Genetic mouse models of the NO receptor 'soluble' guanylyl cyclases

Cellular Factors That Shape the Activity or Function of Nitric Oxide-Stimulated Soluble Guanylyl Cyclase

NO-stimulated guanylyl cyclase (SGC) is a hemoprotein that plays key roles in various physiological functions. SGC is a typical enzyme-linked receptor that combines the functions of a sensor for NO gas and cGMP generator. SGC possesses exclusive selectivity for NO and exhibits a very fast binding of NO, which allows it to function as a sensitive NO receptor. This review describes the effect of various cellular factors, such as additional NO, cell thiols, cell-derived small molecules and proteins on the function of SGC as cellular NO receptor. Due to its vital physiological function SGC is an important drug target. An increasing number of synthetic compounds that affect SGC activity via different mechanisms are discovered and brought to clinical trials and clinics. Cellular factors modifying the activity of SGC constitute an opportunity for improving the effectiveness of existing SGC-directed drugs and/or the creation of new therapeutic strategies.

Hippocampal AMPA- and NMDA-induced cGMP signals are mainly generated by NO-GC2 and are under tight control by PDEs 1 and 2

In the central nervous system, the nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) signalling cascade has an established role in fine-tuning of synaptic transmission. In the present study, we asked which isoform of NO-sensitive guanylyl cyclase, NO-GC1 or NO-GC2, is responsible for generation of N-methyl-d-aspartate (NMDA)- and AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid)-induced cGMP signals and which of the phosphodiesterases (PDEs) is responsible for degradation. To this end, we performed live cell fluorescence measurements of primary hippocampal neurons isolated from NO-GC isoform-deficient mice. Although both isoforms contributed to the NMDA- and AMPA-induced cGMP signals, NO-GC2 clearly played the predominant role. Whereas under PDE-inhibiting conditions the cGMP levels elicited by both glutamatergic ligands were comparable, NMDA-induced cGMP signals were clearly higher than the AMPA-induced ones in the absence of PDE inhibitors. Thus, AMPA-induced cGMP signals are more tightly controlled by PDE-mediated degradation than NMDA-induced signals. In addition, these findings are compatible with the existence of at least two different pools of cGMP in both of which PDE1 and PDE2-known to be highly expressed in the hippocampus-are mainly responsible for cGMP degradation. The finding that distinct pools of cGMP are equipped with different amounts of PDEs highlights the importance of PDEs for the shape of NO-induced cGMP signals in the central nervous system.

AMPA Induces NO-Dependent cGMP Signals in Hippocampal and Cortical Neurons via L-Type Voltage-Gated Calcium Channels

The nitric oxide (NO)/cGMP signaling cascade has an established role in synaptic plasticity. However, with conventional methods, the underlying cGMP signals were barely detectable. Here, we set out to confirm the well-known NMDA-induced cGMP increases, to test the impact of AMPA on those signals, and to identify the relevant phosphodiesterases (PDEs) using a more sensitive fluorescence resonance energy transfer (FRET)-based method. Therefore, a “knock-in” mouse was generated that expresses a FRET-based cGMP indicator (cGi-500) allowing detection of cGMP concentrations between 100 nM and 3 μM. Measurements were performed in cultured hippocampal and cortical neurons as well as acute hippocampal slices. In hippocampal and cortical neurons, NMDA elicited cGMP signals half as high as the ones elicited by exogenous NO. Interestingly, AMPA increased cGMP independently of NMDA receptors and dependent on NO synthase (NOS) activation. NMDA- and AMPA-induced cGMP signals were not additive indicating that both pathways converge on the level of NOS. Accordingly, the same PDEs, PDE1 and PDE2, were responsible for degradation of NMDA- as well as AMPA-induced cGMP signals. Mechanistically, AMPAR induced calcium influx through L-type voltage-gated calcium channels leading to NOS and finally NO-sensitive guanylyl cyclase activation. Our results demonstrate that in addition to NMDA also AMPA triggers endogenous NO formation and hence cGMP production.

cGMP Imaging in Brain Slices Reveals Brain Region-Specific Activity of NO-Sensitive Guanylyl Cyclases (NO-GCs) and NO-GC Stimulators

