Optimization of the human adenosine A2a receptor yields in Saccharomyces cerevisiae

G-protein coupled receptors (GPCRs) have been implicated in many human diseases and have emerged as important drug targets. Despite their medical relevance, knowledge about GPCR structure is limited, mainly due to difficulties associated with producing large amounts of functional protein and isolating this protein in functional form. However, our previous results indicate that when the human adenosine A(2)a receptor (A(2)aR) is expressed in Saccharomyces cerevisiae, high yields can be achieved. In light of these initial results and in anticipation of future purification efforts, experiments were conducted to optimize the system for maximum total protein yield. Emphasis was placed on not only producing large quantities of A(2)aR in each cell but also achieving high cell density in batch culture. Therefore, temperature, media pH, inducer concentration in the media, and induction cell density were tested for their effects on both cell growth (as measured by optical density, OD(600)) and per cell A(2)aR expression levels. For these studies, the A(2)aR expression levels were determined using a previously described A(2)aR-green fluorescent protein (GFP) fusion, so that expression could be monitored by fluorescence. Overall the data indicate that at late times ( approximately 60 h of expression) approximately 75% higher total batch protein yields can be achieved using lower expression temperatures or 60% higher using elevated induction cell density. The highest yields correspond to approximately 28 mg per liter of culture of total A(2)aR. Amounts of functional receptor were shown to increase on a per cell basis by decreasing expression temperature up to 25 h of expression, but at late time points ( approximately 60 h) functional yields did not appreciably improve. When compared to other reports of GPCR expression in yeast it is clear that this system is among those producing the highest GPCR protein yields per culture both before and after optimization.

Influence of haemodialysis and left ventricular failure on peripheral A2A adenosine receptor expression

BACKGROUND

Haemodialysis (HD) sometimes accelerates left ventricular failure (LVF). As adenosine (ADO) is strongly implicated in cardiovascular functions, particularly via A(2A) receptor activation and as changes of peripheral A(2A) receptors mirror changes occurring in the cardiovascular system, we examined the influence of HD and LVF on both ADO plasma concentration and the expression of A(2A) receptors (i.e. Bmax, K(D) and mRNA amount) of peripheral blood mononuclear cells.

METHODS

This cross-sectional study included 61 chronic renal failure (CRF) patients: 41 without LVF (24 haemodialysed and 17 undialysed) and 20 with LVF (9 haemodialysed and 11 undialysed). Ten LVF patients without CRF and 10 healthy subjects were also examined.

RESULTS

(i) Bmax values of CRF patients without LVF were significantly decreased in undialysed patients compared with haemodialysed patients, and compared with controls (69 +/- 25 vs 98 +/- 33 vs 180 +/- 60 fmol/mg of protein, P < 0.05). Bmax values of CRF patients with LVF were lower in undialysed patients than in haemodialysed patients (60 +/- 27 vs 101 +/- 27 fmol/mg of protein, P < 0.05). Bmax values of LVF patients without CRF were lower than in controls (51 +/- 19 vs 180 +/- 60 fmol/mg of protein). (ii) A(2A) mRNA expression was increased in haemodialysed patients compared with controls (20.2 +/- 0.75 vs 17.6 +/- 1.3, P < 0.05). (iii) ADO plasma levels were high in haemodialysed patients and further increased during the HD sessions.

CONCLUSION

The number of A(2A) receptors was decreased by CRF with or without LVF. However, this decrease was less important in haemodialysed patients. The changes in peripheral A(2A) receptor expression suggest a significant inflammatory response to HD and heart or kidney failure. Whether these changes do reflect alterations in cardiomyocytes needs further investigation.

IL-1 beta and TNF-alpha regulation of the adenosine receptor (A2A) expression: differential requirement for NF-kappa B binding to the proximal promoter

Adenosine is a potent endogenous regulator of airway inflammation that acts through specific receptor subtypes that can either cause constriction (A1R, A2BR, and A3R) or relaxation (A2AR) of the airways. We therefore examined the effects of key inflammatory mediators on the expression of the A2AR in a lung epithelial cell line (A549). IL-1beta and TNF-alpha increased the expression of the A2AR gene at the mRNA and protein levels. In contrast, LPS had no effect on A2AR gene expression. IL-1beta and TNF-alpha rapidly activated p50 and p65, but not C-Rel, RelB, or p52, and both IL-1beta- and TNF-alpha-stimulated A2AR expression was inhibited by the IkappaB kinase 2 inhibitor AS602868 in a concentration-dependent manner. Using chromatin immunoprecipitation assays, we demonstrate that IL-1beta can enhance p65 association with putative kappaB binding sites in the A2AR promoter in a temporal manner. In contrast, TNF-alpha failed to enhance p65 binding to these putative sites. Functionally, the two most 5' kappaB sites were important for IL-1beta-, but not TNF-alpha-, induced A2AR promoter reporter gene activity. Finally, neither TNF-alpha nor Il-1beta had any effect on A2AR mRNA transcript degradation. These results directly implicate a major role for NF-kappaB in the regulation of A2AR gene transcription by IL-1beta and TNF-alpha but suggest that the effects of TNF-alpha on A2AR gene transcription are not mediated through the proximal promoter.

Working memory deficits in transgenic rats overexpressing human adenosine A2A receptors in the brain

Adenosine receptors in the central nervous system have been implicated in the modulation of different behavioural patterns and cognitive functions although the specific role of A(2A) receptor (A(2A)R) subtype in learning and memory is still unclear. In the present work we establish a novel transgenic rat strain, TGR(NSEhA2A), overexpressing adenosine A(2A)Rs mainly in the cerebral cortex, the hippocampal formation, and the cerebellum. Thereafter, we explore the relevance of this A(2A)Rs overexpression for learning and memory function. Animals were behaviourally assessed in several learning and memory tasks (6-arms radial tunnel maze, T-maze, object recognition, and several Morris water maze paradigms) and other tests for spontaneous motor activity (open field, hexagonal tunnel maze) and anxiety (plus maze) as modification of these behaviours may interfere with the assessment of cognitive function. Neither motor performance and emotional/anxious-like behaviours were altered by overexpression of A(2A)Rs. TGR(NSEhA2A) showed normal hippocampal-dependent learning of spatial reference memory. However, they presented working memory deficits as detected by performance of constant errors in the blind arms of the 6 arm radial tunnel maze, reduced recognition of a novel object and a lack of learning improvement over four trials on the same day which was not observed over consecutive days in a repeated acquisition paradigm in the Morris water maze. Given the interdependence between adenosinic and dopaminergic function, the present results render the novel TGR(NSEhA2A) as a putative animal model for the working memory deficits and cognitive disruptions related to overstimulation of cortical A(2A)Rs or to dopaminergic prefrontal dysfunction as seen in schizophrenic or Parkinson's disease patients.

A novel mechanism of vasoregulation: ADP-induced relaxation of the porcine isolated coronary artery is mediated via adenosine release

In this study, we have investigated the mechanism of ADP-induced relaxation of porcine coronary artery (PCA) rings. The P2Y receptor agonists ADP and ADPbetaS produced concentration-dependent relaxation of endothelium-denuded PCA smooth muscle with pD2 values of 5.3 and 4.9, respectively. RT-polymerase chain reaction (RT-PCR) and immunoblotting demonstrated mRNA and protein expression of P2Y1 and A2A adenosine receptors in the PCA. The nonselective P2 antagonist PPADS or the P2Y1-selective antagonist MRS2179 failed to alter ADP- or ADPbetaS-induced relaxations. Relaxations to ADP were, however, blocked by the A2A adenosine receptor-selective antagonists ZM241385 and SCH58261 (apparent pK(B) values of 9.2 and 8.9, respectively). We excluded roles for direct occupancy of A2A adenosine receptors by ADP or ADPbetaS as well as metabolism to adenosine as mechanisms for ADP-evoked relaxations. However, ADP responses were significantly enhanced in the presence of the ENT1 nucleoside transporter inhibitors dipyridamole and NBTI and were significantly inhibited by adenosine deaminase, indicating a role for extracellular adenosine. Suprafusion of [3H]-adenine-labeled PCA segments showed that ADP induced the release of a number of purines, including adenosine. These data suggest that ADP mediates relaxation of the PCA via a novel mechanism that involves adenine nucleotide-evoked adenosine release and the subsequent activation of A2A receptors.

