A1 adenosine receptors. Two amino acids are responsible for species differences in ligand recognition

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.

The Adenosinergic System as a Therapeutic Target in the Vasculature: New Ligands and Challenges

Adenosine is an adenine base purine with actions as a modulator of neurotransmission, smooth muscle contraction, and immune response in several systems of the human body, including the cardiovascular system. In the vasculature, four P1-receptors or adenosine receptors—A₁, A_2A, A_2B and A₃—have been identified. Adenosine receptors are membrane G-protein receptors that trigger their actions through several signaling pathways and present differential affinity requirements. Adenosine is an endogenous ligand whose extracellular levels can reach concentrations high enough to activate the adenosine receptors. This nucleoside is a product of enzymatic breakdown of extra and intracellular adenine nucleotides and also of S-adenosylhomocysteine. Adenosine availability is also dependent on the activity of nucleoside transporters (NTs). The interplay between NTs and adenosine receptors’ activities are debated and a particular attention is given to the paramount importance of the disruption of this interplay in vascular pathophysiology, namely in hypertension., The integration of important functional aspects of individual adenosine receptor pharmacology (such as in vasoconstriction/vasodilation) and morphological features (within the three vascular layers) in vessels will be discussed, hopefully clarifying the importance of adenosine receptors/NTs for modulating peripheral mesenteric vascular resistance. In recent years, an increase interest in purine physiology/pharmacology has led to the development of new ligands for adenosine receptors. Some of them have been patented as having promising therapeutic activities and some have been chosen to undergo on clinical trials. Increased levels of endogenous adenosine near a specific subtype can lead to its activation, constituting an indirect receptor targeting approach either by inhibition of NT or, alternatively, by increasing the activity of enzymes responsible for ATP breakdown. These findings highlight the putative role of adenosinergic players as attractive therapeutic targets for cardiovascular pathologies, namely hypertension, heart failure or stroke. Nevertheless, several aspects are still to be explored, creating new challenges to be addressed in future studies, particularly the development of strategies able to circumvent the predicted side effects of these therapies.

Sulfur-containing amino acids in 7TMRs: molecular gears for pharmacology and function

Seven-transmembrane receptors (7TMRs) mediate the majority of physiological responses to hormones and neurotransmitters in higher organisms. Tertiary structure stability and activation of these versatile membrane proteins require formation or disruption of complex networks of well-recognized interactions (such as H-bonds, ionic, or aromatic-aromatic) but also of other type of interactions which have been less studied. In this review, we compile evidence from crystal structure, biophysical, and site-directed mutagenesis data that indicate or support the importance of interactions involving Met and Cys in 7TMRs in terms of pharmacology and function. We show examples of Met/Cys-aromatic and Met-Met interactions participating in ligand binding, in tuning the orientation of functionally important aromatic residues during activation or even in modulating the type of signaling response. Collectively, data presented enlarge the repertoire of interactions governing 7TMR functioning.

The GPCR Network: a large-scale collaboration to determine human GPCR structure and function

G protein-coupled receptors (GPCRs) are targeted by ∼30-40% of marketed drugs, and their key roles in normal physiology and in disease demonstrate that an understanding of their structure and function is valuable to researchers in both basic science and drug discovery. However, until recently, detailed structural information on this protein family was limited by challenges in X-ray crystallographic analysis of such membrane proteins. The GPCR Network was created in 2010 with the goal of structurally characterizing 15-25 representative human GPCRs within 5 years, based on an active outreach programme addressing an interdisciplinary community of scientists interested in GPCR structure, chemistry and biology. Here, we provide an overview of how this collaborative effort has enabled the structural determination and characterization of eight human GPCRs so far, and discuss some of the challenges that remain in gaining more detailed insights into structure-function relationships in this receptor superfamily.

Pyrimidine derivatives as potent and selective A3 adenosine receptor antagonists

Two regioisomeric series of diaryl 2- or 4-amidopyrimidines have been synthesized and their adenosine receptor affinities were determined in radioligand binding assays at the four human adenosine receptors (hARs). Some of the ligands prepared herein exhibit remarkable affinities (K(i) < 10 nm) and, most noticeably, the absence of activity at the A(1), A(2A), and A(2B) receptors. The structural determinants that support the affinity and selectivity profiles of the series were highlighted through an integrated computational approach, combining a 3D-QSAR model built on the second generation of GRid INdependent Descriptors (GRIND2) with a novel homology model of the hA(3) receptor. The robustness of the computational model was subsequently evaluated by the design of new derivatives exploring the alkyl substituent of the exocyclic amide group. The synthesis and evaluation of the novel compounds validated the predictive power of the model, exhibiting excellent agreement between predicted and experimental activities.

Phe369(7.38) at human 5-HT(7) receptors confers interspecies selectivity to antagonists and partial agonists

BACKGROUND AND PURPOSE

Human and rat 5-HT(7) receptors were studied with a particular emphasis on the molecular interactions involved in ligand binding, searching for an explanation to the interspecies selectivity observed for a set of compounds. We performed affinity studies, molecular modelling and site-directed mutagenesis, with special focus on residue Phe(7.38) of the human 5-HT(7) receptor [Cys(7.38) in rat].

