Identification of a ligand-binding site of the human endothelin-A receptor and specific regions required for ligand selectivity

Determination of the endothelin-1 recognition sites of endothelin receptor type A by the directed-degeneration method

G-protein coupled receptors (GPCRs) play indispensable physiological roles in cell proliferation, differentiation, and migration; therefore, identifying the mechanisms by which ligands bind to GPCRs is crucial for developing GPCR-targeting pharmaceutics and for understanding critical biological functions. Although some structural information is available regarding the interactions between GPCRs and their small molecule ligands, knowledge of how GPCRs interact with their corresponding macromolecule ligands, such as peptides and proteins, remains elusive. In this study, we have developed a novel strategy to investigate the precise ligand recognition mechanisms involved in the interaction of endothelin receptor type A (ET_A) with its ligand, endothelin-1 (ET-1); we call this method “directed degeneration” method. Through flow cytometric screening of a randomized ET_A library, statistical analysis of the identified sequences, and biochemical studies, the ligand interaction map was successfully obtained.

Embryonic expression of endothelins and their receptors in lamprey and frog reveals stem vertebrate origins of complex Endothelin signaling

Neural crest cells (NCCs) are highly patterned embryonic cells that migrate along stereotyped routes to give rise to a diverse array of adult tissues and cell types. Modern NCCs are thought to have evolved from migratory neural precursors with limited developmental potential and patterning. How this occurred is poorly understood. Endothelin signaling regulates several aspects of NCC development, including their migration, differentiation, and patterning. In jawed vertebrates, Endothelin signaling involves multiple functionally distinct ligands (Edns) and receptors (Ednrs) expressed in various NCC subpopulations. To test the potential role of endothelin signaling diversification in the evolution of modern, highly patterned NCC, we analyzed the expression of the complete set of endothelin ligands and receptors in the jawless vertebrate, the sea lamprey (Petromyzon marinus). To better understand ancestral features of gnathostome edn and ednr expression, we also analyzed all known Endothelin signaling components in the African clawed frog (Xenopus laevis). We found that the sea lamprey has a gnathsotome-like complement of edn and ednr duplicates, and these genes are expressed in patterns highly reminiscent of their gnathostome counterparts. Our results suggest that the duplication and specialization of vertebrate Endothelin signaling coincided with the appearance of highly patterned and multipotent NCCs in stem vertebrates.

Discovery of Dual ETA/ETB Receptor Antagonists from Traditional Chinese Herbs through in Silico and in Vitro Screening

Endothelin-1 receptors (ETAR and ETBR) act as a pivotal regulator in the biological effects of ET-1 and represent a potential drug target for the treatment of multiple cardiovascular diseases. The purpose of the study is to discover dual ETA/ETB receptor antagonists from traditional Chinese herbs. Ligand- and structure-based virtual screening was performed to screen an in-house database of traditional Chinese herbs, followed by a series of in vitro bioassay evaluation. Aristolochic acid A (AAA) was first confirmed to be a dual ETA/ETB receptor antagonist based intracellular calcium influx assay and impedance-based assay. Dose-response curves showed that AAA can block both ETAR and ETBR with IC₅₀ of 7.91 and 7.40 μM, respectively. Target specificity and cytotoxicity bioassay proved that AAA is a selective dual ETA/ETB receptor antagonist and has no significant cytotoxicity on HEK293/ETAR and HEK293/ETBR cells within 24 h. It is a feasible and effective approach to discover bioactive compounds from traditional Chinese herbs using in silico screening combined with in vitro bioassay evaluation. The structural characteristic of AAA for its activity was especially interpreted, which could provide valuable reference for the further structural modification of AAA.

