Identification of an extracellular segment of the oxytocin receptor providing agonist-specific binding epitopes

Oxytocin in the Male Reproductive Tract; The Therapeutic Potential of Oxytocin-Agonists and-Antagonists

In this review, the role of oxytocin and oxytocin-like agents (acting via the oxytocin receptor and belonging to the oxytocin-family) in the male reproductive tract is considered. Previous research (dating back over 60 years) is revised and connected with recently found aspects of the role oxytocin plays in male reproductive health. The local expression of oxytocin and its receptor in the male reproductive tract of different species is summarized. Colocalization and possible crosstalk to other agents and receptors and their resulting effects are discussed. The role of the newly reported oxytocin focused signaling pathways in the male reproductive tract, other than mediating contractility, is critically examined. The structure and effect of the most promising oxytocin-agonists and -antagonists are reviewed for their potential in treating male disorders with origins in the male reproductive tract such as prostate diseases and ejaculatory disorders.

The oxytocin system in drug discovery for autism: animal models and novel therapeutic strategies

Animal models and behavioral paradigms are critical for elucidating the neural mechanism involved in complex behaviors, including social cognition. Both genotype and phenotype based models have implicated the neuropeptide oxytocin (OT) in the regulation of social behavior. Based on the findings in animal models, alteration of the OT system has been hypothesized to play a role in the social deficits associated with autism and other neuropsychiatric disorders. While the evidence linking the peptide to the etiology of the disorder is not yet conclusive, evidence from multiple animal models suggest modulation of the OT system may be a viable strategy for the pharmacological treatment of social deficits. In this review, we will discuss how animal models have been utilized to understand the role of OT in social cognition and how those findings can be applied to the conceptualization and treatment of the social impairments in ASD. Animal models with genetic alterations of the OT system, like the OT, OT receptor and CD38 knock-out mice, and those with phenotypic variation in social behavior, like BTBR inbred mice and prairie voles, coupled with behavioral paradigms with face and construct validity may prove to have predictive validity for identifying the most efficacious methods of stimulating the OT system to enhance social cognition in humans. The widespread use of strong animal models of social cognition has the potential yield pharmacological, interventions for the treatment social impairments psychiatric disorders. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

Extracellular loop 3 (ECL3) and ECL3-proximal transmembrane domains VI and VII of the mesotocin and vasotocin receptors confer differential ligand selectivity and signaling activity

Mesotocin (MT) and vasotocin (VT) are the nonmammalian orthologs of mammalian oxytocin (OT) and arginine vasopressin (AVP), respectively. The OT/AVP family of peptides has arisen from gene duplication but has evolved to possess high selectivity toward their cognate receptors. The process of molecular evolution of receptors to confer high selectivity to their cognate ligands, however, is poorly understood. We constructed a series of reciprocal chimeras using a pair of bullfrog MT receptor (MTR) and VT1 receptor (VT1R) DNA fragments. Among the MTR/VT1R chimeras, the MTR chimera containing a region from transmembrane domain (TMD) VI to the carboxyl-terminal tail (C-tail) of VT1R showed an increased sensitivity to VT, while a chimeric VT1R containing TMD VI to C-tail of MTR showed an increased sensitivity to MT. Further dissection of domains using additional chimeras demonstrated that the receptor with the fragment containing extracellular loop 3 (ECL3) and ECL3-proximal TMDs VI and VII of MTR increased MT selectivity. This fragment is also important for receptor conformation that permits the signaling ability of the receptor. Particularly, the amino acids Val/Ile(6.54) in TMD VI and Pro/Glu(7.29) in ECL3 appear to be involved in this activity, since double mutation of these amino acids completely blocked signaling activity while maintaining ligand binding activity. Mutations at these residues in human OT and AVP 1a receptors markedly decreased receptor signaling activity. This study provides clues for understanding molecular coevolution of the OT/AVP peptides and their receptors with regard to receptor-ligand binding and receptor signaling activity.

Receptor-based QSAR studies of non-peptide human oxytocin receptor antagonists

In the present study, QSAR calculations were performed on the receptor-based alignment of 58 non-peptide human oxytocin receptor antagonists. With the aid of different scoring functions (AutoDock 3.05 built-in and X-Score 1.2) the evolved receptor-ligand complexes were characterized. By means of various datasets it was confirmed that the scoring functions were not capable to predict the biological activity correctly in compounds containing a rigid derivative in the variable region. To improve the pKi prediction 3D-QSAR calculation was performed. The regions related to the biological activity were determined by using cross-validated r2(q2)-guided region selection (q2-GRS) method. The predictive power of the CoMFA model [r(pred)2=0.89, q2(LMO, five groups)=0.695+/-0.034] allowed prediction of the biological activities of newly synthesized compounds and confirmed the receptor-based alignment.

