Random mutagenesis of the human adenosine A2B receptor followed by growth selection in yeast. Identification of constitutively active and gain of function mutations

Cancer-Related Somatic Mutations in Transmembrane Helices Alter Adenosine A1 Receptor Pharmacology

Overexpression of the adenosine A₁ receptor (A₁AR) has been detected in various cancer cell lines. However, the role of A₁AR in tumor development is still unclear. Thirteen A₁AR mutations were identified in the Cancer Genome Atlas from cancer patient samples. We have investigated the pharmacology of the mutations located at the 7-transmembrane domain using a yeast system. Concentration-growth curves were obtained with the full agonist CPA and compared to the wild type hA₁AR. H78L^3.23 and S246T^6.47 showed increased constitutive activity, while only the constitutive activity of S246T^6.47 could be reduced to wild type levels by the inverse agonist DPCPX. Decreased constitutive activity was observed on five mutant receptors, among which A52V^2.47 and W188C^5.46 showed a diminished potency for CPA. Lastly, a complete loss of activation was observed in five mutant receptors. A selection of mutations was also investigated in a mammalian system, showing comparable effects on receptor activation as in the yeast system, except for residues pointing toward the membrane. Taken together, this study will enrich the view of the receptor structure and function of A₁AR, enlightening the consequences of these mutations in cancer. Ultimately, this may provide an opportunity for precision medicine for cancer patients with pathological phenotypes involving these mutations.

Molecular Effects of Auto-Antibodies on Angiotensin II Type 1 Receptor Signaling and Cell Proliferation

The angiotensin II (Ang II) type 1 receptor (AT1R) is involved in the regulation of blood pressure (through vasoconstriction) and water and ion homeostasis (mediated by interaction with the endogenous agonist). AT1R can also be activated by auto-antibodies (AT1R-Abs), which are associated with manifold diseases, such as obliterative vasculopathy, preeclampsia and systemic sclerosis. Knowledge of the molecular mechanisms related to AT1R-Abs binding and associated signaling cascade (dys-)regulation remains fragmentary. The goal of this study was, therefore, to investigate details of the effects of AT1R-Abs on G-protein signaling and subsequent cell proliferation, as well as the putative contribution of the three extracellular receptor loops (ELs) to Abs-AT1R signaling. AT1R-Abs induced nuclear factor of activated T-cells (NFAT) signaling, which reflects Gq/11 and Gi activation. The impact on cell proliferation was tested in different cell systems, as well as activation-triggered receptor internalization. Blockwise alanine substitutions were designed to potentially investigate the role of ELs in AT1R-Abs-mediated effects. First, we demonstrate that Ang II-mediated internalization of AT1R is impeded by binding of AT1R-Abs. Secondly, exclusive AT1R-Abs-induced Gq/11 activation is most significant for NFAT stimulation and mediates cell proliferation. Interestingly, our studies also reveal that ligand-independent, baseline AT1R activation of Gi signaling has, in turn, a negative effect on cell proliferation. Indeed, inhibition of Gi basal activity potentiates proliferation triggered by AT1R-Abs. Finally, although AT1R containing EL1 and EL3 blockwise alanine mutations were not expressed on the human embryonic kidney293T (HEK293T) cell surface, we at least confirmed that parts of EL2 are involved in interactions between AT1R and Abs. This current study thus provides extended insights into the molecular action of AT1R-Abs and associated mechanisms of interrelated pathogenesis.

Engineering a Model Cell for Rational Tuning of GPCR Signaling

G protein-coupled receptor (GPCR) signaling is the primary method eukaryotes use to respond to specific cues in their environment. However, the relationship between stimulus and response for each GPCR is difficult to predict due to diversity in natural signal transduction architecture and expression. Using genome engineering in yeast, we constructed an insulated, modular GPCR signal transduction system to study how the response to stimuli can be predictably tuned using synthetic tools. We delineated the contributions of a minimal set of key components via computational and experimental refactoring, identifying simple design principles for rationally tuning the dose response. Using five different GPCRs, we demonstrate how this enables cells and consortia to be engineered to respond to desired concentrations of peptides, metabolites, and hormones relevant to human health. This work enables rational tuning of cell sensing while providing a framework to guide reprogramming of GPCR-based signaling in other systems.

