Galectins bind to the multivalent glycoprotein asialofetuin with enhanced affinities and a gradient of decreasing binding constants

CUREs for high-level Galectin-3 expression

Galectins are a large and diverse protein family defined by the presence of a carbohydrate recognition domain (CRD) that binds β-galactosides. They play important roles in early development, tissue regeneration, immune homeostasis, pathogen recognition, and cancer. In many cases, studies that examine galectin biology and the effect of manipulating galectins are aided by, or require the ability to express and purify, specific members of the galectin family. In many cases, E. coli is employed as a heterologous expression system, and galectin expression is induced with isopropyl β-galactoside (IPTG). Here, we show that galectin-3 recognizes IPTG with micromolar affinity and that as IPTG induces expression, newly synthesized galectin can bind and sequester cytosolic IPTG, potentially repressing further expression. To circumvent this putative inhibitory feedback loop, we utilized an autoinduction protocol that lacks IPTG, leading to significantly increased yields of galectin-3. Much of this work was done within the context of a course-based undergraduate research experience, indicating the ease and reproducibility of the resulting expression and purification protocols.

Mechanism of multivalent glycoconjugate-lectin interaction: An update

Lectins are predominantly oligomeric proteins with several binding sites per molecule. Glycoconjugates are their natural ligands, which often possess multiple binding epitopes. Thus, lectin-glycoconjugate interactions are mostly multivalent in nature. The mechanism of multivalent binding is fundamentally different from those described for monovalent interactions in textbooks and research papers. Over the years, binding studies that make use of different lectins and a variety of multivalent glycoconjugate ligands were conducted in order to understand the underlying principles of multivalency. Starting with seemingly simple synthetic multivalent analogs, systematic studies were carried out using natural glycoconjugate ligands with increasing valency and complexity. Those ligands included multivalent glycoproteins, polyvalent polysaccharides, including glycosaminoglycans, as well as supra-valent mucins and proteoglycans. Models and mechanisms of multivalent binding derived from quantitative data are summarized in the present updated review.

Multivalent lectin-carbohydrate interactions: Energetics and mechanisms of binding

The biological signaling properties of lectins, which are carbohydrate-binding proteins, are due to their ability to bind and cross-link multivalent glycoprotein receptors on the surface of normal and transformed cells. While the cross-linking properties of lectins with multivalent carbohydrates and glycoproteins are relatively well understood, the mechanisms of binding of lectins to multivalent glycoconjugates are less well understood. Recently, the thermodynamics of binding of lectins to synthetic clustered glycosides, a multivalent globular glycoprotein, and to linear glycoproteins (mucins) have been described. The results are consistent with a dynamic binding mechanism in which lectins bind and jump from carbohydrate to carbohydrate epitope in these molecules. Importantly, the mechanism of binding of lectins to mucins is similar to that for a variety of protein ligands binding to DNA. Recent analysis also shows that high-affinity lectin-mucin cross-linking interactions are driven by favorable entropy of binding that is associated with the bind and jump mechanism. The results suggest that the binding of ligands to biopolymers, in general, may involve a common mechanism that involves enhanced entropic effects which facilitate binding and subsequent complex formation including enzymology.

Galectin-3 and Epithelial MUC1 Mucin-Interactions Supporting Cancer Development

Aberrant glycosylation of cell surface proteins is a very common feature of many cancers. One of the glycoproteins, which undergoes specific alterations in the glycosylation of tumor cells is epithelial MUC1 mucin, which is highly overexpressed in the malignant state. Such changes lead to the appearance of tumor associated carbohydrate antigens (TACAs) on MUC1, which are rarely seen in healthy cells. One of these structures is the Thomsen-Friedenreich disaccharide Galβ1-3GalNAc (T or TF antigen), which is typical for about 90% of cancers. It was revealed that increased expression of the T antigen has a big impact on promoting cancer progression and metastasis, among others, due to the interaction of this antigen with the β-galactose binding protein galectin-3 (Gal-3). In this review, we summarize current information about the interactions between the T antigen on MUC1 mucin and Gal-3, and their impact on cancer progression and metastasis.

Characterization of Galectin Fusion Proteins with Glycoprotein Affinity Columns and Binding Assays

Galectins are β-galactosyl-binding proteins that fulfill essential physiological functions. In the biotechnological field, galectins are versatile tools, such as in the development of biomaterial coatings or the early-stage diagnosis of cancer diseases. Recently, we introduced galectin-1 (Gal-1) and galectin-3 (Gal-3) as fusion proteins of a His₆-tag, a SNAP-tag, and a fluorescent protein. We characterized their binding in ELISA-type assays and their application in cell-surface binding. In the present study, we have constructed further fusion proteins of galectins with fluorescent protein color code. The fusion proteins of Gal-1, Gal-3, and Gal-8 were purified by affinity chromatography. For this, we have prepared glycoprotein affinity resins based on asialofetuin (ASF) and fetuin and combined this in a two-step purification with Immobilized Metal Affinity chromatography (IMAC) to get pure and active galectins. Purified galectin fractions were analyzed by size-exclusion chromatography. The binding characteristics to ASF of solely His₆-tagged galectins and galectin fusion proteins were compared. As an example, we demonstrate a 1.6-3-fold increase in binding efficiency for HSYGal-3 (His₆-SNAP-yellow fluorescent protein-Gal-3) compared to the HGal-3 (His₆-Gal-3). Our results reveal an apparent higher binding efficiency for galectin SNAP-tag fusion proteins compared to His₆-tagged galectins, which are independent of the purification mode. This is also demonstrated by the binding of galectin fusion proteins to extracellular glycoconjugates laminin, fibronectin, and collagen IV. Our results indicate the probable involvement of the SNAP-tag in apparently higher binding signals, which we discuss in this study.

Isolipoic acid-linked gold nanoparticles bearing the thomsen friedenreich tumor-associated carbohydrate antigen: Stability and in vitro studies

We have previously prepared gold nanoparticles (AuNPs) bearing the Thomsen-Friedenreich antigen disaccharide (TF_ag), a pan-carcinoma, Tumor-Associated Carbohydrate Antigen (TACA), as tools for various assays and biological applications. Conjugation to AuNPs typically involves the use of thiols due to the affinity of sulfur for the gold surface of the nanoparticle. While a use of a single thiol-containing ligand bound to the gold surface is standard practice, several studies have shown that ligands bearing multiple thiols can enhance the strength of the conjugation in a nearly linear fashion. (R)-(+)-α-Lipoic acid (LA), a naturally occurring disulfide-containing organic acid that is used as a cofactor in many enzymatic reactions, has been used as a linker to conjugate various molecules to AuNPs through its branched di-thiol system to enhance nanoparticle stability. We sought to use a similar system to increase nanoparticle stability that was devoid of the chiral center in (R)-(+)-α-lipoic acid. Isolipoic acid, an isomer of LA, where the exocyclic pentanoic acid chain is shifted by one carbon on the dithiolane ring to produce an achiral acid, was thought to act similarly as LA without the risk of any contaminating (L)-(−) isomer. We synthesized AuNPs with ligands of both serine and threonine glycoamino acids bearing the TF_ag linked to isolipoic acid and examined their stability under various conditions. In addition, these particles were shown to bind to Galectin-3 and inhibit the interaction of Galectin-3 with a protein displaying copies of the TF_ag. These agents should prove useful in the design of potential antimetastatic therapeutics that would benefit from achiral linkers that are geometrically linear and achiral.

