The guanylyl cyclase receptors

Guanylyl cyclase/natriuretic peptide receptor-A: Identification, molecular characterization, and physiological genomics

The natriuretic peptides (NPs) hormone family, which consists mainly of atrial, brain, and C-type NPs (ANP, BNP, and CNP), play diverse roles in mammalian species, ranging from renal, cardiac, endocrine, neural, and vascular hemodynamics to metabolic regulations, immune responsiveness, and energy distributions. Over the last four decades, new data has transpired regarding the biochemical and molecular compositions, signaling mechanisms, and physiological and pathophysiological functions of NPs and their receptors. NPs are incremented mainly in eliciting natriuretic, diuretic, endocrine, vasodilatory, and neurological activities, along with antiproliferative, antimitogenic, antiinflammatory, and antifibrotic responses. The main locus responsible in the biological and physiological regulatory actions of NPs (ANP and BNP) is the plasma membrane guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), a member of the growing multi-limbed GC family of receptors. Advances in this field have provided tremendous insights into the critical role of Npr1 (encoding GC-A/NPRA) in the reduction of fluid volume and blood pressure homeostasis, protection against renal and cardiac remodeling, and moderation and mediation of neurological disorders. The generation and use of genetically engineered animals, including gene-targeted (gene-knockout and gene-duplication) and transgenic mutant mouse models has revealed and clarified the varied roles and pleiotropic functions of GC-A/NPRA in vivo in intact animals. This review provides a chronological development of the biochemical, molecular, physiological, and pathophysiological functions of GC-A/NPRA, including signaling pathways, genomics, and gene regulation in both normal and disease states.

Safety and improved efficacy signals following gene therapy in childhood blindness caused by GUCY2D mutations

A first-in-human clinical trial of gene therapy in Leber congenital amaurosis due to mutations in the GUCY2D gene is underway, and early results are summarized. A recombinant adeno-associated virus serotype 5 (rAAV5) vector carrying the human GUCY2D gene was delivered by subretinal injection to one eye in three adult patients with severe visual loss, nystagmus, but preserved retinal structure. Safety and efficacy parameters were monitored for 9 months post-operatively. No systemic toxicity was detected; there were no serious adverse events, and ocular adverse events resolved. P1 and P2 showed statistically significant rod photoreceptor vision improvement by full-field stimulus testing in the treated eye. P1 also showed improvement in pupillary responses. Visual acuity remained stable from baseline in P1 and P2. P3, however, showed a gain of 0.3 logMAR in the treated eye, indicating greater cone-photoreceptor function. The results show safety and both rod- and cone-mediated efficacy of this therapy.

Regulation of retinal membrane guanylyl cyclase (RetGC) by negative calcium feedback and RD3 protein

This article presents a brief overview of the main biochemical and cellular processes involved in regulation of cyclic GMP production in photoreceptors. The main focus is on how the fluctuations of free calcium concentrations in photoreceptors between light and dark regulate the activity of retinal membrane guanylyl cyclase (RetGC) via calcium sensor proteins. The emphasis of the review is on the structure of RetGC and guanylyl cyclase activating proteins (GCAPs) in relation to their functional role in photoreceptors and congenital diseases of photoreceptors. In addition to that, the structure and function of retinal degeneration-3 protein (RD3), which regulates RetGC in a calcium-independent manner, is discussed in detail in connections with its role in photoreceptor biology and inherited retinal blindness.

GUCY2D mutations in retinal guanylyl cyclase 1 provide biochemical reasons for dominant cone-rod dystrophy but not for stationary night blindness

Mutations in the GUCY2D gene coding for the dimeric human retinal membrane guanylyl cyclase (RetGC) isozyme RetGC1 cause various forms of blindness, ranging from rod dysfunction to rod and cone degeneration. We tested how the mutations causing recessive congenital stationary night blindness (CSNB), recessive Leber's congenital amaurosis (LCA1), and dominant cone-rod dystrophy-6 (CORD6) affected RetGC1 activity and regulation by RetGC-activating proteins (GCAPs) and retinal degeneration-3 protein (RD3). CSNB mutations R666W, R761W, and L911F, as well as LCA1 mutations R768W and G982VfsX39, disabled RetGC1 activation by human GCAP1, -2, and -3. The R666W and R761W substitutions compromised binding of GCAP1 with RetGC1 in HEK293 cells. In contrast, G982VfsX39 and L911F RetGC1 retained the ability to bind GCAP1 in cyto but failed to effectively bind RD3. R768W RetGC1 did not bind either GCAP1 or RD3. The co-expression of GUCY2D allelic combinations linked to CSNB did not restore RetGC1 activity in vitro The CORD6 mutation R838S in the RetGC1 dimerization domain strongly dominated the Ca2+ sensitivity of cyclase regulation by GCAP1 in RetGC1 heterodimer produced by co-expression of WT and the R838S subunits. It required higher Ca2+ concentrations to decelerate GCAP-activated RetGC1 heterodimer-6-fold higher than WT and 2-fold higher than the Ser838-harboring homodimer. The heterodimer was also more resistant than homodimers to inhibition by RD3. The observed biochemical changes can explain the dominant CORD6 blindness and recessive LCA1 blindness, both of which affect rods and cones, but they cannot explain the selective loss of rod function in recessive CSNB.

Involvement of natriuretic peptide system in C2C12 myocytes

The natriuretic peptide system, a key regulator of cGMP signaling, comprises three types of natriuretic peptides, osteocrin/musclin (OSTN), and their natriuretic peptide receptors. Although this system plays important roles in many organs, its physiological roles in skeletal muscle have not been clearly described. In the present study, we investigated the role of the natriuretic peptide system in C2C12 myocytes. All three natriuretic peptide receptors were expressed by cells differentiating from myoblasts to myotubes, and natriuretic peptide receptor B (NPR-B) transcripts were detected at the highest levels. Further, higher levels of cGMP were generated in response to stimulation with C-type natriuretic peptide (CNP) versus atrial natriuretic peptide (ANP), which reflected receptor expression levels. A cGMP analog downregulated the expression of a few ER stress-related genes. Furthermore, OSTN gene expression was strongly upregulated after 20 days of differentiation. Augmented gene expression was found to correlate closely with endoplasmic reticulum (ER) stress, and C/EBP [CCAAT-enhancer-binding protein] homologous protein (CHOP), which is known to be activated by ER stress, affected the expression of OSTN. Together, these results suggest a role for natriuretic peptide signaling in the ER stress response of myocytes.

The R838S Mutation in Retinal Guanylyl Cyclase 1 (RetGC1) Alters Calcium Sensitivity of cGMP Synthesis in the Retina and Causes Blindness in Transgenic Mice

Substitutions of Arg⁸³⁸ in the dimerization domain of a human retinal membrane guanylyl cyclase 1 (RetGC1) linked to autosomal dominant cone-rod degeneration type 6 (CORD6) change RetGC1 regulation in vitro by Ca²⁺ In addition, we find that R838S substitution makes RetGC1 less sensitive to inhibition by retinal degeneration-3 protein (RD3). We selectively expressed human R838S RetGC1 in mouse rods and documented the decline in rod vision and rod survival. To verify that changes in rods were specifically caused by the CORD6 mutation, we used for comparison cones, which in the same mice did not express R838S RetGC1 from the transgenic construct. The R838S RetGC1 expression in rod outer segments reduced inhibition of cGMP production in the transgenic mouse retinas at the free calcium concentrations typical for dark-adapted rods. The transgenic mice demonstrated early-onset and rapidly progressed with age decline in visual responses from the targeted rods, in contrast to the longer lasting preservation of function in the non-targeted cones. The decline in rod function in the retina resulted from a progressive degeneration of rods between 1 and 6 months of age, with the severity and pace of the degeneration consistent with the extent to which the Ca²⁺ sensitivity of the retinal cGMP production was affected. Our study presents a new experimental model for exploring cellular mechanisms of the CORD6-related photoreceptor death. This mouse model provides the first direct biochemical and physiological in vivo evidence for the Arg⁸³⁸ substitutions in RetGC1 being the culprit behind the pathogenesis of the CORD6 congenital blindness.

