The responsiveness of a neuron to neurotransmitter released from a presynaptic cell is determined by the type and amount of receptor expressed on the postsynaptic membrane. 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 regulation and localization. We are now studying the regulation of NR2C-containing NMDA receptors, which are specifically enriched in the cerebellum. NR2C is phosphorylated by both PKA and PKB on distinct phosphorylation sites which are not conserved in other NR2 subunits, suggesting a unique role of phosphorylation in regulating NR2C-containing receptors. Interestingly, PKB phosphorylation regulates protein binding and trafficking of NR2C. In another project, we are investigating the phosphorylation of metabotropic glutamate receptors. We have identified several specific residues within mGluR5 that are phosphorylated by protein kinase C. These PKC sites are localized within the proximal one-third of the mGluR5 C-terminal domain. One phosphorylation site, Ser839, determines the regulation of intracellular calcium oscillations in response to mGluR5 activation. A second phosphorylation site that we have identified, Ser901, regulates the binding of the protein calmodulin. In addition, PKC phosphorylation of Ser901 regulates mGluR5 surface expression and endocytosis. These studies precisely defining the phosphorylation of mGluRs by PKC will allow us to study the functional consequences of glutamate receptor phosphorylation and the regulation of intracellular signaling and receptor trafficking. Finally, we have characterized the trafficking of the kainate receptor subunit KA2 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 protein 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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