Modulation of BACE1 by a novel sorting nexin in Alzheimer's disease

beta-site amyloid precursor protein cleaving enzyme 1 (BACE1) cleaves beta-amyloid precursor protein (APP) and initiates the production of amyloid-beta peptides (Abeta), which is thought to trigger Alzheimer's disease (AD). Recent reports demonstrate that BACE1 levels are elevated in brains of AD mouse models as well as in postmortem AD brain tissue, implying a role for aberrant BACE1 activity in AD pathogenesis. However, cellular mechanisms by which the level and activity of BACE1 are controlled are not yet clear. We took a proteomics approach to search for cellular modulators of BACE1 activity. BACE1-harboring protein complexes in SH-SY5Y human neuroblastoma cells were subject to in vivo crosslinking and tandem affinity purification (TAP) followed by mass spectrometry. One of the BACE1-interacting proteins identified is Sorting Nexin 6 (SNX6), a member of the Phox homology (PX) domain-containing trafficking protein family, whose function is implicated in protein trafficking. We show that SNX6 is expressed in neurons of mammalian brains. We also find that reducing SNX6 by RNA interference increases BACE1-mediated APP cleavage, resulting in the increase of Abeta production. Conversely, overexpression of SNX6 decreases BACE1-mediated APP cleavage, leading to a reduction in Abeta. SNX6 mainly localizes to the endosomal compartments, while a subset of SNX6 co-localizes with BACE1 in the trans-Golgi network (TGN). Subcellular fractionation reveals that SNX6 regulates cellular BACE1 levels by modulating BACE1 levels in the TGN. Using BACE1 antibody uptake assays, we show that reducing SNX6 enhances the retrograde transport of BACE1 from cell surface to the TGN. In sum, SNX6 functions as a negative regulator of BACE1-mediated cleavage of APP and Abeta production, illustrating a novel pathway for BACE1 regulation in AD. SNX6 may also represent a novel therapeutic target for the treatment of Alzheimer's disease.