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P R Schofield

Publications and source records attributed to P R Schofield.

At least 73 records · Page 4Linked to original sources
PubMed1995

A mutation in codon 717 of the amyloid precursor protein gene in an Australian family with Alzheimer's disease.

DNA from the probands of seven Australian families with hereditary Alzheimer's disease was screened for the presence of known mutations in the amyloid precursor protein (APP) gene on chromosome 21 using single stranded conformational polymorphism (SSCP) analysis [14]. One subject was found to have a mutation causing a Val-->Ile substitution at position 717. This was confirmed by restriction enzyme digestion and sequencing. The mutation has been found in both the other affected family members available for study and in two at-risk relatives. It was not present in the only living unaffected relative who has passed the usual age of onset in this family. There is so far no evidence that apolipoprotein E (APOE) genotype influences age of onset in this family, though numbers are small. Two other families with autopsy confirmation and age of onset in the fifth decade had no APP mutation and are thought likely to have a mutation on chromosome 14 on the basis of their earlier onset age.

Alzheimer Disease↗
PubMed1995

The unique extracellular disulfide loop of the glycine receptor is a principal ligand binding element.

A loop structure, formed by the putative disulfide bridging of Cys198 and Cys209, is a principal element of the ligand binding site in the glycine receptor (GlyR). Disruption of the loop's tertiary structure by Ser mutations of these Cys residues either prevented receptor assembly on the cell surface, or created receptors unable to be activated by agonists or to bind the competitive antagonist, strychnine. Mutation of residues Lys200, Tyr202 and Thr204 within this loop reduced agonist binding and channel activation sensitivities by up to 55-, 520- and 190-fold, respectively, without altering maximal current sizes, and mutations of Lys200 and Tyr202 abolished strychnine binding to the receptor. Removal of the hydroxyl moiety from Tyr202 by mutation to Phe profoundly reduced agonist sensitivity, whilst removal of the benzene ring abolished strychnine binding, thus demonstrating that Tyr202 is crucial for both agonist and antagonist binding to the GlyR. Tyr202 also influences receptor assembly on the cell surface, with only large chain substitutions (Phe, Leu and Arg, but not Thr, Ser and Ala) forming functional receptors. Our data demonstrate the presence of a second ligand binding site in the GlyR, consistent with the three-loop model of ligand binding to the ligand-gated ion channel superfamily.

Amino Acid Sequence↗
PubMed1995

Mutations affecting the glycine receptor agonist transduction mechanism convert the competitive antagonist, picrotoxin, into an allosteric potentiator.

Contrary to its effects on the gamma-aminobutyric acid type A receptor, picrotoxin antagonism of the alpha 1 subunit of the human glycine receptor is shown to be competitive, not use-dependent, and nonselective between the picrotoxin components, picrotin, and picrotoxinin. Competitive antagonism and non-use dependence are consistent with picrotoxin binding to a site in the extracellular domain. The mutations Arg-->Leu or Arg-->Gln at residue 271 of the glycine receptor alpha 1 subunit, which are both associated with human startle disease, have previously been demonstrated to disrupt the transduction process between agonist binding and channel activation. We show here that these mutations also transform picrotoxin from an allosterically acting competitive antagonist to an allosteric potentiator at low (0.01-3 microM) concentrations and to a noncompetitive antagonist at higher (> or = 3 microM) concentrations. This demonstrates that arginine 271 is involved in the transduction process between picrotoxin binding and its mechanism of action. Thus, the allosteric transduction pathways of both agonists and antagonists converge at a common residue prior to the activation gate of the channel, suggesting that this residue may act as an integration point for information from various extracellular ligand binding sites.

Allosteric Regulation↗
PubMed1995

Mutation of an arginine residue in the human glycine receptor transforms beta-alanine and taurine from agonists into competitive antagonists.

Agonist binding to the inhibitory glycine receptor (GlyR) initiates the opening of a chloride-selective channel that modulates the neuronal membrane potential. Point mutations of the GlyR, substituting Arg-271 with either Leu or Gln, have been shown to underlie the inherited neurological disorder startle disease (hyperekplexia). We show that these substitutions result in the redistribution of GlyR single-channel conductances to lower conductance levels. Additionally, the binding of the glycinergic agonists beta-alanine and taurine to mutated GlyRs does not initiate a chloride current, but instead competitively antagonizes currents activated by glycine. These findings are consistent with mutations of Arg-271 resulting in the uncoupling of the agonist binding process from the channel activation mechanism of the receptor.

Arginine↗
PubMed1995

Both of the beta-subunit carbohydrate residues of follicle-stimulating hormone determine the metabolic clearance rate and in vivo potency.

