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Biomedical subjects

X Peng

Publications and source records attributed to X Peng.

At least 181 records · Page 10Linked to original sources
PubMed1994

Nicotine-induced increase in neuronal nicotinic receptors results from a decrease in the rate of receptor turnover.

Chronic nicotine exposure in tobacco smokers or experimental animals is known to cause an increase in brain binding sites for nicotine. It has been proposed that this is an adaptive response of neurons to accumulation of chronically desensitized receptors. Acetylcholine receptors of the same (alpha 4)2(beta 2)3 subunit composition as the predominant subtype of brain nicotinic receptors with high affinity for nicotine have been expressed in Xenopus oocytes and in a permanently transfected fibroblast cell line. Chronic exposure of these cells to nicotine or another agonist is shown to result in an increase in receptor amount, indicating that nicotine-induced up-regulation reflects properties of the alpha 4 beta 2 receptor protein, rather than being an adaptive response unique to the neurons in which these receptors are normally expressed. The nicotine concentration dependence, time course, and extent of receptor up-regulation are similar to those reported for receptors in brain. Up-regulation does not appear to require ion flow through the ion channel, because it is also caused by mecamylamine, which blocks the ion channel, and because after prolonged exposure to nicotine most receptors become permanently unable to open their channels in response to nicotine binding. The noncompetitive antagonist mecamylamine blocks open channels more effectively, and so it is more effective at blocking channels in the presence of nicotine. Mecamylamine and nicotine are also synergistic in causing receptor up-regulation. Ligands that cause up-regulation appear to induce a conformation of the receptor that is removed from the surface and degraded more slowly.

Animals↗
PubMed1994

Physostigmine and galanthamine: probes for a novel binding site on the alpha 4 beta 2 subtype of neuronal nicotinic acetylcholine receptors stably expressed in fibroblast cells.

In the present study, we demonstrated that the chicken alpha 4 beta 2 neuronal nicotinic receptor stably expressed in transfected mouse fibroblasts (M10 cells) can be activated via the acetylcholine-binding site or via a site that is distinct from that for acetylcholine and recognizes physostigmine and galanthamine as agonists. In outside-out patches excised from dexamethasone-induced M10 cells, (+)-anatoxin-a, physostigmine and galanthamine (each at 1 microM) activated single channels with conductances of 18 and 30 pS. Dihydro-beta-erythroidine (1-30 nM), but not the nicotinic receptor-specific monoclonal antibody FK1, reduced the frequency of channels activated by anatoxin (1 microM). On the other hand, the frequency of channel activity induced by physostigmine (1 microM) was unaffected by dihydro-beta-erythroidine and was markedly decreased by FK1. In uninduced M10 cells and in dexamethasone-treated untransfected fibroblasts, we observed that physostigmine, galanthamine and nicotinic agonists did not evoke whole-cell or single-channel currents. Also, neither [3H]L-nicotine nor FK1 was able to bind to uninduced M10 cells. In dexamethasone-induced M10 cells, the nicotinic agonists acetylcholine, anatoxin, 1,1-dimethyl-4-phenylpiperazinium, (-)-nicotine, and cytisine (each at 100 microM) activated whole-cell currents that showed a marked inward rectification and were sensitive to blockade by dihydro-beta-erythroidine (100 nM). However, neither galanthamine nor physostigmine could evoke whole-cell currents in cells that were responsive to nicotinic agonists. Other effects of physostigmine and galanthamine on the nicotinic receptor that outweight the agonist properties of these compounds could account for their inability to evoke whole-cell currents.

Animals↗
PubMed1994

Human alpha 7 acetylcholine receptor: cloning of the alpha 7 subunit from the SH-SY5Y cell line and determination of pharmacological properties of native receptors and functional alpha 7 homomers expressed in Xenopus oocytes.