Getting from A to B-exploring the activation motifs of the class B adhesion G protein-coupled receptor subfamily G member 4/GPR112

The adhesion G protein-coupled receptors [ADGRs/class B2 G protein-coupled receptors (GPCRs)] constitute an ancient family of GPCRs that have recently been demonstrated to play important roles in cellular and developmental processes. Here, we describe a first insight into the structure-function relationship of ADGRs using the family member ADGR subfamily G member 4 (ADGRG4)/GPR112 as a model receptor. In a bioinformatics approach, we compared conserved, functional elements of the well-characterized class A and class B1 secretin-like GPCRs with the ADGRs. We identified several potential equivalent motifs and subjected those to mutational analysis. The importance of the mutated residues was evaluated by examining their effect on the high constitutive activity of the N-terminally truncated ADGRG4/GPR112 in a 1-receptor-1-G protein Saccharomyces cerevisiae screening system and was further confirmed in a transfected mammalian human embryonic kidney 293 cell line. We evaluated the results in light of the crystal structures of the class A adenosine A2A receptor and the class B1 corticotropin-releasing factor receptor 1. ADGRG4 proved to have functionally important motifs resembling class A, class B, and combined elements, but also a unique highly conserved ADGR motif (H3.33). Given the high conservation of these motifs and residues across the adhesion GPCR family, it can be assumed that these are general elements of ADGR function.-Peeters, M. C., Mos, I., Lenselink, E. B., Lucchesi, M., IJzerman, A. P., Schwartz, T. W. Getting from A to B-exploring the activation motifs of the class B adhesion G protein-coupled receptor subfamily G member 4/GPR112.

Differences in allosteric communication pipelines in the inactive and active states of a GPCR

G-protein-coupled receptors (GPCRs) are membrane proteins that allosterically transduce the signal of ligand binding in the extracellular (EC) domain to couple to proteins in the intracellular (IC) domain. However, the complete pathway of allosteric communication from the EC to the IC domain, including the role of individual amino acids in the pathway is not known. Using the correlation in torsion angle movements calculated from microseconds-long molecular-dynamics simulations, we elucidated the allosteric pathways in three different conformational states of β2-adrenergic receptor (β2AR): 1), the inverse-agonist-bound inactive state; 2), the agonist-bound intermediate state; and (3), the agonist- and G-protein-bound fully active state. The inactive state is less dynamic compared with the intermediate and active states, showing dense clusters of allosteric pathways (allosteric pipelines) connecting the EC with the IC domain. The allosteric pipelines from the EC domain to the IC domain are weakened in the intermediate state, thus decoupling the EC domain from the IC domain and making the receptor more dynamic compared with the other states. Also, the orthosteric ligand-binding site becomes the initiator region for allosteric communication in the intermediate state. This finding agrees with the paradigm that the nature of the agonist governs the specific signaling state of the receptor. These results provide an understanding of the mechanism of allosteric communication in class A GPCRs. In addition, our analysis shows that mutations that affect the ligand efficacy, but not the binding affinity, are located in the allosteric pipelines. This clarifies the role of such mutations, which has hitherto been unexplained.

A role for picomolar concentrations of pregnenolone sulfate in synaptic activity-dependent Ca2+ signaling and CREB activation

Fast excitatory synaptic transmission that is contingent upon N-methyl d-aspartate receptor (NMDAR) function contributes to core information flow in the central nervous system and to the plasticity of neural circuits that underlie cognition. Hypoactivity of excitatory NMDAR-mediated neurotransmission is hypothesized to underlie the pathophysiology of schizophrenia, including the associated cognitive deficits. The neurosteroid pregnenolone (PREG) and its metabolites pregnenolone sulfate (PregS) and allopregnanolone in serum are inversely associated with cognitive improvements after oral PREG therapy, raising the possibility that brain neurosteroid levels may be modulated therapeutically. PregS is derived from PREG, the precursor of all neurosteroids, via a single sulfation step and is present at low nanomolar concentrations in the central nervous system. PregS, but not PREG, augments long-term potentiation and cognitive performance in animal models of learning and memory. In this report, we communicate the first observation that PregS, but not PREG, is a potent (EC50 ∼2 pM) enhancer of intracellular Ca(2+) that is contingent upon neuronal activity, NMDAR-mediated synaptic activity, and L-type Ca(2+) channel activity. Low picomolar PregS similarly activates cAMP response element-binding protein (CREB) phosphorylation (within 10 minutes), an essential memory molecule, via an extracellular-signal-regulated kinase/mitogen-activated protein kinase signal transduction pathway. Taken together, the results are consistent with a novel biologic role for the neurosteroid PregS that acts at picomolar concentrations to intensify the intracellular response to glutamatergic signaling at synaptic but not extrasynaptic, NMDARs by differentially augmenting CREB activation. This provides a genomic signal transduction mechanism by which PregS could participate in memory consolidation of relevance to cognitive function.

