Human recombinant soluble guanylyl cyclase: expression, purification, and regulation

The importance of and difficulties involved in creating molecular probes for a carbon monoxide gasotransmitter

As one of the triumvirate of recognized gasotransmitter molecules, namely NO, H2S, and CO, the physiological effects of CO and its potential as a biomarker have been widely investigated, garnering particular attention due to its reported hypotensive, anti-inflammatory, and cytoprotective properties, making it a promising therapeutic agent. However, the development of CO molecular probes has remained relatively stagnant in comparison with the fluorescent probes for NO and H2S, owing to its inert molecular state under physiological conditions. In this review, starting from elucidating the definition and significance of CO as a gasotransmitter, the imperative for the advancement of CO probes, especially fluorescent probes, is expounded. Subsequently, the current state of development of CO probe methodologies is comprehensively reviewed, with an overview of the challenges and prospects in this burgeoning field of research.

Cellular Factors That Shape the Activity or Function of Nitric Oxide-Stimulated Soluble Guanylyl Cyclase

NO-stimulated guanylyl cyclase (SGC) is a hemoprotein that plays key roles in various physiological functions. SGC is a typical enzyme-linked receptor that combines the functions of a sensor for NO gas and cGMP generator. SGC possesses exclusive selectivity for NO and exhibits a very fast binding of NO, which allows it to function as a sensitive NO receptor. This review describes the effect of various cellular factors, such as additional NO, cell thiols, cell-derived small molecules and proteins on the function of SGC as cellular NO receptor. Due to its vital physiological function SGC is an important drug target. An increasing number of synthetic compounds that affect SGC activity via different mechanisms are discovered and brought to clinical trials and clinics. Cellular factors modifying the activity of SGC constitute an opportunity for improving the effectiveness of existing SGC-directed drugs and/or the creation of new therapeutic strategies.

Co-expression of soluble guanylyl cyclase subunits and PDE5A shRNA to elevate cellular cGMP level: A potential gene therapy for myocardial cell death

BACKGROUND: Genetic manipulation on the NO-sGC-cGMP pathway has been rarely achieved, partially due to complexity of the soluble guanylyl cyclase (sGC) enzyme. OBJECTIVE: We aim to develop gene therapy directly targeting the pathway to circumvent cytotoxicity and tolerance after prolonged use of NO-donors and the insufficiency of PDE inhibitors. METHODS: In this study, we constructed lentivirus vectors expressing GUCY1A3 and GUCY1B3 genes, which encoded the α1 and β1 subunits of soluble guanylyl cyclase (sGC), respectively, to enhance cGMP synthesis. We also constructed lentiviral vector harboring PDE5A shRNA to alleviate phosphodiesterase activity and cGMP degradation. RESULTS: Transductions of human HEK293 cells with the constructs were successful, as indicated by the fluorescent signal and altered gene expression produced by each vector. Overexpression of GUCY1A3 and GUCY1B3 resulted in increased sGC enzyme activity and elevated cGMP level in the cells. Expression of PDE5A shRNA resulted in decreased PDE5A expression and elevated cGMP level. Co-transduction of the three lentiviral vectors resulted in a more significant elevation of cGMP in HEK293 cells without obvious cytotoxicity. CONCLUSION: To the best of our knowledge, this is the first study to show that co-expression of exogenous subunits of the soluble guanylyl cyclase could form functional enzyme and increase cellular cGMP level in mammalian cells. Simultaneous expression of PDE5A shRNA could alleviate feedback up-regulation on PDE5A caused by cGMP elevation. Further studies are required to evaluate the effects of these constructs in vivo.

Soluble guanylyl cyclase: Molecular basis for ligand selectivity and action in vitro and in vivo

Nitric oxide (NO), carbon monoxide (CO), oxygen (O₂), hydrogen sulfide (H₂S) are gaseous molecules that play important roles in the physiology and pathophysiology of eukaryotes. Tissue concentrations of these physiologically relevant gases vary remarkable from nM range for NO to high μM range of O₂. Various hemoproteins play a significant role in sensing and transducing cellular signals encoded by gaseous molecules or in transporting them. Soluble guanylyl cyclase (sGC) is a hemoprotein that plays vital roles in a wide range of physiological functions and combines the functions of gaseous sensor and signal transducer. sGC uniquely evolved to sense low non-toxic levels of NO and respond to elevated NO levels by increasing its catalytic ability to generate the secondary signaling messenger cyclic guanosine monophosphate (cGMP). This review discusses sGC's gaseous ligand selectivity and the molecular basis for sGC function as high-affinity and selectivity NO receptor. The effects of other gaseous molecules and small molecules of cellular origin on sGC's function are also discussed.

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.

Bioactive Catalytic Nanocompartments Integrated into Cell Physiology and Their Amplification of a Native Signaling Cascade

Bioactive nanomaterials have the potential to overcome the limitations of classical pharmacological approaches by taking advantage of native pathways to influence cell behavior, interacting with them and eliciting responses. Herein, we propose a cascade system mediated by two catalytic nanocompartments (CNC) with biological activity. Activated by nitric oxide (NO) produced by inducible nitric oxidase synthase (iNOS), soluble guanylyl cyclase (sGC) produces cyclic guanosine monophosphate (cGMP), a second messenger that modulates a broad range of physiological functions. As alterations in cGMP signaling are implicated in a multitude of pathologies, its signaling cascade represents a viable target for therapeutic intervention. Following along this line, we encapsulated iNOS and sGC in two separate polymeric compartments that function in unison to produce NO and cGMP. Their action was tested in vitro by monitoring the derived changes in cytoplasmic calcium concentrations of HeLa and differentiated C2C12 myocytes, where the produced second messenger influenced the cellular homeostasis.

Loss of smooth muscle CYB5R3 amplifies angiotensin II-induced hypertension by increasing sGC heme oxidation

Nitric oxide regulates BP by binding the reduced heme iron (Fe2+) in soluble guanylyl cyclase (sGC) and relaxing vascular smooth muscle cells (SMCs). We previously showed that sGC heme iron reduction (Fe3+ → Fe2+) is modulated by cytochrome b5 reductase 3 (CYB5R3). However, the in vivo role of SMC CYB5R3 in BP regulation remains elusive. Here, we generated conditional smooth muscle cell-specific Cyb5r3 KO mice (SMC CYB5R3-KO) to test if SMC CYB5R3 loss affects systemic BP in normotension and hypertension via regulation of the sGC redox state. SMC CYB5R3-KO mice exhibited a 5.84-mmHg increase in BP and impaired acetylcholine-induced vasodilation in mesenteric arteries compared with controls. To drive sGC oxidation and elevate BP, we infused mice with angiotensin II. We found that SMC CYB5R3-KO mice exhibited a 14.75-mmHg BP increase, and mesenteric arteries had diminished nitric oxide-dependent vasodilation but increased responsiveness to sGC heme-independent activator BAY 58-2667 over controls. Furthermore, acute injection of BAY 58-2667 in angiotensin II-treated SMC CYB5R3-KO mice showed greater BP reduction compared with controls. Together, these data provide the first in vivo evidence to our knowledge that SMC CYB5R3 is an sGC heme reductase in resistance arteries and provides resilience against systemic hypertension development.

