cGMP stimulation of cystic fibrosis transmembrane conductance regulator Cl- channels co-expressed with cGMP-dependent protein kinase type II but not type Ibeta

Partially differentiated enterocytes in ileal and distal-colonic human F508del-CF-enteroids secrete fluid in response to forskolin and linaclotide

Constipation causes significant morbidity in Cystic Fibrosis (CF) patients. Using CF patient (F508del) derived ex vivo ileal and distal colonic/rectal enteroids as a model and the Forskolin Induced Swelling Assay (FIS), we compared CFTR mediated fluid secretion in human enterocytes across the crypt-villus axis. CFTR expression and FIS decreased as enterocytes differentiated from crypt to become partially differentiated and then mature villus cells . While there was no FIS response in undifferentiated (crypt enterocytes) F508del-CF enteroids, partially differentiated F508del-CF enteroids had a swelling response to forskolin (cAMP) and linaclotide (cGMP) which was ∼48%, and ∼67% of the response in healthy enteroids, respectively and was prevented by a CFTR inhibitor. Also, linaclotide and a general PDE inhibitor independently enhanced combined CFTR-modulator-induced FIS response from partially differentiated F508del-CF enteroids. These findings demonstrate that partially differentiated ileal and distal colonic F508del-CFTR enteroids can be stimulated to secrete fluid by cAMP and cGMP.

Loss of cGMP-dependent protein kinase II alters ultrasonic vocalizations in mice, a model for speech impairment in human microdeletion 4q21 syndrome

Chromosome 4q21 microdeletion leads to a human syndrome that exhibits restricted growth, facial dysmorphisms, mental retardation, and absent or delayed speech. One of the key genes in the affected region of the chromosome is PRKG2, which encodes cGMP-dependent protein kinase II (cGKII). Mice lacking cGKII exhibit restricted growth and deficits in learning and memory, as seen in the human syndrome. However, vocalization impairments in these mice have not been determined. The molecular pathway underlying vocalization impairment in humans is not fully understood. Here, we employed cGKII knockout (KO) mice as a model for the human microdeletion syndrome to test whether vocalizations are affected by loss of the PRKG2 gene. Mice emit ultrasonic vocalizations (USVs) to communicate in social situations, stress, and isolation. We thus recorded ultrasonic vocalizations as a model for human speech. We isolated postnatal day 5-7 pups from the nest to record and analyze USVs and found significant differences in vocalizations of KO mice relative to wild-type and heterozygous mutant mice. KO mice produced fewer calls that were shorter duration and higher frequency. Because neuronal activation in the arcuate nucleus in the hypothalamus is important for the production of animal USVs following isolation from the nest, we assessed neuronal activity in the arcuate nucleus of KO pups following isolation. We found significant reduction of neuronal activation in cGKII KO pups after isolation. Taken together, our studies indicate that cGKII is important for neuronal activation in the arcuate nucleus, which significantly contributes to the production of USVs in neonatal mice. We further suggest cGKII KO mice can be a valuable animal model to investigate pathophysiology of human microdeletion 4q21 syndrome.

Cyclic nucleotide phosphodiesterase inhibitors as therapeutic interventions for cystic fibrosis

Cystic Fibrosis (CF) lung disease results from mutations in the CFTR anion channel that reduce anion and fluid secretion by airway epithelia. Impaired secretion compromises airway innate defence mechanisms and leads to bacterial colonization, excessive inflammation and tissue damage; thus, restoration of CFTR function is the goal of many CF therapies. CFTR channels are activated by cyclic nucleotide-dependent protein kinases. The second messengers 3'5'-cAMP and 3'5'-cGMP are hydrolysed by a large family of cyclic nucleotide phosphodiesterases that provide subcellular spatial and temporal control of cyclic nucleotide-dependent signalling. Selective inhibition of these enzymes elevates cyclic nucleotide levels, leading to activation of CFTR and other downstream effectors. Here we examine members of the PDE family that are likely to regulate CFTR-dependent ion and fluid secretion in the airways and discuss other actions of PDE inhibitors that can influence cyclic nucleotide-regulated mucociliary transport, inflammation and bronchodilation. Finally, we review PDE inhibitors and the potential benefits they could provide as CF therapeutics.

Plumbagin Prevents Secretory Diarrhea by Inhibiting CaCC and CFTR Channel Activities

Secretory diarrhea, which primarily originates through intestinal pathogens and viruses, is a health burden in many regions worldwide. Enterocyte Cl⁻ channels, as the final step in enterotoxin-induced fluid secretion, constitute an attractive class of targets for diarrhea therapy. Chloride channel inhibitors have become a new class of candidates for antisecretion and anti-intestinal motility agents. In the present study, we identified plumbagin as a transmembrane protein 16A (TMEM16A) inhibitor in a cell-based fluorescence-quenching assay, and the IC₅₀ value was ∼12.46 µM. Short-circuit current measurements showed that plumbagin reversibly inhibited the E_act-induced Cl⁻ current on the apical side of TMEM16A-transfected Fischer rat thyroid (FRT) cells with no significant effect on cytoplasmic Ca²⁺ signaling. Notably, plumbagin also inhibited the activity of intestinal epithelial calcium-activated chloride channel (CaCC) and cystic fibrosis transmembrane conductance regulator (CFTR) in both HT-29 cells and mouse colons, but had no effects on the activity of the Na⁺-K⁺ ATPase or K⁺ channels. In in vivo experiments, the administration of plumbagin reduced both Escherichia coli heat-stable enterotoxin (STa)- and cholera toxin (CT)-induced intestinal fluid secretion. In neonatal mouse models of CT- and rotavirus infection-induced diarrhea, 0.4 µg plumbagin inhibited secretory diarrhea by >40% and 50%, respectively, without affecting intestinal epithelial integrity or the rotaviral infection. In addition, plumbagin exerted inhibitory effects on the vasoactive intestinal peptide (VIP)-, prostaglandin E2 (PGE2)-, and 5-hydroxytryptamine (5-HT)-stimulated Cl⁻ currents. In the evaluations of intestinal motility, plumbagin significantly delayed intestinal motility and inhibited intestinal smooth muscle contractility without an evident impact on contractive frequency. Collectively, our results indicate that plumbagin inhibits both Ca²⁺- and cAMP-activated Cl⁻ channels, accounting for the mechanisms of plumbagin inhibition of chloride secretion and intestinal motility. Thus, plumbagin can be a lead compound in the treatment of CT-induced, Traveler’s, and rotaviral diarrhea, as well as other types of secretory diarrhea that result from excessive intestinal fluid secretion and increased intestinal peristalsis.

NHERF3 is necessary for Escherichia coli heat-stable enterotoxin-induced inhibition of NHE3: differences in signaling in mouse small intestine and Caco-2 cells

Enterotoxigenic Escherichia coli (ETEC) is a leading cause of childhood death from diarrhea and the leading cause of Traveler's diarrhea. E. coli heat-stable enterotoxin (ST) is a major virulence factor of ETEC and inhibits the brush border Na/H exchanger NHE3 in producing diarrhea. NHE3 regulation involves multiprotein signaling complexes that form on its COOH terminus. In this study, the hypothesis was tested that ST signals via members of the Na/H exchanger regulatory factor (NHERF) family of scaffolding proteins, NHERF2, which had been previously shown to have a role, and now with concentration on a role for NHERF3. Two models were used: mouse small intestine and Caco-2/BBe cells. In both models, ST rapidly increased intracellular cGMP, inhibited NHE3 activity, and caused a quantitatively similar decrease in apical expression of NHE3. The transport effects were NHERF3 and NHERF2 dependent. Also, mutation of the COOH-terminal amino acids of NHERF3 supported that NHERF3-NHERF2 heterodimerization was likely to account for this dual dependence. The ST increase in cGMP in both models was partially dependent on NHERF3. The intracellular signaling pathways by which ST-cGMP inhibits NHE3 were different in mouse jejunum (activation of cGMP kinase II, cGKII) and Caco-2 cells, which do not express cGKII (elevation of intracellular Ca²⁺ concentration [Ca²⁺]_i). The ST elevation of [Ca²⁺]_i was from intracellular stores and was dependent on NHERF3-NHERF2. This study shows that intracellular signaling in the same diarrheal model in multiple cell types may be different; this has implications for therapeutic strategies, which often assume that models have similar signaling mechanisms.

