ASGR1 and Its Enigmatic Relative, CLEC10A

Abstract

The large family of C-type lectin (CLEC) receptors comprises carbohydrate-binding proteins that require Ca²⁺ to bind a ligand. The prototypic receptor is the asialoglycoprotein receptor-1 (ASGR1, CLEC4H1) that is expressed primarily by hepatocytes. The early work on ASGR1, which is highly specific for N-acetylgalactosamine (GalNAc), established the foundation for understanding the overall function of CLEC receptors. Cells of the immune system generally express more than one CLEC receptor that serve diverse functions such as pathogen-recognition, initiation of cellular signaling, cellular adhesion, glycoprotein turnover, inflammation and immune responses. The receptor CLEC10A (C-type lectin domain family 10 member A, CD301; also called the macrophage galactose-type lectin, MGL) contains a carbohydrate-recognition domain (CRD) that is homologous to the CRD of ASGR1, and thus, is also specific for GalNAc. CLEC10A is most highly expressed on immature DCs, monocyte-derived DCs, and alternatively activated macrophages (subtype M2a) as well as oocytes and progenitor cells at several stages of embryonic development. This receptor is involved in initiation of T_H1, T_H2, and T_H17 immune responses and induction of tolerance in naïve T cells. Ligand-mediated endocytosis of CLEC receptors initiates a Ca²⁺ signal that interestingly has different outcomes depending on ligand properties, concentration, and frequency of administration. This review summarizes studies that have been carried out on these receptors.

Keywords: ASGR1; CLEC10A; IL-10; IL-12; T cell; asialoglycoprotein receptor-1; calcium; dendritic cell; lectin; macrophage; tolerance.

Figure 1

Comparison of the sequences of CLEC10A (isoform 1) and ASGR1. CLEC10A has two additional amino acids at the C-terminus, SH. ASGR1 has four additional amino acids, PPLL, at the C-terminus. The endocytosis motif (positions 5 to 8) and the QPD sequence that correlates with specificity for Gal/GalNAc are boxed.

Figure 2

(A) In silico docking of an arm of sv6D (NQHTPR) to ASGR1 (accession number 1DV8) with CABS-dock (RMSD = 0.7611 Å) [124]. The peptide is enclosed in red shading. (B) The structure of CLEC10A was generated with SWISS-MODEL Deep View from the structure of ASGR1 [125]. Docking was modeled with CABS-dock (RMSD = 1.421 Å) and downloaded into ArgusLab. The position of sv6D in the binding pocket is shown after additional molecular dynamics. The peptide is colored (carbon, grey; nitrogen, blue; oxygen, red) while the binding site is yellow. The QPD sequence (Gln-Pro-Asp) is at the upper left of the binding site. The positions of His²⁷⁴, His²⁸⁴, and His²⁸⁶ of the binding site are indicated (see Figure 1). (C) A lysate of human mo-DCs was incubated with (1) mouse anti-human CLEC10A, which was recovered with magnetic beads coated with Protein A; (2) biotinylated sv6D; or (3) biotinylated svL4, which were recovered with magnetic beads coated with streptavidin. Proteins were eluted from the beads and subjected to electrophoresis with a BioAnalyzer (Agilent) in the presence of dithiothreitol. Molecular mass markers are indicated for IgG heavy chain (50 kDa), IgG light chain (25 kDa), and a streptavidin C1 subunit (13.6 kDa). The top band is an instrument marker.

Figure 3

Effect of dose on response in C57BL/6 mice. The peptides were injected subcutaneously and measured endpoints included proliferation of progenitor peritoneal cells in healthy mice (red circles), inhibition of growth of glioma tumors with cells implanted in the brain (blue circles), and survival of mice with implanted ID8 ovarian cancer cells treated with svL4 (green squares) or sv6D (yellow squares).