Orphan G-protein-coupled receptors and natural ligand discovery

doi:10.1016/S0165-6147(00)01636-9

Trends in Pharmacological Sciences

Volume 22, Issue 3, 1 March 2001, Pages 132-140

https://doi.org/10.1016/S0165-6147(00)01636-9 Get rights and content

Abstract

The superfamily of seven-transmembrane-domain G-protein-coupled receptors (GPCRs) is the largest and most diverse group of transmembrane proteins involved in signal transduction. Each of the ∼1000 family members found in vertebrates responds to stimuli as diverse as hormones, neurotransmitters, odorants and light, which selectively activate intracellular signaling events mediated by heterotrimeric G proteins. Because GPCRs are centrally positioned in the plasma membrane to initiate a cascade of cellular responses by diverse extracellular mediators, it is not surprising that modulation of GPCR function has been successful in the development of many marketed therapeutic agents. It has become clear that GPCRs for which a natural activating ligand has not yet been identified (orphan GPCRs) might provide a path to discovering new cellular substances that are important in human physiology. The process of ‘de-orphanizing’ these novel proteins has accelerated significantly and opened up new avenues for research in human physiology and pharmacology.

Section snippets

Structural and functional characteristics of GPCRs

In addition to seven hydrophobic α-helical domains, GPCRs share several other sequence features as exemplified by the rhodopsin subfamily ³. These include a single conserved cysteine residue in the first two extracellular loops, which is responsible for protein stability, and a GPCR signature triplet sequence (typically DRY) found downstream from TM3, which is involved in G-protein interaction. N-linked glycosylation sites are found in the N-terminal extracellular domain, whereas

oGPCR discovery and ligand identification

The key to harnessing the potential therapeutic benefits of GPCRs is in the ability to identify highly selective pharmaceutical ligands for these receptors (endogenous or otherwise) with the aim of elucidating their function(s) and evaluating their clinical potential. Not surprisingly, GPCRs have received intense scrutiny from pharmaceutical companies and academic investigators with the result that numerous oGPCRs have been characterized. Initial efforts were focused on identification of

Identification of a ligand and determination of oGPCR function

The terms ‘reverse pharmacology’ and ‘orphan receptor strategy’ have been adopted to describe heterodox approaches to drug discovery 11, 31 (Fig. 1). Traditionally, the development of drug candidates often represents a final step in the analysis of a well-characterized disorder. For example, in growth deficiencies and dwarfism the neuroendocrine system that controls groeth hormone (GH) release was partially defined using growth hormone releasing hormone (GHRH) and its GPCR (GHRH receptor).

Application of the oGPCR strategy

A detailed analysis of oGPCRs has led to several significant discoveries, some of which are presented in Table 2. Of particular interest is the sheer number of recent successful orphan searches driven in part by improvements in assay methodology. Since 1988 these include identification of the receptors for 5-HT (⁴⁴), adenosine 24, 25, nociceptin/orphanin FQ (45, 46), anaphylatoxin C3a (⁴⁷), GHS (⁴⁰), prolactin-releasing peptide ⁴⁸, calcitonin gene-related peptide or adrenomedullin 41, 42,

Conventional and reverse pharmacology

Identification of the GHS receptor ⁴⁰ and its natural peptide ligand ghrelin ⁸⁷ provided an interesting example of oGPCR research in which both conventional and reverse pharmacological approaches were used. The circuitous discovery path began with the observation that the naturally occurring opioid peptide Met-enkephalin weakly stimulated the release of GH (⁸⁸). More potent, synthetic peptides were synthesized (growth-hormone-releasing peptides, typified by GHRP-6), which then prompted the

Concluding remarks

The scientific community has witnessed in a short time the pairing of a growing number of biologically active peptides with their long-sought after receptors, thus ‘closing the loop’ in some ligand–receptor systems. This new era of discovery in pharmacology has at its roots the synergy of three elements: genomics, sensitive and versatile high-throughput assays, and large collections of biologically active molecules. The power of genomics has continued to provide even more orphans that await

Acknowledgments

We gratefully acknowledge the outstanding contributions of the many Merck scientists who have participated in the orphan G-protein-coupled receptor effort.