Down-regulation of cell surface cyclic AMP receptors and desensitization of cyclic AMP-stimulated adenylate cyclase by cyclic AMP in Dictyostelium discoideum. Kinetics and concentration dependence

cAMP binds to Dictyostelium discoideum surface receptors and induces a transient activation of adenylatecyclase, which is followed by desensitization. cAMP also induces a loss of detectable surface receptors (down-regulation). Cells were incubated with constant cAMP concentrations, washed free of cAMP, and cAMP binding to surface receptors and cAMP-induced activation of adenylate cyclase were measured. cAMP could induce maximally 65% loss of binding activity and complete desensitization of cAMP-stimulated adenylate cyclase activity. Half-maximal effects for down-regulation were observed at 50 nM cAMP and for desensitization at 5 nM cAMP. Down-regulation was rapid with half-times of 4, 2.5, and 1 min at 0.1, 1, and 10 microM cAMP, respectively. Similar kinetic data have been reported for desensitization (Dinauer, M.C., Steck, T.L., and Devreotes, P.N. (1980) J. Cell Biol. 86, 554-561). Down-regulation and desensitization were not reversible at 0 degrees C. Down-regulation reversed slowly at 20 degrees C with a half-time of about 1 h. Resensitization of adenylate cyclase was biphasic showing half-times of 4 min and about 1 h, respectively; the contribution of the rapidly resensitizing component was diminished when down-regulation of receptors was enhanced. These results suggest that cAMP-induced down-regulation of receptors and desensitization of adenylate cyclase stimulation proceed by at least two steps. One step is rapidly reversible, occurs at low cAMP concentrations, and induces desensitization without down-regulation, while the second step is slowly reversible, requires higher cAMP concentrations, and also induces down-regulation.     

Ligand-induced modification of a surface cAMP receptor of Dictyostelium discoideum does not require its occupancy

In Dictyostelium discoideum amoebae, cAMP-induced phosphorylation of the surface cAMP receptor is associated with a discrete transition in its electrophoretic mobility. The native and modified forms of the receptor are designated R and D (Mr = 40,000 and 43,000). The relationship of the number of receptors which are modified as a function of the receptors which bind cAMP was investigated. Modification was assessed by determining the amounts of R and D forms in Western blots which detect all receptors whether or not they are exposed on the surface. Cyclic AMP or the analog, adenosine 3',5'-monophosphorothioate ((Rp)-cAMPS), induced a loss of cAMP-binding activity (down-regulation), which was not accompanied by a loss of the receptor protein. About 60% of the receptors do not bind cAMP in the absence of Ca2+ and are unmasked by 10 mM Ca2+. However, the fraction of receptors which are modified in response to cAMP is equal in the absence or presence of Ca2+. (Rp)-cAMPs induces down-regulation (50%) but not modification. Addition of cAMP, following down-regulation by (Rp)-cAMPS, causes all receptors to be modified. cAMP induces both down-regulation (80%) and modification. Modification is more readily reversed than down-regulation: 30 min after removal of cAMP, receptors remain down-regulated (57%) but are found in the R form. All receptors shift to the D form when cAMP is readded to the cells. These results indicate that exposed, as well as cryptic and down-regulated receptors, are modified in response to the cAMP stimulus.     

Reducing reagent-induced activation of adenylate cyclase in the cellular slime mold, Dictyostelium discoideum.

