Caenorhabditis elegans N-glycans containing a Gal-Fuc disaccharide unit linked to the innermost GlcNAc residue are recognized by C. elegans galectin LEC-6

We report a detailed structural analysis of the N-glycans of Caenorhabditis elegans recognized by C. elegans galectin LEC-6. Glycoproteins of C. elegans captured by an immobilized LEC-6 affinity adsorbent were isolated. The N-glycans of these glycoproteins were then liberated by hydrazinolysis and labeled with the fluorophore 2-aminopyridine (PA). The derived pyridylaminated (PA)-sugars were further fractionated by rechromatography on immobilized LEC-6 adsorbent and by reversed-phase high-performance liquid chromatography (HPLC). The structures of the PA-sugars thus obtained were analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS/MS) in conjunction with glycosidase digestion. We confirmed that all PA-sugars having affinity for LEC-6 contain a Gal-Fuc disaccharide unit, and that this unit is bound to the innermost GlcNAc residue of the N-glycan chain. The dissociation constants of LEC-6 for these glycans were measured by frontal affinity chromatography. LEC-6 exhibited higher affinity for oligosaccharides having a Gal-Fuc unit linked to position 6 of the innermost GlcNAc residue than for those having Galbeta1-4GlcNAc units. Affinity for the former disappeared, however, following treatment with beta-galactosidase. If the glycan contained a Hex-Fuc disaccharide linked to the penultimate GlcNAc residue, the affinity would be diminished. We propose, therefore, that the galectins of C. elegans utilize the Gal-Fuc disaccharide unit for recognition instead of the Gal-GlcNAc unit that is common in vertebrates.     

Caenorhabditis elegans galectins LEC-1-LEC-11: structural features and sugar-binding properties

Galectins form a large family of beta-galactoside-binding proteins in metazoa and fungi. This report presents a comparative study of the functions of potential galectin genes found in the genome database of Caenorhabditis elegans. We isolated full-length cDNAs of eight potential galectin genes (lec-2-5 and 8-11) from a lambdaZAP cDNA library. Among them, lec-2-5 were found to encode 31-35-kDa polypeptides containing two carbohydrate-recognition domains similar to the previously characterized lec-1, whereas lec-8-11 were found to encode 16-27-kDa polypeptides containing a single carbohydrate-recognition domain and a C-terminal tail of unknown function. Recombinant proteins corresponding to lec-1-4, -6, and 8-10 were expressed in Escherichia coli, and their sugar-binding properties were assessed. Analysis using affinity adsorbents with various beta-galactosides, i.e., N-acetyllactosamine (Galbeta1-4GlcNAc), lacto-N-neotetraose (Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glc), and asialofetuin, demonstrated that LEC-1-4, -6, and -10 have a significant affinity for beta-galactosides, while the others have a relatively lower affinity. These results indicate that the integrity of key amino acid residues responsible for recognition of lactose (Galbeta1-4Glc) or N-acetyllactosamine in vertebrate galectins is also required in C. elegans galectins. However, analysis of their fine oligosaccharide-binding properties by frontal affinity chromatography suggests their divergence towards more specialized functions.     

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.     

Complex N-glycans are the major ligands for galectin-1, -3, and -8 on Chinese hamster ovary cells

