Biomedical subjects

K Chen

Publications and source records attributed to K Chen.

At least 127 records · Page 7Linked to original sources

PubMed2000

Sodium pentobarbital abolishes bursting spontaneous activity of dorsal cochlear nucleus in rat brain slices.

There is evidence that pentobarbital, a commonly used anesthetic, can affect neuronal activity, but its effects on particular neurons of the dorsal cochlear nucleus (DCN) are not well known. Bursting (complex spiking) spontaneous activity has been observed in the DCN in brain slice preparations and in recordings from unanesthetized decerebrate animals, but seldom in experiments with anesthetized animals. This study investigated the effects of pentobarbital on spontaneous activity in the DCN in brain slices. Most extracellularly recorded bursting neurons decreased firing rates and reversibly changed their firing to simple spiking with irregular intervals during pentobarbital. Some reversibly stopped firing after the change to an irregular pattern. Most neurons with regular spontaneous activity (simple spiking) showed decreased firing rates and more irregular intervals during pentobarbital. The results also suggest some involvement of gamma-aminobutyric acid type A receptors in the pentobarbital effects.

Action Potentials↗

PubMed2000

Three Cases of Primary Hyperparathyroidism (PHP) with Prior Failed Surgery Where Culprit Lesions Were Identified by 11C-Methionine Positron Emission Tomography (PET) and Accurately Localized with PET-MRI Coregistration.

This report describes 3 cases of primary hyperparathyroidism (PHP) in which presurgical localization of the culprit lesions was essential. In 2 cases, the initial parathyroid surgery had failed to restore normal levels of either parathyroid hormone (PTH) or calcium (Ca(++)). In each of these cases experienced surgeons performed the initial parathyroid explorations. In one of these 2 cases, a positive 99mTc sestamibi (MIBI) scan was obtained prior to the initial surgery. 11C-methionine-positron emission tomography (PET) scans and magnetic resonance imaging (MRI) studies were performed prior to repeat surgical exploration with these data coregistered (the digital data sets from each modality were spatially aligned). In each case, the 11C-methionine-PET studies identified metabolically active tumors whose anatomic locations were accurately determined on the PET coregistration. The third case had previously undergone a thyroidectomy and modified right radical neck dissection for removal of a thyroid cancer and regional lymph node metastases. Following the diagnosis of PHP, MIBI, thallium, and ultrasound studies were performed and were all negative. 11C-methionine-PET and MRI were performed identifying a parathyroid adenoma in the prior surgical field. In these 3 cases culprit lesions were readily located at surgery, excised and confirmed as parathyroid neoplasms. This 11C-methionine PET-MRI coregistration technique appears to be effective at localizing parathyroid neoplasms in high risk patients with PHP in the setting of initially failed surgery or when PHP occurs following previous regional neck surgery.

Journal Article↗

PubMed2000

Dynamical process of excitation fusion in polymers

Due to the confinement effect in the nondegenerate polymers and the electron-lattice interaction, the electron-hole pair forms a self-trapped single excitation. It is shown that two single excitations in one chain can fuse into one double excitation, and the relaxation time of the fusion process is about 160 fs.

Journal Article↗

PubMed2000

Effects of high-potassium-induced depolarization on amino acid chemistry of the dorsal cochlear nucleus in rat brain slices.

High K+ was used to depolarize glia and neurons in order to study the effects on amino acid release from and concentrations within the dorsal cochlear nucleus (DCN) of brain slices. The release of glutamate, gamma-aminobutyrate (GABA) and glycine increased significantly during exposure to 50 mM K+, while glutamine and serine release decreased significantly during and/or after exposure, respectively. After 10 min of exposure to 50 mM K+, glutamine concentrations increased in all three layers of DCN slices, to more than 5 times the values in unexposed slices. In the presence of a glutamate uptake blocker, L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC), glutamine concentrations in all layers did not increase as much during 50 mM K+. Similar but smaller changes occurred for serine. Mean ATP concentrations were lower in 50 mM K(+)-exposed slices compared to control. The results suggest that depolarization, such as during increased neural activity, can greatly affect amino acid metabolism in the cochlear nucleus.

Adenosine Triphosphate↗

PubMed2000

Thermosensory activation of insular cortex.

