Regulatory properties of magnesium-dependent guanylate cyclase in Dictyostelium discoideum membranes

Analysis of cGMP Signaling in Dictyostelium

Biochemical assays are described to analyze signal transduction by the second messenger cGMP in Dictyostelium. The methods include enzyme assays to measure the activity and regulation of cGMP synthesizing guanylyl cyclases and cGMP-degrading phosphodiesterases. In addition, several methods are described to quantify cGMP levels. The target of cGMP in Dictyostelium is the large protein GbpC that has multiple domains including a Roc domain, a kinase domain, and a cGMP-stimulated Ras-GEF domain. A cGMP-binding assay is described to detect and quantify GbpC.

Ras proteins signaling in the early metazoan Dictyostelium discoideum

Since the discovery of Ras, Ras-mediated transforming activity has been the major investigative area of interest. Soon thereafter it has emerged that Ras family members regulate different biological processes, other than cell growth, like development and fine-tune the balance between cell death and survival. The lower metazoan Dictyostelium discoideum is a powerful and genetically accessible model organism that has been used to elucidate the roles played by different Ras members in some biological processes, such as cell motility and development. In the following chapter we describe some very basic techniques aiming to identify novel Ras signaling components, throughout insertional mutagenesis screening, and to investigate their role(s) in development and chemotaxis processes.

Activation of soluble guanylyl cyclase at the leading edge during Dictyostelium chemotaxis

Dictyostelium contains two guanylyl cyclases, GCA, a 12-transmembrane enzyme, and sGC, a homologue of mammalian soluble adenylyl cyclase. sGC provides nearly all chemoattractant-stimulated cGMP formation and is essential for efficient chemotaxis toward cAMP. We show that in resting cells the major fraction of the sGC-GFP fusion protein localizes to the cytosol, and a small fraction is associated to the cell cortex. With the artificial substrate Mn2+/GTP, sGC activity and protein exhibit a similar distribution between soluble and particulate fraction of cell lysates. However, with the physiological substrate Mg2+/GTP, sGC in the cytosol is nearly inactive, whereas the particulate enzyme shows high enzyme activity. Reconstitution experiments reveal that inactive cytosolic sGC acquires catalytic activity with Mg2+/GTP upon association to the membrane. Stimulation of cells with cAMP results in a twofold increase of membrane-localized sGC-GFP, which is accompanied by an increase of the membrane-associated guanylyl cyclase activity. In a cAMP gradient, sGC-GFP localizes to the anterior cell cortex, suggesting that in chemotacting cells, sGC is activated at the leading edge of the cell.

Evidence for a role for the Dictyostelium Rap1 in cell viability and the response to osmotic stress

The Dictyostelium genome contains a single rapA gene, which encodes a Rap1 monomeric G protein. As attempts at generating rapA-null Dictyostelium cells had been unsuccessful, expression of antisense RNA from the rapA gene under control of the folate repressible discoidin promoter was used to reduce cellular levels of the Rap1 protein. As Rap1 levels gradually decreased following antisense rapA RNA induction, growth rate and cell viability also decreased, a result consistent with the idea that rapA is an essential gene. The Rap1-depleted cells exhibited reduced viability in response to osmotic shock. The accumulation of cGMP in response to 0.4 M sorbitol was reduced after rapA antisense RNA induction and was enhanced in cells expressing the constitutively activated Rap1(G12V) protein, suggesting a role for Rap1 in the generation of cGMP. Dictyostelium Rap1 formed a complex with the Ras-binding domain of RalGDS only when it was in a GTP-bound state. This assay was used to demonstrate that activation of Rap1 in response to 0.4 M sorbitol occurred with initial kinetics similar to those observed for the accumulation of cGMP. Furthermore, the addition of 2 mM EDTA to osmotically shocked cells, a treatment that enhances cGMP accumulation, also enhanced Rap1 activation. These results suggest a direct role for Rap1 in the activation of guanylyl cyclase during the response to hyperosmotic conditions. Rap1 was also activated in response to low temperature but not in response to low osmolarity or high temperature.