Galectins are implicated in a large variety of biological functions, many of which depend on their carbohydrate-binding ability. Fifteen members of the family have been identified in vertebrates based on binding to galactose (Gal) that is mediated by one or two, evolutionarily conserved, carbohydrate-recognition domains (CRDs). Variations in glycan structures expressed on glycoconjugates at the cell surface may, therefore, affect galectin binding and functions. To identify roles for different glycans in the binding of the three types of mammalian galectins to cells, we performed fluorescence cytometry at 4 degrees C with recombinant rat galectin-1, human galectin-3, and three forms of human galectin-8, to Chinese hamster ovary (CHO) cells and 12 different CHO glycosylation mutants. All galectin species bound to parent CHO cells and binding was inhibited >90% by 0.2 M lactose. Galectin-8 isoforms with either a long or a short inter-CRD linker bound similarly to CHO cells. However, a truncated form of galectin-8 containing only the N-terminal CRD bound only weakly to CHO cells and the C-terminal galectin-8 CRD exhibited extremely low binding. Binding of the galectins to the different CHO glycosylation mutants revealed that complex N-glycans are the major ligands for each galectin except the N-terminal CRD of galectins-8, and also identified some fine differences in glycan recognition. Interestingly, increased binding of galectin-1 at 4 degrees C correlated with increased propidium iodide (PI) uptake, whereas galectin-3 or -8 binding did not induce permeability to PI. The CHO glycosylation mutants with various repertoires of cell surface glycans are a useful tool for investigating galectin-cell interactions as they present complex and simple glycans in a natural mixture of multivalent protein and lipid glycoconjugates anchored in a cell membrane.     

Galectin-1, -2, and -3 exhibit differential recognition of sialylated glycans and blood group antigens

Human galectins have functionally divergent roles, although most of the members of the galectin family bind weakly to the simple disaccharide lactose (Galbeta1-4Glc). To assess the specificity of galectin-glycan interactions in more detail, we explored the binding of several important galectins (Gal-1, Gal-2, and Gal-3) using a dose-response approach toward a glycan microarray containing hundreds of structurally diverse glycans, and we compared these results to binding determinants on cells. All three galectins exhibited differences in glycan binding characteristics. On both the microarray and on cells, Gal-2 and Gal-3 exhibited higher binding than Gal-1 to fucose-containing A and B blood group antigens. Gal-2 exhibited significantly reduced binding to all sialylated glycans, whereas Gal-1 bound alpha2-3- but not alpha2-6-sialylated glycans, and Gal-3 bound to some glycans terminating in either alpha2-3- or alpha2-6-sialic acid. The effects of sialylation on Gal-1, Gal-2, and Gal-3 binding to cells also reflected differences in cellular sensitivity to Gal-1-, Gal-2-, and Gal-3-induced phosphatidylserine exposure. Each galectin exhibited higher binding for glycans with poly-N-acetyllactosamine (poly(LacNAc)) sequences (Galbeta1-4GlcNAc)(n) when compared with N-acetyllactosamine (LacNAc) glycans (Galbeta1-4GlcNAc). However, only Gal-3 bound internal LacNAc within poly(LacNAc). These results demonstrate that each of these galectins mechanistically differ in their binding to glycans on the microarrays and that these differences are reflected in the determinants required for cell binding and signaling. The specific glycan recognition by each galectin underscores the basis for differences in their biological activities.     

Crystallization and preliminary x-ray crystallographic analysis of galectin LEC-1 from Caenorhabditis elegans

Galectin LEC-1 isolated from the nematode Caenorhabditis elegans was the first galectin found in invertebrates and also the first tandem-repeat-type galectin identified, containing two homologous carbohydrate-binding sites. This galectin is localized most abundantly in the adult cuticle and possibly plays a role in the formation of epidermal layers. We succeeded in crystallizing LEC-1 composed of 279 amino acids with a calculated molecular weight of 31,809 Da under two independent sets of conditions as a result of extensive screening. The crystals grown under one set of conditions belong to the triclinic space group P1, with unit-cell parameters a = 48.44, b = 52.13, c = 64.24 A, alpha = 108.73, beta= 91.39, and gamma = 98.45 degrees and two protein molecules per unit cell. The crystals grown under the other set of conditions which included lactose belong to the monoclinic space group P2(1), with unit-cell parameters a = 52.90, b = 47.01, c = 66.16 A, and beta= 113.30 degrees and one protein molecule per asymmetric unit.     

