Ligand-dependent cholesterol interactions with the human A(2A) adenosine receptor

We present nearly 10 μs of all-atom simulation data of a G-protein coupled receptor, the human A2A adenosine receptor, bound to four different ligands. Our focus is on binding of cholesterol to the "cholesterol consensus motif," a cluster of five amino acids on the second and fourth transmembrane helices, which interact with two cholesterols in the intracellular leaflet of the bilayer. We find evidence for a ligand-specific interaction between the CCM and cholesterol, mediated by the rotameric dynamics and configuration of Trp129. Binding of the synthetic agonist UK432097 disrupts hydrogen bonding between Trp129 and Ser47, which activates the rotameric dynamics of Trp129 and disrupts the interaction with one of the two cholesterols. We also investigate the effect of four thermostabilizing mutations, three of which are located on helix two. The conformational stability of helix two has been proposed to be sensitive to interaction with cholesterol in the CCM, suggesting a mechanism for the thermostabilization. However, our data are instead suggestive of a force-field dependent "straightening" of helix two, and therefore offer no basis for rationalizing the effect of the quadruple mutant.

Adenosine A2A receptor and TNF-α regulate the circadian machinery of the human monocytic THP-1 cells

Morning stiffness and increased symptoms of inflammatory arthritis are among the most common manifestations of rheumatoid arthritis (RA). Tumor necrosis alpha (TNF-α), an important mediator of inflammation in RA, regulates the circadian expression of clock proteins, and adenosine A(2A) receptors (A(2A)R) mediate many of the anti-inflammatory and antirheumatic actions of methotrexate, the cornerstone drug in the treatment of RA. We found that A(2A)R activation and TNF-α activated the clock core loop of the human monocytic THP-1 cell line. We further observed that interleukin (IL)-10, but not IL-12, mRNA expression fluctuates in a circadian fashion and that TNF-α and A(2A)R stimulation combined increased IL-10 expression. Interestingly, TNF-α, but not CGS21680, dramatically inhibited IL-12 mRNA expression. The demonstration that A(2A)R and TNF-α regulate the intrinsic circadian clock in immune cells provides an explanation for both the pathologic changes in circadian rhythms in RA and for the adverse circadian effects of methotrexate, such as fatigue.

New synaptic and molecular targets for neuroprotection in Parkinson's disease

The defining anatomical feature of Parkinson's disease (PD) is the degeneration of substantia nigra pars compacta (SNc) neurons, resulting in striatal dopamine (DA) deficiency and in the subsequent alteration of basal ganglia physiology. Treatments targeting the dopaminergic system alleviate PD symptoms but are not able to slow the neurodegenerative process that underlies PD progression. The nucleus striatum comprises a complex network of projecting neurons and interneurons that integrates different neural signals to modulate the activity of the basal ganglia circuitry. In this review we describe new potential molecular and synaptic striatal targets for the development of both symptomatic and neuroprotective strategies for PD. In particular, we focus on the interaction between adenosine A2A receptors and dopamine D2 receptors, on the role of a correct assembly of NMDA receptors, and on the sGC/cGMP/PKG pathway. Moreover, we also discuss the possibility to target the cell death program parthanatos and the kinase LRRK2 in order to develop new putative neuroprotective agents for PD acting on dopaminergic nigral neurons as well as on other basal ganglia structures.

Upregulation of adenosine A2A receptors induced by atypical antipsychotics and its correlation with sensory gating in schizophrenia patients

Sensory gating deficits have been found in patients with schizophrenia and their unaffected relatives. However, the underlying neurobiological mechanism of this deficit remains unclear. Pre-clinical studies have implicated adenosine in sensory gating deficits in schizophrenia. Therefore, the current study investigated a possible relationship between peripheral adenosine A2A receptor (ADORA2A) and sensory gating indices (P50 measures) in medication-free schizophrenia (n=31) and healthy (n=21) groups. The effects of six-week antipsychotic treatment were examined. At baseline, schizophrenia patients showed impaired sensory gating compared to healthy controls. However, there was no significant difference in ADORA2A gene expression among groups. In addition, ADORA2A expression was not correlated with sensory gating at any time point. Following treatment, we found a significant upregulation of ADORA2A expression. Intriguingly, we observed a significant positive association between ADORA2A upregulation and baseline P50 amplitudes in the schizophrenia group. A main finding of the current pilot study is the upregulation of ADORA2A expression following treatment with antipsychotics. In addition, this upregulation was predicted by baseline P50 amplitude, an observation that awaits replication in an expanded sample.

