vasodilatation of diseased blood vessels

ROS are a risk factor of several cardiovascular disorders and interfere with NO/soluble guanylyl cyclase/cyclic GMP (NO/sGC/cGMP) signaling through scavenging of NO and formation of the strong oxidant peroxynitrite. Increased oxidative stress affects the heme-containing NO receptor sGC by both decreasing its expression levels and impairing NO-induced activation, making vasodilator therapy with NO donors less effective. Here we show in vivo that oxidative stress and related vascular disease states, including human diabetes mellitus, led to an sGC that was indistinguishable from the in vitro oxidized/heme-free enzyme. This sGC variant represents what we believe to be a novel cGMP signaling entity that is unresponsive to NO and prone to degradation. Whereas high-affinity ligands for the unoccupied heme pocket of sGC such as zinc-protoporphyrin IX and the novel NO-independent sGC activator 4-[((4-carboxybutyl){2-[(4-phenethylbenzyl)oxy]phenethyl}amino) methyl [benzoic]acid (BAY 58-2667) stabilized the enzyme, only the latter activated the NO-insensitive sGC variant. Importantly, in isolated cells, in blood vessels, and in vivo, BAY 58-2667 was more effective and potentiated under pathophysiological and oxidative stress conditions. This therapeutic principle preferentially dilates diseased versus normal blood vessels and may have far-reaching implications for the currently investigated clinical use of BAY 58-2667 as a unique diagnostic tool and highly innovative vascular therapy.

Role of cyclic GMP and calcineurin in homologous and heterologous desensitization of natriuretic peptide receptor-A

The natriuretic peptide receptor-A (NPR-A) mediates natriuretic, hypotensive, and antihypertrophic effects of natriuretic peptides through the production of cGMP. In pathological conditions such as heart failure, these effects are attenuated by homologous and heterologous desensitization mechanisms resulting in the dephosphorylation of the cytosolic portion of the receptor. In contrast with natriuretic peptide-induced desensitization, pressor hormone-induced desensitization is dependent on protein kinase C (PKC) stimulation and (or) cytosolic calcium elevation. Mechanisms by which PKC and Ca(2+) promote NPR-A desensitization are not known. The role of cGMP and of the cytosolic Ca(2+) pathways in NPR-A desensitization were therefore studied. In contrast with the activation of NPR-A by its agonist, activation of soluble guanylyl cyclases of LLC-PK1 cells by sodium nitroprusside also leads to a production of cGMP but without altering NPR-A activation. Consequently, cGMP elevation per se does not appear to mediate homologous desensitization of NPR-A. In addition, cytosolic calcium increase is required only for the heterologous desensitization pathway since the calcium chelator BAPTA-AM blocks only PMA or ionomycin-induced desensitization. Calcineurin inhibitors block the NPR-A guanylyl cyclase heterologous desensitization induced by ionomycin, suggesting an essential role for this Ca(2+)-stimulated phosphatase in NPR-A desensitization. In summary, the present report demonstrates that neither cGMP nor Ca(2+) cytosolic elevation cause NPR-A homologous desensitization. Our results also indicate for the first time a role for calcineurin in NPR-A heterologous desensitization.

Increased intracavernosal pressure response in hypertensive rats after chronic hemin treatment

INTRODUCTION

Erectile dysfunction (ED) is increased in prevalence in patients with arterial hypertension. Whether upregulation of heme oxygenase (HO) expression could improve penile erection has been unknown.

AIMS

To correlate altered expression profiles of HO-1 and soluble guanylyl cyclase (sGC) in penile tissues with low intracavernosal pressure (ICP) in adult spontaneously hypertensive rats (SHR); and to investigate therapeutic effect of hemin-induced upregulation of HO-1 in penile tissues on ED developed in adult SHR.

METHODS AND MAIN OUTCOME MEASURES

Intracavernosal pressure changes after electrical stimulation were monitored in adult SHR and age-matched normotensive Sprague-Dawley (SD) rats after chronic administration of either hemin or hydralazine. Expression levels of HO-1, HO-2, sGC, and phosphodiesterase type 5 (PDE5) were examined with Western blot.

RESULTS

Frequency-dependent ICP changes were reduced in adult SHR. Three weeks after hemin treatment, high blood pressure of SHR was normalized and ICP responses to electrical stimulations in SHR were significantly increased to the level of normotensive rats. Hydralazine-treated SHR had normalized blood pressure but unaltered low ICP response. Expression of HO-1 and sGC was upregulated and that of PDE5 downregulated in hemin-treated, but not hydralazine-treated, SHR.

CONCLUSIONS

Decreased erectile responses in adult SHR can be improved through chronic hemin treatment. Prolonged upregulation of HO-1 and sGC as well as lowered expression of PDE5 may at least partially explain the effect of hemin treatment on ICP. Upregulation of HO-1 may represent a novel therapeutic approach to treat ED.

Activation of soluble guanylate cyclase by nitric oxide is increased in lymphocytes from both rats chronically exposed to 2,5-hexanedione and workers chronically exposed to n-hexane

Although occupational exposure to n-hexane induces neurotoxic effects in the central and peripheral nervous systems, the mechanisms of its neurotoxicity remain unclear. n-Hexane is metabolized to 2,5-hexanedione (2,5-HD), which is the neurotoxic agent and the indicator chosen for the biological monitoring of exposed workers. It has been previously reported that chronic exposure to 2,5-HD impairs the glutamate-nitric oxide-cyclic GMP pathway at the level of activation of soluble guanylate cyclase (sGC) enzyme by nitric oxide (NO), both in cultured neurons and in the cerebellum of rats in vivo. The aim of this study was to assess whether the activation of sGC by NO is also altered in lymphocytes from rats treated with 2,5-HD and/or workers chronically exposed to n-hexane. Lymphocytes were isolated from male Wistar rats treated with 2,5-HD in drinking water, and from blood samples from shoe-factory workers environmentally and chronically exposed to n-hexane. Urine samples were also collected from workers at the end of the shift in order to measure the urinary levels of 2,5-HD. Activation of sGC by NO was significantly higher (p<0.05) in lymphocytes from rats treated with 2,5-HD than in control rats. In isolated lymphocytes from exposed workers the activation of sGC by NO also increases (p<0.05) in contrast to the controls. The results presented here indicate that the activation of lymphocytes could be an indicator of the toxicity produced by being exposed to n-hexane, since the effects observed in workers chronically exposed to n-hexane are similar to those found in rats chronically treated with 2,5-HD in drinking water.

Soluble guanylyl cyclase activation with HMR1766 attenuates platelet activation in diabetic rats

OBJECTIVE

Platelet activation significantly contributes to cardiovascular morbidity and mortality in diabetes. An association between impaired NO-mediated platelet inhibition and platelet activation has recently been demonstrated in experimental diabetes. Guanylyl cyclase activation enhances the reduced signaling via the NO/cGMP pathway. We investigated whether chronic guanylyl cyclase activation would beneficially modulate platelet activation in experimental diabetes mellitus.

METHODS AND RESULTS

Diabetes was induced by streptozotocin-injection in male Wistar rats. After 2 weeks, treatment with either placebo or the guanylyl cyclase activator HMR1766 (10 mg/kg twice daily by gavage) was initiated. Two weeks later, in vivo platelet activation and in vitro platelet reactivity were assessed. Chronic treatment with HMR1766 enhanced NO/cGMP-mediated signaling in platelets from diabetic rats determined by in vivo phosphorylation of platelet vasodilator-stimulated phosphoprotein (VASP) at Ser157 and Ser239. In parallel, platelet-binding of fibrinogen, surface-expression of P-selectin, appearance of platelet-derived microparticles, and platelet-aggregates with other blood cells were significantly reduced by chronic treatment with HMR1766.

CONCLUSIONS

Chronic activation of soluble guanylyl cyclase in diabetic rats improved markers of platelet activation and is a rationale approach for prevention of adverse cardiovascular events in diabetes.

Expression of endothelial nitric oxide synthase in the ovine ovary throughout the estrous cycle

This study was conducted to evaluate the expression of endothelial nitric oxide synthase (eNOS) in ovarian follicles and corpora lutea (CL) throughout the estrous cycle in sheep. Three experiments were conducted to (1) immunolocalize eNOS protein, (2) determine expression of mRNA for eNOS and its receptor guanylate cyclase 1 soluble β3 (GUCY1B3), and (3) co-localize eNOS and vascular endothelial growth factor (VEGF) proteins in the follicles and/or CL throughout the estrous cycle. In experiment 1, ovaries were collected from ewes treated with FSH, to induce follicular growth or atresia. In experiment 2, ovaries were collected from ewes treated with FSH and hCG to induce follicular growth and ovulation. In experiment 3, ovaries were collected from superovulated ewes to generate multiple CL on days 2, 4, 10, and 15 of the estrous cycle. In experiments 1 and 2, the expression of eNOS protein was detected in the blood vessels of the theca externa and interna of healthy ovarian follicles. However, in early and advanced atretic follicles, eNOS protein expression was absent or reduced. During the immediate postovulatory period, eNOS protein expression was detected in thecal-derived cells that appeared to be invading the granulosa layer. Expression of eNOS mRNA tended to increase in granulosa cells at 12 and 24 h, and in theca cells 48 h after hCG injection. In experiment 3, eNOS protein was located in the blood vessels of the CL during the estrous cycle. Dual localization of eNOS and VEGF proteins in the CL demonstrated that both were found in the blood vessels.

