Discovery of Selective and Orally Available Galectin-1 Inhibitors

A new series of orally available α-d-galactopyranosides with high affinity and specificity toward galectin-1 have been discovered. High affinity and specificity were achieved by changing six-membered aryl-triazolyl substituents in a series of recently published galectin-3-selective α-d-thiogalactosides (e.g., GB1107 Kd galectin-1/3 3.7/0.037 μM) for five-membered heterocycles such as thiazoles. The in vitro pharmacokinetic properties were optimized, resulting in several galectin-1 inhibitors with favorable properties. One compound, GB1490 (Kd galectin-1/3 0.4/2.7 μM), was selected for further characterization toward a panel of galectins showing a selectivity of 6- to 320-fold dependent on galectin. The X-ray structure of GB1490 bound to galectin-1 reveals the compound bound in a single conformation in the carbohydrate binding site. GB1490 was shown to reverse galectin-1-induced apoptosis of Jurkat cells at low μM concentrations. No cell cytotoxicity was observed for GB1490 up to 90 μM in the A549 cells. In pharmacokinetic studies in mice, GB1490 showed high oral bioavailability (F% > 99%).

Alkylation of Galectin-1 with Iodoacetamide and Mass Spectrometric Mapping of the Sites of Incorporation

Galectins can display unique sensitivity to oxidative changes that result in significant conformational alterations that prevent carbohydrate recognition. While a variety of approaches can be utilized to prevent galectin oxidation, several of these require inclusion of reducing agents that not only prevent galectins from undergoing oxidative inactivation but can also interfere with normal redox potentials required for fundamental cellular processes. To overcome the limitations associated with placing cells in an artificial reducing environment, cysteine residues on galectins can be directly alkylated with iodoacetamide to form a stable thioether adduct that is resistant to further modification. Iodoacetamide alkylated galectin remains stable over prolonged periods of time and retains the carbohydrate binding and biological activities of the protein. As a result, this approach allows examination of the biological roles of a stabilized form of galectin-1 without introducing the confounding variables that can occur when typical soluble reducing agents are employed.

Cross-Linking Effects Dictate the Preference of Galectins to Bind LacNAc-Decorated HPMA Copolymers

The interaction of multi-LacNAc (Galβ1-4GlcNAc)-containing N-(2-hydroxypropyl) methacrylamide (HPMA) copolymers with human galectin-1 (Gal-1) and the carbohydrate recognition domain (CRD) of human galectin-3 (Gal-3) was analyzed using NMR methods in addition to cryo-electron-microscopy and dynamic light scattering (DLS) experiments. The interaction with individual LacNAc-containing components of the polymer was studied for comparison purposes. For Gal-3 CRD, the NMR data suggest a canonical interaction of the individual small-molecule bi- and trivalent ligands with the lectin binding site and better affinity for the trivalent arrangement due to statistical effects. For the glycopolymers, the interaction was stronger, although no evidence for forming a large supramolecule was obtained. In contrast, for Gal-1, the results indicate the formation of large cross-linked supramolecules in the presence of multivalent LacNAc entities for both the individual building blocks and the polymers. Interestingly, the bivalent and trivalent presentation of LacNAc in the polymer did not produce such an increase, indicating that the multivalency provided by the polymer is sufficient for triggering an efficient binding between the glycopolymer and Gal-1. This hypothesis was further demonstrated by electron microscopy and DLS methods.

Degraded Arabinogalactans and Their Binding Properties to Cancer-Associated Human Galectins

Galectins represent β-galactoside-binding proteins with numerous functions. Due to their role in tumor progression, human galectins-1, -3 and -7 (Gal-1, -3 and -7) are potential targets for cancer therapy. As plant derived glycans might act as galectin inhibitors, we prepared galactans by partial degradation of plant arabinogalactan-proteins. Besides commercially purchased galectins, we produced Gal-1 and -7 in a cell free system and tested binding capacities of the galectins to the galactans by biolayer-interferometry. Results for commercial and cell-free expressed galectins were comparable confirming functionality of the cell-free produced galectins. Our results revealed that galactans from Echinacea purpurea bind to Gal-1 and -7 with K_D values of 1–2 µM and to Gal-3 slightly stronger with K_D values between 0.36 and 0.70 µM depending on the sensor type. Galactans from the seagrass Zostera marina with higher branching of the galactan and higher content of uronic acids showed stronger binding to Gal-3 (0.08–0.28 µM) compared to galactan from Echinacea. The results contribute to knowledge on interactions between plant polysaccharides and galectins. Arabinogalactan-proteins have been identified as a new source for production of galactans with possible capability to act as galectin inhibitors.

Unravelling the Time Scale of Conformational Plasticity and Allostery in Glycan Recognition by Human Galectin-1

The interaction of human galectin-1 with a variety of oligosaccharides, from di-(N-acetyllactosamine) to tetra-saccharides (blood B type-II antigen) has been scrutinized by using a combined approach of different NMR experiments, molecular dynamics (MD) simulations, and isothermal titration calorimetry. Ligand- and receptor-based NMR experiments assisted by computational methods allowed proposing three-dimensional structures for the different complexes, which explained the lack of enthalpy gain when increasing the chemical complexity of the glycan. Interestingly, and independently of the glycan ligand, the entropy term does not oppose the binding event, a rather unusual feature for protein-sugar interactions. CLEANEX-PM and relaxation dispersion experiments revealed that sugar binding affected residues far from the binding site and described significant changes in the dynamics of the protein. In particular, motions in the microsecond-millisecond timescale in residues at the protein dimer interface were identified in the presence of high affinity ligands. The dynamic process was further explored by extensive MD simulations, which provided additional support for the existence of allostery in glycan recognition by human galectin-1.

An amperometric biosensor based on Cu2O@Au nanocomposites for the detection of galectin-1 via lactose-galectin interactions

In this work, a novel label-free electrochemical biosensor is developed for the detection of galectin-1 (Gal-1) based on gold nanoparticle (AuNP) loaded octahedral Cu₂O (Cu₂O@Au) nanocomposites. The AuNPs on the surface of the Cu₂O nanocrystals not only enhance the electrochemical performance, but also serve as the binding sites for the lactose ligand which can specifically bind with Gal-1. The Cu₂O@Au nanocomposites provide the synergic effect of electrochemical signal amplification and lactose-galectin reaction as the recognition strategy. Under optimal conditions, the proposed biosensor exhibits a variation of electrochemical responses to different concentrations of Gal-1 ranging from 0.1 pg ml^-1 to 10 ng ml^-1. This work presents an alternative electrochemical biosensor for the detection of tumor biomarkers based on a simple and economical lactose ligand incorporated Cu₂O@Au biosensor platform.

