Characterization of α2,6-Sialyltransferase Cleavage by Alzheimer's β-Secretase (BACE1)

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100

Extracellular deposition of amyloid-␤ (A␤) aggregates in the brain represents one of the histopathological hallmarks of Alzheimer's disease (AD). A␤ peptides are generated from proteolysis of the amyloid precursor proteins (APPs) by ␤-and ␥-secretases. ␤-Secretase (BACE1) is a type I integral membrane glycoprotein that can cleave APP first to generate C-terminal 99-or 89-amino acid membrane-bound fragments containing the N terminus of A␤ peptides (␤CTF). As BACE1 cleavage is an essential step for A␤ generation, it is proposed as a key therapeutic target for treating AD. In this study, we show that small interfering RNA (siRNA) specifically targeted to BACE1 can suppress BACE1 (but not BACE2) protein expression in different cell systems. Furthermore, BACE1 siRNA reduced APP ␤CTF and A␤ production in primary cortical neurons derived from both wildtype and transgenic mice harboring the Swedish APP mutant. The subcellular distribution of APP and presenilin-1 did not appear to differ in BACE1 suppressed cells. Importantly, pretreating neurons with BACE1 siRNA reduced the neurotoxicity induced by H 2 O 2 oxidative stress. Our results indicate that BACE1 siRNA specifically impacts on ␤-cleavage of APP and may be a potential therapeutic approach for treating AD.With the global increase in human life expectancy, Alzheimer's disease (AD) 1 with its debilitating memory loss and dementia poses an ever increasing burden to society. Despite an intensive search for therapeutic intervention, no drug has proven effective in combating this devastating neurodegenerative disease. The cause of AD is virtually unknown, with age being one of the only clearly defined major risk factors. The pathological hallmarks of AD include severe brain atrophy, neuronal loss, neurofibrillary tangles, and senile plaques composed of aggregated amyloid-␤ (A␤) peptides (1). Interestingly, all known mutations that are associated with early onset AD enhance A␤42 production (2). Thus, the amyloid cascade hypothesis has been proposed based on the speculation that A␤42 production plays an early and critical role in the pathogenesis of AD (3-5). When aggregated, these amyloidogenic peptides may elicit inflammatory and neurotoxic responses in the brain, which are believed to result in the clinical manifestations of AD.A␤ peptides are generated from the sequential cleavage of the amyloid precursor protein (APP), a type I transmembrane protein, by ␤-secretase in the ectodomain and by ␥-secretase in the transmembrane region (6). A C-terminal membranebound fragment of 99 or 89 residues (C99/C89, ␤CTFs) is produced by ␤-secretase cleavage of APP, which is subsequently cleaved by ␥-secretase within the transmembrane domain to release the A␤ peptides and APP intracellular domain. APP can also be cleaved by ␣-secretase in the ectodomain, which precludes its processing by the ␤-secretase pathway. Much effort in drug development has been aimed at reducing the generation or aggregation of A␤ peptides, among which inhibition of ␤-secretase or ␥-secretase activity has bee...

Section: Discussionmentioning

confidence: 99%

BACE1 Suppression by RNA Interference in Primary Cortical Neurons

Kao

Krichevsky

Kosik

et al. 2004

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100

“…Both of BACE1-transgenic mice had elevated levels of plasma ST6Gal I as compared with the control mice (Fig. 2, lower panel) 43 at the amino terminus, and then the three terminal amino acids, Gln-AlaLys, were trimmed by a luminal aminopeptidase(s) to produce the E41 form that is secreted from the cells (26).…”

Section: Plasma St6gal I Levels Were Decreased In Bace1-deficientmentioning

confidence: 99%

“…The majority of serum ST6Gal I is secreted from the liver (21,22), and secretion is enhanced during acute-phase hepatic reactions (23,24). We previously found that BACE1 is involved in the cleavage and secretion of ST6Gal I, at least in cultured cells (25,26), leading to an assumption that BACE1 cleaves ST6Gal I in the liver and triggers its secretion into plasma.…”

Section: A Characteristic Feature Of Alzheimer Disease (Ad)mentioning

confidence: 99%

In Vivo Cleavage of α2,6-Sialyltransferase by Alzheimer β-Secretase

Kitazume

Nakagawa

Oka

et al. 2005

Self Cite

␤-Site amyloid precursor protein-cleaving enzyme 1 (BACE1) is a membrane-bound aspartic protease that cleaves amyloid precursor protein to produce a neurotoxic peptide, A␤, and is implicated in triggering the pathogenesis of Alzheimer disease. We previously reported that BACE1 cleaved rat ␤-galactoside ␣2,6-sialyltransferase (ST6Gal I) that was overexpressed in COS cells and that the NH 2 terminus of ST6Gal I secreted from the cells (E41 form) was Glu 41 . Here we report that BACE1 gene knock-out mice have one third as much plasma ST6Gal I as control mice, indicating that BACE1 is a major protease which is responsible for cleaving ST6Gal I in vivo. We also found that BACE1-transgenic mice have increased level of ST6Gal I in plasma. Secretion of ST6Gal I from the liver into the plasma is known to be up-regulated during the acute-phase response. To investigate the role of BACE1 in ST6Gal I secretion in vivo, we analyzed the levels of BACE1 mRNA in the liver, as well as the plasma levels of ST6Gal I, in a hepatopathological model, i.e. Long-Evans Cinnamon (LEC) rats. This rat is a mutant that spontaneously accumulates copper in the liver and incurs hepatic damage. LEC rats exhibited simultaneous increases in BACE1 mRNA in the liver and in the E41 form of the ST6Gal I protein, the BACE1 product, in plasma as early as 6 weeks of age, again suggesting that BACE1 cleaves ST6Gal I in vivo and controls the secretion of the E41 form.

“…The cytosolic tail/transmembrane domain/stem region of human ST6Gal I is probably implicated in subcellular traffic through functional homodimerization and/or interactions with other proteins, such as COP-I coated vesicle anterograde traffic of GT or COG (103,(107)(108)(109)(110)(111) or Golgi retention (112)(113)(114) and in the modulation of its enzymatic activity through substrate recognition (61). It has also been shown that the cytosolic tail/transmembrane domain/stem region impacts mammalian ST6Gal I secretion via BACE-1 aminopeptidase activity in Alzheimer disease (115)(116)(117)(118). Altogether, our data on the molecular cloning of lower vertebrate st6gal1 genes and their molecular evo-lution raise the question of the evolution of vertebrate ST6Gal I with regard to their subcellular localization, interaction with other glycosyltransferases, and activity in vivo.…”

Section: Discussionmentioning

confidence: 99%

Molecular Phylogeny and Functional Genomics of β-Galactoside α2,6-Sialyltransferases That Explain Ubiquitous Expression of st6gal1 Gene in Amniotes

Petit

Mir

Petit

et al. 2010

Sialyltransferases are key enzymes in the biosynthesis of sialoglycoconjugates that catalyze the transfer of sialic residue from its activated form to an oligosaccharidic acceptor. ␤-Galactoside ␣2,6-sialyltransferases ST6Gal I and ST6Gal II are the two unique members of the ST6Gal family described in higher vertebrates. The availability of genome sequences enabled the identification of more distantly related invertebrates' st6gal gene sequences and allowed us to propose a scenario of their evolution. Using a phylogenomic approach, we present further evidence of an accelerated evolution of the st6gal1 genes both in their genomic regulatory sequences and in their coding sequence in reptiles, birds, and mammals known as amniotes, whereas st6gal2 genes conserve an ancestral profile of expression throughout vertebrate evolution.