Dimerization Domain of Retinal Membrane Guanylyl Cyclase 1 (RetGC1) Is an Essential Part of Guanylyl Cyclase-activating Protein (GCAP) Binding Interface

The photoreceptor-specific proteins guanylyl cyclase-activating proteins (GCAPs) bind and regulate retinal membrane guanylyl cyclase 1 (RetGC1) but not natriuretic peptide receptor A (NPRA). Study of RetGC1 regulation in vitro and its association with fluorescently tagged GCAP in transfected cells showed that R822P substitution in the cyclase dimerization domain causing congenital early onset blindness disrupted RetGC1 ability to bind GCAP but did not eliminate its affinity for another photoreceptor-specific protein, retinal degeneration 3 (RD3). Likewise, the presence of the NPRA dimerization domain in RetGC1/NPRA chimera specifically disabled binding of GCAPs but not of RD3. In subsequent mapping using hybrid dimerization domains in RetGC1/NPRA chimera, multiple RetGC1-specific residues contributed to GCAP binding by the cyclase, but the region around Met(823) was the most crucial. Either positively or negatively charged residues in that position completely blocked GCAP1 and GCAP2 but not RD3 binding similarly to the disease-causing mutation in the neighboring Arg(822). The specificity of GCAP binding imparted by RetGC1 dimerization domain was not directly related to promoting dimerization of the cyclase. The probability of coiled coil dimer formation computed for RetGC1/NPRA chimeras, even those incapable of binding GCAP, remained high, and functional complementation tests showed that the RetGC1 active site, which requires dimerization of the cyclase, was formed even when Met(823) or Arg(822) was mutated. These results directly demonstrate that the interface for GCAP binding on RetGC1 requires not only the kinase homology region but also directly involves the dimerization domain and especially its portion containing Arg(822) and Met(823).

Evaluating the role of retinal membrane guanylyl cyclase 1 (RetGC1) domains in binding guanylyl cyclase-activating proteins (GCAPs)

Retinal membrane guanylyl cyclase 1 (RetGC1) regulated by guanylyl cyclase-activating proteins (GCAPs) controls photoreceptor recovery and when mutated causes blinding disorders. We evaluated the principal models of how GCAP1 and GCAP2 bind RetGC1: through a shared docking interface versus independent binding sites formed by distant portions of the cyclase intracellular domain. At near-saturating concentrations, GCAP1 and GCAP2 activated RetGC1 from HEK293 cells and RetGC2(-/-)GCAPs1,2(-/-) mouse retinas in a non-additive fashion. The M26R GCAP1, which binds but does not activate RetGC1, suppressed activation of recombinant and native RetGC1 by competing with both GCAP1 and GCAP2. Untagged GCAP1 displaced both GCAP1-GFP and GCAP2-GFP from the complex with RetGC1 in HEK293 cells. The intracellular segment of a natriuretic peptide receptor A guanylyl cyclase failed to bind GCAPs, but replacing its kinase homology and dimerization domains with those from RetGC1 restored GCAP1 and GCAP2 binding by the hybrid cyclase and its GCAP-dependent regulation. Deletion of the Tyr(1016)-Ser(1103) fragment in RetGC1 did not block GCAP2 binding to the cyclase. In contrast, substitutions in the kinase homology domain, W708R and I734T, linked to Leber congenital amaurosis prevented binding of both GCAP1-GFP and GCAP2-GFP. Our results demonstrate that GCAPs cannot regulate RetGC1 using independent primary binding sites. Instead, GCAP1 and GCAP2 bind with the cyclase molecule in a mutually exclusive manner using a common or overlapping binding site(s) in the Arg(488)-Arg(851) portion of RetGC1, and mutations in that region causing Leber congenital amaurosis blindness disrupt activation of the cyclase by both GCAP1 and GCAP2.

Determining consequences of retinal membrane guanylyl cyclase (RetGC1) deficiency in human Leber congenital amaurosis en route to therapy: residual cone-photoreceptor vision correlates with biochemical properties of the mutants

The GUCY2D gene encodes retinal membrane guanylyl cyclase (RetGC1), a key component of the phototransduction machinery in photoreceptors. Mutations in GUCY2D cause Leber congenital amaurosis type 1 (LCA1), an autosomal recessive human retinal blinding disease. The effects of RetGC1 deficiency on human rod and cone photoreceptor structure and function are currently unknown. To move LCA1 closer to clinical trials, we characterized a cohort of patients (ages 6 months-37 years) with GUCY2D mutations. In vivo analyses of retinal architecture indicated intact rod photoreceptors in all patients but abnormalities in foveal cones. By functional phenotype, there were patients with and those without detectable cone vision. Rod vision could be retained and did not correlate with the extent of cone vision or age. In patients without cone vision, rod vision functioned unsaturated under bright ambient illumination. In vitro analyses of the mutant alleles showed that in addition to the major truncation of the essential catalytic domain in RetGC1, some missense mutations in LCA1 patients result in a severe loss of function by inactivating its catalytic activity and/or ability to interact with the activator proteins, GCAPs. The differences in rod sensitivities among patients were not explained by the biochemical properties of the mutants. However, the RetGC1 mutant alleles with remaining biochemical activity in vitro were associated with retained cone vision in vivo. We postulate a relationship between the level of RetGC1 activity and the degree of cone vision abnormality, and argue for cone function being the efficacy outcome in clinical trials of gene augmentation therapy in LCA1.

Enzymatic properties and regulation of the native isozymes of retinal membrane guanylyl cyclase (RetGC) from mouse photoreceptors

Mouse photoreceptor function and survival critically depend on Ca(2+)-regulated retinal membrane guanylyl cyclase (RetGC), comprised of two isozymes, RetGC1 and RetGC2. We characterized the content, catalytic constants, and regulation of native RetGC1 and RetGC2 isozymes using mice lacking guanylyl cyclase activating proteins GCAP1 and GCAP2 and deficient for either GUCY2F or GUCY2E genes, respectively. We found that the characteristics of both native RetGC isozymes were considerably different from other reported estimates made for mammalian RetGCs: the content of RetGC1 per mouse rod outer segments (ROS) was at least 3-fold lower, the molar ratio (RetGC2:RetGC1) 6-fold higher, and the catalytic constants of both GCAP-activated isozymes between 12- and 19-fold higher than previously measured in bovine ROS. The native RetGC isozymes had different basal activity and were accelerated 5-28-fold at physiological concentrations of GCAPs. RetGC2 alone was capable of contributing as much as 135-165 μM cGMP s(-1) or almost 23-28% to the maximal cGMP synthesis rate in mouse ROS. At the maximal level of activation by GCAP, this isozyme alone could provide a significantly high rate of cGMP synthesis compared to what is expected for normal recovery of a mouse rod, and this can help explain some of the unresolved paradoxes of rod physiology. GCAP-activated native RetGC1 and RetGC2 were less sensitive to inhibition by Ca(2+) in the presence of GCAP1 (EC(50Ca) ∼132-139 nM) than GCAP2 (EC(50Ca) ∼50-59 nM), thus arguing that Ca(2+) sensor properties of GCAP in a functional RetGC/GCAP complex are defined not by a particular target isozyme but the intrinsic properties of GCAPs themselves.

