Nociception activates Elk-1 and Sap1a following expression of the ORL1 receptor in Chinese hamster ovary cells

Pharmacological Assays for Investigating the NOP Receptor

The nociceptin/orphanin FQ (N/OFQ) peptide receptor (NOP) is a G protein-coupled receptor involved in the regulation of several physiological functions and pathological conditions. Thus, researchers from academia and industry are pursuing NOP to discover and study novel pharmacological entities. In a multidisciplinary effort of pharmacologists, medicinal chemists, and molecular and structural biologists the mechanisms of NOP activation and inhibition have been, at least partially, disentangled. Here, we review the in vitro methodologies employed, which have contributed to our understanding of this target. We hope this chapter guides the reader through the mostly established assay platforms to investigate NOP pharmacology, and gives some hints taking advantage from what has already illuminated the function of other GPCRs. We analyzed the pharmacological results obtained with a large panel of NOP ligands investigated in several assays including receptor binding, stimulation of GTPγS binding, decrease of cAMP levels, calcium flux stimulation via chimeric G proteins, NOP/G protein and NOP/β-arrestin interaction, label-free assays such as dynamic mass redistribution, and bioassays such as the electrically stimulated mouse vas deferens.

Regulation of G Protein βγ Signaling

Heterotrimeric guanine nucleotide-binding proteins (G proteins) deliver external signals to the cell interior, upon activation by the external signal stimulated G protein-coupled receptors (GPCRs).While the activated GPCRs control several pathways independently, activated G proteins control the vast majority of cellular and physiological functions, ranging from vision to cardiovascular homeostasis. Activated GPCRs dissociate GαGDPβγ heterotrimer into GαGTP and free Gβγ. Earlier, GαGTP was recognized as the primary signal transducer of the pathway and Gβγ as a passive signaling modality that facilitates the activity of Gα. However, Gβγ later found to regulate more number of pathways than GαGTP does. Once liberated from the heterotrimer, free Gβγ interacts and activates a diverse range of signaling regulators including kinases, lipases, GTPases, and ion channels, and it does not require any posttranslation modifications. Gβγ family consists of 48 members, which show cell- and tissue-specific expressions, and recent reports show that cells employ the subtype diversity in Gβγ to achieve desired signaling outcomes. In addition to activated GPCRs, which induce free Gβγ generation and the rate of GTP hydrolysis in Gα, which sequester Gβγ in the heterotrimer, terminating Gβγ signaling, additional regulatory mechanisms exist to regulate Gβγ activity. In this chapter, we discuss structure and function, subtype diversity and its significance in signaling regulation, effector activation, regulatory mechanisms as well as the disease relevance of Gβγ in eukaryotes.

Cellular mechanisms of nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor regulation and heterologous regulation by N/OFQ

The nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor is the fourth and most recently discovered member of the opioid receptor superfamily that also includes μ, δ, and κ opioid receptor subtypes (MOR, DOR, and KOR, respectively). The widespread anatomic distribution of the NOP receptor enables the modulation of several physiologic processes by its endogenous agonist, N/OFQ. Accordingly, the NOP receptor has gained a lot of attention as a potential target for the development of ligands with therapeutic use in several pathophysiological states. NOP receptor activation frequently results in effects opposing classic opioid receptor action; therefore, regulation of the NOP receptor and conditions affecting its modulatory tone are important to understand. Mounting evidence reveals a heterologous interaction of the NOP receptor with other G protein-coupled receptors, including MOR, DOR, and KOR, which may subsequently influence their function. Our focus in this review is to summarize and discuss the findings that delineate the cellular mechanisms of NOP receptor signaling and regulation and the regulation of other receptors by N/OFQ and the NOP receptor.

Ternary complex factors: prime nuclear targets for mitogen-activated protein kinases

Ternary complex factors (TCFs), a subgroup of the ETS protein family, were first described in the context of c-fos gene regulation. Subsequently, their early identification as nuclear targets for mitogen-activated protein kinases served to exemplify the fundamental links in eukaryotic cells between growth factor-mediated signalling pathways and gene control. This article provides an overview of recent work on ternary complex factors, addressing their expression and molecular structure, as well as how selective interactions with members of other protein families serve to up-1 regulate or restrict their activity. Although only one genetic study on ternary complex factors has been published to date, unravelling of the underlying molecular events provides a basis for tentative predictions about their biological roles in mammalian organisms.

