Modulating effect of adenosine deaminase on function of adenosine A1 receptors

Activation of Adenosine A1 Receptor in Ischemic Stroke: Neuroprotection by Tetrahydroxy Stilbene Glycoside as an Agonist

Ischemic stroke is the main cause of death/disability, posing a great menace to human health. Though efforts to search for therapeutic drugs are ongoing, few of them have succeeded. Adenosine A₁ receptor (A1R) activation could ameliorate ischemic injury, representing a very tempting target for stroke treatment. Tetrahydroxy stilbene glycoside (TSG), a potent antioxidant from the well-known Chinese herb Polygonum multiflorum Thunb., has been reported to have notable neuroprotective activities but the underlying mechanisms are elusive. This study investigated the mechanism of TSG focusing on A1R. TSG markedly decreased mortality, neurological deficit score, cerebral infarct size and brain water content of MCAO rats, and ameliorated the disorders in purine metabolism, energy metabolism and antioxidative defense system. TSG helped the survival of SH-SY5Y cells in OGD/R by alleviating oxidative stress and glutamate release, and by maintaining calcium homeostasis. TSG effects were abolished by A1R antagonist DPCPX. Docking and binding assays confirmed the binding of TSG with A1R. In addition, TSG upregulated the A1R level lowered by MCAO and OGD/R. The downstream signals of A1R activation, ERK1/2, HIF-1α and NF-κB contributed to the neuroprotection of TSG. Moreover, void of “well-known” cardiovascular side effects of classical A1R agonists, TSG showcased its great potential for stroke treatment.

Expressional Localization and Functionally Identifying an RNA Editing Enzyme BmADARa of the Silkworm Bombyx mori

The most common type of RNA editing in metazoans is the deamination of adenosine into inosine (A-to-I) catalyzed by the adenosine deaminase acting on the RNA (ADAR) family of proteins. The deletion or dysfunction of ADAR enzymes in higher eukaryotes can affect the efficiency of substrate editing and cause neurological disorders. However, the information concerning A-to-I RNA editing and ADAR members in the silkworm, Bombyx mori (BmADAR), is limited. In this study, a first molecular comprehensive cloning and sequence analysis of BmADAR transcripts was presented. A complete open reading frame (ORF) (BmADARa) was obtained using RT-PCR and RACE and its expression pattern, subcellular localization and A-to-I RNA-editing function on the silkworm synaptotagmin I (BmSyt I) were investigated. Subcellular localization analysis observed that BmADARa was mainly localized in the nucleus. To further study the A-to-I RNA-editing function of BmADARa, BmSyt I-pIZ-EGFP was constructed and co-transfected with BmADARa-pIZ-EGFP into BmN cells. The result demonstrates that BmADARa can functionally edit the specific site of BmSyt I. Taken together, this study not only provides insight into the function of the first ADAR enzyme in B. mori, but also lays foundations for further exploration of the functional domain of BmADARa and its editing substrates and target sites.

Molecular Evidence of Adenosine Deaminase Linking Adenosine A2A Receptor and CD26 Proteins

Adenosine is an endogenous purine nucleoside that acts in all living systems as a homeostatic network regulator through many pathways, which are adenosine receptor (AR)-dependent and -independent. From a metabolic point of view, adenosine deaminase (ADA) is an essential protein in the regulation of the total intracellular and extracellular adenosine in a tissue. In addition to its cytosolic localization, ADA is also expressed as an ecto-enzyme on the surface of different cells. Dipeptidyl peptidase IV (CD26) and some ARs act as binding proteins for extracellular ADA in humans. Since CD26 and ARs interact with ADA at opposite sites, we have investigated if ADA can function as a cell-to-cell communication molecule by bridging the anchoring molecules CD26 and A2AR present on the surfaces of the interacting cells. By combining site-directed mutagenesis of ADA amino acids involved in binding to A2AR and a modification of the bioluminescence resonance energy transfer (BRET) technique that allows detection of interactions between two proteins expressed in different cell populations with low steric hindrance (NanoBRET), we show direct evidence of the specific formation of trimeric complexes CD26-ADA-A2AR involving two cells. By dynamic mass redistribution assays and ligand binding experiments, we also demonstrate that A2AR-NanoLuc fusion proteins are functional. The existence of this ternary complex is in good agreement with the hypothesis that ADA could bridge T-cells (expressing CD26) and dendritic cells (expressing A2AR). This is a new metabolic function for ecto-ADA that, being a single chain protein, it has been considered as an example of moonlighting protein, because it performs more than one functional role (as a catalyst, a costimulator, an allosteric modulator and a cell-to-cell connector) without partitioning these functions in different subunits.

