P38α-MAPK phosphorylates Snapin and reduces Snapin-mediated BACE1 transportation in APP-transgenic mice

Amyloid beta peptide (A beta) is the major pathogenic molecule in Alzheimer's disease (AD). BACE1 enzyme is essential for the generation of A beta. Deficiency of p38 alpha-MAPK in neurons increases lysosomal degradation of BACE1 and decreases A beta deposition in the brain of APP-transgenic mice. However, the mechanisms mediating effects of p38 alpha-MAPK are largely unknown. In this study, we used APP-transgenic mice and cultured neurons and observed that deletion of p38 alpha-MAPK specifically in neurons decreased phosphorylation of Snapin at serine, increased retrograde transportation of BACE1 in axons and reduced BACE1 at synaptic terminals, which suggests that p38 alpha-MAPK deficiency promotes axonal transportation of BACE1 from its predominant locations, axonal terminals, to lysosomes in the cell body. In vitro kinase assay revealed that p38 alpha-MAPK directly phosphorylates Snapin. By further performing mass spectrometry analysis and site-directed mutagenic experiments in SH-SY5Y cell lines, we identified serine residue 112 as a p38 alpha-MAPK-phosphorylating site on Snapin. Replacement of serine 112 with alanine did abolish p38 alpha-MAPK knockdown-induced reduction of BACE1 activity and protein level, and transportation to lysosomes in SH-SY5Y cells. Taken together, our study suggests that activation of p38 alpha-MAPK phosphorylates Snapin and inhibits the retrograde transportation of BACE1 in axons, which might exaggerate amyloid pathology in AD brain.