Pigment dispersion in frog melanophores can be induced by a phorbol ester or stimulation of a recombinant receptor that activates phospholipase C

Sleep and cargo reorganization: A hypothesis

Several molecules that act in the nervous system to regulate sleep and wake were first identified based on their transport effects in pigmented cells. I compiled a list of such molecules like melatonin, melanin-concentrating hormone, and pigment dispersing factor, etc. Molecules that induce pigment aggregation promote sleep whereas molecules that induce pigment dispersal promote wake. I call these Sleep and PIgment Regulating Factors SPIRFs. SPIRFs regulate organelle trafficking in both pigmentary models and neurons. I propose that cargo transport fulfills necessary sleep functions such as remodeling synapses and restoring homeostasis in the distribution of cell components. I put forth the hypothesis that sleep-promoting SPIRFs induce states of increased cargo movement towards the cell body, and propose that this function is a critical neuron maintenance task for which animals must sleep.

5-HT receptors as novel targets for optimizing pigmentary responses in dorsal skin melanophores of frog, Hoplobatrachus tigerinus

BACKGROUND AND PURPOSE

Biochemical identification of 5-HT has revealed similar projection patterns across vertebrates. In CNS, 5-HT regulates major physiological functions but its peripheral functions are still emerging. The pharmacology of 5-HT is mediated by a diverse range of receptors that trigger different responses. Interestingly, 5-HT receptors have been detected in pigment cells indicating their role in skin pigmentation. Hence, we investigated the role of this monoaminergic system in amphibian pigment cells, melanophores, to further our understanding of its role in pigmentation biology together with its evolutionary significance.

EXPERIMENTAL APPROACH

Pharmacological profiling of 5-HT receptors was achieved using potent/selective agonists and antagonists. In vitro responses of melanophores were examined by Mean Melanophores Size Index assay. The melanophores of lower vertebrates are highly sensitive to external stimuli. The immediate cellular responses to drugs were defined in terms of pigment translocation within the cells.

KEY RESULTS

5-HT exerted strong concentration-dependent pigment dispersion at threshold dose of 1 × 10(-6) g·mL(-1). Specific 5-HT(1) and 5-HT(2) receptor agonists, sumatriptan and myristicin. also induced dose-dependent dispersion. Yohimbine and metergoline synergistically antagonized sumatriptan-mediated dispersion, whereas trazodone partially blocked myristicin-induced dispersion. Conversely, 5-HT(3) and 5-HT(4) receptor agonists, 1 (3 chlorophenyl) biguanide (1,3 CPB) and 5-methoxytryptamine (5-MT), caused a dose-dependent pigment aggregation. The aggregatory effect of 1,3 CPB was completely blocked by ondansetron, whereas L-lysine partially blocked the effect of 5-MT.

CONCLUSIONS AND IMPLICATIONS

The results suggest that 5-HT-induced physiological effects are mediated via distinct classes of receptors, which possibly participate in the modulation of pigmentary responses in amphibian.

G protein-coupled receptor mediated trimethylamine sensing

A new approach for the detection of trimethylamine (TMA) using a recombinant cell line of Xenopus laevis melanophores was developed. The cells were genetically modified to express the mouse trace amine-associated receptor 5 (mTAAR5), a G protein-coupled receptor from the mouse olfactory epithelium, which conferred high sensitivity to TMA. Cellular responses to TMA were analyzed by two different techniques, either by absorbance measurements using a microplate reader or by cellular imaging via an inverted microscope. A focused chemical screen allowed the discovery of additional, previously unknown stimuli of mTAAR5. The developed cell-based sensor demonstrated no sensitivity to trimethylamine N-oxide (TMAO), making it suitable for a straightforward evaluation of TMA levels in fish tissue extracts. For the detection of TMA vapor, the cells were covered with agarose, which allowed for intact cell viability for at least 6h in air. The developed gas measurement platform was able to detect TMA from 1 to 100 ppm within 35 min.

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.

