Identification of domains of the human A1 adenosine receptor that are important for binding receptor subtype-selective ligands using chimeric A1/A2a adenosine receptors

The Signaling Pathways Involved in the Anticonvulsive Effects of the Adenosine A1 Receptor

Adenosine acts as an endogenous anticonvulsant and seizure terminator in the brain. Many of its anticonvulsive effects are mediated through the activation of the adenosine A₁ receptor, a G protein-coupled receptor with a wide array of targets. Activating A₁ receptors is an effective approach to suppress seizures. This review gives an overview of the neuronal targets of the adenosine A₁ receptor focusing in particular on signaling pathways resulting in neuronal inhibition. These include direct interactions of G protein subunits, the adenyl cyclase pathway and the phospholipase C pathway, which all mediate neuronal hyperpolarization and suppression of synaptic transmission. Additionally, the contribution of the guanyl cyclase and mitogen-activated protein kinase cascades to the seizure-suppressing effects of A₁ receptor activation are discussed. This review ends with the cautionary note that chronic activation of the A₁ receptor might have detrimental effects, which will need to be avoided when pursuing A₁ receptor-based epilepsy therapies.

Sleep Promoting Effect of Luteolin in Mice via Adenosine A1 and A2A Receptors

Luteolin, a widespread flavonoid, has been known to have neuroprotective activity against various neurologic diseases such as epilepsy, and Alzheimer’s disease. However, little information is available regarding the hypnotic effect of luteolin. In this study, we evaluated the hypnotic effect of luteolin and its underlying mechanism. In pentobarbital-induced sleeping mice model, luteolin (1, and 3 mg/kg, p.o.) decreased sleep latency and increased the total sleep time. Through electroencephalogram (EEG) and electromyogram (EMG) recording, we demonstrated that luteolin increased non-rapid eye movement (NREM) sleep time and decreased wake time. To evaluate the underlying mechanism, we examined the effects of various pharmacological antagonists on the hypnotic effect of luteolin. The hypnotic effect of 3 mg/kg of luteolin was not affected by flumazenil, a GABAA receptor-benzodiazepine (GABAAR-BDZ) binding site antagonist, and bicuculine, a GABAAR-GABA binding site antagonist. On the other hand, the hypnotic effect of 3 mg/kg of luteolin was almost completely blocked by caffeine, an antagonist for both adenosine A1 and A2A receptor (A1R and A2AR), 8-Cyclopentyl-1,3-dipropylxanthine (DPCPX), an A1R antagonist, and SCH-58261, an A2AR antagonist. From the binding affinity assay, we have found that luteolin significantly binds to not only A1R but also A2AR with IC₅₀ of 1.19, 0.84 μg/kg, respectively. However, luteolin did not bind to either BDZ-receptor or GABAAR. From these results, it has been suggested that luteolin has hypnotic efficacy through A1R and A2AR binding.

Probing the binding site of the A1 adenosine receptor reengineered for orthogonal recognition by tailored nucleosides

His272 (7.43) in the seventh transmembrane domain (TM7) of the human A3 adenosine receptor (AR) interacts with the 3' position of nucleosides, based on selective affinity enhancement at a H272E mutant A3 AR (neoceptor) of 3'-ureido, but not 3'-OH, adenosine analogues. Here, mutation of the analogous H278 of the human A1 AR to Ala, Asp, Glu, or Leu enhanced the affinity of novel 2'- and 3'-ureido adenosine analogues, such as 10 (N6-cyclopentyl-3'-ureido-3'-deoxyadenosine), by >100-fold, while decreasing the affinity or potency of adenosine and other 3'-OH adenosine analogues. His278 mutant receptors produced a similar enhancement regardless of the charge character of the substituted residue, implicating steric rather than electrostatic factors in the gain of function, a hypothesis supported by rhodopsin-based molecular modeling. It was also demonstrated that this interaction was orientationally specific; i.e., mutations at the neighboring Thr277 did not enhance the affinity for a series of 2'- and 3'-ureido nucleosides. Additionally, H-bonding groups placed on substituents at the N6 or 5' position demonstrated no enhancement in the mutant receptors. These reengineered human A1 ARs revealed orthogonality similar to that of the A3 but not the A2A AR, in which mutation of the corresponding residue, His278, to Asp did not enhance nucleoside affinity. Functionally, the H278D A1 AR was detectable only in a measure of membrane potential and not in calcium mobilization. This neoceptor approach should be useful for the validation of molecular modeling and the dissection of promiscuous GPCR signaling.

