Allosteric interactions between agonists and antagonists within the adenosine A _2A receptor-dopamine D ₂ receptor heterotetramer

Abstract

Adenosine A 2A receptor (A 2A R)-dopamine D 2 receptor (D 2 R) heteromers are key modulators of striatal neuronal function. It has been suggested that the psychostimulant effects of caffeine depend on its ability to block an allosteric modulation within the A 2A R-D 2 R heteromer, by which adenosine decreases the affinity and intrinsic efficacy of dopamine at the D 2 R. We describe novel unsuspected allosteric mechanisms within the heteromer by which not only A 2A R agonists, but also A 2A R antagonists, decrease the affinity and intrinsic efficacy of D 2 R agonists and the affinity of D 2 R antagonists. Strikingly, these allosteric modulations disappear on agonist and antagonist coadministration. This can be explained by a model that considers A 2A R-D 2 R heteromers as heterotetramers, constituted by A 2A R and D 2 R homodimers, as demonstrated by experiments with bioluminescence resonance energy transfer and bimolecular fluorescence and bioluminescence com- Most evidence indicates that G protein-coupled receptors (GPCRs) form homodimers and heteromers. Homodimers seem to be a predominant species, and oligomeric entities can be viewed as multiples of dimers (1). It has been proposed that GPCR heteromers are constituted mainly by heteromers of homodimers (1, 2). Allosteric mechanisms determine a multiplicity of unique pharmacologic properties of GPCR homodimers and heteromers (1, 3). First, binding of a ligand to one of the receptors in the heteromer can modify the affinity of ligands for the other receptor (1, 3, 4). The most widely reproduced allosteric modulation of ligand-binding properties in a GPCR heteromer is the ability of adenosine A 2A receptor (A 2A R) agonists to decrease the affinity of dopamine D 2 receptor (D 2 R) agonists in the A 2A R-D 2 R heteromer (5). A 2A R-D 2 R heteromers have been revealed both in transfected cells (6, 7), striatal neurons in culture (6,8) and in situ, in mammalian striatum (9, 10), where they play an important role in the modulation of GABAergic striatopallidal neuronal function (9, 11).In addition to ligand-binding properties, unique properties for each GPCR oligomer emerge in relation to the varying intrinsic efficacy of ligands for different signaling pathways (1-3). Intrinsic efficacy refers to the power of the agonist to induce a functional response, independent of its affinity for the receptor. Thus, allosteric modulation of an agonist can potentially involve changes in affinity and/or intrinsic efficacy (1, 3). This principle can be observed in the A 2A R-D 2 R heteromer, where a decrease in D 2 R agonist affinity cannot alone explain the ability of an A 2A R agonist to abolish the decreased excitability of GABAergic striatopallidal neurons induced by high concentration of a D 2 R agonist (9), which should overcome the decrease in affinity. Furthermore, a differential effect of allosteric modulations of different agonist-mediated signaling responses (i.e., functional selectivity) can occur within GPCR heteromers (1, 2, 8 It has been hypothesized that the allos...

Evidence for the heterotetrameric structure of the adenosine A2A–dopamine D2 receptor complex

“…In these experiments we found that the presence of the A 2A R agonist CGS 21680 changed the dissociation constant of the radiolabelled A 2A R antagonist [ 3 H]ZM 241385 [21]. This constant did not change in the presence of several A 2A R antagonists, such as caffeine or SCH 58261 (Table 1).…”

“…modulation of ligandbinding properties, is the ability of A 2A R agonists to decrease the affinity of dopamine D 2 receptor (D 2 R) for its agonists in the A 2A R-D 2 R heteromer [17][18][19][20][21]. A 2A R-D 2 R heteromers have been found in transfected cells [22,23], primary cultures of striatal neurons [22] and, in situ, in mammalian striatum [21,[24][25][26], where they play an important role in the modulation of GABAergic striato-pallidal neuronal function [24,27].…”

“…modulation of ligandbinding properties, is the ability of A 2A R agonists to decrease the affinity of dopamine D 2 receptor (D 2 R) for its agonists in the A 2A R-D 2 R heteromer [17][18][19][20][21]. A 2A R-D 2 R heteromers have been found in transfected cells [22,23], primary cultures of striatal neurons [22] and, in situ, in mammalian striatum [21,[24][25][26], where they play an important role in the modulation of GABAergic striato-pallidal neuronal function [24,27]. It has previously been hypothesized that allosteric interactions between A 2A R and D 2 R agonists within the A 2A R-D 2 R heteromer provide a mechanism responsible for both the behavioural depressant effects of adenosine analogues and for the psychostimulant effects of selective adenosine A 2A R antagonists and the non-selective adenosine receptor antagonist caffeine; this mechanism is of particular relevance in several neuropsychiatric disorders such as Parkinson's disease [15,21,[28][29][30][31].…”

A Significant Role of the Truncated Ghrelin Receptor GHS-R1b in Ghrelin-induced Signaling in Neurons

“…Support for heterotetramer formation was obtained by using BRET with double BiLC and BiFC assays (15,16). In this assay, the two BRET sensors, the donor Rluc8 (a more efficient variant of Rluc) and the acceptor YFP Venus (a more efficient variant of YFP), are split into two hemiproteins, with each split sensor being fused to one of the four putative interacting receptors.…”

Underlying side effects of caffeine

“…The chronic use of CFE may reduce dopamine release via the stimulation of A1-A2A heterodimer in the axon terminal of glutamate neurons and leads CFE tolerance. CFE antagonizes A2A receptor in the ventrolateral preoptic area and reduces inhibitory neurotransmission, especially gamma-amino butyric acid (GABA) to the tuberomammillary nucleus, a histaminergic projection nucleus, activation of which may promote the arousal effect of it (Bonaventura et al, 2015). CFE, otherwise, through its phosphodiesterase inhibitory potential, augments the intracellular cyclic AMP (cAMP), and activates protein kinase-A (PK-A), downregulates tumor necrosis factoralpha (TNF-α) and leukotriene synthesis along with a reduction of inflammation and innate immunity.…”