Evolution of endothelin receptors in vertebrates

Endothelin receptors are G protein coupled receptors (GPCRs) of the β-group of rhodopsin receptors that bind to endothelin ligands, which are 21 amino acid long peptides derived from longer prepro-endothelin precursors. The most basal Ednr-like GPCR is found outside vertebrates in the cephalochordate amphioxus, but endothelin ligands are only present among vertebrates, including the lineages of jawless vertebrates (lampreys and hagfishes), cartilaginous vertebrates (sharks, rays, and chimaeras), and bony vertebrates (ray-finned fishes and lobe-finned vertebrates including tetrapods). A bona fide endothelin system is thus a vertebrate-specific innovation with important roles for regulating the cardiovascular system, renal and pulmonary processes, as well as for the development of the vertebrate-specific neural crest cell population and its derivatives. Expectedly, dysregulation of endothelin receptors and the endothelin system leads to a multitude of human diseases. Despite the importance of different types of endothelin receptors for vertebrate development and physiology, current knowledge on endothelin ligand-receptor interactions, on the expression of endothelin receptors and their ligands, and on the functional roles of the endothelin system for embryonic development and in adult vertebrates is very much biased towards amniote vertebrates. Recent analyses from a variety of vertebrate lineages, however, have shown that the endothelin system in lineages such as teleost fish and lampreys is more diverse and is divergent from the mammalian endothelin system. This diversity is mainly based on differential evolution of numerous endothelin system components among vertebrate lineages generated by two rounds of whole genome duplication (three in teleosts) during vertebrate evolution. Here we review current understanding of the evolutionary history of the endothelin receptor family in vertebrates supplemented with surveys on the endothelin receptor gene complement of newly available genome assemblies from phylogenetically informative taxa. Our assessment further highlights the diversity of the vertebrate endothelin system and calls for detailed functional and pharmacological analyses of the endothelin system beyond tetrapods.

Purification and characterization of recombinant human endothelin receptor type A

Human endothelin receptor type A (ET(A)) is a G-protein coupled receptor that mediates vasoconstriction of blood vessels. To determine the structural characteristics and signaling mechanism of ET(A), we have expressed recombinant ET(A) as a fusion protein with p9 envelope protein from phi6 bacteriophage. The His-tag-labeled p9-ET(A) fusion protein was highly expressed in the membrane fraction of Escherichia coli and purified to homogeneity by single affinity chromatography after solubilization with detergents. Purified p9-ET(A) appeared as an oligomer and presented mainly as an α-helical structure. The protein also showed specific binding to endothelin-1 (ET-1) and the alpha subunit of G(q) protein with apparent K(D) values of 17 and 20 nM, respectively. An antagonist of ET(A), bosentan, prevented the interaction between p9-ET(A) and ET-1 in a concentration-dependent manner. These results indicate that recombinant p9-ET(A) has a competent conformation for interactions with ET-1 and the alpha subunit of G(q) protein.

The endothelin system: evolution of vertebrate-specific ligand-receptor interactions by three rounds of genome duplication

Morphological innovations like the acquisition of the neural crest as well as gene family expansions by genome duplication are considered as major leaps in the evolution of the vertebrate lineage. Using comparative genomic analyses, we have reconstructed the evolutionary history of the endothelin system, a signaling pathway consisting of endothelin ligands and their G protein-coupled receptors. The endothelin system plays a key role in cardiovascular regulation as well as in the development of diverse neural crest derivatives like pigment cells and craniofacial bone structures, which are hot spots of diversity in vertebrates. However, little is known about the origin and evolution of the endothelin system in the vertebrate lineage. We show that the endothelin core system, that is, endothelin ligands (Edn) and their receptors (Ednr), is a vertebrate-specific innovation. The components of the endothelin core system in modern vertebrate genomes date back to single genes that have been duplicated during whole-genome duplication events. After two rounds of genome duplication during early vertebrate evolution, the endothelin system of an ancestral gnathostome consisted of four ligand and four receptor genes. The previously unknown fourth endothelin ligand Edn4 has been kept in teleost fish but lost in tetrapods. Bony vertebrates generally possess three receptor genes, EdnrA, EdnrB1, and EdnrB2. EdnrB2 has been lost secondarily in the mammalian lineage from a chromosome that gave rise to the sex chromosomes in therians (marsupials and placentals). The endothelin system of fishes was further expanded by a fish-specific genome duplication and duplicated edn2, edn3, ednrA, and ednrB1 genes have been retained in teleost fishes. Functional divergence analyses suppose that following each round of genome duplication, coevolution of ligands and their binding regions in the receptors has occurred, adjusting the endothelin signaling system to the increase of possible ligand-receptor interactions. Furthermore, duplications of genes involved in the endothelin system are associated with functional specialization for the development of particular neural crest derivatives. Our results support an important role for newly emerging ligands and receptors as components of signaling pathways and their expansion through genome duplications in the evolution of the vertebrate neural crest.

