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L Xu

Publications and source records attributed to L Xu.

At least 235 records · Page 13Linked to original sources
PubMed2000

Wetting and molecular orientation of 8CB on silicon substrates

The wetting properties of 8CB ( 4(')-n-octyl-4-cyanobiphenyl) on silicon wafers have been studied with scanning polarization force microscopy (SPFM). Layer-by-layer spreading of 8CB droplets is observed. With the help of the surface potential mapping capability of SPFM, we found that the molecular dipole of the first monolayer of 8CB is parallel to the surface. A layer of nearly vertical molecular dimers on top of the monolayer has an associated surface potential of 40 mV, which is attributed to a distortion of the dimer. The dimer distortion propagates to the subsequent smectic bilayers, producing an additional 7 mV potential increase in the second layer, 2 mV on the third, and approximately 1 mV on the fourth.

Journal Article↗
PubMed2000

2,3,7,8 Tetrachlorodibenzo-p-dioxin induction of cytochrome P4501A in cultured rat and human hepatocytes.

We report here a novel observation that 2,3,7,8-tetracholorodibenzo-p-dioxin (TCDD) induced predominantly cytochrome P4501A1 (CYP1A1) in rat hepatocytes and predominantly CYP1A2 in human hepatocytes. As part of our research program to evaluate species-differences in response to CYP inducers, we studied the effects of TCDD on CYP1A activity, protein, and gene expression in primary cultures of rat and human hepatocytes. TCDD was found to induce CYP1A activity, measured as ethoxyresorufin-O-deethylase (EROD) activity, in both rat and human hepatocytes. TCDD induction of EROD activity in human hepatocytes (2-5 fold of concurrent solvent control), was significantly lower than that found in rat hepatocytes ( 20-fold of concurrent solvent control). Two structural analogs of TCDD, 2,3,7,8-tetrachlorodibenzofuran (TCDF) and 6-nitro-1,3,8-trichlorodibenzofuran (6-NCDF), were also evaluated. As observed for TCDD, human hepatocytes consistently showed a lower response than rat hepatocytes. As most TCDD-related effects are believed to be mediated via binding of the TCDD-Ah receptor (AhR) complex to DNA, nuclear AhR levels were measured in rat and human hepatocytes after TCDD treatment. We found that the nuclear AhR levels in TCDD-treated rat hepatocytes were approximately 4 times higher than found in TCDD-treated human hepatocytes. However, the estimated binding affinity of [3H]TCDD to nuclear AhR from rat hepatocytes was similar. The species difference in response to TCDD was further evaluated by analysis of CYP1A1 and CYP1A2 mRNA levels using Northern analysis, and P4501A1 and 1A2 protein levels using Western immunoblotting. Results showed that, at both gene expression and protein levels, TCDD induced predominantly CYP1A1 in rat hepatocytes and CYP1A2 in human hepatocytes.

Animals↗
PubMed2000

The electrogenic sodium bicarbonate cotransporter: developmental expression in rat brain and possible role in acid vulnerability.

The electrogenic sodium bicarbonate cotransporter (NBC) is expressed in glial cells in the brain and plays an important role in the regulation of both intracellular and extracellular pH. Differential vulnerability to acidosis between neurons and glia has been noted and may contribute to infarction after cerebral ischemia. Ionic substitution studies and inhibition of injury by 4, 4'-di-isothiocyanostilbene-2,2'-disulfonic acid suggest that NBC is involved in astrocyte vulnerability to acidic injury. Recently two NBC cDNAs differing in 5'-untranslated and N-terminal coding sequence have been cloned from kidney and pancreas. We cloned one of these cDNAs from rat brain and demonstrate here that the clone is functional by expression in Xenopus oocytes. We determined the developmental and regional expression of NBC in the brain by in situ hybridization. Expression was observed in the spinal cord at embryonic day 17, whereas expression in brain was first seen at approximately postnatal day 0 (P0), increased at P15, and persisted in the adult brain. Expression was widespread throughout the cerebellum, cortex, olfactory bulb, and subcortical structures. Cellular resolution of the in situ hybridization signal and double labeling for glial fibrillary acidic protein were consistent with a glial localization for NBC. Expression of NBC in 3T3 cells that do not normally express this transporter rendered them vulnerable to acid injury. The expression profile suggests that this transporter is critical during the later stages of brain development and could be one of the factors contributing to the different patterns of injury seen in perinatal versus adult cerebral ischemia.

3T3 Cells↗
PubMed2000

Coding of sound-source location by ensembles of cortical neurons.

