ReviewAllosteric modulation of G protein-coupled receptors: A pharmacological perspective
Introduction
G protein-coupled receptors (GPCRs) comprise the largest superfamily of cell-surface receptors, accounting for 1–2% of the human genome (Foord, 2002). Despite sharing a structural similarity, namely a characteristic seven-transmembrane-spanning architecture linked by an N-terminal extracellular domain, C-terminal intracellular domain, three extracellular loops and three intracellular loops, GPCRs recognize a vast array of diverse extracellular stimuli (e.g., photons, ions, biogenic amines, peptides, lipids and proteins) and transduce their signals to the intracellular environment via a conformational rearrangement linked to an increasing number of cytosolic interactants that extends well beyond the classic G protein paradigm (Kristiansen, 2004). For decades, the development of ligands in traditional GPCR drug discovery has focused on targeting the orthosteric binding site of the receptor, which is defined as the site where the endogenous ligand binds to elicit signal transduction. This approach has guided the development of classical orthosteric ligands that directly activate the target receptor (agonists) or block the actions of the endogenous ligand (antagonists/inverse agonists). However, it is now well known that the rate of traditional GPCR-based novel drug discovery, in common with drugs for other target classes, is rapidly in decline (Booth and Zemmel, 2004). Part of this decline may relate to the relative intractability of many GPCRs (e.g., peptide receptors) to orthosteric small molecule discovery, as well as the difficulty in selectively targeting orthosteric sites at receptors that display high homology in this region between subtypes.
Accordingly, the last decade and a half has seen a pronounced shift in the acceptance and pursuit of small molecules that target topographically distinct allosteric sites on GPCRs (Christopoulos, 2002, Christopoulos and Kenakin, 2002, May et al., 2007b). Ligands that bind to these allosteric sites are called “allosteric modulators”, because they elicit a conformational change in the receptor while still allowing, in many instances, the concomitant binding of orthosteric ligands – thus modulating the pharmacological characteristics of the latter agent. Allosteric drugs offer enormous potential with regards to greater selectivity of action and novel modes of sculpting GPCR responsiveness in a spatially and temporally advantageous manner. However, despite the (now) broad acceptance of the phenomenon within academia and the pharmaceutical industry, substantial challenges remain in the detection, validation, classification and translation of putative allosteric GPCR drug candidates. This review will briefly consider some of the issues associated with the pharmacological exploitation of allosteric GPCR ligands.
Section snippets
Development of the concept
The allosteric concept, first coined and formalized by Monod, Wyman, Changeux and colleagues (Monod et al., 1963, Monod et al., 1965, Monod and Jacob, 1961), was built on the synthesis of observations related to cooperativity in enzyme behavior and the study of mechanisms governing end-product inhibition by substances that were structurally distinct from compounds that recognized classic enzyme substrate-binding sites. The first model that was proposed to account for allosteric behavior, the
Operational approaches to quantifying GPCR allosterism
Ideally, it is desirable if the parameters describing allosteric modulator effects on GPCR function could be readily determined from experimental observations in order to facilitate studies of modulator structure–activity relationships (SAR). Unfortunately, such parameters are not as readily attainable as in an orthosteric setting, where most potency/selectivity optimization is facilitated by the relatively straightforward determination of compound affinity values. At a minimum, allosteric
Pharmacological characteristics of GPCR allosterism
Whether viewed from a mechanistic or an operational perspective, GPCR allosterism is characterized by a number of properties that reflect the underlying reciprocity of effect between two different ligands, be they small molecules or large proteins, that interact concomitantly via topographically distinct sites on a GPCR. These key properties are: saturability of effect, probe dependence, differential effects on orthosteric ligand affinity vs. efficacy, and stimulus-bias (functional
The best of both worlds: bitopic orthosteric/allosteric ligands
In recent years, studies have focused on an interesting mode of ligand–GPCR engagement that combines the principles of allostery and functional selectivity with a novel twist. Specifically, compounds are being discovered, or designed, that behave as hybrid molecules that engage both orthosteric and allosteric sites on a GPCR to achieve functionally selective signaling (Mohr et al., 2010, Valant et al., 2009). These compounds have been dubbed “bitopic”, “dualsteric” or “multivalent” (Antony
Structure–function analyses of GPCR allosteric modulation
The enormous diversity of orthosteric ligands that act at the GPCR superfamily indicates that, despite sharing a common architecture, these receptors can display multiple orthosteric binding modes. Thus, for biogenic amine GPCRs, the orthosteric binding site comprises a cavity near the top third of the transmembrane helical bundle, whereas for many peptide-recognizing GPCRs, the primary orthosteric site contacts are composed of regions in the extracellular N terminus and/or loops; for the
A word on terminology
Since its introduction, the word “allosteric” has come to be associated with an increasing array of protein behaviors and phenomena, extending well beyond the description of protein structural changes elicited by the transmission of conformational changes between two topographically distinct ligand-binding sites. It has been argued recently that the uses of the term should be restricted to those instances where the free energy of coupling between a protein and a ligand is altered, in a
Conclusions
The study of allosteric modulation of GPCRs has now progressed from the realm of pharmacological rarity to an established paradigm. The potential therapeutic advantages of allosteric ligands as drugs are well accepted, but have yet to be widely realized in practice because the clinical translation of such compounds is still in its early days. The ever-expanding portfolio of complex behaviors ascribed to allosteric ligands also presents both challenges and opportunities to preclinical lead
Acknowledgments
Work in the authors’ laboratory is funded by Program Grant no. 519461 of the National Health and Medical Research Council (NHMRC) of Australia. Arthur Christopoulos is a Senior, and Patrick Sexton a Principal, Research Fellow of the NHMRC. Peter Keov is a recipient of an Australian Postgraduate Award. The authors would like to thank Rick Neubig, Terry Kenakin, Fred Ehlert and Andrew Tobin for helpful discussions on allosteric ligand classification.