Adenosine A2A receptor deficiency up-regulates cystatin F expression in white matter lesions induced by chronic cerebral hypoperfusion

In previous studies, we have shown that the inactivation of the adenosine A2A receptor exacerbates chronic cerebral hypoperfusion-induced white matter lesions (WMLs) by enhancing neuroinflammatory responses. However, the molecular mechanism underlying the effect of the adenosine A2A receptor remains unknown. Recent studies have demonstrated that cystatin F, a potent endogenous cysteine protease inhibitor, is selectively expressed in immune cells in association with inflammatory demyelination in central nervous system diseases. To understand the expression of cystatin F and its potential role in the effect of A2A receptor on WMLs induced through chronic cerebral hypoperfusion, we investigated cystatin F expression in the WMLs of A2A receptor gene knockout mice, the littermate wild-type mice and wild-type mice treated daily with the A2A receptor agonist CGS21680 or both CGS21680 and A2A receptor antagonist SCH58261 after chronic cerebral hypoperfusion. The results of quantitative-PCR and western blot analysis revealed that cystatin F mRNA and protein expression were significantly up-regulated in the WMLs after chronic cerebral hypoperfusion. In addition, cystatin F expression in the corpus callosum was significantly increased in A2A receptor gene knockout mice and markedly decreased in mice treated with CGS21680 on both the mRNA and protein levels. Additionally, SCH58261 counteracted the attenuation of cystatin F expression produced by CGS21680 after chronic cerebral hypoperfusion. Moreover, double immunofluorescence staining revealed that cystatin F was co-localized with the activated microglia marker CD11b. In conclusion, the cystatin F expression in the activated microglia is closely associated with the effect of the A2A receptors, which may be related to the neuroinflammatory responses occurring during the pathological process.

[Sleep disturbance induced by cocaine abstinence involving in A2A receptor over-expression in rat hypothalamus]

Adult rats were implanted with sleep-wake recording electrodes in our experiments. Polygraphic signs of undisturbed sleep-wake activities were recorded for 24 h before cocaine administration, cocaine withdrawal day 1 (acute), day 8 (subacute), and day 14 (subchronic). Western blot method was performed to examine the expression levels of adenosine receptor subtypes in hypothalamus and cerebellum. Non rapid eye movement (NREM) sleep was significantly increased during nighttime (P < 0.01) and daytime (P < 0.05) on withdrawal day 8. The increase of NREM sleep was significant during nighttime (P < 0.01) and slight during daytime on withdrawal day 14, whereas both daytime and nighttime rapid eye movement (REM) sleeps were reduced markedly (P < 0.01) on withdrawal day 8 and 14. In addition, A2A receptor level was significantly enhanced on cocaine withdrawal day 8 and day 14 (P < 0.05), whereas A1 receptor level reduced markedly on withdrawal day 14 (P < 0.05). However, compared with that in the control group, no significant changes existed among adenosine A1, A2A and A2B receptors in rat cerebellum on cocaine withdrawal day 1, day 8 and day 14. Our findings suggest that sleep disorder caused by subacute and subchronic cocaine abstinence may be associated with over-expression of adenosine A2A receptor in rat hypothalamus to some extent.

