Systematic databank-based sequence comparisons with the cDNA sequence for a lens-specific rat protein localized between lens fiber cells disclosed its similarity to galectins. In mammals, two sequence changes occur within the seven positions of amino acids commonly engaged in contacts to the β-galactoside core of glycans. Taking the compilation of GRIFIN genes to the level of diverse vertebrates revealed an exceptional variability: mammals shared alteration at two sites, birds and reptiles at only one site and amphibians and fish presented complete reconstitution. The homodimeric (proto-type) GRIFINs of chicken and zebrafish thus are active lectins. Crystallographical information for chicken GRIFIN illustrates the contact profile to lactose without one otherwise conserved site due to the Arg-to-Val substitution. That GRIFIN is enormously stable in vertebrate lenses, can act like a glue (or a bridge) due to its structure and interacts with α-crystallin (shown for murine GRIFIN) suggests a role in well-ordered packing of lens proteins. Referring to a likely analogy in plants, oligomeric leguminous lectins, β-sandwich proteins as galectins, are also assumed to participate in depositing and spatially organizing cell contents, here storage proteins in protein bodies and their contact to the membrane in carbohydrate-dependent and -independent manners, a likely case of structural and functional convergence.
Galectin-related protein (GRP) is present in vertebrates. Sequence comparisons between GRPs from diverse species reveal an unusually high degree of similarity indicative of a strong positive selection. In solution, human and chicken GRPs are monomers irrespective of the presence of the 36-amino-acid-long extension of the core structure at the N-terminus. They are devoid of ability to bind lactose due to severe deviations from the respective sequence signature. Crystallography disclosed distortion of the binding-site architecture that precludes accommodation of lactose. The recent characterization of expression of chicken GRP (C-GRP) enables complete galectin network analysis in this organism. When tested in a panel of developing and adult organs, C-GRP presence was detected in bursa of Fabricius. Its epithelium and vessels as well as bursal B cells are positive in immunohistochemistry. In the B lymphocytes, C-GRP was predominantly cytoplasmic, whereas the chicken tandem-repeat-type galectin, the second member of the galectin family expressed in these cells, was detected at the surface. Binding of labeled C-GRP to cells and sections was blocked by heparin. These data illustrate disparities in expression and ligand profiles within the galectin family and hereby stimulate interest to perform respective mapping for mammalian GRPs as step to define its physiological function(s).
The recent discovery of the critical involvement of galectins in cancer progression, and in inflammatory and immune responses, has raised this family of β-D-galactoside-binding proteins to the rank of high-priority drug targets by the scientific community. This report will highlight the relevance of glycochemistry toward the efficient development of synthetic galectins inhibitors with high affinity and selectivity, as small molecules or multivalent glycoconjugates.
The galectins are a family of β-galactoside-specific animal lectins that contain a conserved carbohydrate recognition domain (CRD) with approximately 140 amino acid residues. There are 14 members in the mammalian galectin family (galectin-1–10, and 11–15), and they have different specificities for oligosaccharides. X-ray structures of the galectin CRD in complexes with oligosaccharides have provided important clues about the oligosaccharide-recognition mechanisms of galectins giving the different specificities. Galectin is divalent in glycan binding due to the association of two CRDs that crosslink with oligosaccharides. The spatial arrangement of the two CRDs is very important for elucidating the biological functions of galectins. Several different spatial arrangements of CRDs are found in X-ray structures of galectins. I herein examined the three-dimensional structures of galectins relevant for biological functions, based on the protein–ligand interactions related with oligosaccharide-specificity, the cross-linking structure by galectin and oligosaccharides, and the spatial arrangements of CRDs.
It is now recognized that human milk oligosaccharides (HMOs) can function both as prebiotics and as decoy receptors that inhibit the attachment of pathogenic microorganisms to the colonic mucosa. They can also act as immune modulators and as colonic maturation stimulators in breast-fed infants. These functions could be mediated by biological interaction between a variety of HMOs and lectins including galectins, selectins and siglecs. There are more than 100 HMOs; they have structural units such as H type 1: Fucα1-2Galβ1-3GlcNAc, Lewis a: Galβ1-3(Fucα1-4)GlcNAc, Lewis b: Fucα1-2Galβ1-3(Fucα1-4)GlcNAc, Lewis x: Galβ1-4(Fucα1-3)GlcNAc, sialyl Lewis a: Neu5Acα2-3Galβ1-3(Fucα1-4)GlcNAc, and sialyl Lewis x: Neu5Acα2-3Galβ1-4(Fucα1-3)GlcNAc. It can be expected that these units may be utilized as tools for studies on the sugar-binding specificities of lectins including galectins, monoclonal antibodies, virus capsid proteins and bacterial toxins. This mini-review presents the dataset of comprehensive HMO structures, including recently clarified ones, in tabular form, for its utilization in such studies, including those of carbohydrate-binding specificity of galectins. In addition, this review introduces recent in vivo and clinical studies, which may be relevant to the biological functions and future utilization of HMOs.
