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S C Sealfon

Publications and source records attributed to S C Sealfon.

At least 37 records · Page 2Linked to original sources
PubMed1996

Pharmacology of a capacitative Ca2+ entry in Xenopus oocytes.

We have characterized pharmacological properties of inositol trisphosphate (InsP3)-mediated calcium entry pathway in Xenopus oocytes via activation of Ca(2+)-dependent Cl- channels (ICl, Ca) as a sensitive indicator for increase in cytosolic [Ca2+]. This type of Ca2+ entry mechanism is known as a capacitative Ca2+ entry (CCE). Voltage-clamped oocytes were maintained in Ca(2+)-free medium and injected with InsP3 which depleted the InsP3-sensitive Ca2+ stores. 10-20 min later, the oocytes were exposed, at 2-3 min intervals, to 5 mM Ca(2+)-containing medium for 5-10 s which evoked repeated inward Cl- current. No effect of external Ca2+ was apparent before InsP3 injection. To determine the pharmacological characteristics of CCE, oocytes were incubated with various chemical agents in Ca(2+)-free solution and exposed to Ca2+ again in presence of the chemical. It was found that organic Ca2+ channel blockers were relatively ineffective in blocking CCE while the inorganic Ca2+ channel blocker La3+ was most efficient in blocking the current. Attempts to measure conductance increase when the Cl- channels were blocked during activation of Ca2+ influx were unsuccessful. Therefore we tested the hypothesis that the Ca2+ influx is mediated via a Ca-H transporter. Lowering the external pH (to pH 6.5) or application of the protonophore carbonylcyanide p-trifluoromethoxyphenyl hydrazone (EC50 = 2 x 10(-8) M) effectively blocked CCE. Since Ca-H countertransport in the plasma membrane is coupled to Ca2+ extrusion by Ca-ATPase in vascular smooth muscle we suggest that the capacitative Ca2+ entry in Xenopus oocytes may possibly arise from slippage of plasma membrane Ca-ATPase coupled to proton countertransport, a mechanism reported in a variety of cells. Ca2+ slippage may arise from the large Ca2+ gradient produced by the Ca2+ depletion protocol.

Animals↗
PubMed1996

Contribution of a helix 5 locus to selectivity of hallucinogenic and nonhallucinogenic ligands for the human 5-hydroxytryptamine2A and 5-hydroxytryptamine2C receptors: direct and indirect effects on ligand affinity mediated by the same locus.

An important determinant of the neurobehavioral responses induced by a drug is its relative receptor selectivity. The molecular basis of ligand selectivity of hallucinogenic and nonhallucinogenic compounds of varying structural classes for the human 5-hydroxytryptamine (5-HT)2A and 5-HT2C receptors was investigated with the use of reciprocal site-directed mutagenesis. Because these two closely related receptor subtypes differ in the amino acid present at position 5.46 (residues 242 and 222 in the sequences, respectively), the effects of corresponding substitutions in the 5-HT2A[S5.46(242)-->A] and 5-HT2C[A5.46(222)-->S] receptors were studied in tandem. By studying both receptors, the direct and indirect effects of mutations on affinity and selectivity can be distinguished. The ergolines studied, mesulergine (selective for the 5-HT2C receptor) and d-lysergic acid diethylamide (selective for the 5-HT2A receptor), reversed their relative affinity with mutations in each receptor, supporting a direct role of this locus in the selectivity of these ligands. However, interchange mutations in either receptor led to decreased or unchanged affinity for (+/-)-1-)(2,5-dimethoxy-4-iodophenyl)-2-aminopropane and ketanserin, which have higher affinity for the 5-HT2A receptor, consistent with little contribution of this locus to the selectivity of these ligands. The indoleamines studied were affected differently by mutations in each receptor, suggesting that they bind differently to the two receptor subtypes. Mutation of this locus in the 5-HT2A receptor decreased the affinity of all indoleamines, whereas the interchange mutation of the 5-HT2C receptor did not affect indoleamine affinity. These results are consistent with a direct interaction between this side chain and indoleamines for the 5-HT2A receptor but not for the 5-HT2C receptor. Furthermore, this analysis shows that the higher affinity of 5-HT and tryptamine for the 5-HT2C receptor than for the 5-HT2A receptors is not due to the difference at this locus. The hallucinogens studied [d-lysergic acid diethylamide, psilocin, bufotenin, and (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane] fell into different classes in this analysis. For the classes of ligand studied, the side-chain difference at this position directly determines relative ligand selectivity only for ergolines and may contribute to the specific effects of hallucinogens in this class.

