Presenilin 1 regulates the processing of beta-amyloid precursor protein C-terminal fragments and the generation of amyloid beta-protein in endoplasmic reticulum and Golgi

Progressive cerebral deposition of the amyloid beta-protein (A beta) is bel ieved to play a pivotal role in the pathogenesis of Alzheimer's disease (AD ). The highly amyloidogenic 42-residue form of A beta (A beta(42)) is the f irst species to be deposited in both sporadic and familial AD. Mutations in two familial AD-linked genes, presenilins 1 (PS 1) and 2 (PS2), selectivel y increase the production of A beta(42) in cultured cells and the brains of transgenic mice, and gene deletion of PS1 shows that it is required for no rmal gamma-secretase cleavage of the beta-amyloid precursor protein (APP) t o generate A beta. To establish the subcellular localization of the PS1 reg ulation of APP processing to A beta, fibroblasts from PS1 wild-type (wt) or knockout (KO) embryos as well as Chinese hamster ovary (CHO) cells stably transfected with wt or mutant PS1 were subjected to subcellular fractionati on on discontinuous Iodixanol gradients. APP C-terminal fragments (CTF) wer e markedly increased in both endoplasmic reticulum- (ER-) and Golgi-rich fr actions of fibroblasts from KO mice; moreover, similar increases were docum ented directly in KO brain tissue. No change in the subcellular distributio n of full-length APP was detectable in fibroblasts lacking PS1. In CHO cell s, a small portion of APP, principally the N-glycosylated isoform, formed c omplexes with PS1 in both ER- and Golgi-rich fractions, as detected by coim munoprecipitation. When the same fractions were analyzed by enzyme-linked i mmunosorbent, assays for A beta(total) and A beta(42), A beta(42) was the m ajor A beta species in the ER fraction (A beta(42):A beta(total) ratio 0.5- 1.0), whereas absolute levels of both A beta(42) and A beta(40) were higher in the Golgi fraction and the A beta(42):A beta(total) ratio was 0.05-0.16 there. Mutant PS1 significantly increased A beta(42) levels in the Golgi f raction. Our results indicate PS1 and APP can interact in the ER and Golgi, where PS1 is required for proper gamma-secretase processing of APP CTFs, a nd that PS1 mutations augment A beta(42) levels principally in Golgi-like v esicles.