Extrahelical Binding Site for a 1H-Imidazo[4,5-c]quinolin-4-amine A3 Adenosine Receptor Positive Allosteric Modulator on Helix 8 and Distal Portions of Transmembrane Domains 1 and 7

This study describes the localization and computational prediction of a binding site for the A3 adenosine receptor (A3AR) positive allosteric modulator 2-cyclohexyl-1H-imidazo[4,5-c]quinolin-4-(3,4-dichlorophenyl)amine (LUF6000). The work reveals an extrahelical lipid-facing binding pocket disparate from the orthosteric binding site that encompasses transmembrane domain (TMD) 1, TMD7, and Helix (H) 8, which was predicted by molecular modeling and validated by mutagenesis. According to the model, the nearly planar 1H-imidazo[4,5-c]quinolinamine ring system lies parallel to the transmembrane segments, inserted into an aromatic cage formed by π-π stacking interactions with the side chains of Y2847.55 in TMD7 and Y2938.54 in H8 and by π-NH bonding between Y2847.55 and the exocyclic amine. The 2-cyclohexyl group is positioned "upward" within a small hydrophobic subpocket created by residues in TMDs 1 and 7, while the 3,4-dichlorophenyl group extends toward the lipid interface. An H-bond between the N-1 amine of the heterocycle and the carbonyl of G291.49 further stabilizes the interaction. Molecular dynamics simulations predicted two metastable intermediates, one resembling a pose determined by molecular docking and a second involving transient interactions with Y2938.54; in simulations, each of these intermediates converges into the final bound state. Structure-activity-relationships for replacement of either of the identified exocyclic or endocyclic amines with heteroatoms lacking H-bond donating ability were consistent with the hypothetical pose. Thus, we characterized an allosteric pocket for 1H-imidazo[4,5-c]quinolin-4-amines that is consistent with data generated by orthogonal methods, which will aid in the rational design of improved A3AR positive allosteric modulators. SIGNIFICANCE STATEMENT: Orthosteric A3AR agonists have advanced in clinical trials for inflammatory conditions, liver diseases, and cancer. Thus, the clinical appeal of selective receptor activation could extend to allosteric enhancers, which would induce site- and time-specific activation in the affected tissue. By identifying the allosteric site for known positive allosteric modulators, structure-based drug discovery modalities can be enabled to enhance the pharmacological properties of the 1H-imidazo[4,5-c]quinolin-4-amine class of A3AR positive allosteric modulators.

Evidence of Histone H2A.Z Deacetylation and Cardiomyocyte Dedifferentiation in Infarcted/Tip60-depleted Hearts

Myocardial infarction (MI) in the human heart causes death of billions of cardiomyocytes (CMs), resulting in cardiac dysfunction that is incompatible with life or lifestyle. In order to re-muscularize injured myocardium, replacement CMs must be generated via renewed proliferation of surviving CMs. Approaches designed to induce proliferation of CMs after injury have been insufficient. Toward this end, we are targeting the Tip60 acetyltransferase, based on the rationale that its pleiotropic functions conspire to block the CM cell-cycle at several checkpoints. We previously reported that genetic depletion of Tip60 in a mouse model after MI reduces scarring, retains cardiac function, and activates the CM cell-cycle, although it is unclear whether this culminates in the generation of daughter CMs. For pre-existing CMs in the adult heart to resume proliferation, it is becoming widely accepted that they must first dedifferentiate, a process highlighted by loss of maturity, epithelial to mesenchymal transitioning (EMT), and reversion from fatty acid oxidation to glycolytic metabolism, accompanied by softening of the myocardial extracellular matrix. Findings in hematopoietic stem cells, and more recently in neural progenitor cells, have shown that Tip60 induces and maintains the differentiated state via site-specific acetylation of the histone variant H2A.Z. Here, we report that genetic depletion of Tip60 from naïve or infarcted hearts results in the near-complete absence of acetylated H2A.Z in CM nuclei, and that this is accordingly accompanied by altered gene expressions indicative of EMT induction, ECM softening, decreased fatty acid oxidation, and depressed expression of genes that regulate the TCA cycle. These findings, combined with our previous work, support the notion that because Tip60 has multiple targets that combinatorially maintain the differentiated state and inhibit proliferation, its transient therapeutic targeting to ameliorate the effects of cardiac injury should be considered.

First Potent Macrocyclic A3 Adenosine Receptor Agonists Reveal G-Protein and β-Arrestin2 Signaling Preferences

(N)-Methanocarba adenosine derivatives (A3 adenosine receptor (AR) agonists containing bicyclo[3.1.0]hexane replacing furanose) were chain-extended at N6 and C2 positions with terminal alkenes for ring closure. The resulting macrocycles of 17-20 atoms retained affinity, indicating a spatially proximal orientation of these receptor-bound chains, consistent with molecular modeling of 12. C2-Arylethynyl-linked macrocycle 19 was more A3AR-selective than 2-ether-linked macrocycle 12 (both 5'-methylamides, human (h) A3AR affinities (Ki): 22.1 and 25.8 nM, respectively), with lower mouse A3AR affinities. Functional hA3AR comparison of two sets of open/closed analogues in β-arrestin2 and Gi/o protein assays showed certain signaling preferences divergent from reference agonist Cl-IB-MECA 1. The potencies of 1 at all three Gαi isoforms were slightly less than its hA3AR binding affinity (Ki: 1.4 nM), while the Gαi1 and Gαi2 potencies of macrocycle 12 were roughly an order of magnitude higher than its radioligand binding affinity. Gαi2-coupling was enhanced in macrocycle 12 (EC50 2.56 nM, ∼40% greater maximal efficacy than 1). Di-O-allyl precursor 18 cyclized to form 19, increasing the Gαi1 potency by 7.5-fold. The macrocycles 12 and 19 and their open precursors 11 and 18 potently stimulated β-arrestin2 recruitment, with EC50 values (nM) of 5.17, 4.36, 1.30, and 4.35, respectively, and with nearly 50% greater efficacy compared to 1. This example of macrocyclization altering the coupling pathways of small-molecule (nonpeptide) GPCR agonists is the first for potent and selective macrocyclic AR agonists. These initial macrocyclic derivatives can serve as a guide for the future design of macrocyclic AR agonists displaying unanticipated pharmacology.

Species dependence of A3 adenosine receptor pharmacology and function

Efforts to fully understand pharmacological differences between G protein-coupled receptor (GPCR) species homologues are generally not pursued in detail during the drug development process. To date, many GPCRs that have been successfully targeted are relatively well-conserved across species in amino acid sequence and display minimal variability of biological effects. However, the A3 adenosine receptor (AR), an exciting drug target for a multitude of diseases associated with tissue injury, ischemia, and inflammation, displays as little as 70% sequence identity among mammalian species (e.g., rodent vs. primate) commonly used in drug development. Consequently, the pharmacological properties of synthetic A3AR ligands vary widely, not only in binding affinity, selectivity, and signaling efficacy, but to the extent that some function as agonists in some species and antagonists in others. Numerous heterocyclic antagonists that have nM affinity at the human A3AR are inactive or weakly active at the rat and mouse A3ARs. Positive allosteric modulators, including the imidazo [4,5-c]quinolin-4-amine derivative LUF6000, are only active at human and some larger animal species that have been evaluated (rabbit and dog), but not rodents. A3AR agonists evoke systemic degranulation of rodent, but not human mast cells. The rat A3AR undergoes desensitization faster than the human A3AR, but the human homologue can be completely re-sensitized and recycled back to the cell surface. Thus, comprehensive pharmacological evaluation and awareness of potential A3AR species differences are critical in studies to further understand the basic biological functions of this unique AR subtype. Recombinant A3ARs from eight different species have been pharmacologically characterized thus far. In this review, we describe in detail current knowledge of species differences in genetic identity, G protein-coupling, receptor regulation, and both orthosteric and allosteric A3AR pharmacology.

Pharmacological inhibition of the acetyltransferase Tip60 mitigates myocardial infarction injury

Pharmacologic strategies that target factors with both pro-apoptotic and anti-proliferative functions in cardiomyocytes (CMs) may be useful for the treatment of ischemic heart disease. One such multifunctional candidate for drug targeting is the acetyltransferase Tip60, which is known to acetylate both histone and non-histone protein targets that have been shown in cancer cells to promote apoptosis and to initiate the DNA damage response, thereby limiting cellular expansion. Using a murine model, we recently published findings demonstrating that CM-specific disruption of the Kat5 gene encoding Tip60 markedly protects against the damaging effects of myocardial infarction (MI). In the experiments described here, in lieu of genetic targeting, we administered TH1834, an experimental drug designed to specifically inhibit the acetyltransferase domain of Tip60. We report that, similar to the effect of disrupting the Kat5 gene, daily systemic administration of TH1834 beginning 3 days after induction of MI and continuing for 2 weeks of a 4-week timeline resulted in improved systolic function, reduced apoptosis and scarring, and increased activation of the CM cell cycle, effects accompanied by reduced expression of genes that promote apoptosis and inhibit the cell cycle and reduced levels of CMs exhibiting phosphorylated Atm. These results support the possibility that drugs that inhibit the acetyltransferase activity of Tip60 may be useful agents for the treatment of ischemic heart disease.

