Insight into the rescue of oxidized soluble guanylate cyclase by the activator cinaciguat

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.

NosP Detection of Heme Modulates Burkholderia thailandensis Biofilm Formation

Aggregated bacteria embedded within self-secreted extracellular polymeric substances, or biofilms, are resistant to antibiotics and cause chronic infections. As such, they are a significant public health threat. Heme is an abundant iron source for pathogenic bacteria during infection; many bacteria have systems to detect heme assimilated from host cells, which is correlated with the transition between acute and chronic infection states. Here, we investigate the heme-sensing function of a newly discovered multifactorial sensory hemoprotein called NosP and its role in biofilm regulation in the soil-dwelling bacterium Burkholderia thailandensis, the close surrogate of Bio-Safety-Level-3 pathogen Burkholderia pseudomallei. The NosP family protein has previously been shown to exhibit both nitric oxide (NO)- and heme-sensing functions and to regulate biofilms through NosP-associated histidine kinases and two-component systems. Our in vitro studies suggest that BtNosP exhibits heme-binding kinetics and thermodynamics consistent with a labile heme-responsive protein and that the holo-form of BtNosP acts as an inhibitor of its associated histidine kinase BtNahK. Furthermore, our in vivo studies suggest that increasing the concentration of extracellular heme decreases B. thailandensis biofilm formation, and deletion of nosP and nahK abolishes this phenotype, consistent with a model that BtNosP detects heme and exerts an inhibitory effect on BtNahK to decrease the biofilm.

Role of the Coiled-Coil Domain in Allosteric Activity Regulation in Soluble Guanylate Cyclase

Soluble guanylate cyclase (sGC) is the primary nitric oxide (NO) receptor in higher eukaryotes, including humans. NO-dependent signaling via sGC is associated with important physiological effects in the vascular, pulmonary, and neurological systems, and sGC itself is an established drug target for the treatment of pulmonary hypertension due to its central role in vasodilation. Despite isolation in the late 1970s, high-resolution structural information on full-length sGC remained elusive until recent cryo-electron microscopy structures were determined of the protein in both the basal unactivated state and the NO-activated state. These structures revealed large-scale conformational changes upon activation that appear to be centered on rearrangements within the coiled-coil (CC) domains in the enzyme. Here, a structure-guided approach was used to engineer constitutively unactivated and constitutively activated sGC variants through mutagenesis of the CC domains. These results demonstrate that the activation-induced conformational change in the CC domains is necessary and sufficient for determining the level of sGC activity.

BAY58-2667 Activates Different Soluble Guanylyl Cyclase Species by Distinct Mechanisms that Indicate Its Principal Target in Cells is the Heme-Free Soluble Guanylyl Cyclase-Heat Shock Protein 90 Complex

Nitric oxide (NO)-unresponsive forms of soluble guanylyl cyclase (sGC) exist naturally and in disease can disable NO-sGC-cGMP signaling. Agonists like BAY58-2667 (BAY58) target these sGC forms, but their mechanisms of action in living cells are unclear. We studied rat lung fibroblast-6 cells and human airway smooth muscle cells that naturally express sGC and HEK293 cells that we transfected to express sGC and variants. Cells were cultured to build up different forms of sGC, and we used fluorescence and FRET-based measures to monitor BAY58-driven cGMP production and any protein partner exchange or heme loss events that may occur for each sGC species. We found that: (i) BAY58 activated cGMP production by the apo-sGCβ-Hsp90 species after a 5-8 minute delay that was associated with apo-sGCβ exchanging its Hsp90 partner with an sGCα subunit. (ii) In cells containing an artificially constructed heme-free sGC heterodimer, BAY58 initiated an immediate and three times faster cGMP production. However, this behavior was not observed in cells expressing native sGC under any condition. (iii) BAY58 activated cGMP production by ferric heme sGC only after a 30-minute delay, coincident with it initiating a delayed, slow ferric heme loss from sGCβ We conclude that the kinetics favor BAY58 activation of the apo-sGCβ-Hsp90 species over the ferric heme sGC species in living cells. The protein partner exchange events driven by BAY58 account for the initial delay in cGMP production and also limit the speed of subsequent cGMP production in the cells. Our findings clarify how agonists like BAY58 may activate sGC in health and disease. SIGNIFICANCE STATEMENT: A class of agonists can activate cyclic guanosine monophosphate (cGMP) synthesis by forms of soluble guanylyl cyclase (sGC) that do not respond to NO and accumulate in disease, but the mechanisms of action are unclear. This study clarifies what forms of sGC exist in living cells, which of these can be activated by the agonists, and the mechanisms and kinetics by which each form is activated. This information may help to hasten deployment of these agonists for pharmaceutical intervention and clinical therapy.

