Extracellular domains of the bradykinin B2 receptor involved in ligand binding and agonist sensing defined by anti-peptide antibodies

Agonist-induced redistribution of bradykinin B2 receptor in caveolae

Redistribution of receptors within the plasma membrane as well as between the plasma membrane and various cell compartments presents an important way of regulating the cellular responsiveness to their cognate agonists. We have applied immunocytochemical methods to localize the bradykinin B2 receptor and to examine its agonist induced redistribution in A431 cells. In situ labeling with antibodies to ectodomain-2 of the receptor which do not interfere with bradykinin binding of the receptor showed a random distribution of the B2 receptor on the plasma membrane. Stimulation of cells with 20 nM bradykinin markedly reduced the accessibility of the antibody to its corresponding epitope in non-permeabilized cells. Immuno-electron microscopy revealed the presence of receptors in membrane-near vesicles that are surrounded by an electron-transparent halo. Fluorescence microscopic double labeling co-localized the B2 receptor protein with caveolin-1 by a convergent pattern of punctate staining. At the ultrastructural level the B2 receptor protein was found in vesicles that bear the immunolabel of caveolin-1 and display the morphological characteristics of caveolae. We conclude that stimulation of B2 receptors results in their redistribution and sequestration in caveolae, an event that is likely to be implicated in receptor signaling and/or desensitization. The localization of B2 receptors in endosome-like structures after prolonged exposure to bradykinin might indicate that the internalization through caveolae may communicate with other endocytotic pathways of A431 cells.

Kinin receptors

Rapid developments are expected in the molecular pharmacology of both B1, and B2 types of kinin receptors, since the underlying genetic structures are now known and widely studied. The consequences of kinin receptor duality and physiopathological regulation have not yet been fully appreciated. Medicinal chemistry is also an active front of research in kinin pharmacology, as more effective drugs targeted at kinin receptors are regularly reported. Various complementary molecular approaches (the receptor binding, cloning, immunoreacting, mutagenesis, inactivation, the study of regulation, allelic polymorphisms, and so forth) are expanding our knowledge of the role of kinins in allergy, inflammation, and singularly, renal medicine.

Characterization of non-peptide antagonist and peptide agonist binding sites of the NK1 receptor with fluorescent ligands

Ligand recognition of the NK1 receptor (substance P receptor) by peptide agonist and non-peptide antagonist has been investigated and compared by the use of fluorescent ligands and spectrofluorometric methods. Analogues of substance P (SP) labeled with the environment-sensitive fluorescent group 5-dimethylaminonaphthalene-1-sulfonyl (dansyl) at either position 3, 8, or 11 or with fluorescein at the Nalpha position were synthesized and characterized. Peptides modified at the alpha-amino group or at positions 3 or 11 conserved a relatively good affinity for NK1 and agonistic properties. Modification at position 8 resulted in an 18, 000-fold decrease in affinity. A fluorescent dansyl analogue of the non-peptide antagonist CP96,345 was prepared and characterized. The quantum yield of fluorescence for dansyl-CP96,345 was much higher than for any of the dansyl-labeled peptides indicating that the micro-environment of the binding site is more hydrophobic for the non-peptide antagonist than for the peptide agonists. Comparison of collisional quenching of fluorescence by the water-soluble hydroxy-Tempo compound showed that dansyl-CP96,345 is buried and virtually inaccessible to aqueous quenchers, whereas dansyl- or fluoresceinyl-labeled peptides were exposed to the solvent. Anisotropy of all fluorescent ligands increased upon binding to NK1 indicating a restricted motional freedom. However, this increase in anisotropy was more pronounced for the dansyl attached to the non-peptide antagonist CP96,345 than for the fluorescent probes attached to different positions of SP. In conclusion, our data indicate that the environment surrounding non-peptide antagonist and peptide agonists are vastly different when bound to the NK1 receptor. These results support recent observations by mutagenesis and cross-linking work suggesting that peptide agonists have their major interaction points in the N-terminal extension and the loops forming the extracellular face of the NK1 receptor. Our data also suggest that neither the C terminus nor the N terminus of SP appears to penetrate deeply below the extracellular surface in the transmembrane domain of the receptor.

