Binding of Toxoplasma gondii Glycosylphosphatidylinositols to Galectin-3 Is Required for Their Recognition by Macrophages

We showed that the production of tumor necrosis factor (TNF) α by macrophages in response to Toxoplasma gondii glycosylphosphatidylinositols (GPIs) requires the expression of both Toll-like receptors TLR2 and TLR4, but not of their co-receptor CD14. Galectin-3 is a β-galactoside-binding protein with immune-regulatory effects, which associates with TLR2. We demonstrate here by using the surface plasmon resonance method that the GPIs of T. gondii bind to human galectin-3 with strong affinity and in a dose-dependent manner. The use of a synthetic glycan and of the lipid moiety cleaved from the GPIs shows that both parts are involved in the interaction with galectin-3. GPIs of T. gondii also bind to galectin-1 but with a lower affinity and only through the lipid moiety. At the cellular level, the production of TNF-α induced by T. gondii GPIs in macrophages depends on the expression of galectin-3 but not of galectin-1. This study is the first identification of a galectin-3 ligand of T. gondii origin, and galectin-3 might be a co-receptor presenting the GPIs to the TLRs on macrophages.     

The oncofetal Thomsen–Friedenreich carbohydrate antigen in cancer progression

The oncofetal Thomsen–Friedenreich carbohydrate antigen (Galβ1-3GalNAcα1-Ser/Thr TF or T antigen) is a pan-carcinoma antigen highly expressed by about 90% of all human carcinomas. Its broad expression and high specificity in cancer have attracted many investigations into its potential use in cancer diagnosis and immunotherapy. Over the past few years increasing evidence suggests that the increased TF occurrence in cancer cells may be functionally important in cancer progression by allowing increased interaction/communication of the cells with endogenous carbohydrate-binding proteins (lectins), particularly the members of the galactoside-binding galectin family. This review focuses on the recent progress in understanding of the regulation and functional significance of increased TF occurrence in cancer progression and metastasis.     

Phosphorylation of multifunctional galectins by protein kinases CK1, CK2, and PKA

Phosphorylation is known to have a strong impact on protein functions. We analyzed members of the lectin family of multifunctional galectins as targets of the protein kinases CK1, CK2, and PKA. Galectins are potent growth regulators able to bind both glycan and peptide motifs at intra- and extracellular sites. Performing in vitro kinase assays, galectin phosphorylation was detected by phosphoprotein staining and autoradiography. The insertion of phosphoryl groups varied to a large extent depending on the type of kinase applied and the respective galectin substrate. Sites of phosphorylation observed in the recombinant galectins were determined by a strategic combination of phosphopeptide enrichment and nano-ultra-performance liquid chromatography tandem mass spectrometry (nanoUPLC–MS/MS). By in silico modeling, phosphorylation sites were visualized three-dimensionally. Our results reveal galectin-type-specific Ser-/Thr-dependent phosphorylation beyond the known example of galectin-3. These data are the basis for functional studies and also illustrate the analytical sensitivity of the applied methods for further work on human lectins.     

Synthesis and galectin-binding activities of mercaptododecyl glycosides containing a terminal β-galactosyl group

Mercaptododecyl glycosides containing a terminal β-galactosyl group were prepared from d-galactose or from d-lactose via hexa-O-acetyl-lactal (10) as a key intermediate. Interactions of these glycolipids (5 kinds) and galectins (β-galactoside binding lectins, 6 species) were evaluated by surface plasmon resonance (SPR) method. High binding responses were observed for the lactoside, 2-deoxy-lactoside, and lactosaminide with some galectins (Gal-3, -4, -8), whereas the galactoside and 2,3-dideoxy-lactoside showed low binding activities.     

Newcastle disease virus neuraminidase primes neutrophils for stimulation by galectin-3 and formyl-Met-Leu-Phe

Human neutrophils are activated by the β-galactoside-binding lectin galectin-3, provided that the cells are primed by in vivo extravasation or by in vitro preactivation with, for example, LPS. Removal of terminal sialic acid can change neutrophil functionality and responsiveness due to exposure of underlying glycoconjugate receptors or change in surface charge. Here, we investigated whether such alteration of the cell surface carbohydrate composition can alter the responsiveness of the cells to galectin-3. Neutrophils were treated with neuraminidases (NA) of different origins: Clostridium perfringens (CP), Salmonella typhimurium, Vibrio cholerae, and Newcastle disease virus (NDV). In the presence of NDV-NA, but no other NA, the otherwise non-responding neutrophils responded readily to galectin-3 by activation of the NADPH-oxidase. The galectin-3 priming effect was inhibited by the sialidase inhibitor 2,3-dehydro-2-deoxy-N-acetyl-neuraminic acid. Earlier studies have shown that priming of the neutrophil response to galectin-3 with, for example, LPS is paralleled by degranulation of intracellular vesicles and granules and upregulation of potential galectin-3 receptors. Also, NDV-NA (but not CP-NA) treatment induced degranulation, shown as an upregulation of complement receptor 3. Since not only the galectin response but also the response to the chemoattractant fMLF was primed, NDV-NA appears to induce a general priming phenomenon, possibly due to receptor upregulation by degranulation.     

