Structure and function of G protein coupled receptors

Fluorescent proteins and genetically encoded biosensors

The genetically encoded fluorescent sensors convert chemical and physical signals into light. They are powerful tools for the visualisation of physiological processes in living cells and freely moving animals. The fluorescent protein is the reporter module of a genetically encoded biosensor. In this study, we first review the history of the fluorescent protein in full emission spectra on a structural basis. Then, we discuss the design of the genetically encoded biosensor. Finally, we briefly review several major types of genetically encoded biosensors that are currently widely used based on their design and molecular targets, which may be useful for the future design of fluorescent biosensors.

Extracellular Ca2+ induces desensitized cytosolic Ca2+ rise sensitive to phospholipase C inhibitor which suppresses root growth with Ca2+ dependence

Calcium (Ca) is an essential element for all organisms. In animal cells, the plasma membrane-localized Ca receptor CaSR coupled to a phospholipase C (PLC)-dependent signaling cascade monitors extracellular Ca²⁺ concentrations ([Ca²⁺]_ext) and responds with increases in cytosolic calcium concentrations ([Ca²⁺]_cyt). Plant roots encounter variable soil conditions, but how they sense changes in [Ca²⁺]_ext is largely unknown. In this study, we demonstrate that increasing [Ca²⁺]_ext evokes a transient increase in [Ca²⁺] in the cytosol, mitochondria, and nuclei of Arabidopsis thaliana root cells. These increases were strongly desensitized to repeat applications of [Ca²⁺]_ext, a typical feature of receptor-mediated cellular signaling in animal and plant cells. Treatment with gadolinium (Gd³⁺), a CaSR activator in animal cells, induced concentration-dependent increases in [Ca²⁺]_cyt in roots, which showed self-desensitization and cross-desensitization to [Ca²⁺]_ext-induced increases in [Ca²⁺]_cyt (EICC). EICC was sensitive to extracellular H⁺, K⁺, Na⁺, and Mg²⁺ levels. Treatment with the PLC inhibitor neomycin suppressed EICC and Ca accumulation in roots. The inhibitory effect of neomycin on root elongation was fully rescued by increasing [Ca²⁺]_ext but not [Mg²⁺] or [K⁺] in the growth medium. These results suggest that [Ca²⁺]_ext and the movement of Ca²⁺ into the cytosol of plant roots are regulated by a receptor-mediated signaling pathway involving PLC.

CD36 involvement in the olfactory perception of oleic aldehyde, an odour-active volatile compound, in mice

Cluster of differentiation 36 (CD36) is a broadly expressed transmembrane receptor that has multiple ligands. It has been found to occur abundantly on the surface of the olfactory epithelium in mice and postulated to play a role in mammalian olfaction. However, there have been no ethological analyses of the mammalian behaviour showing CD36 involvement in the olfactory perception of a distinct odour-active volatile compound. In this study, we aimed to assess whether mammals perceive oleic aldehyde, an odour-active volatile that serves as a potential CD36 ligand, and if so, whether CD36 is involved in the sensing by following measurements using CD36-knockout mice and their wild-type littermates. In a two-bottle choice test, wild-type mice, but not CD36-knockout mice, discriminated a sucrose solution containing oleic aldehyde from the sucrose solution alone. To assess the importance of the olfactory system in the oleic aldehyde perception, we conducted an exploration test where the animals could rely primarily on the odour of test volatiles for recognition. We found that the wild-type, but not CD36-knockout mice, were aware of the compound. Our results provide behavioural evidence that CD36 plays a role in the perception of specific odour-active volatile compounds in the nasal cavity.

Structural bioinformatics: deriving biological insights from protein structures

Structural bioinformatics can be described as an approach that will help decipher biological insights from protein structures. As an important component of structural biology, this area promises to provide a high resolution understanding of biology by assisting comprehension and interpretation of a large amount of structural data. Biological function of protein molecules can be inferred from their three-dimensional structures by comparing structures, classifying them and transferring function from a related protein or family. It is well known now that the structure space of protein molecules is more conserved than the sequence space, making it important to seek functional associations at the structural level. An added advantage of structural bioinformatics over simpler sequence-based methods is that the former also provides ultimate insights into the mechanisms by which various biological events take place. A bird's eye-view of the different aspects of structural bioinformatics is given here along with various recent advances in the area including how knowledge obtained from structural bioinformatics can be applied in drug discovery.

