AMPA Receptor Structure, Function, and Dynamics

Abstract

AMPA receptors (GluAs) are essential neuronal ligand-gated ion channels involved in learning and memory. The dimeric conformation of the GluA ligand-binding domain is involved in the coupling of agonist binding to channel gating. We have used NMR, crystallography, ITC, and single channel recording to study the mechanism of action of antagonists and partial agonists on the GluA2 receptor. Antagonists form stable but open cleft binding sites with little dynamics, with binding driven largely by enthalpy. On the other hand, considerable dynamics are observed in the binding site in the presence of partial agonists, whose binding, in most cases, has a large entropic component. Allosteric modulators bind to a large surface that is formed by the dimer interface of two ligand-binding domains in the resting and channel activated states. This binding prevents the dissociation of the dimer interface and inhibits desensitization of the receptor. The desensitized conformation is disrupted along the dimer interface; however, little is known about the dynamic equilibrium between the bound/dimerized form and the unbound/monomer forms. Using small angle x-ray scattering (SAXS), crystallography, and NMR spectroscopy, we developed an equilibrium model for modulator dependent dimerization. This model demonstrates that a second modulator-binding site produces both an increase in positive cooperativity and a higher apparent affinity. A combination of the crystal structures of the bound modulators and the binding model developed using SAXS data provide new clues for the development of more effective allosteric modulators that may have cognitive enhancing effects.