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Biomedical subjects

X L Xu

Publications and source records attributed to X L Xu.

At least 37 records · Page 2Linked to original sources
PubMed1999

[Direct gene transfer into rabbit peripheral nerve in vivo].

OBJECTIVE: To probe the possibility of direct transfer of exogenous gene into peripheral nerve and its following expression in vivo. METHODS: The PCMV beta plasmid containing cytomegalovirus (CMV) promoter and Escherichia Coli (E. Coli), beta-Galactosidease (beta-Gal) structural gene (lacZ gene) was constructed and injected into the rabbit sciatic nerve. The control group was injected PBS solution. The injected nerves were sampled and tested by beta-Gal enzyme activity assay of the 5-bromo-4-chloro-3-indolyl-beta-D-galactoside and beta-Gal histochemical stain. RESULTS: In the control group, no beta-Gal enzyme activity was detected in the different stages after operation, and beta-Gal histochemical stains showed positive. In the experimental group, enzyme activity could be detected from 2 days to 30 days after operation, and the histochemical stains showed negative. CONCLUSION: The exogenous gene can be transferred into peripheral nerve and expressed with bioactivity, thus the gene therapy to accelerate the recovery of nerve is practical.

Animals↗
PubMed1998

Differential roles for disulfide bonds in the structural integrity and biological activity of kappa-Bungarotoxin, a neuronal nicotinic acetylcholine receptor antagonist.

kappa-Bungarotoxin, a kappa-neurotoxin derived from the venom of the banded Krait, Bungarus multicinctus, is a homodimeric protein composed of subunits of 66 amino acid residues containing five disulfide bonds. kappa-Bungarotoxin is a potent, selective, and slowly reversible antagonist of alpha3 beta2 neuronal nicotinic acetylcholine receptors. kappa-Bungarotoxin is structurally related to the alpha-neurotoxins, such as alpha-bungarotoxin derived from the same snake, which are monomeric in solution and which effectively antagonize muscle type receptors (alpha1 beta1 gamma delta) and the homopentameric neuronal type receptors (alpha7, alpha8, and alpha9). Like the kappa-neurotoxins, the long alpha-neurotoxins contain the same five conserved disulfide bonds, while the short alpha-neurotoxins only contain four of the five. Systematic removal of single disulfide bonds in kappa-bungarotoxin by site-specific mutagenesis reveals a differential role for each of the disulfide bonds. Removal of either of the two disulfides connecting elements of the carboxy terminal loop of this toxin (Cys 46-Cys 58 and Cys 59-Cys 64) interferes with the ability of the toxin to fold. In contrast, removal of each of the other three disulfides does not interfere with the general folding of the toxin and yields molecules with biological activity. In fact, when either C3-C21 or C14-C42 are removed individually, no loss in biological activity is seen. However, removing both produces a polypeptide chain which fails to fold properly. Removal of the C27-C31 disulfide only reduces the activity of the toxin 46.6-fold. This disulfide may play a role in specific interaction of the toxin with specific neuronal receptors.

Alanine↗
PubMed1998

Probing the regulatory domain interface of D-3-phosphoglycerate dehydrogenase with engineered tryptophan residues.

D-3-Phosphoglycerate dehydrogenase from Escherichia coli is a homotetrameric enzyme which is allosterically regulated by the end product of its pathway, L-serine. The enzyme binds 4 L-serine molecules at two interfaces formed by the noncovalent association of the regulatory domains. The two domains that comprise each interface are related by an approximately 180 degrees axis of symmetry, and two serine molecules bind at each interface by forming a hydrogen bond network between the domains. A model has been proposed that suggests that serine functions by drawing adjacent domains together and that this in turn translates a conformational change to the active site. A tryptophan residue has been engineered into the helices flanking the regulatory interfaces that displays significant quenching in response to serine binding. Residues on the adjacent subunit appear to be primarily responsible for the tryptophan quenching and thus support the hypothesis that serine binding leads to an increase in the proximity between residues on neighboring subunits. Serine binding studies show that this quenching, as well as inhibition of enzymatic activity, are essentially complete when only two of the four serine binding sites are occupied. The requirement for only one serine per interface is consistent with the notion that the interface is formed by relatively rigid domains and that hydrogen bonding at only a single site is all that is required to substantially close the interface. The fluorescence quenching in response to L-serine binding generally correlates with enzymatic inhibition, but there appears to be a slight lag in inhibition relative to quenching at low serine concentrations. The observed fluorescence quenching of residues in the regulatory domains of D-3-phosphoglycerate dehydrogenase provide the first direct evidence for a conformational change in response to effector binding and provide a means to monitor the first step in the allosteric mechanism.

