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Biomedical subjects

C Liu

Publications and source records attributed to C Liu.

At least 109 records · Page 6Linked to original sources
PubMed2001

Role of the M1 receptor in regulating circadian rhythms.

Cholinergic stimuli are potent regulators of the circadian clock in the hypothalamic suprachiasmatic nucleus (SCN). Using a brain slice model, we have found that the SCN clock is subject to muscarinic regulation, a sensitivity expressed only during the night of the clock's 24-h cycle. Pharmacological and signal transduction characteristics are compatible with a response mediated by an M1-like receptor. Molecular manipulation of muscarinic receptors will provide important insights as to the receptor subtype(s) regulating circadian rhythms.

Animals↗
PubMed2001

Thermodynamics of denaturation of hisactophilin, a beta-trefoil protein.

Hisactophilin is a histidine-rich pH-dependent actin-binding protein from Dictyostelium discoideum. The structure of hisactophilin is typical of the beta-trefoil fold, a common structure adopted by diverse proteins with unrelated primary sequences and functions. The thermodynamics of denaturation of hisactophilin have been measured using fluorescence- and CD-monitored equilibrium urea denaturation curves, pH-denaturation, and thermal denaturation curves, as well as differential scanning calorimetry. Urea denaturation is reversible from pH 5.7 to pH 9.7; however, thermal denaturation is highly reversible only below pH approximately 6.2. Reversible denaturation by urea and heat is well fit using a two-state transition between the native and the denatured states. Urea denaturation curves are best fit using a quadratic dependence of the Gibbs free energy of unfolding upon urea concentration. Hisactophilin has moderate, roughly constant stability from pH 7.7 to pH 9.7; however, below pH 7.7, stability decreases markedly, most likely due to protonation of histidine residues. Enthalpic effects of histidine ionization upon unfolding also appear to be involved in the occurrence of cold unfolding of hisactophilin under relatively mild solution conditions. The stability data for hisactophilin are compared with data on hisactophilin function, and with data for two other beta-trefoil proteins, human interleukin-1beta, and basic fibroblast growth factor.

Animals↗
PubMed2001

Microbial reduction of Fe(III) and sorption/precipitation of Fe(II) on Shewanella putrefaciens strain CN32.

The influence of Fe(II) on the dissimilatory bacterial reduction of an Fe(III) aqueous complex (Fe(III)-citrate(aq)) was investigated using Shewanella putrefaciens strain CN32. The sorption of Fe(II) on CN32 followed a Langmuir isotherm. Least-squares fitting gave a maximum sorption capacity of Qmax = 4.19 x 10(-3) mol/10(12) cells (1.19 mmol/m2 of cell surface area) and an affinity coefficient of log K = 3.29. The growth yield of CN32 with respect to Fe(III)aq reduction showed a linear trend with an average value of 5.24 (+/-0.12) x 10(9) cells/mmol of Fe(III). The reduction of Fe(III)aq by CN32 was described by Monod kinetics with respect to the electron acceptor concentration, Fe(III)aq, with a half-saturation constant (Ks) of 29 (+/-3) mM and maximum growth rate (micromax) of 0.32 (+/-0.02) h(-1). However, the pretreatment of CN32 with Fe(II)aq significantly inhibited the reduction of Fe(III)aq, resulting in a lag phase of about 3-30 h depending on initial cell concentrations. Lower initial cell concentration led to longer lag phase duration, and higher cell concentration led to a shorter one. Transmission electron microscopy and energy dispersive spectroscopy revealed that many cells carried surface precipitates of Fe mineral phases (valence unspecified) during the lag phase. These precipitates disappeared after the cells recovered from the lag phase. The cell inhibition and recovery mechanisms from Fe(II)-induced mineral precipitation were not identified by this study, but several alternatives were discussed. A modified Monod model incorporating a lag phase, Fe(II) adsorption, and aqueous complexation reactions was able to describe the experimental results of microbial Fe(III)aq reduction and cell growth when cells were pretreated with Fe(II)aq.

Adsorption↗
PubMed2001

A novel glutathione peroxidase mimic with antioxidant activity.

