Dysfunctional AMPA Receptor Trafficking in Traumatic Brain Injury

Traumatic brain injury (TBI) is a devastating public health problem for patients and their families. The neurodegeneration that follows TBI is complex, but can be broadly subdivided into primary and secondary damage. Though primary damage is irreversible and therefore unsalvageable, extensive literature aimed at understanding the tissue, cellular, inflammatory and subcellular processes following the injury have proven unequivocally that secondary pathophysiological events are delayed and progressive in nature. Understanding these secondary events at the cellular levels is critical in the eventual establishment of targeted therapeutics aimed at limiting progressive injury after an initial trauma. One such secondary event is referred to in the literature as excitotoxicity; a sustained and de-regulated activation of glutamate receptors that leads to rapid cytotoxic edema and calcium overload. Our understanding of excitotoxicity has evolved to include not only a role for elevated extracellular glutamate in mediating neuronal damage, but also post-synaptic receptor modifications that render glutamate profoundly more toxic to injured neurons than healthy tissue. In this thesis, we explored the hypothesis that glutamate excitotoxicity can be perpetuated by trauma-induced post-synaptic modification of the AMPA receptor. Specifically, we used a cortical culture model of TBI as well as the fluid percussion injury device to test the hypothesis that TBI confers a reduction of surface GluR2 protein, an AMPA receptor subunit that limits neuronal calcium permeability. We conjectured that this decrease in the expression of surface GluR2 would increase the expression of calcium-permeable AMPA receptors, thereby rendering neurons vulnerable to secondary excitotoxic injury. We further investigated the subcellular mechanisms responsible for the internalization of surface GluR2, and the phenotypic consequences of GluR2 endocytosis in both models. Our data revealed that both models of TBI resulted in a regulated signaling cascade leading to the phosphorylation and internalization of GluR2. By exogenously interrupting the trafficking of GluR2 protein with an inhibitory peptide, we further observed that GluR2 internalization was mediated by a protein interaction involving protein interacting with C kinase 1 (PICK1) and protein kinase C alpha (PKCα), two PDZ domain-containing proteins that mediate GluR2 trafficking during constitutive synaptic plasticity. We observed that GluR2 endocytosis was NMDA receptor dependent, and resulted in increased neuronal calcium permeability, augmented AMPA receptor-mediated electrophysiological activity and increased susceptibility to delayed cell death. Finally, we demonstrated that the interruption of GluR2 trafficking is cytoprotective, suggesting that sustaining surface GluR2 protein protects neurons against secondary injury. Overall, our findings suggest that experimental TBI promotes the expression of injurious GluR2-lacking AMPA receptors, thereby enhancing cellular vulnerability to secondary excitotoxicity.Ph