An A2A adenosine receptor agonist, ATL313, reduces inflammation and improves survival in murine sepsis models

Background

The pathophysiology of sepsis is due in part to early systemic inflammation. Here we describe molecular and cellular responses, as well as survival, in A_2A adenosine receptor (AR) agonist treated and untreated animals during experimental sepsis.

Methods

Sepsis was induced in mice by intraperitoneal inoculation of live bacteria (Escherichia coli or Staphylococcus aureus) or lipopolysaccharide (LPS). Mice inoculated with live bacteria were treated with an A_2A AR agonist (ATL313) or phosphate buffered saline (PBS), with or without the addition of a dose of ceftriaxone. LPS inoculated mice were treated with ATL313 or PBS. Serum cytokines and chemokines were measured sequentially at 1, 2, 4, 8, and 24 hours after LPS was administered. In survival studies, mice were followed until death or for 7 days.

Results

There was a significant survival benefit in mice infected with live E. coli (100% vs. 20%, p = 0.013) or S. aureus (60% vs. 20%, p = 0.02) when treated with ATL313 in conjunction with an antibiotic versus antibiotic alone. ATL313 also improved survival from endotoxic shock when compared to PBS treatment (90% vs. 40%, p = 0.005). The serum concentrations of TNF-α, MIP-1α, MCP-1, IFN-γ, and IL-17 were decreased by ATL313 after LPS injection (p < 0.05). Additionally, ATL313 increased the concentration of IL-10 under the same conditions (p < 0.05). Circulating white blood cell concentrations were higher in ATL313 treated animals (p < 0.01).

Conclusion

Further studies are warranted to determine the clinical utility of ATL313 as a novel treatment for sepsis.

Timing of adenosine 2A receptor stimulation relative to reperfusion has differential effects on infarct size and cardiac function as assessed in mice by MRI

The activation of adenosine 2A receptors before reperfusion following coronary artery occlusion reduces infarct size and improves ejection fraction (EF). In this study, we examined the effects of delaying treatment with the adenosine 2A receptor agonist ATL146e (ATL) until 1 h postreperfusion. The infarct size and EF were serially assessed by gadolinium-diethylenetriaminepentaacetic acid-enhanced MRI in C57BL/6 mice at 1 and 24 h postreperfusion. The infarct size was also assessed by 2,3,5-triphenyltetrazolium chloride staining at 24 h. Mice were treated with ATL (10 microg/kg ip) either 2 min before reperfusion (early ATL) or 1 h postreperfusion (late ATL) following the 45-min coronary occlusion. The two methods used to assess infarct size at 24 h postreperfusion (MRI and 2,3,5-triphenyltetrazolium chloride) showed an excellent correlation (R=0.96). The risk region, determined at 24 h postreperfusion, was comparable between the control and ATL-treated groups. The infarct size by MRI at 1 versus 24 h postreperfusion was 25+/-1 vs. 26+/-1% of left ventricular mass (means+/-SE) in control mice, 16+/-2 versus 17+/-2% in early-ATL mice, and 24+/-2 versus 25+/-2% in late-ATL mice (intragroup, P=not significant; and intergroup, early ATL vs. control or late ATL, P<0.05). EF was reduced in control mice but was largely preserved between 1 and 24 h in both early-ATL and late-ATL mice (P<0.05). In conclusion, after coronary occlusion in mice, the extent of myocellular death due to ischemia-reperfusion injury is 95% complete within 1 h of reperfusion. The infarct size was significantly reduced by ATL when given just before reperfusion, but not 1 h postreperfusion. Either treatment window helped preserve the EF between 1 and 24 h postreperfusion.

2-aminothienopyridazines as novel adenosine A1 receptor allosteric modulators and antagonists

A pharmacophore-based screen identified 32 compounds including ethyl 5-amino-3-(4- tert-butylphenyl)-4-oxo-3,4-dihydrothieno[3,4- d]pyridazine-1-carboxylate ( 8) as a new allosteric modulator of the adenosine A1 receptor (A1AR). On the basis of this lead, various derivatives were prepared and evaluated for activity at the human A 1AR. A number of the test compounds allosterically stabilized agonist-receptor-G protein ternary complexes in dissociation kinetic assays, but were found to be more potent as antagonists in subsequent functional assays of ERK1/2 phosphorylation. Additional experiments on the most potent antagonist, 13b, investigating A1AR-mediated [(35)S]GTPgammaS binding and [(3)H]CCPA equilibrium binding confirmed its antagonistic mode of action and also identified inverse agonism. This study has thus identified a new class of A1AR antagonists that can also recognize the receptor's allosteric site with lower potency.

A2B adenosine receptor gene deletion attenuates murine colitis

BACKGROUND & AIMS

The A(2B) adenosine receptor (A(2B)AR) is the predominant adenosine receptor expressed in the colonic epithelia. We have previously shown that A(2B)AR mRNA and protein levels are up-regulated during colitis. In this study, we addressed the role of the A(2B)AR in the development of murine colitis and the potential mechanism underlying its effects.

METHODS

Dextran sodium sulfate (DSS), 2,4,6-trinitrobenzene sulfonic acid (TNBS), and Salmonella typhimurium were used to induce colitis in A(2B)AR-null mice (A(2B)AR(-/-)). Colitis was determined using established clinical and histologic scoring. Keratinocyte-derived chemokine (KC) measurements were performed using an enzyme-linked immunosorbent assay.

RESULTS

Colonic inflammation induced by DSS, TNBS, or S typhimurium was attenuated in A(2B)AR(-/-) compared with their wild-type counterparts. Clinical features, histologic score, and myeloperoxidase activity were significantly decreased in A(2B)AR(-/-) mice. However, A(2B)AR(-/-) showed increased susceptibility to systemic Salmonella infection. Tissue levels of the neutrophil chemokine, KC was decreased in colitic A(2B)AR(-/-) mice. In addition, flagellin-induced KC levels were attenuated in A(2B)AR(-/-) mice. Neutrophil chemotaxis in response to exogenous interleukin-8 was preserved in A(2B)AR(-/-) mice, suggesting intact neutrophil migration in response to appropriate stimuli.