EXPERIMENTAL APPROACH

Competition binding studies were performed for seven 5-HT(7) receptor ligands at three different 5-HT(7) receptors. The functional behaviour was evaluated by measuring 5-carboxytryptamine-stimulated cAMP production. Computational simulations were carried out to explore the structural bases in ligand binding observed for these compounds.

KEY RESULTS

Competition experiments showed a remarkable selectivity for the human receptor when compared with the rat receptor. These results indicate that mutating Cys to Phe at position 7.38 profoundly affects the binding affinities at the 5-HT(7) receptor. Computational simulations provide a structural interpretation for this key finding. Pharmacological characterization of compounds mr25020, mr25040 and mr25053 revealed a competitive antagonistic behaviour. Compounds mr22423, mr22433, mr23284 and mr25052 behaved as partial agonists.

CONCLUSIONS AND IMPLICATIONS

We propose that the interspecies difference in binding affinities observed for the compounds at human and rat 5-HT(7) receptors is due to the nature of the residue at position 7.38. Our molecular modelling simulations suggest that Phe(7.38) in the human receptor is integrated in the hydrophobic pocket in the central part of the binding site [Phe(6.51)-Phe(6.52)] and allows a tighter binding of the ligands when compared with the rat receptor.

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.

Molecular modeling of A1 and A2A adenosine receptors: comparison of rhodopsin- and beta2-adrenergic-based homology models through the docking studies

Adenosine receptors (ARs) are members of the superfamily of G protein-coupled receptors. The homology models of adenosine A1 and A2A receptors were constructed. The high-resolution X-ray structure of bovine rhodopsin and crystal structure of beta2-adrenergic receptor were used as templates. The binding sites of the A1 and A2A ARs were constructed by using data obtained from mutagenesis experiments as well as docking simulations of the respective AR antagonsists DPCPX and XAC. To compare rhodopsin- and beta2-adrenergic-based models, the binding mode of A1 (KW-3902, LUF-5437) and A2A (KW-6002, ZM-241385) ARs antagonists were also examined. The differences in the binding ability of both models were noted during the study. The beta2-adrenergic-based A2A AR model was much more capable to stabilize the ligand in the binding site cavity than the corresponding rhodopsin-based A2A AR model, however, such differences were not so clear in case of A1 AR models. It was suggested that for the A1 AR it is possible to use the crystal structure of rhodopsin as a template as well as beta2-adrenergic receptor, but for A2A AR, with the now available beta2-adrenergic receptor X-ray structure, docking studies should be avoided on the rhodopsin-based model. However, taking into account that the beta2AR shares about 31% of the residues with the AR in comparison to 21% in case of bRho, we suggest using beta2-adrenergic-based models for the A1 and A2A ARs for further in silico ligand screening also because of their generally better ability to stabilize ligands inside the binding pocket.

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

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

Molecular modeling of adenosine receptors: new results and trends

Adenosine is a ubiquitous neuromodulator, which carries out its biological task by stimulating four cell surface receptors (A(1), A(2A), A(2B), and A(3)). Adenosine receptors (ARs) are members of the superfamily of G protein-coupled receptors (GPCRs). Their discovery opened up new avenues for potential drug treatment of a variety of conditions such as asthma, neurodegenerative disorders, chronic inflammatory diseases, and many other physiopathological states that are believed to be associated with changes in adenosine levels. Knowledge of the 3D structure of ARs could be of great help in the task of understanding their function and in the rational design of specific ligands. However, since GPCRs are membrane-bound proteins, high-resolution structural characterization is still an extremely difficult task. For this reason, great importance has been placed on molecular modeling studies and, particularly in the last few years, on homology modeling (HM) techniques. The publication of the first high-resolution crystal structure for bovine rhodopsin (bRh), a GPCR superfamily member, provides the option of utilizing HM to generate 3D models based on detailed structural information. In this review we report, analyze, and compare the main experimental data, computational HM procedures and validation methods used for ARs, describing in detail the most successful results.

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

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

A two-entropies analysis to identify functional positions in the transmembrane region of class A G protein-coupled receptors

Residues in the transmembrane region of G protein-coupled receptors (GPCRs) are important for ligand binding and activation, but the function of individual positions is poorly understood. Using a sequence alignment of class A GPCRs (grouped in subfamilies), we propose a so-called "two-entropies analysis" to determine the potential role of individual positions in the transmembrane region of class A GPCRs. In our approach, such positions appear scattered, while largely clustered according to their biological function. Our method appears superior when compared to other bioinformatics approaches, such as the evolutionary trace method, entropy-variability plot, and correlated mutation analysis, both qualitatively and quantitatively.