Adenosine A2A receptor activation reduces hepatic ischemia reperfusion injury by inhibiting CD1d-dependent NKT cell activation

Ischemia reperfusion injury results from tissue damage during ischemia and ongoing inflammation and injury during reperfusion. Liver reperfusion injury is reduced by lymphocyte depletion or activation of adenosine A_2A receptors (A_2ARs) with the selective agonist 4- {3-[6-amino-9-(5-ethylcarbamoyl-3,4-dihydroxy-tetrahydro-furan-2-yl)-9H-purin-2-yl]- prop-2-ynyl}-cyclohexanecarboxylic acid methyl ester (ATL146e). We show that NKT cells are stimulated to produce interferon (IFN)-γ by 2 h after the initiation of reperfusion, and the use of antibodies to deplete NK1.1-positive cells (NK and NKT) or to block CD1d-mediated glycolipid presentation to NKT cells replicates, but is not additive to, the protection afforded by ATL146e, as assessed by serum alanine aminotransferase elevation, histological necrosis, neutrophil accumulation, and serum IFN-γ elevation. Reduced reperfusion injury observed in RAG-1 knockout (KO) mice is restored to the wild-type (WT) level by adoptive transfer of NKT cells purified from WT or A_2AR KO mice but not IFN-γ KO mice. Additionally, animals with transferred A_2AR^−/− NKT cells are not protected from hepatic reperfusion injury by ATL146e. In vitro, ATL146e potently inhibits both anti-CD3 and α-galactosylceramide–triggered production of IFN-γ by NKT cells. These findings suggest that hepatic reperfusion injury is initiated by the CD1d-dependent activation of NKT cells, and the activation of these cells is inhibited by A_2AR activation.

Affinity of corpora amylacea for oligonucleotides: sequence dependency and proteinaceous binding motif

Corpora amylacea (CA) have an affinity to nucleic acids as shown by in situ hybridization experiments. However, little is known about the specificity of this interaction, as well as the mechanism involved. We investigated the ability of different probes of digoxigenin-labeled oligonucleotides corresponding to some specific neuronal receptors, both sense and antisense, to bind to CA from human autopsy brain tissue. The bound nucleotides were detected with antidigoxigenin antibody and the signal was further amplified using the tyramide signal amplification system. The affinity of binding varies with the sequence of nucleotides. The most intense signal is produced by the adenosine-2A receptor antisense probe and the least intense signal is produced by the N-methyl-D-aspartate receptor sense probe. The affinity of binding for the same probe does not depend on the localization of CA in the central nervous system. Complete staining loss by proteinase K pretreatment in higher concentrations shows that the binding motif is partially proteinaceous. The circumferential but not the punctate internal staining is diminished by mild amylglucosidase pretreatment, suggesting a process of progressive apposition and condensation.

Myocardial infarct-sparing effect of adenosine A2A receptor activation is due to its action on CD4+ T lymphocytes

BACKGROUND

We previously used adenosine A2A receptor (A2AR) knockout (KO) mice and bone marrow transplantation to show that the infarct-sparing effect of A2AR activation at reperfusion is primarily due to effects on bone marrow-derived cells. In this study we show that CD4+ but not CD8+ T lymphocytes contribute to myocardial ischemia/reperfusion injury.

METHOD AND RESULTS

After a 45-minute occlusion of the left anterior descending coronary artery and reperfusion, T cells accumulate in the infarct zone within 2 minutes. Addition of 10 microg/kg of the A2AR agonist ATL146e 5 minutes before reperfusion produces a significant reduction in T-cell accumulation and a significant reduction in infarct size (percentage of risk area) measured at 24 hours. In Rag1 KO mice lacking mature lymphocytes, infarct size is significantly smaller than in C57BL/6 mice. Infarct size in Rag1 KO mice is increased to the level of B6 mice by adoptive transfer of 50 million CD4+ T lymphocytes derived from C57BL/6 or A2AR KO but not interferon-gamma KO mice. ATL146e completely blocked the increase in infarct size in Rag1 KO mice reconstituted with B6 but not A2AR KO CD4+ T cells. The number of neutrophils in the reperfused heart at 24 hours after infarction correlated well with the number of lymphocytes and infarct size.

CONCLUSIONS

These results strongly suggest that the infarct-sparing effect of A2AR activation is primarily due to inhibition of CD4+ T-cell accumulation and activation in the reperfused heart.

Association study of adenosine A2a receptor (1976C>T) genetic polymorphism and mood disorders and age of onset

Evidence suggests that the adenosine A2a receptor (A2aAR) may be implicated in the pathogenesis of mood disorders. We tested the hypothesis that the A2aAR 1976C>T genetic variant confers susceptibility to mood disorders by comparing 192 mood disorder patients and 216 normal controls. The distribution of the A2aAR genotypes and alleles did not differ significantly comparing the two groups, suggesting that it is unlikely that the A2aAR 1976C>T polymorphism plays a major role in the development of mood disorders.

Adenosine A2A receptors in diffuse dermal fibrosis: pathogenic role in human dermal fibroblasts and in a murine model of scleroderma

OBJECTIVE

Adenosine regulates inflammation and tissue repair, and adenosine A2A receptors promote wound healing by stimulating collagen matrix production. We therefore examined whether adenosine A2A receptors contribute to the pathogenesis of dermal fibrosis.

METHODS

Collagen production by primary human dermal fibroblasts was analyzed by real-time polymerase chain reaction, 14C-proline incorporation, and Sircol assay. Intracellular signaling for dermal collagen production was investigated using inhibitors of MEK-1 and by demonstration of ERK phosphorylation. In vivo effects were studied in a bleomycin-induced dermal fibrosis model using adenosine A2A receptor-deficient wild-type littermate mice, C57BL/6 mice, and mice treated with adenosine A2A receptor antagonist. Morphometric features and levels of hydroxyproline were determined as measures of dermal fibrosis.

RESULTS

Adenosine A2A receptor occupancy promoted collagen production by primary human dermal fibroblasts, which was blocked by adenosine A2A, but not A1 or A2B, receptor antagonism. Adenosine A2A receptor ligation stimulated ERK phosphorylation, and A2A receptor-mediated collagen production by dermal fibroblasts was blocked by MEK-1 inhibitors. Adenosine A2A receptor-deficient and A2A receptor antagonist-treated mice were protected from developing bleomycin-induced dermal fibrosis.

CONCLUSION

These results demonstrate that adenosine A2A receptors play an active role in the pathogenesis of dermal fibrosis and suggest a novel therapeutic target in the treatment and prevention of dermal fibrosis in diseases such as scleroderma.

Gs activation is time-limiting in initiating receptor-mediated signaling

To analyze individual steps of G(S)-linked signaling in intact cells, we used fluorescence resonance energy transfer (FRET)-based assays for receptor-G protein interaction, G protein activation, and cAMP effector activation. To do so, we developed a FRET-based sensor to directly monitor G(S) activation in living cells. This was done by coexpressing a Galpha(s) mutant, in which a yellow fluorescent protein was inserted, together with cyan fluorescent protein-tagged Gbetagamma subunits and appropriate receptors in HEK293 cells. Together with assays for receptor activation and receptor-G protein interaction, it is possible to characterize large parts of the G(S) signaling cascade. When A(2A)-adenosine or beta(1)-adrenergic receptors are coexpressed with G(S) in HEK293T cells, the receptor-G(S) interaction was on the same time scale as A(2A) receptor activation with a time constant of <50 ms. G(S) activation was markedly slower and around 450 ms with similar kinetics following activation of A(2A)- or beta(1)-receptors. Taken together, our kinetic measurements demonstrate that the rate of G(S) activation limits initiation of G(S)-coupled receptor signaling.

