Subunit Arrangement and Phenylethanolamine Binding in GluN1/GluN2B NMDA Receptors

Since it was unexpectedly discovered that the anti-hypertensive agent, ifenprodil, has neuroprotective activity through effects to N-methyl-D-aspartate (NMDA) receptors(1), enormous efforts have been made to understand the mechanism of action and to develop improved therapeutic compounds based on this knowledge(2–4). Neurotransmission mediated by NMDA receptors is essential for basic brain development and function(5). These receptors form heteromeric ion channels and become activated upon concurrent binding of glycine and glutamate to the GluN1 and GluN2 subunits, respectively. A functional hallmark of NMDA receptors is that their ion channel activity is allosterically regulated by binding of small compounds to the amino terminal domain (ATD) in a subtype specific manner. Ifenprodil and related phenylethanolamine compounds, which specifically inhibit GluN1/GluN2B NMDA receptors(6,7), have been intensely studied for their potential use in treatment of various neurological disorders and diseases including depression, Alzheimer’s disease and Parkinson’s disease(2,4). Despite great enthusiasm, mechanisms underlying recognition of phenylethanolamines and the ATD-mediated allosteric inhibition remain limited due to lack of structural information. Here we report that the GluN1 and GluN2B ATDs form heterodimer and that phenylethanolamine binds at the GluN1-GluN2B subunit interface rather than within the GluN2B cleft. The crystal structure of the GluN1b/GluN2B ATD heterodimer shows a highly distinct pattern of subunit arrangement that is different from those observed in homodimeric non-NMDA receptors and reveals the molecular determinants for phenylethanolamine binding. Restriction of domain movement in the bi-lobed structures of the GluN2B ATD by engineering an inter-subunit disulfide bond dramatically decreases ifenprodil-sensitivity indicating that conformational freedom in the GluN2B ATD is essential for ifenprodil-mediated allosteric inhibition in NMDA receptors.