Modes of peptide binding in G protein-coupled receptors

The G-protein coupled seven transmembrane domain receptors bind a wide variety of ligands of different molecular size ranging from small monoamines to large neuropeptides and peptide hormones. This review summarises data from studies on the localisation of the binding site for a few neuropeptides in their receptors and compares this to the binding pockets for non peptide ligands. The main conclusion is that neuropeptide binding involves residues on the top of several transmembrane domains and in extracellular loops of the receptors while the non peptide type ligands to the same receptors tend to bind deeper in the plane of the membrane, between several transmembrane domains--similarly to monoamines. Thus the antagonism exerted by most of the non peptide type ligands is an allosteric phenomenon whereby binding of these to another site than the peptide binding site stablises a "non agonist" binding, and for signalling inactive, conformation of the 7 TM receptor.

Bradykinin antagonists in human systems: correlation between receptor binding, calcium signalling in isolated cells, and functional activity in isolated ileum

The determination of the relationship between ligand affinity and bioactivity is important for the understanding of receptor function in biological systems and for drug development. Several physiological and pathophysiological functions of bradykinin (BK) are mediated via the B2 receptor. In this study, we have examined the relationship between B2 receptor (soluble and membrane-bound) binding of BK peptidic antagonists, inhibition of calcium signalling at a cellular level, and in vitro inhibition of ileum contraction. Only human systems were employed in the experiments. Good correlations between the studied activities of BK antagonists were observed for a variety of different peptidic structures. The correlation coefficients (r) were in the range of 0.905 to 0.955. In addition, we analyzed the effect of the C-terminal Arg9 removal from BK and its analogs on B2 receptor binding. The ratios of binding constants (Ki(+Arg)/Ki(-Arg)) for the Arg9 containing compounds and the corresponding des-Arg9 analogs varied from about 10 to 250,000. These ratios strongly depend on the chemical structures of the compounds. The highest ratios were observed for two natural agonist pairs, BK/des-Arg9-BK and Lys0-BK/des-Arg9-Lys0-BK.

Localization of the bradykinin B2 receptor in uterus, bladder and Madin-Darby canine kidney cells

Kinins are biologically active peptides that act through specific receptors, B1 and B2. Here we describe the localization of the bradykinin B2 receptor in Madin-Darby canine kidney cells and in the uterus and urinary bladder of rat or human origin. We discuss the suitability of anti-peptide antibodies to assess the tissue distribution of bradykinin B2 receptors.

Evidence for bradykinin B2-receptors on cultured human decidua cells

Bradykinin is known to be present at sites of acute inflammation and to exert its potent inflammatory effects mainly via the bradykinin B2-receptor. Recently, bradykinin dependent processes have been described in cultured human decidual cells, so that bradykinin may expand the list of paracrine factors involved in labour induction. In this paper we present the results of in vitro studies giving evidence that these cells carry the bradykinin B2-receptor. By immunocytochemical methods the receptor protein was localized on decidual cells. Analysis of cellular extracts of cultured decidual cells by RT-PCR showed the presence of the specific mRNA coding for the bradykinin B2-receptor. Binding studies revealed a single, saturable and specific binding site for bradykinin of high affinity (Kd = 0.85 nM, Bmax = 436 fmol/mg protein). Competitive binding studies showed displacement of [3H]-bradykinin by HOE 140, but not by the ligands for the bradykinin B1-receptor, des-Arg10-kallidin and [Leu8]-des-Arg9-bradykinin. The results are consistent with the presence of the bradykinin B2-receptors.