Binding domains of the oxytocin receptor for the selective oxytocin receptor antagonist barusiban in comparison to the agonists oxytocin and carbetocin

We have analyzed binding domains of the oxytocin receptor for barusiban, a highly selective oxytocin receptor antagonist, in comparison to the combined vasopressin V1A/oxytocin receptor antagonist atosiban and the agonists oxytocin and carbetocin. For this purpose, chimeric 'gain-in function' oxytocin/vasopressin V2 receptors were expressed in COS-7 cells. These recombinant receptors have been produced by transfer of domains from the oxytocin receptor into the related vasopressin V2 receptor and have already been successfully employed for the identification of ligand binding domains at the oxytocin receptor (Postina, R., Kojro, E., Fahrenholz, F., 1996. Separate agonist and peptide antagonist binding sites of the oxytocin receptor defined by their transfer into the V2 vasopressin receptor. J. Biol. Chem. 271, 31593-31601). In displacement studies with 10 chimeric receptor constructs, the binding profile of barusiban was compared with the binding profiles of the ligands oxytocin, [Arg8]vasopressin, carbetocin, and atosiban. The binding profiles for the agonists oxytocin and carbetocin were found to be similar. For both agonists, important binding domains were the extracellular N-terminus (=E1) and the extracellular loops E2 and E3 from the oxytocin receptor. For the vasopressin V1A/oxytocin receptor antagonist atosiban, none of the receptor constructs were able to provide a binding with higher affinity than the starting vasopressin V2 receptor. In contrast, the binding of barusiban was significantly improved when the transmembrane domains 1 and 2 were transferred from the oxytocin receptor to the vasopressin V2 receptor. The binding domain of barusiban differs from the binding domain of the agonists and the nonselective oxytocin receptor antagonist d(CH2)5[Tyr-(Me)2,Thr4,Orn8,Tyr9]vasotocin that has been used in previous studies. Overall, the data supported the concept of a central pocket site within the oxytocin receptor.

Synthesis and biological evaluation of oxytocin analogues containing L-alpha-t-butylglycine [Gly(Bu t)] in positions 8 or 9

We report the solid phase synthesis and some pharmacological properties of seventeen new oxytocin (OT) analogues. Basic modification at positions 8 and/or 9 (introduction of L-alpha-t-butylglycine [Gly(Bu(t))]) was combined with D-Cys(6), D-Tyr(Et)(2), Mpa(1) or Pen(1) modifications and their various combinations. We also present properties of two previously reported re-synthesized analogues ([Gly(Bu(t))(8)]OT and [Mpa(1), Gly(Bu(t))(8)]OT). The analogues were tested for rat uterotonic activity in vitro, in the rat pressor assay and for binding affinity to human OTR.

Identification of the glycosylation sites utilized on the V1a vasopressin receptor and assessment of their role in receptor signalling and expression

Most of the large family of G-protein-coupled receptors (GPCRs) possess putative N-glycosylation sites within their N-termini. However, for the vast majority of GPCRs, it has not been determined which, if any, of the consensus glycosylation sites are actually utilized or what the functional ramifications are of modification by oligosaccharide. The occurrence and function of glycosylation of the V(1a) vasopressin receptor (V(1a)R) has been investigated in this study. Using a combination of translation systems that are either glycosylation-competent or do not support glycosylation, we established that of the four putative N-glycosylation sites at Asn(14), Asn(27), Asn(198) and Asn(333) only the first three sites are actually modified by carbohydrate. This was confirmed by disruption of consensus sites by site-directed mutagenesis, individually and in combination. The V(1a)R is not O-glycosylated. The functionality of a series of glycosylation-defective V(1a)R constructs was characterized after expression in HEK 293T cells. It was found that carbohydrate moieties are not required for the receptor to bind any of the four classes of ligand available, or for intracellular signalling. The glycosylation status of the V(1a)R did, however, regulate the level of total receptor expression and also the abundance of receptor at the cell surface. Furthermore, the nature of this regulation (increased or decreased expression) was dictated by the locus of the oligosaccharide modification. Modification of any one of the consensus sites alone, however, was sufficient for wild-type expression, indicating a redundancy within the glycosylation sites. A role for the carbohydrate in the correct folding or stabilization of the V(1a)R is indicated. Glycosylation is not required, however, for efficient trafficking of the receptor to the cell surface. This study establishes the functional importance of N-glycosylation of the V(1a)R.