RGS9-2 is a negative modulator of mu-opioid receptor function

Regulators of G-protein signaling (RGS) 9-2 is a striatal enriched protein that controls G protein coupled receptor signaling duration by accelerating Galpha subunit guanosine triphosphate hydrolysis. We have previously demonstrated that mice lacking the RGS9 gene show enhanced morphine analgesia and delayed development of tolerance. Here we extend these studies to understand the mechanism via which RGS9-2 modulates opiate actions. Our data suggest that RGS9-2 prevents several events triggered by mu-opioid receptor (MOR) activation. In transiently transfected PC12 cells, RGS9-2 delays agonist induced internalization of epitope HA-tagged mu-opioid receptor. This action of RGS9-2 requires localization of the protein near the cell membrane. Co-immunoprecipitation studies reveal that RGS9-2 interacts with HA-tagged mu-opioid receptor, and that this interaction is enhanced by morphine treatment. In addition, morphine promotes the association of RGS9-2 with another essential component of MOR desensitization, beta-arrestin-2. We also show that over-expression of RGS9-2 prevents opiate-induced extracellular signal-regulated kinase phosphorylation. Our data indicate that RGS9-2 plays an essential role in opiate actions, by negatively modulating MOR downstream signaling as well as the rate of MOR endocytosis.

Melanophores: A model system for neuronal transport and exocytosis?

Black pigment cells, melanophores, from lower vertebrates are specialized in bidirectional and coordinated translocation of pigment granules, melanosomes, in the cytoplasm. Melanophores develop from the neuronal crest and are most abundant in the dermal and epidermal layers of the skin, where the intracellular distribution of the pigment significantly influences the color of the animal. The transport of pigment is dependent on an intact cytoskeleton and motor proteins associated with cytoskeletal components. The easily cultured melanophores have proved to be excellent models for organelle transport because the intracellular movements of pigment can be visualized via light microscopy, and the granules move in response to defined chemical signals. The ease of achieving a combination of morphological and functional transport studies is the advantage of the melanophore system, and studies on pigment cells have revealed new components of the transport machinery, including molecular motors, their adapters, and transfer of vesicles to other cells. Many cellular components are transported with a combination of the actin- and microtubule-based transport systems, and, since all eukaryotic organisms rely on functional intracellular transport and an intact cytoskeleton, studies on melanophores are important for many aspects of cell biology, including axonal transport. In this review, we present an overview of the research on the pigment transport system and the potential use of pigment cells as a model system.

Orphan seven transmembrane receptor screening

Drug discovery has successfully exploited the superfamily of seven transmembrane receptors (7TMR), with over 35% of clinically marketed drugs targeting them. However, it is clear that there remains an undefined potential within this protein family for successful drugs of the future. The human genome sequencing project identified approximately 720 genes that belong to the 7TMR superfamily. Around half of these genes encode sensory receptors, while the other half are potential drug targets. Natural ligands have been identified for approximately 215 of these, leaving 155 receptors classified as orphan 7TMRs having no known ligand. Deorphanisation of these receptors by identification of natural ligands has been the traditional method enabling target validation by use of these ligands as tools to define biological relevance and disease association. Such ligands have been paired with their cognate receptor experimentally by screening of small molecule and peptide ligands, reverse pharmacology and the use of bioinformatics to predict candidate ligands. In this manuscript, we review the methodologies developed for the identification of ligands at orphan 7TMRs and exemplify these with case studies.

Dynein-dynactin complex mediates protein kinase A-dependent clustering of Weibel-Palade bodies in endothelial cells

OBJECTIVE

Perinuclear clustering is observed for several different organelles and illustrates dynamic regulation of the secretory pathway and organelle distribution. Previously, we observed that a subset of Weibel-Palade bodies (WPBs), endothelial cell-specific storage organelles, undergo centralization when endothelial cells are stimulated with cAMP-raising agonists of von Willebrand factor (vWF) secretion. In this study, we investigated this phenomenon of WPB clustering in more detail.

