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

Placental galectins: a subfamily of galectins lose the ability to bind β-galactosides with new structural features†

Galectins are a phylogenetically conserved family of soluble β-galactoside binding proteins. There are 16 different of galectins, each with a specific function determined by its distinct distribution and spatial structure. Galectin-13, galectin-14, and galectin-16 are distinct from other galectin members in that they are primarily found in placental tissue. These galectins, also referred to as placental galectins, play critical roles in regulating pregnancy-associated processes, such as placenta formation and maternal immune tolerance to the embedded embryo. The unique structural characteristics and the inability to bind lactose of placental galectins have recently received significant attention. This review primarily examines the novel structural features of placental galectins, which distinguish them from the classic galectins. Furthermore, it explores the correlation between these structural features and the loss of β-galactoside binding ability. In addition, the newly discovered functions of placental galectins in recent years are also summarized in our review. A detailed understanding of the roles of placental galectins may contribute to the discovery of new mechanisms causing numerous pregnancy diseases and enable the development of new diagnostic and therapeutic strategies for the treatment of these diseases, ultimately benefiting the health of mothers and offspring.

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.

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.

Fine-tuning of prototype chicken galectins: structure of CG-2 and structure-activity correlations

The comparatively small number of members of the family of adhesion/growth-regulatory galectins in chicken predestines this system as an attractive model to study the divergence of these lectins after gene duplication. Expression profiling of the three homodimeric (prototype) chicken galectins (CG-1A, CG-1B and CG-2) has raised evidence of distinct functionalities, explaining the interest in a detailed crystallographic analysis of CG-2. As revealed here, marked differences are found in the ligand-binding site and in the contact pattern within the homodimer interface, underlying a characteristic orientation of the two subunits. Notably, a distinctive trimer of dimers that is unique in all galectin crystal structures reported to date forms the core unit of the crystallographic assembly. Combination with spectroscopic and thermodynamic measurements, and comparisons with CG-1A and CG-1B, identify differential changes in the circular-dichroism spectra in the presence of lactose, reflecting the far-reaching impact of the ligand on hydrodynamic behaviour, and inter-galectin differences in both the entropy and the enthalpy of binding. This structural information is a salient step to complete the analysis of the full set of galectins from this model organism.

Governing the invasive trophoblast: current aspects on intra- and extracellular regulation

This review summarizes several aspects especially of regulating factors governing trophoblast invasion. Those include the composition of the extracellular matrix containing a variety of matrix metalloproeinases and their inhibitors, but also intracellular signals. Furthermore, a newly described trophoblast subtype, the endoglandular trophoblast, is presented. Its presence may provide a possible mechanism for opening and connecting uterine glands into the intervillous space. Amongst others, two intracellular signalling pathways are crucial for regulation of trophoblast functions and development: Wnt- and signal transducer and activator of transcription (STAT)3 signalling. Wnt signalling promotes implantation, placentation and trophoblast differentiation. Several Wnt-dependent cascades and regulatory mechanisms display different functions in trophoblast cells. The STAT3 signalling system is fundamental for induction and regulation of invasiveness in physiological trophoblastic cells, but also in tumours. The role of galectins (Gal) in trophoblast regulation and placenta development comes increasingly into focus. The Gal- 1-4, 7-10 and 12-14 have been detected in humans. Detailed information is only available for Gal-1, -2, -3, -4, -9 and -12 in endometrium and decidua. Gal-1, -3 and -13 (-14) have been detected and studied in trophoblast cells.

Cell-type-specific expression of murine multifunctional galectin-3 and its association with follicular atresia/luteolysis in contrast to pro-apoptotic galectins-1 and -7

Galectin-3 is a multifunctional protein with modular design. A distinct expression profile was determined in various murine organs when set into relation to homodimeric galectins-1 and -7. Fittingly, the signature of putative transcription-factor-binding sites in the promoter region of the galectin-3 gene affords a toolbox for a complex combinatorial regulation, distinct from the respective sequence stretches in galectins-1 and -7. A striking example for cell-type specificity was the ovary, where these two lectins were confined to the surface epithelium. Immunohistochemically, galectin-3 was found in macrophages of the cortical interstitium between developing follicles and medullary interstitium, matching the distribution of the F4/80 antigen. With respect to atresia and luteolysis strong signals in granulosa cells of atretic preantral but not antral follicles and increasing positivity in corpora lutea upon regression coincided with DNA fragmentation. Labeled galectin-3 revealed lactose-inhibitable binding to granulosa cells. Also, slender processes of vital granulosa cells which extended into the zona pellucida were positive. This study demonstrates cell-type specificity and cycle-associated regulation for galectin-3 with increased presence in atretic preantral follicles and in late stages of luteolysis.

