Quantitation of the guanine nucleotide binding regulatory protein Gs in S49 cell membranes using antipeptide antibodies to alpha s

Clozapine, atypical antipsychotics, and the benefits of fast-off D2 dopamine receptor antagonism

Drug-receptor interactions are traditionally quantified in terms of affinity and efficacy, but there is increasing awareness that the drug-on-receptor residence time also affects clinical performance. While most interest has hitherto been focused on slow-dissociating drugs, D(2) dopamine receptor antagonists show less extrapyramidal side effects but still have excellent antipsychotic activity when they dissociate swiftly. Fast dissociation of clozapine, the prototype of the "atypical antipsychotics", has been evidenced by distinct radioligand binding approaches both on cell membranes and intact cells. The surmountable nature of clozapine in functional assays with fast-emerging responses like calcium transients is confirmatory. Potential advantages and pitfalls of the hitherto used techniques are discussed, and recommendations are given to obtain more precise dissociation rates for such drugs. Surmountable antagonism is necessary to allow sufficient D(2) receptor stimulation by endogenous dopamine in the striatum. Simulations are presented to find out whether this can be achieved during sub-second bursts in dopamine concentration or rather during much slower, activity-related increases thereof. While the antagonist's dissociation rate is important to distinguish between both mechanisms, this becomes much less so when contemplating time intervals between successive drug intakes, i.e., when pharmacokinetic considerations prevail. Attention is also drawn to the divergent residence times of hydrophobic antagonists like haloperidol when comparing radioligand binding data on cell membranes with those on intact cells and clinical data.

Chapter 11. Subsecond analyses of G-protein coupled-receptor ternary complex dynamics by rapid mix flow cytometry

The binding of full and partial agonist ligands (L) to G-protein-coupled receptors (GPCRs) initiates the formation of ternary complexes with G-proteins (LRG complexes). We describe the assembly of detergent-solubilized LRG complexes on beads. Rapid mix flow cytometry is used to analyze the subsecond dynamics of guanine nucleotide-mediated ternary complex disassembly. Ternary complexes were assembled with three formyl peptide receptor constructs (wild type, FPR-Galpha(i2) fusion, and FPR-GFP fusion) and two isotypes of the alpha subunit (alpha(i2) and alpha(i3)) and betagamma dimer (beta(i)(1)gamma(2) and beta(4)gamma(2)). Experimental evidence suggests that thermodynamic stability of ternary complexes depends on subunit isotype. Comparison of assemblies derived from the three constructs of FPR and G-protein heterotrimers composed of the available subunit isotypes demonstrate that the fast step is associated with the separation of receptor and G-protein and that the dissociation of the ligand or of the alpha and betagamma subunits was slower. These results are compatible with a cell activation model involving G-protein conformational changes rather than disassembly of Galphabetagamma heterotrimer.

Coupling mode of receptors and G proteins

Signaling via G-protein-coupled receptors (GPCRs) is crucial to many physiological and pathophysiological processes in multicellular organisms, and GPCRs themselves are targets for important drugs. Classical cell supplementation experiments suggest a collision coupling model, in which receptors and G proteins diffuse randomly within the cell membrane and interact only if receptors are activated. This model is also backed by kinetic and live cell imaging data. According to the challenging theory, receptors and G proteins are precoupled--meaning they are forming stable complexes in the absence of agonist, which prevail during signaling. This model has been favored on the basis of copurification and coimmunoprecipitation of inactive receptors with G proteins and more recently by some approaches measuring energy transfer between labeled receptors and G proteins. This article reviews key findings regarding the receptor/G protein coupling mode, including most recent findings obtained by optical techniques.

Both ligand- and cell-specific parameters control ligand agonism in a kinetic model of g protein-coupled receptor signaling

G protein–coupled receptors (GPCRs) exist in multiple dynamic states (e.g., ligand-bound, inactive, G protein–coupled) that influence G protein activation and ultimately response generation. In quantitative models of GPCR signaling that incorporate these varied states, parameter values are often uncharacterized or varied over large ranges, making identification of important parameters and signaling outcomes difficult to intuit. Here we identify the ligand- and cell-specific parameters that are important determinants of cell-response behavior in a dynamic model of GPCR signaling using parameter variation and sensitivity analysis. The character of response (i.e., positive/neutral/inverse agonism) is, not surprisingly, significantly influenced by a ligand's ability to bias the receptor into an active conformation. We also find that several cell-specific parameters, including the ratio of active to inactive receptor species, the rate constant for G protein activation, and expression levels of receptors and G proteins also dramatically influence agonism. Expressing either receptor or G protein in numbers several fold above or below endogenous levels may result in system behavior inconsistent with that measured in endogenous systems. Finally, small variations in cell-specific parameters identified by sensitivity analysis as significant determinants of response behavior are found to change ligand-induced responses from positive to negative, a phenomenon termed protean agonism. Our findings offer an explanation for protean agonism reported in β₂--adrenergic and α_2A-adrenergic receptor systems.

G protein–coupled receptors (GPCRs) are transmembrane proteins involved in physiological functions ranging from vasodilation and immune response to memory. The binding of both endogenous ligands (e.g., hormones, neurotransmitters) and exogenous ligands (e.g., pharmaceuticals) to these receptors initiates intracellular events that ultimately lead to cell responses. We describe a dynamic model for G protein activation, an immediate outcome of GPCR signaling, and use it together with efficient parameter variation and sensitivity analysis techniques to identify the key cell- and ligand-specific parameters that influence G protein activation. Our results show that although ligand-specific parameters do strongly influence cell response (either causing increases or decreases in G protein activation), cellular parameters may also dictate the magnitude and direction of G protein activation. We apply our findings to describe how protean agonism, a phenomenon in which the same ligand may induce both positive and negative responses, may result from changes in cell-specific parameters. These findings may be used to understand the molecular basis of different responses of cell types and tissues to pharmacological treatment. In addition, these methods may be applied generally to models of cellular signaling and will help guide experimental resources toward further characterization of the key parameters in these networks.

Some mechanistic insights into GPCR activation from detergent-solubilized ternary complexes on beads

The binding of full and partial agonist ligands (L) to G protein-coupled receptors (GPCRs) initiates the formation of ternary complexes with G proteins [ligand-receptor-G protein (LRG) complexes]. Cyclic ternary complex models are required to account for the thermodynamically plausible complexes. It has recently become possible to assemble solubilized formyl peptide receptor (FPR) and beta(2)-adrenergic receptor (beta(2)AR) ternary complexes for flow cytometric bead-based assays. In these systems, soluble ternary complex formation of the receptors with G proteins allows direct quantitative measurements which can be analyzed in terms of three-dimensional concentrations (molarity). In contrast to the difficulty of analyzing comparable measurements in two-dimensional membrane systems, the output of these flow cytometric experiments can be analyzed via ternary complex simulations in which all of the parameters can be estimated. An outcome from such analysis yielded lower affinity for soluble ternary complex assembly by partial agonists compared with full agonists for the beta(2)AR. In the four-sided ternary complex model, this behavior is consistent with distinct ligand-induced conformational states for full and partial agonists. Rapid mix flow cytometry is used to analyze the subsecond dynamics of guanine nucleotide-mediated ternary complex disassembly. The modular breakup of ternary complex components is highlighted by the finding that the fastest step involves the departure of the ligand-activated GPCR from the intact G protein heterotrimer. The data also show that, under these experimental conditions, G protein subunit dissociation does not occur within the time frame relevant to signaling. The data and concepts are discussed in the context of a review of current literature on signaling mechanism based on structural and spectroscopic (FRET) studies of ternary complex components.

