Relationship between Availability of NMDA Receptor Subunits and Their Expression at the Synapse

In NMDA receptors, GluR⑀/NR2 subunits strictly require the GluR1/NR1 subunit to exit from endoplasmic reticulum (ER) to the cell surface in vitro and to the postsynapse in vivo, whereas C terminus-dependent self-surface delivery has been demonstrated for the GluR1 subunit in vitro. To test whether this leads to C terminus-dependent self-postsynaptic expression in neurons in vivo, we investigated the GluR1 subunit in cerebellar granule cells lacking two major GluR⑀ subunits, GluR⑀1/NR2A and GluR⑀3/NR2C. In the mutant cerebellum, synaptic labeling for the GluR1 subunit containing the C2 (GluR1-C2) or C2Ј (GluR1-C2Ј) cassette was reduced at mossy fiber-granule cell synapses to the extrasynaptic level. The loss was not accompanied by decreased transcription and translation levels, increased extrasynaptic labeling, or ER accumulation. Quantitative immunoblot revealed substantial reductions in the mutant cerebellum of GluR1-C2 and GluR1-C2Ј. The most severe deficit was observed in the postsynaptic density (PSD) fraction: mutant levels relative to the wild-type level were 12.3 Ϯ 3.3% for GluR1-C2 and 17.0 Ϯ 4.6% for GluR1-C2Ј. The GluR1 subunit carrying the C1 cassette (GluR1-C1) was, although low in cerebellar content, also reduced to 12.7 Ϯ 3.5% in the mutant PSD fraction. Considering a trace amount of other GluR⑀ subunits in the mutant cerebellum, the severe reductions thus represent that the GluR1 subunit, by itself, is virtually unable to accumulate at postsynaptic sites, regardless of C-terminal forms. By protein turnover analysis, the degradation of the GluR1 subunit was accelerated in the mutant cerebellum, being particularly rapid for that carrying the C2 cassette. Therefore, accompanying expression of GluR⑀ subunits is essential for postsynaptic localization and protein stability of the GluR1 subunit.

Section: Facilitative Interaction Between Glur⑀ Subunitssupporting

confidence: 89%

NMDA Receptor GluRϵ/NR2 Subunits Are Essential for Postsynaptic Localization and Protein Stability of GluRζ1/NR1 Subunit

Abe¹,

Fukaya²,

Yagi³

et al. 2004

“…NR1 is reportedly synthesized in considerable excess (estimated to be ϳ10-fold) compared with NR2, based on the finding that one pool of NR1 was not assembled with NR2, which showed rapid degradation (Huh and Wenthold, 1999;Wenthold et al, 2003). Consistent with this idea, overexpression of NR2, but not NR1, increased functional receptors in cultured granule cells (Prybylowski et al, 2002). Moreover, overexpression of NR2B in the mouse forebrain increased synaptic NMDA currents in the hippocampus and cortex (Tang et al, 1999;Wei et al, 1999).…”

Section: Discussionmentioning

confidence: 96%

Upregulation of Forebrain NMDA NR2B Receptors Contributes to Behavioral Sensitization after Inflammation

Wu¹,

Toyoda²,

Zhao³

et al. 2005

222

235

Transgenic overexpression of NMDA NR2B receptors in forebrain regions increased behavioral responses to persistent inflammatory pain. However, it is not known whether inflammation leads to the upregulation of NR2B receptors in these regions. Here, we show that peripheral inflammation increased the expression of NMDA NR2B receptors and NR2B receptor-mediated synaptic currents in the anterior cingulate cortex (ACC). In freely moving mice, the increase in NR2B receptors after inflammation contributed to enhanced NMDA receptor-mediated responses in the ACC. Inhibition of NR2B receptors in the ACC selectively reduced behavioral sensitization related to inflammation. Our results demonstrate that the upregulation of NR2B receptors in the ACC contributes to behavioral sensitization caused by inflammation.

“…Subunit composition controls NMDAR synaptic localization (Mori et al, 1998;Steigerwald et al, 2000;Prybylowski et al, 2002) and synaptic stability (Barria and Malinow, 2002). However, the relative contribution of NR1 and NR2 subunits to receptor endocytosis remains unknown.…”

Section: C-terminal Domains Of Nr1 and Nr2b Contribute Independently mentioning

confidence: 99%

Endocytosis and Degradative Sorting of NMDA Receptors by Conserved Membrane-Proximal Signals

Scott¹,

Michailidis²,

Mu³

et al. 2004

113

102

Regulation of the abundance of NMDA receptors (NMDARs) at excitatory synapses is critical during changes in synaptic efficacy underlying learning and memory as well as during synapse formation throughout neural development. However, the molecular signals that govern NMDAR delivery, maintenance, and internalization remain unclear. In this study, we identify a conserved family of membraneproximal endocytic signals, two within the NMDAR type 1 (NR1) subunit and one within the NR2A and NR2B subunits, necessary and sufficient to drive the internalization of NMDARs. These endocytic motifs reside in the region of NMDAR subunits immediately after the fourth membrane segment, a region implicated in use-dependent rundown and NMDA channel inactivation. Although endocytosis driven by the distal C-terminal domain of NR2B is followed by rapid recycling, internalization mediated by membrane-proximal motifs selectively targets receptors to late endosomes and accelerates degradation. These results define a novel conserved signature of NMDARs regulating internalization and postendocytic trafficking.