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L Li

Publications and source records attributed to L Li.

At least 1,801 records · Page 100Linked to original sources
PubMed1992

Site-directed mutagenesis in rat liver 6-phosphofructo-2-kinase. Mutation at the fructose 6-phosphate binding site affects phosphate activation.

To identify those residues involved in fructose 6-phosphate binding to the kinase domain of rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase site-directed mutations were engineered at Lys194, Arg195, Arg230, and Arg238. The mutant enzymes were purified to homogeneity by anion exchange and Blue-Sepharose chromatography and/or substrate elution from phosphocellulose columns. Circular dichroism experiments demonstrated that all of the single amino acid mutations had no effect on the secondary structure of the protein. In addition, when fructose-2,6-bisphosphatase activity was measured, all mutants had Km values for fructose 2,6-bisphosphate, Ki values for fructose 6-phosphate, and maximal velocities similar to that of the wild-type enzyme. Mutation of Arg195----Ala, or His, had little or no effect on the maximal velocity of the kinase but increased the Km for fructose 6-phosphate greater than 3,000-fold. Furthermore, the Ka for phosphate for Arg195Ala was increased 100-fold compared with the wild-type enzyme. Mutation of Lys194----Ala had no effect on maximal velocity or the Km for fructose 6-phosphate. Mutation of either Arg230 or Arg238----Ala increased the maximal velocity and the Km for fructose-6 phosphate of the kinase by 2-3-fold but had no effect on fructose-2,6-bisphosphatase. However, the Km values for ATP of the Arg230Ala and Arg238Ala mutants were 30-40-fold higher than that for the wild-type enzyme. Mutation of Gly48----Ala resulted in a form with no kinase activity, but fructose-2,6-bisphosphatase activity was identical to that of the wild-type enzyme. The results indicate that: 1) Arg195 is a critical residue for the binding of fructose 6-phosphate to the 6-phospho-fructo-2-kinase domain, and that interaction of the sugar phosphate with Arg195 is highly specific since mutation of the adjacent Lys194----Ala had no effect on fructose 6-phosphate binding; 2) Arg195 also play an important role in the binding of inorganic phosphate; and 3) Gly48 is an important residue in the nucleotide binding fold of 6-phosphofructo-2-kinase and that both Arg230 and Arg238 are also involved in ATP binding; and 4) the bifunctional enzyme has two separate and independent fructose 6-phosphate binding sites.

Animals↗
PubMed1992

Human recombinant IL-3 is a growth factor for normal B cells.

IL-3 is a well known hemopoietic cell growth and differentiation factor. However, its functional role in normal B cell differentiation has not been established. We have investigated the effect of IL-3 on the growth and differentiation of human B cells. IL-3 enhanced the proliferation of Staphylococcus aureus Cowan 1 strain-stimulated B cells. The optimal time of IL-3 to stimulate B cell growth was on day 2 to day 3, suggesting that IL-3 was a B cell growth factor acting in the late stage. IL-3 synergized with IL-2 to enhance B cell proliferation and differentiation. Pretreatment of B cells with IL-3 for more than 3 days increased the expression of IL-2R on B cells. However, pretreatment of B cells with IL-2 did not alter the subsequent response to IL-3, suggesting that the synergy between IL-2 and IL-3 may be attributed to the up-regulation of IL-2 response by IL-3. In addition, pretreatment of B cells with IL-4 decreased subsequent response of B cells to IL-3 as well as IL-2, suggesting that IL-3- and IL-2-responding cells passed a similar way during the early stage of B cell activation. It appears that IL-3 and IL-6 mediate normal B cell differentiation via separate mechanisms. IL-3-induced B cell differentiation was mainly mediated by increasing cell growth, whereas IL-6 induced B cell differentiation without affecting proliferation.

B-Lymphocytes↗
PubMed1992

New magnetic resonance techniques for evaluating cerebrovascular disease.

