Modification of cysteine residues within G(o) and other neuronal proteins by exposure to nitric oxide

S-nitrosoglutathione inhibits cerebrovascular angiotensin II-dependent and -independent AT1 receptor responses: A possible role of S-nitrosation

BACKGROUND AND PURPOSE

Angiotensin II (AngII) and NO regulate the cerebral circulation. AngII AT₁ receptors exert ligand-dependent and ligand-independent (myogenic tone [MT]) vasoconstriction of cerebral vessels. NO induces post-translational modifications of proteins such as S-nitrosation (redox modification of cysteine residues). In cultured cells, S-nitrosation decreases AngII's affinity for the AT₁ receptor. The present work evaluated the functional consequences of S-nitrosation on both AngII-dependent and AngII-independent cerebrovascular responses.

EXPERIMENTAL APPROACH

S-Nitrosation was induced in rat isolated middle cerebral arteries by pretreatment with the NO donors, S-nitrosoglutathione (GSNO) or sodium nitroprusside (SNP). Agonist-dependent activation of AT₁ receptors was evaluated by obtaining concentration-response curves to AngII. Ligand-independent activation of AT₁ receptors was evaluated by calculating MT (active vs. passive diameter) at pressures ranging from 20 to 200 mmHg in the presence or not of a selective AT₁ receptor inverse agonist.

KEY RESULTS

GSNO or SNP completely abolished the AngII-dependent AT₁ receptor-mediated vasoconstriction of cerebral arteries. GSNO had no impact on responses to other vasoconstrictors sharing (phenylephrine, U46619) or not (5-HT) the same signalling pathway. MT was reduced by GSNO, and the addition of losartan did not further decrease MT, suggesting that GSNO blocks AT₁ receptor-dependent MT. Ascorbate (which reduces S-nitrosated compounds) restored the response to AngII but not the soluble GC inhibitor ODQ, suggesting that these effects are mediated by S-nitrosation rather than by S-nitrosylation.

CONCLUSIONS AND IMPLICATIONS

In rat middle cerebral arteries, GSNO pretreatment specifically affects the AT₁ receptor and reduces both AngII-dependent and AngII-independent activation, most likely through AT₁ receptor S-nitrosation.

Involvement of neuronal nitric oxide synthase in desensitisation of µ-opioid receptors in the rat locus coeruleus

Nitric oxide (NO) has been recently shown to enhance µ-opioid receptor (MOR) desensitisation in locus coeruleus (LC) neurons. The aim of this study was to evaluate by single-unit extracellular recordings in rat brain slices whether the neuronal NO synthase is involved in MOR desensitisation in LC neurons. As expected, a high concentration of the opioid agonist Met(5)-enkephalin (ME; 10 µM, 10 min) strongly desensitised the inhibition induced by a test application of ME (0.8 µM, 1 min), whereas lower ME concentrations (1 and 3 µM) only weakly desensitised it. The neuronal NO synthase inhibitors 7-nitroindazole (10-100 µM), S-methyl-L-thiocitrulline (0.01-10 µM) and N(ω)-propyl-L-arginine (1-10 µM) attenuated ME (10 µM)-induced opioid desensitisation, although the endothelial NO synthase inhibitor N(5)-(1-iminoethyl)-L-ornithine (3-30 µM) failed to change it. The NO donor sodium nitroprusside (1 mM), but not its inactive analog potassium ferricyanide (1 mM), enhanced the ME (3 µM)-induced desensitisation and prevented the effect of S-methyl-L-thiocitrulline (10 µM). Sodium nitroprusside (1 mM) failed to change the desensitisation of α2-adrenoceptors by noradrenaline (100 µM, 10 min). These results suggest the contribution of NO and a neuronal type of NO synthase in homologous MOR desensitisation in rat LC neurons.

Post-translational modifications and lipid binding profile of insect cell-expressed full-length mammalian synaptotagmin 1

Synaptotagmin 1 (Syt1) is a Ca(2+) sensor for SNARE-mediated, Ca(2+)-triggered synaptic vesicle fusion in neurons. It is composed of luminal, transmembrane, linker, and two Ca(2+)-binding (C2) domains. Here we describe expression and purification of full-length mammalian Syt1 in insect cells along with an extensive biochemical characterization of the purified protein. The expressed and purified protein is properly folded and has increased α-helical content compared to the C2AB fragment alone. Post-translational modifications of Syt1 were analyzed by mass spectrometry, revealing the same modifications of Syt1 that were previously described for Syt1 purified from brain extract or mammalian cell lines, along with a novel modification of Syt1, tyrosine nitration. A lipid binding screen with both full-length Syt1 and the C2AB fragments of Syt1 and Syt3 isoforms revealed new Syt1-lipid interactions. These results suggest a conserved lipid binding mechanism in which Ca(2+)-independent interactions are mediated via a lysine rich region of the C2B domain while Ca(2+)-dependent interactions are mediated via the Ca(2+)-binding loops.

The nitric oxide-cGMP signaling pathway plays a significant role in tolerance to the analgesic effect of morphine

Although the phenomenon of opioid tolerance has been widely investigated, neither opioid nor nonopioid mechanisms are completely understood. The aim of the present study was to investigate the role of the nitric oxide (NO)-cyclic guanosine monophosphate (cGMP) pathway in the development of morphine-induced analgesia tolerance. The study was carried out on male Wistar albino rats (weighing 180-210 g; n = 126). To develop morphine tolerance, animals were given morphine (50 mg/kg; s.c.) once daily for 3 days. After the last dose of morphine was injected on day 4, morphine tolerance was evaluated. The analgesic effects of 3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole (YC-1), BAY 41-2272, S-nitroso-N-acetylpenicillamine (SNAP), N(G)-nitro-L-arginine methyl ester (L-NAME), and morphine were considered at 15 or 30 min intervals (0, 15, 30, 60, 90, and 120 min) by tail-flick and hot-plate analgesia tests (n = 6 in each study group). The results showed that YC-1 and BAY 41-2272, a NO-independent activator of soluble guanylate cyclase (sGC), significantly increased the development and expression of morphine tolerance, and L-NAME, a NO synthase (NOS) inhibitor, significantly decreased the development of morphine tolerance. In conclusion, these data demonstrate that the nitric oxide-cGMP signal pathway plays a pivotal role in developing tolerance to the analgesic effect of morphine.

