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N -methyl-D-aspartate receptors (NMDARs) are ionotropic glutamate receptors that play an essential role in mediating excitatory neurotransmission in the mammalian central nervous system (CNS). Functional NMDARs are tetramers composed of GluN1, GluN2A-D, and/or GluN3A-B subunits, giving rise to a wide variety of NMDAR subtypes with unique functional properties. Here, we examined the surface delivery and functional properties of NMDARs containing mutations in the glycine-binding sites in GluN1 and GluN3A subunits expressed in mammalian cell lines and primary rat hippocampal neurons. We found that the structural features of the glycine-binding sites in both GluN1 and GluN3A subunits are correlated with receptor forward trafficking to the cell surface. In addition, we found that a potentially clinically relevant mutation in the glycine-binding site of the human GluN3A subunit significantly reduces surface delivery of NMDARs. Taken together, these findings provide novel insight into how NMDARs are regulated by their glycine-binding sites and may provide important information regarding the role of NMDARs in both physiological and pathophysiological processes in the mammalian CNS.

Although numerous pathogenic mutations have been identified in various subunits of N -methyl-D-aspartate receptors (NMDARs), ionotropic glutamate receptors that are central to glutamatergic neurotransmission, the functional effects of these mutations are often unknown. Here, we combined in silico modelling with microscopy, biochemistry, and electrophysiology in cultured HEK293 cells and hippocampal neurons to examine how the pathogenic missense mutation S688Y in the GluN1 NMDAR subunit affects receptor function and trafficking. We found that the S688Y mutation significantly increases the EC 50 of both glycine and d -serine in GluN1/GluN2A and GluN1/GluN2B receptors, and significantly slows desensitisation of GluN1/GluN3A receptors. Moreover, the S688Y mutation reduces the surface expression of GluN3A-containing NMDARs in cultured hippocampal neurons, but does not affect the trafficking of GluN2-containing receptors. Finally, we found that the S688Y mutation reduces Ca 2+ influx through NMDARs and reduces NMDA-induced excitotoxicity in cultured hippocampal neurons. These findings provide key insights into the molecular mechanisms that underlie the regulation of NMDAR subtypes containing pathogenic mutations.

Alzheimer’s disease (AD) is a complex disorder with unknown etiology. Currently, only symptomatic therapy of AD is available, comprising cholinesterase inhibitors and N-methyl-d-aspartate (NMDA) receptor antagonists. Drugs targeting only one pathological condition have generated only limited efficacy. Thus, combining two or more therapeutic interventions into one molecule is believed to provide higher benefit for the treatment of AD. In the presented study, we designed, synthesized, and biologically evaluated 15 novel fluoren-9-amine derivatives. The in silico prediction suggested both the oral availability and permeation through the blood–brain barrier (BBB). An initial assessment of the biological profile included determination of the cholinesterase inhibition and NMDA receptor antagonism at the GluN1/GluN2A and GluN1/GluN2B subunits, along with a low cytotoxicity profile in the CHO-K1 cell line. Interestingly, compounds revealed a selective butyrylcholinesterase (BChE) inhibition pattern with antagonistic activity on the NMDARs. Their interaction with butyrylcholinesterase was elucidated by studying enzyme kinetics for compound 3c in tandem with the in silico docking simulation. The docking study showed the interaction of the tricyclic core of new derivatives with Trp82 within the anionic site of the enzyme in a similar way as the template drug tacrine. From the kinetic analysis, it is apparent that 3c is a competitive inhibitor of BChE.

N-methyl-D-aspartate receptors (NMDARs) are ionotropic glutamate receptors essential for excitatory neurotransmission. Previous studies proposed the existence of four disulfide bonds in the GluN1 subunit; however, their role in NMDAR trafficking remains unclear. Our study first confirmed the existence of four disulfide bonds in the GluN1 subunit using biochemistry in human embryonic kidney 293T (HEK293T) cells. Disrupting the individual disulfide bonds by serine replacements produced the following surface expression trend for GluN1/GluN2A, GluN1/GluN2B, and GluN1/GluN3A receptors: wild-type (WT) > GluN1-C744S-C798S > GluN1-C79S-C308S > GluN1-C420S-C454S > GluN1-C436S-C455S subunits. Electrophysiology revealed altered functional properties of NMDARs with disrupted disulfide bonds, specifically an increased probability of opening (Po) at the GluN1-C744S-C798S/GluN2 receptors. Synchronized release from the endoplasmic reticulum confirmed that disruption of disulfide bonds impaired early trafficking of NMDARs in HEK293T cells and primary hippocampal neurons prepared from Wistar rats of both sexes (embryonic day 18). The pathogenic GluN1-C744Y variant, associated with neurodevelopmental disorder and seizures, caused reduced surface expression and increased Po at GluN1/GluN2 receptors, consistent with findings for the GluN1-C744S-C798S subunit. The FDA-approved memantine inhibited GluN1-C744Y/GluN2 receptors more potently and with distinct kinetics compared to WT GluN1/GluN2 receptors. We also observed enhanced NMDA-induced excitotoxicity in hippocampal neurons expressing the GluN1-C744Y subunit, which memantine reduced more effectively compared to the WT GluN1 subunit. Lastly, we demonstrated that the presence of the hGluN1-1a-C744Y subunit counteracted the effect of the hGluN3A subunit on decreasing dendritic spine maturation, consistent with the reduced surface delivery of the NMDARs carrying this variant. Our findings highlight the critical role of disulfide bonds in the GluN1 subunit in regulating trafficking and function of major conventional (GluN1/GluN2A, GluN1/GluN2B) and unconventional (GluN1/GluN3A) diheteromeric NMDAR subtypes in the postnatal forebrain. We further demonstrated that the pathogenic GluN1-C744Y variant reduces surface expression of all studied NMDARs, as well as increases the probability of opening (Po) of the GluN1/GluN2 receptors, leading to heightened NMDA-induced excitotoxicity in hippocampal neurons. Additionally, we introduced an ARIAD-based system for the synchronized release of NMDARs from the endoplasmic reticulum in hippocampal neurons. This system provides a powerful tool for studying pathogenic GRIN gene variants and addresses the current lack of molecular methods for analyzing the early trafficking of NMDARs.