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

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

Physicochemical properties and oxidative inactivation of soluble lectin from water buffalo (Bubalus bubalis) brain

Lectins are carbohydrate-binding proteins present in a wide variety of plants and animals, which serve various important physiological functions. A soluble beta-galactoside binding lectin has been isolated and purified to homogeneity from buffalo brain using ammonium sulphate precipitation (40-70%) and gel permeation chromatography on Sephadex G50-80 column. The molecular weight of buffalo brain lectin (BBL) as determined by SDS-PAGE under reducing and non-reducing conditions was 14.2 kDa, however, with gel filtration it was 28.5 kDa, revealing the dimeric form of protein. The neutral sugar content of the soluble lectin was estimated to be 3.3%. The BBL showed highest affinity for lactose and other sugar moieties in glycosidic form, suggesting it to be a beta-galactoside binding lectin. The association constant for lactose binding as evidenced by Scatchard analysis was 6.6 x 10(3) M(-1) showing two carbohydrate binding sites per lectin molecule. A total inhibition of lectin activity was observed by denaturants like guanidine HCl, thiourea and urea at 6 M concentration. The treatment of BBL with oxidizing agent destroyed its agglutination activity, abolished its fluorescence, and shifted its UV absorption maxima from 282 to 250 nm. The effect of H2O2 was greatly prevented by lactose indicating that BBL is more stable in the presence of its specific ligand. The purified lectin was investigated for its brain cell aggregation properties by testing its ability to agglutinate cells isolated from buffalo and goat brains. Rate of aggregation of buffalo brain cells by purified protein was more than the goat brain cells. The data from above study suggests that the isolated lectin may belong to the galectin-1 family but is glycosylated unlike those purified till date.

Inhibition of galectin-3 mediated cellular interactions by pectic polysaccharides from dietary sources

Pectic polysaccharides from dietary sources such as Decalepis hamiltonii--swallow root (SRPP), Hemidesmus indicus (HPP), Nigella sativa--black cumin (BCPP), Andrographis serpyllifolia-(APP), Zingiber officinale--ginger (GRPP) and, citrus pectin (CPP) were examined for galectin inhibitory activity. Inhibition of (a) galectin-3 of MDA-MB-231 cells induced hemagglutination of red blood cells; (b) galectin-3 mediated interaction between normal/metastatic human buccal cells (NBC)/(MBC) and; (c) invasion of MDA-MB-231 and MBC in the invasive chamber was assessed. Results indicated that SRPP inhibited hemagglutination at Minimum Inhibitory Concentration (MIC) of 1.86 microg ml(-1) equivalent of carbohydrate as apposed to those of BCPP (130 microg ml(-1)), APP (40 microg ml(-1)), HPP (40 microg ml(-1)) and CPP (25 microg ml(-1)). GRPP even at concentration >1-6 mg ml(-1) did not inhibit agglutination. Also SRPP showed approximately 15 and 2 fold potent anti hemagglutination activity relative to that of galectin-3 specific sugars-galactose (MIC-27.1 microg ml(-1)) and lactose (MIC-4.16 microg ml(-1)) respectively. Further, SRPP at 10 microg ml(-1) inhibited agglutination of NBC by galectin-3 of MDA-MB-231 cells. Modified swallow root pectic polysaccharide (MSRPP) of 50 kDa retained anti hemagglutination activity (MIC of 1.03 microg ml(-1)) and inhibited MDA-MB-231 and MBC invasion by 73 and 50% with an IC(50) of 136 and 200 microg ml(-1) respectively. Both SRPP and MSRPP induced apoptosis up to 80% at 100 microg ml(-1) concentration by activating approximately 2 and 8 folds of Caspase-3 activity. Sugar composition analysis and its correlation with the galectin inhibitory property indicated that pectic polysaccharides with higher arabinose and galactose content-arabinogalactan inhibited hemagglutination significantly.

New roles for galectins in brain tumors--from prognostic markers to therapeutic targets

Despite advances in diagnosis and treatment, brain tumors continue to be the leading cause of cancer-related death in patients under 35 years of age, demonstrating the need for better prognostic and therapeutic targets. Galectins, a family of mammalian carbohydrate binding proteins, are involved in many processes important for tumor survival and dissemination, including proliferation, apoptosis, transcriptional regulation, intracellular signaling, cell adhesion, and cell migration. Several galectins are expressed in human brain, with many galectins demonstrating altered expression during tumor progression. Thus, galectins and the functions regulated by this family of proteins are potential targets for the diagnosis and treatment of brain cancer. This review highlights the roles of galectins in cancer and specifically, the developing field of galectins in brain cancer.