Redox-based regulation of signal transduction: principles, pitfalls, and promises

Oxidants are produced as a by-product of aerobic metabolism, and organisms ranging from prokaryotes to mammals have evolved with an elaborate and redundant complement of antioxidant defenses to confer protection against oxidative insults. Compelling data now exist demonstrating that oxidants are used in physiological settings as signaling molecules with important regulatory functions controlling cell division, migration, contraction, and mediator production. These physiological functions are carried out in an exquisitely regulated and compartmentalized manner by mild oxidants, through subtle oxidative events that involve targeted amino acids in proteins. The precise understanding of the physiological relevance of redox signal transduction has been hampered by the lack of specificity of reagents and the need for chemical derivatization to visualize reversible oxidations. In addition, it is difficult to measure these subtle oxidation events in vivo. This article reviews some of the recent findings that illuminate the significance of redox signaling and exciting future perspectives. We also attempt to highlight some of the current pitfalls and the approaches needed to advance this important area of biochemical and biomedical research.

Inhibition of neuronal nitric oxide synthase attenuates the development of morphine tolerance in rats

Our previous results have shown the involvement of nitric oxide in acute opioid desensitization of mu-opioid receptors in vitro. In the present study, we investigated the effect of repeated administration of 7-nitroindazole (7-NI; 30 mg/kg/12 h, i.p., 3 days), an inhibitor of neuronal nitric oxide synthase in vivo, on mu-opioid receptor tolerance induced by subchronic treatment with morphine in rats. The inhibitory effect of the opioid agonist Met5-enkephalin (ME) on the cell firing rate was evaluated by single-unit extracellular recordings of noradrenergic neurons in the locus coeruleus from brain slices, and the antinociceptive effect of morphine was measured by tail-flick techniques. In morphine-treated animals, concentration-effect curves for ME in the locus coeruleus were shifted by 5-fold to the right as compared to those in sham-treated animals, which confirmed the induction of mu-opioid receptor tolerance. However, tolerance to ME in morphine-treated rats was fully prevented by co-administration of 7-NI when compared to the vehicle-morphine group. Likewise, the antinociceptive effect of morphine was reduced in morphine-treated animals as compared to the sham group, whereas the antinociceptive tolerance was partially prevented by co-administration of 7-NI in morphine-treated rats (when compared to the vehicle-morphine group). Finally, 7-NI administration in sham-treated rats failed to change the effect induced by ME on the locus coeruleus or by morphine in the tail-flick test as compared to vehicle groups. These results demonstrate that subchronic administration of a neuronal inhibitor of nitric oxide synthase attenuates the development of morphine tolerance to the cellular and analgesic effects of mu-opioid receptor agonists.

S-nitrosothiols modulate G protein-coupled receptor signaling in a reversible and highly receptor-specific manner

Background

Recent studies indicate that the G protein-coupled receptor (GPCR) signaling machinery can serve as a direct target of reactive oxygen species, including nitric oxide (NO) and S-nitrosothiols (RSNOs). To gain a broader view into the way that receptor-dependent G protein activation – an early step in signal transduction – might be affected by RSNOs, we have studied several receptors coupling to the G_i family of G proteins in their native cellular environment using the powerful functional approach of [³⁵S]GTPγS autoradiography with brain cryostat sections in combination with classical G protein activation assays.

Results

We demonstrate that RSNOs, like S-nitrosoglutathione (GSNO) and S-nitrosocysteine (CysNO), can modulate GPCR signaling via reversible, thiol-sensitive mechanisms probably involving S-nitrosylation. RSNOs are capable of very targeted regulation, as they potentiate the signaling of some receptors (exemplified by the M2/M4 muscarinic cholinergic receptors), inhibit others (P2Y₁₂ purinergic, LPA₁lysophosphatidic acid, and cannabinoid CB₁ receptors), but may only marginally affect signaling of others, such as adenosine A₁, μ-opioid, and opiate related receptors. Amplification of M2/M4 muscarinic responses is explained by an accelerated rate of guanine nucleotide exchange, as well as an increased number of high-affinity [³⁵S]GTPγS binding sites available for the agonist-activated receptor. GSNO amplified human M4 receptor signaling also under heterologous expression in CHO cells, but the effect diminished with increasing constitutive receptor activity. RSNOs markedly inhibited P2Y₁₂ receptor signaling in native tissues (rat brain and human platelets), but failed to affect human P2Y₁₂ receptor signaling under heterologous expression in CHO cells, indicating that the native cellular signaling partners, rather than the P2Y₁₂ receptor protein, act as a molecular target for this action.

Conclusion

These in vitro studies show for the first time in a broader general context that RSNOs are capable of modulating GPCR signaling in a reversible and highly receptor-specific manner. Given that the enzymatic machinery responsible for endogenous NO production is located in close proximity with the GPCR signaling complex, especially with that for several receptors whose signaling is shown here to be modulated by exogenous RSNOs, our data suggest that GPCR signaling in vivo is likely to be subject to substantial, and highly receptor-specific modulation by NO-derived RSNOs.