Structure-Activity Studies of 1H-Imidazo[4,5-c]quinolin-4-amine Derivatives as A3 Adenosine Receptor Positive Allosteric Modulators

We previously reported 1H-imidazo[4,5-c]quinolin-4-amines as A3 adenosine receptor (A3AR) positive allosteric modulators (PAMs). A3AR agonists, but not PAMs, are in clinical trials for inflammatory diseases and liver conditions. We synthesized new analogues to distinguish 2-cyclopropyl antagonist 17 (orthosteric interaction demonstrated by binding and predicted computationally) from PAMs (derivatives with large 2-alkyl/cycloalkyl/bicycloalkyl groups). We predicted PAM binding at a hydrophobic site on the A3AR cytosolic interface. Although having low Caco-2 permeability and high plasma protein binding, hydrophobic 2-cyclohept-4-enyl-N-3,4-dichlorophenyl, MRS7788 18, and 2-heptan-4-yl-N-4-iodophenyl, MRS8054 39, derivatives were orally bioavailable in rat. 2-Heptan-4-yl-N-3,4-dichlorophenyl 14 and 2-cyclononyl-N-3,4-dichlorophenyl 20 derivatives and 39 greatly enhanced Cl-IB-MECA-stimulated [35S]GTPγS binding Emax, with only 12b trending toward decreasing the agonist EC50. A feasible route for radio-iodination at the p-position of a 4-phenylamino substituent suggests a potential radioligand for allosteric site binding. Herein, we advanced an allosteric approach to developing A3AR-activating drugs that are potentially event- and site-specific in action.

Abstract P1011: Pharmacological Targeting Of Tip60 For Heart Regeneration And Repair

In recent decades, many cardiovascular diseases have been ascribed to acetylation-dependent regulatory mechanisms, as revealed by the beneficial effects of targeting de-acetylase proteins. By contrast, we are targeting the acetyltransferase Tip60 (Tat-interactive protein 60 kD), a pleiotropic tumor suppressor encoded by the Kat5 gene. Using a murine genetic model, we recently reported that cardiomyocyte (CM)-specific disruption of Kat5 markedly protected against the damaging effects of myocardial infarction (MI). To establish therapeutic relevance, we have begun to evaluate the potential cardioprotective effects of TH1834, a small MW drug designed to specifically target the acetyltransferase domain of Tip60. Daily systemic administration of TH1834 on days 3-16 post-MI efficiently preserved cardiac function for up to 28 days post-MI, which was accompanied by reduced scar formation, diminished CM apoptosis, and activation of CM cell-cycle in the absence of CM hypertrophy. Subsequent gene ontology enrichment analysis of transcriptome sequencing data revealed that, among a total of 2,051 differentially expressed genes, 154 cell-cycle regulators and 96 apoptotic markers were altered by TH1834 treatment at day 10 post-MI; significant transcriptional changes in genes involved in cardiac muscle contraction, CM differentiation processes, and responses to ischemia/hypoxia were also identified. These data demonstrate that TH1834 administration promotes retention of post-MI function via enhanced remuscularization and by inhibiting cell death, supporting the translational potential of targeting Tip60 as a novel treatment for ischemic heart disease.

Characterization of Dual-Acting A3 Adenosine Receptor Positive Allosteric Modulators That Preferentially Enhance Adenosine-Induced Gαi3 and GαoA Isoprotein Activation

The A₃ adenosine receptor (A₃AR) is a promising therapeutic target for inflammatory diseases, cancer, and chronic neuropathic pain, with agonists already in advanced clinical trials. Here we report an in-depth comparison of the pharmacological properties and structure-activity relationships of existing and expanded compound libraries of 2-substituted 1H-imidazo[4,5-c]quinolin-4-amine and 4-amino-substituted quinoline derivatives that function as A₃AR positive allosteric modulators (PAMs). We also show that our lead compound from each series enhances adenosine-induced A₃AR signaling preferentially toward activation of Gα_i3 and Gα_oA isoproteins, which are coexpressed with the A₃AR in immune cells and spinal cord neurons. Finally, utilizing an extracellular/intracellular chimeric A₃AR approach composed of sequences from a responding (human) and a nonresponding (mouse) species, we provide evidence in support of the idea that the imidazoquinolin-4-amine class of PAMs variably interacts dually with the orthosteric ligand binding site as well as with a separate allosteric site located within the inner/intracellular regions of the receptor. This study has advanced both structural and pharmacological understanding of these two classes of A₃AR PAMs, which includes leads for future pharmaceutical development.

Conditional depletion of the acetyltransferase Tip60 protects against the damaging effects of myocardial infarction

Injury from myocardial infarction (MI) and consequent post-MI remodeling is accompanied by massive loss of cardiomyocytes (CM), a cell type critical for contractile function that is for all practical purposes non-regenerable due to its profound state of proliferative senescence. Identification of factors that limit CM survival and/or constrain CM renewal provides potential therapeutic targets. Tip60, a pan-acetyltransferase encoded by the Kat5 gene, has been reported to activate apoptosis as well as multiple anti-proliferative pathways in non-cardiac cells; however, its role in CMs, wherein it is abundantly expressed, remains unknown. Here, using mice containing floxed Kat5 alleles and a tamoxifen-activated Myh6-MerCreMer recombinase transgene, we report that conditional depletion of Tip60 in CMs three days after MI induced by permanent coronary artery ligation greatly improves functional recovery for up to 28 days. This is accompanied by diminished scarring, activation of cell-cycle transit markers in CMs within the infarct border and remote zones, reduced expression of cell-cycle inhibitors pAtm and p27, and reduced apoptosis in the remote regions. These findings implicate Tip60 as a novel, multifactorial target for limiting the damaging effects of ischemic heart disease.

A3 adenosine receptor agonists containing dopamine moieties for enhanced interspecies affinity

Following our study of 4'-truncated (N)-methanocarba-adenosine derivatives that displayed unusually high mouse (m) A₃AR affinity, we incorporated dopamine-related N⁶ substituents in the full agonist 5'-methylamide series. N⁶-(2-(4-Hydroxy-3-methoxy-phenyl)ethyl) derivative MRS7618 11 displayed K_i (nM) 0.563 at hA₃AR (∼20,000-fold selective) and 1.54 at mA₃AR. 2-Alkyl ethers maintained A₃ affinity, but with less selectivity than 2-alkynes. Parallel functional assays of G protein-dependent and β-arrestin 2 (βarr2)-dependent pathways indicate these are full agonists but not biased. Through use of computational modeling, we hypothesized that phenyl OH/OMe groups interact with polar residues, particularly Gln261, on the mA₃AR extracellular loops as the basis for the affinity enhancement. Although the pharmacokinetics indicated facile clearance of parent O-methyl catechol nucleosides 21 and 31, prolonged mA₃AR activation in vivo was observed in a hypothermia model, suggested potential formation of active metabolites through demethylation. Selected analogues induced mouse hypothermia following i.p. injection, indicative of peripheral A₃AR agonism in vivo.

Evidence that the acetyltransferase Tip60 induces the DNA damage response and cell-cycle arrest in neonatal cardiomyocytes

Tip60, a pan-acetyltransferase encoded by the Kat5 gene, is enriched in the myocardium; however, its function in the heart is unknown. In cancer cells, Tip60 acetylates Atm (Ataxia-telangiectasia mutated), enabling its auto-phosphorylation (pAtm), which activates the DNA damage response (DDR). It was recently reported that activation of pAtm at the time of birth induces the DDR in cardiomyocytes (CMs), resulting in proliferative senescence. We therefore hypothesized that Tip60 initiates this process, and that depletion of Tip60 accordingly diminishes the DDR while extending the duration of CM cell-cycle activation. To test this hypothesis, an experimental model was used wherein a Myh6-driven Cre-recombinase transgene was activated on postnatal day 0 (P0) to recombine floxed Kat5 alleles and induce Tip60 depletion in neonatal CMs, without causing pathogenesis. Depletion of Tip60 resulted in reduced numbers of pAtm-positive CMs during the neonatal period, which correlated with reduced numbers of pH2A.X-positive CMs and decreased expression of genes encoding markers of the DDR as well as inflammation. This was accompanied by decreased expression of the cell-cycle inhibitors Meis1 and p27, activation of the cell-cycle in CMs, reduced CM size, and increased numbers of mononuclear/diploid CMs. Increased expression of fetal markers suggested that Tip60 depletion promotes a fetal-like proliferative state. Finally, infarction of Tip60-depleted hearts at P7 revealed improved cardiac function at P39 accompanied by reduced fibrosis, increased CM cell-cycle activation, and reduced apoptosis in the remote zone. These findings indicate that, among its pleiotropic functions, Tip60 induces the DDR in CMs, contributing to proliferative senescence.

Myh6-driven Cre recombinase activates the DNA damage response and the cell cycle in the myocardium in the absence of loxP sites

Regeneration of muscle in the damaged myocardium is a major objective of cardiovascular research, for which purpose many investigators utilize mice containing transgenes encoding Cre recombinase to recombine loxP-flanked target genes. An unfortunate side effect of the Cre-loxP model is the propensity of Cre recombinase to inflict off-target DNA damage, which has been documented in various eukaryotic cell types including cardiomyocytes (CMs). In the heart, reported effects of Cre recombinase include contractile dysfunction, fibrosis, cellular infiltration and induction of the DNA damage response (DDR). During experiments on adult mice containing a widely used Myh6-merCremer transgene, the protein product of which is activated by tamoxifen, we observed large, transient, off-target effects of merCremer, some of which have not previously been reported. On Day 3 after the first of three daily tamoxifen injections, immunofluorescent microscopy of heart sections revealed that the presence of merCremer protein in myonuclei was nearly uniform, thereafter diminishing to near extinction by Day 6; during this time, cardiac function was depressed as determined by echocardiography. On Day 5, peaks of apoptosis and expression of DDR-regulatory genes were observed, highlighted by >25-fold increased expression of Brca1 Concomitantly, the expression of genes encoding cyclin-A2, cyclin-B2 and cyclin-dependent kinase 1, which regulate the G2/S cell-cycle transition, were dramatically increased (>50- to 100-fold). Importantly, immunofluorescent staining revealed that this was accompanied by peaks in Ki67, 5'-bromodeoxyuridine and phosphohistone H3 labeling in non-CMs, as well as CMs. We further document that tamoxifen-induced activation of merCremer exacerbates cardiac dysfunction following myocardial infarction. These findings, when considered in the context of previous reports, indicate that the presence of merCremer in the nucleus induces DNA damage and unscheduled cell-cycle activation. Although these effects are transient, the inclusion of appropriate controls, coupled with an awareness of the defects caused by Cre recombinase, are required to avoid misinterpreting results when using Cre-loxP models for cardiac regeneration studies.This article has an associated First Person interview with the first author of the paper.

