Biomedical subjects

Xuewu Zhang

Publications and source records attributed to Xuewu Zhang.

7 recordsLinked to original sources

PubMed2006

An allosteric mechanism for activation of the kinase domain of epidermal growth factor receptor.

The mechanism by which the epidermal growth factor receptor (EGFR) is activated upon dimerization has eluded definition. We find that the EGFR kinase domain can be activated by increasing its local concentration or by mutating a leucine (L834R) in the activation loop, the phosphorylation of which is not required for activation. This suggests that the kinase domain is intrinsically autoinhibited, and an intermolecular interaction promotes its activation. Using further mutational analysis and crystallography we demonstrate that the autoinhibited conformation of the EGFR kinase domain resembles that of Src and cyclin-dependent kinases (CDKs). EGFR activation results from the formation of an asymmetric dimer in which the C-terminal lobe of one kinase domain plays a role analogous to that of cyclin in activated CDK/cyclin complexes. The CDK/cyclin-like complex formed by two kinase domains thus explains the activation of EGFR-family receptors by homo- or heterodimerization.

Allosteric Regulation↗

PubMed2004

Structural and functional analysis of the costimulatory receptor programmed death-1.

PD-1, a member of the CD28/CTLA-4/ICOS costimulatory receptor family, delivers negative signals that have profound effects on T and B cell immunity. The 2.0 A crystal structure of the extracellular domain of murine PD-1 reveals an Ig V-type topology with overall similarity to the CTLA-4 monomer; however, there are notable differences in regions relevant to function. Our structural and biophysical data show that PD-1 is monomeric both in solution as well as on cell surface, in contrast to CTLA-4 and other family members that are all disulfide-linked homodimers. Furthermore, our structure-based mutagenesis studies identify the ligand binding surface of PD-1, which displays significant differences compared to those present in the other members of the family.

Amino Acid Sequence↗

PubMed2004

[Study on extraction and isolation of active constituents from Sorbaria sorbifolia and antitumor effect of the constituents in vivo].

OBJECTIVE: To study the constituents from Sorbaria sorbifolia and their antitumor activities. METHODS: The constituents were isolated with silia gel column chromatography and identified by physicochemical properties and spectroscopic analysis. Thirty six mice were inoculated with sarcocarcinoma 180 according to the standard method, Sorbaria sorbifolia ethyl acetate etract was administrated for 10 days, tumor and body weight, the activities of NK cell and the contents of TNF-alpha, IL-2 in serum were detected. RESULTS: Three compounds were isolated from the ethyl acetate extract and identified as 5,2',4'-trihydroxy-6,7,5'-trimethoxyflavone(I), succinic acid(II) and p-hydroxybenzoic acid(III). The tumor of the test groups were lighter than that of control group(P < 0.05). There was no remarkable difference between the control and test groups in body weight (P > 0.05). The activity of NK cell and the contents of TNF-alpha, IL-2 in serum were higher than that of control group (P < 0.05). CONCLUSION: Compounds I and II were isolated from Sorbaria sorbifolia ethyl acetate extract for the first time. The ethyl acetate extract could improve immune function and strengthen antitumor effect in sarcocarcinoma 180.

Animals↗

PubMed2004

[Experimental study on Sorbaria sorbifolia extract against chronic liver damage in rats].

OBJECTIVE: To study the effects of Sorbaria sorbifolia extract (SSE) on DMN-induced chronic liver damage in rats. METHODS: Seventy-two rats were divided into four groups, normal, model, SSE and colchicin group. Serum hyaluronic (HA) and type precollagen (PCIII) were detected by radioimmunoassay. Degrees of fibrosis and alpha-smooth muscle actin (alpha-SMA) were determined by histopathology under microscope. Hydroxyproline (Hyp) and the markers of liver function were examined also. RESULTS: The levels of serum ALT, AST, ALb, AKP in SSE groups were notably improved as compared with the model group. Serum HA, PCIII and Hyp of hepatic tissue in SSE groups were obviously lower than those of model group. The markers of oxidative stress( SOD, MDA, GSH-Px) were strikingly improved also in SSE groups. HE stains and alpha-SMA expression showed that the degrees of liver fibrosis in SSE groups was slighter than that of model group. There was significant difference between the model group and SSE groups (P < 0.05 or P < 0.01). The rats in SSE groups were better than those of colchinicines group. CONCLUSION: SSE is able to protect against DMN-induced chronic liver damage, which may act through oxidative stress.