Binding of an intrinsic agonist (cAMP) to specific receptors on the cell surface induces transmembrane signals for activation and desensitization (adaptation and down regulation) of adenylate cyclase in the cellular slime mold, Dictyostelium discoideum. It is generally believed that dithiothreitol (DTT) induces the activation through interaction between the receptor and gradually accumulated cAMP, since DTT is known to inhibit cAMP-phosphodiesterase which degrades cAMP. In the present paper, we investigated the mechanism of activation of adenylate cyclase by the thiol-reducing agents, DTT and 2,3-dimercapto-1-propanol (BAL). We found that BAL activated adenylate cyclase transiently even under conditions where the intrinsic agonist supersaturated the cAMP-receptors and competitively inhibited phosphodiesterase. This result is inconsistent with the generally accepted notion. We conclude that BAL has an independent effect from those of the intrinsic agonist (cAMP) and phosphodiesterase in activation of adenylate cyclase. Since BAL could induce activation just after the activation induced by a supersaturating concentration of the intrinsic agonist had ceased, the independent effect of BAL is not a simple enhancement of the cAMP-induced activation. Our result also suggests that the cAMP-induced adaptation (but not down regulation) suppresses the BAL-induced activation while BAL itself does not induce adaptation to cAMP or BAL. We propose that the thiol-reducing reagent induces or modifies the transmembrane activation signal for adenylate cyclase.     

Adenosine and its derivatives inhibit the cAMP signaling response in Dictyostelium discoideum

In developmentally competent Dictyostelium discoideum amoebae, binding of cAMP to high-affinity surface receptors produces a rapid activation of adenylate cyclase which adapts within minutes. The result is a transient increase in intracellular cAMP which is rapidly secreted. Adenosine inhibited this cAMP signaling response with an apparent Ki of 300 microM. The apparent Ki's for 2'-O-methyladenosine and 2-chloroadenosine were approximately 30 and 100 microM, respectively. Inhibition by adenosine was rapid, reversible, and depended on the cAMP stimulus concentration. In addition, the adaptation of the cAMP signaling response was blocked by adenosine. As has been previously reported, adenosine inhibits cAMP binding to intact cells. Under the same developmental conditions as in the perfusion studies, we find the binding inhibition depends on both the cAMP and adenosine concentrations, with an apparent Ki of 100 microM. The apparent Ki's for 2'-O-methyl- and 2-chloroadenosine were approximately 8 and 35 microM, respectively. However, with cells developed for short times and with an axenic strain, inhibition was independent of cAMP concentration or cells showed mixed-type binding inhibition. The effect of adenosine on the cAMP signaling response is consistent with the reported effects of adenosine on other cAMP-mediated processes such as chemotaxis and the increase in intracellular cGMP, and may provide an explanation for the reported inhibition of center formation.     

Pertussis toxin inhibits CAMP-induced desensitization of adenylate cyclase in Dictyostelium discoideum

cAMP binds to surface receptors of Dictyostelium discoideum cells, transducing the signal to adenylate cyclase, guanylate cyclase and to chemotaxis. The activation of adenylate cyclase is maximal after 1 min and then declines to basal levels due to desensitization, which is composed of two components: a rapidly reversible adaptation process, and a slowly reversible down-regulation of cAMP receptors. Adaptation is correlated with receptor phosphorylation.The chemotactic response and the cAMP-induced cGMP response were not significantly altered in D. discoideum cells pretreated with pertussis toxin. The initial increase of cAMP levels was identical in control and toxin treated cells, suggesting that activation of adenylate cyclase was also not affected. However, cAMP synthesis continued in toxin treated cells, due to a strongly diminished desensitization. Pertussis toxin inhibited the adaptation of adenylate cyclase stimulation, but not the down-regulation or phosphorylation of the cAMP receptors. Adenylate cyclase in D. discoideum membranes can be stimulated or inhibited by GTP, depending on the conditions used. Pertussis toxin did not affect the stimulation of adenylate cyclase but nullified the inhibition. In membranes from desensitized control cells, stimulation of adenylate cyclase by GTP was lost, whereas inhibition was retained. Stimulation of adenylate cyclase in membranes from desensitized pertussis toxin treated cells was diminished but not absent. These results indicate that receptor phosphorylation is not sufficient for adaptation of adenylate cyclase, and that a pertussis toxin substrate, possibly Gi, is also involved in this process.Abbreviations used ATPS Adenosine 5-0-(3-Thiotriphosphate) - GTPS Guanosine 5-0-(3-thiotri-phosphate) - (Sp)-cAMPS Adenosine 3,5-monophosphorothioate-Sp-isomer - dcAMP 2-deoxyadenosine 3,5-monophosphate - Hepes N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid - DTT Dithiothreitol - buffer A 10 mM KH₂PO₄/Na2HPO₄, pH 6.5 - buffer B 40 mM Hepes/NaOH, 0.5 mM EDTA, 250 mM sucrose, pH 7.7     

