Constitutive activation of A(3) adenosine receptors by site-directed mutagenesis

Efficient G protein coupling is not required for agonist-mediated internalization and membrane reorganization of the adenosine A3 receptor

Organization of G protein‐coupled receptors at the plasma membrane has been the focus of much recent attention. Advanced microscopy techniques have shown that these receptors can be localized to discrete microdomains and reorganization upon ligand activation is crucial in orchestrating their signaling. Here, we have compared the membrane organization and downstream signaling of a mutant (R108A, R3.50A) of the adenosine A₃ receptor (A₃AR) to that of the wild‐type receptor. Fluorescence Correlation Spectroscopy (FCS) studies with a fluorescent agonist (ABEA‐X‐BY630) demonstrated that both wild‐type and mutant receptors bind agonist with high affinity but in subsequent downstream signaling assays the R108A mutation abolished agonist‐mediated inhibition of cAMP production and ERK phosphorylation. In further FCS studies, both A₃AR and A₃AR R108A underwent similar agonist‐induced increases in receptor density and molecular brightness which were accompanied by a decrease in membrane diffusion after agonist treatment. Using bimolecular fluorescence complementation, experiments showed that the R108A mutant retained the ability to recruit β‐arrestin and these receptor/arrestin complexes displayed similar membrane diffusion and organization to that observed with wild‐type receptors. These data demonstrate that effective G protein signaling is not a prerequisite for agonist‐stimulated β‐arrestin recruitment and membrane reorganization of the A₃AR.

Pharmacological insight into the activation of the human neuropeptide FF2 receptor

The neuropeptide FF2 (NPFF₂) receptor, predominantly expressed in the central nervous system, plays an important role in the modulation of sensory input and opioid analgesia, as well as in locomotion, feeding, intestinal motility, reward, and the control of obesity. The NPFF₂ receptor belongs to the RFamide peptide receptor family and to the G protein coupled receptor (GPCR) super family, but contrary to many other class A GPCRs, no 3D structure has been solved. Thus, it is essential to perform mutagenesis to gain information on the fine functioning of the NPFF₂ receptor. In this study, we examined the role of aspartic acid (D) from the "D/ERY/F" motif found in the second intracellular loop (ICL2) and the role of the C-terminal end of the receptor in ligand binding and signal transduction. We found that mutation D3.49A does not impair binding capacities but inhibits G protein activation as well as adenylyl cyclase regulation. Truncation of the C terminal part of the receptor has different effects depending on the position of truncation. When truncation was realized downstream of the putative acylation site, ligand binding and signal transduction capabilities were not lost, contrary to total deletion of the C terminus, which totally impairs the activity of the receptor.

A3 adenosine receptor activation mechanisms: molecular dynamics analysis of inactive, active, and fully active states

We investigated the Gi-coupled A₃ adenosine receptor (A₃AR) activation mechanism by running 7.2 µs of molecular dynamics (MD) simulations. Based on homology to G protein-coupled receptor (GPCR) structures, three constitutively active mutant (CAM) and the wild-type (WT) A₃ARs in the apo form were modeled. Conformational signatures associated with three different receptor states (inactive R, active R*, and bound to Gi protein mimic) were predicted by analyzing and comparing the CAMs with WT receptor and by considering site-directed mutagenesis data. Detected signatures that were correlated with receptor state included: Persistent salt-bridges involving key charged residues for activation (including a novel, putative ionic lock), rotameric state of conserved W^6.48, and Na⁺ ions and water molecules present. Active-coupled state signatures similar to the X-ray structures of β₂ adrenergic receptor-Gs protein and A_2AAR-mini-Gs and the recently solved cryo-EM A₁AR-Gi complexes were found. Our MD analysis suggests that constitutive activation might arise from the D107^3.49-R108^3.50 ionic lock destabilization in R and the D107^3.49-R111^3.53 ionic lock stabilization in R* that presumably lowers the energy barrier associated with an R to R* transition. This study provides new opportunities to understand the underlying interactions of different receptor states of other Gi protein-coupled GPCRs.

