Heterodimerization of G-protein-coupled receptors in the CNS

doi:10.1016/S1471-4892(01)00001-7

Current Opinion in Pharmacology

Volume 1, Issue 1, 1 February 2001, Pages 40-44

https://doi.org/10.1016/S1471-4892(01)00001-7 Get rights and content

Abstract

Over the last year the combinations of G-protein-coupled receptors that are known to form heterodimeric complexes has rapidly increased. For example, dopamine receptors can dimerize with both somatostatin and adenosine receptors. These studies have been aided by improved technologies to monitor protein/protein interactions in living cells. Crosstalk at the level of the receptors might explain some of the known physiological interactions of these neurotransmitter systems and also provide new approaches for therapeutic intervention.

Introduction

The G-protein-coupled receptors (GPCRs) play a central role in cell–cell communication in the central nervous system (CNS). Through activation by a diverse range of neurotransmitter ligands, the receptors regulate the activity of effector molecules including ion channels, enzymes and transcription factors. Sequencing of the human genome has indicated the presence of around 500 non-sensory GPCRs of which greater than half are still to be paired with their cognate ligand. The GPCRs have a proven track record in drug discovery, with >40% of marketed drugs having activity mediated through this family of proteins. The expansion of the target class through bio-informatics is likely to yield substantial numbers of new drug targets for the future.

Several mathematical models have been proposed to describe the activation of GPCRs and their coupling to G proteins; however, these models have generally assumed a stoichiometry of 1:1:1 with ligand, receptor and G protein. Over the past 10 years evidence has accumulated to suggest that GPCRs, like many other cell surface receptors [1], function as dimers or larger oligomers, and that this interaction is a fundamental and essential component of receptor signalling. More surprisingly, recent findings suggest that GPCRs can form heterodimers not only with closely related receptor subtypes but also with more distant GPCRs and even members of other protein classes. Examples of receptors within the CNS that have been shown to dimerize are reviewed here. The implications of heterodimerization for our understanding of receptor signalling, their pharmacology and how they can be further exploited for drug discovery is discussed.

Section snippets

Homodimerization

The evidence for GPCR homodimerization is now compelling [2], [3]. Indeed, it seems likely that receptor dimers are the natural conformation for GPCRs and that most, if not all, members of this superfamily exist or can exist as dimers. Data to support this comes from a wide variety of sources. Molecular species corresponding to dimers can be clearly visualised following sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and in some cases appear to be the major form of the

Heterodimerization between related receptor subtypes

The first GPCR clearly demonstrated to be a heterodimer was the γ-aminobutyric acid (GABA)_B receptor [14], [15], [16], [17], [18]. This complex differs from other GPCR heterodimers described to date in that each partner in the dimer, made up of GABA_B(1) and GABA_B(2), is non-functional when expressed alone. GABA_B(1) contains an endoplasmic reticulum retention motif that prevents cell surface expression [13 radical dot ]. This motif is masked through an interaction with the related protein GABA_B(2), allowing

Heterodimerization between distinct GPCRs

Heterodimerization also occurs between more distantly related GPCRs. The first clear example of this was the report that SSTR5 could heterodimerize with dopamine D2 receptors [7 radical dot ]. This interaction is of particular interest in light of the well-documented physiological interactions of these two neurotransmitters. Both receptor types are expressed in high concentrations in the striatum and limbic structures and both transmitters interact to regulate locomotor activity. In the study by Rocheville

Heterodimerization of GPCRs and other proteins

Heterodimerization of GPCRs with other membrane proteins has also been reported and adds another layer of complexity to the functioning of GPCRs in the membrane. The ligand specificity and pharmacology of members of the secretin family of GPCRs is determined by the interaction with a family of 1TM proteins called RAMPs (receptor activity modifying proteins) [26], [27]. Another 1TM protein, calcyon, alters the coupling of dopamine D1 receptors from G_s to G_q [28].

GPCRs can also heterodimerize

Implications for pharmacology and drug discovery

There is clear evidence that heterodimerization of GPCRs has profound effects on the pharmacology of natural ligands and that this is likely to be of physiological relevance. Crosstalk between receptors may underlie certain disease pathologies and, in doing so, provide an alternative approach for drug intervention. For example, in the treatment of Parkinson's disease it may be of benefit to consider drugs directed at adenosine receptors rather than dopamine receptors [23 radical dot ]. Heterodimerization,

Conclusions

The colocalization of receptors on individual cells within the CNS has been recognised for many years; however, the knowledge that these receptors may physically interact and, in doing so, alter their function is only starting to emerge. Dimerization has effects on receptor trafficking, G-protein coupling and thereby receptor signalling as well as receptor pharmacology. It is likely that over the next few years many other examples of receptor heterodimers will be identified, a process that will

Acknowledgements

I thank Christopher Reynolds and Steve Foord for their comments on the manuscript and I also thank Christopher Reynolds for providing me with the Figure. I thank Jane Pelling for her excellent administrative assistance.

