The Membrane Form of Guanylate Cyclase

What is the core oscillator in the speract-activated pathway of the Strongylocentrotus purpuratus sperm flagellum?

Sperm chemotaxis has an important role in fertilization. Most of our knowledge regarding this phenomenon comes from studies in organisms whose fertilization occurs externally, like sea urchins. Sea urchin spermatozoa respond to sperm-activating peptides, which diffuse from the egg jelly coat and interact with their receptor in the flagellum, triggering several physiological responses: changes in membrane potential, intracellular pH, cyclic nucleotide levels, and intracellular Ca2+ concentration ([Ca2+]). In particular, flagellar [Ca2+] has been shown to oscillate. These [Ca2+] oscillations are correlated with changes in the flagellar shape and so with the regulation of the sperm swimming paths. In this study, we demonstrate, from a mathematical modeling perspective, that the reported speract-activated signaling pathway in Strongylocentrotus purpuratus (speract being a sperm-activating peptide specific to this species) has the necessary elements to replicate the reported [Ca2+] oscillations. We further investigate which elements of this signaling pathway constitute the core oscillator.

A short history of cGMP, guanylyl cyclases, and cGMP-dependent protein kinases

Here, we review the early studies on cGMP, guanylyl cyclases, and cGMP-dependent protein kinases to facilitate understanding of development of this exciting but complex field of research encompassing pharmacology, biochemistry, physiology, and molecular biology of these important regulatory molecules.

Differential expression of nitric oxide signaling components in undifferentiated and differentiated human embryonic stem cells

Nitric oxide (NO) is an uncharged free-radical gas that is involved in a number of physiological and pathological events. We have examined the expression of various subunits of soluble guanylyl cyclase (sGC alpha (1), alpha (2), beta (1), beta (2)), nitric oxide synthase (s) (NOS-1, -2, -3), MLC2 (cardiac marker) and a cardiac-specific transcription factor (Nkx2.5) in human embryonic stem (hES) cells (H-9 cells) and differentiated cells subjected to differentiation in cell suspension using embryoid body (EB) formation. Our results demonstrate a time-dependent increase in the expression of sGC alpha (1) and beta (1) at the mRNA and protein levels in differentiated cells compared to undifferentiated H-9 cells as examined by real-time PCR and western blot analysis. mRNA for sGC alpha (2) also showed a time-dependent increase compared to undifferentiated cells. In contrast, there was a time-dependent decrease in sGC beta (2) mRNA expression in differentiated cells compared to undifferentiated H-9 cells. In contrast to undifferentiated H-9 cells, the maximum mRNA expression of cardiac marker MLC2 and the cardiac-specific transcription factor Nkx2.5 was observed at day 14 of the differentiated H-9 cells. The protein levels of MLC2 were stable up to day 25 compared to mRNA levels, which showed a sharp decline after day 15. Using immunofluorescence, we also demonstrate positive staining of cardiac markers such as troponin I, alpha-actinin, atrial natriuretic peptide, and SGC alpha (1) at days 8-37 post-differentiation. These results clearly demonstrate the role of NO signaling components in differentiation events or physiological processes of human ES or ES cell-derived cardiomyocytes.

NO-cGMP signaling in development and stem cells

Despite the recognition that the NO-cGMP signaling pathway is involved in so many physiological and pathological events, a clear understanding of many of the functions of this signaling pathway remains elusive. Because of its pleiotropic and often transient actions, its modulation for therapeutic purposes in multiple pathological states is a complex issue. Recent work that combines the areas of developmental and stem cell biology and NO-cGMP signaling in various models may help us elucidate some of these functions and even discover novel actions for this signaling paradigm. This review will discuss some of the recent work in these areas, with additional focus on the nitric oxide receptor, soluble guanylyl cyclase.

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.

Nitric oxide-cyclic GMP pathway with some emphasis on cavernosal contractility

Nitric oxide (NO) is formed from the conversion of L-arginine by nitric oxide synthase (NOS), which exists in three isoforms: neuronal (nNOS), endothelial (eNOS), and inducible (iNOS). nNOS is expressed in penile neurons innervating the corpus cavernosum, and eNOS protein expression has been identified primarily in both cavernosal smooth muscle and endothelium. NO is released from nerve endings and endothelial cells and stimulates the activity of soluble guanylate cyclase (sGC), leading to an increase in cyclic guanosine-3',5'-monophosphate (cGMP) and, finally, to calcium depletion from the cytosolic space and cavernous smooth muscle relaxation. The effects of cGMP are mediated by cGMP dependent protein kinases, cGMP-gated ion channels, and cGMP-regulated phosphodiesterases (PDE). Thus, cGMP effect depends on the expression of a cell-specific cGMP-receptor protein in a given cell type. Numerous systemic vasculature diseases that cause erectile dysfunction (ED) are highly associated with endothelial dysfunction, which has been shown to contribute to decreased erectile function in men and a number of animal models of penile erection. Based on the increasing knowledge of intracellular signal propagation in cavernous smooth muscle tone regulation, selective PDE inhibitors have recently been introduced in the treatment of ED. Phosphodiesterase 5 (PDE5) inactivates cGMP, which terminates NO-cGMP-mediated smooth muscle relaxation. Inhibition of PDE5 is expected to enhance penile erection by preventing cGMP degradation. Development of pharmacologic agents with this effect has closely paralleled the emerging science.

