cIMP synthesized by sGC as a mediator of hypoxic contraction of coronary arteries

Skeletal Muscle Microvascular Dysfunction in Obesity-Related Insulin Resistance: Pathophysiological Mechanisms and Therapeutic Perspectives

Obesity is a worrisomely escalating public health problem globally and one of the leading causes of morbidity and mortality from noncommunicable disease. The epidemiological link between obesity and a broad spectrum of cardiometabolic disorders has been well documented; however, the underlying pathophysiological mechanisms are only partially understood, and effective treatment options remain scarce. Given its critical role in glucose metabolism, skeletal muscle has increasingly become a focus of attention in understanding the mechanisms of impaired insulin function in obesity and the associated metabolic sequelae. We examined the current evidence on the relationship between microvascular dysfunction and insulin resistance in obesity. A growing body of evidence suggest an intimate and reciprocal relationship between skeletal muscle microvascular and glucometabolic physiology. The obesity phenotype is characterized by structural and functional changes in the skeletal muscle microcirculation which contribute to insulin dysfunction and disturbed glucose homeostasis. Several interconnected etiologic molecular mechanisms have been suggested, including endothelial dysfunction by several factors, extracellular matrix remodelling, and induction of oxidative stress and the immunoinflammatory phenotype. We further correlated currently available pharmacological agents that have deductive therapeutic relevance to the explored pathophysiological mechanisms, highlighting a potential clinical perspective in obesity treatment.

Putative Nucleotide-Based Second Messengers in the Archaeal Model Organisms Haloferax volcanii and Sulfolobus acidocaldarius

Research on nucleotide-based second messengers began in 1956 with the discovery of cyclic adenosine monophosphate (3′,5′-cAMP) by Earl Wilbur Sutherland and his co-workers. Since then, a broad variety of different signaling molecules composed of nucleotides has been discovered. These molecules fulfill crucial tasks in the context of intracellular signal transduction. The vast majority of the currently available knowledge about nucleotide-based second messengers originates from model organisms belonging either to the domain of eukaryotes or to the domain of bacteria, while the archaeal domain is significantly underrepresented in the field of nucleotide-based second messenger research. For several well-stablished eukaryotic and/or bacterial nucleotide-based second messengers, it is currently not clear whether these signaling molecules are present in archaea. In order to shed some light on this issue, this study analyzed cell extracts of two major archaeal model organisms, the euryarchaeon Haloferax volcanii and the crenarchaeon Sulfolobus acidocaldarius, using a modern mass spectrometry method to detect a broad variety of currently known nucleotide-based second messengers. The nucleotides 3′,5′-cAMP, cyclic guanosine monophosphate (3′,5′-cGMP), 5′-phosphoadenylyl-3′,5′-adenosine (5′-pApA), diadenosine tetraphosphate (Ap₄A) as well as the 2′,3′-cyclic isomers of all four RNA building blocks (2′,3′-cNMPs) were present in both species. In addition, H. volcanii cell extracts also contain cyclic cytosine monophosphate (3′,5′-cCMP), cyclic uridine monophosphate (3′,5′-cUMP) and cyclic diadenosine monophosphate (3′,5′-c-di-AMP). The widely distributed bacterial second messengers cyclic diguanosine monophosphate (3′,5′-c-di-GMP) and guanosine (penta-)/tetraphosphate [(p)ppGpp] could not be detected. In summary, this study gives a comprehensive overview on the presence of a large set of currently established or putative nucleotide-based second messengers in an eury- and a crenarchaeal model organism.

Involvement of 3',5'-cyclic inosine monophosphate in cystathionine γ-lyase-dependent regulation of the vascular tone

BACKGROUND AND PURPOSE

l-cysteine or hydrogen sulfide (H₂ S) donors induce a biphasic effect on precontracted isolated vessels. The contractile effect occurs within a concentration range of 10 nM to 3 μM followed by vasodilatation at 30-100 μM. Here, we have investigated the signalling involved in the H₂ S-induced contraction.

EXPERIMENTAL APPROACH

Vascular response to NaHS or l-cysteine is evaluated on isolated precontracted with phenylephrine vessel rings harvested from wild type, cystathionine γ-lyase (CSE^-/- ), soluble guanylyl cyclase (sGC_α1 ^-/- ) and endothelial nitric oxide synthase (eNOS^-/- ) knock-out mice. The cAMP, cGMP and inosine 3',5'-cyclic monophosphate (cIMP) levels are simultaneously quantified using ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) analysis. The involvement of sGC, phosphodiesterase (PDE) 4A and PDE5 are also evaluated.

KEY RESULTS

CSE-derived H₂ S-induced contraction requires an intact eNOS/NO/sGC pathway and involves cIMP as a second messenger. H₂ S contractile effect involves a transient increase of cGMP and cAMP metabolism caused by PDE5 and PDE4A, thus unmasking cIMP contracting action. The stable cell-permeable analogue of cIMP elicits concentration-dependent contraction on a stable background tone induced by phenylephrine. The lack of cIMP, coupled to the hypocontractility displayed by vessels harvested from CSE^-/- mice, confirms that H₂ S-induced contraction involves cIMP.

CONCLUSION AND IMPLICATIONS

The endothelium dynamically regulates vessel homeostasis by modulating contractile tone. This also involves CSE-derived H₂ S that is mediated by cIMP.

