Abstract
BackgroundG-protein-coupled receptor (GPCR) heteromeric complexes have distinct properties from homomeric GPCRs, giving rise to new receptor functionalities. Adenosine receptors (A1R or A2AR) can form A1R-A2AR heteromers (A1-A2AHet), and their activation leads to canonical G-protein-dependent (adenylate cyclase mediated) and -independent (β-arrestin mediated) signaling. Adenosine has different affinities for A1R and A2AR, allowing the heteromeric receptor to detect its concentration by integrating the downstream Gi- and Gs-dependent signals. cAMP accumulation and β-arrestin recruitment assays have shown that, within the complex, activation of A2AR impedes signaling via A1R.ResultsWe examined the mechanism by which A1-A2AHet integrates Gi- and Gs-dependent signals. A1R blockade by A2AR in the A1-A2AHet is not observed in the absence of A2AR activation by agonists, in the absence of the C-terminal domain of A2AR, or in the presence of synthetic peptides that disrupt the heteromer interface of A1-A2AHet, indicating that signaling mediated by A1R and A2AR is controlled by both Gi and Gs proteins.ConclusionsWe identified a new mechanism of signal transduction that implies a cross-communication between Gi and Gs proteins guided by the C-terminal tail of the A2AR. This mechanism provides the molecular basis for the operation of the A1-A2AHet as an adenosine concentration-sensing device that modulates the signals originating at both A1R and A2AR.
Highlights
G-protein-coupled receptor (GPCR) heteromeric complexes have distinct properties from homomeric G-proteincoupled receptors (GPCRs), giving rise to new receptor functionalities
In the presence of Homodimerization of A1R and A2AR occurs through the transmembrane (TM) 4/5 interface and heterodimerization via the TM5/6 interface in the A1-A2AHet Our recently published bioluminescence resonance energy transfer (BRET)-aided computational model of the A1-A2AHet predicted the TM interfaces involved in homo- (TM4/5) and heterodimerization (TM5/6) [5]
To further confirm this arrangement, we used synthetic peptides with the sequence of TM domains of the A2AR and the A1R, fused to the cell-penetrating HIV transactivator of transcription (TAT) peptide [6], to alter inter-protomer interactions in the A1A2AHet. These peptides were first tested in bimolecular fluorescence complementation (BiFC) assays in HEK293 T cells expressing receptors fused to two complementary halves of YFP
Summary
G-protein-coupled receptor (GPCR) heteromeric complexes have distinct properties from homomeric GPCRs, giving rise to new receptor functionalities. Adenosine receptors (A1R or A2AR) can form A1R-A2AR heteromers (A1-A2AHet), and their activation leads to canonical G-protein-dependent (adenylate cyclase mediated) and -independent (β-arrestin mediated) signaling. Adenosine has different affinities for A1R and A2AR, allowing the heteromeric receptor to detect its concentration by integrating the downstream Gi- and Gs-dependent signals. CAMP accumulation and β-arrestin recruitment assays have shown that, within the complex, activation of A2AR impedes signaling via A1R. Convergent and compelling evidence shows that GPCRs may form complexes constituted by a number of equal (homo) or different (hetero) receptor protomers [2]. As agreed in the field, a GPCR heteromer displays characteristics that are different from those of the constituting protomers, giving rise to novel functional entities [3]. A1R, which is Gi coupled, and A2AR, which is Gs coupled, form a functional heteromer [4]
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