METHODS AND RESULTS

Our results demonstrate that the clustered WPBs are localized at the microtubule organizing center and that cluster formation depends on an intact microtubule network. Disruption of the microtubules by nocodazole completely abolished clustering, whereas treatment with the actin depolymerizing compound cytochalasin B had no effect on WPB clustering. Interfering with the dynein-dynactin interaction by overexpression of the p50 dynamitin subunit or the CC1 domain of the p150glued subunit of the dynactin complex completely inhibited perinuclear clustering of WPBs, suggesting that dynein activity mediates this process. Furthermore, we found that inhibition of dephosphorylation resulted in an increase in clustering, whereas inhibition of protein kinase A (PKA) markedly reduced WPB clustering.

CONCLUSIONS

These results suggest that perinuclear clustering of WPBs involves PKA-dependent regulation of the dynein-dynactin complex. Endothelial cell stimulation with epinephrine results in retrograde movement of a subset of WPBs to the microtubule organizing center. This minus-end directed transport requires an intact microtubular network and is mediated by the motor protein dynein. Together, our results suggest that epinephrine-induced clustering of WPBs involves PKA-dependent regulation of the dynein-dynactin complex.

Identification of signal transduction pathways used by orphan g protein-coupled receptors

The superfamily of GPCRs have diverse biological roles, transducing signals from a range of stimuli, from photon recognition by opsins to neurotransmitter regulation of neuronal function. Of the many identified genes encoding GPCRs, >130 are orphan receptors ( i.e., their endogenous ligands are unknown), and this subset represents putative novel therapeutic targets for pharmaceutical intervention in a variety of diseases. As an initial step toward drug discovery, determining a biological function for these newly identified receptors is of vital importance, and thus identification of a natural ligand(s) is a primary aim. There are several established methods for doing this, but many have drawbacks and usually require some in-depth knowledge about how the receptor functions. The technique described here utilizes a transcription-based reporter assay in live cells. This allows the determination of the signal transduction pathway any given oGPCR uses, without any prior knowledge of the endogenous ligand. This can therefore reduce the redundancy of effort involved in screening ligands at a given receptor in multiple formats (i.e., Galpha(s), Galpha(i/0), and Galpha(q) assays), as well as ensuring that the receptor targeted is capable of signaling if appropriately activated. Such knowledge is often laboriously obtained, and for almost all oGPCRs, this kind of information is not yet available. This technology can also be used to develop inverse agonist as well as agonist sensitive high throughput assays for oGPCRs. The veracity of this approach is demonstrated, using a number of known GPCRs. The likely signaling pathways of the GPR3, GPR12, GPR19, GPR21, and HG55 oGPCRs are shown, and a high throughput assay for GPR26 receptors developed. The methods outlined here for elucidation of the signal transduction pathways for oGPCRs and development of functional assays should speed up the process of identification of ligands for this potentially therapeutically useful group of receptors.

Some sweet and bitter tastants stimulate inhibitory pathway of adenylyl cyclase via melatonin and alpha 2-adrenergic receptors in Xenopus laevis melanophores

The sweeteners saccharin, D-tryptophan, and neohesperidin dihydrochalcone (NHD) and the bitter tastant cyclo(Leu-Trp) stimulated concentration-dependent pigment aggregation in a Xenopus laevis melanophore cell line similar to melatonin. Like melatonin, these tastants inhibited (by 45-92%) cAMP formation in melanophores; pertussis toxin pretreatment almost completely abolished the tastant-induced cAMP inhibition, suggesting the involvement of the inhibitory pathway (Gi) of adenylyl cyclase. The presence of luzindole (melatonin receptor antagonist) almost completely abolished the inhibition of cAMP formation induced by saccharin, D-tryptophan, and cyclo(Leu-Trp) but only slightly affected the inhibitory effect of NHD. In contrast, the presence of an alpha2-adrenergic receptor antagonist, yohimbine, almost completely abolished the inhibition of cAMP formation induced by NHD but had only a minor effect on that induced by the other tastants. Thus saccharin, D-tryptophan, and cyclo(Leu-Trp) are melatonin receptor agonists whereas NHD is an alpha2-adrenergic receptor agonist, but both pathways lead to the same transduction output and cellular response. Formation of D-myo-inositol 1,4,5-trisphosphate (IP3) in melanophores was reduced (15-58%, no concentration dependence) by saccharin, D-tryptophan, and cyclo(Leu-Trp) stimulation but increased by NHD stimulation. Tastant stimulation did not affect cGMP. Although some of the above tastants were found to be membrane permeant, their direct activation of downstream transduction components in this experimental system is questionable. MT1 and MT2 melatonin receptor mRNAs were identified in rat circumvallate papilla taste buds and nonsensory epithelium, suggesting the occurrence of MT1 and MT2 receptors in these tissues. Melatonin stimulation reduced the cellular content of cAMP in taste cells, which may or may not be related to taste sensation.

