Associate editor: Ulrik Gether
Dimers and beyond: The functional puzzles of class C GPCRs

https://doi.org/10.1016/j.pharmthera.2011.01.006Get rights and content

Abstract

Our understanding of G protein-coupled receptor (GPCR) activation has evolved during the last ten years, both at a molecular level thanks to the resolution of several crystal structures, and at a cellular level with the characterization of complexes surrounding the receptor. Class C GPCRs, including receptors for glutamate, γ-aminobutyric acid (GABA), taste compounds, amino acids and Ca2+, have several structural features that make them unique in the GPCR family. First, they possess a large and structurally-defined extracellular domain, which is distal from the transmembrane core and bears the agonist binding site. Second, they form obligatory dimers providing a unique mode of activation compared to GPCRs of other classes. In this article, we aim to provide an overview of the molecular mechanisms of class C GPCR activation as dimeric entities. Furthermore, we discuss the possibility of modulating receptor function through the use of ligands or by association, direct or indirect, with other receptors (GPCRs or not) with the aim to better understand receptor function. Finally, we present the therapeutic scope for the class C GPCRs that highlights the need to fully characterize the functioning of these receptors in their native environment to develop better therapeutic molecules.

Introduction

GPCRs are key cell-surface receptors for cellular adaptation to their environment and are sensitive to a large array of stimuli including light (photons), amino acids and large proteins. Approximately 800 genes (more than 3% of the human genome) encode for such receptors and they represent an important market for pharmaceutical companies. A common feature for these receptors is a large transmembrane core composed of seven helices with an extracellular N-terminus and an intracellular C-terminal tail. Based on the phylogeny of the transmembrane core (7TM), GPCRs have been classified in 3 main families: classes A, B and C. Despite a similar role (converting an extracellular stimulus into a cellular response via activation of G protein or other signaling cascades), the precise activation mechanisms differ from class to class, mainly due to structural differences. The present review will only focus on class C GPCRs.

Class C consists of the receptors for glutamate (mGlu receptors), GABA (GABAB receptors), Ca2+ ions (CaS receptor), sweet and umami tastes (T1Rs), basic amino acids (GPRC6a) and some orphan receptors (GPR156, GPR158, GPR179 and RAIG) (Fig. 1). A structural feature shared by most of these receptors, is a large, structurally-defined extracellular domain called the Venus flytrap (VFT). This domain is constituted of two opposing lobes, separated by a cleft where the endogenous ligands bind. Such particular protein folding is shared with periplasmic binding proteins and is also found in other cell surface receptors such as ligand-gated ion channels, tyrosine kinase or guanylate cyclase receptors. A second characteristic of class C GPCRs is their constitutive dimerization at the cell-surface as either homodimers (mGlu and CaS) or heterodimers (GABAB and T1Rs) by association between two identical or non-identical proteins, respectively. The homodimeric receptors were shown to be covalently linked by a disulphide bridge in the region of their VFT (Romano et al., 1996, Bai et al., 1998). Heterodimeric receptors are not covalently linked, but their heterodimerization is required to obtain a functional dimer as one of the subunits contains the endogenous ligand binding site, whilst the other mediates signal transduction (Galvez et al., 2001, Margeta-Mitrovic et al., 2001, Nelson et al., 2001, Nelson et al., 2002). The significance of dimerization of class C GPCRs on the activation mechanisms has been studied over the last years.

Like many other GPCRs, class C receptors have been described to be part of larger oligomeric complexes, either with themselves, with other GPCRs or with unrelated cell-surface receptors. This can involve direct interaction or participation in complexes with multiple different proteins, either scaffolding or signaling proteins, resulting in cross-talk of signaling pathways. The characterization of these complexes has yet to be fully defined with regard to understanding their function in their native environment. Elucidation of such information would be of critical importance for therapeutic purposes. Indeed, while class C GPCRs are implicated in many physiological functions and pathological disorders, only a few therapeutic compounds target these receptors, mainly due to a lack of knowledge of the precise function.

In this review, we will present the molecular mechanisms of class C GPCRs activation and of their modulation, the functional significance of receptor association in larger complexes, in addition to reporting on the clinical studies conducted on these receptors, in order to illustrate their therapeutic interest.