Redox control of vascular smooth muscle cell function and plasticity

Vascular smooth muscle cells (SMC) play a major role in vascular diseases, such as atherosclerosis and hypertension. It has long been established in vitro that contractile SMC can phenotypically switch to function as proliferative and/or migratory cells in response to stimulation by oxidative stress, growth factors, and inflammatory cytokines. Reactive oxygen species (ROS) are oxidative stressors implicated in driving vascular diseases, shifting cell bioenergetics, and increasing SMC proliferation, migration, and apoptosis. In this review, we summarize our current knowledge of how disruptions to redox balance can functionally change SMC and how this may influence vascular disease pathogenesis. Specifically, we focus on our current understanding of the role of vascular nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOX) 1, 4, and 5 in SMC function. We also review the evidence implicating mitochondrial fission in SMC phenotypic transitions and mitochondrial fusion in maintenance of SMC homeostasis. Finally, we discuss the importance of the redox regulation of the soluble guanylate cyclase (sGC)-cyclic guanosine monophosphate (cGMP)-protein kinase G (PKG) pathway as a potential oxidative and therapeutic target for regulating SMC function.

Maprotiline Prevents Monocrotaline-Induced Pulmonary Arterial Hypertension in Rats

Pulmonary arterial hypertension (PAH) is a progressive disease caused by increased pulmonary artery pressure and pulmonary vascular resistance, eventually leading to right heart failure until death. Soluble guanylate cyclase (sGC) has been regarded as an attractive drug target in treating PAH. In this study, we discovered that maprotiline, a tetracyclic antidepressant, bound to the full-length recombinant sGC with a high affinity (K_D = 0.307 μM). Further study demonstrated that maprotiline concentration-dependently inhibited the proliferation of hypoxia-induced human pulmonary artery smooth muscle cells. Moreover, in a monocrotaline (MCT) rat model of PAH, maprotiline (ip, 10 mg/kg once daily) reduced pulmonary hypertension, inhibited the development of right ventricular hypertrophy and pathological changes of the pulmonary vascular remodeling. Taken together, our studies showed that maprotiline may contribute to attenuate disease progression of pulmonary hypertension.

The nitric oxide-guanylate cyclase pathway and glaucoma

Glaucoma is a prevalent optic neuropathy characterized by the progressive dysfunction and loss of retinal ganglion cells (RGCs) and their optic nerve axons, which leads to irreversible visual field loss. Multiple risk factors for the disease have been identified, but elevated intraocular pressure (IOP) remains the primary risk factor amenable to treatment. Reducing IOP however does not always prevent glaucomatous neurodegeneration, and many patients progress with the disease despite having IOP in the normal range. There is increasing evidence that nitric oxide (NO) is a direct regulator of IOP and that dysfunction of the NO-Guanylate Cyclase (GC) pathway is associated with glaucoma incidence. NO has shown promise as a novel therapeutic with targeted effects that: 1) lower IOP; 2) increase ocular blood flow; and 3) confer neuroprotection. The various effects of NO in the eye appear to be mediated through the activation of the GC- guanosine 3:5'-cyclic monophosphate (cGMP) pathway and its effect on downstream targets, such as protein kinases and Ca²⁺ channels. Although NO-donor compounds are promising as therapeutics for IOP regulation, they may not be ideal to harness the neuroprotective potential of NO signaling. Here we review evidence that supports direct targeting of GC as a novel pleiotrophic treatment for the disease, without the need for direct NO application. The identification and targeting of other factors that contribute to glaucoma would be beneficial to patients, particularly those that do not respond well to IOP-dependent interventions.

An N-terminally truncated form of cyclic GMP-dependent protein kinase Iα (PKG Iα) is monomeric and autoinhibited and provides a model for activation

The type I cGMP-dependent protein kinases (PKG I) serve essential physiological functions, including smooth muscle relaxation, cardiac remodeling, and platelet aggregation. These enzymes form homodimers through their N-terminal dimerization domains, a feature implicated in regulating their cooperative activation. Previous investigations into the activation mechanisms of PKG I isoforms have been largely influenced by structures of the cAMP-dependent protein kinase (PKA). Here, we examined PKG Iα activation by cGMP and cAMP by engineering a monomeric form that lacks N-terminal residues 1-53 (Δ53). We found that the construct exists as a monomer as assessed by whole-protein MS, size-exclusion chromatography, and small-angle X-ray scattering (SAXS). Reconstruction of the SAXS 3D envelope indicates that Δ53 has a similar shape to the heterodimeric RIα-C complex of PKA. Moreover, we found that the Δ53 construct is autoinhibited in its cGMP-free state and can bind to and be activated by cGMP in a manner similar to full-length PKG Iα as assessed by surface plasmon resonance (SPR) spectroscopy. However, we found that the Δ53 variant does not exhibit cooperative activation, and its cyclic nucleotide selectivity is diminished. These findings support a model in which, despite structural similarities, PKG Iα activation is distinct from that of PKA, and its cooperativity is driven by in trans interactions between protomers.

Biomarkers in Pulmonary Vascular Disease: Gauging Response to Therapy

Biomarkers are increasingly being investigated in the treatment of pulmonary vascular disease. In particular, the signaling pathways targeted by therapies for pulmonary arterial hypertension provide biomarkers that potentially can be used to guide therapy and to assess clinical response as an alternative to invasive procedures such as right-sided cardiac catheterization. Moreover, the growing use of combination therapy for both the initial and subsequent treatment of pulmonary arterial hypertension highlights the need for biomarkers in this treatment approach. Currently approved therapies for pulmonary arterial hypertension target 3 major signaling pathways: the nitric oxide-soluble guanylate cyclase-cyclic guanosine monophosphate pathway, the endothelin pathway, and the prostacyclin pathway. Although the main biomarker used in practice and evaluated in clinical trials is N-terminal pro-brain natriuretic peptide, other putative biomarkers include the endogenous nitric oxide (NO) synthase inhibitor asymmetric dimethylarginine, NO metabolites including S-nitrosothiols and nitrite, exhaled NO, endothelins, cyclic guanosine monophosphate, cyclic adenosine monophosphate, and atrial natriuretic peptide. This review describes accessible biomarkers, related to the actual molecules targeted by current therapies, for measuring and predicting response to the individual pulmonary arterial hypertension treatment classes as well as combination therapy.

Cytochrome b5 Reductase 3 Modulates Soluble Guanylate Cyclase Redox State and cGMP Signaling

RATIONALE

Soluble guanylate cyclase (sGC) heme iron, in its oxidized state (Fe³⁺), is desensitized to NO and limits cGMP production needed for downstream activation of protein kinase G-dependent signaling and blood vessel dilation.

OBJECTIVE

Although reactive oxygen species are known to oxidize the sGC heme iron, the basic mechanism(s) governing sGC heme iron recycling to its NO-sensitive, reduced state remain poorly understood.