Impaired NO-cGMP signaling has been linked to several neurological disorders. NO-sensitive guanylyl cyclase (NO-GC), of which two isoforms—NO-GC1 and NO-GC2—are known, represents a promising drug target to increase cGMP in the brain. Drug-like small molecules have been discovered that work synergistically with NO to stimulate NO-GC activity. However, the effects of NO-GC stimulators in the brain are not well understood. In the present study, we used Förster/fluorescence resonance energy transfer (FRET)-based real-time imaging of cGMP in acute brain slices and primary neurons of cGMP sensor mice to comparatively assess the activity of two structurally different NO-GC stimulators, IWP-051 and BAY 41-2272, in the cerebellum, striatum and hippocampus. BAY 41-2272 potentiated an elevation of cGMP induced by the NO donor DEA/NO in all tested brain regions. Interestingly, IWP-051 potentiated DEA/NO-induced cGMP increases in the cerebellum and striatum, but not in the hippocampal CA1 area or primary hippocampal neurons. The brain-region-selective activity of IWP-051 suggested that it might act in a NO-GC isoform-selective manner. Results of mRNA in situ hybridization indicated that the cerebellum and striatum express NO-GC1 and NO-GC2, while the hippocampal CA1 area expresses mainly NO-GC2. IWP-051-potentiated DEA/NO-induced cGMP signals in the striatum of NO-GC2 knockout mice but was ineffective in the striatum of NO-GC1 knockout mice. These results indicate that IWP-051 preferentially stimulates NO-GC1 signaling in brain slices. Interestingly, no evidence for an isoform-specific effect of IWP-051 was observed when the cGMP-forming activity of whole brain homogenates was measured. This apparent discrepancy suggests that the method and conditions of cGMP measurement can influence results with NO-GC stimulators. Nevertheless, it is clear that NO-GC stimulators enhance cGMP signaling in the brain and should be further developed for the treatment of neurological diseases.

Cytochrome b5 Reductase 3 Modulates Soluble Guanylate Cyclase Redox State and cGMP Signaling

RATIONALE

Soluble guanylate cyclase (sGC) heme iron, in its oxidized state (Fe³⁺), is desensitized to NO and limits cGMP production needed for downstream activation of protein kinase G-dependent signaling and blood vessel dilation.

OBJECTIVE

Although reactive oxygen species are known to oxidize the sGC heme iron, the basic mechanism(s) governing sGC heme iron recycling to its NO-sensitive, reduced state remain poorly understood.

METHODS AND RESULTS

Oxidant challenge studies show that vascular smooth muscle cells have an intrinsic ability to reduce oxidized sGC heme iron and form protein-protein complexes between cytochrome b5 reductase 3, also known as methemoglobin reductase, and oxidized sGC. Genetic knockdown and pharmacological inhibition in vascular smooth muscle cells reveal that cytochrome b5 reductase 3 expression and activity is critical for NO-stimulated cGMP production and vasodilation. Mechanistically, we show that cytochrome b5 reductase 3 directly reduces oxidized sGC required for NO sensitization as assessed by biochemical, cellular, and ex vivo assays.

CONCLUSIONS

Together, these findings identify new insights into NO-sGC-cGMP signaling and reveal cytochrome b5 reductase 3 as the first identified physiological sGC heme iron reductase in vascular smooth muscle cells, serving as a critical regulator of cGMP production and protein kinase G-dependent signaling.

cGMP becomes a drug target

Cyclic guanosine 3′,5′-monophosphate (cGMP) serves as a second messenger molecule, which regulates pleiotropic cellular functions in health and disease. cGMP is generated by particulate or soluble guanylyl cyclases upon stimulation with natriuretic peptides or nitric oxide, respectively. Furthermore, the cGMP concentration is modulated by cGMP-degrading phosphodiesterases. Several targets of cGMP are utilized to effect its various cellular functions. These effector molecules comprise cGMP-dependent protein kinases, ion channels, and phosphodiesterases. During the last decade, it emerged that cGMP is a novel drug target for the treatment of pulmonary and cardiovascular disorders. In this respect, several drugs were developed, which are now in clinical phase studies for, e.g., pulmonary hypertension or cardiovascular diseases. These new drugs act NO-independently with/without heme on soluble guanylyl cyclases or induce subtypes of particular guanylyl cyclases and thereby lead to new therapeutic concepts and horizons. In this regard, the fifth cGMP meeting held in June 2011 in Halle, Germany, comprised the new therapeutic challenges with the novel functional and structural concepts of cGMP generating and effector molecules. This report summarizes the new data on molecular mechanisms, (patho)physiological relevance, and therapeutic potentials of the cGMP signaling system that were presented at this meeting.

