Comparative lectinology: Delineating glycan-specificity profiles of the chicken galectins using neoglycoconjugates in a cell assay

A brief history of galectin evolution

Galectins are a family of carbohydrate-binding proteins found in vertebrates in great abundance and diversity in terms of both structure and ligand-binding properties as well as physiological function. Proteins with clear relationships to vertebrate galectins are already found in primitive Bilateria. The increasing amount of accessible well-annotated bilaterian genomes has allowed us to reveal, through synteny analyses, a new hypothesis about the phylogenetic history of the galectin family in this animal group. Thus, we can trace the genomic localization of the putative ancestral Bilateria galectin back to the scallops as a still very primitive slow-evolving bilaterian lineage. Intriguingly, our analyses show that the primordial galectin of the Deuterostomata most likely exhibited galectin-8-like characteristics. This basal standing galectin is characterized by a tandem-repeat type with two carbohydrate recognition domains as well as by a sialic acid binding property of the N-terminal domain, which is typical for galectin-8. With the help of synteny, the amplification of this potential primordial galectin to the broad galectin cosmos of modern jawed vertebrates can be reconstructed. Therefore, it is possible to distinguish between the paralogs resulting from small-scale duplication and the ohnologues generated by whole-genome duplication. Our findings support a substantially new hypothesis about the origin of the various members of the galectin family in vertebrates. This allows us to reveal new theories on the kinship relationships of the galectins of Gnatostomata. In addition, we focus for the first time on the galectines of the Cyclostomata, which as a sister group of jawed vertebrates providing important insights into the evolutionary history of the entire subphylum. Our studies also highlight a previously neglected member of the galectin family, galectin-related protein 2. This protein appears to be a widespread ohnologue of the original tandem-repeat ancestor within Gnathostomata that has not been the focus of galectin research due to its nonclassical galactose binding sequence motif and the fact that it was lost during mammalian evolution.

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

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

Binding of Gold(III) Porphyrin by the Pro-metastatic Regulatory Protein Human Galectin-3

Gold(III) porphyrin presents an attractive alternative to the use of, for example, cisplatin in chemotherapy. However, approaches that allow to selectively target cancer cells are highly sought. Many plant and mammalian lectins have been shown to bind oligosaccharide sequences of the aberrant glycosylation pattern found on cancerous tumors. For example human galectin-3, of the galectin family specific for β-galactoside, is overexpressed in the extracellular matrix of tumorigenous and metastatic tissues. We searched for non-carbohydrate ligands for galectin-3 that can guide a cytotoxic drug to the cancer cells by maintaining its affinity for tumor associated carbohydrate antigens. Previous findings showed that zinc tetrasulfonatophenylporphyrin can bind galectin-3 with sub-micromolar affinity without disturbing lactose binding. Gold(III) porphyrin is not only cytotoxic to cancer cells, it knows also a potential application as photosensitiser in photodynamic therapy. We investigated the binding of gold(III) porphyrin to galectin-3 using different biophysical interaction techniques and demonstrated a low micromolar affinity of human galectin-3 for the cytotoxic compound. Co-crystallization attempts in order to understand the binding mode of gold porphyrin to galectin-3 failed, but molecular docking emphasized a highly populated secondary binding site that does not hinder lactose or Thomsen Friendenreich disaccharide binding. This suggests that gold(III) porphyrin might significantly enhance its concentration and delivery to cancer cells by binding to human galectin-3 that keeps its orientation towards tumor associated carbohydrate antigens.

Chicken lens development: complete signature of expression of galectins during embryogenesis and evidence for their complex formation with α-, β-, δ-, and τ-crystallins, N-CAM, and N-cadherin obtained by affinity chromatography

The emerging multifunctionality of galectins by specific protein-glycan/protein interactions explains the interest to determine their expression during embryogenesis. Complete network analysis of all seven chicken galectins (CGs) is presented in the course of differentiation of eye lens that originates from a single type of progenitor cell. It answers the questions on levels of expression and individual patterns of distribution. A qualitative difference occurs in the CG-1A/B paralogue pair, underscoring conspicuous divergence. Considering different cell phenotypes, lens fiber and also epithelial cells can both express the same CG, with developmental upregulation for CG-3 and CG-8. Except for expression of the lens-specific CG (C-GRIFIN), no other CG appeared to be controlled by the transcription factors L-Maf and Pax6. Studying presence and nature of binding partners for CGs, we tested labeled galectins in histochemistry and in ligand blotting. Mass spectrometric (glyco)protein identification after affinity chromatography prominently yielded four types of crystallins, N-CAM, and, in the cases of CG-3 and CG-8, N-cadherin. Should such pairing be functional in situ, it may be involved in tightly packing intracellular lens proteins and forming membrane contact as well as in gaining plasticity and stability of adhesion processes. The expression of CGs throughout embryogenesis is postulated to give meaning to spatiotemporal alterations in the local glycome.