Nitric oxide preconditioning regulates endothelial monolayer integrity via the heat shock protein 90-soluble guanylate cyclase pathway

Large (pathological) amounts of nitric oxide (NO) induce cell injury, whereas low (physiological) NO concentrations often ameliorate cell injury. We tested the hypotheses that pretreatment of endothelial cells with low concentrations of NO (preconditioning) would prevent injury induced by high NO concentrations. Apoptosis, induced in bovine aortic endothelial cells (BAECs) by exposing them to either 4 mM sodium nitroprusside (SNP) or 0.5 mM N-(2-aminoethyl)-N-(2-hydroxy-2-nitrosohydrazino)-1,2-ethylenediamine (spermine NONOate) for 8 h, was abolished by 24-h pretreatment with either 100 microM SNP, 10 microM spermine NONOate, or 100 microM 8-bromo-cGMP (8-Br-cGMP). Repair of BAECs following wounding, measured as the recovery rate of transendothelial electrical resistance, was delayed by 8-h exposure to 4 mM SNP, and this delay was significantly attenuated by 24-h pretreatment with 100 microM SNP. NO preconditioning produced increased association and expression of soluble guanyl cyclase (sGC) and heat shock protein 90 (HSP90). The protective effect of NO preconditioning, but not the injurious effect of 4 mM SNP, was abolished by either a sGC activity inhibitor 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ) or a HSP90 binding inhibitor (radicicol) and was mimicked by 8-Br-cGMP. We conclude that preconditioning with a low dose of NO donor accelerates repair and maintains endothelial integrity via a mechanism that includes the HSP90/sGC pathway. HSP90/sGC may thus play a role in the protective effects of NO-generating drugs from injurious stimuli.

Androgen regulation of soluble guanylyl cyclasealpha1 mediates prostate cancer cell proliferation

The growth and progression of prostate cancer are dependent on androgens and androgen receptor (AR), which act by modulating gene expression. Utilizing a gene microarray approach, we have identified the alpha1-subunit gene of soluble guanylyl cyclase (sGC) as a novel androgen-regulated gene. A heterodimeric cytoplasmic protein composed of one alpha and one beta subunit, sGC mediates the widespread cellular effects of nitric oxide (NO). We report here that, in prostate cancer cells, androgens stimulate the expression of sGCalpha1. A cloned human sGCalpha1 promoter is activated by androgen in an AR-dependent manner, suggesting that sGCalpha1 may be a direct AR target gene. Disruption of sGCalpha1 expression severely compromises the growth of both androgen-dependent and androgen-independent AR-positive prostate cancer cells. Overexpression of sGCalpha1 alone is sufficient for stimulating prostate cancer cell proliferation. Interestingly, the major growth effect of sGCalpha1 is independent of NO and cyclic guanosine monophosphate, a major mediator of the sGC enzyme. These data strongly suggest that sGCalpha1 acts in prostate cancer via a novel pathway that does not depend on sGCbeta1. Tissue studies show that sGCalpha1 expression is significantly elevated in advanced prostate cancer. Thus, sGCalpha1 may be an important mediator of the procarcinogenic effects of androgens.

Heme oxygenase/carbon monoxide-biliverdin pathway down regulates neutrophil rolling, adhesion and migration in acute inflammation

BACKGROUND AND PURPOSE

Heme oxygenase (HO) activity is known to down-regulate inflammatory events. Here, we address the role of HO and its metabolites, carbon monoxide (CO) and biliverdin (BVD), in leukocyte rolling, adhesion and neutrophil migration during inflammatory processes.

EXPERIMENTAL APPROACH

Intravital microscopy was used to evaluate leukocyte rolling and adhesion in the mesenteric microcirculation of mice. TNFalpha and IL-1beta were determined by ELISA and HO-1 protein expression by Western blot.

KEY RESULTS

Intraperitoneal challenge with carrageenan enhanced HO-1 protein expression in mesentery and bilirubin concentration in peritoneal exudates. Pretreatment of mice with a non-specific inhibitor of HO (ZnDPBG) or with a HO-1 specific inhibitor (ZnPP IX) enhanced neutrophil migration, rolling and adhesion on endothelium induced by carrageenan. In contrast, HO substrate (hemin), CO donor (DMDC) or BVD reduced these parameters. The reduction of neutrophil recruitment promoted by HO metabolites was independent of the production of chemotactic cytokines. Inhibitory effects of CO, but not of BVD, were counteracted by treatment with a soluble guanylate cyclase (sGC) inhibitor, ODQ. Furthermore, inhibition of HO prevented the inhibitory effect of a nitric oxide (NO) donor (SNAP) upon neutrophil migration, while the blockade of NO synthase (NOS) activity by aminoguanidine did not affect the CO or BVD effects.

CONCLUSIONS AND IMPLICATIONS

Metabolites of HO decreased leukocyte rolling, adhesion and neutrophil migration to the inflammatory site by a mechanism partially dependent on sGC. Moreover, inhibition by NO of neutrophil migration was dependent on HO activity.

Protoporphyrin IX generation from δ-aminolevulinic acid elicits pulmonary artery relaxation and soluble guanylate cyclase activation

Protoporphyrin IX is an activator of soluble guanylate cyclase (sGC), but its role as an endogenous regulator of vascular function through cGMP has not been previously reported. In this study we examined whether the heme precursor δ-aminolevulinic acid (ALA) could regulate vascular force through promoting protoporphyrin IX-elicited activation of sGC. Exposure of endothelium-denuded bovine pulmonary arteries (BPA) in organoid culture to increasing concentrations of the heme precursor ALA caused a concentration-dependent increase in BPA epifluorescence, consistent with increased tissue protoporphyrin IX levels, associated with decreased force generation to increasing concentrations of serotonin. The force-depressing actions of 0.1 mM ALA were associated with increased cGMP-associated vasodilator-stimulated phosphoprotein (VASP) phosphorylation and increased sGC activity in homogenates of BPA cultured with ALA. Increasing iron availability with 0.1 mM FeSO4inhibited the decrease in contraction to serotonin and increase in sGC activity caused by ALA, associated with decreased protoporphyrin IX and increased heme. Chelating endogenous iron with 0.1 mM deferoxamine increased the detection of protoporphyrin IX and force depressing activity of 10 μM ALA. The inhibition of sGC activation with the heme oxidant 10 μM 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) attenuated the force depressing actions of an NO donor without altering the actions of ALA. Thus control of endogenous formation of protoporphyrin IX from ALA by the availability of iron is potentially a novel physiological mechanism of controlling vascular function through regulating the activity of sGC.

Nitric-oxide-dependent activation of pig oocytes: the role of the cGMP-signalling pathway

Pig oocytes matured in vitro were parthenogenetically activated (78%) after treatment with 2 mM nitric oxide-donor (+/-)-S-nitroso-N-acetylpenicillamine (SNAP) for 24 h. Inhibition of soluble guanylyl cyclase with the specific inhibitors 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) or 6-anilino-5,8-quinolinequinone (LY83583) suppressed the SNAP-induced activation in a dose-dependent manner (23% of activated oocytes after treatment with 400 microM ODQ; 12% of activated oocytes after treatment with 40 microM LY83583). 8-Bromo-cyclic guanosine monophosphate (8-Br-cGMP), a phosphodiesterase-resistant analogue of cGMP, enhances the effect of suboptimal doses (0.1 or 0.5 mM) of the NO donor SNAP. DT3, a specific inhibitor of cGMP-dependent protein kinase (PKG, PKG), is also able to inhibit the activation of pig oocytes after NO donor treatment. Involvement of the cGMP-dependent signalling pathway is specific for NO-induced oocyte activation, because both the guanylyl cyclase inhibitor ODQ and the PKG inhibitor DT3 are unable to inhibit activation in oocytes treated with the calcium ionophore A23187. These data indicate that the activation of pig oocytes with an NO donor is cGMP-dependent and that PKG plays an important role in this mode of oocyte activation.