Many diseases are associated with the overproduction of hydroperoxides that inflict cell damage. A novel cyclodextrin derivative, 6A,6B-diseleninic acid-6A',6B'-selenium bridged beta-cyclodextrin (6-diSeCD), was synthesized to be a functional mimic of glutathione peroxidase (GPX) that normally removes these hydroperoxides. The mimic had high catalytic GPX activity of 13.5 U/micromol, which is 13.6-fold higher than ebselen (PZ51), and was chemically and biologically stable in vitro. Antioxidant activity was studied by ferrous sulfate/ascorbate-induced mitochondria damage model system. These data show that the mimic has great antioxidant activity. Such mimics may result in better clinical therapies for diseases mediated by hydroperoxides.

Animals↗
PubMed2001

Total synthesis of antitumor depsipeptide (-)-doliculide.

[reaction: see text] (-)-Doliculide, a potent antitumor agent, is synthesized stereoselectively in a convergent manner. The key strategy involves a stereoselective synthesis of the polyketide unit and synthesis of the D-tyrosine derivative, followed by assembly of the fragments by an esterification and cycloamidation reaction sequence. The synthesis of the polyketide fragment was achieved by an iterative asymmetric synthesis to install stereoselectively both 1,3-dimethyl groups and the 1,3-diol unit by utilizing asymmetric cyclopropanations and Sharpless asymmetric epoxidations as the key steps.

Antineoplastic Agents↗
PubMed2001

Effects of nitric oxide in the dorsal motor nucleus of the vagus on the extrahepatic biliary system in rabbits.

To investigate whether nitric oxide (NO) in the dorsal motor nucleus of the vagus (DMV) mediated an influence on the extrahepatic biliary system, we studied the effects of microinjection of NO-producing drugs into DMV on the motilities of the gallbladder (GB) and the sphincter of Oddi (SO) in anesthetized rabbits. Microinjection of the NO precursor L-arginine into the rostral DMV produced an increase in the GB and SO motilities, which can be counteracted by both NG-nitro-L-arginine-methyl (L-NAME), an inhibitor of NOS, and reduced hemoglobin (rHb), a scavenger of NO, and were eliminated by bilateral cervical vagotomy. On the other hand, the NO donor sodium nitroprusside (SNP) was able to mimic the excitatory effect of L-arginine. This effect can be antagonized by rHb, but not by L-NAME, for SNP supplied exogenous NO without activating NOS. These results indicate that NO in the DMV mediates an excitatory effect on the extrahepatic biliary system.

Animals↗
PubMed2001

Characterization of Corn1 mice: Alteration of epithelial and stromal cell gene expression.

PURPOSE: Corn1 is an autosomal recessive mutation characterized by corneal epithelial hyperplasia and stromal neovascularization. The aim of the present study is to examine the expression patterns of specific epithelial and stromal proteins in corn/corn1 mutant mice. METHODS: Immunohistochemistry with antibodies directed against keratins 1, 4, 5, 12, and 14 as well as loricrin, filaggrin, and involucrin were performed in corn1/corn1 and wild type, A.By/SnJ strain, mice at 4 weeks of age. Western blot hybridization was performed to confirm the presence of involucrin in corneas. In situ and northern blot hybridization were used to evaluate the expression of keratin 12, lumican, and keratocan in these mice. RESULTS: In corn1/corn1 mice, focal areas of corneal epithelial hyperplasia alternate with epithelium with normal appearance. Both regions of normal and hyperplastic corneal epithelium were labeled by anti-keratin 12 antibodies through all corneal epithelial layers. The anti-keratin 14 antibody only labeled the basal cell layer in normal epithelial areas, whereas it labeled both basal and suprabasal cell layers in hyperplastic areas. In wild type mice, anti-keratin 12 antibodies labeled all corneal epithelial layers, whereas anti-keratin 14 labeled the basal corneal epithelial cells only. Positive staining by anti-involucrin antibody was demonstrated in the basal corneal epithelial layer of wild type mice and normal areas of corn1/corn1 mice. Similarly, as observed with anti-keratin 14 antibody, the anti-involucrin antibody labeled both basal and suprabasal cell layers of hyperplastic corneal epithelium of corn1/corn1 mice. Antibodies against keratin 1, keratin 4, loricrin, and fillagrin did not label the corneas of wild type mice or corn1/corn1 mice. Northern hybridization indicated that the expressions of keratocan and lumican mRNA levels were up regulated in corn1/corn1 mice, but keratin 12 mRNA remained similar to that of the wild type mice. In situ hybridization revealed that the lumican mRNA was detected in epithelial and stromal cells of corn1/corn1 mice, whereas keratocan mRNA was only detected in stromal cells. CONCLUSIONS: Hyperproliferative epithelial cells of corn1/corn1 mice have increased levels of expression of keratin 14 and involucrin, but do not exhibit the phenotypical characteristics of cornification. These observations indicate that factors associated with the phenotypes of corn1/corn1 mice do not alter the cornea-type epithelial differentiation of keratin 12 expression, but cause aberrant expression of lumican by corneal epithelial cells.

