Soluble guanylate cyclase

Methods to investigate structure and activation dynamics of GC-1/GC-2

Soluble guanylyl cyclase (sGC) is a heterodimeric enzyme consisting of one α and one β subunit. The α1β1 (GC-1) and α2β1 (GC-2) heterodimers are important for NO signaling in humans and catalyse the conversion from GTP to cGMP. Each sGC subunit consists of four domains. Several crystal structures of the isolated domains are available. However, crystals of full-length sGC have failed to materialise. In consequence, the detailed three dimensional structure of sGC remains unknown to date. Different techniques including stopped-flow spectroscopy, Förster-resonance energy transfer, direct fluorescence, analytical ultracentrifugation, chemical cross-linking, small-angle X-ray scattering, electron microscopy, hydrogen-deuterium exchange and protein thermal shift assays, were used to collect indirect information. Taken together, this circumstantial evidence from different groups brings forth a plausible model of sGC domain arrangement, spatial orientation and dynamic rearrangement upon activation. For analysis of the active conformation the stable binding mode of sGC activators has a significant methodological advantage over the transient, elusive, complex and highly concentration dependent effects of NO in many applications. The methods used and the results obtained are reviewed and discussed in this article.

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.

Exploiting cGMP-based therapies for the prevention of left ventricular hypertrophy: NO* and beyond

Left ventricular hypertrophy (LVH), an increased left ventricular (LV) mass, is common to many cardiovascular disorders, initially developing as an adaptive response to maintain myocardial function. In the longer term, this LV remodelling becomes maladaptive, with progressive decline in LV contractility and diastolic function. Indeed LVH is recognised as an important blood-pressure independent predictor of cardiovascular morbidity and mortality. The clinical efficacy of current treatments for LVH is reduced, however, by their tendency to slow disease progression rather than induce its reversal, and thus the development of new therapies for LVH is paramount. The signalling molecule cyclic guanosine-3',5'-monophosphate (cGMP), well-recognised for its role in regulating vascular tone, is now being increasingly identified as an important anti-hypertrophic mediator. This review is focused on the various means by which cGMP can be stimulated in the heart, such as via the natriuretic peptides, to exert anti-hypertrophic actions. In particular we address the limitations of traditional nitric oxide (NO*) donors in the face of the potential therapeutic advantages offered by novel alternatives; NO* siblings, ligands of the cGMP-generating enzymes, soluble (sGC) and particulate guanylyl cyclases (pGC), and phosphodiesterase inhibitors. Further impact of cGMP within the cardiovascular system is also discussed with a view to representing cGMP-based therapies as innovative pharmacotherapy, alone or concurrent with standard care, for the management of LVH.

Biochemical Detection of cGMP From Past to Present: An Overview

Cyclic guanosine monophosphate (cGMP), generated via the guanylate cyclase (GC)-catalyzed conversion from GTP, is unequivocally recognized as crucial second messenger, intimately involved in the regulation of a broad range of physiological processes such as long term potentiation, blood pressure regulation, or platelet aggregation (for review: Hobbs 2000). Since its first identification in rat urine by Ashman and co-workers (1963), various approaches have been conceived and established to quantify cGMP in biological samples, or to detect cGMP as the reaction product of enzymatic assays, allowing the determination of kinetic parameters. These approaches have evolved from laborious handling of small numbers of samples with average sensitivity to highly developed biochemical detection assays allowing the processing of very large numbers of samples. The present article focuses upon the history of biochemical cGMP detection from the pioneering work of the early years to the actual state-of-the-art approaches for the detection of this important biological messenger.

NO- and haem-independent soluble guanylate cyclase activators

Oxidative stress, a risk factor for several cardiovascular disorders, interferes with the NO/sGC/cGMP signalling pathway through scavenging of NO and formation of the strong intermediate oxidant, peroxynitrite. Under these conditions, endothelial and vascular dysfunction develops, culminating in different cardio-renal and pulmonary-vascular diseases. Substituting NO with organic nitrates that release NO (NO donors) has been an important principle in cardiovascular therapy for more than a century. However, the development of nitrate tolerance limits their continuous clinical application and, under oxidative stress and increased formation of peroxynitrite foils the desired therapeutic effect. To overcome these obstacles of nitrate therapy, direct NO- and haem-independent sGC activators have been developed, such as BAY 58-2667 (cinaciguat) and HMR1766 (ataciguat), showing unique biochemical and pharmacological properties. Both compounds are capable of selectively activating the oxidized/haem-free enzyme via binding to the enzyme's haem pocket, causing pronounced vasodilatation. The potential importance of these new drugs resides in the fact that they selectively target a modified state of sGC that is prevalent under disease conditions as shown in several animal models and human disease. Activators of sGC may be beneficial in the treatment of a range of diseases including systemic and pulmonary hypertension (PH), heart failure, atherosclerosis, peripheral arterial occlusive disease (PAOD), thrombosis and renal fibrosis. The sGC activator HMR1766 is currently in clinical development as an oral therapy for patients with PAOD. The sGC activator BAY 58-2667 has demonstrated efficacy in a proof-of-concept study in patients with acute decompensated heart failure (ADHF), reducing pre- and afterload and increasing cardiac output from baseline. A phase IIb clinical study for the indication of ADHF is currently underway.

