Mutations in presenilins are responsible for approximately 40% of all early-onset familial Alzheimer disease (FAD) cases in which a genetic cause has been identified. In addition, a number of mutations in presenilin-1 (PS1) have been suggested to be associated with the occurrence of frontal temporal dementia (FTD). Presenilins are highly conserved transmembrane proteins that support cleavage of the amyloid precursor protein by γ-secretase. Recently, we discovered that presenilins also function as passive ER Ca 2+ leak channels. Here we used planar lipid bilayer reconstitution assays and Ca 2+ imaging experiments with presenilin-null mouse embryonic fibroblasts to analyze ER Ca 2+ leak function of 6 FAD-linked PS1 mutants and 3 known FTD-associated PS1 mutants. We discovered that L166P, A246E, E273A, G384A, and P436Q FAD mutations in PS1 abolished ER Ca 2+ leak function of PS1. In contrast, A79V FAD mutation or FTD-associated mutations (L113P, G183V, and Rins352) did not appear to affect ER Ca 2+ leak function of PS1 in our experiments. We validated our findings in Ca 2+ imaging experiments with primary fibroblasts obtained from an FAD patient possessing mutant PS1-A246E. Our results indicate that many FAD mutations in presenilins are loss-of-function mutations affecting ER Ca 2+ leak activity. In contrast, none of the FTD-associated mutations affected ER Ca 2+ leak function of PS1, indicating that the observed effects are disease specific. Our observations are consistent with the potential role of disturbed Ca 2+ homeostasis in Alzheimer disease pathogenesis.
IntroductionAlzheimer disease (AD) is the most common form of age-related dementia in human beings over the age of 60 years. AD affects about 2% of populations in industrialized countries. The understanding of the molecular processes that lead to the pathogenesis of AD is immensely important in combatting this neurological disease. Most cases of AD are idiopathic and are characterized by late onset (in individuals over 60 years of age). A small fraction of AD cases (familial AD [FAD]) are characterized by an earlier onset and genetic inheritance. Mutations in presenilin-1 (PS1) and PS2 account for about 40% of all known FAD cases in which a genetic cause has been identified (1). Three missense mutations in PS1 have been suggested to be associated with frontal temporal dementia (FTD) (2), a neurological disorder that affects the frontal and temporal lobes of the brain. Presenilins are 50-kD proteins that contain 9 transmembrane domains (3, 4) and reside in the ER membrane (5). The complex of presenilins, that includes aph-1 and pen-2 subunits, is transported to the cell surface and endosomal structures, where it functions as γ-secretase. The γ-secretase cleaves the amyloid precursor protein (APP) and releases the amyloid β-peptide, the principal constituent of the amyloid plaques in the brains of AD patients. Consistent with the role of presenilins as catalytic subunits of γ-secretase (6, 7), FAD mutations in presenilins affect APP processing.