Meta-Substituted Aryl(thio)ethers as Potent Partial Agonists (or Antagonists) for the Histamine H3 Receptor Lacking a Nitrogen Atom in the Side Chain

4-(3-Aryloxypropyl)-1H-imidazoles, which possess a meta-positioned substituent in the aryl ring, have been synthesized and tested for activity at histamine H(3) receptors. The compounds having a CN, Me, or Br substituent were found to be antagonists, whereas CF(3), Et, i-Pr, t-Bu, COCH(3), or NO(2) substituents remarkably afforded partial agonists when tested in vitro on rat cerebral cortex synaptosomes for inhibition of [(3)H]histamine release. The compounds were also active in vivo, and furthermore, the CF(3)-substituted compound trifluproxim (UCL 1470, 7) acted as a potent full agonist in vivo, having ED(50) = 0.6 +/- 0.3 mg/kg per os in mice for inhibition of brain N(tau)-methylhistamine formation. Related structures have also been investigated; homologues 4-[4-(3-(trifluoromethyl)phenoxy)butyl]-1H-imidazole and 4-[2-(3-(trifluoromethyl)phenylthio)ethyl]-1H-imidazole are shown to be partial agonists, whereas the O isostere 4-[2-(3-(trifluoromethyl)phenoxy)ethyl]-1H-imidazole is an antagonist as is the S homologue 4-[3-(3-(trifluoromethyl)phenylthio)propyl]-1H-imidazole and its CH(2) isostere 4-[4-(3-(trifluoromethyl)phenyl)butyl]-1H-imidazole.

Molecular modelling and site-directed mutagenesis of human GALR1 galanin receptor defines determinants of receptor subtype specificity

Human galanin is a 30 amino acid neuropeptide that elicits a range of biological activities by interaction with G protein-coupled receptors. We have generated a model of the human GALR1 galanin receptor subtype (hGALR1) based on the alpha carbon maps of frog rhodopsin and investigated the significance of potential contact residues suggested by the model using site-directed mutagenesis. Mutation of Phe186 within the second extracellular loop to Ala resulted in a 6-fold decrease in affinity for galanin, representing a change in free energy consistent with hydrophobic interaction. Our model suggests interaction between Phe186 of hGALR1 and Ala7 or Leu11 of galanin. Receptor subtype specificity was investigated by replacement of residues in hGALR1 with the corresponding residues in hGALR2 and use of the hGALR2-specific ligands hGalanin(2-30) and [D-Trp2]hGalanin(1-30). The His267Ile mutant receptor exhibited a pharmacological profile corresponding to that of hGALR1, suggesting that His267 is not involved in a receptor-ligand interaction. The mutation Phe115Ala resulted in a decreased binding affinity for hGalanin and for hGALR2-specific analogues, indicating Phe115 to be of structural importance to the ligand binding pocket of hGALR1 but not involved in direct ligand interaction. Analysis of Glu271Trp suggested that Glu271 of hGALR1 interacts with the N-terminus of galanin and that the Trp residue in the corresponding position in hGALR2 is involved in receptor subtype specificity of binding. Our model supports previous reports of Phe282 of hGALR1 interacting with Trp2 of galanin and His264 of hGALR1 interacting with Tyr9 of galanin.

Effect of Ro 61-1790, a selective endothelin-A receptor antagonist, on systemic and uterine hemodynamics and fetal oxygenation in sheep

OBJECTIVE

Endothelin-1 is reportedly elevated in preeclampsia, and studies in our laboratory have shown that infusion of endothelin-1 produces increased mean arterial pressure, hemoconcentration, and proteinuria, while decreasing uterine blood flow. If a role in preeclampsia is confirmed for endothelin-1, therapeutic intervention may involve selective endothelin-A receptor blockers. Thus, this study was designed to determine the effects of Ro 61-1790, a selective endothelin-A receptor inhibitor, on mean arterial pressure, heart rate, and uteroplacental blood flow in pregnant and nonpregnant sheep.

STUDY DESIGN

Seven pregnant and seven nonpregnant sheep were instrumented with indwelling femoral artery and vein catheters and bilateral uterine artery flow probes, allowing determination of mean arterial pressure, heart rate, and uteroplacental blood flow. After baseline administration, animals received intravenous Ro 61-1790 either 1 mg/kg or 3 mg/kg, and hemodynamic responses were monitored for 120 minutes.

RESULTS

Intravenous administration of Ro 61-1790 at 1 mg/kg had no effect on the parameters measured in either group studied. However, at 3 mg/kg, Ro 61-1790 caused an increase in total uterine blood flow in nonpregnant sheep from 22 +/- 6 mL/min to 51 +/- 15 mL/min. This occurred in the absence of significant changes in mean arterial pressure. In pregnant animals, administration of 3 mg/kg Ro 61-1790 decreased mean (+/- SEM) uteroplacental blood flow by 20% (from 771 +/- 130 mL/min to 621 +/- 110 mL/min) and increased uterine vascular resistance transiently. Administration of Ro 61-1790 had no significant effect on fetal mean arterial pressure, heart rate, or oxygenation.

