Alternative titles; symbols
HGNC Approved Gene Symbol: ADORA2A
Cytogenetic location: 22q11.23 Genomic coordinates (GRCh38): 22:24,423,597-24,442,357 (from NCBI)
Adenosine is released from metabolically active cells by facilitated diffusion and is generated extracellularly by degradation of released ATP. Ledent et al. (1997) noted that it is a potent biologic mediator that modulates the activity of numerous cell types, including various neuronal populations, platelets, neutrophils and mast cells, and smooth muscle cells in bronchi and vasculature. Most of these effects help to protect cells and tissues during stress situations such as ischemia. Adenosine mediates its effects through 4 receptor subtypes: the A1 (ADORA1; 102775), A2a (ADORA2A, also called RDC8), A2b (ADORA2B; 600446), and A3 (ADORA3; 600445) receptors. The A2a receptor is abundant in basal ganglia, vasculature, and platelets, and stimulates adenylyl cyclase. It is a major target of caffeine.
Furlong et al. (1992) cloned ADORA2A from human hippocampal cDNA library based on its structural homology to members of its receptor family. Le et al. (1996) isolated ADORA2A and determined that the full-length protein contains 412 amino acids.
Le et al. (1996) determined that the ADORA2A gene contains 2 exons.
MacCollin et al. (1994) localized the ADORA2A gene to chromosome 22 both by analysis of cosmid clones from a human chromosome 22 library and by Southern hybridization with a comprehensive somatic cell hybrid panel. Le et al. (1996) used fluorescence in situ hybridization and PCR analysis of human/hamster hybrid cell panels to localize the ADORA2A gene to chromosome 22q11.2. These studies were in contrast to previous reports (subsequently retracted) which mapped the gene to 11q11-q13; see erratum for Libert et al. (1991) and HISTORY section of this entry.
Furlong et al. (1992) transfected ADORA2A into HEK293 cells and found that the A2a agonist CGI21680 stimulated cAMP production but did not alter intracellular calcium concentrations in transfected HEK293 cells.
By yeast 2-hybrid screening of a human brain cDNA library, Canela et al. (2007) found that the C-terminal tail of A2AR bound specifically to the NHR domain of the calcium-binding protein NECAB2 (618130). Pull-down experiments showed that this binding could be inhibited by calcium ions in a dose-dependent manner. Analysis of cotransfected HEK293 cells revealed an overlap in distribution of the 2 proteins in intracellular aggregates and at the plasma membrane level, further corroborating the interaction of NECAB2 and A2AR. Coimmunoprecipitation analysis showed that both isoforms of human NECAB2 interacted with A2AR in transfected HEK293 cells. Investigation of Necab2 in primary cultures of rat striatum neurons confirmed the observation in HEK293 cells and demonstrated that Necab2 and A2ar were codistributed in the same glutamatergic nerve terminals. Further analysis revealed that NECAB2 and A2AR interacted to modulate cell surface expression, ligand-dependent internalization, and receptor-mediated activation of the MAPK pathway.
In the R6/2 mouse model of Huntington disease (HD; 143100), Chou et al. (2005) showed that CGS21680 (CGS) attenuated neuronal symptoms of HD. Subsequently, Chiang et al. (2009) showed that A2a receptors are present in liver and that CGS also ameliorated a urea cycle deficiency by reducing mouse Htt aggregates in the liver. By suppressing aggregate formation, CGS slowed the hijacking of a crucial transcription factor (HSF1; 140580) and 2 protein chaperones, Hsp27 (HSPB1; 602195) and Hsp70 (HSPA1A; 140550), into hepatic Htt aggregates. The abnormally high levels of high-molecular-mass ubiquitin conjugates in the liver of R6/2 mouse model of HD were also ameliorated by CGS. The protective effect of CGS against mouse Htt-induced aggregate formation was reproduced in 2 cell lines and was prevented by an antagonist of the A2a receptor and a protein kinase A (PKA) inhibitor. The mouse Htt-induced suppression of proteasome activity was also normalized by CGS through PKA (PRKACA; 601639).
Lee and Taylor (2013) found that mice lacking Mc5r (600042) had no increase in an antigen-presenting cell (APC) subpopulation that increased in wildtype mice after the emergence of experimental autoimmune uveitis (EAU). This APC subpopulation was required for induction of tolerance mediated by regulatory T cells (Tregs). The Mc5r-dependent APCs found in spleen required expression of A2ar on T cells to activate the EAU-suppressing Tregs. Lee and Taylor (2013) concluded that in the recovery from autoimmune disease, the ocular microenvironment induces tolerance through a melanocortin-mediated expansion of regulatory APCs in spleen that use the adenosinergic pathway to promote activation of autoantigen-specific Tregs.