Direct Comparison of (N)-Methanocarba and Ribose-Containing 2-Arylalkynyladenosine Derivatives as A3 Receptor Agonists

A side-by-side pharmacological comparison of ribose and (N)-methanocarba (bicyclo[3.1.0]hexane) nucleosides as A₃AR agonists indicated that the bicyclic pseudoribose ring constraint provided higher affinity/selectivity at human and mouse A₃AR. The mean affinity enhancement for 5 pairs of 5'-methylamides was 11-fold at hA₃AR and 42-fold at mA₃AR. Novel C2-(5-fluorothien-2-ylethynyl) substitution enhanced affinity in the methanocarba but not ribose series, with highly hA₃AR-selective 16 (MRS7334) displaying K_i 280 pM and favorable pharmacokinetics and off-target activity profile. Molecular dynamics comparison of 16 and its corresponding riboside 8 suggested a qualitative entropic advantage of 16 in hA₃AR binding. The 5-F substitution tended to increase hA₃AR affinity (cf. 5-Cl) for methanocarba but not ribose derivatives. A representative methanocarba agonist 4 was shown to interact potently exclusively with A₃AR, among 240 GPCRs and 466 kinases. Thus, despite added synthetic difficulty, the (N)-methanocarba modification has distinct advantages for A₃AR agonists, which have translational potential for chronic disease treatment.

Llgl1 regulates zebrafish cardiac development by mediating Yap stability in cardiomyocytes

The Hippo-Yap pathway regulates multiple cellular processes in response to mechanical and other stimuli. In Drosophila, the polarity protein Lethal (2) giant larvae [L(2)gl], negatively regulates Hippo-mediated transcriptional output. However, in vertebrates, little is known about its homolog Llgl1. Here, we define a novel role for vertebrate Llgl1 in regulating Yap stability in cardiomyocytes, which impacts heart development. In contrast to the role of Drosophila L(2)gl, Llgl1 depletion in cultured rat cardiomyocytes decreased Yap protein levels and blunted target gene transcription without affecting Yap transcript abundance. Llgl1 depletion in zebrafish resulted in larger and dysmorphic cardiomyocytes, pericardial effusion, impaired blood flow and aberrant valvulogenesis. Cardiomyocyte Yap protein levels were decreased in llgl1 morphants, whereas Notch, which is regulated by hemodynamic forces and participates in valvulogenesis, was more broadly activated. Consistent with the role of Llgl1 in regulating Yap stability, cardiomyocyte-specific overexpression of Yap in Llgl1-depleted embryos ameliorated pericardial effusion and restored blood flow velocity. Altogether, our data reveal that vertebrate Llgl1 is crucial for Yap stability in cardiomyocytes and its absence impairs cardiac development.

Abstract 399: Tip60 Depletion in Adult Cardiomyocytes Promotes Proliferation and Preserves Cardiac Function After Myocardial Infarction

Cardiac disease and injury are accompanied by significant cardiomyocyte (CM) loss. CMs are essentially non-regenerable, preventing re-muscularization and resulting in permanent dysfunction during the healing process. The identification of factors from multiple layers of inhibitors that induce and maintain CM’s pronounced state of proliferative senescence provides potential therapeutic targets. Tip60 ( T at- i nteractive p rotein, 60 kD), a pan-acetylase tumor suppressor encoded by the Kat5 gene, activates multiple anti-proliferative pathways in other cell types. Tip60 is known to acetylate Atm and p53, which respectively activate the DNA damage response and apoptosis. And, Tip60 regulates intracellular levels of p21 and Tert polymerase in a fashion designed to maintain proliferative senescence. Although these functions promote CM proliferative senescence and Tip60 is strongly expressed in the myocardium, whether Tip60 regulates these activities in CMs remains unknown. This study tests the hypothesis that Tip60 inhibits CM cell-cycle activation, promotes apoptosis in the myocardium, and prevents regeneration after cardiac injury. In adult mice containing LoxP -flanked Kat5 alleles and a tamoxifen-inducible Cre-recombinase transgene driven by Myh6 (Kat5 flox/flox;Myh6-merCremer ) , Tip60 depletion three days after myocardial infarction (MI) preserved cardiac function as assessed by echocardiography at 10, 21, and 28 days post-MI. Histologic evaluation revealed that depletion of Tip60 in CMs significantly diminished scarring and increased CM cell-cycle activation, as indicated by increased numbers of Ki67-, BrdU- and pH3-positive CMs at 28 days post-MI. This was accompanied by the presence of CMs in the infarct border zone expressing smooth muscle α-actin, indicative of CM de-differentiation, and by reduced apoptosis in the remote zone as assessed by TUNEL and cleaved caspase-3 staining, suggesting reduced cardiac remodeling. These findings suggest that genetic depletion of Tip60 protects CMs from ischemia-induced cell death while promoting CM proliferation, implicating Tip60 as a novel candidate for cardiac therapeutics.

Abstract 400: Tip60 Depletion in Neonatal Cardiomyocytes Reduces the DNA Damage Response and Increases Cell-cycle Activation

Tip60 ( T at i nteractive p rotein, 60 kD) is a tumor suppressor encoded by the Kat5 gene that functions as a pan-acetylase. Among its targets, Atm ( a taxia- t elangiectasia m utated) undergoes auto-phosphorylation (pATM) when acetylated by Tip60 to induce the DNA damage response (DDR). Because pAtm-induced activation of the DDR is known to cause proliferative senescence in neonatal cardiomyocytes (CMs), we hypothesize that Tip60 is one of the upstream components in this pathway. Using mice containing floxed Kat5 alleles and a tamoxifen-inducible Cre-recombinase transgene driven by Myh6 (Kat5 flox/flox;Myh6-merCremer ), we assessed the effect of depleting Tip60 from neonatal CMs via a single injection of tamoxifen on postnatal day 0 (P0). Immunofluorescent microscopy revealed reduced pAtm-positive CMs at three postnatal stages (P7, P12, P39), accompanied by activation of the cell-cycle as indicated by significantly increased percentages of 5’-bromodeoxyuridine (BrdU)-, phosphohistone H3 (pH3)- and Ki67-positive CMs. In addition to cell-cycle activation, Tip60 depletion promoted nuclear division followed by cytokinesis, as indicated by increased percentage of mononuclear diploid CMs at P12. Accordingly, qPCR revealed that Tip60 depletion increased expression of mRNAs encoding the fetal markers Nppa, Nppb, Myh7 , and Acta1 at P39, which in the absence of CM hypertrophy (assessed by WGA staining) suggested maintenance of a proliferation-competent state. These effects occurred concomitant with depressed levels of mRNAs encoding cell-cycle inhibitors ( Meis1, p27 ) and a trend toward increased levels of G 2 -phase cell-cycle activators ( cyclins A2, B1 ; Cdk1 ) in Tip60-depleted hearts. These findings suggest that depletion of Tip60 at birth inhibits the DDR and delays replicative senescence in neonatal CMs.

Truncated (N)-Methanocarba Nucleosides as Partial Agonists at Mouse and Human A3 Adenosine Receptors: Affinity Enhancement by N6-(2-Phenylethyl) Substitution

Dopamine-derived N⁶-substituents, compared to N⁶-(2-phenylethyl), in truncated (N)-methanocarba (bicyclo[3.1.0]hexyl) adenosines favored high A₃ adenosine receptor (AR) affinity/selectivity, e.g., C2-phenylethynyl analogue 15 (MRS7591, K_i = 10.9/17.8 nM, at human/mouse A₃AR). 15 was a partial agonist in vitro (hA₃AR, cAMP inhibition, 31% E_max; mA₃AR, [³⁵S]GTP-γ-S binding, 16% E_max) and in vivo and also antagonized hA₃AR in vitro. Distal H-bonding substitutions of the N⁶-(2-phenylethyl) moiety particularly enhanced mA₃AR affinity by polar interactions with the extracellular loops, predicted using docking and molecular dynamics simulation with newly constructed mA₃AR and hA₃AR homology models. These hybrid models were based on an inactive antagonist-bound hA₁AR structure for the upper part of TM2 and an agonist-bound hA_2AAR structure for the remaining TM portions. These species-independent A₃AR-selective nucleosides are low efficacy partial agonists and novel, nuanced modulators of the A₃AR, a drug target of growing interest.

