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UBC Theses and Dissertations

Regulation of apolipoprotein E secretion, lipidation and recycling in the central nervous system Fan, Jianjia


Lipid transport in the brain is coordinated by glia-derived lipoproteins that contain apolipoprotein E (apoE) as their primary protein. ApoE plays an important role in the pathogenesis of neurodegenerative diseases such as Alzheimer's disease (AD). ATP-binding cassette transporter A1 (ABCA1) effluxes cholesterol and phospholipids to apolipoprotein acceptors including apoE. ABCA1 is a key regulator of apoE levels and lipidation in the brain and deficiency of ABCA1 increases amyloid burden in AD mouse models. Translating these findings to potential therapies for AD will require a more thorough understanding of the biochemical nature of nascent apoE particles generated from glia and of lipoprotein remodeling in the CNS in general. In this thesis, I show that apoE is secreted from wild-type primary murine mixed glia as nascent lipoprotein subspecies ranging from 7.5 to 17 nm in diameter. Glia lacking ABCA1 secrete only one species of small particles (~8.1nm), which are poorly lipidated, but can accept lipids to form the full repertoire of wild-type apoE particles. Inhibition of apoE receptor function blocks appearance of the 8.1 nm species, suggesting that this particle may arise through apoE recycling. Selective deletion of the LDL receptor significantly reduces the level of the 8.1 nm particles, suggesting that apoE is preferentially recycled through LDLR. These results suggest that nascent glial apoE lipoproteins are secreted through multiple pathways. Modulating the expression, secretion or function of apoE may provide potential therapeutic approaches to protect the brain from chronic and acute damage. This thesis shows that progesterone and a synthetic progestin, lynestrenol, significantly induce apoE secretion from human CCF-STTG1 astrocytoma cells, whereas estrogens have negligible effects. Intriguingly, lynestrenol also increases expression of ABCA1 in human astrocytoma cells, primary murine glia, and immortalized murine astrocytes that express human apoE3. The progesterone receptor (PR) inhibitor RU486 attenuates the effect of progestins on apoE expression in astrocytoma but has no effect on ABCA1 expression in all glial cell models tested, suggesting that PR may participate in apoE but does not affect ABCA1 regulation. These results suggest that selective reproductive steroid hormones have the potential to influence glial lipid homeostasis through LXR-dependent and PR-dependent pathways.

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