Animals↗
PubMed2001

Observation of coherent optical information storage in an atomic medium using halted light pulses.

Electromagnetically induced transparency is a quantum interference effect that permits the propagation of light through an otherwise opaque atomic medium; a 'coupling' laser is used to create the interference necessary to allow the transmission of resonant pulses from a 'probe' laser. This technique has been used to slow and spatially compress light pulses by seven orders of magnitude, resulting in their complete localization and containment within an atomic cloud. Here we use electromagnetically induced transparency to bring laser pulses to a complete stop in a magnetically trapped, cold cloud of sodium atoms. Within the spatially localized pulse region, the atoms are in a superposition state determined by the amplitudes and phases of the coupling and probe laser fields. Upon sudden turn-off of the coupling laser, the compressed probe pulse is effectively stopped; coherent information initially contained in the laser fields is 'frozen' in the atomic medium for up to 1 ms. The coupling laser is turned back on at a later time and the probe pulse is regenerated: the stored coherence is read out and transferred back into the radiation field. We present a theoretical model that reveals that the system is self-adjusting to minimize dissipative loss during the 'read' and 'write' operations. We anticipate applications of this phenomenon for quantum information processing.

Journal Article↗
PubMed2001

Expression of the platelet receptor GPVI confers signaling via the Fc receptor gamma -chain in response to the snake venom convulxin but not to collagen.

The mechanism of signal transduction underlying the activation of platelets by collagen has been actively investigated for over 30 years, but the receptors involved remain incompletely understood. Studies of human platelets, which are unresponsive to collagen, mouse knockout models, and platelet biochemical studies support the hypothesis that the recently cloned platelet surface protein GPVI functions as a signaling receptor for collagen. To directly test this hypothesis, we have expressed wild-type and mutant forms of GPVI in RBL-2H3 cells, which express the Fcepsilon receptor gamma-chain (Fc Rgamma), the putative signaling co-receptor for GPVI in platelets, but lack GPVI itself. Expression of GPVI in RBL-2H3 cells confers strong adhesive and signaling responses to convulxin (a snake venom protein that directly binds GPVI) and weak responsiveness to collagen-related peptide but no responsiveness to collagen. To elucidate the mechanism of GPVI intracellular signaling, mutations were introduced in the receptor's transmembrane domain and C-terminal tail. Unlike reported studies of other Fc Rgamma partners, these studies reveal that both the GPVI transmembrane arginine and intracellular C-tail are necessary for coupling to Fc Rgamma and for signal transduction. To our knowledge, these studies are the first to demonstrate a direct signaling role for GPVI and the first to directly test the role of GPVI as a collagen receptor. Our results suggest that GPVI may be necessary but not sufficient for collagen signaling and that a distinct ligand-binding collagen receptor such as the alpha(2)beta(1) integrin is likely to play a necessary role for collagen signaling as well as adhesion in platelets.

Amino Acid Sequence↗
PubMed2001

Uncertainties of Monod kinetic parameters nonlinearly estimated from batch experiments.

Monod kinetic parameters (Ks, micromax, and Y) that are estimated from batch experimental data can have large uncertainties due to linear correlations between them. The degree of correlation and the resulting uncertainties of the Monod parameters are functions of the initial experimental conditions, the values of the parameters, the type and magnitude of measurement errors, and the sampling number. Careful manipulation of experimental conditions can reduce the correlations between Monod parameters allowing for the estimation of Monod kinetic parameters with the lowest degree of uncertainty. By dimensionless analysis, the correlation and relative standard deviations of Monod parameters were found to be functions of a few dimensionless variables involving the initial substrate (S0) and cell (X0) concentrations. Quantitative relationships were analyzed between the dimensionless variables and the correlation and the uncertainties of the Monod parameters. This analysis allowed for identification of the optimal experimental conditions for estimating Monod parameters under both no growth and growth conditions coupled with two kinds of measurement errors: those with constant absolute standard deviation and those with constant relative standard deviation. Examples involving the microbial reduction of iron(III) as an electron acceptor are used to illustrate the application of the developed technique.

