Animal lectins

Immunohistochemical expression of the intracellular component of galectin-8 in squamous cell metaplasia of the bronchial epithelium in neoplastic and benign processes

Specified galectins are known to play a role in regulating cell proliferation, differentiation, adhesion and migration. Po66, a mouse IgG1 monoclonal antibody produced by immunization against squamous cell cancer, reacts against a carbohydrate-binding protein (Po66-CBP), recently shown to be a member of the galectin family with a strong homology with galectin-8 (PCTA-1), identified as a human tumor-associated antigen. We studied Po66 in squamous metaplasia of the bronchi in order to determine whether it could be specifically involved in neoplastic conditions and if so, if it would be helpful in distinguishing metaplasia at risk of cancer. Twenty-eight formalin-fixed, paraffin-embedded archival tissues of 17 metaplasias with SCC, 3 metaplasia with distant neoplastic disease and 8 metaplasias with an inflammatory process, were immunostained using a streptavidin biotin peroxydase method. The squamous metaplasias were positively stained in non-neoplastic disease as well as in neoplastic processes. Expression was also observed in stromal and normal cells. Po66-CBP was not associated with a pre-neoplastic character. We discussed the expression of this intra-cellular component of galectin-8 according to the functions of galectins in cellular differentiation, host reaction against tumor, and inflammation.

Primary colorectal carcinomas and their intrapulmonary metastases: clinical, glyco-, immuno- and lectin histochemical, nuclear and syntactic structure analysis with emphasis on correlation with period of occurrence of metastases and survival

BACKGROUND

The aim of the study was to correlate clinical factors (disease-free interval/survival) with growth pattern in terms of structural entropy of patients with primary colorectal carcinomas and secondary lung lesions.

METHODS

Proliferation and apoptosis markers as well as determinants involved in information transfer by protein-carbohydrate interactions were monitored. The clinical history, surgical and histopathological reports, tumor load, survival of the patients with a maximum follow-up of 14 years, and sections of paraffin blocks of 60 colorectal carcinoma specimens and their pulmonary metastases were examined. Measurements of the staining intensities after processing sections of primary and secondary carcinomas with the marker panel and calculations of syntactic structure and stereological parameters were performed.

RESULTS

The majority of primary tumors (80%, 49/60) were surgically treated at advanced tumor stages (pT3/pT4), with detectable lymph node involvement (34/60). Lung metastases were resected after a median disease-free interval of 30.5 months, an average of 3.0 metastases adding up to a mean intrapulmonary tumor load of 9.98 ccm. The median survival was calculated to be 82 months after resection of the colon/rectal carcinomas and 40 months after that of intrapulmonary metastases. It was correlated with certain structural and vascular features such as vascular circumference. The proliferation index and several textural features were strongly associated with vascularization in primary and secondary tumors.

CONCLUSIONS

Despite intra- and interindividual variations, vascularization properties and features such as bcl-2 positivity and CEA- and galectin-3-associated structural entropy in primary tumors or metastases are described as independent prognostic features. Absence of lymph node involvement or limited tumor stages of colon/rectal carcinomas should not exclude patients from thorough postsurgical scrutiny to detect lung metastases.

Isolation and characterization of an acetyl group-recognizing agglutinin from the serum of the Indian white shrimp Fenneropenaeus indicus

A natural agglutinin from the serum of the Indian white shrimp Fenneropenaeus (Penaeus) indicus was purified to electrophoretic homogeneity by a single-step affinity chromatography on N-acetylglucosamine-Sepharose 6B. The expression of hemagglutinating (HA) activity of F. indicus agglutinin (FIA) was independent of the presence of divalent cations and insensitive to their chelators. FIA gave a single symmetrical peak in its native form with a molecular mass estimate of 200 kDa on gel filtration in HPLC, and SDS-PAGE under reducing conditions revealed that it is a homo-oligomer of a 27-kDa subunit protein. The pattern of reactivity of FIA against anti-FIA rabbit serum in immunodiffusion and immunoelectrophoretic analysis provided additional evidence for its purity and homogeneity. HA-inhibition studies documented exclusive specificity of FIA for acetyl groups in carbohydrates independently of the presence of these groups at the C-2 or C-5 position and its stereochemical arrangement in the axial or equatorial orientation. The unique ability of FIA to recognize acetyl groups was also explicitly demonstrated with sialo- and asialo-glycoproteins. Strikingly, FIA also interacted equally with amino acids and chemicals containing acetyl groups, thereby unambiguously demonstrating the exquisite specificity of FIA for an acetyl group, irrespective of the presence of this group in carbohydrate or noncarbohydrate ligands. The susceptibility of HA activity of FIA to inhibition by lipopolysaccharides from diverse gram-negative bacteria as well as its ability to selectively agglutinate several bacterial species isolated from infected shrimps implicate a potential role of this humoral agglutinin of F. indicus in the host immunodefense reactions against microbial invaders.

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.

Cell surface glycoprotein PZR is a major mediator of concanavalin A-induced cell signaling

PZR is an immunoglobulin superfamily cell surface protein containing a pair of immunoreceptor tyrosine-based inhibitory motifs. As a glycoprotein, PZR displays a strong association with concanavalin A (ConA), a member of the plant lectin family. Treatment of several cell lines with ConA caused tyrosine phosphorylation of a major cellular protein. Immunoblotting and immunoprecipitation studies indicated that this protein corresponded to PZR. Tyrosine phosphorylation of PZR was accompanied by recruitment of SHP-2 and was inhibited by PP1, a selective inhibitor of the Src family tyrosine kinases. Furthermore, c-Src was constitutively associated with PZR and was activated upon treatment of cells with ConA. Moreover, tyrosine phosphorylation of PZR was markedly enhanced in v-Src-transformed NIH-3T3 cells and was predominant in Escherichia coli cells co-expressing c-Src. Expression of an intracellular domain-truncated form of PZR in HT-1080 cells affected cell morphology and had a dominant negative effect on ConA-induced tyrosine phosphorylation of PZR, activation of c-Src, and agglutination of the cells. Together, the data indicate that PZR is a major receptor of ConA and has an important role in cell signaling via c-Src. Considering the various biological activities of ConA, the study of PZR may have major therapeutic implications.