Low affinity binding of the classical D1 antagonist SCH23390 in rodent brain: potential interaction with A2A and D2-like receptors

Whereas structurally dissimilar D(1) antagonists competing for [(3)H]-SCH23390 binding recognize primarily one site in striatum, two distinct affinity states are observed in both amygdala and hippocampus. The binding profile of SCH23390 is similar in both of these regions, with the high affinity site (K(D) approximately 0.4 nM) consistent with D(1)/D(5) receptors. The appearance of the low affinity site (K(D) approximately 300 nM) is dependent upon the absence of MgCl(2), but independent of D(1) expression (i.e., still present in D(1) knockout mice). Although the density of high affinity state receptor is lower in hippocampus or amygdala of D(1) knockout mice, some residual binding remains, consistent with the known expression of D(5) receptors in these regions. Remarkably, in hippocampus, the affinity of the low affinity site is shifted rightward in the presence of the D(2) antagonist domperidone and is largely absent in the hippocampus of D(2) knockout animals. Additionally, this site is also shifted rightward in the presence of the A(2A) ligands SCH58261, CSC, or NECA, or in the absence of A(2A) receptors. The affinity of SCH23390 for this low affinity site is greater than seen for SCH23390 binding to D(2) receptors in heterologous expression systems, consistent with the hypothesis that both D(2) and A(2A) receptors are involved in the low affinity binding site. Therefore, we suggest that the heteromerization of D(2) and A(2A) receptors reported previously in vitro also may occur in the brain of both rats and mice.

Exploring an interaction of adenosine A2A receptor variability with coffee and tea intake in Parkinson's disease

Caffeine is an adenosine receptor A1 and A2A receptor antagonist and a putative functional genetic variant of the A2A receptor (2592C > Tins) mediates caffeine-induced anxiety. Here we investigated the potential interaction of this A2A genetic variant with the quantity of coffee and tea intake and their relationship with the risk of PD. A total of 441 subjects consisting of 222 PD and 219 race, gender and age matched controls were included. A multivariate analysis of the variables including the 2592C > Tins A2A genotypes, age of onset, gender, and the quantity of tea and coffee intake, interaction of the A2A genotypes with coffee intake, interaction of A2A genotypes with tea intake demonstrated the quantity of coffee intake to be significantly associated with PD (P < 0.0005, OR = 0.922, 95% CI: 0.881, 0.964). However, there was no significant interaction of the A2A genotypes with the quantity of coffee and tea intake in modulating the risk of PD. The dose dependent protective effect of coffee intake in PD was independent of the 2592C > Tins A2A genotype suggesting that the pharmacogenetic action of caffeine in PD may be mediated differently from other caffeine-induced neurologic syndromes.

Cutting Edge: Critical Role for A2A Adenosine Receptors in the T Cell-Mediated Regulation of Colitis

A(2A) adenosine receptors (A(2A)AR) inhibit inflammation, although the mechanisms through which adenosine exerts its effects remain unclear. Although the transfer of regulatory Th cells blocks colitis induced by pathogenic CD45RB(high) Th cells, we show that CD45RB(low) or CD25+ Th cells from A(2A)AR-deficient mice do not prevent disease. Moreover, CD45RB(high) Th cells from A(2A)AR-deficient mice were not suppressed by control CD45RB(low) Th cells. A(2A)AR agonists suppressed the production of proinflammatory cytokines by CD45RB(high) and CD45RB(low) T cells in association with a loss of mRNA stability. In contrast, anti-inflammatory cytokines, including IL-10 and TGF-beta, were minimally affected. Oral administration of the A(2A)AR agonist ATL313 attenuated disease in mice receiving CD45RB(high) Th cells. These data suggest that A(2A)AR play a novel role in the control of T cell-mediated colitis by suppressing the expression of proinflammatory cytokines while sparing anti-inflammatory activity mediated by IL-10 and TGF-beta.

Cloning and pharmacological characterization of the equine adenosine A2A receptor: a potential therapeutic target for the treatment of equine endotoxemia

The aim of the current study was to clone the equine adenosine A(2A) receptor gene and to establish a heterologous expression system to ascertain its pharmacologic profile via radioligand binding and functional assays. An eA(2A)-R expression construct was generated by ligation of the eA(2A) cDNA into the pcDNA3.1 expression vector, and stably transfected into human embryonic kidney cells (HEK). Binding assays identified those clones expressing the eA(2A)-R, and equilibrium saturation isotherm experiments were utilized to determine dissociation constants (K(D)), and receptor densities (B(max)) of selected clones. Equilibrium competition binding revealed a rank order of agonist potency of ATL > CV-1808 > NECA > 2-CADO > CGS21680, and a rank order of antagonist potency as ZM241385 > 8-phenyltheophylline > p-sulfophenyltheophylline > caffeine. Furthermore, adenylate cyclase assays using selective A(2A)-R agonists revealed that the eA(2A)-R functionally coupled to Galpha(s) as indicated by an increase in intracellular [(3)H]cAMP upon receptor activation. Finally, NF-kappaB reporter gene assays revealed a CGS21680 concentration-dependent inhibition of NF-kappaB activity. These results indicate that the heterologously expressed eA(2A)-R has a pharmacological profile similar to that of other mammalian A(2A) receptors and thus can be utilized for further characterization of the eA(2A)-R to ascertain whether it can serve as a suitable pharmacological target for equine inflammatory disease.

Adenosine A2A receptor modulation of juvenile female rat skeletal muscle microvessel permeability

Little is known of the regulation of skeletal muscle microvascular exchange under resting or stimulating conditions. Adenosine (ADO) levels in skeletal muscle increase during physiological (exercise) and pathological (hypoxia, inflammation, and ischemia) conditions. Later stages of these pathologies are characterized by the loss of vascular barrier integrity. This study focused on determining which ADO receptor mediates the robust reduction in microvessel permeability to rat serum albumin (P(s)(RSA)) observed in juvenile female rats. In microvessels isolated from abdominal skeletal muscle, ADO suffusion induced a concentration-dependent reduction in arteriolar [log(IC(50)) = -9.8 +/- 0.2 M] and venular [log(IC(50)) = -8.4 +/- 0.2 M] P(s)(RSA). RT-PCR and immunoblot analysis demonstrated mRNA and protein expression of ADO A(1), A(2A), A(2B), and A(3) receptors in both vessel types, and immunofluorescence assay revealed expression of the four subtype receptors in the microvascular walls (endothelium and smooth muscle). P(s)(RSA) responses of arterioles and venules to ADO were blocked by 8-(p-sulphophenyl)theophylline, a nonselective A(1) and A(2) antagonist. An A(2A) agonist, CGS21680, was more potent than the A(1) agonist, cyclopentyladenosine, or the most-selective A(2B) agonist, 5'-(N-ethylcarboxamido)adenosine. The ability of CGS21680 or ADO to reduce P(s)(RSA) was abolished by the A(2A) antagonist, ZM241385. An adenylyl cyclase inhibitor, SQ22536, blocked the permeability response to ADO. In aggregate, these results demonstrate that, in juvenile females (before the production of the reproductive hormones), ADO enhances skeletal muscle arteriole and venule barrier function predominantly via A(2A) receptors using activation of adenylyl cyclase-signaling mechanisms.

Mouse spinal cord compression injury is reduced by either activation of the adenosine A2A receptor on bone marrow-derived cells or deletion of the A2A receptor on non-bone marrow-derived cells

Activation of the adenosine A(2A) receptor (A(2A)R) at the time of reperfusion has been shown to reduce ischemia-reperfusion injury in peripheral tissues and spinal cord. In this study we show that treating mice with the A(2A)R agonist, 4-{3-[6-amino-9-(5-cyclopropylcarbamoyl-3,4-dihydroxy-tetrahydro-furan-2-yl)-9H-purin-2-yl]-prop-2-ynyl}-piperidine-1-carboxylic acid methyl ester for four days beginning before or just after the onset of reperfusion after compression-induced spinal cord injury rapidly (within 1 day) and persistently (>42 days) reduces locomotor dysfunction and spinal cord demyelination. Protection is abolished in knockout/wild type bone marrow chimera mice selectively lacking the A(2A)R only on bone marrow-derived cells but retaining receptors on other tissues including blood vessels. Paradoxically, reduced spinal cord injury is also noted in A(2A)R -/- mice, and in wild type/knockout bone marrow chimera mice selectively lacking the A(2A)R on non-bone marrow-derived cells, or in mice treated with the A(2A) antagonist, 4-(2-[7-amino-2-[2-furyl][1,2,4]triazolo[2,3-a][1,3,5]triazin-5-yl-amino]ethyl)phenol. The greatest protection is seen in knockout/wild type bone marrow chimera mice treated with 4-{3-[6-amino-9-(5-cyclopropylcarbamoyl-3,4-dihydroxy-tetrahydro-furan-2-yl)-9H-purin-2-yl]-prop-2-ynyl}-piperidine-1-carboxylic acid methyl ester, i.e. by activating the A(2A)R in mice expressing the receptor only in bone marrow-derived cells. The data suggest that inflammatory bone marrow-derived cells are the primary targets of A(2A) agonist-mediated protection. We conclude that A(2A) agonists or other interventions that inhibit inflammation during and after spinal cord ischemia may be effective in reducing spinal cord injury in patients, but excessive or prolonged stimulation of the A(2A)R may be counterproductive. It may be possible to devise strategies to produce optimal spinal cord protection by exploiting temporal differences in A(2A)R-mediated responses.

