Stimulation Of cGMP Synthesis Research Articles

PMON52 Use of PDE5 Inhibitors as Potential Treatment for Isolated Growth Hormone Deficiency Caused by Alternate Splicing of GH1 Gene

Abstract Mutations in the GH1 gene cause isolated growth hormone deficiency (IGHD) by affecting production, secretion, and stability of growth hormone as well as its binding to GHR. Genetic mutations resulting from splicing errors are known to increase the production of a small 17.5 kD isoform of human growth hormone. This smaller isoform of GH1 is inactive and is linked to IGHD. While studying the impact of small GH isoform on GH production and secretion, we noticed that cells transfect with sequences carrying the mutated GH gene that resulted in splicing defects produce the short 17.5 kD version of GH, looked very different from the control cells with WT GH gene. We found that production of short GH isoform distorts the cell morphology, contributing to detrimental effects seen in the patients with mutations in GH1 gene causing alternate splicing. We hypothesized that cellular distortion caused by small GH isoform and overall GH secretion can be a target and a tool to experimental approaches for reversing the damaging effects of small GH protein, thereby providing potential treatments for IGHD caused by splicing errors. It is known for some time that increasing cAMP enhances 26S proteasome activity and the degradation of cell proteins, including the selective breakdown of misfolded proteins, and recently increase in cGMP levels has also been shown to regulate protein degradation. Inhibitors of phosphodiesterase 5 (PDE5; e.g., Viagra (sildenafil)), which raise cGMP by inhibiting its hydrolysis to GMP, are widely used for the treatment of erectile dysfunction and pulmonary hypertension, and stimulators of soluble guanylyl cyclases (riociguat), which stimulate cGMP synthesis, are used as treatments for pulmonary hypertension and heart failure. Using our model of cellular morphology changes, have investigated the impact of chemicals that enhance cGMP levels by inhibiting PDE5 or activating guanyl cyclases. The cells expressing small isoform of GH consistently show distorted rounded morphology. Treatment with BAY 41-2272, a cGMP inducer slightly improved the cell morphology in small isoform GH expressing cells. BAY 41-2272 did not show any adverse effects of normal and WT GH expressing cells. Treatment of sildenafil to cells expressing 17.5 kD isoform of GH markedly improved the cell morphology compared to untreated cells, especially after 48h when a larger percentage of cells that were distorted (rounder shapes) regained the shape of normal cells expressing WT GH and no damage to cells expressing WT-GH was observed. Another PDE5 inhibitor tadalafil also caused a marked improvement in cell morphology caused by expression of small GH isoform. Overall, it seems the strategy to regulate the cGMP levels in cells transfected in small GH isoform worked and needs further investigation. A detailed analysis of the impact of PDE5 inhibitors on GH expression and secretion will be presented. Presentation: Monday, June 13, 2022 12:30 p.m. - 2:30 p.m.

How Is Peripheral Injury Signaled to Satellite Glial Cells in Sensory Ganglia?

Injury or inflammation in the peripheral branches of neurons of sensory ganglia causes changes in neuronal properties, including excessive firing, which may underlie chronic pain. The main types of glial cell in these ganglia are satellite glial cells (SGCs), which completely surround neuronal somata. SGCs undergo activation following peripheral lesions, which can enhance neuronal firing. How neuronal injury induces SGC activation has been an open question. Moreover, the mechanisms by which the injury is signaled from the periphery to the ganglia are obscure and may include electrical conduction, axonal and humoral transport, and transmission at the spinal level. We found that peripheral inflammation induced SGC activation and that the messenger between injured neurons and SGCs was nitric oxide (NO), acting by elevating cyclic guanosine monophosphate (cGMP) in SGCs. These results, together with work from other laboratories, indicate that a plausible (but not exclusive) mechanism for neuron-SGCs interactions can be formulated as follows: Firing due to peripheral injury induces NO formation in neuronal somata, which diffuses to SGCs. This stimulates cGMP synthesis in SGCs, leading to their activation and to other changes, which contribute to neuronal hyperexcitability and pain. Other mediators such as proinflammatory cytokines probably also contribute to neuron-SGC communications.

Repurposing Riociguat to Target a Novel Paracrine Nitric Oxide-TRPC6 Pathway to Prevent Podocyte Injury.