The influence of G protein subtype on agonist action at D2 dopamine receptors

In previous studies, we have shown that agonists influence the ability of D2 dopamine receptors to couple to G proteins and here we extend this work. The human D2Short dopamine receptor and a natural polymorphism of this D(2Short)(Ser311Cys), have been studied by co-expressing the receptors in insect cells with Gbeta1gamma2 and either Galpha(o), Galpha(i1), Galpha(i2) or Galpha(i3) G protein subunits. These preparations have been used to study the G protein coupling profiles of the two receptors and the influence of agonists. Receptor/G protein coupling was analysed in dopamine/[3H]spiperone competition binding experiments and through stimulation of [35S]GTPgammaS binding. Although the Ser311Cys polymorphism itself had no appreciable effect on the G protein coupling specificity of the D2 receptor, agonist stimulation of [35S]GTPgammaS binding, revealed that both dopamine and (+)-3PPP showed a clear preference for Galpha(o) compared to the Galpha(i) subtypes, but quinpirole did not. These results indicate that agonists are able to stabilise different receptor conformations with different abilities to couple to G proteins.

Differential inhibition of adenylyl cyclase isoforms and soluble guanylyl cyclase by purine and pyrimidine nucleotides

Mammals express nine membranous adenylyl cyclase isoforms (ACs 1-9), a structurally related soluble guanylyl cyclase (sGC) and a soluble AC (sAC). Moreover, Bacillus anthracis and Bacillus pertussis produce the AC toxins, edema factor (EF), and adenylyl cyclase toxin (ACT), respectively. 2'(3')-O-(N-methylanthraniloyl)-guanosine 5'-[gamma-thio]triphosphate is a potent competitive inhibitor of AC in S49 lymphoma cell membranes. These data prompted us to study systematically the effects of 24 nucleotides on AC in S49 and Sf9 insect cell membranes, ACs 1, 2, 5, and 6, expressed in Sf9 membranes and purified catalytic subunits of membranous ACs (C1 of AC5 and C2 of AC2), sAC, sGC, EF, and ACT in the presence of MnCl(2). N-Methylanthraniloyl (MANT)-GTP inhibited C1.C2 with a K(i) of 4.2 nm. Phe-889 and Ile-940 of C2 mediate hydrophobic interactions with the MANT group. MANT-inosine 5'-[gamma-thio]triphosphate potently inhibited C1.C2 and ACs 1, 5, and 6 but exhibited only low affinity for sGC, EF, ACT, and G-proteins. Inosine 5'-[gamma-thio]triphosphate and uridine 5'-[gamma-thio]triphosphate were mixed G-protein activators and AC inhibitors. AC5 was up to 15-fold more sensitive to inhibitors than AC2. EF and ACT exhibited unique inhibitor profiles. At sAC, 2',5'-dideoxyadenosine 3'-triphosphate was the most potent compound (IC(50), 690 nm). Several MANT-adenine and MANT-guanine nucleotides inhibited sGC with K(i) values in the 200-400 nm range. UTP and ATP exhibited similar affinities for sGC as GTP and were mixed sGC substrates and inhibitors. The exchange of MnCl(2) against MgCl(2) reduced inhibitor potencies at ACs and sGC 1.5-250-fold, depending on the nucleotide and cyclase studied. The omission of the NTP-regenerating system from cyclase reactions strongly reduced the potencies of MANT-ADP, indicative for phosphorylation to MANT-ATP by pyruvate kinase. Collectively, AC isoforms and sGC are differentially inhibited by purine and pyrimidine nucleotides.

Different methods of membrane domains isolation result in similar 2-D distribution patterns of membrane domain proteins

Membrane domains are highly specialized parts of the cell plasma membrane, carrying on and augmenting the incoming signals. To study their structural and functional properties, it is crucial to find the least damaging mode of their isolation. Using two different cell lines, epithelial HEK cells (clone E2M11) and S49 lymphoma cells, three methods of membrane domain isolation (i.e., detergent extraction, alkaline treatment, and "drastic" homogenization) were tested for similarity and reproducibility by 2-D electrophoresis. Our data show that the protein composition of membrane domains obtained by different isolation methods is similar and that approximately 60% of the spots are present in all membrane domain preparations. Furthermore, the same degree of similarity of 2-D profiles of the most intensively silver stained spots found in membrane domains of the two cell lines derived from different tissues suggests that the composition of a large part of membrane domains proteins is conservative. We suggest that these proteins may either be involved in the organization of membrane domain structure or represent the conservative component of signal transduction machinery.

2'(3')-O-(N-methylanthraniloyl)-substituted GTP analogs: a novel class of potent competitive adenylyl cyclase inhibitors

2'(3')-O-(N-Methylanthraniloyl)-(MANT)-substituted nucleotides are fluorescent and widely used for the kinetic analysis of enzymes and signaling proteins. We studied the effects of MANT-guanosine 5'-[gamma-thio]triphosphate (MANT-GTP gamma S) and MANT-guanosine 5'-[beta,gamma-imido]triphosphate (MANT-GppNHp) on G alpha(s)- and G alpha(i)-protein-mediated signaling. MANT-GTP gamma S/MANT-GppNHp had lower affinities for G alpha(s) and G alpha(i) than GTP gamma S/GppNHp as assessed by inhibition of GTP hydrolysis of receptor-G alpha fusion proteins. MANT-GTP gamma S was much less effective than GTP gamma S at disrupting the ternary complex between the formyl peptide receptor and G alpha(i2). MANT-GTP gamma S/MANT-GppNHp non-competitively inhibited GTP gamma S/GppNHp-, AlF(4)(-)-, beta(2)-adrenoceptor plus GTP-, cholera toxin plus GTP-, and forskolin-stimulated adenylyl cyclase (AC) in G alpha(s)-expressing Sf9 insect cell membranes and S49 wild-type lymphoma cell membranes. AC inhibition by MANT-GTP gamma S/MANT-GppNHp was not due to G alpha(s) inhibition because it was also observed in G alpha(s)-deficient S49 cyc(-) lymphoma cell membranes. Mn(2+) blocked AC inhibition by GTP gamma S/GppNHp in S49 cyc(-) membranes but enhanced the potency of MANT-GTP gamma S/MANT-GppNHp at inhibiting AC by approximately 4-8-fold. MANT-GTP gamma S and MANT-GppNHp competitively inhibited forskolin/Mn(2+)-stimulated AC in S49 cyc(-) membranes with K(i) values of 53 and 160 nm, respectively. The K(i) value for MANT-GppNHp at insect cell AC was 155 nm. Collectively, MANT-GTP gamma S/MANT-GppNHp bind to G alpha(s)- and G alpha(i)-proteins with low affinity and are ineffective at activating G alpha. Instead, MANT-GTP gamma S/MANT-GppNHp constitute a novel class of potent competitive AC inhibitors.