Rhamnose-binding lectins from steelhead trout (Oncorhynchus mykiss) eggs recognize bacterial lipopolysaccharides and lipoteichoic acid

The interaction between bacteria and three L-rham-nose-binding lectins, named STL1, STL2, and STL3, from steelhead trout (Oncorhynchus mykiss) eggs was investigated. Although STLs bound to most Gram-negative and Gram-positive bacteria, they agglutinated only Escherichia coli K-12 and Bacillus subtilis among the bacteria tested. The binding was inhibited by L-rhamnose. STLs bound to distinct serotypes of lipopolysaccharides (LPSs), and showed much higher binding activities to smooth-type LPSs of Escherichia coli K-12 and Shigella flexneri 1A than to their corresponding rough-type LPSs. STLs also bound to lipoteichoic acid (LTA) of Bacillus subtilis. These results indicate that STLs bound to bacteria by recognizing LPSs or LTA on the cell surfaces.

Craniopharyngioma: a case report and comparative galectin histochemical analysis

Craniopharyngioma is a rare benign tumour originating from Rathke's pouch. This paper reports a tumour case studied with a set of markers defining protein-carbohydrate recognition. Expression of endogenous lectins and their reactive glycoligands is under differentiation-dependent control in many cell types. These parameters can be related to the degree of cell differentiation in tumours. Therefore, the expression patterns of endogenous lectins, namely galectins-1, -3, and -7, in the craniopharyngioma case were determined. Galectins-1 and -3 were also used to reveal glycoconjugates in cells and extracellular matrices, an approach that has heretofore relied largely on plant lectins. The staining pattern of craniopharyngioma is compared with that of two other types of ectodermally derived tumours, namely basal and squamous cell carcinomas. Clusters of polygonal and flattened cells with morphological characteristics of differentiated cells in the craniopharyngioma and the majority of poorly differentiated cells in squamous cell carcinomas were reactive with galectin-3. No binding of this probe was observed in cells of basal cell carcinomas and the majority of craniopharyngioma cells. In view of the lack of accessible binding in the basal layer of normal squamous epithelia where proliferative cells (including stem cells) are located, galectin-3 binding could be used to distinguish basal from suprabasal cells of squamous epithelial cells.

Organization of the mouse macrophage C-type lectin (Mcl) gene and identification of a subgroup of related lectin molecules

A number of genes encoding C-type lectin molecules have been mapped to the natural killer gene complex (NKC) at the distal region of mouse chromosome 6 and to a syntenic region on human chromosome 12p12-p13. In addition to those receptors which regulate NK cell function, related structures expressed on other cells types have also been localized to this chromosomal region. Among these are a number of recently characterized genes, including macrophage C-type lectin (MCL), macrophage-inducible C-type lectin (Mincle), dendritic cell immunoreceptor (DCIR) and dendritic cell-associated lectin-2 (Dectin-2). The amino acid sequences comprising the single C-type lectin domains of MCL, Mincle, DCIR and Dectin-2 are shown here to be closely related to each other. These molecules show overall similarity to two groups of animal C-type lectins, groups II and V, which demonstrate type II transmembrane topology. In this study, sequence analysis suggests that MCL, Mincle, DCIR and Dectin-2 represent a subset of group II-related C-type lectins which may participate in analogous recognition events on macrophages and dendritic cells. The genomic organization of the MCL gene and the sequence of the promoter region, with putative regulatory elements, were determined from a mouse MCL genomic DNA clone and are described here in detail.

Carbohydrate-based interactions of oviductal sperm reservoir formation-studies in the pig

Competitive inhibition of sperm to explants of the oviductal epithelium was used to study the complementary receptor system that may be involved in the establishment of the oviductal sperm reservoir in the pig. Sperm binding to the oviductal explants is expressed as Binding Index (BI = sperm cells/0.01 mm(2)). From a set of glycoproteins with known oligosaccharide structures, only asialofetuin and ovalbumin showed inhibitory activity, indicating that ovalbumin may block high affinity binding sites (IC(50) congruent with 1.3 microM) and asialofetuin low affinity sites (IC(50) congruent with 18 microM) of the complementary receptor systems, whereas fetuin carrying terminal sialic acid has no effect. Ovalbumin glycopeptides were isolated by Con A affinity chromatography and reverse-phase HPLC following tryptic digestion. Glycopeptides and enzymatically released glycans were analyzed by MS, and were shown to represent preferentially the two high mannose type glycans (Man)(5)(GlcNAc)(2) and (Man)(6)(GlcNAc)(2), and as a minor component the hybrid type glycan (Hex)(4)(GlcNAc)(5). Glycopeptides (84% inhibition) and glycans (81% inhibition) significantly reduced sperm-oviduct binding at a concentration of 3 microM, whereas the deglycosylated peptides showed no inhibitory activity. Mannopentaose (IC(50) congruent with 0.8 microM) representing the oligomannose residue of the high mannose glycans of ovalbumin was as effective as ovalbumin. These data indicate that the carbohydrate-based mechanisms underlying the formation of the oviductal sperm reservoir in the pig is the result of the concerted action of at least the high-affinity binding sites for oligomannose or nonreducing terminal mannose residues and low-affinity binding of galactose.

Galectin-1 is overexpressed in nasal polyps under budesonide and inhibits eosinophil migration

Because of the importance of galectins for various cellular activities, the influence of the glucocorticoid budesonide on the level of expression of galectins-1 and -3 was investigated in human nasal polyposis. Ten nasal polyps obtained from surgical resection were maintained for 24 hours in the presence of various concentrations of budesonide. As quantitatively demonstrated by means of computer-assisted microscopy, 250 ng/ml (the highest dose tested) induced a pronounced increase of galectin-1 expression. This feature was observed in nasal polyps from allergic patients but not in those from nonallergic patients. Since eosinophils represent the main inflammatory cell population in nasal polyps, we investigated the effect of galectin-1 on their migration levels by means of quantitative phase-contrast computer-assisted videomicroscopy. Our results show that galectin-1 (coated on plastic supports) markedly reduced the migration levels of eosinophils in comparison to P-selectin. On the cellular level, marked modifications in the polymerization/depolymerization dynamics of the actin cytoskeleton (as revealed by means of computer-assisted fluorescence microscopy) and, to a much lesser extent, an increase in the adhesiveness of eosinophils to tested substrata were detectable. The present study therefore reveals a new galectin-1-mediated mechanism of action for glucocorticoid-mediated anti-inflammatory effects.

