Isolation and characterization of chicken thymic electrolectin

Cysteine Oxidation in Human Galectin-1 Occurs Sequentially via a Folded Intermediate to a Fully Oxidized Unfolded Form

Galectins are multifunctional effectors in cellular homeostasis and dysregulation. Oxidation of human galectin-1 (Gal-1) with its six sulfhydryls produces a disulfide-bridged oxidized form that lacks normal lectin activity yet gains new glycan-independent functionality. Nevertheless, the mechanistic details as to how Gal-1 oxidation occurs remain unclear. Here, we used 15N and 13C HSQC NMR spectroscopy to gain structural insight into the CuSO4-mediated path of Gal-1 oxidation and identified a minimum two-stage conversion process. During the first phase, disulfide bridges form slowly between C16-C88 and/or C42-C66 to produce a partially oxidized, conformationally flexible intermediate that retains the ability to bind lactose. Site-directed mutagenesis of C16 to S16 impedes the onset of this overall slow process. During the second phase, increased motional dynamics of the intermediate enable the relatively distant C2 and C130 residues to form the third and final disulfide bond, leading to an unfolded state and consequent dimer dissociation. This fully oxidized end state loses the ability to bind lactose, as shown by the hemagglutination assay. Consistent with this model, we observed that the Gal-1 C2S mutant maintains intermediate-state structural features with a free sulfhydryl group at C130. Incubation with dithiothreitol reduces all disulfide bonds and allows the lectin to revert to its native state. Thus, the sequential, non-random formation of three disulfide bridges in Gal-1 in an oxidative environment acts as a molecular switch for fundamental changes to its functionality. These data inspire detailed bioactivity analysis of the structurally defined oxidized intermediate in, e.g., acute and chronic inflammation.

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.

The glycosylation status of murine [corrected] postnatal thymus: a study by histochemistry and lectin blotting

During the intrathymic development, the fate of the thymocytes depends largely on variable expression of CD4/CD8 markers and T cell receptor protein expressions. In addition, changes of cell surface glycosylation status also affect the thymocyte maturation. In this study the glycosylation alterations in thymic tissues from 1, 9, 13 and 16 days old mice were evaluated by histochemical and lectin blotting techniques. With alcian blue (AB) at pH 5.7/periodic acid-Schiff (PAS) stainings, it was shown that thymic microenvironments contained carboxlylated and sulfated glycosaminoglycans (GAGs). Strong positivity to AB at pH 2.5, which specific for sialomucins, was seen in some medullary thymocytes. Similarly, it was shown that with Maackia amurensis agglutinin (MAL) medullary thymocytes, but not cortical ones, contained alpha(2 --> 3) linked sialic acid structures. On the other hand, while reaction with peanut agglutinin (PNA), which specific for core disaccharide galactose beta(1 --> 3) N-acetylgalactosamine, was only seen in cortical thymocytes, reaction with Galanthus nivalis agglutinin (GNA), which specific for terminal mannose residues, was seen in both cortex and medulla. However, Datura stramonium agglutinin (DSA), which recognizes galactose beta(1 --> 4) N-acetylglucosamine, was not only cell-specific, but it was bound some thymic vessels. With lectin blotting studies, five glycoprotein bands of molecular weights approximately 39, approximately 54, 100, approximately 110 and approximately 212 were found which reacted with MAL, PNA and DSA as well as GNA. These results suggest that glycosylation patterns of cell surface glycoconjugates are modified during thymocyte selection processes of postnatal days.

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.

Oxidized galectin-1 promotes axonal regeneration in peripheral nerves but does not possess lectin properties

Galectin-1 has recently been identified as a factor that regulates initial axonal growth in peripheral nerves after axotomy. Although galectin-1 is a well-known beta-galactoside-binding lectin, its potential to promote axonal regeneration as a lectin has not been reported. It is essential that the process of initial repair in peripheral nerves after axotomy is well clarified. We therefore undertook to investigate the relation between the structure and axonal regeneration-promoting activity of galectin-1. Recombinant human galectin-1 secreted into the culture supernatant of transfected COS1 cells (rhGAL-1/COS1) was purified under nonreducing conditions and subjected to structural analysis. Mass spectrometric analysis of peptide fragments from rhGAL-1/COS1 revealed that the secreted protein exists as an oxidized form containing three intramolecular disulfide bonds (Cys2-Cys130, Cys16-Cys88 and Cys42-Cys60). Recombinant human galectin-1 (rhGAL-1) and a galectin-1 mutant in which all six cysteine residues were replaced by serine (CSGAL-1) were expressed in and purified from Escherichia coli for further analysis; the purified rhGAL-1 was subjected to oxidation, which induced the same pattern of disulfide linkages as that observed in rhGAL-1/COS1. Oxidized rhGAL-1 enhanced axonal regeneration from the transected nerve sites of adult rat dorsal root ganglion explants with associated nerve stumps (5.0-5000 pg. mL-1), but it lacked lectin activity. In contrast, CSGAL-1 induced hemagglutination of rabbit erythrocytes but lacked axonal regeneration-promoting activity. These results indicate that galectin-1 promotes axonal regeneration only in the oxidized form containing three intramolecular disulfide bonds, not in the reduced form which exhibits lectin activity.

