A Rare Variant Identified Within the GluN2B C-Terminus in a Patient with Autism Affects NMDA Receptor Surface Expression and Spine Density

Abstract

NMDA receptors (NMDARs) are ionotropic glutamate receptors that are crucial for neuronal development and higher cognitive processes. NMDAR dysfunction is involved in a variety of neurological and psychiatric diseases; however, the mechanistic link between the human pathology and NMDAR dysfunction is poorly understood. Rare missense variants within NMDAR subunits have been identified in numerous patients with mental or neurological disorders. We specifically focused on the GluN2B NMDAR subunit, which is highly expressed in the hippocampus and cortex throughout development. We analyzed several variants located in the GluN2B C terminus and found that three variants in patients with autism (S1415L) or schizophrenia (L1424F and S1452F) (S1413L, L1422F, and S1450F in rodents, respectively) displayed impaired binding to membrane-associated guanylate kinase (MAGUK) proteins. In addition, we observed a deficit in surface expression for GluN2B S1413L. Furthermore, there were fewer dendritic spines in GluN2B S1413L-expressing neurons. Importantly, synaptic NMDAR currents in neurons transfected with GluN2B S1413L in GluN2A/B-deficient mouse brain slices revealed only partial rescue of synaptic current amplitude. Functional properties of GluN2B S1413L in recombinant systems revealed no change in receptor properties, consistent with synaptic defects being the result of reduced trafficking and targeting of GluN2B S1413L to the synapse. Therefore, we find that GluN2B S1413L displays deficits in NMDAR trafficking, synaptic currents, and spine density, raising the possibility that this mutation may contribute to the phenotype in this autism patient. More broadly, our research demonstrates that the targeted study of certain residues in NMDARs based on rare variants identified in patients is a powerful approach to studying receptor function.

A GluN2B mutation identified in Autism prevents NMDA receptor trafficking and interferes with dendrite growth

“…Recent genetic analysis of de novo mutations has identified additional ASD-associated mutations in GluN2B. Consistent with the observations reported here, several of these mutations produce NMDA receptors with reduced or abolished channel properties or trafficking to the cell surface in heterologous expression systems (Adams et al, 2014;Fedele et al, 2018;Liu et al, 2017;Swanger et al, 2016;Vyklicky et al, 2018). It will be important in future studies to compare the effects of distinct ASD mutations on NMDAR trafficking and function within neurons and to determine whether abnormal dendrite development is a common phenotypic outcome of ASD-associated GluN2B mutations.…”

“…The recurrence of de novo ASD-associated mutations in GRIN2B and rigorous analyses indicate that GRIN2B is a true ASD-associated gene (De Rubeis et al, 2014;Iossifov et al, 2014;O'Roak et al, 2012b;Stessman et al, 2017). However, we are only beginning to understand the impact of ASD-associated GRIN2B mutations on NMDAR function (Fedele et al, 2018;Liu et al, 2017;Vyklicky et al, 2018), and it is not yet clear how GRIN2B mutations alter neuronal development to cause ASD.…”

Changes in Serotonin Modulation of Glutamate Currents in Pyramidal Offspring Cells of Rats Treated With 5-MT during Gestation

“…[72], postulates that autism could be a hypoglutamatergic disorder; this is based on neuroanatomical and neuroimaging studies indicating alterations in regions rich in glutamatergic neurons, as well as the similarity of symptoms observed in autism and those produced by NMDA antagonists in healthy subjects. In support of this theory, variations of the gene encoding the GluN2B subunit, which does not allow for adequate trafficking and expression to the cell surface, have recently been found in some autistic and schizophrenic patients [73,74]. Concerning this, Yuen et al [75] demonstrated that activation of the 5-HT 1A receptor decreases the traffic and expression of the GluN2B subunit.…”

Chronic Subconvulsive Activity during Early Postnatal Life Produces Autistic Behavior in the Absence of Neurotoxicity in the Juvenile Weanling Period

“…and ablation of voltage-dependent Mg 2+ block that may result in ASD phenotypes [60,61] . Although not studied, these perturbations probably do not occur with the low doses of KA used.…”