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 upregulation
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 posttranscriptional.
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.
Keywords: Alzheimer's disease (AD), amyloid-β , Apoer2, apolipoprotein E4 (ApoE4), targeted replacement mice, Lrp1.
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