Cathepsins B and L Differentially Regulate Amyloid Precursor Protein Processing

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