The alpha-bungarotoxin-binding acetylcholine receptors from the human neuroblastoma cell line SH-SY5Y were found to cross-react with some monoclonal antibodies to alpha 7 subunits of nicotinic acetylcholine receptors from chicken brain. The human alpha 7 subunit cDNA from SH-SY5Y was cloned, revealing 94% amino acid sequence identity to rat alpha 7 subunits and 92% identity to chicken alpha 7 subunits. Native human alpha 7 receptors showed affinities for some ligands similar to those previously observed with native chicken alpha 7 receptors, but for other ligands there were large species-specific differences in binding affinity. These results paralleled properties of alpha 7 homomers expressed in Xenopus oocytes. Human alpha 7 homomers exhibited rapidly desensitizing, inwardly rectifying, agonist-induced, cation currents that triggered Ca(2+)-sensitive Cl- channels in the oocytes. A change in efficacy from partial agonist for chicken alpha 7 homomers to full agonist for human alpha 7 homomers was exhibited by 1,1-dimethyl-4-phenylpiperazinium. This result reveals a large species-specific pharmacological difference, despite small differences in alpha 7 sequences. This is important for understanding the effects of these drugs in humans and for identifying amino acids that may contribute to the acetylcholine binding site, for analysis by in vitro mutagenesis. These results also characterize properties of native alpha 7 receptors and alpha 7 homomers that will provide criteria for functional properties expected of structural subunits, when these can be identified, cloned, and coexpressed with alpha 7 subunits.

Amino Acid Sequence↗
PubMed1993

Reporter epitopes: a novel approach to examine transmembrane topology of integral membrane proteins applied to the alpha 1 subunit of the nicotinic acetylcholine receptor.

The development of a novel immunological method called the "reporter epitope" technique to probe the transmembrane topology of integral membrane proteins is described. Using this method, synthetic oligonucleotides encoding epitopes (reporter epitopes) for well characterized monoclonal antibodies (reporter mAbs) were inserted at various locations within the human acetylcholine receptor (AChR) alpha 1 subunit cDNA. The engineered subunits were then expressed along with Torpedo beta 1, gamma, and delta subunits in Xenopus oocytes, and the transmembrane location of the site of insertion was determined by the binding of the 125I-labeled reporter mAbs to whole oocytes. Control reporter epitope insertions at alpha 347 exhibited the expected cytoplasmic location. Reporter epitopes inserted at alpha 429 are located on the extracellular surface. Reporter epitopes that are 16-48 amino acids long do not disrupt assembly or function of hybrid AChRs when inserted near the carboxy terminus (at alpha 429) or in the large cytoplasmic domain (at alpha 347). However, because two reporter epitopes inserted at alpha 157 obliterated subunit assembly and a third reporter epitope when tolerated at this position was inaccessible from the extracellular surface and only marginally accessible after detergent solubilization of the AChRs, a definitive transmembrane location for this region was not possible. Nonetheless, the use of this approach has been successfully demonstrated, and it may be generally applicable to the study of other integral membrane proteins.

Amino Acid Sequence↗
PubMed1993

Homomeric and native alpha 7 acetylcholine receptors exhibit remarkably similar but non-identical pharmacological properties, suggesting that the native receptor is a heteromeric protein complex.

Sucrose gradient analysis of chick acetylcholine receptor (AChR) alpha 7 subunits expressed in oocytes indicates that they form pharmacologically active homomers of the same size as native alpha 7 AChRs, a size compatible with a complex of five alpha 7 subunits. By immunoisolating the [35S]methionine-labeled alpha 7 subunits we also demonstrate that they do not appear to assemble with endogenous Xenopus AChR subunits. Pharmacological characterization of detergent-solubilized brain alpha 7 AChRs and alpha 7 homomers reveals that they have similar but nonidentical properties. The pharmacological difference is most accentuated for cytisine (approximately 50-fold). Thus, at least in E18 chicken brain, most or all of the native alpha 7 AChRs do not appear to be homomeric.

Animals↗
PubMed1993

Effects of amino acid substitutions on the pressure denaturation of staphylococcal nuclease as monitored by fluorescence and nuclear magnetic resonance spectroscopy.

