Mechanism of AMPA Receptor Modulation by Gamma-8

Abstract

Fast excitatory neurotransmission is mediated by ionotropic glutamate receptor (iGluR). AMPA-subtype ionotropic glutamate receptors (AMPARs) are associated with auxiliary subunits (transmembrane AMPA receptor regulatory proteins (TARPs), cornichons, cysteine knot proteins and GSG1); which regulate AMPAR assembly, trafficking, gating, and pharmacology. Of particular interest is the interactions with gamma-8 which results in re-sensitization of the currents where the transmembrane channel in the receptor initially undergoes closure due to desensitization but reopens at longer time and remain open in the continued presence of agonist. Here we have investigated the mechanism underlying this resensitization process using single molecule FRET and single channel functional recordings. Specifically we have determined changes at the AMPA receptor ligand binding domain dimer interface in presence of gamma-8 and compared it to the changes at this site observed in the presence of cyclothiazide, a drug that stabilizes the open channel state of the receptor. These studies show that while the overall states populated are similar in the two cases, the kinetics of transitions between states are different, with activation barrier between transitions being higher in the presence of gamma-8 . The single channel current recordings reflected these slower kinetics in the presence of gamma-8 and also showed very rapid channel openings and closing which were no reflected at the dimer interface suggesting that these may be localized changes at the transmembrane segments.