A method for evaluating the effects of ligands upon Gs protein-coupled receptors using a recombinant melanophore-based bioassay

N-oleoyldopamine enhances glucose homeostasis through the activation of GPR119

G protein-coupled receptor 119 (GPR119) is largely restricted to pancreatic insulin-producing beta-cells and intestinal glucagon-like peptide-1-producing L-cells. Synthetic agonists of this receptor elicit glucose-dependent release of these endocrine factors, thereby enhancing glycemic control. Oleoylethanolamide also activates GPR119, but it remains unclear whether endogenous production of this lipid modulates GPR119 activity under normal or dysglycemic conditions. We show here that a relatively diverse set of lipid amides activate GPR119. Among these, the endovallinoid N-oleoyldopamine (OLDA) stimulated cAMP accumulation in GPR119-transfected cells as effectively as oleoylethanolamide and the previously described synthetic agonist AR231453. None of these lipid amides increased cAMP in control-transfected cells or in cells transfected with a number of other G protein-coupled receptors. OLDA stimulated both cAMP accumulation and insulin release in HIT-T15 cells, which express GPR119 endogenously, and in GPR119-transfected RIN-5F cells. Oral administration of OLDA to C57bl/6 mice elicited significant improvement in glucose tolerance, whereas GPR119-deficient mice were essentially unresponsive. OLDA also acutely elevated plasma gastric inhibitory peptide levels, a known hallmark of GPR119 activation. OLDA represents a possible paracrine modulator of GPR119 in pancreatic islets, where markers of dopamine synthesis correlated well with GPR119 expression. However, no such correlation was seen in the colon. Collectively, these studies indicate that multiple, distinct classes of lipid amides, acting via GPR119, may be important modulators of glucose homeostasis.

Panax ginseng induces anterograde transport of pigment organelles in Xenopus melanophores

Melanophores from Xenopus laevis are pigmented cells, capable of quick colour changes through cyclic adenosine 3':5'-monophosphate (cAMP) coordinated transport of their intracellular pigment granules, melanosomes. In this study we use the melanophore cell line to evaluate the effects of Panax ginseng extract G115 on organelle transport. Absorbance readings of melanophore-coated microplates, Correlate-EIA direct cAMP enzyme immunoassay kit, and western blot were used to measure the melanosome movement and changes in intracellular signalling. We show that Panax ginseng induces a fast concentration-dependent anterograde transport of the melanosomes. No significant increase in the cAMP level was seen and pre-incubation of melanophores with the protein kinase C (PKC) inhibitor EGF-R Fragment 651-658 (M-EGF) only partly decreased the ginseng-induced dispersion. We also demonstrate that Panax ginseng, endothelin-3 (ET-3) and alpha-melanocyte stimulating hormone (MSH) stimulate an activation of mitogen activated protein kinase (MAPK). Pre-incubation with M-EGF decreased the MAPK activity induced by ET-3 and MSH, but again only marginally affected the response of Panax ginseng. Thus, in melanophores we suggest that Panax ginseng stimulates an anterograde transport of pigment organelles via a non-cAMP and mainly PKC-independent pathway.

A new tool for investigating G protein-coupled receptors

Vertebrate olfactory receptors are members of the seven-transmembrane-domain G protein-coupled receptor family. They utilize intracellular signal transduction pathways which are activated by stimulation of odorant receptors and use the second messengers cAMP and/or inositol 1,4,5-trisphosphate and diacylglycerol. Studies of how odorants bind to and activate the receptors can be considered part of the more general problem of how chemicals interact with G protein-coupled receptors. This review describes the development of a new technique for assessing functional interactions between chemicals and these receptors in only minutes. Predicted uses of the system include structure-function analyses of both G protein-coupled receptors and their ligands, studies of receptor coupling to G proteins and cloning of cDNAs for these receptors.

Modeling of pattern regulation in melanophores

Melanosomes, pigment granules in melanophores, play a principal role in physiological color adaptation of fish and frog. Melanophores regulate melanosome trafficking on cytoskeletal filaments to generate a range of spatiotemporal patterns. Here, we present the first comprehensive model of spatiotemporal evolution of melanosome patterns. The model encompasses both physical and biochemical aspects of melanosome dynamics. It consists of (i) a kinetic description of biochemical reactions involved in intracellular signaling, (ii) a system of macroscopic reaction-diffusion-convection equations for melanosome concentration, and (iii) a set of constitutive relationships for coupling transport with the biochemical network. The model relates molecular-level regulatory actions to cell-level melanosome distribution, allowing unification of existing experimental observations and qualitative hypotheses into an integrated, consistent framework. The model reproduces salient features of melanosome patterns, both during transient and steady state. It gives useful insights into how cells coordinate motor-assisted transport to maintain and adapt spatial organization of intracellular organelles. In particular, we calculate the optimal transition paths from aggregation to dispersion in fish melanophores. The calculations suggest that fish melanophores optimally control intracellular signaling to maximize the efficiency of motor-assisted transport during dispersion.