Galectins are a family of β-galactoside-binding lectins widely distributed among animals and fungi, and Galβ1-4GlcNAc is considered the major recognition unit of vertebrate galectins. On the other hand, more than 10 galectins have been reported in Caenorhabditis elegans that belong to the invertebrate class and these C. elegans galectins have been confirmed to have affinity for Galβ1-4GlcNAc. However, the glycan structure differs among species and glycan containing Galβ1-4GlcNAc has not been reported in C. elegans so far. Therefore, the endogenous ligands of C. elegans galectins remained undetermined. Recent studies uncovered that the structures of endogenous ligand glycans of C. elegans galectins are different from those of vertebrate galectins, for e.g., N-glycan containing Galactoseβ1-4 fucose epitope which is not found in vertebrates. Further, C. elegans galectins play a role in host defense against infection and oxidative stress. The roles of mammalian galectins in host defense have been explored in recent years. Taken together, one of the fundamental functions of galectins is possibly host defense because both C. elegans and mammalian galectins function in host defense despite alterations of their ligand structure in the evolutionary process.
The fruiting body of the edible mushroom Agrocybe cylindracea contains a proto-type galectin named ACG, which shows affinity to a wide range of β-galactosides. Unlike other galectins, it also binds to disaccharides (GalNAcα1-3Gal/GalNAc) found in blood group A and Forssman epitopes. Structurally, ACG lacks an evolutionarily conserved Asn located on the S4 strand (Ala64) but is compensated for by Asn46, which is located on the extended loop region unique to this galectin. A recent site-directed mutagenesis study revealed that the N46A mutant had selective affinity to oligosaccharides containing GalNAcα1-3Gal/GalNAc epitopes (Hu et al., (2013) Biochem. J., 453, 261–270). Taking into consideration the previous observation that Pro45 takes the cis conformation, Hu et al. assumed that ACG has evolved to attain two conformations at the imide group of Pro45: a cis conformation, where it can recognize β-galactosides, and a Pro45-tras conformation while it binds to the unique GalNAcα1-3Gal/GalNAc. The proposed dual recognition mechanism was proved through further site-directed mutagenesis and X-ray crystallography analysis. Notably, N46A recognizes even non-reducing terminal disaccharides, GalNAcα1-3Gal/GalNAc. Thus, the one face of this “Janus-type” lectin fulfills the conventional definition of galectins, whereas the other face does not. A possible scenario of galectin deviation is discussed.
Galectins, β-galactoside-binding lectins, consist of 15 members and are broadly distributed in the mammalian body in organ- and cell-specific manners. This minireview summarizes current knowledge on the cellular localization of galectin subtypes in various organs including the digestive tract, lymphatic system, respiratory system, urinary system, and reproductive system. We also summarize the specific localization of galectins in the epithelium with a focus on the characteristic morphologies of epithelia. In addition, we discuss the functions of galectins in the reproductive and endocrine systems, pathogenic angiogenesis, and the regulation of stem cells. The regulatory mechanism of galectin expression is also discussed based on findings obtained from luteal cells. Various glycoconjugate ligands for galectins have been identified, and notably the ligands for galectins differ in each galectin-expressing cell. The physiological and pathological states of cells affect the expression of galectin and glycoconjugate ligands, and the extracellular environment, such as the concentration of growth factors, nutrients, and oxygen also controls the expression levels. Future studies to identify the cellular and intracellular localization of galectins using histological analyses will provide a better understanding of the potential functions of galectins in healthy and disease states.
The proper function of the immune system entails multiple regulatory pathways aimed at modulating immunogenic and tolerogenic functions of immune cells. Galectins, a family of carbohydrate-binding proteins, control a variety of biological processes involved in activation, differentiation, trafficking and survival of immune cells. In this review we summarize pioneer work and emerging findings highlighting selected functions of galectins as regulatory checkpoints that control innate and adaptive immune cell programs.