Unpredictable Chronic Stress Alters Adenosine Metabolism in Zebrafish Brain

Stress is considered a risk factor for several human disorders. Despite the broad knowledge of stress responses in mammals, data on the relationship between unpredictable chronic stress (UCS) and its effects on purinergic signaling are limited. ATP hydrolysis by ectonucleotidases is an important source of adenosine, and adenosine deaminase (ADA) contributes to the control of the nucleoside concentrations. Considering that some stress models could affect signaling systems, the objective of this study was to investigate whether UCS alters ectonucleotidase and ADA pathway in zebrafish brain. Additionally, we analyzed ATP metabolism as well as ada1, ada2.1, ada2.2, adaL, and adaasi gene expression in zebrafish brain. Our results have demonstrated that UCS did not alter ectonucleotidase and soluble ADA activities. However, ecto-ADA activity was significantly decreased (26.8%) in brain membranes of animals exposed to UCS when compared to the control group. Quantitative reverse transcription PCR (RT-PCR) analysis did not show significant changes on ADA gene expression after the UCS exposure. The brain ATP metabolism showed a marked increase in adenosine levels (ADO) in animals exposed to UCS. These data suggest an increase on extracellular adenosine levels in zebrafish brain. Since this nucleoside has neuromodulatory and anxiolytic effects, changes in adenosine levels could play a role in counteracting the stress, which could be related to a compensatory mechanism in order to restore the homeostasis.

Expression, pharmacology and functional activity of adenosine A1 receptors in genetic models of Huntington's disease

Adenosine A1 receptor (A1R) stimulation exerts beneficial effects in response to various insults to the brain and, although it was found neuroprotective in a lesional model of Huntington's disease (HD), the features of this receptor in genetic models of HD have never been explored. In the present study we characterized the expression, affinity and functional effects of A1Rs in R6/2 mice (the most widely used transgenic model of HD) and in a cellular model of HD. Binding studies revealed that the density of A1Rs was significantly reduced in the cortex and the striatum of R6/2 mice compared to age-matched wild-type (WT), while receptor affinity was unchanged. The selective A1R agonist cyclopentyladenosine (CPA, 300nM) was significantly more effective in reducing synaptic transmission in corticostriatal slices from symptomatic R6/2 than in age-matched WT mice. Such an effect was due to a stronger inhibition of glutamate release from the pre-synaptic terminal. The different functional activities of A1Rs in HD mice were associated also to a different intracellular signaling pathway involved in the synaptic effect of CPA. In fact, while the PKA pathway was involved in both genotypes, p38 MAPK inhibitor SB203580 partially prevented synaptic effects of CPA in R6/2, but not in WT, mice; moreover, CPA differently modulated the phosphorylation status of p38 in the two genotypes. In vitro studies confirmed a different behavior of A1Rs in HD: CPA (100 nM for 5h) modulated cell viability in STHdh(Q111/Q111) (mhttHD cells), without affecting the viability of STHdh(Q7/Q7) (wthtt cells). This effect was prevented by the application of SB203580. Our results demonstrate that in the presence of the HD mutation A1Rs undergo profound changes in terms of expression, pharmacology and functional activity. These changes have to be taken in due account when considering A1Rs as a potential therapeutic target for this disease.

The catalytic site structural gate of adenosine deaminase allosterically modulates ligand binding to adenosine receptors

The enzyme adenosine deaminase (ADA) is a multifunctional protein that can both degrade adenosine and bind extracellularly to adenosine receptors, acting as an allosteric modulator regulating the hormonal effects of adenosine. The molecular regions of ADA responsible for the latter are unknown. In this work, alanine scanning mutagenesis of various ADA amino acid stretches, selected through in silico docking experiments, allowed us to identify regions of the enzyme responsible for modulating both its catalytic activity and its ability to modulate agonist binding to A and A adenosine receptors (AR and AR). The combination of computational and in vitro experiments show that the structural gate to the catalytic site; i.e., the α-1 helix containing residues L58-I72 and the loop containing residues A184-I188 of ADA, were important to maintain both the catalytic efficiency of the enzyme and its action as an allosteric modulator of the adenosine receptors. These data are consistent with a predicted supramolecular assembly, in which ADA bridges AR and CD26 and are in line with the notion that the interaction of ADA with adenosine receptors has an important role in the immunosynapse. We propose that it is the ADA open form, but not the closed one, that is responsible for the functional interaction with A₁R and A₂AR.

