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Yongchang Chang

Publications and source records attributed to Yongchang Chang.

8 recordsLinked to original sources
PubMed2006

Mechanism of action of benzodiazepines on GABAA receptors.

Wild-type and mutant alpha1beta2gamma2 GABA(A) receptors were expressed in Xenopus laevis oocytes and examined using the two-electrode voltage clamp. Dose-response relationships for GABA were compared in the absence and presence of 1 microM diazepam (DZP) or methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM). The dose-current relationships yielded EC(50)'s (concentration for half-maximal activation) of 41.0+/-3.0, 21.7+/-2.7, and 118.3+/-6.8 microM for GABA, GABA plus DZP, and GABA plus DMCM, respectively.DZP- and DMCM-mediated modulation were examined in GABA(A) receptors in which the beta-subunit carries the L259S mutation. This mutation has been shown to produce spontaneous opening and impart a leftward shift in the dose-response relationship. In this case, neither DZP nor DMCM produced a significant alteration in the GABA dose-response relationship with GABA EC(50)'s of 0.078+/-0.005, 0.12+/-0.03, and 0.14+/-0.004 microM for GABA, GABA plus 1 microM DZP, and GABA plus 1 microM DMCM.DZP- and DMCM-mediated modulations were examined in GABA(A) receptors in which the alpha-subunit carries the L263S mutation. This mutation also produced spontaneous opening and a leftward shift of the GABA dose-response relation, but to a lesser extent than that of betaL259S. In this case, the leftward and rightward shifts for DZP and DMCM were still present with EC(50)'s=0.24+/-0.03, 0.14+/-0.02, and 1.2+/-0.04 microM for GABA, GABA plus 1 microM DZP, and GABA plus 1 microM DMCM, respectively.Oocytes expressing ultrahigh levels of wild-type GABA(A) receptors exhibited currents in response to 1 muM DZP alone, whereas DMCM decreased the baseline current. The DZP-mediated activation currents were determined in wild-type receptors as well as receptors in which the GABA binding site was mutated (beta2Y205S). The EC(50)'s for DZP-mediated activation were 72.0+/-2.0 and 115+/-6.2 nM, respectively, similar to the EC(50) for DZP-mediated enhancement of the wild-type GABA-activated current (64.8+/-3.7 nM). Our results support a mechanism in which DZP increases the apparent affinity of the receptor, not by altering the affinity of the closed state, but rather by shifting the equilibrium towards the high-affinity open state.

Algorithms↗
PubMed2006

Evolutionarily conserved allosteric network in the Cys loop family of ligand-gated ion channels revealed by statistical covariance analyses.

The Cys loop family of ligand-gated ion channels mediate fast synaptic transmission for communication between neurons. They are allosteric proteins, in which binding of a neurotransmitter to its binding site in the extracellular amino-terminal domain triggers structural changes in distant transmembrane domains to open a channel for ion flow. Although the locations of binding site and channel gating machinery are well defined, the structural basis of the activation pathway coupling binding and channel opening remains to be determined. In this paper, by analyzing amino acid covariance in a multiple sequence alignment, we have identified an energetically interconnected network in the Cys loop family of ligand-gated ion channels. Statistical coupling and correlated mutational analyses along with clustering revealed a highly coupled cluster. Mapping the positions in the cluster onto a three-dimensional structural model demonstrated that these highly coupled positions form an interconnected network linking experimentally identified binding domains through the coupling region to the gating machinery. In addition, these highly coupled positions are also condensed in the transmembrane domains, which are a recent focus for the sites of action of many allosteric modulators. Thus, our results revealed a genetically interconnected network that potentially plays an important role in the allosteric activation and modulation of the Cys loop family of ligand-gated ion channels.

Allosteric Regulation↗
PubMed2004

Mapping the rho1 GABA(C) receptor agonist binding pocket. Constructing a complete model.

Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian brain. The GABA receptor type C (GABA(C)) is a ligand-gated ion channel with pharmacological properties distinct from the GABA(A) receptor. To date, only three binding domains in the recombinant rho1 GABA(C) receptor have been recognized among six potential regions. In this report, using the substituted cysteine accessibility method, we scanned three potential regions previously unexplored in the rho1 GABA(C) receptor, corresponding to the binding loops A, E, and F in the structural model for ligand-gated ion channels. The cysteine accessibility scanning and agonist/antagonist protection tests have resulted in the identification of residues in loops A and E, but not F, involved in forming the GABA(C) receptor agonist binding pocket. Three of these newly identified residues are in a novel region corresponding to the extended stretch of loop E. In addition, the cysteine accessibility pattern suggests that part of loop A and part of loop E have a beta-strand structure, whereas loop F is a random coil. Finally, when all of the identified ligand binding residues are mapped onto a three-dimensional homology model of the amino-terminal domain of the rho1 GABA(C) receptor, they are facing toward the putative binding pocket. Combined with previous findings, a complete model of the GABA(C) receptor binding pocket was proposed and discussed in comparison with the GABA(A) receptor binding pocket.

Amino Acid Sequence↗
PubMed2004

Growth hormone alters epidermal growth factor receptor binding affinity via activation of extracellular signal-regulated kinases in 3T3-F442A cells.

