GluN1 splice variant control of GluN1/GluN2D NMDA receptors

Key points NMDA receptors are ionotropic glutamate receptors that mediate the slow component of excitatory synaptic transmission in the central nervous system. GluN1‐1a/GluN2D receptors, in which the GluN1 subunit does not contain exon‐5 encoded residues in the amino‐terminal domain, previously have been shown to have a low open probability and an exceptionally slow deactivation time course. In this study, we report that GluN1 splice variants containing residues encoded by exon 5 in their amino‐terminal domain influence agonist potencies, deactivation time course, and single channel properties of GluN2D‐containing NMDA receptors. We also show a model of NMDA receptor gating that can describe the key characteristics of the single channel and macroscopic current responses of GluN1/GluN2D NMDA receptors. These data demonstrate that the GluN1 amino‐terminal domain is an important determinant in the kinetic and pharmacological properties of GluN2D‐containing NMDA receptors. Abstract NMDA receptors are ionotropic glutamate receptors that mediate a slow, Ca 2+ ‐permeable component of excitatory synaptic transmission in the central nervous system. Recombinant GluN1‐1a/GluN2D receptors are characterized by low channel open probability and prolonged deactivation time course following the removal of agonist. Here, we show that the deactivation time course, agonist potency, and single channel properties of GluN2D‐containing NMDA receptors are modulated by alternative RNA splicing of GluN1. Our results demonstrate that GluN1 exon 5, which encodes a 21‐amino‐acid insert in the amino‐terminal domain, is a key determinant of GluN1/GluN2D receptor function. GluN1‐1b/GluN2D receptors, which contain the residues encoded by exon 5, deactivate with a dual exponential time course described by a τ FAST of 410 ms and a τ SLOW of 1100 ms. This time course is 3‐fold more rapid than that for exon 5‐lacking GluN1‐1a/GluN2D, which deactivates with a τ FAST of 1100 ms and a τ SLOW of 3400 ms. Exon 5‐containing NMDA receptors also have a two‐fold higher open probability (0.037) than exon 5‐lacking receptors (0.017). Furthermore, inclusion of exon 5‐encoded residues within the GluN1‐1b subunit decreases the potency for the endogenous agonist l ‐glutamate. Evaluation of receptor kinetics for NMDA receptors containing mutated GluN1‐1b subunits and wild‐type GluN2D identified residue Lys211 in GluN1‐1b as a key determinant of exon 5 control of the deactivation time course and glutamate potency. Evaluation of a kinetic model of GluN1/GluN2D gating suggests that residues encoded by exon 5 influence several rate‐limiting steps. These data demonstrate that the GluN1 subunit is a key determinant of the kinetic and pharmacological properties of GluN2D‐containing NMDA receptors.