GYY4137 and Sodium Hydrogen Sulfide Relaxations Are Inhibited by L-Cysteine and KV7 Channel Blockers in Rat Small Mesenteric Arteries

Donors of H₂S may be beneficial in treating cardiovascular diseases where the plasma levels of H₂S are decreased. Therefore, we investigated the mechanisms involved in relaxation of small arteries induced by GYY4137 [(4-methoxyphenyl)-morpholin-4-yl-sulfanylidene-sulfido-λ5-phosphane;morpholin-4-ium], which is considered a slow-releasing H₂S donor. Sulfides were measured by use of 5,5′-dithiobis-(2-nitro benzoic acid), and small rat mesenteric arteries with internal diameters of 200–250 µm were mounted in microvascular myographs for isometric tension recordings. GYY4137 produced similar low levels of sulfides in the absence and the presence of arteries. In U46619-contracted small mesenteric arteries, GYY4137 (10⁻⁶–10^–3 M) induced concentration-dependent relaxations, while a synthetic, sulfur-free, GYY4137 did not change the vascular tone. L-cysteine (10⁻⁶–10^–3 M) induced only small relaxations reaching 24 ± 6% at 10^–3 M. Premixing L-cysteine (10^–3 M) with Na₂S and GYY4137 decreased Na₂S relaxation and abolished GYY4137 relaxation, an effect prevented by an nitric oxide (NO) synthase inhibitor, L-NAME (N^ω-nitro-L-arginine methyl ester). In arteries without endothelium or in the presence of L-NAME, relaxation curves for GYY4137 were rightward shifted. High extracellular K⁺ concentrations decreased Na₂S and abolished GYY4137 relaxation suggesting potassium channel-independent mechanisms are also involved Na₂S relaxation while potassium channel activation is pivotal for GYY4137 relaxation in small arteries. Blockers of large-conductance calcium-activated (BK_Ca) and voltage-gated type 7 (K_V7) potassium channels also inhibited GYY4137 relaxations. The present findings suggest that L-cysteine by reaction with Na₂S and GYY4137 and formation of sulfides, inhibits relaxations by these compounds. The low rate of release of H₂S species from GYY4137 is reflected by the different sensitivity of these relaxations towards high K⁺ concentration and potassium channel blockers compared with Na₂S. The perspective is that the rate of release of sulfides plays an important for the effects of H₂S salt vs. donors in small arteries, and hence for a beneficial effect of GYY4137 for treatment of cardiovascular disease.

Neural stem cell-specific ITPA deficiency causes neural depolarization and epilepsy

Inosine triphosphate pyrophosphatase (ITPA) hydrolyzes inosine triphosphate (ITP) and other deaminated purine nucleotides to the corresponding nucleoside monophosphates. In humans, ITPA deficiency causes severe encephalopathy with epileptic seizure, microcephaly, and developmental retardation. In this study, we established neural stem cell-specific Itpa-conditional KO mice (Itpa-cKO mice) to clarify the effects of ITPA deficiency on the neural system. The Itpa-cKO mice showed growth retardation and died within 3 weeks of birth. We did not observe any microcephaly in the Itpa-cKO mice, although the female Itpa-cKO mice did show adrenal hypoplasia. The Itpa-cKO mice showed limb-clasping upon tail suspension and spontaneous and/or audiogenic seizure. Whole-cell patch-clamp recordings from entorhinal cortex neurons in brain slices revealed a depolarized resting membrane potential, increased firing, and frequent spontaneous miniature excitatory postsynaptic current and miniature inhibitory postsynaptic current in the Itpa-cKO mice compared with ITPA-proficient controls. Accumulated ITP or its metabolites, such as cyclic inosine monophosphates, or RNA containing inosines may cause membrane depolarization and hyperexcitability in neurons and induce the phenotype of ITPA-deficient mice, including seizure.

PDE1 or PDE5 inhibition augments NO-dependent hypoxic constriction of porcine coronary artery via elevating inosine 3',5'-cyclic monophosphate level

Hypoxic coronary vasospasm may lead to myocardial ischaemia and cardiac dysfunction. Inosine 3',5'-cyclic monophosphate (cIMP) is a putative second messenger to mediate this pathological process. Nevertheless, it remains unclear as to whether levels of cIMP can be regulated in living tissue such as coronary artery and if so, what is the consequence of this regulation on hypoxia-induced vasoconstriction. In the present study, we found that cIMP was a key determinant of hypoxia-induced constriction but not that of the subsequent relaxation response in porcine coronary arteries. Subsequently, coronary arteries were treated with various phosphodiesterase (PDE) inhibitors to identify PDE types that are capable of regulating cIMP levels. We found that inhibition of PDE1 and PDE5 substantially elevated cIMP content in endothelium-denuded coronary artery supplemented with exogenous purified cIMP. However, cGMP levels were far lower than their levels in intact coronary arteries and lower than cIMP levels measured in endothelium-denuded coronary arteries supplemented with exogenous cIMP. The increased cIMP levels induced by PDE1 or PDE5 inhibition further led to augmented hypoxic constriction without apparently affecting the relaxation response. In intact coronary artery, PDE1 or PDE5 inhibition up-regulated cIMP levels under hypoxic condition. Concomitantly, cGMP level increased to a comparable level. Nevertheless, the hypoxia-mediated constriction was enhanced in this situation that was largely compromised by an even stronger inhibition of PDEs. Taken together, these data suggest that cIMP levels in coronary arteries are regulated by PDE1 and PDE5, whose inhibition at a certain level leads to increased cIMP content and enhanced hypoxic constriction.

Metabolic events in HIV-infected patients using abacavir are associated with erythrocyte inosine triphosphatase activity

Objectives

Abacavir use has been associated with an increased risk of cardiovascular disease (CVD) and metabolic events in HIV-infected patients, although this finding was not consistently found. It is unclear whether abacavir only increases this risk in subpopulations of HIV-infected patients. It may be hypothesized that inosine 5'-triphosphate pyrophosphohydrolase (ITPase), an enzyme involved in the metabolism of purine analogues used in HIV treatment, plays a role in the risk of CVD and metabolic events in HIV-infected patients.

Methods

ITPase activity and ITPA genotype were determined in 393 HIV-infected patients. ITPase activity <4 mmol IMP/mmol Hb/h was considered decreased. ITPA polymorphisms tested were: c.94C>A (rs1127354) and c.124 + 21A>C (rs7270101). ORs were determined using generalized estimating equation models for developing CVD in patients who had ever been exposed to abacavir, tenofovir or didanosine and for developing metabolic events in patients currently using these drugs.

Results

In patients using abacavir, metabolic events were associated with ITPase activity. No association was demonstrated for tenofovir or didanosine. The OR for metabolic events was 3.11 in patients using abacavir with normal ITPase activity (95% CI 1.34-7.21; P = 0.008) compared with patients with decreased ITPase activity [adjusted for age, BMI, cumulative duration of combination ART (cART) use and the use of PI and NNRTI]. CVD was not associated with ITPase activity or ITPA genotype.

Conclusions

This study shows, for the first time, that ITPase activity is associated with the occurrence of metabolic events in patients using abacavir. Further studies are needed to confirm this association and to elucidate the possible mechanism.