Epidermal growth factor receptor (EGFR) is a transmembrane protein that binds EGF in its extracellular domain and initiates signaling via intrinsic tyrosine kinase activity in its cytoplasmic domain. EGFR is important in development, cellular proliferation, and cancer. GH is a critical growthpromoting and metabolic regulatory hormone that binds the GH receptor, thereby engaging various signaling pathways, including ERKs. Prior studies suggest cross-talk between the GH receptor and EGFR signaling systems. Using the GH- and EGF-responsive 3T3-F442A preadipocyte, we previously observed that GH, in addition to causing EGFR tyrosine phosphorylation, also induced EGFR phosphorylation that was detected by PTP101, an antibody reactive with ERK consensus phosphorylation sites. This latter phosphorylation was prevented by pretreatment with MAPK kinase (MEK)1 inhibitors, suggesting ERK pathway dependence. Furthermore, GH cotreatment with EGF markedly slowed EGF-induced EGFR degradation and down-regulation, thereby potentiating EGF-induced EGFR signaling. These effects were also MEK1 dependent and suggested ERK pathway-dependent influence of GH on EGF-induced EGFR postendocytic trafficking and signaling. We now explore the impact of GH on cell surface binding of EGF in 3T3-F442A cells. We found that GH pretreatment caused transient, but substantial, lessening of (125)I-EGF binding. Competitive binding experiments revealed that the decreased binding was primarily due to decreased affinity, rather than a change in the number of EGF binding sites. The effect of GH on EGF binding was concentration dependent and temporally correlated with GH-induced ERK activation and EGFR PTP101-reactive phosphorylation. Blockade of the MEK1/ERK but not the protein kinase C pathway, prevented GH's effects on EGF binding, and our results indicate that the mechanisms of GH- and phorbol-12-myristate-13-acetateinduced inhibition of EGF binding differ substantially. Overall, our findings suggest that GH can modulate both EGF binding kinetics and the EGFR's postbinding signaling itinerary in a MEK1/ERK pathway-dependent fashion.

3T3 Cells↗
PubMed2002

Desensitization mechanism of GABA receptors revealed by single oocyte binding and receptor function.

Prolonged exposure of most fast neurotransmitter-operated ion channels to agonist drives the receptors into a nonfunctional, or desensitized, state. Despite extensive investigation, desensitization remains a thoroughly characterized, yet poorly understood, process. Part of the difficulty in elucidating the mechanism of desensitization has been an inability to resolve the kinetics of both agonist binding and functional desensitization in the same set of operable receptors. To overcome this limitation, we applied single oocyte 3H-ligand binding and two-electrode voltage clamp to oocytes expressing recombinant alpha1beta2gamma2 GABA receptors. Using this approach, we report several observations fundamental to the mechanism of desensitization. First, we confirm that desensitization reversibly shifts GABA receptors into a high-affinity state. For [3H]GABA binding, the half-maximal binding of the desensitized state was approximately 0.040 microm. Second, we show that, upon agonist removal, this high-affinity state disappears with a time constant of 127 +/- 12 sec (n = 4), similar to the time constant for functional recovery from desensitization of 124 +/- 26 sec (n = 5). [3H]GABA, however, dissociates fourfold faster (tau = 30 +/- 2 sec; n = 3) than functional recovery, indicating that desensitized receptors need not be bound by GABA. These data provide direct evidence for a cyclical model of receptor desensitization.

Animals↗
PubMed2002

Cloning and characterization of glioma BK, a novel BK channel isoform highly expressed in human glioma cells.

Voltage-dependent large-conductance Ca2+-activated K+ channels (BK channels) are widely expressed in excitable and nonexcitable cells. BK channels exhibit diverse electrophysiological properties, which are attributable in part to alternative splicing of their alpha-subunits. BK currents have been implicated in the growth control of glial cells, and BK channels with novel biophysical properties have recently been characterized in human glioma cells. Here we report the isolation, cloning, and functional characterization of glioma BK (gBK), a novel splice isoform of hSlo, the gene that encodes the alpha-subunits of human BK channels. The primary sequence of gBK is 97% identical to its closest homolog hbr5, but it contains an additional 34-amino-acid exon at splice site 2 in the C-terminal tail of BK channels. hSlo transcripts containing this novel exon are expressed ubiquitously in various normal tissues as well as in neoplasmic samples, suggesting that the novel exon may modulate important physiological functions of BK channels. Expression of gBK in Xenopus oocytes gives rise to iberiotoxin-sensitive (IbTX) currents, with an IC(50) for IbTX of 5.7 nm and a Hill coefficient of 0.76. Single gBK channels have a unitary conductance of similar250 pS, and the currents show significantly slower activation and higher Ca2+ sensitivity than hbr5. Ca2+ sensitivity was enhanced specifically at physiologically relevant [Ca2+]i (100-500 nm). Examination of biopsies from patients with malignant gliomas has revealed specific overexpression of BK channels in gliomas compared with nonmalignant human cortical tissues. Importantly, tumor malignancy grades have correlated positively with BK channel expression, suggesting an important role for the gBK channel in glioma biology.

Alternative Splicing↗
PubMed2002

Site-specific fluorescence reveals distinct structural changes with GABA receptor activation and antagonism.

Neurotransmitter-operated ion channels, such as the GABA (gamma-aminobutyric acid) receptor, are important in fast synaptic transmission between neurons. Using site-specific fluorescent labeling and simultaneous electrophysiological analysis in Xenopus laevis oocytes expressing recombinant rho1 GABA receptors, we identified agonist-mediated molecular rearrangements at three positions within and near the agonist-binding pocket that were highly correlated with receptor activation. We also show that competitive antagonists induced distinct rearrangements on their own that stabilized the receptor in a closed state. Finally, the allosteric antagonist picrotoxin induced a global conformational change that was sensed in the subunit-subunit interface of the amino (N)-terminal domain, distant from its presumed site of action within the transmembrane domains. This first detection in real time of molecular rearrangements of a ligand-activated receptor provides insights into the structural correlates of activation, antagonism and allosteric modulation.

Allosteric Regulation↗