Identification of receptor binding and activation sites in endothelin-1 by use of site-directed mutagenesis

Activation mechanism of endothelin ETB receptor by endothelin-1

Endothelin, a 21-amino-acid peptide, participates in various physiological processes, such as regulation of vascular tone, humoral homeostasis, neural crest cell development and neurotransmission. Endothelin and its G-protein-coupled receptor are involved in the development of various diseases, such as pulmonary arterial hypertension, and thus are important therapeutic targets. Here we report crystal structures of human endothelin type B receptor in the ligand-free form and in complex with the endogenous agonist endothelin-1. The structures and mutation analysis reveal the mechanism for the isopeptide selectivity between endothelin-1 and -3. Transmembrane helices 1, 2, 6 and 7 move and envelop the entire endothelin peptide, in a virtually irreversible manner. The agonist-induced conformational changes are propagated to the receptor core and the cytoplasmic G-protein coupling interface, and probably induce conformational flexibility in TM6. A comparison with the M2 muscarinic receptor suggests a shared mechanism for signal transduction in class A G-protein-coupled receptors.

Pharmacological and structural characterization of long-sarafotoxins, a new family of endothelin-like peptides: Role of the C-terminus extension

Long-sarafotoxins (l-SRTXs) have recently been identified in both the venom of Atractaspis microlepidota and that of Atractaspis irregularis. They are characterized by different C-terminus extensions that follow the invariant Trp21, which plays a crucial role in endothelin-receptor binding. We initially determined the toxicity and three-dimensional structures of two chemically synthesized l-SRTXs that have different C-terminus extensions, namely SRTX-m (24 aa, including extension "D-E-P") and SRTX-i3 (25 aa, including extension "V-N-R-N"). Both peptides were shown to be highly toxic in mice and displayed the cysteine-stabilized α-helical motif that characterizes endothelins and short-SRTXs, to which a longer C-terminus with variable flexibility is added. To discern the functional and pharmacological consequences of the supplementary amino acids, different chimerical as well as truncated forms of SRTX were designed and synthesized. Thus, we either removed the extra-C-terminal residues of SRTX-m or i3, or grafted the latter onto the C-terminal extremity of a short-SRTX (s-SRTX) (ie. SRTX-b). Our competitive binding assays where SRTXs competed for iodinated endothelin-1 binding to cloned ET(A) and ET(B) receptor subtypes over-expressed in CHO cells, revealed the essential role of the C-terminus extensions for ET-receptor recognition. Indeed, l-SRTXs displayed an affinity three to four orders of magnitude lower as compared to SRTX-b for the two receptor subtypes. Moreover, grafting the C-terminus extension to SRTX-b induced a drastic decrease in affinity, while its removal (truncated l-SRTXs) yielded an affinity for ET-receptors similar to that of s-SRTXs. Furthermore, we established by intracellular Ca(2+) measurements that l-SRTXs, as well as s-SRTXs, display agonistic activities. We thus confirmed in these functional assays the major difference in potency for these two SRTX families as well as the crucial role of the C-terminus extension in their various pharmacological profiles. Finally, one of the chimeric toxin synthesized in this study appears to be one of the most potent and selective ligand of the ET(B) receptor known to date.

Endothelin degradation by vascular smooth muscle cells

The mechanism(s) of degradation of the potent vasoconstrictor endothelin-1 (ET-1) by rat vascular smooth muscle A-10 cells, which possess the ETA receptor subtype, was investigated by incubating [125I]ET-1 (0.1 nM) with cells for 0-4 h at 37 degrees C in the presence and absence of lysosomal enzyme inhibitors, NH4Cl and chloroquine, and a neutral endopeptidase inhibitor, phosphoramidon. The assay buffer and cell extracts were analyzed by reverse-phase HPLC, and the radioactivity in the fractions was measured. In the absence of inhibitors, most of the radioactivity in the medium was in the form of [125I]Tyr after a 4 h incubation. When [125I]ET-1 was incubated with A-10 cells at 4 degrees C, six radiolabeled peaks, including some [125I]Tyr and about 30% of the original [125I]ET-1, were present in the medium. In the presence of 5 microM chloroquine there was no [125I]Tyr peak in the medium, indicating that internalization and putative lysosomal degradation of ET-1 were blocked. NH4Cl (50 and 100 mM) also reduced the amount of [125I]Tyr formed. The presence of ET-1 fragments indicated that, in addition to lysosomal degradation, some of the ligand is metabolized by enzymes located on the cell membrane; we demonstrated, however, that secreted proteases from A-10 cells are not involved in the degradation of ET-1. The neutral endopeptidase inhibitor, phosphoramidon, did not completely inhibit the metabolism of [125I]ET-1 to [125I]Tyr. These results establish that various cell-associated enzymes are capable of degrading ET-1 in A-10 cells. Moreover, analysis of the cell lysates indicated the presence of a relatively stable pool of ET-1-occupied receptors or compartmentalized ET-1, protected from cell proteases, which may contribute to the potent contractility of ET-1.