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

. of special interest
. of outstanding interest

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However, we were aware that besides direct interactions also indirect interactions involving, e.g., adapter proteins could mediate receptor–receptor interactions (Agnati et al., 1995, 2005b; Fuxe et al., 2007a). Now there exists a long list of transmitter heteromers and homomers (Agnati et al., 2003a; Bouvier, 2001; George et al., 2002; Jordan and Devi, 1999; Marshall et al., 1999, 2001; Prinster et al., 2005) showing that this is a fundamental mechanism in the brain and in the periphery (Fuxe et al., 2007a). It is important to underline that receptor–receptor interactions are only one type of the vast class of protein–protein interactions occurring at membrane level.
Future therapies for diseases associated with altered dopaminergic signaling, including Parkinson's disease, schizophrenia and drug addiction or drug dependence may substantially build on the existence of intramembrane receptor–receptor interactions within dopamine receptor containing receptor mosaics (RM; dimeric or high-order receptor oligomers) where it is believed that the dopamine D₂ receptor may operate as the ‘hub receptor’ within these complexes. The constitutive adenosine A_2A/dopamine D₂ RM, located in the dorsal striato-pallidal GABA neurons, are of particular interest in view of the demonstrated antagonistic A_2A/D₂ interaction within these heteromers; an interaction that led to the suggestion and later demonstration that A_2A antagonists could be used as novel anti-Parkinsonian drugs. Based on the likely existence of A_2A/D₂/mGluR5 RM located both extrasynaptically on striato-pallidal GABA neurons and on cortico-striatal glutamate terminals, multiple receptor–receptor interactions within this RM involving synergism between A_2A/mGluR5 to counteract D₂ signaling, has led to the proposal of using combined mGluR5 and A_2A antagonists as a future anti-Parkinsonian treatment. Based on the same RM in the ventral striato-pallidal GABA pathways, novel strategies for the treatment of schizophrenia, building on the idea that A_2A agonists and/or mGluR5 agonists will help reduce the increased dopaminergic signaling associated with this disease, have been suggested. Such treatment may ensure the proper glutamatergic drive from the mediodorsal thalamic nucleus to the prefrontal cortex, one which is believed to be reduced in schizophrenia due to a dominance of D₂-like signaling in the ventral striatum. Recently, A_2A receptors also have been shown to counteract the locomotor and sensitizing actions of cocaine and increases in A_2A receptors have also been observed in the nucleus accumbens after extended cocaine self-administration, probably representing a compensatory up-regulation to counteract the cocaine-induced increases in dopamine D₂ and D₃ signaling. Therefore, A_2A agonists, through antagonizing D₂ and D₃ signaling within A_2A/D₂ and A_2A/D₃ RM heteromers in the nucleus accumbens, may be found useful as a treatment for cocaine dependence. Furthermore, antagonistic cannabinoid CB₁/D₂ interactions requiring A_2A receptors have also been discovered and possibly operate in CB₁/D₂/A_2A RM located principally on striatal glutamate terminals but also on some ventral striato-pallidal GABA neurons, thereby opening up a new mechanism for the integration of endocannabinoid, DA and adenosine mediated signals. Thus, A_2A, mGluR5 and/or CB₁ receptors can form integrative units with D₂ receptors within RM displaying different compositions, topography and localization. Also galaninR/5-HT_1A RM probably participates in the transmission of the ascending 5-hydroxytryptamine neurons, where galanin receptors antagonize 5-HT_1A recognition and signaling. Subtype specific galanin receptor antagonists may therefore represent novel antidepressant drugs. These results suggest the importance of a complete understanding of the function of these RM with regard to disease. Ultimately receptor–receptor interactions within RM that modify dopaminergic and serotonergic signaling may give new strategies for treatment of a wide range of diseases associated with altered dopaminergic and serotonergic signaling.
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Also, to our knowledge, heteromers of catalytic receptors or ionotropic receptors, or heteromers of catalytic and ionotropic receptors have not yet been described. Different molecular mechanisms for neurotransmitter receptor homomerization and heteromerization have been proposed, such as covalent disulfide bonds of extracellular domains, coiled–coil interactions and epitope–epitope electrostatic interactions of intracellular domains, and interactions of transmembrane domains (by contact or domain swapping) [13,15,16,21]. Most probably, different molecular mechanisms are implicated in each receptor heteromerization.
The existence of neurotransmitter receptor heteromers is becoming broadly accepted and their functional significance is being revealed. Heteromerization of neurotransmitter receptors produces functional entities that possess different biochemical characteristics with respect to the individual components of the heteromer. Neurotransmitter receptor heteromers can function as processors of computations that modulate cell signaling. Thus, the quantitative or qualitative aspects of the signaling generated by stimulation of any of the individual receptor units in the heteromer are different from those obtained during coactivation. Furthermore, recent studies demonstrate that some neurotransmitter receptor heteromers can exert an effect as processors of computations that directly modulate both pre- and postsynaptic neurotransmission. This is illustrated by the analysis of striatal receptor heteromers that control striatal glutamatergic neurotransmission.
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ReviewHeterodimerization of G-protein-coupled receptors in the CNS

Abstract

Introduction

Section snippets

Homodimerization

Heterodimerization between related receptor subtypes

Heterodimerization between distinct GPCRs

Heterodimerization of GPCRs and other proteins

Implications for pharmacology and drug discovery

Conclusions

Acknowledgements

References and recommended reading

Cell

Trends Endocrinol Metab

J Biol Chem

Curr Biol

J Biol Chem

J Biol Chem

Trends Pharmacol Sci

J Biol Chem

FEBS Letters

J Biol Chem

J Biol Chem

Structural and functional aspects of G protein-coupled receptor oligomerization

Biochem Cell Biol

Coexpression studies with mutant muscarinic/adrenergic receptors provide evidence for intermolecular ‘cross-talk’ between G-protein-linked receptors

Proc Natl Acad Sci USA

Receptors for dopamine and somatostatin: formation of hetero-oligomers with enhanced functional activity

Science

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

Proc Natl Acad Sci USA

Review
Heterodimerization of G-protein-coupled receptors in the CNS