Natriuretic peptides in brain physiology

Natriuretic peptides (NPs) regulate salt and water homeostasis by inducing natriuresis and diuresis in the kidney. These actions in addition to those via the heart and vascular system play important roles in the regulation of blood pressure. In the central nervous system NPs play a significant role in neuronal development, synaptic transmission and neuroprotection. Currently, six different human NPs have been described: atrial natriuretic peptide (ANP), urodilatin (URO, renal natriuretic peptide), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP) as well as guanylin and uroguanylin. ANP, URO and BNP activate the natriuretic peptide receptor A (NPR-A or guanylate cyclase A (GC-A)) while CNP activates natriuretic peptide receptor B (NPR-B or guanylate cyclase B (GC-B)). Guanylin and uroguanylin are known to activate guanylate cyclase C (GC-C). The receptors GC-A, GC-B, and GC-C are widely expressed in the human body. Currently, GC-B and CNP seems to have the highest expression in central nervous system compared to other NPs and their receptors. All known NPs generate intracellular cyclic GMP (cGMP) by activating their specific guanylate cyclase receptors. Subsequently, cGMP is able to activate protein kinase I or II (PKG I or II) and/or directly regulate transmembrane proteins such as ion channels, transporters and pumps. NPs also bind to the natriuretic peptide receptor C (also called clearance receptor NPR-C) which is a major pathway for the degradation of NPs and has no guanylate cyclase activity. In this review we will focus on new insights regarding the physiological effects of NPs in the brain, especially specific areas of their signaling pathways in neurons and glial cells.

Mg2+/Ca2+ cation binding cycle of guanylyl cyclase activating proteins (GCAPs): role in regulation of photoreceptor guanylyl cyclase

Photon absorption by photoreceptors activates hydrolysis of cGMP, which shuts down cGMP-gated channels and decreases free Ca(2+) concentrations in outer segment. Suppression of Ca(2+) influx through the cGMP channel by light activates retinal guanylyl cyclase through guanylyl cyclase activating proteins (GCAPs) and thus expedites photoreceptors recovery from excitation and restores their light sensitivity. GCAP1 and GCAP2, two ubiquitous among vertebrate species isoforms of GCAPs that activate retGC during rod response to light, are myristoylated Ca(2+)/Mg(2+)-binding proteins of the EF-hand superfamily. They consist of one non-metal binding EF-hand-like domain and three other EF-hands, each capable of binding Ca(2+) and Mg(2+). In the metal binding EF-hands of GCAP1, different point mutations can selectively block binding of Ca(2+) or both Ca(2+) and Mg(2+) altogether. Activation of retGC at low Ca(2+) (light adaptation) or its inhibition at high Ca(2+) (dark adaptation) follows a cycle of Ca(2+)/Mg(2+) exchange in GCAPs, rather than release of Ca(2+) and its binding by apo-GCAPs. The Mg(2+) binding in two of the EF-hands controls docking of GCAP1 with retGC1 in the conditions of light adaptation and is essential for activation of retGC. Mg(2+) binding in a C-terminal EF-hand contributes to neither retGC1 docking with the cyclase nor its subsequent activation in the light, but is specifically required for switching the cyclase off in the conditions of dark adaptation by binding Ca(2+). The Mg(2+)/Ca(2+) exchange in GCAP1 and 2 operates within different range of intracellular Ca(2+) concentrations and provides a two-step activation of the cyclase during rod recovery.

A role for GCAP2 in regulating the photoresponse. Guanylyl cyclase activation and rod electrophysiology in GUCA1B knock-out mice

Cyclic GMP serves as the second messenger in visual transduction, linking photon absorption by rhodopsin to the activity of ion channels. Synthesis of cGMP in photoreceptors is supported by a pair of retina-specific guanylyl cyclases, retGC1 and -2. Two neuronal calcium sensors, GCAP1 and GCAP2, confer Ca(2+) sensitivity to guanylyl cyclase activity, but the importance and the contribution of each GCAP is controversial. To explore this issue, the gene GUCA1B, coding for GCAP2, was disrupted in mice, and the capacity for knock-out rods to regulate retGC and generate photoresponses was tested. The knock-out did not compromise rod viability or alter outer segment ultrastructure. Levels of retGC1, retGC2, and GCAP-1 expression did not undergo compensatory changes, but the absence of GCAP2 affected guanylyl cyclase activity in two ways; (a) the maximal rate of cGMP synthesis at low [Ca(2+)] dropped 2-fold and (b) the half-maximal rate of cGMP synthesis was attained at a higher than normal [Ca(2+)]. The addition of an antibody raised against mouse GCAP2 produced similar effects on the guanylyl cyclase activity in wild type retinas. Flash responses of GCAP2 knock-out rods recovered more slowly than normal. Knock-out rods became more sensitive to flashes and to steps of illumination but tended to saturate at lower intensities, as compared with wild type rods. Therefore, GCAP2 regulation of guanylyl cyclase activity quickens the recovery of flash and step responses and adjusts the operating range of rods to higher intensities of ambient illumination.

Inhibition of cGMP-dependent protein kinase reduces the response to sonic hedgehog in neuralized embryoid bodies

Previously, we have shown that increasing the intracellular cGMP concentration enhances the sonic hedgehog (Shh) response in neural plate cells. The use of two mouse embryonic stem (ES) cell lines allowed a highly sensitive and reproducible quantification of the Shh response in neuralized embryoid bodies. Here we demonstrate that the specific, membrane-permeable cGMP-dependent protein kinase G-Ialpha (PKG-Ialpha) inhibitor DT-2 prevents an efficient Shh response, indicating that the effects of cGMP on the Shh response are mediated via PKG. We also demonstrate that the PKG acts upon the Shh response upstream of the Ptc1 promoter, which is up-regulated invariably and early in response to Shh, significantly limiting the targets for PKG phosphorylation to molecules involved in the early steps of the Shh response. These effects of cGMP and PKG are antagonistic to those of cAMP and PKA, and thus provide a mechanism by which the sensitivity of cells to the effects of Shh can be regulated, by modulating the intracellular cyclic nucleotide concentration.