Magnetic resonance (MR) imaging of acute stroke has made important contributions to diagnosis. Several novel MR technologies, now in preclinical and clinical development, will contribute to stroke diagnosis and perhaps help to guide therapy. MR angiography is the most clinically advanced new MR technology and offers the clinician a method to image noninvasively the extra- and intracranial vasculature. Diffusion-weighted MR imaging can demonstrate ischemic lesions quantitatively within minutes of onset in experimental stroke models, and human application is proceeding. Perfusion MR studies can reveal the presence or absence of cerebral perfusion in specific arterial territories. MR spectroscopy can assess tissue metabolites in vivo and reveal changes in these metabolites associated with ischemic injury. The combination of these new MR techniques should provide a plethora of information about the extent of ischemic lesions, associated vascular and perfusion deficits, and metabolic consequences. This information will afford the clinician the opportunity to assess and subtype ischemic stroke patients more rapidly and could be used to monitor therapeutic responses.

Animals↗
PubMed1992

MR imaging of anisotropic and restricted diffusion by simultaneous use of spin and stimulated echoes.

A new magnetic resonance imaging technique for evaluating anisotropic and restricted diffusion effects in a single experiment is described. The method is based on a pulse sequence that simultaneously excites a spin echo (SE) and a stimulated echo (STE) and contains two pairs of diffusion-sensitive gradient pulses. The diffusion attenuation of the SE and STE can be controlled independently by separate adjustment of the parameters associated with each pair of the diffusion-sensitive gradient pulses. This method can be more time efficient than acquiring separate SE and STE images for the same study. Furthermore, the pulse sequence minimizes the effect of motion and instrumental and physiological variations that occur between separate image acquisitions which can give rise to artifactual results when difference imaging is employed. In addition to applications to anisotropic and restricted diffusion, this technique can be used for: (1) simultaneous acquisition of T2-weighted and diffusion-weighted images during cerebral ischemia studies; (2) calculation of diffusion coefficients from a pair of diffusion-weighted images; (3) separating fast and slow diffusion components in a single imaging experiment; and (4) two-dimensional velocity-encoded imaging.

Animals↗
PubMed1992

Somatic cell mapping of T-cell receptor CD3 complex and CD8 genes in cattle.

Bovine genes encoding T-cell receptor, CD3, and CD8 molecules have been mapped to syntenic groups using bovine x rodent hybrid somatic cells. T-cell receptor alpha and delta chains were assigned to bovine syntenic group U5, and the beta and gamma genes were syntenic with each other and with markers on U13. CD3E and CD3D genes were syntenic with each other and located to bovine syntenic group U19. CD8 was most concordant with markers of syntenic group U16, although the concordancy was only 85% and the assignment must be regarded as tentative. The comparative gene maps of human chromosome 7, bovine syntenic group U13, and mouse chromosomes 6 and 13 suggest extensive evolutionary conservation.

Animals↗
PubMed1992

Nuclear receptor sites for vitamin D-soltriol in midbrain and hindbrain of Siberian hamster (Phodopus sungorus) assessed by autoradiography.

Autoradiograms were prepared from midbrains and hindbrains of male and female Siberian hamsters (Phodopus sungorus), kept under short-day or long-day illumination, after injection of tritium-labeled 1,25-dihydroxycholecalciferol (vitamin D, soltriol). Concentration and retention of radioactivity was noted in nuclei of certain neurons, glial cells, and ependymal cells, and in choroid epithelium. Labeled neurons of varying intensity were found throughout the brainstem in distinct populations at characteristic topographical sites, which include cranial nerve motor nuclei, the nucleus (n.) reticularis tegmenti pontis, the caudoventral region of the n. raphe dorsalis, the n. trapezoides, the n. vestibularis lateralis and n. vestibularis superior, neurons in the various nuclei of the sensory trigeminus, accessory optic nuclei, scattered neurons in nuclei of the reticular formation, the n. ambiguus, certain cells in the area postrema, and many others. Glial cells with nuclear labeling, probably microglia, were scattered predominantly in or near myelinated nerve fascicles. The choroid epithelium showed strong nuclear labeling throughout the ventricle. Nuclear labeling of ependyma was variable and weak, mainly at ventral and lateral extensions (recesses) of the ventricle. The extensive presence of nuclear binding in select neural structures indicates that vitamin D exerts specific genomic effects on cell populations that are known to be involved in the regulation of motor, sensory, autonomic, neuroendocrine, metabolic, and immune functions. The results of these studies, in conjunction with those from other brain and peripheral tissues, recognize vitamin D-soltriol as a steroid hormone with a wide scope of hormone-specific target cells, similar to estrogen, androgen, and adrenal steroids, and which are topographically distinct and characteristic for its functions as the steroid hormone of sunlight.

