Inhibition of ACAT as a Therapeutic Target for Alzheimer’s Disease is Independent of ApoE4-lipidation

Abstract

<b>Abstract ID 24886</b> <b>Poster Board 10</b> <b>Background:</b> While the mechanism(s) underlying <i>APOE4</i>-induced AD risk remains unclear, apoE4 levels in the brains of humans and transgenic mice (Tg) expressing human <i>APOE4</i> are lower than those expressing <i>APOE3</i>, and apoE4-lipoproteins are poorly lipidated compared to apoE3-lipoproteins. Increasing the lipidation of apoE4 can stabilize apoE4-lipoproteins, thus making this process an attractive therapeutic target. Several enzymes affect CNS cholesterol levels including ACAT (acyl-CoA:cholesterol-acyltransferase), which catalyzes the formation of intracellular cholesteryl-ester droplets, reducing the intracellular free cholesterol (FC) pool. Inhibition of ACAT increases the FC pool, making the lipids available for secretion to extracellular apoE-containing lipoproteins. Previous studies using commercially available ACAT inhibitors, including Avasimibe (AVAS), as well as <i>ACAT</i>-knock out (KO) mice, demonstrate reduced plaque burden, gliosis, amyloid-beta peptide (Ab), and altered amyloid precursor protein (APP) processing in familial AD (FAD)-Tg mouse models. However, the effects of AVAS in the presence of human apoE4 remain unknown. Our hypothesis is that AVAS, via inhibition of ACAT, will increase the FC pool, promoting ABCA1-mediated efflux of cholesterol to nascent apoE4-lipoproteins. This correction of apoE4 structure via lipidation will restore its function in the CNS, including clearance of oAβ and reducing neuroinflammation. <b>Methods:</b> Male E4FAD-Tg mice (5xFAD^+/-/<i>APOE4</i>^+/+) were treated by oral gavage with AVAS from 6-8 months. Synaptic proteins were measured by Western blot and learning and memory by Morris Water Maze (MWM). Lipid droplets/cell were analyzed by staining with LipidSpot. Ab pathology was evaluated by measuring Ab solubility (3-step sequential extraction followed by ELISA) and Ab/amyloid deposition (immunostaining). Neuroinflammation (astrogliosis and microgliosis) were evaluated by immuhistochemistry. ApoE levels and lipidation were analyzed by apoE-ELISA and native gels. <b>Results:</b> AVAS treatment significantly reduced intracellular lipid droplets, demonstrating indirect target engagement, increased MWM measures of memory and postsynaptic protein levels, indicating surrogate efficacy, and reduced pathological changes in Aβ solubility/deposition, and neuroinflammation, all critical components of <i>APOE4</i>-modulated AD pathology. However, there was no increase in apoE4 levels or lipidation, while amyloidogenic processing of APP was significantly reduced. <b>Conclusion:</b> This study suggests that the AVAS-induced reduction in Aβ via reduced APP processing was sufficient to reduce AD pathology, while apoE4-lipoproteins remained poorly lipidated. This study was partially sponsored by AbbVie, Inc. AbbVie contributed to the study design, research, and interpretation of data, reviewing, and approving the manuscript.