Norbin: A Critical Regulator of group I metabotropic glutamate receptor internalization and synaptic AMPA receptor endocytosis

Abstract

G protein-coupled receptors (GPCRs) are seven transmembrane receptors that transduce information provided by the extracellular stimuli into intracellular signals via their coupling to Gproteins. Due to the diversity in GPCR regulation, each GPCR is unique and an extensively studied GPCR may not provide all the details about other GPCRs. Glutamate is a major excitatory neurotransmitter in the central nervous system. It activates three types of receptors, viz., NMDARs, AMPARs and metabotropic glutamate receptors (mGluRs). Among them, mGluRs belong to the GPCR family. Group I mGluR family consists of mGluR1 and mGluR5. Group I mGluRs are believed to be involved in multiple forms of experience dependent synaptic plasticity including learning and memory. In addition, they also have been implicated in various neuropsychiatric disorders like Fragile X syndrome, autism etc. Like many other GPCRs, group I mGluRs get desensitized subsequent to the ligand exposure and undergo rapid internalization. These receptors are localized in a protein dense region at the post-synaptic membrane called the post-synaptic density (PSD). The post-synaptic density of excitatory synapses is very complex in composition and dynamic in nature. The involvement of post-synaptic density proteins in the ligand-mediated trafficking of group I mGluRs is not well understood. Norbin is a neuronal cytoplasmic protein that interacts with group I mGluRs and affects mGluR5 activity. However, the mechanism by which it modulates mGluR function is largely unknown. Using an experimentally tractable system that can closely mimic the in vivo trafficking events, i.e., dissociated primary hippocampal neurons from mice, we show that Norbin is a scaffolding protein that is crucial for the ligand-mediated internalization of group I mGluRs. We report that Norbin, through its N-terminus, associates with protein kinase A (PKA) and anchors mGluR5 through its C-terminus, both of which are necessary for the ligand-mediated mGluR5 endocytosis. We also found a novel role for Norbin in the regulation of mGluR-mediated AMPAR endocytosis. Our results suggest that recruitment of PKA to AMPARs through Norbin upon mGluR activation is an important mechanism specifically for mGluR-triggered AMPAR endocytosis and consequently for mGluR-mediated synaptic plasticity in the hippocampus. Thus, Norbin is crucial for the spatiotemporal regulation of group I mGluRs and it also provides a control over mGluR-mediated AMPAR endocytosis that may be relevant for neuropsychiatric disorders that report a dysregulation of mGluR5 and Norbin