Improved spatial recognition memory in mice lacking adenosine A2A receptors

Adenosine receptors play an important role in learning and memory as their antagonists have been found to facilitate learning and memory in various tasks in rodents. However, few studies have examined the effect of adenosine A2A receptor deficiency on cognition. In the present study, we therefore used the Y-maze, a simple two-trial recognition test to measure spatial recognition memory in mice lacking adenosine A2A receptors. The results showed that adenosine A2A receptor knockout mice had a higher percentage of novel arm visits as first choice than wild-type CD1 mice. Moreover, these mice showed longer duration of visits in the novel arm when compared with controls, suggesting that the lack of adenosine A2A receptors improved spatial recognition memory. On the other hand, mice lacking the adenosine A2A receptors had low scores in the number of arm visits, suggesting that they were hypoactive. In conclusion, these data suggest the involvement of adenosine receptors in modulating spatial recognition memory in mice, consistent with earlier findings using adenosine receptor antagonists.

The allosteric enhancer PD81,723 increases chimaeric A1/A2A adenosine receptor coupling with Gs

PD81,723 {(2-amino-4,5-dimethyl-3-thienyl)-[3-(trifluromethyl)-phenyl]methanone} is a selective allosteric enhancer of the G(i)-coupled A1 AR (adenosine receptor) that is without effect on G(s)-coupled A2A ARs. PD81,723 elicits a decrease in the dissociation kinetics of A1 AR agonist radioligands and an increase in functional agonist potency. In the present study, we sought to determine whether enhancer sensitivity is dependent on coupling domains or G-protein specificity of the A1 AR. Using six chimaeric A1/A2A ARs, we show that the allosteric effect of PD81,723 is maintained in a chimaera in which the predominant G-protein-coupling domain of the A1 receptor, the 3ICL (third intracellular loop), is replaced with A2A sequence. These chimaeric receptors are dually coupled with G(s) and G(i), and PD81,723 increases the potency of N6-cyclopentyladenosine to augment cAMP accumulation with or without pretreatment of cells with pertussis toxin. Thus PD81,723 has similar functional effects on chimaeric receptors with A1 transmembrane sequences that couple with G(i) or G(s). This is the first demonstration that an allosteric regulator can function in the context of a switch in G-protein-coupling specificity. There is no enhancement by PD81,723 of G(i)-coupled A2A chimaeric receptors with A1 sequence replacing A2A sequence in the 3ICL. The results suggest that the recognition site for PD81,723 is on the A1 receptor and that the enhancer acts to directly stabilize the receptor to a conformational state capable of coupling with G(i) or G(s).

Effects of targeted deletion of A1 adenosine receptors on postischemic cardiac function and expression of adenosine receptor subtypes

To examine ischemic tolerance in the absence of A(1) adenosine receptors (A(1)ARs), isolated wild-type (WT) and A(1)AR knockout (A(1)KO) murine hearts underwent global ischemia-reperfusion, and injury was measured in terms of functional recovery and efflux of lactate dehydrogenase (LDH). Hearts were analyzed by real-time RT-PCR both at baseline and at intervals during ischemia-reperfusion to determine whether compensatory expression of other adenosine receptor subtypes occurs with either A(1)AR deletion and/or ischemia-reperfusion. A(1)KO hearts had higher baseline coronary flow (CF) and left ventricular developed pressure (LVDP) than WT hearts, whereas heart rate was unchanged by A(1)AR deletion. After 20 min of ischemia, CF was attenuated in A(1)KO compared with WT hearts, and this reduction persisted throughout reperfusion. Final recovery of LVDP was decreased in A(1)KO hearts (54.4 +/- 5.1 vs. WT 81.1 +/- 3.4% preischemic baseline) and correlated with higher diastolic pressure during reperfusion. Postischemic efflux of LDH was greater in A(1)KO compared with WT hearts. Real-time RT-PCR demonstrated the absence of A(1)AR transcript in A(1)KO hearts, and the message for A(2A), A(2B), and A(3) adenosine receptors was similar in uninstrumented A(1)KO and WT hearts. Ischemia-reperfusion increased A(2B) mRNA expression 2.5-fold in both WT and A(1)KO hearts without changing A(1) or A(3) expression. In WT hearts, ischemia transiently doubled A(2A) mRNA, which returned to preischemic level upon reperfusion, a pattern not observed in A(1)KO hearts. Together, these data affirm the cardioprotective role of A(1)ARs and suggest that induced expression of other adenosine receptor subtypes may participate in the response to ischemia-reperfusion in isolated murine hearts.

Adenosine A2A receptor inactivation increases survival in polymicrobial sepsis

The mechanisms governing the impairment of bacterial clearance and immune function in sepsis are not known. Adenosine levels are elevated during tissue hypoxia and damage associated with sepsis. Adenosine has strong immunosuppressive effects, many of which are mediated by A(2A) receptors (A(2A)R) expressed on immune cells. We examined whether A(2A)R are involved in the regulation of immune function in cecal ligation and puncture-induced murine polymicrobial sepsis by genetically or pharmacologically inactivating A(2A)R. A(2A)R knockout (KO) mice were protected from the lethal effect of sepsis and had improved bacterial clearance compared with wild-type animals. cDNA microarray analysis and flow cytometry revealed increased MHC II expression in A(2A)-inactivated mice, suggesting improved Ag presentation as a mechanism of protection. Apoptosis was attenuated in the spleen of A(2A) KO mice indicating preserved lymphocyte function. Levels of the immunosuppressive cytokines IL-10 and IL-6 were markedly lower following A(2A)R blockade. Similar to observations with A(2A)R KO mice, an A(2A)R antagonist increased survival even when administered in a delayed fashion. These studies demonstrate that A(2A)R blockade may be useful in the treatment of infection and sepsis.

The lack of A2A adenosine receptors diminishes the reinforcing efficacy of cocaine

Adenosine is an endogenous purine nucleoside, which acts as a neuromodulator in the central nervous system. A2A adenosine and D2 dopamine receptors are colocalized in the same neurons in discrete brain areas, and the dopaminergic transmission plays a crucial role in the addictive properties of drugs of abuse, such as cocaine. In the present study, we have investigated the specific role of A2A adenosine receptors in cocaine-induced behavioral responses related to its addictive properties. For this purpose, we have evaluated the acute locomotor effects produced by cocaine and the development of locomotor sensitization by repeated cocaine administration. In addition, we have also examined cocaine acute rewarding properties using the conditioned place preference. Finally, we used the intravenous drug self-administration paradigm to investigate the acquisition of an operant response maintained by cocaine self-administration and the reinforcing efficacy of the drug in these knockout animals. Acute cocaine induced a similar increase of locomotor activity in mice lacking A2A adenosine receptors and wild-type littermates. Cocaine-induced locomotor sensitization and conditioned place preference were also maintained in A2A knockout mice. Nevertheless, these knockout mice showed a lower rate of cocaine self-administration than wild-type mice in both fixed ratio 1 and 3 schedules of reinforcement. Moreover, a reduction in the maximal effort to obtain a cocaine infusion was found in A2A knockout mice under a progressive ratio schedule. In addition, a vertical shift of the cocaine dose-response curve was observed in mice lacking A2A adenosine receptors in comparison with wild-type littermates. Our study demonstrates that A2A adenosine receptors play an important role in cocaine addictive properties, and these receptors seem to be required to develop the addictive effects of this drug.

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

RATIONALE

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.