Increased expression and activity of the Ca2+ channel transient receptor potential channel 6 (TRPC6) is associated with focal segmental glomerulosclerosis, but therapeutic strategies to target TRPC6 are currently lacking. Nitric oxide (NO) is crucial for normal glomerular function and plays a protective role in preventing glomerular diseases. We investigated if NO prevents podocyte injury by inhibiting injurious TRPC6-mediated signaling in a soluble guanylate cyclase (sGC)-dependent manner and studied the therapeutic potential of the sGC stimulator Riociguat. Experiments were performed using human glomerular endothelial cells and podocytes. Podocyte injury was induced by Adriamycin incubation for 24 h, with or without the NO-donor S-Nitroso-N-acetyl-DL-penicillamine (SNAP), the sGC stimulator Riociguat or the TRPC6 inhibitor Larixyl Acetate (LA). NO and Riociguat stimulated cGMP synthesis in podocytes, decreased Adriamycin-induced TRPC6 expression, inhibited the Adriamycin-induced TRPC6-mediated Ca2+ influx and reduced podocyte injury. The protective effects of Riociguat and NO were blocked when sGC activity was inhibited with 1H-[1,2,4]Oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) or when TRPC6 activity was inhibited by LA. Our data demonstrate a glomerular (e)NOS-NO-sGC-cGMP-TRPC6 pathway that prevents podocyte injury, which can be translated to future clinical use by, e.g., repurposing the market-approved drug Riociguat.

Muscarinic regulation of the neuronal Na+/K+‐ATPase in rat hippocampus

Stimulation of postsynaptic muscarinic receptors was shown to excite principal hippocampal neurons by modulating several membrane ion conductances. We show here that activation of postsynaptic muscarinic receptors also causes neuronal excitation by inhibiting Na+ /K+ -ATPase activity. Muscarinic Na+ /K+ -ATPase inhibition is mediated by two separate signalling pathways that lead downstream to enhanced Na+ /K+ -ATPase phosphorylation by activating protein kinase C and protein kinase G. Muscarinic excitation through Na+ /K+ -ATPase inhibition is probably involved in cholinergic modulation of hippocampal activity and may turn out to be a widespread mechanism of neuronal excitation in the brain. Stimulation of muscarinic cholinergic receptors on principal hippocampal neurons enhances intrinsic neuronal excitability by modulating several membrane ion conductances. The electrogenic Na+ /K+ -ATPase (NKA; the 'Na+ pump') is a ubiquitous regulator of intrinsic neuronal excitability, generating a hyperpolarizing current to thwart excessive neuronal firing. Using electrophysiological and pharmacological methodologies in rat hippocampal slices, we show that neuronal NKA pumping activity is also subjected to cholinergic regulation. Stimulation of postsynaptic muscarinic, but not nicotinic, cholinergic receptors activates membrane-bound phospholipase C and hydrolysis of membrane-integral phosphatidylinositol 4,5-bisphosphate into diacylglycerol (DAG) and inositol 1,4,5-triphosphate (IP3 ). Along one signalling pathway, DAG activates protein kinase C (PKC). Along a second signalling pathway, IP3 causes Ca2+ release from the endoplasmic reticulum, facilitating nitric oxide (NO) production. The rise in NO levels stimulates cGMP synthesis by guanylate-cyclase, activating protein kinase G (PKG). The two pathways converge to cause partial NKA inhibition through enzyme phosphorylation by PKC and PKG, leading to a marked increase in intrinsic neuronal excitability. This novel mechanism of neuronal NKA regulation probably contributes to the cholinergic modulation of hippocampal activity in spatial navigation, learning and memory.

Male mice with elevated C-type natriuretic peptide-dependent guanylyl cyclase-B activity have increased osteoblasts, bone mass and bone strength.