Molecular cloning of a putative membrane form guanylyl cyclase from the crayfish Procambarus clarkii

Available data indicate that crustacean hyperglycemic hormone (CHH) stimulates membrane-bound guanylyl cyclase (GC), producing cyclic guanosine 3',5'-monophosphate, which in turn mediates the effect of CHH on carbohydrate metabolism. In the present study, we report the cloning of a cDNA (PcGC-M2) encoding a putative membrane form GC from the muscle of the crayfish, Procambarus clarkii. Analysis of the deduced amino acid sequence shows that PcGC-M2 contains the signature domains characteristic of membrane form GCs, including an extracellular ligand-binding domain, a single transmembrane, and intracellular kinase-like and cyclase catalytic domains. In addition, a C-terminal domain of 247 residues is present following the cyclase catalytic domain. PcGC-M2 is most closely related (33% identity) to a Drosophila membrane form GC (DrGC-1), and an Anopheles gambiae membrane form GC (AgaGC); the three GCs also share a similar distribution pattern of conserved cysteine residues in the extracellular domain. The PcGC-M2 transcript is expressed in several CHH target tissues, including muscle, hepatopancreas, heart, ovary, testis, and gill, suggesting that PcGC-M2 may participate in the signaling cascade activated by CHH.

Regulation of nitric oxide and soluble guanylyl cyclase

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

Ligands and receptors mediating signal transduction in sea urchin spermatozoa

Sea urchins have long been a model system for the study of fertilization. Much has been learned about how sea urchin sperm locate and fertilize the egg. Sperm and eggs are spawned simultaneously into the surrounding seawater. Sperm signaling pathways lead to downstream events that ensure fertilization. Upon spawning, sperm must acquire motility and then they must swim towards or respond to the egg in some way. Finally, they must undergo a terminal exocytotic event known as the acrosome reaction that allows the sperm to bind to the vitelline layer of the egg and then to fuse with the egg plasma membrane. Motility is stimulated by exposure to seawater, while later events are orchestrated by factors from the egg. The sperm signaling pathways are exquisitely tuned to bring the sperm to the egg, bind, and fuse the two cells as quickly as possible.

A sperm-activating peptide controls a cGMP-signaling pathway in starfish sperm

Peptides released from eggs of marine invertebrates play a central role in fertilization. About 80 different peptides from various phyla have been isolated, however, with one exception, their respective receptors on the sperm surface have not been unequivocally identified and the pertinent signaling pathways remain ill defined. Using rapid mixing techniques and novel membrane-permeable caged compounds of cyclic nucleotides, we show that the sperm-activating peptide asterosap evokes a fast and transient increase of the cGMP concentration in sperm of the starfish Asterias amurensis, followed by a transient cGMP-stimulated increase in the Ca(2+) concentration. In contrast, cAMP levels did not change significantly and the Ca(2+) response evoked by photolysis of caged cAMP was significantly smaller than that using caged cGMP. By cloning of cDNA and chemical crosslinking, we identified a receptor-type guanylyl cyclase in the sperm flagellum as the asterosap-binding protein. Sperm respond exquisitely sensitive to picomolar concentrations of asterosap, suggesting that the peptide serves a chemosensory function like resact, a peptide involved in chemotaxis of sperm of the sea urchin Arbacia punctulata. A unifying principle emerges that chemosensory transduction in sperm of marine invertebrates uses cGMP as the primary messenger, although there may be variations in the detail.

Soluble guanylate cyclase is allosterically inhibited by direct interaction with 2-substituted adenine nucleotides

Nitric oxide (NO), the principal endogenous ligand for soluble guanylate cyclase (sGC), stimulates that enzyme and accumulation of intracellular cGMP, which mediates many of the (patho) physiological effects of NO. Previous studies demonstrated that 2-substituted adenine nucleotides, including 2-methylthioATP (2MeSATP) and 2-chloroATP (2ClATP), allosterically inhibit guanylate cyclase C, the membrane-bound receptor for the Escherichia coli heat-stable enterotoxin in the intestine. The present study examined the effects of 2-substituted adenine nucleotides on crude and purified sGC. 2-Substituted nucleotides inhibited basal and NO-activated crude and purified sGC, when Mg2+ served as the substrate cation cofactor. Similarly, 2-substituted adenine nucleotides inhibited those enzymes when Mn2+, which activates sGC in a ligand-independent fashion, served as the substrate cation cofactor. Inhibition of sGC by 2-substituted nucleotides was associated with a decrease in Vmax, consistent with a noncompetitive mechanism. In contrast to guanylate cyclase C, 2-substituted nucleotides inhibited sGC by a guanine nucleotide-independent mechanism. These studies demonstrate that 2-substituted adenine nucleotides allosterically inhibit basal and ligand-stimulated sGC. They support the suggestion that allosteric inhibition by adenine nucleotides is a general characteristic of the family of guanylate cyclases. This allosteric inhibition is mediated by direct interaction of adenine nucleotides with sGC, likely at the catalytic domain in a region outside the substrate-binding site.