CONCLUSIONS

These data demonstrate, for the first time, that the A(2B)AR plays a proinflammatory role in colitis. A(2B) receptor antagonism may be an effective treatment for acute inflammatory intestinal diseases such as acute flare of inflammatory bowel disease.

Adenosine A2A receptor activation reduces infarct size in the isolated, perfused mouse heart by inhibiting resident cardiac mast cell degranulation

Mast cells are found in the heart and contribute to reperfusion injury following myocardial ischemia. Since the activation of A2A adenosine receptors (A2AARs) inhibits reperfusion injury, we hypothesized that ATL146e (a selective A2AAR agonist) might protect hearts in part by reducing cardiac mast cell degranulation. Hearts were isolated from five groups of congenic mice: A2AAR+/+ mice, A2AAR(-/-) mice, mast cell-deficient (Kit(W-sh/W-sh)) mice, and chimeric mice prepared by transplanting bone marrow from A2AAR(-/-) or A2AAR+/+ mice to radiation-ablated A2AAR+/+ mice. Six weeks after bone marrow transplantation, cardiac mast cells were repopulated with >90% donor cells. In isolated, perfused hearts subjected to ischemia-reperfusion injury, ATL146e or CGS-21680 (100 nmol/l) decreased infarct size (IS; percent area at risk) from 38 +/- 2% to 24 +/- 2% and 22 +/- 2% in ATL146e- and CGS-21680-treated hearts, respectively (P < 0.05) and significantly reduced mast cell degranulation, measured as tryptase release into reperfusion buffer. These changes were absent in A2AAR(-/-) hearts and in hearts from chimeric mice with A2AAR(-/-) bone marrow. Vehicle-treated Kit(W-sh/W-sh) mice had lower IS (11 +/- 3%) than WT mice, and ATL146e had no significant protective effect (16 +/- 3%). These data suggest that in ex vivo, buffer-perfused hearts, mast cell degranulation contributes to ischemia-reperfusion injury. In addition, our data suggest that A2AAR activation is cardioprotective in the isolated heart, at least in part by attenuating resident mast cell degranulation.

Early adenosine receptor activation ameliorates spinal cord reperfusion injury

OBJECTIVES

Adenosine receptor activation at reperfusion has been shown to ameliorate ischemia-reperfusion injury of the spinal cord, but the effects of therapy given in response to ischemic injury are unknown. We hypothesized that adenosine receptor activation with ATL-146e would produce similar protection from ischemic spinal cord injury, whether given at reperfusion or in a delayed fashion.

METHODS

Twenty-two New Zealand white rabbits were divided into three groups. All three groups, including the ischemia-reperfusion group (IR, n = 8), underwent 45 min of infrarenal aortic occlusion. The early treatment group (early, n = 8) received 0.06 mug/kg/min of ATL-146e for 3 h beginning 10 min prior to reperfusion. The delayed treatment group (delayed, n = 6) received ATL-146e starting 1 h after reperfusion. After 48 h, hind limb function was graded using the Tarlov score. Finally, lumbar spinal cord neuronal cytoarchitecture was evaluated.

RESULTS

Hemodynamic parameters were similar among the groups. Hind limb function at 48 h was significantly better in the early group (3.5 +/- 1.0) compared to the IR group (0.625 +/- 0.5, P < or = 0.01). There was a trend towards better hind limb function in the early group compared to the delayed group (2.4 +/- 1.1, P = 0.08). Hind limb function was similar between delayed and IR groups. Hematoxylin-eosin spinal cord sections demonstrated preservation of viable motor neurons in the early group compared to the delayed and IR groups.

CONCLUSIONS

Early therapy with ATL-146e provided better protection in this study; therefore, therapy should not be delayed until there is evidence of ischemic neurological deficit. This study suggests that adenosine receptor activation is most effective as a preventive strategy at reperfusion for optimal protection in spinal cord ischemia-reperfusion injury.

Adenosine A2A receptor antagonists exert motor and neuroprotective effects by distinct cellular mechanisms

OBJECTIVE

To investigate whether the motor and neuroprotective effects of adenosine A(2A) receptor (A(2A)R) antagonists are mediated by distinct cell types in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of Parkinson's disease.

METHODS

We used the forebrain A(2A)R knock-out mice coupled with flow cytometric analyses and intracerebroventricular injection to determine the contribution of A(2A)Rs in forebrain neurons and glial cells to A(2A)R antagonist-mediated motor and neuroprotective effects.

RESULTS

The selective deletion of A(2A)Rs in forebrain neurons abolished the motor stimulant effects of the A(2A)R antagonist KW-6002 but did not affect acute MPTP neurotoxicity. Intracerebroventricular administration of KW-6002 into forebrain A(2A)R knock-out mice reinstated protection against acute MPTP-induced dopaminergic neurotoxicity and attenuated MPTP-induced striatal microglial and astroglial activation.

INTERPRETATION

A(2A)R activity in forebrain neurons is critical to the control of motor activity, whereas brain cells other than forebrain neurons (likely glial cells) are important components for protection against acute MPTP toxicity.

Activation of adenosine 2A receptors preserves structure and function of podocytes

Adenosine 2A receptor (A(2A)R) activation was recently shown to be renoprotective in diabetic nephropathy. A(2A)R are found in glomeruli and have been shown to associate with the podocyte cytoskeletal protein alpha-actinin-4, but the effect of their activation on podocyte structure and function is unknown. Podocyte injury was induced in C57BL/6 mice with puromycin aminonucleoside, and the selective A(2A)R agonist ATL313 was found to attenuate the resulting albuminuria and foot process fusion. The selective A(2A)R antagonist ZM241385 reversed the effects of ATL313. In vitro, A(2A)R mRNA and protein were expressed in a conditionally immortalized podocyte cell line, and A(2A)R-like immunoreactivity co-localized with the actin cytoskeleton. Treatment with ATL313 also blocked the increased podocyte permeability to albumin and disruption of the actin cytoskeleton that accompanied puromycin aminonucleoside-induced injury in vitro. ATL313 was ineffective, however, in the presence of the A(2A)R antagonist and in A(2A)R-deficient podocytes. It was concluded that A(2A)R activation reduces glomerular proteinuria, at least in part, by preserving the normal structure of podocyte foot processes, slit diaphragms, and actin cytoskeleton.