Transmembrane domain V of the endothelin-A receptor is a binding domain of ETA-selective TTA-386-derived photoprobes

On the basis of the structure of TTA-386, a specific antagonist of the endothelin-A receptor subtype (ET(A)), photosensitive analogues were developed to investigate the binding domain of the receptor. Among those, a derivative containing, in position 6, the photoreactive amino acid D- or L-p-benzoyl-phenylalanine showed pharmacological properties very similar to those of TTA-386. Affinity of the probes were also evaluated on transfected CHO cells overexpressing the human ET(A) receptor. Data showed that binding of the radiolabeled peptides were inhibited by ET-1 and BQ-610. Therefore, these photolabile probes were used to label the ET(A) receptor found in CHO cells. Photolabeling produced a ligand-protein complex appearing on SDS-PAGE at around 66 kDa. An excess of ET-1 or BQ-610 completely abolished the formation of the complex showing the selectivity of the photoprobes. Digestions of the [125I-Tyr5, D- or L-Bpa6]TTA-386-ET(A) complex were carried out, and receptor fragments were analyzed to define the region of the receptor where the ligand interacted. Results showed that Endo Lys-C digestion gave a 4.8 kDa fragment corresponding to the Asp256-Lys299 segment, whereas migration after V8 digestion revealed a fragment of 2.9 kDa. Because the fragments of these two digestions must overlap, the latter would be the Trp257-Glu281 stretch. A cleavage with CNBr confirmed the identity of the binding domain by giving a fragment of 3.9 kDa corresponding to Glu249-Met278. Thus, the combined cleavage data strongly suggested that the binding domain of ET(A) includes a portion of the fifth transmembrane domain, between residues Trp257 and Met278.

Decreased wild-type full-length Et-A and -B receptors in neuroblastoma and Ewing sarcoma cells

BACKGROUND

Endothelins and their receptors, Et-A and Et-B, play an essential role in differentiation and migration of neural crest cells. Expression of endothelin receptors has been examined in neuroblastoma and Ewing sarcoma cell lines.

PROCEDURE

RNA was amplified for Et-A and Et-B by RT-PCR. Amplified products were cloned into the expression vector pLNCX, which was used to transfect CHO cells. Binding characteristics of transfected CHO cells were examined.

RESULTS

Full-length Et-A mRNA was identified in all cell lines, in addition to a truncated Et-A product. CHO cells expressing full-length Et-A bound to endothelin, but cells expressing truncated Et-A did not. Full length Et-B mRNA was not detected, but two smaller molecular weight products were amplified. These are as yet uncharacterised.

CONCLUSIONS

These results suggest that endothelins and their receptors may be important in the development and biology of neuroblastoma and Ewing sarcoma.

Modeling and docking the endothelin G-protein-coupled receptor

A model of the endothelin G-protein-coupled receptor (ET(A)) has been constructed using a segmented approach. The model was produced using a bovine rhodopsin model as a template for the seven transmembrane alpha-helices. The three cytoplasmic loop regions and the C-terminal region were modeled on NMR structures of corresponding segments from bovine rhodopsin. The three extracellular loops were modeled on homologous loop regions in other proteins of known structure. The N-terminal region was modeled as a three-helix domain based on its homology with a hydrolase protein. To test the model, the FTDOCK algorithm was used to predict the ligand-binding site for the crystal structure of human endothelin. The site of docking is consistent with mutational and biochemical data. The principal sites of interaction in the endothelin ligand all lie on one face of a helix that has been implicated by structure-activity relationship studies as being essential for binding. As further support for the model, attempts to dock bigET, an inactive precursor to endothelin that does not bind to the receptor, found no sites for tight binding. The model of the receptor-ligand complex produced forms a basis for rational drug design of agonists and antagonists for this G-protein-coupled receptor.