Nitric oxide-evoked cGMP production in Purkinje cells in rat cerebellum: an immunocytochemical and pharmacological study

The cerebellar cells that account for glutamate-dependent cyclic GMP (cGMP) production, involving activation of the ionotropic glutamate receptors/nitric oxide synthase/soluble guanylyl cyclase pathway, are not fully established. In the present paper we have searched for the localisation of the cGMP response to the nitric oxide (NO) donor S-nitroso-penicillamine (SNAP 1muM), expected to generate local NO concentrations in the low nanomolar physiological range and evoking a cGMP response dependent on glutamate release and on the consequent activation of ionotropic glutamate NMDA/non-NMDA receptors, in cerebellar slices from adult rat. We have found that low concentration of exogenous NO evoked cGMP accumulation in Purkinje cells in an ionotropic glutamate receptor-dependent and tetrodotoxin-sensitive manner. Such immunocytochemical localisation appears consistent with functional evidence for physiologically relevant glutamate-dependent cGMP production in Purkinje cells in rat cerebellar cortex.

Endothelin signaling pathways in rat adrenal medulla

1. We further characterized the effect of endothelins (ETs) on receptor-mediated phosphoinositide (PI) turnover, nitric oxide synthase (NOS) activation, and cGMP formation in whole rat adrenal medulla. 2. The PI hydrolysis was assessed as accumulation of inositol monophosphates (InsP(1)) in the presence of 10 mM LiCl in whole tissue and the analysis of inositol-1-phosphate by Dowex anion exchange chromatography. NOS activity was assayed by monitoring the conversion of radiolabeled L-arginine to L-citrulline. Cyclic GMP formation was assessed as accumulation of cGMP in whole tissue in the presence of phosphodiesterase inhibition, and the amount of cGMP formed was determined by radioimmuno-antibody procedure. 3. ET-1 and ET-3 increased PI turnover by 30% in whole adrenal medulla prelabeled with [(3)H] myoinositol. Both ETs isoforms, at equimolar doses, increased NOS activity and cGMP levels in similar degree. The selective ET(B) receptor agonist, IRL-1620, also increased cGMP formation, mimicking the effects of ETs, while IRL-1620 did not alter the PI metabolism. ETs-induced InsP(1) accumulation and cGMP was dependent on extracellular calcium. The effect of ETs on PI turnover was inhibited by neomycin. The L-arginine analogue, N-nitro-L-arginine (L-NAME), and two inhibitors of soluble guanylyl cyclase, methylene blue and ODQ, significantly inhibited the increase in cGMP production induced by ETs or IRL-1620. The selective ET(A) receptor antagonist, BQ 123, inhibited the ETs-induced increase in PI turnover, while the selective ET(B) receptor antagonist, BQ 788, was ineffective. Likewise, BQ 788, significantly inhibited ET-1- or ET-3-induced NOS activation and cGMP generation but not ETs-induced InsP(1) accumulation. 4. Our data indicate that stimulation of PI turnover and NO-induced cGMP generation constitutes ETs signaling pathways in rat adrenal medulla. The former action is mediated through activation of ET(A) receptor, while the latter through the activation of ET(B) receptor. These results support the role of endothelins in the regulation of adrenal medulla function.

Genetic overexpression of eNOS attenuates hepatic ischemia-reperfusion injury

Previous studies have shown that endothelial nitric oxide (NO) synthase (eNOS)-derived NO is an important signaling molecule in ischemia-reperfusion (I-R) injury. Deficiency of eNOS-derived NO has been shown to exacerbate injury in hepatic and myocardial models of I-R. We hypothesized that transgenic overexpression of eNOS (eNOS-TG) would reduce hepatic I-R injury. We subjected two strains of eNOS-TG mice to 45 min of hepatic ischemia and 5 h of reperfusion. Both strains were protected from hepatic I-R injury compared with wild-type littermates. Because the mechanism for this protection is still unclear, additional studies were performed by using inhibitors and activators of both soluble guanylyl cyclase (sGC) and heme oxygenase-1 (HO-1) enzymes. Blocking sGC with 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) and HO-1 with zinc (III) deuteroporphyrin IX-2,4-bisethyleneglycol (ZnDPBG) in wild-type mice increased hepatic I-R injury, whereas pharmacologically activating these enzymes significantly attenuated I-R injury in wild-type mice. Interestingly, ODQ abolished the protective effects of eNOS overexpression, whereas ZnDPBG had no effect. These results suggest that hepatic protection in eNOS-TG mice may be mediated in part by NO signaling via the sGC-cGMP pathway and is independent of HO-1 signal transduction pathways.

Interaction between endogenously produced carbon monoxide and nitric oxide in regulation of renal afferent arterioles

Heme oxygenases (HO-1 and HO-2) catalyze the conversion of heme to carbon monoxide (CO), iron, and biliverdin. CO causes vasorelaxation via stimulation of soluble guanylate cyclase (sGC) and/or activation of calcium-activated potassium channels. Because nitric oxide (NO) exerts effects via the same pathways, we tested the interaction between CO and NO on rat afferent arterioles (AAs) using the blood-perfused juxtamedullary nephron preparation. AAs were superfused with either tricarbonyldichlororuthenium (II) dimer, known as CO releasing molecule (CORM-2), 10 micromol/l CO solution, or 15 micromol/l chromium mesoporphyrin (CrMP, HO inhibitor). AAs were also superfused with 1 mmol/l N(omega)-nitro-L-arginine (L-NNA) to inhibit NO synthase (NOS) or 10 micromol/l 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one to inhibit sGC, and then CrMP was superfused during NOS inhibition or sGC inhibition. Treatment with 150 and 300 micromol/l CORM-2 or with CO (10 micromol/l) significantly dilated AAs (22.0 +/- 0.9 and 22.8 +/- 0.9 vs. 18.3 +/- 0.9 microm, n = 5, P < 0.05; and 26.0 +/- 1.4 vs. 18.8 +/- 0.7 microm, n = 5, P < 0.05). In untreated vessels, HO inhibition did not alter AA diameter (17.5 +/- 0.7 vs. 17.2 +/- 0.6 microm, n = 7, P > 0.05); however, during inhibition of NO production, which constricted arterioles to 14.6 +/- 1.2 microm, n = 6, P < 0.05, concurrent HO inhibition led to further vasoconstriction (11.7 +/- 1.6 microm, n = 6, P < 0.05). CORM-2 attenuated the L-NNA-induced vasoconstriction. Inhibition of sGC caused significant constriction (15.7 +/- 0.4 vs. 18.8 +/- 0.4 microm, n = 6, P < 0.05). HO inhibition during sGC inhibition did not cause further change in AAs (15.5 +/- 0.7 microm, n = 6). We conclude that endogenously produced CO does not exert a perceptible influence on AA diameter in the presence of intact NO system; however, when NO production is inhibited, CO serves as an important renoprotective reserve mechanism to prevent excess afferent arteriolar constriction.

Cerebellar localization of the NO-receptive soluble guanylyl cyclase subunits-alpha(2)/beta (1) in non-human primates

Nitric-oxide-sensitive guanylyl cyclase (NO-sGC) plays a pivotal role in many second messenger cascades. Neurotransmission- and neuropathology-related changes in NO-sGC have been suggested. However, the cellular localization of NO-sGC in primate brains, including humans, remains unknown. Biochemical evidence has linked the alpha(2)-subunit of NO-sGC directly to neurotransmission in rodents. Here, we have used a recently characterized subunit-specific antibody for the localization of the alpha(2)-subunit on sections from the cerebelli of the common marmoset (Callithrix jacchus; New World monkey) and macaque monkeys (Macaca mulatta, M. fascicularis; Old World monkeys). In contrast to the more ubiquitous cytoplasmic presence of subunit-beta(1), the alpha(2)-subunit is mainly confined to the somato-dendritic membrane including the spines of the Purkinje cells. Only limited colocalization with presynaptically localized synaptophysin has been seen under our staining conditions, indicating a higher abundance of subunit-alpha(2) at the postsynaptic site. This localization indicates that subunit-alpha(2) links NO-sGC to neurotransmission, whereas subunit-beta(1) may act as a cytoplasmic regulator/activator by contributing to active heterodimer formation via translocation from the cytoplasm to the cell membrane. The last-mentioned action may be a prerequisite for generating nitric-oxide-dependent, subcellular, and postsynaptically localized cGMP signals along neuronal processes.