Heat-Stable Enterotoxins of Enterotoxigenic Escherichia coli and Their Impact on Host Immunity

Enterotoxigenic Escherichia coli (ETEC) are an important diarrhea-causing pathogen and are regarded as a global threat for humans and farm animals. ETEC possess several virulence factors to infect its host, including colonization factors and enterotoxins. Production of heat-stable enterotoxins (STs) by most ETEC plays an essential role in triggering diarrhea and ETEC pathogenesis. In this review, we summarize the heat-stable enterotoxins of ETEC strains from different species as well as the molecular mechanisms used by these heat-stable enterotoxins to trigger diarrhea. As recently described, intestinal epithelial cells are important modulators of the intestinal immune system. Thus, we also discuss the impact of the heat-stable enterotoxins on this role of the intestinal epithelium and how these enterotoxins might affect intestinal immune cells. Finally, the latest developments in vaccination strategies to protect against infections with ST secreting ETEC strains are discussed. This review might inform and guide future research on heat-stable enterotoxins to further unravel their molecular pathogenesis, as well as to accelerate vaccine design.

SLC6A14, an amino acid transporter, modifies the primary CF defect in fluid secretion

The severity of intestinal disease associated with Cystic Fibrosis (CF) is variable in the patient population and this variability is partially conferred by the influence of modifier genes. Genome-wide association studies have identified SLC6A14, an electrogenic amino acid transporter, as a genetic modifier of CF-associated meconium ileus. The purpose of the current work was to determine the biological role of Slc6a14, by disrupting its expression in CF mice bearing the major mutation, F508del. We found that disruption of Slc6a14 worsened the intestinal fluid secretion defect, characteristic of these mice. In vitro studies of mouse intestinal organoids revealed that exacerbation of the primary defect was associated with reduced arginine uptake across the apical membrane, with aberrant nitric oxide and cyclic GMP-mediated regulation of the major CF-causing mutant protein. Together, these studies highlight the role of this apical transporter in modifying cellular nitric oxide levels, residual function of the major CF mutant and potentially, its promise as a therapeutic target.

AMP-activated protein kinase and adenosine are both metabolic modulators that regulate chloride secretion in the shark rectal gland ( Squalus acanthias)

The production of endogenous adenosine during secretagogue stimulation of CFTR leads to feedback inhibition limiting further chloride secretion in the rectal gland of the dogfish shark (Squalus acanthias). In the present study, we examined the role of AMP-kinase (AMPK) as an energy sensor also modulating chloride secretion through CFTR. We found that glands perfused with forskolin and isobutylmethylxanthine (F + I), potent stimulators of chloride secretion in this ancient model, caused significant phosphorylation of the catalytic subunit Thr172 of AMPK. These findings indicate that AMPK is activated during energy-requiring stimulated chloride secretion. In molecular studies, we confirmed that the activating Thr172 site is indeed present in the α-catalytic subunit of AMPK in this ancient gland, which reveals striking homology to AMPKα subunits sequenced in other vertebrates. When perfused rectal glands stimulated with F + I were subjected to severe hypoxic stress or perfused with pharmacologic inhibitors of metabolism (FCCP or oligomycin), phosphorylation of AMPK Thr172 was further increased and chloride secretion was dramatically diminished. The pharmacologic activation of AMPK with AICAR-inhibited chloride secretion, as measured by short-circuit current, when applied to the apical side of shark rectal gland monolayers in primary culture. These results indicate that that activated AMPK, similar to adenosine, transmits an inhibitory signal from metabolism, that limits chloride secretion in the shark rectal gland.

Transepithelial Fluid and Salt Re-Absorption Regulated by cGK2 Signals

Transepithelial fluid and salt re-absorption in epithelial tissues play an important role in fluid and salt homeostasis. In absorptive epithelium, fluid and salt flux is controlled by machinery mainly composed of epithelial sodium channels (ENaC), cystic fibrosis transmembrane conductance regulator (CFTR), Na⁺/H⁺ exchanger (NHE), aquaporin, and sodium potassium adenosine triphosphatase (Na⁺/K⁺-ATPase). Dysregulation of fluid and salt transport across epithelium contributes to the pathogenesis of many diseases, such as pulmonary edema and cystic fibrosis. Intracellular and extracellular signals, i.e., hormones and protein kinases, regulate fluid and salt turnover and resolution. Increasing evidence demonstrates that transepithelial fluid transport is regulated by cyclic guanosine monophosphate-dependent protein kinase (cGK) signals. cGK2 was originally identified and cloned from intestinal specimens, the presence of which has also been confirmed in the kidney and the lung. cGK2 regulates fluid and salt through ENaC, CFTR and NHE. Deficient cGK2 regulation of transepithelial ion transport was seen in acute lung injury, and cGK2 could be a novel druggable target to restore edematous disorder in epithelial tissues.

Cyclic GMP kinase II (cGKII) inhibits NHE3 by altering its trafficking and phosphorylating NHE3 at three required sites: identification of a multifunctional phosphorylation site

The epithelial brush-border Na(+)/H(+) exchanger NHE3 is acutely inhibited by cGKII/cGMP, but how cGKII inhibits NHE3 is unknown. This study tested the hypothesis that cGMP inhibits NHE3 by phosphorylating it and altering its membrane trafficking. Studies were carried out in PS120/NHERF2 and in Caco-2/Bbe cells overexpressing HA-NHE3 and cGKII, and in mouse ileum. NHE3 activity was measured with 2',7'-bis(carboxyethyl)-S-(and 6)carboxyfluorescein acetoxy methylester/fluorometry. Surface NHE3 was determined by cell surface biotinylation. Identification of NHE3 phosphorylation sites was by iTRAQ/LC-MS/MS with TiO2 enrichment and immunoblotting with specific anti-phospho-NHE3 antibodies. cGMP/cGKII rapidly inhibited NHE3, which was associated with reduced surface NHE3. cGMP/cGKII increased NHE3 phosphorylation at three sites (rabbit Ser(554), Ser(607), and Ser(663), equivalent to mouse Ser(552), Ser(605), and Ser(659)), all of which had to be present at the same time for cGMP to inhibit NHE3. NHE3-Ser(663) phosphorylation was not necessary for cAMP inhibition of NHE3. Dexamethasone (4 h) stimulated wild type NHE3 activity and increased surface expression but failed to stimulate NHE3 activity or increase surface expression when NHE3 was mutated to either S663A or S663D. We conclude that 1) cGMP inhibition of NHE3 is associated with phosphorylation of NHE3 at Ser(554), Ser(607), and Ser(663), all of which are necessary for cGMP/cGKII to inhibit NHE3. 2) Dexamethasone stimulates NHE3 by phosphorylation of a single site, Ser(663). The requirement for three phosphorylation sites in NHE3 for cGKII inhibition, and for phosphorylation of one of these sites for dexamethasone stimulation of NHE3, is a unique example of regulation by phosphorylation.

cGMP inhibition of type 3 phosphodiesterase is the major mechanism by which C-type natriuretic peptide activates CFTR in the shark rectal gland

The in vitro perfused rectal gland of the dogfish shark (Squalus acanthias) and filter-grown monolayers of primary cultures of shark rectal gland (SRG) epithelial cells were used to analyze the signal transduction pathway by which C-type natriuretic peptide (CNP) stimulates chloride secretion. CNP binds to natriuretic receptors in the basolateral membrane, elevates cellular cGMP, and opens cystic fibrosis transmembrane conductance regulator (CFTR) chloride channels in the apical membrane. CNP-provoked chloride secretion was completely inhibitable by the nonspecific protein kinase inhibitor staurosporine and the PKA inhibitor H89 but insensitive to H8, an inhibitor of type I and II isoforms of cGMP-dependent protein kinase (cGKI and cGKII). CNP-induced secretion could not be mimicked by nonhydrolyzable cGMP analogs added alone or in combination with the protein kinase C activator phorbolester, arguing against a role for cGK or for cGMP-induced PKC signaling. We failed to detect a dogfish ortholog of cGKII by molecular cloning and affinity chromatography. However, inhibitors of the cGMP-inhibitable isoform of phosphodiesterase (PDE3) including milrinone, amrinone, and cilostamide but not inhibitors of other PDE isoenzymes mimicked the effect of CNP on chloride secretion in perfused glands and monolayers. CNP raised cGMP and cAMP levels in the SRG epithelial cells. This rise in cAMP as well as the CNP and amrinone-provoked chloride secretion, but not the rise in cGMP, was almost completely blocked by the Gαi-coupled adenylyl cyclase inhibitor somatostatin, arguing against a role for cGMP cross-activation of PKA in CNP action. These data provide molecular, functional, and pharmacological evidence for a CNP/cGMP/PDE3/cAMP/PKA signaling cascade coupled to CFTR in the SRG.

