NMDA receptor subunits: function and pharmacology

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N-methyl-D-aspartate receptors (NMDARs) are glutamate-gated ion channels widely expressed in the central nervous system that play key roles in excitatory synaptic transmission. Because of their involvement in numerous neurological disorders, NMDARs are also targets of therapeutic interest. NMDARs occur as multiple subtypes which differ in their subunit composition and in their biophysical and pharmacological properties. In particular, NMDARs contain a diversity of sites at which endogenous ligands or pharmacological agents can act to modulate receptor activity in a subunit-selective manner, and recent structural and functional data have started to reveal the molecular determinants for this subunit selectivity. These include the binding sites for glutamate, the ion-channel pore and the recently identified allosteric sites on the N-terminal domain. Other potential sites yet unexplored by medicinal chemistry programs are also considered, in particular at the interface between subunits. Given the growing body of evidence that diverse brain disorders implicate different NMDAR subtypes, such as NR2B in pain or NR3A in white matter injury, there is a growing interest in exploiting the pharmacological heterogeneity of NMDARs for the development of novel NMDAR subtype-selective compounds.

Introduction

Within the large family of excitatory ionotropic glutamate receptors (iGluRs), N-methyl-d-aspartate receptors (NMDARs) constitute a subfamily identified by specific molecular composition and unique pharmacological and functional properties [1, 2]. Of particular importance is the high permeability to calcium ions, which confers on NMDARs a central role in both synaptic plasticity under physiological conditions and neuronal death under excitotoxic pathological conditions. Because they are built by heteromeric assembly from a relatively large pool of homologous subunits, NMDARs exist as diverse subtypes endowed with distinctive functional properties and patterns of expression [3]. Since the cloning of the different subunit isoforms, relating particular functions to NMDAR subtypes has been a continuous challenge [2]. In this review, we concentrate on recent structural and pharmacological data that could help in revealing detailed NMDAR functions.

Section snippets

Molecular organization and operation of NMDARs

NMDARs are heteromeric complexes incorporating different subunits within a repertoire of three subtypes: NR1, NR2 and NR3. There are eight different NR1 subunits generated by alternative splicing from a single gene, four different NR2 subunits (A, B, C and D) and two NR3 subunits (A and B); the NR2 and NR3 subunits are encoded by six separate genes [1]. Expression of functional recombinant NMDARs in mammalian cells requires the co-expression of at least one NR1 and one NR2 subtype. The

Functional domains in NMDAR subunits

NMDAR subunits all share a common membrane topology (Figure 1) characterized by a large extracellular N-terminus, a membrane region comprising three transmembrane segments (TM1, 3 and 4) plus a re-entrant pore loop (M2), an extracellular loop between TM3 and TM4, and a cytoplasmic C-terminus, which varies in size depending upon the subunit and provides multiple sites of interaction with numerous intracellular proteins [1, 5•].

The extracellular region of NMDAR subunits (like that of other

Structural aspects of NMDAR activation

The crystallographic studies of Gouaux and colleagues [13] on the ABD of GluR2 (an α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor [AMPAR] subunit) have markedly improved our understanding of the initial steps in iGluR activation. The available structures, which now include NR1 and NR2A ABDs, have fully confirmed the predicted structural and functional homologies between bacterial glutamine-binding protein and iGluR ABDs [9•, 14]. These latter domains show the typical two-lobe

Pharmacology of NMDAR subtypes

Ever since the pioneering work of Watkins and colleagues showing that NMDA selectively activates a subclass of glutamate receptors (hence coined NMDARs), extensive efforts have been made to discover potent and selective NMDAR antagonists. The 1980s saw the development of the first broad-spectrum competitive antagonists and high-affinity channel blockers. The cloning of NMDAR subunits in the early 1990s and the subsequent finding that NMDARs occur in vivo as multiple subtypes with distinct