Cloning and functional analysis of a polymorphic variant of the ovine Mel 1a melatonin receptor

We have isolated a novel variant of the Mel 1a melatonin receptor from an ovine PT cDNA library. Relative to the reported sequence for the Mel 1a melatonin receptor there are 8 changes in the DNA sequence. Only 3 of these result in amino acid substitutions, one in extracellular loop 3 and two in the carboxy-terminal tail. We have designated the novel variant of the sheep Mel 1a receptor Mel 1a(beta), and correspondingly the previously reported variant Mel 1a(alpha). As minor changes in the primary amino acid sequence of G-protein-coupled receptors can influence their functional characteristics we have accordingly characterized this novel variant of the Mel 1a melatonin receptor. This melatonin receptor displays high affinity binding and inhibits the cAMP second messenger pathway in transfected L-cells demonstrating that this receptor is fully functional. PCR analysis shows Mel 1a(beta) is present in several breeds of sheep and suggests that the Mel 1a(beta) receptor was established early in the evolution of the sheep species.

Cloning and functional characterization of the ornithokinin receptor. Recognition of the major kinin receptor antagonist, HOE140, as a full agonist

Kinins are proinflammatory peptides that dilate vessels, increase vascular permeability, contract smooth muscles, and provoke pain. The known mammalian kinin receptors are classified as two subtypes, i.e. the B1 receptor triggered by [des-Arg9]bradykinin and inhibited by [des-Arg9,Leu8]bradykinin, and the B2 receptor stimulated by bradykinin and antagonized by HOE140. Here we report the cloning of a non-mammalian kinin receptor gene amplified from genomic chicken DNA. The protein predicted from the open reading frame shows 31 and 49% sequence identity to the human B1 and B2 receptors, respectively, suggesting that it represents a G protein-coupled receptor of the kinin receptor family. The recombinantly expressed chicken receptor had IC50 values of 4.7 nM for the authentic ligand, ornithokinin ([Thr6,Leu8]bradykinin), 3.8 nM for HOE140, and >/=10 microM for bradykinin, [des-Arg9]bradykinin, and [des-Arg9,Leu8]bradykinin. Ornithokinin and HOE140 at nanomolar concentrations stimulated intracellular inositol phosphate accumulation and induced a significant transient rise in intracelluar free Ca2+, whereas bradykinin was ineffective even at 100 nM. Hence the principal B2 receptor antagonist HOE140 is a potent agonist of the chicken kinin receptor. This unique pharmacological profile classifies the ornithokinin receptor as a novel subtype among kinin receptors and will facilitate further molecular studies on ligand binding and receptor activation.

An NMR study of the interaction of 15N-labelled bradykinin with an antibody mimic of the bradykinin B2 receptor

An isotope-edited NMR study of the peptide hormone bradykinin (RPPGFSPFR) bound to the Fab fragment of a monoclonal antibody against bradykinin (MBK3) is reported. MBK3 was previously shown to provide a binding site model of the B2 bradykinin receptor [Haasemann, M., Buschko, J., Faussner, A., Roscher, A. A., Hoebeke, J., Burch, R. M. & Muller-Esterl, W. (1991) Anti-idiotypic antibodies bearing the internal image of a bradykinin epitope, J. Immunol. 147, 3882-3892]. Bradykinin was obtained in a uniformly 15N-labelled form using recombinant expression of a fusion protein consisting of the glutathione-binding domain of glutathione S-transferase fused to residues 354-375 of the high-molecular-mass kininogen from which bradykinin was released by proteolytic digestion with its natural protease plasma kallikrein. Bradykinin forms a complex with the Fab fragment of MBK3 which exchanges slowly on the NMR time scale. The 15N and 1H resonances of the tightly bound residues of bradykinin show appreciable changes in chemical shift with respect to the free form, while the 15N and 1H linewidths indicate that the hydrodynamic behaviour of bound bradykinin is dominated by the high-molecular-mass Fab fragment. The NMR data indicate that essentially the entire nonapeptide is involved in binding. The kinetics of the ligand-exchange process, together with resonance assignments obtained via exchange spectroscopy. indicate that bradykinin binds to MBK3 only in the all-trans conformation at all three Xaa-Pro amide bonds. NH-NH NOE connectivities suggest that bradykinin is bound in an extended conformation. The spectroscopic data obtained from this study are compared to recently proposed computational models of the conformation of bradykinin bound to the B2 receptor.