Synthesis of novel neutrophil-specific imaging agents for Positron Emission Tomography (PET) imaging

A neutrophil-specific peptide, cinnamoyl-F(D)LF(D)LFK (cFLFLFK), was conjugated consecutively with a polyethylene glycol moiety (3.4K) and 2,2',2'',2'''-(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetraacetic acid (DOTA) to form cFLFLFK-PEG-DOTA. After (64)Cu labeling, Positron Emission Tomography (PET) imaging was successfully able to detect mouse lung inflammation.

Dual acting antioxidant A1 adenosine receptor agonists

Herein we report the synthesis and biological evaluation of some potent and selective A(1) adenosine receptor agonists, which incorporate a functionalised linker attached to an antioxidant moiety. N(6)-(2,2,5,5-Tetramethylpyrrolidin-1-yloxyl-3-ylmethyl)adenosine (VCP28, 2e) proved to be an agonist with high affinity (K(i)=50nM) and good selectivity (A(3)/A(1) > or = 400) for the A(1) adenosine receptor. N(6)-[4-[2-[1,1,3,3-Tetramethylisoindolin-2-yloxyl-5-amido]ethyl]phenyl]adenosine (VCP102, 5a) has higher binding affinity (K(i)=7 nM), but lower selectivity (A(3)/A(1)= approximately 3). All compounds bind weakly (K(i)>1 microM) to A(2A) and A(2B) receptors. The combination of A(1) agonist activity and antioxidant activity has the potential to produce cardioprotective effects.

A2A receptor signaling promotes peripheral tolerance by inducing T-cell anergy and the generation of adaptive regulatory T cells

Tissue-derived adenosine, acting via the adenosine A(2A) receptor (A(2A)R), is emerging as an important negative regulator of T-cell function. In this report, we demonstrate that A(2A)R stimulation not only inhibits the generation of adaptive effector T cells but also promotes the induction of adaptive regulatory T cells. In vitro, antigen recognition in the setting of A(2A)R engagement induces T-cell anergy, even in the presence of costimulation. T cells initially stimulated in the presence of an A(2A)R agonist fail to proliferate and produce interleukin-2 and interferon (IFN)-gamma when rechallenged in the absence of A(2A)R stimulation. Likewise, in an in vivo model of autoimmunity, tissue-derived adenosine promotes anergy and abrogates tissue destruction. Indeed, A(2A)R stimulation inhibits interleukin-6 expression while enhancing the production of transforming growth factor-beta. Accordingly, treating mice with A(2A)R agonists not only inhibits Th1 and Th17 effector cell generation but also promotes the generation of Foxp3(+) and LAG-3(+) regulatory T cells. In this regard, A(2A)R agonists fail to prevent autoimmunity by LAG-3(-/-) clonotypic T cells, implicating an important role for LAG-3 in adenosine-mediated peripheral tolerance. Overall, our findings demonstrate that extracellular adenosine stimulates the A(2A)R to promote long-term T-cell anergy and the generation of adaptive regulatory T cells.

A1 adenosine receptor activation promotes angiogenesis and release of VEGF from monocytes

Adenosine is a proangiogenic purine nucleoside released from ischemic and hypoxic tissues. Of the 4 adenosine receptor (AR) subtypes (A1, A2A, A2B, and A3), the A2 and A3 have been previously linked to the modulation of angiogenesis. We used the chicken chorioallantoic membrane (CAM) model to determine whether A1 AR activation affects angiogenesis. We cloned and pharmacologically characterized chicken AR subtypes to evaluate the selectivity of various agonists and antagonists. Application of the A1 AR-selective agonist N6-cyclopentyladenosine (CPA; 100 nmol/L) to the CAM resulted in a 40% increase in blood vessel number (P<0.01), which was blocked by the A1 AR-selective antagonist C8-(N-methylisopropyl)-amino-N6-(5'-endohydroxy)-endonorbornan-2-yl-9-methyladenine (WRC-0571; 1 micromol/L). Selective A2A AR agonists did not stimulate angiogenesis in the CAM. In an ex vivo rat aortic ring model of angiogenesis that includes cocultured endothelial cells, fibroblasts, and smooth muscle cells, 50 nmol/L CPA did not directly stimulate capillary formation; however, medium from human mononuclear cells pretreated with CPA, but not vehicle, increased capillary formation by 48% (P<0.05). This effect was blocked by WRC-0571 (1.5 micromol/L) or anti-VEGF antibody (1 microg/mL). CPA (5 nmol/L) stimulated a 1.7-fold increase in VEGF release from the mononuclear cells. This is the first study to show that A1 AR activation induces angiogenesis. Stimulation of A2 ARs on endothelial cells results in proliferation and tube formation, and A2 and A3 ARs on inflammatory cells modulate release of angiogenic factors. We conclude that adenosine promotes a coordinated angiogenic response through its interactions with multiple receptors on multiple cell types.

