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Biomedical subjects

X Lin

Publications and source records attributed to X Lin.

At least 19 recordsLinked to original sources
PubMed1998

BDNF accelerates gene expression in cultured cerebellar granule neurons.

This study reports that in purified cultures of postnatal cerebellar granule cells, BDNF significantly accelerated GABAA receptor alpha 6 subunit (GABAA alpha 6) mRNA expression, a marker for terminally differentiated cerebellar granule neurons, and also accelerated p21cip1 expression. p21cip1 is a general cyclin-dependent kinase (Cdk) inhibitor that can inhibit progression through the cell cycle. Alternatively, the expression of p27kip1, another Cdk inhibitor closely related to p21cip1, is not modified by BDNF. In cultured granule cells, the increase in p21cip1 expression induced by BDNF occurred after dividing granule cells had left the cell cycle and thus was not required to direct granule neuron precursors out of the cell cycle. p21cip1 may have an alterative function during granule neuron terminal differentiation, separate from its ability to regulate cell cycle exit. This report shows that, in vitro, BDNF accelerates granule cell gene expression and may thus modulate cerebellar granule cell differentiation.

Animals↗
PubMed1998

Use of generalized linear mixed models in analyzing mutant frequency data from the transgenic mouse assay.

The transgenic mouse assay is now widely used for the study of mutagenesis in diverse rodent tissues and to test chemicals for genotoxic potential. This kind of assay generally involves nested observations at several levels of sampling, e.g., animals, packaging reactions, and plates. Due to the common origin, the mutant frequency (MF) in tissues from the same animal are likely to be positively correlated, inducing extra variation relative to the common binomial variation. In this article, a generalized linear mixed model is used to analyze the overdispersed binomial data on mutant frequency from the transgenic mouse assay, with a random effect for each level of the sampling hierarchy. This is a comprehensive framework within which different sources of variation in the data can be evaluated in nested factorial experiments and treatment effects can be assessed simultaneously. It avoids the current practice of repeated testing for excess binomial variability at each level of the sampling hierarchy and aggregating data up the levels, but fits the data with one single model. Parameters associated with the fixed effects, particularly dose, and the variance components for the random effects (e.g., animals, packages, and plates) can be estimated and tested for significance. Data previously reported in the literature involving the lacl gene from the Big Blue mouse are used to illustrate the proposed method.

Animals↗
PubMed1998

Effect of berbamine on T-cell mediated immunity and the prevention of rejection on skin transplants in mice.

Berbamine, an ingredient of Berberis, which itself is widely utilized in Chinese folk-medicine has been used as a source of leukogenics, anti-arrhythmics and anti-hypertensives. In recent years, the immunosuppressive effects of berbamine has been demonstrated. In order to further investigate the value of berbamine as an immunosuppressive agent, the delayed type hypersensitivity reaction (DTH) response with sheep red blood cells (SRBC), the mixed lymphocyte reaction (MLR) and a skin model of allograft rejection on mice were studied. Berbamine showed suppressive effects on DTH and MLR and significantly prolonged allograft survival compared with untreated transplanted mice. The results indicate that berbamine may be a potential agent in clinical transplantation.

Alkaloids↗
PubMed1998

Molecular characterization of a nondemyelinating variant of Daniel's strain of Theiler's virus isolated from a persistently infected glioma cell line.