Crystal Structure
Jaakola et al. (2008) determined the crystal structure of the human A2A adenosine receptor, in complex with a high affinity subtype-selective antagonist, ZM241385, to 2.6-angstrom resolution. Four disulfide bridges in the extracellular domain, combined with a subtle repacking of the transmembrane helices relative to the adrenergic and rhodopsin (180380) receptor structures, define a pocket distinct from that of other structurally determined GPCRs. The arrangement allows for the binding of the antagonist in an extended conformation, perpendicular to the membrane plane. Identification of the binding site highlighted an integral role for the extracellular loops, together with the helical core, in ligand recognition by this class of GPCRs and suggested a role for the high affinity subtype-selective antagonist in restricting the movement of a tryptophan residue important in the activation mechanism of the class A receptors.
Xu et al. (2011) reported the crystal structure of the A2A adenosine receptor (A2AAR) bound to an agonist, UK-432097, at 2.7-angstrom resolution. Relative to inactive, antagonist-bound A2AAR, the agonist-bound structure displays an outward tilt and rotation of the cytoplasmic half of helix VI, a movement of helix V, and an axial shift of helix III, resembling the changes associated with the active-state opsin structure. Additionally, a seesaw movement of helix VII and a shift of extracellular loop 3 are likely specific to A2AAR and its ligand. The results defined the molecule UK-432097 as a 'conformationally selective agonist' capable of receptor stabilization in a specific active-state configuration.
Lebon et al. (2011) presented 2 crystal structures of the thermostabilized human adenosine A2A receptor bound to its endogenous agonist adenosine and the synthetic agonist NECA. The structures represent an intermediate conformation between the inactive and active states, because they share all the features of G protein-coupled receptors that are thought to be in a fully activated state, except that the cytoplasmic end of transmembrane helix 6 partially occludes the G protein-binding site. The adenine substituent of the agonists binds in a similar fashion to the chemically related region of the inverse agonist.
Mass Spectrometry
Using mass spectrometry to identify endogenous lipids bound to 3 G protein-coupled receptors of class A, Yen et al. (2018) observed preferential binding of phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2) over related lipids and confirmed that the intracellular surface of the receptors contain hotspots for PtdIns(4,5)P2 binding. Endogenous lipids were also observed bound directly to the trimeric G-alpha(s)-beta-gamma protein complex (see 139320) of the adenosine A2A receptor (ADORA2A) in the gas phase. Using engineered G-alpha subunits (mini-G-alpha(s), 139320; mini-G-alpha(i), see 139310; and mini-G-alpha(12), 604394), Yen et al. (2018) demonstrated that the complex of mini-G-alpha(s) with the beta-1 adrenergic receptor (ADRB1; 109630) is stabilized by the binding of 2 PtdIns(4,5)P2 molecules. By contrast, PtdIns(4,5)P2 does not stabilize coupling between ADRB1 and other G-alpha subunits (mini-G-alpha(i) or mini-G-alpha(12)) or a high-affinity nanobody. Other endogenous lipids that bind to these receptors had no effect on coupling, highlighting the specificity of PtdIns(4,5)P2.
By homologous recombination, Ledent et al. (1997) disrupted the Adora2a gene in embryonic stem cells of mice and bred homozygous Adora2a-deficient mice. The mice were viable and bred normally. Their exploratory activity was reduced, however, and caffeine, which normally stimulates exploratory behavior, became a depressant of exploratory activity. Knockout animals scored higher in anxiety tests, and male mice were more aggressive toward intruders. Their response to acute pain stimuli was slower. Blood pressure and heart rate were increased, as well as platelet aggregation. The specific A2a agonist CGS21680 lost its biologic activity in all systems tested.
Chen et al. (1999) generated mice deficient in the A2a adenosine receptor and observed that these mice had decreased volume of cerebral infarction and decreased neurologic deficit following transient focal ischemia of the middle cerebral artery when compared with wildtype littermates. This neuroprotective phenotype of A2a receptor-deficient mice was observed in different genetic backgrounds, confirming A2a receptor disruption as its cause.
Ohta and Sitkovsky (2001) demonstrated that A2a adenosine receptors have a nonredundant role in the attenuation of inflammation and tissue damage in vivo. Subthreshold doses of an inflammatory stimulus that caused minimal tissue damage in wildtype mice were sufficient to induce extensive tissue damage, more prolonged and higher levels of proinflammatory cytokines, and death of male animals deficient in the A2a adenosine receptor. Similar observations were made in studies of 3 different models of inflammation and liver damage as well as during bacterial endotoxin-induced septic shock. Ohta and Sitkovsky (2001) suggested that A2a adenosine receptors are a critical part of the physiologic negative feedback mechanism for limitation and termination of both tissue-specific and systemic inflammatory responses.