Amino acid sequence and carbohydrate-binding analysis of the N-acetyl-D-galactosamine-specific C-type lectin, CEL-I, from the Holothuroidea, Cucumaria echinata

CEL-I is one of the Ca2+-dependent lectins that has been isolated from the sea cucumber, Cucumaria echinata. This protein is composed of two identical subunits held by a single disulfide bond. The complete amino acid sequence of CEL-I was determined by sequencing the peptides produced by proteolytic fragmentation of S-pyridylethylated CEL-I. A subunit of CEL-I is composed of 140 amino acid residues. Two intrachain (Cys3-Cys14 and Cys31-Cys135) and one interchain (Cys36) disulfide bonds were also identified from an analysis of the cystine-containing peptides obtained from the intact protein. The similarity between the sequence of CEL-I and that of other C-type lectins was low, while the C-terminal region, including the putative Ca2+ and carbohydrate-binding sites, was relatively well conserved. When the carbohydrate-binding activity was examined by a solid-phase microplate assay, CEL-I showed much higher affinity for N-acetyl-D-galactosamine than for other galactose-related carbohydrates. The association constant of CEL-I for p-nitrophenyl N-acetyl-beta-D-galactosaminide (NP-GalNAc) was determined to be 2.3 x 10(4) M(-1), and the maximum number of bound NP-GalNAc was estimated to be 1.6 by an equilibrium dialysis experiment.

NMR investigations of protein-carbohydrate interactions: insights into the topology of the bound conformation of a lactose isomer and beta-galactosyl xyloses to mistletoe lectin and galectin-1

A hallmark of oligosaccharides is their often limited spatial flexibility, allowing them to access a distinct set of conformers in solution. Viewing each individual or even the complete ensemble of conformations as potential binding partner(s) for lectins in protein-carbohydrate interactions, it is pertinent to address the question on the characteristics of bound state conformation(s) in solution. Also, it is possible that entering the lectin's binding site distorts the low-energy topology of a glycosidic linkage. As a step to delineate the strategy of ligand selection for galactosides, a common physiological docking point, we have performed a NMR study on two non-homologous lectins showing identical monosaccharide specificity. Thus, the conformation of lactose analogues bound to bovine heart galectin-1 and to mistletoe lectin in solution has been determined by transferred nuclear Overhauser effect measurements. It is demonstrated that the lectins select the syn conformation of lactose and various structural analogues (Galbeta(1-->4)Xyl, Galbeta(1-->3)Xyl, Galbeta(1-->2)Xyl, and Galbeta(1-->3)Glc) from the ensemble of presented conformations. No evidence for conformational distortion was obtained. Docking of the analogues to the modeled binding sites furnishes explanations, in structural terms, for exclusive recognition of the syn conformer despite the non-homologous design of the binding sites.

Expression of LSLCL, a new C-type lectin, is closely restricted, in bone marrow, to immature neutrophils

In vitro, LSLCL is expressed by numerous myeloid, promyelocytic, and T or B lymphoblastoid cell lines. In vivo, LSLCL is strongly expressed in bone marrow and only faintly in lymphoid organs. We show here that, in bone marrow, LSLCL is detected: (i) concentrated in the cytoplasm of immature neutrophils but not in myeloblasts nor in mature neutrophils, (ii) in extracellular bone marrow fluid. Besides, numerous cDNAs, similar to LSLCL (identity of 93-99%), are found in 'expressed sequence tags' databases from various origins, mostly fetal and undifferentiated tumour tissues. Since LSLCL and various closely related cDNAs are expressed at definite stages of cellular maturation processes, we hypothesize that this class of proteins could play an important role in the control of cellular differentiation.

Detection of galectin-3 in tear fluid at disease states and immunohistochemical and lectin histochemical analysis in human corneal and conjunctival epithelium

BACKGROUND/AIM

Components of the tear fluid contribute to the biochemical defence system of the eye. To reveal whether the immune mediator and lipopolysaccharide binding galectin-3 is present in tears, tear samples were collected from eyes in healthy and pathological states. Investigation of expression of galectin-3 and galectin-3 reactive glycoligands in normal human conjunctival and corneal epithelia was also initiated as a step to understand the role of galectin-3 in ocular surface pathology.

METHODS

Immunoblot analysis using either a rabbit polyclonal or a mouse monoclonal antibody against galectin-3 was employed to detect galectin-3 in tear fluid. Galectin-3 expression in tissue specimens was detected by immunocytochemistry employing A1D6 mouse monoclonal antibody, and galectin-3 reactive glycoligands were visualised by lectin histochemistry using labelled galectin-3.

RESULTS

Galectin-3 was found only in tears from patients with ocular surface disorders. It was expressed in normal corneal and conjunctival epithelia but not in lacrimal glands. Inflammatory leucocytes and goblet cells found in galectin-3 containing tear fluid also expressed galectin-3. Galectin-3 binding sites were detected on the surface of conjunctival and corneal epithelial cells co-localising with desmoglein.

CONCLUSIONS

This study revealed expression of galectin-3 in tear fluid obtained from patients with eye diseases. The role of this endogenous lectin (produced by inflammatory as well as epithelial cells) in antimicrobial action and inflammation modulation could be expected.

PFA, a novel mollusk agglutinin, is structurally related to the ribosome-inactivating protein superfamily

The structural organization of PFA, a novel beta-galactose-specific agglutinin from the snail Pomacea flagellata, was partially characterized. Using mass spectrometry, the molecular weight of this glycoprotein was determined as 32,444 Da (7.4% carbohydrate). The agglutinin was found to form very large aggregates in solution, which were dissociated to monodisperse native-like dimers upon addition of polyethyleneglycol. The identity of the first 38 and the last 11 residues of the polypeptide chain was determined. It was found that PFA and the N-glycosidase subunit of ricin, a heterodimeric cytotoxin isolated from castor bean seeds, are homologous to each other in the N-terminal region. Furthermore, the far-UV circular dichroism spectra of these proteins were found to be nearly superimposable, evidencing that they share common general features in their secondary and tertiary structures. On the basis of these similarities, it can be concluded that PFA is structurally related to the ribosome-inactivating protein superfamily.

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.