Functional coupling of the human dopamine D2 receptor with G alpha i1, G alpha i2, G alpha i3 and G alpha o G proteins: evidence for agonist regulation of G protein selectivity

(1) The human dopamine D(2long) (D(2L)) receptor was expressed with four different G proteins in Sf9 cells using the baculovirus expression system. When co-expressed with G(i)/G(o) G proteins (G(i1)alpha, G(i2)alpha, G(i3)alpha, or G(o)alpha, plus Gbeta(1) and Ggamma(2)), the receptor displayed a high-affinity binding site for the agonists (dopamine and NPA), which was sensitive to GTP (100 micro M), demonstrating interaction between the receptor and the different G proteins. (2) The receptor to G protein ratio (R : G ratio) was evaluated using [(3)H]-spiperone saturation binding (R) and [(35)S]-GTPgammaS saturation binding (G). R : G ratios of 1 : 12, 1 : 3, 1 : 14 and 1 : 5 were found for G(i1), G(i2), G(i3), and G(o) preparations, respectively. However, when R : G ratios of 1 : 2 and 1 : 12 were compared for G(i2) and G(o), no difference was found for the stimulation of [(35)S]-GTPgammaS binding. (3) Several agonists were tested for their ability to stimulate [(35)S]-GTPgammaS binding to membranes co-expressing the receptor and various G proteins. All the compounds tested showed agonist activity in preparations expressing G(i3) and G(o). However, for G(i2) and G(i1) preparations, compounds such as S-(-)-3-PPP and p-tyramine were unable to stimulate [(35)S]-GTPgammaS binding. (4) Most of the compounds showed higher relative efficacies (compared to dopamine) and higher potencies in the preparation expressing G(o). Comparison of the effects of different agonists in the different preparations showed that each agonist differentially activates the four G proteins. (5) We conclude that the degree of selectivity of G protein activation by the D(2L) receptor can depend on the conformation of the receptor stabilised by an agonist.

Distinct interactions of GTP, UTP, and CTP with G(s) proteins

Early studies showed that in addition to GTP, the pyrimidine nucleotides UTP and CTP support activation of the adenylyl cyclase (AC)-stimulating G(s) protein. The aim of this study was to elucidate the mechanism by which UTP and CTP support G(s) activation. As models, we used S49 wild-type lymphoma cells, representing a physiologically relevant system in which the beta(2)-adrenoreceptor (beta(2)AR) couples to G(s), and Sf9 insect cell membranes expressing beta(2)AR-Galpha(s) fusion proteins. Fusion proteins provide a higher sensitivity for the analysis of beta(2)AR-G(s) coupling than native systems. Nucleoside 5'-triphosphates (NTPs) supported agonist-stimulated AC activity in the two systems and basal AC activity in membranes from cholera toxin-treated S49 cells in the order of efficacy GTP > or = UTP > CTP > ATP (ineffective). NTPs disrupted high affinity agonist binding in beta(2)AR-Galpha(s) in the order of efficacy GTP > UTP > CTP > ATP (ineffective). In contrast, the order of efficacy of NTPs as substrates for nucleoside diphosphokinase, catalyzing the formation of GTP from GDP and NTP was ATP > or = UTP > or = CTP > or = GTP. NTPs inhibited beta(2)AR-Galpha(s)-catalyzed [gamma-(32)P]GTP hydrolysis in the order of potency GTP > UTP > CTP. Molecular dynamics simulations revealed that UTP is accommodated more easily within the binding pocket of Galpha(s) than CTP. Collectively, our data indicate that GTP, UTP, and CTP interact differentially with G(s) proteins and that transphosphorylation of GDP to GTP is not involved in this G protein activation. In certain cell systems, intracellular UTP and CTP concentrations reach approximately 10 nmol/mg of protein and are higher than intracellular GTP concentrations, indicating that G protein activation by UTP and CTP can occur physiologically. G protein activation by UTP and CTP could be of particular importance in pathological conditions such as cholera and Lesch-Nyhan syndrome.

Agonist regulation of D(2) dopamine receptor/G protein interaction. Evidence for agonist selection of G protein subtype

The D(2) dopamine receptor has been expressed in Sf21 insect cells together with the G proteins G(o) and G(i2), using the baculovirus system. Expression levels of receptor and G protein (alpha, beta, and gamma subunits) in the two preparations were similar as shown by binding of [(3)H]spiperone and quantitative Western blot, respectively. For several agonists, binding data were fitted best by a two-binding site model in either preparation, showing interaction of expressed receptor and G protein. For some agonists, binding to the higher affinity site was of higher affinity in D(2)/G(o) than in the D(2)/G(i2) preparation. Some agonists exhibited binding data that were best fitted by a two-binding site model in D(2)/G(o) and a one-binding site model in D(2)/G(i2). Therefore, receptor/G protein interaction seemed to be stronger in the D(2)/G(o) preparation. Agonist stimulation of [(35)S]GTP gamma S (guanosine 5'-3-O-(thio)triphosphate) binding in the two preparations also gave evidence for higher affinity D(2)/G(o) interaction. In the D(2)/G(o) preparation, agonist stimulation of [(35)S]GTP gamma S binding occurred at higher potency for several agonists, and a higher stimulation (relative to dopamine) was achieved in D(2)/G(o) compared with D(2)/G(i2). Some agonists were able to stimulate [(35)S]GTP gamma S binding in the D(2)/G(o) preparation but not in D(2)/G(i2). The extent of D(2) receptor selectivity for G(o) over G(i2) is therefore dependent on the agonist used, and thus agonists may stabilize different conformations of the receptor with different abilities to couple to and activate G proteins.

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.

Molecular analysis of beta(2)-adrenoceptor coupling to G(s)-, G(i)-, and G(q)-proteins

The beta(2)-adrenoceptor (beta(2)AR) couples to the G-protein G(s) to activate adenylyl cyclase. Intriguingly, several studies have demonstrated that the beta(2)AR can also interact with G-proteins of the G(i)- and G(q)-family. To assess the efficiency of beta(2)AR interaction with various G-protein alpha-subunits (G(xalpha)), we expressed fusion proteins of the beta(2)AR with the long (G(salphaL)) and short (G(salphaS)) splice variants of G(salpha), the G(i)-proteins G(ialpha2) and G(ialpha3), and the G(q)-proteins G(qalpha) and G(16alpha) in Sf9 cells. Fusion proteins provide a rigorous approach for comparing the coupling of a given receptor to G(xalpha) because of the defined 1:1 stoichiometry of receptor and G-protein and the efficient coupling. Here, we show that the beta(2)AR couples to G(s)-, G(i)-, and G(q)-proteins as assessed by ternary complex formation and ligand-regulated guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) binding. The combined analysis of ternary complex formation, GTPgammaS binding, agonist efficacies, and agonist potencies revealed substantial differences in the interaction of the beta(2)AR with the various classes of G-proteins. Comparison of the coupling of the beta(2)AR and formyl peptide receptor to G(ialpha2) revealed receptor-specific differences in the kinetics of GTPgammaS binding. We also detected highly efficient stimulation of GTPgammaS dissociation from G(salphaL), but not from G(qalpha) and G(16alpha), by a beta(2)AR agonist. Moreover, we show that the 1:1 stoichiometry of receptor to G-protein in fusion proteins reflects the in vivo stoichiometry of receptor/G-protein coupling more closely than was previously assumed. Collectively, our data show 1) that the beta(2)AR couples differentially to G(s)-, G(i)-, and G(q)-proteins, 2) that there is ligand-specific coupling of the beta(2)AR to G-proteins, 3) that receptor-specific G-protein conformational states may exist, and 4) that nucleotide dissociation is an important mechanism for G-protein deactivation.

