Compartmentalization of beta-secretase (Asp2) into low-buoyant density, noncaveolar lipid rafts

Recent epidemiological studies show a reduced prevalence of Alzheimer's dis ease (AD) in patients treated with inhibitors of cholesterol biosynthesis [ 1, 2]. Moreover, the cholesterol-transport protein, apolipoprotein E4, and elevated cholesterol are important risk factors for AD [3-5]. Additionally, in vitro and in vivo studies show that intracellular cholesterol levels ca n modulate the processing of amyloid precursor protein (APP) to beta -amylo id [6-11], the major constituent of senile plaques [12-14]. Cholesterol pla ys a crucial role in maintaining lipid rafts in a functional state [15]. Li pid rafts are cholesterol-enriched membrane microdomains implicated in sign al transduction, protein trafficking, and proteolytic processing [15-18]. S ince APP, beta -amyloid, and the putative gamma -secretase, presenilin-1 (P S-1), have all been found in lipid rafts [12, 19-21], we hypothesized that the recently identified beta -secretase, Asp2 (BACE1) [13], might also be p resent in rafts. Here, we report that recombinant Asp2 expressed in three d istinct cell lines is raft associated. Using both detergent and nondetergen t methods, Asp2 protein and activity were found in a light membrane raft fr action that also contained other components of the amyloidogenic pathway. I mmunoisolation of caveolin-containing vesicles indicated that Asp2 was pres ent in a unique raft population distinct from caveolae. Finally, depletion of raft cholesterol abrogated association of Asp2 with the light membrane f raction. These observations are consistent with the raft localization of AP P processing and suggest that the partitioning of Asp2 into lipid rafts may underlie the cholesterol sensitivity of beta -amyloid production.