Wild-type Daniel's strain of Theiler's virus (wt-DA) induces a chronic demyelination in susceptible mice which is similar to multiple sclerosis. A variant of wt-DA (designated DA-P12) generated during the 12th passage of persistent infection of a G26-20 glioma cell line failed to persist and induce demyelination in SJL/J mice. To identify the determinants responsible for this change in phenotype, we sequenced the capsid coding sequence (nucleotides [nt] 2991 to 3994) and found three mutations in VP1: residues 99 (Gly to Ser), 100 (Gly to Asp), and 103 (Asn to Lys). To study the role of these mutations in neurovirulence and demyelination, we prepared a recombinant virus, DAP-1C-2A/DA, with replacement of wt-DA nt 2991 to 3994 with the corresponding region of DA-P12, and viruses with individual point mutations at VP1 residues 99(Ser), 100(Asp), and 103(Lys). DAP-1C-2A/DA and viruses with a mutation at VP1 residue 99 or 100 (but not 103) completely attenuated the ability of wt-DA to induce demyelination. Failure to induce demyelination was not due to a general failure in growth, since DA-P12 and other mutant viruses lysed L-2 cells in vitro as effectively as wt-DA. The change in disease phenotype was independent of the specific B- or T-cell immune recognition because a decrease in the neurovirulence of mutant viruses was observed in neonatal mice and immune-deficient RAG1 -/- mice. This difference in neurovirulence is not the complete explanation for the failure of DA-P12 to demyelinate, since virus with a mutation at residue 103(Lys) had decreased neurovirulence but did induce demyelination. Therefore, point mutation at VP1 residue 99 or 100 altered the ability of wt-DA to demyelinate, perhaps related to a disruption in interaction between virus and receptor on certain neural cells.

Animals↗
PubMed1998

Addition of catfish gonadotropin-releasing hormone (GnRH) receptor intracellular carboxyl-terminal tail to rat GnRH receptor alters receptor expression and regulation.

Mammalian GnRH receptor (GnRHR) is unique among G protein-coupled seven-transmembrane segment receptors due to the absence of an intracellular C-terminal tail frequently important for internalization and/or desensitization of other G protein-coupled receptors. The recent cloning of nonmammalian (i.e. catfish, goldfish, frog, and chicken) GnRHRs shows that these contain an intracellular C terminus. Addition of the 51-amino acid intracellular C terminus from catfish GnRHR (cfGnRHR) to rat GnRHR (rGnRHR) did not affect rGnRHR binding affinity but elevated receptor expression by about 5-fold. Truncation of the added C terminus impaired the elevated receptor-binding sites by 3- to 8-fold, depending on the truncation site. In addition, introducing the C terminus to rGnRHR altered the pattern of receptor regulation from biphasic down-regulation and recovery to monophasic down-regulation. The extent of down-regulation was also enhanced. The alteration in receptor regulation due to the addition of a C terminus was reversed by truncation of the added C terminus. Furthermore, addition of the cfGnRHR C terminus to rGnRHR significantly augmented the inositol phospholipid (IP) response of transfected cells to Buserelin, but this did not result from the elevation of receptor-binding sites. Addition of the C terminus did not affect Buserelin-stimulated cAMP and PRL release. GH3 cells transfected with wild-type cfGnRHR did not show measurable Buserelin binding or significant stimulation of IP, cAMP, or PRL in response to Buserelin (10[-13]-10[-9] M). GH3 cells transfected with C terminus-truncated cfGnRHR showed no IP response to Buserelin (10[-13]-10[-7] M). These results suggest that addition of the cfGnRHR intracellular C terminus to rGnRHR has a significant impact on rGnRHR expression and regulation and efficiency of differential receptor coupling to G proteins.

Amino Acid Sequence↗
PubMed1997

A Study on the Chemical Heterogeneity of Active Surface of Solid Catalysts

A method that can be used to characterize the chemical heterogeneity of active surface of solid catalyst is given. The chemical adsorption behavior of three adsorbates on Ni-W/F-Al2O3-SiO2 catalyst and supported Ni catalyst has been investigated. On the basis of fractal geometry, the roughness fractal dimension (Dr) and the chemisorption fractal dimension (Dc) of two kinds of catalyst are estimated. We use the difference between Dc and Dr to characterize the chemical heterogeneity of active sites on the catalytic surface. As a result, the chemical heterogeneity of the acid sites of Ni-W/F-Al2O3-SiO2 catalyst is very great. As for supported Ni catalyst, the chemical heterogeneity of active sites decreased when impregnating operation times in preparation of the catalyst increased. After impregnated four times, the chemical heterogeneity of its active surface of supported Ni catalyst is eliminated. Copyright 1997 Academic Press. Copyright 1997Academic Press

Journal Article↗
PubMed1997

Alteration of glycosylation renders HIV sensitive to inactivation by normal human serum.