Noradrenaline- and melatonin-mediated regulation of pigment aggregation in fish melanophores

The effects of melatonin and noradrenaline (NA) on bi-directional melanosome transport were analysed in primary cultures of melanophores from the Atlantic cod. Both agents mediated rapid melanosome aggregation, and by using receptor antagonists, melatonin was found to bind to a melatonin receptor whereas NA binds to an alpha2-adrenoceptor. It has previously been stated that melatonin-mediated melanosome aggregation in Xenopus is coupled with tyrosine phosphorylation of a so far unidentified high molecular weight protein and we show that although acting through different receptors and through somewhat different downstream signalling events, tyrosine phosphorylation is of the utmost importance for melanosome aggregation mediated by both NA and melatonin in cod melanophores. Together with cyclic adenosine 3-phosphate-fluctuations, tyrosine phosphorylation functions as a switch signal for melanosome aggregation and dispersion in these cells.

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'.

An endogenous 5-HT(7) receptor mediates pigment granule dispersion in Xenopus laevis melanophores

Melatonin (5-methoxy N-acetyltryptamine) and serotonin (5-HT) exert rapid, but opposite effects on pigment granule distribution in Xenopus laevis melanophores. Low concentrations of melatonin (10(-11) - 10(-9) M) cause a dramatic perinuclear aggregation of the melanin-containing granules, while 5-HT (10(-8) - 10(-5) M) disperses pigment granules throughout the cell. The present study found that pharmacological doses of melatonin (> or =10(-6) M) induced a time- and concentration-dependent pigment granule dispersion, which was mediated by an endogenous melanophore 5-HT receptor. 5-HT produced a concentration-dependent elevation of melanophore cyclic AMP, and 5-HT-induced dispersion was blocked by H89 (10(-4) M), an inhibitor of protein kinase A (PKA), but not by a PKC inhibitor (Ro 31-8220, 10(-5) M), indicating a vital role for cyclic AMP in 5-HT-induced dispersion. 5-HT-mediated dispersion was not blocked by antagonists selective for G(s)-coupled 5-HT(4) (GR113808) or 5-HT(6) (Ro 04-6790, Ro 63-0563, olanzepine) receptors, nor by 5-HT(1 - 3) (pindolol, ketanserine, metoclopramide, MDL72222, tropisetron) receptor antagonists, but was inhibited by a selective 5-HT(7) receptor antagonist, DR4004, and other antagonists with a high affinity for 5-HT(7) receptors. The rank order of antagonist potency was: risperidone (mean pK(B) 7.82)>methiothepin (7.43)>DR4004 (6.92)>mesulergine (6.83)>methysergide (6.60)>[+/-]-sulpiride (5.81)>spiperone (5.52). The agonist potency order [mean pEC(50), 5-CT (8.68)>5-HT (7.13)>5-MT (6.94)>8-OH-DPAT (4.79)>sumatriptan (<4)] was also consistent with an action on 5-HT(7) receptors. RT - PCR confirmed that melanophores express 5-HT(7) receptor mRNA. The pigment dispersing effect of high melatonin concentrations in melanophores is most likely mediated by activation of 5-HT(7) receptors. Conceivably some of the effects attributed to pharmacological doses of melatonin in mammals may be mediated by activation of 5-HT(7) receptors.