Hippocampal neurons and recombinant galectins as tools for systematic carbohydrate structure-function studies in neuronal differentiation

Membrane glycoconjugates play a central role in neuronal interactions and regulation. To define the precise links between membrane polysaccharides and neuronal functions, two main requirements must be fulfilled: (1) the availability of molecular tools able to finely discriminate among carbohydrate structures and (2) the use of an experimental system suitable for systematic and quantitative studies of particular neuronal processes. In this work, we used two chicken proto-type galectins, i.e., monomeric CG-14 and dimeric CG-16, with very similar carbohydrate affinities, and rat hippocampal neurons in culture to quantitatively measure the involvement of carbohydrate-protein interaction in axonal growth and directionality, neurite sprouting and axon regenerative capacity after section. CG-16 potently stimulated axonal growth and guidance. Neurite sprouting was enhanced by immobilized CG-16 and, notably, reduced by lectin in solution. Overall, cross-linking CG-16 invariably excelled CG-14 in these functional assays, although none of them were able to improve axon regenerative capacity when compared to mammalian galectin-1. Our results demonstrate the potential of the experimental set-up to perform a systematic study of galectin functionality in neuronal differentiation. In view of the concept of the sugar code, the presented results indicate that biological effects triggered by glycan binding engaging an endogenous lectin can be modulated by carbohydrate affinity and/or by other factors like differential cross-linking capacity.

Determination of structural and functional overlap/divergence of five proto-type galectins by analysis of the growth-regulatory interaction with ganglioside GM1in silicoandin vitroon human neuroblastoma cells

The growth-regulatory interplay between ganglioside GM1 on human SK-N-MC neuroblastoma cells and an endogenous lectin provides a telling example for glycan (polysaccharide) functionality. Galectin-1 is the essential link between the sugar signal and the intracellular response. The emerging intrafamily complexity of galectins raises the question on defining extent of their structural and functional overlap/divergence. We address this problem for proto-type galectins in this system: ganglioside GM1 as ligand, neuroblastoma cells as target. Using the way human galectin-1 interacts with this complex natural ligand as template, we first defined equivalent positioning for distinct substitutions in the other tested proto-type galectins, e.g., Lys63 vs. Leu60/Gln72 in galectins-2 and -5. As predicted from our in silico work, the tested proto-type galectins have affinity for the pentasaccharide of ganglioside GM1. In contrast to solid-phase assays, cell surface presentation of the ganglioside did not support binding of galectin-5, revealing the first level of regulation. Next, a monomeric proto-type galectin (CG-14) can impair galectin-1-dependent negative growth control by competitively blocking access to the shared ligand without acting as effector. Thus, the quaternary structure of proto-type galectins is an efficient means to give rise to functional divergence. The identification of this second level of regulation is relevant for diagnostic monitoring. It might be exploited therapeutically by producing galectin variants tailored to interfere with galectin activities associated with the malignant phenotype. Moreover, the given strategy for comparative computational analysis of extended binding sites has implications for the rational design of galectin-type-specific ligands.

Expression and localization of galectin 4 in rat stomach during postnatal development

Galectins are lectins implicated in cell-cell or cell-matrix adhesion, cell growth, the cell cycle, transcription processes, and apoptosis, and some of them are differentially regulated during pre- or post-natal development. The purpose of the present study was to determine whether the expression of galectin 4 is relevant to developmental processes during postnatal development in the rat stomach. Galectin 4 expression in the rat gastric mucosa, between birth and adulthood, was studied at the protein and mRNA levels by western and northern blotting, respectively. This lectin was localized precisely by immunoelectron microscopy. In the gastric mucosa, galectin 4 protein was present at lower levels in suckling than in weaned rats, but mRNA levels did not change significantly during postnatal development. This suggests possible differences in mRNA stability or in the translation regulation. Immunocytochemical examination of galectin 4 confirmed more highly elevated levels of the protein in endocrine, parietal, and chief cells in weaned rats than in suckling rats. Galectin 4 was more strongly localized in weaned rats than in suckling rats in the nuclei of all cell types and in or over secretory granules in endocrine and chief cells, suggesting that galectin 4 is implicated in nuclear events and perhaps in secretory processes.