Multicomponent reaction derived small di- and tri-carbohydrate-based glycomimetics as tools for probing lectin specificity

Lectins, carbohydrate-binding proteins, play important functions in all forms of life from bacteria and viruses to plants, animals, and humans, participating in cell-cell communication and pathogen binding. In an attempt to modify lectin functions, artificial lectin ligands were made usually as big dendrimeric or cluster multivalent glycomimetic structures. Here we synthesized a novel set of glycomimetic ligands through protection/deprotection multicomponent reactions (MCR) approach. Multivalent di-and tri-carbohydrate glycomimetics containing D-fructose, D-galactose, and D-allose moieties were prepared in 63-96% yield. MCR glycomimetics demonstrated different binding abilities for plant lectins Con A and UEA I, and human galectin-3. Information gained about the influence of molecule structure, multivalency and optical purity on the lectin binding ability can be used in lectin detection and sensitivity measurements to further facilitate understanding of carbohydrate recognition process.

Mechanism of Mucin Recognition by Lectins: A Thermodynamic Study

Isothermal titration microcalorimetry (ITC) can directly determine the thermodynamic binding parameters of biological molecules including affinity constant, binding stoichiometry, heat of binding (enthalpy) and indirectly the entropy, and free energy of binding. ITC has been extensively used to study the binding of lectins to mono- and oligosaccharides, but limitedly in applications to lectin-glycoprotein interactions. Inherent experimental challenges to ITC include sample precipitation during the experiment and relative high amount of sample required, but careful design of experiments can minimize these problems and allow valuable information to be obtained. For example, the thermodynamics of binding of lectins to multivalent globular and linear glycoproteins (mucins) have been described. The results are consistent with a dynamic binding mechanism in which lectins bind and jump from carbohydrate to carbohydrate epitope in these molecules leading to increased affinity. Importantly, the mechanism of binding of lectins to mucins appears similar to that for a variety of protein ligands binding to DNA. Recent results also show that high-affinity lectin-mucin cross-linking interactions are driven by favorable entropy of binding that is associated with the bind and jump mechanism. The results suggest that the binding of ligands to biopolymers, in general, may involve a common mechanism that involves enhanced entropic effects that facilitate binding interactions.

Evaluation of Galectin Binding by Surface Plasmon Resonance

Surface plasmon resonance (SPR) instruments, like the BIAcore 3000, are useful for studying the binding between macromolecules in real time. The high sensitivity and low sample consumption in the Biacore enables the measurement of rapid kinetics and low affinities characteristics of many biological interactions. This chapter describes the affinity measurement of Galectins-1, -2 and -3 and their glycoside ligands using this approach.

Structural Characterization of Rat Galectin-5, an N-Tailed Monomeric Proto-Type-like Galectin

Galectins are multi-purpose effectors acting via interactions with distinct counterreceptors based on protein-glycan/protein recognition. These processes are emerging to involve several regions on the protein so that the availability of a detailed structural characterization of a full-length galectin is essential. We report here the first crystallographic information on the N-terminal extension of the carbohydrate recognition domain of rat galectin-5, which is precisely described as an N-tailed proto-type-like galectin. In the ligand-free protein, the three amino-acid stretch from Ser2 to Ser5 is revealed to form an extra β-strand (F0), and the residues from Thr6 to Asn12 are part of a loop protruding from strands S1 and F0. In the ligand-bound structure, amino acids Ser2–Tyr10 switch position and are aligned to the edge of the β-sandwich. Interestingly, the signal profile in our glycan array screening shows the sugar-binding site to preferentially accommodate the histo-blood-group B (type 2) tetrasaccharide and N-acetyllactosamine-based di- and oligomers. The crystal structures revealed the characteristically preformed structural organization around the central Trp77 of the CRD with involvement of the sequence signature’s amino acids in binding. Ligand binding was also characterized calorimetrically. The presented data shows that the N-terminal extension can adopt an ordered structure and shapes the hypothesis that a ligand-induced shift in the equilibrium between flexible and ordered conformers potentially acts as a molecular switch, enabling new contacts in this region.

What is the Sugar Code?

A code is defined by the nature of the symbols, which are used to generate information‐storing combinations (e. g. oligo‐ and polymers). Like nucleic acids and proteins, oligo‐ and polysaccharides are ubiquitous, and they are a biochemical platform for establishing molecular messages. Of note, the letters of the sugar code system (third alphabet of life) excel in coding capacity by making an unsurpassed versatility for isomer (code word) formation possible by variability in anomery and linkage position of the glycosidic bond, ring size and branching. The enzymatic machinery for glycan biosynthesis (writers) realizes this enormous potential for building a large vocabulary. It includes possibilities for dynamic editing/erasing as known from nucleic acids and proteins. Matching the glycome diversity, a large panel of sugar receptors (lectins) has developed based on more than a dozen folds. Lectins ‘read’ the glycan‐encoded information. Hydrogen/coordination bonding and ionic pairing together with stacking and C−H/π‐interactions as well as modes of spatial glycan presentation underlie the selectivity and specificity of glycan‐lectin recognition. Modular design of lectins together with glycan display and the nature of the cognate glycoconjugate account for the large number of post‐binding events. They give an entry to the glycan vocabulary its functional, often context‐dependent meaning(s), hereby building the dictionary of the sugar code.

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.

