Common synaptic phenotypes arising from diverse mutations in the human NMDA receptor subunit GluN2A

Abstract

Dominant mutations in the human gene GRIN2A, encoding NMDA receptor (NMDAR) subunit GluN2A, make a significant and growing contribution to the catalogue of published single-gene epilepsies. Understanding the disease mechanism in these epilepsy patients is complicated by the surprising diversity of effects that the mutations have on NMDARs. We have examined the cell-autonomous impact of 5 severe GluN2A mutations by measuring NMDAR-mediated synaptic currents (NMDAR-EPSCs) in CA1 pyramidal neurons following rescue with human GluN2A mutants. Surprisingly, prolonged NMDAR-EPSC current decay and smaller peak amplitudes were common features of both gain- and loss-of-function mutants despite there being drastic differences between their effects on receptor function and enrichment at synapses. Modelling of NMDARs with mutant properties in CA1 neurons indicates that mutant NMDARs may contribute to broadening of depolarizations during bursts of high-frequency synaptic activity. Overall, the implication is that similar therapeutic approaches may be more widely applicable to patients with GRIN2A-related disorders irrespective of their molecular defect.

Complex functional phenotypes of NMDA receptor disease variants

“…For a multifunctional protein with numerous physiological outputs such as the NMDA receptor, a single mutation may exhibit both gain- and loss-of-function properties under different stimuli for different physiological outputs (Figure 4). This may explain recent observations showing that disease variants on GluN2A and GluN2B classified as either gain- or loss-of-function based on microscopic parameters behavior largely indistinguishably in vivo 32, 33 . This will impact how variants should be classified and, thus, how to tailor treatment to individuals harboring a specific mutation.…”

“…The copyright holder for this preprint (which this version posted July 2, 2022. ; https://doi.org/10.1101/2022.07.01.498520 doi: bioRxiv preprint acterization and in vivo behavior difficult 32,33 . To begin to explain how NMDA receptor mutations alter receptor functions and how to restore pharmacologically their normal operation, it is necessary to outline the mechanism by which individual residues contribute to the receptor's normal operation and how their substitution alters receptor responses.…”

“…Of the patient-derived variants that have been classified as pathogenic, fewer than half have been examined functionally, and even fewer have a proposed mechanism 1,5,6,[19][20][21][22][23][24][25][26][27][28][29] . In part, this is because the activation mechanism of NMDA receptors is complex and insufficiently understood 30,31 making correlation between in vitro char-acterization and in vivo behavior difficult 32,33 . To begin to explain how NMDA receptor mutations alter receptor functions and how to restore pharmacologically their normal operation, it is necessary to outline the mechanism by which individual residues contribute to the receptor's normal operation and how their substitution alters receptor responses.…”

Complex functional phenotypes of NMDA receptor disease variants

“…For a multifunctional protein with numerous physiological outputs such as the NMDA receptor, a single mutation may exhibit both gain-and loss-of-function properties under different stimuli for different physiological outputs (Figure 4). This may explain recent observations showing that disease variants on GluN2A and GluN2B classi ed as either gain-or loss-of-function based on microscopic parameters behavior largely indistinguishably in vivo 32,33 . This will impact how variants should be classi ed and, thus, how to tailor treatment to individuals harboring a speci c mutation.…”

“…Of the patient-derived variants that have been classi ed as pathogenic, fewer than half have been examined functionally, and even fewer have a proposed mechanism 1,5,6,[19][20][21][22][23][24][25][26][27][28][29] . In part, this is because the activation mechanism of NMDA receptors is complex and insu ciently understood 30,31 making correlation between in vitro characterization and in vivo behavior di cult 32,33 . To begin to explain how NMDA receptor mutations alter receptor functions and how to restore pharmacologically their normal operation, it is necessary to outline the mechanism by which individual residues contribute to the receptor's normal operation and how their substitution alters receptor responses.…”