Ca2+ and Mg2+ Binding Properties of GCAP-1

Guanylyl cyclase-activating protein 1 (GCAP-1) is an EF-hand protein that activates retinal guanylyl cyclase (RetGC) in photoreceptors at low free Ca2+ in the light and inhibits it in the dark when Ca2+ concentrations rise. We present the first direct evidence that Mg2+-bound form of GCAP-1, not its cation-free form, is the true activator of RetGC-1 under physiological conditions. Of four EF-hand structures in GCAP-1, three bound Ca2+ ions and could exchange Ca2+ for Mg2+. At concentrations of free Ca2+ and Mg2+ typical for the light-adapted photoreceptors, all three metal-binding EF-hands were predominantly occupied by Mg2, and the presence of bound Mg2+ in GCAP-1 was essential for its ability to stimulate RetGC-1. In the Mg2+-bound form of GCAP-1 all three Trp residues became more exposed to the polar environment compared with its apo form. The replacement of Mg2+ by Ca2+ in the EF-hands 2 and 3 further exposed Trp-21 to the solution in a non-metal-binding EF-hand domain 1 that interacts with RetGC. Contrary to that, replacement of Mg2+ by Ca2+ in the EF-hand 4 moved Trp-94 in the entering alpha-helix of the EF-hand 3 back to the non-polar environment. Our results demonstrate that Mg2+ regulates GCAP-1 not only by adjusting its Ca2+ sensitivity to the physiological conditions in photoreceptors but also by creating the conformation required for RetGC stimulation.

Cellular effects of guanylin and uroguanylin

Ingestion of a salty meal induces secretion of guanylin (GN) and uroguanylin (UGN) into the intestinal lumen, where they inhibit Na+ absorption and induce Cl-, HCO3-, and water secretion. Simultaneously, these hormones stimulate renal electrolyte excretion by inducing natriuresis, kaliuresis, and diuresis. GN and UGN therefore participate in the prevention of hypernatremia and hypervolemia after salty meals. The signaling pathway of GN and UGN in the intestine is well known. They activate enterocytes via guanylate cyclase C (GC-C), which leads to cGMP-dependent inhibition of Na+/H+ exchange and activation of the cystic fibrosis transmembrane regulator. In GC-C-deficient mice, GN and UGN still produce renal natriuresis, kaliuresis, and diuresis, suggesting different signaling pathways in the kidney compared with the intestine. Signaling pathways for GN and UGN in the kidney differ along the various nephron segments. In proximal tubule cells, a cGMP- and GC-C-dependent signaling was demonstrated for both peptides. In addition, UGN activates a pertussis toxin-sensitive G-protein-coupled receptor. A similar dual signaling pathway is also known for atrial natriuretic peptide. Recently, a cGMP-independent signaling pathway for GN and UGN was also shown in principal cells of the human and mouse cortical collecting duct. Because GN and UGN activate different signaling pathways in specific organs and even within the kidney, this review focuses on more recent findings on cellular effects and signaling mechanisms of these peptides and their pathophysiologic implications in the intestine and the kidney.

Nitric oxide sensitivity in pulmonary artery and airway smooth muscle: a possible role for cGMP responsiveness

We aimed to assess intrinsic smooth muscle mechanisms contributing to greater nitric oxide (NO) responsiveness in pulmonary vascular vs. airway smooth muscle. Porcine pulmonary artery smooth muscle (PASM) and tracheal smooth muscle (TSM) strips were used in concentration-response studies to the NO donor (Z)-1-[N-2-aminoethyl-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (DETA-NO). PASM consistently exhibited greater relaxation at a given DETA-NO concentration (NO responsiveness) than TSM NO responsiveness, with DETA-NO log EC(50) being -6.55 +/- 0.11 and -5.37 +/- 0.13 for PASM and TSM, respectively (P < 0.01). We determined relationships between tissue cGMP concentration ([cGMP](i)) and relaxation using the particulate guanylyl cyclase agonist atrial natriuretic peptide. Atrial natriuretic peptide resulted in nearly complete relaxation, with no detectable increase in [cGMP](i) in PASM and only 20% relaxation (10-fold increase in [cGMP](i)) in TSM, indicating that TSM is less cGMP responsive than PASM. Total cGMP-dependent protein kinase I (cGKI) mRNA expression was greater in PASM than in TSM (2.23 +/- 0.36 vs. 0.93 +/- 0.31 amol mRNA/mug total RNA, respectively; P < 0.01), but total cGKI protein expression was not significantly different (0.56 +/- 0.07 and 0.49 +/- 0.04 ng cGKI/mug protein, respectively). The phosphotransferase assay for the soluble fraction of tissue homogenates demonstrated no difference in the cGMP EC(50) between PASM and TSM. The maximal phosphotransferase activity indexed to the amount of total cGKI in the homogenate differed significantly between PASM and TSM (1.61 +/- 0.15 and 1.04 +/- pmol.min(-1).ng cGKI(-1), respectively; P < 0.05), suggesting that cGKI may be regulated differently in the two tissues. A novel intrinsic smooth muscle mechanism accounting for greater NO responsiveness in PASM vs. TSM is thus greater cGMP responsiveness from increased cGKI-specific activity in PASM.

Drosophila soluble guanylyl cyclase mutants exhibit increased foraging locomotion: behavioral and genomic investigations

Genetic variation in the gene foraging (for) is associated with a natural behavioral dimorphism in the fruit fly, Drosophila melanogaster. Some larvae, called 'rovers', have increased foraging locomotion compared to others, called 'sitters', and this difference is directly related to for-encoded cGMP-dependent protein kinase (PKG) activity. Here we report that larvae with mutations in the gene dgcalpha1, which encodes a soluble guanylyl cyclase (sGC) subunit, have increases in both PKG activity and foraging locomotion. This is contrary to our original prediction that, based on the role of sGC in the synthesis of cGMP, dgcalpha1 mutant larvae would have deficient cGMP production leading to decreased PKG activation and thus reduced larval foraging locomotion. We performed DNA microarray analyses to compare transcriptional changes induced by a dgcalpha1 mutation in both rover and sitter wildtype genetic backgrounds. In either background, we identified many genes that are differentially transcribed, and interestingly, relatively few are affected in both backgrounds. Furthermore, several of these commonly affected genes are enhanced or suppressed in a background-dependent manner. Thus, genetic background has a critical influence on the molecular effects of this mutation. These findings will support future investigations of Drosophila foraging behavior.

Mechanisms of NO release by N1-nitrosomelatonin: nucleophilic attack versus reducing pathways

A new type of physiologically relevant nitrosamines have been recently recognized, the N(1)-nitrosoindoles. The possible pathways by which N(1)-nitrosomelatonin (NOMel) can react in physiological environments have been studied. Our results show that NOMel slowly decomposes spontaneously in aqueous solution, generating melatonin as the main organic product (k = (3.7 +/- 1.1) x 10(-5) s(-1), Tris-HCl (0.2 M) buffer, pH 7.4 at 37 degrees C, anaerobic). This rate is accelerated by acidification (k(pH 5.8) = (4.5 +/- 0.7) x 10(-4) s(-1), k(pH 8.8) = (3.9 +/- 0.6) x 10(-6) s(-1), Tris-HCl (0.2 M) buffer at 37 degrees C), by the presence of O(2) (k(o) = (9.8 +/- 0.1) x 10(-5) s(-1), pH 7.4, 37 degrees C, [NOMel] = 0.1 mM, P(O(2)) = 1 atm), and by the presence of the spin trap TEMPO (2,2,6,6-tetramethylpiperidine 1-oxyl; k(o) = (2.0 +/- 0.1) x 10(-4) s(-1), pH 7.4, 37 degrees C, [NOMel] = 0.1 mM, [TEMPO] = 9 mM). We also found that NOMel can transnitrosate to l-cysteinate, producing S-nitrosocysteine and melatonin (k = 0.127 +/- 0.002 M(-1) s(-1), Tris-HCl (0.2 M) buffer, pH 7.4 at 37 degrees C). The reaction of NOMel with ascorbic acid as a reducing agent has also been studied. This rapid reaction produces nitric oxide and melatonin. The saturation of the observed rate constant (k = (1.08 +/- 0.04) x 10(-3) s(-1), Tris-HCl (0.2 M) buffer, pH 7.4 at 37 degrees C) at high ascorbic acid concentration (100-fold with respect to NOMel) and the pH independence of this reaction in the pH range 7-9 indicate that the reactive species are ascorbate and melatonyl radical originated from the reversible homolysis of NOMel. Taking into account kinetic and DFT calculation data, a comprehensive mechanism for the denitrosation of NOMel is proposed. On the basis of our kinetics results, we conclude that under physiological conditions NOMel mainly reacts with endogenous reducing agents (such as ascorbic acid), producing nitric oxide and melatonin.