Prenatal hypothyroidism diminished exogenous NO-mediated diastolic effects in fetal rat thoracic aorta smooth muscle via increased oxidative stress

Maternal hypothyroidism is an important problem of modern healthcare and is reported to increase the risk of cardiovascular diseases in the offspring later in life. However, it is unclear whether hypothyroidism during pregnancy causes vascular damage in the fetal period. We established the prenatal hypothyroidism rat model and collected the fetuses at the 21th day of gestation (GD21). Thyroid hormone concentrations in maternal and offspring blood serum were assessed by enzyme-linked immunosorbent assay (ELISA). The thoracic aortas of the fetuses were isolated for microvessel functional testing and histochemical stainings. qPCR and Western blot were performed to access mRNA and protein expression. We found that the concentrations of thyroid hormones in the serum of pregnant rats and fetuses were significantly suppressed at GD21. The responses of the fetal thoracic aortas to SNP were significantly attenuated in the PTU group. However, no statistical difference was found between the two groups when treated with either inhibitor (ODQ) or activator (BAY58-2667) of sGC. The production of O₂^-• in the arterial wall was significantly increased in hypothyroid fetuses. Moreover, the level of NADPH oxidase (NOX) was increased, while superoxide dismutase 2 (SOD2) was down-regulated in the PTU group, ultimately contributing to the increased production of superoxide. Additionally, decreased SNP-mediated vasodilation found in fetal vessels was improved by either NOX inhibitor (Apocynin) or SOD mimic (Tempol). These results indicate that increased oxidative stress is probably the cause of the diminished diastolic effect of exogenous NO in the thoracic artery of prenatal hypothyroidism exposed fetuses.

Factors influencing the soluble guanylate cyclase heme redox state in blood vessels

Soluble guanylate cyclase (sGC) plays an important role in maintaining vascular homeostasis, as an acceptor for the biological messenger nitric oxide (NO). However, only reduced sGC (with a ferrous heme) can be activated by NO; oxidized (ferric heme) and apo (absent heme) sGC cannot. In addition, the proportions of reduced, oxidized, and apo sGC change under pathological conditions. Although diseased blood vessels often show decreased NO bioavailability in the vascular wall, a shift of sGC heme redox balance in favor of the oxidized/apo forms can also occur. Therefore, sGC is of growing interest as a drug target for various cardiovascular diseases. Notably, the balance between NO-sensitive reduced sGC and NO-insensitive oxidized/apo sGC in the body is regulated in a reversible manner by various biological molecules and proteins. Many studies have attempted to identify endogenous factors and determinants that influence this redox state. For example, various reactive nitrogen and oxygen species are capable of inducing the oxidation of sGC heme. Conversely, a heme reductase and some antioxidants reduce the ferric heme in sGC to the ferrous state. This review summarizes the factors and mechanisms identified by these studies that operate to regulate the sGC heme redox state.

