Mitogenicity and binding properties of beta-galactoside-binding lectin from chick-embryo kidney

Glycobiology of developing chicken kidney: Profiling the galectin family and selected β-galactosides

The concept of the sugar code interprets the cellular glycophenotype as a rich source of information read by glycan-lectin recognition in situ. This study's aim is the comprehensive characterization of galectin expression by immunohistochemistry during chicken nephrogenesis along with mapping binding sites by (ga)lectin histochemistry. Light and two-color fluorescence microscopy were used. First, six plant/fungal lectins that are specific for galectin-binding parts of N- and O-glycans were applied. The spatiotemporally regulated distributions of these glycans in meso- and metanephros equip cells with potential binding partners for the galectins. Complete galectin profiling from HH Stage 20 (about 70-72 hr) onward revealed cell-, galectin-, and stage-dependent expression patterns. Representatives of all three types of modular architecture of the galectin family are detectable, and overlaps of signal distribution in light and two-color fluorescence microscopy illustrate a possibility for functional cooperation among them. Performing systematic galectin histochemistry facilitated comparisons between staining profiles of plant lectins and galectins. They revealed several cases for differences so that tissue lectins appear to be selective among the β-galactosides. Notably, selectivity is also disclosed in intrafamily comparison. Thus, combining experimental series with plant and tissue lectins is a means to characterize target populations of glycans presented by cellular glycoconjugates for individual galectins. Our results document the presence and sophisticated level of elaboration among β-galactosides and among the members of the family of galectins during organogenesis, using chicken galectins and kidney as model. Thus, they provide a clear guideline for functional assays using supramolecular tools, cells, and organ cultures.

How galectins have become multifunctional proteins

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

"Stuck on sugars - how carbohydrates regulate cell adhesion, recognition, and signaling"

We have explored the fundamental biological processes by which complex carbohydrates expressed on cellular glycoproteins and glycolipids and in secretions of cells promote cell adhesion and signaling. We have also explored processes by which animal pathogens, such as viruses, bacteria, and parasites adhere to glycans of animal cells and initiate disease. Glycans important in cell signaling and adhesion, such as key O-glycans, are essential for proper animal development and cellular differentiation, but they are also involved in many pathogenic processes, including inflammation, tumorigenesis and metastasis, and microbial and parasitic pathogenesis. The overall hypothesis guiding these studies is that glycoconjugates are recognized and bound by a growing class of proteins called glycan-binding proteins (GBPs or lectins) expressed by all types of cells. There is an incredible variety and diversity of GBPs in animal cells involved in binding N- and O-glycans, glycosphingolipids, and proteoglycan/glycosaminoglycans. We have specifically studied such molecular determinants recognized by selectins, galectins, and many other C-type lectins, involved in leukocyte recruitment to sites of inflammation in human tissues, lymphocyte trafficking, adhesion of human viruses to human cells, structure and immunogenicity of glycoproteins on the surfaces of human parasites. We have also explored the molecular basis of glycoconjugate biosynthesis by exploring the enzymes and molecular chaperones required for correct protein glycosylation. From these studies opportunities for translational biology have arisen, involving production of function-blocking antibodies, anti-glycan specific antibodies, and synthetic glycoconjugates, e.g. glycosulfopeptides, that specifically are recognized by GBPs. This invited short review is based in part on my presentation for the IGO Award 2019 given by the International Glycoconjugate Organization in Milan.