The role of the second and third extracellular loops of the adenosine A1 receptor in activation and allosteric modulation

The adenosine A1 receptor is a member of the large membrane protein family that signals through G proteins, the G protein-coupled receptors (GPCRs). GPCRs consist of seven transmembrane domains connected by three intracellular and three extracellular loops. Their N-terminus is extracellular, the C-terminal tail is in the cytoplasm. The transmembrane domains in receptor subfamilies that bind the same endogenous ligand, such as dopamine or adenosine, tend to be highly similar. In contrast, the loop regions can vary greatly, both in sequence and in length, and the role these loops have in the activation mechanism of the receptors remains unclear. Here, we investigated the activating role of the second and third extracellular loop of the human adenosine A1 receptor. By means of an (Ala)3 mutagenic scan in which consecutive sets of three amino acids were mutated into alanine residues in EL2 and a classical alanine scan in EL3, we revealed a strong regulatory role for the second extracellular loop (EL2) of the human adenosine A1 receptor. Besides many residues in the second and the third extracellular loops important for adenosine A1 receptor activation, we also identified two residues in EL2, a tryptophan and a glutamate, that affect the influence of the allosteric modulator PD81,723. These results, combined with a comparison of the different receptor loop regions, provide insight in the activation mechanism of this typical class A GPCR and further emphasize the unique pharmacological profile the loops can provide to individual receptors, even within subfamilies of GPCRs.

Three "hotspots" important for adenosine A(2B) receptor activation: a mutational analysis of transmembrane domains 4 and 5 and the second extracellular loop

G protein-coupled receptors (GPCRs) are a major drug target and can be activated by a range of stimuli, from photons to proteins. Despite the progress made in the last decade in molecular and structural biology, their exact activation mechanism is still unknown. Here we describe new insights in specific regions essential in adenosine A_2B receptor activation (A_2BR), a typical class A GPCR. We applied unbiased random mutagenesis on the middle part of the human adenosine A_2BR, consisting of transmembrane domains 4 and 5 (TM4 and TM5) linked by extracellular loop 2 (EL2), and subsequently screened in a medium-throughput manner for gain-of-function and constitutively active mutants. For that purpose, we used a genetically engineered yeast strain (Saccharomyces cerevisiae MMY24) with growth as a read-out parameter. From the random mutagenesis screen, 12 different mutant receptors were identified that form three distinct clusters; at the top of TM4, in a cysteine-rich region in EL2, and at the intracellular side of TM5. All mutant receptors show a vast increase in agonist potency and most also displayed a significant increase in constitutive activity. None of these residues are supposedly involved in ligand binding directly. As a consequence, it appears that disrupting the relatively “silent” configuration of the wild-type receptor in each of the three clusters readily causes spontaneous receptor activity.

Signaling of the Human P2Y(1) Receptor Measured by a Yeast Growth Assay with Comparisons to Assays of Phospholipase C and Calcium Mobilization in 1321N1 Human Astrocytoma Cells

The human P2Y(1) receptor was expressed in the yeast Saccharomyces cerevisiae strain MPY578q5, which is engineered to couple to mammalian G protein-coupled receptors (GPCRs) and requires agonist-induced activation for growth. A range of known P2Y(1) receptor agonists were examined with the yeast growth assay system, and the results were validated by comparing with potencies in the transfected 1321N1 astrocytoma cell line, in which calcium mobilization was measured with a FLIPR (fluorescence-imaging plate reader). The data were also compared with those from phospholipase C activation and radioligand binding with the use of a newly available radioligand [H]MRS2279 (2-chloro- N-methyl-(N)-methanocarba-2'-deoxyadenosine-3',5'-bisphosphate). In the yeast growth assay, the rank order of potency of 2-MeSADP (2-methylthioadenosine 5'-diphosphate), ADP (adenosine 5'-diphosphate), and ATP (adenosine 5'-triphosphate) is the same as those in other assay systems, i.e., 2-MeSADP>ADP>ATP. The P2Y(1)-selective antagonist MRS2179 (N-methyl-2-deoxyadenosine-3',5'-bisphosphate) was shown to act as an antagonist with similar potency in all systems. The results suggest that the yeast expression system is suitable for screening P2Y(1) receptor ligands, both agonists and antagonists. The yeast system should be useful for random mutagenesis of GPCRs to identify mutants with certain properties, such as selective potency enhancement for small synthetic molecules and constitutive activity.

