The Ca2+ overload hypothesis in the familial form of Alzheimer's disease: New light from presenilin 2 mutations.

Background: The presenilin 2 (PS2) mutation M239I is associated with late onset and relatively mild progression of Familial Alzheimer Disease (FAD) (1). In fibroblasts of FAD patients carrying this PS2 mutation, and in cell lines (HeLa and HEK293) both transiently and stably transfected with the cDNA for PS2-M239I, Ca2+ release in response to application of the SERCA inhibitor cyclopiazonic acid is significantly reduced compared to the corresponding controls (fibroblasts from age-matched, healthy individuals and non transfected cell lines, respectively). In addition, the capacitative Ca2+ influx (CCE), that is associated with store depletion, showed a tendency to a reduction, however not significant (2). Objective: In this study we aim at establishing whether other FAD-linked PS2 mutations (N141I and T122R) induce a similar dysregulation of Ca2+ homeostasis. Methods: By means of different techniques, such as the fluorescent probe fura-2, and the cytosolic or ER-targeted aequorins, that allow to measure the Ca2+ concentration of the cytosol and the endoplasmic reticulum (ER), we analyzed how Ca2+ is handled by human fibroblasts from FAD patients carrying the PS2-T122R mutation and by different cell lines either stably or transiently expressing the same PS2 mutant or the PS2-N141I. Results: Preliminary data suggest that also these two other PS2 mutants significantly reduce the ER Ca2+ content and cause a mild reduction of CCE. The molecular target(s) through which these FAD-associated PS2 mutants exert their effect is under investigation. Conclusions: Our findings confirm that Ca2+ dysregulation is linked to FAD but contrast with the general idea that mutant presenilins cause an ER “Ca2+ overload”. We are therefore testing with the same previously described approach, also three FAD-associated PS1 mutations (M146L, A246E, L286V) together with the loss of function mutation PS1-D257A. Differences in onset and progression of the disease have been reported between FAD-linked PS1 and PS2 mutants suggesting the possibility that opposite effects on Ca2+ handling by mutant PS1 and PS2 may impact on APP processing, Ab42 release and neuronal survival. 1. Finckh U. et al. (2000) Neurology 23:2006-8. 2. Zatti G. et al. (2004) Neurobiol. Disease (in press)