Imitating evolution's tinkering by protein engineering reveals extension of human galectin-7 activity

Wild-type lectins have distinct types of modular design. As a step to explain the physiological importance of their special status, hypothesis-driven protein engineering is used to generate variants. Concerning adhesion/growth-regulatory galectins, non-covalently associated homodimers are commonly encountered in vertebrates. The homodimeric galectin-7 (Gal-7) is a multifunctional context-dependent modulator. Since the possibility of conversion from the homodimer to hybrids with other galectin domains, i.e. from Gal-1 and Gal-3, has recently been discovered, we designed Gal-7-based constructs, i.e. stable (covalently linked) homo- and heterodimers. They were produced and purified by affinity chromatography, and the sugar-binding activity of each lectin unit proven by calorimetry. Inspection of profiles of binding of labeled galectins to an array-like platform with various cell types, i.e. sections of murine epididymis and jejunum, and impact on neuroblastoma cell proliferation revealed no major difference between natural and artificial (stable) homodimers. When analyzing heterodimers, acquisition of altered properties was seen. Remarkably, binding properties and activity as effector can depend on the order of arrangement of lectin domains (from N- to C-termini) and on the linker length. After dissociation of the homodimer, the Gal-7 domain can build new functionally active hybrids with other partners. This study provides a clear direction for research on defining the full range of Gal-7 functionality and offers the perspective of testing applications for engineered heterodimers.

Characterizing ligand-induced conformational changes in clinically relevant galectin-1 by HN/H2O (D2O) exchange

Glycans of cellular glycoconjugates serve as biochemical signals for a multitude of (patho)physiological processes via binding to their receptors (e.g. lectins). In the case of human adhesion/growth-regulatory galectin-1 (Gal-1), small angle neutron scattering and fluorescence correlation spectroscopy have revealed a significant decrease of its gyration radius and increase of its diffusion coefficient upon binding lactose, posing the pertinent question on the nature and region(s) involved in the underlying structural alterations. Requiring neither a neutron source nor labeling, diffusion measurements by ¹H NMR spectroscopy are shown here to be sufficiently sensitive to detect this ligand-induced change. In order to figure out which region(s) of Gal-1 is (are) affected at the level of peptides, we first explored the use of H/D exchange mass spectrometry (HDX MS). Hereby, we found a reduction in proton exchange kinetics beyond the lactose-binding site. The measurement of fast H^N/H₂O exchange by phase-modulated NMR clean chemical exchange (CLEANEX) NMR on ¹⁵N-labeled Gal-1 then increased the spatial resolution to the level of individual amino acids. The mapped regions with increased protection from H^N/H₂O (D₂O) exchange that include the reduction of solvent exposure around the interface can underlie the protein's compaction. These structural changes have potential to modulate this galectin's role in lattice formation on the cell surface and its interaction(s) with protein(s) at the F-face.

Burkholderia cenocepacia BCAM2418-induced antibody inhibits bacterial adhesion, confers protection to infection and enables identification of host glycans as adhesin targets

Trimeric Autotransporter Adhesins (TAA) found in Gram-negative bacteria play a key role in virulence. This is the case of Burkholderia cepacia complex (Bcc), a group of related bacteria able to cause infections in patients with cystic fibrosis. These bacteria use TAAs, among other virulence factors, to bind to host protein receptors and their carbohydrate ligands. Blocking such contacts is an attractive approach to inhibit Bcc infections. In this study, using an antibody produced against the TAA BCAM2418 from the epidemic strain Burkholderia cenocepacia K56-2, we were able to uncover its roles as an adhesin and the type of host glycan structures that serve as recognition targets. The neutralisation of BCAM2418 was found to cause a reduction in the adhesion of the bacteria to bronchial cells and mucins. Moreover, in vivo studies have shown that the anti-BCAM2418 antibody exerted an inhibitory effect during infection in Galleria mellonella. Finally, inferred by glycan arrays, we were able to predict for the first time, host glycan epitopes for a TAA. We show that BCAM2418 favoured binding to 3'sialyl-3-fucosyllactose, histo-blood group A, α-(1,2)-linked Fuc-containing structures, Lewis structures and GM1 gangliosides. In addition, the glycan microarrays demonstrated similar specificities of Burkholderia species for their most intensely binding carbohydrates.

Exploration of Galectin Ligands Displayed on Gram-Negative Respiratory Bacterial Pathogens with Different Cell Surface Architectures

Galectins bind various pathogens through recognition of distinct carbohydrate structures. In this work, we examined the binding of four human galectins to the Gram-negative bacteria Klebsiella pneumoniae (Kpn) and non-typeable Haemophilus influenzae (NTHi), which display different surface glycans. In particular, Kpn cells are covered by a polysaccharide capsule and display an O-chain-containing lipopolysaccharide (LPS), whereas NTHi is not capsulated and its LPS, termed lipooligosacccharide (LOS), does not contain O-chain. Binding assays to microarray-printed bacteria revealed that galectins-3, -4, and -8, but not galectin-1, bind to Kpn and NTHi cells, and confocal microscopy attested binding to bacterial cells in suspension. The three galectins bound to array-printed Kpn LPS. Moreover, analysis of galectin binding to mutant Kpn cells evidenced that the O-chain is the docking point for galectins on wild type Kpn. Galectins-3, -4, and -8 also bound the NTHi LOS. Microarray-assisted comparison of the binding to full-length and truncated LOSs, as well as to wild type and mutant cells, supported LOS involvement in galectin binding to NTHi. However, deletion of the entire LOS oligosaccharide chain actually increased binding to NTHi cells, indicating the availability of other ligands on the bacterial surface, as similarly inferred for Kpn cells devoid of both O-chain and capsule. Altogether, the results illustrate galectins’ versatility for recognizing different bacterial structures, and point out the occurrence of so far overlooked galectin ligands on bacterial surfaces.

Placental Glycoredox Dysregulation Associated with Disease Progression in an Animal Model of Superimposed Preeclampsia

Pregnancies carried by women with chronic hypertension are at increased risk of superimposed preeclampsia, but the placental pathways involved in disease progression remain poorly understood. In this study, we used the stroke-prone spontaneously hypertensive rat (SHRSP) model to investigate the placental mechanisms promoting superimposed preeclampsia, with focus on cellular stress and its influence on galectin-glycan circuits. Our analysis revealed that SHRSP placentas are characterized by a sustained activation of the cellular stress response, displaying significantly increased levels of markers of lipid peroxidation (i.e., thiobarbituric acid reactive substances (TBARS)) and protein nitration and defective antioxidant enzyme expression as early as gestation day 14 (which marks disease onset). Further, lectin profiling showed that such redox imbalance was associated with marked alterations of the placental glycocode, including a prominent decrease of core 1 O-glycan expression in trophoblasts and increased decidual levels of sialylation in SHRSP placentas. We also observed significant changes in the expression of galectins 1, 3 and 9 with pregnancy progression, highlighting the important role of the galectin signature as dynamic interpreters of placental microenvironmental challenges. Collectively, our findings uncover a new role for the glycoredox balance in the pathogenesis of superimposed preeclampsia representing a promising target for interventions in hypertensive disorders of pregnancy.

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.