Pharmacological developments in antihypertensive treatment through nitric oxide-cGMP modulation

Treatment of hypertension until now has been directed at inhibition of vasoconstriction, of cardiac contractility and of blood volume regulation. Despite the arsenal of drugs available for this purpose, the control of target blood pressure is still a difficult goal to reach in outpatients. The nitric oxide-cyclic guanosine monophosphate signaling is one of the most important mediators of vasodilation. It might therefore be a potential and most welcome drug target for optimization of the treatment of hypertension. In this chapter we review the problems that can occur in this signaling system, the attempts that have been made to correct these problems, and those that are still under investigation. Recently developed, clinically safe medicines that are currently approved for other applications, such as myocardial infarction, await to be tested for essential systemic hypertension. We conclude that despite many years of research without translation, stimulation of nitric oxide-cyclic guanosine monophosphate is still a viable strategy in the prevention of the health risk posed by chronic hypertension.

Smooth muscle cell CYB5R3 preserves cardiac and vascular function under chronic hypoxic stress

Chronic hypoxia is a major driver of cardiovascular complications, including heart failure. The nitric oxide (NO) - soluble guanylyl cyclase (sGC) - cyclic guanosine monophosphate (cGMP) pathway is integral to vascular tone maintenance. Specifically, NO binds its receptor sGC within vascular smooth muscle cells (SMC) in its reduced heme (Fe2+) form to increase intracellular cGMP production, activate protein kinase G (PKG) signaling, and induce vessel relaxation. Under chronic hypoxia, oxidative stress drives oxidation of sGC heme (Fe2+→Fe3+), rendering it NO-insensitive. We previously showed that cytochrome b5 reductase 3 (CYB5R3) in SMC is a sGC reductase important for maintaining NO-dependent vasodilation and conferring resilience to systemic hypertension and sickle cell disease-associated pulmonary hypertension. To test whether CYB5R3 may be protective in the context of chronic hypoxia, we subjected SMC-specific CYB5R3 knockout mice (SMC CYB5R3 KO) to 3 weeks hypoxia and assessed vascular and cardiac function using echocardiography, pressure volume loops and wire myography. Hypoxic stress caused 1) biventricular hypertrophy in both WT and SMC CYB5R3 KO, but to a larger degree in KO mice, 2) blunted vasodilation to NO-dependent activation of sGC in coronary and pulmonary arteries of KO mice, and 3) decreased, albeit still normal, cardiac function in KO mice. Overall, these data indicate that SMC CYB5R3 deficiency potentiates bilateral ventricular hypertrophy and blunts NO-dependent vasodilation under chronic hypoxia conditions. This implicates that SMC CYB5R3 KO mice post 3-week hypoxia have early stages of cardiac remodeling and functional changes that could foretell significantly impaired cardiac function with longer exposure to hypoxia.

Activation mechanism of human soluble guanylate cyclase by stimulators and activators

Soluble guanylate cyclase (sGC) is the receptor for nitric oxide (NO) in human. It is an important validated drug target for cardiovascular diseases. sGC can be pharmacologically activated by stimulators and activators. However, the detailed structural mechanisms, through which sGC is recognized and positively modulated by these drugs at high spacial resolution, are poorly understood. Here, we present cryo-electron microscopy structures of human sGC in complex with NO and sGC stimulators, YC-1 and riociguat, and also in complex with the activator cinaciguat. These structures uncover the molecular details of how stimulators interact with residues from both β H-NOX and CC domains, to stabilize sGC in the extended active conformation. In contrast, cinaciguat occupies the haem pocket in the β H-NOX domain and sGC shows both inactive and active conformations. These structures suggest a converged mechanism of sGC activation by pharmacological compounds.

Soluble guanylate cyclase (sGC) is a validated drug target for cardiovascular diseases. Here, the authors report structures of human sGC in complex with NO and sGC stimulators or activator, providing insight into the mechanism of sGC activation by pharmacological compounds.