Retroviruses from various mammalian species, excluding humans, are effectively inactivated in normal human serum (NHS). Recent studies have shown that NHS inactivation of retroviruses occurs through natural Ab recognition of a terminal glycosidic moiety on the viral envelope that is acquired during replication in the host cell. This carbohydrate structure (the alpha-galactosyl epitope) is expressed on the cells of most mammals, with the exception of humans and other Old World primates. In this study, NHS sensitivity of HIV was assessed following viral propagation in human cells that were manipulated to express the alpha-galactosyl epitope. HUT-78 cells were transduced with an exogenous alpha1-3-galactosyl transferase gene, which codes for the terminal glycosyl transferase responsible for generation of the alpha-galactosyl epitope. The transduced HUT-78 cells expressed high levels of the alpha-galactosyl epitope on their membrane surface, rendering them sensitive to killing in NHS. Similarly, HIV passaged through these cells acquired the alpha-galactosyl epitope in association with the envelope glycoprotein gp120 and was also effectively inactivated in NHS. Viral inactivation was abolished by the addition of a synthetic disaccharide that contains the alpha-galactosyl epitope, indicating that virolysis is mediated by anti-alpha-galactosyl natural Ab. These results demonstrate that, like other retroviruses bearing the alpha-galactosyl epitope, HIV modified to express this epitope is inactivated in NHS. Furthermore, these data suggest that expression of the alpha-galactosyl epitope on the surface of viruses may have implications in the interspecies transmission of such viruses to humans.

Animals↗
PubMed1997

Kinetic properties of saquinavir-resistant mutants of human immunodeficiency virus type 1 protease and their implications in drug resistance in vivo.

In order to study the basis of resistance of human immunodeficiency virus, type 1 (HIV-1), to HIV-1 protease inhibitor saquinavir, the catalytic and inhibition properties of the wild-type HIV-1 protease and three saquinavir resistant mutants, G48V, L90M, and G48V/L90M, were compared. The kinetic parameter kcat/Km was determined for these proteases using eight peptide substrates whose sequences were derived from the natural processing site sequences of HIV-1. The kcat/Km values were determined using conventional steady-state kinetics as well as initial velocities of mixed substrate cleavages under the condition where the substrate concentrations [S]o << Km. The independently determined kcat and Km values for some of the substrates confirmed the accuracy of the mixed-substrate method and also permitted the calculation in all cases of true rather than relative kcat/Km values. The Ki values were also determined. Using a previously described kinetic model [Tang, J., & Hartsuck, J. A. (1995) FEBS Lett. 367, 112-116], the relative processing activities of HIV-1 protease variants were estimated in the saquinavir concentration range of 0-10(-7) M. Although the protease activity of G48V, L90M, and G48V/L90M are only about 10, 7, and 3% of that of the wild-type HIV-1 protease in the absence of inhibitor, the resistance tendencies of the three mutants are clearly manifest by relatively less activity loss as inhibitor concentration becomes higher. Also, the ratios of the activities of the four protease species at certain saquinavir concentrations appear to correlate with the population ratios of the four protease species at different time points of clinical trials. This correlation suggests that the population ratio of the protease species is driven by in vivo saquinavir concentration, which appears to be in the range 10(-10)-10(-9) M during the clinical trials.

Dose-Response Relationship, Drug↗
PubMed1997

Ethnic differences in the length of the menstrual cycle during the postmenarcheal period.

In 1989, 125 African-American and 123 European-American girls aged 12-14 years were enrolled in a 2-year study in which they maintained a menstrual calendar, recording the date and amount of menstrual bleeding. Weight, exercise, and stress during the previous week were recorded at the start of each menstrual cycle. Although only minor ethnic differences were observed in expected cycle length (29.3 vs. 28.8 days for European-American and African-American girls, respectively), more prominent differences were observed in the between-subjects standard deviation of cycle length (2.9 vs. 2.2 days, respectively) and in the odds of having a cycle longer than 45 days (odds ratio=1.86, 95% confidence interval 1.17-2.97) for European-American compared with African-American girls. Low weight for height and high levels of exercise increased the probability of having a cycle longer than 45 days and decreased expected cycle length of 13- to 45-day cycles. Additional investigation of potential ethnic differences in menstrual cycle characteristics is warranted.