Viral-mediated gene delivery of constitutively activated G alpha s alters vasoreactivity

1. Decline in beta-adrenoceptor (beta-AR)-mediated function occurs with increasing age, as well as in multiple disease conditions. The mechanisms responsible for this decline include alterations in beta-AR itself, beta-AR coupling proteins, such as G-proteins, or other beta-AR-linked proteins, such as G-protein receptor kinases and/or phosphatases. 2. The present study examines the physiological effects of in vitro transfer of constitutively activated G alpha s (G alpha s-Q227L) to both cultured vascular smooth muscle cells (VSMC) and whole aortic tissue of 6-month-old (adult) animals via a replication-deficient Herpes simplex virus (HSV) vector. These studies were conducted to provide a model for future examination of the role of G alpha s in the age-related decline in beta-AR-mediated vasorelaxation. 3. Gene transfer was confirmed by western blotting for specific proteins. Aortic tissue infected with HSV-G alpha s-Q227L had reduced phenylephrine-induced contraction and enhanced isoproterenol-stimulated vasorelaxation. Infection of cultured VSMC with HSV-G alpha s-Q227L increased both basal- and isoproterenol-stimulated cAMP accumulation, whereas forskolin-stimulated cAMP production was unchanged. 4. These results implicate G alpha s as a target for further investigation in age-related changes in vascular reactivity and support the use of viral-mediated gene transfer as an effective tool to study adrenergic signal transduction and physiology in vascular tissue.

Quantitative analysis of formyl peptide receptor coupling to g(i)alpha(1), g(i)alpha(2), and g(i)alpha(3)

The human formyl peptide receptor (FPR) is a prototypical G(i) protein-coupled receptor, but little is known about quantitative aspects of FPR-G(i) protein coupling. To address this issue, we fused the FPR to G(i)alpha(1), G(i)alpha(2), and G(i)alpha(3) and expressed the fusion proteins in Sf9 insect cells. Fusion of a receptor to Galpha ensures a defined 1:1 stoichiometry of the signaling partners. By analyzing high affinity agonist binding, the kinetics of agonist- and inverse agonist-regulated guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) binding and GTP hydrolysis and photolabeling of Galpha, we demonstrate highly efficient coupling of the FPR to fused G(i)alpha(1), G(i)alpha(2), and G(i)alpha(3) without cross-talk of the receptor to insect cell G proteins. The FPR displayed high constitutive activity when coupled to all three G(i)alpha isoforms. The K(d) values of high affinity agonist binding were approximately 100-fold lower than the EC(50) (concentration that gives half-maximal stimulation) values of agonist for GTPase activation. Based on the B(max) values of agonist saturation binding and ligand-regulated GTPgammaS binding, it was previously proposed that the FPR activates G proteins catalytically, i.e. one FPR activates several G(i) proteins. Analysis of agonist saturation binding, ligand-regulated GTPgammaS saturation binding and quantitative immunoblotting with membranes expressing FPR-G(i)alpha fusion proteins and nonfused FPR now reveals that FPR agonist binding greatly underestimates the actual FPR expression level. Our data show the following: (i) the FPR couples to G(i)alpha(1), G(i)alpha(2), and G(i)alpha(3) with similar efficiency; (ii) the FPR can exist in a state of low agonist affinity that couples efficiently to G proteins; and (iii) in contrast to the previously held view, the FPR appears to activate G(i) proteins linearly and not catalytically.

GPCR-Galpha fusion proteins: molecular analysis of receptor-G-protein coupling

The efficiency of interactions between G-protein-coupled receptors (GPCRs) and heterotrimeric guanine nucleotide-binding proteins (G proteins) is greatly influenced by the absolute and relative densities of these proteins in the plasma membrane. The study of these interactions has been facilitated by the use of GPCR-Galpha fusion proteins, which are formed by the fusion of GPCR to Galpha. These fusion proteins ensure a defined 1:1 stoichiometry of GPCR to Galpha and force the physical proximity of the signalling partners. Thus, fusion of GPCR to Galpha enhances coupling efficiency can be used to study aspects of receptor-G-protein coupling that could not otherwise be examined by co-expressing GPCRs and G proteins as separate proteins. The results of studies that have made use of GPCR-Galpha fusion proteins will be discussed in this article, along with the strengths and limitations of this approach.

Examining the efficiency of receptor/G-protein coupling with a cleavable beta2-adrenoceptor-gsalpha fusion protein

Reconstitution of high-affinity agonist binding at the beta2-adrenoceptor (beta2AR) expressed in Sf9 insect cells requires a large excess of the stimulatory G-protein of adenylyl cyclase, Gsalpha, relative to receptor [R. Seifert, T. W. Lee, V. T. Lam & B. K. Kobilka, (1998) Eur. J. Biochem. 255, 369-382]. In a fusion protein of the beta2AR and Gsalpha (beta2AR-Gsalpha), which has only a 1 : 1 stoichiometry of receptor and G-protein, high-affinity agonist binding and agonist-stimulated GTP hydrolysis, guanosine 5'-O-(3-thiotriphosphate) (GTP[S]) binding and adenylyl cyclase (AC) activation are more efficient than in the nonfused coexpression system. In order to analyze the stability of the receptor/G-protein interaction, we constructed a fusion protein with a thrombin-cleavage site between beta2AR and Gsalpha (beta2AR-TS-Gsalpha). beta2AR-TS-Gsalpha efficiently reconstituted high-affinity agonist binding, agonist-stimulated GTP hydrolysis, GTP[S] binding and AC activation. Thrombin cleaves approximately 70% of beta2AR-TS-Gsalpha molecules in Sf9 membranes. Thrombin cleavage did not impair high-affinity agonist binding and GTP[S] binding but strongly reduced ligand-regulated GTPase activity and AC activity. We conclude that fusion of the beta2AR to Gsalpha promotes tight physical association of the two partners and that this association remains stable for a single activation/deactivation cycle even after cleavage of the link between the receptor and G-protein. Dilution of Gsalpha in the membrane and release of activated Gsalpha into the cytosol can both prevent cleaved beta2AR-TS-Gsalpha from undergoing multiple activation/deactivation cycles.

