AB-type lectin (toxin/agglutinin) from mistletoe: differences in affinity of the two galactoside-binding Trp/Tyr-sites and regulation of their functionality by monomer/dimer equilibrium

Development of a carbohydrate-binding protein prediction algorithm using structural features of stacking aromatic rings

Carbohydrate-protein interactions play fundamental roles in numerous aspects of biological activities, and the search for new carbohydrate (CHO)-binding proteins (CBPs) has long been a research focus. In this study, through the analysis of CBP structures, we identified significant enrichment of aromatic residues in CHO-binding regions. We further summarized the structural features of these aromatic rings within the CHO-stacking region, namely "exposing" and "proximity" features, and developed a screening algorithm that can identify CHO-stacking Trp (tryptophan) residues based on these two features. Our Trp screening algorithm can achieve high accuracy in both CBP (specificity score 0.93) and CBS (Carbohydrate binding site, precision score 0.77) prediction using experimentally determined protein structures. We also applied our screening algorithm on AlphaGO pan-species predicted models and observed significant enrichment of carbohydrate-related functions in predicted CBP candidates across different species. Moreover, through carbohydrate arrays, we experimentally verified the CHO-binding ability of four candidate proteins, which further confirms the robustness of the algorithm. This study provides another perspective on proteome-wide CBP and CBS prediction. Our results not only help to reveal the structural mechanism of CHO-binding, but also provide a pan-species CBP dataset for future CHO-protein interaction exploration.

Isolation, purification, and characterization of lectins from medicinal plant Combretum glutinosum seeds endowed with analgesic and antiulcer properties

In the pursuit of safer and more effective treatments, there is a growing interest in plant-derived compounds, particularly lectins, because of their diverse pharmacological properties. This study focused on isolating, purifying, and characterizing lectin from Combretum glutinosum seeds (CGSLs) to assess its potential as an analgesic and antiulcer agent. CGSL extraction involved defatting and buffer extraction, followed by purification using ammonium sulfate fractionation and fetuin-agarose affinity column chromatography. The isolectins (iso-CGSLs), each consisting of 60 kDa and 57 kDa heterodimeric subunits, displayed glycoprotein properties with a 40 % neutral sugar content. They exhibited peak activity at 55 °C and remained stable for up to the fifth day at room temperature. The activity exhibited a pH dependence, peaking between 7.5 and 10.5, and all seemingly operated independently of metal ions. CGSL, at optimal doses ranging from 6 to 12 mg/kg, had significant analgesic effects on acetic acid-induced writhing and hot plate tests in mice. Evaluation using 0.7 % acetic acid resulted in notable pain reduction across all doses (P < 0.05). The analgesic effect of lectin was partially reversed by naloxone (a morphine antagonist), indicating partial involvement of the opioid receptor system. Furthermore, CGSL exhibited antiulcer effects in ethanol-induced gastric ulcer models in rats, highlighting its therapeutic potential as a natural alternative for analgesic and antiulcer treatments.

Specificity of viscumin revised. As probed with a printed glycan array

Viscumin, a lectin used in anti-cancer therapy, was originally considered as βGal recognizing protein; later, an ability to bind 6'-sialyl N-acetyllactosamine (6'SLN) terminated gangliosides was found. Here we probed viscumin with a printed glycan array (PGA) containing a large number of mammalian sulfated glycans, and found a strong binding to glycans with 6-O-SuGal moiety as lactose, N-acetyllactosamine (LN), di-N-acetyllactosamine (LacdiNAc), and even 6-O-SuGalNAcα (but not SiaTn). Also, the ability to bind some of αGal terminated glycans, including Galα1-3Galβ1-4GlcNAc, was observed. Unexpectedly, only weak interaction was detected with parent neutral β-galactosides including LN-LN-LN and branched (LN)₂LN oligolactosamines; in the light of these data, one should not confidently classify viscumin as a β-galactoside-binding lectin. Carrying out PGA in the presence of neutral or sulfated/sialylated glycan, together with sequential elution from lactose-sepharose and consideration of the protein structure, lead to the conclusion that two glycan-binding sites of viscumin have different specificities, one of which prefers charged sulfated and sialylated moieties.