Adolescent↗
PubMed1997

The 90-kDa ribosomal S6 kinase (pp90rsk) phosphorylates the N-terminal regulatory domain of IkappaBalpha and stimulates its degradation in vitro.

Nuclear factor kappaB (NF-kappaB) is a eukaryotic member of the Rel family of transcription factors whose biological activity is post-translationally regulated by its assembly with various ankyrin-rich cytoplasmic inhibitors, including IkappaBalpha. Expression of NF-kappaB in the nucleus occurs after signal-induced phosphorylation, ubiquitination, and proteasome-mediated degradation of IkappaBalpha. The induced proteolysis of IkappaBalpha unmasks the nuclear localization signal within NF-kappaB, allowing its rapid migration into the nucleus, where it activates the transcription of many target genes. At present, the identity of the IkappaBalpha kinase(s) that triggers the first step in IkappaBalpha degradation remains unknown. We have investigated the potential function of the 90-kDa ribosomal S6 kinase, or pp90(rsk), as a signal-inducible IkappaBalpha kinase. pp90(rsk) lies downstream of mitogen-activated protein (MAP) kinase in the well characterized Ras-Raf-MEK-MAP kinase pathway that is induced by various growth factors and phorbol ester. We now show that pp90(rsk), but not pp70(S6K) or MAP kinase, phosphorylates the regulatory N terminus of IkappaBalpha principally on serine 32 and triggers effective IkappaBalpha degradation in vitro. When co-expressed in vivo in COS cells, IkappaBalpha and pp90(rsk) readily assemble into a complex that is immunoprecipitated with antibodies specific for either partner. While phorbol 12-myristate 13-acetate produced rapid activation of pp90(rsk), in vivo, other potent NF-kappaB inducers, including tumor necrosis factor alpha and the Tax transactivator of human T-cell lymphotrophic virus, type I, failed to activate pp90(rsk). These data suggest that more than a single IkappaBalpha kinase exists within the cell and that these IkappaBalpha kinases are differentially activated by different NF-kappaB inducers.

Animals↗
PubMed1997

Reexpression of the major protein kinase C substrate, SSeCKS, suppresses v-src-induced morphological transformation and tumorigenesis.

SSeCKS (pronounced essex) encodes a major protein kinase C substrate, the expression of which is down-regulated in src- and ras-transformed rodent fibroblasts but not in raf-transformed rodent fibroblasts (X. Lin et al., Mol. Cell. Biol., 15: 2754-2762, 1995). Using a panel of ras-transformed or revertant Rat-6 cells that exhibit selective parameters of transformation, we show that down-regulation of SSeCKS correlates with anchorage-independent growth. Cotransfection of NIH3T3 fibroblasts with an SSeCKS expression plasmid decreased 6-30-fold the ability of a v-src expressor plasmid to induce colonies in soft agar. To differentiate between possible tumor suppressive or growth-inhibitory effects of SSeCKS, we developed conditionally transformed cell lines (expressing ts72v-src) with tetracycline-regulated SSeCKS expression. SSeCKS suppressed the ability of v-src to induce increased cellular refractility, focus formation, soft agar colony formation, in vitro invasiveness in Matrigel, and growth in low serum (0.5%) but did not inhibit cell proliferation in high serum (10%) at the permissive (35 degrees C) temperature for src kinase activity. However, at the nonpermissive (39.5 degrees C) temperature, SSeCKS induced growth arrest. SSeCKS expression did not affect: (a) the protein level, in vivo or in vitro kinase activity of ts72src; (b) the activity of jun NH2-terminal kinase; and (c) the level of mitogen-activated protein kinase (extracellular signal-regulated kinase 2) protein. However, extracellular signal-regulated kinase 2 activity was induced 5-10-fold by SSeCKS in the presence of active src. SSeCKS reversed the ability of v-src to decrease the formation of vinculin-associated adhesion plaques, actin-based stress fibers, and filopodia structures. These data suggest a tumor suppressive role for SSeCKS via the control of cytoskeletal architecture and cell signaling.