Prototype chicken galectins revisited: characterization of a third protein with distinctive hydrodynamic behaviour and expression pattern in organs of adult animals

Prototype galectins are versatile modulators of cell adhesion and growth via their reactivity to certain carbohydrate and protein ligands. These functions and the galectins' marked developmental regulation explain their attractiveness as models to dissect divergent evolution after gene duplication. Only two members have so far been assumed to constitute this group in chicken, namely the embryonic muscle/liver form {C-16 or CLL-I [16 kDa; chicken lactose lectin, later named CG-16 (chicken galectin-16)]} and the embryonic skin/intestine form (CLL-II or C-14; later named CG-14). In the present study, we report on the cloning and expression of a third prototype CG. It has deceptively similar electrophoretic mobility compared with recombinant C-14, the protein first isolated from embryonic skin, and turned out to be identical with the intestinal protein. Hydrodynamic properties unusual for a homodimeric galectin and characteristic traits in the proximal promoter region set it apart from the two already known CGs. Their structural vicinity to galectin-1 prompts their classification as CG-1A (CG-16)/CG-1B (CG-14), whereas sequence similarity to mammalian galectin-2 gives reason to refer to the intestinal protein as CG-2. The expression profiling by immunohistochemistry with specific antibodies discerned non-overlapping expression patterns for the three CGs in several organs of adult animals. Overall, the results reveal a network of three prototype galectins in chicken.

Galectin-1: a bifunctional regulator of cellular proliferation

Galectin-1 has demonstrated a diverse range of activities in relation to cell survival and proliferation. In different circumstances, it acts as a mitogen, as an inhibitor of cell proliferation, and as a promoter of cellular apoptosis. Many of these activities, particularly the mitogenic and apoptotic responses, follow from the interaction of galectin-1 with cell-surface beta-galactoside ligands, but there is increasing evidence for protein-protein interactions involving galectin-1, and for a beta-galactoside-independent cytostatic mechanism. The bifunctional nature of galectin-1, in conjunction with other experimental variables, makes it difficult to assess the overall outcomes and significance of the growth-regulatory actions in many previous investigations. There is thus a need for well-defined experimental cross-correlation of observations, for which specific loss-of-function galectin-1 mutants will be invaluable. Unsurprisingly, in view of this background, the interpretation of the actions of galectin-1 in developmental situations, both normal and neoplastic, is often very complex.

Dimeric galectin-1 induces surface exposure of phosphatidylserine and phagocytic recognition of leukocytes without inducing apoptosis

We report that human galectin-1 (dGal-1), a small dimeric beta-galactoside-binding protein, induces phosphatidylserine (PS) exposure, measured by Annexin V staining, on human promyelocytic HL-60 cells, T leukemic MOLT-4 cells, and fMet-Leu-Phe-activated, but not resting, human neutrophils. This effect of dGal-1 on HL-60 and MOLT-4 cells is enhanced by pretreatment of the cells with neuraminidase, but treatment of resting neutrophils with neuraminidase does not enhance their sensitivity to dGal-1. Although the induction of staining with Annexin V is often associated with apoptosis, the dGal-1-treated HL-60 cells, MOLT-4 cells, and activated neutrophils do not undergo apoptosis, and there is no detectable DNA fragmentation. HL-60 and MOLT-4 cells treated with dGal-1 continue to grow normally. By contrast, camptothecin-treated HL-60 cells, etoposide-treated MOLT-4 cells, and anti-Fas-treated neutrophils exhibit extensive DNA fragmentation and/or cell death. Lactose inhibits the dGal-1-induced effects, indicating that dGal-1-induced signaling requires binding to cell surface beta-galactosides. The dimeric form of Gal-1 is required for signaling, because a monomeric mutant form of Gal-1, termed mGal-1, binds to cells but does not cause these effects. Importantly, dGal-1, but not mGal-1, treatment of HL-60 cells and activated human neutrophils significantly promotes their phagocytosis by activated mouse macrophages. These dGal-1-induced effects are distinguishable from apoptosis, but like apoptotic agents, prepare cells for phagocytic removal. Such effects of dGal-1 may contribute to leukocyte homeostasis.

Partial identification by site-directed mutagenesis of a cell growth inhibitory site on the human galectin-1 molecule

BACKGROUND

Previous work, by us and others, has shown that mammalian galectins-1 have a growth-inhibitory activity for mammalian cells which is apparently independent of their beta-galactoside binding site.

