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Published on: July 26, 2011
Apolipoprotein E ε4 Mediates Myelin Breakdown by Targeting Oligodendrocytes in Sporadic Alzheimer Disease
Gerald Wai-Yeung Cheng1, Kingston King-Shi Mok1, Sunny Hoi-Sang Yeung1
1From the Department of Health Technology and Informatics, The Hong Kong Polytechnic University, Kowloon, Hong Kong SAR.
Abstract:
White matter degradation in the frontal lobe is one of the earliest detectable changes in aging and Alzheimer disease. The ε4 allele of apolipoprotein E (APOE4) is strongly associated with such myelin pathology but the underlying cellular mechanisms remain obscure. We hypothesized that, as a lipid transporter, APOE4 directly triggers pathology in the cholesterol-rich myelin sheath independent of AD pathology. To test this, we performed immunohistochemistry on brain tissues from healthy controls, sporadic, and familial Alzheimer disease subjects. While myelin basic protein expression was largely unchanged, in frontal cortex the number of oligodendrocytes (OLs) was significantly reduced in APOE4 brains independent of their Braak stage or NIA-RI criteria. This high vulnerability of OLs was confirmed in humanized APOE3 or APOE4 transgenic mice. A gradual decline of OL numbers was found in the aging brain without associated neuronal loss. Importantly, the application of lipidated human APOE4, but not APOE3, proteins significantly reduced the formation of myelinating OL in primary cell culture derived from Apoe-knockout mice, especially in cholesterol-depleted conditions. Our findings suggest that the disruption of myelination in APOE4 carriers may represent a direct OL pathology, rather than an indirect consequence of amyloid plaque formation or neuronal loss.
Insights
The apolipoprotein E4 (APOE4) protein directly damages myelin-producing oligodendrocytes (OLs), independent of Alzheimer's disease pathology. This direct OL pathology contributes to white matter degradation in APOE4 carriers.
Area of Science:
- Neuroscience
- Neurodegenerative Diseases
- Cell Biology
Background:
- White matter degradation in the frontal lobe is an early indicator of aging and Alzheimer's disease (AD).
- The apolipoprotein E4 (APOE4) allele is linked to myelin pathology, but the cellular mechanisms are unclear.
- APOE4's role as a lipid transporter suggests potential direct effects on myelin.
Purpose of the Study:
- To investigate whether APOE4 directly causes oligodendrocyte (OL) pathology and myelin damage.
- To determine if APOE4-induced myelin pathology occurs independently of AD hallmarks like amyloid plaques.
Main Methods:
- Immunohistochemistry on human brain tissues from controls and AD patients.
- Analysis of humanized APOE3 and APOE4 transgenic mouse models.
- Primary cell culture experiments using oligodendrocytes from Apoe-knockout mice exposed to lipidated APOE3 or APOE4.
Main Results:
- Oligodendrocyte (OL) numbers were significantly reduced in the frontal cortex of APOE4 brains, irrespective of AD severity.
- This OL vulnerability was replicated in APOE4 transgenic mice and observed in aging brains without neuronal loss.
- Lipidated APOE4, but not APOE3, inhibited myelination in primary OL cultures, particularly under cholesterol-depleted conditions.
Conclusions:
- APOE4 directly induces oligodendrocyte pathology, leading to myelin disruption.
- This direct cellular effect may underlie white matter degradation in APOE4 carriers, separate from AD-related neurodegeneration.
- Targeting APOE4-mediated oligodendrocyte dysfunction could be a therapeutic strategy for myelin disorders.
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