Guanylyl cyclase protein and cGMP product independently control front and back of chemotaxing Dictyostelium cells

Chemotaxis of amoeboid cells is driven by actin filaments in leading pseudopodia and actin-myosin filaments in the back and at the side of the cell to suppress pseudopodia. In Dictyostelium, cGMP plays an important role during chemotaxis and is produced predominantly by a soluble guanylyl cyclase (sGC). The sGC protein is enriched in extending pseudopodia at the leading edge of the cell during chemotaxis. We show here that the sGC protein and the cGMP product have different functions during chemotaxis, using two mutants that lose either catalytic activity (sGCDelta cat) or localization to the leading edge (sGCDeltaN). Cells expressing sGCDeltaN exhibit excellent cGMP formation and myosin localization in the back of the cell, but they exhibit poor orientation at the leading edge. Cells expressing the catalytically dead sGCDelta cat mutant show poor myosin localization at the back, but excellent localization of the sGC protein at the leading edge, where it enhances the probability that a new pseudopod is made in proximity to previous pseudopodia, resulting in a decrease of the degree of turning. Thus cGMP suppresses pseudopod formation in the back of the cell, whereas the sGC protein refines pseudopod formation at the leading edge.

Characterization of NO/cGMP-mediated responses in identified motoneurons

1. Nitric oxide (NO) is thought to play a neuromodulatory role in the nervous system of vertebrate and invertebrate species. In the hornworm Manduca sexta, NO-mediated signaling has been implicated in behavioral and developmental processes, but its exact function in neurons is unknown. In this study, we identify specific neurons in the CNS of Manduca larvae that accumulate cGMP in response to treatment with NO donors in the presence of cGMP-phosphodiesterase inhibitors. Subsets of these neurons were identified as motoneuron-12 (MN12) and intersegmental motoneurons (ISMs), which innervate dorsal oblique muscles of the larvae. 2. To investigate the physiological role of NO-evoked increases in cGMP in these motoneurons we performed intracellular recordings; we found that application of NO donors caused an increase in neuronal excitability that was characterized by an increase in the spontaneous firing frequency. When action potentials and EPSPs were blocked, NO treatment evoked a depolarization of the resting membrane potential and a decrease in the measured input resistance in both MN12 and the ISMs. 3. Additional experiments with MN12 showed that treatment with the cGMP analogue, 8-Br-cGMP mimicked the NO effect on the resting potential and the input resistance. Furthermore, MN12 incubation with the NOS inhibitor, L-NNA, resulted in a small hyperpolarization of the resting potential and an increase in the input resistance, and incubation with the sGC inhibitor, ODQ blocked the NO-evoked depolarization of MN12. Finally, NO treatment during voltage clamping of MN12 evoked an inward positive current. 4. Taken together, these results suggest that NO can act as a "gain control" of neuronal excitability, which might have an important role in insect behavior.

Localization of soluble guanylyl cyclase in the superficial dorsal horn

Nitric oxide (NO) has been implicated in pain processing at the spinal level, but the mechanisms mediating its effects remain unclear. In the present work, we studied the organization of the major downstream effector of NO, soluble guanylyl cyclase (sGC), in the superficial dorsal horn of rat. Almost all neurokinin 1 (NK1) receptor-positive neurons in lamina I (a major source of ascending projections) were strongly immunopositive for sGC. Many local circuit neurons in laminae I-II also stained for sGC, but less intensely. Numerous fibers, presumably of unmyelinated primary afferent (C fiber) origin, stained for calcitonin gene-related peptide or isolectin B4, but none of these was immunopositive for sGC. These data, along with immunoelectron microscopy results, imply that unmyelinated primary afferent fibers terminating in the superficial dorsal horn lack sGC. Double labeling showed that neuronal nitric oxide synthase (nNOS) seldom colocalized with sGC, but nNOS-positive structures were frequently closely apposed to sGC-positive structures, suggesting that in the superficial dorsal horn NO acts mainly in a paracrine manner. Our data suggest that the NK1 receptor-positive projection neurons in lamina I are a major target of NO released in superficial dorsal horn. NO may also influence local circuit neurons, but it does not act on unmyelinated primary afferent terminals via sGC.

Involvement of nitric oxide-soluble guanylyl cyclase pathway in the control of maximal dentate gyrus activation in the rat

Nitric oxide/soluble Guanylyl cyclase (NO/sGC) pathway on the maximal dentate gyrus activation (MDA) was studied in rats. The cerebral NO levels were modified by administrating 7-Nitroindazole (7-NI), a selective inhibitor of neuronal NOS, and L-arginine, a precursor of the synthesis of NO. 1H-[1,2,4]Oxadiazole[4,3-a]quinoxalin-1-one (ODQ), a specific inhibitor of the NO-sGC pathway, was administered to study the involvement of cGMP pathway. The epileptic activity of the dentate gyrus was obtained through the repetitive stimulation of the angular bundle; MDA parameters studied were: onset time, MDA duration and post-stimulus afterdischarge (AD) duration. 7-NI caused an increase of MDA onset time and a decrease of MDA and AD duration. L-arginine, induced an aggravation of the epileptiform phenomena. ODQ induced modifications of MDA parameters as those caused by 7-NI. Our results indicate that the nitrergic neurotransmission exerts a modulatory role in the proneness to the epileptogenic phenomena through the activation of sGC metabolic pathway.

Thromboxane Receptor Stimulation Suppresses Guanylate Cyclase-Mediated Relaxation of Radial Arteries

BACKGROUND

The internal mammary artery (IMA) and the radial artery (RA) are routinely used in coronary artery bypass grafting. However, RA grafts have a higher incidence of postoperative vasospasm and comparatively poor patency rates. The present study was undertaken to investigate the signaling pathways mediating contraction and relaxation in the IMA and RA with the aim of better understanding the mechanism underlying the propensity of RA grafts to spasm.

METHODS

We examined the contractile responses of the IMA and RA to KCl (a depolarizing agent), phenylephrine (an alpha-adrenergic agonist), and U46619 (a thromboxane analogue).

RESULTS

Contractions induced by KCl or U46619 did not significantly differ in IMA and RA. By contrast, phenylephrine evoked significantly greater contraction of the IMA than the RA. Contractions induced by both phenylephrine and U46619 were dose-dependently inhibited by nifedipine (an L-type calcium channel blocker). Estimation of thromboxane A2 (TxA2) and prostacyclin (PGI2) synthesis revealed that the TxA2 to PGI2 ratio in the RA was twice that in the IMA. Moreover, acetylcholine-induced and nitroglycerin-induced relaxation of RA precontracted with U46619 was significantly impaired, as compared with RA precontracted with phenylephrine. These data suggest that inhibition of nitroglycerin-induced soluble guanylate cyclase activity by U46619 was at least partially responsible for the diminished vasodilatory response of RA to nitric oxide.

CONCLUSIONS

Our findings suggest that by reducing nitric oxide-stimulated soluble guanylate cyclase activity, the higher TxA2 to PGI2 ratios in RA, and the elevated serum TxA2 levels seen during coronary artery bypass grafting operations, may underlie the vasospasm and poor patency rates seen with the RA.

PI3K-dependent lysosome exocytosis in nitric oxide-preconditioned hepatocytes

We investigated the signal mediators and the cellular events involved in the nitric oxide (NO)-induced hepatocyte resistance to oxygen deprivation in isolated hepatocytes treated with the NO donor (Z)-1-(N-methyl-N-[6-(N-methylammoniohexyl)amino])diazen-1-ium-1,2-diolate (NOC-9). NOC-9 greatly induced PI3K activation, as tested by phosphorylation of PKB/Akt. This effect was prevented by either 1H-(1,2,4)-oxadiazolo-(4,3)-quinoxalin-1-one, an inhibitor of the soluble guanylate cyclase (sGC), or KT5823, an inhibitor of cGMP-dependent kinase (cGK), as well as by farnesyl protein transferase inhibitor, which blocks the function of Ras GTPase. Bafilomycin A, an inhibitor of the lysosome-type vacuolar H+-ATPase, cytochalasin D, which disrupts the cytoskeleton-dependent organelle traffic, and wortmannin, which inhibits the PI3K-dependent traffic of lysosomes, all abolished the NOC-9-induced hepatocyte protection. The treatment with NOC-9 was associated with the PI3K-dependent peripheral translocation and fusion with the plasma membrane of lysosomes and the appearance at the cell surface of the vacuolar H+-ATPase. Inhibition of sGC, cGK, and Ras, as well as the inhibition of PI3K by wortmannin, prevented the exocytosis of lysosomes and concomitantly abolished the protective effect of NOC-9 on hypoxia-induced pHi and [Na+]i alterations and cell death. These data indicate that NO increases hepatocyte resistance to hypoxic injury by activating a pathway involving Ras, sGC, and cGK that determines PI3K-dependent exocytosis of lysosomes.

NO-cGMP signaling in development and stem cells

Despite the recognition that the NO-cGMP signaling pathway is involved in so many physiological and pathological events, a clear understanding of many of the functions of this signaling pathway remains elusive. Because of its pleiotropic and often transient actions, its modulation for therapeutic purposes in multiple pathological states is a complex issue. Recent work that combines the areas of developmental and stem cell biology and NO-cGMP signaling in various models may help us elucidate some of these functions and even discover novel actions for this signaling paradigm. This review will discuss some of the recent work in these areas, with additional focus on the nitric oxide receptor, soluble guanylyl cyclase.

