Biomedical subjects

G Zhu

Publications and source records attributed to G Zhu.

At least 19 recordsLinked to original sources

PubMed1999

Sonic hedgehog and BMP2 exert opposing actions on proliferation and differentiation of embryonic neural progenitor cells.

Although Sonic Hedgehog (Shh) plays a critical role in brain development, its actions on neural progenitor cell proliferation and differentiation have not been clearly defined. Transcripts for the putative Shh-receptor genes patched (Ptc) and smoothened (Smo) are expressed by embryonic, postnatal, and adult progenitor cells, suggesting that Shh can act directly on these cells. The recombinant human amino-terminal fragment of Shh protein (Shh-N) alone did not support the survival of cultured progenitor cells, but treatment with Shh-N in the presence of bFGF increased progenitor cell proliferation. Furthermore, treatment of embryonic rat progenitor cells propagated either in primary culture or after mitogen expansion significantly increased the proportions of both beta-tubulin- (neuronal marker) and O4- (oligodendroglial marker) immunoreactive cells and reduced the proportion of nestin- (uncommitted neural progenitor cell marker) immunoreactive cells. By contrast Shh-N had no effect on the elaboration of GFAP- (astroglial marker) immunoreactive cells. Cotreatment with Shh-N and bone morphogenetic protein-2 (BMP2) inhibited the anti-proliferative, astroglial-inductive, and oligodendroglial-suppressive effects of BMP2. Our observations suggest that Shh-N selectively promotes the elaboration of both neuronal and oligodendroglial lineage species and inhibits the effects of BMP2 on progenitor cell proliferation and astroglial differentiation.

Aging↗

PubMed1999

Cryptosporidium parvum possesses a short-type replication protein A large subunit that differs from its host.

Replication protein A (RPA) consisting of three subunits is a eukaryotic single-stranded DNA (ssDNA)-binding protein involved in DNA replication, repair and recombination. We report here the identification and characterization of a RPA large subunit (CpRPA1) gene from the apicomplexan Cryptosporidium parvum. The CpRPA1 gene encodes a 53.9-kDa peptide that is remarkably smaller than that from other eukaryotes (i.e. approximately 70 kDa) and is actively expressed in both free sporozoites and parasite intracellular stages. This short-type RPA large subunit has also been characterized from one other protist, Crithidia fasciculata. Three distinct domains have been identified in the RPA large subunit of humans and yeasts: an N-terminal protein interaction domain, a central ssDNA-binding area, and a C-terminal subunit-interacting region. Sequence analysis reveals that the short-type RPA large subunit differs from that of other eukaryotes in that only the domains required for ssDNA binding and heterotrimer formation are present. It lacks the N-terminal domain necessary for the binding of proteins mainly involved in DNA repair and recombination. This major structural difference suggests that the mechanism for DNA repair and recombination in some protists differs from that of other eukaryotes. Since replication proteins play an essential role in the cell cycle, the fact that RPA proteins of C. parvum differ from those of its host suggests that RPA be explored as a potential chemotherapeutic target for controlling cryptosporidiosis and/or diseases caused by other apicomplexans.

Amino Acid Sequence↗

PubMed1999

Electrochemical studies of cytochrome c disulfide at gold electrodes.

The electrochemistry of disulfide in cytochrome c on gold electrodes was reported. The observed electrochemical response was used to explain why the electrochemical reaction of cytochrome c is irreversible at gold electrodes. Disulfide bonds in cytochrome c were strongly adsorbed onto the surface of gold electrodes and caused slow rate of electron transfer of the heme group. It was found that the presence of disulfides in cytochrome c was responsible for the lack of electrochemical response of the heme group on a gold electrode. The mechanisms for this effect were studied using electrochemistry and photoelectron spectroscopy.

Amino Acids↗

PubMed1999

Identification of a critical motif responsible for gating of Kir2.3 channel by intracellular protons.

