Complex information processing by the transmembrane signaling system involving G proteins

Functional and Structural Analysis of the Toxin-Binding Site of the Cadherin G-Protein-Coupled Receptor, BT-R1, for Cry1A Toxins of Bacillus thuringiensis

The G-protein-coupled receptor BT-R₁ in the moth Manduca sexta represents a class of single-membrane-spanning α-helical proteins within the cadherin family that regulate intercellular adhesion and contribute to important signaling activities that control cellular homeostasis. The Cry1A toxins, Cry1Aa, Cry1Ab, and Cry1Ac, produced by Bacillus thuringiensis bind BT-R₁ very tightly (K_d = 1.1 nM) and trigger a Mg²⁺-dependent signaling pathway that involves the stimulation of G-protein α-subunit, which subsequently launches a coordinated signaling cascade, resulting in insect death. The three Cry1A toxins compete for the same binding site on BT-R₁, and the pattern of inhibition of insecticidal activity against M. sexta is strikingly similar for all three toxins. The binding domain is localized in the 12th cadherin repeat (EC12: Asp1349 to Arg1460, ¹³⁴⁹DR¹⁴⁶⁰) in BT-R₁ and to various truncation fragments derived therefrom. Fine mapping of EC12 revealed that the smallest fragment capable of binding is a highly conserved 94-amino acid polypeptide bounded by Ile1363 and Ser1456 (¹³⁶³IS¹⁴⁵⁶), designated as the toxin-binding site (TBS). Logistical regression analysis revealed that binding of an EC12 truncation fragment containing the TBS is antagonistic to each of the Cry1A toxins and completely inhibits the insecticidal activity of all three. Elucidation of the EC12 motif of the TBS by X-ray crystallography at a 1.9 Å resolution combined with results of competitive binding analyses, live cell experiments, and whole insect bioassays substantiate the exclusive involvement of BT-R₁ in initiating insect cell death and demonstrate that the natural receptor BT-R₁ contains a single TBS.

Sarcolemmal α2-adrenoceptors in feedback control of myocardial response to sympathetic challenge

α2-adrenoceptor (α2-AR) isoforms, abundant in sympathetic synapses and noradrenergic neurons of the central nervous system, are integral in the presynaptic feed-back loop mechanism that moderates norepinephrine surges. We recently identified that postsynaptic α2-ARs, found in the myocellular sarcolemma, also contribute to a muscle-delimited feedback control capable of attenuating mobilization of intracellular Ca²⁺ and myocardial contractility. This previously unrecognized α2-AR-dependent rheostat is able to counteract competing adrenergic receptor actions in cardiac muscle. Specifically, in ventricular myocytes, nitric oxide (NO) and cGMP are the intracellular messengers of α2-AR signal transduction pathways that gauge the kinase-phosphatase balance and manage cellular Ca²⁺ handling preventing catecholamine-induced Ca²⁺ overload. Moreover, α2-AR signaling counterbalances phospholipase C - PKC-dependent mechanisms underscoring a broader cardioprotective potential under sympathoadrenergic and angiotensinergic challenge. Recruitment of such tissue-specific features of α2-AR under sustained sympathoadrenergic drive may, in principle, be harnessed to mitigate or prevent cardiac malfunction. However, cardiovascular disease may compromise peripheral α2-AR signaling limiting pharmacological targeting of these receptors. Prospective cardiac-specific gene or cell-based therapeutic approaches aimed at repairing or improving stress-protective α2-AR signaling may offer an alternative towards enhanced preservation of cardiac muscle structure and function.

Modulation of hippocampal excitability via the hydroxycarboxylic acid receptor 1

In addition to its prominent role as an energetic substrate in the brain, lactate is emerging as a signaling molecule capable of controlling neuronal excitability. The finding that the lactate-activated receptor (hydroxycarboxylic acid receptor 1; HCA1) is widely expressed in the brain opened up the possibility that lactate exerts modulation of neuronal activity via a transmembranal receptor-linked mechanism. Here, we show that lactate causes biphasic modulation of the intrinsic excitability of CA1 pyramidal cells. In the low millimolar range, lactate or the HCA1 agonist 3,5-DHBA reduced the input resistance and membrane time constant. In addition, activation of HCA1 significantly blocked the fast inactivating sodium current and increased the delay from inactivation to a conducting state of the sodium channel. As the observed actions occurred in the presence of 4-CIN, a blocker of the neuronal monocarboxylate transporter, the possibility that lactate acted via neuronal metabolism is unlikely. Consistently, modulation of the intrinsic excitability was abolished when CA1 pyramidal cells were dialyzed with pertussis toxin, indicating the dependency of a G_αi/o -protein-coupled receptor. The activation of HCA1 appears to serve as a restraining mechanism during enhanced network activity and may function as a negative feedback for the astrocytic production of lactate.

Beyond the wiring diagram: signalling through complex neuromodulator networks

During the computations performed by the nervous system, its 'wiring diagram'--the map of its neurons and synaptic connections--is dynamically modified and supplemented by multiple actions of neuromodulators that can be so complex that they can be thought of as constituting a biochemical network that combines with the neuronal network to perform the computation. Thus, the neuronal wiring diagram alone is not sufficient to specify, and permit us to understand, the computation that underlies behaviour. Here I review how such modulatory networks operate, the problems that their existence poses for the experimental study and conceptual understanding of the computations performed by the nervous system, and how these problems may perhaps be solved and the computations understood by considering the structural and functional 'logic' of the modulatory networks.

Design, synthesis, and preliminary pharmacological evaluation of a set of small molecules that directly activate gi proteins

Heterotrimeric G proteins play a pivotal role in the communication of cells with the environment. G proteins are stimulated by cell surface receptors (GPCR) that catalyze the exchange of GDP, bound to Galpha subunit, with GTP and can per se be the target of drugs. Based on the structure of two nonpeptidic modulators of Gi proteins, a series of new molecules characterized by a long hydrophobic chain and at least two nitrogen atoms protonated at physiological pH was designed. The compounds were tested for their ability to stimulate binding of GTPgammaS to recombinant Gi proteins. Gi activation properties were also evaluated by inhibition of adenylyl cyclase activity in intact lymphocytes. Most compounds were able to stimulate GTPgammaS binding and to inhibit cAMP production at micromolar doses. Among the active compounds, 34 showed good efficacy and was the most potent compound studied, particularly on alpha(o) subtype; its regioisomer, 36, was the most efficacious one. Compound 7 showed also an interesting profile as it showed selectivity toward the alpha(o) subtype, in both efficacy and potency. Some of the compounds synthesized and found to be active may be useful leads to develop more potent and selective Gi protein modulators.

An approach to the complexity of the brain

The establishment of ordered neuronal connections is supposed to take place under the control of specific cell adhesion molecules (CAM) which guide neuroblasts and axons to their appropriate destination. The extreme complexity of the nervous system does not provide a favorable medium for the development of deterministic connections. Simon's [112] theorems offer a mean to approach the high level of complexity of the nervous system. The basic tenet is that complex systems are hierarchically organized and decomposable. Such systems can arise by selective trial and error mechanisms. Subsystems in complex systems only interact in an aggregate manner, and no significant information is lost if the detail of aggregate interactions is ignored. A number of nervous activities, which qualify for these requirements, are shown. The following sources of selection are considered: internal and external feedbacks, previous experience, plasticity in simple structures, and the characteristic geometry of dendrites. The role played by CAMs and other membrane-associated molecules is discussed in the sense that they are either inductor molecules that turn on different homeobox genes, or downstream products of genes, or both. These molecules control cellular and tissular differentiation in the developing brain creating sources of selection required for the trial and error process in the organization of the nervous tissue.