C-type natriuretic peptide (CNP) activation of guanylyl cyclase (GC)-B, also known as NPR2, stimulates cGMP synthesis and bone elongation. CNP activation requires the phosphorylation of multiple GC-B residues and dephosphorylation inactivates the receptor. GC-B7E/7E knockin mice, expressing a glutamate-substituted, "pseudophosphorylated," form of GC-B, exhibit increased CNP-dependent GC activity. Since mutations that constitutively activate GC-B in the absence of CNP result in low bone mineral density in humans, we determined the skeletal phenotype of 9-week old male GC-B7E/7E mice. Unexpectedly, GC-B7E/7E mice have significantly greater tibial and L5 vertebral trabecular bone volume fraction, tibial trabecular number, and tibial bone mineral density. Cortical cross-sectional area, cortical thickness, periosteal diameter and cortical cross-sectional moment of inertia were also significantly increased in GC-B7E/7E tibiae. Three-point bending measurements demonstrated that the mutant tibias and femurs had greater ultimate load, stiffness, energy to ultimate load, and energy to failure. No differences in microhardness indicated similar bone quality at the tissue level between the mutant and wildtype bones. Procollagen 1 N-terminal propeptide and osteocalcin were elevated in serum, and osteoblast number per bone perimeter and osteoid width per bone perimeter were elevated in tibias from the mutant mice. In contrast to mutations that constitutively activate GC-B, we report that mutations that enhance GC-B activity only in the presence of its natural ligand, increase bone mass, bone strength, and the number of active osteoblasts at the bone surface.

Contribution of Thyrotropin-Releasing Hormone to Cerebellar Long-Term Depression and Motor Learning.

Thyrotropin-releasing hormone (TRH) regulates various physiological activities through activation of receptors expressed in a broad range of cells in the central nervous system. The cerebellum expresses TRH receptors in granule cells and molecular layer interneurons. However, the function of TRH in the cerebellum remains to be clarified. Here, using TRH knockout (KO) mice we studied the role of TRH in the cerebellum. Immunohistochemistry showed no gross morphological differences between KO mice and wild-type (WT) littermates in the cerebellum. In the rotarod test, the initial performance of KO mice was comparable to that of WT littermates, but the learning speed of KO mice was significantly lower than that of WT littermates, suggesting impaired motor learning. The motor learning deficit in KO mice was rescued by intraperitoneal injection of TRH. Electrophysiology revealed absence of long-term depression (LTD) at parallel fiber-Purkinje cell synapses in KO mice, which was rescued by bath-application of TRH. TRH was shown to increase cyclic guanosine monophosphate (cGMP) content in the cerebellum. Since nitric oxide (NO) stimulates cGMP synthesis in the cerebellum, we examined whether NO-cGMP pathway was involved in TRH-mediated LTD rescue in KO mice. Pharmacological blockade of NO synthase and subsequent cGMP production prevented TRH-induced LTD expression in KO mice, whereas increase in cGMP signal in Purkinje cells by 8-bromoguanosine cyclic 3’,5’-monophosphate, a membrane-permeable cGMP analog, restored LTD without TRH application. These results suggest that TRH is involved in cerebellar LTD presumably by upregulating the basal cGMP level in Purkinje cells, and, consequently, in motor learning.

Angiotensin II-mediated hypertension impairs nitric oxide-induced NKCC2 inhibition in thick ascending limbs.

Δ-41 ± 10%). NO increased cGMP by 2.08 ± 0.36 fmol/μg protein in THALs from vehicle-treated rats but only 1.06 ± 0.25 fmol/μg protein in ANG II-hypertensive rats (P < 0.04). Vardenafil (25 nM), a PDE5 inhibitor, restored NO's ability to inhibit NKCC2 activity in THALs from ANG II-hypertensive rats (Δ: -60 ± 9%, P < 0.003). Similarly, NO's stimulation of cGMP was also restored by vardenafil (vehicle-treated: 1.89 ± 0.71 vs. ANG II-hypertensive: 2.02 ± 0.32 fmol/μg protein). PDE5 expression did not differ between vehicle-treated and ANG II-hypertensive rats. We conclude that NO-induced inhibition of NKCC2 and increases in cGMP are blunted in ANG II-hypertensive rats due to PDE5 activation. Defects in the response of THALs to NO may enhance NaCl retention in ANG II-induced hypertension.