Effect of nitric oxide on the sinusoidal uptake of organic cations and anions by isolated hepatocytes

The issue of whether or not the presence NOx (NO and oxidized metabolites) in the hepatocytes at pathological levels affects the functional activity of transport systems within the sinusoidal membrane was investigated. For this purpose, the effect of the pretreatment of isolated hepatocytes with sodium nitroprusside (SNP), a spontaneous NO donor, on the sinusoidal uptake of tributylmethylammonium (TBuMA) and triethylmethyl ammonium (TEMA), representative substrates of the organic cation transporter (OCT), and taurocholate, a representative substrate of the Na+/taurocholate cotransporting polypeptide (NTCP), was measured. The uptake of TBuMA and TEMA was not affected by the pretreatment, as demonstrated by the nearly identical kinetic parameters for the uptake (i.e., Vmax, Km and CL(linear)). The uptake of mannitol into hepatocytes was not affected, demonstrating that the membrane integrity remained constant, irregardless of the SNP pretreatment. On the contrary, the uptake of taurocholate was significantly inhibited by the pretreatment, resulting in a significant decrease in Vmax, thus providing a clear demonstration that NOx preferentially affects the function of NTCP rather than OCT on the sinusoidal membrane. A direct interaction between NOx and NTCP or a decrease in Na+/K+ ATPase activity as the result of SNP pretreatment might be responsible for this selective effect of NOx.

Mechanism of the reduced elimination clearance of benzylpenicillin from cerebrospinal fluid in rats with intracisternal administration of lipopolysaccharide

The mechanism responsible for the reduced clearance of benzylpenicillin (BPC) from the cerebrospinal fluid (CSF) was investigated in rats that received an intracisternal administration of lipopolysaccharide (LPS). BPC was intraventricularly injected and its elimination from the CSF studied. During the inflammation created by the LPS administration to the cisterna magna, the clearance of BPC and taurine from the CSF was significantly reduced but reverted to the control level when N-nitro-L-arginine, a nitric oxide (NO) synthase inhibitor, was intracisternally administered. The in vitro uptake of BPC and taurine was significantly reduced in the choroid plexus (CP, the blood-CSF barrier) of rats with experimental inflammation and in control CP that had been pretreated with sodium nitroprusside (SNP, an NO donor). Interestingly, the clearance and CP uptake of formycin B, a substrate for a nucleoside transporter, were not affected by the experimental inflammation or by pretreatement with SNP. These observations suggest that the BPC transporter, and probably other transport systems as well, is functionally sensitive to NO in the blood-CSF barrier. Therefore, functional impairment of BPC transport in the CP by NO may be partly responsible for the increase in BPC concentration in the CSF during inflammation such as that caused by meningitis.

Nitric oxide and the other cyclic nucleotide

Nitric oxide (NO) participates in the regulation of the daily activities of cells as well as in cytotoxic events. Elucidating the mechanism(s) by which NO carries out its diverse functions has been the goal of numerous laboratories. In the cardiovascular system, evidence indicates that NO mediates its effects via an activation of soluble guanylyl cyclase (sGC). In other tissues, it is not clear if sGC is an exclusive target for NO or what the functions of cGMP might be. It is also unlikely that the diversity of NO actions is explained solely by changes in cGMP. This review focuses on the evidence that NO modulates cAMP signalling, with specific attention to the effects of NO on adenylyl cyclase (AC) as the target of NO regulation.

The neurobiology of opiate tolerance, dependence and sensitization: mechanisms of NMDA receptor-dependent synaptic plasticity

Long-term administration of opiates leads to changes in the effects of these drugs, including tolerance, sensitization and physical dependence. There is, as yet, incomplete understanding of the neural mechanisms that underlie these phenomena. Tolerance, sensitization and physical dependence can be considered adaptive processes similar to other experience-dependent changes in the brain, such as learning and neural development. There is considerable evidence demonstrating that N-methyl-D-aspartate (NMDA) receptors and downstream signaling cascades may have an important role in different forms of experience-dependent changes in the brain and behavior. This review will explore evidence indicating that NMDA receptors and downstream messengers may be involved in opiate tolerance, sensitization and physical dependence. This evidence has been used to develop a cellular model of NMDA receptor/opiate interactions. According to this model, mu opioid receptor stimulation leads to a protein kinase C-mediated activation of NMDA receptors. Activation of NMDA receptors leads to influx of calcium and activation of calcium-dependent processes. These calcium-dependent processes have the ability to produce critical changes in opioid-responsive neurons, including inhibition of opioid receptor/second messenger coupling. This model is similar to cellular models of learning and neural development in which NMDA receptors have a central role. Together, the evidence suggests that the mechanisms that underlie changes in the brain and behavior produced by long-term opiate use may be similar to other central nervous system adaptations. The experimental findings and the resulting model may have implications for the treatment of pain and addiction.

Pulmonary vasoconstriction by serotonin is inhibited by S-nitrosoglutathione

Nitric oxide (NO) functions as an endothelium-derived relaxing factor by activating guanylate cyclase to increase cGMP levels. However, NO and related species may also regulate vascular tone by cGMP-independent mechanisms. We hypothesized that naturally occurring NO donors could decrease the pulmonary vascular response to serotonin (5-HT) in the intact lung through chemical interactions with 5-HT(2) receptors. In isolated rabbit lung preparations and isolated pulmonary artery (PA) rings, 50-250 microM S-nitrosoglutathione (GSNO) inhibited the response to 0.01-10 microM 5-HT. The vasoconstrictor response to 5-HT was mediated by 5-HT(2) receptors in the lung, since it could be blocked completely by the selective inhibitor ketanserin (10 microM). GSNO inhibited the response to 5-HT by 77% in intact lung and 82% in PA rings. In PA rings, inhibition by GSNO could be reversed by treatment with the thiol reductant dithiothreitol (10 mM). 3-Morpholinosydnonimine (100-500 microM), which releases NO and O simultaneously, also blocked the response to 5-HT. Its chemical effects, however, were distinct from those of GSNO, because 5-HT-mediated vasoconstriction was not restored in isolated rings by dithiothreitol. In the intact lung, neither NO donor altered the vascular response to endothelin, which activates the same second-messenger vasoconstrictor system as 5-HT. These findings, which did not depend on guanylate cyclase, are consistent with chemical modification by NO of the 5-HT(2) G protein-coupled receptor system to inhibit vasoconstriction, possibly by S-nitrosylation of the receptor or a related protein. This study demonstrates that GSNO can regulate vascular tone in the intact lung by a reversible mechanism involving inhibition of the response to 5-HT.