METHODS AND RESULTS

Oxidant challenge studies show that vascular smooth muscle cells have an intrinsic ability to reduce oxidized sGC heme iron and form protein-protein complexes between cytochrome b5 reductase 3, also known as methemoglobin reductase, and oxidized sGC. Genetic knockdown and pharmacological inhibition in vascular smooth muscle cells reveal that cytochrome b5 reductase 3 expression and activity is critical for NO-stimulated cGMP production and vasodilation. Mechanistically, we show that cytochrome b5 reductase 3 directly reduces oxidized sGC required for NO sensitization as assessed by biochemical, cellular, and ex vivo assays.

CONCLUSIONS

Together, these findings identify new insights into NO-sGC-cGMP signaling and reveal cytochrome b5 reductase 3 as the first identified physiological sGC heme iron reductase in vascular smooth muscle cells, serving as a critical regulator of cGMP production and protein kinase G-dependent signaling.

Haem-Based Sensors of O2: Lessons and Perspectives

Haem-based sensors have emerged during the last 15 years as being a large family of proteins that occur in all kingdoms of life. These sensors are responsible mainly for detecting binding of O₂, CO and NO and reporting the ligation status to an output domain with an enzymatic or macromolecule-binding property. A myriad of biological functions have been associated with these sensors, which are involved in vasodilation, bacterial symbiosis, chemotaxis and biofilm formation, among others. Here, we critically review several bacterial systems for O₂ sensing that are extensively studied in many respects, focusing on the lessons that are important to advance the field.

Inhibition of soluble guanylyl cyclase by small molecules targeting the catalytic domain

Soluble guanylyl cyclase (sGC) plays a crucial role in cyclic nucleotide signaling that regulates numerous important physiological processes. To identify new sGC inhibitors that may prevent the formation of the active catalytic domain conformation, we carried out an in silico docking screen targeting a 'backside pocket' of the inactive sGC catalytic domain structure. Compounds 1 and 2 were discovered to inhibit sGC even at high/saturating nitric oxide concentrations. Both compounds also inhibit the BAY 58-2667-activated sGC as well as BAY 41-2272-stimulated sGC activity. Additional biochemical analyses showed that compound 2 also inhibits the isolated catalytic domain, thus demonstrating functional binding to this domain. Both compounds have micromolar affinity for sGC and are potential leads to develop more potent sGC inhibitors.

Erythrocytes do not activate purified and platelet soluble guanylate cyclases even in conditions favourable for NO synthesis

Background

Direct interaction between Red blood cells (RBCs) and platelets is known for a long time. The bleeding time is prolonged in anemic patients independent of their platelet count and could be corrected by transfusion of RBCs, which indicates that RBCs play an important role in hemostasis and platelet activation. However, in the last few years, opposing mechanisms of platelet inhibition by RBCs derived nitric oxide (NO) were proposed. The aim of our study was to identify whether RBCs could produce NO and activate soluble guanylate cyclase (sGC) in platelets.

Methods

To test whether RBCs could activate sGC under different conditions (whole blood, under hypoxia, or even loaded with NO), we used our well-established and highly sensitive models of NO-dependent sGC activation in platelets and activation of purified sGC. The activation of sGC was monitored by detecting the phosphorylation of Vasodilator Stimulated Phosphoprotein (VASP^S239) by flow cytometry and Western blot. ANOVA followed by Bonferroni’s test and Student’s t-test were used as appropriate.

Results

We show that in the whole blood, RBCs prevent NO-mediated inhibition of ADP and TRAP6-induced platelet activation. Likewise, coincubation of RBCs with platelets results in strong inhibition of NO-induced sGC activation. Under hypoxic conditions, incubation of RBCs with NO donor leads to Hb-NO formation which inhibits sGC activation in platelets. Similarly, RBCs inhibit activation of purified sGC, even under conditions optimal for RBC-mediated generation of NO from nitrite.

Conclusions

All our experiments demonstrate that RBCs act as strong NO scavengers and prevent NO-mediated inhibition of activated platelets. In all tested conditions, RBCs were not able to activate platelet or purified sGC.

Renal Integrin-Linked Kinase Depletion Induces Kidney cGMP-Axis Upregulation: Consequences on Basal and Acutely Damaged Renal Function

Soluble guanylyl cyclase (sGC) is activated by nitric oxide (NO) and produces cGMP, which activates cGMP-dependent protein kinases (PKG) and is hydrolyzed by specific phosphodiesterases (PDE). The vasodilatory and cytoprotective capacity of cGMP-axis activation results in a therapeutic strategy for several pathologies. Integrin-linked kinase (ILK), a major scaffold protein between the extracellular matrix and intracellular signaling pathways, may modulate the expression and functionality of the cGMP-axis-related proteins. We introduce ILK as a novel modulator in renal homeostasis as well as a potential target for cisplatin (CIS)-induced acute kidney injury (AKI) improvement. We used an adult mice model of depletion of ILK (cKD-ILK), which showed basal increase of sGC and PKG expressions and activities in renal cortex when compared with wildtype (WT) littermates. Twenty-four h activation of sGC activation with NO enhanced the filtration rate in cKD-ILK. During AKI, cKD-ILK maintained the cGMP-axis upregulation with consequent filtration rates enhancement and ameliorated CIS-dependent tubular epithelial-to-mesenchymal transition and inflammation and markers. To emphasize the role of cGMP-axis upregulation due to ILK depletion, we modulated the cGMP axis under AKI in vivo and in renal cultured cells. A suboptimal dose of the PDE inhibitor ZAP enhanced the beneficial effects of the ILK depletion in AKI mice. On the other hand, CIS increased contractility-related events in cultured glomerular mesangial cells and necrosis rates in cultured tubular cells; ILK depletion protected the cells while sGC blockade with ODQ fully recovered the damage.

Pyridine Hemochromagen Assay for Determining the Concentration of Heme in Purified Protein Solutions

Heme is a common cofactor in proteins, found in hemoglobin, myoglobin, cytochrome P450, DGCR8, and nitric oxide synthase, among others. This protocol describes a method for quantifying heme that works best in purified protein samples. This protocol might be used to, for example, determine whether a given heme-binding protein is fully occupied by heme, thus allowing correlation of heme content with activity. This requires the absolute heme concentration and an accurate protein concentration. Another use is to determine the extinction coefficients of a heme-bound protein. This assay is fast, easy, and reproducible if done correctly.

Primary and secondary lymphatic valve development: molecular, functional and mechanical insights

Fluid homeostasis in vertebrates critically relies on the lymphatic system forming a hierarchical network of lymphatic capillaries and collecting lymphatics, for the efficient drainage and transport of extravasated fluid back to the cardiovascular system. Blind-ended lymphatic capillaries employ specialized junctions and anchoring filaments to encourage a unidirectional flow of the interstitial fluid into the initial lymphatic vessels, whereas collecting lymphatics are responsible for the active propulsion of the lymph to the venous circulation via the combined action of lymphatic muscle cells and intraluminal valves. Here we describe recent findings on molecular and physical factors regulating the development and maturation of these two types of valves and examine their role in tissue-fluid homeostasis.

