Abstract
Like protein phosphorylation, O-GlcNAcylation is a common post-translational protein modification. We already reported that O-GlcNAcylation of amyloid precursor protein (APP) in response to insulin signaling reduces neurotoxic amyloid-β (Aβ) production via inhibition of APP endocytosis. Internalized APP is delivered to endosomes and lysosomes where Aβ is produced. However, the molecular mechanism involved in the effect of APP O-GlcNAcylation on APP trafficking remains unknown. To investigate the relationship between APP O-GlcNAcylation and APP endocytosis, we tested the effects of insulin on neuroblastoma SH-SY5Y cells overexpressing APP and BACE1, and cultured rat hippocampal neurons. The present study showed that APP O-GlcNAcylation translocated APP from lipid raft to non-raft microdomains in the plasma membrane by using immunocytochemistry and discontinuous sucrose gradients method. By using the biotinylation method, we also found that APP preferentially underwent endocytosis from lipid rafts and that the amount of internalized APP from lipid rafts was specifically reduced by O-GlcNAcylation. These results indicate that O-GlcNAcylation can regulate lipid raft-dependent APP endocytosis via translocation of APP into non-raft microdomains. Our findings showed a new functional role of O-GlcNAcylation for the regulation of APP trafficking, offering new mechanistic insight for Aβ production.
Highlights
Alzheimer’s disease (AD) is accompanied by the loss of memory functions and other symptoms such as speech and language problems, loss of motivation, and behavioral disorders [1,2]
We showed that insulin increases cell surface amyloid precursor proteins (APP) by decreasing APP endocytosis rate
The coefficient was significantly decreased by 22.4% (n = 40, number of cells) in insulin-treated cells compared to control cells, indicating that insulin decreased APP localization in lipid rafts
Summary
Alzheimer’s disease (AD) is accompanied by the loss of memory functions and other symptoms such as speech and language problems, loss of motivation, and behavioral disorders [1,2]. Aβ peptides are products of sequential proteolytic cleavage of amyloid precursor proteins (APP) by β- and γ-secretases. APP is synthesized at the endoplasmic reticulum (ER) and transported to the trans-Golgi network (TGN) and the plasma membrane [5]. APP is cleaved by α-secretase at the cell surface most of the time [6]. Some APPs are internalized to endosomes or lysosomes via endocytosis. Β-site amyloid precursor protein cleaving enzyme 1 (BACE1), a key enzyme for generating Aβ, is mostly localized in endosomes, lysosomes, and TGN [7,8,9]. Because APP metabolism is affected by APP trafficking and localization, it is very important to understand the association between APP and trafficking factors [10,11,12]
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