Profile and Regulation of Apolipoprotein E (ApoE) Expression in the CNS in Mice with Targeting of Green Fluorescent Protein Gene to the ApoE Locus

Abstract

To study the profile and regulation of apolipoprotein E (apoE) expression in the CNS, we generated mice in which apoE expression can be detected in vivo with unprecedented sensitivity and resolution. cDNA encoding enhanced green fluorescent protein (EGFP) with a stop codon was inserted by gene targeting into the apoE gene locus (EGFP_apoE) immediately after the translation initiation site. Insertion of EGFP into one apoE allele provides a real-time location marker of apoE expression in vivo; the remaining allele is sufficient to maintain normal cellular physiology. In heterozygous EGFP_apoE mice, EGFP was highly expressed in hepatocytes and peritoneal macrophages. EGFP was also expressed in brain astrocytes; however some astrocytes (∼25%) expressed no EGFP, suggesting that a subset of these cells does not express apoE. EGFP was expressed in in situ hybridization of mouse apoE mRNA and by anti-apoE immunostaining. Smooth muscle cells of large blood vessels and cells surrounding small vessels in the CNS also strongly expressed EGFP, as did cells in the choroid plexus. EGFP_apoE reporter mice will be useful for studying the regulation of apoE expression in the CNS and might provide insights into the diverse mechanisms of apoE4-related neurodegeneration.