Compounds acting at the agonist binding domains

The first NMDAR antagonists were competitive antagonists acting at the glutamate binding site on the NR2 subunits. They are usually conformationally constrained amino acid derivatives containing an ω-phosphonic group [21]. One of the first compounds discovered, (R)-2-amino-5-phosphonopentanoate (R-AP5), remains widely used because it displays strong preference for NMDARs over all other iGluRs. These compounds show some selectivity between the different NR2 subunits (affinity ranking typically

Compounds acting in the pore

A large number of organic compounds inhibit NMDARs by occluding the ion channel pore [21]. These compounds are uncompetitive antagonists because their action requires prior activation of the receptor (i.e. pore opening). Moreover, although structurally diverse, they are all positively charged and act in a voltage-dependent manner. NMDAR pore blockers usually discriminate poorly between NMDAR subtypes. This is indeed the case for the dissociative anaesthetics phencyclidine (PCP),

Compounds acting at the NR2 N-terminal domains

The only known organic compounds that display a high NMDAR subtype selectivity are ifenprodil and derivatives, which are selective antagonists of NR2B-containing receptors [18]. Because of the important therapeutic promise of these antagonists (see below), significant efforts to identify novel derivatives have been made during the past decade [31], and some compounds with affinities and selectivities greater than that of ifenprodil have been found (Table 3). However, it is only recently that

Other potential sites for ligand binding

There are other hypothetical sites where extracellular ligands could act to modulate NMDAR activity. Besides the NR1 NTD (Figure 1, site 2), the ABD dimer interface might provide another site for new allosteric modulators (Figure 1, site 3). In AMPARs, this interface binds positive allosteric modulators such as cyclothiazide and aniracetam [16, 38]. These agents reduce AMPAR desensitization and slow channel deactivation by stabilizing the ABD interface and the closed-cleft active conformation

Triheteromeric NMDARs complicate native NMDAR pharmacology

There is compelling evidence that NMDARs are not always simple binary assemblies of NR1 with only one type of NR2 subunit, and that some receptors can incorporate two types of NR2 subunits [1, 2]. Such triheteromeric assemblies have been observed in many brain regions, such as NR1/NR2A/NR2B in the forebrain and NR1/NR2A/NR2C in the cerebellum (see [40•]). In vivo, the NR3 subunit is believed to form ternary complexes by co-assembling with NR1 and NR2 subunits [4]. The heterogeneity of

Renaissance of NMDARs as targets of therapeutic interest

NMDARs have always triggered an intense interest as potential therapeutic drug targets because they are involved in many brain disorders [41]. Traditionally, NMDARs are best known for their role in excitotoxicity, a process during which excessive glutamate release causes overactivation of NMDARs, accumulation of intracellular calcium and, eventually, neuronal death. Excitotoxicity occurs during cerebral ischemia (following stroke or brain trauma) and in neurodegenerative disorders such as

NR2B-selective antagonists

One explanation for the failures of first-generation NMDAR antagonists is their lack of subunit specificity. By targeting the ABD (competitive antagonists) or the channel pore (channel blockers), these compounds do not discriminate between the various NMDAR subtypes. NMDAR antagonists with improved tolerability have now been identified. The most promising compounds are ifenprodil derivatives, which selectively inhibit NR2B-containing NMDARs by binding to the NR2B N-terminal domain (see above) [

NR3A subunit and the myelin sheath

It was generally assumed that NMDAR expression in the central nervous system was restricted to neurons, with no (or very little) expression in glial cells. Several recent studies, however, indicate that NMDARs are present on both astrocytes and oligodendrocytes. Oligodendrocytes — cells in the white matter that produce the myelin sheath surrounding axons — are damaged by an excess of glutamate, and loss of the myelin sheath is observed in multiple neurological disorders including cerebral

NMDAR enhancers against NMDAR hypofunction in schizophrenia

Several lines of evidence indicate that hypofunction of NMDARs may be a key feature in major human cognitive disorders, particularly schizophrenia. Non-selective NMDAR channel blockers (e.g. PCP or ketamine) disrupt memory formation and cause a schizophrenia-like syndrome in humans, recapitulating both positive and negative symptoms and cognitive impairments [50]. Transgenic mice with reduced NMDAR expression or impaired NMDAR function display behaviours related to schizophrenia [51, 52].

