Forskolin: unique diterpene activator of adenylate cyclase in membranes and in intact cells

The diterpene, forskolin [half-maximal effective concentration (EC50), 5-10 microM] activates adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] in rat cerebral cortical membranes in a rapid and reversible manner. Activation is not dependent on exogenous guanyl nucleotides and is not inhibited by guanosine 5'-O-(2-thiodiphosphate) when assayed with adenosine 5'-[beta, gamma-imido]triphosphate as substrate. GTP and GDP potentiate responses to forskolin. The activations of adenylate cyclase by forskolin and guanosine 5'-[beta, gamma-imido]triphosphate p[NH]ppG are not additive, whereas activations by forskolin and fluoride are additive or partially additive. The responses of adenylate cyclase to forskolin or fluoride are not inhibited by manganese ions, whereas the response to p[NH]ppG is completely blocked. Activation of adenylate cyclase by forskolin is considerably greater than the activation by fluoride in membranes from rat cerebellum, striatum, heart, and liver, while being about equal or less than the activation by fluoride in other tissues. Forskolin (EC50, 25 microM) causes a rapid and readily reversible 35-fold elevation of cyclic AMP in rat cerebral cortical slices that is not blocked by a variety of neurotransmitter antagonists. Low concentrations of forskolin (1 microM) augment the response of cyclic AMP-generating systems in brain slices to norepinephrine, isoproterenol, histamine, adenosine, prostaglandin E2, and vasoactive intestinal peptide. Forskolin would appear to activate adenylate cyclase through a unique mechanism involving both direct activation of the enzyme and facilitation or potentiation of the modulation of enzyme activity by receptors or the guanyl nucleotide-binding subunit, or both.

Dopamine D4 receptors elevated in schizophrenia

Although the biological basis of schizophrenia is not known, possible causes include genetic defects, viruses, amines, brain structure and metabolism, neuroreceptors, and G proteins. The hypothesis of dopamine overactivity in schizophrenia is based on the fact that neuroleptics block dopamine D2 receptors in direct relation to their clinical antipsychotic potencies. Moreover, dopamine D2 or D2-like receptors are elevated in postmortem schizophrenia brain tissue. This elevation, however, is only found in vivo using [11C]methylspiperone but not [11C]raclopride. The dopamine D4 receptor gene has not yet been excluded in schizophrenia because the 21 gene variants of D4 have not yet been tested. Because the link between D1 and D2 receptors is reduced in schizophrenia tissue, we tested whether one component of this link was sensitive to guanine nucleotide. We report here that the binding of [3H]raclopride to D2 receptors in schizophrenia was not sensitive to guanine nucleotide. This finding permitted analysis of data on the binding of [3H]emonapride to the D2, D3 and D4 receptors. We conclude that the combined density of D2 and D3 receptors (labelled by [3H]raclopride) is increased by only 10% in schizophrenia brain, as found by Farde et al., but that it is the density of dopamine D4 receptors which is sixfold elevated in schizophrenia. These findings resolve the apparent discrepancy, mentioned above, wherein the density of [11C]methylspiperone-labelled sites (D2, D3 and D4), but not that of [11C]raclopride-labelled sites (D2 and D3), was found elevated in the schizophrenia striatum.

Oligomerization of mu- and delta-opioid receptors. Generation of novel functional properties

The existence of dimers and oligomers for many G protein-coupled receptors has been described by us and others. Since many G protein-coupled receptor subtypes are highly homologous to each other, we examined whether closely related receptors may interact with each other directly and thus have the potential to create novel signaling units. Using mu- and delta-opioid receptors, we show that each receptor expressed individually was pharmacologically distinct and could be visualized following electrophoresis as monomers, homodimers, homotetramers, and higher molecular mass oligomers. When mu- and delta-opioid receptors were coexpressed, the highly selective synthetic agonists for each had reduced potency and altered rank order, whereas endomorphin-1 and Leu-enkephalin had enhanced affinity, suggesting the formation of a novel binding pocket. No heterodimers were visualized in the membranes coexpressing mu- and delta-receptors by the methods available. However, hetero-oligomers were identified by the ability to co-immunoprecipitate mu-receptors with delta-receptors and vice versa using differentially epitope-tagged receptors. In contrast to the individually expressed mu- and delta-receptors, the coexpressed receptors showed insensitivity to pertussis toxin and continued signal transduction, likely due to interaction with a different subtype of G protein. In this study, we provide, for the first time, evidence for the direct interaction of mu- and delta-opioid receptors to form oligomers, with the generation of novel pharmacology and G protein coupling properties.