Nitric oxide reduces the palmitoylation of rat myelin proteolipid protein by an indirect mechanism

Brain slices from 20-day-old rats were incubated with [3H]palmitate for 2 hours in the absence or presence of the NO-donors S-nitroso-N-acetyl-penicillamine (SNAP), ethyl-2-[hydroxyimino]-5-nitro-3-hexeneamide (NOR-3), 4-phenyl-3-furoxan carbonitrile (PFC) and sodium nitroprusside (SNP). Each of these drugs reduced the incorporation of [3H]palmitate into myelin proteolipid protein (PLP) in a concentration-dependent manner, SNP being the most active. The effect of SNAP was prevented by the NO-scavenger PTIO (2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide). Furthermore, decayed-SNAP, sodium nitrite and N- nitrosopyrrolidine were inactive, suggesting that free NO and/or some of its direct oxidation products are the active molecular species. The amount of fatty acids bound to PLP and the rate of deacylation were unaffected by NO. Although NO diminished the number of thiols in brain and myelin proteins, with the formation of both nitrosothiols and disulfides, these changes did not parallel those in PLP acylation. In contrast, NO was effective at reducing the palmitoylation of brain and myelin lipids, and this effect along with that of PLP, was ascribed to a decrease in palmitoyl-CoA levels. The NO-induced reduction in acyl-CoA concentration was due to the decline in ATP levels, while the amount of [3H]palmitate incorporated into the tissue, the activity of palmitoyl-CoA ligase and palmitoyl-CoA hydrolase, and the concentration of CoASH were unaltered by the drugs. Experiments with endogenously-synthesized [18O]fatty acids confirmed that NO affects predominantly the ATP-dependent palmitoylation of PLP. In conclusion, the inhibitory action of NO on the fatty acylation of PLP is indirect and caused by energy depletion.

Is nitric oxide overproduction the target of choice for the management of septic shock?

Sepsis is a heterogeneous class of syndromes caused by a systemic inflammatory response to infection. Septic shock, a severe form of sepsis, is associated with the development of progressive damage in multiple organs, and is a leading cause of patient mortality in intensive care units. Despite important advances in understanding its pathophysiology, therapy remains largely symptomatic and supportive. A decade ago, the overproduction of nitric oxide (NO) had been discovered as a potentially important event in this condition. As a result, great hopes arose that the pharmacological inhibition of NO synthesis could be developed into an efficient, mechanism-based therapeutic approach. Since then, an extraordinary effort by the scientific community has brought a deeper insight regarding the feasibility of this goal. Here we present in summary form the present state of knowledge of the biological chemistry and physiology of NO. We then proceed to a systematic review of experimental and clinical data, indicating an up-regulation of NO production in septic shock; information on the role of NO in septic shock, as provided by experiments in transgenic mice that lack the ability to up-regulate NO production; effects of pharmacological inhibitors of NO production in various experimental models of septic shock; and relevant clinical experience. The accrued evidence suggests that the contribution of NO to the pathophysiology of septic shock is highly heterogeneous and, therefore, difficult to target therapeutically without appropriate monitoring tools, which do not exist at present.

Effects of S-nitroso-cysteine on proteins that regulate exocytosis in PC12 cells: inhibitory effects on translocation of synaptophysin and ADP-ribosylation of GTP-binding proteins

S-Nitroso-cysteine (SNC) inhibits Ca2+-induced noradrenaline (NA) release from PC12 cells. Since SNC stimulated Ca2+ mobilization from intracellular Ca2+ pools and SNC-induced inhibition of NA release was not washed-out, SNC may modify exocytosis-related proteins that overcome Ca2+ mobilization. In the present study, we investigated the effects of SNC on exocytosis-related proteins in PC12 cells. Ionomycin stimulated NA release and increased the immunoreactivity of synaptophysin in the cytosol fraction. A 25-kDa synaptosome-associated protein (SNAP-25), which localizes to plasma membranes and vesicles, increased in the cytosol fraction after stimulation. The increases in these proteins by ionomycin were inhibited in PC12 cells treated with 0.6 mM SNC. Synaptobrevin and synapsin-1 in the cytosol fraction, and syntaxin and 43 kDa growth-associated protein in the membrane fraction were not affected by ionomycin or SNC. Incubation of each protein with SNC did not affect antibody immunoreactivity. [32P]ADP-ribosylation of GTP-binding proteins (Gi/Go) by pertussis toxin, but not Gs by cholera toxin, was inhibited in SNC-treated PC12 cells and by co-addition of SNC to the assay mixture. These findings suggest that 1) SNC inhibits translocation of vesicles containing synaptophysin and SNAP-25, and 2) SNC reacts with cysteine residues in Gi/Go, causing inhibition of ADP-ribosylation by pertussis toxin.

The role of reactive nitrogen species in secondary spinal cord injury: formation of nitric oxide, peroxynitrite, and nitrated protein

To determine whether reactive nitrogen species contribute to secondary damage in CNS injury, the time courses of nitric oxide, peroxynitrite, and nitrotyrosine production were measured following impact injury to the rat spinal cord. The concentration of nitric oxide measured by a nitric oxide-selective electrode dramatically increased immediately following injury and then quickly declined. Nitro-L-arginine reduced nitric oxide production. The extracellular concentration of peroxynitrite, measured by perfusing tyrosine through a microdialysis fiber into the cord and quantifying nitrotyrosine in the microdialysates, significantly increased after injury to 3.5 times the basal level, and superoxide dismutase and nitro-L-arginine completely blocked peroxynitrite production. Tyrosine nitration examined immunohistochemically significantly increased at 12 and 24 h postinjury, but not in sham-control sections. Mn(III) tetrakis(4-benzoic acid)-porphyrin (a novel cell-permeable superoxide dismutase mimetic) and nitro-L-arginine significantly reduced the numbers of nitrotyrosine-positive cells. Protein-bound nitrotyrosine was significantly higher in the injured tissue than in the sham-operated controls. These results demonstrate that traumatic injury increases nitric oxide and peroxynitrite production, thereby nitrating tyrosine, including protein-bound tyrosine. Together with our previous report that trauma increases superoxide, our results suggest that reactive nitrogen species cause secondary damage by nitrating protein through the pathway superoxide + nitric oxide peroxynitrite protein nitration.