Glycosylation, galectins and cellular signaling

Glycosylation is a common posttranslational modification of proteins and lipids of the secretory pathway that generates binding sites for galactose-specific lectins or galectins. Branching of Asn-linked (N-)glycans by the N-acetylglucosaminyltransferases (Mgat genes) increases affinity for galectins. Both tissue-specific expression of the enzymes and the metabolic supply of sugar-nucleotides to the ER and Golgi regulate glycan distribution while protein sequences specify NXS/T site multiplicity, providing metabolic and genetic contributions to galectin-glycoprotein interactions. Galectins cross-link glycoproteins forming dynamic microdomains or lattices that regulate various mediators of cell adhesion, migration, proliferation, survival and differentiation. There are a similar number of galactose-specific galectins in C. elegans and humans, but expression of higher-affinity branched N-glycans are a more recent feature of vertebrate evolution. Galectins might be considered a reading code for repetition of the minimal units of binding [Gal(NAc)β1-3/4GlcNAc] and NXS/T site multiplicity in proteins. The rapidly evolving and structurally complex Golgi modifications to surface receptors are interpreted through affinity for the lattice, which regulates receptor levels as a function of the cellular environment, and thereby the probability of various cell fates. Many important questions remain concerning the regulation of the galectins, the glycan ligands and lattice interaction with other membrane domains and endocytic pathways.     

Ligand interactions of the Coprinopsis cinerea galectins

The basidiomycete Coprinopsis cinerea (Coprinus cinereus) expresses two fruiting body-specific isolectins (CGL1 and CGL2) that belong to the family of galectins. Understanding the role of these beta-galactoside binding lectins is still in the beginning. Even though the prerequisites for substrate binding are well understood, it is not known how discrimination between potential substrates is achieved and what kind of influence this has on the function in a distinct cellular context. Precise knowledge of the expression of galectins and their ligands will aid in elucidating their function. In Coprinopsis, the developmentally regulated ligands for galectins co-localise with galectin expression in the veil surrounding the developing primordium and the outer cells of the young stipe. In addition, galectin ligands are observed in the hymenium. The subcellular localisation of the galectin ligands suggests these to be present in cellular compartments distinct from galectin transport. The sensitivity of the in situ interactions with exogenous galectin towards detergents and organic solvents infers that these ligands are lipid-borne. Accordingly, lipid fractions from primordia are shown to contain galectin-binding compounds. Based on these results and the determined binding specificity towards substituted beta-galactosides we hypothesise that beta-galactoside-containing lipids (basidiolipids) found in mushrooms are physiological ligands for the galectins in C. cinerea.     

Characterization of wheat germ agglutinin ligand on soluble glycoproteins in Caenorhabditis elegans

Some mutants of Caenorhabditis elegans show altered patterns of ectopic binding with wheat germ agglutinin (WGA). Some of these mutants also have defects of morphogenesis and movement during development. To clarify the structures of WGA-ligands in C. elegans that may be involved in developmental events, we have analyzed glycan structures capable of binding WGA. We isolated glycoproteins from wild-type C. elegans by WGA-affinity chromatography, and analyzed their glycan structures by a combination of hydrazine degradation and fluorescent labeling. The glycoproteins had oligomannose-type and complex-type N-glycans that included agalacto-biantenna and agalacto-tetraantenna glycans. Although the complex-type glycans carried beta-GlcNAc residues at their non-reducing ends, they did not bind to the WGA-agarose-resin. Thus, it was suggested that these N-glycans were not responsible for WGA-binding of the isolated glycoproteins. Hydrazinolysis of the glycoproteins also released a considerable amount of GalNAc monosaccharide. It was surmised that N-acetylgalactosamine was derived from mucin-type O-glycans with the Tn-antigen structure (GalNAcalpha1-O-Ser/Thr). WGA-blotting assay of neoglycoproteins revealed that a cluster of Tn-antigens was a good ligand for WGA. These results suggested that the WGA-ligand in C. elegans is a cluster of alpha-GalNAc monosaccharides linked to mucin-like glycoprotein(s). The observations reported in this paper emphasize the possible significance of mucin-type O-glycans in the development of a multicellular organism.     