We examined the coding of sound-source location by ensembles of neurons in the auditory cortex. Broadband noise bursts were presented from loudspeakers throughout 360 degrees in the horizontal plane. Sound levels varied from 20 to 40 dB above neural thresholds. We recorded temporal spike patterns simultaneously at 16 recording sites in area A2 of alpha-chloralose-anesthetized cats. Spike patterns of individual units varied in spike counts and in spike timing as a function of sound-source location. Ensembles of up to 19 units recorded simultaneously demonstrated additional location sensitivity in the form of relative spike counts and relative spike timing among neurons. We used an artificial neural network (ANN) algorithm to recognize ensemble spike patterns and, thereby, to infer the locations of sound sources. The ANN could estimate stimulus locations based on ensemble responses to single-stimulus presentations. Median errors (MEs) averaged 49.2 +/- 11.9 degrees (mean +/- SD; n = 34; chance level, 90 degrees ). The ANN maintained better-than-chance performance even when input spike patterns were expressed as relative spike counts across units (i.e., no information available from absolute spike counts of individual units; ME, 63.0 +/- 11.8 degrees ) or when spike latencies were represented as time relative to the first spike for each trial (i.e., no external time reference available; ME, 54.3 +/- 12.4 degrees ). The ANN performance improved monotonically as the sizes of ensemble patterns were increased by combining patterns across the entire unit sample. The performance by ensembles of 128 units approached the level of localization performance of behaving cats.

Action Potentials↗
PubMed2000

Equine bullous pemphigoid IgG autoantibodies target linear epitopes in the NC16A ectodomain of collagen XVII (BP180, BPAG2).

Bullous pemphigoid (BP) is an autoimmune subepithelial blistering dermatosis of humans, dogs, cats and pigs. It is characterized by skin-fixed and circulating IgG autoantibodies that target one or both BP antigens. An immunological homologue of BP in humans was diagnosed in two horses with cutaneous and mucosal ulcerations as well as microscopic subepithelial vesiculation. Immunological investigations revealed similar findings for both the horses. Direct immunofluorescence demonstrated the presence of IgG deposited linearly at the dermoepidermal junction in mucosal and skin biopsy specimens. Indirect immunofluorescence testing confirmed the existence of circulating basement membrane-specific IgG autoantibodies. Using intact and salt-split epithelial substrates, serum IgG were shown to target antigens situated not only at the basal, but also at the lateral and apical aspects of stratum basale keratinocytes. Immunoblotting and ELISA corroborated that the IgG from affected horses, but not those from normal controls, exhibited high immunoreactivity against the NC16A extracellular domain of type XVII collagen (BPAG2, BP180). Equine BP could be proposed, therefore, as another spontaneous model of this most common basement membrane autoimmune dermatosis of humans.

Animals↗
PubMed2000

Molecular cloning of canine bullous pemphigoid antigen 2 cDNA and immunomapping of NC16A domain by canine bullous pemphigoid autoantibodies.

The autoantibody-mediated subepidermal blistering skin disease bullous pemphigoid affects both humans and dogs. We previously demonstrated that canine bullous pemphigoid patient's autoantibodies targeted skin basement membrane component and a 180-kDa keratinocyte protein. We extend our works to partially isolate the cDNA encoding canine bullous pemphigoid antigen 2 (BPAg2, BP180). Total RNA extracted from a papillomavirus-immortalized canine keratinocyte cell line and a cultured canine squamous carcinoma cell line SCC 2/88 were used to isolate fragments of cDNA encoding BPAg2 by reverse transcription-PCR and 5'-rapid amplification of cDNA end. The isolated sequence included the 5'-untranslated region, the entire intracellular, transmembranous, and extracellular NC16A autoantigenic domains, plus a small segment of the collagenous domain. Sequence analyses of the isolated cDNA showed 87 and 85% identities between canine and human at the nucleotide sequence and at the deduced amino acid sequence levels, respectively. The canine BPAg2 sequence was confirmed by a rabbit antibody raised against a 18-amino acid peptide deduced from the canine NC16A nucleotide sequence. Autoantibodies from canine bullous pemphigoid patients' sera recognized epitopes within the human NC16A domain. The cloning of the cDNA encoding this disease-associated protein may allow us to develop a canine model in dissecting the immunopathologic mechanism underlying bullous pemphigoid.

Animals↗
PubMed2000

Electrospray ionization mass spectra of amino acid phosphoramidates of adenosine.