Molecular Dynamics Simulations of the Allosteric Modulation of the Adenosine A2A Receptor by a Mini-G Protein

Through their coupling to G proteins, G Protein-Coupled Receptors (GPCRs) trigger cellular responses to various signals. Some recent experiments have interestingly demonstrated that the G protein can also act on the receptor by favoring a closed conformation of its orthosteric site, even in the absence of a bound agonist. In this work, we explored such an allosteric modulation by performing extensive molecular dynamics simulations on the adenosine A2 receptor (A2aR) coupled to the Mini-Gs protein. In the presence of the Mini-Gs, we confirmed a restriction of the receptor’s agonist binding site that can be explained by a modulation of the intrinsic network of contacts of the receptor. Of interest, we observed similar effects with the C-terminal helix of the Mini-Gs, showing that the observed effect on the binding pocket results from direct local contacts with the bound protein partner that cause a rewiring of the whole receptor’s interaction network.

Intracellular Receptor Modulation: Novel Approach to Target GPCRs

Recent crystal structures of multiple G protein-coupled receptors (GPCRs) have revealed a highly conserved intracellular pocket that can be used to modulate these receptors from the inside. This novel intracellular site partially overlaps with the G protein and β-arrestin binding site, providing a new manner of pharmacological intervention. Here we provide an update of the architecture and function of the intracellular region of GPCRs, until now portrayed as the signaling domain. We review the available evidence on the presence of intracellular binding sites among chemokine receptors and other class A GPCRs, as well as different strategies to target it, including small molecules, pepducins, and nanobodies. Finally, the potential advantages of intracellular (allosteric) ligands over orthosteric ligands are also discussed.

Molecular dissection of the human A3 adenosine receptor coupling with β-arrestin2

Besides classical G protein coupling, G protein-coupled receptors (GPCRs) are nowadays well known to show significant signalling via other adaptor proteins, such as β-arrestin2 (βarr2). The elucidation of the molecular mechanism of the GPCR-βarr2 interaction is a prerequisite for the structure-activity based design of biased ligands, which introduces a new chapter in drug discovery. The general mechanism of the interaction is believed to rely on phosphorylation sites, exposed upon agonist binding. However, it is not known whether this mechanism is universal throughout the GPCR family or if GPCR-specific patterns are involved. In recent years, promising orally active agonists for the human A₃ adenosine receptor (A₃AR), a GPCR highly expressed in inflammatory and cancer cells, have been evaluated in clinical trials for the treatment of rheumatoid arthritis, psoriasis, and hepatocellular carcinoma. In this study, the effect of cytoplasmic modifications of the A₃AR on βarr2 recruitment was evaluated in transiently transfected HEK293T cells, using a live-cell split-reporter system (NanoBit®, Promega), based on the structural complementation of NanoLuc luciferase, allowing real-time βarr2 monitoring. The A₃AR-selective reference agonist 2-Cl-IB-MECA yielded a robust, concentration dependent (5 nM-1 µM) recruitment of βarr2 (logEC50: -7.798 ± 0.076). The role of putative phosphorylation sites, located in the C-terminal part and cytoplasmic loops, and the role of the 'DRY' motif was evaluated. It was shown that the A₃AR C-terminus was dispensable for βarr2 recruitment. This contrasts with studies in the past for the rat A₃AR, which pointed at crucial C-terminal phosphorylation sites. When combining truncation of the A₃AR with modification of the 'DRY' motif to 'AAY', the βarr2 recruitment was drastically reduced. Recruitment could be partly rescued by back-mutation to 'NQY', or by extending the C-terminus again. In conclusion, other parts of the human A₃AR, either cytosolic or exposed upon receptor activation, rather than the C-terminus alone, are responsible for βarr2 recruitment in a complementary or synergistic way.