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.

Constitutive receptor systems for drug discovery

This paper discusses the use of constitutively active G-protein-coupled receptor systems for drug discovery. Specifically, the ternary complex model is used to define the two major theoretical advantages of constitutive receptor screening-namely, the ability to detect antagonists as well as agonists directly and the fact that constitutive systems are more sensitive to agonists. In experimental studies, transient transfection of Chinese hamster ovary cyclic AMP response element (CRE) luciferase reporter cells with cDNA for human parathyroid hormone receptor, glucagon receptor, and glucagon-like peptide (GLP-1) receptor showed cDNA concentration-dependent constitutive activity with parathyroid hormone (PTH-1) and glucagon. In contrast, no constitutive activity was observed for GLP-1 receptor, yet responses to GLP-1 indicated that receptor expression had taken place. In another functional system, Xenopus laevi melanophores transfected with cDNA for human calcitonin receptor showed constitutive activity. Nine ligands for the calcitonin receptor either increased or decreased constitutive activity in this assay. The sensitivity of the system to human calcitonin increased with increasing constitutive activity. These data indicate that, for those receptors which naturally produce constitutive activity, screening in this mode could be advantageous over other methods.

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.

Physico-chemical and biological analysis of true combinatorial libraries

Combinatorial libraries offer new sources of compounds for the research of pharmacological agents such as receptor ligands, enzyme inhibitors or substrates and antibody-binding epitopes. The present review stresses the main roles played by both physico-chemical analysis, particularly when complex mixture of compounds are synthesized as libraries, and biological analysis from which active compounds are identified. After a brief discussion of semantic problems related to the designation of the product mixtures, the physico-chemical analysis of mixtures is reviewed with special emphasis on mass spectrometric techniques. These methods are able both to give a representative view of a library composition and to identify single critical compounds in large libraries. Then the biological screening of such combinatorial libraries is critically discussed with respect to the power and limitations of the methods used for the identification of the active components. Special attention is given to the complex process of library deconvolution. It is pointed out that while combinatorial techniques have evolved towards sophisticated high-tech methods, simple and robust biochemical tests should be used to deconvolute. From a large panel of published examples, a set of trends are identified which should help investigators to choose the most appropriate assay for the discovery of new entities.

Leukotriene binding, signaling, and analysis of HIV coreceptor function in mouse and human leukotriene B4 receptor-transfected cells

The mouse leukotriene B4 receptor (m-BLTR) gene was cloned. Membrane fractions of human embryonic kidney 293 cells stably expressing m-BLTR demonstrated a high affinity and specific binding for leukotriene B4 (LTB4, Kd = 0.24 +/- 0.03 nM). In competition binding experiments, LTB4 was the most potent competitor (Ki = 0.23 +/- 0.05 nM) followed by 20-hydroxy-LTB4 (Ki = 1.1 +/- 0.2 nM) and by 6-trans-12-epi-LTB4 and LTD4 (Ki > 1 microM). In stably transfected Chinese hamster ovary cells, LTB4 inhibited forskolin-activated cAMP production and induced an increase of intracellular calcium, suggesting that this receptor is coupled to Gi- and Go-like proteins. In Xenopus laevis melanophores transiently expressing m-BLTR, LTB4 induced the aggregation of pigment granules, confirming the inhibition of cAMP production induced by LTB4. BLT receptors share significant sequence homology with chemokine receptors (CCR5 and CXCR4) that act as human immunodeficiency virus (HIV) coreceptors. However, among the 16 HIV/SIV strains tested, the human BLT receptor did not act as a coreceptor for virus entry into CD4-expressing cells based on infection and cell-cell fusion assays. In 5-lipoxygenase-deficient mice, the absence of leukotriene B4 biosynthesis did not detectably alter m-BLT receptor binding in membranes obtained from glycogen-elicited neutrophils. Isolation of the m-BLTR gene will form the basis of future experiments to elucidate the selective role of LTB4, as opposed to cysteinyl-leukotrienes, in murine models of inflammation.