Conclusions

The NMDAR complex contains several potential binding sites for extracellular modulators. In this review, we have advertised the choice of the NTDs of NMDAR subunits as an interesting therapeutic target. These domains indeed bind allosteric modulators in two NMDAR subunits (NR2A and NR2B) with strong subunit selectivity, and it is tempting to speculate that other selective modulators may be found that bind NTDs of other NMDAR subunits (and more generally those of other iGluRs). More knowledge on

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

This work was supported by INSERM and ANR (PP). We thank Boris Barbour and Marc Gielen for comments on the manuscript.

References (61)

  • B.A. Chizh et al.

    NMDA antagonists and neuropathic pain–multiple drug targets and multiple uses

    Curr Pharm Des

    (2005)
  • J.N. Kew et al.

    A novel mechanism of activity-dependent NMDA receptor antagonism describes the effect of ifenprodil in rat cultured cortical neurones

    J Physiol

    (1996)
  • M.G. Salter et al.

    NMDA receptors are expressed in developing oligodendrocyte processes and mediate injury

    Nature

    (2005)
  • T. Yamakura et al.

    Different sensitivities of NMDA receptor channel subtypes to non-competitive antagonists

    Neuroreport

    (1993)
  • G. Fischer et al.

    Ro 25-6981, a highly potent and selective blocker of N-methyl-D-aspartate receptors containing the NR2B subunit. Characterization in vitro

    J Pharmacol Exp Ther

    (1997)
  • R. Dingledine et al.

    The glutamate receptor ion channels

    Pharmacol Rev

    (1999)
  • S.G. Cull-Candy et al.

    Role of distinct NMDA receptor subtypes at central synapses

    Sci STKE

    (2004)
  • Y.F. Sasaki et al.

    Characterization and comparison of the NR3A subunit of the NMDA receptor in recombinant systems and primary cortical neurons

    J Neurophysiol

    (2002)
  • M.L. Mayer

    Glutamate receptors at atomic resolution

    Nature

    (2006)
  • T. Masuko et al.

    A regulatory domain (R1-R2) in the amino terminus of the N-methyl-d-aspartate receptor: effects of spermine, protons, and ifenprodil, and structural similarity to bacterial leucine/isoleucine/valine binding protein

    Mol Pharmacol

    (1999)
  • P. Paoletti et al.

    Molecular organization of a zinc binding N-terminal modulatory domain in a NMDA receptor subunit

    Neuron

    (2000)
  • Y. Yao et al.

    Characterization of a soluble ligand binding domain of the NMDA receptor regulatory subunit NR3A

    J Neurosci

    (2006)
  • L.P. Wollmuth et al.

    Structure and gating of the glutamate receptor ion channel

    Trends Neurosci

    (2004)
  • K. Matsuda et al.

    Cloning and characterization of a novel NMDA receptor subunit NR3B: a dominant subunit that reduces calcium permeability

    Brain Res Mol Brain Res

    (2002)
  • N. Armstrong et al.

    Structure of a glutamate-receptor ligand-binding core in complex with kainate

    Nature

    (1998)
  • Y. Sun et al.

    Mechanism of glutamate receptor desensitization

    Nature

    (2002)
  • S. Schorge et al.

    Studies of NMDA receptor function and stoichiometry with truncated and tandem subunits

    J Neurosci

    (2003)
  • P. Paoletti et al.

    High-affinity zinc inhibition of NMDA NR1-NR2A receptors

    J Neurosci

    (1997)
  • S.F. Traynelis et al.

    Control of voltage-independent zinc inhibition of NMDA receptors by the NR1 subunit

    J Neurosci

    (1998)
  • J.N. Kew et al.

    Ionotropic and metabotropic glutamate receptor structure and pharmacology

    Psychopharmacology (Berl)

    (2005)

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