Structural Probing and Molecular Modeling of the A₃ Adenosine Receptor: A Focus on Agonist Binding

Adenosine is an endogenous modulator exerting its functions through the activation of four adenosine receptor (AR) subtypes, termed A₁, A_2A, A_2B and A₃, which belong to the G protein-coupled receptor (GPCR) superfamily. The human A₃AR (hA₃AR) subtype is implicated in several cytoprotective functions. Therefore, hA₃AR modulators, and in particular agonists, are sought for their potential application as anti-inflammatory, anticancer, and cardioprotective agents. Structure-based molecular modeling techniques have been applied over the years to rationalize the structure–activity relationships (SARs) of newly emerged A₃AR ligands, guide the subsequent lead optimization, and interpret site-directed mutagenesis (SDM) data from a molecular perspective. In this review, we showcase selected modeling-based and guided strategies that were applied to elucidate the binding of agonists to the A₃AR and discuss the challenges associated with an accurate prediction of the receptor extracellular vestibule through homology modeling from the available X-ray templates.

Selectivity in the Use of Gi/o Proteins Is Determined by the DRF Motif in CXCR6 and Is Cell-Type Specific

CXCR6, the receptor for CXCL16, is expressed on multiple cell types and can be a coreceptor for human immunodeficiency virus 1. Except for CXCR6, all human chemokine receptors contain the D(3.49)R(3.50)Y(3.51) sequence, and all but two contain A(3.53) at the cytoplasmic terminus of the third transmembrane helix (H3C), a region within class A G protein-coupled receptors that contacts G proteins. In CXCR6, H3C contains D(3.49)R(3.50)F(3.51)I(3.52)V(3.53) at positions 126-130. We investigated the importance and interdependence of the canonical D126 and the noncanonical F128 and V130 in CXCR6 by mutating D126 to Y, F128 to Y, and V130 to A singly and in combination. For comparison, we mutated the analogous positions D142, Y144, and A146 to Y, F, and V, respectively, in CCR6, a related receptor containing the canonical sequences. Mutants were analyzed in both human embryonic kidney 293T and Jurkat E6-1 cells. Our data show that for CXCR6 and/or CCR6, mutations in H3C can affect both receptor signaling and chemokine binding; noncanonical H3C sequences are functionally linked, with dual changes mitigating the effects of single mutations; mutations in H3C that compromise receptor activity show selective defects in the use of individual Gi/o proteins; and the effects of mutations in H3C on receptor function and selectivity in Gi/o protein use can be cell-type specific. Our findings indicate that the ability of CXCR6 to make promiscuous use of the available Gi/o proteins is exquisitely dependent on sequences within the H3C and suggest that the native sequence allows for preservation of this function across different cellular environments.

A yeast screening method to decipher the interaction between the adenosine A2B receptor and the C-terminus of different G protein α-subunits

The expression of human G protein-coupled receptors (GPCRs) in Saccharomyces cerevisiae containing chimeric yeast/mammalian Gα subunits provides a useful tool for the study of GPCR activation. In this study, we used a one-GPCR-one-G protein yeast screening method in combination with molecular modeling and mutagenesis studies to decipher the interaction between GPCRs and the C-terminus of different α-subunits of G proteins. We chose the human adenosine A2B receptor (hA2BR) as a paradigm, a typical class A GPCR that shows promiscuous behavior in G protein coupling in this yeast system. The wild-type hA2BR and five mutant receptors were expressed in 8 yeast strains with different humanized G proteins, covering the four major classes: Gαi, Gαs, Gαq, and Gα12. Our experiments showed that a tyrosine residue (Y) at the C-terminus of the Gα subunit plays an important role in controlling the activation of GPCRs. Receptor residues R103(3.50) and I107(3.54) are vital too in G protein-coupling and the activation of the hA2BR, whereas L213(IL3) is more important in G protein inactivation. Substitution of S235(6.36) to alanine provided the most divergent G protein-coupling profile. Finally, L236(6.37) substitution decreased receptor activation in all G protein pathways, although to a different extent. In conclusion, our findings shed light on the selectivity of receptor/G protein coupling, which may help in further understanding GPCR signaling.