The essential role of N-methyl-D-aspartate receptors (NMDARs) in excitatory neurotransmission is underscored by numerous pathogenic variants in the GluN subunits, including those identified in their ligand-binding domains (LBDs). The prevailing hypothesis postulates that the endoplasmic reticulum (ER) quality control machinery verifies the agonist occupancy of NMDARs; however, whether it controls the structure of LBDs or the functionality of NMDARs is unknown. Using alanine substitutions combined with microscopy and electrophysiology, we found that surface expression of GluN1/GluN2A receptors, the primary NMDAR subtype in the adult forebrain, strongly correlates with EC50 values for glycine and L-glutamate. Interestingly, co-expression of both GluN1 and GluN2A subunits with alanine substitutions led to an additive reduction in the surface number of GluN1/GluN2A receptors, as did co-expression of both GluN1 and GluN2A subunits containing closed cleft conformation of LBDs. The synchronized ER release confirmed the altered regulation of early trafficking of GluN1/GluN2A receptors bearing alanine substitutions in the LBDs. Furthermore, the human versions of GluN1/GluN2A receptors containing pathogenic GluN1-S688Y, GluN1-S688P, GluN1-D732E, GluN2A-S511L, and GluN2A-T690M variants exhibited distinct surface expression compared to the corresponding alanine substitutions. Mutant cycles of GluN1-S688, GluN1-D732, GluN2A-S511, and GluN2A-T690 residues revealed, in most cases, a weak correlation between surface expression of the mutant GluN1/GluN2A receptors and their EC50 values for glycine or L-glutamate. Consistent with our experimental data, molecular modeling and dynamics showed that the ER quality control machinery likely perceives structural changes of the LBDs but not the functionality of GluN1/GluN2A receptors. Our study showed that structural changes in LBDs independently regulate the early trafficking of GluN1/GluN2A receptors and that the surface numbers of mutant GluN1/GluN2A receptors do not necessarily correlate with their agonist sensitivity. In addition, we validated a novel system of synchronized release of GluN1/GluN2A receptors from the ER. Together, our experimental and in silico findings support the urgency of further detailed research on the regulation of early trafficking of NMDARs, as it may open the avenue to targeted therapeutic intervention of CNS disorders associated with pathogenic variants in GluN subunits.

N-methyl-D-aspartate receptors (NMDARs) play a significant role in developing several central nervous system (CNS) disorders. Currently, memantine, used for treating Alzheimer’s disease, and ketamine, known for its anesthetic and antidepressant properties, are two clinically used NMDAR open-channel blockers. However, despite extensive research into NMDAR modulators, many have shown either harmful side effects or inadequate effectiveness. For instance, dizocilpine (MK-801) is recognized for its powerful psychomimetic effects due to its high-affinity and nearly irreversible inhibition of the GluN1/GluN2 NMDAR subtypes. Unlike ketamine, memantine and MK-801 also act through a unique, low-affinity “membrane-to-channel inhibition” (MCI). We aimed to develop an open-channel blocker based on MK-801 with distinct inhibitory characteristics from memantine and MK-801. Our novel compound, K2060, demonstrated effective voltage-dependent inhibition in the micromolar range at key NMDAR subtypes, GluN1/GluN2A and GluN1/GluN2B, even in the presence of Mg2+. K2060 showed reversible inhibitory dynamics and a partially trapping open-channel blocking mechanism with a significantly stronger MCI than memantine. Using hippocampal slices, 30 µM K2060 inhibited excitatory postsynaptic currents in CA1 hippocampal neurons by ∼51%, outperforming 30 µM memantine (∼21% inhibition). K2060 exhibited No Observed Adverse Effect Level (NOAEL) of 15 mg/kg upon intraperitoneal administration in mice. Administering K2060 at a 10 mg/kg dosage resulted in brain concentrations of approximately 2 µM, with peak concentrations (Tmax) achieved within 15 minutes. Finally, applying K2060 with trimedoxime and atropine in mice exposed to tabun improved treatment outcomes. These results underscore K2060’s potential as a therapeutic agent for CNS disorders linked to NMDAR dysfunction.