G(q/11) and G(i/o) activation profiles in CHO cells expressing human muscarinic acetylcholine receptors: dependence on agonist as well as receptor-subtype

1. Profiles of G protein activation have been assessed using a [35S]-GTPgammaS binding/immunoprecipitation strategy in Chinese hamster ovary cells expressing either M1, M2, M3 or M4 muscarinic acetylcholine (mACh) receptor subtypes, where expression levels of M1 and M3, or M2 and M4 receptors were approximately equal. 2. Maximal [35S]-GTPgammaS binding to G(q/11)alpha stimulated by M1/M3 receptors, or G(i1-3)alpha stimulated by M2/M4 receptors occurred within approximately 2 min of agonist addition. The increases in G(q/11)alpha-[35S]-GTPgammaS binding after M1 and M3 receptor stimulation differed substantially, with M1 receptors causing a 2-3 fold greater increase in [35S]-GTPgammaS binding and requiring 5 fold lower concentrations of methacholine to stimulate a half-maximal response. 3. Comparison of M2 and M4 receptor-mediated G(i1-3)alpha-[35S]-GTPgammaS binding also revealed differences, with M2 receptors causing a greater increase in G(i1-3)alpha activation and requiring 10 fold lower concentrations of methacholine to stimulate a half-maximal response. 4. Comparison of methacholine- and pilocarpine-mediated effects revealed that the latter partial agonist is more effective in activating G(i3)alpha compared to G(i1/2)alpha for both M2 and M4 receptors. More marked agonist/partial agonist differences were observed with respect to M1/M3-mediated stimulations of G(q/11)alpha- and G(i1-3)alpha-[35S]-GTPgammaS binding. Whereas coupling to these Galpha subclasses decreased proportionately for M1 receptor stimulation by these agonists, pilocarpine possesses a greater intrinsic activity at M3 receptors for G(i)alpha versus G(q/11)alpha activation. 5. These data demonstrate that mACh receptor subtype and the nature of the agonist used govern the repertoire of G proteins activated. They also provide insights into how the diversity of coupling can be pharmacologically exploited, and provide a basis for a better understanding of how multiple receptor subtypes can be differentially regulated.

Analytical pharmacology of G protein-coupled receptors by stoichiometric expression of the receptor and G(alpha) protein subunits

The description of a new family of recombinant proteins, which are constructed by the covalent fusion of the cDNA encoding a G protein-coupled receptor with that of a G(alpha) protein subunit, has recently been introduced as an original strategy to explore receptor pharmacology under defined experimental conditions. As such, a controlled 1:1 stoichiometry of receptor and G(alpha) protein expression can be achieved, as well as a forced spatial proximity to each other. Fusion proteins have been revealed as active at the receptor ligand binding level and functional at the G(alpha) protein and effector level. Insights on analytical pharmacological data are discussed for wild-type and mutant receptors interacting with a given G(alpha) protein subunit and different subtypes of either wild-type or mutant G(alpha) proteins activated by a single receptor subtype. A possible alteration of the receptor:G(alpha) protein selectivity may occur due either to the spatial proximity of both protein partners or to a constraint receptor state unable to accommodate to different G(alpha) protein states. Coactivation of endogenous G(alpha) proteins in host cells expressing a fusion protein has also been observed, but depends mainly on the coupling efficiency of the receptor and G(alpha) protein engaged in the fusion process. The ligand's apparent intrinsic activity has been shown to be either enhanced, attenuated, or unmodified when the functional responses of a fusion protein are compared to the coexpression of both fusion protein partners.

G protein beta 3 subunit 825T allele, hypertension, obesity, and diabetic nephropathy

The 825T allele of the gene GNB3 which encodes the beta 3 subunit of heterotrimeric G proteins is associated with enhanced signal transduction via G proteins through the generation of a splice variant termed Gbeta3s. It was detected following a classical candidate gene approach using cell lines from patients with enhanced signal transduction and essential hypertension. The high frequency of the 825T allele in 'old' ethnicities, e.g. bushmen and Australian aborigines as well as in black populations, together with its strong association with obesity suggests that the 825T allele is a true 'thrifty genotype'. Development of obesity associated with the 825T allele is strongly influenced by lifestyle, e.g. physical activity, and other exogenous influences like pregnancy. In hypertension the 825T allele is associated with low renin activity and appears to strongly predict the development of left ventricular hypertrophy. In type 2 diabetes the 825T allele was reported to be predispose for end-stage renal disease, whereas this effect has not yet been confirmed for patients with type 1 diabetes.

alpha(1)-adrenoceptor subtypes differentially couple to growth promotion and inhibition in Chinese hamster ovary cells

We have compared the coupling of human alpha(1A)-, alpha(1B)-, and alpha(1D)-adrenoceptors (expressed at approximately 2000 fmol/mg protein in Chinese hamster ovary cells) to cellular growth promotion (as assessed by [(3)H]thymidine incorporation) and related signaling mechanisms. Maximum elevation of intracellular Ca(2+) by the three subtypes occurred with the rank order alpha(1A) (1691 nM) > alpha(1D) (1215 nM) > alpha(1B) (360 nM). In contrast, activation of the ERK, JNK, and p38 forms of mitogen-activated protein kinases occurred with the rank order alpha(1D) > alpha(1A) > alpha(1B). alpha(1A)-Adrenoceptor stimulation inhibited basal and growth factor-stimulated [(3)H]thymidine incorporation by 74%, and this was mitigated by p38 inhibition. In contrast, alpha(1D)-adrenoceptor stimulation enhanced cellular growth by 136%, and this was blocked by two distinct inhibitors of ERK activation. We conclude that within a given cell type alpha(1)-adrenoceptor subtypes can have opposite effects on cellular growth, although their proximal signal transduction displays only quantitative differences.

[Genetic polymorphism of the G-protein beta3 subunit, obesity and essential hypertension]

Following a classical candidate gene approach we have detected a C825T polymorphism in the gene GNB3 which encodes the G beta 3 subunit of heterotrimeric G proteins. The 825T allele causes alternative splicing of the gene and the generation of a truncated but functionally active splice variant of G beta 3 which is referred to as G beta 3s. Thus, genotyping for the C825T polymorphism is predictive for the activation of certain G proteins in humans. The 825T allele is significantly associated with an increased risk for hypertension in Caucasians, most likely "low renin hypertension" and it accumulates significantly in individuals with a strong family history of hypertension. Highest frequencies of the 825T allele (up to 80%) are found in old ethnicities, e.g. black Africans, African Americans, bushmen, and Australian aborigines. This suggests that enhanced G protein activation represents a thrifty genotype which might have facilitated survival in our ancestors. Frequencies of the 825T allele are significant lower in Asians (approximately 40 to 50%) and Caucasians (30%). More recent studies show that young 825T allele carriers are predisposed for obesity and this association could be confirmed across different ethnicities including young Germans, as well as Chinese and black African individuals. Thus, genotyping at the GNB3 locus represents an ideal tool for preventive medicine in that individuals at risk for obesity and hypertension can be identified early and counteract their genetic predisposition through changes in lifestyle. In individuals with borderline hypertension genotyping can facilitate the decision for medical treatment as a positive test result confirms an inherited form of hypertension.