Ontogeny of bradykinin B2 receptors in the rat kidney: implications for segmental nephron maturation

Kinins modulate renal function, yet their role in the developing kidney is largely unknown. To explore the developmental role of the kallikrein-kinin system, we examined the postnatal ontogeny and intrarenal localization of B2 receptors in the rat. Northern blot analysis and RT-PCR documented the expression of B2 receptor mRNA in the kidney and extrarenal tissues of fetal, neonatal and adult animals. The abundance of B2 receptor mRNA is 10- to 30-fold higher in neonatal than adult tissues in the following order: kidney > heart > aorta > lung > brain. Receptor autoradiography revealed a gradual shift in the localization of bradykinin binding sites from the outer cortex in the newborn to the outer medulla in weanling and maturing rats. The almost complete displacement of [125I]tyr(zero)-bradykinin by HOE-140 indicates that the majority of kinin receptors in the developing kidney belong to the B2 type. Immunolocalization studies using antipeptide antibodies directed against various portions of the receptor revealed that B2 receptors are first expressed on the luminal aspect of the upper limb of S-shaped bodies and differentiating cortical collecting ducts. In marked contrast, the metanephric mesenchyme, pretubular aggregates and glomeruli display weak or no B2 receptor immunoreactivity. Following completion of nephrogenesis, B2 receptor expression shifts to both luminal and basolateral aspects of connecting tubules and collecting ducts. The results demonstrate that bradykinin B2 receptor gene expression is activated in the developing kidney and cardiovascular system. The spatially restricted expression of B2 receptors in the differentiating epithelium of the distal nephron, the site of kinin formation, supports the hypothesis that kinins are paracrine modulators of segmental nephron maturation.

The sixth transmembrane domains of the human B1 and B2 bradykinin receptors are structurally compatible and involved in discriminating between subtype-selective agonists

In order to investigate the molecular basis for the ability of the human B1 and B2 bradykinin (BK) receptor subtypes to discriminate between subtype-selective ligands, we constructed chimeric proteins in which the sixth transmembrane domains (TM-VI) of these receptors were exchanged. The pharmacological profiles of the constructs were analyzed by radioligand binding in particulate preparations of transiently transfected HEK293 cells using the agonist [3H]des-Arg10-kallidin and the antagonist [3H]NPC17731. The ability of these constructs to transmit an intracellular signal was measured in transiently transfected A10 cells, a vascular smooth muscle cell line, by single cell Ca2+ imaging. Substitution of B1 TM-VI into the B2 receptor (B2(B1VI)) dramatically reduced the affinity of the B2-selective agonist BK, whereas the affinity of the B2-selective antagonist NPC17731 was unaltered. High affinity BK binding was fully regained when two residues, Tyr259 and Ala263, near the extracellular surface of TM-VI in B2(B1VI), were replaced with the corresponding residues in the wild-type B2 receptor, which are Phe259 and Thr263. The construct B1(B2VI), produced by substitution of B2 TM-VI into the B1 receptor, did not support high affinity binding of the B1-selective agonist des-Arg10-kallidin. In contrast to BK and des-Arg10-kallidin, the binding of the less subtype-selective agonist kallidin showed little sensitivity to TM-VI exchange. These results show that TM-VI in the human B1 and B2 BK receptor subtypes, although only 36% identical, are structurally compatible. Furthermore, this domain contributes significantly to the ability of these receptors to discriminate between the subtype-selective agonists BK and des-Arg10-kallidin.