Differential induction of heat shock, SOS, and oxidation stress regulons and accumulation of nucleotides in Escherichia coli

Heat and various inhibitory chemicals were tested in Escherichia coli for the ability to cause accumulation of adenylylated nucleotides and to induce proteins of the heat shock (htpR-controlled), the oxidation stress (oxyR-controlled), and the SOS (lexA-controlled) regulons. Under the conditions used, heat and ethanol initiated solely a heat shock response, hydrogen peroxide and 6-amino-7-chloro-5,8-dioxoquinoline (ACDQ) induced primarily an oxidation stress response and secondarily an SOS response, nalidixic acid and puromycin induced primarily an SOS and secondarily a heat shock response, isoleucine restriction induced a poor heat shock response, and CdCl2 strongly induced all three stress responses. ACDQ, CdCl2, and H2O2 each stimulated the synthesis of approximately 35 proteins by factors of 5- to 50-fold, and the heat shock, oxidation stress, and SOS regulons constituted a minor fraction of the overall cellular response. The pattern of accumulation of adenylylated nucleotides during these treatments was inconsistent with a simple role for these nucleotides as alarmones sufficient for triggering the heat shock response, but was consistent with a role in the oxyR-mediated response.

Purification of a RAS-responsive adenylyl cyclase complex from Saccharomyces cerevisiae by use of an epitope addition method

We developed a method for immunoaffinity purification of Saccharomyces cerevisiae adenylyl cyclase based on creating a fusion with a small peptide epitope. Using oligonucleotide technology to encode the peptide epitope we constructed a plasmid that expressed the fusion protein from the S. cerevisiae alcohol dehydrogenase promoter ADH1. A monoclonal antibody previously raised against the peptide was used to purify adenylyl cyclase by affinity chromatography. The purified enzyme appeared to be a multisubunit complex consisting of the 200-kilodalton adenylyl cyclase fusion protein and an unidentified 70-kilodalton protein. The purified protein could be activated by RAS proteins. Activation had an absolute requirement for a guanine nucleoside triphosphate.

Guanine nucleotide-dependent assembly of FtsZ into filaments

FtsZ is an essential cell division protein that is localized to the leading edge of the bacterial septum in a cytokinetic ring. It contains the tubulin signature motif and is a GTP binding protein with a GTPase activity. Further comparison of FtsZ with eukaryotic tubulins revealed some additional sequence similarities, perhaps indicating a similar GTP binding site. Examination of FtsZ incubated in vitro by electron microscopy revealed a guanine nucleotide-dependent assembly into protein filaments, supporting the hypothesis that the FtsZ ring is formed through self-assembly. FtsZ3, which is unable to bind GTP, does not polymerize, whereas FtsZ2, which binds GTP but is deficient in GTP hydrolysis, is capable of polymerization.

Two roles for guanine nucleotides in the stimulus-secretion sequence of neutrophils

The term 'stimulus-secretion coupling' has, since first enunciated, been held to involve the mobilization of cytosol Ca2+, which in turn is sufficient to trigger exocytotic secretory processes in metabolically competent cells. However, recent studies on a wide range of secretory cell types indicate that a role for Ca2+ can be obviated: examples are stimulation with phorbol ester (phorbol myristate acetate, PMA) which causes the activation of protein kinase C or the stimulation of platelets with collagen. Ca2+-independent exocytosis also occurs when analogues of GTP are injected through the lumen of patch pipettes directly into the cytosol of mass cells. The results presented here suggest that GTP analogues can activate secretory processes by actions at two distinct locations: one may be at the level of the receptor involving the activation of polyphosphoinositide (PPI) phosphodiesterase with consequent liberation of diacylglycerol (DG); the other involves direct activation of the exocytotic mechanism. These conclusions are based on measurements of exocytotic secretion from permeabilized neutrophils into which we have been able to introduce, individually and in combination, Ca2+ chelators (EGTA and BAPTA), Ca2+ (buffered at micromolar concentrations with EGTA), analogues of GTP and GDP and the direct activator of protein kinase C, PMA.

