Allostery in NMDA Receptors

Abstract

Ionotropic glutamate receptors, of which the N-methyl-D-aspartate receptor (NMDAR) is one subtype, play a crucial role in excitatory neurotransmission within the mammalian central nervous system. In order to fully understand the role the NMDAR plays in neurotransmission, the conformational states and transitions that the receptor undergoes must be understood. Although the structures of the various conformational states of the NMDAR are known, the conformational changes by which the NMDAR transitions from state to state are not well understood. One question concerning NMDAR dynamics that remains to be answered is that of the nature of allosteric communication between the glycine binding sites and glutamate binding sites on the various subunits of the heterotetrameric receptor. In order to understand this allosteric communication, we utilized both electrophysiology and single molecule Förster Resonance Energy Transfer (smFRET) to examine the changes in the ligand binding domain (LBD) brought about by the binding of glycine and/or glutamate. The electrophysiological studies showed that there is negative cooperativity between the glycine and glutamate binding sites. The smFRET studies allowed the observation of the various states that a single receptor adopts under various ligand conditions. These single molecule experiments demonstrated that the conformational dynamics of the NMDAR LBD change when the receptor is in the presence or absence of various ligands; the NMDAR exhibits altered conformational dynamics when it is bound to glycine, glutamate, both, or neither. This data illustrates a potential mechanism for the negative allosteric effects between the glycine and glutamate binding sites that was observed in the electrophysiological studies.