Structure of baby hamster kidney carbohydrate-binding protein CBP30, an S-type animal lectin

The interrelation of galectins and autophagy

Autophagy is a vital physiological process that maintains intracellular homeostasis by removing damaged organelles and senescent or misfolded molecules. However, excessive autophagy results in cell death and apoptosis, which will lead to a variety of diseases. Galectins are a type of animal lectin that binds to β-galactosides and can bind to the cell surface or extracellular matrix glycans, affecting a variety of immune processes in vivo and being linked to the development of many diseases. In many cases, galectins and autophagy both play important regulatory roles in the cellular life course, yet our understanding of the relationship between them is still incomplete. Galectins and autophagy may share common etiological cofactors for some diseases. Hence, we summarize the relationship between galectins and autophagy, aiming to draw attention to the existence of multiple associations between galectins and autophagy in a variety of physiological and pathological processes, which provide new ideas for etiological diagnosis, drug development, and therapeutic targets for related diseases.

Proteolytic processing of galectin-3 by meprin metalloproteases is crucial for host-microbiome homeostasis

The metalloproteases meprin α and meprin β are highly expressed in the healthy gut but significantly decreased in inflammatory bowel disease, implicating a protective role in mucosal homeostasis. In the colon, meprin α and meprin β form covalently linked heterodimers tethering meprin α to the plasma membrane, therefore presenting dual proteolytic activity in a unique enzyme complex. To unravel its function, we applied N-terminomics and identified galectin-3 as the major intestinal substrate for meprin α/β heterodimers. Galectin-3-deficient and meprin α/β double knockout mice show similar alterations in their microbiome in comparison to wild-type mice. We further demonstrate that meprin α/β heterodimers differentially process galectin-3 upon bacterial infection, in germ-free, conventionally housed (specific pathogen-free), or wildling mice, which in turn regulates the bacterial agglutination properties of galectin-3. Thus, the constitutive cleavage of galectin-3 by meprin α/β heterodimers may play a key role in colon host-microbiome homeostasis.

Liquid-liquid phase separation: Galectin-3 in nuclear speckles and ribonucleoprotein complexes

Nuclear speckles are subcellular structures originally characterized by punctate immunofluorescence staining of the monoclonal antibody SC35, which recognizes an epitope on SRRM2 (serine/arginine repetitive matrix protein 2) and Sfrs2, a member of the SR (serine/arginine-rich) family of splicing factors. Galectin-3 co-localizes with SC35 in nuclear speckles, which represent one group of nuclear bodies that include the nucleolus, Cajal bodies and gems, paraspeckles, etc. Although they appear to have well-delineated physical boundaries, these nuclear bodies are not membrane-bound structures but represent macromolecular assemblies arising from a phenomenon called liquid-liquid phase separation. There has been much recent interest in liquid phase condensation as a newly recognized mechanism by which a cell can organize and compartmentalize subcellular structures with distinct composition. The punctate/speckled staining of galectin-3 with SC3 demonstrates their co-localization in a phase-separated body in vivo, under conditions endogenous to the cell. The purpose of the present review is to summarize the studies that document three key features of galectin-3 for its localization in liquid phase condensates: (a) an intrinsically disordered domain; (b) oligomer formation for multivalent binding; and (c) association with RNA and ribonucleoprotein complexes.

Galectin-3 as a biomarker in breast neoplasms: Mechanisms and applications in patient care

Galectin-3 is a member of the lectin family encoded by the LGALS3 gene on chromosome 14. It is secreted by a wide range of immune cells and mammary tumor cells. Through its activity on the tumor microenvironment, in particular on tumor-infiltrating leukocytes, galectin-3 improves the proliferation, survival, and colonizing ability of mammary neoplastic cells. Consequently, galectin-3 expression in the tumor microenvironment could worsen therapeutic outcomes of breast neoplasms and become a biomarker and a therapeutic target in combined immunotherapy in breast neoplasms. There is a limited amount of information that is available on galectin-3 in breast cancer in Africa. In this review, we analyze how galectin-3 influences the tumor microenvironment and its potential as a biomarker and therapeutic target in breast neoplasms. We aim to emphasize the significance of investigating galectin-3 in breast neoplasms in Africa based on the results of studies conducted elsewhere.

Galectin-3: a key player in microglia-mediated neuroinflammation and Alzheimer's disease

Alzheimer’s disease (AD) is the most common cause of dementia and is characterized by the deposition of extracellular aggregates of amyloid-β (Aβ), the formation of intraneuronal tau neurofibrillary tangles and microglial activation-mediated neuroinflammation. One of the key molecules involved in microglial activation is galectin-3 (Gal-3). In recent years, extensive studies have dissected the mechanisms by which Gal-3 modulates microglial activation, impacting Aβ deposition, in both animal models and human studies. In this review article, we focus on the emerging role of Gal-3 in biology and pathobiology, including its origin, its functions in regulating microglial activation and neuroinflammation, and its emergence as a biomarker in AD and other neurodegenerative diseases. These aspects are important to elucidate the involvement of Gal-3 in AD pathogenesis and may provide novel insights into the use of Gal-3 for AD diagnosis and therapy.

Molecular Identification and mRNA Expression Profiles of Galectin-9 Gene in Red Sea Bream (Pagrus major) Infected with Pathogens

Galectin (Gal) is a member of a family of β-galactoside-binding lectin. The members of this family play important roles in the recognition of carbohydrate ligands and in various other biological processes. In this study, we identified the gene encoding Gal-9 in Pagrus major (PmGal-9) and analyzed its expression in various tissues after pathogen challenge. Alignment analysis revealed that the two galactose-binding lectin domains of the deduced protein were highly conserved among all the teleosts. Phylogenetic analysis revealed that PmGal-9 is most closely related to the Gal-9 gene of gilthead sea bream. PmGal-9 was ubiquitously expressed in all tissues analyzed but was predominantly expressed in the spleen, head kidney, and intestine. After challenges with major microbial pathogens (Edwardsiella piscicida, Streptococcus iniae, or red sea bream iridovirus) of red sea bream, PmGal-9 mRNA expression was significantly regulated in most immune-related tissues. These results suggested that PmGal-9 not only plays an important role in the immune system of red sea bream but is also a possible inflammatory marker for pathogenic diseases.

Influence of protein (human galectin-3) design on aspects of lectin activity

The concept of biomedical significance of the functional pairing between tissue lectins and their glycoconjugate counterreceptors has reached the mainstream of research on the flow of biological information. A major challenge now is to identify the principles of structure–activity relationships that underlie specificity of recognition and the ensuing post-binding processes. Toward this end, we focus on a distinct feature on the side of the lectin, i.e. its architecture to present the carbohydrate recognition domain (CRD). Working with a multifunctional human lectin, i.e. galectin-3, as model, its CRD is used in protein engineering to build variants with different modular assembly. Hereby, it becomes possible to compare activity features of the natural design, i.e. CRD attached to an N-terminal tail, with those of homo- and heterodimers and the tail-free protein. Thermodynamics of binding disaccharides proved full activity of all proteins at very similar affinity. The following glycan array testing revealed maintained preferential contact formation with N-acetyllactosamine oligomers and histo-blood group ABH epitopes irrespective of variant design. The study of carbohydrate-inhibitable binding of the test panel disclosed up to qualitative cell-type-dependent differences in sections of fixed murine epididymis and especially jejunum. By probing topological aspects of binding, the susceptibility to inhibition by a tetravalent glycocluster was markedly different for the wild-type vs the homodimeric variant proteins. The results teach the salient lesson that protein design matters: the type of CRD presentation can have a profound bearing on whether basically suited oligosaccharides, which for example tested positively in an array, will become binding partners in situ. When lectin-glycoconjugate aggregates (lattices) are formed, their structural organization will depend on this parameter. Further testing (ga)lectin variants will thus be instrumental (i) to define the full range of impact of altering protein assembly and (ii) to explain why certain types of design have been favored during the course of evolution, besides opening biomedical perspectives for potential applications of the novel galectin forms.