Glucagon increases cellular uptake and plasma membrane binding of L-carnitine in S49 lymphoma cells

The carnitine carrier was investigated in S49 lymphoma cells, a murine cell type cultured in suspension culture and used widely in signal transduction studies. Carnitine uptake in S49 lymphoma cells was stimulated almost twofold by pretreatment of intact cells by 0.5 microM glucagon for 4 h. Plasma membranes derived from S49 lymphoma cells bound 556 +/- 81 pmol/mg protein whereas pretreatment by 0.5 microM glucagon for 4 h of cells, before cell harvesting and preparation of plasma membranes, increased the number of carnitine binding sites to 1196 +/- 52 pmol/mg protein. The glucagon pretreatment also altered the carnitine binding characteristics from a two site model to a single binding site. S49 lymphoma cells were further shown to contain 50.9 +/- 2.6 fmol glucagon receptors per 10(6) cells. We conclude that glucagon stimulated cellular uptake of carnitine by a mechanism that at least partially operated through increasing the number of available carnitine binding sites in plasma membranes.

Gsalpha-mediated regulation of the carnitine carrier in S49 lymphoma cells

Carnitine is essential for mitochondrial oxidation of long-chain fatty acids. Peripheral cells rely on plasma transport of carnitine which is taken up by an active mechanism in the plasma membrane. This project investigated the plasma membrane bound carnitine carrier in cultured S49 lymphoma cells. We investigated wild-type cells and two mutant cells lines showing deficient activity of adenylyl cyclase, cyc- lacking and H21a containing a deficient Gsalpha. Plasma membranes derived from cyc- cells displayed six times more carnitine binding sites and a 1.35 times faster uptake rate than plasma membranes from wild-type cells. In vitro mixing of plasma membranes from cyc- and wild-type cells transferred a factor reducing the number of expected carnitine binding sites by about 30%. Cyclic AMP could not substitute for wild-type membranes as the inhibitor of carnitine binding to plasma membranes derived from cyc- cells. Cholera toxin induced ADP-ribosylation of Gsalpha causing activation of Gsalpha present in wild-type but not in cyc- cells, further reducing carnitine uptake and carnitine binding to plasma membranes. Our findings thus supported the notion that Gsalpha by a mechanism not involving cyclic AMP inhibited cellular uptake of carnitine by reducing the number of available carnitine binding sites in plasma membranes.

Examination of the effects of increasing Gs protein on beta2-adrenergic receptor, Gs, and adenylyl cyclase interactions

We have examined the effect of increased Gs protein levels on the abilities of three different beta2-agonists to induce GTP shifts and stimulate adenylyl cyclase response in an effort to investigate the kinetic association between the beta2-adrenergic receptor Gs and adenylyl cyclase. Agonist competition binding analysis and adenylyl cyclase concentration-response assays revealed that increases in Gs protein resulted in proportional increases in the areas of the GTP shift and adenylyl cyclase activity. Changes in the magnitude of the GTP shift were evaluated with a novel and straightforward approach for analyzing the GTP shift data that allowed us to determine the proportion of high agonist affinity binding receptor population and the apparent dissociation constant between the agonist bound receptor and Gs, regardless of the Gs protein level or the type of beta2-agonist. Using this method, we concluded that increased Gs results in the accumulation of the receptor population displaying high affinity towards agonist (HRGs) by increasing the number of receptor-Gs complexes (to a receptor:Gs protein ratio of about 0.7 at maximal Gs expression) without affecting the affinity between hormone bound receptor and Gs. Using the Gs protein levels determined with our novel analysis, we ran simulations using the theoretical shuttle model equation that relates the EC50 to available Gs. Fitting the simulations to experimental data required a receptor to catalytic unit ratio of 0.45 and revealed at least two distinct stages for beta2-agonist-stimulated adenylyl cyclase activity, namely, the activation of Gs by the beta2-adrenergic receptor (a step whose rate is dependent on the type of agonist used to stimulate activity), and the activation of adenylyl cyclase by active Gs (a step whose rate is independent of the type of agonist).

Differential changes of myocardial beta-adrenoceptor subtypes and G-proteins in dogs with right-sided congestive heart failure

We have shown previously in dogs with right heart failure that the reduction of myocardial beta-adrenoceptor density occurs only in the failing right ventricle, while cardiac inotropic responses to beta-adrenergic stimulation are reduced in both the right and left ventricles. The purpose of the present study was to determine whether a post-receptor defect in the guanine nucleotide-binding regulatory proteins (G-proteins) existed which would explain, at least in part, the adrenergic subsensitivity in both ventricles of the heart failure dogs. Using both immunoblotting technique and the bacterial toxin-mediated ADP ribosylation assays, we found that the stimulatory G-protein (Gs) was reduced in both ventricles of the heart failure dogs. In contrast, there were no changes in the inhibitory G-protein (Gi). In addition, receptor subtype analysis showed that only beta(1)-adrenoceptors were reduced in the failing right ventricle of the heart failure animals. This study demonstrated that the reduction of beta-adrenoceptors in right heart failure was chamber-specific whereas the reduction of Gs was non-selective, occurring in both ventricles of right heart failure dogs. The findings further suggest that the reduction of Gs probably was caused by systemic neurohormonal activation, independent of local ventricular stress.

The stoichiometry of expression of protein components of the stimulatory adenylyl cyclase cascade and the regulation of information transfer

Quantitative analysis of the proteins which compromise the stimulatory arm of the adenylyl cyclase cascade indicate that the adenylyl cyclase catalytic component is usually the least highly expressed. The effects on both potency of agonist ligands and maximal output resulting from targetted alterations in expression levels of each element of this cascade are discussed.

Adaptive changes in the number of Gs- and Gi-proteins underlie adenylyl cyclase sensitization in morphine-treated rat striatal neurons

In the present study we investigated the possible role of changes in the number of membrane-bound G-proteins in the sensitization of dopamine D1 receptor-stimulated adenylyl cyclase, observed in primary cultures of rat striatal neurons chronically exposed to morphine. Whereas exposure of these neurons to 10 microM morphine for 7 days caused a profound increase in cyclic AMP production, induced by the dopamine D1 receptor agonist SKF 38393 (1 microM), Scatchard analysis of [125I]SCH 23982 binding to membrane preparations revealed that neither the Bmax nor the Kd values of dopamine D1 receptor binding sites were affected. Interestingly, immunoblotting experiments revealed an increase (of more than 50%) in the number of stimulatory G-proteins (G alpha s) in neurons displaying an enhanced adenylyl cyclase activity. In morphine-treated neurons, the number of inhibitory G-proteins (G alpha i) appeared to be slightly reduced (by about 16%). Moreover, the observation that cholera toxin (0.1 nM)-stimulated cyclic AMP production, unlike that induced by forskolin (1 microM), was enhanced in morphine-treated neurons, indicates a causal relationship between the reciprocal changes in G-protein number and the increase of dopamine D1 receptor-stimulated adenylyl cyclase activity. The possible role of these changes in G-protein number in the development of morphine tolerance and dependence is discussed.

