Elevation of β-Amyloid Peptide 2-42 in Sporadic and Familial Alzheimer's Disease and Its Generation in PS1 Knockout Cells

Abstract

Urea-based β-amyloid (Aβ) SDS-polyacrylamide gel electrophoresis and immunoblots were used to analyze the generation of Aβ peptides in conditioned medium from primary mouse neurons and a neuroglioma cell line, as well as in human cerebrospinal fluid. A comparable and highly conserved pattern of Aβ peptides, namely, 1-40/42 and carboxyl-terminal-truncated 1-37, 1-38, and 1-39, was found. Besides Aβ1-42, we also observed a consistent elevation of amino-terminal-truncated Aβ2-42 in a detergent-soluble pool in brains of subjects with Alzheimer's disease. Aβ2-42 was also specifically elevated in cerebrospinal fluid samples of Alzheimer's disease patients. To decipher the contribution of potential different γ-secretases (presenilins (PSs)) in generating the amino-terminal- and carboxyl-terminal-truncated Aβ peptides, we overexpressed β-amyloid precursor protein (APP)-trafficking mutants in PS1+/+ and PS1-/- neurons. As compared with APP-WT (primary neurons from control or PS1-deficient mice infected with Semliki Forest virus), PS1-/- neurons and PS1+/+ neurons overexpressing APP-Δct (a slow-internalizing mutant) show a decrease of all secreted Aβ peptide species, as expected, because this mutant is processed mainly by α-secretase. This drop is even more pronounced for the APP-KK construct (APP mutant carrying an endoplasmic reticulum retention motif). Surprisingly, Aβ2-42 is significantly less affected in PS1-/- neurons and in neurons transfected with the endocytosis-deficient APP-Δct construct. Our data confirm that PS1 is closely involved in the production of Aβ1-40/42 and the carboxyl-terminal-truncated Aβ1-37, Aβ1-38, and Aβ1-39, but the amino-terminal-truncated and carboxyl-terminal-elongated Aβ2-42 seems to be less affected by PS1 deficiency. Moreover, our results indicate that the latter Aβ peptide species could be generated by a βAsp/Ala-secretase activity.