Identification of novel hexapeptide agonists at the Xenopus laevis melanophore melanocortin receptor

We used a combinatorial chemical approach to identify novel agonists for the endogenous melanocortin receptor expressed in Xenopus laevis melanophores. A random one-bead one-compound hexapeptide library was screened to detect new molecules able to induce pigment dispersion in melanophores. Our approach led to the discovery of seven related novel peptides able to stimulate pigment dispersion with EC50 in the range of 0.1-10 microM. Their action was inhibited by the amphibian melanocortin receptor antagonist dWRL. These novel peptides share no significant sequence homology with known melanocortins. This study may aid in the understanding of the chemical interaction between the melanocortin receptors and their ligands.

Generation of biochemical response patterns of different substances using a whole cell assay with multiple signaling pathways

Distinctive generation of biochemical response patterns of eight different substances, using an assay based on pigment containing cells, was demonstrated. Xenopus laevis melanophores, transfected with human beta(2)-adrenergic receptor, were seeded in a 96 well microplate and used to generate individual biochemical images through a two transient measuring protocol that contributes to highlight the response signatures of the agents. Adequate signal processing creates distinctive patterns in a time-concentration response space suitable for substance classification. The concept of biochemical images is introduced here. The assays were evaluated both with a standard microplate reader and with a computer screen photo-assisted technique (CSPT) yielding similar results. Since CSPT platforms only demand standard computer sets and web cameras as measuring setup, applications for these kind of assays outside main-laboratories were discussed.

Phosphoinositide 3-kinase is involved in Xenopus and Labrus melanophore aggregation

Melanophores are pigmented cells capable of quick colour changes through coordinated transport of their intracellular pigment granules. We demonstrate the involvement of phosphoinositide 3-kinase (PI3-K) in Xenopus and Labrus aggregation by the use of the PI3-K inhibitor, LY-294002. In Xenopus, wortmannin-insensitive PI3-K was found to be essential for the aggregation, mitogen-activated protein kinase (MAPK) activation and tyrosine phosphorylation of a 280-kDa protein, and for the maintenance of low cyclic adenosine 3':5'-monophosphate (cAMP) during the aggregated state. Pre-aggregated cells disperse completely to LY-294002 at 50-100 muM, involving a transient elevation in cAMP due to adenylate cyclase (AC) stimulation or to inhibition of cyclic nucleotide phosphodiesterase (PDE). The inactive analogue LY-303511 did not induce dispersion at the same concentrations. PDE4 and/or PDE2 was found to be involved in melanosome aggregation. The similar kinetics of LY-294002 and various PDE inhibitors indicates that the elevation of cAMP might be due to inhibition of PDE. In Labrus melanophores, LY-294002 had a less dramatic effect, probably due to less dependence on PDE in regulation of cAMP levels. In Xenopus aggregation, we suggest that melatonin stimulation of the Mel1c receptor via G(beta gamma) activates PI3-K that, directly or indirectly via MAPK, activates PDE.

Target validation of G-protein coupled receptors

G-protein coupled receptors (GPCRs) represent possibly the most important target class of proteins for drug discovery. Over 30% of clinically marketed drugs are active at this receptor family. These drugs exhibit their activity at <10% of all known GPCRs. A major challenge for the pharmaceutical industry is to associate the many novel GPCRs with disease to identify the drugs of the future. This process consists of a collection of experimental paradigms that together can be loosely labelled 'target validation'.

Discovery of cytokine mimics using cell-based systems

The successful cloning and subsequent clinical application of recombinant cytokines and/or growth factors has generated a number of important therapeutics. In contrast to the G-protein-coupled receptors, identification of small-molecule agonists of the cytokine and/or growth factor receptor family has proved difficult. The first small peptides and non-peptidic small-molecule agonists for several receptors have recently been reported. The initial identification and/or crucial characterization of these molecules as true mimics was dependent on the use of cell-based functional assays. This article will review recent cell-based assay technologies that are suitable for HTS and that are being applied to the discovery of novel cytokine and growth factor mimics.

