cGMP signaling through cAMP- and cGMP-dependent protein kinases

Fine tuning by protein kinases of CaV1.2 channel current in rat tail artery myocytes

Seventeen compounds, rather selective, direct or indirect inhibitors and activators of PKA, PKG, and PKC, were analysed for effects on vascular Ca_V1.2 channel current (I_Ca1.2) by using the patch-clamp technique in single rat tail artery myocytes. The aim was to investigate how PKs regulate I_Ca1.2 and disclose any unexpected modulation of Ca_V1.2 channel function by these agents. The cAMP analogues 8-Br-cAMP and 6-Bnz-cAMP partially reduced I_Ca1.2 in dialysed cells, while weakly increasing it under the perforated configuration. The β-adrenoceptor agonist isoproterenol and the adenylate cyclase activator forskolin concentration-dependently increased I_Ca1.2; this effect was reversed by PKA inhibitors H-89 and KT5720, but not by PKI 6-22. The cGMP analogue 8-Br-cGMP, similarly to the NO-donor SNP, moderately reduced I_Ca1.2, this effect being reversed to a slight stimulation under the perforated configuration. Among PKG inhibitors, Rp-8-Br-PET-cGMPS decreased current amplitude in a concentration-dependent manner while Rp-8-Br-cGMPS was ineffective. The non-specific phosphodiesterase inhibitor IBMX increased I_Ca1.2, while H-89, KT5720, and PKI 6-22 antagonized this effect. The PKC activator PMA, but not the diacylglycerol analogue OAG, stimulated I_Ca1.2 in a concentration-dependent manner; conversely, the PKCα inhibitor Gö6976 markedly reduced basal I_Ca1.2 and, similarly to the PKCδ (rottlerin) and PKCε translocation inhibitors antagonised PMA-induced current stimulation. The ensemble of findings indicates that the stimulation of cAMP/PKA, in spite of the paradoxical effect of both 8-Br-cAMP and 6-Bnz-cAMP, or PKC pathways enhanced, while that of cGMP/PKG weakly inhibited I_Ca1.2 in rat tail artery myocytes. Since Rp-8-Br-PET-cGMPS and Gö6976 appeared to block directly Ca_V1.2 channel, their docking to the channel protein was investigated. Both compounds appeared to bind the α_1C subunit in a region involved in Ca_V1.2 channel inactivation, forming an interaction network comparable to that of Ca_V1.2 channel blockers. Therefore, caution should accompany the use of these agents as pharmacological tools to elucidate the mechanism of action of drugs on vascular preparations.

Control of Motility in the Internal Anal Sphincter

The internal anal sphincter (IAS) plays an important role in the maintenance of fecal continence since it generates tone and is responsible for > 70% of resting anal pressure. During normal defecation the IAS relaxes. Historically, tone generation in gastrointestinal muscles was attributed to mechanisms arising directly from smooth muscle cells, ie, myogenic activity. However, slow waves are now known to play a fundamental role in regulating gastrointestinal motility and these electrical events are generated by the interstitial cells of Cajal. Recently, interstitial cells of Cajal, as well as slow waves, have also been identified in the IAS making them viable candidates for tone generation. In this review we discuss four different mechanisms that likely contribute to tone generation in the IAS. Three of these involve membrane potential, L-type Ca²⁺ channels and electromechanical coupling (ie, summation of asynchronous phasic activity, partial tetanus, and window current), whereas the fourth involves the regulation of myofilament Ca²⁺ sensitivity. Contractile activity in the IAS is also modulated by sympathetic motor neurons that significantly increase tone and anal pressure, as well as inhibitory motor neurons (particularly nitrergic and vasoactive intestinal peptidergic) that abolish contraction and assist with normal defecation. Alterations in IAS motility are associated with disorders such as fecal incontinence and anal fissures that significantly decrease the quality of life. Understanding in greater detail how tone is regulated in the IAS is important for developing more effective treatment strategies for these debilitating defecation disorders.

Papaverine Prevents Vasospasm by Regulation of Myosin Light Chain Phosphorylation and Actin Polymerization in Human Saphenous Vein

Objective

Papaverine is used to prevent vasospasm in human saphenous veins (HSV) during vein graft preparation prior to implantation as a bypass conduit. Papaverine is a nonspecific inhibitor of phosphodiesterases, leading to increases in both intracellular cGMP and cAMP. We hypothesized that papaverine reduces force by decreasing intracellular calcium concentrations ([Ca²⁺]_i) and myosin light chain phosphorylation, and increasing actin depolymerization via regulation of actin regulatory protein phosphorylation.

Approach and Results

HSV was equilibrated in a muscle bath, pre-treated with 1 mM papaverine followed by 5 μM norepinephrine, and force along with [Ca²⁺]_i levels were concurrently measured. Filamentous actin (F-actin) level was measured by an in vitro actin assay. Tissue was snap frozen to measure myosin light chain and actin regulatory protein phosphorylation. Pre-treatment with papaverine completely inhibited norepinephrine-induced force generation, blocked increases in [Ca²⁺]_i and led to a decrease in the phosphorylation of myosin light chain. Papaverine pre-treatment also led to increased phosphorylation of the heat shock-related protein 20 (HSPB6) and the vasodilator stimulated phosphoprotein (VASP), as well as decreased filamentous actin (F-actin) levels suggesting depolymerization of actin.

Conclusions

These results suggest that papaverine-induced force inhibition of HSV involves [Ca²⁺]_i-mediated inhibition of myosin light chain phosphorylation and actin regulatory protein phosphorylation-mediated actin depolymerization. Thus, papaverine induces sustained inhibition of contraction of HSV by the modulation of both myosin cross-bridge formation and actin cytoskeletal dynamics and is a pharmacological alternative to high pressure distention to prevent vasospasm.

Nitrergic neuromuscular transmission in the mouse internal anal sphincter is accomplished by multiple pathways and postjunctional effector cells

The effector cells and second messengers participating in nitrergic neuromuscular transmission (NMT) were investigated in the mouse internal anal sphincter (IAS). Protein expression of guanylate cyclase (GCα, GCβ) and cyclic GMP-dependent protein kinase I (cGKI) were examined in cryostat sections with dual-labeling immunohistochemical techniques in PDGFRα(+) cells, interstitial cells of Cajal (ICC), and smooth muscle cells (SMC). Gene expression levels were determined with quantitative PCR of dispersed cells from Pdgfrα(egfp/+), Kit(copGFP/+), and smMHC(Cre-egfp) mice sorted with FACS. The relative gene and protein expression levels of GCα and GCβ were PDGFRα(+) cells > ICC ≫ SMC. In contrast, cGKI gene expression sequence was SMC = ICC > PDGFRα(+) cells whereas cGKI protein expression sequence was neurons > SMC ≫ ICC = PDGFRα(+) cells. The functional role of cGKI was investigated in cGKI(-/-) mice. Relaxation with 8-bromo (8-Br)-cGMP was greatly reduced in cGKI(-/-) mice whereas responses to sodium nitroprusside (SNP) were partially reduced and forskolin responses were unchanged. A nitrergic relaxation occurred with nerve stimulation (NS, 5 Hz, 60 s) in cGKI(+/+) and cGKI(-/-) mice although there was a small reduction in the cGKI(-/-) mouse. N(ω)-nitro-l-arginine (l-NNA) abolished responses during the first 20-30 s of NS in both animals. The GC inhibitor ODQ greatly reduced or abolished SNP and nitrergic NS responses in both animals. These data confirm an essential role for GC in NO-induced relaxation in the IAS. However, the expression of GC and cGKI by all three cell types suggests that each may participate in coordinating muscular responses to NO. The persistence of nitrergic NMT in the cGKI(-/-) mouse suggests the presence of a significant GC-dependent, cGKI-independent pathway.

Differential expression of multidrug resistance protein 5 and phosphodiesterase 5 and regulation of cGMP levels in phasic and tonic smooth muscle

Previous studies have identified differences in the expression of proteins that regulate myosin light chain phosphorylation and contraction in tonic and phasic smooth muscle. cGMP plays a critical role in smooth muscle relaxation and is important for optimal function of phasic and tonic smooth muscle. The intracellular cGMP levels are regulated by its hydrolysis via phosphodiesterase 5 (PDE5) and efflux via novel multidrug resistance protein 5 (MRP5). In the present study we tested the hypothesis that the differences in the phasic and tonic behavior of smooth muscles may be related to differences in mechanisms that terminate cGMP signaling. Expression of PDE5 and MRP5 was significantly (more than 2-fold) higher in fundus compared with antrum. The NO donor S-nitrosoglutathione (GSNO) caused an increase in PDE5 activity and intra- and extracellular cGMP levels in both fundus and antrum. Stimulation of PDE5 activity and increase in extracellular cGMP were significantly higher in fundus, whereas increase in intracellular cGMP was significantly higher in antrum. GSNO-induced increase in extracellular cGMP was blocked in dispersed cells by the cyclic nucleotide export blocker probenecid and in cultured muscle cells by depletion of ATP or suppression of MRP5 by siRNA, providing evidence that cGMP efflux was mediated by ATP-dependent export via MRP5. Consistent with the higher expression and activity levels of PDE5 and MRP5, GSNO-induced PKG activity and muscle relaxation were significantly lower in muscle cells from fundus compared with antrum. Thus higher expression of PDE5 and MRP5 in muscle cells from fundus correlates with tonic phenotype of muscle.