Galectin-3 modulation of T-cell activation: mechanisms of membrane remodelling

Galectin-3 (Gal3) is a multifaceted protein which belongs to a family of lectins and binds β-galactosides. Gal3 expression is altered in many types of cancer, with increased expression generally associated with poor prognosis. Although the mechanisms remain unknown, Gal3 has been implicated in several biological processes involved in cancer progression, including suppression of T cell-mediated immune responses. Extracellular Gal3 binding to the plasma membrane of T cells alters membrane organization and the formation of an immunological synapse. Its multivalent capacity allows Gal3 to interact specifically with different membrane proteins and lipids, influencing endocytosis, trafficking and T cell receptor signalling. The ability of Gal3 to inhibit T cell responses may provide a mechanism by which Gal3 aids in cancer progression. In this review, we seek to give an overview of the mechanisms by which Gal3 alters the spatial organization of cell membranes and how these processes impact on T cell activation.

The Role of Galectins as Modulators of Metabolism and Inflammation

Galectins are β-galcotosid-binding lectins. The function of galectins varies with their tissue-specific and subcellular location, and their binding to carbohydrates makes them key players in several intra- and extracellular processes where they bind to glycosylated proteins and lipids. In humans, there are 12 identified galectins, some with tissue-specific distribution. Galectins are found inside cells and in the nucleus, cytosol, and organelles, as well as extracellularly. Galectin-1, -2, -3, -4, -7, -8, -9, and -12 can all induce T-cell apoptosis and modulate inflammation. In the context of metabolic control and loss of the same in, for example, diabetes, galectin-1, -2, -3, -9, and -12 are especially interesting. This review presents information on galectins relevant to the control of inflammation and metabolism and the potential to target galectins for therapeutic purposes.

The intrinsically disordered N-terminal domain of galectin-3 dynamically mediates multisite self-association of the protein through fuzzy interactions

Galectins are a family of lectins that bind β-galactosides through their conserved carbohydrate recognition domain (CRD) and can induce aggregation with glycoproteins or glycolipids on the cell surface and thereby regulate cell activation, migration, adhesion, and signaling. Galectin-3 has an intrinsically disordered N-terminal domain and a canonical CRD. Unlike the other 14 known galectins in mammalian cells, which have dimeric or tandem-repeated CRDs enabling multivalency for various functions, galectin-3 is monomeric, and its functional multivalency therefore is somewhat of a mystery. Here, we used NMR spectroscopy, mutagenesis, small-angle X-ray scattering, and computational modeling to study the self-association-related multivalency of galectin-3 at the residue-specific level. We show that the disordered N-terminal domain (residues ∼20-100) interacts with itself and with a part of the CRD not involved in carbohydrate recognition (β-strands 7-9; residues ∼200-220), forming a fuzzy complex via inter- and intramolecular interactions, mainly through hydrophobicity. These fuzzy interactions are characteristic of intrinsically disordered proteins to achieve liquid-liquid phase separation, and we demonstrated that galectin-3 can also undergo liquid-liquid phase separation. We propose that galectin-3 may achieve multivalency through this multisite self-association mechanism facilitated by fuzzy interactions.

Galectin-3 Determines Tumor Cell Adaptive Strategies in Stressed Tumor Microenvironments

Galectin-3 is a member of the β-galactoside-binding lectin family, whose expression is often dysregulated in cancers. While galectin-3 is usually an intracellular protein found in the nucleus and in the cytoplasm, under certain conditions, galectin-3 can be secreted by an yet unknown mechanism. Under stressing conditions (e.g., hypoxia and nutrient deprivation) galectin-3 is upregulated, through the activity of transcription factors, such as HIF-1α and NF-κB. Here, we review evidence that indicates a positive role for galectin-3 in MAPK family signal transduction, leading to cell proliferation and cell survival. Galectin-3 serves as a scaffold protein, which favors the spatial organization of signaling proteins as K-RAS. Upon secretion, extracellular galectin-3 interacts with a variety of cell surface glycoproteins, such as growth factor receptors, integrins, cadherins, and members of the Notch family, among other glycoproteins, besides different extracellular matrix molecules. Through its ability to oligomerize, galectin-3 forms lectin lattices that act as scaffolds that sustain the spatial organization of signaling receptors on the cell surface, dictating its maintenance on the plasma membrane or their endocytosis. Galectin-3 induces tumor cell, endothelial cell, and leukocyte migration, favoring either the exit of tumor cells from a stressed microenvironment or the entry of endothelial cells and leukocytes, such as monocytes/macrophages into the tumor organoid. Therefore, galectin-3 plays homeostatic roles in tumors, as (i) it favors tumor cell adaptation for survival in stressed conditions; (ii) upon secretion, galectin-3 induces tumor cell detachment and migration; and (iii) it attracts monocyte/macrophage and endothelial cells to the tumor mass, inducing both directly and indirectly the process of angiogenesis. The two latter activities are potentially targetable, and specific interventions may be designed to counteract the protumoral role of extracellular galectin-3.

Intra- and intermolecular interactions of human galectin-3: assessment by full-assignment-based NMR

Galectin-3 is an adhesion/growth-regulatory protein with a modular design comprising an N-terminal tail (NT, residues 1-111) and the conserved carbohydrate recognition domain (CRD, residues 112-250). The chimera-type galectin interacts with both glycan and peptide motifs. Complete (13)C/(15)N-assignment of the human protein makes NMR-based analysis of its structure beyond the CRD possible. Using two synthetic NT polypeptides covering residues 1-50 and 51-107, evidence for transient secondary structure was found with helical conformation from residues 5 to 15 as well as proline-mediated, multi-turn structure from residues 18 to 32 and around PGAYP repeats. Intramolecular interactions occur between the CRD F-face (the 5-stranded β-sheet behind the canonical carbohydrate-binding 6-stranded β-sheet of the S-face) and NT in full-length galectin-3, with the sequence P(23)GAW(26)…P(37)GASYPGAY(45) defining the primary binding epitope within the NT. Work with designed peptides indicates that the PGAX motif is crucial for self-interactions between NT/CRD. Phosphorylation at position Ser6 (and Ser12) (a physiological modification) and the influence of ligand binding have minimal effect on this interaction. Finally, galectin-3 molecules can interact weakly with each other via the F-faces of their CRDs, an interaction that appears to be assisted by their NTs. Overall, our results add insight to defining binding sites on galectin-3 beyond the canonical contact area for β-galactosides.