Molecular characterization, tissue distribution and functional analysis of galectin 1-like 2 in grass carp (Ctenopharyngodon idella)

Galectins, as an evolutionary conserved group of lectin superfamily, has the functions of pathogen recognition, anti-bacteria and anti-virus. In this study, a 405 bp cDNA sequence of galectin 1-like 2 (CiGal1-L2) was obtained from grass carp (Ctenopharyngodon idella), which encoded 134 amino acids with a predicted molecular mass of 15.143 kDa and an isoelectric point of 5.33. The sugar binding motifs (H-N-R, V-N and W--E-R) were detected in carbohydrate-binding domain (CRD). The amino acid sequence similarity showed that CiGal1-L2 was 40.30-42.54% and 66.42-81.20% similarity to mammalian and fish counterparts, respectively. The phylogenetic tree showed that CiGal1-L2 was clustered with fish galectin-1s and closely related to Cyprinus carpio. Real-time quantitative PCR (RT-qPCR) analysis revealed that CiGal1-L2 was widely expressed in all tested tissues. In addition, the expression of CiGal1-L2 was differentially up-regulated challenged with grass carp reovirus (GCRV), lipopolysaccharide (LPS) and polyinosinic:polycytidylic acid (poly I:C). The fluorescence of CiGal1-L2-GFP was distributed in the cytoplasm and nucleus of HEK 293T cells and showed a trend of nuclear translocation after LPS and poly I:C treatment. Finally, the recombinant CiGal1-L2 (rCiGal1-L2) protein showed strong binding ability to LPS. In conclusion, the results provided further insight into the immune roles of galectin-1 in teleost.

How presence of a signal peptide affects human galectins-1 and -4: Clues to explain common absence of a leader sequence among adhesion/growth-regulatory galectins

BACKGROUND

Galectins are multifunctional effectors, which all share absence of a signal sequence. It is not clear why galectins belong to the small set of proteins, which avoid the classical export route.

METHODS

Products of recombinant galectin expression in P. pastoris were analyzed by haemagglutination, gel filtration and electrophoresis and lectin blotting as well as mass spectrometry on the level of tryptic peptides and purified glycopeptides(s). Density gradient centrifugation and confocal laser scanning microscopy facilitated localization in transfected human and rat cells, proliferation assays determined activity as growth mediator.

RESULTS

Directing galectin-1 to the classical secretory pathway in yeast produces N-glycosylated protein that is active. It cofractionates and -localizes with calnexin in human cells, only Gal-4 is secreted. Presence of N-glycan(s) reduces affinity of cell binding and growth regulation by Gal-1.

CONCLUSIONS

Folding and activity of a galectin are maintained in signal-peptide-directed routing, N-glycosylation occurs. This pathway would deplete cytoplasm and nucleus of galectin, presence of N-glycans appears to interfere with lattice formation.

GENERAL SIGNIFICANCE

Availability of glycosylated galectins facilitates functional assays to contribute to explain why galectins invariably avoid classical routing for export.

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.

Design-functionality relationships for adhesion/growth-regulatory galectins

Glycan-lectin recognition is assumed to elicit its broad range of (patho)physiological functions via a combination of specific contact formation with generation of complexes of distinct signal-triggering topology on biomembranes. Faced with the challenge to understand why evolution has led to three particular modes of modular architecture for adhesion/growth-regulatory galectins in vertebrates, here we introduce protein engineering to enable design switches. The impact of changes is measured in assays on cell growth and on bridging fully synthetic nanovesicles (glycodendrimersomes) with a chemically programmable surface. Using the example of homodimeric galectin-1 and monomeric galectin-3, the mutual design conversion caused qualitative differences, i.e., from bridging effector to antagonist/from antagonist to growth inhibitor and vice versa. In addition to attaining proof-of-principle evidence for the hypothesis that chimera-type galectin-3 design makes functional antagonism possible, we underscore the value of versatile surface programming with a derivative of the pan-galectin ligand lactose. Aggregation assays with N,N'-diacetyllactosamine establishing a parasite-like surface signature revealed marked selectivity among the family of galectins and bridging potency of homodimers. These findings provide fundamental insights into design-functionality relationships of galectins. Moreover, our strategy generates the tools to identify biofunctional lattice formation on biomembranes and galectin-reagents with therapeutic potential.

An introduction to the sugar code

Carbohydrates have physiological importance far beyond their roles as source of energy (glycolysis) and activated hydrogen for synthesis (pentosephosphate pathway) or as constituent of the backbone of nucleic acids and of cell wall polysaccharides. The extent of compositional and structural variability of their oligomers (glycans) is unsurpassed in Nature due to the unique property of independently combining the following parameters with sequence: anomeric status, linkage positions, ring size, addition of branches and site-specific introduction of substitutions. The monosaccharides (letters of the third alphabet of life) thus generate 'words' (signals) of high-density coding capacity. These 'words' are part of the glycans on proteins and lipids, and the glycome represented by these 'words' in their entirety has cell type-dependent features. The often limited intramolecular flexibility of oligosaccharides along with an abundance of contact points for intermolecular interactions is ideal for binding processes. Glycan-based 'words' can thus be 'read,' and their message translated into cellular effects by receptors called lectins. This journal's special issue covers central aspects of the concept of the sugar code.

Molecular cloning and characterization of a galectin-1 homolog in orange-spotted grouper, Epinephelus coioides

As a member of animal lectin family, galectin has the functions of pathogen recognition, anti-bacteria and anti-virus. In the present study, a galectin-1 homolog (EcGel-1) from grouper (Epinephelus coioides) was cloned and its possible role in fish immunity was analyzed. The full length cDNA of EcGel-1 is 504 bp, including a 408 bp open reading frame (ORF) which encodes 135 amino acids with a molecular mass of 15.19 kDa. Quantitative real-time PCR analysis indicated that EcGel-1 was constitutively expressed in all analyzed tissues of healthy grouper. The expression of EcGel-1 in the spleen of grouper was differentially up-regulated challenged with Singapore grouper iridovirus (SGIV), poly (I:C), and LPS. EcGel-1 was abundantly distributed in the cytoplasm in GS cells. Recombinant EcGel-1(rEcGel-1) protein can make chicken erythrocyte aggregation, and combine with gram negative bacteria and gram positive bacteria in the presence of 2-Mercaptoethanol (β-ME). Taken together, the results showed that EcGel-1 may be an important molecule involved in pathogen recognition and pathogen elimination in the innate immunity of grouper.

Galectin-1 dimers can scaffold Raf-effectors to increase H-ras nanoclustering

Galectin-1 (Gal-1) dimers crosslink carbohydrates on cell surface receptors. Carbohydrate-derived inhibitors have been developed for cancer treatment. Intracellularly, Gal-1 was suggested to interact with the farnesylated C-terminus of Ras thus specifically stabilizing GTP-H-ras nanoscale signalling hubs in the membrane, termed nanoclusters. The latter activity may present an alternative mechanism for how overexpressed Gal-1 stimulates tumourigenesis. Here we revise the current model for the interaction of Gal-1 with H-ras. We show that it indirectly forms a complex with GTP-H-ras via a high-affinity interaction with the Ras binding domain (RBD) of Ras effectors. A computationally generated model of the Gal-1/C-Raf-RBD complex is validated by mutational analysis. Both cellular FRET as well as proximity ligation assay experiments confirm interaction of Gal-1 with Raf proteins in mammalian cells. Consistently, interference with H-rasG12V-effector interactions basically abolishes H-ras nanoclustering. In addition, an intact dimer interface of Gal-1 is required for it to positively regulate H-rasG12V nanoclustering, but negatively K-rasG12V nanoclustering. Our findings suggest stacked dimers of H-ras, Raf and Gal-1 as building blocks of GTP-H-ras-nanocluster at high Gal-1 levels. Based on our results the Gal-1/effector interface represents a potential drug target site in diseases with aberrant Ras signalling.