TF-containing MUC1 glycopeptides fail to entice Galectin-1 recognition of tumor-associated Thomsen-Freidenreich (TF) antigen (CD176) in solution

Aberrant Mucin-1 (MUC1) glycosylation with the Thomsen-Friedenreich (TF) tumor-associated antigen (CD176) is a hallmark of epithelial carcinoma progression and poor patient prognosis. Recognition of TF by glycan-binding proteins, such as galectins, enables the pathological repercussions of this glycan presentation, yet the underlying binding specificities of different members of the galectin family is a matter of continual investigation. While Galectin-3 (Gal-3) recognition of TF has been well-documented at both the cellular and molecular level, Galectin-1 (Gal-1) recognition of TF has only truly been alluded to in cell-based platforms. Immunohistochemical analyses have purported Gal-1 binding to TF on MUC1 at the cell surface, however binding at the molecular level was inconclusive. We hypothesize that glycan scaffold (MUC1's tandem repeat peptide sequence) and/or multivalency play a role in the binding recognition of TF antigen by Gal-1. In this study we have developed a method for large-scale expression of Gal-1 and its histidine-tagged analog for use in binding studies by isothermal titration calorimetry (ITC) and development of an analytical method based on AlphaScreen technology to screen for Gal-1 inhibitors. Surprisingly, neither glycan scaffold or multivalent presentation of TF antigen on the scaffold was able to entice Gal-1 recognition to the level of affinity expected for functional significance. Future evaluations of the Gal-1/TF binding interaction in order to draw connections between immunohistochemical data and analytical measurements are warranted.

Structural insight into the binding of human galectins to corneal keratan sulfate, its desulfated form and related saccharides

Glycosaminoglycan chains of keratan sulfate proteoglycans appear to be physiologically significant by pairing with tissue lectins. Here, we used NMR spectroscopy and molecular dynamics (MD) simulations to characterize interactions of corneal keratan sulfate (KS), its desulfated form, as well as di-, tetra- (N-acetyllactosamine and lacto-N-tetraose) and octasaccharides with adhesion/growth-regulatory galectins, in particular galectin-3 (Gal-3). The KS contact region involves the lectin canonical binding site, with estimated K_D values in the low µM range and stoichiometry of ~ 8 to ~ 20 galectin molecules binding per polysaccharide chain. Compared to Gal-3, the affinity to Gal-7 is relatively low, signaling preferences among galectins. The importance of the sulfate groups was delineated by using desulfated analogs that exhibit relatively reduced affinity. Binding studies with two related di- and tetrasaccharides revealed a similar decrease that underscores affinity enhancement by repetitive arrangement of disaccharide units. MD-based binding energies of KS oligosaccharide-loaded galectins support experimental data on Gal-3 and -7, and extend the scope of KS binding to Gal-1 and -9N. Overall, our results provide strong incentive to further probe the relevance of molecular recognition of KS by galectins in terms of physiological processes in situ, e.g. maintaining integrity of mucosal barriers, intermolecular (lattice-like) gluing within the extracellular meshwork or synaptogenesis.

Pro4 prolyl peptide bond isomerization in human galectin-7 modulates the monomer-dimer equilibrum to affect function

Human galectin-7 (Gal-7; also termed p53-induced gene 1 product) is a multifunctional effector by productive pairing with distinct glycoconjugates and protein counter-receptors in the cytoplasm and nucleus, as well as on the cell surface. Its structural analysis by NMR spectroscopy detected doubling of a set of particular resonances, an indicator of Gal-7 existing in two conformational states in slow exchange on the chemical shift time scale. Structural positioning of this set of amino acids around the P4 residue and loss of this phenomenon in the bioactive P4L mutant indicated cis-trans isomerization at this site. Respective resonance assignments confirmed our proposal of two Gal-7 conformers. Mapping hydrogen bonds and considering van der Waals interactions in molecular dynamics simulations revealed a structural difference for the N-terminal peptide, with the trans-state being more exposed to solvent and more mobile than the cis-state. Affinity for lactose or glycan-inhibitable neuroblastoma cell surface contact formation was not affected, because both conformers associated with an overall increase in order parameters (S2). At low µM concentrations, homodimer dissociation is more favored for the cis-state of the protein than its trans-state. These findings give direction to mapping binding sites for protein counter-receptors of Gal-7, such as Bcl-2, JNK1, p53 or Smad3, and to run functional assays at low concentration to test the hypothesis that this isomerization process provides a (patho)physiologically important molecular switch for Gal-7.

Sequence-Defined Heteromultivalent Precision Glycomacromolecules Bearing Sulfonated/Sulfated Nonglycosidic Moieties Preferentially Bind Galectin-3 and Delay Wound Healing of a Galectin-3 Positive Tumor Cell Line in an In Vitro Wound Scratch Assay

Within this work, a new class of sequence-defined heteromultivalent glycomacromolecules bearing lactose residues and nonglycosidic motifs for probing glycoconjugate recognition in carbohydrate recognition domain (CRD) of galectin-3 is presented. Galectins, a family of β-galactoside-binding proteins, are known to play crucial roles in different signaling pathways involved in tumor biology. Thus, research has focused on the design and synthesis of galectin-targeting ligands for use as diagnostic markers or potential therapeutics. Heteromultivalent precision glycomacromolecules have the potential to serve as ligands for galectins. In this work, multivalency and the introduction of nonglycosidic motifs bearing either neutral, amine, or sulfonated/sulfated groups are used to better understand binding in the galectin-3 CRD. Enzyme-linked immunosorbent assays and surface plasmon resonance studies are performed, revealing a positive impact of the sulfonated/sulfated nonglycosidic motifs on galectin-3 binding but not on galectin-1 binding. Selected compounds are then tested with galectin-3 positive MCF 7 breast cancer cells using an in vitro would scratch assay. Preliminary results demonstrate a differential biological effect on MCF 7 cells with high galectin-3 expression in comparison to an HEK 293 control with low galectin-3 expression, indicating the potential for sulfonated/sulfated heteromultivalent glycomacromolecules to serve as preferential ligands for galectin-3 targeting.