Transcriptional and post-transcriptional regulation of cGMP-dependent protein kinase (PKG-I): pathophysiological significance

The ability of the endothelium to produce nitric oxide, which induces generation of cyclic guanosine monophosphate (cGMP) that activates cGMP-dependent protein kinase (PKG-I), in vascular smooth muscle cells (VSMCs), is essential for the maintenance of vascular homeostasis. Yet, disturbance of this nitric oxide/cGMP/PKG-I pathway has been shown to play an important role in many cardiovascular diseases. In the last two decades, in vitro and in vivo models of vascular injury have shown that PKG-I is suppressed following nitric oxide, cGMP, cytokine, and growth factor stimulation. The molecular basis for these changes in PKG-I expression is still poorly understood, and they are likely to be mediated by a number of processes, including changes in gene transcription, mRNA stability, protein synthesis, or protein degradation. Emerging studies have begun to define mechanisms responsible for changes in PKG-I expression and have identified cis- and trans-acting regulatory elements, with a plausible role being attributed to post-translational control of PKG-I protein levels. This review will focus mainly on recent advances in understanding of the regulation of PKG-I expression in VSMCs, with an emphasis on the physiological and pathological significance of PKG-I down-regulation in VSMCs in certain circumstances.

Epac in cardiac calcium signaling

Epac, exchange protein directly activated by cAMP, is emerging as a new regulator of cardiac physiopathology. Although its effects are much less known than the classical cAMP effector, PKA, several studies have investigated the cardiac role of Epac, providing evidences that Epac modulates intracellular Ca(2+). In one of the first analyses, it was shown that Epac can increase the frequency of spontaneous Ca(2+) oscillations in cultured rat cardiomyocytes. Later on, in adult cardiomyocytes, it was shown that Epac can induce sarcoplasmic reticulum (SR) Ca(2+) release in a PKA independent manner. The pathway identified involved phospholipase C (PLC) and Ca(2+)/calmodulin kinase II (CaMKII). The latter phosphorylates the ryanodine receptor (RyR), increasing the Ca(2+) spark probability. The RyR, Ca(2+) release channel located in the SR membrane, is a key element in the excitation-contraction coupling. Thus Epac participates in the excitation-contraction coupling. Moreover, by inducing RyR phosphorylation, Epac is arrhythmogenic. A detailed analysis of Ca(2+) mobilization in different microdomains showed that Epac preferently elevated Ca(2+) in the nucleoplasm ([Ca(2+)]n). This effect, besides PLC and CaMKII, required inositol 1,4,5 trisphosphate receptor (IP3R) activation. IP3R is other Ca(2+) release channel located mainly in the perinuclear area in the adult ventricular myocytes, where it has been shown to participate in the excitation-transcription coupling (the process by which Ca(2+) activates transcription). If Epac activation is maintained for some time, the histone deacetylase (HDAC) is translocated out of the nucleus de-repressing the transcription factor myocyte enhancer factor (MEF2). These evidences also pointed to Epac role in activating the excitation-transcription coupling. In fact, it has been shown that Epac induces cardiomyocyte hypertrophy. Epac activation for several hours, even before the cell hypertrophies, induces a profound modulation of the excitation-contraction coupling: increasing the [Ca(2+)]i transient amplitude and cellular contraction. Thus Epac actions are rapid but time and microdomain dependent in the cardiac myocyte. Taken together the results collected indicate that Epac may have an important role in the cardiac response to stress.

cGMP-dependent protein kinases and cGMP phosphodiesterases in nitric oxide and cGMP action

To date, studies suggest that biological signaling by nitric oxide (NO) is primarily mediated by cGMP, which is synthesized by NO-activated guanylyl cyclases and broken down by cyclic nucleotide phosphodiesterases (PDEs). Effects of cGMP occur through three main groups of cellular targets: cGMP-dependent protein kinases (PKGs), cGMP-gated cation channels, and PDEs. cGMP binding activates PKG, which phosphorylates serines and threonines on many cellular proteins, frequently resulting in changes in activity or function, subcellular localization, or regulatory features. The proteins that are so modified by PKG commonly regulate calcium homeostasis, calcium sensitivity of cellular proteins, platelet activation and adhesion, smooth muscle contraction, cardiac function, gene expression, feedback of the NO-signaling pathway, and other processes. Current therapies that have successfully targeted the NO-signaling pathway include nitrovasodilators (nitroglycerin), PDE5 inhibitors [sildenafil (Viagra and Revatio), vardenafil (Levitra), and tadalafil (Cialis and Adcirca)] for treatment of a number of vascular diseases including angina pectoris, erectile dysfunction, and pulmonary hypertension; the PDE3 inhibitors [cilostazol (Pletal) and milrinone (Primacor)] are used for treatment of intermittent claudication and acute heart failure, respectively. Potential for use of these medications in the treatment of other maladies continues to emerge.

Myristoylation of cGMP-dependent protein kinase dictates isoform specificity for serotonin transporter regulation

By transporting serotonin (5-HT) into neurons and other cells, serotonin transporter (SERT) modulates the action of 5-HT at cell surface receptors. SERT itself is modulated by several processes, including the cGMP signaling pathway. Activation of SERT by cGMP requires the cGMP-dependent protein kinase (PKG). Here we show that in HeLa cells lacking endogenous PKG, expression of PKGIα or PKGIβ was required for 8-bromoguanosine-3',5'-cyclic monophosphate (8-Br-cGMP) to stimulate SERT phosphorylation and 5-HT influx. Catalytically inactive PKG mutants and wild-type PKGII did not support this stimulation. However, a mutant PKGII (G2A) that was not myristoylated substituted for functional PKGI, suggesting that myristoylation and subsequent membrane association blocked productive interaction with SERT. PKG also influenced SERT expression and localization. PKGI isoforms increased total and cell surface SERT levels, and PKGII decreased cell surface SERT without altering total expression. Remarkably, these changes did not require 8-Br-cGMP or functional kinase activity and were also observed with a SERT mutant resistant to activation by PKG. Both PKGIα and PKGIβ formed detergent-stable complexes with SERT, and this association did not require catalytic activity. The nonmyristoylated PKGII G2A mutant stimulated SERT expression similar to PKGI isoforms. These results suggest multiple mechanisms by which PKG can modulate SERT and demonstrate that the functional difference between PKG isoforms results from myristoylation of PKGII.

Inhibition of the mitochondrial permeability transition by protein kinase A in rat liver mitochondria and hepatocytes

NO and cGMP administered at reperfusion after ischaemia prevent injury to hepatocytes mediated by the MPT (mitochondrial permeability transition). To characterize further the mechanism of protection, the ability of hepatic cytosol in combination with cyclic nucleotides to delay onset of the calcium-induced MPT was evaluated in isolated rat liver mitochondria. Liver cytosol plus cGMP or cAMP dose-dependently inhibited the MPT, required ATP hydrolysis for inhibition and did not inhibit mitochondrial calcium uptake. Specific peptide inhibitors for PKA (protein kinase A), but not PKG (protein kinase G), abolished cytosol-induced inhibition of MPT onset. Activity assays showed a cGMP- and cAMP-stimulated protein kinase activity in liver cytosol that was completely inhibited by PKI, a PKA peptide inhibitor. Size-exclusion chromatography of liver cytosol produced a single peak of cGMP/cAMP-stimulated kinase activity with an estimated protein size of 180-220 kDa. This fraction was PKI-sensitive and delayed onset of the MPT. Incubation of active catalytic PKA subunit directly with mitochondria in the absence of cytosol and cyclic nucleotide also delayed MPT onset, and incubation with purified outer membranes led to phosphorylation of a major 31 kDa band. After ischaemia, administration at reperfusion of membrane-permeant cAMPs and cAMP-mobilizing glucagon prevented reperfusion injury to hepatocytes. In conclusion, PKA in liver cytosol activated by cGMP or cAMP acts directly on mitochondria to delay onset of the MPT and protect hepatocytes from cell death after ischaemia/reperfusion.