FSH is a glycoprotein hormone required for the development and maturation of the ovarian follicle and for spermatogenesis. FSH is glycosylated at asparagine residues 52 and 78 on the alpha-subunit and residues 7 and 24 on the beta-subunit. In vitro, the carbohydrate residue at position alpha 52 is required for signal transduction. To define the contribution of the carbohydrate residues to FSH potency in vivo, we assessed the MCR and in vivo bioactivity of site-specifically deglycosylated recombinant human FSH variants. The removal of the beta-subunit carbohydrate residues significantly (P < 0.05) affected the MCR and resulted in significantly (P < 0.05) reduced in vivo bioactivity. For all recombinant human FSH variants, a strong correlation (r = 0.90; P < 0.01) was observed between MCR and in vivo potency, indicating that the circulatory half-life of the hormone appears to be the primary determinant of in vivo bioactivity. Although the beta-subunit carbohydrate residues have the greatest effect in determining FSH potency in vivo; the alpha 52 residue, important in vitro, has no effect on either MCR or in vivo potency. This study highlights the difficulties of translating in vitro results to whole animal physiology.

Animals↗
PubMed1995

Molecular mechanisms of inherited startle syndromes.

Inherited neurological disorders involving an exaggerated startle response to unexpected sensory stimuli have been identified in mice, cows, dogs, horses and humans. Recent studies of the molecular pathology of a number of these startle syndromes have revealed that they are caused by defects in the inhibitory glycinergic pathways that mediate reciprocal and recurrent inhibition in the spinal cord. These defects arise from various mutations of the receptor for glycine, which either impair its sensitivity to agonists or reduce its expression in vivo. The emergent models of the molecular mechanisms that underlie startle disorders illustrate how diverse mutations can converge physiologically to produce a common phenotype.

Amino Acid Sequence↗
PubMed1994

Startle disease mutations reduce the agonist sensitivity of the human inhibitory glycine receptor.

The receptor for the inhibitory neurotransmitter glycine is a member of the ligand-gated ion channel receptor superfamily. Point mutations in the gene encoding the alpha 1 subunit of the glycine receptor-channel complex (GlyR) have recently been identified in pedigrees with the autosomal dominant neurological disorder, startle disease (hyperekplexia). These mutations result in the substitution of leucine or glutamine for arginine 271. This charged residue is located near the ion channel region and is predicted to affect chloride permeation through the GlyR. We found little evidence for this role from the anion/cation selectivity and lack of pronounced rectification of currents flowing through recombinant human alpha 1 subunit GlyRs containing the startle disease mutations. We reveal, however, that the startle disease mutations profoundly disrupt GlyR function by causing 230-410-fold decreases in the sensitivity of receptor currents activated by the agonist glycine. Additionally, we report corresponding 56- and 120-fold reductions in the apparent binding affinity (Ki) of glycine to the mutant GlyRs, but no change in the binding affinity of the competitive antagonist, strychnine. Thus, startle disease reduces the efficacy of glycinergic inhibitory neurotransmission by producing GlyRs with diminished agonist responsiveness. Our results show that startle disease mutations define a novel receptor activation site.

Humans↗
PubMed1994

A threonine residue in the seventh transmembrane domain of the human A1 adenosine receptor mediates specific agonist binding.

The A1 adenosine receptor is a member of the seven-transmembrane G protein-coupled, receptor superfamily. This receptor binds the purine nucleoside adenosine with high affinity and inhibits the activity of adenylate cyclase. We have used site-directed mutagenesis and functional expression studies to examine the role of the threonine residue, located at position 277 in transmembrane domain VII of the human A1 receptor. Mutation of Thr-277 to either serine or alanine resulted in the expression of receptors that had essentially no change in binding affinity for the A1 selective antagonist 8-cyclo-pentyl-1,3-dipropylxanthine. Mutation of Thr-277 to serine resulted in modest (4.4-8.6-fold) but significant increases in the observed Ki values for three adenosine agonists, namely N-(1-methyl-2-phenethyl)adenosine (R-PIA or S-PIA) and 1-(6-amino-9H-purin-9-yl)-1-deoxy-N-ethyl-beta-L- ribofuranuronamide) (NECA). However, mutation of Thr-277 to alanine resulted in no significant changes in the Ki for R-PIA or S-PIA but did result in a highly significant 437-fold increase in the Ki for NECA. This demonstrates that the hydroxyl moiety of Thr-277 mediates agonist but not antagonist binding and, more specifically, that this residue forms a probable molecular contact site with the 5' substitution found in NECA.

Adenosine↗
PubMed1994

Specific roles for the asparagine-linked carbohydrate residues of recombinant human follicle stimulating hormone in receptor binding and signal transduction.