Articulatin B chain induced dendritic cells maturation and driven type I T helper cells and cytotoxic T cells activation

AIMS

Articulatin (AT), purified from the Chinese mistletoe (Viscum articulatum), belongs to the family of type II ribosome-inactivating proteins (RIPs) that contain two subunits, the A and B chains. The B chain of AT is believed to function by means of interacting with the galactose moiety of glycoproteins or glycolipids on the cell membrane and is internalized into cells through endocytosis. In the study, we aim to investigate the immunomodulatory effects of recombinant articulatin B chain (rATB) on mouse bone marrow-derived dendritic cells (BM-DCs).

MAIN METHODS

Detection of surface markers expression on BM-DCs by flow cytometry. Analysis of RNA and protein expression by RNAseq and Western blotting assays. Assessment of the adaptive immune responses using an in vivo mouse model.

KEY FINDING

Our study presents novel results showing the activation of mouse BM-DCs by rATB, which leads to the induction of CD80, CD86, and MHC II expression as well as primed type I CD4+ T cell differentiation and CD8+ T cell activation. RNAseq and Western blotting assays revealed rATB-induced BM-DC activation to be dependent on the MAPK and NF-κB signaling pathways. In a mouse model, rATB was observed to have adjuvant effects that induced an antigen-specific Th1 immune response.

SIGNIFICANCE

Based on in vitro and in vivo assays, this study shows rATB acting as a potential adjuvant that induces BM-DC activation and antigen-specific Th1 related immune response. rATB might have potential applicability in the development of vaccines against pathogens and tumors.

Lectinology 4.0: Altering modular (ga)lectin display for functional analysis and biomedical applications

BACKGROUND

Recognition of glycans by lectins is emerging as (patho)physiologically broadly used mode of cellular information transfer. Whereas the direct ligand-receptor contact is often already thoroughly characterized, the functional relevance of aspects of architecture such as modular design and valence of lectins is less well defined.

SCOPE OF REVIEW

Following an introduction to modular lectin design, three levels of methodology are then reviewed that delineate lectin structure-activity relationships beyond glycan binding, with emphasis on domain shuffling.

MAJOR CONCLUSIONS

Engineering of variants by modular transplantation facilitates versatile Nature-inspired design switches and access to new combinations with translational potential, as exemplified for human adhesion/growth-regulatory galectins.

GENERAL SIGNIFICANCE

To gain an understanding of the functional significance of natural variations in quaternary structure and modular design within a protein family is a current challenge. Strategic application of methods of the described phases is a means to respond to this challenge.

Differential recognition of Haemophilus influenzae whole bacterial cells and isolated lipooligosaccharides by galactose-specific lectins

Bacterial surfaces are decorated with carbohydrate structures that may serve as ligands for host receptors. Based on their ability to recognize specific sugar epitopes, plant lectins are extensively used for bacteria typing. We previously observed that the galactose-specific agglutinins from Ricinus communis (RCA) and Viscum album (VAA) exhibited differential binding to nontypeable Haemophilus influenzae (NTHi) clinical isolates, their binding being distinctly affected by truncation of the lipooligosaccharide (LOS). Here, we examined their binding to the structurally similar LOS molecules isolated from strains NTHi375 and RdKW20, using microarray binding assays, saturation transfer difference NMR, and molecular dynamics simulations. RCA bound the LOS_RdKW20 glycoform displaying terminal Galβ(1,4)Glcβ, whereas VAA recognized the Galα(1,4)Galβ(1,4)Glcβ epitope in LOS_NTHi375 but not in LOS_RdKW20, unveiling a different presentation. Binding assays to whole bacterial cells were consistent with LOS_NTHi375 serving as ligand for VAA, and also suggested recognition of the glycoprotein HMW1. Regarding RCA, comparable binding to NTHi375 and RdKW20 cells was observed. Interestingly, an increase in LOS_NTHi375 abundance or expression of HMW1 in RdKW20 impaired RCA binding. Overall, the results revealed that, besides the LOS, other carbohydrate structures on the bacterial surface serve as lectin ligands, and highlighted the impact of the specific display of cell surface components on lectin binding.

A guide into glycosciences: How chemistry, biochemistry and biology cooperate to crack the sugar code

BACKGROUND

The most demanding challenge in research on molecular aspects within the flow of biological information is posed by the complex carbohydrates (glycan part of cellular glycoconjugates). How the 'message' encoded in carbohydrate 'letters' is 'read' and 'translated' can only be unraveled by interdisciplinary efforts.

SCOPE OF REVIEW

This review provides a didactic step-by-step survey of the concept of the sugar code and the way strategic combination of experimental approaches characterizes structure-function relationships, with resources for teaching.