Heterologous expression of the adenosine A1 receptor in transgenic mouse retina

Traditional cell-based systems used to express integral membrane receptors have yet to produce protein samples of sufficient quality for structural study. Herein we report an in vivo method that harnesses the photoreceptor system of the retina to heterologously express G protein-coupled receptors in a biochemically homogeneous and pharmacologically functional conformation. As an example we show that the adenosine A1 receptor, when placed under the influence of the mouse opsin promoter and rhodopsin rod outer segment targeting sequence, localized to the photoreceptor cells of transgenic retina. The resulting receptor protein was uniformly glycosylated and pharmacologically well behaved. By comparison, we demonstrated in a control experiment that opsin, when expressed in the liver, had a complex pattern of glycosylation. Upon solubilization, the retinal adenosine A1 receptor retained binding characteristics similar to its starting material. This expression method may prove generally useful for generating high-quality G protein-coupled receptors for structural studies.

Adenosine receptor modelling. A1/A2a selectivity

Three-dimensional models of the A(1) and A(2a) adenosine receptors (AR) were constructed by means of a homology procedure, using bovine rhodopsin as a template. In order to validate the two models, a docking analysis of selective agonists was carried out. The study shows that A(1)/A(2a) selectivity is mainly influenced by the different ability of the two receptors to give lipophilic interactions, instead of giving different H bonds. The binding site cavity of the A(1)AR is smaller than that of the A(2a)AR, and for this reason, less bulky ligands like CPA are able to give close interactions with the A(1)AR, unlike larger ligands such as CGS-21680. The different dimensions of the binding site cavity could be due to the presence of three residues of proline, which cause a different rearrangement of the TM, thus modifying the side chain disposition inside the inter-helix channel.

Coupling of the human A1 adenosine receptor to different heterotrimeric G proteins: evidence for agonist-specific G protein activation

The present study investigates the effect of varying ligand structure on the ability of agonists to activate guanine nucleotide-binding proteins of the Gi, Gs and Gq families via the A(1) adenosine receptor. In CHO cells expressing this receptor, inhibition or potentiation of forskolin-stimulated cAMP accumulation was used as an end point to measure the activation of Gi and, in Pertussis toxin (PTX)-treated cells, Gs, respectively. Stimulation of inositol phosphate accumulation in PTX-treated cells was used as an index of Gq activation. CPA (N(6)-cyclopentyladenosine), NECA (5'-N-ethyl-carboxyamidoadenosine) and eight analogues of these ligands presented a range of guanine nucleotide-binding protein (G-protein)-activating profiles. Some ligands could only activate Gi (e.g. 2'deoxyCPA), some primarily Gi and Gs (and only weakly Gq) (e.g. 3'deoxyCPA), highlighting the importance of the ribose hydroxyls in agonist activation of multiple G proteins. CHA (N(6)-cyclohexyladenosine) activated Gi, Gs and Gq, but was more efficacious than CPA in activating Gs. The NECA analogues 5'-N-cyclopropyl-carboxamidoadenosine, 5'-N-cyclobutyl-carboxamidoadenosine and 5'-N-cyclopentyl-carboxamidoadenosine (CPeCA) also activated all three G proteins, although their ability to activate Gs and Gq (relative to CPA) was reduced with increasing substituent size, such that CPeCA produced only a small stimulation (at 100 microM) at Gq, but was a full agonist, relative to CPA, at Gi and Gs. This study suggests that the A(1) adenosine receptor can adopt agonist-specific conformations, arising from small changes in ligand structure, which lead to the differential activation of Gi, Gs and Gq.

