Traumatic brain injury (TBI) is one of the most disabling injuries in the population, with 1.5 million Americans new cases each year and 5.3 million Americans overall requiring long-term daily care as a result of their injuries. One critical aspect in developing effective treatments for TBI is determining if new, specific receptor populations emerge in the early phase after injury that can subsequently be targeted to reduce neuronal death after injury. One specific glutamate receptor subtype, the calcium-permeable AMPA receptor (CP-AMPAR), is becoming increasingly recognized for its role in physiological and pathophysiological processes. Although present in relatively low levels in the mature brain, recent studies show that CP-AMPARs can appear following ischemic brain injury or status epilepticus, and the mechanisms that regulate the appearance of these receptors include alterations in transcription, RNA editing, and receptor trafficking. In this report, we use an in vitro model of TBI to show a gradual appearance of CP-AMPARs four hours following injury to cortical neurons. Moreover, the appearance of these receptors is mediated by the phosphorylation of CaMKIIα following injury. Selectively blocking CP-AMPARs after mechanical injury leads to a significant reduction in the cell death that occurs 24 h following injury in untreated controls, and is similar in protection offered by broad-spectrum NMDA and AMPA receptor antagonists. These data point to a potentially new and more targeted therapeutic approach for treating TBI.

Previous studies have shown that cathepsins control amyloid beta (A␤) levels in chromaffin cells via a regulated secretory pathway. In the present study, this concept was extended to investigations in primary hippocampal neurons to test whether A␤ release was coregulated by cathepsins and electrical activity, proposed components of a regulated secretory pathway. Inhibition of cathepsin B (catB) activity with CA074Me or attenuation of catB expression through small interfering RNA produced decreases in A␤ release, similar to levels produced with suppression of ␤-site APP-cleaving enzyme 1 (BACE1) expression. To test whether the catB-dependent release of A␤ was linked to ongoing electrical activity, neurons were treated with tetrodotoxin (TTX) and CA074Me. These comparisons demonstrated no additivity between decreases in A␤ release produced by TTX and CA074Me. In contrast, pharmacological inhibition of cathepsin L (catL) selectively elevated A␤42 levels but not A␤40 or total A␤. Mechanistic studies measuring C-terminal fragments of amyloid precursor protein (APP) suggested that catL elevated ␣-secretase activity, thereby suppressing A␤42 levels. The mechanism of catB-mediated regulation of A␤ release remains unclear but may involve elevation of ␤-secretase. In summary, these studies provide evidence for a significant alternative pathway for APP processing that involves catB and activity-dependent release of A␤ in a regulated secretory pathway for primary neurons.