Truncated human endothelin receptor A produced by alternative splicing and its expression in melanoma

In this study, reverse transcriptase polymerase chain reaction was used to amplify human endothelin receptor A (ETA) and ETB receptor mRNA. A truncated ETA receptor transcript with exons 3 and 4 skipped was found. The skipping of these two exons results in 109 amino acids being deleted from the receptor. The truncated receptor was expressed in all tissues and cells examined, but the level of expression varied. In melanoma cell lines and melanoma tissues, the truncated receptor gene was the major species, whereas the wild-type ETA was predominant in other tissues. A 1.9-kb ETA transcript was identified in melanoma cell lines by Northern blot, which was much smaller than the transcript in heart and in other tissues reported previously (4.3 kb). The cDNA coding regions of the truncated and wild-type ETA receptors were stably transfected into Chinese hamster ovary (CHO) cells. The truncated ETA receptor-transfected CHO cells did not show binding affinity to endothelin 1 (ET-1) or endothelin 3 (ET-3). The function and biological significance of this truncated ETA receptor is not clear, but it may have regulatory roles for cell responses to ETs.

Human endothelin receptors ET(A) and ET(B) expressed in baculovirus-infected insect cells--direct application for signal transduction analysis

We expressed human endothelin receptors, ET(A) and ET(B), in insect Sf9 cells infected by recombinant baculoviruses that contained the respective cDNAs. Ligand-binding experiments showed that the two expressed receptors have the same affinities as observed for the receptors in mammalian cells, i.e. the ET(A) receptor showed an affinity order of ET-1 > or = ET-2 >> ET-3, and the ET(B) receptor remained nonselective for three isopeptide ligands. The ET(B) receptor was purified by affinity chromatography with K9-biotinyl-ET-1 without losing the ligand-binding activity from the membrane of infected Sf9 cells. Protein chemical analysis of the purified ET(B) receptor showed that it is glycosylated, and that the N-terminal 38-amino-acid peptide is susceptible to proteolytic digestion, resulting in a small 35-kDa receptor like that found in the human placenta. Surprisingly, the infected and unlysed cells showed a strong intracellular Ca2+ concentration increase ([Ca2+]i), which was generated by a unique signal-transduction pathway consisting of the insect GTP-binding protein and human endothelin receptors expressed in the late phase of virus infection. Due mainly to an efficient expression (over 200,000 receptors/cell), to a low background owing to no endogenous homolog receptor in insect Sf9 cells, and to a sensitive fluorescent reagent Fura-2, this insect Sf9 cell system can detect the [Ca2+]i induced by picomolar levels of endothelin-receptor. We propose that this highly sensitive system be used to screen for potential antagonists/agonists of endothelin receptors.

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.