The four cysteine residues in the second extracellular loop of the human adenosine A2B receptor: role in ligand binding and receptor function

The adenosine A(2B) receptor is of considerable interest as a new drug target for the treatment of asthma, inflammatory diseases, pain, and cancer. In the present study we investigated the role of the cysteine residues in the extracellular loop 2 (ECL2) of the receptor, which is particularly cysteine-rich, by a combination of mutagenesis, molecular modeling, chemical and pharmacological experiments. Pretreatment of CHO cells recombinantly expressing the human A(2B) receptor with dithiothreitol led to a 74-fold increase in the EC(50) value of the agonist NECA in cyclic AMP accumulation. In the C78(3.25)S and the C171(45.50)S mutant high-affinity binding of the A(2B) antagonist radioligand [(3)H]PSB-603 was abolished and agonists were virtually inactive in cAMP assays. This indicates that the C3.25-C45.50 disulfide bond, which is highly conserved in GPCRs, is also important for binding and function of A(2B) receptors. In contrast, the C166(45.45)S and the C167(45.46)S mutant as well as the C166(45.45)S-C167(45.46)S double mutant behaved like the wild-type receptor, while in the C154(45.33)S mutant significant, although more subtle effects on cAMP accumulation were observed - decrease (BAY60-6583) or increase (NECA) - depending on the structure of the investigated agonist. In contrast to the X-ray structure of the closely related A(2A) receptor, which showed four disulfide bonds, the present data indicate that in the A(2B) receptor only the C3.25-C45.50 disulfide bond is essential for ligand binding and receptor activation. Thus, the cysteine residues in the ECL2 of the A(2B) receptor not involved in stabilization of the receptor structure may have other functions.

Expression of human melanocortin 4 receptor in Saccharomyces cerevisiae

The melanocortin 4 receptor (MC4R) is involved in the regulation of energy homeostasis and is known as one of the major hypothalamic regulators of food intake. Several studies have shown that replacement of aspartic acid at position 126 of the MC4R abolishes the ligand binding. We used the modified yeast Saccharomyces cerevisiae strain MMY28 to functionally express the MC4R and characterise the importance of this amino acid for ligand based activation of the receptor. The efficiency of the functional expression system was estimated by activation with αMSH, ACTH and THIQ and compared with cAMP response in mammalian cells. We generated the library of MC4R mutants randomised at the amino acid position 126. Recombinant MC4R clones were screened for the αMSH induced activity in yeast. From 9 different amino acids obtained only the natural aspartic acid displayed the ligand dependent activity of MC4R. The MC4R variants with glutamic acid and leucine at position 126, however, displayed higher background activity than other amino acid substitutions. The results suggest that the yeast expression system is suitable for screening of the MC4R receptor ligands and that the substitution of aspartic acid at position 126 of MC4R by different amino acids functionally inactivates the receptor.

The resurgence of A2B adenosine receptor signaling

Since its discovery as a low-affinity adenosine receptor (AR), the A2B receptor (A2BAR), has proven enigmatic in its function. The previous discovery of the A2AAR, which shares many similarities with the A2BAR but demonstrates significantly greater affinity for its endogenous ligand, led to the original perception that the A2BAR was not of substantial physiologic relevance. In addition, lack of specific pharmacological agents targeting the A2BAR made its initial characterization challenging. However, the importance of this receptor was reconsidered when it was observed that the A2BAR is highly transcriptionally regulated by factors implicated in inflammatory hypoxia. Moreover, the notion that during ischemia or inflammation extracellular adenosine is dramatically elevated to levels sufficient for A2BAR activation, indicated that A2BAR signaling may be important to dampen inflammation particularly during tissue hypoxia. In addition, the recent advent of techniques for murine genetic manipulation along with development of pharmacological agents with enhanced A2BAR specificity has provided invaluable tools for focused studies on the explicit role of A2BAR signaling in different disease models. Currently, studies performed with combined genetic and pharmacological approaches have demonstrated that A2BAR signaling plays a tissue protective role in many models of acute diseases e.g. myocardial ischemia, or acute lung injury. These studies indicate that the A2BAR is expressed on a wide variety of cell types and exerts tissue/cell specific effects. This is an important consideration for future studies where tissue or cell type specific targeting of the A2BAR may be used as therapeutic approach.