Amino acid phosphoramidates of adenosine were synthesized and determined by positive and negative ion electrospray ionization mass spectrometry (ESI-MS) in conjunction with tandem mass spectrometry (MS/MS). The fragmentation pathways were investigated. In the positive ion mass spectra abundant characteristic fragment ions appeared, and many complementary ions were found. In the negative ion mass spectra only a few fragment ions were observed, and most of them contained phosphoryl groups. The results show that ESI-MS is a useful tool for structural determination of amino acid phosphoramidates of nucleosides.

Adenosine↗
PubMed2000

Identification of a novel mechanism for endotoxin-mediated down-modulation of CC chemokine receptor expression.

In the present study, we explored the molecular mechanisms by which bacterial endotoxin (LPS) mediates the down-regulation of CCR2 receptors on human monocytes. We found that LPS induced a marked reduction in CCR2 cell surface protein levels which was blocked by pretreatment with the tyrosine kinase inhibitors genistein and herbimycin A. The effector mechanism underlying LPS-induced CCR2 down-modulation appears to involve the enzymatic activity of proteinases since Western blot analysis of LPS-stimulated monocytes revealed the degradation of a 38-kDa species corresponding to the CCR2B monomer. In RBL cells expressing the CCR2B-green fluorescent protein (GFP) fusion chemokine receptor, LPS stimulated the internalization and degradation of CCR2. The serine proteinase inhibitor N-alpha-p-tosyl-L-lysine chloromethyl ketone blocked LPS-induced down-modulation of CCR2 in monocytes and CCR2B-GFP in RBL cells. This work describes a previously uncharacterized mechanism for CC chemokine receptor down-modulation that is dependent upon tyrosine kinase activation and serine proteinase-mediated receptor degradation and may provide further insight into the mechanisms of leukocyte regulation during immunological and inflammatory responses.

Chemokine CCL2↗
PubMed2000

Mammalian-cell-produced neurturin (NTN) is more potent than purified Escherichia coli-produced NTN.

Neurturin (NTN) is a recently identified homologue of glial-cell-line-derived neurotrophic factor. Both factors promote the survival of dopaminergic (DA) neurons. We investigated the biological activity of mammalian-cell-produced NTN versus purified Escherichia coli-produced NTN. Baby hamster kidney cells were engineered to stably secrete mature human NTN. Mammalian-cell-derived NTN enhanced the activity of embryonic DA neurons in vitro, with greater potency (maximum effect achieved in the picogram range) than purified E. coli-produced NTN. Cell-based delivery of NTN (less than 10 ng/day) was also shown to be biologically active in vivo. These results suggest that mammalian-cell-derived NTN, synthesized de novo and delivered in small quantities to the parenchyma at the target site, may be as active as much larger quantities of purified, E. coli-produced NTN, delivered by other means.

Animals↗
PubMed2000

Crystallization and preliminary X-ray analysis of receptor-binding protein P2 of bacteriophage PRD1.

Bacteriophage PRD1 has remarkable structural similarities to adenovirus, but is unusual in containing a membrane beneath its icosahedral capsid. Its monomeric receptor-binding protein, P2, is part of a complex at each capsid vertex and so is the functional equivalent of adenovirus fiber. P2 has been crystallized by the "hanging-drop" method of vapor diffusion and two different crystal forms were obtained. Macroseeding, used to increase the size of the initial small needles, gave rod-shaped crystals. These grew to a size of 0.08 x 0.08 x 0.50 mm(3) and diffracted to 2.6 A resolution. They have the orthorhombic space group P222(1), with unit cell dimensions a = 137.8 A, b = 46.5 A, c = 136.4 A. A few single crystals of a second form were grown without seeding under slightly different conditions. A parallelepiped crystal (0.10 x 0.10 x 0.35 mm(3)), with space group C222(1) and unit cell dimensions a = 182.3 A, b = 204.8 A, c = 133.3 A, diffracted to 3.5 A resolution. A rotation function for the second form revealed that four monomers of P2 are related by a noncrystallographic twofold axis. The structure of P2 will reveal how this arrangement relates to the trimeric adenovirus fiber.

Capsid↗
PubMed2000

A spontaneously arising porcine model of bullous pemphigoid.