Role of the cAMP-binding protein Epac in cardiovascular physiology and pathophysiology

Exchange proteins directly activated by cyclic AMP (Epac) were discovered 10 years ago as new sensors for the second messenger cyclic AMP (cAMP). Epac family, including Epac1 and Epac2, are guanine nucleotide exchange factors for the Ras-like small GTPases Rap1 and Rap2 and function independently of protein kinase A. Given the importance of cAMP in the cardiovascular system, numerous molecular and cellular studies using specific Epac agonists have analyzed the role and the regulation of Epac proteins in cardiovascular physiology and pathophysiology. The specific functions of Epac proteins may depend upon their microcellular environments as well as their expression and localization. This review discusses recent data showing the involvement of Epac in vascular cell migration, endothelial permeability, and inflammation through specific signaling pathways. In addition, we present evidence that Epac regulates the activity of various cellular compartments of the cardiac myocyte and influences calcium handling and excitation-contraction coupling. The potential role of Epac in cardiovascular disorders such as cardiac hypertrophy and remodeling is also discussed.

Novel techniques for real-time monitoring of cGMP in living cells

Recent developments of biophysical and electrophysiological techniques have enabled researchers to monitor levels of free intracellular cGMP in real-time and in intact living cells. These techniques are based on the use of cGMP sensors, which respond to cGMP with changes in transmembrane ion current or changes in fluorescence. Here, we describe the principles of these techniques, compare them in terms of sensitivity and discuss possible application for current cell biology and physiology.

Reduced rather than enhanced cholinergic airway constriction in mice with ablation of the large conductance Ca2+-activated K+ channel

The unique voltage- and Ca2+-dependent K+ (BK) channel, prominently expressed in airway smooth muscle cells, has been suggested as an important effector in controlling airway contractility. Its deletion in mice depolarized resting membrane potential of tracheal cells, suggesting an increased open-probability of voltage-gated Ca2+ channels. While carbachol concentration-dependently increased the tonic tension of wild-type (WT) trachea, mutant trachea showed a different response with rapid tension development followed by phasic contractions superimposed on a tonic component. Tonic contractions were substantially more dependent on L-type Ca2+ current in mutant than in WT trachea, even though L-type Ca2+ channels were not up-regulated. In the absence of L-type Ca2+ current, half-maximal contraction of trachea was shifted from 0.51 to 1.7 microM. In agreement, cholinergic bronchoconstriction was reduced in mutant lung slices, isolated-perfused lungs and, most impressively, in mutant mice analyzed by body plethysmography. Furthermore, isoprenaline-mediated airway relaxation was enhanced in mutants. In-depth analysis of cAMP and cGMP signaling revealed up-regulation of the cGMP pathway in mutant tracheal muscle. Inhibition of cGMP kinase reestablished normal sensitivity toward carbachol, indicating that up-regulation of cGMP signaling counterbalances for BK channel ablation, pointing to a predominant role of BK channel in regulation of airway tone.

Nitric oxide preconditioning regulates endothelial monolayer integrity via the heat shock protein 90-soluble guanylate cyclase pathway

Large (pathological) amounts of nitric oxide (NO) induce cell injury, whereas low (physiological) NO concentrations often ameliorate cell injury. We tested the hypotheses that pretreatment of endothelial cells with low concentrations of NO (preconditioning) would prevent injury induced by high NO concentrations. Apoptosis, induced in bovine aortic endothelial cells (BAECs) by exposing them to either 4 mM sodium nitroprusside (SNP) or 0.5 mM N-(2-aminoethyl)-N-(2-hydroxy-2-nitrosohydrazino)-1,2-ethylenediamine (spermine NONOate) for 8 h, was abolished by 24-h pretreatment with either 100 microM SNP, 10 microM spermine NONOate, or 100 microM 8-bromo-cGMP (8-Br-cGMP). Repair of BAECs following wounding, measured as the recovery rate of transendothelial electrical resistance, was delayed by 8-h exposure to 4 mM SNP, and this delay was significantly attenuated by 24-h pretreatment with 100 microM SNP. NO preconditioning produced increased association and expression of soluble guanyl cyclase (sGC) and heat shock protein 90 (HSP90). The protective effect of NO preconditioning, but not the injurious effect of 4 mM SNP, was abolished by either a sGC activity inhibitor 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ) or a HSP90 binding inhibitor (radicicol) and was mimicked by 8-Br-cGMP. We conclude that preconditioning with a low dose of NO donor accelerates repair and maintains endothelial integrity via a mechanism that includes the HSP90/sGC pathway. HSP90/sGC may thus play a role in the protective effects of NO-generating drugs from injurious stimuli.

Phosphorylation and activation of a transducible recombinant form of human HSP20 in Escherichia coli

Protein-based cellular therapeutics have been limited by getting molecules into cells and the fact that many proteins require post-translational modifications for activation. Protein transduction domains (PTDs), including that from the HIV TAT protein (TAT), are small arginine rich peptides that carry molecules across the cell membrane. We have shown that the heat shock-related protein, HSP20 is a downstream-mediator of cyclic nucleotide-dependent relaxation of vascular smooth muscle and is activated by phosphorylation. In this study, we co-expressed in Escherichia coli the cDNAs encoding the catalytic subunit of protein kinase G and a TAT-HSP20 fusion protein composed of the TAT PTD (-YGRKKRRQRRR-) fused to the N-terminus of human HSP20. Immunoblot and HPLC-ESI-MS/MS analysis of the purified TAT-HSP20 demonstrated that it was phosphorylated at serine 40 (equivalent to serine 16 in wild-type human HSP20). This phosphorylated TAT-HSP20 was physiologically active in intact smooth muscles in that it inhibited 5-hydroxytryptamine-induced contractions by 57%+/-4.5. The recombinant phosphorylated protein also led to changes in actin cytoskeletal morphology in 3T3 cells. These results delineate strategies for the expression and activation of therapeutic molecules for intracellular protein based therapeutics.

cGMP signals mainly through cAMP kinase in permeabilized murine aorta

GMP affects vascular tone by multiple mechanisms, including inhibition of the Rho/Rho kinase-mediated Ca(2+) sensitization, a process identified as Ca(2+) desensitization. Ca(2+) desensitization is mediated probably by both cGMP- and cAMP-dependent protein kinases (cGKI and PKA). We investigate to which extent Ca(2+) desensitization is initiated by cGKI and PKA. cGMP/cAMP-induced relaxation was studied at constant [Ca(2+)] in permeabilized aortas from wild-type and cGKI-deficient mice. [Ca(2+)] increased aortic tone in the absence and presence of 50 microM GTPgammaS with EC(50) values of 160 and 30 nM, respectively. In the absence of GTPgammaS, the EC(50) for [Ca(2+)] was shifted rightward from 0.16 microM to 0.43 and 0.82 microM by 1 and 300 microM 8-bromo-cGMP (8-Br-cGMP), and to 8 microM by 10 microM Y-27632. Contractions induced by 300 nM [Ca(2+)] were relaxed by 8-Br-cGMP with an EC(50) of 2.6 microM. Surprisingly, [Ca(2+)]-induced contractions were also relaxed by 8-Br-cGMP in aortas from cGKI(-/-) mice (EC(50) of 19 microM). Western blot analysis of the vasodilator-stimulated phosphoprotein indicated "cross"-activation of PKA by 1 mM 8-Br-cGMP in aortic smooth muscle cells from cGKI(-/-) mice. Indeed, the PKA inhibitor peptide (PKI 5-24) completely abolished the relaxant effect of 8-Br-cGMP in muscles from cGKI(-/-) mice and to 65% in wild-type aortas. The thromboxane analogue U-46619 induced contraction at constant [Ca(2+)], which was only partially relaxed by 8-Br-cGMP but completely relaxed by Y-27632. The effect of 8-Br-cGMP on U-46619-induced contraction was attenuated by PKI 5-24. These results show that cGKI has only a small inhibitory effect on Ca(2+) sensitization in murine aortas.

Compartmentation of cyclic nucleotide signaling in the heart: the role of A-kinase anchoring proteins

The activation of the cyclic nucleotide protein kinase A (PKA) and PKG by their respective second messengers is responsible for the modulation of many cellular functions in the heart including cardiac hypertrophy, strength of contraction, and ion flux. However, several studies have revealed that a general increase in cyclic nucleotide concentration in the cell is not sufficient for the specific regulation of target proteins. These studies found that PKA and PKG must be colocalized with their targets to ensure spatial-temporal control of substrate phosphorylation. This compartmentation of cyclic nucleotide signaling is accomplished by tethering the protein kinases with their respective substrates through the association with scaffolding proteins. For cAMP signaling, A-kinase anchoring proteins (AKAPs) provide a molecular mechanism for cAMP compartmentation, allowing for the precise control of PKA-mediated phosphorylation events. (cAMP, PKA, AKAP, PKG).

