Galectin-Glycan Interactions: Guidelines for Monitoring by 77 Se NMR Spectroscopy, and Solvent (H2 O/D2 O) Impact on Binding

Functional pairing between cellular glycoconjugates and tissue lectins like galectins has wide (patho)physiological significance. Their study is facilitated by nonhydrolysable derivatives of the natural O‐glycans, such as S‐ and Se‐glycosides. The latter enable extensive analyses by specific ⁷⁷Se NMR spectroscopy, but still remain underexplored. By using the example of selenodigalactoside (SeDG) and the human galectin‐1 and ‐3, we have evaluated diverse ⁷⁷Se NMR detection methods and propose selective ¹H,⁷⁷Se heteronuclear Hartmann–Hahn transfer for efficient use in competitive NMR screening against a selenoglycoside spy ligand. By fluorescence anisotropy, circular dichroism, and isothermal titration calorimetry (ITC), we show that the affinity and thermodynamics of SeDG binding by galectins are similar to thiodigalactoside (TDG) and N‐acetyllactosamine (LacNAc), confirming that Se substitution has no major impact. ITC data in D₂O versus H₂O are similar for TDG and LacNAc binding by both galectins, but a solvent effect, indicating solvent rearrangement at the binding site, is hinted at for SeDG and clearly observed for LacNAc dimers with extended chain length.

Placental protein 13: An important biological protein in preeclampsia

Placental protein 13 (PP13), a glycan binding protein predominantly expressed in syncytiotrophoblast, dimeric in nature, lacks N-terminal signal peptide, bypasses the endoplasmic reticulum, and secretes into maternal circulation as exosomes or microvesicles. PP13 has jelly roll fold conformation with conserved carbohydrate recognition domain which specifically binds to β-galactosides of the glycan receptors during placentation. PP13 binds to glycosylated receptors on human erythrocytes and brings about hemagglutination by the property of lectin activity; other functions are immunoregulation and vasodilation during placentation and vascularization. The gene LGALS13 located on 19q13.2 comprising four exons expresses a 32-kDa protein with 139 amino acid residues, PP13. Impaired expression due to mutation in the gene leads to a nonfunctional truncated PP13. The low serum levels predict high risk for the onset of preeclampsia or obstetric complications. Hence, PP13 turned to be an early marker for risk assessment of preeclampsia. The recombinant PP13 and monoclonal antibodies availability help for replenishing PP13 in conditions with low serum levels and for detection and prevention of preeclampsia, respectively.

Poly-N-Acetyllactosamine Neo-Glycoproteins as Nanomolar Ligands of Human Galectin-3: Binding Kinetics and Modeling

Galectin-3 (Gal-3) is recognized as a prognostic marker in several cancer types. Its involvement in tumor development and proliferation makes this lectin a promising target for early cancer diagnosis and anti-cancer therapies. Gal-3 recognizes poly-N-acetyllactosamine (LacNAc)-based carbohydrate motifs of glycoproteins and glycolipids with a high specificity for internal LacNAc epitopes. This study analyzes the mode and kinetics of binding of Gal-3 to a series of multivalent neo-glycoproteins presenting complex poly-LacNAc-based oligosaccharide ligands on a scaffold of bovine serum albumin. These neo-glycoproteins rank among the strongest Gal-3 ligands reported, with K_d reaching sub-nanomolar values as determined by surface plasmon resonance. Significant differences in the binding kinetics were observed within the ligand series, showing the tetrasaccharide capped with N,N'-diacetyllactosamine (LacdiNAc) as the strongest ligand of Gal-3 in this study. A molecular model of the Gal-3 carbohydrate recognition domain with docked oligosaccharide ligands is presented that shows the relations in the binding site at the molecular level. The neo-glycoproteins presented herein may be applied for selective recognition of Gal-3 both on the cell surface and in blood serum.

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.

Structural Basis Underlying the Binding Preference of Human Galectins-1, -3 and -7 for Galβ1-3/4GlcNAc

Galectins represent β-galactoside-binding proteins and are known to bind Galβ1-3/4GlcNAc disaccharides (abbreviated as LN1 and LN2, respectively). Despite high sequence and structural homology shared by the carbohydrate recognition domain (CRD) of all galectin members, how each galectin displays different sugar-binding specificity still remains ambiguous. Herein we provided the first structural evidence of human galectins-1, 3-CRD and 7 in complex with LN1. Galectins-1 and 3 were shown to have higher affinity for LN2 than for LN1, while galectin-7 displayed the reversed specificity. In comparison with the previous LN2-complexed structures, the results indicated that the average glycosidic torsion angle of galectin-bound LN1 (ψ^LN1 ≈ 135°) was significantly differed from that of galectin-bound LN2 (ψ^LN2 ≈ -108°), i.e. the GlcNAc moiety adopted a different orientation to maintain essential interactions. Furthermore, we also identified an Arg-Asp/Glu-Glu-Arg salt-bridge network and the corresponding loop (to position the second Asp/Glu residue) critical for the LN1/2-binding preference.