Bullous pemphigoid (BP) is an IgG-mediated autoimmune blistering disease targeting the hemidesmosomal proteins bullous pemphigoid antigens 1 and 2. Currently, there is no active animal model in which to dissect the immunopathogenic mechanism. We noticed that cutaneous blistering arose spontaneously in 12 adult Yucatan minipigs. Skin lesions consisted of turgid, isolated or clustered vesicles that occasionally evolved from erythematous and pruritic patches. Histopathological examination revealed subepidermal vesicles rich in intact and degranulated eosinophils. Antigen mapping and transmission electron microscopy confirmed that dermoepidermal separation took place in the lamina lucida of the epidermal basement membrane zone. Direct immunofluorescence revealed the presence of IgG deposited linearly at the dermoepidermal junction in seven of nine skin specimens examined. Indirect immunofluorescence testing confirmed the presence, in the serum from eight of eight affected pigs, of circulating basement membrane-specific IgG autoantibodies (titers 1 : 50 to 1 : 250). Using uncleaved and salt-split lip substrates, the autoantibodies were shown to target antigens situated not only at the basal, but also at the lateral and apical aspects of stratum basale keratinocytes. Immunoelectron microscopy confirmed that circulating IgG autoantibodies recognized hemidesmosomal antigen(s). ELISA, immunoblotting and immunoadsorption demonstrated that five of eight serum samples exhibited high immunoreactivity against BPAG2-NC16A peptides. This novel porcine acquired blistering dermatosis could be proposed as a valuable model to conduct immunomechanistic studies on the natural progression of BP, correlation of autoreactive T cells or autoantibodies with disease activity, and the role of eosinophils in the blistering process, as these diseases cannot be modeled easily in human patients or in murine passive transfer models.

Amino Acid Sequence↗
PubMed2000

Identification of differentially expressed genes in nasopharyngeal carcinoma by means of the Atlas human cancer cDNA expression array.

PURPOSE: To investigate genes of critical areas, including cell cycle/growth control, apoptosis, oncogene/tumor suppressors and growth factor/cytokines, that are differentially expressed in nasopharyngeal carcinoma. METHODS: The Human Cancer cDNA Atlas, which contains 588 genes relating to tumor biology, was used to screen normal nasopharyngeal tissue, nasopharyngeal cancer (NPC). The reverse transcription/polymerase chain reaction was used to confirm the expression pattern of some genes identified by Atlas hybridization. RESULTS: The differentially expressed cell cycle/growth control regulators in NPC showed a stronger tendency toward cell proliferation with the up-regulation of cyclin D1, cyclin D2 etc. The expression pattern of apoptosis-related genes demonstrated the up-regulation of both anti-apoptotic factors such as the BCL-2-related protein A1, TRAF3, the inhibitor of apoptosis protein A1 (IAPI) and apoptotic pathway elements such as Fas/Apo-1, Apo-2 ligand etc. Among oncogenes/tumor suppressors, MDM2, STAT1 and STAT2 were found to be up-regulated in NPC. The expression profile of growth factors/cytokines showed the up-regulation of many growth-enhancing factors such as EGR1, tumor-derived growth factor 1, platelet-derived growth factor A chain etc. as well as Th1-type cytokines e.g. interleukin-1beta and interferons. A smaller number of genes were down-regulated in nasopharyngeal cancer, such as those encoding ERK1, Raf, secreted apoptosis-related protein 1, CD27BP, transforming growth factor beta2, pre-B-cell-stimulating factor homologue etc. CONCLUSION: The consistent tendency toward cell proliferation, the possibility of a stronger antiapoptotic force that operates on the normal apoptotic pathway, or the autocrine or paracrine growth factors may account for the development of NPC. Some genes are reported for the first time to have changed expression in nasopharyngeal carcinoma. The simple, quick, and high-throughput method of profiling gene expression by cDNA array hybridization provides us with a quick overview of key factors that may be involved in NPC, and may identify genes suitable for further study of carcinogenesis mechanism or targets for possible molecular diagnosis or therapy.

Apoptosis↗
PubMed2000

Evidence for a role of the lumenal M3-M4 loop in skeletal muscle Ca(2+) release channel (ryanodine receptor) activity and conductance.

We tested the hypothesis that part of the lumenal amino acid segment between the two most C-terminal membrane segments of the skeletal muscle ryanodine receptor (RyR1) is important for channel activity and conductance. Eleven mutants were generated and expressed in HEK293 cells focusing on amino acid residue I4897 homologous to the selectivity filter of K(+) channels and six other residues in the M3-M4 lumenal loop. Mutations of amino acids not absolutely conserved in RyRs and IP(3)Rs (D4903A and D4907A) showed cellular Ca(2+) release in response to caffeine, Ca(2+)-dependent [(3)H]ryanodine binding, and single-channel K(+) and Ca(2+) conductances not significantly different from wild-type RyR1. Mutants with an I4897 to A, L, or V or D4917 to A substitution showed a decreased single-channel conductance, loss of high-affinity [(3)H]ryanodine binding and regulation by Ca(2+), and an altered caffeine-induced Ca(2+) release in intact cells. Mutant channels with amino acid residue substitutions that are identical in the RyR and IP(3)R families (D4899A, D4899R, and R4913E) exhibited a decreased K(+) conductance and showed a loss of high-affinity [(3)H]ryanodine binding and loss of single-channel pharmacology but maintained their response to caffeine in a cellular assay. Two mutations (G4894A and D4899N) were able to maintain pharmacological regulation both in intact cells and in vitro but had lower single-channel K(+) and Ca(2+) conductances than the wild-type channel. The results support the hypothesis that amino acid residues in the lumenal loop region between the two most C-terminal membrane segments constitute a part of the ion-conducting pore of RyR1.