Cyclic nucleotide signaling in cavernous smooth muscle

INTRODUCTION

Penile erection depends on cavernous smooth muscle relaxation that is principally regulated by cyclic nucleotide signaling. It is hoped that a comprehensive review of publications relevant to this subject will be helpful to both scientists and clinicians who are interested in the sciences of erectile function/dysfunction. AIMS. To review the roles of extracellular signaling molecules, their receptors, intracellular effectors, and phosphodiesterases in cyclic nucleotide signaling that leads to cavernous smooth muscle relaxation. The involvement of these molecules in the development of erectile dysfunction and the possibility of using them as therapeutic agents or targets are also discussed.

METHODS

Entrez, the search engine for life sciences, was used to search for publications relevant to the topics of this review. Keywords used in the searches included vascular, cavernous, penis, smooth muscle, signaling molecules (adenosine, nitric oxide, etc.), and key elements in the cyclic nucleotide signaling pathways (cAMP, cGMP, cyclases, PKG, PKA, etc.). Articles that are dedicated to the study of erectile function/dysfunction were prioritized for citation.

RESULTS

More than 1,000 articles were identified, many of which are studies of the vascular system and are therefore reviewed but not cited. Studies on erectile function have identified both cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) signaling pathways in cavernous smooth muscle. Many signaling molecules of these two pathways have been shown capable of inducing erection when administered intracavernously. However, for sexually induced erection, nitric oxide (NO) is the responsible signaling molecule and it passes on the signal through soluble guanyl cyclase (sGC), cGMP, and protein kinase G (PKG).

CONCLUSIONS

The NO/sGC/cGMP/PKG pathway is principally responsible for sexually stimulated erection. Detumescence is mainly carried out by the degradation of cGMP by phosphodiesterase 5. Both cAMP and cGMP signaling pathways are susceptible to genetic and biochemical alterations in association with erectile dysfunction. Several key elements along these pathways are potential therapeutic targets.

Chronic L-arginine supplementation improves endothelial cell vasoactive functions in hypercholesterolemic and atherosclerotic monkeys

Chronic exposure to L-arginine results in regression of atherosclerotic lesions and reversal of endothelial dysfunction. We investigated whether chronic L-arginine supplementation induces regression of atherosclerotic lesions and reversal of endothelial dysfunction in atherogenic rhesus monkeys and the mechanism which leads to these effects. About 12 male rhesus monkeys were fed 1% cholesterol and 18 g butter for 6 months to create an experimental model of hypercholesterolaemia and atherosclerosis (Group I) and 12 monkeys were fed standard stock diet for 6 months (Group II). After, 6 months these two groups were further divided into 2 sub-groups which in addition to their respective diets were fed 2.5% L-arginine in drinking water for additional 6 months (Group III and Group IV). Systemic nitric oxide (NO) formation was assessed as plasma nitrite and cGMP formation every 3 months. Oxygen free radical (OFR) generation and malondialdehyde production as an index of lipid peroxidation were determined. Changes in isometric tension were compared in isolated ring segments of thoracic aorta from normal and hypercholesterolemic animals. Cholesterol feeding progressively reduced plasma nitrite and cGMP generation (p < 0.05). Dietary L-arginine partly restored the levels of plasma nitrite and cGMP (p < 0.05) but did not change plasma cholesterol levels. L-arginine significantly reduced aortic intimal thickening, blocked the production of carotid and coronary intimal plaques and completely preserved endothelium-dependent vasodilator function. Further, L-arginine significantly inhibited generation of the reactive oxygen species (ROS) generation and lipid peroxidation. Chronic oral supplementation with L-arginine blocks the progression of plaques via restoration of nitric oxide synthase substrate availability and reduction of vascular oxidative stress.

Peptide blockers of PKG inhibit ROS generation by acetylcholine and bradykinin in cardiomyocytes but fail to block protection in the whole heart

Bradykinin and acetylcholine (ACh) trigger preconditioning by ATP-sensitive K(+) (K(ATP)) channel-dependent production of reactive oxygen species (ROS). Recent evidence suggests that ROS production may in turn be influenced by cGMP-dependent protein kinase (PKG). This study utilized DT-2 and DT-3 peptides, highly specific membrane-permeable blockers of PKG. Rabbit cardiomyocytes were incubated for 15 min in reduced MitoTracker red, which becomes fluorescent only after exposure to ROS. Bradykinin (400 nM) and ACh (250 microM) caused a 49.9 +/- 5.9% and 46.8 +/- 1.7% increase in ROS production, respectively (P < 0.005 vs. untreated cells). Coincubation with DT-3 (250 nM) abolished both the ACh- and bradykinin-induced ROS signal, whereas a nonpermeable form of the peptide (W45) had no effect on ACh-induced ROS production. DT-3 was unable to block ROS production from diazoxide (100 microM), a selective opener of mitochondrial K(ATP) channels, suggesting that these channels are downstream of PKG. DT-2 (125 nM) also prevented ACh from triggering ROS production. 8-(4-Chlorophenylthio)-guanosine 3',5'-cyclic monophosphate (100 microM), a cGMP analog and potent direct activator of PKG, increased ROS production of cardiomyocytes by 44.7 +/- 7.1% (P < 0.001 vs. untreated cells). This increase was blocked by DT-2. Neither DT-2 nor DT-3 could block the anti-infarct effect of bradykinin in isolated rabbit hearts. Studies with fluorescent-tagged DT-3 revealed that it was confined to endothelial cells and never reached the myocytes. We conclude that both bradykinin and ACh trigger ROS generation by a pathway that includes PKG. Although the peptides may be inappropriate for a whole heart model, they are likely to become important tool drugs for elucidation of signal transduction pathways in cell preparations.

Nitric oxide promotes mitogen-induced dna synthesis in human dermal fibroblasts through cGMP

1. Nitric oxide (NO) is a free radical with multiple functions in cellular pathophysiology. Nitric oxide has been proven to play an important role in wound healing; however, the mechanisms by which NO may promote wound healing are not clearly understood. We have investigated the effect of NO on growth factor-induced DNA synthesis in human dermal fibroblasts to suggest interactions between growth factors and NO as a possible mechanism for the role of NO in wound healing. 2. The NO donor sodium nitroprusside (SNP) significantly (P < 0.001) increased fetal bovine serum-induced thymidine incorporation into the DNA of human dermal fibroblasts. The maximal comitogenic concentration of SNP (100 micro mol/L) was also found to significantly (twofold; P < 0.01) enhance fibroblast growth factor- or platelet-derived growth factor-induced DNA synthesis. 3. Nitric oxide treatment significantly increased the production of cGMP. 8-Bromo-cGMP, a stable structural analogue of cGMP, was found to markedly potentiate (P < 0.001) the growth factor-induced DNA synthesis. 4. This study concludes that NO and cGMP promote growth factor-induced DNA synthesis in dermal fibroblasts, suggesting another possible mechanism by which NO may promote skin wound healing.

Regulation of nitric oxide and soluble guanylyl cyclase

Since the discoveries that have verified nitric oxide (NO) as an endogenously produced cell signaling molecule, research surrounding its production and mechanisms of action have been studied at an exponentially increasing rate. NO is produced by a family of enzymes termed the NO synthases (NOS), which are regulated independently by various stimuli. Once produced, NO can solicit numerous biological events by reacting with various metals, thiols, and oxygen species to modify proteins, DNA and lipids. One of the most biologically relevant actions of NO is its binding to the heme moiety in the heterodimeric enzyme, soluble guanylyl cyclase (sGC). Activation of sGC by NO results in the production of the second messenger molecule, 3',5'-cyclic guanosine monophosphate (cGMP), which can regulate numerous physiological events such as vasodilatation and neurotransmission. Here we will review the synthesis and fate of NO, and discuss the activation and regulation of the NO receptor, sGC.

NRSF regulates the fetal cardiac gene program and maintains normal cardiac structure and function

Reactivation of the fetal cardiac gene program is a characteristic feature of hypertrophied and failing hearts that correlates with impaired cardiac function and poor prognosis. However, the mechanism governing the reversible expression of fetal cardiac genes remains unresolved. Here we show that neuron-restrictive silencer factor (NRSF), a transcriptional repressor, selectively regulates expression of multiple fetal cardiac genes, including those for atrial natriuretic peptide, brain natriuretic peptide and alpha-skeletal actin, and plays a role in molecular pathways leading to the re-expression of those genes in ventricular myocytes. Moreover, transgenic mice expressing a dominant-negative mutant of NRSF in their hearts exhibit dilated cardiomyopathy, high susceptibility to arrhythmias and sudden death. We demonstrate that genes encoding two ion channels that carry the fetal cardiac currents I(f) and I(Ca,T), which are induced in these mice and are potentially responsible for both the cardiac dysfunction and the arrhythmogenesis, are regulated by NRSF. Our results indicate NRSF to be a key transcriptional regulator of the fetal cardiac gene program and suggest an important role for NRSF in maintaining normal cardiac structure and function.