Affinity capturing and gene assignment of soluble glycoproteins produced by the nematode Caenorhabditis elegans

Protein glycosylation is a central issue for post-genomic (proteomic) sciences. We have taken a systematic approach for analyzing soluble glycoproteins produced in the nematode Caenorhabditis elegans. The approach aims at assigning (i) genes that encode glycoproteins, (ii) sites where glycosylation occurs, and (iii) types of attached glycan structures. A soluble extract of C. elegans, as a starting material, was applied first to a concanavalin A (ConA) column (specific for high-mannose type N-glycans), and then the flow-through fraction was applied to a galectin LEC-6 (GaL6) column (specific for complex-type N-glycans). The adsorbed glycoproteins were digested with lysylendopeptidase, and the resultant glycopeptides were selectively recaptured with the same lectin columns. The glycopeptides were separated by reversed-phase chromatography and then subjected to sequence determination. As a result, 44 and 23 glycopeptides captured by the ConA and GaL6 columns, respectively, were successfully analyzed and assigned to 32 and 16 corresponding genes, respectively. For these glycopeptides, 49 N-glycosylation sites were experimentally confirmed, whereas 21 sites remained as potential sites. Of the identified genes, about 80% had apparent homologues in other species, as represented by typical secreted proteins. However, the two sets of genes assigned for the ConA and GaL6-recognized glycopeptides showed only 1 overlap with each other. Proof of the practical applicability of the glyco-catch method to a model organism, C. elegans, directs us to explore more complex multicellular organisms.     

Emerging role of alpha2,6-sialic acid as a negative regulator of galectin binding and function

Galectins are β-galactoside-binding lectins that regulate diverse cell behaviors, including adhesion, migration, proliferation, and apoptosis. Galectins can be expressed both intracellularly and extracellularly, and extracellular galectins mediate their effects by associating with cell-surface oligosaccharides. Despite intensive current interest in galectins, strikingly few studies have focused on a key enzyme that acts to inhibit galectin signaling, namely β-galactoside α2,6-sialyltransferase (ST6Gal-I). ST6Gal-I adds an α2,6-linked sialic acid to the terminal galactose of N-linked glycans, and this modification blocks galectin binding to β-galactosides. This minireview summarizes the evidence suggesting that ST6Gal-I activity serves as an "off switch" for galectin function.     

Sugar binding properties of the two lectin domains of the tandem repeat-type galectin LEC-1 (N32) of Caenorhabditis elegans. Detailed analysis by an improved frontal affinity chromatography method

The 32-kDa galectin (LEC-1 or N32) of the nematode Caenorhabditis elegans is the first example of a tandem repeat-type galectin and is composed of two domains, each of which is homologous to typical vertebrate 14-kDa-type galectins. To elucidate the biological meaning of this unique structure containing two probable sugar binding sites in one molecule, we analyzed in detail the sugar binding properties of the two domains by using a newly improved frontal affinity chromatography system. The whole molecule (LEC-1), the N-terminal lectin domain (Nh), and the C-terminal lectin domain (Ch) were expressed in Escherichia coli, purified, and immobilized on HiTrap gel agarose columns, and the extent of retardation of various sugars by the columns was measured. To raise the sensitivity of the system, we used 35 different fluorescence-labeled oligosaccharides (pyridylaminated (PA) sugars). All immobilized proteins showed affinity for N-acetyllactosamine-containing N-linked complex-type sugar chains, and the binding was stronger for more branched sugars. Ch showed 2-5-fold stronger binding toward all complex-type sugars compared with Nh. Both Nh and Ch preferred Galbeta1-3GlcNAc to Galbeta1-4GlcNAc. Because the Fucalpha1-2Galbeta1-3GlcNAc (H antigen) structure was found to interact with all immobilized protein columns significantly, the K(d) value of pentasaccharide Fucalpha1-2Galbeta1-3GlcNAcbeta1-3Galbeta1-4Glc-PA for each column was determined by analyzing the concentration dependence. Obtained values for immobilized LEC-1, Nh, and Ch were 6.0 x 10(-5), 1.3 x 10(-4), and 6.5 x 10(-5) m, respectively. The most significant difference between Nh and Ch was in their affinity for GalNAcalpha1-3(Fucalpha1-2)Galbeta1-3GlcNAcbeta1-3Galbeta1-4Glc-PA, which contains the blood group A antigen; the K(d) value for immobilized Nh was 4.8 x 10(-5) m, and that for Ch was 8.1 x 10(-4) m. The present results clearly indicate that the two sugar binding sites of LEC-1 have different sugar binding properties.     