The role of cAMP in nerve growth factor-promoted neurite outgrowth in PC12 cells

Nerve growth factor (NGF)-mediated neurite outgrowth in rat pheochromocytoma PC12 cells has been described to be synergistically potentiated by the simultaneous addition of dibutyryl cAMP. To elucidate further the role of cAMP in NGF-induced neurite outgrowth we have used the adenylate cyclase activator forskolin, cAMP, and a set of chemically modified cAMP analogues, including the adenosine cyclic 3',5'-phosphorothioates (cAMPS) (Rp)-cAMPS and (Sp)-cAMPS. These diastereomers have differential effects on the activation of cAMP-dependent protein kinases, i.e., (Sp)-cAMPS behaves as a cAMP agonist and (Rp)-cAMPS behaves as a cAMP antagonist. Our data show that the establishment of a neuritic network, as observed from PC12 cells treated with NGF alone, could not be induced by either forskolin, cAMP, or cAMP analogues alone. The presence of NGF in combination with forskolin or cAMP or its agonistic analogues potentiated the initiation of neurite outgrowth from PC12 cells. The (Sp)-cAMPS-induced stimulation of NGF-mediated process formation was successfully blocked by the (Rp)-cAMPS diastereomer. On the other hand, NGF-stimulated neurite outgrowth was not inhibited by the presence of the cAMP antagonist (Rp)-cAMPS. We conclude that the morphological differentiation of PC12 cells stimulated by NGF does not require cAMP as a second messenger. The constant increase of intracellular cAMP, caused by either forskolin or cAMP and the analogues, in combination with NGF, not only rapidly stimulated early neurite outgrowth but also exerted a maintaining effect on the neuronal network established by NGF.     

Viable mouse thymocytes as a model system for studying the onset of hormone-induced cellular refractoriness

Mouse thymocytes are characterized as a model cellular system for studying the onset of hormone-induced cellular refractoriness (desensitization). This system has the following combination of useful features. (a) The cells can be isolated without the use of digestive enzymes, avoiding possible damage to surface receptors or to other exposed membranal constituents. (b) They can be kept viable for several hours, a period during which both stimulation and desensitization get well under way. (c) They can be stimulated by a variety of hormones which function via cAMP (β-agonists, prostaglandin E₁ and specific thymic humoral factors). (d) Their desensitization is receptor-specific. (e) They can be readily ruptured under mild conditions so as to allow a physiologically relevant biochemical analysis of hormonal stimulation and desensitization. (f) The hormonal response of these cells can be monitored simultaneously by the activation of adenylate cyclase, by the intracellular level of cAMP, and by the activation of cAMP-dependent protein kinase (which functions as a metabolic sensor for cAMP). In this cellular system, desensitization does not involve processes such as the efflux of cAMP, the activation of cAMP-phosphodiesterase or the synthesis of a protein mediator. On the other hand, desensitization can be accounted for by a hormone-triggered inactivation of the adenylate cyclase system. The immediate desensitization of thymocytes is reversible and occurs without apparent loss of functional receptors. Continuous presence of hormone is shown to be required not only for triggering the chain of events which leads to the readily reversible desensitization, but also for the process which transfers the cells to the subsequent, ‘locked’ desensitized state.     

Identifying the molecular switches that determine whether (Rp)-cAMPS functions as an antagonist or an agonist in the activation of cAMP-dependent protein kinase I.

