PubMed Health⌕ Search

Biomedical subjects

Robin Sellar

Publications and source records attributed to Robin Sellar.

8 recordsLinked to original sources
PubMed2006

Bovine and ovine gonadotropin-releasing hormone (GnRH)-II ligand precursors and type II GnRH receptor genes are functionally inactivated.

The decapeptide sequence of GnRH-II is conserved in all jawed vertebrate species studied to date. New data for cattle (Bos taurus) indicates a gene encoding GnRH-II decapeptide possessing arginine (codon: CGG) rather than tryptophan (TGG) at position three in the mature peptide. This substitution is unique. We confirmed the DNA sequence after cloning part of the bovine prepro-GnRH-II gene. Bovine GnRH-II peptide was synthesized and pharmacologically characterized. It did not bind to mammalian GnRH receptors expressed in different types of cell nor did it exhibit agonist or antagonist properties on types I or II GnRH receptors expressed in COS-7 cells. Bovine primers facilitated cloning of ovine GnRH-II DNA. A premature stop codon (TGA) replaces the expected tryptophan codon at position seven of GnRH-II in sheep DNA. Thus, both species possess prepro-GnRH-II genes encoding inactive peptides, as previously described for chimpanzee GnRH-II. The updated bovine type II GnRH receptor gene sequence revealed inactivation by frame shifts, premature stop codons, and nucleotide changes specifying nonconservative replacement of amino acid residues, similar to inactivation of sheep type II GnRH receptor. Spliced RNA transcripts from the disrupted receptor gene were not detected in bovine pituitary. In contrast, bovine prepro-GnRH-I and type I GnRH receptor genes are intact, encoding well-conserved protein sequences. These findings, and previous descriptions of inactivation of the human type II GnRH receptor and deletions of prepro-GnRH-II and type II GnRH receptor in laboratory rodents, suggest the GnRH-II system has been replaced by the GnRH-I system or is redundant in certain mammals.

Amino Acid Sequence↗
PubMed2006

Interobserver agreement in magnetic resonance brain and neck imaging.

AIM: To investigate the interobserver reliability of brain and neck magnetic resonance imaging (MRI) in older community subjects previously recruited to a case-control study of dizziness. METHODS: Subjects aged over 65 years were identified through a local survey and advertising. Several investigations were performed, including MRI of the head and neck using a Siemens 1.5 Tesla machine. Images were coded independently by two consultant neuroradiologists. Interobserver agreement was calculated using kappa statistics. RESULTS: Of the 246 participants, scans from 84 dizzy subjects and 79 non-dizzy subjects were available for coding by two neuroradiologists. Agreement was "fair" for cerebral atrophy, "good" for white matter lesions (WML) in the cerebral hemispheres, and moderate for WML in the posterior fossa. Agreement for cervical spine disease ranged from "moderate" (severity of cord compression) to "poor" (vertebral artery occlusion). CONCLUSION: Interobserver variability in the evaluation of brain and cervical cord MRI was substantial. The development of standardized scores, particularly for disease of the cervical cord, may improve interobserver reliability.

Aged↗
PubMed2005

Bifunctional gonadotropin-releasing hormone antagonist-progesterone analogs with increased efficacy and duration of action.

GnRH peptide analogs are widely used to treat diverse clinical conditions. However, they have poor oral activity and exhibit rapid metabolic clearance, thus requiring injection and depot formulation. Because steroid hormones are bound to plasma proteins, we explored the possibility of conjugating hydroxylated progesterones to GnRH analogs to reduce metabolic clearance of the peptides. Conjugation of [D-Lys6]GnRH agonist to the alpha11-hydroxyl of alpha11-hydroxyl progesterone via a hemi-succinate bridge increased the plasma half-life after iv injection in rabbits by 3.6-fold while retaining high binding affinity, thus providing proof of concept. Five GnRH antagonists were then synthesized with 21-hydroxyprogesterone conjugated via C21-hydroxyl to positions six (conjugates A and B) and position seven (conjugates C and D) of GnRH antagonists. In the fifth compound the NH2 terminus of a GnRH antagonist lacking the first two amino acids was conjugated via the C21-hydroxyl to 21-hydroxyprogesterone (conjugate E). All five analogs bound to guinea pig progesterone binding globulin with relatively high affinities (264-1020 nM). Moreover, all five conjugates retained high progestogenic activity in stimulating a progesterone-response-element-driven chloramphenicol acetyltransferase reporter gene in the T47D breast cancer cell line. Conjugation via the epsilon-amino function of D-Lys6 (conjugates A and B) produced compounds with high binding affinity for the human GnRH receptor (15 and 7 nM) comparable to that of the unconjugated GnRH antagonists (4 and 26 nM). Conjugation via the epsilon-amino function of Lys7 (conjugates C and D) or the NH2 terminus of an N-terminally truncated antagonist (conjugate E) produced compounds of low binding affinity. Conjugates A and B also exhibited high functional antagonism of GnRH stimulation of inositol phosphate production in COS-7 cells expressing the human GnRH receptor (2.6 and 16 nM) compared with the unconjugated antagonists (1.3 and 122 nM). In accordance with their poor receptor binding affinity, conjugates C, D, and E had poor functional antagonism. Preliminary dose-finding studies in female marmosets showed transitory progesterone inhibition by 0.25 mg and prolonged suppression of 12 and 17 d by 0.5- and 1.0-mg doses. Injection of conjugate A in adult male marmosets (0.5 mg sc) rapidly suppressed plasma testosterone levels, which remained suppressed for at least 3 d. In contrast, the unconjugated parent antagonist alone or with progesterone suppressed testosterone for only 8 h to 1 d. The findings demonstrate that conjugation of progesterone to GnRH antagonists conveys plasma binding and progestogenic properties and increases their efficacy and duration of action in vivo. These new GnRH antagonists show promise as therapeutic agents for hormone-dependent diseases and as contraceptives.

