New insight into the functioning of nitric oxide-receptive guanylyl cyclase: physiological and pharmacological implications

Rapid and slow nitric oxide responses during conducted vasodilation in the in vivo intestine and brain cortex microvasculatures

Conduction of arteriolar vasodilation is initiated by activation of nitric oxide (NO) mechanisms, but dependent on conduction of hyperpolarization. Most studies have used brief (<1 second) activation of the initial vasodilation to evaluate the fast conduction processes. However, most arteriolar mechanisms involving NO production persist for minutes. In this study, fast and slower components of arteriolar conduction in the in vivo rat brain and small intestine were compared using three-minute stimulation of NO-dependent vasodilation and measurement of [NO] at the distal sites. Within 10-15 seconds, both vasculatures had a rapidly conducted vasodilation and dilation at distance had a fast but small [NO] component. A slower but larger distal vasodilation occurred after 60-90 seconds in the intestine, but not the brain, and was associated with a substantial increase in [NO]. This slowly developed dilation appeared to be caused by flow mediated responses of larger arterioles as smaller arterioles dilated to lower downstream resistance. These results indicate while the intestinal and cerebral arterioles have a fast conducted vasodilation system, the intestinal arterioles also have a slower but larger dilation of major arterioles that is NO related and dependent on the conduction of vasodilation between small arterioles.

Platelet inhibition by nitrite is dependent on erythrocytes and deoxygenation

Background

Nitrite is a nitric oxide (NO) metabolite in tissues and blood, which can be converted to NO under hypoxia to facilitate tissue perfusion. Although nitrite is known to cause vasodilation following its reduction to NO, the effect of nitrite on platelet activity remains unclear. In this study, the effect of nitrite and nitrite+erythrocytes, with and without deoxygenation, on platelet activity was investigated.

Methodology/Finding

Platelet aggregation was studied in platelet-rich plasma (PRP) and PRP+erythrocytes by turbidimetric and impedance aggregometry, respectively. In PRP, DEANONOate inhibited platelet aggregation induced by ADP while nitrite had no effect on platelets. In PRP+erythrocytes, the inhibitory effect of DEANONOate on platelets decreased whereas nitrite at physiologic concentration (0.1 µM) inhibited platelet aggregation and ATP release. The effect of nitrite+erythrocytes on platelets was abrogated by C-PTIO (a membrane-impermeable NO scavenger), suggesting an NO-mediated action. Furthermore, deoxygenation enhanced the effect of nitrite as observed from a decrease of P-selectin expression and increase of the cGMP levels in platelets. The ADP-induced platelet aggregation in whole blood showed inverse correlations with the nitrite levels in whole blood and erythrocytes.

Conclusion

Nitrite alone at physiological levels has no effect on platelets in plasma. Nitrite in the presence of erythrocytes inhibits platelets through its reduction to NO, which is promoted by deoxygenation. Nitrite may have role in modulating platelet activity in the circulation, especially during hypoxia.

CO metabolism, sensing, and signaling

CO is a colorless and odorless gas produced by the incomplete combustion of hydrocarbons, both of natural and anthropogenic origin. Several microorganisms, including aerobic and anaerobic bacteria and anaerobic archaea, use exogenous CO as a source of carbon and energy for growth. On the other hand, eukaryotic organisms use endogenous CO, produced during heme degradation, as a neurotransmitter and as a signal molecule. CO sensors act as signal transducers by coupling a "regulatory" heme-binding domain to a "functional" signal transmitter. Although high CO concentrations inhibit generally heme-protein actions, low CO levels can influence several signaling pathways, including those regulated by soluble guanylate cyclase and/or mitogen-activated protein kinases. This review summarizes recent insights into CO metabolism, sensing, and signaling.

Identification of residues in the heme domain of soluble guanylyl cyclase that are important for basal and stimulated catalytic activity

Nitric oxide signals through activation of soluble guanylyl cyclase (sGC), a heme-containing heterodimer. NO binds to the heme domain located in the N-terminal part of the β subunit of sGC resulting in increased production of cGMP in the catalytic domain located at the C-terminal part of sGC. Little is known about the mechanism by which the NO signaling is propagated from the receptor domain (heme domain) to the effector domain (catalytic domain), in particular events subsequent to the breakage of the bond between the heme iron and Histidine 105 (H105) of the β subunit. Our modeling of the heme-binding domain as well as previous homologous heme domain structures in different states point to two regions that could be critical for propagation of the NO activation signal. Structure-based mutational analysis of these regions revealed that residues T110 and R116 in the αF helix-β1 strand, and residues I41 and R40 in the αB-αC loop mediate propagation of activation between the heme domain and the catalytic domain. Biochemical analysis of these heme mutants allows refinement of the map of the residues that are critical for heme stability and propagation of the NO/YC-1 activation signal in sGC.