Gas what: NO is not the only answer to sexual function

The ability to get and keep an erection is important to men for several reasons and the inability is known as erectile dysfunction (ED). ED has started to be accepted as an early indicator of systemic endothelial dysfunction and subsequently of cardiovascular diseases. The role of NO in endothelial relaxation and erectile function is well accepted. The discovery of NO as a small signalling gasotransmitter led to the investigation of the role of other endogenously derived gases, carbon monoxide (CO) and hydrogen sulphide (H2 S) in physiological and pathophysiological conditions. The role of NO and CO in sexual function and dysfunction has been investigated more extensively and, recently, the involvement of H2 S in erectile function has also been confirmed. In this review, we focus on the role of these three sister gasotransmitters in the physiology, pharmacology and pathophysiology of sexual function in man, specifically erectile function. We have also reviewed the role of soluble guanylyl cyclase/cGMP pathway as a common target of these gasotransmitters. Several studies have proposed alternative therapies targeting different mechanisms in addition to PDE-5 inhibition for ED treatment, since some patients do not respond to these drugs. This review highlights complementary and possible coordinated roles for these mediators and treatments targeting these gasotransmitters in erectile function/ED.

Ilk conditional deletion in adult animals increases cyclic GMP-dependent vasorelaxation

AIMS

Integrin-linked kinase (ILK) regulates proliferation, differentiation, cell adhesion, and motility in many cell types and has been related to cancer progression, fibrosis, and vascular diseases. We designed the present study to directly explore the effect of ILK deletion on the regulation of vascular tone through the soluble guanylate cyclase (sGC) /protein kinase G (PKG) pathway in healthy adult mice.

METHODS AND RESULTS

Experiments were carried out using a tamoxifen-inducible CRE-LOX system to conditionally delete the ILK gene in adult mice. Mice lacking ILK expression (cKO) presented increased vascular content and increased activity of sGC and PKG, resulting in a more intense vasodilatory response to a single dose of a nitric oxide (NO) donor [sodium nitroprusside (SNP)] or PKG agonist [8-bromoguanosine 3',5'-cyclic monophosphate sodium salt (8-Br)]. Five minutes after SNP or 8-Br administration the reduction in the systolic arterial pressure was enhanced in cKO mice (SNP WT: -7.4 ± 1.2 mmHG; SNP cKO: -14.0 ± 2.5; 8-Br WT: -2.9 ± 1.5 mmHG; 8-Br cKO: -10.0 ± 3.4 mmHG). ILK deletion restored the vascular response to SNP after chronic oral nitrite administration. In addition, ILK deletion also increased hypotensive SNP effect in angiotensin II-treated animals, suggesting a role for ILK in basal and pathological states.

CONCLUSION

Deletion of ILK in adult animals increased the vascular response to NO. These findings show, for the first time, a requirement for ILK in regulating sGC-PKG expression in vivo.

Structure and regulation of soluble guanylate cyclase

Nitric oxide (NO) is an essential signaling molecule in biological systems. In mammals, the diatomic gas is critical to the cyclic guanosine monophosphate (cGMP) pathway as it functions as the primary activator of soluble guanylate cyclase (sGC). NO is synthesized from l-arginine and oxygen (O(2)) by the enzyme nitric oxide synthase (NOS). Once produced, NO rapidly diffuses across cell membranes and binds to the heme cofactor of sGC. sGC forms a stable complex with NO and carbon monoxide (CO), but not with O(2). The binding of NO to sGC leads to significant increases in cGMP levels. The second messenger then directly modulates phosphodiesterases (PDEs), ion-gated channels, or cGMP-dependent protein kinases to regulate physiological functions, including vasodilation, platelet aggregation, and neurotransmission. Many studies are focused on elucidating the molecular mechanism of sGC activation and deactivation with a goal of therapeutic intervention in diseases involving the NO/cGMP-signaling pathway. This review summarizes the current understanding of sGC structure and regulation as well as recent developments in NO signaling.

Nitric oxide-cyclic GMP signaling in stem cell differentiation

The nitric oxide-cyclic GMP (NO-cGMP) pathway mediates important physiological functions associated with various integrative body systems including the cardiovascular and nervous systems. Furthermore, NO regulates cell growth, survival, apoptosis, proliferation, and differentiation at the cellular level. To understand the significance of the NO-cGMP pathway in development and differentiation, studies have been conducted both in developing embryos and in stem cells. Manipulation of the NO-cGMP pathway, by employing activators and inhibitors as pharmacological probes, and genetic manipulation of NO signaling components have implicated the involvement of this pathway in the regulation of stem cell differentiation. This review focuses on some of the work pertaining to the role of NO-cGMP in the differentiation of stem cells into cells of various lineages, particularly into myocardial cells, and in stem cell-based therapy.

Mechanisms of penile erection and basis for pharmacological treatment of erectile dysfunction

Erection is basically a spinal reflex that can be initiated by recruitment of penile afferents, both autonomic and somatic, and supraspinal influences from visual, olfactory, and imaginary stimuli. Several central transmitters are involved in the erectile control. Dopamine, acetylcholine, nitric oxide (NO), and peptides, such as oxytocin and adrenocorticotropin/α-melanocyte-stimulating hormone, have a facilitatory role, whereas serotonin may be either facilitatory or inhibitory, and enkephalins are inhibitory. The balance between contractant and relaxant factors controls the degree of contraction of the smooth muscle of the corpora cavernosa (CC) and determines the functional state of the penis. Noradrenaline contracts both CC and penile vessels via stimulation of α₁-adrenoceptors. Neurogenic NO is considered the most important factor for relaxation of penile vessels and CC. The role of other mediators, released from nerves or endothelium, has not been definitely established. Erectile dysfunction (ED), defined as the "inability to achieve or maintain an erection adequate for sexual satisfaction," may have multiple causes and can be classified as psychogenic, vasculogenic or organic, neurologic, and endocrinologic. Many patients with ED respond well to the pharmacological treatments that are currently available, but there are still groups of patients in whom the response is unsatisfactory. The drugs used are able to substitute, partially or completely, the malfunctioning endogenous mechanisms that control penile erection. Most drugs have a direct action on penile tissue facilitating penile smooth muscle relaxation, including oral phosphodiesterase inhibitors and intracavernosal injections of prostaglandin E₁. Irrespective of the underlying cause, these drugs are effective in the majority of cases. Drugs with a central site of action have so far not been very successful. There is a need for therapeutic alternatives. This requires identification of new therapeutic targets and design of new approaches. Research in the field is expanding, and several promising new targets for future drugs have been identified.

Application of nitric oxide in drug discovery and development

INTRODUCTION

It is becoming increasingly clear that many diseases are characterized or associated with perturbations in nitric oxide (NO) production/signaling. Therapeutics or strategies designed to restore normal NO homeostasis will likely have broad application and utility in human health. This highly complex and multi-step pathway for NO production and subsequent target activation provides many steps in the endogenous pathway that may be useful targets for drug development. Important therapeutic areas for NO-based therapies are inflammatory disorders, cardiovascular diseases, erectile dysfunction and metabolic disorders.

AREAS COVERED

The following review will discuss the endogenous NO pathway, highlight the current market and indications for NO-based therapeutics, as well as identify pathway targets currently under drug development. Each step along the NO pathway will be discussed including exogenous sources of NO, use of precursors to promote NO production and downstream pathways affected by NO production with advantages and disadvantages highlighted for each.