Influence of protein (human galectin-3) design on aspects of lectin activity

The concept of biomedical significance of the functional pairing between tissue lectins and their glycoconjugate counterreceptors has reached the mainstream of research on the flow of biological information. A major challenge now is to identify the principles of structure–activity relationships that underlie specificity of recognition and the ensuing post-binding processes. Toward this end, we focus on a distinct feature on the side of the lectin, i.e. its architecture to present the carbohydrate recognition domain (CRD). Working with a multifunctional human lectin, i.e. galectin-3, as model, its CRD is used in protein engineering to build variants with different modular assembly. Hereby, it becomes possible to compare activity features of the natural design, i.e. CRD attached to an N-terminal tail, with those of homo- and heterodimers and the tail-free protein. Thermodynamics of binding disaccharides proved full activity of all proteins at very similar affinity. The following glycan array testing revealed maintained preferential contact formation with N-acetyllactosamine oligomers and histo-blood group ABH epitopes irrespective of variant design. The study of carbohydrate-inhibitable binding of the test panel disclosed up to qualitative cell-type-dependent differences in sections of fixed murine epididymis and especially jejunum. By probing topological aspects of binding, the susceptibility to inhibition by a tetravalent glycocluster was markedly different for the wild-type vs the homodimeric variant proteins. The results teach the salient lesson that protein design matters: the type of CRD presentation can have a profound bearing on whether basically suited oligosaccharides, which for example tested positively in an array, will become binding partners in situ. When lectin-glycoconjugate aggregates (lattices) are formed, their structural organization will depend on this parameter. Further testing (ga)lectin variants will thus be instrumental (i) to define the full range of impact of altering protein assembly and (ii) to explain why certain types of design have been favored during the course of evolution, besides opening biomedical perspectives for potential applications of the novel galectin forms.

Glycan characterization of pregnancy-specific glycoprotein 1 and its identification as a novel Galectin-1 ligand

Pregnancy-specific beta 1 glycoprotein (PSG1) is secreted from trophoblast cells of the human placenta in increasing concentrations as pregnancy progresses, becoming one of the most abundant proteins in maternal serum in the third trimester. PSG1 has seven potential N-linked glycosylation sites across its four domains. We carried out glycomic and glycoproteomic studies to characterize the glycan composition of PSG1 purified from serum of pregnant women and identified the presence of complex N-glycans containing poly LacNAc epitopes with α2,3 sialyation at four sites. Using different techniques, we explored whether PSG1 can bind to galectin-1 (Gal-1) as these two proteins were previously shown to participate in processes required for a successful pregnancy. We confirmed that PSG1 binds to Gal-1 in a carbohydrate-dependent manner with an affinity of the interaction of 0.13 μM. In addition, we determined that out of the three N-glycosylation-carrying domains, only the N and A2 domains of recombinant PSG1 interact with Gal-1. Lastly, we observed that the interaction between PSG1 and Gal-1 protects this lectin from oxidative inactivation and that PSG1 competes the ability of Gal-1 to bind to some but not all of its glycoprotein ligands.

Chemokines and galectins form heterodimers to modulate inflammation

Chemokines and galectins are simultaneously upregulated and mediate leukocyte recruitment during inflammation. Until now, these effector molecules have been considered to function independently. Here, we tested the hypothesis that they form molecular hybrids. By systematically screening chemokines for their ability to bind galectin‐1 and galectin‐3, we identified several interacting pairs, such as CXCL12 and galectin‐3. Based on NMR and MD studies of the CXCL12/galectin‐3 heterodimer, we identified contact sites between CXCL12 β‐strand 1 and Gal‐3 F‐face residues. Mutagenesis of galectin‐3 residues involved in heterodimer formation resulted in reduced binding to CXCL12, enabling testing of functional activity comparatively. Galectin‐3, but not its mutants, inhibited CXCL12‐induced chemotaxis of leukocytes and their recruitment into the mouse peritoneum. Moreover, galectin‐3 attenuated CXCL12‐stimulated signaling via its receptor CXCR4 in a ternary complex with the chemokine and receptor, consistent with our structural model. This first report of heterodimerization between chemokines and galectins reveals a new type of interaction between inflammatory mediators that can underlie a novel immunoregulatory mechanism in inflammation. Thus, further exploration of the chemokine/galectin interactome is warranted.

How galectins have become multifunctional proteins

Having identified glycans of cellular glycoconjugates as versatile molecular messages, their recognition by sugar receptors (lectins) is a fundamental mechanism within the flow of biological information. This type of molecular interplay is increasingly revealed to be involved in a wide range of (patho)physiological processes. To do so, it is a vital prerequisite that a lectin (and its expression) can develop more than a single skill, that is the general ability to bind glycans. By studying the example of vertebrate galectins as a model, a total of five relevant characteristics is disclosed: i) access to intra- and extracellular sites, ii) fine-tuned gene regulation (with evidence for co-regulation of counterreceptors) including the existence of variants due to alternative splicing or single nucleotide polymorphisms, iii) specificity to distinct glycans from the glycome with different molecular meaning, iv) binding capacity also to peptide motifs at different sites on the protein and v) diversity of modular architecture. They combine to endow these lectins with the capacity to serve as multi-purpose tools. Underscoring the arising broad-scale significance of tissue lectins, their numbers in terms of known families and group members have steadily grown by respective research that therefore unveiled a well-stocked toolbox. The generation of a network of (ga)lectins by evolutionary diversification affords the opportunity for additive/synergistic or antagonistic interplay in situ, an emerging aspect of (ga)lectin functionality. It warrants close scrutiny. The realization of the enormous potential of combinatorial permutations using the five listed features gives further efforts to understand the rules of functional glycomics/lectinomics a clear direction.

Binding of Gold(III) Porphyrin by the Pro-metastatic Regulatory Protein Human Galectin-3

Gold(III) porphyrin presents an attractive alternative to the use of, for example, cisplatin in chemotherapy. However, approaches that allow to selectively target cancer cells are highly sought. Many plant and mammalian lectins have been shown to bind oligosaccharide sequences of the aberrant glycosylation pattern found on cancerous tumors. For example human galectin-3, of the galectin family specific for β-galactoside, is overexpressed in the extracellular matrix of tumorigenous and metastatic tissues. We searched for non-carbohydrate ligands for galectin-3 that can guide a cytotoxic drug to the cancer cells by maintaining its affinity for tumor associated carbohydrate antigens. Previous findings showed that zinc tetrasulfonatophenylporphyrin can bind galectin-3 with sub-micromolar affinity without disturbing lactose binding. Gold(III) porphyrin is not only cytotoxic to cancer cells, it knows also a potential application as photosensitiser in photodynamic therapy. We investigated the binding of gold(III) porphyrin to galectin-3 using different biophysical interaction techniques and demonstrated a low micromolar affinity of human galectin-3 for the cytotoxic compound. Co-crystallization attempts in order to understand the binding mode of gold porphyrin to galectin-3 failed, but molecular docking emphasized a highly populated secondary binding site that does not hinder lactose or Thomsen Friendenreich disaccharide binding. This suggests that gold(III) porphyrin might significantly enhance its concentration and delivery to cancer cells by binding to human galectin-3 that keeps its orientation towards tumor associated carbohydrate antigens.

Galectin-3: is this member of a large family of multifunctional lectins (already) a therapeutic target?