Therapeutic anti-inflammatory effects of myeloid cell adenosine receptor A2a stimulation in lipopolysaccharide-induced lung injury

To determine the role of the adenosine receptor A2a in a murine model of LPS-induced lung injury, migration of polymorphonuclear leukocytes (PMNs) into the different compartments of the lung was determined by flow cytometry, microvascular permeability was assessed by the extravasation of Evans blue, and the release of chemotactic cytokines into the alveolar airspace was determined by ELISA. Measurements were performed in wild-type and A2a gene-deficient mice (A2a(-/-)). To differentiate the role of A2a on hemopoietic and nonhemopoietic cells, we created chimeric mice by transfer of bone marrow (BM) between wild-type and A2a(-/-) mice and used mice that lacked A2a expression selectively on myeloid cells (A2a(flox/flox) x LysM-cre). A specific A2a receptor agonist (ATL202) was used to evaluate its potential to reduce lung injury in vivo. In wild-type mice, therapeutic treatment with ATL202 reduced LPS-induced PMN recruitment, and release of cytokines. Pretreatment, but not posttreatment, also reduced Evans blue extravasation. In the BM chimeric mice lacking A2a on BM-derived cells, PMN migration into the alveolar space was increased by approximately 50%. These findings were confirmed in A2a(flox/flox) x LysM-cre mice. ATL202 was only effective when A2a was present on BM-derived cells. A2a agonists may be effective at curbing inflammatory lung tissue damage.

Adenosine A2A receptor agonists inhibit lipopolysaccharide-induced production of tumor necrosis factor-alpha by equine monocytes

Adenosine is an endogenous nucleoside that regulates many physiological processes by activating one or more adenosine receptor subtypes, namely A1, A2A, A2B and A3. The results of previous studies indicate that adenosine analogues inhibit lipopolysaccharide (LPS)-induced production of reactive oxygen species (ROS) by equine neutrophils primarily through activation of A2A receptors. Because peripheral blood monocytes produce cytokines that are responsible for many of the deleterious effects of LPS, the current study was performed to evaluate the effects of an array of novel adenosine receptor agonists on LPS-induced production of tumor necrosis factor-alpha (TNF-alpha), and to assess the selectively of these agonists for equine adenosine A2A over the A1 receptor. Radioligand binding studies performed with equine tissues expressing adenosine A1 and A2A receptor subtypes yielded a rank order of affinity for the equine A2A receptor of ATL307>ATL309 approximately ATL310 approximately ATL313>ATL202 approximately ATL361 approximately ATL376>ATL372>CGS21680>NECA. Co-incubation of equine peripheral blood monocytes with LPS and these agonists resulted in inhibition of TNF-alpha production with a rank order of potency that strongly correlated with their binding affinities for equine adenosine A2A receptors. Results of experiments performed with one of the adenosine receptor agonists (ATL313) and selective adenosine receptor antagonists confirmed that inhibition of LPS-induced production of TNF-alpha occurred via stimulation of A2A receptors. Although incubation of monocytes with IB-MECA, a compound purported to act as an adenosine A3 receptor agonist, reduced LPS-induced TNF-alpha production, this effect of IB-MECA was inhibited by the A2A selective antagonist ZM241385 but not by the A3 receptor antagonist MRS1220. These results indicate that the adenosine receptor subtype responsible for regulation of LPS-induced cytokine production by equine monocytes is the A2A receptor. To address the signal transduction mechanism responsible for the anti-inflammatory effects of ATL313 in equine monocytes, production of cAMP was compared in the presence and absence of either the adenosine A2A receptor antagonist ZM241385 or the adenosine A2B receptor antagonist MRS1706. In the absence of the antagonists, ATL313 increased production of cAMP; ZM241385 inhibited this effect of ATL313, whereas MRS1706 did not. Furthermore, incubation of monocytes with either the stable analogue of cAMP, dibutyryl cAMP, or forskolin, an activator of adenylyl cyclase, also inhibited LPS-induced production of TNF-alpha production by equine monocytes. Collectively, the results of the current study indicate that adenosine analogues inhibit LPS-induced production of TNF-alpha by equine monocytes primarily via activation of adenosine A2A receptors and do so in a cAMP-dependent manner. The results of this study indicate that stable adenosine analogues that are selective for adenosine A2A receptors may be suitable for development as anti-inflammatory drugs in horses.

Attenuation of reperfusion lung injury and apoptosis by A2A adenosine receptor activation is associated with modulation of Bcl-2 and Bax expression and activation of extracellular signal-regulated kinases

Adenosine receptors (AR) and extracellular signal-regulated kinases (ERK) have been implicated in tissue protection and apoptosis regulation during ischemia/reperfusion (I/R) injury. This study tests the hypothesis that reduction of reperfusion lung injury after A2A AR activation is associated with attenuation of apoptosis, modulation of ERK activation, and alterations in antiapoptotic and proapoptotic protein expression (Bcl-2 and Bax, respectively). Experiments were performed in intact-chest, spontaneously breathing cats in which the arterial branch of the left lower lung lobe was occluded for 2 h and reperfused for 3 h (I/R group). Animals were treated with the selective A2A AR agonist ATL313 given 5 min before reperfusion alone or in combination with the selective A2A AR antagonist ZM241385. Western blot analysis showed significant reduction in expression of Bcl-2 and increase in expression of Bax after reperfusion, compared with control lungs. Phosphorylated ERK1/2 levels were also increased after reperfusion. Compared with the I/R group, ATL313 markedly (P < 0.01) attenuated indices of injury and apoptosis including the percentage of injured alveoli, wet-dry weight ratio, myeloperoxidase activity, in situ terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end-labeling-positive cells, and caspase 3 activity and expression. Furthermore, compared with reperfused lungs, in ATL313-pretreated lungs, Western blot analysis demonstrated substantial ERK1/2 activation, increased expression of Bcl-2, and attenuated expression of Bax. The protective effects of ATL313 were blocked by pretreatment with ZM241385. In summary, the present study shows that in vivo activation of A2A AR confers protection against reperfusion lung injury. This protection is associated with decreased apoptosis and involves ERK1/2 activation and alterations in antiapoptotic Bcl-2 and proapoptotic Bax proteins.