S-Nitrosocysteine increases palmitate turnover on Ha-Ras in NIH 3T3 cells

Ha-Ras is modified by isoprenoid on Cys(186) and by reversibly attached palmitates at Cys(181) and Cys(184). Ha-Ras loses 90% of its transforming activity if Cys(181) and Cys(184) are changed to serines, implying that palmitates make important contributions to oncogenicity. However, study of dynamic acylation is hampered by an absence of methods for acutely manipulating Ha-Ras palmitoylation in living cells. S-nitrosocysteine (SNC) and, to a more modest extent, S-nitrosoglutathione were found to rapidly increase [(3)H]palmitate incorporation into cellular or oncogenic Ha-Ras in NIH 3T3 cells. In contrast, SNC decreased [(3)H]palmitate labeling of the transferrin receptor and caveolin. SNC accelerated loss of [(3)H]palmitate from Ha-Ras, implying that SNC stimulated deacylation and permitted subsequent reacylation of Ha-Ras. SNC also decreased Ha-Ras GTP binding and inhibited phosphorylation of the kinases ERK1 and ERK2 in NIH 3T3 cells. Thus, SNC altered two important properties of Ha-Ras activation state and lipidation. These results identify SNC as a new tool for manipulating palmitate turnover on Ha-Ras and for studying requirements of repalmitoylation and the relationship between palmitate cycling, membrane localization, and signaling by Ha-Ras.

Biology of nitric oxide signaling

The free radical nitric oxide (NO) has emerged in recent years as a fundamental signaling molecule for the maintenance of homeostasis, as well as a potent cytotoxic effector involved in the pathogenesis of a wide range of human diseases. Although this paradoxical fate has generated confusion, separating the biological actions of NO on the basis of its physiologic chemistry provides a conceptual framework which helps to distinguish between the beneficial and toxic consequences of NO, and to envision potential therapeutic strategies for the future. Under normal conditions, NO produced in low concentration acts as a messenger and cytoprotective (antioxidant) factor, via direct interactions with transition metals and other free radicals. Alternatively, when the circumstances allow the formation of substantial amounts of NO and modify the cellular microenvironment (formation of the superoxide radical), the chemistry of NO will turn into indirect effects consecutive to the formation of dinitrogen trioxide and peroxynitrite. These "reactive nitrogen species" will, in turn, mediate both oxidative and nitrosative stresses, which form the basis of the cytotoxicity generally attributed to NO, relevant to the pathophysiology of inflammation, circulatory shock, and ischemia-reperfusion injury.

Involvement of Hg2+-sensitive sulfhydryl groups in regulating noradrenaline release induced by S-nitrosocysteine in rat brain slices

Nitric oxide has been shown to regulate neurotransmitter release. Previously, we reported that S-nitrosothiols such as S-nitrosocysteine (SNC) stimulate noradrenaline (NA) release in rat hippocampus in vivo and in vitro. To examine the role of sulfhydryl groups in SNC-induced NA release, the effects of metal ions such as Hg2+ and N-ethylmaleimide (NEM, a sulfhydryl alkylating agent) on [3H]NA release from labeled rat brain slices (hippocampus and cerebral cortex) were studied and compared with the effects of SNC. The addition of 200 microM HgCl2, but not Pb2+, Zn2+, or Cd2+, stimulated [3H]NA release from both types of slices in the presence of extracellular CaCl2. p-Chloromercuribenzoic acid (p-CMBA) also stimulated [3H]NA release. NEM stimulated [3H]NA release from both types of slices in the presence and absence of extracellular CaCl2. The effect of 200 microM NEM was enhanced, but the effect of 200 microM SNC was inhibited by co-addition of 200 microM p-CMBA in the absence of extracellular CaCl2. The concentration-response curve of SNC shifted to the right after co-addition of 200 microM p-CMBA or 100 microM HgCl2, although the effect of 200 microM NEM was additive to the effect of SNC. These findings demonstrate that SNC acts as a sulfhydryl agent on proteins that regulate NA release, and that SNC may share the same sulfhydryl groups with Hg compounds. The effect of T-588 ¿(R)-(-)-(benzo[b]thiophen-5-yl)-2-[2-(N,N-diethylamino)ethoxy]eth anol hydrochloride¿, a novel cognitive enhancer and a stimulator of NA release, was compared with the effects of sulfhydryl reagents.

Nitric oxide modulates beta(2)-adrenergic receptor palmitoylation and signaling

To determine whether nitric oxide (NO) modulates the beta-adrenergic signaling pathway, we treated cells expressing beta(2)-adrenergic receptors (beta(2)AR) with the NO donors, 3-morpholinosydnonimine (SIN-1) and 1,2,3,4-oxatriazolium, 5-amino-3-(3-chloro-2-methylphenyl)chloride and determined the intracellular production of cAMP after exposure to beta-adrenergic receptor agonists, cholera toxin and forskolin. NO significantly decreased the potency of the beta-adrenergic agonist, isoproterenol, to stimulate cAMP production without affecting the stimulatory action of forskolin and cholera toxin, which directly activate adenylyl cyclase and G(s), respectively. Treatment with the NO donor increased the guanyl nucleotide-sensitive high affinity constant for the agonist, isoproterenol, thus suggesting that it reduced functional coupling between the receptor and G(s). Stimulation of endogenous NO production by lipopolysaccharide in RAW 264.7 macrophages also caused a significant increase in the EC(50) for isoproterenol-stimulated cAMP production. SIN-1 treatment also led to a reduction in both basal and isoproterenol-stimulated incorporation of [(3)H]palmitate into the beta(2)AR. Signaling through the nonpalmitoylated, Gly(341)beta(2)AR mutant was unchanged by SIN-1 treatment. Given the link between beta(2)AR palmitoylation and its responsiveness to agonist, these results suggest that the primary action of NO was depalmitoylation of the beta(2)AR resulting in decreased signaling through the beta(2)AR.