Plasma-membrane-independent pool of the alpha subunit of the stimulatory guanine-nucleotide-binding regulatory protein in a low-density-membrane fraction of S49 lymphoma cells

We report that compartmentalisation of the stimulatory guanine-nucleotide-binding regulatory protein (Gs) exists in S49 lymphoma cells. In addition to the previously reported cytosolic form of the alpha subunit of Gs (Gs alpha) [Ransnäs, L. A., Svoboda P., Jasper, J. R. & Insel, P. A. (1989) Proc. Natl Acad. Sci. USA 86, 7900-7903], three membrane-bound forms of Gs alpha were identified through rate-zonal centrifugation in sucrose density gradients, Gs alpha-specific anti-peptide serum and an adenylate cyclase complementation assay. The sedimentation profile of the first pool of Gs alpha in the high-density portion of the gradient (1.13-1.16 g/cm3) is identical with that of beta-adrenergic-receptor binding, Na/K-ATPase and adenylate cyclase activity, and may therefore be identified as plasma-membrane fragments. The second pool, which was recovered in the middle portion of the gradient (1.09-1.11 g/cm3), contains a much lower total amount of Gs alpha and correlates with the endoplasmic reticulum (microsomal) enzyme markers, NADPH-cytochrome-c reductase and glucose-6-phosphatase. The identity of the third pool of Gs alpha located at the top of the gradient (1.06-1.08 g/cm3), is unknown. The Golgi apparatus marker, UDPgalactose:N-acetylglucosamine glycosyltransferase, was partially recovered in this area; however, this enzyme was also present in the high-density portion of the gradient. Complete absence of specific adenylate cyclase and Na/K-ATPase activity indicates that this low-density (light) membrane form of Gs alpha is distinct from any plasma-membrane fragments. Furthermore, sedimentation at 100,000 x g proves its particulate (membrane) character. The light membrane form of Gs alpha subunit is functionally active in an adenylate cyclase complementation assay using cyc- membranes devoid of Gs alpha. Overall, our data indicates that a substantial portion of Gs alpha is localized in membrane pools other than plasma membrane.

Down-regulation of beta-adrenergic receptors by pindolol in Gs alpha-transfected S49 cyc- murine lymphoma cells

The role of the alpha subunit of the guanine nucleotide-binding regulatory protein that stimulates adenylyl cyclase (GS alpha) in the down-regulation of beta-adrenergic receptors by pindolol was studied in S49 cyc- cells (normally GS alpha-deficient) transfected to express functional recombinant rat GS alpha. An inducible cell line (S49 GS alpha IND) was derived from S49 cyc- cells transfected with a vector containing the full-length coding sequence of GS alpha under the inducible control of the mouse mammary tumor virus long-terminal repeat promoter. GS alpha was not detectable in S49 GS alpha IND cells by immunoblot or by ADP-ribosylation in the presence of cholera toxin and [alpha-32P]NAD. When cells were grown in 100 nM dexamethasone, isoproterenol-stimulated cyclic AMP accumulation increased within 3 h. After 15 h, GS alpha was present at a level 40-50% of that found in S49 wild-type (WT) cells as measured either by immunoblot analysis or by [alpha-32P]ADP-ribosylation. Membranes prepared from GS alpha IND cells grown in the presence of dexamethasone bound agonist with high affinity, and this binding was sensitive to guanine nucleotides. A second vector, DzbGS alpha +, contained the coding sequence of GS alpha under the constitutive regulatory control of the SV40 early promoter. This vector was introduced into cyc- cells, and the resulting cells, S49 GS alpha CST cells, expressed GS alpha at a level comparable to that found in S49 WT cells as measured by immunoblot analysis. Isoproterenol-stimulated cyclic AMP accumulation in S49 GS alpha CST cells was at least as great as in S49 WT cells. When cells were grown in the presence of dexamethasone, exposure to 50 nM pindolol for 12 h down-regulated the density of beta-adrenergic receptors in S49 WT cells to 60% of that in cells grown in the absence of pindolol, but pindolol had no effect on the density of receptors on cyc- or GS alpha IND cells. When GS alpha CST cells were exposed to 50 nM pindolol for 12 h, the density of beta-adrenergic receptors was down-regulated by the same amount as in S49 WT cells. These results suggest that GS alpha is necessary to restore the ability of pindolol to down-regulate beta-adrenergic receptors in S49 cyc- cells and that the protein must be expressed at a level comparable to that found in S49 WT cells.

Myocardial beta-adrenergic receptor expression and signal transduction after chronic volume-overload hypertrophy and circulatory congestion

BACKGROUND

The volume-overload, high-output state induced by aortocaval fistula is unique because it is not generally associated with marked abnormalities of contractile function. Thus, changes in beta-adrenergic receptor (beta AR) expression should reflect more directly the influence of neurohumoral adrenergic tone, clarifying the manner in which peripheral (neurohumoral) versus primary myocardial factors are operative in decreased beta AR-dependent signal transduction.

METHODS AND RESULTS

We examined the beta-adrenergic receptor-responsive adenylyl cyclase pathway in hearts from pigs subjected to volume-overload hypertrophy with circulatory congestion. Nine pigs underwent initial pharmacological and hemodynamic studies, and, 5 weeks after aortocaval fistula placement, when signs of circulatory congestion were evident, these measurements were repeated. Biochemical analyses of plasma and myocardium from these animals and seven normal animals were compared. Experimental animals showed signs of circulatory congestion (tachypnea, weight gain, pulmonary rales) within 3-4 weeks of fistula placement. Necropsy showed ascites and biventricular cardiac hypertrophy, but no fibrosis or inflammation was present on histological inspection. Heart rate responsiveness to beta AR stimulation was blunted, with ED50, for isoproterenol increased 133% (p less than 0.001) after development of circulatory congestion. Biochemical analyses of the beta AR-responsive adenylyl cyclase pathway showed uniform decreases in beta AR number in right atrium, right ventricle, and left ventricle (36-41% decreases, p less than 0.005). Downregulation was selective for beta 1-receptors, and remaining receptors in the right and left ventricles showed low-affinity agonist binding, suggesting an uncoupling from Gs. All measures of adenylyl cyclase activity were diminished significantly in membrane homogenates from the right atrium (mean reduction, 50 +/- 10%) and left ventricle (mean reduction, 44 +/- 8%) after volume overload. Finally, we found that amounts of cardiac Gs, as measured in reconstitution assays, were decreased in both the right atrium (p less than 0.02) and the left ventricle (p less than 0.01) of volume-overloaded animals but that levels of pertussis toxin substrate were unchanged.

CONCLUSIONS

Biochemical findings occurred in the absence of myocardial inflammation or fibrosis and without pharmacological interventions, suggesting that circulatory congestion, with attendant elevation in plasma norepinephrine, may be a sufficient stimulus to induce such changes. The data are compatible with a catecholamine-driven beta AR pathway desensitization. Thus, a primary defect in intrinsic contractile function is not a necessary component for abnormalities of the myocardial beta AR-responsive adenylyl cyclase pathway.