CONCLUSIONS

These data suggest that endogenous endothelin-1 may play an important role in regulating uterine vascular tone in pregnant and nonpregnant sheep and that inhibition of endogenous endothelin-1 may lead to reductions in uteroplacental perfusion in pregnant animals.

Recovery and expression of messenger RNA from postmortem human brain tissue

The Bryan Alzheimer's Disease Research Center Rapid Autopsy Program at Duke University Medical Center obtains postmortem human brain tissue for experimental investigations. We evaluated 19 brains for RNA integrity and mRNA gene expression. Nine were from patients diagnosed with Alzheimer's disease, and ten were from nondemented controls. In all cases, the following variables were recorded: postmortem procurement delay (range, 1 hour and 10 minutes to 14 hours), pH of cerebrospinal fluid, premortem fever or sepsis, provision of supplemental oxygen in the agonal period, and temporal relation to time of death (either sudden death or protracted illness). Total RNA was extracted, quantified, and evaluated by agarose gel electrophoresis and quantitative gene expression analysis of 18S rRNA and edg-1 using TaqMan technology. All samples appeared to yield intact RNA without significant degradation, and expression of the edg-1 gene was detected by the real time reverse transcriptase polymerase chain reaction in all cases. We conclude that intact RNA can be obtained from postmortem human brain tissue, even in patients with severe premortem illnesses and delayed postmortem tissue procurement intervals. However, we caution that the successful expression of certain genes from postmortem brain tissue may require enhanced procurement efforts to maximize RNA integrity.

A fluorescent reporter assay for the detection of ligands acting through Gi protein-coupled receptors

Accompanying the advances in basic biology of G protein-coupled receptors (GPCRs) is the practical need among biopharmaceutical companies for sensitive assays to assess GPCR function, particularly formats that are compatible with high-throughput drug screening. Here we describe a novel cell-based assay format for the high-throughput detection of ligands for Gi protein-coupled receptors. Two Gi-GPCRs, mu-opioid receptor (mu-OPR) and 5-hydroxytryptamine receptor la (5HT1aR) are employed as model receptor targets. The key feature of this assay system is the isolation of stable, clonal Chinese hamster ovary (CHO) cell lines that carry three separate expression plasmids: (1) a chimeric Gq/i5 protein (which re-directs a negative Gi-type signal to a positive Gq-type response), (2) a given Gi-GPCR, and (3) a beta-lactamase (beta1a) reporter gene responsive to Gi-GPCR signaling. Cell-based assays built using this format show appropriate rank order of potency among a reference set of receptor agonist and antagonist compounds. Such assays are also robust, reliable, and can be used for industrial-scale applications such as high-throughput screening for drug leads.

Murine cytomegalovirus homologues of cellular immunomodulatory genes

The study of 'molecular mimicry' or 'genetic piracy', with respect to the utilisation of cellular genes captured and modified during the course of virus evolution, has been an area of increasing research with the expansion in virus genome sequencing. Examples of cellular immunomodulatory genes which have been captured from hosts have been identified in a number of viruses. This review concentrates upon studies of murine cytomegalovirus (MCMV), investigating the functions of viral genes homologous to G protein-coupled receptors, MHC class I and chemokines. The study of recombinant MCMV engineered with specific disruptions of these genes has revealed their significance during virus replication and dissemination within the host. In the case of the latter two classes of genes, evidence suggests they interfere with cellular immune responses, although the detailed mechanisms underlying this interference have yet to be delineated.

Thrombin receptor-activating peptides (TRAPs): investigation of bioactive conformations via structure-activity, spectroscopic, and computational studies