Molecular basis of opioid dependence: role of signal regulation by G-proteins

1. Morphine and opiate narcotics are potent analgesics that have a high propensity to induce tolerance and physical dependence following their repeated administration. 2. The molecular basis of opiate dependence has not been completely elucidated, although the participation of opioid receptors is a prerequisite. Cellular dependence on opioids is believed to result from the chronic stimulation of opioid-regulated signalling networks. 3. As G-protein-coupled receptors, the opioid receptors must rely on heterotrimeric G-proteins for signal transduction. Recent advances in our understanding of G-protein signalling have unveiled novel signalling molecules and mechanisms, some of which may be intricately involved in the manifestation of opiate dependence. 4. In the present review, we will attempt to trace chronic opioid signals along elaborate G-protein-regulated pathways.

Development of a homogeneous MAP kinase reporter gene screen for the identification of agonists and antagonists at the CXCR1 chemokine receptor

Agonist activity at G protein-coupled receptors (GPCRs) that regulate heterotrimeric G proteins of the Galpha(i/o) or Galpha(q) families has been shown to result in activation of the mitogen-activated protein (MAP) kinase cascade. To facilitate compound screening for these classes of GPCR, we have developed a reporter gene that detects the activation of the ternary complex transcription factor Sap1a following MAP kinase activation. In contrast to other reporter gene assays for Galpha(i/o)-coupled GPCRs, the MAP kinase reporter generates an increase in signal in the presence of agonist. The reporter gene has been transfected into Chinese hamster ovary cells to generate a "host" reporter gene-containing cell line. The Galpha(i)-coupled human CXCR1 chemokine receptor was subsequently transfected into this cell line in order to develop a 384-well format screen for both agonists and antagonists of this receptor. Agonists activated the reporter gene with the expected rank order of potency and with similar concentration dependence as seen with the regulation of other signal transduction cascades in mammalian cells: interleukin-8 (IL-8) (pEC(50) = 7.0 +/- 0.1) > GCP-2 (pEC(50) = 6.3 +/- 0.1) > NAP-2 (pEC(50) < 6). CXCR1-mediated activation of MAP kinase was inhibited by pertussis toxin and the MEK inhibitor PD98059, demonstrating that receptor activation of MAP kinase is due to pertussis toxin-sensitive Galpha(i/o)-family G proteins to cause the activation of MEK kinase. Using the 384-well format, assay performance was unaffected by solvent concentrations of 0.5% ethanol, 0.15% glycerol, or 1% DMSO. Signal crosstalk between adjacent wells was less than 1%. The assay exhibited a Z factor of 0.53 and a coefficient of variation of response to repeated application of IL-8 (100 nM) of 15.9%.

Development of a facile method for high throughput screening with reporter gene assays

This report describes a facile methodology for high throughput screening with stable mammalian cell reporter gene assays. We have adapted a 96-well adherent cell method to an assay in which cells propagated in suspension are dispensed into 96- or 384-well plates containing test compounds in 100% DMSO. The validation of a stable CHO cell line that expresses 6xCRE-luciferase for use as a reporter gene host cell line is described. The reporter gene, when expressed in this particular CHO cell line, appears to respond specifically to modulation of cAMP levels, thus the cell line is appropriate for screening and pharmacological analysis of Galpha(s)- and Galpha(i)-coupled seven-transmembrane receptors. The development of the new suspension cell assay in both 96- and 384-well formats was performed using a derivative of the CHO host reporter cell line that was stably transfected with human melanocortin-1 receptor. The response of this cell line to NDP-alpha-melanocyte-stimulating hormone and forskolin was nearly identical between the adherent and suspension methods. The new method offers improvements in cost, throughput, cell culture effort, compound stability, accuracy of compound delivery, and hands-on time. The 384-well assay can be performed at high capacity in any laboratory without the use of expensive automation systems such that a single person can screen 100 plates per day with 3.5-4 h hands-on time. Although the system has been validated using Galpha(s)-coupled receptor-mediated activation of a cAMP response element, the method can be applied to other types of targets and/or transcriptional response elements.

Activation of heterotrimeric G-protein signaling by a ras-related protein. Implications for signal integration

Utilizing a functional screen in the yeast Saccharomyces cerevisiae we identified mammalian proteins that activate heterotrimeric G-protein signaling pathways in a receptor-independent fashion. One of the identified activators, termed AGS1 (for activator of G-protein signaling), is a human Ras-related G-protein that defines a distinct subgroup of the Ras superfamily. Expression of AGS1 in yeast and in mammalian cells results in specific activation of Galpha(i)/Galpha(o) heterotrimeric signaling pathways. In addition, the in vivo and in vitro properties of AGS1 are consistent with it functioning as a direct guanine nucleotide exchange factor for Galpha(i)/Galpha(o). AGS1 thus presents a unique mechanism for signal integration via heterotrimeric G-protein signaling pathways.

