Reconstitution of receptor/GTP-binding protein interactions

A genetically encoded sensor with improved fluorescence intensity for opioid detection at cellular resolution

The mu-opioid receptor (MOR) regulates the neuronal pathways involved in pain, reward, and respiration. To increase our understanding of MOR's roles in these pathways, there is a need to detect opioids at cellular resolution. Here, we engineered an improved opioid-sensor, called M-SPOTIT2, which is 11x brighter than our previously engineered M-SPOTIT1.1. We engineered M-SPOTIT2 by adding the amino acids YNSH, located near the fluorophore of the enhanced green fluorescent protein, to the circular permuted green fluorescent protein in M-SPOTIT2. M-SPOTIT2 is 11x brighter than our previously engineered M-SPOTIT1.1 in HEK293T cell culture and 2.7x brighter in neuronal culture. M-SPOTIT2 will potentially be useful for the detection of opioids in cell culture for drug screening and the detection of opioids at cellular resolution in animal tissues. By using M-SPOTIT2, researchers can gain more understanding about the mechanisms of addiction, respiratory suppression, and pain-modulation involved in opioid signaling.

Designing a Single Protein-Chain Reporter for Opioid Detection at Cellular Resolution

Mu-opioid receptor (MOR) signaling regulates multiple neuronal pathways, including those involved in pain, reward, and respiration. To advance the understanding of MOR's roles in pain modulation, there is a need for high-throughput screening methods of opioids in vitro and high-resolution mapping of opioids in the brain. To fill this need, we designed and characterized a genetically encoded fluorescent reporter, called Single-chain Protein-based Opioid Transmission Indicator Tool for MOR (M-SPOTIT). M-SPOTIT represents a new and unique mechanism for fluorescent reporter design and can detect MOR activation, leaving a persistent green fluorescence mark for image analysis. M-SPOTIT showed an opioid-dependent signal to noise ratio (S/N) up to 12.5 and was able to detect as fast as a 30-second opioid exposure in HEK293T cell culture. Additionally, it showed an opioid-dependent S/N up to 4.6 in neuronal culture and detected fentanyl with an EC₅₀ of 15 nM. M-SPOTIT will potentially be useful for high-throughput detection of opioids in cell cultures and cellular-resolution detection of opioids in vivo. M-SPOTIT's novel mechanism can be used as a platform to design other G-protein-coupled receptor-based sensors.

Biosensor-based affinities and binding kinetics of small molecule antagonists to the adenosine A(2A) receptor reconstituted in HDL like particles

The options for investigating solubilised G protein-coupled receptors (GPCRs) by biophysical techniques have long been hampered by their instability. A thermostabilised adenosine A2A receptor expressed in insect cells, purified in detergent and reconstituted into high-density lipoprotein (HDL) particles was immobilised onto a Surface Plasmon Resonance sensor chip. This allowed measurement of affinities and kinetics for A2A antagonists with affinities ranging from 50 pM to almost 2 μM. Compared with other formats, reproduction of affinities, and dissociation and association rate constants are good, reasonable and poor respectively, indicating stabilised receptors in HDL particles are useful for investigating specific aspects of GPCR-ligand interactions.

Comparative pharmacology of adrenergic alpha(2C) receptors coupled to Ca(2+) signaling through different Galpha proteins