Introduction: The discoveries that sugars are a highly versatile platform to generate biochemical messages and that glycan-specific receptors (lectins) are a link between these signals and their bioactivity explain the interest in endogenous lectins such as galectins. Their analysis is a highly dynamic field. It is often referred to as being promising for innovative drug design. Area covered: We present a primer to the concept of the sugar code by glycan-(ga)lectin recognition, followed by a survey on galectin-3 (considering common and distinct features within this family of multifunctional proteins expressed at various cellular sites and cell types). Finally, we discuss strategies capable of blocking (ga)lectin activity, with an eye on current challenges and inherent obstacles. Expert opinion: The emerging broad profile of homeostatic and pathophysiological bioactivities stimulates further efforts to explore galectin (Gal-3) functionality, alone and then in mixtures. Like thoroughly assessing the pros and cons of blocking approaches for a multifunctional protein active at different sites, identifying a clinical situation, in which the galectin is essential in the disease process, will be critical.

The sugar code: letters and vocabulary, writers, editors and readers and biosignificance of functional glycan-lectin pairing

Ubiquitous occurrence in Nature, abundant presence at strategically important places such as the cell surface and dynamic shifts in their profile by diverse molecular switches qualifies the glycans to serve as versatile biochemical signals. However, their exceptional structural complexity often prevents one noting how simple the rules of objective-driven assembly of glycan-encoded messages are. This review is intended to provide a tutorial for a broad readership. The principles of why carbohydrates meet all demands to be the coding section of an information transfer system, and this at unsurpassed high density, are explained. Despite appearing to be a random assortment of sugars and their substitutions, seemingly subtle structural variations in glycan chains by a sophisticated enzymatic machinery have emerged to account for their specific biological meaning. Acting as 'readers' of glycan-encoded information, carbohydrate-specific receptors (lectins) are a means to turn the glycans' potential to serve as signals into a multitude of (patho)physiologically relevant responses. Once the far-reaching significance of this type of functional pairing has become clear, the various modes of spatial presentation of glycans and of carbohydrate recognition domains in lectins can be explored and rationalized. These discoveries are continuously revealing the intricacies of mutually adaptable routes to achieve essential selectivity and specificity. Equipped with these insights, readers will gain a fundamental understanding why carbohydrates form the third alphabet of life, joining the ranks of nucleotides and amino acids, and will also become aware of the importance of cellular communication via glycan-lectin recognition.

A chimeric, multivalent assembly of galectin-1 and galectin-3 with enhanced extracellular activity

Galectins are attractive therapeutic candidates to control aberrant immune system activation because they can alter the phenotype and function of various innate and adaptive immune cells. However, use of exogenous galectin-1 ("G1") and galectin-3 ("G3") as immunomodulators is challenged by their high dosing requirements and dynamic quaternary structures. Here we report a chimeric assembly of G1 and G3 with enhanced extracellular activity ("G1/G3 Zipper"), which was created by recombinant fusion of G1 and G3 via a peptide linker that forms a two-stranded α-helical coiled-coil. G1/G3 Zipper had higher apparent binding affinity for immobilized lactose and a lower concentration threshold for inducing soluble glycoprotein crosslinking than G1, a recombinant fusion of G1 and G3 with a flexible peptide linker ("G1/G3"), or a recently reported stable G1 dimer crosslinked by poly(ethylene glycol) diacrylate ("G1-PEG-G1"). As a result, G1/G3 Zipper was more effective at inducing Jurkat T cell apoptosis in media containing serum, and was the only variant that could induce apoptosis at low concentrations under serum-free conditions. The monomeric G1/G3 fusion protein lacked extracellular activity under all conditions tested, suggesting that the enhanced activity of G1/G3 Zipper was due to its quaternary structure and increased carbohydrate-recognition domain valency. Thus, combining G1 and G3 into a non-native chimeric assembly provides a new candidate therapeutic with greater immunomodulatory potency than the wild-type proteins and previously reported engineered variants.

How altering the modular architecture affects aspects of lectin activity: case study on human galectin-1

Discoveries on involvement of glycan-protein recognition in many (patho)physiological processes are directing attention to exploring the significance of a fundamental structural aspect of sugar receptors beyond glycan specificity, i.e., occurrence of distinct types of modular architecture. In order to trace clues for defining design-functionality relationships in human lectins, a lectin's structural unit has been used as source material for engineering custom-made variants of the wild-type protein. Their availability facilitates comparative analysis toward the stated aim. With adhesion/growth-regulatory human galectin-1 as example, the strategy of evaluating how changes of its design (here, from the homodimer of non-covalently associated domains to (i) linker-connected di- and tetramers and (ii) a galectin-3-like protein) affect activity is illustrated by using three assay systems of increasing degree of glycan complexity. Whereas calorimetry with two cognate disaccharides and array testing with 647 (glyco)compounds disclosed no major changes, galectin histochemical staining profiles of tissue sections that present natural glycome complexity revealed differences between wild-type and linker-connected homo-oligomers as well as between the galectin-3-like variant and wild-type galectin-3 for cell-type positivity, level of intensity at the same site and susceptibility for inhibition by a bivalent glycocompound. These results underscore the strength of the documented approach. Moreover, they give direction to proceed to (i) extending its application to other members of this lectin family, especially galectin-3 and (ii) then analyzing impact of architectural alterations on cell surface lattice formation and ensuing biosignaling systematically, considering the variants' potential for translational medicine.

Placental Galectins Are Key Players in Regulating the Maternal Adaptive Immune Response

Galectins are potent immunomodulators that regulate maternal immune responses in pregnancy and prevent the rejection of the semi-allogeneic fetus that also occurs in miscarriages. We previously identified a gene cluster on Chromosome 19 that expresses a subfamily of galectins, including galectin-13 (Gal-13) and galectin-14 (Gal-14), which emerged in anthropoid primates. These galectins are expressed only by the placenta and induce the apoptosis of activated T lymphocytes, possibly contributing to a shifted maternal immune balance in pregnancy. The placental expression of Gal-13 and Gal-14 is decreased in preeclampsia, a life-threatening obstetrical syndrome partly attributed to maternal anti-fetal rejection. This study is aimed at revealing the effects of Gal-13 and Gal-14 on T cell functions and comparing the expression of these galectins in placentas from healthy pregnancies and miscarriages. First-trimester placentas were collected from miscarriages and elective termination of pregnancies, tissue microarrays were constructed, and then the expression of Gal-13 and Gal-14 was analyzed by immunohistochemistry and immunoscoring. Recombinant Gal-13 and Gal-14 were expressed and purified, and their effects were investigated on primary peripheral blood T cells. The binding of Gal-13 and Gal-14 to T cells and the effects of these galectins on apoptosis, activation marker (CD25, CD71, CD95, HLA-DR) expression and cytokine (IL-1β, IL-6, IL-8, IL-10, IFNγ) production of T cells were examined by flow cytometry. Gal-13 and Gal-14 are primarily expressed by the syncytiotrophoblast at the maternal-fetal interface in the first trimester, and their placental expression is decreased in miscarriages compared to first-trimester controls. Recombinant Gal-13 and Gal-14 bind to T cells in a population- and activation-dependent manner. Gal-13 and Gal-14 induce apoptosis of Th and Tc cell populations, regardless of their activation status. Out of the investigated activation markers, Gal-14 decreases the cell surface expression of CD71, Gal-13 increases the expression of CD25, and both galectins increase the expression of CD95 on T cells. Non-activated T cells produce larger amounts of IL-8 in the presence of Gal-13 or Gal-14. In conclusion, these results show that Gal-13 and Gal-14 already provide an immunoprivileged environment at the maternal-fetal interface during early pregnancy, and their reduced expression is related to miscarriages.

