G protein-coupled receptors (GPCRs) represent the largest family of cell-surface receptors. These receptors are natural allosteric proteins because agonist-mediated signaling by GPCRs requires a conformational change in the receptor protein transmitted between two topographically distinct binding sites, one for the agonist and another for the G protein. It is now becoming increasingly recognized, however, that the agonist-bound GPCR can also form ternary complexes with other ligands or “accessory” proteins and display altered binding and/or signaling properties in relation to the binary agonist-receptor complex. Allosteric sites on GPCRs represent novel drug targets because allosteric modulators possess a number of theoretical advantages over classic orthosteric ligands, such as a ceiling level to the allosteric effect and a potential for greater GPCR subtype-selectivity. Because of the noncompetitive nature of allosteric phenomena, the detection and quantification of such effects often relies on a combination of equilibrium binding, nonequilibrium kinetic, and functional signaling assays. This review discusses the development and properties of allosteric receptor models for GPCRs and the detection and quantification of allosteric effects. Moreover, we provide an overview of the current knowledge regarding the location of possible allosteric sites on GPCRs and candidate endogenous allosteric modulators. Finally, we discuss the potential for allosteric effects arising from the formation of GPCR oligomers or GPCRs complexed with accessory cellular proteins. It is proposed that the study of allosteric phenomena will become of progressively greater import to the drug discovery process due to the advent of newer and more sensitive GPCR screening technologies.

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G Protein-Coupled Receptor Allosterism and Complexing

Somatostatin and dopamine are two major neurotransmitter systems that share a number of structural and functional characteristics. Somatostatin receptors and dopamine receptors are colocalized in neuronal subgroups, and somatostatin is involved in modulating dopamine-mediated control of motor activity. However, the molecular basis for such interaction between the two systems is unclear. Here, we show that dopamine receptor D2R and somatostatin receptor SSTR5 interact physically through hetero-oligomerization to create a novel receptor with enhanced functional activity. Our results provide evidence that receptors from different G protein (heterotrimeric guanine nucleotide binding protein)–coupled receptor families interact through oligomerization. Such direct intramembrane association defines a new level of molecular crosstalk between related G protein–coupled receptor subfamilies.

https://science.sciencemag.org/content/sci/288/5463/154.full.pdf

Receptors for Dopamine and Somatostatin: Formation of Hetero-Oligomers with Enhanced Functional Activity

GABA(B) receptors are broadly expressed in the nervous system and have been implicated in a wide variety of neurological and psychiatric disorders. The cloning of the first GABA(B) receptor cDNAs in 1997 revived interest in these receptors and their potential as therapeutic targets. With the availability of molecular tools, rapid progress was made in our understanding of the GABA(B) system. This led to the surprising discovery that GABA(B) receptors need to assemble from distinct subunits to function and provided exciting new insights into the structure of G protein-coupled receptors (GPCRs) in general. As a consequence of this discovery, it is now widely accepted that GPCRs can exist as heterodimers. The cloning of GABA(B) receptors allowed some important questions in the field to be answered. It is now clear that molecular studies do not support the existence of pharmacologically distinct GABA(B) receptors, as predicted by work on native receptors. Advances were also made in clarifying the relationship between GABA(B) receptors and the receptors for gamma-hydroxybutyrate, an emerging drug of abuse. There are now the first indications linking GABA(B) receptor polymorphisms to epilepsy. Significantly, the cloning of GABA(B) receptors enabled identification of the first allosteric GABA(B) receptor compounds, which is expected to broaden the spectrum of therapeutic applications. Here we review current concepts on the molecular composition and function of GABA(B) receptors and discuss ongoing drug-discovery efforts.

Molecular Structure and Physiological Functions of GABAB Receptors

Metabotropic glutamate (mGlu) receptors, which include mGlu1-8 receptors, are a heterogeneous family of G-protein-coupled receptors which function to modulate brain excitability via presynaptic, postsynaptic and glial mechanisms. Certain members of this receptor family have been shown to function as presynaptic regulatory mechanisms to control release of neurotransmitters. In general, Gi-coupled mGlu receptor subtypes appear to negatively modulate excitatory (and possibly also inhibitory) neurotransmitter output when activated. Localization studies have shown that mGlu7 is restricted to the presynaptic grid at the site of vesicle fusion. These studies along with other evidence suggest that mGlu7 is the nerve terminal autoreceptor that regulates physiological release of glutamate. Other mGlu subtypes, in particular mGlu2, mGlu8, and possibly mGlu4, are also localized presynaptically, but at perisynaptic sites outside the active zone of neurotransmitter release. Gi-coupled mGlu receptors also may exist on presynaptic elements of neighboring gamma-aminobutyric acid (GABA) neurons where they play a role in heterosynaptic suppressions of GABA release. This suggests that these receptors may have evolved to monitor glutamate that has "spilled" out of the synapse. Thus, they may serve as the brain's evolutionary mechanism to prevent pathological changes in neuronal excitability and thus maintain homeostasis. Recent progress on the molecular and pharmacological aspects of these presynaptic mGlu receptors is unveiling their functions and the therapeutic directions of agents designed for these novel glutamate receptor targets.

Unveiling the Functions of Presynaptic Metabotropic Glutamate Receptors in the Central Nervous System

The classical idea that G-protein-coupled receptors (GPCRs) function as monomeric entities has been unsettled by the emerging concept of GPCR dimerization. Recent findings have indicated not only that many GPCRs exist as homodimers and heterodimers, but also that their oligomeric assembly could have important functional roles. Several studies have shown that dimerization occurs early after biosynthesis, suggesting that it has a primary role in receptor maturation. G-protein coupling, downstream signalling and regulatory processes such as internalization have also been shown to be influenced by the dimeric nature of the receptors. In addition to raising fundamental questions about GPCR function, the concept of dimerization could be important in the development and screening of drugs that act through this receptor class. In particular, the changes in ligand-binding and signalling properties that accompany heterodimerization could give rise to an unexpected pharmacological diversity that would need to be considered.

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Kenneth A Jones

Papers

GABAB receptors - the first 7TM heterodimers.