RESULTS

We have made recombinant human galectin-1 as a bacterial fusion protein with an N-terminal hexahistidine tag. This protein displays both haemagglutination and growth-inhibitory activities, even in the presence of the hexahistidine tag. Site-directed mutagenesis of this protein has confirmed the independent nature of the protein sites responsible for the two biological activities. Mutant proteins were created, which displayed each activity in the absence of the other.

CONCLUSIONS

Human galectin-1 possesses a growth-inhibitory site, which is not part of the beta-galactoside binding site. A surface loop, comprising amino acid residues 25-30, and joining two internal beta-strands, forms part of the growth-inhibitory site. This region is relatively close to the N-terminus of the protein, and N-terminal substitutions or extensions also affect growth-inhibitory activity. Further experiments will be necessary to fully define this site.

Negative regulation of neuroblastoma cell growth by carbohydrate-dependent surface binding of galectin-1 and functional divergence from galectin-3

The cell density-dependent growth inhibition of human SK-N-MC neuroblastoma cells is initiated by increased ganglioside sialidase activity leading to elevated cell surface presentation of ganglioside GM1, a ligand of galectin-1. We herein show that the extent of the cell surface expression of the galectin coincides with marked increases of the sialidase activity. Reverse transcriptase-polymerase chain reaction analysis excludes a regulation at the transcriptional level. Exposure of cells to purified galectin-1 reveals its carbohydrate-dependent activity to reduce cell proliferation. Assays to detect DNA fragmentation biochemically and cytometrically and to block caspases render it unlikely that galectin-1 acts as a classical proapoptotic factor on these cells. Because the chimeric galectin-3 shares binding sites and binding parameters with galectin-1 for these cells, we tested whether this galectin will elicit the same response as the homodimeric cross-linking galectin-1. Evidently, galectin-3 fails to affect cell growth by itself but interferes with galectin-1 upon coincubation. Its proteolytically truncated variant, the C-terminal lectin domain with impaired capacity to form aggregates when surface bound, has only weak binding properties. Thus, the way in which the galectin-1 interacts topologically with an apparently common set of ligands relative to galectin-3 is crucial for eliciting post-binding events. We conclude that galectin-1 is a probable effector in the sialidase-dependent growth control in this system. Moreover, the experiments with galectin-3 reveal functional divergence, most probably based on different topologies of presentation of homologous carbohydrate-binding sites.

Elucidation of the mechanism of interaction of sheep spleen galectin-1 with splenocytes and its role in cell-matrix adhesion

The binding of a 14 kDa beta-galactoside animal lectin to splenocytes has been studied in detail. The binding data show that there are two classes of binding sites on the cells for the lectin: a high-affinity site with a K(a) ranging from 1.1 x 10(6) to 5.1 x 10(5) M (-1) and a low affinity binding site with a K(a) ranging from 7.7 x 10(4) to 3.4 x 10(4) M (-1). The number of receptors per cell for the high- and low-affinity sites is 9 +/- 3 x 10(6) and 2.5 +/- 0.5 x 10(6), respectively. The temperature dependence of the K value yielded the thermodynamic parameters. The energetics of this interaction shows that, although this interaction is essentially enthalpically driven (DeltaH - 21 kJ lambdamol(-1)) for the high-affinity sites, there is a very favorable entropy contribution to the free energy of this interaction (-TDeltaS - 17.5 Jmol(-1)), suggesting that hydrophobic interaction may also be playing a role in this interaction. Lactose brought about a 20% inhibition of this interaction, whereas the glycoprotein asialofetuin brought about a 75% inhibition, suggesting that complex carbohydrate structures are involved in the binding of galectin-1 to splenocytes. Galectin-1 also mediated the binding and adhesion of splenocytes to the extracellular matrix glycoprotein laminin, suggesting a role for it in cell-matrix interactions.