Progress in the pursuit of therapeutic adenosine receptor antagonists

Ever since the discovery of the hypotensive and bradycardiac effects of adenosine, adenosine receptors continue to represent promising drug targets. First, this is due to the fact that the receptors are expressed in a large variety of tissues. In particular, the actions of adenosine (or methylxanthine antagonists) in the central nervous system, in the circulation, on immune cells, and on other tissues can be beneficial in certain disorders. Second, there exists a large number of ligands, which have been generated by introducing several modifications in the structure of the lead compounds (adenosine and methylxanthine), some of them highly specific. Four adenosine receptor subtypes (A1, A2A, A2B, and A3) have been cloned and pharmacologically characterized, all of which are G protein-coupled receptors. Adenosine receptors can be distinguished according to their preferred mechanism of signal transduction: A1 and A3 receptors interact with pertussis toxin-sensitive G proteins of the Gi and Go family; the canonical signaling mechanism of the A2A and of the A2B receptors is stimulation of adenylyl cyclase via Gs proteins. In addition to the coupling to adenylyl cyclase, all four subtypes may positively couple to phospholipase C via different G protein subunits. The development of new ligands, in particular, potent and selective antagonists, for all subtypes of adenosine receptors has so far been directed by traditional medicinal chemistry. The availability of genetic information promises to facilitate understanding of the drug-receptor interaction leading to the rational design of a potentially therapeutically important class of drugs. Moreover, molecular modeling may further rationalize observed interactions between the receptors and their ligands. In this review, we will summarize the most relevant progress in developing new therapeutic adenosine receptor antagonists.

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

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

Sphingosine 1-phosphate analogue recognition and selectivity at S1P4 within the endothelial differentiation gene family of receptors

Synergistic computational and experimental studies provided previously unforeseen details concerning the structural basis of S1P (sphingosine 1-phosphate) recognition by the S1P4 G-protein-coupled receptor. Similarly to reports on the S1P1 receptor, cationic and anionic residues in the third transmembrane domain (R3.28 and E3.29 at positions 124 and 125) form ion pairs with the phosphate and ammonium of S1P, and alanine mutations at these positions abolished specific S1P binding, S1P-induced receptor activation and cell migration. Unlike findings on the S1P1 receptor, no cationic residue in the seventh transmembrane domain interacts with the phosphate. Additionally, two previously undiscovered interactions with the S1P polar headgroup have been identified. Trp186 at position 4.64 in the fourth transmembrane domain interacts by a cation-pi interaction with the ammonium group of S1P. Lys204 at position 5.38 forms an ion pair with the S1P. The S1P4 and S1P1 receptors show differences in binding-pocket shape and electrostatic distributions that correlate with the published structure-activity relationships. In particular, the binding pocket of mS1P4 (mouse S1P4) has recognition sites for the anionic phosphate and cationic ammonium groups that are equidistant from the end of the non-polar tail. In contrast, the binding pocket of hS1P1 (human S1P4) places the ammonium recognition site 2 A (1 A=0.1 nm) closer to the end of the non-polar tail than the phosphate recognition site.

Novel mitogenic effect of adenosine on coronary artery smooth muscle cells: role for the A1 adenosine receptor

Adenosine is a vascular endothelial cell mitogen, but anti-mitogenic for aortic smooth muscle cells and fibroblasts when acting via the A2B adenosine receptor. However, we show that adenosine increases porcine coronary artery smooth muscle cell (CASMC) number, cellular DNA content, protein synthesis, and PCNA staining. RT-PCR analysis indicates that porcine CASMC express A1, A2A, A3, and barely detectable levels of A2B receptor mRNAs. The mitogenic effect of adenosine is mimicked by NECA, CCPA, and R-PIA, but not by CGS21680and 2-Cl-IB-MECA, and is inhibited by DPCPX, indicating a prominent role for the A1 receptor. This interpretation is supported by the finding that adenosine- and CCPA-induced DNA synthesis is significantly inhibited by pertussis toxin, but substantially potentiated by PD81723, an allosteric enhancer of the A1 receptor. When a cDNA encoding the porcine A1 receptor was cloned and expressed in COS-1 cells, A1 receptor pharmacology is confirmed. Anti-sense oligonucleotides to the cloned sequence dramatically suppress the mitogenic effect of adenosine and CCPA. Conversely, over-expression of the cloned A1 receptor in CASMC increases adenosine- and CCPA-induced DNA synthesis. Furthermore, stimulation with adenosine or CCPA of intact coronary arteries in an organ culture model of vascular disease increases cellular DNA synthesis, which was abolished by DPCPX. We conclude that adenosine acts as a novel mitogen in porcine CASMC that express the A1 adenosine receptor, possibly contributing to the development of coronary artery disease.

