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Biomedical subjects

M Li

Publications and source records attributed to M Li.

At least 937 records · Page 52Linked to original sources
PubMed1995

Placental teratoma. A case report and review of the literature.

A case of teratoma from the placenta is described. Sixteen previous reports of this entity are briefly reviewed, and the histological features, differential diagnosis and pathogenesis of the placental teratoma are discussed. Authors consider the placental teratoma a true tumor and not a fetus amorphus. Placental teratomas are extremely rare tumors. Sixteen cases have been reported since first described by Morville in 1925. Placental teratoma has not been reported in China.

Adult↗
PubMed1994

Purification of (-)-epigallocatechin from enzymatic hydrolysate of its gallate using high-speed counter-current chromatography.

Epigallocatechin gallate (EGCG) was hydrolyzed at various concentrations of tannase under pH 6.0 at 35 degrees C, and the reaction mixtures were separated by high-speed counter-current chromatography with a two-phase solvent system composed of hexane-ethyl acetate-water (1:13:20). The best results were obtained when 2 mg/ml of the enzyme buffer solution was added to 0.3 M EGCG buffers at a rate of 0.1 ml/min. Using 10 mg of the enzyme, 342 mg of epigallocatechin were obtained at a purity of 99.1%.

Carboxylic Ester Hydrolases↗
PubMed1994

Modulation of brain Na+ channels by a G-protein-coupled pathway.

Na+ channels in acutely dissociated rat hippocampal neurons and in Chinese hamster ovary (CHO) cells transfected with a cDNA encoding the alpha subunit of rat brain type IIA Na+ channel (CNaIIA-1 cells) are modulated by guanine nucleotide binding protein (G protein)-coupled pathways under conditions of whole-cell voltage clamp. Activation of G proteins by 0.2-0.5 mM guanosine 5'-[gamma-thio]triphosphate (GTP[gamma S]), a nonhydrolyzable GTP analog, increased Na+ currents recorded in both cell types. The increase in current amplitude was caused by an 8- to 10-mV negative shift in the voltage dependence of both activation and inactivation. The effects of G-protein activators were blocked by treatment with pertussis toxin or guanosine 5'-[beta-thio]diphosphate (GDP[beta S]), a nonhydrolyzable GDP analog, but not by cholera toxin. GDP[beta S] (2 mM) alone had effects opposite those of GTP[gamma S], shifting Na(+)-channel gating 8-10 mV toward more-positive membrane potentials and suggesting that basal activation of G proteins in the absence of stimulation is sufficient to modulate Na+ channels. In CNaIIA-1 cells, thrombin, which activates pertussis toxin-sensitive G proteins in CHO cells, caused a further negative shift in the voltage dependence of Na(+)-channel activation and inactivation beyond that observed with GTP alone. The results in CNaIIA-1 cells indicate that the alpha subunit of the Na+ channel alone is sufficient to mediate G protein effects on gating. The modulation of Na+ channels via a G-protein-coupled pathway acting on Na(+)-channel alpha subunits may regulate electrical excitability through integration of different G-protein-coupled synaptic inputs.

Animals↗
PubMed1994

Autophosphorylation-activated protein kinase inactivates the protein tyrosine phosphatase activity of protein phosphatase 2A.

Phosphorylation of the catalytic subunit of protein phosphatase 2A (PP2A) on threonines with a distinct autophosphorylation-activated protein kinase [Guo and Damuni (1993) Proc. Natl. Acad. Sci. USA 90, 2500-2504] inactivated the phosphatase with 32P-labelled myelin basic protein prepared by incubation with the kinase domain of the epidermal growth factor receptor, the src-family protein kinases p56lck and p60c-src, myelin basic protein kinase-1, or protamine kinase. Phosphoamino acid analysis demonstrated that the kinase domain of the epidermal growth factor receptor, p56lck and p60c-src phosphorylated myelin basic protein on tyrosines, that the protamine kinase phosphorylated myelin basic protein on serines, and that myelin basic protein kinase-1 phosphorylated myelin basic protein on threonines. The results demonstrate that the autophosphorylation-activated protein kinase not only inactivates the protein serine/threonine phosphatase, but also the protein tyrosine phosphatase activity of PP2A. This autophosphorylation-activated protein kinase-mediated inactivation of PP2A may, in response to extracellular stimuli, not only contribute to the enhanced phosphorylation of cellular proteins on serines and threonines but also on tyrosines.

