The binding specificity of normal and variant rat Kupffer cell (lectin) receptors expressed in COS cells

CLEC4F is an inducible C-type lectin in F4/80-positive cells and is involved in alpha-galactosylceramide presentation in liver

CLEC4F, a member of C-type lectin, was first purified from rat liver extract with high binding affinity to fucose, galactose (Gal), N-acetylgalactosamine (GalNAc), and un-sialylated glucosphingolipids with GalNAc or Gal terminus. However, the biological functions of CLEC4F have not been elucidated. To address this question, we examined the expression and distribution of murine CLEC4F, determined its binding specificity by glycan array, and investigated its function using CLEC4F knockout (Clec4f-/-) mice. We found that CLEC4F is a heavily glycosylated membrane protein co-expressed with F4/80 on Kupffer cells. In contrast to F4/80, CLEC4F is detectable in fetal livers at embryonic day 11.5 (E11.5) but not in yolk sac, suggesting the expression of CLEC4F is induced as cells migrate from yolk cells to the liver. Even though CLEC4F is not detectable in tissues outside liver, both residential Kupffer cells and infiltrating mononuclear cells surrounding liver abscesses are CLEC4F-positive upon Listeria monocytogenes (L. monocytogenes) infection. While CLEC4F has strong binding to Gal and GalNAc, terminal fucosylation inhibits CLEC4F recognition to several glycans such as Fucosyl GM1, Globo H, Bb3∼4 and other fucosyl-glycans. Moreover, CLEC4F interacts with alpha-galactosylceramide (α-GalCer) in a calcium-dependent manner and participates in the presentation of α-GalCer to natural killer T (NKT) cells. This suggests that CLEC4F is a C-type lectin with diverse binding specificity expressed on residential Kupffer cells and infiltrating monocytes in the liver, and may play an important role to modulate glycolipids presentation on Kupffer cells.

Molecular characterization of the rat Kupffer cell glycoprotein receptor

The Kupffer cell receptor for glycoproteins has been reported to have a role in clearance of galactose- and fucose-terminated glycoproteins from circulation. Although the gene and a cDNA encoding the receptor have been described, there has been little study of the receptor protein. To address some questions about possible ligands and functions for this receptor, fragments representing portions of the extracellular domain have been expressed and characterized. The extracellular domain consists of a trimer stabilized by an extended coiled-coil of alpha-helices. The receptor displays monosaccharide-binding characteristics similar to the hepatic asialoglycoprotein receptor, but with somewhat less selectivity. The two best monosaccharide ligands are GalNAc and galactose. alpha-Methyl fucoside is a particularly poor ligand. Analysis of Kupffer cell receptor binding to glycoproteins and oligosaccharides released from them reveals highest affinity for desialylated, complex N-linked glycans. The best glycoprotein ligands contain multiple highly branched oligosaccharides. A human ortholog of the rat receptor gene does not encode a full-length protein and is not expressed in liver. These characteristics suggest that the receptor may have functions parallel to those of the hepatocyte asialoglycoprotein receptor in some (but not all) mammalian species.

Specific binding of glucose-derivatized polymers to the asialoglycoprotein receptor of mouse primary hepatocytes