Protons are involved in gating Kir2.3. To identify the molecular motif in the Kir2.3 channel protein that is responsible for this process, experiments were performed using wild-type and mutated Kir2. 3 and Kir2.1. CO2 and low pHi strongly inhibited wild-type Kir2.3 but not Kir2.1 in whole cell voltage clamp and excised inside-out patches. This CO2/pH sensitivity was completely eliminated in a mutant Kir2.3 in which the N terminus was substituted with that in Kir2.1, whereas a similar replacement of its C terminus had no effect. Site-specific mutations of all titratable residues in the N terminus, however, did not change the CO2/pH sensitivity. Using several chimeras generated systematically in the N terminus, a 10-residue motif near the M1 region was identified in which only three amino acids are different between Kir2.3 and Kir2.1. Mutations of these residues, especially Thr53, dramatically reduced the pH sensitivity of Kir2.3. Introducing these residues or even a single threonine to the corresponding positions of Kir2.1 made the mutant channel pH-sensitive. Thus, a critical motif responsible for gating Kir2.3 by protons was identified in the N terminus, which contained about 10 residues centered by Thr53.

Amino Acid Sequence↗

PubMed1999

Effects of intra- and extracellular acidifications on single channel Kir2.3 currents.

1. The inward rectifier K+ channel Kir2.3 is inhibited by hypercapnia, and this inhibition may be mediated by decreases in intra- and extracellular pH. To understand whether Kir2.3 has two distinct pH sensors and whether cytosol-soluble factors are involved in the modulation of this channel during intracellular acidification, single channel currents were studied by expressing Kir2.3 in Xenopus oocytes. 2. In excised inside-out patches, Kir2.3 currents had a high baseline channel open-state probability (Po, at pH 7.4) with a strong inward rectification. Single channel conductance at hyperpolarizing membrane potential was about 17 pS with 150 mM K+ applied to both sides of the membrane. The channel showed a substate conductance of about 8 pS. 3. Reduction of intracellular pH (pHi) produced a fast and reversible inhibition of single channel Kir2.3 currents in inside-out patches. The extent of this inhibition is concentration dependent. A clear reduction in Kir2.3 currents was seen at pHi 7.0, and channel activity was completely suppressed at pHi 6.2 with mid-point inhibition (pK) at pH 6.77. 4. The effect of low pHi on Kir2.3 currents was due to a strong inhibition of Po and a moderate suppression of single channel conductance. The pK values for these single channel properties were pH 6.78 and 6.67, respectively. 5. The decrease in Po with low pHi resulted from an increase in the channel mean closed time without significant changes in the mean open time. Substate conductance was not seen during low pHi. 6. Decrease in extracellular pH (pHo) also caused inhibition of single channel activity of Kir2.3 currents in excised outside-out patches. This effect, however, was clearly different from that of pHi: the pK (pH 6.70) was about 0.1 pH units lower; more than 50 % channel activity was retained at pHo 5.8; and low pHo affected mainly single channel conductance. 7. These results therefore indicate that (1) there are two distinct pH sensors in Kir2.3, (2) different mechanisms are involved in the modulation of Kir2.3 through these two pH sensors, and (3) cytosol-soluble factors do not appear to be engaged in this modulation.

Algorithms↗

PubMed1999

Suppression of Kir2.3 activity by protein kinase C phosphorylation of the channel protein at threonine 53.

Kir2.3 plays an important part in the maintenance of membrane potential in neurons and myocardium. Identification of intracellular signaling molecules controlling this channel thus may lead to an understanding of the regulation of membrane excitability. To determine whether Kir2.3 is modulated by direct phosphorylation of its channel protein and identify the phosphorylation site of protein kinase C (PKC), we performed experiments using several recombinant and mutant Kir2.3 channels. Whole-cell Kir2.3 currents were inhibited by phorbol 12-myristate 13-acetate (PMA) in Xenopus oocytes. When the N-terminal region of Kir2.3 was replaced with that of Kir2.1, another member in the Kir2 family that is insensitive to PMA, the chimerical channel lost its PMA sensitivity. However, substitution of the C terminus was ineffective. Four potential PKC phosphorylation sites in the N terminus were studied by comparing mutations of serine or threonine with their counterpart residues in Kir2.1. Whereas substitutions of serine residues at positions 5, 36, and 39 had no effect on the channel sensitivity to PMA, mutation of threonine 53 completely eliminated the channel response to PMA. Interestingly, creation of this threonine residue at the corresponding position (I79T) in Kir2.1 lent the mutant channel a PMA sensitivity almost identical to the wild-type Kir2.3. These results therefore indicate that Kir2.3 is directly modulated by PKC phosphorylation of its channel protein and threonine 53 is the PKC phosphorylation site in Kir2.3.