The role of purinergic signaling in depressive disorders

The purinergic signaling system consists of transporters, enzymes and receptors responsible for the synthesis, release, action and extracellular inactivation of adenosine 5'-triphosphate (ATP) and its extracellular breakdown product adenosine. The actions of ATP are mediated ionotropic P2X and metabotropic P2Y receptor subfamilies, whilst the actions of adenosine are mediated by P1 adenosine receptors. Purinergic signaling pathways are widely expressed in the central nervous system (CNS) and participate in its normal and pathological functions. Among P2X receptors, the P2X7 receptor (P2rx7) has received considerable interest in both basic and clinical neuropsychiatric research because of its profound effects in animal CNS pathology and its potential involvement as a susceptibility gene in mood disorders. Although genetic findings were not always consistently replicated, several studies demonstrated that single nucleotide polymorphisms (SNPs) in the human P2X7 gene (P2RX7) show significant association with major depressive disorder and bipolar disorder. Animal studies revealed that the genetic knock-down or pharmacological antagonism leads to reduced depressive-like behavior, attenuated response in mania-model and alterations in stress reactivity. A potential mechanism of P2rx7 activation on mood related behavior is increased glutamate release, activation of extrasynaptic NMDA receptors and subsequent enduring changes in neuroplasticity. In addition, dysregulation of monoaminergic transmission and HPA axis reactivity could also contribute to the observed changes in behavior. Besides P2rx7, the inhibition of adenosine A1 and A2A receptors also mediate antidepressant-like effects in animal experiments. In conclusion, despite contradictions between existing data, these findings point to the therapeutic potential of the purinergic signaling system in mood disorders.

Adenosine A2A antagonists in Parkinson's disease: what's next?

Parkinson's disease (PD) is the second most prevalent neurodegenerative disorder, affecting up to 10 million people worldwide. Current treatment primarily involves symptom management with dopaminergic replacement therapy. Levodopa remains the most effective oral treatment, although long-term use is associated with complications such as wearing off, dyskinesias, and on-off fluctuations. Non-dopaminergic medications that improve PD symptoms and motor fluctuations are in demand. Adenosine A2A receptors are abundantly expressed within the basal ganglia and offer a unique target to modify abnormal striatal signaling associated with PD. Preclinical animal models have shown the ability of adenosine A2A receptor antagonists to improve PD motor symptoms, reduce motor fluctuations and dyskinesia, as well as protect against toxin-induced neuronal degeneration. Both istradefylline and preladenant have demonstrated moderate efficacy in reducing off time in PD patients with motor fluctuations. The safety and efficacy of this class of compounds continues to be defined and future studies should focus on non-motor symptoms, dyskinesias, and neuroprotection.

Molecular determinants of A2AR-D2R allosterism: role of the intracellular loop 3 of the D2R

In the CNS, an antagonistic interaction has been shown between adenosine A(2A) and dopamine D(2) receptors (A(2A)Rs and D(2)Rs) that may be relevant both in normal and pathological conditions (i.e., Parkinson's disease). Thus, the molecular determinants mediating this receptor-receptor interaction have recently been explored, as the fine tuning of this target (namely the A(2A)R/D(2)R oligomer) could possibly improve the treatment of certain CNS diseases. Here, we used a fluorescence resonance energy transfer-based approach to examine the allosteric modulation of the D(2)R within the A(2A)R/D(2)R oligomer and the dependence of this receptor-receptor interaction on two regions rich in positive charges on intracellular loop 3 of the D(2)R. Interestingly, we observed a negative allosteric effect of the D(2)R agonist quinpirole on A(2A)R ligand binding and activation. However, these allosteric effects were abolished upon mutation of specific arginine residues (217-222 and 267-269) on intracellular loop 3 of the D(2)R, thus demonstrating a major role of these positively charged residues in mediating the observed receptor-receptor interaction. Overall, these results provide structural insights to better understand the functioning of the A(2A)R/D(2)R oligomer in living cells.

Adenosine, dopamine and serotonin receptors imbalance in lymphocytes of Lesch-Nyhan patients

Lesch-Nyhan disease (LND) is caused by complete deficiency of the hypoxanthine-guanine phosphoribosyltransferase enzyme. It is characterized by overproduction of uric acid, jointly with severe motor disability and self-injurious behaviour which physiopathology is unknown. These neurological manifestations suggest a dysfunction in the basal ganglia, and three neurotransmitters have been implicated in the pathogenesis of the disease: dopamine, adenosine and serotonin. All of them are implicated in motor function and behaviour, and act by binding to specific G-protein coupled receptors in the synaptic membrane where they seem to be integrated through receptor-receptor interactions. In this work we have confirmed at protein level the previously reported increased expression of DRD5 and the variably aberrant expression of ADORA2A, in LND PBL respect to control PBL. We have also described, for the first time, a decreased expression and protein level of 5-HTR1A in LND PBL respect to control PBL. If these results were confirmed in the Lesch-Nyhan patients basal ganglia cells, this would support the hypothesis that pathogenesis of neurological manifestations of Lesch-Nyhan patients may be related to an imbalance of neurotransmitters, rather than to the isolated disturbance of one of the neurotransmitters, and this fact should be taken into account in the design of pharmacologic treatment for their motor and behavioural disturbances.

