Cellular and Molecular Biology of Orphan G Protein‐Coupled Receptors
Introduction
G protein‐coupled receptors (GPCRs) are the largest family of cell surface molecules. They allow tissues to respond to a wide variety of extracellular signaling molecules (Bockaert 1999, Gether 2000, Kristiansen 2004). The GPCR superfamily participates in a variety of physiological processes such as reproduction, growth, homeostasis, metabolism, food intake, behaviors, sleep, and so on. Therefore, many members of this superfamily are major targets of pharmaceutical drugs (Wilson and Bergsma, 2000).
Rhodopsin was the first GPCR whose primary amino acid sequence and possible topological structure were identified (Nathans and Hogness, 1983). The subsequent identification of the β2‐adrenergic receptor sequence gave rise to the idea that receptors that couple to G protein share a similar seven‐helix topology (Dixon et al., 1986). This sequence information about these two receptors allowed homology‐based screening approaches such as the degenerate polymerase chain reaction (PCR) and low‐stringency hybridization, and led to the identification of new GPCR members (Bunzow 1988, Libert 1989, O'Dowd 1997). The database of expressed sequence‐tagged cDNAs (ESTs) also permitted further expansion of the GPCR superfamily (Lee 2001, Marchese 1999a), and completion of the first draft of the human genome revealed the sequences of almost all GPCRs, including those of unknown function and with unknown ligands (Lander 2001, Venter 2001). This led to efforts to reclassify the GPCR subfamily and to identify novel uncharacterized GPCRs (Fredriksson 2003a, Kristiansen 2004, Takeda 2002, Vassilatis 2003). Currently, it is thought that the human genome contains approximately 853 genes of the GPCR superfamily (Fredriksson 2005, Gloriam 2005, Young 2002). Among these, about 478 encode olfactory and gustatory GPCRs referred to as chemosensory GPCRs because they recognize signals of external origin sensed as odors, pheromones, and tastes (Young 2002, Zozulya 2001). Thus, the human genome contains approximately 375 nonchemosensory/transmitter GPCRs that bind a variety of ligands including biogenic amines, amino acids, short and long peptides, proteins, nucleotides, and lipids (Fredriksson 2003a, Vassilatis 2003).
The nonchemosensory/transmitter GPCRs include many whose endogenous ligands are unknown, the so‐called orphans (orphan GPCRs) (Marchese 1999b, Wilson 2000). Because identification of the ligands for orphan GPCRs is important for understanding the roles of these receptors, and for providing a rich source of potential drug candidates, efforts have been made to deorphan these receptors (Civelli 2005, Robas 2003a, Wise 2004). About 87 orphan GPCRs have been paired with about 70 different ligands (Table I). Most of the deorphaned receptors respond to a single compound, but some share a single ligand, or respond to more than two different compounds with different affinities (Civelli 2005, Wise 2004). Currently, about 120 orphan GPCRs remain to be deorphaned (Table I).
Various strategies have been used to increase the chances of discovering relevant ligands (Robas et al., 2003a). Classification of orphan GPCRs together with insight into the structure and function of related GPCRs may help in predicting the nature of the ligand. Knowledge of the various signal transduction pathways activated by GPCRs has led to the development of a variety of screening systems. In addition, clarification of receptor activation mechanisms as agonist dependent or agonist independent provides strategies for identifying inverse agonists and allosteric agonists. This review describes the classification of GPCRs, their activation mechanisms and associated signal transduction pathways, and current screening systems for the ligands of orphan GPCRs.