A digital data interpretation method for hemagglutination inhibition assay by using a plate reader

Hemagglutination inhibition (HAI) assay is a simple method quantifying relative binding activities of glycan-lectin interactions. Currently, interpretation of HAI data remains a manual task depending on visual observation. In this study we developed a digital data reading method for HAI assay by using the area scanning function of a microplate reader. This was based on galectin-3-induced hemagglutination inhibition assay. OD values showed a four-parameter logistic correlation with concentrations of galectin-3 inhibitors (R² ≥ 0.97), and IC₅₀ values were obtained from the curve fitting. The method provides an objective and robust data interpretation for HAI assays conducted with chicken erythrocytes.

Effects of linker and liposome anchoring on lactose-functionalized glycomacromolecules as multivalent ligands for binding galectin-3

We combine multivalent presentation of glycan ligands on sequence-defined oligo(amidoamines) and liposomes to achieve high avidity ligands targeting galectin-3.

Locally anchoring enzymes to tissues via extracellular glycan recognition

Success of enzymes as drugs requires that they persist within target tissues over therapeutically effective time frames. Here we report a general strategy to anchor enzymes at injection sites via fusion to galectin-3 (G3), a carbohydrate-binding protein. Fusing G3 to luciferase extended bioluminescence in subcutaneous tissue to ~7 days, whereas unmodified luciferase was undetectable within hours. Engineering G3-luciferase fusions to self-assemble into a trimeric architecture extended bioluminescence in subcutaneous tissue to 14 days, and intramuscularly to 3 days. The longer local half-life of the trimeric assembly was likely due to its higher carbohydrate-binding affinity compared to the monomeric fusion. G3 fusions and trimeric assemblies lacked extracellular signaling activity of wild-type G3 and did not accumulate in blood after subcutaneous injection, suggesting low potential for deleterious off-site effects. G3-mediated anchoring to common tissue glycans is expected to be broadly applicable for improving local pharmacokinetics of various existing and emerging enzyme drugs.

The use of enzymes as drugs requires that they persist within target tissues over therapeutically relevant time frames. Here the authors use a galectin-3 fusion to anchor enzymes at injection sites for up to 14 days.

Genetically Engineered Mucoadhesive Spider Silk

Mucoadhesion is defined as the adhesion of a material to the mucus gel covering the mucous membranes. The mechanisms controlling mucoadhesion include nonspecific electrostatic interactions and specific interactions between the materials and the mucins, the heavily glycosylated proteins that form the mucus gel. Mucoadhesive materials can be used to develop mucosal wound dressings and noninvasive transmucosal drug delivery systems. Spider silk, which is strong, biocompatible, biodegradable, nontoxic, and lightweight would serve as an excellent base for the development of such materials. Here, we investigated two variants of the partial spider silk protein 4RepCT genetically engineered in order to functionalize them with mucoadhesive properties. The pLys-4RepCT variant was functionalized with six cationically charged lysines, aiming to provide nonspecific adhesion from electrostatic interactions with the anionically charged mucins, while the hGal3-4RepCT variant was genetically fused with the Human Galectin-3 Carbohydrate Recognition Domain which specifically binds the mucin glycans Galβ1-3GlcNAc and Galβ1-4GlcNAc. First, we demonstrated that coatings, fibers, meshes, and foams can be readily made from both silk variants. Measured by the adsorption of both bovine submaxillary mucin and pig gastric mucin, the newly produced silk materials showed enhanced mucin binding properties compared with materials of wild-type (4RepCT) silk. Moreover, we showed that pLys-4RepCT silk coatings bind mucins through electrostatic interactions, while hGal3-4RepCT silk coatings bind mucins through specific glycan-protein interactions. We envision that the two new mucoadhesive silk variants pLys-4RepCT and hGal3-4RepCT, alone or combined with other biofunctional silk proteins, constitute useful new building blocks for a range of silk protein-based materials for mucosal treatments.

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.

Presenting Precision Glycomacromolecules on Gold Nanoparticles for Increased Lectin Binding

Glyco-functionalized gold nanoparticles have great potential as biosensors and as inhibitors due to their increased binding to carbohydrate-recognizing receptors such as the lectins. Here we apply previously developed solid phase polymer synthesis to obtain a series of precision glycomacromolecules that allows for straightforward variation of their chemical structure as well as functionalization of gold nanoparticles by ligand exchange. A novel building block is introduced allowing for the change of spacer building blocks within the macromolecular scaffold going from an ethylene glycol unit to an aliphatic spacer. Furthermore, the valency and overall length of the glycomacromolecule is varied. All glyco-functionalized gold nanoparticles show high degree of functionalization along with high stability in buffer solution. Therefore, a series of measurements applying UV-Vis spectroscopy, dynamic light scattering (DLS) and surface plasmon resonance (SPR) were performed studying the aggregation behavior of the glyco-functionalized gold nanoparticles in presence of model lectin Concanavalin A. While the multivalent presentation of glycomacromolecules on gold nanoparticles (AuNPs) showed a strong increase in binding compared to the free ligands, we also observed an influence of the chemical structure of the ligand such as its valency or hydrophobicity on the resulting lectin interactions. The straightforward variation of the chemical structure of the precision glycomacromolecule thus gives access to tailor-made glyco-gold nanoparticles (glyco-AuNPs) and fine-tuning of their lectin binding properties.

Key regulators of galectin-glycan interactions

Protein-ligand interactions serve as fundamental regulators of numerous biological processes. Among protein-ligand pairs, glycan binding proteins (GBPs) and the glycans they recognize represent unique and highly complex interactions implicated in a broad range of regulatory activities. With few exceptions, cell surface receptors and secreted proteins are heavily glycosylated. As these glycans often represent highly regulatable post-translational modifications, alterations in glycosylation can fundamentally impact GBP recognition. Among GBPs, galectins in particular appear to engage a diverse set of glycan determinants to impact a broad range of biological processes. In this review, we will explore factors that impact galectin activity, including the effect of glycan modification on galectin-glycan interactions.