Disulfide bond structure of the atrial natriuretic peptide receptor extracellular domain: conserved disulfide bonds among guanylate cyclase-coupled receptors

The disulfide bond structure of the extracellular domain of rat atrial natriuretic peptide (ANP) receptor (NPR-ECD) has been determined by mass spectrometry (MS) and Edman sequencing. Recombinant NPR-ECD expressed in COS-1 cells and purified from the culture medium binds ANP with as high affinity as the natural ANP receptor. Reaction with iodoacetic acid yielded no S-carboxymethylcysteine, indicating that all six Cys residues in NPR-ECD are involved in disulfide bonds. Electrospray ionization MS of NPR-ECD deglycosylated by peptide-N-glycosidase F gave a molecular mass of 48377.5+/-1.6 Da, which was consistent with the presence of three disulfide bonds. Liquid chromatography MS analysis of a lysylendopeptidase digest yielded three cystine-containing fragments with disulfide bonds Cys(60)-Cys(86), Cys(164)-Cys(213) and Cys(423)-Cys(432) based on their observed masses. These bonds were confirmed by Edman sequencing of each of the three fragments. No evidence for an inter-molecular disulfide bond was found. The six Cys residues in NPR-ECD, forming a 1-2, 3-4, 5-6 disulfide pairing pattern, are strictly conserved among A-type natriuretic peptide receptors and are similar in B-type receptors. We found that in other families of guanylate cyclase-coupled receptors, the Cys residues involved in 1-2 and 5-6 disulfide pairs are conserved in nearly all, suggesting an important contribution of these disulfide bonds to the receptor's structure and function.

Molecular aspects of soluble guanylyl cyclase regulation

Soluble guanylyl cyclase (sGC) is a heterodimeric enzyme (comprised of alpha and beta subunits) that generates the intracellular second messenger cyclic guanosine monophosphate (cGMP) from guanosine triphosphate (GTP). cGMP is subsequently important for the regulation of protein kinases, ion channels, and phosphodiesterases. Since recent evidence has demonstrated that heterodimerization of the alpha/beta subunits is essential for basal and stimulated enzymatic activity, the existence of several types of isoforms for each of the two subunits, along with their varying degrees of expression in different tissues, implies that multiple regulatory mechanisms exist for sGC. Yet, progress in studying and clarifying the regulatory processes that can alter sGC expression and activity has only slowly started being elucidated. In the following paper, we elaborate on sGC structure, function, and distribution along with recently described signaling pathways that modulate sGC gene expression.

Ligand binding-dependent limited proteolysis of the atrial natriuretic peptide receptor: juxtamembrane hinge structure essential for transmembrane signal transduction

The atrial natriuretic peptide (ANP) receptor is a 130-kDa transmembrane protein containing an extracellular ANP-binding domain, a single transmembrane sequence, an intracellular kinase-homologous domain, and a guanylate cyclase (GCase) domain. We observed that the receptor, when bound with ANP, was rapidly cleaved by endogenous or exogenously added protease to yield a 65-kDa ANP-binding fragment. No cleavage occurred without bound ANP. This ligand-induced cleavage abolished GCase activation by ANP. Cleavage occurred in an extracellular, juxtamembrane region containing six closely spaced Pro residues and a disulfide bond. Such structural features are shared among the A-type and B-type ANP receptors but not by ANP clearance receptors. The potential role of the hinge structure was examined by mutagenesis experiments. Mutation of Pro(417), but not other Pro residues, to Ala abolished GCase activation by ANP. Elimination of the disulfide bond by Cys to Ser mutations yielded a constitutively active receptor. Pro(417), and Cys(423) and Cys(432) forming the disulfide bond are strictly conserved among GCase-coupled receptors, while other residues are largely variable. The conserved Pro(417) and the disulfide bond may represent a consensus signaling motif in the juxtamembrane hinge structure that undergoes a marked conformational change upon ligand binding and apparently mediates transmembrane signal transduction.

