Allosteric modulators of NR2B-containing NMDA receptors: molecular mechanisms and therapeutic potential

Abstract

N-methyl-D-aspartate receptors (NMDARs) are ion channels gated by glutamate, the major excitatory neurotransmitter in the mammalian central nervous system (CNS). They are widespread in the CNS and are involved in numerous physiological and pathological processes including synaptic plasticity, chronic pain and psychosis. Aberrant NMDAR activity also plays an important role in the neuronal loss associated with ischaemic insults and major degenerative disorders including Parkinson's and Alzheimer's disease. Agents that target and alter NMDAR function may, thus, have therapeutic benefit. Interestingly, NMDARs are endowed with multiple extracellular regulatory sites that recognize ions or small molecule ligands, some of which are likely to regulate receptor function in vivo. These allosteric sites, which differ from agonist-binding and channel-permeation sites, provide means to modulate, either positively or negatively, NMDAR activity. The present review focuses on allosteric modulation of NMDARs containing the NR2B subunit. Indeed, the NR2B subunit confers a particularly rich pharmacology with distinct recognition sites for exogenous and endogenous allosteric ligands. Moreover, NR2B-containing receptors, compared with other NMDAR subtypes, appear to contribute preferentially to pathological processes linked to overexcitation of glutamatergic pathways. The actions of extracellular H+, Mg2+, Zn2+, of polyamines and neurosteroids, and of the synthetic compounds ifenprodil and derivatives ('prodils') are presented. Particular emphasis is put upon the structural determinants and molecular mechanisms that underlie the effects exerted by these agents. A better understanding of how NR2B-containing NMDARs (and NMDARs in general) operate and how they can be modulated should help define new strategies to counteract the deleterious effects of dysregulated NMDAR activity.

Figure 1

Organization and expression of NMDAR subunits. (A) Schematic representation of the NR2B subunit. It is composed of an NTD that binds allosteric inhibitors such as zinc and ifenprodil, an S1–S2 ABD (binds glutamate), a TM region (TM 1, 2, 3 and a re-entrant loop) that forms the ion channel and a C-terminal cytoplasmic region. (B) Distribution of NMDAR subunit mRNAs in the mouse brain at post-natal day 21 (figure reproduced with permission from Watanabe et al., 1993). NR2A (panel A), NR2B (panel B), NR2C (panel C), NR2D (panel D) and NR1 (panel E). Note the restricted expression of the NR2B subunit in the forebrain. ABD, agonist-binding domain; AC, anterior cingulate cortex; Cx, cerebral cortex; Cb, cerebellum; CPu, caudate-putamen; Hi, hippocampal formation; MB, midbrain; NMDAR, N-methyl-D-aspartate; NTD, N-terminal domain; OB, olfactory bulb; S, septum; Th, thalamus; TM, transmembrane.

Figure 4

Spermine, but not histamine, potentiates NR1a/NR2B receptors. (A) Typical current traces obtained from oocytes expressing the NR1-1a subunit (NR1a) with the NR2B (left panel) or the NR2A (right panel) subunit. Spermine was applied at a concentration of 200 µM and histamine at 100 µM, each during an application of agonists (100 µM glutamate and glycine, saturating concentrations). Holding potential −40 mV (left panel) and −30 mV (right panel). The bars above the current traces indicate the duration of agonist, spermine and histamine applications. Note that application of 100 µM histamine has no effect on NR1a/NR2B receptors while spermine does (same cell). Note also that spermine potentiation is absent on NR1a/NR2A receptors. (B) Two hypothetical mechanisms of how a polyamine could potentiate NR2B-containing receptors. These models are based on the mechanism proposed for allosteric inhibition of NR1/NR2A receptors (Gielen et al., 2008). While the full receptor is a tetramer, only a NR1/NR2B heterodimer is shown. It is hypothesized that NTDs dimerize, and that closures of the NTDs can inactivate the receptors (i.e. induce channel gate closure) by pulling apart the ABD dimer interface (‘desensitized’ state). Mechanism (1): the polyamine molecule binds between the NR1 and NR2 NTD lobes II, making NTD closure, and ABD dimer interface disruption, more difficult. Mechanism (2): the polyamine molecule directly binds and stabilizes the ABD dimer interface. Entry into the ‘desensitized’ state is thus disfavoured. This mechanism resembles that described for cyclothiazide-induced suppression of desensitization at AMPA receptors (Sun et al., 2002; Mayer, 2006). In both models, proton binding, which stabilizes a closed state of the channel (Banke et al., 2005), is proposed to be closely associated with ABD dimer interface breaking (Gielen et al., 2008). ABD, agonist-binding domain; AMPA, α-amino-3-hydroxy-5-methyl-4-isoxazole-propionate; Glu, glutamate; Gly, glycine; NTD, N-terminal domain.

Figure 2

Positive and negative allosteric modulators of NR2B-containing NMDARs. (A) Allosteric modulators that inhibit NR2B-containing receptors. (B) Allosteric modulators that potentiate NR2B-containing receptors. The agents that selectively modulate NMDARs incorporating the NR2B subunit are named in red. NMDAR, N-methyl-D-aspartate receptors; 3α5βS, 3α-hydroxy-5β-pregnan-20-one sulphate.

Figure 3

Structure of NR2B-selective NMDAR antagonists. (A) ‘Second generation’ compounds closely related in structure to the prototypical NR2B antagonist ifenprodil. (B) The latest generation of NR2B-selective antagonists and new structural templates. This represents a current perspective based on publications, patents, company press releases and analyst information; literature references, where available, are cited in the text. NMDAR, N-methyl-D-aspartate receptors.