Adenosine Triphosphate↗
PubMed1994

cAMP-dependent differential regulation of extracellular matrix (ECM) gene expression in cultured rat Schwann cells.

cAMP-dependent regulation of the steady-state mRNA levels for the ECM components, laminin A, B1 and B2 chains, collagen types I, III and IV were examined by Northern blot analysis in cultured rat Schwann cells. ECM mRNAs of laminin B1 chain and collagen types I and IV were expressed at high levels in the control Schwann cells, while laminin B2 chain and collagen type III mRNA levels were low, and laminin A chain mRNA was not detectable. When Schwann cells were treated with forskolin or cAMP derivatives, the gene expression for the ECM molecules constituting the Schwann cell basement membrane, laminin B1 and B2 chains, and collagen type IV, was enhanced in time- and dose-dependent manners for exogenously administered forskolin or cAMP derivatives, while the mRNA levels for the ECM molecules, which are not the major components of the basement membrane, fibrillary collagen types I and III were significantly suppressed. This cAMP-dependent differential regulation of Schwann cell ECM gene expression may be related to the role of each ECM molecule in the peripheral nerve development and regeneration.

Adrenergic beta-Agonists↗
PubMed1994

Okadaic acid and microcystin-LR directly inhibit the methylation of protein phosphatase 2A by its specific methyltransferase.

The catalytic C subunit of protein phosphatase 2A2 was methylated with an apparent km of about 0.1 microM by purified preparations of a methyltransferase from bovine brain. This methylation was inhibited by okadaic acid and microcystin-LR half-maximally at 40 nM and 60 nM, respectively. The extent of inhibition depended on the protein phosphatase concentration in the incubations, but was independent of the methyltransferase concentration. The results demonstrate that okadaic acid and microcystin-LR directly inhibit the methylation of protein phosphatase 2A. The results are consistent with the idea that okadaic acid and microcystin-LR act, at least in part, by binding to the carboxyl terminus of the C subunit of protein phosphatase 2A thereby preventing access of the methyltransferase to its target site, the C subunit carboxyl terminal Leu309.

Animals↗
PubMed1994

The effect of Met-->Leu mutations on calmodulin's ability to activate cyclic nucleotide phosphodiesterase.

Calmodulin (CaM) has two hydrophobic surface patches that are particularly rich in Met residues, and these are the major contact areas where CaM interacts with its target enzymes. The amino acid Leu has been introduced by site-directed mutagenesis to replace all the Met residues in CaM. All nine individual Met-->Leu mutants of CaM as well as some double and quadruple mutants were expressed in Escherichia coli. All mutants could be purified by calcium-dependent hydrophobic affinity chromatography, indicating that they still expose their hydrophobic surfaces upon binding calcium. Each single Met-->Leu mutation in the C-terminal domain of the protein had little effect on its ability to activate phosphodiesterase (PDE), while a quadruple mutant with four C-terminal Leu residues instead of Met has a significantly lower affinity for PDE. The M36L mutant is a poor activator compared with the other three N-terminal single Met-->Leu mutants, which have a slightly lower affinity for PDE than wild-type CaM. The introduction of a positively charged Arg for Met-145 resulted in an almost complete loss of CaM's ability to activate PDE. Nuclear magnetic resonance spectroscopy was used to show that most CaM mutants retain their overall three-dimensional structure. Thus, the altered activation properties appear to arise from differences in the flexibility and polarizability of the Met and Leu sidechains, rather than from structural perturbations.

3',5'-Cyclic-AMP Phosphodiesterases↗
PubMed1994

Dicistronic targeting constructs: reporters and modifiers of mammalian gene expression.