A constitutively "phosphorylated" guanylyl cyclase-linked atrial natriuretic peptide receptor mutant is resistant to desensitization

Dephosphorylation of the natriuretic peptide receptor-A (NPR-A) is hypothesized to mediate its desensitization in response to atrial natriuretic peptide (ANP) binding. Recently, we identified six phosphorylation sites within the kinase homology domain of NPR-A and determined that the conversion of these residues to alanine abolished the ability of the receptor to be phosphorylated or to be activated by ANP and ATP. In an attempt to generate a form of NPR-A that mimics a fully phosphorylated receptor but that is resistant to dephosphorylation, we engineered a receptor variant (NPR-A-6E) containing glutamate substitutions at all six phosphorylation sites. Consistent with the known ability of negatively charged glutamate residues to substitute functionally, in some cases, for phosphorylated residues, we found that NPR-A-6E was activated 10-fold by ANP and ATP. As determined by guanylyl cyclase assays, the hormone-stimulated activity of the wild-type receptor declined over time in membrane preparations in vitro, and this loss was blocked by the serine/threonine protein phosphatase inhibitor microcystin. In contrast, the activity of NPR-A-6E was more linear with time and was unaffected by microcystin. The nonhydrolyzable ATP analogue adenosine 5'-(beta,gamma-imino)-triphosphate was half as effective as ATP in stimulating the wild-type receptor but was equally as potent in stimulating NPR-A-6E, suggesting that ATP is required to keep the wild-type but not 6E variant phosphorylated. Finally, the desensitization of NPR-A-6E in whole cells was markedly blunted compared with that of the wild-type receptor, consistent with its inability to shed the negative charge from its kinase homology domain via dephosphorylation. These data provide the first direct test of the requirement for dephosphorylation in guanylyl cyclase desensitization and they indicate that it is an essential component of this process.

Corin, a mosaic transmembrane serine protease encoded by a novel cDNA from human heart

A novel cDNA has been identified from human heart that encodes an unusual mosaic serine protease, designated corin. Corin has a predicted structure of a type II transmembrane protein and contains two frizzled-like cysteine-rich motifs, seven low density lipoprotein receptor repeats, a macrophage scavenger receptor-like domain, and a trypsin-like protease domain in the extracellular region. Northern analysis showed that corin mRNA was highly expressed in the human heart. In mice, corin mRNA was detected by in situ hybridization in the cardiac myocytes of the embryonic heart as early as embryonic day (E) 9.5. By E11.5-13.5, corin mRNA was most abundant in the primary atrial septum and the trabecular ventricular compartment. Expression in the heart was maintained through the adult. In addition, mouse corin mRNA was also detected in the prehypertrophic chrondrocytes in developing bones. By fluorescent in situ hybridization analysis, the human corin gene was mapped to 4p12-13 where a congenital heart disease locus, total anomalous pulmonary venous return, had been previously localized. The unique domain structure and specific embryonic expression pattern suggest that corin may have a function in cell differentiation during development. The chromosomal localization of the human corin gene makes it an attractive candidate gene for total anomalous pulmonary venous return.

Soluble guanylate cyclase: the forgotten sibling

Despite widespread distribution in most mammalian cells, the role of soluble guanylate cyclase has, until recently, been poorly defined, especially when compared with its more illustrious sibling, adenylate cyclase. In this review Adrian Hobbs outlines some of the reasons why the soluble guanylate cyclase-cGMP pathway has remained outside the signalling spotlight for much of the past 30 years. He goes on to describe how new molecular biological and biochemical approaches have facilitated a characterization of soluble guanylate cyclase and how this enzyme has acquired a profound physiological significance, and much research attention, as the intracellular 'receptor' for nitric oxide.

Signal transduction: activation of the guanylate cyclase-cyclic guanosine-3'-5' monophosphate system by hormones and free radicals

Intracellular communication and transmission of messages for many hormones and free radicals occur after the hormones and free radicals bind to their receptors by enhancing the activity of guanylate cyclase, the enzyme that catalyzes the conversion of guanosine triphosphate to the intracellular messenger cyclic guanosine-3'-5' monophosphate (cyclic GMP). The guanylate cyclase-linked receptors exist intracellularly (ie, cytoplasmic) and in membrane-bound forms. Enhancement of guanylate cyclase by hormones or free radicals increases intracellular cyclic GMP, which closes cation channels in the kidney while activating cation channels in the retina and olfactory cilia, either directly or by cyclic GMP-dependent protein kinase. Cyclic GMP also has potent blood pressure lowering properties. Cyclic GMP promotes growth by increasing DNA, RNA, and protein synthesis. Overactivity of this system is observed in Traveler's diarrhea, whereas underactivity occurs in Chediak-Higashi syndrome in which lysosomal enzyme release and chemotaxis are defective and can be corrected in vitro by addition of cyclic GMP.