EXPERT OPINION

Development of NO-based therapeutics is and will continue to be a major focus of biotech and pharmaceutical companies. Understanding and utilizing dietary and nutritional strategies to restore NO homeostasis could allow for safer, quicker marketing of products that may be just as efficacious as drugs designed against specific targets.

Carbon monoxide and the eye: Implications for glaucoma therapy

In the late 1990s, the scientific community witnessed a very peculiar phenomenon: the transformation of nitric oxide (NO) from a noxious gas into a key chemical messenger. The importance of NO in biology and medicine was highlighted in 1998 when the Nobel Prize was awarded in Physiology and Medicine to Robert Furchgott, Louis Ignarro and Ferid Murad for their pioneering work on the role of NO in the nervous, cardiovascular and immune systems. In this same time period, carbon monoxide (CO), another gas usually associated with environmental pollution, air poisoning and suicidal behavior, was also undergoing a similar change in image, although not as closely followed. It had been known for several decades that the human body generated CO upon the decomposition of hemoglobin, which was determined by the discovery that heme oxygenase (HO) is the enzymatic source of CO. However, CO's role as an endogenous neurotransmitter was established only in the early 1990s. Since then, many biological activities of CO have been demonstrated in studies using different tools, such as the pharmacological induction of HO by hemin, the direct administration of CO or the use of pro-drugs that generate CO. This review focuses on CO as a fine modulator of intraocular pressure and on its potential implications in glaucoma.

Cyclic nucleotide metabolism including nitric oxide and phosphodiesterase-related targets in the lower urinary tract

The clinical data on the use of the orally active phosphodiesterase (PDE) type 5 inhibitors sildenafil (VIAGRA™), vardenafil (LEVITRA™), and tadalafil (CIALIS™) for the treatment of male erectile dysfunction have boosted research activities on the physiology and pharmacology of the organs of the lower urinary tract (LUT). This includes both intracellular signal transduction in the prostate, urinary bladder (detrusor), and urethra, as well as central brain and spinal cord pathways controlling the function of the LUT. Such efforts provided the basis for the development of new therapeutic modalities into the management of dysfunctions/ syndromes of the LUT, some of which are already offered to the patients. The pharmacological treatment of the overactive bladder and the so-called benign prostatic syndrome, including LUT symptomatology and bladder outlet obstruction secondary to benign prostatic enlargement, has primarily focused on selective, orally available drugs acting by influencing intracellular regulatory mechanisms. These agents are regarded efficacious, have a fast onset of drug action in the target tissue and an improved effect-to-side-effect ratio. Better understanding of the functional significance of proteins related to cyclic nucleotide-dependent pathways, such as nitric oxide synthase, cytosolic and membrane-bound guanylyl cyclases, PDE isoenzymes and cyclic AMP- and cyclic GMP-binding protein kinases, the relative distribution in tissues of the LUT, and the consequences for urogenital function, seems to be of particular interest in order to identify new or more selective pharmacological approaches to manage disorders of the LUT. The present review focuses on cyclic nucleotide-related targets involved in the control of the function of the bladder, prostate, and urethra and the significance of those proteins in the process of evolving new pharmacological options for the treatment of LUT symptoms secondary to benign prostatic hyperplasia as well as dysfunctions of the storage and voiding capability of the urinary bladder.

Fluorescent fusion proteins of soluble guanylyl cyclase indicate proximity of the heme nitric oxide domain and catalytic domain

BACKGROUND

To examine the structural organisation of heterodimeric soluble guanylyl cyclase (sGC) Förster resonance energy transfer (FRET) was measured between fluorescent proteins fused to the amino- and carboxy-terminal ends of the sGC beta1 and alpha subunits.

METHODOLOGY/PRINCIPAL FINDINGS

Cyan fluorescent protein (CFP) was used as FRET donor and yellow fluorescent protein (YFP) as FRET acceptor. After generation of recombinant baculovirus, fluorescent-tagged sGC subunits were co-expressed in Sf9 cells. Fluorescent variants of sGC were analyzed in vitro in cytosolic fractions by sensitized emission FRET. Co-expression of the amino-terminally tagged alpha subunits with the carboxy-terminally tagged beta1 subunit resulted in an enzyme complex that showed a FRET efficiency of 10% similar to fluorescent proteins separated by a helix of only 48 amino acids. Because these findings indicated that the amino-terminus of the alpha subunits is close to the carboxy-terminus of the beta1 subunit we constructed fusion proteins where both subunits are connected by a fluorescent protein. The resulting constructs were not only fluorescent, they also showed preserved enzyme activity and regulation by NO.

CONCLUSIONS/SIGNIFICANCE

Based on the ability of an amino-terminal fragment of the beta1 subunit to inhibit activity of an heterodimer consisting only of the catalytic domains (alphacatbetacat), Winger and Marletta (Biochemistry 2005, 44:4083-90) have proposed a direct interaction of the amino-terminal region of beta1 with the catalytic domains. In support of such a concept of "trans" regulation of sGC activity by the H-NOX domains our results indicate that the domains within sGC are organized in a way that allows for direct interaction of the amino-terminal regulatory domains with the carboxy-terminal catalytic region. In addition, we constructed "fluorescent-conjoined" sGC's by fusion of the alpha amino-terminus to the beta1 carboxy-terminus leading to a monomeric, fluorescent and functional enzyme complex. To our knowledge this represents the first example where a fluorescent protein links two different subunits of a higher ordered complex to yield a stoichometrically fixed functionally active monomer.

Anatomy, physiology, and pathophysiology of erectile dysfunction

INTRODUCTION

Significant scientific advances during the past 3 decades have deepened our understanding of the physiology and pathophysiology of penile erection. A critical evaluation of the current state of knowledge is essential to provide perspective for future research and development of new therapies.

AIM

To develop an evidence-based, state-of-the-art consensus report on the anatomy, physiology, and pathophysiology of erectile dysfunction (ED).

METHODS

Consensus process over a period of 16 months, representing the opinions of 12 experts from seven countries.

MAIN OUTCOME MEASURE

Expert opinion was based on the grading of scientific and evidence-based medical literature, internal committee discussion, public presentation, and debate.

RESULTS

ED occurs from multifaceted, complex mechanisms that can involve disruptions in neural, vascular, and hormonal signaling. Research on central neural regulation of penile erection is progressing rapidly with the identification of key neurotransmitters and the association of neural structures with both spinal and supraspinal pathways that regulate sexual function. In parallel to advances in cardiovascular physiology, the most extensive efforts in the physiology of penile erection have focused on elucidating mechanisms that regulate the functions of the endothelium and vascular smooth muscle of the corpus cavernosum. Major health concerns such as atherosclerosis, hyperlipidemia, hypertension, diabetes, and metabolic syndrome (MetS) have become well integrated into the investigation of ED.

CONCLUSIONS

Despite the efficacy of current therapies, they remain insufficient to address growing patient populations, such as those with diabetes and MetS. In addition, increasing awareness of the adverse side effects of commonly prescribed medications on sexual function provides a rationale for developing new treatment strategies that minimize the likelihood of causing sexual dysfunction. Many basic questions with regard to erectile function remain unanswered and further laboratory and clinical studies are necessary.