Biology of natriuretic peptides and their receptors

Increasing evidence suggests that natriuretic peptides (NPs) play diverse roles in mammals, including renal hemodynamics, neuroendocrine, and cardiovascular functions. Collectively, NPs are classified as hypotensive hormones; the main actions of NPs are implicated in eliciting natriuretic, diuretic, steroidogenic, antiproliferative, and vasorelaxant effects, important factors in the control of body fluid volume and blood pressure homeostasis. One of the principal loci involved in the regulatory actions of NPs is their cognate plasma membrane receptor molecules, which are activated by binding with specific NPs. Interaction of NPs with their receptors plays a central role in physiology and pathophysiology of hypertension and cardiovascular disorders. Gaining insight into the intricacies of NPs-specific receptor signaling pathways is of pivotal importance for understanding both hormone-receptor biology and the disease states arising from abnormal hormone receptor interplay. During the last decade there has been a surge in interest in NP receptors; consequently, a wealth of information has emerged concerning molecular structure and function, signaling mechanisms, and use of transgenics and gene-targeted mouse models. The objective of this present review is to summarize and document the previous findings and recent discoveries in the field of the natriuretic peptide hormone family and receptor systems with emphasis on the structure-function relationship, signaling mechanisms, and the physiological and pathophysiological significance in health and disease.

ATP-independent activation of natriuretic peptide receptors

Natriuretic peptide receptor A (NPR-A) is an essential cardiovascular regulator that is stimulated by atrial natriuretic peptide and B-type natriuretic peptide, whereas natriuretic peptide receptor B (NPR-B) stimulates long bone growth in a C-type natriuretic peptide-dependent manner. Many reports indicate that ATP is essential for NPR-A and NPR-B activation. Current models suggest that natriuretic peptide binding to receptor extracellular domains causes ATP binding to intracellular kinase homology domains, which derepresses adjacent catalytic domains. Here, we report 100-fold activations of natriuretic peptide receptors in the absence of ATP. The addition of a nonhydrolyzable ATP analog had no effect at early time periods (measured in seconds) but increased cGMP production about 2-fold after longer incubations (measured in minutes), consistent with a stabilization, not activation, mechanism. These data indicate that ATP does not activate natriuretic peptide receptors as has been repeatedly reported. Instead, ATP increases activity primarily by maintaining proper receptor phosphorylation status but also serves a previously unappreciated enzyme stabilizing function.

The Y99C mutation in guanylyl cyclase-activating protein 1 increases intracellular Ca2+ and causes photoreceptor degeneration in transgenic mice

Guanylyl cyclase-activating proteins (GCAPs) are Ca2+-binding proteins that activate guanylyl cyclase when free Ca2+ concentrations in retinal rods and cones fall after illumination and inhibit the cyclase when free Ca2+ reaches its resting level in the dark. Several forms of retinal dystrophy are caused by mutations in GUCA1A, the gene coding for GCAP1. To investigate the cellular mechanisms affected by the diseased state, we created transgenic mice that express GCAP1 with a Tyr99Cys substitution (Y99C GCAP1) found in human patients with a late-onset retinal dystrophy (Payne et al., 1998). Y99C GCAP1 shifted the Ca2+ sensitivity of the guanylyl cyclase in photoreceptors, keeping it partially active at 250 nM free Ca2+, the normal resting Ca2+ concentration in darkness. The enhanced activity of the cyclase in the dark increased cyclic nucleotide-gated channel activity and elevated the rod outer segment Ca2+ concentration in darkness, measured by using fluo-5F and laser spot microscopy. In different lines of transgenic mice the magnitude of this effect rose with the Y99C GCAP1 expression. Surprisingly, there was little change in the rod photoresponse, indicating that dynamic Ca2+-dependent regulation of cGMP synthesis was preserved. However, the photoreceptors in these mice degenerated, and the rate of the cell loss increased with the level of the transgene expression, unlike in transgenic mice that overexpressed normal GCAP1. These results provide the first direct evidence that a mutation linked to congenital blindness increases Ca2+ in the outer segment, which may trigger the apoptotic process.

Molecular cloning of a novel variant of the rat soluble guanylate cyclase beta2 subunit

Soluble guanylate cyclases (sGCs) are heterodimeric enzymes consisting of alpha and beta subunits and catalyze the formation of cGMP from GTP. The beta1 subunit has been characterized in detail, whereas the function and physiological role of the beta2 subunit are poorly understood. In this study, I isolated two distinct cDNA fragments for the beta2 subunit of sGC (beta2a and beta2b) from a rat brain cDNA library by 3' rapid amplification of cDNA ends using degenerate sense primers based on amino acid sequences conserved among membrane-bound guanylate cyclases. The deduced amino acid sequence of beta2a is identical with the corresponding sequence of the previously described beta2 subunit, whereas that of beta2b is C-terminally shorter by 46 amino acids and thus does not contain a consensus sequence for isoprenylation/carboxymethylation. Reverse transcription-polymerase chain reaction (RT-PCR) analysis demonstrated that both variants are expressed in various tissues, including kidney, liver, and brain. Although the functional significance of the C-terminal region containing the consensus sequence for isoprenylation/carboxymethylation of beta2a remains unclear yet, it is likely that these beta2 subunits play some physiological or pathophysiological role in various tissues.

Forced Homodimerization by Site-Directed Mutagenesis Alters Guanylyl Cyclase Activity of Natriuretic Peptide Receptor B

Natriuretic peptides mediate their physiologic effects through activation of membrane-bound, guanylyl cyclase–coupled receptors (NPRs). Receptor dimerization is an important feature of signal transduction. This study was aimed at characterizing structurally important residues of the extracellular ligand-binding domain of NPR-B for receptor dimerization and cGMP generation. Deletion mutagenesis was used to replace cysteine residues at positions 53 (C53S), 417 (C417S), and 426 (C426S) by serine. Receptor expression, dimerization, whole-cell cGMP response, and guanylyl cyclase activity of membrane fractions were determined in stably transfected COS-7 cells. C53S, C417S, and C426S mutants were expressed and found to form disulfide-bridged covalent dimers. In contrast to NPR-B and C53S, C417S and C426S mutants displayed constitutive activity in whole cells (C417S, 146±12%, P <0.01; C426S, 153±7% of ligand-independent NPR-B cGMP generation, P <0.01). The cGMP response of C417S and C426S mutants in whole cells was dose dependent and ≈4 times lower than that in NPR-B, whereas it was blunted in C53S-transfected cells (1 μmol/L CNP, NPR-B 2868±436%; C53S, 206±16% of control, P <0.001 vs NPR-B, C417S, and C426S). Guanylyl cyclase assay in transfected cells confirmed the constitutive activity of C417S and C426S mutants. These data suggest that receptor dimerization by covalent disulfide bridges alters ligand-independent as well as ligand-dependent receptor activity. Localization of the crosslink in relation to the cell membrane is important for configuration of the extracellular domain and the consecutive signal transduction.