The A3 adenosine receptor as multifaceted therapeutic target: pharmacology, medicinal chemistry, and in silico approaches

Adenosine is an ubiquitous local modulator that regulates various physiological and pathological functions by stimulating four membrane receptors, namely A(1), A(2A), A(2B), and A(3). Among these G protein-coupled receptors, the A(3) subtype is found mainly in the lung, liver, heart, eyes, and brain in our body. It has been associated with cerebroprotection and cardioprotection, as well as modulation of cellular growth upon its selective activation. On the other hand, its inhibition by selective antagonists has been reported to be potentially useful in the treatment of pathological conditions including glaucoma, inflammatory diseases, and cancer. In this review, we focused on the pharmacology and the therapeutic implications of the human (h)A(3) adenosine receptor (AR), together with an overview on the progress of hA(3) AR agonists, antagonists, allosteric modulators, and radioligands, as well as on the recent advances pertaining to the computational approaches (e.g., quantitative structure-activity relationships, homology modeling, molecular docking, and molecular dynamics simulations) applied to the modeling of hA(3) AR and drug design. 

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.

Molecular modeling of the second extracellular loop of G-protein coupled receptors and its implication on structure-based virtual screening

The current study describes the validation of high-throughput modeling procedures for the construction of the second extracellular loop (ecl2) of all nonolfactory human G Protein-coupled receptors. Our modeling flowchart is based on the alignment of essential residues determining the particular ecl2 fold observed in the bovine rhodopsin (bRho) crystal structure. For a set of GPCR targets, the dopamine D2 receptor (DRD2), adenosine A3 receptor (AA3R), and the thromboxane A2 receptor (TA2R), the implications of including ecl2 atomic coordinates is evaluated in terms of structure-based virtual screening accuracy: the suitability of the 3D models to distinguish between known antagonists and randomly chosen decoys using automated docking approaches. The virtual screening results of different models describing increasingly exhaustive receptor representations (seven helices only, seven helices and ecl2 loop, full model) have been compared. Explicit modeling of the ecl2 loop was found to be important in only one of three test cases whereas a loopless model was shown to be accurate enough in the two other receptors. An exhaustive comparison of ecl2 loops of 365 receptors to that of bRho suggests that explicit ecl2 loop modeling should be reserved to receptors where loop building can be guided by experimental restraints.

Functional analysis of the murine cytomegalovirus chemokine receptor homologue M33: ablation of constitutive signaling is associated with an attenuated phenotype in vivo

The murine cytomegalovirus (MCMV) M33 gene is conserved among all betaherpesviruses and encodes a homologue of seven-transmembrane receptors (7TMR) with the capacity for constitutive signaling. Previous studies have demonstrated that M33 is important for MCMV dissemination to or replication within the salivary glands. In this study, we probed N- and C-terminal regions of M33 as well as known 7TMR signature motifs in transmembrane (TM) II and TM III to determine the impact on cell surface expression, constitutive signaling, and in vivo phenotype. The region between amino acids R(340) and A(353) of the C terminus was found to be important for CREB- and NFAT-mediated signaling, although not essential for phosphatidylinositol turnover. Tagging or truncation of the N terminus of M33 resulted in loss of cell surface expression. Within TM II, an F79D mutation abolished constitutive signaling, demonstrating a role, as in other cellular and viral 7TMR, of TM II in receptor activation. In TM III, the arginine (but not the asparagine) residue of the NRY motif (the counterpart of the common DRY motif in cellular 7TMR) was found to be essential for constitutive signaling. Selected mutations incorporated into recombinant MCMV showed that disruption of constitutive signaling for a viral 7TMR homologue resulted in a reduced capacity to disseminate to or replicate in the salivary glands. In addition, HCMV UL33 was found to partially compensate for the lack of M33 in vivo, suggesting conserved biological roles of the UL33 gene family.