Activation of adenosine 2A receptors attenuates allograft rejection and alloantigen recognition

The current studies investigated the in vitro and in vivo effect of adenosine 2A receptor (A(2A)R) agonists to attenuate allogenic immune activation. We performed MLRs with spleen T lymphocytes and APCs isolated from wild-type and A(2A)R knockout mice of both C57BL/6 and BALB/c background strains. Two-way MLR-stimulated T cell proliferation was reduced by ATL313, a selective A(2A)R agonist in a dose-responsive manner (approximately 70%; 10 nM), an effect reversed by the A(2A)R antagonist ZM241385 (100 nM). By one-way MLRs, we observed that ATL313's inhibitory effect was due to effects on both T cells and APCs. ATL313 suppressed the activation markers CD25 and CD40L and the release of inflammatory cytokines IFN-gamma, RANTES, IL-12P(70), and IL-2. ATL313 also increased negative costimulatory molecules programmed death-1 and CTLA-4 expressed on T cells. In lymphocytes activated with anti-CD3e mAb, ATL313 inhibited the phosphorylation of Zap70, an effect that was reversed by the protein kinase A inhibitor H-89. In skin transplants, allograft survival was enhanced with ATL313, an effect blocked by ZM241385. These results indicate that A(2A)R agonists attenuate allogenic recognition by action on both T lymphocytes and APCs in vitro and delayed acute rejection in vivo. We conclude that A(2A)R agonists may represent a new class of compounds for induction therapy in organ transplantation.

Adenosine generation catalyzed by CD39 and CD73 expressed on regulatory T cells mediates immune suppression

The study of T regulatory cells (T reg cells) has been limited by the lack of specific surface markers and an inability to define mechanisms of suppression. We show that the expression of CD39/ENTPD1 in concert with CD73/ecto-5′-nucleotidase distinguishes CD4⁺/CD25⁺/Foxp3⁺ T reg cells from other T cells. These ectoenzymes generate pericellular adenosine from extracellular nucleotides. The coordinated expression of CD39/CD73 on T reg cells and the adenosine A2A receptor on activated T effector cells generates immunosuppressive loops, indicating roles in the inhibitory function of T reg cells. Consequently, T reg cells from Cd39-null mice show impaired suppressive properties in vitro and fail to block allograft rejection in vivo. We conclude that CD39 and CD73 are surface markers of T reg cells that impart a specific biochemical signature characterized by adenosine generation that has functional relevance for cellular immunoregulation.

Cardioprotection by adenosine A2A agonists in a canine model of myocardial stunning produced by multiple episodes of transient ischemia

We sought to determine whether administration of a very low, nonvasodilating dose of a highly selective adenosine A(2A) receptor agonist (ATL-193 or ATL-146e) would be cardioprotective in a canine model of myocardial stunning produced by multiple episodes of transient ischemia. Twenty-four anesthetized open-chest dogs underwent either 4 (n=12) or 10 cycles (n=12) of 5-min left anterior descending coronary artery (LAD) occlusions interspersed by 5 or 10 min of reperfusion. Left ventricular thickening was measured from baseline through 180 min after the last occlusion-reperfusion cycle. Regional flow was measured with microspheres. In 12 of 24 dogs, A(2A) receptor agonist was infused intravenously beginning 2 min prior to the first occlusion and continuing throughout reperfusion at a dose below that which produces vasodilatation (0.01 microg x kg(-1) x min(-1)). Myocardial flow was similar between control and A(2A) receptor agonist-treated animals, confirming the absence of A(2) receptor agonist-induced vasodilatation. During occlusion, there was severe dyskinesis with marked LAD zone thinning in all animals. After 180 min of reperfusion following the last cycle, significantly greater recovery of LAD zone thickening was observed in A(2A) receptor agonist-treated vs. control animals in both the 4-cycle (91 +/- 7 vs. 56 +/- 12%, respectively; P<0.05) and the 10-cycle (65 +/- 9 vs. 8 +/- 16%, respectively; P<0.05) occlusion groups. The striking amount of functional recovery observed with administration of low, nonvasodilating doses of adenosine A(2A) agonist ATL-193 or ATL-146e supports their further evaluation for the attenuation of postischemic stunning in the clinical setting.

Potent and orally bioavailable 8-bicyclo[2.2.2]octylxanthines as adenosine A1 receptor antagonists

In the search for a selective adenosine A1 receptor antagonist with greater aqueous solubility than the compounds currently in clinical trials as diuretics, a series of 1,4-substituted 8-cyclohexyl and 8-bicyclo[2.2.2]octylxanthines were investigated. The binding affinities of a variety of cyclohexyl and bicyclo[2.2.2]octylxanthines for the rat and human adenosine A1, A2A, A2B, and A3 receptors are presented. Bicyclo[2.2.2]octylxanthine 16 exhibited good pharmaceutical properties and in vivo activity in a rat diuresis model (ED50=0.3 mg/kg po). Optimization of the bridgehead substituent led to propionic acid 29 (BG9928), which retained high potency (hA1, Ki=7 nM) and selectivity for the adenosine A1 receptor (915-fold versus adenosine A2A receptor; 12-fold versus adenosine A2B receptor) with improved oral efficacy in the rat diuresis model (ED50=0.01 mg/kg) as well as high oral bioavailability in rat, dog, and cynomolgus monkey.

Subcellular distributions of adenosine A1 and A2A receptors in the rat dorsomedial nucleus of the solitary tract at the level of the area postrema

Adenosine A1 and A2A receptors mediate distinct cardiovascular components of defense reactions that are ascribed, in part, to opposing actions within the nucleus tractus solitarius. To assess the cellular sites of relevance to these actions, we examined the light and electron microscopic immunolabeling of adenosine A1 and A2A receptors in the rat dorsomedial nucleus of the solitary tract at the level of the area postrema (dmNTS-AP), a region crucial for cardiovascular regulation involving vagal baroreceptor afferents. Immunoreactivity for each receptor was independently localized to distinct segments of plasma membranes and endomembranes in somatodendritic, axonal, and glial profiles. The dendritic labeling for each receptor also was detected within and near asymmetric, excitatory-type synapses. Of all peroxidase labeled profiles exclusive of somata, approximately 58% were A1- and 39% were A2A-labeled dendrites. Dendrites and astrocytic glia were the profiles that most often expressed both subtypes of adenosine receptors. The axonal labeling for A2A receptors was seen mainly in unmyelinated axons, whereas the A1 receptors were prominently localized within axon terminals. These terminals often formed single or multisynaptic excitatory-type junctions or single symmetric synapses on dendrites, a few of which expressed A1 and A2A receptors. These results provide the first ultrastructural evidence that A1 and A2A receptors have distributions conductive to their dual involvement in modulating the output of single neurons and glial function in the dmNTS-AP, where the predominate presynaptic effects of adenosine are mediated through A1 receptors.

