Biomedical subjects

J Xing

Publications and source records attributed to J Xing.

At least 19 recordsLinked to original sources

PubMed2001

Measurement and modeling of center-surround suppression and enhancement.

The apparent contrast of a central stimulus is affected by the presence of surrounding stimuli. For some stimulus conditions, the apparent contrast is suppressed and for other conditions the apparent contrast is enhanced. This report is intended to offer a coherent description of the stimulus factors that influence suppression and enhancement. Using a contrast-matching protocol, we measured the contrast dependence of center-surround interactions by systematically varying the suprathreshold contrasts of the central and surround gratings. Different spatial configurations of the surround stimuli were studied. Our results confirmed previous findings that (1) a surround stimulus could produce either contrast enhancement or contrast suppression depending on the balance of the central and surround contrasts; (2) suppression varied with the width of the surround stimulus and was strongly orientation-specific; and (3) enhancement was less sensitive to changes in surround configurations (in particular, enhancement did not depend on the colinearity of the central and surround gratings). Based on the experimental data, we developed a computational model to account for center-surround suppression and enhancement.

Contrast Sensitivity↗

PubMed2001

Development of NMR and raman spectroscopic methods for the determination of the degree of substitution of maleate in modified starches.

In this paper we report the application of NMR spectroscopy and Raman spectroscopy to determine the degree of maleate substitution in maleinated starches. Five kinds of maleinated starches were investigated and calibration sets were constructed to derive linear regression equations that may be used to predict the degree of maleate substitution for starch samples with unknown amounts of chemical modification. The calibration sets reported have very high linearity (r > 0.99) for both the NMR and Raman methods. The NMR and Raman calibration sets allow fast and nondestructive measurement of the degree of maleate substitution for different starches with little need of sample preparation.

Calibration↗

PubMed2001

p53 Stimulates TFIID-TFIIA-promoter complex assembly, and p53-T antigen complex inhibits TATA binding protein-TATA interaction.

Simian virus 40 large T antigen has been shown to inhibit p53-mediated transcription once tethered to p53-responsive promoters through interaction with p53. In this study we report that p53 stimulates transcription by enhancing the recruitment of the basal transcription factors, TFIIA and TFIID, on the promoter (the DA complex) and by inducing a conformational change in the DA complex. Significantly, we have demonstrated that T antigen inhibits p53-mediated transcription by blocking this ability of p53. We investigated the mechanism for this inhibition and found that DA complex formation was resistant to T-antigen repression when the TFIID-DNA complex was formed prior to addition of p53-T antigen complex, indicating that the T antigen, once tethered to the promoter by p53, targets TFIID. Further, we have shown that the p53-T antigen complex prevents the TATA binding protein from binding to the TATA box. Thus, these data suggest a detailed mechanism by which p53 activates transcription and by which T antigen inhibits p53-mediated transcription.

Antigens, Polyomavirus Transforming↗

PubMed2000

Alterations of prostate biomarker expression and testosterone utilization in human LNCaP prostatic carcinoma cells by garlic-derived S-allylmercaptocysteine.

BACKGROUND: This study determined the effects of S-allylmercaptocysteine (SAMC), a phytoconstituent from garlic, on the expression of androgen-responsive biomarkers, prostate specific antigen (PSA), and prostate specific membrane antigen (PSMA), in human prostatic carcinoma cells (LNCaP). METHODS: Secretion of PSA was determined as well as the activity of PSMA measured as a function of its ability to hydrolyze poly-gamma-glutamated folate and N-acetylaspartylglutamate (NAAG). Folate hydrolase capacity was also determined in SAMC-treated cells grown in charcoal stripped fetal calf serum (CS-FCS). In addition, testosterone disappearance was measured from culture media of SAMC-treated LNCaP and PC-3 cells as well as from cell free lysates. RESULTS: PSA secretions were significantly decreased compared to control values at 1 day (8.4 +/- 2.6 vs. 18.9 +/- 1.7, P < 0.01), 4 days (18.9 +/- 5.3 vs. 73.8 +/- 4. 4, P < 0.001), and 6 days (35.6 +/- 2.1 vs. 96.5 +/- 17.9 ng/10(5) cells, P < 0.01; mean +/- SD). By contrast, PSMA activity measured as either folate hydrolase or NAAG dipeptidase (NAALADase) activity increased in cells treated with SAMC. PSMA-folate hydrolase activity in SAMC-treated cells grown in CS-FCS increased beyond that observed in cells grown in CS-FCS alone. Pre-exposure of LNCaP cells to SAMC resulted in enhanced rate of testosterone disappearance from culture media at 6 hr (P < 0.01) and at 48 hr (P < 0.001) compared to media from cells not previously exposed to SAMC. Results similar to these were also observed in androgen-independent PC-3 cells treated with SAMC. In lysates of SAMC-treated LNCaP cells, the rate of testosterone catabolism was twice that from phosphate buffered saline (PBS)-treated cells. SAMC-treated LNCaP cells grown in media supplemented with testosterone temporarily exhibited enhanced growth over a 2 day period but cell numbers declined later to levels similar to those of SAMC treatment. CONCLUSIONS: These results show that SAMC exhibits differential effects on recognized biomarkers for LNCaP cells similar to those produced by androgen deprivation and strongly suggests that this effect may be mediated, in part, by diminishing the trophic effects of testosterone, likely by converting it to metabolites less reactive toward androgen receptors.

