An Increased Percentage of Long Amyloid β Protein Secreted by Familial Amyloid β Protein Precursor (βApp ₇₁₇ ) Mutants

TLDR

Normal processing of βAPP produces a soluble 4‑kDa Aβ that forms insoluble fibrils in Alzheimer’s disease, and longer Aβ species such as Aβ1‑42 aggregate more rapidly. The study compared wild‑type and βAPP 717 mutant βAPP expressed in human neuroblastoma cells by isolating metabolically labeled 4‑kDa Aβ from conditioned medium, analyzing its C‑terminal peptides after cyanogen bromide digestion, and measuring Aβ1‑40 versus Aβ1‑42 with specific ELISAs. Wild‑type βAPP mainly secretes Aβ1‑40, whereas βAPP 717 mutants increase the proportion of longer Aβ species by 1.5‑ to 1.9‑fold, suggesting that elevated long Aβ secretion promotes amyloid deposition in Alzheimer’s disease.

Abstract

Normal processing of the amyloid β protein precursor (βAPP) results in secretion of a soluble 4-kilodalton protein essentially identical to the amyloid β protein (Aβ) that forms insoluble fibrillar deposits in Alzheimer's disease. Human neuroblastoma (M17) cells transfected with constructs expressing wild-type βAPP or the βAPP 717 mutants linked to familial Alzheimer's disease were compared by (i) isolation of metabolically labeled 4-kilodalton Aβ from conditioned medium, digestion with cyanogen bromide, and analysis of the carboxyl-terminal peptides released, or (ii) analysis of the Aβ in conditioned medium with sandwich enzyme-linked immunosorbent assays that discriminate Aβ 1-40 from the longer Aβ 1-42 . Both methods demonstrated that the 4-kilodalton Aβ released from wild-type βAPP is primarily but not exclusively Aβ 1-40 . The βAPP 717 mutations, which are located three residues carboxyl to Aβ 43 , consistently caused a 1.5- to 1.9-fold increase in the percentage of longer Aβ generated. Long Aβ (for example, Aβ 1-42 ) forms insoluble amyloid fibrils more rapidly than Aβ 1-40 . Thus, the βAPP 717 mutants may cause Alzheimer's disease because they secrete increased amounts of long Aβ, thereby fostering amyloid deposition.