Search for new antagonist ligands for adenosine receptors from QSAR point of view. How close are we?

In view of the large libraries of nucleoside analogues that are now being handled in organic synthesis, the identification of drug biological activity is advisable prior to synthesis and this can be achieved by employing predictive biological property methods. In this sense, Quantitative Structure-Activity Relationships (QSAR) or docking approaches have emerged as promising tools. Although a large number of in silico approaches have been described in the literature for the prediction of different biological activities, the use of QSAR applications to develop adenosine receptor (AR) antagonists is not common as for the case of the antibiotics and anticancer compounds for instance. The intention of this review is to summarize the present knowledge concerning computational predictions of new molecules as adenosine receptor antagonists.

Adipokines and the immune system: an adipocentric view

There is increasing evidence of close interactions between the adipose and the immune systems. Adipocytes secrete multiple factors, including adipokines such as leptin and adiponectin that have both pro- and anti-inflammatory effects, and influence diseases involving the immune system. Further, adipose tissue also secretes various chemokines and cytokines, derived from either the adipocytes themselves, or the neighbouring cells including both resident and infiltrating macrophages. This close physical and paracrine interaction results in reciprocal actions of adipocytes, preadipocytes and macrophages within the microenvironment of the adipose tissue. Adipose tissue is a source of Acylation Stimulating Protein (ASP)/C3adesArg which interacts with the receptor C5L2 to stimulate triglyceride synthesis and glucose transport. C5L2, present on adipocytes, preadipocytes, macrophages, and numerous other myeloid and non-myeloid cells is also postulated to be a decoy receptor for C5a in immune cells. Several reviews within the past year have recently examined the role of C5L2 in C5a-mediated physiology. The present mini-review is an adipocentric view with emphasis on the role of ASP and C5L2 in lipid metabolism. C5L2 may play a role in mediating, on one hand, ASP stimulation of triglyceride synthesis in adipose, and, on the other hand, a role as mediator of C5a immune function. Both roles remain controversial, and will only be resolved with further studies.

Common structural requirements for heptahelical domain function in class A and class C G protein-coupled receptors

G protein-coupled receptors (GPCRs) are key players in cell communication. Several classes of such receptors have been identified. Although all GPCRs possess a heptahelical domain directly activating G proteins, important structural and sequence differences within receptors from different classes suggested distinct activation mechanisms. Here we show that highly conserved charged residues likely involved in an interaction network between transmembrane domains (TM) 3 and 6 at the cytoplasmic side of class C GPCRs are critical for activation of the gamma-aminobutyric acid type B receptor. Indeed, the loss of function resulting from the mutation of the conserved lysine residue into aspartate or glutamate in the TM3 of gamma-aminobutyric acid type B(2) can be partly rescued by mutating the conserved acidic residue of TM6 into either lysine or arginine. In addition, mutation of the conserved lysine into an acidic residue leads to a nonfunctional receptor that displays a high agonist affinity. This is reminiscent of a similar ionic network that constitutes a lock stabilizing the inactive state of many class A rhodopsin-like GPCRs. These data reveal that despite their original structure, class C GPCRs share with class A receptors at least some common structural feature controlling G protein activation.

Beta-blockers show inverse agonism to a novel constitutively active mutant of beta1-adrenoceptor

We obtained a new mutant of the beta(1)-adrenergic receptor (beta(1)-AR) by point mutations that can constitutively activate beta(1)-AR. Aspartate104 of the beta(1)-AR in the 2nd transmembrane was replaced with alanine. The beta(1)-AR mutant expressed in human embryonic kidney (HEK)-293 cells displayed high level of constitutive activity with respect to wild-type (P<0.05), which could be partially inhibited by some beta-blockers. The constitutive activity of the mutant was confirmed by the finding that the enhanced activity is dependent on the level of receptor expression. The results of this study might have interesting implications for future studies aiming at elucidating the activation process of the beta(1)-AR as well as the mechanism of action of beta-blockers.