Synthesis, biological evaluation, and molecular modeling of ribose-modified adenosine analogues as adenosine receptor agonists

A number of 3'-C-methyl analogues of selective adenosine receptor agonists such as CPA, CHA, CCPA, 2'-Me-CCPA, NECA, and IB-MECA was synthesized to further investigate the subdomain of the receptor that binds the ribose moiety of the ligands. Affinity data at A(1), A(2A), and A(3) receptors in bovine brain membranes showed that the 3'-C-modification in adenosine resulted in a decrease of the affinity at all three receptor subtypes. When this modification was combined with N(6)-substitution with groups that induce high potency and selectivity at A(1) receptor, the affinity and selectivity were increased. However, all 3'-C-methyl derivatives proved to be very less active than the corresponding 2'-C-methyl analogues. The most active compound was found to be 3'-Me-CPA which displayed a K(i) value of 0.35 microM at A(1) receptor and a selectivity for A(1) vs A(2A) and A(3) receptors higher than 28-fold. 2'-Me-CCPA was confirmed to be the most selective, high affinity agonist so far known also at human A(1) receptor with a K(i) value of 3.3 nM and 2903- and 341-fold selective vs human A(2A) and A(3) receptors, respectively. In functional assay, 3'-Me-CPA, 3'-Me-CCPA, and 2-Cl-3'-Me-IB-MECA inhibited forskolin-stimulated adenylyl cyclase activity with IC(50) values ranging from 0.3 to 4.9 microM, acting as full agonists. A rhodopsin-based model of the bovine A(1)AR was built to rationalize the higher affinity and selectivity of 2'-C-methyl derivatives of N(6)-substituted-adenosine compared to that of 3'-C-methyl analogues. In the docking exploration, it was found that 2'-Me-CCPA was able to form a number of interactions with several polar residues in the transmembrane helices TM-3, TM-6, and TM-7 of bA(1)AR which were not preserved in the molecular dynamics simulation of 3'-Me-CCPA/bA(1)AR complex.

Adenosine receptor agonists: from basic medicinal chemistry to clinical development

Adenosine is a physiological nucleoside which acts as an autocoid and activates G protein-coupled membrane receptors, designated A(1), A(2A), A(2B) and A(3). Adenosine plays an important role in many (patho)physiological conditions in the CNS as well as in peripheral organs and tissues. Adenosine receptors are present on virtually every cell. However, receptor subtype distribution and densities vary greatly. Adenosine itself is used as a therapeutic agent for the treatment of supraventricular paroxysmal tachycardia and arrhythmias and as a vasodilatatory agent in cardiac imaging. During the past 20 years, a number of selective agonists for A(1), A(2A) and A(3) adenosine receptors have been developed, all of them structurally derived from adenosine. Several such compounds are currently undergoing clinical trials for the treatment of cardiovascular diseases (A(1)and A(2A)), pain (A(1)), wound healing (A(2A)), diabetic foot ulcers (A(2A)), colorectal cancer (A(3)) and rheumatoid arthritis (A(3)). Clinical evaluation of some A(1) and A(2A) adenosine receptor agonists has been discontinued. Major problems include side effects due to the wide distribution of adenosine receptors; low brain penetration, which is important for the targeting of CNS diseases; short half-lifes of compounds; or a lack of effects, in some cases perhaps due to receptor desensitisation or to low receptor density in the targeted tissue. Partial agonists, inhibitors of adenosine metabolism (adenosine kinase and deaminase inhibitors) or allosteric activators of adenosine receptors may be advantageous for certain indications, as they may exhibit fewer side effects.

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.

Comparative Analysis of Putative Agonist-Binding Modes in the Human A1 Adenosine Receptor

A recent study reported a model of the human A(1) adenosine receptor and its agonist binding site, proposing two putative binding modes in the same binding site for the natural agonist, adenosine. The present work investigates the flexibility of this binding site by exhaustive exploration with the natural agonist and with three other adenosine derivatives: N6-cyclopentyladenosine (CPA), 2-chloro-N6-cyclopentyladenosine (CCPA), and 5'-N-ethylcarboxamidoadenosine (NECA). Our aim was to find a common binding mode for agonists that would explain the role in the binding process of the different substitutions allowed at the 2, N6, and 5' positions of adenosine. This problem was addressed through docking simulations, molecular dynamics studies, and estimations of the ligand-binding free energy with both the AUTODOCK scoring function and the linear interaction energy (LIE) approach. The results point to a single receptor-binding position that explains the effects of the different chemical modifications on the adenosine derivatives considered here.

Modeling the adenosine receptors: comparison of the binding domains of A2A agonists and antagonists