Amino Acid Sequence↗
PubMed2000

Effects of repeated nicotine administration and footshock stress on rat mesoprefrontal dopamine systems: Evidence for opioid mechanisms.

We have examined the effects of nicotine pre-treatment on mesoprefrontal dopamine (DA) function in the presence and absence of acute stress, and the involvement of endogenous opiate peptide systems (EOPS). Acute electrical footshock stress preferentially increases DA utilization in medial prefrontal cortex (mPFC) compared to nucleus accumbens (NAS) and striatal terminal fields, and this is correlated with profound locomotor immobility. Our recent studies have demonstrated that repeated, but not acute, nicotine pre-treatment significantly reduced mPFC DA utilization and footshock stress-induced immobility responses. There is increasing evidence that the biochemical and behavioral effects of nicotine are mediated by EOPS, and we hypothesized that the stress-reducing effects of repeated nicotine administration in these studies were mediated by EOPS. Accordingly, rats pre-treated subcutaneously with repeated nicotine were given a single dose of the opiate receptor antagonist naloxone (0.1-10.0 mg/kg, i.p.) or saline as a co-treatment with nicotine or saline 10 min prior to acute footshock stress. Naloxone had no effects on non-stressed or acute footshock stress-induced mPFC DA utilization, but dose-dependently antagonized repeated nicotine's attenuation of stress-induced mesoprefrontal DA utilization and immobility responses. Furthermore, naloxone dose-dependently blocked repeated nicotine's augmentation of accumbal DA utilization. These results suggest that EOPS may be involved in mediating repeated nicotine administration effects on mesoprefrontal dopaminergic and immobility responses to acute footshock stress.

Animals↗
PubMed2000

Development and validation of a calcium intake questionnaire for postmenopausal women in China.

PURPOSE: Develop and evaluate a food frequency questionnaire (FFQ) for assessing calcium intake in Chinese postmenopausal women. METHODS: An interviewer-conducted FFQ that asks respondents about their consumption of 110 calcium-containing foods over the previous week was developed and administered to 21 randomly selected postmenopausal women from Chengdu. This FFQ uses both the traditional weight-estimation method of assessing portion size in China and a new volume-estimation method. Test-retest reliability was assessed by re-administering the questionnaire two weeks later and validity was assessed by comparing the computed daily calcium intake to that obtained by a researcher-conducted 4-day food record. RESULTS: Estimates of calcium intake derived from the volume-estimation method were significantly lower than estimates derived from the traditional weight-estimation method (median calcium intakes = 270 mg/day versus 570 mg/day), but were quite similar to those obtained from the 4-day food record (median calcium intake = 275 mg/day). The test-retest reliability and validity of the volume-estimation method (intraclass correlation coefficient (ICC) = 0.73 and 0.86, respectively) were better than those of the weight-estimation method (ICC = 0.58 and 0.39, respectively). CONCLUSIONS: The reliability and validity of the traditional weight-estimation FFQ method of assessing dietary intake of nutrients in China is poor; this method may significantly overestimate dietary intakes. The proposed volume-estimation FFQ method for assessing calcium intake is a simple, interviewer-conducted method that is both reliable and valid. The calcium intake of postmenopausal women in urban China is much lower than the recommended daily allowance of 800 mg.

Aged↗
PubMed2000

Regulation of tyrosine kinase activation and granule release through beta-arrestin by CXCRI.

Chemoattractant-stimulated granule release from neutrophils, basophils and eosinophils is critical for the innate immune response against infectious bacteria. Interleukin 8 (IL-8) activation of the chemokine receptor CXCRI was found to stimulate rapid formation of beta-arrestin complexes with Hck or c-Fgr. Formation of beta-arrestin-Hck complexes led to Hck activation and trafficking of the complexes to granule-rich regions. Granulocytes expressing a dominant-negative beta-arrestin-mutant did not release granules or activate tyrosine kinases after IL-8 stimulation. Thus, beta-arrestins regulate chemokine-induced granule exocytosis, indicating a broader role for beta-arrestins in the regulation of cellular functions than was previously suspected.

Amino Acid Sequence↗