The positive allosteric modulator GNE-9278 increases gating, conductance, and Ca2+ permeability for GluN2D-containing NMDA receptors

Positive allosteric modulators (PAMs) of NMDA receptors hold promise to alleviate functional deficits associated with neuropsychiatric disorders. GNE-9278 is a synthetic PAM that binds to the obligatory GluN1 subunit of NMDA receptors, yet it displays substantial subtype-specific efficacies. To examine the underlying mechanism by which GNE-9278 potentiates NMDA receptor responses, we examined electrical activity from endogenous and recombinant NMDA receptors and measured changes in kinetics, unitary conductance, and Ca²⁺ permeability for GluN2A- and GluN2D-containing receptors. We found that GNE-9278 stabilized the open states of both receptor types and increased their open state occupancies at equilibrium. This change was small for GluN2A receptors, whose open probability (Po), remained around 0.5, and was substantial for GluN2D, which increased four-fold to 0.24. Moreover, specifically when acting on GluN2D receptors, GNE-9278 increased the channel unitary conductance (γ_Na), from 43 pS to 56 pS, and their Ca²⁺ permeability (P_Ca/P_Na), from 1.4 to 2.9. These results show large and complex effects of GNE-9278 on GluN2D receptors, which together boost these receptors' excitatory signal and impact on synaptic plasticity and cellular fate. This new information should guide the use of drugs acting at the GNE-9278-binding site for research and therapeutic purposes.