Molecular Bases of Disease
Full-length amyloid precursor protein regulates lipoprotein metabolism and amyloid-β clearance in human astrocytes

https://doi.org/10.1074/jbc.RA117.000441Get rights and content
Under a Creative Commons license
open access

Mounting evidence suggests that alterations in cholesterol homeostasis are involved in Alzheimer's disease (AD) pathogenesis. Amyloid precursor protein (APP) or multiple fragments generated by proteolytic processing of APP have previously been implicated in the regulation of cholesterol metabolism. However, the physiological function of APP in regulating lipoprotein homeostasis in astrocytes, which are responsible for de novo cholesterol biosynthesis and regulation in the brain, remains unclear. To address this, here we used CRISPR/Cas9 genome editing to generate isogenic APP-knockout (KO) human induced pluripotent stem cells (hiPSCs) and differentiated them into human astrocytes. We found that APP-KO astrocytes have reduced cholesterol and elevated levels of sterol regulatory element-binding protein (SREBP) target gene transcripts and proteins, which were both downstream consequences of reduced lipoprotein endocytosis. To elucidate which APP fragments regulate cholesterol homeostasis and to examine whether familial AD mutations in APP affect lipoprotein metabolism, we analyzed an isogenic allelic series harboring the APP Swedish and APP V717F variants. Only astrocytes homozygous for the APP Swedish (APPSwe/Swe) mutation, which had reduced full-length APP (FL APP) due to increased β-secretase cleavage, recapitulated the APP-KO phenotypes. Astrocytic internalization of β-amyloid (Aβ), another ligand for low-density lipoprotein (LDL) receptors, was also impaired in APP-KO and APPSwe/Swe astrocytes. Finally, impairing cleavage of FL APP through β-secretase inhibition in APPSwe/Swe astrocytes reversed the LDL and Aβ endocytosis defects. In conclusion, FL APP is involved in the endocytosis of LDL receptor ligands and is required for proper cholesterol homeostasis and Aβ clearance in human astrocytes.

astrocyte
amyloid precursor protein (APP)
amyloid-beta (Aβ)
Alzheimer disease
cholesterol metabolism
induced pluripotent stem cell (iPS cell) (iPSC)
lipoprotein receptor
lipoprotein metabolism

Cited by (0)

This work was supported in whole or part by National Institutes of Health Predoctoral Training Grant T32 GM008666; Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Grant 2013/18028-9, and National Institutes of Health Grants RF1 AG048083-01 and 2P50 AG005131-31 from NIA. The authors declare that they have no conflicts of interest with the contents of this article. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

2

The abbreviations used are:

    AD

    Alzheimer's disease

    APP

    amyloid precursor protein

    CTF

    C-terminal fragment

    amyloid-β

    KO

    knockout

    hiPSC

    human induced pluripotent stem cell

    iPSC

    induced pluripotent stem cells

    LDL

    low density lipoprotein

    LDLR

    low density lipoprotein receptor

    SREBP

    sterol-regulatory element-binding protein

    BBR

    berberine

    SREBP

    sterol regulatory element-binding protein

    qRT-PCR

    quantitative RT-PCR

    NPC

    neural precursor cell

    HMGCR

    HMG-CoA reductase

    sAPP

    soluble APP

    FL

    full length

    Tfn

    transferrin

    SAD

    sporadic Alzheimer's disease

    FAD

    familial Alzheimer's disease

    BSI

    β-secretase inhibitor

    DMEM

    Dulbecco's modified Eagle's medium

    RI

    ROCK inhibitor

    FBS

    fetal bovine serum

    BisTris

    2-[bis(2-hydroxyethyl)amino]-2-(hydroxymethyl)propane-1,3-diol

    MEF

    mouse embryonic fibroblast.