Structural and Functional Variations in Human Apolipoprotein E3 and E4

Abstract

There are three major apolipoprotein E (apoE) isoforms. Although APOE-⑀3 is considered a longevity gene, APOE-⑀4 is a dual risk factor to atherosclerosis and Alzheimer disease. We have expressed full-length and N-and C-terminal truncated apoE3 and apoE4 tailored to eliminate helix and domain interactions to unveil structural and functional disturbances. The N-terminal truncated apoE4-(72-299) and C-terminal truncated apoE4-(1-231) showed more complicated or aggregated species than those of the corresponding apoE3 counterparts. This isoformic structural variation did not exist in the presence of dihexanoylphosphatidylcholine. The C-terminal truncated apoE-(1-191) and apoE-(1-231) proteins greatly lost lipid binding ability as illustrated by the dimyristoylphosphatidylcholine turbidity clearance. The low density lipoprotein (LDL) receptor binding ability, determined by a competition binding assay of 3 H-LDL to the LDL receptor of HepG2 cells, showed that apoE4 proteins with N-terminal (apoE4-(72-299)), C-terminal (apoE4-(1-231)), or complete C-terminal truncation (apoE4-(1-191)) maintained greater receptor binding abilities than their apoE3 counterparts. The cholesterol-lowering abilities of apoE3-(72-299) and apoE3-(1-231) in apoE-deficient mice were decreased significantly. The structural preference of apoE4 to remain functional in solution may explain the enhanced opportunity of apoE4 isoform to display its pathophysiologic functions in atherosclerosis and Alzheimer disease. Human apolipoprotein E (apoE)2 is a 299-amino acid protein with a molecular mass of 34 kDa. ApoE is encoded by the three alleles (APOE-⑀2, APOE-⑀3, and APOE-⑀4) of a gene on chromosome 19q13.2 determining the three major isoforms, namely apoE2, apoE3, and apoE4 in six phenotypes (1). The three apoE isoforms differ from each other only by a single amino acid substitution involving cysteine-arginine replacement at residues 112 and 158, i.e. apoE2 (Cys 112 /Cys 158 ), apoE3 (Cys 112 / Arg 158 ), and apoE4 (Arg 112 /Arg 158 ) (Fig. 1A). Genetically, the APOE-⑀4 allele is associated with both familial late-onset and sporadic Alzheimer disease (AD) and atherosclerosis (2-4). AD patients carrying the APOE-⑀4 allele have more profound deposition of ␤-amyloid peptides (A␤) in their brains than those carrying APOE-⑀2 and APOE-⑀3 alleles (5, 6). Considerable evidence supports the view that apoE4 increases the risk of AD by accelerating the plaque formation and by impairing the neurons. ApoE4 appears to modulate amyloid precursor protein processing and A␤ production through both the LDL receptor-related protein pathway and domain interaction (7). Strong correlation of APOE-⑀4 allele with dyslipidemia and atherosclerosis, the major underlying mechanism of coronary heart disease, has been demonstrated (8). Human apoE4 represents a dual risk factor for these two major degenerative diseases.ApoE contains two independently folded domains (N-terminal domain, residues 20 -165, and C-terminal domain, residues 225-299) that are separated by a large nonstructura...

Structural Basis for the Recognition and Cross-linking of Amyloid Fibrils by Human Apolipoprotein E

“…Sedimentation velocity studies confirmed that apoE3 and apoE4 were primarily tetrameric and monomeric in the presence of sub-micellar DHPC (4 mM) and micellar DHPC (20 mM), respectively (data not shown) (13,18). Fig.…”

“…The 10-kDa C-terminal domain (residues 225-299) consists of three putative amphipathic ␣-helices. This C-terminal domain contains the major lipid binding region and in the absence of lipid mediates the self-association of apoE (12,17,18). In the absence of lipid, apoE and the isolated C-terminal domain exist primarily as tetramers, whereas the isolated N-terminal domain of apoE is monomeric (12,13,18).…”

“…This C-terminal domain contains the major lipid binding region and in the absence of lipid mediates the self-association of apoE (12,17,18). In the absence of lipid, apoE and the isolated C-terminal domain exist primarily as tetramers, whereas the isolated N-terminal domain of apoE is monomeric (12,13,18). The N-terminal domain undergoes a conformational change in the presence of lipid where the four-helix bundle opens and helix one moves with respect to helix three (19 -21).…”

Parental history of Alzheimer disease associated with lower plasma apolipoprotein E levels

“…ApoE4 has the highest affinity for the low-density lipoprotein receptor and is cleared fastest, whereas apoE2 has the lowest affinity for the low-density lipoprotein receptor and is cleared at a much lower rate, explaining the variation in plasma apoE levels. 26,27 Besides the influence of genetic variation on plasma apoE levels, phenotypic variation, such as dietary intake, has also been shown to influence plasma apoE levels. Several studies have reported increasing plasma apoE levels after the administration of diets containing saturated fatty acids, or fish and vegetable oil, 28,29 although others did not report such an association.…”

Using Prior Knowledge in the Determination of Macromolecular Size-Distributions by Analytical Ultracentrifugation

“…In a hierarchical approach, some of this knowledge has been incorporated into a secondary analysis in the form of a fixed constraint (for example, with the hybrid discrete/continuous distribution 29,[53][54][55], in combination of statistical analysis of the quality of fits obtained. However, in the present work we examined a more subtle approach, which permits the experimental data to override our expectation if they contain sufficient information contrary to our expectation.…”