The role of gasotransmitters in neonatal physiology

The gasotransmitters, nitric oxide (NO), hydrogen sulfide (H₂S), and carbon monoxide (CO), are endogenously-produced volatile molecules that perform signaling functions throughout the body. In biological tissues, these small, lipid-permeable molecules exist in free gaseous form for only seconds or less, and thus they are ideal for paracrine signaling that can be controlled rapidly by changes in their rates of production or consumption. In addition, tissue concentrations of the gasotransmitters are influenced by fluctuations in the level of O₂ and reactive oxygen species (ROS). The normal transition from fetus to newborn involves a several-fold increase in tissue O₂ tensions and ROS, and requires rapid morphological and functional adaptations to the extrauterine environment. This review summarizes the role of gasotransmitters as it pertains to newborn physiology. Particular focus is given to the vasculature, ventilatory, and gastrointestinal systems, each of which uniquely illustrate the function of gasotransmitters in the birth transition and newborn periods. Moreover, given the relative lack of studies on the role that gasotransmitters play in the newborn, particularly that of H₂S and CO, important gaps in knowledge are highlighted throughout the review.

Endothelial Dysfunction in Primary Aldosteronism

Primary aldosteronism (PA) is characterized by excess production of aldosterone from the adrenal glands and is the most common and treatable cause of secondary hypertension. Aldosterone is a mineralocorticoid hormone that participates in the regulation of electrolyte balance, blood pressure, and tissue remodeling. The excess of aldosterone caused by PA results in an increase in cardiovascular and cerebrovascular complications, including coronary artery disease, myocardial infarction, stroke, transient ischemic attack, and even arrhythmia and heart failure. Endothelial dysfunction is a well-established fundamental cause of cardiovascular diseases and also a predictor of worse clinical outcomes. Accumulating evidence indicates that aldosterone plays an important role in the initiation and progression of endothelial dysfunction. Several mechanisms have been shown to contribute to aldosterone-induced endothelial dysfunction, including aldosterone-mediated vascular tone dysfunction, aldosterone- and endothelium-mediated vascular inflammation, aldosterone-related atherosclerosis, and vascular remodeling. These mechanisms are activated by aldosterone through genomic and nongenomic pathways in mineralocorticoid receptor-dependent and independent manners. In addition, other cells have also been shown to participate in these mechanisms. The complex interactions among endothelium, inflammatory cells, vascular smooth muscle cells and fibroblasts are crucial for aldosterone-mediated endothelial dysregulation. In this review, we discuss the association between aldosterone and endothelial function and the complex mechanisms from a molecular aspect. Furthermore, we also review current clinical research of endothelial dysfunction in patients with PA.

Hypoxic augmentation: The tale of a strange contraction

Almost fifty years ago, experiments on isolated veins showed that acute hypoxia augments venoconstrictor responses in vitro and that such facilitation relied on anaerobic glycolysis. Over the years, this phenomenon was extended to a number of arterial preparations of different species and revisited, from a mechanistic point of view, with the successive demonstration that it depends on calcium handling in the vascular smooth muscle cells, is endothelium-dependent and requires the production of nitric oxide (NO) by endothelial nitric oxide synthase (eNOS) and the activation of soluble guanylyl cyclase (sGC). However, rather than the vasodilator cyclic nucleotide 3',5'-cyclic guanosine monophosphate (cGMP), its canonical product, the latter enzyme produces 3',5'-cyclic inosine monophosphate (cIMP) instead during acute hypoxia; this non-canonical cyclic nucleotide facilitates the contractile process in the vascular smooth muscle cells. This 'biased' activity of soluble guanylyl cyclase appears to involve stimulation of NAD(P)H:quinone oxidoreductase 1 (NQO-1). The exact interactions between hypoxia, anaerobic metabolism and NQO-1 leading to biased activity of soluble guanylyl cyclase remain to be established.

Urapidil, but not dihydropyridine calcium channel inhibitors, preserves the hypoxic pulmonary vasoconstriction: an experimental study in pig arteries

Hypoxic pulmonary vasoconstriction (HPV) is a protective mechanism maintaining blood oxygenation by redirecting blood flow from poorly ventilated to well-ventilated areas in the lung. Such a beneficial effect is blunted by antihypertensive treatment with dihydropyridine calcium channel inhibitors. The aim of the present study was to evaluate the effect of urapidil, an antihypertensive agent acting as an α₁ adrenergic antagonist and a partial 5-HT_1A agonist, on HPV in porcine proximal and distal pulmonary artery rings, and to characterize underlying mechanisms. Rings from proximal and distal porcine pulmonary artery were suspended in organ chambers and aerated with a 95% O₂ + 5% CO₂ gas mixture. HPV was induced by changing the gas to a 95% N₂ + 5% CO₂ mixture following a low level of pre-contraction with U46619. Hypoxia induced a contractile response in both proximal and distal pulmonary artery rings. This effect is observed in the presence of a functional endothelium and is inhibited by a soluble guanylyl cyclase inhibitor (ODQ), a NO scavenger (carboxy-PTIO), and by catalase in proximal pulmonary artery rings. The endothelium-dependent HPV is prevented by nicardipine and clevidipine but remained unaffected by urapidil in both proximal and distal pulmonary artery rings. These findings indicate that urapidil, in contrast to nicardipine and clevidipine, preserves the hypoxia-triggered vasoconstriction in isolated pulmonary arteries. They further indicate the involvement of the NO-guanylyl cyclase pathway and H₂ O₂ in HPV. Further research is warranted to determine the potential clinical relevance of the preserved hypoxia-induced pulmonary vasoconstriction by urapidil.

