Isolation of the endothelin B receptor from bovine lung. Structure, signal sequence, and binding site

Up-regulation of the Endothelin Receptor A in Placental Tissue From First Trimester Delayed Miscarriages

OBJECTIVE

This study tested the hypothesis that the endothelin (ET)/ET receptor (ETR) system in biologic fluids and in the human placenta is altered in delayed miscarriages as compared to apparently normal early pregnancies (reference group).

METHODS

Immunoreactive ET (irET) concentrations were measured in plasma, urine, and cervical smears from 57 pregnant women in the weeks 6 to 14 of gestation (46 delayed miscarriages, 11 references) with radioimmunoassay (RIA). ET-1, ETR-A, and ETR-B mRNA, and ETR protein expression were measured in placental tissue of 45 early pregnancies (31 delayed miscarriages, 14 references) using semiquantitative reverse-transcription polymerase chain reaction (RT-PCR) and immunoblotting, respectively.

RESULTS

irET levels in plasma, urine, and cervical smears did not differ between groups. Two prevailing ETR-A and ETR-B proteins were found at 45 and 55 kd, and were distributed similarly in delayed miscarriages and references. ETR-A protein and mRNA levels were 54% (P = .009) and threefold (P = .021) higher, respectively, in delayed miscarriages versus references. There was no difference in placental ETR-B and ET-1 mRNA levels between groups.

CONCLUSION

Neither irET nor ET-1 mRNA levels differ between delayed miscarriages and normal early pregnancies. Pregnancies at risk for miscarriage cannot be identified by measurement of ET in plasma, urine, or cervical smears. Within the ET/ETR system, ETR-A is selectively up-regulated in placental tissue of delayed miscarriages as compared to normal pregnancies. ETR protein processing is similar in both groups.

Molecular modeling of the complex of endothelin-1 (ET-1) with the endothelin type A (ET(A)) receptor and the rational design of a peptide antagonist

ET-1 is the most potent vasoconstrictor known to date, causing vasoconstriction when bound to the ET(A) receptor. Inhibitors of the binding of ET-1 to the ET(A) receptor would be of immense value as potential therapeutic agents in the treatment of cardiovascular disorders such as angina and hypertension. We present here the rational design of such an inhibitor, which is arrived at on the basis of a model of the ET-1/ET(A) receptor complex proposed by us. The model is found to be consistent with binding and mutagenesis studies of ET-1 as well as of BQ123, a known, potent ET(A)-selective antagonist which competes with ET-1 for receptor binding. BQ123 is a peptidic antagonist which is constrained to adopt a definite conformation on account of its cyclic nature. The noncyclic peptide antagonist designed by us also has a unique conformation because it contains two dehydro-Alanine (deltaAla) residues which, on account of their planarity, cause the peptide backbone to bend in a specific and predictable manner. The folding rules for peptides containing deltaAla were derived in our earlier studies. Energy minimization and modelling of the complex of the designed peptide with the ET(A) receptor indicate that the antagonist is ET(A)-selective and the binding is more stable and more specific as compared to that of BQ123.

Endothelins and asthma

In the decade since endothelin-1 (ET-1) and related endogenous peptides were first identified as vascular endothelium-derived spasmogens, with potential pathophysiological roles in vascular diseases, there has been a significant accumulation of evidence pointing to mediator roles in obstructive respiratory diseases such as asthma. Critical pieces of evidence for this concept include the fact that ET-1 is an extremely potent spasmogen in human and animal airway smooth muscle and that it is synthesised in and released from the bronchial epithelium. Importantly, symptomatic asthma involves a marked enhancement of these processes, whereas asthmatics treated with anti-inflammatory glucocorticoids exhibit reductions in these previously elevated indices. Despite this profile, a causal link between ET-1 and asthma has not been definitively established. This review attempts to bring together some of the evidence suggesting the potential mediator roles for ET-1 in this disease.

