Functional assessment of triheteromeric NMDA receptors containing a human variant associated with epilepsy

Abstract

Key points: NMDA receptors are neurotransmitter-gated ion channels that are critically involved in brain cell communication Variations in genes encoding NMDA receptor subunits have been found in a range of neurodevelopmental disorders. We investigated a de novo genetic variant found in patients with epileptic encephalopathy that changes a residue located in the ion channel pore of the GluN2A NMDA receptor subunit. We found that this variant (GluN2A^N615K ) impairs physiologically important receptor properties: it markedly reduces Mg²⁺ blockade and channel conductance, even for receptors in which one GluN2A^N615K is co-assembled with one wild-type GluN2A subunit. Our findings are consistent with the GluN2A^N615K mutation being the primary cause of the severe neurodevelopmental disorder in carriers.

Abstract: NMDA receptors are ionotropic calcium-permeable glutamate receptors with a voltage-dependence mediated by blockade by Mg²⁺ . Their activation is important in signal transduction, as well as synapse formation and maintenance. Two unrelated individuals with epileptic encephalopathy carry a de novo variant in the gene encoding the GluN2A NMDA receptor subunit: a N615K missense variant in the M2 pore helix (GRIN2A^C1845A ). We hypothesized that this variant underlies the neurodevelopmental disorders in carriers and explored its functional consequences by electrophysiological analysis in heterologous systems. We focused on GluN2A^N615K co-expressed with wild-type GluN2 subunits in physiologically relevant triheteromeric NMDA receptors containing two GluN1 and two distinct GluN2 subunits, whereas previous studies have investigated the impact of the variant in diheteromeric NMDA receptors with two GluN1 and two identical GluN2 subunits. We found that GluN2A^N615K -containing triheteromers showed markedly reduced Mg²⁺ blockade, with a value intermediate between GluN2A^N615K diheteromers and wild-type NMDA receptors. Single-channel conductance was reduced by four-fold in GluN2A^N615K diheteromers, again with an intermediate value in GluN2A^N615K -containing triheteromers. Glutamate deactivation rates were unaffected. Furthermore, we expressed GluN2A^N615K in cultured primary mouse cortical neurons, observing a decrease in Mg²⁺ blockade and reduction in current density, confirming that the variant continues to have significant functional impact in neuronal systems. Our results demonstrate that the GluN2A^N615K variant has substantial effects on NMDA receptor properties fundamental to the roles of the receptor in synaptic plasticity, even when expressed alongside wild-type subunits. This work strengthens the evidence indicating that the GluN2A^N615K variant underlies the disabling neurodevelopmental phenotype in carriers.

Keywords: Epilepsy; Genetics; Human; electrophysiology; intellectual disability.