Functional consequences of naturally occurring DRY motif variants in the mammalian chemoattractant receptor GPR33

Most members of the large family of rhodopsin-like G-protein-coupled receptors possess an evolutionarily conserved Asp-Arg-Tyr (DRY) motif in the C-terminal region of the third transmembrane domain. Mutations of residues within this motif usually abolish receptor function and, when they occur naturally, can even cause human diseases. By analyzing over 100 mammalian orthologs of the chemoattractant receptor GPR33 we identified several polymorphic and fixed sequence variations within the DRY motif. Unexpectedly, the naturally occurring mutation of Arg(3.50) to His in mouse GPR33 showed no difference from the wild-type receptor in several functional tests. Sequence analysis of GPR33 from Asian house mice revealed the polymorphic existence of Arg(3.50) and His(3.50) alleles in wild-trapped populations, further supporting the functional equivalence of both allelic variants. In contrast, the Arg(3.50) to Gly mutation found in hamster GPR33 inactivates the receptor and may have contributed to pseudogenization of this gene in this species. Functional data with GPR33 variants indicate different receptor- and context-specific consequences of DRY mutations. Our study also reveals GPR33 as a new example illustrating missense mutations as a first step in the pseudogenization process.

A GPCR that is not "DRY"

The conserved "DRY" motif (Asp-Arg-Tyr) at the cytosolic surface of rhodopsin-like G protein-coupled receptors has been the subject of much work attempting to understand the mechanisms of receptor activation and interaction with G proteins. Both the acidic (Asp) and basic (Arg) residues of this motif are important for isomerization of receptors between inactive and activated conformations. In this issue of Molecular Pharmacology, Rosenkilde et al. (pp. 11-19) show that a novel wild-type receptor, ORF74-EHV2, which lacks the Arg residue, is fully functional, showing both constitutive and ligand-induced activation of G protein signaling. Reintroducing the DRY motif by mutagenesis decreased constitutive activity while retaining ligand-inducible function. This work shows that the conserved Arg side chain is not required for receptor function, but it is important for stabilizing receptors in the inactive conformation.

High Constitutive Activity of a Virus-Encoded Seven Transmembrane Receptor in the Absence of the Conserved DRY Motif (Asp-Arg-Tyr) in Transmembrane Helix 3

The highly conserved Arg in the so-called DRY motif (Asp-Arg-Tyr) at the intracellular end of transmembrane helix 3 is in general considered as an essential residue for G protein coupling in rhodopsin-like seven transmembrane (7TM) receptors. In the open reading frame 74 (ORF74) receptor encoded by equine herpesvirus 2 (EHV2), the DRY motif is substituted with a DTW motif. Nevertheless, this receptor signaled with high constitutive activity through Gi as determined by a receptor-mediated inhibition of forskolin-induced cAMP-production and by an induction of the serum response element-driven transcriptional activity through a pertussis toxin-sensitive manner. Gs and Gq were not activated constitutively as determined by the lack of inositol phosphate turnover and activities of the three transcription factors: cAMP response element-binding protein (CREB), nuclear factor-kappaB, and nuclear factor of activated T cells. Coexpression of the ORF74-EHV2 receptor with the promiscuous G protein Gqi4myr supported the constitutive Gi activation as determined by inositol phosphate turnover and CREB activation. The constitutive activity was inhibited by nonpeptide inverse agonists with micromolar potencies, and the chemokine CXCL6 acted as a high-affinity agonist. It is noteworthy that reconstitution of the DRY motif resulted in a 4- to 5-fold decrease of the constitutive activity. Both the wild type and the receptor with the reconstituted DRY motif were expressed at the cell surface as indicated by immunohistochemistry and enzyme-linked immunosorbent assay analysis. It is concluded that the Arg of the DRY motif in transmembrane helix 3 is not essential for G protein coupling based on the constitutive as well as the ligand-mediated activity observed for ORF74-EHV2.

