The 6' and 7' Residue of the Second Transmembrane Domain of Ligand-Gated Ion Channels Influence Gating and Picrotoxin Sensitivity

Abstract

Gonzales, Eric B., The 6’ and 7’ residue of the second transmembrane domain of ligand-gated ion channels influence gating and picrotoxin sensitivity. Doctor of Philosophy (Pharmacology and Neuroscience), July 2005, pp213, 4 tables, 33 illustrations, 89 titles. The GABAA and glycine receptor are members of the Cys-loop family of ion channels. These receptors mediate rapid neurotransmission in the nervous system. Picrotoxin (PTX) interacts within the channel near the TM2 2’-6’ position, with the most critical interaction at the 6’ position. The present studied addressed the stoichiometric dependence and molecular requirements of the TM2 6’ position on PTX sensitivity. I hypothesized that there is not a stoichiometric dependence and that residues with a hydroxyl group remain sensitive to PTX. Further, work previously completed in the laboratory demonstrated that the TM2 7’ position influences channel kinetics in the serotonin type-3 receptor. However, similar work has not been performed in other members of the Cys-loop family of receptors. I hypothesize that the TM2 7’ position influences both gating in the glycine α1 receptor similarly to that in the serotonin type-3 receptor. Additionally, the TM2 7’ position could influence the PTX in the glycine α1 receptor. Picrotoxin sensitivity was determined to not be stoichiometrically dependent on the subunit location of the T6’F mutation in α1β2 or α1β2γ2 GABAA receptors, a single T6’F mutation was sufficient to eliminate PTX sensitivity. The α1(T6’F) β2 receptor showed PTX concentration-dependent stimulation. Picrotoxin sensitivity had a rank order of potency in α1β2(mutant) as follows: Serine [greater than] Threonine = Alanine [greater than] Cysteine [greater than] Tryptophan. In several cases, the kinetics of the T6’F mutant receptors exhibited rapid desensitization during prolonged application of agonist. In combinations of subunits with the T6’F mutation, two or fewer mutant receptors appeared to have normal phenotypes. Three or four T6’F mutations exhibited rapid desensitization, and strongly suggests that the stoichiometery of the α1β2 GABAA receptor is two α subunits and three β subunits in the receptor. The Gly α1(T7’L) mutation exhibited enhanced glycine sensitivity with slower gating kinetics than the wild type (approximately 3-fold slower); the T7’A mutation had significantly reduced glycine affinity. The glycine EC50 kinetics of the α1(T7’A) mutant receptor was more complex than the wild type. There appears to be a complex interaction between agonist binding and gating of the channel that is disrupted by the 7’ position mutations. Picrotoxin sensitivity in the mutation were enhanced with either a T7’A or T7’L substitution. The data suggest that the 7’ residue may play an accessory role in shaping the PTX site. Finally, these residues are also critical in gating of the receptor. Residues critical for gating and PTX sensitivity may be coupled in the normal function of the Cys-loop family of receptors.