Adrenergic alpha(1), alpha(2) and beta receptors are members of the G-protein-coupled receptor families (GPCRs) mediating physiological responses to adrenaline (epinephrine) and noradrenaline (norepinephrine). Since GPCRs are major targets for potential therapeutic agents, development of robust, reliable and cost effective functional screening methods for these receptors is in the focus of pharmacological research. For this reason, the aim of the present study was to develop an intracellular calcium assay for investigating the pharmacology of the alpha(2C) type of adrenergic receptors (alpha(2C)-AR). Although activation of alpha(2C)-AR is not linked to calcium mobilization, co-expression of these receptors with the chimeric Galpha(qi5) protein, containing the five carboxyl-terminal amino acids from G(i), or promiscuosus Galpha(16) protein can divert receptor signaling to the G(q) pathway generating Ca(2+) release from intracellular stores. In order to assess the functional potency of alpha(2)-AR agonists and antagonists, we established a fluorometric Ca(2+) assay using cell lines stably and constitutively co-expressing alpha(2C)-AR and Galpha(qi5) or Galpha(16) proteins (Galpha(qi5)/alpha(2C) and Galpha(16)/alpha(2C)). As part of the pharmacological characterization, we measured the changes in cytoplasmic Ca(2+) levels due to activation of the chimeric Galpha(qi5) or Galpha(16) coupled recombinant alpha(2C) receptors as a function of increasing concentration of several agonists (noradrenaline, brimonidine, oxymetazoline, clonidine, moxonidine) and antagonists (MK912, yohimbine). The binding affinities of alpha(2)-AR agonist and antagonists and the inhibition of the forskolin-stimulated cAMP accumulation in alpha(2C)-AR expressing cells were also measured. These results confirmed that the Galpha(qi5)/alpha(2C) and Galpha(16)/alpha(2C) recombinant systems can be useful for modelling the native G(i)-coupled system. Our results indicate that a plate-reader based fluorometric Ca(2+) assay may be suitable in high-throughput screening for alpha(2C)-AR ligands as well.

Functional reconstitution of β2-adrenergic receptors utilizing self-assembling Nanodisc technology

Integral membrane G protein-coupled receptors (GPCRs) compose the single most prolific class of drug targets, yet significant functional and structural questions remain unanswered for this superfamily. A primary reason for this gap in understanding arises from the difficulty of forming soluble, monodisperse receptor membrane preparations that maintain the trans-membrane signaling activity of the receptor and provide robust biophysical and biochemical assay systems. Here we report a technique for self-assembling functional 2-adrenergic receptor (β2AR) into a nanoscale phospholipid bilayer system (Nanodisc) that is highly soluble in aqueous solution. The approximately 10-nm nanobilayer particles contain β2AR in a native-like phospholipid bilayer domain of approximately 100 phospholipid molecules circumferentially bound by a membrane scaffold protein (MSP). The resulting construct allows for access to the physiologically intracellular and extracellular faces of the receptor and thus allows unrestricted access of antagonists, agonists, and G proteins. These Nanodisc-solubilized GPCRs can be directly purified by normal chromatographic procedures. We define the resultant Nanodisc-embedded monomeric β2AR by antagonist and agonist binding isotherms and demonstrate faithful G protein coupling.

Inhibitory effects of a dominant-interfering form of the Rho-GTPase Cdc42 in the chemoattractant-elicited signaling pathways leading to NADPH oxidase activation in differentiated HL-60 cells

A tetracycline-controlled expression system was adapted to the human promyelocytic HL-60 cell line by placement of the transactivator (tTA-off) sequence under the control of the human EF-1alpha promoter region. Constitutively active and dominant-inhibitory forms of Cdc42 (Cdc42V12 and Cdc42N17, respectively) were conditionally expressed in this system. The expression of Cdc42V12 had no marked effect on chemoattractant-mediated superoxide production, corroborating previous results indicating that the guanosine 5'-triphosphate (GTP)-bound form of Cdc42 is ineffective in directly activating nicotinamide adenine dinucleotide phosphate (NADPH) oxidase in a cell-free system. However, the N17 mutant potently inhibited chemoattractant-induced superoxide production. The expression of Cdc42N17 interfered with the GTP-loading of Rac and Ras and with the activation of the MAP-kinase pathway. A drastic reduction of chemoattractant-induced inositol-1,4,5-trisphosphate formation and calcium mobilization was observed, corroborating previous in vitro study results identifying PLCbeta2 as a Rac/Cdc42 effector. Cdc42N17 was also found to inhibit the translocation of Ras-GRF2, a guanine nucleotide exchange factor for Ras and Rac but not for Cdc42. Thus, the dominant-inhibitory mutant Cdc42N17 was found to interfere at multiple levels in the signaling pathways. The pleiotropic inhibitory effects of Cdc42N17 illustrate the potential pitfalls of using dominant-inhibitory proteins to study the function of Ras-family GTPases. In this regard, a number of conclusions drawn from the use of dominant-inhibitory mutants in myeloid cells might have to be reconsidered.

