Detection and functions of mammalian lectins--with emphasis on membrane lectins

What is the Sugar Code?

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

Glycans in autophagy, endocytosis and lysosomal functions

Glycans have been shown to function as versatile molecular signals in cells. This prompted us to look at their roles in endocytosis, endolysosomal system and autophagy. We start by introducing the cell biological aspects of these pathways, the concept of the sugar code, and provide an overview on the role of glycans in the targeting of lysosomal proteins and in lysosomal functions. Moreover, we review evidence on the regulation of endocytosis and autophagy by glycans. Finally, we discuss the emerging concept that cytosolic exposure of luminal glycans, and their detection by endogenous lectins, provides a mechanism for the surveillance of the integrity of the endolysosomal compartments, and serves their eventual repair or disposal.

Glycan Chains of Gangliosides: Functional Ligands for Tissue Lectins (Siglecs/Galectins)

Molecular signals on the cell surface are responsible for adhesion and communication. Of relevance in this respect, their chemical properties endow carbohydrates with the capacity to store a maximum of information in a minimum of space. One way to present glycans on the cell surface is their covalent conjugation to a ceramide anchor. Among the resulting glycosphingolipids, gangliosides are special due to the presence of at least one sialic acid in the glycan chains. Their spatial accessibility and the dynamic regulation of their profile are factors that argue in favor of a role of glycans of gangliosides as ligands (counterreceptors) for carbohydrate-binding proteins (lectins). Indeed, as discovered first for a bacterial toxin, tissue lectins bind gangliosides and mediate contact formation (trans) and signaling (cis). While siglecs have a preference for higher sialylated glycans, certain galectins also target the monosialylated pentasaccharide of ganglioside GM1. Enzymatic interconversion of ganglioside glycans by sialidase action, relevant for neuroblastoma cell differentiation and growth control in vitro, for axonogenesis and axon regeneration, as well as for proper communication between effector and regulatory T cells, changes lectin-binding affinity profoundly. The GD1a-to-GM1 "editing" is recognized by such lectins, for example, myelin-associated glycoprotein (siglec-4) losing affinity and galectin-1 gaining reactivity, and then translated into postbinding signaling. Orchestrations of loss/gain of affinity, of ganglioside/lectin expression, and of lectin presence in a network offer ample opportunities for fine-tuning. Thus glycans of gangliosides such as GD1a and GM1 are functional counterreceptors by a pairing with tissue lectins, an emerging aspect of ganglioside and lectin functionality.

A Panel of Recombinant Mucins Carrying a Repertoire of Sialylated O-Glycans Based on Different Core Chains for Studies of Glycan Binding Proteins

Sialylated glycans serve as key elements of receptors for many viruses, bacteria, and bacterial toxins. The microbial recognition and their binding specificity can be affected by the linkage of the terminal sugar residue, types of underlying sugar chains, and the nature of the entire glycoconjugate. Owing to the pathobiological significance of sialylated glycans, we have engineered Chinese hamster ovary (CHO) cells to secrete mucin-type immunoglobulin-fused proteins carrying terminal α2,3- or α2,6-linked sialic acid on defined O-glycan core saccharide chains. Besides stably expressing P-selectin glycoprotein ligand-1/mouse immunoglobulin G2b cDNA (PSGL-1/mIgG2b), CHO cells were stably transfected with plasmids encoding glycosyltransferases to synthesize core 2 (GCNT1), core 3 (B3GNT6), core 4 (GCNT1 and B3GNT6), or extended core 1 (B3GNT3) chains with or without the type 1 chain-encoding enzyme B3GALT5 and ST6GAL1. Western blot and liquid chromatography-mass spectrometry analysis confirmed the presence of core 1, 2, 3, 4, and extended core 1 chains carrying either type 1 (Galb3GlcNAc) or type 2 (Galb4GlcNAc) outer chains with or without α2,6-linked sialic acids. This panel of recombinant mucins carrying a repertoire of sialylated O-glycans will be important tools in studies aiming at determining the fine O-glycan binding specificity of sialic acid-specific microbial adhesins and mammalian lectins.

Lectins: getting familiar with translators of the sugar code

The view on the significance of the presence of glycans in glycoconjugates is undergoing a paradigmatic change. Initially mostly considered to be rather inert and passive, the concept of the sugar code identifies glycans as highly versatile platform to store information. Their chemical properties endow carbohydrates to form oligomers with unsurpassed structural variability. Owing to their capacity to engage in hydrogen (and coordination) bonding and C-H/π-interactions these “code words” can be “read” (in Latin, legere) by specific receptors. A distinct class of carbohydrate-binding proteins are the lectins. More than a dozen protein folds have developed carbohydrate-binding capacity in vertebrates. Taking galectins as an example, distinct expression patterns are traced. The availability of labeled endogenous lectins facilitates monitoring of tissue reactivity, extending the scope of lectin histochemistry beyond that which traditionally involved plant lectins. Presentation of glycan and its cognate lectin can be orchestrated, making a glycan-based effector pathway in growth control of tumor and activated T cells possible. In order to unravel the structural basis of lectin specificity for particular glycoconjugates mimetics of branched glycans and programmable models of cell surfaces are being developed by strategic combination of lectin research with synthetic and supramolecular chemistry.

