The membrane-associated guanylate kinases [Chapsyn-110/postsynaptic density-93 (PSD-93), synapse-associated protein-90 (SAP-90)/PSD-95, and SAP-102] are believed to cluster and anchor NMDA receptors at the synapse and to play a role in signal transduction. We have investigated the developmental changes in expression of these proteins in rat hippocampus using biochemical analyses and quantitative immunogold electron microscopy. At postnatal day 2 (P2), SAP-102 was highly expressed, whereas PSD-93 and PSD-95 were low. SAP-102 expression increased during the first week, stayed stable through P35, and showed a reduced expression at 6 months. From P2 through 6 months, PSD-93 and PSD-95 increased. For PSD-95, the percent of labeled synapses increased almost threefold with age, whereas the number of gold particles per labeled synapse did not change significantly, suggesting that the increase in PSD-95 is attributable primarily to an increase in the number of synapses containing PSD-95. In contrast, for SAP-102, both percent labeled synapses and the number of gold particles per labeled synapse decreased during this time. From Western blots of hippocampus and immunogold analysis of CA1 synapses, the high expression of NR2B at P2 coincides with the high level of SAP-102 at synapses, whereas the later expression of NR2A coincides with that of PSD-93 and PSD-95. To determine whether the changes in PSD-93/95 and SAP-102 reflect preferred associations with NR2A and NR2B, respectively, we measured co-immunoprecipitation in the adult hippocampus. These studies suggest that there is a preference for complexes of NR2A/PSD-93/95 and NR2B/SAP-102. These results indicate that individual receptor-associated proteins may have specific functions that are critical to synapse development.

The NMDA receptor (NMDAR) is a component of excitatory synapses and a key participant in synaptic plasticity. We investigated the role of two domains in the C terminus of the NR2B subunit--the PDZ binding domain and the clathrin adaptor protein (AP-2) binding motif--in the synaptic localization of NMDA receptors. NR2B subunits lacking functional PDZ binding are excluded from the synapse. Mutations in the AP-2 binding motif, YEKL, significantly increase the number of synaptic receptors and allow the synaptic localization of NR2B subunits lacking PDZ binding. Peptides corresponding to YEKL increase the synaptic response within minutes. In contrast, the NR2A subunit localizes to the synapse in the absence of PDZ binding and is not altered by mutations in its motif corresponding to YEKL of NR2B. This study identifies a dynamic regulation of synaptic NR2B-containing NMDARs through PDZ protein-mediated stabilization and AP-2-mediated internalization that is modulated by phosphorylation by Fyn kinase.

Planar cell polarity (PCP) is a process in which cells