A shift in protein S-palmitoylation, with persistence of growth-associated substrates, marks a critical period for synaptic plasticity in developing brain

In the mammalian cortex, the initial formation of synaptic connections is followed by a prolonged period during which synaptic circuits are functional, but retain an elevated capacity for activity-dependent remodeling and functional plasticity. During this period, synaptic terminals appear fully mature, morphologically and physiologically. We show here, however, that synaptic terminals during this period are distinguished by their simultaneous accumulation of multiple growth-associated proteins at levels characteristic of axonal growth cones, and proteins involved in synaptic transmitter release at levels characteristic of adult synapses. We show further that newly formed synapses undergo a switch in the dynamic S-palmitoylation of proteins early in the critical period, which includes a large and specific decrease in the palmitoylation of GAP-43 and other major substrates characteristic of growth cones. Previous studies have shown that a similar reduction in ongoing palmitoylation of growth cone proteins is sufficient to stop advancing axons in vitro, suggesting that a developmental switch in protein S-palmitoylation serves to disengage the molecular machinery for axon extension in the absence of local triggers for remodeling during the critical period. Only much later does a decline in the availability of major growth cone components mark the molecular maturation of cortical synapses at the close of the critical period.

Effects of superoxide anions on endothelial Ca2+ signaling pathways

Although the involvement of free radicals in the development of endothelial dysfunction under pathological conditions, like diabetes and hypercholesterolemia, has been proposed frequently, there is limited knowledge as to how superoxide anions (O2-) might affect endothelial signal transduction. In this study, we investigated the effects of preincubation with the O2(-)-generating system xanthine oxidase/hypoxanthine (XO/HX) on mechanisms for Ca2+ signaling in cultured porcine aortic endothelial cells. Incubation of cells with XO/HX yielded increased intracellular Ca2+ release and capacitative Ca2+ entry in response to bradykinin and ATP in a time- and concentration-dependent manner. This effect was prevented by superoxide dismutase but not by the tyrosine kinase inhibitor tyrphostin A48. In addition, capacitative Ca2+ entry induced by the receptor-independent stimulus 2,5-di-(tert-butyl)-1,4-benzohydroquinone or thapsigargin was enhanced in O2(-)-exposed cells (+38% and +32%, respectively). Increased Ca2+ release in response to bradykinin in XO/HX-pretreated cells might be due to enhanced formation of inositol-1,4,5-trisphosphate (+140%). Exposure to XO/HX also affected other signal transduction mechanisms involved in endothelial Ca2+ signaling, such as microsomal cytochrome P450 epoxygenase and membrane hyperpolarization to Ca2+ store depletion with thapsigargin (+103% and +48%, respectively) and tyrosine kinase activity (+97%). A comparison of bradykinin-initiated intracellular Ca2+ release and thapsigargin-induced hyperpolarization with membrane viscosity modulated by XO/HX (decrease in viscosity) or cholesterol (increase in viscosity) reflected a negative correlation between bradykinin-initiated Ca2+ release and membrane viscosity. Because intracellular Ca2+ is a main regulator of endothelial vascular function, our data suggest that O2- anions are involved in regulation of the vascular endothelium.

Flow-mediated regulation of G-protein expression in cocultured vascular smooth muscle and endothelial cells

G-proteins have been implicated in the transduction of a number of flow-induced responses. We determined whether flow can modulate vascular endothelial or smooth muscle cell G-protein signaling. By use of a perfused transcapillary coculture system that permits the chronic exposure of cultured endothelial cells (ECs) and smooth muscle cells (SMCs) to physiological shear stresses, cocultures were exposed to stepwise increases in flow up to (1) 2 mL/min (low flow: 0.5 dyne/cm2), or (2) 44 mL/min (high flow: 15 dyne/cm2) and maintained for 72 hours before SMCs and ECs were harvested separately. Using Western blot analysis, EC Gi alpha3 expression was significantly increased (41+/-2.9%) by high-flow conditions compared with low-flow. The changes in G-protein expression were associated with a significant increase in endothelial nitric oxide synthase (eNOS) activity, elevated prostacyclin levels in the perfusing media, increased pertussis toxin-catalyzed ADP ribosylation of Gi alpha substrates, and enhanced agonist-stimulated GTPase activity in cocultured ECs. In contrast, high flow induced a significant decrease in Gi alpha1-2 expression (57 5%) in SMCs cocultured with ECs, an effect that was endothelium dependent, inhibited by indomethacin, and correlated with a decrease in pertussis toxin-catalyzed ADP ribosylation of Gi alpha substrates, reduced agonist-stimulated GTPase activity, and enhanced basal and G-protein-stimulated adenylyl cyclase activity. These data demonstrate that flow mediates selective changes in EC and SMC G-protein expression concomitant with changes in G-protein functionality and cellular signaling capacity. Moreover, flow-induced changes in SMC G-protein signaling capacity are endothelium dependent and require a cyclooxygenase product. G-protein modulation may thus represent an important mechanism whereby hemodynamic forces regulate vessel wall function.