Section snippets

Class C GPCRs: family members

The first members of the family to be cloned were the metabotropic glutamate receptors (Houamed et al., 1991, Masu et al., 1991). These receptors are activated by glutamate, the major excitatory neurotransmitter of the central nervous system (CNS), and thus participate in regulating the cell excitability and synaptic transmission. Eight subtypes of receptors have been identified so far and are classified in three groups based on their sequence, localization and signaling pathways. Group-I (mGlu1

Class C GPCRs mechanisms of action

As previously mentioned, class C GPCRs form constitutive dimers. It is becoming more apparent that dimer formation is mandatory for the function of these receptors, making them complex allosteric proteins where each domain influences the neighboring ones, such that a conformational change in one domain will facilitate changes in others. As abovementioned, in addition to the common 7TM most class C GPCRs are characterized by the presence of a large VFT extracellular domain (Fig. 1A). This domain

Class C GPCR complexes

Oligomerization of GPCRs has been described for about 20 years now, but the functional impact of that phenomenon have been the subject of intense debate (for review see (Chabre et al., 2009, Gurevich and Gurevich, 2008a, Gurevich and Gurevich, 2008b). Despite that in a purified lipid environment all studied receptors could activate G proteins as a monomer, transcomplementation experiments performed in cells support that dimers are indeed the signaling entities. Regarding class C GPCRs, their

Therapeutic potential of class C GPCRs

These past few years many reviews have been written about the implication in physiopathology of receptors belonging to class C, especially mGlu, GABAB and CaS receptors. In the present chapter, the therapeutic interest of these receptors will be illustrated through examples of clinical studies targeting class C GPCRs. For more details on preclinical studies involving these receptors, we invite the reader to refer to the review articles cited below.

Concluding remarks

As described here, the unique structural features of class C GPCRs are reflected in their particular molecular mechanisms of action. In a simplistic view, these receptors are dimeric with each protomer composed of two or three well separate domains, but all communicate as an ensemble to activate or inhibit their cognate signaling molecules. Class C GPCRs signaling can also be modulated by the presence of other receptors in the local environment. Whilst there remains much to do in order to

Acknowledgments

We would like to thank Dr. Gregory D. Stewart for critical reading of the manuscript and English editing. This work was supported by CNRS, INSERM, Cisbio and by grants from the French Ministry of Research, Action Concertée Incitative “Biologie Cellulaire Moléculaire et Structurale” (ACI-BCMS 328), the Agence Nationale de la Recherche (ANR-05-PRIB-02502, ANR-BLAN06-3_135092, ANR-05-NEUR-035 and ANR-NT09-481664), and by an unrestricted grant from Senomyx (La Jolla, CA, USA).

References (178)

  • H. Brauner-Osborne et al.

    The agonist-binding domain of the calcium-sensing receptor is located at the amino-terminal domain

    J Biol Chem

    (1999)
  • C. Brock et al.

    Activation of a dimeric metabotropic glutamate receptor by intersubunit rearrangement

    J Biol Chem

    (2007)
  • E.M. Brown

    Anti-parathyroid and anti-calcium sensing receptor antibodies in autoimmune hypoparathyroidism

    Endocrinol Metab Clin North Am

    (2009)
  • E.M. Brown

    Clinical utility of calcimimetics targeting the extracellular calcium-sensing receptor (CaSR)

    Biochem Pharmacol

    (2010)
  • M. Chabre et al.

    The apparent cooperativity of some GPCRs does not necessarily imply dimerization

    Trends Pharmacol Sci

    (2009)
  • W. Chang et al.

    Complex formation with the Type B gamma-aminobutyric acid receptor affects the expression and signal transduction of the extracellular calcium-sensing receptor. Studies with HEK-293 cells and neurons

    J Biol Chem

    (2007)
  • F. Ciruela et al.

    Metabotropic glutamate 1alpha and adenosine A1 receptors assemble into functionally interacting complexes

    J Biol Chem

    (2001)
  • P.J. Conn et al.

    Activation of metabotropic glutamate receptors as a novel approach for the treatment of schizophrenia

    Trends Pharmacol Sci

    (2009)
  • J.F. Cryan et al.

    Don't worry ‘B’ happy!: a role for GABA(B) receptors in anxiety and depression

    Trends Pharmacol Sci

    (2005)
  • G. Dolen et al.

    Mechanism-based approaches to treating fragile X

    Pharmacol Ther

    (2010)
  • B. Duthey et al.

    A single subunit (GB2) is required for G-protein activation by the heterodimeric GABA(B) receptor

    J Biol Chem

    (2002)
  • S. Ferre et al.

    Functional relevance of neurotransmitter receptor heteromers in the central nervous system

    Trends Neurosci

    (2007)
  • T. Galvez et al.

    Mapping the agonist-binding site of GABAB type 1 subunit sheds light on the activation process of GABAB receptors

    J Biol Chem

    (2000)
  • L. Gama et al.

    Heterodimerization of calcium sensing receptors with metabotropic glutamate receptors in neurons

    J Biol Chem

    (2001)
  • J. Gonzalez-Maeso et al.