Differential effects of ADORA2A gene variations in pre-attentive visual sensory memory subprocesses

The ADORA2A gene encodes the adenosine A(2A) receptor that is highly expressed in the striatum where it plays a role in modulating glutamatergic and dopaminergic transmission. Glutamatergic signaling has been suggested to play a pivotal role in cognitive functions related to the pre-attentive processing of external stimuli. Yet, the precise molecular mechanism of these processes is poorly understood. Therefore, we aimed to investigate whether ADORA2A gene variation has modulating effects on visual pre-attentive sensory memory processing. Studying two polymorphisms, rs5751876 and rs2298383, in 199 healthy control subjects who performed a partial-report paradigm, we find that ADORA2A variation is associated with differences in the efficiency of pre-attentive sensory memory sub-processes. We show that especially the initial visual availability of stimulus information is rendered more efficiently in the homozygous rare genotype groups. Processes related to the transfer of information into working memory and the duration of visual sensory (iconic) memory are compromised in the homozygous rare genotype groups. Our results show a differential genotype-dependent modulation of pre-attentive sensory memory sub-processes. Hence, we assume that this modulation may be due to differential effects of increased adenosine A(2A) receptor signaling on glutamatergic transmission and striatal medium spiny neuron (MSN) interaction.

BF066, a novel dual target antiplatelet agent without significant bleeding

In this study, we report BF066, a novel adenine derivative, inhibits platelet activation and thrombosis via the adenosine receptor (A(2A)) activation and phosphodiesterase (PDE) inhibition. BF066 inhibits platelet aggregation and ATP releasing induced by multiple platelet agonists in a dose-dependent manner. The inhibition of BF066 on ADP-induced aggregation is potentiated by adenosine and can be dramatically antagonized by the A(2A) antagonist SCH58261. BF066 also inhibits the PDE activity and platelet spreading on fibrinogen. In FeCl(3)-injured mouse mesenteric arterial thrombosis model, BF066 prevents thrombus formation effectively, similar to clopidogrel. Intriguingly, at dose achieving similar antithrombotic effect compared to clopidogrel, BF066 does not increase bleeding significantly. Taken together, these results suggest that BF066 may be an effective and safe antiplatelet agent targeting both PDE and A(2A). Considering the successful use of combined antiplatelet therapy, BF066 may be further developed as a novel dual target antiplatelet agent.

The anti-tumor effect of A3 adenosine receptors is potentiated by pulsed electromagnetic fields in cultured neural cancer cells

A₃ adenosine receptors (ARs) play a pivotal role in the development of cancer and their activation is involved in the inhibition of tumor growth. The effects of pulsed electromagnetic fields (PEMFs) on cancer have been controversially discussed and the detailed mechanisms are not yet fully understood. In the past we have demonstrated that PEMFs increased A_2A and A₃AR density and functionality in human neutrophils, human and bovine synoviocytes, and bovine chondrocytes. In the same cells, PEMF exposure increased the anti-inflammatory effect mediated by A_2A and/or A₃ARs. The primary aim of the present study was to evaluate if PEMF exposure potentiated the anti-tumor effect of A₃ARs in PC12 rat adrenal pheochromocytoma and U87MG human glioblastoma cell lines in comparison with rat cortical neurons. Saturation binding assays and mRNA analysis revealed that PEMF exposure up-regulated A_2A and A₃ARs that are well coupled to adenylate cyclase activity and cAMP production. The activation of A_2A and A₃ARs resulted in the decrease of nuclear factor-kappa B (NF-kB) levels in tumor cells, whilst only A₃ARs are involved in the increase of p53 expression. A₃AR stimulation mediated an inhibition of tumor cell proliferation evaluated by thymidine incorporation. An increase of cytotoxicity by lactate dehydrogenase (LDH) release and apoptosis by caspase-3 activation in PC12 and U87MG cells, but not in cortical neurons, was observed following A₃AR activation. The effect of the A₃AR agonist in tumor cells was enhanced in the presence of PEMFs and blocked by using a well-known selective antagonist. Together these results demonstrated that PEMF exposure significantly increases the anti-tumor effect modulated by A₃ARs.