A guide into glycosciences: How chemistry, biochemistry and biology cooperate to crack the sugar code

BACKGROUND

The most demanding challenge in research on molecular aspects within the flow of biological information is posed by the complex carbohydrates (glycan part of cellular glycoconjugates). How the 'message' encoded in carbohydrate 'letters' is 'read' and 'translated' can only be unraveled by interdisciplinary efforts.

SCOPE OF REVIEW

This review provides a didactic step-by-step survey of the concept of the sugar code and the way strategic combination of experimental approaches characterizes structure-function relationships, with resources for teaching.

MAJOR CONCLUSIONS

The unsurpassed coding capacity of glycans is an ideal platform for generating a broad range of molecular 'messages'. Structural and functional analyses of complex carbohydrates have been made possible by advances in chemical synthesis, rendering production of oligosaccharides, glycoclusters and neoglycoconjugates possible. This availability facilitates to test the glycans as ligands for natural sugar receptors (lectins). Their interaction is a means to turn sugar-encoded information into cellular effects. Glycan/lectin structures and their spatial modes of presentation underlie the exquisite specificity of the endogenous lectins in counterreceptor selection, that is, to home in on certain cellular glycoproteins or glycolipids.

GENERAL SIGNIFICANCE

Understanding how sugar-encoded 'messages' are 'read' and 'translated' by lectins provides insights into fundamental mechanisms of life, with potential for medical applications.

Structural studies on mannose-selective glycoprotein receptors using molecular modeling techniques

Glycoproteins play important roles in various cellular events and their presence in appropriate locations in proper active conformations is essential for many biochemical functions. Recent evidences suggest that some glycoproteins may require sorting receptors for efficient exit from the endoplasmic reticulum. These receptors need the presence of calcium or other metal ions for their native activity. The three-dimensional structure of such a receptor, p58/ERGIC-53, has been recently solved by x-ray crystallography, which is a mannose-selective lectin and contains two Ca(2+) ions. Homology search in the sequence databases indicates a large number of proteins which bear varying degrees of homology in a wide spectrum of species with this receptor. In this study we have systematically searched for such genes which are potential candidates for acting as mannose-mediated glycoprotein receptors in various species as initially inferred from their amino acid sequence homology. Structures of a number of proteins have been predicted using knowledge-based homology modeling, and their ability to act as the glycoprotein receptor has been explored by examining the nature of sugar-binding site. Tetramer of mannose was docked in the binding pockets of the modeled structures followed by energy minimization and molecular dynamics to obtain most probable structures of the complexes. Properties of these modeled complexes were studied to examine the nature of physicochemical forces involved in the complex formation and compared with p58/ERGIC-53-mannose complex.

Lectins: from basic science to clinical application in cancer prevention

Many physiological functions are attributable to lectin-carbohydrate interactions. Lectins are currently being studied for their ability to destroy tumour growth by binding to specific carbohydrate motifs on cancer cells. Cell-surface molecules, including growth factor receptors are often glycosylated, and lectins may act by binding to these. Certain lectins effect the proliferation of intestinal epithelial cells. This effect is cell-type and lectin specific and occurs in the intestine of intact animals, in human colonic explants and colorectal cancer cell lines. Lectins present in mammalian tissue are involved in cell-matrix adhesion, differentiation, lymphocyte circulation and immunomodulation. Mammalian lectins contribute to detection, diagnosis and prognosis of tumour cells, and can be targeted for therapy. New lectins of plant and mammalian origin that have one or more of these functions are currently being developed as tools that could be used to target tumour cells.

Chemical biology of the sugar code

A high-density coding system is essential to allow cells to communicate efficiently and swiftly through complex surface interactions. All the structural requirements for forming a wide array of signals with a system of minimal size are met by oligomers of carbohydrates. These molecules surpass amino acids and nucleotides by far in information-storing capacity and serve as ligands in biorecognition processes for the transfer of information. The results of work aiming to reveal the intricate ways in which oligosaccharide determinants of cellular glycoconjugates interact with tissue lectins and thereby trigger multifarious cellular responses (e.g. in adhesion or growth regulation) are teaching amazing lessons about the range of finely tuned activities involved. The ability of enzymes to generate an enormous diversity of biochemical signals is matched by receptor proteins (lectins), which are equally elaborate. The multiformity of lectins ensures accurate signal decoding and transmission. The exquisite refinement of both sides of the protein-carbohydrate recognition system turns the structural complexity of glycans--a demanding but essentially mastered problem for analytical chemistry--into a biochemical virtue. The emerging medical importance of protein-carbohydrate recognition, for example in combating infection and the spread of tumors or in targeting drugs, also explains why this interaction system is no longer below industrial radarscopes. Our review sketches the concept of the sugar code, with a solid description of the historical background. We also place emphasis on a distinctive feature of the code, that is, the potential of a carbohydrate ligand to adopt various defined shapes, each with its own particular ligand properties (differential conformer selection). Proper consideration of the structure and shape of the ligand enables us to envision the chemical design of potent binding partners for a target (in lectin-mediated drug delivery) or ways to block lectins of medical importance (in infection, tumor spread, or inflammation).