Heterotrimeric kinesin II is the microtubule motor protein responsible for pigment dispersion in Xenopus melanophores

Melanophores move pigment organelles (melanosomes) from the cell center to the periphery and vice-versa. These bidirectional movements require cytoplasmic microtubules and microfilaments and depend on the function of microtubule motors and a myosin. Earlier we found that melanosomes purified from Xenopus melanophores contain the plus end microtubule motor kinesin II, indicating that it may be involved in dispersion (Rogers, S.L., I.S. Tint, P.C. Fanapour, and V.I. Gelfand. 1997. Proc. Natl. Acad. Sci. USA. 94: 3720-3725). Here, we generated a dominant-negative construct encoding green fluorescent protein fused to the stalk-tail region of Xenopus kinesin-like protein 3 (Xklp3), the 95-kD motor subunit of Xenopus kinesin II, and introduced it into melanophores. Overexpression of the fusion protein inhibited pigment dispersion but had no effect on aggregation. To control for the specificity of this effect, we studied the kinesin-dependent movement of lysosomes. Neither dispersion of lysosomes in acidic conditions nor their clustering under alkaline conditions was affected by the mutant Xklp3. Furthermore, microinjection of melanophores with SUK4, a function-blocking kinesin antibody, inhibited dispersion of lysosomes but had no effect on melanosome transport. We conclude that melanosome dispersion is powered by kinesin II and not by conventional kinesin. This paper demonstrates that kinesin II moves membrane-bound organelles.

Regulation of organelle movement in melanophores by protein kinase A (PKA), protein kinase C (PKC), and protein phosphatase 2A (PP2A)

We used melanophores, cells specialized for regulated organelle transport, to study signaling pathways involved in the regulation of transport. We transfected immortalized Xenopus melanophores with plasmids encoding epitope-tagged inhibitors of protein phosphatases and protein kinases or control plasmids encoding inactive analogues of these inhibitors. Expression of a recombinant inhibitor of protein kinase A (PKA) results in spontaneous pigment aggregation. alpha-Melanocyte-stimulating hormone (MSH), a stimulus which increases intracellular cAMP, cannot disperse pigment in these cells. However, melanosomes in these cells can be partially dispersed by PMA, an activator of protein kinase C (PKC). When a recombinant inhibitor of PKC is expressed in melanophores, PMA-induced pigment dispersion is inhibited, but not dispersion induced by MSH. We conclude that PKA and PKC activate two different pathways for melanosome dispersion. When melanophores express the small t antigen of SV-40 virus, a specific inhibitor of protein phosphatase 2A (PP2A), aggregation is completely prevented. Conversely, overexpression of PP2A inhibits pigment dispersion by MSH. Inhibitors of protein phosphatase 1 and protein phosphatase 2B (PP2B) do not affect pigment movement. Therefore, melanosome aggregation is mediated by PP2A.

Neuromedin B activates phospholipase D through both PKC-dependent and PKC-independent mechanisms