Increased effects of C-type natriuretic peptide on contractility and calcium regulation in murine hearts overexpressing cyclic GMP-dependent protein kinase I

1. C-type natriuretic peptide (CNP) and its receptor guanylyl cyclase (GC-B) are expressed in the heart and modulate cardiac contractility in a cGMP-dependent manner. Since the distal cellular signalling pathways remain unclear, we evaluated the peptide effects on cardiac function and calcium regulation in wild-type (WT) and transgenic mice with cardiac overexpression of cGMP-dependent protein kinase I (PKG ITG). 2. In isolated, perfused working WT hearts, CNP (10 nm) provoked an immediate increase in the maximal rates of contraction and relaxation, a small increase in the left ventricular systolic pressure and a decrease in the time of relaxation. These changes in cardiac function were accompanied by a marked increase in the levels of Ser16-phosphorylated phospholamban (PLB). 3. In PKG ITG hearts, the effects of CNP on cardiac contractility and relaxation as well as on PLB phosphorylation were markedly enhanced. 4. CNP increased cell shortening and systolic Cai2+ levels, and accelerated Cai2+ decay in isolated, Indo-1/AM-loaded WT cardiomyocytes, and these effects were enhanced in PKG I-overexpressing cardiomyocytes. 5. 8-pCPT-cGMP, a membrane-permeable PKG activator, mimicked the contractile and molecular actions of CNP, the effects again being more pronounced in PKG ITG hearts. In contrast, the cardiac responses to beta-adrenergic stimulation were not different between genotypes. 6. Taken together, our data indicate that PKG I is a downstream target activated by the CNP/GC-B/cGMP-signalling pathway in cardiac myocytes. cGMP/PKG I-stimulated phosphorylation of PLB and subsequent activation of the sarcoplasmic reticulum Ca2+ pump appear to mediate the positive inotropic and lusitropic responses to CNP.

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.

Interaction of guanylyl cyclase C with SH3 domain of Src tyrosine kinase. Yet another mechanism for desensitization

Protein-protein interactions mediated by the Src homology 3 (SH3) domain have been implicated in the regulation of receptor functions for subcellular localization of proteins and the reorganization of cytoskeleton. The experiments described in this article begin to identify the interaction of the SH3 domain of Src tyrosine kinase with the guanylyl cyclase C receptor after activation with Escherichia coli heat-stable enterotoxin (ST). Only one of two post-translationally modified forms of guanylyl cyclase C from T84 colonic carcinoma cells bind to GST-SH3 fusion protein of Src and Hck tyrosine kinases. Interestingly, the GST-Src-SH3 fusion protein showed 2-fold more affinity to native guanylyl cyclase C in solution than the GST-Hck-SH3 fusion protein. The affinity of the GST-Src-SH3 fusion protein to guanylyl cyclase C increased on desensitization of receptor in vivo. An in vitro cyclase assay in the presence of GST-Src-SH3 fusion protein indicated inhibition of the catalytic activity of guanylyl cyclase C. The catalytic domain recombinant protein (GST-GCD) of guanylyl cyclase C could pull-down a 60-kDa protein that reacted with Src tyrosine antibody and also showed autophosphorylation. These data suggest that SH3 domain-mediated protein-protein interaction with the catalytic domain of guanylyl cyclase C inhibited the cyclase activity and that such an interaction, possibly mediated by Src tyrosine kinase or additional proteins, might be pivotal for the desensitization phenomenon of the guanylyl cyclase C receptor.

Natriuretic peptide receptor A activation stabilizes a membrane-distal dimer interface

We have shown previously (Rondeau, J.-J., McNicoll, N., Gagnon, J., Bouchard, N., Ong, H., and De Léan, A. (1995) Biochemistry 34, 2130-2136) that atrial natriuretic peptide (ANP) stabilizes a dimeric form of the natriuretic peptide receptor A (NPRA) by simultaneously interacting with both receptor subunits. However, the first crystallographic study of unliganded NPRA extracellular domain documented a V-shaped dimer involving a membrane-proximal dimer interface and separate binding sites for ANP on each monomer. We explored the possibility of an alternative A-shaped dimer involving a membrane-distal dimer interface by substituting an unpaired solvent-exposed cysteine for Trp(74) in the amino-terminal lobe of full-length NPRA. The predicted spacing between Trp(74) from both subunits drastically differs, depending on whether the V-shaped (84 A) or the A-shaped (8 A) dimer model is considered. In contrast with the expected results for the reported V-shaped dimer, the NPRA(W74C) mutant was constitutively covalently dimeric. Also, the subunits spontaneously reassociated following transient disulfide reduction by dithiothreitol and reoxidation. However, ANP could neither bind to nor activate NPRA(W74C). Permanent disulfide opening by reduction with dithiothreitol and alkylation with N-ethylmaleimide rescued ANP binding to NPRA(W74C). The NPRA mutant could be maintained as a covalent dimer while preserving its function by crosslinking with the bifunctional alkylating agent phenylenedimaleimides (PDM), the ortho-substituted oPDM being more efficient than mPDM or pPDM. These results indicate that the membrane-distal lobe of the NPRAM extracellular domains are dynamically interfacing in the unliganded state and that ANP binding stabilizes the receptor dimer with more stringent spacing at the dimer interface.

Smooth muscle-selective deletion of guanylyl cyclase-A prevents the acute but not chronic effects of ANP on blood pressure

Atrial natriuretic peptide (ANP) is an important regulator of arterial blood pressure. The mechanisms mediating its hypotensive effects are complex and involve the inhibition of the sympathetic and renin-angiotensin-aldosterone (RAA) systems, increased diuresis/natriuresis, vasodilation, and enhanced vascular permeability. In particular, the contribution of the direct vasodilating effect of ANP to the hypotensive actions remains controversial, because variable levels of the ANP receptor, guanylyl cyclase A (GC-A), are expressed in different vascular beds. The objective of our study was to determine whether a selective deletion of GC-A in vascular smooth muscle would affect the hypotensive actions of ANP. We first created a mutant allele of mouse GC-A by flanking a required exon with loxP sequences. Crossing floxed GC-A with SM22-Cre transgene mice expressing Cre recombinase in smooth muscle cells (SMC) resulted in mice in which vascular GC-A mRNA expression was reduced by approximately 80%. Accordingly, the relaxing effects of ANP on isolated vessels from these mice were abolished; despite this fact, chronic arterial blood pressure of awake SMC GC-A KO mice was normal. Infusion of ANP caused immediate decreases in blood pressure in floxed GC-A but not in SMC GC-A knockout mice. Furthermore, acute vascular volume expansion, which causes release of cardiac ANP, did not affect resting blood pressure of floxed GC-A mice, but rapidly and significantly increased blood pressure of SMC GC-A knockout mice. We conclude that vascular GC-A is dispensable in the chronic and critical in the acute moderation of arterial blood pressure by ANP.