Nitric oxide signaling via nuclearized endothelial nitric-oxide synthase modulates expression of the immediate early genes iNOS and mPGES-1

Stimulation of freshly isolated rat hepatocytes with lysophosphatidic acid (LPA) resulted in LPA1 receptor-mediated and nitricoxide-dependent up-regulation of the immediate early genes iNOS (inducible nitric-oxide synthase (NOS)) and mPGES-1 (microsomal prostaglandin E synthase-1). Because LPA is a ligand for both cell surface and intracellular receptor sites and a potent endothelial NOS (eNOS) activator, we hypothesized that NO derived from activated nuclearized eNOS might participate in gene regulation. Herein we show, by confocal microscopy performed on porcine cerebral endothelial cells expressing native LPA1-receptor and eNOS and on HTC4 rat hepatoma cells co-transfected with recombinant human LPA1-receptor and fused eNOS-GFP cDNA, a dynamic eNOS translocation from peripheral to nuclear regions upon stimulation with LPA. Nuclear localization of eNOS and its downstream effector, soluble guanylate cyclase, were demonstrated in situ in rat liver specimens by immunogold labeling using specific antibodies. Stimulation of this nuclear fraction with LPA and the NO donor sodium nitroprusside resulted, respectively, in increased production of nitrite (and eNOS phosphorylation) and cGMP; these separate responses were also correspondingly blocked by NOS inhibitor L-NAME and soluble guanylate cyclase inhibitor ODQ. In addition, sodium nitroprusside evoked a sequential increase in nuclear Ca2+ transients, activation of p42 MAPK, NF-kappaB binding to DNA consensus sequence, and dependent iNOS RNA. This study describes a hitherto unrecognized molecular mechanism by which nuclear eNOS through ensuing NO modulates nuclear calcium homeostasis involved in gene transcription-associated events. Moreover, our findings strongly support the concept of the nucleus as an autonomous signaling compartment.

Brain regional alterations in the modulation of the glutamate-nitric oxide-cGMP pathway in liver cirrhosis. Role of hyperammonemia and cell types involved

Hepatic encephalopathy is a complex neuropsychiatric syndrome present in patients with liver disease that includes impaired intellectual function and alterations in personality and neuromuscular coordination. Hyperammonemia and liver failure result in altered glutamatergic neurotransmission, which contributes to hepatic encephalopathy. Alterations in the function of the glutamate-nitric oxide-cGMP pathway may be responsible for some of the neurological alterations found in hepatic encephalopathy. The function of this pathway is altered in brain from patients died with liver cirrhosis and one altered step of the pathway is the activation of soluble guanylate cyclase by nitric oxide, which is increased in cerebral cortex and reduced in cerebellum from these patients. Portacaval anastomosis and bile duct ligation plus hyperammonemia in rats reproduce the alterations in the activation of soluble guanylate cyclase by NO both in cerebellum and cerebral cortex. We assessed whether hyperammonemia is responsible for the region-selective alterations in guanylate cyclase modulation in liver cirrhosis and whether the alteration occurs in neurons or in astrocytes. Activation of guanylate cyclase by nitric oxide is lower in cerebellar neurons exposed to ammonia (1.5-fold) than in control neurons (3.3-fold). The activation of guanylate cyclase by nitric oxide is higher in cortical neurons exposed to ammonia (8.7-fold) than in control neurons (5.5-fold). The activation is not affected in cerebellar or cortical astrocytes. These findings indicate that hyperammonemia is responsible for the differential alterations in the modulation of soluble guanylate cyclase by nitric oxide in cerebellum and cerebral cortex of cirrhotic patients. Moreover, under the conditions used, the alterations occur selectively in neurons and not in astrocytes.

Ammonia affects the activity and expression of soluble and particulate GC in cultured rat astrocytes

Neurotoxic effects of ammonia are mediated by increased accumulation of nitric oxide (NO), which combines with free radicals to form a highly toxic compound, peroxynitrite. Previous experiments in vivo and in vitro have suggested that this phenomenon engages neuron-derived NO and is coupled to changes in the accumulation of cGMP. The present study accounted for the facts that: (i) astrocytes, not neurons are the morphological target of ammonia, and (ii) both NO-dependent, soluble (sGC) and NO-independent, particulate guanylate cyclase (pGC) mediate cGMP production in the cells. Neocortical rat astrocytes were treated for 1 or 24 h with 5 mM ammonium chloride ("ammonia") and then subjected to: (i) cGMP measurement, and (ii) mRNA and/or protein expression analysis of alpha1 and beta1 subunits of sGC and two pGC forms: pGC-A and pGC-B. Treatment with ammonia for 1h increased accumulation of cGMP and sGCbeta1 mRNA expression, without producing significant changes in the protein expression. This was followed by a decrease of cGMP level at 24 h treatment, associated with a decreased expression of sGCbeta1 and sGCalpha1 mRNA and sGCbeta1 protein. Expression of pGC-A and pGC-B mRNA was elevated in ammonia-treated astrocytes after 24 h. Accordingly, increased cGMP accumulation was noted in the presence of a specific sGC inhibitor (ODQ). The results show that ammonia affects cGMP production in astrocytes, and that this may involve not only sGC but also pGC.

Effects of hsp90 binding inhibitors on sGC-mediated vascular relaxation

Vascular soluble guanylate cyclase (sGC) exists in multimeric complexes with endothelial nitric oxide (NO) synthase (eNOS) and heat shock protein 90 (hsp90). Whereas disruption of hsp90-eNOS complexes clearly attenuates eNOS-dependent vascular relaxation, the contribution of sGC-hsp90 complexes to eNOS- or NO donor-dependent relaxations remains unclear. Isolated rat thoracic aortic rings were preincubated with structurally diverse hsp90 binding inhibitors, radicicol (RA) or geldanamycin (GA), or vehicle for 0.5, 1, or 15 h. Preconstricted vessels were exposed to ACh, 8-bromo-cGMP (8-BrcGMP), forskolin, or one of three NO donors: nitroglycerin (NTG), sodium nitroprusside, or spermine NONOate (SNN). Both RA and GA inhibited endothelium-dependent relaxations dose dependently. Indomethacin or the antioxidant tiron did not affect the inhibition of ACh-induced relaxations by GA. Long-term (15 h) exposure to RA inhibited all NO donor-induced relaxations; however, GA inhibited SNN-induced relaxation only. The effects of GA and RA appeared to be selective because 15-h treatment with either agent did not affect forskolin-induced relaxations and only slightly decreased 8-BrcGMP-induced relaxations. Similarly to their effects on NO-donor-induced relaxation, 15-h exposure to RA, but not to GA, decreased hsp90-bound sGC protein expression and NTG-stimulated cGMP formation in aortic rings, whereas RA more than GA reduced SNN-stimulated cGMP formation. We conclude that RA, much more so than GA, selectively inhibits sGC-dependent relaxations of aortic rings by reducing sGC expression, disrupting sGC-hsp90 complex formation and decreasing cGMP formation. These studies suggest that hsp90 regulates both eNOS- and sGC-dependent relaxations.

KMUP-1 activates BKCa channels in basilar artery myocytes via cyclic nucleotide-dependent protein kinases

This study investigated whether KMUP-1, a synthetic xanthine-based derivative, augments the delayed-rectifier potassium (K(DR))- or large-conductance Ca2+-activated potassium (BKCa) channel activity in rat basilar arteries through protein kinase-dependent and -independent mechanisms. Cerebral smooth muscle cells were enzymatically dissociated from rat basilar arteries. Conventional whole cell, perforated and inside-out patch-clamp electrophysiology was used to monitor K+- and Ca2+ channel activities. KMUP-1 (1 microM) had no effect on the K(DR) current but dramatically enhanced BKCa channel activity. This increased BKCa current activity was abolished by charybdotoxin (100 nM) and iberiotoxin (100 nM). Like KMUP-1, the membrane-permeable analogs of cGMP (8-Br-cGMP) and cAMP (8-Br-cAMP) enhanced the BKCa current. BKCa current activation by KMUP-1 was markedly inhibited by a soluble guanylate cyclase inhibitor (ODQ 10 microM), an adenylate cyclase inhibitor (SQ 22536 10 microM), competitive antagonists of cGMP and cAMP (Rp-cGMP, 100 microM and Rp-cAMP, 100 microM), and cGMP- and cAMP-dependent protein kinase inhibitors (KT5823, 300 nM and KT5720, 300 nM). Voltage-dependent L-type Ca2+ current was significantly suppressed by KMUP-1 (1 microM), and nearly abolished by a calcium channel blocker (nifedipine, 1 microM). In conclusion, KMUP-1 stimulates BKCa currents by enhancing the activity of cGMP-dependent protein kinase, and in part this is due to increasing cAMP-dependent protein kinase. Physiologically, this activation would result in the closure of voltage-dependent calcium channels and the relaxation of cerebral arteries.