Adenosine A2A receptors in bone marrow-derived cells but not in forebrain neurons are important contributors to 3-nitropropionic acid-induced striatal damage as revealed by cell-type-selective inactivation

Endogenous adenosine acting at the adenosine A2A receptor (A2AR) can modify brain injury in a variety of neurological disorder models. However, both A2AR activation and inactivation have been shown to be neuroprotective in different situations, raising the intriguing possibility that A2ARs in distinct cellular elements may have different and even opposing effects. In this study, we developed three novel transgenic models to dissect out cell-type-specific actions of A2ARs on striatal damage by the mitochondrial toxin 3-nitropropionic acid (3-NP). Whereas global inactivation of A2ARs exacerbated 3-NP-induced neurological deficit behaviors and striatal damage, selective inactivation of A2ARs in forebrain neurons (using the Cre/loxP strategy) did not affect neurological deficit or striatal damage after the acute systemic treatment of 3-NP and intrastriatal injection of malonate. However, selective inactivation of A2ARs in bone marrow-derived cells (BMDCs) by transplanting bone marrow cells from global A2AR knock-out (KO) mice into wild-type C57BL/6 mice produced a similar phenotype of global A2AR KO mice, i.e., exacerbation of 3-NP-induced striatal damage. Thus, cell-type-selective inactivation of A2ARs reveals that A2ARs in BMDCs but not in forebrain neurons are an important contributor to striatal damage induced by mitochondrial dysfunction.

Influence of differential stability of G protein βγ dimers containing the γ11 subunit on functional activity at the M1 muscarinic receptor, A1 adenosine receptor, and phospholipase C-β

Ggamma11 is an unusual guanine nucleotide-binding regulatory protein (G protein) subunit. To study the effect of different Gbeta-binding partners on gamma11 function, four recombinant betagamma dimers, beta1gamma2, beta4gamma2, beta1gamma11, and beta4gamma11, were characterized in a receptor reconstitution assay with the G(q)-linked M1 muscarinic and the G(i1)-linked A1 adenosine receptors. The beta4gamma11 dimer was up to 30-fold less efficient than beta4gamma2 at promoting agonist-dependent binding of [35S]GTPgammaS to either alpha(q) or alpha(i1). Using a competition assay to measure relative affinities of purified betagamma dimers for alpha, the beta4gamma11 dimer had a 15-fold lower affinity for G(i1) alpha than beta4gamma2. Chromatographic characterization of the beta4gamma11 dimer revealed that the betagamma is stable in a heterotrimeric complex with G(i1) alpha; however, upon activation of alpha with MgCl2 and GTPgammaS under nondenaturing conditions, the beta4 and gamma11 subunits dissociate. Activation of purified G(i1) alpha:beta4gamma11 with Mg+2/GTPgammaS following reconstitution into lipid vesicles and incubation with phospholipase C (PLC)-beta resulted in stimulation of PLC-beta activity; however, when this activation preceded reconstitution into vesicles, PLC-beta activity was markedly diminished. In a membrane coupling assay designed to measure the ability of G protein to promote a high-affinity agonist-binding conformation of the A1 adenosine receptor, beta4gamma11 was as effective as beta4gamma2 when coexpressed with G(i1) alpha and receptor. However, G(i1) alpha:beta4gamma11-induced high-affinity binding was up to 20-fold more sensitive to GTPgammaS than G(i1) alpha:beta4gamma2-induced high-affinity binding. These results suggest that the stability of the beta4gamma11 dimer can modulate G protein activity at the receptor and effector.

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.

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.

New insights into the regulation of inflammation by adenosine

Adenosine, long known as a regulator of cardiovascular function, has recently been identified as a significant paracrine inhibitor of inflammation that acts primarily by activation of A2A adenosine receptors (A2AARs) on lymphoid or myeloid cells. In this issue of the JCI, Yang et al. describe a proinflammatory phenotype resulting from deletion of the gene encoding the A2B adenosine receptor (A2BAR) in the mouse, suggesting that activation of the A2BAR can also have antiinflammatory effects (see the related article beginning on page 1913). Nevertheless, the role of the A2BAR remains enigmatic since its activation can either stimulate or inhibit the release of proinflammatory cytokines in different cells and tissues.

Functional and cytoarchitectural spinal cord protection by ATL-146e after ischemia/reperfusion is mediated by adenosine receptor agonism

BACKGROUND

ATL-146e protects the spinal cord from ischemia/reperfusion injury, presumably via adenosine A(2A) receptor activation, but this relationship remains unproven. We hypothesized that spinal cord functional and cytoarchitectural preservation from ATL-146e would be lost with simultaneous administration of the specific adenosine A(2A) antagonist ZM241385 (ZM), thus proving that adenosine A(2A) receptor activation is responsible for the protective effects of this compound.

METHODS

New Zealand White rabbits underwent 45 minutes of infrarenal aortic cross-clamping. Groups (n = 10) included sham, ischemia, ischemia plus ATL-146e (ATL-146E), ischemia plus ZM, or ischemia with both compounds (agonist-antagonist). Tarlov scores were recorded every 12 hours. After 48 hours, the spinal cord was fixed for histology and microtubule-associated protein 2 immunohistochemistry.

RESULTS

Tarlov scores at 48 hours were significantly better in the sham and ATL-146E groups (5.0 and 3.9, respectively) compared with the other three groups (all < or =1.3; P < .001). On hematoxylin and eosin, neuronal viability was higher in the sham, ATL-146E, and agonist-antagonist groups compared with the control and ZM groups (P < .05). Microtubule-associated protein 2 expression was preserved in the sham and ATL-146E groups but was lost in the ATL + ZM, ZM241385, and control groups.