Animals↗

PubMed1999

Developmental changes in progenitor cell responsiveness to cytokines.

Multipotent progenitor cells have been identified within periventricular generative zones of the developing and adult brain. To determine whether the environmental responsiveness of these cells changes during development, progenitor cells were cultured from embryonic, postnatal, and adult rat brain in the presence of either basic fibroblast growth factor (bFGF) or epidermal growth factor (EGF). Embryonic cells cultured as intact progenitor neurospheres proliferated more robustly in response to bFGF than to EGF, whereas proliferation of postnatal and adult progenitor cells was enhanced more by EGF than bFGF. Progenitor cells generated in the presence of either bFGF or EGF had the capacity to generate neurons, astrocytes, and oligodendrocytes at all developmental stages. Most embryonic and neonatal bFGF-generated cells differentiated predominantly into neurons, whereas late stage embryonic and neonatal EGF-generated progenitors largely remained in an undifferentiated state. However, later postnatal and adult progenitor species, irrespective of whether they were generated in the presence of bFGF or EGF, gave rise preferentially to astrocytes. Treatment with bone morphogenetic protein (BMP)2 or BMP7 enhanced astroglial differentiation and suppressed oligodendroglial differentiation of both EGF- and bFGF-generated progenitor species, suggesting that the effects of the BMPs are not dependent on EGF receptor activation. Thus, while central nervous system (CNS) progenitor cells retain multipotent capacity and responsiveness to the BMPs throughout development, they exhibit significant changes in other cellular response properties, perhaps reflecting differences in the requirements for specific generative versus regenerative events.

Animals↗

PubMed1999

Genetic and environmental contributions to size, color, shape, and other characteristics of melanocytic naevi in a sample of adolescent twins.

The presence of melanocytic naevi is the strongest known risk factor for malignant melanoma. We have developed a computer imaging system with which it is possible to make quantitative measures of the size, color, and shape of pigmented lesions. The objective of this study was to examine the genetic and environmental contributions to these characteristics of naevi as measured by computer image analysis in a sample of adolescent twins. We captured video images of the 5 most atypical pigmented skin lesions (i.e., the largest, darkest, or most irregularly shaped) on each individual from 322 Australian adolescent twin pairs. Features extracted by computer image analysis for each lesion included color, size, symmetry, elongation, boundary irregularity, and edge distinctness. We found major genetic influences on the color and size of lesions accounting for between 40 and 80% of total variance. There were significant components of shared environmental influence (22-45% of total variance) for the color variables, with sun exposure the most obvious explanation. Differences between individuals in naevus color and size are largely genetic in origin although there are significant environmental contributions to color as well.

Adolescent↗

PubMed1999

Pharmacokinetics and tissue disposition in monkeys of an antisense oligonucleotide inhibitor of Ha-ras encapsulated in stealth liposomes.

PURPOSE: This study examined the pharmacokinetics and tissue distribution of an antisense oligonucleotide ISIS 2503, formulated in stealth (pegylated) liposomes (encapsulated) or in phosphate-buffered saline (unencapsulated). METHODS: Encapsulated or unencapsulated ISIS 2503 was administered to rhesus monkeys by intravenous infusion. The concentrations of ISIS 2503 and metabolites in blood, plasma, and tissue samples were determined by capillary gel electrophoresis. RESULTS: Plasma concentrations of encapsulated ISIS 2503 decreased mono-exponentially after infusion with a mean half-life of 57.8 hours. In contrast, the concentration of unencapsulated ISIS 2503 in plasma decreased rapidly with a mean half-life of 1.07 hours. Both encapsulated and unencapsulated ISIS 2503 distributed widely into tissues. Encapsulated ISIS 2503 distributed primarily to the reticulo-endothelial system and there were few metabolites observed. In contrast, unencapsulated ISIS 2503 distributed rapidly to tissue with highest concentration seen in kidney and liver. Nuclease-mediated metabolism was extensive for unencapsulated oligonucleotide in plasma and tissues. CONCLUSIONS: The data suggest that stealth liposomes protect ISIS 2503 from nucleases in blood and tissues, slow tissue uptake, and slow the rate of clearance from the systemic circulation. These attributes may make these formulations attractive for delivering oligonucleotides to sites with increased vasculature permeability such as tumors or sites of inflammation.