The type II cGMP dependent protein kinase regulates GluA1 levels at the plasma membrane of developing cerebellar granule cells

Trafficking of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) is regulated by specific interactions with other proteins and by post-translational mechanisms, such as phosphorylation. We have found that the type II cGMP-dependent protein kinase (cGKII) phosphorylates GluA1 (formerly GluR1) at S845, augmenting the surface expression of AMPARs at both synaptic and extrasynaptic sites. Activation of cGKII by 8-Br-cGMP enhances the surface expression of GluA1, whereas its inhibition or suppression effectively diminished the expression of this protein at the cell surface. In granule cells, NMDA receptor activation (NMDAR) stimulates nitric oxide and cGMP production, which in turn activates cGKII and induces the phosphorylation of GluA1, promoting its accumulation in the plasma membrane. GluA1 is mainly incorporated into calcium permeable AMPARs as exposure to 8-Br-cGMP or NMDA activation enhanced AMPA-elicited calcium responses that are sensitive to NASPM inhibition. We summarize evidence for an increase of calcium permeable AMPA receptors downstream of NMDA receptor activation that might be relevant for granule cell development and plasticity.

MRP4 and CFTR in the regulation of cAMP and β-adrenergic contraction in cardiac myocytes

Spatiotemporal regulation of cAMP in cardiac myocytes is integral to regulating the diverse functions downstream of β-adrenergic stimulation. The activities of cAMP phosphodiesterases modulate critical and well-studied cellular processes. Recently, in epithelial and smooth muscle cells, it was found that the multi-drug resistant protein 4 (MRP4) acts as a cAMP efflux pump to regulate intracellular cAMP levels and alter effector function, including activation of the cAMP-stimulated Cl(-) channel, CFTR (cystic fibrosis transmembrane conductance regulator). In the current study we investigated the potential role of MRP4 in regulating intracellular cAMP and β-adrenergic stimulated contraction rate in cardiac myocytes. Cultured neonatal ventricular myocytes were used for all experiments. In addition to wildtype mice, β(1)-, β(2)-, and β(1)/β(2)-adrenoceptor, and CFTR knockout mice were used. MRP4 expression was probed via Western blot, intracellular cAMP was measured by fluorescence resonance energy transfer, while the functional role of MRP4 was assayed via monitoring of isoproterenol-stimulated contraction rate. We found that MRP4 is expressed in mouse neonatal ventricular myocytes. A pharmacological inhibitor of MRP4, MK571, potentiated submaximal isoproterenol-stimulated cAMP accumulation and cardiomyocyte contraction rate via β(1)-adrenoceptors. CFTR expression was critical for submaximal isoproterenol-stimulated contraction rate. Interestingly, MRP4-dependent changes in contraction rate were CFTR-dependent, however, PDE4-dependent potentiation of contraction rate was CFTR-independent. We have shown, for the first time, a role for MRP4 in the regulation of cAMP in cardiac myocytes and involvement of CFTR in β-adrenergic stimulated contraction. Together with phosphodiesterases, MRP4 must be considered when examining cAMP regulation in cardiac myocytes.

Cure and curse: E. coli heat-stable enterotoxin and its receptor guanylyl cyclase C

Enterotoxigenic Escherichia coli (ETEC) associated diarrhea is responsible for roughly half a million deaths per year, the majority taking place in developing countries. The main agent responsible for these diseases is the bacterial heat-stable enterotoxin STa. STa is secreted by ETEC and after secretion binds to the intestinal receptor guanylyl cyclase C (GC-C), thus triggering a signaling cascade that eventually leads to the release of electrolytes and water in the intestine. Additionally, GC-C is a specific marker for colorectal carcinoma and STa is suggested to have an inhibitory effect on intestinal carcinogenesis. To understand the conformational events involved in ligand binding to GC-C and to devise therapeutic strategies to treat both diarrheal diseases and colorectal cancer, it is paramount to obtain structural information on the receptor ligand system. Here we summarize the currently available structural data and report on physiological consequences of STa binding to GC-C in intestinal epithelia and colorectal carcinoma cells.

Toxin mediated diarrhea in the 21 century: the pathophysiology of intestinal ion transport in the course of ETEC, V. cholerae and rotavirus infection

An estimated 4 billion episodes of diarrhea occur each year. As a result, 2–3 million children and 0.5–1 million adults succumb to the consequences of this major healthcare concern. The majority of these deaths can be attributed to toxin mediated diarrhea by infectious agents, such as E. coli, V. cholerae or Rotavirus. Our understanding of the pathophysiological processes underlying these infectious diseases has notably improved over the last years. This review will focus on the cellular mechanism of action of the most common enterotoxins and the latest specific therapeutic approaches that have been developed to contain their lethal effects.

Receptor guanylyl cyclase C (GC-C): regulation and signal transduction

Receptor guanylyl cyclase C (GC-C) is the target for the gastrointestinal hormones, guanylin, and uroguanylin as well as the bacterial heat-stable enterotoxins. The major site of expression of GC-C is in the gastrointestinal tract, although this receptor and its ligands play a role in ion secretion in other tissues as well. GC-C shares the domain organization seen in other members of the family of receptor guanylyl cyclases, though subtle differences highlight some of the unique features of GC-C. Gene knock outs in mice for GC-C or its ligands do not lead to embryonic lethality, but modulate responses of these mice to stable toxin peptides, dietary intake of salts, and development and differentiation of intestinal cells. It is clear that there is much to learn in future about the role of this evolutionarily conserved receptor, and its properties in intestinal and extra-intestinal tissues.

Regulation of epithelial sodium channels by cGMP/PKGII

Airway and alveolar fluid clearance is mainly governed by vectorial salt movement via apically located rate-limiting Na(+) channels (ENaC) and basolateral Na(+)/K(+)-ATPases. ENaC is regulated by a spectrum of protein kinases, i.e. protein kinase A (PKA), C (PKC), and G (PKG). However, the molecular mechanisms for the regulation of ENaC by cGMP/PKG remain to be elucidated. In the present study, we studied the pharmacological responses of native epithelial Na(+) channels in human Clara cells and human alphabetagammadelta ENaCs expressed in oocytes to cGMP. 8-pCPT-cGMP increased amiloride-sensitive short-circuit current (I(sc)) across H441 monolayers and heterologously expressed alphabetagammadelta ENaC activity in a dose-dependent manner. Similarly, 8-pCPT-cGMP (a PKGII activator) but not 8-Br-cGMP (a PKGI activator) increased amiloride-sensitive whole cell currents in H441 cells in the presence of CFTRinh-172 and diltiazem. In all cases, the cGMP-activated Na(+) channel activity was inhibited by Rp-8-pCPT-cGMP, a specific PKGII inhibitor. This was substantiated by the evidence that PKGII was the sole isoform expressed in H441 cells at the protein level. Importantly, intratracheal instillation of 8-pCPT-cGMP in BALB/c mice increased amiloride-sensitive alveolar fluid clearance by approximately 30%, consistent with the in vitro results. We therefore conclude that PKGII is an activator of lung epithelial Na(+) channels, which may expedite the resolution of oedematous fluid in alveolar sacs.

cGK substrates

Signalling of cGK (cGMP-dependent protein kinases) are mediated through phosphorylation of specific substrates. Several substrates of cGKI and cGKII were identified meanwhile. Some cGKI substrates are specifically regulated by the cGKIalpha or the cGKIbeta isozyme. In various cells and tissues, different cGK substrates exist that are essential for the regulation of diverse functions comprising tissue contractility, cell motility, cell contact, cellular secretion, cell proliferation, and cell differentiation. On the molecular level, cGKI substrates fulfill various cellular functions regulating e.g. the intracellular calcium and potassium concentration, the calcium sensitivity, and the organisation of the intracellular cytoskeleton. cGKII substrates are involved e.g. in chloride transport, sodium/proton transport and transcriptional regulation. The understanding of cGK signalling and function depends strongly on the identification of further specific substrates. In the last years, diverse approaches ranging from biochemistry to genetic deletion lead to the identification and establishment of several substrates, which raised new insights in the molecular mechanisms of cGK functions and elucidated new cellular cGK functions. However, the analysis of the dynamic signalling of cGK in tissues and cells will be necessary to discover new signalling pathways and substrates.

Trafficking of cGMP-dependent protein kinase II via interaction with Rab11

cGMP-dependent protein kinase II (cGK-II) is implicated in several physiological functions including intestinal secretion, bone growth, and learning and memory, but the detailed mechanisms are still unclear. To identify proteins that are involved in cGMP/cGK-II signaling, we performed yeast two-hybrid screening and identified Rab11b as a cGK-II-interacting protein that regulates the slow-recycling pathway. Interestingly, cGK-II interacted with the GDP-bound form of Rab11b (Rab11b S25N), but not the GTP-bound form, in mammalian cells. Immunofluorescence staining revealed that Rab11b S25N promoted the translocation of cGK-II from the plasma membrane to the cytoplasm and that the localization of cGK-II extensively overlapped with Rab11b. Furthermore, treatment with a membrane-permeable cGMP analog caused the rapid retranslocation of cGK-II and Rab11b S25N to the membrane. These data indicate that Rab11b is necessary for the trafficking of cGK-II and that the cGMP/cGK-II signaling pathway is closely related to Rab11b recycling pathway.