Carbohydrate specificity of a galectin from chicken liver (CG-16)

Owing to the expression of more than one type of galectin in animal tissues, the delineation of the functions of individual members of this lectin family requires the precise definition of their carbohydrate specificities. Thus, the binding properties of chicken liver galectin (CG-16) to glycoproteins (gps) and Streptococcus pneumoniae type 14 polysaccharide were studied by the biotin/avidin-mediated microtitre-plate lectin-binding assay and by the inhibition of lectin-glycan interactions with sugar ligands. Among 33 glycans tested for lectin binding, CG-16 reacted best with human blood group ABO (H) precursor gps and their equivalent gps, which contain a high density of D-galactopyranose(beta1-4)2-acetamido-2-deoxy-D-glucopyranose [Gal(beta1-4)GlcNAc] and Gal(beta1-3)GlcNAc residues at the non-reducing end, but this lectin reacted weakly or not at all with A-,H-type and sialylated gps. Among the oligosaccharides tested by the inhibition assay, the tri-antennary Gal(beta1-4)GlcNAc (Tri-II) was the best. It was 2.1x10(3) nM and 3.0 times more potent than Gal and Gal(beta1-4)GlcNAc (II)/Gal(beta1-3)GlcNAc(beta1-3)Gal(beta1-4)Glc (lacto-N-tetraose) respectively. CG-16 has a preference for the beta-anomer of Gal at the non-reducing end of oligosaccharides with a Gal(beta1-4) linkage >Gal(beta1-3)> or =Gal(beta1-6). From the results, it can be concluded that the combining site of this agglutinin should be a cavity type, and that a hydrophobic interaction in the vicinity of the binding site for sugar accommodation increases the affinity. The binding site of CG-16 is as large as a tetrasaccharide of the beta-anomer of Gal, and is most complementary to lacto-N-tetraose and Gal(beta1-4)GlcNAc related sequences.

Fluorescent carbohydrate probes for cell lectins

Fluorescein labeled carbohydrate (Glyc) probes were synthesized as analytical tools for the study of cellular lectins, i.e. SiaLe(x)-PAA-flu, Sia2-PAA-flu, GlcNAc2-PAA-flu, LacNAc-PAA-flu and a number of similar ones, with PAA a soluble polyacrylamide carrier. The binding of SiaLe(x)-PAA-flu was assessed using CHO cells transfected with E-selectin, and the binding of Sia2-PAA-flu was assessed by COS cells transfected with siglec-9. In flow cytometry assays, the fluorescein probes demonstrated a specific binding to the lectin-transfected cells that was inhibited by unlabeled carbohydrate ligands. The intense binding of SiaLe(x)-PAA-3H to the E-selectin transfected cells and the lack of binding to both native and permeabilized control cells lead to the conclusion that the polyacrylamide carrier itself and the spacer arm connecting the carbohydrate moiety with PAA did not contribute anymore to the binding. Tumors were obtained from nude mice by injection of CHO E-selectin or mock transfected cells. The fluorescent SiaLe(x)-PAA-flu probe could bind to the tumor sections from E-selectin positive CHO cells, but not from the control ones. Thus, these probes can be used to reveal specifically the carbohydrate binding sites on cells in culture as well as cells in tissue sections. The use of the confocal spectral imaging technique with Glyc-PAA-flu probes offered the unique possibility to detect lectins in different cells, even when the level of lectin expression was rather low. The confocal mode of spectrum recording provided an analysis of the probe localization with 3D submicron resolution. The spectral analysis (as a constituent part of the confocal spectral imaging technique) enabled interfering signals of the probe and intrinsic cellular fluorescence to be accurately separated, the distribution of the probe to be revealed and its local concentration to be measured.

Detection of macrophage migration inhibitory factor (MIF) in human cholesteatomas and functional implications of correlations to recurrence status and to expression of matrix metalloproteinases-3/9, retinoic acid receptor-beta, and anti-apoptotic galectin-3

OBJECTIVES

To investigate whether the expression of the macrophage migration inhibitory factor (MIF) 1) is detectable, 2) changes in relation to recurrence and infection status, and 3) relates to the levels of expression of growth regulators/differentiation markers, including galectin-1, -3, and -8, retinoid acid receptors (RAR)]-alpha, -beta, and -gamma, binding sites for sarcolectin, and invasion markers (cathepsins -B and -D, and matrix metalloproteinases [MMP]-2, -3, and -9) in human cholesteatomas.

STUDY DESIGN

An analysis of 56 cholesteatomas resected by the same surgeon using canal wall up and canal wall down surgical procedures.

METHODS

The immunohistochemical levels of expression of MIF and the proteases were quantitatively determined (using computer-assisted microscopy) on routine histologic slides by specific antibodies, and statistically correlated to parameters of the other markers determined previously in conjunction with data on apoptosis/proliferation.

RESULTS

MIF expression was detected. It was significantly higher in the epithelium (P =.002) and vessels (P =.04) of the connective tissues (but not in the connective tissue itself) of recurrent as opposed to non-recurrent cholesteatomas. The MIF expression is significantly correlated (P =.006) to the RAR beta expression in non-infected cholesteatomas, and to MMP-3 (P <.01) and anti-apoptotic galectin-3 (P =.01) in infected cholesteatomas. The level of MIF expression was also correlated significantly to MMP-9 (P = 0.003), RAR beta (P <.001), and galectin-8 (P =.003) expression in the cholesteatomas regardless of their infection status.

CONCLUSIONS

MIF expression in human cholesteatomas is related to the levels of biologic aggressiveness reflected in their recurrence status and MMP expression, and to the differentiation status reflected in their galactin and RAR beta expressions. Together with galectin-3, it could cooperate to form an anti-apoptotic feedback loop.

Plant lectins: occurrence, biochemistry, functions and applications

Growing insights into the many roles of glycoconjugates in biorecognition as ligands for lectins indicates a need to compare plant and animal lectins. Furthermore, the popularity of plant lectins as laboratory tools for glycan detection and characterization is an incentive to start this review with a brief introduction to landmarks in the history of lectinology. Based on carbohydrate recognition by lectins, initially described for concanavalin A in 1936, the chemical nature of the ABH-blood group system was unraveled, which was a key factor in introducing the term lectin in 1954. How these versatile probes are produced in plants and how they are swiftly and efficiently purified are outlined, and insights into the diversity of plant lectin structures are also given. The current status of understanding their functions calls for dividing them into external activities, such as harmful effects on aggressors, and internal roles, for example in the transport and assembly of appropriate ligands, or in the targeting of enzymatic activities. As stated above, attention is given to intriguing parallels in structural/functional aspects of plant and animal lectins as well as to explaining caveats and concerns regarding their application in crop protection or in tumor therapy by immunomodulation. Integrating the research from these two lectin superfamilies, the concepts are discussed on the role of information-bearing glycan epitopes and functional consequences of lectin binding as translation of the sugar code (functional glycomics).