Allosteric Regulation↗
PubMed1997

Cell cycle-dependent colocalization of BARD1 and BRCA1 proteins in discrete nuclear domains.

Germ-line mutations of the BRCA1 gene predispose women to early-onset breast and ovarian cancer by compromising the gene's presumptive function as a tumor suppressor. Although the biochemical properties of BRCA1 polypeptides are not understood, their expression pattern and subcellular localization suggest a role in cell-cycle regulation. When resting cells are induced to proliferate, the steady-state levels of BRCA1 increase in late G1 and reach a maximum during S phase. Moreover, in S phase cells, BRCA1 polypeptides are hyperphosphorylated and accumulate into discrete subnuclear foci termed "BRCA1 nuclear dots." BRCA1 associates in vivo with a structurally related protein termed BARD1. Here we show that the steady-state levels of BARD1, unlike those of BRCA1, remain relatively constant during cell cycle progression. However, immunostaining revealed that BARD1 resides within BRCA1 nuclear dots during S phase of the cell cycle, but not during the G1 phase. Nevertheless, BARD1 polypeptides are found exclusively in the nuclear fractions of both G1- and S-phase cells. Therefore, progression to S phase is accompanied by the aggregation of nuclear BARD1 polypeptides into BRCA1 nuclear dots. This cell cycle-dependent colocalization of BARD1 and BRCA1 indicates a role for BARD1 in BRCA1-mediated tumor suppression.

BRCA1 Protein↗
PubMed1997

Critical interactions at the dimer interface of kappa-bungarotoxin, a neuronal nicotinic acetylcholine receptor antagonist.

The alpha- and kappa-neurotoxins are polypeptide antagonists of nicotinic acetylcholine receptors derived from snake venom. They are structurally very similar but differ in their specificity for receptor subtype and in their native aggregation state. While the alpha-neurotoxins are monomeric, the kappa-neurotoxins occur as homodimers. The crystal structure shows that there is a correlation in the distance between essential arginine residues in the kappa-bungarotoxin dimer and the distance between the acetylcholine binding sites in the pentameric receptor. This has lead to an investigation of the critical interactions at the dimer interface of kappa-bungarotoxin. Mutations of residues that the crystal structure indicates participate in dimer interaction were found to fall into two general groups: those that do not affect the dimerization state or activity of kappa-bungarotoxin as single mutants, and those that interfere with it to such an extent that the protein is no longer able to fold properly. In general, those residues that fall into the latter group are found to be invariant in kappa-neurotoxins and not found in alpha-neurotoxins. The results suggest that the extent of both the main chain-main chain beta-sheet hydrogen bond interaction and van der Waals interactions between Phe 49 and Ile 20 are required for dimer formation. These studies provide a basis for understanding why the kappa-neurotoxins readily dimerize in solution and the alpha-neurotoxins do not and also suggest that there is a possible interrelationship between dimer formation and protein folding in kappa-bungarotoxin.

Amino Acid Sequence↗
PubMed1997

Regulation of the twist target gene tinman by modular cis-regulatory elements during early mesoderm development.