MAJOR CONCLUSIONS

The unsurpassed coding capacity of glycans is an ideal platform for generating a broad range of molecular 'messages'. Structural and functional analyses of complex carbohydrates have been made possible by advances in chemical synthesis, rendering production of oligosaccharides, glycoclusters and neoglycoconjugates possible. This availability facilitates to test the glycans as ligands for natural sugar receptors (lectins). Their interaction is a means to turn sugar-encoded information into cellular effects. Glycan/lectin structures and their spatial modes of presentation underlie the exquisite specificity of the endogenous lectins in counterreceptor selection, that is, to home in on certain cellular glycoproteins or glycolipids.

GENERAL SIGNIFICANCE

Understanding how sugar-encoded 'messages' are 'read' and 'translated' by lectins provides insights into fundamental mechanisms of life, with potential for medical applications.

Molecular recognition of complex-type biantennary N-glycans by protein receptors: a three-dimensional view on epitope selection by NMR

The current surge in defining glycobiomarkers by applying lectins rekindles interest in definition of the sugar-binding sites of lectins at high resolution. Natural complex-type N-glycans can present more than one potential binding motif, posing the question of the actual mode of interaction when interpreting, for example, lectin array data. By strategically combining N-glycan preparation with saturation-transfer difference NMR and modeling, we illustrate that epitope recognition depends on the structural context of both the sugar and the lectin (here, wheat germ agglutinin and a single hevein domain) and cannot always be predicted from simplified model systems studied in the solid state. We also monitor branch-end substitutions by this strategy and describe a three-dimensional structure that accounts for the accommodation of the α2,6-sialylated terminus of a biantennary N-glycan by viscumin. In addition, we provide a structural explanation for the role of terminal α2,6-sialylation in precluding the interaction of natural N-glycans with lectin from Maackia amurensis . The approach described is thus capable of pinpointing lectin-binding motifs in natural N-glycans and providing detailed structural explanations for lectin selectivity.

Studies related to Norway spruce galactoglucomannans: chemical synthesis, conformation analysis, NMR spectroscopic characterization, and molecular recognition of model compounds

Galactoglucomannan (GGM) is a polysaccharide mainly consisting of mannose, glucose, and galactose. GGM is the most abundant hemicellulose in the Norway spruce (Picea abies), but is also found in the cell wall of flax seeds, tobacco plants, and kiwifruit. Although several applications for GGM polysaccharides have been developed in pulp and paper manufacturing and the food and medical industries, attempts to synthesize and study distinct fragments of this polysaccharide have not been reported previously. Herein, the synthesis of one of the core trisaccharide units of GGM together with a less-abundant tetrasaccharide fragment is described. In addition, detailed NMR spectroscopic characterization of the model compounds, comparison of the spectral data with natural GGM, investigation of the acetyl-group migration phenomena that takes place in the polysaccharide by using small model compounds, and a binding study between the tetrasaccharide model fragment and a galactose-binding protein (the toxin viscumin) are reported.

Bi- to tetravalent glycoclusters: synthesis, structure-activity profiles as lectin inhibitors and impact of combining both valency and headgroup tailoring on selectivity

The emerging functional versatility of cellular glycans makes research on the design of synthetic inhibitors a timely topic. In detail, the combination of ligand (or headgroup or contact site) structure with spatial parameters that depend on topological and geometrical factors underlies the physiological selectivity of glycan-protein (lectin) recognition. We herein tested a panel of bi-, tri- and tetravalent compounds against two plant agglutinins and adhesion/growth-regulatory lectins (galectins). In addition, we examined the impact of headgroup tailoring (converting lactose to 2'-fucosyllactose) in combination with valency increase in two assay types of increasing biorelevance (from solid-phase binding to cell binding). Compounds were prepared using copper-catalysed azide alkyne cycloaddition from peracetylated lactosyl or 2'-fucosyllactosyl azides. Significant inhibition was achieved for the plant toxin with a tetravalent compound. Different levels of sensitivity were noted for the three groups of the galectin family. The headgroup extension to 2'-fucosyllactose led to a selectivity gain, especially for the chimera-type galectin-3. Valency increase established discrimination against the homodimeric proteins, whereas the combination of valency with the headgroup extension led to discrimination against the tandem-repeat-type galectin-8 for chicken galectins but not human galectins-3 and -4. Thus, detailed structure-activity profiling of glycoclusters combined with suitably modifying the contact site for the targeted lectin will help minimize cross-reactivity among this class of closely related proteins.