Maturation, inactivation, and recovery mechanisms of soluble guanylyl cyclase

Soluble guanylate cyclase (sGC) is a heme-containing heterodimeric enzyme that generates many molecules of cGMP in response to its ligand nitric oxide (NO); sGC thereby acts as an amplifier in NO-driven biological signaling cascades. Because sGC helps regulate the cardiovascular, neuronal, and gastrointestinal systems through its cGMP production, boosting sGC activity and preventing or reversing sGC inactivation are important therapeutic and pharmacologic goals. Work over the last two decades is uncovering the processes by which sGC matures to become functional, how sGC is inactivated, and how sGC is rescued from damage. A diverse group of small molecules and proteins have been implicated in these processes, including NO itself, reactive oxygen species, cellular heme, cell chaperone Hsp90, and various redox enzymes as well as pharmacologic sGC agonists. This review highlights their participation and provides an update on the processes that enable sGC maturation, drive its inactivation, or assist in its recovery in various settings within the cell, in hopes of reaching a better understanding of how sGC function is regulated in health and disease.

Multi-Compartment, Early Disruption of cGMP and cAMP Signalling in Cardiac Myocytes from the mdx Model of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is the most frequent and severe form of muscular dystrophy. The disease presents with progressive body-wide muscle deterioration and, with recent advances in respiratory care, cardiac involvement is an important cause of morbidity and mortality. DMD is caused by mutations in the dystrophin gene resulting in the absence of dystrophin and, consequently, disturbance of other proteins that form the dystrophin-associated protein complex (DAPC), including neuronal nitric oxide synthase (nNOS). The molecular mechanisms that link the absence of dystrophin with the alteration of cardiac function remain poorly understood but disruption of NO-cGMP signalling, mishandling of calcium and mitochondrial disturbances have been hypothesized to play a role. cGMP and cAMP are second messengers that are key in the regulation of cardiac myocyte function and disruption of cyclic nucleotide signalling leads to cardiomyopathy. cGMP and cAMP signals are compartmentalised and local regulation relies on the activity of phosphodiesterases (PDEs). Here, using genetically encoded FRET reporters targeted to distinct subcellular compartments of neonatal cardiac myocytes from the DMD mouse model mdx, we investigate whether lack of dystrophin disrupts local cyclic nucleotide signalling, thus potentially providing an early trigger for the development of cardiomyopathy. Our data show a significant alteration of both basal and stimulated cyclic nucleotide levels in all compartments investigated, as well as a complex reorganization of local PDE activities.

Heme inhibits the activity of a c-di-GMP phosphodiesterase in Vibrio cholerae

Heme, a complex of iron and protoporphyrin IX, plays an essential role in numerous biological processes including oxygen transport, oxygen storage, and electron transfer. The role of heme as a prosthetic group in bacterial hemoprotein gas sensors, which utilize heme as a cofactor for the binding of diatomic gas molecules, has been well studied. Less well known is the role of protein sensors of heme. In this report, we characterize the heme binding properties of a phosphodiesterase, CdpA, from Vibrio cholerae. We demonstrate that the N-terminal domain of CdpA is a NosP domain capable of heme binding, which consequently inhibits the c-di-GMP hydrolysis activity of the C-terminal phosphodiesterase domain. Further evidence for CdpA as a heme responsive sensor is supported by a relatively fast rate of heme dissociation. This study provides insight into an emerging class of heme-responsive sensor proteins.