Human chimera-type galectin-3: defining the critical tail length for high-affinity glycoprotein/cell surface binding and functional competition with galectin-1 in neuroblastoma cell growth regulation

Many human proteins have a modular design with receptor and structural domains. Using adhesion/growth-regulatory galectin-3 as model, we describe an interdisciplinary strategy to define the functional significance of its tail established by nine non-triple helical collagen-like repeats (I-IX) and the N-terminal peptide. Genetic engineering with sophisticated mass spectrometric product analysis provided the tools for biotesting, i.e. eight protein variants with different degrees of tail truncation. Evidently,various aspects of galectin-3 activity (cis binding and cell bridging) are affected by tail shortening in a different manner. Thus, this combined approach reveals an unsuspected complexity of structure-function relationship, encouraging further application beyond this chimera-type galectin.

pH-dependent recycling of galectin-3 at the apical membrane of epithelial cells

The β-galactoside binding protein galectin-3 is highly expressed in a variety of epithelial cell lines. Polarized MDCK cells secrete this lectin predominantly into the apical medium by non-classical secretion. Once within the apical extracellular milieu, galectin-3 can re-enter the cell followed by passage through endosomal organelles and modulate apical protein sorting. Here, we could show that galectin-3 is internalized by non-clathrin mediated endocytosis. Within endosomal organelles this pool associates with newly synthesized neurotrophin receptor in the biosynthetic pathway and assists in its membrane targeting. This recycling process is accompanied by transient interaction of galectin-3 with detergent insoluble membrane microdomains in a lactose- and pH-dependent manner. Moreover, in the presence of lactose, apical sorting of the neurotrophin receptor is affected following endosomal deacidification. Taken together, our results suggest that internalized galectin-3 directs the subcellular targeting of apical glycoproteins by membrane recycling.

Galectin-3 mediates oligomerization of secreted hensin using its carbohydrate-recognition domain

A multidomain, multifunctional 230-kDa extracellular matrix (ECM) protein, hensin, regulates the adaptation of rabbit kidney to metabolic acidosis by remodeling collecting duct intercalated cells. Conditional deletion of hensin in intercalated cells of the mouse kidney leads to distal renal tubular acidosis and to a significant reduction in the number of cells expressing the basolateral chloride-bicarbonate exchanger kAE1, a characteristic marker of α-intercalated cells. Although hensin is secreted as a monomer, its polymerization and ECM assembly are essential for its role in the adaptation of the kidney to metabolic acidosis. Galectin-3, a unique lectin with specific affinity for β-galactoside glycoconjugates, directly interacts with hensin. Acidotic rabbits had a significant increase in the number of cells expressing galectin-3 in the collecting duct and exhibited colocalization of galectin-3 with hensin in the ECM of microdissected tubules. In this study, we confirmed the increased expression of galectin-3 in acidotic rabbit kidneys by real-time RT-PCR. Galectin-3 interacted with hensin in vitro via its carbohydrate-binding COOH-terminal domain, and the interaction was competitively inhibited by lactose, removal of the COOH-terminal domain of galectin-3, and deglycosylation of hensin. Galectin-9, a lectin with two carbohydrate-recognition domains, is also present in the rabbit kidney; galectin-9 partially oligomerized hensin in vitro. Our results demonstrate that galectin-3 plays a critical role in hensin ECM assembly by oligomerizing secreted monomeric hensin. Both the NH₂-terminal and COOH-terminal domains are required for this function. We suggest that in the case of galectin-3-null mice galectin-9 may partially substitute for the function of galectin-3.

Galectin-3C inhibits tumor growth and increases the anticancer activity of bortezomib in a murine model of human multiple myeloma

Galectin-3 is a human lectin involved in many cellular processes including differentiation, apoptosis, angiogenesis, neoplastic transformation, and metastasis. We evaluated galectin-3C, an N-terminally truncated form of galectin-3 that is thought to act as a dominant negative inhibitor, as a potential treatment for multiple myeloma (MM). Galectin-3 was expressed at varying levels by all 9 human MM cell lines tested. In vitro galectin-3C exhibited modest anti-proliferative effects on MM cells and inhibited chemotaxis and invasion of U266 MM cells induced by stromal cell-derived factor (SDF)-1α. Galectin-3C facilitated the anticancer activity of bortezomib, a proteasome inhibitor approved by the FDA for MM treatment. Galectin-3C and bortezomib also synergistically inhibited MM-induced angiogenesis activity in vitro. Delivery of galectin-3C intravenously via an osmotic pump in a subcutaneous U266 cell NOD/SCID mouse model of MM significantly inhibited tumor growth. The average tumor volume of bortezomib-treated animals was 19.6% and of galectin-3C treated animals was 13.5% of the average volume of the untreated controls at day 35. The maximal effect was obtained with the combination of galectin-3C with bortezomib that afforded a reduction of 94% in the mean tumor volume compared to the untreated controls at day 35. In conclusion, this is the first study to show that inhibition of galectin-3 is efficacious in a murine model of human MM. Our results demonstrated that galectin-3C alone was efficacious in a xenograft mouse model of human MM, and that it enhanced the anti-tumor activity of bortezomib in vitro and in vivo. These data provide the rationale for continued testing of galectin-3C towards initiation of clinical trials for treatment of MM.

The role of galectins in protein trafficking

The galectins, a family of lectins, modulate distinct cellular processes, such as cancer progression, immune response and cellular development, through their specific binding to extracellular or intracellular ligands. In the past few years, research has unravelled interactions of different galectins with lipids and glycoproteins in the outer milieu or in the secretory pathway of cells. Interestingly, these lectins do not possess a signalling sequence to enter the endoplasmic reticulum as a starting point for the classical secretory pathway. Instead they use a so-called non-classical mechanism for translocation across the plasma membrane and/or into the lumen of transport vesicles. Here, they stabilize transport platforms for apical trafficking or sort apical glycoproteins into specific vesicle populations. Modes of ligand interaction as well as the modulation of binding activities and trafficking pathways are discussed in this review.

Galectins: novel anti-inflammatory drug targets

Galectins are a protein family defined by their affinity for beta-galactosides and consensus sequences. They are pleiotropic regulators involved in a multitude of functions, both in and out of the cell. Extracellularly, they have the potential to bind to various surface receptors on a variety of cell types as well as extracellular matrix (ECM) proteins, thus causing cell activation or apoptosis, modulating cell adhesion and inducing cell migration. Intracellularly, they can regulate cell growth, apoptosis and cell cycle progression. Galectins are either pro-inflammatory or anti-inflammatory. Some, such as galectin-1, may be employed as anti-inflammatory agents, while others, such as galectin-3, are evidently suitable targets for anti-inflammatory drugs. The extracellular functions of galectins involve their lectin-carbohydrate interactions and thus their carbohydrate ligands or mimetics would be suitable inhibitors. While the intracellular functions of galectins do not appear to engage lectin-carbohydrate interactions, the carbohydrate-binding sites of these proteins may still be involved. Therefore, the same inhibitors may be used regardless of whether intracellular or extracellular galectins are to be targeted.