Three-step monitoring of glycan and galectin profiles in the anterior segment of the adult chicken eye

A histochemical three-step approach is applied for processing a panel of sections that covers the different regions of fixed anterior segment of the adult chicken eye. This analysis gains insight into the presence of binding partners for functional pairing by galectin/lectin recognition in situ. Glycophenotyping with 11 fungal and plant lectins (step 1) revealed a complex pattern of reactivity with regional as well as glycan- and cell-type-dependent differences. When characterizing expression of the complete set of the seven adhesion/growth-regulatory chicken galectins immunohistochemically (step 2), the same holds true, clearly demonstrating profiles with individual properties, even for the CG-1A/B paralogue pair. Testing this set of labeled tissue lectins as probes (step 3) detected binding sites in a galectin-type-dependent manner. The results of steps 2 and 3 reflect the divergence of sequences and argue against functional redundancy among the galectins. These data shape the concept of an in situ network of galectins. As consequence, experimental in vitro studies will need to be performed from the level of testing a single protein to work with mixtures that mimic the (patho)physiological situation, a key message of this report.

Network analysis of adhesion/growth-regulatory galectins and their binding sites in adult chicken retina and choroid

The highly ordered multilayered organization of the adult chicken retina is a suitable test model for examining zonal distribution of the members of a bioeffector family. Based on the concept of the sugar code, the functional pairing of glycan epitopes with cognate receptors (lectins) is emerging as a means to explain the control of diverse physiological activities. Having recently completed the biochemical characterization of all seven adhesion/growth-regulatory galectins present in chicken, it was possible to establish how the individual characteristics of their expression profiles add up to shape the galectin network, which until now has not been defined at this level of complexity. This information will also have relevance in explaining the region-specific presence of glycan determinants in the retina, as illustrated in the first part of this study using a panel of nine plant/fungal agglutinins. The following systematic monitoring of the galectins yielded patterns for which quantitative and qualitative differences were detected. Obviously, positivity in distinct layers is not confined to a single protein of this family, e.g. CG-1A, CG-3 or CG-8. These results underline the requirement for network analysis for these proteins that can functionally interact in additive or antagonistic modes. Labeling of the tissue galectins facilitated profiling of their accessible binding sites. It also revealed differences among the galectin family members, highlighting the ability of this method to define binding properties on the level of tissue sections. Methodologically, the detection of endogenous lectins intimates that cognate glycans can become inaccessible, a notable caveat for lectin histochemical studies.

Specificity of human galectins on cell surfaces

Galectins are β-galactoside-binding proteins sharing homology in amino acid sequence of their carbohydrate-recognition domain. Their carbohydrate specificity outside cells has been studied previously. The main conclusion of these studies was that several levels of glycan ligand recognition exist for galectins: (i) disaccharide Galβ1-4GlcNAc (LN, N-acetyllactosamine) binds stronger than β-galactopyranose; (ii) substitution at O-2 and O-3 of galactose residue as well as core fragments ("right" from GlcNAc) provides significant increase in affinity; (iii) similarly glycosylated proteins can differ significantly in affinity to galectins. Information about the natural cellular receptors of galectins is limited. Until recently, it was impossible to study specificity of cell-bound galectins. A model based on controlled incorporation of a single protein into glycocalyx of cells and subsequent interaction of loaded cells with synthetic glycoprobes measured by flow cytometry made this possible recently. In this review, data about glycan specificity of proto-, chimera-, and tandem-repeat type galectins on the cell surface are systematized, and comparative analysis of the results with data on specificity of galectins in artificial systems was performed. The following conclusions from these studies were made: (i) cellular galectins have practically no ability to bind disaccharide LNn, but display affinity to 3'-substituted oligolactosamines and oligomers LNn; (ii) tandem-repeat type galectins recognize another disaccharide, namely Galβ1-3GlcNAc (Le(c)); (iii) on the cell surface, tandem-repeat type galectins conserve the ability to display high affinity to blood group antigens of ABH system; (iv) in general, when galectins are immersed into glycocalyx, they are more selective regarding glycan interactions. Thus, we conclude that competitive interaction of galectins with cell microenvironment (endogenous cell glycans) is the main factor providing selectivity of galectins in vivo.

