New Insights into the Not-So-New NR 3 Subunits of N-Methyl-D-aspartate Receptor : Localization , Structure , and Function

The NR3 subunits (NR3A and NR3B) are new players in a well established field of N-methyl-D-aspartate (NMDA) receptors, previously involving the NR1 and NR2 subunits. Their incorporation into conventional NMDA receptors forms glutamate-activated NR1/NR2/NR3 triheteromers, whereas the omission of the glutamate-binding NR2 subunits results in excitatory glycine-activated NR1/NR3 diheteromers. These NR3-containing NMDA receptors exhibit several differences in receptor properties compared with the conventional NR1/NR2 receptors. This review highlights the major landmarks that have been achieved in the past decade or so involving NR3 subunit research in four key areas: the spatiotemporal mapping of NR3 protein, the structural elucidation of NR3 domains, pharmacological characterization of NR3-containing receptors, and the successful generation of NR3 knockout/transgenic animals. It is expected that further characterization of their functional roles coupled with the identification of endogenous and exogenous ligands will eventually advance the understanding of the basic pharmacology and the complex role of NMDA receptors in higher brain functions and neurological disorders. The N-methyl-D-aspartate (NMDA) receptor is a subtype of the ionotropic glutamate (iGlu) receptors, a family of ligand-gated ion channels that mediates the majority of excitatory neurotransmission in the mammalian central nervous system. Named after the agonist originally used to differentiate this receptor from two other iGlu receptor members [ -amino-3-hydroxy-5-methyl-4-isoazolepropionic acid (AMPA) receptor and the 2-carboxy-3-carboxymethyl-4-isopropenylpyrrolidine (kainate) receptor], the NMDA receptor is unlike the other subtypes in three important ways. First, it requires both L-glutamate (agonist) and glycine (coagonist) for maximum activation (Johnson and Ascher, 1987; Kleckner and Dingledine, 1988; Dalkara et al., 1992). Second, the unique presence of an Mg channel blockade brings about a voltage-dependence property absent in other iGlu receptor members (Mayer et al., 1984; Nowak et al., 1984; Kupper et al., 1998). Third, the NMDA receptor exhibits relatively higher permeability to calcium ion (Ca ) than AMPA and kainate receptors, which links it to a variety of neurophysiological processes including neurodevelopment and cognition, and neuropathological conditions such as bipolar disorder, Parkinson’s disease, and stroke (Dingledine et al., 1999; Mayer and Armstrong, 2004). This work was supported by the ARF(NUS) [Grant 184-000-160-112]; the National Medical Research Council [Grant 1024/2005]; and the Biomedical Research Council [Grants 05/1/21/19/376, 09/1/21/19/617]. Article, publication date, and citation information can be found at http://molpharm.aspetjournals. org. doi:10.1124/mol.110. 064006. ABBREVIATIONS: NMDA, N-methyl-D-aspartate; AMPA, -amino-3-hydroxy-5-methyl-4-isoazolepropionic acid; iGlu, ionotropic glutamate; ATD, amino-terminal domain; CTD, cytoplasmic terminal domain; S1S2, ligand binding domain; PSD, postsynaptic density; HEK, human embryonic kidney; DCS, D-cycloserine; ACPC, 1-aminocyclopropane-1-carboxylic acid; ACBC, 1-aminocyclobutane-1-carboxylic acid; 5,7-DCKA, 5,7dichlorokynurenic acid; bp, base pair(s); Tg, transgenic; LTP, long-term potentiation; AP5, 2-amino-5-phosphonopentanoic acid; L-689560, trans-2-carboxy-5,7-dichloro-4-phenylaminocarbonylamino-1,2,3,4-tetrahydroquinoline; MK-801, 5H-dibenzo[a,d]cyclohepten-5,10-imine (dizocilpine maleate); L701324, 7-chloro-4-hydroxy-3-(3-phenoxy)phenyl-2(1H)-quinolinone; L689560, trans-2-carboxy-5,7-dichloro-4-phenylaminocarbonylamino-1,2,3,4-tetrahydroquinoline; CGP78608, [(1S)-1-[[(7-bromo-1,2,3,4-tetrahydro-2,3-dioxo-5-quino xalinyl)methyl]amino]ethyl]phosphonic acid; MDL-29951, 3-(4,6-dichloro-2-carboxyindol-3-yl)propionic acid; ALX-1393, O-[(2-benzyloxyphenyl-3-flurophenyl)methyl]-L-serine. 0026-895X/10/7801-1–11$20.00 MOLECULAR PHARMACOLOGY Vol. 78, No. 1 Copyright © 2010 The American Society for Pharmacology and Experimental Therapeutics 64006/3596087 Mol Pharmacol 78:1–11, 2010 Printed in U.S.A. 1 at A PE T Jornals on A ril 9, 2017 m oharm . aspeurnals.org D ow nladed from The NMDA receptor consists of three subunits: glycinebinding NR1, glutamate-binding NR2, and the most recently discovered glycine-binding NR3 (Dingledine et al., 1999) (Fig. 1A). Each NMDA receptor subunit includes the following: 1) an amino-terminal domain (ATD); 2) three transmembrane domains (M1, M3, M4) with a pore channel-forming re-entrant loop; 3) a bilobed agonist binding domain formed by the remaining distal segment after the ATD (termed as S1 domain) and the large extracellular loop linking M3 and M4 (termed as S2 domain); and 4) the cytoplasmic terminal domain (CTD) (Dingledine et al., 1999; Mayer and Armstrong, 2004) (Fig. 1B). In the heteromeric NMDA receptor complex, NR1 is the essential subunit that coassembles with various combinations of NR2 and/or NR3 subunits (Ciabarra et al., 1995; Sucher et al., 1995; Al-Hallaq et al., 2002; Schüler et al., 2008). The successful cloning of the third subunit of NMDA receptor, NR3, 15 years ago has taken the complexity of NMDA receptors to a new level. A broad overview on NR3 subunits first appeared in 2008 (Cavara and Hollmann, 2008). This minireview focuses primarily on NR3 localization, structural and functional understanding gained through immunocytochemistry, structural biology, and electrophysiology in the recent years.