Influence of mercury chloride on adenosine deaminase activity and gene expression in zebrafish (Danio rerio) brain

Mercury is a widespread environmental contaminant that is neurotoxic even at very low concentrations. In this study we investigated the effects of mercury chloride on soluble and membrane adenosine deaminase (ADA) activity and gene expression in zebrafish brain. Inhibition of ADA activity was observed in the soluble fraction at 5-250 microM HgCl(2) (84.6-92.6%, respectively), whereas inhibition occurred at 50-250 microM in membrane fractions (20.9-26%, respectively). We performed in vitro experiments with chelants (EDTA and DTT) to test if these compounds prevented or reversed the inhibition caused by HgCl(2) and found that the inhibition was partially or fully abolished. The effect on ADA activity in soluble and membrane fractions was evaluated after acute (24h) and subchronic (96h) in vivo exposure of zebrafish to 20 microg/l HgCl(2). ADA activity in the soluble fraction was decreased after both acute (24.5%) and subchronic (40.8%) exposures, whereas in brain membranes the enzyme was inhibited only after subchronic exposure (21.9%). Semiquantitative RT-PCR analysis showed that HgCl(2) did not alter ADA gene expression. This study demonstrated that ADA activity was inhibited by mercury and this effect might be related to the neurotoxicity of this heavy metal.

Lipopolysaccharide and TNF-alpha modify adenosine A(2A) receptor expression and function in equine monocytes

Stimulation of adenosine A(2A) receptors results in anti-inflammatory effects in a variety of cell types. Lipopolysaccharide (LPS) and pro-inflammatory cytokines, such as TNF-alpha and IL-1, have been reported to up-regulate the expression of adenosine A(2A) receptors and thereby enhance the functional activity of adenosine A(2A) receptors in human and murine monocyte/macrophage cell lines and in monocytes/macrophages isolated from those species. In this study, we investigated the effects of LPS and TNF-alpha on the expression and functional activity of adenosine A(2A) receptors in isolated equine peripheral blood monocytes. The results of this study indicate that LPS and TNF-alpha up-regulate the transcription of adenosine A(2A) receptors for up to 24h; the response to LPS was of greater magnitude than the response to TNF-alpha. In this study, incubation with LPS, but not with TNF-alpha, resulted in down-regulation of adenosine A(3) receptor mRNA expression. Furthermore, incubation of these cells with LPS significantly increases the surface density of adenosine A(2A) receptors, and incubation with low concentrations of either LPS or TNF-alpha significantly increases the potency of the adenosine A(2A) receptor agonist, ATL313, to inhibit LPS-induced production of TNF-alpha. These findings suggest that the increased expression of adenosine A(2A) receptors and the enhanced functional potency of adenosine A(2A) receptor agonists after exposure to pro-inflammatory substances such as LPS or TNF-alpha may render adenosine A(2A) receptor agonists particularly important in the treatment of the systemic inflammatory response syndrome that occurs secondary to endotoxemia and bacterial infections in adult horses and neonatal foals.

Differential modulation of lipopolysaccharide-induced expression of inflammatory genes in equine monocytes through activation of adenosine A2A receptors