Cyclic Nucleotide-Directed Protein Kinases in Cardiovascular Inflammation and Growth

Cardiovascular disease (CVD), including myocardial infarction (MI) and peripheral or coronary artery disease (PAD, CAD), remains the number one killer of individuals in the United States and worldwide, accounting for nearly 18 million (>30%) global deaths annually. Despite considerable basic science and clinical investigation aimed at identifying key etiologic components of and potential therapeutic targets for CVD, the number of individuals afflicted with these dreaded diseases continues to rise. Of the many biochemical, molecular, and cellular elements and processes characterized to date that have potential to control foundational facets of CVD, the multifaceted cyclic nucleotide pathways continue to be of primary basic science and clinical interest. Cyclic adenosine monophosphate (cyclic AMP) and cyclic guanosine monophosphate (cyclic GMP) and their plethora of downstream protein kinase effectors serve ubiquitous roles not only in cardiovascular homeostasis but also in the pathogenesis of CVD. Already a major target for clinical pharmacotherapy for CVD as well as other pathologies, novel and potentially clinically appealing actions of cyclic nucleotides and their downstream targets are still being discovered. With this in mind, this review article focuses on our current state of knowledge of the cyclic nucleotide-driven serine (Ser)/threonine (Thr) protein kinases in CVD with particular emphasis on cyclic AMP-dependent protein kinase (PKA) and cyclic GMP-dependent protein kinase (PKG). Attention is given to the regulatory interactions of these kinases with inflammatory components including interleukin 6 signals, with G protein-coupled receptor and growth factor signals, and with growth and synthetic transcriptional platforms underlying CVD pathogenesis. This article concludes with a brief discussion of potential future directions and highlights the importance for continued basic science and clinical study of cyclic nucleotide-directed protein kinases as emerging and crucial controllers of cardiac and vascular disease pathologies.

Nitric Oxide: From Good to Bad

This essay summarizes a lecture presented on October 19th, 2017, during the 58th Annual Meeting of the Japanese College of Angiology in Nagoya, Japan. The lecture summarizes several instances where the absence of relaxations of isolated blood vessels in response to endothelium-dependent vasodilator agonists, which cause activation of endothelial nitric oxide synthase (eNOS) and consequent production of endothelium-derived nitric oxide (NO) and stimulation of soluble guanylyl cyclase (sGC) in underlying vascular smooth muscle, or hypoxia are curtailed or reversed to endothelium-dependent contractions. Chosen examples include selective dysfunction of eNOS activation in regenerated endothelial cells, unresponsiveness of vascular smooth muscle cells to NO during subarachnoid hemorrhage, and biased activation of sGC in vascular smooth muscle cells during acute exposure to hypoxia. The main message of this essay is that absence, blunting, or reversal of endothelium-dependent relaxations in response to vasodilator agonists cannot necessarily be interpreted as a sign of endothelial dysfunction. (This is a review article based on the invited lecture of the 58th Annual Meeting of Japanese College of Angiology.)

Inactivation of Non-canonical Cyclic Nucleotides: Hydrolysis and Transport

This chapter addresses cNMP hydrolysis by phosphodiesterases (PDEs) and export by multidrug resistance associated proteins (MRPs). Both mechanisms are well-established for the canonical cNMPs, cAMP, and cGMP. Increasing evidence shows that non-canonical cNMPs (specifically cCMP, cUMP) are also PDE and MRP substrates. Hydrolysis of cUMP is achieved by PDE 3A, 3B, and 9A, which possibly explains the cUMP-degrading activities previously reported for heart, adipose tissue, and brain. Regarding cCMP, the only known "conventional" (class I) PDE that hydrolyzes cCMP is PDE7A. Older reports describe cCMP-degrading PDE-like activities in mammalian tissues, bacteria, and plants, but the molecular identity of these enzymes is not clear. High K _M and V _max values, insensitivity to common inhibitors, and unusually broad substrate specificities indicate that these activities probably do not represent class I PDEs. Moreover, the older results have to be interpreted with caution, since the historical analytical methods were not as reliable as modern highly sensitive and specific techniques like HPLC-MS/MS. Besides PDEs, the transporters MRP4 and 5 are of major importance for cAMP and cGMP disposal. Additionally, both MRPs also export cUMP, while cCMP is only exported by MRP5. Much less data are available for the non-canonical cNMPs, cIMP, cXMP, and cTMP. None of these cNMPs has been examined as MRP substrate. It was shown, however, that they are hydrolyzed by several conventional class I PDEs. Finally, this chapter reveals that there are still large gaps in our knowledge about PDE and MRP activities for canonical and non-canonical cNMPs. Future research should perform a comprehensive characterization of the known PDEs and MRPs with the physiologically most important cNMP substrates.

3',5'-cIMP as Potential Second Messenger in the Vascular Wall

Traditionally, only the 3',5'-cyclic monophosphates of adenosine and guanosine (produced by adenylyl cyclase and guanylyl cyclase, respectively) are regarded as true "second messengers" in the vascular wall, despite the presence of other cyclic nucleotides in different tissues. Among these noncanonical cyclic nucleotides, inosine 3',5'-cyclic monophosphate (cIMP) is synthesized by soluble guanylyl cyclase in porcine coronary arteries in response to hypoxia, when the enzyme is activated by endothelium-derived nitric oxide. Its production is associated with augmentation of vascular contraction mediated by stimulation of Rho kinase. Based on these findings, cIMP appears to meet most, if not all, of the criteria required for it to be accepted as a "second messenger," at least in the vascular wall.

cAMP-Dependent Protein Kinase and cGMP-Dependent Protein Kinase as Cyclic Nucleotide Effectors

The cAMP-dependent protein kinase (PKA) and the cGMP-dependent protein kinase (PKG) are homologous enzymes with different binding and activation specificities for cyclic nucleotides. Both enzymes harbor conserved cyclic nucleotide-binding (CNB) domains. Differences in amino acid composition of these CNB domains mediate cyclic nucleotide selectivity in PKA and PKG, respectively. Recently, the presence of the noncanonical cyclic nucleotides cCMP and cUMP in eukaryotic cells has been proven, while the existence of cellular cIMP and cXMP remains unclear. It was shown that the main effectors of cyclic nucleotide signaling, PKA and PKG, can be activated by each of these noncanonical cyclic nucleotides. With unique effector proteins still missing, such cross-activation effects might have physiological relevance. Therefore, we approach PKA and PKG as cyclic nucleotide effectors in this chapter. The focus of this chapter is the general cyclic nucleotide-binding properties of both kinases as well as the selectivity for cAMP or cGMP, respectively. Furthermore, we discuss the binding affinities and activation potencies of noncanonical cyclic nucleotides.

