Presenilin‐2 dampens intracellular Ca 2+ stores by increasing Ca 2+ leakage and reducing Ca 2+ uptake

We have previously shown that familial Alzheimer’s disease mutants of presenilin‐2 (PS2) and, to a lesser extent, of presenilin‐1 (PS1) lower the Ca 2+ concentration of intracellular stores. We here examined the mechanism by which wild‐type and mutant PS2 affect store Ca 2+ handling. By using HeLa, SH‐SY5Y and MEFs as model cells, and recombinant aequorins as Ca 2+ probes, we show evidence that transient expression of either wild‐type or mutant PS2 increases the passive Ca 2+ leakage: both ryanodine‐ and IP 3 ‐receptors contribute to Ca 2+ exit out of the ER, whereas the ribosome translocon complex is not involved. In SH‐SY5Y cells and MEFs, wild‐type and mutant PS2 potently reduce the uptake of Ca 2+ inside the stores, an effect that can be counteracted by over‐expression of SERCA‐2B. On this line, in wild‐type MEFs, lowering the endogenous level of PS2 by RNA interference, increases the Ca 2+ ‐loading capability of intracellular stores. Furthermore, we show that in PS double knockout MEFs, reduction of Ca 2+ stores is mimicked by the expression of PS2‐D366A, a loss‐of‐function mutant, uncleaved because also devoid of presenilinase activity but not by co‐expression of the two catalytic active fragments of PS2. In summary, both physiological and increased levels of wild‐type and mutant PS2 reduce the Ca 2+ uptake by intracellular stores. To exert this newly described function, PS2 needs to be in its full‐length form, even if it can subsequently be cleaved.