Characterization of two unusual guanylyl cyclases from dictyostelium

Guanylyl cyclase A (GCA) and soluble guanylyl cyclase (sGC) encode GCs in Dictyostelium and have a topology similar to 12-transmembrane and soluble adenylyl cyclase, respectively. We demonstrate that all detectable GC activity is lost in a cell line in which both genes have been inactivated. Cell lines with one gene inactivated were used to characterize the other guanylyl cyclase (i.e. GCA in sgc(minus sign) null cells and sGC in gca(minus sign) null cells). Despite the different topologies, the enzymes have many properties in common. In vivo, extracellular cAMP activates both enzymes via a G-protein-coupled receptor. In vitro, both enzymes are activated by GTPgammaS (K(a) = 11 and 8 microm for GCA and sGC, respectively). The addition of GTPgammaS leads to a 1.5-fold increase of V(max) and a 3.5-fold increase of the affinity for GTP. Ca(2+) inhibits both GCA and sGC with K(i) of about 50 and 200 nm, respectively. Other biochemical properties are very different; GCA is expressed mainly during growth and multicellular development, whereas sGC is expressed mainly during cell aggregation. Folic acid and cAMP activate GCA maximally about 2.5-fold, whereas sGC is activated about 8-fold. Osmotic stress strongly stimulates sGC but has no effect on GCA activity. Finally, GCA is exclusively membrane-bound and is active mainly with Mg(2+), whereas sGC is predominantly soluble and more active with Mn(2+).

GTPgammaS regulation of a 12-transmembrane guanylyl cyclase is retained after mutation to an adenylyl cyclase

DdGCA is a Dictyostelium guanylyl cyclase with a topology typical for mammalian adenylyl cyclases containing 12 transmembrane-spanning regions and two cyclase domain. In Dictyostelium cells heterotrimeric G-proteins are essential for guanylyl cyclase activation by extracellular cAMP. In lysates, guanylyl cyclase activity is strongly stimulated by guanosine 5'-3-O-(thio) triphosphate (GTPgammaS), which is also a substrate of the enzyme. DdGCA was converted to an adenylyl cyclase by introducing three point mutations. Expression of the obtained DdGCA(kqd) in adenylyl cyclase-defective cells restored the phenotype of the mutant. GTPgammaS stimulated the adenylyl cyclase activity of DdGCA(kqd) with properties similar to those of the wild-type enzyme (decrease of K(m) and increase of V(max)), demonstrating that GTPgammaS stimulation is independent of substrate specificity. Furthermore, GTPgammaS activation of DdGCA(kqd) is retained in several null mutants of Galpha and Gbeta proteins, indicating that GTPgammaS activation is not mediated by a heterotrimeric G-protein but possibly by a monomeric G-protein.

A novel adenylyl cyclase detected in rapidly developing mutants of Dictyostelium

Disruption of either the RDEA or REGA genes leads to rapid development in Dictyostelium. The RDEA gene product displays homology to certain H2-type phosphotransferases, while REGA encodes a cAMP phosphodiesterase with an associated response regulator. It has been proposed that RDEA activates REGA in a multistep phosphorelay. To test this proposal, we examined cAMP accumulation in rdeA and regA null mutants and found that these mutants show a pronounced accumulation of cAMP at the vegetative stage that is not observed in wild-type cells. This accumulation was due to a novel adenylyl cyclase and not to the known Dictyostelium adenylyl cyclases, aggregation stage adenylyl cyclase (ACA) or germination stage adenylyl cyclase (ACG), since it occurred in an acaA/rdeA double mutant and, unlike ACG, was inhibited by high osmolarity. The novel adenylyl cyclase was not regulated by G-proteins and was relatively insensitive to stimulation by Mn2+ ions. Addition of the cAMP phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX) permitted detection of the novel adenylyl cyclase activity in lysates of an acaA/acgA double mutant. The fact that disruption of the RDEA gene as well as inhibition of the REGA-phosphodiesterase by IBMX permitted detection of the novel AC activity supports the hypothesis that RDEA activates REGA.