Chromosomal aberrations, profiles of expression of growth-related markers including galectins and environmental hazards in relation to the incidence of chondroid pulmonary hamartomas

This prospective study includes 103 cases of chondroid pulmonary hamartomas, resected over a period of nearly six years. Genes encoding proteins of the high motility group (HMGI-C, (Y), chromosomes 12q15 and 6p21) were analyzed cytogenetically. Furthermore, we examined the expression of growth-regulatory markers, including galectins-1, -3, -8, heparin-binding lectin (HBL), calcyclin (S100A6) and macrophage migration inhibitory factor (MIF), as well as that of Ki-67 (MIB-1). Syntactic structure analysis was applied to automated classification of stained histological slides and for the detection of topological properties in hamartomas and disease-free lung. These data were set in relation to clinical features, including environmental hazards, smoking habit, and the occurrence of heart-lung disease. Men and women contributed to the study in 61 and 42 cases, respectively. Smoking was frequent (75% men and 54% women), with a mean tobacco consumption of 36 pack years. Aberrations affecting exclusively the HMGI-C gene and the HGMI(Y) gene were seen in 46 cases (44.7%) and in 22 cases (21.3%), respectively. Both genes were affected in only one case. Abnormalities most frequently occurred in chromosomal bands 6p12 and 12q14. Genetic aberrations were significantly increased in men exposed to environmental (occupational) risk factors, excluding smoking (p < 0.05), and in tumors larger than average hamartomas. There were significant differences in staining profiles, particularly for calcyclin and MIF. The mean proliferation index was Nv = 9.9 +/- 6.4%; structural entropy was similar in all markers applied. Owing to their remarkably high values (from 142 to 148), these data were in contrast to a low current of entropy seen in most markers applied. The staining profile identified several markers that delimited cell positivity from normal parenchymal cells. These results contribute to the definition of biochemical characteristics in hamartomas and can be useful for distinguishing them from chronic degenerative disorders.

Animal lectins: from initial description to elaborated structural and functional classification

The genetic code connects the two biochemical dimensions of nucleic acids and proteins. Theoretical calculations on coding capacity reveal that oligosaccharides as hardware surpass peptides by more than seven orders of magnitude based on hexamer synthesis. Thus, the sugar code establishes the third dimension of biological information transfer. Using carbohydrate-binding proteins (lectins, enzymes and antibodies) the information content of such epitopes is decoded. Currently, five families of animal lectins are defined in structural terms. i.e. the C-type, I-type and P-type groups, the galectins and the pentraxins. They are involved in intra- and intercellular glycan routing using oligosaccharides as postal-code equivalents and acting as defense molecules homing in on foreign or aberrant glycosignatures, as crosslinking agent in biosignaling and as coordinator of transient or firm cell-cell/cell-matrix contacts. By delineating the driving forces toward complex formation, knowledge about the causes for specificity can be turned into design of custom-made high-affinity ligands for clinical application, e.g. in anti-adhesion therapy, drug targeting or diagnostic histopathology.

Analysis of binding of mannosides in relation to Langerin (CD207) in Langerhans cells of normal and transformed epithelia

Tandem-repeat C-type lectins (pattern-recognition receptors) with specificity for mannosides are intimately involved in antigen recognition, uptake, routing and presentation in macrophages and dendritic cells. In Langerhans cells, Langerin (CD207), a type-II transmembrane protein with a single C-type carbohydrate recognition domain attached to a heptad repeat in the neck region, which is likely to establish oligomers with an alpha-coiled-coil stalk, has been implicated in endocytosis and the formation of Birbeck granules. The structure of Langerin harbours essential motifs for Ca2+-binding and sugar accommodation. Lectin activity has previously been inferred by diminished antibody binding to cells in the presence of the glycan ligand mannan. In view of the complexity of the C-type lectin/lectin-like network, it is unclear what role Langerin plays for Langerhans cells in binding mannosides. In order to reveal in frozen tissue sections to what extent mannose-binding activity co-localizes with Langerin, we have used a synthetic marker, i.e. a neoglycoprotein carrying mannose maxiclusters, as a histochemical ligand, and computer-assisted fluorescence monitoring in a double-labelling procedure. Mannoside-binding capacity was detected in normal epithelial cells. Double labelling ensured the unambiguous assessment of the binding of the neoglycoprotein in Langerhans cells. Light-microscopically, its localization profile resembled the pattern of immunohistochemical detection of Langerin. This result has implications for suggesting rigorous controls in histochemical analysis of this cell type, because binding of kit reagents, i.e. mannose-rich glycoproteins horseradish peroxidase or avidin, to Langerin (or a spatially closely associated lectin) could yield false-positive signals. To show that recognition of carbohydrate ligands in dendritic cells is not restricted to mannose clusters, we have also documented binding of carrier-immobilized histo-blood group A trisaccharide, a ligand of galectin-3, which was not affected by the presence of a blocking antibody to Langerin. Remarkably, access to the carbohydrate recognition domain of Langerin appeared to be impaired in proliferatively active environments (malignancies, hair follicles), indicating presence of an endogenous ligand with high affinity to saturate the C-type lectin under these conditions.

