A conserved glycine harboring disease-associated mutations is required for slow deactivation and high Ca²⁺ permeability in NMDA receptors

Abstract

A variety of de novo and inherited missense mutations associated with neurological disorders are found in the NMDA receptor M4 transmembrane helices, which are peripheral to the pore domain in eukaryotic ionotropic glutamate receptors. Subsets of these mutations affect receptor gating with dramatic effects, including in one instance halting it, occurring at a conserved glycine near the extracellular end of M4. Functional experiments and molecular dynamic simulations of constructs with and without substitutions at this glycine indicate that it acts as a hinge, permitting the intracellular portion of the ion channel to laterally expand. This expansion stabilizes long-lived open states leading to slow deactivation and high Ca 2+ permeability. Our studies provide a functional and structural framework for the effect of missense mutations on NMDARs at central synapses and highlight how the M4 segment may represent a pathway for intracellular modulation of NMDA receptor function. Word count: 144

GRIN2B-related neurodevelopmental disorder: current understanding of pathophysiological mechanisms

“…Most NMDARs in the cortex and hippocampus contain GluN2B, with levels peaking during the third postnatal week. A variety of strategies have been used to examine whether mutant GluN2B is properly trafficked, including surface immunolabeling of unpermeabilized (or mildly permeabilized) cells (Liu et al, 2017;Sceniak et al, 2019;Santos-Gómez et al, 2021), surface biotinylation-based approaches (Ogden et al, 2017), and fluorescent/luminescent/colorimetric reporter methods (Swanger et al, 2016;Amin et al, 2018;Li et al, 2019;Kellner et al, 2021). These studies showed that some variants produce GluN2B that is not trafficked to the cell surface, while others are trafficked less efficiently than or equivalent to wildtype.…”

“…Receptors that are never delivered to the cell surface cannot function as plasma membrane receptors and ion channels; for receptors that reach the cell surface, it is necessary to measure their channel properties directly. Electrophysiology and calcium imaging have been used to evaluate channel function (Swanger et al, 2016;Ogden et al, 2017;Amin et al, 2018;Fedele et al, 2018;Vyklicky et al, 2018;Li et al, 2019;Sceniak et al, 2019;Shin et al, 2020;Kellner et al, 2021;Santos-Gómez et al, 2021;Elmasri et al, 2022). Some mutants form NMDARs possessing channels that are completely non-functional.…”

“…Recent experiments have begun to address these questions by examining mutant localization and function in the presence of wild-type subunits. Several studies have examined the effects of disease-associated GluN2B variants in heterologous systems that co-express wild-type and mutant GluN2B (Swanger et al, 2016;Amin et al, 2018;Li et al, 2019;Kellner et al, 2021;Elmasri et al, 2022). Together, these studies suggest that GluN2B variants are capable of co-assembly with wild-type GluN2B subunits.…”