Melatonin-induced organelle movement in melanophores is coupled to tyrosine phosphorylation of a high molecular weight protein

Melanophores, brown to black pigment cells from, for example, Xenopus laevis, contain mobile melanin filled organelles, and are well suited for studies on organelle movement. The intracellular regulation of the movement seems to be controlled by serine and threonine phosphorylations and dephosphorylations. Melatonin induces aggregation of the melanosomes to the cell centre through a G(i/o)-protein-coupled receptor, Mel1c, which leads to an inhibition of PKA and a stimulation of PP2A. However, this study shows that the melatonin-induced aggregation of melanosomes is also accompanied by tyrosine phosphorylation of a protein with a molecular weight of approximately 280 kDa. Cells pre-incubated with genistein, an inhibitor of tyrosine phosphorylations, showed inhibited melanosome movement after melatonin stimulation, and a lower degree of tyrosine phosphorylation of the approximately 280 kDa protein. The adenylyl cyclase activator forskolin, and the G(i/o) protein inhibitor pertussis toxin, also inhibited tyrosine phosphorylation of the approximately 280 kDa protein. The results indicate that melatonin stimulation generates tyrosine phosphorylation of a high molecular weight protein, an event that seems to be essential for melanosome aggregation.

A Melanophore-Based Screening Assay for Erythropoietin Receptors

A rapid, functional assay in frog melanophore cells for the erythropoietin receptor (EPOR), a member of the cytokine receptor family, is demonstrated. A chimeric receptor that comprised the extracellular portion of the murine EPOR and the transmembrane and intracellular domains of the human epidermal growth factor receptor (EGFR) was subcloned into the expression vector pJG3.6. When the full-length EGFR was expressed in melanophores, EGF but not EPO mediated pigment dispersion in a time- and dose-dependent manner with an EC50 of 12.6 6 2.9 pM. However, when the chimeric EPOR/EGFR was expressed, EPO but not EGF stimulated pigment dispersion in a time- and dose-dependent manner with an EC50 of 380 6 107 pM. Neither EGF nor EPO had any effect on pigment dispersion in wild-type melanophores. EGF- and EPO-mediated pigment dispersion was blocked by the bis-indolylmaleimide protein kinase C inhibitor Ro 31-8220. This study extends the use of the melanophore-based bioassay to include cytokine receptors in addition to G protein­ and tyrosine kinase­coupled receptors. It represents a potentially powerful method for screening of combinatorial libraries to identify novel small molecule agonists and antagonists to this clinically important class of binding sites as well as performing studies of functional ligand-receptor interactions.

Amphibian Melanophore Technology as a Functional Screen for Antagonists of G-Protein Coupled 7-Transmembrane Receptors

Xenopus laevis melanophores stably expressing 7-transmembrane G-protein-coupled receptors were established and evaluated, either as a primary screening utility for antagonists of the human calcium receptor, or as a screen to assign function to binding inhibitors of human cannabinoid receptors. Stably or transiently expressing melanophores responded selectively to respective effectors of the human calcium, cannabinoid, and neurokinin-1 receptors. Several selective cannabinoid receptor-binding inhibitors of known potency were characterized as agonists or antagonists of the human peripheral cannabinoid (CB(2)) receptor. The results were consistent with changes in cAMP content of hCB(2)-transfected human embryonic kidney (HEK) cells challenged with the same CB(2)-binding antagonists. A stable melanophore cell line expressing the human calcium receptor was used to screen a compound collection directly for functional antagonists, several of which were confirmed as antagonists in secondary screens by stimulating parathyroid hormone (PTH) secretion from bovine parathyroid cells. The percentage of hits in this cell-based screen was reasonably low (1.2%), indicating minimal interference due to toxic effects and validating melanophores as a primary screening modality. Also described is the development of a novel procedure for cryopreservation and reconstitution of cells retaining functional human receptors. ()

Pharmacological characterization of receptor-activity-modifying proteins (RAMPs) and the human calcitonin receptor