Abstract 152: Tip60 Depletion Promotes Cardiomyocyte Proliferation and Attenuates Ischemic Injury in the Adult Heart

The pathogenesis of myocardial infarction (MI) is largely attributed to the loss of cardiomyocytes (CMs) and their insufficient regeneration. Inducing the proliferation of pre-existing CMs has emerged as a potential therapeutic strategy for cardiac repair. Results in our laboratory indicate that Tip60 (Tat-interactive protein 60 kD), a pan-acetylase protein encoded by the Kat5 gene, inhibits CM proliferation consequent to its induction of the DNA damage response (DDR) at neonatal stages, which has recently been shown to cause CM replicative senescence. To determine whether Tip60 depletion permits re-entry of adult CMs into the cell-cycle and confers protection from MI, we are employing a line of Kat5 flox/flox mice wherein Tip60 is conditionally and specifically depleted in CMs via tamoxifen-induced activation of a Myh6 -driven merCremer transgene. In uninjured hearts, Tip60 depletion results in transient thickening of the left ventricular walls, accompanied by markedly increased expression of G 2 /M-phase cell cycle regulators (cyclins A2 & B1, Cdk1) and de-differentiation markers (Myh7, Osm, OsmR, Runx1), diminished CM size, decreased expression of cell-cycle inhibitors (p27, Meis1), and remarkable increases in Ki67 and pH3-positive CMs as well as non-CMs. In hearts infarcted by permanent ligation, tamoxifen administration increases fractional shortening, ejection fraction, and anterior wall thickening within 7 days, conditions that are sustained for at least 18 additional days, when reduced scarring is indicated by trichrome staining. Taken together, these results indicate that Tip60 depletion in adult heart may preserve cardiac performance after MI by inducing CM regeneration. These findings should help advance our understanding of the molecular mechanisms that keep CMs in replicative senescence, establishing a possible therapeutic target for maintaining and restoring cardiac muscle after MI.

Ability of CP-532,903 to protect mouse hearts from ischemia/reperfusion injury is dependent on expression of A3 adenosine receptors in cardiomyoyctes

A₃ adenosine receptor (A₃AR) agonists are effective at limiting injury caused by ischemia/reperfusion injury of the heart in experimental animal models. However, understanding of their mechanism of action, which is likely multifactorial, remains incomplete. In prior studies, it has been demonstrated that A₃AR-mediated ischemic protection is blocked by glibenclamide and is absent in Kir6.2 gene ablated mice that lack the pore-forming subunit of the ATP-sensitive potassium (K_ATP) channel, suggesting one contributing mechanism may involve accelerated activation of K_ATP channels. However, presence of A₃ARs in the myocardium has yet to be established. Utilizing a whole-cell recording technique, in this study we confirm functional expression of the A₃AR in adult mouse ventricular cardiomyocytes, coupled to activation of ATP-dependent potassium (K_ATP) channels via G_i inhibitory proteins. We further show that ischemic protection provided by the selective A₃AR agonist CP-532,903 in an isolated, buffer-perfused heart model is lost completely in Adora3^{LoxP/LoxP;Myh6-Cre} mice, which is a newly developed model developed and comprehensively described herein whereby the A₃AR gene (Adora3) is deleted exclusively in cardiomyocytes. Our findings, taken together with previously published work, are consistent with the hypothesis that A₃AR agonists provide ischemic tolerance, at least in part, by facilitating opening of myocardial K_ATP channels.

Design and in Vivo Characterization of A1 Adenosine Receptor Agonists in the Native Ribose and Conformationally Constrained (N)-Methanocarba Series

(N)-Methanocarba ([3.1.0]bicyclohexyl) adenosines and corresponding ribosides were synthesized to identify novel A₁ adenosine receptor (A₁AR) agonists for CNS or peripheral applications. Human and mouse AR binding was determined to assess the constrained ring system's A₁AR compatibility. N⁶-Dicyclobutylmethyl ribose agonist (9, MRS7469, >2000-fold selective for A₁AR) and known truncated N⁶-dicyclopropylmethyl methanocarba 7 (MRS5474) were drug-like. The pure diastereoisomer of known riboside 4 displayed high hA₁AR selectivity. Methanocarba modification reduced A₁AR selectivity of N⁶-dicyclopropylmethyl and endo-norbornyladenosines but increased ribavirin selectivity. Most analogues tested (ip) were inactive or weak in inducing mouse hypothermia, despite mA₁AR full agonism and variable mA₃AR efficacy, but strong hypothermia by 9 depended on A₁AR, which reflects CNS activity (determined using A₁AR or A₃AR null mice). Conserved hA₁AR interactions were preserved in modeling of 9 and methanocarba equivalent 24 (∼400-fold A₁AR-selective). Thus, we identified, and characterized in vivo, ribose and methanocarba nucleosides, including with A₁AR-enhancing N⁶-dicyclobutylmethyl-adenine and 1,2,4-triazole-3-carboxamide (40, MRS7451) nucleobases.

Repurposing of a Nucleoside Scaffold from Adenosine Receptor Agonists to Opioid Receptor Antagonists

While screening off-target effects of rigid (N)-methanocarba-adenosine 5'-methylamides as A₃ adenosine receptor (AR) agonists, we discovered μM binding hits at the δ-opioid receptor (DOR) and translocator protein (TSPO). In an effort to increase OR and decrease AR affinity by structure activity analysis of this series, antagonist activity at κ-(K)OR appeared in 5'-esters (ethyl 24 and propyl 30), which retained TSPO interaction (μM). 7-Deaza modification of C2-(arylethynyl)-5'-esters but not 4'-truncation enhanced KOR affinity (MRS7299 28 and 29, K _i ≈ 40 nM), revealed μ-OR and DOR binding, and reduced AR affinity. Molecular docking and dynamics simulations located a putative KOR binding mode consistent with the observed affinities, placing C7 in a hydrophobic region. 3-Deaza modification permitted TSPO but not OR binding, and 1-deaza was permissive to both; ribose-restored analogues were inactive at both. Thus, we have repurposed a known AR nucleoside scaffold for OR antagonism, with a detailed hypothesis for KOR recognition.

IL-13 promotes in vivo neonatal cardiomyocyte cell cycle activity and heart regeneration

There is great interest in identifying signaling mechanisms by which cardiomyocytes (CMs) can enter the cell cycle and promote endogenous cardiac repair. We have previously demonstrated that IL-13 stimulated cell cycle activity of neonatal CMs in vitro. However, the signaling events that occur downstream of IL-13 in CMs and the role of IL-13 in CM proliferation and regeneration in vivo have not been explored. Here, we tested the role of IL-13 in promoting neonatal CM cell cycle activity and heart regeneration in vivo and investigated the signaling pathway(s) downstream of IL-13 specifically in CMs. Compared with control, CMs from neonatal IL-13 knockout (IL-13^-/-) mice showed decreased proliferative markers and coincident upregulation of the hypertrophic marker brain natriuretic peptide ( Nppb) and increased CM nuclear size. After apical resection in anesthetized newborn mice, heart regeneration was significantly impaired in IL-13^-/- mice compared with wild-type mice. Administration of recombinant IL-13 reversed these phenotypes by increasing CM proliferation markers and decreasing Nppb expression. RNA sequencing on primary neonatal CMs treated with IL-13 revealed activation of gene networks regulated by ERK1/2 and Akt. Western blot confirmed strong phosphorylation of ERK1/2 and Akt in both neonatal and adult cultured CMs in response to IL-13. Our data demonstrated a role for endogenous IL-13 in neonatal CM cell cycle and heart regeneration. ERK1/2 and Akt signaling are important pathways known to promote CM proliferation and protect against apoptosis, respectively; thus, targeting IL-13 transmembrane receptor signaling or administering recombinant IL-13 may be therapeutic approaches for activating proregenerative and survival pathways in the heart. NEW & NOTEWORTHY Here, we demonstrate, for the first time, that IL-13 is involved in neonatal cardiomyocyte cell cycle activity and heart regeneration in vivo. Prior work has shown that IL-13 promotes cardiomyocyte cell cycle activity in vitro; however, the signaling pathways were unknown. We used RNA sequencing to identify the signaling pathways activated downstream of IL-13 in cardiomyocytes and found that ERK1/2 and Akt signaling was activated in response to IL-13.

Structure activity relationship of 2-arylalkynyl-adenine derivatives as human A3 adenosine receptor antagonists

Recognition of nucleosides at adenosine receptors (ARs) is supported by multiple X-ray structures, but the structure of an adenine complex is unknown. We examined the selectivity of predicted A₁AR and A₃AR adenine antagonists that incorporated known agonist affinity-enhancing N ⁶ and C2 substituents. Adenines with A₁AR-favoring N ⁶-alkyl, cycloalkyl and arylalkyl substitutions combined with an A₃AR-favoring 2-((5-chlorothiophen-2-yl)ethynyl) group were human (h) A₃AR-selective, e.g. MRS7497 17 (∼1000-fold over A₁AR). In addition, binding selectivity over hA_2AAR and hA_2BAR and functional A₃AR antagonism were demonstrated. 17 was subjected to computational docking and molecular dynamics simulation in a hA₃AR homology model to predict interactions. The SAR of nucleoside AR agonists was not recapitulated in adenine AR antagonists, and modeling suggested an alternative, inverted binding mode with the key N250^6.55 H-bonding to the adenine N ³ and N ⁹, instead of N ⁶ and N ⁷ as in adenosine agonists.

Activation of adenosine A2A or A2B receptors causes hypothermia in mice

Extracellular adenosine is a danger/injury signal that initiates protective physiology, such as hypothermia. Adenosine has been shown to trigger hypothermia via agonism at A1 and A3 adenosine receptors (A1AR, A3AR). Here, we find that adenosine continues to elicit hypothermia in mice null for A1AR and A3AR and investigated the effect of agonism at A2AAR or A2BAR. The poorly brain penetrant A2AAR agonists CGS-21680 and PSB-0777 caused hypothermia, which was not seen in mice lacking A2AAR. MRS7352, a likely non-brain penetrant A2AAR antagonist, inhibited PSB-0777 hypothermia. While vasodilation is probably a contributory mechanism, A2AAR agonism also caused hypometabolism, indicating that vasodilation is not the sole mechanism. The A2BAR agonist BAY60-6583 elicited hypothermia, which was lost in mice null for A2BAR. Low intracerebroventricular doses of BAY60-6583 also caused hypothermia, indicating a brain site of action, with neuronal activation in the preoptic area and paraventricular nucleus of the hypothalamus. Thus, agonism at any one of the canonical adenosine receptors, A1AR, A2AAR, A2BAR, or A3AR, can cause hypothermia. This four-fold redundancy in adenosine-mediated initiation of hypothermia may reflect the centrality of hypothermia as a protective response.

