Abstract
The modulation of transmembrane signaling by G protein-coupled receptors (GPCRs) constitutes the single most important therapeutic target in medicine. Drugs acting on GPCRs have traditionally been classified as agonists, partial agonists, or antagonists based on a two-state model of receptor function embodied in the ternary complex model. Over the past decade, however, many lines of investigation have shown that GPCR signaling exhibits greater diversity and “texture” than previously appreciated. Signal diversity arises from numerous factors, among which are the ability of receptors to adopt multiple “active” states with different effector-coupling profiles; the formation of receptor dimers that exhibit unique pharmacology, signaling, and trafficking; the dissociation of receptor “activation” from desensitization and internalization; and the discovery that non-G protein effectors mediate some aspects of GPCR signaling. At the same time, clustering of GPCRs with their downstream effectors in membrane microdomains and interactions between receptors and a plethora of multidomain scaffolding proteins and accessory/chaperone molecules confer signal preorganization, efficiency, and specificity. In this context, the concept of agonist-selective trafficking of receptor signaling, which recognizes that a bound ligand may select between a menu of active receptor conformations and induce only a subset of the possible response profile, presents the opportunity to develop drugs that change the quality as well as the quantity of efficacy. As a more comprehensive understanding of the complexity of GPCR signaling is developed, the rational design of ligands possessing increased specific efficacy and attenuated side effects may become the standard mode of drug development.
Footnotes
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This research was supported by National Institutes of Health Grants DK55524, DK58283, and DK64353 (L.M.L.).
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doi:10.1124/jpet.105.083121.
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ABBREVIATIONS: GPCR, G protein-coupled receptor; GEF, guanine nucleotide exchange factor; 5-HT, 5-hydroxytryptamine; NK, neurokinin; CCK, cholecystokinin; GnRH, gonadotropin-releasing hormone; ICI-118-551, (±)-1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)-amino]-2-butanol hydrochloride; Ang, angiotensin; MAP, mitogen-activated protein; ERK1/2, extracellular signal-regulated kinases 1 and 2; RAMP, receptor activity modifying protein; RCP, receptor component protein; CRLR, calcitonin receptor-like receptor; GRK, G protein-coupled receptor kinase; mGluR, metabotropic glutamate receptor; PDZ, postsynaptic density protein of 95 kDa/disc-large/zona occludens-1; EGF, epidermal growth factor; HIV, human immunodeficiency virus; Ant135-25, Ac-d-Nal(2)-d-4-ClPhe-d-Pal-Ser-1-MePal-d-IsopropylLys-Leu-IsopropylLys-Pro-d-AlaNH2.
- Received January 22, 2005.
- Accepted March 31, 2005.
- The American Society for Pharmacology and Experimental Therapeutics
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