Section snippets
General Structure of GPCR Families
GPCRs share a similar topology, with seven transmembrane helices (TMHs) connected by three extracellular loops (ECLs), and three intracellular loops (ICLs); the N terminus is on the extracellular side of the membrane, and the C terminus is on the cytoplasmic side (Baldwin 1993, Donnelly 1994). Determination of the crystal structure of bovine rhodopsin showed that the polypeptide folds into seven helical segments spanning the membrane, and that these TMHs are largely α‐helical, but each is bent
GPCR Activation and G Protein Coupling
Various regulatory molecules bind to the specific domains of GPCRs. In general, small ligands, such as amines, bind primarily to the core of TMHs, middle‐size peptides to the ECLs and TMHs, and large peptides and proteins to the N termini and ECLs of their specific receptors. Ligand binding induces conformational changes of the receptors involving movement of the TMHs (Gether 1995, Schwartz 1994). Such changes are likely to induce alterations in the conformation of the ICLs, and therefore
Deorphaned GPCRs
The identification of 207 novel orphan GPCRs led to efforts to identify their ligands by various strategies. These included prediction of the ligand nature based on receptor sequence homology, choice of tissue extracts and synthetic compound libraries for ligand activity, and a variety of functional screening assays. The receptor sequence homology‐based approaches help predict the nature of the ligand for an orphan GPCR. Many GPCRs are clustered within a subfamily and have a high degree of
Concluding Remarks
Because known GPCRs have often been successfully used as therapeutic targets, orphan GPCRs may serve as a rich source of potential targets for drug discovery. Thus, many companies and academic investigators have invested heavily in the analysis of orphan GPCRs, with the aim of identifying new receptor sequences, analyzing their expression, discovering their cognate and/or surrogate ligands, and assigning the functions of novel receptor–ligand pairs. About 70 novel or known molecules such as
Acknowledgments
This study was supported by a grant (M103KV010004 03K2201 00410) from the Brain Research Center of the 21st Century Frontier Research Program. Address all correspondence to Jae Young Seong, Ph.D., Laboratory of G Protein Coupled Receptors, Korea University College of Medicine, Seoul 136‐705, Republic of Korea, Tel: +82‐2‐920‐6090, Fax: +82‐2‐921‐4355, [email protected].
References (295)
- et al.
Differential desensitization and internalization of three different bullfrog gonadotropin‐releasing hormone receptors
Mol. Cells
(2002) - et al.
Identification of amino acid residues that direct differential ligand selectivity of mammalian and nonmammalian V1a type receptors for arginine vasopressin and vasotocin. Insights into molecular coevolution of V1a type receptors and their ligands
J. Biol. Chem.
(2004) - et al.
Src‐mediated tyrosine phosphorylation of dynamin is required for beta2‐adrenergic receptor internalization and mitogen‐activated protein kinase signaling
J. Biol. Chem.
(1999) - et al.
Molecular cloning and characterization of the human anaphylatoxin C3a receptor
J. Biol. Chem.
(1996) - et al.
Molecular cloning of the human Edg2 protein and its identification as a functional cellular receptor for lysophosphatidic acid
Biochem. Biophys. Res. Commun.
(1997) - et al.
Identification of cDNAs encoding two G protein‐coupled receptors for lysosphingolipids
FEBS Lett.
(1997) - et al.
Homologous and heterologous phosphorylation of the vasopressin V1a receptor
Cell. Signal.
(1999) - et al.
Mediation of cyclic AMP signaling by the first intracellular loop of the gonadotropin‐releasing hormone receptor
J. Biol. Chem.
(1998) - et al.
Beta‐arrestin2, a novel member of the arrestin/beta‐arrestin gene family
J. Biol. Chem.
(1992) - et al.
Integrated methods for the construction of three‐dimensional models and computational probing of structure‐function relations in G protein coupled receptors
Methods Neurosci.
(1995)
Molecular cloning and characterization of a novel human G‐protein‐coupled receptor, EDG7, for lysophosphatidic acid
J. Biol. Chem.
Real‐time visualization of the cellular redistribution of G protein‐coupled receptor kinase 2 and beta‐arrestin 2 during homologous desensitization of the substance P receptor
J. Biol. Chem.
Frizzled proteins constitute a novel family of G protein‐coupled receptors, most closely related to the secretin family
Trends Pharmacol. Sci.
Agonist‐independent activation of Gz by the 5‐hydroxytryptamine1A receptor co‐expressed in Spodoptera frugiperda cells. Distinguishing inverse agonists from neutral antagonists
J. Biol. Chem.
The dynamin‐dependent, arrestin‐independent internalization of 5‐hydroxytryptamine 2A (5‐HT2A) serotonin receptors reveals differential sorting of arrestins and 5‐HT2A receptors during endocytosis
J. Biol. Chem.
The human and mouse repertoire of the adhesion family of G‐protein‐coupled receptors
Genomics
Sequence and expression pattern of a novel human orphan G‐protein‐coupled receptor, GPRC5B, a family C receptor with a short amino‐terminal domain
Genomics
The orphan G protein‐coupled receptors GPR41 and GPR43 are activated by propionate and other short chain carboxylic acids
J. Biol. Chem.
G‐protein‐coupled receptors: Turn‐ons and turn‐offs
Curr. Opin. Neurobiol.
