Involvement of the Apoer2 and Lrp1 receptors in mediating the pathological effects of ApoE4 in vivo

Abstract

This study investigated the possible role of the ApoE receptors Lrp1 and Apoer2 in mediating the pathological effects of ApoE4 in ApoE-targeted-replacement mice expressing either the human ApoE3 or ApoE4 allele. In this study we show that activation of the amyloid cascade by inhibition of the Aβ-degrading enzyme neprilysin results in up-regulation of the ApoE receptor Lrp1 in the CA1 hippocampal neurons of 4-month-old ApoE4 mice, but not in the corresponding ApoE3 or ApoE-deficient (KO) mice. These results are in accordance with the previous findings that activation of the amyloid cascade induces Aβ accumulation in the CA1 neurons of ApoE4 mice, but not in ApoE3 or ApoE-KO mice. This suggests that the apoE4-driven elevation of Lrp1 is mediated via a gain of function mechanism and may play a role in mediating the effects of ApoE4 on Aβ. In contrast, no changes were observed in the levels of the corresponding Apoer2 receptor following the neprilysin inhibition. The ApoE receptors of naive ApoE4 mice were also affected differentially and isoform specifically by ApoE4. However, under these conditions, the effect was an ApoE4-driven reduction in the levels of Apoer2 in CA1 and CA3 pyramidal neurons, whereas the levels of Lrp1 were not affected. RT-PCR measurements revealed that the levels of Apoer2 and Lrp1 mRNA in the hippocampus of naïve and neprilysin-inhibited mice were not affected by ApoE4, suggesting that the observed effects of ApoE4 on the levels of these receptors is post-transcriptional. In conclusion, this study shows that the levels of hippocampal ApoE receptors Lrp1 and Apoer2 in vivo are affected isoform specifically by ApoE4 and that the type of receptor affected is context dependent.

Reversal of apoE4-Driven Brain Pathology and Behavioral Deficits by Bexarotene

“…Immunohistochemistry was performed as described previously (Belinson et al, 2008). Sections were washed with 10 mM PBS, pH 7.4, and blocked for 1 h in 20% serum diluted in PBS with 0.1% Triton X-100 (PBST), after which the primary Ab, diluted in PBST containing 2% of the appropriate serum, was applied overnight at 4°C.…”

“…Control experiments revealed no staining in sections lacking the first Ab. The intensities of immunofluorescence staining were calculated using the Image-Pro Plus system (version 5.1; Media Cybernetics) as described previously (Belinson et al, 2008). All images for each immunostaining were obtained under identical conditions and their quantitative analyses were performed with no further handling.…”

“…The homogenates were then aliquoted and stored at Ϫ70°C. For SDSelectrophoresis, the samples were boiled for 10 min with 0.5% SDS and immunoblotted as described previously (Belinson et al, 2008;Haas et al, 2012). The following Abs were used: mouse anti-VGluT1 (1:1000; Millipore); goat anti-apoE (1:10,000, Millipore); rabbit anti-ABCA1 (1:500; Novus); rabbit anti-ABCG1 (1:2000; Novus); and mouse anti-GAPDH (1:1000; Abcam).…”

“…Freshly excised hippocampai from control and bexarotene-treated apoE4 mice and their corresponding apoE3 mice were gently pressed through a 40 m cell strainer using a plunger from a 1 ml syringe (Evans et al, 2014). The extract was briefly centrifuged to dispose of crude debris and then run on a nondenaturing 3-16% gradient gel.…”

“…ABCA1 preferentially stimulates the efflux of cholesterol and its binding to lipid-free acceptors, such as lipid-free apoE, whereas ABCG1 is more selective for partially lipidated lipoprotein complexes (Karten et al, 2006;Kim et al, 2008). The retinoid X receptor (RXR) system regulates the expression of ABCA1 and ABCG1, as well as that of apoE, by forming heterodimers with the liver X receptor (LXR), which then binds to distinct promoter sequences, thereby activating gene expression (Chawla et al, 2001;Laffitte et al, 2001;Liang et al, 2004). It has been shown recently that treatment of APP/PS1-transgenic mice with the RXR agonist bexarotene stimulates the synthesis of mouse ABCA1, ABCG1, and apoE and that this leads to the clearance of brain A␤ and reversal of cognitive deficits .…”