Animals↗

PubMed1999

A major quantitative-trait locus for mole density is linked to the familial melanoma gene CDKN2A: a maximum-likelihood combined linkage and association analysis in twins and their sibs.

Important risk factors for melanoma are densely clustered melanocytic nevi (common moles) and mutations in the p16 (CDKN2A) gene. Nevi may be subclassified as raised or flat. In our sample, raised nevi were 27% of the total, and the two kinds had a correlation of.33. Correlations for total-nevus count (TNC) in 153 MZ and 199 DZ twin pairs were.94 and.60, respectively, which are compatible with a very-high degree of genetic determination. We hypothesized that some of the genetic variance might be due to variation in the p16 gene. Analysis of linkage to a highly polymorphic marker (D9S942), located close to p16, detected quantitative-trait-loci (QTL) effects accounting for 27% of variance in TNC, rising to 33% if flat but not raised moles were considered. Total heritability was higher for raised (.69) than for flat (.42) moles, but QTL linkage was 0 for raised moles, whereas it accounted for 80% of the heritability of flat moles; additionally, family environment accounted for only 15% of variance in raised versus 46% in flat moles. These findings suggest that raised and flat nevi have very different etiologies. Longer alleles at D9S942 were associated with higher flat-mole counts, and a novel modification to a within-sibship association test showed that this association is genuine and not due to population stratification, although it accounts for only 1% of total variance. Since germline mutations in the exons of CDKN2A are rare, it is likely that variants in the noncoding regions of this gene, or in another gene nearby, are responsible for this major determinant of moliness and, hence, of melanoma risk.

Child↗

PubMed1999

Reconstitution of a bacterial/plant polyamine biosynthesis pathway in Saccharomyces cerevisiae.

Polyamine synthesis in most organisms is initiated by the decarboxylation of ornithine to form putrescine via ornithine decarboxylase (ODC). Plants, some bacteria and some fungi and protozoa generate putrescine from arginine, via arginine decarboxylase (ADC) and agmatine ureohydrolase (AUH) or agmatine iminohydrolase. A polyamine-requiring strain of Saccharomyces cerevisiae with a mutation in the gene encoding ODC was transformed with plasmids bearing genes encoding Escherichia coli ADC and AUH. Transformants regained the ability to grow in the absence of exogenous polyamines and contained enzyme activities consistent with the presence of both prokaryotic enzymes. Similar results were obtained when a plasmid containing a gene encoding oat (Avena sativa L.) ADC was substituted for the E. coli gene. These data demonstrate the successful complementation of a yeast biosynthetic polyamine synthesis defect by genes encoding an alternative pathway found in bacteria; they also show that plant ADC can substitute for the bacterial enzyme in this pathway. The recombinant yeast provides a tool for the study of the functional properties of these enzymes and for discovery of compounds that specifically inhibit this pathway.

Avena↗

PubMed1998

The fork head transcription factor Hcm1p participates in the regulation of SPC110, which encodes the calmodulin-binding protein in the yeast spindle pole body.

