Apo E structure determines VLDL clearance and atherosclerosis risk in mice

Abstract

Apo E is a 34-kDa plasma protein important for the metabolism of plasma lipoproteins (1). Like other apolipoproteins, apo E contains multiple 22-amino acid repeats that form amphipathic helices, enabling it to associate with the surface of plasma lipoproteins. Apo E also contains a stretch of basic residues (136-150) that is important for high-affinity binding to the LDL receptor and subsequent endocytosis of the associated lipoprotein particle (2). In addition, apo E mediates lipoprotein interactions with LDL receptor-related protein (LRP) (3), the VLDL receptor (4), other lipoprotein receptors (5), endothelial heparin sulfate (6), and plasma lipases (7,8). The phenotype of severe hyperlipidemia and spontaneous development of atherosclerosis in mice lacking apo E clearly demonstrates the central role of apo E in mammalian lipid metabolism (9, 10).In humans, the APOE gene is polymorphic and has 3 alleles: APOE*2, APOE*3, and APOE*4. These alleles have frequencies of 7%, 77%, and 15%, respectively, in the general population (11). The APOE*3 allele codes for cysteine at position 112 and for arginine at 158. The APOE*2 allele codes cysteines at both positions, whereas the APOE*4 allele codes for arginines at both positions. Various population-based studies have suggested that the different APOE alleles have distinct influences on lipid metabolism in humans. Possession of at least 1 copy of the APOE*2 allele has been associated with higher plasma apo E (12) and lower plasma cholesterol, LDL cholesterol, and apo B levels (11) when compared with APOE*3 homozygotes. The APOE*2 allele is also associated with lower risk of coronary artery disease (13), except in 5-10% of APOE*2 homozygotes who develop type III hyperlipoproteinemia and premature atherosclerosis (14). On the other hand, the presence of at least 1 APOE*4 allele is associated with lower plasma apo E (12) and increased plasma cholesterol, LDL cholesterol, and apo B levels (11), and a greater risk of coronary artery disease (13), when compared with APOE*3 homozygotes. Davignon et al. (11) estimate that the apo E polymorphism accounts for 2.8% of the variation of risk for atherosclerosis, which is a large contribution for a single locus in this complex, polygenic disease.The availability of a well-defined model system should benefit studies of the role of the human apo E polymorphism in lipid metabolism and atherosclerosis. To develop such a model, we have used gene targeting to replace the murine Apoe gene with the 3 human APOE alleles. These mice retain the murine Apoe regulatory sequences and solely produce human apo E proteins with different We have generated mice expressing the human apo E4 isoform in place of the endogenous murine apo E protein and have compared them with mice expressing the human apo E3 isoform. Plasma lipid and apolipoprotein levels in the mice expressing only the apo E4 isoform (4/4) did not differ significantly from those in mice with the apo E3 isoform (3/3) on chow and were equally elevated in response to increased lipid and choles...

ApoE4 disrupts sterol and sphingolipid metabolism in Alzheimer's but not normal brain

“…Although ApoE4 binds lipoprotein receptors and clears triglyceride-rich lipoprotein remnants with similar efficacy to ApoE3 (Gregg et al 1986;Bohnet et al 1996;Knouff et al 1999), compared with ApoE3, ApoE4 is less efficient at promoting cholesterol efflux in fibroblasts and astrocytes (Huang et al 1995;Michikawa et al 2000), binds preferentially very low density lipoproteins (Weisgraber 1990;Dong et al 1994) and is recycled inefficiently after internalization (Heeren et al 2004). Despite these deficits, ApoE4 may effectively transport cholesterol in brain under steady state conditions when cholesterol turnover is low.…”

Anatomical differences and atherosclerosis in apolipoprotein E-deficient mice with 129/SvEv and C57BL/6 genetic backgrounds

“…Lipoproteins were separated from pooled plasma from at least six mice (1 ml) into density fractions by ultracentrifugation, and subjected to SDS-PAGE analysis as described (15).…”

Effect of human apolipoprotein E genotype on the pathogenesis of experimental ocular HSV-1

“…In these transgenic mice, the coding sequence for murine apoE has been replaced by the human apoE3 or apoE4 under the regulation of murine regulatory sequences. As a result, the mice express only the human apoE isoforms, with tissue distribution and levels very similar to those of endogenous mouse apoE (Knouff et al, 1999;Sullivan et al, 1997). We also measured apoE expression in uninfected cornea, TG, and brain tissue and found no significant differences.…”

“…The E4 allele that is associated with higher low-density lipoprotein cholesterol is considered pro-atherogenic (Davignon et al, 1999;Mahley, 1988;Song et al, 2004). The link between apoE4 and pathological vascularization has not been well elucidated; however, apoE4 is accepted as a risk factor for atherosclerosis (Altenburg et al, 2007;Knouff et al, 1999;Scuteri et al, 2005). In animal model studies, murine apoE is known to influence pathological vascularization (Couffinhal et al, 1999;Pola et al, 2003).…”

“…These mice express the transgene under the regulation of the mouse apoE promoter. Levels of expression of apoE3 or apoE4 have been characterized and are not significantly different between the apoE3 TR and apoE4 TR mice Knouff et al, 1999;Piedrahita et al, 1992;Sullivan et al, 1997;Sullivan et al, 1998).…”