Temperature sensation is regarded as a submodality of touch, but evidence suggests involvement of insular cortex rather than parietal somatosensory cortices. Using positron emission tomography (PET), we found contralateral activity correlated with graded cooling stimuli only in the dorsal margin of the middle/posterior insula in humans. This corresponds to the thermoreceptive- and nociceptive-specific lamina I spinothalamocortical pathway in monkeys, and can be considered an enteroceptive area within limbic sensory cortex. Because lesions at this site can produce the post-stroke central pain syndrome, this finding supports the proposal that central pain results from loss of the normal inhibition of pain by cold. Notably, perceived thermal intensity was well correlated with activation in the right (ipsilateral) anterior insular and orbitofrontal cortices.

Adult↗

PubMed2000

Gene-target recognition among members of the myc superfamily and implications for oncogenesis.

Myc and Mad family proteins regulate multiple biological processes through their capacity to influence gene expression directly. Here we show that the basic regions of Myc and Mad proteins are not functionally equivalent in oncogenesis, have separable E-box-binding activities and engage both common and distinct gene targets. Our data support the view that the opposing biological actions of Myc and Mxi1 extend beyond reciprocal regulation of common gene targets. Identification of differentially regulated gene targets provides a framework for understanding the mechanism through which the Myc superfamily governs the growth, proliferation and survival of normal and neoplastic cells.

3T3 Cells↗

PubMed2000

Neurons overexpressing heme oxygenase-1 resist oxidative stress-mediated cell death.

This is the first report on the protective effect of heme oxygenase-1 (HO-1) overexpression against oxidative stress-mediated neuronal cell death and demonstration of a decreased production of oxygen free radicals when HO-1 levels are increased. HO-1 is the heat shock/stress cognate of the heat shock protein 32 family of proteins. A known function of these proteins is alpha-meso bridge-specific cleavage of the heme molecule. For the present study, we used cerebellar granular neurons (CGNs) isolated from homozygous transgenic (Tg) mice that overexpress HO-1 under neuron-specific enolase control and nontransgenic (Ntg) littermates. The Tg mouse CGNs were characterized by increased levels of HO-1 mRNA and protein, a lower resting intracellular calcium concentration, and a reduced HO-1 transcriptional response to glutamate-mediated oxidative stress. Compared with the Ntg neurons, when exposed to glutamate (30 microM or 3 mM), the magnitude of cell viability was increased and the number of cells exhibiting membrane permeability and chromatin condensation were significantly decreased in the Tg CGN cultures. The population of neurons surviving glutamate toxicity decreased when HO-1 activity was inhibited by a peptide inhibitor. The neuroprotective effect by HO-1 was extended to H(2)O(2)-induced cell death. The mechanism of protection may involve in part a reduced production of reactive oxygen species upon exposure to glutamate. We suggest that induction of HO-1 by pharmacological means may be a novel approach to amelioration of oxidative insults to neurons.

Animals↗

PubMed2000

Phe(208) and Ile(199) in human monoamine oxidase A and B do not determine substrate and inhibitor specificities as in rat.

It has been reported previously that reciprocally switching Phe(208) and Ile(199) in rat monoamine oxidase (MAO) A and B, respectively, was sufficient to switch their substrate and inhibitor preferences. In this study, the same mutants were made in the human forms of MAO. When compared with MAO A, MAO A-F208I showed a sixfold decrease in the specificity constant k(cat)/K(m) for both the MAO A- and the MAO B-preferring substrates 5-hydroxytryptamine and beta-phenylethylamine, respectively. The reciprocal point mutant MAO B-I199F had no effect on substrate affinity. To investigate if the region neighboring these two residues is responsible for conferring preferences, we have also made chimeric constructs by reciprocally switching the corresponding amino acid segments 159-214 in MAO A and 150-205 in MAO B. Chimerics MAO AB(159-214)A and MAO BA(150-205)B had small changes in K(m) and IC(50) values when compared with MAO A and B, respectively, but did not exhibit a preference switch. The results suggest that Phe(208) in MAO A and amino acid segments 159-214 and 150-205 in MAO A and B, respectively, influence the enzyme active site. However, substrate and inhibitor preferences of human MAO A and B are not determined by the respective residues Phe(108) and Ile(199) as in rat MAO nor by their neighboring regions.

Animals↗

PubMed2000

Extent of smoking and nicotine dependence in the United States: 1991-1993.

