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

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.

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.

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.

cCMP and cUMP Across the Tree of Life: From cCMP and cUMP Generators to cCMP- and cUMP-Regulated Cell Functions

The cyclic purine nucleotides cAMP and cGMP are well-established second messenger molecules that are generated by distinct nucleotidyl cyclases (NCs) and regulate numerous cell functions via specific effector molecules. In contrast, the existence of the cyclic pyrimidine nucleotides cCMP and cUMP has been controversial for many years. The development of highly specific and sensitive mass spectrometry methods has enabled the unequivocal detection and quantitation of cCMP and cUMP in biological systems. These cNMPs occur broadly in numerous mammalian cell lines and primary cells. cCMP has also been detected in mouse organs, and both cCMP and cUMP occur in various developmental stages of the zebrafish Danio rerio. So far, the soluble guanylyl cyclase (sGC) and soluble adenylyl cyclase (sAC) have been identified as cCMP and cUMP generators. Dissociations in the expression patterns of sAC and sGC relative to cCMP and cUMP abundance may point to the existence of hitherto unidentified cCMP- and cUMP-generating NCs. The broad occurrence of cCMP and cUMP in vertebrates and the distinct cNMP patterns suggest specific roles of these cNMPs in the regulation of numerous cell functions.

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.