Different roles of the heterodimer architecture of galectin-4 in selective recognition of oligosaccharides and lipopolysaccharides having ABH antigens

The dimeric architecture of tandem-repeat type galectins, such as galectin-4 (Gal-4), modulates their biological activities, although the underlying molecular mechanisms have remained elusive. Emerging evidence show that tandem-repeat galectins play an important role in innate immunity by recognizing carbohydrate antigens present on the surface of certain pathogens, which very often mimic the structures of the human self-glycan antigens. Herein, we have analyzed the binding preferences of the C-domain of Gal-4 (Gal-4C) toward the ABH-carbohydrate histo-blood antigens with different core presentations and their recognition features have been rationalized by using a combined experimental approach including NMR, solid-phase and hemagglutination assays, and molecular modeling. The data show that Gal-4C prefers A over B antigens (two-fold in affinity), contrary to the N-domain (Gal-4N), although both domains share the same preference for the type-6 presentations. The behavior of the full-length Gal-4 (Gal-4FL) tandem-repeat form has been additionally scrutinized. Isothermal titration calorimetry and NMR data demonstrate that both domains within full-length Gal-4 bind to the histo-blood antigens independently of each other, with no communication between them. In this context, the heterodimeric architecture does not play any major role, apart from the complementary A and B antigen binding preferences. However, upon binding to a bacterial lipopolysaccharide containing a multivalent version of an H-antigen mimetic as O-antigen, the significance of the galectin architecture was revealed. Indeed, our data point to the linker peptide domain and the F-face of the C-domain as key elements that provide Gal-4 with the ability to cross-link multivalent ligands, beyond the glycan binding capacity of the dimer.

Structural insights in galectin-1-glycan recognition: Relevance of the glycosidic linkage and the N-acetylation pattern of sugar moieties

Galectins, soluble lectins widely expressed intra- and extracellularly in different cell types, play major roles in deciphering the cellular glycocode. Galectin-1 (Gal-1), a prototype member of this family, presents a carbohydrate recognition domain (CRD) with specific affinity for β-galactosides such as N-acetyllactosamine (β-d-Galp-(1 → 4)-d-GlcpNAc), and mediate numerous physiological and pathological processes. In this work, Gal-1 binding affinity for β-(1 → 6) galactosides, including β-d-Galp-(1 → 6)-β-d-GlcpNAc-(1 → 4)-d-GlcpNAc was evaluated, and their performance was compared to that of β-(1 → 4) and β-(1 → 3) galactosides. To this end, the trisaccharide β-d-Galp-(1 → 6)-β-d-GlcpNAc-(1 → 4)-d-GlcpNAc was enzymatically synthesized, purified and structurally characterized. To evaluate the affinity of Gal-1 for the galactosides, competitive solid phase assays (SPA) and isothermal titration calorimetry (ITC) studies were carried out. The experimental dissociation constants and binding energies obtained were compared to those calculated by molecular docking. These analyses evidenced the critical role of the glycosidic linkage between the terminal galactopyranoside residue and the adjacent monosaccharide, as galactosides bearing β-(1 → 6) glycosidic linkages showed dissociation constants six- and seven-fold higher than those involving β-(1 → 4) and β-(1 → 3) linkages, respectively. Moreover, docking experiments revealed the presence of hydrogen bond interactions between the N-acetyl group of the glucosaminopyranose moiety of the evaluated galactosides and specific amino acid residues of Gal-1, relevant for galectin-glycan affinity. Noticeably, the binding free energies (ΔG_bind^calc) derived from the molecular docking were in good agreement with experimental values determined by ITC measurements (ΔG_bind^exp), evidencing a good correlation between theoretical and experimental approaches, which validates the in silico simulations and constitutes an important tool for the rational design of future optimized ligands.

Recombinant SARS-CoV-2 S Protein Binds to Glycans of the Lactosamine Family in vitro

Many viruses, beside binding to their main cell target, interact with other molecules that promote virus adhesion to the cell; often, these additional targets are glycans. The main receptor for SARS-CoV-2 is a peptide motif in the ACE2 protein. We studied interaction of the recombinant SARS-CoV-2 spike (S) protein with an array of glycoconjugates, including various sialylated, sulfated, and other glycans, and found that the S protein binds some (but not all) glycans of the lactosamine family. We suggest that parallel influenza infection will promote SARS-CoV-2 adhesion to the respiratory epithelial cells due to the unmasking of lactosamine chains by the influenza virus neuraminidase.