The actions of neuromedin B (NMB), a recently discovered mammalian bombesin-related peptide, are mediated by interacting with a distinct receptor; however, little is known about its cellular basis of action. Recent studies show activation of phospholipase D (PLD) is an important transduction cascade for a number of GI hormones, especially for stimulation of growth and protein sorting. The purpose of the present study was to determine whether activation of the NMB receptor causes activation of PLD and to explore whether this activation was coupled to PLC activation. Rat C6 glioblastoma cells (C6 cells), which contain a low density of native NMB receptors and BALB 3T3 cells stably transfected with rat NMB receptors, were used. NMB caused a 3-fold increase in C6 cells and an 11-fold increase in rNMB-R transfected cells in PLD activity. Increases in PLD activity were rapid and NMB was 100-fold more potent than gastrin-releasing peptide (GRP). NMB caused a half-maximal increase in [Ca2+]i at 0.2 nM, in [3H]IP and PLD at 1 nM, and half-maximal receptor occupation at 1.2 nM. TPA increased PLD dose-dependently with a half-maximal effect at 60 nM. The calcium ionophore A23187 (1 microM) alone did not increase PLD activity but potentiated the effect of TPA. The Ca2+-ATPase inhibitor, thapsigargin, did not affect NMB- or TPA-stimulated PLD activities, although it blocked completely the NMB-induced increase in [Ca2+]i. The PKC inhibitor GF109203X completely abolished TPA-induced PLD activity, however, it only inhibited NMB-induced PLD activity by 20%. The combination of thapsigargin and GF109203X had the same effect as GF109203X alone. These data indicate that NMB receptor activation is coupled to both PLC and PLD. In contrast to a number of other phospholipase C-coupled receptors, NMB receptor stimulated changes in [Ca2+]i do not contribute to PLD activation. Both PKC-dependent and PKC-independent mechanisms are involved in the NMB-stimulated PLD activation with the PKC-independent pathway predominating.

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.

Neuromedin B receptor activation causes tyrosine phosphorylation of p125FAK by a phospholipase C independent mechanism which requires p21rho and integrity of the actin cytoskeleton

Recent studies show that tyrosine phosphorylation by a number of neuropeptides may be an important intracellular pathway in mediating changes in cell function, particularly related to growth. Neuromedin B (NMB), a mammalian bombesin related peptide, functions through a distinct receptor, the neuromedin B receptor (NMB-R), of which little is known about its cellular basis of action. In the present study we explored the ability of NMB-R activation to cause tyrosine phosphorylation of focal adhesion kinase (p125(FAK)), an important substrate for tyrosine phosphorylation by other neuropeptides. NMB caused rapid increases in p125(FAK) phosphorylation which reached maximum at 2 min in both rat C6 glioblastoma cells which possess native NMB-Rs and rat neuromedin B receptor (rNMR-R) transfected BALB 3T3 cells. NMB had a half-maximal effect was at 0.4 nM and was 30-fold more potent than gastrin-releasing peptide (GRP). The stoichiometric relationships between increased p125(FAK) tyrosine phosphorylation and other cellular processes was similar in both C6 cells and rNMB-R transfected cells. TPA (1 microM) caused 45% and the calcium ionophore, A23187, 11% of maximal tyrosine phosphorylation of p125(FAK) seen with NMB. A23187 potentiated the effect of TPA. Pretreatment with the selective PKC inhibitor, GF109203X, inhibited TPA-induced p125(FAK) tyrosine phosphorylation, but it had no effect on the NMB stimulation. Pretreatment with thapsigargin completely inhibited NMB-stimulated increases in [Ca2+]i, but had no effect on NMB-stimulation of p125(FAK) phosphorylation either alone or with GF109203X. The tyrosine kinase inhibitor, tyrphostin A25, inhibited NMB-induced phosphorylation of p125(FAK) by 52%. However, tyrphostin A25 did not inhibit NMB-stimulated increases in [3H]inositol phosphates. Cytochalasin D, an agent which disrupts actin microfilaments, inhibited BN- and TPA-induced tyrosine phosphorylation of p125(FAK) completely. In contrast, colchicine, an agent which disrupts microtubules, had no effect. Pretreatment with Clostridium botulinum C3 exoenzyme which inactivates the small GTP-binding protein rho p21, also inhibited tyrosine phosphorylation of p125(FAK) by 55%. These results demonstrate that activation of NMB-R can cause rapid tyrosine phosphorylation of p125(FAK). NMB-induced tyrosine phosphorylation of p125(FAK) is independent of NMB-induced changes in [Ca2+]i or PKC. The integrity of the actin cytoskeleton but not of microtubules is necessary for NMB-stimulated phosphorylation of p125(FAK). The ras-related small GTP-binding protein rho p21 is at least partially involved in mediating NMB-induced tyrosine phosphorylation of p125(FAK). These results suggest that similar to some other neuropeptides, activation of this pathway may be an important mechanism in mediating cellular changes by this receptor such as growth.