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

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

The levels of retinoid RARβ receptors correlate with galectin-1, -3 and -8 expression in human cholesteatomas

Cholesteatoma is a benign disease characterized by the presence of an unrestrained growth and the accumulation of keratin debris in the middle ear cavity. This often recurs, even when surgical resection is thought to be complete. In a previous study we showed that cholesteatomas with the highest apoptotic indices recurred more rapidly and also exhibited a high level of p53 immunopositive cells. In view of their relevance to the characterization of the cell differentiation status, the present study focuses on the expression of retinoid acid receptors (RARs) and galectins in human cholesteatomas. Retinoids control the differentiation processes in keratinocytes while galectins play strikingly modulatory roles at apoptosis and cell adhesion levels in a wide variety of tissue (embryonic, normal and neoplastic). To clarify the expression of these two protein families in human cholesteatomas we examined and quantified the levels of immunohistochemical expression of RARalpha, beta and gamma, and also galectin-1, -3 and -8 in a series of 70 human cholesteatomas. Our data show clearly that predominantly RARbeta and galectin-1 were expressed. The RARgamma concentration was significantly lower than that of the RARalpha; this was also observed for the galectin-8 concentration in comparison with the galectin-3 one. Furthermore, the level of RARbeta expression correlated highly (P=0.00001) with the level of galectin-8 expression, which also correlated significantly with the level of RARalpha and RARgamma expression. In addition, this parameter also correlated with the level of galectin-1 and galectin-3 expression. These data suggest that cholesteatomas may originate in an undifferentiated population of keratinocytes, and that a relation may exist between retinoid activity and galectins.

The levels of expression of galectin-3, but not of galectin-1 and galectin-8, correlate with apoptosis in human cholesteatomas

OBJECTIVES

To investigate whether galectins 1, 3, and 8 are expressed in human cholesteatomas and whether any such expression does correlate with the level of apoptosis, which is, as we have previously shown, predictive of recurrence.7

STUDY DESIGN

The analysis of 52 cholesteatomas resected by the same surgeon by means of canal wall up and canal wall down procedures.

METHODS

The immunohistochemical levels of expression of galectins 1, 3, and 8 were quantitatively determined (using computer-assisted microscopy) on conventional histological slides by means of specific anti-galectin-1, anti-galectin-3, and anti-galectin-8 antibodies. The level of apoptosis in each cholesteatoma under study had already been determined 7 by means of the in situ labeling of nuclear DNA fragmentation (Tolt-mediated dUTP nick end labeling [TUNEL] staining).

RESULTS

Galectin-1 was expressed markedly in both the epithelial and the connective tissue areas of all the cholesteatomas under study. The levels of expression of galectin-3 and galectin-8 were considerably lower than that of galectin-1. The level of expression of galectin-3 correlated both highly and positively with the level of apoptosis.

CONCLUSIONS

An upregulation of galectin-3 (known to have an antiapoptotic and antianoikis effect in certain model systems) expression, which is associated with pronounced apoptotic activity, could have a physiologically protective effect against the characteristically substantial apoptotic features occurring in recurrent cholesteatomas.

A novel rhamnose-binding lectin family from eggs of steelhead trout (Oncorhynchus mykiss) with different structures and tissue distribution

An L-rhamnose-binding isolectin named STL3 (subunit Mr, 21.5 k) was isolated from eggs of the steelhead trout (Oncorhynchus mykiss) in addition to STL1 (subunit Mr, 31.4 k) and STL2 (subunit Mr, 21.3 k) that had been already isolated. STLs were composed of noncovalently linked subunits. The primary structures of STL1 and STL3 were analyzed by the combined use of protein sequencing and cDNA sequencing. A cDNA encoding STL2, of which the protein sequence had been previously studied, was also analyzed. The STL1 subunit (289 amino acid residues) had different structural properties compared to those of the STL2 subunit (195 amino acid residues) and the STL3 subunit (195 amino acid residues); e.g., the number of repeated domain (three for STL1, and two for STL2 and STL3), although all of them were composed of tandemly repeated homologous domains (40 to 53% identities). The lectin levels in various tissues and during the embryonic development showed that STL1 had different distribution and expression profiles from those of STL2 and STL3. Although STL1 could be detected in several tissues and serum of both male and female steelhead trout, STL2 and STL3 were only abundant in the ovary. STL2 and STL3 levels dramatically decreased just after hatching, however, the STL1 level increased temporarily. These results indicate that the multiple lectins from eggs of the steelhead trout form a novel rhamnose-binding lectin family with different structures and tissue distribution to share distinct functions in eggs.

The polycystin-1 C-type lectin domain binds carbohydrate in a calcium-dependent manner, and interacts with extracellular matrix proteins in vitro

Mutations in the PKD1 gene are responsible for 85% of cases of autosomal dominant polycystic kidney disease (ADPKD). This gene encodes a large membrane associated glycoprotein, polycystin-1, which is predicted to contain a number of extracellular protein motifs, including a C-type lectin domain between amino acids 403--532. We have cloned and expressed the PKD1 C-type lectin domain, and have demonstrated that it binds carbohydrate matrices in vitro, and that Ca(2+) is required for this interaction. This domain also binds to collagens type I, II and IV in vitro. This binding is greatly enhanced in the presence of Ca(2+) and can be inhibited by soluble carbohydrates such as 2-deoxyglucose and dextran. These results suggest that polycystin-1 may be involved in protein-carbohydrate interactions in vivo. The data presented indicate that there may a direct interaction between the PKD1 gene product and an ubiquitous extracellular matrix (ECM) protein.

A novel osteoblast-derived C-type lectin that inhibits osteoclast formation

We have cloned and expressed murine osteoclast inhibitory lectin (mOCIL), a 207-amino acid type II transmembrane C-type lectin. In osteoclast formation assays of primary murine calvarial osteoblasts with bone marrow cells, antisense oligonucleotides for mOCIL increased tartrate-resistant acid phosphatase-positive mononucleate cell formation by 3-5-fold, whereas control oligonucleotides had no effect. The extracellular domain of mOCIL, expressed as a recombinant protein in Escherichia coli, dose-dependently inhibited multinucleate osteoclast formation in murine osteoblast and spleen cell co-cultures as well as in spleen cell cultures treated with RANKL and macrophage colony-stimulating factor. Furthermore, mOCIL acted directly on macrophage/monocyte cells as evidenced by its inhibitory action on adherent spleen cell cultures, which were depleted of stromal and lymphocytic cells. mOCIL completely inhibited osteoclast formation during the proliferative phase of osteoclast formation and resulted in 70% inhibition during the differentiation phase. Osteoblast OCIL mRNA expression was enhanced by parathyroid hormone, calcitriol, interleukin-1alpha and -11, and retinoic acid. In rodent tissues, Northern blotting, in situ hybridization, and immunohistochemistry demonstrated OCIL expression in osteoblasts and chondrocytes as well as in a variety of extraskeletal tissues. The overlapping tissue distribution of OCIL mRNA and protein with that of RANKL strongly suggests an interaction between these molecules in the skeleton and in extraskeletal tissues.