Chemotactic and osmotic signals share a cGMP transduction pathway in Dictyostelium discoideum

In the ameboid eukaryote Dictyostelium discoideum, chemotactic stimulation by cAMP induces an increase of intracellular cGMP and subsequently the phosphorylation of myosin heavy chain II. Resistance to high osmotic stress also requires transient increases of intracellular cGMP and phosphorylation of myosin heavy chain II, although the kinetics is much slower than for chemotaxis. To examine if chemotaxis and osmotic stress share common signaling components we systematically analyzed the osmotic cGMP response and survival in chemotactic mutants with altered cGMP signaling. Null mutants with deletions of cell surface cAMP receptors or the associated GTP-binding proteins Galpha2 and Gbeta show no cAMP-induced cGMP response and chemotaxis; in contrast, osmotic stress induces the normal cGMP accumulation and survival. The same result was obtained with the non-chemotactic mutant KI-10, which lacks the activation of guanylyl cyclase by cAMP. This indicates that these components are required for chemotaxis but not osmotic cGMP signaling and survival. The potential guanylyl cyclase null mutant KI-8 shows no chemotaxis, no osmotic cGMP increase and reduced survival in high osmolarity. Two types of cGMP-binding protein mutants, KI-4 and KI-7, also show reduced tolerance during high osmotic stress. Taken together, these observations clarify that chemotactic and osmotic signals are detected by different mechanisms, but share a cGMP signaling pathway.

cGMP potentiates receptor-stimulated Ca2+ influx in Dictyostelium discoideum

Binding of extracellular cAMP to surface receptors induces at least two responses in Dictyostelium discoideum, the G-protein-dependent activation of guanylyl cyclase, and the opening of a plasma membrane Ca2+ channel. Some experiments suggest that intracellular cGMP opens the Ca2+ channel, while others demonstrate that the channel can open in the absence of functional G-proteins (and thus in the absence of cGMP formation). We have analysed 45Ca2+ uptake in three mutants with altered cGMP formation. Mutant stmF shows a prolonged cGMP response due to deletion of an intracellular phosphodiesterase. Uptake of receptor-stimulated 45Ca2+ is enhanced about two-fold in this mutant if compared to wild-type cells, suggesting that cGMP regulates the opening of the channel. Mutant KI-7 has very low levels of surface cAMP receptors, but nevertheless an enhanced receptor-stimulated cGMP response due to a defect in the turn-off of guanylyl cyclase. This mutant shows poor receptor-stimulated 45Ca2+ uptake, suggesting that cGMP alone is not sufficient to open the Ca2+ channel. Finally, mutant KI-8 has no cGMP due to the absence of nearly all guanylyl cyclase activity. The mutant shows significant but reduced 45Ca2+ uptake (19% of wild-type; 60% if corrected for the reduced level of surface cAMP receptors), suggesting that the channel can open in the absence of cGMP. Taken together, the results demonstrate that receptor-stimulated Ca2+ influx is not directly induced by cGMP formation; it can occur in the absence of cGMP, but is potentiated two- to four-fold by cGMP.

Regulation of guanylyl cyclase by a cGMP-binding protein during chemotaxis in Dictyostelium discoideum

Chemoattractants transiently activate guanylyl cyclase in Dictyostelium discoideum cells. Mutant analysis demonstrates that the produced cGMP plays an essential role in chemotactic signal transduction, controlling the actomyosin-dependent motive force. Guanylyl cyclase activity is associated with the particulate fraction of a cell homogenate. The addition of the cytosol stimulates guanylyl cyclase activity, whereas the cytosol plus ATP/Mg2+ inhibits enzyme activity. We have analyzed the regulation of guanylyl cyclase in chemotactic mutants and present evidence that a cGMP-binding protein mediates both stimulation and ATP-dependent inhibition of guanylyl cyclase. Upon chromatography of cytosolic proteins, cGMP binding activity co-elutes with both guanylyl cyclase-stimulating and ATP-dependent-inhibiting activities. In addition, ATP-dependent inhibition of guanylyl cyclase activity is enhanced by the cGMP analogue 8-Br-cGMP, suggesting that a cGMP-binding protein regulates guanylyl cyclase activity. Mutant KI-4 has an aberrant cGMP binding activity with very low Kd and shows a very small chemoattractant-mediated cGMP response; the cytosol from this mutant does not stimulate guanylyl cyclase. In contrast to KI-4, the aberrant cGMP binding activity of mutant KI-7 has a very high Kd and chemoattractants induce a prolonged cGMP response. The cytosol of this mutant stimulates guanylyl cyclase activity, but ATP does not inhibit the enzyme. Thus, two previously isolated chemotactic mutants are defective in the activation and inhibition of guanylyl cyclase, respectively. The positive and negative regulation of guanylyl cyclase by its product cGMP may well explain how cells process the temporospatial information of chemotactic signals, which is necessary for sensing the direction of the chemoattractant.