Antimicrobial response of galectin-1 from rock bream Oplegnathus fasciatus: Molecular, transcriptional, and biological characterization

In this study, we describe the identification and characterization of a proto type galectin, galectin-1, from rock bream Oplegnathus fasciatus (OfGal-1). Galectins are evolutionarily conserved carbohydrate binding lectins that show a wide range of functions related to development and immune physiology. They have been identified as pattern recognition receptors of innate immune system that recognize a broad range of microbes. OfGal-1 cDNA comprised of 993 bp with an open reading frame of 408 bp that encodes 135 amino acids. A single carbohydrate recognition domain was present in the OfGal-1 amino acid sequence. The sequence comparison by multiple and pairwise alignments and the phylogenetic tree emphasized the strong evolutionary conservation of Gal-1. The typical β-sandwich structure was identified from the predicted tertiary structure. The constitutive expression of mRNA transcripts was detected in a wide range of tissues examined, with the highest expression in the heart. Immune challenges with live bacteria (Edwardsiella tarda and Streptococcus iniae), rock bream irido virus, and mitogens (lipopolysaccharide and poly I:C) modulated the expression of OfGal-1 mRNAs in the gills, head kidney, and liver. The recombinant OfGal-1 (rOfGal-1) strongly agglutinatinated the human erythrocytes, and this hemagglutination was inhibited by lactose and D-galactose. A wide range of bacteria (S. iniae, S. parauberis, Escherichia coli, Edwardsiella tarda, Vibrio anguillarum, Vibrio harveyi, and Vibrio tapetis) and a ciliate (Miamiensis avidus) were also effectively recognized by rOfGal-1. Significant antiviral activity against rock bream irido virus was also demonstrated by rOfGal-1. Collectively, results from the present study indicate that OfGal-1 can recognize a wide range of microbes and is a vital pattern recognition receptor in the innate immune system of rock bream.

Anti-Retroviral Lectins Have Modest Effects on Adherence of Trichomonas vaginalis to Epithelial Cells In Vitro and on Recovery of Tritrichomonas foetus in a Mouse Vaginal Model

Trichomonas vaginalis causes vaginitis and increases the risk of HIV transmission by heterosexual sex, while Tritrichomonas foetus causes premature abortion in cattle. Our goals were to determine the effects, if any, of anti-retroviral lectins, which are designed to prevent heterosexual transmission of HIV, on adherence of Trichomonas to ectocervical cells and on Tritrichomonas infections in a mouse model. We show that Trichomonas Asn-linked glycans (N-glycans), like those of HIV, bind the mannose-binding lectin (MBL) that is part of the innate immune system. N-glycans of Trichomonas and Tritrichomonas bind anti-retroviral lectins (cyanovirin-N and griffithsin) and the 2G12 monoclonal antibody, each of which binds HIV N-glycans. Binding of cyanovirin-N appears to be independent of susceptibility to metronidazole, the major drug used to treat Trichomonas. Anti-retroviral lectins, MBL, and galectin-1 cause Trichomonas to self-aggregate and precipitate. The anti-retroviral lectins also increase adherence of ricin-resistant mutants, which are less adherent than parent cells, to ectocervical cell monolayers and to organotypic EpiVaginal tissue cells. Topical application of either anti-retroviral lectins or yeast N-glycans decreases by 40 to 70% the recovery of Tritrichomonas from the mouse vagina. These results, which are explained by a few simple models, suggest that the anti-retroviral lectins have a modest potential for preventing or treating human infections with Trichomonas.

Alkylation of galectin-1 with iodoacetamide and mass spectrometric mapping of the sites of incorporation

Galectins can display unique sensitivity to oxidative changes that result in significant conformational alterations that prevent carbohydrate recognition. While a variety of approaches can be utilized to prevent galectin oxidation, several of these require inclusion of reducing agents that not only prevent galectins from undergoing oxidative inactivation, but can also interfere with normal redox potentials required for fundamental cellular processes. To overcome limitations associated with placing cells in an artificial reducing environment, cysteine residues on galectins can be directly alkylated with iodoacetamide to form a stable thioether adduct that is resistant to further modification. Iodoacetamide alkylated galectin remains stable over prolonged periods of time and retains the carbohydrate binding and biological activities of the native protein. As a result, this approach allows examination of the biological roles of a stabilized form of galectin-1 without introducing the confounding variables that can occur when typical soluble reducing agents are employed.

Anti-viral activity of galectin-1 from flounder Paralichthys olivaceus

Galectins are a family of Ca(2+)-independent soluble lectins characterized by their affinity to β-galactosides. Mammalian galectins have been shown to play a defense role against certain bacteria, fungi and viruses. However, the immunological functions of galectins in fish is poorly characterized. Here we demonstrated that the expression of galectin-1 gene from the flounder Paralichthys olivaceus was decreased in the initial 8 h after challenge with poly I:C, then increased markedly from 24 h onwards, and the recombinant galectin-1 was able to neutralize the lymphocystis disease virus (LCDV), inhibiting the formation of cytopathic effects. In addition, the recombinant galectin had a potential anti-inflammatory activity against infection by LCDV, and was able to restrain the overexpression of the anti-viral protein gene mx against virus infection. These results indicate that flounder galectin-1 has an anti-viral activity, capable of reducing LCDV pathogenicity.

Structural features for α-galactomannan binding to galectin-1

Galectins have a highly conserved carbohydrate-binding domain to which a variety of galactose-containing saccharides, both β- and α-galactosides, can interact with varying degrees of affinity. Recently, we demonstrated that the relatively large α(1 → 6)-D-galacto-β(1 → 4)-D-mannan (Davanat) binds galectin-1 (gal-1) primarily at an alternative carbohydrate-binding domain. Here, we used a series of α-galactomannans (GMs) that vary in their mannose-to-galactose ratios for insight into an optimal structural signature for GM binding to gal-1. Heteronuclear single-quantum coherence nuclear magnetic resonance spectroscopy with (15)N-labeled gal-1 and statistical modeling suggest that the optimal signature consists of α-D-galactopyranosyl doublets surrounded by regions of about four or more "naked" mannose residues. These relatively large and complex GMs all appear to interact with varying degrees at essentially the same binding surface on gal-1 that includes the Davanat alternative binding site and elements of the canonical β-galactoside-binding region. The use of two small, well-defined GMs [6(1)-α(1 → 6)-D-galactosyl-β-D-mannotriaose and 6(3),6(4)-di-α(1 → 6)-D-galactosyl-β-D-mannopentaose] helped characterize how GMs, in general, interact in part with the canonical site. Overall, our findings contribute to better understanding interactions of gal-1 with larger, complex polysaccharides and to the development of GM-based therapeutics for clinical use.

Galectin-1-binding glycoforms of haptoglobin with altered intracellular trafficking, and increase in metastatic breast cancer patients

Sera from 25 metastatic breast cancer patients and 25 healthy controls were subjected to affinity chromatography using immobilized galectin-1. Serum from the healthy subjects contained on average 1.2 mg per ml (range 0.7-2.2) galectin-1 binding glycoproteins, whereas serum from the breast cancer patients contained on average 2.2 mg/ml (range 0.8-3.9), with a higher average for large primary tumours. The major bound glycoproteins were α-2-macroglobulin, IgM and haptoglobin. Both the IgM and haptoglobin concentrations were similar in cancer compared to control sera, but the percentage bound to galectin-1 was lower for IgM and higher for haptoglobin: about 50% (range 20-80) in cancer sera and about 30% (range 25-50) in healthy sera. Galectin-1 binding and non-binding fractions were separated by affinity chromatography from pooled haptoglobin from healthy sera. The N-glycans of each fraction were analyzed by mass spectrometry, and the structural differences and galectin-1 mutants were used to identify possible galectin-1 binding sites. Galectin-1 binding and non-binding fractions were also analyzed regarding their haptoglobin function. Both were similar in forming complex with haemoglobin and mediate its uptake into alternatively activated macrophages. However, after uptake there was a dramatic difference in intracellular targeting, with the galectin-1 non-binding fraction going to a LAMP-2 positive compartment (lysosomes), while the galectin-1 binding fraction went to larger galectin-1 positive granules. In conclusion, galectin-1 detects a new type of functional biomarker for cancer: a specific type of glycoform of haptoglobin, and possibly other serum glycoproteins, with a different function after uptake into tissue cells.

