Galectins as self/non-self recognition receptors in innate and adaptive immunity: an unresolved paradox

Abstract

Galectins are characterized by their binding affinity for β-galactosides, a unique binding site sequence motif, and wide taxonomic distribution and structural conservation in vertebrates, invertebrates, protista, and fungi. Since their initial description, galectins were considered to bind endogenous ("self") glycans and mediate developmental processes and cancer. In the past few years, however, numerous studies have described the diverse effects of galectins on cells involved in both innate and adaptive immune responses, and the mechanistic aspects of their regulatory roles in immune homeostasis. More recently, however, evidence has accumulated to suggest that galectins also bind exogenous ("non-self") glycans on the surface of potentially pathogenic microbes, parasites, and fungi, suggesting that galectins can function as pattern recognition receptors (PRRs) in innate immunity. Thus, a perplexing paradox arises by the fact that galectins also recognize lactosamine-containing glycans on the host cell surface during developmental processes and regulation of immune responses. According to the currently accepted model for non-self recognition, PRRs recognize pathogens via highly conserved microbial surface molecules of wide distribution such as LPS or peptidoglycan (pathogen-associated molecular patterns; PAMPs), which are absent in the host. Hence, this would not apply to galectins, which apparently bind similar self/non-self molecular patterns on host and microbial cells. This paradox underscores first, an oversimplification in the use of the PRR/PAMP terminology. Second, and most importantly, it reveals significant gaps in our knowledge about the diversity of the host galectin repertoire, and the subcellular targeting, localization, and secretion. Furthermore, our knowledge about the structural and biophysical aspects of their interactions with the host and microbial carbohydrate moieties is fragmentary, and warrants further investigation.

Keywords: C-type lectin; galectin; glycan ligands; microbial recognition.

FIGURE 1

Recognition and effector activities of the mannose-binding lectin (MBL). (A) A schematic representation of CTLD organization; (B) Crystallographic model of CTLD; (C) Ca²⁺-dependent carbohydrate binding of CTLD. The CRD recognizes equatorial hydroxyls on C3 and C4 of non-reducing terminal mannose with participation of the Ca²⁺ atom. (D) The MBL trimer binds to ligands that are displayed about 45 Å apart on the microbial surface, and via association with the MASP) may activate the complement cascade, leading to opsonization or lysis of the microbe.

FIGURE 2

Galectin recognition. (A) Schematic representation of galectin domain organization; (B) Schematic illustration of cis-interactions of proto, chimera, and tandem repeat galectins with host cell surface glycans. (C) Schematic illustration of trans-interactions of proto, chimera, and tandem repeat galectins with host cell surface and microbial glycans. Proto- and trandem repeat-type galectins can cross-link host and microbial glycans. Chimera galectins (galectin-3) can recognize microbial glycans but it is not clear that they can cross-link them to the host cell surface.

FIGURE 3

Gene organization and phylogeny of galectins. (A) Genomic structures of galectin-1 or galectin-1 like protein from human, murine, chicken, and zebrafish (Danio rerio). Coding sequences (exons) are represented by boxes, with their sizes noted above each box. Intron sizes are shown below. (B) Phylogenetic analysis of invertebrate galectins modified based on Tasumi and Vasta (2007). The unrooted tree constructed by the N-J distance method is shown.

FIGURE 4

Structures of bovine galectin-1 and Bufo arenarum galectin-1 like protein. (A) The ribbon diagram shows the overlap of the toad (B. arenarum) galectin-1 like protein (blue, PDB 1GAN) and bovine (Bos taurus) galectin-1 (yellow, PDB 1slt) in complex with LacNAc (stick representation). (B) Carbohydrate-binding sites of B. arenarum galectin-1 like protein (blue) and bovine galectin-1 (yellow). The interactions of amino acid residues with LacNAc are shown for the bovine galectin-1. The OH at C4’ of Gal (in LacNAc) makes hydrogen bond interactions with the highly conserved residues His44, Asn46, and Arg48. The OH at C6’ makes similar interactions with the Asn61 and Glu71. Trp68 participates in a stacking interaction with the Gal ring carbons and restricts orientation of the OH at C4’ to the axial form. In GlcNAc moiety of the LacNAc, the hydrogen bond interactions are involved with the protein through the C3-OH with Agr48, Glu71, and Arg73. Additional interactions are involved via a water molecule that bridges the nitrogen of the NAc group with His52, Asp54, and Arg73. (C) Drgal1-L2 (green) was modeled at the SWISS-MODEL Protein Modeling Server (http://swissmodel.expasy.org) based on the bovine galectin-1 structure (yellow, PDB 1slt). All nine residues that form the carbohydrate-binding cassette in mammalian galectin-1 are present in the putative binding site of Drgal1-L2. All side chains of these residues were within 0.5 Å of the equivalent side chains of the bovine galectin-1. (D) C. elegans 16-kDa galectin (Lec-6; shown in green) was modeled at the SWISS-MODEL Protein Modeling Server (http://swissmodel.expasy.org) based on the bovine galectin-1 structure (shown in yellow, PDB 1slt). The model reveals that a shorter loop (indicated by arrow) between strands 4 and 5 is responsible for its unique binding profile. (E) Alignment of bovine galectin-1, Ce16 (C. elegans 16 kDa galectin), and CRD1 to -4 of CvGal. (F) Homology modeling of CvGal CRDs. Bovine galectin-1, CRD-1, -2, -3, and -4 are shown in white, blue, yellow, red, and green, respectively. Numbering of amino acid residues is based on bovine galectin-1.

FIGURE 5

Expression, secretion, and functional diversification of galectins: (1) Galectin transcripts are translated in the cytoplasm, and the proteins can be translocated into the nucleus (2) where they can associate with ribonucleoproteins. Via unconventional mechanism(s), galectins can be secreted to the extracellular space (3) where they can function as pattern recognition receptors for microbial glycans (4), bind to the host cell surface glycans (5), and cross-link them with ECM glycans (6) thereby, for example, promoting cell migration. Galectins can also cross-link cell surface glycans and induce clustering of microdomains and lattice formation at the cell surface (7) that can trigger signaling cascades, or cross-link neighboring cells (8) and promote cell–cell interactions/adhesion.