Receptor-activity-modifying proteins (RAMPs) are a family of single transmembrane domain proteins shown to be important for the transport and ligand specificity of the calcitonin gene-related peptide (CGRP) receptor. In this report, we describe the analysis of pharmacological properties of the human calcitonin receptor (hCTR) coexpressed with different RAMPs with the use of the Xenopus laevis melanophore expression system. We show that coexpression of RAMP3 with human calcitonin receptor changed the relative potency of hCTR to human calcitonin (hCAL) and rat amylin. RAMP1 and RAMP2, in contrast, had little effect on the change of hCTR potency to hCAL or rat amylin. When coexpressed with RAMP3, hCTR reversed the relative potency by a 3.5-fold loss in sensitivity to hCAL and a 19-fold increase in sensitivity to rat amylin. AC66, an inverse agonist, produced apparent simple competitive antagonism of hCAL and rat amylin, as indicated by linear Schild regressions. The potency of AC66 was changed in the blockade of rat amylin but not hCAL responses with RAMP3 coexpression. The mean pK(B) for AC66 to hCAL was 9.4 +/- 0.3 without RAMP3 and 9.45 +/- 0.07 with RAMP3. For the antagonism of AC66 to rat amylin, the pK(B) was 9.25 +/- 0.15 without RAMP3 and 8.2 +/- 0.35 with RAMP3. The finding suggests that RAMP3 might modify the active states of calcitonin receptor in such a way as to create a new receptor phenotype that is "amylin-like." Irrespective of the physiological association of the new receptor species, the finding that a coexpressed membrane protein can completely change agonist and antagonist affinities for a receptor raises implications for screening in recombinant receptor systems.

Use of a cell-based, lawn format assay to rapidly screen a 442,368 bead-based peptide library

A cell-based, lawn format assay utilizing an in situ photocleavage method has been developed that allows the rapid examination of large bead-based compound libraries as discrete molecules. The format uses frog melanophore cells in a contiguous, adherent, confluent layer in small petri dishes covered with a 0.5-1-mm layer of agarose containing 130 micron diameter TentaGel beads at a density of 2-20 beads/mm2. Employing this technique a 9-mer, 442,368-member peptide library (designed around the 13 amino acid alpha-MSH peptide sequence) made up of 12 separate pools of 36,864 peptides/pool was assayed. Initially, a fraction (approximately 10%) of each pool was scanned (approximately 3700 beads from each pool) in 60-mm petri dishes to identify the most active pools. Upon direct photocleavage of the beads with UV light (365 nm), each petri dish was photographed over a 60-min period with a CCD camera to record changes in light intensity as an index of melanosome dispersion. Active beads were those that were surrounded by a localized decrease in light transmittance indicating melanosome dispersed cells. Upon examination with a dissecting microscope, single beads centrally located to a circular array of dispersed cells were identified and removed from the agarose and sequenced by Edman degradation to determine the peptide sequence. Re-synthesized peptides were re-examined against alpha-MSH receptor to confirm and quantify the activity. Several 9-mer peptides were identified with potencies similar to the natural 13-mer peptide. This method allows for the rapid screening of large bead-based photo-cleavable peptide libraries with the advantage that each compound is screened as a discrete molecule in a well-less format.

Molecular mechanisms of pigment transport in melanophores

We present an overview of the research on intracellular transport in pigment cells, with emphasis on the most recent discoveries. Pigment cells of lower vertebrates have been traditionally used as a model for studies of intracellular transport mechanisms, because these cells transport pigment organelles to the center or to the periphery of the cell in a highly co-ordinated fashion. It is now well established that both aggregation and dispersion of pigment in melanophores require two elements of the cytoskeleton: microtubules and actin filaments. Melanosomes are moved along these cytoskeletal tracks by motor proteins. Recent studies have identified the motors responsible for pigment dispersion and aggregation in melanophores. We propose a model for the possible roles of the two cytoskeletal transport systems and how they might interact. We also discuss the putative mechanisms of regulation of pigment transport, especially phosphorylation. Last, we suggest areas of research that will receive attention in the future in order to elucidate the mechanisms of organelle transport.

Novel GPCRs and their endogenous ligands: expanding the boundaries of physiology and pharmacology

Nearly all molecules known to signal cells via G proteins have been assigned a cloned G-protein-coupled-receptor (GPCR) gene. This has been the result of a decade-long genetic search that has also identified some receptors for which ligands are unknown; these receptors are described as orphans (oGPCRs). More than 80 of these novel receptor systems have been identified and the emphasis has shifted to searching for novel signalling molecules. Thus, multiple neurotransmitter systems have eluded pharmacological detection by conventional means and the tremendous physiological implications and potential for these novel systems as targets for drug discovery remains unexploited. The discovery of all the GPCR genes in the genome and the identification of the unsolved receptor-transmitter systems, by determining the endogenous ligands, represents one of the most important tasks in modern pharmacology.