Amino Acid Sequence↗
PubMed1995

A locus of the gonadotropin-releasing hormone receptor that differentiates agonist and antagonist binding sites.

The decapeptide gonadotropin-releasing hormone controls reproductive function via interaction with a heptahelical G protein-coupled receptor. Because of molecular model of the receptor predicts that Lys121 in the third transmembrane helix contributes to the binding pocket, the function of this side chain was studied by site-directed mutagenesis. Substitution of Arg at this position preserved high affinity agonist binding, whereas Gln at this position reduced binding below the limits of detection. Leu and Asp at this locus abolished both binding and detectable signal transduction. The EC50 of concentration-response curves for coupling to phosphatidyl inositol hydrolysis obtained with the Gln121 receptor was more than 3 orders of magnitude higher than that obtained for the wild-type receptor. In order to determine whether the increased EC50 obtained with this mutant reflects an altered receptor affinity, the effect of decreases in wild-type receptor density on concentration-response curves was determined by irreversible antagonism. Progressively decreasing the concentration of the wild-type receptor increased the EC50 values obtained to a maximal level of 2.4 +/- 0.2 nM. Comparison of this value with the EC50 of 282 +/- 52 nM observed with the Gln121 receptor mutant indicates that the agonist affinity for this mutant is reduced more than 100-fold. In contrast, antagonist had comparable high affinities for the wild-type, Arg121, and Gln121 mutants. The results indicate that a charge-strengthened hydrogen bond donor is required at this locus for high affinity agonist binding but not for high affinity antagonist binding.

Amino Acid Sequence↗
PubMed1995

Related contribution of specific helix 2 and 7 residues to conformational activation of the serotonin 5-HT2A receptor.

A conserved helix 2 Asp is required for the proper function of many G-protein-coupled receptors. To reveal the structural basis for the role of this residue, the additive effects of mutations at this locus and at a conserved helix 7 locus were investigated in the 5-HT2A receptor. All mutant receptors studied retained high affinity agonist and antagonist binding. Whereas an Asp-->Asn mutation in helix 2 eliminated coupling, interchanging the residues at the two positions by a second mutation of Asn-->Asp in helix 7 restored receptor function. These data suggest that these residues are adjacent in space and interact. The loss of function observed with Ala at either position is consistent with each side chain forming hydrogen bonds. Molecular dynamics simulations were performed on three-dimensional computational models of agonist-receptor complexes of both the wild-type receptor and the Asp-->Asn mutant receptor. Consonant with the lack of coupling observed for the mutant construct, introducing the mutation into the computational model produced a conformational change in a direction opposite to that seen from computational simulations of activation of the wild-type receptor model. These results implicate both loci in a common hydrogen-bonding network underlying receptor activation by agonist.

Mutation↗
PubMed1995

Functional domains of the gonadotropin-releasing hormone receptor.

1. The cloning of the mammalian gonadotropin-releasing hormone receptor sets the stage for rapid progress in understanding the structure of the receptor, its interaction with ligand, and its mechanisms of activation. 2. The receptor is a 327 to 328-amino acid seven-transmembrane domain G protein-coupled receptor. 3. Recent site-direct mutagenesis studies have provided considerable insight into glycosylation of the receptor, the arrangement of the helices, and the ligand binding domains.

Amino Acid Sequence↗
PubMed1995

Identification of N-glycosylation sites in the gonadotropin-releasing hormone receptor: role in receptor expression but not ligand binding.

