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.