Myocardial beta-adrenergic adenylate cyclase complex in a canine model of chagasic cardiomyopathy

Infection of beagles with an opossum-derived strain of Trypanosoma cruzi (Tc-O) results in features of early and chronic chagasic cardiomyopathy, that is, increases in PR interval, atrioventricular block, and frequent ventricular premature contractions, ventricular tachycardia, and decreased left ventricular ejection fraction. These signs are not observed in animals infected with a canine strain of T. cruzi (Tc-D). To understand the biochemical basis for these early cardiac effects, we examined the beta-adrenergic adenylate cyclase complex in myocardial membranes prepared from animals infected with either of the two strains. In animals infected with Tc-O (symptomatic), the maximum velocity (Vmax) decreased and concentration of agonist resulting in 50% of Vmax (Kact) increased for isoproterenol-dependent adenylate cyclase activity; in animals infected with Tc-D (asymptomatic), Vmax and Kact for isoproterenol were unchanged from control, uninfected animals. beta-Receptor density decreased by 20% in symptomatic animals with no change in affinity, whereas no differences were observed between uninfected and infected asymptomatic animals. A complex pattern of changes was apparent in the guanine nucleotide binding protein, Gs, in the setting of infection. Alterations in cholera toxin-dependent ADP-ribosylation patterns as well as immunochemical detection with anti-G alpha s antisera suggested a change in the biochemical nature of the Gs species and not necessarily a physical loss of this protein. Reconstitution of adenylate cyclase activity in cyc- membranes demonstrated a decrease in hormone-sensitive Gs activity in membranes prepared from symptomatic animals without a change in activity demonstrable in the presence of Gpp(NH)p. Collectively, the results suggest that the depression in beta-adrenergic adenylate cyclase activity associated with symptomatic infection of beagles with T. cruzi occurs primarily as a result of changes in the Gs protein complex, most likely resulting in an uncoupling of the beta-adrenergic receptor from the Gs protein.

1,25-dihydroxyvitamin D reduces parathyroid hormone receptor number in ROS 17/2.8 cells and prevents the glucocorticoid-induced increase in these receptors: relationship to adenylate cyclase activation

We have previously shown that 1,25-dihydroxyvitamin D [1,25-(OH)2D3] and glucocorticoid modulate adenylate cyclase activation by PTH in osteoblast-like cells. Here we examine whether steroid effects on PTH receptor density explain the modulation of PTH action. Receptor assays were performed on late logarithmicphase monolayers of ROS 17/2.8 cells using human PTH-like peptide (hPLP) as radioligand. Kd and receptor density were computed from competition of tracer amounts of [125I-Tyr36] hPLP-(1-36) with unlabeled hPLP-(1-36) (0.1-30 nM). Steroid treatment had little or no effect on affinity for ligand. Pretreating cells with 10 nM 1,25-(OH)2D3 for 48 h decreased PTH receptor number to 17% of control values. Treating cells with 10 nM of the glucocorticoid triamcinolone acetonide (TRM) increased receptor number 10-fold, but simultaneous treatment with 1,25-(OH)2D3 (10 nM) completely prevented this receptor increase. Steroid effects required 13-18 h of treatment. Dose-response relationships for steroid modulation, determined from binding at 0.17 nM radioligand, indicated an EC50 of 0.3 nM for glucocorticoid augmentation of PTH receptor number and 0.02 nM for 1,25-(OH)2D3 reduction of receptor number in the presence of absence of the maximum TRM effect. The initial rate of cAMP production by receptor-saturating concentrations of PTH was 11,500 molecules per receptor per minute in untreated cells, comparable to reported turnover numbers for mammalian adenylate cyclase. Control experiments were validated measuring cAMP in intact cells as an indicator of adenylate cyclase activity. Cyclic AMP production was reduced 63% by 1,25-(OH)2D3 (10 nM) treatment. Glucocorticoid (10 nM) enhanced cAMP production twofold but reduced cAMP generation per receptor by 80%.(ABSTRACT TRUNCATED AT 250 WORDS)

Beta-adrenergic receptors and the Gs protein in myocardial ischemia and injury

The purpose of this study was to explore alterations in the life cycle of adrenergic receptors and the Gs protein in the heart of ischemic animals. In initial experiments left anterior descending coronary artery occlusion was performed in guinea pigs. Sarcolemmal (SL) and light vesicle (LV) (presumably intracellular) fractions were prepared. Both fractions contained a substantial number of beta-adrenergic receptors and alpha 1-adrenergic receptors: the relative proportion of beta-adrenergic receptors in LV/SL was greater than for alpha 1-adrenergic receptors. Myocardial ischemia produced a rapid externalization of beta-adrenergic receptors from LV to SL. alpha 1-adrenergic receptors also increased in SL but without an apparent externalization from LV. Pretreatment of animals with either the non-selective beta-antagonist propranolol or the beta 1-selective antagonist atenolol increased the number of SL beta-receptors and blunted the ischemia-induced increase in SL beta-adrenergic receptors. Treatment with the partial agonist pindolol did not cause up-regulation of beta-receptors, and did not block ischemia-induced externalization. In the second part of this study, we have begun to examine post-receptor events in a rat model of myocardial ischemia. Ligation of the distal left main coronary artery in the rat led to an increase in SL beta-receptors. As G proteins play a pivotal role in transducing receptor occupancy to activation of effector molecules, we measured levels of Gs which stimulates adenylate cyclase activity, using an ELISA technique. In rat SL the amount of alpha s markedly decreased within 15 min of coronary occlusion. There was no transfer of Gs activity to the light vesicle fraction. These studies indicate the dynamic nature of adrenergic receptors and the alpha s protein in the sarcolemma in myocardial ischemia. Changes in adrenergic receptor number and in G protein expression may contribute to the altered pathophysiology of the ischemic heart.

Regulation of beta-adrenergic responses during in vitro differentiation of mouse erythroleukemia cells

Dimethyl sulfoxide (DMSO)-induced erythroid differentiation of Friend mouse erythroleukemia (MEL) cells is associated with a marked transient modulation of catecholamine sensitivity. Within 24 h after induction and well before the onset of hemoglobin synthesis, we observed a 3-fold increase in beta-receptor density and a more than 10-fold increase in receptor-coupled cAMP formation. During the following 4 days, in parallel with the development of normoblast-like cells, receptor numbers returned to preinduction levels while catecholamine-dependent cAMP formation remained significantly elevated. Simultaneously, the apparent potency of the beta-adrenoceptor agonist isoprenaline increased 10-fold. Improved receptor-cyclase coupling is probably due to a major shift in the expression of Gi and Gs regulatory proteins. Bacterial toxin-mediated ADP-ribosylation of membrane proteins suggests that the dominating species in native cells is Gi (Gsa:Gia = 1.7). By contrast, Gs predominates in differentiated cells (Gsa:Gia = 1.8:1). Receptor-independent forskolin-stimulated cAMP formation showed a pronounced, albeit transient, decrease during differentiation. We suggest that these changes in cellular cAMP responses may be important for transient positive or negative cooperative interactions between hormones and growth factors in the course of erythroid cell development.