Effect of hereditary obesity on renal expressions of NO synthase, caveolin-1, AKt, guanylate cyclase, and calmodulin

BACKGROUND

Obesity has emerged as a major cause of diabetes, cardiovascular disease, and renal insufficiency worldwide. Obese Zucker rats exhibit hyperphagia, obesity, insulin resistance, hyperlipidemia, and glomerulosclerosis and are frequently used as a model to study hereditary form of metabolic syndrome. Nitric oxide plays a major role in preservation of renal function and structure. The present study was designed to test the hypothesis that renal disease in this model may be associated with down-regulation of endothelial (eNOS) and neuromal NO synthases (nNOS) in the kidney. The study further sought to explore expressions of caveolin-1, phospho AKt, and calmodulin, which regulate activities of constituitive NOS isoforms, as well as soluble guanylate cyclase (sGC), which is involved in NO signaling.

METHODS

Twenty-two-week-old male obese and lean Zucker rats were studied. Body weight, serum lipids, urine albumin excretion, and renal tissue abundance of the above proteins were determined.

RESULTS

Serum glucose and arterial pressure were unchanged, whereas urinary NO metabolite (NO(chi)) excretion and renal tissue nitrotyrosine abundance were markedly reduced (denoting depressed NO production) in the obese versus lean Zucker rats. This was accompanied by significant glomerulosclerosis, tubulointerstitial damage, renal immune cell infiltration, marked down-regulations of renal tissue eNOS and nNOS, mild reduction of caveolin-1, and unchanged calmodulin, phospho-AKt, and sGC.

CONCLUSION

Hereditary obesity can result in down-regulations of kidney eNOS and nNOS, marked reduction of NO production, and glomerulosclerosis prior to the onset of frank diabetes and hypertension.

Effects of nitroglycerin/L-cysteine on soluble guanylate cyclase: evidence for an activation/inactivation equilibrium controlled by nitric oxide binding and haem oxidation

GTN (nitroglycerin; glycerol trinitrate) causes dilation of blood vessels via activation of nitric oxide (NO)-sensitive sGC (soluble guanylate cyclase), a heterodimeric haem protein that catalyses the conversion of GTP into cGMP. Activation of sGC by GTN requires enzymatic or non-enzymatic bioactivation of the nitrate. Based on insufficient NO release and lack of spectroscopic evidence for formation of NO-sGC, the cysteine (Cys)-dependent activation of sGC by GTN was proposed to occur in an NO-independent manner. This extraordinary claim is questioned by the present findings. First, the effect of GTN/Cys was blocked by the NO scavenger oxyhaemoglobin, the superoxide-generating compound flavin mononucleotide and the haem-site sGC inhibitor ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one). Secondly, at equi-effective concentrations, GTN/Cys and the NO donor 2,2-diethyl-1-nitroso-oxyhydrazine released identical amounts of NO. Finally, at sufficiently high rates of NO release, activation of sGC by GTN/Cys was accompanied by a shift of the Soret band from 431 to 399 nm, indicating formation of NO-sGC. In the absence of Cys, GTN caused haem oxidation, apparent as a shift of the Soret band to 392 nm, which was accompanied by inactivation of the NO-stimulated enzyme. These results suggest that the effect of GTN/Cys is the result of an activation/inactivation equilibrium that is controlled by the rate of NO release and haem oxidation.

Commutators of PAR-1 signaling in cancer cell invasion reveal an essential role of the Rho-Rho kinase axis and tumor microenvironment

We recently reported that proteinase-activated receptors type I (PAR-1) are coupled to both negative and positive invasion pathways in colonic and kidney cancer cells cultured on collagen type I gels. Here, we found that treatments with the cell-permeant analog 8-Br-cGMP and the soluble guanylate cyclase activator BAY41-2272, and Rho kinase (ROK) inhibition by Y27632 or a dominant negative form of ROK lead to PAR-1-mediated invasion through differential Rac1 and Cdc42 signaling. Hypoxia or the counteradhesive matricellular protein SPARC/BM-40 (SPARC: secreted protein acidic rich in cysteine) overexpressed during cancer progression also commutated PAR-1 to cellular invasion through the cGMP/protein kinase G (PKG) cascade, RhoA inactivation, and Rac1-dependent or -independent signaling. Cultured primary cancer cells isolated from peritoneal and pleural effusions from patients with colon cancer or other malignant tumors harbored PAR-1, as shown by RT-PCR and FACS analyses. These malignant effusions also contained high levels of activated thrombin and fibrin, and induced a proinvasive response in HCT8/S11 human colorectal cancer cells. Our data underline the essential role of the tumor microenvironment and of several commutators targeting cGMP/PKG signaling and the RhoA-ROK axis in the control of PAR-1 proinvasive activity and metastatic potential of cancer cells in distant organs and peritoneal or pleural cavities. We also add new insights into the mechanisms linking the coagulation mediators thrombin and PAR-1 in the context of blood coagulation disorders and venous thrombosis often observed in cancer patients, as described in 1865 by Armand Trousseau.

Kv channels contribute to nitric oxide- and atrial natriuretic peptide-induced relaxation of a rat conduit artery

The role of K(+) channels in nitric oxide (NO)-induced vasorelaxation has been largely investigated in resistance vessels where iberiotoxin-sensitive MaxiK channels play a predominant role. However, the nature of the K(+) channel(s) involved in the relaxation triggered by NO-releasing compounds [nitroglycerin, NTG; NOR 3 [(+/-)-(E)-4-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexenamide]] or atrial natriuretic peptide (ANP) in the conduit vessel aorta has remained elusive. We now demonstrate that, in rat aorta, the relaxation due to these vasorelaxants is not affected by the MaxiK channel blocker iberiotoxin (10(-7)-10(-6) M) as was the control vascular bed used (mesenteric artery). The inability of iberiotoxin to prevent NO/ANP-induced aortic relaxations was not due to lower expression of MaxiK in aorta or due to the predominance of iberiotoxin-resistant channels in this conduit vessel. Aortic relaxations were strongly diminished by 4-aminopyridine (4-AP) (> or =5 x 10(-3) M) or by tetraethylammonium (>2 x 10(-3) M) at concentrations known to inhibit voltage-dependent K(+) (K(v)) 2-type channels but not by other K(+) channel inhibitors, glibenclamide, apamin, charybdotoxin, tertiapin, or E-4031 N-[4-[[1-[2-(6-methyl-2-pyridinyl)ethyl]-4-piperidinyl-]carbonyl]phenyl]methanesulfonamide dihydrochloride). Consistent with a role of K(v)2-type channels, K(v) currents in A7r5 aortic myocytes were stimulated by NTG and inhibited by > or =5 x 10(-3) M 4-AP. Furthermore, immunocytochemistry, immunoblot, and real-time polymerase chain reaction analyses confirmed the presence of K(v)2.1 channels in aorta. K(v)2.1 transcripts were approximately 100-fold more abundant than K(v)2.2. Our results support low-affinity 4-AP-sensitive K(v) channels, assembled at least partially by K(v)2.1 subunit, as downstream effectors of NO/ANP-signaling cascade regulating aortic vasorelaxation and further demonstrate vessel-specific K(+) channel involvement in NO/ANP-induced relaxation.

Nitric oxide and the vascular endothelium

The vascular endothelium synthesises the vasodilator and anti-aggregatory mediator nitric oxide (NO) from L-arginine. This action is catalysed by the action of NO synthases, of which two forms are present in the endothelium. Endothelial (e)NOS is highly regulated, constitutively active and generates NO in response to shear stress and other physiological stimuli. Inducible (i)NOS is expressed in response to immunological stimuli, is transcriptionally regulated and, once activated, generates large amounts of NO that contribute to pathological conditions. The physiological actions of NO include the regulation of vascular tone and blood pressure, prevention of platelet aggregation and inhibition of vascular smooth muscle proliferation. Many of these actions are a result of the activation by NO of the soluble guanylate cyclase and consequent generation of cyclic guanosine monophosphate (cGMP). An additional target of NO is the cytochrome c oxidase, the terminal enzyme in the electron transport chain, which is inhibited by NO in a manner that is reversible and competitive with oxygen. The consequent reduction of cytochrome c oxidase leads to the release of superoxide anion. This may be an NO-regulated cell signalling system which, under certain circumstances, may lead to the formation of the powerful oxidant species, peroxynitrite, that is associated with a variety of vascular diseases.