Alternative splicing of cGMP-dependent protein kinase I in angiotensin-hypertension: novel mechanism for nitrate tolerance in vascular smooth muscle

Nitrate tolerance (NT) in hypertension is attributed to reduced activity of soluble guanylyl cyclase (sGC). We examined NT in basilar artery vascular smooth muscle cells (VSMCs) from control rats, rats infused with angiotensin II (Ang; 240 microg/kg per hour for 4 days), which were normotensive, and Ang-hypertensive rats (AHR; 240 microg/kg per hour for 28 days). Ca2+-activated K+ (Maxi-K) channels in VSMCs from AHR showed reduced activation by NO donor, consistent with NT. The concentration-response relationship for 8-Br-cGMP was shifted 2.5-fold to the right, indicating that abnormal sGC alone could not account for NT. Inside-out patches from AHR showed normal activation with exogenous cGMP-dependent protein kinase I (cGKI), suggesting no abnormality downstream of cGKI. We hypothesized that the reduction in apparent affinity of 8-Br-cGMP for cGKI in AHR might be due to a change in relative amounts of cGKIalpha versus cGKIbeta, since cGKIbeta is less sensitive to cGMP activators than cGKIalpha. This was substantiated by showing the following in AHR: (1) reduced effect of the cGKIalpha-selective activator 8-APT-cGMP; (2) reduced total cGKI protein (both isoforms), but an increase in cGKIbeta protein in quantitative immunofluorescence and Western blots; (3) similar changes in cGKI isoforms immunoisolated with Maxi-K channels; and (4) a large increase in cGKIbeta mRNA and a decrease in cGKIalpha mRNA in real-time PCR and Northern blots. Upregulation of cytosolic cGKIbeta was evident 4 days after Ang infusion, before development of hypertension. Our data identify a functional role for cGKIbeta in VSMCs previously ascribed exclusively to cGKIalpha. Ang-induced alternative splicing of cGKI represents a novel mechanism for reducing sensitivity to NO/cGMP.

Cyclic nucleotide phosphodiesterase activity, expression, and targeting in cells of the cardiovascular system

Cyclic AMP (cAMP) and cGMP regulate a myriad of cellular functions, such as metabolism, contractility, motility, and transcription in virtually all cell types, including those of the cardiovascular system. Considerable effort over the last 20 years has allowed identification of the cellular components involved in the synthesis of cyclic nucleotides, as well as effectors of cyclic nucleotide-mediated signaling. More recently, a central role for cyclic nucleotide phosphodiesterase (PDE) has also been elaborated in many cell types, including those involved in regulating the activities of the cardiovascular system. In this review, we introduce the PDE families whose members are expressed in cells of the cardiovascular system including cardiomyocytes, vascular smooth muscle cells, and vascular endothelial cells. Because cell behavior is a dynamic process influenced by numerous factors, we will attempt to emphasize how changes in the activity, expression, and targeting of PDE influence cyclic nucleotide-mediated regulation of the behavior of these cells.

Dissociation of cGMP accumulation and relaxation in myometrial smooth muscle: effects of S-nitroso-N-acetylpenicillamine and 3-morpholinosyndonimine

In guinea pig, primate and man, nitric oxide (NO)-induced regulation of myometrial smooth muscle contraction is distinct from other smooth muscles because cyclic guanosine 3',5'-cyclic monophosphate (cGMP) accumulation is neither necessary nor sufficient to relax the tissue. To further our understanding of the mechanism of action of NO in myometrium, we employed the NO donors, S-nitroso-N-acetylpenicillamine (SNAP), and 3-morpholinosyndonimine (SIN-1) proposed to relax airway smooth muscle by disparate mechanisms involving elevation in intracellular calcium ([Ca(2+)](i)) or cGMP accumulation, respectively. Treatment of guinea pig myometrial smooth muscle with either NO donor at concentrations thought to produce maximal relaxation of smooth muscles resulted in significant elevations in cGMP that were accompanied by phosphorylation of the cGMP-dependent protein kinase substrate vasodilator-stimulated phosphoprotein (VASP), shown here for the first time to be present and phosphorylated in myometrium. Stimulation of myometrial strips with oxytocin (OT, 1 microM) produced an immediate increase in contractile force that persisted in the continued presence of the agonist. Addition of SNAP (100 microM) in the presence of OT relaxed the tissue completely as might be expected of an NO donor. SIN-1 failed to relax the myometrium at any concentration tested up to 300 microM. In Fura-2 loaded myometrial cells prepared from guinea pig, addition of SNAP (100 microM) in the absence of other agonists caused a significant, reproducible elevation of intracellular calcium while SIN-1 employed under the same conditions did not. Our data further support the notion that NO action in myometrium is distinct from that in other smooth muscles and underscores the possibility that discrete regional changes in [Ca(2+)](i), rather than cGMP, signal NO-induced relaxation of the muscle.

A novel cyclic GMP-dependent protein kinase is expressed in the ring stage of the Plasmodium falciparum life cycle

Cyclic GMP-dependent protein kinases (PKGs) are the major mediators of the cGMP signal transduction pathway and regulate a variety of physiological effects. We report here the characterization of an unusual PKG from the human malaria parasite Plasmodium falciparum (designated PfPKG). The 97.5 kDa protein contains some of the structural features of mammalian PKGs but, uniquely, contains a third predicted cGMP binding site and a degenerate fourth. Using both protein kinase activity assays and Western blotting with native P. falciparum proteins, we demonstrate here that PfPKG is expressed predominantly in the ring stage of the life cycle, suggesting a role in the development of asexual blood stage parasites. An Escherichia coli-derived recombinant protein (PfPKG2, Met115-Phe853) was purified and shown to have phosphotransferase activity in terms of both substrate phosphorylation and auto-phosphorylation. This activity was stimulated at least fivefold by 1.0 microM cyclic GMP, but was not stimulated by cAMP or by 8-pCPT-cGMP, which is a potent activator of mammalian PKGs. Several protein kinase inhibitors exhibited a range of inhibitory effects on PfPKG activity. Biochemical analysis therefore shows that PfPKG is distinct from mammalian PKGs with respect to both cyclic nucleotide analogue activation and inhibition profiles.

Exploring the mechanisms of vascular smooth muscle tone with highly specific, membrane-permeable inhibitors of cyclic GMP-dependent protein kinase Ialpha

The structural similarity of cyclic GMP-dependent protein kinase (cGPK) and cyclic AMP-dependent protein kinase (cAPK) has made it difficult to study cGPK pathways independent of those mediated by cAPK, primarily due to the lack of potent and selective cGPK inhibitors. We recently reported a novel peptide library screen specifically designed to select for tight-binding peptides that identified selective inhibitors of cGPK [Proc Natl Acad Sci USA, 97 (2000) 14772]. Iterative deconvolution of octameric library arrays on paper identified the sequence LRK(5)H (W45). Binding of W45 to cGPK resulted in selective inhibition of the kinase, with K(i) values of 0.8 microM and 560 microM for cGPK and cAPK, respectively. Cellular internalization of highly charged W45 was accomplished by N-terminal fusion of membrane translocation sequences from either the human immunodeficiency virus tyrosine aminotransferase protein (47-59) DT-2 or from the Drosophila Antennapedia homeodomain (43-58) DT-3, respectively. For both fusion peptides, DT-2 and DT-3, we observed a potentiating effect with respect to the inhibitory potency, with K(i) values 40- to 80-fold lower than W45. Fluorescein-labeled DT-2 and DT-3 demonstrated rapid translocation through the cytosol and nuclei in a time-dependent manner using cultured cells and intact tissue samples (cerebral arteries). The physiological effects of DT-2 and DT-3 as selective cGPK inhibitors in smooth muscle were studied in small intact arteries. Nitric oxide, a cyclic GMP/cGPK activator, elicited a concentration-dependent dilation of isolated rat cerebral arteries, which was markedly inhibited by DT-2 and DT-3. Collectively, these results indicate that DT-2 and DT-3 effectively inhibit nitric oxide-induced vasodilation, further emphasizing the central role for cGPK in the modulation of vascular contractility.

Nitric oxide signaling pathway regulates potassium chloride cotransporter-1 mRNA expression in vascular smooth muscle cells

Rat vascular smooth muscle cells (VSMCs) express at least two mRNAs for K-Cl cotransporters (KCC): KCC1 and KCC3. cGMP-dependent protein kinase I regulates KCC3 mRNA expression in these cells. Here, we show evidence implicating the nitric oxide (NO)/cGMP signaling pathway in the expression of KCC1 mRNA, considered to be the major cell volume regulator. VSMCs, expressing soluble guanylyl cyclase (sGC) and PKG-I isoforms showed a time- and concentration-dependent increase in KCC1 mRNA levels after treatment with sodium nitroprusside as demonstrated by semiquantitative RT-PCR. sGC-dependent regulation of KCC1 mRNA expression was confirmed using YC-1, a NO-independent sGC stimulator. The sGC inhibitor LY83583 blocked the effects of sodium nitroprusside and YC-1. Moreover, 8-Br-cGMP increased KCC1 mRNA expression in a concentration- and time-dependent fashion. The 8-Br-cGMP effect was partially blocked by KT5823 but not by actinomycin D. However, actinomycin D and cycloheximide increased basal KCC1 mRNA in an additive manner, suggesting different mechanisms of action for both drugs. These findings suggest that in VSMCs, the NO/cGMP-signaling pathway participates in KCC1 mRNA regulation at the post-transcriptional level.

