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NMDA receptor NR2A/B subunits have PDZ-binding domains on their extreme C-termini that are known to interact with the PSD-95 family and other PDZ proteins. We explore the interactions between PSD-95 family proteins and the NR2A/B cytoplasmic tails, and the consequences of these interactions, from the endoplasmic reticulum (ER) through delivery to the synapse in primary rat hippocampal and cortical cultured neurons. We find that the NR2A/B cytoplasmic tails cluster very early in the secretory pathway and interact serially with SAP102 beginning at the intermediate compartment, and then PSD-95. We further establish that colocalization of the distal C-terminus of NR2B and PSD-95 begins at the trans-Golgi Network (TGN). Formation of NR2B/PSD-95/SAP102 complexes is dependent on the PDZ binding domain of NR2B subunits, but association with SAP102 and PSD-95 plays no distinguishable role in cluster pre-formation or initial targeting to the vicinity of the synapse. Instead the PDZ binding domain plays a role in restricting cell-surface clusters to postsynaptic targets.

The NMDA receptor (NMDAR) plays a central role in the function of excitatory synapses. Recent studies have provided interesting insights into several aspects of the trafficking of this receptor in neurons. The NMDAR is not a static resident of the synapse. Rather, the number and composition of synaptic NMDARs can be modulated by several factors. The interaction of PDZ proteins, generally thought to occur at the synapse, appears to occur early in the secretory pathway; this interaction may play a role in the assembly of the receptor complex and its exit from the endoplasmic reticulum. This review addresses recent advances in our understanding of NMDAR trafficking and its synaptic delivery and maintenance.

Although synaptic AMPA receptors have been shown to rapidly internalize, synaptic NMDA receptors are reported to be static. It is not certain whether NMDA receptor stability at synaptic sites is an inherent property of the receptor, or is due to stabilization by scaffolding proteins. In this study, we demonstrate that NMDA receptors are internalized in both heterologous cells and neurons, and we define an internalization motif, YEKL, on the distal C-terminus of NR2B. In addition, we show that the synaptic protein PSD-95 inhibits NR2B-mediated internalization, and that deletion of the PDZ-binding domain of NR2B increases internalization in neurons. This suggests an involvement for PSD-95 in NMDA receptor regulation and an explanation for NMDA receptor stability at synaptic sites.

The NMDA receptor NR1 subunit has four splice variants that differ in their C-terminal, cytoplasmic domain. We investigated the contribution of the C-terminal cassettes, C0, C1, C2, and C2', to trafficking of NR1 in heterologous cells and neurons. We identified an ER retention signal (RRR) in the C1 cassette of NR1, which is similar to the RXR motif in ATP-sensitive K(+) channels (Zerangue et al., 1999). We found that surface expression of NR1-3, which contains C1, is due to a site on the C2' cassette, which includes the terminal 4 amino acid PDZ-interacting domain. This site suppresses ER retention of the C1 cassette and leads to surface expression. These findings suggest a role for PDZ proteins in facilitating the transition of receptors from an intracellular pool to the surface of the neuron.