Synergistic interaction between adenosine A2A and glutamate mGlu5 receptors: implications for striatal neuronal function

The physiological meaning of the coexpression of adenosine A2A receptors and group I metabotropic glutamate receptors in gamma- aminobutyric acid (GABA)ergic striatal neurons is intriguing. Here we provide in vitro and in vivo evidence for a synergism between adenosine and glutamate based on subtype 5 metabotropic glutamate (mGluR5) and adenosine A2A (A2AR) receptor/receptor interactions. Colocalization of A2AR and mGluR5 at the membrane level was demonstrated in nonpermeabilized human embryonic kidney (HEK)-293 cells transiently cotransfected with both receptors by confocal laser microscopy. Complexes containing A2AR and mGluR5 were demonstrated by Western blotting of immunoprecipitates of either Flag-A2AR or hemagglutinin-mGluR5 in membrane preparations from cotransfected HEK-293 cells and of native A2AR and mGluR5 in rat striatal membrane preparations. In cotransfected HEK-293 cells a synergistic effect on extracellular signal-regulated kinase 1/2 phosphorylation and c-fos expression was demonstrated upon A2AR/mGluR5 costimulation. No synergistic effect was observed at the second messenger level (cAMP accumulation and intracellular calcium mobilization). Accordingly, a synergistic effect on c-fos expression in striatal sections and on counteracting phencyclidine-induced motor activation was also demonstrated after the central coadministration of A2AR and mGluR5 agonists to rats with intact dopaminergic innervation. The results suggest that a functional mGluR5/A2AR interaction is required to overcome the well-known strong tonic inhibitory effect of dopamine on striatal adenosine A2AR function.

Influences of adenosine on the fetus and newborn

Few signaling molecules have the potential to influence the developing mammal as the nucleoside adenosine. In contrast to most neurotransmitters, adenosine is released by all cells and is present in all tissues. The adenosinergic system is therefore not dependent on the presence of mature synaptic structures or an intact autonomic nervous system for its release. However, similar to other signaling molecules, adenosine levels are dynamically regulated and increase with increased tissue activity, hypoxia, or stress. Local adenosine concentrations thus provide a "humoral barometer" of acute changes in cellular physiology. The receptors that transduce adenosine action include A1, A2a, A2b, and A3 adenosine receptors. These receptors differ in their affinities for adenosine and in patterns of tissues expression. During development A1 adenosine receptors (A1ARs) are especially important, and A1ARs are among the earliest receptors expressed in the embryonic brain and heart. In the developing heart, the adenosinergic system is the dominant regulator of fetal cardiac function and A1AR activation inhibits cardiac cell division leading to cardiac hypoplasia. In the forming central nervous system, A1AR activation potently inhibits the development of axons and can lead to leukomalacia. These recent data suggest that adenosine is an important modulator of mammalian development.

Differential effects of the allosteric enhancer (2-amino-4,5-dimethyl-trienyl)[3-trifluoromethyl) phenyl]methanone (PD81,723) on agonist and antagonist binding and function at the human wild-type and a mutant (T277A) adenosine A1 receptor

The 2-amino-benzoylthiophene derivative PD81,723 [(2-amino-4,5-dimethyl-trienyl)[3-(trifluoromethyl) phenyl]methanone] has been shown to allosterically enhance agonist binding and function at the adenosine A(1) receptor. The aim of the present study was to elucidate the effects of PD81,723 both as an allosteric enhancer and as an antagonist on the adenosine A(1) receptor. We investigated its effect on the human wild-type in relation to a mutant (T277A) adenosine A(1) receptor for which agonists have a greatly diminished affinity. Binding (saturation and displacement experiments) and functional adenosine 3',5'-cyclic monophosphate studies were performed, and differential effects of allosteric enhancer PD81,723 on agonists and antagonists were observed on the wild-type (wt) and mutant adenosine A(1) receptor. Our results showed opposite effects of PD81,723 on the binding of agonists and antagonists. Within the concept of a simplified two-state receptor model, it is possible that the effects of PD81,723 are mainly "allosteric", enhancing the binding of adenosine A(1) agonists and inhibiting the binding of antagonists/inverse agonists. However, the suggestion that PD81,723 acts as an allosteric inhibitor of DPCPX (1,3-dipropyl-8-cyclopentylxanthine) binding cannot be confirmed by kinetic studies, since PD81,723 does not seem to affect the dissociation kinetics of [(3)H]DPCPX. Nevertheless, our results show that the action of PD81,723 on DPCPX binding is due to more than mere competitive antagonistic activity, i.e. binding to the ligand-binding site and competing with the binding of DPCPX, as suggested previously. The effect of PD81,723 on the mutant receptor was much less pronounced. Mutation of Thr277 to Ala not only decreased agonist affinity but also inhibited the effects of PD81,723. Insensitivity of the mutT277A to PD81,723 may be linked to the fact that this mutant appears to be uncoupled from G proteins. It further supported a differential binding mode of PD81,723 compared to competitive antagonists for the adenosine A(1) receptor.