The asparagine residues of the three N-glycosylation consensus sequences in the mouse gonadotropin-releasing hormone receptor were mutated to determine which residues were glycosylated and the function of glycosylation. Photoaffinity labelled Gln4 and Gln18 receptor mutants exhibited lower apparent molecular weight on SDS polyacrylamide gel electrophoresis, while the Gln102 receptor showed wildtype mobility. This indicates that the receptor is glycosylated at Asn4 and Asn18 but not at Asn102. Binding affinities of all the mutant receptors were normal, indicating that carbohydrate moieties are not involved in ligand binding interactions. However, expression of the Gln4 and Gln18 receptors were substantially decreased, indicating a role for glycosylation in receptor expression or stability. All the glycosylation site mutants were capable of normal signal transduction, as indicated by their ability to stimulate inositol phosphate production.

Animals↗
PubMed1995

Translational regulation of the gonadotropin-releasing hormone receptor in alpha T3-1 cells.

Prolonged exposure of the GnRH receptor to high concentrations of GnRH leads to receptor down-regulation. The role of altered receptor biosynthesis in this agonist-induced receptor down-regulation was investigated in the mouse gonadotrope cell line, alpha T3-1 cells. After exposure to 1 microM GnRH for 24 h, the number of GnRH receptor-binding sites in alpha T3-1 cells decreased to 25 +/- 6% of the control levels. No corresponding changes were observed in GnRH receptor messenger RNA (mRNA) using either quantitative ribonuclease protection/solution hybridization assay or Northern blot analysis. However, when the ability of this RNA to direct the synthesis of functional GnRH receptors was examined by quantitative assessment of the voltage clamp response in Xenopus oocytes, GnRH-induced currents in oocytes injected with RNA isolated from down-regulated cells was reduced to 40 +/- 13% of the response obtained after the injection of RNA from control alpha T3-1 cells. Thus, although GnRH receptor mRNA levels were not altered, the ability of cellular RNA isolated from alpha T3-1 cells to direct the synthesis of functional GnRH receptors was regulated in concert with receptor binding. To investigate the possibility that GnRH receptor mRNA translational efficiency was reduced, the distribution of polyribosome-associated GnRH receptor mRNA was studied. Polyribosome-associated mRNA was separated by linear sucrose gradient, and GnRH receptor mRNA distribution was determined by ribonuclease protection assay. GnRH receptor mRNA distribution shifted from the largest to smaller polyribosome and monosome fractions in cells exposed to GnRH compared to controls. The weighted mean of GnRH receptor mRNA distribution among eight fractions shifted from fraction 5.924 +/- 0.06 in control polysomes to fraction 5.45 +/- 0.219 for polysomes from down-regulated cells (P < 0.05). These results demonstrate that GnRH receptor down-regulation is accompanied by decreased GnRH receptor mRNA translation in the absence of any change in GnRH receptor mRNA levels. These data suggest that decreased efficiency of GnRH receptor mRNA translation contributes to the down-regulation of this receptor in alpha T3-1 cells.

Animals↗
PubMed1995

A new family of Boucher-Neuhäuser syndrome: coexistence of Holmes type cerebellar atrophy, hypogonadotropic hypogonadism and retinochoroidal degeneration: case reports and review of literature.

The association of familial hypogonadism with progressive cerebellar ataxia is only rarely encountered, and the exact link between the symptoms remains unknown. We report here two sisters presenting with Holmes type cerebellar ataxia, hypogonadotropic hypogonadism and retinochoroidal degeneration recently diagnosed as Boucher-Neuhäuser syndrome. There was consanguinity between the parents of the affected individuals and the condition seemed to be inherited as an autosomal recessive defect. On endocrinological examinations, in both cases, the responses of LH and FSH to LH-RH (100 micrograms) were impaired even after repetitive stimulation with LH-RH (400 micrograms, 7 days), suggesting that the hypogonadism was due to a primary pituitary disturbance. Impaired GH responses to GRF (100 micrograms) and insulin-induced hypoglycemia (0.1 U/kg) were also noted. The two sisters shared an almost identical clinical and endocrinological picture. Their karyotypes were 46, XX. They had been treated for primary and secondary amenorrhea at the age of 20 years and neurological problems had started at the age of 30 years. This unique family displays clinical evidence of a possible common mechanism responsible for a progressive hypothalamo-pituitary and cerebellar impairment of late onset.