Afferent arteriolar vasodilator effect of adenosine predominantly involves adenosine A2B receptor activation

Adenosine is an important paracrine agent regulating renal vascular tone via adenosine A(1) and A(2) receptors. While A(2B) receptor message and protein have been localized to preglomerular vessels, functional evidence on the role of A(2B) receptors in mediating the vasodilator action of adenosine on afferent arterioles is not available. The present study determined the role of A(2B) receptors in mediating the afferent arteriolar dilation and compared the effects of A(2B) and A(2A) receptor blockade on afferent arterioles. We used the rat in vitro blood-perfused juxtamedullary nephron technique combined with videomicroscopy. Single afferent arterioles of Sprague-Dawley rats were visualized and superfused with solutions containing adenosine or adenosine A(2) receptor agonist (CV-1808) along with adenosine A(2B) and A(2A) receptor blockers. Adenosine (10 micromol/l) caused modest constriction and subsequent superfusion with SCH-58261 (SCH), an A(2A) receptor blocker, at concentrations up 10 micromol/l elicited only slight additional decreases in afferent arteriolar diameter with maximum effect at a concentration of 1 micromol/l (-11.0 +/- 2.5%, n = 6, P < 0.05). However, superfusion of adenosine-treated vessels with MRS-1754 (MRS), an A(2B) receptor blocker, elicited greater decreases in afferent arteriolar diameter (-26.0 +/- 4.7%, n = 5, P < 0.01). SCH did not significantly augment the adenosine-mediated afferent constriction elicited by MRS; however, adding MRS after SCH caused further significant vasoconstriction. Superfusion with CV-1808 dilated afferent arterioles (17.2 +/- 2.4%, n = 6, P < 0.01). This effect was markedly attenuated by MRS (-22.6 +/- 2.0%, n = 5, P < 0.01) but only slightly reduced by SCH (-9.0 +/- 1.1%, n = 5, P < 0.05) and completely prevented by adding MRS after SCH (-24.7 +/- 1.8%, n = 5, P < 0.01). These results indicate that, while both A(2A) and A(2B) receptors are functionally expressed in juxtamedullary afferent arterioles, the powerful vasodilating action of adenosine predominantly involves A(2B) receptor activation, which counteracts A(1) receptor-mediated vasoconstriction.

Homology modelling of the human adenosine A2B receptor based on X-ray structures of bovine rhodopsin, the beta2-adrenergic receptor and the human adenosine A2A receptor

A three-dimensional model of the human adenosine A2B receptor was generated by means of homology modelling, using the crystal structures of bovine rhodopsin, the beta2-adrenergic receptor, and the human adenosine A2A receptor as templates. In order to compare the three resulting models, the binding modes of the adenosine A2B receptor antagonists theophylline, ZM241385, MRS1706, and PSB601 were investigated. The A2A-based model was much better able to stabilize the ligands in the binding site than the other models reflecting the high degree of similarity between A2A and A2B receptors: while the A2B receptor shares about 21% of the residues with rhodopsin, and 31% with the beta2-adrenergic receptor, it is 56% identical to the adenosine A2A receptor. The A2A-based model was used for further studies. The model included the transmembrane domains, the extracellular and the intracellular hydrophilic loops as well as the terminal domains. In order to validate the usefulness of this model, a docking analysis of several selective and nonselective agonists and antagonists was carried out including a study of binding affinities and selectivities of these ligands with respect to the adenosine A2A and A2B receptors. A common binding site is proposed for antagonists and agonists based on homology modelling combined with site-directed mutagenesis and a comparison between experimental and calculated affinity data. The new, validated A2B receptor model may serve as a basis for developing more potent and selective drugs.

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.

Characterization of the relaxant response to N,N'-dipropyl-1,2-bis(2,6-dichloro-4-hydroxyphenyl)ethylenediamine in porcine coronary arteries