OBJECTIVES

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Adenosine A2a receptors induce heterologous desensitization of chemokine receptors

Adenosine, released by cells in an injurious or hypoxic environment, possesses potent anti-inflammatory effects by inhibiting the production of proinflammatory cytokines and superoxide anions (O2-). We hypothesized that adenosine compounds also induced heterologous desensitization of chemokine receptors, which played a critical role in leukocyte trafficking. Our studies using adenosine receptor subtype-specific agonists revealed that pretreatment with adenosine compounds suppressed RANTES-induced chemotaxis and Ca2+ flux through activation of A2a adenosine receptor. Adenosine compounds also desensitized IL-8- and MCP-1-induced chemotaxis, but not that induced by fMLP. Activation of protein kinase A (PKA), a component of the signaling pathway induced by the A2a receptor, was sufficient to desensitize RANTES-induced chemotaxis. Inhibition of PKA reversed the desensitization effects of adenosine compounds, suggesting that PKA was necessary for A2a receptor-mediated heterologous desensitization. In a mouse model, prior activation of A2a receptors blocked RANTES-induced recruitment of leukocytes in an air pouch. Moreover, the A2a receptor-induced cross-desensitization also reduced the susceptibility of monocytes to infection by an R5 strain of HIV-1. Our results suggest that activation of A2a adenosine receptors suppresses chemokine receptor function, and such receptor cross-talk was based on the simple mechanism of PKA-mediated heterologous desensitization, thus contributing to the antiinflammatory activity of adenosine.

Receptor crosstalk: characterization of mice deficient in dopamine D1 and adenosine A2A receptors

Here we report the development of D1A2A receptor knockout mice to investigate whether interactions between dopamine D1 and adenosine A2A receptors participate in reward-related behavior. The combined deletion of D1 and A2A receptors resulted in mice with decreased weight and appetitive processes, reduced rearing and exploratory behaviors, increased anxiety, and a significantly poorer performance on the rotarod, compared to wild-type littermates. D1A2A receptor knockout mice shared phenotypic similarities with mice deficient in D1 receptors, while also paralleling behavioral deficits seen in A2A receptor knockout mice, indicating individual components of the behavioral phenotype of the D1A2A receptor knockout attributable to the loss of both receptors. In contrast, ethanol and saccharin preference in D1A2A receptor knockout mice were distinctly different from that observed in derivative D1 or A2A receptor-deficient mice. Compared to wild types, preference and consumption of ethanol were decreased in D1A2A receptor knockout mice, the reduction in ethanol consumption greater even than that seen in D1 receptor-deficient mice. Preference and consumption of saccharin were also reduced in D1A2A receptor knockout mice, whereas saccharin preference was similar in wild-type, D1, and A2A receptor knockout mice. These data suggest an interaction of D1 and A2A receptors in the reinforcement processes underlying the intake of rewarding substances, whereby the A2A receptor seems involved in goal-directed behavior and the motor functions underlying the expression of such behaviors, and the D1 receptor is confirmed as essential in mediating motivational processes related to the repeated intake of novel substances and drugs.

Identifying interactions between transmembrane helices from the adenosine A2A receptor

The human adenosine A(2A) receptor (A(2A)R) is an integral membrane protein and a member of the G-protein-coupled receptor (GPCR) superfamily, characterized by seven transmembrane (TM) helices. Although helix-helix association in the lipid bilayer is known to be an essential step in the folding of GPCRs, the determinants of their structures, folding, and assembly in the cell membrane are poorly understood. Previous studies in our group showed that while peptides corresponding to all seven TM domains of A(2A)R form stable helical structures in detergent micelles and lipid vesicles, they display significant variability in their helical propensity. This finding suggested to us that some TM domains might need to interact with other domains to properly insert and fold in hydrophobic environments. In this study, we assessed the ability of TM peptides to interact in pairwise combinations. We analyzed peptide interactions in hydrophobic milieus using circular dichroism spectroscopy and Förster resonance energy transfer. We find that specific interactions between TM helices occur, leading to additional helical content, especially in weakly helical TM domains, suggesting that some TM domains need a partner for proper folding in the membrane. The approach developed in this study will enable complete analysis of the TM domain interactions and the modeling of a folding pathway for A(2A)R.

Lipopolysaccharide rapidly modifies adenosine receptor transcripts in murine and human macrophages: role of NF-kappaB in A(2A) adenosine receptor induction

The A(2A) adenosine receptor (A(2A)AR) mediates anti-inflammatory actions of adenosine in a variety of cell types. LPS (lipopolysaccharide) was reported to induce a small (<2-fold) increase in the expression of A(2A)AR mRNA in human monocytes and monocytic cell lines. We investigated the effects of LPS on the expression of adenosine receptor mRNAs in primary mouse IPMPhi (intraperitoneal macrophages), human macrophages and Wehi-3 cells. Treatment with 10 ng/ml LPS for 4 h produced a >100-fold increase in A(2A)AR mRNA. LPS-induced increases in mRNA for A(2A)AR and TNFalpha (tumour necrosis factor alpha) are reduced by 90% in IPMPhi pretreated with the NF-kappaB (nuclear factor kappaB) inhibitor, BAY 11-7082 {(E)3-[(4-methylphenyl)sulphonyl]-2-propenenitrile; 10 microM}. In Wehi-3 cells exposed to LPS, A(2A)AR and A(2B)AR transcripts are elevated by 290- and 10-fold respectively, the A(1)AR transcript is unchanged and the A(3)AR transcript is decreased by 67%. The induction of A(2A)AR mRNA by LPS is detectable after 1 h, reaches a peak at 6 h at 600 times control and remains elevated beyond 24 h. The ED50 (effective dose) of LPS is 2.3 ng/ml. A(2A)AR receptor number, measured by 125I-ZM241385 binding to whole cells, is undetectable in naïve cells and increases linearly at a rate of 23 receptors x cell(-1) x min(-1) to a B(max) of 348 fmol/mg (28000 receptors/cell) in 20 h. The increase in receptor number is correlated with an increase in the potency of an A(2A) agonist (4-{3-[6-amino-9-(5-ethylcarbamoyl-3,4-dihydroxy-tetrahydro-furan-2-yl)-9H-purin-2-yl]-prop-2-ynyl}-cyclohexanecarboxylic acid methyl ester; referred to as ATL146e) to stimulate cAMP in these cells. After LPS pretreatment, the potency of the A(2A) agonist, ATL146e, to reduce TNFalpha release from IPMPhi was increased by 200-fold. The results support the hypothesis that regulation of adenosine receptor expression, especially up-regulation of the A(2A)AR, is part of a delayed feedback mechanism initiated through NF-kappaB to terminate the activation of human and mouse macrophages.

Modulation of paracetamol antinociception by caffeine and by selective adenosine A2 receptor antagonists in mice

This study investigated the involvement of adenosine receptors in the interaction between paracetamol and caffeine in mice, using the adenosine A2A receptor antagonist 5-amino-7-(2-phenylethyl)-2-(2-furyl)pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine (SCH58261) and the adenosine A2B receptor antagonist 1-propyl-8-p-sulfophenylxanthine (PSB1115), in the tail immersion and hot-plate tests. Paracetamol (10-200 mg/kg) was antinociceptive in both tests, but, in contrast to previous studies, caffeine (10 mg/kg) was pronociceptive in the tail immersion test, and reduced the effects of paracetamol in both tests. SCH58261 (3 mg/kg) was antinociceptive in both tests and in its presence paracetamol (50 mg/kg) had no further effect. PSB1115 (10 mg/kg) had little effect alone but potentiated the effect of paracetamol (50 mg/kg) in the hot-plate test and abolished it in the tail immersion test. These results suggest that adenosine A2B receptors may be involved in the action of paracetamol in a pathway-dependent manner, and also support the existence of pronociceptive adenosine A2A receptors.

Genetic interdependence of adenosine and dopamine receptors: Evidence from receptor knockout mice

Dopamine and adenosine receptors are known to share a considerable overlap in their regional distribution, being especially rich in the basal ganglia. Dopamine and adenosine receptors have been demonstrated to exhibit a parallel distribution on certain neuronal populations, and even when not directly co-localized, relationships (both antagonistic and synergistic) have been described. This study was designed to investigate dopaminergic and purinergic systems in mice with ablations of individual dopamine or adenosine receptors. In situ hybridization histochemistry and autoradiography was used to examine the level of mRNA and protein expression of specific receptors and transporters in dopaminergic pathways. Expression of the mRNA encoding the dopamine D2 receptor was elevated in the caudate putamen of D1, D3 and A2A receptor knockout mice; this was mirrored by an increase in D2 receptor protein in D1 and D3 receptor knockout mice, but not in A2A knockout mice. Dopamine D1 receptor binding was decreased in the caudate putamen, nucleus accumbens, olfactory tubercle and ventral pallidum of D2 receptor knockout mice. In substantia nigra pars compacta, dopamine transporter mRNA expression was dramatically decreased in D3 receptor knockout mice, but elevated in A2A receptor knockout mice. All dopamine receptor knockout mice examined exhibited increased A2A receptor binding in the caudate putamen, nucleus accumbens and olfactory tubercle. These data are consistent with the existence of functional interactions between dopaminergic and purinergic systems in these reward and motor-related brain regions.