7-Deazaadenines bearing polar substituents: structure-activity relationships of new A(1) and A(3) adenosine receptor antagonists

A series of 28 new pyrrolo[2,3-d]pyrimidine-4-amines, pyrimido[4, 5-b]indole-4-amines, and tetrahydropyrimido[4,5-b]indole-4-amines was synthesized and their adenosine receptor affinity determined in radioligand binding assays at rat A(1) and A(2A) adenosine receptors (ARs). Selected compounds were additionally investigated in binding assays at recombinant A(3) ARs. The 2-phenyl residue in (R)-7-(1-methylbenzyl)-2-phenylpyrrolo[2,3-d]pyrimidine-4-amine (ADPEP, 1) and in the corresponding pyrimido[4,5-b]indole (APEPI, 3) could be bioisosterically replaced by heterocyclic rings, such as 2-thienyl and 4-pyridyl. The resulting compounds retained high affinity and selectivity for A(1) ARs. Judging from the investigation of selected compounds, it appears that they are also potent at human A(1) ARs and selective not only versus A(2A) ARs but also highly selective versus A(2B) and A(3) ARs. The p-pyridyl-substituted derivatives 11 and 27 (APPPI) may be interesting pharmacological tools due to their fluorescent properties. Pyrrolo[2,3-d]pyrimidine-4-amine derivatives which were simultaneously substituted at N7 and N(4), combining the substitution pattern of ADPEP (1) and DPEAP (2), showed very low affinity for A(1) ARs. This finding supports our previously published hypothesis of different binding modes for pyrrolopyrimidines, such as ADPEP (1) and DPEAP (2). DPEAP (2), a pyrrolo[2,3-d]pyrimidine-4-amine substituted at the amino group (N(4)), was found to exhibit high affinity for human A(3) ARs (K(i) = 28 nM), whereas N(4)-unsubstituted analogues were inactive. DPEAP (2) and related compounds provide new leads for the development of antagonists for the human A(3) AR.

The role of receptor structure in determining adenosine receptor activity

Adenosine produces a wide variety of physiological effects through the activation of cell surface adenosine receptors (ARs). ARs are members of the G-protein-coupled receptor family, and currently, four subtypes, the A1AR, A2AAR, A2BAR, and A3AR, are recognized. This review focuses on the role of receptor structure in governing various facets of AR activity. Ligand-binding properties of ARs are primarily dictated by amino acids in the transmembrane domains of the receptors, although a role for extracellular domains of certain ARs has been suggested. Studies have identified certain amino acids conserved amongst AR subtypes that are critical for ligand recognition, as well as additional residues that may differentiate between agonist and antagonist ligands. Receptor regions responsible for activation of Gs have been identified for the A2AAR. The location of these intracellular sites is consistent with findings described for other G-protein-coupled receptors. Site-directed mutagenesis has been employed to analyze the structural basis for the differences in the kinetics of the desensitization response displayed by various AR subtypes. For the A2AAR and A3AR, agonist-stimulated phosphorylation of the AR, presumably via a G-protein receptor kinase, has been shown to occur. For these AR subtypes, intracellular regions or individual amino acids that may be targets for this phosphorylation have been identified. Finally, the role of A1AR gene structure in regulating the expression of this AR subtype is reviewed.