Adenocarcinoma↗

PubMed2000

Structural mapping of single cysteine mutants of cardiac troponin I.

The global conformation of cardiac muscle troponin I (cTnI) was investigated with single-cysteine mutants by using a combination of sulfhydryl reactivity and fluorescence resonance energy transfer (FRET) to determine cysteine accessibility and intersite distances. The reactivity was determined with a fluorescent reagent for its reaction with cysteine residues singly located at positions 5, 40, 81, 98, 115, 133, 150, 167, and 192. FRET measurements were made by using the endogenous single Trp-192 as the energy donor and an acceptor probe covalently attached to the cysteines as energy acceptor. The results suggest an open and extended conformation of cTnI with a large curvature in which the cysteines are highly exposed to the solvent. These conformational features are largely retained in the segment between residues 40 and 192 upon phosphorylation at Ser-23 and Ser-24. The sulfhydryl groups of the Cys-133 and Cys-150 of the cTnI incorporated into the binary cTnC-cTnI and fully reconstituted troponin complexes experience large reduced exposure resulting from the binding of Ca(2+) to the regulatory site of cTnC, suggesting that key regions of cTnI involved in activation become highly shielded upon activation. In the cTnC-cTnI complex, every intramolecular distance in the cTnI is lengthened and the overall conformation of the bound cTnI remains elongated with reduced exposure for the cysteines. The global conformation of the troponin C-troponin I complex from cardiac muscle has an elongated shape with constrained flexibility. The highly flexible nature of the N-terminal extension of cTnI is preserved in the complex, suggesting that this segment of cTnI is either not bound or only loosely bound to the C-domain of cTnC.

Amino Acid Substitution↗

PubMed2000

Kinesin has three nucleotide-dependent conformations. Implications for strain-dependent release.

Although crystallographic information is available on several nucleotide-induced states in myosin, little is known about the corresponding structural changes in kinesin, since a crystallographic model is only available for the kinesin:ADP complex. This makes it difficult to characterize at a molecular level the structural changes that occur in this motor through the course of its ATPase cycle. In this study, we report on the production of a series of single tryptophan mutants of a monomeric human kinesin motor domain, which demonstrate nucleotide-dependent changes in microtubule affinity that are similar to wild type. We have used these mutations to measure intramolecular distances in both strong and weak binding states, using fluorescence resonance energy transfer. This work provides direct evidence that movement of the switch II loop and helix are essential to mediate communication between the catalytic and microtubule binding sites, evidence that is supported as well by molecular modeling. Kinetic studies of fluorescent nucleotide binding to these mutants are consistent with these distance changes, and demonstrate as well that binding of ADP produces two structural transitions, neither of which are identical to that produced by the binding of ATP. This study provides a basis for understanding current structural models of the kinesin mechanochemical cycle.

Adenosine Diphosphate↗

PubMed2000

Kinetic and spectroscopic evidence for three actomyosin:ADP states in smooth muscle.

Smooth muscle myosin II undergoes an additional movement of the regulatory domain with ADP release that is not seen with fast skeletal muscle myosin II. In this study, we have examined the interactions of smooth muscle myosin subfragment 1 with ADP to see if this additional movement corresponds to an identifiable state change. These studies indicate that for this myosin:ADP, both the catalytic site and the actin-binding site can each assume one of two conformations. Relatively loose coupling between these two binding sites leads to three discrete actin-associated ADP states. Following an initial, weakly bound state, binding of myosin:ADP to actin shifts the equilibrium toward a mixture of two states that each bind actin strongly but differ in the conformation of their catalytic sites. By contrast, fast myosins, including Dictyostelium myosin II, have reciprocal coupling between the actin- and ADP-binding sites, so that either actin or nucleotide, but not both, can be tightly bound. This uncoupling, which generates a second strongly bound actomyosin ADP state in smooth muscle, would prolong the fraction of the ATPase cycle time that this actomyosin spends in a force-generating conformation and may be central to explaining the physiologic differences between this and other myosins.