Effects of lead on adenylate cyclase activity in rat cerebral cortex

Lead decreased in a dose dependent manner the basal AC activity in membranes of rat cerebral cortex (IC50 = 2.5 +/- 0.1 microM). In membranes preincubated under basal conditions, AC activity was stimulated by approximately two and fourfold by 10 microM Gpp(NH)p or forskolin, respectively. Under basal conditions, lead (3 microM) inhibited enzyme activity up to 50%, but was not able to inhibit the Gpp(NH)p- or the forskolin-stimulated AC activity. However, in membranes preincubated with Gpp(NH)p (10 microM), lead (3 microM) had no significant effect on enzyme activity, but it partly blocked the stimulation of AC activity elicited by forskolin (10 microM). In membranes preincubated with 10 microM lead, the addition of 10 microM Gpp(NH)p or forskolin in the incubation medium did not stimulate AC activity. However, when added together in the incubation medium Gpp(NH)p + forskolin produced an increase in enzyme activity. In membranes preincubated with 10 microM lead + 10 microM Gpp(NH)p, Gpp(NH)p (10 microM) or forskolin (10 microM) added alone or in combination to the incubation medium did not stimulate AC activity. Moreover, under these latter conditions lead had no further effect on enzyme activity. These results indicate that lead may interact with G-proteins and with the catalytic subunit of cerebral cortical AC to produce inhibition of the enzyme activity.

Signaling pathways mediating gastrin's growth-promoting effects

In addition to its fundamental role in stimulating gastric acid secretion, the peptide hormone gastrin induces growth-promoting effects on diversity of target cells. Various mechanisms, including endocrine, paracrine, and autocrine, have been proposed for gastrin's growth-promoting actions. The mitogenic effects of gastrin are mediated by specific cell surface receptors activated after gastrin binding. The functionally defined receptors for gastrin include cholecystokinin A (CCKA) receptor, which is discriminating for sulfated CCK8; cholecystokinin B (CCKB)/gastrin receptor, which binds gastrin17 sulfated, and nonsulfated CCK8 with nearly equal affinities; cholecystokinin C (CCKC), which is a low-affinity gastrin binding protein; and novel, high-affinity receptors selective for amidated gastrin, processing intermediates of gastrin, or both. The signaling pathways mediating gastrin's stimulation of the CCKB/gastrin receptor have been progressively outlined, and the pathways mediating other receptors have been slowly emerging. Engagement of the gastrin receptor initiates various biochemical and molecular events, including recruitment and activation of tyrosine kinases, activation of the phospholipase C signaling pathway leading to phosphoinositide breakdown, intracellular calcium mobilization and protein kinase C stimulation, activation of the mitogen-activated protein kinase pathway, and induction of early response genes. Current emphasis is on understanding the functional significance of processing intermediate forms of gastrin, and the receptor subtypes and pathways that promote the trophic/mitogenic effects of the different molecular forms of gastrin.

Interactions of phosducin with the subunits of G-proteins. Binding to the alpha as well as the betagamma subunits

The high affinity interactions of phosducin with G-proteins involve binding of phosducin to the G-protein betagamma subunits. Here we have investigated whether phosducin interacts also with G-protein alpha subunits. Interactions of phosducin with the individual subunits of Go were measured by retaining phosducin-G-protein subunit complexes on columns containing immobilized anti-phosducin antibodies. Both the alpha and the beta subunits of trimeric Go were specifically retained by the antibodies in the presence of phosducin. This binding was almost completely abolished for both subunits following protein kinase A-mediated phosphorylation of phosducin and was reduced, more for alpha than for beta subunits, by the stable GTP analog guanosine 5'-(3-O-thio)triphosphate. Isolated alphao was also retained on the columns in the presence of phosducin but not in the presence of protein kinase A-phosphorylated phosducin. Likewise, purified G-protein betagamma subunit complexes as well as purified alpha subunits of Go and Gt were precipitated together with His6-tagged phosducin with nickel-agarose; this co-precipitation occurred concentration-dependently, with apparent affinities for phosducin of 55 nM (Gbetagamma), 110 nM (alphao), and 200 nM (alphat). In functional experiments, the steady state GTPase activity of isolated alphao was inhibited by phosducin by approximately 60% with an IC50 value of approximately 300 nM, whereas the GTPase activity of trimeric Go was inhibited by approximately 90% with an IC50 value of approximately 10 nM. Phosducin did not inhibit the GTP-hydrolytic activity of isolated alphao as measured by single-turnover assays, but it inhibited the release of GDP from alphao; the rate constant of GDP release was decreased approximately 40% by 500 nM phosducin, and the inhibition occurred with an IC50 value for phosducin of approximately 100 nM. These data suggest that phosducin binds with high affinity to G-protein betagamma subunits and with lower affinity to G-protein alpha subunits. We propose that the alpha subunit-mediated effects of phosducin might increase both the extent and the rapidity of its inhibitory effects compared with an action via the betagamma subunit complex alone.

Gsalpha-selective G protein antagonists

Suramin acts as a G protein inhibitor because it inhibits the rate-limiting step in activation of the Galpha subunit, i.e., the exchange of GDP for GTP. Here, we have searched for analogues that are selective for Gsalpha. Two compounds have been identified: NF449 (4,4',4",4'"-[carbonyl-bis[imino-5,1,3-benzenetriyl bis-(carbonylimino)]]tetrakis-(benzene-1,3-disulfonate) and NF503 (4, 4'-[carbonylbis[imino-3,1-phenylene-(2, 5-benzimidazolylene)carbonylimino]]bis-benzenesulfonate). These compounds (i) suppress the association rate of guanosine 5'-[gamma-thio]triphosphate ([35S]GTP[gammaS]) binding to Gsalpha-s but not to Gialpha-1, (ii) inhibit stimulation of adenylyl cyclase activity in S49 cyc- membranes (deficient in endogenous Gsalpha) by exogenously added Gsalpha-s, and (iii) block the coupling of beta-adrenergic receptors to Gs with half-maximum effects in the low micromolar range. In contrast to suramin, which is not selective, NF503 and NF449 disrupt the interaction of the A1-adenosine receptor with its cognate G proteins (Gi/Go) at concentrations that are >30-fold higher than those required for uncoupling of beta-adrenergic receptor/Gs tandems; similarly, the angiotensin II type-1 receptor (a prototypical Gq-coupled receptor) is barely affected by the compounds. Thus, NF503 and NF449 fulfill essential criteria for Gsalpha-selective antagonists. The observations demonstrate the feasibility of subtype-selective G protein inhibition.

Human substance P receptor expressed in Sf9 cells couples with multiple endogenous G proteins

To identify the G proteins involved in the function of human substance P receptor (hSPR), the receptor was expressed in Sf9 cells using the baculovirus expression system. Maximal hSPR expression was up to 65 pmol/mg membrane protein. The following data indicated that hSPR in Sf9 membranes is coupled to endogenous G proteins: 1) binding of agonist radioligand [125I]BHSP to the receptor was sensitive to guanine nucleotides; and 2) stimulation of the receptor increased [35S]GTPgammaS binding. The hSPR-associated G proteins were identified by photoaffinity labeling with [alpha-32P]-azidoanilido GTP ([alpha-32P]AAGTP), followed by immunoprecipitation of the labeled G proteins with antibodies specific for various Galpha-subunits. These experiments showed that stimulation of hSPR in Sf9 membranes activated multiple endogenous G proteins including Galpha(o), Galpha(q/11), and Galpha(s). While hSPR's ability to associate with Gq/11 is well-documented, the present study provides the first evidence of hSPR's potential to activate Galpha(o) and Galpha(s).

