The metabotropic glutamate receptor mGluR5 is endocytosed by a clathrin-independent pathway

Metabotropic glutamate receptors 5 (mGluR5) are members of the growing group C G protein-coupled receptor family. Widely expressed in mammalian brain, they are involved in modulation of the glutamate transmission. By means of transfection of mGluR5 receptors in COS-7 cells and primary hippocampal neurons in culture followed by immunocytochemistry and quantitative image analysis and by a biochemical assay, we have studied the internalization of mGluR5 splice variants. mGluR5a and -5b were endocytosed in COS-7 cells as well as in axons and dendrites of cultured neurons. Endocytosis occurred even in the absence of receptor activity, because receptors mutated in the glutamate binding site were still internalized as well as receptors in which endogenous activity had been inhibited by an inverse agonist. We have measured a constitutive rate of endocytosis of 11.7%/min for mGluR5a. We report for the first time the endocytosis pathway of mGluR5. Internalization of mGluR5 is not mediated by clathrin-coated pits. Indeed, inhibition of this pathway by Eps15 dominant negative mutants did not disturb their endocytosis. However, the large GTPase dynamin 2 is implicated in the endocytosis of mGluR5 in COS-7. mGluR5 is the first shown member of the group C G-protein coupled receptor family internalized by a nonconventional pathway.