Previous investigations revealed that under physiological conditions in the presence of MgATP the phosphorothioate analogue of cAMP, (Rp)-cAMPS, is a competitive inhibitor and antagonist for cAMP for cAMP-dependent protein kinases I and II [DeWit et al., (1984) Eur. J. Biochem. 142, 255-260]. For the type I holoenzyme, the antagonist properties of (Rp)-cAMPS are shown here to be absolutely dependent on MgATP. In the absence of MgATP, (Rp)-cAMPS serves as a weak agonist with a Ka of 7.9 microM. The high-affinity binding of MgATP imposes a barrier on cAMP-induced activation of the homoenzyme--a barrier that both cAMP and (Sp)-cAMPS, but not (Rp)-cAMPS, can overcome. In the absence of MgATP, this barrier no longer exists, and (Rp)-cAMPS functions as an agonist. The holoenzyme also was formed with mutant regulatory subunits. Replacing the essential arginine, predicted to bind the exocyclic oxygens of cAMP, in site A with lysine abolishes high-affinity binding of cAMP to site A. The holoenzyme formed with this mutant R-subunit is activated by (Rp)-cAMPS in both the presence and absence of MgATP. These results suggest that the stereospecific requirements for holoenzyme activation involve this guanidinium side chain. Mutations that eliminate the high-affinity binding of MgATP, such as the introduction of an autophosphorylation site in the autoinhibitory domain, also generate a holoenzyme that can be activated by (Rp)-cAMPS. In the case of the type II holoenzyme, (Rp)-cAMPS is an antagonist in both the presence and absence of MgATP, emphasizing distinct roles for MgATP in these two forms of cAMP-dependent protein kinase.     

Enzymatic synthesis of the cAMP antagonist (Rp)-adenosine 3',5'-monophosphorothioate on a preparative scale

(Rp)-Adenosine 3',5'-monophosphorothioate ((Rp)-cAMPS) is a highly specific antagonist of the cAMP-dependent protein kinase from eukaryotic cells and is a very poor substrate for phosphodiesterases. It is therefore a useful tool for investigating the role of cAMP as a second messenger in a variety of biological systems. Taking advantage of stereospecific inversion of configuration around the alpha-phosphate during the adenylate cyclase reaction, we have developed a method for the preparative enzymatic synthesis of the Rp diastereomer of adenosine 3',5'-monophosphorothioate ((Rp)-cAMPS) from the Sp diastereomer of adenosine 5'-O-(1-thiotriphosphate) ((Sp)-ATP alpha S). The adenylate cyclase from Bordetella pertussis, partially purified by calmodulin affinity chromatography, cyclizes (Sp)-ATP alpha S approximately 40-fold more slowly than ATP, but binds (Sp)-ATP alpha S with about 10-fold higher affinity than ATP. The triethylammonium salt of the reaction product can be purified by elution from a gravity flow reversed-phase C18 column with a linear gradient of increasing concentrations of methanol. Yields of the pure (Rp)-cAMPS product of a synthesis with 2 mg of substrate are about 75%.     

Surface receptor-mediated activation of adenylate cyclase in Dictyostelium. Regulation by guanine nucleotides in wild-type cells and aggregation deficient mutants

GTP and GTP analogs produced significant (up to 17-fold) and persistent activation of adenylate cyclase in lysates of Dictyostelium discoideum amoeba. The activation was enhanced 2- to 4-fold by cAMP (the agonist for receptor-mediated adenylate cyclase activation), was specific for guanine nucleoside triphosphates, and was inhibited by guanosine 5'-(O-2-thio)diphosphate. The order of potency of guanine nucleotides was guanosine 5'-(O-3-thio)triphosphate greater than guanyl-5'-yl imidodiphosphate greater than GTP; half-maximal activation was observed with 1-10 microM guanine nucleotide. Maximal activation occurred when the guanine nucleotide was added within seconds after cell lysis and the lysate was preincubated for 5 min prior to assay. Under these optimal in vitro conditions, the capacity of guanine nucleotides to activate decreased, closely correlating with adaptation or desensitization induced by exposure of intact cells to cAMP during a period of 10 min. These data strongly support that regulation of adenylate cyclase in Dictyostelium occurs via a receptor-linked GTP/GDP exchange protein. Two mutants, designated synag 7 and 49 were isolated in which cAMP and/or guanine nucleotides were not sufficient to activate adenylate cyclase. The wild-type pattern of guanine nucleotide regulation was restored to synag 7 lysates by the addition of a high-speed supernatant from wild-type cells. Characterization of these mutants demonstrates that activation of adenylate cyclase is not required for growth or cell-type specific differentiation but is essential for cellular aggregation and influences morphogenesis and pattern formation. This suggests that Dictyostelium may provide a model suitable for detailed genetic analysis of surface receptor-guanine nucleotide-binding regulatory protein linked adenylate cyclase systems and for determining the role of these systems in development.     