Adenosine is an endogenous nucleoside that has potent receptor-mediated immunomodulatory effects on macrophage/monocyte function. In this study, we determined the effects of an adenosine A(2A) receptor agonist, ATL313, on the expression of mRNAs for four pro-inflammatory mediators, IL-1beta, IL-8, COX-2, and TNF-alpha, and the mRNA and protein for the anti-inflammatory cytokine, IL-10 in equine monocytes incubated with lipopolysaccharide (LPS). The results indicate that ATL313 significantly reduces LPS-induced expression of COX-2 and TNF-alpha, enhances the expression of IL-10 and IL-8, but does not alter the expression of IL-1beta. These effects of ATL313 were reversed by co-incubation with the selective adenosine A(2A) antagonist ZM241385, and were mimicked by the cAMP analogue dibutyryl cAMP. These differential effects of adenosine A(2A) receptor activation were in contrast to those obtained using the P38 MAPK inhibitor, SB203580, which nearly abolished all LPS-induced changes in mRNA expression as well as the production of TNF-alpha protein. These findings, which indicate that adenosine A(2A) receptor activation modulates the transcription of several, but not all, pro-inflammatory mediators and exerts a synergistic effect on the induction of at least one anti-inflammatory cytokine, suggest that selective adenosine A(2A) agonists may reduce the early pro-inflammatory effects of endotoxemia in horses.

Adenosine deamination to inosine in isolated basolateral membrane from kidney proximal tubule: implications for modulation of the membrane-associated protein kinase A

In this work, the metabolism of adenosine by isolated BLM associated-enzymes and the implications of this process for the cAMP-signaling pathway are investigated. Inosine was identified as the major metabolic product, suggesting the presence of adenosine deaminase (ADA) activity in the BLM. This was confirmed by immunoblotting and ADA-specific enzyme assay. Implications for the enzymatic deamination of adenosine on the receptor-modulated cAMP-signaling pathway were also investigated. We observed that inosine induced a 2-fold increase in [(35)S] GTPgammaS binding to the BLM and it was inhibited by 10(-6)M DPCPX, an A(1) receptor-selective antagonist. Inosine (10(-7)M) inhibited protein kinase A activity in a DPCPX-sensitive manner. Molecular association between ADA and G(alphai-3) protein-coupled A(1) receptor was demonstrated by co-immunoprecipitation assay. These data show that adenosine is deaminated by A(1) receptor-associated ADA to inosine, which in turn modulates PKA in the BLM through A(1) receptor-mediated inhibition of adenylyl cyclase.

Human adenosine deaminase as an allosteric modulator of human A(1) adenosine receptor: abolishment of negative cooperativity for [H](R)-pia binding to the caudate nucleus

It has been shown that adenosine deaminase (ADA; EC 3.5.4.4) behaves as an ecto-enzyme anchored to membrane proteins, among them A(1) adenosine receptors (A(1)Rs). Bovine ADA interacts with A(1)Rs from many species and regulates agonists binding to receptors in an activity-independent form. However, it was not known whether human ADA exerted any effect on the agonist binding to human A(1)Rs, because of both technical difficulties in obtaining pure human ADA and tissues containing human A(1)Rs. In this study, human ADA was purified to homogeneity. Taking in consideration that A(1)Rs form homodimers and taking advantage of a new procedure to fit binding data to receptors dimers, which allows to calculate ligand dissociation constants and the degree of cooperativity between the two subunits in the dimer, here it is demonstrated that human ADA markedly enhances the agonist and antagonist affinity and abolishes the negative cooperativity on agonist binding to human striatal A(1)Rs. ADA also increases the ability of the agonist to decrease the forskolin-induced cAMP levels. The results show that human ADA, apart from reducing the adenosine concentration and thus preventing A(1)R desensitization, binds to A(1)R behaving as an allosteric effector that markedly enhances agonist affinity and increases receptor functionality. The physiological role of the interaction is to make receptors more sensitive to adenosine. This powerful regulation has important implications for the physiology and pharmacology of neuronal A(1)Rs.

Kinetic characterization of adenosine deaminase activity in zebrafish (Danio rerio) brain

Adenosine deaminase (ADA; EC 3.5.4.4) activity is responsible for cleaving adenosine to inosine. In this study we described the biochemical properties of adenosine deamination in soluble and membrane fractions of zebrafish (Danio rerio) brain. The optimum pH for ADA activity was in the range of 6.0-7.0 in soluble fraction and reached 5.0 in brain membranes. A decrease of 31.3% on adenosine deamination in membranes was observed in the presence of 5 mM Zn(2+), which was prevented by 5 mM EDTA. The apparent K(m) values for adenosine deamination were 0.22+/-0.03 and 0.19+/-0.04 mM for soluble and membrane fractions, respectively. The apparent V(max) value for soluble ADA activity was 12.3+/-0.73 nmol NH(3) min(-1) mg(-1) of protein whereas V(max) value in brain membranes was 17.5+/-0.51 nmol NH(3) min(-1) mg(-1) of protein. Adenosine and 2'-deoxyadenosine were deaminated in higher rates when compared to guanine nucleosides in both fractions. Furthermore, a significant inhibition on adenosine deamination in both soluble and membrane fractions was observed in the presence of 0.1 mM of erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA). The presence of ADA activity in zebrafish brain may be important to regulate the adenosine/inosine levels in the CNS of this species.