Dopaminergic modulation of inhibitory glutamate receptors in the lobster stomatogastric ganglion

The intrinsic rhythmicity of the spiny lobster stomatogastric ganglion (STG) is strongly influenced by the strengths of the graded synapses between identified cells within the neural network. These synaptic strengths can be powerfully influenced by chemical neuromodulators such as dopamine and serotonin. Most of the intraganglionic chemical synapses in the STG are mediated by postsynaptic inhibitory glutamate receptors (IGluRs). To determine whether or not direct effects on these IGluRs contribute to the modulation of synaptic strength, unidentified STG neurons were extracted into primary culture and the effects of these aminergic neuromodulators on the glutamate-evoked membrane current were assessed. Dopamine (100 microM) reliably and significantly reduced the whole cell slope conductance of all IGluRs tested. Serotonin (20 microM) never affected the IGlu response, although it clearly altered other cellular membrane properties. Although all identified STG neurons may not conform to these observations, the data reveal a specific dopamine-activated modulatory pathway within cultured neurons that reduces IGluR slope conductance. The relationship between IGluR modulation and net synaptic modulation in situ contributes to an emerging model in which synaptic strengths can be multiply modulated at different functional sites, yielding a complex, distributed, and state-dependent regulatory structure.

Denervation-induced supersensitivity to forskolin and pinacidil is not related to changes in the adenylate cyclase transduction pathway in the rabbit femoral artery

1. The present study investigates whether chemical sympathectomy compromises the relaxation of the rabbit femoral artery precontracted with serotonin. The vasodilating agents promoted cyclic adenosine monophosphate (cAMP) accumulation or opening of the potassium channels. The effect of denervation on the adenylyl cyclase transduction pathway was also studied. 2. 6-Hydroxydopamine treatment did not impair the relaxation to adenosine, 8-bromoadenosine-cAMP (a membrane-permeable analog of cAMP) and 3-isobutyl-1-methylxanthine (a cAMP phosphodiesterase inhibitor). Moreover, denervation enhanced the relaxation to forskolin (a direct Gs-type protein activator) and pinacidil (a potassium channel opener). 3. Denervation modified neither adenosine diphosphate ribosylation of Gs- and Gi-proteins nor adenylyl cyclase activity.

Receptors couple to L-type calcium channels via distinct Go proteins in rat neuroendocrine cell lines

1. The present study examines the hypothesis of G protein subtype selectivity in receptor-induced inhibition of calcium channel currents (ICa) in the insulin-secreting RINm5F and pituitary GH3 rat cell lines. Specificity of receptor coupling to G proteins was studied by infusion of purified G alpha isoforms into cells via a patch pipette. 2. In RINm5F cells, the neuropeptide galanin inhibited dihydropyridine (DHP)- and omega-conotoxin-sensitive components of ICa and slowed down their activation kinetics. In GH3 cells, DHP-sensitive ICa was inhibited by galanin, as well as by somatostatin and carbachol. Agonist-induced ICa inhibition was suppressed by pertussis toxin (PTX) pretreatment of the cells. In PTX-pretreated cells of either cell line, the response to galanin was restored only by the G alpha o1 subunit. Following PTX treatment of GH3 cells, only the G alpha o1 subunit restored carbachol-induced inhibition of ICa, whereas only the G alpha o2 subunit restored somatostatin-induced inhibition of ICa. G(i) subtypes had no effect on ICa inhibition. 3. Both cell lines expressed two distinct immunoreactive Go proteins. Whereas in RINm5F cell membranes Go1 was found to be the predominant isoform, we detected more Go2 than Go1 in GH3 cell membranes. Nevertheless, all agonists stimulated incorporation of the photoreactive GTP analogue [alpha-32P]GTP azidoanilide into both G(o) isoforms. 4. The results indicate that the same Go subtype, i.e. Go1, mediates galanin-induced inhibition of ICa in both cell lines and that the Go subtype specificity of receptor-G protein coupling is confined to intact cells.

The P2U purinoceptor obligatorily engages the heterotrimeric G protein G16 to mobilize intracellular Ca2+ in human erythroleukemia cells

To assess the role of G16, a trimeric G protein exclusively expressed in hematopoietic cells, Galpha16 antisense RNA was stably expressed in human erythroleukemia (HEL) cells. Western blot analysis showed that in transfected cell lines, the expression of endogenous Galpha16 protein was suppressed, but the expression of Galphaq/11, Galphai2, and Galphai3 remained unaffected. Suppression of Galpha16 in transfected HEL cells did not interfere with transient elevations of intracellular free Ca2+ concentrations induced by prostaglandin E1 (PGE1), platelet-activating factor, or thrombin. In parental HEL cells, UTP and ATP mobilized Ca2+ from intracellular stores with half-maximum effective concentrations of 3. 6 +/- 0.7 and 4.7 +/- 1.6 microM, respectively, apparently by stimulating P2U purinoceptors. By contrast, Ca2+ mobilization by UTP or ATP was completely abrogated in Galpha16-suppressed cells, indicating specific coupling of G16 to P2U purinoceptors. Pertussis toxin inhibited the effect of UTP in parental HEL cells by 57.6 +/- 4.9%. These data indicate that signaling by the P2U purinoceptor obligatorily requires G16 but may be modulated further by activation of Gi. Priming of HEL cells with UTP or ATP prior to stimulation with PGE1 markedly enhanced the PGE1-induced intracellular Ca2+ release. This indirect, potentiating effect of UTP and ATP was not impaired in Galpha16-suppressed cells but was inhibited by pertussis toxin, indicating that functional P2U purinoceptors are present on these cells and that the potentiating effect primarily depends on Gi. The data demonstrate (i) that Galpha16 antisense RNA selectively inhibits endogenous Galpha16 protein expression in HEL cells; (ii) that stimulation of endogenous P2U (P2Y2) purinoceptors leads to the mobilization of intracellular Ca2+ by a mechanism that strictly depends on Galpha16; and (iii) that P2U purinoceptors in HEL cells can communicate with two distinct signaling pathways diverging at the G protein level.

G protein heterotrimer Galpha13beta1gamma3 couples the angiotensin AT1A receptor to increases in cytoplasmic Ca2+ in rat portal vein myocytes

The subunit composition of angiotensin AT1 receptor-activated G protein was identified by using antisense oligonucleotide injection into the nucleus of rat portal vein myocytes. In these cells, we have previously shown that increases in the cytoplasmic calcium concentration ([Ca2+]i) induced by activation of angiotensin AT1 receptors were dependent on extracellular Ca2+ entry by L-type Ca2+ channels and subsequent Ca2+-induced Ca2+ release from the intracellular stores. The angiotensin AT1 receptor-activated increases in [Ca2+]i were selectively inhibited by injection of antisense oligonucleotides directed against the mRNAs coding for the alpha13, beta1, and gamma3 subunits. A correlating reduction in Galpha13, Gbeta1, and Ggamma3 protein expression was confirmed by immunocytochemistry. In addition, anti-alpha13 antibody and synthetic peptide corresponding to the carboxyl terminus of the Galpha13 subunit inhibited, in a concentration-dependent manner, the angiotensin AT1 receptor-mediated Ca2+ response. Reverse transcription-polymerase chain reaction analysis showed that only the angiotensin AT1A receptor was expressed in rat portal vein smooth muscle. Furthermore, injection of anti-AT1A oligonucleotides selectively inhibited the angiotensin II-induced increase in [Ca2+]i. We conclude that the receptor-activated signal leading to increases in [Ca2+]i is transduced by the heterotrimeric G13 protein composed of alpha13/beta1/gamma3 subunits and that the carboxyl terminus of the Galpha13 subunit interacts with the angiotensin AT1A receptor.

