Phosphorylation of the BACE1 DISLL sorting motif in HeLa cells and primary neurons modulates intracellular trafficking and reduces Aβ production

WH Toh and PA Gleeson

Department of Biochemistry and Molecular Biology and Bio21 Institute, University of Melbourne, Victoria 3010, Australia

BACE1 initiates the cleavage of the amyloid precursor protein (APP) to produce amyloid β-peptides (Aβ), a major component of amyloid plaques in Alzheimers disease. A key factor in the APP processing by BACE1 is the intracellular trafficking of both BACE1 and APP and the compartments where cleavage occurs. We have shown that BACE1 is rapidly internalized and traffics to the early endosomes and recycling endosomes in different cell lines (Chia et al., 2013). However, the mechanisms regulating endosomal sorting of BACE1 to the recycling endosome is less defined. A phosphorylated DISLL motif has been identified in the BACE1 cytoplasmic tail in vivo (Walter et al., 2001) and we explored whether this phosphorylation has a role in early endosome-to-recycling endosome trafficking. Expression of BACE1 phosphomimetic S498D and dephosphomimetic S498A mutants in Hela cells and primary neurons resulted in the S498A mutant predominantly localized to the early endosomes while the S498D showed increased levels in the recycling endosomes. To further investigate this difference in distribution, using antibody internalization assays, we showed that the S498D mutant traffics to the recycling endosomes from early endosomes at a faster rate compared with wild-type BACE1 while the S498A mutant traffics to recycling endosomes at a slower rate. Expression of BACE1S498A resulted in increased Aβ production while expression of BACE1S498D reduced Aβ production compared with wild-type BACE1. These findings indicate that phosphorylation of the DISLL motif can alter residency time of BACE1 in the early endosomes which in turns affects Aβ production. Currently, molecular machinery that regulate phosphorylated BACE1 to the recycling endosomes is being assessed.