Cathepsins B and L Differentially Regulate Amyloid Precursor Protein Processing

Previous studies have shown that cathepsins control amyloid beta (Aβ) 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 Aβ 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 Aβ release, similar to levels produced with suppression of β-site APP-cleaving enzyme 1 (BACE1) expression. To test whether the catB-dependent release of Aβ was linked to ongoing electrical activity, neurons were treated with tetrodotoxin (TTX) and CA074Me. These comparisons demonstrated no additivity between decreases in Aβ release produced by TTX and CA074Me. In contrast, pharmacological inhibition of cathepsin L (catL) selectively elevated Aβ42 levels but not Aβ40 or total Aβ. Mechanistic studies measuring C-terminal fragments of amyloid precursor protein (APP) suggested that catL elevated α-secretase activity, thereby suppressing Aβ42 levels. The mechanism of catB-mediated regulation of Aβ release remains unclear but may involve elevation of β-secretase. In summary, these studies provide evidence for a significant alternative pathway for APP processing that involves catB and activity-dependent release of Aβ in a regulated secretory pathway for primary neurons.