Molecular Recognition in C-Type Lectins: The Cases of DC-SIGN, Langerin, MGL, and L-Sectin

Abstract

Carbohydrates play a pivotal role in intercellular communication processes. In particular, glycan antigens are key for sustaining homeostasis, helping leukocytes to distinguish damaged tissues and invading pathogens from healthy tissues. From a structural perspective, this cross-talk is fairly complex, and multiple membrane proteins guide these recognition processes, including lectins and Toll-like receptors. Since the beginning of this century, lectins have become potential targets for therapeutics for controlling and/or avoiding the progression of pathologies derived from an incorrect immune outcome, including infectious processes, cancer, or autoimmune diseases. Therefore, a detailed knowledge of these receptors is mandatory for the development of specific treatments. In this review, we summarize the current knowledge about four key C-type lectins whose importance has been steadily growing in recent years, focusing in particular on how glycan recognition takes place at the molecular level, but also looking at recent progresses in the quest for therapeutics.

Keywords: drug discovery; glycans; lectins; molecular recognition.

Figure 1

General classification of animal lectins.[ ^2b , ^2c ] The 16 groups of C‐type lectins are on the right.[ ⁷ , ¹⁰ ]

Figure 2

Characteristic structural features of C‐type lectins. A) Cartoon representation of five representative CLRs belonging to five different C‐type lectin groups (II, III, IV, V and VI).[ ⁶ , ¹⁰ ] B) Structural comparison between the CRDs of the same five lectins: human langerin (PDB ID: 5G6U), human surfactant protein D (PDB ID: 4E52), human L‐selectin (PDB ID: 3CFW), murine dectin‐1 (PDB ID: 2CL8), and human macrophage mannose receptor 1 (CRD2, PDB ID: 5XTS). Calcium ions are depicted in each case. C) Common structural motifs present in the CTLD fold (model: DC‐SIGN CRD, PDB ID: 1SL5). Right: the main secondary structure elements; left: typical conserved residues among different CTLDs and species. Calcium ions are shown as black spheres.[ ^7a , ¹¹ ]

Figure 3

Binding poses experimentally described for the DC‐SIGN CRD and four typical mannosylated and fucosylated oligosaccharides. Structures A, B and D have been solved by X‐ray crystallography. Model C is a representative structure obtained by MD and supported by NMR data. Sugars are colored as follows: Man: green, Fuc: magenta, Gal: yellow, Glc: blue. Residues F313, V351 and K368 are shown as sticks in all cases. A) Man₄ (PDB ID: 1SL4). B) GlcNAc₂Man₃ (PDB ID: 1K9I). C) Blood group A type VI. D) Lacto‐N‐fucopentaose III (PDB ID: 1SL5).

Figure 4

Representative Man‐ and Fuc‐containing epitopes and their recognition by DC‐SIGN according to published array data (refs. [85, 89–91, 93, 98]).

Figure 5

Synthetic modifications progressively introduced in the Manα1‐2Man scaffold in order to increase receptor affinity and DC‐SIGN selectivity over langerin. Also, two examples of low millimolar multivalent structures bearing these mimetics are shown.

Figure 6

Some non‐sugar inhibitors described for DC‐SIGN and their affinities. A) and B) Active compounds found by fluorescent assays. C) Active fragment detected and validated by NMR. Note, it still binds to DC‐SIGN in the absence of Ca²⁺, thus suggesting that the interaction is not established at the primary lectin site.

Figure 7

Qualitative comparison between the sugar preferences of DC‐SIGN and langerin. The central area displays those epitopes similarly recognized by both receptors. To clarify, Gal has not been included as free sugar, as the dissociation constants are rather high in both cases. Only langerin recognizes sulfated moieties, including Gal and Glc, although it preferentially targets outer Man residues in complex glycans. Also, DC‐SIGN interacts with a wider range of fucosylated structures and more easily accommodates Man residues from highly branched scaffolds.

Figure 8

Crystallographic models obtained for langerin interacting with different mono‐ and oligosaccharides. Sugars are colored as follows: Man: green, Fuc: magenta, Gal: yellow, Glc: blue. Residues A289, K299, K313, F315, and V351 are shown as sticks in all structures, other relevant amino acids are labeled in particular cases. A) Man₂ (PDB ID: 3P5F). B) Blood group B trisaccharide (PDB ID: 3P5G). C) 6S‐LacNAc (PDB ID: 3P5I). D) GlcNS6S (PDB ID: 5G6U). E) α‐OMe‐GlcNAc (PDB ID: 4N32); on the right is a mimetic scaffold based on the binding pose of GlcNAc, bearing an aromatic moiety to establish aliphatic contacts with nearby side chains (F315).

Figure 9

Schematic structure of the most relevant glycosaminoglycans (GAGs). All GAGs consist of a repeated disaccharide unit constituted by an acetylated sugar and an uronic acid. Chondroitin sulfate (CS) and dermatan sulfate (DS) share a GalNAc unit that can be sulfated at O4 and/or O6. Keratan sulfate (KS) displays a sulfated Gal instead of an uronic unit. Heparins and heparan sulfate (HS) are fairly heterogeneous, they can contain variable amounts of IdoA and GlcA. Often, heparins are highly sulfated (up to three sulfate groups per disaccharide) and preferentially contain IdoA.

Figure 10

A) Summary of the cell‐based fragment screening assay (Cell‐Fy) developed to directly screen compounds against lectin‐expressing cells. As depicted, the detected hits are further validated by NMR techniques. B) Langerin binding to Le^B and Le^Y using different ligand formulations. Langerin ECD‐Fc can recognize both antigens coated on a plate and linked to the tumor‐associated peptide MART‐1. Moreover, the latter are successfully internalized by LCs. In contrast, only Le^Y‐coated glycoliposomes are targeted by langerin but no internalization by LCs is observed.

Figure 11

Schematic overview of the MUC‐1 structure.^[243] On the top, the 20‐amino acid sequence that constitute one tandem repeat. On the bottom, the most common tumor‐associated antigens as compared to normal glycosylation. Copyright: 2014, Elsevier Ltd.

Figure 12

Schematic representation of the STD profiles described for different MGL ligands.^{[217,227,232]} To clarify, for each antigen only those STDs above 50 % in relative scale are depicted as red circles. In all cases, recombinant soluble MGL‐ECD has been used for data recording (in B, it was additionally tagged with myc and associated to anti‐myc AB and streptavidin). A) α‐OMe‐GalNAc. B) sialyl‐Tn antigen. C) Blood group A trisaccharide. D) Forsmann antigen. E) asialo‐GM2. F) GM2. G) GalNAc linked to a MUC1 repeat. In this case, the peptide residues displaying weaker STD effects are also highlighted in orange.

Figure 13

Schematic cartoon of the Gram‐positive bacterial cell wall displaying WTA and LTA chains. The GalNAc‐containing WTA structure is depicted at the top.

Figure 14

Structure of the LOS decasaccharide of E. coli R1.