Multiple G-protein coupling of the dog thyrotropin receptor

We investigated, in dog thyroid membranes, the ability of the dog thyrotropin (TSH) receptor to interact with the endogenous G proteins expressed in this tissue. Activation of the receptor led to increased incorporation of the photoreactive GTP analog [alpha-(32)P]GTP azidoanilide into immunoprecipitated alpha subunits of three G protein families: G(s), G(q/11), G(i/o). This effect was not due to a general loss of receptor G protein specificity since carbamylcholine, in the same membrane preparations, only stimulated the binding of the GTP analog to the alpha subunits of G(q/11) proteins. To investigate the multiple coupling of the dog TSH receptor in intact cells, cyclic AMP accumulation, IP(3) formation and (45)Ca2+ efflux experiments were performed. When thyrocytes were pretreated with pertussis toxin (PTX), the TSH receptor-mediated accumulation of cAMP increased by approximately 45% with TSH at 1 mU/ml, suggesting that the TSH receptor coupled to both G(s) and G(i) in vivo. On the other hand, no increase in IP(3) accumulation nor Ca2+ efflux was observed in the presence of thyrotropin. These data in intact cells are thus in contradiction with those obtained in membranes, suggesting that receptor-mediated transmembrane signalling may implicate a specificity which itself may reflect a localization and organization of the different components (receptors, G proteins, ...) in the plasma membrane of intact cells. As in some cells, G(i) activates mitogenesis by hormone activated G-protein-coupled receptors, we tested its role in the stimulation by TSH of the proliferation of thyrocytes. This was not affected by PTX, suggesting that the mitogenic effect of TSH does not involve G(i)-proteins.

Distinct functions of Gq and G11 proteins in coupling alpha1-adrenoreceptors to Ca2+ release and Ca2+ entry in rat portal vein myocytes

In this study, we identified the subunit composition of Gq and G11 proteins coupling alpha1-adrenoreceptors to increase in cytoplasmic Ca2+ concentration ([Ca2+]i) in rat portal vein myocytes maintained in short-term primary culture. We used intranuclear antisense oligonucleotide injection to inhibit selectively the expression of subunits of G protein. Increases in [Ca2+]i were measured in response to activation of alpha1-adrenoreceptors, angiotensin AT1 receptors, and caffeine. Antisense oligonucleotides directed against the mRNAs coding for alphaq, alpha11, beta1, beta3, gamma2, and gamma3 subunits selectively inhibited the increase in [Ca2+]i activated by alpha1-adrenoreceptors. A corresponding reduction of the expression of these G protein subunits was immunochemically confirmed. In experiments performed in Ca2+-free solution only cells injected with anti-alphaq antisense oligonucleotides displayed a reduction of the alpha1-adrenoreceptor-induced Ca2+ release. In contrast, in Ca2+-containing solution, injection of anti-alpha11 antisense oligonucleotides suppressed the alpha1-adrenoreceptor-induced stimulation of the store-operated Ca2+ influx. Agents that specifically bound Gbetagamma subunits (anti-betacom antibody and overexpression of a beta-adrenergic receptor kinase carboxyl-terminal fragment) had no effect on the alpha1-adrenoreceptor-induced signal transduction. Taken together, these results suggest that alpha1-adrenoreceptors utilize two different Galpha subunits to increase [Ca2+]i. Galphaq may activate phosphatidylinositol 4,5-bisphosphate hydrolysis and induce release of Ca2+ from intracellular stores. Galpha11 may enhance the Ca2+-activated Ca2+ influx that replenishes intracellular Ca2+ stores.

Functional and structural complexity of signal transduction via G-protein-coupled receptors

A prerequisite for the maintenance of homeostasis in a living organism is fine-tuned communication between different cells. The majority of extracellular signaling molecules, such as hormones and neurotransmitters, interact with a three-protein transmembrane signaling system consisting of a receptor, a G protein, and an effector. These single components interact sequentially and reversibly. Considering that hundreds of G-protein-coupled receptors interact with a limited repertoire of G proteins, the question of coupling specificity is worth considering. G-protein-mediated signal transduction is a complex signaling network with diverging and converging transduction steps at each coupling interface. The recent realization that classical signaling pathways are intimately intertwined with growth-factor-signaling cascades adds another level of complexity. Elaborate studies have significantly enhanced our knowledge of the functional anatomy of G-protein-coupled receptors, and the concept has emerged that receptor function can be modulated with high specificity by coexpressed receptor fragments. These results may have significant clinical impact in the future.

Expression of phosducin in a phosducin-negative cell line reveals functions of a Gbetagamma-binding protein

Phosducin is a member of the large group of proteins that bind to G-protein betagamma-subunits (Gbetagamma) and whose biological functions are often unknown. Human A431 cells do not contain detectable amounts of phosducin. We generated A431 cells expressing phosducin at a level of approximately 1 pmol/mg of cytosolic protein, which is approximately 10% of the phosducin level in brain. cAMP accumulation in response to beta2-adrenergic receptor agonists was enhanced at early times in phosducin-expressing cells, but reached a lower plateau than in control cells. Permeabilization of the cells with digitonin did not change this pattern, but allowed the introduction of specific inhibitors: antibodies to phosducin abolished all differences between the two cell lines. Inhibitors of the beta-adrenergic receptor kinase abolished the differences at early time points. An almost complete loss of beta2-adrenergic receptor desensitization in the phosducin-expressing cells was also observed when intact cells were desensitized and receptor function was then determined in membrane preparations. Inhibition of protein kinase A accentuated the effects of phosducin, suggesting that also in vivo phosducin is regulated by this kinase. These data indicate that phosducin affects G-protein-mediated signaling in at least two ways: it dampens the overall responsiveness, and it impairs the rapid desensitization mediated by the beta-adrenergic receptor kinase.

A database of mutants and effects of site-directed mutagenesis experiments on G protein-coupled receptors

A database system and computer programs for storage and retrieval of information about guanine nucleotide-binding protein (G protein) -coupled receptor mutants and associated biological effects have been developed. Mutation data on the receptors were collected from the literature and a database of mutants and effects of mutations was developed. The G protein-coupled receptor, family A, point mutation database (GRAP) provides detailed information on ligand-binding and signal transduction properties of more than 2130 receptor mutants. The amino acid sequences of receptors for which mutation experiments have been reported were aligned, and from this alignment mutation data may be retrieved. Alternatively, a search form allowing detailed specification of which mutants to retrieve may be used, for example, to search for specific amino acid substitutions, substitutions in specific protein domains or reported biological effects. Furthermore, ligand and bibliographic oriented queries may be performed. GRAP is available on the Internet (URL: http://www-grap.fagmed.uit.no/GRAP/+ +homepage.html) using the World-Wide Web system.

Na+/H+ exchange in hypertension and in diabetes mellitus--facts and hypotheses

An enhancement of Na+/H+ exchange (NHE) in blood cells of selected patients with essential hypertension and with diabetic nephropathy has been described by various investigators. Recent studies have shown that enhanced NHE activity persists in immortalized lymphoblasts from these patients after prolonged cell culture and, thus, appears to be under genetic control. Available evidence strongly argues against a mutation in the encoding gene or an overexpression of the NHE. Immortalized cells from hypertensive patients with enhanced NHE activity display two-fold enhanced agonist-induced rises of the cytosolic free Ca2+ concentration and the underlying reason was identified as an increased activation of pertussis toxin (PTX)-sensitive G proteins. The molecular mechanism(s) of this phenomenon have not yet been elucidated. It appears likely that similar changes contribute to the enhanced NHE activity phenotype in diabetic nephropathy, although experimental evidence for this is still lacking. An enhanced activation of PTX-sensitive G proteins could explain many of the hitherto unexplained phenomena in essential hypertension, e.g. inheritance, increased vasoconstriction, hypertrophy of remodeling of arterial blood vessels and the heart, enhanced platelet aggregation etc. In diabetes the same defect could provide the basis for the susceptibility to nephropathy, e.g. by enhancing the deleterious effects of autocrine and paracrine growth factors. Thus, the experimental approach of immortalizing blood cells from patients with essential hypertension and diabetic nephropathy has opened new horizons in the identification of genetically fixed abnormalities in intracellular signal transduction which could contribute to both pathologies and which can now be studied without the confounding influences of the diabetic or hypertensive in vivo milieu.

