The structure–energy landscape of NMDA receptor gating

Abstract

N-methyl-D-aspartate (NMDA) receptors are the main calcium-permeable excitatory receptors in the mammalian central nervous system. The NMDA receptor gating is complex, exhibiting multiple closed, open, and desensitized states; however, the central questions regarding the conformations and energetics of the transmembrane domains as they relate to the gating states are still unanswered. Here, using single molecule Förster Resonance Energy Transfer (smFRET), we map the energy landscape of the first transmembrane segment of the Rattus norvegicus NMDA receptor under resting and various liganded conditions. These results show kinetically and structurally distinct changes associated with apo, agonist-bound, and inhibited receptors linked by a linear mechanism of gating at this site. Furthermore, the smFRET data suggest that allosteric inhibition by zinc occurs by an uncoupling of the agonist-induced changes at the extracellular domains from the gating motions leading to an apo-like state, while dizocilpine, a pore blocker, stabilizes multiple closely packed transmembrane states.

Kinetic models for activation and modulation of NMDA receptor subtypes

“…In addition to rendering channels impermeable, the blocker changed transition rates in the activation sequence, indicative of a trapping block mechanism. This kinetic model identified blocker-sensitive transition rates, blocker concentration-dependence of receptor open probability, and stimulus-dependence of blocker efficacy thus supporting the premise that in addition to occluding permeation this ligand also alters activation kinetics [17 • ]. Delineating blocker mechanisms with similar detail will be necessary to understand why different blockers affect NMDA receptor responses differentially, a vexing unanswered question.…”

“…This linear activation sequence depicts the most rapid pathway by which resting receptors can open (activation) and open receptors can deactivate (deactivation). Recently, single-molecule FRET data have supported this linear arrangement of states [17 • ]. Its transition rates set the rise and decay timecourse for synaptic NMDA receptor currents (Figure 1b) and the decay timecourse of the excitatory postsynaptic current (EPSC).…”

Mechanisms for Zinc and Proton Inhibition of the GluN1/GluN2A NMDA Receptor

“…Here we show how the GluN1/GluN2A receptor populates an ensemble of structural states depending on zinc and proton concentrations (Amico-Ruvio et al, 2011; Dolino et al, 2017; Erreger and Traynelis, 2008). The 1-knuckle conformation, one of the structural conformations that the receptor adopts when inhibited by zinc and protons, resembles the GluN1/GluN2B receptor bound by ifenprodil or Ro25–6891 (Tajima et al, 2016; Zhu et al, 2016).…”

“…Consistent with zinc and ifenprodil inhibiting NMDARs through similar structural mechanisms, ifenprodil enhances zinc and proton sensitivity (Amico-Ruvio et al, 2012; Mott et al, 1998; Pahk and Williams, 1997) and zinc enhances ifenprodil sensitivity and the extent of inhibition (Hansen et al, 2014). Indeed, single molecule studies support an ensemble of electrophysiologically silent states, where the NMDA receptor populates pre-open or low P O states in the presence of zinc (Amico-Ruvio et al, 2011; Christine and Choi, 1990; Dolino et al, 2017; Erreger and Traynelis, 2008). …”

An inter‐dimer allosteric switch controls NMDA receptor activity

“…In studies examining how the NTD‐binding allosteric inhibitors zinc and ifenprodil affect the gating reaction of NMDARs, the same pre‐open to open transition is preferentially affected (Amico‐Ruvio et al , , ). Single‐molecule FRET analysis also indicates that zinc inhibition of NMDARs occurs by uncoupling of the agonist‐induced changes at the ABD domains from the gating motions (Dolino et al , ). Our work provides the novel information that this uncoupling occurs through a quaternary rearrangement involving the unrolling of the two constitutive ABD dimers.…”