N-methyl-d-aspartate (NMDA) receptors (NMDARs) are a subtype of ionotropic glutamate receptor with important roles in CNS function. Since excessive NMDAR activity can lead to neuronal cell death and epilepsy, there is interest in developing NMDAR negative allosteric modulators (NAMs) as neuroprotective agents. In this study, we characterize the inhibitory properties of a novel NMDAR antagonist, UBP792. This compound displays partial subtype-selectivity by having a varied maximal inhibition of GluN2A-, GluN2B-, GluN2C-, and GluN2D-containing receptors (52%, 70%, 87%, 89%, respectively) with IC50s 4-10 μM. UBP792 inhibited NMDAR responses by reducing l-glutamate and glycine potencies and efficacies. Consistent with non-competitive inhibition, increasing agonist concentrations 30-fold did not reduce UBP792 potency. UBP792 inhibition was also not competitive with the structurally-related positive allosteric modulator (PAM) UBP684. UBP792 activity was voltage-independent, unaffected by GluN1's exon-5, and reduced at low pH (except for GluN1/GluN2A receptors which were more sensitive at acidic pH). UBP792 binding appeared independent of agonist binding and may be entering the plasma membrane to gain access to its binding site. Inhibition by UBP792 is reduced when the ligand-binding domain (LBD) of the GluN2 subunit, but not that of the GluN1 subunit, is cross-linked in the closed-cleft, activated conformation. Thus, UBP792 may be inhibiting by stabilizing an open GluN2-LBD cleft associated with channel inactivation or by stabilizing downstream closed channel conformations allosterically-coupled to the GluN2-LBD. These findings further expand the repertoire displayed by NMDAR NAMs thus expanding the opportunities for developing NMDAR modulators with the most appropriate selectivity and physiological actions for specific therapeutic indications.
Keywords: GluN2C; GluN2D; Glutamate; Glycine; Ligand-binding domain; N-methyl-d-aspartate receptor.
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