GAPDH delivers heme to soluble guanylyl cyclase

Soluble guanylyl cyclase (sGC) is a key component of NO-cGMP signaling in mammals. Although heme must bind in the sGC β1 subunit (sGCβ) for sGC to function, how heme is delivered to sGCβ remains unknown. Given that GAPDH displays properties of a heme chaperone for inducible NO synthase, here we investigated whether heme delivery to apo-sGCβ involves GAPDH. We utilized an sGCβ reporter construct, tetra-Cys sGCβ, whose heme insertion can be followed by fluorescence quenching in live cells, assessed how lowering cell GAPDH expression impacts heme delivery, and examined whether expressing WT GAPDH or a GAPDH variant defective in heme binding recovers heme delivery. We also studied interaction between GAPDH and sGCβ in cells and their complex formation and potential heme transfer using purified proteins. We found that heme delivery to apo-sGCβ correlates with cellular GAPDH expression levels and depends on the ability of GAPDH to bind intracellular heme, that apo-sGCβ associates with GAPDH in cells and dissociates when heme binds sGCβ, and that the purified GAPDH-heme complex binds to apo-sGCβ and transfers its heme to sGCβ. On the basis of these results, we propose a model where GAPDH obtains mitochondrial heme and then forms a complex with apo-sGCβ to accomplish heme delivery to sGCβ. Our findings illuminate a critical step in sGC maturation and uncover an additional mechanism that regulates its activity in health and disease.

Nitric Oxide-Independent Soluble Guanylate Cyclase Activation Improves Vascular Function and Cardiac Remodeling in Sickle Cell Disease

Sickle cell disease (SCD) is associated with intravascular hemolysis and oxidative inhibition of nitric oxide (NO) signaling. BAY 54-6544 is a small-molecule activator of oxidized soluble guanylate cyclase (sGC), which, unlike endogenous NO and the sGC stimulator, BAY 41-8543, preferentially binds and activates heme-free, NO-insensitive sGC to restore enzymatic cGMP production. We tested orally delivered sGC activator, BAY 54-6544 (17 mg/kg/d), sGC stimulator, BAY 41-8543, sildenafil, and placebo for 4-12 weeks in the Berkeley transgenic mouse model of SCD (BERK-SCD) and their hemizygous (Hemi) littermate controls (BERK-Hemi). Right ventricular (RV) maximum systolic pressure (RVmaxSP) was measured using micro right-heart catheterization. RV hypertrophy (RVH) was determined using Fulton's index and RV corrected weight (ratio of RV to tibia). Pulmonary artery vasoreactivity was tested for endothelium-dependent and -independent vessel relaxation. Right-heart catheterization revealed higher RVmaxSP and RVH in BERK-SCD versus BERK-Hemi, which worsened with age. Treatment with the sGC activator more effectively lowered RVmaxSP and RVH, with 90-day treatment delivering superior results, when compared with other treatments and placebo groups. In myography experiments, acetylcholine-induced (endothelium-dependent) and sodium-nitroprusside-induced (endothelium-independent NO donor) relaxation of the pulmonary artery harvested from placebo-treated BERK-SCD was impaired relative to BERK-Hemi but improved after therapy with sGC activator. By contrast, no significant effect for sGC stimulator or sildenafil was observed in BERK-SCD. These findings suggest that sGC is oxidized in the pulmonary arteries of transgenic SCD mice, leading to blunted responses to NO, and that the sGC activator, BAY 54-6544, may represent a novel therapy for SCD-associated pulmonary arterial hypertension and cardiac remodeling.

A Randomized, Controlled Phase I/II Study to Evaluate the Safety and Efficacy of MGV354 for Ocular Hypertension or Glaucoma

PURPOSE

To assess the clinical safety, tolerability, and efficacy of topically administered MGV354, a soluble guanylate cyclase (sGC) activator, in patients with ocular hypertension (OH) or glaucoma.

DESIGN

Double-masked, randomized, and vehicle-controlled study.

METHODS

Parts 1 and 2 evaluated safety and tolerability to identify the maximum tolerated dose (MTD) of once-daily MGV354 in 32 healthy volunteers (Part 1) and 16 patients with OH or glaucoma (Part 2) at a single clinical site. Part 3 was a multisite trial that evaluated intraocular pressure (IOP)-lowering efficacy of the MTD administered nightly for 1 week in 50 patients with minimum IOP of 24 mm Hg at 8 AM, with a main outcome measure of mean diurnal IOP at day 8 compared to baseline (ClinicalTrials.govNCT02743780).