Glycohistochemistry: the why and how of detection and localization of endogenous lectins

The central dogma of molecular biology limits the downstream flow of genetic information to proteins. Progress from the last two decades of research on cellular glycoconjugates justifies adding the enzymatic production of glycan antennae with information-bearing determinants to this famous and basic pathway. An impressive variety of regulatory processes including cell growth and apoptosis, folding and routing of glycoproteins and cell adhesion/migration have been unravelled and found to be mediated or modulated by specific protein (lectin)-carbohydrate interactions. The conclusion has emerged that it would have meant missing manifold opportunities not to recruit the sugar code to cellular information transfer. Currently, the potential for medical applications in anti-adhesion therapy or drug targeting is one of the major driving forces fuelling progress in glycosciences. In histochemistry, this concept has prompted the introduction of carrier-immobilized carbohydrate ligands (neoglycoconjugates) to visualize the cells' capacity to be engaged in oligosaccharide recognition. After their isolation these tissue lectins will be tested for ligand analysis. Since fine specificities of different lectins can differ despite identical monosaccharide binding, the tissue lectins will eventually replace plant agglutinins to move from glycan profiling and localization to functional considerations. Namely, these two marker types, i.e. neoglycoconjugates and tissue lectins, track down accessible binding sites with relevance for involvement in interactions in situ. The documented interplay of synthetic organic chemistry and biochemistry with cyto- and histochemistry nourishes the optimism that the application of this set of innovative custom-prepared tools will provide important insights into the ways in which glycans can act as hardware in transmitting information during normal tissue development and pathological situations.

Endogenous lectins as targets for drug delivery

To minimize side effects of drugs it would be ideal to target them exclusively to those cell types which require treatment. As a means to this end prototypical cellular recognition systems pique our interest to devise biomimetic strategies. Since oligosaccharides of glycoconjugates outmatch other information-carrying biomolecules (proteins, nucleic acids) in theoretical storage capacity by far, work on the sugar code can spark off development of effective targeting devices. Conjugation of custom-made glycan epitopes to proteins or biocompatible non-immunogenic polymeric scaffolds produces neoglycoconjugates with purpose-adaptable properties. In the interplay with endogenous receptors such as lectins, suitable oligosaccharides such as histo-blood group trisaccharides as parts of neoglycoconjugates have already proven their practical applications in histopathology. Elucidation of the structure of cell lectins with currently five main families aids to tailor ligand characteristics rationally. They include the types of functional groups and their topological presentation to optimize the bimolecular binding as well as the optimal spatial clustering and spacer characteristics to exploit cooperativity. Indeed, the potent trivalent cluster glycosides designed for the C-type asialoglycoprotein receptors furnish an instructive example how to turn the theoretical guideline on ligand modification into nM-affinity. By placing emphasis on tissue lectins as targets of neoglycoconjugate-mediated drug delivery, the long-term perspective is opened to likewise test members of these families themselves for routing of therapeutic payloads, aiming at cell addressins. This review illustrates the conceivable potential which work on the sugar code with custom-made neoglycoconjugates and tissue lectins can have in store for drug delivery.

Lectin-mediated drug targeting: selection of valency, sugar type (Gal/Lac), and spacer length for cluster glycosides as parameters to distinguish ligand binding to C-type asialoglycoprotein receptors and galectins

PURPOSE

Common oligosaccharides of cellular glycoconjugates are ligands for more than one type of endogenous lectin. Overlapping specificities to beta-galactosides of C-type lectins and galectins can reduce target selectivity of carbohydrate-ligand-dependent drug targeting. The purpose of this study is to explore distinct features of ligand presentation and structure for design of cluster glycosides to distinguish between asialoglycoprotein-specific (C-type) lectins and galectins.

METHODS

Extent of binding of labeled sugar receptors to two types of matrix-immobilized (neo)glycoproteins and to cells was evaluated in the absence and presence of competitive inhibitors. This panel comprised synthetic mono-, bi-, and trivalent glycosides with two spacer lengths and galactose or lactose as ligand part.

RESULTS

In contrast to C-type lectins of hepatocytes and macrophages, bi- and trivalent glycosides do not yield a notable glycoside cluster effect for galectins-1 and -3. Also, these Ca2+-independent galactoside-binding proteins prefer to home in on lactose-bearing glycosides relative to galactose as ligand, while spacer length requirements were rather similar.

CONCLUSIONS

Trivalent cluster glycosides with Gal/GalNAc as ligand markedly distinguish between C-type lectins and galectins. Undesired side reactivities to galectins for C-type lectin drug delivery will thus be minimal.

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.

TIN2, a new regulator of telomere length in human cells

Telomeres are DNA-protein structures that cap linear chromosomes and are essential for maintaining genomic stability and cell phenotype. We identified a novel human telomere-associated protein, TIN2, by interaction cloning using the telomeric DNA-binding-protein TRF1 as a bait. TIN2 interacted with TRF1 in vitro and in cells, and co-localized with TRF1 in nuclei and metaphase chromosomes. A mutant TIN2 that lacks amino-terminal sequences effects elongated human telomeres in a telomerase-dependent manner. Our findings suggest that TRF1 is insufficient for control of telomere length in human cells, and that TIN2 is an essential mediator of TRF1 function.

Sequence and expression of the monkey homologue of the ER-golgi intermediate compartment lectin, ERGIC-53

We obtained the cDNA sequence of the monkey homologue of the intermediate compartment protein ERGIC-53 by both cDNA library screening and RT-PCR amplification. The final sequence of 2422 nts of the monkey ERGIC-53 cDNA is 96.2% identical to the human ERGIC-53 cDNA and 87% and 67% identical to the rat and amphibian cDNA, respectively. The translated CV1 ERGIC-53 protein is 96.47% identical to the human ERGIC-53, 87% identical to the rat p58 and 66. 98% to the Xenopus laevis protein. Southern blot analysis of multiple genomic DNAs shows the presence of sequences similar to ERGIC-53 in different species. ERGIC-53 is expressed as a major transcript of about 5.5 kb in either monkey CV1 or in human CaCo2. A shorter transcript of 2.3 kb was detected in both cell lines and in mRNAs derived from human pancreas and placenta.