Guanylate cyclase activity in permeabilized Dictyostelium discoideum cells

Dictyostelium discoideum cells respond to chemoattractants by transient activation of guanylate cyclase. Cyclic GMP is a second messenger that transduces the chemotactic signal. We used an electropermeabilized cell system to investigate the regulation of guanylate cyclase. Enzyme activity in permeabilized cells was dependent on the presence of a nonhydrolysable GTP analogue (e.g., GTP gamma S), which could not be replaced by GTP, GDP, or GMP. After the initiation of the guanylate cyclase reaction in permeabilized cells only a short burst of activity is observed, because the enzyme is inactivated with a t1/2 of about 15 s. We show that inactivation is not due to lack of substrate, resealing of the pores in the cell membrane, product inhibition by cGMP, or intrinsic instability of the enzyme. Physiological concentrations of Ca2+ ions inhibited the enzyme (half-maximal effect at 0.3 microM), whereas InsP3 had no effect. Once inactivated, the enzyme could only be reactivated after homogenization of the permeabilized cells and removal of the soluble cell fraction. This suggests that a soluble factor is involved in an autonomous process that inactivates guanylate cyclase and is triggered only after the enzyme is activated. The initial rate of guanylate cyclase activity in permeabilized cells is similar to that in intact, chemotactically activated cells. Moreover, the rate of inactivation of the enzyme in permeabilized cells and that due to adaptation in vivo are about equal. This suggests that the activation and inactivation of guanylate cyclase observed in this permeabilized cell system is related to that of chemotactic activation and adaptation in intact cells.

Transduction of the chemotactic cAMP signal across the plasma membrane of Dictyostelium cells

Aggregating Dictyostelium cells secrete cAMP during cell aggregation. cAMP induces two fast responses, the production of more cAMP (relay) and directed cell locomotion (chemotaxis). Extracellular cAMP binds to G-protein-coupled receptors leading to the activation of second messenger pathways, including the activation of adenylyl cyclase, guanylyl cyclase, phospholipase C and the opening of plasma membrane Ca2+ channels. Many genes encoding these sensory transduction proteins have been cloned and null mutants of nearly all components have been characterized in detail. Undoubtedly, activation of adenylyl cyclase is the most complex, involving G-proteins, a soluble protein called CRAC and components of the MAP kinase pathway. Null mutants in this pathway do not aggregate, but can exhibit chemotaxis and develop normally when supplied with exogenous cAMP. The pathways leading to the activation of phospholipase C were identified, but unexpectedly, deletion of the phospholipase C gene has no effect on chemotaxis and development, nor on intracellular Ins(1,4,5)P3 levels; the metabolism of this second messenger will be discussed in some detail. Activation of guanylyl cyclase is G-protein-dependent and essential for chemotaxis. Analysis of a collection of chemotactic mutants reveals that most mutants are defective in either the production or intracellular detection of cGMP, thereby placing this second messenger at the center of chemotactic signal transduction. Analysis of the cAMP-mediated opening of plasma membrane calcium channels in signal transduction mutants suggests that it has two components, one that depends on G-proteins and intracellular cGMP and one that is G-protein-independent.

Endogenous ADP-ribosylation of glyceraldehyde-3-phosphate dehydrogenase that is not regulated by nitric oxide in Dictyostelium discoideum

A 41,000 M(r) cytosolic protein (p41) in Dictyostelium discoideum was shown to be modified by ADP-ribosylation that was not regulated by nitric oxide (NO). This endogenous ADP-ribosylation was optimal at conditions distinct from those optimal for the NO-stimulated ADP-ribosylation of p41. These two activities were also differentially sensitive to reducing agents and modified different amino acids. The addition of haemoglobin, which sequesters NO, and of NO synthase inhibitors failed to block the endogenous ADP-ribosylation. P41 was purified to homogeneity. The N-terminal sequence of the purified protein was shown to be highly homologous to glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Both endogenous and NO-stimulated activities ADP-ribosylated three isoforms of the protein, with pI values of 6.6, 6.8 and 7.0. In each case, the isoform with pI 6.8 was preferentially modified. Experiments using purified GAPDH indicate that both the endogenous and NO-stimulated ADP-ribosylation are self-catalysed modifications.

