<p class="first" id="d5467238e273">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.
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<b>SIGNIFICANCE STATEMENT</b> We have used a “bedside-to-bench” approach to investigate
the functional regulation
of NMDA receptors (NMDARs). Using information from deep sequencing of patients with
neurological or psychiatric disorders, we investigated missense variants identified
in the intracellular C-terminal domain of the GluN2B NMDAR subunit. We found several
variants that displayed altered properties. In particular, one variant identified
in a patient with autism, human GluN2B S1415L, displayed reduced surface expression
and binding to PSD-95. Furthermore expression of GluN2B S1415L (S1413L in mouse) showed
a deficit in rescue of synaptic NMDAR currents and fewer dendritic spines, consistent
with other reports of spine abnormalities being associated with autism. More broadly,
we demonstrate that using patient data is an effective approach to probing the structure/function
relationship of NMDARs.
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