Endothelin expression and responsiveness in human ovarian carcinoma cell lines

To elucidate the potential role of endothelins (ETs) as growth regulators in ovarian carcinoma cells in culture, expression of endothelins and their receptors were measured in two ovarian cancer cell lines (PEO4 and PEO14), together with the effect of the exogenous addition of endothelins on the growth of these cell lines in vitro. RT-PCR analysis of mRNA prepared from PEO4 and PEO14 indicated the presence of ET-1 and ET-3 mRNA. Immunoreactive ET-1-like peptide was found in media from cultures of both PEO4 (1.7 +/- 0.4 fmol/10(6) cells/72 h) and PEO14 (20.2 +/- 6.8 fmol/10(6) cells/72 h) cell lines. Radioligand binding studies using 125I-ET-1 and membrane fractions were consistent with PEO4 cells having two receptor sites of either high affinity (Kd = 0.065 nM, Bmax = 0.047 pmol/mg protein) or lower affinity sites (Kd = 0.49 nM, Bmax = 0.23 pmol/mg protein). Studies using membrane fractions of PEO14 cells indicated that this cell line has only a single lower affinity binding site (Kd = 0.56 nM, Bmax = 0.31 pmol/mg protein). However, RT-PCR analysis indicated the presence of mRNA from both ETA and ETB receptors in PEO4 and PEO14 cell lines. Exogenous addition of ETs to PEO4 and PEO14 cells at concentrations of 10(-10)-10(-7)M resulted in specific dose-dependent increases in cell number for ET-1 (with maximum effects at 10(-10) and 10(-9)M for PEO4 and PEO14, respectively) and ET-2 (maximum effects at 10(-8) and 10(-9)M for PEO4 and PEO14, respectively) but not for ET-3. Experiments on the growth of PEO14 cells using BQ123 (ETA-R) antagonist and "antisense" oligonucleotide against the ETA-R, in the absence of exogenous ETs, suggested that immunoreactive ET-1-like material secreted by PEO14 cells can affect their growth in an autocrine manner. These results would be consistent with ET-1 acting as a possible autocrine growth regulator in human ovarian carcinoma cells.

Influence of endothelin ET(A) and ET(B) receptor antagonists on endothelin-induced contractions of the guinea pig isolated gall bladder

The receptors mediating guinea pig gall bladder (GPGB) contractions induced by endothelin-1 (ET-1) and related peptides were characterized using various ET receptor antagonists. As all ET-receptor agonists used, except sarafotoxin S6c (SRTX), failed to induce a clear-cut maximal response at the highest concentration tested (i.e. 100 nM), their potencies are expressed in terms of a CK50 (i.e. the concentration causing 50% of the response to 80 mM KCl). ET-1 (CK50 0.8 nM) was equipotent to ET-2 and SRTX (selective ET(B) receptor agonist), but more potent than ET-3 (5-fold) or IRL 1620 (selective ET(B) receptor agonist). BQ-123 (0.3 microM, peptidic ET(A) receptor antagonist) did not alter responses to ET-1, ET-3 or SRTX. BQ-788 (1 microM, peptidic ET(B) receptor antagonist) reduced the potency of ET-3 (9-fold at the CK50 level) and SRTX ( > 20-fold), but not ET-1. SRTX responses were unaffected by RES-701-1 (3 microM, peptidic ET(B) receptor antagonist). The combination BQ-123 (0.3 microM) plus BQ-788 (1 microM) did not modify responses to ET-1, inhibited SRTX responses similarly to BQ-788 alone and abolished ET-3 responses. Bosentan (1 microM, non-peptidic ET(A)/ET(B) receptor antagonist) reduced the potency of ET-1 (15-fold). ET-3 (9-fold) and SRTX (4-fold). In rat aorta, the antagonists blocked ET-1-induced contractions (BQ-123 and bosentan) or SRTX-induced endothelium-dependent relaxations (BQ-788, RES-701-1 and bosentan). Thus, the GPGB expresses both ET(A) and ET(B) receptors. As BQ-123 only blocked responses to ET-3 in the presence of BQ-788, there appears to be cross-talk between both receptor types. Also, the binding sites of ET-1 and ET-3 on the ET(A) receptor may not coincide entirely, as BQ-123, even in presence of BQ-788, did not affect ET-1-induced contractions.