Endothelium-derived hyperpolarising factors and associated pathways: a synopsis

The term endothelium-derived hyperpolarising factor (EDHF) was introduced in 1987 to describe the hypothetical factor responsible for myocyte hyperpolarisations not associated with nitric oxide (EDRF) or prostacyclin. Two broad categories of EDHF response exist. The classical EDHF pathway is blocked by apamin plus TRAM-34 but not by apamin plus iberiotoxin and is associated with endothelial cell hyperpolarisation. This follows an increase in intracellular [Ca(2+)] and the opening of endothelial SK(Ca) and IK(Ca) channels preferentially located in caveolae and in endothelial cell projections through the internal elastic lamina, respectively. In some vessels, endothelial hyperpolarisations are transmitted to myocytes through myoendothelial gap junctions without involving any EDHF. In others, the K(+) that effluxes through SK(Ca) activates myocytic and endothelial Ba(2+)-sensitive K(IR) channels leading to myocyte hyperpolarisation. K(+) effluxing through IK(Ca) activates ouabain-sensitive Na(+)/K(+)-ATPases generating further myocyte hyperpolarisation. For the classical pathway, the hyperpolarising "factor" involved is the K(+) that effluxes through endothelial K(Ca) channels. During vessel contraction, K(+) efflux through activated myocyte BK(Ca) channels generates intravascular K(+) clouds. These compromise activation of Na(+)/K(+)-ATPases and K(IR) channels by endothelium-derived K(+) and increase the importance of gap junctional electrical coupling in myocyte hyperpolarisations. The second category of EDHF pathway does not require endothelial hyperpolarisation. It involves the endothelial release of factors that include NO, HNO, H(2)O(2) and vasoactive peptides as well as prostacyclin and epoxyeicosatrienoic acids. These hyperpolarise myocytes by opening various populations of myocyte potassium channels, but predominantly BK(Ca) and/or K(ATP), which are sensitive to blockade by iberiotoxin or glibenclamide, respectively.

A real-time fluorescent assay of the purified nitric oxide receptor, guanylyl cyclase

Nitric oxide (NO) mediates intercellular signaling through activation of its receptor, soluble guanylyl cyclase (sGC), leading to elevation of intracellular guanosine 3',5'-cyclic monophosphate (cGMP) levels. Through this signal transduction pathway, NO regulates a diverse range of physiological effects, from vasodilatation and platelet disaggregation to synaptic plasticity. Measurement of sGC activity has traditionally been carried out using end-point assays of cGMP accumulation or by transfection of cells with "detector" proteins such as fluorescent proteins coupled to cGMP binding domains or cyclic nucleotide gated channels. Here we report a simpler approach: the use of a fluorescently labeled substrate analog, mant-GTP (2'-O-(N-methylanthraniloyl) guanosine 5'-triphosphate), which gives an increase in emission intensity after enzymatic cyclization to mant-cGMP. Activation of purified recombinant sGC by NO led to a rapid rise in fluorescence intensity within seconds, reaching a maximal 1.6- to 1.8-fold increase above basal levels. The sGC inhibitor, ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one), eliminated the fluorescence increase due to NO, and the synergistic activator of sGC, BAY 41-2272 (3-(4-amino-5-cyclopropylpyrimidin-2-yl)-1-(2-fluorobenzyl)-1H-pyrazolo[3,4-b]pyridine), increased the rate at which the maximal fluorescence increase was attained. High-performance liquid chromatography (HPLC) confirmed the formation of mant-cGMP product. This real-time assay allows the progress of purified sGC activation to be quantified precisely and, with refinement, could be optimized for use in a cellular environment.

Thrombospondin-1-CD47 blockade and exogenous nitrite enhance ischemic tissue survival, blood flow and angiogenesis via coupled NO-cGMP pathway activation

Tissue ischemia and ischemia-reperfusion (I/R) remain sources of cell and tissue death. Inability to restore blood flow and limit reperfusion injury represents a challenge in surgical tissue repair and transplantation. Nitric oxide (NO) is a central regulator of blood flow, reperfusion signaling and angiogenesis. De novo NO synthesis requires oxygen and is limited in ischemic vascular territories. Nitrite (NO(2-)) has been discovered to convert to NO via heme-based reduction during hypoxia, providing a NO synthase independent and oxygen-independent NO source. Furthermore, blockade of the matrix protein thrombospondin-1 (TSP1) or its receptor CD47 has been shown to promote downstream NO signaling via soluble guanylate cyclase (sGC) and cGMP-dependant kinase. We hypothesized that nitrite would provide an ischemic NO source that could be potentiated by TSP1-CD47 blockade enhancing ischemic tissue survival, blood flow and angiogenesis. Both low dose nitrite and direct blockade of TSP1-CD47 interaction using antibodies or gene silencing increased acute blood flow and late tissue survival in ischemic full thickness flaps. Nitrite and TSP1 blockade both enhanced in vitro and in vivo angiogenic responses. The nitrite effect could be abolished by inhibition of sGC and cGMP signaling. Potential therapeutic synergy was tested in a more severe ischemic flap model. We found that combined therapy with nitrite and TSP1-CD47 blockade enhanced flap perfusion, survival and angiogenesis to a greater extent than either agent alone, providing approximately 100% flap survival. These data provide a new therapeutic paradigm for hypoxic NO signaling through enhanced cGMP mediated by TSP1-CD47 blockade and nitrite delivery.

Signaling by gasotransmitters

Nitric oxide is well established as a major signaling molecule. Evidence is accumulating that carbon monoxide and hydrogen sulfide also are physiologic mediators in the cardiovascular, immune, and nervous systems. This Review focuses on mechanisms whereby they signal by binding to metal centers in metalloproteins, such as in guanylyl cyclase, or modifying sulfhydryl groups in protein targets.

Characterization of oxidized guanosine 5'-triphosphate as a viable inhibitor of soluble guanylyl cyclase

The guanine base is prone to oxidation by free radicals regardless of the cellular moiety it is bound to. However, under conditions of oxidative stress, 8-oxoguanosine triphosphate (oxo(8)GTP) formation has been shown to occur without oxidation of the guanine base in DNA. In vitro studies have suggested that oxo(8)GTP could impact G-protein signaling and RNA synthesis. Whether increased levels of oxo(8)GTP translate into cellular malfunction is unknown. Data presented herein show that oxo(8)GTP is formed in cell-free preparations as well as in PC12 cells after exposure to physiologically relevant oxidative conditions generated with 10 microM copper sulfate and 1 mM L-ascorbic acid (Cu/Asc). We also determined that oxo(8)GTP has biological activity as a potent inhibitor of nitric oxide-stimulated soluble guanylyl cyclase (sGC). The increase in oxo(8)GTP formation in purified GTP and PC12 cells exposed to Cu/Asc caused a significant reduction in the product of sGC activity, cGMP. This oxidation of GTP was attenuated by the addition of reduced glutathione under these same Cu/Asc conditions, thus preventing the decrease in sGC activity. This suggests that oxo(8)GTP is produced by free radicals in vivo and could have significant impact on cell functions regulated by sGC activity such as synaptic plasticity in the central nervous system.