Atrophy↗
PubMed1994

Glutamate 301 of the mouse gonadotropin-releasing hormone receptor confers specificity for arginine 8 of mammalian gonadotropin-releasing hormone.

The Arg residue at position 8 of mammalian GnRH is necessary for high affinity binding to mammalian GnRH receptors. This requirement has been postulated to derive from an electrostatic interaction of Arg8 with a negatively charged receptor residue. In order to identify such a residue, 8 conserved acidic residues of the mouse GnRH receptor were mutated to isosteric Asn or Gln. Mutant receptors were tested for decreased preference for Arg8-containing ligands by ligand binding and inositol phosphate production. One of the mutants, in which the Glu301 residue was mutated to Gln, exhibited a 56-fold decrease in apparent affinity for mammalian GnRH. The mutant receptor also exhibited decreased affinity for [Lys8]GnRH, but its affinity for [Gln8]GnRH was unchanged compared with the wild type receptor. The apparent affinity of the mutant receptor for the acidic analogue, [Glu8]GnRH, was increased more than 10-fold. The mutant receptor did not, therefore, distinguish mammalian GnRH from analogues with amino acid substitutions at position 8 as effectively as the wild type receptor. This loss of discrimination was specific for the residue at position 8, because the mutant receptor did distinguish mammalian GnRH from analogues with favorable substitutions at positions 5, 6, and 7. These findings show that Glu301 of the GnRH receptor plays a role in receptor recognition of Arg8 in the ligand and are consistent with an electrostatic interaction between these 2 residues.

Amino Acid Sequence↗
PubMed1994

Structure of the mouse gonadotropin-releasing hormone receptor gene: variant transcripts generated by alternative processing.

The mouse gonadotropin-releasing hormone receptor (GnRHR) is unique among G-protein-coupled receptors in its lack of a putative intracellular carboxy-terminal domain. A gonadotrope cell line cDNA library was screened in a search for alternative forms of the receptor transcript and 42 clones were obtained, representing a number of variant cDNAs. To determine the origin of these transcripts, the structure of the mouse gene was mapped from 11 distinct genomic clones. The gene contains three exons, spanning more than 22 kb. Exons 1, 2, and 3 encode, respectively, nucleotides +1 to +522, +523 to +739, and +740 to +981 of the open reading frame of the cDNA for the functional mouse GnRHR. Southern blot analysis with genomic DNA is consistent with the presence of a single gene. By comparison with the genomic sequence, the origins of the variant cDNAs isolated can be clarified. All the cDNAs contain the first exon and the majority (71%) encode the functional 327-amino-acid receptor previously reported. One group of clones (14%), which contains exons 1 and 2, continues 700 bp past the exon 2 splice donor of the wild-type receptor. These clones terminate after a polyadenylation signal and have an open reading frame encoding a protein of only 261 amino acids. In a different group of transcripts (5%), exon 2 is absent, resulting in a shift in the reading frame and encoding a protein of 177 amino acids. These data support alternative processing of the mouse GnRHR gene.

Alternative Splicing↗
PubMed1994

Gonadal hormones and gonadotropin-releasing hormone (GnRH) alter messenger ribonucleic acid levels for GnRH receptors in sheep.