Identification of CD8 as a peanut agglutinin (PNA) receptor molecule on immature thymocytes

Differentiation of most T lymphocytes occurs within the thymus and is characterized by variable expression of CD4/CD8 coreceptor molecules, increased surface density of T cell antigen receptor (TCR) alpha beta proteins, and decreased expression of glycan chains recognized by the galactose-specific lectin peanut agglutinin (PNA). Although appreciated for several decades that PNA agglutination is useful for the physical separation of immature and mature thymocyte sub-populations, the identity of specific PNA-binding glycoproteins expressed on immature thymocytes remains to be determined. In the current report, we studied the expression of PNA-specific glycans on immature and mature T cells and used lectin affinity chromatography and immunoprecipitation techniques to characterize PNA-binding glycoproteins on thymocytes. Our data demonstrate that PNA-specific glycans are localized on a relatively small subset of thymocyte surface proteins, several of which were specifically identified, including CD43, CD45, and suprisingly, CD8 molecules. CD8 alpha and CD8 alpha' proteins bound to PNA in the absence of CD8 beta expression showing that O-glycans on CD8 beta glycoproteins are not necessary for PNA binding and that glycosylation of CD8 alpha and CD8 alpha' proteins proceeds effectively in the absence of CD8 beta. Finally, we demonstrate that PNA binding of CD8 is developmentally regulated by sialic acid addition as CD8 proteins from mature T cells bound to PNA only after sialidase treatment. These studies identify CD8 as a PNA receptor molecule on immature thymocytes and show that PNA binding of CD8 on immature and mature T cells is developmentally regulated by sialic acid modification.

ESR studies on the effect of ionic radii on displacement of Mn2+ bound to a soluble beta-galactoside binding hepatic lectin

Binding of divalent metal ions to hepatic soluble beta-galactoside binding lectin was studied using electron spin resonance (ESR) spectroscopy. The Mn2+ bound to hepatic lectin could be displaced by Mg2+, Cu2+, Ni2+ and Ca2+ but not by Sr2+. As the ionic radii of Mg2+ (0.65 A), Cu2+ (0.73 A) and Ni2+ (0.72 A) are appreciably smaller than Ca2+ (0.99 A), it appears that the Mn2+ binding site is more accessible to Mg2+, Cu2+, and Ni2+ as compared to Ca2+, the ionic radius of Mn2+ being 0.80 A. Sr2+ with an ionic radius of 1.13 is thus unable to displace bound Mn2+. Surprisingly, the presence of specific sugars like alpha-lactose, or alpha-D-galactose facilitated the displacement of bound Mn2+ by metal ions whereas non-specific sugars, i.e. alpha-D-glucose, beta-D-fructose and alpha-D-ribose had no effect. It appears that minor perturbations in the saccharide binding site significantly affect the ability of the metal binding site to ligate bivalent metals.

Human thymic epithelial cells express an endogenous lectin, galectin-1, which binds to core 2 O-glycans on thymocytes and T lymphoblastoid cells

Thymic epithelial cells play a crucial role in the selection of developing thymocytes. Thymocyte-epithelial cell interactions involve a number of adhesion molecules, including members of the integrin and immunoglobulin superfamilies. We found that human thymic epithelial cells synthesize an endogenous lectin, galectin-1, which binds to oligosaccharide ligands on the surface of thymocytes and T lymphoblastoid cells. Binding of T lymphoblastoid cells to thymic epithelial cells was inhibited by antibody to galectin-1 on the epithelial cells, and by two antibodies, T305 and 2B11, that recognize carbohydrate epitopes on the T cell surface glycoproteins CD43 and CD45, respectively. T lymphoblastoid cells and thymocytes bound recombinant galectin-1, as demonstrated by flow cytometric analysis, and lectin binding was completely inhibited in the presence of lactose. The degree of galectin-1 binding to thymocytes correlated with the maturation stage of the cells, as immature thymocytes bound more galectin-1 than did mature thymocytes. Preferential binding of galectin-1 to immature thymocytes may result from regulated expression of preferred oligosaccharide ligands on those cells, since we found that the epitope recognized by the T305 antibody, the core 2 O-glycan structure on CD43, was expressed on cortical, but not medullary cells. The level of expression of the UDP-GlcNAc:Gal beta 1,3GalNAc-R beta 1, 6GlcNAc transferase (core 2 beta 1, 6 GlcNAc transferase, or C2GnT), which creates the core 2 O-glycan structure, correlated with the glycosylation change between cortical and medullary cells. Expression of mRNA encoding the C2GnT was high in subcapsular and cortical thymocytes and low in medullary thymocytes, as demonstrated by in situ hybridization. These results suggest that galectin-1 participates in thymocyte-thymic epithelial cell interactions, and that this interaction may be regulated by expression of relevant oligosaccharide ligands on the thymocyte cell surface.