A three-dimensional model of the human A(2A) adenosine receptor (AR) and its docked ligands was built by homology to rhodopsin and validated with site-directed mutagenesis and the synthesis of chemically complementary agonists. Different binding modes of A(2A)AR antagonists and agonists were compared by using the FlexiDock automated docking procedure, with manual adjustment. Putative binding regions for the 9H-purine ring in agonist NECA 3 and the 1H-[1,2,4]triazolo[1,5-c]quinazoline ring in antagonist CGS15943 1 overlapped, and the exocyclic amino groups of each were H-bonded to the side chain of N(6.55). For bound agonist, H-bonds formed between the ribose 3'- and 5'-substituents and the hydrophilic amino acids T(3.36), S(7.42), and H(7.43), and the terminal methyl group of the 5'-uronamide interacted with the hydrophobic side chain of F(6.44). Formation of the agonist complex destabilized the ground-state structure of the A(2A)AR, which was stabilized through a network of H-bonding and hydrophobic interactions in the transmembrane helical domain (TM) regions, facilitating a conformational change upon activation. Both flexibility of the ribose moiety, required for the movement of TM6, and its H-bonding to the receptor were important for agonism. Two sets of interhelical H-bonds involved residues conserved among ARs but not in rhodopsin: (1) E13(1.39) and H278(7.43) and (2) D52(2.50), with the highly conserved amino acids N280(7.45) and S281(7.46), and N284(7.49) with S91(3.39). Most of the amino acid residues lining the putative binding site(s) were conserved among the four AR subtypes. The A(2A)AR/3 complex showed a preference for an intermediate conformation about the glycosidic bond, unlike in the A(3)AR/3 complex, which featured an anti-conformation. Hydrophilic amino acids of TMs 3 and 7 (ribose-binding region) were replaced with anionic residues for enhanced binding to amine-derivatized agonists. We identified new neoceptor (T88D)-neoligand pairs that were consistent with the model.

Comparison of three different A1 adenosine receptor antagonists on infarct size and multiple cycle ischemic preconditioning in anesthetized dogs

A(1) adenosine receptor (AR) antagonists are effective diuretic agents that may be useful for treating fluid retention disorders including congestive heart failure. However, antagonism of A(1)ARs is potentially a concern when using these agents in patients with ischemic heart disease. To address this concern, the present study was designed to compare the actions of the A(1)AR antagonists CPX (1,3-dipropyl-8-cyclopentylxanthine), BG 9719 (1,3-dipropyl-8-[2-(5,6-epoxynorbornyl)]xanthine), and BG 9928 (1,3-dipropyl-8-[1-(4-propionate)-bicyclo-[2,2,2]octyl]xanthine) on acute myocardial ischemia/reperfusion injury and ischemic preconditioning (IPC) in an in vivo dog model of infarction. Barbital-anesthetized dogs were subjected to 60 min of left anterior descending coronary artery occlusion followed by 3 h of reperfusion, after which infarct size was assessed by staining with triphenyltetrazolium chloride. IPC was elicited by four 5-min occlusion/5-min reperfusion cycles produced 10 min before the 60-min occlusion. Multiple-cycle IPC produced a robust reduction ( approximately 65%) in infarct size; this effect of IPC on infarct size was not abrogated in dogs pretreated with any of the three AR antagonists. Surprisingly, in the absence of IPC, pretreatment with CPX or BG 9928 before occlusion or immediately before reperfusion resulted in significant reductions ( approximately 40-50%) in myocardial infarct size. However, treatment with an equivalent dose of BG 9719 had no similar effect. We conclude that the A(1)AR antagonists BG 9719, BG 9928, and CPX do not exacerbate cardiac injury and do not interfere with IPC induced by multiple ischemia/reperfusion cycles. We discuss the possibility that the cardioprotective actions of CPX and BG 9928 may be related to antagonism of A(2B)ARs.

Synthesis, molecular modeling studies, and pharmacological activity of selective A(1) receptor antagonists

We present a combined computational study aimed at identifying the three-dimensional structural properties required for different classes of compounds to show antagonistic activity toward the A(1) adenosine receptor (AR). Particularly, an approach combining pharmacophore mapping, molecular alignment, and pseudoreceptor generation was applied to derive a hypothesis of the interaction pathway between a set of A(1) AR antagonists taken from the literature and a model of the putative A(1) receptor. The pharmacophore model consists of seven features and represents an improvement of the N(6)-C8 model, generally reported as the most probable pharmacophore model for A(1) AR agonists and antagonists. It was used to build up a pseudoreceptor model able to rationalize the relationships between structural properties and biological data of, and external to, the training set. In fact, to further assess its statistical significance and predictive power, the pseudoreceptor was employed to predict the free energy of binding associated with compounds constituting a test set. While part of these molecules was also taken from the literature, the remaining compounds were designed and synthesized by our research group. All of the new compounds were tested for their affinity toward A(1), A(2a), and A(3) AR, showing interesting antagonistic activity and A(1) selectivity.

Identification by site-directed mutagenesis of residues involved in ligand recognition and activation of the human A3 adenosine receptor

Ligand recognition has been extensively explored in G protein-coupled A(1), A(2A), and A(2B) adenosine receptors but not in the A(3) receptor, which is cerebroprotective and cardioprotective. We mutated several residues of the human A(3) adenosine receptor within transmembrane domains 3 and 6 and the second extracellular loop, which have been predicted by previous molecular modeling to be involved in the ligand recognition, including His(95), Trp(243), Leu(244), Ser(247), Asn(250), and Lys(152). The N250A mutant receptor lost the ability to bind both radiolabeled agonist and antagonist. The H95A mutation significantly reduced affinity of both agonists and antagonists. In contrast, the K152A (EL2), W243A (6.48), and W243F (6.48) mutations did not significantly affect the agonist binding but decreased antagonist affinity by approximately 3-38-fold, suggesting that these residues were critical for the high affinity of A(3) adenosine receptor antagonists. Activation of phospholipase C by wild type (WT) and mutant receptors was measured. The A(3) agonist 2-chloro-N(6)-(3-iodobenzyl)-5'-N-methylcarbamoyladenosine stimulated phosphoinositide turnover in the WT but failed to evoke a response in cells expressing W243A and W243F mutant receptors, in which agonist binding was less sensitive to guanosine 5'-gamma-thiotriphosphate than in WT. Thus, although not important for agonist binding, Trp(243) was critical for receptor activation. The results were interpreted using a rhodopsin-based model of ligand-A(3) receptor interactions.