Holistic Methods for the Analysis of cNMP Effects

Cyclic nucleotide monophosphates (cNMPs) typify the archetype second messenger in living cells and serve as molecular switches with broad functionality. cAMP and cGMP are the best-described cNMPs; however, there is a growing body of evidence indicating that also cCMP and cUMP play a substantial role in signal transduction. Despite research efforts, to date, relatively little is known about the biology of these noncanonical cNMPs, which is due, at least in part, to methodological issues in the past entailing setbacks of the entire field. Only recently, with the use of state-of-the-art techniques, it was possible to revive noncanonical cNMP research. While high-sensitive detection methods disclosed relevant levels of cCMP and cUMP in mammalian cells, knowledge about the biological effectors and their physiological interplay is still incomplete. Holistic biophysical readouts capture cell responses label-free and in an unbiased fashion with the advantage to detect concealed aspects of cell signaling that are arduous to access via traditional biochemical assay approaches. In this chapter, we introduce the dynamic mass redistribution (DMR) technology to explore cell signaling beyond established receptor-controlled mechanisms. Both common and distinctive features in the signaling structure of cCMP and cUMP were identified. Moreover, the integrated response of whole live cells revealed a hitherto undisclosed additional effector of the noncanonical cNMPs. Future studies will show how holistic methods will become integrated into the methodological arsenal of contemporary cNMP research.

Cyclic Nucleotide Monophosphates in Plants and Plant Signaling

Cyclic nucleotide monophosphates (cNMPs) and the enzymes that can generate them are of increasing interest in the plant sciences. Arguably, the major recent advance came with the release of the complete Arabidopsis thaliana genome that has enabled the systematic search for adenylate (ACs) or guanylate cyclases (GCs) and did eventually lead to the discovery of a number of GCs in higher plants. Many of these proteins have complex domain architectures with AC or GC centers moonlighting within cytosolic kinase domains. Recent reports indicated the presence of not just the canonical cNMPs (i.e., cAMP and cGMP), but also the noncanonical cCMP, cUMP, cIMP, and cdTMP in plant tissues, and this raises several questions. Firstly, what are the functions of these cNMPs, and, secondly, which enzymes can convert the substrate triphosphates into the respective noncanonical cNMPs? The first question is addressed here by comparing the reactive oxygen species (ROS) response of cAMP and cGMP to that elicited by the noncanonical cCMP or cIMP. The results show that particularly cIMP can induce significant ROS production. To answer, at least in part, the second question, we have evaluated homology models of experimentally confirmed plant GCs probing the substrate specificity by molecular docking simulations to determine if they can conceivably catalytically convert substrates other than ATP or GTP. In summary, molecular modeling and substrate docking simulations can contribute to the evaluation of cyclases for noncanonical cyclic mononucleotides and thereby further our understanding of the molecular mechanism that underlie cNMP-dependent signaling in planta.

Conventional and Unconventional Mechanisms for Soluble Guanylyl Cyclase Signaling

Soluble guanylyl cyclase (sGC) is the principal enzyme in mediating the biological actions of nitric oxide. On activation, sGC converts guanosine triphosphate to guanosine 3',5'-cyclic monophosphate (cGMP), which mediates diverse physiological processes including vasodilation, platelet aggregation, and myocardial functions predominantly by acting on cGMP-dependent protein kinases. Cyclic GMP has long been considered as the sole second messenger for sGC action. However, emerging evidence suggests that, in addition to cGMP, other nucleoside 3',5'-cyclic monophosphates (cNMPs) are synthesized by sGC in response to nitric oxide stimulation, and some of these nucleoside 3',5'-cyclic monophosphates are involved in various physiological activities. For example, inosine 3',5'-cyclic monophosphate synthesized by sGC may play a critical role in hypoxic augmentation of vasoconstriction. The involvement of cytidine 3',5'-cyclic monophosphate and uridine 3',5'-cyclic monophosphate in certain cardiovascular activities is also implicated.

Distinct Signaling Roles of cIMP, cCMP, and cUMP

The cyclic purine nucleotide cIMP and the cyclic pyrimidine nucleotides cCMP and cUMP are emerging second messengers. These cNMPs show different biological effects, but the molecular mechanisms remain elusive. In this issue of Structure, Ng et al. (2016) provide structural evidence for distinct interactions of cIMP, cCMP, and cUMP with ion channels.

Biased activity of soluble guanylyl cyclase: the Janus face of thymoquinone

The natural compound thymoquinone, extracted from Nigella sativa (black cumin), is widely used in humans for its anti-oxidative properties. Thymoquinone is known for its acute endothelium-independent vasodilator effects in isolated rat aortae and pulmonary arteries, depending in part on activation of adenosine triphosphate-sensitive potassium channels and inhibition of voltage-dependent calcium channels. The compound also improves endothelial dysfunction in mesenteric arteries of ageing rodents and in aortae of rabbits treated with pyrogallol, by inhibiting oxidative stress. Serendipitously, thymoquinone was found to augment contractions in isolated arteries with endothelium of both rats and pigs. The endothelium-dependent augmentation it causes counterintuitively depends on biased activation of soluble guanylyl cyclase (sGC) producing inosine 3',5'-cyclic monophosphate (cyclic IMP) rather than guanosine 3',5'-cyclic monophosphate. This phenomenon shows a striking mechanistic similarity to the hypoxic augmentation previously observed in porcine coronary arteries. The cyclic IMP preferentially produced under thymoquinone exposure causes an increased contractility of arterial smooth muscle by interfering with calcium homeostasis. This brief review summarizes the vascular pharmacology of thymoquinone, focussing in particular on how the compound causes endothelium-dependent contractions by biasing the activity of sGC.

Novel protein kinase targets in vascular smooth muscle therapeutics

Many signaling factors have been identified over the years that serve as mechanistic foundations for the pathogenesis and/or maintenance of cardiovascular disease (CVD). Of these, cyclic nucleotide-driven protein kinases in vascular smooth muscle (VSM) are of essential importance. Comprised primarily of cyclic AMP-dependent and cyclic GMP-dependent protein kinases, these ubiquitous signaling molecules have capacity to operate through numerous downstream effectors including vasodilator-stimulated phosphoprotein (VASP) to control aberrant VSM growth elemental to CVD. As more information is gathered regarding genetic, biochemical, molecular and cellular makeup of CVD including VSM cyclic nucleotide-dependent protein kinases and VASP, advances will be made in precision medicine by identifying more precise therapeutic targets to enhance clinical decision making.