Synergistic interactions between human transfected adenosine A1 receptors and endogenous cholecystokinin receptors in CHO cells

The effect of Gi coupled receptor activation (adenosine A1 and 5-HT1B receptors) on cholecystokinin receptor-stimulated inositol phosphate accumulation has been investigated in Chinese hamster ovary cells transfected with the human adenosine A1 receptor cDNA (CHO-A1). CHO cells constitutively express the 5-HT1B receptor [Berg, Clarke, Sailstad, Saltzman and Maayani (1994) Mol. Pharmacol. 46, 477-484]. Our previous studies using CHO-A1 cells have revealed that both the adenosine A1 and 5-HT1B receptor are negatively coupled to adenylyl cyclase activity and stimulate increases in [Ca2+]i, through a pertussis toxin-sensitive pathway. In the present study the selective adenosine A1 receptor agonist N6-cyclopentyladenosine stimulated a pertussis toxin-sensitive increase in total [3H]inositol phosphate accumulation. The sulphated C-terminal octapeptide of cholecystokinin (CCK-8) stimulated a robust and pertussis toxin-insensitive increase in [3H]inositol phosphate accumulation through the activation of CCKA receptors. Co-stimulation of CHO-A1 cells with N6-cyclopentyladenosine and CCK-8 produced a synergistic increase in [3H]inositol phosphate accumulation. The synergistic interaction between N6-cyclopentyladenosine and CCK-8 was abolished in pertussis toxin-treated cells. Synergy between N6-cyclopentyladenosine and CCK-8 still occurred in the absence of extracellular calcium. The 5-HT1B receptor agonist 5-carboxyamidotryptamine did not stimulate a measurable increase in [3H]inositol phosphate accumulation. Furthermore, 5-carboxyamidotryptamine had no significant effect on CCK-8 mediated [3H]inositol phosphate production. Activation of endogenous P2U receptors (Gq/Gll coupled) with ATP gamma S produced a significant increase in [3H]inositol phosphate accumulation. Co-stimulation of CHO-A1 cells with ATP gamma S and CCK-8 produced additive increases in [3H]inositol phosphate accumulation. These data indicate that CHO-A1 cells may prove a useful model system in which to investigate further the mechanisms underlying the intracellular 'cross-talk' between phospholipase C coupled receptors (Gq/Gll linked) and Gi/Go coupled receptors.

Inhibition of G-protein betagamma-subunit functions by phosducin-like protein

Phosducin is a cytosolic protein predominantly expressed in the retina and the pineal gland that can interact with the betagamma subunits of guanine nucleotide binding proteins (G proteins) and thereby may regulate transmembrane signaling. A cDNA encoding a phosducin-like protein (PhLP) has recently been isolated from rat brain [Miles, M. F., Barhite, S., Sganga, M. & Elliott, M. (1993) Proc. Natl. Acad. Sci. USA 90, 10831-10835. Here we report the expression of PhLP in Escherichia coli and its purification. Recombinant purified PUP inhibited multiple effects of G-protein betagamma subunits. First, it inhibited the betagamma-subunit-dependent ADP-ribosylation of purified alpha(o) by pertussis toxin. Second, it inhibited the GTPase activity of purified G(o). The IC50 value of PhLP in the latter assay was 89 nM, whereas phosducin caused half-maximal inhibition at 17 nM. And finally, PhLP antagonized the enhancement of rhodopsin phosphorylation by purified betagamma subunits. The N terminus of PhLP shows no similarity to the much longer N terminus of phosducin, the region shown to be critical for phosducin-betagamma-subunit interactions. Therefore, PhLP appears to bind to G-protein betagamma subunits by an as yet unknown mode of interaction and may represent an endogenous regulator of G-protein function.

Protein kinase cross-talk: membrane targeting of the beta-adrenergic receptor kinase by protein kinase C

The beta-adrenergic receptor kinase (betaARK) is the prototypical member of the family of cytosolic kinases that phosphorylate guanine nucleotide binding-protein-coupled receptors and thereby trigger uncoupling between receptors and guanine nucleotide binding proteins. Herein we show that this kinase is subject to phosphorylation and regulation by protein kinase C (PKC). In cell lines stably expressing alpha1B- adrenergic receptors, activation of these receptors by epinephrine resulted in an activation of cytosolic betaARK. Similar data were obtained in 293 cells transiently coexpressing alpha1B- adrenergic receptors and betaARK-1. Direct activation of PKC with phorbol esters in these cells caused not only an activation of cytosolic betaARK-1 but also a translocation of betaARK immunoreactivity from the cytosol to the membrane fraction. A PKC preparation purified from rat brain phospborylated purified recombinant betaARK-1 to a stoichiometry of 0.86 phosphate per betaARK-1. This phosphorylation resulted in an increased activity of betaARK-1 when membrane-bound rhodopsin served as its substrate but in no increase of its activity toward a soluble peptide substrate. The site of phosphorylation was mapped to the C terminus of betaARK-1. We conclude that PKC activates betaARK by enhancing its translocation to the plasma membrane.

A heterotrimeric G protein complex couples the muscarinic m1 receptor to phospholipase C-beta

We addressed the question as to which subtypes of G protein subunits mediate the activation of phospholipase C-beta by the muscarinic m1 receptor. We used the rat basophilic leukemia cell line RBL-2H3-hm1 stably transfected with the human muscarinic m1 receptor cDNA. We microinjected antisense oligonucleotides into the nuclei of the cells to inhibit selectively the expression of G protein subunits; 48 hr later muscarinic receptors were activated by carbachol, and the increase in free cytosolic calcium concentration ([Ca2+]i) was measured. Antisense oligonucleotides directed against the mRNA coding for alpha(q) and alpha11 subunits both suppressed the carbachol-induced increase in [Ca2+]i. In cells injected with antisense oligonucleotides directed against alpha(o1) and alpha14 subunits, the carbachol effect was unchanged. A corresponding reduction of Galpha(q), and Galpha11 proteins by 70-80% compared to uninjected cells was immunochemically detected 2 days after injection of a mixture of alpha(q) and alpha11 antisense oligonucleotides. Expression of Galpha(q) and Galpha11 completely recovered after 4 days. Cells injected with antisense oligonucleotides directed against the mRNAs encoding for beta1, beta4, and gamma4 subunits showed a suppression of the carbachol-induced increase in [Ca2+]i compared to uninjected cells measured at the same time from the same coverslip, whereas in cells injected with antisense oligonucleotides directed against the beta2, beta3, gamma1, gamma2, gamma3, gamma5, and gamma7 subunits, no suppression of carbachol effect was observed. In summary, the results from RBL-2H3-hm1 cells indicate that the m1 receptor utilizes a G protein complex composed of the subunits alpha(q), alpha11, beta1, beta4, and gamma4 to activate phospholipase C.