3T3 Cells↗
PubMed1997

Dose- and time-dependent relations between infective Anopheles inoculation and outcomes of Plasmodium falciparum parasitemia among children in western Kenya.

Blood-stage level Plasmodium falciparum infection (parasitemia density) is generally elevated prior to, or at the time of, clinical presentation of severe pediatric malaria episodes. Intensity of exposure to infective Anopheles mosquito bites is a suspected determinant of higher density parasitemia. Analyses of entomologic and parasitologic data collected in 1986-1987 were conducted to investigate whether the dose of infective bites predicted the incidence or degree of P. falciparum parasitemia in Kenyan children < 6 years old. At 21 consecutive 30-day intervals, a new cohort (n approximately 50 each) was enrolled, cured of malaria parasites, and monitored over 84 days for recurrent parasitemia. Outcomes included time to parasitemia, time to parasitemia > or = 5,000/microliter, and parasitemia density. Ecologic and individual-level analyses were conducted. The mean infective bite exposure experienced by each cohort was significantly associated with the incidence of parasitemia (age-adjusted r2 = 0.38, p = 0.022) and more strongly associated with the incidence of parasitemia > or = 5,000/microliter (age-adjusted r2 = 0.72, p < 0.001). The infective bite dose, analyzed as a time-dependent covariate, was associated with a 2.8 times higher rate of parasitemia > or = 5,000/microliter among children exposed to > or = 1 infective bite per day as compared with the referent (rate ratio (RR) = 2.82, 95% confidence interval (CI) 2.24-3.56). Cumulative infective bite exposure, exposure duration, and age were significant predictors of recurrent parasitemia density in multiple linear regression analyses. The results support the contention that reductions in P. falciparum transmission intensity, in the absence of complete elimination, will reduce higher level parasitemia among African children.

Animals↗
PubMed1997

Insulin-like growth factor I (IGF-I) is a critical trophic factor for developing cerebellar granule cells.

In this study we showed that insulin-like growth factor I (IGF-I) directly increased cell survival in pure cerebellar granule cell cultures established from postnatal day 7 (P7) mice. The maximal survival-promoting effect could be obtained at low IGF-I concentrations (3-5 ng/ml). Withdrawal of IGF-I from differentiated granule neurons resulted in neuronal death which suggests that IGF-I has a survival-promoting effect on differentiated granule neurons. Furthermore, the survival-promoting effect of IGF-I was not attenuated by the addition of K252a, a selective blocker of Trk signaling, indicating that the survival-promoting effect of IGF-I did not require or was not mediated by endogenously produced neurotrophins, such as BDNF and NT3. Further experiments also suggest that IGF-I stimulates proliferation of granule cell precursors and allows terminal granule neuron differentiation to occur, as indicated by the expression of terminal differentiation markers MEF2A and GABA(A) alpha6. Thus, IGF-I could potentially function as both a mitogen and a trophic factor for developing granule cells. This dual action of IGF-I may be important in regulating granule neuron number.

Animals↗
PubMed1997

Acetylcholine, outer hair cell electromotility, and the cochlear amplifier.

The dominant efferent innervation of the cochlea terminates on outer hair cells (OHCs), with acetylcholine (ACh) being its principal neurotransmitter. OHCs respond with a somatic shape change to alterations in their membrane potential, and this electromotile response is believed to provide mechanical feedback to the basilar membrane. We examine the effects of ACh on electromotile responses in isolated OHCs and attempt to deduce the mechanism of ACh action. Axial electromotile amplitude and cell compliance increase in the presence of the ligand. This response occurs with a significantly greater latency than membrane current and potential changes attributable to ACh and is contemporaneous with Ca2+ release from intracellular stores. It is likely that increased axial compliance largely accounts for the increase in motility. The mechanical responses are probably related to a recently demonstrated slow efferent effect. The implications of the present findings related to commonly assumed efferent behavior in vivo are considered.

Acetylcholine↗