Species differences and mechanism of action of A3 adenosine receptor allosteric modulators

Activity of the A₃ adenosine receptor (AR) allosteric modulators LUF6000 (2-cyclohexyl-N-(3,4-dichlorophenyl)-1H-imidazo [4,5-c]quinolin-4-amine) and LUF6096 (N-{2-[(3,4-dichlorophenyl)amino]quinolin-4-yl}cyclohexanecarbox-amide) was compared at four A₃AR species homologs used in preclinical drug development. In guanosine 5'-[γ-[³⁵S]thio]triphosphate ([³⁵S]GTPγS) binding assays with cell membranes isolated from human embryonic kidney cells stably expressing recombinant A₃ARs, both modulators substantially enhanced agonist efficacy at human, dog, and rabbit A₃ARs but provided only weak activity at mouse A₃ARs. For human, dog, and rabbit, both modulators increased the maximal efficacy of the A₃AR agonist 2-chloro-N ⁶-(3-iodobenzyl)adenosine-5'-N-methylcarboxamide as well as adenosine > 2-fold, while slightly reducing potency in human and dog. Based on results from N ⁶-(4-amino-3-[¹²⁵I]iodobenzyl)adenosine-5'-N-methylcarboxamide ([¹²⁵I]I-AB-MECA) binding assays, we hypothesize that potency reduction is explained by an allosterically induced slowing in orthosteric ligand binding kinetics that reduces the rate of formation of ligand-receptor complexes. Mutation of four amino acid residues of the human A₃AR to the murine sequence identified the extracellular loop 1 (EL1) region as being important in selectively controlling the allosteric actions of LUF6096 on [¹²⁵I]I-AB-MECA binding kinetics. Homology modeling suggested interaction between species-variable EL1 and agonist-contacting EL2. These results indicate that A₃AR allostery is species-dependent and provide mechanistic insights into this therapeutically promising class of agents.

Bitopic fluorescent antagonists of the A2A adenosine receptor based on pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidin-5-amine functionalized congeners

A pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidin-5-amine antagonist of the A_2A adenosine receptor (AR) was functionalized as amine congeners, fluorescent conjugates and a sulfonate, and the A_2AAR binding modes were predicted computationally. The optimal n-butyl spacer was incorporated into the following A_2AAR-selective (K_i, nM) conjugates: BODIPY630/650 derivative 11 (MRS7396, 24.6) and AlexaFluor488 derivative 12 (MRS7416, 30.3). Flow cytometry of 12 in hA_2AAR-expressing HEK-293 cells displayed saturable binding (low nonspecific) and inhibition by known A_2AAR antagonists. Water-soluble sulfonate 13 was a highly potent (K_i = 6.2 nM) and selective A_2AAR antagonist based on binding and functional assays. Docking and molecular dynamics simulations predicted the regions of interaction of the distal portions of these chain-extended ligands with the A_2AAR. The BODIPY630/650 fluorophore of 11 was buried in a hydrophobic interhelical (TM1/TM7) region, while AlexaFluor488 of 12 associated with the hydrophilic extracellular loops. In conclusion, we have identified novel high affinity antagonist probes for A_2AAR drug discovery and characterization.

Scaffold Repurposing of Nucleosides (Adenosine Receptor Agonists): Enhanced Activity at the Human Dopamine and Norepinephrine Sodium Symporters

We have repurposed (N)-methanocarba adenosine derivatives (A₃ adenosine receptor (AR) agonists) to enhance radioligand binding allosterically at the human dopamine (DA) transporter (DAT) and inhibit DA uptake. We extended the structure-activity relationship of this series with small N⁶-alkyl substitution, 5'-esters, deaza modifications of adenine, and ribose restored in place of methanocarba. C2-(5-Halothien-2-yl)-ethynyl 5'-methyl 9 (MRS7292) and 5'-ethyl 10 (MRS7232) esters enhanced binding at DAT (EC₅₀ ∼ 35 nM) and at the norepinephrine transporter (NET). 9 and 10 were selective for DAT compared to A₃AR in the mouse but not in humans. At DAT, the binding of two structurally dissimilar radioligands was enhanced; NET binding of only one radioligand was enhanced; SERT radioligand binding was minimally affected. 10 was more potent than cocaine at inhibiting DA uptake (IC₅₀ = 107 nM). Ribose analogues were weaker in DAT interaction than the corresponding bicyclics. Thus, we enhanced the neurotransmitter transporter activity of rigid nucleosides while reducing A₃AR affinity.

Hypothermia in mouse is caused by adenosine A1 and A3 receptor agonists and AMP via three distinct mechanisms

Small mammals have the ability to enter torpor, a hypothermic, hypometabolic state, allowing impressive energy conservation. Administration of adenosine or adenosine 5'-monophosphate (AMP) can trigger a hypothermic, torpor-like state. We investigated the mechanisms for hypothermia using telemetric monitoring of body temperature in wild type and receptor knock out (Adora1^-/-, Adora3^-/-) mice. Confirming prior data, stimulation of the A₃ adenosine receptor (AR) induced hypothermia via peripheral mast cell degranulation, histamine release, and activation of central histamine H₁ receptors. In contrast, A₁AR agonists and AMP both acted centrally to cause hypothermia. Commonly used, selective A₁AR agonists, including N⁶-cyclopentyladenosine (CPA), N⁶-cyclohexyladenosine (CHA), and MRS5474, caused hypothermia via both A₁AR and A₃AR when given intraperitoneally. Intracerebroventricular dosing, low peripheral doses of Cl-ENBA [(±)-5'-chloro-5'-deoxy-N⁶-endo-norbornyladenosine], or using Adora3^-/- mice allowed selective stimulation of A₁AR. AMP-stimulated hypothermia can occur independently of A₁AR, A₃AR, and mast cells. A₁AR and A₃AR agonists and AMP cause regulated hypothermia that was characterized by a drop in total energy expenditure, physical inactivity, and preference for cooler environmental temperatures, indicating a reduced body temperature set point. Neither A₁AR nor A₃AR was required for fasting-induced torpor. A₁AR and A₃AR agonists and AMP trigger regulated hypothermia via three distinct mechanisms.

Purine (N)-Methanocarba Nucleoside Derivatives Lacking an Exocyclic Amine as Selective A3 Adenosine Receptor Agonists

Purine (N)-methanocarba-5′-N-alkyluronamidoriboside A₃ adenosine receptor (A₃AR) agonists lacking an exocyclic amine resulted from an unexpected reaction during a Sonogashira coupling and subsequent aminolysis. Because the initial C6-Me and C6-styryl derivatives had unexpectedly high A₃AR affinity, other rigid nucleoside analogues lacking an exocyclic amine were prepared. Of these, the C6-Me-(2-phenylethynyl) and C2-(5-chlorothienylethynyl) analogues were particularly potent, with human A₃AR K_i values of 6 and 42 nM, respectively. Additionally, the C2-(5-chlorothienyl)-6-H analogue was potent and selective at A₃AR (MRS7220, K_i 60 nM) and also completely reversed mouse sciatic nerve mechanoallodynia (in vivo, 3 μmol/kg, po). The lack of a C6 H-bond donor while maintaining A₃AR affinity and efficacy could be rationalized by homology modeling and docking of these hypermodified nucleosides. The modeling suggests that a suitable combination of stabilizing features can partially compensate for the lack of an exocyclic amine, an otherwise important contributor to recognition in the A₃AR binding site.

Characterization of Dahl salt-sensitive rats with genetic disruption of the A2B adenosine receptor gene: implications for A2B adenosine receptor signaling during hypertension

The A(2B) adenosine receptor (AR) has emerged as a unique member of the AR family with contrasting roles during acute and chronic disease states. We utilized zinc-finger nuclease technology to create A(2B)AR gene (Adora2b)-disrupted rats on the Dahl salt-sensitive (SS) genetic background. This strategy yielded a rat strain (SS-Adora2b mutant rats) with a 162-base pair in-frame deletion of Adora2b that included the start codon. Disruption of A(2B)AR function in SS-Adora2b mutant rats was confirmed by loss of agonist (BAY 60-6583 or NECA)-induced cAMP accumulation and loss of interleukin-6 release from isolated fibroblasts. In addition, BAY 60-6583 produced a dose-dependent increase in glucose mobilization that was absent in SS-Adora2b mutants. Upon initial characterization, SS-Adora2b mutant rats were found to exhibit increased body weight, a transient delay in glucose clearance, and reduced proinflammatory cytokine production following challenge with lipopolysaccharide (LPS). In addition, blood pressure was elevated to a greater extent (∼15-20 mmHg) in SS-Adora2b mutants as they aged from 7 to 21 weeks. In contrast, hypertension augmented by Ang II infusion was attenuated in SS-Adora2b mutant rats. Despite differences in blood pressure, indices of renal and cardiac injury were similar in SS-Adora2b mutants during Ang II-augmented hypertension. We have successfully created and validated a new animal model that will be valuable for investigating the biology of the A(2B)AR. Our data indicate varying roles for A(2B)AR signaling in regulating blood pressure in SS rats, playing both anti- and prohypertensive roles depending on the pathogenic mechanisms that contribute to blood pressure elevation.

