Presenilins mediate endosomal recycling via ARF6 to maintain proper lysosomal clearance

Autophagic and endosomal dysfunctions are prominently observed at preclinical stages of Alzheimer's disease (AD) as well as in presenilin (PSEN) 1-deficient mice and neurons. In the latter, the defects relate to the γ-secretase-independent role of PSEN in lysosomal fusion and organelle turnover. While we demonstrated previously that the impaired capacity of lysosomal fusion is associated with a significant reduction in lysosomal calcium storage/release, the underlying mechanism remained unexplored. Here we demonstrate that PSEN-deficient cells are impaired in endosomal recycling of several cargo proteins reminiscent of clathrin-independent carriers and lipid rafts. This is accompanied by the accumulation of cholesterol in LAMP1-positive organelles. The small GTPase ARF6, an important regulator of lipid raft recycling, fully rescues the endo-lysosomal abnormalities in PSEN-/- cells, suggesting that defective recycling is upstream of lysosomal dysfunction. PSEN-/- cells and neurons present significantly reduced ARF6 expression levels. Importantly, similar decreased ARF6 levels are observed in aging murine neurons and brain and are even more pronounced in AD brains, suggesting a particular vulnerability of ARF6-mediated recycling in the early etiology of AD. 4 cell lines were analyzed from 3 biological replicates. These samples include MEF wild-type, MEF PSEN1&2-/-, MEF PSEN1&2-/- retrovirally transduced with hARF6, and MEF PSEN1&2-/- retrovirally transduced with hPSEN1 D257A/D385A mutant.