Animals↗
PubMed2005

Evolution of constrained gonadotropin-releasing hormone ligand conformation and receptor selectivity.

Gonadotropin-releasing hormone (GnRH) is the central regulator of reproduction in vertebrates. GnRHs have recently been identified in protochordates and retain the conserved N- and C-terminal domains involved in receptor binding and activation. GnRHs of the jawed vertebrates have a central achiral amino acid (glycine) that favors a type II' beta-turn such that the N- and C-terminal domains are closely apposed in binding the GnRH receptor. However, protochordate GnRHs have a chiral amino acid in this position, suggesting that they bind their receptors in a more extended form. We demonstrate here that a protochordate GnRH receptor does not distinguish GnRHs with achiral or chiral amino acids, whereas GnRH receptors of jawed vertebrates are highly selective for GnRHs with the central achiral glycine. The poor activity of the protochordate GnRH was increased >10-fold at vertebrate receptors by replacement of the chiral amino acid with glycine or a d-amino acid, which favor the type II' beta-turn. Structural analysis of the GnRHs using ion mobility-mass spectrometry and molecular modeling showed a greater propensity for a type II' beta-turn in GnRHs with glycine or a d-amino acid, which correlates with binding affinity at vertebrate receptors. These findings indicate that the substitution of glycine for a chiral amino acid in GnRH during evolution allows a more constrained conformation for receptor binding and that this subtle single amino acid substitution in a site remote from the ligand functional domains has marked effects on its structure and activity.

Amino Acid Sequence↗
PubMed2005

Mutations remote from the human gonadotropin-releasing hormone (GnRH) receptor-binding sites specifically increase binding affinity for GnRH II but not GnRH I: evidence for ligand-selective, receptor-active conformations.

The human gonadotropin-releasing hormone (GnRH) receptor is evolutionarily configured for high affinity binding of GnRH I ([Tyr(5),Leu(7),Arg(8)]GnRH) but at lower affinity for GnRH II ([His(5),Trp(7),Tyr(8)]GnRH). GnRH I is more potent in the activation of the G(q/11) protein in the gonadotrope; however, GnRH II is more potent in the stimulation of apoptosis and antiproliferative effects through activating G(i) protein-mediated signaling, implying that GnRH I and II selectively stabilize different receptor-active conformations that preferentially couple to different signaling pathways. Receptor activation involves ligand induction or conformational selection, but the molecular basis of the communication between ligand-binding sites and receptor allosteric sites remains unclear. We have sought conformational coupling between receptor-ligand intermolecular interactions and intramolecular interaction networks in the human GnRH receptor by mutating remote residues that induce differential ligand binding affinity shifts for GnRH I and II. We have demonstrated that certain Ala mutations in the intracellular segments of transmembrane domains 3 (Met(132)), 5 (Met(227)), 6 (Phe(272) and Phe(276)), and 7 (Ile(322) and Tyr(323)) of the human GnRH receptor allosterically increased ligand binding affinity for GnRH II but had little effect on GnRH I binding affinity. We examined the role of the three amino acids that differ in these two ligands, and we found that Tyr(8) in GnRH II plays a dominant role for the increased affinity of the receptor mutants for GnRH II. We propose that creation of a high affinity binding site for GnRH II accompanies receptor conformational changes, i.e."induced fit" or "conformational selection," mainly determined by the intermolecular interactions between Tyr(8) and the receptor contact residues, which can be facilitated by disruption of particular sets of receptor-stabilizing intramolecular interactions. The findings suggest that GnRH I and II binding may selectively stabilize different receptor-active conformations and therefore different ligand-induced selective signaling described previously for these ligands.

Animals↗
PubMed2003

The ISAT trial.

Explore the source record for details and available documents.

Angioplasty↗
PubMed2003

A transcriptionally active human type II gonadotropin-releasing hormone receptor gene homolog overlaps two genes in the antisense orientation on chromosome 1q.12.

GnRH-II peptide hormone exhibits complete sequence conservation across vertebrate species, including man. Type-II GnRH receptor genes have been characterized recently in nonhuman primates, but the human receptor gene homolog contains a frameshift, a premature stop codon (UGA), and a 3' overlap of the RBM8A gene on chromosome 1q.12. A retrotransposed pseudogene, RBM8B, retains partial receptor sequence. In this study, bioinformatics show that the human receptor gene promoter overlaps the peroxisomal protein 11-beta gene promoter and the premature UGA is positionally conserved in chimpanzee. A CGA [arginine (Arg)] occurs in porcine DNA, but UGA is shifted one codon to the 5' direction in bovine DNA, suggesting independent evolution of premature stop codons. In contrast to marmoset tissue RNA, exon- and strand-specific probes are required to distinguish differently spliced human receptor gene transcripts in cell lines (HP75, IMR-32). RBM8B is not transcribed. Sequencing of cDNAs for spliced receptor mRNAs showed no evidence for alteration of the premature UGA by RNA editing, but alternative splicing circumvents the frameshift to encode a two-membrane-domain protein before this UGA. A stem-loop motif resembling a selenocysteine insertion sequence and a potential alternative translation initiation site might enable expression of further proteins involved in interactions within the GnRH system.

Adenoma↗