Galectin-3: an open-ended story

Galectins, an ancient lectin family, are characterized by specific binding of beta-galactosides through evolutionary conserved sequence elements of carbohydrate-recognition domain (CRD). A structurally unique member of the family is galectin-3; in addition to the CRD it contains a proline- and glycine-rich N-terminal domain (ND) through which is able to form oligomers. Galectin-3 is widely spread among different types of cells and tissues, found intracellularly in nucleus and cytoplasm or secreted via non-classical pathway outside of cell, thus being found on the cell surface or in the extracellular space. Through specific interactions with a variety of intra- and extracellular proteins galectin-3 affects numerous biological processes and seems to be involved in different physiological and pathophysiological conditions, such as development, immune reactions, and neoplastic transformation and metastasis. The review attempts to summarize the existing information on structural, biochemical and intriguing functional properties of galectin-3.

The binding of mycolic acids to galectin-3: a novel interaction between a host soluble lectin and trafficking mycobacterial lipids?

Understanding the molecular mechanism of host-pathogen interactions is the basis for drug design and vaccine development. The fine composition of mycolic acids (MA), the major constituents of Mycobacterium tuberculosis (Mtb) cell envelope, as well as other cell wall-associated lipids, contribute to determine the virulence of a given strain. However, endogenous receptors for mycolic acids on susceptible cells exposed to mycobacterial infections have not been fully identified. Here, we show that galectin-3, a multifunctional beta-galactoside binding lectin present mainly in the cytoplasm of inflammatory cells and also present on the cell surface, can recognize mycobacterial mycolic acids. MA can inhibit the lectin self-association but not its carbohydrate-binding abilities and can selectively interfere in the interaction of the lectin with its receptors on temperature-sensitive dendritic cell line, suggesting that galectin-3 could be involved in the recognition of trafficking mycolic acids and participate in their interaction with host cells.

Alterations in galectin-3 expression and distribution correlate with breast cancer progression: functional analysis of galectin-3 in breast epithelial-endothelial interactions

To define the role of galectin-3 in breast cancer progression, we have used a novel three-dimensional co-culture system that recapitulates in vivo reciprocal functional breast epithelial-endothelial cell-cell and cell-matrix interactions, and examined the expression of galectin-3 mRNA and protein in human breast tumors and xenografts. Galectin-3 is required for the stabilization of epithelial-endothelial interaction networks because immunoneutralization with galectin-3 antibodies abolishes the interactions in a dose-dependent manner. Co-culture of epithelial cells with endothelial cells results in increase in levels of secreted galectin-3 and presence of proteolytically processed form of galectin-3 in the conditioned media. In contrast, intracellular galectin-3 predominantly exists in the intact form. This difference in sensitivity to proteolytic processing of secreted versus intracellular galectin-3 probably arises from differences in accessibility of protease-sensitive sites, levels, and/or type of activated protease(s), and may be indicative of different functional roles for intact and processed galectin-3. To determine whether the proteolytically cleaved galectin-3 retains its ability to bind to endothelial cells, binding assays were performed with the full-length and matrix metallopeoteinase-2-cleaved recombinant galectin-3. Although a dose-dependent increase in binding to human umbilical vein endothelial cells was observed with both full-length and cleaved galectin-3, proteolytically cleaved galectin-3 displayed approximately 20-fold higher affinity for human umbilical vein endothelial cells as compared to the full-length protein. Examination of galectin-3 expression in breast tumors and xenografts revealed elevated levels of galectin-3 mRNA and protein in the luminal epithelial cells of normal and benign ducts, down-regulation in early grades of ductal carcinoma in situ (DCIS), and re-expression in peripheral tumor cells as DCIS lesions progressed to comedo-DCIS and invasive carcinomas. These data suggest that galectin-3 expression is associated with specific morphological precursor subtypes of breast cancer and undergoes a transitional shift in expression from luminal to peripheral cells as tumors progressed to comedo-DCIS or invasive carcinomas. Such a localized expression of galectin-3 in cancer cells proximal to the stroma could lead to increased invasive potential by inducing novel or better interactions with the stromal counterparts.

LacdiNAc-Glycans Constitute a Parasite Pattern for Galectin-3-Mediated Immune Recognition

Although Gal beta 1-4GlcNAc (LacNAc) moieties are the most common constituents of N-linked glycans on vertebrate proteins, GalNAc beta 1-4GlcNAc (LacdiNAc, LDN)-containing glycans are widespread in invertebrates, such as helminths. We postulated that LDN might be a molecular pattern for recognition of helminth parasites by the immune system. Using LDN-based affinity chromatography and mass spectrometry, we have identified galectin-3 as the major LDN-binding protein in macrophages. By contrast, LDN binding was not observed with galectin-1. Surface plasmon resonance (SPR) analysis and a solid phase binding assay demonstrated that galectin-3 binds directly to neoglycoconjugates carrying LDN glycans. In addition, galectin-3 bound to Schistosoma mansoni soluble egg Ags and a mAb against the LDN glycan inhibited this binding, suggesting that LDN glycans within S. mansoni soluble egg Ags contribute to galectin-3 binding. Immunocytochemistry demonstrated high levels of galectin-3 in liver granulomas of S. mansoni-infected hamsters, and a colocalization of galectin-3 and LDN glycans was observed on the parasite eggshells. Finally, we demonstrate that galectin-3 can mediate recognition and phagocytosis of LDN-coated particles by macrophages. These findings provide evidence that LDN-glycans constitute a parasite pattern for galectin-3-mediated immune recognition.

Galectin-3 precipitates as a pentamer with synthetic multivalent carbohydrates and forms heterogeneous cross-linked complexes

Galectin-3 is unique among the galectin family of animal lectins in its biological activities and structure. Most members of the galectin family including galectin-1 possess apoptotic activities, whereas galectin-3 possesses anti-apoptotic activity. Galectin-3 is also the only chimera type galectin and consists of a nonlectin N-terminal domain and a C-terminal carbohydrate-binding domain. Recent sedimentation equilibrium and velocity studies show that murine galectin-3 is a monomer in the absence and presence of LacNAc, a monovalent sugar. However, quantitative precipitation studies in the present report indicate that galectin-3 precipitates as a pentamer with a series of divalent pentasaccharides with terminal LacNAc residues. Furthermore, the kinetics of precipitation are fast, on the order of seconds. This indicates that although the majority of galectin-3 in solution is a monomer, a rapid equilibrium exists between the monomer and a small percentage of pentamer. The latter, in turn, precipitates with the divalent oligosaccharides, resulting in rapid conversion of monomer to pentamer by mass action equilibria. Mixed quantitative precipitation experiments and electron microscopy suggest that galectin-3 forms heterogenous, disorganized cross-linking complexes with the multivalent carbohydrates. This contrasts with galectin-1 and many plant lectins that form homogeneous, organized cross-linked complexes. The results are discussed in terms of the biological properties of galectin-3.

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.