Differential synergy with nitric oxide across a panel of soluble guanylate cyclase stimulators

Soluble guanylate cyclase (sGC) is the endogenous receptor for nitric oxide (NO). Upon binding of NO, sGC catalyzes the synthesis of cGMP which is involved in numerous downstream processes, including vasorelaxation, fibrosis, and inflammation. Dysregulation of this pathway has been implicated in multiple pathologies. sGC stimulators are a class of molecules which are capable of increasing the activity of the enzyme. While these stimulators are capable of activating the enzyme in the absence of NO, their effect is greatly amplified in the presence of NO. However, it is unclear if all stimulators synergize with NO equally. To examine differences in NO synergy across stimulators, we assessed a panel of compounds for their ability to stimulate cGMP synthesis with or without NO in a human whole cell assay. All compounds selected for this analysis were potent in vitro under high NO conditions (with EC50 values ranging from 10 to 200nM) and were effective at lowering blood pressure in rodents at doses between 1 and 10 mg/kg. We found that all compounds tested had similar EC50 values (~5µM) when examined in the absence of NO. However, in the presence of high NO the compounds exhibited a wide range of potency shifts relative to the NO‐free state (16‐320 fold). Emax values for the compounds were also differentially affected by NO, with some compounds reaching the same Emax regardless of the presence of NO, and others exhibiting a 4‐6 fold increase in Emax upon addition of NO. These data suggest that synergy between NO and sGC stimulators is complex and varies considerably between compounds. We hypothesize that differences between compounds in NO synergy may be harnessed to maximize their therapeutic value in a variety of disease states and patient populations.

Sildenafil does not enhance but rather attenuates vasorelaxant effects of antidiabetic agents

Type 2 diabetic men commonly experience erectile dysfunction for which phosphodiesterase-5 (PDE5) inhibitors like sildenafil (Viagra) are often recommended. By preventing degradation of cyclic guanosine monophosphate (cGMP) in vascular smooth muscle, these inhibitors also enhance arterial vasorelaxant effects of nitric oxide donors (which stimulate cGMP synthesis). In the present work, we confirmed this enhancing effect after co-administration of sildenafil with nitroprusside to freshly-isolated rat tail arterial tissues. However, in the same tissues we also observed that sildenafil does not enhance but rather attenuates vasorelaxant effects of three commonly-used antidiabetic drugs, i.e. the biguanide metformin and the thiazolidinediones pioglitazone and rosiglitazone. Indeed, sildenafil completely blocked vasorelaxant effects of low concentrations of these drugs. In addition, we found that this same novel anti-vasorelaxant interaction of sildenafil with these agents was abolished by either 1) omitting extracellular glucose or 2) inhibiting specific smooth muscle glycolytic pathways; pathways known to preferentially utilize extracellular glucose to fuel certain adenosine triphosphate (ATP)-dependent ion transporters: e.g. ATP-sensitive K channels, sarcoplasmic reticulum Ca-ATPase, plasma membrane Ca-ATPase and Na/K-ATPase. Accordingly, we suspect that altered activity of one or more of these ion transporters mediates the observed attenuating (anti-vasorelaxant) interaction of sildenafil with the antidiabetic drugs. The present results are relevant because hypertension is so common and difficult to control in Type 2 diabetes. The present data suggest that sildenafil might interfere with the known antihypertensive potential of metformin and the thiazolidinediones. However, they do not suggest that it will interact with them to cause life-threatening episodes of severe hypotension, as can occur when it is co-administered with nitrates.

Phosphodiesterase-2 Is Up-Regulated in Human Failing Hearts and Blunts β-Adrenergic Responses in Cardiomyocytes

This study investigated whether myocardial phosphodiesterase-2 (PDE2) is altered in heart failure (HF) and determined PDE2-mediated effects on beta-adrenergic receptor (β-AR) signaling in healthy and diseased cardiomyocytes. Diminished cyclic adenosine monophosphate (cAMP) and augmented cyclic guanosine monophosphate (cGMP) signaling is characteristic for failing hearts. Among the PDE superfamily, PDE2 has the unique property of being able to be stimulated by cGMP, thus leading to a remarkable increase in cAMP hydrolysis mediating a negative cross talk between cGMP and cAMP signaling. However, the role of PDE2 in HF is poorly understood. Immunoblotting, radioenzymatic- and fluorescence resonance energy transfer-based assays, video edge detection, epifluorescence microscopy, and L-type Ca2(+) current measurements were performed in myocardial tissues and/or isolated cardiomyocytes from human and/or experimental HF, respectively. Myocardial PDE2 expression and activity were ~2-fold higher in advanced human HF. Chronic β-AR stimulation via catecholamine infusions in rats enhanced PDE2 expression ~2-fold and cAMP hydrolytic activity ~4-fold, which correlated with blunted cardiac β-AR responsiveness. In diseased cardiomyocytes, higher PDE2 activity could be further enhanced by stimulation of cGMP synthesis via nitric oxide donors, whereas specific PDE2 inhibition partially restored β-AR responsiveness. Accordingly, PDE2 overexpression in healthy cardiomyocytes reduced the rise in cAMP levels and L-type Ca2(+) current amplitude, and abolished the inotropic effect following acute β-AR stimulation, without affecting basal contractility. Importantly, PDE2-overexpressing cardiomyocytes showed marked protection from norepinephrine-induced hypertrophic responses. PDE2 is markedly up-regulated in failing hearts and desensitizes against acute β-AR stimulation. This may constitute an important defense mechanism during cardiac stress, for example, by antagonizing excessive β-AR drive. Thus, activating myocardial PDE2 may represent a novel intracellular antiadrenergic therapeutic strategy in HF.