The amino-terminal extracellular domain of the MCP-1 receptor, but not the RANTES/MIP-1alpha receptor, confers chemokine selectivity. Evidence for a two-step mechanism for MCP-1 receptor activation

The chemoattractant cytokines, MCP-1 (monocyte chemoattractant protein) and MIP-1alpha (macrophage inflammatory protein), are recognized by highly homologous but distinct receptors. To identify receptor domains involved in determining ligand specificity, we created a series of chimeric MCP-1 and RANTES (regulated on activation, normal T cell expressed and secreted)/MIP-1alpha receptors that progressively interchanged the amino terminus and each of the three extracellular loops. Radiolabeled MCP-1 bound with high affinity to the wild-type MCP-1 receptor, but not to the RANTES/MIP-1alpha receptor (C-C CKR-1). Chimeras that retained the amino-terminal extension of the MCP-1 receptor bound MCP-1 with high affinity. In contrast, chimeric MCP-1 receptors, in which the wild-type amino terminus was replaced with the corresponding portion of the RANTES/MIP-1alpha receptor, bound MCP-1 with low affinity. These data indicate that the amino terminus of the MCP-1 receptor is necessary for high affinity binding of the ligand. Very different results were obtained using the RANTES/MIP-1alpha receptor. Radiolabeled MIP-1alpha bound with high affinity to chimeras that expressed the extracellular loops of the RANTES/MIP-1alpha receptor. In contrast to the MCP-1 receptor, substitution of the wild-type amino-terminal extension had little or no effect on MIP-1alpha binding. For the MCP-1, but not the RANTES/MIP-1alpha receptor, the presence of the wild-type amino terminus also significantly lowered the ligand concentration required for maximal signaling. We conclude that the amino-terminal extension of the MCP-1 receptor, but not the RANTES/MIP-1alpha receptor, is critically involved in ligand binding and signal transduction. These data reveal significant functional differences between the two C-C chemokine receptors and suggest a two-step mechanism for activation of the MCP-1 receptor.

Mutational analysis of the endothelin type A receptor (ETA): interactions and model of selective ETA antagonist BMS-182874 with putative ETA receptor binding cavity

Endothelin (ET) receptor antagonism is a potential therapeutic intervention in the treatment of vascular diseases. To elucidate the mechanism of antagonist-ET receptor complex formation, the interactions of four chemically distinct antagonists were investigated using a combination of genetic and biochemical approaches. By site-specific mutagenesis we previously demonstrated that Tyr129 in the second transmembrane domain was critical for high-affinity, subtype-selective binding to the A subtype of ET (ETA) receptors [Krystek et al. (1994) J. Biol. Chem. 269, 12383-12386]. Affinities of the constrained cyclic pentapeptide BQ-123, the pyrimidinylbenzenesulfonamide bosentan, the indancarboxlic acid SB 209670, and the naphthalenesulfonamide BMS-182874 were decreased 20-1000-fold in Tyr129Ala, Tyr129Ser, and Tyr129His ETA receptor mutants. Substitution of Tyr129 with Phe or Trp did not alter the high-affinity binding of BQ-123, bosentan, or SB 209670. BMS-182874 binding affinity was decreased 10-fold in Tyr129Phe and Tyr129trp ET receptors. These data indicate a role of aromatic interactions in the binding of these antagonists to ETA receptors an, in the case of BMS-182874, also suggested a hydrogen bond with the tyrosine hydroxyl. This hypothesis was supported by structure-activity data with analogs of BMS-182874 that varied the C-5 dimethylamino substituent on the naphthalene ring. Mutation of Asp126 and Asp133 also altered binding of BMS-182874 and C-5 analogs. In all cases, naphthalenesulfonamide binding was more severely affected by mutation of Asp133 than by mutation of Asp126. Phosphoinositide hydrolysis and extracellular acidification rate studies demonstrated the importance of Tyr129 to ETA-mediated signal transduction. On the basis of these data, two plausible models of the docked conformation of BMS-182874 in the ETA receptor are proposed as a starting point for further delineation of interactions that underlie antagonist-ETA receptor complex formation.