Homocysteine ameliorates the endothelium-independent hypoxic vasoconstriction via the suppression of phosphatidylinositol 3-kinase/Akt pathway in porcine coronary arteries

OBJECTIVE

Endothelium-independent coronary vasoconstriction induced by continuous hypoxia contributes to the development of ischemic heart diseases. Acute elevation of homocysteine (Hcy) has a potent of vasodilation. The present study aims to investigate the role of Hcy in endothelium-independent hypoxic coronary vasoconstriction and its underlying mechanisms.

METHODS AND RESULTS

Vessel tension of isolated porcine coronary arteries was measured by organ chamber study and the protein expression were detected by western blot. A sustained contraction of porcine coronary artery was induced when exposed to prolonged hypoxia for more than 15 min, which was significantly reduced by Hcy in a dose-dependent manner but not affected by cysteine or N-acetyl-l-cysteine. Phosphorylated myosin light chain (MLC-p) at Ser19 was decreased when exposure to hypoxia for 15 min, and could be reversed by prolonged hypoxia for 30 and 60 min. The recovery of MLC-p at Ser19 by hypoxia for more than 30 min could be abolished by Hcy. The protein levels of phosphorylated Akt at Ser473 and phosphorylated P85 at Tyr508 were decreased by Hcy in normoxia, and were also reduced exposure to hypoxia for 15 min and then augmented by prolonged hypoxia for more than 30 min, which could be prevented by Hcy. The protein level of P110α was not affected by Hcy or prolonged hypoxia.

CONCLUSIONS

This study demonstrates that Hcy can ameliorate the endothelium-independent hypoxic coronary vasoconstriction, in which the inhibition of PI3K/Akt signaling pathway may be involved.

Endothelial dysfunction and vascular disease - a 30th anniversary update

The endothelium can evoke relaxations of the underlying vascular smooth muscle, by releasing vasodilator substances. The best-characterized endothelium-derived relaxing factor (EDRF) is nitric oxide (NO) which activates soluble guanylyl cyclase in the vascular smooth muscle cells, with the production of cyclic guanosine monophosphate (cGMP) initiating relaxation. The endothelial cells also evoke hyperpolarization of the cell membrane of vascular smooth muscle (endothelium-dependent hyperpolarizations, EDH-mediated responses). As regards the latter, hydrogen peroxide (H₂ O₂ ) now appears to play a dominant role. Endothelium-dependent relaxations involve both pertussis toxin-sensitive G_i (e.g. responses to α₂ -adrenergic agonists, serotonin, and thrombin) and pertussis toxin-insensitive G_q (e.g. adenosine diphosphate and bradykinin) coupling proteins. New stimulators (e.g. insulin, adiponectin) of the release of EDRFs have emerged. In recent years, evidence has also accumulated, confirming that the release of NO by the endothelial cell can chronically be upregulated (e.g. by oestrogens, exercise and dietary factors) and downregulated (e.g. oxidative stress, smoking, pollution and oxidized low-density lipoproteins) and that it is reduced with ageing and in the course of vascular disease (e.g. diabetes and hypertension). Arteries covered with regenerated endothelium (e.g. following angioplasty) selectively lose the pertussis toxin-sensitive pathway for NO release which favours vasospasm, thrombosis, penetration of macrophages, cellular growth and the inflammatory reaction leading to atherosclerosis. In addition to the release of NO (and EDH, in particular those due to H₂ O₂ ), endothelial cells also can evoke contraction of the underlying vascular smooth muscle cells by releasing endothelium-derived contracting factors. Recent evidence confirms that most endothelium-dependent acute increases in contractile force are due to the formation of vasoconstrictor prostanoids (endoperoxides and prostacyclin) which activate TP receptors of the vascular smooth muscle cells and that prostacyclin plays a key role in such responses. Endothelium-dependent contractions are exacerbated when the production of nitric oxide is impaired (e.g. by oxidative stress, ageing, spontaneous hypertension and diabetes). They contribute to the blunting of endothelium-dependent vasodilatations in aged subjects and essential hypertensive and diabetic patients. In addition, recent data confirm that the release of endothelin-1 can contribute to endothelial dysfunction and that the peptide appears to be an important contributor to vascular dysfunction. Finally, it has become clear that nitric oxide itself, under certain conditions (e.g. hypoxia), can cause biased activation of soluble guanylyl cyclase leading to the production of cyclic inosine monophosphate (cIMP) rather than cGMP and hence causes contraction rather than relaxation of the underlying vascular smooth muscle.

What has passed is prolog: new cellular and physiological roles of G6PD

G6PD deficiency has been the most pervasive inherited disorder in the world since having been discovered. G6PD has an antioxidant role by functioning as a major nicotinamide adenine dinucleotide phosphate (NADPH) provider to reduce excessive oxidative stress. NADPH can produce reactive oxygen species (ROS) and reactive nitrogen species (RNS) mediated by NADPH oxidase (NOX) and nitric oxide synthase (NOS), respectively. Hence, G6PD also has a pro-oxidant role. Research in the past has focused on the enhanced susceptibility of G6PD-deficient cells or individuals to oxidative challenge. The cytoregulatory role of G6PD has largely been overlooked. By using a metabolomic approach, it is noted that upon oxidant challenge, G6PD-deficient cells will reprogram the GSH metabolism from regeneration to synthesis with exhaustive energy consumption. Recently, new cellular/physiologic roles of G6PD have been discovered. By using a proteomic approach, it has been found that G6PD plays a regulatory role in xenobiotic metabolism possibly via NOX and the redox-sensitive Nrf2-signaling pathway to modulate the expression of xenobiotic-metabolizing enzymes. Since G6PD is a key regulator responsible for intracellular redox homeostasis, G6PD deficiency can alter redox balance leading to many abnormal cellular effects such as the cellular inflammatory and immune response against viral infection. G6PD may play an important role in embryogenesis as G6PD-knockdown mouse cannot produce offspring and G6PD-deficient C. elegans with defective egg production and hatching. This array of findings indicates that the cellular and physiologic roles of G6PD, other than the classical role as an antioxidant enzyme, deserve further attention.