Rates of nicotine dependence vary in different sociodemographic groups in the population. These group differences remain to be understood. The objective of this study is to specify the relationship between cigarette consumption and nicotine dependence and whether group differences are explained by quantity smoked or differential sensitivity to nicotine. The data used derived from National Household Surveys on Drug Abuse (1991-1993), multistage area probability samples of the US population 12 years old and over, and anonymous structured household interviews, considering subjects who were last-month smokers (12,550 White, 4903 Black, 4839 Hispanic). The main outcome measure was last-year nicotine dependence based on symptoms of dependence and drug-related problems approximating DSM-IV dependence criteria. Rates are higher among females than males, Whites than minorities and the lowest among older adults. Dependence rates increase sharply up to half a pack of cigarettes smoked per day. At higher quantities, the increased risk for dependence is minimal. The higher rate of dependence among females than males results from a greater number of symptoms at the same quantity smoked. The similarity of adolescent and middle-adult rates results from the fact that adolescents smoke significantly fewer cigarettes than adults, but experience higher rates of dependence at the same levels of use. Although adults 50 and over are the heaviest smokers, they experience the lowest rates of dependence because of hypothesized lower sensitivity to increased quantity of nicotine intake. The higher rates among Whites than Blacks appear to result from heavier smoking and greater sensitivity to nicotine effects. It was concluded that adolescents, women and Whites are particularly vulnerable to becoming dependent on nicotine. Group-specific thresholds may be more appropriate criteria than an absolute threshold to define the risk for nicotine dependence.

Adolescent↗

PubMed2000

NOTUNG: a program for dating gene duplications and optimizing gene family trees.

Large scale gene duplication is a major force driving the evolution of genetic functional innovation. Whole genome duplications are widely believed to have played an important role in the evolution of the maize, yeast, and vertebrate genomes. The use of evolutionary trees to analyze the history of gene duplication and estimate duplication times provides a powerful tool for studying this process. Many studies in the molecular evolution literature have used this approach on small data sets, using analyses performed by hand. The rapid growth of genetic sequence data will soon allow similar studies on a genomic scale, but such studies will be limited unless the analysis can be automated. Even existing data sets admit alternative hypotheses that would be too tedious to consider without automation. In this paper, we describe a program called NOTUNG that facilitates large scale analysis, using both rooted and unrooted trees. When tested on trees analyzed in the literature, NOTUNG consistently yielded results that agree with the assessments in the original publications. Thus, NOTUNG provides a basic building block for inferring duplication dates from gene trees automatically and can also be used as an exploratory analysis tool for evaluating alternative hypotheses.

Algorithms↗

PubMed2000

Ginkgo biloba abolishes aggression in mice lacking MAO A.

Mice deficient in monoamine oxidase A (MAO A) have increased brain levels of serotonin (5-HT) and norepinephrine and show enhanced aggression. We used MAO A knock-out (KO) mice as a model to study the effect of ginkgo biloba (EGb) on aggression. When EGb was administered to MAO A KO mice, their aggressive behavior in resident-intruder confrontations was reduced to levels seen in wild types. EGb did not affect the locomotive behavior of MAO A KO mice, which suggests that its effects on aggression were not due to sedation. EGb caused a significant 16.9% decrease in [3H]ketanserin binding to 5-HT2A receptors in the frontal cortex of MAO A KO mice but did not change the receptor affinity for [3H]ketanserin. This suggests that the antiaggressive effect of EGb may be mediated by 5-HT2A receptors and that EGb may be developed as a novel antiaggressive agent.

Aggression↗

PubMed2000

Apoptosis of thyrocytes and effector cells during induction and resolution of granulomatous experimental autoimmune thyroiditis.

Experimental autoimmune thyroiditis (EAT) with granulomatous histopathology (G-EAT) can be induced by cells from mouse thyroglobulin (MTg)-immunized donors activated in vitro with MTg and IL-12. G-EAT lesions reach maximum severity 18-21 days after cell transfer and, if some thyroid follicles remain, lesions almost completely resolve by day 35. CD8(+) cells are required for G-EAT resolution. To begin to determine the mechanisms involved in G-EAT resolution, apoptosis in thyroids was analyzed by TUNEL staining. Apoptotic thyrocytes and inflammatory cells were present in the thyroids of both CD8(+) and CD8-depleted recipient mice at day 19-21. By day 35, apoptotic cells were rare in thyroids of mice whose lesions had resolved; the few apoptotic inflammatory cells were generally in close proximity to thyroid follicles. Thyroids of CD8-depleted mice had ongoing inflammation at day 35 and most apoptotic cells were thyroid follicular cells. The expression of Fas and Fas ligand (FasL) mRNA in thyroids was also determined by RT-PCR in both CD8(+) and CD8-depleted recipient mice. Fas was expressed in normal thyroids and its expression was relatively constant throughout the course of disease. FasL mRNA was not expressed in normal thyroids. FasL mRNA expression generally correlated with G-EAT severity, being maximal at day 21 and diminishing as lesions resolved. However, FasL mRNA expression in thyroids of CD8-depleted mice in which resolution was delayed was decreased compared to thyroids of CD8(+) mice with comparable disease severity, suggesting that FasL expressed by CD8(+) cells may play a role in G-EAT resolution.