IMP and AMP deaminase in reperfusion injury down-regulates neutrophil recruitment

We examined gene regulation in murine lungs after hind-limb vessel occlusion and reperfusion. A rapid increase of transcript for the AMP deaminase 3 gene (AMPD3) and its enzymatic activity (EC) generating inosine monophosphate (IMP) were identified with transcripts located in bronchial and alveolar epithelium. AMP deaminase inhibitor decreased IMP levels and significantly enhanced neutrophil recruitment within lung tissue during reperfusion. In addition, IMP inhibited cytokine-initiated neutrophil infiltration in vivo and selectively attenuated neutrophil rolling by 90% in microvessels. We prepared labeled IMP and demonstrated that IMP specifically binds to neutrophils. IMP also stimulated binding of gamma-[(35)S]thio-GTP, suggesting that IMP is a potent regulator of neutrophils. Taken together, these results elucidate a previously unrecognized mechanism that protects tissues from the potentially deleterious consequences of aberrant neutrophil accumulation. Moreover, they are relevant for new therapeutic approaches to regulate neutrophil responses in inflammation and vascular disease.

A GppNHp-insensitivity factor modulates the activation of beta-adrenoceptor-coupled Gs protein in rat cortex and cerebellum

The effect known as the GTP-shift refers to the complete conversion of receptors from the high- to the low agonist-affinity state in the presence of an excess of GTP or one of its analogs. 5'-Guanylylimidodiphosphate (GppNHp) was able to fully suppress the high (-)-isoproterenol-affinity of beta-adrenoceptors (beta AR) in cultured rat brain astrocytes. In contrast, a proportion of beta AR in rat cortex and cerebellum synaptosomes was found to be insensitive to this GTP analog. This GppNHp-insensitivity was due to a membrane-associated factor, presumably interacting with Gs proteins and not present in a functional form in cultured astrocytes. Here we assessed the effect of this factor on the beta AR-mediated activation of Gs proteins. The removal of the GppNHp-insensitivity factor from the synaptosomes was achieved using 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS), a mild detergent. The activation of Gs proteins was monitored by the binding of another non-hydrolysable GTP-analog, guanylyl 5'-[gamma-[35S]thio]-triphosphate ([35S]GTP gamma S). The beta AR-Gs protein coupling was at least twofold less efficient in synaptosomes relative to cultured astrocytes. The CHAPS treatment induced a twofold increase in the coupling efficiency in cortex and cerebellum synaptosomes, but had no effect in cultured astrocytes. It undoubtedly indicated the inhibitory effect of the GppNHp-insensitivity factor on the activation of Gs proteins in the synaptosomes. Using CHAPS-soluble material extracted from synaptosomes, it was possible to reconstitute the GppNHp-insensitivity of CHAPS-treated membranes or even to induce it in cultured astrocytes. This effect correlated with the amount of CHAPS-soluble material according to a sigmoid curve, but it was abolished by the heat of CHAPS-soluble material. Successful crossed reconstitutions of the GppNHp-insensitivity suggest that the GppNHp-insensitivity factor is the same regardless of its originating area, and that it might play a general role in the central nervous system. Further investigations should help to identify the GppNHp-insensitivity factor.

Activation of various subtypes of G-protein alpha subunits by partial agonists of the adenosine A1 receptor