In a chimeric mouse model, Yu et al. (2004) transplanted wildtype bone marrow cells into Adora2a knockout mice and observed that ischemic brain injury volume was largely reinstated, to 84% of that seen in wildtype mice. Conversely, transplanting wildtype mice with Adora2a knockout bone marrow cells attenuated infarct volumes and ischemia-induced expression of several proinflammatory cytokines in the brain, but exacerbated ischemic liver injury. Yu et al. (2004) concluded that the A2a receptor-stimulated cascade in bone marrow derived cells is an important modulator of ischemic brain injury and that ischemic brain and liver injuries are distinctly regulated by A2a receptors on bone marrow derived cells.
Huang et al. (2005) found that caffeine increased wakefulness in both wildtype and Adora1-null mice, but not in Adora2a-null mice. The findings indicated that caffeine-induced wakefulness depends on adenosine A2a receptors.
Kachroo and Schwarzschild (2012) generated transgenic mice who were null for Adora2a and carried compound pathogenic mutations in the SNCA gene (A53T, 163890.0001 and A30P, 163890.0002), both of which cause Parkinson disease (168601). Knockout of Adora2a completely prevented loss of dopamine and dopaminergic neurons caused by the mutant SNCA without altering levels of SNCA expression. The findings suggested that the Adora2a receptor is required for neurotoxicity in a mutant SNCA model of PD, and indirectly supported the neuroprotective potential of caffeine in PD and possibly ADORA2A antagonists.
By in situ hybridization, Libert et al. (1991) assigned the RDC8 gene to 11q11-q13. Szepetowski et al. (1993) used amplification-based mapping of the 11q13 region to demonstrate that the ADORA2 gene is located in that band proximal to BCL1 (168461). It was found to be in the coamplification group closest to BCL1 in 11q13 along with PPP1A (176875) and GST3 (134660). Physical mapping by hybridization of the same probes to DNA fragments generated by rare-cutting restriction endonucleases and separated by pulsed field gel electrophoresis confirmed the findings. MacCollin et al. (1994) suggested that the assignment to chromosome 11 was in error; they localized the gene to chromosome 22 both by analysis of cosmid clones from a human chromosome 22 library and by Southern hybridization with a comprehensive somatic cell hybrid panel. Libert et al. (1991) and Szepetowski et al. (1993) were clearly mapping the same locus since they used the same RDC8 probe. Although the probe used by MacCollin et al. (1994) was reportedly very similar in sequence, it must in fact have come from a different locus (Gusella, 1994; Gaudray, 1994).
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Chen, J.-F., Huang, Z., Ma, J., Zhu, J., Moratalla, R., Standaert, D., Moskowitz, M. A., Fink, J. S., Schwarzschild, M. A. A2A adenosine receptor deficiency attenuates brain injury induced by transient focal ischemia in mice. J. Neurosci. 19: 9192-9200, 1999. [PubMed: 10531422] [Full Text: https://doi.org/10.1523/JNEUROSCI.19-21-09192.1999]
Chiang, M.-C., Chen, H.-M., Lai, H.-L., Chen, H.-W., Chou, S.-Y., Chen, C.-M., Tsai, F.-J., Chern, Y. The A(2A) adenosine receptor rescues the urea cycle deficiency of Huntington's disease by enhancing the activity of the ubiquitin-proteasome system. Hum. Molec. Genet. 18: 2929-2942, 2009. [PubMed: 19443488] [Full Text: https://doi.org/10.1093/hmg/ddp230]
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Lee, D. J., Taylor, A. W. Both MC5r and A2Ar are required for protective regulatory immunity in the spleen of post-experimental autoimmune uveitis in mice. J. Immun. 191: 4103-4111, 2013. [PubMed: 24043903] [Full Text: https://doi.org/10.4049/jimmunol.1300182]
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Szepetowski, P., Perucca-Lostanlen, D., Gaudray, P. Mapping genes according to their amplification status in tumor cells: contribution to the map of 11q13. Genomics 16: 745-750, 1993. [PubMed: 8325649] [Full Text: https://doi.org/10.1006/geno.1993.1257]
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Yen, H.-Y., Hoi, K. K., Liko, I., Hedger, G., Horrell, M. R., Song, W., Wu, D., Heine, P., Warne, T., Lee, Y., Carpenter, B., Pluckthun, A., Tate, C. G., Sansom, M. S. P., Robinson, C. V. PtdIns(4,5)P2 stabilizes active states of GPCRs and enhances selectivity of G-protein coupling. Nature 559: 423-427, 2018. [PubMed: 29995853] [Full Text: https://doi.org/10.1038/s41586-018-0325-6]
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