The specificity of the cAMP receptor mediating activation of adenylate cyclase in Dictyostelium discoideum

In Dictyostelium discoideum amoebae, binding of cyclic AMP (cAMP) to surface receptors elicits numerous responses including chemotaxis, cyclic GMP (cGMP) accumulation, and activation of adenylate cyclase. The specificity of the surface cAMP receptor which mediates activation of adenylate cyclase and cAMP secretion was determined by testing the relative effectiveness of a series of 10 cAMP analogs. Each of the 10 analogs elicited cAMP secretion, chemotaxis, and cGMP accumulation in the same dose range. The order of potency for eliciting these responses (cAMP greater than 2'-H-cAMP greater than N1-O-cAMP greater than cAMPS(Sp) greater than 6-Cl-cAMP greater than cAMPN(CH3)2(Sp) greater than 3'-NH-cAMP greater than 8-Br-cAMP greater than cAMPS(Rp) greater than cAMPN(CH3)2(Rp] matches that for binding to the major cell surface cAMP binding sites and differs from that of the cell surface phosphodiesterase and the major intracellular cAMP binding protein.     

Regulation of adenylate cyclase in electropermeabilized Dictyostelium discoideum cells.

In Dictyostelium discoideum cells the enzyme adenylate cyclase is functionally coupled to cell surface receptors for cAMP. Coupling is known to involve one or more G-proteins. Receptor-mediated activation of adenylate cyclase is subject to adaptation. In this study we employ an electropermeabilized cell system to investigate regulation of D. discoideum adenylate cyclase. Conditions for selective permeabilization of the plasma membrane have been described by C.D. Schoen, J. C. Arents, T. Bruin, and R. Van Driel (1989, Exp. Cell Res. 181, 51-62). Only small pores are created in the membrane, allowing exchange of exclusively low molecular weight substances like nucleotides, and preventing the loss of macromolecules. Under these conditions functional protein-protein interactions are likely to remain intact. Adenylate cyclase in permeabilized cells was activated by the cAMP receptor agonist 2'-deoxy cAMP and by the nonhydrolyzable GTP-analogue GTP gamma S, which activates G-proteins. The time course of the adenylate cyclase reaction in permeabilized cells was similar to that of intact cells. Maximal adenylate cyclase activity was observed if cAMP receptor agonist or GTP-analogue was added just before cell permeabilization. If these activators were added after permeabilization adenylate cyclase was stimulated in a suboptimal way. The sensitivity of adenylate cyclase activity for receptor occupation was found to decay more rapidly than that for G-protein activation. Importantly, the adenylate cyclase reaction in permeabilized cells was subject to an adaptation-like process that was characterized by a time course similar to adaptation in vivo. In vitro adaptation was not affected by cAMP receptor agonists or by G-protein activation. Evidently electropermeabilized cells constitute an excellent system for investigating the positive and negative regulation of D. discoideum adenylate cyclase.     