Neoceptor concept based on molecular complementarity in GPCRs: a mutant adenosine A(3) receptor with selectively enhanced affinity for amine-modified nucleosides

Adenosine A(3) receptors are of interest in the treatment of cardiac ischemia, inflammation, and neurodegenerative diseases. In an effort to create a unique receptor mutant that would be activated by tailor-made synthetic ligands, we mutated the human A(3) receptor at the site of a critical His residue in TM7, previously proposed to be involved in ligand recognition through interaction with the ribose moiety. The H272E mutant receptor displayed reduced affinity for most of the uncharged A(3) receptor agonists and antagonists examined. For example, the nonselective agonist 1a was 19-fold less potent at the mutant receptor than at the wild-type receptor. The introduction of an amino group on the ribose moiety of adenosine resulted in either equipotency or enhanced binding affinity at the H272E mutant relative to wild-type A(3) receptors, depending on the position of the amino group. 3'-Amino-3'-deoxyadenosine proved to be 7-fold more potent at the H272E mutant receptor than at the wild-type receptor, while the corresponding 2'- and 5'-amino analogues did not display significantly enhanced affinities. An 3'-amino-N(6)-iodobenzyl analogue showed only a small enhancement at the mutant (K(i) = 320 nM) vs wild-type receptors. The 3'-amino group was intended for a direct electrostatic interaction with the negatively charged ribose-binding region of the mutant receptor, yet molecular modeling did not support this notion. This design approach is an example of engineering the structure of mutant receptors to recognize synthetic ligands for which they are selectively matched on the basis of molecular complementarity between the mutant receptor and the ligand. We have termed such engineered receptors "neoceptors", since the ligand recognition profile of such mutant receptors need not correspond to the profile of the parent, native receptor.

Molecular approach to adenosine receptors: receptor-mediated mechanisms of tissue protection

Adenosine accumulation during ischemia and inflammation protects tissues from injury. In ischemic tissues adenosine accumulates due to inhibition of adenosine kinase, and in inflamed tissues adenosine is formed from adenine nucleotides that are released from many cells including platelets, mast cells, nerves, and endothelium. Nucleotides are rapidly converted to adenosine by a family of ecto-nucleotidases including CD39 and CD73. Activation of A(1) and possibly A(3) adenosine receptors (ARs) protects heart and other tissues by preconditioning through a pathway including protein kinase C and mitochondrial K(ATP) channels. Activation of A(2A) receptors limits reperfusion injury by inhibiting inflammatory processes in neutrophils, platelets, macrophages and T cells. Adenosine produces proinflammatory responses mediated by receptors that vary among species; A(3) and A(2B) receptors mediate degranulation of rodent and human or canine mast cells, respectively. Novel adenosine receptor subtype-selective ligands have recently been developed. These include MRS1754 (A(2B) blocker), MRS1220 (A(3) blocker), MRE 3008F20 (human A(3) blocker), MRS1523 (rat A(3) blocker), and ATL146e (A(2A) agonist). These new pharmacologic tools will help investigators to sort out how adenosine protects tissues from injury and to identify new therapeutic agents that hold promise for the treatment of inflammatory and ischemic diseases.

Determination of amino acid residues that are accessible from the ligand binding crevice in the seventh transmembrane-spanning region of the human A(1) adenosine receptor

The substituted-cysteine accessibility method (SCAM) was applied to transmembrane span seven of the human A(1) adenosine receptor (hA(1)AR) to reveal a subset of amino acids that are exposed to the ligand-binding crevice. The SCAM approach involved a systematic probe of receptor structure by individual substitutions of residues K265 (7.30) to R296 (7.61) with cysteine. In most cases, hA(1)AR substituted-cysteine mutant membranes displayed antagonist dissociation binding constants that did not differ significantly from wild-type (WT). Radioligand binding assays were used to compare cell membranes that were treated with hydrophilic, sulfhydryl-specific methanethiosulfonate derivatives with control cell membranes. Position H278 was previously reported to be required for A(1)AR ligand binding; however, that report did not establish that H278 represents a contact point for ligands. Cysteine-substitution at H278 yields membrane preparations with greatly decreased receptor density compared with WT membranes from cells in the same transfection experiment. However, H278C membranes retain a measurable fraction of antagonist binding. This observation allows for the investigation of binding-crevice accessibility at position 278 and suggests that H278 may not be required for binding of antagonist ligands. Our data reveal the binding-crevice accessibility of residues T270 (7.35), A273 (7.38), I274 (7.39), T277 (7.42), H278 (7.43), N284 (7.49), and Y288 (7.53) in the hA(1)AR. These data are consistent with the high-resolution structure of bovine rhodopsin that features three alpha-helical turns in this region that are interrupted by an elongated, nonhelical structure from positions 7.43 to 7.48 in the primary amino acid sequence.