To investigate the activity of candidate regulatory molecules in mammalian embryogenesis, we have developed a general strategy for modifying and reporting resident chromosomal gene expression. The picornaviral internal ribosome-entry site was incorporated into gene targeting constructs to provide cap-independent translation of a selectable marker from fusion transcripts generated following homologous recombination. These promoterless constructs were highly efficient and have been used both to inactivate the stem-cell-specific transcription factor Oct-4 and to introduce a quantitative regulatory modification into the gene for a stem-cell maintenance factor, differentiation-inhibiting activity. In addition, the inclusion of a beta-galactosidase reporter gene in the constructs enabled accurate and sensitive detection of cellular sites of transcription. This has allowed visualization of putative "stem-cell niches" in which sources of elevated expression of differentiation-inhibiting activity were localized to the differentiated cells surrounding colonies of stem cells.

Animals↗
PubMed1994

FcR gamma chain deletion results in pleiotrophic effector cell defects.

The gamma subunit of immunoglobulin Fc receptors is an essential component of the high-affinity receptor for IgG (Fc gamma RIII) and is associated with the high-affinity receptor for IgG (Fc gamma RI) and the T cell receptor-CD3 complex. It is required for both receptor assembly and signal transduction. Targeted disruption of this subunit results in immunocompromised mice. Activated macrophages from gamma chain-deficient mice unexpectedly lack the ability to phagocytose antibody-coated particles, despite normal binding. Defects in NK cell-mediated antibody-dependent cytotoxicity and mast cell-mediated allergic responses are evident in these animals, establishing the indispensable role of FcRs in these responses. However, loss of gamma chain does not appear to perturb T cell development, since both thymic and peripheral T cell populations appear normal. These mice thus represent an important tool for evaluating the role of these receptors in humoral and cellular immune responses.

Animals↗
PubMed1994

Images of purified Shaker potassium channels.

BACKGROUND: Voltage-gated K+ channels play an important role in the control of neuronal excitability and synaptic plasticity. Their low abundance and extraordinary heterogeneity have rendered their purification from natural sources difficult. We have previously cloned a voltage-gated K(+)-channel gene, Shaker, from Drosophila. The Shaker K(+)-channel polypeptide resembles one of the four internal repeats of a Na(+)- or Ca(2+)-channel alpha subunit, suggesting that this example of a K+ channel contains four identical or homologous subunits. Similar K(+)-channel polypeptides have been characterized from mammals, other vertebrate and invertebrate species, and from plants. Electrophysiological studies of K+ channels expressed in Xenopus oocytes suggest that they are indeed tetramers, and heteromultimeric K+ channels have been found in the mammalian brain. Until now, however, no K+ channel, nor any other member of the superfamily of voltage-gated ion channels, has been characterized by electron microscopy or other structural analysis. RESULTS: We have purified Shaker K+ channels, expressed in insect Sf9 cells, to apparent homogeneity, and imaged them using the electron microscope. The physical dimensions of these molecules, as well as their biochemical characteristics, are consistent with a tetrameric subunit composition. Moreover, the Shaker channel revealed by negative staining has the appearance of a four-fold symmetric tetramer, with a large, central vestibule that presumably constitutes part of the pathway for ions. CONCLUSION: These first clear images of a voltage-gated ion channel reveal a marked four-fold symmetry. The integrity of the purified tetrameric complex indicates that the purification scheme used in this study may be further developed for future structural analysis of voltage-gated K+ channels.

Animals↗
PubMed1994

Target of the transcriptional activation function of phage lambda cI protein.

Activation of transcription initiation by the cI protein of phage lambda is thought to be mediated by a direct interaction between cl and RNA polymerase at the PRM promoter. Two negatively charged amino acid residues in the DNA binding domain of cI play a key role in activation, suggesting that these residues contact RNA polymerase. The subunit of RNA polymerase involved was identified by selecting polymerase mutants that restored the activation function of a mutant form of cI protein. Although previous studies suggest that several activators interact with the alpha subunit of RNA polymerase, the results here suggest that cI interacts with the sigma subunit. An arginine to histidine change near the carboxyl terminus of sigma specifically suppresses an aspartic acid to asparagine change in the activation region of cI. This finding supports the direct-contact model and suggests that a cluster of positively charged residues near the carboxyl terminus of sigma is the target of the negatively charged activation region of cI.

Bacteriophage P22↗