Sildenafil Potentiates a cGMP-Dependent Pathway to Promote Melanoma Growth

Sildenafil, an inhibitor of the cGMP-degrading phosphodiesterase 5 that is used to treat erectile dysfunction, has been linked to an increased risk of melanoma. Here, we have examined the potential connection between cGMP-dependent signaling cascades and melanoma growth. Using a combination of biochemical assays and real-time monitoring of melanoma cells, we report a cGMP-dependent growth-promoting pathway in murine and human melanoma cells. We document that C-type natriuretic peptide (CNP), a ligand of the membrane-bound guanylate cyclase B, enhances the activity of cGMP-dependent protein kinase I (cGKI) in melanoma cells by increasing the intracellular levels of cGMP. Activation of this cGMP pathway promotes melanoma cell growth and migration in a p44/42 MAPK-dependent manner. Sildenafil treatment further increases intracellular cGMP concentrations, potentiating activation of this pathway. Collectively, our data identify this cGMP-cGKI pathway as the link between sildenafil usage and increased melanoma risk.

Role of CB2 receptors and cGMP pathway on the cannabinoid-dependent antiepileptic effects in an in vivo model of partial epilepsy

This study aimed at providing an insight on the possible role of cannabinoid (CB) type 2 receptors (CB2R) and cGMP pathway in the antiepileptic activity of WIN 55,212-2, (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl) pyrrolo[1,2,3-de]-1,4-benzoxazin-6-Yl]-1-naphthalenylmethanone, a non-selective CB agonist, in the maximal dentate activation (MDA) model of partial epilepsy in adult male rats. We evaluated the activity of a CB2 antagonist/inverse agonist AM630, [6-iodo-2-methyl-1-[2-(4-morpholinyl)ethyl]-1H-indol-3-yl](4-methoxyphenyl)methanone or 6-iodopravadoline, alone or in co-administration with WIN 55,212-2. Also, in the MDA model it was investigated the co-treatment of WIN 55,212-2 and 1H-[1,2,4]Oxadiazole[4,3-a]quinoxalin-1-one (ODQ), a specific inhibitor of the nitric oxide (NO)-activated soluble guanylyl cyclase (sGC), the cGMP producing enzyme. The WIN 55,212-2-dependent (21mg/kg) antiepileptic effects were significantly increased by the co-administration with AM630 and by the co-treatment with ODQ (10mg/kg). Whereas, the administration of AM630 (2mg/kg), alone exerts no effects on hippocampal hyperexcitability. Our data show that pharmacological blockade of CB2 receptors and of sGC seems to cooperate with WIN in its antiepileptic action. These findings shed light on CB signaling mechanisms, hinting that the modulation of the effects of CB agonist in the hyperexcitability phenomena may be exerted both by targeting CB receptors and their possible downstream effectors, such as nitrergic-dependent cGMP pathway.

Specific inhibitory effects of the NO donor MAHMA/NONOate on human platelets

Nitric oxide (NO) is a physiological inhibitor of platelet function and has vaso-dilating effects. Therefore, synthesized NO releasing agents are used e.g. in cardiovascular medicine. The aim of this study was to characterise specific effects of the short living agent MAHMA/NONOate, a NO donor of the diazeniumdiolate class, on human platelets. Whole blood was obtained from healthy volunteers. In washed human platelets, the MAHMA/NONOate induced phosphorylation of the vasodilator-stimulated phosphoprotein (VASP) and cyclic nucleotide production were studied by Western Blot and by enzyme immunoassay kits. Agonist induced aggregation was measured in platelet rich plasma. Paired Student׳s t-test was used for statistical analysis. MAHMA/NONOate significantly stimulated platelet VASP phosphorylation in a concentration dependent manner and increased intracellular cGMP, but not cAMP levels, transiently. ODQ, a specific inhibitor of the soluble guanylyl cyclase, completely prevented VASP phosphorylation induced by low MAHMA/NONOate concentrations (5nM-15nM). The effects of higher concentrations (30-200nM) were only partially inhibited by ODQ. MAHMA/NONOate reduced platelet aggregation induced by low doses of agonists (2µM ADP, 0.5µg/mL collagen, 5µM TRAP-6) in a concentration dependent manner. MAHMA/NONOate leads to a rapid and transient activation of platelet inhibitory systems, accompanied by decreased platelet aggregation induced by low dose agonists. At low MAHMA/NONOate concentrations, the effects are cGMP dependent and at higher concentrations additionally cGMP independent. The substance could be of interest for clinical situations requiring transient and subtotal inhibition of platelet function.