Endothelin receptor synthetic N-terminal fragment interacts with the receptor itself

Endothelin binds to receptors belonging to the family of G-protein-coupled receptors with an N-terminal extracellular domain that is suspected to be part of the binding site. We have synthesized different peptides of this N-terminal extracellular domain and analyzed the increase in calcium concentration ([Ca2+]i) induced by these peptides in the MEG-01 cell line and their influence on the ET-1 concentration-effect response. Nt (20-79) exhibited a partial agonistic effect on [Ca2+]i and blunted the functional response of ET-1 in MEG-01 cells, but was not able to compete with radiolabeled ET-1 binding. The agonist effect was inhibited by the ET receptor antagonists PD 142893 and BQ123, suggesting an interaction between Nt (20-79) and the ETA receptor at a site that could be different from the one of ET-1.

The endothelin system and its potential as a therapeutic target in cardiovascular disease

Endothelin (ET)-1, an endothelium-derived peptide, is the most potent vasoconstrictor agent described to date. ET-1 also has positive inotropic and chronotropic effects in the heart and is a co-mitogen in both cardiac and vascular myocytes. The major elements of the system involved in formation of ET-1 and its isopeptides, as well as the receptors mediating their effects, have been cloned and characterised. Antagonists of the ET receptors are now available, and selective inhibitors of the ET-converting enzymes are being developed. Early studies using receptor antagonists support the involvement of ET-1 in the pathophysiology of several cardiovascular diseases. The relative merits of ET-converting enzyme inhibitors and receptor antagonists for the treatment of cardiovascular disease are discussed.

G protein-coupled receptor structure and function: the impact of disease-causing mutations

Just as the discovery of 'inborn errors of metabolism' in humans contributed to our basic understanding of normal enzymatic pathways, so can genetic defects in signal transduction help to elucidate the functions normally subserved by different GPCR pathways. Identification and characterization of naturally occurring GPCR mutations not only has inherent value in understanding the molecular basis of disease, but can also accelerate progress in understanding the fundamental mechanisms involved in GPCR synthesis, transport to the membrane, ligand binding, activation and deactivation.

Endothelin receptor subtypes and their role in transmembrane signaling mechanisms

The endothelins (ETs) are potent vasoactive peptides that appear to be involved in diverse biological actions, for example, contraction, neuromodulation, and neurotransmission, as well as in various pathophysiological conditions, such as renal and heart failure. The diversity of actions of ETs may be explained in terms of (1) the existence of several receptor subtypes and (2) the activation of different signal transduction pathways. This review summarizes the state of the art in this intensively studied field, with particular focus on structural aspects, receptor heterogeneity, coupling of receptors to G-proteins, and signal transduction mechanisms mediated by the activation of ET-receptors.

Endothelin receptor antagonism

Following the original report by Yanagisawa et al. (1988) more than 7 years ago, compelling evidence that ET plays an important role in the local regulation of smooth muscle tone and cell growth has been reported. In addition, many studies point to a significant role for endothelin in nonvascular function. The investigation of the endothelin system has been greatly advanced in the last 2 to 3 years through significant advances in the development of potent and selective ET receptor antagonists. These agents have proven to be essential tools for elucidating the biological significance of the ET system, leading to the realization that antagonism of the ET system may have significant therapeutic potential. As emphasized in this review, the importance of chronic blockade of the ET system may be a critical aspect of future research in this exciting area. Confounding issues remain the lack of information about the role of the ETB receptor, the apparent pharmacological evidence for additional ET receptor subtypes, and species variation in the tissue distribution of ET isoforms and receptor subtypes. Along with the greater ability to understand the endothelin system provided by potent and selective pharmacological agents, is the important contribution of modern molecular biology techniques, highlighted by the insights gained from recent reports of results from ET gene disruption studies. Kurihara et al. (1994) found that ET-1-deficient homozygous mice die at birth of apparent respiratory failure secondary to severe craniofacial abnormalities. Subsequently, Yanagisawa's laboratory has presented and published a series of complementary gene disruption studies. First, Hosoda et al. (1994) demonstrated remarkably, that ETA receptor knockout mice bear morphological abnormalities nearly identical to ET-1 knockout mice. Second, they found that disruption of the ET-3 peptide and ETB receptor genes result in homozygous mice that share identical phenotypic traits (i.e., coloration changes and aganglionic megacolon) which are similar to a previously known natural mutation, the Piebald-Lethal mouse (Hosoda et al., 1994; Baynash et al., 1994). This phenotype has a human corollary known as Hirschsprung's Disease and it is now known that the disease, though multigenic, results from a missense mutation of the ETB receptor gene in some individuals (Puffenberger et al., 1994). Taken together these data indicate that the endothelin system is essential to correct embryonic neural crest development, a completely novel finding within the superfamily of guanine-protein-linked receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of a region of the human endothelin ETA receptor required for interaction with bosentan