Primary structure and differential gene expression of three membrane forms of guanylyl cyclase found in the eye of the teleost Oryzias latipes

Three cDNAs (OlGC3, OlGC4, and OlGC5) encoding membrane guanylyl cyclases were isolated from a medaka (Oryzias latipes) eye cDNA library. An open reading frame for OlGC3 predicted a protein of 1057 amino acids, and those for OlGC4 and OlGC5, 1134 and 1151, respectively. These proteins consist of an apparent signal peptide (21 residues for OlGC3, 50 residues for OlGC4, and 48 residues for OlGC5) and a single transmembrane domain that divides the protein into an amino-terminal extracellular domain and a carboxyl-terminal intracellular domain that further divides into a kinase-like domain and a cyclase catalytic domain. Phylogenetic analysis with amino acid sequences of OlGC3, OlGC4, and OlGC5, as well as those of other membrane guanylyl cyclases, indicated that OlGC3, OlGC4, and OlGC5 are members of the sensory organ-specific guanylyl cyclase family. Reverse transcription-polymerase chain reaction and Northern blot analyses demonstrated that OlGC3, OlGC4, and OlGC5 transcripts are present in the eye, which contains more cGMP than the other organs. In addition to being expressed in the eye, OlGC3 transcripts are also present in the brain, heart, liver, pancreas, and ovary, while OlGC4 is present in the liver and OlGC5 in the heart. Reverse transcription-polymerase chain reaction analysis with RNA from unfertilized eggs and embryos showed that OlGC3 and OlGC5 are expressed both maternally and zygotically, while OlGC4 is expressed only zygotically, and that the zygotic expression of these three genes is differentially activated. These results suggest a structural and functional diversity of sensory organ-specific guanylyl cyclases in vertebrates.

Guanylyl cyclase receptors and guanylin-like peptides in reptilian intestine

Receptors for guanylin and uroguanylin were identified on the mucosal surface of enterocytes lining the intestine of the bobtail skink (Tiliqua rugosa), king's skink (Egernia kingii), and knight anole (Anolis equestris) by receptor autoradiography using 125I-ST (Escherichia coli heat-stable enterotoxin) as the radioligand. Specific, high-affinity binding of 125I-ST to receptors was found on the microvillus border of enterocytes and little or no specific binding of 125I-ST was observed in other strata comprising the gut wall. The American alligator (Alligator mississippensis) also exhibited receptor binding, but unlike the other three species had relatively high levels of apparent nonspecific binding. A comparison of intestinal cGMP accumulation responses between the American alligator and the knight anole demonstrated a greater magnitude of cGMP responses to ST and guanylin in vitro in the knight anole relative to the tissue cGMP accumulation responses of alligators. Treatment with ST resulted in markedly greater tissue cGMP accumulation responses in both species compared to treatment with guanylin. To complete a paracrine signaling pathway in reptilian intestine, guanylin-like peptides that stimulated cGMP accumulation in human T84 intestinal cells were isolated from the intestinal mucosa of alligators. We conclude that functional receptor-guanylyl cyclases and one or more endogenous guanylin/uroguanylin-like peptides occur in the intestinal tract of reptiles as well as in the intestines of mammals and birds. Thus, higher vertebrates have a conserved signaling pathway that regulates intestinal function through the first-messenger peptides, guanylin and/or uroguanylin, and the intracellular second messenger, cGMP.

Molecular characterization of two Drosophila guanylate cyclases expressed in the nervous system

We have isolated, by interspecies hybridization, two classes of Drosophila cDNA each encoding a different guanylate cyclase (GC). One of them encodes an alpha subunit homolog of soluble GC, designated DGC alpha 1, and the other encodes a receptor-type GC, designated DrGC. The dgc alpha 1 cDNA encodes a protein of 676 amino acids and maps to 99B. In situ hybridization to adult tissue sections showed that dgc alpha 1 mRNA is found mainly in the cell bodies of the optic lobe, central brain, and thoracic ganglia. The DGC alpha 1 protein was also localized primarily to the nervous system by immunocytochemical staining, consistent with results of in situ hybridization. However, no detectable expression of this protein was found in the retina. The other class of cDNA, drgc, maps to 76C and encodes a 1525-amino acid protein displaying structural features similar to other known receptor-type guanylate cyclases. However, it has a C-terminal 430 amino acid region that has no homology to any known proteins. drgc RNA is expressed at low levels throughout development and in adult heads and bodies. In situ hybridizations to adult tissue sections showed that drgc mRNA is expressed in a wide range of tissues, including the optic lobe, central brain, thoracic ganglia, digestive tract, and the oocyte.

Mutational inactivation of the catalytic domain of guanylate cyclase-A receptor

Guanylate cyclase-A, the receptor for atrial natriuretic factor, contains a protein kinase-like domain and a catalytic domain in the intracellular region. To investigate the active site (the catalytic cavity) of guanylate cyclase-A, we amplified the catalytic domain plus three amino acids from the kinase-like domain of guanylate cyclase-A (GC-c) with polymerase chain reaction (PCR) and expressed it in Escherichia coli. During the screening of the PCR-cloned gene products with guanylate cyclase assay, a mutant that lacks enzyme activity was identified. Results of cDNA sequencing revealed that Leu 817 was replaced by an Arg residue in the mutated protein. The mutated GC-c bound to GTP-agarose as well as the wild-type protein, indicating that the binding capability of mutated GC-c to GTP is not significantly affected by the Arg substitution. Gel-filtration column chromatography showed that, like the wild-type GC-c, the mutated protein also formed a high-molecular-weight complex. Since mutation of Leu 817 to Arg abolishes the catalytic activity, Leu 817 is likely located near the active site of guanylate cyclase-A. These results demonstrate that the carboxyl fragment of guanylate cyclase-A is an ideal system for studying the active site of guanylate cyclase-A.