An adipose tissue galectin controls endothelial cell function via preferential recognition of 3-fucosylated glycans

Upon overnutrition, adipocytes activate a homeostatic program to adjust anabolic pressure. An inflammatory response enables adipose tissue (AT) expansion with concomitant enlargement of its capillary network, and reduces energy storage by increasing insulin resistance. Galectin-12 (Gal-12), an endogenous lectin preferentially expressed in AT, plays a key role in adipocyte differentiation, lipolysis, and glucose homeostasis. Here, we reveal biochemical and biophysical determinants of Gal-12 structure, including its preferential recognition of 3-fucosylated structures, a unique feature among members of the galectin family. Furthermore, we identify a previously unanticipated role for this lectin in the regulation of angiogenesis within AT. Gal-12 showed preferential localization within the inner side of lipid droplets, and its expression was upregulated under hypoxic conditions. Through glycosylation-dependent binding to endothelial cells, Gal-12 promoted in vitro angiogenesis. Moreover, analysis of in vivo AT vasculature showed reduced vascular networks in Gal-12-deficient (Lgals12^-/-) compared to wild-type mice, supporting a role for this lectin in AT angiogenesis. In conclusion, this study unveils biochemical, topological, and functional features of a hypoxia-regulated galectin in AT, which modulates endothelial cell function through recognition of 3-fucosylated glycans. Thus, glycosylation-dependent programs may control AT homeostasis by modulating endothelial cell biology with critical implications in metabolic disorders and inflammation.

Specificity of human galectins on cell surfaces

Galectins are β-galactoside-binding proteins sharing homology in amino acid sequence of their carbohydrate-recognition domain. Their carbohydrate specificity outside cells has been studied previously. The main conclusion of these studies was that several levels of glycan ligand recognition exist for galectins: (i) disaccharide Galβ1-4GlcNAc (LN, N-acetyllactosamine) binds stronger than β-galactopyranose; (ii) substitution at O-2 and O-3 of galactose residue as well as core fragments ("right" from GlcNAc) provides significant increase in affinity; (iii) similarly glycosylated proteins can differ significantly in affinity to galectins. Information about the natural cellular receptors of galectins is limited. Until recently, it was impossible to study specificity of cell-bound galectins. A model based on controlled incorporation of a single protein into glycocalyx of cells and subsequent interaction of loaded cells with synthetic glycoprobes measured by flow cytometry made this possible recently. In this review, data about glycan specificity of proto-, chimera-, and tandem-repeat type galectins on the cell surface are systematized, and comparative analysis of the results with data on specificity of galectins in artificial systems was performed. The following conclusions from these studies were made: (i) cellular galectins have practically no ability to bind disaccharide LNn, but display affinity to 3'-substituted oligolactosamines and oligomers LNn; (ii) tandem-repeat type galectins recognize another disaccharide, namely Galβ1-3GlcNAc (Le(c)); (iii) on the cell surface, tandem-repeat type galectins conserve the ability to display high affinity to blood group antigens of ABH system; (iv) in general, when galectins are immersed into glycocalyx, they are more selective regarding glycan interactions. Thus, we conclude that competitive interaction of galectins with cell microenvironment (endogenous cell glycans) is the main factor providing selectivity of galectins in vivo.

Lactose-functionalized dendrimers arbitrate the interaction of galectin-3/MUC1 mediated cancer cellular aggregation

By using lactose-functionalized poly(amidoamine) dendrimers as a tunable multivalent platform, we studied cancer cell aggregation in three different cell lines (A549, DU-145, and HT-1080) with galectin-3. We found that small lactose-functionalized G(2)-dendrimer 1 inhibited galectin-3-induced aggregation of the cancer cells. In contrast, dendrimer 4 (a larger, generation 6 dendrimer with 100 carbohydrate end groups) caused cancer cells to aggregate through a galectin-3 pathway. This study indicates that inhibition of cellular aggregation occurred because 1 provided competitive binding sites for galectin-3 (compared to its putative cancer cell ligand, TF-antigen on MUC1). Dendrimer 4, in contrast, provided an excess of ligands for galectin-3 binding; this caused crosslinking and aggregation of cells to be increased.