The human thyrotropin receptor: a heptahelical receptor capable of stimulating members of all four G protein families

Thyrotropin is the primary hormone that, via one heptahelical receptor, regulates thyroid cell functions such as secretion, specific gene expression, and growth. In human thyroid, thyrotropin receptor activation leads to stimulation of the adenylyl cyclase and phospholipase C cascades. However, the G proteins involved in thyrotropin receptor action have been only partially defined. In membranes of human thyroid gland, we immunologically identified alpha subunits of the G proteins Gs short, Gs long, Gi1, Gi2, Gi3, G(o) (Go2 and another form of Go, presumably Go1), Gq, G11, G12, and G13. Activation of the thyrotropin (TSH) receptor by bovine TSH led to increased incorporation of the photoreactive GTP analogue [alpha-32P]GTP azidoanilide into immunoprecipitated alpha subunits of all G proteins detected in thyroid membranes. This effect was receptor-dependent and not due to direct G protein stimulation because it was mimicked by TSH receptor-stimulating antibodies of patients suffering from Grave disease and was abolished by a receptor-blocking antiserum from a patient with autoimmune hypothyroidism. The TSH-induced activation of individual G proteins occurred with EC50 values of 5-50 milliunits/ml, indicating that the activated TSH receptor coupled with similar potency to different G proteins. When human thyroid slices were pretreated with pertussis toxin, the TSH receptor-mediated accumulation of cAMP increased by approximately 35% with TSH at 1 milliunits/ml, indicating that the TSH receptor coupled to Gs and G(i). Taken together, these findings show that, at least in human thyroid membranes, in which the protein is expressed at its physiological levels, the TSH receptor resembles a naturally occurring example of a general G protein-activating receptor.

G-protein-coupled receptors in HL-60 human leukemia cells

1. HL-60 human leukemia cells are a widely employed model system for the analysis of signal transduction processes mediated via regulatory heterotrimeric guanine nucleotide-binding proteins (G-proteins). HL-60 promyelocytes are pluripotent and can be differentiated into neutrophilic or monocytic cells. 2. HL-60 cells express formyl peptide-, complement C5a-, leukotriene B4 (LTB4)- and platelet-activating factor receptors, receptors for purine and pyrimidine nucleotides, histamine H1- and H2-receptors, beta 2-adrenoceptors and prostaglandin receptors. 3. The major G-proteins in HL-60 cells are pertussis toxin (PTX)-sensitive Gi-proteins (Gi2 > Gi3). Gs-proteins and G-proteins of the Gq-family (e.g., G16) are expressed, too. 4. G-protein-regulated effector systems in HL-60 cells are adenylyl cyclase and phospholipase C-beta 2 (PLC-beta 2) and, possibly, phospholipase D (PLD), nonselective cation (NSC) channels and NADPH oxidase. 5. The expression of signal transduction pathways in HL-60 cells strongly depends on the differentiation state of cells. 6. Formyl peptides, via Gi-proteins, mediate activation of PLC, PLD, NSC channels, NADPH oxidase and azurophilic granule release and are referred to as full secretagogues. In dibutyryl cAMP (Bt2cAMP)-differentiated HL-60 cells, C5a and LTB4 are partial and incomplete secretagogues, respectively. There are substantial differences in the Gi-protein activations induced by formyl peptides, C5a and LTB4. 7. In HL-60 promyelocytes, purine and pyrimidine nucleotides mediate activation of PLC and NSC channels largely via PTX-insensitive G-proteins and induce functional differentiation. In Bt2cAMP-differentiated HL-60 cells, they additionally activate PLD, NADPH oxidase and granule release via PTX-sensitive and -insensitive pathways. ATP and UTP are partial secretagogues. Multiple types of receptors (i.e., P2Y- and P2U-receptors and pyrimidinocyeptors) may mediate the effects of nucleotides in HL-60 cells. 8. Bt2cAMP- and 1 alpha,25-dihydroxycholecalciferol-differentiated HL-60 cells express H1-receptors coupled to Gi-proteins and PTX-insensitive G-proteins. In the former cells, histamine mediates activation of PLC and NSC channels, and in the latter, activation of NSC channels. Histamine is an incomplete secretagogue in these cells. 9. HL-60 promyelocytes express H2-receptors coupled to adenylyl cyclase, PLC, and NSC channels. There are substantial differences in the agonist/antagonist profiles of H2-receptor-mediated cAMP formation and rises in cytosolic Ca2+ concentration, indicative of the involvement of different H2-receptor subtypes. H2-receptors mediate functional differentiation of HL-60 cells. 10. Certain cationic-amphiphilic histamine receptor ligands (i.e., 2-substituted histamines, lipophilic guanidines, and a histamine trifluoromethyl-toluidide derivative) show stimulatory effects in HL-60 cells that are attributable to receptor-independent activation of Gi-proteins.

Coupling of an endogenous 5-HT1B-like receptor to increases in intracellular calcium through a pertussis toxin-sensitive mechanism in CHO-K1 cells

1. Chinese hamster ovary cells (CHO-K1) express an endogenous 5-hydroxytryptamine (5-HT)1B-like receptor that is negatively coupled to adenylyl cyclase through a pertussis toxin (PTX)-sensitive mechanism. Furthermore, the human adenosine A1 receptor when expressed in CHO-K1 cells (CHO-A1) has been shown to mobilize intracellular Ca2+ through a PTX-sensitive mechanism. Therefore the aim of this investigation was to determine whether the endogenous 5-HT1B-like receptor was able to stimulate increases in intracellular free [Ca2+] ([Ca2+]i) in CHO-A1 cells. 2. In agreement with previous studies using CHO cells, 5-hydroxytryptamine (5-HT) elicited a concentration-dependent inhibition of forskolin-stimulated [3H]-cyclic AMP production in CHO-A1 cells (p[EC50] = 7.73 +/- 0.13). 5-HT (1 microM) inhibited 47 +/- 5% of the [3H]-cyclic AMP accumulation induced by 3 microM forskolin. Forskolin stimulated [3H]-cyclic AMP accumulation was also inhibited by the 5-HT1 receptor agonists (p[EC50] values) 5-carboxyamidotryptamine (5-CT; 8.07 +/- 0.08), RU 24969 (8.12 +/- 0.33) and sumatriptan (5.80 +/- 0.31). 3. 5-HT elicited a concentration-dependent increase in [Ca2+]i in CHO-A1 cells (p[EC50] = 8.07 +/- 0.05). In the presence of 2 mM extracellular Ca2+, 5-HT (1 microM) increased [Ca2+]i from 174 +/- 17 nM to 376 +/- 22 nM. The 5-HT1 receptor agonists (p[EC50] values), 5-carboxyamidotryptamine (5-CT; 7.9 +/- 0.02), RU 24969 (8.1 +/- 0.07) and sumatriptan (5.9 +/- 0.11) all elicited concentration-dependent increases in [Ca2+]i. Similar maximal increases in [Ca2+]i were obtained with each agonist. The selective 5-HT1A receptor agonist, 8-OH-DPAT (10 microM) did not stimulate increases in [Ca2+]i. 5-HT (100 microM) and 5-CT (10 microM) did not stimulate a measurable increase in [3H]-inositol phosphate accumulation in CHO-A1 cells. 4. 5-HT (1 microM)-mediated increases in [Ca2+]i were insensitive to the 5-HT receptor antagonist, ritanserin (5-HT2; 100 nM), ketanserin (5-HT2; 100 nM), LY-278,584 (5-HT3; 1 microM) and WAY 100635 (5-HT1A; 1 microM). The response to 5-HT (100 nM) was antagonized by the non-selective 5-HT1 antagonist, methiothepin (pKb = 8.90 +/- 0.09) and the 5-HT1D antagonist GR 127935 (pKb = 10.44 +/- 0.06). 5. Pretreatment with PTX (200 ng ml-1 for 4 h) completely attenuated the Ca2+ response to 100 microM 5-HT. 6. In untransfected CHO-K1 cells, 5-HT (1 microM), RU 24969 (1 microM), and 5-CT (1 microM) elicited increases in [Ca2+]i similar to those observed in CHO-A1 cells. 7. These data demonstrate that in CHO-K1 cells the endogenously expressed 5-HT1B-like receptor couples to the phospholipase C/Ca2+ signalling pathway through a PTX-sensitive pathway, suggesting the involvement of Gi/Go protein(s).