Distinct cholecystokinin receptors in brain and pancreas

125I-Labeled (Bolton-Hunter) cholecystokinin triacontatriapeptide (CCK-33) binds saturably and reversibly to distinct receptors in brain and pancreatic membranes. The peptide specificity of pancreatic CCK binding is the same as that for pancreatic amylase release. In brain, gastrin and pentagastrin display nanomolar affinity for binding sites, whereas in pancreas these two peptides are virtually inactive. Though these differences indicate that brain and pancreas possess distinct CCK receptors, the two tissues show some similarities. Both pancreas and brain receptors show greater sensitivity to sulfated than to desulfated COOH-terminal octapeptide of CCK and display dissociation constants of 0.3-9.5 nM. The pancreas possesses about 300 times more binding sites than does brain. CCK binding in both brain and pancreas is enhanced by divalent cations and reduced by monovalent cations. Receptor binding in both tissues is regulated in a selective fashion by guanyl nucleotides.

Differential activation of yeast adenylate cyclase by wild-type and mutant RAS proteins

In these experiments we demonstrate that purified RAS proteins, whether derived from the yeast RAS1 or RAS2 or the human H-ras genes, activate yeast adenylate cyclase in the presence of guanine nucleotides. These results confirm the prediction of earlier genetic and biochemical data and for the first time provide a complete biochemical assay for RAS protein function. Furthermore, we observe a biochemical difference between the RAS2 and RAS2val19 proteins in their ability to activate adenylate cyclase after preincubation with GTP.

Calcium channel currents and their inhibition by (-)-baclofen in rat sensory neurones: modulation by guanine nucleotides

1. The effect of intracellular application of the hydrolysis-resistant GTP and GDP analogues, guanosine 5'-O-3-thiotriphosphate (GTP-gamma-S), and guanosine 5'-O-2-thiodiphosphate (GDP-beta-S) has been examined on voltage-activated calcium-channel currents and the ability of the gamma-aminobutyric acid B agonist baclofen to inhibit them, in cultured rat dorsal root ganglion (d.r.g.) neurones. 2. Under control conditions, the calcium-channel current, recorded using the whole-cell patch technique with Ba2+ rather than Ca2+ as the permeant divalent cation, consists of an inactivating and a sustained current. In the presence of 500 microM-GTP-gamma-S included in the patch pipette, the calcium-channel current was activated more slowly and was largely non-inactivating during the 100 ms depolarization voltage step. The effects of GTP-gamma-S were abolished by pre-treatment of cells with pertussis toxin. 3. The calcium-channel current recorded in the presence of 500 microM-GDP-beta-S had a more marked transient component than the control calcium-channel current. The proportion of transient calcium-channel current in the presence of GDP-beta-S was not reduced in Na+-free medium. 4. No statistically significant effects of GTP-gamma-S and GDP-beta-S were observed on the calcium-activated potassium current IK(Ca), the transient outward potassium current activated in Ca2+-free medium, or on the inwardly rectifying current (Ih) activated by hyperpolarization. 5. GTP-gamma-S increased the ability of baclofen to inhibit calcium-channel currents, whereas this was decreased by GDP-beta-S and by pre-treatment of cells with pertussis toxin. The half-maximal effective dose (EC50) for baclofen was 2 microM in the presence of GTP-gamma-S, 15 microM for control and 50 microM in the presence of GDP-beta-S. Comparable results were obtained using a single concentration of the adenosine agonist 2-chloroadenosine (2-CA, 0.05 microM) to inhibit calcium-channel currents; its effect was significantly increased by GTP-gamma-S and reduced by GDP-beta-S. 6. The ability of baclofen to inhibit calcium-channel currents was not affected by 1 microM-forskolin or 50 microM-intracellular cyclic AMP. 7. It is concluded that calcium channels in d.r.g.s are associated with a nucleotide binding protein, and that this mediates the effect of baclofen and 2-CA on calcium-channel currents. The ability of GTP-gamma-S to inhibit the transient component of calcium-channel currents in the absence of agonist may represent a means of differentially regulating calcium-channel activity.