NMR solution studies of hamster galectin-3 and electron microscopic visualization of surface-adsorbed complexes: evidence for interactions between the N- and C-terminal domains

Galectin-3, a beta-galactoside binding protein, contains a C-terminal carbohydrate recognition domain (CRD) and an N-terminal domain that includes several repeats of a proline-tyrosine-glycine-rich motif. Earlier work based on a crystal structure of human galectin-3 CRD, and modeling and mutagenesis studies of the closely homologous hamster galectin-3, suggested that N-terminal tail residues immediately preceding the CRD might interfere with the canonical subunit interaction site of dimeric galectin-1 and -2, explaining the monomeric status of galectin-3 in solution. Here we describe high-resolution NMR studies of hamster galectin-3 (residues 1--245) and several of its fragments. The results indicate that the recombinant N-terminal fragment Delta 126--245 (residues 1--125) is an unfolded, extended structure. However, in the intact galectin-3 and fragment Delta 1--93 (residues 94--245), N-terminal domain residues lying between positions 94 and 113 have significantly reduced mobility values compared with those expected for bulk N-terminal tail residues, consistent with an interaction of this segment with the CRD domain. In contrast to the monomeric status of galectin-3 (and fragment Delta 1--93) in solution, electron microscopy of negatively stained and rotary shadowed samples of hamster galectin-3 as well as the CRD fragment Delta 1--103 (residues 104--245) show the presence of a significant proportion (up to 30%) of oligomers. Similar imaging of the N-terminal tail fragment Delta 126--245 reveals the presence of fibrils formed by intermolecular interactions between extended polypeptide subunits. Oligomerization of substratum-adsorbed galectin-3, through N- and C-terminal domain interactions, could be relevant to the positive cooperativity observed in binding of the lectin to immobilized multiglycosylated proteins such as laminin.

Galectins structure and function--a synopsis

The 20 or so galectins expected to be found in man, and their many possible functional effects promise a rich and fruitful research field in the future. At present, the biomedically most promising areas for use of galectins or their ligands are in inflammation, immunity, and cancer. Many good stories can be formulated, but the field lacks the cohesion of knowing basic galectin function. The only basic common denominators among galectins are beta-galactoside binding, and the unusual combination of intra- and extracellular expression with non-classical secretion in between. Maybe that is all there is, and nature has used these properties for multiple, otherwise unrelated functions. Then again, maybe there is some deeper common function that has so far been overlooked. If it exists, this probably lies somewhere in the detailed integration of galectin activity in the complexities of cell physiology.

Galectin-3 modulates ureteric bud branching in organ culture of the developing mouse kidney

Galectin-3 is a mammalian beta-galactoside-specific lectin with functions in cell growth, adhesion, and neoplastic transformation. On the basis of expression patterns in humans, it is proposed that galectin-3 modulates fetal collecting duct growth. This article provides evidence that galectin-3 can modulate branching morphogenesis of the mouse ureteric bud/collecting duct lineage. With the use of immunohistochemistry, galectin-3 was not detected in early metanephrogenesis but was upregulated later in fetal kidney maturation when the protein was prominent in basal domains of medullary collecting ducts. Addition of galectin-3 to embryonic days 11 and 12 whole metanephric cultures inhibited ureteric bud branching, whereas galectin-1 did not perturb morphogenesis, nor did a galectin-3 mutant lacking wild-type high-affinity binding to extended oligosaccharides. Exogenous galectin-3 retarded conversion of renal mesenchyme to nephrons in whole metanephric explants but did not affect nephron induction by spinal cord in isolated renal mesenchymes. Finally, addition of a blocking antiserum to galectin-3 caused dilation and distortion of developing epithelia in embryonic day 12 metanephroi cultured for 1 wk. The upregulation of galectin-3 protein during kidney maturation, predominantly at sites where it could mediate cell/matrix interactions, seems to modulate growth of the ureteric tree.

Phosphorylation of the beta-galactoside-binding protein galectin-3 modulates binding to its ligands

The beta-galactoside-binding protein galectin-3 has pleiotropic biological functions and has been implicated in cell growth, differentiation, adhesion, RNA processing, apoptosis, and malignant transformation. Galectin-3 may be phosphorylated at N-terminal Ser(6), but the role of phosphorylation in determining interactions of this endogenous lectin with its ligands remains to be elucidated. We therefore studied the effect of phosphorylation on binding of galectin-3 to two of its reported ligands, laminin and purified colon cancer mucin. Human recombinant galectin-3 was phosphorylated in vitro by casein kinase I, and separated from the native species by isoelectric focusing for use in solid phase binding assays. Non-phosphorylated galectin-3 bound to laminin and asialomucin in a dose-dependent manner with half-maximal binding at 1.5 microg/ml. Phosphorylation reduced saturation binding to each ligand by >85%. Ligand binding could be fully restored by dephosphorylation with protein phosphatase type 1. Mutation of galectin-3 at Ser(6) (Ser to Glu) did not alter galectin ligand binding. Metabolic labeling or separation by isoelectric focusing confirmed the presence of phosphorylated galectin-3 species in vivo in the cytosol of human colon cancer cells from which ligand mucin was purified. Phosphorylation significantly reduces the interaction of galectin-3 with its ligands. The process by which phosphorylation modulates protein-carbohydrate interactions has important implications for understanding the biological functions of this protein, and may serve as an "on/off" switch for its sugar binding capabilities.

Neutrophil gelatinase B potentiates interleukin-8 tenfold by aminoterminal processing, whereas it degrades CTAP-III, PF-4, and GRO-α and leaves RANTES and MCP-2 intact

Chemokines are mediators in inflammatory and autoimmune disorders. Aminoterminal truncation of chemokines results in altered specific activities and receptor recognition patterns. Truncated forms of the CXC chemokine interleukin (IL)-8 are more active than full-length IL-8 (1-77), provided the Glu-Leu-Arg (ELR) motif remains intact. Here, a positive feedback loop is demonstrated between gelatinase B, a major secreted matrix metalloproteinase (MMP-9) from neutrophils, and IL-8, the prototype chemokine active on neutrophils. Natural human neutrophil progelatinase B was purified to homogeneity and activated by stromelysin-1. Gelatinase B truncated IL-8(1-77) into IL-8(7-77), resulting in a 10- to 27-fold higher potency in neutrophil activation, as measured by the increase in intracellular Ca++concentration, secretion of gelatinase B, and neutrophil chemotaxis. This potentiation correlated with enhanced binding to neutrophils and increased signaling through CXC chemokine receptor-1 (CXCR1), but it was significantly less pronounced on a CXCR2-expressing cell line. Three other CXC chemokines—connective tissue-activating peptide-III (CTAP-III), platelet factor-4 (PF-4), and GRO-α—were degraded by gelatinase B. In contrast, the CC chemokines RANTES and monocyte chemotactic protein-2 (MCP-2) were not digested by this enzyme. The observation of differing effects of neutrophil gelatinase B on the proteolysis of IL-8 versus other CXC chemokines and on CXC receptor usage by processed IL-8 yielded insights into the relative activities of chemokines. This led to a better understanding of regulator (IL-8) and effector molecules (gelatinase B) of neutrophils and of mechanisms underlying leukocytosis, shock syndromes, and stem cell mobilization by IL-8.