NO-independent stimulators and activators of soluble guanylate cyclase: discovery and therapeutic potential

Soluble guanylate cyclase (sGC) is a key signal-transduction enzyme activated by nitric oxide (NO). Impaired bioavailability and/or responsiveness to endogenous NO has been implicated in the pathogenesis of cardiovascular and other diseases. Current therapies that involve the use of organic nitrates and other NO donors have limitations, including non-specific interactions of NO with various biomolecules, lack of response and the development of tolerance following prolonged administration. Compounds that activate sGC in an NO-independent manner might therefore provide considerable therapeutic advantages. Here we review the discovery, biochemistry, pharmacology and clinical potential of haem-dependent sGC stimulators (including YC-1, BAY 41-2272, BAY 41-8543, CFM-1571 and A-350619) and haem-independent sGC activators (including BAY 58-2667 and HMR-1766).

Residues stabilizing the heme moiety of the nitric oxide sensor soluble guanylate cyclase

Soluble guanylate cyclase, a heterodimer consisting of an alpha- and a heme-containing beta-subunit, is the major receptor for the biological messenger nitric oxide (NO) and is involved in various signal transduction pathways. The heme moiety of the enzyme is bound between the axial heme ligand histidine(105) and the recently identified counterparts of the heme propionic acids, tyrosine(135) and arginine(139). The latter residues together with an invariant serine(137) form the unique heme binding motif Y-x-S-x-R. In this work, we show that replacement of the serine(137) with alanine destabilizes the binding of the heme moiety and impairs NO-mediated soluble guanylate cyclase activation.

Identification of residues crucially involved in the binding of the heme moiety of soluble guanylate cyclase

Soluble guanylate cyclase (sGC), a heterodimeric hemeprotein, is the only receptor for the biological messenger nitric oxide (NO) identified to date and is intimately involved in various signal transduction pathways. By using the recently discovered NO- and heme-independent sGC activator BAY 58-2667 and a novel cGMP reporter cell, we could distinguish between heme-containing and heme-free sGC in an intact cellular system. Using these novel tools, we identified the invariant amino acids tyrosine 135 and arginine 139 of the beta(1)-subunit as crucially important for both the binding of the heme moiety and the activation of sGC by BAY 58-2667. The heme is displaced by BAY 58-2667 due to a competition between the carboxylic groups of this compound and the heme propionic acids for the identified residues tyrosine 135 and arginine 139. This displacement results in the release of the axial heme ligand histidine 105 and to the observed activation of sGC. Based on these findings we postulate a signal transmission triad composed of histidine 105, tyrosine 135, and arginine 139 responsible for the enzyme activation by this compound and probably also for transducing changes in heme status and porphyrin geometry upon NO binding into alterations of sGC catalytic activity.

Preparation of heme-free soluble guanylate cyclase

Soluble guanylate cyclase (sGC), a heterodimer consisting of alpha- and beta-subunit, is the key enzyme of the NO/cGMP signaling pathway. The heme moiety ligated to the beta-subunit via His(105) is crucial for the activation of the enzyme by NO. In addition to this NO binding capability, the heme status of the enzyme influences the activity of non-NO sGC activators and sGC inhibitors. Different sGC activity profiles were observed in the presence, absence, or the oxidized form of heme. Modulating the heme status is therefore crucial for the investigation of the mechanism of sGC activation. Here, we present a simple and reliable procedure for the removal of the heme moiety of sGC that is capable of eliminating any traces of unbound heme and detergent from the sample mixture in one single step. Samples containing 15 microg sGC and the non-ionic detergent Tween 20 (2%) were incubated at 37 degrees C for 10 min and loaded onto centrifugal ion exchange columns. After centrifugation, heme was bound entirely to the ion exchanger and could not be eluted, even after incubation with 1M NaCl. Tween 20 was found completely within the flowthrough. Heme-free sGC was eluted from the ion exchanger after application of 300 mM NaCl. The absence of the heme moiety was confirmed by UV/Vis spectra and determination of the enzymatic activity. In summary, the described procedure is suitable for the preparation of very small amounts of highly purified heme-free sGC for the investigation of the mechanism of action of different types of sGC activators.