Stimulation of the soluble guanylate cyclase (sGC) inhibits fibrosis by blocking non-canonical TGFβ signalling

OBJECTIVES

We have previously described the antifibrotic role of the soluble guanylate cyclase (sGC). The mode of action, however, remained elusive. In the present study, we describe a novel link between sGC signalling and transforming growth factor β (TGFβ) signalling that mediates the antifibrotic effects of the sGC.

METHODS

Human fibroblasts and murine sGC knockout fibroblasts were treated with the sGC stimulator BAY 41-2272 or the stable cyclic guanosine monophosphate (cGMP) analogue 8-Bromo-cGMP and stimulated with TGFβ. sGC knockout fibroblasts were isolated from sGCI(fl/fl) mice, and recombination was induced by Cre-adenovirus. In vivo, we studied the antifibrotic effects of BAY 41-2272 in mice overexpressing a constitutively active TGF-β1 receptor.

RESULTS

sGC stimulation inhibited TGFβ-dependent fibroblast activation and collagen release. sGC knockout fibroblasts confirmed that the sGC is essential for the antifibrotic effects of BAY 41-2272. Furthermore, 8-Bromo-cGMP reduced TGFβ-dependent collagen release. While nuclear p-SMAD2 and 3 levels, SMAD reporter activity and transcription of classical TGFβ target genes remained unchanged, sGC stimulation blocked the phosphorylation of ERK. In vivo, sGC stimulation inhibited TGFβ-driven dermal fibrosis but did not change p-SMAD2 and 3 levels and TGFβ target gene expression, confirming that non-canonical TGFβ pathways mediate the antifibrotic sGC activity.

CONCLUSIONS

We elucidated the antifibrotic mode of action of the sGC that increases cGMP levels, blocks non-canonical TGFβ signalling and inhibits experimental fibrosis. Since sGC stimulators have shown excellent efficacy and tolerability in phase 3 clinical trials for pulmonary arterial hypertension, they may be further developed for the simultaneous treatment of fibrosis and vascular disease in systemic sclerosis.

Crystal structures of the catalytic domain of human soluble guanylate cyclase

Soluble guanylate cyclase (sGC) catalyses the synthesis of cyclic GMP in response to nitric oxide. The enzyme is a heterodimer of homologous α and β subunits, each of which is composed of multiple domains. We present here crystal structures of a heterodimer of the catalytic domains of the α and β subunits, as well as an inactive homodimer of β subunits. This first structure of a metazoan, heteromeric cyclase provides several observations. First, the structures resemble known structures of adenylate cyclases and other guanylate cyclases in overall fold and in the arrangement of conserved active-site residues, which are contributed by both subunits at the interface. Second, the subunit interaction surface is promiscuous, allowing both homodimeric and heteromeric association; the preference of the full-length enzyme for heterodimer formation must derive from the combined contribution of other interaction interfaces. Third, the heterodimeric structure is in an inactive conformation, but can be superposed onto an active conformation of adenylate cyclase by a structural transition involving a 26° rigid-body rotation of the α subunit. In the modelled active conformation, most active site residues in the subunit interface are precisely aligned with those of adenylate cyclase. Finally, the modelled active conformation also reveals a cavity related to the active site by pseudo-symmetry. The pseudosymmetric site lacks key active site residues, but may bind allosteric regulators in a manner analogous to the binding of forskolin to adenylate cyclase. This indicates the possibility of developing a new class of small-molecule modulators of guanylate cyclase activity targeting the catalytic domain.