Trk and cAMP-dependent survival activity of adenosine A(2A) agonist CGS21680 on rat motoneurons in culture

The survival activity of adenosine A(2A) agonist CGS21680 on motoneurons in culture through the transactivation of neurotrophin receptor TrkB has been reported previously; however, since adenosine A(2A) receptor belongs to a Gs-protein-coupled receptor, we investigated the involvement of the cAMP pathway in the survival activity of CGS21680 using purified motoneurons in culture. CGS21680 alone showed only small survival activity, but the activity was significantly enhanced by the addition of a phosphodiesterase inhibitor, IBMX. This survival activity was partially inhibited by a protein kinase A inhibitor H89 or a neurotrophin receptor tyrosine kinase inhibitor K252a, and was completely inhibited by their combination. These results indicate that the survival activity of CGS21680 on motoneurons is exerted by the mixed effect of the adenylate cyclase-cAMP-PKA pathway and transactivation of Trk neurotrophin receptor. Under conditions in which the maximum survival of motoneurons was supported by sufficient concentrations of brain-derived neurotrophic factor (BDNF), a TrkB ligand, the addition of 100μM AMPA for 3 days led to significant cell death. Treatment with CGS21680 and IBMX protected motoneurons from the toxicity of AMPA, further supporting the presence of a TrkB-independent pathway of CGS21680 activity and suggesting a novel therapeutic approach to motoneuron diseases such as amyotrophic lateral sclerosis.

A2A adenosine receptor signaling in lymphocytes and the central nervous system regulates inflammation during experimental autoimmune encephalomyelitis

Extracellular adenosine has an important role in regulating the severity of inflammation during an immune response. Although there are four adenosine receptor (AR) subtypes, the A2AAR is both highly expressed on lymphocytes and known as a prime mediator of adenosine's anti-inflammatory effects. To define the importance of A2AAR signaling during neuroinflammatory disease progression, we used the experimental autoimmune encephalomyelitis (EAE) animal model for multiple sclerosis. In EAE induction experiments, A2AAR antagonist treatment protected mice from disease development and its associated CNS lymphocyte infiltration. However, A2AAR(-/-) mice developed a more severe acute EAE phenotype characterized by more proinflammatory lymphocytes and activated microglia/macrophages. Interestingly, very high levels of A2AAR were expressed on the choroid plexus, a well-established CNS lymphocyte entry point. To determine the contribution of A2AAR signaling in lymphocytes and the CNS during EAE, we used bone marrow chimeric mice. Remarkably, A2AAR(-/-) donor hematopoietic cells potentiated severe EAE, whereas lack of A2AAR expression on nonhematopoietic cells protected against disease development. Although no defect in the suppressive ability of A2AAR(-/-) regulatory T cells was observed, A2AAR(-/-) lymphocytes were shown to proliferate more and produced more IFN-γ following stimulation. Despite this more proinflammatory phenotype, A2AAR antagonist treatment still protected against EAE when A2AAR(-/-) lymphocytes were adoptively transferred to T cell-deficient A2AAR(+/+) mice. These results indicate that A2AAR expression on nonimmune cells (likely in the CNS) is required for efficient EAE development, while A2AAR lymphocyte expression is essential for limiting the severity of the inflammatory response.

Optimization of adenosine 5'-carboxamide derivatives as adenosine receptor agonists using structure-based ligand design and fragment screening

Structures of G protein-coupled receptors (GPCRs) have a proven utility in the discovery of new antagonists and inverse agonists modulating signaling of this important family of clinical targets. Applicability of active-state GPCR structures to virtual screening and rational optimization of agonists, however, remains to be assessed. In this study of adenosine 5' derivatives, we evaluated the performance of an agonist-bound A(2A) adenosine receptor (AR) structure in retrieval of known agonists and then employed the structure to screen for new fragments optimally fitting the corresponding subpocket. Biochemical and functional assays demonstrate high affinity of new derivatives that include polar heterocycles. The binding models also explain modest selectivity gain for some substituents toward the closely related A(1)AR subtype and the modified agonist efficacy of some of these ligands. The study suggests further applicability of in silico fragment screening to rational lead optimization in GPCRs.