Structure-Based Design, Synthesis by Click Chemistry and in Vivo Activity of Highly Selective A3 Adenosine Receptor Agonists

2-Arylethynyl derivatives of (N)-methanocarba adenosine 5'-uronamides are selective A3AR (adenosine receptor) agonists. Here we substitute a 1,2,3-triazol-1-yl linker in place of the rigid, linear ethynyl group to eliminate its potential metabolic liability. Docking of nucleosides containing possible short linker moieties at the adenine C2 position using a hybrid molecular model of the A3AR (based on the A2AAR agonist-bound structure) correctly predicted that a triazole would maintain the A3AR selectivity, due to its ability to fit a narrow cleft in the receptor. The analogues with various N6 and C2-aryltriazolyl substitution were synthesized and characterized in binding (Ki at hA3AR 0.3 - 12 nM) and in vivo to demonstrate efficacy in controlling chronic neuropathic pain (chronic constriction injury). Among N6-methyl derivatives, a terminal pyrimidin-2-yl group in 9 (MRS7116) increased duration of action (36% pain protection at 3 h) in vivo. N6-Ethyl 5-chlorothien-2-yl analogue 15 (MRS7126) preserved in vivo efficacy (85% protection at 1 h) with short duration. Larger N6 groups, e.g. 17 (MRS7138, >90% protection at 1 and 3 h), greatly enhanced in vivo activity. Thus, we have combined structure-based methods and phenotypic screening to identify nucleoside derivatives having translational potential.

Rigidified A3 Adenosine Receptor Agonists: 1-Deazaadenine Modification Maintains High in Vivo Efficacy

Substitution of rigidified A3 adenosine receptor (AR) agonists with a 2-((5-chlorothiophen-2-yl)ethynyl) or a 2-(4-(5-chlorothiophen-2-yl)-1H-1,2,3-triazol-1-yl) group provides prolonged protection in a model of chronic neuropathic pain. These agonists contain a bicyclo[3.1.0]hexane ((N)-methanocarba) ring system in place of ribose, which adopts a receptor-preferred conformation. N (6)-Small alkyl derivatives were newly optimized for A3AR affinity and the effects of a 1-deaza-adenine modification probed. 1-Deaza-N (6)-ethyl alkyne 20 (MRS7144, K i 1.7 nM) and 1-aza N (6)-propyl alkyne 12 (MRS7154, K i 1.1 nM) were highly efficacious in vivo. Thus, the presence of N1 is not required for nanomolar binding affinity or potent, long-lasting functional activity. Docking of 1-deaza compounds to a receptor homology model confirmed a similar binding mode as previously reported 1-aza derivatives. This is the first demonstration in nonribose adenosine analogues that the 1-deaza modification can maintain high A3AR affinity, selectivity, and efficacy.

In vivo phenotypic screening for treating chronic neuropathic pain: modification of C2-arylethynyl group of conformationally constrained A3 adenosine receptor agonists

(N)-Methanocarba adenosine 5'-methyluronamides containing 2-arylethynyl groups were synthesized as A3 adenosine receptor (AR) agonists and screened in vivo (po) for reduction of neuropathic pain. A small N(6)-methyl group maintained binding affinity, with human > mouse A3AR and MW < 500 and other favorable physicochemical properties. Emax (maximal efficacy in a mouse chronic constriction injury pain model) of previously characterized A3AR agonist, 2-(3,4-difluorophenylethynyl)-N(6)-(3-chlorobenzyl) derivative 6a, MRS5698, was surpassed. More efficacious analogues (in vivo) contained the following C2-arylethynyl groups: pyrazin-2-yl 23 (binding Ki, hA3AR, nM 1.8), fur-2-yl 27 (0.6), thien-2-yl 32 (0.6) and its 5-chloro 33, MRS5980 (0.7) and 5-bromo 34 (0.4) equivalents, and physiologically unstable ferrocene 36, MRS5979 (2.7). 33 and 36 displayed particularly long in vivo duration (>3 h). Selected analogues were docked to an A3AR homology model to explore the environment of receptor-bound C2 and N(6) groups. Various analogues bound with μM affinity at off-target biogenic amine (M2, 5HT2A, β3, 5HT2B, 5HT2C, and α2C) or other receptors. Thus, we have expanded the structural range of orally active A3AR agonists for chronic pain treatment.

Rap1b in smooth muscle and endothelium is required for maintenance of vascular tone and normal blood pressure

OBJECTIVE

Small GTPase Ras-related protein 1 (Rap1b) controls several basic cellular phenomena, and its deletion in mice leads to several cardiovascular defects, including impaired adhesion of blood cells and defective angiogenesis. We found that Rap1b(-/-) mice develop cardiac hypertrophy and hypertension. Therefore, we examined the function of Rap1b in regulation of blood pressure.

APPROACH AND RESULTS

Rap1b(-/-) mice developed cardiac hypertrophy and elevated blood pressure, but maintained a normal heart rate. Correcting elevated blood pressure with losartan, an angiotensin II type 1 receptor antagonist, alleviated cardiac hypertrophy in Rap1b(-/-) mice, suggesting a possibility that cardiac hypertrophy develops secondary to hypertension. The indices of renal function and plasma renin activity were normal in Rap1b(-/-) mice. Ex vivo, we examined whether the effect of Rap1b deletion on smooth muscle-mediated vessel contraction and endothelium-dependent vessel dilation, 2 major mechanisms controlling basal vascular tone, was the basis for the hypertension. We found increased contractility on stimulation with a thromboxane analog or angiotensin II or phenylephrine along with increased inhibitory phosphorylation of myosin phosphatase under basal conditions consistent with elevated basal tone and the observed hypertension. Cyclic adenosine monophosphate-dependent relaxation in response to Rap1 activator, Epac, was decreased in vessels from Rap1b(-/-) mice. Defective endothelial release of dilatory nitric oxide in response to elevated blood flow leads to hypertension. We found that nitric oxide-dependent vasodilation was significantly inhibited in Rap1b-deficient vessels.

CONCLUSIONS

This is the first report to indicate that Rap1b in both smooth muscle and endothelium plays a key role in maintaining blood pressure by controlling normal vascular tone.

Rational design of sulfonated A3 adenosine receptor-selective nucleosides as pharmacological tools to study chronic neuropathic pain

(N)-Methanocarba(bicyclo[3.1.0]hexane)adenosine derivatives were probed for sites of charged sulfonate substitution, which precludes diffusion across biological membranes, e.g., blood-brain barrier. Molecular modeling predicted that sulfonate groups on C2-phenylethynyl substituents would provide high affinity at both mouse (m) and human (h) A3 adenosine receptors (ARs), while a N(6)-p-sulfophenylethyl substituent would determine higher hA3AR vs mA3AR affinity. These modeling predictions, based on steric fitting of the binding cavity and crucial interactions with key residues, were confirmed by binding/efficacy studies of synthesized sulfonates. N(6)-3-Chlorobenzyl-2-(3-sulfophenylethynyl) derivative 7 (MRS5841) bound selectively to h/m A3ARs (Ki(hA3AR) = 1.9 nM) as agonist, while corresponding p-sulfo isomer 6 (MRS5701) displayed mixed A1/A3AR agonism. Both nucleosides administered ip reduced mouse chronic neuropathic pain that was ascribed to either A3AR or A1/A3AR using A3AR genetic deletion. Thus, rational design methods based on A3AR homology models successfully predicted sites for sulfonate incorporation, for delineating adenosine's CNS vs peripheral actions.

An adenosine-mediated signaling pathway suppresses prenylation of the GTPase Rap1B and promotes cell scattering

During metastasis, cancer cells acquire the ability to dissociate from each other and migrate, which is recapitulated in vitro as cell scattering. The small guanosine triphosphatase (GTPase) Rap1 opposes cell scattering by promoting cell-cell adhesion, a function that requires its prenylation, or posttranslational modification with a carboxyl-terminal isoprenoid moiety, to enable its localization at cell membranes. Thus, signaling cascades that regulate the prenylation of Rap1 offer a mechanism to control the membrane localization of Rap1. We identified a signaling cascade initiated by adenosine A2B receptors that suppressed the prenylation of Rap1B through phosphorylation of Rap1B, which decreased its interaction with the chaperone protein SmgGDS (small GTPase guanosine diphosphate dissociation stimulator). These events promoted the cytosolic and nuclear accumulation of nonprenylated Rap1B and diminished cell-cell adhesion, resulting in cell scattering. We found that nonprenylated Rap1 was more abundant in mammary tumors than in normal mammary tissue in rats and that activation of adenosine receptors delayed Rap1B prenylation in breast, lung, and pancreatic cancer cell lines. Our findings support a model in which high concentrations of extracellular adenosine, such as those that arise in the tumor microenvironment, can chronically activate A2B receptors to suppress Rap1B prenylation and signaling at the cell membrane, resulting in reduced cell-cell contact and promoting cell scattering. Inhibiting A2B receptors may be an effective method to prevent metastasis.

Abstract 206: Small GTPase Rap1 Transmits Mechanical Signals to Control Vascular Tone and Blood Pressure

Smooth muscle (SM)-mediated vessel contraction and endothelium-dependent vessel dilation are major mechanisms controlling basal vascular tone. Increased vascular tone leads to arterial hypertension, a risk factor for cardiovascular diseases. Agonist-induced, Ca2+-dependent SM contraction is modulated by intracellular cAMP, which promotes SM relaxation by inhibition of Rho-mediated myosin regulatory light chain (RLC20) phosphorylation and actomyosin contractility. In response to elevated blood flow, endothelium releases dilator substances that relax vascular SM, and in particular defects in production of nitric oxide (NO) lead to hypertension.