Crosstalk between thrombin and adenylyl cyclase-stimulating agonists in proliferating human erythroid progenitor cells

Human erythroid progenitor cells grown in a suspension culture system were used to study possible interactions between different guanine nucleotide-binding protein (G-protein)-coupled receptor-effector systems during normal cell differentiation. Agonist-stimulated adenylyl cyclase was not inhibited by any one of a panel of ligands (ADP, UTP, platelet-activating factor, thrombin, alpha2-adrenoceptor agonists, interleukin 8, lysophosphatidic acid) most of which are known, in other cells, to reduce cAMP formation by a Gi-mediated, pertussis toxin-sensitive mechanism. The first four of these ligands are also known to cause transient changes in intracellular [Ca2+] in erythroid cells. Rather than inhibiting, thrombin (but not ADP, UTP or PAF) specifically caused a fivefold increase in the maximum adenosine- or prostaglandin E1-stimulated cAMP formation, without any shift of the concentration/response curves. Thrombin did not enhance forskolin- and AlF4-stimulated cyclase activity and had only a marginal effect on isoprenaline-dependent stimulation. The effect of thrombin seemed to be unrelated to intracellular Ca2+ release but could be partially mimicked by phorbol ester (PMA)-induced stimulation of protein kinase C (PKC) and was inhibited by staurosporin or by inactivation of PKC after long-term incubation with PMA. The activity of thrombin was restricted to proliferating, colony-forming progenitor cells while proerythroblasts were completely unresponsive. Our results suggest that the interaction of thrombin with Gs-linked receptors requires phosphorylation of a target protein that is different from adenylyl cyclase, Gs or Gi but may be involved in the regulation of receptor desensitization.

Differential calcium signalling by m2 and m3 muscarinic acetylcholine receptors in a single cell type

We have compared muscarinic acetylcholine receptor (mAChR) coupling to phospholipase C (PLC) and increases in cytoplasmic Ca2+ concentration [Ca2+]i in human embryonic kidney (HEK) cells, stably expressing either the human m3 or m2 receptor subtype. In m3 mAChR-expressing cells, carbachol stimulated inositol phosphate (InsP) formation and increased [Ca2+]i with EC50 values of about 2 microM and 30 nM, respectively. Maximal inositol 1,4,5-trisphosphate (InsP3) production (about fourfold) was rapid (15 s) and stable for 2 min. Maximal increases in [Ca2+]i were 300-350 nM and mainly, almost 90%, due to influx of extracellular Ca2+. The efficacy of pilocarpine for stimulating InsP and Ca2+ responses was not significantly different from that of carbachol. All m3 mAChR-mediated responses were pertussis toxin (PTX)-insensitive. In m2 mAChR-expressing cells, carbachol stimulated InsP formation and increased [Ca2+]i with EC50 values of about 20 microM and 7 microM, respectively. Maximal InsP formation was only 10-15% of that observed in m3 mAChR-expressing cells, whereas maximal elevations of [Ca2+]i were similar in both cell types. Formation of InsP3 was rapid (15 s to 2 min) and about twofold above basal. In contrast to m3 mAChR activation, [Ca2+]i increases induced by m2 mAChR activation were exclusively due to Ca2+ mobilization from intracellular stores. The efficacy of pilocarpine for stimulating InsP and Ca2+ responses was 50% and 20% of the efficacy of carbachol, respectively. PTX treatment did not affect m2 mAChR-induced PLC stimulation, but reduced the m2 mAChR-mediated increases in [Ca2+]i to 50%. In conclusion, m3 and m2 mAChRs stably expressed in HEK cells can induce similar cellular responses; however, they do so by activating apparently distinct signalling pathways. While coupling of m2 mAChR to PLC occurs in a PTX-insensitive manner, coupling to mobilization of Ca2+ from intracellular stores is partly PTX-sensitive and this may occur at least partly independent of PLC activation.

A novel G protein alpha subunit containing atypical guanine nucleotide-binding domains is differentially expressed in a molluscan nervous system

We described the characterization of a novel G protein alpha subunit, G alpha a. cDNA encoding this subunit was cloned from the central nervous system of the mollusc Lymnaea stagnalis. The deduced protein contains all characteristic guanine nucleotide-binding domains of G alpha subunits but shares only a limited degree of overall sequence identity with known subtypes (approximately 30%). Moreover, two of the nucleotide-binding domains exhibit salient deviations from corresponding sequences in other G protein alpha subunits. The A domain, determining kinetic features of the GTPase cycle, contains a markedly unique amino acid sequence (ILIIGGPGAGK). In addition, the C domain is also clearly distinct (DVAGQRSL). The presence of a leucine in this motif, instead of glutamic acid, has important implications for hypotheses concerning the GTPase mechanism. In contrast to other G alpha subtypes, G alpha a has no appropriate N-terminal residues that could be acylated. It does contain the strictly conserved arginine residue that serves as a cholera toxin substrate in G alpha s and G alpha t but lacks a site for ADP-ribosylation by pertussis toxin. In situ hybridization experiments indicate that G alpha a-encoding mRNA is expressed in a limited subpopulation of neurons within the Lymnaea brain. These data suggest that G alpha a defines a separate class of G proteins with cell type-specific functions.

Direct and indirect receptor-independent G-protein activation by cationic-amphiphilic substances. Studies with mast cells, HL-60 human leukemic cells and purified G-proteins

Studies from several laboratories have revealed that structurally diverse substances including the wasp venom, mastoparan (MP), activate purified regulatory heterotrimeric guanine nucleotide-binding proteins (G-proteins) in a receptor-independent manner, presumably by mimicking the effects of heptahelical receptors. Mast cells and differentiated HL-60 human leukemic cells are useful model systems for the analysis of receptor-independent G-protein activation. We compared the effects of 2-phenylhistamines which are cationic-amphiphilic, too, and of MP on G-protein activation in dibutyryl cAMP-differentiated HL-60 cells and in the rat basophilic leukemia cell line, RBL 2H3. In HL-60 cells, 2-phenylhistamines show stimulatory effects which resemble those of formyl peptide receptor agonists but which cannot be attributed to agonism at classical receptors. 2-phenylhistamines do not, however, activate RBL 2H3 cells and various other myeloid cell types, pointing to cell type-specificity of receptor-independent G-protein activation. In HL-60 cells, MP shows effects on G-protein activation which differ substantially from those of formyl peptides. In RBL 2H3 membranes, MP shows similar effects on G-protein activation as in HL-60 membranes. We develop a model according to which receptor-independent G-protein activation can be subdivided into direct and indirect receptor-independent G-protein activation. In case of the former mechanism, substances like 2-phenylhistamines interact with G-protein alpha-subunits and in case of the latter mechanism, substances like MP interact with nucleoside diphosphate kinase which catalyzes the formation of GTP. This newly formed GTP is then transferred to, and cleaved by, G-protein alpha-subunits.(ABSTRACT TRUNCATED AT 250 WORDS)