Temporal and spatial regulation of expression of two galectins during kidney development of the chicken

Organogenesis and the establishment of the mature phenotype require an interplay between diverse recognition systems. Concerning protein-carbohydrate interactions, galectins are known to be involved in several extra- and intracellular functions. Due to the occurrence of two avian galectins in liver (chicken galectin-16 CG-16) and intestine (chicken galectin-14; CG-14) with different developmental regulation. the questions addressed are to what extent and where these galectins are present during chicken kidney development. Using Western blot analysis, the presence of both activities in tissue extracts was ascertained. A solid-phase assay showed peak levels at day 12 followed by a decline. A histochemical analysis was carried out in combination with routine staining. Epithelial cells of the mesonephric proximal tubules were immunoreactive in the cytoplasm for CG-14 from day 5 of incubation onwards. Additionally, epithelial cells of the metanephric collecting ducts were stained. For CG-16 a rather similar pattern of staining was seen, additional positivity in early glomerular podocytes being notable. At the electron microscopical level, a diffuse staining for CG-14 was seen in the cytoplasm, whereas immunoreactivity for CG-16 was observed mainly in mitochondria. These results demonstrate quantitative differences in the developmental regulation of the two avian galectins with obvious similarities in the cell-type pattern but with a disparate intracellular localisation profile.

Galectin-3 stimulates cell proliferation

Galectin-3, a beta-galactoside-binding protein, has been shown to be involved in multiple biological processes through interaction with its complementary glycoconjugates. Here we provide the first evidence of galectin-3 as a mitogen. Incubation of quiescent cultures of normal human lung fibroblast IMR-90 cells with recombinant galectin-3 (rgalectin-3) stimulated DNA synthesis as well as cell proliferation in a dose-dependent manner. This mitogenic activity was dependent on the lectin property of galectin-3, as it could be significantly inhibited by lactose, a disaccharide competitive for carbohydrate-binding by galectin-3. Chemical cross-linking and affinity-purification experiments identified binding of rgalectin-3 to cell surface glycoproteins, which were not recognized by antibodies directed against lysosome-associated membrane proteins (LAMPs), putative cellular ligands for galectin-3. Moreover, pulse-chase analysis revealed no secretion of galectin-3 by IMR-90 cells. These results indicate that galectin-3 is a mitogen capable of stimulating fibroblast cell proliferation in a paracrine fashion through interaction with cell surface glycoconjugates different from LAMPs and suggest a possible involvement of galectin-3 in tissue remodeling.

Biphasic modulation of cell growth by recombinant human galectin-1

Human soluble galactose-binding lectin (galectin-1) has been expressed as an Escherichia coli fusion protein, following the amplification by polymerase chain reaction of cDNA prepared from a human osteosarcoma cell line. The fusion protein is a functional beta-galactoside-binding lectin, as is the recombinant galectin when purified from the cleaved fusion protein. The recombinant galectin has a biphasic effect on cell proliferation. Unlike the fusion protein, it functions as a human cell growth inhibitor, confirming earlier findings with natural human galectin-1, though it is less effective than the natural galectin. This reaction is not significantly inhibited by lactose, and is thus largely independent of the beta-galactoside-binding site. At lower concentrations, recombinant galectin-1 is mitogenic, this activity being susceptible to inhibition by lactose, and thus attributable to the beta-galactoside-binding ability of the protein. Some tumour cells are susceptible to the growth-inhibitory effect, and the galectin-1 gene is expressed in both normal and tumour cells.