Neutrophil gelatinase B potentiates interleukin-8 tenfold by aminoterminal processing, whereas it degrades CTAP-III, PF-4, and GRO-α and leaves RANTES and MCP-2 intact

Chemokines are mediators in inflammatory and autoimmune disorders. Aminoterminal truncation of chemokines results in altered specific activities and receptor recognition patterns. Truncated forms of the CXC chemokine interleukin (IL)-8 are more active than full-length IL-8 (1-77), provided the Glu-Leu-Arg (ELR) motif remains intact. Here, a positive feedback loop is demonstrated between gelatinase B, a major secreted matrix metalloproteinase (MMP-9) from neutrophils, and IL-8, the prototype chemokine active on neutrophils. Natural human neutrophil progelatinase B was purified to homogeneity and activated by stromelysin-1. Gelatinase B truncated IL-8(1-77) into IL-8(7-77), resulting in a 10- to 27-fold higher potency in neutrophil activation, as measured by the increase in intracellular Ca++concentration, secretion of gelatinase B, and neutrophil chemotaxis. This potentiation correlated with enhanced binding to neutrophils and increased signaling through CXC chemokine receptor-1 (CXCR1), but it was significantly less pronounced on a CXCR2-expressing cell line. Three other CXC chemokines—connective tissue-activating peptide-III (CTAP-III), platelet factor-4 (PF-4), and GRO-α—were degraded by gelatinase B. In contrast, the CC chemokines RANTES and monocyte chemotactic protein-2 (MCP-2) were not digested by this enzyme. The observation of differing effects of neutrophil gelatinase B on the proteolysis of IL-8 versus other CXC chemokines and on CXC receptor usage by processed IL-8 yielded insights into the relative activities of chemokines. This led to a better understanding of regulator (IL-8) and effector molecules (gelatinase B) of neutrophils and of mechanisms underlying leukocytosis, shock syndromes, and stem cell mobilization by IL-8.

The role of weak protein-protein interactions in multivalent lectin-carbohydrate binding: crystal structure of cross-linked FRIL

Binding of multivalent glycoconjugates by lectins often leads to the formation of cross-linked complexes. Type I cross-links, which are one-dimensional, are formed by a divalent lectin and a divalent glycoconjugate. Type II cross-links, which are two or three-dimensional, occur when a lectin or glycoconjugate has a valence greater than two. Type II complexes are a source of additional specificity, since homogeneous type II complexes are formed in the presence of mixtures of lectins and glycoconjugates. This additional specificity is thought to become important when a lectin interacts with clusters of glycoconjugates, e.g. as is present on the cell surface. The cryst1al structure of the Glc/Man binding legume lectin FRIL in complex with a trisaccharide provides a molecular snapshot of how weak protein-protein interactions, which are not observed in solution, can become important when a cross-linked complex is formed. In solution, FRIL is a divalent dimer, but in the crystal FRIL forms a tetramer, which allows for the formation of an intricate type II cross-linked complex with the divalent trisaccharide. The dependence on weak protein-protein interactions can ensure that a specific type II cross-linked complex and its associated specificity can occur only under stringent conditions, which explains why lectins are often found forming higher-order oligomers.

Interaction of a novel cysteine and histidine-rich cytoplasmic protein with galectin-3 in a carbohydrate-independent manner

We have used the yeast two-hybrid system to search for cytoplasmic proteins that might assist in the intracellular trafficking of the soluble beta-galactoside-binding protein, galectin-3. We utilised as bait murine full-length galectin-3 to screen a murine 3T3 cDNA library. Several interacting clones were found to encode a partial open reading frame and a full-length clone was obtained by rapid amplification of cDNA ends methodology. In various assays in vitro the novel protein was shown to bind galectin-3 in a carbohydrate-independent manner. The novel protein contains an unusually high content of cysteine and histidine residues and shows significant sequence homologies with several metal ion-binding motifs present in known proteins. Confocal immunofluorescence microscopy of permeabilised 3T3 cells shows a prominent perinuclear, as well as cytoplasmic, localisation of the novel protein.

Secretion of the galectin family of mammalian carbohydrate-binding proteins

Galectins are cytosolic proteins that lack any signal sequence for transport into the endoplasmic reticulum and are not glycosylated, although several galectins contain consensus sites for N-glycosylation, indicating that these proteins do not traverse the ER-Golgi network. However, there is abundant evidence for the extracellular localisation of some galectins at cell surfaces, in the extracellular matrix and in cell secretions consistent with other evidence for extracellular roles of galectins as modulators of cell adhesion and signalling. How then are galectins secreted if not through the classical secretory pathway? Do all galectins share the same secretory pathway? Can a particular galectin utilise more than one secretory pathway? If galectins play important extracellular roles how is their secretion regulated in relation to function? These are still largely unanswered questions but recent studies are beginning to give glimpses into some novel aspects of the secretion of these intriguing proteins.

Lactose-containing starburst dendrimers: influence of dendrimer generation and binding-site orientation of receptors (plant/animal lectins and immunoglobulins) on binding properties

Starburst glycodendrimers offer the potential to serve as high-affinity ligands for clinically relevant sugar receptors. In order to define areas of application, their binding behavior towards sugar receptors with differential binding-site orientation but identical monosaccharide specificity must be evaluated. Using poly(amidoamine) starburst dendrimers of five generations, which contain the p-isothiocyanato derivative of p-aminophenyl-beta-D-lactoside as ligand group, four different types of galactoside-binding proteins were chosen for this purpose, i.e., the (AB)(2)-toxic agglutinin from mistletoe, a human immunoglobulin G fraction, the homodimeric galectin-1 with its two binding sites at opposite ends of the jelly-roll-motif-harboring protein and monomeric galectin-3. Direct solid-phase assays with surface-immobilized glycodendrimers resulted in obvious affinity enhancements by progressive core branching for the plant agglutinin and less pronounced for the antibody and galectin-1. High density of binding of galectin-3 with modest affinity increases only from the level of the 32-mer onwards points to favorable protein-protein interactions of the monomeric lectin and a spherical display of the end groups without a major share of backfolding. When the inhibitory potency of these probes was evaluated as competitor of receptor binding to an immobilized neoglycoprotein or to asialofetuin, a marked selectivity was detected. The 32- and 64-mers were second to none as inhibitors for the plant agglutinin against both ligand-exposing matrices and for galectin-1 on the matrix with a heterogeneous array of interglycoside distances even on the per-sugar basis. In contrast, a neoglycoprotein with the same end group was superior in the case of the antibody and, less pronounced, monomeric galectin-3. Intimate details of topological binding-site presentation and the ligand display on different generations of core assembly are major operative factors which determine the potential of dendrimers for applications as lectin-targeting device, as attested by these observations.

Determinants in the N-terminal domains of galectin-3 for secretion by a novel pathway circumventing the endoplasmic reticulum-Golgi complex

Galectin-3 is a beta-galactoside-binding protein that is secreted from many cells although the protein lacks a signal sequence for transfer into the endoplasmic reticulum and Golgi compartments and entry into classical secretory pathways. Previously it was shown that attachment of the first 120 amino acid residues of the N-terminal sequence of hamster galectin-3 to the cytoplasmic protein chloramphenicol acetyltransferase (CAT) supported the rapid secretion of the fusion protein from transiently transfected Cos cells under conditions in which CAT protein was not secreted. Here we report that progressive N-terminal truncation gradually reduced secretion of the fusion proteins, eventually to very low levels compared with the starting product, but did not totally eliminate secretion until a significant majority of the sequence was removed. Mutant CAT fusion proteins containing internal deletions in residues 97-120 of the galectin-3 N-terminal sequence were also secreted to a similar extent to the starting product, but further deletion of residues 89-96 abolished detectable secretion. Proline to alanine mutagenesis of the sequence YP(90)SAP(93)GAY in two secretion-competent CAT fusion proteins greatly reduced or abolished their secretion, whereas similar mutagenesis of proline pairings present elsewhere in the galectin-3 N-terminal segments of these proteins had no effect. The results indicate that this sequence is one essential determinant for secretion of galectin-3-CAT fusion proteins and by inference galectin-3, at least from transfected Cos cells. However, the short sequence of residues 89-96 by itself is insufficient to direct secretion of CAT fusion proteins and appears to be active only in the context of a larger portion of the galectin-3 N-terminal sequence.