The lectin from the mushroom Pleurotus ostreatus: a phosphatase-activating protein that is closely associated with an alpha-galactosidase activity. A part of this paper has been presented as a preliminary report at the 17th Interlec. Meeting 1997 in Würzburg, Germany

The lectin from the mushroom Pleurotus ostreatus described earlier [F. Conrad, H. Rüdiger, The lectin from Pleurotus ostreatus: purification, characterization and interaction with a phosphatase, Phytochemistry 36 (1994) 277-283] was further characterized. Determination of the isoelectric point by capillary electrophoresis gave a value of 7.6. The dissociation constant of the lectin-alpha-lactose-1-phosphate complex determined by capillary electrophoresis is 3 mM. The activation of an endogenous phosphatase by the lectin as found earlier for the pseudosubstrate p-nitrophenylphosphate was confirmed also for naturally occurring substrates as ADP and ATP. We observed that at all purification steps the lectin is accompanied by an alpha-galactosidase activity. Both activities could neither be resolved by electrophoresis under non-denaturing conditions nor by affinity chromatography. However, carbohydrate binding by the lectin and carbohydrate processing by the enzyme are not due to the same site since: (i) the lectin accepts both alpha- and beta-glycosides whereas the enzyme activity is restricted to the alpha-anomer; (ii) the interaction with erythrocytes leads to a stable agglutinate, i.e. no 'clot-dissolving activity' [C.N. Hankins, J.I. Kindinger, L.M. Shannon, Legume alpha-galactosidases which have hemagglutinin properties, Plant Physiol. 65 (1980) 618-622] is observed; (iii) the alpha-galactosidase activity is inhibited by galactose but not by a beta-galactoside. Therefore, lectin and enzymatic activities are either properties of two tightly associated proteins, or of just one molecule. The kinetic parameters of the lectin-associated alpha-galactosidase activity for p-nitrophenyl-alpha-galactopyranoside are: K(M)=2.5 mM, k(cat)=66 s(-1), and K(I)=20mM for the inhibitor D-galactose.

Galectin-1 is highly expressed in human gliomas with relevance for modulation of invasion of tumor astrocytes into the brain parenchyma

Protein (lectin)-carbohydrate interaction is supposed to be relevant for tumor cell behavior. The aims of the present work are to investigate whether galectin-1 modulates migration/invasion features in human gliomas in vitro, whether it can be detected in human gliomas immunohistochemically, and whether its expression is attributable to certain glioma subgroups with respect to invasion and prognosis. For this purpose, we quantitatively determined (by computer-assisted microscopy) the immunohistochemical expression of galectin-1 in 220 gliomas, including 151 astrocytic, 38 oligodendroglial, and 31 ependymal tumors obtained from surgical resection. We also xenografted three human glioblastoma cell lines (the H4, U87, and U373 models) into the brains of nude mice in order to characterize the in vivo galectin-1 expression pattern in relation to tumor invasion of the normal brain parenchyma. In addition, we characterized the role in vitro of galectin-1 in U373 tumor astrocyte migration and kinetics. Our data reveal expression of galectin-1 in all human glioma types with no striking differences between astrocytic, oligodendroglial, and ependymal tumors. The level of galectin-1 expression correlated with the grade in the group of astrocytic tumors only. Furthermore, immunopositivity of high-grade astrocytic tumors from patients with short-term survival periods was stronger than that of tumors from patients with long-term survivals. In human glioblastoma xenografts, galectin-1 was preferentially expressed in the more invasive parts of these xenografts. In vitro experiments revealed that galectin-1 stimulates migration of U373 astrocytes.

Glyco- and immunohistochemical refinement of the differential diagnosis between mesothelioma and metastatic carcinoma and survival analysis of patients

The aim of this study was to analyse the diagnostic value of selected glyco- and immunohistochemical probes for discrimination between mesotheliomas and metastatic carcinomas within the pleura, and to evaluate prognostic indicators in the tested panel. A panel of nine markers (five antibodies, two neoglycoproteins, and labelled hyaluronic acid) was applied to a total of 264 specimens with mesotheliomas (118 cases) and metastatic carcinomas in the pleura (146 cases); the material consisted exclusively of surgical specimens. The diagnosis obtained by standard procedures was further substantiated through a detailed follow-up and clear-cut descriptions of primary sites. The metastatic tumours originated from the lung (82 cases), breast (47 cases), colon (three cases), and kidney (two cases); in 12 cases, however, the tumour origin could not be ascertained. In detail, the probes tested included antibodies against carcinoembryonic antigen (CEA), vimentin, calretinin, mesothelial cells (HBME-1), calcyclin and keratin-5; and also biotinylated neoglycoproteins with ganglioside GM1 and N-acetyl-D-glucosamine (GlcNAc) as the ligand part, and hyaluronic acid. Carrier-immobilized ganglioside GM1 and hyaluronic acid displayed the highest specificity and sensitivity for mesotheliomas, followed by calretinin and HBME-1, whereas keratin-5 and vimentin were of low specificity (43% and 52%, respectively). Metastatic carcinomas could be discerned by CEA detection and application of GlcNAc-bearing neoglycoprotein with similar sensitivity (76% and 72%, respectively) and specificity (91% and 86%, respectively). In cases of breast carcinoma, the maximum specificity (59%) and sensitivity (67%) were low for all markers. Patients with mesothelioma survived longer than those with metastatic carcinoma, especially those with detectable binding sites for hyaluronic acid. No association of tumour type and binding properties of the other applied probes with survival of the patients could be found at a statistically significant level. It is concluded that in routine practice, the application of carrier-immobilized GM1, hyaluronic acid, and antibodies against calretinin and HBME-1 is useful for confirmation of mesothelioma, whereas the detection of CEA and GlcNAc-specific binding sites is useful for distinguishing metastatic carcinoma from mesothelioma. Despite the rather infrequent occurrence of mesotheliomas in women, particular attention should be given to exclude or confirm metastatic breast carcinoma in cases of unknown history or long metastatic interval.

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.

