The GluN3-containing NMDA receptors

Abstract

N-methyl-D-aspartate receptors (NMDARs) are heterotetrameric ion channels that play crucial roles in brain function. Among all the NMDAR subtypes, GluN1-N3 receptors exhibit unique agonist binding and gating properties. Unlike "conventional" GluN1-N2 receptors, which require both glycine and glutamate for activation, GluN1-N3 receptors are activated solely by glycine. Furthermore, GluN1-N3 receptors display faster desensitization, reduced Ca²⁺ permeability, and lower sensitivity to Mg²⁺ blockage compared to GluN1-N2 receptors. Due to these characteristics, GluN1-N3 receptors are thought to play critical roles in eliminating redundant synapses and pruning spines in early stages of brain development. Recent studies have advanced pharmacological tools for specifically targeting GluN1-N3 receptors and provided direct evidence of these glycine-activated excitatory receptors in native brain tissue. The structural basis of GluN1-N3 receptors has also been elucidated through cryo-EM and artificial intelligence. These findings highlight that GluN1-N3 receptors are not only involved in essential brain functions but also present potential targets for drug development.

Keywords: Ionotropic glutamate receptors; ligand-gated ion channels; pathology and physiology; protein prediction; synaptic transmission.

Figure 1.

Subunit homology of iGluRs, sequence alignment of GluN1 and GluN3, schematic model of di- or tri-heteromeric GluN3-containing NMDARs. a. The relationship between the iGluRs subunits. The iGluRs are classified into the NMDARs, AMPARs (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors), KARs (Kainate receptors), and GluDRs. The sequences Q05586, Q12879.1, Q13224.3, Q14957.3, Q15399.2, Q9R1M7.1, O60391.2, P42261.2, P42262.3, P42263.2, P48058.2, P39086.1, Q13002.1, Q13003.3, Q16099.2, Q16478.2, Q9ULK0.2, O43424.2, were analyzed. b. Schematic structure of the GluN1, GluN3A, and GluN3B subunits. c. Schematic models of several di- and tri-heterotetrameric GluN3-containing NMDARs with “1-3-1-3” or “1-2-1-3” stoichiometries.

Figure 2.

The spatial and subcellular expression pattern of GluN3. a. A schematic diagram illustrates GluN3 and other NMDAR subunits expression pattern in the adult brain. b. Schematic representation of changes in GluN3A and GluN3B expression during brain development and comparison of expression in major neuronal cells. c. GluN3B is expressed in the cultured neurons and co-located with the synaptophysin. Electron micrograph demonstrating GluN3B localized in the dendritic spine. rA/C, recurrent associational-commissural terminal; sp, dendritic spine. d. GluN1-GluN3 can be in both the presynaptic and postsynaptic areas. e. t-distributed stochastic neighbor embedding (t-sne) plots for the frontal cortex, the hippocampus, and the thalamus global expression patterns of Grin1, Grin3a, and Grin3b. Excerpt from [20] (a), [23, 28] (c), [25] (e), and these figures have been reproduced with permission from Oxford University Press (a), Springer nature (c), Elsevier (c), and Frontiers (e), respectively.

Figure 3.

Structural basis, sequence alignment, and Ligplot⁺ analysis of GluN1-N3 NMDA receptors. a. Structural model of GluN1-GluN3. b. Particle sequence alignment of GluN1, GluN2, and GluN3 (the predicted loop was shown in green and blue, respectively). c. A schematic illustrating ligand-protein interactions of glycine/CGP-78608-GluN1-lbd and glycine-GluN3A LBD. d, e structural model of glycine bound (d) or CNQX bound GluN1-N3A(e). f. density map and structure model of GluN1-N2A-N3A. g. predicted GluN1-N3A, GluN1-N3B, and GluN1-N3A-N3B structure models. Excerpt from PDB(8JF7) (f), [25] (g). Panel (g) has been reproduced with permission from Frontiers.

Figure 4.

Representative channel currents of GluN1-N3 and activation model. a. The current of GluN1-N3 was measured from the Xenopus laevis oocytes. b. The concentration-response relationship of GluN1-N3A with or without F484A, T518L, or both mutants in the Xenopus laevis oocytes. c. Native GluN1-N3A recording in the juvenile hippocampus from wild-type or GluN3A-KO animal by preincubation of CGP-67808. d. The activation schematic diagram of GluN1-N2A. e. The activation schematic model of GluN1-N3A with or without the presence of double mutants or CGP-78608. Excerpt from [7] (a), [40] (b), [45] (c), and these figures have been reproduced with permission from Springer nature (a), Elsevier (b), Nature Portfolio (c).

Figure 5.

Evidence for the presence of GluN3-containing triheteromeric NMDARs. a. The GluN1, GluN2A, and GluN2B subunits were immunoprecipitated with GluN3A (fused with GFP). b. The GluN1 and GluN2A subunits can be immunoprecipitated with GluN3B. c. The current of GluN1-GluN3A, GluN1-GluN3B, and GluN1-N3A-N3B are expressed in the Xenopus laevis oocytes. d. The assembly of GluN1, N3A, and N3B subunits. Excerpt from [55] (a), [52] (b), [47] (c), [57] (d), and these figures have been reproduced with permission from Elsevier (a), Society for Neuroscience (b), ASPET (c). and National Academy of Sciences (d).