Actins↗

PubMed2000

Investigations of the active site of Saccharomyces cerevisiae dolichyl-phosphate-mannose synthase using fluorescent labeled dolichyl-phosphate derivatives.

Dolichol-phosphate mannose (Dol-P-Man) is a key mannosyl donor for the biosynthesis of N-linked oligosaccharides as well as for O-linked oligosaccharides on yeast glycoproteins, and for the synthesis of the glycosyl-phosphatidylinositol anchor found on many cell surface glycoproteins. It is synthesized by Dol-P-Man synthase which is the only glycosyltransferase in the dolichol pathway that has been expressed as an active protein, solubilized and purified in large enough quantities for structural investigations. Earlier studies showed that the enzyme is closely associated with membranes of endoplasmic reticulum with unique lipid requirements for its maximal activity. This potential target of antibiotic therapy is now being investigated at the molecular level to establish information about the structure of the enzyme as well as determine the nature and properties of the enzyme-phospholipid interactions. In this paper, we have determined the activities of the fluorescent labeled dolichyl-phosphate derivatives as well as the intramolecular distances between amino acid residues near the active site and/or the fluorophores of the substrate derivatives using fluorescence energy resonance transfer. These results also show that the conserved consensus sequence is not required by Dol-P-Man synthase neither for the recognition of Dol-P nor for the catalytic activity.

Amino Acid Sequence↗

PubMed2000

Synthesis of dolichyl phosphate derivatives with fluorescent label at the omega-end of the chain, new tools to study protein glycosylation.

Derivatives of dolichyl phosphate (Dol-P) with 2-aminopyridine or 1-aminonaphtalene fluorophore groups at the omega-end of the chain were synthesized. These products serve as substrates for recombinant yeast Dol-P-mannose synthase. Fluorescence resonance energy transfer between a Trp residue of the enzyme and the 1-aminonaphtalene group of the Dol-P analogue was demonstrated.

1-Naphthylamine↗

PubMed2000

Center-surround interactions in foveal and peripheral vision.

The perceived contrast of a central stimulus can be decreased (surround suppression) or increased (surround facilitation) by the presence of surround stimuli. In this report we examined center-surround interactions in foveal and peripheral vision using contrast-matching tasks. We found that: (1) surround suppression became markedly stronger as the center-surround stimulus was moved toward the periphery; (2) surround facilitation diminished in the periphery; and (3) the suppression in the periphery was less orientation- and frequency-specific than that in the fovea, so that significant suppression was induced even when the central and surround gratings had very different orientations and spatial frequencies. The different center-surround interactions in the fovea and periphery can not be accounted for by cortical magnification, suggesting that center-surround interactions in the fovea and periphery are incommensurable and play different functional roles in human image processing.

Contrast Sensitivity↗

PubMed2000

Allograft tolerance induced by cyclophosphamide without prior inoculation of donor cells--immune suppression and redirection.

OBJECTIVES: To determine the possibility and cellular mechanism of inducing allograft tolerance by multiple injection of a lower dose of cyclophosphamide without prior infusion of donor cells. METHODS AND RESULTS: Heterotopic heart grafts were performed in MHC mismatched strain combinations (C57/B6 vs. BALB/c). Cyclophosphamide (40 mg/kg) was given intravenously on days 0, 2, 4 and 7 without prior infusion of donor cells. Long-term (> 100 days) allograft survival with normal histology was achieved. The long-term survivors accepted the donor skin grafts, but rejected the third-party skin grafts. Cyclophosphamide treatment initially led to profound lymphocytopenia, inhibition of spontaneous blastogenesis and low levels of lymphocyte proliferation response to both donor and third-party antigens. Ultimately, donor-specific tolerance occurred demonstrated by normal levels of peripheral lymphocytes, spontaneous blastogenesis and lymphocyte proliferation response to third-party antigens, and low levels of lymphocyte proliferation response to donor antigen. A switch of cytokines from IFNgamma dominant to IL-4 dominant, a low level of IgM and a high level of IgG1 were found in tolerant mice. CONCLUSIONS: Allograft tolerance can be induced by a short course of cyclophosphamide without prior donor cell inoculation. Tolerance induced is characterized initially by non-specific immunosuppression, which progresses to donor-specific hyporesponsiveness associated with the development of a Th2 dominant cytokine response.