The Drosophila tinman homeobox gene has a major role in early mesoderm patterning and determines the formation of visceral mesoderm, heart progenitors, specific somatic muscle precursors and glia-like mesodermal cells. These functions of tinman are reflected in its dynamic pattern of expression, which is characterized by initial widespread expression in the trunk mesoderm, then refinement to a broad dorsal mesodermal domain, and finally restricted expression in heart progenitors. Here we show that each of these phases of expression is driven by a discrete enhancer element, the first being active in the early mesoderm, the second in the dorsal mesoderm and the third in cardioblasts. We provide evidence that the early-active enhancer element is a direct target of twist, a gene encoding a basic helix-loop-helix (bHLH) protein, which is necessary for tinman activation. This 180 bp enhancer includes three E-box sequences which bind Twist protein in vitro and are essential for enhancer activity in vivo. Ectodermal misexpression of twist causes ectopic activation of this enhancer in ectodermal cells, indicating that twist is the only mesoderm-specific activator of early tinman expression. We further show that the 180 bp enhancer also includes negatively acting sequences. Binding of Even-skipped to these sequences appears to reduce twist-dependent activation in a periodic fashion, thus producing a striped tinman pattern in the early mesoderm. In addition, these sequences prevent activation of tinman by twist in a defined portion of the head mesoderm that gives rise to hemocytes. We find that this repression requires the function of buttonhead, a head-patterning gene, and that buttonhead is necessary for normal activation of the hematopoietic differentiation gene serpent in the same area. Together, our results show that tinman is controlled by an array of discrete enhancer elements that are activated successively by differential genetic inputs, as well as by closely linked activator and repressor binding sites within an early-acting enhancer, which restrict twist activity to specific areas within the twist expression domain.

Animals↗
PubMed1996

Identification of a RING protein that can interact in vivo with the BRCA1 gene product.

The hereditary breast and ovarian cancer gene, BRCA1, encodes a large polypeptide that contains the cysteine-rich RING motif, a zinc-binding domain found in a variety of regulatory proteins. Here we describe a novel protein that interacts in vivo with the N-terminal region of BRCA1. This BRCA1-associated RING domain (BARD1) protein contains an N-terminal RING motif, three tandem ankyrin repeats, and a C-terminal sequence with significant homology to the phylogenetically conserved BRCT domains that lie near the C terminus of BRCA1. The BARD1/BRCA1 interaction is disrupted by BRCA1 missense mutations that segregate with breast cancer susceptibility, indicating that BARD1 may be involved in mediating tumour suppression by BRCA1.

Amino Acid Sequence↗
PubMed1995

CD45-cross-linking stimulates IFN-gamma production in NK cells.

The in vitro demonstration of the ability of NK cells to secrete cytokines prompted in vivo studies that illustrated the importance of NK cell-derived cytokines in regulating immune responses. Cross-linking of CD16 on NK cells can stimulate cytokine production. CD16-independent interactions capable of stimulating cytokine production have also been described, but molecules mediating such stimulations remain to be biochemically defined. We report here that cross-linking of CD45 specifically stimulates IFN-gamma production in human NK cells. The NK cells used were IL-2-activated adherent NK cells and from the NK3.3 cell line. The ability of CD45 mAbs to stimulate NK cells appears not to be dependent on CD16, as CD45 mAbs of both IgG1 and IgG2a isotypes were equally stimulatory, as were F(ab')2 compared with whole anti-CD45 mAbs. Resting NK cells, like T cells, express predominantly CD45RA, whereas IL-2 activated adherent NK cells acquire expression of CD45RO. Abs specific for CD45RO, but not CD45RA, were able to stimulate IFN-gamma production in NK cells. It has been reported that one ligand for CD45RO is CD22 beta. We tested the ability of CD22-expressing transfectants to bind to and stimulate NK cells. Whereas NK cells bound to CD22 alpha and CD22 beta transfectants, this interaction was not inhibited by CD45RO Abs. In addition, neither of the CD22-transfectants were able to stimulate NK3.3 cells to secrete IFN-gamma. These observations collectively suggest that binding of NK3.3 cells to CD22 may be independent of CD45RO on NK3.3 cells.