The activation of different G protein subtypes by the rat adenosine A1 receptor initiated by stimulation with the full agonist 2-chloro-N6-cyclopentyladenosine (CCPA) and by six structurally distinct partial agonists of this receptor was investigated. Endogenous G protein alpha subunits in rat cortical membranes were inactivated by N-ethylmaleimide (NEM). Activation of rat recombinant myristoylated alpha(o), alpha(i1), alpha(i2) and alpha(i3) by partial agonists in comparison to the full agonist was assessed by guanosine-5'-(gamma-[35S]thio)triphosphate ([35S]GTPgammaS) binding after reconstitution of G protein alpha subunits with the adenosine A1 receptor in N-ethylmaleimide-treated membranes. 2-Chloro-N6-cyclopentyladenosine and 3' -deoxy-N6-cyclopentyladenosine (3'-d-CPA), the partial agonist with the highest intrinsic activity, were significantly more potent in activation of alpha(i) subtypes than alpha(o). In contrast, 5'-methylthioadenosine (MeSA), 2'-deoxy-2-chloroadenosine (cladribine), 2'-deoxy-N6-cyclopentyladenosine (2'-d-CPA), 2-phenylaminoadenosine (CV 1808) and C8-aminopropyl-N6-cyclopentyladenosine (C8-aminopropyl-CPA) did not exhibit higher potency for Go or any Gi subtype. All partial agonists, although carrying structurally different modifications, showed higher relative intrinsic activities in activation of Gi than of Go, indicating that Gi-coupled pathways may be activated selectively via the A1 receptor by partial agonists, but not Go-mediated responses.

Cytoskeletal regulation of the signal transduction of prostaglandin EP4 receptor

Prostaglandin (PG) EP4 receptor is coupled to Gs, stimulating adenylate cyclase. We tested whether cytoskeleton modulates the signal transduction of the EP4 receptor. A microtubule depolymerizing agent, colcemid, enhanced the PGE2-induced cAMP formation in the cloned EP4 receptor-expressing Chinese hamster ovary cells, but enhanced neither NaF plus AlCl3 nor forskolin-induced cAMP formation. Other microtubule depolymerizing agents, including colchicine, also induced the enhancement. These effects stemmed from the action of the agents on microtubules, because beta-lumicolchicine, an inactive isomer of colchicine, had no effect. In contrast, the microfilament depolymerizing agents did not affect the PGE2-induced cAMP formation but potentiated the enhancing effect of colcemid. This enhancement by colcemid was not due to the suppression of the desensitization of the EP4 receptor. The enhancing effect of colcemid was also observed in another Gs-coupled PGE receptor subtype, EP2 receptor. These results demonstrate that the state of microtubule assembly modulates the signal transduction of the EP4 receptor in concert with microfilament.

Recognition and binding of mitochondrial presequences during the import of proteins into mitochondria

Nuclear-encoded mitochondrial proteins are imported into mitochondria due to the presence of a targeting sequence, the presequence, on their amino termini. Presequences, which are typically proteolyzed after a protein has been imported into a mitochondrion, lack any strictly conserved primary structure but are positively charged and are predicted to form amphiphilic alpha-helices. Studies with synthetic peptides corresponding to various presequences argue that presequences can partition nonspecifically into the mitochondrial outer membrane and that the specificity of translocation of precursors into mitochondria may depend on interactions of the presequence with the electrical potential of the inner membrane. Although proteins of the outer membrane that are necessary for the translocation of precursor proteins have been proposed to function as receptors for presequences, the binding of presequences to these proteins has not been demonstrated directly. Proteins of the mitochondrial outer membrane may not be responsible for the specificity of translocation of precursors but may instead function, together with cytosolic molecular chaperones, to maintain precursor proteins in conformations that are competent for translocation as the precursors associate with the mitochondrial surface.

Functional reconstitution in situ of 5-hydroxytryptamine2c (5HT2c) receptors with alphaq and inverse agonism of 5HT2c receptor antagonists