A1 adenosine receptor of human and mouse adipose tissues: cloning, expression, and characterization

The aberrant functioning of the A1 adenosine receptor of adipose tissue has been implicated as a factor in obesity. To begin to address questions concerning this relationship, the possibility of a unique A1 adenosine receptor in adipose tissue must be investigated. Therefore, cDNAs encoding the A1 adenosine receptors of adipose tissues of a mouse and an obese human were isolated, sequenced, and expressed in eukaryotic cells. We found their sequences to be 90% identical and each identical to published sequences of the receptors in brain preparations of the two species. The two cDNAs were transiently expressed in 293T cells, a human kidney cell line. Despite the 90% identity in their sequences, the ligand binding properties of the human and mouse cDNAs expressed in the 293T cell line differed markedly. With respect to amino acid differences in the extracellular loops, four occur in the second extracellular loop, which has been implicated in binding by other studies. The ligand binding characteristics of the recombinant receptors matched those of native receptors from human and mouse adipose tissue. The human A1 receptor cDNA was also expressed in ob17 preadipocyte cells to investigate reported influences of cellular environment on binding characteristics. We compared ligand binding of the expressed receptor in the two cell lines (ob17 and 293T). We also compared ligand binding of native receptors from mouse brain and adipose tissue preparations. In both studies, cellular environment had no affect on binding characteristics. This conclusive evidence resolves earlier conflicting reports in the literature.

Expression of A2B adenosine receptors in human lymphocytes: their role in T cell activation

Extracellular adenosine has a key role in the development and function of the cells of the immune system. Many of the adenosine actions seem to be mediated by specific surface receptors positively coupled to adenylate cyclase: A2A and A2B. Despite the fact that A2A receptors (A2ARs) can be easily studied due to the availability of the specific agonist CGS21680, a pharmacological and physiological characterization of adenosine A2B receptors (A2BRs) in lymphocytes has not been possible due to the lack of suitable reagents. Here we report the generation and characterization of a polyclonal antipeptide antibody raised against the third extracellular loop of the A2BR human clone which is useful for immunocytochemical studies. This antibody has permitted the detection of A2BR+ cells in lymphocyte samples isolated from human peripheral blood. The pharmacology of cAMP-producing compounds is consistent with the presence of functional A2BRs but not of A2A receptors in these human cells. The percentage of A2BR-expressing cells was similar in the CD4(+) or CD8(+) T cell subpopulations. Interestingly activation signals delivered by either phytohemagglutinin or anti-T cell receptor/CD3 complex antibodies led to a significant increase in both the percentage of cells expressing the receptor and the intensity of the labeling. These receptors are functional since interleukin-2 production in these cells is reduced by NECA but not by R-PIA or CGS21680. These results show that A2BR expression is regulated in T cell activation and suggest that the role of adenosine in lymphocyte deactivation is mediated by A2BRs.

Identification of the adenine binding site of the human A1 adenosine receptor

To provide new insights into ligand-A1 adenosine receptor (A1AR) interactions, site-directed mutagenesis was used to test the role of several residues in the first four transmembrane domains of the human A1AR. First, we replaced eight unique A1AR residues with amino acids present at corresponding transmembrane (TM) positions of A2AARs. We also tested the role of carboxamide amino acids in TMs 1-4, and the roles of Val-87, Leu-88, and Thr-91 in TM3. Following conversion of Gly-14 in TM1 to Thr-14, the affinity for adenosine agonists increased 100-fold, and after Pro-25 in TM1 was converted to Leu-25, the affinity for agonists fell. After conversion of TM3 sites Thr-91 to Ala-91, and Gln-92 to Ala-92, the affinity for N6-substituted agonists was reduced, and binding of ligands without N6 substituents was eliminated. When Leu-88 was converted to Ala-88, the binding of ligands with N6 substituents was reduced to a greater extent than ligands without N6 substituents. Following conversion of Pro-86 to Phe-86, the affinity for N6-substituted agonists was lost, and the affinity for ligands without N6 substitution was reduced. These observations strongly suggest that Thr-91 and Gln-92 in TM3 interact with the adenosine adenine moiety, and Leu-88 and Pro-86 play roles in conferring specificity for A1AR selective compounds. Using computer modeling based on the structure of rhodopsin, a revised model of adenosine-A1AR interactions is proposed with the N6-adenine position oriented toward the top of TM3 and the ribose group interacting with the bottom half of TMs 3 and 7.