Structural basis for distinct binding properties of the human galectins to Thomsen-Friedenreich antigen

The Thomsen-Friedenreich (TF or T) antigen, Galβ1-3GalNAcα1-O-Ser/Thr, is the core 1 structure of O-linked mucin type glycans appearing in tumor-associated glycosylation. The TF antigen occurs in about 90% of human cancer cells and is a potential ligand for the human endogenous galectins. It has been reported that human galectin-1 (Gal-1) and galectin-3 (Gal-3) can perform their cancer-related functions via specifically recognizing TF antigen. However, the detailed binding properties have not been clarified and structurally characterized. In this work, first we identified the distinct TF-binding abilities of Gal-1 and Gal-3. The affinity to TF antigen for Gal-3 is two orders of magnitude higher than that for Gal-1. The structures of Gal-3 carbohydrate recognition domain (CRD) complexed with TF antigen and derivatives, TFN and GM1, were then determined. These structures show a unique Glu-water-Arg-water motif-based mode as previously observed in the mushroom galectin AAL. The observation demonstrates that this recognition mode is commonly adopted by TF-binding galectins, either as endogenous or exogenous ones. The detailed structural comparisons between Gal-1 and Gal-3 CRD and mutagenesis experiments reveal that a pentad residue motif (⁵¹AHGDA⁵⁵) at the loop (g1-L4) connecting β-strands 4 and 5 of Gal-1 produces a serious steric hindrance for TF binding. This motif is the main structural basis for Gal-1 with the low affinity to TF antigen. These findings provide the intrinsic structural elements for regulating the TF-binding activity of Gal-1 in some special conditions and also show certain target and approach for mediating some tumor-related bioactivities of human galectins.

Disaccharide binding to galectin-1: free energy calculations and molecular recognition mechanism

Galectin-1, a member of the conserved family of carbohydrate-binding proteins with affinity for β-galactosides, is a key modulator of diverse cell functions such as immune response and regulation. The binding affinity and specificity of galectin-1 for eight different β-galactosyl terminal disaccharides was studied using molecular-dynamics simulations in which the ligand was pulled away from the binding site using a mechanical force. We present what we believe to be a novel procedure, based on combinations of multistep trajectories, that was used to estimate the binding free energy (ΔG) of each disaccharide. The computed binding free energy differences show excellent correlation with experimental values determined previously. The small differences in affinity among the disaccharides are the result of an exquisite balance between the strengths of the galectin-sugar H-bonds and the H-bonds the protein and the disaccharides make with the solvent. Analysis of the free energies along the reaction coordinate shows that disaccharide unbinding/binding presents no energetic barrier and, therefore, is diffusion-limited. In addition, the calculations revealed that as the ligand is undocked from the binding site, breaking of protein-disaccharide H-bonds takes place in stages with intermediate states in which the interactions are bridged by water molecules.

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

The expression of recombinant proteins in Escherichia coli often leads to inactive aggregated proteins known as the inclusion bodies. To date, the best available tool has been the use of fusion tags, including the carbohydrate-binding protein; e.g., the maltose-binding protein (MBP) that enhances the solubility of recombinant proteins. However, none of these fusion tags work universally with every partner protein. We hypothesized that galectins, which are also carbohydrate-binding proteins, may help as fusion partners in folding the mammalian proteins in E. coli. Here we show for the first time that a small soluble lectin, human galectin-1, one member of a large galectin family, can function as a fusion partner to produce soluble folded recombinant human glycosyltransferase, beta-1,4-galactosyltransferase-7 (beta4Gal-T7), in E. coli. The enzyme beta4Gal-T7 transfers galactose to xylose during the synthesis of the tetrasaccharide linker sequence attached to a Ser residue of proteoglycans. Without a fusion partner, beta4Gal-T7 is expressed in E. coli as inclusion bodies. We have designed a new vector construct, pLgals1, from pET-23a that includes the sequence for human galectin-1, followed by the Tev protease cleavage site, a 6x His-coding sequence, and a multi-cloning site where a cloned gene is inserted. After lactose affinity column purification of galectin-1-beta4Gal-T7 fusion protein, the unique protease cleavage site allows the protein beta4Gal-T7 to be cleaved from galectin-1 that binds and elutes from UDP-agarose column. The eluted protein is enzymatically active, and shows CD spectra comparable to the folded beta4Gal-T1. The engineered galectin-1 vector could prove to be a valuable tool for expressing other proteins in E. coli.

NMR and MD investigations of human galectin-1/oligosaccharide complexes

The specific recognition of carbohydrates by lectins plays a major role in many cellular processes. Galectin-1 belongs to a family of 15 structurally related beta-galactoside binding proteins that are able to control a variety of cellular events, including cell cycle regulation, adhesion, proliferation, and apoptosis. The three-dimensional structure of galectin-1 has been solved by x-ray crystallography in the free form and in complex with various carbohydrate ligands. In this work, we used a combination of two-dimensional NMR titration experiments and molecular-dynamics simulations with explicit solvent to study the mode of interaction between human galectin-1 and five galactose-containing ligands. Isothermal titration calorimetry measurements were performed to determine their affinities for galectin-1. The contribution of the different hexopyranose units in the protein-carbohydrate interaction was given particular consideration. Although the galactose moiety of each oligosaccharide is necessary for binding, it is not sufficient by itself. The nature of both the reducing sugar in the disaccharide and the interglycosidic linkage play essential roles in the binding to human galectin-1.

Synergistic targeting of alphavbeta3 integrin and galectin-1 with heteromultivalent paramagnetic liposomes for combined MR imaging and treatment of angiogenesis

Effective and specific targeting of nanoparticles is of paramount importance in the fields of targeted therapeutics and diagnostics. In the current study, we investigated the targeting efficacy of nanoparticles that were functionalized with two angiogenesis-specific targeting ligands, an alpha(v)beta(3) integrin-specific and a galectin-1-specific peptide. We show in vitro, using optical techniques and MRI, that the dual-targeting approach produces synergistic targeting effects, causing a dramatically elevated uptake of nanoparticles as compared to single ligand targeting.

1H, 13C, and 15N backbone and side-chain chemical shift assignments for the 29 kDa human galectin-1 protein dimer

Galectin-1 is an important regulator of leukocyte function and tumor angiogenesis. Recently, this lectin has been identified as a molecular target for the potent angiogenesis inhibitor anginex. Here, we report (1)H, (13)C, and (15)N chemical shift assignments for human galectin-1 as determined by using heteronuclear triple resonance NMR spectroscopy.