Novel therapies for cyclic GMP control of vascular smooth muscle growth

Cyclic GMP, guanosine 3',5'-cyclic monophosphate, is a critical and multifunctional second-messenger molecule that mediates diverse physiological and pathophysiological functions in cardiac and vascular tissues. Synthesized through nitric oxide, carbon monoxide, and/or natriuretic peptide-mediated guanylate cyclase stimulation and guanosine triphosphate dephosphorylation, cyclic GMP is capable of stimulating a cascade of serine/threonine kinase events, including signaling through cyclic GMP- and/or cyclic AMP-dependent protein kinases, eliciting protein kinase-independent actions such as modulation of ion channels or transporters, or undergoing hydrolytic degradation through actions of cyclic GMP-regulated phosphodiesterases. Substrates, enzymes, cofactors, and associated variables in this multifaceted system have historically been targets of vital pharmacotherapies with perhaps most common the use of vascular smooth muscle-targeting organonitrates in cardiac patients and phosphodiesterase inhibitors in individuals with erectile dysfunction. Accumulating basic science and clinical evidence, however, suggests that cyclic GMP signaling is compromised under conditions of disease or elevated physiological stresses. Moreover, nitric oxide can stimulate an array of cytotoxic effects and nitric oxide-based therapies can be limited by diminished bioactivity and the development of tachyphylaxis or tolerance after prolonged use. Consequently, an emerging area for clinical drug development and therapeutic drug evaluation for conditions of cardiovascular adversity has focused on identification of cyclic GMP signaling pathways that act under oxidized or nitric oxide-unresponsive conditions and/or that operate irrespective of nitric oxide-induced complications. The aim of this therapeutic review is to describe novel, nitric oxide-alternate avenues for cyclic GMP signaling in vascular smooth muscle growth with particular emphasis on pharmacotherapeutics of recently characterized cyclic GMP-specific approaches.

The titerless infected-cells preservation and scale-up (TIPS) method for large-scale production of NO-sensitive human soluble guanylate cyclase (sGC) from insect cells infected with recombinant baculovirus

Compounds capable of stimulating soluble guanylate cyclase (sGC) activity might become important new tools to treat hypertension. While rational design of these drugs would be aided by elucidation of the sGC three-dimensional structure and molecular mechanism of activation, such efforts also require quantities of high quality enzyme that are challenging to produce. We implemented the titerless infected-cells preservation and scale-up (TIPS) methodology to express the heterodimeric sGC. In the TIPS method, small-scale insect cell cultures were first incubated with a recombinant baculovirus which replicated in the cells. The baculovirus-infected insect cells (BIIC) were harvested and frozen prior to cell lysis and the subsequent escape of the newly replicated virus into the culture supernatant. Thawed BIIC stocks were ultimately used for subsequent scale up. As little as 1 mL of BIIC was needed to infect a 100-L insect cell culture, in contrast to the usual 1L of high-titer, virus stock supernatants. The TIPS method eliminates the need and protracted time for titering virus supernatants, and provides stable, concentrated storage of recombinant baculovirus in the form of infected cells. The latter is particularly advantageous for virus stocks which are unstable, such as those for sGC, and provides a highly efficient alternative for baculovirus storage and expression. The TIPS process enabled efficient scale up to 100-L batches, each producing about 200mg of active sGC. Careful adjustment of expression culture conditions over the course of several 100-L runs provided uniform starting titers, specific activity, and composition of contaminating proteins that facilitated development of a process that reproducibly yielded highly active, purified sGC.

Purification and characterization of recombinant human soluble guanylate cyclase produced from baculovirus-infected insect cells

Soluble guanylate cyclase (sGC) has been purified from 100 L cell culture infected by baculovirus using the newer and highly effective titerless infected-cells preservation and scale-up (TIPS) method. Successive passage of the enzyme through DEAE, Ni(2+)-NTA, and POROS Q columns obtained approximately 100mg of protein. The sGC obtained by this procedure was already about 90% pure and suitable for various studies which include high throughput screening (HTS) and hit follow-up. However, in order to obtain enzyme of greater homogeneity and purity for crystallographic and high precision spectroscopic and kinetic studies of sGC with select stimulators, the sGC solution after the POROS Q step was further purified by GTP-agarose affinity chromatography. This additional step led to the generation of 26 mg of enzyme that was about 99% pure. This highly pure and active enzyme exhibited a M(r)=144,933 by static light scattering supportive of a dimeric structure. It migrated as a two-band protein, each of equal intensity, on SDS-PAGE corresponding to the alpha (M(r) approximately 77,000) and beta (M(r) approximately 70,000) sGC subunits. It showed an A(430)/A(280)=1.01, indicating one heme per heterodimer, and a maximum of the Soret band at 430 nm indicative of a penta-coordinated ferrous heme with a histidine as the axial ligand. The Soret band shifted to 398 nm in the presence of an NO donor as expected for the formation of a penta-coordinated nitrosyl-heme complex. Non-stimulated sGC had k(cat)/K(m)=1.7 x 10(-3)s(-1)microM(-1) that increased to 5.8 x 10(-1)s(-1)microM(-1) upon stimulation with an NO donor which represents a 340-fold increase due to stimulation. The novel combination of using the TIPS method for co-expression of a heterodimeric heme-containing enzyme, along with the application of a reproducible ligand affinity purification method, has enabled us to obtain recombinant human sGC of both the quality and quantity needed to study structure-function relationships.

NO-independent, haem-dependent soluble guanylate cyclase stimulators

The nitric oxide (NO) signalling pathway is altered in cardiovascular diseases, including systemic and pulmonary hypertension, stroke, and atherosclerosis. The vasodilatory properties of NO have been exploited for over a century in cardiovascular disease, but NO donor drugs and inhaled NO are associated with significant shortcomings, including resistance to NO in some disease states, the development of tolerance during long-term treatment, and non-specific effects such as post-translational modification of proteins. The development of pharmacological agents capable of directly stimulating the NO receptor, soluble guanylate cyclase (sGC), is therefore highly desirable. The benzylindazole compound YC-1 was the first sGC stimulator to be identified; this compound formed a lead structure for the development of optimized sGC stimulators with improved potency and specificity for sGC, including CFM-1571, BAY 41-2272, BAY 41-8543, and BAY 63-2521. In contrast to the NO- and haem-independent sGC activators such as BAY 58-2667, these compounds stimulate sGC activity independent of NO and also act in synergy with NO to produce anti-aggregatory, anti-proliferative, and vasodilatory effects. Recently, aryl-acrylamide compounds were identified independent of YC-1 as sGC stimulators; although structurally dissimilar to YC-1, they have a similar mode of action and promote smooth muscle relaxation. Pharmacological stimulators of sGC may be beneficial in the treatment of a range of diseases, including systemic and pulmonary hypertension, heart failure, atherosclerosis, erectile dysfunction, and renal fibrosis. An sGC stimulator, BAY 63-2521, is currently in clinical development as an oral therapy for patients with pulmonary hypertension. It has demonstrated efficacy in a proof-of-concept study, reducing pulmonary vascular resistance and increasing cardiac output from baseline. A full, phase 2 trial of BAY 63-2521 in pulmonary hypertension is underway.