Biphasic roles for soluble guanylyl cyclase (sGC) in platelet activation

AbstractNitric oxide (NO) stimulates cGMP synthesis by activating its intracellular receptor, soluble guanylyl cyclase (sGC). It is a currently prevailing concept that No and cGMP inhibits platelet function. However, the data supporting the inhibitory role of NO/sGC/cGMP in platelets have been obtained either in vitro or using whole body gene deletion that affects vessel wall function. Here we have generated mice with sGC gene deleted only in megakaryocytes and platelets. Using the megakaryocyte- and platelet-specific sGC-deficient mice, we identify a stimulatory role of sGC in platelet activation and in thrombosis in vivo. Deletion of sGC in platelets abolished cGMP production induced by either NO donors or platelet agonists, caused a marked defect in aggregation and attenuated secretion in response to low doses of collagen or thrombin. Importantly, megakaryocyte- and platelet-specific sGC deficient mice showed prolonged tail-bleeding times and impaired FeCl3-induced carotid artery thrombosis in vivo. Interestingly, the inhibitory effect of the NO donor SNP on platelet activation was sGC-dependent only at micromolar concentrations, but sGC-independent at millimolar concentrations. Together, our data demonstrate important roles of sGC in stimulating platelet activation and in vivo thrombosis and hemostasis, and sGC-dependent and -independent inhibition of platelets by NO donors.

SGCα1 mediates the negative inotropic effects of NO in cardiac myocytes independent of changes in calcium handling

In the heart, nitric oxide (NO) modulates contractile function; however, the mechanisms responsible for this effect are incompletely understood. NO can elicit effects via a variety of mechanisms including S-nitrosylation and stimulation of cGMP synthesis by soluble guanylate cyclase (sGC). sGC is a heterodimer comprised of a β(1)- and an α(1)- or α(2)-subunit. sGCα(1)β(1) is the predominant isoform in the heart. To characterize the role of sGC in the regulation of cardiac contractile function by NO, we compared left ventricular cardiac myocytes (CM) isolated from adult mice deficient in the sGC α(1)-subunit (sGCα(1)(-/-)) and from wild-type (WT) mice. Sarcomere shortening under basal conditions was less in sGCα(1)(-/-) CM than in WT CM. To activate endogenous NO synthesis from NO synthase 3, CM were incubated with the β(3)-adrenergic receptor (β(3)-AR) agonist BRL 37344. BRL 37344 decreased cardiac contractility in WT CM but not in sGCα(1)(-/-) myocytes. Administration of spermine NONOate, an NO donor compound, did not affect sarcomeric shortening in CM of either genotype; however, in the presence of isoproterenol, addition of spermine NONOate reduced sarcomere shortening in WT but not in sGCα(1)(-/-) CM. Neither BRL 37344 nor spermine NONOate altered calcium handling in CM of either genotype. These findings suggest that sGCα(1) exerts a positive inotropic effect under basal conditions, as well as mediates the negative inotropic effect of β(3)-AR signaling. Additionally, our work demonstrates that sGCα(1)β(1) is required for NO to depress β(1)/β(2)-AR-stimulated cardiac contractility and that this modulation is independent of changes in calcium handling.

