Can Activation and Desensitization Properties of iGluRs Be Predicted and Understood by Studying the LBD Dimer Dynamics?

Abstract

Ionotropic glutamate receptors (iGluRs) are vital for the function of our central nervous system (CNS), e.g. in learning and memory formation, and thus implicated in many CNS disorders. The tetrameric iGluRs contain a glutamate-gated cation channel with the extracellular ligand binding domains (LBD) forming a dimer of dimers. Subsequent to channel opening, iGluRs undergo conformational changes to a desensitized state with channel closure while glutamate remains bound. The LBD dimer interface plays important roles in activation and desensitization. The first step in desensitization is believed to involve opening and rearrangement of this dimer interface. The three major iGluR subtypes, AMPA, kainate and NMDA receptors, are similar in structure and sequence, yet show large diversity in activation and desensitization kinetics. AMPA receptors activate and desensitize rapidly whereas NMDA receptors have much slower kinetics. Sequence variation at the LBD dimer interface may play key roles in determining these properties of specific receptors. E.g., as opposed to other iGluRs, activation of GluK2 homotetrameric receptors requires ion binding to the LBD dimer interface. Using computational methods, equilibrium and biased molecular dynamics simulations at atomic resolution, we have characterized the influence of subtype differences and interface mutations on the dynamics of LBD dimers of GluA2 AMPA receptors, GluK2 kainate receptors and GluN1/GluN2A NMDA receptors. Surprising differences are revealed, e.g. that K759 of GluA2, which structurally appears to function like sodium in GluK2 by positioning the positive charge in the “cation site” of GluA2, dynamically has a destabilizing effect. Furthermore, interface stability has been investigated using steered molecular dynamics simulations. For the majority of cases, the amount of work required to open the dimer interface predicts whether a mutation has a destabilizing effect, thus producing a less active receptor.