Rap1 is a small GTPase that integrates signals from multiple receptors and two Rap1 isoforms, Rap1a and Rap1b are ubiquitously expressed. We found global Rap1b deletion leads to cardiac hypertrophy and hypertension and examined the effect of Rap1-deficiency on vascular tone. Using SM and endothelium tissue-restricted Rap1 knockout mice we show that deletion of Rap1, via distinct mechanisms in the two tissues, contributes to elevated vascular tone. In SM Rap1 regulates basal contraction level and mediates cAMP-induced desensitization of contraction. Rap1 suppresses RhoA-mediated RLC20 phosphorylation, Ca+2 sensitization and relaxation, signaling that is further enhanced upon cAMP-dependent activation of Rap1 by Rap1 GEF Epac.

In endothelium, we find Rap1 is essential for regulation of NO-dependent vasodilation, as deletion of three of the four Rap1 alleles leads to a significant decrease in NO-dependent vasodilation. Significantly, we find that Rap1 is required for normal shear stress-induced NO release and Rap1-deficency in endothelium leads to elevated blood pressure in mice. Mechanistically, we show that Rap1 is required for transducing signals from the endothelial mechanosensing complex triggering signaling leading to NO production. Because shear stress from flowing blood is the main determinant of NO release, this novel finding positions Rap1 as a key regulator of endothelial function.

In conclusion, Rap1 in SM and endothelium, via distinct mechanisms, plays a key role in maintaining normal vascular tone and blood pressure.

Characterization by flow cytometry of fluorescent, selective agonist probes of the A(3) adenosine receptor

Various fluorescent nucleoside agonists of the A3 adenosine receptor (AR) were compared as high affinity probes using radioligands and flow cytometry (FCM). They contained a fluorophore linked through the C2 or N(6) position and rigid A3AR-enhancing (N)-methanocarba modification. A hydrophobic C2-(1-pyrenyl) derivative MRS5704 bound nonselectively. C2-Tethered cyanine5-dye labeled MRS5218 bound selectively to hA3AR expressed in whole CHO cells and membranes. By FCM, binding was A3AR-mediated (blocked by A3AR antagonist, at least half through internalization), with t1/2 for association 38min in mA3AR-HEK293 cells; 26.4min in sucrose-treated hA3AR-CHO cells (Kd 31nM). Membrane binding indicated moderate mA3AR affinity, but not selectivity. Specific accumulation of fluorescence (50nM MRS5218) occurred in cells expressing mA3AR, but not other mouse ARs. Evidence was provided suggesting that MRS5218 detects endogenous expression of the A3AR in the human promyelocytic leukemic HL-60 cell line. Therefore, MRS5218 promises to be a useful tool for characterizing the A3AR.

Structure-guided design of A(3) adenosine receptor-selective nucleosides: combination of 2-arylethynyl and bicyclo[3.1.0]hexane substitutions

(N)-Methanocarba adenosine 5'-methyluronamides containing known A(3) AR (adenosine receptor)-enhancing modifications, i.e., 2-(arylethynyl)adenine and N(6)-methyl or N(6)-(3-substituted-benzyl), were nanomolar full agonists of human (h) A(3)AR and highly selective (K(i) ∼0.6 nM, N(6)-methyl 2-(halophenylethynyl) analogues 13 and 14). Combined 2-arylethynyl-N(6)-3-chlorobenzyl substitutions preserved A(3)AR affinity/selectivity in the (N)-methanocarba series (e.g., 3,4-difluoro full agonist MRS5698 31, K(i) 3 nM, human and mouse A(3)) better than that for ribosides. Polyaromatic 2-ethynyl N(6)-3-chlorobenzyl analogues, such as potent linearly extended 2-p-biphenylethynyl MRS5679 34 (K(i) hA(3) 3.1 nM; A(1), A(2A), inactive) and fluorescent 1-pyrene adduct MRS5704 35 (K(i) hA(3) 68.3 nM), were conformationally rigid; receptor docking identified a large, mainly hydrophobic binding region. The vicinity of receptor-bound C2 groups was probed by homology modeling based on recent X-ray structure of an agonist-bound A(2A)AR, with a predicted helical rearrangement requiring an agonist-specific outward displacement of TM2 resembling opsin. Thus, the X-ray structure of related A(2A)AR is useful in guiding the design of new A(3)AR agonists.

Polyamidoamine (PAMAM) dendrimer conjugate specifically activates the A3 adenosine receptor to improve post-ischemic/reperfusion function in isolated mouse hearts

Background

When stimulated by small molecular agonists, the A₃ adenosine receptor (AR) mediates cardioprotective effects without inducing detrimental hemodynamic side effects. We have examined pharmacologically the protective properties of a multivalent dendrimeric conjugate of a nucleoside as a selective multivalent agonist for the mouse A₃AR.

Results

A PAMAM dendrimer fully substituted by click chemistry on its peripheral groups with 64 moieties of a nucleoside agonist was shown to be potent and selective in binding to the mouse A₃AR and effective in cardioprotection in an isolated mouse heart model of ischemia/reperfusion (I/R) injury. This conjugate MRS5246 and a structurally related model compound MRS5233 displayed binding K_i values of 0.04 and 3.94 nM, respectively, and were potent in in vitro functional assays to inhibit cAMP production. A methanocarba (bicyclo[3.1.0]hexane) ring system in place of ribose maintained a North conformation that is preferred at the A₃AR. These analogues also contained a triazole linker along with 5'-N-methyl-carboxamido and 2-alkynyl substitution, previously shown to be associated with species-independent A₃AR selectivity. Both MRS5233 and MRS5246 (1 and 10 nM) were effective at increasing functional recovery of isolated mouse hearts after 20 min ischemia followed by 45 min reperfusion. A statistically significant greater improvement in the left ventricular developed pressure (LVDP) by MRS5246 compared to MRS5233 occurred when the hearts were observed throughout reperfusion. Unliganded PAMAM dendrimer alone did not have any effect on functional recovery of isolated perfused mouse hearts. 10 nM MRS5246 did not improve functional recovery after I/R in hearts from A₃AR gene "knock-out" (A₃KO) mice compared to control, indicating the effects of MRS5246 were A₃AR-specific.

Conclusions

Covalent conjugation to a versatile drug carrier enhanced the functional potency and selectivity at the mouse A₃AR and maintained the cardioprotective properties. Thus, this large molecular weight conjugate is not prevented from extravasation through the coronary microvasculature.

Protection from myocardial ischemia/reperfusion injury by a positive allosteric modulator of the A₃ adenosine receptor

Adenosine is increased in ischemic tissues where it serves a protective role by activating adenosine receptors (ARs), including the A₃ AR subtype. We investigated the effect of N-{2-[(3,4-dichlorophenyl)amino]quinolin-4-yl}cyclohexanecarboxamide (LUF6096), a positive allosteric modulator of the A₃ AR, on infarct size in a barbital-anesthetized dog model of myocardial ischemia/reperfusion injury. Dogs were subjected to 60 min of coronary artery occlusion and 3 h of reperfusion. Infarct size was assessed by macrohistochemical staining. Three experimental groups were included in the study. Groups I and II received two doses of vehicle or LUF6096 (0.5 mg/kg i.v. bolus), one administered before ischemia and the other immediately before reperfusion. Group III received a single dose of LUF6096 (1 mg/kg i.v. bolus) immediately before reperfusion. In preliminary in vitro studies, LUF6096 was found to exert potent enhancing activity (EC₅₀ 114.3 ± 15.9 nM) with the canine A₃ AR in a guanosine 5'-[γ-[³⁵S]thio]triphosphate binding assay. LUF6096 increased the maximal efficacy of the partial A₃ AR agonist 2-chloro-N⁶-(3-iodobenzyl)adenosine-5'-N-methylcarboxamide and the native agonist adenosine more than 2-fold while producing a slight decrease in potency. In the dog studies, administration of LUF6096 had no effect on any hemodynamic parameter measured. Pretreatment with LUF6096 before coronary occlusion and during reperfusion in group II dogs produced a marked reduction in infarct size (∼50% reduction) compared with group I vehicle-treated dogs. An equivalent reduction in infarct size was observed when LUF6096 was administered immediately before reperfusion in group III dogs. This is the first study to demonstrate efficacy of an A₃ AR allosteric enhancer in an in vivo model of infarction.

A role for the low-affinity A2B adenosine receptor in regulating superoxide generation by murine neutrophils

The formation of adenosine dampens inflammation by inhibiting most cells of the immune system. Among its actions on neutrophils, adenosine suppresses superoxide generation and regulates chemotactic activity. To date, most evidence implicates the G(s) protein-coupled A(2A) adenosine receptor (AR) as the primary AR subtype responsible for mediating the actions of adenosine on neutrophils by stimulating cAMP production. Given that the A(2B)AR is now known to be expressed in neutrophils and that it is a G(s) protein-coupled receptor, we examined in this study whether it signals to suppress neutrophil activities by using 2-[6-amino-3,5-dicyano-4-[4-(cyclopropylmethoxy)phenyl]pyridin-2-ylsulfanyl]acetamide (BAY 60-6583), a new agonist for the human A(2B)AR that was confirmed in preliminary studies to be a potent and highly selective agonist for the murine A(2B)AR. We found that treating mouse neutrophils with low concentrations (10(-9) and 10(-8) M) of BAY 60-6583 inhibited formylated-methionine-leucine-phenylalanine (fMLP)-stimulated superoxide production by either naive neutrophils, tumor necrosis factor-α-primed neutrophils, or neutrophils isolated from mice treated systemically with lipopolysaccharide. This inhibitory action of BAY 60-6583 was confirmed to involve the A(2B)AR in experiments using neutrophils obtained from A(2B)AR gene knockout mice. It is noteworthy that BAY 60-6583 increased fMLP-stimulated superoxide production at higher concentrations (>1 μM), which was attributed to an AR-independent effect. In a standard Boyden chamber migration assay, BAY 60-6583 alone did not stimulate neutrophil chemotaxis or influence chemotaxis in response to fMLP. These results indicate that the A(2B)AR signals to suppress oxidase activity by murine neutrophils, supporting the idea that this low-affinity receptor for adenosine participates along with the A(2A)AR in regulating the proinflammatory actions of neutrophils.