Protein kinase G regulates potassium chloride cotransporter-4 [corrected] expression in primary cultures of rat vascular smooth muscle cells

K-Cl cotransport (KCC) is activated by nitric oxide donors and appears to be regulated by the cGMP signaling pathway. Expression of KCC mRNAs (KCC1-KCC4) in rat vascular smooth muscle cells (VSMCs) is unknown. We have reported the presence of KCC1 and KCC3 mRNAs in primary cultures of VSMCs by specific reverse transcription-polymerase chain reaction. KCC2 mRNA appeared at extremely low levels. KCC4 mRNA was undetectable. Semiquantitative reverse transcription-polymerase chain reaction revealed a 2:1 KCC1/KCC3 mRNA ratio in VSMCs. Depletion of protein kinase G (PKG)-1 from VSMCs did not change KCC3 mRNA expression. Analogous results were obtained with PKG-1-catalytic domain- and vector only-transfected VSMCs lacking endogenous PKG, suggesting no involvement of PKG-1 in the maintenance of basal KCC3 mRNA expression. However, 8-bromo-cGMP, a PKG stimulator, acutely increased KCC3 mRNA expression in a concentration- and time-dependent fashion; this effect was blocked by the PKG inhibitor KT5823 but not by actinomycin D. These findings show that VSMCs express mainly two mRNA isoforms, KCC1 and KCC3, and suggest that PKG participates post-transcriptionally in the acute KCC3 mRNA regulation. The role of KCC3 on cell volume and electrolyte homeostasis in response to PKG modulators remains to be determined.

Spatiotemporal dynamics of guanosine 3',5'-cyclic monophosphate revealed by a genetically encoded, fluorescent indicator

To investigate the dynamics of guanosine 3',5'-cyclic monophosphate (cGMP) in single living cells, we constructed genetically encoded, fluorescent cGMP indicators by bracketing cGMP-dependent protein kinase (cGPK), minus residues 1-77, between cyan and yellow mutants of green fluorescent protein. cGMP decreased fluorescence resonance energy transfer (FRET) and increased the ratio of cyan to yellow emissions by up to 1.5-fold with apparent dissociation constants of approximately 2 microM and >100:1 selectivity for cGMP over cAMP. To eliminate constitutive kinase activity, Thr(516) of cGPK was mutated to Ala. Emission ratio imaging of the indicators transfected into rat fetal lung fibroblast (RFL)-6 showed cGMP transients resulting from activation of soluble and particulate guanylyl cyclase, respectively, by nitric oxide (NO) and C-type natriuretic peptide (CNP). Whereas all naive cells tested responded to CNP, only 68% responded to NO. Both sets of signals showed large and variable (0.5-4 min) latencies. The phosphodiesterase (PDE) inhibitor 3-isobutyl-1-methylxanthine (IBMX) did not elevate cGMP on its own but consistently amplified responses to NO or CNP, suggesting that basal activity of guanylate cyclase is very low and emphasizing the importance of PDEs in cGMP recycling. A fraction of RFL cells showed slowly propagating tides of cGMP spreading across the cell in response to delocalized application of NO. Biolistically transfected Purkinje neurons showed cGMP responses to parallel fiber activity and NO donors, confirming that single-cell increases in cGMP occur under conditions appropriate to cause synaptic plasticity.

Highly specific, membrane-permeant peptide blockers of cGMP-dependent protein kinase Ialpha inhibit NO-induced cerebral dilation

Arrays of octameric peptide libraries on cellulose paper were screened by using (32)P-autophosphorylated cGMP-dependent protein kinase Ialpha (cGPK) to identify peptide sequences with high binding affinity for cGPK. Iterative deconvolution of every amino acid position in the peptides identified the sequence LRK(5)H (W45) as having the highest binding affinity. Binding of W45 to cGPK resulted in selective inhibition of the kinase with K(i) values of 0.8 microM and 560 microM for cGPK and cAMP-dependent protein kinase (cAPK), respectively. Fusion of W45 to membrane translocation signals from HIV-1 tat protein (YGRKKRRQRRRPP-LRK(5)H, DT-2) or Drosophila Antennapedia homeo-domain (RQIKIWFQNRRMKWKK-LRK(5)H, DT-3) proved to be an efficient method for intracellular delivery of these highly charged peptides. Rapid translocation of the peptides into intact cerebral arteries was demonstrated by using fluorescein-labeled DT-2 and DT-3. The inhibitory potency of the fusion peptides was even greater than that for W45, with K(i) values of 12.5 nM and 25 nM for DT-2 and DT-3, respectively. Both peptides were still poor inhibitors of cAPK. Selective inhibition of cGPK by DT-2 or DT-3 in the presence of cAPK was demonstrated in vitro. In pressurized cerebral arteries, DT-2 and DT-3 substantially decreased NO-induced dilation. This study provides functional characterization of a class of selective cGPK inhibitor peptides in vascular smooth muscle and reveals a central role for cGPK in the modulation of vascular contractility.

Mechanisms of NO/cGMP-dependent vasorelaxation

Both cGMP-dependent and -independent mechanisms have been implicated in the regulation of vascular tone by NO. We analyzed acetylcholine (ACh)- and NO-induced relaxation in pressurized small arteries and aortic rings from wild-type (wt) and cGMP kinase I-deficient (cGKI(-/-)) mice. Low concentrations of NO and ACh decreased the spontaneous myogenic tone in wt but not in cGKI(-/-) arteries. However, contractions of cGKI(-/-) arteries and aortic rings were reduced by high concentrations (10 micromol/L) of 2-(N:, N-diethylamino)-diazenolate-2-oxide (DEA-NO). Iberiotoxin, a specific blocker of Ca(2+)-activated K(+) (BK(Ca)) channels, only partially prevented the relaxation induced by DEA-NO or ACh in pressurized vessels and aortic rings. DEA-NO increased the activity of BK(Ca) channels only in vascular smooth muscle cells isolated from wt cGKI(+/+) mice. These results suggest that low physiological concentrations of NO decrease vascular tone through activation of cGKI, whereas high concentrations of DEA-NO relax vascular smooth muscle independent of cGKI and BK(Ca). NO-stimulated, cGKI-independent relaxation was antagonized by the inhibition of soluble guanylyl cyclase or cAMP kinase (cAK). DEA-NO increased cGMP to levels that are sufficient to activate cAK. cAMP-dependent relaxation was unperturbed in cGKI(-/-) vessels. In conclusion, low concentrations of NO relax vessels by activation of cGKI, whereas in the absence of cGKI, NO can relax small and large vessels by cGMP-dependent activation of cAK.

KT5823 inhibits cGMP-dependent protein kinase activity in vitro but not in intact human platelets and rat mesangial cells

Many signal transduction pathways are mediated by the second messengers cGMP and cAMP, cGMP- and cAMP-dependent protein kinases (cGK and PKA), phosphodiesterases, and ion channels. To distinguish among the different cGMP effectors, inhibitors of cGK and PKA have been developed including the K-252 compound KT5823 and the isoquinolinesulfonamide H89. KT5823, an in vitro inhibitor of cGK, has also been used in numerous studies with intact cells to implicate or rule out the involvement of this protein kinase in a given cellular response. However, the efficacy and specificity of KT5823 as cGK inhibitor in intact cells or tissues have never been demonstrated. Here, we analyzed the effects of both KT5823 and H89 on cyclic-nucleotide-mediated phosphorylation of vasodilator-stimulated phosphoprotein (VASP) in intact human platelets and rat mesangial cells. These two cell types both express high levels of cGK. KT5823 inhibited purified cGK. However, with both intact human platelets and rat mesangial cells, KT5823 failed to inhibit cGK-mediated serine 157 and serine 239 phosphorylation of VASP induced by nitric oxide, atrial natriuretic peptide, or the membrane-permeant cGMP analog, 8-pCPT-cGMP. KT5823 enhanced 8-pCPT-cGMP-stimulated VASP phosphorylation in platelets and did not inhibit forskolin-stimulated VASP phosphorylation in either platelets or mesangial cells. In contrast H89, an inhibitor of both PKA and cGK, clearly inhibited 8-pCPT-cGMP and forskolin-stimulated VASP phosphorylation in the two cell types. The data indicate that KT5823 inhibits purified cGK but does not affect a cGK-mediated response in the two different cell types expressing cGK I. These observations indicate that data that interpret the effects of KT5823 in intact cells as the major or only criteria supporting the involvement of cGK clearly need to be reconsidered.