Endothelial and Smooth Muscle Cell Interactions in the Pathobiology of Pulmonary Hypertension

In the pulmonary vasculature, the endothelial and smooth muscle cells are two key cell types that play a major role in the pathobiology of pulmonary vascular disease and pulmonary hypertension. The normal interactions between these two cell types are important for the homeostasis of the pulmonary circulation, and any aberrant interaction between them may lead to various disease states including pulmonary vascular remodeling and pulmonary hypertension. It is well recognized that the endothelial cell can regulate the function of the underlying smooth muscle cell by releasing various bioactive agents such as nitric oxide and endothelin-1. In addition to such paracrine regulation, other mechanisms exist by which there is cross-talk between these two cell types, including communication via the myoendothelial injunctions and information transfer via extracellular vesicles. Emerging evidence suggests that these nonparacrine mechanisms play an important role in the regulation of pulmonary vascular tone and the determination of cell phenotype and that they are critically involved in the pathobiology of pulmonary hypertension.

Endothelium-Dependent Contractions of Isolated Arteries to Thymoquinone Require Biased Activity of Soluble Guanylyl Cyclase with Subsequent Cyclic IMP Production

Preliminary experiments on isolated rat arteries demonstrated that thymoquinone, a compound widely used for its antioxidant properties and believed to facilitate endothelium-dependent relaxations, as a matter of fact caused endothelium-dependent contractions. The present experiments were designed to determine the mechanisms underlying this unexpected response. Isometric tension was measured in rings (with and without endothelium) of rat mesenteric arteries and aortae and of porcine coronary arteries. Precontracted preparations were exposed to increasing concentrations of thymoquinone, which caused concentration-dependent, sustained further increases in tension (augmentations) that were prevented by endothelium removal, Nω-nitro-L-arginine methyl ester [L-NAME; nitric oxide (NO) synthase inhibitor], and 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; soluble guanylyl cyclase [sGC] inhibitor). In L-NAME-treated rings, the NO-donor diethylenetriamine NONOate restored the thymoquinone-induced augmentations; 5-[1-(phenylmethyl)-1H-indazol-3-yl]-2-furanmethanol (sGC activator) and cyclic IMP (cIMP) caused similar restorations. By contrast, in ODQ-treated preparations, the cell-permeable cGMP analog did not restore the augmentation by thymoquinone. The compound augmented the content (measured with ultra-high performance liquid chromatography-tandem mass spectrometry) of cIMP, but not that of cGMP; these increases in cIMP content were prevented by endothelium removal, L-NAME, and ODQ. The augmentation of contractions caused by thymoquinone was prevented in porcine arteries, but not in rat arteries, by 1-(5-isoquinolinylsulfonyl)homopiperazine dihydrochloride and trans-4-[(1R)-1-aminoethyl]-N-4-pyridinylcyclohexanecarboxamide dihydrochloride (Rho-kinase inhibitors); in the latter, but not in the former, it was reduced by 3,5-dichloro-N-[[(1α,5α,6-exo,6α)-3-(3,3-dimethylbutyl)-3-azabicyclo[3.1.0]hex-6-yl]methyl]-benzamide hydrochloride (T-type calcium channel inhibitor), demonstrating species/vascular bed differences in the impact of cIMP on calcium handling. Thymoquinone is the first pharmacological agent that causes endothelium-dependent augmentation of contractions of isolated arteries, which requires endothelium-derived NO and biased sGC activation, resulting in the augmented production of cIMP favoring the contractile process.

Hypoxic Vasospasm Mediated by cIMP: When Soluble Guanylyl Cyclase Turns Bad

In a number of isolated blood vessel types, hypoxia causes an acute contraction that is dependent on the presence of nitric oxide and activation of soluble guanylyl cyclase. It is more pronounced when the preparations are constricted and is therefore termed hypoxic augmentation of vasoconstriction. This hypoxic response is accompanied by increases in the intracellular level of inosine 5′-triphosphate and in the synthesis of inosine 3′,5′-cyclic monophosphate (cIMP) by soluble guanylyl cyclase. The administration of exogenous cIMP or inosine 5′-triphosphate causes augmented vasoconstriction to hypoxia. Furthermore, the vasoconstriction evoked by hypoxia and cIMP is associated with increased activity of Rho kinase (ROCK), indicating that cIMP may mediate the hypoxic effect by sensitizing the myofilaments to Ca²⁺ through ROCK. Hypoxia is implicated in exaggerated vasoconstriction in the pathogenesis of coronary artery disease, myocardial infarction, hypertension, and stroke. The newly found role of cIMP may help to identify unique therapeutic targets for certain cardiovascular disorders.

Endothelium-Independent Hypoxic Contraction Is Prevented Specifically by Nitroglycerin via Inhibition of Akt Kinase in Porcine Coronary Artery

Objective. Hypoxia-induced sustained contraction of porcine coronary artery is endothelium-independent and mediated by PI3K/Akt/Rho kinase. Nitroglycerin (NTG) is a vasodilator used to treat angina pectoris and acute heart failure. The present study was to determine the role of NTG in hypoxia-induced endothelium-independent contraction and the underlying mechanism. Methods and Results. Organ chamber technique was used to measure the isometric vessel tension of isolated porcine coronary arteries. Protein levels of phosphorylated and total Akt were determined by western blot. A sustained contraction of porcine coronary arteries induced by hypoxia was significantly reduced by NTG but not by isoproterenol. This contraction was also inhibited by DETA NONOate, 8-Br-cGMP, which can be reversed by ODQ, and Rp-8-Br-PET-cGMPS. The restored contraction was blocked by LY294002. The reduction of Akt-p at Ser-473 by NTG, DETA NONOate, and 8-Br-cGMP was significantly inhibited by ODQ, PKG-I. The decrease in Akt-p level by NTG and 8-Br-cGMP was prevented by calyculin A but not by okadaic acid. Conclusions. These results demonstrated that the endothelium-independent sustained hypoxic vasoconstriction can be prevented by NTG and that the inhibition of PI3K/Akt signaling pathway may be involved.