Thermodynamics of full agonist, partial agonist, and antagonist binding to wild-type and mutant adenosine A1 receptors

A thermodynamic analysis of the binding of a full agonist (N6-cyclopentyladenosine), a partial agonist (8-butylamino-N6-cyclopentyladenosine) and an antagonist (8-cyclopentyltheophylline) to human wild-type and mutant (mutation of a threonine (Thr) to an alanine (Ala) residue at position 277) adenosine A1 receptors expressed on Chinese hamster ovary (CHO) cells, and to rat brain adenosine A1 receptors was undertaken. The thermodynamic parameters deltaGo (standard free energy), deltaHo (standard enthalpy) and deltaSo (standard entropy) of the binding equilibrium to rat brain receptors were determined by means of affinity measurements carried out at four different temperatures (0, 10, 20 and 25 degrees) and van't Hoff plots. Two temperatures (0 and 25 degrees) were considered for human receptors. Affinity constants were obtained from inhibition assays on membrane preparations of rat brain and CHO cells by use of the antagonist [3H]1,3-dipropyl-8-cyclopentylxanthine ([3H]DPCPX) as selective adenosine A1 receptor radioligand. As for rat brain receptors, full agonist binding was totally entropy driven, whereas antagonist binding was essentially enthalpy driven. Partial agonist binding appeared both enthalpy and entropy driven. As for human receptors, full agonist affinity was highly dependent on the presence of Thr277. Moreover, affinity to both wild-type and mutant receptors was enhanced by temperature increase, suggesting a totally entropy-driven binding. Antagonist binding did not depend on the presence of Thr277. Antagonist affinity decreased with an increase in temperature, suggesting a mainly enthalpy-driven binding. Partial agonist binding was significantly dependent on the presence of Thr277 at 25 degrees, whereas such a dependence was not evident at 0 degrees. It is concluded that Thr277 contributes only to the binding of adenosine derivatives and that its role changes drastically with the receptor conformation and with the type of agonist (full or partial) interacting with the adenosine A1 receptors.

Adenosine receptors: new opportunities for future drugs

This review summarises current knowledge on adenosine receptors, an important G protein-coupled receptor. The four known adenosine receptor subtypes A1, A2A, A2B, and A3 are discussed with special reference to the opportunities for drug development.

Species differences in brain adenosine A1 receptor pharmacology revealed by use of xanthine and pyrazolopyridine based antagonists

1. The pharmacological profile of adenosine A1 receptors in human, guinea-pig, rat and mouse brain membranes was characterized in a radioligand binding assay by use of the receptor selective antagonist, [3H]-8-cyclopentyl-1,3-dipropylxanthine ([3H]-DPCPX). 2. The affinity of [3H]-DPCPX binding sites in rat cortical and hippocampal membranes was similar. Binding site affinity was higher in rat cortical membranes than in membranes prepared from guinea-pig cortex and hippocampus, mouse cortex and human cortex. pKD values (M) were 9.55, 9.44, 8.85, 8.94, 8.67, 9.39 and 8.67, respectively. The binding site density (Bmax) was lower in rat cortical membranes than in guinea-pig or human cortical membranes. 3. The rank order of potency of seven adenosine receptor agonists was identical in each species. With the exception of 5'-N-ethylcarboxamidoadenosine (NECA), agonist affinity was 3.5-26.2 fold higher in rat cortical membranes than in human and guinea-pig brain membranes; affinity in rat and mouse brain membranes was similar. While NECA exhibited 9.3 fold higher affinity in rat compared to human cortical membranes, affinity in other species was comparable. The stable GTP analogue, Gpp(NH)p (100 microM) reduced 2-chloro-N6-cyclopentyladenosine (CCPA) affinity 7-13.9 fold, whereas the affinity of DPCPX was unaffected. 4. The affinity of six xanthine-based adenosine receptor antagonists was 2.2-15.9 fold higher in rat cortical membranes compared with human or guinea-pig membranes. The rank order of potency was species-independent. In contrast, three pyrazolopyridine derivatives, (R)-1-[(E)-3-(2-phenylpyrazolo[1,5-a]pyridin-3-yl) acryloyl]-2-piperidine ethanol (FK453), (R)-1-[(E)-3-(2-phenylpyrazolo[1,5-a]pyridin-3-yl) acryloyl]-piperidin-2-yl acetic acid (FK352) and 6-oxo-3-(2-phenylpyrazolo[1,5-a]pyridin-3-yl)-1(6H)-pyridazinebutyric acid (FK838) exhibited similar affinity in human, guinea-pig, rat and mouse brain membranes. pKi values (M) for [3H]-DPCPX binding sites in human cortical membranes were 9.31, 7.52 and 7.92, respectively. 5. Drug affinity for adenosine A2A receptors was determined in a [3H]-2-[4-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamido ade nosine ([3H]-CGS 21680) binding assay in rat striatal membranes. The pyrazolopyridine derivatives, FK453, FK838 and FK352 exhibited pKi values (M) of 5.90, 5.92 and 4.31, respectively, compared with pKi values of 9.31, 8.18 and 7.57 determined in the [3H]-DPCPX binding assay in rat cortical membranes. These novel pyrazolopyridine derivatives therefore represent high affinity, adenosine A1 receptor selective drugs that, in contrast to xanthine based antagonists, exhibit similar affinity for [3H]-DPCPX binding sites in human, rat, mouse and guinea-pig brain membranes.