Differential expression of adenosine receptors in human neutrophils: up-regulation by specific Th1 cytokines and lipopolysaccharide

Four types of adenosine receptors have been identified in different tissues and cell types, namely, A1, A2A, A2B, and A3 receptors. We report that A2AR but not A2BR mRNA in freshly isolated polymorphonuclear neutrophils (PMN) is maximally up-regulated after 4 h stimulation with lipopolysaccharide (LPS) or tumor necrosis factor alpha (TNF-alpha) and to a lesser extent, with interleukin (IL)-1beta. These effects were maintained up to 21 h. Consistent with changes in A2AR mRNA expression, up-regulation of A2AR protein was also detected after 4 h of LPS or TNF-alpha exposure. Up-regulation of A2AR protein expression was transient and returned to near basal levels after 12 h or 16 h stimulation with TNF-alpha or LPS, respectively. Conversely, IL-1beta failed to promote A2AR protein expression. Suppression of thapsigargin-induced leukotriene synthesis by the selective A2AR agonist CGS-21680 was found to be more pronounced when PMN were cultured for 4 h with LPS or TNF-alpha. In contrast, the up-regulation of A2AR has no impact on CGS-21680-induced inhibition of phospholipase D activation and superoxide production in response to formyl-Met-Leu-Phe. These results demonstrate that the A2AR is up-regulated by specific T helper cell type 1 cytokines and LPS. Although this could represent a potential feedback mechanism to control inflammation, the effect of A2AR up-regulation varied depending on the stimulus used to stimulate PMN functional responses after their incubation with proinflammatory mediators.

Adenosine A2A receptors promote adenosine-stimulated wound healing in bronchial epithelial cells

Adenosine produces a wide variety of physiological effects through the activation of specific adenosine receptors (A(1), A(2A), A(2B), A(3)). Adenosine, acting particularly at the A(2A) adenosine receptor (A(2A)AR), is a potent endogenous anti-inflammatory agent and sensor of inflammatory tissue damage. The complete healing of wounds is the final step in a highly regulated response to injury. Recent studies on epidermal wounds have identified the A(2A)AR as the main adenosine receptor responsible for altering the kinetics of wound closure. We hypothesized that A(2A)AR promotes wound healing in bronchial epithelial cells (BECs). To test this hypothesis, the human BEC line BEAS-2B and bovine BECs (BBECs) were used. Real-time RT-PCR of RNA from unstimulated BEAS-2B cells revealed transcriptional expression of A(1), A(2A), A(2B) and A(3) receptors. Western blot analysis of lysates from BEAS-2B cells and BBECs detected a single band at 44.7 kDa in both the BECs, indicating the presence of A(2A)AR. In a wound healing model, we found that adenosine stimulated wound repair in cultured BBECs in a concentration-dependent manner, with an optimal closure rate observed between 4 and 6 h. Similarly, the A(2A)AR agonist 5'-(N-cyclopropyl)carboxamidoadenosine (CPCA) augmented wound closure, with a maximal closure rate occurring between 4 and 6 h. Inhibition of A(2A)AR with ZM-241385, a known A(2A)AR antagonist, impeded wound healing. In addition, ZM-241385 also attenuated adenosine-mediated wound repair. Kinase studies revealed that adenosine-stimulated airway repair activates PKA by ligating A(2A)AR. Collectively, the data suggest that the A(2A)AR is involved in BEC adenosine-stimulated wound healing and may prove useful in understanding purinergic-mediated actions on airway epithelial repair.

Adenosine inhibits tumor necrosis factor-alpha release from mouse peritoneal macrophages via A2A and A2B but not the A3 adenosine receptor

Adenosine is elaborated in injured tissues where it suppresses inflammatory responses of essentially all immune cells, including production of proinflammatory cytokines such as tumor necrosis factor-alpha (TNF-alpha). Most of the anti-inflammatory actions of adenosine have been attributed to signaling through the A(2A) adenosine receptor (A(2A)AR). Previously, however, it has been shown that the A(3)AR agonist N(6)-(3-iodobenzyl)adenosine-5'-N-methylcarboxamide (IB-MECA) potently inhibited TNF-alpha release from macrophages obtained from A(2A)AR "knockout" (A(2A)KO) mice, suggesting that the A(3)AR may also regulate cytokine expression. Here, we confirmed that the A(2A)AR is the primary AR subtype that suppresses TNF-alpha release from thioglycollate-elicited mouse peritoneal macrophages induced by both Toll-like receptor-dependent (TLR) and TLR-independent stimuli, but we determined that the A(2B)AR rather than the A(3)AR mediates the non-A(2A)AR actions of adenosine since 1) the ability of IB-MECA to inhibit TNF-alpha release was not altered in macrophages isolated from A(3)KO mice, and 2) the A(2B)AR antagonist 1,3-dipropyl-8-[4-[((4-cyanophenyl)carbamoylmethyl)oxy]phenyl]xanthine (MRS 1754) blocked the ability of the nonselective AR agonist adenosine-5'-N-ethylcarboxamide (NECA) to inhibit TNF-alpha release from macrophages isolated from A(2A)KO mice. Although A(2B)ARs seem capable of inhibiting TNF-alpha release, the A(2A)AR plays a dominant suppressive role since MRS 1754 did not block the ability of NECA to inhibit TNF-alpha release from macrophages isolated from wild-type (WT) mice. Furthermore, the potency and efficacy of adenosine to inhibit TNF-alpha release from WT macrophages were not influenced by blocking A(2B)ARs with MRS 1754. The data indicate that adenosine suppresses TNF-alpha release from macrophages primarily via A(2A)ARs, although the A(2B)AR seems to play an underlying inhibitory role that may contribute to the anti-inflammatory actions of adenosine under select circumstances.

Altered expression of adenosine receptors in heart of diabetic rat

Diabetes results in functional, biochemical, and morphological abnormalities in the heart. Some of these changes may be attributed to altered adenosine action. This study aimed to examine the expression level of adenosine receptors (AR) in heart of streptozotocin-induced diabetic rat. Performed analyses revealed detectable levels of A1-AR, A2a-AR, A2b-AR, A3-AR mRNA and protein in whole heart and isolated cardiac myocytes. An increase in A1-AR protein content with no changes in mRNA level was observed in isolated cardiac myocytes. Diabetes resulted in an increase of A3-AR mRNA and protein levels in heart and in cardiac myocytes. The level of A2a-AR mRNA was increased in whole diabetic heart, but it decreased in cardiac myocytes with no detectable changes in protein content. We did not observe any changes in expression level of A2b-AR in diabetic heart and isolated cardiac myocytes. Administration of insulin to diabetic rat for four days resulted in returning of the ARs mRNA and protein to the levels observed in heart of normal rat. These changes in ARs genes expression, and receptors protein content correspond to some abnormalities characteristic of the diabetic heart, suggesting involvement in pathogenesis of diabetic cardiomyopathy.

Immunomodulatory impact of the A2A adenosine receptor on the profile of chemokines produced by neutrophils

In LPS-stimulated human neutrophils, engagement of the adenosine A2A receptor selectively prevented the expression and release of TNF-alpha, MIP-1alpha/CCL3, MIP-1beta/CCL4, MIP-2alpha/CXCL2, and MIP-3alpha/CCL20. In mice lacking the A2A receptor, granulocytes that migrated into the air pouch 4 h after LPS injection expressed higher mRNA levels of TNF-alpha, MIP-1alpha, and MIP-1beta than PMNs from wild-type mice. In mononuclear cells present in the air pouch 72 h after LPS injection, expression of IL-1beta, TNF-alpha, IL-6, and MCP-2/CCL6 was higher in A2AR knockout mice. In addition to highlighting neutrophils as an early and pivotal target for mediating adenosine anti-inflammatory activities, these results identify TNF-alpha and the MIP chemokine family as gene products whose expression is pivotally affected by activation of A2AR in LPS-activated PMNs. Modulation by A2AR in the production of inflammatory signals by PMNs may thus influence the evolution of an inflammatory response by reducing the activation status of inflammatory cells.