Protein kinase G as a therapeutic target for the treatment of metastatic colorectal cancer

Colorectal cancer is a leading cause of cancer-related death in the world and there is an urgent need for new strategies to combat this disease. Findings from several independent laboratories have converged on cGMP signaling as an exciting new therapeutic target, but the mechanisms remain controversial. A key intracellular effector of cGMP is protein kinase G (PKG). This article reviews the scientific literature concerning PKG effects on tumor development and progression, and discusses possible strategies for its exploitation in future cancer therapies. Studies from several independent laboratories have described novel anti-tumor effects of PKG in colon cancer cells that include inhibition of tumor growth and angiogenesis. While more preclinical research is warranted to better understand signaling mechanisms, these properties support the notion that PKG is a novel cancer target.

Association of VASP with TRPC4 in PKG-mediated inhibition of the store-operated calcium response in mesangial cells

We tested the hypotheses that the NO-cGMP-PKG pathway mediates inhibition of the store-operated cation channel (SOC) in human glomerular mesangial cells (HMC) and that TRPC4, a molecular component of SOC in HMC, is associated with PKG-phosphorylated vasodilator-stimulated phosphoprotein (VASP). Using fura 2 ratiometry, we measured intracellular Ca(2+) concentration [Ca(2+)](i) to determine whether sodium nitroprusside (SNP), an NO donor, and 8-Br-cGMP affected SOC-TRPC4 via PKG. We found that the SOC response in HMC was attenuated in the presence of 100 microM SNP, an NO donor, or 100 microM 8-Br-cGMP. Addition of DT-3 (2.5 microM), a specific PKG-1alpha inhibitor, reversed the effects of 8-Br-cGMP on the SOC response. Application of 100 microM cAMP did not significantly inhibit the SOC response. RT-PCR and Western blotting revealed PKG-1alpha transcript and protein in HMC. Immunocytochemical analysis localized PKG-1alpha to the cytoplasm and plasma membrane of HMC. Previous studies have shown that PKG-mediated phosphorylation of VASP attenuates cellular Ca(2+) entry, resulting in altered growth and proliferation. Therefore, we used Western blotting and immunocytochemistry to determine whether PKG-phosphorylated VASP associates with TRPC4. Western blot analysis revealed that 8-Br-cGMP enhanced the phosphorylation of VASP at serine 239 (Ser239), a known PKG phosphorylation site, in HMC within 5 min. Coimmunoprecipitation and coimmunostaining showed that P-Ser239-VASP associated with TRPC4. However, VASP that was unphosphorylated at Ser239 was not associated with TRPC4. These results indicate that VASP has a role in the NO/PKG-1alpha-mediated inhibition of the TRPC4-SOC response in HMC.

The alpha2beta1 isoform of guanylyl cyclase mediates plasma membrane localized nitric oxide signalling

Nitric oxide (NO) is a mediator of copious biological processes, in many cases through the production of cGMP from the enzyme nitric oxide-sensitive guanylyl cyclase. Natriuretic peptides also elevate cGMP, often with distinct biological effects, raising the issue of how specificity is achieved. Here we show that a recently described alpha(2)beta(1) isoform of guanylyl cyclase is expressed in a number of epithelia, where it is localized to the apical plasma membrane. We measured the functional properties of the alpha(2)beta(1) isoform by utilizing the NO-dependent activation of the ion channel cystic fibrosis transmembrane conductance regulator (CFTR), which occurs by phosphorylation via the membrane-bound type II isoform of cGMP-dependent protein kinase. We found that cGMP generated by NO activation of the alpha(2)beta(1) isoform of guanylyl cyclase is an exceptionally efficient mediator of nitric oxide action on membrane targets, activating CFTR far more effectively than the cytoplasmically located alpha(1)beta(1) guanylyl cyclase isoform. Targeting the alpha(1)beta(1) isoform of guanylyl cyclase to the membrane also dramatically enhanced the effects of nitric oxide on CFTR within the membrane. This was not due to increased enzymatic activity of guanylyl cyclase in a membrane location, but to production of a localised membrane pool of cGMP by membrane-localized NO-dependent guanylyl cyclase that was resistant to degradation by phosphodiesterases. Selective effects of cGMP produced from this enzyme in response to NO are directed at membrane targets and suggest that drugs selectively activating or inhibiting this alpha(2)beta(1) isoform of guanylyl cyclase may have unique pharmacological properties.

Cyclic Guanosine 5′-Monophosphate-Dependent Protein Kinase II Is Induced by Luteinizing Hormone and Progesterone Receptor-Dependent Mechanisms in Granulosa Cells and Cumulus Oocyte Complexes of Ovulating Follicles

Cyclic GMP (cGMP)-dependent protein kinase II (Prkg2, cGK II) was identified as a potential target of the progesterone receptor (Nr3c3) in the mouse ovary based on microarray analyses. To document this further, the expression patterns of cGK II and other components of the cGMP signaling pathway were analyzed during follicular development and ovulation using the pregnant mare serum gonadotropin (PMSG)-human chorionic gonadotropin (hCG)-primed immature mice. Levels of cGK II mRNA were low in ovaries of immature mice, increased 4-fold in response to pregnant mare serum gonadotropin and 5-fold more within 12 h after hCG, the time of ovulation. In situ hybridization localized cGK II mRNA to granulosa cells and cumulus oocyte complexes of periovulatory follicles. In progesterone receptor (PR) null mice, cGK II mRNA was reduced significantly at 12 h after hCG in contrast to heterozygous littermates. In primary granulosa cell cultures, cGK II mRNA was induced by phorbol 12-myristate 13-acetate enhanced by adenoviral expression of PR-A and blocked by RU486 and trilostane. PR-A in the absence of phorbol 12-myristate 13-acetate was insufficient to induce cGK II. Expression of cGK I (Prkg1) was restricted to the residual tissue and not regulated by hormones. Guanylate cyclase-A (Npr1; GC-A) mRNA expression increased 6-fold by 4 h after hCG treatment in contrast to pregnant mare serum gonadotropin alone and was localized to granulosa cells of preovulatory follicles. Collectively, these data show for the first time that cGK II (not cGK I) and GC-A are selectively induced in granulosa cells of preovulatory follicles by LH- and PR-dependent mechanisms, thereby providing a pathway for cGMP function during ovulation.

MKP-1 expression and stabilization and cGK Iα prevent diabetes- associated abnormalities in VSMC migration

Diabetes mellitus is a major risk factor in the development of atherosclerosis and cardiovascular disease conditions, involving intimal injury and enhanced vascular smooth muscle cell (VSMC) migration. We report a mechanistic basis for divergences between insulin’s inhibitory effects on migration of aortic VSMC from control Wistar Kyoto (WKY) rats versus Goto-Kakizaki (GK) diabetic rats. In normal WKY VSMC, insulin increased MAPK phosphatase-1 (MKP-1) expression as well as MKP-1 phosphorylation, which stabilizes it, and inhibited PDGF-mediated MAPK phosphorylation and cell migration. In contrast, basal migration was elevated in GK diabetic VSMCs, and all of insulin’s effects on MKP-1 expression and phosphorylation, MAPK phosphorylation, and PDGF-stimulated migration were markedly inhibited. The critical importance of MKP-1 in insulin inhibition of VSMC migration was evident from several observations. MKP-1 small interfering RNA inhibited MKP-1 expression and abolished insulin inhibition of PDGF-induced VSMC migration. Conversely, adenoviral expression of MKP-1 decreased MAPK phosphorylation and basal migration rate and restored insulin's ability to inhibit PDGF-directed migration in GK diabetic VSMCs. Also, the proteasomal inhibitors lactacystin and MG132 partially restored MKP-1 protein levels in GK diabetic VSMCs and inhibited their migration. Furthermore, GK diabetic aortic VSMCs had reduced cGMP-dependent protein kinase Iα (cGK Iα) levels as well as insulin-dependent, but not sodium nitroprusside-dependent, stimulation of cGMP. Adenoviral expression of cGK Iα enhanced MKP-1 inhibition of MAPK phosphorylation and VSMC migration. We conclude that enhanced VSMC migration in GK diabetic rats is due at least in part to a failure of insulin-stimulated cGMP/cGK Iα signaling, MKP-1 expression, and stabilization and thus MAPK inactivation.