Interaction of Azospirillum lipoferum with wheat germ agglutinin stimulates nitrogen fixation

In vitro, the nitrogen fixation capability of A. lipoferum is efficiently increased in the presence of wheat germ agglutinin (WGA). A putative WGA-binding receptor, a 32-kDa protein, was detected in the cell capsule. The stimulatory effect required N-acetyl-D-glucosamine dimer (GlcNAcdi) terminated sugar side chains of the receptor and was dependent on the number of GlcNAcdi links involved in receptor-WGA interface. Binding to the primary sugar binding sites on WGA had a larger stimulatory effect than binding to the secondary sites. The WGA-receptor complex generated stimulus led to elevated transcription of the nifH and nifA genes and of the glnBA gene cluster but not of the glnA gene from its own promoter. There may well be a signalling cascade contributing to the regulation of nitrogen fixation.

A parallel association between differentiation and induction of galectin-1, and inhibition of galectin-3 by retinoic acid in mouse embryonal carcinoma F9 cells

Soluble endogenous lactoside-binding lectins, galectins, have been implicated in cell adhesion, growth, differentiation, neoplastic transformation, and metastasis. Two major classes of these lectins, galectin-1 and galectin-3, are developmentally regulated. To explore the mechanisms by which the expression of the galectins is regulated and to examine their association with the differentiation processes induced by all-trans retinoic acid (RA), dibutyryl cyclic AMP (Bt2cAMP) and their combination, we used the murine embryonal carcinoma (EC) cell line F9 and its RA-resistant mutant, RA-3-10. RA induced endodermal differentiation and a concurrent induction of galectin-1 and its complementary glycoconjugates (laminin and lysosomal-associated membrane protein, LAMP) in the F9 wild-type (wt) line, but failed to induce differentiation and had no effects on or even reduced the expression of galectin-1, laminin, and LAMP in the RA-3-10 line. On the other hand, RA inhibited expression of galectin-3 in the wild-type line but had no effect on the RA-3-10 line. The galectin-1 gene is at least partially regulated at the transcriptional level. These results demonstrate a parallel association between differentiation and induction of galectin-1, and inhibition of galectin-3 in F9 cells by RA. The study suggests that a regulated expression of galectins and their complementary glycoconjugates is involved in the differentiation pathway induced by RA in F9 cells.

Lectin histochemistry of astrocytic tumors and in vitro characterization of lectin-induced modifications on the proliferation of the SW1088, U373 and U87 human astrocytic cell lines

The role of lectins as biosignalling molecules or as markers of human astrocytic tumors remains relatively unexplored. The aim of the present work is to investigate (1) whether or not human astrocytic tumors express specific glycans, evidenced experimentally by means of lectin histochemistry, and (2) whether, in turn, these lectins can significantly modulate astrocytic tumor cell proliferation. Using a cell image processor, we therefore began by quantitatively measuring the histochemical binding pattern of 5 lectins (WGA, PNA, PHA-L, GSA-IA4 and Con A) in 5 astrocytomas, 5 anaplastic astrocytomas and 5 glioblastomas. Secondly, we measured the influence of these 5 lectins on the proliferation of 3 astrocytic tumor cell lines (SW1088, U373 and U87) growing in vitro as monolayers. Cell proliferation was assessed by means of the colorimetric MTT assay. The histochemical lectin staining markedly varied intra- and inter-group. However, some constant results were obtained. Indeed, the staining increased markedly from GSA-IA4 and PHA-L through WGA and PNA to ConA in the three histopathological groups. The assessment of cell proliferation demonstrated that WGA, Con A and PHA-L very significantly decreased proliferation in the 3 astrocytic cell lines in a dose-dependent manner. Astrocytic tumor cells in the confluent growth phase were less sensitive to the WGA, Con A and PHA-L lectin-induced effects than cells in the log growth phase. The GSA-IA4 and PNA lectins had globally very weak effects on the proliferation of the astrocytic tumor cell lines. Increasing the fetal calf serum from 1% to 10% in the culture media significantly antagonized the WGA-, Con A- and PHA-L-induced cell proliferation decrease in the 3 astrocytic cell lines. In conclusion, the present data strongly suggest that some lectins (including WGA, Con A and PHA-L) significantly influence the proliferation of astrocytic tumor cells.

Animal lectins

Protein and lipid glycosylation is no longer considered as a topic whose appeal is restricted to a limited number of analytical experts perseveringly pursuing the comprehensive cataloguing of structural variants. It is in fact arousing curiosity in various areas of basic and applied bioscience. Well founded by the conspicuous coding potential of the sugar part of cellular glycoconjugates which surpasses the storage capacity of oligonucleotide- or oligopeptide-based code systems, recognition of distinct oligosaccharide ligands by endogenous receptors, i.e. lectins and sugar-binding enzymes or antibodies, is increasingly being discovered to play salient roles in animal physiology. Having inevitably started with a descriptive stage, research on animal lectins has now undubitably reached maturity. Besides listing the current categories for lectin classification and providing presentations of the individual families and their presently delineated physiological significance, this review places special emphasis on tracing common structural and functional themes which appear to reverberate in nominally separated lectin and animal categories as well as lines of research which may come to fruition for medical sciences.