Nitric oxide and peroxynitrite affect differently acetylcholine release, choline acetyltransferase activity, synthesis, and compartmentation of newly formed acetylcholine in Torpedo marmorata synaptosomes

Recent reports proposed that nitric oxide was a modulator of cholinergic transmission. Here, we examined the role of NO on cholinergic metabolism in a model of the peripheral cholinergic nervous synapse: synaptosomes from Torpedo electric organ. The presence of NO synthase was immunodetected in the cell bodies, in the nerve ending area of nerve-electroplate tissue and in the electroplates. Exogenous source of NO was provided from SIN1, a donor of NO and O2-., and an end-derivative peroxynitrite (ONOO-). SIN1 increased calcium-dependent acetylcholine (ACh) release induced by KCl depolarization or a calcium ionophore A23187. The formation of ONOO- was continuously followed by a new chemiluminescent assay. The addition of superoxide dismutase, that decreases the formation of ONOO-, did not impair the stimulation of ACh release, suggesting that NO itself was the main stimulating agent. When the endogenous source of NO was blocked by proadifen, an inhibitor of cytochrome P450 activity of NO synthase, both KCl- and A23187-induced ACh release were abolished; nevertheless, the inhibitor Ng-monomethyl-L-arginine did not modify ACh release when applied in a short time duration of action. Both NO synthase inhibitors reduced the synthesis of ACh from the radioactive precursor acetate and its incorporation into synaptic vesicles as did ONOO- chemically synthesized or formed from SIN1. In addition, choline acetyltransferase activity was strongly inhibited by ONOO- and SIN1 but not by the NO donors SNAP and SNP or, by NO synthase inhibitors. Altogether these results indicate that NO and ONOO modulate presynaptic cholinergic metabolism in the micromolar range, NO (up to 100 microM) being a stimulating agent of ACh release and ONOO- being an inhibitor of ACh synthesis and choline acetyltransferase activity.

Modulation of bradykinin receptor ligand binding affinity and its coupled G-proteins by nitric oxide

To determine whether nitric oxide (NO) can modulate bradykinin (BK) signaling pathways, we treated endothelial cells with an NO donor, S-nitrosoglutathione (GSNO), to determine its effect(s) on G-proteins (Gi and Gq) that are coupled to the type II kinin (BK2) receptor. Radioligand binding assays and Western analyses showed that GSNO (10-500 microM, 0-72 h) did not alter the expression of BK2 receptor, Gi, or Gq. However, GSNO caused a 6-fold increase in basal cGMP production and decreased high affinity BK bindings sites and GTPase activity by 74 and 85%, respectively. The cGMP analogue, dibutyryl-cGMP, also inhibited BK-stimulated GTPase activity by 74% suggesting that some of the effects of NO may be mediated through activation of guanylyl cyclase. The NO synthase inhibitor, Nomega-monomethyl-L-arginine, inhibited endogenous NO synthase activity and cGMP production by 91 and 76%, respectively, but increased BK-stimulated GTPase activity by 61%. To determine which G-proteins are affected by NO, we performed GTP binding assays with [35S]GTPgammaS followed by immunoprecipitation with specific G-protein antisera. Both GSNO and dibutyryl-cGMP increased basal G-protein GTP binding activities by 18-26%. However, GSNO decreased BK-stimulated Galphai2, Galphai3, and Galphaq/11 GTP binding activity by 93, 61, and 90%, respectively, whereas epinephrine-stimulated Galphas GTP binding activity was unaffected. These results suggest that NO can modulate BK signaling pathways by selectively inhibiting G-proteins of the Gi and Gq family.

Synergies and coincidence requirements between NO, cGMP, and Ca2+ in the induction of cerebellar long-term depression

Parallel fiber synapses onto Purkinje neurons in acute cerebellar slices undergo long-term depression (LTD) when presynaptic activity coincides with postsynaptic depolarization. These electrical inputs can be respectively replaced by nitric oxide (NO) and Ca2+ photolytically released inside the Purkinje neuron, showing that these two messengers are sufficient for LTD induction. NO acts via cGMP production because inhibitors of guanylate cyclase prevent LTD but can be circumvented by photoreleased cGMP combined with Ca2+ elevation. Three inhibitors of cGMP-dependent protein kinase, Rp-8Br-PET-cGMPS, KT5823, and a novel pseudosubstrate peptide, all block LTD. LTD induction permits <10 ms gap between NO release and Ca2+ elevation, whereas 200-300 ms is allowed between uncaged cGMP and Ca2+ increase. This surprising difference in timing precision can be explained either by tighter localization and faster decay of cGMP when generated by NO rather than uncaging, or by two independent coincidence detectors in series.

Opposite actions of nitric oxide on cholinergic synapses: which pathways?

Nitric oxide (NO) produced opposite effects on acetylcholine (ACh) release in identified neuroneuronal Aplysia synapses depending on the excitatory or the inhibitory nature of the synapse. Extracellular application of the NO donor, SIN-1, depressed the inhibitory postsynaptic currents (IPSCs) and enhanced the excitatory postsynaptic currents (EPSCs) evoked by presynaptic action potentials (1/60 Hz). Application of a membrane-permeant cGMP analog mimicked the effect of SIN-1 suggesting the participation of guanylate cyclase in the NO pathway. The guanylate cyclase inhibitor, methylene blue, blocked the NO-induced enhancement of EPSCs but only reduced the inhibition of IPSCs indicating that an additional mechanism participates to the depression of synaptic transmission by NO. Using nicotinamide, an inhibitor of ADP-ribosylation, we found that the NO-induced depression of ACh release on the inhibitory synapse also involves ADP-ribosylation mechanism(s). Furthermore, application of SIN-1 paired with cGMP-dependent protein kinase (cGMP-PK) inhibitors showed that cGMP-PK could play a role in the potentiating but not in the depressing effect of NO on ACh release. Increasing the frequency of stimulation of the presynaptic neuron from 1/60 Hz to 0.25 or 1 Hz potentiated the EPSCs and reduced the IPSCs. In these conditions, the potentiating effect of NO on the excitatory synapse was reduced, whereas its depressing effect on the inhibitory synapse was unaffected. Moreover the frequency-dependent enhancement of ACh release in the excitatory synapse was greatly reduced by the inhibition of NO synthase. Our results indicate that NO may be involved in different ways of modulation of synaptic transmission depending on the type of the synapse including synaptic plasticity.