Cell Division↗
PubMed2001

Effects of consanguinity, exposure to pregnant females, and stimulation from young on male gerbils' responses to pups.

In three experiments investigating variables affecting responses of male Mongolian gerbils to conspecific young, we compared the behavior directed towards pups of natural fathers, virgin foster fathers, and sexually experienced foster fathers (Experiment 1); males either previously exposed or not exposed to pregnant females (Experiment 2); and males provided or not provided with extra opportunities to huddle over pups (Experiment 3). We found no difference in responses to pups among natural fathers, virgin foster fathers, and foster fathers that had fathered litters. On the other hand, both a week of exposure to a pregnant female and opportunity to huddle over pups for an extra 15 min/day had significant effects on males' subsequent responses to conspecific young. We speculate on the reasons why a male's response to pups might be affected by his exposure to a pregnant female and stimuli from pups, but not by the probability that the pups were his own offspring.

Animals↗
PubMed2001

Expression of aminopeptidase N in bile canaliculi: a predictor of clinical outcome in biliary atresia and a potential tool to implicate the mechanism of biliary atresia.

BACKGROUND: Only a few studies on extrahepatic biliary atresia (BA) have reported that the morphological changes of bile canaliculi could predict the clinical outcome after portoenterostomy and provide differential diagnosis of neonatal jaundice. Aminopeptidase N (APN) is an ectoenzyme of bile canaliculi that is involved in bile secretion. In this study, we tried to see whether APN of bile canaliculi had a significant role in BA. PATIENTS AND METHODS: We used monoclonal antibody 9B2 to compare the expression of APN in livers with BA, neonatal hepatitis, and choledochal cysts, as well as in nontumorous portions of pediatric hepatic livers with tumors. The expression of APN in fetuses, preterm babies, and term neonates was also studied. RESULTS: A high degree of 9B2 expression in BA was closely related to poor outcome. Cholestasis in choledochal cysts, rather than neonatal hepatitis, made 9B2 expression stronger. Increasing expression of 9B2 from fetuses to neonates was noted and the degree of 9B2 expression was similar between term neonates and nontumorous portions of pediatric livers with tumors. Interestingly, some cases of BA had 9B2 expression like that of preterm babies. CONCLUSIONS: APN of bile canaliculi progressively develops from fetuses to neonates and is well developed in neonates. APN can be induced to stronger expression by obstructive jaundice. The amount of expression of APN of bile canaliculi in BA is a predictor of clinical outcome and may be a tool for implicating the mechanism of BA.

Antibodies, Monoclonal↗
PubMed2001

Large-scale plant micropropagation.

Plant micropropagation is an efficient method of propagating disease-free, genetically uniform and massive amounts of plants in vitro. The micropropagation from cells can be achieved by direct organogenesis from hairy roots or regeneration via somatic tissue. Once the availability of embryogenic cell and hairy root systems based on liquid media has been demonstrated, the scale-up of the whole process should be established by an economically feasible technology for their large-scale production in appropriate bioreactors. It is necessary to design a suitable bioreactor configuration that can provide adequate mixing and mass transfer while minimizing the intensity of shear stress and hydrodynamic pressure. Automatic selection of embryogenic calli and regenerated plantlets using an image analysis procedure should be associated with the system. Using the above systems, it will be possible to establish an advanced plant micropropagation system in which the plantlets can be propagated without soil under optimal conditions controlled in plant factory. The aim of this review is to identify the problems related to large-scale plant micropropagation via somatic embryogenesis and hairy roots, and to summarize the most recent developments in bioreactor design. Emphasis is placed on micropropagation technology and computer-aided image analysis, including the successful results obtained in our laboratories.

Bioreactors↗
PubMed2001

Cytokine-induced cell surface expression of adhesion molecules in vascular endothelial cells in vitro.