Mechanisms of nitric oxide independent activation of soluble guanylyl cyclase

The heterodimeric heme-protein soluble guanylyl cyclase (sGC) is the only proven receptor for nitric oxide (NO). Recently, two different types of NO-independent soluble guanylyl cyclase stimulators have been discovered. The heme-dependent stimulator 2-[1-[2-fluorophenyl)methyl]-1H-pyrazolo[3,4-b]pyridin-3-yl]-5(4-morpholinyl)-4,6-pyrimidinediamine (BAY 41-8543) stimulates the enzyme in a synergistic fashion when combined with NO, requires the presence of the heme group and can be blocked by the soluble guanylyl cyclase inhibitor 1H-(1,2,4)-Oxadiazole-(4,3-a)-quinoxalin-1-one (ODQ). The heme-independent activator 4-[((4-carboxybutyl)[2-[(4-phenethylbenzol) oxy]phenethyl]amino)methyl[benzoic]acid (BAY 58-2667) activates soluble guanylyl cyclase even in the presence of ODQ or rendered heme-deficient. In the present study, BAY 41-8543, BAY 58-2667 and NO strongly increased V(max). Combination of BAY 58-2667 and NO increased V(max) in an additive manner, whereas the synergistic effect of BAY 41-8543 and NO on enzyme activation was reflected in an overadditive increase of V(max). ODQ potentiated V(max) of BAY 58-2667-stimulated soluble guanylyl cyclase. BAY 41-8543 prolonged the half-life of the nitrosyl-heme complex of NO-activated enzyme, an effect that was not observed with BAY 58-2667. These results show the different activation patterns of both compounds and demonstrate their value as tools to investigate the mechanisms that underlie soluble guanylyl cyclase activation.

NO- and haem-independent activation of soluble guanylyl cyclase: molecular basis and cardiovascular implications of a new pharmacological principle

1. Soluble guanylyl cyclase (sGC) is the only proven receptor for the ubiquitous biological messenger nitric oxide (NO) and is intimately involved in many signal transduction pathways, most notably in regulating vascular tone and platelet function. sGC is a heterodimeric (alpha/ss) protein that converts GTP to cyclic GMP; NO binds to its prosthetic haem group. Here, we report the discovery of a novel sGC activating compound, its interaction with a previously unrecognized regulatory site and its therapeutic implications. 2. Through a high-throughput screen we identified BAY 58-2667, an amino dicarboxylic acid which potently activates sGC in an NO-independent manner. In contrast to NO, YC-1 and BAY 41-2272, the sGC stimulators described recently, BAY 58-2667 activates the enzyme even after it has been oxidized by the sGC inhibitor ODQ or rendered haem deficient. 3. Binding studies with radiolabelled BAY 58-2667 show a high affinity site on the enzyme. 4. Using photoaffinity labelling studies we identified the amino acids 371 (alpha-subunit) and 231 - 310 (ss-subunit) as target regions for BAY 58-2667. 5. sGC activation by BAY 58-2667 results in an antiplatelet activity both in vitro and in vivo and a potent vasorelaxation which is not influenced by nitrate tolerance. 6. BAY 58-2667 shows a potent antihypertensive effect in conscious spontaneously hypertensive rats. In anaesthetized dogs the hemodynamic effects of BAY 58-2667 and GTN are very similar on the arterial and venous system. 7. This novel type of sGC activator is a valuable research tool and may offer a new approach for treating cardiovascular diseases.

Pharmacological actions of a novel NO-independent guanylyl cyclase stimulator, BAY 41-8543: in vitro studies

BAY 41-8543 is a novel, highly specific and so far the most potent NO-independent stimulator of sGC. Here we report the effects of BAY 41-8543 on the isolated enzyme, endothelial cells, platelets, isolated vessels and Langendorff heart preparation. BAY 41-8543 stimulates the recombinant sGC concentration-dependently from 0.0001 microM to 100 microM up to 92-fold. In combination, BAY 41-8543 and NO have synergistic effects over a wide range of concentrations. Similar results are shown in implying that BAY 41-8543 stimulates the sGC directly and furthermore makes the enzyme more sensitive to its endogenous activator NO. In vitro, BAY 41-8543 is a potent relaxing agent of aortas, saphenous arteries, coronary arteries and veins with IC(50)-values in the nM range. In the rat heart Langendorff preparation, BAY 41-8543 potently reduces coronary perfusion pressure from 10(-9) to 10(-6) g ml(-1) without any effect on left ventricular pressure and heart rate. BAY 41-8543 is effective even under nitrate tolerance conditions proved by the same vasorelaxing effect on aortic rings taken either from normal or nitrate-tolerant rats. BAY 41-8543 is a potent inhibitor of collagen-mediated aggregation in washed human platelets (IC(50)=0.09 microM). In plasma, BAY 41-8543 inhibits collagen-mediated aggregation better than ADP-induced aggregation, but has no effect on the thrombin pathway. BAY 41-8543 is also a potent direct stimulator of the cyclic GMP/PKG/VASP pathway in platelets and synergizes with NO over a wide range of concentrations. These results suggest that BAY 41-8543 is on the one hand an invaluable tool for studying sGC signaling in vitro and on the other hand its unique profile may offer a novel approach for treating cardiovascular diseases.