Cloning and characterization of RGS9-2: a striatal-enriched alternatively spliced product of the RGS9 gene

Regulators of G-protein signaling (RGS) proteins act as GTPase-activating proteins (GAPs) for alpha subunits of heterotrimeric G-proteins. Previous in situ hybridization analysis of mRNAs encoding RGS3-RGS11 revealed region-specific expression patterns in rat brain. RGS9 showed a particularly striking pattern of almost exclusive enrichment in striatum. In a parallel study, RGS9 cDNA, here referred to as RGS9-1, was cloned from retinal cDNA libraries, and the encoded protein was identified as a GAP for transducin (Galphat) in rod outer segments. In the present study we identify a novel splice variant of RGS9, RGS9-2, cloned from a mouse forebrain cDNA library, which encodes a striatal-specific isoform of the protein. RGS9-2 is 191 amino acids longer than the retinal isoform, has a unique 3' untranslated region, and is highly enriched in striatum, with much lower levels seen in other brain regions and no expression detectable in retina. Immunohistochemistry showed that RGS9-2 protein is restricted to striatal neuropil and absent in striatal terminal fields. The functional activity of RGS9-2 is supported by the finding that it, but not RGS9-1, dampens the Gi/o-coupled mu-opioid receptor response in vitro. Characterization of a bacterial artificial chromosome genomic clone of approximately 200 kb indicates that these isoforms represent alternatively spliced mRNAs from a single gene and that the RGS domain, conserved among all known RGS members, is encoded over three distinct exons. The distinct C-terminal domains of RGS9-2 and RGS9-1 presumably contribute to unique regulatory properties in the neural and retinal cells in which these proteins are selectively expressed.

Functional characterization of a receptor for vasoactive-intestinal-peptide-related peptides in cultured dermal melanophores from Xenopus laevis

A receptor for vasoactive-intestinal-peptide (VIP)-related peptides was functionally characterized in a cell line derived from Xenopus melanophores using a recently described microtiter-plate-based bioassay. Activation of the melanophore VIP receptor by VIP or the peptides pituitary-adenylate-cyclase-activating polypeptide (PACAP 38), PACAP 27, and helodermin stimulated intracellular 3'-5' cyclic adenosine monophosphate (cAMP) accumulation and pigment dispersion in the cells. Helodermin, with an EC50 (concentration of peptide inducing half-maximal melanosome dispersion) of 46.5 pM, was the most potent activator of pigment dispersion, followed by PACAP 38 > VIP > PACAP 27. A similar order of potencies was observed for the peptides to induce cAMP accumulation. The responses to VIP agonists were selectively inhibited by the VIP antagonists PACAP-(6-27) and (N-Ac-Tyr(1)-D-Phe2)-growth-hormone-releasing factor[GRF](1-29)-NH2. Taken together, the results suggest that the melanophores express a VIP receptor that shares certain characteristics of, but also differs significantly from, other previously identified VIP receptors.

Cloning of a novel G-protein-coupled receptor GPR 51 resembling GABAB receptors expressed predominantly in nervous tissues and mapped proximal to the hereditary sensory neuropathy type 1 locus on chromosome 9

Query of the expressed sequence tag database with the rat metabotropic GABABR1A receptor amino acid sequence using the TFASTA algorithm revealed two partial cDNA fragments whose sequence information was then used to isolate by PCR a novel full-length human cDNA encoding a putative G-protein-coupled receptor (GPCR), termed GPR 51. Sequence analysis revealed that it encoded a protein of 941 amino acids, similar in size and homology to GABAB receptors followed by metabotropic glutamate receptors but not other GPCRs. GPR 51 expressed in COS-1 cells showed no specific binding for [3H](+)baclofen and when expressed in Xenopus oocyte and Xenopus melanophore functional assays showed no activity to GABA, (-)baclofen, and glutamic acid. Northern blot analysis and in situ hybridization revealed that GPR 51 transcripts were predominantly expressed in the central nervous system with highest abundance in the cortex, thalamus, hippocampus, amygdala, cerebellum, and spinal cord. In contrast, GPR 51 receptor transcripts were almost not detected in the peripheral tissues. Gene GPR 51 was localized by radiation hybrid mapping to chromosome 9, 4.81 cR from the WI-8684 marker, and proximal to the hereditary sensory neuropathy type 1 locus.

Recombinant human CXC-chemokine receptor-4 in melanophores are linked to Gi protein: seven transmembrane coreceptors for human immunodeficiency virus entry into cells

This article describes the transient expression of the CXC chemokine receptor-4 in Xenopus laevis melanophores and the resulting functional assay for the endogenous ligand for this receptor stromal cell-derived factor (SDF)-1alpha. Specifically, it will be shown that SDF-1alpha produces increased light transmittance in transfected cells that is consistent with the activation of Gi protein. This stimulus pathway is further implicated by the abolition of this response after pretreatment of the cells with pertussis toxin, a known method for the inactivation of Gi protein. The fact that SDF-1alpha does not produce responses in nontransfected cells and that treatment of the cells with 12G5, an antibody specific for the CXC chemokine receptor-4, eliminates this response indicates that this ligand produces responses by activation of this receptor in these cells. The possible relevance to human immunodeficiency virus (HIV) entry into cells was explored by observing the effects of SDF-1alpha on HIV-mediated cell fusion. It was found that SDF-1alpha blocked cell-to-cell fusion (as has been previously reported) at concentrations 1200-fold greater than those required to produce Gi protein mediated responses. The implications of the functional assay to screening for new drugs to block HIV-mediated fusion is discussed.