The thrombin receptor (PAR-1) is an unusual transmembrane G-protein coupled receptor in that it is activated by serine protease cleavage of its extracellular N-terminus to expose an agonist peptide ligand, which is tethered to the receptor itself. Synthetic peptides containing the agonist motif, such as SFLLRN for human PAR-1, are capable of causing full receptor activation. We have probed the possible bioactive conformations of thrombin receptor-activating peptides (TRAPs) by systematic introduction of certain conformational perturbations, involving alpha-methyl, ester psi(COO), and reduced-amide psi(CH2N) scans, into the minimum-essential agonist sequence (SFLLR) to probe the importance of the backbone conformation and amide NH hydrogen bonding. We performed extensive conformational searches of representative pentapeptides to derive families of putative bioactive structures. In addition, we employed 1H NMR and circular dichroism (CD) to characterize the conformational disposition of certain pentapeptide analogues experimentally. Activation of platelet aggregation by our pentapeptide analogues afforded a structure-function correlation for PAR-1 agonist activity. This correlation was assisted by PAR-1 receptor binding data, which gauged the affinity of peptide ligands for the thrombin receptor independent of a functional cellular response derived from receptor activation (i.e. a pure molecular recognition event). Series of alanine-, proline-, and N-methyl-scan peptides were also evaluated for comparison. Along with the known structural features for PAR-1 agonist peptides, our work adds to the understanding of peptide topography relative to platelet functional activity and PAR-1 binding. The absolute requirement of a positively charged N-terminus for strong agonist activity was contradicted by the N-terminal hydroxyl peptide psi(HO)S-FLLR-NH2. The amide nitrogen between residues 1 and 2 was found to be a determinant of receptor recognition and the carbonyl groups along the backbone may be involved in hydrogen bonding with the receptor. Position 3 (P3) of TRAP-5 is known to tolerate a wide variety of side chains, but we also found that the amide nitrogen at this position can be substituted by an oxygen, as in SF-psi(COO)-LLR-NH2, without diminishing activity. However, this peptide bond is sensitive to conformational changes in that SFPLR-NH2 was active, whereas SF-NMeL-LR-NH2 was not. Additionally, we found that position 3 does not tolerate rigid spacers, such as 3-aminocyclohexane-1-carboxylic acid and 2-aminocycloalkane-1-carboxylic acid, as analogues 1A, 1B, 2A, 2B, 3, 4, 5A and 5B lack agonist activity. On the basis of our results, we suggest that an extended structure of the agonist peptide is principally responsible for receptor recognition (i.e. binding) and that hydrophobic contact may occur between the side chains of the second (Phe) and fourth (Leu) residues (i.e. P2-P4 interaction).

Design, synthesis, and biological characterization of a peptide-mimetic antagonist for a tethered-ligand receptor

Protease-activated receptors (PARs) represent a unique family of seven-transmembrane G protein-coupled receptors, which are enzymatically cleaved to expose a truncated extracellular N terminus that acts as a tethered activating ligand. PAR-1 is cleaved and activated by the serine protease alpha-thrombin, is expressed in various tissues (e.g., platelets and vascular cells), and is involved in cellular responses associated with hemostasis, proliferation, and tissue injury. We have discovered a series of potent peptide-mimetic antagonists of PAR-1, exemplified by RWJ-56110. Spatial relationships between important functional groups of the PAR-1 agonist peptide epitope SFLLRN were employed to design and synthesize candidate ligands with appropriate groups attached to a rigid molecular scaffold. Prototype RWJ-53052 was identified and optimized via solid-phase parallel synthesis of chemical libraries. RWJ-56110 emerged as a potent, selective PAR-1 antagonist, devoid of PAR-1 agonist and thrombin inhibitory activity. It binds to PAR-1, interferes with PAR-1 calcium mobilization and cellular function (platelet aggregation; cell proliferation), and has no effect on PAR-2, PAR-3, or PAR-4. By flow cytometry, RWJ-56110 was confirmed as a direct inhibitor of PAR-1 activation and internalization, without affecting N-terminal cleavage. At high concentrations of alpha-thrombin, RWJ-56110 fully blocked activation responses in human vascular cells, albeit not in human platelets; whereas, at high concentrations of SFLLRN-NH(2), RWJ-56110 blocked activation responses in both cell types. Thus, thrombin activates human platelets independently of PAR-1, i.e., through PAR-4, which we confirmed by PCR analysis. Selective PAR-1 antagonists, such as RWJ-56110, should serve as useful tools to study PARs and may have therapeutic potential for treating thrombosis and restenosis.

Endothelin receptor mRNAs in human fetal membranes, chorionic vessels, and decidua parietalis

Endothelin-1 (ET-1) is synthesized in avascular human amnion; and, immunoreactive ET-1 is present in amniotic fluid in concentrations 10 to 100 times those in blood. ET-1 acts, most commonly, in a local or paracrine manner; therefore, it is possible that amnion/amniotic fluid ET-1 acts on contiguous tissues, namely chorion laeve or placental surface (chorionic) vessels, or in an autocrine fashion on amnion cells. To address these possibilities, the levels of ET(A) and ET(B) receptor mRNAs were evaluated in amnion, chorion laeve, decidua parietalis, placenta, and chorionic vessel tissues. By Northern analysis of total RNA (20 microg), ET(A) and ET(B) receptor mRNAs were detected in decidua (n = 18), placenta (n = 14), and chorionic vessels (n = 13). In chorion laeve (n = 24), ET(B) receptor mRNA but not ET(A) receptor mRNA was detected by Northern analysis of total RNA. Northern analysis of chorion laeve poly(A)+ mRNA (1.5-2.5 microg) revealed ET(A) receptor mRNA at low levels. Neither ET(A) nor ET(B) receptor mRNAs were detected in amnion tissue by Northern analysis of total RNA (n = 30; placental and reflected amnion from 15 pregnancies) or by Northern analysis of poly(A)+ mRNA (1.5-2.5 microg). Moreover, there was no demonstrable dose-dependent effect of ET-1 on prostaglandin E2 production or DNA synthesis in amnion epithelial cells in primary culture. The findings of this investigation are indicative that ET-1 in amniotic fluid or secreted from amnion may act in a paracrine fashion on chorion laeve by way of the ET(B) receptor and on chorionic vessels by way of ET(A) and ET(B) receptors.