Animals↗

PubMed2000

Scale-dependent dimension in the forest fire model

The forest fire model is a reaction-diffusion model where energy, in the form of trees, is injected uniformly, and burned (dissipated) locally. We show that the spatial distribution of fires forms a geometric structure where the fractal dimension varies continuously with the length scale. In the three-dimensional model, the dimensions vary from zero to three, proportional with ln(l), as the length scale increases from l approximately 1 to a correlation length l=xi. Beyond the correlation length, which diverges with the growth rate p as xi approximately p(-2/3), the distribution becomes homogeneous. We suggest that this picture applies to the "intermediate range" of turbulence where it provides a natural interpretation of the extended scaling that has been observed at small length scales. Unexpectedly, it might also be applicable to the spatial distribution of luminous matter in the universe. In the two-dimensional version, the dimension increases to D=1 at a length scale l approximately 1/p, where there is a crossover to homogeneity, i.e., a jump from D=1 to D=2.

Journal Article↗

PubMed2000

Initial binding of preproteins involving the Toc159 receptor can be bypassed during protein import into chloroplasts.

Two integral outer envelope GTPases, Toc34 and Toc86, are proposed to regulate the recognition and translocation of nuclear-encoded preproteins during the early stages of protein import into chloroplasts. Defining the precise roles of Toc86 and Toc34 has been complicated by the inability to distinguish their GTPase activities. Furthermore, the assignment of Toc86 function is rendered equivocal by recent reports suggesting that the standard protocol for the isolation of chloroplasts results in significant proteolysis of Toc86 (B. Bolter, T. May, J. Soll [1998] FEBS Lett 441: 59-62; G. Schatz [1998] Nature 395: 439-440). We demonstrate that Toc86 corresponds to a native protein of 159 kD in pea (Pisum sativum), designated Toc159. We take advantage of the proteolytic sensitivity of Toc159 to selectively remove its 100-kD cytoplasmic GTPase domain and thereby distinguish its activities from other import components. Proteolysis eliminates detectable binding of preproteins at the chloroplast surface, which is consistent with the proposed role of Toc159 as a receptor component. Remarkably, preprotein translocation across the outer membrane can occur in the absence of the Toc159 cytoplasmic domain, suggesting that binding can be bypassed. Translocation remains sensitive to GTP analogs in the absence of the Toc159 GTP-binding domain, providing evidence that Toc34 plays a key role in the regulation of translocation by GTP.

Amino Acid Sequence↗

PubMed2000

Optimal image sampling schedule for both image-derived input and output functions in PET cardiac studies.

Optimal sampling schedule (OSS) design for both image-derived input and output functions in tracer kinetic modeling with positron emission tomography (PET) is investigated. This problem is very important in noninvasive PET dynamic cardiac studies where both the input function, i.e., the plasma time-activity curve (PTAC), and the output function, i.e., the tissue time-activity curve (TTAC), are obtained simultaneously from the same sequence of PET images. The integral PET measurement is used in this study. The spillover correction for the cross contaminations in cardiac studies is incorporated into the OSS design procedure. A new target function based on the D-optimal criterion involving both the input and output sensitivity functions is proposed. The fluorodeoxyglucose (FDG) model and a six-parameter PTAC model are used to illustrate the simultaneous OSS design for both the PTAC and TTAC. An OSS design consisting of six different scanning intervals is derived. Computer simulations are performed based on the estimated parameters from real studies to evaluate the effectiveness of the OSS. The double modeling approach is used in parameter estimation to simultaneously estimate the parameters involved. The results have shown that, for a wide range of parameter variations, the OSS is as effective as a conventional sampling schedule (CSS) and comparable parameter estimates can be obtained. Compared with the use of the CSS, the use of the OSS leads to an approximately 70% reduction in the storage space and data processing time.

Computer Simulation↗

PubMed2000

Optical computed tomography in a turbid medium using early arriving photons.