Animals↗
PubMed1992

Modulation of nicotinic acetylcholine receptor channel by pH: a difference in pH sensitivity of Torpedo and mouse receptors expressed in Xenopus oocytes.

1. In this study, effects of pH on the ion channel function of nicotinic acetylcholine receptor (nAChR) from Torpedo californica electroplax and mouse muscle BC3H-1 cells were investigated using Xenopus laevis oocytes injected with in vitro synthesized RNA transcripts. The acetylcholine (ACh)-induced whole-cell peak current responses and slow desensitization rates were measured by voltage-clamp. 2. The ACh-induced peak currents of Torpedo nAChRs were reversibly diminished when extracellular pH was reduced from 7.4 to 5.0 and increased at pH greater than 7.4. This pH dependence had an apparent pKa of 7.0. In contrast, the peak current of mouse muscle nAChRs were reversibly decreased at both acidic and alkaline pH's. This bell-shaped pH profile with a maximum at pH 7.4 had two apparent pKa values of 5.6 and 9.2. 3. The peak current responses of four mouse-Torpedo nAChR hybrids consisting of three Torpedo subunits and one mouse nAChR subunit displayed similar pH profiles at acidic pH, with a pKa of 6.0 to 6.5, which lay between the pKa 5.6 for mouse and the pKa 7.0 for Torpedo nAChRs. Two of these combinations, alpha M beta T gamma T delta T and alpha T beta T gamma M delta T, also had an alkaline pH dependence of the current response similar to that of mouse receptor, with a pKa of 9.3 to 9.5. 4. The slow desensitization rate of Torpedo nAChRs increased at both acidic and alkaline pH's, with two pKa values of 6.5 and 9.5, whereas that of mouse muscle receptors remained unchanged from pH 6.5 to pH 9.0 and increased at pH less than 6.0, with a pKa of 4.7. 5. Substitution of each subunit of Torpedo nAChR with a mouse counterpart resulted in a pH dependence pattern (pKa 6.0 to 6.4 and pKa 9.1 to 9.3) similar to that of Torpedo nAChR except the substitution with mouse beta subunit (pKa 4.8 and pKa 8.9), which appears to carry the characteristic acidic group determining the pH dependence of mouse nAChR desensitization. 6. The apparent pKa values obtained from the pH dependence studies of nAChR channel activation and desensitization suggest the involvement of different amino acid residues in determining channel functions of Torpedo and mouse nAChRs. The groups with pKa 7.0 and 6.5 on Torpedo nAChR can be tentatively identified as His residues, whereas those with pKa 5.6 and 4.7 on mouse receptor as Glu or Asp residues. The alkaline pKa of 8.9 to 9.5 may be Cys, Tyr, or Lys.

Allosteric Regulation↗
PubMed1992

Sensory nerve endings in the puborectalis and anal region of the fetus and newborn.