Animals↗

PubMed2000

Determination of the degree of succinylation in diverse modified starches by raman spectroscopy.

A method using Raman spectroscopy was recently developed for the determination of the degree of substitution of succinate in waxy maize starch. In this paper it is demonstrated that the method can be generalized to a wide range of starches of different amylose contents and botanical origins. Raman calibration sets were used to form regression equations for five types of succinylated starches, that is, waxy, regular, and two high-amylose maize samples (47 and 66% amylose, respectively) and wheat. The derived calibration curves can be used to find the degree of substitution in samples with unknown levels of succinylation. The Raman calibration lines had linear correlation coefficients of 0.995 or better and enable the fast and nondestructive determination of the degree of substitution of succinate for different types of starches with minimal sample preparation. Also discussed is the potential utility of Raman spectroscopy to simultaneously determine the degree of substitution of succinate and amylose content, using previously determined calibration curves developed for the amylose content of maize starches.

Amylose↗

PubMed2000

Effect of urea on volatile generation from Maillard reaction of cysteine and ribose.

Urea occurs naturally in many food products, and its presence affects food quality. However, little is known about its impact on flavor generation in food production. In this study, the urea contents in beef, pork, and chicken were determined. The effects of urea and pH on thermal flavor generation were investigated using the model system of cysteine with ribose, which was heated to the roasting temperature of 180 degrees C for 2 h at pH 5 and pH 8.5. The results revealed relatively large amounts of urea in these meats and demonstrated that pH affects aroma generation. Volatiles identified from the reaction system of ribose and cysteine showed that sulfur-containing compounds such as thiophenes, thiazoles, and thiophenethiols were the most abundant compounds. The addition of urea into the reaction mixture caused the disappearance or reduction in content of some sulfur-containing compounds but resulted in the generation of several important nitrogen-containing volatiles, like pyrazine, methylpyrazine, 2,5- (and 2,6-)dimethylpyrazine and other alkylpyrazines, which are known to elicit roasty, nutty flavor notes. A plausible explanation for this phenomenon is that ammonia can be released from urea upon heating and the formed ammonia competes with hydrogen sulfide to react with Maillard reaction precursors to produce nitrogen-containing compounds such as alkylpyrazines.

Cysteine↗

PubMed2000

An interdomain distance in cardiac troponin C determined by fluorescence spectroscopy.

The distance between Ca2+-binding site III in the C-terminal domain and Cys35 in the N-terminal domain in cardiac muscle troponin C (cTnC) was determined with a single-tryptophan mutant using bound Tb3+ as the energy donor and iodoacetamidotetramethylrhodamine linked to the cysteine residue as energy acceptor. The luminescence of bound Tb3+ was generated through sensitization by the tryptophan located in the 12-residue binding loop of site III upon irradiation at 295 nm, and this sensitized luminescence was the donor signal transferred to the acceptor. In the absence of bound cation at site II, the mean interdomain distance was found to be 48-49 A regardless of whether the cTnC was unbound or bound to cardiac troponin I, or reconstituted into cardiac troponin. These results suggest that cTnC retains its overall length in the presence of bound target proteins. The distribution of the distances was wide (half-width >9 A) and suggests considerable interdomain flexibility in isolated cTnC, but the distributions became narrower for cTnC in the complexes with the other subunits. In the presence of bound cation at the regulatory site II, the interdomain distance was shortened by 6 A for cTnC, but without an effect on the half-width. The decrease in the mean distance was much smaller or negligible when cTnC was complexed with cTnI or cTnI and cTnT under the same conditions. Although free cTnC has considerable interdomain flexibility, this dynamics is slightly reduced in troponin. These results indicate that the transition from the relaxed state to an activated state in cardiac muscle is not accompanied by a gross alteration of the cTnC conformation in cardiac troponin.

Amino Acid Substitution↗

PubMed2000

Memory activity of LIP neurons for sequential eye movements simulated with neural networks.

Many neurons in macaque lateral intraparietal cortex (LIP) maintain elevated activity induced by visual or auditory targets during tasks in which monkeys are required to withhold one or more planned eye movements. We studied the mechanisms for such memory activity with neural network modeling. Recurrent connections among simulated LIP neurons were used to model memory responses of LIP neurons. The connection weights were computed using an optimization procedure to produce desired outputs in memory-saccade tasks. One constraint for the training process is the "single-purpose" rule, which mimics the fact that once LIP neurons hold the memory activity of a saccade, they are insensitive to further stimuli until the motor action is completed. After training, excitatory connections were developed between units with similar preferred saccade directions, while inhibitory connections were formed between units with dissimilar directions. This "push-pull" mechanism enables the network to encode the next intended eye movement and is essential for programming sequential saccades. In simulating double saccades, the push-pull connections locked the on-going activity in the network for the first saccade until the saccade was made, then a new population of units became active to prepare for the second saccade. The simulated LIP neurons exhibited sensory responses and memory activities similar to those recorded in LIP neurons. We propose that push-pull recurrent connections might be the basic structure mediating the memory activity of area LIP in planning sequential eye movements.