Novel carbohydrate specificity of the 16-kDa galectin from Caenorhabditis elegans: binding to blood group precursor oligosaccharides (type 1, type 2, Talpha,and Tbeta) and gangliosides

Galectins, a family of soluble beta-galactosyl-binding lectins, are believed to mediate cell-cell and cell-extracellular matrix interactions during development, inflammation, apoptosis, and tumor metastasis. However, neither the detailed mechanisms of their function(s) nor the identities of their natural ligands have been unequivocally elucidated. Of the several galectins present in the nematode Caenorhabditis elegans, the 16-kDa "proto" type and the 32-kDa "tandem-repeat" type are the best characterized so far, but their carbohydrate specificities have not been examined in detail. Here, we report the carbohydrate-binding specificity of the recombinant C. elegans 16-kDa galectin and the structural analysis of its binding site by homology modeling. Our results indicate that unlike the galectins characterized so far, the C. elegans 16-kDa galectin interacts with most blood group precursor oligosaccharides (type 1, Galbeta1,3GlcNAc, and type 2, Galbeta1,4GlcNAc; Talpha, Galbeta1,3GalNAcalpha; Tbeta, Galbeta1,3GalNAcbeta) and gangliosides containing the Tbeta structure. Homology modeling of the C. elegans 16-kDa galectin CRD revealed that a shorter loop containing residues 66-69, which enables interactions of Glu(67) with both axial and equatorial -OH at C-3 of GlcNAc (in Galbeta1,4GlcNAc) or at C-4 of GalNAc (in Galbeta1,3GalNAc), provides the structural basis for this novel carbohydrate specificity.     

Solid-phase assays for study of carbohydrate specificity of galectins

We have recently shown that the carbohydrate-binding pattern of galectins in cells differs from that determined in artificial (non-cellular) test-systems. To understand the observed discrepancy, we compared several test-systems differing in the mode of galectin presentation on solid phase. The most representative system was an assay where the binding of galectin (human galectins-1 and -3 were studied) to asialofetuin immobilized on solid phase was inhibited by polyacrylamide glycoconjugates, Glyc-PAA. This approach permits us to range quantitatively glycans (Glyc) by their affinity to galectin, i.e. to study both high and low affinity ligands. Our attempts to imitate the cell system by solid-phase assay were not successful. In the cell system galectin binds glycoconjugates by one carbohydrate-recognizing domain (CRD), and after that the binding to the remaining non-bound CRD is studied by means of fluorescein-labeled Glyc-PAA. In an “imitation” variant when galectins are loaded on adsorbed asialofetuin or Glyc-PAA followed by revealing of binding by the second Glyc-PAA, the interaction was not observed or glycans were ordered poorly, unlike in the inhibitory assay. When galectins were adsorbed on corresponding antibodies (when all CRDs were free for recognition by carbohydrate), a good concentration dependence was observed and patterns of specificities were similar (though not identical) for the two methods; notably, this system does not reflect the situation in the cell. Besides the above-mentioned, other variants of solid-phase analysis of galectin specificity were tested. The results elucidate the mechanism and consequence of galectin CRD cis-masking on cell surface.     