Cyclic AMP- and Ins(1,4,5)P3-induced Ca2+ fluxes in permeabilised cells of Dictyostelium discoideum: cGMP regulates Ca2+ entry across the plasma membrane

Transduction of chemotactic signals in Dictyostelium discoideum apparently involves a precise regulation of the cytosolic Ca2+ concentration. Cyclic AMP stimulation causes Ca2+ influx followed by Ca2+ extrusion, the magnitude of extrusion depending on the state of differentiation. Here, we show that the cAMP receptor controls Ca2+ influx both at the level of entry across the plasma membrane and at the level of transport into Ca2+-sequestering organelles. The use of permeabilised cells allowed us to discriminate between both fluxes. Permeabilised cells still showed the cAMP-induced Ca2+ uptake. The flux across the plasma membrane was more sensitive to Ba2+ and Mn2+, respectively, than Ca2+ sequestration. We have shown previously, using stmF mutants, that cGMP regulates Ca2+ influx. We confirmed this result with the membrane-permeant cGMP-analogue, Sp-8-Br-cGMPS, which enhanced the cAMP-induced Ca2+ influx in intact cells but not the uptake in permeabilised cells, indicating that cGMP regulates Ca2+ influx across the plasma membrane. Occasionally, a fast transient Ca2+ efflux, preceding the influx, occurred in intact cells. A small cAMP-induced Ca2+ release was also found in permeabilised cells. A similarly sized Ca2+ release was elicited by Ins(1,4,5)P3 and could be substituted for by GTP or GTPgammaS. This result suggests that rapid Ca2+ release can be mediated by Ins(1,4,5)P3.

Intercellular guanosine-5'-0-(3-thiotriphosphate) blocks chemotactic motility of Dictyostelium discoideum amoebae

Starving amoebae of the cellular slime mold Dictyostelium discoideum react chemotactically towards the attractant cAMP. In this study, the effect of nonhydrolyzable analogs of GTP and GDP on the chemotactic behavior was analyzed with light microscopic techniques. Guanosine-5'-0-(2-thiotriphosphate) (GTP gamma S) or guanosine-5'-0-(2-thiodiphosphate) (GDP beta S) was scrape-loaded into the cytoplasm of cells, together with a fluorescent marker. Stimulation with a cAMP-filled glass capillary revealed a reduced capacity of loaded cells to migrate towards the capillary tip. Most cells still protruded filopods in the direction of the capillary tip, but full extension of pseudopods was inhibited in a dose-dependent and reversible manner. This indicates that in the presence of the analogs, chemotactic sensing still occurs, and that a more distal step of the cascade of events leading to the formation of the pseudopod is impaired. In cells loaded with the analogs together with the calcium indicator fura-2, stimulation with 10 microM cAMP led to a transient change in the intracellular free calcium concentration ([Ca2+]i), which was detectable in 28% of the cells. Furthermore, large vacuoles were found containing high amounts of calcium. On the other hand, clamping of [Ca2+]i at low levels with 1,2-bis(2-aminophenoxy) ethane N,N,N',N'-tetraacetic acid (BAPTA) also inhibited motility, with neither filopods nor pseudopods formed. The data suggest that chemotactic migratory activity involves GTP-dependent processes that participate in the regulation of the Ca2+ homeostasis of the cell and in the regulation of membrane traffic that contributes to the directed locomotion.

Lithium, an inhibitor of cAMP-induced inositol 1,4,5-trisphosphate accumulation in Dictyostelium discoideum, inhibits activation of guanine-nucleotide-binding regulatory proteins, reduces activation of adenylylcyclase, but potentiates activation of guanylyl cyclase by cAMP

Li+ drastically alters pattern formation in Dictyostelium by inhibiting cAMP-induced prespore-gene expression and promoting cAMP-induced prestalk-gene expression. We reported previously that Li+ inhibits inositol monophosphatases in this organism and strongly reduces basal and cAMP-stimulated inositol 1,4,5-trisphosphate levels. We show here that Li+ also reduces cAMP-induced accumulation of cAMP, but promotes cAMP-induced accumulation of cGMP. This effect is not due to inhibition of cGMP hydrolysis or inhibition of adaptation and may therefore reflect stimulation of guanylyl-cyclase activation. Li+ does not affect the binding of cAMP to surface receptors but interferes with the interaction between receptors and guanine-nucleotide-binding regulatory (G) proteins. These effects are complex; in the absence of Mg2+, Li+ increases guanosine 5'-[gamma-thio]triphosphate(GTP[S])-binding activity to similar levels as 1 mM Mg2+. However, while Mg2+ potentiates cAMP-induced stimulation of GTP[S]-binding activity, Li+ effectively inhibits stimulation. Li+ also inhibits cAMP-stimulated, but not basal high-affinity GTP-ase activity, indicating an inhibitory effect on cAMP-induced activation of G-proteins. Our data suggest that in addition to inositolphosphate metabolism, the activation of G-proteins may be a second biochemical target for Li+ effects on pattern formation and signal transduction in Dictyostelium.

