Alteration of Ca²⁺ permeability and sensitivity to Mg²⁺ and channel blockers by a single amino acid substitution in the N-methyl-D-aspartate receptor

The N-methyl-D-aspartate (NMDA) receptor plays an important role in glutamate-mediated neuronal plasticity and neurotoxicity in the central nervous system. This receptor is composed of a fundamental subunit (NMDAR1) and its potentiating subunits (NMDAR2A-NMDAR2D). The NMDA receptor is distinct from other glutamate receptor channels because of its high Ca²⁺ permeability and inhibition by selective cationic channel blockers such as Mg²⁺, Zn²⁺, and MK-801. In this study, we investigated the structural features that control Ca²⁺ permeation and channel blockade of the NMDA receptor by in vitro mutagenesis and expression in Xenopus oocytes. We constructed a series of mutations with single amino acid substitutions in the second transmembrane segment of NMDAR1 and examined channel properties of the resultant mutants in combined expression with the NMDAR2A subunit. Substitution of the asparagine with either glutamine or arginine altered both the Ca²⁺ permeability and the sensitivity to blockades by Mg²⁺ and MK- 801. These mutations also reduced the inhibitory effects of Zn²⁺ and an antidepressant, desipramine. Based on these results, we concluded that an asparagine ring formed in the central part of the channel-forming second transmembrane segments plays a critical role in determining the Ca²⁺ permeability and the inhibition of open channel blockers.