Probing the V1a vasopressin receptor binding site with pyroglutamate-substituted linear antagonists

Novel mastoparan analogs induce differential secretion from mast cells

Cationic amphiphilic peptides stimulate secretion via a receptor-independent action upon G proteins. We have previously utilized chimeric analogs of mastoparan (MP), including galparan (galanin(1-13)-MP ), as molecular probes of secretion. Here, we further resolve the structure-activity relationship of peptidyl secretagogs, including rationally designed chimeric MP analogs. The secretory efficacies of 10 MP analogs were significantly higher than 45 unrelated basic peptides. Comparative studies identified MP analogs that are differential secretagogs for 5-hydroxytryptamine (5-HT) and beta-hexosaminidase. Peptide-induced activation of phospholipase D (PLD), an enzyme intimately involved in regulated exocytosis [5], correlated with the secretion of beta-hexosaminidase but not 5-HT. Thus, these data indicate that different mechanisms are responsible for the exocytosis of 5-HT and beta-hexosaminidase, respectively. Moreover, mastoparan analogs are novel tools for probing the molecular details of exocytosis and other biological phenomena.

Critical role of a subdomain of the N-terminus of the V1a vasopressin receptor for binding agonists but not antagonists; functional rescue by the oxytocin receptor N-terminus

A fundamental issue in molecular pharmacology is to define how agonist:receptor interaction differs from that of antagonist:receptor. The V(1a) receptor (V(1a)R) is a member of a family of related G-protein-coupled receptors that are activated by the neurohypophysial peptide hormone arginine-vasopressin (AVP). Here we define a short subdomain of the N-terminus of the V(1a)R from Glu(37) to Asn(47) that is an absolute requirement for binding AVP and other agonists. In marked contrast to the situation for agonists, deleting this segment has little or no effect on the binding of either peptide or non-peptide antagonists. In addition, we established that this subdomain was crucial for receptor activation and second messenger generation. The oxytocin receptor (OTR) also binds AVP with high affinity but exhibits a different pharmacological profile to the V(1a)R. Substitution of the N-terminus of the V(1a)R with the corresponding sequence from the OTR generated a chimeric receptor (OTR(N)-V(1a)R). The presence of the OTR N-terminus recovered high affinity agonist binding such that the OTR(N)-V(1a)R possessed almost wild-type V(1a)R pharmacology and signaling. Consequently, a domain within the N-terminus is required for agonist binding but it does not provide the molecular discriminator for subtype-selective agonist recognition. Cotransfection and peptide mimetic studies demonstrated that this N-terminal subdomain had to be contiguous with the receptor polypeptide to be functional. This study establishes that a segment of the V(1a)R N-terminus has a pivotal role in the mechanism of agonist binding and provides molecular insight into key differences between the interaction of agonists and antagonists with a peptide receptor family.

Effects of vasopressin-mastoparan chimeric peptides on insulin release and G-protein activity

Two chimeric peptides, consisting of the linear vasopressin receptor V1 antagonist PhAc-D-Tyr(Me)-Phe-Gln-Asn-Arg-Pro-Arg-Tyr, in the N-terminus and mastoparan in the C-terminus connected directly (M375) or via 6-aminohexanoic acid (M391), have been synthesised. At 10 microM concentration, these novel peptides increased insulin secretion from isolated rat pancreatic islet cells 18-26-fold at 3.3 mM glucose and 3.5-5-fold at 16.7 mM glucose. PTX pretreatment of the islets decreased the peptide-induced insulin release. M375 and M391 bind to V1a vasopressin receptors with affinities lower than the unmodified vasopressin antagonist, but with K(D) values of 3.76 nM and 9.02 nM, respectively, both chimeras are high affinity ligands. The GTPase activity and GTPgammaS binding in the presence of these peptides has been characterised in Rin m5F cells. Comparison of the influence of the peptides M375 and M391 on GTPase activity in native and pertussis toxin-treated cells suggests a selective activation of G alpha(i)/G alpha(o) subunits, combined with a suppression of other GTPases, primarily G alpha(s). However, the GTPgammaS binding data show that the peptides retain some of the activating property even in PTX-treated cell membranes. In conclusion, the conjugation of mastoparan with the V1a receptor antagonists produce peptides with properties different from the parent peptides that could be used to elucidate the role of different G proteins in insulin release.