Animals↗

PubMed2003

Crystal structure of the receptor-binding domain of human B7-2: insights into organization and signaling.

B7-1 and B7-2 are homologous costimulatory ligands expressed on the surfaces of antigen-presenting cells. Their interactions with CD28/CTLA-4 receptors expressed on T cell surfaces are crucial for the proper regulation of T cell activity. B7-1 and B7-2 display distinct roles in immune regulation, although they are usually considered to have redundant functions. Here, we report the crystal structure of the receptor-binding (Ig V-type) domain of human B7-2 at 2.7-A resolution. Structures of unliganded and liganded B7-1 and B7-2 suggest a physical-chemical basis for the observed functional similarities and differences between these two costimulatory ligands. Of particular note, whereas the majority of the residues mediating B7-1 dimerization are hydrophobic, the B7-2 dimer observed in the B7-2/CTLA-4 complex displays a very hydrophilic dimer interface. These differences provide a mechanism for preventing the formation of B7-1/B7-2 heterodimers. The divergence at the putative dimer interface is also consistent with the lower tendency of B7-2 to dimerize, as shown by the monomeric state of unliganded B7-2 both in solution and crystalline form, and may result in detailed differences in signaling mechanisms associated with B7-1 and B7-2.

Abatacept↗

PubMed2002

Expression, refolding, purification, molecular characterization, crystallization, and preliminary X-ray analysis of the receptor binding domain of human B7-2.

The cell-mediated immune response involves a series of specific molecular interactions between cell surface molecules on T cells and antigen-presenting cells. Of particular importance for the regulation of T cell activity is the interaction of the B7 isoforms, B7-1 and B7-2, with the T cell surface costimulatory receptors, CD28 and CTLA-4. The binding of CD28 by B7-1/B7-2 results in an enhancement of T cell responses initiated by the interaction between a clonotypic T cell receptor and its specific, antigenic MHC-peptide complex, whereas the subsequent engagement of CTLA-4 by B7-1/B7-2 leads to a down-regulation of the response. Here we report the expression, refolding, purification, characterization, and crystallization of the receptor-binding domain of human B7-2. The receptor-binding domain of human B7-2 was overexpressed in Escherichia coli as inclusion bodies, solubilized in 6 M guanidine-hydrochloride, and then refolded in vitro by rapid dilution into a renaturing buffer. Refolded B7-2 was subsequently purified to homogeneity by anion-exchange chromatography. Gel-filtration chromatography and native PAGE analysis showed that the receptor-binding domain of B7-2 is exclusively monomeric in solution. Purified B7-2 binds tightly to bacterially expressed monomeric and disulfide-linked homodimeric human CTLA-4 as shown by gel-filtration chromatography and native PAGE. This suggests that glycosylation is not important for the proper folding of the receptor-binding domain of B7-2 nor for its binding to CTLA-4. In addition, these results suggest that refolded B7-2 is biologically active and may be a useful therapeutic and experimental reagent for regulating T cell activity. Refolded and purified B7-2 was crystallized by the hanging-drop vapor diffusion method, allowing for the initiation of an X-ray crystallographic study.

Abatacept↗

PubMed2002

Structural mechanisms of costimulation.

Recent studies have highlighted the structural requirements for T cell costimulation and have revealed unusual modes of dimerization for the cytolytic T lymphocyte-associated antigen 4 (CTLA-4) costimulatory receptor and its B7 ligands. These distinctive quaternary structures potentially endow both receptor and ligand with bivalent binding properties, which suggests a number of mechanistic features relevant to signaling. These include the potential to form a highly ordered, alternating network of CTLA-4 and B7 homodimers that may represent the organization of these molecules and their associated signaling partners within the immunological synapse. Primary sequence and structural considerations suggest that some aspects of the organizational and mechanistic features associated with the CTLA-4-B7 complexes may extend to other members of the costimulatory receptor-ligand family. An examination of the signaling mechanisms within the costimulatory receptor-ligand family provides an excellent framework to consider the general principles that are relevant to cell surface receptor-mediated signaling events.

Abatacept↗