Role Of A2A Receptors Research Articles

Molecular and Structural Insight into Adenosine A2A Receptor in Neurodegenerative Disorders: A Significant Target for Efficient Treatment Approach.

All biological tissues and bodily fluids include the autacoid adenosine. The P1 class of purinergic receptors includes adenosine receptors. Four distinct G-protein-coupled receptors on the cellular membrane mediate the effects of adenosine, whose cytoplasmic content is regulated by producing/degrading enzymes and nucleoside transporters. A2A receptor has received a great deal of attention in recent years because it has a wide range of potential therapeutic uses. A2B and, more significantly, A2A receptors regulate numerous physiological mechanisms in the central nervous system (CNS). The inferior targetability of A2B receptors towards adenosine points that they might portray a promising medicinal target since they are triggered only under pharmacological circumstances (when adenosine levels rise up to micromolar concentrations). The accessibility of specific ligands for A2B receptors would permit the exploration of such a theory. A2A receptors mediate both potentially neurotoxic and neuroprotective actions. Hence, it is debatable to what extent they play a role in neurodegenerative illnesses. However, A2A receptor blockers have demonstrated clear antiparkinsonian consequences, and a significant attraction exists in the role of A2A receptors in other neurodegenerative disorders. Amyloid peptide extracellular accumulation and tau hyperphosphorylation are the pathogenic components of AD that lead to neuronal cell death, cognitive impairment, and memory loss. Interestingly, in vitro and in vivo research has shown that A2A adenosine receptor antagonists may block each of these clinical symptoms, offering a crucial new approach to combat a condition for which, regrettably, only symptomatic medications are currently available. At least two requirements must be met to determine whether such receptors are a target for diseases of the CNS: a complete understanding of the mechanisms governing A2A-dependent processes and the availability of ligands that can distinguish between the various receptor populations. This review concisely summarises the biological effects mediated by A2A adenosine receptors in neurodegenerative disorders and discusses the chemical characteristics of A2A adenosine receptor antagonists undergoing clinical trials. Selective A2A receptor blocker against neurodegenerative disorders.

Upregulation of Src Family Tyrosine Kinases in the Rat Striatum by Adenosine A2A Receptors.

Adenosine A2A receptors are Golf-coupled receptors and are predominantly expressed in the striatum of mammalian brains. As a mostly postsynaptic receptor, A2A receptors are implicated in the regulation of a variety of intracellular signaling pathways in striatopallidal output neurons and are linked to the pathogenesis of various neuropsychiatric and neurological disorders. This study investigated the possible role of A2A receptors in the modulation of the Src family kinase (SFK) in the adult rat striatum. In acutely prepared striatal slices, adding the A2A receptor agonist PSB-0777 induced a significant increase in phosphorylation of SFKs at a conserved autophosphorylation site (Y416) in the caudate putamen (CPu). This increase was also seen in the nucleus accumbens (NAc). Another A2A agonist CGS-21680 showed the similar ability to elevate SFK Y416 phosphorylation in the striatum. Treatment with the A2A receptor antagonist KW-6002 blocked the effect of PSB-0777 on SFK Y416 phosphorylation. In addition, PSB-0777 enhanced kinase activity of two key SFK members (Src and Fyn) immunoprecipitated from the striatum. These data demonstrate a positive linkage from A2A receptors to the SFK signaling pathway in striatal neurons. Activation of A2A receptors leads to the upregulation of phosphorylation of SFKs (Src and Fyn) at an activation-associated autophosphorylation site and kinase activity of these SFK members.