Use of a dual firefly and Renilla luciferase reporter gene assay to simultaneously determine drug selectivity at human corticotrophin releasing hormone 1 and 2 receptors

The aim of this study was to investigate and validate the use of a dual glow-signal luciferase reporter gene assay to simultaneously evaluate drug activity at two different seven-transmembrane receptor subtypes. Stable cell lines (CHO) transfected with either human corticotrophin releasing hormone 1 (hCRH1) receptors and a firefly luciferase reporter gene or hCRH2 and a Renilla luciferase reporter gene were created to provide different luciferase readouts for CRH1 and CRH2 receptors, respectively. Cells were combined for stimulation and measurement of luciferase luminescence in a 96-well plate format assay. The nonselective CRH agonists rat/human CRH and sauvagine caused concentration-dependent increases in luminescence via activation of CRH1 (firefly luciferase; pEC50 = 8.40 +/- 0.06 and 8.39 +/- 0.08, respectively, n = 8) and CRH2 (Renilla luciferase; pEC50 = 8.89 +/- 0.14 and 8.92 +/- 0.13, respectively, n = 8) receptors. The nonselective CRH antagonist astressin blocked these agonist-induced increases in luciferase at both CRH1 and CRH2 receptors. The selective CRH1 antagonist CP154,526 blocked r/hCRH- and sauvagine-induced increases in luciferase at CRH1 receptors only. These data report the expected pharmacology for CRH1 and CRH2 receptors. This assay enabled two receptor subtypes to be studied simultaneously in the same 96-well plate and generated robust data with low variability. It has the potential advantage of significant time and cost savings, with application to both basic research and compound screening.

Characterisation of somatostatin sst2 receptor splice variants

Somatostatin is a peptide with a multitude of functions in the central nervous system and the periphery. It mediates its actions by binding to high-affinity G-protein coupled receptors, genes for five of which (sst1-sst5) have recently been cloned. The somatostatin sst2 receptor exists as two splice variants, sst2(a) and sst2(b) receptors, which differ in length and composition of their intracellular carboxy-termini. In this review, we describe the localisation of the two receptor isoforms in the central nervous system, the periphery and also in tumour tissue. Furthermore, we summarise and discuss the data on the functional properties of the recombinant splice variants that have been generated so far, which include activation of extracellular acidification rates, inhibition of adenylate cyclase and activation of MAP-kinases as well as the transcription factor Elk-1.

A combination assay for simultaneous assessment of multiple signaling pathways

We have developed an assay in which modulation of two or more signaling pathways can be assessed concurrently by combining reporter gene systems with fluorescent probe technology. The validation of this method was achieved by indirect analysis of adenylyl cyclase activation with the use of a cyclic AMP response element (CRE)-luciferase reporter system in combination with the measurement of calcium mobilization by Calcium Green-1 AM fluorescence on a fluorescent imaging plate reader. To demonstrate the utility of the method in studying the pharmacology of receptors that couple to more than one G protein, Chinese hamster ovary (CHO) cells, which stably expressed both the CRE-luciferase reporter gene and the human pituitary adenylyl cyclase-activating peptide (PACAP) receptor, were treated with PACAP 1-27 and 1-38. Calcium mobilization and the induction of adenylyl cyclase activity in response to each concentration of peptide were assessed in individuals wells. This assay may also be used to screen for ligands of two or more unrelated receptors simultaneously without compromising the assessment of either signaling pathway. To illustrate this point, Rat-1 fibroblasts, which expressed human alpha1A receptors, were cocultured with CRE-luciferase CHO cells, which expressed human GLP-1 receptors. Calcium mobilization elicited by phenylephrine agonism of the alpha1A receptor was assessed in the same assay as GLP-1-induced activation of adenylyl cyclase. The pEC(50) for each agonist was similar to that observed when the cell lines were not cocultured. The number of different receptors that can be screened per well is limited only by the ability to distinguish different reporter gene signals and fluorescent indicators.

Functional analysis of a human A(1) adenosine receptor/green fluorescent protein/G(i1)alpha fusion protein following stable expression in CHO cells

Fusion proteins between the human A(1) adenosine receptor and the pertussis toxin resistant (Cys351Gly) mutant of the G-protein alpha subunit G(i1)alpha (A1/Gi), and between the human A(1) adenosine receptor, the Aequorea victoria green fluorescent protein (GFP) and Cys351Gly G(i1)alpha (A1/GFP/Gi), were expressed in CHO cells. The agonist NECA caused a stimulation of [(35)S]GTPgammaS binding at both fusion proteins with similar concentration dependence as at the native receptor. However in the presence of pertussis toxin NECA stimulation of [(35)S]GTPgammaS binding was only seen at the A1/GFP/Gi fusion protein. The regulation of the adenylyl cyclase and MAP kinase effector systems by both fusion proteins was attenuated following pertussis toxin treatment. These studies demonstrate for the first time the characterisation of a fusion protein between a G-protein coupled receptor, GFP and a G-protein alpha subunit.