Stimulation of soluble guanylyl cyclase by BAY 41-2272 relaxes anococcygeus muscle: interaction with nitric oxide

The compound BAY 41-2272 stimulates the soluble guanylyl cyclase in a nitric oxide (NO)-independent manner. We have investigated the potency and efficacy of BAY 41-2272 in the rat anococcygeus muscle, as well as the effects of BAY 41-2272 on NO-mediated anococcygeus relaxations. BAY 41-2272 (0.01-10 microM) potently relaxed precontracted anococcygeus muscle strips, with a pEC(50) value of 6.44 +/- 0.03 and maximum response of 100 +/- 2%. The soluble guanylyl cyclase inhibitor 1H-[1,2,4]-oxidiazolo[4,3-a] quinoxalin-1-one (ODQ, 1 microM) and the NO inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME, 100 microM) caused significant rightward shifts in the concentration-response curves to BAY 41-2272. The phosphodiesterase type-5 inhibitor tadalafil (0.1 microM) markedly enhanced the relaxations evoked by BAY 41-2272. In addition, BAY 41-2272 increased the duration of nitrergic relaxations by approximately 55%. The relaxations induced by glyceryl trinitrate were also significantly potentiated by BAY 41-2272. In conclusion, BAY 41-2272 interacts with endogenous and exogenous NO causing a potent relaxation of rat anococcygeus muscle.

Effects of potassium channel inhibitors in the forced swimming test: Possible involvement of l-arginine-nitric oxide-soluble guanylate cyclase pathway

The effects of inhibitors of different subtypes of potassium (K+) channels were investigated in the mouse forced swimming test (FST). The treatment of animals with tetraethylammonium (TEA, a non-specific inhibitor of potassium channels, 0.25-2.5 ng/site, intracerebroventricular, i.c.v.), glibenclamide (an ATP-sensitive potassium channels (K(ATP) inhibitor, 0.05-5 ng/site, i.c.v.), apamine (a small conductance calcium-activated potassium channels inhibitor (SKCa), 0.1-1 ng/site, i.c.v.), charybdotoxin (a large- (big, BK) and intermediate- (IK) conductance calcium-activated potassium channels inhibitor, 2.5-25 ng/site, i.c.v.) produced an anti-depressant-like effect in the FST. At the highest effective doses, none of the drugs affected the locomotor activity in an open-field. Besides that, the pre-treatment of animals with l-arginine (a nitric oxide (NO) precursor, 750 mg/kg, intraperitoneal, i.p.) or sildenafil (a specific phosphodiesterase type 5 (PDE5) inhibitor, 5 mg/kg, i.p.) prevented the anti-depressant-like effect of all K+ channel inhibitors. The present results demonstrate that the decrease in the immobility time in the FST elicited by the inhibition of several subtypes of K+ channels is also dependent on the inhibition of NO-cGMP synthesis.

Mechanisms of Increased Vascular Superoxide Production in an Experimental Model of Idiopathic Dilated Cardiomyopathy

Objective— In the present study, we sought to identify mechanisms underlying increased oxidative stress in vascular tissue in an experimental animal model of chronic congestive heart failure (CHF).

Methods and Results— Superoxide and nitric oxide (NO) was measured in vessels from cardiomyopathic hamsters (CHF hamsters) and golden Syrian hamsters. We also determined expression of endothelial nitric oxide synthase (NOSIII), the soluble guanylyl cyclase, the cGMP-dependent kinase, and the NADPH oxidase. To analyze the contribution of the renin-angiotensin system to oxidative stress, CHF hamsters were treated with the angiotensin-converting enzyme inhibitor captopril for 200 days (120 mg · kg −1 · d −1 ). CHF led to increased superoxide production by NOSIII and the NADPH oxidase. Decreased NO production in CHF was associated with a decrease in the expression of NOSIII and an inhibition of NO downstream signaling in the aorta. NOSIII expression was increased within the left ventricle. Captopril treatment normalized NOSIII expression in vessels and the myocardium, reduced superoxide levels, and prevented NOSIII uncoupling. Accordingly, endothelial function, NO production, and downstream signaling were improved in CHF vessels.

Conclusions— Oxidative stress in CHF is mediated by NADPH oxidase and an uncoupled NOSIII secondary to an activation of the renin-angiotensin system leading to impaired NO downstream signaling.

Formalin injection causes a coordinated spinal cord CO/NO-cGMP signaling system response

Background

The CO/NO-cGMP signalling system participates in the regulation of many physiological processes. The roles this system plays in spinal cord nociceptive signalling are particularly important. While individual components have been examined in isolation, little study has been dedicated to understanding the regulation and functioning of the system as a whole.

Results

In these studies we examined the time course of expression of 13 genes coding for components of this system including isoforms of the heme oxygenase (HO), nitric oxide synthase (NOS), soluble guanylate cyclase (sGC), cGMP dependent protein kinase (PKG) and phosphodiesterase (PDE) enzyme systems. Of the 13 genes studied, 11 had spinal cord mRNA levels elevated at one or more time points up to 48 hours after hindpaw formalin injection. Of the 11 with elevated mRNA, 8 had elevated protein levels 48 hours after formalin injection when mechanical allodynia was maximal. No component had an increased protein level which did not have an increased mRNA level at one or more time points. Injection of morphine 10 mg/kg prior to formalin completely abolished the acute nociceptive behaviours, but did not alter the degree of sensitivity which developed in the formalin treated hind paws during the subsequent 48 hours. Morphine treatment did, however, eliminate formalin induced increases in enzyme protein levels.

Conclusion

Our results indicate that the expression of the components of the CO/NO-cGMP signalling system seems to be coordinated in such a way that a generalized multi-level enhancement rather than a tightly limited step specific response occurs with noxious stimulation. Furthermore, the analgesic morphine administered prior to noxious stimulation can prevent long-term changes in gene expression though not necessarily nociceptive sensitisation.

Stimulatory Roles of Nitric-oxide Synthase 3 and Guanylyl Cyclase in Platelet Activation

Nitric oxide (NO) stimulates soluble guanylyl cyclase and, thus, enhances cyclic guanosine monophosphate (cGMP) levels. It is a currently prevailing concept that NO inhibits platelet activation. This concept, however, does not fully explain why platelet agonists stimulate NO production. Here we show that a major platelet NO synthase (NOS) isoform, NOS3, plays a stimulatory role in platelet secretion and aggregation induced by low doses of platelet agonists. Furthermore, we show that NOS3 promotes thrombosis in vivo. The stimulatory role of NOS is mediated by soluble guanylyl cyclase and results from a cGMP-dependent stimulation of platelet granule secretion. These findings delineate a novel signaling pathway in which agonists sequentially activate NOS3, elevate cGMP, and induce platelet secretion and aggregation. Our data also suggest that NO plays a biphasic role in platelet activation, a stimulatory role at low NO concentrations and an inhibitory role at high NO concentrations.

Ligand discrimination in soluble guanylate cyclase and the H-NOX family of heme sensor proteins

Soluble guanylate cyclases (s GC s) are eukaryotic heme sensor proteins that selectively bind NO in the presence of a large excess of the similar diatomic gas, O(2); this discrimination is essential for NO signaling. Recent discoveries place sGC in the H-NOX (heme nitric oxide and/or oxygen binding domain) family that includes bacterial proteins. The defining characteristic of this family is that some H-NOX proteins tightly bind O(2) whereas others, such as sGC, show no measurable affinity for O(2). A molecular basis for this ligand selectivity has now been established. A distal pocket tyrosine is requisite for O(2) binding and is used to kinetically distinguish between NO and O(2). In the absence of this tyrosine, the O(2) dissociation rate is so fast that the O(2) complex is never formed, whereas the rate of NO dissociation remains essentially unchanged, thus providing discrimination.

Stimulation of soluble guanylate cyclase slows progression in anti-thy1-induced chronic glomerulosclerosis

BACKGROUND

A critical role of soluble guanylate cyclase and nitric oxide-dependent cyclic 3',5'-guanosine monophosphate (cGMP) production for glomerular matrix expansion has recently been documented in a rat model of acute anti-thy1 glomerulonephritis. The present study analyzes the renal activity of the nitric oxide-cGMP signaling cascade in and the effect of the specific soluble guanylate cyclase stimulator Bay 41-2272 on a progressive model of anti-thy1-induced chronic glomerulosclerosis.

METHODS

Anti-thy1 glomerulosclerosis was induced by injection of anti-thy1 antibody into uninephrectomized rats. One week after disease induction, animals were randomly assigned to chronic glomerulosclerosis, chronic glomerulosclerosis plus Bay 41-2272 (10 mg/kg body weight/day) or chronic glomerulosclerosis plus hydralazine (15 mg/kg body weight/day). In week 16, analysis included effects on systolic blood pressure, proteinuria, kidney function, glomerular and tubulointerstitial matrix protein accumulation, expression of transforming growth factor-beta1 (TGF-beta1), fibronectin and plasminogen activator inhibitor type 1 (PAI-1), macrophage infiltration, cell proliferation, basal and nitric oxide-stimulated cGMP production as well as tubulointerstitial mRNA expression of alpha 1 and beta 1 soluble guanylate cyclase.