Analysis of Drosophila cGMP-dependent Protein Kinases and Assessment of Their in Vivo Roles by Targeted Expression in a Renal Transporting Epithelium

cGMP-dependent protein kinase (cGK) forms encoded by the dg2 (for) gene are implicated in behavior and epithelial transport in Drosophila melanogaster. Here, we provide the first biochemical characterization and cellular localization of cGKs encoded by the major transcripts of dg2: dg2P1 and dg2P2. cGMP stimulates kinase activity of DG2P1 (EC(50): 0.13 +/- 0.039 microm) and DG2P2 (EC(50): 0.32 +/- 0.14 microm) in Malpighian tubule and S2 cell extracts. DG2P1 and DG2P2 are magnesium-requiring enzymes and were inhibited by 10 and 100 microm of a cGK inhibitor, 8-(4-chlorophenylthio)guanosine-3',5'-cyclic monophosphorothioate, Rp isomer; whereas DG1, the cGK encoded by the D. melanogaster dg1 gene, was unaffected. DG2P1 and DG2P2 were localized in the plasma membrane in S2 cells, whereas DG1 was localized in the cytosol. The D. melanogaster fluid-transporting Malpighian tubule was used as an organotypic model to analyze cGK localization and function in vivo. Targeted expression of DG2P2, DG2P1, and DG1 in tubule cells via the UAS/GAL4 system in transgenic flies revealed differential localization of all three cGKs in vivo: DG2P2 expression at the apical membrane; DG2P1 expression at both the apical and basolateral membranes; and DG1 expression at the basolateral membrane and in the cytosol. Transgenic tubules for all three cGKs displayed enhanced cGK activity compared with wild-type tubules. The physiological impact of targeted expression of individual cGKs in tubule principal cells was assessed by measuring basal and stimulated rates of fluid transport. DG1 expression greatly enhanced fluid transport by the tubule in response to exogenous cGMP, whereas DG2P2 expression significantly increased fluid transport in response to the nitridergic neuropeptide, capa-1. Thus, dg2-encoded proteins are bona fide cGKs, which have differential roles in epithelial fluid transport, as assessed by in vivo studies. Furthermore, a novel epithelial role is suggested for DG1, which is considerably more responsive to cGMP than to capa-1 stimulation.

Insulin inhibits PDGF-directed VSMC migration via NO/ cGMP increase of MKP-1 and its inactivation of MAPKs

In this study, we examined the role of insulin in the control of vascular smooth muscle cell (VSMC) migration in the normal vasculature. Platelet-derived growth factor (PDGF) increased VSMC migration, which was inhibited by pretreatment with insulin in a dose-dependent manner. Insulin also caused a 60% decrease in PDGF-stimulated mitogen-activated protein kinase (MAPK) phosphorylation and activation. Insulin inhibition of MAPK was accompanied by a rapid induction of MAPK phosphatase (MKP-1), which inactivates MAPKs by dephosphorylation. Pretreatment with inhibitors of the nitric oxide (NO)/cGMP pathway, blocked insulin-induced MKP-1 expression and restored PDGF-stimulated MAPK activation and migration. In contrast, adenoviral infection of VSMCs with MKP-1 or cGMP-dependent protein kinase Ialpha (cGK Ialpha), the downstream effector of cGMP signaling, blocked the activation of MAPK and prevented PDGF-directed VSMC migration. Expression of antisense MKP-1 RNA prevented insulin's inhibitory effect and restored PDGF-directed VSMC migration and MAPK phosphorylation. We conclude that insulin inhibition of VSMC migration may be mediated in part by NO/cGMP/cGK Ialpha induction of MKP-1 and consequent inactivation of MAPKs.

Modulation of Cl-/OH- exchange activity in Caco-2 cells by nitric oxide

The present studies were undertaken to determine the direct effects of nitric oxide (NO) released from an exogenous donor, S-nitroso-N-acetyl pencillamine (SNAP) on Cl-/OH- exchange activity in human Caco-2 cells. Our results demonstrate that NO inhibits Cl-/OH- exchange activity in Caco-2 cells via cGMP-dependent protein kinases G (PKG) and C (PKC) signal-transduction pathways. Our data in support of this conclusion can be outlined as follows: 1) incubation of Caco-2 cells with SNAP (500 microM) for 30 min resulted in approximately 50% inhibition of DIDS-sensitive 36Cl uptake; 2) soluble guanylate cyclase inhibitors Ly-83583 and (1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one significantly blocked the inhibition of Cl-/OH- exchange activity by SNAP; 3) addition of 8-bromo-cGMP (8-BrcGMP) mimicked the effects of SNAP; 4) specific PKG inhibitor KT-5823 significantly inhibited the decrease in Cl-/OH- exchange activity in response to either SNAP or 8-BrcGMP; 5) Cl-/OH-exchange activity in Caco-2 cells in response to SNAP was not altered in the presence of protein kinase A (PKA) inhibitor (Rp-cAMPS), demonstrating that the PKA pathway was not involved; 6) the effect of NO on Cl-/OH- exchange activity was mediated by PKC, because each of the two PKC inhibitors chelerythrine chloride and calphostin C blocked the SNAP-mediated inhibition of Cl-/OH- exchange activity; 7) SO/OH- exchange in Caco-2 cells was unaffected by SNAP. Our results suggest that NO-induced inhibition of Cl-/OH- exchange may play an important role in the pathophysiology of diarrhea associated with inflammatory bowel diseases.

Inhibition of cGMP-dependent protein kinase II by its own splice isoform

cGMP- and cAMP-dependent protein kinases (cGK I, cGK II, and cAK) are important mediators of many signaling pathways that increase cyclic nucleotide concentrations and ultimately phosphorylation of substrates vital to cellular functions. Here we demonstrate a novel mRNA splice isoform of cGK II arising from alternative 5' splicing within exon 11. The novel splice variant encodes a protein (cGK II Delta(441-469)) lacking 29 amino acids of the cGK II Mg-ATP-binding/catalytic domain, including the conserved glycine-rich loop consensus motif Gly-x-Gly-x-x-Gly-x-Val which interacts with ATP in the protein kinase family of enzymes. cGK II Delta(441-469) has no intrinsic enzymatic activity itself, however, it antagonizes cGK II and cGK I, but not cAK. Thus, the activation and cellular functions of cGK II may be determined not only by intracellular cGMP levels but also by alternative splicing which may regulate the balance of expression of cGK II versus its own inhibitor, cGK II Delta(441-469).

Active Rho kinase (ROK-alpha ) associates with insulin receptor substrate-1 and inhibits insulin signaling in vascular smooth muscle cells

Recent studies from our laboratory have shown that insulin stimulates myosin-bound phosphatase (MBP) in vascular smooth muscle cells (VSMCs) by decreasing site-specific phosphorylation of the myosin-bound subunit (MBS) of MBP via nitric oxide/cGMP-mediated Rho/Rho kinase inactivation. Here we tested potential interactions between Rho kinase and insulin signaling pathways. In control VSMCs, insulin inactivates ROK-alpha, the major Rho kinase isoform in VSMCs, and inhibits thrombin-induced increase in ROK-alpha association with the insulin receptor substrate-1 (IRS-1). Hypertension (in spontaneous hypertensive rats) or expression of an active RhoA(V14) up-regulates Rho kinase activity and increases ROK-alpha/IRS-1 association resulting in IRS-1 serine phosphorylation that leads to inhibition of both insulin-induced IRS-1 tyrosine phosphorylation and phosphatidylinositol 3-kinase (PI3-kinase) activation. In contrast, expression of dominant negative RhoA or cGMP-dependent protein kinase type I alpha inactivates Rho kinase, abolishes ROK-alpha/IRS-1 association, and potentiates insulin-induced tyrosine phosphorylation and PI3-kinase activation leading to decreased MBS(T695) phosphorylation and decreased MBP inhibition. Collectively, these results suggest a novel function for ROK-alpha in insulin signal transduction at the level of IRS-1 and potential cross-talk between cGMP-dependent protein kinase type I alpha, Rho/Rho kinase signaling, and insulin signaling at the level of IRS-1/PI3-kinase.