RESULTS

There was no difference in favor of MGV354 for IOP lowering; change from baseline to day 8 in mean diurnal IOP was -0.6 mm Hg for MGV354-treated patients and -1.1 mm Hg for vehicle-treated patients in Part 3, with a confidence interval of -0.7 to 1.7. The most common adverse events reported after MGV354 administration were conjunctival and ocular hyperemia.

CONCLUSIONS

Overall, MGV354 0.1% demonstrated no statistically significant effect compared to vehicle in lowering IOP based on the study's main outcome measure. MGV354 produced ocular hyperemia consistent with its pharmacology.

Physiological activation and deactivation of soluble guanylate cyclase

Soluble guanylate cyclase (sGC) is responsible for transducing the gaseous signaling molecule nitric oxide (NO) into the ubiquitous secondary signaling messenger cyclic guanosine monophosphate in eukaryotic organisms. sGC is exquisitely tuned to respond to low levels of NO, allowing cells to respond to non-toxic levels of NO. In this review, the structure of sGC is discussed in the context of sGC activation and deactivation. The sequence of events in the activation pathway are described into a comprehensive model of in vivo sGC activation as elucidated both from studies with purified enzyme and those done in cells. This model is then used to discuss the deactivation of sGC, as well as the molecular mechanisms of pathophysiological deactivation.

Irreversible Activation and Stabilization of Soluble Guanylate Cyclase by the Protoporphyrin IX Mimetic Cinaciguat

Belonging to the class of so-called soluble guanylate cyclase (sGC) activators, cinaciguat and BAY 60-2770 are interesting therapeutic tools for the treatment of various cardiovascular pathologies. The drugs are supposed to preferentially stimulate oxidized or heme-depleted, but not native sGC. Since this concept has been challenged by studies demonstrating complete relaxation of nondiseased vessels, this study was designed to reinvestigate the mode of action in greater detail. To this purpose, the effect of cinaciguat was studied on vessel tone of porcine coronary arteries and rat thoracic aortas. Organ bath studies showed that the compound caused time- and concentration-dependent relaxation of precontracted vessels with a maximal effect observed at 90 minutes. The dilatory response was not affected by extensive washout of the drug. Cinaciguat-induced vasodilation was associated with a time- and concentration-dependent increase of cGMP levels. Experiments with purified sGC in the presence of Tween 20 showed that cinaciguat activates the heme-free enzyme in a concentration-dependent manner with an EC50 value of ∼0.2 µM and maximal cGMP formation at 10 µM. By contrast, the effect of cinaciguat on 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one-oxidized (ferric) sGC was moderate, reaching ∼10%-15% of maximal activity. Dilution experiments of cinaciguat/Tween 20-preincubated sGC revealed the irreversible character of the drug. Assuming a sensitive balance between heme-free, ferric, and nitric oxide-sensitive ferrous sGC in cells and tissues, we propose that cinaciguat by virtue of its irreversible mode of action is capable of shifting this equilibrium toward the heme-free apo-sGC species.

The molecular mechanism of heme loss from oxidized soluble guanylate cyclase induced by conformational change

Heme oxidation and loss of soluble guanylate cyclase (sGC) is thought to be an important contributor to the development of cardiovascular diseases. Nevertheless, it remains unknown why the heme loses readily in oxidized sGC. In the current study, the conformational change of sGC upon heme oxidation by ODQ was studied based on the fluorescence resonance energy transfer (FRET) between the heme and a fluorophore fluorescein arsenical helix binder (FlAsH-EDT2) labeled at different domains of sGC β1. This study provides an opportunity to monitor the domain movement of sGC relative to the heme. The results indicated that heme oxidation by ODQ in truncated sCC induced the heme-associated αF helix moving away from the heme, the Per/Arnt/Sim domain (PAS) domain moving closer to the heme, but led the helical domain going further from the heme. We proposed that the synergistic effect of these conformational changes of the discrete region upon heme oxidation forces the heme pocket open, and subsequent heme loss readily. Furthermore, the kinetic studies suggested that the heme oxidation was a fast process and the conformational change was a relatively slow process. The kinetics of heme loss from oxidized sGC was monitored by a new method based on the heme group de-quenching the fluorescence of FlAsH-EDT2.