Influence of intraperitoneal injection of three types of hydrogel beads on expression of carbohydrate-binding sites in spleen macrophages

The influence of the chemical structure of polymer implants on selected characteristics of macrophages was studied to improve our understanding of the mechanisms of non-self recognition of synthetic materials. Three types of polymers differing in net charge were prepared to compare in vivo responses. Beads from preparation of poly(2-hydroxyethyl methacrylate), a copolymer of 2-hydroxyethyl methacrylate with sodium methacrylate, and a copolymer of 2-hydroxyethyl methacrylate and dimethylaminoethyl methacrylate were injected intraperitoneally into rats and harvested 48 h later. The effects of these polymers on the presence of inflammatory cells in the peritoneal exudate, on the adhesion of macrophages to individual batches of the different types of beads and on distinct molecular aspects of macrophages in the red pulp of spleen were evaluated. Beads from both types of copolymer caused an elevation in the number of macrophages in the exudate, in contrast to the situation in rats treated with poly(2-hydroxyethyl methacrylate) beads and physiological saline solution as control. The molecular design of the implant had no significant influence on the extent of macrophage adhesion to beads or on the expression of selected carbohydrate-binding sites. Since important cellular functions such as cell adhesion and glycoprotein routing depend on the sugar part of glycoconjugates, labelled neoglycoproteins were employed to analyse this aspect of macrophages in the tested animals. The beads of the copolymer of 2-hydroxyethyl methacrylate with dimethylaminoethyl methacrylate clearly led to an elevation of the expression of specific binding sites for beta-galactoside-terminating structures which are presented by asialofetuin, for mannose, fucose, sialic acid and N-acetylgalactosamine, which had been used as the ligand parts of biotinylated neoglycoproteins, in spleen macrophages whereas the levels of sites which recognize mannose-6-phosphate were unaffected. Expression of sites with specificity to N-acetylglucosamine was lessened. The effect of beads from the copolymer of 2-hydroxyethyl methacrylate with sodium methacrylate on the measured glycobiological features in the splenic macrophages was only negligible. These results suggest the possibility of systemic effects of implanted polymers on the distinct recognitive functions of macrophages.

Lectin-like receptor for alpha 1-acid glycoprotein in the epithelium of the rat prostate gland and seminal vesicles

BACKGROUND

A receptor for alpha 1-acid glycoprotein glycoforms AGP-B and AGP-C in the epithelium of the rat prostate gland and seminal vesicles is described.

METHODS

The interaction between AGP-glycoforms and their receptor is a lectin-like interaction confirmed by inhibition of the binding by mannose and N-Acetyl-D-glucosamine.

RESULTS

In vitro the receptor was also inhibited by the steroid hormones cortisone, aldosterone, progesterone, and estradiol, but not by testosterone. A significant regional variation in the expression of AGP-lectin receptor and in the localization of AGP was seen in rat prostate and seminal vesicles. The localization of the AGP lectin receptor is compared to the localization of glycoprotein AGP, and small differences are found.

CONCLUSIONS

It is proposed the AGP receptors in the prostate and seminal vesicles belong to a group of lectins in the control of differentiation and organ formation.

Acetylcholinesterase-induced respiratory burst in macrophages: evidence for the involvement of the macrophage mannose-fucose receptor

It has long been suggested that acetylcholinesterase is capable of functioning in a non-cholinergic manner. However, very little is known about the molecular structures which mediate the interaction between this protein and the cellular membrane. Previously it was demonstrated that acetylcholinesterase interacted in a carbohydrate-specific manner with peritoneal macrophages and induced the 'respiratory burst' [1]. This study aimed to establish whether a carbohydrate-binding site exists on the acetylcholinesterase molecule itself, or alternatively, whether the macrophage carbohydrate-binding receptor is involved. No carbohydrate binding properties intrinsic to acetylcholinesterase were detected using affinity chromatography with immobilised monosaccharides, erythrocyte agglutination and gel-diffusion techniques. The interaction between acetylcholinesterase and several monosaccharide columns observed in this study appeared to be due to ionic interactions. Moreover, it was shown that a specific inhibitor of the enzymatic activity of AChE, BW284C51, could inhibit the peritoneal cell response not only to acetylcholinesterase, but also to several other stimuli, thus exhibiting a non-specific effect on macrophages. However, the inhibitory effects of specific ligands of the macrophage mannose-fucose receptor and the inability of non-glycosylated acetylcholinesterase to interact with macrophages suggested that the effect of acetylcholinesterase on peritoneal cells is most probably mediated by the macrophage mannose-fucose receptor. The role of the mannose-fucose receptor in triggering the respiratory burst response was supported by the fact that several ligands of these receptors were capable of inducing the functional response of macrophages.

Mapping of endogenous lectins in macrophages colonizing an implanted polymer surface--effect of polymer structure

The design of synthetic polymers may affect structural features of inflammatory cells, for example the expression of endogenous sugar receptor molecules like lectins in macrophages and foreign body giant multinucleate cells. This characteristic was studied histochemically by use of biotinylated (neo)glycoproteins. The results demonstrate the influence of polymer structure on the phenotypic expression of molecules recognizing carbohydrates in these cells. Very strong activity was observed for carrier-immobilized beta-galactose in cells colonizing hydrophobic polystyrene implants. Sialic acid moieties were not recognized by cells located on the surface of all types of polymeric implant. This study encourages investigation of the influence of polymer design on the differentiation of macrophages.