Localization of cGMP-dependent protein kinase type II in rat brain

In brain, signaling pathways initiated by atrial natriuretic peptide, or transmitters which stimulate nitric oxide synthesis, increase cGMP as their second messenger. One important class of target molecules for cGMP is cGMP-dependent protein kinases, and in the present study, biochemical and immunocytochemical analyses demonstrate the widespread distribution of type II cGMP-dependent protein kinase in rat brain, from the cerebral cortex to the brainstem and cerebellum. Also, colocalization of cGMP-dependent protein kinase type II with its activator, cGMP, was found in several brain regions examined after in vitro stimulation of brain slices with sodium nitroprusside. In western blots, cGMP-dependent protein kinase type II was observed in all brain regions examined, although cerebellar cortex and pituitary contained comparatively less of the kinase. Immunocytochemistry revealed cGMP-dependent protein kinase type II in certain neurons, and occasionally in putative oligodendrocytes and astrocytes, however, its most striking and predominant localization was in neuropil. Electron microscopy examination of neuropil in the medial habenula showed localization of the kinase in both axon terminals and dendrites. As a membrane-associated protein, cGMP-dependent protein kinase type II often appeared to be transported to cell processes to a greater extent than being retained in the cell body. Thus, immunocytochemical labeling of cGMP-dependent protein kinase type II often did not coincide with the localization of kinase mRNA previously observed by others using in situ hybridization. We conclude that in contrast to cGMP-dependent protein kinase type I, which has a very restricted localization to cerebellar Purkinje cells and a few other sites, cGMP-dependent protein kinase type II is a very ubiquitous brain protein kinase and thus a more likely candidate for relaying myriad cGMP effects in brain requiring protein phosphorylation.

Electrolyte transport in the mammalian colon: mechanisms and implications for disease

The colonic epithelium has both absorptive and secretory functions. The transport is characterized by a net absorption of NaCl, short-chain fatty acids (SCFA), and water, allowing extrusion of a feces with very little water and salt content. In addition, the epithelium does secret mucus, bicarbonate, and KCl. Polarized distribution of transport proteins in both luminal and basolateral membranes enables efficient salt transport in both directions, probably even within an individual cell. Meanwhile, most of the participating transport proteins have been identified, and their function has been studied in detail. Absorption of NaCl is a rather steady process that is controlled by steroid hormones regulating the expression of epithelial Na(+) channels (ENaC), the Na(+)-K(+)-ATPase, and additional modulating factors such as the serum- and glucocorticoid-regulated kinase SGK. Acute regulation of absorption may occur by a Na(+) feedback mechanism and the cystic fibrosis transmembrane conductance regulator (CFTR). Cl(-) secretion in the adult colon relies on luminal CFTR, which is a cAMP-regulated Cl(-) channel and a regulator of other transport proteins. As a consequence, mutations in CFTR result in both impaired Cl(-) secretion and enhanced Na(+) absorption in the colon of cystic fibrosis (CF) patients. Ca(2+)- and cAMP-activated basolateral K(+) channels support both secretion and absorption of electrolytes and work in concert with additional regulatory proteins, which determine their functional and pharmacological profile. Knowledge of the mechanisms of electrolyte transport in the colon enables the development of new strategies for the treatment of CF and secretory diarrhea. It will also lead to a better understanding of the pathophysiological events during inflammatory bowel disease and development of colonic carcinoma.

Gene transfer of cGMP-dependent protein kinase I enhances the antihypertrophic effects of nitric oxide in cardiomyocytes

NO acting through soluble guanylyl cyclase and cGMP formation is a negative regulator of cardiomyocyte hypertrophy. Downstream targets mediating the inhibitory effects of NO/cGMP on cardiomyocyte hypertrophy have not been elucidated. In addition to its antihypertrophic effects, NO promotes apoptosis in cardiomyocytes, presumably through cGMP-independent pathways. We investigated the role of cGMP-dependent protein kinase (PKG) in the antihypertrophic and proapoptotic effects of NO. Incubation of neonatal rat cardiomyocytes with the NO donor S-nitroso-N-acetyl-D,L-penicillamine (SNAP) (250 micromol/L) or the PKG-selective cGMP analog 8-pCPT-cGMP (500 micromol/L) activated endogenous PKG type I, as shown by the site-specific phosphorylation of vasodilator-stimulated phosphoprotein, a well-characterized PKG substrate. SNAP (250 micromol/L) and 8-pCPT-cGMP (500 micromol/L) modestly attenuated the hypertrophic response to alpha(1)-adrenergic stimulation with phenylephrine. Although a high concentration of SNAP (1000 micromol/L) promoted apoptosis in cardiomyocytes, as evidenced by the formation of histone-associated DNA fragments, antihypertrophic concentrations of SNAP (250 micromol/L) and 8-pCPT-cGMP (500 micromol/L) did not promote cell death. Because chronic activation downregulated endogenous PKG I, we explored whether gene transfer of PKG I would enhance the sensitivity of cardiomyocytes to the antihypertrophic effects of NO/cGMP. Indeed, after adenoviral overexpression of PKG Ibeta, SNAP (250 micromol/L) and 8-pCPT-cGMP (500 micromol/L) completely suppressed the hypertrophic response to alpha(1)-adrenergic stimulation. As observed in noninfected cells, SNAP (250 micromol/L) and 8-pCPT-cGMP (500 micromol/L) did not promote apoptosis in cardiomyocytes overexpressing PKG Ibeta. Moreover, overexpression of PKG Ibeta did not enhance the proapoptotic effects of 1000 micromol/L SNAP, implying PKG-independent effects of NO on apoptosis. Endogenous PKG I mediates antihypertrophic but not proapoptotic effects of NO in a cell culture model of cardiomyocyte hypertrophy. Adenoviral gene transfer of PKG I selectively enhances the antihypertrophic effects of NO without increasing the susceptibility to apoptosis.

Overexpression of protein kinase G using adenovirus inhibits cyclin E transcription and mesangial cell cycle

The cGMP-cGMP-dependent protein kinase (protein kinase G) system plays an important role in the pathogenesis of mesangial proliferative glomerulonephritis. However, the molecular mechanisms of the inhibitory effects of the cGMP-protein kinase G system in the cell cycle progression of mesangial cells are not well known. To determine the inhibitory pathway of cGMP-protein kinase G in cultured mesangial cells, we investigated the effects of cGMP- and adenovirus-mediated overexpression of protein kinase G on the promoter activities of cyclin E, cyclin D1, and cyclin A. 8-Bromo-cGMP (8-BrcGMP) and overexpression of protein kinase G reduced [(3)H]thymidine uptake, reduced the numbers of cells in S and G(2)/M phases, and decreased the phosphorylation of retinoblastoma (Rb) protein. 8-BrcGMP (10(-3) M), protein kinase G adenovirus (Ad-cGKIbeta; 10(10) plaque-forming units/ml), atrial natriuretic peptide (ANP), and C-type natriuretic peptide (CNP) inhibited the promoter activity of cyclin E to 49, 57, 77, and 78%, respectively. On the other hand, the promoter activities of cyclin D1 and cyclin A were not changed significantly. In Western blot analysis, 8-BrcGMP, Ad-cGKIbeta, ANP, and CNP also inhibited cyclin E protein expression dose and time dependently. The p44/p42 mitogen-activated protein kinase (MAPK) kinase 1-p44/p42 MAPK had no effect on cyclin E promoter activities, and the cGMP-protein kinase G pathway did not change MAPK activity. In conclusion, our findings suggest that the reduction of the cyclin E promoter activity that downregulates G(1)/S transition plays a dominant role in the cGMP- and protein kinase G-induced inhibition of mesangial cell proliferation.

Tyrosine phosphorylation of the human guanylyl cyclase C receptor

Tyrosine phosphorylation events are key components of several cellular signal transduction pathways. This study describes a novel method for identification of substrates for tyrosine kinases. Co-expression of the tyrosine kinase EphB1 with the intracellular domain of guanylyl cyclase C (GCC) in Escherichia coli cells resulted in tyrosine phosphorylation of GCC, indicating that GCC is a potential substrate for tyrosine kinases. Indeed, GCC expressed in mammalian cells is tyrosine phosphorylated, suggesting that tyrosine phosphorylation may play a role in regulation of GCC signalling. This is the first demonstration of tyrosine phosphorylation of any member of the family of membrane-associated guanylyl cyclases.

The role of nitric oxide in the regulation of ion channels in airway epithelium: implications for diseases of the lung

The human respiratory tract is covered with airway surface liquid (ASL) that is essential for lung defense and normal airway function. The quantity and composition of ASL is regulated by active ion transport across the airway epithelium. Abnormal electrolyte transport produces changes in ASL volume and composition, inhibits mucociliary clearance and leads to chronic infection of airway surfaces, as is evident in cystic fibrosis. Agonists that induce intracellular increases in cAMP or Ca2+ are generally associated with electrolyte secretion. While these mechanisms have been studied in detail for many years, modulation of ion channels by nitric oxide (NO) has emerged only recently as a significant determinant of ion channel function. NO is a physiological regulator of transepithelial ion movement and alterations of its generation and action may play an important role in the pathogenesis of lung disorders characterized by hypersecretion of ASL. This review presents the current understanding of regulation of airway epithelial ion channels by NO and attempts to highlight the importance of this regulation for lung defense.

