Mini-review
Allosteric modulation of G-protein coupled receptors

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Abstract

The superfamily of G-protein coupled receptors (GPCRs) has more than 1000 members and is the largest family of proteins in the body. GPCRs mediate signalling of stimuli as diverse as light, ions, small molecules, peptides and proteins and are the targets for many pharmaceuticals. Most GPCR ligands are believed to activate (agonists) or inhibit (competitive antagonists) receptor signalling by binding the receptor at the same site as the endogenous agonist, the orthosteric site. In contrast, allosteric ligands modulate receptor function by binding to different regions in the receptor, allosteric sites. In recent years, combinatorial chemistry and high throughput screening have helped identify several allosteric GPCR modulators with novel structures, several of which already have become valuable pharmacological tools and may be candidates for clinical testing in the near future. This mini review outlines the current status and perspectives of allosteric modulation of GPCR function with emphasis on the pharmacology of endogenous and synthesised modulators, their receptor interactions and the therapeutic prospects of allosteric ligands compared to orthosteric ligands.

Introduction

Cell surface receptors are membrane bound proteins that translate extracellular signals delivered as neurotransmitters or hormones into intracellular cascades and events such as phosphorylation, calcium mobilisation and opening of ion channels. The three major classes of cell surface receptors are ligand-gated ion channels, tyrosine kinase receptors and G-protein coupled receptors (GPCRs). These receptor classes constitute the majority of the therapeutic targets identified today, with drugs targeted to GPCRs accounting for $18,548 million in sales during 2000 (Bailey, 2001).

GPCR ligands generally act by stimulating a response (agonists) or blocking the activity of an endogenous agonist (competitive antagonists). Inverse agonists also block the constitutive activity of a GPCR. Since most ligands appear unable to bind simultaneously with each other (e.g. in radioligand binding), and are similarly affected by mutations in the receptors, it is generally assumed that they act through the binding site of endogenous transmitter at the receptor, the orthosteric site.

In recent years, there has been increasing therapeutic interest in ligands that bind to other sites on the receptor, allosteric sites. Many allosteric modulators have little or no agonist or inverse agonist activity themselves, but they affect the receptor’s response to endogenous agonists and other ligands. Classical examples of allosteric modulators include benzodiazepines and barbiturates, which enhance or inhibit GABAA receptor function, and memantine which negatively modulates NMDA receptors (Kohl and Dannhardt, 2001, Mohler et al., 2002). These receptors are ion channels. In contrast, the vast majority of clinically-used GPCR ligands are believed to act orthosterically. Recently, rapid developments in combinatorial chemistry and high throughput screening techniques have helped identify numerous allosteric ligands, several of which already have become valuable pharmacological tools and may be candidates for clinical testing in the near future.

In this review, we discuss the pharmacological characteristics of allosteric modulators, the interactions of specific allosteric modulators with their respective GPCRs, and the potential therapeutic advantages of allosteric versus orthosteric ligands.

Section snippets

GPCR function and allosteric modulation

The term “GPCR” originates from these receptors’ ability to associate with heterotrimeric G-proteins and through them stimulate or inhibit the formation of second messengers such as inositol-3,4,5-tris-phosphate and cyclic AMP and modulate the function of ion channels (Gether, 2000, Pierce et al., 2002). They are also capable of interacting directly with ion channels and other effector molecules (Pierce et al., 2002). The GPCRs do not share an overall amino acid sequence identity and as shown

Endogenous GPCR modulators

The modulation of GPCRs by endogenous substrates could be perceived as one of nature’s subtle ways of fine-tuning neurotransmitter/hormone signalling. A classic case of endogenous GPCR modulation is the electrostatic interaction between a sodium ion and a highly conserved aspartate residue located in cytoplasmic part of TM2 of a number of family A GPCRs (Fig. 4) (Horstman et al., 1990, Neve et al., 1991, Neve et al., 2001, Martin et al., 1999, Schetz and Sibley, 2001, Gao et al., 2003).

GPCRs, GPCR dimers and GPCR-protein complexes as targets for allosteric modulators

It is clear that GPCRs can be activated by mechanisms that do not involve agonist binding to the orthosteric ligand binding site. Schwartz and coworkers have engineered artificial agonistic Zn2+ binding sites in analogous positions in the β2-adrenergic receptor and the distantly related tachykinin NK1 receptor (Elling et al., 1999, Holst et al., 2000). This is remarkable considering the different binding modes of the orthosteric ligands to the two receptors (Fig. 1). Furthermore, there are

Nature of response

Allosteric modulators with little inherent intrinsic activity that act by enhancing or attenuating the response elicited by the endogenous transmitter hold several advantages to the actions of conventional agonists and antagonists. Firstly, the effects of the allosteric modulator are often saturable and are therefore less likely to elicit adverse effects from overdose. Secondly, the effects of the modulator are closely linked with the physiological pulse of synaptic signalling, and thus it will

Conclusion

Increasing numbers of allosteric GPCR ligands are being published. Subtype-selective allosteric ligands have already proven valuable pharmacological tools in in vivo investigations of the physiological roles of GPCRs, and delineation of the interactions of these ligands with their respective receptors have contributed significantly to the understanding of the signal transduction through the receptors. Considering the continued extensive use of high throughput technologies in the drug discovery

Acknowledgements

The Lundbeck Foundation is thanked for its financial support of AAJ.

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