Adenosine 2A receptor inhibition protects phrenic motor neurons from cell death induced by protein synthesis inhibition

Respiratory motor neuron survival is critical for maintenance of adequate ventilation and airway clearance, preventing dependence to mechanical ventilation and respiratory tract infections. Phrenic motor neurons are highly vulnerable in rodent models of motor neuron disease versus accessory inspiratory motor pools (e.g. intercostals, scalenus). Thus, strategies that promote phrenic motor neuron survival when faced with disease and/or toxic insults are needed to help preserve breathing ability, airway defense and ventilator independence. Adenosine 2A receptors (A2A) are emerging as a potential target to promote neuroprotection, although their activation can have both beneficial and pathogenic effects. Since the role of A2A receptors in the phrenic motor neuron survival/death is not known, we tested the hypothesis that A2A receptor antagonism promotes phrenic motor neuron survival and preserves diaphragm function when faced with toxic, neurodegenerative insults that lead to phrenic motor neuron death. We utilized a novel neurotoxic model of respiratory motor neuron death recently developed in our laboratory: intrapleural injections of cholera toxin B subunit (CtB) conjugated to the ribosomal toxin, saporin (CtB-Saporin). We demonstrate that intrapleural CtB-Saporin causes: 1) profound phrenic motor neuron death (~5% survival); 2) ~7-fold increase in phrenic motor neuron A2A receptor expression prior to cell death; and 3) diaphragm muscle paralysis (inactive in most rats; ~7% residual diaphragm EMG amplitude during room air breathing). The A2A receptor antagonist istradefylline given after CtB-Saporin: 1) reduced phrenic motor neuron death (~20% survival) and 2) preserved diaphragm EMG activity (~46%). Thus, A2A receptors contribute to neurotoxic phrenic motor neuron death, an effect mitigated by A2A receptor antagonism.

Abstract 523: Implications of the A2a receptor (A2aR) on tumor microenvironment in non-small cell lung cancer (NSCLC)

Abstract Hypoxia-driven adenosine accumulation in tumor microenvironment is believed to play an important role in tumor evasion of immune response. Adenosine via interaction with its A2a receptor, expressed on immune cells, acts as a negative regulator of cytotoxic T-cell immune activity. Increased A2aR expression was observed in tumor infiltrating lymphocytes in freshly resected NSCLC specimens. Higher expression of A2aR was associated with worse prognostic findings in head and neck squamous cell carcinoma. Conversely, it correlated with favorable clinical outcome in non-metastatic lung adenocarcinoma. It has been hypothesized that A2aR antagonism could lead to enhanced efficacy for immune checkpoint blockade. A2aR antagonists have demonstrated favorable in vitro and ex vivo response in NSCLC and ongoing clinical trials are testing these drugs in conjunction with established immunotherapy. However, little is known about the role of A2aR in NSCLC patients. We investigated how A2aR expression affected tumor immune landscape, PD-L1 expression and clinical outcomes in a large pool of NSCLC patients. Methods: We obtained A2aR expression for NSCLC patients from TCGA; adenocarcinoma (ADC), n=586; squamous cell carcinoma (SCC), n=517. The data was arranged into 4 quartiles according to A2aR expression mRNA-seq z-scores, defining the highest quartile as A2aR-high and the lowest quartile as A2aR-low. We evaluated how A2aR expression correlated with a) immune landscape, b) PD-L1 expression, c) tumor mutational burden (TMB) and neoantigen burden, and d) clinical outcome. Results: In ADC, A2aR-high was significantly associated with lower infiltration of activated CD4 and CD8 T cells when compared with A2aR-low patients (12% vs 48% ; 9% vs 47%, all p<0.001). In SCC, similar results were observed (12% vs 61%; 9% vs 52%, all p<0.001). In addition, higher PD-L1 (CD274) expression was observed in A2aR-high ADC (p<0.05). There was no difference on TMB or neoantigen burden between groups. In ADC, A2aR-high was associated with more favorable overall survival (OS) and progression free survival when compared with A2aR-low (median 71 vs 41 months and 84 vs 27 months, respectively, all p<0.05). An inverse trend was noted in SCC, with A2aR-high patients having significantly shorter OS (median 39 vs 71 months, p<0.05). Conclusion: Higher A2aR expression was associated with lower activated CD4 and CD8 T-cells in NSCLC and higher PD-L1 expression in lung ADC, suggesting a potential target for developing new strategies for immunotherapy in NSCLC. Citation Format: Pedro Viveiros, Bhoomika Sukhadia, Pratyusha Nunna, Keon Woo Park, Jeffrey Chuang, Lihua Zou, Young Kwang Chae. Implications of the A2a receptor (A2aR) on tumor microenvironment in non-small cell lung cancer (NSCLC) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 523.