Energetics of carbohydrate binding by a 14 kDa S-type mammalian lectin

The thermodynamics of the binding of derivatives of galactose and lactose to a 14 kDa beta-galactoside-binding lectin (L-14) from sheep spleen has been studied in 10 nM phosphate/150 mM NaCl/10 mM beta-mercaptoethanol buffer, pH 7.4, and in the temperature range 285-300 K using titration calorimetry. The single-site binding constants of various sugars for the lectin were in the following order: N-acetyl-lactosamine thiodigalactoside > 4-methylumbelliferyl lactoside > lactose > 4-methylumbelliferyl alpha-D-galactoside > methyl-alpha-galactose > methyl-beta-galactose. Reactions were essentially enthalpically driven with the binding enthalpies ranging from -53.8 kJ/mol for thiodigalactoside at 301 K to -2.2 kJ/mol for galactose at 300 K, indicating that hydrogen-bonding and van der Waals interactions provide the major stabilization for these reactions. However, the binding of 4-methylumbelliferyl-alpha-D-galactose displays relatively favourable entropic contributions, indicating the existence of a non-polar site adjacent to the galactose-binding subsite. From the increments in the enthalpies for the binding of lactose, N-acetyl-lactosamine and thiodigalactoside relative to methyl-beta-galactose, the contribution of glucose binding in the subsite adjacent to that for galactose shows that glucose makes a major contribution to the stability of L-14 disaccharide complexes. Observation of enthalpy-entropy compensation for the recognition of saccharides such as lactose by L-14 and the absence of it for monosaccharides such as galactose, together with the lack of appreciable changes in the heat capacity (delta Cp), indicate that reorganization of water plays an important role in these reactions.

On the possible role of endogenous lectins in early animal development

In this review I have tried to summarize the information available on the lectins of developing embryos. The emerging evidence indicates that during fertilization carbohydrate-binding proteins play a role in sperm adhesion and in the reorganization of the extracellular matrix of the fertilized egg. Results also indicate that in adult tissues lectins participate in cell recognition and adhesion, and that several galactose-binding lectins function as receptors for laminin and, in principle could also interact with polylactosamine groups of other extracellular matrix glycoproteins. Since in developing embryos lectins are located at the cell surface, and colocalize with extracellular matrix glycoproteins, they could play a role in transitory adhesive interactions and in the segregation of organ primordia. On the basis of experiments in cultured cell lines, it has been suggested that lectins are involved in lysosomal and nuclear glycoprotein transport. These carbohydrate-binding proteins could also regulate development by modulating these processes in the embryo. Since galactose-binding lectins are mitogenic, and are present in high concentration in the chick yolk sac, these proteins could be released into the embryonic circulation, bind to cells expressing appropriate receptors, and act as growth regulators, by modulating cell division of specific cell lineages.

Evidence for export of a muscle lectin from cytosol to extracellular matrix and for a novel secretory mechanism

A soluble lactose-binding lectin with subunit Mr of 14,500 is believed to function by interacting with extracellular glycoconjugates, because it has been detected extracellularly by immunohistochemistry. This localization has been questioned, however, since the lectin lacks a secretion signal sequence, which challenges the contention that it is secreted. We have demonstrated externalization of this lectin from C2 mouse muscle cells by both immunoprecipitation of metabolically labeled protein and immunohistochemical localization. We further show that externalization of the lectin is a developmentally regulated process that accompanies myoblast differentiation and that the lectin codistributes with laminin in myotube extracellular matrix. Immunohistochemical localization during intermediate stages of externalization suggests that the lectin becomes concentrated in evaginations of plasma membrane, which pinch off to form labile lectin-rich extracellular vesicles. This suggests a possible mechanism for lectin export from the cytosol to the extracellular matrix.

Lectin activity and distribution of chicken lactose lectin I in the extracellular matrix of the chick developing kidney

A lectin activity inhibitable by thiodigalactose, N-acetyllactosamine, lactulose, lactose and by an antibody raised against CLL I (chicken-lactose lectin I) has been investigated in the chick embryo developing kidney. At post-induction stages this activity was found in both mesonephros and metanephros. In immunofluorescence and immunoelectron microscopy, the extracellular distribution of CLL I was similar in the mesonephros and the metanephros. The lectin was never found intracellularly; cultured kidney cells did not express any endogenous lectin but were rich in lectin-receptor sites, which led to the hyphothesis that CLL I is not produced in situ but could be adsorbed on renal cells. Potential physiological roles for embryonic lectins are discussed.

