A novel reciprocal and biphasic relationship between membrane cholesterol and β‐secretase activity in SH‐SY5Y cells and in human platelets

Abstract

Research into the cause of Alzheimer's disease (AD) has identified strong connections to cholesterol. Cholesterol and cholesterol esters can modulate amyloid precursor protein (APP) processing, thus altering production of the Ab peptides that deposit in cortical amyloid plaques. Processing depends on the encounter between APP and cellular secretases, and is thus subject to the influence of cholesterol-dependent factors including protein trafficking, and distribution between membrane subdomains. We have directly investigated endogenous membrane b-secretase activity in the presence of a range of membrane cholesterol levels in SH-SY5Y human neuroblastoma cells and human platelets. Membrane cholesterol significantly influenced membrane b-secretase activity in a biphasic manner, with positive correlations at higher membrane cholesterol levels, and negative correlations at lower membrane cholesterol levels. Platelets from individuals with AD or mild cognitive impairment (n = 172) were significantly more likely to lie within the negative correlation zone than control platelets (n = 171). Pharmacological inhibition of SH-SY5Y b-secretase activity resulted in increased membrane cholesterol levels. Our findings are consistent with the existence of a homeostatic feedback loop between membrane cholesterol level and membrane b-secretase activity, and suggest that this regulatory mechanism is disrupted in platelets from individuals with cognitive impairment. Keywords: Alzheimer's disease, amyloid precursor protein, cognitive impairment, platelet, protease, statin. all studies have supported these findings, reviewed by Rockwood 2006). We have studied APP cleavage at the N-terminus of Ab (bsecretase site), the rate-limiting step for Ab formation. Two 'b-secretase' proteases have been identified: b-site APP cleaving enzymes 1 and 2 (BACE1/memapsin 2/EC 3.4.23.46 and BACE2/memapsin 1/EC 3.4.23.45) (Sinha et al. 1999;Vassar et al. 1999;Farzan et al. 2000). Elevations in brain b-secretase activity have been found in AD (Fukumoto et al. 2002;Holsinger et al. 2002;Li et al. 2004;Stockley et al. 2006;Zhao et al. 2007); and in CSF (Holsinger et al. 2004). However, little is currently known about endogenous regulation of b-secretase activity. Most previous studies have indicated that cellular cholesterol influenced generation of Ab, but crucially, the studies examined APP processing or Ab production rather than secretase activity per se. As APP processing is dependent on APP encountering the appropriate secretases within the cell, it is likely to be sensitive to cholesterol-dependent events, such as protein trafficking and distribution of proteins between membrane subdomains. One study addressed this by using recombinant BACE1 in an artificial membrane system, reporting lipid-mediated effects on proteolytic activity (Kalvodova et al. 2005). We and another group (Grimm et al. 2008) studied a more physiological system by directly investigating endogenous membrane b-secretase activity. We have also simultaneously measured membrane...

Platelet Alpha- and Beta- Secretase Activities are not Significantly Affected by Dementia or Mild Cognitive Impairment in Swedish Patients

“…Very recently Liu et al have reported that the absolute cholesterol levels may be important for the platelet BACE activity [30]. In the present study we could not detect any significant correlations between circulating cholesterol levels and platelet -or -secretase activity.…”

“…There was a higher proportion of female subjects in the healthy control group than the MCI or AD groups (Table 1) but there was no significant effect of gender on the platelet secretase activity in any of the groups when tested using Students two tailed t-Test. The patients sampled in this study had a range of MMSE score (15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30), indicating relative heterogeneity in terms of disease progression (Table 1).…”

Docosahexaenoic Acid Reduces Amyloid β Production via Multiple Pleiotropic Mechanisms

“…DHA Influences Cholesterol de Novo Synthesis and HMGCR Activity-The effect of cholesterol on degenerative processes in AD is well known. Elevated cholesterol levels are linked to increased amyloid plaque formation in animal models and A␤ aggregation in vitro (52)(53)(54)(55)(56)(57)(58)(59)(60). On the cellular level, cholesterol increases A␤ production by increasing ␤-and ␥-secretase cleavage (44).…”

The role of APP proteolytic processing in lipid metabolism

“…1 Schematic overview of APP processing and generated cleavage products which reduces A level, decreases cholesterol levels in lipid rafts, where APP is preferentially cleaved by -secretase (Grimm et al 2011c). Investigating -secretase activity in neuroblastoma cells with a broad range of diVerent membrane cholesterol levels revealed membrane cholesterol to signiWcantly inXuence -secretase activity (Liu et al 2009). In return, pharmacological inhibition of -secretase in SH-SY5Y cells leads to increased membrane cholesterol levels (Liu et al 2009).…”

“…Investigating -secretase activity in neuroblastoma cells with a broad range of diVerent membrane cholesterol levels revealed membrane cholesterol to signiWcantly inXuence -secretase activity (Liu et al 2009). In return, pharmacological inhibition of -secretase in SH-SY5Y cells leads to increased membrane cholesterol levels (Liu et al 2009). Fibroblasts deWcient in APP and APLP2 or lacking a functional -secretase show an increase in cellular cholesterol levels, which can be reversed by treatment with A 40 .…”