In this study, we designed a novel amphiphilic poly-(p-N-vinylbenzyl-D-glucuronamide) (PV6Gna) modified at the 6-OH position of glucose for hepatocyte recognition to address the mechanism of the interaction between mouse primary hepatocytes and the PV6Gna. PV6Gna bound to lectins specific for glucose but not galactose as did other glucose-derivatized polymers. However, hepatocyte adhesion onto the PV6Gna surface was inhibited in the presence of galactose and its analogues but not in the presence of glucose and its analogues. We also showed that hepatocyte adhesion to the PV6Gna surface was inhibited dose dependently by asialofetuin (ASF). Interactions between soluble PV6Gna and hepatocytes were inhibited by GalNAc, ASF, and EGTA in flow cytometry analysis using fluorescein isothiocyanate-conjugated PV6Gna. Hepatocyte adhesion to the PV6Gna surface was inhibited more effectively by GalNAc than by methyl beta-D-galactose. In flow cytometry analysis and cell adhesion assay, ASF competed for the inhibition of interaction between PV6Gna and hepatocytes 0.5-4 x 10(5)-fold more effectively than did GalNAc. These results demonstrate involvement of asialoglycoprotein receptors (ASGPRs) in the interaction between PV6Gna and hepatocytes. Furthermore, to clarify the mechanism of the interaction between glycopolymers modified at the 6-OH position of glucose and the hepatocyte, we prepared a gel particle containing 6-O-methacryloyl-d-glucose (PMglc) synthesized by an enzymatic method. ASGPRs could be detected using Western blot analysis following precipitation with PMglc in hepatocyte cell lysate. The precipitation of ASGPRs was inhibited in the presence of galactose, ASF, PV6Gna, and EGTA. The precipitation was inhibited more effectively by GalNAc than by methyl beta-d-galactose. ASGPRs were rarely precipitated by PMglc in the cell lysate that had been treated with ASF-conjugated Sepharose. Taken together, we suggest that mouse primary hepatocytes adhere to the PV6Gna surface mediated by ASGPRs, which specifically interacted with the glycopolymers modified at the C-6 position of glucose.

Sialic acids in molecular and cellular interactions

Sialic acids (Sias) are terminal components of many glycoproteins and glycolipids especially of higher animals. In this exposed position they contribute significantly to the structural properties of these molecules, both in solution and on cell surfaces. Therefore, it is not surprising that Sias are important regulators of cellular and molecular interactions, in which they play a dual role. They can either mask recognition sites or serve as recognition determinants. Whereas the role of Sias in masking and in binding of pathogens to host cells has been documented over many years, their role in nonpathological cellular interaction has only been shown recently. The aim of this chapter is to summarize our knowledge about Sias in masking, for example, galactose residues, and to review the progress made during the past few years with respect to Sias as recognition determinants in the adhesion of pathogenic viruses, bacteria, and protozoa, and particularly as binding sites for endogenous cellular interaction molecules. Finally, perspectives for future research on these topics are discussed.

Neisseria gonorrhoeae utilizes and enhances the biosynthesis of the asialoglycoprotein receptor expressed on the surface of the hepatic HepG2 cell line

One of the lipooligosaccharide (LOS) structures of Neisseria gonorrhoeae contains a terminal Gal(beta 1-4)GlcNAc residue which is a good candidate to serve as a ligand for human asialoglycoprotein receptors (ASGP-R). These receptors have been shown to be present on macrophages, sperm cells, and hepatocytes. The human tissue culture cell line used most often to study this receptor, HepG2, was used in our investigations only as a model. We also chose N. gonorrhoeae 1291 for these studies because, unlike many other gonococcal strains, this strain expresses one main species of LOS. The LOS structure expressed by this strain has also been fully characterized. Using well-established assays for the utilization of the ASGP-R, we found that incubation of HepG2 cells with gonococci expressing the terminal Gal(beta 1-4)GlcNAc asialo-LOS carbohydrate structure competitively inhibited the ASGP-R from binding to one of its well-known ligands, asialo-alpha-acid-1-glycoprotein. The inhibition was specific to the ASGP-R, since binding of two other ligands to their specific receptors in the same model cell system was not affected. Immunoblot analysis for the ASGP-R suggested that gonococci seemed to stimulate the HepG2 cells to increase the expression of the major (46-kDa) receptor species. This observation was confirmed both by functional analysis, which showed that the concentration of total receptor molecules, as well as surface receptors, was about 60% higher after incubation with gonococci than in control cells and by Northern (RNA) blot analysis using a cDNA probe of the major human H1 subunit. Poly(A) RNA purified from control and HepG2 cells exposed to gonococci indicated the presence of increased amounts of mRNA coding for the ASGP-R after incubation with gonococci. This result supports the idea that the molecular mechanism controlling the receptor level after gonococcal exposure is under transcriptional regulation.