N,N'-Dialkyl-1,2-bis(2,6-dichloro-4-hydroxyphenyl)ethylenediamines show structural analogy with estrogens and selective estrogen receptor modulators. Because the vasodilator properties of these compounds are unknown, we investigated their potential to relax porcine coronary arteries and determined the mechanism(s) of relaxation. Isolated porcine coronary arterial rings were suspended in organ chambers, precontracted with KCl (30 mM), and the relaxant response was determined by measurement of changes in isometric force. Dependent on the chemical structure, the drugs induced concentration-dependent relaxation in rings with and without endothelium. N,N'-Dipropyl-1,2-bis(2,6-dichloro-4-hydroxyphenyl)ethylenediamine (8) was most potent and showed a 12- to 15-fold higher vasodilatory effect than 17beta-estradiol (E2). The vasorelaxation was independent of endothelium. Calcium concentration-dependent contractions in high-potassium depolarizing medium were insurmountably inhibited by 8. The effect of the L-type Ca2+ channel activator (S)-(-)-Bay K 8644 [(S)-(-)-1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)phenyl]-3-pyridine-carboxylic acid methyl ester], which induced a leftward shift of Ca2+ contraction, was blocked by 8. The relaxant response to 8 was unaffected by the estrogen receptor antagonist ICI 182,780 (7alpha-[9-[(4,4,5,5,5-pentafluoropentyl]-sulfinyl]nonyl]-estra-1,3,5(10)-triene-3,17beta-diol) and K+ channel blockers, i.e., TEA, glibenclamide, and 4-aminopyridine. Furthermore, the vasodilatory effect of 8 was unaffected by the adenylyl cyclase inhibitor SQ 22536 [9-(tetrahydro-2-furanyl)-9H-purin-6-amine], the guanylyl cyclase inhibitor ODQ [1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one], the protein kinase A inhibitor KT 5720 [(9S,10S,12R)-2,3,9,10,11,12-hexahydro-10-hydroxy-9-methyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg: 3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid hexyl ester], the protein kinase G inhibitor KT 5823 [(9S,10R,12R)-2,3,9,10,11,12-hexahydro-10-methoxy-2,9-dimethyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid methyl ester], and the p38 mitogen-activated protein kinase (MAPK) inhibitor SB 203580 [4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)-1H-imidazole]. Western blot analysis demonstrated that 8, unlike E2, raloxifene, and tamoxifen, failed to stimulate p38 MAPK. It is concluded that N,N'-dipropyl-1,2-bis(2,6-dichloro-4-hydroxyphenyl)ethylenediamine induces endothelium-independent relaxation of coronary arteries; the mechanism apparently involves inhibition of L-type Ca2+ channels. The drug may be protective against cardiovascular diseases.

Neoceptors: reengineering GPCRs to recognize tailored ligands

Efforts to model and reengineer the putative binding sites of G-protein-coupled receptors (GPCRs) have led to an approach that combines small-molecule 'classical' medicinal chemistry and gene therapy. In this approach, complementary structural changes (e.g. based on novel ionic or H-bonds) are made in the receptor and ligand for the selective enhancement of affinity. Thus, a modified receptor (neoceptor) is designed for activation by tailor-made agonists that do not interact with the native receptor. The neoceptor is no longer activated by the native agonist, but rather functions as a scaffold for the docking of novel small molecules (neoligands). In theory, the approach could verify the accuracy of GPCR molecular modeling, the investigation of signaling, the design of small molecules to rescue disease-related mutations, and small-molecule-directed gene therapy. The neoceptor-neoligand pairing could offer spatial specificity by delivering the neoceptor to a target site, and temporal specificity by administering neoligand when needed.

Computer aided comparative analysis of the binding modes of the adenosine receptor agonists for all known subtypes of adenosine receptors

Molecular models of all known subtypes (A1, A2A, A2B, and A3) of the human adenosine receptors were built in homology with bovine rhodopsin. These models include the transmembrane domain as well as all extracellular and intracellular hydrophilic loops and terminal domains. The molecular docking of adenosine and 46 selected derivatives was performed for each receptor subtype. A binding mode common for all studied agonists was proposed, and possible explanations for differences in the ligand activities were suggested.

ZM241385, DPCPX, MRS1706 are inverse agonists with different relative intrinsic efficacies on constitutively active mutants of the human adenosine A2B receptor

