Mechanism of receptor trafficking regulation by mGluR5 phosphorylation

Abstract

Glutamate plays an important role in CNS as an excitatory neurotransmitter, and exerts its action through the ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs). mGluR5, a subtype in the group I mGluRs, is widely expressed in hippocampus and pre-frontal cortex, and modulates synaptic transmission. It was shown that mGluR5 serine 901 (S901) is a novel phosphorylation site of PKC. The surface expression of mGluR5 was reduced by S901 phosphorylation by inhibiting the binding of CaM to the receptor. The underlying mechanism of CaM-dependent mGluR5 trafficking, however, was poorly understood. Recent works have shown that seven in absentia homolog-1A (Siah-1A) mediates the ubiquitination and degradation of the group I mGluRs, and that Siah-1A competes with calmodulin (CaM) for binding to the group I mGluRs.In this study, a novel trafficking mechanism of mGluR5, which is regulated by competitive interaction between CaM and Siah-1A, an E3 protein ubiquitin ligase, is presented. It was found that the protein stability of mGluR5 S901D is lower than that of WT, and that the stability of S901D is affected by the Siah-1A binding. It was also found that S901 phosphorylation induces CaM displacement and Siah-1A binding to mGluR5, and that the competitive interaction between CaM and Siah-1A affects the CaM-dependent regulation of mGluR5 trafficking. Important residues were identified on the mGluR5 C-terminus for Siah-1A binding, and it was shown that Siah-1A binding is a critical factor for the regulation of mGluR5 trafficking. Siah-1A binding decreased the membrane stability of mGluR5 in the hippocampal neurons, and Siah-1A binding to the receptor affects the mGluR5 endosomal trafficking. Taken together, these data indicate that CaM regulates mGluR5 trafficking through the PKC-dependent regulation of the receptor-binding proteins. Furthermore, it expects that these results could enhance to understand the GPCR functions and the pathophysiology of mGluR5-related diseases in the CNS.