In the present study we have used high hydrostatic pressure coupled with either time-resolved and steady-state fluorescence or NMR spectroscopy in order to investigate the effects of amino acid substitutions on the high-pressure denaturation properties of staphylococcal nuclease. This protein has been shown previously to be structurally heterogeneous in its native state. On the NMR time scale, four distinct interconverting conformational forms arise from the population of both cis and trans Xaa-Pro peptide bonds (His46-Pro47 and Lys116-Pro117) [Evans et al. (1989) Biochemistry 28, 362; Loh et al. (1991) in Techniques in Protein Chemistry II, pp 275-282, Academic Press, New York]. Mutations in the protein sequence have been shown to change the distribution among the various forms [Alexandrescu et al. (1989) Biochemistry 28, 204; Alexandrescu et al. (1990) Biochemistry 29, 4516]. Time-resolved fluorescence on a series of mutants with altered equilibria for cis/trans isomerism about the 116-117 peptide bond did not reveal any simple relationship between the position of the cis/trans equilibrium in the folded state and the heterogeneity of the fluorescence decay. However, the specific dynamic properties of each mutant, as revealed by time-resolved fluorescence, do appear to be correlated with their partial molar volume changes of denaturation. A striking finding is that mutation of either (or both) of the prolines that exhibits structural heterogeneity to glycine greatly alters the stability of the protein to pressure. These mutations also result in decreased chain mobility as assessed by time-resolved fluorescence. It appears that packing defects, which allow for peptide bond cis/trans heterogeneity in the wild-type protein, are removed by the Pro-->Gly substitutions.

Amino Acid Sequence↗
PubMed1993

Molten-globule conformation of Arc repressor monomers determined by high-pressure 1H NMR spectroscopy.

The conformation of the pressure-dissociated monomer of Arc repressor was characterized by 1H NMR spectroscopy. The NMR spectra of the monomer under pressure (up to 5.0 kbar; 1 bar = 100 kPa) are typical of a molten globule and they are considerably different from those of the native dimer and thermally denatured monomer. The two-dimensional nuclear Overhauser effect spectra suggest that the pressure-induced molten globule retains some secondary structure. The presence of nuclear Overhauser effects in the beta-sheet region in the dissociated state suggests that the intermonomer beta-sheet (residues 8-14) in the native dimer is replaced by an intramonomer beta-sheet. Changes in one-dimensional and two-dimensional NMR spectra prior to pressure dissociation were found and suggest the existence of a "predissociated" state.

Amino Acid Sequence↗
PubMed1993

Papillomas and carcinomas in transgenic rabbits carrying EJ-ras DNA and cottontail rabbit papillomavirus DNA.

Two transgenic rabbits (TRI and TRIII) that carried cottontail rabbit papillomavirus (CRPV) DNA alone were identified; another (TRII) carried both CRPV DNA and EJ-ras. TRI and TRIII developed extensive skin papillomas at about 1 month of age, and transcripts of CRPV DNA were detectable only in skin and/or papillomas. TRII developed extensive squamous carcinomas of the skin at a very early age. Transcription of both CRPV DNA and EJ-ras was found in the skin cancers. Thus, the tissue specificity of CRPV DNA expression in transgenic rabbits was the same as in virion-infected animals. The expression of EJ-ras could be dependent on the expression of certain CRPV genes and may be a critical cofactor of CRPV DNA in the progression of carcinomas.

Animals↗
PubMed1993

Thyrotrophin-dependent desensitization by Chinese hamster ovary cells that express the recombinant human thyrotrophin receptor.

To determine whether thyrotrophin (TSH)-induced desensitization requires a thyroid-specific factor(s), the human TSH (hTSH) receptor was expressed in Chinese hamster ovary cells. The first incubation of the cells with TSH decreased the subsequent response of adenosine 3',5'-cyclic monophosphate to freshly added TSH in the second incubation. This homologous desensitization was observed as early as after 3 h of the first incubation. The lowest dose of TSH that elicited desensitization was 0.1 nmol/l. The desensitization was not overcome by adding higher doses of TSH in the second incubation. A 125I-labelled TSH-binding study revealed a decrease in the number of high-affinity binding sites but not in that of low-affinity binding sites. The data suggest that TSH-induced desensitization in hTSH receptor-transfected cells is caused, at least in part, by a decrease in the number of TSH receptors on the cell surface. The evidence demonstrates, contrary to an earlier report, that a thyroid-specific factor(s) is not required for hTSH receptor desensitization.

