Date Published: January 25, 2019
Publisher: Public Library of Science
Author(s): Yoon Young Cho, Oh-Hoon Kwon, Myoung Kyu Park, Tae-Wan Kim, Sungkwon Chung, Madepalli K. Lakshmana.
Familial Alzheimer’s disease (FAD)-associated presenilin 1 (PS1) serves as a catalytic subunit of γ-secretase complex, which mediates the proteolytic liberation of β-amyloid (Aβ) from β-amyloid precursor protein (APP). In addition to its proteolytic role, PS1 is involved in non-proteolytic functions such as protein trafficking and ion channel regulation. Furthermore, postmortem AD brains as well as AD patients showed dysregulation of cholesterol metabolism. Since cholesterol has been implicated in regulating Aβ production, we investigated whether the FAD PS1-associated cholesterol elevation could influence APP processing. We found that in CHO cells stably expressing FAD-associated PS1 ΔE9, total cholesterol levels are elevated compared to cells expressing wild-type PS1. We also found that localization of APP in cholesterol-enriched lipid rafts is substantially increased in the mutant cells. Reducing the cholesterol levels by either methyl-β-cyclodextrin or an inhibitor of CYP51, an enzyme mediating the elevated cholesterol in PS1 ΔE9-expressing cells, significantly reduced lipid raft-associated APP. In contrast, exogenous cholesterol increased lipid raft-associated APP. These data suggest that in the FAD PS1 ΔE9 cells, the elevated cellular cholesterol level contributes to the altered APP processing by increasing APP localized in lipid rafts.
Alzheimer’s disease (AD) is characterized by the accumulation of β-amyloid peptide (Aβ) and the formation of neurofibrillary tangles in the brain; the highly amyloidogenic 42-residue Aβ (Aβ42) is the first species to be deposited in both sporadic and familial AD (FAD). Aβ42 and 40-residue Aβ (Aβ40) are produced by the interplay between β-amyloid precursor protein (APP) and the key enzymes. APP is first cleaved at its amino terminus of Aβ sequence by an aspartyl protease β-site-APP-cleaving enzyme (BACE1, or β-secretase), releasing a large N-terminal truncated APP (sAPPβ) and a membrane-associated carboxy terminal fragment (CTFβ, or C99) [1, 2]. The remaining CTFβ is cleaved by a γ-secretase complex that comprises presenilin 1 (PS1), presenilin enhancer 2 (PEN-2), nicastrin, and anterior pharynx defective 1 (APH1). This sequential cleavage produces Aβ40 and Aβ42 as well as APP intracellular domain (AICD) [3–5]. Alternatively, APP can be cleaved by α-secretase followed by γ-secretase, precluding Aβ production. It was initially established that mutations in FAD-associated PS1 and PS2 increase the ratio of Aβ42 to Aβ40 both in vivo and in vitro as well as in AD patients [6–9], although recent studies indicate that the increase in Aβ42 levels relative to Aβ40 may not be a universal phenotype associated with presenilin FADs . Much evidence shows that cholesterol plays an important role in the pathogenesis of AD. Changes in lipid composition including increased cholesterol level are observed in AD patients and in postmortem AD brains [11, 12]. Most brain regions in AD patients show significantly increased cholesterol levels, and the intermediates involved in cholesterol biosynthesis such as lanosterol, demosterol, 24-hyroxycholesterol, and 27-hydroxycholesterol are also altered in AD patients and transgenic AD mouse model [13, 14]. Furthermore, APP processing and Aβ generation are influenced by cholesterol, likely because of the localization of the catalytic core of γ-secretase complex within the transmembrane domain [15–17].
Specific molecular phenotypes associated with FAD PS have long been under intense investigation. Owing to the PS’s role as a catalytic subunit, it has been postulated that FAD-linked mutations in the PS genes universally affect the γ-secretase activity resulting in the increase of Aβ42 relative to Aβ40 (Aβ42/Aβ40 ratio) [41, 42]. However, recent studies revealed that the reconstituted γ-secretase complex harboring FAD mutant forms of PS1 do not overproduce Aβ42 or change the Aβ42/Aβ40 ratio in vitro, suggesting that additional cellular mechanisms may be involved with the altered γ-secretase activity observed in the FAD PS1 cells . In addition to the catalytic function, PS1 is involved with several non-catalytic cellular functions, such as Ca2+ influx and protein trafficking, in a γ-secretase-independent manner [25–27].