Galectin-3 modulates rat mesangial cell proliferation and matrix synthesis during experimental glomerulonephritis induced by anti-Thy1.1 antibodies

Galectin-3 is a beta-galactoside-binding protein synthesized by macrophages and other inflammatory cells and expressed in various branching epithelia, including the developing kidney. The expression of galectin-3 has been studied in a rat model of acute mesangial proliferative glomerulonephritis in which a single injection of anti-Thy1.1 antibodies leads to destruction of mesangial cells expressing a Thy1.1 epitope on their surface. The glomerular lesion is characterized by expansion of the mesangial matrix, especially laminin and collagen type IV, and mesangial hypercellularity. Galectin-3 expression, which is sparse in mature rat kidney and confined to the apical face of some distal tubules, is increased within 1-3 days following antibody administration, with the recruitment of glomerular macrophages and pronounced neo-expression in the cytoplasm and at the basal face of distal tubules. At later times, galectin-3 is detected immunohistochemically in the repopulating mesangial cell mass, preceding the extensive mesangial deposition of laminin and collagen type IV. Mesangial cells in culture do not produce appreciable amounts of galectin-3 but do bind and endocytose exogenously added lectin. Addition of galectin-3 to primary cultures of mesangial cells prepared from normal rats induces a 1.5-fold increase in the synthesis of collagen type IV and it also acts in synergy with a quantitatively similar stimulatory effect of transforming growth factor beta (TGF-beta) on matrix synthesis. Exogenous galectin-3 prolongs the survival of mesangial cells in serum-free cultures and also protects these cells against cytotoxic effects of TGF-beta. The data support the notion that the increased expression and secretion of galectin-3 in infiltrating macrophages and in distal tubular epithelia, together with up-regulation of IL-1beta and TGF-beta genes, play a role in mesangial hypercellularity in the progression of one model of inflammatory renal disease.

Matrix metalloproteinase 2 (MMP2) and MMP9 are produced by kidney collecting duct principal cells but are differentially regulated by SV40 large-T, arginine vasopressin, and epidermal growth factor

We analyzed the expression and regulation of matrix metalloproteinase 2 (MMP2) and MMP9 gelatinases in a rabbit kidney collecting duct principal cell line (RC.SVtsA58) (Prié, D., Ronco, P. M., Baudouin, B., Géniteau-Legendre, M., Antoine, M., Piedagnel, R., Estrade, S., Lelongt, B., Verroust, P. J., Cassingéna, R., and Vandewalle, A. (1991) J. Cell Biol. 113, 951-962) infected with the temperature-sensitive (ts) SV40 strain tsA58. At the permissive temperature (33 degreesC), cells produced only MMP2. Shifting cells to a nonpermissive temperature (39.5 degreesC) induced a marked increase in total gelatinolytic activity due to an increase of MMP2 and an induction of MMP9 synthesis. This effect was attributed to large-T inactivation at 39.5 degreesC because it was abolished by re-infecting the cells with wild-type SV40 strain LP. Run-on experiments showed that negative regulation of MMP2 and MMP9 by large-T was transcriptional and posttranscriptional, respectively. MMP2 and MMP9 were also produced by primary cultures of collecting duct cells. In rabbit kidney, both MMP2 and MMP9 were almost exclusively expressed in collecting duct cells, where an unexpected apical localization was observed. Arginine vasopressin and epidermal growth factor, which exert opposite hydroosmotic effects in the collecting duct, also exhibited contrasted effects on MMP9 synthesis. Epidermal growth factor increased but arginine vasopressin suppressed MMP9 at a posttranscriptional level, whereas MMP2 was not affected. These results suggest a specific physiological role of MMP2 and MMP9 in principal cells of renal collecting duct.

Lymphocyte triggering via L-selectin leads to enhanced galectin-3-mediated binding to dendritic cells

For proper immune surveillance, naive lymphocytes are recruited from the blood into secondary lymphoid organs. L-selectin expressed on lymphocytes plays an important role in the initial attachment of these cells to high endothelial venules (HEV) in lymph nodes. Previously, we found that triggering via L-selectin resulted in activation of lymphocytes, followed by an alteration in their adhesion capacity. This suggested that L-selectin triggering might play a role in cell-cell interactions after lymph node entry. Here, we identify a novel adhesion mechanism involving L-selectin-triggered lymphocytes and dendritic cells, and we show that enhanced binding to dendritic cells is mediated by galectin-3 and not by integrins. Furthermore, it was shown that L-selectin-triggered T lymphocytes exhibited enhanced proliferation in an allogeneic mixed lymphocyte reaction. It is concluded that, in addition to a role for L-selectin in tethering and rolling on endothelium, triggering of the molecule on the lymphocyte surface leads to changes that are pertinent for the function of the cell after passing the HEV. We argue that the described adhesion mechanism plays a role in optimizing the initial interaction between dendritic cells and lymphocytes.

X-ray crystal structure of the human galectin-3 carbohydrate recognition domain at 2.1-A resolution

Galectins are a family of lectins which share similar carbohydrate recognition domains (CRDs) and affinity for small beta-galactosides, but which show significant differences in binding specificity for more complex glycoconjugates. We report here the x-ray crystal structure of the human galectin-3 CRD, in complex with lactose and N-acetyllactosamine, at 2.1-A resolution. This structure represents the first example of a CRD determined from a galectin which does not show the canonical 2-fold symmetric dimer organization. Comparison with the published structures of galectins-1 and -2 provides an explanation for the differences in carbohydrate-binding specificity shown by galectin-3, and for the fact that it fails to form dimers by analogous CRD-CRD interactions.

Role of the carboxyl-terminal lectin domain in self-association of galectin-3

Galectin-3 is a member of a large family of beta-galactoside-binding animal lectins and is composed of a carboxyl-terminal lectin domain connected to an amino-terminal nonlectin part. Previous experimental results suggest that, when bound to multivalent glycoconjugates, galectin-3 self-associates through intermolecular interactions involving the amino-terminal domain. In this study, we obtained evidence suggesting that the protein self-associates in the absence of its saccharide ligands, in a manner that is dependent on the carboxyl-terminal domain. This mode of self-association is inhibitable by the lectin's saccharide ligands. Specifically, recombinant human galectin-3 was found to bind to galectin-3C (the carboxyl-terminal domain fragment) conjugated to Sepharose 4B and the binding was inhibitable by lactose. In addition, biotinylated galectin-3 bound to galectin-3 immobilized on plastic surfaces and the binding could also be inhibited by various saccharide ligands of the lectin. A mutant with a tryptophan to leucine replacement in the carboxyl-terminal domain, which exhibited diminished carbohydrate-binding activity, did not bind to galectin-3C-Sepharose 4B. Furthermore, galectin-3C formed covalent homodimers when it was treated with a chemical cross-linker and the dimer formation was completely inhibited by lactose. Therefore, galectin-3 can self-associate through intermolecular interactions involving both the amino- and the carboxyl-terminal domains and the relative contribution of each depends on whether the lectin is bound to its saccharide ligands.