RESULTS

The moderately elevated systolic blood pressure seen in the chronic glomerulosclerosis group was comparably decreased by both treatments. Compared to normal controls, soluble guanylate cyclase mRNA expression and nitric oxide-stimulated cGMP production were up-regulated in the tubulointerstitium of the untreated chronic glomerulosclerosis animals, while its activity was decreased in glomeruli. Bay 41-2272 treatment enhanced glomerular and tubulointerstitial nitric oxide-cGMP signaling significantly. This went along with markedly reduced glomerular and tubulointerstitial macrophage infiltration, number of proliferating cells, matrix expression and accumulation, as well as improved kidney function. In contrast, hydralazine therapy did not significantly affect renal nitric oxide-cGMP signaling, macrophage number, cell proliferation, matrix protein expression and accumulation.

CONCLUSION

Glomerular and tubulointerstitial soluble guanylate cyclase activity are discordantly altered in anti-thy1-induced chronic glomerulosclerosis. Stimulation of soluble guanylate cyclase signaling by Bay 41-2272 limits the progressive course of this model toward tubulointerstitial fibrosis and impaired renal function at least in part in a blood pressure-independent manner. The results suggest that soluble guanylate cyclase activation counteracts fibrosis and progression in chronic renal disease.

Soluble guanylyl cyclase expression is reduced in allergic asthma

Soluble guanylyl cyclase (sGC) is an enzyme highly expressed in the lung that generates cGMP contributing to airway smooth muscle relaxation. To determine whether the bronchoconstriction observed in asthma is accompanied by changes in sGC expression, we used a well-established murine model of allergic asthma. Histological and biochemical analyses confirmed the presence of inflammation in the lungs of mice sensitized and challenged with ovalbumin (OVA). Moreover, mice sensitized and challenged with OVA exhibited airway hyperreactivity to methacholine inhalation. Steady-state mRNA levels for all sGC subunits (alpha1, alpha2, and beta1) were reduced in the lungs of mice with allergic asthma by 60-80%, as estimated by real-time PCR. These changes in mRNA were paralleled by changes at the protein level: alpha1, alpha2, and beta1 expression was reduced by 50-80% as determined by Western blotting. Reduced alpha1 and beta1 expression in bronchial smooth muscle cells was demonstrated by immunohistochemistry. To study if sGC inhibition mimics the airway hyperreactivity seen in asthma, we treated naïve mice with a selective sGC inhibitor. Indeed, in mice receiving ODQ the methacholine dose response was shifted to the left. We conclude that sGC expression is reduced in experimental asthma contributing to the observed airway hyperreactivity.

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.

Expression of soluble guanylyl cyclase in rat cerebral cortex during postnatal development

Soluble guanylyl cyclase (sGC), the principle "receptor" for nitric oxide (NO), catalyzes the formation of cyclic guanosine monophosphate (cGMP), an intracellular second messenger. Studies in invertebrates have shown that the NO/cGMP pathway is involved in several aspects of neural development, including neuronal migration, dendritic and axonal outgrowth, and synaptogenesis. In vitro studies suggest a developmental role also in mammals. To investigate whether the NO/cGMP pathway might mediate these processes in vivo, we performed immunohistochemistry for sGC on sections from postnatal rat cerebral cortex. Early in postnatal development, migrating neurons in the cortical plate were immunonegative, whereas neurons deeper in the cortex that had completed migration were immunopositive. At the subcellular level, sGC preferentially stained dendrites rather than axons, but, at postnatal day 1 (PND1), sGC was found in a large fraction of axonal growth cones, especially those oriented toward the pial surface. At PND10-20 (the period of maximal synaptogenesis), sGC immunostaining was located mainly in dendritic shafts and was only occasionally associated with spines or axon terminals. These results support a role for the NO/cGMP pathway in dendritic development but argue against a major role in neuronal migration and synaptogenesis.

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.

Interaction between the 90-kDa heat shock protein and soluble guanylyl cyclase: physiological significance and mapping of the domains mediating binding

The 90-kDa heat shock protein (hsp90) regulates the stability and function of many client proteins, including members of the NO-cGMP signaling pathway. Soluble guanylyl cyclase (sGC), an NO receptor, was recently reported to be an hsp90-interacting partner. In the present study, we show that hsp90 binds to both subunits of the most common sGC form (alpha(1)beta(1)) when these are expressed individually but only interacts with beta(1) in the heterodimeric form of the enzyme. Characterization of the region of hsp90 required to bind each subunit in immunoprecipitation experiments revealed that residues 310 to 456 of hsp90 interact with the sGC subunits. The region of beta(1) responsible for binding to hsp90beta was mapped using in vitro binding assays and immunoprecipitation experiments and was found to lie in the regulatory domain. The physiological importance of the hsp90/sGC interaction was investigated by treating rat smooth muscle cells with the hsp90 inhibitors radicicol and geldanamycin (GA) and determining both sGC activity and protein levels. Long-term (24 or 48 h) inhibition of hsp90 resulted in a strong decrease of both alpha(1) and beta(1) protein levels and sGC activity. Moreover, incubation of smooth muscle cells with the proteasome inhibitor N-benzoyloxycarbonyl (Z)-Leu-Leu-leucinal (MG132) blocked the GA-induced down-regulation of sGC. We conclude that the N-terminal region of the beta(1) subunit mediates binding of the heterodimeric form of sGC to hsp90 and that this interaction involves the M domain of hsp90. Hsp90 binding to sGC regulates the pool of active enzymes by affecting the protein levels of the two subunits.

Amphipathic α-Helix Mediates the Heterodimerization of Soluble Guanylyl Cyclase

Soluble guanylyl cyclase (soluble GC) is an enzyme consisting of alpha and beta subunits and catalyzes the conversion of GTP to cGMP. The formation of the heterodimer is essential for the activity of soluble GC. Each subunit of soluble GC has been shown to comprize three functionally different parts: a C-terminal catalytic domain, a central dimerization domain, and an N-terminal regulatory domain. The central dimerization domain of the beta(1) subunit, which contains an N-terminal binding site (NBS) and a C-terminal binding site (CBS), has been postulated to be responsible for the formation of alpha/ beta heterodimer. In this study, we analyzed heterodimerization by the pull-down assay using the affinity between a histidine tag and Ni(2+) Sepharose after co-expression of various N- and C-terminally truncated FLAG-tagged mutants of the alpha(1) subunit and the histidine-tagged wild type of the beta(1) subunit in the vaculovirus/Sf9 system, and demonstrated that the CBS-like sequence of the alpha(1) subunit is critical for the formation of the heterodimer with the beta(1) subunit and the NBS-like sequence of the alpha(1) subunit is essential for the formation of the enzymatically active heterodimer, although this particular sequence was not involved in heterodimerization. The analysis of the secondary structure of the alpha(1) subunit predicted the existence of an amphipathic alpha-helix in residues 431-464. Experiments with site-directed alpha(1) subunit mutant proteins demonstrated that the amphipathicity of the alpha-helix is important for the formation of the heterodimer, and Leu(463) in the alpha-helix region plays a critical role in the formation of a properly arranged active center in the dimer.

Localization and characterization of cGMP-immunoreactive structures in rat brain slices after NO-dependent and NO-independent stimulation of soluble guanylyl cyclase

Possible differences in the localization of the cGMP response were investigated in rat brain coronal slices after in vitro incubation and NO-dependent or NO-independent stimulation of soluble guanylyl cyclase (sGC). Dose-dependent stimulation of cGMP synthesis by the NO donors, sodium nitroprusside, S-nitrosoglutathione, 3-morpholinosydnonimine and diethylamino NONOate was studied in the somatoparietal cortex, the hippocampus and the thalamus. cGMP accumulation was evaluated using a radioimmunoassay and by measuring cGMP-immunofluorescence using image analysis. All four NO donors induced similar cGMP staining patterns in the somatoparietal cortex, the hippocampus and the thalamus. NO-mediated cGMP synthesis in the cortical areas colocalized predominantly with the acetylcholine transporter and occasionally with parvalbumin (GABAergic cells) or the neuronal glutamate transporter. Incubation of the slices in the combined presence of a NO donor and the NO-independent activators YC-1 or BAY 41-2272 strongly potentiated cGMP synthesis and induced abundant cGMP-immunoreactivity in cortical GABAergic and glutamatergic cells. These findings indicate that the mechanism of NO release from the NO donors used does not determine the location of the cGMP response. The results suggest that YC-1 and BAY 41-2272 trigger a NO-sensing mechanism in cells in which the sGC is otherwise not sensitive to NO.