We previously identified HCM1 as a dosage-dependent suppressor of a calmodulin temperature-sensitive mutant (cmd1-1). Calmodulin performs multiple functions in yeast. Here we demonstrate that the effects of HCM1 are specific to the role of calmodulin at the spindle pole body. Overexpression of HCM1 fully suppresses the temperature sensitivity of a calmodulin mutant (cmd1-3) that only has defects in assembly of the spindle pole body but does not suppress the temperature sensitivity of a calmodulin mutant (cmd1-8) that only affects other functions of calmodulin. The DNA binding specificity of Hcm1p was determined by a selection, amplification and binding protocol. The consensus sequence for an Hcmlp binding site is WAAYAAACAAW. Mutations in the DNA binding domain of Hcm1p abolish the ability of Hcmlp to specifically recognize this binding site and abolish the ability of Hcm1p to act as a suppressor of calmodulin mutants. The promoter of SPC110 contains a match to the consensus binding site. Deletion of HCM1 does not affect the basal level of SPC110 transcription, but reduces the induction that occurs late in G1 of the cell cycle.

Amino Acid Sequence↗

PubMed1998

X-ray studies on two forms of bovine beta-trypsin crystals in neat cyclohexane.

Two orthorhombic forms (Vm values are 2.3 and 3.0 A3/Da) of bovine beta-trypsin crystals in neat cyclohexane were determined to 1.93 A resolution, by X-ray diffraction. Both structures in organic solvent are similar to those in aqueous solution. In the high packing density form, one cyclohexane molecule is found in a hydrophobic site near the active center. One sulfate locates at the active site with hydrogen or salt bond to the Ser-His catalytic diad, and five more sulfates bind on the molecular surface. The conformation of the side chains near the sulfates changed greatly. In the low packing density form, one cyclohexane and three sulfates are found. In both structures, one benzamidine molecule locates at the hydrophobic pocket of the active center. Most water molecules on the enzyme surface are retained except some with high temperature factors.

Animals↗

PubMed1998

Differentiation of oligodendroglial progenitors derived from cortical multipotent cells requires extrinsic signals including activation of gp130/LIFbeta receptors.

We have previously isolated epidermal growth factor (EGF)-responsive multipotent progenitor cells from the early postnatal rodent cerebral cortex independent of generative zones. In this study we have examined the mechanisms regulating the generation of differentiated oligodendrocytes (OLs) from these multipotent cells. Although cultures of primary cortical OL progenitor cells propagated at clonal density spontaneously gave rise to differentiated OLs in defined medium, cultures of multipotent progenitors isolated from identical regions supported the elaboration of OL progenitors but not differentiated OLs. These observations indicate that the terminal maturation of OL progenitors derived from multipotent cells is dependent on signals present within the cellular environment. Application of cytokines such as basic fibroblast growth factor (bFGF), platelet-derived growth factor (PDGF), or neurotrophin 3 (NT3) to clonal density cultures of cortical multipotent progenitors increased the proportion of OL progenitors but failed to support the generation of differentiated OLs. By contrast, application of factors that activate gp130/leukemia inhibitory factor beta (LIFbeta) heterodimeric receptors, such as ciliary neurotrophic factor (CNTF), activated signal transducers and activators of transcription-3 in these OL progenitor cells and promoted the generation of differentiated OLs. Clonal analysis also demonstrated that CNTF directly targets OL progenitors derived from the multipotent cells. These observations suggest that two distinct progenitor cell pathways contribute to the generation of differentiated OLs during postnatal cortical gliogenesis. Although oligodendroglial maturation of classical OL progenitor cells is driven by cell autonomous mechanisms, our findings demonstrate that the generation of differentiated OLs from cortical multipotent progenitor cells is dependent on environmental cues, including activation of gp130/LIFbeta receptors.

Animals↗

PubMed1998

Identification of endogenous outward currents in the human embryonic kidney (HEK 293) cell line.

