Lipid rafts play an important role in Aβ biogenesis by regulating the β-secretase pathway

Abstract

The Alzheimer's amyloid beta protein (A beta) precursor (APP) is proteolytically cleaved by beta-secretase to N- and C-terminal fragments sAPPbeta and CTFbeta, respectively. Subsequently, CTFbeta is cleaved by gamma-secretase to generate A beta. We previously showed that the levels of secreted A beta and sAPPbeta were significantly reduced upon removal of glycosylphosphatidylinositol (GPI)-anchored proteins from either primary brain cells or Chinese hamster ovary cultures. The results indicated that GPI-anchored proteins facilitated beta-secretase activity. In this report, we strengthen the previous findings by demonstrating that CTFbeta, like sAPPbeta, is also reduced upon removal of GPI-anchored proteins and that sAPPbeta does not accumulate in an intracellular compartment. This facilitation pathway does not appear to be important for the processing of a disease-linked mutant form of APP (670NL), known to be a superior beta-secretase substrate. A novel aspartyl protease, BACE, responsible for beta-secretase activity in the brain is not GPI-anchored. However, BACE in brain membranes accumulate in lipid rafts, a compartment marked by the accumulation of GPI-anchored proteins. This finding is consistent with the hypothesis that BACE interacts with GPI-anchored proteins that facilitate its activity possibly by chaperoning it into lipid rafts.

Altered β‐secretase enzyme kinetics and levels of both BACE1 and BACE2 in the Alzheimer's disease brain

“…Many factors besides actual BACE1 levels have been proposed to regulate b-secretase activity, and may be important in further understanding the functional relationship between BACE1, and the increased V max for the enzyme detected in this study in AD. These include: heparan sulphate proteoglycans [45,46], cholesterol [47,48] lipid rafts [36,37,49], reticulon family members [50,51] and PAR-4 [52]. Changed V max in the presence of unchanged K m may reflect the loss of a non-competitive inhibitor of b-secretase activity in AD brain, and increased V max for b-secretase has been observed with BACE1 dimerisation [53].…”

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

“…Internalization of M2 ED Is Influenced by Cell Surface GPIanchored Proteins-It has been shown that the removal of GPI-anchored cell surface proteins reduced the endocytosis of cellular expressed native memapsin 2 (19). We therefore sought to determine if the same effect is also seen for exogenously added M2 ED .…”

“…This interaction apparently serves to package memapsin 2 into clathrin-coated vesicles for transportation in the endocytic or/and recycling pathways (17,18). 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).…”

“…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).…”

Cholesterol and Alzheimer's disease: A still poorly understood correlation

“…Because the enzymatic activity of b-and c-secretases is cholesterol-dependent (16,17), it is likely that the production of Ab reflects a biological response to the increased cholesterol availability.…”