Protein 1a: a major wheat germ agglutinin binding protein on the surface of human granulocytes associated with the cytoskeleton

Lectin-receptors on leukocyte and endothelial surfaces are becoming more important in the light of increasing evidence which implicates lectin-carbohydrate interactions in diverse physiological phenomena. This study reports the identification of a major 118 kDa granulocyte surface protein, (Protein 1 a) which binds the lectin wheat germ agglutinin (WGA), and is distinctly different from reported WGA binding granulocyte membrane proteins. Protein 1 a has been isolated from the Triton-soluble and Triton-insoluble lysates of normal individuals and patients with Chronic Myeloid Leukemia (CML) using a combination of differential solubilization, lectin affinity, ion exchange chromatography and HPLC. The protein from the detergent lysates of both normal and CML granulocytes has similar pI values, lectin affinities, and hydrophobicity. However, its solubility in Triton is different in the two cell types. In 71% of CML cases examined, Protein 1 a exhibits decreased Triton solubility suggesting its increased association with the cytoskeleton (CSK). Stimulation of normal granulocytes with WGA leads to the translocation of the soluble form of Protein 1 a to the Triton-insoluble fraction. This cytoskeletal recruitment of Protein 1 a is sustained only under conditions of excess WGA and occupied receptor. The CSK disruptive agent dihydrocytochalasin B (H2CB) releases the insoluble form of the receptor into the Triton-soluble fraction. Investigation of a CSK-involving process such as ligand internalization revealed that CML granulocytes exhibit slower kinetics of internalization of fluorescent WGA molecules.(ABSTRACT TRUNCATED AT 250 WORDS)

Use of lectins as diagnostic and therapeutic tools for cancer

Within the past few years, lectins have become a well-established means for understanding varied aspects of cancer and metastasis. Evidence is now emerging that lectins are dynamic contributors to tumor cell recognition (surface markers), cell adhesion and localization, signal transduction across membranes, mitogenic stimulation, augmentation of host immune defense, cytotoxicity, and apoptosis. To advance understanding of these lectin-dependent processes, attempts are being made to discover new lectins that have one or more of these functions and to develop lectin- (or glycoconjugate-) based tools that could be used to home in on tumor cells. This review will summarize current research on the lectins and recent advances in the development of lectin-based diagnostic and therapeutic tools for cancer. Additionally, the future potential of lectin-based diagnosis and therapy is discussed.

Water-soluble poly(acrylamide-allylamine) derivatives of saccharides for protein-saccharide binding studies

Water-soluble poly(acrylamide-allylamine) copolymers containing covalently bound amino groups, prepared by copolymerization of acrylamide and allylamine, can be used as general carriers for coupling of different types of saccharides or saccharide derivatives. The water-soluble macromolecular carbohydrate derivatives can be easily labelled and used in various solid-phase techniques to study protein-saccharide interaction. Two types of coupling reaction were used to prepare polyacrylamide derivatives of saccharides: reductive amination was applied to couple the reducing disaccharides and a carbodiimide reaction was used to couple heparin via its carboxyl groups to the amino groups of the poly(acrylamide-allylamine) derivative. Peroxidase labelled or biotinylated derivatives were shown to be useful in studies on the binding properties of lectins and proteins from boar seminal plasma.

Major organ-specific glycoproteins in isolated brain and kidney membranes identified as Na,K-ATPase subunits by combined glycan-, lectin-, and immunoblotting

In the present work combined glycan-, lectin-, and immunoblotting of isolated brain and kidney membranes shows that the alpha and beta subunits of Na,K-ATPase are the most abundant glycoproteins. Further, Datura stramonium and Galanthus nivalis agglutinins recognize the Na,K-ATPase subunits in a mutually exclusive manner in membranes from human, rabbit and rat brain or human, rabbit, rat, pig and dog kidney indicating the presence of species-independent organ-typical glycoforms. The glycosylation status is not related to the ouabain-sensitivity. Taken together, the data reveals organ-specific glycoforms of Na,K-ATPase which might have roles for organ identification and recognition.

Differential response of the epidermal growth factor receptor tyrosine kinase activity to several plant and mammalian lectins

Biosignalling via lectins may involve modulation of protein kinase activities. This aspect of the biological action of mammalian and plant lectins has been investigated for their effect on the activity of the isolated epidermal growth factor receptor (EGFR). The constitutive tyrosine kinase activity of the epidermal growth factor receptor from rat liver, isolated by calmodulin-affinity chromatography, was activated by concanvalin A (ConA), and wheat germ agglutinin (WGA) to a similar extent as the measured enhancement induced by EGF. In contrast, two mannose-specific lectins, the mannan-binding protein (MBP) and serum amyloid P component (SAP), isolated from human serum, have inhibitory effects, both in the absence and presence of EGF. The differential effects of these lectins were tested using as phosphorylatable substrates a co-polymer of glutamic acid-tyrosine, as well as calmodulin. However, two galactoside-specific lectins, the laminin-binding beta-galactoside-binding 14 kDa lectin, isolated from bovine heart (14K-BHL), and the alpha/beta-galactoside-binding lectin, isolated from mistletoe (Viscum album L.) leaves (VAA), do not inhibit the EGFR tyrosine kinase activity. The sugar dependence of the lectin-mediated action was studied by inhibition assays. Mannose and a mannose-containing neoglycoprotein prevent the activating effect of ConA, and N-acetyl-D-glucosamine partially prevents the activation produced by WGA. However, mannose and mannose-containing neoglycoprotein were ineffective to reduce the inhibitory effect of MBP or SAP.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of galectins on microvessel endothelial cells and their involvement in tumour cell adhesion