Chronic Activation of Heme Free Guanylate Cyclase Leads to Renal Protection in Dahl Salt-Sensitive Rats

The nitric oxide (NO)/soluble guanylate cyclase (sGC)/cyclic guanosine monophasphate (cGMP)-signalling pathway is impaired under oxidative stress conditions due to oxidation and subsequent loss of the prosthetic sGC heme group as observed in particular in chronic renal failure. Thus, the pool of heme free sGC is increased under pathological conditions. sGC activators such as cinaciguat selectively activate the heme free form of sGC and target the disease associated enzyme. In this study, a therapeutic effect of long-term activation of heme free sGC by the sGC activator cinaciguat was investigated in an experimental model of salt-sensitive hypertension, a condition that is associated with increased oxidative stress, heme loss from sGC and development of chronic renal failure. For that purpose Dahl/ss rats, which develop severe hypertension upon high salt intake, were fed a high salt diet (8% NaCl) containing either placebo or cinaciguat for 21 weeks. Cinaciguat markedly improved survival and ameliorated the salt-induced increase in blood pressure upon treatment with cinaciguat compared to placebo. Renal function was significantly improved in the cinaciguat group compared to the placebo group as indicated by a significantly improved glomerular filtration rate and reduced urinary protein excretion. This was due to anti-fibrotic and anti-inflammatory effects of the cinaciguat treatment. Taken together, this is the first study showing that long-term activation of heme free sGC leads to renal protection in an experimental model of hypertension and chronic kidney disease. These results underline the promising potential of cinaciguat to treat renal diseases by targeting the disease associated heme free form of sGC.

Direct sGC activation bypasses NO scavenging reactions of intravascular free oxy-hemoglobin and limits vasoconstriction

AIMS

Hemoglobin-based oxygen carriers (HBOC) provide a potential alternative to red blood cell (RBC) transfusion. Their clinical application has been limited by adverse effects, in large part thought to be mediated by the intravascular scavenging of the vasodilator nitric oxide (NO) by cell-free plasma oxy-hemoglobin. Free hemoglobin may also cause endothelial dysfunction and platelet activation in hemolytic diseases and after transfusion of aged stored RBCs. The new soluble guanylate cyclase (sGC) stimulator Bay 41-8543 and sGC activator Bay 60-2770 directly modulate sGC, independent of NO bioavailability, providing a potential therapeutic mechanism to bypass hemoglobin-mediated NO inactivation.

RESULTS

Infusions of human hemoglobin solutions and the HBOC Oxyglobin into rats produced a severe hypertensive response, even at low plasma heme concentrations approaching 10 μM. These reactions were only observed for ferrous oxy-hemoglobin and not analogs that do not rapidly scavenge NO. Infusions of L-NG-Nitroarginine methyl ester (L-NAME), a competitive NO synthase inhibitor, after hemoglobin infusion did not produce additive vasoconstriction, suggesting that vasoconstriction is related to scavenging of vascular NO. Open-chest hemodynamic studies confirmed that hypertension occurred secondary to direct effects on increasing vascular resistance, with limited negative cardiac inotropic effects. Intravascular hemoglobin reduced the vasodilatory potency of sodium nitroprusside (SNP) and sildenafil, but had no effect on vasodilatation by direct NO-independent activation of sGC by BAY 41-8543 and BAY 60-2770.

INNOVATION AND CONCLUSION

These data suggest that both sGC stimulators and sGC activators could be used to restore cyclic guanosine monophosphate-dependent vasodilation in conditions where cell-free plasma hemoglobin is sufficient to inhibit endogenous NO signaling.