Adenosine A2A receptor deletion affects social behaviors and anxiety in mice: Involvement of anterior cingulate cortex and amygdala

Blockade of adenosine A2A receptors can potentiate motivation to work for natural reinforcers such as food. Conspecific interaction is a potent natural reinforcer in social animals that can be manifested as preference for social exploration versus other sources of novel stimulation. Deficiencies in this type of motivated behavior (social withdrawal) have been seen in several pathologies such as autism and depression. However, the role of A2A receptors in motivation for social interaction has not been widely explored. Social interaction paradigms evaluate the natural preference of animals for exploring other conspecifics, and the ability to differentiate between familiar versus novel ones. Anxiety is one of the factors that can induce avoidance of social interaction. In the present study, adenosine A2A knockout (A2AKO) and wild-type (WT) mice were assessed for social and anxiety-related behaviors. c-Fos immunoreactivity was evaluated as a measure of neuronal activation in brain areas involved in different aspects of motivation and emotional processes. Although A2AKO mice showed an anxious profile, they displayed higher levels of sociability and were less sensitive to social novelty. WT mice displayed a typical pattern of social recognition 24h later, but not A2AKO mice, which explored equally both conspecifics. There were no differences between strains in aggressiveness, perseverance or social odor preferences. c-Fos immunoreactivity in A2AKO mice was higher in anterior cingulate and amygdala compared to WT mice. Thus, A2A receptors appear to be potential targets for the improvement of pathologies related to social function.

Adenosine A2A receptor promotes collagen type III synthesis via β-catenin activation in human dermal fibroblasts.

Adenosine A2A receptor stimulation promotes the synthesis of collagen type I and type III (Col1 and Col3), mediators of fibrosis and scarring. The A2A receptor modulates collagen balance via cAMP/PKA/p38-MAPK/Akt pathways. Wnt signalling is important in fibrosis and the cAMP and Wnt pathways converge. Because the A2A receptor is Gs-linked and increases cAMP, we determined whether A2A receptors and Wnt signalling interact. Total β-catenin, de-phosphorylated β-catenin (canonical activation, de-phospho β-catenin) and phosphorylated β-catenin at Ser552 (non-canonical activation, p-Ser552 β-catenin) levels were determined in primary human dermal fibroblasts, cytosol and nucleus, by western blot analysis and fluorescence microscopy, before and after stimulation by A2A receptor-selective agonist CGS21680, with/without A2A receptor-selective antagonist (SCH56261) pretreatment. β-Catenin was knocked down by transfection with scrambled-siRNA or specific-siRNA, and Col1 and Col3 levels determined by western blots. CGS21680 stimulation rapidly (15min) increased cellular β-catenin levels. Both de-phospho β-catenin and p-Ser552 β-catenin levels were also increased. CGS21680 stimulated the translocation of total de-phospho and p-Ser552 β-catenin to the nucleus. A2A receptor-stimulation increased Col1 synthesis similarly in β-catenin knockeddown and scrambled cells. However, β-catenin knockdown abolished the increase in Col3 synthesis induced in A2A receptor-stimulated fibroblasts. A2A receptor stimulation promotes Col3 synthesis via the activation of canonical and non-canonical β-catenin, consistent with a role for A2A receptors in dermal fibrosis and scarring.

Abstract 062: Up-regulation Of Adenosine A2a Receptor Contributes To Epoxyeicosatrienoic Acids-induced Improvement In Endothelial Function And Decreases In Renal Inflammation In Obese Mice