Galectins and other endogenous carbohydrate-binding proteins of animal bladder

Defects in the glycocalyx of the bladder epithelium may be related to the development of bladder diseases including interstitial cystitis which is a chronic bladder disease of unknown etiology. Indirect evidence has implicated alterations in the bladder epithelial glycoconjugates in interstitial cystitis and vesicaler instillation of glycosaminoglycans is promoted as treatments. However, information on the nature of the glycoconjugates of the bladder epithelium and lectins that may interact with the exogenous instilled glycoconjugates is very limited. We have examined the endogenous lectin associated with bladder epithelium by immunohistochemistry using biotinylated neoglycoconjugates. The strong calcium-independent binding of beta-D-galactose probe suggested the presence of galectins in rabbit and human bladder. Extracts of rabbit bladder organ cultures metabolically labeled with [14C]-amino acids were subjected to affinity chromatography on immobilized lactose and the specifically bound material eluted with 0.2 M lactose. SDS-PAGE of the recovered proteins revealed a major band of approximately 30 kDa and a minor band of 21 kDa. Polymerase chain reaction and northern blot analysis showed that both galectin-3 and galectin-4 are expressed in rabbit bladder. Since galectin-3 from rabbit had been previously cloned, we cloned and sequenced galectin-4 from rabbit bladder. The deduced full length sequence of 328 amino acids revealed four distinct regions: a N-terminal peptide of 19 residues, two carbohydrate recognition domains of 130 residues each, and a linker region of 49 residues. Comparison of the rabbit galectin-4 sequence with those of human, pig, rat, and mouse revealed two invariant peptide motifs that are proposed as signature sequences for identifying related galectins.

Distinct differences in binding capacity to saccharide epitopes in supratentorial pilocytic astrocytomas, astrocytomas, anaplastic astrocytomas, and glioblastomas

We monitored the expression of glycan-binding sites on a panel of 10 biotinylated neoglycoconjugates by means of quantitative computer-assisted microscopy to further study the molecular mechanisms in the extensive infiltration of the surrounding brain parenchyma by most astrocytic tumors. Three distinct histological compartments were analyzed for each of the 108 astrocytic tumors (15 pilocytic astrocytomas (WHO grade I), 25 astrocytomas (WHO grade II), 30 anaplastic astrocytomas (WHO grade III), and 38 glioblastomas (WHO grade IV) included in our series. These compartments were tumors (nonperivascular tumor astrocytes), perivascular tumor astrocytes, and blood vessel walls. Clear differences were observed between the pilocytic and the diffuse astrocytic tumors. Furthermore, malignant progression in the latter category was paralleled by a decrease in cells' ability to bind distinct sugar epitopes, especially the D-GalNAc(alpha1-3)-D-GalNAc-beta1-R determinant of the Forssman pentasaccharide in tumors, the alpha-L-fucose in perivascular tumor areas, and the beta-D-glucose in tumor vessel walls. Markedly, the level of binding site expression for alpha-D-mannose decreased in the tumors, the perivascular tumor areas, and the vessel walls. These glycohistochemical results imply the functional relevance of protein-carbohydrate interactions in this tumor system.

Determination of the disulfide bond pattern of a novel C-type lectin from snake venom by mass spectrometry

The disulfide bond pattern of Trimeresurus stejnegeri lectin (TSL), a new member of the C-type lectin family, was determined by mass spectrometry. Four intrachain disulfide bonds of TSL, Cys(3)-Cys(14), Cys(31)-Cys(131), Cys(38)-Cys(133) and Cys(106)-Cys(123), and two interchain linkages, Cys(2)-Cys(2) and Cys(86)-Cys(86), were determined. Three strategies were used in this work. One intrachain (Cys(106)-Cys(123)) and one interchain (Cys(86)-Cys(86)) disulfide linkages were detected by standard MS methods. The disulfide bonds Cys(2)-Cys(2) and Cys(3)-Cys(14) were analyzed using a modified partial reduction procedure and MS/MS. The last two disulfide bonds were characterized by a MS/MS/MS technique. The strategies developed in this work could be applied more generally to detection of disulfide bond patterns.

Description of a monomeric prototype galectin from the lizard Podarcis hispanica

Galectins are a continuously expanding family of beta-galactoside-binding lectins present in a variety of evolutionarily divergent animal species. Here we report, for the first time, that expression of galectins extends to the reptilia lineage of lizards. Up to five lactose-binding proteins were isolated from the lizard Podarcis hispanica by affinity chromatography on asialofetuin-Sepharose. The main component, which is most abundantly expressed in skin, was purified from this tissue and further characterized. Under native conditions the protein behaved as a monomer with a molecular mass of 14,500 Da and an isoelectric point of 6.3. Based on sequence homology of the 58 N-terminal amino acid residues with galectins, and on its demonstrated galactoside-binding activity, this lectin we named LG-14 (from Lizard Galectin and 14 kDa) is classified as a new member of the galectin family. LG-14 falls into and strengthen the still thinly populated category of monomeric prototype galectins.

Solution conformations of a trimannoside from nuclear magnetic resonance and molecular dynamics simulations

N-linked oligosaccharides often act as ligands for receptor proteins in a variety of cell recognition processes. Knowledge of the solution conformations, as well as protein-bound conformations, of these oligosaccharides is required to understand these important interactions. In this paper we present a model for the solution conformations sampled by a simple trimannoside, methyl 3, 6-di-O-(alpha-D-mannopyranosyl)-alpha-D-mannopyranoside, which contains two of the most commonly found glycosidic linkages in N-linked oligosaccharides. This model was derived from simulated annealing protocols incorporating distance restraints extracted from NOESY spectra along with torsional restraints computed from three-bond (1)H-(13)C coupling constants measured across the glycosidic bonds. The model was refined in light of unrestrained molecular dynamics simulations conducted in the presence of solvent water. The resulting model depicts a molecule undergoing conformational averaging in solution, adopting four major and two minor conformations. The four major conformations arise from a pair of two-state transitions, one each at the alpha(1-->3) and alpha(1-->6) linkages, whereas the minor conformations result from an additional transition of the alpha(1-->6) linkage. Our data also suggest that the alpha(1-->3) transition is fast and changes the molecular shape slightly, whereas the alpha(1-->6) is much slower and alters the molecular shape dramatically.

