Cryo-EM structures of the triheteromeric NMDA receptor and its allosteric modulation

Abstract

Summary N-methyl-D-aspartate receptors (NMDARs) are heterotetrameric ion channels assembled as diheteromeric or triheteromeric complexes. Here we report structures of the triheteromeric GluN1/GluN2A/GluN2B receptor in the absence or presence of the GluN2B-specific allosteric modulator Ro 25-6981 (Ro), determined by cryo-EM. In the absence of Ro, the GluN2A and GluN2B amino terminal domains (ATD) adopt “closed” and “open” clefts, respectively. Upon binding Ro, the GluN2B ATD clamshell transitions from an open to a closed conformation. Consistent with a predominance of the GluN2A subunit in ion channel gating, the GluN2A subunit interacts more extensively with GluN1 subunits throughout the receptor, in comparison to the GluN2B subunit. Differences in the conformation of the pseudo 2-fold related GluN1 subunits further reflect receptor asymmetry. The triheteromeric NMDAR structures provide the first view of the most common NMDA receptor assembly and show how incorporation of two different GluN2 subunits modifies receptor symmetry and subunit interactions, allowing each subunit to uniquely influence receptor structure and function, thus increasing receptor complexity.

Allosteric Interactions between NMDA Receptor Subunits Shape the Developmental Shift in Channel Properties

“…3, Table 1). This dominant role of the GluN2A subunit is consistent with structural interactions between the ATD and LBD of triheteromeric GluN2A/GluN2B-containing receptors described in (Lü et al, 2017). …”

“…As demonstrated in this study, unexpected properties can arise from combining different subunits into the NMDAR complex. Furthermore, our findings support the possibility that unique intra- and inter-subunit interfaces exist in triheteromeric GluN1/2A/2B NMDARs, which are not present in diheteromeric GluN1/2A and GluN1/2B NMDARs (Lü et al, 2017). The distinct structure and function of GluN1/2A/2B NMDARs create opportunities for the development of novel modulators, which are selective for triheteromeric over diheteromeric receptors and could provide a path forward for selectively targeting GluN1/2A/2B NMDARs in the treatment of neurological and psychiatric disorders.…”

“…Our study shows a direct functional consequence of losing this structural symmetry: GluN2A and GluN2B interact asymmetrically, and GluN2A dominates this interaction resulting in a high open probability and fast deactivating receptor. We therefore identify functional consequences of asymmetry in GluN1/2A/2B NMDARs that were not detected in previous reports describing triheteromeric receptor function (Hansen et al, 2014; Stroebel et al, 2014; Tovar et al, 2013) and cannot be determined from static cryo-EM protein structures (Lü et al, 2017). Since the cloning of NMDA receptor subunits 25 years ago, these allosteric interactions between glutamate binding GluN2 subunits have not been revealed, and because of this, the GluN2 subunits have been assumed to operate independently (Clements and Westbrook, 1991).…”

“…Recent cryo-EM data suggest that structure of triheteromeric GluN1/2A/2B NMDARs is unique because it breaks the two-fold symmetry of diheteromeric GluN1/2B receptors (Lü et al, 2017). Our study shows a direct functional consequence of losing this structural symmetry: GluN2A and GluN2B interact asymmetrically, and GluN2A dominates this interaction resulting in a high open probability and fast deactivating receptor.…”

Functional and pharmacological properties of triheteromeric GluN1/2B/2D NMDA receptors

“…; Lü et al . ). By contrast, GluN2A is not governing the function of GluN1/2A/2C receptors that display deactivation kinetics intermediate to those of GluN1/2A and GluN1/2C receptors (Bhattacharya et al .…”