Adenosine A2A receptor dysfunction correlates with age at onset anticipation in blood platelets of subjects with Huntington's disease

Huntington's disease (HD) may manifest at an earlier age in affected offspring than in transmitting parents. Earlier onset in successive generations (anticipation) only partially depends on intergenerational parent-child elongation of the CAG expanded mutation. An aberrant amplification of adenosine A(2A) receptor signaling documented in peripheral blood cells of subjects with HD implies that this cellular dysfunction may be related to clinical and genetic features. Prompted by evidence of higher receptor densities in siblings of HD subjects with stronger onset anticipation, in this study we investigated a possible relationship between A(2A) receptor densities and age at onset. We measured adenosine A(2A) receptor densities in blood cell platelets from 32 patients with HD and healthy control siblings, and sought a possible linear correlation between maximum platelet A(2A) receptor binding (B(max)) values for the whole cohort of HD subjects and anticipation in years. The increased B(max) values for the 32 subjects with HD (220 in patients vs. 137 in healthy control subjects, P = 0.0001) correlated significantly with anticipation in years (r2, 0.48, P = 0.0001 by linear correlation analysis). An increased platelet A(2A) receptor B(max) may belong in a cascade of toxic events leading to earlier onset of HD: as such it could be a useful marker of onset anticipation.

Interindividual variation in anxiety response to amphetamine: possible role for adenosine A2A receptor gene variants

Amphetamine is thought to produce its stimulant effects mainly via the dopamine system, but its effects may also be influenced by other systems. Dopamine D1 and D2 receptors exist as heterodimers with adenosine A1 and A(2A) receptors, which modulate their responsiveness, suggesting that responses to amphetamine may also depend on adenosinergic function. We therefore studied the relevance of one adenosine A1 and three adenosine A(2A) receptor gene polymorphisms for the interindividual variability in amphetamine response in 99 healthy volunteers who received placebo or d-amphetamine (10 or 20 mg). The 1976C/T and 2592C/T(ins) polymorphisms of the adenosine receptor gene were associated with increases in anxiety at both doses. This is consistent with recent observations indicating a role for adenosine A(2A) receptor gene polymorphisms in anxiety.

Reduced appetite for caffeine in adenosine A(2A) receptor knockout mice

Adenosine A(2A) receptor knockout mice (A(2A)R KO) were compared to wild-type controls (A(2A)R WT) in a caffeine intake paradigm. When mice had ad libitum access to caffeine (0.3 g/l) and water in a two-bottle paradigm for 12 consecutive days, adenosine A(2A)R KO mice drank less caffeinated solution, demonstrating a reduced appetite for caffeine as compared to adenosine A(2A)R WT mice. These data reveal an important role for the adenosine A(2A) receptor in the appetitive properties of caffeine.

Exceptional total and functional yields of the human adenosine (A2a) receptor expressed in the yeast Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae was used to express a medically relevant G-protein coupled receptor (GPCR), the human adenosine (A2a) receptor, with a C-terminal green fluorescent protein (GFP) fusion tag. In prior studies, we established an expression system for A2a-GFP. Here, we quantified the total A2a-GFP expression levels by correlating GFP levels as detected by fluorescence and densitometry to A2a-GFP molecules overexpressed in the system. We also quantified A2a-GFP functional levels by classical radioligand binding assays. Approximately, 120,000 functional A2a-GFP molecules per cell were present on the plasma membrane as determined by radioligand binding. Using whole cell GFP fluorescence, 340,000 A2a-GFP molecules per cell were detected; approximately 70% of those molecules were plasma membrane localized, as determined by using confocal microscopy analysis. These results show that a significant portion of the total expressed protein is functional. In addition, the quick and inexpensive whole cell fluorescence appears to provide a good approximation of functional receptor numbers for this case. Importantly, the amount of functionally expressed A2a-GFP per culture ( approximately 4 mg/L) is among the highest reported for any GPCR in any expression system.

New 8-substituted xanthiene derivatives as potent bronchodilators

The synthesis and structure determination of 8-aryl /alkyl aryl 1, 3-dimethyl-3, 7-dihydropurin-2, 6-dione derivatives (1-13), was carried out in this study. Bronchodilator activity is investigated using isolated guinea-pig tracheal strips, pre-contracted by acetylcholine and histamine. Spasmolytic activity of the compounds was compared to theophylline. Synthesized compounds (1-13) did not inhibit the acetylcholine-induced pre-contractions except compound (8) at 10(-5) M concentration. In contrast, some of the compounds, especially (7), (11), (12) at 10(-5) M and (3), (4), (9) and (11) in 10(-4) M displayed inhibitory activity on the tracheal strips pre-contracted by histamine. The potency of the compounds at human adenosine receptors was evaluated using radioligand binding assay and a cyclic AMP functional assay in CHO cells expressing these receptors. Compound (11) displayed the greatest activity against radioligand binding of specific agonists to A2A and A2B receptors. The compounds were relatively selective for both A2A and A2B compared with A1 and A3 receptors. All compounds were also tested for the inhibition of NECA-induced cAMP accumulation mediated by the A2B adenosine receptor and compound (11) was found to be the most effective. Our results showed that these compounds are acting as selective adenosine antagonists, especially for adenosine A2B receptors, and are promising as potent anti-inflammatory agents rather than bronchodilator for the treatment of asthma.

Adenosine-dopamine interactions revealed in knockout mice

Neurochemical and pharmacological evidence obtained over the past 30 yr has indicated that adenosine and dopamine interact functionally in the basal ganglia and that such interactions have pathophysiological and therapeutic implications. The receptors implicated are adenosine A1 and A2A, and dopamine D1 and D2. There is evidence that dopamine D2 receptor activation in vivo antagonizes tonic activation of adenosine A2A receptors. Thus, acute blockade of dopamine D2 receptors, or disruption of dopamine transmission, unmasks strong adenosine A2A activation. Effects of dopamine D2 blockade are different after adenosine A2A blockade or in A2A knockout mice. Possibly as an adaptation to this increase in adenosine A2A signaling, there is a decreased coupling of A2A receptors to biological effects in dopamine D2 knockout mice. Compared to wild-type mice, adenosine A2A knockout mice show decreased neurodegeneration after treatment with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and show improved motor performance in models of Parkinson's disease Adenosine A1 receptors are not specifically located with any dopamine receptor, as is the A2A receptor with D2 receptors. Many A1 receptors are located presynaptically, where they regulate transmitter release. In A1 knockout mice, glutamatergic and dopaminergic transmission is therefore modified.

Arterial pressure and heart rate increase during REM sleep in adenosine A2A-receptor knockout mice, but not in wild-type mice

Rapid eye movement (REM)-sleep related changes in arterial pressure (AP) and heart rate (HR) were observed in homozygous and heterozygous adenosine A(2A) receptor (A2AR) knockout (KO) mice, and the corresponding wild-type mice. During REM sleep, the mean AP (MAP) and HR were clearly increased in the homozygous A2AR KO mice, while, in the wild-type mice, they were decreased or maintained at the same level. Neither homozygous nor heterozygous A2AR KO mice showed significant difference in diurnal pattern and the hourly values of MAP and HR compared to the wild-type mice. From these findings, it is likely that the adenosine A2AR is involved in autonomic regulation during REM sleep.

Differential coronary microvascular exchange responses to adenosine: roles of receptor and microvessel subtypes

OBJECTIVE

To assess the role of adenosine receptors in the regulation of coronary microvascular permeability to porcine serum albumin (P(s)(PSA)).

METHODS

Solute flux was measured in single perfused arterioles and venules isolated from pig hearts using fluorescent dye-labeled probes by microspectro-fluorometry. Messenger RNA, protein, and cellular distribution of adenosine receptors in arterioles and venules were analyzed by RT-PCR, immunoblot, and immunofluorescence.

RESULTS

Control venule P(s)(PSA) (10.7 +/- 4.8 x 10(- 7) cm x s(- 1)) was greater than that of arterioles (6.4+/- 2.8 x 10(-7) cm . s(-1); p < .05). Arteriolar P(s)(PSA) decreased (p < .05) with adenosine suffusion over the range from 10(- 8) to 10(-5) M, while venular P(s)(PSA) did not change. The nonselective A(1) and A(2) receptor antagonist, 8-(p-sulfophenyl) theophylline, blocked the adenosine-induced decrease in arteriolar P(s)(PSA). Messenger RNA for adenosine A(1), A(2A), A(2B), and A(3) receptors was expressed in arterioles and venules. Protein for A(1), A(2A), and A(2B), but not A(3), was detected in both microvessel types and was further demonstrated on vascular endothelial cells.

