Cathepsins B and L Differentially Regulate Amyloid Precursor Protein Processing

Abstract

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.

Lysosomal Modulatory Drugs for a Broad Strategy Against Protein Accumulation Disorders

“…Genetic inactivation of cathepsin B was also found to increase the abundance of A 1-42 and worsen the pathology. In striking contrast however, while the above studies strongly implicate cathepsin B activity in A clearance, others report that inhibition of cathepsin B reduces A production and release in chromaffin cells, primary hippocampal neurons, and hAPP mice expressing the wild-type -secretase site [89][90][91][92]. The mouse model exhibited a reduction in the characteristic memory deficit in association with the inhibitor's effect on A 1-42 , CTFs, and amyloid load.…”

“…With such evidence that cathepsin B contributes to the formation of amyloidogenic peptides, perhaps PADK, being both a cathepsin inhibitor and a positive modulator, can express multiple protective actions including blocking cathepsin B's production of toxic peptides through its inhibitory effect, and up-regulating cathepsin L levels to suppress the peptides. The latter stems from the report in which cathepsin L was found to reduce A levels [92], and it is of further interest that combined deficiency in cathepsins B and L leads to pathogenic accumulations, axonopathy, and subsequent neuronal loss [93]. It is important to note that PADK reduces CTFs and other accumulation events that are pre-existing in the slice model and whose production has been ceased by removal of chloroquine, thus suggesting that the lysosomal modulator primarily improves clearance rather than blocking cathepsin B's or other cathepsins' ability to generate amyloidogenic species.…”

Pharmacogenetic features of cathepsin B inhibitors that improve memory deficit and reduce β-amyloid related to Alzheimer's disease

“…Furthermore, gene silencing of cathepsin B by siRNA in normal WT hippocampal brain neurons (in primary culture) reduces the amount of Aβ in the regulated secretory pathway (Klein et al, 2009). These studies also showed that the CA074Me inhibitor reduces Aβ in the regulated secretory pathway of rat hippocampal neurons, indicating a role for cathepsin B in Aβ production.…”

“…Notably, inhibitors of cathepsin B reduced production of Aβ in the isolated regulated secretory vesicles (Hook et al, 2005). These inhibitors of cathepsin B also reduced production of Aβ in the regulated secretory pathway of hippocampal neurons (rat) (Klein et al, 2009). But these inhibitors had no effect on Aβ in the constitutive secretory pathway.…”

“…Aβ production in regulated secretory vesicles by cathepsin B has been demonstrated in several neuronal and animal model systems consisting of neuronal systems in culture and in animal models from bovine, guinea pig, mouse, and rat (summarized in Table 2) (Hook et al, 2005, 2007a/b, 2008a,b, 2009; Bohme et al, 2008; Klein et al, 2009). These studies support the hypothesis that cathepsin B may produce Aβ from APP via β-secretase activity in the regulated secretory pathway, but not the constitutive secretory pathway.…”