Quantifying protein interface footprinting by hydroxyl radical oxidation and molecular dynamics simulation: application to galectin-1

Biomolecular surface mapping methods offer an important alternative method for characterizing protein-protein and protein-ligand interactions in cases in which it is not possible to determine high-resolution three-dimensional (3D) structures of complexes. Hydroxyl radical footprinting offers a significant advance in footprint resolution compared with traditional chemical derivatization. Here we present results of footprinting performed with hydroxyl radicals generated on the nanosecond time scale by laser-induced photodissociation of hydrogen peroxide. We applied this emerging method to a carbohydrate-binding protein, galectin-1. Since galectin-1 occurs as a homodimer, footprinting was employed to characterize the interface of the monomeric subunits. Efficient analysis of the mass spectrometry data for the oxidized protein was achieved with the recently developed ByOnic (Palo Alto, CA) software that was altered to handle the large number of modifications arising from side-chain oxidation. Quantification of the level of oxidation has been achieved by employing spectral intensities for all of the observed oxidation states on a per-residue basis. The level of accuracy achievable from spectral intensities was determined by examination of mixtures of synthetic peptides related to those present after oxidation and tryptic digestion of galectin-1. A direct relationship between side-chain solvent accessibility and level of oxidation emerged, which enabled the prediction of the level of oxidation given the 3D structure of the protein. The precision of this relationship was enhanced through the use of average solvent accessibilities computed from 10 ns molecular dynamics simulations of the protein.

Varied expression and localization of multiple galectins in different cancer cell lines

Galectins play a key role in oncogenic processes. Although several galectins are known, their relative expression at the mRNA and protein levels, the subcellular localization, and their relationship to the oncogenic manifestation remains unclear. Herein we report a comprehensive characterization of the expression of major galectins in human breast cancer (drug-sensitive MCF-7 and drug-resistant MCF-7/Adr(R)), colon cancer (HCT-116 and HT-29), and glioma (T98G) cell lines, as these cells are common model systems for studying oncogenic processes. The expected approximately 14.5 kDa galectin-1, predominantly cytosolic, was detected in the cancer and normal cell lines. Notably, two different molecular forms of galectin-1 with molecular masses of approximately 13.5 and 15 kDa were detected in T98G cells, the latter being in the extracellular medium, perhaps a result of post-translational processing. Immunocytochemistry indicated that the extracellular galectin-1 bound to the cell surface was punctated in appearance, suggesting that it was bound to specific receptors. Immunohistological studies indicated that metastasizing carcinomas express high levels of galectin-1. On the other hand, galectin-3 was readily detectable in all cancer cell lines but undetectable in normal cell lines, indicating that galectin-3 is a cancer-specific biomarker protein. Galectin-3 was a cytosolic protein but was not detected in the extracellular medium, indicating that cancer cells do not secrete this galectin. Finally, despite the RT-PCR analysis suggesting the presence of two transcripts of galectin-8 in all cancer cell lines, the corresponding approximately 36 kDa protein was only detectable in the nuclear and cytosolic fractions upon cell fractionation. Notably, a different molecular form of galectin-8 of approximately 18 kDa was immunoprecipitated from the extracellular media, suggesting that the secreted galectin-8 undergoes post-translational processing. These results highlight the expression of galectins in different molecular forms in cancers, warranting caution in interpreting the results of functional studies of individual galectins, particularly because these proteins function redundantly in cancer pathways.

Crystallization and preliminary crystallographic analysis of recombinant human galectin-1

Galectin-1 is considered to be a regulator protein as it is ubiquitously expressed throughout the adult body and is responsible for a broad range of cellular regulatory functions. Interest in galectin-1 from a drug-design perspective is founded on evidence of its overexpression by many cancers and its immunomodulatory properties. The development of galectin-1-specific inhibitors is a rational approach to the fight against cancer because although galectin-1 induces a plethora of effects, null mice appear normal. X-ray crystallographic structure determination will aid the structure-based design of galectin-1 inhibitors. Here, the crystallization and preliminary diffraction analysis of human galectin-1 crystals generated under six different conditions is reported. X-ray diffraction data enabled the assignment of unit-cell parameters for crystals grown under two conditions, one belongs to a tetragonal crystal system and the other was determined as monoclinic P2(1), representing two new crystal forms of human galectin-1.

Multivalent interactions between lectins and supramolecular complexes: Galectin-1 and self-assembled pseudopolyrotaxanes

Supramolecular chemistry has been employed to develop flexible and adaptable multivalent neoglycoconjugates for binding galectin-1 (Gal-1). Gal-1, a dimeric lectin with two galactoside-binding sites, regulates cancer progression and immune responses. Self-assembled pseudopolyrotaxanes consisting of lactoside-displaying cyclodextrin (LCD) "beads" threaded onto polyviologen "strings" display mobile ligands as a result of cyclodextrin rotation about, and limited translation along, the polymer chain. The pseudopolyrotaxanes rapidly and efficiently precipitate Gal-1 and provide valency-corrected enhancements of up to 30-fold compared to native lactose and 20-fold over free LCD in a T-cell agglutination assay. A supramolecular statistical effect was observed, wherein the efficacy of Gal-1 inhibition correlates with the number of ligands connected to each other solely through mechanical and noncovalent interactions. Such flexible and adaptable self-assembled pseudopolyrotaxanes show promise for the study of multivalent interactions and targeting of therapeutically relevant lectins.

Carbohydrate chain of ganglioside GM1 as a ligand: identification of the binding strategies of three 15 mer peptides and their divergence from the binding modes of growth-regulatory galectin-1 and cholera toxin

The branched pentasaccharide chain of ganglioside GM1 is a prominent cell surface ligand, for example, for cholera toxin or tumor growth-regulatory homodimeric galectins. This activity profile via protein recognition prompted us to examine the binding properties of peptides with this specificity. Our study provides insights into the mechanism of molecular interaction of this thus far unexplored size limit of the protein part. We used three pentadecapeptides in a combined approach of mass spectrometry, NMR spectroscopy and molecular modelling to analyze the ligand binding in solution. Availability of charged and hydrophobic functionalities affected the intramolecular flexibility of the peptides differently. Backfolding led to restrictions in two cases; the flexibility was not reduced significantly by association of the ligand in its energetically privileged conformations. Major contributions to the interaction energy arise from the sialic acid moiety contacting Arg/Lys residues and the N-terminal charge. Considerable involvement of stacking between the monovalent ligand and aromatic rings could not be detected. This carbohydrate binding strategy is similar to how an adenoviral fiber knob targets sialylated glycans. Rational manipulation for an affinity enhancement can now be directed to reduce the flexibility, exploit the potential for stacking and acquire the cross-linking capacity of the natural lectins by peptide attachment to a suitable scaffold.

Characterization of the galectin-1 carbohydrate recognition domain in terms of solvent occupancy

Human galectin-1, a galactosil-terminal sugar binding soluble protein, is a potent multifunctional effector that participates in specific protein-carbohydrate and protein-protein interactions. Recent studies revealed that it plays a key role as a modulator of cellular differentiation and immunological response. In this work, we have investigated the solvation properties of the carbohydrate recognition domain of Gal-1 by means of molecular dynamics simulations. Water sites (ws) were identified in terms of radial and angular distribution functions, and properties such as water residence times, interaction energies, and free-energy contributions were evaluated for those sites. Our results allowed us to correlate the thermodynamic properties of the ws and their binding pattern with the N-acetilgalactoside ligand. These results let us further infer that the water molecules located at the ws, which exhibit much more favorable binding, are the ones replaced by -OH groups of the sugar.

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.