Functional proteomics of signal transduction by membrane receptors

Functional proteomic methods have been developed and applied to the investigation of signal transduction systems involving platelet-derived growth factor (PDGF), endothelin and bradykinin receptors. Mouse fibroblast cells have been stimulated with PDGF or endothelin. Phosphorylation/dephosphorylation of several hundred proteins has been followed as a function of time following stimulation using 2-D gel electrophoresis and anti-phosphotyrosine or anti-phosphoserine antibodies. Up to 100 of these proteins showed strong changes in phosphorylation with minutes of receptor stimulation. Identification of some of these proteins by mass fingerprinting using matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry and by partial peptide sequencing with ion trap electrospray mass spectrometry has identified proteins which were previously known to be associated with PDGF signaling, proteins which have been shown to be involved in other signaling pathways, but not PDGF and proteins not previously associated with signal transduction. Parallel to these studies, new methods for rapid, single-step isolation of peptide receptors using a peptide coupled to a (dA)30 oligonucleotide have been developed and applied to mass spectrometric studies of post-translational modifications of the endothelin B and bradykinin B2 receptors under in vivo conditions. Both receptors have been shown to undergo extensive phosphorylation as well as palmitoylation. The patterns of post-translational modifications are more complex than previously recognized and provide new indications of possible roles for these modifications in the regulation and response of these receptors.

Conformational analysis of biologically active truncated linear analogs of endothelin-1 using NMR and molecular modeling

Some linear truncated analogs of endothelin-1 display potent agonistic activity at the ET(B) receptor, especially when the side chain of Trp21 is N-formylated. Then, the three-dimensional arrangements of six structurally reduced linear analogs, three formylated and three nonformylated, have been investigated by high resolution NMR spectroscopy and molecular modeling, in order to pinpoint the conformational features related to the biological activity. Two-dimensional double-quantum-filtered correlation spectroscopy (DQFCOSY), total correlation spectroscopy (TOCSY) and nuclear Overhauser enhancement spectroscopy (NOESY) were recorded and analyzed for each molecule. Interspatial distance constraints were derived from the intensity of the NOESY connectivities. The formation of hydrogen bonding was monitored from the temperature dependence of the NH chemical shifts. Molecular models calculated by means of distance geometry, simulated annealing and energy minimization, using the NMR constraints, strongly suggested a global elongated structure for the formylated analogs exhibiting biological activity, and a folded arrangement for the unformylated derivatives. Homology comparisons allowed the identification of a beta-turn-like folding of the C-terminal segment Asp18-Trp21 as a probable key-factor for activity.

Post-translational modifications of endothelin receptor B from bovine lungs analyzed by mass spectrometry

A new mild experimental approach for isolation of peptide membrane receptors and subsequent analysis of post-translational modifications is described. Endothelin receptors A and B were isolated on oligo(dT)-cellulose using N-(epsilon-maleimidocaproyloxy)succinimide endothelin coupled to a protected (dA)-30-mer. This allowed a one-step isolation of the receptor from oligo(dT)-cellulose via variation solely of salt concentration. The identity of the receptor was confirmed by direct amino acid sequencing of electroblotted samples or by using antibodies against ETA and ETB receptors. The method used here is very fast, requires only very mild elution conditions and, for the first time, gave both ETA and ETB receptors concurrently in very good yield. Following enzymatic in-gel digestion, MALDI, and electrospray ion trap mass spectrometric analysis of the isolated endothelin B receptor showed phosphorylation at Ser-304, -418, -438, -439, -440, and -441. Further phosphorylation at either Ser-434 or -435 was observed. The endothelin B receptor is also palmitoylated at Cys residues 402 and 404. Phosphorylation of Ser304 may play a role in Hirschsprung's disease.

Structure of the bradykinin B2 receptors' amino terminus

The peptide hormone bradykinin exerts important biological functions by binding to and activating bradykinin B2 receptors. B2 receptors belong to the seven transmembrane domain (7TM) receptor family. Cloning of the cDNA sequences for the rat, human, and mouse bradykinin B2 receptor revealed several in-frame AUG triplets as potential initiation sites for translation. Due to "Kozak-like" consensus nucleotides, the AUG codon closest to transmembrane domain 1 was assumed the preferred initiation site for translation, but the real amino terminus of the B2 receptor protein was unknown. The amino terminus of several 7TM receptors has been shown to be essentially involved in receptor activation and/or ligand binding. Therefore we determined the amino terminus of the human and of the rat B2 receptor using domain-specific antipeptide antibodies, amino acid sequence analysis, and in vitro transcription/translation. We report that the human and rat B2 receptor protein start with the methionine which is translated from the first in-frame AUG. This start site extends the known amino terminus of the human and rat B2 receptors by 27 or 30 amino acid residues, respectively. Antibodies raised against a peptide of the initial 27 amino acids of the human B2 receptor stained B2 receptors on intact cells. This finding excludes the existence of a signal sequence for this receptor.