Animals↗

PubMed2000

Models of the posterior parietal cortex which perform multimodal integration and represent space in several coordinate frames.

Many neurons in the posterior-parietal cortex (PPC) have saccadic responses to visual and auditory targets. The responses are modulated by eye position and head position. These findings suggest that PPC integrates multisensory inputs and may provide information about saccadic targets represented in different coordinate frames. In addition to an eyecentered output representation, PPC may also project to brain areas which contain head-centered and body-centered representations of the space. In this report, possible coordinate transformations in PPC were examined by comparing several sets of models of PPC, each having different representations in the output layer: (i) an eye-centered map only; (ii) a head-centered map only; (iii) an eye-centered map and a head-centered map; and (iv) an eye-centered map, a head-centered map, and a body-centered map. These output maps correctly encoded saccades to visual and auditory targets through training. The units in the hidden layers of the models exhibited the following properties: (1) The units had gain fields (GFs) for eye position, and also for head position if the model had a body-centered output representation; (2) As the result of the GF and the nonlinear activation function of the units, the hidden layers often employed "intermediate" coding, e.g., the hidden units coded targets partially in eye-centered coordinates and, partially, in head-centered coordinates; (3) Different types of coordinate transformations in these models were carried out by different relationships between the receptive fields (RFs) and the GFs of the hidden units; and (4) The properties of PPC neurons are in better accordance with the hidden units of the models that had multiple-output representations than the models that had only one single-output representation. In conclusion, the results show that the GF is an effective mechanism for performing coordinate transformations. The models also suggest that neurons with intermediate coding are to be expected in the process of coordinate transformations.

Algorithms↗

PubMed2000

[Recognition of multiple information of human auricular points by linear model].

The three variables of the electric characteristic of human auricular points can be regarded as observed targets that reflect the physiologic or pathologic changes. Medical principle design follows the theory of the channels and collaterals and the viscera-state doctrine of traditional Chinese medicine and that of the pattern recognition. The linear integration model can be established by drawing the characteristic variables of the correlation groups of the auricular points and applying the multi-variable normalization. With this model, the recognition of the three variations of the auricular points can be of use for differential diagnosis. In this paper, the test to identify the cases of Upper-Digestive tract achieves good result. This research, based on the integration of modern science with traditional Chinese medicine, provides a method for building the specialist system of the auricular point diagnosis, and probably has some prospect in clinical application.

Acupuncture Points↗

PubMed1999

Complement inhibition with an anti-C5 monoclonal antibody prevents hyperacute rejection in a xenograft heart transplantation model.

BACKGROUND: The present study was undertaken to determine whether anti-complement 5 (C5) monoclonal antibodies (mAb) prevent hyperacute rejection (HAR) in a rat-to-presensitized mouse heart transplantation model and whether these mAb, combined with cyclosporine (CsA) and cyclophosphamide (CyP), can achieve long-term graft survival. METHODS: BALB/c mice were presensitized with 2x10(7) splenocytes from Lewis rats 14 days before grafting. Heart grafts from Lewis rats were heterotopically transplanted into BALB/c mice. Presensitized mice were treated with either anti-C5 mAb or a combination of anti-C5 mAb, CsA, and CyP. Controls included: presensitized mice with no treatment, presensitized mice treated with either CsA + CyP or IgG, and nonpresensitized mice with either no treatment or with CsA + CyP treatment. RESULTS: Although typical features of HAR were evident in the presensitized grafts, the mAb completely inhibited complement activation and successfully prevented HAR. Despite complement inactivation, the graft was rejected on postoperative day 6 with acute vascular rejection (AVR) also known as delayed xenograft rejection (DXR). Notably, this type of rejection cannot be effectively overcome by CsA and CyP. CONCLUSIONS: We conclude that (1) anti-C5 mAb prevents HAR, (2) AVR/DXR still occurs when HAR is prevented by complement inactivation, and (3) AVR/DXR cannot be overcome by conventional immunosuppression. These data suggest that anti-C5 mAb may be valuable for preventing HAR in future clinical xenotransplantation and that additional interventions may be required to address AVR/DXR.

Acute Disease↗