Myocardial protection against reperfusion injury: The cGMP pathway

Reperfusion injury may cause myocardial cell death and limit the benefit achieved by restoration of coronary artery patency in patients with acute myocardial infarction. The mechanism includes altered Ca(2+) handling with cytosolic and mitochondrial Ca(2+) overload, Ca(2+)- and ATP-dependent hypercontraction, cytoskeletal fragility, mitochondrial permeability transition and gap junction-mediated propagation of cell death, as well as alterations in non-cardiomyocyte cells, in particular platelets and endothelial cells. cGMP modulates favorably all these mechanism, mainly through PKG-mediated actions, but cGMP synthesis is altered in reperfused cardiomyocytes and endothelial cells by mechanisms that are only partially understood. Stimulation of cGMP synthesis during initial reperfusion by means of natriuretic peptides has been found protective in different animal models and in patients. Moreover, increasing evidence indicates that cGMP is an important step in signal transduction of endogenous cardioprotection. Thus, the cGMP pathway appears as a key element in the pathophysiology of myocardial ischaemia-reperfusion and as a promising therapeutic target in patients with acute myocardial infarction.

Ammonia inhibits the C-type natriuretic peptide-dependent cyclic GMP synthesis and calcium accumulation in a rat brain endothelial cell line

Recently we reported a decrease of C-type natriuretic peptide (CNP)-dependent, natriuretic peptide receptor 2 (NPR2)-mediated cyclic GMP (cGMP) synthesis in a non-neuronal compartment of cerebral cortical slices of hyperammonemic rats [Zielińska, M., Fresko, I., Konopacka, A., Felipo, V., Albrecht, J., 2007. Hyperammonemia inhibits the natriuretic peptide receptor 2 (NPR2)-mediated cyclic GMP synthesis in the astrocytic compartment of rat cerebral cortex slices. Neurotoxicology 28, 1260–1263]. Here we accounted for the possible involvement of cerebral capillary endothelial cells in this response by measuring the effect of ammonia on the CNP-mediated cGMP formation and intracellular calcium ([Ca 2+] i ) accumulation in a rat cerebral endothelial cell line (RBE-4). We first established that stimulation of cGMP synthesis in RBE-4 cells was coupled to protein kinase G (PKG)-mediated Ca 2+ influx from the medium which was inhibited by an L-type channel blocker nimodipine. Ammonia treatment (1 h, 5 mM NH 4Cl) evoked a substantial decrease of CNP-stimulated cGMP synthesis which was related to a decreased binding of CNP to NPR2 receptors, and depressed the CNP-dependent [Ca 2+] i accumulation in these cells. Ammonia also abolished the CNP-dependent Ca 2+ accumulation in the absence of Na +. In cells incubated with ammonia in the absence of Ca 2+ a slight CNP-dependent increase of [Ca 2+] i was observed, most likely representing Ca 2+ release from intracellular stores. Depression of CNP-dependent cGMP-mediated [Ca 2+] i accumulation may contribute to cerebral vascular endothelial dysfunction associated with hyperammonemia or hepatic encephalopathy.

Rapid monitoring of intracellular cGMP

In order to observe the spatial and temporal pattern of intracellular signalling in intact cells, we have developed a variety of sensors that are based on fluorescence resonance energy transfer (FRET) between CFP and YFP, or similar fluorophores, fused to various signaling proteins [1]. For the detection of the second messengers we have developed similar sensors based on cAMP or cGMP binding domains derived from various proteins that were likewise fused to CFP and YFP or analogous fluorescent proteins. These sensors respond to changes in intracellular second messenger concentrations with changes in FRET. For cGMP, various sensors were developed based on fragments of cGMP-dependent protein kinase or regulatory GAF-domains, which differed in amplitude, speed of reaction and in sensitivity to cGMP [2]. When expressed in primary cells or cell culture lines, these sensors allowed spatially and temporally resolved imaging of cGMP concentrations in intact cells. More rapid variants of these sensors also permit the monitoring of oscillating changes in cGMP levels, generated, for example, by pulsatile stimulation of cGMP synthesis by sodium nitroprusside.