The study was carried out in seven fetuses (ovulation ages were 16, 18, 20, 22, 26, 28, and 31 weeks, respectively) and in five full-term newborn babies. The modified Bielschowsky method and Barker's block silver impregnation were used. It was found that the muscle spindle was arranged in the puborectalis and the external sphincter muscles from the 20th week onward; the tendon organ was located in association with the musculo-tendinous junction of the puborectalis from the 20th week onward; the Pacinian corpuscle lay in the plane of cleavage between the internal anal sphincter and the external anal sphincter from the 22nd week onward, and in the presacral space from the 28th week onward, and the later lagged behind the former in development; the globular endings lay in the presacral space from the 28th week onward; and the free nerve ending was distributed in the epithelium of the anal mucosa and the epidermis around the anus from the 31st week onward. These sensory nerve endings tend to mature gradually with increasing age. It was concluded that these endings may be responsible for the reflex of the external anal sphincter and the anal sensation.

Anal Canal↗
PubMed1992

Maintenance of opsin and S-antigen gene expression in RCS dystrophic rats following RPE transplantation.

The effects of retinal pigment epithelium (RPE) transplantation in rescuing photoreceptor cells (PRC) were studied in RCS rats bearing the rdy mutation. RPE preparations from Long-Evans rats were transplanted into the subretinal space of post-natal (P) 17-19-day-old RCS rats. Age-matched RCS animals in the control group received injections of the transplantation vehicle buffer alone. The animals were killed at various post-natal ages after RPE transplantation. Immunocytochemical studies using antibodies to opsin and S-antigen indicated intact photoreceptor cells in the transplanted retinas. In contrast age-matched sham-injected animals did not show any rescue of photoreceptor cells at P90. Single RPE transplantation of 60,000 cells in each eye resulted in restoration of near normal mRNA levels for opsin, and S-antigen proteins. Comparisons of mRNA levels of two visual proteins using cDNA probes demonstrated a therapeutic effect of RPE transplantation in preventing a progressive decline in mRNA levels due to retinal degeneration. In contrast, P109 sham-injected controls showed no detectable mRNA levels for these proteins. In vitro protein synthesis in RPE-transplanted retinas implied further competence of these retinas. These data suggest that RPE transplantation not only rescues photoreceptors from degeneration, but more importantly, it allows normal transcription and translation in these cells rendering them capable of participating in the visual and signal transduction cascade.

Animals↗
PubMed1992

t(13q;17p) and del(5q): possibly specific changes in Chinese patients with colorectal cancers.

Cytogenetic study of 18 colorectal carcinomas confirmed the extensive heterogeneity and the complexity of the karyotypic picture in this type of tumor. Karyotypic analysis showed that chromosomes 17p and 5q, in both numerical and structural aspects, were the most frequently involved chromosomes and prone to losses. The most common structurally rearranged forms were translocations of 17p with other chromosomes, especially t(13q;17p), which constituted over 50% of all 17p rearrangements, and an interstitial deletion of 5q that made up as much as 73% of all structural abnormalities of 5q. According to the results, we conclude that chromosomes 17 and 5 may play important roles in the evolution of colorectal cancer and t(13q;17p) and del(5q) may be possibly specific to Chinese patients with colorectal cancer.

Adult↗
PubMed1992

Binaural responses in rat inferior colliculus following kainic acid lesions of the superior olive: interaural intensity difference functions.

The binaural responses of cells in the rat's inferior colliculus were determined following either unilateral or bilateral lesions of the superior olivary complex (SOC). The lesions were made by local injection of kainic acid through a glass micropipette lowered stereotaxically into the auditory brain stem. After a recovery period of at least two weeks, neural responses to pure tone pulses were recorded from the inferior colliculus with tungsten microelectrodes inserted into the central nucleus. Attention was focused on neurons that exhibited binaural suppression, ie., were excited by contralateral and inhibited by bilateral stimulation. Binaural suppression responses were still present following complete unilateral destruction of the superior olive either ipsilateral or contralateral to the recording site. Binaural responses were also present following bilateral lesions of the SOC. Furthermore, the shape and slope of interaural intensity difference functions were not statistically different for normal animals and animals with either unilateral or bilateral SOC lesions. No differences were found among lesion groups or between recording sites ipsilateral or contralateral to the lesions. These data suggest that the superior olive is not essential for binaural interaction and that supraolivary structures play a significant role in shaping binaural responses in the inferior colliculus.