Epoxyeicosatrienoic acids (EETs) and adenosine activation of A2A receptor have anti-inflammatory properties and play a role in the regulation of vascular tone. During salt loading, renal response to A2A receptor activation is exacerbated via increased EETs production. However, potential EETs activation of adenosine signaling remains elusive. We hypothesize that increased EETs levels, via inhibition of soluble epoxide hydrolase, (sEH), improve endothelial function in type 2 diabetes through the up-regulation of A2A receptor to increase NO production. Obese db/db mice were treated with the sEH inhibitor t-AUCB (10 mg/L) alone or plus the A2A receptor antagonist SCH58261 (10 mg/kg, biweekly) for 12 weeks. Inhibition of sEH up-regulated renal A2A receptor expression and improved aortic ring relaxation to acetylcholine in obese db/db mice compared to lean (p<0.05). Administration of t-AUCB also reduced renal T-cell infiltration and apoptosis and increased renal eNOS expression in db/db mice. Co-administration of SCH58261 with t-AUCB reduced t-AUCB’s ability to improve aortic ring relaxation to acetylcholine, decreased renal eNOS expression and increased renal T-cell infiltration and apoptosis in db/db mice. Inhibition of sEH also restored the decrease in renal nitrate/nitrite and cGMP levels in db/db mice (renal nitrate/nitrite was 6.7±1 in db/db plus t-AUCB vs. 3.6± 0.7 nmol/mg in db/db and renal cGMP was 3.5± 0.4 in db/db plus t-AUCB vs. 2 ± 0.2 pmol/mg in db/db, p<0.05) and these changes were decreased by co-administration of SCH58261. To further explore the role of A2A receptor in EETs-induced improvement in endothelial function and decreases in renal inflammation, wild type control (WT) and A2A knock-out (KO) mice were fed high fat diet (HF) for 12 weeks with or without t-AUCB treatment. A2A KO mice fed HF displayed lesser aortic endothelial relaxation to acetylcholine and a decrease in renal eNOS expression compared to HF fed WT mice and inhibition of sEH improved aortic ring relaxation to acetylcholine in WT but not in A2A KO mice fed HF. These data suggest that EETs up-regulation of A2A receptor improves endothelial function and decreases inflammation via eNOS-dependent activation of NO production.

Chapter Fifteen - Involvement of Adenosine A2A Receptors in Depression and Anxiety

When administered to normal healthy patients, a nonselective adenosine A1/A2A antagonist, caffeine, tended to improve anxiety and depression at low doses and to exacerbate anxiety at high doses. Caffeine also appears to enhance anxiety-related symptoms in patients with panic disorder, and A2A receptor-deficient mice have been reported to exhibit higher anxiety-like behaviors, as well as a lower incidence of depression-like behaviors. Some selective A2A antagonists were reported to ameliorate anxiety-like behaviors in rodents, while others did not affect these behaviors. In addition, most A2A antagonists showed inhibitory effects on depression-like behaviors. The mechanisms underlying the relationship between A2A receptor antagonists and anxiety and depression remain unclear at the present time, although many studies have produced hypotheses. Given that a selective A2A receptor antagonist has recently become available for use in humans, research on the role of A2A receptors in the treatment of mental illness should progress in the near future.

Distribution of the vegetative nervous system at the level of the right ventricular outflow tract in a murine model

increase of A2A mRNA expression levels ( CTA2A 16,8±2.72 vs. 11,2±2.64; p<0.001), and a steady maintenance of the A1 mRNA levels. No differences between right and left atrium were observed. Conclusions: Sustained atrial high-rate stimulation induces an early overexpression of A2A receptors, suggesting an impairment of the cross-talk between atrial A1 and A2A receptors. This can be associated with a role for A2A receptors in atrial Ca2+ kinetics and atrial remodeling.

Overexpression of atrial adenosine2A receptors induced by high-rate pacing stimulation: a new step in the molecular remodeling of the atria