A cGMP-dependent protein kinase (PKG) controls synaptic transmission tolerance to acute oxidative stress at the Drosophila larval neuromuscular junction

Increasing evidence demonstrates that modulating the cGMP-dependent protein kinase G (PKG) pathway produces an array of behavioral phenotypes in the fruit fly, Drosophila melanogaster. Altering PKG activity, either genetically via the foraging ( for) gene or using pharmacology modifies tolerance to acute abiotic stresses such as hyperthermia and hypoxia. PKG signaling has been shown to modulate neuroprotection in many experimental paradigms of acute brain trauma and chronic neurodegenerative diseases. However, relatively little is known about how this stress-induced neuroprotective mechanism affects neural communication. In this study, we investigated the role PKG activity has on synaptic transmission at the Drosophila larval neuromuscular junction (NMJ) during acute oxidative stress and found that the application of 2.25 mM hydrogen peroxide (H2O2) disrupts synaptic function by rapidly increasing the rate of neuronal failure. Here, we report that reducing PKG activity through either natural genetic variation or an induced mutation of the for gene increases synaptic tolerance during acute oxidative conditions. Furthermore, pharmacological manipulations revealed that neurotransmission is significantly extended during acute H2O2 exposure upon inhibition of the PKG pathway. Conversely, activation of this signaling cascade using either genetics or pharmacology significantly reduced the time until synaptic failure. Therefore, these findings suggest a potential role for PKG activity to regulate the tolerance of synaptic transmission during acute oxidative stress, where inhibition promotes functional protection while activation increases susceptibility to neurotransmission breakdown.

Contributions of nitric oxide synthases, dietary nitrite/nitrate, and other sources to the formation of NO signaling products

Mice lacking all three nitric oxide synthase (NOS) genes remain viable even though deletion of the major downstream target of NO, soluble guanylyl cyclase, is associated with a dramatically shortened life expectancy. Moreover, findings of relatively normal flow responses in eNOS knockouts are generally attributed to compensatory mechanisms including upregulation of remaining NOS isoforms, but the alternative possibility that dietary nitrite/nitrate (NOx) may contribute to basal levels of NO signaling has never been investigated.

AIM

The aim of the present study was to examine how NO signaling products (nitrosated and nitrosylated proteins) and NO metabolites (nitrite, nitrate) are affected by single NOS deletions and whether dietary NOx plays a compensatory role in any deficiency. Specifically, we sought to ascertain whether profound alterations of these products arise upon genetic deletion of either NOS isoform, inhibition of all NOS activity, NOx restriction, or all of the above.

RESULTS

Our results indicate that while some significant changes do indeed occur, they are surprisingly moderate and compartmentalized to specific tissues. Unexpectedly, even after pharmacological inhibition of all NOSs and restriction of dietary NOx intake in eNOS knockout mice significant levels of NO-related products remain. Innovation/Conclusion: These findings suggest that a yet unidentified source of NO, unrelated to NOSs or dietary NOx, may be sustaining basal NO signaling in tissues. Given the significance of NO for redox regulation in health and disease, it would seem to be important to identify the nature of this additional source of NO products as it may offer new therapeutic avenues for correcting NO deficiencies.

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.

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.

Presynaptic nitric oxide/cGMP facilitates glutamate release via hyperpolarization-activated cyclic nucleotide-gated channels in the hippocampus

In hippocampal neurons, synaptic transmission is affected by a variety of modulators, including nitric oxide (NO), which was proposed as a retrograde messenger as long as two decades ago. NO signals via two NO-sensitive guanylyl cyclases (NO-GCs) (NO-GC1 and NO-GC2) and the subsequent increase in cGMP. Lack of long-term potentiation in mice deficient in either one of the two NO-GCs demonstrates the involvement of both NO-GCs in synaptic transmission. However, the physiological consequences of NO/cGMP and the cellular mechanisms involved are unknown. Here, we analyzed glutamatergic synaptic transmission, most likely reflecting glutamate release, in the hippocampal CA1 region of NO-GC knockout mice by single-cell recording, and found glutamate release to be reduced under basal and stimulated conditions in the NO-GC1 knockout mice, but restorable to wild-type-like levels with a cGMP analog. Conversely, an inhibitor of NO/cGMP signaling, ODQ, reduced glutamate release in wild-type mice to knockout-like levels; thus, we conclude that presynaptic cGMP formed by NO-GC1 facilitates glutamate release. In this pathway, NO is supplied by endothelial NO synthase. In search of a cGMP target, we found that two mechanistically distinct blockers of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels (ZD7288 and DK-AH269) abolished the cGMP-induced increase in glutamate release, suggesting that cGMP either directly or indirectly signals via HCN channels. In summary, we unravel a presynaptic role of NO/cGMP most likely in glutamate release and propose that HCN channels act as effectors for cGMP.