    Hallucinogens recruit specific cortical 5-HT(2A) receptor-mediated signaling pathways to affect behavior

    Neuron

    (2007)
  • C. Goudet et al.

    Asymmetric functioning of dimeric metabotropic glutamate receptors disclosed by positive allosteric modulators

    J Biol Chem

    (2005)
  • C. Goudet et al.

    Metabotropic receptors for glutamate and GABA in pain

    Brain Res Rev

    (2009)
  • V.V. Gurevich et al.

    GPCR monomers and oligomers: it takes all kinds

    Trends Neurosci

    (2008)
  • V.V. Gurevich et al.

    How and why do GPCRs dimerize?

    Trends Pharmacol Sci

    (2008)
  • K.P. Hofmann et al.

    A G protein-coupled receptor at work: the rhodopsin model

    Trends Biochem Sci

    (2009)
  • J. Hu et al.

    Human Ca2+ receptor cysteine-rich domain. Analysis of function of mutant and chimeric receptors

    J Biol Chem

    (2000)
  • J. Hu et al.

    A region in the seven-transmembrane domain of the human Ca2+ receptor critical for response to Ca2+

    J Biol Chem

    (2005)
  • J. Hu et al.

    Identification of acidic residues in the extracellular loops of the seven-transmembrane domain of the human Ca2+ receptor critical for response to Ca2+ and a positive allosteric modulator

    J Biol Chem

    (2002)
  • P. Jiang et al.

    The cysteine-rich region of T1R3 determines responses to intensely sweet proteins

    J Biol Chem

    (2004)
  • E. Lancaster et al.

    Antibodies to the GABA(B) receptor in limbic encephalitis with seizures: case series and characterisation of the antigen

    Lancet Neurol

    (2010)
  • A. Lehmann

    GABAB receptors as drug targets to treat gastroesophageal reflux disease

    Pharmacol Ther

    (2009)
  • L.F. Agnati et al.

    Molecular mechanisms and therapeutical implications of intramembrane receptor/receptor interactions among heptahelical receptors with examples from the striatopallidal GABA neurons

    Pharmacol Rev

    (2003)
  • S. Angers et al.

    Detection of beta 2-adrenergic receptor dimerization in living cells using bioluminescence resonance energy transfer (BRET)

    Proc Natl Acad Sci USA

    (2000)
  • L. Aniksztejn et al.

    Quisqualate metabotropic receptors modulate NMDA currents and facilitate induction of long-term potentiation through protein kinase C

    Eur J Neurosci

    (1992)
  • K. Azdad et al.

    Dopamine D2 and adenosine A2A receptors regulate NMDA-mediated excitation in accumbens neurons through A2A-D2 receptor heteromerization

    Neuropsychopharmacology

    (2009)
  • M. Bai et al.

    Intermolecular interactions between dimeric calcium-sensing receptor monomers are important for its normal function

    Proc Natl Acad Sci USA

    (1999)
  • M.A. Benneyworth et al.

    Chronic phenethylamine hallucinogen treatment alters behavioral sensitivity to a metabotropic glutamate 2/3 receptor agonist

    Neuropsychopharmacology

    (2007)
  • M.A. Benneyworth et al.

    A selective positive allosteric modulator of metabotropic glutamate receptor subtype 2 blocks a hallucinogenic drug model of psychosis

    Mol Pharmacol

    (2007)
  • E. Berry-Kravis et al.

    A pilot open label, single dose trial of fenobam in adults with fragile X syndrome

    J Med Genet

    (2009)
  • A.S. Bessis et al.

    Three-dimensional model of the extracellular domain of the type 4a metabotropic glutamate receptor: new insights into the activation process

    Protein Sci

    (2000)
  • A.S. Bessis et al.

    Closure of the Venus flytrap module of mGlu8 receptor and the activation process: insights from mutations converting antagonists into agonists

    Proc Natl Acad Sci USA

    (2002)
  • C. Beurrier et al.

    Electrophysiological and behavioral evidence that modulation of metabotropic glutamate receptor 4 with a new agonist reverses experimental parkinsonism

    FASEB J

    (2009)
  • G.A. Block et al.

    Cinacalcet for secondary hyperparathyroidism in patients receiving hemodialysis

    N Engl J Med

    (2004)
  • H. Brauner-Osborne et al.

    Structure, pharmacology and therapeutic prospects of family C G-protein coupled receptors

    Curr Drug Targets

    (2007)
  • E.M. Brown

    Clinical lessons from the calcium-sensing receptor

    Nat Clin Pract Endocrinol Metab

    (2007)
  • Cited by (0)

    View full text