Nitric oxide modulates synaptic vesicle docking fusion reactions

Nitric oxide (NO) stimulates calcium-independent neurotransmitter release from synaptosomes. NO-stimulated release was found to be inhibited by Botulinum neurotoxins that inactivate the core complex of synaptic proteins involved in the docking and fusion of synaptic vesicles. In experiments using recombinant proteins, NO donors increased formation of the VAMP/SNAP-25/syntaxin 1a core complex and inhibited the binding of n-sec1 to syntaxin 1a. The combined effects of these activities is predicted to promote vesicle docking/fusion. The sulfhydryl reagent NEM inhibited the binding of n-sec1 to syntaxin 1a, while beta-ME could reverse the NO-enhanced association of VAMP/SNAP-25/syntaxin 1a. These data suggest that post-translational modification of sulfhydryl groups by a nitrogen monoxide (likely to be NO+) alters the synaptic protein interactions that regulate neurotransmitter release and synaptic plasticity.

Nitric oxide synthase-cyclo-oxygenase pathways in organum vasculosum laminae terminalis: possible role in pyrogenic fever in rabbits

1. Fever was induced in rabbits by administration of Escherichia coli endotoxin (lipopolysaccharide; LPS; 0.001-10 micrograms) into the organum vasculosum laminae terminalis (OVLT). Deep body temperature was evaluated over a period of 7 h. 2. The LPS-induced febrile response was mimicked by intra-OVLT injection of the nitric oxide (NO) donors, S-nitroso-acetylpenicillamine (SNAP, 1-10 micrograms), sodium nitroprusside (SNP, 50 micrograms), or hydroxylamine (10 micrograms), the cyclic GMP analogue 8-bromo-cyclic GMP (8-Br-cyclic GMP, 10-100 micrograms), or prostaglandin E2 (PGE2, 0.2 micrograms). 3. Dexamethasone (Dex, a potent inhibitor of the transcription of inducible NO synthase, iNOS, 10 micrograms), anisomycin (a protein synthesis inhibitor, 100 micrograms), L-N5-(1-iminoethyl)ornithine (L-NIO; an irreversible NOS inhibitor, 10-200 micrograms), aminoguanidine (a specific iNOS inhibitor, 1000 micrograms), or NG-methyl-L-arginine acetate (L-NMMA, a NOS inhibitor, 100 micrograms) inhibited fever induced by LPS when injected into the OVLT 1 h before LPS injection. An intra-OVLT dose of 1000 micrograms of NG-nitro-L-arginine methyl ester (L-NAME, a potent inhibitor of constitutive NOS) did not exhibit antipyretic effects. 4. Methylene blue (an inhibitor of NOS and soluble guanylate cyclase, 1-10 micrograms), 6-(phenylamino)-5,8-quinolinedione (LY-83583; an inhibitor of soluble guanylate cyclase and NO release, 20 micrograms), or indomethacin (an inhibitor of cyclo-oxygenase, COX, 400 micrograms) inhibited fever induced by LPS when injected into the OVLT 1 h before LPS injection. Pretreatment with methylene blue or haemoglobin (a NO scavenger, 100 micrograms) attenuated the fever induced by intra-OVLT injection of SNAP. 5. The PGE2-induced fever was potentiated, rather then attenuated, by pretreatment with an intra-OVLT dose of animoguanidine (1000 micrograms), L-NMMA (100 micrograms) or L-NIO (200 micrograms). 6. These results suggest that iNOS-COX pathways in the OVLT represent an important mechanism for modulation of pyrogenic fever in rabbits.

Exogenous nitric oxide causes collapse of retinal ganglion cell axonal growth cones in vitro

We show that nitric oxide (NO) from applied NO-donating chemicals induces collapse of ganglion cell axonal growth cones extending from explants of tadpole retina in culture. Peroxynitrite, a neurotoxic product of NO and superoxide reaction, did not induce collapse, and oxyhemoglobin, which binds NO, blocked the highly effective collapsing activity of the NO donor S-nitrosocysteine. Membrane-permeable analogs of cyclic guanosine monophosphate had no collapsing activity. Inhibitors of NO synthase did not induce collapse. NO is a potential retrograde messenger through which postsynaptic neurons signal to their inputs to modify synaptic efficacy following NMDA receptor activation. Our results suggest a role for NO as such a messenger during development of the retinotectal projection.

Cell protective role of melatonin in the brain

In recent years an increasing amount of data has been published involving melatonin in the control of brain function. The pineal gland exerts a depressive influence on CNS excitability. This activity is linked to melatonin, since pharmacological doses of the hormone prevent seizures in several animal models. In addition, melatonin also has analgesic properties in these species. However, the sites and mechanism of melatonin action are not known. A role for the pineal gland and its hormone melatonin as a homeostatic system controlling brain excitability has been proposed, and GABA-containing neurons may be involved in some central action of melatonin. There is evidence supporting a role of melatonin in the regulation of the GABA-benzodiazepine receptor complex, and it appears that melatonin potentiates this inhibitory neurotransmitter system in brain. Melatonin does not bind to GABA or benzodiazepine binding sites themselves, because in vitro binding data showed that melatonin is a weak competitor of benzodiazepine binding in brain membranes at concentrations greater than 10(-5) M. The effect of melatonin on brain activity also involves the participation of corticotropic and opioid peptides, and the existence of an opioid-antiopioid homeostatic system is proposed, with the GABA-benzodiazepine receptor complex as an effector. Moreover, the interaction of melatonin with corticotropic peptides and mitochondrial benzodiazepine receptors may result in a participation of neurosteroids in the control of GABA activity and function. The most recently available data from biochemical and electrophysiological studies support the possibility that the anticonvulsant and depressive effects of melatonin on neuron activity may depend on its antioxidant and antiexcitotoxic roles, acting as a free radical scavenger and regulating brain glutamate receptors. The full characterization of the nuclear melatonin receptor explains the genomic effects of melatonin, opening a new perspective regarding actions and roles of melatonin as a cellular protector.