Evaluation of the therapeutic utility of phosphodiesterase 5A inhibition in the mdx mouse model of duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a devastating and ultimately fatal disease characterized by progressive muscle wasting and weakness. DMD is caused by the absence of a functional dystrophin protein, which in turn leads to reduced expression and mislocalization of dystrophin-associated proteins including neuronal nitric oxide (NO) synthase mu (nNOSμ). Disruption of nNOSμ signaling results in muscle fatigue and unopposed sympathetic vasoconstriction during exercise, thereby increasing contraction-induced damage in dystrophin-deficient muscles. The loss of normal nNOSμ signaling during exercise is central to the vascular dysfunction proposed over 40 years ago to be an important pathogenic mechanism in DMD. Recent preclinical studies focused on circumventing defective nNOSμ signaling in dystrophic skeletal and cardiac muscle by inhibiting phosphodiesterase 5A (PDE5A) have shown promising results. This review addresses nNOS signaling in normal and dystrophin-deficient muscles and the potential of PDE5A inhibition as a therapeutic approach for the treatment of cardiovascular deficits in DMD.

Anti-aggregating effect of BAY 58-2667, an activator of soluble guanylyl cyclase

The purpose of the present study was to determine whether an activator of soluble guanylyl cyclase (sGC), BAY 58-2667, inhibits platelet aggregation and to clarify its mechanism of action. Blood was collected from anesthetized WKY rats. The aggregation of washed platelet was measured and the production of cAMP and cGMP was determined. BAY 58-2667 produced a partial inhibition of the ADP- and collagen-induced platelet aggregation, but did not significantly affect thrombin-induced aggregation. In ADP-induced platelet aggregation, the inhibitory effects of BAY 58-2667 were associated with an increased level of both cGMP and cAMP while that of the prostacyclin analogue, beraprost, was correlated only with an increase in cAMP. The inhibitor of sGC, ODQ, enhanced the effects of BAY 58-2667. The presence of L-nitroarginine, an inhibitor of NO-synthase, hydroxocobalamin, a scavenger of NO, or that of three different NO-donors did not affect the anti-aggregating effect of BAY 58-2667. However, the anti-aggregating effects of beraprost were potentiated by BAY 58-2667. Therefore, the platelet inhibitory effects of BAY 58-2667 are associated with the generation of cGMP and a secondary increase in cAMP, both being totally NO-independent. When the sGC is oxidized, BAY 58-2667 becomes a relevant anti-aggregating agent, which synergizes with the cAMP-dependent pathway.

What is the real physiological NO concentration in vivo?

Clarity about the nitric oxide (NO) concentrations existing physiologically is essential for developing a quantitative understanding of NO signalling, for performing experiments with NO that emulate reality, and for knowing whether or not NO concentrations become abnormal in disease states. A decade ago, a value of about 1 μM seemed reasonable based on early electrode measurements and a provisional estimate of the potency of NO for its guanylyl cyclase-coupled receptors, which mediate physiological NO signal transduction. Since then, numerous efforts to measure NO concentrations directly using electrodes in cells and tissues have yielded an irreconcilably large spread of values. In compensation, data from several alternative approaches have now converged to provide a more coherent picture. These approaches include the quantitative analysis of NO-activated guanylyl cyclase, computer modelling based on the type, activity and amount of NO synthase enzyme contained in cells, the use of novel biosensors to monitor NO release from single endothelial cells and neurones, and the use of guanylyl cyclase as an endogenous NO biosensor in tissue subjected to a variety of challenges. All these independent lines of evidence suggest the physiological NO concentration range to be 100 pM (or below) up to ∼5 nM, orders of magnitude lower than was once thought.