Bivalent Llama Single-Domain Antibody Fragments against Tumor Necrosis Factor Have Picomolar Potencies due to Intramolecular Interactions

The activity of tumor necrosis factor (TNF), a cytokine involved in inflammatory pathologies, can be inhibited by antibodies or trap molecules. Herein, llama-derived variable heavy-chain domains of heavy-chain antibody (VHH, also called Nanobodies™) were generated for the engineering of bivalent constructs, which antagonize the binding of TNF to its receptors with picomolar potencies. Three monomeric VHHs (VHH#1, VHH#2, and VHH#3) were characterized in detail and found to bind TNF with sub-nanomolar affinities. The crystal structures of the TNF-VHH complexes demonstrate that VHH#1 and VHH#2 share the same epitope, at the center of the interaction area of TNF with its TNFRs, while VHH#3 binds to a different, but partially overlapping epitope. These structures rationalize our results obtained with bivalent constructs in which two VHHs were coupled via linkers of different lengths. Contrary to conventional antibodies, these bivalent Nanobody™ constructs can bind to a single trimeric TNF, thus binding with avidity and blocking two of the three receptor binding sites in the cytokine. The different mode of binding to antigen and the engineering into bivalent constructs supports the design of highly potent VHH-based therapeutic entities.

Network analysis of adhesion/growth-regulatory galectins and their binding sites in adult chicken retina and choroid

The highly ordered multilayered organization of the adult chicken retina is a suitable test model for examining zonal distribution of the members of a bioeffector family. Based on the concept of the sugar code, the functional pairing of glycan epitopes with cognate receptors (lectins) is emerging as a means to explain the control of diverse physiological activities. Having recently completed the biochemical characterization of all seven adhesion/growth-regulatory galectins present in chicken, it was possible to establish how the individual characteristics of their expression profiles add up to shape the galectin network, which until now has not been defined at this level of complexity. This information will also have relevance in explaining the region-specific presence of glycan determinants in the retina, as illustrated in the first part of this study using a panel of nine plant/fungal agglutinins. The following systematic monitoring of the galectins yielded patterns for which quantitative and qualitative differences were detected. Obviously, positivity in distinct layers is not confined to a single protein of this family, e.g. CG-1A, CG-3 or CG-8. These results underline the requirement for network analysis for these proteins that can functionally interact in additive or antagonistic modes. Labeling of the tissue galectins facilitated profiling of their accessible binding sites. It also revealed differences among the galectin family members, highlighting the ability of this method to define binding properties on the level of tissue sections. Methodologically, the detection of endogenous lectins intimates that cognate glycans can become inaccessible, a notable caveat for lectin histochemical studies.

Biotinylated N-Acetyllactosamine- and N,N-Diacetyllactosamine-Based Oligosaccharides as Novel Ligands for Human Galectin-3

Galectin inhibitor design is an emerging research field due to the involvement of galectins in cancer. Galectin-3, in particular, plays an important role in tumor progression. To generate inhibitors, modifications of the glycan structure can be introduced. Conjugation of hydrophobic compounds to saccharides has proven to be promising as increased binding of galectin-3 can be observed. In the present study, we report on neo-glycans carrying hydrophobic biotin as novel ligands for human galectin-3. We modified N-acetyllactosamine- and N,N-diacetyllactosamine-based tetrasaccharides at the C6-position of the terminal saccharide unit using selective enzymatic oxidation and subsequent chemical conjugation of biotinamidohexanoic acid hydrazide. These neo-glycans were much better bound by galectin-3 than the unmodified counterparts. High selectivity for galectin-3 over galectin-1 was also proven. We generated multivalent neo-glycoproteins by conjugation of neo-glycans to bovine serum albumin showing high affinity for galectin-3. Compared to non-biotinylated neo-glycoproteins, we achieved high binding levels of galectin-3 with a lesser amount of conjugated neo-glycans. Multivalent ligand presentation of neo-glycoproteins significantly increased the inhibitory potency towards galectin-3 binding to asialofetuin when compared to free monovalent glycans. Our findings show the positive impact of 6-biotinylation of tetrasaccharides on galectin-3 binding, which broadens the recent design approaches for producing high-affinity ligands.

Teaming up synthetic chemistry and histochemistry for activity screening in galectin-directed inhibitor design

A hallmark of endogenous lectins is their ability to select a few distinct glycoconjugates as counterreceptors for functional pairing from the natural abundance of cellular glycoproteins and glycolipids. As a consequence, assays to assess inhibition of lectin binding should necessarily come as close as possible to the physiological situation, to characterize an impact of a synthetic compound on biorelevant binding with pharmaceutical perspective. We here introduce in a proof-of-principle manner work with sections of paraffin-embedded tissue (jejunum, epididymis) and labeled adhesion/growth-regulatory galectins, harboring one (galectin-1 and galectin-3) or two (galectin-8) types of lectin domain. Six pairs of synthetic lactosides from tailoring of the headgroup (3'-O-sulfation) and the aglycone (β-methyl to aromatic S- and O-linked extensions) as well as three bi- to tetravalent glycoclusters were used as test compounds. Varying extents of reduction in staining intensity by synthetic compounds relative to unsubstituted/free lactose proved the applicability and sensitivity of the method. Flanking cytofluorimetric assays on lectin binding to native cells gave similar grading, excluding a major impact of tissue fixation. The experiments revealed cell/tissue binding of galectin-8 preferentially via one domain, depending on the cell type so that the effect of an inhibitor in a certain context cannot be extrapolated to other cells/tissues. Moreover, the work with the other galectins attests that this assay enables comprehensive analysis of the galectin network in serial tissue sections to determine overlaps and regional differences in inhibitory profiles.

Innate immunity against molecular mimicry: Examining galectin-mediated antimicrobial activity

Adaptive immunity provides the unique ability to respond to a nearly infinite range of antigenic determinants. Given the inherent plasticity of the adaptive immune system, a series of tolerance mechanisms exist to reduce reactivity toward self. While this reduces the probability of autoimmunity, it also creates an important gap in adaptive immunity: the ability to recognize microbes that look like self. As a variety of microbes decorate themselves in self-like carbohydrate antigens and tolerance reduces the ability of adaptive immunity to react with self-like structures, protection against molecular mimicry likely resides within the innate arm of immunity. In this review, we will explore the potential consequences of microbial molecular mimicry, including factors within innate immunity that appear to specifically target microbes expressing self-like antigens, and therefore provide protection against molecular mimicry.