Evidence for the presence of cGMP-dependent protein kinase-II in human distal colon and in T84, the colonic cell line

Heat-stable enterotoxin (STa) stimulates intestinal Cl(-) secretion by activating guanylate cyclase C (GCC) to increase intracellular cyclic GMP (cGMP). In the colon, cGMP action could involve protein kinase (PK) G-II or PKA pathways, depending on the segment and species. In the human colon, both PKG and PKA pathways have been implicated, and, therefore, the present study examined the mechanism of cGMP-mediated Cl(-) transport in primary cultures of human distal colonocytes and in T84, the colonic cell line. Both cell preparations express mRNA for CFTR, Na(+)-K(+)-2Cl(-) cotransporter (NKCC1), GCC and PKG-II as detected by RT-PCR. The effects of STa and the PKG-specific cGMP analogues, 8Br-cGMP and 8pCPT-cGMP, on Cl(-) transport were measured using a halide-sensitive probe. In primary human colonocytes and T84 cells, STa, the cGMP analogues and the cAMP-dependent secretagogue, prostaglandin E(1) (PGE(1)), enhanced Cl(-) transport. The effects of 8Br-cGMP and 8pCPT-cGMP suggested the involvement of PKG, and this was explored further in T84 cells. The effects of 8pCPT-cGMP were dose-dependent and sensitive to the PKG inhibitor, H8 (70 microM), but H8 had no effect on PGE(1)-induced Cl(-) secretion. In contrast, a PKA inhibitor, H7 (50 microM), blocked PGE(1)-mediated but not 8pCPT-cGMP-induced Cl(-) transport. 8pCPT-cGMP enhanced phosphorylation of the PKG-specific substrate, 2A3, by T84 membranes in vitro. This phosphorylation was inhibited by H8. These results strongly suggest that cGMP activates Cl(-) transport through a PKG-II pathway in primary cells and in the T84 cell line of the human colon.

Mechanisms of guanylin action via cyclic GMP in the kidney

Guanylin, uroguanylin, and lymphoguanylin are small peptides that activate cell-surface guanylate cyclase receptors and influence cellular function via intracellular cGMP. Guanylins activate two receptors, GC-C and OK-GC, which are expressed in intestine and/or kidney. Elevation of cGMP in the intestine elicits an increase in electrolyte and water secretion. Activation of renal receptors by uroguanylin stimulates urine flow and excretion of sodium, chloride, and potassium. Intracellular cGMP pathways for guanylins include activation of PKG-II and/or indirect stimulation of PKA-II. The result is activation of CFTR and/or C1C-2 channel proteins to enhance the electrogenic secretion of chloride and bicarbonate. Similar cellular mechanisms may be involved in the renal responses to guanylin peptides. Uroguanylin serves as an intestinal natriuretic hormone in postprandial states, thus linking the digestive and renal organ systems in a novel endocrine axis. Therefore, uroguanylin participates in the complex physiological processes underlying the saliuresis that is elicited by a salty meal.

Regulation of human endothelial cell focal adhesion sites and migration by cGMP-dependent protein kinase I

cGMP-dependent protein kinase type I (cGK I), a major constituent of the atrial natriuretic peptide (ANP)/nitric oxide/cGMP signal transduction pathway, phosphorylates the vasodilator-stimulated phosphoprotein (VASP), a member of the Ena/VASP family of proteins involved in regulation of the actin cytoskeleton. Here we demonstrate that stimulation of human umbilical vein endothelial cells (HUVECs) by both ANP and 8-(4-chlorophenylthio)guanosine 3':5'-monophosphate (8-pCPT-cGMP) activates transfected cGK I and causes detachment of VASP and its known binding partner (zyxin) from focal adhesions in >60% of cells after 30 min. The ANP effects, but not the 8-pCPT-cGMP effects, reversed after 3 h of treatment. In contrast, a catalytically inactive cGK Ibeta mutant (cGK Ibeta-K405A) was incapable of mediating these effects. VASP mutated (Ser/Thr to Ala) at all three of its established phosphorylation sites (vesicular stomatitis virus-tagged VASP-AAA mutant) was not phosphorylated by cGK I and was resistant to detaching from HUVEC focal adhesions in response to 8-pCPT-cGMP. Furthermore, activation of cGK I, but not of mutant cGK Ibeta-K405A, caused a 1.5-2-fold inhibition of HUVEC migration, a dynamic process highly dependent on focal adhesion formation and disassembly. These results indicate that cGK I phosphorylation of VASP results in loss of VASP and zyxin from focal adhesions, a response that could contribute to cGK alteration of cytoskeleton-regulated processes such as cell migration.

Endothelial nitric-oxide synthase (type III) is activated and becomes calcium independent upon phosphorylation by cyclic nucleotide-dependent protein kinases

Endothelial nitric-oxide synthase (NOS-III) is defined as being strictly dependent on Ca(2+)/calmodulin (CaM) for activity, although NO release from endothelial cells has been reported to also occur at intracellular free Ca(2+) levels that are substimulatory for the purified enzyme. We demonstrate here that NOS-III, but neither NOS-I nor -II, is rapidly and strongly activated and phosphorylated on both Ser and Thr in the presence of cGMP-dependent protein kinase II (cGK II) and the catalytic subunit of cAMP-dependent protein kinase (cAK) in vitro. Phosphopeptide analysis by mass spectrometry identified Ser(1177), as well as Ser(633) which is situated in a recently defined CaM autoinhibitory domain within the flavin-binding region of human NOS-III. Phosphoamino acid analysis identified a putative phosphorylation site at Thr(495) in the CaM-binding domain. Importantly, both cAK and cGK phosphorylation of NOS-III in vitro caused a highly reproducible partial (10-20%) NOS-III activation which was independent of Ca(2+)/CaM, and as much as a 4-fold increase in V(max) in the presence of Ca(2+)/CaM. cAK stimulation in intact endothelial cells also increased both Ca(2+/)CaM-independent and -dependent activation of NOS-III. These data collectively provide new evidence for cAK and cGK stimulation of both Ca(2+)/CaM-independent and -dependent NOS-III activity, and suggest possible cross-talk between the NO and prostaglandin I(2) pathways and a positive feedback mechanism for NO/cGMP signaling.

Guanylin peptides: renal actions mediated by cyclic GMP

The guanylin family of cGMP-regulating peptides has three subclasses of peptides containing either three intramolecular disulfides found in bacterial heat-stable enterotoxins (ST), or two disulfides observed in guanylin and uroguanylin, or a single disulfide exemplified by lymphoguanylin. These small, heat-stable peptides bind to and activate cell-surface receptors that have intrinsic guanylate cyclase (GC) activity. Two receptor GC signaling molecules have been identified that are highly expressed in the intestine (GC-C) and/or the kidney (OK-GC) and are selectively activated by the guanylin peptides. Stimulation of cGMP production in renal target cells by guanylin peptides in vivo or ex vivo elicits a long-lived diuresis, natriuresis, and kaliuresis. Activation of GC-C receptors in target cells of intestinal mucosa markedly stimulates the transepithelial secretion of Cl(-) and HCO(-)/(3), causing enhanced secretion of fluid and electrolytes into the intestinal lumen. Bacterial ST peptides act as mimics of guanylin and uroguanylin in the intestine, which provide a cellular mechanism underlying the diarrhea caused by ST-secreting strains of Escherichia coli. Uroguanylin and guanylin may participate in a novel endocrine axis linking the digestive system and kidney as a physiological mechanism that influences Na(+) homeostasis. Guanylin, uroguanylin, and/or lymphoguanylin may also serve within intrarenal signaling pathways controlling cGMP production in renal target cells. Thus we propose that guanylin regulatory peptides participate in a complex multifactorial biological process that evolved to regulate the urinary excretion of NaCl when dietary salt levels exceed the body's physiological requirements. This highly integrated and redundant mechanism allows the organism to maintain sodium balance by eliminating excess NaCl in the urine. Uroguanylin, in particular, may be a prototypical "intestinal natriuretic hormone."

Differential function of the two nucleotide binding domains on cystic fibrosis transmembrane conductance regulator

The genetic disease cystic fibrosis is caused by defects in the chloride channel cystic fibrosis transmembrane conductance regulator (CFTR). CFTR belongs to the family of ABC transporters. In contrast to most other members of this family which transport substrates actively across a membrane, the main function of CFTR is to regulate passive flux of substrates across the plasma membrane. Chloride channel activity of CFTR is dependent on protein phosphorylation and presence of nucleoside triphosphates. From electrophysiological studies of CFTR detailed models of its regulation by phosphorylation and nucleotide interaction have evolved. These investigations provide ample evidence that ATP hydrolysis is crucial for CFTR gating. It becomes apparent that the two nucleotide binding domains on CFTR not only diverge strongly in sequence, but also in function. Based on previous models and taking into account new data from pre-steady-state experiments, a refined model for the action of nucleotides at two nucleotide binding domains was recently proposed.

