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Articles by X Niu
Total Records ( 4 ) for X Niu
  X Ren , J Zhang , X Gong , X Niu , X Zhang , P Chen and X. Zhang

The kidney is formed from two tissue populations derived from the intermediate mesoderm, the ureteric bud, and the metanephric mesenchyme. Metanephric mesenchyme is a pluripotent renal stem population, and conversion of renal mesenchyme into epithelia depends on the ureteric bud in vivo and in vitro. Embryonic stem (ES) cells have been induced to differentiate into a broad spectrum of specialized cell types in vitro, including hematopoietic, pancreatic, and neuronal cells. Such ES-derived cells can provide a valuable source of progenitor cells. However, whether ES cells can be stimulated by factors secreted from the fetal renal cells to differentiate into renal precursor cells in vitro has not been reported. In this study, we showed that murine ES cells can give rise to embryoid bodies in the absence of leukemia inhibitory factor. Culture conditions were optimized [6 days, 10 ng/ml activin and 10–7 M retinoic acid (RA)] to generate maximal mesoderm populations specifically expressing Pax2 and brachyury. Results showed that 72% of the cells were brachyury positive by fluorescent activated cell sorter on Day 6 of EB cell differentiation. Conditioned medium collected from cultures of ureteric bud cells from renal cells of a 13-day-old fetus was added to the culture medium. Mesoderm cells were cultured for up to 10 days before showing expression of renal markers, initiation of nephrogenesis (WT-1 and Pax2), and terminally differentiated renal cell types (POD-1 and E-cadherin). This study suggests that ES cells pre-treated by RA and activin can interact with secreted molecules of the fetal renal cells to specifically differentiate into renal precursor cells. Our results provide an experimental basis for the development of in vitro assays to steer differentiation of ES cells toward renal lineages.

  X Wang , Q Li , X Niu , H Chen , L Xu and C. Qi

Sspg1d, one of endopolygalacturonases, is an important fungal effector secreted by the necrotrophic fungus Sclerotinia sclerotiorum during early infection. Using sspg1d as bait, a small C2 domain protein (designated as IPG-1) was identified by yeast two-hybrid screening of a canola cDNA library. Deletion analysis confirmed that the C-terminus of IPG-1 is responsible for its interaction with sspg1d in the yeast two-hybrid assay. The sspg1d/IPG-1 interaction was further confirmed in plant cells by a biomolecular fluorescence complementation (BiFC) assay. A transient expression assay showed that the IPG-1–GFP fusion protein was targeted to the plasma membrane and nucleus in onion epidermal cells. Following treatment with a Ca2+ ionophore, it was distributed throughout the cytosol. Real-time PCR assay demonstrated that IPG-1 was highly induced by Sclerotinia sclerotiorum in canola leaves and stems. Southern blot analysis indicated the presence of about five homologues of IPG-1 in the canola genome. Two additional members of the IPG-1gene family were isolated by RT-PCR. Their sequence similarity with IPG-1 is as high as 95%. However, they did not interact with sspg1d in the yeast two-hybrid assay. Possible roles of IPG-1 and its association with sspg1d in the defence signalling pathway were discussed.

  G Liu , X Niu , R. S Wu , N Chudasama , Y Yao , X Jin , R Weinberg , S. I Zakharov , H Motoike , S. O Marx and A. Karlin

Large-conductance voltage- and calcium-activated potassium (BK) channels contain four pore-forming subunits and four modulatory β subunits. From the extents of disulfide cross-linking in channels on the cell surface between cysteine (Cys) substituted for residues in the first turns in the membrane of the S0 transmembrane (TM) helix, unique to BK , and of the voltage-sensing domain TM helices S1–S4, we infer that S0 is next to S3 and S4, but not to S1 and S2. Furthermore, of the two β1 TM helices, TM2 is next to S0, and TM1 is next to TM2. Coexpression of with two substituted Cys’s, one in S0 and one in S2, and β1 also with two substituted Cys’s, one in TM1 and one in TM2, resulted in two s cross-linked by one β. Thus, each β lies between and can interact with the voltage-sensing domains of two adjacent subunits.

  C Shelley , X Niu , Y Geng and K. L. Magleby

Voltage-dependent gating mechanisms of large conductance Ca2+ and voltage-activated (BK) channels were investigated using two-dimensional maximum likelihood analysis of single-channel open and closed intervals. To obtain sufficient data at negative as well as positive voltages, single-channel currents were recorded at saturating Ca2+ from BK channels mutated to remove the RCK1 Ca2+ and Mg2+ sensors. The saturating Ca2+ acting on the Ca2+ bowl sensors of the resulting BKB channels increased channel activity while driving the gating into a reduced number of states, simplifying the model. Five highly constrained idealized gating mechanisms based on extensions of the Monod-Wyman-Changeux model for allosteric proteins were examined. A 10-state model without coupling between the voltage sensors and the opening/closing transitions partially described the voltage dependence of Po but not the single-channel kinetics. With allowed coupling, the model gave improved descriptions of Po and approximated the single-channel kinetics; each activated voltage sensor increased the opening rate approximately an additional 23-fold while having little effect on the closing rate. Allowing cooperativity among voltage sensors further improved the description of the data: each activated voltage sensor increased the activation rate of the remaining voltage sensors approximately fourfold, with little effect on the deactivation rate. The coupling factor was decreased in models with cooperativity from ~23 to ~18. Whether the apparent cooperativity among voltage sensors arises from imposing highly idealized models or from actual cooperativity will require additional studies to resolve. For both cooperative and noncooperative models, allowing transitions to five additional brief (flicker) closed states further improved the description of the data. These observations show that the voltage-dependent single-channel kinetics of BKB channels can be approximated by highly idealized allosteric models in which voltage sensor movement increases Po mainly through an increase in channel opening rates, with limited effects on closing rates.

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