Crystal structures of the catalytic domain of human soluble guanylate cyclase

Soluble guanylate cyclase (sGC) catalyses the synthesis of cyclic GMP in response to nitric oxide. The enzyme is a heterodimer of homologous α and β subunits, each of which is composed of multiple domains. We present here crystal structures of a heterodimer of the catalytic domains of the α and β subunits, as well as an inactive homodimer of β subunits. This first structure of a metazoan, heteromeric cyclase provides several observations. First, the structures resemble known structures of adenylate cyclases and other guanylate cyclases in overall fold and in the arrangement of conserved active-site residues, which are contributed by both subunits at the interface. Second, the subunit interaction surface is promiscuous, allowing both homodimeric and heteromeric association; the preference of the full-length enzyme for heterodimer formation must derive from the combined contribution of other interaction interfaces. Third, the heterodimeric structure is in an inactive conformation, but can be superposed onto an active conformation of adenylate cyclase by a structural transition involving a 26° rigid-body rotation of the α subunit. In the modelled active conformation, most active site residues in the subunit interface are precisely aligned with those of adenylate cyclase. Finally, the modelled active conformation also reveals a cavity related to the active site by pseudo-symmetry. The pseudosymmetric site lacks key active site residues, but may bind allosteric regulators in a manner analogous to the binding of forskolin to adenylate cyclase. This indicates the possibility of developing a new class of small-molecule modulators of guanylate cyclase activity targeting the catalytic domain.

Activation of haem-oxidized soluble guanylyl cyclase with BAY 60-2770 in human platelets lead to overstimulation of the cyclic GMP signaling pathway

BACKGROUND AND AIMS

Nitric oxide-independent soluble guanylyl cyclase (sGC) activators reactivate the haem-oxidized enzyme in vascular diseases. This study was undertaken to investigate the anti-platelet mechanisms of the haem-independent sGC activator BAY 60-2770 in human washed platelets. The hypothesis that sGC oxidation potentiates the anti-platelet activities of BAY 60-2770 has been tested.

METHODS

Human washed platelet aggregation and adhesion assays, as well as flow cytometry for α(IIb)β(3) integrin activation and Western blot for α1 and β1 sGC subunits were performed. Intracellular calcium levels were monitored in platelets loaded with a fluorogenic calcium-binding dye (FluoForte).

RESULTS

BAY 60-2770 (0.001-10 µM) produced significant inhibition of collagen (2 µg/ml)- and thrombin (0.1 U/ml)-induced platelet aggregation that was markedly potentiated by the sGC inhibitor ODQ (10 µM). In fibrinogen-coated plates, BAY 60-2770 significantly inhibited platelet adhesion, an effect potentiated by ODQ. BAY 60-2770 increased the cGMP levels and reduced the intracellular Ca(2+) levels, both of which were potentiated by ODQ. The cell-permeable cGMP analogue 8-Br-cGMP (100 µM) inhibited platelet aggregation and Ca(2+) levels in an ODQ-insensitive manner. The cAMP levels remained unchanged by BAY 60-2770. Collagen- and thrombin-induced α(IIb)β(3) activation was markedly inhibited by BAY 60-2770 that was further inhibited by ODQ. The effects of sodium nitroprusside (3 µM) were all prevented by ODQ. Incubation with ODQ (10 µM) significantly reduced the protein levels of α1 and β1 sGC subunits, which were prevented by BAY 60-2770.

CONCLUSION

The inhibitory effects of BAY 60-2770 on aggregation, adhesion, intracellular Ca(2+) levels and α(IIb)β(3) activation are all potentiated in haem-oxidizing conditions. BAY 60-2770 prevents ODQ-induced decrease in sGC protein levels. BAY 60-2770 could be of therapeutic interest in cardiovascular diseases associated with thrombotic complications.