Heparan sulfate regulates amyloid precursor protein processing by BACE1, the Alzheimer's β-secretase

Abstract

Cleavage of amyloid precursor protein (APP) by the Alzheimer's β-secretase (BACE1) is a key step in generating amyloid β-peptide, the main component of amyloid plaques. Here we report evidence that heparan sulfate (HS) interacts with β-site APP-cleaving enzyme (BACE) 1 and regulates its cleavage of APP. We show that HS and heparin interact directly with BACE1 and inhibit in vitro processing of peptide and APP substrates. Inhibitory activity is dependent on saccharide size and specific structural characteristics, and the mechanism of action involves blocking access of substrate to the active site. In cellular assays, HS specifically inhibits BACE1 cleavage of APP but not alternative cleavage by α-secretase. Endogenous HS immunoprecipitates with BACE1 and colocalizes with BACE1 in the Golgi complex and at the cell surface, two of its putative sites of action. Furthermore, inhibition of cellular HS synthesis results in enhanced BACE1 activity. Our findings identify HS as a natural regulator of BACE1 and suggest a novel mechanism for control of APP processing.

Internalization of Exogenously Added Memapsin 2 (β-Secretase) Ectodomain by Cells Is Mediated by Amyloid Precursor Protein

“…Being located in the lipid raft at the plasma membrane (19,20), memapsin 2 endocytosis is influenced by other components having direct or indirect contact with the protease. Such components include the glycosylphosphatidylinositol (GPI)-anchored proteins (19,36), scramblase (21), heparan sulfate (22), and cholesterol (20). It has been shown recently that memapsin 2 and APP are in close contact on the cell surface and throughout the endocytic process (23).…”

Crystal Structure of the E2 Domain of Amyloid Precursor Protein-like Protein 1 in Complex with Sucrose Octasulfate

“…Inhibition of endogenous HS synthesis caused increased A␤ secretion from cells (33). Because the binding of HS to the E2 domain could affect APP in ways that might hinder its cleavage by ␤-secretase (for example, the HS-induced dimerization could cause clustering of the molecule or retain the precursor at sites where local HS concentrations were high; HS was known to inhibit ␤-secretase activity (33)), we wanted to determine whether the loss of E2-HS interaction could contribute to the observed increase in A␤ secretion.…”

“…Besides their direct interactions with A␤, heparin and HS were also known to affect A␤ production itself (30 -33). Some of the effects of heparin on A␤ production in cell cultures were thought to be due to its binding to ␤-secretase, which resulted in inhibition of the protease (33). Whether the binding of heparin and HS to APP might also influence the proteolytic processing of the precursor has not been studied before.…”

Regulation of β cleavage of amyloid precursor protein

“…Heparan sulfate interacts and co-localizes with BACE1 in the Golgi and at the cell surface [70] . sAPP and A levels are downregulated by overexpression of heparan sulfate, or increased when endogenous heparan sulfate synthesis is inhibited.…”