Recently, an association between atrial fibrillation (AF) and a local increase in sarcoplasmic calcium release was suggested as a mechanism able to generate afterdepolarizations and promote arrhythmias recurrence. The adenosine A2A receptors (A2A) are expressed in the atria, modulate the frequency of spontaneous calcium release, and may be involved in the remodeling process during AF. Aim: To investigate the role of atrial adenosine receptors by analyzing A1 and A2A receptor protein levels through changes in their transcriptional control during atrial high-rate. Methods: The high-right atrium of anesthetized, artificially ventilated rats (n=10; age 12 weeks) was paced epicardially for 2 hours (50Hz). Animals were sacrificed and separate samples of the atria collected for A1 and A2A mRNA Real Time PCR quantification. The housekeeping gene 18SrRNA was used as internal control for the quantification of receptors mRNA. A sham group was used as control (matching number, age and sex). CT values were calculated for each sample, and converted into relative quantities with the ΔCTmethod (Eq.1: ΔCT=CTAdora-CT18SrRNA; Eq.2: ΔΔCT=ΔCTcontrols- ΔCTStimulation). The effects of high-rate pacing were evaluated by one way analysis of variance (significance if P<0.05). Results: After 2 hours of high-rate pacing, real time PCR showed a significant increase of A2A mRNA expression levels (ΔCTA2A 16,8±2.72 vs. 11,2±2.64; p<0.001), and a steady maintenance of the A1 mRNA levels. No differences between right and left atrium were observed. ![Figure][1] Figure 1. Adenosine receptors remodeling Conclusions: Sustained atrial high-rate stimulation induces an early overexpression of A2A receptors, suggesting an impairment of the cross-talk between atrial A1 and A2A receptors. This can be associated with a role for A2A receptors in atrial Ca2+ kinetics and atrial remodeling. [1]: pending:yes

Cannabidiol provides long-lasting protection against the deleterious effects of inflammation in a viral model of multiple sclerosis: A role for A2A receptors

Inflammation in the central nervous system (CNS) is a complex process that involves a multitude of molecules and effectors, and it requires the transmigration of blood leukocytes across the blood–brain barrier (BBB) and the activation of resident immune cells. Cannabidiol (CBD), a non-psychotropic cannabinoid constituent of Cannabis sativa, has potent anti-inflammatory and immunosuppressive properties. Yet, how this compound modifies the deleterious effects of inflammation in TMEV-induced demyelinating disease (TMEV-IDD) remains unknown. Using this viral model of multiple sclerosis (MS), we demonstrate that CBD decreases the transmigration of blood leukocytes by downregulating the expression of vascular cell adhesion molecule-1 (VCAM-1), chemokines (CCL2 and CCL5) and the proinflammatory cytokine IL-1β, as well as by attenuating the activation of microglia. Moreover, CBD administration at the time of viral infection exerts long-lasting effects, ameliorating motor deficits in the chronic phase of the disease in conjunction with reduced microglial activation and pro-inflammatory cytokine production. Adenosine A2A receptors participate in some of the anti-inflammatory effects of CBD, as the A2A antagonist ZM241385 partially blocks the protective effects of CBD in the initial stages of inflammation. Together, our findings highlight the anti-inflammatory effects of CBD in this viral model of MS and demonstrate the significant therapeutic potential of this compound for the treatment of pathologies with an inflammatory component.

Potential Therapeutic Relevance of Adenosine A2B and A2A Receptors in the Central Nervous System

Adenosine A2B and, much more importantly, adenosine A2A receptors modulate many physiological and pathological processes in the brain. In this review, the most recent evidence concerning the role of such receptors and their potential therapeutic relevance is discussed. The low affinity of A2B receptors for adenosine implies that they might represent a good therapeutic target, since they are activated only under pathological conditions (when adenosine levels raise up to micromolar concentrations). The availability of selective ligands for A2B receptors would allow exploration of such an hypothesis. Since adenosine A2A receptors mediate both potentially neuroprotective and potentially neurotoxic effects, their role in neurodegenerative diseases is highly controversial. Nevertheless, A2A receptor antagonists have shown clear antiparkinsonian effects, and a great interest exists on the role of A2A receptors in Alzheimer's disease, brain ischaemia, spinal cord injury, drug addiction and other conditions. In order to establish whether such receptors represent a target for CNS diseases, at least two conditions are needed: the full comprehension of A2A-dependent mechanisms and the availability of ligands capable of discriminating among the different receptor populations.