Inhibition of choriogonadotropin-activated steroidogenesis in cultured Leydig tumor cells by the Rp diastereoisomer of adenosine 3',5'-cyclic phosphorothioate

The diastereoisomers of adenosine 3',5'-cyclic phosphorothioate, (Sp)-cAMPS and (Rp)-cAMPS, have been previously shown to act as agonists and antagonists, respectively, in the activation of several mammalian cAMP-dependent protein kinases. In an effort to characterize further the involvement of cAMP in the activation of Leydig cell steroidogenesis by lutropin/choriogonadotropin (LH/CG), we examined the effects of these cyclic nucleotide analogues on a clonal strain of cultured murine Leydig tumor cells (designated MA-10). Our results show that (i) (Sp)-cAMPS activates and (Rp)-cAMPS inhibits the isolated cAMP-dependent protein kinase of the MA-10 cells; (ii) both analogues inhibit the isolated cAMP phosphodiesterase(s); (iii) (Sp)-cAMPS activates steroid biosynthesis in intact cells, but (Rp)-cAMPS does not; and (iv) (Rp)-cAMPS is a competitive inhibitor of the activation of steroidogenesis by (Sp)-cAMPS, 8-bromo-cAMP, human CG, cholera toxin, and forskolin. However, (Rp)-cAMPS is a more effective inhibitor when steroidogenesis is activated by (Sp)-cAMPS or 8-bromo-cAMP than when it is activated by human CG, cholera toxin, or forskolin. This difference appears to be related to the combined effects of (Rp)-cAMPS on the cAMP-dependent protein kinases and cAMP phosphodiesterase(s). We conclude that cAMP is a quantitatively important mediator of the activation of steroidogenesis by LH/CG even at low concentrations of hormone where an increase in steroid biosynthesis cannot be easily correlated with increased cAMP accumulation. Thus, our data indicate that if other second messengers are involved in the activation of steroidogenesis by LH/CG, they must do so by acting together with, rather than independently of, cAMP.     

Catecholamine-induced desensitization in turkey erythrocytes: cAMP mediated impairment of high affinity agonist binding without alteration in receptor number

Desensitization of turkey erythrocyte adenylate cyclase by exposure of these cells to the beta-adrenergic agonist isoproterenol leads to a decrease in subsequent adenylate cyclase stimulation by isoproterenol, F-, or Gpp(NH)p without any apparent loss or down regulation of receptors (B.B. Hoffman et al. J. Cyclic Nucl. Res. 5: 363-366, 1979). We now report that the desensitization is associated with a functional "uncoupling" of the beta-adrenergic receptor. This is evidenced by an impaired ability of receptors to form a high affinity, guanine nucleotide sensitive complex with agonist as assessed by computer analysis of radioligand binding data. The changes in adenylate cyclase responsiveness as well as the alterations in receptor affinity for agonists are reproduced by incubation of turkey erythrocytes with the cAMP analog 8-Bromo-adenosine 3':5'- cyclic monophosphate. These findings suggest that one possible mechanism for the development of desensitization in adenylate cyclase systems may be a cAMP mediated alteration of a component(s) of the beta-adrenergic receptor-adenylate cyclase complex which results in impaired receptor-cyclase coupling.     

Ring-deleted analogs of atrial natriuretic factor inhibit adenylate cyclase/cAMP system. Possible coupling of clearance atrial natriuretic factor receptors to adenylate cyclase/cAMP signal transduction system

We have recently shown that atrial natriuretic factor (ANF) inhibits adenylate cyclase activity in rat platelets where only one population of ANF receptors (ANF-R2) is present, indicating that ANF-R2 receptors may be coupled to the adenylate cyclase/cAMP system. In the present studies, we have used ring-deleted peptides which have been reported to interact with ANF-R2 receptors also called clearance receptors (C-ANF) without affecting the guanylate cyclase/cGMP system, to examine if these peptides can also inhibit the adenylate cyclase/cAMP system. Ring-deleted analog C-ANF4-23 like ANF99-126 inhibited the adenylate cyclase activity in a concentration-dependent manner in rat aorta, brain striatum, anterior pituitary, and adrenal cortical membranes. The maximal inhibition was about 50-60% with an apparent Ki between 0.1 and 1 nM. In addition, C-ANF4-23 also decreased the cAMP levels in vascular smooth muscle cells in a concentration-dependent manner without affecting the cGMP levels. The maximal decrease observed was about 60% with an apparent Ki of about 1 nM. Furthermore, C-ANF4-23 was also able to inhibit cAMP levels and progesterone secretion stimulated by luteinizing hormone in MA-10 cell line. Other smaller fragments of ANF with ring deletions were also able to inhibit the adenylate cyclase activity as well as cAMP levels. Furthermore, the stimulatory effects of various agonists such as 5'-(N-ethyl)carboxamidoadenosine, dopamine, and forskolin on adenylate cyclase activity and cAMP levels were also significantly inhibited by C-ANF4-23. The inhibitory effect of C-ANF4-23 on adenylate cyclase was dependent on the presence of GTP and was attenuated by pertussis toxin treatment. These results indicate that ANF-R2 receptors or so-called C-ANF receptors are coupled to the adenylate cyclase/cAMP signal transduction system through inhibitory guanine nucleotide regulatory protein.     