FSH comprises two distinct subunits, both of which contain asparagine-linked carbohydrate residues, located at positions 52 and 78 on the alpha-subunit and positions 7 and 24 on the beta-subunit. These carbohydrate chains have been shown to regulate the biological activity of FSH, including signal transduction and receptor binding. However, the specific roles of the individual carbohydrate chains have been poorly defined. Using site-directed mutagenesis we disrupted the consensus sequences for glycosylation and expressed the mutated cDNAs in Chinese Hamster Ovary cells. Specifically deglycosylated FSH variants were secreted from all clonal cell lines expressing the mutated FSH cDNAs except for the cell line that lacked all four glycosylation sites. Analysis of the singly or doubly deglycosylated FSH mutants revealed that removal of the carbohydrate residue at position 78 on the alpha-subunit significantly increased the receptor binding affinity of human FSH by 72%. Removal of the other carbohydrate residues had no significant effect on receptor binding. The carbohydrate residue at position 52 on the alpha-subunit was found to play an essential role in signal transduction as its removal resulted in a significant decrease in potency to 26% of wild type levels. The other individual carbohydrate residues appear to play a minor role in signal transduction, although removal of each residue results in reduced maximal response. The removal of both alpha-subunit carbohydrates resulted in a significant decrease in biopotency, to 41% of wild type levels; whereas, the removal of both beta-subunit carbohydrate chains resulted in a significant increase in biopotency, to 216% of wild type levels. These studies have allowed the identification of site-specific roles for the carbohydrate residues of human FSH. Our data suggest that the carbohydrate residues play a greater role in determining the biological activity of FSH than has been suggested in similar studies of other glycoprotein hormones.

Animals↗
PubMed1993

Isoelectric charge of recombinant human follicle-stimulating hormone isoforms determines receptor affinity and in vitro bioactivity.

Recombinant human FSH (rhFSH) was obtained by expressing the human FSH alpha- and beta-subunit complementary DNAs in the chinese hamster ovary cell line. Isoforms of rhFSH were resolved into specific isoelectric (pI) fractions by chromatofocusing. rhFSH isoforms ranged from pI 3.0-5.5 with a modal value of pI 4.2. Analysis of the biological activity of specific pI isoforms of rhFSH was undertaken using both the rat granulosa cell aromatase (in vitro) bioassay and a RRA. More acidic isoforms (e.g. pI 3.5) showed significantly lower affinity (P < 0.05) for rat testicular FSH receptors than did the less acidic isoforms (e.g. pI 4.8). Consistent with the receptor binding affinity data, the more acidic fractions resulted in significantly less activation (P < 0.05) of rat granulosa cell aromatase activity, as measured by estrogen production, than did the less acidic isoforms. The observed bioactivities and their correlation with the pI values of the rhFSH isoforms are consistent with observations of differing bioactivities seen in both pituitary and urinary FSH isoforms. These results demonstrate that rhFSH, made in the chinese hamster ovary cell line, is both biologically active and has isoform profiles, and presumably carbohydrate structures, that closely resemble those seen in natural hFSH.

Animals↗
PubMed1993

The extracellular disulfide loop motif of the inhibitory glycine receptor does not form the agonist binding site.

Inhibitory (glycine and gamma-aminobutyric acid type A) and excitatory (nicotinic acetylcholine and serotonin 5-hydroxytryptamine type 3) receptors of the ligand-gated ion channel superfamily are related by both structural similarities and primary sequence identity. One invariant feature of all members of this receptor superfamily is the presence of an extracellular disulfide loop motif. This structural motif has been modeled, and Cockcroft et al. [Proteins 8:386-397 (1990)] have suggested that it forms the agonist binding site of the ligand-gated ion channel receptors. Using site-directed mutagenesis of the inhibitory glycine receptor (GlyR), we have specifically tested this hypothesis. The lysine residue at position 143 is proposed to form the binding site for the negatively charged carboxyl group of the agonist glycine. Differing residues at this position in other ligand-gated receptors are proposed to confer agonist specificity. The aspartic acid residue at position 148 is an invariant residue in every known subunit of the ligand-gated ion channel receptor superfamily. This residue has been proposed as the binding site for the positively charged amino group of the various agonists. Mutation of the lysine at position 143 to alanine resulted in essentially unaltered GlyRs, showing only modest decreases in strychnine affinity (Kd, 8.1 +/- 1.4 nM versus 13.4 +/- 1.3 nM), glycine displacement of strychnine binding (Ki, 25 +/- 5 microM versus 49 +/- 9 microM), and glycine activation of chloride currents (EC50, 27 +/- 6 microM versus 114 +/- 14 microM). Thus, we conclude that Lys-143 does not play a major role in either agonist or antagonist binding or agonist activation of the GlyR. Mutation of Asp-148 to either alanine or asparagine disrupted the expression and/or assembly of the receptor, and no binding sites or ion channels were expressed on the cell surface. The conservative mutation of the aspartic acid at position 148 to glutamic acid (D148E) allowed the expression of receptors, although with reduced efficiency. The D148E GlyRs showed a 1 order of magnitude decrease in strychnine affinity (Kd, 8.1 +/- 1.4 nM versus 82 +/- 21 nM), without any change in the glycine displacement of strychnine binding (Ki, 25 +/- 5 microM versus 29 +/- 8 microM) or glycine activation of chloride currents (EC50, 27 +/- 6 microM versus 20 +/- 1 microM).(ABSTRACT TRUNCATED AT 400 WORDS)

