Structural Basis of Functional Transitions in Mammalian NMDA Receptors

Abstract

N-methyl-D-aspartate receptors (NMDARs) are ligand-gated ion channels that mediate calcium influx in excitatory synapses and play a crucial role in synaptic plasticity, learning, and memory function. In mammalian brains, the majority of NMDARs are heterotetramers composed of two copies each of GluN1 and GluN2 subunits to form a functional channel. Activation of NMDARs requires simultaneous binding of glycine and glutamate due to its heterotetrameric assembly. Conversely, competitive inhibition of NMDARs can be evoked by binding of either a glycine antagonist on GluN1 or a glutamate antagonist on GluN2. Despite the extensive efforts on unraveling the mechanism of channel gating and competitive inhibition of NMDARs, it remains elusive due to the lack of detailed structural information. Here, we present multiple cryo-EM structures in distinct ligand states at 4 Å or better. The structures elucidate conformational transitions and inter-subunit/domain reorientations across different ligand binding states, providing great insights of ligand-gating and subunit dependent competitive inhibition. Also, the structures reveal that activation and competitive inhibition of NMDARs is controlled by the tension of the linker between the ligand-binding domain and the transmembrane ion channel of the GluN2 subunit. Our results provide detailed mechanistic insights into NMDAR pharmacology, activation, and inhibition.