Structural aspects of binding of α-linked digalactosides to human galectin-1

By definition, adhesion/growth-regulatory galectins are known for their ability to bind β-galactosides such as Galβ(1 → 4)Glc (lactose). Indications for affinity of human galectin-1 to α-linked digalactosides pose questions on the interaction profile with such bound ligands and selection of the galactose moiety for CH-π stacking. These issues are resolved by a combination of (15)N-(1)H heteronuclear single quantum coherence (HSQC) chemical shift and saturation transfer difference nuclear magnetic resonance (STD NMR) epitope mappings with docking analysis, using the α(1 → 3/4)-linked digalactosides and also Galα(1 → 6)Glc (melibiose) as test compounds. The experimental part revealed interaction with the canonical lectin site, and this preferentially via the non-reducing-end galactose moiety. Low-energy conformers appear to be selected without notable distortion, as shown by molecular dynamics simulations. With the α(1 → 4) disaccharide, however, the typical CH-π interaction is significantly diminished, yet binding appears to be partially compensated for by hydrogen bonding. Overall, these findings reveal that the type of α-linkage in digalactosides has an impact on maintaining CH-π interactions and the pattern of hydrogen bonding, explaining preference for the α(1 → 3) linkage. Thus, this lectin is able to accommodate both α- and β-linked galactosides at the same site, with major contacts to the non-reducing-end sugar unit.

Symmetric dithiodigalactoside: strategic combination of binding studies and detection of selectivity between a plant toxin and human lectins

Thioglycosides offer the advantage over O-glycosides to be resistant to hydrolysis. Based on initial evidence of this recognition ability for glycosyldisulfides by screening dynamic combinatorial libraries, we have now systematically studied dithiodigalactoside on a plant toxin (Viscum album agglutinin) and five human lectins (adhesion/growth-regulatory galectins with medical relevance e.g. in tumor progression and spread). Inhibition assays with surface-presented neoglycoprotein and in solution monitored by saturation transfer difference NMR spectroscopy, flanked by epitope mapping, as well as isothermal titration calorimetry revealed binding properties to VAA (K(a): 1560 ± 20 M(-1)). They were reflected by the structural model and the affinity on the level of toxin-exposed cells. In comparison, galectins were considerably less reactive, with intrafamily grading down to very minor reactivity for tandem-repeat-type galectins, as quantitated by radioassays for both domains of galectin-4. Model building indicated contact formation to be restricted to only one galactose moiety, in contrast to thiodigalactoside. The tested glycosyldisulfide exhibits selectivity between the plant toxin and the tested human lectins, and also between these proteins. Therefore, glycosyldisulfides have potential as chemical platform for inhibitor design.

Carbamate-linked lactose: design of clusters and evidence for selectivity to block binding of human lectins to (neo)glycoproteins with increasing degree of branching and to tumor cells

Various pathogenic processes are driven by protein(lectin)-glycan interactions, especially involving beta-galactosides at branch ends of cellular glycans. These emerging insights fuel the interest to design potent inhibitors to block lectins. As a step toward this aim, we prepared a series of ten mono- to tetravalent glycocompounds with lactose as a common headgroup. To obtain activated carbonate for ensuing carbamate formation, conditions for the facile synthesis of pure isomers from anomerically unprotected lactose were identified. To probe for the often encountered intrafamily diversity of human lectins, we selected representative members from the three subgroups of adhesion/growth-regulatory galectins as receptors. Diversity of the glycan display was accounted for by using four (neo)glycoproteins with different degrees of glycan branching as matrices in solid-phase assays. Cases of increased inhibitory potency of lactose clusters compared to free lactose were revealed. Extent of relative inhibition was not directly associated with valency in the glycocompound and depended on the lectin type. Of note for screening protocols, efficacy of blocking appeared to decrease with increased degree of glycan branching in matrix glycoproteins. Binding to tumor cells was impaired with selectivity for galectins-3 and -4. Representative compounds did not impair growth of carcinoma cells up to a concentration of 5 mM of lactose moieties (valence-corrected value) per assay. The reported bioactivity and the delineation of its modulation by structural parameters of lectins and glycans set instructive examples for the further design of selective inhibitors and assay procedures.