Aspartic Acid↗
PubMed1992

Antagonism of ligand-gated ion channel receptors: two domains of the glycine receptor alpha subunit form the strychnine-binding site.

The inhibitory glycine receptor (GlyR) is a member of the ligand-gated ion channel receptor superfamily. Glycine activation of the receptor is antagonized by the convulsant alkaloid strychnine. Using in vitro mutagenesis and functional analysis of the cDNA encoding the alpha 1 subunit of the human GlyR, we have identified several amino acid residues that form the strychnine-binding site. These residues were identified by transient expression of mutated cDNAs in mammalian (293) cells and examination of resultant [3H]strychnine binding, glycine displacement of [3H]strychnine, and electrophysiological responses to the application of glycine and strychnine. This mutational analysis revealed that residues from two separate domains within the alpha 1 subunit form the binding site for the antagonist strychnine. The first domain includes the amino acid residues Gly-160 and Tyr-161, and the second domain includes the residues Lys-200 and Tyr-202. These results, combined with analyses of other ligand-gated ion channel receptors, suggest a conserved tertiary structure and a common mechanism for antagonism in this receptor superfamily.

Amino Acid Sequence↗
PubMed1992

Distinct agonist- and antagonist-binding sites on the glycine receptor.

The distinction between receptor-binding sites for agonists and antagonists underpins the pharmacological differences between these two classes of ligands. In the glycine receptor, antagonist (strychnine) binding requires an interaction with residues Lys-200 and Tyr-202. We now demonstrate that the agonist-binding site of this receptor is located at the residue Thr-204. The agonist-binding site interaction is thus likely to be mediated by hydrogen bonding and not by ionic interactions. Our results demonstrate that, in contrast to other studies of ligand-gated ion channel receptors, agonist- and antagonist-binding sites are composed of distinct amino acid residues.

Alanine↗
PubMed1991

Amino-terminal leucine-rich repeats in gonadotropin receptors determine hormone selectivity.

Recombinant expression of truncated receptors for luteinizing hormone/chorionic gonadotropin (LH/CG) revealed that the amino-terminal leucine-rich repeats 1-8 of the extracellular receptor domain bind human chorionic gonadotropin (hCG) with an affinity (Kd = 0.72 +/- 0.2 nM) similar to that of the native LH/CG receptor (Kd = 0.48 +/- 0.05 nM). LH/CG receptor leucine-rich repeats 1-8 were used to replace homologous sequences in the closely related receptor for follicle stimulating hormone (FSH). Cells expressing such chimeric LH/CG-FSH receptors bind hCG and show elevated cylic AMP levels when stimulated by hCG but not by recombinant human FSH (rhFSH). Similarly, a chimeric LH/CG receptor in which leucine-rich repeats 1-11 originated from the FSH receptor is activated by rhFSH but not by hCG. For this chimera, no residual [125I] hCG binding was observed in a range of 2 pM to 10 nM. Our results demonstrate that specificity of gonadotropin receptors is determined by a high affinity hormone binding site formed by the amino-terminal leucine-rich receptor repeats.

Base Sequence↗
PubMed1990

Structural and functional characterization of the gamma 1 subunit of GABAA/benzodiazepine receptors.

The GABAA receptor gamma 1 subunit of human, rat and bovine origin was molecularly cloned and compared with the gamma 2 subunit in structure and function. Both gamma subunit variants share 74% sequence similarity and are prominently synthesized in often distinct areas of the central nervous system as documented by in situ hybridization. When co-expressed with alpha and beta subunits in Xenopus oocytes and mammalian cells, the gamma variants mediate the potentiation of GABA evoked currents by benzodiazepines and help generate high-affinity binding sites for these drugs. However, these sites show disparate pharmacological properties which, for receptors assembled from alpha 1, beta 1 and gamma 1 subunits, are characterized by the conspicuous loss in affinity for neutral antagonists (e.g. flumazenil) and negative modulators (e.g. DMCM). These findings reveal a pronounced effect of gamma subunit variants on GABAA/benzodiazepine receptor pharmacology.

Amino Acid Sequence↗