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.

The enhancement of dopamine D1 receptor desensitization by adenosine A1 receptor activation

The present study was designed to examine the effects of adenosine A(1) receptor on dopamine D(1) receptor desensitization in a human embryonic kidney 293 cell line stably cotransfected with human adenosine A(1) receptor and dopamine D(1) receptor cDNAs (A(1)D(1) cells) by means of cAMP accumulation assay. Long-term exposure of A(1)D(1) cells to dopamine D(1) receptor agonist (+/-)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol hydrochloride (SKF38393) caused a rapid desensitization of dopamine D(1) receptor. Coadministration of adenosine A(1) receptor agonist N(6)-cyclopentyladenosine (CPA) potentiated the effect of SKF38393. This enhancement effect of CPA was blocked by adenosine A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) but not by pertussis toxin, indicating that this effect of CPA was mediated by adenosine A(1) receptor and was G(i) protein independent. Furthermore, the blockade of endogenous adenosine by adenosine deaminase or DPCPX attenuated dopamine D(1) receptor desensitization. Collectively, these results suggest that adenosine A(1) receptor plays an important role in the regulation of dopamine D(1) receptor by potentiating ligand-induced desensitization.

Activation of adenosine A1 receptor modulates dopamine D1 receptor activity in stably cotransfected human embryonic kidney 293 cells

The antagonistic interactions between adenosine A1 receptors and dopamine D1 receptors were studied in a human embryonic kidney 293 cell line stably cotransfected with human adenosine A1 receptor and dopamine D1 receptor cDNAs. In the cotransfected cells, but not in control cells only transfected with dopamine D1 receptors, adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA, 10 microM) increased the Kd of dopamine D1 receptor antagonist [N-methyl-3H]R(+)-7-Chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine ([3H]SCH23390) without affecting the Bmax. Moreover, CPA induced a concentration-dependent decrease in the affinity of dopamine D1 receptors for the agonist (+/-)-1-Phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol hydrochloride (SKF38393) and inhibited dopamine D1 receptor-mediated cyclic AMP response element recruitment. Furthermore, pertussis toxin treatment completely counteracted the effects of low concentrations of CPA but only partially counteracted the effects of high concentrations of CPA. These results suggest that adenosine A1 receptors antagonistically modulate dopamine D1 receptors at the level of receptor binding and the second messenger generation. Furthermore, the antagonistic interactions between these two receptors induced by low concentrations of CPA might have a different manner with those induced by high concentrations of CPA.

Role of the second transmembrane domain of rat adenosine A1 receptor in ligand-receptor interaction

Initial mutagenesis studies exploring the ligand recognition model of A1 adenosine receptor (A1R) mainly focused on the residues in the 5th-7th transmembrane domains (TMs5-7). Little is known about the role of residues in TM2. To explore the importance of reserved hydrophobic region in TM2 of A1R, we mutated the hydrophobic residues at positions 65 and 69 to hydrophilic residues (L65T, Leu-65 to Thr-65; I69T, Ile-69 to Thr-69; I69S, Ile-69 to Ser-69) to change the hydrophobicity at the outer end of TM2. Binding assays showed that the affinities of mutant receptors were significantly decreased for ribose group-containing agonists (2-chloro-N6-cyclopentyladenosine (CCPA) and 5'-N-ethyl-carboxamidoadenosine (NECA)) but not for antagonists, N6-cyclopentyl-9-methyladenine (N-0840), an adenine derivative lacking ribose group, and 8-cyclopentyl-1, 3-dipropylxanthine (DPCPX), a xanthine derivative. This observation suggests that the hydrophobic region at the outer end of TM2 may mediate the recognition of the ribose group of CCPA and NECA.