CONCLUSIONS

ATL-146e preserves the spinal cord in terms of both cytoarchitecture and function after reperfusion of the ischemic spinal cord, but this preservation is not completely blocked by competitive adenosine A(2A) receptor antagonism. Although ATL-146e does seem to partially function through activation of the adenosine A(2A) receptor, the neuroprotective mechanism may not be limited to this particular receptor.

Comparison of systemic and retrograde delivery of adenosine A2A agonist for attenuation of spinal cord injury after thoracic aortic cross-clamping

BACKGROUND

Paraplegia remains a devastating complication of thoracic aortic surgery, which has been attenuated by retrograde adenosine and systemic adenosine A2A receptor activation. We hypothesized that despite retrograde spinal perfusion of an adenosine A2A agonist (ATL-146e), systemic therapy produces superior spinal cord protection with reduced inflammation.

METHODS

Forty pigs underwent 30-minute thoracic aortic cross-clamping. Pigs received: no therapy (control); retrograde saline (retrograde control); retrograde ATL-146e; systemic ATL-146e; systemic ATL-146e with retrograde saline; or systemic and retrograde ATL-146e. Retrograde therapies were given during ischemia. Systemic ATL-146e (0.06 microg.kg(-1).min(-1)) was given intravenously for 3 hours at reperfusion. At 24 hours, motor function was assessed using the Tarlov scale. Tissue was analyzed for neuronal viability, microtubule-associated protein-2 expression, and neutrophil sequestration (myeloperoxidase activity).

RESULTS

Four pigs received retrograde barium showing both radiographic and histologic spinal cord perfusion. Tarlov scores at 24 hours were significantly improved versus both control groups in all ATL groups except the combined ATL-146e group (all p < 0.05). Neuronal viability by hematoxylin and eosin stain was significantly preserved in systemic ATL groups compared with both control groups (all p < 0.05). Microtubule-associated protein-2 expression was significantly preserved compared with both control groups in all systemic ATL groups. Systemic ATL significantly lowered myeloperoxidase activity versus both control groups (p < 0.01).

CONCLUSIONS

Both retrograde and systemic ATL-146e therapies attenuate ischemic spinal cord injury, but combining the two routes was less effective. Given comparable results between the two routes and the simplicity of systemic delivery, peripheral venous ATL-146e at reperfusion should be preferred for spinal cord protection in thoracic aortic surgery.

A selective adenosine A2A receptor agonist, ATL-146e, prevents concanavalin A-induced acute liver injury in mice

BACKGROUND AND AIMS

Concanavalin A (Con A) activates T lymphocytes and induces CD4+ T cell-mediated hepatic injury in mice. Pro-inflammatory cytokines, such as tumor necrosis factor-alpha (TNF-alpha), interferon-gamma (IFN-gamma), and interleukin-6 (IL-6), are critical mediators in this experimental model. Activation of adenosine A2A receptors reduces the production of various pro-inflammatory cytokines and suppresses T cell activation. A selective adenosine A2A receptor agonist (ATL-146e) has been shown to be a potent inhibitor of inflammation by increasing intracellular cyclic AMP (cAMP) in leukocytes. The aim of the present study was to determine whether ATL-146e could ameliorate Con A-induced hepatic injury, reduction of pro-inflammatory cytokine production.

METHODS

Balb/c mice were injected with 25mg/kg Con A with or without a single injection of ATL-146e (0.5-50 microg/kg), 5 min prior to Con A administration. Liver enzymes, histology, and serum levels of tumor necrosis factor-alpha, interferon-gamma, and interleukin-6 were examined. We also assessed the effects of ATL-146e on pro-inflammatory cytokine production with CD4+ T cell.

RESULTS

Pretreatment with ATL-146e significantly reduced serum levels of liver enzymes (P<0.001). The serum pro-inflammatory cytokines were all increased after Con A administration and reduced to near normal levels by ATL-146e. ATL-146e also inhibited CD4+ T cell pro-inflammatory cytokine production.

CONCLUSION

A selective adenosine A2A receptor agonist, ATL-146e, can prevent concanavalin A-induced hepatic injury that is presumably mediated by its anti-inflammatory properties.

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.

Effects of cis-9, trans-11, CLA in rats at intake levels reported for breast-fed infants

CLA intake in exclusively breast-fed infants is close to levels found to have physiological effects in animals. However, in the majority of studies mixtures of CLA isomers have been used and the independent effects of the major CLA isomer in human milk, cis-9,trans-11 CLA, at the intake level in exclusively breast-fed infants have hardly been studied. We therefore studied the effects of cis-9,trans-11 CLA on plasma lipids and glucose, immune function, and bone metabolism in growing rats. Thirty male Sprague-Dawley rats (n = 10/group) were fed either 20 mg/kg/d cis-9,trans-11 CLA and 20 mg/kg/d sunflower oil (CLA20), 40 mg/kg/d cis-9,trans-11 CLA (CLA40), or 40 mg/kg/d sunflower oil (placebo) for 8 wk. No significant differences between groups were found in plasma lipids, glucose, insulin, C-reactive protein, or lipid peroxidation. Liver fat content was lowest in the CLA20 group. In vitro interleukin 2 (IL-2) production increased, and tumor necrosis factor alpha, IL-1beta, prostaglandin E2, and leukotriene B4 production decreased in the CLA20 group. No differences between groups were detected in IL-4, IL-6, or interferon gamma production, plasma osteocalcin, insulin-like growth factor, or urinary deoxypyridinoline crosslinks. Plasma tartrate-resistant acid phosphatase 5b activity was significantly increased in the CLA40 group. The results indicate anti-inflammatory effects and enhanced T-cell function for the CLA20 group. No adverse effects were seen in the CLA20 group, whereas indications of increased bone resorption rate were observed in the CLA40 group.

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.