3-Aryl[1,2,4]triazino[4,3-a]benzimidazol-4(10H)-ones: a new class of selective A1 adenosine receptor antagonists

Radioligand binding assays using bovine cortical membrane preparations and biochemical in vitro studies revealed that various 3-aryl[1,2,4]triazino[4,3-a]benzimidazol-4(10H)-one (ATBI) derivatives, previously reported by us as ligands of the central benzodiazepine receptor (BzR) (Primofiore, G.; et al. J. Med. Chem. 2000, 43, 96-102), behaved as antagonists at the A1 adenosine receptor (A1AR). Alkylation of the nitrogen at position 10 of the triazinobenzimidazole nucleus conferred selectivity for the A1AR vs the BzR. The most potent ligand of the ATBI series (10-methyl-3-phenyl[1,2,4]triazino[4,3-a]benzimidazol-4(10H)-one 12) displayed a Ki value of 63 nM at the A1AR without binding appreciably to the adenosine A2A and A3 nor to the benzodiazepine receptor. Pharmacophore-based modeling studies in which 12 was compared against a set of well-established A1AR antagonists suggested that three hydrogen bonding sites (HB1 acceptor, HB2 and HB3 donors) and three lipophilic pockets (L1, L2, and L3) might be available to antagonists within the A1AR binding cleft. According to the proposed pharmacophore scheme, the lead compound 12 engages interactions with the HB2 site (via the N2 nitrogen) as well as with the L2 and L3 sites (through the pendant and the fused benzene rings). The results of these studies prompted the replacement of the methyl with more lipophilic groups at the 10-position (to fill the putative L1 lipophilic pocket) as a strategy to improve A1AR affinity. Among the new compounds synthesized and tested, the 3,10-diphenyl[1,2,4]triazino[4,3-a]benzimidazol-4(10H)-one (23) was characterized by a Ki value of 18 nM which represents a 3.5-fold gain of A1AR affinity compared with the lead 12. A rhodopsin-based model of the bovine adenosine A1AR was built to highlight the binding mode of 23 and two well-known A1AR antagonists (III and VII) and to guide future lead optimization projects. In our docking simulations, 23 receives a hydrogen bond (via the N1 nitrogen) from the side chain of Asn247 (corresponding to the HB1 and HB2 sites) and fills the L1, L2, and L3 lipophilic pockets with the 10-phenyl, 3-phenyl, and fused benzene rings, respectively.

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.

Site-directed mutagenesis studies of human A(2A) adenosine receptors: involvement of glu(13) and his(278) in ligand binding and sodium modulation

To provide insights into interactions between ligands and A(2A) adenosine receptors, site-directed mutagenesis was used to test the roles of a glutamic acid residue in the first transmembrane domain (Glu13) and a histidine residue in the seventh transmembrane domain (His278). The two residues, which have been suggested to be closely linked in molecular modeling studies, were mutated to glutamine (E13Q) and tyrosine (H278Y), respectively. Saturation experiments revealed that [(3)H]ZM241385 (4-2-[7-amino-2-(2-furyl)-1,2, 4-triazolo[1,5-a][1,3,5]triazin-5-yl-amino]ethylphenol) bound wild-type and mutant receptors in membranes from COS-7 cells expressing human A(2A) adenosine receptors with high affinity and low non-specific binding. It was found from the competition experiments that the affinity of the A(2A) adenosine receptor agonists for the mutant receptors was 3- to 200-fold lower than for the wild-type receptor. Among antagonist competitors of binding at E13Q and H278Y mutant receptors, there was variation in the affinity depending on their different structures, although changes were relatively minor (<3-fold) except in the case of theophylline, whose affinity was decreased approximately 20 times on the H278Y mutant. The possible involvement of the two residues in sodium ion regulation was also tested. The agonist competition curves for [(3)H]ZM241385 were shifted to the right in both wild-type and mutant receptors in the presence of 1 M sodium ions, but the extent of shift (2- to 27-fold) in wild-type receptor was generally larger than for the mutant receptors. Sodium ions also decreased [(3)H]ZM241385 dissociation from both wild-type and mutant receptors, being more influential on the former than the latter. The results suggest that the two closely linked residues Glu13 and His278 in A(2A) adenosine receptor are most important for agonist recognition and are partly responsible for the allosteric regulation by sodium ions.