Effect of cGMP analogues and protein kinase G blocker on secretory activity, apoptosis and the cAMP/protein kinase A system in porcine ovarian granulosa cells in vitro

The aim of the present study was to examine the role of cGMP-dependent intracellular mechanisms in control of ovarian functions. In the first series of experiments we studied the effects of the cGMP analogues 8-pCPT-cGMP (0.001-100 nM), Rp-8-pCPT-cGMPS (0. 01-100 nM), Rp-8-Br-cGMPS (0.01-100 nM), and Rp-8-Br-PET-cGMPS (0.01-100 nM) on the release of progesterone, insulin-like growth factor I (IGF-I) and oxytocin by cultured porcine granulosa cells. In a second series of experiments, the effects of Rp-8-Br-PET-cGMPS (50 nM) and KT5822 (100 ng/ml), specific inhibitor of cGMP-dependent protein kinase (PKG), on cAMP, PKA, oxytocin and the occurrence of apoptosis in cultured cells were compared. The release of hormones and IGF-I into the culture medium was evaluated using a RIA, while the percentage of cells containing visible oxytocin, cAMP, as well as the regulatory and catalytic subunits of PKA was assessed using immunocytochemistry. Occurrence of apoptosis in these cells was detected using the TUNEL method. The stimulatory (8-pCPT-cGMP and Rp-8-pCPT-cGMPS), inhibitory (Rp-8-Br-cGMPS) and biphasic (Rp-8-Br-PET-cGMPS) effect of cGMP analogues on progesterone release was observed. All cGMP analogues used suppressed IGF-I release. All cGMP analogues decreased oxytocin release, but 8-pCPT-cGMP and Rp-8-Br-cGMPS, when given at low doses (0.01-0.1 and 1-10 nM, respectively) stimulated oxytocin output. Both, Rp-8-Br-PET-cGMPS and KT5822 increased the rate of incidence of apoptosis and percentage of cells containing immunoreactive cAMP. Both Rp-8-Br-PET-cGMPS and KT5822 decreased the proportion of cells containing immunoreactive oxytocin and regulatory subunit of PAK KT5822, but not Rp-8-Br-PET-cGMPS, increased the number of cells containing catalytic subunit of PKA. The present observations suggest the involvement of cGMP and PKG in control of the production of steroid, nonapeptide hormone, growth factor, cAMP and cAMP-dependent PKA, as well as the induction of apoptosis in porcine ovarian cells.

Functional characteristics of urinary tract smooth muscles in mice lacking cGMP protein kinase type I

Nitric oxide (NO)-mediated smooth muscle relaxation is mediated by cGMP through activation of cGMP-dependent protein kinase I (cGKI). We studied the importance of cGKI for lower urinary tract function in mice lacking the gene for cGKI (cGKI-/-) and in litter-matched wild-type mice (cGKI+/+) in vitro and in vivo. cGKI deficiency did not result in any changes in bladder gross morphology or weight. Urethral strips from cGKI-/- mice showed an impaired relaxant response to nerve-derived NO. The cGMP analog 8-bromo-cGMP (8-BrcGMP) and the NO-donor SIN-1 relaxed the wild-type urethra (50-60%) but had only marginal effects in the cGKI-deficient urethra. Bladder strips from cGKI-/- mice responded normally to electrical field stimulation and to carbachol but not to 8-BrcGMP. In vivo, the cGKI-deficient mice showed bladder hyperactivity characterized by decreased intercontraction intervals and nonvoiding bladder contractions. Loss of cGKI abolishes NO-cGMP-dependent relaxations of urethral smooth muscle and results in hyperactive voiding. These data suggest that certain voiding disturbances may be associated with impaired NO-cGKI signaling.

Nitric oxide-induced cGMP accumulation in the mouse bladder is not related to smooth muscle relaxation

The functional role of nitric oxide (NO) and the guanylate cyclase/cGMP second messenger system was investigated in the mouse bladder. Electrical field stimulation and the NO-donor 3-morpholino-sydnonimin hydrochloride (SIN-1) did not induce relaxation of the carbachol-precontracted bladder. However, sodium nitroprusside (10(-3) M) was found to enhance the contractile response to electrical field stimulation by 24+/-6% (n=8; P<0.05) without affecting the contractile response to carbachol. The enhancement of bladder contractility evoked by sodium nitroprusside was inhibited by the guanylate cyclase inhibitor 1H-[1,2, 4]oxadiazolo[4,3-a]quinoxalime-1-one (ODQ; 10(-6) M). Incubation of bladder strips with SIN-1 and sodium nitroprusside caused an increase in cGMP accumulation as measured by radioimmunoassay. Immunohistochemical studies showed cGMP-immunoreactivity in nerve fibres and in stromal cells, but not in smooth muscle bundles after exposure to NO-donors. The results show that NO-donors have no inhibitory effect on smooth muscle tone in the mouse bladder, but that NO may have a functional role as an excitatory neuromodulator. The targets of endogenous NO in the bladder may be the demonstrated cGMP-positive structures, i.e., nerves and stromal cells.

The molecular basis of hypertension

More than 50 million Americans display blood pressures outside the safe physiological range. Unfortunately for most individuals, the molecular basis of hypertension is unknown, in part because pathological elevations of blood pressure are the result of abnormal expression of multiple genes. This review identifies a number of important blood pressure regulatory genes including their loci in the human, mouse, and rat genome. Phenotypes of gene deletions and overexpression in mice are summarized. More detailed discussion of selected gene products follows, beginning with proteins involved in ion transport, specifically the epithelial sodium channel and sodium proton exchangers. Next, proteins involved in vasodilation/natriuresis are discussed with emphasis on natriuretic peptides, guanylin/uroguanylin, and nitric oxide. The renin angiotensin aldosterone system has an important role antagonizing the vasodilatory cyclic GMP system.

A(2A) adenosine receptor mediated potassium channel activation in rat epididymal smooth muscle

The effects of A(2) adenosine receptor agonists upon phenylephrine-stimulated contractility in preparations of rat epididymis were investigated. Preparations responded to phenylephrine (3 microM) with submaximal contractions. Adenosine and the stable agonists 5'-N-ethylcarboxamido-adenosine (NECA) and 2-p-(2-carboxyethyl) phenethylamino-N-ethylcarboxamide adenosine (CGS 21680) inhibited phenylephrine-induced contractions (potency order, NECA>CGS 21680>adenosine). The A(2A) receptor-selective antagonist, 4-(2-[7-amino-2-(2-furyl)[1,2,4]-triazolo-[2,3-a][1,3, 5]triazin-5-ylamino]ethyl)phenol (ZM 241385, 30 microM) blocked the response to NECA. The A(2A) adenosine receptor-mediated inhibitory responses to NECA were reduced by the K(ATP) channel blocker, glibenclamide (3 microM) and abolished by charybdotoxin (100 nM). The diterpene forskolin elicited a concentration-dependent inhibition of phenylephrine (3 microM)-stimulated contractility (by 62+/-8% of control at 100 microM). Charybdotoxin (100 nM), but not glibenclamide (3 microM) blocked the forskolin (10 microM) inhibition of phenylephrine-stimulated contractility. NECA elicited concentration-dependent increases in both cyclic AMP and cyclic GMP accumulation which were antagonized by ZM 241385 (30 nM). The protein kinase G activator, APT-cyclic GMP (8-(-Aminophenylthio) guanosine-3',5'-cyclic monophosphate) and the protein kinase A activator (Sp)-8-bromoadenosine-3',5'-cyclic monophosphorothioate (Sp-8-Br-cyclic AMPs), inhibited phenylephrine (3 microM) induced contractions of rat epididymis. Glibenclamide (3 microM), but not charybdotoxin (100 nM), inhibited ATP-cyclic GMP responses. Charybdotoxin (100 nM), but not glibenclamide (3 microM) reduced the effect of Sp-8-Br-cyclic AMPs. This study shows that the A(2A) adenosine receptor inhibition of epididymal contractility may be mediated through the activation of charybdotoxin- and glibenclamide-sensitive potassium channels and may involve the activation of both protein kinases A and G.

cAMP-dependent vasodilators cross-activate the cGMP-dependent protein kinase to stimulate BK(Ca) channel activity in coronary artery smooth muscle cells

cAMP-dependent vasodilators are used to treat a variety of cardiovascular disorders; however, the signal transduction pathways and effector mechanisms stimulated by these agents are not fully understood. In the present study we demonstrate that cAMP-stimulating agents enhance the activity of the large-conductance, calcium-activated potassium (BK(Ca)) channel in single myocytes from coronary arteries by "cross-activation" of the cGMP-dependent protein kinase (protein kinase G, PKG). Single-channel patch-clamp data revealed that 10 micromol/L isoproterenol, forskolin, or dopamine opens BK(Ca) channels in coronary myocytes and that this effect is attenuated by inhibitors of PKG (KT5823; Rp-8-pCPT-cGMPS), but not by inhibiting the cAMP-dependent protein kinase (protein kinase A, PKA). In addition, a membrane-permeable analog, CPT-cAMP, also opened BK(Ca) channels in these myocytes, and this effect was reversed by KT5823. Direct biochemical measurement confirmed that dopamine or forskolin stimulates PKG activity in coronary arteries but does not elevate cGMP. Finally, the stimulatory effect of cAMP on BK(Ca) channels was reconstituted in a cell-free, inside-out patch by addition of purified PKG activated by either cGMP or cAMP. In contrast, channel gating was unaffected by exposure to the purified catalytic subunit of PKA. In summary, findings from on-cell and cell-free patch-clamp experiments provide direct evidence that cAMP-dependent vasodilators open BK(Ca) channels in coronary myocytes by cross-activation of PKG (but not via PKA). Biochemical assay confirmed this cross-activation mechanism of cAMP action in these arteries. This signaling pathway is a novel mechanism for regulation of potassium channel activity in vascular smooth muscle and other cells.