Cell functions in Drosophila oogenesis

We are studying Drosophila oogenesis by analysing at genetic and molecular levels several female-sterile mutations. Some (hold up, wavoid-like and abnormal oocyte) have been isolated by L. Sandler in region 32 of the second chromosome; others have been isolated by us and their phenotype is presented for the first time in this paper. We performed chromosome walking in 32D-32E-F(250 Kb) and 32A-B(100 Kb) and in the last years we molecularly identified several genes with specific maternal expression patterns. We will review here our studies on two of these genes: the Vitelline Membrane Protein gene 32E and the gene coding for a receptor form of Guanylate Cyclase.

A human gene encoding a putative basic helix-loop-helix phosphoprotein whose mRNA increases rapidly in cycloheximide-treated blood mononuclear cells

G0S8 is a member of a set of putative G0/G1 switch regulatory genes (G0S genes) selected by screening cDNA libraries prepared from blood mononuclear cells cultured for 2 hr with lectin and cycloheximide. Comparison of a full-length cDNA sequence with the corresponding genomic sequence reveals an open reading frame of 211 amino acids, distributed across 5 exons. The 24-kD protein has a basic domain preceding a potential helix-loop-helix domain which contains a QTK motif found about 60 amino acids from the carboxyl terminus in the loop region of several helix-loop-helix proteins. There are potential phosphorylation sites for protein kinase C, creatine kinase II, and protein tyrosine kinases and regions of sequence similarity to helix-loop-helix proteins, tyrosine phosphatases, and RNA and DNA polymerases. The genomic sequence contains a CpG island, suggesting expression in the germ line. Potential binding sites for transcription factors are present in the 5' flank and introns; these include Zif268/NGFI-A/EGR1/G0S30, NGFI-B, Ap1, and factors that react with retroviral long terminal repeats (LTRs). There are several potential interferon response elements and a serum response element in the 3' flank overlapping a region of similarity to a cytomegalovirus immediate-early gene enhancer. Many of these motifs are found in immediate-early G0/G1 switch genes; however, we were unable to demonstrate an increase in G0S8 mRNA in response to lectin alone. Sequence similarities are noted between G0S8 and a variety of genes involved in the immune system, in the regulation of retroviruses, and in the cell cycle.

Cloning of guanylyl cyclase isoforms

The cloning of particulate and soluble guanylyl cyclases is summarized in Table I. With respect to transmembrane signal transduction systems, guanylyl and adenylyl cyclases can be grouped together with some protein tyrosine kinases and protein tyrosine phosphatases to form a diverse protein family with various structural and functional similarities (Garbers, 1989, 1991, 1992; Koesling et al., 1991; Chinkers and Garbers, 1991; Fig. 1). Particulate guanylyl cyclase contains a single transmembrane domain, and the peptide-binding portion (ligand receptor) is on the exterior surface and the catalytic region on the interior, similar to the protein tyrosine kinase/receptor and the protein tyrosine phosphatase/receptor families (Yarden et al., 1986; Charbonneau et al., 1988; Tonks et al., 1988). Protein tyrosine kinases and phosphatases are also activated by ligand binding to the extracellular domain, which in turn results in phosphorylation or dephosphorylation. On the other hand, soluble guanylyl cyclase exists as a heterodimer with two putative catalytic domains, and both subunits are essential for enzyme activity and activation by nitric oxide. It is thus particularly interesting that adenylyl cyclase also contains two catalytic domains, which are both necessary for catalytic activity (Tang et al., 1991). It is possible that particulate guanylyl cyclase may also dimerize on hormonal stimulation and two catalytic domains from two monomers form a functional catalytic center capable of forming cyclic GMP. The catalytic core of GC-A expressed in bacteria was shown to form a homodimer with positively cooperative kinetics (Thorpe et al., 1991). The physiological significance of the existence of multiple forms of soluble guanylyl cyclase subunits remains unclear. Future studies should reveal the differences in tissue distribution and activation by nitrovasodilators in various heterodimers of soluble guanylyl cyclase.

Effects of cyclic GMP on smooth muscle relaxation

Cyclic GMP levels within smooth muscle are affected then by a number of different pathways. Physiologically NO and ANF are probably the two most important regulators for smooth muscle function, but a variety of other mediators and pharmacological agents may also influence this system. Because of the important role that cyclic GMP plays in the control of smooth muscle tone, which clearly includes vascular smooth muscle, it is now and will continue to be in the future an important physiological and biochemical target for research and a pharmacological target for therapeutic agents.