A luminescence-based assay for sensitive nitric oxide detection

Nitric oxide (NO) plays an important role in the protection against the onset and progression of various cardiovascular diseases, including hypertension and atherosclerosis, which are associated with an apparently reduced NO bioavailability. Therefore, the NO/cGMP signaling pathway has gained considerable attention and has become a target for new drug development. We have established a rapid, homogeneous, cell-based and highly sensitive nitric oxide reporter assay which is suitable for ultra-high-throughput screening. In our coculture system, endothelial nitric oxide synthase (eNOS) mediated NO generation is monitored in living cells via soluble guanylyl cyclase (sGC) activation and calcium influx through the olfactory cyclic nucleotide-gated (CNG) cation channel CNGA2, acting as the intracellular cGMP sensor [1]. Using this NO reporter assay, a fully automated highthroughput screening campaign for stimulators of NO synthesis was performed. The coculture system reflects most aspects of the natural NO/cGMP signaling pathway. Namely, Ca2+-dependent and Ca2+-independent regulation of eNOS activity by G protein-coupled receptor agonists, oxidative stress, phosphorylation, and cofactor availability, as well as NO-mediated stimulation of cGMP synthesis by sGC activation. The reporter assay allows the real-time detection of NO synthesis within living cells and makes it possible to identify and characterize activators and inhibitors of enzymes involved in the NO/cGMP pathway.

Hyperammonemia inhibits the natriuretic peptide receptor 2 (NPR-2)-mediated cyclic GMP synthesis in the astrocytic compartment of rat cerebral cortex slices

The decrease of cyclic GMP (cGMP) level in the brain, contributing to cognitive and memory deficit in hyperammonemia (HA), has been attributed to the interference of ammonia with the NMDA/nitric oxide/soluble guanylate cyclase (GC)/cGMP pathway in neurons. The present study tested the hypotheses that (a) HA also affects cGMP synthesis elicited by stimulation of the natriuretic peptide receptor 2 (NPR-2) with its natural ligand, C-type natriuretic peptide (CNP) and (b) the latter effect may involve astrocytes, the ammonia-sensitive cells. In the cerebral cortical slices of control rats, CNP stimulated cGMP synthesis in a degree comparable to the NO donor, S-nitroso- N-acetylpenicillamine (SNAP) used at an optimal concentration. Fluoroacetate (FA), a metabolic inhibitor specifically affecting astrocytic mitochondria, inhibited the CNP-dependent cGMP synthesis by about 50%. Ammonium acetate-induced HA decreased by 68% the CNP-dependent cGMP generation in slices incubated in the absence of FA. In slices incubated in the presence of FA, cGMP synthesis in slices derived from HA rats did not differ from that in control slices. The results indicate that HA inhibits CNP-dependent cGMP synthesis in the FA-vulnerable, astrocytic compartment, but not in the FA-resistant compartment(s) of the brain. HA did not affect the expression of NPR-2 mRNA in the cerebral cortex tissue as tested using real-time PCR, indicating that the effect of ammonia involves as yet unidentified events occurring posttranscriptionally. Deregulation of NPR-2 function in astrocytes by ammonia may contribute to neurophysiological symptoms of HA.

Thrombospondin-1 Inhibits Nitric Oxide Signaling via CD36 by Inhibiting Myristic Acid Uptake

Although CD36 is generally recognized to be an inhibitory signaling receptor for thrombospondin-1 (TSP1), the molecular mechanism for transduction of this signal remains unclear. Based on evidence that myristic acid and TSP1 each modulate endothelial cell nitric oxide signaling in a CD36-dependent manner, we examined the ability of TSP1 to modulate the fatty acid translocase activity of CD36. TSP1 and a CD36 antibody that mimics the activity of TSP1 inhibited myristate uptake. Recombinant TSP1 type 1 repeats were weakly inhibitory, but an anti-angiogenic peptide derived from this domain potently inhibited myristate uptake. This peptide also inhibited membrane translocation of the myristoylated CD36 signaling target Fyn and activation of Src family kinases. Myristate uptake stimulated cGMP synthesis via endothelial nitric-oxide synthase and soluble guanylyl cyclase. CD36 ligands blocked myristate-stimulated cGMP accumulation in proportion to their ability to inhibit myristate uptake. TSP1 also inhibited myristate-stimulated cGMP synthesis by engaging its receptor CD47. Myristate stimulated endothelial and vascular smooth muscle cell adhesion on type I collagen via the NO/cGMP pathway, and CD36 ligands that inhibit myristate uptake blocked this response. Therefore, the fatty acid translocase activity of CD36 elicits proangiogenic signaling in vascular cells, and TSP1 inhibits this response by simultaneously inhibiting fatty acid uptake via CD36 and downstream cGMP signaling via CD47.