Identification of novel adenosine A(2A) receptor antagonists by virtual screening

Virtual screening was performed against experimentally enabled homology models of the adenosine A(2A) receptor, identifying a diverse range of ligand efficient antagonists (hit rate 9%). By use of ligand docking and Biophysical Mapping (BPM), hits 1 and 5 were optimized to potent and selective lead molecules (11-13 from 5, pK(I) = 7.5-8.5, 13- to >100-fold selective versus adenosine A(1); 14-16 from 1, pK(I) = 7.9-9.0, 19- to 59-fold selective).

Role of ω-hydroxylase in adenosine-mediated aortic response through MAP kinase using A2A-receptor knockout mice

Previously, we have shown that A(2A) adenosine receptor (A(2A)AR) knockout mice (KO) have increased contraction to adenosine. The signaling mechanism(s) for A(2A)AR is still not fully understood. In this study, we hypothesize that, in the absence of A(2A)AR, ω-hydroxylase (Cyp4a) induces vasoconstriction through mitogen-activated protein kinase (MAPK) via upregulation of adenosine A(1) receptor (A(1)AR) and protein kinase C (PKC). Organ bath and Western blot experiments were done using isolated aorta from A(2A)KO and corresponding wild-type (WT) mice. Isolated aortic rings from WT and A(2A)KO mice were precontracted with submaximal dose of phenylephrine (10(-6) M), and concentration responses for selective A(1)AR, A(2A)AR agonists, angiotensin II and cytochrome P-450-epoxygenase, 20-hydroxyeicosatrienoic acid (20-HETE) PKC, PKC-α, and ERK1/2 inhibitors were obtained. 2-p-(2-Carboxyethyl)-phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride (CGS-21680, A(2A)AR agonist) induced concentration-dependent relaxation in WT, which was blocked by methylsulfonyl-propargyloxyphenylhexanamide (cytochrome P-450-epoxygenase inhibitor; 10(-5) M) and also with removal of endothelium. A(1) agonist, 2-chloro-N(6)-cyclopentyladenosine (CCPA) produced higher contraction in A(2A)KO aorta than WT (49.2 ± 8.5 vs. 27 ± 5.9% at 10(-6) M, P < 0.05). 20-HETE produced higher contraction in A(2A)KO than WT (50.6 ± 8.8 vs. 21.1 ± 3.3% at 10(-7) M, P < 0.05). Contraction to CCPA in WT and A(2A)KO aorta was inhibited by PD-98059 (p42/p44 MAPK inhibitor; 10(-6) M), chelerythrine chloride (nonselective PKC blocker; 10(-6) M), Gö-6976 (selective PKC-α inhibitor; 10(-7) M), and HET0016 (20-HETE inhibitor; 10(-5) M). Also, contraction to 20-HETE in WT and A(2A)KO aorta was inhibited by PD-98059 and Gö-6976. Western blot analysis indicated the upregulation of A(1)AR, Cyp4a, PKC-α, and phosphorylated-ERK1/2 in A(2A)KO compared with WT (P < 0.05), while expression of Cyp2c29 was significantly higher in WT. CCPA (10(-6) M) increased the protein expression of PKC-α and phosphorylated-ERK1/2, while HET0016 significantly reduced the CCPA-induced increase in expression of these proteins. These data suggest that, in the absence of A(2A)AR, Cyp4a induces vasoconstriction through MAPK via upregulation of A(1)AR and PKC-α.

Toward rational design of protein detergent complexes: determinants of mixed micelles that are critical for the in vitro stabilization of a G-protein coupled receptor

Although reconstitution of membrane proteins within protein detergent complexes is often used to enable their structural or biophysical characterization, it is unclear how one should rationally choose the appropriate micellar environment to preserve native protein folding. Here, we investigated model mixed micelles consisting of a nonionic glucosylated alkane surfactant from the maltoside and thiomaltoside families, bile salt surfactant, and the steryl derivative cholesteryl hemisuccinate. We correlated several key attributes of these micelles with the in vitro ligand-binding activity of hA(2)aR in these systems. Through small-angle neutron scattering and radioligand-binding analysis, we found several key aspects of mixed micellar systems that preserve the activity of hA(2)aR, including a critical amount of cholesteryl hemisuccinate per micelle, and an optimal hydrophobic thickness of the micelle that is analogous to the thickness of native mammalian bilayers. These features are closely linked to the headgroup chemistry of the surfactant and the hydrocarbon chain length, which influence both the morphology and composition of resulting micelles. This study should serve as a general guide for selecting the appropriate mixed surfactant systems to stabilize membrane proteins for biophysical analysis.

