ABSTRACT
BACKGROUND: The Alzheimer's disease (AD)-associated amyloid-beta protein precursor (AßPP) can be cleaved by ß-site AßPP cleaving enzyme 1 (BACE1) and the γ-secretase complex to yield neurotoxic amyloid-ß (Aß) peptides. However, AßPP can also be cleaved in a 'non-amyloidogenic' manner either by α-secretase to produce soluble AßPP alpha (sAßPPα) (a fragment with neuroprotective/neurogenic functions) or through alternative BACE1-mediated 'beta prime' activity yielding soluble AßPP beta prime (sAßPPß'). OBJECTIVE: To determine whether sAßPPα depletion, as opposed to Aß peptide accumulation, contributes to cytotoxicity in AD-relevant SH-SY5Y neuroblastoma cell models. METHODS: AßPP proteolysis was characterized by immunoblotting in mock-, wild-type AßPP (wtAßPP)-, BACE1-, and Swedish mutant AßPP (SweAßPP)-transfected cells. AßPP beta prime cleavage was confirmed through secretase inhibitor studies and C-terminal fragment analysis. The roles of sAßPPα and sAßPPß' in cell viability were confirmed by overexpression studies. RESULTS: Despite producing enhanced Aß peptide levels, wtAßPP- and SweAßPP-transfected cells did not exhibit reduced viability whereas BACE1-transfected cells did. sAßPPα generation in SH-SY5Y-BACE1 cells was virtually ablated in lieu of BACE1-mediated sAßPPß' production. sAßPPα overexpression in SH-SY5Y-BACE1 cells restored viability whereas sAßPPß' overexpression decreased viability further. The anti-AßPP 6E10 antibody was shown to cross-react with sAßPPß'. CONCLUSION: sAßPPα depletion and/or sAßPPß' accumulation, but not elevated Aß peptide levels, represent the cytotoxic mechanism following BACE1 overexpression in SH-SY5Y cells. These data support the novel concept that competitive sAßPPα depletion by BACE1 beta prime activity might contribute to AD. The cross-reactivity of 6E10 with AßPPß'also questions whether previous studies assessing sAßPPα as a biomarker using this antibody should be revisited.