Soluble fusion proteins between single transmembrane photoreceptor guanylyl cyclases and their activators

Among single-spanning transmembrane receptors (sTMRs), two guanylyl cyclase receptors, GC1 and GC2, are critically important during phototransduction in vertebrate retinal photoreceptor cells. Ca(2+)-free forms of guanylyl cyclase-activating proteins (GCAPs) stimulate GCs intracellularly by a molecular mechanism that is not fully understood. To gain further insight into the mechanism of activation and specificity among these proteins, for the first time, several soluble and active truncated GCs and fusion proteins between intracellular domains of GCs and full-length GCAPs were generated. The GC activity of myristoylated GCAP--(437-1054)GC displayed typical [Ca(2+)] dependence, and was further enhanced by ATP and inhibited by guanylyl cyclase inhibitor protein (GCIP). The myristoyl group of GCAP1 appeared to be critical for the inhibition of GCs at high [Ca(2+)], even without membranes. In contrast, calmodulin (CaM)--(437-1054)GC1 fusion protein was inactive, but could be stimulated by exogenous GCAP1. In a series of experiments, we showed that the activation of GCs by linked GCAPs involved intra- and intermolecular mechanisms. The catalytically productive GCAP1--(437-1054)GC1 complex can dissociate, allowing binding and stimulation of the GC1 fusion protein by free GCAP1. This suggests that the intramolecular interactions within the fusion protein have low affinity and are mimicking the native system. We present evidence that the mechanism of GC activation by GCAPs involves a dimeric form of GCs, involves direct interaction between GCs and GCAPs, and does not require membrane components. Thus, fusion proteins may provide an important advance for further structural studies of photoreceptor GCs and other sTMRs with and without different forms of regulatory proteins.

Instead of binding calcium, one of the EF-hand structures in guanylyl cyclase activating protein-2 is required for targeting photoreceptor guanylyl cyclase

Guanylyl cyclase activator proteins (GCAPs) are calcium-binding proteins closely related to recoverin, neurocalcin, and many other neuronal Ca(2+)-sensor proteins of the EF-hand superfamily. GCAP-1 and GCAP-2 interact with the intracellular portion of photoreceptor membrane guanylyl cyclase and stimulate its activity by promoting tight dimerization of the cyclase subunits. At low free Ca(2+) concentrations, the activator form of GCAP-2 associates into a dimer, which dissociates when GCAP-2 binds Ca(2+) and becomes inhibitor of the cyclase. GCAP-2 is known to have three active EF-hands and one additional EF-hand-like structure, EF-1, that deviates form the EF-hand consensus sequence. We have found that various point mutations within the EF-1 domain can specifically affect the ability of GCAP-2 to interact with the target cyclase but do not hamper the ability of GCAP-2 to undergo reversible Ca(2+)-sensitive dimerization. Point mutations within the EF-1 region can interfere with both the activation of the cyclase by the Ca(2+)-free form of GCAP-2 and the inhibition of retGC basal activity by the Ca(2+)-loaded GCAP-2. Our results strongly indicate that evolutionary conserved and GCAP-specific amino acid residues within the EF-1 can create a contact surface for binding GCAP-2 to the cyclase. Apparently, in the course of evolution GCAP-2 exchanged the ability of its first EF-hand motif to bind Ca(2+) for the ability to interact with the target enzyme.

cGMP enhances the sonic hedgehog response in neural plate cells

The elaboration of distinct cell types during development is dependent on a small number of inductive molecules. Among these inducers is Sonic hedgehog (Shh), which, in combination with other factors, patterns the dorsoventral (DV) axis of the nervous system. The response of a cell is dependent in part on its complement of cyclic nucleotides. cAMP antagonizes Shh signaling, and we examined the influence of cGMP on the Shh response. Cells in chick neural plate explants respond to Shh by differentiating into ventral neural-cell types. Exposure of intermediate-zone explants to cGMP analogs enhanced their response to Shh in a dose-dependent manner. The Shh response was also enhanced in dorsal-zone explants exposed to chick natriuretic peptide (chNP), which stimulates cGMP production by membrane-bound guanylate cyclase (mGC). Addition of chNP to intermediate-zone explants did not enhance the Shh response, consistent with a reported lack of mGC in this region of the neural tube. Finally, the presence of a nitric oxide (NO)-sensitive guanylate cyclase (GC) was established by demonstrating cGMP immunoreactivity in neural tissue following NO stimulation of whole chick embryos. Intracellular levels of cGMP and cAMP may thus provide a mechanism through which other factors modulate the Shh response during neural development.

Detailed analysis of the atrial natriuretic factor receptor hormone-binding domain crystal structure

The X-ray crystal structure of the dimerized atrial natriuretic factor (ANF) receptor hormone-binding domain has provided a first structural view of this anti-hypertensive receptor. The structure reveals a surprising evolutionary link to the periplasmic-binding protein fold family. Furthermore, the presence of a chloride ion in the membrane distal domain and the presence of a second putative effector pocket suggests that the extracellular domain of this receptor is allosterically regulated. The scope of this article is to extensively review the data published on this receptor and to correlate it with the hormone-binding domain structure. In addition, a more detailed description is provided of the important features of this structure including the different binding sites for the ANF hormone, chloride ion, putative effector pocket, glycosylation sites, and dimer interface.Key words: crystal structure, periplasmic-binding protein fold, guanylyl cyclase, hormone receptor.

Allosteric regulatory step and configuration of the ATP-binding pocket in atrial natriuretic factor receptor guanylate cyclase transduction mechanism

The atrial natriuretic factor (ANF) signal transduction mechanism consists of the transformation of the signal information into the production of cyclic GMP. The binding of ANF to its receptor, which is also a guanylate cyclase, generates the signal. This cyclase has been termed atrial natriuretic factor receptor guanylate cyclase, ANF-RGC. ANF-RGC is a single transmembrane-spanning protein. The ANF receptor domain resides in the extracellular region of the protein, and the catalytic domain is located in the intracellular region at the C-terminus of the protein. Thus, the signal is relayed progressively from the receptor domain to the catalytic domain, where it is converted into the formation of cyclic GMP. The first transduction step is the direct binding of ATP with ANF-RGC. This causes allosteric regulation of the enzyme and primes it for the activation of its catalytic moiety. The partial structural motif of the ATP binding domain in ANF-RGC has been elucidated, and it has been named ATP regulatory module (ARM). In this presentation, we provide a brief review of the ATP-regulated transduction mechanism and the ARM model. The model depicts a configuration of the ATP-binding pocket that has been experimentally validated, and the model shows that the ATP-dependent transduction process is a two- (or more) step event. The first step involves the binding of ATP with its ARM. This partially activates the cyclase and prepares it for the subsequent steps, which are consistent with its being phosphorylated and attaining the fully activated state.Key words: ANF, ANF-receptor guanylate cyclase (ANF-RGC), ATP, ATP-regulatory module (ARM).