Temporal and organ-specific detection of cNMPs including cUMP in the zebrafish

The cyclic pyrimidine nucleotides cCMP and cUMP occur in mammalian cell lines. Recently, cCMP was also identified in mouse organs. Due to technical difficulties, it has not been possible to detect cUMP in organs or tissues yet. Here, we have generated a temporal profile of the occurrence of nucleoside 3',5'-cyclic monophosphates during different developmental stages of embryogenesis and in different organs of the adult zebrafish Danio rerio. Cyclic nucleotides were quantified by high performance liquid chromatography quadrupole tandem mass spectrometry. The identity of cCMP and cUMP in the zebrafish was confirmed by high performance liquid chromatography quadrupole time-of-flight mass spectrometry. We show for the first time that cUMP can be detected during embryogenesis and in adult organs of this vertebrate model system.

Vascular nitric oxide: Beyond eNOS

As the first discovered gaseous signaling molecule, nitric oxide (NO) affects a number of cellular processes, including those involving vascular cells. This brief review summarizes the contribution of NO to the regulation of vascular tone and its sources in the blood vessel wall. NO regulates the degree of contraction of vascular smooth muscle cells mainly by stimulating soluble guanylyl cyclase (sGC) to produce cyclic guanosine monophosphate (cGMP), although cGMP-independent signaling [S-nitrosylation of target proteins, activation of sarco/endoplasmic reticulum calcium ATPase (SERCA) or production of cyclic inosine monophosphate (cIMP)] also can be involved. In the blood vessel wall, NO is produced mainly from l-arginine by the enzyme endothelial nitric oxide synthase (eNOS) but it can also be released non-enzymatically from S-nitrosothiols or from nitrate/nitrite. Dysfunction in the production and/or the bioavailability of NO characterizes endothelial dysfunction, which is associated with cardiovascular diseases such as hypertension and atherosclerosis.

Selectivity of pharmacological tools: implications for use in cell physiology. A review in the theme: Cell signaling: proteins, pathways and mechanisms

Pharmacological inhibitors are frequently used to identify the receptors, receptor subtypes, and associated signaling pathways involved in physiological cell responses. Based on the effects of such inhibitors conclusions are drawn about the involvement of their assumed target or lack thereof. While such inhibitors can be useful tools for a better physiological understanding, their uncritical use can lead to incorrect conclusions. This article reviews the concept of inhibitor selectivity and its implication for cell physiology. Specifically, we discuss the implications of using inhibitor vs. activator approaches, issues of direct vs. indirect pathway modulation, implications of inverse agonism and biased signaling, and those of orthosteric vs. allosteric, competitive vs. noncompetitive, and reversible vs. irreversible inhibition. Additional problems can result from inconsistent estimates of inhibitor potency and differences in potency between cell-free systems and intact cells. These concepts are illustrated by several examples of inhibitors displaying affinity for related but distinct targets or even unrelated targets. Of note, many of the issues being addressed are also applicable to genetic inhibition strategies. The main practical conclusion following from these concepts is that investigators should be critical in the choice of inhibitor, its concentrations, and its mode of application. When this advice is adhered to, small-molecule pharmacological inhibitors can be important experimental tools in the hand of physiologists.

From canonical to non-canonical cyclic nucleotides as second messengers: pharmacological implications

This review summarizes our knowledge on the non-canonical cyclic nucleotides cCMP, cUMP, cIMP, cXMP and cTMP. We place the field into a historic context and discuss unresolved questions and future directions of research. We discuss the implications of non-canonical cyclic nucleotides for experimental and clinical pharmacology, focusing on bacterial infections, cardiovascular and neuropsychiatric disorders and reproduction medicine. The canonical cyclic purine nucleotides cAMP and cGMP fulfill the criteria of second messengers. (i) cAMP and cGMP are synthesized by specific generators, i.e. adenylyl and guanylyl cyclases, respectively. (ii) cAMP and cGMP activate specific effector proteins, e.g. protein kinases. (iii) cAMP and cGMP exert specific biological effects. (iv) The biological effects of cAMP and cGMP are terminated by phosphodiesterases and export. The effects of cAMP and cGMP are mimicked by (v) membrane-permeable cyclic nucleotide analogs and (vi) bacterial toxins. For decades, the existence and relevance of cCMP and cUMP have been controversial. Modern mass-spectrometric methods have unequivocally demonstrated the existence of cCMP and cUMP in mammalian cells. For both, cCMP and cUMP, the criteria for second messenger molecules are now fulfilled as well. There are specific patterns by which nucleotidyl cyclases generate cNMPs and how they are degraded and exported, resulting in unique cNMP signatures in biological systems. cNMP signaling systems, specifically at the level of soluble guanylyl cyclase, soluble adenylyl cyclase and ExoY from Pseudomonas aeruginosa are more promiscuous than previously appreciated. cUMP and cCMP are evolutionary new molecules, probably reflecting an adaption to signaling requirements in higher organisms.

A facile and sensitive method for quantification of cyclic nucleotide monophosphates in mammalian organs: basal levels of eight cNMPs and identification of 2',3'-cIMP

A sensitive, versatile and economical method to extract and quantify cyclic nucleotide monophosphates (cNMPs) using LC-MS/MS, including both 3',5'-cNMPs and 2',3'-cNMPs, in mammalian tissues and cellular systems has been developed. Problems, such as matrix effects from complex biological samples, are addressed and have been optimized. This protocol allows for comparison of multiple cNMPs in the same system and was used to examine the relationship between tissue levels of cNMPs in a panel of rat organs. In addition, the study reports the first identification and quantification of 2',3'-cIMP. The developed method will allow for quantification of cNMPs levels in cells and tissues with varying disease states, which will provide insight into the role(s) and interplay of cNMP signalling pathways.

Rho-kinase as a therapeutic target in vascular diseases: striking nitric oxide signaling

Rho GTPases are a globular, monomeric group of small signaling G-protein molecules. Rho-associated protein kinase/Rho-kinase (ROCK) is a downstream effector protein of the Rho GTPase. Rho-kinases are the potential therapeutic targets in the treatment of cardiovascular diseases. Here, we have primarily discussed the intriguing roles of ROCK in cardiovascular health in relation to nitric oxide signaling. Further, we highlighted the biphasic effects of Y-27632, a ROCK inhibitor under shear stress, which acts as an agonist of nitric oxide production in endothelial cells. The biphasic effects of this inhibitor raised the question of safety of the drug usage in treating cardiovascular diseases.