Membranes prepared after infection of Sf9 cells with recombinant baculovirus containing the rat 5HT2c receptor DNA, but not after infection with wild-type virus, expressed high affinity binding sites for 125I-lysergic acid diethylamide and [3H]mesulergine. The receptor site density reached an optimum of 50-70 pmol/mg membrane protein at 60 h postinfection. Extraction of peripheral membrane proteins from the postnuclear membrane fraction with 6 M urea depleted GTPgammaS-binding 4-fold without decreasing 5HT2c receptor binding activity. Urea-extracted Sf9 membranes expressing the 5HT2c receptor catalyzed the activation of squid retinal alphaq but not bovine retinal alphat or bovine alphao/alphai. Productive interaction of 5HT2c receptors with squid alphaq was enhanced by the addition of betagamma dimers prepared from either bovine brain or bovine rod outer segment discs. While the addition of serotonin increased 5HT2c receptor-catalyzed GTPgammaS binding to alphaq, the unoccupied receptor was also catalytically active. The 5HT2c receptor antagonists, mesulergine, mianserin, and ketanserin competitively inhibited 5HT activation of the receptor with predicted rank-order affinities; and mianserin and ketanserin markedly inhibited basal 5HT2c receptor activity. Interestingly, this "inverse agonist" efficacy did not correlate with antagonist affinity for the 5HT2c receptor. Baculoviral expression of the 5HT2c receptor and urea extraction of postnuclear Sf9 cell membranes have provided a high density of in situ, uncoupled, G-protein-linked receptor useful for reconstitution with purified G-protein subunits. This has allowed for independent manipulation of receptor and G-protein chemical concentrations and has revealed that a G-protein-linked receptor can possess a significant basal catalytic activity and that antagonist compounds can act as inverse agonists of this basal activity at the level of receptor activation of G-proteins.

Role of heterotrimeric G-proteins in epidermal growth factor signalling

Since in 1986 it was reported that a pertussis toxin-sensitive substrate was involved in the Ca2+ signal induced by epidermal growth factor (EGF) in rat hepatocytes, much evidence accumulated to implicate heterotrimeric G-proteins in EGF action. EGF can also induce a cyclic AMP signal, but while the generation of a Ca2+ signal appears to be quite general in EGF action, the increase in cyclic AMP occurs only in few cell types. In non-transformed cell types these effects appear to involve G-proteins. EGF not only induces cell proliferation but also interacts with hormones in the short-term control of cell function in quiescent cells. Most of the known interactions are on cyclic AMP mediated hormone effects, and in many cases, the interaction between EGF and hormones involves G-proteins. Here we review the evidence accumulated in recent years that implicate G-proteins in EGF action. An understanding of the mechanisms involved may reveal new mechanisms of G-protein regulation and will contribute to our knowledge of EGF function and signal transduction.

S-prenylated cysteine analogues inhibit receptor-mediated G protein activation in native human granulocyte and reconstituted bovine retinal rod outer segment membranes

We have previously shown that the S-prenylated cysteine analogue N-acetyl-S-trans,trans-farnesyl-L-cysteine (L-AFC) inhibits basal and formyl peptide receptor-stimulated binding of guanosine 5'-O-(3-thiotriphosphate) (GTP[S]) to and hydrolysis of GTP by membranes of HL-60 granulocytes and have presented evidence suggesting that this inhibition was not caused by reduced protein carboxyl methylation [Scheer, A., & Gierschik, P. (1993) FEBS Lett. 319, 110-114]. We now report a detailed analysis of the structural properties of S-prenylated cysteine analogues required for this inhibition and demonstrate that S-prenylcysteines also suppress basal and receptor-stimulated GTP[S] binding to human peripheral neutrophil and HL-60 granulocyte membranes when stimulated by formyl peptide and complement C5a, respectively. S-Prenylcysteines did not affect pertussis toxin-mediated [32P]ADP-ribosylation of Gi proteins. The inhibitory effect of L-AFC was reversible and was not mimicked by farnesylic acid. L-AFC also interfered with GTP[S] binding to retinal transducin when stimulated by light-activated rhodopsin in a reconstituted system. This inhibitory effect was fully reversed upon increasing the concentration of either the G protein beta gamma dimer or the activated receptor. On the basis of these results, we suggest that S-prenylated cysteine analogues like L-AFC inhibit receptor-mediated G protein activation by specifically and reversibly interfering with the interaction of activated receptors with G proteins, most likely with their beta gamma dimers, rather than by inhibiting alpha.beta gamma heterotrimer formation.