cAMP signalling in Trypanosoma brucei

Cyclic AMP was the first second messenger to be identified. After five decades of research, much is currently known about its biological functions and clinical implications. Several components of the cAMP signalling pathways, such as the G-protein coupled receptors and the phosphodiesterases, have become sensitive and specific drug targets for a host of clinical applications. Surprisingly, very little effort has been invested so far into the study of cAMP signalling in parasites, and its significance in host/parasite interaction. Our laboratory has embarked on a study of cAMP signalling in Trypanosoma brucei. A newly identified adenylyl cyclase, GRESAG4.4B, a member of a small family of closely related genes, is being used as a model molecule for investigating the mechanisms which control cyclase activity in the T. brucei cell. On the other hand, a number of genes for different families of cAMP-specific phosphodiesterases have been identified and characterised. One enzyme, TbPDE1, is coded for by a single-copy gene. Knock-outs of this gene display an almost normal phenotype in culture, indicating that TbPDE1 is not an essential enzyme under culture conditions. A second phosphodiesterase which is being studied in detail, TbPDE2A, is clearly different from TbPDE1, and it is coded for by a member of a small gene family containing about six similar, but non-identical genes. TbPDE2A, as TbPDE1, is specific for cAMP. In its N-terminal, it contains a GAF domain which may represent an allosteric cGMP-binding site. The other members of the TbPDE2 family all exhibit strongly conserved catalytic domains, but vary widely in their N-terminal regulatory domains. With regard to downstream signalling by the cAMP generated through the interplay of adenylyl cyclases and phosphodiesterases, we have recently identified a single-copy gene (TbRSU1) which codes for a putative regulatory subunit of the cAMP-regulated protein kinase A. This protein exhibits considerable similarity with its mammalian counterparts. Immunoprecipitation co-precipitates a protein kinase activity with the characteristics of protein kinase A.

Agonistic induction of a covalent dimer in a mutant of natriuretic peptide receptor-A documents a juxtamembrane interaction that accompanies receptor activation

The natriuretic peptide receptor-A (NPR-A) is composed of an extracellular domain with a ligand binding site, a transmembrane-spanning domain, a kinase homology domain, and a guanylyl cyclase domain. In response to agonists (atrial natriuretic peptide (ANP) and brain natriuretic peptide), the kinase homology domain-mediated guanylate cyclase repression is removed, which allows the production of cyclic GMP. Previous work from our laboratory strongly indicated that agonists are exerting their effects through the induction of a juxtamembrane dimeric contact. However, a direct demonstration of this mechanism remains to be provided. As a tool, we are now using the properties of a new mutation, D435C. It introduces a cysteine at a position in NPR-A corresponding to a supplementary cysteine found in NPR-C6, another receptor of this family (a disulfide-linked dimer). Although this D435C mutation only leads to trace levels of NPR-A disulfide-linked dimer at basal state, covalent dimerization can be induced by a treatment with rat ANP or with other agonists. The NPR-A(D435C) mutant has not been subjected to significant structural alterations, since it shares with the wild type receptor a similar dose-response pattern of cellular guanylyl cyclase activation. However, a persistent activation accompanies NPR-A(D435C) dimer formation after the removal of the inducer agonist. On the other hand, a construction where the intracellular domain of NPR-A(D435C) has been truncated (DeltaKC(D435C)) displays a spontaneous and complete covalent dimerization. In addition, the elimination of the intracellular domain in wild type DeltaKC and DeltaKC(D435C) is associated with an increase of agonist binding affinity, this effect being more pronounced with the weak agonist pBNP. Also, a D435C secreted extracellular domain remains unlinked even after incubation with rat ANP. In summary, these results demonstrate, in a dynamic fashion, the agonistic induction of a dimeric contact in the juxtamembrane domain of NPR-A. In addition, this process seems to require membrane attachment of the receptor. Finally, the intracellular domain represses this contact at the basal state, showing its potent influence on the outer juxtamembrane domain.

Molecular pathways of cyclic nucleotide-induced inhibition of arterial smooth muscle cell proliferation

Cyclic adenosine 3',5'-monophosphate (cAMP) and cyclic guanosine 3',5'-monophosphate (cGMP) are second messengers involved in the intracellular signal transduction of a wide variety of extracellular stimuli. These signals regulate many biological processes including cell proliferation, differentiation, migration, and apoptosis. Recently, significant progress has been achieved in the molecular basis underlying cyclic nucleotide regulation of cell proliferation. This review summarizes our knowledge of the signaling pathways regulated by cyclic nucleotides in arterial smooth muscle cells.

Biochemical characterization of the intracellular domain of the human guanylyl cyclase C receptor provides evidence for a catalytically active homotrimer

Guanylyl cyclase C (GCC) is the receptor for the family of guanylin peptides and bacterial heat-stable enterotoxins (ST). The receptor is composed of an extracellular, ligand-binding domain and an intracellular domain with a region of homology to protein kinases and a guanylyl cyclase catalytic domain. We have expressed the entire intracellular domain of GCC in insect cells and purified the recombinant protein, GCC-IDbac, to study its catalytic activity and regulation. Kinetic properties of the purified protein were similar to that of full-length GCC, and high activity was observed when MnGTP was used as the substrate. Nonionic detergents, which stimulate the guanylyl cyclase activity of membrane-associated GCC, did not appreciably increase the activity of GCC-IDbac, indicating that activation of the receptor by Lubrol involved conformational changes that required the transmembrane and/or the extracellular domain. The guanylyl cyclase activity of GCC-IDbac was inhibited by Zn(2+), at concentrations shown to inhibit adenylyl cyclase, suggesting a structural homology between the two enzymes. Covalent cross-linking of GCC-IDbac indicated that the protein could associate as a dimer, but a large fraction was present as a trimer. Gel filtration analysis also showed that the major fraction of the protein eluted at a molecular size of a trimer, suggesting that the dimer detected by cross-linking represented subtle differences in the juxtaposition of the individual polypeptide chains. We therefore provide evidence that the trimeric state of GCC is catalytically active, and sequences required to generate the trimer are present in the intracellular domain of GCC.

Activation of protein kinase C stimulates the dephosphorylation of natriuretic peptide receptor-B at a single serine residue: a possible mechanism of heterologous desensitization

The binding of atrial natriuretic peptide and C-type natriuretic peptide (CNP) to the guanylyl cyclase-linked natriuretic peptide receptors A and B (NPR-A and -B), respectively, stimulates increases in intracellular cGMP concentrations. The vasoactive peptides vasopressin, angiotensin II, and endothelin inhibit natriuretic peptide-dependent cGMP elevations by activating protein kinase C (PKC). Recently, we identified six in vivo phosphorylation sites for NPR-A and five sites for NPR-B and demonstrated that the phosphorylation of these sites is required for ligand-dependent receptor activation. Here, we show that phorbol 12-myristate 13-acetate, a direct activator of PKC, causes the dephosphorylation and desensitization of NPR-B. In contrast to the CNP-dependent desensitization process, which results in coordinate dephosphorylation of all five sites in the receptor, phorbol 12-myristate 13-acetate treatment causes the dephosphorylation of only one site, which we have identified as Ser(523). The conversion of this residue to alanine or glutamate did not reduce the amount of mature receptor protein as indicated by detergent-dependent guanylyl cyclase activities or Western blot analysis but completely blocked the ability of PKC to induce the dephosphorylation and desensitization of NPR-B. Thus, in contrast to previous reports suggesting that PKC directly phosphorylates and inhibits guanylyl cyclase-linked natriuretic peptide receptors, we show that PKC-dependent dephosphorylation of NPR-B at Ser(523) provides a possible molecular explanation for how pressor hormones inhibit CNP signaling.