Functional analysis by imaging of melanophore pigment dispersion of chimeric receptors constructed by recombinant polymerase chain reaction

Analysis of functional aspects of the molecular structure of proteins often requires a means to selectively alter structure and subsequently analyze function. We have adapted a method of overlap extension polymerase chain reaction (PCR) to generate multiple domain replacements in G-protein coupled receptors. The examples described herein are beta 2-adrenergic receptors whose G-protein coupling domains have been replaced by homologous domains of olfactory receptors, but the procedure has also been used to produce constructs with mutations, deletions, and fusions of two complete open reading frames. The chimeric olfactory-adrenergic receptors were assayed by functional expression in clonal lines of Xenopus melanophores. The ability of G-protein coupled second messenger pathways to cause translocation of pigment organelles within melanophores allows the use of video microscopy to assay the function of the chimeric receptors. Digital automation of microscope stage, camera, and image processing allows multiple parallel experiments to be performed. Melanophores allow responses mediated by the Gs, Gq and Gi pathways to be assayed with equal efficiency and the specificity of the coupling between chimera (or receptor) and G-protein subtypes can be rapidly determined.

Pigment cell signalling for physiological color change

The cellular signalling pathways participating in physiological color change are reviewed, particularly in crustaceans, teleosts, amphibians, and reptiles. This review is an attempt to summarize what is known and to raise some hypotheses about basic questions still to be elucidated. The first picture that emerges from the literature is that the transduction pathways are identical in the various types of chromatophores of a single species, except for the iridophore. The cAMP-dependent pathway has been well conserved throughout evolution: cAMP increase is the pigment dispersion signal whereas the nucleotide decrease leads to granule aggregation. On the other hand, the Ca(-2)-dependent pathways evoke pigment aggregation in teleosts and crustaceans, and dispersion in amphibians and probably reptiles as well. Another interesting point is the ultimate convergence of the signalling pathways of different agonists inducing the same response in one chromatophore type. A hypothesis is raised about why different chromatophores behave differently in the absence of agonists, that is, why some are punctate, whereas others are stellate.

Gene expression systems in the development of high-throughput screens

Recent advances in the development of combinatorial automated chemical synthesis, robotic sample handling, and data collection and analysis have significantly increased the number of compounds available for screening against potential therapeutic targets. The implementation of highly sensitive in vitro biochemical and cell-based high-throughput screening assays is essential to facilitate the rapid identification of selective and potent lead molecules from compound libraries. The ability to easily produce functional proteins in sufficient quantities for in vitro biochemical assays and to devise useful cell-based systems is dependent on the successful application of a variety of gene expression systems.

Functional expression of olfactory-adrenergic receptor chimeras and intracellular retention of heterologously expressed olfactory receptors

Replacing the G-protein-coupling domains of the beta 2-adrenergic receptor with homologous domains of putative olfactory receptors produced chimeric receptors which were able to stimulate pigment dispersion in Xenopus melanophores, a G-protein-mediated pathway. A multiple replacement chimera containing the second, third and C-terminal cytoplasmic domains of receptor OR5 elevated cyclic adenosine 3':5'-monophosphate (cAMP) and suppressed production of inositol phosphates. Co-expression of G alpha olf did not alter the strength of response of this chimera. A novel rat olfactory receptor cDNA (U131) was isolated and sequenced. Expression of U131 and OR5 constructs containing an N-terminal epitope-tag or C-terminal fusion to green fluorescent protein occurred in an intracellular network but not in the plasma membrane of heterologous cells. Similarly treated beta 2-adrenergic receptors were functional and were observed in the plasma membrane and the intracellular network. These results demonstrate that the putative cytoplasmic domains of olfactory receptors are capable of functional interaction with heterologous G-proteins of the G alpha s subtype. Instead, the absence of these receptors from the plasma membrane of heterologous cells appears to explain our inability to determine if odorants can activate the olfactory receptor clones. We hypothesize that the olfactory receptors have requirements for maturation and targeting to the plasma membrane that are different from most other G-protein-coupled receptors.