YC-1 stimulates the expression of gaseous monoxide-generating enzymes in vascular smooth muscle cells

The benzylindazole derivative 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole (YC-1) is an allosteric stimulator of soluble guanylate cyclase (sGC) that sensitizes the enzyme to the gaseous ligands carbon monoxide (CO) and nitric oxide (NO). In this study, we examined whether YC-1 also promotes the production of these gaseous monoxides by stimulating the expression of the inducible isoforms of heme oxygenase (HO-1) and NO synthase (iNOS) in vascular smooth muscle cells (SMCs). YC-1 increased HO-1 mRNA, protein, and promoter activity and potentiated cytokine-mediated expression of iNOS protein and NO synthesis by SMCs. The induction of HO-1 by YC-1 was unchanged by the sGC inhibitor, 1H-(1,2,4)oxadiazolo[4,3-alpha]quinozalin-1-one (ODQ) or by the protein kinase G inhibitors (8R,9S,11S)-(-)-2-methyl-9-methoxyl-9-methoxycarbonyl-8-methyl-2,3,9,10-tetrahydro-8,11-epoxy-1H,8H,11H-2,7b,11a-triazadibenzo(a,g)cyclocta9(cde)trinen-1-one (KT 5823) and YGRKKRRQRRRPPLRKKKKKH-amide (DT-2) and was not duplicated by 8-bromo-cGMP or the NO-independent sGC stimulator 5-cyclopropyl-2[1-(2-fluorobenzyl)-1H-pyrazolo [3,4-b] pyridine-3-yl] pyrimidin-4-ylamine (BAY 41-2272). However, the YC-1-mediated induction of HO-1 was inhibited by the phosphatidylinositol-3-kinase (PI3K) inhibitors wortmannin and 2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride (LY294002). In contrast, the enhancement of cytokine-stimulated iNOS expression and NO production by YC-1 was prevented by ODQ and the protein kinase A inhibitor (9S,10S, 12R)-2,3,9,10,11,12-hexahydro-10-hydroxy-9-methyl-1-oxo-9, 12-epoxy-1H-diindolo(1,2,3-fg:3',2',1'-kl)pyrrolo(3,4-i)(1,6)-benzodiazocine-10-carboxylic acid hexyl ester (KT 5720) and was mimicked by 8-bromo-cGMP and BAY 41-2272. In conclusion, these studies demonstrate that YC-1 stimulates the expression of HO-1 and iNOS in vascular SMCs via the PI3K and sGC-cGMP-protein kinase A pathway, respectively. The ability of YC-1 to sensitize sGC to gaseous monoxides and simultaneously stimulate their production through the induction of HO-1 and iNOS provides a potent mechanism by which the cGMP-dependent and -independent biological actions of this agent are amplified.

An internal ribosome entry site mediates the initiation of soluble guanylyl cyclase beta2 mRNA translation

The soluble guanylyl cyclases (sGC), the receptor for nitric oxide, are heterodimers consisting of an alpha- and beta-subunit. This study aimed to investigate the translational mechanism of the sGC beta2-subunit. Two mRNA species for sGC beta2 were isolated from human kidney. These transcripts had dissimilar 5'-untranslated regions (5'-UTRs). The most abundant sGC beta2 mRNA showed numerous upstream open reading frames (ORFs) and stable secondary structures that inhibited in vivo and in vitro translation. To evaluate whether these 5'-UTRs harbored an internal ribosome entry site (IRES) that allows translation by an alternative mechanism, we inserted these regions between the two luciferase genes of a bicistronic vector. Transfection of those genetic constructs into HeLa cells demonstrated that both sGC beta2 leaders had IRES activity in a cell-type dependent manner. Finally, the secondary structural model of the sGC beta2 5'-UTR predicts a Y-type pseudoknot that characterizes the IRES of cellular mRNAs. In conclusion, our findings suggest that sGC beta2 5'-UTRs have IRES activity that may permit sGC beta2 expression under conditions that are not optimal for scanning-dependent translation.

Protein kinase G phosphorylates soluble guanylyl cyclase on serine 64 and inhibits its activity

OBJECTIVE

Binding of nitric oxide (NO) to soluble guanylyl cyclase (sGC) leads to increased cGMP synthesis that activates cGMP-dependent protein kinase (PKG). Herein, we tested whether sGC activity is regulated by PKG.

METHODS AND RESULTS

Overexpression of a constitutively active form of PKG (DeltaPKG) stimulated (32)P incorporation into the alpha1 subunit. Serine to alanine mutation of putative sites revealed that Ser64 is the main phosphorylation site for PKG. Using a phospho-specific antibody we observed that endogenous sGC phosphorylation on Ser 64 increases in cells and tissues exposed to NO, in a PKG-inhibitable manner. Wild-type (wt) sGC coexpressed with DeltaPKG exhibited lower basal and NO-stimulated cGMP accumulation, whereas the S64A alpha1/beta1 sGC was resistant to the PKG-induced reduction in activity. Using purified sGC we observed that the S64D alpha1 phosphomimetic /beta1 dimer exhibited lower Vmax; moreover, the decrease in Km after NO stimulation was less pronounced in S64D alpha1/beta1 compared to wild-type sGC. Expression of a phosphorylation-deficient sGC showed enhanced responsiveness to endothelium-derived NO, reduced desensitization to acute NO exposure, and allowed for greater VASP phosphorylation.

CONCLUSIONS

We conclude that PKG phosphorylates sGC on Ser64 of the alpha1 subunit and that phosphorylation inhibits sGC activity, establishing a negative feedback loop.

Allostery in recombinant soluble guanylyl cyclase from Manduca sexta

Soluble guanylyl/guanylate cyclase (sGC), the primary biological receptor for nitric oxide, is required for proper development and health in all animals. We have expressed heterodimeric full-length and N-terminal fragments of Manduca sexta sGC in Escherichia coli, the first time this has been accomplished for any sGC, and have performed the first functional analyses of an insect sGC. Manduca sGC behaves much like its mammalian counterparts, displaying a 170-fold stimulation by NO and sensitivity to compound YC-1. YC-1 reduces the NO and CO off-rates for the approximately 100-kDa N-terminal heterodimeric fragment and increases the CO affinity by approximately 50-fold to 1.7 microm. Binding of NO leads to a transient six-coordinate intermediate, followed by release of the proximal histidine to yield a five-coordinate nitrosyl complex (k(6-5) = 12.8 s(-1)). The conversion rate is insensitive to nucleotides, YC-1, and changes in NO concentration up to approximately 30 microm. NO release is biphasic in the absence of YC-1 (k(off1) = 0.10 s(-1) and k(off2) = 0.0015 s(-1)); binding of YC-1 eliminates the fast phase but has little effect on the slower phase. Our data are consistent with a model for allosteric activation in which sGC undergoes a simple switch between two conformations, with an open or a closed heme pocket, integrating the influence of numerous effectors to give the final catalytic rate. Importantly, YC-1 binding occurs in the N-terminal two-thirds of the protein. Homology modeling and mutagenesis experiments suggest the presence of an H-NOX domain in the alpha subunit with importance for heme binding.