Animals↗
PubMed1993

Cloning of coat protein gene of soybean mosaic virus and its expression in Escherichia coli.

The 3'-terminal genomic region of the Beijing isolate of soybean mosaic virus (SMV-BJ) has been cloned through technique of polymerase chain reaction (PCR). We have analyzed the nucleotide sequence of 3' region of SMV-BJ genome. Comparisons of the nucleotide and deduced amino acid sequence of SMV-BJ coat protein gene with those of SMV-N strain show 93.4% and 98.5% identity between them, respectively. Alignments of the 3' non-coding sequence in pairs with that of SMV-N strain show homology of 88.8%. It has been found that the SMV coat protein gene is expressed in E. coli by western blot analysis. The coat protein produced in E. coli has the same electrophoretic mobility as the authentic coat protein.

Amino Acid Sequence↗
PubMed1993

Pharmacological characterization of alpha-bungarotoxin-sensitive acetylcholine receptors immunoisolated from chick retina: contrasting properties of alpha 7 and alpha 8 subunit-containing subtypes.

At least three subtypes of alpha-bungarotoxin-sensitive acetylcholine receptors (alpha Bgt-sensitive AChRs) exist in chick brain and retina. All may contain previously unknown structural subunits. One subtype contains alpha 7 subunits. Another contains alpha 8 subunits. A third contains both alpha 7 and alpha 8 subunits. In this article, we describe, for the first time, the pharmacological characterization of alpha 7 AChRs and alpha 8 AChRs immunoisolated from chick retina. Pharmacologically, the alpha 8 AChRs exhibit two classes of binding sites, the high affinity of which have higher affinity for most cholinergic ligands than do alpha 7 AChRs. These differences are most accentuated for ACh (approximately 5400-fold), decamethonium (approximately 1400-fold), 1,1,-dimethyl-4 phenylpiperazinium (approximately 200-fold), atropine (approximately 200-fold), nicotine (approximately 100-fold), and tetramethylammonium (approximately 100-fold). The alpha 8 AChR low affinity sites exhibit affinities that are similar but not identical to that of alpha 7 AChRs. Many of the pharmacological differences between the alpha 7 AChRs and alpha 8 AChRs can be attributed to the limited differences between the amino acid sequences of the N-terminal region of the alpha 7 and alpha 8 subunits because expressed alpha 7 homomers and alpha 8 homomers also exhibit these characteristic differences.

Animals↗
PubMed1993

[Localization of E(wa) in Drosophila melanogaster using rearrangement].

The white-apricot (wa) allele differs from the wild type white gene by the presence of the retrovirus-like transposable element copia within the transcription unit. The activity of wa is reduced in trans by a semidominant mutation in the gene Enhancer-of white-apricot (E(wa)). The map position of E(wa) is refined by analyzing recombination among three genes: px, E(wa) and sp. The recombinational distance between E(wa) and sp is 0.2 cM, which places E(wa) at 2-106.8. To localize E(wa) cytologically, four Y; 2 translocations, one 1;2 translocation and three 2R deficiency stocks were crossed to the suitable E(wa) stocks and the wa sons were scored for intensity of pigment and the speck at the axis of wings. The E(wa) was placed in the distal portion of 60B. The one E(wa) mutation and two sp mutations derived from our gamma-ray mutagenesis were used for examination of polytene chromosome. The former was a revertant which has a cytologically visible insertion of material from an unknown source at 60B5-13, most likely in the 60B8-9. It is likely that this insertion is within the E(wa) gene. The latter shows breakpoint at 60C1-2, where is likely the location of the sp gene. Combining the data of both cytological localization and recombinational mapping, the E(wa) was localized at 60B5-13, most likely in the 60B8-9, the left of sp but very close to it.

Animals↗
PubMed1992

High-pressure 31P NMR study of dipalmitoylphosphatidylcholine bilayers.