Chicken GRIFIN: A homodimeric member of the galectin network with canonical properties and a unique expression profile

Occurrence of the adhesion/growth-regulatory galectins as family sets the challenge to achieve a complete network analysis. Along this route taken for a well-suited model organism (chicken), we fill the remaining gap to characterize its seventh member known from rat as galectin-related inter-fiber protein (GRIFIN) in the lens. Its single-copy gene is common to vertebrates, with one or more deviations from the so-called signature sequence for ligand (lactose) contact. The chicken protein is a homodimeric agglutinin with capacity to bind β-galactosides, especially the histo-blood group B tetrasaccharide, shown by solid-phase/cell assays and a glycan microarray. Mass spectrometric identification of two lactose-binding peptides after tryptic on-bead fragmentation suggests an interaction at the canonical region despite a sequence change from Arg to Val at the site, which impairs reactivity of human galectin-1. RT-PCR and Western blot analyses of specimen from adult chicken organs reveal restriction of expression to the lens, here immunohistochemically throughout its main body. This report sets the stage for detailed structure-activity studies to define factors relevant for affinity beyond the signature sequence and to perform the first complete network analysis of the galectin family in developing and adult organs of a vertebrate.

Multivalent Carbohydrate-Lectin Interactions: How Synthetic Chemistry Enables Insights into Nanometric Recognition

Glycan recognition by sugar receptors (lectins) is intimately involved in many aspects of cell physiology. However, the factors explaining the exquisite selectivity of their functional pairing are not yet fully understood. Studies toward this aim will also help appraise the potential for lectin-directed drug design. With the network of adhesion/growth-regulatory galectins as therapeutic targets, the strategy to recruit synthetic chemistry to systematically elucidate structure-activity relationships is outlined, from monovalent compounds to glyco-clusters and glycodendrimers to biomimetic surfaces. The versatility of the synthetic procedures enables to take examining structural and spatial parameters, alone and in combination, to its limits, for example with the aim to produce inhibitors for distinct galectin(s) that exhibit minimal reactivity to other members of this group. Shaping spatial architectures similar to glycoconjugate aggregates, microdomains or vesicles provides attractive tools to disclose the often still hidden significance of nanometric aspects of the different modes of lectin design (sequence divergence at the lectin site, differences of spatial type of lectin-site presentation). Of note, testing the effectors alone or in combination simulating (patho)physiological conditions, is sure to bring about new insights into the cooperation between lectins and the regulation of their activity.

Sweet complementarity: the functional pairing of glycans with lectins

Carbohydrates establish the third alphabet of life. As part of cellular glycoconjugates, the glycans generate a multitude of signals in a minimum of space. The presence of distinct glycotopes and the glycome diversity are mapped by sugar receptors (antibodies and lectins). Endogenous (tissue) lectins can read the sugar-encoded information and translate it into functional aspects of cell sociology. Illustrated by instructive examples, each glycan has its own ligand properties. Lectins with different folds can converge to target the same epitope, while intrafamily diversification enables functional cooperation and antagonism. The emerging evidence for the concept of a network calls for a detailed fingerprinting. Due to the high degree of plasticity and dynamics of the display of genes for lectins the validity of extrapolations between different organisms of the phylogenetic tree yet is inevitably limited.

Network monitoring of adhesion/growth-regulatory galectins: localization of the five canonical chicken proteins in embryonic and maturing bone and cartilage and their introduction as histochemical tools

Divergence from an ancestral gene leads to a family of homologous proteins. Whether they are physiologically distinct, similar, or even redundant is an open question in each case. Defining profiles of tissue localization is a step toward giving diversity a functional meaning. Due to the significance of endogenous sugar receptors (lectins) as effectors for a wide range of cellular activities we have focused on galectins. The comparatively low level of network complexity constituted by only five canonical proteins makes chicken galectins (CGs) an attractive choice to perform comprehensive analysis, here studied on bone/cartilage as organ system. Galectin expression was monitored by Western blotting and immunohistochemistry using non-cross-reactive antibodies. Overall, three galectins (CG-1B, CG-3, CG-8) were present with individual expression patterns, one was found exclusively in the mesenchyme (CG-1A), the fifth (CG-2) not being detectable. The documented extents of separation are a sign for functional divergence; in cases with overlapping stainings, as for example in the osteoprogenitor layer or periosteum, cooperation may also be possible. Recombinant production enabled the introduction of the endogenous lectins as tools for binding-site localization. Their testing revealed developmental regulation and cell-type-specific staining. Of relevance for research on mammalian galectins, this study illustrates that certain cell types can express more than one galectin, letting functional interrelationships appear likely. Thus, complete network analysis irrespective of its degree of complexity is mandatory.