Kininogen binding protein p33/gC1qR is localized in the vesicular fraction of endothelial cells

The endothelial protein p33/gC1qR is thought to mediate the assembly of components of the kinin-forming and complement-activating pathways on the surface of cardiovascular cells. FACS analysis of intact human umbilical vein endothelial cells using specific antibodies to p33 revealed a minor fluorescence on the cell surface whereas permeabilized cells showed a bright fluorescence indicative of an intracellular localization of p33. Immunostaining of fixed cells confirmed the predominant intracellular localization of p33. Fractionation studies demonstrated that the vesicular but not the membrane fraction of EA.hy926 cells is rich in p33. We conclude that externalization of p33 must precede its complex formation with target proteins on the endothelial cell surface.

Ligand-induced phosphorylation/dephosphorylation of the endogenous bradykinin B2 receptor from human fibroblasts

We have studied the ligand-induced phosphorylation/dephosphorylation of the bradykinin B2 receptor endogenously expressed in human HF-15 fibroblasts. An antiserum (AS346) to a synthetic peptide (CRS36), derived from the extreme carboxyl terminus of the human B2 receptor, precipitated the receptor from solubilized membranes of HF-15 cells that had been labeled with [32P]orthophosphate. A low basal level of B2 receptor phosphorylation was found in the absence of a ligand. Stimulation of the cells with the B2 receptor agonists bradykinin, [Lys0,Hyp3]bradykinin, kallidin, and T-kinin resulted in a rapid and efficient phosphorylation of the receptor. The B2 receptor antagonist HOE140 and the B1 receptor agonist des-Arg9-bradykinin failed to induce significant phosphorylation of the B2 receptor. Phosphoamino acid analysis revealed that the B2 receptor is phosphorylated on serine and threonine, but not on tyrosine residues. The ligand-induced phosphorylation of the receptor was concentration-dependent, with an apparent EC50 of 33 nM, and peaked at 1 min after challenge. The kinin-stimulated phosphorylation of the B2 receptor was rapid and transient and paralleled the kinetics of desensitization/resensitization of the receptor as followed by [Ca2+]i release and radioligand binding assay, respectively. The ligand-induced phosphorylation of the B2 receptor was independent of the protein kinase C pathway. In vitro experiments suggest betaARK1 (beta-adrenergic receptor kinase) as a candidate kinase that could mediate the homologous B2 receptor phosphorylation. Inhibitors of protein phosphatases 1 and 2A effectively blocked the dephosphorylation, but did not affect the internalization of the B2 receptor, whereas inhibitors of receptor internalization delayed its dephosphorylation. These finding point to a role of ligand-induced phosphorylation in the desensitization and redistribution of the bradykinin receptor in human fibroblasts.

The N terminus of bradykinin when bound to the human bradykinin B2 receptor is adjacent to extracellular Cys20 and Cys277 in the receptor

Chemical cross-linking combined with site-directed mutagenesis was used to evaluate the role of extracellular cysteines and their positions relative to the binding site for the agonist bradykinin (BK) in the human BK B2 receptor. All extracellular cysteines, Cys20, Cys103, Cys184, and Cys277, in the receptor were mutated to serines, and single and double mutants were transfected into COS-7 cells. The Ser20 and Ser277 single mutants and the Ser20/Ser277 double mutant bound [3H]BK and the antagonist [3H]NPC17731 with pharmacological profiles identical to the wild-type B2 receptor. In contrast, the Ser103 and Ser184 single mutants were unable to bind either of the two radioligands. However, these mutants were still expressed as determined by immunoblotting with anti-B2 receptor antibodies. Previous studies on the bovine B2 receptor showed that bifunctional reagents, which are reactive to amines at one end and to free sulfhydryls in the opposite end, cross-link the N terminus of receptor-bound BK to a sulfhydryl in the receptor (Herzig, M. C. S., and Leeb-Lundberg, L. M. F. (1995) J. Biol. Chem. 270, 20591-20598). Here, we show that m-maleimidobenzoyl-N-hydroxysuccinimide ester and 1,5-difluoro-2, 4-dinitrobenzene cross-linked BK to the wild-type human B2 receptor and the Ser20 and Ser277 single mutant receptors, whereas these reagents were unable to cross-link BK to the Ser20/Ser277 double mutant. These results show that Cys103 and Cys184 are both required for expression of high affinity agonist and antagonist binding sites in the human B2 receptor, while Cys20 and Cys277 are not required. Furthermore, the results provide direct biochemical evidence that the N terminus of BK, when bound to the B2 receptor, is adjacent to Cys277 in extracellular domain 4 and Cys20 in extracellular domain 1 of the receptor.