Regulation of the adhesion molecules expression by cytokine in vascular endothelial cells was investigated. Human umbilical vein endothelial cells (HUVEC) were stimulated with cytokines, TNF-alpha (1-250 U/ml) or IL-1 beta (0.1-50 U/ml) for 24 h. HUVEC were also cultured with cytokines, TNF-alpha (100 U/ml) or IL-1 beta (10 U/ml), for 4-72 h, cell surface expression of adhesion molecules (ICAM-1 and VCAM-1) were detected and quantitated by immunocytochemical methods and computerized imaging analysis technique. Adhesion molecules expression were up-regulated by TNF-alpha, IL-1 beta in a concentration- and time-dependent manner. Some significant differences were observed between the effects of cytokines on the ICAM-1 and on VCAM-1 expression. Cytokines might directly induce the expression of ICAM-1 and VCAM-1 in vascular endothelial cells. Our observations indicate differential functions of the two adhesion molecules during the evolution of inflammatory responses in stroke.

Cells, Cultured↗
PubMed2001

Muscarinic agonists and antagonists in the treatment of Alzheimer's disease.

Alzheimer's disease (AD) is a neurodegenerative disease characterized by cognitive impairment and personality changes. The development of drugs for the treatment of the cognitive deficits of AD has focused on agents which counteract loss in cholinergic activity. Although symptoms of AD have been successfully treated with acetylcholinesterase inhibitors (tacrine, donepezil. rivastigmine, galanthamine), limited success has been achieved with direct M1 agonists, probably due to their lack of selectivity versus other muscarinic receptor subtypes. Muscarinic M2 antagonists have been reported to increase synaptic levels of acetylcholine after oral administration to rats (e.g. BIBN-99, SCH-57790), but their selectivity versus other muscarinic receptor subtypes is modest. Exploration of a series of piperidinylpiperidines has yielded the potent and selective M2 antagonist SCH-217443. This antagonist has excellent bioavailability in rats and dogs and shows activity in a rat model of cognition.

Alzheimer Disease↗
PubMed2001

Does MMP-2 expression and secretion change with increasing serial passage of keratocytes in culture?

The effects of ageing on matrix metalloprotease degradation of the extracellular matrix during corneal wound healing are largely unknown. The following study used an in vitro model of ageing to assess changes in MMP-2 RNA expression and protein secretion. Early passage (EP) EK1.BR keratocyte cultures from 14 to 18 cumulative population doublings (cpds) and late passage (LP) cultures from 40 to 47 cpds were used to isolate protein and mRNA samples. Total protein from EP and LP cultures was measured using the Bradford protein assay. Zymographic analysis of EP and LP samples was carried out to compare MMP-2 activity. Northern blot analysis was used to assess changes in MMP-2 mRNA expression by EP and LP cultures, using a digoxigenin (DIG) based chemiluminescent detection system. LP cultures secreted more total protein per cell. MMP-2 but not MMP-9 activity was detected in keratocyte cultures. Densitometric analysis of zymograms and calculation of MMP-2 activity indicated a significant increase in MMP-2 activity per cell (P<0.05, n=11). No difference was observed in the levels of MMP-2 mRNA expressed by EP and LP cultures. An increase in MMP-2 activity per cell by LP cultures suggests that senescent keratocytes increase their degradative capacity. Similar changes in the keratocyte phenotype within the ageing cornea may alter the balanced response necessary for adequate wound healing and may have implications for the therapeutic use of MMP inhibitors in the eye.

Cell Division↗
PubMed2001

Kinetics of hydroxyapatite precipitation at pH 10 to 11.

The mechanism and kinetics of hydroxyapatite (HAP) precipitation from aqueous solution at the conditions of pH 10-11 and at calcium ion concentrations of more than 0.5 mol/l were investigated. The results show that the reaction undergoes the following process: transferring from octacalcium phosphate (OCP) to amorphous calcium phosphate (ACP) rapidly, and then from ACP to calcium-deficient hydroxyapatite (DAP) and HAP. DAP was the non-stoichiometric material that ACP converts to HAP. Reaction temperature greatly affects the reaction rate of the conversion from ACP to HAP. It takes about 24 h to form pure-phase HAP at 25 degrees C while it takes only 5 min at 60 degrees C. The temperature also has a great deal of influence on the particle size and morphology of precipitated hydroxyapatite. The reaction of transformation from ACP to DAP was second order and the activation energy was 95 kJ/mol (22.7 kCal/mol) and therefore it was inferred that the conversion reaction was a surface control process.

Calcium Phosphates↗