Inhibition of receptor-stimulated guanylyl cyclase by intracellular calcium ions in Dictyostelium cells

In Dictyostelium discoideum extracellular cAMP stimulates guanylyl cyclase and phospholipase C; the latter enzyme produces Ins(1,4,5)P3 which releases Ca2+ from internal stores. The following data indicate that intracellular Ca2+ ions inhibit guanylyl cyclase activity. 1) In vitro, Ca2+ inhibits guanylyl cyclase with IC50 = 41 nM Ca2+ and Hill-coefficient of 2.1. 2) Extracellular Ca2+ does not affect basal cGMP levels of intact cells. In electro-permeabilized cells, however, cGMP levels are reduced by 85% within 45 s after addition of 10(-6) M Ca2+ to the medium; halfmaximal reduction occurs at 200 nM extracellular Ca2+. 3) Receptor-stimulated activation of guanylyl cyclase in electro-permeabilized cells is also inhibited by extracellular Ca2+ with half-maximal effect at 200 nM Ca2+. 4) In several mutants an inverse correlation exists between receptor-stimulated Ins(1,4,5)P3 production and cGMP formation. We conclude that receptor-stimulated cytosolic Ca2+ elevation is a negative regulator of receptor-stimulated guanylyl cyclase.

A soluble factor and GTP gamma S are required for Dictyostelium discoideum guanylate cyclase activity

Amoeba of Dictyostelium discoideum show a rapid, transient cGMP synthesis in response to chemotactic stimulation. Using Mg(2+)-GTP as a substrate, guanylate cyclase (E.C. 4.6.1.2.) activity is found exclusively in the particulate fraction of Dictyostelium cells. Here we show that the activity is dependent on the presence of the non-hydrolysable GTP-analogue GTP gamma S, which itself is only a poor substrate for the enzyme under the prevailing conditions. Evidence is presented that a transient exposure of the enzyme to GTP gamma S is sufficient to constitutively activate the enzyme. GTP gamma S-dependent activity is found to require a factor that can be separated from the enzyme by washing the particulate fraction with low salt buffer. Addition of the soluble cell fraction to these washed membranes restores enzyme activity.

Endothelium-dependent and -independent effects of exogenous ATP, adenosine, GTP and guanosine on vascular tone and cyclic nucleotide accumulation of rat mesenteric artery

1. The effects of exogenous guanosine 5'-triphosphate (GTP) and guanosine on vascular tone and cyclic nucleotide accumulation of noradrenaline-precontracted endothelium-intact and endothelium-denuded rat mesenteric artery rings were compared with the effects of the known purinoceptor agonists adenosine 5'-triphosphate (ATP) and adenosine. 2. GTP (10 microM-1 mM) dose-dependently relaxed endothelium-intact mesenteric artery rings by producing a rapid initial response followed by sustained relaxation resembling the relaxant response to acetylcholine. GTP also slightly relaxed endothelium-denuded artery rings. The acetylcholine- and GTP-induced relaxations of endothelium-intact rings were attenuated by NG-nitro L-arginine methyl ester (L-NAME, 330 microM) which attenuation was reversed with L-arginine (1 mM). 3. Guanosine (10 microM-1 mM) relaxed both endothelium-intact and -denuded artery rings in a dose-dependent manner. The relaxations were more pronounced in endothelium-intact preparations and were only slightly attenuated by L-NAME (330 microM). 4. ATP (1 microM-1 mM) and adenosine (10 microM-1 mM) dose-dependently relaxed endothelium-intact and -denuded artery rings. The responses were more pronounced in endothelium-intact vascular preparations. 5. GTP (100 microM) and guanosine (100 microM) increased guanosine 3':5'-cyclic monophosphate (cyclic GMP) accumulation in both endothelium-intact and -denuded artery rings corresponding to the relaxations observed. The concentrations of adenosine 3':5'-cyclic monophosphate (cyclic AMP) were not affected. 6. ATP (100 microM) increased cyclic GMP concentration of endothelium-intact artery rings. The concentrations of cyclic AMP were not affected by ATP (100 microM) and adenosine (100 microM) in endothelium-intact and -denuded vascular preparations.7. These results provide evidence that exogenous GTP and guanosine relax precontracted endothelium-intact and -denuded rat mesenteric artery rings by increasing cyclic GMP accumulation. The response to GTP of endothelium-intact rings can mainly be explained by the release of endothelium-derived relaxing factor (EDRF), but that of guanosine is only partly due to EDRF, and is a combination of endothelium-dependent and -independent effects. The endothelium-independent response of GTP and guanosine is a direct, unknown effect on smooth muscle and guanylate cyclase.