A mutational analysis of residues essential for ligand recognition at the human P2Y1 receptor

We conducted a mutational analysis of residues potentially involved in the adenine nucleotide binding pocket of the human P2Y1 receptor. Mutated receptors were expressed in COS-7 cells with an epitope tag that permitted confirmation of expression in the plasma membrane, and agonist-promoted inositol phosphate accumulation was assessed as a measure of receptor activity. Residues in transmembrane helical domains (TMs) 3, 5, 6, and 7 predicted by molecular modeling to be involved in ligand recognition were replaced with alanine and, in some cases, by other amino acids. The potent P2Y1 receptor agonist 2-methylthio-ATP (2-MeSATP) had no activity in cells expressing the R128A, R310A, and S314A mutant receptors, and a markedly reduced potency of 2-MeSATP was observed with the K280A and Q307A mutants. These results suggest that residues on the exofacial side of TM3 and TM7 are critical determinants of the ATP binding pocket. In contrast, there was no change in the potency or maximal effect of 2-MeSATP with the S317A mutant receptor. Alanine replacement of F131, H132, Y136, F226, or H277 resulted in mutant receptors that exhibited a 7-18-fold reduction in potency compared with that observed with the wild-type receptor. These residues thus seem to subserve a less important modulatory role in ligand binding to the P2Y1 receptor. Because changes in the potency of 2-methylthio-ADP and 2-(hexylthio)-AMP paralleled the changes in potency of 2-MeSATP at these mutant receptors, the beta- and gamma-phosphates of the adenine nucleotides seem to be less important than the alpha-phosphate in ligand/P2Y1 receptor interactions. However, T221A and T222A mutant receptors exhibited much larger reductions in triphosphate (89- and 33-fold versus wild-type receptors, respectively) than in diphosphate or monophosphate potency. This result may be indicative of a greater role of these TM5 residues in gamma-phosphate recognition. Taken together, the results suggest that the adenosine and alpha-phosphate moieties of ATP bind to critical residues in TM3 and TM7 on the exofacial side of the human P2Y1 receptor.

Mutagenesis reveals structure-activity parallels between human A2A adenosine receptors and biogenic amine G protein-coupled receptors

Structure-affinity relationships for ligand binding at the human A2A adenosine receptor have been probed using site-directed mutagenesis in the transmembrane helical domains (TMs). The mutant receptors were expressed in COS-7 cells and characterized by binding of the radioligands [3H]CGS21680, [3H]NECA, and [3H]XAC. Three residues, at positions essential for ligand binding in other G protein-coupled receptors, were individually mutated. The residue V(3.32) in the A2A receptor that is homologous to the essential aspartate residue of TM3 in the biogenic amine receptors, i.e., V84(3.32), may be substituted with L (present in the A3 receptor) but not with D (in biogenic amine receptors) or A. H250(6.52), homologous to the critical N507 of rat m3 muscarinic acetylcholine receptors, may be substituted with other aromatic residues or with N but not with A (Kim et al. J. Biol. Chem. 1995, 270, 13987-13997). H278(7.43), homologous to the covalent ligand anchor site in rhodopsin, may not be substituted with either A, K, or N. Both V84L(3.32) and H250N(6.52) mutant receptors were highly variable in their effect on ligand competition depending on the structural class of the ligand. Adenosine-5'-uronamide derivatives were more potent at the H250N(6.52) mutant receptor than at wild type receptors. Xanthines tended to be close in potency (H250N(6.52)) or less potent (V84L(3.32)) than at wild type receptors. The affinity of CGS21680 increased as the pH was lowered to 5.5 in both the wild type and H250N(6.52) mutant receptors. Thus, protonation of H250(6.52) is not involved in this pH dependence. These data are consistent with a molecular model predicting the proximity of bound agonist ligands to TM3, TM5, TM6, and TM7.