CONCLUSION

Arteriolar P(s)(PSA) decreases with adenosine suffusion but not venular P(s)(PSA). Adenosine A(1), A(2A), and A(2B) receptors are expressed in both arterioles and venules. Selective receptor-linked cellular signaling mechanisms underlying the regulation of permeability remain to be determined.

Decreases in yeast expression yields of the human adenosine A2a receptor are a result of translational or post-translational events

The human adenosine receptor (A2a), a G-protein-coupled receptor (GPCR), was C-terminally tagged with the green fluorescent protein (GFP) and expressed in the yeast Saccharomyces cerevisiae to gain an understanding of the expression limitations of this medically relevant class of membrane proteins. The A2a-GFP protein was able to bind adenosine analogs indicating that the GFP tag did not alter the ligand binding activity of the receptor. A screen based on whole cell fluorescence was developed and a library of clones with various gene copy numbers was screened via flow cytometry to isolate clones with the highest protein expression levels. All clones studied exhibited a decrease in the net A2a-GFP protein production rate over time as determined by whole cell fluorescence, Western blotting, confocal microscopy, and ligand binding. Quantitative PCR showed that A2a-GFP mRNA levels remained relatively high even as the protein production rate decreased. A cycloheximide chase experiment showed that the mature protein was stable over time and was not significantly degraded. Taken together, these results suggest that heterologous expression of GPCRs is limited by a translational or post-translational bottleneck that is unique from expression limitations seen for soluble proteins.

Optimisation of insect cell growth in deep-well blocks: development of a high-throughput insect cell expression screen

This report describes a method to culture insects cells in 24 deep-well blocks for the routine small-scale optimisation of baculovirus-mediated protein expression experiments. Miniaturisation of this process provides the necessary reduction in terms of resource allocation, reagents, and labour to allow extensive and rapid optimisation of expression conditions, with the concomitant reduction in lead-time before commencement of large-scale bioreactor experiments. This therefore greatly simplifies the optimisation process and allows the use of liquid handling robotics in much of the initial optimisation stages of the process, thereby greatly increasing the throughput of the laboratory. We present several examples of the use of deep-well block expression studies in the optimisation of therapeutically relevant protein targets. We also discuss how the enhanced throughput offered by this approach can be adapted to robotic handling systems and the implications this has on the capacity to conduct multi-parallel protein expression studies.

Long-term Effect of Convulsive Behavior on the Density of Adenosine A1 and A2A Receptors in the Rat Cerebral Cortex

PURPOSE

Adenosine is a neuromodulator that has been proposed to act as an anticonvulsant mainly via inhibitory A1 receptors, but recent data show that genetic deletion of facilitatory A 2A receptors might also attenuate convulsions. Since both A1 and A 2A receptors are prone to down- and upregulation in different stressful situations, we investigated if convulsive behavior leads to a long-term change in A1 and A 2A receptor density in the rat cerebral cortex.

METHODS

Stage 4-5 convulsions (Racine's scale) were induced in adult Wistar rats either through amygdala stimulation (kindling) or by intraperitoneal injection of kainate (10 mg/ml). Rats were killed after 4 weeks to evaluate adenosine A1 and A 2A receptor density in the cerebral cortex using both Western blot and membrane binding assays.

RESULTS

The binding density of the A1 antagonist, 3H-DPCPX, decreased by 40. +/- 4.4% and by 20.7 +/- 0.5% after kindling or kainate injection. Likewise, A1 receptor immunoreactivity in cortical membranes from kindled or kainate-injected rats decreased by 19.1 +/- 3.3% and 12.7 +/- 5.7%, respectively. In contrast, the binding density of the A 2A receptor antagonist 3H-SCH 58261 increased by 293 +/- 34% and by 159 +/- 32% in cortical membranes from kindled or kainate-injected rats, and A 2A receptor immunoreactivity also increased by 151 +/- 12% and 79.6 +/- 7.0%.

CONCLUSIONS

This indicates that after convulsive behavior there is a long-term decrease of A1 receptors accompanied by an increased density of A 2A receptors, suggesting that A 2A antagonists rather than A1 agonists may be more promising anticonvulsive drugs.

Oligomerization of the fifth transmembrane domain from the adenosine A2A receptor

The human adenosine A2A receptor (A(2A)R) belongs to one of the largest family of membrane proteins, the G-protein coupled receptors (GPCRs), characterized by seven transmembrane (TM) helices. Little is known about the determinants of their structures, folding, assembly, activation mechanisms, and oligomeric states. Previous studies in our group showed that peptides corresponding to all seven TM domains form stable helical structures in detergent micelles and lipid vesicles. However, the peptides behave differently; TM5 is the only peptide to have a ratio [theta]222/[theta]208 obtained by circular dichroism (CD) spectroscopy>1. This finding suggested to us that TM5 might self-associate. In the present study, we investigate the unique properties of the TM5 domain. We performed detailed analyses of TM5 peptide behavior in membrane-mimetic environments using CD spectroscopy, fluorescence spectroscopy and Förster resonance energy transfer, and gel electrophoresis. We find that TM5 peptide has the ability to self-associate to form oligomeric structures in various hydrophobic milieus and that these oligomers are highly resistant to temperature and chemical denaturation. We also find that mutation of the full-length A(2A)R at position M193, which is located in the fifth TM domain, noticeably alters A(2A)R monomer: dimer ratio as observed on SDS-PAGE. Our results suggest that parallel association of TM5 dimers may play a role in the known adenosine A2A receptor dimerization. This study represents the first evidence of an individual GPCR transmembrane domain self-association.

Hooray for hypoxia?

The mortality of critically ill patients rises steadily as the partial pressure of arterial oxygen falls below about 11 kPa (80 mm Hg). A new animal study in the May 2005 issue of PLoS Biology showing a potential benefit for hypoxia is thus a challenge to current thinking.

A2A adenosine receptors on bone marrow-derived cells protect liver from ischemia-reperfusion injury

Activation of the A2A adenosine receptor (A(2A)R) during reperfusion of various tissues has been found to markedly reduce ischemia-reperfusion injury. In this study, we used bone marrow transplantation (BMT) to create chimeric mice that either selectively lack or selectively express the A(2A)R on bone marrow-derived cells. Bolus i.p. injection of the selective A2A agonist, 4-[3-[6-amino-9-(5-cyclopropylcarbamoyl-3,4-dihydroxy-tetrahydro-furan-2-yl)-9H-purin-2-yl]-prop-2-ynyl]-piperidine-1-carboxylic acid methyl ester (ATL313; 3 microg/kg), at the time of reperfusion protects wild-type (wt) mice from liver ischemia-reperfusion injury. ATL313 also protects wt/wt (donor/recipient BMT mouse chimera) and wt/knockout chimera but produces modest protection of knockout/wt chimera as assessed by alanine aminotransferase activity, induction of cytokine transcripts (RANTES, IFN-gamma-inducible protein-10, IL-1alpha, IL-1-beta, IL-1Ralpha, IL-18, IL-6, and IFN-gamma), or histological criteria. ATL313, which is highly selective for the A(2A)R, produces more liver protection of chimeric BMT mice than 4-[3-[6-amino-9-(5-ethylcarbamoyl-3,4-dihydroxy-tetrahydro-furan-2-yl)-9H-purin-2-yl]-prop-2-ynyl]-cyclohexanecarboxylic acid methyl ester, which is rapidly metabolized in mice to produce 4-[3-[6-amino-9-(5-ethylcarbamoyl-3,4-dihydroxy-tetrahydro-furan-2-yl)-9H-purin-2-yl]-prop-2-ynyl]-cyclohexanecarboxylic acid, which has similar affinity for the A(2A)R and the proinflammatory A3 adenosine receptor. GFP chimera mice were created to show that vascular endothelial cells in the injured liver do not account for liver protection because they are not derived by transdifferentiation of bone marrow precursors. The data suggest that activation of the A(2A)R on bone marrow-derived cells is primarily responsible for protecting the liver from reperfusion injury.