The unique distribution of receptors and their subtypes within a single cell and throughout the brain requires highly selective intracellular targeting mechanisms. My laboratory studies the regulation of glutamate receptor trafficking and localization using a combination of biochemical and molecular techniques. We are investigating the differential sorting of NMDA receptor subunits following endocytosis from the plasma membrane. Using both heterologous cells and primary hippocampal cultures, we have examined the fate of internalized receptors. The NR2B subunit, which is highly expressed early in development, is sorted into recycling endosomes; whereas the NR2A subunit, which is highly expressed in adult animals, is sorted into the late endosomal/lysosomal pathway. These data support unique contributions of the individual NMDA receptor subunits to NMDA receptor trafficking and localization. We are now studying the specific regulation of NR2A and NR2B by the PSD-95 family of proteins. Our results support a unique role for SAP102 in regulating NR2B-containing NMDA receptors. In addition, we are examining the subunit-specific properties of NR2C-containing NMDA receptors, which are specifically enriched in the cerebellum. We find that NR2C is phosphorylated by both PKA (Ser1244) and PKB (Ser1096) on distinct phosphorylation sites that are not conserved in other NR2 subunits, suggesting a unique role of phosphorylation in regulating NR2C-containing receptors. Interestingly, Akt/PKB phosphorylation regulates NR2C binding to 14-3-3 proteins and trafficking of NR2C to the cell surface. Furthermore, IGF-1, an important molecule regulating the survival of cerebellar granule cells, stimulates a signaling cascade that increases phosphorylation of NR2C on Ser1096 and increases NR2C surface expression. In addition to NMDA receptors, we have characterized the trafficking of kainate receptors through the secretory pathway en route to the plasma membrane. We have identified a novel ER-retention motif encoded within an intracellular loop of the kainate receptor subunit KA2 that regulates intracellular transport and surface expression. These data demonstrate that regions of kainate receptors other than the C-terminal domains regulate receptor trafficking.
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