Galectin-9 (Gal-9)/Ecalectin was first identified as a T cell-derived eosinophil chemoattractant. We found that Gal-9 plays a role in not only the accumulation but also the activation of eosinophils in experimental allergic models and human allergic patients; Gal-9 was shown to induce eosinophil chemotaxis in vitro and in vivo and activated eosinophils in various aspects. Recent studies, however, showed that Gal-9 has other functions in the differentiation, maturation, aggregation, adhesion, and death of various cells. Presently, we and other groups are in the process of investigating the function of Gal-9 in a variety of physiological and pathological conditions. In this article, we will show the in vivo therapeutic effects of Gal-9 in various disease models (both hyper-immune and immune-compromised conditions), suggesting that Gal-9’s immune function changes according to the context. Indeed, the accumulated evidence suggests that Gal-9 orchestrates a variety of biological phenomena to maintain homeostasis.
Galectin-8 (Gal-8) is a member of the galectin family of animal lectins that regulate a myriad of biological processes including cell growth, cell transformation, embryogenesis, apoptosis, cell adhesion and immune responses. Gal-8 expression increases in several, though not all, cancerous tissues including lung, bladder, kidney, prostate, and breast tissues. Based on its prevalence, an estimated ~500,000 newly diagnosed cancer patients/year are expected to possess an amplified Gal-8 gene. Yet, the molecular mechanisms underlying its role in cancer growth and metastasis remain incompletely understood. Here we describe potential modes of action of Gal-8 that might account for its central role in cancer biology. The evidence, gathered thus far, implicates Gal-8 as a driver of a ‘vicious cycle,’ whereby cancer cells that overexpress and secrete Gal-8, benefit from its potential to promote their own growth; potentiate epithelial mesenchymal transition, and induce secretion of metastasis-promoting agents at the metastatic niche that induce further recruitment and seeding of cancer cells. Further in-depth studies related to its mode of action, are expected to support ongoing efforts aimed at implementing Gal-8-targeted therapies for the treatment of cancer patients.
The proteins now called galectins were discovered about 1975 based on their galactoside-binding activity, in a quest to find proteins that decode complex cell-surface glycans, to take part in cell adhesion. They were defined and named in 1994 based on conserved β-galactoside binding sites found within their characteristic ~130 amino acid (aa) carbohydrate recognition domains (CRDs). However, already at their initial discovery, it was also realized that galectins reside in the cytosol or nucleus for much of their life time, and reach their galactoside ligands only after non-classical secretion that bypasses the Golgi apparatus. Here some of the early studies (mainly before 1994) will be summarized, and exemplified with some galectin stories. The phylogenetic relationships of vertebrate galectins will be summarized as a background. The galectin field has developed rapidly after 1994 in many directions. A few basic outstanding questions will be raised and briefly discussed. What determines galectin binding affinity and specificity for natural glycoconjugate ligands? What is the functional role of galectin fine specificity for carbohydrates? Is there a functional connection between on one hand the cytosolic and nuclear galectin functions and on the other extracellular/intravesicular activities? Are there regulatory loops?
To understand the biological functions of lectins, it is important to investigate their sugar-binding specificity. Although galectins are characterized as β-galactoside-binding proteins comprising evolutionarily conserved amino-acid sequences, they have significantly divergent specificities depending on their individual carbohydrate-recognition domains (CRDs). Of the various methods available to analyze lectin-glycan interactions, frontal affinity chromatography is unique in that it provides a quantitative set of dissociation constants (Kd’s) between immobilized lectins and a panel of (>100) fluorescently labeled oligosaccharides. In this article, we provide an overview of the features of galectin specificities with a focus on human galectins based on published data. From the data obtained, comprehensive features of individual CRDs can be systematically understood in terms of branching, and 3′-modifications including sialylation, sulfation, αGal/GalNAc substitutions, β1-3Gal extension, and N-acetyllactosamine repeats. Additionally, we analyze evolutionarily more distant galectin molecules of non-human origins. These findings provide not only basic knowledge but also useful information for their applications: e.g., for engineering superior galectins improved in their specificity and affinity and developing galectin-targeted drugs.