Identification of a key region of kinin B(1) receptor for high affinity binding of peptide antagonists

To investigate the molecular basis for the specificity of ligand recognition in human kinin B(1) (B(1)R) and B(2) (B(2)R) receptors, we constructed a series of chimeric receptors by progressively replacing, from the N to the C terminus, the human B(2)R domains by their B(1) counterparts. The chimeric construct possessing the C-terminal tail and the transmembrane domain VII (TM VII) of the B(2)R (construct 6) displayed 7- and 20- fold decreased affinities for the B(1) agonist [(3)H]desArg(10)-kallidin (desArg(10)-KD) and the B(1) antagonist [(3)H]desArg(10)-[Leu(9)]-KD respectively, as compared with the wild-type B(1)R. Moreover, the substitution of the B(1) TM VII by its B(2) homologue TM increased the affinity for the pseudopeptide antagonists, Hoe140 and NPC 567. High affinity for desArg(10)-KD binding was fully regained when the B(2) residue Thr(287) was replaced in construct 6 by the corresponding B(1) Leu(294) residue. When the B(2) residue Tyr(295) was exchanged with the corresponding B(1) Phe(302), high affinity binding for both agonist and antagonist was recovered. Moreover, the L294T and F302Y mutant B(1)R exhibited 69- and 6.5-fold increases, respectively, in their affinities for the B(2) receptor antagonist, Hoe140. Therefore we proposed that Leu(294) and Phe(302) residues, which may not be directly involved in the binding of B(1)R ligands and, hence, their Thr(287) and Tyr(295) B(2) counterparts, are localized in a receptor region, which plays a pivotal role in the binding selectivity of the peptide or pseudopeptide kinin ligands.

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.

Differential effects of the adenosine A(1) receptor allosteric enhancer PD 81,723 on agonist binding to brain and adipocyte membranes

The benzoylthiophene analog, PD 81,723, has been shown to allosterically enhance agonist binding and functional activation of the mammalian adenosine (ADO) A(1) receptor subtype by putatively maintaining the receptor in a high affinity state. The present studies were conducted to evaluate the ability of PD 81,723 to enhance the binding of [3H]cyclohexyladenosine ([3H]CHA) to A(1) receptors of neural (cerebral cortex) and non-neural (adipocyte) origin in three different species; rat, guinea pig and dog. PD 81, 723 (0.3-100 microM) produced a concentration-dependent enhancement of [3H]CHA binding to rat brain A(1) receptors. These effects were also species-dependent with larger enhancements (150-200% of control) observed in guinea pig and dog brain membranes as compared to the rat (120% of control). In contrast, PD 81,723 did not produce any enhancement of [3H]CHA binding to A(1) receptors in adipocyte membranes from any of the species examined. Additional binding studies were conducted using pharmacological manipulations that have previously been shown to enhance the allosteric effects of PD 81,723. In the presence of 1 mM GTP, the allosteric effects of PD 81,723 (15 microM) were increased in rat, guinea pig and dog brain membranes, however, in adipocyte membranes from each species, no significant alteration in agonist binding was observed. Similarly, the A(1) receptor selective antagonist 8-cyclopentyl-1, 3-dipropylxanthine (added to effectively reduce the intrinsic antagonist properties of PD 81,723) was found to enhance the allosteric effects of PD 81,723 (15 microM) in brain, but produce no alteration of agonist binding in adipocyte membranes from each species. Examination of the dissociation kinetics of [3H]CHA binding from rat brain and adipocyte membranes revealed that PD 81,723 (15 microM) differentially slowed agonist dissociation from brain, but not adipocyte, membranes. Taken together, the present data support the hypothesis that in tissues that are sensitive to PD 81,723, this benzyolthiophene functions to maintain the A(1) receptor in a high-affinity state and that the relative proportions of high-affinity A(1) receptors present in specific tissues may contribute, at least in part, to the apparent differential effects of PD 81,723 on agonist binding. The tissue specific modulation of A(1) receptor function by PD 81,723 also illustrates the possibility that the locus of allosteric modulation by PD 81,723 may be manifest via a specific, but indirect and tissue-dependent, interaction with the A(1) receptor.

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.

The cloned guinea pig neuropeptide Y receptor Y1 conforms to other mammalian Y1 receptors

We have cloned the guinea pig neuropeptide Y (NPY) Y1 receptor and found it to be 92-93% identical to other cloned mammalian Y1 receptors. Porcine NPY and peptide YY (PYY) displayed affinities of 43 pM and 48 pM, respectively. NPY2-36 and NPY3-36 had 6- and 46-fold lower affinity, respectively, than intact NPY. Functional coupling was measured by using a microphysiometer. Human NPY and PYY were equipotent in causing extracellular acidification with EC50 values of 0.59 nM and 0.69 nM, respectively, whereas NPY2-36 and NPY3-36 were about 15-fold and 500-fold less potent, respectively, than NPY. The present study shows that the cloned guinea pig Y1 receptor is very similar to its orthologues in other mammals, both with respect to sequence and pharmacology. Thus, results from previous studies on guinea pig NPY receptors might imply the existence of an additional Y1-like receptor sensitive to B1BP3226.

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.

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.