The role of the DRY motif of human MC4R for receptor activation

We constructed several mutant human MC4R cDNAs by site directed mutagenesis and expressed these receptors in COS-1 cells. The conserved DRY motif among GPCRs was mutated to generate eight mutants. While no MC4R ligand binding was detected in any of the mutants, one mutant, D146A, resulted in higher cAMP production in cells than the wild-type receptor without ligand stimulation.

Lysophosphatidylcholine-induced surface redistribution regulates signaling of the murine G protein-coupled receptor G2A

Intracellular trafficking and spatial dynamics of membrane receptors critically regulate receptor function. Using microscopic and subcellular fractionation analysis, we studied the localization of the murine G protein-coupled receptor G2A (muG2A). Evaluating green fluorescent protein-tagged, exogenously expressed as well as the endogenous muG2A, we observed that this receptor was spontaneously internalized and accumulated in endosomal compartments, whereas its surface expression was enhanced and stabilized by lysophosphatidylcholine (LPC) treatment. Monensin, a general inhibitor of recycling pathways, blocked LPC-regulated surface localization of muG2A as well as muG2A-dependent extracellular signal-regulated kinase (ERK) activation and cell migration induced by LPC treatment. Mutation of the conserved DRY motif (R-->A) enhanced the surface expression of muG2A, resulting in its resistance to monensin inhibition of ERK activation. Our data suggest that intracellular sequestration and surface expression regulated by LPC, rather than direct agonistic activity control the signaling responses of murine G2A toward LPC.

Purine receptors: GPCR structure and agonist design

An integrated approach to the study of drug-receptor interactions has been applied to adenosine receptors (ARs) and P2Y nucleotide receptors. This approach includes probing the receptor structure through site-directed mutagenesis and molecular modeling, in concert with altering the structure of the agonist ligands. Goals of this structural approach are to generate a testable hypothesis for location of the binding site and subsequently to enable the rational design of new agonists and antagonists. In this manner, receptor subtype selectivity has been increased, and agonists have been converted into partial agonists and antagonists. An approach to receptor engineering (neoceptors) has been explored, in which synthetic small molecule agonists (neoligands) are specifically tailored to activate only receptors in which the putative binding sites have been modified. This orthogonal approach to receptor activation, intended for eventual gene therapy, has been demonstrated for A3 and A2A ARs.

Purine and deazapurine nucleosides: synthetic approaches, molecular modelling and biological activity

A number of ligands for the adenosine binding sites has been obtained by using nucleoside convergent and divergent synthesis. Most of our nucleosides have been synthesized by coupling 2,6-dichloropurine (1), 2,6-dichloro-1-deazapurine (2), 2,6-dichloro-3-deazapurine (3) with ribose, 2- and 3-deoxyribose and 2,3-dideoxyribose derivatives. The use of these versatile synthons allowed the introduction of various substituents in 2- and/or 6-positions. The glycosylation site and the anomeric configuration of the obtained nucleosides were assigned on the basis of spectroscopic studies and confirmed by molecular models. A series of potent adenosine receptor ligands has been obtained by using divergent approaches, mostly starting from guanosine. Substitutions in 2, 6, 8, and 5' position of adenosine molecule led to ligands selective for the different adenosine receptor subtypes. Furthermore, we investigated the molecular bases of the different behavior of 2- and 8-alkynyl adenosines, by means of NMR experiments and molecular modeling studies. With docking experiments, we demonstrated that the two class of molecules should have different binding modes that explain their different degree of affinity and the shift of their activity from agonistic (2-substituted derivatives) to antagonistic (8-substituted derivatives).