Coupling of Ca(2+) to CREB activation and gene expression in intact cerebral arteries from mouse : roles of ryanodine receptors and voltage-dependent Ca(2+) channels

Pathological changes of the vasculature are characterized by changes in Ca(2+) handling and alterations in gene expression. In neurons and other cell types, [Ca(2+)](i) often drives changes in gene expression. However, the relationship between Ca(2+) signaling and gene expression in vascular smooth muscle is not well understood. This study examines the ability of Ca(2+) influx through voltage-dependent, L-type Ca(2+) channels (VDCCs) and Ca(2+) release through ryanodine receptors (RyRs) to activate the transcription factor, cAMP-responsive element binding protein (CREB), and increase c-fos levels in intact cerebral arteries. Membrane depolarization increased the fraction of nuclei staining for phosphorylated CREB (P-CREB) and levels of c-fos mRNA in intact mouse cerebral arteries. Ryanodine, which inhibits RyRs, increased P-CREB staining and c-fos levels. Forskolin, an activator of adenylyl cyclase, and sodium nitroprusside, an NO donor, increased P-CREB and c-fos levels. Nisoldipine, an inhibitor of VDCCs, reversed the effects of depolarization and ryanodine on P-CREB and c-fos levels, but not the effects of forskolin or sodium nitroprusside. Inhibition of Ca(2+)/calmodulin-dependent protein kinase (CaM kinase) blocked increases in P-CREB and c-fos levels seen with membrane depolarization, suggesting that CaM kinase has an important role in the pathway leading from Ca(2+) influx to CREB-mediated changes in c-fos levels. Our data suggest that membrane depolarization increases [Ca(2+)](i) through activation of VDCCs, leading to increased P-CREB and c-fos, and that RyRs have a profound effect on this pathway by indirectly regulating Ca(2+) entry through VDCCs. These results provide the first evidence of Ca(2+) regulation of CREB and c-fos in arterial smooth muscle.

Erectile dysfunction in cyclic GMP-dependent kinase I-deficient mice

The generation of nitric oxide (NO) in penile erectile tissue and the subsequent elevation of cyclic GMP (cGMP) levels are important for normal penile erection. Current treatments of erectile dysfunction elevate either cGMP levels by blocking cGMP degrading phosphodiesterase 5 or cyclic AMP (cAMP) levels by intrapenile injection of prostaglandin E1. The molecular target or targets of cGMP in erectile tissue and the role of cAMP for normal penile erection are not known. Herein, we report that mice lacking cGMP-dependent kinase I (cGKI) have a very low ability to reproduce and that their corpora cavernosa fail to relax on activation of the NO/cGMP signaling cascade. Elevation of cAMP by forskolin, however, induces similar relaxation in normal and cGKI-null corpus cavernosum. In addition, sperm derived from cGKI-null mice is normal, can undergo acrosomal reactions, and can efficiently fertilize eggs. Altogether, these data identify cGKI as the downstream target of cGMP in erectile tissue and provide evidence that cAMP signaling cannot compensate for the absence of the cGMP/cGKI signaling cascade in vivo.

Alleviation of graft-versus-host disease after conditioning with cobalt-protoporphyrin, an inducer of heme oxygenase-1

BACKGROUND

Recently, we demonstrated that elevated expression of heme oxygenase-1 (HO-1 or Hsp-32) resulted in the modulation of several immune effector functions. Here we evaluated whether induction of HO-1 after administration of cobalt protoporphyrin (CoPP) can prevent the development of acute graft-versus-host-disease (GVHD).

METHODS

Acute GVHD was initiated by injection of unfractionated spleen cells from C57BL/6 into B6D2/F1 mice.

RESULTS

Administration of CoPP resulted in increased survival: 85% of CoPP-treated animals survived for >100 days compared with only 29% of saline-treated control animals (P<0.05). In contrast, administration of ZnPP, a well-known inhibitor of HO, accelerated GVHD development. The protective effect of CoPP therapy seemed to be caused by immunomodulation of donor cells, because treatment of cell donors prevented development of acute GVHD in 80% of recipients compared with 0% in control animals. Spontaneous lymphocyte proliferation could be measured with splenocytes harvested from animals developing GVHD but not with splenocytes from recipients of CoPP-treated donor cells. CoPP-treatment had no effect on interleukin-2 or interleukin-4 synthesis but inhibited interferon-gamma production. Mice with active GVHD demonstrated a defective lympho-proliferative response to alloantigens or concanavalin A. However, spleen cells isolated from survivors (on day 100) responded normally. Flow cytometric analysis of splenic T cell populations revealed a severe reduction in recipient type (H-2b,d) cells in mice with active GVHD, whereas in protected mice the number of cells remained normal.

CONCLUSION

The results from this study confirmed our previous observation that up-regulation of HO-1 activity is associated with down-regulation of several immune effector functions. This resulted in protection from acute GVHD in a parent into F1 mouse model.

Cysteine-rich protein 2, a novel substrate for cGMP kinase I in enteric neurons and intestinal smooth muscle

Nitric oxide/cGMP/cGMP kinase I (cGKI) signaling causes relaxation of intestinal smooth muscle. In the gastrointestinal tract substrates of cGKI have not been identified yet. In the present study a protein interacting with cGKIbeta has been isolated from a rat intestinal cDNA library using the yeast two-hybrid system. The protein was identified as cysteine-rich protein 2 (CRP2), recently cloned from rat brain (Okano, I., Yamamoto, T., Kaji, A., Kimura, T., Mizuno, K., and Nakamura, T. (1993) FEBS Lett. 333, 51-55). Recombinant CRP2 is specifically phosphorylated by cGKs but not by cAMP kinase in vitro. Co-transfection of CRP2 and cGKIbeta into COS cells confirmed the phosphorylation of CRP2 in vivo. Cyclic GMP kinase I phosphorylated CRP2 at Ser-104, because the mutation to Ala completely prevented the in vivo phosphorylation. Immunohistochemical analysis using confocal laser scan microscopy showed a co-localization of CRP2 and cGKI in the inner part of the circular muscle layer, in the muscularis mucosae, and in specific neurons of the myenteric and submucosal plexus. The co-localization together with the specific phosphorylation of CRP2 by cGKI in vitro and in vivo suggests that CRP2 is a novel substrate of cGKI in neurons and smooth muscle of the small intestine.

Regulation of fetal rat bone growth by C-type natriuretic peptide and cGMP

C-type natriuretic peptide (CNP) and its high affinity receptor-B are expressed in fetal bones. Here we show that CNP accelerates longitudinal growth of fetal rat metatarsal bones in organ culture by several mechanisms. First, CNP stimulates chondrocyte proliferation in the proliferative zone as assessed by [3H]thymidine incorporation. Second, CNP stimulates cell hypertrophy as assessed by quantitative histology. Third, CNP stimulates cartilage matrix production as assessed by incorporation of 35SO4 into glycosaminoglycans. Natriuretic peptide receptor-B contains an intracellular guanylyl cyclase catalytic domain. We therefore hypothesized that cyclic GMP (cGMP) would reproduce the effects of CNP on fetal bones. Consistent with this hypothesis, we found that 8-Br-cGMP, like CNP, stimulates longitudinal growth and glycosaminoglycan synthesis. However, unlike CNP, cGMP inhibits proliferation of growth plate chondrocytes and has no effect on hypertrophy. We conclude that CNP stimulates longitudinal bone growth by increasing chondrocyte proliferation, chondrocyte hypertrophy, and cartilage matrix production. cGMP, a second messenger for CNP, reproduces some but not all of the effects of CNP, suggesting that other signal transduction mechanisms may also be involved.