Opioid agonists binding and responses in SH-SY5Y cells.

SH-SY5Y (human neuroblastoma) cultured cells, known to have mu-opioid receptors, have been used to assess and compare the ability of eight representative mu-selective compounds from diverse opioid families to recognize and activate these receptors. A wide range of receptor affinities spanning a factor of 10,000 was found between the highest affinity fentanyl analogs (Ki = 0.1nM) and the lowest affinity analog, meperidine (Ki = 1 microM). A similar range was found for inhibition of PGE1-stimulated cAMP accumulation with a rank order of activities that closely paralleled binding affinities. Maximum inhibition of cAMP accumulation by each compound was about 80%. Maximum stimulation of GTPase activity (approximately 50%) was also similar for all compounds except the lowest affinity meperidine. Both effects were naloxone reversible. These results provide further evidence that mu-receptors are coupled to inhibition of adenylate cyclase and that the SH-SY5Y cell line is a good system for assessment of mu-agonists functional responses.     

Cyclic 3'', 5''-AMP relay in Dictyostelium discoideum: adaptation is independent of activation of adenylate cyclase

In Dictyostelium discoideum, binding of cAMP to high affinity surface receptors leads to a rapid activation of adenylate cyclase followed by subsequent adaptation within several minutes. The rate of secretion of [ 3H ]cAMP, which reflects the state of activation of the enzyme, was measured. Caffeine noncompetitively inhibited the response to cAMP. Inhibition was rapidly reversible and pretreatment of cells with caffeine for up to 22 min had little effect on the subsequent responsiveness to cAMP. However, cells pretreated with caffeine plus cAMP for greater than or equal to 8 min did not respond when caffeine was removed and the same concentration of cAMP was applied. The following observations indicate that both adaptation and deadaptation to cAMP occurred to the same extent and at the same rate whether or not cAMP synthesis was inhibited. First, when cells were pretreated with 10(-9)-10(-6) M cAMP in the presence or absence of caffeine and the stimulus was switched to a saturating dose of cAMP, the response to the increment was the same whether or not the initial response was blocked. Second, cells progressively lost responsiveness to 10(-6) M cAMP as pretreatment with 10(-6) M cAMP plus caffeine was extended from 0 to 8 min with the same time course as for those pretreated with 10(-6) M cAMP alone. Third, cells which were adapted in the presence of caffeine and cAMP deadapted within the same time period as controls when cAMP was removed. These observations demonstrate that while some part of the activation process is inhibited by caffeine the adaptation process is unaffected. Our conclusion is that adaptation does not depend on the activation of adenylate cyclase.     