The ligand-binding domain in metabotropic glutamate receptors is related to bacterial periplasmic binding proteins

Receptors for the major excitatory neurotransmitter glutamate include metabotropic (G protein-coupled) and ionotropic (glutamate-gated ion channel) types. These receptors have large, presumably extracellular, amino-terminal domains. Sensitive sequence analysis techniques indicate that the metabotropic receptor extracellular domain is similar to bacterial periplasmic amino acid binding proteins. A structural model built using the observed similarity predicts a ligand-binding site, and mutants with conservative amino acid substitutions at this site are shown to have reduced ligand affinity. The metabotropic receptor extracellular domain is a member of a family of structural domains linked to a variety of receptor types, including ionotropic glutamate receptors.

Two classes of receptor specific for sperm-activating peptide III in sand-dollar spermatozoa

We characterized receptors specific for sperm-activating peptide III (SAP-III: DSDSAQNLIQ) in spermatozoa of the sand dollar, Clypeaster japonicus, using both binding and cross-linking techniques. Analyses of the data obtained from the equilibrium binding of a radiolabeled SAP-III analogueto C. japonicus spermatozoa, using Klotz, Scatchard and Hill plots, showed the presence of two classes of receptors specific for SAP-III in the spermatozoa. One of the receptors (high-affinity) had a Kd of 3.4 nM and 3.4 x 10(4) binding sites/spermatozoon. The other receptor (low-affinity) had a Kd of 48 nM, with 6.1 x 10(4) binding sites/spermatozoon. The Kd of the high-affinity receptor was comparable to the median effective concentration of the intracellular-pH-increasing activity of SAP-III and that of the low-affinity receptor was comparable to the median effective concentration of the cellular-cGMP-elevating activity of the peptide. In addition, Scatchard and Hill plots of the data suggested the existence of positive cooperativity between the high-affinity members. Similar results were also obtained from a binding experiment using a sperm-membrane fraction prepared from C. japonicus spermatozoa. The incubation of intact spermatozoa or sperm plasma membranes with the radioiodinated SAP-III analogue and a chemical cross-linking reagent, disuccinimidyl suberate, resulted in the radiolabeling of three proteins with molecular masses of 126, 87 and 64 kDa, estimated by SDS/PAGE under reducing conditions.

Overexpression of dimeric guanylyl cyclase cores of an atrial natriuretic peptide receptor

Using a bacterial expression system, large amounts of the catalytic core of an atrial natriuretic peptide receptor guanylyl cyclase were produced and purified. After refolding the protein from a buffer containing urea, the enzyme had positively cooperative kinetics with a Hill coefficient, nH = 1.42 +/- 0.08. Size exclusion chromatography and denaturing polyacrylamide gel electrophoresis demonstrated that the enzyme is composed of homodimers with interacting catalytic sites.

Isolation and expression of a guanylate cyclase-coupled heat stable enterotoxin receptor cDNA from a human colonic cell line

Heat stable enterotoxins (STs) are low molecular-weight peptides secreted by enterotoxigenic bacteria. One type of these enterotoxins (STa) induces intestinal secretion leading to acute diarrhea by binding to a membrane form of guanylate cyclase. We have isolated a cDNA from a human colonic cell line, T84, encoding for a guanylate cyclase-coupled enterotoxin receptor (STaR). The predicted amino acid sequence of the human STa receptor is 81% identical with the previously cloned enterotoxin receptor (GC-C) from rat intestine. COS-7 cells transiently transfected with the cloned cDNA expressed specific concentration-dependent response to STa as measured by cyclic GMP accumulation and is about 20 times more sensitive to the stimulation by STa than has been shown for GC-C.

Sequence of a human brain adenylyl cyclase partial cDNA: evidence for a consensus cyclase specific domain

A cDNA coding for a human brain adenylyl cyclase was isolated and sequenced. The deduced partial 675 amino-acid sequence was compared with those of other known adenylyl and guanylyl cyclases. Comparison of this predicted amino-acid sequence with that of bovine brain (type I) and rat olfactory (type III) adenylyl cyclase indicated a significant homology with the carboxyl-terminal halves of both enzymes. The homology between the human adenylyl cyclase and the other two mammalian adenylyl cyclase also appears at the topographic level. Indeed, the human enzyme includes a extremely hydrophobic region containing six potential membrane-spanning segments followed by a large hydrophilic domain. At the beginning of the hydrophilic domain, there is a 250 amino-acid region which shows not only a striking homology with the bovine and rat adenylyl cyclase (86% of similarity and 57% of identity), but also a significant homology with non-mammalian adenylyl cyclase and guanylyl cyclases. We found that this 250 amino-acid domain contains a sequence of about 165 amino-acids which is highly conserved in most of the known nucleotide cyclases suggesting that it includes residues that are critical for the function of the enzymes.