Crystal Structure of Bovine Coronavirus Spike Protein Lectin Domain

The spike protein N-terminal domains (NTDs) of bovine coronavirus (BCoV) and mouse hepatitis coronavirus (MHV) recognize sugar and protein receptors, respectively, despite their significant sequence homology. We recently determined the crystal structure of MHV NTD complexed with its protein receptor murine carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1), which surprisingly revealed a human galectin (galactose-binding lectin) fold in MHV NTD. Here, we have determined at 1.55 Å resolution the crystal structure of BCoV NTD, which also has the human galectin fold. Using mutagenesis, we have located the sugar-binding site in BCoV NTD, which overlaps with the galactose-binding site in human galectins. Using a glycan array screen, we have identified 5-N-acetyl-9-O-acetylneuraminic acid as the preferred sugar substrate for BCoV NTD. Subtle structural differences between BCoV and MHV NTDs, primarily involving different conformations of receptor-binding loops, explain why BCoV NTD does not bind CEACAM1 and why MHV NTD does not bind sugar. These results suggest a successful viral evolution strategy in which coronaviruses stole a galectin from hosts, incorporated it into their spike protein, and evolved it into viral receptor-binding domains with altered sugar specificity in contemporary BCoV or novel protein specificity in contemporary MHV.     

Search for additional influenza virus to cell interactions

Sialyl oligosaccharides have long been considered to be the sole receptors for influenza virus. However, according to [1] some viruses are able to grow in sialic-free MDCK cells. Here we attempted to reveal a possible second, non-sialic receptor, hypothesizing the involvement of additional carbohydrate lectin recognition in influenza virus reception process, first of all in situations when a lectin of the host cell could recognize the viral carbohydrate ligand. We tested the presence of galactose- and sialic acid-binding lectins, as well as mannoside- and sulfo-N-acetyllactosamine-recognizing properties of MDCK and Vero cells using polyacrylamide neoglycoconjugates and antibodies. MDCK cells bind galactoside probes stronger than Vero cells, whereas Vero cells bind preferentially sialoside, mannoside and various sulfo-oligosaccharide probes. The probing of viruses with the neoglycoconjugates revealed specific 6′-HSO ₃ LacNAc (but not other sulfated oligosaccharides) binding property of A and B human strains. Affinity of 6′-HSO ₃ LacNAc probe was comparable with affinity of 6′-SiaLac probe but the binding was not inhibited by the sialooligosaccharide.     

Integration of lectin–glycan recognition systems and immune cell networks in CNS inflammation

Multiple sclerosis (MS) is a progressive degenerative disorder of the central nervous system (CNS), characterized by inflammation, demyelination and axonal loss. While the majority of MS patients experience relapsing-remitting symptoms followed by a secondary progressive phase, about 10–15% patients exhibit a primary progressive disease involving continuous progression from its onset. Here we review the role of lectin–glycan recognition systems, including those concerning siglecs, C-type lectins and galectins in the pathogenesis of MS and experimental autoimmune encephalomyelitis. Particularly, we will focus on the role of galectins in the fate of T cells, dendritic cells and CNS cell populations. Understanding the regulatory circuits governed by lectin–glycan interactions and their association with disease-associated cytokine networks will contribute to develop novel therapeutic strategies in MS.     

COVID-19: disease,or no disease? - that is the question. It’s the dose stupid!

The COVID-19 pandemic has raised many issues not the least of which is the reason for its high variability in consequences to the infected person. In this opinion letter, we advocate that the dose and presentation of the infecting virus is a major factor that affects whether the outcome is subclinical, tissue damaging or even lethal following infection. We briefly describe the known effects of virus dose on the course COVID-19 and discuss practical maneuvers as well as largely untested procedures that can raise the threshold dose needed to break through barriers of resistance.     

Galectin-1 as a fusion partner for the production of soluble and folded human β-1,4-galactosyltransferase-T7 in E. coli

The expression of recombinant proteins in Escherichia coli often leads to inactive aggregated proteins known as the inclusion bodies. To date, the best available tool has been the use of fusion tags, including the carbohydrate-binding protein; e.g., the maltose-binding protein (MBP) that enhances the solubility of recombinant proteins. However, none of these fusion tags work universally with every partner protein. We hypothesized that galectins, which are also carbohydrate-binding proteins, may help as fusion partners in folding the mammalian proteins in E. coli. Here we show for the first time that a small soluble lectin, human galectin-1, one member of a large galectin family, can function as a fusion partner to produce soluble folded recombinant human glycosyltransferase, β-1,4-galactosyltransferase-7 (β4Gal-T7), in E. coli. The enzyme β4Gal-T7 transfers galactose to xylose during the synthesis of the tetrasaccharide linker sequence attached to a Ser residue of proteoglycans. Without a fusion partner, β4Gal-T7 is expressed in E. coli as inclusion bodies. We have designed a new vector construct, pLgals1, from pET-23a that includes the sequence for human galectin-1, followed by the Tev protease cleavage site, a 6× His-coding sequence, and a multi-cloning site where a cloned gene is inserted. After lactose affinity column purification of galectin-1-β4Gal-T7 fusion protein, the unique protease cleavage site allows the protein β4Gal-T7 to be cleaved from galectin-1 that binds and elutes from UDP-agarose column. The eluted protein is enzymatically active, and shows CD spectra comparable to the folded β4Gal-T1. The engineered galectin-1 vector could prove to be a valuable tool for expressing other proteins in E. coli.