Comparative analysis of galactoside-binding lectins isolated from mammalian spleens

Galactoside-inhibitable lectins have been isolated from rabbit, rat, mouse, pig, lamb, calf, and human spleens. Native molecular mass, subunit structure, pI, and hemagglutinating activity have been compared for these lectins. The yields of lectin varied from 1.8 mg/kg for rabbit spleen to 79 mg/kg for lamb spleen. Pig, lamb, calf, and human spleen lectins yielded single protein peaks when subjected to Superose 12 fast-protein liquid chromatography. The apparent molecular mass for these lectins was 33-34 kDa. In contrast, rat and mouse spleen lectin preparations were separated into three components ranging from 8.4 to 34 kDa. Superose 12 chromatography of rabbit spleen lectin revealed the presence of at least six components. Gradient slab gel sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed the presence of single polypeptides for pig, calf, lamb, and human lectins corresponding to a molecular mass of 14-14.5 kDa. Multiple polypeptides were detected for the mouse, rat, and rabbit lectins. The molecular mass of the major polypeptides were 15, 15, and 17 kDa for rat, mouse, and rabbit, respectively. The presence of isolectins in all preparations was shown by isoelectric focusing. The major isolectins were acidic proteins with pI 4.38-4.80. Hemagglutination and hemagglutination inhibition assays demonstrated similarities as well as differences among the lectin preparations. Hemagglutinating activity could not be demonstrated in rabbit spleen extracts nor for isolated putative lectin. Human buffy coat cells were reversibly agglutinated by calf and human spleen lectins, demonstrating the presence of leucocyte cell surface lectin receptors.

Differential expression of endogenous sugar-binding proteins (lectins) in murine tumor model systems with metastatic capacity

In order to investigate possible differences in sugar binding activities of strongly versus weakly metastatic tumors, sugar-binding molecules (endogenous lectins) of murine tumor cells differing in metastatic capacity were analyzed by affinity chromatography on supports with immobilized sugars or glycoproteins and compared. After elution with specific sugar in the absence of Ca2+-ions, the proteins were separated by sodium dodecyl sulfate-polyacrylamide slab gel electrophoresis. In comparison to a weakly metastatic subline (Eb) spontaneous strongly metastatic variants (ESb) of a murine lymphoma contained additional sugar receptors for N-acetylglucosamine (Mr 30 kDa) and maltose (Mr 64 kDa, 62 kDa, 54 kDa and 32 kDa), and lacked one sugar receptor for myoinositol (Mr 85 kDa), N-acetylglucosamine (Mr 23 kDa) and maltose (Mr 22 kDa), respectively. The strongly metastatic variant ESb expressed the common beta-galactoside-specific lectin to a higher extent and receptors for myo-inositol, melibiose and mannan to a lower extent. In another model system derived from the murine mastocytoma cell line P 815 X 2A, biochemical analysis of the liver-metastasizing variant P 815 X 2B revealed additional characteristic N-acetylgalactosamine- and maltose-specific binding proteins. This variant had reduced amounts of receptors for beta-galactosides and fucose in comparison to the parental clone. In a third tumor system a similar qualitative difference was disclosed: a metastatic variant derived from spleen metastases displayed a sugar receptor profile with 5 additional beta-galactoside-binding proteins when compared to its parental clone 6-6#3 + F, which is a virally transformed fibroblast line. The results show that metastatic variants of 3 murine tumor models consisting of lymphomas, mastocytomas and sarcomas are characterized by qualitative and quantitative alterations in the profiles of sugar-binding proteins.

Mitogenic heparin-binding lectin-like protein from cloned thymic myoid cells

A mitogenic heparin-binding (reactive) lectin-like protein (HBP) was purified from the extract of a cloned rat thymic myoid cell R615B2 by a one-step procedure of affinity chromatography on a heparin--Sepharose CL-6B column. Four distinct peptide bands with molecular weights of 10,000, 13,000, 13,700, and 14,600 were detected on SDS-polyacrylamide gel electrophoresis. This protein is mitogenic at concentrations of as low as 1.1-70.0 ng/ml for peanut lectin-nonagglutinated thymocytes and splenocytes from euthymic mice and rats but not for splenocytes from nude mice. These results indicate that thymic myoid cell-derived HBP is an important signal for one particular step in T-cell differentiation.