GnRH regulates the synthesis and secretion of pituitary gonadotropins. The number of receptors for GnRH (GnRH-rec) can vary from 500 to 15,000-20,000/gonadotrope in ovine pituitary cultures after treatment with physiologically relevant combinations of gonadal hormones. This large range suggests that regulation of GnRH-rec expression may be an important control point in GnRH action at the pituitary level. Reported here are the changes in GnRH-rec mRNA associated with pituitary treatments (48 h) of 17 beta-estradiol (E), progesterone (P), and an enriched preparation of porcine follicular inhibin (IN). Northern blot analysis was used to detect 3 species of GnRH-rec mRNA in primary ovine pituitary culture [5.5 kilobases (kb; 32%), 3.6 kb (51%), and 1.4 kb (17%)]; all were changed in parallel by E, P, and IN. GnRH-rec mRNAs were increased 190% over control levels after treatment with either E or IN, and 400% with E and IN combined; when E and IN were added along with P, the increase was only 50% (P caused an 87% inhibition of E plus IN induction). The addition of P in the absence of any other treatment reduced levels of GnRH-rec mRNA by 50%. Studies were also conducted with GnRH agonists (GnRH-A) due to their widespread clinical use for down-regulating reproductive function in men and women. The addition of GnRH-A to cultures was as effective as P in blocking E plus IN induction of GnRH-rec mRNA. In vivo studies in wethers showed that 7 days of chronic treatment with GnRH-A decreased all sizes of ovine GnRH-rec mRNA by 84-89%. These data indicate that E, P, and IN change GnRH-rec levels at least in part by changing levels of GnRH-rec mRNAs. They also show that GnRH-A can almost entirely block E plus IN induction of GnRH-rec mRNA in vitro and decrease levels of GnRH-rec mRNA in vivo in wethers.

Animals↗
PubMed1994

A reciprocal mutation supports helix 2 and helix 7 proximity in the gonadotropin-releasing hormone receptor.

Activation of the pituitary gonadotropin-releasing hormone receptor, a member of the seven-transmembrane G protein-coupled receptor (GPCR) family, triggers a cascade of events leading to gonadotropin release and stimulation of the reproductive system. An unusual feature of this receptor, observed in mice, rats, and humans, is the presence of Asn87 in the second putative transmembrane helix at the location of a highly conserved aspartate in the GPCR family and of Asp318 in the putative seventh transmembrane helix where nearly all other GPCRs have asparagine. The possibility that these residues interact was suggested by this reciprocal pattern and by a three-dimensional model of the gonadotropin-releasing hormone receptor and was investigated by site-directed mutagenesis. Replacing Asn87 in the second transmembrane domain by aspartate eliminated detectable ligand binding. A second mutation, generating the double-mutant receptor Asp87Asn318, recreated the arrangement found in other GPCRs and re-established high affinity agonist and antagonist binding. The restoration of binding by a reciprocal mutation indicates that these two specific residues in helices 2 and 7 are adjacent in space and provides an empirical basis to refine the model of the transmembrane helix bundle of the receptor.

Amino Acid Sequence↗
PubMed1993

Comparative sequence analysis and functional characterization of the cloned sheep gonadotropin-releasing hormone receptor reveal differences in primary structure and ligand specificity among mammalian receptors.

The cloned sheep gonadotropin-releasing hormone (GnRH) receptor was analysed for sequence homology/differences among mammalian receptors and its pharmacology characterized in COS-1 cells. Transmembrane domains TM2, TM3, TM5, TM6 and TM7, and extracellular loop 1 are most highly conserved (> 90%) in this G-protein coupled receptor. The Kd of the sheep receptor in binding assays (4.9 nM) was similar to the human and rat receptors, but lower than the mouse receptor. The rank order of potency of a series of GnRH analogues for binding and inositol phosphate stimulation in transfected COS-1 cells was identical to that of the receptor characterized in sheep pituitary gonadotropes. Northern blot analysis identified four transcripts sized 5.4 kb, 3.6 kb, 2.3 kb and 1.3 kb in sheep pituitaries which were upregulated by castration.

Amino Acid Sequence↗
PubMed1993

Heterogeneous distribution of D1, D2 and D5 receptor mRNAs in monkey striatum.