Alpha-O-linked glycopeptide mimetics: synthesis, conformation analysis, and interactions with viscumin, a galactoside-binding model lectin

Efficient cycloaddition of a silylidene-protected galactal with a suitable heterodiene yielded the basis for a facile diastereoselective route to a glycopeptide-mimetic scaffold. Its carbohydrate part was further extended by beta1-3-linked galactosylation. The pyranose rings retain their (4)C(1) chair conformation, as shown by molecular modeling and NMR spectroscopy, and the typical exo-anomeric geometry was observed for the disaccharide. The expected bioactivity was ascertained by saturation-transfer-difference NMR spectroscopy by using the galactoside-specific plant toxin viscumin as a model lectin. The experimental part was complemented by molecular docking. The described synthetic route and the strategic combination of computational and experimental techniques to reveal conformational properties and bioactivity establish the prepared alpha-O-linked glycopeptide mimetics as promising candidates for further exploitation of this scaffold to give O-glycans for lectin blocking and vaccination.

In vitro immunoregulatory effects of Korean mistletoe lectin on functional activation of monocytic and macrophage-like cells

Korean mistletoe lectin (KML) is one of the major active components in Viscum album var. (coloratum), displaying various biological effects such as anti-tumor and anti-metastatic activities. Even though it has been shown to boost host immune defense mechanisms, the immunomodulatory effects of KML on specific immune responses mediated by macrophages have not been fully elucidated. Therefore, in this study, we aimed to demonstrate KML's regulatory roles on macrophage-mediated immune responses. KML clearly blocked lipopolysaccharide (LPS)-induced events [expression of interleukin (IL)-10, nitric oxide (NO) production and phagocytic uptake], and suppressed the normal expression levels of IL-10 (at 2 ng/ml) and tumor necrosis factor (TNF)-alpha (at 10 ng/ml). In contrast, (1) the expression of cytokine (TNF-alpha) and (2) the generation of reactive oxygen species (ROS) induced by LPS were significantly up-regulated with KML co-treatment. In addition, KML itself increased the mRNA levels of IL-3 and IL-23; phagocytic uptake; the surface levels of co-stimulatory molecules (CD80 and CD86), pattern recognition receptors (PRRs) [such as dectin-1 and toll like receptor (TLR)-2] and adhesion molecules [beta1-integrins (CD29) and CD43]; and CD29-mediated cell adhesion events. Finally, according to co-treatment of D-galactose with KML under LPS-induced NO production conditions, KML inhibition seems to be mediated by binding to proteins with D-galactose. Therefore, these data suggest that KML may participate in regulating various macrophage-mediated innate and adaptive responses via binding to surface protein with D-galactose and that some of these may deserve in KML's therapeutic activities such as anti-tumor and anti-microbial effects.

Photoderivatized polymer thin films at quartz crystal microbalance surfaces: sensors for carbohydrate-protein interactions

Photoderivatized polymer-coated gold surfaces have been developed following a perfluorophenylazide-based double ligation strategy. Gold-plated quartz crystal microbalance (QCM) crystals were initially covalently functionalized with a monolayer of poly(ethylene glycol) (PEG), using photo- or thermolytic nitrene formation and insertion. The polymer surfaces were subsequently used as substrates for photoinsertion of carbohydrate-derivatized photoprobes, yielding different recognition motifs for selective protein binding. The resulting robust and biocompatible sensor surfaces were applied to a flow-through QCM instrument for monitoring lectin-carbohydrate interactions in real time. The results clearly show the predicted lectin selectivity, demonstrating the applicability of the approach.

The conformation of the C-glycosyl analogue of N-acetyl-lactosamine in the free state and bound to a toxic plant agglutinin and human adhesion/growth-regulatory galectin-1

The conformational behavior of the C-glycoside analogue of N-acetyl-lactosamine, beta-C-Gal-(1-->4)-beta-GlcNAc-OMe, 1, has been studied using a combination of molecular mechanics calculations and NMR spectroscopy (J and NOE data). It is shown that the C-disaccharide populates three distinctive conformational families in solution, the major one being the anti-psi conformation. Of note, this conformation is only marginally populated for the O-disaccharide. Due to its conspicuous role in the regulation of adhesion, growth and tissue invasion of tumors and its avid binding to N-acetyl-lactosamine human, galectin-1 was tested as a receptor. This endogenous lectin recognizes a local minimum of 1, the syn-PhiPsi conformer, and thus a conformational selection process is correlated with the molecular recognition event.