Different immunoreactivities of anti-soluble lactose lectin antisera to tissues from early chick embryos: a histochemical study

The location of soluble lactose-binding proteins (S-lac lectins) has been studied by immunohistochemical methods during morphogenesis of the chick embryo, when segregation and early differentiation of organ primordia was occurring. Using a panel of polyclonal antisera raised to various purified lectin preparations, we observed striking differences in the antigenic properties of these antisera, indicating that diverse versions of the lectins may be expressed during development. The antisera referred to as anti-L-16, anti-M-16, anti-S-14 and anti-I-14 were respectively raised to native or denatured 16 kDa lectins from adult liver and embryonic muscle and to 14 kDa lectins from embryonic skin and adult intestine. Having determined the optimal immunohistochemical conditions in the preparation of embryo sections (fixation, embedding, sectioning) we show that anti-L-16, anti-S-14 and anti-I-14 mostly bind the lectins expressed at the cell surface, in the extracellular matrix and in some released secretion. As previously shown, anti-L-16 and anti-S-14 are also able to recognize the cytoplasmic form of some migrative lectin-rich cells (primitive streak, neural crest cells, germ cells). Anti-M-16 was bound exclusively to the cytoplasmic form of the 16 kDa lectin in the same cell lines as above and also in some others, such as in the notochord, the myotomal part of the somites, the pharyngeal endoderm and the cardiac muscle. These different antigenic properties may be applied to the accurate mapping of various lectin isoforms and evaluation of the respective contribution of their intra- and extracellular variants during development and differentiation.

Isolation and characterization of a galactose-specific carbohydrate binding protein from human lymphoblastic cells

A carbohydrate binding protein of Mr = 32,000 (CBP 32) has been isolated from detergent extracts of human B and T lymphoblastoid cells. CBP 32 binds specifically to glycoproteins containing asparagine-linked complex oligosaccharides, and can be eluted from a fetuin affinity matrix by beta-lactose. Binding is not thiol dependent, nor are divalent cations necessary for binding. Native CBP 32 appears to exist as a monomer, with a pI of 8.2. Purified CBP 32 can bind detergent, as shown by charge-shift electrophoresis, and thus appears to be an integral membrane protein.

Extensive amino acid sequence homologies between animal lectins

We have established the amino acid sequence of the beta-D-galactoside binding lectin from the electric eel and the sequences of several peptides from a similar lectin isolated from human placenta. These sequences were compared with the published sequences of peptides derived from the beta-D-galactoside binding lectin from human lung and with sequences deduced from cDNAs assigned to the beta-D-galactoside binding lectins from chicken embryo skin and human hepatomas. Significant homologies were observed. One of the highly conserved regions that contains a tryptophan residue and two glutamic acid residues is probably part of the beta-D-galactoside binding site, which, on the basis of spectroscopic studies of the electric eel lectin, is expected to contain such residues. The similarity of the hydropathy profiles and the predicted secondary structure of the lectins from chicken skin and electric eel, in spite of differences in their amino acid sequences, strongly suggests that these proteins have maintained structural homologies during evolution and together with the other beta-D-galactoside binding lectins were derived from a common ancestor gene.

Oxidation and chemical modification of lung beta-galactoside-specific lectin

Galaptins are small, soluble, lectins with a specificity for beta-galactose residues. Many galaptins are inactivated by atmospheric oxygen and are protected by disulphide-reducing reagents. We find that each subunit of rat lung galaptin contains one residue of tryptophan and six of cysteine. Oxygen inactivates rat lung galaptin by oxidation of the cysteine residues. During oxidation, the normal dimeric structure is maintained and all disulphide bonds are formed within individual subunits. Exogenous thiols protect against inactivation, but oxidized thiols accelerate inactivation. Human lung fibroblast galaptin is almost completely inactivated within 1 h in tissue culture medium at 37 degrees C. Alkylation of native rat lung galaptin with iodoacetate or ethyleneimine causes substantial loss of activity. The dimeric galaptin structure is maintained. In contrast, alkylation with iodoacetamide yields carboxamidomethyl-galaptin, which is fully active and stable to atmospheric oxygen in the absence of disulphide-reducing reagents. This derivative is very useful for studies of galaptin properties and function.