Inhibition of herpes simplex virus-induced DNA polymerase activity and viral DNA replication by 9-(2-hydroxyethoxymethyl)guanine and its triphosphate

The effect of the nucleoside analog 9-(2-hydroxyethoxymethyl)guanine (acycloguanosine) on herpes simplex virus type 1 DNA synthesis was examined. Acycloguanosine inhibited herpesvirus DNA synthesis in virus-infected cells. The synthesis of host cell DNA was only partially inhibited in actively growing cells at acycloguanosine concentrations several hundred-fold greater than the 50% effective dose for herpes simplex virus type 1. Studies using partially purified enzymes revealed that the triphosphate of this compound inhibited the virus-induced DNA polymerases (DNA nucleotidyltransferases) to a greater degree than the DNA polymerase of the host cell, that the inhibition was dependent upon the base composition of the template, and that the triphosphate was a better substrate for the virus-induced polymerases than for the alpha cellular DNA polymerases.

Biochemical dissection of AP-1 recruitment onto Golgi membranes

Recruitment of the Golgi-specific AP-1 adaptor complex onto Golgi membranes is thought to be a prerequisite for clathrin coat assembly on the TGN. We have used an in vitro assay to examine the translocation of cytosolic AP-1 onto purified Golgi membranes. Association of AP-1 with the membranes required GTP or GTP analogues and was inhibited by the fungal metabolite, brefeldin A. In the presence of GTP gamma S, binding of AP-1 to Golgi membranes was strictly dependent on the concentration of cytosol added to the assay. AP-1 recruitment was also found to be temperature dependent, and relatively rapid at 37 degrees C, following a lag period of 3 to 4 min. Using only an adaptor-enriched fraction from cytosol, purified myristoylated ARF1, and Golgi membranes, the GTP gamma S-dependent recruitment of AP-1 could be reconstituted. Our results show that the association of the AP-1 complex with Golgi membranes, like the coatomer complex, requires ARF, which accounts for the sensitivity of both to brefeldin A. In addition, they provide the basis for a model for the early biochemical events that lead to clathrin-coated vesicle formation on the TGN.

5'-Guanylylimidodiphosphate, a potent activator of adenylate cyclase systems in eukaryotic cells

5'-Guanylylimidodiphosphate (Gpp(NH)-p) stimulates adenylate cyclase [ATP-pyrophosphate-lyase (cyclizing), EC 4.6.1.1] activity in plasma membranes isolated from frog and salmon erythrocytes, from rat adrenal, hepatic, and fat cells, and from bovine thyroid cells. The nucleotide acts cooperatively with the various hormones (glucagon, secretin, ACTH, thyrotropin, and catecholamines) that stimulate these adenylate cyclase systems with resultant activities that equal or exceed those obtained with hormone plus GTP or with fluoride ion. In the absence of hormones, Gpp(NH)p is a considerably more effective activator than GTP, and, under certain conditions of incubation, stimulates rat fat cell adenylate cyclase to levels of activity (about 20 nmoles of 3',5'-adenosine monophosphate mg protein per min) far higher than reported hitherto for any adenylate cyclase system examined. The nucleotide activates frog erythrocyte adenylate cyclase when the catecholamine receptor is blocked by the competitive antagonist, propranolol, and activates the enzyme from an adrenal tumor cell line which lacks functional ACTH receptors. In contrast, Gpp(NH)p does not stimulate adenylate cyclase in extracts from Escherichia coli B. Gpp(NH)p appears to be a useful probe for investigating the mechanism of hormone and nucleotide action on adenylate cyclase systems in eukaryotic cells.

Guanine nucleotides decrease the free [Ca2+] required for secretion of serotonin from permeabilized blood platelets. Evidence of a role for a GTP-binding protein in platelet activation

