Structural Basis of Ligand-gated Ion Channel Function
The biological actions of serotonin (5-HT) are mediated by at least 14 different receptors in seven different families (5-HT1 to 5-HT7). Unlike the other 5-HT receptors, which are G-protein coupled receptors (GPCR's), the 5-HT3R is a ligand-gated cation channel. The 5-HT3R has been implicated in a number of physiological and pathophysiological processes. 5-HT3R antagonists such as granisetron have selective antiemetic activity when nausea is induced by anesthesia, cytotoxic chemotherapy, or radiation. This activity allows patients to tolerate higher doses and longer treatment with chemotherapy agents, and, along with the prevention of post-operative nausea and vomiting, represents the main therapeutic use of 5-HT3R antagonists. 5-HT3R antagonists, however, are not without side effects, including arrhythmias. As these side effects are most likely mediated through interactions with other targets, there is a need for more selective antagonists, which is where elucidation of the structural features of the ligand-binding domain may provide useful information.
Ligand-gated ion channels such as the nicotinic acetylcholine receptor (AChR), the GABA type A (GABAAR) and type C (GABACR) receptors, the glycine receptor (GlyR), and the 5-HT3R play important roles in signaling in the central and peripheral nervous systems, and are targets for a number of therapeutic agents. These receptors exhibit a great deal of homology between each other and are considered to be members of a single gene family- the cys-loop ligand-gated ion channel (LGIC) family. The members of this family are transmembrane proteins that transduce the binding of a small-molecule agonist into a series of conformational changes that ultimately result in the opening of an ion-selective channel. A central question concerning cys-loop LGIC's is what is the architecture of the ligand-binding site, and how does agonist binding to a site far removed from the ion channel induce channel opening?
We use an approach that combines molecular biology, biochemistry, electrophysiology, and (eventually) structural biology to analyze the structural basis of ligand-gated ion channel function, in particular the architecture of the ligand-binding domains. A series of well-defined, mutant channels or receptors, which differ from the wild-type by one or two amino acids, are generated using site-directed mutagenesis. The mutant receptors are then expressed in transfected mammalian cells, where their properties are studied using ligand-binding and electrophysiological assays. Detailed comparison of the functional properties of the mutant receptors with those of the wild-type can elucidate the structural features that underlie various aspects of ion channel or receptor function and expression. Our current studies focus on architecture of the ligand-binding domain of the 5-HT3R. A great deal of work has been done on the nicotinic AChR by a number of different laboratories, and important structural details have emerged concerning the transduction of agonist binding to AChR gating. However, it is not clear to what extent these features are conserved among other members of the cys-loop LGIC family, and so we have focused our efforts on another member of the gene family, the 5-HT3R, in order to determine common vs receptor-specific aspects of the transduction process.
Selected Publications:
"The role of Loop F residues in determining differential d-tubocurarine potencies in mouse and human 5-hydroxytryptamine3A receptors"
Zhang, R, Wen, X, Militante, J, Hester, B, Rhubottom, HE, Sun, H, Leidenheimer, NJ, Yan, D, White, MM, and Machu, TK
Biochemistry 46:1194-1204 (2007).
"Pretty subunits all in a row: Using concatenated subunit constructs to force the expression of receptors with defined subunit stoichiometry and spatial arrangement"
White, MM
Mol. Pharmacol. 68:365-371 (2006).
"Mapping residues in the ligand-binding domain of the 5-HT3 receptor onto d-tubocurarine structure"
Yan, D, Meyer, J, and White, MM
Mol. Pharmacol. 70:571-578 (2006).
"Spatial orientation of the antagonist granisetron in the ligand-binding site of the 5-HT3 receptor"
Yan, D and White, MM
Mol. Pharmacol. 68:365-371 (2005).
"Interaction of d-tubocurarine analogs with mutant 5-HT3 receptors"
Yan, D and White, MM
Neuropharmacology 43: 367-373 (2002).
"Acetylcholine receptor gating is influenced by the polarity of amino acids at position 9’ in the M2 domain"
Kosolapov, A, Filatov, GN, and White, MM
J. Membr. Biol. 174:191-197 (2000).
"Structural features of the ligand-binding domain of the serotonin 5HT3 receptors"
Yan, D, Schulte, MK, Bloom, KE, and White, MM
J. Biol. Chem. 274:5537-5541 (1999).
"Interaction of d-tubocurarine analogs with the 5HT3 receptor"
Yan, D, Pedersen, SE, and White, MM
Neuropharmacology 37:251-257 (1998).
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