New Insights into the Heparan Sulfate Proteoglycan-binding Activity of Apolipoprotein E

Abstract

Defective binding of apolipoprotein E (apoE) to heparan sulfate proteoglycans (HSPGs) is associated with increased risk of atherosclerosis due to inefficient clearance of lipoprotein remnants by the liver. The interaction of apoE with HSPGs has also been implicated in the pathogenesis of Alzheimer's disease and may play a role in neuronal repair. To identify which residues in the heparin-binding site of apoE and which structural elements of heparan sulfate interact, we used a variety of approaches, including glycosaminoglycan specificity assays, 13C nuclear magnetic resonance, and heparin affinity chromatography. The formation of the high affinity complex required Arg-142, Lys-143, Arg-145, Lys-146, and Arg-147 from apoE and N-and 6-O-sulfo groups of the glucosamine units from the heparin fragment. As shown by molecular modeling, using a high affinity binding octasaccharide fragment of heparin, these findings are consistent with a binding mode in which five saccharide residues of fully sulfated heparan sulfate lie in a shallow groove of the ␣-helix that contains the HSPG-binding site (helix 4 of the four-helix bundle of the 22-kDa fragment). This groove is lined with residues Arg-136, Ser-139, His-140, Arg-142, Lys-143, Arg-145, Lys-146, and Arg-147. In the model, all of these residues make direct contact with either the 2-O-sulfo groups of the iduronic acid monosaccharides or the N-and 6-O-sulfo groups of the glucosamine sulfate monosaccharides. This model indicates that apoE has an HSPG-binding site highly complementary to heparan sulfate rich in N-and O-sulfo groups such as that found in the liver and the brain. Human apolipoprotein E (apoE)1 is a 299-residue polymorphic protein that facilitates the transport and metabolism of lipids (1). ApoE is a ligand for members of the low density lipoprotein (LDL) receptor family, heparin, and heparan sulfate proteoglycans (HSPGs) (2, 3). It is composed of two domains: a 22-kDa NH 2 -terminal domain (residues 1-191) and a 10-kDa COOH-terminal domain (residues 216 -299) (4). The 22-kDa NH 2 -terminal domain contains the primary HSPGbinding site (residues 140 -150) (5) colocalized with the LDL receptor binding site (6 -8).Binding of apoE to HSPG is an initial step in the localization of apoE-containing lipoproteins to the surface of several different types of cells (9). After localization, the apoE-containing lipoproteins are transported into the cell by pathways dependent on either the LDL receptor or the LDL receptor-related protein (LRP) or by direct uptake of an apoE-containing lipoprotein-HSPG complex (10). Binding of apoE to HSPG affects neurite extension in neurons (11, 12) and localizes secreted apoE to the surface of macrophages (13). Binding of apoE to HSPG may also play a role in Alzheimer's disease through either competition between apoE and the amyloid precursor protein (APP) for HSPG-binding sites or by modulation of the HSPG/LRP uptake pathway (14 -17).The best understood physiological role of the binding of apoE to HSPG is in lipoprotein remnant clearance....

Isolation and characterization of heparan sulfate from various murine tissues

“…Our examination of porcine, bovine, and human liver HS [8,35] show that it is rich in highly sulfated sequences and that these are responsible for ApoE interaction [34]. Furthermore, these highly charged liver HS chains also appear to act as receptors for pathogens such as dengue virus [36] and malaria circumsporozoite [37].…”

“…Liver HS is believed to act as a receptor for apoE in lipid clearance [33,34]. Our examination of porcine, bovine, and human liver HS [8,35] show that it is rich in highly sulfated sequences and that these are responsible for ApoE interaction [34].…”

Role of the N- and C-Terminal Domains in Binding of Apolipoprotein E Isoforms to Heparan Sulfate and Dermatan Sulfate: A Surface Plasmon Resonance Study

“…Compared to heparin, the less sulfated HS and DS showed a slower first association rate and, consequently, a significant decrease in the favorable free energy of binding for the first step. This indicates that the degree of sulfation in GAG has a major effect on the electrostatic interaction with apoE (26).…”

“…Although the variations in the distribution of acidic groups in different types of GAG (31) are likely to influence the interaction with apoE (32,33), quantitative information on the interaction with various GAG molecules is limited (26,34). In the present study, we extended the SPR approach to explore the contributions of the N-and C-terminal domains of apoE to its interaction with more physiological GAG, such as HS and DS.…”

“…The N-terminal domain is folded into a four-helix bundle of amphipathic α-helices and contains the LDLR binding region (around residues 136-150 in helix 4) (20), whereas the C-terminal domain contains amphipathic α-helices that are involved in binding to lipoproteins with high affinity (21,22). Both the N-and C-terminal domains contain a heparin binding site (23,24): the N-terminal domain site is located between residues 136-147, overlapping with the LDLR binding region (25,26), whereas the C-terminal site involves basic residues around lysine 233 (27). Although both sites are functional in the separated fragments, only the N-terminal site is available for interaction in both the lipid-free and lipidated states of the intact apoE molecule (27,28).…”