Proteomics discovery of metalloproteinase substrates in the cellular context by iTRAQ labeling reveals a diverse MMP-2 substrate degradome

Elucidation of protease substrate degradomes is essential for understanding the function of proteolytic pathways in the protease web and how proteases regulate cell function. We identified matrix metalloproteinase-2 (MMP-2) cleaved proteins, solubilized pericellular matrix, and shed cellular ectodomains in the cellular context using a new multiplex proteomics approach. Tryptic peptides of intact and cleaved proteins, collected from conditioned culture medium of Mmp2(-/-) fibroblasts expressing low levels of transfected active human MMP-2 at different time points, were amine-labeled with iTRAQ mass tags. Peptide identification and relative quantitation between active and inactive protease transfectants were achieved following tag fragmentation during tandem MS. Known substrates of MMP-2 were identified thereby validating this technique with many novel MMP-2 substrates including the CX(3)CL1 chemokine fractalkine, osteopontin, galectin-1, and HSP90alpha also being identified and biochemically confirmed. In comparison with ICAT-labeling and quantitation, 8-9-fold more proteins and substrates were identified by iTRAQ. "Peptide mapping," the location of multiple peptides identified within a particular protein by iTRAQ in combination with their relative abundance ratios, enabled the domain shed and general location of the cleavage site to be identified in the native cellular substrate. Hence this advance in degradomics cell-based screens for native protein substrates casts new light on the roles for proteases in cell function.

Murine homodimeric adhesion/growth-regulatory galectins-1, -2 and -7: comparative profiling of gene/ promoter sequences by database mining, of expression by RT-PCR/immunohistochemistry and of contact sites for carbohydrate ligands by computational chemistry

Following the detection of individual members of the family of galectins it is an obvious challenge to define the extent of functional overlap/divergence among these proteins. As a step to address this issue a comparative profiling has been started in the mouse as a model organism, combining sequence analysis, expression patterns and structural features in the cases of the homodimeric galectins-1, -2 and -7. Close relationship was apparent at the level of global gene organization. Scrutiny of the proximal promoter regions for putative transcription-factor-binding sites by two search algorithms uncovered qualitative and quantitative differences with potential to influence the combinatorial functionality of regulatory sequences. RT-PCR mapping with samples from an array of 17 organs revealed significant differences, separating rather ubiquitous gene expression of galectin-1 from the more restricted individual patterns of galectins-2 and -7. Using specific antisera obtained by affinity depletion including stringent controls to ascertain lack of cross-reactivity these results were corroborated at the level of galectin localization in fixed tissue sections. Nuclear presence was seen in the case of galectin-1. In addition to nonidentical expression profiles the mapping of the carbohydrate recognition domains of galectins-1 and -7 by homology modelling and docking of naturally occurring complex tetra- and pentasaccharides disclosed a series of sequence deviations which may underlie disparate affinities for cell surface glycans/glycomimetic peptides. In view of applicability the presented data can serve as useful reference to delineate changes with respect to disease and in genetically engineered models. To enable more general conclusions on the galectin network it is warranted to further pursue this combined approach within this lectin family.

Crosslinking of low-affinity glycoprotein ligands to galectin LEC-1 using a photoactivatable sulfhydryl reagent

The N-terminal lectin domain (Nh) of the tandem repeat-type nematode galectin LEC-1 has a lower affinity for sugars than the C-terminal lectin domain. To confirm that LEC-1 forms a complex with N-acetyllactosamine-containing glycoproteins, we used several mutants of LEC-1 in which a unique cysteine residue was introduced into the Nh domain and examined their binding to bovine asialofetuin with a photoactivatable sulfhydryl crosslinking reagent. A crosslinked product was formed with the Q38C mutant, strongly suggesting the low-affinity interaction of Nh with the glycoprotein could be detected with this system.

Haploinsufficiency of C2GnT-I glycosyltransferase renders T lymphoma cells resistant to cell death

Neoplastic T cells in mycosis fungoides (MF) are resistant to apoptotic agents, including galectin-1 that is abundant in skin. Although MF cells are typically CD7-, and thus galectin-1 resistant, CD7+ HH cells, derived from a patient with MF, were also resistant to galectin-1. HH cells demonstrate altered cell surface glycosylation, with loss of core 2 O-glycan ligands for galectin-1 created by core 2 beta1,6-N-acetylglucosaminyltransferase (C2GnT-I). Loss of core 2 O-glycans on tumor cells was also seen in primary CD7+ MF lesions. Surprisingly, HH cells are heterozygous for a C2GnT-I point mutation, yet this mutation resulted in a dramatic reduction in cellular glycosyltransferase activity. Expression of wild-type C2GnT-I in human HH cells, or murine lymphoma cells that lack C2GnT-I, restored core 2 O-glycan expression and susceptibility to galectin-1, whereas mutant enzyme lacked activity and did not restore core 2 O-glycan expression or susceptibility to galectin-1. Mutant enzyme did not have a dominant negative effect by affecting dimerization or activity of wild-type enzyme; rather, C2GnT-I haploinsufficiency is sufficient for loss of core 2 O-glycan expression and galectin-1 resistance. Thus, glycosyltransferase haploinsufficiency results in altered cellular glycosylation and resistance to cell death, identifying a new survival mechanism for T-lymphoma cells.

Spatiotemporal organization of Ras signaling: rasosomes and the galectin switch

1. Ras signaling and oncogenesis depend on the dynamic interplay of Ras with distinctive plasma membrane (PM) microdomains and various intracellular compartments. Such interaction is dictated by individual elements in the carboxy-terminal domain of the Ras proteins, including a farnesyl isoprenoid group, sequences in the hypervariable region (hvr)-linker, and palmitoyl groups in H/N-Ras isoforms. 2. The farnesyl group acts as a specific recognition unit that interacts with prenyl-binding pockets in galectin-1 (Gal-1), galectin-3 (Gal-3), and cGMP phosphodiesterase delta. This interaction appears to contribute to the prolongation of Ras signals in the PM, the determination of Ras effector usage, and perhaps also the transport of cytoplasmic Ras. Gal-1 promotes H-Ras signaling to Raf at the expense of phosphoinositide 3-kinase (PI3-K) and Ral guanine nucleotide exchange factor (RalGEF), while galectin-3 promotes K-Ras signaling to both Raf and PI3-K. 3. The hvr-linker and the palmitates of H-Ras and N-Ras determine the micro- and macro-localizations of these proteins in the PM and in the Golgi, as well as in 'rasosomes', randomly moving nanoparticles that carry palmitoylated Ras proteins and their signal through the cytoplasm.4. The dynamic compartmentalization of Ras proteins contributes to the spatial organization of Ras signaling, promotes redistribution of Ras, and provides an additional level of selectivity to the signal output of this regulatory GTPase.

Characterization of the Interaction of Galectin-1 with Sodium Arsenite

We previously showed that galectin-1 (GAL1) is an arsenic-binding protein. In the current study, we further characterize the interaction of GAL1 with sodium arsenite (As(III)). The GALl-As(III) complex was prepared from the cell extracts of GAL1-transfected Escherichia coli (E. coli) that were pretreated with As(III). The results of the circular dichroism (CD) spectrum of GAL1-As(III) exhibited a negative signal at around 205-210 nm, whereas that of GAL1 showed a negative signal at around 215-220 nm. This shift in the CD spectrum is indicative of a substantial change in the secondary structure arising from the binding of As(III) to the GAL1 protein. The UV absorptive spectrum of the GAL1-As(III) complex was significantly lower than that of GAL1 itself. A mobility shift binding assay showed that the GAL1-As(III) complex migrated closer than GAL1 toward the anode. Capillary electrophoretic analysis also showed that As(III) binding decreased the mobility of GAL1. These results further confirmed the structural change of the GAL1 complex with As(III). Furthermore, isothermal titration microcalometric studies showed that As(III) titration into the GAL1 protein solution was an endothermic process with absorption enthalpy (DeltaH(abs)) around 8-10 kJ/mol As(III). The affinity constant (K(d)) of As(III) toward GAL1 was around 8.239 +/- 2.627 microM as estimated by tryptophan (Trp) fluorescence quenching. However, the binding of As(III) did not significantly affect the biological activity of GAL1, since the GAL1-As(III) complex only partially lost its lectin activity. In addition, we show that GAL1-transfected KB cells accumulated more arsenic than did the parental cells. Taken together, these results suggest that GAL1 might serve as a target protein of As(III) in vivo, and the binding of GAL1 with As(III) could interfere with the excretion of As(III).

