Impact of cholesterol level upon APP and BACE proximity and APP cleavage

Deparment of Neurology, Ulm University, Albert-Einstein-Allee 11, 89081 Ulm, Germany.
Biochemical and Biophysical Research Communications (Impact Factor: 2.3). 06/2008; 370(2):207-12. DOI: 10.1016/j.bbrc.2008.03.047
Source: PubMed


Cleavage of APP by BACE is the first proteolytic step in the production of Amyloid beta (Abeta, which accumulates in senile plaques in Alzheimer's disease. BACE-cleavage of APP is thought to happen in endosomes. However, there are controversial data whether APP and BACE can already interact on the cell surface dependent on the cholesterol level. To examine whether APP and BACE come into close proximity on the cell surface in living cells, we employed a novel technique by combining time-resolved Förster resonance energy transfer (FRET) measurements with total internal reflection microscopy (TIRET microscopy). Our data indicate that BACE and APP come into close proximity within the cell, but probably not on the cell surface. To analyze the impact of alterations in cholesterol level upon BACE-cleavage, we measured sAPP secretion. Alteration of APP processing and BACE proximity by cholesterol might be explained by alterations in cell membrane fluidity.

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    • "A␤PP/BACE1 association was better correlated with the viscosity at the aqueous interface than at hydrophobic core of the bilayer, suggesting interactions of their respective transmembrane segments with long chain saturated fatty acids, which in turn are facilitated for interaction upon reduction of membrane phospholipids polyunsaturation. Obviously, factors promoting physical contact between A␤PP and ␤-secretase within rafts are expected to enhance CTF␤99 intermediate formation and to increase substrate availability for further cleavage by ␥-secretase [5] [41] [43]. Therefore , we postulate that it is the viscous lipid rafts environment, rather than their lipid components themselves , what determines the ease of amyloidogenic processing of A␤PP within rafts. "
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    ABSTRACT: In the present study, we have assessed the biophysical properties of lipid rafts from different brain areas in subjects exhibiting early neuropathological stages of Alzheimer's disease (AD). By means of steady-state fluorescence polarization analyses using two environment-sensitive fluorescent probes, we demonstrate that lipid rafts from cerebellum, and frontal and entorhinal cortices, exhibit different biophysical behaviors depending on the stage of the disease. Thus, while membrane anisotropies were similar in the cerebellum along stages, lipid rafts from frontal and entorhinal cortices at AD stages I/II and AD III were significantly more liquid-ordered than in control subjects, both at the aqueous interface and hydrophobic core of the raft membrane. Thermotropic analyses demonstrated the presence of Arrhenius breakpoints between 28.3-32.0°C, which were not influenced by the disease stage. However, analyses of membrane microviscosity (ηapp) demonstrate that frontal and entorhinal lipid rafts are notably more viscous and liquid-ordered all across the membrane from early stages of the disease. These physicochemical alterations in lipid rafts do not correlate with changes in cholesterol or sphingomyelin levels, but to reduced unsaturation index and increased saturate/polyunsaturated ratios in phospholipid acyl chains. Moreover, we demonstrate that β-secretase/AβPP (amyloid-β protein precursor) interaction and lipid raft microviscosity are strongly, and positively, correlated in AD frontal and entorhinal cortices. These observations strengthens the hypothesis that physical properties of these microdomains modulate the convergence of amyloidogenic machinery toward lipid rafts, and also points to a critical role of polyunsaturated fatty acids in amyloidogenic processing of AβPP.
    Full-text · Article · Aug 2014 · Journal of Alzheimer's disease: JAD
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    • "We found that various paradigms inducing activity in cultured neurons also led to increased colocalization of APP/BACE-1, as well as a routing of APP into recycling endosomes containing BACE-1 (Figures 3 and 4), along with increased b-cleavage of APP (Figure 4F). Early studies in cell lines suggested that APP/BACE-1 convergence occurs at or perhaps near the plasma membrane (Kinoshita et al., 2003; von Arnim et al., 2008), but more recent data (also mostly in nonneuronal cells or neuronal cell lines) suggest that these two proteins converge within early endosomes (Rajendran et al., 2006; Sannerud et al., 2011). Other studies show that APP and Rab5 may colocalize in presynaptic terminals (Ikin et al., 1996; Sabo et al., 2003). "
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    ABSTRACT: The convergence of APP (substrate) and BACE-1 (enzyme) is a rate-limiting, obligatory event triggering the amyloidogenic pathway-a key step in Alzheimer's disease (AD) pathology. However, as both APP/BACE-1 are highly expressed in brain, mechanisms precluding their unabated convergence are unclear. Exploring dynamic localization of APP/BACE-1 in cultured hippocampal neurons, we found that after synthesis via the secretory pathway, dendritic APP/BACE-1-containing vesicles are largely segregated in physiologic states. While BACE-1 is sorted into acidic recycling endosomes, APP is conveyed in Golgi-derived vesicles. However, upon activity induction-a known trigger of the amyloidogenic pathway-APP is routed into BACE-1-positive recycling endosomes via a clathrin-dependent mechanism. A partitioning/convergence of APP/BACE-1 vesicles is also apparent in control/AD brains, respectively. Considering BACE-1 is optimally active in an acidic environment, our experiments suggest that neurons have evolved trafficking strategies that normally limit APP/BACE-1 proximity and also uncover a pathway routing APP into BACE-1-containing organelles, triggering amyloidogenesis.
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    • "Many studies support the notion that Aβ production occurs in endosomes [22, 73–77]. Since AβPP is a transmembrane protein, its internalization from the plasma membrane is regulated by key regulators of endocytosis, such as Rab5, and this process has been found to enhance AβPP cleavage by β-secretase leading to increased Aβ levels [78]. "
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