Acoustic Stimulation↗
PubMed1992

Left-handed first-episode, neuroleptic-naive schizophrenic patients have a higher prevalence of autoantibodies.

Using standard immunological techniques, sera from first-episode, neuroleptic-naive schizophrenic patients (n = 51) and age, race and sex matched healthy controls (n = 51) were screened for seven common autoantibodies. Significantly more left-handed (67%) than right-handed (23%) schizophrenic patients had autoantibodies (p = 0.011). Left-handed schizophrenic patients were six times more likely than right-handed patients or controls and four times more likely than left-handed controls to test positive for autoantibodies (p = 0.012). These data suggest that disease and sinistrality contribute to the excess of autoantibodies in schizophrenia.

Adult↗
PubMed1992

EPR and 1H and 13C dynamic nuclear polarization studies of a molecularly doped polymer: bisphenol A polycarbonate doped with trianisylamine and trianisylaminium perchlorate.

We have investigated the EPR and DNP behavior of a molecularly doped polymer modeling those used to transport electronic charge in electrophotography. The EPR spectra show no evidence of the superexchange reported for a closely related system based on tri-p-tolylamine. The difference may be due to larger charge-transfer matrix elements in the latter system. An unambiguous interpretation of the observed 1H DNP was rendered difficult by the unanticipated asymmetry of the EPR spectra. We report extensive data on the "three-spin" effect evident in the DNP-enhanced 13C NMR spectra, and comment on its potential for characterizing polymer interfaces.

Aniline Compounds↗
PubMed1992

Retinal pigment epithelial cell transplantation in RCS rats: normal metabolism in rescued photoreceptors.

Photoreceptor cells in Royal College of Surgeons (RCS) rats with inherited retinal dystrophy can be rescued by the transplantation of normal, wild-type retinal pigment epithelial (RPE) cells, if done before photoreceptor cell death. In the present study, we have examined several metabolic features of rescued photoreceptors and transplanted RPE cells at 2.3-3.3 months after transplantation. Rescued photoreceptors with a structurally normal RPE interface showed a rod outer segment renewal rate similar to that of normal control rats of 2.2 microns day-1, as measured in autoradiograms. Rod outer segment disc shedding had values indistinguishable from those in normal controls, as measured by the number of phagosomes in the transplanted RPE cells both during the burst of disc shedding soon after the onset of light in the morning and during the middle of the light cycle when disc shedding is low. The interphotoreceptor matrix, which is synthesized by both photoreceptors and the RPE, was distributed normally in the regions where normal-appearing photoreceptors were present underlying normal, transplanted RPE cells. Thus, the rescued photoreceptors show normal metabolic rates and normal interactions with the RPE in each of the parameters examined. These findings, combined with the previous demonstration of opsin and Na+,K(+)-ATPase expression by the rescued photoreceptors, support our interpretation that the surviving, normal-appearing photoreceptors may function normally. Moreover, transplantation of normal RPE cells reversed pathological changes in the photoreceptors that had already occurred by the time of transplantation.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

Regulation of muscle cell growth and differentiation by the MyoD family of helix-loop-helix proteins.

The skeletal muscle cell system provides a powerful model for exploring the mechanistic basis for the antagonism between cell growth and differentiation. The recent identification of the MyoD family of muscle-specific transcription factors now offers opportunities to dissect at the molecular level of the mechanisms through which defined cell type-specific transcription factors can activate an entire differentiation program as well as to unravel the mechanisms through which growth factor and oncogenic signals can disrupt cellular differentiation. Because the mechanisms for growth factor signaling and induction of cell proliferation are conserved in diverse cell types, it is tempting to speculate that the molecular mechanisms responsible for the antagonism between cell proliferation and differentiation in muscle cells are also operative in other cell types. Resolution of this question, however, must await identification of the regulatory factors that specify cell fate in other lineages.

Amino Acid Sequence↗

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