BACE overexpression alters the subcellular processing of APP and inhibits Aβ deposition in vivo

Abstract

Introducing mutations within the amyloid precursor protein (APP) that affect β- and γ-secretase cleavages results in amyloid plaque formation in vivo. However, the relationship between β-amyloid deposition and the subcellular site of Aβ production is unknown. To determine the effect of increasing β-secretase (BACE) activity on Aβ deposition, we generated transgenic mice overexpressing human BACE. Although modest overexpression enhanced amyloid deposition, high BACE overexpression inhibited amyloid formation despite increased β-cleavage of APP. However, high BACE expression shifted the subcellular location of APP cleavage to the neuronal perikarya early in the secretory pathway. These results suggest that the production, clearance, and aggregation of Aβ peptides are highly dependent on the specific neuronal subcellular domain wherein Aβ is generated and highlight the importance of perikaryal versus axonal APP proteolysis in the development of Aβ amyloid pathology in Alzheimer's disease.

Alternative Selection of β-Site APP-Cleaving Enzyme 1 (BACE1) Cleavage Sites in Amyloid β-Protein Precursor (APP) Harboring Protective and Pathogenic Mutations within the Aβ Sequence

“…Overexpression of BACE1 decreases the level of mature APP by disturbing axonal transport in the brain in vivo (36); however, based on our observations in undifferentiated N2a cells (Fig. 5A), it is reasonable to speculate that the decrease in the level of mature hAPP is due to proteolysis by overexpressed BACE1 along with elevated generation of CTF␤/ C99 and CTF␤Ј/C89.…”

“…Third, we demonstrated that elevation of BACE1 activity due to overexpression of the enzyme results in secondary cleavage of CTF␤/C99 at the ␤Ј-site, dramatically increasing A␤(11-XX) production. This is true, at least in the cell study, even for APP harboring the Swedish mutation, which predominantly produces A␤(1-XX), although a pilot study in a BACE-overexpressing transgenic mouse suggested that reduction in A␤ deposition is mediated by another mechanism (36). Finally, we showed that A␤(11-XX) may be more metabolically labile than A␤(1-XX) in vivo and in vitro.…”

“…These results clearly showed that CTF␤/C99, the product of hAPP primarily cleaved at the ␤-site by BACE1, can be further cleaved at the ␤Ј-site by excess BACE1 activity. These data suggest that excess BACE1 activity functions amyloidolytic instead of amyloidogenic, at least under our experimental conditions, rather than by decreasing the levels of its substrates (mature APP and CTF␤/C99) by altering the intracellular APP transport system in in vivo neurons (36).…”

“…Because BACE1 can use CTF␤/ C99 as well as APP as substrates (12), we investigated whether elevated BACE1 activity can cleave CTF␤/C99 at the ␤Ј-site subsequent to the primary cleavage. BACE1 overexpression was previously reported to decrease brain A␤ levels in vivo (36), but the mechanism remains controversial.…”

Alzheimer Disease Aβ Production in the Absence of S-Palmitoylation-dependent Targeting of BACE1 to Lipid Rafts

“…Short pulse labeling with [ 35 S]Met/Cys showed that wtAPP expression is similar in the stable pools analyzed (data not shown). Continuous labeling for 3 h showed a marked reduction in the levels of mature APP in stable cells overexpressing wtBACE1, indicative of efficient processing by BACE1, as observed in previous studies (61). To confirm this notion, we examined the proteolytic products derived from BACE1 cleavage of APP, i.e.…”