Extracellular matrix derived from Trypanosoma cruzi infected endothelial cells directs phenotypic expression

Infection of confluent human umbilical vein endothelial cells by the parasite Trypanosoma cruzi results in the appearance of an altered heparan sulfate proteoglycan (HSPG) isolated from the extracellular matrix of infected endothelial cells (ECMi). HSPG from ECMi differed from HSPG obtained from the extracellular matrix of uninfected endothelial cells (ECMu) by virtue of an 8-10-fold increase in sulfation and a different elution pattern using DEAE Sepharose chromatography. Analysis of the HSPG that binds to acidic fibroblast growth factor (aFGF) revealed that infection increased the proportion of HSPG that binds to aFGF by 35%. Heparitinase and alkaline borohydride treatment of aFGF-binding HSPG and chromatographic resolution on Sepharose CL4B column revealed an infection-associated 10-fold increase in sulfation of the GAG side chain with no significant change in the migration of the core protein. In addition, the aFGF binding HSPG isolated from ECMi demonstrated a markedly attenuated synergistic mitogenic activity with aFGF in a cell proliferation assay. All of the infection associated changes in HSPG could be demonstrated in HSPG obtained from uninfected endothelial cell cultures grown on ECMi. Hence, the ECMi is associated with signals capable of modulating the ECM associated metabolism of uninfected endothelial cells. This facility of ECMi was also shown to extend to patterns of Gs protein synthesis as revealed by Western blot analysis. The observation that the ECM produced by infected endothelial cells can direct the synthetic patterns of uninfected endothelial cells in a manner uniquely observed in infected endothelial cells suggests a plausible pathway by which infection of only a few cells can ultimately result in the coordinate responses of neighboring uninfected cells.

Sensorineural hearing loss owing to deficient G proteins in patients with pseudohypoparathyroidism: results of a multicentre study

Pseudohypoparathyroidism (PHP) is a rare disorder that might be caused by an hereditary defect in the G protein system. These membrane-bound proteins are responsible for the transduction of biological signals through the outer cell membrane. The investigation of 22 patients with PHP showed a symmetric sensorineural hearing loss in 63.6% of the subjects. In erythrocyte membrane preparations from blood samples of 15 of these patients, we measured the concentration of the stimulatory Gs protein and the inhibitory Gi protein by means of the Western blot analysis using selective antibodies against alpha-subunits of G proteins. In nine of the 15 cases (60%), we found a distinct decrease in the amount of the Gs protein with a partial preponderance of the Gi protein. These patients had a considerable symmetric sensorineural hearing loss. Pathophysiological mechanisms and the possible role of G proteins in the inner ear are discussed.

Chronic exposure of NG 108-15 cells to opiate agonists does not alter the amount of the guanine nucleotide-binding proteins Gi and Go

We have characterized the pertussis toxin substrate in NG 108-15 cell membranes using site-specific antisera and ADP-ribosylation. Cell membranes contain two pertussis toxin-sensitive guanine nucleotide-binding protein alpha-subunits (G alpha) whose Rf values in gel electrophoresis coincide with those of G alpha o and G alpha i2. The total quantity of Gi and Go immunoreactivity amounted to 24.3 +/- 2.8 pmol/mg, whereas only 1.5 +/- 0.2 pmol/mg are capable of undergoing ADP-ribosylation catalyzed by pertussis toxin. Pretreatment of cells with the agonist [D-Ala2,D-Leu2]-enkephalin (DADLE) for 24 h and DADLE or morphine for 72 h did not alter the incorporation of ADP-ribose or the immunoreactive amount of Gi and Go subunits. However, pretreatment for 72 h with naloxone increased the incorporation of ADP-ribose without an apparent change in affinity or in the immunochemically determined protein levels of Gi and Go. This indicates that the process of down-regulation and desensitization of the delta-opioid receptor neither requires quantitative alterations in the levels of Gi and Go nor changes in the degree of coupling among their subunits. In contrast, chronic exposure to antagonists seems to alter the degree of precoupling between alpha- and beta-subunits of Gi and/or Go.

Insulin-like growth factor-I and insulin increase the stimulatory guanine nucleotide binding protein (Gs) in cultured bovine adrenal cells

The present study shows that pretreatment of BAC cells with insulin or insulin-like growth factor-I (IGF-I) enhances the cAMP response to maximal concentrations of ACTH and cholera toxin. However, the effects of IGF-I at a nanomolar concentration (50 ng/ml) were higher than for insulin at the same concentration but similar for insulin at a micromolar concentration (10 micrograms/ml). We have investigated whether the effects of the two peptides can be related to some modifications of the guanine nucleotide regulatory binding protein Gs. Insulin enhanced Gs as observed by ADP ribosylation and immunoblotting but the effects were approximately the same at nanomolar and micromolar concentrations; again, the effects of IGF-I (50 ng/ml) were higher. These results indicate that both IGF-I and insulin increase the Gs complex of adenylate cyclase, but IGF-I is more potent than insulin at physiological concentrations.

Reconstitution of cyc- S49 membranes by in vitro translated Gs alpha. Membrane anchorage and functional implications

After ADP-ribosylation by cholera toxin which promotes dissociation of the subunits, the alpha-subunit of Gs (Gs alpha) remained strongly associated with plasma membranes of wild-type S49 cells, since its interaction with the membrane was insensitive to 1 M KCl. Its association with the membrane was partially disrupted by 6 M urea and totally abolished by treatment with alkali at pH greater than or equal to 11.5. In vitro translated Gs alpha could interact with plasma membranes from the cyc- mutant of S49 cells as revealed by its cosedimentation with the membrane fraction and incubation of reconstituted membranes with GTP gamma S did not alter anchorage of Gs alpha. The characteristics of the association of in vitro translated Gs alpha with cyc- membranes after GTP gamma S treatment, i.e. sensitivity to 1 M KCl, 6 M urea and alkali treatment, were very similar to those described for the ADP-ribosylated form in wild-type membranes. Restoration of the coupling between the adrenergic receptor and adenylate cyclase further confirmed the vectorial reconstitution of cyc- membranes by in vitro translated alpha-subunit of Gs.

Quantitation of a guanine nucleotide binding regulatory protein by an enzyme-linked immunosorbent competition assay

We have developed a new method using a competitive enzyme-linked immunosorbent assay, ELISA, for the determination of levels of the stimulatory guanine nucleotide binding protein, Gs, in membrane extracts. The method is based on the use of antipeptide antibodies generated in rabbits directed against amino acids 28-42 in the alpha-subunit, alpha s, of Gs. The peptide is utilized as the stationary phase in the ELISA and anti-alpha s antibody bound to the microtiter plate is assessed by a peroxidase-coupled anti-rabbit immunoglobulin G antibody that yields detectable color development at 490 nm. Gs purified from rabbit liver is utilized as the standard to assess the ability of Gs present in cholate extracts of membrane samples to compete with bound peptides for primary antibody. This assay provides a direct means to quantify changes in levels of native Gs in membranes and cells.

Chronic opiate receptor activation in vivo alters the level of G-protein subunits in guinea-pig myenteric plexus

Studies with the longitudinal muscle-myenteric plexus preparation of the guinea-pig ileum were undertaken to investigate the relationship between guanine nucleotide-binding proteins (G-proteins) and chronic opioid receptor activation in vivo. Treatment with the narcotic agonist fentanyl, at doses which render the preparation tolerant and dependent, led to an increase of pertussis toxin-catalysed incorporation of ADP-ribose in a protein of approximately 40,000 mol. wt. Quantitative immunoblotting, using site-directed antisera, demonstrated an upregulation of G alpha i/G alpha o and, to an even greater degree, of G beta. However, the level of G alpha s was decreased by the same treatment. All alterations observed were abolished by the concomitant presence of the antagonist naloxone. The implications of this differential regulation of G-protein subunits for opiate-induced tolerance and dependence are discussed.