Evidence for subsites in the galectins involved in sugar binding at the nonreducing end of the central galactose of oligosaccharide ligands: sequence analysis, homology modeling and mutagenesis studies of hamster galectin-3

A model of the carbohydrate recognition domain CRD, residues 111-245, of hamster galectin-3 has been made using homology modeling and dynamics minimization methods. The model is based on the known x-ray structures of bovine galectin-1 and human galectin-2. The oligosaccharides NeuNAc-alpha2,3-Gal-beta1,4-Glc and GalNAc-alpha1, 3-[Fuc-alpha1,2]-Gal-beta1,4-Glc, known to be specific high-affinity ligands for galectin-3, as well as lactose recognized by all galectins were docked in the galectin-3 CRD model structure and a minimized binding conformation found in each case. These studies indicate a putative extended carbohydrate-binding subsite in the hamster galectin-3 involving Arg139, Glu230, and Ser232 for NeuNAc-alpha2,3-; Arg139 and Glu160 for fucose-alpha1,2-; and Arg139 and Ile141 for GalNAc-alpha1,3- substituents on the primary galactose. Each of these positions is variable within the whole galectin family. Two of these residues, Arg139 and Ser232, were selected for mutagenesis to probe their importance in this newly identified putative subsite. Residue 139 adopts main-chain dihedral angles characteristic of an isolated bridge structural feature, while residue 232 is the C-terminal residue of beta-strand-11, and is followed immediately by an inverse gamma-turn. A systematic series of mutant proteins have been prepared to represent the residue variation present in the aligned sequences of galectins-1, -2, and -3. Minimized docked models were generated for each mutant in complex with NeuNAc-alpha2,3-Gal-beta1,4-Glc, GalNAc-alpha1, 3-[Fuc-alpha1,2]-Gal-beta1,4- Glc, and Gal-beta1,4-Glc. Correlation of the computed protein-carbohydrate interaction energies for each lectin-oligosaccharide pair with the experimentally determined binding affinities for fetuin and asialofetuin or the relative potencies of lactose and sialyllactose in inhibiting binding to asiolofetuin is consistent with the postulated key importance of Arg139 in recognition of the extended sialylated ligand.

Detection and distribution of the carbohydrate binding protein galectin-3 in human notochord, intervertebral disc and chordoma

Galectin-3 is an endogenous carbohydrate-binding protein which plays a role in cell differentiation, morphogenesis and cancer biology. We investigated the occurrence and distribution of galectin-3 in the embryonic and fetal human notochord, the developing human vertebral column, adult intervertebral discs and in six chordomas, which are tumors thought to originate from notochordal remnants. By means of Western blots, the expression of galectin-3 was confirmed in tissue probes from the vertebral column region beginning with the 8th gestational week. These results were supported by immunohistochemical data which revealed the presence of galectin-3 in the cytoplasm of cells of the notochord also from the 8th gestational week onwards. Notochordal immunostaining became stronger with increasing gestational age. A persisting notochordal remnant in an adult intervertebral disc and various cells of the nucleus pulposus also contained galectin-3. All chordomas showed moderate or strong immunoreactivity irrespective of their cellular composition. Subcellularly, galectin-3 was localized mostly in the cytoplasm, while a subset of tumor cells also showed nuclear distribution. Differences in staining patterns of chordoma cells could not, in general, be correlated to any histological features of these tumors.

Plasma membrane targetting, vesicular budding and release of galectin 3 from the cytoplasm of mammalian cells during secretion

Galectin 3, a 30 kDa galactoside-binding protein distributed widely in epithelial and immune cells, contains no signal sequence and is externalized by a mechanism independent of the endoplasmic reticulum (ER)-Golgi complex. We show here that hamster galectin 3 overexpressed in transfected cos-7 cells is secreted at a very low rate. A chimaera of galectin 3 fused to the N-terminal acylation sequence of protein tyrosine kinase p56(lck), Nt-p56(lck)-galectin 3, which is myristoylated and palmitoylated and rapidly transported to plasma membrane domains, is efficiently released from transfected cells indicating that movement of cytoplasmic galectin 3 to plasma membrane domains is a rate limiting step in lectin secretion. N-terminal acylation is not sufficient for protein secretion since p56(lck) and the chimaera Nt-p56(lck)-CAT are not secreted from transfected cells. The amino-terminal half of galectin 3 is sufficient to direct export of a chimaeric CAT protein indicating that part of the signal for plasma membrane translocation lies in the N-terminal domains of the lectin. Immunofluorescence studies show that Nt-p56(lck)-galectin 3 aggregates underneath the plasma membrane and is released by membrane blebbing. Vesicles of low buoyant density isolated from conditioned medium are enriched in galectin 3. The lectin is initially protected from exogenous collagenase but is later released in soluble protease-sensitive form from the lectin-loaded vesicles. Using murine macrophages, which secrete their endogenous galectin 3 at a moderate rate especially in the presence of Ca2+-ionophores, we were also able to trap a galectin 3-loaded vesicular fraction which was released into the culture supernatant.

Retargeting of cytosolic proteins to the plasma membrane by the Lck protein tyrosine kinase dual acylation motif

Several members of the Src family of protein tyrosine kinases have a N-terminal dual acylation motif which specifies their myristoylation and S-acylation. These lipid modifications are necessary for correct intracellular localisation to the plasma membrane and to detergent-resistant glycolipid-enriched membrane domains (GEMs). Using chimaeras of the Lck dual acylation motif with two normally cytosolic proteins (chloramphenicol acetyl transferase and galectin-3), we show here that this motif is sufficient to encode correct lipid modification and to target these chimaeras to the plasma membrane, as demonstrated by subcellular fractionation and confocal immunofluorescence microscopy of transiently transfected COS cells. In addition, the chimaeras are resistant to extraction with cold non-ionic detergent, indicating targeting to GEM subdomains in the plasma membrane. The dual acylation motif has potential for targeting proteins to specific plasma membrane subdomains involved in signalling.

Galectin-3 stimulates uptake of extracellular Ca2+ in human Jurkat T-cells

Galectin-3, a mammalian galactoside-binding protein, is not expressed in the Jurkat T-lymphoblastoid cell line. However, Jurkat cells express surface glycoprotein receptors for galectin-3, one of which is shown to be the glycosylated heavy chain of CD98 (4F2 antigen), a T-cell activation marker. Addition of galectin-3 to Jurkat cells triggers a sustained influx of extracellular Ca2+ in a concentration dependent manner. The induced increase in cytosolic [Ca2+]i is blocked by sugar hapten inhibitors of galectin-3. The galectin-3-induced effect is insensitive to voltage-gated Ca2+ channel antagonists such as prenylamine, nifedipine and diltiazem and to pertussis toxin but is inhibited by cholera toxin. The results suggest that galectin-3 released by accessory cells such as macrophages may bind in vivo to T-cell activation antigens and also participate in Ca2+ signalling.