Ca2+/calmodulin-binding proteins in Dictyostelium discoideum

We have initiated a systematic study of Ca2+/calmodulin-regulated enzymes in the cellular slime mold Dictyostelium discoideum. Using 125I-labelled D. discoideum calmodulin (CaM) as a functional probe, several Ca2+/CaM-binding proteins were detected in crude cell lysates. Proteins with apparent molecular weights of 22 kDa and 78-80 kDa, respectively, were found in the soluble fraction. In addition, membrane-bound high molecular weight CaM-binding proteins were identified. Binding of CaM to all of the proteins required the presence of Ca2+ ions and competed efficiently with nonradioactive CaM from both Dictyostelium and bovine brain. The CaM antagonists melittin, W-7 and R24571 inhibited CaM binding. With a functional cloning approach, we previously obtained cDNA clones by screening a lambda gt11 lysogen expression library; in this paper, we report the analysis of CaM-binding activity by one of the recombinant cDNA clones in Escherichia coli. When rabbit antiserum was raised against it, the antiserum recognized a 78-80-kDa protein in Dictyostelium extracts which comigrated on SDS-polyacrylamide gels with 78-80-kDa CaM-binding activity.

Involvement of cyclic AMP cell surface receptors and G-proteins in signal transduction during slug migration of Dictyostelium discoideum

The presence of G-proteins, interacting with cAMP surface receptors, was investigated in vegetative cells, aggregation-competent cells, and migrating slugs of Dictyostelium discoideum. Our results indicate that G-proteins are present in all stages. In vegetative cells there is a limited number of cAMP receptors but no effect of GTP tau S on cAMP binding could be detected; in addition, no effect of cAMP on GTP tau S binding or GTPase activity was observed. In both aggregation-competent cells and slugs GTP tau S inhibits cAMP binding, while cAMP stimulates GTP tau S binding and high-affinity GTPase. Since the presence of G-proteins coupled to cAMP receptors could be demonstrated in slugs, the involvement of the effector enzymes adenylate cyclase and phospholipase C was investigated. The results show that adenylate cyclase activity is stimulated by GTP tau S in both stages and that in cells from migrating slugs the Ins(1,4,5)P3 production is increased upon stimulation with cAMP. The possible involvement of G-proteins in signal transduction during the slug stage of D. discoideum is discussed.

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.

Exogenous GTP enhances the effects of sodium nitrite on cyclic GMP accumulation, vascular smooth muscle relaxation and platelet aggregation

Exogenous guanosine triphosphate (GTP) (1-2 x 10(-4)M) resulted in increased concentrations of cyclic GMP both in endothelium denuded rat mesenteric artery (RMA) and in human ADP-stimulated platelets. Sodium nitrite (3.3 x 10(-4)M) relaxed precontracted RMA by 34%. When the arteries were preincubated with GTP (2 x 10(-4)M) sodium nitrite administration resulted in a significantly greater relaxation (58%) of the RMA with concomitant 2-fold increase in cGMP. Sodium nitrite (1 x 10(-4)M) had an inhibitory effect on ADP-induced platelet aggregation. Preincubation with GTP enhanced significantly the sodium nitrite-induced inhibition of ADP-induced platelet aggregation with a simultaneous 5-fold increase in cGMP. These results indicate that exogenous GTP enhances the sodium nitrite-induced stimulation of guanylate cyclase and thus enhances the effects of sodium nitrite on arterial smooth muscle and platelets.