Biphasic Ca2+ response of adenylate cyclase. The role of calmodulin in its activation by Ca2+ ions

The Ca2+-dependent regulation of human platelet membrane adenylate cyclase has been studied. This enzyme exhibited a biphasic response to Ca2+ within a narrow range of Ca2+ concentrations (0.1-1.0 microM). At low Ca2+ (0.08-0.3 microM) adenylate cyclase was stimulated (Ka = 0.10 microM), whereas at higher Ca2+ (greater than 0.3 microM) the enzyme was inhibited to 70-80% control (Ki = 0.8 microM). Membrane fractions, prepared by washing in the presence of LaCl3 to remove endogenous calmodulin (approximately equal to 70-80% depletion), exhibited no stimulation of adenylate cyclase by Ca2+ but did show the inhibitory phase (Ki = 0.4 microM). The activation phase could be restored to La3+-washed membranes by addition of calmodulin (Ka = 3.0 nM). Under these conditions it was apparent that calmodulin reduced the sensitivity of adenylate cyclase to Ca2+ (Ki = 0.8 microM). Prostaglandin E1 (PGE1) did not alter Ki or Ka values for Ca2+. Calmodulin did not alter the EC50 for PGE1 stimulation of adenylate cyclase but increased the Vmax (1.5-fold). The calmodulin antagonist trifluoperazine potently inhibited adenylate cyclase in native membranes (80%) and to a much lesser extent in La3+-washed membranes (15%). This inhibition was due to interaction of trifluoperazine with endogenous calmodulin since trifluoperazine competitively antagonized the stimulatory effect of calmodulin on adenylate cyclase in La3+-washed membranes. We propose that biphasic Ca2+ regulation of platelet adenylate cyclase functions to both dampen (low Ca2+) and facilitate (high Ca2+) the haemostatic function of platelets.     

Signal transduction in Dictyostelium fgd A mutants with a defective interaction between surface cAMP receptors and a GTP-binding regulatory protein [published erratum appears in J Cell Biol 1988 Dec;107(6 Pt 1):following 2463]

Transmembrane signal transduction was investigated in four Dictyostelium discoideum mutants that belong to the fgd A complementation group. The results show the following. (a) Cell surface cAMP receptors are present in fgd A mutants, but cAMP does not induce any of the intracellular responses, including the activation of adenylate or guanylate cyclase and chemotaxis. (b) cAMP induces down-regulation and the covalent modification (presumably phosphorylation) of the cAMP receptor. (c) The inhibitory effects of GTP gamma S and GDP beta S on cAMP binding are reduced; the stimulatory effect of cAMP on GTP gamma S binding is lost in fgd A mutants. (d) Basal high-affinity GTPase activity is reduced 40% and the stimulatory effect of cAMP is decreased from 40% in wild type to 30% in fgd A. (e) GTP-mediated stimulation and inhibition of adenylate cyclase is normal in mutant membranes. The results suggest a defective interaction between cell surface cAMP receptors and a specific G-protein in fgd A mutants. This interaction appears to be essential for nearly all signal transduction pathways in Dictyostelium discoideum.     

Characterization of myometrial desensitization to beta-adrenergic agonists

Continuous exposure of ovine myometrial strips exposed to isoproterenol (10 microM) resulted in only transient inhibition with contractions returning within 60 min. Rechallenging these strips with isoproterenol failed to induce inhibition, confirming the occurrence of desensitization. In contrast, exposure of myometrial tissue to isoproterenol for only 5 min did not result in desensitization. Myometrial strips exposed to isoproterenol demonstrated a significant increase in cAMP content associated with inhibition of contractile activity and a subsequent fall in cAMP content upon desensitization. Elevation of endogenous cAMP levels by either inhibition of cAMP-dependent phosphodiesterase activity (0.5 mM isobutylmethylxanthine, in ovine strips) or direct activation of adenylyl cyclase (10 microM forskolin, in rat strips) induced a rapid and significant inhibition of myometrial contractile activity in desensitized tissue. Scatchard analysis of the binding of the beta-adrenoceptor antagonist, [125I]iodocyanopindolol, revealed a significant reduction in the concentration of beta-adrenergic receptors (but no change in binding affinity) in desensitized myometrial tissue. Incubation of desensitized tissue with fresh buffer for 3 h induced only a partial recovery in responsiveness to isoproterenol. These data suggest that prolonged, but not acute, exposure of the myometrium to beta-adrenergic agonists induces a state of desensitization that is associated with a down-regulation of beta-adrenoceptors but maintenance of postreceptor function.