Distinct Roles of NR2A and NR2B Cytoplasmic Tails in Long-Term Potentiation

Abstract

NMDA receptors (NMDARs) are critical mediators of activity-dependent synaptic plasticity, but the differential roles of NR2A-versus NR2B-containing NMDARs have been controversial. Here, we investigate the roles of NR2A and NR2B in long-term potentiation (LTP) in organotypic hippocampal slice cultures using RNA interference (RNAi) and overexpression, to complement pharmacological approaches. In young slices, when NR2B is the predominant subunit expressed, LTP is blocked by the NR2B-selective antagonist Ro25-6981 [R-(R,S)-␣-(4-hydroxyphenyl)-␤-methyl-4-(phenylmethyl)-1-piperidine propranol]. As slices mature and NR2A expression rises, activation of NR2B receptors became no longer necessary for LTP induction. LTP was blocked, however, by RNAi knockdown of NR2B, and this was rescued by coexpression of an RNAi-resistant NR2B (NR2B*) cDNA. Interestingly, a chimeric NR2B subunit in which the C-terminal cytoplasmic tail was replaced by that of NR2A failed to rescue LTP, whereas the reverse chimera, NR2A channel with NR2B tail, was able to restore LTP. Thus, expression of NR2B with its intact cytoplasmic tail is required for LTP induction, at an age when channel activity of NR2B-NMDARs is not required for LTP. Overexpression of wild-type NR2A failed to rescue LTP in neurons transfected with the NR2B-RNAi construct, despite restoring NMDA-EPSC amplitude to a similar level as NR2B*. Surprisingly, an NR2A construct lacking its entire C-terminal cytoplasmic tail regained its ability to restore LTP. Together, these data suggest that the NR2B subunit plays a critical role for LTP, presumably by recruiting relevant molecules important for LTP via its cytoplasmic tail. In contrast, NR2A is not essential for LTP, and its cytoplasmic tail seems to carry inhibitory factors for LTP.

Interaction Between CaMKII and GluN2B Controls ERK-Dependent Plasticity

“…Since ERK1/2 activation and LTP induction are linked (English and Sweatt, 1997;Di Cristo et al, 2001;Zhu et al, 2002), we might expect that the GluN2A/2B to ERK1/2 activation is also developmentally regulated. An important study that helped in teasing out the distinct roles of GluN2B and GluN2A in the signaling of LTP is from Foster et al (2010), which showed, by exchanging the cytoplasmic tails of each subunit, that the one of GluN2B is playing a critical role in LTP. Meanwhile, the increased ratio of GluN2A/GluN2B, later in development, may confer a more important role for GluN2A in gating the postsynaptic Ca 2ϩ necessary for LTP.…”

“…In addition, their cytoplasmic tails have distinct features allowing each subunit to engage in different signaling pathways (Köhr, 2006). Some of these features may specify the impact of the receptor on synaptic plasticity (Foster et al, 2010) and neurotoxicity (Xiao et al, 2011). Meanwhile, studies addressing the role of NMDAR subunit composition in ERK1/2 signaling pathway in synaptic plasticity and memory have yielded seemingly contrary results.…”

Studying Signal Transduction in Single Dendritic Spines

“…It has been suggested that LTP is mediated via nanodomain signal transduction at specific subunits of NMDARs because, under some conditions, LTP induction has been found to be more sensitive to BAPTA and EGTA (Hoffman et al 2002). Furthermore, pharmacological inhibition of NR2A and NR2B containing NMDARs can differentially impact LTP, suggesting that there may be subunit-specific signal transduction (Liu et al 2004;Yashiro and Philpot 2008;Foster et al 2010). Using CaMKII and Ras imaging techniques, we can now directly examine if the activation of these pathways is mediated via nanodomain signaling.…”

Metabotropic NMDA receptor function is required for β-amyloid–induced synaptic depression

“…Whereas GluN2B is predominant in synapses of the early postnatal brain, GluN2A numbers progressively increase with age and eventually outnumber GluN2Bs (22)(23)(24)(25). This developmental switch from GluN2B-to GluN2A-rich synapses has important implications for the induction of NMDAR-mediated plasticity (26)(27)(28)(29).…”