Given the critical role of galectins in cancer and other diseases, considerable efforts have been deployed towards the development of carbohydrate-based inhibitors that limit the binding of galectins to glycosylated residues on cell surface receptors. However, despite decades of research, progress in this field has not met expectations. In this article, we discuss the rationale justifying the development of a new class of galectin-specific peptide inhibitors that disrupt the formation of a prototypic galectin and its protumorigenic functions. These dimer interfering peptides (DIPs) represent an interesting alternative—and possibly a complementary avenue—to neutralize galectin-mediated protumoral functions. If validated, the approach could broaden the classes of galectin inhibitors that can be readily generated against other prototypic galectins, and possibly all other galectin subtypes.
Galectins interact with N-acetyllactosamine (LacNAc) epitopes in transmembrane glycoproteins at the cell surface in a multivalent manner forming a “lattice.” The term “galectin lattice” was first used to describe the impact of galectin-3 on immune synapse formation, T cell activation and autoimmunity (Demetriou et al. (2001) Nature 409, 733). The galectin lattice displays rapid exchange of binding partners or stochastic-binding, thereby acting as an intermediary between free diffusion of glycoproteins and stable complexes in the membrane. This includes (i) slowing diffusion and loss of receptor and transporters to coated-pit endocytosis and/or caveolin domains, (ii) slowing the integration of transmembrane phosphatases with signaling microdomains and (iii) promoting turnover (i.e., opposing stability) of cell-cell and focal adhesion complexes. The lattice model classifies galectins as adaptors of glycoprotein functions; regulating their localization, trafficking and thereby activity thresholds. The lattice model has been validated in immune regulation, cell adhesion and motility, and glucose homeostasis in mice. Here we review physical attributes of galectins and their N-glycan ligands and apply logical inference, coupled with convergence of biochemical, cell biology and genetic evidence that provide a strong Bayesian probability for greater utility of the lattice model.
Galectins are β-galactoside-recognizing animal lectins distributed both extracellularly and intracellularly. Extracellular galectins can recognize cell surface carbohydrates and initiate cellular responses. Intracellular galectins can control cellular functions independently of carbohydrate recognition. Recently, studies have suggested that intracellular galectins accumulate around damaged endocytic vesicles through the recognition of host glycans exposed to the cytosol, and accumulated galectins further mediate cellular responses. Here, we summarize the current understanding of the molecular mechanisms underlying how intracellular galectins regulate cellular responses through the recognition of glycans in damaged endocytic vesicles.
Galectins regulate a wide variety of biological processes. However, one of the earliest and most common galectin activities is likely their ability to recognize microbes. Galectin binding to microbes can result in direct microbial killing and activation of host immunity, eventually enhancing the ability of a host to eliminate microbes. However, microbes appear to have also evolved the ability to utilize galectins to enhance host attachment, ultimately leading to increased risk for infection. The ability of galectins to directly engage microbes, coupled with their role in regulating host immune function, positions these carbohydrate binding proteins as key factors that can dictate the consequence of microbial exposure. In this way, galectins represent a highly pleiotropic protein family involved in the regulation of a broad range of host-microbial interactions.
The glycocalyx is a layer of glycoconjugates found on the surface of both host cells and microorganisms. Information presented on some of the glycans on glycoconjugates is recognized by mammalian glycan-binding proteins, lectins, and these interactions modulate various physiological processes, including host innate immune responses. Lectins and host glycoconjugates are synthesized in the same secretory pathway. One notable exception is a family of soluble β-galactoside-binding lectins, galectins, which are synthesized and accumulated in the cytosol and thereby segregated from their glycan ligands. In cases where pathogenic infection persists and tissue injury occurs, galectins are passively released from injured cells. In addition, galectins are actively secreted through unconventional secretory pathways by inflammation-activated or differentiating cells. Thus, extracellular emergence of galectins is associated with the presence of pathogenic hazards. Evidence from a series of studies suggests that galectins exert multiple immunological effects. Extracellular galectin-3 acts as a damage-associated molecular pattern (DAMP) and adhesion molecule for neutrophils in lungs to initiate a proinflammatory response and to mediate rapid neutrophil migration in lungs infected with pathogenic microorganisms. In this review, the roles of galectin-3 in initial innate immune responses and resolution are discussed together with a historical overview of research on galectins in the secretory pathway and innate immunology.
Galectins comprise a relatively large family of carbohydrate-binding proteins that recognize and bind to β-D-galactoside residues expressed on a variety of different molecules. Advances in the study of their biological activities demonstrate potential functions in cancer progression, inflammatory, immune, and fibrotic responses which have recently translated to target the galectins as novel therapeutic strategies to address unmet medical needs. This review will summarize the therapeutic applications of galectin antagonists to treat human diseases currently in clinical trials.