Cloning, expression, and gene structure of a G protein-coupled glutamate receptor from rat brain

A complementary DNA encoding a G protein-coupled glutamate receptor from rat brain, GluGR, was cloned by functional expression in Xenopus oocytes. The complementary DNA encodes a protein of 1199 amino acids containing a seven-transmembrane motif, flanked by large amino- and carboxyl-terminal domains. This receptor lacks any amino acid sequence similarity with other G protein-coupled receptors, suggesting that it may be a member of a new subfamily. The presence of two introns flanking the central core suggests that GluGR may have evolved by exon shuffling. Expressed in oocytes, GluGR is activated by quisqualate greater than glutamate greater than ibotenate greater than trans-1-aminocyclopentyl-1,3-dicarboxylate, and it is inhibited by 2-amino-3-phosphonopropionate. Activation is blocked by Bordella pertussis toxin. These properties are typical of some metabotropic glutamate receptors.

The expanding family of guanylyl cyclases

The guanylyl cyclase receptor family contains members that exist in both the particulate and soluble fractions of cell homogenates. Soluble forms of the enzyme recognize EDRF (nitric oxide) or similar molecules, while diversity within the extracellular domain of the plasma membrane forms has resulted in a series of guanylyl cyclases that are specifically activated by different ligands. David Garbers and colleagues describe the recent cloning of some of these molecules.

Sensory transduction in eukaryotes. A comparison between Dictyostelium and vertebrate cells

The organization of multicellular organisms depends on cell-cell communication. The signal molecules are often soluble components in the extracellular fluid, but also include odors and light. A large array of surface receptors is involved in the detection of these signals. Signals are then transduced across the plasma membrane so that enzymes at the inner face of the membrane are activated, producing second messengers, which by a complex network of interactions activate target proteins or genes. Vertebrate cells have been used to study hormone and neurotransmitter action, vision, the regulation of cell growth and differentiation. Sensory transduction in lower eukaryotes is predominantly used for other functions, notably cell attraction for mating and food seeking. By comparing sensory transduction in lower and higher eukaryotes general principles may be recognized that are found in all organisms and deviations that are present in specialised systems. This may also help to understand the differences between cell types within one organism and the importance of a particular pathway that may or may not be general. In a practical sense, microorganisms have the advantage of their easy genetic manipulation, which is especially advantageous for the identification of the function of large families of signal transducing components.

Guanylyl cyclase-linked receptors

The guanylyl cyclase receptor family contains members that exist in both the particulate and soluble fractions of cell homogenates. Based on cloning studies, proteins with guanylyl cyclase activity contain a single transmembrane domain, or exist as heterodimers with no apparent transmembrane domains. The members containing the single transmembrane domain appear to act as cell surface receptors for peptides such as natriuretic peptides and bacterial heat-stable enterotoxins, while the heterodimeric forms are activated by nitric oxide. The concentrations of the intracellular messenger, cyclic GMP, then, are regulated by multiple primary signaling molecules, all of which appear to bind directly to the guanylyl cyclase enzyme.

A new form of guanylyl cyclase is preferentially expressed in rat kidney

On the basis of the conserved amino acid sequences of the catalytic domain of both soluble and plasma membrane forms of guanylyl cyclase, we have used the polymerase chain reaction to identify a new form of guanylyl cyclase that is expressed principally in kidney. The cDNA for this new form (GC-S beta 2) codes for a 76.3-kDa protein, which most closely resembles a 70-kDa subunit (GC-S beta 1) of the lung soluble guanylyl cyclase. The mRNA for GC-S beta 1 is preferentially expressed in lung and brain, whereas GC-S beta 2 mRNA is more abundant in kidney and liver. An 86 amino acid carboxyl-terminal region extends beyond the C-terminus of GC-S beta 1 and contains a consensus sequence (-C-V-V-L) for isoprenylation/carboxymethylation. This is the first demonstration of heterogeneity among the heterodimeric forms of guanylyl cyclase and suggests differential regulation.

Guanylyl cyclase is a heat-stable enterotoxin receptor

Plasma membrane forms of guanylyl cyclase have been shown to function as natriuretic peptide receptors. We describe a new clone (GC-C) encoding a guanylyl cyclase receptor for heat-stable enterotoxin. GC-C encodes a protein containing an extracellular amino acid sequence divergent from that of previously cloned guanylyl cyclases; however, the protein retains the intracellular protein kinase-like and cyclase catalytic domains. Expression of GC-C in COS-7 cells results in high guanylyl cyclase activity. In addition, heat-stable enterotoxin from E. coli, but not natriuretic peptides, causes marked elevations of cyclic GMP and is specifically bound by cells transfected with GC-C. The enterotoxin fails to elevate cyclic GMP in nontransfected cells or in cells transfected with the natriuretic peptide/guanylyl cyclase receptors. These results show that a heat-stable enterotoxin receptor responsible for acute diarrhea is a plasma membrane form of guanylyl cyclase.