Two eye guanylyl cyclases are expressed in the same photoreceptor cells and form homomers in preference to heteromers

We recently described two eye guanylyl cyclases (GC-E and GC-F) that contain an apparent extracellular domain potentially capable of binding ligands (Yang, R.-B., Foster, D. C., Garbers, D. L., and Fülle, H.-J. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 602-606). Here, Northern and Western analyses showed that both cyclases are expressed in the retina and enriched in photoreceptor outer segments. By the use of specific GC-E and GC-F antibodies coupled to different sized gold particles both cyclases were colocalized within the same photoreceptor cells raising the possibility of homomeric and/or heteromeric interactions. A point mutant of GC-E (D878A) was constructed and expressed; it contained no detectable cyclase activity but acted in a dominant negative fashion to abolish the activity of native GC-E and GC-F in coexpression studies. These results suggested that GC-E and GC-F could form either homomers or heteromers, at least when overexpressed in COS-7 cells. Immunoprecipitation with GC-E and GC-F antibody followed by Western analysis confirmed that both homomers and heteromers could be formed. However, similar experiments using retina or outer segments revealed that a vast majority of GC-E and GC-F were precipitated as homomers in the eye. Therefore, like other members of the membrane guanylyl cyclase subfamily, GC-E and GC-F appear to preferentially form homomers.

Acute and chronic actions of ethanol on CA1 hippocampal responses to serotonin

The effects of acute or chronic ethanol on serotonin (5-HT)-induced membrane hyperpolarization and inhibition of the slow Ca2(+)-dependent after hyperpolarization (sAHP) were recorded in rat CA1 pyramidal neurons in hippocampal slices using sharp intracellular electrodes. 5-HT (1-100 microM) caused concentration-dependent hyperpolarization of the membrane that was not altered by simultaneous 30 mM ethanol treatment, but blunted by 10 microM buspirone, a weak 5-HT1A agonist. 5-HT (1-30 microM) also partially inhibited (approximately 40%) the sAHP following a burst of five or more action potentials. Initially ethanol (30 mM) alone did not alter the sAHP, but over a period of 38 min, a slow increase in amplitude (approximately 40%) was observed. 5-HT-mediated inhibition of the sAHP was significantly greater with ethanol present, regardless of the length of exposure. Pyramidal neurons in hippocampal slices prepared from ethanol-dependent animals showed no obvious signs of withdrawal related hyperexcitability and neither concentration-dependent membrane hyperpolarization nor sAHP inhibition caused by 5-HT were significantly changed from responses in controls. These results suggest that hyperpolarizing responses to 5-HT in hippocampal CA1 pyramidal neurons are functionally resistant to acute or chronic ethanol treatment. 5-HT-mediated inhibition of the sAHP is enhanced by ethanol acutely, but does not show an adaptive change as a result of ethanol dependence.

Overexpression, solubilization and purification of rat and human olfactory receptors

The superfamily of olfactory receptor genes, whose products are thought to be activated by odorant ligands, is critical for odor recognition. Two olfactory receptors, olp4 from rat and OR17-4 from human, were overexpressed in Sf9 insect cells. The presence of the proteins in cell membranes was monitored by immunoblotting with peptide-specific polyclonal antibodies directed against the C-terminal sequences of these receptors and with a mAb against an N-terminal octapeptide epitope tag. A DNA sequence that codes for a His6 tag, which binds tightly to a Ni2+-chelate-affinity column, was incorporated into the N-termini of both genes. The expressed olfactory receptors were found mainly in the cell-membrane fraction. The proteins were difficult to solubilize by many detergents and only lysophosphatidylcholine was found to be both suitable for efficient solubilization of the overexpressed olfactory receptors and compatible with the purification system used. After solubilization, the olfactory receptors were purified to near homogeneity by affinity chromatography on nickel nitrilotriacetic acid resin and by cation-exchange chromatography. Electrophoresis of the purified proteins and visualization with Coomassie Blue staining or by immunoblotting with specific antibodies, revealed bands of 32, 69 and 94 kDa, which were identified as the monomeric, dimeric and trimeric forms of the receptor proteins. The oligomeric forms were resistant to reduction and alkylation, and are therefore thought to be held together by non-covalent hydrophobic interactions that are resistant to SDS. This finding is similar to previous observations for other guanine-nucleotide-binding-regulatory-protein-coupled receptors. Reconstitution in phospholipid vesicles showed that the purified olfactory receptors insert specifically into the lipid bilayer. This provides a means to study functional reconstitution with putative transduction components such as olfactory guanine-nucleotide-binding-regulatory protein.