Synthesis and characterization of bivalent peptide ligands targeted to G-protein-coupled receptors

BACKGROUND

Through the effects of avidity, multivalency can increase the apparent affinity of a ligand for its binding site. Low molecular weight, high affinity, multivalent ligands theoretically could be used to deliver a variety of agents to specific cell subtypes. In order to target specific G-protein-coupled receptors, a series of monospecific peptide dimers were synthesized that are designed to bind to two adjacent receptor sites.

RESULTS

Three dimers, consisting of a ligand region, a short, flexible, uncharged spacer, a longer, polylysine spacer and a single cysteine residue to permit dimerization, and the corresponding monomers were synthesized by solid-phase peptide synthesis. The ligand domain was either alpha-melanocyte stimulating hormone (alpha-MSH), an alpha-MSH receptor antagonist (alpha-MSH-ANT), or bombesin. These ligands were characterized in a functional melanocyte dispersion assay. In wild-type melanophores, the alpha-MSH dimer stimulated dispersion with an EC50 approximately seven-fold lower than that of the corresponding monomer. Similarly, in cells transfected with bombesin receptor cDNA, the bombesin dimer was approximately five-fold more potent than the monomer. The alpha-MSH-ANT monomer specifically inhibited alpha-MSH-mediated dispersion with no significant agonist activity, but the dimer acted predominantly as an agonist.

CONCLUSIONS

Peptide dimers can be synthesized easily and have enhanced functional activity; monospecific dimers have greater avidity and bispecific dimers are likely to have greater selectivity. They may therefore have practical potential as specific cell-targeting agents.

Melanophore pigment dispersion responses to agonists show two patterns of sensitivity to inhibitors of cAMP-dependent protein kinase and protein kinase C

Melanophore pigment dispersion is a sensitive bioassay for activation of the adenylyl cyclase and phospholipase C second-messenger pathways. The necessity of protein kinase activation in causing pigment dispersion was confirmed for eight agonists of endogenous melanophore receptors and for two transfected receptors. All agonists and receptors previously shown to elevate intracellular cAMP in melanophores--melanocyte stimulating hormone, light, (-) norepinephrine, 5-hydroxytrptamine, and the beta2-adrenergic receptor--were able to stimulate pigment dispersion in the presence of Ro31-8220, a potent inhibitor of protein kinase C, but were blocked in the presence of H89, an inhibitor of cAMP-dependent protein kinase. The bombesin receptor, which elevates intracellular IP3 in melanophores, was unable to stimulate pigment dispersion in the presence of Ro31-8220 or H89. Agonists whose mechanism of activation of pigment dispersion are unknown were also tested. Endothelin 3 responses were blocked by both H89 and Ro31-8220, predicting coupling to phospholipase C. Vasoactive intestinal polypeptide, oxytocin, and calcitonin gene-related peptide beta responses were blocked only by H89, predicting coupling to adenylyl cyclase.

Pharmacology of muscarinic acetylcholine receptor subtypes (m1-m5): high throughput assays in mammalian cells

Based on the ability of many receptors to amplify NIH 3T3 cells, we developed a high throughput assay of cloned receptor pharmacology. In this assay, receptors are transiently co-expressed with the marker enzyme beta-galactosidase. Receptors that induce cellular proliferation select and amplify the cells that also express the marker, thus the ability of ligands to alter receptor activity are reported as changes in enzyme activity. We used this assay to evaluate the pharmacology of agonist and antagonist ligands for five cloned human muscarinic receptor subtypes (m1-m5). When cells were transfected with subtypes that prefer the G-protein Gq (m1, m3, m5) robust increases in enzyme activity were observed. The subtypes that prefer Gi (m2 and m4) only induced beta-galactosidase when co-expressed with a chimera between the G-proteins Gq and Gi (Gq-i5). Overall, the rank-order of potency and intrinsic activity of most of the tested ligands were in remarkably good agreement with earlier results using cloned cell lines and isolated tissues. These data demonstrate that a high throughput colorimetric assay performed in 96-well plates can be used to evaluate subtle differences the pharmacology of ligands for cloned muscarinic receptor subtypes.

Prospects for rational approaches to anthelmintic discovery

Rational approaches to anthelmintic discovery include the design of screens for compounds directed at specific proteins in helminths that are pharmacologically distinguishable from their vertebrate homologues. The existence of several anthelmintics that selectively target the neuromusculature of helminths (e.g. levamisole, ivermectin, praziquantel, metrifonate), together with recent basic research in helminth physiology, have contributed to the recognition that neurobiology distinguishes these organisms from their vertebrate hosts. In this survey, we focus on mechanism-based screening and its application to anthelmintic discovery, with particular emphasis on targets in the neuromusculature of helminths. Few of these proteins have been exploited in chemotherapy. However, recent studies in comparative pharmacology and molecular biology, including the C. elegans genome project, have provided insights on potential new targets and, in some cases, molecular probes useful for their incorporation in mechanism-based screens.