The human adenosine A(2B) receptor belongs to class A G protein-coupled receptors (GPCRs). In our previous work, constitutively active mutant (CAM) human adenosine A(2B) receptors were identified from a random mutation bank. In the current study, three known A(2B) receptor antagonists, 4-{2-[7-amino-2-(2-furyl)[1,2,4]triazolo-[2,3-a][1,3,5]triazin-5-yl-amino]ethyl}phenol (ZM241385), 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), and N-(4-acetylphenyl)-2-[4-(2,3,6,7-tetrahydro-2,6-dioxo-1,3-dipropyl-1H-purin-8-yl)phenoxy]acetamide (MRS1706) were tested on wild-type and nine CAM A(2B) receptors with different levels of constitutive activity in a yeast growth assay. All three compounds turned out to be inverse agonists for the adenosine A(2B) receptor because they were able to fully reverse the basal activity of four low-level constitutively active A(2B) receptor mutants and to partially reverse the basal activity of three medium-level constitutively active A(2B) receptor mutants. We also discovered two highly constitutively active mutants whose basal activity could not be reversed by any of the three compounds. A two-state receptor model was used to explain the experimental observations; fitting these yielded the following relative intrinsic efficacies for the three inverse agonists ZM241385, DPCPX, and MRS1706: 0.14 +/- 0.03, 0.35 +/- 0.03, and 0.31 +/- 0.02, respectively. Moreover, varying L, the ratio of active versus inactive receptors in this model, from 0.11 for mutant F84L to 999 for two highly constitutively active mutants yielded simulated dose-response curves that mimicked the experimental curves. This study is the first description of inverse agonists for the human adenosine A(2B) receptor. Moreover, the use of receptor mutants with varying levels of constitutive activity enabled us to determine the relative intrinsic efficacy of these inverse agonists.

Pharmacological and Electrophysiological Characterization of Store-Operated Currents and Capacitative Ca2+Entry in Vascular Smooth Muscle Cells

Capacitative Ca(2+) entry (CCE) in vascular smooth muscle cells contributes to vasoconstrictor and mitogenic effects of vasoactive hormones. In A7r5 rat aortic smooth muscle cells, measurements of cytosolic free Ca(2+) concentration ([Ca(2+)](i)) have demonstrated that depletion of intracellular Ca(2+) stores activates CCE. However, there is disagreement in published studies regarding the regulation of this mechanism by the vasoconstrictor hormone [Arg(8)]-vasopressin (AVP). We have employed electrophysiological methods to characterize the membrane currents activated by store depletion [store-operated current (I(SOC))]. Because of different recording conditions, it has not been previously determined whether I(SOC) corresponds to CCE measured using fura-2; nor has the channel protein responsible for CCE been identified. In the present study, the pharmacological characteristics of I(SOC), including its sensitivity to blockade by 2-aminoethoxydiphenylborane, diethylstilbestrol, or micromolar Gd(3+), were found to parallel the effects of these drugs on thapsigargin- or AVP-activated CCE measured under identical external ionic conditions using fura-2. Thapsigargin-stimulated I(SOC) was also measured in freshly isolated rat mesenteric artery smooth muscle cells (MASMC). Members of the transient receptor potential (TRP) family of nonselective cation channels, TRPC1, TRPC4, and TRPC6, were detected by reverse transcription-polymerase chain reaction and Western blot in both A7r5 cells and MASMC. TRPC1 expression was reduced in a stable A7r5 cell line expressing a small interfering RNA (siRNA) or by infection of A7r5 cells with an adenovirus expressing a TRPC1 antisense nucleotide sequence. Thapsigargin-stimulated I(SOC) was reduced in both the TRPC1 siRNA- and TRPC1 antisense-expressing cells, suggesting that the TRPC1 channel contributes to the I(SOC)/CCE pathway.

Allosteric modulators affect the internalization of human adenosine A1 receptors

To study the effect of allosteric modulators on the internalization of human adenosine A(1) receptors, the receptor was equipped with a C-terminal yellow fluorescent protein tag. The introduction of this tag did not affect the radioligand binding properties of the receptor. CHO cells stably expressing this receptor were subjected during 16 h to varying concentrations of the agonist N(6)-cyclopentyladenosine (CPA) in the absence or presence of 10 microM of the allosteric enhancer PD 81,723 ((2-amino-4,5-dimethyl-3-thienyl)-[3-(trifluoromethyl)phenyl]methanone) or the allosteric inhibitor SCH-202676 (N-(2,3-diphenyl-1,2,4-thiadiazol-5(2H)-ylidene)methanamine). CPA itself was able to internalize 25% and 40% of the receptors at a concentration of 400 nM or 4 muM, respectively. Addition of either PD 81,723 or SCH-202676 alone had no effect on internalization. However, with PD 81,723 a slight amount of internalization was obtained already at 40 nM of CPA and at 400 nM CPA 59% of the receptors internalized. SCH-202676 on the other hand effectively prevented CPA-induced internalization of the receptor.