Characterization of [125I]ZM 241385 binding to adenosine A2A receptors in the pineal of sheep brain

Adenosine is a ubiquitous neuromodulator and homeostatic regulator that exerts its physiologic actions through activation of A(1), A(2A), A(2B) and A(3) adenosine receptor subtypes. In the central nervous system, adenosine's action in neurons is manifested in its modulation of tonic inhibitory control. Adenosine released in the brain during hypoxia has critical depressant effects on breathing in fetal and newborn mammals, an action suggested to be mediated by A(2A) receptors in the posteromedial thalamus. In an effort to more accurately define the spatial distribution of adenosine A(2A) receptors in fetal sheep diencephalon, we have used a receptor autoradiographic technique utilizing an iodinated radioligand [(125)I]ZM 241385, which has greater sensitivity and resolution than the tritiated compound. The distribution of ligand binding sites in the fetal sheep diencephalon indicated that the highest levels of binding were in select thalamic nuclei, including those implicated in hypoxic depression of fetal breathing, and the pineal. Given the high density of labeled A(2A) receptors in the pineal, these sites were characterized more fully in homogenate radioligand binding assays. These data indicate that [(125)I]ZM 241385 binding sites display a pharmacological signature consistent with that of adenosine A(2A) receptors and are expressed at similar levels in fetal, lamb and adult ovine brain. The adenosine A(2A) receptor pharmacologic signature of the [(125)I]ZM 241385 binding site in pineal cell membranes generalized to the site characterized in membranes derived from other portions of the lamb thalamus, including the sector involved in hypoxic inhibition of fetal breathing. These results have important implications for the functional roles of adenosine A(2A) receptors in the thalamus and pineal of sheep brain.

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).

Effect of novel A2A adenosine receptor agonist ATL 313 on Clostridium difficile toxin A-induced murine ileal enteritis

Clostridium difficile is a spore-forming, anaerobic, gram-positive bacillus that releases two main virulence factors: toxins A and B. Toxin A plays an important pathogenic role in antibiotic-induced diarrhea and pseudomembranous colitis, a condition characterized by intense mucosal inflammation and secretion. Agonist activity at A2A adenosine receptors attenuates inflammation and damage in many tissues. This study evaluated the effects of a new selective A2A adenosine receptor agonist (ATL 313) on toxin A-induced injury in murine ileal loops. ATL 313 (0.5 to 5 nM) and/or the A2A adenosine receptor antagonist (ZM241385; 5 nM) or phosphate-buffered saline (PBS) were injected into ileal loops immediately prior to challenge with toxin A (1 to 10 microg/loop) or PBS. Intestinal fluid volume/length and weight/length ratios were calculated 3 h later. Ileal tissues were collected for the measurement of myeloperoxidase, adenosine deaminase activity, tumor necrosis factor alpha (TNF-alpha) production, histopathology, and detection of cell death by the TUNEL (terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling) method. Toxin A significantly increased volume/length and weight/length ratios in a dose-dependent fashion. ATL 313 treatment significantly (P < 0.05) reduced toxin A-induced secretion and edema, prevented mucosal disruption, and neutrophil infiltration as measured by myeloperoxidase activity. ATL 313 also reduced the toxin A-induced TNF-alpha production and adenosine deaminase activity and prevented toxin A-induced cell death. These protective effects of ATL 313 were reversed by ZM241385. In conclusion, the A2A adenosine receptor agonist, ATL 313, reduces tissue injury and inflammation in mice with toxin A-induced enteritis. The finding of increased ileal adenosine deaminase activity following the administration of toxin A is new and might contribute to the pathogenesis of the toxin A-induced enteritis by deaminating endogenous adenosine.

N6-Ethyl-2-alkynyl NECAs, selective human A3 adenosine receptor agonists

A series of N6-ethyl-2-alkynyl NECA (5'-N-ethylcarboxamidoadenosine) analogs were synthesized and their binding affinity with the four human adenosine receptors was evaluated. One of the compounds ZR1121 shows high affinity with hA3 receptor and its selectivity over hA1 receptor is 1-2 log orders greater than IB-MECA or Cl-IB-MECA, the currently employed selective A3 agonists.

Renal ischemia-reperfusion injury and adenosine 2A receptor-mediated tissue protection: the role of CD4+ T cells and IFN-gamma

A(2A) adenosine receptor (A(2A)R)-expressing bone marrow (BM)-derived cells contribute to the renal protective effect of A(2A) agonists in renal ischemia-reperfusion injury (IRI). We performed IRI in mice lacking T and B cells to determine whether A(2A)R expressed in CD4+ cells mediate protection from IRI. Rag-1 knockout (KO) mice were protected in comparison to wild-type (WT) mice when subjected to IRI. ATL146e, a selective A(2A) agonist, did not confer additional protection. IFN-gamma is an important early signal in IRI and is thought to contribute to reperfusion injury. Because IFN-gamma is produced by kidney cells and T cells we performed IRI in BM chimeras in which the BM of WT mice was reconstituted with BM from IFN-gamma KO mice (IFN-gamma KO-->WT chimera). We observed marked reduction in IRI in comparison to WT-->WT chimeras providing additional indirect support for the role of T cells. To confirm the role of CD4+ A(2A)R in mediating protection from IRI, Rag-1 KO mice were subjected to ischemia-reperfusion. The protection observed in Rag-1 KO mice was reversed in Rag-1 KO mice that were adoptively transferred WT CD4+ cells (WT CD4+-->Rag-1 KO) or A(2A) KO CD4+ cells (A(2A) KO CD4+-->Rag-1 KO). ATL146e reduced injury in WT CD4+-->Rag-1 KO mice but not in A(2A) KO CD4+-->Rag-1 KO mice. Rag-1 KO mice reconstituted with CD4+ cells derived from IFN-gamma KO mice (IFN-gamma CD4+-->Rag-1 KO) were protected from IRI; ATL146e conferred no additional protection. These studies demonstrate that CD4+ IFN-gamma contributes to IRI and that A(2A) agonists mediate protection from IRI through action on CD4+ cells.