A G protein‐coupled receptor for UDP‐glucose
J. Biol. Chem.
Kinetic control of guanine nucleotide binding to soluble Galpha(q)
Biochem. Pharmacol.
GPCR deorphanizations: The novel, the known and the unexpected transmitters
Trends Pharmacol. Sci.
Cloning of a human purinergic P2Y receptor coupled to phospholipase C and adenylyl cyclase
J. Biol. Chem.
Identification of a novel human ADP receptor coupled to G(i)
J. Biol. Chem.
Structural elements of G alpha subunits that interact with G beta gamma, receptors, and effectors
Cell
Discrete amino acid sequences of the alpha 1‐adrenergic receptor determine the selectivity of coupling to phosphatidylinositol hydrolysis
J. Biol. Chem.
Chimeric G proteins allow a high‐throughput signaling assay of Gi‐coupled receptors
Anal. Biochem.
Transfer of M2 muscarinic acetylcholine receptors to clathrin‐derived early endosomes following clathrin‐independent endocytosis
J. Biol. Chem.
A ternary complex model explains the agonist‐specific binding properties of the adenylate cyclase‐coupled beta‐adrenergic receptor
J. Biol. Chem.
A diverse family of GPCRs expressed in specific subsets of nociceptive sensory neurons
Cell
Principles: A model for the allosteric interactions between ligand binding sites within a dimeric GPCR
Trends Pharmacol. Sci.
Receptor for the pain modulatory neuropeptides FF and AF is an orphan G protein‐coupled receptor
J. Biol. Chem.
Seven evolutionarily conserved human rhodopsin G protein‐coupled receptors lacking close relatives
FEBS Lett.
A new peptidic ligand and its receptor regulating adrenal function in rats
J. Biol. Chem.
Fluorescent labeling of purified beta 2 adrenergic receptor. Evidence for ligand‐specific conformational changes
J. Biol. Chem.
Nine new human rhodopsin family G‐protein coupled receptors: Identification, sequence characterisation and evolutionary relationship
Biochim. Biophys. Acta
The second intracellular loop of metabotropic glutamate receptor 1 cooperates with the other intracellular domains to control coupling to G‐proteins
J. Biol. Chem.
Visualization of agonist‐induced association and trafficking of green fluorescent protein‐tagged forms of both beta‐arrestin‐1 and the thyrotropin‐releasing hormone receptor‐1
J. Biol. Chem.
Casein kinase II sites in the intracellular C‐terminal domain of the thyrotropin‐releasing hormone receptor and chimeric gonadotropin‐releasing hormone receptors contribute to beta‐arrestin‐dependent internalization
J. Biol. Chem.
cDNA cloning of a putative G protein‐coupled receptor from brain
Biochim. Biophys. Acta
Characterization of the human cysteinyl leukotriene 2 receptor
J. Biol. Chem.
Biochemical and pharmacological control of the multiplicity of coupling at G‐protein‐coupled receptors
Pharmacol. Ther.
Reporter‐gene systems for the study of G‐protein‐coupled receptors
Curr. Opin. Pharmacol.
Molecular cloning and functional characterization of MCH2, a novel human MCH receptor
J. Biol. Chem.
Molecular characterization of CCR6: Involvement of multiple domains in ligand binding and receptor signaling
J. Biomed. Sci.
G‐protein‐coupled receptor genes as protooncogenes: Constitutively activating mutation of the alpha 1B‐adrenergic receptor enhances mitogenesis and tumorigenicity
Proc. Natl. Acad. Sci. USA
Human urotensin‐II is a potent vasoconstrictor and agonist for the orphan receptor GPR14
Nature
Signaling mechanisms and molecular characteristics of G protein‐coupled receptors for lysophosphatidic acid and sphingosine 1‐phosphate
J. Cell. Biochem. suppl.
G proteins and phototransduction
Annu. Rev. Physiol.
Structural insights into the amino‐terminus of the secretin receptor: I. Status of cysteine and cystine residues
Mol. Pharmacol.
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