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

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.