A cDNA encoding the calcitonin gene-related peptide type 1 receptor

Calcitonin gene-related peptide (CGRP) is a neuropeptide with diverse biological effects including potent vasodilator activity. We report here the cloning of a complementary DNA (cDNA) encoding a human CGRP1 receptor, which shares significant peptide sequence homology with the human calcitonin receptor, a member of the G-protein-coupled receptor superfamily. Northern blot analysis revealed that the messenger RNA for this receptor is predominantly expressed in the lung and heart. In situ studies showed specific localization of the receptor mRNA to alveolar cells in the lung and to cardiac myocytes in the heart. Stable expression of the cDNA in human embryonic kidney 293 (HEK 293) cells produced specific, high affinity binding sites for CGRP that displayed pharmacological and functional properties very similar to native human CGRP1 receptor. Exposure of these cells to CGRP resulted in a 60-fold increase in cAMP production, which was inhibited in a competitive manner by the CGRP1 receptor antagonist, CGRP-(8-37).

Changes in collagen metabolism in response to endothelin-1: evidence for fibroblast heterogeneity

Endothelin-1 (Et-1) is a 21-amino acid peptide primarily synthesized by endothelial cells. It was originally classified as a potent vasoconstrictor but recent evidence suggests that it also possesses a wide variety of non-vascular actions. It stimulates fibroblast and smooth muscle cell proliferation and it has been shown to stimulate fibroblast collagen metabolism. However, studies on its ability to regulate collagen production remain incomplete, and its effect on post-translational processing of procollagen has not been studied. This report details the effect of Et-1 on the rates of procollagen synthesis and degradation in two fibroblast cell lines; human foetal lung (HFL-1) and whole foetal rat fibroblasts (Rat 2). Fibroblast cultures were incubated for 24 hr in the presence or absence of Et-1 before procollagen metabolism was determined by measuring hydroxyproline. Non-collagen metabolism was also determined in these cultures from the uptake of tritiated phenylalanine. Et-1 stimulated procollagen synthesis in HFL-1 fibroblasts and reduced synthesis in Rat 2 cells. The response was dose dependent with the greatest effect at 1.10(-6) M Et-1 for both cell types (155 +/- 6% of control (mean +/- SD, n = 6, P < 0.01) and 61 +/- 4% of control (n = 4, P < 0.01) for HFL-1 and Rat 2 fibroblasts, respectively). Non-collagen protein synthesis was increased to 148 +/- 5% of control (P < 0.05) at 1.10(-6) M Et-1. Non-collagen protein synthesis remained unaffected in the HFL-1 fibroblast cultures. Procollagen degradation, expressed as a proportion of total procollagen synthesis, was decreased in HFL-1 fibroblasts (control, 29 +/- 2%; Et-1, 1.10(-6) M; 21 +/- 2%; P < 0.01), and increased in Rat 2 fibroblasts (control 42 +/- 1%; Et-1, 1.10(-6) M; 49 +/- 1%; P < 0.01). Blocking of the EtA receptor for Et-1, using the receptor antagonist-BQ123, abolished the effect of Et-1 on procollagen metabolism in both cell types. These results suggest that different populations of fibroblasts exhibit heterogeneous responses to Et-1. It is concluded that Et-1 may play an important role in the extent and distribution of fibrosis seen in diseases associated with the overproduction of Et-1.

The extracellular calcium receptor

The importance of intracellular calcium in regulating cell function is well recognized. No less important, but less well understood (and probably appreciated), is the fundamental role played by extracellular calcium, Ca2+o, in the modulation of cell function. The recent cloning of Ca2+o-sensing, G-protein-coupled receptors from bovine (and human) parathyroid and rat kidney (and brain) has clearly demonstrated that Ca2+o can function as a traditional 'first messenger'. The identification of 'inactivating' and 'activating' mutations in this Ca2+o-sensing receptor in two hypercalcemic disorders and in an autosomal dominant form of hypocalcemia, respectively, has underscored the physiological relevance of this receptor in Ca2+ homeostasis in man. These advances have significantly enhanced our understanding of the molecular mechanisms involved in extracellular calcium sensing in parathyroid and kidney. Moreover, the localization of the Ca2+o-sensing receptor in tissues previously not known to have Ca2+o-sensing capability has suggested novel and potentially quite important roles for Ca2+o in regulating the function of cells not apparently directly involved in Ca2+ homeostasis.

