Excitotoxicity mediated by glutamate receptors plays crucial roles in ischemia and other neurodegenerative diseases. Whereas overactivation of ionotropic glutamate receptors is neurotoxic, the role of metabotropic glutamate receptors (mGluRs), and especially mGluR1, remains equivocal. Here we report that activation of NMDA receptors results in calpain-mediated truncation of the C-terminal domain of mGluR1α at Ser936. The truncated mGluR1α maintains its ability to increase cytosolic calcium while it no longer activates the neuroprotective PI3K-Akt signaling pathways. Full-length and truncated forms of mGluR1α play distinct roles in excitotoxic neuronal degeneration in cultured neurons. A fusion peptide derived from the calpain cleavage site of mGluR1α efficiently blocks NMDA-induced truncation of mGluR1α in primary neuronal cultures and exhibits neuroprotection against excitotoxicity both in vitro and in vivo. These findings shed light on the relationship between NMDA and mGluR1α and indicate the existence of a positive feedback regulation in excitotoxicity involving calpain and mGluR1α.
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