Absence of adenosine-mediated aortic relaxation in A2A adenosine receptor knockout mice

Adenosine mediates vascular responses through four receptor subtypes: A(1), A(2A), A(2B), and A(3). The role of A(2A) receptors in aortic vascular tone was investigated using A(2A) adenosine receptor (AR) knockout (A(2A)KO) and corresponding wild-type (A(2A)WT) mice. Isolated aortic rings from A(2A)WT and A(2A)KO mice were precontracted with phenylephrine (10(-7) M), and concentration responses for adenosine analogs and selective agonists/antagonists were obtained. Nonselective adenosine analog (NECA; EC(50) = 6.78 microM) and CGS-21680 (A(2A)AR selective agonist; EC(50) = 0.013 microM) produced concentration-dependent relaxation (maximum of 25% and 28% relaxation at 10(-5) M NECA and CGS-21680, respectively) in A(2A)WT aorta. In A(2A)KO aorta, NECA (EC(50) = 0.075 microM) induced concentration-dependent contraction (maximum contraction of 47% at 10(-6) M; P < 0.05 compared with A(2A)WT), whereas CGS-21680 produced no response. SCH-58261 (10(-6) M; A(2A)AR selective antagonist) abolished both NECA- and CGS-21680-mediated vasorelaxation in A(2A)WT (P < 0.05), whereas no change was observed in A(2A)KO. When DPCPX (10(-5) M; A(1) selective antagonist) was used in NECA concentration response, greater vasorelaxation was observed in A(2A)WT (50% vs. 25% in controls at 10(-5) M; P < 0.05), whereas lower contraction was seen in A(2A)KO tissues (5% vs. 47% in controls at 10(-6) M; P < 0.05). Aortic endothelial function, determined by response to acetylcholine, was significantly higher in WT compared with KO (66% vs. 51%; P < 0.05). BAY 60-6583 (A(2B) selective agonist) produced similar relaxation in both KO and WT tissues. In conclusion, A(2A)AR KO mice had significantly lower aortic relaxation and endothelial function, suggesting that the A(2A)AR plays an important role in vasorelaxation, probably through an endothelium-dependent mechanism.

Adenosine receptor antagonists for cognitive dysfunction: a review of animal studies

Over the last decade, adenosine receptors in the central nervous system have been implicated in the modulation of cognitive functions. Despite the general view that endogenous adenosine modulates cognition through the activation of adenosine A1 receptors, evidence is now emerging on a possible role of A2A receptors in learning and memory. The present review attempts to examine results reported in different studies using diverse animal models, to provide a comprehensive picture of the recent evidence of a relationship between adenosinergic function and memory deficits. The present data suggest that caffeine (a nonselective adenosine receptor antagonist) and selective adenosine A2A receptor antagonists can improve memory performance in rodents evaluated through different tasks. They might also afford protection against memory dysfunction elicited in experimental models of aging, Alzheimer's disease, Parkinson's disease and, in spontaneously hypertensive rats (SHR), a putative genetic model of attention deficit hyperactivity disorder (ADHD).

Absence of quasi-morphine withdrawal syndrome in adenosine A2A receptor knockout mice

Caffeine and other methylxanthines induce behavioral activation and anxiety responses in mice via antagonist action at A2A adenosine receptors. When combined with the opioid antagonist naloxone, methylxanthines produce a characteristic quasi-morphine withdrawal syndrome (QMWS) in opiate-naive animals. The aim of this study was to establish the role of A2A receptors in the quasi-morphine withdrawal syndrome induced by co-administration of caffeine and naloxone and in the behavioral effects of caffeine. We have used A2A receptor knockout (A(2A)R(-/-)) mice in comparison with their wild-type and heterozygous littermates to measure locomotor activity in the open field and withdrawal symptoms induced by caffeine and naloxone. Naïve wild-type and knockout mice were also examined for enkephalin and dynorphin mRNA expression by in situ hybridization and for mu-opiate receptor by ligand binding autoradiography to check for possible opiate receptor changes induced by A2A receptor inactivation. Caffeine increases locomotion and anxiety in wild-type animals, but it has no psychomotor effects in A(2A)R(-/-) mice. Co-administration of caffeine (20 mg/kg) and naloxone (2 mg/kg) resulted in a severe quasi-morphine withdrawal syndrome in wild-type mice that was almost completely abolished in A(2A)R(-/-) mice. Heterozygous animals exhibited a 40% reduction in withdrawal symptoms, suggesting that there is no genetic/developmental compensation for the inactivation of one of the A(2A)R alleles. A(2A)R(-/-) and wild-type mice have similar levels of striatal mu-opioid receptors, thus the effect is not due to altered opioid receptor expression. Our results demonstrate that A2A receptors are required for the induction of quasi-morphine withdrawal syndrome by co-administration of caffeine and naloxone and implicate striatal A2A receptors and mu-opiate receptors in tonic inhibition of motor activity in the striatum.