Cloning and expression of a second photoreceptor-specific membrane retina guanylyl cyclase (RetGC), RetGC-2

One of the membrane guanylyl cyclases (GCs), RetGC, is expressed predominantly in photoreceptors. No extracellular ligand has been described for RetGC, but it is sensitive to activation by a soluble 24-kDa protein (p24) and is inhibited by Ca2+. This enzyme is, therefore, thought to play a role in resynthesizing cGMP for photoreceptor recovery or adaptation. By screening a human retinal cDNA library at low stringency with the cytoplasmic domains from four cyclases, we cloned cDNAs encoding a membrane CG that is most closely related to RetGC. We have named this GC RetGC-2, and now term the initially described RetGC RetGC-1. By in situ hybridization, mRNA encoding RetGC-2 is found only in the outer nuclear layer and inner segments of photoreceptor cells. By using synthetic peptide antiserum specific for each RetGC subtype, RetGC-2 can be distinguished from RetGC-1 as a slightly smaller protein in immunoblots of bovine rod outer segments. Membrane GC activity of recombinant RetGC-2 expressed in human embryonic kidney 293 cells is stimulated by the activator p24 and is inhibited by Ca2+ with an EC50 value of 50-100 nM. Our data reveal a previously unappreciated diversity of photoreceptor GCs.

Sensing of extracellular Ca2+ by parathyroid and kidney cells: cloning and characterization of an extracellular Ca(2+)-sensing receptor

The ability of the parathyroid cell to sense minute fluctuations in the extracellular ionized calcium concentration (Ca2+ o) is essential for maintaining mineral ion homeostasis. However, the mechanism(s) through which the parathyroid cell and other cells recognize and respond to changes in Ca2+ o has remained unclear. We recently isolated a cDNA encoding a Ca2+ o-sensing receptor from bovine parathyroid using expression cloning in Xenopus laevis oocytes. The receptor shows pharmacologic properties that are almost identical to those of the receptor on the parathyroid cell and, like the latter, stimulates phospholipase C in a G-protein-dependent manner. The amino acid sequence of the cloned receptor deduced from this cDNA predicts a protein with a molecular mass of 121 kd, which has three principal structural domains. The first is a 613 amino acid, putatively extracellular amino terminus which has several regions rich in acidic amino acids that may potentially be involved in binding Ca2+ and other polycationic agonists. The second comprises seven membrane-spanning segments that are characteristic of the superfamily of G-protein-coupled receptors, and the third is a 222 amino acid cytoplasmic tail. Transcripts for this Ca2+ o-sensing receptor are present in the parathyroid as well as in the kidney, thyroid, and brain. We next investigated the hypercalcemic disorders, familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism, as possible examples of inherited abnormalities in this Ca2+ o-sensing receptor, since both disorders show abnormal Ca2+ o-sensing and/or handling in the kidney and parathyroid.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth of Chinese hamster ovary (CHO) cells expressing the 5-HT2 serotonin receptor in suspension culture: an efficient method for large-scale acquisition of membrane protein for drug evaluation

Chinese hamster ovary (CHO) cells expressing the rat 5-HT2 serotonin receptor were grown and evaluated in suspension culture to provide an efficient method of producing membrane-bound receptors for drug discovery. Expression of the 5-HT2 receptor in cells grown in batch suspension culture fluctuated as a function of culture age. Both receptor expression and receptor G-protein coupling were the highest early (1.9 pmol/mg membrane protein) but declined rapidly as the culture increased in age. However, addition of fresh serum-containing medium to stationary-phase cells reversed the decline and, after 24 h of growth, resulted in maximal receptor density and G-protein coupling. This serum response was found to be reproducible and lead to the establishment of a semi-continuous batch culture system in which cells were maintained in a growth state that supported high levels of membrane-incorporated and G protein-coupled receptors. In this system, 50% of the culture volume could be removed at 24-h intervals for membrane preparation and the lost volume replenished with fresh medium, resulting in a continuous supply of high-quality membrane preparations.

Single cells as biosensors for chemical separations

A biosensor system based on the response of living cells was demonstrated that can detect specific components of a complex mixture fractionated by a microcolumn separation technique. This system uses ligand-receptor binding and signal-transduction pathways to biochemically amplify the presence of an analyte after electrophoretic separation. The transduced signal was measured by means of two approaches: (i) fluorescence determination of intracellular calcium concentrations in one or more rat PC-12 cells and (ii) measurement of transmembrane current in a Xenopus laevis oocyte microinjected with messenger RNA that encodes a specific receptor. This analysis system has the potential to identify biologically active ligands present in a complex mixture with exceptional sensitivity and selectivity.

