β-Galactoside α2,6-Sialyltransferase I Cleavage by BACE1 Enhances the Sialylation of Soluble Glycoproteins

Ritsuko Oka,Naoyuki Taniguchi,Ichiro Sugimoto,Yasuhiro Hashimoto,Satoshi Futakawa,Shinobu Kitazume,Kazuko Ogawa,Jamey D. Marth,Eiji Miyoshi

doi:10.1074/jbc.m704766200

Abstract

BACE1 (beta-site amyloid precursor protein-cleaving enzyme-1) is a membrane-bound aspartic protease that cleaves amyloid precursor protein to produce a neurotoxic peptide, amyloid beta-peptide, and has been implicated in triggering the pathogenesis of Alzheimer disease. We showed previously that BACE1 cleaves beta-galactoside alpha2,6-sialyltransferase I (ST6Gal I) to initiate its secretion, but it remained unclear how BACE1 affects the cellular level of alpha2,6-sialylation. Here, we found that BACE1 overexpression in Hep3B cells increased the sialylation of soluble secreted glycoproteins, but did not affect cell-surface sialylation. The sialylation of soluble glycoproteins was not increased by ST6Gal I overexpression alone, but was increased by co-overexpression of ST6Gal I and BACE1 or by expression of the soluble form of ST6Gal I, suggesting that soluble ST6Gal I produced by BACE1 plays, at least in part, a role in the sialylation of soluble glycoproteins. We also found that plasma glycoproteins from BACE1-deficient mice exhibited reduced levels of alpha2,6-sialylation compared with those from wild-type mice. We propose a novel regulatory mechanism in which cleavage and secretion of ST6Gal I enhance the sialylation of soluble glycoprotein substrates.

Highlights

The generation and deposition of the amyloid ␤-peptide in the brain are a hallmark of Alzheimer disease and have been implicated in the pathogenesis of Alzheimer disease [1]
Our previous study using Long-Evans Cinnamon (LEC) rats revealed that the hepatic BACE1 mRNA level is increased before the hepatitis stage and that plasma ST6Gal I increases at the same time [8], thereby suggesting a relationship between altered BACE1 expression and the level of sialylation in vivo
After Hep3B cells were cultured in serum-free media, proteins secreted into the media fractions were concentrated and analyzed by blotting with Sambucus sieboldiana agglutinin (SSA) lectin, which recognizes Sia␣2,6Gal epitopes

Summary

Introduction

The generation and deposition of the amyloid ␤-peptide in the brain are a hallmark of Alzheimer disease and have been implicated in the pathogenesis of Alzheimer disease [1]. We found that BACE1 overexpression in Hep3B cells increased the sialylation of soluble secreted glycoproteins, but did not affect cellsurface sialylation. After detection of intracellular ST6Gal I in the Hep3B cell lysates, the membrane was washed with Restore Western blot stripping buffer (Pierce) and incubated with anti-GAPDH antibody.

Results

Conclusion