Structural determinants for agonist binding affinity to thromboxane/prostaglandin endoperoxide (TP) receptors. Analysis of chimeric rat/human TP receptors

The two most extensively characterized thromboxane/prostaglandin endoperoxide (TP) receptors, from human platelets and rat vascular smooth muscle, exhibit thromboxane agonist [15-(1alpha,2beta(5Z), 3alpha-(1E,3S), 4alpha)]-7-[3-hydroxy-4-(p-iodophenoxy)-1-butenyl-7-oxabi cyclohepteno ic acid (I-BOP) binding affinities that differ by an order of magnitude, rat TP having the higher affinity. We utilized this difference in I-BOP affinity to identify structural determinants of TP receptor heterogeneity. No significant difference was found in the rank order of affinities for a series of thromboxane receptor ligands to bind to cloned human TPalpha versus rat TP, indicating that these represent species homologs, not distinct TP subtypes. Structural determinants for observed differences in I-BOP binding Kd were localized by creating chimeric human/rat TP followed by mutational substitution of specific critical amino acids. Initially, seven chimeric receptors with splice sites in transmembranes 1, 2, 4, or 7 were constructed and expressed in HEK293 cells for analysis of ligand binding properties. Substitution of any part except the carboxyl tail of the human TP into the rat TP resulted in a receptor with I-BOP binding affinity intermediate between the two. Analysis of chimeras in which only the extracellular amino terminus and a portion of transmembrane 1 were switched localized the determinant of high affinity binding to the region between amino acids 3 and 40. Using this chimera, amino acids in the human portion (extracellular amino terminus and part of transmembrane 1) were replaced with analogous amino acids from rat TP to regain high affinity I-BOP binding. Only when amino acid Val37 and either Val36 or Ala40 were reverted to their respective rat TP counterparts (Ala36, Leu37, and Gly40, respectively) was high affinity I-BOP binding recovered. The mechanism for the increased I-BOP affinity may be the lengthening of the amino acid side chain at position 37, thus extending this group further into the putative I-BOP binding pocket, with compensatory shortening of side chains in spatially adjacent amino acids.

Derivatives of the triazoloquinazoline adenosine antagonist (CGS15943) are selective for the human A3 receptor subtype

The adenosine antagonist 9-chloro-2-(2-furanyl)[1,2,4]triazolo[1,5-c]quinazolin-5-amine (CGS15943) binds to human A3 receptors with high affinity (Ki = 14 nM), while it lacks affinity at rat A3 receptors. Acylated derivatives of the 5-amino group and other modifications were prepared in an effort to provide A3 subtype selectivity. Affinity was determined in radioligand binding assays at rat brain A1 and A2A receptors using [3H]-(R)-PIA ([3H]-(R)-N6-(phenylisopropyl)-adenosine) and [3H]CGS 21680 ([3H]-2-[[4-(2-carboxy ethyl)phenyl]ethylaminol]-5'-(N- ethyl- carbamoyl)adenosine), respectively. Affinity was determined at cloned human A3 receptors using [125I]AB-MECA (N6-(4-amino-3-iodobenzyl)-5'-(N-methylcarbamoyl)adenosine). A series of straight chain alkyl amides demonstrated that the optimal chain length occurs with the 5-N-propionyl derivative, 3, which had a Ki value of 7.7 nM at human A3 receptors, and was 40- and 14-fold selective vs rat A1 and A2A receptors, respectively. The 5-N-benzoyl derivative, 10, displayed Ki values of 680 and 273 nM at rat A1 and A2A receptors, respectively, and 3.0 nM at human A3 receptors. A 5-N-phenylacetyl derivative, 12, was 470-fold selective for human A3 vs rat A1 receptors with a Ki value of 0.65 nM. A conjugate of Boc-gamma-aminobutyric acid was also prepared but was nonselective. Conversion of the 5-amino group of CGS15943 to an oxo function resulted in lower affinity but 15-fold selectivity for human A3 receptors.