Adaptive Regulation at the Cell Surface by N-Glycosylation

The association of receptors and solute transporters with components of the endocytic machinery regulates their surface levels, and thereby cellular sensitivity to cytokines, ligands and nutrients in the extracellular environment. Most transmembrane receptors and solute transporters are glycoproteins, and the Asn ( N )-linked oligosaccharides ( N -glycans) can bind animal lectins, forming multivalent lattices or microdomains that regulate glycoprotein mobility in the plane of membrane. The N -glycan number (sequence-encoded NXS/T) and context-dependent Golgi N -glycan branching cooperate to regulate glycoprotein affinities for the galectin family of lectins. Galectin-3 binding reduces EGF receptor trafficking into clathrin-coated pits and caveolae lipid rafts, decreases ligand-independent receptor activation and promotes α5β1 integrin remodelling in focal adhesions. N -glycan branching in the medial Golgi increases glycan affinity for galectins, and the Golgi pathway is sensitive to uridine diphosphate-N-acetylglucosamine (UDP-GlcNAc) supply, in turn hexosamine pathway metabolites (fructose-6-P, glutamine and acetyl-CoA). Thus, lattice avidity and cellular responsiveness to extracellular cues are regulated in an adaptive manner by metabolism and Golgi modification to glycoproteins. Computational modelling of the hexosamine/Golgi/lattice has provided new insight on cell surface adaptation in cancer and autoimmune disease.     

Nutrient-responsive O-GlcNAcylation dynamically modulates the secretion of glycan-binding protein galectin 3

Endomembrane glycosylation and cytoplasmic O-GlcNAcylation each play essential roles in nutrient sensing, and characteristic changes in glycan patterns have been described in disease states such as diabetes and cancer. These changes in glycosylation have important functional roles and can drive disease progression. However, little is known about the molecular mechanisms underlying how these signals are integrated and transduced into biological effects. Galectins are proteins that bind glycans and that are secreted by a poorly characterized nonclassical secretory mechanism. Once outside the cell, galectins bind to the terminal galactose residues of cell surface glycans and modulate numerous extracellular functions, such as clathrin-independent endocytosis (CIE). Originating in the cytoplasm, galectins are predicted substrates for O-GlcNAc addition and removal; and as we have shown, galectin 3 is a substrate for O-GlcNAc transferase. In this study, we also show that galectin 3 secretion is sensitive to changes in O-GlcNAc levels. We determined using immunoprecipitation and Western blotting that there is a significant difference in O-GlcNAcylation status between cytoplasmic and secreted galectin 3. We observed dramatic alterations in galectin 3 secretion in response to nutrient conditions, which were dependent on dynamic O-GlcNAcylation. Importantly, we showed that these O-GlcNAc-driven alterations in galectin 3 secretion also facilitated changes in CIE. These results indicate that dynamic O-GlcNAcylation of galectin 3 plays a role in modulating its secretion and can tune its function in transducing nutrient-sensing information coded in cell surface glycosylation into biological effects.     

Galectins and urological cancer

Galectins are a family of proteins defined by their affinity for beta-galactoside and by their conserved sequence. Each galectins exhibits a specific expression pattern in various tissues and their expression is regulated during development. Their expression is altered in many types of cancers and non-cancerous disorders. They interact with glycoproteins in both extracellular and intracellular milieu and regulate various biological phenomenon including cell growth, cell differentiation, cell adhesion, and apoptosis. A series of experimental and clinical evidences have been reported to support correlation between galectin expressions and neoplastic transformation. The recent findings show that expressions of galectins are elevated with neoplastic progression in certain malignancies, and therefore, galectins are expected to serve as reliable tumor markers. In this review, we describe the expression and role of galectins in urological cancers and their clinical applications for diagnostic and therapeutic use.     

Correspondence of gradual developmental increases of expression of galectin-reactive glycoconjugates with alterations of the total contents of the two differentially regulated galectins in chicken intestine and liver as indication for overlapping functions.

