Full-length and not mature presenilin 2 increases the leakage of intracellular calcium stores: Specific mechanisms and targets.

Background. We have previously shown that fibroblasts from FAD patients carrying mutations in either PS1 (M146L, P117L) or PS2 (M239I, T122R) showed a reduced instead of an exaggerated calcium release from intracellular stores (1-3). These and other PS1/2 mutants (PS1-A246E, PS1-L286V, PS2-N141I), as well as the wild type (wt) forms of these proteins share the same feature when transiently over-expressed in different cell lines and rat primary neurons. Whereas PS1 mutants cause modest reductions under transient expression, PS2 mutants dramatically reduce the store calcium content. Accordingly, the latter also cause lower steady-state calcium concentrations at the level of the endoplasmic reticulum (ER) and Golgi apparatus (3). Methods. By employing both Fura-2 and organelle-targeted aequorins to monitor calcium concentrations in the cytosolic and the lumen, we here investigate in HeLa, SH-SY5Y and MEFs - either wt or devoid of endogenous PSs (DKO MEFs) – the mechanisms by which PS2 variants alter store calcium handling. Results. We provide evidence that: i) expression of wt or mutant PS2 reduces the store calcium content both by increasing the ER calcium leakage and by reducing the ER calcium uptake by SERCA pumps; ii) the full-length (FL) but not the mature form of the protein is directly involved in store calcium handling; iii) the effect is completely independent of -secretase activity since it is not rescued by pre-incubation with enzyme inhibitors or by expression of a loss-of-function mutant (PS2-D366A); iv) the ryanodine receptor and the ribosome translocon complex are not directly involved. Whether FL PS2 variants represent by themselves a leakage pathway (4) or whether their effect is mediated by IP3 receptors is now under investigation. Conclusions. We suggest that, at variance with the majority of FL PS1 mutants that leave unchanged or even overload intracellular calcium stores, making the cells more susceptible to toxic stimuli, FL PS2 mutants, by lowering ER/Golgi calcium levels result in a less aggressive FAD phenotypes. 1.Giacomello et al., Neurobiology of Disease 18, 638-648, 2005. 2. Zatti et al., Neurobiology of Disease 15, 269-278, 2004. 3. Zatti et al., Cell Calcium 39, 539-550, 2006. 4. Tu et al. Cell 126, 981-993, 2006.