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.

Exploring NMR methods as a tool to select suitable fluorescent nucleotide analogues

Fluorescent analogues provide important tools for biochemical/biophysical research. However, the analogues contain chemical modifications much larger than those known to affect ligand-binding, such as the inversion of a carbon centre or substitution of an atom. We lack experimental tools and protocols to select the most appropriate fluorescent analogue. Herein, we use several NMR spectroscopy methods, including Saturation Transfer Difference (STD), STD competition and transferred nuclear Overhauser effect spectroscopy (Tr-NOESY), as tools to select appropriate fluorescent probes. Annexin A6 (AnxA6) is a ubiquitous protein that forms in vitro GTP-induced ion channels. We used this protein as a model and screened guanosine triphosphate (GTP) and four fluorescent analogues against AnxA6. STD reported that the GTP moiety of all ligands made similar contacts with the protein, despite additional interactions between the fluorescent tags and AnxA6. Competition STD experiments verified that the analogues and GTP bind to the same site. Tr-NOESY indicated that the bound conformation of the base relative to ribose is altered for some analogues compared to GTP. MANT-GTP or the BODIPY thioester of guanosine 5'-O-(3-thiotriphosphate) are the most suitable fluorescent analogues for AnxA6, according to NMR. These results reveal NMR as a useful technique to select and design proper fluorescent tags for biochemical/biophysical assays.

Adaptable synthesis of C-lactosyl glycoclusters and their binding properties with galectin-3

The synthesis of mono- to tetravalent C-β-lactosyl glycoclusters has been achieved in good yield. The K_D values of glycoclusters against galectin-3 were tested by SPR assay, and the structure–activity relationship has been summarized in detail.

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.

From lectin structure to functional glycomics: principles of the sugar code

Lectins are carbohydrate-binding proteins which lack enzymatic activity on their ligand and are distinct from antibodies and free mono- and oligosaccharide sensor/transport proteins. Emerging insights into the functional dimension of lectin binding to cellular glycans have strongly contributed to the shaping of the 'sugar code'. Fittingly, over a dozen folds and a broad spectrum of binding site architecture, ranging from shallow grooves to deep pockets, have developed sugar-binding capacity. A central question is how the exquisite target specificity of endogenous lectins for certain cellular glycans can be explained. In this regard, affinity regulation is first systematically dissected into six levels. Experimentally, the strategic combination of methods to monitor distinct aspects of the lectin-glycan interplay offers a promising perspective to answer this question.

Inhibitory potential of chemical substitutions at bioinspired sites of β-D-galactopyranose on neoglycoprotein/cell surface binding of two classes of medically relevant lectins

Galactose is the key contact site for plant AB-toxins and the human adhesion/growth-regulatory galectins. Natural anomeric extensions and 3'-substitutions enhance its reactivity, thus prompting us to test the potential of respective chemical substitutions of galactose in the quest to develop potent inhibitors. Biochemical screening of a respective glycoside library with 60 substances in a solid-phase assay was followed by examining the compounds' activity to protect cells from lectin binding. By testing 32 anomeric extensions, 18 compounds with additional 3'-substitution, three lactosides and two Lewis-type trisaccharides rather mild effects compared to the common haptenic inhibitor lactose were detected in both assays. When using trivalent glycoclusters marked enhancements with 6- to 8-fold increases were revealed for the toxin and three of four tested galectins. Since the most potent compound and also 3'-substituted thiogalactosides reduced cell growth of a human tumor line at millimolar concentrations, biocompatible substitutions and scaffolds will be required for further developments. The synthesis of suitable glycoclusters, presenting headgroups which exploit differences in ligand selection in interlectin comparison to reduce cross-reactivity, and the documented strategic combination of initial biochemical screening with cell assays are considered instrumental to advance inhibitor design.

Glycocluster design for improved avidity and selectivity in blocking human lectin/plant toxin binding to glycoproteins and cells

Blocking lectin/toxin binding to human cells by suitable inhibitors can therapeutically protect them from harmful effects. Clustered design of ligand presentation holds the promise of affinity increase relative to the free sugar and inherent selectivity among lectin targets. Using first a solid-phase assay with a glycoprotein presenting N-glycans as lectin-reactive probe, we assessed the inhibitory potency of bi- to tetravalent clusters on a plant toxin and three human adhesion/growth-regulatory lectins. Enhanced avidity relative to the free sugar was detected together with lectin-type selectivity. These effects were confirmed on the level of cells in vitro, also for two leguminous lectins. The lack of toxicity in cell proliferation assays excluded concerns to further work on these compounds. The given cluster design and the strategic combination of the two assay systems of increasing biorelevance will thus be helpful to take the next steps in drug development, e.g. tailoring the sugar headgroup.