Human embryonic kidney cells (HEK 293) are widely used as an expression system in studies of ion channels. However, their endogenous ionic currents remain largely unidentified. To characterize these currents, we performed patch clamp experiments on this expression system. In whole-cell voltage clamp mode, the HEK 293 cells showed mainly outward currents using physiological concentrations of Na+ and K+ and symmetric concentrations of Cl- (150 mM) across the plasma membranes. K+ currents contributed to a small portion of these outward currents, since a shift of the reversal potentials of only approximately 20 mV was seen with a change of extracellular K+ concentration from 3 to 150 mM. In contrast, the reversal potential shifted approximately 25 mV when extracellular Cl- was reduced to 50 mM, indicating that most of the outward currents are carried by Cl-. In inside-out patches, several distinct Cl- currents were identified. They were: (1) 350 pS Cl- current, which was voltage-activated and had a moderate outward rectification; (2) 240 pS Cl- current with a weak outward rectification; and (3) 55 pS Cl- current, which was voltage-activated, sensitive to DIDS, and showed a strong outward rectification. Activation of these Cl- currents did not require an elevation of free Ca2+ level in the cytosol. Besides these three currents, we observed two other Cl- currents with much smaller conductances (25 and 16 pS, respectively). Two different K+ currents were seen in the HEK 293 cells, with one of them (125 pS) showing inward rectification and the other (70 pS) outward rectification. Moreover, a 50 pS cation channel was recorded in these cells. The presence of a variety of ion channels in the HEK 293 cells suggests that a great precaution needs to be taken when this expression system is used in studies of several similar ion channels.

Calcium↗

PubMed1998

X-ray studies on cross-linked lysozyme crystals in acetonitrile-water mixture.

Tetragonal crystals of hen egg white lysozyme were cross-linked and subjected to X-ray diffraction study in acetonitrile-water media with different acetonitrile concentrations. Crystals in neat acetonitrile did not scatter X-ray well. Structures of crystals in neat water, in 90% and 95% acetonitrile, and crystal back-soaked from acetonitrile to water, were determined to about 2 A resolution. For crystals in both 90% acetonitrile, and crystal back-soaked from acetonitrile to water, were determined to about 2 A resolution. For crystals in both 90% and 95% acetonitrile, only one protein-bond acetonitrile molecule is found in the active site cleft, and its location and binding-protein mode is similar to the C subunit of polysaccharide. The alteration in conformation and hydrogen-bond pattern involving water as solvent causes the reduction of the protein's flexibility in organic media. The back-soaked crystal regained its ordinary three-dimensional structure in water.

Acetonitriles↗

PubMed1998

In vivo degradation of a poly(propylene fumarate)/beta-tricalcium phosphate injectable composite scaffold.

This study was designed to investigate the in vivo biodegration and biocompatibility of a poly(propylene fumarate) (PPF)-based orthopedic biomaterial. The effects of varying the PPF to N-vinyl pyrrolidinone ratio and PPF to beta-tricalcium phosphate content were studied. The composite mechanical properties and local tissue interactions were analyzed over 12 weeks. An initial increase in both compressive modulus and strength was seen for composite formulations that incorporated beta-tricalcium phosphate. The samples incorporating a higher PPF to N-vinyl pyrrolidinone ratio reached a maximal compressive strength of 7.7 MPa and a maximal compressive modulus of 191.4 MPa at 3 weeks. The lower PPF to N-vinyl pyrrolidinone ratio samples gained a maximum compressive strength of 7.5 MPa initially and a compressive modulus of 134.0 MPa at 1 week. At 6 weeks, all samples for formulations incorporating beta-tricalcium phosphate crumbled upon removal and were not mechanically tested. Samples that did not incorporate beta-tricalcium phosphate were very weak and insufficient for bone replacement at the 4-day time point and beyond. Tissue interactions resulted in a mild inflammatory response at the initial time points and mature fibrous encapsulation by 12 weeks.

Animals↗

PubMed1998

Spectral parameter estimation by an iterative quadratic maximum likelihood method.

An iterative quadratic maximum likelihood (IQML) method is applied to spectral parameter estimation of 1D NMR data. A careful comparison of the linear prediction (LP) method based on the singular value decomposition, the total least squares (TLS) method, and IQML has clearly demonstrated that IQML is superior to both the LP and TLS methods in terms of the accuracy and bias of the estimation. The superiority of the IQML method lies in the fact that constraints on the NMR signal can easily be incorporated into the iterative process. The iterative quadratic maximum likelihood method can be used to analyze NMR data directly or to provide a starting point for further data refinement.

Algorithms↗