Group I mGluRs have diverse functions in some fundamental neuronal processes, including modulation of synaptic plasticity; and dysregulation of these receptors could lead to various neuropsychiatric disorders. Trafficking of Group I mGluRs plays critical roles in controlling the precise spatiotemporal localization and activity of these receptors, both of which contribute to proper downstream signaling. Using "molecular replacement" approach in hippocampal neurons derived from mice of both sexes, we demonstrate a critical role for the
postsynaptic density protein Norbin in regulating the
ligand-induced internalization of Group I mGluRs. We show that
Norbin associates with
protein kinase A (PKA) through its N-terminus and anchors mGluR5 through its C-terminus, both of which are necessary for the
ligand-mediated endocytosis of mGluR5, a member of the Group I mGluR family. A point mutation (A687G) at the C-terminus of
Norbin inhibits the binding of
Norbin to mGluR5 and blocks mGluR5 endocytosis. Finally, we demonstrate an important mechanism by which
Norbin regulates mGluR-mediated AMPAR endocytosis in hippocampal neurons, a cellular correlate for mGluR-dependent synaptic plasticity.
Norbin, through its PKA-binding regions, recruits PKA to AMPARs on activation of mGluRs; and deletion of the PKA-binding regions of
Norbin inhibits mGluR-triggered AMPAR endocytosis. We further report that
Norbin is important specifically for the mGluR-mediated AMPAR endocytosis, but not for NMDAR-dependent AMPAR endocytosis. Thus, this study unravels a novel role for
Norbin in the internalization of mGluRs and mGluR-mediated AMPAR endocytosis that can have clinical relevance to the function of Group I mGluRs in
pathologic processes.SIGNIFICANCE STATEMENT The postsynaptic
protein Norbin interacts with mGluR5, and both of them have been implicated in disorders, such as
schizophrenia. However, the mechanistic basis underlying the regulation of mGluRs by
Norbin remains elusive. We have identified
Norbin as an essential mediator of
ligand-mediated endocytosis of Group I mGluRs. Mechanistically,
Norbin N-terminus associates with
protein kinase-A (PKA) and C-terminus binds to mGluR5 to coordinate receptor internalization. A point mutation NorA687G inhibits endocytosis by disrupting this interaction. Additionally,
Norbin is critical for the recruitment of PKA to AMPARs on activation of Group I mGluRs that assists in mGluR-mediated AMPAR endocytosis. Thus,
Norbin has a dual function in the hippocampus: regulation of mGluR internalization and PKA-dependent modulation of mGluR-mediated AMPAR endocytosis, a prerequisite for mGluR-mediated synaptic plasticity.