Bosentan (Ro 47-0203, 4-tert-butyl-N-[6-(2-hydroxy-ethoxy)-5-(2- methoxy-phenoxy)-2,2'-bipyrimidin-4-yl]-benzenesulfonamide) is a new non-peptidic mixed antagonist of endothelin receptors whose binding activity was two orders higher for the endothelin ETA receptor than that for the endothelin ETB receptor. To identify which region of the human endothelin ETA receptor interacts with bosentan, we created various chimeric endothelin receptors containing domains from the endothelin ETA and ETB receptors in Chinese hamster ovary cells and studied the effect of bosentan on the binding of endothelin-1 to the chimeric receptors. We found that the chimeric endothelin ETB receptor containing domains from the endothelin ETA receptor, the second extracellular region including the proximal transmembrane region (B-region) revealed an affinity toward bosentan which was similar to that of the endothelin ETA receptor. In contrast, the chimeric endothelin ETA receptor, containing the B-region of the endothelin ETB receptor, reduced the binding affinity to the level of the endothelin ETB receptor. Since bosentan competes with endothelin-1 for binding to the endothelin ETA receptor, this receptor antagonist seems to interact with the (140-144) KLLAG sequence located at the carboxylterminus of the second transmembrane region of the endothelin ETA receptor, required for the natural ligand binding.

Identification of specific regions of the human endothelin-B receptor required for high affinity binding with endothelin-3

To investigate the endothelin-3 (ET-3) binding region of the endothelin-B (ETB) receptor, we have transiently produced various chimeric endothelin receptors in transfected Chinese hamster ovary cells. Using 125I-ET-1 as the radioactive ligand in the displacement experiment, the replacement of both the second and third extracellular regions including the flanking transmembranes of the ETB receptor with the corresponding domains of the endothelin-A (ETA) receptor, increased the apparent Ki value for ET-3 from 5 x 10(-11) M to 10(-8) M. The affinity of this chimeric receptor, ETB-BC, for ET-3 was about two orders lower than ETB yet one order higher than ETA. Previously we have reported the involvement of Lys-140 located in the C-terminus of the second transmembrane region of the ETA receptor for ET-1 binding (Eur. J. Biochem., 220, 37-43, 1994). To assess the importance of the corresponding Lys-161 of the ETB receptor in ET-3 binding, we have replaced it with Ile in the ETB receptor. The mutant receptor had a 5.6-fold reduction in its affinity for ET-3, but its affinity for ET-1 remained similar. These results demonstrate that Lys-161 of the receptor is important for high affinity binding with ET-3 which, in part, confers the non-selective binding characteristics of the ETB receptor for ET isopeptides.

Locating ligand-binding sites in 7TM receptors by protein engineering

Over the past year, mutational analysis of peptide receptors has started to change our understanding of the interaction between G protein coupled receptors and their ligands, an area previously almost totally dominated by results from studies of monoamine receptors. A picture is currently emerging, in which small ligands appear to bind in three (more or less) overlapping ligand-binding pockets in between the transmembrane segments. In contrast, contact residues for peptide and protein ligands have mainly been found in exterior regions of peptide and protein receptors. It is also becoming increasingly clear that agonists and antagonists may interact in vastly different manners, even though they are competitive ligands for a common receptor.