Lactoside-binding lectins (galectins) with molecular weights of about 14.5 kDa (galectin-1) and 29-35 kDa (galectin-3) bind preferentially to polylactosaminoglycan-containing glycoconjugates and have been found on the surface of tumour cells and implicated in cell-cell and cell-extracellular matrix adhesion and metastasis. We have demonstrated by immunoblotting that both galectin-1 and galectin-3 are present in extracts of endothelial cells cultured from bovine aorta, rat lung, mouse lung and mouse brain microvessels, whereas mouse hepatic sinusoidal endothelial cells expressed primarily galectin-1. These galectins were also localized by indirect immunofluorescent labelling on the surface of the different endothelial cells in culture and by immunohistochemical staining in human tissues in vivo. Anti-galectin-1 antibodies inhibited the adhesion of liver-preferring murine RAW117-H10 large-cell lymphoma cells to hepatic sinusoidal endothelial cells or lung microvessel endothelial cells in vitro. The data indicate that galectin-1 is expressed on the extracellular surface of endothelial cells and can mediate in part the adhesion of RAW117-H10 cells to liver microvessel endothelial cells.

Expression of galactoside-specific endogenous lectins and their ligands in human oral squamous cell carcinoma

Human endogenous lectins have a wide spectrum of biological functions. The present study analyses the expression of beta-galactoside specific and N-acetyl-D-galactosamine specific endogenous lectins in oral squamous cell carcinomas using biotinylated neoglycoproteins. The expression pattern of beta-galactosyl-containing glycoconjugates or ligands of beta-galactoside specific lectins in these tissues was also studied using an endogenous biotinylated lectin, the human 14-kDa lectin. For comparison a galactoside specific plant lectin from mistletoe, Viscum album was also employed. The results demonstrate that oral squamous cell carcinomas mainly express accessible binding sites for lactosylated neoglycoprotein (90%) while few carcinomas expressed mild amount of N-acetyl-D-galactosamine specific binding sites (40%). There was no difference in the binding patterns of these probes between well and less differentiated carcinomas. Expression of these neoglycoprotein binding sites were mostly concentrated in immature basaloid cells, indicating a possible association with cell proliferation. The binding pattern of D-galactosyl specific lectins (human 14-kDa and mistletoe lectins) showed conspicuous differences. This feature emphasizes the caution that needs to be exercised in interpreting the biological significance of results attained using plant lectins on human tissue.

Distribution patterns in glycoconjugate expression during the development of the rat palate

The distribution of complex carbohydrate structures during the embryonic development of the rat palate was analysed by examining lectin-binding patterns in serial paraffin and cryostat sections. With few exceptions, the binding patterns showed a general increase in lectin receptors in the more developed stages of palatogenesis. High mannose oligosaccharides were especially amplified during development. Terminal fucose molecules were not expressed. In contrast, terminal sialic acid molecules were ubiquitously distributed in epithelial and mesenchymal tissues. Non-sialylated terminal N-acetylglucosamine was specifically restricted to evolving bone matrix. Before palatal fusion, quantitative but not qualitative differences were detected between oral, nasal, and medial-edge epithelial surfaces. The only exception was LCA, which specifically marked epithelial cells at the tip of palatal shelves. A very selective affinity for Jacalin was demonstrated in the oral epithelium of the palate after day 16, suggesting the presence of sialylated terminal galactose-(beta-1,3)-N-acetylgalactosamine. PNA specifically marked the basal lamina of the oral side of palatal processes. The binding patterns of DBA, GSL IA, SBA, and VVA indicated that the epithelium of the tongue is characterized by terminal alpha- and beta-galactose residues, whereas palatine cells possess only molecules with beta-anomery. During palatogenesis, glycosaminoglycans patterns were significantly modified. Our data suggest that alteration of complex carbohydrate structures may play a central role in modulating cell-cell and cell-matrix interactions. The significance of these findings, however, remains to be elucidated.

Cell type-dependent alterations of binding of synthetic blood group antigen-related oligosaccharides in lung cancer

Blood group antigen-related oligosaccharides have been implicated in growth regulation, cell mobility control and adhesion; we are therefore interested in the localization of receptors for these oligosaccharides in tumour cells. Labelled neoglycoconjugates that carry synthetic sugar structures are suitable tools to determine: whether such binding sites are present in human lung cancer; whether structural alterations of the glycoligand part will affect extent of binding; and whether cell type-associated alterations can be detected. Sections from 121 cases of lung cancer, representing small cell and non-small cell lung carcinoma, mesothelioma and metastases from extrapulmonary primary carcinomas were used to study the binding of nine synthetic AH- and Le-related oligosaccharides. Probes with fucose-alpha 1-3/4-N-acetylglucosamine-beta 1-R, an A-like disaccharide and 3'-sulfated galactose as ligand appear to bind less well to small cell than to non-small cell lung cancer cases, whereas Lec-disaccharide distinguishes mesothelioma from metastatic carcinoma. The latter ligand, A-like disaccharide and H (type III)-like trisaccharide exhibit evident cell type-associated differences in extent of binding. Thus, tailor-made neoglycoconjugates constitute a promising class of histopathological tools that warrants further study.