Natriuretic peptides and nitric oxide stimulate cGMP synthesis in different cellular compartments.

Cyclic nucleotide-gated (CNG) channels are a family of ion channels activated by the binding of cyclic nucleotides. Endogenous channels have been used to measure cyclic nucleotide signals in photoreceptor outer segments and olfactory cilia for decades. Here we have investigated the subcellular localization of cGMP signals by monitoring CNG channel activity in response to agonists that activate either particulate or soluble guanylyl cyclase. CNG channels were heterologously expressed in either human embryonic kidney (HEK)-293 cells that stably overexpress a particulate guanylyl cyclase (HEK-NPRA cells), or cultured vascular smooth muscle cells (VSMCs). Atrial natriuretic peptide (ANP) was used to activate the particulate guanylyl cyclase and the nitric oxide donor S-nitroso-n-acetylpenicillamine (SNAP) was used to activate the soluble guanylyl cyclase. CNG channel activity was monitored by measuring Ca2+ or Mn2+ influx through the channels using the fluorescent dye, fura-2. We found that in HEK-NPRA cells, ANP-induced increases in cGMP levels activated CNG channels in a dose-dependent manner (0.05–10 nM), whereas SNAP (0.01–100 μM) induced increases in cGMP levels triggered little or no activation of CNG channels (P < 0.01). After pretreatment with 100 μM 3-isobutyl-1-methylxanthine (IBMX), a nonspecific phosphodiesterase inhibitor, ANP-induced Mn2+ influx through CNG channels was significantly enhanced, while SNAP-induced Mn2+ influx remained small. In contrast, we found that in the presence of IBMX, both 1 nM ANP and 100 μM SNAP triggered similar increases in total cGMP levels. We next sought to determine if cGMP signals are compartmentalized in VSMCs, which endogenously express particulate and soluble guanylyl cyclase. We found that 10 nM ANP induced activation of CNG channels more readily than 100 μM SNAP; whereas 100 μM SNAP triggered higher levels of total cellular cGMP accumulation. These results suggest that cGMP signals are spatially segregated within cells, and that the functional compartmentalization of cGMP signals may underlie the unique actions of ANP and nitric oxide.

CGMP signalling: from bench to bedside

The 2nd International Conference on cGMP Generators, Effectors and Therapeutic Implications took place in Potsdam, Germany, during 10–12 June 2005, and was organized by L. Ignarro, F. Hofmann, H. Schmidt and J.P. Stasch (www.cyclicgmp.net). The next cGMP meeting will be held in Dresden, Germany, in June 2007. ![][1] Cyclic nucleotide research originated in the 1960s, when cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) were first identified as natural products (reviewed in Beavo & Brunton, 2002). The discovery of cAMP led to the formulation of the second‐messenger concept in hormone signalling by Earl W. Sutherland, who was duly rewarded with the Nobel Prize in 1971. Consequently, cAMP research surged ahead, whereas cGMP attracted little attention. Indeed, the biological role of cGMP was unknown until the 1980s when two key discoveries were made. First, it was found that a peptide synthesized in the heart, atrial natriuretic peptide (ANP), could stimulate cGMP synthesis by binding to a transmembrane receptor, the particulate guanylyl cyclase (pGC). Second, the elusive endothelium‐derived relaxing factor was identified as nitric oxide (NO), which stimulates soluble guanylyl cyclase (sGC) in smooth muscle cells to synthesize cGMP, thereby causing vasorelaxation. Subsequently, other components of cGMP metabolism and signal transduction were also identified (Fig 1). It is now known that cGMP‐hydrolysing phosphodiesterases (PDEs) are responsible for cGMP breakdown, and at least three types of cGMP‐binding protein transduce the cGMP signal to alter cellular function. These three types are cGMP‐modulated cation channels, cGMP‐dependent protein kinases (cGKs) and cGMP‐regulated PDEs that degrade cAMP and/or cGMP. Figure 1. Current concepts of cGMP signalling. cGMP generators (green) and effectors (red), as well as some downstream pathways and cellular functions (grey boxes) that are involved in the effects of endogenous cGMP and/or cGMP‐elevating drugs (blue), are shown. The lower part shows some current (blue) as well as potential … [1]: /embed/graphic-1.gif