Differences in zero-force and force-driven kinetics of ligand dissociation from beta-galactoside-specific proteins (plant and animal lectins, immunoglobulin G) monitored by plasmon resonance and dynamic single molecule force microscopy

Protein-carbohydrate interactions are involved in diverse regulatory processes. To help understand the mechanics and kinetics of dissociation of receptor-ligand complexes, we have analyzed the separation of lactose and the N-glycan chains of asialofetuin (ASF) from three lectins and an immunoglobulin G fraction by surface plasmon resonance at zero force and by atomic force microscopy with variations of the external force. While the (AB)2 agglutinins from Ricinus communis (RCA) and Viscum album (VAA) show structural homology, the homodimeric galectin-1 from bovine heart (BHL) has no similarity to the two plant lectins except for sharing this monosaccharide specificity. The beta-galactoside-binding immunoglobulin G (IgG) fraction from human serum provides a further model system with distinct binding-site architecture. The k(off) constants for the two plant agglutinins were independent of the nature of the ligand at 1.1-1.3 x 10(-3) s(-1), whereas the geometry of ligand and binding site presentation affected this parameter for BHL (0.5 x 10(-3) s(-1) for lactose and 1 x 10(-3) s(-1) for ASF) and IgG (1.3 x 10(-3) s(-1) for lactose and 0.55 x 10(-3) s(-1) for ASF). When assessing comparatively the rupture forces at a loading rate of 3 nN/s with lactose as ligand, 34 +/- 6 pN (BHL), 36 +/- 4 pN (IgG), 47 +/- 7 pN (VAA), and 58 +/- 9 pN (RCA) were measured. For the same loading rate the rupture forces for the receptor-ASF interactions were found to be 37 +/- 3 pN (BHL), 43 +/- 5 pN (VAA), 45 +/- 6 pN (IgG), and 65 +/- 9 pN (RCA). The variation of the pulling velocity revealed in all cases a linear dependence between the rupture force and the natural logarithm of the loading rate. Performing probability density and Monte Carlo calculations, the potential barrier widths, which determine the inverse dynamic dependence with the rate of force elevation, increased from 4 A (RCA) and 7 A (VAA and IgG) to 10 A (BHL) for the receptor-lactose interactions. Presenting ASF as ligand potential widths of 4 A for RCA and IgG and 6 A for VAA and BHL were obtained. Since the dissociation kinetics at zero force apparently cannot predict the behavior in force-driven experiments, these results reveal new insights into biological functions. The dissociation kinetics under force helps to explain the difference in the toxic potency of VAA and RCA and points to a function of the galectin in cis-crosslinking and in transient trans-bridging.

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.

The amino-acid sequence of the abalone (Haliotis laevigata) nacre protein perlucin. Detection of a functional C-type lectin domain with galactose/mannose specificity

Perlucin isolated from abalone nacre consists of 155 amino acids including a glycosylated asparagine. The sequence of the first 130 amino acids shows a high similarity to the C-type carbohydrate-recognition domains of asialoglycoprotein receptors and other members of the group of C-type lectins but also a weaker similarity to related proteins without carbohydrate-binding activity. This C-type module is followed by a short C-terminal domain containing two almost identical sequence repeats with a length of 10 amino acids. Solid phase assays show a divalent metal ion-dependent binding of perlucin to (neo)glycoproteins containing D-galactose or D-mannose/D-glucose indicating that perlucin is a functional C-type lectin with broad carbohydrate-binding specificity. Our results also indicate that it may be difficult to predict carbohydrate-binding specificity and the occurrence of alternative binding configurations by amino-acid sequence comparisons and homology modeling.

NMR investigations of protein-carbohydrate interactions binding studies and refined three-dimensional solution structure of the complex between the B domain of wheat germ agglutinin and N,N', N"-triacetylchitotriose

The specific interaction of the isolated B domain of wheat germ agglutinin (WGA-B) with N,N',N"-triacetylchitotriose has been analyzed by 1H-NMR spectroscopy. The association constants for the binding of WGA-B to this trisaccharide have been determined from both 1H-NMR titration experiments and microcalorimetry methods. Entropy and enthalpy of binding have been obtained. The driving force for the binding process is provided by a negative DeltaH which is partially compensated by negative DeltaS. These negative signs indicate that hydrogen bonding and van der Waals forces are the major interactions stabilizing the complex. NOESY NMR experiments in water solution provided 327 protein proton-proton distance constraints. All the experimental constraints were used in a refinement protocol including restrained molecular dynamics in order to determine the refined solution conformation of this protein/carbohydrate complex. With regard to the NMR structure of the free protein, no important changes in the protein NOEs were observed, indicating that carbohydrate-induced conformational changes are small. The average backbone rmsd of the 35 refined structures was 1.05 A, while the heavy atom rmsd was 2.10 A. Focusing on the bound ligand, two different orientations of the trisaccharide within WGA-B binding site are possible. It can be deduced that both hydrogen bonds and van der Waals contacts confer stability to both complexes. A comparison of the three-dimensional structure of WGA-B in solution to that reported in the solid state and to those deduced for hevein and pseudohevein in solution has also been performed.

Structural basis for chitin recognition by defense proteins: GlcNAc residues are bound in a multivalent fashion by extended binding sites in hevein domains

BACKGROUND

Many plants respond to pathogenic attack by producing defense proteins that are capable of reversible binding to chitin, a polysaccharide present in the cell wall of fungi and the exoskeleton of insects. Most of these chitin-binding proteins include a common structural motif of 30 to 43 residues organized around a conserved four-disulfide core, known as the 'hevein domain' or 'chitin-binding' motif. Although a number of structural and thermodynamic studies on hevein-type domains have been reported, these studies do not clarify how chitin recognition is achieved.

RESULTS

The specific interaction of hevein with several (GlcNAc)(n) oligomers has been studied using nuclear magnetic resonance (NMR), analytical ultracentrifugation and isothermal titration microcalorimetry (ITC). The data demonstrate that hevein binds (GlcNAc)(2-4) in 1:1 stoichiometry with millimolar affinity. In contrast, for (GlcNAc)(5), a significant increase in binding affinity is observed. Analytical ultracentrifugation studies on the hevein-(GlcNAc)(5,8) interaction allowed detection of protein-carbohydrate complexes with a ratio of 2:1 in solution. NMR structural studies on the hevein-(GlcNAc)(5) complex showed the existence of an extended binding site with at least five GlcNAc units directly involved in protein-sugar contacts.

CONCLUSIONS

The first detailed structural model for the hevein-chitin complex is presented on the basis of the analysis of NMR data. The resulting model, in combination with ITC and analytical ultracentrifugation data, conclusively shows that recognition of chitin by hevein domains is a dynamic process, which is not exclusively restricted to the binding of the nonreducing end of the polymer as previously thought. This allows chitin to bind with high affinity to a variable number of protein molecules, depending on the polysaccharide chain length. The biological process is multivalent.

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.