High throughput assays of cloned adrenergic, muscarinic, neurokinin, and neurotrophin receptors in living mammalian cells

Many receptors stimulate proliferation of NIH 3T3 cells in a ligand dependent fashion. Based on this observation, we developed a high throughput assay of cloned receptor pharmacology. In this assay, receptors are transiently co-expressed with the marker enzyme beta-galactosidase. Receptors that induce cellular proliferation select and amplify the cells that also express the marker, thus the ability of ligands to alter receptor activity are reported as changes in enzyme activity. In the present study, we used this assay to evaluate the ability of agonist ligands to stimulate four cloned receptors. The agonists phenylephrine, carbachol, substance P and nerve growth factor selectively stimulated cells transfected with the alpha-1b adrenergic, m4 muscarinic, NK1 neurokinin and trkA neurotrophin receptors, respectively. These data demonstrate that a high throughput colorimetric assay performed in 96 well plates can be used to evaluate the pharmacology of ligands for cloned receptors belonging to a wide range of functional and pharmacological classes.

Combinatorial diffusion assay used to identify topically active melanocyte-stimulating hormone receptor antagonists

alpha-Melanocyte-stimulating hormone (alpha-MSH) is implicated in pigmentation, central nervous system and immune system functions, growth, mitogenesis, and melanoma. Evaluation of these roles has been hindered by the lack of alpha-MSH antagonists. A combinatorial chemistry-based diffusion assay is used to find random tripeptides that antagonize normal frog and human melanoma MSH receptors and to identify pharmacological groups responsible for receptor interaction. The alpha-MSH antagonist D-Trp-Arg-Leu-NH2 is used to demonstrate directly the contribution of MSH to normal skin tone in frogs following injection or topical application.

A rapid bioassay for platelet-derived growth factor beta-receptor tyrosine kinase function

We have extended a melanophore-based bioassay for G-protein coupled receptors to include the functional expression of the murine platelet-derived growth factor (PDGF) beta-receptor. The homodimeric ligand PDGF-BB induced activation of the transiently expressed receptor in melanophore cells. This led to dose dependent pigment dispersion whereas it did not induce pigment dispersion in wild type cells. The effective concentration of PDGF-BB giving half-maximal pigment dispersion (EC50) was 1nM after 30 minutes exposure. PDGF-AA had no ability to induce pigment dispersion in melanophore cells transiently expressing the beta-PDGF receptor. PDGF-BB-induced pigment dispersion could be blocked by the bis-indolylmaleimide Ro 31-8220 which is an inhibitor of protein kinase C isoenzymes. Functional expression of the PDGF beta-receptor extends the use of the pigment translocation assay to include transmembrane signaling receptor tyrosine kinases. It opens the opportunity for the discovery of potent agonists and antagonists through massive drug screening and investigations of functional ligand-receptor interactions for single transmembrane domain receptors.

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.

Creation and functional screening of a multi-use peptide library

Studies of functional interactions between transmembrane proteins such as G-protein-coupled receptors and ligands would benefit from the ability to utilize synthetic molecule libraries. This is realized here by the construction and application of a multi-use combinatorial peptide library (MUPL). Peptides are liberated from their supports in a dry state so that the problem of signal interference due to mixing of peptide molecules, particularly agonists and antagonists, is avoided. In addition, the peptides are released from their supports in a controlled manner so that fractions are available for multiple independent tests, thus eliminating the need for iterative library analysis and resynthesis. The MUPL concept was validated with a functional screen which detects agonists to G-protein-coupled receptors and led to the discovery of new ligands. It is expected that combining MUPLs with functional assays will enhance both basic scientific research and the rates of drug discovery and development.

Characterization of a serotonin receptor endogenous to frog melanophores

The response of a cell line of Xenopus laevis melanophores to serotonin was examined. Serotonin increased intracellular levels of cAMP and induced pigment dispersion in the cells. The responses depended on both the concentration of serotonin applied and on the time for which the cells were exposed to serotonin. Using a recently described, microtiter-plate-based bioassay, a series of serotonin receptor ligands were evaluated as agonists or antagonists at the melanophore serotonin receptor. The pharmacological profile suggests the presence of a receptor which shares some properties with but appears different from other previously described serotonin receptors.