Characterization of growth hormone-releasing hormone (GHRH) binding to cloned porcine GHRH receptor

To study structure-activity relationships of growth hormone-releasing hormone (GHRH), a competitive binding assay was developed using cloned porcine adenopituitary GHRH receptors expressed in human kidney 293 cells. Specific binding of [His1, 125I-Tyr10,Nle27]hGHRH(1-32)-NH2 increased linearly with protein concentration (10-45 micrograms protein/ tube). Binding reached equilibrium after 90 min at 30 degrees C and remained constant for at least 240 min. Binding was reversible to one class of high-affinity sites (Kd = 1.04 +/- 0.19 nM, Bmax = 3.9 +/- 0.53 pmol/mg protein). Binding was selective with a rank order of affinity (IC50) for porcine GHRH (2.8 +/- 0.51 nM), rat GHRH (3.1 +/- 0.69 nM), [N-Ac-Tyr1, D-Arg2]hGHRH(3-29)-NH2 (3.9 +/- 0.58 nM), and [D-Thr7]GHRH(1-29)-NH2 (189.7 +/- 14.3 nM), consistent with their binding to a GHRH receptor. Nonhydrolyzable guanine nucleotides inhibited binding. These data describe a selective and reliable method for a competitive GHRH binding assay that for the first time utilizes rapid filtration to terminate the binding assay.

Liver beta-adrenergic receptors, G proteins, and adenylyl cyclase activity in obesity-diabetes syndromes

The ob and db genes produce similar hormonal anomalies in mice. Although the expression of the syndromes diverges with age, at 8-12 wk both ob/ob and db/db mice are hyperglycemic and hyperinsulinemic and show evidence of hypercorticoidism. Nevertheless, membranes isolated from livers of ob/ob and db/db mice behave differently in terms of adenylyl cyclase activity and beta-adrenergic receptor function. There are three times as many beta 2-adrenergic receptor binding sites and a threefold increase in the response to catecholamines in ob/ob mouse liver membranes than in comparable preparations from normal controls or db/db mice. By contrast, the two main G proteins of liver membranes (Gs alpha and Gi alpha 2) are less abundant in the mutants, ob/ob and db/db, than in their respective lean controls. Adrenalectomy normalizes the exaggerated response to beta-adrenergic agonists and the number of beta-adrenergic binding sites in the ob/ob mouse. This shows that the enhanced beta-adrenergic receptor response is linked to hypercorticoidism. Cellular maturation and differentiation (D. C. Watkins, J. K. Northrup, and C. C. Malbon, J. Biol. Chem. 262: 10651-10657, 1987) and diseases such as obesity and diabetes (cf. N. McFarlane-Anderson, J. Bailly, and N. Bégin-Heick, Biochem. J. 282: 15-23, 1992) have been associated with modifications in the complement of G proteins detected in cells. However, the relationship among levels, types, and intracellular localization of G proteins in tissues and their influence on the transduction of the message to an effector system, such as adenylyl cyclase, are not yet well understood.

Pharmacological characterization of acetylcholine-stimulated [35S]-GTP gamma S binding mediated by human muscarinic m1-m4 receptors: antagonist studies

1. We have used dose-ratio analysis to estimate functionally the affinity constants (pKb) and Schild slope factors of a range of selective or atypical antagonists at human muscarinic m1-m4 receptors. 2. The functional response was the stimulation by acetylcholine of [35S]-GTP gamma S binding to membranes from Chinese hamster ovary (CHO) cells stably expressing individual receptor subtypes. 3. A novel experimental design and analysis was used which allowed the estimation of affinity and Schild slope factor from a single antagonist inhibition curve, and the results were compared with other methods of analysis, both theoretically valid and invalid. 4. In general, the affinity estimates were very similar to previously reported values obtained in binding studies with animal tissues and cloned human receptors and the Schild slope factors were close to unity. 5. These results demonstrate the validity of the assay and provide no evidence for species differences in antagonist affinity for muscarinic receptor subtypes. 6. The results confirm both the utility of himbacine in distinguishing between m1 and m4 receptors and a previously reported modest m4-selectivity for tropicamide and secoverine. 7. The cholinesterase inhibitor, tacrine (THA), had a potency profile similar to that of gallamine but with less selectivity. Its affinity could not be determined since it had Schild slope factors of about 2 at all subtypes. 8. o-Methoxy-sila-hexocyclium had only a modest selectivity for the m1 subtype.