Adenosine A2A receptor antagonism and genetic deletion attenuate the effects of dopamine D2 antagonism on effort-based decision making in mice

Brain dopamine (DA) and adenosine interact in the regulation of behavioral activation and effort-related processes. In the present studies, a T-maze task was developed in mice for the assessment of effort-related decision making. With this task, the two arms of the maze have different reinforcement densities, and a vertical barrier is positioned in the arm with the higher density (HD), presenting the animal with an effort-related challenge. Under control conditions mice prefer the HD arm, and climb the barrier to obtain the larger amount of food. The DA D(2) receptor antagonist haloperidol decreased selection of the HD arm and increased selection of the arm with the low density of reinforcement. However, the HD arm was still the preferred choice in haloperidol-treated mice trained with barriers in both arms. Pre-feeding the mice to reduce food motivation dramatically increased omissions, an effect that was distinct from the actions of haloperidol. Co-administration of theophylline, a nonselective adenosine receptor antagonist, partially reversed the effects of haloperidol. This effect seems to be mediated by the A(2A) receptor but not the A(1) receptor, since the A(2A) antagonist MSX-3, but not the A(1) antagonist CPT, dose dependently reversed the effects of haloperidol on effort-related choice and on c-Fos expression in the dorsal striatum and nucleus accumbens. In addition, adenosine A(2A) receptor knockout mice were resistant to the effects of haloperidol on effort-related choice in the maze. These results indicate that DA D(2) and adenosine A(2A) receptors interact to regulate effort-related decision making and effort expenditure in mice.

Adenosine promotes alternative macrophage activation via A2A and A2B receptors

Adenosine has been implicated in suppressing the proinflammatory responses of classically activated macrophages induced by Th1 cytokines. Alternative macrophage activation is induced by the Th2 cytokines interleukin (IL)-4 and IL-13; however, the role of adenosine in governing alternative macrophage activation is unknown. We show here that adenosine treatment of IL-4- or IL-13-activated macrophages augments the expression of alternative macrophage markers arginase-1, tissue inhibitor of matrix metalloproteinase-1 (TIMP-1), and macrophage galactose-type C-type lectin-1. The stimulatory effect of adenosine required primarily A(2B) receptors because the nonselective adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA) increased both arginase activity (EC(50)=261.8 nM) and TIMP-1 production (EC(50)=80.67 nM), and both pharmacologic and genetic blockade of A(2B) receptors prevented the effect of NECA. A(2A) receptors also contributed to the adenosine augmentation of IL-4-induced TIMP-1 release, as both adenosine and NECA were less efficacious in augmenting TIMP-1 release by A(2A) receptor-deficient than control macrophages. Of the transcription factors known to drive alternative macrophage activation, CCAAT-enhancer-binding protein β was required, while cAMP response element-binding protein and signal transducer and activator of transcription 6 were dispensable in mediating the effect of adenosine. We propose that adenosine receptor activation suppresses inflammation and promotes tissue restitution, in part, by promoting alternative macrophage activation.

A2A and A3 adenosine receptor expression in rheumatoid arthritis: upregulation, inverse correlation with disease activity score and suppression of inflammatory cytokine and metalloproteinase release

INTRODUCTION

The reduction of the inflammatory status represents one of the most important targets in rheumatoid arthritis (RA). A central role of A2A and A3 adenosine receptors (ARs) in mechanisms of inflammation has been reported in different pathologies. The primary aim of this study was to investigate the A2A and A3ARs and their involvement in RA progression measured by Disease Activity Score in 28 or 44 joints (DAS28 or DAS).

