Carboxyl-terminal fragments of alzheimer β-amlyloid precursor protein accumulate in restricted and unpredicted intracellular compartments in presenilin 1-deficient cells

Absence of functional presenilin 1 (PS1) protein leads to loss of γ-secretase cleavage of the amyloid precursor protein (βAPP), resulting in a dramatic reduction in amyloid β peptide (Aβ) production and accumulation of α- or β-secretase-cleaved COOH-terminal fragments of βAPP (α- or β-CTFs). The major COOH-terminal fragment (CTF) in brain was identified as αAPP-CTF-(11-98), which is consistent with the observation that cultured neurons generate primarily Aβ-(11-40). In PS1(-/-) murine neurons and fibroblasts expressing the loss-of-function PS1(D385A) mutant, CTFs accumulated in the endoplasmic reticulum, Golgi, and lysosomes, but not late endosomes. There were some subtle differences in the subcellular distribution of CTFs in PS1(-/-) neurons as compared with PS1(D385A) mutant fibroblasts. However, there was no obvious redistribution of full-length βAPP or of markers of other organelles in either mutant. Blockade of endoplasmic reticulum-to-Golgi trafficking indicated that in PS1(-/-) neurons (as in normal cells) trafficking of βAPP to the Golgi compartment is necessary before α- and β-secretase cleavages occur. Thus, although we cannot exclude a specific role for PS1 in trafficking of CTFs, these data argue against a major role in general protein trafficking. These results are more compatible with a role for PS1 either as the actual γ-secretase catalytic activity or in other functions indirectly related to γ-secretase catalysis (e.g. an activator of γ-secretase, a substrate adaptor for γ-secretase, or delivery of γ-secretase to βAPP-containing compartments).