The duplication of genes for recognition molecules and the ensuing diversification of the members of such families generate complex groups of homologous proteins. One example are galactoside-specific lectins whose sequences display constant features related to sugar binding, the galectins. Based on the inverse abundance of the chicken galectins CG-14 and CG-16 in adult intestine and liver, these two lectins represent a model to comparatively study expression of the related proteins and the galectin-reactive sites (glycoproteins and glycolipids) biochemically and histochemically. Functional overlap and/or acquisition of distinct functions would be reflected in qualitative and/or quantitative aspects of ligand display. Using five different stages of embryogenesis, differential regulation of the two galectins was detected in liver and intestine. The clear preference for one galectin (CG-14) was observed in intestine already at rather early stages, whereas equivalence for both proteins was noted in liver from day 12 to day 18 prior to hatching, as seen by ELISA assays and Western blot analysis. Presentation of galectin-reactive glycoproteins showed a tendency for gradual increase in both organs. Galectin-blotting analysis revealed primarily very similar patterns of positive bands at the different stages of development and only few quantitative and qualitative changes. The reactivity of glycolipids in a solid-phase assay was more variable, even surpassing the response of extracts of the adult organ at several embryonic stages. While the localization patterns of the galectins and galectin-reactive sites were nearly indistinguishable in the liver, intestinal tissue differed with respect to the placement and accessibility of binding sites. Thus, the results suggest a differential regulation of galectin activities in the two organs. As a sum they resemble the course of development of availability of glycoprotein ligands in vitro. These findings support the notion for a partial functional redundancy in this family. The described approach to employ galectin-specific antibodies and the labeled galectins as tools to assess presentation of ligands is suggested to be of general relevance to address the question of distinct vs. overlapping functions of related recognition molecules.     

Ah,sweet mystery of death! Galectins and control of cell fate

Control of cell death is critical in eukaryotic development, immune system homeostasis, and control of tumorigenesis. The galectin family of lectins is implicated in all of these processes. Other families of molecules function as death receptors or death effectors, but galectins are uniquely capable of acting both extracellularly and intracellularly to control cell death. Extracellularly, galectins cross-link glycan ligands to transduce signals that lead directly to death or that influence other signals regulating cell fate. Intracellular expression of galectins can modulate other signals controlling cell viability. Individual galectins can act on multiple cell types, and multiple galectins can act on the same cell. Understanding how galectins regulate cell viability and function will broaden our knowledge of the roles of galectins in basic biological processes and facilitate development of therapeutic applications for galectins in autoimmunity, transplant-related disease, and cancer.     

Probing the cons and pros of lectin-induced immunomodulation: case studies for the mistletoe lectin and galectin-1

When imagining to monitor animal cells through a microscope with resolution at the molecular level, a salient attribute of their surfaces will be the abundance of glycan chains. They present galactosides at their termini widely extending like tentacles into the extracellular space. Their spatial accessibility and their potential for structural variability endow especially these glycan parts with capacity to act as docking points for molecular sensors (sugar receptors such as lectins). Binding and ligand clustering account for transmission of post-binding signals into the cell interior. The range of triggered activities has turned plant lectins into popular tools in cell biology and immunology. Potential for clinical application has been investigated rigorously only in recent years. As documented in vitro and in vivo for the galactoside-specific mistletoe lectin, its apparent immunomodulatory capacity reflected in upregulation of production of proinflammatory cytokines will not necessarily be clinically favorable but a double-edged sword. In fact, lectin application has been shown to stimulate tumor growth in cell lines, histocultures of human tumors and in two animal models using chemical carcinogenesis or tumor transplantation. When testing immunological effects of the endogenous lectin galectin-1, protection against disorders mediated by activated T cells came up for consideration. Elimination of these cells via CD7-dependent induction of apoptosis, and a shift to the Th2 response by the galectin, are factors to ameliorate disease states. This result encourages further efforts with other galectins. Functional redundancy, synergism, diversity or antagonism among galectins are being explored to understand the actual role of this class of endogenous lectins in inflammation. Regardless of the results of further preclinical testing for galectin-1, these two case studies break new ground in our understanding how glycans as ligands for lectins convey reactivity to immune cells, with impact on the course of a tumor or autoimmune disease.