Inhibition of neuronal nitric oxide synthase prevents alterations in medial prefrontal cortex excitability induced by repeated cocaine administration

RATIONALE

The medial prefrontal cortex (mPFC), a forebrain region that regulates cognitive function and reward-motivated behaviors, has been implicated in the neuropathological mechanisms of drug addiction and withdrawal. In cocaine-abstinent human addicts, neuronal activity of the mPFC is increased in response to cocaine re-exposure or drug-associated cues. Additionally, repeated cocaine exposure alters the membrane properties and ion channel function of mPFC pyramidal neurons in drug-withdrawn rats, leading to an increased firing in response to excitatory stimuli. Nitric oxide (NO), a diffusible neuromodulator of neuronal excitability, may play a role in initiating and maintaining behavioral effects of psychostimulants. However, the role of NO in the mechanisms by which cocaine affects membrane excitability is not well clarified.

OBJECTIVES

In this study, we attempted to determine whether inhibition of neuronal nitric oxide synthase (nNOS) altered the changes induced by repeated cocaine exposure and withdrawal.

METHODS

Visualized whole-cell current clamp recordings in brain slices containing the mPFC of rats administered (once per day for 5 days) with either vehicle (10% Cremophor EL in saline 0.9%), cocaine (15 mg/kg, i.p.), or cocaine and the nNOS inhibitor 7-NI (50 mg/kg, i.p.) were employed.

RESULTS

We found that nNOS inhibition prevented cocaine sensitization and the increased membrane excitability of pyramidal cells, evidenced by an increased number of evoked spikes and reductions in inward rectification observed after short-term withdrawal from cocaine.

CONCLUSIONS

These findings suggest that NO plays an important role in chronic cocaine-induced deregulation of the mPFC activity that may contribute to the development of behavioral sensitization and cocaine withdrawal.

Differentiation of cGMP-dependent and -independent nitric oxide effects on platelet apoptosis and reactive oxygen species production using platelets lacking soluble guanylyl cyclase

Platelet activation is an irreversible process resulting in platelet apoptosis and necrosis, and circulating platelets contain many components of the apoptotic machinery. Cyclic guanosine monophosphate (cGMP) generated by nitric oxide (NO) activated soluble guanylyl cyclase (sGC) plays a crucial role in preventing platelet activation. However, in addition to activation of sGC, cGMP-independent NO effects in platelets have been described. To differentiate between cGMP-dependent and -independent NO effects on platelet apoptosis and reactive oxygen species (ROS) production, we generated platelet-specific sGC-deficient mice (PS-GCKO). Platelet apoptosis was induced by a combination of thrombin/convulxin (Thr/Cvx) and assessed by phosphatidylserine (PS) surface exposure, and loss of the mitochondrial membrane potential. NO-induced inhibition of PS externalisation was mediated only by cGMP-dependent mechanisms. Inhibition of the mitochondrial membrane potential decrease at low NO concentration was also cGMP-dependent but became cGMP-independent at high NO concentrations. In contrast, inhibition of ROS formation at any NO concentration was mediated by cGMP-independent mechanisms, very likely due to direct radical scavenging. NO inhibits platelet apoptosis by cGMP-dependent mechanisms and ROS production by cGMP-independent mechanisms. The PS-GCKO mouse model is an important tool for the differentiation of cGMP-dependent and -independent NO effects on platelets.

Role of neuronal nitric oxide synthase in colonic distension-induced hyperalgesia in distal colon of neonatal maternal separated male rats

BACKGROUND

Nitric oxide (NO) is implicated in the pathogenesis of irritable bowel syndrome (IBS) but the underlying mechanism is unclear. Thus, the aim of the present study is to examine the role of NO synthase (NOS) expression in the distal colon of neonatal maternal separation (NMS) model rats employed in IBS studies.

METHODS

Male neonates of Sprague-Dawley rats were randomly assigned into NMS and normal control (N) groups. Rats of NMS group were subjected to 3 h daily maternal separation on postnatal day 2-21. Rats were administrated non-selective NOS inhibitor l-NAME (100 mg kg(-1) ), selective neuronal NOS (nNOS) inhibitor 7-NINA (10mgkg(-1) ), selective inducible NOS (iNOS) inhibitor, endothelial NOS (eNOS) inhibitor (10mgkg(-1) ) or Vehicle (Veh; distilled water) intraperitoneally 1h prior to the experiment for the test and control groups, respectively.