References (80)
- et al.
Allosteric receptors after 30 years
Neuron
(1998) - et al.
Pharmacological analysis of the mode of interaction of McN-A-343 at atrial muscarinic M2 receptors
European Journal of Pharmacology
(1997) - et al.
A ternary complex model explains the agonist-specific binding properties of the adenylate cyclase-coupled beta-adrenergic receptor
Journal of Biological Chemistry
(1980) Allostery: an illustrated definition for the ‘second secret of life’
Trends in Biochemical Sciences
(2008)Receptor classification: post genome
Current Opinion in Pharmacology
(2002)Mechanisms of peptide and nonpeptide ligand binding to class B G-protein-coupled receptors
Drug Discovery Today
(2005)Agonist-receptor efficacy I: mechanisms of efficacy and receptor promiscuity
Trends in Pharmacological Sciences
(1995)Collateral efficacy in new drug discovery: taking advantage of the good (allosteric) nature of 7TM receptors
Trends in Pharmacological Sciences
(2007)7TM receptor allostery: putting numbers to shapeshifting proteins
Trends in Pharmacological Sciences
(2009)- et al.
Constitutive activation of the a1B-adrenergic receptor by all amino acid substitutions at a single site. Evidence for a region which constrains receptor activation
Journal of Biological Chemistry
(1992)
Molecular mechanisms of ligand binding, signaling, and regulation within the superfamily of G-protein-coupled receptors: molecular modeling and mutagenesis approaches to receptor structure and function
Pharmacology & Therapeutics
Allosteric GPCR modulators: taking advantage of permissive receptor pharmacology
Trends in Pharmacological Sciences
Allosteric proteins and cellular control systems
Journal of Molecular Biology
On the nature of allosteric transitions: a plausible model
Journal of Molecular Biology
Structural determinants of allosteric agonism and modulation at the M4 muscarinic acetylcholine receptor: identification of ligand-specific and global activation mechanisms
Journal of Biological Chemistry
A novel mechanism of G protein-coupled receptor functional selectivity
Journal of Biological Chemistry
β-Arrestin-biased ligands at seven-transmembrane receptors
Trends in Pharmacological Sciences
Dualsteric GPCR targeting: a novel route to binding and signaling pathway selectivity
FASEB Journal
Allosteric modulators of the adenosine A1 receptor: synthesis and pharmacological evaluation of 4-substituted 2-amino-3-benzoylthiophenes
Journal of Medicinal Chemistry
Orthosteric and allosteric modes of interaction of novel selective agonists of the M1muscarinic acetylcholine receptor
Molecular Pharmacology
Agonist and inverse agonist actions of β-blockers at the human β2-adrenoceptor provide evidence for agonist-directed signaling
Molecular Pharmacology
The effect of McN-A-343 on muscarinic receptors in the cerebral cortex and heart
British Journal of Pharmacology
Allosterism at muscarinic receptors: ligands and mechanisms
Mini Reviews in Medicinal Chemistry
Operational models of pharmacological agonism
Proceedings of the Royal Society of London, Series B: Biological Sciences
Prospects for productivity
Nature Reviews: Drug Discovery
Allosteric modulation of the muscarinic M4 receptor as an approach to treating schizophrenia
Proceedings of the National Academy of Sciences of the United States of America
Quantification of allosteric interactions at G protein coupled receptors using radioligand binding assays
Allosteric binding sites on cell-surface receptors: novel targets for drug discovery
Nature Reviews: Drug Discovery
G protein-coupled receptor allosterism and complexing
Pharmacological Reviews
The inhibitory effect of gallamine on muscarinic receptors
British Journal of Pharmacology
Allosteric modulators of GPCRs: a novel approach for the treatment of CNS disorders
Nature Reviews: Drug Discovery
Antagonists with negative intrinsic activity at δ opioid receptors coupled to GTP-binding proteins
Proceedings of the National Academy of Sciences of the United States of America
Design, synthesis, and action of oxotremorine-related hybrid-type allosteric modulators of muscarinic acetylcholine receptors
Journal of Medicinal Chemistry
The relationship between muscarinic receptor occupancy and adenylate cyclase inhibition in the rabbit myocardium
Molecular Pharmacology
Estimation of the affinities of allosteric ligands using radioligand binding and pharmacological null methods
Molecular Pharmacology
Analysis of allosterism in functional assays
Journal of Pharmacology and Experimental Therapeutics
Binding and functional characterization of the cardioselective muscarinic antagonist methoctramine
Journal of Pharmacology and Experimental Therapeutics
Allosteric modulation of muscarinic acetylcholine receptors
Current Neuropharmacology
Estimation of agonist activity at G protein-coupled receptors: analysis of M2 muscarinic receptor signaling through Gi/o, Gs, and G15
Journal of Pharmacology and Experimental Therapeutics
Modeling the functional effects of allosteric modulators at pharmacological receptors: an extension of the two-state model of receptor activation
Molecular Pharmacology
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