PSEN2 (presenilin 2) mutants linked to familial Alzheimer disease impair autophagy by altering Ca2+ homeostasis.

PS2 (presenilin 2) is one of the 3 proteins that, when mutated, causes early onset familial Alzheimer disease (FAD) cases. In addition to its well-known role within the -secretase complex (the enzyme ultimately responsible for A peptides formation), PS2 is endowed with some -secretase-independent functions in distinct cell signaling pathways, such as the modulation of intracellular Ca2+ homeostasis. Here, by using different FAD-PS2 cell models, we demonstrate that mutated PS2 impairs autophagy by causing a block in the degradative flux at the level of the autophagosome-lysosome fusion step. The defect does not depend on an altered lysosomal functionality but rather on a decreased recruitment of the small GTPase RAB7 to autophagosomes, a key event for normal autophagy progression. Importantly, FAD-PS2 action on autophagy is unrelated to its -secretase activity but depends on its previously reported ability to partially deplete ER Ca2+ content, thus reducing cytosolic Ca2+ response upon IP3-linked cell stimulations. Our data sustain the pivotal role for Ca2+ signaling in autophagy and reveal a novel mechanism by which FAD-linked PSs alter the degradative process, reinforcing the view of a causative role for a dysfunctional quality control pathway in AD neurodegeneration.