KEY RESULTS

The amount of NO was significantly higher in the NMS Veh rats compared with unseparated N rats. Western-blotting and real-time quantitative PCR studies showed that protein and mRNA expression of nNOS were higher in the NMS group than that in the N rats; whereas no significant change in iNOS and eNOS was found in either groups. Neonatal maternal separation Veh rats showed low pain threshold and increased electromyogram (EMG) activity in response to colonic distension stimuli. l-NAME and 7-Nitroindazole monosodium salt (7-NINA) increased pain threshold pressure and attenuated EMG activity in the NMS rats. In addition, l-NAME and 7-NINA substantially reduced oxidative marker malondialdehyde level in NMS rats.

CONCLUSIONS & INFERENCES

Neonatal maternal separation increased the NO generation by nNOS upregulation that interact with reactive oxygen species contributing to the visceral hypersensitivity in IBS.

Behavioral responses to hypoxia and hyperoxia in Drosophila larvae: molecular and neuronal sensors

The ability to detect changes in oxygen concentration in the environment is critical to the survival of all animals. This requires cells to express a molecular oxygen sensor that can detect shifts in oxygen levels and transmit a signal that leads to the appropriate cellular response. Recent biochemical, genetic and behavioral studies have shown that the atypical soluble guanylyl cyclases function as oxygen detectors in Drosophila larvae triggering a behavioral escape response when exposed to hypoxia. These studies also identified the sensory neurons that innervate the terminal sensory cones as likely chemosensors that mediate this response. Here I summarize the data that led to these conclusions and also highlight evidence that suggests additional, as yet unidentified, proteins are also required for detecting increases and decreases in oxygen concentrations.

Transmembrane guanylate cyclase in intestinal pathophysiology

PURPOSE OF REVIEW

Production of cyclic guanosine monophosphate (cGMP) by guanylate cyclase is of critical importance to gastrointestinal physiology. Tight regulation of cGMP concentration is necessary for proper intestinal secretion and intestinal epithelial cell proliferative and apoptotic homeostasis. This review focuses on recent work detailing the role of a subset of transmembrane guanylate cyclases in the pathophysiology of intestinal secretory and motility disorders and intestinal epithelial cell transformation. Also considered is the potential for therapeutic manipulation of intestinal guanylate cyclase/cGMP signaling for the correction of chronic constipation and gastrointestinal cancer.

RECENT FINDINGS

Recent work in mice and humans suggests a role for transmembrane guanylate cyclases in intestinal fluid secretion as well as hormonal enteric-renal signaling which mediates postprandial natriuresis. Transmembrane guanylate cyclases are also important in gastrointestinal transit rate and motility. Ongoing clinical trials have found that guanylate cyclase activating peptides are safe and effective in the treatment of constipation-predominant irritable bowel syndrome and chronic constipation. In addition, accumulating evidence indicates that membrane-associated guanylate cyclase receptors regulate intestinal epithelial cell homeostatic proliferation and apoptosis as well as gastrointestinal malignancy. The anticancer activity of cGMP signaling in animal studies suggests additional therapeutic applications for guanylate cyclase agonists.

SUMMARY

Progress toward understanding gastrointestinal transmembrane guanylate cyclase/cGMP physiology has recently accelerated due to definitive in-vitro studies and work using gene-targeted animal models and has facilitated the development of safe and effective drugs designed to regulate cGMP production in the intestine. Current work should be directed toward a detailed understanding of cGMP effector pathways and the manner in which subcellular concentrations of cGMP regulate them to influence intestinal health and disease.

The anti-aggregating effect of BAY 41-2272, a stimulator of soluble guanylyl cyclase, requires the presence of nitric oxide

BACKGROUND AND PURPOSE

The purpose of the present study was to determine whether a stimulator of soluble guanylyl cyclase, BAY 41-2272, inhibits platelet aggregation and to clarify its interaction with nitric oxide (NO).

EXPERIMENTAL APPROACH

Blood was collected from anaesthetized Wistar Kyoto rats. The aggregation of washed platelets was measured and the production of cAMP and cGMP was determined.