GABAB receptors

GABAB receptors are a distinct subclass of receptors for the major inhibitory transmitter 4-aminobutanoic acid (GABA) that mediate depression of synaptic transmission and contribute to the inhibition controlling neuronal excitability. The development of specific agonists and antagonists for these receptors has led to a better understanding of their physiology and pharmacology, highlighting their diverse coupling to different intracellular effectors through Gi/G(o) proteins. This review emphasises our current knowledge of the neurophysiology and neurochemistry of GABAB receptors, including their heterogeneity, as well as the therapeutic potential of drugs acting at these sites.

Cloning of the rat edg-1 immediate-early gene: expression pattern suggests diverse functions

The edg-1 immediate-early gene encodes a G-protein-coupled receptor homolog implicated in endothelial cell differentiation. We report the cloning of the rat edg-1 gene. Our Northern analyses indicate that edg-1 is much more widely expressed than previously thought. edg-1 mRNA was found in many organs at several stages of development with relatively high levels present in adult brain. edg-1 transcripts were also detected in several cell lines. Expression of edg-1 mRNA in the PC12 cell model of neuronal differentiation was unaffected by agents that cause PC12 cells to differentiate or proliferate. Therefore, edg-1 may play a cell-type-specific role in differentiation and also participate in neurotransmission.

Tools for investigating functional interactions between ligands and G-protein-coupled receptors

A general assay for evaluating functional interactions between ligands and G-protein-coupled receptors within minutes has been developed. The system uses the principles employed by animals such as reptiles, amphibians and fish to control their colors. In nature, activation of G-protein-coupled receptors expressed by skin cells called chromatophores effects pigment redistribution within the cells to change an animal's coloration. The in vitro 'chameleon in a dish' equivalent can use essentially any cloned G-protein-coupled receptor.

Disruption of potential alpha-helix in the G loop of the guinea pig 5-hydroxytryptamine2 receptor does not prevent receptor coupling to phosphoinositide hydrolysis

Heterogeneity of the 5-hydroxytryptamine2 (5-HT2) receptor across species has been implicated in several pharmacological and physiological studies. Although 5-HT2 receptors in the rat have been linked to increases in phosphoinositide (PI) hydrolysis, little evidence exists to support the association of guinea pig 5-HT2 receptors with PI hydrolysis, the second messenger generally linked with 5-HT2 receptors. In the present study, we have taken a molecular and biochemical approach to determining whether species differences in brain 5-HT2 receptors exist between rat and guinea pig. First, we isolated partial cortical 5-HT2 receptor cDNA clones that encompassed the third intracellular loop, a receptor area putatively important in receptor-effector coupling. The amino acid sequences deduced from the cDNA clones for rat and guinea pig brain 5-HT2 receptor were 97% homologous. However, the guinea pig 5-HT2 receptor had two tandem substitutions that disrupted a potential alpha helix in the region of the third cytoplasmic loop, which theoretically could alter the intracellular coupling of the guinea pig cortical 5-HT2 receptor. Because of these molecular differences, we examined further the pharmacological activation of the brain 5-HT2 receptor from guinea pig. 5-HT and the 5-HT2 receptor agonist alpha-methyl-5-HT increased PI hydrolysis in guinea pig cortical slices whereas the 5-HT1C receptor agonist 5-methyltryptamine was significantly less potent. In addition, the 5-HT2 receptor antagonists LY53857, ketanserin, and spiperone blocked 5-HT-stimulated PI hydrolysis. These pharmacological data suggested that activation of the 5-HT2 receptor in guinea pig cortical slices was associated with PI hydrolysis. Thus, although areas of the guinea pig brain 5-HT2 receptor that influence receptor-effector coupling were different from the rat, such differences were not critical to receptor-effector coupling because, as in the rat, guinea pig brain 5-HT2 receptors were also coupled to PI hydrolysis.

Pertussis toxin sensitive photoaggregation of pigment in isolated Xenopus tail-fin melanophores

Direct illumination of Xenopus laevis tail-fin melanophores results in rapid, reversible translocation of intracellular pigment granules to a perinuclear location, an effect distinct from and opposite to the photodispersion of pigment found in melanophores isolated from Xenopus embryos. In this report we show that both pertussis toxin and dibutyryl-adenosine-3',5'-monophosphate block the ability of light to cause photoaggregation of pigment in cultured tail-fin melanophores, whereas dibutyryl-guanosine-3',5'-monophosphate is without effect.