The primate striatum has a compartmental organization reflected both in the topography of its afferent projections and in the segregation of its morphologically similar but neurochemically distinct efferent neurons. Discretely projecting mesostriatal neurons release dopamine (DA) which modulates the responses of striatal neurons to other afferent inputs. Multiple DA receptor (DAR) subtypes have been cloned and characterized and mapping their cellular expression is crucial for understanding the influence of DA on striatal function. We report the distribution of mRNAs for D1, D2 and D5 DAR subtypes (D2R, D2R and D5R) in the striatum of cynomolgus monkeys (Macaca fascicularis) studied by in situ hybridization histochemistry (ISH) using monkey-specific cRNA probes. Adjacent sections were stained for calbindin immunoreactivity to distinguish striosomal and matrix compartments for comparison with the patterns obtained with ISH. In the caudate nucleus, D1R mRNA was concentrated in calbindin-poor striosomes where dense grain clusters were seen overlying the majority of medium-sized neurons (diameter approximately 15 microns). D1R mRNA localization was relatively homogeneous in the putamen. By contrast, the distributions of D2R and D5R mRNAs showed no clear preference for the striosomal or matrix compartments of either caudate nucleus or putamen. In the ventral striatum (nucleus accumbens, olfactory tubercle and ventral portions of caudate nucleus and putamen), expression of D1R and D2R mRNA was sparse relative to dorsal striatum, while D5R mRNA expression was roughly equal in ventral and dorsal striatum. Circumscribed zones of hybridization associated with islands of tightly packed small cells occurred with all three DAR mRNA subtypes in the ventral striatum.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

Regulation of dopamine D2 receptor mRNA expression in the olfactory tubercle by cocaine.

The influence of acute and chronic cocaine treatment on the expression of dopamine D2 receptor (D2R) mRNA was examined in the rat basal ganglia by in situ hybridization histochemistry. No significant alterations in D2R mRNA levels were observed in the striatum or the nucleus accumbens following acute or chronic cocaine. In the olfactory tubercle, acute cocaine (20 and 40 mg/kg, i.p.) was found to decrease D2R mRNA expression. Chronic cocaine (20 mg/kg, i.p. x 15 days), however, produced an increase in D2R mRNA levels in this region which was detected 24 h, but not 7 days after withdrawal. Acute and chronic cocaine have different effects on the regulation of D2R gene expression in the olfactory tubercle.

Animals↗
PubMed1993

Cloning and characterization of the human GnRH receptor.

A cDNA encoding the human GnRH receptor (GnRHR) has been cloned and functionally expressed in both Xenopus oocytes and COS-1 cells. The 2160 bp cDNA encodes a 328 amino acid protein with a predicted amino acid sequence that is 90% identical to that of the mouse GnRHR (Tsutsumi et al. (1992) Mol. Endocrinol. 6, 1163-1169). Injection of synthetic RNA transcript into oocytes led to the development of a depolarizing response to agonists when assayed by voltage-clamp electrophysiology. Consistent with the expression of a mammalian GnRHR, the response was blocked by GnRH antagonists. Following expression of the human GnRHR in COS-1 cells, agonists and an antagonist displaced [125I]GnRH agonist from membrane isolates with nanomolar range dissociation constants similar to those described for displacement from human pituitary membranes. Transfected COS-1 cells manifested a GnRH-stimulated increase in phosphoinositol turnover, with an EC50 of approximately 3 nM, which was inhibited by GnRH antagonists. Northern blot analysis revealed a single band of approximately 4.7 kb expressed in human pituitary which was not detected in testis. The predicted structure of the human GnRHR is similar to that previously reported for the mouse receptor. Although the mammalian GnRHR is a seven transmembrane domain receptor, it differs from other G-protein coupled receptors in several respects, most notably the lack of a cytoplasmic C-terminal domain. The present study demonstrates that the cDNA isolated encodes the human GnRHR and suggests that several unique features conserved among mammalian GnRHRs may be essential for receptor function and/or regulatory control.

Amino Acid Sequence↗