2- and 8-alkynyladenosines: conformational studies and docking to human adenosine A3 receptor can explain their different biological behavior

Adenosine (Ado) derivatives substituted at the C2 position with an alkynyl chain are endowed with high affinity for A(1), A(2A) and A(3) human adenosine receptors, while being less active at the low affinity A(2B) subtype. On the other hand, the introduction of an alkynyl chain at the C8 position of adenosine is detrimental for the affinity and potency at A(1), A(2A), and A(2B) receptors, while is more tolerated by the A(3) receptor. The evaluation of the stimulation of [35S]GTPgammaS binding revealed that 2-alkynyladenosines behave as adenosine receptors agonists while, on the contrary, 8-alkynyladenosines behave as antagonists. With this work we demonstrated, by means of an NMR-based and a computational conformational analysis, that 8-alkynyladenosines, differently from 2-alkynyladenosines, cannot adopt the sugar-base anti conformation required for adenosine receptor activation.Furthermore, using the recently reported X-ray crystal structure of bovine rhodopsin as template, we built a 3D model of the seven transmembrane domains of the human adenosine A(3) receptor with the homology modeling. After identification of the binding site we carried out docking experiments, demonstrating that the two class of molecules have different binding modes that explain their different degree of affinity and the shift of their activity from agonism to antagonism.

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.

2-Chloro-N(6)-cyclopentyladenosine, adenosine A(1) receptor agonist, antagonizes the adenosine A(3) receptor

The potent adenosine A(1) receptor agonists, N(6)-cyclopentyladenosine (CPA) and 2-chloro-N(6)-cyclopentyladenosine (CCPA), were studied in Chinese hamster ovary (CHO) cells expressing the human adenosine A(3) receptor. CPA, but not CCPA, induced phosphoinositide turnover. CPA inhibited forskolin-stimulated cyclic AMP production (EC(50) value of 242+/-47 nM). CCPA competitively antagonized the effects of agonist Cl-IB-MECA (2-chloro-N(6)-(3-iodobenzyl)-5'-N-methylcarbamoyladenosine) with K(B) value of 5.0 nM. CPA competition curves versus the A(3) antagonist radioligand [3H]PSB-11 (8-ethyl-4-methyl-2-phenyl-(8R)-4,5,7,8-tetrahydro-1H-imidazo[2.1-i]purin-5-one) were right-shifted four-fold by 100 microM GTP, which had no effect on binding of CCPA or the antagonist MRS 1220 (N-[9-chloro-2-(2-furanyl)[1,2,4]triazolo[1,5-c]quinazolin-5-yl]benzene-acetamide). Thus, CCPA is a moderately potent antagonist (K(i)=38 nM) of the human A(3) adenosine receptor.

Role of the conserved DRY motif on G protein activation of rat angiotensin II receptor type 1A

To delineate the functional importance of the highly conserved triplet amino acid sequence, Asp-Arg-Tyr (DRY) among G protein-coupled receptors in the second intracellular loop, these residues of rat angiotensin II (Ang II) receptor type 1A (AT(1A)) were changed by alanine or glycine by site-directed mutagenesis. These mutant receptors were stably expressed in CHO-K1 cells, and the binding of Ang II, GTP effect, InsP(3) production, and the acidification of the medium in response to Ang II were determined. The effects of GTPgammaS on Ang II binding in the mutant receptors D125A and D125G were markedly reduced. InsP(3) production of the mutant D125A, D125G, R126A, and R126G was markedly reduced. Extracellular acidification of D125A was not distinguishable from untransfected CHO-K1 cells. Mutant Y127A was able to produce InsP(3) and acidify medium comparable with wild type AT(1A). These results indicate as follows; Asp(125) is essential for intracellular signal transduction involving G protein coupling, Arg(126) is essential for coupling of G(q) protein but not other G proteins, and Tyr(127) is not important for G protein coupling.

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.