We employ photon migration to image absorbing objects embedded in a turbid medium. For improved resolution, we use early arriving photons (a few hundred picoseconds in excess of the time of flight), a regime in which the diffusion approximation breaks down. Our image reconstruction method is based on extension of x-ray computed tomography (CT) to the optical regime. The CT algorithm must be generalized to take into account the distributions of photon paths. We express the point spread function (PSF) in terms of the Green's function for the transport equation. This PSF then provides weighting functions for use in a generalized series expansion method of x-ray CT. Experiments were performed on a turbid medium with scattering and absorption properties similar to those of human breast tissue. Multiple absorbers were embedded into the medium to mimic tumors. Coaxial transmission scans were collected in two projections, and the early-time portions were analyzed. Through accurate modeling, we could remove the blurring associated with multiple scattering and obtain high-resolution images. Our results show that the diffusion approximation PSF is inadequate to describe the early arriving photons. A PSF incorporating causality is required to reconstruct accurate images of turbid media.

Algorithms↗

PubMed2000

Loss of cyclosporin and azidopine binding are associated with altered ATPase activity by a mutant P-glycoprotein with deleted phe(335).

In this study, we further characterize a mutant P-glycoprotein (P-gp) that has a deletion of Phe(335) and is resistant to inhibition by cyclosporins. Photoaffinity labeling with [(3)H]cyclosporine and [(3)H]azidopine revealed markedly decreased binding to the mutant P-gp compared with wild-type P-gp. Expression of the mutant P-gp in multidrug-resistant variant cell line MES-SA/DxP (DxP) cells was associated with a 2-fold higher basal ATPase activity relative to multidrug-resistant cell line MES-SA/Dx5 (Dx5) cells with wild-type P-gp. Cyclosporine inhibited ATPase activity in both cell types, whereas the cyclosporin D analog valspodar (PSC 833), vinblastine, and dactinomycin stimulated ATPase activity in Dx5 but not in mutant DxP cells. Moreover, the cell lines differed in their responses to verapamil, which produced greater stimulation of ATPase in Dx5 than DxP cells. Verapamil significantly reversed the [(3)H]daunorubicin accumulation defect in wild-type Dx5 cells, but it had no significant effect on [(3)H]daunorubicin accumulation in the mutant DxP cells. Verapamil was not transported by cells expressing either mutant or wild-type P-gp. Vanadate trapping of azido-ATP was markedly impaired in mutant P-gp. In conclusion, our data demonstrate that Phe(335) of transmembrane 6 is an important amino acid residue for the formation of cyclosporine and azidopine drug-binding site(s). Phe(335) also plays a role in the coupling of verapamil binding and modulation of daunorubicin intracellular accumulation in wild-type P-gp. In addition, Phe(335) in transmembrane 6 may play a role in coupling drug binding to ATPase activity. The deletion of Phe(335) results in a significant increase in the basal ATPase activity with a concomitant decrease in its ability to trap ATP and transport some P-gp substrates.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMed2000

Analysis of the pharmacological and molecular heterogeneity of I(2)-imidazoline-binding proteins using monoamine oxidase-deficient mouse models.

The I(2) subgroup of imidazoline-binding sites was identified as monoamine oxidases (MAOs), but it is unclear whether there are I(2)-binding sites located on proteins distinct from MAOs. To address this issue, we characterized I(2)-binding proteins in liver and brain of wild-type and MAO A- and MAO B-deficient mice. I(2)-binding sites were identified using [(3)H]idazoxan and the photoaffinity adduct 2-[3-azido-4-[(125)I]iodophenoxyl]methylimidazoline ([(125)I]AZIPI). [(3)H]Idazoxan labeled binding sites with ligand recognition properties typical of I(2) sites in both brain and liver of wild-type mice. High-affinity, specific [(3)H]idazoxan binding were not altered in MAO A knockout (KO) mice. In contrast, [(3)H]idazoxan binding was completely abolished in both liver and brain of MAO B KO mice. In wild-type mice, [(125)I]AZIPI photolabeled three proteins with apparent molecular masses of approximately 28 (liver), approximately 61 (brain), and approximately 55 kDa (liver and brain). The photolabeling of each protein was blocked by the imidazoline cirazoline (10 microM). Photolabeling of the approximately 61- and approximately 55-kDa proteins was not observed in MAO A and B KO mice, respectively. In contrast, photolabeling of the liver approximately 28-kDa protein was still observed in MAO-deficient mice, indicating that this protein is unrelated to MAOs. These data indicate that I(2) imidazoline-binding sites identified by [(3)H]idazoxan reside solely on MAO B. The binding sites on MAO A and the liver approximately 28-kDa protein may represent additional subtypes of the family of the imidazoline-binding sites.

Adrenergic alpha-Antagonists↗