Evidence that the acute phase of ischemic preconditioning does not require signaling by the A 2B adenosine receptor

Ischemic preconditioning (IPC) is a protective phenomenon in which brief ischemia renders the myocardium resistant to subsequent ischemic insults. Here, we used A(2B)AR gene knock-out (A(2B)KO)/β-galactosidase reporter gene knock-in mice and the A(2B)AR antagonist ATL-801 to investigate the potential involvement of the A(2B)AR in IPC, focusing on the acute phase of protection. Cardioprotection provided by acute IPC elicited by two 3-min occlusion/3-min reperfusion cycles was readily apparent in an isolated, Langendorff-perfused mouse heart model in studies using hearts from A(2B)KO mice. IPC equivalently improved the recovery of contractile function following 20 min of global ischemia and 45 min of reperfusion in both WT and A(2B)KO hearts by ~30-40%, and equivalently decreased the release of cardiac troponin I during the reperfusion period (from 5969 ± 925 to 1595 ± 674 ng/g and 4376 ± 739 to 2278 ± 462 ng/g using WT and A(2B)KO hearts, respectively). Similarly, the infarct size-reducing capacity of acute IPC in an in vivo model of infarction was fully manifested in experiments using A(2B)KO mice, as well as in experiments using rats pretreated with ATL-801. We did observe, however, a marked reduction in infarct size in rats following administration of the selective A(2B)AR agonist BAY 60-6583 (~25% reduction at a dose of 1.0mg/kg). While supportive of its concept as a cardioprotective receptor, these experiments indicate that the mechanism of the early phase of IPC is not dependent on signaling by the A(2B)AR. We present the idea that the A(2B)AR may contribute to the later stages of IPC dependent on the induction of stress-responsive genes.

Activation of the A(3) adenosine receptor inhibits fMLP-induced Rac activation in mouse bone marrow neutrophils

Adenosine is released from injured or hypoxic tissues where it exerts numerous anti-inflammatory effects including suppression of neutrophil functions. Although most previous work has implicated the A(2A)AR, we have recently shown that selective activation of the abundantly expressed A(3)AR inhibits neutrophil superoxide production and chemotaxis providing a potential mechanistic explanation for the efficacy of A(3)AR agonists in experimental animal models of inflammation. In this study, we hypothesized that the A(3)AR suppresses neutrophil functions by inhibiting the monomeric GTPase Rac, a central regulator of chemokine-directed neutrophil migration and superoxide production. We found that pre-treating neutrophils with the highly selective A(3)AR agonist CP-532,903 reduced fMLP-induced Rac activation using an ELISA-based assay that detects all three Rac isoforms. CP-532,903 also inhibited fMLP-induced F-actin formation, a downstream effector function of Rac relevant to neutrophil migration, but not activation of ERK1/2 or p38. Pre-treating neutrophils with CP-532,903 did not stimulate cAMP production or alter fMLP-induced calcium transients, implicating that A(3)AR stimulation does not inhibit Rac activation or neutrophil activities by suppressing Ca(2+) signaling, elevating the intracellular concentration of cAMP, or by cross-desensitizing fMLP receptors. Our results suggest that activation of the A(3)AR signals to suppress neutrophil functions by interfering with the monomeric GTPase Rac, thus contributing to the ant-inflammatory actions of adenosine.

Synthesis and pharmacological characterization of [(125)I]MRS5127, a high affinity, selective agonist radioligand for the A3 adenosine receptor

A recently reported selective agonist of the human A(3) adenosine receptor (hA(3)AR), MRS5127 (1'R,2'R,3'S,4'R,5'S)-4'-[2-chloro-6-(3-iodobenzylamino)-purine]-2',3'-O-dihydroxy-bicyclo-[3.1.0]hexane, was radioiodinated and characterized pharmacologically. It contains a rigid bicyclic ring system in place of a 5'-truncated ribose moiety, and was selected for radiolabeling due to its nanomolar binding affinity at both human and rat A(3)ARs. The radioiodination of the N(6)-3-iodobenzyl substituent by iododestannylation of a 3-(trimethylstannyl)benzyl precursor was achieved in 73% yield, measured after purification by HPLC. [(125)I]MRS5127 bound to the human A(3)AR expressed in membranes of stably transfected HEK 293 cells. Specific binding was saturable, competitive, and followed a one-site binding model, with a K(d) value of 5.74+/-0.97nM. At a concentration equivalent to its K(d), non-specific binding comprised 27+/-2% of total binding. In kinetic studies, [(125)I]MRS5127 rapidly associated with the hA(3)AR (t(1/2)=0.514+/-0.014min), and the affinity calculated from association and dissociation rate constants was 3.50+/-1.46nM. The pharmacological profile of ligands in competition experiments with [(125)I]MRS5127 was consistent with the known structure-activity-relationship profile of the hA(3)AR. [(125)I]MRS5127 bound with similar high affinity (K(d), nM) to recombinant A(3)ARs from mouse (4.90+/-0.77), rabbit (2.53+/-0.11), and dog (3.35+/-0.54). For all of the species tested, MRS5127 exhibited A(3)AR agonist activity based on negative coupling to cAMP production. Thus, [(125)I]MRS5127 represents a new species-independent agonist radioligand for the A(3)AR. The major advantage of [(125)I]MRS5127 compared with previously used A(3)AR radioligands is its high affinity, low degree of non-specific binding, and improved A(3)AR selectivity.

Adenosine suppresses lipopolysaccharide-induced tumor necrosis factor-alpha production by murine macrophages through a protein kinase A- and exchange protein activated by cAMP-independent signaling pathway

Adenosine is generated during tissue hypoxia and stress, which reduces inflammation by suppressing the activity of most immune cells. Among its various actions, adenosine suppresses the production of proinflammatory cytokines including tumor necrosis factor (TNF)-alpha, through the cAMP-elevating A(2A) adenosine receptor (AR) subtype. In this study, we examined the signaling mechanisms by which A(2A)AR activation inhibits TNF-alpha production in thioglycollate-elicited mouse peritoneal macrophages. Pretreating murine macrophages with the nonselective AR agonist adenosine-5'-N-ethylcarboxamide (NECA), the A(2A)AR agonist 2-[p-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamidoadenosine (CGS 21680), or the cAMP-elevating agent forskolin reduced TNF-alpha production in response to lipopolysaccharide (LPS) by greater than 60%. All of these agents increased cAMP production in macrophages and activated protein kinase A (PKA). However, we were surprised to find that treating macrophages with three different PKA inhibitors or small interfering RNA-mediated knockdown of the exchange protein activated by cAMP (Epac-1) failed to block the suppressive actions of NECA or forskolin on LPS-induced TNF-alpha release. Instead, okadaic acid was effective at low concentrations that selectively inhibit protein serine/threonine phosphatases. Subsequent studies showed that NECA and forskolin decreased LPS-induced steady-state TNF-alpha mRNA levels; this effect was due to a decreased rate of transcription based on assays examining the rate of generation of primary TNF-alpha transcripts. Treatment with NECA or forskolin did not interfere with LPS-induced translocation or DNA binding of the RelA/p65 subunit of nuclear factor-kappaB or phosphorylation of inhibitor of nuclear factor-kappaB-alpha, extracellular signal-regulated kinase 1/2, c-Jun NH(2)-terminal kinase, or p38 kinase. Our results suggest that AR activation inhibits LPS-induced TNF-alpha production by murine macrophages at the level of gene transcription through a unique cAMP-dependent, but PKA- and Epac-independent, signaling pathway involving protein phosphatase activity.

Characterization of the A2B adenosine receptor from mouse, rabbit, and dog

We have cloned and pharmacologically characterized the A(2B) adenosine receptor (AR) from the dog, rabbit, and mouse. The full coding regions of the dog and mouse A(2B)AR were obtained by reverse transcriptase-polymerase chain reaction, and the rabbit A(2B)AR cDNA was obtained by screening a rabbit brain cDNA library. It is noteworthy that an additional clone was isolated by library screening that was identical in sequence to the full-length rabbit A(2B)AR, with the exception of a 27-base pair deletion in the region encoding amino acids 103 to 111 (A(2B)AR(103-111)). This 9 amino acid deletion is located in the second intracellular loop at the only known splice junction of the A(2B)AR and seems to result from the use of an additional 5' donor site found in the rabbit and dog but not in the human, rat, or mouse sequences. [(3)H]3-Isobutyl-8-pyrrolidinoxanthine and 8-[4-[((4-cyano-[2,6-(3)H]-phenyl)carbamoylmethyl)oxy]phenyl]-1,3-di(n-propyl)xanthine ([(3)H]MRS 1754) bound with high affinity to membranes prepared from human embryonic kidney (HEK) 293 cells expressing mouse, rabbit, and dog A(2B)ARs. Competition binding studies performed with a panel of agonist (adenosine and 2-amino-3,5-dicyano-4-phenylpyridine analogs) and antagonist ligands identified similar potency orders for the A(2B)AR orthologs, although most xanthine antagonists displayed lower binding affinity for the dog A(2B)AR compared with A(2B)ARs from rabbit and mouse. No specific binding could be detected with membranes prepared from HEK 293 cells expressing the rabbit A(2B)AR(103-111) variant. Furthermore, the variant failed to stimulate adenylyl cyclase or calcium mobilization. We conclude that significant differences in antagonist pharmacology of the A(2B)AR exist between species and that some species express nonfunctional variants of the A(2B)AR due to "leaky" splicing.