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.

Analysis of the transmembrane topology and membrane assembly of the GAT-1 gamma-aminobutyric acid transporter

The transmembrane topology of the Na+- and Cl--dependent gamma-aminobutyric acid transporter GAT-1 has been studied using protein chimeras in Xenopus oocytes. A series of COOH-terminal truncations was generated to which a prolactin epitope was fused. Following expression of transporter-prolactin chimeras in Xenopus oocytes, the transmembrane orientation of each chimera was determined by testing for protease sensitivity in an oocyte membrane preparation. Data from protease protection assays with GAT-1-prolactin chimeras has shown that residues in the loops connecting hydrophobic domain (HD)3 and HD4 and HD7 and HD8 are accessible to protease in the cytoplasm and suggest the presence of pore loop structures which extend into the membrane from the extracellular face. Such pore loop structures may be involved in the formation of the substrate-binding pocket. Studies presented herein confirm that the NH2 and COOH termini are cytosolic and hydrophobic domains span the membrane in a manner consistent with the predicted hydropathy model for Na+- and Cl--dependent transporters. These data also provide insight into GAT-1 transmembrane assembly and suggest that a complex series of topogenic sequences directs this process. A potential pause-transfer sequence has been identified and may be responsible for the translocational pausing observed in this study.

Melatonin agonists induce phosphoinositide hydrolysis in Xenopus laevis melanophores

Melatonin, the principal hormone of the vertebrate pineal gland, has been implicated in a variety of neurobiological processes such as circadian rhythmicity and reproductive function. One of the earliest described actions of melatonin was its ability to cause pigment translocation in the dermal melanophores of amphibians. Melatonin binding sites have been identified in the brain of many species and in pigmented tumour cell lines; however, the dermal melanophores of the frog Xenopus Laevis possess the highest known density of melatonin binding sites. These cells are the source from which a melatonin receptor has been cloned and provide an excellent model to study melatonin-mediated signal transduction in an isolated cell system. In Xenopus melanophores, melatonin induces a rapid perinuclear aggregation of intracellular pigment which is associated with a pertussis toxin-sensitive inhibition of cAMP. We have previously demonstrated that a subtype of melatonin binding sites found in selected regions of the pigeon brain and in Syrian Hamster RPMI 1846 melatonin cells are functionally coupled to phosphoinositide hydrolysis as a second messenger. Here we now present evidence to suggest that Xenopus Laevis melanophores also possess melatonin binding sites which are functionally linked to phosphoinositide hydrolysis. Melatonin agonists induced phosphoinositide hydrolysis in melanophores in a concentration-dependent manner with a rank order of potency of 2-iodomelatonin > 6-chloromelatonin > N-acetylserotonin > melatonin. Stimulatory response of 2-iodomelatonin was blocked by the melatonin antagonist N-acetyltryptamine and the alpha-adrenergic antagonist prazosin, which has been shown to have high affinity for melatonin binding sites. Phosphoinositide hydrolysis induced by melatonin agonists was not blocked by the serotonin antagonist ketanserin or by phentolamine, an alpha-adrenergic antagonist, indicating that the response observed was not due to stimulation of 5-HT2a/2c receptors or alpha-adrenergic receptors. Furthermore, incubation of melanophores with the non-hydrolyzable G-protein source GTP-gamma-S attenuated the phosphoinositide dose response induced by 2-iodomelatonin, and pre-incubation of the cells with pertussis toxin had no effect on 2-iodomelatonin-induced phosphoinositide hydrolysis. The present data suggest that Xenopus Laevis Melanophores possess G-protein linked pertussis toxin-insensitive melatonin binding sites which are functionally coupled to phosphoinositide hydrolysis as a signal transduction mechanism.

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.

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.

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.