Mutations in the B2 bradykinin receptor reveal a different pattern of contacts for peptidic agonists and peptidic antagonists

The B2 bradykinin receptor, a seven-helix transmembrane receptor, binds the inflammatory mediator bradykinin (BK) and the structurally related peptide antagonist HOE-140. The binding of HOE-140 and the binding of bradykinin are mutually exclusive and competitive. Fifty-four site-specific receptor mutations were made. BK's affinity is reduced 2200-fold by F261A, 490-fold by T265A, 60-fold by D286A, and 3-10-fold by N200A, D268A, and Q290A. In contrast, HOE-140 affinity is reduced less than 7-fold by F254A, F261A, Y297A, and Q262A. The almost complete discordance of mutations that affect BK binding versus HOE-140 binding is surprising, but it was paralleled by the effect of single changes in BK and HOE-140. [Ala9]BK and [Ala6]BK are reduced in receptor binding affinity 27,000- and 150-fold, respectively, while [Ala9]HOE-140 affinity is reduced 7-fold and [Ala6]HOE-140 affinity is unchanged. NMR spectroscopy of all of the peptidic analogs of BK or HOE-140 revealed a beta-turn at the C terminus. Models of the receptor-ligand complex suggested that bradykinin is bound partially inside the helical bundle of the receptor with the amino terminus emerging from the extracellular side of helical bundle. In these models a salt bridge occurs between Arg9 and Asp286; the models also place Phe8 in a hydrophobic pocket midway through the transmembrane region. Models of HOE-140 binding to the receptor place its beta-turn one alpha-helical turn deeper and closer to helix 7 and helix 1 as compared with bradykinin-receptor complex models.

The N-terminal amino group of [Tyr8]bradykinin is bound adjacent to analogous amino acids of the human and rat B2 receptor

To obtain data of the bradykinin B2 receptor's agonist binding site, we used a combined approach of affinity labeling and "immunoidentification" of receptor fragments generated by cyanogen bromide cleavage. Domain-specific antibodies to the various extracellular receptor domains were applied to detect receptor fragments with covalently attached [125I-Tyr8]bradykinin. As a cross-linker we used the homobifunctional reagent disuccinimidyl tartarate (DST), which reacts preferentially with primary amines. With this technique a [125I-Tyr8]bradykinin-labeled receptor fragment derived from the third extracellular domain was identified. The epsilon-amino group of lysine (Lys172) of the human B2 receptor provides the only primary amino group within this receptor fragment. This strongly suggests that DST attached the N-terminal amino group of [Tyr8]bradykinin to Lys172 of the human B2 receptor. Next we asked whether DST attaches [Tyr8]bradykinin to the analogous residue, Lys174 of the rat B2 receptor, which is 81% identical to the human B2 receptor, and we attempted to label the wild-type rat B2 receptor and a rat B2 receptor mutant where Lys174 had been exchanged for alanine. Affinity labeling of the wild-type rat B2 receptor worked efficiently, whereas DST did not attach detectable amounts of [125I-Tyr8]bradykinin to the K174A rat B2 receptor mutant. Taken together these observations indicate that the N-terminal amino group of [Tyr8]bradykinin is bound to analogous positions of the rat and of the human B2 receptor, i.e. [Tyr8]bradykinin's N terminus is bound adjacent to Lys172 of the human and Lys174 of the rat B2 receptor.