Human platelets containing granule-bound [14C]serotonin were permeabilized, equilibrated at 0 degrees C with ATP and with various Ca2+ buffers and guanine nucleotides, and then incubated at 25 degrees C with or without a stimulatory agonist. Ca2+ alone induced the ATP-dependent secretion of [14C]serotonin (50% at a pCa of 5.1) but the sensitivity of secretion to Ca2+ was greatly enhanced by guanine nucleotides [6-fold by 100 microM GTP, 100-fold by 100 microM guanyl-5'-yl imidodiphosphate and greater than 500-fold by 100 microM guanosine 5'-O-(3-thiotriphosphate)] or by stimulatory agonists (10-fold by 2 units thrombin/ml and 4-fold by 1 microM 1-O-octadecyl-2-O-acetyl-sn-glyceryl-3-phosphorylcholine). When both GTP and a stimulatory agonist were added, they had synergistic effects on secretion. Cyclic GMP and GMP acted similarly to GTP. The effects of all these guanine nucleotides were inhibited by guanosine 5'-O-(2-thiodiphosphate), whereas those of stimulatory agonists were not. Our results demonstrate the presence in platelets of guanine nucleotide-dependent and independent mechanisms regulating the sensitivity of secretion to Ca2+.

Activation of a muscarinic receptor selectively inhibits a rapidly inactivated Ca2+ current in rat sympathetic neurons

Sympathetic neurons dissociated from the superior cervical ganglion of 2-day-old rats were studied by whole-cell patch clamp and by fura-2 measurements of the cytosolic free Ca2+ concentration, [Ca2+]i. Step depolarizations in the presence of tetrodotoxin and hexamethonium triggered two Ca2+ currents that differed in the voltage dependence of activation and kinetics of inactivation. These currents resemble the L and N currents previously described in chicken sensory neurons [Nowycky, M. C., Fox, A. P. & Tsien, R. W. (1985) Nature (London) 316, 440-442]. Treatment with acetylcholine resulted in the rapid (within seconds), selective, and reversible inhibition of the rapidly inactivated, N-type current, whereas the long-lasting L-type current remained unaffected. The high sensitivity to blocker drugs (atropine, pirenzepine) indicated that this effect of acetylcholine was due to a muscarinic M1 receptor. Intracellular perfusion with nonhydrolyzable guanine nucleotide analogs or pretreatment of the neurons with pertussis toxin had profound effects on the Ca2+ current modulation. Guanosine 5'-[gamma-thio]triphosphate caused the disappearance of the N-type current (an effect akin to that of acetylcholine, but irreversible), whereas guanosine 5'-[beta-thio]diphosphate and pertussis toxin pretreatment prevented the acetylcholine-induced inhibition. In contrast, cAMP, applied intracellularly together with 3-isobutyl-1-methylxanthine, as well as activators and inhibitors of protein kinase C, were without effect. Acetylcholine caused shortening of action potentials in neurons treated with tetraethylammonium to partially block K+ channels. Moreover, when applied to neurons loaded with the fluorescent indicator fura-2, acetylcholine failed to appreciably modify [Ca2+]i at rest but caused a partial blunting of the initial [Ca2+]i peak induced by depolarization with high K+. This effect was blocked by muscarinic antagonists and pertussis toxin and was unaffected by protein kinase activators. Thus, muscarinic modulation of the N-type Ca2+ channels appears to be mediated by a pertussis toxin-sensitive guanine nucleotide-binding protein and independent of both cAMP-dependent protein kinase and protein kinase C.

Guanine nucleotides modulate the binding affinity of the oligopeptide chemoattractant receptor on human polymorphonuclear leukocytes

The oligopeptide chemoattractant receptor on human polymorphonuclear leukocyte (PMN) membranes exists in two affinity states. Since guanine nucleotides regulate the binding affinity and transductional activity of several other types of receptors, we examined the effect of nucleotides on the binding of N-formyl-methionyl peptides to their receptors on human PMN membranes. The addition of guanylylimidodiphosphate (0.1 mM), a nonhydrolyzable derivative of guanosine triphosphate (GTP), to PMN membrane preparations reduced the fraction of high-affinity receptors detected in equilibrium binding studies from 21.3 +/- 0.13 to 11.8 +/- 0.05% (P less than 0.03), without altering the binding affinities. Since the total number of receptors remained unchanged, the effect of guanylylimidodiphosphate was to convert a portion of the receptors from the high-affinity state to the low-affinity state. At the maximal concentration of guanine nucleotide tested, approximately 50% of the high-affinity sites were converted to low-affinity sites. The findings obtained by equilibrium binding were supported by kinetic studies since the dissociation of the radiolabeled oligopeptide chemoattractant N-formyl-methionyl-leucyl-[3H]phenylalanine from PMN membranes was accelerated in the presence of guanine nucleotide. The effect of guanine nucleotides was reversed upon washing, indicating that affinity conversion is bidirectional. The guanine nucleotide effects were greatest with nonhydrolyzable derivatives of GTP followed by GTP then guanosine diphosphate. Neither guanosine monophosphate nor any adenine nucleotide tested had an effect on receptor binding. These data suggest a role for guanine nucleotides in the regulation of stimulus-receptor coupling of chemoattractant receptors on human PMN.

