C-Terminal Truncation of NR2A Subunits Impairs Synaptic But Not Extrasynaptic Localization of NMDA Receptors

Abstract

NMDA receptors interact via the extended intracellular C-terminal domain of the NR2 subunits with constituents of the postsynaptic density for purposes of retention, clustering, and functional regulation at central excitatory synapses. To examine the role of the C-terminal domain of NR2A in the synaptic localization and function of NR2A-containing NMDA receptors in hippocampal Schaffer collateral-CA1 pyramidal cell synapses, we analyzed mice which express NR2A only in its C-terminally truncated form. In CA1 cell somata, the levels, activation, and deactivation kinetics of extrasynaptic NMDA receptor channels were comparable in wild-type and mutant NR2A(Delta)(C/)(Delta)(C) mice. At CA1 cell synapses, however, the truncated receptors were less concentrated than their full-length counterparts, as indicated by immunodetection in cultured neurons, synaptosomes, and postsynaptic densities. In the mutant, the NMDA component of evoked EPSCs was reduced in a developmentally progressing manner and was even more reduced in miniature EPSCs (mEPSCs) elicited by spontaneous glutamate release. Moreover, pharmacologically isolated NMDA currents evoked by synaptic stimulation had longer latencies and displayed slower rise and decay times, even in the presence of an NR2B-specific antagonist. These data strongly suggest that the C-terminal domain of NR2A subunits is important for the precise synaptic arrangement of NMDA receptors.

An NMDA Receptor ER Retention Signal Regulated by Phosphorylation and Alternative Splicing

“…Although this study has focused on the NR subunit, it is likely that efficient assembly and trafficking of NMDA receptors through the secretory pathway will depend on additional as yet unidentified domains in both NR1 and NR2. Indeed, C-terminal domains of NR2 are required for efficient synaptic targeting of NMDA receptors (Mori et al, 1998;Sprengel et al, 1998;Steigerwald et al, 2000), and N-terminal extracellular regions are required for efficient assembly and ER exit of AMPA receptor subunits (Leuschner and Hoch, 1999).…”

cAMP-Dependent Protein Kinase Mediates Activity-Regulated Synaptic Targeting of NMDA Receptors

“…Synaptic anchoring of the NMDA receptor does not require actin filaments or microtubules (Allison et al, 2000) but presumably requires binding of NMDA receptor subunits to other postsynaptic density proteins. Analysis of neurons from mice bearing NR2 genes with C-terminal truncations indicates that the C-terminal domains of NR2A and NR2B contribute to synaptic localization along with additional, as yet unidentified, domains of the receptor (Mori et al, 1998;Steigerwald et al, 2000). Candidate anchoring proteins include the PDZ domain proteins PSD-95, chapsyn-110/PSD-93, SAP102, and S-SCAM (Kornau et al, 1997;Hirao et al, 1998), the actin-binding proteins ␣-actin and spectrin (Wyszynski et al, 1997;Wechsler and Teichberg, 1998), and the AKAP yotiao (Westphal et al, 1999).…”

In vivo blockade of N-methyl-d-aspartate receptors induces rapid trafficking of NR2B subunits away from synapses and out of spines and terminals in adult cortex

“…In vitro and fewer in vivo preparations have been used to demonstrate that NMDA receptors, too, are removed from or inserted into synaptic membrane (Rao and Craig, 1997;Liao et al, 1999;Quinlan et al, 1999;Heynen et al, 2000;Barria and Malinow, 2002;Grosshans et al, 2002;Tovar and Westbrook, 2002). However, since nearly all of these studies have used neonatal tissues, whether or not such dynamic properties of NMDA receptor subunits persist at mature synapses, in vivo, is a topic that remains relatively unexplored.Among the NMDA receptor subunits, the NR1 is necessary for NMDA receptor function, but the NR2 subunits may be relatively more central in determining NMDA receptor localization (Mori et al, 1998;Steigerwald et al, 2000;Mohrmann et al, 2002) because their C-termini are longer and permit interaction with postsynaptic density (PSD) scaffolding proteins, such as PSD-95 (Niethammer et al, 1996;Bassand et al, 1999;Sheng and Pak, 2000). Among the NR2 subunits, the NR2A and NR2B subtypes are the most prevalent in the cerebral cortex (Watanabe et al, 1992), and of the two, the NR2B subunits are shown to be particularly influential in learning and memory (Tang et al, 1999;Tang and Schuman, 2002) as they prolong NMDA receptor currents, thus allowing greater Ca influx (Mori and Mishina, 1995;Dingledine et al, 1999).…”

“…Among the NMDA receptor subunits, the NR1 is necessary for NMDA receptor function, but the NR2 subunits may be relatively more central in determining NMDA receptor localization (Mori et al, 1998;Steigerwald et al, 2000;Mohrmann et al, 2002) because their C-termini are longer and permit interaction with postsynaptic density (PSD) scaffolding proteins, such as PSD-95 (Niethammer et al, 1996;Bassand et al, 1999;Sheng and Pak, 2000). Among the NR2 subunits, the NR2A and NR2B subtypes are the most prevalent in the cerebral cortex (Watanabe et al, 1992), and of the two, the NR2B subunits are shown to be particularly influential in learning and memory (Tang et al, 1999;Tang and Schuman, 2002) as they prolong NMDA receptor currents, thus allowing greater Ca influx (Mori and Mishina, 1995;Dingledine et al, 1999).…”