Characterization of G protein-coupled receptors expressed by ECV304 human endothelial cells

The aims of this study were to characterize G protein-coupled receptors endogenously expressed by ECV304 human endothelial cells, and to determine the utility of this transformed cell line as a vehicle for the expression of cloned receptors. Cellular responses to a broad range of agonists were determined by measuring changes in the intracellular content of second messengers (inositol phosphates and cyclic adenosine monophosphate). These studies identified H1 histamine receptors, P2U-purinoceptors and lysophosphatidic acid receptors which are functionally coupled to phosphoinositidase C. G protein-coupled receptors which bind adenosine (A2 receptor), calcitonin, and adrenaline (beta-adrenoceptor), and markedly stimulate adenylyl cyclase, are also endogenously expressed by ECV304. Agonists which did not stimulate ECV304 cells are: angiotensin II, angiotensin1-7, bombesin, bradykinin, desArg9-bradykinin, carbachol, endothelin-1, neurotensin, serotonin, substance K, substance P, thrombin and vasopressin. The rat Via vasopressin receptor was expressed by lipofection in two antibiotic-resistant clonal lines and expression confirmed by measuring agonist-induced changes in inostol phosphate production. We conclude that the ECV304 cell line is a suitable in vitro system to study the signal transduction pathways of some endogenous G protein-coupled receptors known to modulate endothelial function in vivo. ECV304 is also appropriate for the expression and functional characterization of cloned receptor proteins.

Biochemical mechanisms of calcium mobilisation induced by mastoparan and chimeric hormone-mastoparan constructs

Ca2+ efflux, Ca(2+)-ATPase, and membrane permeability measurements were used to investigate the biochemical mechanisms of Ca2+ release induced by mastoparan (MP) and the chimeric hormone-MP constructs incorporating galanin (galparan) or vasopressin antagonist (M375 and M391) moieties. Comparative studies utilised preparations of porcine cerebellar microsomes and rabbit skeletal muscle sarcoplasmic reticulum (SR). MP and chimeric peptides galparan, M375 and M391 induce Ca2+ release over a range of concentrations from 0.3-10 microM. Comparison of MP and three chimeric, N-terminal extended, constructs indicates that N-terminal extension modifies the biological properties of MP, producing changes in efficacy which are enzyme-isoform-specific. Biochemical studies indicate that the chimeric analogues and MP inhibit Ca(2+)-ATPases and directly activate the ryanodine receptor (RyR) to release Ca2+ from both heavy SR (HSR) and microsomes. The same peptides have no effect on the InsP3 receptor (InsP3R). Other actions that include modest changes in membrane permeability may also contribute to the Ca(2+)-mobilising action of MP and chimeric constructs.

Renal bradykinin and vasopressin receptors: ligand selectivity and classification

We studied the specific binding of radiolabeled bradykinin ([3H]BK) and vasopressin ([3H]AVP) to membrane preparations of bovine and porcine kidney medulla. [3H]BK reversibly labeled a single site (Kd = 1.06 nM) in bovine kidney medulla independently of [Mg2+]. The number of BK receptors in bovine kidney medulla, Bmax = 122 fmol/mg protein, is markedly (2- to 3-fold) higher than that reported in other tissues. Further characterization by ligand binding indicated that the bovine bradykinin receptor was the B2a subtype, pharmacologically related to B2a receptors expressed by human and rabbit tissues. In contrast, the specific binding of [3H]BK, but not [3H]AVP, to porcine kidney medulla (Kd = 0.32 nM, Bmax = 45 fmol/mg) was dependent upon the presence of enzyme inhibitors to prevent the rapid and selective degradation of bradykinin. Interspecies differences were revealed for renal medulla V2 vasopressin receptors with respect to their abundance and their affinity for several V2-selective ligands. In summary, (i) bovine kidney medulla is a convenient source of tissue for studying the B2a bradykinin receptor subtype; (ii) there are significant species-dependent differences in both the abundance of renal medulla B2a and V2 receptors and the ligand selectivity of V2 receptors; and (iii) these findings are significant in relation to the physiological and pathological roles of renal kinins and their interaction with the neurohypophysial peptide hormone system.