Evidence that the inositol phospholipids are necessary for exocytosis. Loss of inositol phospholipids and inhibition of secretion in permeabilized cells caused by a bacterial phospholipase C and removal of ATP

We directly manipulated the levels of PtdIns, PtdInsP and PtdInsP2 in digitonin-treated adrenal chromaffin cells with a bacterial phospholipase C (PLC) from Bacillus thuringiensis and by removal of ATP. The PtdIns-PLC acted intracellularly to cause a large decrease in [3H]inositol- or [32P]phosphate-labelled PtdIns, but did not directly hydrolyse PtdInsP or PtdInsP2. [3H]PtdInsP and [3H]PtdInsP2 levels declined markedly, probably because of the action of phosphatases in the absence of synthesis. Removal of ATP also caused marked decreases in [3H]PtdInsP and [3H]PtdInsP2. The decrease in polyphosphoinositide levels by PtdIns-PLC treatment or ATP removal was reflected by the inhibition of the production of inositol phosphates upon subsequent activation of the endogenous PLC by Ca2(+)-dependent catecholamine secretion from permeabilized cells was strongly inhibited by PtdIns-PLC treatment and by ATP removal. Ca2(+)-dependent secretion was similarly correlated with the sum of PtdInsP and PtdInsP2 when the level of these lipids was changed by either manipulation. PtdIns-PLC inhibited only the ATP-dependent component of secretion and did not affect ATP-dependent secretion. Both PtdIns-PLC and ATP removal inhibited the late slow phase of secretion, but had little effect on the initial rapid phase. Although we found a tight correlation between polyphosphoinositide levels and secretion, endogenous phospholipase C activity (stimulated by Ca2+, guanine nucleotides and related agents) was not correlated with secretion. Additional experiments indicated that neither the products of the PtdIns-PLC reaction (diacylglycerol and InsP1) nor the inability to generate products by subsequent activation of the endogenous PLC is likely to account for the inhibition of secretion. Incubation of permeabilized cells with neomycin in the absence of ATP maintained the level of polyphosphoinositides and more than doubled subsequent Ca2(+)-dependent secretion. The data suggest that: (1) Ca2(+)-dependent secretion has a requirement for the presence of inositol phospholipids; (2) the enhancement of secretion by ATP results in part from increased polyphosphoinositide levels; and (3) the role for inositol phospholipids in secretion revealed in these experiments is independent of their being substrates for the generation of diacylglycerol and InsP3.

Control of rRNA and tRNA syntheses in Escherichia coli by guanosine tetraphosphate

The expression of stable RNA (rRNA and tRNA) genes and the concentration of guanosine tetraphosphate (ppGpp) were measured in an isogenic pair of relA+ and relA derivatives of Escherichia coli B/r. The cells were either growing exponentially at different rates or subject to amino acid starvation when they were measured. The specific stable RNA gene activity (rs/rt, the rate of rRNA and tRNA synthesis relative to the total instantaneous rate of RNA synthesis) was found to decrease from 1.0 at a ppGpp concentration of 0 (extrapolated value) to 0.24 at saturating concentrations of ppGpp (above 100 pmoles per optical density at 460 nm unit of cell mass). The same relationship between the rs/rt ratio and ppGpp concentration was obtained independent of the physiological state of the bacteria (i.e., independent of the growth rate or of amino acid starvation) and independent of the relA allele. It can be concluded that ppGpp is an effector for stable RNA gene control and that stable RNA genes are not controlled by factors other than the ppGpp-mediated system. The results were shown to be qualitatively and quantitatively consistent with data on in vitro rRNA gene control by ppGpp, and they were interpreted in the light of reported ideas derived from those in vitro experiments.