Molecular basis of positive allosteric modulation of GluN2B NMDA receptors by polyamines

Abstract

NMDA receptors (NMDARs) form glutamate-gated ion channels that have central roles in neuronal communication and plasticity throughout the brain. Dysfunctions of NMDARs are involved in several central nervous system disorders, including stroke, chronic pain and schizophrenia. One hallmark of NMDARs is that their activity can be allosterically regulated by a variety of extracellular small ligands. While much has been learned recently regarding allosteric inhibition of NMDARs, the structural determinants underlying positive allosteric modulation of these receptors remain poorly defined. Here, we show that polyamines, naturally occurring polycations that selectively enhance NMDARs containing the GluN2B subunit, bind at a dimer interface between GluN1 and GluN2B subunit N-terminal domains (NTDs). Polyamines act by shielding negative charges present on GluN1 and GluN2B NTD lower lobes, allowing their close apposition, an effect that in turn prevents NTD clamshell closure. Our work reveals the mechanistic basis for positive allosteric modulation of NMDARs. It provides the first example of an intersubunit binding site in this class of receptors, a discovery that holds promise for future drug interventions.

Subtype-dependent N-Methyl-d-aspartate Receptor Amino-terminal Domain Conformations and Modulation by Spermine

“…Propping open the ATD of either the GluN1 or the GluN2 subunit increases the channel open probability (2,22). Because spermine acts as potentiator, this mechanism suggests that spermine stabilizes an open conformation of the GluN2B ATD (9). Consistent with this, the extent of spermine potentiation is much less when the GluN2B ATD is already braced open by a thiol-reactive MTS (methanethiosulfonate) reagent (9).…”

“…ATD structure with suggested spermine-binding site and mutation sites for LRET experiments. A, crystal structure of the GluN1-GluN2B ATD dimer structure is shown highlighting the suggested spermine-binding residues in blue (PDB code 4PE5) (9,10). B, schematics of individual subunits are shown indicating the locations of mutations and the resulting amino acid sequence.…”

“…Because spermine acts as potentiator, this mechanism suggests that spermine stabilizes an open conformation of the GluN2B ATD (9). Consistent with this, the extent of spermine potentiation is much less when the GluN2B ATD is already braced open by a thiol-reactive MTS (methanethiosulfonate) reagent (9). These studies provide indirect evidence that spermine potentiates the receptor by opening the GluN2B cleft; however, at present there is no direct study showing and quantifying the conformational changes in the ATDs upon spermine binding.…”

“…Based on mutational studies, spermine is thought to bind to acidic residues on the lower lobes of both GluN1 and GluN2B ATDs (blue residues in Fig. 1A) (9,20). Interestingly, GluN1 subunits that include exon 5, a 21-residue stretch in the ATD lower lobe containing six basic residues, are no longer sensitive to spermine modulation.…”

Genetically encoding a light switch in an ionotropic glutamate receptor reveals subunit-specific interfaces

“…1A). Besides having essential functions in receptor assembly (10,11), recent studies of the NTDs have also revealed that the individual GluN2 (12)(13)(14) and GluN1 (15) NTDs fine-tune NMDAR gating and pharmacological properties by undergoing large-range conformational changes. The recent X-ray crystal structure of a GluN1/GluN2B NTD complex reveals a unique arrangement of the two NTD protomers with intersubunit interactions distinct from those observed in AMPA and kainate receptors (16,17).…”