Modulating Amyloid beta Clearance by Altering Cellular and Whole Brain Apolipoprotein E Metabolism.

摘要

The aggregation and accumulation of the amyloid beta peptide in the brain is hypothesized to be an initial necessary event in the pathogenesis of Alzheimer's disease (AD). Since the level of monomeric soluble amyloid beta as well as amyloid beta-binding molecules determine the onset and amount of amyloid beta aggregation, significant attention has been devoted to defining the molecular and systemic pathways that modulate amyloid beta synthesis and clearance in the brain. Extensive evidence exists that both the isoform and amount of apolipoprotein E (apoE), an amyloid beta-binding molecule, influence amyloid beta aggregation and clearance from the brain. Therefore, studying how the molecular mechanisms that modulate apoE levels in the brain affect amyloid beta clearance will enhance our insight into the disease process. The apoE receptor low-density lipoprotein receptor (LDLR) and ATP-binding cassette transporter A1 (ABCA1), a protein that regulates apoE lipidation, have previously been shown to modulate brain amyloid beta levels. In the work presented in this dissertation, we found that increasing LDLR levels enhanced the cellular uptake and degradation of amyloid beta by primary astrocytes, and increased amyloid beta transport to lysosomes. The effect of LDLR on amyloid beta uptake and clearance occurred independently of apoE, and potentially involved a direct interaction between amyloid beta and LDLR. To measure the clearance of apoE and amyloid beta in the mouse brain, we developed a technique that couples stable isotope-labeling kinetics (SILK) with mass spectrometry. We validated this technique by demonstrating that apoE clearance is enhanced in the brains of mice overexpressing LDLR. We also applied this technique to measure apoE clearance rates in the brains of human apoE targeted-replacement mice. Finally, we analyzed the effect of ABCA1 on apoE and amyloid beta clearance from the mouse brain. The fractional clearance rate of apoE was increased in amyloid precursor protein (APP) transgenic mice that either lacked or overexpressed ABCA1, while ABCA1 levels had no effect on amyloid beta clearance. Therefore, ABCA1 likely influences amyloid beta aggregation in vivo through a process other than modulating amyloid beta clearance. These data further our understanding of how proteins involved in apoE metabolism influence AD pathogenesis, and have important implications for future therapeutic strategies that target brain apoE levels and function.