Medawar’s PostEra: Galectins Emerged as Key Players During Fetal-Maternal Glycoimmune Adaptation

Lectin-glycan interactions, in particular those mediated by the galectin family, regulate many processes required for a successful pregnancy. Over the past decades, increasing evidence gathered from in vitro and in vivo experiments indicate that members of the galectin family specifically bind to both intracellular and membrane bound carbohydrate ligands regulating angiogenesis, immune-cell adaptations required to tolerate the fetal semi-allograft and mammalian embryogenesis. Therefore, galectins play important roles in fetal development and placentation contributing to maternal and fetal health. This review discusses the expression and role of galectins during the course of pregnancy, with an emphasis on maternal immune adaptions and galectin-glycan interactions uncovered in the recent years. In addition, we summarize the galectin fingerprints associated with pathological gestation with particular focus on preeclampsia.

GAG Multivalent Systems to interact with Langerin

: Langerin is a C-type Lectin expressed at the surface of Langerhans cells, which play a pivotal role in protecting organisms against pathogen infections. To address this aim, Langerin presents at least two recognition sites, one Ca2+-dependent and another one independent, capable of recognizing a variety of carbohydrate ligands. In contrast to other lectins, Langerin recognizes sulfated glycosaminoglycans (GAGs), a family of complex and heterogeneous polysaccharides present in the cell membrane and the extracellular matrix at the interphase generated in the trimeric form of Langerin but absent in the monomeric form. The complexity of these oligosaccharides has impeded the development of well-defined monodisperse structures to study these interaction processes. However, in the last few decades, an improvement of synthetic developments to achieve the preparation of carbohydrate multivalent systems mimicking the GAGs has been described. Despite all these contributions, very few examples are reported where the GAG multivalent structures are used to evaluate the interaction with Langerin. These molecules should pave the way to explore these GAG-Langerin interactions.

Reversible Lectin Binding to Glycan-Functionalized Graphene

The monolayer character of two-dimensional materials predestines them for application as active layers of sensors. However, their inherent high sensitivity is always accompanied by a low selectivity. Chemical functionalization of two-dimensional materials has emerged as a promising way to overcome the selectivity issues. Here, we demonstrate efficient graphene functionalization with carbohydrate ligands—chitooligomers, which bind proteins of the lectin family with high selectivity. Successful grafting of a chitooligomer library was thoroughly characterized, and glycan binding to wheat germ agglutinin was studied by a series of methods. The results demonstrate that the protein quaternary structure remains intact after binding to the functionalized graphene, and that the lectin can be liberated from the surface by the addition of a binding competitor. The chemoenzymatic assay with a horseradish peroxidase conjugate also confirmed the intact catalytic properties of the enzyme. The present approach thus paves the way towards graphene-based sensors for carbohydrate–lectin binding.

A Comprehensive Phylogenetic and Bioinformatics Survey of Lectins in the Fungal kingdom

Fungal lectins are a large family of glycan-binding proteins, with no enzymatic activity. They play fundamental biological roles in the interactions of fungi with their environment and are found in many different species throughout the fungal kingdom. In particular, their contribution to defence against feeders has been emphasized and extracellular lectins may be involved in the recognition of bacteria, fungal competitors and specific host plants. Their carbohydrate specificities and quaternary structures vary widely, but evidence for an evolutionary relationship within the different classes of lectins is provided by the high degree of amino acid sequence identity shared by the different fungal lectins. The UniLectin3D database contains 194 3D structures of fungal lectins, of which 129 are characterized with their carbohydrate ligand. UniLectin3D lectin classes from all origins were used to construct 107 lectin motifs in 26 folding configurations and to screen 1,223 species deposited in the genomic portal MycoCosm of the Joint Genome Institute. The resulting 33 485 protein sequences of putative lectins are organized in MycoLec, a publicly available and searchable database. The charac-terization of the lectin candidates in fungal genomes is based on systematic statistics regarding po-tential carbohydrate ligands, protein lengths, signal peptides, relative motif positions and amino acid compositions of fungal lectins. These results shed light on the evolution of the lectin gene families.

Corynebacterium matruchotii Demography and Adhesion Determinants in the Oral Cavity of Healthy Individuals

Corynebacterium matruchotii may be key in tooth biofilm formation, but information about demographics, bacterial partners, and binding ligands is limited. The aims of this study were to explore C. matruchotii’s demography by age and colonization site (plaque and saliva), in vitro bacterial–bacterial interactions in coaggregation and coadhesion assays, and glycolipids as potential binding ligands in thin-layer chromatogram binding assays. C. matruchotii prevalence increased from 3 months to 18 years old, with 90% and 100% prevalence in saliva and tooth biofilm, respectively. C. matruchotii aggregated in saliva in a dose-dependent manner but lacked the ability to bind to saliva-coated hydroxyapatite. In vivo, C. matruchotii abundance paralleled that of Actinomyces naeslundii, Capnocytophaga sp. HMT 326, Fusobacterium nucleatum subsp. polymorphum, and Tannerella sp. HMT 286. In vitro, C. matruchotii bound both planktonic and surface-bound A. naeslundii, Actinomyces odontolyticus, and F. nucleatum. In addition, C. matruchotii exhibited the ability to bind glycolipids isolated from human erythrocytes (blood group O), human granulocytes, rabbit intestine, human meconium, and rat intestine. Binding assays identified candidate carbohydrate ligands as isoglobotriaosylceramide, Galα3-isoglobotriaosylceramide, lactotriaosylceramide, lactotetraosylceramide, neolactotetraosylceramide, and neolactohexaosylceramide. Thus, C. matruchotii likely uses specific plaque bacteria to adhere to the biofilm and may interact with human tissues through carbohydrate interactions.

