Enhanced production and oligomerization of the 42-residue amyloid β- protein by chinese hamster ovary cells stably expressing mutant presenilins

Mutations in the presenilin 1 (PS1) and presenilin 2 (PS2) genes cause the most common and aggressive form of early onset familial Alzheimer's disease. To elucidate their pathogenic mechanism, wild-type (wt) or mutant (M146L, C410Y) PS1 and wt or mutant (M239V) PS2 genes were stably transfected into Chinese hamster ovary cells that overexpress the β-amyloid precursor protein (APP). The identity of the 43-45-kDa PSI holoproteins was confirmed by N-terminal radiosequencing. PSI was rapidly processed (t(1/2) = 40 min) in the endoplasmic reticulum into stable fragments. Wild-type and mutant PS2 holoproteins exhibited similar half lives (1.5 h); however, their endoproteolytic fragments showed both mutation-specific and cell type- specific differences. Mutant PS1 or PS2 consistently induced a 1.4-2.5-fold increase (p < 0.001) in the relative production of the highly amyloidogenic 42-residue form of amyloid β-protein (Aβ₄₂) as determined by quantitative immunoprecipitation and by enzyme-linked immunosorbent assay. In mutant PS1 and PS2 cell lines with high increases in Aβ₄₂/Aβ(total) ratios, spontaneous formation of low molecular weight oligomers of Aβ₄₂ was observed in media, suggesting enhanced Aβ aggregation from the elevation of Aβ₄₂. We conclude that mutant PS1 and PS2 proteins enhance the proteolysis of β-amyloid precursor protein by the γ-secretase cleaving at Aβ residue 42, thereby promoting amyloidogenesis.