Regulation of the neuronal fate by DeltaFosB and its downstream target, galectin-1

In mammals, the regulation of the cell fate to either proliferate, differentiate, arrest cell growth, or initiate programmed cell death is the most fundamental mechanism for maintaining normal cell function and tissue homeostasis. Under multiple signaling pathways, Jun and Fos family proteins are known to play important roles as components of an AP-1 (activator protein-1) complex, to regulate the transcription of various genes involved in cell proliferation, differentiation and programmed cell death. DeltaFosB, one of the AP-1 subunits encoded by alternatively spliced fosB mRNA, triggers one round of proliferation in quiescent rat embryo cell lines, followed by a different cell fate such as morphological alteration or delayed cell death. As one of the downstream targets of the DeltaFosB in rat3Y1 cell line, we identified rat galectin-1 and its novel variant, galectin-1beta, and demonstrated that the expression of galectin-1 is required for the proliferative activation of quiescent rat1A cells by DeltaFosB, thus indicating that galectin-1 is one of functional targets of DeltaFosB. The expression of DeltaFosB is highly inducible in the adult brain in response to various insults such as ischemic reperfusion injury, seizure induced by electric stimulation or cocaine administration. On the other hand, galectin-1 has also been shown to be involved in the regeneration of damaged axons in the peripheral nerve, as well as in neurite outgrowth or synaptic connectivity in the olfactory system during development. We herein propose that DeltaFosB together with galectin-1, may therefore mediate neuroprotection and neurogenesis in response to brain damage.

The role of Galectin-1 in the interaction between chondrocytes and a lactose-modified chitosan

Evidences for the involvement of the Galectin-1 in the interaction of pig chondrocytes with a lactose-modified chitosan, namely Chitlac, are reported. The Chitlac glycopolymer has been shown to promote pig chondrocyte aggregation and to induce extracellular matrix production. Highly pure Galectin-1 was obtained from pig spleen by affinity chromatography and its identity was determined by ion spray mass spectrometry analysis of tryptic peptide fragments obtained after in-gel digestion. The complete sequence of pig Galectin-1 CDS was obtained by screening a pig EST database using human Galectin-1 sequence as template. The Galectin-1 cDNA was cloned into a pGEX-4T-1 expression vector and the recombinant protein was purified, characterized and used to produce a rabbit anti-serum. Recombinant Galectin-1 interacts in a dose-dependent manner with Chitlac as determined with ELISA assay. Expression level of galectin-1 gene, quantified by real-time PCR, was significantly higher in chondrocytes cultivated on Chitlac. In the same way, the presence of Chitlac stimulates secretion of Galectin-1 in culture medium that, by immunohistochemical analysis, revealed to be clustered on the surface of Chitlac-induced aggregates. These data indicate the role of Galectin-1 as a bridging agent between Chitlac and chondrocyte aggregates.

Structural and Functional Studies of Galectin-1: A Novel Axonal Regeneration-Promoting Activity for Oxidized Galectin-1

Recently, we discovered oxidized galectin-1 as a factor that regulates initial axonal growth in the peripheral nerve after axotomy. Galectin-1 is a member of the galectins, a family of animal lectins ranging from Caenorhabditis elegans to humans, which is defined by their affinity for beta-galactosides and by significant sequence similarity in the carbohydrate-binding site. Galectin-1 is a homodimer with a subunit molecular mass of 14.5 kDa, which contains six cysteine residues per subunit. The cysteine residues should be in a free state in order to maintain a molecular structure that is capable of showing lectin activity. However, our structural analysis revealed that the axonal regeneration-promoting factor exists as an oxidized form of galectin-1, containing three intramolecular disulfide bonds. The oxidized galectin-1 exhibited marked peripheral nerve regeneration-promoting activity, although it showed no lectin activity. It was also revealed that oxidized galectin-1 exists as a monomer in a physiological solution. Galectin-1 seems to have a variety of biological functions. These functions could vary according to the time at which a biological function is taking place, as well as the site in which a biological function is taking place. In addition, these functions could vary according to the structure of galectin-1 by which a particular biological function is taking place. Disulfide bond formation alters the structure of galectin-1, so as to confer the novel ability to promote axonal regeneration. Oxidized galectin-1 likely acts as an autocrine or paracrine factor to promote axonal regeneration, functioning more like a cytokine than as a lectin.

Contributions of Ca2+ to galectin-1-induced exposure of phosphatidylserine on activated neutrophils

Apoptotic cells redistribute phosphatidylserine (PS) to the cell surface by both Ca(2+)-dependent and -independent mechanisms. Binding of dimeric galectin-1 (dGal-1) to glycoconjugates on N-formyl-Met-Leu-Phe (fMLP)-activated neutrophils exposes PS and facilitates neutrophil phagocytosis by macrophages, yet it does not initiate apoptosis. We asked whether dGal-1 initiated Ca(2+) fluxes that are required to redistribute PS to the surface of activated neutrophils. Prolonged occupancy by dGal-1 was required to maximally mobilize PS to the surfaces of fMLP-activated neutrophils. Like fMLP, dGal-1 rapidly elevated cytosolic Ca(2+) levels in Fluo-4-loaded neutrophils. An initial Ca(2+) mobilization from intracellular stores was followed by movement of extracellular Ca(2+) to the cytosolic compartment, with return to basal Ca(2+) levels within 10 min. Chelation of extracellular Ca(2+) did not prevent PS mobilization. Chelation of intracellular Ca(2+) revealed that fMLP and dGal-1 independently release Ca(2+) from intracellular stores that cooperate to induce optimal redistribution of PS. Ca(2+) mobilization by ionomycin did not permit dGal-1 to mobilize PS, indicating that fMLP initiated both Ca(2+)-dependent and -independent signals that facilitated dGal-1-induced exposure of PS. dGal-1 elevated cytosolic Ca(2+) and mobilized PS through a pathway that required action of Src kinases and phospholipase Cgamma. These results demonstrate that transient Ca(2+) fluxes contribute to a sustained redistribution of PS on neutrophils activated with fMLP and dGal-1.

IgA1 is the premier serum glycoprotein recognized by human galectin-1 since T antigen (Galbeta1-->3GalNAc-) is far superior to non-repeating N-acetyl lactosamine as ligand

Human heart galectin-1 (HHL) was separated by high pressure liquid chromatography from endogenous glycoproteins co-purified with it during affinity chromatography. These glycoproteins offered excellent ligands for HHL binding and were rich in T antigen (Galbeta1-->3 GalNAc-) of O-linked oligosaccharides. In enzyme linked lectin assay and hemagglutination inhibition assay, human IgA1, bovine fetuin and other O-glycosylated T antigen-bearing glycoproteins bound to the lectin efficiently in contrast to single N-acetyl lactosamine (LacNAc)-bearing N-linked oligosaccharides released from them and to IgG which is not O-glycosylated. HHL binding to IgA1 and fetuin was unaffected by removal of their N-linked oligosaccharides by alpha-mannosidase. When immobilized, O-glycosylated serum proteins but not IgG could capture HHL from its solutions. Desialylated or polymeric IgA1 was better inhibitor than monomeric IgA1. The findings suggest a possible role for galectin-1 in anchoring of microbial and cancer cells known to be rich in T antigen, in high serum IgA1 turn over and in tissue sequestering of IgA1 immune complexes especially after their microbial desialylation in IgA nephropathy and other immune complex-mediated disorders.