Physiological regulation of acute inflammation by A2A adenosine receptor

The activation of Gs-coupled A2A adenosine receptor on the surface of immune cells increases levels of immunosuppressive cyclic AMP and leads to inhibition of secretion of proinflammatory cytokines, which, in turn, results in downregulation of acute inflammation. The role of A2A receptors in the regulation of inflammation appears to be non-redundant. Small doses of inflammatory stimulus induce high levels of proinflammatory cytokines and extensive tissue injury in A2A receptor-deficient (Adora2a−/−) mice leading to death due to unregulated inflammatory damage. The gene-dose effect of A2A receptor expression in T cells suggests that there are no spare A2A receptors and that the final outcome of immune cell activation may be dependent on the number of these receptors on the individual immune cells. Heterozygous (Adora2a+/−) mice express decreased levels of A2A receptor on the surface and adenosine-induced cAMP-accumulation of immune cells was lower than in wild-type. Thus, the number of A2A receptors determines cAMP response to adenosine and may pre-determine the immunosuppressive role of adenosine. Drug Dev Res 64:172–177, 2005. © 2005 Wiley-Liss, Inc.

Modulation of GABAergic transmission in the striatopallidal system by adenosine A2A receptors: a potential mechanism for the antiparkinsonian effects of A2A antagonists.

The selective localization of adenosine A2A receptors to the striatopallidal system suggested a new therapeutic approach to the management of Parkinson's disease (PD). The results of behavioral studies using A2A receptor-specific agents in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys highlight the therapeutic potential of A2A antagonists as a novel treatment for PD. However, little is known about the role of A2A receptors in basal ganglia function or their pathophysiologic role in PD. Recently, the authors found that presynaptic A2A receptors modulate GABAergic synaptic transmission in the striatum and globus pallidus (GP), suggesting an A2A receptor-mediated dual modulation of the striatopallidal system. Striatal A2A receptors may increase the excitability of medium spiny neurons (MSNs) by modulating an intrastriatal GABAergic network. In addition, pallidal modulation occurs at striatopallidal MSN terminals located at the GP, enhancing GABA release onto GP projection neurons and directly suppressing their activity. Blockade of these modulatory functions by A2A antagonists could counteract excessive striatopallidal neuronal activity provoked by striatal dopamine depletion in patients with PD, leading to a reversal of parkinsonian motor deficits.

Limitation of myocardial reperfusion injury by AMP579, an adenosine A1/A2A receptor agonist: role of A2A receptor and Erk1/2.