Antibodies, Monoclonal↗
PubMed1993

[Studies on the chemical constituents of the leaves of Lantana camara].

Six compounds were isolated from leaves of Lantana camara. On the basis of chemical and spectral (UV, IR, EI-MS, 1HNMR 13CNMR) analysis, they were identified as oleanonic acid (I), lantadene A (II), lantadene B (III), lantanilic acid (IV), icterogenin (V) and 4',5-dihydroxy-3,7-dimethoxyflavone-4'-O-beta-D-glucopyranoside (VI). VI is a new compound named camaroside.

Drugs, Chinese Herbal↗
PubMed1990

Host range determinant in the late region of SV40 and RF virus affecting growth in human cells.

WtSV40 and its variant EL-SV40 (contains two complementing defective genomes) fail to productively infect human embryonic kidney cells or human fibroblasts. However, early SV40 (E-SV40) genomes can propagate in human cells when complemented by a particular late RF virus (L-RFV) genome or the closely related wtBKV genome. The L-RFV genome (L-RFV clone H) contains a deleted early region, a complete set of BKV capsid genes, and a single SV40 regulatory region (acquired by recombination). In contrast, it was not possible to make the reciprocal genome cross in human cells; late SV40 genomes containing a deleted early region do not complement early RFV or early BKV DNAs. The L-RFV clone H genome was also shown to complement wtSV40 in human cells. However, wtSV40 DNA was rapidly lost and replaced by a defective SV40 genome. The SV40 defective (E-SV40 alpha) contained a deletion of the late region, an intact early region, and paired with L-RFV clone H DNA to form a new hybrid virus. In human cells wtSV40 was also complemented by wtBKV DNA, but after two serial passages SV40 DNA disappeared. These findings indicate that SV40 late or capsid gene sequences, but not SV40 early sequences, generate a block to SV40 growth in human cells. When the SV40 late region is replaced by a RFV or a BKV late region, E-SV40 DNA propagates efficiently in human cells and in some cases more rapidly than wtBKV. Northern blot hybridization indicates that SV40 DNA is poorly transcribed in human cells when the SV40 late region is present.

Antigens, Viral, Tumor↗
PubMed1989

Sodium and potassium levels in hypertensive children.

In a baseline survey of 4,936 school children aged 6-16 years, 199 children with systolic blood pressure (SBP) values equal or greater than the 95-th percentiles for age and sex were chosen as the hypertensive group (HBP), and were matched for age and sex with 197 children with SBP from the 5-th through the 50-th percentiles as the control (normotensive) group (NBP). For both groups the intra-RBC and plasma sodium and potassium contents, 8-hour night urinary sodium, potassium and creatinine excretions for three days, and an oral saline-water load test were performed. The results show that (1) intra-RBC potassium level in the HBP was lower than that in NBP. The level in those with positive hypertension family history (FH+) was lower than that with negatives (FH-). The intra-RBC potassium contents correlated inversely with diastolic BP. No correlation between intra-RBC sodium and BP was found; (2) Plasma sodium concentration in HBP was much lower than that in NBP. No difference was found between the FH+ and FH- in the plasma sodium concentrations; (3) Mean 8-hour night urinary potassium excretion expressed as mmol/g creatinine, was lower in HBP than in NBP; (4) After the saline load test the 4-hour urinary sodium excretion was significantly higher in HBP. Of those children with FH- the 4-hour sodium excretion in HBP was higher than that in NBP, but no significant difference was found between HBP and NBP of the FH+ children in the 4-hour urinary sodium excretions.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed1989

[Sodium and potassium in children with hypertension].