Fluorescence polarization as an analytical tool to evaluate galectin-ligand interactions

Galectins are a family of beta-galactose binding lectins associated with functions such as immunological and malignant events. To study the binding affinity of galectins for natural and artificial saccharides and glycoconjugates we have developed an assay using fluorescence polarization. A collection of fluorescein-conjugated saccharides was synthesized and used as probes with galectins-1 and -3 and the two carbohydrate recognition domains of galectin-4. Direct binding of a fixed probe amount with different amounts of each galectin defined specificity and selectivity and permitted selection of the optimal probe for inhibition studies. Then fixed amounts of galectin and selected probe were used to screen the inhibitory potency of a library of nonfluorescent compounds. As the assay is in solution and does not require separation of free and bound probe, it is simple and rapid and can easily be applied to different unlabeled galectins. As all interaction components are known, K(d) values for galectin-inhibitor interaction can be directly calculated without approximation other than the assumption of a simple one-site competition.

Physicochemical properties and amino acid sequence of sheep brain galectin-1

A beta-galactoside-specific soluble 14-kD lectin from sheep brain was isolated, sequenced, and compared with similar galectins from other species. Percent identities of amino acid sequence and the carbohydrate recognition domain (CRD) revealed that the isolated galectin shares all the absolutely preserved and critical residues of the mammalian galectin-1 subfamily. The isolated sheep brain galectin (SBG) showed more than 90% amino acid sequence (92%) and carbohydrate recognition domain identity (96%) with human brain galectin-1. Conformational changes were found induced by interaction of the protein with its specific disaccharide and oxidizing agent (hydrogen peroxide). Upon oxidation a drastic change in the environment of aromatic residues and conformation of the galectin was observed with the loss of hemagglutination activity, while no significant change was observed upon addition of D-lactose (Gal(beta1-4)Glc) in the far-UV and near-UV spectra, suggesting no significant modification in the secondary as well as tertiary structures of sheep brain galectin. But the functional integrity of the CRD is found to be affected in the presence of oxidizing agent, indicating intramolecular disulfide bonds and requirement of complete polypeptide chain for functional integrity of the carbohydrate recognition domain.

Growth-regulatory human galectin-1: crystallographic characterisation of the structural changes induced by single-site mutations and their impact on the thermodynamics of ligand binding

Human galectin-1 is a potent multifunctional effector that participates in specific protein-carbohydrate and protein-protein (lipid) interactions. By determining its X-ray structure, we provide the basis to define the structure of its ligand-binding pocket and to perform rational drug design. We have also analysed whether single-site mutations introduced at some distance from the carbohydrate recognition domain can affect the lectin fold and influence sugar binding. Both the substitutions introduced in the C2S and R111H mutants altered the presentation of the loop, harbouring Asp123 in the common "jelly-roll" fold. The orientation of the side-chain was inverted 180 degrees and the positions of two key residues in the sugar-binding site of the R111H mutant were notably shifted, i.e. His52 and Trp68. Titration calorimetry was used to define the decrease in ligand affinity in both mutants and a significant increase in the entropic penalty was found to outweigh a slight enhancement of the enthalpic contribution. The position of the SH-groups in the galectin appeared to considerably restrict the potential to form intramolecular disulphide bridges and was assumed to be the reason for the unstable lectin activity in the absence of reducing agent. However, this offers no obvious explanation for the improved stability of the C2S mutant under oxidative conditions. The noted long-range effects in single-site mutants are relevant for the functional divergence of closely related galectins and in more general terms, the functionality definition of distinct amino acids.

Dimeric galectin-1 binds with high affinity to alpha2,3-sialylated and non-sialylated terminal N-acetyllactosamine units on surface-bound extended glycans

Galectin-1 is a member of the galectin family of glycan-binding proteins and occurs as an approximately 29.5-kDa noncovalent homodimer (dGal-1) that is widely expressed in many tissues. Here, we report that human recombinant dGal-1 bound preferentially and with high affinity (apparent K(d) approximately 2-4 microM) to immobilized extended glycans containing terminal N-acetyllactosamine (LN; Galbeta1-4GlcNAc) sequences on poly-N-acetyllactosamine (PL; (-3Galbeta1-4GlcNAcbeta1-)(n)) sequences, complex-type biantennary N-glycans, or novel chitin-derived glycans modified to contain terminal LN. Although terminal Gal residues are important for dGal-1 recognition, dGal-1 bound similarly to alpha3-sialylated and alpha2-fucosylated terminal LN, but not to alpha6-sialylated and alpha3-fucosylated terminal LN. The binding specificity of human recombinant dGal-1 was similar to that observed with purified bovine heart-derived dGal-1. Unexpectedly, dGal-1 bound free ligands in solution with relatively low affinity and displayed no preference for extended glycans, indicating that dGal-1 preferentially recognizes extended glycans only when they are surface-bound, such as found on cell surfaces. Human dGal-1 also bound to both native and desialylated human promyelocytic HL-60 cells with similar affinity as observed for immobilized long chain PL. Binding to these cells was reduced upon treatment with endo-beta-galactosidase, which cleaves PL sequences, indicating that cell-surface PLs are ligands. To test the role of dimerization in dGal-1 binding, we examined the binding of a mutated form of dGal-1 that weakly dimerizes (monomeric Gal-1 (mGal-1)) and a covalently dimerized (chemically cross-linked) form of mGal-1 (cd-mGal-1). dGal-1 and cd-mGal-1 had similar affinities that were both approximately 3.5-fold higher for immobilized PL than observed for mGal-1, suggesting that dGal-1 acts as a dimer to cross-link terminal LN units on immobilized PL. These results indicate that dGal-1 functions as a dimer to recognize LN units on extended PLs on cell surfaces.

A Self-Assembled Multivalent Pseudopolyrotaxane for Binding Galectin-1

A self-assembled pseudopolyrotaxane consisting of lactoside-displaying cyclodextrin (CD) "beads" threaded onto a linear polyviologen "string" was investigated for its ability to inhibit galectin-1-mediated T-cell agglutination. The CDs of the pseudopolyrotaxane are able to spin around the axis of the polymer chain as well as to move back and forth along its backbone to alter the presentation of its ligand. This supramolecular superstructure incorporates all the advantages of polymeric structures, such as the ability to span large distances, along with a distinctively dynamic presentation of its lactoside ligands to afford a neoglycoconjugate that can adjust to the relative stereochemistries of the lectin's binding sites. The pseudopolyrotaxane exhibited a valency-corrected 10-fold enhancement over native lactose in the agglutination assay, which was greater than the enhancements observed for lactoside-bearing trivalent glycoclusters and a lactoside-bearing chitosan polymer tested using the same assay. The experimental results indicate that supramolecular architectures, such as the pseudopolyrotaxane, provide tools for investigating protein-carbohydrate interactions.