AMP579, an adenosine A(1)/A(2A) receptor agonist, protects against myocardial infarction when given at the onset of reperfusion. However, it is unclear which receptor subtype mediates its protective actions. Anaesthetised rabbits were subjected to 30 min regional ischaemia/180 min reperfusion in vivo. AMP579 (30 microg kg(-1) bolus + 3 microg kg(-1) min(-1) for 70 min) reduced heart rate and mean arterial blood pressure with the latter being abolished with ZM241385 (a selective A(2A) receptor antagonist). AMP579 reduced infarct size from 46.0 +/- 3.4% in vehicle control hearts to 29.6 +/- 3.5% (P < 0.05), an effect that was attenuated in the presence of ZM241385, in a dose-dependent manner (38.2 +/- 4.9% at 1 mg kg(-1); 45.1 +/- 4.2% at 2.5 mg kg(-1)). CGS21680 (a selective A(2A) agonist, 30 microg kg(-1) bolus + 3 microg kg(-1) min(-1) for 70 min), or CCPA (a selective A(1) agonist, 50 microg kg(-1)), alone or in combination showed no protection (44.7 +/- 5.8%; 39.8 +/- 2.8%; 39.1 +/- 5.1%, respectively) when given at the commencement of reperfusion. Furthermore, we hypothesized that the prosurvival MEK1/2-Erk1/2 pathway was involved in the downstream mechanism of cardioprotection afforded by AMP579. PD098059, an inhibitor of MEK1/2 showed a dose dependent attenuation on infarct size (39.9 +/- 5.3% at 2 mg kg(-1); 48.3 +/- 5.7% at 4 mg kg(-1), i.v., respectively). PD098059 alone had no effect on infarct size (44.7 +/- 5.8%, 2 mg kg(-1), i.v.). We conclude that AMP579 limits myocardial infarction by activating A(2A) adenosine receptors that might be linked to further downstream kinases such as Erk1/2.

A Dual Role of Adenosine A2AReceptors in 3-Nitropropionic Acid-Induced Striatal Lesions: Implications for the Neuroprotective Potential of A2AAntagonists

Reduction of A2A receptor expression is one of the earliest events occurring in both Huntington's disease (HD) patients and mice overexpressing the N-terminal part of mutated huntingtin. Interestingly, increased activity of A2A receptors has been found in striatal cells prone to degenerate in experimental models of this neurodegenerative disease. However, the role of A2A receptors in the pathogenesis of HD remains obscure. In the present study, using A2A-/- mice and pharmacological compounds in rat, we demonstrate that striatal neurodegeneration induced by the mitochondrial toxin 3-nitropropionic acid (3NP) is regulated by A2A receptors. Our results show that the striatal outcome induced by 3NP depends on a balance between the deleterious activity of presynaptic A2A receptors and the protective activity of postsynaptic A2A receptors. Moreover, microdialysis data demonstrate that this balance is anatomically determined, because the A2A presynaptic control on striatal glutamate release is absent within the posterior striatum. Therefore, because blockade of A2A receptors has differential effects on striatal cell death in vivo depending on its ability to modulate presynaptic over postsynaptic receptor activity, therapeutic use of A2A antagonists in Huntington's as well as in other neurodegenerative diseases could exhibit undesirable biphasic neuroprotective-neurotoxic effects.

Occupancy of adenosine A2a receptors promotes fMLP-induced cyclic AMP accumulation in human neutrophils: impact on phospholipase D activity and recruitment of small GTPases to membranes

The aim of this study was to assess in human neutrophils the implication of an adenosine 3',5'-cyclic monophosphate (cAMP)-dependent pathway in the inhibitory effects of A2a receptor engagement. We found that Ro20-1724, a cAMP phosphodiesterase inhibitor, in the presence of adenosine deaminase (ADA) or A2a receptor antagonists rendered transient the fMLP-induced sustained increases in cAMP levels. The role of A2a receptor stimulation was demonstrated by the ability of the A2a receptor agonist, CGS21680, to prevent ADA-mediated reduction of the persistent cAMP elevation induced by fMLP. Persistent cAMP elevation correlated with inhibition of fMLP-induced PLD activation and recruitment of Arf, RhoA, and PKC to membranes. The suppressive effect of CGS21680 or isoproterenol, a beta-adrenergic receptor agonist, was increased by Ro20-1724 or by the adenylyl cyclase activator, forskolin, and reversed, at least in part, by the inhibitor of adenylyl cyclase, 2',5'-dideoxyadenosine. The activator of protein kinase A (PKA), Sp-cAMP inhibited fMLP-induced PLD activation and translocation of Arf and RhoA to membranes. In contrast, the suppression by A2a receptor stimulation of fMLP-induced PLD activation and cofactor recruitment was antagonized by PKA inhibitors, Rp-cAMP and H89. In conclusion, A2a receptor occupancy by extracellular adenosine inhibits fMLP-induced neutrophil activation via cAMP and PKA-regulated events.