On a baseline survey of 4936 school children, age ranged from 6 to 16 years, 199 children with the systolic blood pressure (SBP) value equal or greater than 95-th percentiles for age and sex were chosen as the hypertensive group (HBP), and the same number of children with the SBP lower than 50-th percentiles were matched as the normotensive group (NBP). For both groups the intra-RBC and plasma Na and K content, three sequential nights 8-hour urinary Na, K and creatinine excretion, and an oral saline-load test were performed. The results show that: (1) The intra-RBC K in the HBP was lower than in NBP, 259 +/- 57 vs 291 +/- 78 mumol/g-protein, P less than 0.01. That in those with positive hypertensive family history (FH+) was lower than in those of negatives (FH-). The intra-RBC K content was correlated inversely with diastolic BP, r = -0.43, P less than 0.001. No correlation between intra-RBC Na and BP was found; (2) Plasma Na concentration in HBP was much lower than in NBP, 137.5 +/- 7.9 vs 142.6 +/- 6.9 mmol/L, P less than 0.001. No difference was found between FH+ and FH-; (3) Mean 8-hour urinary K excretion at night was lower in HBP than in NBP, 23.9 +/- 11.5 vs. 28.2 +/- 14.7 mmol/g-creatinine, P less than 0.01; (4) After a load of oral saline the 4-hour urinary Na excretion was significantly higher in HBP than in NBP, 39.5 +/- 29.1 vs 30.8 +/- 23.2 mmol, P less than 0.01. Of those children with FH-, the 4-hour Na excretion in HBP was higher than in NBP, 39.8 +/- 29.4 vs 29.3 +/- 21.5 mmol, P less than 0.001, but no significant difference was found between HBP and NBP in children with FH+. We believed that the above changes in electrolytes in the children with their BP at higher-level percentiles represent an abnormal relation between potassium and arterial BP in the early stage of hypertension, as well as the early compensatory natriuresis of the kidneys.

Adolescent↗
PubMed1988

A study on the expectational model for tumor growth.

Based on the biological assumptions of the logistic and the Bertalanffy models, the paper makes a microscopic analysis to confirm that the Gompertz equation is the expectational model for tumor growth, which had been asserted by Steel in 1977 (Growth of Kinetic Tumors, Oxford University Press) but could hardly be verified experimentally. It is also known that the logistic and the Bertalanffy equations are two deviated models in the two sides of the expectational model and the logisitic is too biased to be a good model.

Cell Division↗
PubMed1987

Activated c-Ha-ras oncogene with a guanine to thymine transversion at the twelfth codon in a human stomach cancer cell line.

The rat fibroblast cell line Rat 1 was transfected with total DNA of a gastrocarcinoma cell line, BGC-823. The transforming gene was cloned from the genomic library of the secondary transformants using in situ hybridization with a probe of the human Alu repeat sequence. This cloned gene is homologous to the protooncogene c-Ha-ras. The activation lesion of the transforming gene was identified by sequence analysis as a single nucleotide substitution of thymine for guanine in the 12th codon. This results in the substitution of valine for glycine at the 12th amino acid of the Mr 21,000 protein.

Adenocarcinoma↗
PubMed1987

The biological foundation of the Gompertz model.

The paper examines the biological reasoning of the Gompertz model for tumor growth: the increase and loss rates of the tumor volume are both proportional to the volume itself. The statement is proved mathematically. The theoretical foundation is therefore established, this ensures the application of the Gompertz model as an effective aid for the understanding of the experimental and clinical tumor process.

Animals↗
PubMed1987

Molecular cloning and nucleotide sequence analysis of a transforming gene from human gastrocarcinoma cell line.

Mouse and rat fibroblasts were transfected with total DNA from human gastrocarcinoma cell line BGC-823. It was shown by hybridization assay that the genome of one of the rat secondary foci contains transforming genes from the human gastrocarcinoma cell line, which are homologous to the protooncogene c-Ha-ras in the normal cells. The genomic library of the rat secondary foci was constructed, using lambda phage EMBL3 as the vector. The transforming gene Ha-ras of the human gastrocarcinoma cell was thus cloned by screening the library with the probes of human Alu repeat sequence and c-Ha-ras. The nucleotide sequences of the first and second exons were analysed by M13-dideoxy method. The result shows that the nucleotide sequence of the transforming gene is the same as that of the normal protooncogene except one nucleotide difference in the first exon.

Amino Acid Sequence↗