Expression of coxsackie adenovirus receptor and alphav-integrin does not correlate with adenovector targeting in vivo indicating anatomical vector barriers

Recombinant adenoviral vectors are broadly applied in gene therapy protocols. However, adenovector-mediated gene transfer has limitations in vivo. One of these is the low gene transfer rate into organs other than the liver after systemic intravenous vector injection. Local direct injection into the target organ has been used as one possible solution, but increases necessary equipment and methodology and is traumatic to the target. Wild-type adenovirus infection as well as adenovector-mediated gene transfer depends on virus interaction with the Coxsackie adenovirus receptor (CAR) mediating virus attachment to the cell surface, and on interaction with alphavbeta3 and alphavbeta5 integrins mediating virus entry into the cell. In order to assess the receptor-associated potential of different tissues to act as adenovector targets, we have therefore determined CAR and alphav-integrin expression in multiple organs from different species. In addition, we have newly determined several human, rat, pig and dog CAR-mRNA sequences. Sequence comparison and structural analyses of known and of newly determined sequences suggests a potential adenovirus binding site between amino acids 29 and 128 of the CAR. With respect to the virus receptor expression patterns we found that CAR-mRNA expression was extremely variable between different tissues, with the highest levels in the liver, whereas alphav-integrin expression was far more homogenous among different organs. Both CAR and alphav-integrin showed similar expression patterns among different species. There was no correlation, however, between the adenovector expression patterns after intravenous, intracardiac and aortic root injection, respectively, and the virus receptor patterns. In summary, many organs carry both receptors required to make them potential adenovector targets. In sharp contrast, their actual targeting clearly indicates that adenovirus receptor expression is necessary but not sufficient for vector transfer after systemic injection. The apparently very important role of anatomical barriers, in particular the endothelium, requires close attention when developing non-traumatic, organ-specific gene therapy protocols.

Cyclic nucleotide-dependent protein kinases: intracellular receptors for cAMP and cGMP action

Intracellular cAMP and cGMP levels are increased in response to a variety of hormonal and chemical stimuli; these nucleotides play key roles as second messenger signals in modulating myriad physiological processes. The cAMP-dependent protein kinase and cGMP-dependent protein kinase are major intracellular receptors for these nucleotides, and the actions of these enzymes account for much of the cellular responses to increased levels of cAMP or cGMP. This review summarizes many studies that have contributed significantly to an improved understanding of the catalytic, regulatory, and structural properties of these protein kinases. These accumulated findings provide insights into the mechanisms by which these enzymes produce their specific physiological effects and are helpful in considering the actions of other protein kinases as well.

'Autoreplication' of the vector genome in recombinant adenoviral vectors with different E1 region deletions and transgenes

High transgene stabilities of 1 year and more have been reported in immunodeficient hosts after adenovirus-mediated gene transfer. Transgene persistence of this duration could be due to inherently high stability of the episomal viral vector DNA. An alternative explanation would be limited 'autoreplication' of transgenic vector DNA, just sufficient to counteract slow but continuous degradation within the host cells. Autoreplication could occur in the absence of any production of infectious virus particles, based on residual activity of the adenoviral DNA replication system only. To test this hypothesis, a series of DNA metabolic labeling studies in non-permissive cells cultures transfected with different vectors was conducted. Due to extensive E1 region deletions none of the vectors was able to produce viral progeny in non-permissive cells. Vectors fell into two categories, however, with respect to their autoreplication potential. Neosynthesis of vector DNA in non-permissive vector-transfected cells was readily detectable in 'type A', but not in 'type B' vectors. In addition to their different transgene expression cassettes, vector DNA sequencing showed a less extensive E1 deletion in type A (nucleotides 453-3333 of wild-type virus) as compared to type B vectors (nucleotides 325-3523). Autoreplication was also associated with high transcriptional activity of several viral genes (E1B-14k, adenoviral DNA polymerase, single-strand DNA-binding protein, E4-25k), in contrast to type B vectors. In addition to these 'wild-type' transcripts, 'irregular' recombinant transcripts were detected in autoreplication vectors which contained the transgenic cDNA in conjunction with adenoviral vector sequences. Exogenous or cryptic promotors may (under certain conditions) enhance the transcriptional activity of a vector in such a way that autoreplication occurs. Conditions determining the level of transcriptional enhancement (extent of E1 deletion, type of promoter and transgene, etc) need to be further defined before rational design of adenovectors with high autoreplication capacity becomes possible. In summary, we have shown autoreplication to be a novel feature of certain E1-deleted adenovectors with likely relevance for their stability in vivo, but also with possibly adverse consequences for target cell function or vector immunogenicity. Full characterization of adenoviral vector systems should therefore include a description of their autoreplication capacity.

Guanylin regulatory peptides: structures, biological activities mediated by cyclic GMP and pathobiology

The guanylin family of bioactive peptides consists of three endogenous peptides, including guanylin, uroguanylin and lymphoguanylin, and one exogenous peptide toxin produced by enteric bacteria. These small cysteine-rich peptides activate cell-surface receptors, which have intrinsic guanylate cyclase activity, thus modulating cellular function via the intracellular second messenger, cyclic GMP. Membrane guanylate cyclase-C is an intestinal receptor for guanylin and uroguanylin that is responsible for stimulation of Cl- and HCO3- secretion into the intestinal lumen. Guanylin and uroguanylin are produced within the intestinal mucosa to serve in a paracrine mechanism for regulation of intestinal fluid and electrolyte secretion. Enteric bacteria secrete peptide toxin mimics of uroguanylin and guanylin that activate the intestinal receptors in an uncontrolled fashion to produce secretory diarrhea. Opossum kidney guanylate cyclase is a key receptor in the kidney that may be responsible for the diuretic and natriuretic actions of uroguanylin in vivo. Uroguanylin serves in an endocrine axis linking the intestine and kidney where its natriuretic and diuretic actions contribute to the maintenance of Na+ balance following oral ingestion of NaCl. Lymphoguanylin is highly expressed in the kidney and myocardium where this unique peptide may act locally to regulate cyclic GMP levels in target cells. Lymphoguanylin is also produced in cells of the lymphoid-immune system where other physiological functions may be influenced by intracellular cyclic GMP. Observations of nature are providing insights into cellular mechanisms involving guanylin peptides in intestinal diseases such as colon cancer and diarrhea and in chronic renal diseases or cardiac disorders such as congestive heart failure where guanylin and/or uroguanylin levels in the circulation and/or urine are pathologically elevated. Guanylin peptides are clearly involved in the regulation of salt and water homeostasis, but new findings indicate that these novel peptides have diverse physiological roles in addition to those previously documented for control of intestinal and renal function.

Atrial natriuretic peptide-stimulated Ca2+ reabsorption in rabbit kidney requires membrane-targeted, cGMP-dependent protein kinase type II

Atrial natriuretic peptide (ANP) and nitric oxide (NO) are key regulators of ion and water transport in the kidney. Here, we report that these cGMP-elevating hormones stimulate Ca2+ reabsorption via a novel mechanism specifically involving type II cGMP-dependent protein kinase (cGK II). ANP and the NO donor, sodium nitroprusside (SNP), markedly increased Ca2+ uptake in freshly immunodissected rabbit connecting tubules (CNT) and cortical collecting ducts (CCD). Although readily increasing cGMP, ANP and SNP did not affect Ca2+ and Na+ reabsorption in primary cultures of these segments. Immunoblot analysis demonstrated that cGK II, and not cGK I, was present in freshly isolated CNT and CCD but underwent a complete down-regulation during the primary cell culture. However, upon adenoviral reexpression of cGK II in primary cultures, ANP, SNP, and 8-Br-cGMP readily increased Ca2+ reabsorption. In contrast, no cGMP-dependent effect on electrogenic Na+ transport was observed. The membrane localization of cGK II proved to be crucial for its action, because a nonmyristoylated cGK II mutant that was shown to be localized in the cytosol failed to mediate ANP-stimulated Ca2+ transport. The Ca2+-regulatory function of cGK II appeared isotype-specific because no cGMP-mediated increase in Ca2+ transport was observed after expression of the cytosolic cGK Ibeta or a membrane-bound cGK II/Ibeta chimer. These results demonstrate that ANP- and NO-stimulated Ca2+ reabsorption requires membrane-targeted cGK II.