Familial Alzheimer’s Disease presenilin mutants reduce calcium levels of intracellular stores. A critical revaluation of the “calcium overload” hypothesis

Background: Presenilin-1 and -2 (PS1 and PS2) mutations, the major cause of Familial Alzheimer’s Disease (FAD), have been causally implicated in the pathogenesis of neuronal cell death through a perturbation of cellular Ca2+ homeostasis. We have recently shown that, at variance with previous suggestions obtained in cells expressing other FAD-linked PS mutations, PS2-M239I and PS2-T122R cause a reduction and not an increase in cytosolic Ca2+ rises induced by Ca2+ release from stores (1,2). Objective(s): In this study we aim at investigating whether other FAD-linked PS mutations induce a similar dysregulation of Ca2+ homeostasis. Methods: Different cell models have been used: human fibroblasts from controls and FAD patients, cell lines (SH-SY5Y, HeLa, HEK293, MEFs) and rat primary neurons expressing a number of PS mutations, e.g. P117L, M146L, L286V, and A246E in PS1 and M239I, T122R, and N141I in PS2. The effects of FAD-linked PS mutations on cytosolic Ca2+ changes have been monitored either by using fura-2 or recombinant cytosolic aequorin as the probe. Results: Independently of the cell model or the employed probe, the cytosolic Ca2+ increases, caused by agonist stimulation or full store depletion by drug treatment, were reduced or unchanged in cells expressing the PS mutations. Using aequorins, targeted to the endoplasmic reticulum or the Golgi apparatus, we here show that FAD-linked PS mutants lower the Ca2+ content of intracellular stores. The phenomenon was most prominent in cells expressing PS2 mutants, and was observed also in cells expressing the non-pathogenic, “loss-of-function” PS2-D366A mutation. Conclusions: Taken as a whole, our findings, while confirming the capability of presenilins to modify Ca2+ homeostasis, suggest a re-evaluation of the “Ca2+ hypothesis” in AD and a new working hypothesis is presented. 1. Zatti G, Ghidoni R, Barbiero L, Binetti G, Pozzan T, Fasolato C, Pizzo P. (2004). Neurobiology of disease, 15, 269-278. 2. Giacomello M, Barbiero L, Zatti G, Squitti R, Binetti G, Pozzan T, Fasolato C, Ghidoni R, Pizzo P. (2005). Neurobiology of disease, 18, 638-648.