Mechanism of differential control of NMDA receptor activity by NR2 subunits

Abstract

Members of the NMDA family of glutamate-gated ion channels, which play an important role in the control of synaptic plasticity and excitotoxic neuronal death in the brain, exhibit different opening probabilities and pharmacological profiles according to the NR2 subunit that they possess (NR2A-NR2D). Gielen et al. show that the degree of closure of the clamshell-like N-terminal domains of NR2 subunits determines this functional diversity. N-methyl-D-aspartate (NMDA) receptors have different opening probabilities and pharmacological profiles according to the type of NR2 subunit they possess (NR2A to NR2D). The region formed by the NR2 amino-terminal domain, and the short linker connecting this to the agonist-binding domain, are now shown to be responsible for controlling the subunit-specific gating of NMDA receptors. N-methyl-d-aspartate (NMDA) receptors (NMDARs) are a major class of excitatory neurotransmitter receptors in the central nervous system. They form glutamate-gated ion channels that are highly permeable to calcium and mediate activity-dependent synaptic plasticity1. NMDAR dysfunction is implicated in multiple brain disorders, including stroke, chronic pain and schizophrenia2. NMDARs exist as multiple subtypes with distinct pharmacological and biophysical properties that are largely determined by the type of NR2 subunit (NR2A to NR2D) incorporated in the heteromeric NR1/NR2 complex1,3,4. A fundamental difference between NMDAR subtypes is their channel maximal open probability (Po), which spans a 50-fold range from about 0.5 for NR2A-containing receptors to about 0.01 for receptors containing NR2C and NR2D; NR2B-containing receptors have an intermediate value (about 0.1)5,6,7,8,9. These differences in Po confer unique charge transfer capacities and signalling properties on each receptor subtype4,6,10,11. The molecular basis for this profound difference in activity between NMDAR subtypes is unknown. Here we show that the subunit-specific gating of NMDARs is controlled by the region formed by the NR2 amino-terminal domain (NTD), an extracellular clamshell-like domain previously shown to bind allosteric inhibitors12,13,14,15, and the short linker connecting the NTD to the agonist-binding domain (ABD). The subtype specificity of NMDAR Po largely reflects differences in the spontaneous (ligand-independent) equilibrium between open-cleft and closed-cleft conformations of the NR2-NTD. This NTD-driven gating control also affects pharmacological properties by setting the sensitivity to the endogenous inhibitors zinc and protons. Our results provide a proof of concept for a drug-based bidirectional control of NMDAR activity by using molecules acting either as NR2-NTD ‘closers’ or ‘openers’ promoting receptor inhibition or potentiation, respectively.