Techniques: How to boost GPCR mutagenesis studies using yeast

G-protein-coupled receptors (GPCRs) are the major targets of today's medicines. To elucidate the mechanism of activation of GPCRs and the interaction of these receptors with their G proteins, mutagenesis studies have proven to be a powerful tool and have provided insight into the structure and function of GPCRs. Random mutagenesis is useful in this respect particularly when combined with a robust screening assay that is based on the functional properties of the mutants. In this article, the use of random mutagenesis combined with a functional screening assay in yeast is described and compared with alternative approaches such as site-directed mutagenesis per se, alanine/cysteine scanning and another screening assay, receptor selection and amplification technology (R-SAT). Screening in yeast of randomly mutated GPCRs has proven successful in the identification of ligands for orphan receptors and in high-throughput approaches. Moreover, it has provided substantial insight into G-protein coupling and receptor activation.

Functional analysis of heterologous GPCR signalling pathways in yeast

G protein-coupled receptors (GPCRs) regulate diverse biological processes in eukaryotes and such conservation allows an almost unrestricted interchange of signalling components between different cell types. Yeasts are attractive hosts in which to study GPCRs--they are amenable to both genetic and biochemical manipulation and their robustness, low cost and our ability to create strains that lack endogenous GPCRs make them ideal starting points for the development of assays suitable for high-throughput screening. Here we introduce readers to the possibilities of using yeast to analyse GPCRs describing the endogenous signalling pathways, the development of assays for heterologous GPCRs and the technology to elucidate GPCR structure and activity, focusing on the budding yeast Saccharomyces cerevisiae and recent developments using the fission yeast Schizosaccharomyces pombe.

A constitutively active GPCR retains its G protein specificity and the ability to form dimers

G protein-coupled receptors (GPCRs) are cell surface proteins which help to regulate the physiology of all the major organ systems within higher eukaryotes. They are stimulated by multiple ligands and activate a range of effector molecules to bring about changes in cell behaviour. The use of constitutively active mutants (CAMs) of GPCRs has enabled a better understanding of receptor activation as CAMs exhibit ligand-independent signalling negating the use of ligands. Here we introduce the fission yeast Schizosaccharomyces pombe as a host for producing CAMs, by describing the isolation and characterization of constitutive mutants of the P-factor receptor (Mam2). One mutant Mam2[P261L] contained a single-amino-acid substitution (Pro261 to Leu) within a region of high homology in GPCRs. Substitution of this proline leads to an 18-fold increase in ligand-independent signalling. We utilized Mam2[P261L] to investigate CAM activity by demonstrating that Mam2[P261L] is efficiently trafficked to the cell surface where it can form fully functional oligomeric complexes with the native receptor. Mam2[P261L] also retains the G protein specificity (RG-profile) of the native receptor and only induces constitutive signalling in the same G proteins. Finally, evidence is provided to indicate that CAM activity results from a reduction in the kinetics of G protein binding. This is the first time that S. pombe has been utilized for isolating and characterizing CAMs and the techniques employed will complement the current systems available for studying these important receptors.

Development of a semi-quantitative plate-based ?-galactosidase gene reporter forSchizosaccharomyces pombe and its use to isolate a constitutively active Mam2

To extend the tools available for biochemical and genetical analysis in the fission yeast Schizosaccharomyces pombe we have investigated the development of gene reporter systems using the secreted alpha-galactosidase encoded by the Sz. pombe ORF SPAC869.07c (CAB60017), which we propose naming Mel1p to reflect its structural and functional similarity to MEL1p in Saccharomyces cerevisiae. The alpha-galactosidase activity can be monitored in liquid assays and converted the colourless substrate 5-bromo-4-chloro-3-indolyl-alpha-D-galactopyranoside (X-alpha-gal) into an insoluble blue product that was suitable for semi quantitative plate-based assays; colonies expressing the highest levels of alpha-galactosidase developed the most intense blue colour. Unlike assays based on beta-galactosidase, the Sz. pombe colonies develop the blue colouration under normal growth conditions, avoiding the need to replicate colonies to fresh plates for analysis. It is therefore suitable for screening large numbers of colonies. To illustrate the use of mel1 as a reporter we linked expression to the sxa2 gene promoter to provide a convenient readout for signalling through the pheromone response pathway. The sxa2 > mel1 strain identified constitutively active Mam2 pheromone receptors from a randomly mutagenised library. There was an approximate correlation between the intensity of the blue colour developed by each mutant colony and its level of constitutive activity and we identified a subset of mutants with low constitutive activity that could not have been isolated by a previous screen using nutritional selection. The mel1 alpha-galactosidase activity identified and characterised in this study can be easily adapted to provide a gene reporter for many biological processes and is a new addition to the research tools available in Sz. pombe.