KEY RESULTS

In adenosine 5'-diphosphate (ADP)-induced platelet aggregation, the anti-aggregating effects of BAY 41-2272, nitroglycerin, sodium nitroprusside and DEA-NONOate were associated with increased levels of cGMP while that of beraprost, a prostacyclin analogue, was correlated with an increase in cAMP. 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) prevented the effects of BAY 41-2272 and that of nitroglycerin and sodium nitroprusside, but only inhibited the increase in cGMP produced by of DEA-NONOate. Hydroxocobalamin, an NO scavenger, inhibited the effects of the three NO donors and BAY 41-2272 but did not affect those of beraprost. ADP-induced aggregation and the effects of BAY 41-2272 were not affected by L-nitroarginine. A positive interaction was observed between BAY 41-2272 and the three NO donors. BAY 41-2272 potentiated also the anti-aggregating effects of beraprost, and again this potentiation was inhibited by hydroxocobalamin.

CONCLUSIONS AND IMPLICATIONS

Inhibition of platelet aggregation by BAY 41-2272 requires the reduced form of soluble guanylyl cyclase and the presence of NO. The positive interaction observed between BAY 41-2272 and various NO donors is qualitatively similar whatever the mechanism involved in NO release. Furthermore, a potent synergism is observed between BAY 41-2272 and a prostacyclin analogue, but only in the presence of NO.

Signaling via IRAG is essential for NO/cGMP-dependent inhibition of platelet activation

Platelet activation is strongly affected by nitric oxide/cyclic GMP (NO/cGMP) signaling involving cGMP-dependent protein kinase I (cGKI). Previously it was shown that interaction of the cGKI substrate IRAG with InsP(3)RI is essential for NO/cguanosine monophosphate (GMP)-dependent inhibition of platelet aggregation in vitro and in vivo. However, the role of Inositol-trisphosphate receptor associated cGMP kinase substrate (IRAG) for platelet adhesion or granule secretion was unknown. Here, we analysed the functional role of IRAG for platelet activation. Murine IRAG-deficient platelets displayed enhanced aggregability towards several agonists (collagen, thrombin and TxA2). NO- or cGMP-dependent inhibition of agonist induced ATP- or 5-HT secretion from dense granules, and P-selectin secretion from alpha granules was severely affected in IRAG-deficient platelets. Concomitantly, the effect of NO/cGMP on platelet aggregation was strongly reduced in IRAG-deficient platelets. Furthermore, GPIIb/IIIa-mediated adhesion of platelets to fibrinogen could only weakly be inhibited in IRAG-deficient mice contrary to wild-type (WT) mice. Our results suggest that signaling via IRAG is essential for NO/cGMP-dependent inhibition of platelet activation regarding granule secretion, aggregation and adhesion. This platelet disorder might cause that the bleeding time of IRAG-deficient mice was reduced.

Nitric oxide as key mediator of neuron-to-neuron and endothelia-to-glia communication involved in the neuroendocrine control of reproduction

Nitric oxide (NO) is a peculiar chemical transmitter that freely diffuses through aqueous and lipid environments and plays a role in major aspects of brain function. Within the hypothalamus, NO exerts critical effects upon the gonadotropin-releasing hormone (GnRH) network to maintain fertility. Here, we review recent evidence that NO regulates major aspects of the GnRH neuron physiology. Far more active than once thought, NO powerfully controls GnRH neuronal activity, GnRH release and structural plasticity at the neurohemal junction. In the preoptic region, neuronal nitric oxide synthase (nNOS) activity is tightly regulated by estrogens and is found to be maximal at the proestrus stage. Natural fluctuations of estrogens control both the differential coupling of this Ca²+-activated enzyme to glutamate N-methyl-D-aspartic acid receptor channels and phosphorylation-mediated nNOS activation. Furthermore, NO endogenously produced by neurons expressing nNOS acutely and directly suppresses spontaneous firing in GnRH neurons, which suggests that neuronal NO may serve as a synchronizing switch within the preoptic region. At the median eminence, NO is spontaneously released from an endothelial source and follows a pulsatile and cyclic pattern of secretion. Importantly, GnRH release appears to be causally related to endothelial NO release. NO is also highly involved in mediating the dialogue set in motion between vascular endothelial cells and tanycytes that control the direct access of GnRH neurons to the pituitary portal blood during the estrous cycle. Altogether, these data raise the intriguing possibility that the neuroendocrine brain uses NO to coordinate both GnRH neuronal activity and GnRH release at key stages of reproductive physiology.

Information processing deficits and nitric oxide signalling in the phencyclidine model of schizophrenia

RATIONALE

Schizophrenia-like cognitive deficits induced by phencyclidine (PCP), a drug commonly used to model schizophrenia in experimental animals, are attenuated by nitric oxide (NO) synthase inhibitors. Furthermore, PCP increases NO levels and sGC/cGMP signalling in the prefrontal cortex in rodents. Hence, a cortical NO/sGC/cGMP signalling pathway may constitute a target for novel pharmacological therapies in schizophrenia.

