PICK1-mediated GluR2 endocytosis contributes to cellular injury after neuronal trauma.

Constitutive and activity-dependent regulation of the AMPA receptor GluR2 content is recognized as an important mediator of both neuronal plasticity and vulnerability to excitotoxic neuron death. In the latter case, inclusion of GluR2 protects against glutamate excitotoxicity in CNS disease by lowering receptor single-channel conductance and preventing deleterious calcium influx. We investigated the hypothesis that aberrations in GluR2 trafficking after in vitro and in vivo cerebral trauma contribute to excitotoxicity and associated calcium-dependent cell death processes. First, in an in vitro model of traumatic brain injury (TBI), we observed PICK1 and N-methyl-D-aspartic acid (NMDA) receptor-dependent phosphorylation and internalization of GluR2. The contributing cell signaling mechanisms involved enhanced binding between PKCalpha (the kinase that phosphorylates GluR2) and PICK1 (its PDZ-binding partner), and a novel protein interaction between PKCalpha and the NMDA receptor scaffolding protein PSD-95. Functionally, these phenomena enhanced single cell AMPAR mEPSCs and protracted calcium extrusion. In vivo TBI similarly promoted GluR2 phosphorylation and internalization, with enhanced expression of calcium-permeable AMPARs in the injured hippocampus. Peptide-mediated perturbation of the PKCalpha/PICK1 protein interaction after trauma preserved surface GluR2 expression, attenuated AMPAR-mediated toxicity, and occluded the sensitivity of neuronal physiology to calcium-permeable AMPAR antagonists. These findings suggest that experimental TBI promotes the expression of injurious GluR2-lacking AMPARs, thereby enhancing cellular vulnerability to secondary excitotoxicity.