L1CAM as an E-selectin Ligand in Colon Cancer

Metastasis is the main cause of death among colorectal cancer (CRC) patients. E-selectin and its carbohydrate ligands, including sialyl Lewis X (sLeX) antigen, are key players in the binding of circulating tumor cells to the endothelium, which is one of the major events leading to organ invasion. Nevertheless, the identity of the glycoprotein scaffolds presenting these glycans in CRC remains unclear. In this study, we firstly have characterized the glycoengineered cell line SW620 transfected with the fucosyltransferase 6 (FUT6) coding for the α1,3-fucosyltransferase 6 (FUT6), which is the main enzyme responsible for the synthesis of sLeX in CRC. The SW620FUT6 cell line expressed high levels of sLeX antigen and E-selectin ligands. Moreover, it displayed increased migration ability. E-selectin ligand glycoproteins were isolated from the SW620FUT6 cell line, identified by mass spectrometry, and validated by flow cytometry and Western blot (WB). The most prominent E-selectin ligand we identified was the neural cell adhesion molecule L1 (L1CAM). Previous studies have shown association of L1CAM with metastasis in cancer, thus the novel role as E-selectin counter-receptor contributes to understand the molecular mechanism involving L1CAM in metastasis formation.

Carbohydrate Immune Adjuvants in Subunit Vaccines

Modern subunit vaccines are composed of antigens and a delivery system and/or adjuvant (immune stimulator) that triggers the desired immune responses. Adjuvants mimic pathogen-associated molecular patterns (PAMPs) that are typically associated with infections. Carbohydrates displayed on the surface of pathogens are often recognized as PAMPs by receptors on antigen-presenting cells (APCs). Consequently, carbohydrates and their analogues have been used as adjuvants and delivery systems to promote antigen transport to APCs. Carbohydrates are biocompatible, usually nontoxic, biodegradable, and some are mucoadhesive. As such, carbohydrates and their derivatives have been intensively explored for the development of new adjuvants. This review assesses the immunological functions of carbohydrate ligands and their ability to enhance systemic and mucosal immune responses against co-administered antigens. The role of carbohydrate-based adjuvants/delivery systems in the development of subunit vaccines is discussed in detail.

A detailed picture of a protein–carbohydrate hydrogen-bonding network revealed by NMR and MD simulations

Cyanovirin-N (CV-N) is a cyanobacterial lectin with antiviral activity towards HIV and several other viruses. Here, we identify mannoside hydroxyl protons that are hydrogen bonded to the protein backbone of the CV-N domain B binding site, using NMR spectroscopy. For the two carbohydrate ligands Manα(1→2)ManαOMe and Manα(1→2) Manα(1→6)ManαOMe five hydroxyl protons are involved in hydrogen-bonding networks. Comparison with previous crystallographic results revealed that four of these hydroxyl protons donate hydrogen bonds to protein backbone carbonyl oxygens in solution and in the crystal. Hydrogen bonds were not detected between the side chains of Glu41 and Arg76 with sugar hydroxyls, as previously proposed for CV-N binding of mannosides. Molecular dynamics simulations of the CV-N/Manα(1→2)Manα(1→6)ManαOMe complex confirmed the NMR-determined hydrogen-bonding network. Detailed characterization of CV-N/mannoside complexes provides a better understanding of lectin-carbohydrate interactions and opens up to the use of CV-N and similar lectins as antiviral agents.

Molecular basis for recognition of Listeria cell wall teichoic acid by the pseudo-symmetric SH3b-like repeats of a bacteriophage endolysin

Endolysins are bacteriophage-encoded peptidoglycan hydrolases targeting the cell wall of host bacteria via their cell wall-binding domains (CBDs). The molecular basis for selective recognition of surface carbohydrate ligands by CBDs remains elusive. Here, we describe, in atomic detail, the interaction between the Listeria phage endolysin domain CBD500 and its cell wall teichoic acid (WTA) ligands. We show that 3’ O-acetylated GlcNAc residues integrated into the WTA polymer chain are the key epitope recognized by a CBD binding cavity located at the interface of tandem copies of beta-barrel, pseudo-symmetric SH3b-like repeats. This cavity consists of multiple aromatic residues making extensive interactions with two GlcNAc acetyl groups via hydrogen bonds and van der Waals contacts, while permitting the docking of the diastereomorphic ligands. The multidisciplinary approach described here delineates a previously unknown recognition mechanism by which a phage endolysin specifically recognizes and targets WTA, suggesting an adaptable model for regulation of endolysin specificity.