High-pressure 31P NMR was used for the first time to investigate the effects of pressure on the structure and dynamics of the phosphocholine headgroup in pure 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) multilamellar aqueous dispersions and in DPPC bilayers containing the positively charged form of the local anesthetic tetracaine (TTC). The 31P chemical shift anisotropies, delta sigma, and the 31P spin-lattice relaxation times, T1, were measured as a function of pressure from 1 bar to 5 kbar at 50 degrees C for both pure DPPC and DPPC/TTC bilayers. This pressure range permitted us to explore the rich phase behavior of DPPC from the liquid-crystalline (LC) phase through various gel phases such as gel I (P beta'), gel II (L beta'), gel III, gel IV, gel X, and the interdigitated, Gi, gel phase. For pure DPPC bilayers, pressure had an ordering effect on the phospholipid headgroup within the same phase and induced an interdigitated Gi gel phase which was formed between the gel I (P beta') and gel II (L beta') phases. The 31P spin-lattice relaxation time measurements showed that the main phase transition (LC to gel I) was accompanied by the transition between the fast and slow correlation time regimes. Axially symmetric 31P NMR lineshapes were observed at pressures up to approximately 3 kbar but changed to characteristic axially asymmetric rigid lattice lineshapes at higher pressures (3.1-5.1 kbar).(ABSTRACT TRUNCATED AT 250 WORDS)

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1990

Cystitis, urolithiasis and cystic calculi in ageing guineapigs.

Nineteen cases of cystitis were diagnosed at necropsy and/or by histology in a group of 170 (96 females, 74 males) guineapigs (11.2%). Seventeen of the 19 cases (89.4%) were females. The mean age of guineapigs with cystitis was 34.7 months, which was higher than the mean age of 24 months of the 170 members of the study group. In addition, 6 cases of urolithiasis and cystic calculi in 5 females and one male were also found in the 170 guineapigs (3.5%). The mean age of the 6 cases was 30 months, which was also higher than the mean age of the 170 animals. The study suggests that aged female guineapigs were much more predisposed to cystitis and urolithiasis or cystic calculi than male and young guineapigs. The cause may be related to infection with Escherichia coli and Staphylococcus sp, cystic calculi, diabetes mellitus and female guineapig urogenital anatomy and function.

Aging↗
PubMed1989

Effect of cage population density on plasma corticosterone and peripheral lymphocyte populations of laboratory mice.

The effect of different population densities of mice per cage on plasma corticosterone, peripheral lymphocytes and specific lymphocyte subpopulations was investigated. The animals were housed in groups of 2, 4 or 8 mice per cage and the blood samples were taken from each animal of these groups on days one, 7 and 14. A significant elevation (P less than 0.05) in plasma corticosterone concentration was observed in the group of 8 mice per cage on days one and 7 as compared with those of 2 or 4 mice per cage. The number of peripheral lymphocytes was significantly decreased in the groups of 2 (P less than 0.01) and 8 (P less than 0.05) mice per cage as compared with the group of 4 mice per cage on day one. A significantly decreased number of lymphocytes (P less than 0.01) in the group of 8 mice per cage continued to day 7. There were no significant differences in specific lymphocyte subpopulations observed among these groups. The results of this study suggest that a population density of 4 mice per cage induced minimal stress compared to that induced by the population densities of 2 or 8 mice per cage. Since stress is known to induce alteration in a variety of biological functions, the population density of mice per cage should be considered in the interpretation of research data.

Animals↗
PubMedDate not supplied

Polymeric gadolinium chelate magnetic resonance imaging contrast agents: design, synthesis, and properties.

We have synthesized and evaluated five series of polymeric gadolinium chelates which are of interest as potential MRI blood pool contrast agents. The polymers were designed so that important physical properties including molecular weight, relaxivity, metal content, viscosity, and chelate stability could be varied. We have shown that, by selecting polymers of the appropriate MW, extended blood pool retention can be achieved. In addition, relaxivity can be manipulated by changing the polymer rigidity, metal content affected by monomer selection, viscosity by polymer shape, and chelate stability by chelator selection.

Animals↗