METHODS

ARs were analyzed by saturation binding assays, mRNA and Western blotting analysis in lymphocytes from early and established RA patients. The effect of A2A and A3AR agonists in nuclear factor kB (NF-kB) pathway was evaluated. Tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β) and interleukin-6 (IL-6) release was carried out by A2A and A3AR activation. AR pharmacological regulation in matrix metalloproteinase-1 (MMP-1) and metalloproteinase-3 (MMP-3) release was also studied.

RESULTS

In lymphocytes obtained from RA patients, A2A and A3ARs were up-regulated if compared with healthy controls. A2A and A3AR activation inhibited the NF-kB pathway and diminished inflammatory cytokines such as TNF-α, IL-1β and IL-6. A2A and A3AR agonists mediated a reduction of MMP-1 and MMP-3 release. A2A and A3AR density inversely correlated with DAS28 and DAS suggesting a direct role of the endogenous activation of these receptors in the control of RA joint inflammation.

CONCLUSIONS

Taken together these data demonstrate that the inflammatory and clinical responses in RA are regulated by A2A and A3ARs and support the use of A2A and/or A3AR agonists as novel and effective pharmacological treatment in RA patients.

CD73-generated extracellular adenosine in chronic lymphocytic leukemia creates local conditions counteracting drug-induced cell death

Extracellular adenosine (ADO), generated from ATP or ADP through the concerted action of the ectoenzymes CD39 and CD73, elicits autocrine and paracrine effects mediated by type 1 purinergic receptors. We have tested whether the expression of CD39 and CD73 by chronic lymphocytic leukemia (CLL) cells activates an adenosinergic axis affecting growth and survival. By immunohistochemistry, CD39 is widely expressed in CLL lymph nodes, whereas CD73 is restricted to proliferation centers. CD73 expression is highest on Ki-67(+) CLL cells, adjacent to T lymphocytes, and is further localized to perivascular areas. CD39(+)/CD73(+) CLL cells generate ADO from ADP in a time- and concentration-dependent manner. In peripheral blood, CD73 expression occurs in 97/299 (32%) CLL patients and pairs with CD38 and ZAP-70 expression. CD73-generated extracellular ADO activates type 1 purinergic A2A receptors that are constitutively expressed by CLL cells and that are further elevated in proliferating neoplastic cells. Activation of the ADO receptors increases cytoplasmic cAMP levels, inhibiting chemotaxis and limiting spontaneous drug-induced apoptosis of CLL cells. These data are consistent with the existence of an autocrine adenosinergic loop, and support engraftment of leukemic cells in growth-favorable niches, while simultaneously protecting from the action of chemotherapeutic agents.

Adenosine A2A and A2B receptor expression in neuroendocrine tumours: potential targets for therapy

The clinical management of neuroendocrine tumours is complex. Such tumours are highly vascular suggesting tumour-related angiogenesis. Adenosine, released during cellular stress, damage and hypoxia, is a major regulator of angiogenesis. Herein, we describe the expression and function of adenosine receptors (A(1), A(2A), A(2B) and A(3)) in neuroendocrine tumours. Expression of adenosine receptors was investigated in archival human neuroendocrine tumour sections and in two human tumour cell lines, BON-1 (pancreatic) and KRJ-I (intestinal). Their function, with respect to growth and chromogranin A secretion was carried out in vitro. Immunocytochemical data showed that A(2A) and A(2B) receptors were strongly expressed in 15/15 and 13/18 archival tumour sections. Staining for A(1) (4/18) and A(3) (6/18) receptors was either very weak or absent. In vitro data showed that adenosine stimulated a three- to fourfold increase in cAMP levels in BON-1 and KRJ-1 cells. The non-selective adenosine receptor agonist (adenosine-5'N-ethylcarboxamide, NECA) and the A(2A)R agonist (CGS21680) stimulated cell proliferation by up to 20-40% which was attenuated by A(2B) (PSB603 and MRS1754) and A(2A) (SCH442416) receptor selective antagonists but not by the A(1) receptor antagonist (PSB36). Adenosine and NECA stimulated a twofold increase in chromogranin A secretion in BON-1 cells. Our data suggest that neuroendocrine tumours predominantly express A(2A) and A(2B) adenosine receptors; their activation leads to increased proliferation and secretion of chromogranin A. Targeting adenosine signal pathways, specifically inhibition of A(2) receptors, may thus be a useful addition to the therapeutic management of neuroendocrine tumours.