OBJECTIVES

The objective of this study was to further investigate the role of NO signalling for a PCP-induced deficit in pre-attentive information processing.

MATERIALS AND METHODS

Male Sprague-Dawley rats were surgically implanted with NO-selective amperometric microsensors aimed at the prefrontal cortex, ventral hippocampus or nucleus accumbens, and NO levels and prepulse inhibition (PPI) were simultaneously assessed.

RESULTS

PCP treatment increased NO levels in the prefrontal cortex and ventral hippocampus, but not in the nucleus accumbens. The increase in NO levels was not temporally correlated to the deficit in PPI induced by PCP. Furthermore, pretreatment with the neuronal NO synthase inhibitor N-propyl-L-arginine dose-dependently attenuated both the increase in prefrontal cortex NO levels and the deficit in PPI.

CONCLUSIONS

These findings support a demonstrated role of NO in the behavioural and neurochemical effects of PCP. Furthermore, this effect is brain region-specific and mainly involves the neuronal isoform of NOS. However, a temporal correlation between a PCP-induced disruption of PPI and an increase in prefrontal cortex NO levels was not demonstrated, suggesting that the interaction between PCP and the NO system is more complex than previously thought.

Copper and homocysteine in cardiovascular diseases

High blood copper (Cu) and homocysteine (Hcy) concentrations have been independently reported as risk factors for cardiovascular diseases. When they are simultaneously measured, a concomitant increase in both parameters in association with vascular dysfunction has been observed. Cu chelator penicillamine can significantly diminish the inhibitory effect of Hcy on endothelial function, which has led to the interpretation that Cu mediates the deleterious effect of Hcy. However, Cu itself has been shown to be beneficial to the cardiovascular system. In particular, Cu promotion of angiogenesis has been well documented. Cu stimulates endothelial cell proliferation and differentiation and promotes microtubule formation in cultured saphenous veins. High levels of Hcy do not affect the process of microtubule formation, but the combination of Cu and Hcy leads to a significant inhibitory effect. Under other conditions, Cu does not affect, but Hcy inhibits, the endothelium-dependent relaxation of blood vessels and the combination of both augments the inhibition. Why does Cu produce adverse effects when it co-exists with Hcy? Cu forms complexes with Hcy and the Cu-Hcy complexes possess a deleterious potential due to their redox properties. Cu chelation can remove Cu from the Cu-Hcy complexes, but leaves behind high levels of Hcy and produces Cu deficiency. An alternative approach should focus on the reduction of Hcy, but maintenance of Cu, making detrimental Cu beneficial. A comprehensive understanding of Cu speciation and a development of selective modulation of Cu coordination to Cu-binding molecules to avoid Cu-Hcy complex formation would effectively improve the condition of cardiovascular disease.

Protein modifications involved in neurotransmitter and gasotransmitter signaling

Covalent modifications of intracellular proteins, such as phosphorylation, are generally thought to occur as secondary or tertiary responses to neurotransmitters, following the intermediation of membrane receptors and second messengers such as cyclic AMP. By contrast, the gasotransmitter nitric oxide directly S-nitrosylates cysteine residues in diverse intracellular proteins. Recently, hydrogen sulfide has been acknowledged as a gasotransmitter, which analogously sulfhydrates cysteine residues in proteins. Cysteine residues are also modified by palmitoylation in response to neurotransmitter signaling, possibly in reciprocity with S-nitrosylation. Neurotransmission also elicits sumoylation and acetylation of lysine residues within diverse proteins. This review addresses how these recently appreciated protein modifications impact our thinking about ways in which neurotransmission regulates intracellular protein disposition.

What part of NO don't you understand? Some answers to the cardinal questions in nitric oxide biology

Nitric oxide (NO) regulates biological processes through signaling mechanisms that exploit its unique biochemical properties as a free radical. For the last several decades, the key aspects of the chemical properties of NO relevant to biological systems have been defined, but it has been a challenge to assign these to specific cellular processes. Nevertheless, it is now clear that the high affinity of NO for transition metal centers, particularly iron, and the rapid reaction of NO with oxygen-derived free radicals can explain many of its biological and pathological properties. Emerging studies also highlight a growing importance of the secondary metabolites of NO-dependent reactions in the post-translational modification of key metabolic and signaling proteins. In this minireview, we emphasize the current understanding of the biochemistry of NO and place it in a biological context.

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

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